Abstracts No.59: 15th AGC Understanding Planet Earth, 2000, Sydney

15ih Australian Geplogical Convention Sydney, July 3rci-7tii, 2 0 0 0 University of Teciinoiogy, Sydney

ABSTRACTS No: 59

Geological Society of Australia

ABSTRACTS Number 59

SEARCHING FOR A SUSTAINABLE FUTURE

15"" Australian Geological Convention Sydney,

- 7'" July, 2000

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ISSN 0729-011X © Geological Society of Australia Incorporated, 2000 Copies of this publication may be obtained from the Geological Society of Australia Incorporated, 301 George Street, Sydney, NSW, Australia, 2000.

This volume should be cited as: Skilbeck, C.G. and Hubble, T.C.T. (editors), 2000. Understanding Planet Earth: Searching for a Sustainable Future. Abstracts of the 15th Australian Geological Convention, University of Technology, Sydney, NSW, Australia. July 3-7, No 59, 577p.

Example citation for papers in this volume Zwingmann H., Wilson T., and Offler R., 2000. K-Ar dating of fluid flow and fault movement in clay rich gouge: an example from the northern Sydney Basin. In Skilbeck, C.G. and Hubble, T.C.T. (editors) Understanding Planet Earth: Searching for a Sustainable Future. Abstracts of the 15th Australian Geological Convention, University of Technology, Sydney, NSW, Australia. July 3-7, No 59, p.567.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Contents of Abstract Volume Listed Alphabetically by First Author under Convention Themes (k) keynote speaker; (p) oral and poster presentation; (author in brackets) nominated speaker where this is not the first author et al. used for all papers with more than one author For a full list of authors see the Author Index given at the rear of the volume.

Theme 1: The Australian Plate Friday, Rm 6/328 BERNARDEL, G et al. Recent swath mapping of the southern Macquarie Ridge complex: poster seafloor characteristics and tectonic development BROWN, BJ et al. Revisiting the tectonic evolution of the Australian southern margin poster poster Testing Solomon Sea subduction models using geodynamics CLARK, S et al. PM2 Crustal thickness patterns in the Australian continent COLLINS, C et al. AM2 Earth-science datasets for the Australian continent online: the COX, SJD AGCRC map-maker, Open GIS and beyond poster Microcontinent formation around Australia GAINA, C et al. PMl Fission track imaging of the Australian continent GLEADOW, AJW et aL AM2 Towards improved understanding of the deep structure of the GONCHAROV, A Australian northwest margin from combined refraction/reflection et aL seismic studies poster GRAEBER, F et al. The three-dimensional upper mantle p-wave velocity structure of western Victoria poster Aeromagnetic evidence of possible hotspot activity in Australia GUNN, PJ AMI Geodynamic history of the Australian region GURNIS, M (k) AM2 Mesozoic-Tertiary evolution of the northern Australian margin HILL, KC PMl The Australian stress map HILLIS, Ret al. HOUSEMAN, GA Stability of continental lithosphere under transpression: the northern AM2 margin of the Australian continent. poster HOUSEMAN, GA MALT: the Murray Basin, Adelaide and Lachlan Fold Belt et al. teleseismic survey (1998-2000) PM2 The mantle beneath Australia KENNETT, BNL PM2 The electrical conductivity structure of the Australian continent a LILLEY, FEM et al. review AM2 McCUE, KF et al. Models of earthquake processes for seismic hazard assessment in Australia AMI The dynamic evolution of the Australian continent since the early MORESI, L et al. Archaean: what can we learn from dynamic modelling? AMI The Big Rip: Tectonic consequences of east-west Antarctic MULLER, RD et al. separation in the Cenozoic PMl O'REILLY, SY. et al. 4-D lithosphere mapping in Australia and global comparisons Regional lithospheric domains in Australia from gravity modelling PM2 POUDJOM DJOMANI, Y et al. SANDIFORD, M Neotectonics of southeast Australia and the origin of the intraplate PMl stress field. TREGONING, P Intraplate deformation of the Australia, Pacific and South Bismarck AMI Plates from GPS observations ZHANG, M et al. Large-scale global convection in the mantle beneath Australia from poster 55 Ma to now

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

Theme 2: Plate and Intraplate Processes on Planet Earth and Present Day Plate Boundaries Topic: 2.1: SE Asia, SW Pacific and PNG Plate Reconstructions Thursday, Rm 6/320 Paleomagnetic data from the oceanic Poya and Koh terranes, New AM2 ALI, J et al. Caledonia, and their significance for modelling the tectonic history of the southwest Pacific SHRIMP zircon dating of granitoids from Myanmar: constraints on AMI BARLEY, ME et al. the tectonic evolution of southeast Asia PMl Geochemical comparison between the Oligocene volcanic BINNS, RA et al. sequences in the Finisterre Mountains, New Britain and New Ireland, Papua New Guinea BURBIDGE, DR et al. Numerical models of the evolution of accretionary wedges and fold- poster and-thrust belts FINDLAY, RH et al. The Finisterre Volcanics and tectonics of northern Papua New AM2 Guinea - the true story HALL, R (k) Cenozoic history of southeast Asia and the southwest Pacific: origin AMI and evolution of island arcs poster JAMES, P Some geological fallacies PMl MAGEE, C et al. Diamond and chromite geochemical constraints on the Dachine Complex, French Guiana AMI MUSGRAVE, RJ The Australia-Pacific plate boundary at 25 Ma AM2 PARR, JM et al. Lead isotope characteristics of volcanic rocks and hydrothermal sulfide deposits from the eastern manus Basin and Franklin Seamount, Papua New Guinea: evidence for the Pacific - Indian mantle boundary AM2 PEREMBO, RCB et al. Port Morseby basement geology: a mid-Cainozoic accretionary prism AM2 RAWLING, TJ et al. Tertiary ophiolite emplacement, mountain building and high pressure metamorphism in New Caledonia, eastern Australian plate margin Thermobarometry of clinoproxene in ankaramite lava from the poster ROACH, I Monaro Volcanic Province, southeastern N.S.W. WHATTAM, S et al. Trace element geochemistry of the Northland Ophiolite, northern poster New Zealand: new constraints on tectonic environment of formation AMI WINDLEY, BF et al. Terrane accretion in central Asia: constraints from Mongolia

Topic: 2.2: Plate Tectonics and Convergent Margin Processes Thursday, Rm 6/320 JAGODZINSKI, E The Turondale Formation, Hill End Trough, N.S.W.: facies interpretation and palaeogeography JAMES, P Some myths of mobilism KIMURA, GK (k) The life of a subduction zone - mass and tectonic balance in the subduction zone MEFFRE, S et al. Ridge subduction in the New Hebrides intra-oceanic island arc: implications for the recognition of collision events in ancient arcrelated sequences. SCHELLART, WP Analogue modelling of back-arc extension et al.

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Topic: 2.3: Geochemistry and Petrology Monday, Rm 6/320 A characteristic trace element signature in the Jurassic midalkaline Poster ALLEN, TC gabbros of south-eastern Australia AMI Use and abuse of the term calcalkaline ARCULUS, RJ AM2 Felsic volcanism and related sedimentation in a late Archaean BROWN, S et al. convergent margin environment. Eastern Goldfields Province, Western Australia AMI An Ockham's razor style review of the inappropriate use of CAPRARELLI, G geochemical diagrams in tectonic reconstructions GILMORE, T et al. Petrological and geochemical significance of felsic veins within the AMI gabbroic lower ocean crust drilled during ODP Leg 176 at the southwest Indian ridge The north Australian low-Ti Antrim Plateau flood basalt province AM2 HANLEY, L

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MACKIE, S et al. McBRIDE, J et al. NEUMANN, N et al. SIVELL, W et al. SQUIRE, R

Abundances and distribution of trace elements in the lower ocean crust: a study of seismic layer 3 gabbros drilled during ODP Leg 176 at the southwest Indian Ridge Re-Os isotopic evidence for crust-mantle interaction in the genesis of continental intraplate basalts from the newer volcanics province, southeastern Australia Tracking the evolution of heat-producing elements in the South Australian Proterozoic Geochemistry of the Ebor intraplate central volcano: implications for plume-lithosphere interaction. Pliocene volcanics in Fiji: a window to understanding the Ordovician volcanics of the Lachlan Fold Belt

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Topic 2.4: Volcanic Fades and Processes Monday, Rm 6/320 Water-settling and resedimentation of submarine rhyolitic pumice at PMl ALLEN, S et al. Yali, Greece JAGODZINSKI, E Crystal-rich volcaniclastic megaturbidites of the Turondale and PMl et al. Merrions Formations, Hill End Trough, N.S.W.: arguments for pyroclastic flow disintegration upon entering the sea. McPHIE, J et al. Extensive felsic lavas in intraplate volcanic provinces PMl MOORE, CL et al. Products of explosive felsic volcanism within the basalt lava pile of poster the Kerguelen Plateau Large Igneous Province, southern Indian Ocean MOORE, CL et al. Characteristics and origin of marine ash layers from the Kerguelen poster Plateau, southern Indian Ocean (ODP Leg 183) POLONSKA, D Intraplate seismicity within south-western part of Kanto Plain, Japan PM2 - manifested in heterogeneous subsurface zones, revealed through geomorphological hypotheses SIMPSON, C et al. The Goonummbla Caldera, Parkes, N.S.W.: fact or fiction? PMl Subduction beneath the North Island of New Zealand: geodynamics PM2 UPTON, P et al. of compression and extension The Phanerozoic thermotectonic evolution of the northern Western PM2 WEBER, U et al. Australian shield: evidence from apatite fission track thermochronology

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Theme 3: Orogenic Belts as a Record of Past Processes of Planet Earth Topic: 3.1 Precambrian and Phanerozoic Orogenic Belts Tuesday, Rm 6/322 BLACK, LP et al. The quest for a high-quality zircon standard for microbeam Pb-U-Thposter geochronology poster BLEWETT, RS Horizontal vs. vertical tectonics and the dome-and-basin geometry of the Archaean Pilbara Craton, Western Australia: Implications for mineral deposits CAMACHO, A et al. Intracratonic strike-slip partitioned transpression and the formation AM2 and exhumation of eclogite facies rocks: An example from the Musgrave Block, Australia PM2 CHEN, SF et al. Archaean tectonism through time in Western Australia poster CHO, M et al. Polycyclic orogeny in central Ogcheon Metamorphic Belt, Korea: evidence from 40ar/39ar hornblende ages PMl CONOR, C et al. The Walter-Outalpa shear zone eastern Weekaroo Inlier, Olary Block, South Australia AM2 FORSTER, M et al. Extensional shear zones formed during Gondwana Breakup in the Otago Schist, New Zealand AM2 GIBSON, G Low-angle structures in the Broken Hill block, NSW: legacy of thrust tectonics or extensional collapse? AMI GOLEBY, BR et al. 3D crustal structure- thrust stacks and gold pathways within the Archaean granite-greenstones of the eastern goldfields. Western Australia: as revealed by deep seismic reflection profiling poster GOLEBY, BR et al. Towards the third dimension in the eastern Goldfields, WA from combined seismic reflection and potential fields GREENFIELD, JE Relative age of felsic magmatism and deformation of the central and PM2 Et al. eastern Yilgam Craton, Western Australia PM2 HENDRICKX, MA The Tanami Region: a possible transpressed Palaeoproterozoic et al. passive margin in northern Australia. PMl Relative timing of deformation, metamorphism and mineralisation HILLS, Q et al. within the Willyama Complex. AMI High-pressure metamorphism in the Zambezi Belt, northern JOHNSON, S Zimbabwe: evidence for pan African-aged continental collision between the Zimbabwe and Congo cratons. AMI KELLY, NM et al. Proterozoic reworking of Archaean crust in the Rayner Complex, east Antarctica AMI LISTER, GS et al. Episodicity during orogenesis AM2 MCLAREN, S et al. Carboniferous tectonism at Mount Painter-the Alice Springs Orogeny moves south poster OLIVER, R et al. Mafic dykes in Antarctica and southern Eyre Peninsula AM2 PAGE, RW et al. New SHRIMP zircon results from Broken Hill: towards robust stratigraphic and event timing PMl RAETZ, M et al. Three tectonostratigraphic cycles in the Broken Hill Terrane based on a review of SHRIMP data - comparison with northern Australia REDDY, SM et al. Dynamic linkage between internal zone extension and external zone AMI shortening in the European Alps REDDY, SM et al. Palaeoproterozoic kinematics in the southern Capricorn Orogen, poster Western Australia SCRIMGEOUR, I High-t, low-p metamorphism in the Mopunga Range, eastern AruntaPM2 et al. TAYLOR, WR et al. Inlier: an upper crustal expression of the Late Strangways event Geochemistry of mantle xenoliths and xenocrysts from the Skerring poster Kimberlite, Western Australia - evidence for a - 8 0 0 ma lithosphere re-fertilization event TEALE, GS et al. Hydrothermally altered Archaean felsic volcanics from the Mt. poster Hope area. Eyre Peninsula, South Australia. WATERS, CL et al. AMS fabric in granulites: Mount Hay-Mount Chappie region, poster Arunta Block, central Australia ZANG, W-L et al. Timing of deformations and magmatism of the southeastern Gawler PMl Craton, Yorke Peninsula, South Australia

Topic 3.2: Processesy Geometries and Rates of Deformation Processes Monday, Rm 6/322 BELL, B et al. Detailed gravity as an aid for constraining crustal structures interpreted from deep seismic reflection profiling with the eastern goldfields. Western Australia

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

BIERMEIER, C et al. The rotation rate of garnets during simple shear AM2 COX, SF et aL Fluid -driven faulting processes in an intrusive-related hydrothermal AM2 system, Porgera, Papua New Guinea FINLAYSON, DM Monitoring plate margin earthquake and volcanic events: a major poster et al. upgrade of the Philippines seismic recording network HERMANN, J et al. Exhumation as fast as subduction? AM2 REDDY, SM et al. The systematic assessment of deformation histories in fracture AMI systems REY, PF et al. Was the continental lithosphere desperately flat in the Archaean? AM2 ROBIN, P-Y et al. Low viscosity shear zones within broader ductile transpression AMI zones SCHELLART, WP Cohesion and fi-iction coefficients of dry granular materials poster SMITH, JV Kinematics of brittle deformation in a pluton roof zone, northeast AMI New South Wales WILSON, T et al. Interpretation of brittle microstructures AMI ZWINGMANN, H K-Ar dating of fluid flow and fault movement in clay rich gouge: an AMI et al. example from the northern Sydney Basin

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Symposium 3A: Precambrian Supercontinents Monday, Rm 6/344 BETTS, PG et al. 1.8 Ga to 1.1 Ga evolution of the Australian continent: a northern, AMI p.34 central and eastern Australian perspective BURRETT, CF et al. Proterozoic Australia-Laurentia Fits: AUSWUS An Alternative To AM2 p.66 Sweat COLLINS, AS et al. The tectonic architecture of central Madagascar - disentangling the PMl p.90 amalgamation of Gondwana DALZIEL, IWD (k) Global palaeotectonics: reconstructing a credible supercontinent PM2 p.ll4 EVANS, DAD et al. Toward a quantitative reconstruction of Kenorland, the ArcheanAMI p. 144 Paleoproterozoic supercontinent FAGAN, RK Archaean obduction tectonics poster p. 147 FITZSIMONS, I The circum east Antarctic mobile belt—three different GrenvilleAM2 p.l5I age orogens juxtaposed during Gondwana assembly GILES, D et al. The Early to Middle Proterozoic configuration of Australia and its AM2 p. 174 implications for Australian-US relationships HUTTON, L et al. Late Mesoproterozoic and Neoproterozoic to Cambrian crust in PM2 p.245 north Queensland- implications for continental reconstructions KEMP, TIS et al. Genesis of peraluminous granitic rocks: a source-based perspective poster p.274 KINNY, PD et al. The Lewisian Complex-a collage of disparate terranes assembled AMI p.279 during the Palaeoproterozoic era LI, Z-X et al. Neoproterozoic magmatism, crustal unroofing, and continental PM2 p.302 rifting in Australia and south China: results of a mantle plume at the start of Rodinia breakup MOORES, E (k) Pre-Rodinian ophiolites: a progress report AM2 p.353 PISAREVSKY, SA Rodinia: building blocks and palaeomagnetic constraints PMl p.392 et al. POWELL, C.McA (k) 2000 IGCP lecture: The lost Mozambique Ocean PM1 p.397 POWELL, C.McA 2000 Mawson Lecture: The Search for Supercontinents Evening p.398 TAKANO, K et al. Meso-late Proterozoic igneous activity in the southwest periphery of poster p.488 the Siberian Craton. WINDLEY, BF (k) A supercontinent at the Archaean-Proterozoic boundary AM 1 p.546 ZHAO, G et al. Review of 2.1-1.8 Ga orogens and cratons in North America, poster p.565 Baltica, Siberia, central Australia, Antarctica, and north China: a pre-Rodinia supercontinent?

Symposium 3B: Evolution of the Eastern Australian Tasmanides After Breakup Tuesday-Wednesday-Thursday, Rm 6/344 ANDERSON, K A new mid-Carboniferous palaeomagnetic pole from northern poster p. 10 Australia: further constraints on Gondwana's apparent polar wander path BERTON, JR et al. Classification of sedimentary units using radiometric and DTM poster p.33 data: trial application to middle-upper Devonian sediments of the Barka Basin, NSW BROWN, MC Late Devonian and Carboniferous deformations in east-central Tu-PMl p.59 Victoria

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CAPRARELLI, G et al. CAYLEY, R et al.

CLARK, DA et al.

COLLINS, WJ et al. COOK, M CROSS, AJ et al. DIREEN, NG et al. DIREEN, NG (P Lyons) et al. DURNEY, DW EVERARD, JL FODEN, J FUREY-GREIG, T et al. GLEN, RA et al.

A Mariana arc analogue for Late Devonian metabasites in the W-PMl p.70 Nundle district (NSW) The western Lachlan Fold Belt, southeast Australia -a doubly Tup.73 vergent fold belt formed by continental collision with an intervening AM2 weak zone Early Permian palaeomagnetic poles from Australia: the Mount W-PMl p.81 Leyshon intrusive complex and the Tuckers Igneous Complex, north Queensland Late Ordovician-Early Silurian crustal thickening in the east/central Tu-PM2 p.92 Lachlan Fold Belt: Implication for S-type granite genesis p.94 ThHigh arsenic granites in the New England Batholith AMl An interpretation of the eastern Lachlan region using the PREDICT W-AM2 p . l l O geological information system W-AM2 p. 132 "Annelidical geometry": A 3-D geological and geophysical appraisal of major structures in the eastern Lachlan Orogen using multi-scale edge analysis W-AM2p.l33 The turn of the worms: mineral systems implications of crustal structures in the eastern Lachlan Orogen derived from multiscale edge analysis W-AMl p.I38 Problems of deformation style in the northern Lachlan Fold Belt p. 145 Neoproterozoic basalts from northwest Tasmania: multiple mantle TuAM2 sources during continental rifting. p. 153 Crust-Mantle Interaction and the Formation of Granitic Terranes in TuAMI eastern Gondwanaland Early Palaeozoic arc basin sequence in the Tamworth Belt: W-PM2 p. 163 constraints for east Gondwana tectonics

Seismic evidence for a major arc-backarc collision in the Lachlan W-AMl p.177 Orogen p. 189 TuEastern Australia crustal evolution: timing and temporal links AMl between Delamerian and Lachlan Orogens, including Tasmania p.l88 Geology of the Glenelg River Complex Segment of the Delamerian TuGRAY, CM et al. AMl Orogenic Belt W-PM2 p.220 Hunter-Bowen orogenesis in the Hodgkinson province of the HENDERSON, RA northern Tasmanides et a l Tup.234 A structural and metamorphic profile of the Arthur lineament and HOLM, OH AM2 surrounds, northwest Tasmania W-AMl p.235 Sequence and kinematics of multiple deformation in the Taemas HOOD, DAI et al. Bridge area, N.S.W.: a preliminary outline W-AMl p.247 Re-evaluation of the geochronology of Nymagee 1:250,000 sheet ISAACS, D et al. Tup.275 The link between crustal anatexis and mafic magmatism for the KEMP, TIS et al. AMI petrogenesis of granitic rocks: an example from western Victoria W-PMl p.280 Late Palaeozoic evolution of the southern New England Orogen: KLOOTWIJK, C palaeomagnetic constraints from the rocky creek block of the Tamworth Belt A deep seismic reflection profile across the Gilmore Fault, eastern W-AM2p.281 KORSCH, RJ et al. Lachlan Orogen LACKIE, MA et al. Synthesis of palaeomagnetic results from Siluro-Ordovician rocks ofTu-PM2 p.289 the Lachlan Fold Belt LACKIE, MA Palaeomagnetic investigation of late Palaeozoic volcanic units from poster p.288 the northern New England Orogen, Queensland LANDENBERGER, Gabbroids, basalts & lamprophyres of the New England Batholith: a Thp.290 BB et al. legacy Late Carboniferous - Middle Triassic arc migration AMI LEITCH, EC et al. The structural geology of the Kindee district and the age and timing W-PM2 p.297 of displacement of the Hastings Block, southern New England Fold Belt W-AMl p.298 LENNOX, PG et al. Using geophysics and geology to determine the upper crustal architecture in the northern part of the eastern Lachlan Fold Belt poster p.315 LYONS, P et al. Fault controls on gold mineralisation in the Young Granodiorite poster p.316 MAAS, R et al. Quenched melt inclusions in phenociysts of a Silurian dacite (Lachlan Fold Belt): the petrogenesis of mafic S-type magmas W-PMl p.368 OFFLER, R et al. Change in tectonic setting of volcanics in the New England Fold Belt ,1Q during the Late Silurian to1 Late Devonian 40 /in . . ^ , , , . , , . • , Ar/^^Ar and K-Ar whole rock age constraints on the timing of Tu-PM2 p.387 PHILLIPS, D et al. regional southeastern Australia: problems and implications. RAYMOND, O et al. The Jemalong Trough: an extension of Silurian-Devonian rifting in W-AM2 p.409 the eastern Lachlan Fold Belt GRAY, D et al.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

REED, A

Timing and deformation of allochthonous Proterozoic and Tup.413 Palaeozoic rocks in the Badger Head region, northern Tasmania. AM2 New 1:25,000 geological map of the eastern Tamworth Belt, poster p.449 Manilla, southern New England Fold Belt SIRCOMBE, KN et al. Characterisation of zircons in Palaeozoic red beds at Bombala, poster p.453 N.S.W.: implications for sediment age and provenance SIVELL, W et al. Nd isotopic constraints on the evolution of the Gympie Province: a W-PM2 p.455 portion of an island arc-Gondwana rim accretion zone SIVELL, W et al. Geochemical and petrogenetic relations among Triassic Stanthorpe Thp.456 I-type granitoids AMl SPAGGIARI, CV et al. Oceanic setting and subduction-related tectonics for the central Tu-PMl p.468 Lachlan Orogen, southeastern Australia. SPRIGG, AG et al. The Bermagui dykes: geochemical affinities and relationships to poster p.469 structure in the se Lachlan Fold Belt TALUSANI, RVR The Wateranga Intrusion, southeast Queensland, Australia: a high- poster p.490 et al. Al and low-K mafic, tholeiitic layered intrusion TAYLOR, DH et al. The Delamerian Orogeny in western Victoria: consequence of arc- Tup.492 continent collision AM2 VANDENBERG, Volcanic history of the Buchan Rift, southeastern Lachlan Fold Belt Tu-PMl p.509 AHM WATANABE, T et al. The Tasman Fold Belt System in the Late Neoproterozoic: insights Tup.520 from study of Mt Arrowsmith volcanics and Attunga eclogite AMl WILLMAN, CE et al. Evolution of the eastern Lachlan Fold Belt in Victoria-implications Tu-PMl p.543 for oblique convergence WITHNALL, IW et al. Late Palaeozoic magmatism in the northern New England Orogen - Thp.550 evidence from u-Pb shrimp dating in the Yarrol and Connors AMl provinces, central Queensland SHARP, TR et al.

Topic 3C: Pacific Connections of Gondwana in the Palaeozoic and Mesozoic Thursdayy Rm 6/344 ADAMS, CJ Provenance of Permian-Triassic volcaniclastic terranes in New PM1 Zealand from their detrital zircon age patterns BASSETT, K Isotopic provenance and terrane tectonics: a warning about sediment poster transport distances BRADSHAW, JD Intrusion of the Balloon Melange: age, origin and implications for AM2 et al. Cambro-Ordovician regional tectonics BRADSHAW, JD Termination of subduction on the Australian-New ZealandPM 1 Antarctic margin in the Late Mesozoic CRAWFORD, AJ et al. Cambrian arc evolution along the SE Gondwana active margin: A AM2 Tasmania-New Zealand perspective DALZIEL, IWD et al. Plumes, orogenesis and supercontinental fragmentation PM2 KIM, H et al. Thermal metamorphism of volcanic rocks on Barton Peninsula, PM2 King George Island, Antarctica MALLOCH, KR et al. Cape Foulwind Supersuite: Carboniferous A-type granites of AM2 Western New Zealand and Pacific margin connections NORVICK, MS et al. The separation of Australia from Antarctica and its stratigraphic PM2 signal in southern Australian basins NORVICK, MS et al. A comparison of chronostratigraphic cross sections through the deep poster water Otway and Bight Basins, southern Australia OCH, D et al. The nature and tectonic evolution of the Port Macquarie Complex, AM2 northeast New South Wales RAPELA, CW et al. Age and origin of coeval TTG, I- and S-type granites in the AM2 Famatinian Belt of northwest Argentina ROSSELLO, E et al. The Cretaceous Trans-Argentine aborted rift corridor and its PM2 associated alkaline volcanism. SIVELL, W et al. Geochemistry and Nd-isotope systematics of sediments from the poster Dun Mountain Ophiolite: I - Chemical Sediments SIVELL, W et al. Geochemistry and Nd-isotope systematics of sediments from the poster Dun Mountain Ophiolite: II - Terrigenous Sediments STEVENSON, JA (p) Contact relationships and post-intrusion loading of the western PM 1 et al. Fiordland orthogneiss, southwest New Zealand WANDRES, AM et al. Significance of igneous clasts - The Torlesse Terrane, South Island, PM 1 New Zealand. WANDRES, AM et al. A load of old cobbles - evidence of recycling of a terrane. The poster Torlesse Terrane, South Island, New Zealand

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Symposium 3D: The Ron Vernon Symposium Topic 3D1: Microstructural Processes Tuesday, Rm 6/320 CESARE, B et al. Growth of myrmekite coronas by contact metamorphism of granitic poster ultramylonites in the aureole of Cima Di Vila (Eastern Alps - Italy). AMI GOODWIN, L et al. Competency contrast, strain localisation and microstructural evolution at a variety of crustal levels poster Deformation gradients and rate of metamorphic transformation: a key to identify tectonometamorphic units (Austroalpine units, Central Alps, Italy) JESSELL, MW Numerical simulation of transient and steady-state microstructures AM2 AM2 JOHNSON, SE Porphyroblast microstructures - some central debated issues PM2 KRUHL, JH Foam textures in quartz, plagioclase and analogue material: an investigation in textural equilibrium poster LAVERY, PE et al. Fluid flow within the marbles and calc-silicates of the Harts Range Group, central Australia PMl MANCKTELOW, NS Deformation of partially molten rock: numerical modelling of the interplay between strain localization and melt formation poster MARMO, BA Bulk rock fractionation during garnet growth and its effect on the assemblage of eclogite from Pam Peninsula, New Caledonia. poster McLELLAND, J et al. Quartz-sillimanite veins and nodules associated with the late leucogranites, Adirondack Highlands, New York State, USA. PMl MEANS, W Infiltration melting in an analog system PATERSON, SR et al. Inclusion trail patterns in porphyroblasts from the foothills terrane, AM2 California: a record of orogenesis or local strain heterogeneity? poster PENNACCHIONI, G Strain insensitive preferred orientation of porphyroclasts in Mont et al. Mary mylonites AMI PRIOR, D et al. Rock microstructures in the new millenium ROBIN, P-Y et al. Modelling the propagation of pressure solution cleavage seams and AMI their interactions PMl RUSHMER, T Fractures and flow: an experimental look at melt segregation GOSSO, G et al.

processes in the earth's crust TIKOFF, B et al. Competency contrast, kinematics, and development of foliations and AMI lineations WILLIAMS, PF et al. Garnet rotation and the interpretation of the l a y Nappe AM2 WILLIAMS, M et al. Micromapping, age mapping, and chemical dating on the electron AM2 microprobe: powerful tools for microstructural analysis WINTSCH, RP et al. The origin of foliation: microstructural and microchemical evidence AMI for the role of pressure solution and pressure solution creep WORLEY, BA et al. Deformation and the preservation of pressures: relative barometry poster constraints from Peterman age shear zones in the western Musgrave Block. PMl YOSHINOBU, AS Deformation of partially molten gabbro beneath oceanic spreading et al. centers: olivine as a rheological "Rosetta Stone" poster Reaction-enhanced permeability during decarbonation of calcite + ZHANG, S et al. quartz = wollastonite + carbon dioxide poster Three-stage symplectite or corona growth in the Hengshan highZHAO, G et al. pressure granulites (retrograded eclogites). North China Craton and tectonic implications

Topic 3D2: Metamorphic Processes Wednesday, Rm6/322 A geochemical classification scheme for granitic rocks poster BARNES, CG et al. Magmatic fabrics in plutons as kinematic indicators in convergent poster BENN, K orogens: examples from the cascades core (Washington state) and the Meguma Terrane (Nova Scotia) poster Magma migration and mixing processes of felsic magma in the BONNAY, M et al. middle crust, example of Mount Hay, central Australia AMI Prograde-Retrograde Evolution in Relation to P-T-t paths, the BROWN, M Metamorphic Volatile Phase and Melting Regimes CARMICHAEL, DM Deformation of Slate and Schist by Difftision Metasomatism during PM2 Prograde Metamorphism Melting of Ti -rich biotitie in natural, quartz present and quartz free AM2 CESARE, B assemblages: microstructures, phase chemistry and melting reactions.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15''^ Australian Geological Convention, Sydney, July 2000

CLARKE, GL et al.

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Equilibration in eclogite and blueschist facies rocks: constraints PMl from cation maps Geochemical contrast between igneous phases within the poster Croajingolong Plutonic Complex, southeastern Australia: implications for magma genesis DEGELING, H et al. Zircon formation during garnet breakdown in Rogaland, south AM2 western Norway GIBSON, RL et al. Multiple mechanisms of melting in granulites in the Vredefort AM 1 Dome, South Africa HARLEY, S Cordierite as a sensor of high-grade metamorphic processes AM2 HOADLEY, E et al. Magma mingling, mixing, and filter-press fractionation within an poster active shear zone, Ravenswood Batholith, northeast Queensland. HUTTON, L (p) High-temperature medium-pressure crustal anatexis in the Cape AM2 River area - evidence for Ordovician orogenesis in the western Lolworth-Ravenswood Province. JAMIESON, RA et al. Interaction of metamorphism and deformation: crystal- to crustalAMI scale KRABBENDAM, MM Reaction-hardening during exhumation of felsic eclogite PM2 MORRISON, J et al. Origin and significance of late- to post-tectonic leucogranite and PM1 associated quartz-sillimanite nodules in the Adirondack Mountains, NY, USA PATTISON, DRM Figuring out Ron's world: metapelites at low pressure PMl et al. Multistage magma mixing: evidence from an Archaean composite poster PAWLEY, MJ et al. laccolith Calculation of phase relations involving haplogranitic melts using AM2 POWELL, R et al. an internally-consistent thermodynamic data set The smectite-to-chlorite transformation: a model for contrasting PM2 ROBINSON, D reaction pathways and reaction progress Albitization of pelitic and psammitic rocks in the Sanke Creek PM2 RUBENACH, MJ et al. Anticline, Mt Isa Inlier - structural controls and p-t-t Heat production distributions, tectonic styles and the thermal AM 1 SANDIFORD, M evolution of the continental lithosphere VIGNERESSE, JL Regional stress field interaction with magma emplacement poster et al. Metamorphic processes revisited: assessing the extent of AM 1 WATERS, D et al. disequilibrium in prograde reactions in metapelites Mineral equilibria calculations in the system K 2 0 - F e 0 - M g 0 PM 1 WHITE, R et al. Al203-Si02-H20-Ti02-Fe203 and application to Fe-rich metapelitic granulites of the Musgrave Block, central Australia.

Topic 3D3: Magma Emplacement Processes Thursday, Rm 6/322 BARNES, CG et al. Evidence for magma mixing in the three cordilleran plutons, western north America CLARKE, B et al. The Mouton Shear Zone: Interaction of a crustal scale fracture with a crystallising granitoid pluton DACZKO, NR et al. Simultaneous melting and fracturing of the lower crust, Fiordland, New Zealand. FLOOD, RH et al. The Walcha Road Adamellite: A large zoned pluton in the New England Batholith Australia GREEN, TH Pressure effect on accessory phase saturation in evolved granitic melts with differing K/Na ratios. GRIFFIN, WL et al. Tracking magma mixing: in-situ Hf-isotope analysis of zircons HAWKINS, DP et al. Geological signatures of explosive eruptions in granitic plutons: examples from coastal Maine, USA HEALY, B et al. Chemical effects of squeezing a granitic crystal mush during compressive deformation: the Kameruka Pluton, Lachlan Fold Belt Crystallization of orbicular granite HOGAN, JP et al. METCALF, RVR et al. Coeval tertiary volcanic and plutonic rocks, Colorado River extensional corridor (CREC), USA: implications for crust-mantle interaction and magma mixing as a petrologic process MILLER, CF Contrasting stratified plutons exposed in tilt blocks, Eldorado Mountains, Colorado River Rift, Nevada, USA POLI, G et al. The role of chaotic dynamics in the petrogenesis of granitoid rocks: multiscale observations and analyses

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RUSHMER, T et al. SAWYER, E SEAMAN, S et al.

Textures in melt-solid systems: experimental deformation of partially molten rocks. Diatexite or metatexite? The importance of melt generation rate Survival or fragmentation of basaltic enclaves during pyroclastic eruptions and the origin of andesitic ignimbrites Crustal melt segregation: the need for deformation

VIGNERESSE, JL et aL WEINBERG, RF et al. Compositional and thermal convection within the Tavares Pluton, northeast Brazil WIEBE, R Crystal accumulation in granitic plutons: solidification by sedimentation

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Topic 3D4: Magma Chamber Processes: Controls on Granite Petrogenesis Friday, Rm 6/322 BENN, K et al. Analogue scale models of pluton emplacement during transpression AM2 in brittle and ductile crust BROWN, M The switch from magma ascent to emplacement during orogeny AM 1 CARR, S et al. Relationship between Middle Jurassic and Cretaceous - Middle AM2 Eocene deformation and plutonism in mid-crustal rocks of the Omineca Belt, southern Canadian Cordillera JOHNSON, SE Ring complexes - subvolcanic magma plumbing systems AM2 LOPEZ DE LUCHI, M Rock fabric and magnetostructure constraints on the emplacement AM 1 et al. of the Renca Batholith (Sierra De San Luis, Argentina) MILLER, RB et al. Pluton construction and emplacement in arcs: implications from the AMI Cretaceous Cascades Arc PATERSON, SR et al. The construction of magmatic systems in arcs: melt migration in AM2 disordered porous media independent of regional faulting RICHARDS, S W et al. The Cooma Complex: a crustal scale magma transfer zone of the AM 1 Murrumbidgee Batholith WEINBERG, RF Plutons and shear zones in northeast Brazil: a self organizing system AM 1 YOSHINOBU, AS Magma emplacement in the lower crust at oceanic spreading centers AM2

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Theme 4: The Mineral Resources of Australia and Its Near Neighbours Topic 4.1: Gold - Archaean and Palaeozoic Systems

Monday, Rm 4/G02 (University Hall) ADABI, MH BAKER, DE et al.

Carbonate-hosted gold deposit in Tasmania, Australia PM2 Geodynamic framework and characteristics of mineralizing fluids at poster the McPhee's Gold Deposit, Pilbara Craton, Western Australia BIERLEIN, FP et al. Disseminated sediment-hosted gold mineralisation - a new type of PM2 ore deposit in slate belts? BIERLEFN, FP et al. Comparisons between turbidite-hosted gold deposits in the Lachlan poster Fold Belt, the Otago Schist and Buller Terrane and the Meguma Terrane BRATHWAITE, RL Telescoping of porphyry Cu-Au mineralisation, advanced argillic poster alteration and polymetallic sulphide-gold-quartz-anhydrite veins in et al. the Thames Goldfield, New Zealand BUCCI, L et al. Two-Stage gold mineralisation at the Archaean Chalice gold PMl deposit, Yilgam Craton, Western Australia COOKE, DR et al. Phreatic explosions, breccia deposits and gold mineralisation in low poster sulfidation epithermal environments CORBETT, G poster Breccia environments in Pacific rim gold-copper deposits DAVIES, AGS et al. Overlaps between phreatic and phreatomagmatic brecciation AMI implications for the development of hydrothermal systems. An example from the Kelian Gold Deposit, East Kalimantan, Indonesia AM2 DAVIS, BK et al. Kanowna Belle Gold Mine - Anatomy and history of a plumbing system AM2 DUURING, P et al. A thrust-ramp model for gold mineralisation at the Archaean trondhjemite-hosted Tarmoola deposit, Leonora Camp, Western Australia FRICK, LR et al. Re-Os isotope systematics of the Radio Hill Ni-Cu-PGE complex, poster west Pilbara Craton PM2 HAGEMANN, S et al. Hematite in orogenic Archaean lode-gold deposits: vector to bonanza type gold mineralisation? HUSTON, M et al. Hydrothermal veins and alteration. Marvel Loch gold mine. Western poster Australia Golden cities - a new type of gold deposit in Archaean granite poster KEHAL, HS et al. Rethinking orogenic gold in the southern Lachlan Fold Belt PM2 MAHER, S et al. Exploration models for Cu-Au skams AMI MEINERT, LD (k) NEUMAYR, P et al. Stable isotope and fluid inclusion signatures of hydrothermal fluids AM2 in transcrustal fault zones: significance for orogenic, Archaean lodegold mineralisation Structural controls of fluid pathways during gold deposition at the AM2 RUMING, K et al. victory complex St Ives Goldfields, Kambalda, Western Australia Tectonic evolution and mineralisation of the east Kimberley region, Poster SHEPPARD, S Western Australia (L Hassan) et al. Poster SWEETAPPLE, MT Characteristics of tantalum-niobium-tin oxide minerals from the Wodgina and Mt. Cassiterite Pegmatites, Pilbara Craton, Western et al. Australia. AMI Porphyry and sedimentary-hosted gold deposits. Cygnet area, TAHERI, J et al. southeast Tasmania Repeated mobilisation of gold as a polymetallic melt at the TOMKINS, A et al. Poster Challenger Deposit, South Australia TRIPP, G et al. PMl Gold mineralised Archaean fault-fracture networks, Ora Banda, Western Australia VICKERY, N AM2 Plutonic Gold Mine WILDE, A et al. AMI The giant Muruntau Gold Deposit: Why is it so large? PM2 WITHAM, B The Magdala Gold Deposit, Stawell: tracing the evolution of the dynamics of a mesothermal gold-quartz system. Two separate hydrothermal events at Marvel Loch Gold Deposit, PMl WITT, WK et al. Western Australia

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Topic 4.2: Hydrothermal Fluids and Ore-Forming Processes Tuesday, Rm 4/G02 (University Hall) BASTRAKOV, E et al. Gold transport at Archaean lode gold deposits: gold solubility AMI along the rock-buffered pathways BROWN, M et al. Geometry, timing and temperatures of fluid flow in the Hamersley AM2 Province, Western Australia. AMI DAVIDSON, P et al. Melt inclusion studies of immiscible magmatic fluids: the first stage of hydrothermal fluids? DOUGLAS, N e t aL The Liquid Bismuth Collector model: an alternative gold AM2 deposition mechanism ELLISTON, JN The hydration of silica and its role in the formation of quartz veins AM2 JOHNSTON, A et al. Molybdenite trace elements as an indicator of granitic source AMI LIU, W et al. The stability of copper (I) acetate complexes in hydrothermal AM2 solutions between 50°c and 150°c MERNAGH, T et al. The source of gold in the western Lachlan Fold Belt: constraints AMI from mass transfer modelling RIDLEY, J Why geochemical tracers have failed us in our search for the AM I mesothermal gold ore fluid WEBB, AS et al. From BIF to ore: the chemistry of the Dales Gorge Member and AM2 surrounding shales across the Hamersley Province, Western Australia

Topic 4.3: Intrusive-Related Mineralisation - General and Eastern Australia Tuesday, Rm 4/G02 (University Hall) BEAMS, S et al. Mt Mackenzie: a large high sulphidation, advanced argillic gold PM2 system, central Queensland CORBETT, G (k) et al. Styles of Pacific rim gold-copper mineralisation PM1 FORSTER, Db et al. Skam Mineralisation at the World-Class Cadia Porphyry Au-Cu PM2 Deposits, NSW, Australia. HUSTON, DL et al. Archaean epithermal deposits in the Pilbara Craton, Western PMl Australia LICKFOLD, V The intrusive history of the Goonumbla porphyry Cu-Au deposits, PM2 N.S.W. LOWDER, GG et al. Tooloom - a newly recognised, intrusive related, epithermal gold PM2 field in northeastern New South Wales MEINERT, LD The role of magmatic fluids in skam and porphyry systems PMl

Topic 4.4: Proterozoic Mineralisation Thursday, Rm 4/G02 (University Hall) The enigma of the Broken Hill quartz-albitites AM 1 FORBES, C et al. AM 1 GUNTON, CG et al. Determination of fluid origins from trace element analysis of molybdenite at Ernest Henry, northwest Queensland Mount Kelly: a Mount Isa-type copper-gold deposit in the western AMI MAIDEN, K et al. Mount Isa Block, Queensland Partial melting of the Broken Hill ores: the smoking gun AM 1 MAVROGENES, J et al. SKIRROW, RG et al. Timing of Cu-Au (-Mo) and regional sodic-calcic alteration in the AMI Olary - Broken Hill region: molybdenite Re-Os and titanite U-Pb dating constraints

Topic 4.5: Modern and Ancient Seafloor Systems Thursday, Rm 4/G02 (University Hall) AGNEW, M Late Silurian setting of the Lewis Ponds massive sulphide deposit, poster New South Wales AM2 BINNS, RA Anatomy of an active seafloor hydrothermal system hosted by felsic volcanic rocks: Ocean Drilling Program Leg 193 AM2 DAVIDSON, GJ et al. The ocean crust dyke/basalt hydrothermal sulfur anomaly— insights into its growth and geometry during crustal extension on Macquarie Island, Southern Ocean. DOWNES, PM et al. A review of sulphur isotope results from late Silurian VHMS poster mineralisation. Hill End Trough, NSW.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PMl Effects of seawater and magmatic fluids in the Panorama VMS district, Pilbara Craton: A stable isotope study AM2 Sulfur isotope characteristics of actively forming, subaqueous epithermal mineralisation, conical seamount, P.N.G. HEATH, S et al. Fe-Si-Mn oxides of the Pacmanus seafloor massive sulphide field, AM2 eastern Manus Basin, Papua New Guinea SEBASTIAN, S et al. Bulk geochemistry and mineralogy of sediments from the eastern poster Manus Basin, Papua New Guinea poster SECCOMBE, PK et al Late Silurian VHMS mineralisation at the John Fardy Mine, Peelwood district, southern N.S.W. SOLOMON, M et al. Explaining variation in VHMS ores (e.g. Hokuroku, Hellyer and PMl Mt. Lyell): the likely role of fractures, deep footwall permeability, and rock buffering capacity AM2 Advanced argillic alteration associated with actively forming, YEATS, C et al. submarine polymetallic sulfide mineralisation in the eastern Manus Basin, Papua New Guinea FOLKERT, SL et al.

GEMMELL, JB et al.

Topic 4.6: Industrial Minerals Thursday, Rm 4/G02 (University Hall) AL-BU SAIDI, S et al. Preparation and characterization of feldspar products from some granitic sources in Oman and eastern Australia SWEETAPPLE, MT Controls on regional distribution of rare metal pegamatites in the et al. Archaean Pilbara Craton, Western Australia

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Topic 4.7: Gemstones Thursday, Rm 2/410 AMI Features of diamonds from Copeton-Bingara, NSW, Australia BARRON, L et al. AM2 Cretaceous microbes - producer of black opal at lightning ridge, BEHR, H et al. NSW, Australia. poster Diamond-sapphire zircon association in Victoria BIRCH, WD AMI Multiple origins of the eastern Australian alluvial diamonds DAVIES, RM et al. Gem corundums from basaltic eruptives Tumbarumba Field, N S W AMI GRAHAM, IT et al. poster LIMTRAKUN, P et al. Formation of the Denchai gem sapphires, northern Thailand: mineral chemistry and fluid/melt characteristics AMI Laser probe '^^Ar/^^Ar step-heating analyses of single PHILLIPS, D et al. clinopyroxene inclusions extracted from Jwaneng, Orapa and Mbuji-Mayi diamonds. AMI Colourful corundums from volcanic fiimaces, eastern Australia SUTHERLAND, L and elsewhere et al.

Topic 4.8: Geochemistry and Chemical Mineralogy Thursday, Rm 2/410 COHEN, DR et al. Characterisation of residual and transported regolith profiles using poster PIMA II The limits of chemistry in the supergene environment: chromium AM2 CRANE, MJ (vi) species and related minerals (P Williams) et al. Distribution of gold and base metals in regolith materials at AM2 DE SOUZA Mandamah prospect, central western New South Wales KOVACS, N et al. Field spectrometry and hyperspectral imagery for regolith mappingposter DEHAAN, R et al. at Mrangelli, near Cobar, NSW. McQUEEN, KG et al. Mineralogical and geochemical features of palaeochannel poster sediments at Gidji, WA: Implications for landscape history and gold exploration The Syerston Deposit - lateritic nickel cobalt platinum over an AM2 MOTTERAM, G (A Spinks) et al. Alaskan intrusion TOMKINS, A et al. Variation in the distribution of gold within arsenopyrite and AM2 loellingite during pro- and retrograde metamorphism

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Topic 4.9: Predicting Thursday, Rm 2/410 BARNES, R (H Henley) et aL BAXTER, JL COHEN, DR et al. HILL, KC et al. JAIRETH, S (I Lambert) et al.

Mineralisation Grafton - Maclean metallogenic map, northern NSW

poster

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Complex adaptive systems - an exciting paradigm for ore deposit PMl models A comparison of unsupervised neural networks and K-means PM1 clustering of multielement stream sediment data, northeastern New South Wales Predicting copper-gold mineralisation in New Guinea PM 1 Mineral potential of Australia PM 1

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Topic 4.10: New and Other Deposit Types - International Thursday, Rm 2/410 KEAYS, RR et al. The enigma of the metal sources for the Ni-Cu-PGE sulphide PM2 deposits of the Sudbury Igneous Complex: implications for mineral exploration SPRY, PG et al. The geology, mineralogy, and geochemistry of the Bemers Bay PM2 gold-silver telluride district, Juneau Gold Belt, Alaska TALUSANI, RVR Geology and geochemistry of the sediment-hosted gold Poster mineralisation at Barhi and Jhal, Madhya Pradesh, India VIELREICHER, RM The Jian Cha Ling Ni-Au-deposit: listvenite-hosted, epizonal gold PM2 et al. mineralisation in the west Qinling Mountains, central China

Topic 4.11: New Exploration Techniques Friday, Rm 4/G02 (University Hall) BOSCHETTI, F et al. Enhanced visual interpretation of potential field maps JACKSON, SE et al. Determination of copper isotope ratios by LA-MC-ICP-MS LAWRIE, KC et al. Mapping porphyry Au-Cu and epithermal Au mineral systems under complex regolith cover, Lachlan Fold Belt, N.S.W. Interactive inversion in geology MORESI, L et al. RYAN, CG et al. High resolution, high sensitivity imaging and analysis of minerals and inclusions (fluid and melt) using the new CSIRO-GEMOC nuclear microprobe TAYLOR, GR Semi-quantitative mineral and lithology mapping of drill core pulps using visible and infrared spectroscopy.

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Topic 4.12: Cobar Mineralisation Revisited Friday, Rm 4/G02 (University Hall) DAVID, V et al. A new look at the Elura deposit, NSW AM2 JIANG, Z-Y et al. Significance of fluid inclusions within sulphide minerals - an AM2 example from The Peak and Elura deposits, Cobar, N.S.W. JIANG, Z-Y et al. Source of ore-forming components at the Peak Mine, Cobar poster N.S.W. - evidence from isotope studies Comparison of an empirical alteration index, Pearce Element Ratio poster WHITBREAD, M Analysis and the Isocon method: as applied to navigating et al. lithogeochemical alteration surrounding the Elura Zn-Pb-Ag deposit

Topic 4.13: Modelling and Mechanics of Mineralising Systems Friday, Rm 4/G02 (University Hall) FREIJ-AYOUB, R Toward a quantitative holistic understanding of rock alteration et al. patterns GOW, PA et al. Controls on porphyry-related copper-gold mineralisation in the New Guinea Fold Belt - insights from geodynamic modelling Geodynamic modelling of the Broken Hill mineralising system HOBBS, B et al. Mantle dynamics and the formation of major ore deposits MORESI, L et al. Geodynamic modelling of the Century Zinc Mineralising System: ORD, A et al. three scenarios

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RICE, A SCHAUBS, P et al. SORJONEN-WARD, P et al. WALSHE, JL et al. ZHANG, Y et al. ZHAO, C et al. RICE, A HUYNH, T et al.

How to form a Rustenburg layered suite from a single pulse of magma Numerical models of gold deposit formation in the BendigoBallarat zone, central Victoria Numerical modelling of orogenic processes and gold mineralisation in the Yilgam The nature of the third dimension in mineralising systems Interaction between deformation, thermal structure and fluid flow in thrusting regimes and its relevance to mineralisation Predictive modelling of ore deposits in hydrothermal/sedimentary basins Finite element modelling of the formation of komatiite hostednickel deposits and the non-role of thermal erosion Geostatistical characterisation of petrophysical properties in 3D geological and geophysical modelling

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East Asia-west Pacific: the best ground PM2 Area selection in the Yilgam II AM2 Area selection in Australia - Risk reduction and value creation in AMI mineral exploration Area selection and government regional surveys PM2 Late timing of Yilgam gold mineralisation and significance to area AM2 selection. Area selection in the Yilgam I AM2 Area selection in minerals exploration: a global perspective AMI Area selection in Australia AMI Ground selection for stratiform zinc deposits in north Australian PM 1 Proterozoic basins: sources, aquifers, seals, faults, traps and halos. Area selection in the Gawler and Cumamona PM 1 Application of risk analysis to area selection in Pasminco AM 1 Area selection in the Proterozoic of Northem Australia PMl Area selection in the Tasmanides PM 1

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Symposium 4A: Area Selection in Exploration

Wednesday, Rm 4/G02 (University Hall) BAKER, M et al. ESHUYS, E ETHERIDGE, MA et al. GEE,RD GROVES, DI HALL, GC HAYNES, DW (k) KAY, BD LARGE, R et al. PARKER, AJ ROBERTS, P et al. VALENTA, RK WOOD, DG

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Symposium 4B: Bicentennial Gold

Thursday, Rm 4/G02 (University Hall) BROWN, W et al. KOTSONIS, A et al. PIKE, G et al. SCHAUBS, P SKABAR, A

Mineral prospectivity mapping: a comparison of MLP neural PM2 network, fuzzy logic and weights of evidence methods Weathering and secondary gold, Bendigo region, central Victoria, PM2 Australia PM2 Enigmatic metal sulphide deposits of the Whim Creek Belt, Pilbara Craton, WA: the geological constraints PM2 Mineralogical and geochemical aspects of gold mineralisation along the Deborah Anticline, Bendigo, Victoria Application of data mining and knowledge discovery techniques to PM2 mineral potential mapping in a GIS environment

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Theme 5: Sedimentary Basins and Resources Topic 5.1: Sedimentary Basins Tuesday, Rm 2/413 A flexural tectonic mechanism for Third-Order Sequences in the AM 1 ABBOTT, S middle Roper Group (Mesoproterozoic), Northern Territory BAROVICH, K et al. Nd isotope constraints on sources of Palaeozoic petroleum-bearing AM 1 rocks in the Amadeus Basin, central Australia Chronological constraints on a rift sequence underlying the AM 1 CLOSE, D et al. southwest Amadeus Basin, central Australia Australian-wide transcurrent faults and their role in the creation of AM2 DAIM, FM et al. sedimentary basins AM 1 HOCKING, RM et al. Stacked Proterozoic basins in central Western Australia Geomorpher: digital seismic reconstruction as an integral part of poster HOFFMAN, N the modem interpretation process Post-rift tectonic subsidence and palaeo-water depths in the poster KAIKO, AR et al. Northern Carnarvon Basin; Western Australia Tectonic significance of late-stage sedimentary sequences in the AM2 KRAPEZ, BB Archaean Eastern Goldfields Province of Western Australia Neogene sedimentary basin evolution in northern Papua New AM2 LIU, K et al. Guinea: a model for convergent margin basin evolution Jinggu strike slip basin of Tertiary in southwest Yunnan, China: an AM2 LIU, S et al. effect of the collision between Indian and Asian continents Devonian fluvial deposition and tectonics adjacent to the AM2 NEEF, G et al. Koonenberry Fault at Moona Vale Station, far western New South Wales PIRAJNO, F (J Jones) Revised stratigraphy of the Palaeoproterozoic Earaheedy Group: AM 1 implications for the tectonic evolution of the Earaheedy Basin, et al. Western Australia A new look at the structural and tectonic evolution of the PM1 POWER, M et al. Gippsland Basin: preliminary results STAGG, HMJ (P Structural elements of the Lord Howe Rise poster Symonds) et al. VAN DE BEUQUE, S Diapirism in the Fairway Basin, northeast Lord Howe jR^ise: PMl et al. implications for basin formation and geological evolution XU, M et al. Cretaceous tectonic-sedimenta^ evolution and related petroleum poster systems in basins of western Liaoning, northern Liaoning and Songliao areas, China Topic 5.2: Continental Margins and Tectonics Tuesday-Wednesday, Rm 2/413 Tectonic provinces of the Kerguelen Plateau: integration of poster BORISSOVA, I (J seismic data and ODP drilling Sayers) et al. Evolutionary and climatic consequences of opening the Tasmanian W-AM1 EXON, N et al. seaway: Results from Ocean drilling A new view of continental margins Tu-PM 1 GARDNER, JV (k) Swath mapping Australia's southeast continental margin for Tu-PM2 HILL, PJ et al. geoscience and environmental management: Austrea survey 2000 Swath-mapping as a tool for ground truthing studies of continental Tu-PM2 KEENE, J et al. margins, Gippsland Basin, southeast Australia ODP drilling in the Great Australian Bight reveals the evolution W-AM 1 LI, Q et al. history of the southern margin Summary of initial results from ODP Leg 188 - history of east W-AM 1 O'BRIEN, PE et al. Antarctic glaciation from drilling in Prydz Bay Sequence stratigraphy, biostratigraphy and depositional system poster POLLOCK, R et al. analysis - an integrated study in the Gambier Basin, southern Australia Biostratigraphy of ODP Leg 188 and its contribution to Tu-PM2 QUILTY, PG et al. understanding of the "Amery Basin" Seabed mapping in the Great Australian Bight Marine Park: Tu-PM2 ROLLET, N et al. preliminary results from the Austrea-1 Cruise Nature of the continent-ocean transition on the non-volcanic rifted W-AMl SAYERS, J et al. margin of the central Great Australian Bight Australia's volcanic margin poster SYMONDS, P Swath mapping of the Norfolk/Three Kings Ridge region poster SYMONDS, P

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Topic 5.3: Sedimentary Environments and Depositional

Systems

Wednesday, Rm 2/413 AMINI, Z

The affect of oceanography on sedimentology and geochemistry of AM2 the temperate carbonates of Bass Strait, Australia. AMI Southeast Australia - a classic wave dominated margin? BOYD, R et al. AM2 CANTALAMESSA, G A Plio-Pleistocene composite third order depositional sequence generated by ridge subduction along the Ecuadorian convergent et al. margin ERIKSSON, K et al. Tidal rhythmites as high-resolution time clocks in basin analysis: AM2 examples from the Appalachian Basin, USA Physical property changes as a monitor of east Antarctic poster FORSBERG, CF (P sedimentation history: first results from OOP Leg 188 (Prydz Bay) O'Brien) et al. Discovery of a Holocene drift deposit on the George Vth poster HARRIS, P et al. continental shelf, east Antarctica - an expanded record of regional sea ice and Antarctic bottom water production? HUGHES, MG et al. Sorting of heavy mineral grains on beaches: assessment of existing AM2 hypotheses Cyclicity in backreef carbonates of the Devonian reefal platforms PMl JELL, JS et al. of the Lennard Shelf, Canning Basin, Western Australia: implications for reef development and palaeobathymetry poster Lithostratigraphy of continental shelf, trough-mouth fan and KAIKO, AR et al. sediment drift deposits, ODP Leg 188, Prydz Bay, east Antarctica Preliminary geochemical results from ODP Leg 188: Prydz Bay, poster KRYC, KA (P Antarctica O'Brien) et al. Miocene palaeoenvironmental and palaeoceanographic evolution AM2 SMITH, AJ et al. of the Seaspray Group, Gippsland Basin, southeast Australia PMl Cyclicity at a carbonate platform margin, Menyous Gap, Pillara WEBB, GE et al. Range, Canning Basin, Western Australia: implications for reef development and palaeobathymetry Biostratigraphic and palaeontological summary: ODP Leg 188, poster WHALEN, P (P Prydz Bay Antarctica Quilty) et al.

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Topic 5.4: Maturation and Basin Modelling

Monday, Rm 2/413 PM2 CROWHURST, PV et (U-Th)/He apatite ages from the Taranaki Basin, New Zealand: implications for cooling and denudation in the Pliocene. al. PMl Microprobe techniques for the assessment of petroleum source FAIZ, M et al. rocks - a case study from the Timor Sea PM2 Mid-Miocene cooling in the northern Qilian Shan, northeastern GEORGE, AD et al. margin of the Tibetan Plateau, revealed by apatite fission-track and Vr analysis Depositional Effects on Coal Rank Indicators in the San Juan PM1 GURBA, LW et al. Basin (USA) - Comparison to Australian Coal Basins HE, S et al. Study of anomalous low vitrinite reflectance profiles in the Barrow poster Sub-Basin, Western Australia: implications of two effects related to perhydrous suppression and overpressure retardation MARSHALL, C et al. Development of laser Raman and x-ray photoelectron PM2 spectroscopic parameters as an additional thermal maturity indicator to the conodont alteration index 3 D thermal subsidence modelling PM 1 MIDDLETON, MF Thermal maturation pattern in the southern Bowen and northern PM 1 OTHMAN, R et al. Gunnedah Basins and the overlying Surat Basin sequence, northern New South Wales

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Symposium 5A: Hydrocarbon Exploration in Australia and PNG

Monday, Rm 2/413 BALDWIN, S CHEN, G et al. GEORGE, SC et al. GRICE, K et al. GURBA, LW et al.

A method to quantify the development of a deep sedimentary AM 1 basin: the Bonaparte Basin, northwest Australia. 3D palaeo-migration pathway analysis for the Skua and Swift AM 1 structures, Vulcan Sub-basin, Timor Sea Oil-bearing fluid inclusions: geochemical analysis techniques and AM2 geological applications Diamondoid hydrocarbon ratios as indicators of biodegradation in AM2 Australian crude oils Coalbed methane gas generation in the Gloucester Basin, N.S.W. AMI

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Geological Convention, Sydney, July 2000

3D structural modelling of the Moran Oilfield, Papua New Guinea poster KEETLEY, JT et al. compared with Cape Liptrap McCONACHIE, BA et Foreland to Fold Belt: controls on orogenesis and petroleum traps AMI al. in New Guinea New insights on the Green River petroleum system of the Uinta AM2 RUBLE, TE et al. Basin fi-om hydrous pyrolysis experiments Potential gas hydrates in Australia's marine zones poster STAGG, HMJ (P Symonds) et al. STAGPOOLE, V et al. Hydrocarbon prospectivity of deep-water ft-ontier basins northwest AMI of New Zealand AM2 VAN AARSSEN, BG Reconstruction of the geological history of Australian crude oils et al. AM2 VAN AARSSEN, BG Palaeovegetation changes during Jurassic times as determined from higher-plant-derived biomarkers (R Alexander) et al. Northern Lord Howe Rise: tectonic setting and possibilities for poster VAN DE BEUQUE, S et al. hydrocarbon indications Symposium 5B: 34th Newcastle Symposium Thursday, Rm 2/413 BANN, G et al. BIGGS, M BOYD, R et al. BOYD, R et al. CHALMERS, G CREECH, M CREECH, M DIESSEL, C et al. HARWOOD, C et al. HILL, MBL et al. LEE, J LINDSAY, G et al. McCABE, PJ (k) WARD, CR (k) WILSON, GA et al.

The onset of Permian volcanism in the south-eastern Sydney Basin PM2 Investigations into the prediction and modelling of total sulphur in PMl coal seams, German Creek Mines, Central Queensland. Developing a model for non-marine sequence stratigraphy AM 1 The Greta Coal Measures in the Muswellbrook Anticline Area, AM2 NSW Cyclic correlation and significant surfaces in low accommodation AMI coal So, how does your peat grow? AM 1 The Wollombi Coal Measures-Refugees from Newcastle AM2 Significant surfaces and accommodation trends in paralic coal AM 1 seams Sedimentology of the Shoalhaven group along the western margin PM2 of the Southern Coalfield In search of the Woonona Seam: a reappraisal of the geology of AMI the south and west of the Southern Coalfield Hydrogeology of the Hawkesbury Sandstone in the southern PM2 highlands of N.S.W. in relation to Mesozoic horst-graben tectonics and stratigraphy Coal and conglomerate in the Newcastle Coal Measures - lateral AM2 facies or coevally unrelated? The evolving global energy mix: impacts for Australia in the 21st AM2 century Mineral matter in coal seams: foreign invaders or indigenous PMl community? Modelling a coal subcrop using the impedance method PMl

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Theme 6: Civilising Geology: Geotechnical and Environmental Studies Topic 6.1: Geological Hazards Friday, Rm 2/411 Tsunami at Aitape - "Is it safe to go back"? DAVIES, HL et al. Quantifying enhanced soil cohesion due to the roots of Casuarina DOCKER, BB et al. cunninghamiana. Lake Tahoe debris avalanche GARDNER, JV Vegetation, dredging and river bank stability: examples from the HUBBLE, TC Nepean River, near Camden N.S.W. The 1997 Thredbo landslide SULLIVAN, T (k) Eruptive potential, metropolitan New South Wales or the bush? SUTHERLAND, L

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Topic 6.2: Hydrogeology and Groundwater Management Wednesday, Rm 2/41 It PMl Applications of geophysical techniques to characterization of BAKER, S groundwater in river banks contaminated with mine tailings, king river, Tasmania AM2 A New Approach to Groundwater Management for Alumina COOLING, D (R Residue Storage Facilities in Kwinana, WA by Alcoa of Australia Colman) et al. Ltd. DEHAAN, R et al. Field and image-derived spectral end members as indicators of soil PMl salinity DENT, B AM2 Cemetery decay product profiles: two cases in Australian unconfmed sandy aquifers. HABERMEHL, MA Hydrogeology and environmental geology of the Great Artesian AMI Basin, Australia HORN, AM Determining the effects of land use on water quality in the Fitzroy AMI River Basin. Opportunities for geoscience based technology. KAZEMI, GA et al. Salt balance of the Buckinbah Creek catchment, Macquarie River PMl Basin, New South Wales, Australia LaBRECQUE, JJ et al. The application of soil-gas geochemistry to precisely locate La AMI Victoria Fault near Paracotos (Venezuela) MERRICK, NP (k) Now is the time for applying optimisation techniques to AM2 groundwater resource management Identification of the Murringo Bottleneck, central N.S.W.: MOORE, CL poster implications for dryland salinity hazard mitigation PALMER, D (p) Detailed geological mapping of bedrock using seismic refraction - PM2 new and simple processing for an old method RAYMOND, O Depth-to-bedrock modelling using drilling and the ANUDEM poster topographic modelling program RUXTON, BP Ultrafme particle movement in the regolith: field and experimental PMl evidence SAPPAL, KK et al. Geological implications in rehabilitation of mined voids. Collie PM2 Basin, WA SWANE, IP et al. Local recharge and discharge areas in the Wimmera region, AMI southern Murray Basin, Victoria: their role in determining groundwater quality. poster Controls on groundwater chemistry and the vulnerability and TWEED, S et al. surface water to contamination, Yarra Catchment, Victoria AM2 Mineral water composition and groundwater flow in cold CO2 WEAVER, TR et al. bearing mineral springs, central highlands, Victoria, Australia AMI Use of water chemistry to identify flow conduits in the porous WEBB, JA et al. Gambier Limestone, southeast South Australia Assessment of landfill leachate contamination of groundwater in a poster WEBB, JA porous sand aquifer Topic 6.3: Geotoxicology and Environmental Geochemistry Thursday, Rm 2/411 ASHLEY, PM et al. Heavy metal loadings in the Macleay River catchments, poster northeastern New South Wales BIRCH, G et al. Possible biological significance of contaminated sediments in Port AM2 Jackson, Sydney. CLARK, MW et al. Sulphide mineral oxidation in the regolith: Implications for trace AM 1 metal dispersion and acid retention. COSTELLOE, MT et Environmental review of the Mary Kathleen uranium mine site, poster al. northwest Queensland

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

Tidal re-suspension of cohesive sediments in the Parramatta River AM2 estuary: Implications for contaminant dispersal. Metal concentration and speciation trends in Cockle Creek, Lake AMI DAVIESMcCONCHIE, F et al. Macquarie, N.S.W. Australia. Abandoned alluvial tin mines in NE Tasmania: characterisation for AMI de JONG, JM et al. successfiil remediation The development of a method for assessing effluent toxicity using PMl GOLDING, C et al. the doughboy scallop Investigation of the spatial and temporal pollution of the Tonalli AM 1 HARRISON, J et al. River (N.S.W.): preliminary geochemical data Geosciences in the protection of marine biodiversity PM2 JENKINS, CJ JOHNSTONE, R (k) The need for using collaborative and multidisciplinary approaches PMl to research the biogeochemistry of coastal zone ecosystems AM2 Sediment quality guidelines: their development, use and LONG, ER (k) implementation McCONCHIE, D et al. Treatment of acid mine waters using seawater-neutralised bauxite AMI refinery residues PMl SCHNEIDER, P et al. Accumulation of metals and pesticides in the sediments of Iron Cove PM2 SCOLLEN, A et al. Road dust and its impact on estuarine sediments in the intensely urbanised catchment of Iron Cove, Sydney, N.S.W., Australia PM2 SNOWDEN, R et al. The geochemistry of soils in the Iron Cove catchment. SUH, JY et al. Heavy metal distribution in reclaimed lands of Homebush Bay, the AM2 venue of the 2000 Olympic Games, Sydney, prior to remediations WALSH, SR Determination of trace elements in sedimentary phosphorites with PM2 ultraviolet laser ablation ICP-MS COUTTS-SMITH, A

Symposium 6A: Sydney - The Sandstone City Friday, Rms 2/413 and 2/410 Offshore sand as a building material ALBANI, A et al. BRANAGAN, DF The Hawkesbury Sandstone: Its origins and later life BRANAGAN, DF Structural geology of the Hawkesbury sandstone in the city of Sydney DRAGOVICH, D Weathering processes and rates EMERSON, D Petrophysical properties of the Hawkesbury Sandstone FRANKLIN, B Sydney dimension sandstone: the value of petrography in stone selection and assessing durability. FRANKLIN, B et al. Modulus of rupture test HEIMAN, J Testing of Sydney Sandstone HEIMAN, J et al. Salt attack and its treatment on sandstone & brick buildings JEFFERY, R Crushed sandstone use in pavement design and construction McNALLY, G Geotechnical investigations and reclamation of sandstone and shale quarries in the Sydney region McNALLY, G Sandstone earthfill and rockfill McNALLY, G et al. Geomechanical properties of sandstones and what they mean McNALLY, G Sandstone aggregate, roadbase and ballast MCQUEEN, LB Stress relief effects in sandstones in underground and deep excavations in Sydney MOELLE, K Microfabrics of the Hawkesbury Sandstone PELLS, P (k) Geological controls on tunnel design - a tale of two cities RANNARD, TM The selection and durability of Hawkesbury Sandstone for breakwater armorstone and coastal protection works. The Sydney Sandstone industry - past and present RAY,H Hydrogeology & environmental problems of fractured sandstone SMITH, P aquifers Sydney Sandstone: an overview SPRY, A (k) Chemical consolidation of sandstone: the Sydney project SPRY, A Quantitative mineralogical analysis of sandstones using x-ray WARD, CR et al. diffraction techniques Contour delamination as a form of deterioration of Sydney WEST, D sandstone in buildings Brard's test into the 21st century: sodium sulphate testing of WEST, D dimension sandstone WHITEHOUSE, J et Friable sandstone resources of the Sydney Planning Region and nearby areas al. Sandstone's role in the architecture of Sydney WILKINS, P Geomorphology of sandstones in the Sydney region YOUNG, B

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Theme 7: Getting the Message Across - Geoscience Communication, Past, Present and Future

Friday. Rm 6/320 CLARKE, GL et aL COUSINS, S et al. COUSINS, S et al.

HyperPET: a web-based optical petrology tutorial package poster Developing a promotions and education program at the GSA AMI Earthworks - Earth science, technology and minerals learning poster centre HAFNER, R Geology is alive and well in NSW post-compulsory education AM2 HUBBLE, TC Helping them through: the use of formative assessment, trial AM2 exams and WEBMCQ to assist students survive and excel in their first university exams. LAWRIE, KC (P Earth Science Mentor program: a joint initiative of the Geological AM2 Society of Australia and Science Educators Association of the Lyons) et aL Australian capital territory LEWIS, GB AGSO's teacher resources kits and training sessions AM2 MAJOR, B Charles Darwin - the geologist AMI McNAMARA, GC AM2 The AGSO Earth Science Education Centre PEMBERTON, M Conserving geodiversity, the importance of valuing our geological AMI heritage SIMPSON, A Opportunities and obstacles for campus-based museums in earth AMI science outreach programs STACY, S (M Yeung) Interpretation - the key to making geology meaningful to everyone AMI

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

Theme 8: Environmental Change and Landscape Evolution: The Recent Past and Clues for the Future

Tuesday, Rm 2/410

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PM2 Seasonal variation of calcareous epiphyte on artificial seagrass: preliminary studies. West Island, South Australia. Sea level and environmental changes since the last interglacial in PM2 the Gulf of Carpentaria AM2 Megaclast emplacement and movement on shore platforms: an CROOK, K et al. index of extreme events at the land-sea interface. poster GARCIA, A et al. Repetitive marine to lacustrine faunal changes in the Gulf of Carpentaria poster GRANT, KM et al. The Late Miocene-Early Pliocene biogenic bloom at DSDP Site 590 and ODP Site 1125, southwest Pacific ^ AM2 GREENWOOD, DR et Early Palaeogene warm climates and vegetation in southeastern Australia. al ^ Cainozoic multiple impact event in South Australia and its inferred AM2 HAINES, PW signature within terminal Eocene stratigraphy of the St Vincent Basin poster HANCOCK, HJ et al. Early Palaeogene planktic foraminiferal and carbon isotope stratigraphy. Hole 762C, Exmouth Plateau, northwest Australian margin. Salt dynamics in a complex landscape: taking a multidisciplinary PMl LAWRIE, KC (T approach in central-west N.S.W. Munday) et al. poster Neotectonic disruption of Cainozoic volcanics in southeastern LEWIS, AC et al. Australia: implications for landscape evolution PMl Origin and diagenesis of a Quaternary ironstone in Babanango LIU, KW area, northern Kwazulu-Natal, South Africa AMI Productivity in the Australian maritime region: are there direct OPDYKE, B et al. links between Milankovitch forcing and marine productivity in Australia PM2 OSBORNE, A Petrography of lithified cave sediments PILLANS, B et al. Palaeomagnetic dating of Phanerozoic weathering imprints, Mount PMl Percy Mine, Kalgoorlie, Western Australia Apatite fission track thermochronology and southeast Australian AM2 ROACH, I landscape evolution-can exaggerated denudation rates be reconciled? SKILBECK, G et al. Time correlation of magnetic susceptibility data from Myall Lakes poster and the record of sediment and climate cyclicity AMI Possible causes and consequences of Palaeogene warm climates SLOAN, L et al. poster The regolith/landform map and its applications for mineral SPRY, MJ et al. exploration, an example from Cobar, NSW. The morphology and chemistry of gold grains at Portia Prospect, PMl TAN, KP et al. Cumamona, South Australia AM2 Weathering: cyclical or continuous? A southern perspective. TAYLOR, G et al. GIS-based approach to mapping salt movement and storage using poster WILFORD, J et al. catchment analysis of terrain attributes, airborne gamma-ray spectrometry, geology and hydrology AMI Nature and consequences of rapid global warming in the early ZACHOS, JC (k) Cenozoic

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CONTENTS Abstracts (in alphabetical order of first authors) Author Index

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A FLEXURAL TECTONIC MECHANISM FOR THIRD-ORDER SEQUENCES IN THE MIDDLE ROPER GROUP (MESOPROTEROZOIC), NORTHERN TERRITORY Stephen T. Abbott School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia. The Roper Group is a cyclic, predominantly marine, siliciclastic succession with a distribution of at least 145,000 km^ and a maximum known thickness of approximately 5,000 m. In its type area in the Roper River district, the middle Roper Group is c. 1300 m thick, and can be divided into six large-scale coarseningupward cycles, between 175 m and 330 m thick, that are interpreted as third-order depositional sequences. Age control is poor and existing radiometric dates indicate deposition of the six sequences spanned a period of time as short as 30 million years and as long as 90 million years. Recurring facies composition and sequence architecture in Roper sequences have been resolved into a generalised Roper sequence motif. The components of the Roper motif are listed in ascending stratigraphic order as follows: • • • • • •

An apparently conformable or, less commonly, erosional basal contact (sequence boundary), A fming-upward interval of distal shelf facies that may include a basal redbed interval characterised by the presence of ooidal ironstone (transgressive systems tract), Organic-rich, basinal black shale (upper transgressive systems tract/lower highstand systems tract), A sanding- and thickening-upward storm-dominated shelf succession (highstand systems tract), An erosional contact (intrasequence erosion surface), A tidal-platform sandstone, in some cases capped by a thin, probably fluvial, red bed succession (highstand systems tract).

Roper sequences accumulated on a vast siliciclastic ramp within the Roper Superbasin and the mainly conformable nature of sequence boundaries indicates that sequences, apart from the perturbation expressed as intrasequence erosion surfaces, were generally deposited in the context of varying rates of relative sea-level rise. Sequence architecture is supply-driven as indicated by the regional filling of accommodation during each sea-level cycle and resulted in the "layercake" aspect of Roper Group stratigraphy. The features of the Roper motif and analogy with similar successions suggest a basin setting adjacent to a tectonically active hinterland. Flexural tectonics driven by episodic tectonic loading of the crust can generate accommodation cycles on 10^ y timescales (Beaumont et at. 1988; Ettensohn 1994). This process generates a "two-step" accommodation cycle in the adjacent basin. The lower part of the cycle, generated by subsidence associated with crustal relaxation, is characterised by a deep to shallow marine regressive succession. The upper part, generated by subsidence associated with erosional crustal unloading, is characterised by a shallow marine to non-marine succession. The contacts between these two parts, and some sequence bounding surfaces, are generated by migration of flexural bulges. The "two-step" flexural mechanism reconciles well with the Roper motif (Abbott & Sweet 2000) and in particular furnishes a mechanism for the erosion surfaces at the bases of Roper Group tidal sandstones. Identification of the orogen responsible for episodic sedimentation in the Roper Superbasin awaits detailed analysis of the entire basin fill to establish its palaeogeographic disposition and refined dating of both the basin succession and tectonic events in the adjacent orogenic belts. However, we note that the Isan Orogeny and the Anmatjira Uplift (central Australia) are of similar age to the Roper Group, and may have generated the episodic subsidence that lead to the deposition of the third-order sequences described here. References ABBOTT, S. T. & SWEET, L P. 2000. Tectonic origin of third order sequences in a siliciclastic ramp-style basin: an example from the Roper Superbasin (Mesoproterozoic), Australia. Australian Journal of Earth Sciences 47, 000-000, in press. BEAUMONT, C., QUINLAN, G. M. & HAMILTON, J. 1988. Orogeny and stratigraphy: Numerical models of the Paleozoic in the eastern interior of North America. Tectonics 7, 389-416. ETTENSOHN, F. R. 1994. Tectonic control on formation and cyclicity of major Appalachian unconformities and associated stratigraphic sequences. In: Dennison, J. M. & Ettensohn, F. R. eds. Tectonic and eustatic controls on sedimentary cyclicity, pp. 217-242. SEPM (Society for Sedimentary Geology) Concepts in Sedimentology and Paleontology 4.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CARBONATE HOSTED GOLD DEPOSIT IN TASMANIA, AUSTRALIA M. H. Adabi School of Earth Sciences, Shahid Beheshti University, Tehran, Iran This study uses elemental and isotopic composition of carbonates associated with gold from Henty and Beaconsfield in Tasmania, Australia, to illustrate source of gold-bearing fluids, salinity, temperature and dissolution and reprecipitation of carbonate. The Beaconsfield and Henty gold mines are located in northern and western Tasmania respectively. Gold mineralisation in Beaconsfield occurs within the quartz-carbonate Tasmania Reef (Lower to Middle Palaeozoic sequence, Hills, 1998). The Henty gold mine is located at the base of the Cambrian Tyndall Group (volcano-sedimentary succession, White and McPhie, 1996) close to Henty Fault. Gold in carbonate samples from Henty ranges from 7.7 to 9360 ppm and in Beaconsfield ranges from 0.01 to 434 ppm. The amount of carbonate in samples from Henty and Beaconsfield gold mines varies from approximately 24 to 99.8%. Bivariate plot of Ca relative to total amounts of Mg, Fe and Mn illustrates that the major carbonate minerals at Beaconsfield and Henty gold mines are magnesian ankerite and calcite. The difference in carbonate mineralogy, at Henty and Beaconsfield gold mines, is attributed to the composition of fluids responsible for carbonate alteration. Gold and magnesium in Beaconsfield ankerite are derived from the leaching of Cambrian ultramafic rocks during the Devonian by the passage of meteoric fluids through tectonically affected Ordovician carbonates (Rao and Adabi, 1999). The total concentration of Fe and Mn are low (0.5 to 2%) in Henty and high (1 to 17.5%) in Beaconsfield ankerite, possibly due to oxidising conditions at Henty and reducing conditions at Beaconsfield gold mines during gold mineralisation. Variation of Sr values between Beaconsfield ankerite and Henty calcite is related to dissolution of limestone that increase Sr concentrations in gold mineralising fluids. Na values in both Beaconsfield (20 to 1100 ppm) and Henty carbonates (25 to 1650 ppm) suggest low salinity fluids responsible for gold mineralisation. Oxygen isotope values of gold-bearing carbonate from Beaconsfield (-6.3 to -17.7%o VPDB) and Henty (20.98 to -21.49%o. VPDB) are negative compared to modem marine calcite (+0.5 to -l%o. VPDB). The very depleted oxygen isotope values along with elemental compositions and palaeotemperature calculation indicate the presence of hot meteoric fluids during gold mineralisation (Rao and Adabi, 1999).Carbon isotope values in both Beaconsfield and Henty carbonates are lower (-0.4 to -5.3%o. VPDB) than modem marine calcite (0 to +4%o. VPDB) due to dissolution of large concentration of carbonate prior to gold mineralisation and this is likely to have increased PH of gold-bearing fluid and promoted gold precipitation. References HILLS P.B. 1998. Tasmania gold deposit, Beaconsfield. In Berkman D. A. and Mackenzie D. H. eds. Geology of Mineral Deposit of Australian and Papua New Guinean , 467-472. RAO C. P. & ADABI M. H. 1999. Geochemistry of gold-bearing carbonates, Beaconsfield gold mine, Tasmania, Australia. Carbonates and Evaporites, in press. WHITE M. J. & MCPHIE J. 1996. Stratigraphy and Palaeovolcanology of the Cambrian Tyndall Group, Mt Read Volcanic, western Tasmania. Australian Journal of Earth Sciences 43, 147-159.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PROVENANCE OF PERMIAN-TRIASSIC VOLCANICLASTIC TERRANES IN NEW ZEALAND FROM THEIR DETRITAL ZIRCON AGE PATTERNS C.J.Adams^ and M.E.Barley^ ^ Institute of Geological & Nuclear Sciences, PO Box 31312, Lower Hutt, New Zealand. ^Centre for Global Metallogeny, University of Western Australia, Nedlands, Western Australia 6907.

Permian-Triassic sediments make up accretionary complexes in several terranes of the Eastern Province of New Zealand. The easternmost of these accretionary prisms (Torlesse Superterrane) is quartzofeldspathic, continent-derived, but separated from the Gondwanaland margin by basic to intermediate volcanic rocks and volcaniclastic sedimentary rocks (Permian to Jurassic) of the Brook Street, Dun Mountain-Maitai and Murihiku Terranes. U-Pb SHRIMP detrital zircon age patterns have been obtained from Permian to Triassic sandstones and greywackes in these volcaniclastic-dominated terranes, to help characterise distinctive source area components and highlight any similarities. In the Late Permian part of the Dun Mountain-Maitai Terrane, Tramway Formation sandstones have detrital zircon age patterns dominated by a major young component, 250280 Ma, reflecting penecontemporaneous Permian volcanism, and an older Middle Devonian to Early Carboniferous component, 320-380 Ma, reflecting hinterland basement rocks. These latter match well with the extensive Devonian to Carboniferous granitoiddominated part of the Lachlan Fold Belt of southeastern Australia. Proterozoic to Early Paleozoic components are minor. In contrast the Momus Sandstone of the Caples Terrane (the westernmost, volcaniclasticdominated terrane, part of the Torlesse Superterrane) has a single detrital zircon age component in the range 210-250 Ma (Middle to Late Triassic), again probably reflecting a contemporary, dominant Middle Triassic volcanic source (and stratigraphic age), remote from any other older basement sources. Both of these detrital zircon age patterns show an important Permian to Triassic input, of which the most likely source in Eastern Gondwanaland is at the convergent plate margin of the New England Orogen of NE Australia, from its contemporary island arc volcanic sources or associated calc-alkaline granitoid complexes. In this respect they share features with the voluminous quartzofeldspathic greywackes of Permian-Triassic, Rakaia Terrane (Torlesse Superterrane) but lack their distinctive minor Early Paleozoic and Proterozoic components, which possibly originate from older basement of NE Queensland. However, the Dun Mountain-Maitai sandstones contain significant Devonian-Carboniferous zircons possibly originating in the Lachlan Fold Belt and which are absent in Permian Torlesse rocks. It is concluded the Permian-Triassic, Dun Mountain - Maitai depocentre had access to the Lachlan Fold Belt basement; was remote from the oldest Queensland basement but still within or adjacent to the New England Orogen. A position adjacent to the northern end of the Lachlan Fold Belt (east of northern New South Wales) is suggested. The Triassic part of the Caples Terrane depocentre, although originating within the New England Orogen, is clearly isolated from contemporary continental sources, in an island arc setting

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LATE SILURIAN SETTING OF THE LEWIS PONDS MASSIVE SULPHIDE DEPOSIT, NEW SOUTH WALES Michael W. Agnew Centre for Ore Deposit Research, University of Tasmania

Lewis Ponds is a polymetallic sheet-style massive and disseminated sulphide deposit, situated on the western margin of the Hill End Trough near Orange in New South Wales (Valliant & Meares, 1998). The host sequence forms part of a Late Silurian succession of volcanic and marine sedimentary rocks (Mumbil Group), interpreted by Pogson and Watkins (1998) to record a progressively deepening environment of deposition. The massive sulphide lenses occur within a sequence of siltstone, crinoidal limestone and quartz-feldspar crystal-rich volcaniclastics (Transitional Unit). This has been intruded or overlain by several coherent bodies of the Mullions Range Volcanics (Eastern and Western porphyritic volcanic succession), previously considered to be tuffaceous volcaniclastic rocks. Sihstone within the Transitional Unit is interbedded with one to ten metre thick, massive to normally graded quartz crystal-rich sandstone beds with sharp bases and tops, interpreted to result from decelerating, turbidity currents. Limestone occurs as large massive 'blocks' up to 100 m in thickness with sharp, conformable or faulted contacts. The limestone is composed of crinoid and shelly fragments, conodonts and corals with some detrital quartz where primary textures are preserved. Given the absence of associated shallow-water sedimentary structures or facies, these relatively pure carbonate rocks are likely to represent allochthonous megaclasts sourced from the basin margin; possibly from the lower crinoidal limestone member of the Anson Formation. The Western porphyritic succession occurs in the footwall of the deposit as a thick, massive, semi-concordant quartz-feldspar phyric unit comprising 20-40% euhedral to subhedral quartz, plagioclase, potassium feldspar and biotite phenocrysts in a variably altered microcrystalline groundmass. Immobile element compositions suggest that it was derived from at least two chemically distinct magmas- a rhyolitic (Ti/Zr = 10 to 13) and dacitic (Ti/Zr = 20 to 22) melt. The Lewis Ponds host sequence is interpreted to have been deposited in a relatively deep, below wave-base, partially anoxic slope or base-of-slope environment, where the fine grained ambient sedimentation was periodically disrupted by mass flows and allochthonous limestone megaclasts derived from the basin margin. These sediments were later intruded or overlain by rhyolitic to dacitic bodies, providing a source of heat for the circulation of hydrothermal fluids and precipitation of metals. References POGSON D. J. & WATKINS J. J. 1998. Bathurst 1:250,000 Geological Sheet SI/55-8: Explanatory Notes. Geological Survey of N e w South Wales, Sydney. VALLIANT R. I. & MEARES R. M. D. 1998. Lewis Ponds gold-silver-copper-lead-zinc deposits. In: Berkman A. D. & Mackenzie D. H. eds. Geology of Australian and Papua New Guinean Mineral Deposits, pp. 635-640. The Australasian Institute of Mining and Metallurgy, Melbourne.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PREPARATION AND CHARACTERIZATION OF FELDSPAR PRODUCTS FROM SOME GRANITIC SOURCES IN OMAN AND EASTERN AUSTRALIA S. Al-bu Saidi, A.C. Dunlop and C.R. Ward School of Geology, University of New South Wales, Sydney, NSW 2052, Australia

Five granitic intrusions, including Precambrian leucocratic granite, microgranite and Kfeldspar porphyritic granite bodies near Ja'alan Bani bu Hasan (Sultanate of Oman) and Carboniferous alaskite granite and biotite-rich granite bodies near Oberon (NSW, Australia) have been investigated as sources of feldspar for the glass and ceramic industries. The granites typically have mineral assemblages with 50-60% feldspar, 25-30 % quartz, <10% muscovite and/or biotite ± chlorite, and accessories include garnet, apatite, sericite, ilmenite, zircon and sphene. Multi step froth flotation and high intensity magnetic separation were used on -475|LI materials to concentrate the feldspar from the granites to industry specifications. Mica was recovered by conditioning the circuit with a cationic collector (an amine-Armac T) at pH 3 using sulfuric acid. Heavy minerals such as garnet, magnetite and ilmenite were then removed using an anionic collector (petroleum sulfonate), again at pH 3, and discarded as tailings. The final flotation step was to separate the feldspar from the quartz, again at pH 3, using the same cationic amine as used in the mica float, but with HF to control the pH and depress the quartz. The dried feldspar product was cleaned by high intensity magnetic separation, to eliminate any retained iron-bearing minerals and any iron abraded from the grinding equipment. Saleable muscovite and quartz are stocked as co-products. The leucocratic granite, the microgranite and the alaskite were found to be the most suitable granites for use as feldspar source materials. The alumina and alkali contents of the products from these granites conform to commercial feldspar specifications of >17% AI2O3, >11% (Na20 + K2O), and 0.05 to 0.15% FeiOa. A feldspar recovery rate of more than 60% was achieved in the flotation tests. The chemical characteristics of the feldspar products were found to be suitable for general proposes, such as making ceramic bodies, glazes and some glass manufacture. The feldspar products were heated over an interval from 1150 tol350^C to establish their firing properties. All samples showed onset of vitrification at around 1200^C, with complete beading at 1250°C. A range of colors from white to yellow was developed in the fired feldspar products. Fine speckling with minute dark spots was also noted in some of the fired feldspar products. Electron microprobe studies show a small but significant proportion of iron (Fe) in the feldspar of some samples from porphyritic and biotite granites, probably due to substitution of Fe^^ for A1 in the crystal lattice. This is probably responsible for much of the color variation. Most of the feldspar contains different amount of accessories such as garnet, magnetite and ilmenite. However when these iron-rich accessories, which are not removed during processing, they appear to give rise to the speckling in the fired feldspar materials. The alaskite, leucogranite, and to a lesser extent the microgranite, however, show a whiter color (brightness 86-93%) and are free of speckling, and thus provide better quality feldspar products.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

PALEOMAGNETIC DATA FROM THE OCEANIC POYA AND KOH TERRANES, NEW CALEDONIA, AND THEIR SIGNIFICANCE FOR MODELLING THE TECTONIC HISTORY OF THE SW PACIFIC Jason R. Ali and Jonathan C. Aitchison Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China. The 19,000 km^ island of New Caledonia provides key information on the complex tectonic evolution of the SW Pacific. It is centrally located and has a geological history that extends back to at least the Carboniferous (Aitchison et al. 1998). Evidence of several regionally significant post 100 Ma tectonic events is preserved on the island including the accretion of Late Paleozoic-Mesozoic subduction-related terranes to east Gondwana in the Early Cretaceous (Meffi-e et al. 1996) rifting-passive margin development related to east Gondwana break-up in the Late Cretaceous-Paleocene (e.g. Falvey & Mutter, 1981), NE-SW emplacement of two separate and distinct oceanic nappes in the latter part of the Eocene (Aitchison et al. 1995, Eissen et al. 1998), and unroofing of metamorphic core complexes caused by the late-stage gravitational collapse of the obducted ophiolite pile (e.g. Clarke et al. 1997). A paleomagnetic study has recently been carried out on two oceanic terranes, Poya and Koh, that outcrop on the island. The presentation will focus on the Poya Terrane (work on Late Paleozoic ophiolitic rocks of the Koh Terrane is almost complete and a summary and interpretation of the data will be included in the presentation). Pillow basalts and associated pelagic sediments of the Late Cretaceous-Paleocene Poya Terrane were sampled for the study. The terrane forms a nappe that was thrust SW over the island, i.e. fi-om the Pacific side, in the middle Eocene. Data from four outcrops (nine sites), spread over 250 km, yield a normal polarity tilt corrected mean inclination of -51.2\ where a95 = 11.2^ and k = 67.4, which suggests formation at 37.8^ (±12.r) S (Ali & Aitchison, in press). The between outcrop declinations are, unfortunately, too scattered to be used in regional tectonic discussions; they indicate rotations that are localised and sometimes very large. Following reference to SW Pacific Late Cretaceous-Paleogene plate tectonic reconstructions (Yan & Kroenke, 1993), the inclination data suggest that the Poya Terrane formed close to the New Caledonian portion of the Indo-Australia plate. This interpretation is consistent with tectonic models where the Poya Terrane is generated in a marginal basin immediately to the NE of New Caledonia during the break-up of east Gondwana (Aitchison et al. 1995; Eissen et al. 1998). The terrane was thrust over New Caledonia in the middle Eocene, possibly in response to a change in relative motion of the Australian and Pacific Plates at about this time (Yan & Kroenke, 1993). A short time later it was overthrust by a supra-subduction zone ultramafic nappe and together they have since drifted northwards as passengers on the Australia Plate. References

AITCHISON, J.C, CLARKE, G.L., MEFFRE, S, & CLUZEL, D. 1995. Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution. Geology 23, 161164. AITCHISON, J.C., IRELAND, T.R., CLARKE, G.L. CLUZEL, D., DAVIS, A.M. & MEFFRE, S. 1998. Regional implications of U/Pb SHRIMP age constraints on the tectonic evolution of New Caledonia, Tectonophysics 299, 333-343. ALI, J.R. & AITCHISON, J.C. In press. Significance of paleomagnetic data from the oceanic Poya Terrane, New Caledonia, for SW Pacific tectonic models. Earth Planet. Sci. Lett. CLARKE, G.L., AITCHISON, J.C. & CLUZEL, D. 1997. Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: a reappraisal. Journal of Petrology 36, 539-553. EISSEN, J.-P. CRAWFORD, A.J., COTTON, J., MEFFRE, S., BELLON, H. & DELAUNE, M. 1998. Geochemistry and tectonic significance of basalts in the Poya Terrane, New Caledonia, Tectonophysics 284, 203-219. FALVEY, D.A. & MUTTER, J.C. 1981.Regional plate tectonics and the evolution of Australia's passive continental margins, BMR Journal of Australian Geology and Geophysics 6, 1-29. MEFFRE, S., AITCHISON, S. CRAWFORD, A.J. 1996. Geochemical and tectonic significance of boninites and tholeites from the Koh ophiolite. New Caledonia, Tectonics 15, 67-83. YAN, C.Y. & KROENKE, L.W. 1993. A plate reconstruction of the southwest Pacific, 0-100 Ma, Proc. ODP Scientific Results 130, 697-709.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

WATER-SETTLING AND RESEDIMENTATION OF SUBMARINE RHYOLITIC PUMICE AT YALI, GREECE S.R. Allen and J. McPhie Centre for Ore Deposit Research, University of Tasmania, G P O Box 252-79, Hobart, Tasmania 7001, Australia

The Yali pumice breccia is a very thick (>150 m), Quaternary succession of submarine pumice that has been upHfted and exposed in the southern part of YaH island in the eastern Aegean, Greece. The pumice breccia comprises moderately- to well-sorted, 0.03-3 m-thick beds of loose pumice clasts that are matrix-poor (predominantly <1-10 wt% <1 mm) and include only rare (<1 vol%) dense juvenile and basement lithic clasts. The dominant pumice type is white aphyric rhyoHte with subordinate grey and speckled porphyritic dacitic and rhyolitic clasts. Pumice vesicularity ranges from 36-86 vol% and vesicles are dominantly tubes or uneven ovoids. The larger pumice clasts (cobbles and boulders) are prismatic with quenched margins and internal polyhedral joints. In comparison, the smaller pumice clasts are polyhedral, angular to subrounded, blocky pebbles and granules. These smaller clasts lack quenched margins and have sharp curviplanar surfaces that cross-cut vesicle boundaries. We interpret the larger pumice clasts to be the products of spalling and explosive fragmentation of a small extrusion of submarine pumiceous lava. The smaller pumice clasts were generated by a combination of: (1) passive and explosive disintegration of the larger pumice clasts, and (2) phreatomagmatic explosions. Phreatomagmatic explosions also generated a minor component of non- or poorly vesicular juvenile and basement-derived lithic clasts. The Yali pumice breccia includes four major facies: cobble-boulder, pebble, mixed cobbledominant and mixed pebble-dominant. The cobble-boulder facies occurs in well sorted, massive, tabular beds of large (64 mm-1.5 m) pumice up to 3 m thick that lack both lithic clasts and matrix. This facies has textural and lithological features consistent with deposition of large pumice clasts by means of water-settling from suspension. The three remaining facies are comparatively less well sorted (although still fines poor) and include rare lithic clasts. They exhibit massive, or internally diffusely stratified, wedging, 0.1-2 mthick beds of coarser grained (medium pebble to cobble) pumice clasts that onlap or are separated by wedge-shaped, thinner (<0.1 m) beds of finer grained (fine pebble to granule) pumice clasts. These components of the pebble facies resemble the products of fallout which are interpreted to have been resedimented submarine modified grain flows of waterlogged, cohesionless pumice and by suspension settling. Mixed facies probably resulted from downslope resedimentation of unstable pebble and cobble-boulders facies and from pumice cobbles and boulders settling from suspension onto, and incorporated into, the active gravity flows.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AN UNUSUAL TRACE ELEMENT SIGNATURE IN THE JURASSIC MIDALKALINE GABBROS OF SOUTHEASTERN AUSTRALIA. Trent C. Allen Division of Geology and Geophysics, School of Geosciences, University of Sydney NSW 2006 Teschenites are midalkaline gabbros which contain analcite but little or no nepheline. Southeastern Australia is the site of a volumetrically minor but regionally extensive province of such rocks. In New South Wales, teschenites range in age from Permo-Triassic to Cenozoic; but many formed during a peak in magmatic activity in the Early-to-Middle Jurassic. Radiometrically (K-Ar) dated midalkaline gabbros from this period include those at Sutton Forest (202±8 Ma; Carr & Facer 1980), Prospect (172 Ma; Evemden & Richards 1961), and Tong Bong Mtn near Rylstone (181±2 to 189±8 Ma; Dulhunty 1976, and Embleton et al 1985). The average age of these rocks is 186 Ma. A suite of teschenites from the central Gunnedah Basin, the Glenrowan Intrusives (Bean 1974), may also be of Jurassic age. Aside from likenesses in their petrography and major element geochemistry, which they share with many midalkaline gabbros, many of the southeastern Australian teschenites have closely similar incompatible element 'spidergrams'. These are characterised by a concave-downward (OIB-like) configuration, with a significant trough at Th and a peak at Ta. This low thorium or L-Th pattern matches that shown by Early to Middle Jurassic (191.4±3.9 Ma) midalkaline gabbroic dykes from Freestone in eastern Victoria (Soesoo et al. 1999). A negative Th anomaly is an unusual feature of unaltered igneous rocks. Other rocks which exhibit relative depletions in Th include some oceanic island tholeiites (e.g. Sun & McDonough 1989), and olivine tholeiites and alkali basalts of the Dubbo Province in central New South Wales (Zhang & O'Reilly 1997). None of the common rock-forming minerals are capable of creating a selective depletion in Th, either by removal during fractional crystallisation, or by retention in the source during partial melting. Contamination by Th-poor crustal material could cause a negative thorium anomaly; but the Jurassic teschenites have mantle-like ^^Sr/^^Sr and '"^^Nd/^'^'^Nd ratios which shows them to be free of such contamination (Allen 1999). It is proposed, therefore, that the Jurassic teschenites inherited their distinctive L-Th signature from a Th-poor and Ta-rich mantle source. This specific trace element signature has not so far been detected in any other igneous rocks in southeastern Australia, either of Jurassic age or otherwise, and appears to be unique to the Early-to-Middle Jurassic teschenites. In spite of this apparent temporal restriction on its availability for melting, the L-Th mantle source must have existed over a wide geographical area. One possible type of source which satisfies these two conditions, i.e. relative transience and broad geographical extent, is a mantle plume. This contention is supported by the coincidence of the Middle Jurassic peak in magmatism in southeastern Australia with a widespread period of plume-related igneous activity on the proto-Pacific margin of Gondwana, which led to the formation of the extensive Karoo-Ferrar-Tasman flood basalt province. Recent Ar-Ar and U-Pb radiometric dating has shown that the latter activity, which immediately preceded and may have partly triggered the Gondwana break-up, was concentrated at around 183 Ma (e.g. Storey & Kyle 1997). It is suggested that re-dating of the Jurassic teschenites of southeastern Australia using Ar-Ar or U-Pb isotopic techniques would reveal a greater degree of cohesion in their ages; and that these dates would fall close to, or perhaps just prior to, the critical age of 183 Ma. References ALLEN T. C. 1999. Petrogenesis of the Jurassic Igneous Rocks of Southeastern Australia PhD thesis, University of Sydney (unpubl.). BEAN J. M. 1974. The geology and petrology of the Mullaley area of New South Wales. Journal of the Geological Society of Australia 2(1), 63-72. CARR P. F. & FACER R. A. 1980. Radiometric ages of some igneous rocks from the Southern and Southwestern Coalfields of New South Wales. Search 11,382-383. DULHUNTY J. A. 1976. Potassium-Argon Ages of Igneous Rocks in the Wollar-Rylstone Region, New South Wales. Journal <S: Proceedings, Royal Society of New South Wales 109, 35-39. EMBLETON B. J. J., SCHMIDT P. W., HAMILTON L. H. & RILEY G. H. 1985. Dating volcanism in the Sydney Basin: evidence from K-Ar ages and palaeomagnetism. In: Sutherland F. L., Franklin B. J. & Waltho A.E. Volcanism in Eastern Australia with case histories from New South Wales, pp. 59-72, P' Publication of the Geological Society of Australia, New South Wales Division. EVERNDEN J. F. & RICHARDS J. R. 1961. Potassium-argon ages in eastern Australia. Journal of the Geological Society of Australia 9, 1-49. SOESOO A., BONS P.D., & ELBURG M.A. 1999. Freestone dykes—an alkali-rich Jurassic dyke population in eastern Victoria. Australian Journal of Earth Sciences 46, 1-9. STOREY B. C. & KYLE P. R. 1997. An active mantle mechanism for Gondwana breakup. South African Journal of Geology, 100(4), 283-290. SUN S.-s. & MCDONOUGH W. F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders A. D. & Norrey M. J. eds. Magmatism in the Ocean Basins, pp. 313-345. Geological Society Special Publication, 42. ZHANG M. & O'REILLY S. Y. 1997. Multiple sources for basaUic rocks from Dubbo, Eastern Australia: geochemical evidence for plume- lithospheric mantle interaction. Chemical Geology 136, 33-54.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE AFFECT OF OCEANOGRAPHY ON SEDIMENTOLOGY AND GEOCHEMISTRY OF THE TEMPERATE CARBONATES OF BASS STRAIT, AUSTRALIA. Zahra Z. Amini School of Earth Sciences, University of Tasmania, GRO BOX 252-79 Hobart, Tasmania, 7001, Australia. Modem cool temperate carbonates occur on the shallow shelf (50 -70 m) of Bass Strait in an area of approximately 85,000 Km^ between latitudes of 38^ to 40.5° S and longitudes of about 143^30' to 149° E. Bass Strait carbonates are mainly affected by different water masses that consist of, to the west, the warm, saline Leeuwin Current and low salinity cold sub-Antarctic water, while to the east a weak intrusion of high salinity, warm. East Australian Current and relatively low salinity, cool, Tasman Sea water. To recognise the physical and chemical affect of these water masses on the sediments of the region, samples fi-om eastern and western Bass Strait have been selected The gravel size fractions are mostly distributed in the shallow shelf areas surrounding the islands. The higher gravel concentration nears the islands is attributed to the input of terrigenous materials from these islands and also occurrence of large skeletal grains such as molluscs. The sand size fractions are distributed throughout the area due to changes in water energy in different parts of the shelf. In Bass Strait a combination of bryozoans, molluscs and to some extent foraminifera, comprise the main components of the bulk sediments. The proportion of bryozoans is higher in the eastern rather than western Bass Strait. This is due to the more stable oceanographic conditions to the east, where the water energy is less, temperature and salinity are more uniform and the water contains higher concentration of nutrients. The carbonate mineralogy in Bass Strait is influenced by seawater temperature, and this is influenced by the water currents rather than by changes in depth or latitude. The Ca and Mg contour maps correlate well in the eastern and western of Bass Strait, due to the formation of higher amounts of high-Mg calcite in these areas. The Sr concentration is mainly related to carbonate mineralogy. In eastern Bass Strait, the relatively high Sr content is related to aragonite mineralogy. High Sr content in a few localised areas in the west of Bass Strait are correlated with higher accumulations of fauna. Na values have a wider range in the western compared to the eastern Bass Strait. This may reflect salinity variations in the depositional environments due to the presence of Leeuwin and sub-Antarctic water masses in western Bass Strait. Fe and Mn concentrations are higher around island and island groups, as a result of high terrigenous input, sourced from these islands. The oxygen isotope values of bulk sediments are strongly related to the seawater temperature and in some extent the salinity of the water (possible evaporation). It also reflects mineralogy of the sediment in some areas. The values of the bulk sediments are higher in western compared to the eastern Bass strait, due to the influence of cool sub-Antarctic water to the western Bass Strait. The higher Oxygen isotopic values in the north of western Bass Strait, are related to the presence of higher amounts of aragonite mineralogy. Variation in 5 ^^C values in Bass Strait is largely due to mixing water masses and water temperature. The values, increase with decreasing temperature north to south west of Flinders Island. The depleted values in the central parts of western Bass Strait can be related to mixing of deep sub-Antarctic water with shallow waters.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A NEW MID-CARBONIFEROUS PALEOMAGNETIC POLE FROM NORTHERN AUSTRALIA: FURTHER CONSTRAINTS ON GONDWANA'S APWP Kari Anderson School of Earth Sciences, Macquarie Univesity, Macquarie University, Sydney, 2109, Australia Queensland's Thomson Fold Belt, the northern extent of the Tasman Orogen, is the structural marker of Gondwana's trailing edge during the late Paleozoic. The Newcastle Range (18.3°S, 143.7°E) is a composite volcano-tectonic cauldron in the center of the Thomson Fold Belt that has experienced relatively minor deformation, making it well suited for paleomagnetic investigation. Welded rhyolitic ignimbrites dominate the Newcastle Range Volcanics but contemporaneous lava flows and microgranitoid intrusions are common. The mid-Carboniferous segment of Gondwana's APWP is still poorly defined and it is suggested that data presented herein could provide a key pole in the definition of this part of the path. Three mid-Carboniferous ( - 3 2 7 Ma) volcanic formations have yielded an internally consistent paleomagnetic pole, mean declination and inclination of D=189°, 1=60.3° with N=13 sites (n=72 samples), k=109.8, a95=4.6 with corresponding palaeopole at 65.7°S, 127.1°E. Examples of Gondwanan poles that agree with this NRV pole, when rotated into Australian coordinates, include the Ain Ech-Chebbi, Hassi Bashir (Morocco, western Gondwana) pole located at 54.°6S, 121.3°E and the La Colina Basalt (Argentina, cratonic Gondwana) pole of 51.2°S, 119.4°E, mid-Carboniferous paleopoles that are significantly south of previously published APWPs for Paleozoic Australia. Specimens from the Bousey Rhyolite, Kitchen Creek Rhyolite and the Routh Creek Dacite are characterized by a distinctive south and moderately down direction, reversed polarity and univectorial thermal demagnetization curves. Samples fi-om the Bousey Rhyolite and a site in the Routh Creek Dacite where an associated ring dyke is exposed display both the reversed direction (ignimbrites) and a complementary normal polarity (dyke samples). Rock magnetic tests suggest the primary magnetic carriers are PSD titanomagnetites and titanohematites, particularly stable combinations and providers of additional evidence that the characteristic directions are primary. A less well defined Permian pole (Brodies Gap Rhyolite) determined during this investigation has a mean declination and inclination of: D=215.1°, 1=80.0°, N=4, k=39.36, a95=14.8. The paleopole location, 33.7°S, 130.4°E, although poorly constrained, agrees with previously published Permian poles for cratonic Australia. The dominant magnetic carrier in the Permian

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15'^ Australian Geological Convention, Sydney, July 2000

USE AND ABUSE OF THE TERM CALCALKALINE Richard J. Arculus Department of Geology, Australian National University, Canberra, ACT 0200

We are collectively in trouble as petrologists and geochemists with the use of the term calcalkaline (or calcalkalic) because: 1. The term was one of several originally defined by Peacock (1931) applied to a specific group of rocks within the spectrum of volcanic rock series; the terms (alkalic, alkali-calcic, calc-alkalic, and calcic) were applied to arbitrary divisions of wt% Si02 ranges within which the wt% CaO = wt% (Na20 + K2O) The continued use of the term calc-alkalic (or variations in spelling thereof) as per Peacock's original definition has only been retained by granite petrologists - note that feldspar compositions (readily determinable with a petrologic miccroscope) were regarded as particularly significant in early classification systems, and there was obvious merit in using discriminants based on combinations of alkalies vs. lime. 2. The majority of geochemists and petrologists (e.g., see Gill, 1981) engaged in studies of volcanic rocks having employed the "total alkali - silica [TAS] diagram" (Le Bas et al., 1986) to firstly distinguish between subalkaline and alkaline series, then employ either: i. Miyashiro's (1974) arbitrary discriminant based on FeO*/MgO vs wt% Si02; ii. Irvine and Baragar's (1971) or Kuno's (1968) discriminant boundaries in (Na20 + K2O) vs. FeO* vs. MgO (AFM) diagrams; to distinguish between tholeiitic and calcalkalic rocks. Note that "lime" (or "calc") does not feature directly in any of the TAS, FeO*/MgO vs wt% Si02 or AFM diagrams; 3. Calcalkaline has become synonymous in many geologists' minds variously with: i. medium-K (as opposed to low-K = tholeiific, or high-K = shoshonitic/alkalic) volcanic rock series; ii. derivative from high-Al basalt (rather than "low-Al = tholeiitic"); iii. "hypersthenic" as opposed to "pigeonitic" rock series (associated with low-K tholeiitic parental basalts) (Kuno, 1968); iv. moderately light rare earth vs. heavy rare earth element (REE) enriched cf tholeiitic suites with low LREE/HREE. 4. Wherever calcalkaline rocks are to be found, it is assumed that there must be (or have been) an island or continental arc! Whenever the same term is being used to describe objects that differ from one another in definable, different, and interesting ways then we have a clear scientific problem, and communication between scientists inevitably will be at cross-purposes. The purpose of igneous rock classification is to identify distinctive members of the compositional spectrum, and to establish bounding parameters that constrain genetic theories for particular rock types. The discriminant between alkaline and subalkaline rocks in the TAS diagram has particular significance because: i. many volcanic suites of distinctive geochemical and mineralogical composition clearly diverge from the discriminant boundary (rather than crossing or straddling it); ii. the position of the boundary in wt% (Na20 + K2O) vs. Si02 space results from the projection in this compositional space of the low-pressure (105Pa to 1 GPa) thermal divide corresponding to the stability of olivine-clinopyroxene-plagioclase that separates melts that fractionate to feldspathoid-normative from hypersthene- (and ultimately) quartz-normative compositions respectively. Within the realm of subalkaline rocks, it has proven useful to distinguish suites characterised by varying degrees of Fe-enrichment at equivalent SiOs contents (or some other measure of magmatic evolution). The key factors controlling this variability are: i. water content (controls the timing of phase appearance of plagioclase compared with ferromagnesian minerals); ii. oxidation state (controlling inter alia: a. the appearance and persistence of a spinel phase in the compositional range Al-bearing chromite to magnetite; b. solubility of S and hence sulfide/sulfate stability). Note that there is a clear economic significance to the influence of redox state. However, it has not yet been established that any fundamental physico-chemical equivalent of the olivineclinopyroxene-plagioclase low-pressure thermal divide exists to separate high- from low-FeO*/MgO rock suites; rather we should assume at present, that a continuum exists (but see below). Granted the usefulness however, of distinguishing variations of FeO*/MgO (at specific fractionation variable), I propose we restrict the use of the term calcalkaline only for those suites that conform to Peacock's original criterion, and introduce terms such as "low-, medium- and high- FeO*/MgO (abbreviated loFe, meFe and hiFe) subalkaline" suites based on arbitrary divisions of the natural subalkaline compositional spectrum, radiating from a common point at 49 wt% Si02 and FeO*/MgO = 1.0

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HEAVY METAL LOADINGS IN THE MACLEAY RIVER CATCHMENT, NORTHEASTERN NEW SOUTH WALES P.M. Ashley and B.P. Graham Earth Sciences, University of New England, Armidale, NSW 2351 Heavy metal contamination of river systems can occur from a variety of sources including the erosion of naturally mineralised rocks, mining activities and from urban and industrial wastes and effluents. We have investigated heavy metal loadings in the Macleay River catchment, one of the major coastal river systems in northeastern NSW. Sources, transport and sinks of metals have been studied and potential environmental consequences considered. The catchment covers 11,500 km2 and includes parts of the Northern Tablelands, the eastern escarpment or gorge country, the lower Macleay valley and estuary. Higher parts of the catchment on the tablelands are commonly underlain by Tertiary basalt. Late Palaeozoic-early Mesozoic granitoids occupy -10% of the catchment and the remainder is dominated by late Palaeozoic metasediments (mostly accretionary complex). Eloodplain and estuary sediments occupy only - 5 % of the catchment, which also contains several hundred mineral deposits. Except for low-energy tracts on the tablelands, most of the catchment contains a high-energy gravel-bed stream system, maintained until the tidal limit near Kempsey. In this study, 490 stream sediments and 70 waters have been collected, with sampling of all major subcatchments. Sample density was increased at and downstream of major mineralised areas (Hillgrove [SbAsAu], Rockvale [polymetallic]. Halls Peak [PbZnAgCuAsSbCd], Enmore-Melrose [AuAs], Mungay Ck [SbAs]) and urban areas (Armidale, Kempsey, Walcha). Sieved (-180 fim) stream sediments were analysed for Cu, Pb, Zn, Cd, As, Sb, Ee and Mn. Filtered waters were analysed for Cu, Pb, Zn, Cd, As, Sb, Na, K, Ca, Mg, CI and S04, with pH, conductivity and salinity also being measured. Selected stream sediments have been analysed for Pb isotopic composition and sequential leach tests performed to determine metal siting. Results indicate that urban areas, including major traffic arteries, have a limited and local effect on the catchment. Enrichments of Pb, Zn and Sb are up to lOx background, with lesser enrichments of Cu and As, but downstream dispersion extends only a few kilometres. Urban Pb is partly derived from petrol and Zn from galvanised iron and detergents. Many mineral deposits are small and have negligible metal dispersion detectable at the scale of sampling. However, some mineral fields have caused strong and lengthy contamination trails in stream sediments, and locally in waters, although acid mine drainage effects are very restricted (Halls Peak and Rockvale). Moderate Sb(As) anomalies occur downstream of Mungay Ck for several kilometres and a minor As anomaly is evident at Enmore-Melrose. Local AsCuPb anomalies occur at Rockvale, with As detectable for 25 km in the Wollomombi R. Strong PbZn(CuAsSbCd) contamination has occurred at Halls Peak, with up to 50x background Pb values in the Chandler R. The Pb(Zn) anomaly in the Chandler R-Macleay R system can be detected for at least 40 km downstream from Halls Peak. The Hillgrove mineral field has caused strong and extensive Sb(As) contamination of stream sediments in the Bakers Ck-Macleay R system, with anomalous As being detectable for -100 km downstream and Sb (at 5xlOOx background values) being detectable to the Pacific Ocean (-300 km). Sediment cores from the Macleay floodplain and estuary show up to lOx background Sb enrichment and 4x background As enrichment in the most-recently deposited (top 20-50 cm) sediments, but not at greater depth. The anomaly from Hillgrove has been partly derived from past (1870's-1950's) mine waste disposal practices and is unlikely to have been significantly sourced from present mining operations. It is also derived from erosion of a heavily mineralised system and from mine waters. Sb(As) contents of stream sediments and waters in Bakers Ck are up to 10200x NWQMS guideline values. Above-background (4-8 ppb) values of Sb occur in waters of the trunk Macleay R to downstream of Kempsey. The long Sb(As) dispersion train from Hillgrove (and other Sb-rich mineralisation in NE NSW) indicates substantial physical and chemical transport, with subsequent entrapment of metals in fine sediments (clays, organics, pyrite). It attests to the high mobility of these potential toxicants in the fluvial environment. Organic and acid sulphate soil processes in the Macleay floodplain and estuary may facilitate further mobility of metals. Acknowledgements: This project has been supported by the University of New England, Macleay Catchment Management Committee, Department of Mineral Resources and Armidale City Council.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEODYNAMIC FRAMEWORK AND CHARACTERISTICS OF MINERALISING FLUIDS AT THE MCPHEE'S GOLD DEPOSIT, PILBARA CRATON, WESTERN AUSTRALIA D. E. L. Baker, P. K. Seccombe and W. J. Collins School of Geosciences, The University of Newcastle, Callaghan, N S W 2308

The McPhee's deposit, a recent gold producer in the Archaean Pilbara Craton, is located within a multiply deformed succession of metavolcanic rocks. A structural, alteration and fluid inclusion study was undertaken to examine the nature of fluids associated with Au mineralisation and the timing and relationship of mineralisation to deformational events. Regional deformational occurred during 5 stages (D1-D5), of which the later four affected the McPhee's area as evidenced by mesoscopic structures within and surrounding the deposit. A ubiquitous, variably developed tectonic fabric formed during D2 and is defined by peak metamorphic minerals. Mineralisation is hosted within, but post-dates, a 400mwide D2 high strain zone comprised chiefly of talc-chlorite-carbonate schist (deformed and metamorphosed komatiite and peridotite) which envelops relatively competent boudins of actinolite-chlorite schist (deformed meta-basalt). Regional open folding and sinistral shearing occurred during D4 deformation which also controlled syn-kinematic intrusion of pegmatitic granite at ca. 2880 Ma (P.D. Kinny pers. comm. 1999). Narrow, sinistral, NNEtrending D4 shears cut earlier structures and were conduits for mineralising fluids, which reacted with and preferentially mineralised mafic rocks. Gold is disseminated in schistose wallrock and within veins composed of actinolite, quartz and albite, and minor tourmaline, talc, pyrrhotite and magnesite; vein selvedges consist of albite and tourmaline. The veins were not significantly affected by subsequent deformation. Fluid inclusions in quartz include early carbonic (H20-C02-6CH4-6NaCl) inclusions and later aqueous (H20-NaCl-MgCl2) and mixed CO2-CH4-N2 inclusions. Early inclusions are variably gas-rich (10-100 vol. % CO2-CH4) and of variable salinity (3-21 eq. wt% NaCl, mean 10 eq. wt%). Homogenization and decrepitation of inclusions indicate minimum temperatures of trapping at 260-420''C for carbonic inclusions and at 100-360''C for aqueous inclusions, which have a bimodal distribution of salinity values. Lower salinity aqueous inclusions generally have higher minimum temperatures of trapping and locally appear coeval with early carbonic inclusions. Higher salinity aqueous inclusions homogenise at lower temperatures and invariably occur in planar arrays cutting recrystallised quartz. Early carbonic and co-existing, higher temperature aqueous inclusions are considered to contain primary mineralising fluids. Structural relations indicate that mineralisation at McPhee's was controlled by relatively competent mafic lithologies during late, post-peak metamorphic deformation, associated with intrusion of 2880 Ma old sheet-like pegmatitic granite bodies. Relatively high salinities of mineralising fluid, and timing of mineralisation well after peak metamorphism suggests that these hydrothermal fluids were probably derived from a magmatic source, with the coeval pegmatite bodies being the best candidate.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EAST ASIA-WEST PACIFIC: THE BEST GROUND Max Baker^ and Vic WalP ' E.M. Baker and Associates, 191 Hilder Rd The Gap, Brisbane, Queensland 4061 ^Taylor Wall and Associates, 67 St Pauls Terracce, Spring Hill, Brisbane Queensland 4000 Given the current state of the industry, large and small minerals companies are focused on acquisition of more advanced projects and mining entities. In this context "informed opportunism" is the key to selecting the best ground. This strategy requires an active information system, selective geographic and geological focus, as well as the knowledge and ability to pick winners and make valuations which lead to effective investment decisions. Over the past thirty years the application of conventional exploration approaches and models for porphyry and epithermal styles in Asia-Pacific belts has met with considerable success, evidenced by the discovery of world class systems such as Grasberg-Ertsberg, Panguna, Ok-Tedi, Porgera, Lihir, Hishikari and numerous other significant deposits which attest to the copper and gold endowment of the region. However over the past decade the rates and tenor of discovery (and recently, exploration activity) have declined. More effective selection and evaluation of opportunities at prospect and district scales requires a confident working knowledge of the essentials of ore-forming systems at such scales. Many of the current plethora of ore deposit models are basic "recognition" tools, with limited predictive capacity and little regard for the range of possibilities among key structural, lithological and geochemical factors such as intrusion geometries, country rock character and structures, erosion level, etc. We illustrate with models for the essentials of 'Porgera', 'Hishikari', porphyry- and pluton-related systems applicable in Asia-Pacific. At broader scales, opportunistic or more systematic exploration activities should focus on belts which have the potential for world class resources or low capital, low cost production. Discovery history and mineral occurrence distributions provide important tools for selecting the better regions but these need to be prioritised in terms of geological character and evolution. In Asia-Pacific some relatively evolved arcs and back arc regions are most prospective for copper and gold reflecting the evolution and reactivation structural systems (eg arc) and of deformation regimes which control the magmatic and hydrothermal plumbing systems involved. Such regions commonly exhibit the products of a range of geological events and also erosion levels which give rise to a range of target styles. In back arc regions, particularly, some of these target styles (pluton-related copper and gold systems) have been neglected. Furthermore some underexplored arcs or arc segments have architectures, geodynamic histories and mineral occurrences indicative of high copper and gold potential. "Being there f i r s f or early in the exploration history of such 'forgotten' arcs will, as elsewhere, prove to be a potent strategy for companies with longer term vision. It is inevitable that the number of quality resource properties available in Asia Pacific will decline over the next three to five years, requiring re-invigoration of early stage exploration. Smart companies will be well prepared for this, having done their homework in the intervening period. Government agencies and world organisations fostering economic growth in less developed countries can contribute to increasing exploration and mining activity in such regions. World class mining and investment regulations, geological, geophysical and metallogenic datasets as well as specifically targeted initiatives will be essential to attract exploration and mining dollars to even the best ground.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

APPLICATIONS OF GEOPHYSICAL TECHNIQUES TO CHARACTERIZATION OF GROUNDWATER IN RIVERBANKS CONTAMINATED WITH MINE TAILINGS, KING RIVER, TASMANIA. Simon F. Baker Department o f Earth Sciences, Monash University, PO Box 28E, Monash University, Melbourne.

In this study a riverbank covered in fluvially deposited mine tailings containing pyrite was monitored to determine seasonal variation in impacts of the riverbank on river contamiant loading. Riverbanks containing predominantly sands have high values of hydraulic conductivity and fluctuations in river levels and rainfall can produce large changes in groundwater levels. Sediments containing pyrite become highly reactive when exposed to oxidizing conditions, and dissolution and deposition of metals associated with tailings may occur. We installed 42 piezometers for measuring porewater and sediment chemistry, groundwater levels and hydraulic conductivity. We found hydraulic conductivity vary from 5.3 X 10 -06 to 8.0x 10 -08 in a few meters and groundwater level fluctuations of up to 3 meters, over 110% of height above river levels. Groundwater contours in winter were parallel with the river but in summer regions of recharge and discharge occurred along the 800-meter section of bank investigated. In summer we found a slight net recharge of the riverbank occurred, and in winter, discharge of 35kL/day. Riverbank porewater had pH values as low as 3.7 and TDS of 1200 mg/1. Electrical conductivity values varied from 800mS to 3000mS. Large variation in these values between near-by piezometers indicated a need for much higher resolution for monitoring purposes. In order to determine contribution of the groundwater to the total river contaminant flow, we needed to constrain the depth of the contaminant zone. Electrical geophysical methods were used including self-potential, resistivity, GPR and time domain EM in order to provide a high-resolution groundwater and geological profile of the bank. Vertical soundings and surface contours of self-potential using stainless steel electrodes gave vertical gradients of 1V / m, and a variation of IV along the length of the bank. Vertical gradients were attributed to chemical gradients generated by oxidation reactions of porewater with pyrite. Horizontal gradients in the upstream section of the bank were due to streaming potential effects, although the downstream contours appeared to indicate salinity gradients. Resistivity soundings using a Wenner array gave values of 10 to 30 Ohm.m near the surface and made measurements of the deeper less contaminated layer difficult. GPR surveys gave inconsistent indications of the original bank layer, and time domain EM signals were dominated by paramagnetic effects close to the river. Magnetic susceptibility of sediments was about 100 x 10-5 SI near the water table, about a meter deep. And higher in the vicinity of hardpan layers near the river.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A METHOD TO QUANTIFY THE DEVELOPMENT OF A DEEP SEDIMENTARY BASIN: THE BONAPARTE BASIN, NW AUSTRALIA Stephanie Baldwin Cambridge University, Department of Earth Sciences, Bullard Laboratories, UK Now at: Shell International Exploration and Production, UK The Bonaparte Basin on the North West Shelf of Australia, contains >25 km of Devonian to Recent sedimentary succession. This vast depth to basement is uncommon compared to the average depth of sedimentary basins worldwide, which are typically <15 km deep. However, several examples of deep basins exist globally which are defined by the extensional range: 2 < p< oo. In order to understand the evolution of deep sedimentary basins, the spatial and temporal development of the Bonaparte Basin has been studied in detail. Over 7,500 km of seismic data, including 2,500 km of deep seismic data imaging to 14 seconds twoway-travel-time, were interpreted in an attempt to constrain the depth and structure of basement across the entire basin area. A database of over 100 exploration wells was used to determine the ages and lithologies of a total of 19 interpreted seismic horizons. To define periods of structural activity during basin development, the styles, spatial distribution and timing of faulting were documented. Normal faults dominate the structural style, suggesting that the basin formed by extension of the lithosphere, however, examples of localised strike-slip and reactivated inverted faults are also evident. The observed fault pattern is used to constrain models explaining the presence of a gravity high in the centre of the basin. Since gravity modelling is non-unique the origin of the gravity high cannot be determined unequivocally, however a model is presented here which explains the observed anomaly in terms of a thinned basin margin signature. In order to compare the Bonaparte Basin with other sedimentary basins worldwide, a quantitative method to automatically extract the temporal variation in tectonic subsidence and strain rate fi-om well data was used. The amount of crustal stretching, p, required to obtain the observed amount of total tectonic subsidence is also calculated by this technique. Eleven discreet extension episodes are identifiable across this basin. Their spatial and temporal distribution is observed as a northward progression in rifting from the Bonaparte Basin to the North West Shelf These modelled results for the number, timing and duration of tectonic events correlate with independent observations of fault offset. To quantify the subsidence and strain rate for the deep parts of the basin not directly sampled by exploration wells an additional database of 115 synthetic well points was derived from interpreted seismic data. These synthetic data show much higher amounts of total tectonic subsidence and strain rate than the global average for sedimentary basins. However, these large values are not unexpected and are predicted by the uniform crustal stretching model, as the maximum subsidence expected (for a given total p) at the time of lithospheric re-equilibration. This consistency between observation and theory suggests that it is possible that nothing more than lithospheric extension need be invoked to account for this large amount of total tectonic subsidence in the Bonaparte Basin, and possibly for deep basins worldwide. Results for the Bonaparte Basin show that the current global 'average' values of the expected subsidence and strain rate variation for sedimentary basins, are heavily skewed towards a minima since the global data sample the areas of sedimentary basins where basement <4 km. The Bonaparte Basin data lie in the same trend as the global data, but extend the range of the expected limits of p. This continuity suggests that the global estimates for average subsidence and strain rate are not representative of the full range recorded in sedimentary basins worldwide. Results presented here show that a more accurate range in this global variation is obtained if the total area covered by sedimentary basins can be quantitatively measured.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15''^ Australian Geological Convention, Sydney, July 2000

THE ONSET OF PERMIAN VOLCANISM IN THE SOUTH-EASTERN SYDNEY BASIN Glen Bann and Brian Jones, School of Geosciences, University of Wollongong, NSW, 2522, Australia

Shallow marine sediments of the mid-Permian upper Shoalhaven Group were derived from an associated andesitic to basahic (or mafic) shoshonitic volcanic arc. New evidence, using geophysical and sedimentological data, indicates that this volcanic arc consisted of a number of large volcanoes, the largest of which was situated 40km south-south-east offshore from Jervis Bay. Jervis Bay itself was an active volcano. A dyke/sill which intruded wet, unconsolidated sediments of the lower Berry Formation, an ash band towards the top of the Berry Formation and a new lower member of the Gerringong Volcanics, the "Coolangatta Latite Member", indicates volcanism was penecontemporaneous with deposition by this period (i.e. mid-Permian). Evidence indicates a relatively proximal source from volcanoes ranging from mild Strombolian to the violently explosive Vulcanian or Plinean phreatomagmatic eruptions. Volcanic detritus swamped sediment derived from the Lachlan Fold Belt and resulted in high rates of sediment accumulation in the eastern half of the basin. This caused the conformable coarsening up succession from the Berry Siltstone to the Broughton Formation. Deposition was dominated by episodic storm activity in cold climate conditions with periodic coastal ice sheets depositing volcanic-derived clasts and megaclasts (dropstones) in the east. Glacio-eustatic sea level transgressions and regressions were overprinted by the influence of volcanism and tectonism in this part of the Sydney Basin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SHRIMP ZIRCON DATING OF GRANITOIDS FROM MYANMAR: CONSTRAINTS ON THE TECTONIC EVOLUTION OF SOUTHEAST ASIA Mark E. Barley, April L. Pickard and Khin Zaw Centre for Global Metallogeny, University of Western Australia, Nedlands, WA 6907 Centre for Ore Deposit Research, University of Tasmania, Hobart, Tasmania, 7001

Situated south of the eastern syntaxis of the Himalayas, Myanmar occupies a key position in the tectonic evolution of Southeast Asia. However, there is almost no modem geochronology for this region. In this contribution we present new SHRIMP zircon dates for granitoids from the Shan Scarp (Mogok Metamorphic Belt), Taninthayi (Tenasserim) Region in the Myeik (Mergui) Archipelago and Central Valley (Western Myanmar) regions of Myanmar. The oldest ages obtained were from Jurassic granitoids, gneisses and amphibolites interlayered with marbles that were metamorphosed and deformed during the EoceneOligiocene and Miocene in the Mogok Metamorphic Belt. The occurrence of mid-Jurassic metamorphosed igneous rocks in the Mogok Metamorphic Belt is similar to that in the Hunza Karakoram and confirms interpretations that the southern margin of Asia became an Andean-type convergent margin at that time. Ages between 120 and 80 Ma for I-type granitoids intruding the Mogok Metamorphic Belt, Myeik Archipelago and Western Myanmar confirm that an up to 200km wide mid Cretaceous magmatic belt extended along the Eurasian margin from Tibet to Sumatra. Fractionated I-Type granitoids, that locally host Sn-W mineralisation, were emplaced in the Myeik Archipelago (and adjacent Thailand) in the latest Cretaceous to Early Eocene (80 to 50 Ma). These granitoids formed a wide convergent margin magmatic belt as the Indian plate rapidly approached Eurasia. Deformation and high-grade metamorphism occurred in the Mogok Metamorphic Belt during the Eocene-Oligiocene as the collision between India and Eurasia initiated crustal thickening prior to extrusion, or rotation, of Indochina and northward movement of Western Myanmar. Arc magmatism continued in Western Myanmar with emplacement of granitoids in the Central Valley. Deformation, extensional uplift and further granitoid magmatism occurred in the Mogok Metamorphic Belt during the Early Miocene northward movement of Western Myanmar.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A GEOCHEMICAL CLASSIFICATION SCHEME FOR GRANITIC ROCKS Calvin G. Barnes Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053, USA.

We present a geochemical classification of granitic rocks which is based upon three variables that have been used in the petrologic community for decades. These are, in decreasing order of importance, the FeO/(FeO+MgO) ratio of the rock (or Fe#), the alkaliUme index of Peacock (1931) and the aluminum saturation index of Shand (1927). The primary classification consists of three series: for rocks with <70% SiOi, or suites that contain rocks with silica contents below 70%, the ferrous series have Fe# that lies above a boundary modified from Miyashiro (1970); the magnesian series have Fe# that lies below that line. For rocks that have >70% Si02 and are not obviously a fractionated part of the above, we designate the leucogranitoid series, which have such low concentrations of the elements CaO, FeO, and MgO, that small changes become meaningless. A secondary classification divides the magnesian and ferrous series into four classes: alkalic, alkalicalcic, calc-alkalic, or calcic using the variable Na20+K20-Ca0, which extends the Peacock (1931) classification to high silica rocks (Si02>61%). Within each class (and the leucogranitoid series), the tertiary classification is peraluminous, metaluminous or peralkaline. This classification leads to 19 possible groups of granitic rocks, though not all of the categories are populated with known occurrences. The classification scheme distinguishes well between various types of granitic rocks from differing tectonic associations. The granitoids of the Cordillerian batholiths are part of the magnesian series, are calcic and calc-alkalic in character, and tend to be either metaluminous or peraluminous. We suggest they be called calc-magnesian granitoids. Shoshonitic granites are part of the magnesian series. Most of these are alkali-calcic, but alkalic varieties are known. We propose to call these alkali-magnesian granites. The within-plate or "anorogenic" granitoids are part of the ferrous series and are dominantly alkali-calcic, though associated monzonites and alkali granites may be alkalic. We propose calling them alkali-ferrous granitoids. This classification scheme is preferred to any present granite classification because it is purely chemical, it requires no a priori inference as to the origin of granites, and it relies upon well-recognized major element discriminators.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15''^ Australian Geological Convention, Sydney, July 2000

EVIDENCE FOR MAGMA MIXING IN THREE CORDILLERAN PLUTONS, WESTERN NORTH AMERICA Calvin G. Barnes and Kenneth Johnson Department Of Geosciences, Texas Tech University, Lubbock, Tx 79409-1053, USA. Field evidence for magma mingling is widespread in Cordilleran plutons. Synplutonic dikes and mafic microgranular enclaves (hereafter "enclaves") are generally accepted as the result of injection of mafic and intermediate magmas into magma chambers. Disruption of synplutonic dikes can result in enclave swarms and, at extreme levels of disruption, in schlieren bands. In volcanic rocks, similar styles of disruption result in clots of mafic rock in felsic hosts and locally in banded pumice. One of the outstanding questions in granite {s.l) petrology is whether magma mixing logically accompanies magma mingling. If mixing is thorough, field evidence is, by definition, destroyed. Thus, if magma mixing is important in granite petrogenesis, other lines of evidence must be used. Rapid cooling of volcanic rocks commonly preserves mixing evidence in the form of (1) disequilibrium phenocryst assemblages (Anderson, 1976; JVGR). Simple mixing also results in (2) linear compositional arrays in binary plots and hyperbolic arrays in ratio-ratio plots (Langmuir et al., 1978; EPSL). Such mineralogical and chemical evidence is sometimes preserved in plutonic suites, assuming one can see past the effects of other processes, such as crystal accumulation. We apply these ideas to three examples of mixing in Cordilleran plutons. The Wooley Creek batholith (161 Ma) was emplaced into a package of thrust-bounded oceanic terranes of the Klamath Mountain province. Subsequent tilting exposed an ~6 km-thick section of the pluton, which is upwardly zoned from diorite to granite. The central part of the pluton is characterized by enclave swarms and synplutonic dikes in various stages of disruption. Below this level, the pluton is isotopically homogenous and shows trace element patterns consistent with recharge-fractional crystallization (RFC), whereas above, the pluton lacks evidence for RFC. Roof dikes derived from the lower level contain plagioclase with prominent zoning reversals, as would be expected of an origin in a zone of magma mixing. The central mingling zone probably formed at a density trap within the pluton, where dense mafic magmas stalled and were disrupted. Below the density trap, magmas such as those seen in the roof dikes were thoroughly mixed. The coeval, gabbroic to tonalitic Ma Grayback pluton was emplaced in the same setting as the Wooley Creek batholith. Composite basalt-granodiorite and basalt-tonalite dikes demonstrate the presence of a number of mafic and felsic components in the magma system. Geochemical variation among hybrids in these dikes shows characteristic mixing trends that are also preserved in the host pluton. In fact, curvature of the hyperbolic mixing lines serves to identify at least one of the mafic end members of the suite. In addition, the main stage preserves Mg-rich cpx and calcic plagioclase inherited from injection and mixing of H20-rich basaltic magmas. The 36 Ma Harrison Pass pluton intruded the infrastructure—suprastructure boundary of the Ruby Mountains core complex (NE Nevada). Early-stage tonalite to granite in this pluton contains plagioclase with a prominent zoning reversal, biotite rimmed by hornblende, and linear compositional variation. The elemental variation is consistent with simple mixing of tonalitic and monzogranitic end members, modified by crystal accumulation. Temperature of emplacement of the early stage was low (circa 850°C), which suggests that tonalite-into-monzogranite mixing could not occur due to viscosity constraints. Instead, we propose that monzogranitic magma intruded a tonalitic magma, with consequent vigorous mixing (cf Weinberg & Leitch, 1998; EPSL). We conclude that magma mixing in a variety of plutons can be discerned with the use of mineral disequilibrium relations. In ideal cases, linear compositional variation can also be used to document magma mixing. In well-stirred magmas, evidence for RFC can be preserved as enrichments of trace element above that expected from fractional crystallization.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GRAFTON - MACLEAN METALLOGENIC MAP, NORTHERN NSW R.G. Barnes, R.E. Brown, J.W. Brownlow, H.F. Henley and W.J. Stroud Geological Survey of New South Wales, Dept Mineral Resources, PO Box 65, Armidale, 2350 A metallogenic map covering the Grafton and Maclean 1:250,000 sheets has been prepared by the Geological Survey of New South Wales for publication in 2000-2001. This map and accompanying notes cover an intensely mineralised area in north-eastern New South Wales and add to the major NSW metallogenic map series. The map and accompanying notes, data sheets and mineral deposit database information record the locations and geological characteristics of over 2000 mineral deposits and occurrences. These data have been progressively released as exploration data packages as they have been acquired. The metallogenic map and notes integrate this knowledge into a condensed package. The Grafton Maclean metallogenic map extends over a large part of the northern tablelands, across the Great Dividing Range and to the coast. Mineral production from the map sheet area exceeds 89000 t of tin concentrate, 3.5 t gold, a large part of the 500 M carats of sapphires from the combined Glen Innes and Inverell field, 4770 t arsenic oxides, 1700 t chromium oxide, >89000 t of heavy mineral sand concentrate (zircon, rutile, ilmenite, but also gold, platinum, osmiridium and tin), >470 t tungsten, >200 t molybdenite, 200 t bismuth, >500 kg mercury, 19400 t asbestos and 324,000 t coal. Major identified resources in the area are tin at Taronga (37.5 Mt at 0.153% Sn), silexite at Torrington (7.3 Mt quartz-topaz rock, topaz >5%), gold at Timbarra (14.45 Mt at 0.8g/t Au), Mo W Bi at Glen Eden (potential resource >10M t 0.15% Mo+Sn+W) and coal (Nymboida, >650,000 t coal). Geological environments and deposits present on the map include: • •

• • •

• • • •

Siluro-Devonian island arcs and associated basins (host to the enigmatic Cangai copper - gold mine). Carboniferous oceanic crustal rocks and associated deposits now in accretionary complexes. These rocks contain basalt-associated copper and manganese mineralisation. They are also the dominant hosts for metahydrothermal lowsulphide gold-quartz veins (Dalmorton). Basic and ultrabasic ophiolite complex rocks with chromite lenses and minor copper mineralisation (Gordonbrook Serpentinite). Late Permian sub-aerial and marginal marine volcanics (Drake Volcanics) hosting epithermal precious metal and basemetal deposits. Extensive Permo-Triassic granitoids in several geochemically distinct suites with associated mineralisation. The deposit types include: • tin and base-metal veins and disseminations associated with fractionated leucogranites (Mole Granite, Glen Elgin). The Mole Granite is the major mineralising system with hundreds of deposits occurring in and surrounding the granite. In addition to the well- known tin deposits, the granite has produced a multitude of commodities including arsenic, topaz, lead, silver and emerald. • molybdenum, tungsten, bismuth and tin in quartz pipes and breccias (Kingsgate, Glen Eden). • gold disseminated in leucocratic granite (Timbarra). • magnetite and copper skams associated with Clarence River Suite plutons (Fine Flower). Intracontinental basin sediments of the Triassic-Cretaceous Clarence Moreton Basin and infrabasins with contained coal, and potential for coal seam methane and petroleum. Tertiary sediments and weathering surfaces with extensive accumulations of tin (Emmaville). Tertiary volcanics with sapphire accumulations in epiclastic and volcaniclastic rocks (Glen Innes). Quaternary and recent sediments with gold (Timbarra), sapphire (Glen Innes) and tin (Emmaville) accumulations, and coastal heavy mineral sands.

The mineral exploration interest in the sheet area is focussed on: • • • • •

gold in leucocratic granite, gold-copper porphyry systems, tin skams, disseminations and multi veins deposits and molybdenum porphyry systems. sapphires, particularly paler coloured varieties east of Glen Innes and other gemstone and topaz occurrences elsewhere. metahydrothermal gold quartz vein systems. construction materials in populated coastal areas. coal seam methane, petroleum and coal in the Clarence Moreton Basin.

The Grafton Maclean metallogenic map and its supporting data and interpretative information provide a core geoscience data set for the region supporting mineral exploration, geoscientific enquiry, geochemical and geophysical interpretations, and environmental and natural resource studies. The data will assist in land use decisions and add significantly to regional and national mineral potential studies. Acknowledgement: Published with permission of the Director General, NSW Department of Mineral Resources.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

Nd ISOTOPE CONSTRAINTS ON SOURCES OF PALAEOZOIC PETROLEUM BEARING ROCKS IN THE AMADEUS BASIN, CENTRAL AUSTRALIA Karin M. Barovich and John Foden Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005 The Amadeus Basin, central Australia, is one of a number of shallow intracratonic basins across central and southern Australia formed during the Late Proterozoic through Early Palaeozoic. The basin is bounded to the north and south by Proterozoic basement, the Arunta and Musgrave Blocks, respectively. The Mereenie and Palm Valley oil and gas fields in the basin currently yield hydrocarbons. Formations of the Ordovician Larapinta Group are both the primary hydrocarbon source and reservoir rocks. Previous Nd isotope constraints on the provenance of Cambrian Pertaoorrta Group sedimentary rocks in the basin (Zhao et al., 1992), which are restricted largely to sub-basins in the north, indicate these rocks were derived from a localized Musgrave Block source. As discussed by the authors, these results are in agreement with sedimentological studies that propose a localized source region to the southwest during the Cambrian, which developed due to uplift of the Musgrave Block during the Petermann Ranges Orogeny (see Lindsay and Korsch, 1991). Their isotope data can also be used to support suggestions that the northeastern margin of the Amadeus Basin during that time was not a topographic high. New Nd isotope data place constraints on the provenance of the hydrocarbon source and reservoir rocks of the shallow marine Ordovician Larapinta Group, and reveal a widespread averaged continental source terrane, with a signature reflecting contributions from both the younger Musgrave and older Arunta Blocks. Initial 8Nd values for the Pacoota Sandstone, the Horn Valley Siltstone and the Stairway Sandstone of the Larapinta Group range from -11 to -13.5. Average Musgrave Block at Ordovician time was around -9.5, while average Arunta Block was around -15 (Zhao et al., 1992). The shift to more negative initial 8Nd values from the Cambrian to the Ordovician sedimentary rocks documents a change from a localized uplifted Musgrave Block source to a more widespread regional source, which included both Musgrave and Arunta Block material. The continuous nature and slow rate of the Ordovician sedimentation and the sediment maturity also suggest a more averaged continental source terrane. Evidence of such a widespread source terrane for the petroleum bearing and petroleum source units of the Larapinta Group suggests similar sedimentary origins for identifiable correlative units in other central Australian sedimentary basins. These results are encouraging for hydrocarbon potential of correlative Ordovician rocks in the formerly contiguous Georgina, Ngalia and Officer Basins. References

LINDSAY, J.F. AND KORSCH, R.J. 1991. The evolution of the Amadeus Basin, central Australia. Aust. Bureau ofMin. Res., Geol and Geophys. Bull 236, 7-32. ZHAO, J.X., MCCULLOCH, M.T. AND BENNETT, V.C. 1992. Sm-Nd and U-Pb zircon isotopic constraints on the provenance of sediments from the Amadeus Basin, central Australia: Evidence for REE fractionation. Geochim et Cosmochim Acta 56, 921-940.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney, July 2000

FEATURES OF DIAMONDS FROM COPETON-BINGARA, NSW, AUSTRALIA L.M. Barron\ S.R. Lishmund\ B.J. Barron^ and G.M. Oakes' 'NSW Department of Mineral Resources, 29-57 Christie Street, St. Leonards, NSW 2065, Australia. ^Consulting Petrologist, 7 Fairview Avenue, St. Ives, NSW 2075, Australia.

NSW historic production totals about two million alluvial diamonds, and production at Copeton was much greater than at Bingara, which was much greater than at Mount Airly and near Wellington. Small numbers of alluvial diamonds are known from hundreds of other sites across eastern Australia. A parcel of 226 diamonds from the principal Copeton mine (Star of the South) shows some key tribal features: the parcel is a mixture of three subdivisions based on size and colour groupings; 89% have abundant negative microdisk scars on their surfaces; 61% have euhedral large indentations; >60% are yellow (unzoned crystals presumed rich in nitrogen); <40% are white (strongly zoned, presumed low to moderate levels of nitrogen), « 1 % have coloured spots due to radiation bums, « 1 % have percussion marks; and most have minimal wear on the surfaces of the predominantly dodecahedral crystals. Bingara diamonds, based on several small historic parcels and a Diamond Ventures N.L. parcel of 768 diamonds, show remarkably similar characteristic features. The Copeton Bingara (CB) stones were historically classified as industrial diamonds because their toughness made them difficult to cut, but modem cutting techniques have solved this problem and more than 90% are now regarded as gem quality. Although the parcel mode of Bingara diamonds (0.16 cts) is smaller than for Copeton diamonds (0.25 cts), the subdivision intervals are similar. The characteristic CB features are anomalous with respect to alluvial diamonds recovered from elsewhere in NSW, Victoria, Queensland, Westem Australia and Africa where the converse is characteristic. Unique physical, chemical and textural features of CB diamonds suggest an unconventional local source. Apparently, similar diamonds have recently been recovered by Russian Minerals N.L. in Eastem Siberia from a geological setting analogous to the CB region (Phanerozoic accretionary volcanic arc). The nature of microdisks on diamonds has been used as an exploration measure. Microdisks are a strong identifier of CB diamonds: most CB stones have them in great abundance; they are so well defined optically at 125x magnification that a sequence of overprinting can be determined. The abundance and size of microdisks decrease rapidly down the face of a large euhedral indent on a Copeton diamond, suggesting microdisks form as a xenolith texture. This is compatible with published experiments on diamonds which show that etching at about 950''?C produces microdisks, while trigons form if the temperature is significantly different. Despite the great abundance of microdisks on CB diamonds, the feature is difficult to image with a scanning electron microscope, especially for yellow stones, so microdisk relief must be at or below resolution on CB stones. Microdisks are much easier to SEM-image for diamonds from other areas. For instance, SEM images of a Smoke Creek alluvial diamond (Argyle) show trails of overlapping microdisks.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ISOTOPIC PROVENANCE ANALYSIS AND TERRANE TECTONICS: A WARNING ABOUT SEDIMENT TRANSPORT DISTANCES Dr. Kari N. Bassett Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand In the last 10 years the field of provenance analysis has undergone a revolution with the development of single-crystal isotopic dating techniques, the most common being U/Pb zircon and 40Ar/39Ar techniques. These have allowed age determination of single crystals thus providing more detail about probable provenance of each individual grain rather than an averaged population of grains. The usefulness for resolving complex terrane accretion and translation histories was immediately obvious and there have been many studies in many different regions aimed at tracking terrane motions by provenance of individual grains upward through the stratigraphy of a basin. Recent research in the North American Cordilleran terranes and in the New Zealand Torlesse Superterrane show how widely used and powerful these provenance analysis techniques are. However, isotopic provenance analysis has often been presented as key information to resolve controversies around terrane translation histories with very little discussion of the context of sedimentary facies and sediment transport mechanisms. An example is the recent use of U/Pb detrital zircon ages as the supposedly controversy-ending evidence for the amount of lateral translation of the Insular Superterrane in British Columbia (Baja BC) (Mahoney et al., 1999). The zircon grains were separated fi-om fme-grained turbidite deposits and could easily have been transported very large distances by a variety of mechanisms; yet they were presented as definitively resolving the Baja BC controversy. Modem examples illustrate the problem of using the provenance of fine grained sediment to constrain terrane tectonics. Sediment in the tip of the Bengal submarine fan was transported - 3 0 0 0 km fi-om source, first by fluvial processes then by sediment gravity flow in the submarine fan. The detrital isotopic ages of single grains are the same as the depositional ages indicating a very rapid unroofing and transport rate with minimal temporary storage. Sediment fi-om the Amazon River can be found in the Barbados trench, transported - 2 0 0 0 km northward by near shore currents after more than 2500 km by the Amazon River fi-om source. It's interesting to note that while large fluvial systems are helpful for transporting sediment very large distances, it is also possible by other mechanisms. Both the Bengal fan and the Barbados sediment have been transported at least 1500 km by purely marine sediment transport mechanisms. Similar transport distances occur in the Hikurangi submarine channel off the eastern coast of New Zealand (Lewis, 1999). There fine sands are transported - 2 0 0 0 km northward into the Kermadec trench by sediment gravity flow and deep water currents. Such modem examples of long distance sediment transport should be kept in mind when interpreting details of terrane translation. The possibility that the fine grained fraction may have been transported 2000-3000 km from their source makes their provenance of very little use when attempting to resolve controversies where distances of tectonic translation are on the same order of magnitude (Insular and Torlesse Superterranes). Thus detailed histories of terrane translation are best developed using coarse rather than fine size fractions. Gravels are transported over 10s to 100s of kilometers rather than the 1000s of kilometers possible for fine grained sediments. In addition, more detailed isotopic and geochemical provenance analysis is possible due to the larger grain size available allowing more specific characterization of the source area. However, truly effective characterization of sediment source rocks can only be accomplished as part of multidisciplinary research projects that would include provenance analysis by a variety of methods combined with basin analysis to understand facies relations and sediment dispersal paths.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GOLD TRANSPORT AT ARCHAEAN LODE GOLD DEPOSITS: GOLD SOLUBILITY ALONG THE ROCK-BUFFERED PATHWAYS Evgeniy N Bastrakov^ Kevin F Cassidy' and Yuri V Shvarov^ ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^Department of Geology, Moscow State University, Moscow 119899, Russia Archaean lode gold deposits of the Yilgam Craton formed under a wide range of temperature-pressure conditions (200 to > 600°C, < 1 - 4 kbar) and are hosted by a range of lithologies with assemblages corresponding from sub-greenschist to the upper amphibolite - lower granulite facies metamorphic conditions. The deposits formed at different crustal levels and have broad similarities in ore-fluid compositions with respect to major molecular components, isotope ratios, and ore geochemistry (Ridley et ai 1996). These data support genetic models invoking long-distance advection of hydrothermal fluid at depths of 3 to 18 km. Whether gold can be both effectively transported and precipitated over the significant range of crustal temperatures and pressures is still largely unexplored. The recent progress in experimental studies of gold solubilities {e.g., see review in Gibert et ai 1998) and interpretation of their results (our study) provide a favourable opportunity to simulate processes of gold mobilisation, transport, and deposition in a quantitative way. Here we report the results of numerical modelling of the generation of gold-bearing fluids in rockdominated systems prior to their structural focussing. The available fluid inclusion data show that the gold-bearing fluids in Archaean lode gold deposits are represented by low-salinity mixed aqueous-carbonic fluids. Their ultimate origin is still debatable, and two main hypotheses have been proposed: (1) metamorphic devolatilisation of rocks during prograde metamorphism, and (2) devolatilisation of granitic rocks during crystallisation. Whatever the ultimate origin, the long fluid travel paths will result in modification of the fluid compositions through fluid-rock interaction. In the extreme case of pervasive fluid flow in a rock-dominated system, the major chemical features of the hydrothermal fluids will be defined almost exclusively by the fluid-rock reactions. We have completed calculations of the gold solubility in aqueous fluids in equilibrium with three different rock types (tholeiitic basalt, mafic granite, and low-Ca granite). Chemical equilibria were modelled for a 15-component system H-O-Cl-C-S-Na-K-Ca-Mg-Fe-Si-Al-Ti-Ag-Au using the HCh package for geochemical modelling. The chosen chemical system was sufficient to reproduce the rock-forming mineral assemblages that can control or affect gold solubility in natural environments. The aqueous speciation model for gold incorporated AuOH^, AuCb", AuHS^ and Au(HS)2' species. The modelling was performed in the TP region of 300-600°C and 1-3 kbar. The initial water-to-rock ratios were specified either by the reactions of metamorphic devolatilisation or by setting them to reasonably small values (~ 0.01-0.03). For each calculation point, our calculations determined the chemical speciation (the combination, abundance, and composition of all stable phases) in a closed multi-component system at equilibrium. The results of our modelling imply different efficiency of gold mobilisation and precipitation at different crustal levels. At low water-to-rock ratios and high temperatures (>400°C), calculated solubility of gold is high enough {e.g., > 1000 ppb) to completely scavenge native gold from the magmatic host-rocks. Our calculations support earlier speculations that high-temperature Archaean ore fluids were probably undersaturated in gold in their source region (Mikucki 1998). Two major implications are as follows. (1) Gold-undersaturation of rockbuffered fluids would permit gold transport over long distances, preventing gold precipitation during pervasive fluid flow in the high temperature (>400°C) low crustal levels. (2) Primary gold-undersaturation in typical fluids of amphibolite-facies terrains might account for the relatively low gold production fi-om amphibolite-facies terrains as opposed to those of greenschist-facies. Gold deposition in high-grade metamorphic terrains may require special conditions, being a subject for further research.

References 1998. Gold solubility and speciation in hydrothermal solutions: Experimental study of the stability of hydrosulphide complex of gold (AuHS°) at 3 5 0 to 4 5 0 ° C and 5 0 0 bars. Geochimica et Cosmochimica Acta, 62, 2 9 3 1 - 2 9 4 7 . MIKUCKL E. J. 1998. Hydrothermal transport and depositional processes in Archean lode-gold systems: A review. Ore Geology Reviews, 13, 3 0 7 - 3 2 1 . RIDLEY J., MIKUCKI E. J., & GROVES D. I. 1996. Archean lode-gold deposits: fluid flow and chemical evolution in vertically extensive hydrothermal systems. Ore Geology Reviews, 10, 2 7 9 - 2 9 3 .

GIBERT F., PASCAL M . L., & PICHAVANT M

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

COMPLEX ADAPTIVE SYSTEMS AN EXCITING PARADIGM FOR ORE DEPOSIT MODELS John L. Baxter Continental Resource Management, P.O. Box 307, Belmont Western Australia 6984 Complex systems are a collection of interacting agents functioning as a whole, which may develop a previously unseen emergent property that is generally irreversible. Ore deposits emerge as a result of the interaction of physical, chemical and temporal agents in the geological environment. The complex adaptive systems approach is a powerful tool in establishing, or assessing, ore deposit models. A system is considered to be complex if it has a significant number of the following characteristics: • • • • • • • •

Agent based: characterized by activities of the individual geological agents; Heterogeneous: agents differ in important characteristics eg. rheology; Dynamic: agents change with time as they respond to local and general factors; Feedback: positive and negative feedback affects agent interaction; Fractal: spatial patterns display scaling, endless repetition and growth of simple units; Organisation: hierarchical agents influence the evolution of the underlying system; Stable disequilibrium: systems rarely attain equilibrium for any period of time; Emergence: coherent macroscopic new properties appear with time.

The system is said to be a complex adaptive system when it evolves through time through a series of usually irreversible steps. Prograde and retrograde alteration system are examples of complex adaptive systems. The study of ore processes involves the scientific investigation of both the nature of systems that are far from equilibrium and the nature of systems which express some measure of self-organisation. Heavy mineral (HM) deposits are the results of emergent properties in a complex adaptive system. The process of formation is relatively simple within a coastal barrier. However, economic grades are not uniform so application of reductionist analysis based on river mouths or long-shore current does not predict economic concentration whereas analysis of the interaction of agents can help. An example is the Minninup Shoreline (WA) which has economic mineralisation at Koombana Bay, Minninup Beach, Wonnerup and Bunker Bay, but little concentration of HM sands has occured between those deposits. Economic grades are created from the interaction of sediment supply, heavy minerals present, transgression and regression, swell and storm waves, coastal geomorphology and preservation. Swell and storm waves and their strength and directions emerge from interaction of agents like the prevailing currents, weather patterns, slope of the continental shelf and presence and shape of offshore islands or promontories. Individual agents do not determine the location, size and grade of a HM deposit and may not even be critical to the overall emergence of an economic deposit but all contribute to the development of the complex adaptive system that forms the deposit. Mesothermal gold deposits form as the result of interaction between such agents as host rock composition, rheology contrast, differential and mean stress, deformation style, fluid pressure, fluid composition, confining pressure and temperature. However, the tenor and type of ore deposit emerging from interaction of the same agents vary markedly from extension quartz veins systems (Bellevue), to compressive shear zones with no significant quartz (Golden Mile). The study of the mineralising system has identified several attractors to ore and also identified agents that do not appear to directly contribute to the emergence of an ore deposit. These include metamorphic grade, vein type and style of shearing (dip-slip or strike-slip) although these factors appear to contribute to the emergence of secondary agents. "Technical success" in exploration is the identification of all or most of the agents, but where emergence of mineralisation has not occurred. Complex adaptive systems may resist the reductionist analysis. Breaking the deposit into component parts may destroy the critical observation of the emergent property. Ore deposits may be viewed as complex systems that evolved toward order with self-reinforcing trends and identifying this emergent behaviour will provide an effective exploration tool. Techniques such as critical path, critical mass, matrix and force field analysis will provide vehicles for effective establishment of ore deposit models formed in complex adaptive systems.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MT MACKENZIE: A LARGE HIGH SULPHIDATION, ADVANCED ARGILLIC GOLD SYSTEM, CENTRAL QUEENSLAND S.D. Beams^ K.J. Harvey^ and J.P. Laurie^ 'Managing Director/Principal Geologist, Terra Search Pty Ltd, PO Box 981, Hyde Park Qld 4812 ^Consulting Geologist, KJ Harvey and Associates Pty Ltd, 25 Famworth Street, Chappel Hill, Qld 4069 ^Exploration Director, Coolgardie Gold NL, PO Box 1026, West Perth, WA 6872

Mt Mackenzie in central Queensland is one of the largest areas of high sulphidation, advanced argillic alteration in Eastern Australia. Down faulting associated with the extensional event that formed the Permian to Triassic Bowen Basin, to the west of Mt Mackenzie, apparently has preserved the more prospective, high level volcanic related hydrothermal system along the western side of the Connors Magmatic Arc, a late Palaeozoic convergent plate margin. High level systems including porphyry coppers, low sulphidation epithermal veins and advanced argillic systems are all represented along this "preserved" belt. The deposit is hosted in Carboniferous andesitic lithic tuffs, volcaniclastics and lavas. These rocks are overlain by Lower Permian dacitic volcaniclastics and lithic tuffs. Recent geologically constrained age dating has placed the mineralisation in the late Carboniferous (300 + 3 Ma). At Mt Mackenzie, the mineralising and alteration system has not been closed off to the north west, south or down dip to the west. The geological model and geochemical data indicate that the intensity of alteration is increasing under Permian volcanic cover to the west. There is therefore a strong likelihood that the center of the system could well be further west than the current drilling. Therefore the major mineral potential within the system could be untested at this stage. Mt Mackenzie displays the typical zonal relationships of a high sulphidation deposit, where the alteration zones decrease in intensity outwards from the mineralised zone of silica-pyrite, to zones of quartz-alunite (very low pH), pyrophyllite (low pH), illite (moderately low pH) and smectite neutral pH) alteration. Au grades are associated with mineralised silica and quartz alunite zones. Away from the siliceous feeder zones the alteration becomes dominated by illite-smectite assemblages of intermediate argillic alteration type and are accompanied by lower Au grades. Mt Mackenzie is considered to have significant potential for the discovery of a major porphyry style deposit adjacent to the large intense advanced argillic alteration system. Such a deposit, if it is preserved under cover to the west of Mt Mackenzie, could be expected to have its attendant palaeoweathering and surficial enrichment profile preserved. Mt Mackenzie is a drill-ready property enhanced by high quality geological, geochemical and geophysical data sets. The next phase of the project requires a substantial drilling program accompanied by deep probing EM and IP geophysics to tighten target delineation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CRETACEOUS MICROBES - PRODUCER OF BLACK OPAL AT LIGHTNING RIDGE, NSW, AUSTRALIA. H. J. Behr^ K. Behr^ and J.J. Watkins^ ' Institute for Geology and Lithospheric Dynamics, Gottingen University. ^ New South Wales Department of Mineral Resources.

Black opal from Lightning Ridge is the most sought after of all opal varieties, and, at the high end of the market, the most expensive. It occurs in geode-like nobbies up to many cm in diameter, in seam-like structures and in irregular patches in an Early Cretaceous volcaniclastic hostrock. The hostrock at Lightning Ridge consists of a finely laminated silty claystone that often has a high content of organic detritis. Strong bioturbation by nematodes is common as are opalised macrofossils. Recent studies have shown that microbe communities exist within both the hostrock and opal in cell numbers up to lO^-loVcm^ The most common microbes are the aerobic bacteria Actinomycetes (Nocardia, Streptomyces, Micromonospora) and Myxobacteria - although 13 other species were also identified. The microbes (or their negatives) occur in the form of mycels, mats, biofilms, globular colonies, networks, swarms and single individuals. The cell forms are mostly rod-shaped, ovoid and coccoid and generally range from 2-5|Lim but may exceed 100|Lim. Small globular spores may contain organic residues with strong red fluorescence. All microbes are autochtonous and of the same age as the opal. The type of microbal communities found in Lightning Ridge opal generally occur in soil or in organic muds deposited under still conditions or in a surface-fouling biomass. The microbes require a nutrient-rich (cellulose and chitin) near-surface aerobic environment with a temterature range of 23 -35^C and neutral Ph. In this environment, large populations of relative large bacteria grew along grain boundaries of montmorillonite (smectite). The microbes excreted acids and enzymes that resulted in the biochemical weathering of clay minerals and feldspar. This process begins below biomats that are growing at the bottom and on the walls of cavaties and beside biofilms on the hostrock and produces Si (0H)4, AlO (OH), and Fe oxides which form colloidal particles that are stored in the compaction water of the mud. Many nobbies were former gas bubbles originally filled with pore and compaction water which has been altered into a colloidal sol (dispersion, suspension). During the initial stage of sol formation, spherulites of opal-CT are formed, probably by devitrification of volcanic glass from the hostrock. At this stage, the hostrock is also silicifled by irregular and nodular opal-CT crystallisation. Volume-stable sols then begin to form in cavities and open pores by the electrostatic adsorption of organic molecules. Previous laboratory experiments (Lagaly et al, 1999) have show that strongly stabilised silica sols can exist for many years. These sols therefore are an excellent cultural medium for the development of bacteria much like agar agar. The size of the opal spheres increase with increasing Ph and when they reach a size of greater than 60 nm, the opal spheres settle out into horizontal layers. At the bottom of the cavities, horizontal layers of potch are often formed with different amounts of pollutants (colloidal clay, Fe-oxides, AlO (OH), organics). The dark brown-violet to black coloured particles in black opal are present if microbes are present that produce macromolecules of melanin. In the top of the sedimented layers, clear residual sols are accumulated which have a lower density and viscosity and are the source of the precious opal. During this process, more and more H2O is bound into the silica colloids and the salinity in the residual water increases. With increasing salinity, gelification of the sol and the sediments commences by bridging between the particles. The remaining water accumulates above a meniscus developed on the gel and is partly preserved in fluid inclusions. The model suggested here, for the origin of black opal at Lightning Ridge, can be summarised as a selfproducing metabolic system controlled by biochemical weathering and colloid chemical processes. The unique sedimentary black opal is very rare in the geological record because the metabolic processes are extremely sensitive. References LAGALY G., SCHULZ O. & ZIMENL D. 1999. Dispersions and Emulsions. Steinkopf, Darmstadt (Germany) 300S.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DETAILED GRAVITY AS AN AID FOR CONSTRAINING CRUSTAL STRUCTURES INTERPRETED FROM DEEP SEISMIC REFLECTION PROFILING WITHIN THE EASTERN GOLDFIELDS, WESTERN AUSTRALIA. B. B e l l B.R. Goleby, T. Fomin, A.J. Owen and R.J. Korsch, Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601, Australia.

Within the granite-greenstone terrain of the Eastern Goldfields, an understanding of the structural environment is essential for gold exploration and effective ore genesis modelling. Recent two-dimensional modelling of detailed gravity data in conjunction with deep seismic reflection profiling has provided significant constraints in the interpretation of the Eastern Goldfields' crustal structure. Initial seismic reflection data produced unclear information on the lateral and depth extent of the Goongarrie-Mt Pleasant and ScotiaKanowna monzogranites resulting from an absence of truncated reflections at the granitegreenstone contact. Gravity modelling across these granitoids refined the interpretation due to the notable density contrast between the granitoids and the surrounding greenstones. The results suggest both monzogranites are trapezoidal in shape with a limited depth extent not exceeding three kilometres. The modelled thickness are consistent with previous interpretations. However, the geometries are contrary to earlier interpretations, which considered these bodies to be steep sided. Potential field modelling along the seismic traverse also indicates an absence of significant mafic material beneath the syn-tectonic granitoids and suggests that the greenstone sequence within the Kalgoorlie Terrane does not exceed nine kilometres in depth. This implies that the decollement or detachment zone, the interpreted base of the greenstone sequence onto which most of the major shears sole, also occurs at depths no greater than nine kilometres. Knowledge of the position of the decollement and its relationship to through-cutting shear zones (eg, Bardoc Shear) is important for exploration, as it is suggested that the Bardoc Shear - decollement relationship influenced the distribution of gold deposits within the Eastern Goldfields. Another crustal feature implied by the modelling of the detailed gravity is the presence of a large volume of felsic, possibly gneissic, material beneath the greenstone sequence. The gravity models, combined with the deep seismic interpretations and aeromagnetic models, have provided the first stage in the developing of a three-dimensional understanding of the region and will provide useful information on gold mineralisation within this economically significant region. Acknowledgments: The Authors publish with the permission of the Chief Executive Officer, AGSO and the Director, AGCRC.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MAGMATIC FABRICS IN PLUTONS AS KINEMATIC INDICATORS IN CONVERGENT OROGENS: EXAMPLES FROM THE CASCADES CORE (WASHINGTON STATE) AND THE MEGUMA TERRANE (NOVA SCOTIA) Keith Benn Ottawa-Carleton Geoscience. Centre and Department of Earth Sciences, University of Ottawa, Ottawa, Canada

Fabric data compiled during field mapping and anisotropy of magnetic susceptibility (AMS) measurements are used to propose that the igneous fabrics preserved within syntectonic plutons may be used to determine the directions of regional shortening and stretching, and kinematics, in contractional orogens. One data set comes from the Cretaceous Mt. Stuart batholith that was emplaced within amphibolite and greenschist grade metamorphic rocks of the Cascades Crystalline Core. The data were collected in the northern part of the batholith, which defines a NW-SE trending antiformal fold. The AMS provides measurements of the preferred orientations of the Fe-rich minerals (biotite ± hornblende ± traces of pyrrhotite, magnetite and ilmenite) which are consistent with field measurements of the mesocopic foliation and lineation defined by plagioclase, biotite and hornblende. The magmatic fabrics are consistent with the orientations of folds and fabrics that record tectonic flattening and stretching in the margin of the pluton, which has undergone high temperature subsolidus strain, and in the host rocks. The lineations formed during the crystallization of the Mt. Stuart batholith are perpendicular to the bulk regional shortening direction and parallel to a direction of important bulk regional stretching parallel to fold axes, that occurred during the convergent (transpressive?) deformation of the magmatic arc. Another data set comes from the South Mountain batholith and the Barrington Passage pluton, both emplaced with metasedimentary rocks of the Meguma Terrane, Nova Scotia. In both cases studied in the Meguma Terrane, the magnetic lineations are parallel to the regional fold axes. They are interpreted to be perpendicular to the bulk regional shortening during pluton emplacement, and to record fold axis-parallel stretching. Based on these results, it is proposed that the magmatic fabrics preserved within syntectonic plutons can serve as markers of the synmag! matic strain fields and kinematics of convergent arcs and orogens. Comparison of the magmatic fabrics with structures such as joints and dikes that indicate the paleo-stress field late during pluton emplacement suggest the fabric information may also allow determination of paleo-stress axes.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ANALOGUE SCALE MODELS OF PLUTON EMPLACEMENT DURING TRANSPRESSION IN BRITTLE AND DUCTILE CRUST Keith Benn', Francis Odonne^, Sharon K. Y. Lee^ and Ken Darcovich"^ ' Ottawa-Carleton Geosci. Centre and Dept. of Earth Sciences, Univ. of Ottawa, Ottawa, Canada, ^ Laboratoire de Dynamique des Bassins Sedimentaires, Univ. Paul-Sabatier, 31400 Toulouse, France ^ Dept. of Earth Sciences, Univ. of Ottawa, Ottawa, Canada "^National Research Council of Canada - ICPET, Ottawa, Ontario K1A0R6, Canada

Analogue experiments were used to investigate pluton emplacement during transpression in a layered brittle-ductile crust. Models consisted of 1) a silicone gum-PbO suspension as analogue magma, 2) a silicone gum-Pb suspension representing a basal ductile crustal layer and 3) an overlying sand pack representing brittle crust. The models were transpressed at 3 mm/hr causing the extrusion of the analogue magma from a progressively closing slot, and its emplacement into the ductile layer. The thicknesses of the brittle and ductile layers were critical in controlling the shapes of intrusions and the structures that developed in the brittle overburden. Thicker sand packs led to flattened, symmetrical laccolith-shaped intrusions and the late nucleation of one oblique thrust in the sand pack above the extremity of the intrusion. Thinner sand packs led to thicker, asymmetrical laccolith-like intrusions with uplift of the overburden on an oblique thrust that nucleated early in the experiments, and the formation of a shallow transtensional graben in the extrados of a bending fold. Reducing the thickness of the basal ductile layer leads to the development of a larger number of shear zones in the sand pack, and structural geometries approaching those produced in earlier experiments involving only a brittle analogue crust and no ductile layer. Shear zones in the sand pack were localized by intrusions, and also played a key role in displacing analogue brittle crust to make space for intrusions. The results suggest that tectonic forces may play an important role in displacing blocks of crust to make space for growing plutons emplaced within transpressional belts. They also suggest that pluton shapes, and the geometries and kinematics of emplacement-related shear zones and faults may depend on the depth of emplacement. In nature, depending on the structural level exposed in the map plane, faults and shear zones that were formed during, and that helped make space for, pluton emplacement may not appear to be spatially associated with the pluton.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RECENT SWATH MAPPING OF THE SOUTHERN MACQUARIE RIDGE COMPLEX: SEAFLOOR CHARACTERISTICS AND TECTONIC DEVELOPMENT G. Bemardel^ M.B. Alcock', P. A. Symonds', P. Petkovic' and M. F. Coffin^ 'Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 Australia ^Institute for Geophysics, The University of Texas at Austin, 4412 Spicewood Springs Rd., Austin, TX 78759-8500, USA

As part of the process of developing a Regional Marine Plan for southeast Australia, and to aid definition of Australia's marine jurisdiction south of Macquarie Is., the Australian Geological Survey Organisation conducted a seafloor swath-mapping survey in the region from January-February 2000 on behalf of Environment Australia and the National Oceans Office. This was the second of two such surveys in Australia's southeast marine jurisdictional zone using the French multibeam swath-mapping vessel VAtalante. A total of about 60,000 km^ of multibeam swath bathymetry and backscatter, and 4,300 line-km of 6-channel seismic reflection, gravity, magnetics and 3.5 kHz sub-bottom profiling data were acquired between latitudes 56^ S and 61^ S over the arcuate-shaped Hjort Trench and adjacent Hjort Ridge - the southern extension of the 1600 km-long Macquarie Ridge Complex. Significant features of the seafloor morphology of the area include: • •

•

• •

The deep (6500 m), mostly sedimented Hjort Trench, the best developed trench along the entire Macquarie Ridge Complex; A southward broadening and deepening of the Hjort Ridge segment of the Macquarie Ridge Complex with its crest characterised by tectonic fabric, created by seafloor spreading; A southward change in the character of the Macquarie Ridge, and the presence of a broader, higher relief axial valley, in the segment between and parallel to the Hjort Trench and Hjort Ridge; A chain of seamounts approximately 180 km northeast of the deepest portion of the Hjort Trench. A narrow, north-northwest-trending linear trough/ridge feature that obliquely truncates the southern end of the Hjort Trench and Ridge.

The mapping provides new insights into the tectonic development of the poorly known Hjort segment of the Macquarie Ridge Complex. Prominent spreading fabric is present over much of the survey area, and indicates that the crust of the Hjort Ridge was created by seafloor spreading. This crust was subsequently uplifted and deformed, although not severely enough to obliterate the tectonic fabric. The axial valley along the crest of the southward prolongation of the Macquarie Ridge segment broadens and deepens considerably. It is probably a continuation of the Macquarie Fault Zone identified to the north, and is interpreted to represent the present-day strike-slip Australia-Pacific plate boundary. The Hjort Trench represents past northeastward underthrusting of the Australian plate beneath the Pacific plate. The unsampled chain of seamounts northeast of the Hjort Trench may be the result of either hot-spot volcanism, or limited subduction of the Australian plate. The Hjort Trench and Ridge, as well as the intervening bifurcated prolongation of the crest of the Macquarie Ridge, clearly terminate against a younger (fracture zone?) lineament, which is aligned with active transform faults along the PacificAntarctic and Southeast Indian ridges.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CLASSIFICATION OF SEDIMENTARY UNITS USING RADIOMETRIC AND DTM DATA: TRIAL APPLICATION TO MIDDLE-UPPER DEVONIAN SEDIMENTS OF THE BARKA BASIN, NSW J.R. Berton, D.W. Dumey and M.A. Lackie Department of Earth and Planetary Sciences, Macquarie University, NSW 2109 Current methods of utilizing airborne radiometric data to delineate soil and rock types are essentially qualitative in that they rely on visual inspection of 3-band ( K - U - T h ) , colour-composite, images. Quantitative (classification) methods have generally met with little success. This study investigates classification methods flirther using an area of Devonian rocks that seemed to have potential for yielding results. Advantages to be gained from this approach are: (a) frill utilization of data sets with more than 3 bands, (b) quantification of radiometric or other image properties of specified soil and rock types, and (c) a potential to map large areas semi-automatically. Issues that needed to be addressed were: (1) background noise, (2) differentiation of transported material, (3) selection of supplementary data, and (4) selection of units or criteria for classification. The study area is the Rankins Springs 1:100,000 Sheet in the southern Barka Basin north of Griffith. Recent ternary radiometric plus total-count and elevation (DTM) image data were supplied from the New South Wales Department of Mineral Resources 'Discovery 2000 Exploration Initiative' and processed in ER Mapper® (Berton 1999). The area comprises broadly folded, mostly fluvial, mature quartz-rich to immature volcanolithic, variably iron-cemented, ?Middle to ?Upper Devonian, pebble-conglomerates, sandstones and siltstones. The units were deposited in a series of sheet-like upward-coarse-to-fme cycles, with good topographic expression of the sandstone units and extensive cover on the finer ones (Conolly 1962). Most of the radioelement content can be attributed to accessory white-mica (K) and zircon (U), and iron-cement (Th). Weathering is not thought to be a serious problem as the sediments were extensively weathered prior to deposition. Supervised classification, based on new stratigraphical and photogeological fieldwork with better subdivision of map units (17 vs. the previous 7), was chosen in an attempt to generate geologically recognizable classes that could be evaluated on that basis. Training areas for defining unit signatures were selected on the basis of this mapping, as were the evaluation areas. Noise suppression was addressed with a broad low-pass filter (at the cost of some detail). However, differentiation of transported cover could be achieved only by supplementation of the radiometric data with an elevation layer. This was further supplemented by a Laplacian-of-elevation layer, which represents convexity of landform and thus differentiates ridge-forming units from other exposed units and from gently concave drainage basins. Finally, confusion between units was reduced somewhat by amalgamation of similar signatures, though, again, at the expense of some detail. The results obtained suggest that the method is feasible for outlining units of formation and member size and some smaller ones. Further improvements could undoubtedly be achieved, with possible applications to regolith study as well as geology. References

BERTON J. 1999. Unpublished BSc. Hons, thesis, Macquarie University, Sydney. CONOLLY J.R. 1962. Unpublished PhD. thesis, v.l. University of NSW, Sydney

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

1.8 Ga to 1.1 Ga EVOLUTION OF THE AUSTRALIAN CONTINENT A NORTHERN, CENTRAL AND EASTERN AUSTRALIAN PERSPECTIVE Peter Betts and David Giles Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton, VIC 3168 Our perception of how the Australian continent has evolved during the Palaeo- to Mesoproterozoic has been underpinned by recognition of discrete orogenic cycles within the various terranes that make up the continent. Whilst Proterozoic Australia is thought to have been amalgamated by Rodinia-times, attempts to understand the evolution that led to this is poorly constrained (although see Myers et al. 1996). We present our view as to how Proterozoic Australia evolved leading to the formation of Rodinia Supercontinent. The tectonic evolution of the Proterozoic Australia after the Barramundi Orogeny (ca 1.9-1.87 Ga) records evidence for the collisional tectonics in which the Kimban Orogen, Eyre Peninsula (ca 1.82-1.70 Ga; Daly et al., 1998) and the Strangeways Orogen, Central Australia (ca 1.78-1.73 Ga; Collins and Shaw, 1995) probably formed a continuous orogenic belt above a north-dipping subduction zone along the southern margin of the Northern Australia Craton. Arc magmatism occurred along this margin during development of the Strangeways Orogen (Zhao and McCulloch, 1995). The Gawler Craton was accreted onto the southern part of the belt during the Kimban Orogeny. The subduction system then shifted westward, and accretion and arc-magmatism continued along the Ifould Complex, western Gawler Craton (ca 1.70-1.67 Ga; Teasdale, 1997). Contemporaneous with the accretion along the southern margin of the Northern Australian Craton there was protracted and episodic extension in the continent interior (ca 1.8-1.6 Ga) which resulted in the formation of the numerous superimposed and unconformity bounded basins (e.g. McArthur Basin, Leichhardt and Isa Superbasins). Large tracts of the Northern Australian Craton share similar depositional and extensional history. This extensional evolution was intracratonic in the strict sense but was controlled by plate boundary interactions (cf Basin and Range). Basin formation was interrupted by a short-lived midbasin inversion (ca 1730 Ma) suggesting relatively rapid oscillation between extension and shortening, probably associated with shifts in relative plate convergence rates. A tectonothermal event at ca 1.6 Ga is manifest as voluminous magmatism at middle and upper crustal levels (Gawler Range Volcanics, ca 1.621.60 Ga; Hiltaba Granites, cal.58), and as high-grade metamorphism at middle and lower crustal levels (eastern Mount Isa Inlier metamorphism, ca 1.60-1.58 Ga; Olarian Orogeny, ca 1.60-1.58 Ga). In the Mount Isa Inlier extension was accommodated along mid-crustal detachment faults whereas the upper crustal architecture in the Gawler Craton preserves evidence for dextral offset along NW-oriented (present orientation) transverse faults. After the Hiltaba Event a west-dipping subduction zone developed to the east of the Georgetown Inlier leading ultimately to the closure of an oceanic basin (ca 1.58-1.50 Ga) that separated Australia and North America (Giles and Betts, this volume). This event resulted in the development of a 1000km wide - N - S oriented orogenic belt which included the Isan Orogeny (Mount Isa and Georgetown Inliers), the Willyama Orogeny (Broken Hill Block), and the Kararan Orogeny (northern Gawler Craton). Voluminous magmatism in the Georgetown Inlier (Esmeralda and Forsayth Granites, Croydon Volcanics, ca 1.55 Ga; Black and Withnall, 1993) occurred in an arc/back-arc setting. Thereafter, Mesoproterozoic (ca 1.50-1.20 Ga) eastern Australia records an era of uplift and exhumation probably associated with renewed extension. The rock record for this era is sparse. 40Ar/39Ar thermochronology indicates that rocks at mid-crustal levels were exhumed in the Mount Isa terrane (Spikings, 1998). The Roper River Group was deposited in the McArthur Basin and the Pandurra Formation (ca 1.45 Ga) was deposited in the Cariewerloo Basin on the western margin of the Stuart Shelf At this time the Gawler Craton and the Broken Hill Block broke away from the remainder of the Northern Australian Craton only to be reattached in their present position during the Musgravian Orogeny (ca 1.20-1.15 Ga). References BLACK, L.P. WITHNALL, LW. 1993. The ages of Proterozoic granites in the Georgetown Inlier of northeastern Australia, and their relevance to the dating of tectonothermal events. AGSO Journal of Australian Geology and Geophysics 14, 331-341. COLLINS, W.J., SHAW, R.D. 1995. Geochronology constraints on orogenic events in the Arunta Inlier: a review. Precambrian Research 71,315-346. DALY, S.J., FANNING, M., FAIRCLOUGH, C.M. 1998. Tectonic evolutionand exploration potential of the Gawler craton. South Australia. AGSO Journal of Australian Geology and Geophysics 17, 145-168. MYERS, J.S. SHAW, R.D. TYLER, I.M. 1996. Tectonic Evolution of Proterozoic Australia. Tectonics 15, 1431-1446. SPIKINGS, R. 1998. The fission track and 40Ar/39Ar thermochronology of he mount Isa Inlier and surrounding areas. PhD Thesis (unpublished) Latrobe University. TEASDALE, J. 1997. Methods for understanding Poorly Exposed terranes: The Interpretative geology and tectonothermal Evolution of the Western Gawler Craton. PhD Thesis (unpublished) University of Adelaide. ZHAO, J-X., MCCULLOCH, M.T. 1995. Geochemical and Nd isotopic systematics of granites from the Arunta Inlier, central Australia: Implications for Proterozoic crustal evolution. Precambrian Research 71, 265-299.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

COMPARISONS BETWEEN TURBIDITE-HOSTED GOLD DEPOSITS IN THE LACHLAN FOLD BELT, THE OTAGO SCHIST AND BULLER TERRANE, AND THE MEGUMA TERRANE Frank P. Bierlein\ Antony Christie^ Paul K. Smith^ and Dennis Ame^ Victorian Institute of Earth and Planetary Sciences, Monash University, Clayton VIC 3800 ^Western Australian School of Mines, Curtin University, Kalgoorlie WA 6430 ^Institute of Geological and Nuclear Sciences, PO Box 31-312, Lower Hutt, New Zealand "^Department of Natural Resources, PO Box 698, Halifax Nova Scotia B3J 2T9, Canada Gold mineralisation in the Palaeozoic sedimentary successions of central Victoria, the Otago Schist and Buller Terrane in South Island, New Zealand, and the Meguma Terrane in Nova Scotia, Canada, are characterised by striking similarities regarding provenance and structural development of host rocks, crustal characteristics, mineralisation style, and physico-chemical properties of the ore-bearing fluids. Mesothermal gold mineralisation in each of these slate-belt provinces is commonly associated with structurally controlled dilatational features. Although their origin is still poorly understood, these occurrences are well documented and this mineralisation style is characteristic of most of the major reef gold producers in Central Victoria, New Zealand and Nova Scotia. The laminated to massive, in part brecciated, quartz veins are related to contraction faults, saddle reefs, sub-horizontal extension veins, and en-echelon gashes. These mineralising sites are demonstrated by incremental, brittle-dominated fracturing and appear to be intimately related to high fluid flux regimes characteristically developed late during crustal shortening and thickening. However, closer examination and comparison of the lode gold deposits reveal that some significant differences exist between the three provinces. The timing of auriferous vein formation relative to peak deformation and greenschist metamorphism generally varies fi-om deposits in the Meguma (syn-peak metamorphism; ?terrane-wide) to those in the Otago Schist (transitional, brittle-ductile shear zone-hosted), the Buller Terrane (late but significant brittle post-ore deformation) and in central Victoria (late-stage, multiple episodes). Conversely, the folding appears to be much tighter in the vicinity of the Victorian deposits than for the deposits in the Buller Terrane and Nova Scotia. Although disseminated sandstonehosted gold mineralisation has largely gone unnoticed in Victoria and New Zealand, similar occurrences in Nova Scotia are considered to form an integral part of lode gold formation and are attracting significant exploration interest. The primary source of the gold remains controversial, although recent studies demonstrated that Cambrian greenstones and exhalative interflow sediments known to underlie the Palaeozoic succession in Victoria could readily account for the majority of the enormous quantities of turbidite-hosted gold in central Victoria. Such a potentially Tertile' basement has also been identified in at least part of the Otago Schist (Aspiring Terrane) and Meguma Terrane (Liscomb Complex), but remains unrecognised in the Buller Terrane. On the other hand, granitic intrusives are abundant in the Meguma and the Buller terranes, and also in the Lachlan Fold Belt, but are virtually absent fi-om the Otago Schist. Additional regional differences include an apparent higher proportion of sulphides in the Nova Scotia deposits compared with deposits in the Buller Terrane and in Victoria, a higher pyrite to arsenopyrite ratio in the Victorian deposits than in Nova Scotia, and the abundance of stibnite in the Buller deposits. In Victoria and the Otago Schist, pyrite is regarded as a good sign for gold mineralisation, but in most Nova Scotia deposits arsenopyrite signals better gold values. Many of the veins in Nova Scotia have coarse K-mica, not seen in veins fi-om the other terranes. These variations are considered to represent controlling factors not only on the style of gold deposit that develop, but also the overall gold endowment of a slate belt province under investigation. Thus, recognising these determinants has important implications for ore genesis models and assessing the 'fertility' of a given slate belt. The three regions represent prime geological targets for further gold exploration and provide an ideal environment in which to study the characteristics of ore-forming processes and the origin of turbiditehosted gold in a variety of deposit styles.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DISSEMINATED SEDIMENT-HOSTED GOLD MINERALISATION - A NEW TYPE OF ORE DEPOSIT IN SLATE BELTS? Frank P. Bierlein^ Andrew W i l d e \ Simon Maher^ Russell Fulton^ Stafford McKnight^ and Paul K. Smith^ Victorian Institute of Earth and Planetary Sciences, Monash University, Clayton VIC 3800 ^Geological Survey of Victoria, 250 Victoria Parade, Fitzroy VIC 3065 ^PO Box 81, Bridport TAS 7262 ^Minerals Industry Research Institute, University of Ballarat, PO Box 663, Ballarat VIC 3353 ^Department of Natural Resources, PO Box 698, Halifax Nova Scotia B3J 2T9, Canada Sediment-hosted gold mineralisation in the Lachlan Orogen and in other Phanerozoic slate belt provinces has generally been assumed to be confined entirely to massive to laminated quartz veins. Evidence is emerging however, from the former Soviet Union, New Zealand, Nova Scotia and indeed central Victoria and Tasmania, that disseminated gold forms a significant component of many of those orogenic lode gold deposits. Quartz veins in this class of gold deposits yield high gold grades but relatively low tonnages of ore. Furthermore, gold within quartz veins typically exhibits erratic spatial distribution, which creates difficulty in prediction and mining. Disseminated ore, on the other hand, is usually of lower grade but adds large tonnages and is often more evenly distributed. The presence of such ore is of obvious economic significance and a better understanding of the genetic relationship between disseminated and vein-type ore will be of great value in grass-roots exploration, in evaluating known prospects and in optimising existing mining operations in slate belt provinces worldwide. Based on preliminary results of current research into disseminated gold accumulations in meta-sedimentary rocks (Lachlan Orogen in central Victoria and NE Tasmania; Otago Schist, New Zealand; Meguma Terrane, Canada; Tien Shan mountains, Kyrgyzstan), potential for new discoveries of such deposits in existing slate belt-hosted goldfields is considered to be excellent. Non-quartz vein associated, disseminated gold mineralisation in these deposits is commonly hosted by variably altered greenschist facies metamorphosed meta-siltstone and meta-greywackes. On the other hand, occurrences of extremely fine-grained gold in shear zone-hosted deposits tend to be sited in highly strained carbonaceous pelitic phyllite. In the Meguma Terrane, native gold grains are generally less than 100 [im in maximum dimension and occur as either high purity Au enclosed in coarse arsenopyrite porphyroblasts or as composite spheres enclosed in larger irregular and nuggety grains with an electrum core (± As, Fe, Cu, Pb) surrounded by base metal sulphides. Ag, Cu, W, Fe, Sb, native Pb, and Sn are present in mineralised samples. Although the possible origin and timing of emplacement of disseminated gold in meta-siltstones and -shales associated with orogenic lode gold deposits range from sedimentary, diagenetic and hydrothermal to supergene enrichment, features of alteration and strong structural control in all of the studied occurrences point to a close genetic association with quartz veinhosted mineralisation. Another important feature of this group of gold deposits is the almost ubiquitous association of gold with organic matter, a feature shared with deposits of the Witswatersrand- and Carlin-styles. The carbonaceous matter is thought to be important in localisation of gold through reduction reactions, implying introduction of gold in relatively oxidised solutions, possibly as chloride complexes. Analysis of carbonaceous shale samples from a number of goldfields throughout Victoria by NAA and ICP-MS revealed highly anomalous concentrations of platinum group elements and gold (Pt: 24ppb; Pd: 14ppb; Ir: 0.125ppb; Au: 5.2ppm). Furthermore, the commonly poorly crystalline carbon is an efficient adsorber of Au cyanocomplexes and is the prime cause of gold loss to tailings ('preg robbing') in several mineral processing operations. Further work is planned via SPIRT/AMIRA grant schemes.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE ROTATION RATE OF GARNETS DURING SIMPLE SHEAR Christian Biermeier and Kurt Stuwe Department of Geology and Palaeontology, Karl-Franzens-University of Graz, Austria,

Deformation rates are one of the least-constrained parameters of intraplate processes. However, in principle, they may be derived from inclusion trails in garnets if two parameters are know: (i) the relationship between rotation-rate of the crystal and deformation rate of the rock and (ii) the growth rate of the garnet. The growth rate of the crystal may be determined from petrogenetic pseudosections, if the PT path of the rock is known. However, the rotation rate is much harder to determine, as it strongly depends on the distance of the rotating crystal from the effective shear zone boundary, for example a neighbouring crystal, or the distance from an area of less strain. In order to constrain this rotational behaviour, we have calculated the rotational behaviour of cylindrical objects during simple shear using a two dimensional finite element simulation. We can show that the rotation rate varies dramatically as a function of four parameters: 1.) as a function of object size relative to the shear zone width, L; 2.) as a function of the power law exponent of the object, n; 3.) as a function of the rheology contrast between object and matrix, V and 4.) as a function of the assumed boundary conditions for simple shear. For n = 1 and high V, our results confirm analytical descriptions for small L, in that the total angular change of the object is half of the bulk shear strain rate. For some rheologies, the rotation rate decreases to < 0.3 at intermediate L. However, for 0.1 > L < 0.9, the rigid body rotation as a function of L can have a minimum, a maximum, or it can decrease or increase. These results have important consequences on the interpretation of the rotational behaviour of garnets in deforming rocks and ultimately for the interpretation of deformation rates. In particular, in coarse-grained garnet-bearing mica schists the effective shear zone boundaries to a given garnet crystal may be given by its neighbouring crystal. Thus, different crystals may hinder or enhance their relative rotation rates. For some rheologies, this effect is maximised when garnets constitute about 20% of the bulk rock (this corresponds to L=0.5). If garnets grow during deformation and the distance between neighbouring crystals changes, there may be complicated interactions and fluctuations in the rotation rate. In view of this we suggest that one should refrain from making the simple-minded assumption that the rotation rate of crystals is half of the bulk shear strain rate. However, we suggest that it may be possible to derive deformation rates from the inclusion trails in garnets by a combined microstructural and petrological approach. (This study is supported by FWFP12846-GEO)

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

INVESTIGATIONS INTO THE PREDICTION AND MODELLING OF TOTAL SULPHUR IN COAL SEAMS, GERMAN CREEK MINES, CENTRAL QUEENSLAND. Mark Biggs Capricorn Coal Management Pty. Ltd., PMB, Middlemount. QLD. 4746. Background The German Creek Formation contains five laterally persistent coal seam intervals known as the German Creek, Corvus, Tieri, Aquila and Pleiades in ascending order. The principal seam is the German Creek seam but the Aquila seam and, to a lesser extent, the Tieri seam have also been the focus of past open cut mining operations. The coal measures dip generally eastwards at to and the German Creek seam reaches depths of 600m to 800m at the eastern lease boundary. The majority of German Creek's production is from the German Creek seam, an ortho-bituminous, low ash, medium rank, hard coking coal, primarily for export. Most of the shallow coal in the German Creek sequence has been mined except in the north of the leases where thinner, poorer quality seams remain. Present day production is from three separate mines, German Creek Open-cut, Central and Southern Collieries. Statement of the Problem There is a systematic variation in rank across the German Creek mining lease, which is exemplified by a regular decrease in volatile matter in the coal from southwest to northeast. Because many of the coal's coking properties are governed by rank, there is a long-range reduction of coking parameters, such as fluidity and dilatation, towards the northeast. By comparison, the sulphur content of some coal seams can, on occasions, widely vary in both a vertical and lateral sense within and between seams, particularly near dykes and seam washout areas. This short-range variability creates problems in predicting the sulphur content of washed product coal, with the possibility of coal shipments to customers being rejected if sulphur levels exceed pre-defmed limits. Therefore, if patterns in this variability could be determined and related to some geological property or process of formation of the coal seams, then areas of high sulphur might be predicted in advance of mining. Discussion The author instigated a background coal characterisation study, as it was seen as an integral part of ongoing investigations aimed at determining lithological and structural controls to the sulphur distribution in the coal. To this end, a representative selection of exploration slimcore and face channel samples from all production areas were subjected to coal petrographic, coal and parting mineral matter by XRD and/or SEM using SIROQUANT techniques, and coal ash trace element analyses by ICP, AAS or XRF techniques. Additional information was gained by comparing quantitative XRD analyses (SIROQUANT^^) with mineral matter determinations obtained using SEDNORM. Various modelling methodologies were employed, ranging from geostatistics (ordinary kriging) to polygons of influence so as to best represent the short-range variations present in the seam data sets. Though this proved moderately satisfactory for raw insitu data, the influence of float/sink washability testing was also examined, as it was found that borecore size influences the clean total sulphur result. Discrepancies between raw and clean sulphur values from the models so constructed still need to be reconciled. In non-cored boreholes it was hoped that some sensible use could be made of available downhole geophysical logging. Whilst providing a wealth of information on coal quality and thickness, data on sulphur distribution from these typical log responses was found to be limited. Trials will soon be conducted using CSIRO's SulphaLOG downhole geophysical logging sonde. This logging technology employs the prompt neutron-gamma method (neutron-capture), for the determination of sulphur and other elements in coal seams and partings. The characterisation study information from exploration and channel sampling enabled significant updating of the existing quality database to occur, which in turn allowed rigorous statistical analyses of available values (by seam) to be conducted. These analyses are essential building blocks to allow areas of high sulphur to be predicted in advance of mining. Eventually, a methodology for predicting sulphur mineral concentrations in advance of mining and reconciling "as-sold" coal quality to that predicted will be developed. These prediction theories will be assessed at the coalface as mining progresses through high-sulphur zones.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ANATOMY OF AN ACTIVE SEAFLOOR HYDROTHERMAL SYSTEM HOSTED BY FELSIC VOLCANIC ROCKS: OCEAN DRILLING PROGRAM LEG 193 Raymond A Binns CSIRO Exploration and Mining, PO Box 136, North Ryde 1670

Drilling into an active hydrothermal system linked to convergent-margin felsic magmatism is a priority objective of the Ocean Drilling Program (ODP) that will be achieved late in 2000 by Leg 193 to the PACMANUS site in the Bismarck Sea, PNG. The expedition will address many fundamental geoscience issues, for example solid and fluid products of felsic rock-water interaction and the consequences for global chemical fluxes. These will differ from those in basaltic mid-ocean ridge environments already drilled by ODP, as also will the sources of hydrothermal fluids and of their contained metals and ligands. PACMANUS includes fields of focussed, high-temperature "smoker" venting with Cu+Au-rich massive sulfide deposits, and a large field of diffuse, lower temperature venting through intensely altered dacite where modelling indicates significant subsurface mineralisation. Its geological setting is effectively destined to become continental crust hence the site is a close analogue of ancient orebody environments. Unravelling the volcanic architecture, the lateral and vertical variability in wallrock alteration and sulfide mineralisation patterns, the volcanological and structural controls on fluid pathways, and the coupling between all the processes involved, will have major implications for economic geology and land-based mineral exploration strategies. A first-order hypothesis to be tested, stating that fluids derived from volatile-rich magmas are more important than recirculated seawater in convergent-margin hydrothermal systems, is increasingly appealed to for creation of "world-class" hydrothermal orebodies. Another hypothesis proposes "subhalative" massive sulfide deposition within hydraulically expanded volcanic products as a factor in generating large massive sulfide orebodies. Many second-order hypotheses such as leaching fractionated volcanics to cause gold enrichment in back-arc systems will also be tested. Leg 193 will drill below both focussed and diffuse outflow zones, into a likely seawater inflow zone for the PACMANUS hydrothermal system, and at a "background" reference site, thereby establishing the internal volcanic and hydrothermal anatomy of the system. A number of new technological challenges will be addressed. Rough seabed topography may prevent deployment of guide funnels used to achieve deep penetrations in previous hardrock operations, so bare hole re-entries will be attempted using precise navigational capabilities. An advanced diamond coring system will be used with the aim of improved core recovery. High temperatures are anticipated at depth, for which new monitoring instrumentation is being prepared. Geophysical logging of cored holes by wireline will be supplemented by use of "logging while drilling" equipment in parallel uncored holes. Sophisticated resistivity and radiometric imaging of the hole walls will enable quantitative assessments of fracture density and orientation, and the mapping of alteration and vein systems to supplement core observations. Efforts are being made to develop a hightemperature fluid sampling system, which will enable the first direct comparisons between hydrothermal fiuid compositions at depth and those at seabed vents and chimneys. Leg 193 will also provide the first tests for presence of a deep microbial biosphere in a convergent-margin hydrothermal system, with special interest attached to the nature and extent of subsurface hyperthermophilic assemblages.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

GEOCHEMICAL COMPARISON BETWEEN THE OLIGOCENE VOLCANIC SEQUENCES IN THE FINISTERRE MOUNTAINS, NEW BRITAIN AND NEW IRELAND, PAPUA NEW GUINEA Raymond A Binns^ and Robert H Findlay ' CSIRO Exploration and Mining, PO Box 136, North Ryde NSW 1670 ^Geological Survey of PNG, Private Mail Bag, Port Moresby, PNG Since 1994, fieldwork by the GSPNG in the Finisterre / Sarawaget Ranges (FSM) and West New Britain has cast considerable doubt on a long-standing paradigm of PNG's geology. This states that the earlier Neogene volcanic sequences of those two provinces (as well as those of Bougainville, New Ireland, New Hanover and Manus Island) developed in a single, formerly continuous island-arc which has collided with the Australian continental plate in Miocene or Pliocene times, after being disturbed by a poorly understood sequence of back-arc spreading and rotation. However, the Finisterre Volcanics of the FSM interdigitate with terrigenous sediments derived from continental outcrops south of the Ramu-Markham Valley: thus they could not have formed in an island-arc which was initially far removed from the rocks of the Scrapland terrane which had accreted to Australia by no later than Middle Miocene times. This relationship leads us to question the prevailing paradigm concerning the accretion history of the so-called Melanesian arc and to test correlations of units within this supposed arc. Geochemistry provides a means of comparison of the tectonic environment of volcanic rocks and our results demonstrate that the Finisterre Volcanics and the Baining Volcanics (New Britain) are not geochemical correlates; rather, our collections from the two areas demonstrate marked geochemical differences between the two sequences. The contempoary Jaulu Volcanics of southern New Ireland appear different again. The Baining Volcanics of New Britain, which host several gold and porphyry copper prospects, have not previously been characterised geochemically. Our study of new collections from West New Britain shows a predominance of plagioclase and clinopyroxene-phyric andesites and basaltic andesites, with typical island arc or calc-alkali major-element and trace-element geochemistry. Although lacking hornblende and biotite phenocrysts, they represent the kind of volcanic sequence expected in a mature island-arc like that which presumably formed during the earlier Neogene subduction along the now inactive Manus-Kilinilau trench north of New Ireland. In total contrast, the majority of outcrop samples from the Finisterre/Sarawaget Ranges are a most unusual kind of undersaturated, highly potassic picritic basalt with abundant olivine phenocrysts. These are accompanied by equally potassic basalts (shoshonites) that can be modelled as a simple fractionation product of the picrites. Using mostly stream boulders Jaques (1976, Geo! Soc Amer Bull, v87, p861) had previously recognised a shoshonitic affinity for the Finisterre and Adelbert Ranges, though he described only one picrite. Interestingly, it has become petro-tectonic folklore that shoshonites form either during early rifting of continental or arc crust, or during the early immature stages of arc volcanism. The latter interpretation may involve an element of circular argument if the Finisterre Volcanics are cited in evidence. Although their unusual geochemistry means that we cannot identify with precision a modem geochemical analogue of the Finisterre Volcanics, our combined stratigraphic and geochemical data suggest the likelihood of a back-arc setting related to an ?Eocene -Oligocene microcontinent \ island-arc system to the south. It is notable that the Finisterre Volcanics contain few mineral prospects and no significant orebodies. Three samples of the Jaulu Volcanics from southern New Ireland were collected for this study by an ANU geodetic field party. Two of these proved suitable for geochemical assessments. They are low-K basalt and basaltic andesite whose closest geochemical affinity is with mid-ocean ridge basalts (MORB), though such an assessment might be dangerous based on only two samples from the same proximity. These reconnaissance results imply a low prospectivity for significant orebodies and do not allow a geochemical correlation with the Finisterre Volcanics. Clearly more work is needed to amplify these preliminary results and extend the comparisons to the Gazelle Peninsula, other parts of New Ireland, and the other islands of the chain. The sequences studied so far all exhibit low grade burial metamorphism, and it is important to combine careflil field observations and petrography to screen out unsuitable samples from geochemical programs. Even a small amount of carbonate metasomatism can have severe consequences for trace element characterisation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

POSSIBLE BIOLOGICAL SIGNIFICANCE OF CONTAMINATED SEDIMENTS IN PORT JACKSON, SYDNEY. Gavin Birch and Stuart Taylor Environmental Geology Group, School of Geosciences, The University of Sydney, NSW, 2006, Australia.

Comprehensive investigations of estuaries in central New South Wales have identified Port Jackson as the most contaminated in this part of Australia. Extensive areas of the estuary are mantled in high concentrations of a large range of metallic and organic contaminants. Although extensive, this database does not provide an effective basis for determining the potential adverse effects of chemicals on living resources. In the absence of any ecotoxicological information, the recently published (1999) draft Australian and New Zealand Environmental Conservation Council (ANZECC) sediment quality guidelines have been used to assess possible adverse biological effects of these toxicants. The ANZECC guidelines use the lower effects range of the widely used U. S. National Atmospheric Administration (NOAA) scheme to identify potentially contaminated sediment and as a threshold to trigger additional investigative work. This guideline level has been used in the current study to assess possible toxicity of contaminated sediments in Port Jackson. It is estimated that approximately 26% of the estuary, mainly the upper parts of the harbour and much of the central harbour, has a 67% probability of being toxic. The central harbour and a major tributary, the Middle Harbour, comprising about 19% of the estuary, have a 13% to 25 % probability of toxicity. All sediments in the harbour, except at the mouth of the estuary, would require additional environmental assessment based on criteria proposed by the draft ANZECC sediment quality guidelines.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DIAMOND-SAPPHIRE-ZIRCON ASSOCIATION IN VICTORIA William D. Birch Department o f Mineralogy and Petrology, Museum Victoria

Alluvial diamonds have been found in localities across Victoria but in considerably lower concentrations than in other eastern Australian (especially New South Wales) fields. The sources of the diamonds are unknown, despite extensive exploration focussed on kimberlitic indicator minerals. In Victoria, alluvial sapphires are more widespread than diamonds. They show the full range of habits and colours typical of the more extensive deposits in eastern Australia. They also show regional distinctions, and many exhibit magmatic resorption suggesting they are derived from nearby volcaniclastic deposits. Zircons in each alluvial field often show a range of ages, indicating mixed sources, but with the oldest age often coinciding with the known age of Cainozoic basaltic lavas in the region. Recently a subduction model has been developed for the formation of eastern Australian diamonds. This would allow for diamonds to crystallise during episodes of Palaeozoic subduction, at half the pressures associated with kimberlitic sources, and preserve them for transport to the surface during Cainozoic volcanism. Corundum (including gem sapphires) which formed in different crustal environments, and zircons which crystallised in felsic intrusives, could be brought to the surface in the same volcanic episodes. This model ultimately provides an explanation for the close coincidence of the NSW diamond fields with commercial deposits of gem sapphires found in volcaniclastic deposits associated with Cainozoic alkali basalt volcanism. The relationship between diamond, sapphire and zircon in Victoria therefore tends to support a regional subduction-diamond model. While the kimberlitic model should not be abandoned in Victoria, sapphires and dated zircons should be considered as additional diamond 'indicators'.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE QUEST FOR A HIGH-QUALITY ZIRCON STANDARD FOR MICROBEAM PB-U-TH GEOCHRONOLOGY Lance P. Black\ Sandra L. Kamo^ Ian S. Williams^ Chris Foudoulis', Jonathon C. Claoue-Long\ Russell J. Korsch\ and Donald W. Davis^ ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^Jack Satterly Geochronology Laboratory, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario, M5S 2C6, Canada ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200

Many, if not most, geological studies are critically dependent on effective geochronological control, and microbeam technology is playing an increasingly important role in this respect. Like other microbeam geochronology techniques, SHRIMP Pb-U-Th zircon dating is currently totally dependent on calibrating the analyses of unknown zircons against zircon of known age (the standard). The ideal standard must meet several strict criteria. Naturally, it must have been dated by a technique quite independent of SHRIMP, and that independent age measurement must be both highly accurate and precise. The standard must also represent only a single generation of zircon growth, and have constant Pb/U (and preferably Pb/Th) on all scales from submicron to intergranular. There must have been no post-crystallisation chemical or isotopic disturbance. The standard should also be sufficiently abundant to last indefinitely, and its quality should be so obvious that it will be readily adopted by other laboratories. Over the course of the past two decades about a dozen different zircon samples from a wide range of rocks around the world have been trialed as reference materials at the RSES/AGSO laboratory. A few of these have been selected as in-house standards. As knowledge of both SHRIMP itself and those standards has advanced, the production of progressively more precise data has allowed the question of standard homogeneity to be addressed, and at least most of these standards have proved to be less than ideal in this regard. A recent outcome of this testing process has been the identification of a promising new standard from a small, high-level gabbroic diorite plug near Temora, in the Lachlan Fold Belt, eastern Australia. A reconnaissance sample from this pluton contains 10 ppm of relatively coarse-grained zircon crystals which appear to have remained geologically and isotopically undisturbed since the diorite was emplaced 417 Ma ago. The excellent uniformity of the zircon Pb/U originally identified by SHRIMP has been confirmed by more than a dozen IDTIMS analyses. This isotopic integrity, and apparent lack of any inherited component, are very encouraging. The main outstanding issue is to determine whether the high-quality zircon is present in sufficient quantities for widespread distribution to other laboratories. This is currently being assessed by SHRIMP and IDTIMS analyses of a range of samples from other, more zircon-rich, sites.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HORIZONTAL VS VERTICAL TECTONICS AND THE DOME &-BASIN GEOMETRY OF THE ARCHAEAN PILBARA CRATON, WESTERN AUSTRALIA: IMPLICATIONS FOR MINERAL DEPOSITS. Richard S. Blewett Minerals Division, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601, Australia Analysis of regional geophysical datasets combined with regional and detailed mapping by the Australian Geological Survey Organisation and the Geological Survey of Western Australia, has outlined the 3D tectonic framework of the Archaean Pilbara Craton. The results of these studies have major implications for tectonic models, as well as mineralisation. Published models of tectonic evolution range from accretionary (horizontal tectonics) to intraplate and diapiric (vertical) tectonics. The 3.65-2.85 Ga Pilbara Craton is a longlived granite-greenstone belt that was built up by punctuated plutonism and greenstone deposition. The Pilbara Craton appears to be relatively unique in its geophysical properties, especially the significant depth of the greenstone belts. This uniqueness needs to be considered before tectonic models from other Archaean granite-greenstone belts are adopted. This paper will outline some of the constraints on the applicability of these various models, and comment on the implications for mineral deposits. The early Archaean granitegreenstone rocks of the Pilbara have two structural styles. In the east are large (up to 120 km), intact ovoid granitoid batholiths, with intervening tightly folded greenstone synclines. This compares to the west, and possibly north, where the batholiths are more elongate, the greenstone belt structure is less related to the intrusions, and major shear zones transect the craton, and control structural development. The 3D structure of the Pilbara upper crust was studied using gravity, magnetic and seismic refraction data (Wellman, 1999). The results have been consistent in defining a major change in the crust at about 14 km depth, marking the base of the granitoid batholiths and greenstone belts. The data show that the east Pilbara dome-&-basin bounding surfaces dip steeply, and penetrate to 14 km. The ovoid granitoid batholiths are flattened cylinders with diameter to depth ratios ranging from 4:1 to 8:1. They are composite multi-phase bodies emplaced from 3.65 to 2.85 Ga, comprising -13 felsic magmatic events that were built up by successive sheet-like intrusion into the centres of older phases of the batholiths. In the east Pilbara, the greenstones occupy large amplitude synclines up to 14 km deep, that were established after 3240 Ma (BIF deposition). The Pilbara Supergroup has up to 7 greenstone packages (3.5 to 2.9 Ga) separated by local to regional unconformities on previously folded greenstones. Greenstone deposition (including volcanism) developed in an extensional setting that was mostly coeval with felsic magmatism (batholith development). The 3D geometry of the Pilbara granitegreenstone terrane has important implications for possible tectonic models. Detailed and regional structural studies, literature compilations and published geochronology has resulted in the definition of up to 13 phases of penetrative deformation over an 800 Ma period (at micro- to mega-scales), with multiple overprinting relationships mostly about steeply dipping planar fabric elements. The correlation of these early to mid Archaean deformation events across the whole Pilbara implies far-field horizontal tectonics were occurring, rather than random diapiric processes. A model is presented where the dome-&-basin geometry is developed and controlled by large-scale fold interference during multiple deformation events, and later modified by fold tightening and amplification (steepened and deepened) by periodic 'diapirism', possibly accommodated by post-orogenic extensional collapse. The challenge for structural geologists is to separate the effects of the obvious repeated horizontal shortening events from the more subtle vertical accommodation due to diapirism. The Pilbara Craton contains numerous small and diverse (setting and metal type) mineral deposits, but the question remains, where are the big ones? Prospective lithologies, which were periodically rejuvenated, extend to 14 km and create a large volume of rock to scavenge metals. The steep to vertical contacts between granitoid complexes are commonly faulted, which may also extend to 14 km, thus providing suitable plumbing systems. In contrast, the metal-rich Eastern Goldfields Province is dominated by low-angle decollement zones, which may be more effective in tapping fluids and metals, despite smaller volumes per surface area. Reference

Wellman, P. 1999. Granitoid complexes and greenstone belts in the Pilbara Craton interpreted to extend down to the mid-crustal boundary at 14km. AGSO Research Newsletter 30, 15-17.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MAGMA MIGRATION AND MIXING PROCESSES OF FELSIC MAGMA IN THE MIDDLE CRUST, EXAMPLE OF MOUNT HAY, CENTRAL AUSTRALIA M. Bonnay and E.W. Sawyer Sciences de la Terre, Universite du Quebec, Chicoutimi, Qc, G7H 2B1, Canada Magma migration and mixing processes are well illustrated by the Early Proterozoic middle-lower crust at Mount Hay. Study of exhumed hot crustal zones, where magmas have not undergone rapid freezing, have revealed a range of magma ascent mechanisms characterised by mesoscale pervasive magma flow. The ability of dyking to drain partially molten source-rocks depends critically on several variables, such as the characteristics of the transported felsic magmas (viscosity, composition), the geometry of the system, and the permeability of the source. Mount Hay represents a crustal section containing locally derived and externally injected felsic magmas which underwent mixing during the formation of the igneous protolith at - 1 8 2 0 Ma (predeformational stage) and during the subsequent deformation at - 1 7 7 0 Ma (syndeformational stage). In the latter stage, closure of a back arc system commenced and marked the beginning of a contractional event sweeping from the north, when the protolith underwent metamorphism and anatexis to granulite facies (825-875°C and P=6-7 kbar). In situ partial melting of mafic, intermediate and felsic granulites produced localised tonalite leucosomes (cm scale) whereas melting of metasediments produced more abundant leucogranites (cm to m scale). Partial melting of deeper crust also occurred during contractional deformation and resulted in the generation of chamockite magma, which pervasively intruded the granulites at Mount Hay. This study documents the melt path geometry in the magma transfer network and provides a realistic estimate of parameters such as permeability and volume-flow rate relative to dyke connectivity, during magma movement through the crust. It does not deal with melt segregation from partially melted sources. A simple case of a single magma and a homogeneous pathway of veins and dykes is not really applicable to Mount Hay, where geochemical arguments indicate three felsic magma components existed. Two magma transfer systems are proposed and presented for Mount Hay. In the first case, each magma used its own network and developed a multi-sourced, homogeneous pathway of chamockite-leucograniteleucosome veins and dykes. Migration of the different magmas is independent with no interaction, but subject to contamination and fractionnation. In the second case, they are linked and interlayered, forming a multi-sourced heterogeneous pathway of chamockite-leucogranite-leucosome- hybrid veins and dykes. This system allows for mixing of components during transportation, forming hybrid composition veins. The magmas may also have been modified by fractionation and contamination. The geochemical diversity at Mount Hay indicates magma mixing between chamockite and leucogranite, suggesting that the second system is more applicable. Moreover, Mount Hay was subject to a progressive overthrusting event during magma transfer, which could have closed and opened dykes and veins, modifying the pathway systems by shear-zone and fracture propagation. The tectonics control the development of the geometry of the transport system (shear zones, folds, fractures) and in doing so, develop and influence the pervasive migration of the felsic magmas.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TECTONIC PROVINCES OF THE KERGUELEN PLATEAU: INTEGRATION OF SEISMIC DATA AND ODP DRILLING 'l. Borissova, 'A.M.G. Moore, ^J. Savers, ^M.F. Coffin, ^P.A. Symonds and Teliatnikov 'Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 Australia ^Institute for Geophysics, The University of Texas at Austin, 4412 Spicewood Springs Rd., Austin TX 78759-8500, USA ^Deptartment of Geology and Geophysics, University of Sydney, NSW 2006 Australia The Kerguelen Plateau extends for more than 2500 km north-northwestwards from near the Antarctic margin in the southern Indian Ocean. A large part of the plateau lies within Australia's Exclusive Economic Zone and the 'extended continental shelf, as defined under the United Nations Convention on the Law of the Sea. Data from three multi-channel seismic surveys conducted by the Australian Geological Survey Organisation in 1985 and 1997 have been interpreted and integrated with results from Ocean Drilling Program (ODP) Legs 119, 120 and 183. Several distinct structural provinces have previously been identified beneath the plateau, including: the northern (not considered here), central, and southern Kerguelen Plateau; Elan Bank; and the Labuan Basin. This study elucidates some of the differences in crustal structure, basement age and geological evolution between these provinces. The central Kerguelen Plateau (50°-55°S) includes the volcanically active Heard and McDonald Islands and contains a major sedimentary basin (the Kerguelen-Heard Basin). Recent results from Ar/Ar step heating experiments on ODP samples show that basalts from the top of volcanic basement are of Albian age (102 Ma). The Kerguelen-Heard Basin is a 'sag' basin, more than 40 000 km^ in area and containing more than 2000m of Cainozoic sediments. A prominent seismic reflector (the 'acoustic discordance') within the basin appears to be caused partly by a diagenetic 'front' associated with a Late Paleogene - Early Neogene unconformity. Apparent deformation of the section above this unconformity is caused by sedimentary structuring related to vigorous bottom current activity during the Neogene. The southern Kerguelen Plateau (south of 55°S) is tectonically complex. 'Basement' is again of basaltic composition and is estimated to be of Aptian-Albian age (119-110 Ma). There are several large basement uplifts and evidence of multiple stages of normal and oblique/strike-slip faulting from the Early Cretaceous to at least the Paleocene. At the intersection of the major 77^ Graben with the 59^ Rift, a much younger (Miocene?) tectonic event is identified, at which time the Upper Cretaceous section was uplifted and locally eroded. Elan Bank extends westward from the boundary between the central and southern Kerguelen Plateau. Gneissic metamorphic and felsic igneous clasts recovered from the primarily basaltic basement complex at ODP Site 1137 on the bank reveal its continental origins. The margins of the bank are characterised by massive lava flows and highly reflective layered crust at its base. These volcanics have obvious similarities with volcanic sequences identified on volcanic passive margins (eg the Wallaby and Exmouth Plateaus off Western Australia). The volcanic sequences on Elan Bank may have formed either during the Valanginian(?) breakup of India/Elan Bank and Antarctica, or during later Albian breakup of India and Elan Bank, when the bank was probably transferred from the Indian plate to the Antarctic plate via a ridge jump. In either case, massive Albian volcanism has overprinted and radically altered the continental sliver forming the core of the Elan Bank. The Labuan Basin flanks the eastern margin of the Kerguelen Plateau and contains up to 3 s TWT {ca 4 km) of sediment. Basement is extensively faulted and intruded(?), and the structural style and sedimentary fill of the basin are highly variable. A cross-cutting reflector within the upper sedimentary section is interpreted to be a 'bottom-simulating reflector' (BSRs), generally considered to be diagnostic of the presence of gas hydrates in the shallow sediments. Our initial interpretation of seismic and magnetic data suggests that the basin is floored by a combination of highly extended continental?) crust to the west and non-magnetised peridotite(?) to the east. A large volume of metamorphic and granitic rock dredged from one basement block within the province could support this interpretation. In the northern part of the Labuan Basin there is some evidence of tectonism that post-dates the initial extension and basin formation. This tectonism could be associated with the uplift of the Williams Ridge to the northeast, preceding the breakup between Kerguelen Plateau and Broken Ridge.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ENHANCED VISUAL INTERPRETATION OF POTENTIAL FIELD MAPS Fabio Boschetti\ Frank Horowitz', Peter Hornby' and Nick Archibald^ CSIRO, Division of Exploration and Mining, Nedlands, Western Australia Fractal Graphics, 39 Fairway Nedlands, Western Australia 6009 We present a new tool for the analysis and interpretation of magnetic and gravity data. Its major feature lies in the precise and mathematically rigorous meaning given to the 'worms', i.e. the lines that geoscientists traditionally draw on potential field maps, highlighting major geological signatures. These features can now be automatically detected, thus removing subjectivity from the analysis and simplifying and enhancing the visual interpretation. Also, algorithms for image processing and inversion can be written based directly on the 'worms', thus providing the geologist with an easy-to-use set of geophysical tools that avoid mathematically abstract and geologically unrealistic parameterisation, such as pixels and Fourier basis. The method. The worms consist of the collection of edges in the potential field data at different heights from the earth surface. On a horizontal plane these edges form individual, isolated 'strings', which are closely related to the lines commonly drawn by geoscientists in the traditional visual analysis of potential field maps. When derived in 3D these strings form surfaces whose shape and mathematical properties are a function of the underground causative sources. Accordingly such surfaces provide a powerful tool for an insight to the geometries of geological structures at depth. The greatest strength of the worms lies in the fact that sophisticated maths is implemented and represented in terms of features of intuitive visual and geological significance. Geologists can 'see' the geophysics and interact with it through their geological knowledge, free from the abstraction of traditional processing algorithms. This is not a black box tool designed to remove the interpreter from the analysis, rather it is designed to facilitate and enhance the experienced geologists in their interpretation by letting the 'geophysics' speaking an understandable 'geological' language. Visual Interpretation. Certain features are easier to interpret from their 'worms' response than from traditional images. For example the dip of faults is easily detected by the bending of the worms in 3D . Cross cutting relation between causative sources, hard to discriminate directly from the anomalies, are also maintained in the worm response. Sources of different shape, but resulting in very similar anomalies, often give quite different 'worm' response. Geoscientists both in CSIRO and Fractal Graphics have been trained to interpret worm maps by studying the worm response of various synthetic models generated with the Noddy forward modeling code. In this way the signatures of both isolated and interacting bodies can be understood and recognized on real maps. Together with the major anomalies, the worms also outline structures in the "flat" parts of maps, where no feature is apparent through visual inspection. The worms act as a sort of magnifying glass at multiple resolution. Features at different scales (different level of upward continuation) and of different magnitude are represented in the same visualization and can be analyzed at the same time. Examples on Real Data: from Mine to Regional, Continental and Planetary Scale: The technique is scale-independent. Datasets at different geographical scales have been analyzed and will be described in the presentation. These include prospect-scale aeromagnetic datasets, aeromagnetic data covering 1:100,000 scale map sheets (representing the district-scale), continent-scale maps such as the Australian gravity dataset (see Figure 1), and the worm map of the gravity field over the globe, representing the planetary scale (see Figure 2). The results produce different information at the different scales. At the continent-scale the multiscale edges allow discrimination of different tectonic styles, and comparison of the significance of crustal and lithospheric-scale structures. At the district- to prospect-scale the edges can be used for geological mapping purposes such as to map subtle changes in sedimentary sequences, map alteration patterns, and constrain pluton geometries at depth. The worms over the globe seem to indicate intra-plate relationships and deep mantle structures. More information about the underlying theory and applications can be found at the web address: http://www.agcrc.csiro.au/projects/3054CO.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DEVELOPING A MODEL FOR NON-MARINE SEQUENCE STRATIGRAPHY Ron Boyd^ Glaus Diessel^ Jennifer Wadsworth\ Murray Little^ Dale Leckie^, and Brian Zaitlin^ ' School of Geoscience, University of Newcastle, NSW, 2308 ^ Canadian Occidental Petroleum, Calgary, Alberta, Canada, ^ PanCanadian Petroleum Limited Sequence stratigraphy has been increasingly used in the marine environment to provide a framework for correlation and interpretation. Until recently, it was not clear how the technique might be extended to terrestrial settings where marine units are not available for correlation. Sedimentation is governed by the rate of sediment flux supplied to a depositional system and the rate of space made available for potential sediment accumulation (termed accommodation). In most natural systems there is a form of equilibrium response that governs the accumulation and preservation of sediments. In the non-marine setting that equilibrium response is mostly provided by the graded stream concept. Additional equilibrium control is provided by the groundwater table which in turn controls or influences the formation of organic deposits, soils, and lacustrine and floodplain water bodies. In the case of fluvial systems, a low accommodation to sediment flux ratio results in low rates of accumulation. This in turn results in the erosive migrating base of channel systems returning close to their earlier stratigraphic level, producing a succession that contains numerous erosional surfaces and preferential preservation of the lowermost part of fluvial successions as amalgamated channel deposits. Multiple cycles may be found over a short vertical range of stratigraphy, separated by unconformities (sequence boundaries). The geometry of the preserved sediments will be that of a fluvial sheet sand or an incised valley. The lack of accommodation to enable vertical accretion results in underlying structure and paleotopography exerting continued control on the site of deposition. Low accommodation to sediment supply ratios result in the low preservability of organic deposits. In the extreme case, oxidising conditions and low water tables may remove all organic traces. Increasing accommodation will result in preservation of root traces and increased darker organic content, and ultimately result in the development of coal seams. However, when developed, these coal seams may split frequently, with splits that correlate laterally to incised valleys, and the seams may contain internal unconformities that result in compound coals. Low accommodation is also conducive to the development of multiple deep, long lived soil profiles, resulting in compound or amalgamated soils. In a sequence stratigraphic context, low accommodation to sediment supply ratios in terrestrial settings promote the preservation of lowstand system tract deposits, as these are preferentially deposited in stratigraphically low positions such as the base of incised valleys. Transgressive systems tracts will also have potential for preservation, especially if deposited within backfilling valleys, but highstand systems tracts will be more exposed and subject to removal by subsequent cycles of erosion. The basic sequence stratigraphic correlation surface in terrestrial settings will be the sequence boundary, often located at the base of incised valleys and correlated laterally onto interfluves at the position of well-developed soil profiles. Additional correlation may result from recognition of accommodation reversal surfaces. Increasing accommodation to sediment flux ratios result in a greater vertical distance between successive lateral migration paths of channel bases. This in turn allows for the greater preservation of higher stratigraphic elements in the fluvial system such as upper bar forms, channel fills and in particular, levee and floodplain deposits. Hence, under increased accommodation conditions, fluvial processes will preserve not only amalgamated channel deposits but will show increasing preservation of intervening floodplain facies, making the channel sand bodies less interconnected and more ribbon-like in geometry. Under increasing accommodation conditions, both soils and coals will be less complex. Peat preservation is rare in times of low groundwater table. As water table elevations rise, organic preservation and darker colouration begins and culminates with peat formation when the water table is consistently high. Individual coal units or plies exhibit wetting upward cycles defined by trends of increasing mineral matter and characteristic maceral response. Individual coal seams will have fewer internal hiatuses in higher accommodation settings and will eventually develop a seam for each cycle of accommodation. Increasing accommodation results in sediment aggradation and the development of thicker stratigraphic sections, in turn removing deposits from the direct influence of underlying structure and paleotopography. Concurrently, the potential for marine influence increases and preserved transgressive and highstand deposits may exhibit brackish influence and possible tidal signatures, as well as deposition in open water bodies on alluvial floodplains.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE GRETA COAL MEASURES IN THE MUSWELLBROOK ANTICLINE AREA, NSW Ron Boyd' and Dale Leckie^ 'School of Geoscience, University of Newcastle, NSW, 2308 ^Canadian Occidental Petroleum, Calgary, Alberta, Canada

The Greta Coal Measures are the lower of two main coal-bearing intervals in the Permian northern Sydney Basin. High quality outcrop and continuous core data are available from the Muswellbrook Anticline area in the Hunter Valley, enabling a sequence stratigraphic interpretation of the Greta Coal Measures to be presented for the first time. Age and core relationships indicate an unconformity at the base and the top of the Greta Coal Measures. A correlation between dated tuffs in the upper Greta Coal Measures in the Muswellbrook area and the Maitland Group in the Cessnock area establishes a clear diachronous upper boundary for the Greta Coal Measures resulting from a northwestward marine transgression. The Greta Coal Measures are interpreted to occupy a single sequence in which the lower fluvial and lacustrine Skelatar Formation makes up a transgressive systems tract, the Ayrdale Sandstone Member is an estuarine unit around the maximum flooding surface, and the upper fluvial to deltaic Rowan Formation occupies a highstand systems tract. The overlying Jasdec Park Sandstone Member of the Maitland Group infills incised valleys above a sequence boundary and then occurs as a transgressive shoreline system before passing into the glacial marine Branxton Formation. The Greta Coal Measures represent high accommodation where subsidence and sediment supply were approximately balanced over more than 100 m of accumulation and the development of 14 recognisable coal seams in a single sequence.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SOUTHEAST AUSTRALIA - A CLASSIC WAVE-DOMINATED MARGIN? Ron Boyd^ Peter Roy^ and Jennifer Wadsworth^ 'School of Geoscience, University of Newcastle, NSW, 2308 ^ Geological Survey of New South Wales Southeast Australia has long been regarded as a textbook example of a wave dominated clastic coast, based largely on studies of highstand barriers and estuaries. Recent approaches to clastic sedimentology have taken a sequence stratigraphic approach. When applied to Quaternary passive margin coastlines similar to SE Australia, such the US Gulf and Atlantic coasts, the sequence stratigraphic approach has identified the presence of modem shorelines and river systems linked to incised valleys crossing the continental shelf and feeding lowstand deltas and shorelines on the outer shelf, and submarine canyons and fans further seaward in deeper water. Our recent investigations on the SE Australian margin have allowed us to test these concepts on the NSW coast and shelf Although the NSW coast has a classic distribution of barrier and estuarine deposits at the current highstand shoreline, it provides some surprises when viewed as a composite accumulation of Quaternary sediments deposited over a number of sea level cycles: 1)

The modem coastline and inner shelf consist of a laterally stacked composite of barrier and shoreface deposits accumulated over at least the last five interglacial highstands. Quatemary barriers are composed of sand reworked onshore from the continental shelf Rates of clastic sand supply from the land are low.

2)

There is a remarkable modem divided sediment dispersal system in which onshore and northward wave induced longshore transport occurs inshore of the shoreface base, and offshore and southward transport occurs on the outer shelf and upper slope in response to the East Australia Current. This situation is equivalent to that in South Africa with the corresponding Agulhas Currrent.

3)

The shelf is divisible into an inner zone of clastic deposition and an outer zone of temperate carbonate deposition or a hardground of no deposition. Carbonate mounds or reefs are locally developed along the shelf edge.

4)

Quatemary incised valleys that are well developed on the coastal zone terminate seaward at the base of the shoreface or on the adjacent inner shelf

5)

There are no well-developed lowstand shorelines or deltas. On the southem and central sections, the lowstand shoreline is landward of the shelf break. Modem river valleys do not connect seaward with submarine canyons and do not appear to have supplied sediments to canyons and fans at lowstand. Carbonate sediments appear to have aggraded at low rates and have also been swept off the outer shelf and prograded the margin at lowstand, similar to ODP results reported from the Great Australian Bight.

6)

Transgressive sedimentation in estuaries is limited to the modem coastal plain. On the shelf, transgression produced deep reworking of former highstand shorelines by wave ravinement processes and tidal inlet excavation and tidal delta infilling.

The southeast Australian margin is thus a mixed clastic and carbonate margin. It is a complex depositional environment dominated by tectonic stability, low accommodation and sea level cyclicity. The margin is partitioned into a coastal plain, an inner shelf and an outer shelf, each of which is controlled by different processes and has accumulated highly contrasting sedimentary deposits.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TERMINATION OF SUBDUCTION ON THE AUSTRALIAN-NEW ZEALANDANTARCTIC MARGIN IN THE LATE MESOZOIC J. D. Bradshaw Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand.

It is widely accepted that the cessation of convergent-margin tectonics along the AustraliaNew Zealand-Antarctic sector of the Pacific margin was due to the approach and death of the Phoenix-Pacific spreading ridge. This occurred diachronously in a counter-clockwise direction progressively from northeastern Australia in the Early Cretaceous to the Antarctic Peninsula in the Cenozoic The change is marked geologically by a rapid switch from subduction-related deformation and magmatism to crustal extension and within-plate magmatism. The rapid change to extension and eventual continental separation can be explained by the progressive welding of the Gondwana margin to the receding Pacific plate. In the New Zealand-West Antarctica sector there are anomalies in the general pattern that may relate to the commonly postulated distinction between 'northern New Zealand' and 'southern New Zealand' in the late Mesozoic. For example, subduction related deformation on the eastern margin of the South Island ends at approximately 105+5 Ma, somewhat later than the onset of continental extension in western New Zealand. Similarly, structural relationships, detrital zircon (SHRIMP) ages, and apatite and zircon fission track data suggest that subduction persisted after 100 Ma in the eastern North Island, possibly until 90 Ma, an interpretation that is inconsistent with data from sedimentary basins and withinplate magmatism in the South Island and Antarctica. A possible explanation may lie in either one or more long transforms in the Pacific-Phoenix ridge resulting in a remnant of the Phoenix plate persisting for 10+ million years east of the present North Island and bounded by an isolated segment of a subduction zone. Subduction plus subduction zone roll-back may have continued in northern New Zealand while extension and within plate magmatism were occurring in southern New Zealand and western Marie Byrd Land, Antarctica. The present Aluk (Phoenix) plate remnant near the northern tip of the Antarctic Peninsula presents a similar relationship. The Juan de Fuca plate is a larger example and represents a relic of the Farallon plate still bounded by an active subduction zone and a major transform.

51

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

INTRUSION OF THE BALLOON MELANGE: AGE ORIGIN AND IMPLICATIONS FOR CAMBRO-ORDOVICIAN REGIONAL TECTONICS J. D. Bradshaw, R. Jongens and A. Fowler Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand.

The Balloon Melange is a regional scale body of disrupted rocks within the Takaka terrane, NW Nelson, New Zealand. It comprises a sheared sandstone and mudstone matrix that encloses exotic blocks of Cambrian formations. New work suggests that the characteristic fabric had a two-stage origin. The first stage is dominated by layer parallel extension in weakly lithified rocks. Recent location of intrusive contacts between melange and Middle to Late Cambrian volcanic arc assemblage rocks suggests that initial disruption was followed by diapiric intrusion. Mesoscopic and microscopic structural examination shows the primary fabric was extensively modified during the development of the regional SI cleavage in the mid-Paleozoic. This tectonic fabric, the basis of the previous interpretation as 'tectonic melange', postdates diagenesis and ankerite metasomatism in both melange and host. Close lithologic similarity suggests that the protolith of the Balloon Melange matrix is the quartzofeldspathic Junction Formation of Middle Cambrian age. The latter is so different in composition from contemporaneous arc rocks that it has been suggested that it represents either rocks of an accretionary complex, or deposits of a backarc basin, probably formed at some distance from the arc and brought into contact with arc rocks by crustal shortening. Our evidence supports crustal shortening and suggests that the arc was thrust over the quartzofeldspathic succession prior to melange diapirism. Intrusion may have taken place during a period of stress relaxation after thrusting. Melange formation is thought to have occurred in the Late Cambrian, a precursor to the change to 'passive margin' type sedimentation that characterises the latest Cambrian to Devonian. Blocks of the 'passive margin' assemblage do not occur within the melange, though it should be noted that a primary sedimentary contact between the arc-related and 'passive margin' assemblages cannot be demonstrated., a Late cambrian age is consistent with evidence that the Gendarme Dolerite dikes with Ar/Ar ages of 484+25 Ma cut the melange and early folds. Preliminary results from dating detrital zircons from the Junction Formation and the melange indicate the presence of the typical Australasian Paleozoic Gondwana margin assemblage in both units with a strong peak at -550 Ma. The rocks however are coarser and more feldspathic than dominant Cambro-Ordovician 'turbidite apron' and may represent an older and more proximal sedimentary belt. A similar change from active margin rocks to a quartzitic passive margin assemblage takes place in northern Victoria Land and Tasmania in the latest Cambrian but sedimentary diapirism is not recorded.

52

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE HAWKESBURY SANDSTONE: ITS ORIGINS AND LATER LIFE D. F. Branagan Division of Geology and Geophysics, School of Geosciences, University of Sydney

The Triassic Haw^kesbury Sandstone, although well known, is difficult to pin down in the field, both its lower and upper boundaries being difficult to map. Its characteristic features include slumps, some of which may occur on specific horizons, representing slight tectonic events during deposition. Large sand waves and sand channels are also notable features. The sedimentary structures and features such as relatively extensive shale units suggest a possible Copper Creek/Diamantina semi-arid type of depositional environment. The postdepositional history of the Hawkesbury Sandstone is by no means clear, the original depth of burial, type and thickness of cover and amount of erosion are disputed. This paper argues for very limited cover, slow rate of erosion, and a non-catastrophic history of postdepositional igneous activity. Post-depositional structural features include widelydistributed normal, reverse and strike-slip faults and crush zones, often present in close juxtaposition in the sandstone. The earliest formed of these were a product of the final phase of the Hunter-Bowen tectonic event. The weaknesses thus formed at the end of the Triassic were the loci for later deformation, which took place mainly about the end of the Cretaceous with the opening of the Tasman Sea. The sandstone still retains stored stress. Although its rate of release is generally steady, there have been at least three explosive failures in the past thirty years. Such failures might have previously played a significant part in developing the characteristic valley cross-sections of the Sydney region, with large sandstone blocks irregularly distributed on slopes below cliff-lines. The sandstone landscape has strongly influenced the lives of both Aboriginal and European inhabitants.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

STRUCTURAL GEOLOGY OF THE HAWKESBURY SANDSTONE REGION, WITH PARTICULAR REFERENCE TO THE CITY OF SYDNEY AND NEARBY AREAS D.F. Branagan School of Geosciences, University of Sydney

Aspects of the post-depositional history of the Triassic Hawkesbury Sandstone are discussed in this paper. The post-depositional structures consist of folds, joints and faults. Some structural features date back to the initial period of uplift of the sandstone. Dyke intrusions, explosive vent eruptions and shallow pie-shaped intrusions disrupt the sandstone in places. The dykes are closely related to jointing which occurs in several preferred directions. Dyke intrusion is more common in the WNW-ESE orientation, although some dykes occur trending about N-S. Brittle failure features include widely-distributed normal, reverse and strike-slip faults and crush zones, often present in close juxtaposition in the sandstone. The earliest formed of these were a product of the final phase of the Hunter-Bowen tectonic event. The weaknesses thus formed at the end of the Triassic were the loci for later deformation, which took place mainly about the end of the Cretaceous with the opening of the Tasman Sea. The sandstone still retains stored stress. Although its rate of release is generally steady, there have been at least three explosive failures in the past thirty years. Such failures might have previously played a significant part in developing the characteristic valley cross-sections of the Sydney region, with large sandstone blocks irregularly distributed on slopes below clifflines. Earthquakes with a maximum intensity of 6 on the Richter Scale are to be expected in the Sydney region, but the periodicity is still unpredictable. A tentative post-depositional history of the Hawkesbury Sandstone is presented in a table.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TELESCOPING OF PORPHYRY CU-AU MINERALISATION, ADVANCED ARGILLIC ALTERATION AND POLYMETALLIC SULFIDE-GOLD-QUARTZANHYDRITE VEINS IN THE THAMES GOLDFIELD, NEW ZEALAND R.L. Brathwaite, M.P. Simpson, K. Faure and D.N.B. Skinner Institute of Geological and Nuclear Sciences Ltd, PO Box 31-312, Lower Hutt, New Zealand Porphyry Cu-Au mineralisation and potassic/phyllic alteration, advanced argillic alteration and polymetallic sulfide-gold-quartz-anhydrite veins are telescoped within a vertical interval of 400-800m on the northeast margin of the Thames goldfield. The pological setting is a Jurassic greywacke basement overlain by Late Miocene andesite-dacite volcanics, which are extensively altered to propylitic and argillic assemblages. The porphyry Cu-Au mineralisation and alteration is hosted in dacite porphyry associated with porphyry intrusion breccias and overlying andesites and dacites. Diamond drilling intersected up to 300m of low grade Cu-Au (0.12-0.20% Cu, 0.23-0.4 g/t Au) mineralisation. An adjacent high-sulfidation, advanced argillic alteration cap contains quartz-alunite±dickite and pyrophyllite-dickite±diaspore assemblages. There is a zonation from potassic K-feldspar-magnetite±biotite alteration at depth in the dacite porphyry, to phyllic quartz-sericite-pyriteanhydrite alteration, which is overprinted by pyrophyllite and andalusite at the base of the overlying advanced argillic cap. Early quartz stockwork veinlets associated with the potassic alteration contain fluid inclusions that trapped a highly saline (50-80 wt% NaCl equiv) fluid at high temperatures (450 to > 6 0 0 T ) indicative of a magmatic fluid. Some chalcopyrite is associated with slightly later quartz-magnetite veinlets, but the bulk of the copper occurs as disseminated chalcopyrite associated with the phyllic alteration. At depth the potassic and phyllic alteration zones grade outwards into propylitic chlorite±epidote alteration. Secondary lower temperature (300-400''C) and salinity (3-24 wt% NaCl equiv) fluid inclusions in the early quartz veins and quartz phenocrysts may represent a late stage magmatic fluid that caused the phyllic alteration. Later sphalerite-galena-pyritechalcopyrite-acanthite-gold-quartz-anhydrite veins occur within and around the margins of the porphyry system, and were mined for gold in the Sylvia-Kaiser lode. Fluid inclusions associated with these late veins were formed at temperatures of about 250-320^C from low salinity (1-3 wt% NaCl equiv) fluids. 5*^0 analyses of quartz in the early quartz veinlets are in the range of 7.8-10.0%o and yield calculated 5^^0h2o values of 6.3-7.6%o that are indicative of a magmatic source. Calculated 5 ^ ^ 0 H 2 O values for the fluid that deposited the late sulfldequartz veins are close to 0%o, and are consistent with mixing of a magmatic fluid (7-8%o) with local meteoric water (-6%o). The spatial association between porphyry Cu-Au mineralisation, high-sulfidation advanced argillic alteration and polymetallic sulflde-gold veins suggests that they are genetically part of the same hydrothermal system. This is consistent with reconnaissance K-Ar dates of 11.6-10.7 Ma for the intrusive porphyry, alunite in the advanced argillic alteration, and sericite selvedges from gold-quartz veins in the Thames goldfield. Paleo-depth reconstruction from stratigraphy and fluid inclusion temperature-pressure estimates, indicates that the telescoping of the late polymetallic sulflde-quartz veins on to the early high-temperature quartz stockwork veins was a result of the paleosurface being lowered by about 700m. The lowering of the paleosurface and consequent telescoping of mineralisation may be due to rapid erosion or sector collapse of an overlying stratovolcano, as is common in volcano-plutonic arcs of the Circum-Paciflc region.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

REVISITING THE TECTONIC EVOLUTION OF THE AUSTRALIAN SOUTHERN MARGIN B. J. Brown', R. D. Muller', H. 1. M. Struckmeyer^, P. A. Symonds^ and J. Sayers^ ' School of Geosciences, F05, University of Sydney, NSW, 2006, Australia ^ Australian Geological Survey Organisation

Recent deep seismic reflection and sonobuoy data present an opportunity to revisit the early evolution of the southern margin of Australia. These data have been used together with older seismic and well data to analyse the tectonic structure and subsidence history. The Great Australian Bight is characterised by an extremely wide zone of highly stretched crust (up to 630 km wide), which includes lineated magnetic anomalies. Several lines of evidence support a pure shear model of extension, including the symmetry of marginparallel gravity anomalies on Australia and Antarctica; and matching geophysical profiles of the conjugate margins. Beta factors (1.1-1.2) derived from data from two wells from the irmer shelf roughly agree with beta factors (1.1-1.3) derived from crustal thickness (sonobuoy) data, indicating similar extension factors for the crust and lithosphere. The margin's structural similarity to the west Iberian serpentinite margin also supports a pureshear origin. Tectonic subsidence analysis has revealed the combined influence of Cretaceous dynamic mantle topography and lithospheric extension on tectonic subsidence in the Great Australian Bight. Negative dynamic topography from 120-95 Ma was caused by the eastward motion of the Great Australian Bight region over a sinking slab in the mantle, enhancing rift-phase subsidence. Subsequent lithospheric rebound reduced post-rift thermal subsidence. This result has implications for thermal geohistory modelling, as geothermal gradients will generally be smaller than those based on the simple lithospheric thirming model. Seismic and well data provide evidence for flexure of the South Australian margin in the Late Tertiary. Subsidence analysis reveals a consistent pattern of a tectonic event between approximately 25-0 Ma. This subsidence is not in response to extension and thinning like the rift related subsidence observed in the Early Cretaceous, as across the margin virtually no deformation in Late Tertiary sequences is observed in seismic profiles. This indicates that Late Tertiary tectonic subsidence is related to an elastic plate response to changes in far field plate stresses.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

SEASONAL VARIATION OF CALCAREOUS EPIPHYTE ON ARTIFICIAL SEAGRASS: PRELIMINARY STUDIES, WEST ISLAND, SOUTH AUSTRALIA. Kirstv M. Brown\ Yvonne Bom\ Anthony Cheshire^ and Noel James^ ^Department of Geology and Geophysics, University of Adelaide, Adelaide, South Australia. 5005 ^South Australian research and development institute (SARDI), 2 Hamra Ave, West Beach, South Australia ^Department of Geological Sciences, Queens University, Kingston, Ontario K7L 3N6, Canada Definition: Calcareous epiphytes are sessile marine organisms (plant or animal) with calcareous skeletons which, for the dominant part of their lives (if not the entire) are attached to the living outer tissues of a plant. They do not draw water or food from the living tissue. Previous studies (Brown et al, 1999, 1998) have shown that calcareous epiphytes produce significant quantities of material that contribute to the sediment off the coast of South Australia. The present study was established to i) determine whether the contribution of calcareous epiphytes to the sediment varies throughout the year ii) determine whether the relationship to the seagrass is purely that of a substrate, iii) examine the favoured sites and iv) establish the rate of recruitment. The artificial seagrass set-ups include Posidonia and Amphibolis simulated grass, comprising of 4 different materials for the Posidonia, 3 different materials for the Amphibolis stem and 1 for the Amphibolis blades. Most of these materials have been used successfully on previous artifical seagrass experiments to look at algal epiphytes, but have not been compared. The location selected for this experiment was West Island. This island is located approximately 5km to the south-west of Victor Harbor. S.A. and is a pristine area, as it is designated marine reserve. The water type is a mix of open marine from the Southern Ocean and minor bottom currents from the Gulf St. Vincent. Up to 16 sets of artificial seagrass were deployed at various times from 05/99 to 06/99. The set-ups were placed in a mixed Amphibolis / Posidonia seagrass bed, at a depth of approximately 6 m. Two set-ups are collected every 3 months, i.e. at the end of each season. Other set-ups were left for 3, 6, 9 and 12 months to check the seasonal variation by an alternative method and also because Amphibolis stems continue to grow for up to 2 years. Recovery of the experiments has been 80% successful, with only a few Amphiolis stems having been dislodged. The study has revealed many interesting factors including a discrepancy in the number of calcareous epiphytes found on real Posidonia / Amphibolis versus the artificial Posidonia / Amphibolis. One example is that only 1 species of Bryozoa has been found on the artifical seagrass (both Posidonia and Amphibolis) whereas on the real seagrass, up to 8 species have been found in the same area. Seasonal variation is observed, with apparent higher productivity of calcareous organisms in the summer and winter months. Other results from these experiments have shown that there are favoured recruitment sites for the organisms. These observations have been quantified using a spatial analysis program developed for this project. The experiment is on going, with the presentation representing only 1/2 of the total data hoped to be obtained.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOMETRY, TIMING AND TEMPERATURES OF REGIONAL FLUID FLOW IN THE HAMERSLEY PROVINCE, WESTERN AUSTRALIA. M. Brown, N. Oliver and J. Dickens Economic Research Unit, School of Earth Sciences, James Cook University.

The Hamersley Iron Province, Western Australia, outcrops over 40,000 Km^ and contains some of the largest iron ore deposits in the world. The huge Fe deposits (E.g. Mt Whaleback, Mt Tom Price and Paraburdoo) are hosted within a sequence of Banded Iron Formation (BIF) of the Mount Bruce Supergroup. Two major deformation events have affected the Hamersley Province, the Opthalmian Orogeny (2300-2200 Ma) and the Capricorn Orogeny (1820-1650Ma). The northern portion of the Pilbara is relatively undeformed and the level of deformation increases toward the south. The majority of the Fe deposits lie within the southern most, deformed portion of the Hamersley Province suggesting a possible connection between mineralisation and deformation, which this research is examining in detail. Eight separate stages of fracturing and veining have been identified from the rocks of the southern Hamersley Province. These stages of veining can be related to the regional structural history of the Province. This veining is indicative of several periods of fluid overpressure, consistent with one or more cycles of diagenesis, metamorphism and deformation. From the timing, geometry and distribution of these veins it can be seen that there was a substantial fluid flow and mass transfer event (both Si and Fe) during and directly after the peak metamorphic D2 deformation event of the Opthalmian Orogeny. It can also be seen that the majority of fluid flow occurred in the base of the Brockman Iron Formation centered on the Dales Gorge Member. Fluid inclusion homogenisation temperatures were between ITS^'C to 300°C and the dominant fluids were of low to moderate salinity. These are the characteristics of moderately heated basinal or metamorphic fluids, also consistent with our preliminary oxygen isotope data (Powell et.al, 1999). Two other fluid types are observed; a high temperature, moderate salinity fluid and a high salinity, moderate temperature fluid that are possibly associated with post D2 dolerite dyke intrusion. Small patches of microplaty hematite mineralisation can be seen in relation to some of these veins (Oliver & Dickens, 1999) where fluids have converted magnetite to microplaty hematite, and removed silica, some time after the D2 deformation event. References OLIVER, N. H. S. & DICKENS, G. R., 1999: Hematite ores of Australia formed by syntectonic heated meteoric fluids, in C. J. Stanley et al. (eds.). Mineral Deposits: Processes to Processing, Balkema, Rotterdam, pp. 889 - 892. POWELL, C. MCA., OLIVER, N. H., LI, Z. X., MARTIN, D. MCB. AND RONASZEKI, J., 1999: Synorogenic hydrothermal origin for giant Hamersley iron oxide ore bodies. Geology, 27:175-178.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LATE DEVONIAN AND CARBONIFEROUS DEFORMATIONS IN EASTCENTRAL VICTORIA Max C.Brown 51 Debenham St., Mawson, A.C.T. 2607.

Most accounts of the geology of the Melbourne Zone of the Lachlan Fold Belt in eastcentral Victoria conclude that a major Middle Devonian orogeny, producing N-S trending folds and thrusts in early Palaeozoic marine sediments, was the last significant deformation - unlike the eastern Lachlan, where there was a later Carboniferous orogeny. This is an oversimplification. There is clear evidence for significant Late Devonian and Carboniferous compressional deformation near the eastern margin of the Melbourne Zone, where the early Palaeozoic rocks are overlain by the Mount Howitt Province of Late Devonian to Early Carboniferous felsic volcanics and fluvial sediments. In the north of the province Middle - Late Devonian felsic volcanics and sediments near Tatong are folded along N-S axes and overlain unconformably by Late Devonian felsic caldera volcanics. Folded and cleaved Late Devonian fluvial sediments of the Jamison Syncline near Mansfield are also overlain unconformably by Late Devonian volcanics. The Late Devonian volcanics and co-magmatic granites and the Jamison Syncline sediments are overlain unconformably by Early Carboniferous fluvial red beds. Where they rest on the rigid block of granitoids and caldera volcanics east and north of Mansfield the Carboniferous red beds are near-horizontal except that, as in the rest of the Mount Howitt province, they are sharply upturned and locally folded adjacent to major N-S thrust faults. Elsewhere in the province the Carboniferous red beds and underlying Late Devonian volcanics and sediments overlie deformed Early Palaeozoic sediments, and are typically folded along N-S axes into widely- spaced folds with dips on fold limbs of 10-45 degrees. They are most strongly deformed near Licola in the south west of the province, where red mudstones have a close spaced near vertical cleavage, refracting to bedding-normal cleavage in sandstones. The intensity of deformation in the Early Carboniferous red beds near Licola is similar to that in the more highly deformed Late Devonian-Carboniferous sediment outcrops in the eastern Lachlan Mobile Zone. There is evidence for Late Devonian or younger compressional deformation in the Melbourne Zone west of the Mount Howitt Province. Mine sections show that dykes of the "post-orogenic" Middle Devonian Woods Point Dyke Swarm are displaced by thrust fauhs. Axes of the Middle Devonian folds bend around Late Devonian cross-cutting granitoid plutons and volcanic calderas in a manner consistent with later east-west crustal shortening of 10-20%; during which the plutons and caldera volcanics, as they did in the Mount Howitt Province, acted as rigid blocks

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PROGRADE-RETROGRADE EVOLUTION IN RELATION TO P-T-t PATHS, THE METAMORPHIC VOLATILE PHASE AND MELTING REGIMES Michael Brown Lab. for Crustal Petrology, Department of Geology, University of Maryland, College Park, M D 20742, U S A Rocks are flight recorders recovered from the wreckage of an orogen, from which we can retrieve something of the history of the passage of material through the orogen. Field relations, rock fabrics and microstructures, mineral assemblages, reaction textures, mineral compositional variation and isotope data provide a record of the history. What happens to a rock on the flight through P-T space is a function of the reaction lines crossed, whether any of these reaction lines are re-crossed along the retrograde (decreasing 7) segment, and whether any fluid evolved along the prograde (increasing T) segment is retained to allow reaction reversal during the retrograde stage. Our ability to read the evidence preserved in rocks is critical to interpret their history, constrain alternative models of orogenesis, and understand better orogenic processes. Improved calibration of mineral equilibria, development of techniques to retrieve close-to-peak P-T conditions and ability to date points along the flight have allowed us to constrain better the P-T-t path followed by individual rocks. Nonetheless, much of what we do in petrology remains a matter of interpretation, and therefore, it is open to discussion and re-evaluation. This is an area in which Ron Vernon has been an active participant throughout his career. In any discussion of reaction history, we must consider what is the phase assemblage, how it may change, how change may be recorded and whether any fluid phase was lost from the equilibration volume. Untested assumptions, for example that particular textures record unique processes, must be challenged. Challenging accepted dogma has been a characteristic of Ron Vernon's career. The use of corona textures for the derivation of P-T conditions and P-T vectors in migmatites is such an example; commonly regarded as recording decompression (decrease in P\ coronas also may result from a heating-cooling cycle without substantial change in P, The type of P-T-t path and availability of fluid (water-rich metamorphic volatile phase or melt) are important variables in metamorphism. Reactions involving fluid may be fluid generating, conserving or consuming. Back reaction sensu sthcto occurs between products of a prograde reaction as the same equilibrium is re-crossed in the retrograde sense. In the case of fluid consuming reactions, the continued presence of the fluid phase is critical to reversing the reaction during the retrograde stage. Above the solidus, melt producing reactions may be reversed upon cooling, and back reaction between crystallizing melt and residue may occur. However, melting in compressional orogens is a syntectonic process. In migmatites, segregation into leucosome and melanosome implies melt migration from the immediate generation site. Commonly, melt migrates into local dilational sinks of lower melt pressure on length scales greater than the equilibration volume, which has implications for reversal of reactions. If melt has been lost from the equilibration volume, back reaction may or may not occur, and in any case, back reaction cannot return the residual rock completely to its sub-solidus mineralogy. In a model system, if a P-T-t evolution crosses sequentially divariant reactions around an invariant point, each reaction is crossed only once in a reaction sequence. For back reaction to occur, the P-T-t path should approximate isobaric heating and cooling. However, P-T-t paths in compressional orogens are clockwise, involving decompression while close to peak T. Melting may occur on the prograde and decompression portions of the path, and melt extraction may occur along any part of the path where melt volume is greater than the percolation threshold. Nonetheless, melt is likely to be present in the deep crust of orogens for tens of millions of years, and back reaction sensu lata (i.e. crossing a melt-consuming reaction as T decreases, but at lower P than the prograde evolution) may occur during the retrograde stage of the metamorphic evolution. There are two principal melting regimes, contact melting (regime 1 - essentially isobaric heating and cooling paths, producing contact migmatites) and regional melting (regime 2 essentially clockwise P-T-t paths, producing regional migmatites). Regional melting is subdivided into: 2A the wet melting regime; and, 2B the dehydration melting regime. Back reaction is restricted to regime 1, although back reaction s.l can occur in regime 2 providing the cooling path does not pass below the P of any invariant point terminating the melt-producing reaction. Whether back reaction occurs is determined by: the dPIdT slope of the retrograde segment of the P-T-t path, and the amount of decompression before the beginning of cooling, in relation to invariant points in the chemical system under consideration; the Clapeyron slopes of the various equilibria crossed; and, the availability of fluid (water-rich metamorphic volatile phase or melt) to allow reversal of fluid producing reactions. One feature that has been interpreted in various ways, the most recent of which is recording back-reaction, is the biotite-rich melanosome in stromatic migmatite. In some cases, however, erosion features along the contact between the biotite-rich melanosome and leucosome with melt-present crystallization microstructures precludes an origin by back-reaction. Water dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled water may promote wet melting at supra-solidus conditions and retrogression at sub-solidus conditions. Growth of 'late' muscovite over sillimanite may be the result of this process, and influx of exogeneous water may be unnecessary.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15'^ Australian Geological Convention, Sydney, July 2000

THE SWITCH FROM MAGMA ASCENT TO EMPLACEMENT DURING OROGENY Michael Brown Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, M D 20742, USA Earth's continental crust is differentiated because granite magmatism dehydrates and makes refractory the lower crust, enriching upper crust in felsic and heat producing elements. How this occurs is fundamental to understanding crustal evolution. Granite magmatism is made up of four stages, generation, segregation, ascent and emplacement, with length scales that vary by up to in a time span of <lmyr. Plate boundary stresses drive deformation, the record of which is the accumulated strain in orogens. Melt flow is driven by regional stress and buoyancy, but in compressional orogens ascent of magma usually is controlled by the structures, particularly those that localize strain. Assuming availability of melt in storage porosity within an anatectic zone limited by the granite wet solidus, and an ascent column through which magma is flowing by pervasive and focused mechanisms, we may ask: What controls the switch from magma ascent to emplacement, and how does this happen? To answer this we must realize the coupled nature of orogenic systems, with feedback relations among stress, deformation, thermal evolution, strength, strain rate, melt flow, etc. These systems are deterministic at large length scales, controlled by the external forces acting on them, but stochastic at small length scales, governed by the laws of probability. In such non-linear dynamical systems, large effects are generated from small fluctuations. The approach adopted here reconciles into one general model contradictory views put forth by proponents of the dike - horizontal tabular pluton, migmatite - hemi-ellipsoidal pluton root/composite tabular pluton head, and visco-elastic magmatic diapir ascent and emplacement models. The perception that emplacement occurs at a 'level of neutral buoyancy' is inconsistent with the density of granite liquid (-2.44 kg m^ at 1 GPa, 800EC) and the range of emplacement depths. Further, the generalization that emplacement is controlled primarily by structural interactions between ascending melt and anisotropics in upper crust is only part of the story. Although plutons may be formed by magma expansion into an evolving structural trap, or by multiple MTPs acting locally, the systematic variation in 3-D shape with depth, from horizontal tabular to blobby to vertical lozenge, suggests that most emplacements occur by two principal mechanisms, according to host rock behavior (a function of thermal gradient, strain rate, etc.): in the brittle regime, vertical inflation (lifting the roof/depressing the floor, accommodation mechanism(s) unspecified) after horizontal flow in a fracture or preexisting anisotropy; and, in the ductile regime, lateral expansion (swelling out like a balloon, accommodation mechanism(s) undefined) localized by some instability. In the brittle regime, magma may be arrested by a structure or 'crack stopper', some instability or thermal death. Emplacement occurs when mainly vertical flow switches to predominantly horizontal flow. Analysis of the length V5. thickness of horizontal tabular plutons suggests they inflate according to a power law relationship, interpreted to mean that vertical thickening only occurs after magma has traveled horizontally some critical minimum distance. Inflation is assimilated by depression of the floor and/or lifting of the roof. Although sagging of the floor can be accommodated because magma has been extracted at depth, no simple relationship exists since the volume of melt in the pluton is from an order of magnitude larger source. To lift the roof, Fpieit must overcome lithostatic load and tectonic overpressure. In the ductile regime, amplification of naturally occurring instabilities in the system likely causes the switch from ascent to emplacement. Instabilities may be internal to the ascent column, such as fluctuations in permeability or magma flow rate (or changes in cross-sectional shape), or external to the ascent column, such as variations in strength or state of stress in host rock. They are not mutually exclusive, and feedback relations are similar whatever the initial instability. Consider an ascent column with zones of higher permeability, these are conduits of higher magma flux, but higher flux increases permeability, and also heats and weakens the host rock surrounding these conduits, which in turn increases the strain rate, and so on. Magma in the preferred ascent conduit will exploit the weakening and may expand into the host rock, switching ascent to emplacement and forming a vertical lozenge pluton. A similar phenomenon occurs if the instability is due to differences in the strength of the host rock or the stress field around the ascent column. Magma exploits the weaker/lower stress sectors, swelling out of the ascent column into those sites, heating and weakening the host rocks, which enables further lateral expansion, forming a blobby pluton. Above the anatectic zone, differences in flow rate or cross-sectional shape of the ascent conduit may lead to fluctuations around the critical width for flow without freezing. If freezing occurs in the slower/narrower parts of the conduit, flow will focus in the faster/wider parts. Heating of the host rock will cause weakening that will facilitate swelling of the conduit. The switch to emplacement may lead to formation of a pluton with a horizontal tabular head, formed by inflation of a sub-horizontal magma fracture, and a hemi-ellipsoidal root that passes down into a migmatite zone through which magma was transferred. Acknowledgement. Discussion with many people, most recently Sandy Cruden, Nick Petford, Ed Sawyer and Roberto Weinberg; my ideas are irretrievably entangled with theirs, but any mistakes most likely are mine!

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

FELSIC VOLCANISM AND RELATED SEDIMENTATION IN A LATE ARCHAEAN CONVERGENT MARGIN ENVIRONMENT, EASTERN GOLDFIELDS PROVINCE, WESTERN AUSTRALIA Stuart J. A. Brown^ M. E. Barley^ B. Krapez\ J. Hand^ and R.A.F. Cas^ ^Department of Geology and Geophysics, University of Western Australia, Nedlands 6907, Western Australia; ^Department of Earth Sciences, Monash University, Clayton 3168, Victoria

Felsic volcanic and associated sedimentary rocks are significant component of Late Archaean greenstone sequences, and provide important constraints on depositional environments, and tectonic processes. Three principal felsic volcano-sedimentary associations have been recognised in the Eastern Goldfields Provine of Western Australia. 1) Early arc-related andesitic complexes with associated quartz-poor epiclastic sediments (>2700 Ma). Proximal facies of this association define discrete volcanic centres (andesitic complexes) comprising intermediate lavas (with minor dacite and rhyolite), lenticular coarse mass-flow deposits, epiclastic sandstones, and mafic to intermediate sill complexes. Enveloping epiclastic sedimentary sequences grade laterally from coarse proximal submarine-fan deposits to more distal turbiditic sequences interlayered with tholeiitic pillowed basalt and black shale. The andesite centres are interpreted to represent subaqueous to emergent stratovolcanoes that were rapidly eroded and detritus resedimented into volcano-adjacent basins similar to modem intra-arc or inter-arc basins. 2) Na and Sr enriched, HREE-depleted (TTD) andesitic to rhyolitic volcaniclastic rocks with subordinate lavas (c.2700 to <2670 Ma). The association is dominated by sand- and mud-prone turbidite facies, with local thick proximal breccias and conglomerates, and rare coherent lava facies deposited in a deep submarine basin environment. The association represents an extended and complex period of deposition in an arc-related submarine (back-arc or intra-arc) basin. 3) Bimodal Association (c. 2692-2680 Ma). This association is characterised by pillowed basahs and mafic hyaloclastites interlayered with rhyolitic to dacitic lavas and volcaniclastic rocks, and appears to represent both subaqueous volcanism and reworking of pyroclastic material derived from shallow subaqueous to subaerial effusive and explosive volcanism. The compositional range, eruptive styles and association with late mafic sill complexes are typical of intra-arc rift settings. All three volcanic associations have facies associations that are consistent with formation in a convergent margin setting over a period of at least 50 Myr (2720 Ma to <2670 Ma). However differences in the geochemistry of the volcanic rocks and their present geographic distributions, suggest that each association represents a distinct basin type, possibly along the same convergent margin. All known felsic volcaniclastic sequences in the Eastern Goldfields record either subaqueous (mostly effusive) volcanic activity or reworking of subaerial volcanic rocks into fringing sedimentary basins. Subaerial volcanic facies are not directly represented (or are extremely rare), reflecting the poor preservation potential of subaerial volcanic facies.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MINERAL PROSPECTIVITY MAPPING: A COMPARISON OF MLP NEURAL NETWORK, FUZZY LOGIC AND WEIGHTS OF EVIDENCE METHODS Warick M. Brown J Tamas D. Gedeon,^ David 1. Groves^ and Robert G. Barnes^ ' Centre for Global Metallogeny, Department of Geology and Geophysics, University of Western Australia, Perth, WA 6907 ^School of Information Technology, Murdoch University, Perth, WA 6150 ^Geological Survey of NSW, NSW Department of Mineral Resources, PO Box 65, Armidale, NSW 2350

A multilayer perceptron (MLP) neural network is used to estimate the favourability for placer and disseminated primary gold deposits using a raster GIS database for the Tenterfield 1:100,000 sheet area, NSW. The database consists of solid geology, regional faults and shear zones, airborne magnetic and gamma-ray data survey (U, Th, K, and total count channels), and 63 deposit and occurrence locations. Input to the neural network consists of feature vectors formed by combining the values from co-registered grid cells in each GIS thematic layer. The network contains two hidden-layer neurons and a single output-layer neuron, resulting in an 18-2-1 topology. Linear scaling is applied to all network inputs using the maximum and minimum values contained in each thematic layer. The network was trained using binary target values; that is, one and zero to indicate the presence or absence of known deposits, respectively. Although the neural network was trained as a binary classifier, output values of the trained network indicate the degree of similarity of each input vector to a composite of all the deposit vectors used in training. The output values also represent a measure of favourability for mineral deposits of the type included in the training data. These values are re-scaled to produce a multi-class prospectivity map. To validate and assess the effectiveness of the neural-network method, mineral prospectivity maps are also prepared using the empirical weights of evidence and conceptual fuzzy-logic methods. The neural network produces a geologically-plausible mineral-prospectivity map similar to the fuzzy logic and weights of evidence maps. The quality of prospectivity maps can be quantitatively compared using the Chi-square statistic, Spearman's and Kendall's rank correlation coefficients, the conditional probability of a deposit, given that the location corresponds to a particular prospectivity map class, and the ratio of the conditional probability to the prior probability of a known deposit. The results of this study indicate that the use of neural networks for the integration of large multisource data sets used in regional mineral exploration, and for prediction of mineral prospectivity, offers several advantages over existing methods. These include the ability of neural networks to; 1) respond to critical combinations of parameters rather than increase the estimated prospectivity in response to each individual favourable parameter, 2) combine data sets without the loss of information inherent in existing methods, and 3) produce results that are relatively unaffected by redundant data, spurious data and data containing multiple populations. Statistical measures of map quality indicate that the neural network method performs as well as, or better than, existing methods while using approximately a third less data than the weights of evidence method. Acknowledgements: WMB thanks the AusIMM for supporting this study in the form of a grant from the Bicentennial Gold '88 Endowment Fund.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TWO-STAGE GOLD MINERALIZATION AT THE ARCHEAN CHALICE GOLD DEPOSIT, YILGARN CRATON, WESTERN AUSTRALIA L. A. Bucci, S.G. Hagemann, D.L Groves and N.J. McNaughton Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, The University of Western Australia, Perth, W.A., Australia, 6079. The 0.6 Moz Chalice gold deposit, in the Norseman-Wiluna greenstone belt of the Yilgam Craton, W.A., is hosted in a sequence of intercalated mafic and ultramafic amphibolites, which have undergone five generations of deformation. Peak-metamorphic conditions of the amphibolites reached 500°±25°C and 3-4 kbars (hornblende-feldspar geothermobarometry) during the D1 event. The mafic/ultramafic sequence is intruded by four generations of monzogranite dyke, which are spatially associated with gold mineralization, retain their igneous texture, and can be temporally differentiated based on cross-cutting relationships. Stage 1 gold mineralization occurred during the D2 event, in a broad ductile shear-zone developed in mafic amphibolite, and is associated with quartz-albite-diopside-titanite-homblende-gamet-Au-pyrrhotite hydrothermal alteration (2643±8Ma, SHRIMP U-Pb on titanite), manifest as pervasive replacement of wallrock, as well as discrete shear veins. Stage 2 gold mineralizati! on is coeval with the D3 intrusion of a quartz-feldspar-biotite monzogranite. Dyke 2 (2625±9Ma, SHRIMP U-Pb on zircon), and is characterized by a homblende-Au, diopside-Au and molybdenite-Bismuthotelluride-scheelite-Au alteration assemblage (2621±9.6Ma, Re-Os on molybdenite). Stage 2 gold mineralization also occurs as "fi-ee gold" interstitial to quartz-feldspar grains in the Dyke 2 monzogranite. D4 and D5 are post-mineralization events. Stage 1 calcsilicate hydrothermal alteration and gold mineralization at Chalice is not unlike that of other Archean lodegold deposits in hypozonal settings (see Mueller and Groves, 1991). In contrast. Stage 2 gold mineralization appears to be related to the 2625±9Ma quartz-feldspar-biotite Dyke 2 monzogranite, with both diopsidehomblende-molybdenite-Bismuthotelluride-scheelite-Au "exoskam" in the amphibolite, and titanite-calciteAu "endoskam" in the Dyke 2 monzogranite, akin to gold-skam deposits in Phanerozoic terranes. However, there are several significant differences between Phanerozoic gold-skams and Stage 2 gold mineralization at Chalice. These include the: (1) mid-amphibolite facies metamorphic grade of the Chalice host rocks; most Phanerozoic gold skams are located in very low-grade metamorphic rocks; (2) temperature of host rocks; most granitic plutons related to skam mineralization intrude into "cold" host rocks (commonly unmetamorphosed calcareous sedimen! tary rocks); although the absolute timing of peak-metamorphism is somewhat ambiguous, Yeats et al. (1999) interpret a 2630Ma timing for peak-metamorphism in amphibolite facies domains in the Eastern Goldfields. Based on hornblende-feldspar geothermometry. Dyke 2 (2625±9Ma) intruded into mafic amphibolites which reached 500°±25°C; (3) depth of emplacement; hornblende-feldspar geobarometry at Chalice indicates crustal depths of 12-14km, whereas the characteristic depth of the majority of Phanerozoic skams is between 2 and 5km, and; (4) intrusions related to Phanerozoic skams commonly display a prominent homfels aureole within the adjacent country rocks, whereas no contact thermal effect can be distinguished between Dyke 2 and adjacent amphibolite. The aforementioned problems, with respect to a skam classification for Stage 2 gold mineralization, need to be addressed in this on-going research, in order to develop a genetic model for both stages of gold mineralization at Chalice. The depth of emplacement may be resolved via fluid inclusion analysis on primary magmatic quartz in Dyke 2 to constrain P-T conditions of crystallization. Detailed fluid inclusion analysis of silicate phases, in equilibrium with gold, will constrain P-T conditions and fluid composition (eg salinity, x C 0 2 ) of the hydrothermal system. Argon-Ar geochronology on peak-metamorphic homblende may constrain the absolute timing of peak-metamorphism. In summary, empirical observations of Stage 2 mineralization at Chalice, suggest a close genetic link to the 2625±9Ma Dyke 2 monzogranite. However, a paucity of data with respect to the timing of peak-metamorphism and the depth of emplacement of Dyke 2, preclude an unequivocal skam classification for Chalice, at the current level of understanding. References MUELLER A. G., & GROVES D. 1. 1991. The classification of Westem Australian greenstone-hosted gold deposits according to wallrock-alteration mineral assemblages. Ore Geology Reviews 6, 291-331. YEATS C. J., McNAUGHTON N. J., RtJETTGER D., BATEMAN R., GROVES D. I., HARRIS J. L. & KOHLER E. 1999. Evidence for diachronous Archean lode gold mineralization in the Yilgam Craton, Westem Australia: A SHRIMP U-PB study of intmsive rocks. Economic Geology 94, 1259-1276.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NUMERICAL MODELS OF THE EVOLUTION OF ACCRETIONARY WEDGES AND FOLD-AND-THRUST BELTS David Burbidge and Jean Braun Geodynamics Group, Research School of Earth Sciences, Australian National University

The results of a two-dimensional numerical model of the evolution of accretionary wedges and fold-and-thrust belts are presented for a range of rheologies using the distinct element method (DEM). Unlike many continuum numerical models, DEM allows localization to occur even after substantial amounts of deformation. In principle, the method is similar to using sand in analogue models. However, since the method is numerical, it is more flexible and the rheology can be made more realistic. The method is used to study the evolution of simple accretionary wedges with a rigid backstop and base. Experiments are done with a large range of co-efficients of inter-element friction (mu_e) and element-wall friction (mu_b). Two modes of deformation which depend mainly on mu_b are observed. For the weak base case (low mu_b) the dominant mode is frontal accretion by "pop-up" structures at or near the toe of the wedge. For the strong base case (high mu_b), uplift is concentrated near the back of the wedge, and is accompanied by underthrusting along a flat-ramp-flat (or "staircase") thrust fault structure. At intermediate values of mu_b, the wedge oscillates between the two modes of deformation. Both frontal and rear accretion modes are commonly observed in accretionary prisms (Piatt, 1986). Oscillation in mode have recently been observed in analogue models and may be one explanation for variations along the strike of an accretionary prism (different points may be out of phase (Gutshcer et al, 1998)). During periods of frontal accretion, normal faulting sometimes occurs in regions where the material has thickened considerably. The transition between the two modes of deformation is found to be a strong function of mu_b but a weak function of mu_e. A simple explanation of the experimental results is made using the principle of work minimization. Comparisons between the results and some accretionary wedges/fold-and-thrust belts are also made. References CRESPI, J. M., CHAN, Y.-C. & SWAIM, M. S., 1996. Synorogenic extension and exhumation of the Taiwan hinterland. Geology, 24, 247-250 GUTSCHER, M-A., KUKOWSKI, N., MALAVIELLE, J. & LALLEMAND, S., 1998a. Episodic imbricate thrusting and underthrusting: Analog experiments and mechanical analysis applied to Alaskan Accretionary Wedge, J. Geophys. Res., 103, 10161-10176. PLATT, J. P., 1986. Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks, Geo. Soc. Am. Bull., 97, 1037-1053.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"'Australian Geological Convention, Sydney, July 2000

PROTEROZOIC AUSTRALIA-LAURENTIA FITS: AUSWUS AN ALTERNATIVE TO SWEAT Clive Burrett and Ron Berry School of Earth Sciences, University of Tasmania, Box 252-79, Hobart, Tasmania, Australia 7001

Many authors have placed Laurentia adjacent to AustraHa in Proterozoic reconstructions. The most popular of these is the 1991 SWEAT reconstruction of Eldredge Moores and Ian Dalziel. Burrett and Berry (2000) and Karlstrom et al (1999) have independently come to the conclusion that other reconstructions are possible and that a reconstruction placing California against central eastern Australia is just as credible as SWEAT. This alternative hypothesis is termed AUSWUS (Australia-Western United States). The problem with SWEAT is that most of the margin-matching of Laurentia is with poorly known areas of Antarctica. The SWEAT hypothesis was based on a critical piercing point in Antarctica which subsequent work has shown to be incorrect. We have compared the geological congruence of the two continental margins by digitising the major Precambrian features of Laurentia and Australia and assessing the many different possible reconstructions. The reconstruction that provides the greatest number of Laurentia-Australia matches is very similar to that of Brookfield (1993). The Burrett and Berry AUSWUS reconstruction fits the Mojave Terrane of California and Nevada into an embayment in the Tasman Line between the Mt Isa blocks and Broken Hill. However, Karistrom et al (1999) prefer to place Mexican terranes between these two areas. The Precambrian history of Mojavia is surprisingly similar to that of the Broken Hill block and includes series of pre-orogenic gneisses and post-orogenic granitoids of statistically indistinguishable ages and overlapping crustal Nd model ages. Similar arguments can be used to exclude South China from a position between Laurentia and Australia. We prefer a placement of South China to the present day west of Australia which fits well with its preferred biogeographically-based placement in the Early Palaeozoic. References BROOKFIELD, M., 1993. Neoproterozoic Laurentia-Australia fit. Geology 21, 683-686. BURRETT, C. and BERRY, R., 2000 Proterozoic Australia-Western United States (AUSWUS) fit between Laurentia and Australia. Geology 28, 103-106. KARLSTROM, K., HARLAN, S., WILLIAMS, M., MCLELLAND, J GEISMAN J and AHALL, K., 1999. Refining Rodinia: geologic evidence for the Australia-western U.S. connection in the Proterozoic GSA Today 9 1-7.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

INTRACRATONIC, STRIKE-SLIP PARTITIONED TRANSPRESSION AND THE FORMATION AND EXHUMATION OF ECLOGITE FACIES ROCKS: AN EXAMPLE FROM THE MUSGRAVE BLOCK, AUSTRALIA. A. Camacho* and 1. McDougall Research School of Earth Sciences, The Australian National University, Canberra, A.C.T. 0200, Australia. * Present address: School of Geology, University of New South Wales, Sydney 2052, NSW, Australia. Introduction High-pressure (>10 kbar) metamorphic rocks generally are confined either to subduction zone settings or to orogenic belts associated with plate margin collisional situations. Rarely are high-pressure rocks recognised in orogenic belts that formed in intracratonic settings. Intracratonic deformation generally is believed to be localised along zones of previous structural weakness (e.g., Korsch et al., 1998). The mechanisms that may promote crustal-scale reactivation of old fault zones are less clear, but Sandiford & Hand (1998) proposed that, for central Australia, there is a link between sediment blanketing and fault reactivation. Orogenic activity and basin formation in central Australia is considered to have occurred in an intraplate setting, well away fi-om plate boundaries (e.g., Shaw et al., 1991). The Proterozoic Musgrave and Arunta Blocks are separated by a series of correctable intracratonic sedimentary basins of Neoproterozoic to early Phanerozoic age comprising the Officer, Amadeus, Ngalia and Georgina Basins (e.g., Shaw et al., 1991). These basins constitute remnants of the once continuous Centralian Superbasin (Walter et al., 1995). Tectonic evolution of the Musgrave Block at - 5 5 0 Ma The intracratonic Petermann Orogeny (-550 Ma) in the Musgrave Block appears to have features that are comparable with modem orogenic areas elsewhere (e.g., central Alps). Mesoproterozoic (-1180 Ma) granulite and amphibolite facies gneisses of the Musgrave Block were heterogeneously overprinted during the Petermann Orogeny, with zones of high-strain concentrated along broadly east-west trending shear zones. These high-strain zones formed under eclogite to greenschist facies conditions and have strike-slip and reverse movement directions. Eclogite facies (T -650°C and P - 1 2 kbar) deformation is confined to localised regions in granulite facies gneisses, whereas greenschist facies shear zones that formed at pressures of - 5 kbar and temperatures of ~400°C overprint all rock types. Outside the high-pressure shear zones minerals with low closure temperatures such as biotite (-350°C in the 40Ar-39Ar and Rb-Sr systems), preserve ages >700 Ma, suggesting that these rocks did not experience temperatures greater than about 350°C at - 5 5 0 Ma for any extended period. For these gneisses not to have been thermally equilibrated to temperatures of >350°C at a depth equivalent to a pressure o f - 1 2 kbar requires either an extraordinarily low geothermal gradient (<9°C km-1) or if the geothermal gradient was more typical (-20°C km-1), then transport to that depth and subsequent exhumation occurred in a short time interval (*40 Ma). Orogenic activity appears to have terminated by about 530 Ma, by which time - 2 3 km of exhumation had occurred over an interval o f - 1 5 Ma. The shear zones may form part of a strike-slip related, crustal-scale flower type structure. This kind of model can account for the burial and exhumation of the granulite facies gneisses during the Petermann Orogeny. References KORSCH R. J., GOLEBY B. R., LEVEN J. H. & DRUMMOND B. J. 1998. Crustal architecture of central Australia based on deep seismic reflection profiling. Tectonophysics, 288, 57-69. SANDIFORD M. & HAND M. 1998. Controls on the locus of intraplate deformation in central Australia. Earth & Planetary Science Letters, 162, 97-110. SHAW R. D., ETHERIDGE M. A. & LAMBECK K. 1991. Development of the late Proterozoic to midPaleozoic, intracratonic Amadeus Basin in central Australia: a key to understanding tectonic forces in plate interiors. Tectonics, 10,688-721. WALTER M. R., VEEVERS J. J., CALVER C. R. & GREY K. 1995. Neoproterozoic stratigraphy of the Centralian Superbasin, Australia. Precambrian Research, 73, 173-195.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A PLIO-PLEISTOCENE COMPOSITE THIRD ORDER DEPOSITIONAL SEQUENCE GENERATED BY RIDGE SUBDUCTION ALONG THE ECUADORIAN CONVERGENT MARGIN G. Cantalamessa\ C. Di Celma^, G. Bianucci G. Camevale^ W. Landing, L. Ragaini^ and G. Valleri^ 'Universita degli Studi di Camerino, Italy, ^Universita degli Studi di Pisa, Italy. ^Universita degli Studi di Firenze, Italy During Late Pliocene, the arrival of the aseismic Carnegie Ridge at the Ecuadorian subduction zone (Lonsdale, 1978) generated conditions for the triggering of heavy subsidence (Lallemand et al., 1992) followed, during Pleistocene, by alternating periods of uplift and subsidence of the same order of fi-equency as those connected with contemporaneous glacio-eustatic sea-level fluctuations (Flint et al., 1991). In the coastal area of central-southern Ecuador, interaction between tectonics and eustacy produced the necessary conditions for the depositing of a sedimentary succession traditionally subdivided into two distinct formations: the Canoa Formation in Late Pliocene, and the Tablazo Formation in Pleistocene. Taken together, this represents a third order composite depositional sequence (sensu Mitchum & Van Wagoner, 1991), made up of a great number of depositional sequences of differing duration, characterized by complex stratigraphic relationships and considerable angular unconformity (Cantalamessa et al., 2000). The Canoa Formation (Sheppard, 1930) is a complete third order depositional sequence, deposited on the shelf during a phase of marked subsidence, which has completely masked the effects of the contemporaneous glacio-eustatic sea-level oscillations. Analysis of the facies and stratigraphic-sequential interpretation of the sediments cropping out along a stretch of cliff on the southern coast of Cabo San Lorenzo, have revealed that its transgressive portion represents the early transgressive stage, and that on the whole its emplacement occurred distally with respect to the maximum ingression of the coastline (Cantalamessa et al., 1999). Thus, the Canoa Formation constitutes the transgressive depositional sequence inside the third order composite sequence. The Pleistocene uplifting, which took place in various moments, produced the progressive seaward shift of the tectonic hinge and shorelines of maximum ingression. In this way, the Pleistocene depositional sequences characterizing the Tablazo Formation, and which rest on the Canoa Formation along the same stratigraphic section, assumed a prograding stacking pattern; that is to say they point to ever later transgressive phases and, taken together, represent the highstand sequence set of the third order composite sequence. References CANTALAMESSA, G., DI CELMA, C., BIANUCCI, G., CARNEVALE, G., RAGAINI, L., LANDINI, W. & VALLERI G. (2000) Pleistocene sediments along the Central Ecuador convergent margin. II Latin American Sedimentological Congress-VII Argentinian Meeting of Sedimentology, Mar del Plata (Ar), 14-17 March. CANTALAMESSA, G., DI CELMA, C., CIMARELLI, M., BIANUCCI, G., LANDINI, W., RAGAINI, L. & VALLERI G. (1999) The Canoa Formation (Manabi Basin) western Ecuador: an example of Late Pliocene shelf asymmetrical sequence. Proc. 14th Argentine Geological Congress. 19-24 September, Salta (Ar.), I, 496-499. FLINT, S., TURNER, P. & JOLLEY, E.J. (1991) Depositional architecture of Quaternary fan-delta deposits of the Andean fore-arc: relative sea-level changes as a response to aseismic ridge subduction. Int. Ass. Sediment., Spec. Pubis. 12, 91-103. LALLEMAND, S.E., MALAVIEILLE, J. & CALASSOU, S. (1992) Effects of the oceanic ridge subduction on accretionary wedges: experimental modeling and marine observations. Tectonics, 11.6, 1301-1313. LONSDALE, P. (1978) Equatorian Subduction System. A.A.P.G. Bull., 62, 2454-2477. MITCHUM, R.M. JR. & VAN WAGONER, J.C. (1991) High-fi-equency sequences and their stacking patterns: sequence-stratigraphic evidence of high-fi-equency eustatic cycles. Sedimentary Geology, 70, 131-160. SHEPPARD, G. (1930) The geology of South Western Ecuador. A.A.P.G. Bull., 14.3, 263-309.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AN OCKHAM'S RAZOR STYLE REVIEW OF THE INAPPOPRIATE USE OF GEOCHEMICAL DIAGRAMS IN TECTONIC RECONSTRUCTIONS Graziella Caprarelli Department of Environmental Sciences, UTS, PO Box 123, Broadway, NSW 2007 Tectonic discrimination diagrams based on the abundances of selected major and trace elements in basalts have been used quite successfully in a number of geological and petrological studies. However, initial success has encouraged the proliferation and indiscriminate use of such diagrams to justify esoteric, and often unrealistic, tectonic reconstructions. Papers warning of the shortcomings of tectonic discrimination diagrams have been published, seemingly without effect. The mirage of obtaining a quick answer to the question of what kind of setting a certain igneous rock originated in, renders otherwise conscientious scientists less prudent than is required in the application of these diagrams. The large number of commercially available software packages that involve just plugging-in data that are then plotted in the trendiest diagrams, thus perpetuating a "black-box" behaviour to the use of discrimination diagrams, makes the problem even more difficult to address. At the risk of repeating what already has been pointed out in other publications, fKistration caused by the widespread problems derived by the "easy" use of tectonic discrimination diagrams prompted the submission of this abstract. Shortcomings inherent in some tectonic discrimination diagrams, based on the statistical analysis underlying the diagrams themselves, have already been outlined in the scientific literature. Additional problems related to rock freshness have also been dealt with extensively. Those arguments are therefore not reiterated here. Even when the rocks are absolutely fresh, the statistics are correct, the boundaries among various fields in the diagram are drawn at a 95% level of confidence, a large number of rocks have been analysed, fi-om a representative range of tectonic environments, and the analytical results are accurate and reliable, there remains no justification to use the diagrams uncritically, because, according to this author, the problems with the use of tectonic discrimination diagrams are inherently petrological. First of all, the assumption underlying the use of tectonic discrimination diagrams based on basalts is that the rocks reflect the composition of the mantle from which they originated, and that such mantle composition is unique to a particular tectonic setting. This double-assumption is wishful thinking at best, and absolutely flawed at worst, as any geochemist knows only too well. A second major petrological error is the use of major elements in some diagrams. Major elements being the major constituents of mineral phases, have abundances strongly dependent on: (a) the eutectic composition at the source of the melt; (b) whether the rocks are more or less primitive; (c) fractional crystallisation and abundance of phenocrysts. These problems also affect diagrams based upon mineral compositions, for example those based on clinopyroxene analyses. Calculations demonstrate that clinopyroxene compositions cannot be uncritically considered to be proxies of the magma bulk composition unless the order of crystallisation of mineral phases in the rocks is clear. Provisos accounting for these problems are usually mentioned in the "instructions" to the users of the diagrams, which recommend plotting aphyric rocks data. Discrimination diagrams based on trace element abundances theoretically overcome the problems outlined in the last paragraph. However, discrimination diagrams in which incompatible elements (representative of the processes leading to the formation of the melt from the mantle rock) are coupled with compatible elements (whose concentrations are influenced by the solid) are bound to cause major conftision if not properly interpreted. Elementary differential equations demonstrate that using compatible elements to model partial melting processes is futile. Consequently, it is unreasonable to use those very elements in tectonic discrimination diagrams. Calculations show that contrasting interpretations can be obtained using the same set of geochemical data, by slightly varying source compositions, melting processes, or crystallisation sequences. This being the case, the risk is that the uncritical use of tectonic discrimination diagrams leads to the formulation of ad hoc tectonic models. If the use of tectonic discrimination diagrams is unavoidable, then Ockham's razor must be rigorously appplied and the simplest tectonic model resulting fi-om their interpretation is the only one appropriate.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A MARIANA ARC ANALOGUE FOR LATE DEVONIAN METABASITES IN THE NUNDLE DISTRICT (NSW) Graziella Caprarelli and Evan C. Leitch Department of Environmental Sciences, UTS, PO Box 123, Broadway, NSW 2007 Metabasites in the Nundle district (NSW) are part of the Tamworth Group as defined by Cawood (1983), a major Devonian unit of the Tamworth Belt, in the southern New England Fold Belt. Leitch and Cawood (1987) and Morris (1988) argued that the rocks originated in a forearc setting, whereas Aitchison and Flood (1995) interpreted them as the products of arc rifting. We present major and trace element abundances for samples of metabasites cropping out around Bowling Alley Point, about 10 km north of Nundle. The rocks have AI2O3 abundances of 13.16 to 16.04 wt% and Ti02 concentrations of 0.15 to 2.40 wt%. The Mg numbers, calculated assuming a Fe203/Fe0 ratio of 0.2, range from 20 to 55, with a mode of 42.5, which indicates that the magmas are differentiated. This is supported by the concentrations of the mantle-compatible elements Ni and Cr which are, in all samples, below typical values for primary magmas. Plotting of data on a Zr/Ti02-Nb/Y diagram indicates that the samples have subalkaline affinity. The use of appropriate tectonic discrimination diagrams based on the relatively immobile incompatible elements Zr, Y and Nb indicates a normal MORB affinity for all rocks. Chondrite normalized spider diagrams of the more magnesian samples closely follow the MORB pattern, except for elements vulnerable to mobilisation. No depletion of high field strength elements is observed. A comparison of the Nundle metabasites with mafic rocks of MORB affinity found in the Mariana forearc (Johnson and Fryer, 1990) shows broad geochemical similarities. On a log(Ni)-log(Zr) diagram all available data plot along a linear trend whose slope is compatible with low pressure olivine fractionation. Thus, we have used olivine-control trends to constrain the compositional analogue of the primary magma. This procedure resulted in an estimated MgO content of 12 wt% and a Ni content of 336 ppm in the primary magma, which suggests an origin by partial melting of a dry mantle peridotite. Despite the fact that the limited data collected so far on the Nundle rocks leave the problem of the origin of the magmas and their tectonic significance largely undetermined, petrological constraints and their geochemical similarities with Mariana forearc rocks allow for interesting speculations. The origin of the Mariana forearc mafic rocks of MORB affinity is still debated, and some workers have interpreted them as portions of oceanic crust accreted during subduction. However, the geological setting of the Nundle rocks indicates that these magmas were intruded in a near-arc setting. By extrapolation, a local origin for the MORB-like rocks of the Mariana forearc is thus plausible. Forearc mantle is a suitable reservoir for the production of partial melts of chemical compositions like those inferred for the Nundle primary magmas, because the depth of the subducting slab under the forearc region is insufficient to reach the pressure at which break-down of amphibole commences, leaving the asthenosphere dry. References AITCHISON J.C. & FLOOD P.O. 1995. Gamilaroi Terrane: A Devonian rifted intra-oceanic island-arc assemblage, NSW, Australia. In: Smellie J.L. ed. Volcanism Associated with Extension at Consuming Plate Margins, pp. 155-168. Geological Society Special Publication 81. CAWOOD P.A. 1983. Modal composition and detrital clinopyroxene geochemistry of lithic sandstones from the New England Fold belt (east Australia): A Palaeozoic forearc terrane. Geological Society of America Bulletin 94, 1199-1214. JOHNSON L.E. & FRYER P. 1990. The first evidence for MORB-like lavas from the outer Mariana forearc: geochemistry, petrography and tectonic implications. Earth and Planetary Science Letters 100, 304-316. LEITCH E.C. & C A W O O D P.A. 1987. Provenance determination of volcaniclastic rocks: the nature and tectonic significance of a Cambrian conglomerate from the New England Fold Belt, eastern Australia. Journal of Sedimentary Petrology 57, 630-638. MORRIS P.A. 1988. Petrogenesis of fore-arc metabasites from the Paleozoic of New England, eastern Australia. Mineralogy and Petrology 38, 1-16.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1S""Australian Geological Convention, Sydney, July 2000

DEFORMATION OF SLATE AND SCHIST BY DIFFUSION METASOMATISM DURING PROGRADE METAMORPHISM Dugald M. Carmichael Q u e e n ' s University, Kingston ON K 7 L 3 N 6 , Canada

Field evidence shows clearly that greenschist-facies metapelites and calcshists are significantly less competent than associated quartzite and marble. During prograde metamorphism in greenschist facies, it is postulated that metapelites deform primarily by diffusion metasomatism at the microstructural scale. At a muscovite/albite interface, for example, there is a continuum of possible diffusion-metasomatic reactions, the extremes being dissolution or growth of either grain without involvement of the other. However, due to the relatively low diffusivity of the component AI2O3, a reaction approximating to constant-aluminum replacement, 3 NaAlSijOg + K^ + 2 H^ + 12 H2O = KAl2AlSi30,o(OH)2 + 3 Na^ + 6 Si(0H)4 3 NaAlSiaOg + KCl + 2 HCl + 12 H2O = KAl2AlSi30,o(OH)2 + 3 NaCl + 6 Si(0H)4

(1), or (2)

is postulated to be kinetically favoured. This type of reaction is possible only in the presence of at least a small excess of intergranular H2O, which greatly facilitates the necessary grain-boundary diffusion. Because the volume-change of reaction is -53%, it is predicted that muscovite should replace albite at interfaces normal to ai, permitting local uniaxial shortening, and albite should replace muscovite at interfaces normal to 03, permitting local uniaxial extension. A planar albite/albite interface in the same rock would be stable if normal to a3, but if normal to G\ it is a likely site for nucleation and growth of muscovite. A muscovite/muscovite interface would be stable if normal to ai, but if normal to a3 it is a likely site for nucleation and growth of albite. To the extent that these and other local metasomatic reactions function as source and sink for one another, the rock can deform without change in its bulk composition, and with minimal need for grain-boundary diffusion of aluminum. To the extent that monomineralic domains may form (e.g, in the m-domains of crenulation cleavage), the flow strength of the rock will tend to increase. In amphibolite facies, although higher T will facilitate diffusion, the creep strength of metapelites may actually increase due to larger grain size, growth of porphyroblasts, weaker cationic partitioning among the coexisting minerals, and higher concentration of titanium in biotite. The relatively high creep strength of metabasites (inferred from boudinage etc) is postulated to be due to incorporation of most of the easily diffuseable cations into hornblende and plagioclase, two minerals with almost the same volumetric concentration of aluminum. At the onset of cooling in both facies, the intergranular aqueous vapour is consumed by back-hydration, and a very important increase in the creep strength of most rock types is postulated.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RELATIONSHIP BETWEEN MIDDLE JURASSIC AND CRETACEOUS MIDDLE EOCENE DEFORMATION AND PLUTONISM IN MID-CRUSTAL ROCKS OF THE OMINECA BELT, SOUTHERN CANADIAN CORDILLERA Sharon D. Carr^ and Philip S. Simony^ 'Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada, KIS 5B6 ^Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada, T2N 1N4

The southern Omineca Belt is characterized by stacked and folded thrust sheets in the east, gneiss complexes of the Shuswap core complex in the west, and Paleozoic, Middle Jurassic, middle Cretaceous, and early Tertiary plutonic suites. The tectonic events that formed most of the Omineca Belt were initiated in the Early Jurassic. These included episodes of obduction of offshore terranes, thrusting, folding, crustal thickening and metamorphism in response to westward underthrusting of the North American plate. Contractional deformation continued in the Cretaceous and Paleogene in the Omineca and Foreland belts, concomitant with accretion of terranes, thrusting and strike slip faulting in the western Cordillera. In the Shuswap core complex of the western Omineca Belt, denudation of basement rocks and of deep-seated ductile thrust faults and packages of middle crustal rocks, that have relatively young deformation history, occurred during regional extension in the Early Tertiary. The 400 km x -150 km Shuswap complex, one of the largest of North America's Cordilleran metamorphic core complexes, is bounded by regional-scale ductile, brittleductile and brittle extensional fault systems that were active in the Early - Middle Eocene, and overprinted compressional structures. The exhumed lower plate of the core complex is a window into mid-crustal migmatitic amphibolite-facies rocks containing structures that were mainly formed during mid-Cretaceous - Paleocene crustal shortening and thickening of the orogen. The predominant structure at this crustal level is a diachronous transposition foliation. It is overprinted in places by polyphase folding and zones of intense strain, all interpreted to have formed in a compressional regime. Penetrative ductile fabrics related to Eocene extension are also prevalent in parts of the lower plate. At higher structural levels, in the upper plate of the Shuswap complex, are thrust bounded panels of polydeformed greenschist- to amphibolite-facies rocks that were predominantly deformed and metamorphosed in the Middle Jurrasic and Early Cretaceous, and have been carried in the hanging walls of younger thrust faults and dissected by later Eocene normal fault systems. At the north and south ends of the Shuswap core complex, where displacement on extensional faults is < 5 km, relationships are preserved that are not seen elsewhere: 1) ductile extensional faults are discrete and their relationship to pre-extensional geology and granitoids can be mapped, 2) a downward transition from Middle Jurassic structures into Cretaceous transposition foliation is exposed without a significant tectonic break, and 3) it is possible (with structural, petrological and geochronological studies) to work out the spatial distribution and timing of shear zones and transposition foliation using ca. 165 Ma, ca. 100 Ma, 75 Ma, 63 Ma, 59-54 Ma and 52.5 Ma plutonic suites, and suites of leucocratic pegmatites and aplites that range from 140 - 54 Ma. This enables us to look at processes that facilitate localization and propagation of shear zones. The presence and orientation of pre-existing structures has served to guide younger structures and influence disposition of magma. Foliations that range in age over 120 m.y. may all be statistically parallel. Where emplacement of magmas or channelized fluid flow is localized at particular structural levels, deformation may be strongly thermally controlled and confined to particular levels.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

THE WESTERN LACHLAN FOLD BELT, SOUTHEAST AUSTRALIA-A DOUBLY VERGENT FOLD BELT FORMED BY CONTINENTAL COLLISION WITH AN INTERVENING WEAK ZONE Ross Cayley, David Taylor, Pons VandenBerg and David Moore Geological Survey of Victoria Geological Survey of Victoria, P.O. Box 500 East Melbourne, Victoria 3002 Geological Survey of Victoria mapping and geophysical data interpretation has improved the understanding of the western Lachlan (Stawell, Bendigo and Melbourne zones). The Lachlan Fold Belt has traditionally been considered an atypical fold belt, with no documented craton-directed thrust belts, no substantial metamorphic hinterlands, no preserved platform successions and no fragments of older basement. The extent of the monotonous Palaeozoic turbidite succession appeared unusual and its plate tectonic setting unresolved. Intraplate tectonic settings, typical of the palaeo-Pacific rim, have been proposed, but missing and anomalous elements were problems. We now recognise many of these "missing" elements. We propose an intraplate setting for the western Lachlan (Cayley et al., in prep), with deformation contemporaneous with-and linked to-deformation of the eastern Lachlan Fold Belt. East-west shortening of the Stawell and Bendigo zones is Late Ordovician-Silurian. At the Stawell Zone western margin, the east-dipping Moyston Fault separates the Lachlan and Delamerian fold belts. It is a major craton-directed thrust, solving many of the problems of earlier interpretations. The Moomambool Metamorphic Complex (MMC) in its immediate hangingwall represents deeper levels of the Lachlan orogen and is a metamorphic hinterland to this major western Lachlan fault. Farther west, the Grampians allochthon is a foreland fold-and-thrust belt to the Lachlan orogen, a deformed platform sequence lying para-autochthonously upon Delamerian crust. East of the MMC, the Stawell and Bendigo zones are essentially a single stratigraphic and structural entity, consisting of chevronfolded Cambro-Ordovician turbidites cut by spaced, predominantly east-verging thrusts. A major stratigraphic and structural break occurs along the Heathcote Fault Zone, a group of west-dipping thrust faults separating the Bendigo and Melbourne zones. The Melbourne Zone is anomalous with respect to the flanking structural zones, but similar to NE Tasmania. An older crustal block beneath the zoney4the Selwyn Blocky4is exposed along the Victorian south coast. Magnetic data show it to be a northern continuation of Proterozoic/Cambrian Tasmanian crust. Cambrian-Ordovician cover rocks at Waratah Bay correlate with platform sequences in western Tasmania and constrain the underlying deformation to the Cambrian Delamerian/Tyennan Orogeny. The Selwyn Block appears to underlie the entire Melbourne Zone, exposed in several windows that may have been a structural high since Ordovician times. During Silurian overthrusting of the Bendigo Zone along the Heathcote Fault Zone, the Melbourne Zone acted as a foreland-type basin on the downloaded cratonic Selwyn Block, with no interruption to submarine sedimentation. Like NE Tasmania, its cover sequence was deformed in the Middle Devonian Tabberabberan Orogeny during the final amalgamation of the Lachlan Fold Belt. At this time the Melbourne Zone sequence detached from the underlying Selwyn Block and was thrust eastwards against a basement high. The detachment is exposed as the Thomas Fault in the Mount Useful Fault Zone. This deformation was driven by renewed overthrusting of the Bendigo Zone over the Selwyn Block along the reactivated Heathcote Fault Zone (the Mount William Fault). The Heathcote Fault Zone is therefore, like the Moyston Fault, a major craton-verging fault. This realisation highlights an essential symmetry to the Stawell and Bendigo zones, enclosed between the Delamerian Fold Belt and the Selwyn Block. In this new perspective on the western Lachlan, we view it as a passive marginal basin adjacent to the Delamerian continent margin, incorporating an old continental fragment (Tasmania and the Selwyn Block) throughout the Cambro-Ordovician. The marginal basin was closed during strong outboard convergence in the Silurian to form a doubly-vergent craton-directed fold-and-thrust belt, with underthrusting from both the west (Delamerian Fold Belt underthrusting along the Moyston Fault) and the east (Selwyn Block underthrusting along the Heathcote Fault Zone), and forming a foreland basin sediment wedge upon the Selwyn Block-the Melbourne Zone. In effect this is a "vice" scenario of convergence between two competent blocks with an intervening weak zone. In this model some deformation may occur early at both margins, but the bulk of the deformation will initiate where the intervening zone is weakest (which for the western Lachlan is obviously in the vicinity of the Moyston Fault) and propagate from that point. Reference CAYLEY, R.A. & TAYLOR, D.H., VANDENBERG, A.H.M. & MOORE, D.H., in review. Proterozoic rocks and the Tyennan Orogeny in central Victoria, and tectonic implications. Submitted AJES 2/99.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MELTING OF TI-RICH BIOTITE IN NATURAL, QTZ-PRESENT AND QTZFREE ASSEMBLAGES: MICROSTRUCTURES, PHASE CHEMISTRY AND MELTING REACTIONS B. Cesare Dipartimento di Mineralogia e Petrologia, Universita di Padova - Italy The restitic xenoliths in the dacite of El Joyazo (Cerro de Hoyazo, SE Spain) exhibit reaction textures related to the incongruent melting of biotite in both Qtz-present and Qtz-free assemblages. The former textures are found in Qtz-Crd xenoliths, and display orthopyroxene among the products; the latter occur in Grt-Bt-Sil restites, and contain hercynitic spinel. In the Qtz-Crd xenoliths, biotite is resorbed and displays reaction rims composed of orthopyroxene, ilmenite and rhyolitic glass; the same glass also fills embayments in biotite. Reaction rims are developed at Bt-Qtz contacts, whereas they are absent at Bt-Crd interfaces. Biotite is Ti-rich (0.56 atoms) with XMg = 0.34; orthopyroxene has restricted XMg (0.62±0.04), but scattered A1 (0.08 to 0.62 atoms) indicative of diffusion-controlled crystallization and disequilibrium. Cordierite has XMg = 0.51; the glass is silica-rich (up to 77 wt.%) and felsic (FeO+MgO < 2.5 wt.%). Mass balance calculations in the simplified KFMASH-Ti system indicate that the reaction inferred from textural analysis: Bt + Qtz = melt + Opx + Ilm cannot be balanced with the measured phase compositions, unless Crd and/or Sil are included as reactants, suggesting that the preferred location of reaction rims at Qtz-Bt intefaces is related to kinetic controls on reaction mechanisms. Further complexity to the natural melting reaction is then required by the additional components Na, Ca and Mn. Qualitative thermobarometry based on orthopyroxene composition indicates equilibration conditions of 4-5 kbar, > 900

In the Qtz-free Grt-Bt-Sil xenoliths, melting microstructures are very similar to those described above, and consist of reaction rims between biotites and aggregates („mix") of fibrolite and melt. In these textures, resorbed biotite crystals of about 1 mm length are rimmed by a layer of glass <200 mm in thickness, containing euhedral crystals of spinel <100 mm in size and smaller ilmenite needles. Plagioclase and garnet are located >5 mm away from the reaction texture. Biotite is zoned. Cores (Btl) have XMg = 0.35±0.02 and Ti = 0.58±0.01 atoms; whereas the rims (Bt2) have XMg = 0.45±0.01 and Ti up to 0.68 atoms. The hercynite-rich spinel has low ZnO content (<0.80 wt%), and XMg = 0.26±0.04. The chemical compositions of the mix aggregate represent linear combinations between sillimanite and a silica-rich melt. Garnet (Grt) has low Ca and Mn, and XMg = 0.14. Plagioclase shows large homogeneous cores (Pll, An31±2) and more calcic rims (P12, An49±6). Matrix analysis in the 9-component (Al-Ca-Fe-K-Mg-Mn-Na-Si-Ti), 9-phase (Btl-Bt2-Grt-Spl-Ilm-melt-mix-Pll-P12) system provides the mass balance: 0.78Btl+1.95Pll+2.82mix+0.32Grt = 0.25Ilm + 5.00melt+0.30Bt2+2.53Spl+1.03P12. This relationship is in agreement with the observed textures and can be considered a good model for the incongruent melting of biotite in the Qtz-free xenoliths. As it requires garnet and plagioclase as reactants, the reaction volume must be larger than the melt production site. The melting of biotite is constrained at T = 900950 °C and P * 5 kbar.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GROWTH OF MYRMEKITE CORONAS BY CONTACT METAMORPHISM OF GRANITIC ULTRAMYLONITES IN THE AUREOLE OF CIMA DI VILA (EASTERN ALPS - ITALY) B. Cesare', C. Marchesi' and J.A.D. Connolly^ ' Dipartimento di Mineralogia e Petrologia, Universita di Padova - Italy ^ IMP-ETH Zurich - Switzerland

In the contact aureole of the Oligocene granodiorite of Cima di Vila, granitic pegmatites of Variscan age are strongly deformed, and show local development of ultramylonitic fabric. The mylonitic deformation (of Eo-alpine age?) predates emplacement of the tonalite, as indicated by widespread static recrystallization and by andalusite porphyroblasts overgrowing the mylonitic foliation in interbedded pelitic layers. Porphyroclasts in the ultramylonites include K-feldspar, plagioclase, mica-fish of biotite and muscovite, and rare garnet. The fine-grained matrix consists of quartz, plagioclase, biotite and muscovite. Despite the marked compositional layering, characterized by alternating Qtz- and mica-rich ribbons, textures of matrix minerals such as initiation of polygonal grain aggregates of quartz and decussate arrangements of micas, indicate that dynamic recrystallization related to mylonite development was followed by extensive static recrystallisation and grain growth by grain boundary area reduction. K-feldspar porphyroclasts up to 1cm diametre are mantled by myrmekite, that forms a continuous corona with fairly constant thickness of =1 mm. In both ZY and XY sections, myrmekite bulbs are symmetrically distributed in the corona, and plagioclase hosts have random crystallographic orientation. Myrmekite development can be modelled from the P-T-t evolution of the pegmatites, considering that contact metamorphism occurred at c 550 2.75 kbar, and assuming that ultramylonite developed during Eoalpine metamorphism at c 450 °C, 7.5 kbar. Phase diagram pseudosections calculated from the measured bulk composition of granitic pegmatites indicate that the equilibrium assemblage should change from Qtz-PheAb±Zo±Cpx±Kfs during the ultramylonite stage to Qtz-Pl(An25-30)-Ms-Kfs-Bt(Ann55) during the stage of myrmekite growth. The thermodynamic prediction of increasing plagioclase and anorthite content, change of white mica composition, and growth of biotite are in agreement with the observed textures and analysed phase compositions. This supports the model, based on microstructural evidence, that myrmekite formation took place under static conditions, coeval with the widespread static recrystallization of the rock, which occurred during the contact metamorphic event, the last metamorphism recorded in the area.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CYCLIC CORRELATION AND SIGNIFICANT SURFACES IN LOW ACCOMMODATION COAL Gareth Chalmers School of Geoscience, University of Newcastle, NSW, 2308

Problems arise when trying to establish the sequence of depositional events that occur in low accommodation, non-marine depositional environments. This is due to non-marine environments having additional factors affecting base level changes. They are also more susceptible to sub-aerial exposure and erosion. Coal-bearing strata has become a focus in recent years due to the recognition of the sensitivity that peat-forming environments exhibit due to subtle changes in base level. These changes are reflected in the variation in coal petrographic composition. Significant surfaces can be identified within coal seams, which are equivalent to sequence stratigraphic surfaces of down-dip, co-eval marine strata. The coal-bearing strata of the Lower Cretaceous Mannville Group of the Lloydminster heavy oilfield in eastern Alberta is situated on the cratonic margin of the western Canadian sedimentary basin (WCSB). Accommodation is relatively low and sequences have become thinner and more condensed in comparison to the rest of the basin. The study area is constrained by the Wainright and Kindersley palaeohighs to the northwest and southeast, respectively, affecting Lower Mannville sedimentation and creating a small subbasin. Another structural control on the sedimentation is the syndepositional faulting of the basement, with concomitant incised valley development and coal splitting observed in these zones. Previous workers have traced linear channels of incised valleys in the central WCSB and have concluded that syndepositional faulting has influenced drainage patterns. The irregular palaeotopography, resulting from the major unconformity lying beneath the Mannville Group also has had an influence on total Mannville strata thickness. One of the objectives of this study is to identify wetting-up and drying-up cycles in coal seams and their associated interseam sediments, in an attempt to correlate the coal across the study area. Three diamond drill cores 13-3-40-1W4M, 7-14-39-3W4M and 15-19-41-3W4M were sampled and petrographically analysed in order to define internal trends. Defining of such cycles is ascertained by identifying changes to the mire ecosystem - e.g. a progression from a flooded carbonaceous shale horizon to a limnotelmatic, rheotrophic mire to an ombrotrophic, raised mire shows a drying-up cycle. The determination of cycles is also dependant on the style of initiation of peat deposition. Peat accumulation can begin by two different styles: (1) when base level rises dry land is replaced with a mire; (2) when accommodation and water depth are decreasing due to continuous sedimentation, peat will develop by the process of terrestrialisation. The boundaries between the floor and the coal seam are termed a paludification and terrestrialisation surface, respectively. As the separation of the three coal core sites is less than 50 km, it is expected that the coal seam over this distance will experience similar conditions, and respond in a similar and contemporaneous manner. This is the basis of the correlation for the coal seams at this stratigraphic level. Results show that it is not always a simple task to correlate a coal seam across a basin, particular one that has developed in a low accommodation setting. There are at least three sequence boundaries identified in only 20 metres of strata, based on tracing incised valley erosional surfaces identified in geophysical wireline logs. These sequence boundaries can be traced to the top and an internal surface of the amalgamated coal seam in the east. A palaeosol at the base of the Cummings coal identifies a third sequence boundary. The deposition history is shown to be quite complex in this sub-basin during the time of the coal seam development. Initially the 13-3-40-1W4M site was excluded from sedimentation and developed a palaeosol. Peat deposition occurred in the west with thick drying-up cycles found in 15-19-41-3W4M. Following this, a thick wetting-up cycle developed on the eastern flank (13-3-40-1W4M) of the basin which may be explained by a higher magnitude of base level flooding, or by syndepositional faulting increasing local accommodation. Research in the nearby Medicine River area has shown that there was a major re-organisation of the basin during the deposition of the stratigraphically equivalent Glauconite Formation. This re-organisation was identified by a major change in sediment composition, indicating a change in provenance and a change in palaeoslope from the northeast to the northwest. This event may be represented in the study area by the change in local accommodation, showing thicker cycles in the west initially and then switching to the east during the deposition of the Lloydminster coal seam. The 15-19-41-3W4M core site only developed the Cummings coal seam, with sedimentation being restricted during the development of the Lloydminster coal seam to the southeastern area of the sub-basin. It appears that a major marine transgression occurs above the Lloydminster coal seam, resulting in the deposition of a shoreface across the whole sub-basin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

3D PALAEO-MIGRATION PATHWAY ANALYSIS FOR THE SKUA AND SWIFT STRUCTURES, VULCAN SUB-BASIN, TIMOR SEA Gang Chen, Kevin Hill and Nick Hoffman Australian Geodynamic CRC, Earth Sciences, La Trobe University, Melbourne, 3083

The Swan Graben and Cartier Trough are considered to be the main source kitchens in the Vulcan Sub-Basin. The focus of this work is to assess the migration pathway geometries at different times leading out of these source kitchens to the Plover Formation reservoir in the Skua area. A detailed 3D-depth model was constructed from the Skua 3D seismic datasets. The Plover Formation was then restored and decompacted to the Top Miocene and to the Top Eocene, which were the main times of hydrocarbon migration. This 3D restoration allows determination of the kinematic evolution of the structures so that the 3D structure of intermediate stages of development can be seen. Unlike other methods involving simple flattening, this analysis involves lateral movements to compensate for fault heave, as well as decompaction of the sediments and adjustment for palaeo-bathymetry, giving the true depth and geometry of the basin at any time. Using these true depths and assuming regional heat flow of ~40°/km, the correct position of the actively generating source kitchen was determined at Miocene and Eocene times. Applying the updip migration analysis tool in 3D Move allows determination of the migration paths from the source kitchens. By comparing these palaeo-migration vectors, it was noted that the migration surface itself was also changed by unfaulting faults and unfolding beds to the palaeo-bathymetry, as well as general changes in dip, due to the evolution of structures through time. Combined with the positions of prospects from other data, traps that are favorably positioned to receive oil charge were determined. More detailed analysis in the vicinity of existing discoveries confirms that the Skua trap was favourably located in term of oil migration and that the Swift trap is a higher risk prospect based on migration path vectors. Acknowledgment: BHP Petroleum provided digital 3D seismic data. Interpretation software is sourced from Paradigm Geophysical and restoration tools from Midland Valley Software. This research was conducted as part of the Australian Geodynamic CRC.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ARCHAEAN TECTONISM THROUGH TIME IN WESTERN AUSTRALIA She Fa Chen\ Songfa Liu^ and Arthur Hickman^ 'Geological Survey of Western Australia, Kalgoorlie Regional Office, P.O. Box 1664, Kalgoorlie, WA 6430 ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^ Geological Survey of Western Australia, 100 Plain Street, east Perth, WA 6004 Significant geological and tectonic differences between the eastern Pilbara and eastern Yilgam Cratons of Western Australia have been highlighted by recent geological mapping combined with structural, geochronological and geophysical studies. The early-mid Archaean eastern Pilbara Craton (3.66-3.20 Ga) is characterised by domal granitoid complexes, between 50 and 120 km across, arcuate greenstone synclines with no preferred structural trend, and lack of regional-scale shear zones. Well-preserved greenstone successions up to 20 km thick are dominated by basalt, but also include calc-alkaline volcanics, units of banded iron-formation (BIF), and thick clastic sedimentary rocks. Thick ultramafic extrusive sequences are rare. These successions contain several local unconformities, and some regional unconformities. Multi-stage emplacements of TTG-type and potassic granites were accompanied by coeval felsic volcanism. The granites commonly show intrusive relations to the greenstones. Where granite-greenstone contacts are sheared they usually dip steeply towards greenstones and have a normal movement sense. In greenstone synclines, bedding typically dips away from the granite domes, and most asymmetric folds verge away from them. The diapiric rise of large, elliptical granitoid batholiths, and contemporaneous sinking of intervening synformal greenstones, occurred episodically over a period of at least 400 Ma, and produced most of the structural features seen today. Tectonism was mainly vertical, although horizontal compressional structures, especially after 3.0 Ga, have also been recognised. In contrast, the late Archaean eastern Yilgam Craton (2.72-2.62 Ga) is characterised by a prominent NNW tectonic grain that is defined by elongate granite bodies, narrow greenstone belts, regional-scale shear zones, large- and small-scale upright folds, and axial planar foliations. The distribution of greenstone successions varies from a western succession characterised by widespread komatiite, abundant pillowed basalt, and a paucity of calc-alkaline volcanics and BIF, to an eastern succession characterised by more widespread calcalkaline volcanics, significant BIF, and less komatiite and pillowed basalt. Greenstone successions are typically less than 7 km thick and few unconformities have been recognised. TTG-type granites appear to be largely absent, and granitoids are dominated by voluminous, elongate monzogranite complexes that extend up to 300 km. Although there is evidence for granitoid diapirism or doming, most of the dominant structures formed under horizontal compression. Early N-S compression produced E-trending large-scale thrust and duplex structures, with associated layer-parallel foliation and tight to isoclinal folds. ENE-WSW regional shortening produced the dominant NNW-trending structures. It was accompanied by peak metamorphism of greenschist to amphibolite facies and intrusion of abundant monzogranite at c. 2.67-2.65 Ga. Late transpression resulted in sinistral strike-slip on the NNW-trending regional-scale shear zones and contemporaneous contraction in the intervening greenstone belts. Current tectonic models for Archaean granite-greenstone terrains differ with regard to the importance of vertical versus horizontal tectonism. Evidence from Western Australia suggests that these different interpretations may be explained by significant temporal changes in tectonism, from the early to late Archaean. The major tectonic style of the early-mid Archaean eastern Pilbara Craton can be explained by vertical tectonism, whereas the dominant structural style in the late Archaean eastern Yilgam Craton is consistent with horizontal compression. Two possible factors that may have contributed to this temporal change in tectonism were: (1) the amount of heat generated by radioactive decay and the geothermal gradient decreased from the early to late Archaean (Pollack, 1997), and (2) global-scale catastrophic convective overtum of the Earth's mantle played a dominant role in cmst-formation during the early Archaean, whereas plate tectonic processes became increasingly dominant during the late Archaean (Nelson, 1998). REFERENCES POLLACK H. N., 1997. Thermal characteristics of the Archaean. In: de Wit, M.J. and Ashwal, L.D. eds. Greenstone Belts. Clarendon Press, Oxford, 223-232. NELSON D. R., 1998. Granite-greenstone crust formation on the Archaean Earth: a consequence of two superimposed processes. Earth and Planetary Science Letters 158, 109-119.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

SEA-LEVEL AND ENVIRONMENTAL CHANGES SINCE THE LAST INTERGLACIAL IN THE GULF OF CARPENTARIA Allan R. Chivas^ Adriana Garcia^ Sander van der Kaars^ Martine J.J. Couapel', Sabine Holt\ Jessica M. Reeves', David J. Wheeler^ Adam D. Switzer\ Colin V. Murray-Wallace\ Debabrata Banerjee\ David M. Price\ Sue X. Wang', Grant Pearson', N. Terry Edgar^ Luc Beaufort\ Patrick De Deckker^ Ewan Lawson^, and C. Blaine Cecif 'School of Geosciences, University of Wollongong, NSW 2522, Australia ^School of Geography and Environmental Science, Monash University, Clayton VIC 3168, Australia ^U.S. Geological Survey, 600 Street South, St. Petersburg, FL 33701, USA '^Centre Europeen de Recherche et d'Enseignement des Geosciences de I'Environnement (CNRS-CEREGE), BP 80 Aix-en-Provence 13540 Cedex 4, France ^Department of Geology, The Australian National University, Canberra ACT 0200, Australia ^Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights Research Laboratories, PMB 1, Menai NSW 2234, Australia ^U.S. Geological Survey, MS-956, Reston, VA 20192 , USA

The Gulf of Carpentaria is an epicontinental sea (maximum depth 70m) between Australia and New Guinea, bordered to the east by Torres Strait (currently 12m deep) and to the west by the Arafura Sill (53m below present sea level). Throughout the Quaternary, during times of low sea-level, the Gulf has been separated from the open waters of the Indian and Pacific Oceans, forming Lake Carpentaria, an isolation basin, perched above contemporaneous sea-level with outlet channels to the Arafura Sea. More than 7000km of high-resolution seismic survey have been completed. A United States Geological Survey seismic survey of 1993/94 delineated up to 17 reflectors, of which about ten occur in the uppermost 150m of sediment and are marked by abundant incised channels, and erosional unconformities. This sequence is interpreted to indicate at least 14, possibly 17, major transgressive/regressive events, within the 3m resolution of the seismic system employed. A preliminary interpretation is presented of the palaeoenvironments represented in six sediment cores collected by the IMAGES (International MArine Global changE Study) program in the Gulf of Carpentaria. The longest core (approx. 15m) spans the past 130,000 years and includes a record of sea-level/lake-level changes, with particular complexity in the 80 to 40ka interval when sea-level repeatedly breached and withdrew from Gulf/Lake Carpentaria. Evidence from biotic remains (foraminifers, ostracods, pollen), sedimentology and geochemistry (carbon contents, C/N ratios and ^^C values of organic matter) clearly identifies a marine transgression at about 9.7ka (radiocarbon years). Prior to this transgression. Lake Carpentaria was surrounded by grassland, was near full, and may have had a surface area approaching 600km x 300km and a depth of about 15m. The earlier rise in sea-level at about 13 Oka is constrained by sedimentological and biotic evidence and dated by optical- and thermoluminescence and amino acid racemisation methods.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

POLYCYCLIC OROGENY IN CENTRAL OGCHEON MET AMORPHIC BELT, KOREA: EVIDENCE FROM 40Ar/39Ar HORNBLENDE AGES Moonsup Cho Hyeoncheol Kim \ Ching-Hua Lo ^ Kyoungwon Min ^ and Jung Ho Ahn ^ ' School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea ^ Institute of Geology, National Taiwan University, Taipei, Taiwan, R.O.C. ^ Department, of Geology and Geophysics, Univ. of California at Berkeley, CA 94720, USA ^ Department, of Earth and Environmental Sci., Chungbuk National Univ., Cheongju 361-763, Korea

The Ogcheon fold-thrust belt is one of the key Phanerozoic belts for delineating the regional correlation in East Asia including mainland China and Japan. This NE-trending belt consists of the Ogcheon metamorphic belt (0MB) in the southwest and the Taebaeksan sedimentary zone in the northeast. The 0MB comprises Late (?) Proterozoic to Paleozoic metasedimentary and metavolcanic sequences intruded by Mesozoic granitoids. The age of initial intracontinental rifting is constrained to be earlier than 756 Ma by the U-Pb zircon age of a felsic metavolcanic rock (Lee et al., 1998, Precamb. Res., 89, 47-57). Polyphase tectonism in the 0MB is characterised by the Middle Paleozoic Ogcheon orogeny responsible for the ductile stacking of thrust nappes and the Triassic Songrim orogeny occurring at shallower depths under the brittle-ductile transitional regime (Cluzel et al., 1990, Tectonophysics, 183, 41-56). Peak metamorphic conditions of the former are in the range of 5-8 kbar and 520-590''C, whereas those of the latter are 1-3 kbar and 350-500^C (Min and Cho, 1998, Lithos, 43, 31-51). Major ambiguity in the polycyclic model of the 0MB resuhs from the lack of isotopic age data. In order to determine the age of peak metamorphism, we analysed hornblende crystals of quartzose amphibole schists in central 0MB, using the ^W^^Ar dating technique. Mineral assemblages of amphibole schists are represented by calcic amphiboles + plagioclase + epidote + garnet + quartz + titanite + Fe-Ti oxide (magnetite or ilmenite). Amphibole separates commonly display discordant age spectra. Nevertheless, amphiboles of five analysed samples yield Ar/ Ar intercept dates ranging from 432+4 Ma to 499±10 Ma. Three of them yield plateau dates of 440-492 Ma, defined by more than three contiguous steps accounting for >50% of the total ^^ArK released. Two of these plateau dates (465±4 Ma and 492+4 Ma) are from hornblende fractions of different grain size in a gamet-homblende schist of the structurally lower Poeun unit. In addition, one sample of the upper Pibanryeong unit gives a plateau date of 263.5+24 Ma with an intercept date of 222±23 Ma. The appearance of the hornblende ^^Axl^^Ax dates of 432 - 499 Ma suggests that the timing of major metamorphic event in the 0MB is close to ca. 500 Ma. This resuh substantiates the presence of long-debated Caledonian disturbance in central Ogcheon belt. On the other hand, one sample recording an intercept date of 222+23 Ma is the product of Triassic thermal event that is apparently mild in central part of the 0MB. Our results together with the P-T estimates of Min and Cho (1998) suggest that the Ogcheon metamorphic belt is the product of Ordovician Barrovian-type metamorphism, and that this polycyclic belt is not correlated with the Triassic continental collision belt between Sino-Korean and Yangtze cratons in east-central China. Moreover, it is likely that the 0MB has a prolonged history of tectono-metamorphic evolution in contrast to the Paleozoic sequences of the Taebaeksan sedimentary zone. Hence, we conclude that the Ogcheon belt represents a collage of at least two separate terranes.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

EARLY PERMIAN PALAEOMAGNETIC POLES FROM AUSTRALIA: THE MOUNT LEYSHON INTRUSIVE COMPLEX AND THE TUCKERS IGNEOUS COMPLEX, NORTH QUEENSLAND D. A. Clark^ and M.A. Lackie^ 'CSIRO Exploration and Mining, PO Box 136, North Ryde NSW 2113, Australia ^Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109, Australia This study provides reliable, well-dated and precisely defined Early Permian (285 ± 5 Ma) palaeomagnetic poles for Australia from the Mount Leyshon Igneous Complex (MLIC) and the Tuckers Igneous Complex (TIC). These data indicate that Australia occupied high latitudes in the Early Permian. The mean direction obtained from the MLIC is Dec - 196.3°; Inc = +77.4° (N - 34; k = 53.5; a95 = 3.4°), while the mean direction obtained from the intrusive phases and the aureole of the TIC is Dec = 186°, Inc = +74° (N = 4; k = 216; a95 = 6.3°). The pole positions are: MLIC: Lat = 43°S, Long = 137°E (dp = 6.0°, dm = 6.4°), TIC: Lat = 49°S, Long = 142°E (dp - 10.3°, dm = 11.4°). The primary nature of the Early Permian palaeomagnetic signature is established by full baked contact/aureole tests at both localities. Adjacent to the intrusions the host rocks are completely remagnetised and exhibit reversed polarity palaeomagnetic directions that are identical to those of the intrusions. At somewhat greater distances the country rocks bear a Permian overprint on a more ancient component and at sufficiently large distances the host rocks show no signs of Permian overprinting. Permian overprinting is detectable at considerable distances from the MLIC (2-3 km), well beyond the zone of visible alteration. Proximal overprinting is generally associated with secondary magnetite, but distal overprinting appears to be associated with precipitation of secondary hematite. A similar pattern of overprinting is found at the TIC, where the pronounced Permian overprinting persists well outboard of the biotite/secondary amphibole homfels zone. However, the distal overprinting associated with the TIC appears to be purely thermal, with no evidence of secondary magnetic minerals outside the mappable homfels zone.

Silurian-Cretaceous Apparent Polar Wander Path for Australia. Poles are shown with associated errors. TR, Tuckers Igneous Complex. MTL, Mount Leyshon Igneous Complex.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SULPHIDE MINERAL OXIDATION IN THE REGOLITH: IMPLICATIONS FOR TRACE METAL DISPERSION AND ACID RETENTION. M.W. Clark, D. McConchie, F. Davies-McConchie and C. Lin Centre for Coastal Management, Southern Cross University, P.O. Box 5125, East Lismore, NSW, 2480

Most sulphide minerals in the regolith oxidise readily when exposed to atmospheric oxygen and in the process they release trace metals and sulphuric acid; the acid can leach additional trace metals from other minerals. During periods of high rainfall, both the acid and some of the trace metals may be flushed from the site of sulphide oxidation to aquatic environments where they can have several undesirable ecological impacts. However, in this paper, we examine ways in which both the acid and the trace metals, produced during sulphide mineral oxidation, can be retained in the regolith. Secondary oxidation products that can store acidity are usually in the form of oxyhydroxides (e.g. goethite, lepidocrocite, ferrihydrite), which may later dehydrate to hematite, or sulphate minerals (e.g. jarosite, brochantite, copiapite, chalcanthite, anglesite, etc.); many of these minerals can act as a store for acidity either through incomplete oxidation (e.g. melanterite), or by retaining hydrogen ions within the crystal structure (e.g. jarosite). Carbonates and hydroxycarbonates (e.g. siderite, malachite, azurite, cerussite, smithsonite, etc.) can form where carbonate activities are high and acid produced during sulphide oxidation is neutralised by reaction with other minerals in the regolith or during mixing with groundwaters that have some alkalinity, but these minerals are not considered in this paper because there is no acid storage in the system. In this paper we examine trace metal and acid retention by secondary oxidation products of sulphide minerals from field sites and sulphide mineral samples in specimen collections and we evaluate the environmental implications of the secondary mineral formation at each site. The different assemblage of oxidation products at each site reflects subtle differences in geochemical, biogeochemical and hydrological conditions between the sites. Oxidation rates for sulphide minerals are influenced by the grain size, composition and mineralogy of the sulphide minerals, the oxidation pathway, geochemical gradients near the oxidising surface (particularly the concentration of oxygen and the removal of reaction products in solution) and the presence of catalysing agents (e.g. bacteria, and ferric iron for pyrite oxidation). These influences may have a similar effect over a large area of a site or they may be expressed as very fine scale (sub-millimetre scales) changes in oxidation rates or in the composition of the oxidation products. For example, iron-oxyhydroxide coatings on pyrite grains can act as a barrier to oxygen and need only be one atomic layer thick to prevent or substantially slow further oxidation; very thin iron-oxyhydroxide coatings also have a major influence on trace metal trapping. The precipitation of some secondary oxidation minerals also requires the development of micro-gradients of acidity, oxygen supply and hydration, and these gradients may greatly affect the type of secondary oxidation products produced, the concentration of the stored trace metals, and the amount of stored acidity. Given time, the secondary oxidation minerals may be dissolved, or transformed, resulting in the release of incorporated trace metals and acidity to the environment.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TESTING SOLOMON SEA SUBDUCTION MODELS USING GEODYNAMICS Stuart Clark', M. Gumis^ and R.D. Muller' ' Department of Geology and Geophysics, University of Sydney, NSW 2006, Seismological Laboratory, Califomian ^Institute of Technology, Pasadena, CA 91125, USA

The plate driving forces acting on Australia have changed dramatically though time. Understanding these changes and the associated tectonic events on Australia's interior and its margins requires an unraveling of the subduction history north and east of Australia. The tectonic history of the Solomon Plate north of Australia is particularly elusive, as most of this plate has been recycled back into the mantle. Here, we test two published endmember scenarios using a semi-dynamic mantle convection model. For each hypothesis a kinematic plate formulation is generated from 40 Ma to the present. In the first model (double-subduction case) the Solomon Sea plate (SSP) moves northward at half the rate of the Australian plate. It is subducted orthogonally to the north underneath the Caroline Plate from 40 to 25 Ma and obliquely to the south from 40 Ma to the present, whilst the Pacific plate subducts underneath the SSP obliquely as it moves westward. In the second model (single subduction/transform case) the SSP moves west laterally with respect to the Australian plate from 40 to 25 Ma, creating a transform boundary between Papua New Guinea and the SSP. At the same time, the SSP is subducted to the north underneath the Caroline Plate. The Pacific Plate moves westward faster than the SSP, also creating subduction on the eastern boundary of the SSP. After 25 Ma, the transform boundary changes to a convergent margin, and northward subduction underneath the Caroline Plate ceases. The constraints for modeling the motion of the SSP include the accretion of the Albert-Finisterre Arc to Papua New Guinea in the mid-Oligocene and the Solomon Islands collision with the Ontong-Java Plateau in the early Miocene. The two kinematic plate formulations are self-consistent and constrain the surface velocity boundary condition for the mantle convection model using the rules of plate tectonics. The temperature and viscosity fields produced by the mantle convection models are compared to the present-day location of subducted slabs, the fast velocity regions in seismic tomography. Each model also produces a dynamic topographic history that is verified by comparison to tectonic subsidence records.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE MOUTON SHEAR ZONE: INTERSECTION OF A CRUSTAL-SCALE FRACTURE WITH A CRYSTALLIZING GRANITOID PLUTON D. Barrie Clarke^ Krista L. McCuish', Ron H. Vemon^ Victor Maksaev^ and Brent V. Miller^ ' Department of Earth Sciences, Dalhousie University, Halifax, NS B3H 3J5, ^ School of Earth and Planetary Sciences and ARC Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC), Macquarie University, Sydney, NSW 2109, Australia ^ Departamento de Geologia, Universidad de Chile, Santiago, Chile ^ Department of Geological Sciences, University of North Carolina at Chapel Hill, NC, USA 27599-3315 The peraluminous tonalite-granodiorite-monzogranite Port Mouton Pluton (PMP) is a petrological, geochemical, and structural anomaly among the many Late Devonian (373 ± 1 Ma) granitoid intrusions of the Meguma Zone of southern Nova Scotia. In this contribution, we examine a possible genetic connection among several previously unrelated structural and textural features: well-developed linear schlieren-banded monzogranites, chaotic schlieren-banded granodiorites, foliated monzogranites, mafic-felsic magma mingling, a swarm of aligned angular xenoliths, and a breccia pipe. The most remarkable structural feature of the PMP is a wide (up to 1 km) zone of strongly foliated (040/subvertical) monzogranites culminating in a central, narrow (10-30 m), straight, en echelon, centimetre-decimetre-scale, segregation-banded zone that extends for at least 3 km along strike. The segregation-banded monzogranites have parallel aligned feldspars developed during relatively high-melt-fraction magmatic flow. They also show strong modal variations in micas and feldspars between bands indicating efficient mineral-melt and mineral-mineral sorting, fractured feldspars with quartz fillings, and a change from homogeneous to heterogeneous strain partitioning, all suggesting continuing deformation under low melt-fraction conditions. Internal deformation of feldspars, polygonized and ribbonized quartz, microcline twinning, and kinking of micas indicate an overprinting by high-temperature solid-state flow. This major structural feature is effectively colinear with the only two outcrops of Late Devonian mafic intrusions (376 ± 2 Ma) in the area, one of which is synplutonic with well-developed mingling textures in the marginal tonalite of the pluton, a granitoid dyke that extends well beyond the outer contract of the PMP, a swarm of large aligned angular xenoliths, a zone of contorted schlieren banding involving refractory biotite drawn off the ends of disintegrating country rock xenoliths, and a breccia pipe with abundant gametiferous pelitic xenoliths. We believe that the fracture which delivered the mafic magma and the breccia pipe to the upper crust also intersected the partially crystalline PMP resulting in: mafic-felsic magma mingling textures and turbulent stirring of the felsic magmas to produce the chaotic schlieren in the most liquid parts of the pluton, deformation and segregation to form the zone of foliated and banded granitoids in the nearly crystalline part of the pluton, and brittle fracturing of the roof to release the angular xenolith swarm from the solid carapace of the pluton. This structural feature, here termed the Mouton Shear Zone, is parallel to the postulated post-intrusive Tobeatic Fault, and to two much younger Triassic-Jurassic features, the Shelbume Dyke and the Fundy Rift. These four lineaments may be old planes of weakness developed during docking of the Meguma Zone against North America.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EQUILIBRATION IN ECLOGUE AND BLUESCHIST FACIES ROCKS: CONSTRAINTS FROM CATION MAPS G. L. Clarke\ N.R. Daczko' andC.Noccolds^ ^School of Geosciences F05, University of Sydney, Sydney, NSW 2006 Australia ^Electron Microscope Unit F09, University of Sydney, Sydney, NSW 2006 Australia.

Matlab scripts are presented that apply the Bence & Albee (1968) matrix correction algorithm to X-ray intensity data collected as element maps on a Cameca SX-50 microprobe. Once generated, large data sets of mapped oxide weight per cent values or cation numbers that retain spatial information can be used in routine petrological plots. An example of the technique evaluates the compositional range of barrositic amphibole in an eclogite from New Caledonia. Cation maps are generated for a sample to examine aspects of equilibration during the partial hydration of the eclogite facies assemblage. Reference Bence, A.E. & Albee, A.L., 1968. Empirical correction factors for the electron microanalysis of silicates and oxides. Journal of Geology, 76, 382-403.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HYPERPET: A WEB-BASED OPTICAL PETROLOGY TUTORIAL PACKAGE Geoffrey L. Clarke, Thomas C.T. Hubble and Daniel D. Dunkley School of Geosciences, The University of Sydney, New South Wales, Australia, 2006.

In order to assist our second-year students master the complex tasks involved in the microscopic identification of rocks and minerals a set of CD-Rom based tutorials have been created. These focus on the set of practical petrology classes used in second year. The tutorial package utilises the capabilities of the Netscape Web Browser to present its material as a cascade of hotlinked buttons. Short Quicktime movies of the microscope eyepiece view of thin-sections as they are rotated through a full 360o in both planepolarised and cross-polarised light provide a perfect way to view the optical properties of minerals. Minerals of specific interest are generally located in the centre of the movie's field of view or are obviously identifiable by their colour. The HYPERPET package is, in effect, a virtual microscope that comes complete with a library of full-colour Quicktime movies of petrographic slides from the collection the students work on. The Quicktime movies are linked by html and Java-scripts to sets of notes that describe the various optical properties in general and the minerals in particular. A birefringence chart is also provided. Properties such as colour, pleochroism, birefringence, extinction angle and even relief can be clearly demonstrated to students on a standard home computer. The ability of the Quicktime movies to present the variation in the appearance of a mineral as the stage is rotated is an enormous advance over conventional still images provided in textbooks and photo atlases of rocks and minerals. The mineral movies have been digitally recorded using a video camera attachment on a high-quality petrographic microscope. The size of the individual movie-files, two to three megabytes, generally requires a fairly fast computer to play them. The size of these files and current modem speeds has also required that the package be delivered by CD-Rom rather than by placing the material on a server. We find that the minimum machine requirement is to run HYPERPET is a Pentium 75MHZ PC or equivalent. The use of the Netscape browser as the means of presentation in combination with the flexibility of HTML, Java-scripts and Quicktime ensures that HYPERPET is platform independent. We are not sure if the use of Hyperpet has improved student learning of mineral optics in a rigorous statistical sense because we have no formal control group. The optical mineralogy class sizes are relatively small and the ability of the group of students in these classes varies quite a lot from year to year. Our impression is that students are more confident when using a petrographic microscope and more successful at identifying minerals. The versatility of the Hyperpet package can only improve their ability to revise, especially as students are not tied to a real microscope but can use Hyperpet's virtual microscope instead.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CHRONOLOGICAL CONSTRAINTS ON A RIFT SEQUENCE UNDERLYING THE SOUTHWEST AMADEUS BASIN, CENTRAL AUSTRALIA Dorothy Closed Christine Edgoose\ Peter Haines^ ' Northern Territory Geological Survey, P.O. Box 2655, Alice Springs, NT, 0871. ^ School of Earth Sciences, University of Tasmania, Hobart, Tas, 7001 The Neoproterozoic to Palaeozoic Amadeus Basin is an extensive intracratonic basin in central Australia. The stratigraphy and tectonic development of this basin is relatively well documented, especially on the northeastern margin which overlies basement rocks of the Arunta Inlier. In contrast, the southwestern region that overlies Musgrave Block basement is less well known due to remoteness and access constraints. Eorman (1966) first recognised a sequence of bimodal volcanics and sediments that underlie this southwestern margin, however the exact nature and timing of this sequence has remained unclear, preventing a fiill appraisal of its relationship to the initiation of the overlying basin. Recent mapping and subsequent geochronology by the NT Geological Survey in the northwestern Musgrave Block has established a stratigraphic framework for this sequence, the Wataru Group. The basal unit of the rift sequence is a 5 metre thick quartzite layer that locally overlies the 1190-1150 Ma Pottoyu Suite granites of the Musgrave Block basement. Quartz pebble lag deposits and winnowed heavy mineral horizons suggest this basal quartzite was deposited in a shoreline setting. Overlying the quartzite is a sequence of interlayered felsic and mafic volcanics, volcaniclastics, quartzites and phyllites. Poorly preserved, relic pillow structures in the basalt and the interlayering of sediments indicate subaqueaous extrusion. The rift basin was subsequently starved of sediments whilst mafic volcanism continued, and is characterised by alternation of pervasively silicified and epidotised amygdaloidal flows with unaltered, amygdaloidal-poor flows. This layering is thought to represent alteration of flow tops due to contact with seawater. A minimum age for the basalt is constrained by the local intrusion of shallow level granites that yield U-Pb SHRIMP zircon ages of 1084 ± 9 Ma. Both the basalt and the granites are cut by dolerite dykes of the - 1 0 7 8 Ma Alcurra Dyke Swarm. The basalt flow is overlain by rhyolite that has recently been dated at 1075 ± 2 Ma using the Pb-Pb Kober technique. This correlates well with the age of 1078 ± 5 Ma for the Tollu Volcanics of the Bentley Supergroup that outcrop to the southwest in Western Australia. In situ erosion of this rhyolite formed polymictic conglomerate and redbeds that grade laterally east into an alluvial sequence comprising lensiodal quartz sandstone channel deposits within an arkosic succession. Zircon Pb-Pb ages from a felsic volcanic unit within the Petermann Ranges region yields an age of 1041 ± 2 Ma suggesting that volcanism continued episodically during the evolution of the rift sequence. The basal units of the Amadeus Basin unconformably overlie the alluvial sediments. This work documents a prolonged period of extension, volcanism and sedimentation that predating the onset of the Amadeus Basin proper. Known correlatives of this pre-Amadeus rift sequence extend well into Western Australia and possibly underlie the northwestern Amadeus Basin. It's tectonic influence on the evolution of the Amadeus Basin is yet to be defined. Reference

Eorman, D. J., 1966. The geology of the southwestern margin of the Amadeus Basin, Central Australia. Bureau of Mineral Resources, Report 87

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A COMPARISON OF UNSUPERVISED NEURAL NETWORKS AND K-MEANS CLUSTERING OF MULTI-ELEMENT STREAM SEDIMENT DATA, NORTHEASTERN NEW SOUTH WALES. D.R. Cohen^'^ and A.P. Clare^ ^School of Geology, University of New South Wales, Sydney, NSW 2052. ^Apache Energy, 256 St.Georges Terrace Perth WA 6000. ^CRC LEME Associate. Isolation of complex patterns of correlation between variables, associations among samples and the identification of anomaly through conventional multivariate parametric statistical procedures, may be biased or obscured by the presence of multivariate outliers and non-normal variable distributions. Such procedures generally require substantial pre-processing of data prior to modelling. Unsupervised neural networks (UNN) have the capacity to organise multivariate data into a set of unique classes without a priori knowledge or the need for substantial data pre-processing. Patterns in n-dimensional space may be transformed to lower dimensional space that is topologically ordered and easily visualised. UNN clustering, using a modified form of the standard unsupervised Kohonen self-organising map, is nonlinear, non-parametric and rapid. The number of clusters into which samples are allocated is determined by the unsupervised neural network and is directly dependent upon the original input data. Though similar in objective to UNN clustering, k-means clustering is affected by the original data distribution, the presence of outliers and the initial approximations to the k group centroids. UNN and k-means clustering was performed on stream sediment geochemical data from 1677 subcatchments in the northeast region of NSW. Fourteen high-density data clouds (common patterns or clusters) and one much smaller composite anomalous group were obtained after UNN clustering of the 33 variable data set. Element associations within the clusters can be related to a feldspar suite (Na, K, Ba, Sr, Rb, Pb and Eu), siderophiles and metals that strongly adsorbed to Fe-oxides (Fe, Co, Cr, Cu and Ni), a base metal mineralisation suite (Cu, Pb, Zn) and various combinations of metals associated with heavy minerals (REE, Nb, Ta, Th, Hf and Zr). The clusters are closely related to sub-catchment geology and topography. UNN clustering of the Cu-Pb-Zn suite produced 10 clusters and 17 outliers that were mainly from sub-catchments containing major economic mineral occurrences. K-means clustering of transformed Cu, Pb and Zn yielded five major clusters and 19 anomalies contained within a further five clusters. The catchments only identified as anomalous by UNN clustering included some mineralised areas in the west and south of the region (including the Drake mineral field) and some catchments with anomalously low metal contents in the Clarence-Moreton Basin. The UNN has proved capable of extracting both common geochemical patterns and the subset of anomalous patterns by reducing the complex higher dimensionality raw data layers into lower dimensionality ordered groups. The study demonstrates unsupervised neural networks to be viable alternatives to conventional statistical approaches in the modelling of multivariate geochemical data.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CHARACTERIZATION OF RESIDUAL AND TRANSPORTED REGOLITH PROFILES USING THE PIMA II D.R. Cohen^ ^ X.C. Shen^ and A.C. Dunlop^'^ 'School of Geology, University of New South Wales, Sydney NSW 2052. ^CRC LEME Associate.

In areas of thick regolith with a significant transported component, interpretation of trace element geochemical patterns is partly dependent on the differentiation of geochemical signatures associated with residual and transported materials. Differentiation is a problem in terrains where there has been partial or complete stripping of the regolith profile down to the saprolite, the gross mineral composition of residual and transported regolith are similar and where subsurface sampling is conducted using drilling methods that do not preserve gross regolith fabrics. In characterising geochemical patterns in transported cover within thick regolith in the Cobar region, an investigation has been made of infrared spectrometry to measure subtle differences between hydrated mineral phase (principally phyllosilicate) characteristics and abundance in the transported and residual portion of the regolith. PIMA II is a field-portable infrared spectrometer, which can measure the short wave infrared (1300-2500 nm) reflectance spectra. PIMA II has a spectral sample interval of 2 or 4 nm and a spectral resolution of 7-10 nm. The PIMA provides a fast means of identifying minerals, their crystallinity and compositional variations. Slates, siltstones and sandstones of the Devonian Cobar Supergroup host structurally controlled Cu mineralisation at the nearby CSA Mine. The deep weathering profile has been partially stripped. Up to 30 m of transported regolith overlies saprolite, often in the form of channel fills related to both present and palaeodrainages, and is dominated by fine grained quartz and kaolinite (with a significant aeolian component). PIMA spectra were recorded for the <63 |im fraction of 617 air dried profile samples, collected at 1 m intervals from 24 reverse circulation (RC) drill holes, on four traverses near the CSA Mine. Two spectral types were detected; Type I corresponding to residual regolith and containing an assemblage of kaolinite + muscovite/illite ± halloysite ± montmorillonite and Type II for transported regolith and containing muscovite/illite+kaolinite (the mineralogy being confirmed by quantitative XRD and normative XRF mineralogy). The residual regolith contains more muscovite/illite than kaolinite. The transition from Type I to Type II occurs at depths ranging from 3 to 25 m below surface. The higher kaolinite/muscovite ratio in the transported materials is attributed to erosion of kaolinite-rich materials from upper saprolite and deposition on a truncated weathering profile. Selected hull-quotient absorption peaks were extracted from the data and subjected to Kmeans clustering and discriminant analysis. K-means clustering achieved an 82 % correct classification and discriminant analysis a 90 % correct classification of residual versus transported regolith, for the a priori sample groupings. The errors in classification by these methods may include problems with mixed spectra for samples from drill intervals that spanned the transported-residual boundary or incorrect initial sample classification.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE TECTONIC ARCHITECTURE OF CENTRAL MADAGASCAR DISENTANGLING THE AMALGAMATION OF GONDWANA. A. S. Collins^ A. Kroner^ T. Razakamanana^ and B. F. Windley^ ^Tectonic Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth 6845, Western Australia ^Institut fiir Geowissenschaften, Universitat Mainz, D-55099 Mainz, Germany. ^Departement des Sciences de la Terre, Universite de Toliara, Toliara, Madagascar. '^Orogenic Processes Group, Department of Geology, University of Leicester, University Road, Leicester, LEI 7RH, United Kingdom.

Madagascar is a key component of the East African Orogen (EAO), one of the largest orogenic belts on the globe and fundamental in unravelling the complex interactions between Neoproterozoic plates. By coupling extensive field and structural studies with zircon geochronology, we have constructed a testable tectonic framework, from which is derived a model for the closure of a strand of the Mozambique Ocean and amalgamation of this part of Gondwana. Five tectonic units are identified in central and northern Madagascar. Rocks within these units have similar histories and are separated by shear zones or regionally significant unconformities. These are: 1) the Antongil block, Archaean gneiss intruded by late Archaean granite non-metamorphosed since earliest Proterozoic times; 2) the Antananarivo block, Archaean gneiss cut by 820-740 Ma granitoid and gabbro intrusions, pervasively structurally and thermally reworked between 700-550 Ma; 3) the Itremo sheet, Mesoproterozoic sediments thrust east to west over, and imbricated with, the Antananarivo block; 4) the Tsaratanana thrust sheet, Archaean gneiss cut by 800-760 Ma gabbro intrusions; 5) Bemarivo orogenic belt, E-W striking metasediments, granites and gneisses, overlain by contractionally deformed metavolcanics. The Tsaratanana thrust sheet has a mylonite zone at its base that separates it from the Antananarivo block. As both the Tsaratanana thrust sheet and the Antananarivo block are cut by 820-740 Ma igneous rocks, formation of this mylonite zone and tectonic emplacement of the Tsaratanana thrust sheet probably occurred before -820 Ma. The Itremo sheet is locally thrust imbricated with the Antananarivo block, however, at its eastern boundary the overlying Itremo sheet is separated from the Antananarivo block by the Betsileo shear zone, a crustal-scale extensional detachment. The Itremo sheet preserves evidence for post 1850 Ma, pre-800 Ma deformation, creating large-scale (<20 km amplitude) recumbent folds. The Antananarivo block was thrust over the Antongil block between -700 and 550 Ma. A paragneiss belt with peridotite bodies marks the boundary zone between these tectonic units. This belt is interpreted as a strand of the Mozambique Ocean suture that separated the Dharwar craton (i.e. the Antongil block) from the Antananarivo block. The Bemarivo orogenic belt truncates the other units and was metamorphosed to granulite-facies conditions in Cambrian times. It is preserves evidence for contractional, top-to-the-south deformation overprinted by later extensional deformation and represents a terminal stage of mountain building in the East African orogenic cycle.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CRUSTAL THICKNESS PATTERNS IN THE AUSTRALIAN CONTINENT Clive Collins and Barry Drummond Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation GPO Box 378, Canberra, ACT 2601, Australia The thickness of the crust is usually defined seismologically by the depth to the Mohorovicic discontinuity or Moho. The Moho is defined as the boundary below which the seismic p-wave velocity of the rocks increases to values above about 7.8 km/s. These velocities are typical for mantle rocks under the expected temperature and pressure conditions. There are arguments that the Moho may not always be the base of the crust from a petrological point of view. In any case, the transition from crustal to mantle compositions may be gradational, and this is reflected by the common observation of a velocity gradient at the base of the crust. There are significant variations in the depth of the Moho under Australia. In general, within the Archean regions of Western Australia the Moho is relatively shallow with a large velocity contrast at the transition between the crust and the mantle. It is significantly deeper under the Proterozoic North Australian Platform, under Central Australia and Phanerozoic Southeastern Australia. Thicker crust in general is reflected in higher surface elevation, although the relationship between crustal thickness and elevation is not linear. Where the Moho is deep there is a very broad transition from crustal to mantle velocities. Other regions of Australia for which data exist generally have average depths to Moho. While the Moho is often mapped as a continuous sub-horizontal or gently dipping boundary, short wavelength features have been imaged by seismic reflection profiling in places where the data are sufficiently detailed. Offsets have been observed where major crustal faulting intersects or soles out at the Moho, for example in eastern Tasmania, and Central Australia. Major variations occur in extensional areas where various mechanisms of rifting have thinned the crust. For example, crustal thinning has occurred beneath the Bass Strait and Northwest Shelf Basins, and culminates in highly attenuated crust at the continent/ocean boundary. Crustal thickness patterns reflect the mechanisms of continental growth and tectonic evolution. The relative thickness of the upper and lower crust are characteristic of various styles of extensional or compressional tectonics and other processes such as underplating. Images across the Australian continental margin clearly demonstrate the role that upper and lower crustal extension have on crustal growth, by sedimentary deposition and magmatic emplacement on and within the attenuated crust. In cratonic areas, recent examples of good quality reflection profiling suggest crustal growth is dominated by thrusting and stacking in a compressional environment. Where the crustal thickness is large the average density of the crust is greater than in areas of 'normal' thickness. This is primarily due to a thicker and denser lower crust, with little difference in the mid- and upper crust. Models of crustal growth must explain the emplacement of this layer while preserving the characteristics of 'normal' crust above. A first order observation that for the postArchean Australian continent lower crustal thickness increases with tectonic age implies lower crustal growth with time. Archean crust does not follow this trend.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LATE ORDOVICIAN-EARLY SILURIAN CRUSTAL THICKENING IN THE EAST/CENTRAL LACHLAN FOLD BELT: IMPLICATIONS FOR S-TYPE GRANITE GENESIS W.J. Collins^ and B.E. Hobbs^ ' Department of Geology, School of Geosciences, University of Newcastle, Newcastle, NSW, 2308, Australia ^Australian Geodynamics CRC, CSIRO Exploration and Mining, Private Bag, Wembley, WA, 6014, Australia.

One of the most significant, but poorly understood, tectonic events in the east Lachlan Fold Belt (LFB) is that which caused the shift from mafic, mantle-derived calcalkaline/shoshonitic volcanism in the Late Ordovician to silicic (S-type) plutonism and volcanism in the late Early Silurian. We suggest this chemical/isotopic shift requires major changes in crustal architecture, but not tectonic setting, and simply involved ongoing subduction-related magmatism following burial of the pre-existing, active intraoceanic arc by overthrusting Ordovician sediments during Late Ordovician-Early Silurian (early Benambran) deformation, associated with regional NECSW shortening. A review of "type" Benambran deformation from the type area (central LFB) show it is constrained to a NNW-trending belt at -430 Ma (late Early Silurian), associated with high-grade metamorphism and S-type granite generation. Similar features were associated with -430 Ma deformation in east LFB, highlighted by the Cooma complex, but these formed within a separate N-trending belt that included the Kosciuszko, Young and Wyangala batholiths. As Ordovician turbidites were partially melted at -430 Ma, they must have been already buried to -20 km before the "type" Benambran deformation. We suggest this occurred during earlier NE-SW shortening, during development of regional oblique folds and thrusts, loosely referred to as latitudinal- or E-W structures. They also caused earliest Silurian uplift in the central LFB (Benambran highlands). This uplift pre-dated the "type" Benambran deformation and is constrained as latest Ordovician-earliest Silurian (-450-440 Ma) in age. We term these "pre"- and "type"-Benambran events as "early" and "late" for historical reasons, though we do not consider they are necessarily related. Heat flow modelling suggests that burial of "average" Ordovician turbidites during early Benambran deformation at 450-440 Ma, to form a 30 km thick crustal pile, cannot provide sufficient heat to induce mid-crustal mehing at -430 Ma by internal heat generation alone. An external, mantle heat source is required, best illustrated by the mafic -430 Ma old, Micalong Swamp Igneous Complex in the S-type Young Batholith. Modem heat flow constraints also indicate that the lower crust cannot be felsic and, along with petrological evidence, seem to preclude older continental "basement terranes" as sources for the S-type granites. Restriction of the S-type batholiths to two discrete, oblique, linear belts in the central and east LFB supports a model of separate magmatic arc/subduction zone complexes, consistent with the existence of adjacent, structurally imbricated turbidite zones with opposite tectonic vergence, inferred by other workers to be independent accretionary prisms. Arc magmas associated with this "double convergent" subduction system in the east LFB were heavily contaminated by Ordovician sediment, recently buried during the early Benambran deformation, causing the shift from mafic to silicic (S-type) magmatism. In contrast, the central LFB magmatic arc, represented by the Wagga-Omeo zone, only began in the Early Silurian, in response to subduction associated with the early Benambran NE-SW shortening. The model requires that the S-type and subsequent I-type (Late Silurian-Devonian) granites of the LFB be subduction-related.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE WALTER-OUTALPA SHEAR ZONE EASTERN WEEKAROO INLIER, OLARY BLOCK, SOUTH AUSTRALIA Colin Conor ^ and Pat James ^ ^Geological Survey, Primary Industries and Resources SA, Grenfell St, Adelaide, SA 5000 ^Department of Geology and Geophysics, University of Adelaide, SA, 5005 The three basement Weekeroo Inliers in eastern South Australia are principally fault and unconformity bounded structural outliers of the more extensive Palaeoproterozoic Olary and Broken Hill Domains. Basement units, comprising a moderately well recognised sequence of paragneisses ( - 1 7 0 5 Ma), schists, calc silicates and pelites, are intruded by at least three sets of granitoids. Neoproterozoic Adelaidean cover strata lie in frequently equivocal adjacence to the basement, with contacts varying from relatively undisturbed angular, and often overlapping unconformities to brittle low angle detachment faults and shear zones. Three ductile "Olarian" deformations (fold phases F1 to 3) recognised in the basement have left a signature of high-grade layer-parallel foliations and isoclinal folds and an overall luniform-geometry of superposed interference patterns. Subsequent major retrograde shear zones either terminate at the basement-cover unconformity, or involve Adelaidean rocks. The "Olarian" event was followed by the Ordovician Delamerian event, which included at least two stages of folding (F4 & 5); Olarian F3 folds in the northern part of the inlier appear to have been further tightened by the co-axial Delamerian F5. Delamerian folds are parallel in style and upright, and locally there are associated internal detachment and imbricate faults. The relative influence of late Olarian semi ductile shearing and Delamerian faulting, including fault reactivation, is the principal focus of this paper. In the northwest section of the largest eastern Weekeroo Inlier, the basement lithologies include a complete overturned metamorphic complex comprising metasediments, metabasites (the Weekeroo Amphibolite), felsic metavolcanics and granites. A largely concordant but obviously intrusive and variably foliated granite sheet comprises at least two magma varieties including a marginal granodiorite sheet and a central peraluminous granite. Similar granites dated elsewhere give ages o f - 1 7 0 5 Ma for the older and - 1 5 8 0 Ma for the younger. All components of the metamorphic complex curve into and are truncated, offset and highly deformed by the Walter Outalpa Shear Zone. This zone is obvious as a major, discrete WNW trending, linear feature, clearly visible on airborne and satellite images, which has a trace length of about 10km and is up to a few hundred metres in width. At the N W extremity the shear zone passes through an angle of 135° and, running southward, controls the western margin of the inlier with the enclosing Adelaidean metasediments. The N-S western margin is a tectonic melange, not only of Willyama and Adelaidean rock, but also of metadolerite (commonly deformed to chlorite schist). The metadolerite suggests at least one extensional event. Within the shear zone, the low angle SE pitching mineral and stretching lineation, together with a clockwise fabric rotation and internal kinematic indicators indicate oblique slip transpressional displacement. The kinematic indicators, together with the overall angular form of the inlier suggests that the N W comer has been driven upwards, piston- like, from the SE. Up to 3.5 km of displacement has been calculated from large-scale marker offset and scaling-up of slip vectors measured on minor faults, using piercing points on fault planes. Internal deformation within the shear zone is by both brittle minor imbrication and ductile flow. Minor structures include arrays of bookshelf-tectonic style riedel and antiriedel fractures and asymmetric foliation boudinage. Protomylonitic and mylonitic fabrics contain a range of S-C composite foliations, shear folds and asymmetric winged porphyroclast microstructures. All of these reinforce the sense of displacement on the zone. There is conflicting evidence for timing, because although the western portion displays involvement of Adelaidean rocks, the SE termination shows little or no displacement of the Adelaidean unconformity ( - 7 8 0 Ma). Mica-garnet pairs give a Sm-Nd Delamerian age; the garnet was stabilised by the influx of metasomatic Mn, and this alteration is possibly broadly coeval with significant Cu-Au mineralisation along the length of the shear zone.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HIGH ARSENIC GRANITES IN THE NEW ENGLAND BATHOLITH Michael Cook Dept of Earth and Planetary Sciences, Macquarie University, NSW, 2109

Granites of the New England Batholith (NEB) have significantly higher average arsenic concentrations (7.35 ppm average from 502 analyses) than previously reported averages for granitic rocks worldwide («1.5 ppm). Within the NEB, 16% (81/502) of analyses contain in excess of 10 ppm As. Comparatively, in granites from the Lachlan Fold Belt, only 1% (2212200 analyses) contain As levels in excess of 1 0 ppm (Chappell, pers. comm.) Elevated arsenic levels occur in both S-type supersuites, and two of the three 1type supersuites in the NEB with only the 1-type Clarence River Supersuite exhibiting "normal" levels. The geochemical behaviour of arsenic is cryptic and shows poor correlation (calculated correlation coefficients < 0. 1) with any other measured elements. The highest arsenic plutons are largely confined to two narrow (< 30km wide) corridors that significantly affect the overall average. The two corridors intersect near the location of the Hillgrove Au-Sb mineral field. One is oriented NW-SE while the other runs NNE-SSW and both correspond to previously described zones of hydrothermal fluid movement and mineralisation suggesting that the high arsenic values are not magmatic characteristics of the granites, but have been overprinted by hydrothermal enrichment. Arsenic enrichment along these corridors has affected numerous individual plutons regardless of supersuite, granite chemistry or inferred level of emplacement. Arsenic concentrations may vary widely (up to two orders of magnitude) within otherwise geochemically homogeneous plutons. 'Me enrichment in arsenic along these corridors is thought to be related to similar. Late Permian post-magmatic hydrothermal activity identified as the source of Au-Sb mineralisation in the Hillgrove and Rockvale mining districts as both fall within the corridors. Removal of all analyses within the two corridors significantly reduces the overall average arsenic value in the NEB. The overall average is reduced to 3.24 ppm, with a systematic difference between the I- and S-type suites apparent, with S-types significantly enriched (6.81 ppm As avg.) relative to 1-types (2.61 ppm avg.). A systematic correlation is found between As levels and reported and Sr/Sr values for the various supercrustal suites (ONeill et al, 1977), with arsenic concentrations increasing as the granites become more obviously S-type. This variation is consistent with the significantly higher arsenic contents that are reported in likely sedimentary source rocks than in any igneous source (eg Baur and Onishi, 1978). References BAUR, W.H. AND ONISHI, H. (1978). Arsenic 33. In Handbook of Geochemistry 11-3 (ed. K.H. Wedepohl). Springer Verlag. Berlin. O'NEIL, J.R., SHAW, S.E. & FLOOD, R.H. (1977). Oxygen and Hydrogen isotope compositions as indicators of granite genesis in the New England Batholith, Australia. Contrib. Mineral. PetroL 62, 313-328.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PHREATIC EXPLOSIONS, BRECCIA DEPOSITS AND GOLD MINERALISATION IN LOW SULFIDATION EPITHERMAL ENVIRONMENTS David R. Cooke and Andrew G.S. Davies Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, 7001 Hydrothermal (phreatic) explosions are an important facet of some high temperature (200 - 300°C) low sulfidation epithermal and modem subaerial geothermal systems. When a catastrophic brittle failure event occurs, hot water flashes to steam as confining pressures decrease. Breccia formation is an essentially instantaneous event that can occur once or several times in the life of a geothermal system (10,000 - 100,000 years?). Even though brecciation events are uncommon, they may be critical for precious metal deposition in breccia-hosted low sulfidation epithermal deposits (eg., Lihir, Kelian). If the surface is breached during a phreatic explosion (ie. a phreatic eruption), water, steam and rock fragments will be discharged from the vent, and unconsolidated phreatic breccia deposits will form an ejecta rim. These low aspect ratio breccia deposits are readily eroded, and need to be cemented or buried rapidly to be preserved in the geologic record. Phreatic explosions can generate high aspect ratio breccia deposits in the subsurface. Breccia pipes, breccia veins or irregular zones of brecciation may all result from catastrophic failure and subsurface steam expansion. As the phreatic explosion progresses, brecciation may propagate both upwards and, to a lesser extent, downwards from the initial site of brittle failure. When the site of initial brecciation is too deep and the energy released is insufficient to completely excavate the overlying rock column, blind subsurface phreatic breccia deposits may form, which have no related surficial breccia deposits. Phreatic breccia formation irreversibly modifies the hydrology of a hydrothermal system. Porous subsurface breccia deposits are natural foci for hydrothermal fluid flow, and may be favourable sites for fluid mixing. Cementation of fragments by hydrothermal minerals can produce distinctive cockade textures, and can improve the preservation potential of the subsurface breccia deposits. However, in some cases this may result in self-sealing of the hydrothermal system, potentially leading to subsequent explosions and deposition of multi-stage hydrothermal breccia deposits. There are several mechanisms that can cause water to flash to steam and trigger phreatic breccia formation in a boiling or near-boiling geothermal system, including: (1) seismic rupture of an overpressured hydrothermal system; (2) sudden increase in thermal gradient due to proximal magma intrusion; (3) explosive magmawater interaction (phreatomagmatic explosions); (4) instantaneous decrease in confining pressure (eg., sector collapse of a volcanic edifice, or draining of a glacially-dammed lake); (5) increasing fluid flux in a confined system. In most cases, it is not possible to determine the exact mechanism(s) of phreatic breccia formation, because the 3-dimensional geometry is not known. Subsurface breccia deposits can be sites for deposition of high grade precious metal mineralisation. Phreatic explosions cause an instantaneous decrease in confining pressure, leading to boiling of non-boiling water, or in the case of a hydrothermal fluid that was already undergoing phase separation, causing an increase in steam production. After hydrothermal explosions have ceased, near-surface groundwaters can descend into the breccia pipe or breccia vein, and mix with ascending auriferous chloride waters, resulting in ore deposition through mixing-induced oxidation. We have conducted numerical simulations of mineral deposition, which show that both of the proposed ore depositional mechanisms (boiling, fluid mixing) are chemically viable. The hydrothermal cements are predicted to have distinctive gangue assemblages (eg., quartz - adularia - carbonate cements associated with boiling waters, quartz-pyrite ± muscovite cements associated mixing with groundwater), which should allow discrimination of the ore depositional mechanism for a given mineralised phreatic breccia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A NEW APPROACH TO GROUNDWATER MANAGEMENT FOR ALUMINA RESIDUE STORAGE FACILITIES IN KWINANA, WA, BY ALCOA OF AUSTRALIA LTD. D. Cooling', R.W. Colman^ S.R. Jeffries^ ' Alcoa of Australia Ltd, Kwinana Alumina Refinery, Cockbum Rd, Kwinana, WA 6167 ^Principal Hydrogeologist, URS, Hyatt Centre, Level 3, 20 Terrace Rd, East Perth, WA 6004 ^Senior Hydrogeologist, Hydrosolutions Pty Ltd, 1/109 Apsley Rd, Willetton, WA 6155 Alcoa has been operating an alumina refinery and storing process residues from the refining process at Kwinana since the early 1960s. The residues comprise a coarse sand fraction and a fine mud fraction entrained in an alkaline solution. This residue is pumped as a slurry into residue storage areas (RSA's) located approximately 2.5 kilometres southeast of the refinery. The older RSA's were lined with clay to limit the potential for seepage of the alkaline water from the storage areas. The clay liners were constructed to design standards considered best practice at that time. However, defects in the older RSA linings have led to localised groundwater contamination by the alkaline water and associated trace elements. To date, this contamination has been managed through a combination of pumped recovery and repairs to the clay liner utilising grout injection. The potential for an increase in contamination has been identified due the long-term seepage of the alkaline water through the clay seal. This has prompted Alcoa to review its long-term groundwater management strategy. The RSA's overlie superficial sand and Tamala Limestone of Quaternary age, underlain at between 37 and 45 metres by interbedded shales and sands of the Mirrabooka Member, Osborne Formation, of Mesozoic age. Unconfined groundwater is present in the upper superficial sand, and is semi-confined by basal silts and clays within the underlying Tamala Limestone. An unsaturated zone of approximately 18m underlies the RSA's. Groundwater flow occurs from the Jandakot Mound to the northeast to discharge at the coast in Cockbum Sound. Flow occurs to the northwest beneath the site due partly to the hydraulic conductivity contrast between the superficial sand (10 to 50m/d) and the Tamala Limestone (100 to +1000m/d). Groundwater resources within the superficial aquifer are used mainly for industrial and horticultural purposes, and are fully allocated under existing abstraction licensing. The likely impact on groundwater quality due to leachate seepage was examined using the MODFLOW numerical groundwater and solute transport model. A hydrochemical model was developed to predict contaminant concentrations at points of use downgradient of the RSA's. Options for future management of groundwater, under a range of potential future contaminant loadings, were examined using a risk based approach. These options included: • • •

A minimal management scenario that provided a base case against which the effectiveness of other management options could be evaluated. Options for the reduction of leachate seepage through leak sealing; dewatering and capping of the RSA's were examined using the Hydrologic Evaluation of Landfill Performance (HELP) model. Options for management of groundwater contamination through pumped recovery.

The risk-based approach to the study indicated that ongoing groundwater contaminant recovery provided the best balance between cost effectiveness and environmental acceptability. The most effective recovery bore configuration was shown to be bores located down the long axis of the plume. Plume management was shown to be effective in protecting existing and future beneficial uses of the groundwater resource.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

BRECCIA ENVIRONMENTS IN PACIFIC RIM GOLD-COPPER DEPOSITS Greg Corbett Corbett Geological Services, 29 Carr St, North Sydney, NSW, 2060 Australia Breccias are clastic rocks comprising fragments, matrix (including mineralization) and local open space. Although common to all Pacific rim gold-copper deposits, breccias vary according to the environment of formation and display relationships to the style of mineralization. Magmatic hydrothermal breccias (e.g., Kidston, tourmaline breccia pipes) may display a strong association with intrusive rocks, form at greatest depths, need not vent to the surface and include upper collapse slab breccias and deeper intrusion breccias. Mineralization is commonly quartz-sulfide gold + copper style. Phreatomagmatic breccias occur in low sulfidation carbonate-base metal gold (e.g., Acupan, Kelian, Wau) and high sulfidation gold-copper-silver (e.g., Yanacocha, Lepanto, Veladero) deposits. Venting diatreme eruptions are characterised by surficial maars, endogenous domes, tuff ring deposits and comprise mostly milled matrix fluidized breccias. Eruption initiated by contact of rising magma with groundwaters predates mineralization and commonly produces ore-hosting permeability and taps the magma source. Eruption breccias result from venting depressurised fluids and are most common in adularia-sericite epithermal gold-silver deposits (e.g., McLaughlin, Twin Hills, Toka Tindung) and include blocks of sinter in an intensely silicified matrix which may overlie mineralized vein systems (e.g., McLaughlin, Yamada). Expansion breccias form in dilatant structural environments as rock fragments are moved apart and filled with hydrothermal mineral growth. Hydrothermal minerals may also occur in fluidized injection breccias which are classified according to fragment/matrix ratio as rotational, mosaic, fluidized, and crackle breccias. Other breccias form by collapse in sediment hosted replacement Au deposits during calcite dissolution and alteration dolomite, or as a result of the overprinting of retrograde hydrothermal alteration. As hydrothermal fluids exploit fault zones there may be transitions between tectonic and hydrothermal breccias.

PHREATIC eruption BRECCIA

PHREATOMAGMATIC BRECCIAS •^r-rry-B-^—

maar volcano/diatreme breccia

MAGMATIC HYDROTHERMAL BRECCIAS sub-volcanic breccia pipe

HYDROTHERMAL COLLAPSE BRECCIAS

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

STYLES OF PACIFIC RIM GOLD-COPPER MINERALIZATION Greg Corbett^ and Terry Leach^ 'Corbett Geological Services, 29 Carr St, North Sydney, NSW, 2060, Australia. ^Terry Leach and Co, Suite 9, Fl, 673 Great North Rd, Grey Lynn, New Zealand This classification is based on crustal levels - porphyry, mesothermal, epithermal, and fluid chemistry - high or low sulfidation (Corbett and Leach, 1998). Porphyry copper-gold systems (e.g., North Parkes, Ridgeway, Ok Tedi, Grasberg, Batu Hijau) have the highest metal budget, and are characterised by repeated intrusion at 1-2 km depths with vein and sulfide deposition associated with zoned and overprinting alteration. Magmatic low sulfidation systems form from meteoric-rich, near neutral pH fluids that are classified with increasing distance from the source as: quartz-sulfide gold ± copper (e.g., Lihir, Kidston, Hamata, Adelong), including a transition to porphyry systems (e.g., Cadia), carbonate-base metal gold (e.g., Porgera, Kelian, Mt Leyshon, Acupan), and epithermal quartz gold-silver (e.g., Porgera Zone Vll, Emperor, Thames) at highest levels. Styles may be zoned in time and space with shallower styles overprinting the deeper ones (e.g., Porgera, Lake Cowal, Kelian), and metals vary as high copper at depth, to high silver, and bonanza gold grades at elevated settings. Low sulfidation adularia-sericite epithermal gold-silver systems (e.g., PajingoVera Nancy, Waihi, Hishikari) form at shallow crustal levels in back arc or rift settings with metal deposition dominated by fluid mixing, whereas boiling accounts for much gangue formation. Sediment hosted replacement gold deposits develop from low sulfidation fluids in reactive carbonate rocks (e.g., Carlin, Sapon, Mesel, Bau). High sulfidation systems develop from the reaction of hot acidic magmatic fluids to produce characteristic zoned alteration, contain copper-gold in the SW Pacific (e.g., Nena, Peak Hill, Temora), also silver in the Americas (e.g.. La Coipa, Yanacocha, Pierina), or occur as massive sulfide veins (e.g.. El Indio). Barren alteration (e.g., Vuda, Alum Mt, Horse-lvaal, Lookout Rocks) forms in near-porphyry environments.

LOW SULFIDATION

HIGH SULFIDATION

acid sulfate alteration

eruption breccia

MINERALIZATION Tension vein ^^^

Banded vein Sheeted vein

y ^ t

CO2 - rich acid sulfate waters

^ ^^ giP

Bonanza vein Breccia infill/ structural control Lithological control

^ 7

Breccia

.;.;•

Disseminated

(J)

Jasper

Stockworkvein

Evolving gases

^

Meteoric recharge

» \

Magmatic mineralized fluids

Corbett 3/2000

Reference Corbett, G.J., and Leach, T.M., 1998, Southwest Pacific rim gold-copper systems: Structure, alteration and mineralization: Economic Geology, Special Publication 6, 238 p.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ENVIRONMENTAL REVIEW OF THE MARY KATHLEEN URANIUM MINESITE, NORTHWEST QUEENSLAND M.T. CosteIIoe\ B.C. Lottermoser\ P.M. Ashley^ ^School of Earth Sciences, James Cook University, P.O. Box 6811, Caims, Qld 4870, Australia ^Division of Earth Sciences, University of New England, Armidale, N.S.W. 2351, Australia The Mary Kathleen uranium deposit, in northwest Queensland, was discovered in 1954 and mined in 19561963 and 1976-1982. Rehabilitation of the site was completed in 1985 and the work won an award for environmental excellence. In 1999 gamma-ray data, plus stream sediment, soil, rock chip, mineral efflorescence, vegetation and water samples were collected from selected sites to assist in the examination of the current environmental status of the rehabilitated area. This paper presents preliminary results and interpretations. In the Mark Kathleen open pit, skam type U-Th-REE mineralisation is hosted in amphibolite grade metamorphosed calc-silicate, mafic to intermediate igneous and sedimentary rocks. Remnant ore zones are composed of medium to coarse grained garnet and clinopyroxene, with accessory allanite, plagioclase, pyrrhotite, chalcopyrite and uraninite. Later retrograde alteration to chlorite, calcite, sericite, epidote and scapolite occurs. Fine grained uraninite is enclosed in allanite, and is partly replaced by metamict products and traces of galena. Elevated gamma-ray readings in the open pit correspond to exposed ore lenses, the former haul road and abandoned ore stockpiles (up to 16 mSv/year). Surficial oxidation of ore and adjacent sulphide-bearing calc-silicate rocks has led to contemporary precipitation of yellow, orange, green and white mineral efflorescences on the pit walls. Wallrock oxidation of reactive sulphides (mainly pyrrhotite breakdown) produces acidic solutions, however, buffering reactions of these fluids with gangue calc-silicates and carbonate phases prevent low pH conditions from developing. The open pit lake is approximately 40m deep and contains saline (0.15%) surface waters which are Ca-, S04-rich with elevated Cu, Fe, Mn, Ni, U and Zn at a pH of 6.11. Waste rock piles are up to 30m thick and have been covered by a thin veneer of benign waste. However, there are high radiation levels on several waste rock piles (up to 20 mSv/year) which were only partly covered or were ripped for seeding. Biogeochemical analyses indicate that Enneapogon lindleyanus (grass), Cymbopogon bombycinns (grass), Aerva javanica (kapok bush), Aristida longicollis (poaceae) and Acacia chisholmii (wattle) accumulate Cu, Pb, Zn, As, Ni, La, Ce, U, Th and Y at mined and disturbed areas compared to background sites. The former tailings dam has been rehabilitated using a multibarrier system (clay and waste rock layers) and gamma-ray measurements demonstrate an intact cover. However, seepage of acid (pH 5.86), saline (0.31%) waters occurs from the toe of the tailings dam into the evaporation ponds and local drainage system. Thus acid-producing reactions are not sufficiently buffered by acid-neutralising reactions within the tailings storage area. Seepage waters are Ca-, S04-rich with elevated Fe, Mn, Ni, U and Zn and precipitate abundant sulphate efflorescences and Fe-oxyhydroxide flocculants with elevated radiation and high As, La, Ce and U levels. Thus radionuclides are mobilised into surface seepage waters, yet are immediately coprecipitated with Fe flocculants. Soils were sampled widely throughout the area and soil analyses show that soils of mined and disturbed areas are enriched in Mn, Cu, Pb, Zn, As, Ni, La, Ce and U compared to background soils. Stream sediments accumulating below waste rock piles are enriched in Cu, Zn, As, Ni, La, Ce and U indicating active weathering and erosion of waste materials into the local drainage system. Cameron Creek is the main drainage from the mine area. Seepage of saline waters occurs from the tailings dam and evaporation ponds into Cameron Creek via surface and subsurface flows as indicated by salt-encrusted creek banks. This stream and its tributaries are usually active only in the wet season. At other times few permanent water holes exist, although there is some sustenance of flow from the tailings dam seepage. When sampled during the dry season, pools in the Cameron Creek system were shallow, saline (0.3-3%), alkaline (pH 8.3-8.6), and strongly enriched in S04 (up to 25.8g/L) and U (up to 5.1mg/L) but locally sustained fish and reeds in lower salinity regimes. The preliminary data suggest that elevated metal loadings of soils and sediments, radionuclide mobility, weathering and erosion of waste dumps and bioaccumulation of elements do not occur beyond the former mine site. Measured radiation levels are at or below Australian Radiation Protection Standards (20 mSv/year averaged over five consecutive years). In contrast, seepage of waters from the tailings storage area and evaporation ponds causes seasonal salinisation and impacts on the water quality of Cameron Creek during the dry season. Acknowledgments: Support for this project was given by the Australian Research Council, James Cook University and the Queensland Department of Mines and Energy. David Trezise (DME Qld), Geoff Bradford (EPA Qld) and Ross Costelloe are thanked for their assistance and support.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DEVELOPING A PROMOTIONS AND EDUCATION PROGRAM AT THE GSA Sonia Cousins and Misha Frankel Geological Society of Australia, Suite 706, 301 George St, Sydney, NSW, 2000 The Geological Society of Australia was established in 1952 with the initial objective to foster the publication of geological papers within a learned society. Its affairs were administered by the National Executive Committee, with Committees in each Division taking care of local matters. All positions were maintained entirely on a voluntary basis. The Society currently has around 3,100 members and the business office in Sydney is maintained by a full-time Business Manager, Promotions & Education Manager, and parttime Administrative Assistant. Active Divisions exist in every mainland State and Territory, each administering its own activities. Prior to 1998, limited promotional and educational projects were administered by the Business Manager. Also, some Divisions with Education Sub-Committees produced leaflets and booklets such as roadside geology guides. A Specialist Group in Geological Education was established in 1996 as a means of sharing information and ideas about geoscience education. Over the last two years, promotional and educational projects undertaken by the Society have expanded at a seemingly exponential rate. Major examples follow.

EARTHWORKS - Earth Science, Technology & Minerals Learning Centre The most significant project undertaken by the Society since February 1998 was the establishment of a new teaching and learning centre in Sydney. This initiative is a joint project between the Society, the NSW Minerals Council and the NSW Department of Mineral Resources. (See also Cousins et at. in this volume)

Public Lecture Series During Earth Science Week in October 1999, the GSA launched its inaugural Public Lecture Series, with the aim of presenting a stimulating geoscience topic with a solid basis in credible scientific research. We achieved a considerable level of public and media interest and intend to host similar events in future. For example, a major new event during July 2000 will be a Public Symposium at the 1 A G C .

Mentor Program A joint initiative of the GSA ACT Division and the Science Educators Association of the ACT, the Mentor Program (detailed in Lawrie et aL, also in this volume) involves professional earth scientists "mentoring" teachers and schools in their local area.

Media articles On an ad hoc basis, the Promotions and Education Manager has written earth science articles for Australasian Science magazine, Australia's only monthly science publication for a general readership. Many articles are based on research papers published in the Australian Journal of Earth Sciences.

Political lobbying The GSA is able to fulfil this role through its membership of the Australian Geoscience Council (AGC), which in turn is a member of the Federation of Australian Scientific and Technological Societies (FASTS), and by submitting reports and statements to various government reviews and inquiries.

Summary We need to rationalise our involvement and commitment to future educational and promotional projects to remain within the aims and objectives of a learned society. While we may not have the financial or time resources to produce glossy educational packages or products ourselves, we can utilise the strengths of our membership through programs such as mentoring, public lectures and political lobbying. We can also support, either financially or in kind, the production of educational packages and resources by other kindred organisations that have expertise in producing these. By working together constructively with like-minded organisations we can expand the reach of our projects and achieve our overall objectives "to promote, advance and support the earth sciences within the scientific and wider communities'".

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

EARTHWORKS - EARTH SCIENCE, TECHNOLOGY & MINERALS LEARNING CENTRE Sonia Cousins^ Susan Mills^ Simon Andrews^ John Leeks^ ^Geological Society of Australia, Suite 706, 301 George St, Sydney NSW 2000 ^NSW Minerals Council, PO Box A244, Sydney South NSW 1235 ^NSW Department of Mineral Resources, PO Box 536, St Leonards NSW 1590

EARTHWORKS is a joint initiative of the Geological Society of Australia, the NSW Minerals Council and the NSW Department of Mineral Resources. Now in its second year of operation, the centre offers structured, hands-on learning experiences for upper Primary students (aged 9-12 years) and their teachers. Students work in small groups to complete the activities that are grouped into a number of themes. Activity sheets and supporting material are sent to teachers prior to their visit so they can prepare for the session. An experienced teacher was recently employed to facilitate the sessions and manage the day-to-day operation of the centre. EARTHWORXS offers a range of stimulating, interactive science and technology activities. These demonstrate the importance of the earth sciences and how they relate to other physical sciences such as chemistry, physics and biology. Teachers can either focus in individual subject areas or integrate various themes into a broader study of earth science and minerals technology. The ten themes currently available are: Tracks Through Time (palaeontology & geological time) The Savage Earth (natural hazards) • The World Around Us (broad environmental theme) Geology: Shaping A Continent (plate tectonics, rocks & minerals) Gold Mines - Past & Present (investigating old and modem mining techniques) What is a Mineral Anyway? (minerals in everyday life) Finding Minerals (exploration methods) Mining Minerals (mining technology) Separating Minerals (mineral separation & processing techniques) Changing Energy (coal mining and use for energy generation) EARTHWORKS is run on a very small budget and has relied heavily on contributions of equipment and educational resources from the three partner organisations. It is located in a disused laboratory of the Department of Mineral Resources, providing an authentic setting for scientific activities and experiments. One of the benefits of this location is the existence of an extensive collection of rocks, mineral samples and fossils that can be accessed and utilised for special events in the Centre. A more practical benefit is its proximity to the demographic centre of metropolitan Sydney. We have conducted an ongoing evaluation of the Centre and its activities and have received greater than 85% positive feedback from teachers, students and parents. The NSW Department of Education & Training have supported the Centre throughout its development and operation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TIDAL RE SUSPENSION OF COHESIVE SEDIMENTS IN THE PARRAMATTA RIVER ESTUARY: IMPLICATIONS FOR CONTAMINANT DISPERSAL. Aaron Coutts-Smith School of Geosciences, The University of Sydney, Sydney, 2006, N.S.W., Australia

The longitudinal variations in physical properties of the Parramatta River estuary show the estuary is generally well-mixed due to low freshwater input. The exception to this rule is during small to moderate flood events when the estuary becomes stratified and it is hypothesised that gravitational circulation can develop to produce a zone of mud accumulation between 23.43 and 25.4km from the estuary mouth. Tide behaviour in the estuary is best described as a co-oscillating wave. The tide wave has a more progressive character at the mouth of the estuary, which changes to a standing wave at the estuary head. Shoaling of the tide wave overwhelms frictional dissipation and amplitudes grow toward the estuary head, where it is hypothesised frictional dissipation becomes significant and amplitudes start to decay. Under these conditions a maximum in tidal flood velocities occurs between Meadowbank and Silverwater leading to the formation of a turbidity maximum zone during periods of low freshwater input (between 23.43 and 25.4km from the estuary mouth). Measurements of turbulent fluctuations of the horizontal and vertical components of velocity and of suspended sediment at 50cm above the bed have been made in the upper reaches of the Parramatta River. These fluctuations displayed an intermittency akin to those of laboratory and estuarine experiments in which burst/sweep events were present. These events made the largest contribution to bed shear stress and were responsible for the re-suspension of sediment over a hydraulically smooth bed. The phase relationship between suspended sediment concentration (SSC) and tidal velocity clearly indicates the effect of threshold and erosion lags, features characteristic of cohesive sediment dynamics. The magnitude of Zn, Cu and Pb loadings on suspended sediment was investigated over a semi-diumal tide cycle. Of these three metals only Zn varied significantly and correlated positively with changes in SSC. It is hypothesised that the variation of Zn was due to changes in sorption upon re-suspension of bed sediments. Residual transport of sediment over the semi-diumal tide cycle in the upper estuary was towards the head of the estuary. Similarly, residual contaminant transport was also landward, thus making the upper estuary a sink for contaminants under low flow conditions

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EARTH-SCIENCE DATASETS FOR THE AUSTRALIAN CONTINENT ONLINE: THE AGCRC MAP-MAKER, OPEN CIS AND BEYOND Simon Cox AGCRC, CSIRO Exploration and Mining, PO Box 437, Nedlands, WA 6009. About 100 broad-scale datasets showing various aspects of the geology and geophysics of the Australian continent and surrounds are available as combinable map-layers on the AGCRC website http://www.agcrc.csiro.au/4dgm/. These have been collected from a variety of public agencies, including AGSO, AUSLIG and NOAA, and from more informal sources, such as university researchers and private sector organisations. The datasets can be viewed as maps which combine one "raster" or image layer with an unlimited number of layers containing line-work and point-locations. The location and scale of the map is user-adjustable. The value of the raster, and observations anchored to the point-locations, such as earthquakes, heatflow, and in-situ stress, can be retrieved with a mouse-click. Profiles of values from rasterlayers can be displayed. While not a fully featured GIS, the system provides useful "backdrops" to more detailed studies, and the ability to display a variety of themes simultaneously allows some synthetic investigations to be carried out as-is. For example, comparing surface heatflow with crustal thickness and crustal elements reveals some interesting contrasts between tectonic units. AGCRC has permission from the datasets owners to provide display versions of the data as images on the website, which can be downloaded by users, but in general does not redistribute the raw data. In some cases the owner of the dataset maintains a more detailed version than displayed by AGCRC. For most datasets a link to the original source is provided via the map-legend, which provides a full description of each dataset. Standard Dublin Core metadata fields are also provided in these descriptions for harvesting by metadataaware web-indexers. The datasets are normally delivered as gif format images in a webpage. We also provide an alternative interface using the GETMAP protocol developed as part of the Web Mapping Testbed by the Open GIS Consortium (OGC). GETMAP is a vendor-neutral interface, through which a map-server delivers the data in standard formats regardless of the underlying database technology used. The client is then free to combine maps from several distributed servers. The AGCRC map-server will be migrated so that it also uses the GETMAP interface internally. Under this regime it will not be necessary to have local copies of datasets that are provided on other GETMAP servers, since those could be retrieved at runtime. For example, datasets for which AGSO is the custodian are expected to be available at higher resolution and in more current versions from an AGSO map-server. The OGC initiative is part of a broader project to facilitate easy interchange of geographic information. Underlying this is a set of standards developed by ISO which, as well as defining the basic geometric elements, are establishing methods for communities to define domain-specific profiles and vocabularies for applications based on geospatial data. Thus, the mineral exploration sector could define a profile for geological entities such as rock units, structural features, drill-holes, etc, which can then be encoded according to the ISO/OGC standards allowing data to be shared by a variety of application software. All of the major GIS vendors support the OGC. Widespread use of such standards would thus encourage routine reuse of exploration data. The statutory reporting requirements under the Mines Acts are a possible lever in moving to such a desirable state of affairs. Acknowledgement: Work reported here was conducted as part of the Australian Geodynamics Cooperative Research Centre and this paper is published with the permission of the Director, AGCRC.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

FLUID-DRIVEN FAULTING PROCESSES IN AN INTRUSIVE-RELATED HYDROTHERMAL SYSTEM, PORGERA, PAPUA NEW GUINEA Stephen F Cox' and Stuart M Munroe^ ^Research School of Earth Sciences and Department of Geology, The Australian National University, Canberra, ACT 0200, Australia; ^SRK Consulting, PO Box 422, Milsons Point, NSW 2061, Australia The Roamane Fault Zone (RFZ) formed within an active hydrothermal system during cooling of the 6 Ma Porgera Intrusive Complex. Faulting occurred at temperatures around 200°C, and at a depth of approximately 3 km. The fault and its associated fracture systems localised the deposition of more than 350 t of Au. The extent of hydrothermal alteration associated with the RFZ indicates the structure strongly focused fluid flow above the intrusive complex. The steeply SSE-dipping RFZ intitiated by dextral strike-slip movement, but underwent a period of normal slip prior to reverting to a dextral strike-slip regime in the waning stages of the hydrothermal system. The fault is continuous along strike for at least one kilometre; the net slip has not been determined. The principal displacement zone (pdz) of the RFZ comprises a core, up to 2 m wide, containing multiple generations of massive to banded cataclasite, wear breccia, and minor foliated cataclasite, injection cataclasite and veins. Localised polished slip surfaces are also present. A 15m wide damage zone of coarse implosion breccia is present in the fault footwall. The intensity of brecciation decreases progressively away from the pdz. Repeated episodes of brecciation and multiple generations of cataclasite indicate episodic and possibly seismic slip in the RFZ. Foliated cataclasites are interpreted to have formed during aseismic creep. Early growth of the RFZ was associated with formation of steeply-dipping, E-W trending breccia-veins which splay predominantly from the pdz into the fault footwall. These structures are dilational normal faults, with lengths up to 200 m and net slips up to 30 cm. They are interpeted as wing cracks formed in association with dextral slip on the RFZ. Internal structures in breccia-veins indicate formation by wall-rock implosion into dilating fractures, driven either by concurrent slip on the pdz, or by rapid, aftershock-related fracture propagation from the tip of slip patches on the pdz. Localised suprahydrostatic fluid pressures are interpreted to be a major factor initiating growth of the RFZ in an active hydrothermal system. Competition between episodic, slip-induced porosity-creation and interseismic hydrothermal pore sealing promoted fluctuations in fault zone permeability thoughout the slip history. Associated repeated fluctuations in fluid pressure and shear stress must have influenced fault shear strength and rupture nucleation. Co-seismic dilatancy on the pdz, and possibly within the large footwall wing cracks, was potentially important in controlling rupture arrest.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE LIMITS OF CHEMISTRY IN THE SUPERGENE ENVIRONMENT: CHROMIUM(VI) SPECIES AND RELATED MINERALS Martin J. Crane, Peter Leverett, Lindsay R. Shaddick and Peter A. Williams School of Science, University of Western Sydney, PO Box 10 Kingswood, NSW 2747

Surprisingly extreme chemical conditions can be achieved in the supergene zone. Acid environments associated with oxidizing sulfides can reach pH values less than zero and redox potentials can reach, and sometimes exceed, the limits of the stability of water. As far as the latter is concerned, the existence of oxyanions such as iodate (lOa"), periodate and perchlorate (104", C104"), chromate and dichromate (Cr04^', Cr207^') and nitrate (NOs") provides compelling evidence for chemistry at extreme limits in the natural environment. Their preservation, for the most part, depends on a lack of substrates, especially organics, which easily reduce them to lower oxidation states. The chemistry involved in the formation of these highly oxidized species is well understood and is developed using an equilibrium approach (Williams, 1990). Desert environments, with a paucity of organic matter, provide situations where these species can persist. Perhaps the most spectacular known examples concern the caliche deposits of the Atacama Desert of northern Chile, where all of the above anions form various species in association and where complex Cu(II)- and Pb(II)- IO3" mineralization is reported from oxidized sulfide ores. Chromate is a powerful oxidant but forms a number of secondary minerals. The most stunning of these is crocoite (PbCr04), an abundant species in the mines of the Dundas region and the Magnet mine, northwestern Tasmania. Here chromium is derived principally from the weathering of stichtite (Mg6Cr2C03(0H)i6.4H20) in ultramafic rocks and is maintained in solution as Cr(VI) by the oxidizing influence of secondary manganese oxides. Crocoite is developed as a result of the interaction of transported chromate ions with oxidizing, fault-hosted base metal orebodies. Apart from crocoite, which buffers Pb(II) and Cr(VI) dispersion in groundwaters, chromate enters the lattice of more familiar secondary lead minerals such as anglesite (PbS04) and cerussite (PbCOs). In other settings, chromate is buffered by the crystallization of alkaline earth, transition and other heavy metal-bearing phases, all of which are involved in complex solid-solution series. Examples include hashemite (BaCr04), chromatite (CaCr04), vauquelinite (CuPb2(Cr04)(P04)0H), embreyite (Pb5(Cr04)2(P04h.H20) and iranite-hemihedrite (CuPbio(Cr04)6(Si04)2(OH)2ZnPbio(Cr04)6(Si04)2F2). The chemistry of some of these systems is described. Adequate models for the transport of chromium in groundwaters require the incorporation of such species, both in natural settings and polluted environments. Origins of highly oxidized assemblages are discussed in some detail, including an assessment of inorganic and microbiological influences. Examples are drawn from a number of overseas deposits, Cr-rich ores in Tasmania, South Australia and Western Australia, and highly unusual base metal-nitrate mineralization in oxidized ores from the Mt Isa Block, northwest Queensland. Reference

WILLIAMS, p.A., 1990. Oxide Zone Geochemistry. Ellis Horwood, Chichester.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CAMBRIAN ARC EVOLUTION ALONG THE SE GONDWANA ACTIVE MARGIN: A TASMANIA-NEW ZEALAND PERSPETIVE 1 2 Anthony J. Crawford and Carsten Munker ^ Centre for Ore Deposit Research, School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Australia. ^ Zentrallabor fur Geochronologie, Universitat MUnster, Corrensstr. 24, 48149 Munster, Germany

Belts of Cambrian rocks with arc affinities in eastern Australia, Tasmania, New Zealand and Antarctica are part of a single convergent margin, active over 30-40 Ma from the latest Early Cambrian to the Late Cambrian. Two of the most complete sequences are exposed in western Tasmania and the northern South Island of New Zealand (Takaka Terrane), and are used here as a basis for correlation between these locations now ca. 2000 km apart. Throughout the Middle Cambrian, magmatism in these two regions, and in the Lachlan Fold Belt (SEAustralia) and the Bowers Terrane (Antarctica), is represented by intraoceanic arc and back-arc sequences. In the mid Middle Cambrian, collision of these arc segments with the proto-Gondwana continent is recorded by obducted boninite-bearing ophiolites in Tasmania and SE Australia. Post-collisional magmatism of latest Middle to early Late Cambrian age (e.g. Mount Read Volcanics of Tasmania, Stavely Volcanic Complex of Victoria) terminates convergent tectonics in SE Australia and Tasmania. In contrast, no post-collisional volcanism is known from the Bowers Terrane in Antarctica and from New Zealand. In the Bowers Terrane, subduction-related igneous activity ends in the Latest Middle Cambrian (Glasgow Volcanics), and in New Zealand an intra-oceanic arc setting continues into the Late Cambrian (Devil River Volcanics). In Antarctica, Cambrian igneous activity in the Wilson Terrane and Transantarctic Mountains (Granite Harbour Intrusives) formed in an active continental margin setting and lasted through the Middle and Late Cambrian. This suggests the Granite Harbour Intrusives to be the lateral continuation on continental basement of the Bowers and Takaka Terrane arcs. A change in collision style from continental against oceanic crust (SE Australia, Transantarctic Mountains, Wilson Terrane, Tasmania) to oceanic against oceanic crust (Lachlan Fold Belt, New Zealand/Antarctica) may explain these differences between the Australian/Tasmanian and New Zealand/Antarctic arc segments. This in turn, may result from a change in subduction polarity along the arc chain, suggested by structural features, sedimentation patterns and isotope systematics. In the SE Australian and Tasmanian arc segments, the proto-Gondwana plate subducted beneath the Pacific plate, whereas subduction in the Antarctic and New Zealand segments was of opposite polarity. Common to most Cambrian fragments in SE proto-Gondwana is the tectonic overprint by the RossDelamerian orogeny from the Middle Cambrian to Early Ordovician, thus paleogeographically linking all the fragments by the end of the Cambrian. The overall synchroneity of tectonomagmatic events in the SE proto-Gondwana fragments, however, suggests that they were always part of a single arc system. This is further supported by faunal and isotopic constraints. No exotic fragments, e.g. originating from Laurentia or South America, both located nearby in Cambrian time, have been identified. The presence of early Cambrian intra-oceanic rocks along the SE proto-Gondwana margin suggests separation of Laurentia from Gondwana prior to the Early Cambrian.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SO HOW DOES YOUR PEAT GROW? Michael Creech Powercoal Pty Ltd, PO B o x 1000 Toronto 2283.

A database consisting of 1200 boreholes, spanning the entire Newcastle Coalfield (NC) provides a unique opportunity to travel back in time to when the Great Northern and Fassifem seams were accumulating. In addition, correlation of numerous tonstein bands within the lower Fassifem seam allows the sequential viewing of several plies of the seam accumulating through time. Most obvious is a several kilometre wide fluvial channel system migrating to the east, with peat formation continuing either side in a series of isolated areas. There is also a more widespread split which occurs midway in the seam across the southern half of the coalfield. The numerous tonstein bands within the Fassifem seam display remarkable continuity across the coalfield, and can be mapped across the entire coalfield using 5mm isopachs. These bands generally varying in thickness by +/ - 5cm reflecting a peat environment which is either underwater (protecting the ash from redistribution by rainfall and mnoff) or has a topographic relief of only 20 to 30cm across the entire coalfield (assuming a 2-3:1 compaction ratio). Recent drilling at Broke by the DMR, and at Denman by Powercoal have identified the same relationships between seams and tonsteins over 100 km from the NC. The Awaba Tuff, a thick volcanoclastic deposit, dominates the interval between the Fassifem and Great Northem seams. This unit varies considerably in thickness up to 30 metres and averaging 8 metres. This variation could be due to either compaction of the underlying peat, proximity to the source or variation in topographic relief Although thicker portions do occur over the best developed peat, differential compaction alone cannot account for the observed distribution. Similarly, the variation in thickness cannot be accounted for by proximity to the source. Redistribution due to topographic relief (of the order lO's of metres) is inconsistent with the preservation of the tonsteins unless both the tonsteins and the Awaba Tuff were deposited in water bodies of variable depth. This interpretation is also consistent with sedimentary features indicating the Awaba Tuff was laid down in deep brackish water. These features indicate widespread peat and tuff accumulation in a subaqueous environment and challenge conventional views on coal formation.The author would like to thank the Dept. Mineral Resources, Powercoal, COAL and Oceanic Coal for access to this data.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE WOLLOMBI COAL MEASURES - REFUGEES FROM NEWCASTLE Michael Creech Powercoal Pty Ltd, PO Box 1000 ,Toronto, New South Wales, 2283.

The Wollombi Coal Measures (WCM) are the uppermost coal bearing sequence within the Hunter Valley (HV) and to date have been considered uneconomic due to their banded nature and poor continuity. They have long been recognised as stratigraphically equivalent to the better known Newcastle Coal Measures (NCM) from east of the Lochinvar Anticline, however this association has not been fully appreciated. Recent exploration by Powercoal at the western extremity of the Hunter Valley has identified a large resource of both the Great Northern and Fasssifem seams 100km west of their Newcastle home. Correlation of NCM units across the HV has implications for exploration of the WCM and also current ideas on coal formation in the Sydney Basin. By utilising known NCM relationships when assessing the WCM, it can be deduced that the economic targets are restricted to the upper seams (including the Great Northern and Fassifem) and the lower West Borehole seam (WBH) and it's equivalents. All but a hand full of bores drilled in the HV have intersected the upper seams of the WCM (requiring collaring of the hole in Triassic strata) or economic sections of the lower seams which are likely to be more restricted in their development. A low interest in the WCM is understandable if one imagines correlating widely spaced boreholes in the Newcastle Coalfield which are all collared below the upper seams and where the WBH is not well developed. The intervening seams (Pilot to Fern Valley seams) are all banded, high ash, laterally discontinuous and of limited prospectivity. The identification of the Anvil Hill Exploration Area (north of Denman) as prospective for Powercoal came as a result of long distance correlations of the Fassifem seam across the HV by the author. The Fassifem seam has a distinctive profile composed of numerous tonsteins and coal plies of highly variable ash content. The continuity of this profile including thin carbonaceous shales, across thousands of square kilometres, suggests a subaqueous environment of coal formation (a series of lakes) with a laterally constant flux of airbome dust and volcanic ash which fluctuates with time. The preservation and continuity of the numerous thin volcanic ash falls also suggests accumulation underwater, receiving protection from redistribution by rainfall and surface mnoff Maceral and chemical evidence suggests that they accumulated in the normal coal forming environment, there being no evidence of flooding immediately above or below these bands. Each tonstein is also laterally continuous and does not crosscut individual coal plies over long distances.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MEGACLAST EMPLACEMENT AND MOVEMENT ON SHORE PLATFORMS: AN INDEX OF EXTREME EVENTS AT THE LAND-SEA INTERFACE. Keith A W C r o o k \ Riko Nooimets^ E Anne Felton^ Louise Minty^ and Dennis Franklin' 'Hawaii Undersea Research Laboratory, University of Hawai i, Honolulu HI 96822 USA ^Department o f Geology and Geochemistry, University of Stockholm, 106 91 Stockholm, Sweden. ^Hawaii Institute of Geophysics and Planetology, University o f Hawai i, Honolulu HI 96822 U S A ''Bureau of Meterology, GPO Box 1289K, Melbourne VIC 3001, Australia

Extreme events are an important feature of environmental change and landscape evolution, because the energy expended is likely leave a physical record. Being located at the landsea interface, shorelines are particularly important repositories of such records, which include the emplacement and movement of megaclasts on rocky shoreline platforms. Two distinct kinds of extreme events are known to be responsible for megaclast emplacement and movement: giant storm waves, from nearby or distant storms; and tsunami, of local or trans-oceanic origin. Although megaclast-derived criteria for distinguishing between these two emplacement mechanisms have yet to be established, understanding the physical and hydrodynamic differences between tsunami and storm waves may provide the needed insights. Both storm and tsunami wave emplacement mechanisms of megaclasts onto shorelines are important from the perspective of natural disaster preparedness. However, for assessing the consequences of environmental change, the record of storm wave effects is particularly important and needs to be distinguished from tsunami wave effects. This task is just beginning. Here we describe evidence from a shoreline terrace near Sunset Beach, O'ahu, Hawai'i, for the emplacement and movement of megaclasts up to 90 tonnes by storm waves and by tsunami between 1927 and 1996; and for emplacement on the rock platform at Ben Buckler, north of Bondi, NSW, of the 235 tonne megaclast "Mermaids Rock" by a storm wave in 1912. These observations, tied to known events, provide valuable qualitative constraints on the physics of the entrainment processes. For example, the orientation and packing of megaclasts is of equal importance or even outweighs the size of the megaclasts as an indicator of wave power at the shoreline. Quantitative investigations of these processes are now under way.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AN INTERPRETATION OF THE EASTERN LACHLAN REGION USING THE PREDICT GEOLOGICAL INFORMATION SYSTEM A.J. Cross, R.J. Korsch, O.L. Raymond, P. Lyons and A.W. Mills Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601 PREDICT is a WWW-based geological information system that has been designed to provide key information about the nature, timing and distribution of tectonic events that have shaped and characterised terranes and sedimentary basins which can be host to mineral resources and petroleum systems. PREDICT provides this information via html forms and active graphical outputs that represent a dynamic interaction with an Oracle database. Information delivered in this fashion has the potential to greatly support the decisions made by exploration managers and explorers. The objectives of the PREDICT system are to: • provide access to the critical information for decisions • provide a quantitative estimate of the quality and adequacy of the work • document the reasoning behind the interpretation • document the source of the information • document the nature, timing and character of the petroleum and mineral systems • document the impact of geological events and processes on resource distribution, timing and character PREDICT has been developed by the AGCRC and its information is stored in a series of Oracle relational database tables that reside on the Oracle server 'mica' at the Australian Geological Survey Organisation (AGSO). Data entry and scrutiny of information in PREDICT is either through dynamically generated html forms, 'clickable' maps or active graphical outputs. PREDICT can produce active graphical time-space plots that display the tectonostratigraphic, time-event and metallogenic character of a geological region. By comparing the time-space histories of any number of geological regions throughout Australia, it may be possible to improve our understanding of the tectonic development of Australia and also Australia's accumulations of world class resources, which would in turn, improve our exploration models. Geological syntheses in PREDICT are controlled by a hierarchical system of geological information management, at the apex of which is the geological province. Provinces are then subdivided into tectonostratigraphic terranes or basins, these in turn, can be further subdivided into lithological terrane or basin units. The character and timing of magmatic, metamorphic, deformational and mineralising events that characterise a particular terrane or basin are also inserted into the PREDICT geological information system. PREDICT interpretations rest upon relevant literature, knowledge from recognised experts for a given region, and, importantly from primary data housed in AGSO databases such as OZROX, OZCHEM, OZCHRON, OZMIN, STRATNAMES, GEODX and STRATDAT. These databases can be directly accessed using PREDICT. PREDICT geological syntheses are therefore stratigraphic and time-event-based interpretations of geological regions that can reflect a project or an individuals interpretation or current understanding of the geological development of a given region. Any number of alternative interpretations for a given geological region can also be stored in PREDICT and latter updated if required. A PREDICT interpretation for the Eastern Lachlan Orogen in New South Wales has been carried out as a part of developing an explorationn model for this region by the AGCRC. The Eastern Lachlan region, which is covered by the Nyngan, Gilgandra, Narromine, Dubbo, Forbes, Bathurst, Cootamundra, and Goulbum sheets, was divided into tectonostratigraphic packages of rock that can be correlated with major Lachlan tectonic episodes. The character of the magmatic, metamorphic, deformational, and mineralising events which have affected each terrane was also inserted into PREDICT. From this synthesis, an active graphical time-space plot can be easily produced that clearly represents the AGCRC interpretation for the geological development for the Eastern Lachlan region.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15""Australian Geological Convention, Sydney, July 2000

(U-TH)/HE APATITE AGES FROM THE TARANAKI BASIN, NEW ZEALAND IMPLICATIONS FOR COOLING AND DENUDATION IN THE PLIOCENE. P.V.Crowhurst'. P.JJ. Kamp^ and P.P. Green^ ' CSIRO Petroleum Resources, PO Box 136, North Ryde, NSW, 1670. ^ Department of Earth Sciences, The University of Waikato, Private Bag 3 1 05, Hamilton, New Zealand. ^ Geotrack International Pty Ltd., 37 Melville Rd., Brunswick West, Victoria, 3055.

The Taranaki basin, located in central-western New Zealand, contains a predominantly terrigenous Late Cretaceous to Holocene terrestrial to marine succession, generally 3-5 km thick and with an estimated maximum thickness of 8 km. This basin is of particular interest because it provides most of New Zealand's commercial hydrocarbon discoveries. Previously published apatite fission track analysis of four well sections in the Taranaki basin indicated that in the southern part cooling from elevated palacotemperatures was effected by the initiation of Late Miocene uplift and erosion ranging from 1-3 km of section (Kamp and Green, 1990). However, the timing of when the denudation ended is poorly defined, being constrained by poorly dated Quaternary sediments overlying mid Miocene deposits. (U-Th)/He apatite age analysis has been applied initially to samples from one well section (1 Fresne) in the southern part of the Taranaki basin. The ages indicate cooling below 75°C occurred in the upper section in the early Pliocene, whereas deeper samples record progressively younger ages as they currently reside in the He partial retention zone (-4085°C; Wolf et al., 1998). This interpretation is based on the assumption that the samples have resided at the current down hole temperatures since the Pliocene. These data possibly imply that cooling and denudation of the 1 Fresne well section extended beyond the end of the Late Miocene and into the Pliocene. This work is to be extended to other well sections in the basin that intersect other inversion structures. The implications of this work for the hydrocarbon prospectivity of the basin is that it better defines the timing of the formation of the potential trapping structures in relation to the timing of maturation. References KAMP, P.J.J, and GREEN, P.P. 1990. Thennal and tectonic history of selected Taranaki Basin (New Zealand) wells assessed by apatite fission track analysis. The American Association of Petroleum Geologists Bulletin, 74,No.9, 1401-1419. WOLF, R.A., FARLEY, K.A. and KASS, D.M. 1998. Modelling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer. Chemical Geology, 148,105-114.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SIMULTANEOUS MELTING AND FRACTURING OF THE LOWER CRUST, FIORDLAND, NEW ZEALAND N.R. Daczko, G.L. Clarke, and K.A. Klepeis. Division of Geology and Geophysics, School of Geosciences, University of Sydney, NSW 2006 Australia.

ABSTRACT Granulite facies gabbroic and dioritic gneiss in the Pembroke Valley, Milford Sound, New Zealand, are cut by vertical garnet reaction zones in rectilinear patterns. In gabbroic gneiss, narrow dykes of anorthositic leucosome are surrounded by fine-grained garnet granulite that patchily recrystallized the host-two-pyroxene-amphibole granulite at conditions of T>750''C and P=14 kbar. Major and trace element whole rock geochemical data indicate that the recrystallization was essentially isochemical. The garnet reaction zones cut contacts between the gabbroic gneiss and dioritic gneiss, but change at the contacts to zones with a septum of coarse-grained garnet surrounded by anorthositic leucosome. The dioritic gneiss additionally contains isolated garnet grains enclosed by leucosome, and short planar trains of garnet grains linked by restricted leucosome. Partial melting of the dioritic gneiss, mostly controlled by biotite breakdown at waterundersaturated conditions, is inferred to have generated the leucosomes. The form of the leucosomes is consistent with melt segregation and transport having been aided by fracture propagation; limited retrogression suggests that there was considerable melt escape. Dyking and melt escape is inferred to have propagated fractures into the gabbroic gneiss, where the dioritic gneiss-sourced melt scavenged water from surrounding rocks and induced the limited recrystallization to garnet granulite.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AUSTRALIAN-WIDE TRANSCURRENT FAULTS AND THEIR ROLE IN CREATION OF THE SEDIMENTARY BASINS Falih M. Daim and Paul G. Lennox School of Geology, The University of New South Wales, P.O. Box 1 Sydney 2052, Australia

Geological and geophysical evidence show that master west-northwest trending transcurrent faults expressed at the surface as lineaments subdivide Australia. These faults have been active intermittently since the Proterozoic (Daim & Lennox, 1998; Glen & Walshe, 1999). Another sets of faults, which are orthogonal to the transcurrent faults, are evident too. The orthogonal faults are confined by the transcurrent faults. Differential mobility (due to subduction and/or rifting rates variation) of the different blocks defined by these lineaments created decompression at the lower crust along the curvilinear transcurrent faults. The block that is occupied, at its northwestern end, by the Canning Basin was more mobile than the adjacent blocks. Because of the curvilinear shape of the bordering transcurrent faults, transtension and transpression are expected when these blocks were in motion. Major extension at a releasing bend triggers melting of the lower crust, leading to mantle upwelling. This is represented by the volcanic rocks along the transcurrent faults and associated pull-apart rhomb-shaped and elongated basins. A further decompression enhances lateral movement of the lower crustal material to balance the pressure from the nearby blocks (Daim & Lennox, 1998). This model is being tested in the Northern Carnarvon Basin and the preliminary results are encouraging. Two types of sedimentary basins are recognized: the pull-apart transcurrent basins (e.g. Canning, Amadeus, Cooper and Gunnedah Basins) and the V-shaped orthogonal basins (e.g. Carnarvon, Perth, Wiso, Officer, Darling, Bowen, Sydney and Clarence-Moreton Basins). The creation of the pull-apart transcurrent basins is well known and related directly to simple extension at a releasing bend, due to differential movement of the blocks and sub-blocks. Whereas the creation of the orthogonal basins were by multi-stage ductile movement of the lower crustal material, towards the decompression zones along the bounding faults of the more mobile block. The lower crustal ductile movement was associated with wrenching in the overlying brittle cover, represented by sinistral and dextral faults, which are orthogonal in trend to the major transcurrent faults. The transcurrent and orthogonal faults appear to determine the basins architecture and control their hydrocarbon habitat. References DAIM F. M. & LENNOX P. G. 1998. A new tectonic model for the evolution of the Northern Carnarvon Basin, Western Australia. In: Purcell P. G. & Purcell R. R. eds. The Sedimentary Basins of Western Australia 2: Proceedings of Petroleum Exploration Society of Australia Symposium, Perth, 1998, pp. 435-446. PES A Limited, WA Branch, Perth. GLEN R. A. & WALSHE J. L. 1999. Cross-structures in the Lachlan Orogen: the Lachlan Transverse Zone example. Australian Journal of Earth Sciences 46, 641-658.

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GLOBAL PALEOTECTONICS: RECONSTRUCTING A CREDIBLE SUPERCONTINENT Ian W. D. Dalziel Institute for Geophysics, University of Texas at Austin 44112 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759-8500 Also affiliated with the Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia

Early attempts to reconstruct pre-Pangea supercontinents were based on poorly constrained paleomagnetic and geochronologic data and on intracratonic stratigraphic correlations of dubious paleotectonic significance. During the 1980's, however, thermal subsidence analysis indicated the likely contemporeneity of late Precambrian rift-drift transitions along the margins of several cratons, apparently demanding the breakout of Laurentia from within a Neoproterozoic supercontinent. A scenario has now been proposed involving an early Neoproterozoic supercontinent, Rodinia, and a latest Neoproterozoic supercontinent, Pannotia, before and after the opening of the Pacific Ocean basin and Gondwanaland amalgamation, and prior to the opening of the Early Paleozoic lapetus Ocean basin by Laurentia-Gondwanaland separation (for review, see Dalziel, Geol. Soc. America Bulletin, 1997). With the establishment of IGCP Project #440 that seeks to 'reconstruct' Rodinia, it is timely to review the principles of global paleotectonics on which a reconstruction would rest. I do so with reference to the reconstruction of Late Paleozoic - early Mesozoic Pangea, now accurately reconstructed on the basis of seafloor spreading data unavailable for earlier cratonic assemblages. A critique of the Rodinia-Parmotia, Pangea scenario points the way for future research in a topic fundamental to an integrated time-space understanding of Earth history.

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PLUMES, OROGENESIS, AND SUPERCONTINENTAL FRAGMENTATION I.W.D. Dalziel^L.A. Lawver^ and J.B. Murphy^ institute for Geophysics, University of Texas at Austin, 4412 Spicewood Springs Road, Austin, Texas, USA 78759-8500 ^Also affiliated with Department of Geological Sciences, University of Texas at Austin, and Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia ^Department of Geology, St Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada

A time-space relationship between large igneous provinces (LIPS), present day hot spots, and the fragmentation of Pangea has been documented over several decades, but the cause of fragmentation has remained elusive. LIPS are regarded either as the result of impingement of a mantle plume on the base of the lithosphere, or as the initial products of adiabatic decompression melting of anomalously hot mantle. Do LIPS therefore constitute evidence of an active role for plumes from the deep mantle in supercontinental fragmentation, or are they merely the first indications of a large-scale but near-surface tectonic process? Two long recognized and enigmatic orogenic events may offer a solution to this geologically important 'chicken or egg' conundrum. The reconstructed early Mesozoic Gondwanide fold beh of South America, southern Africa, and Antarctica, could have resulted from 'plume-modified orogeny', flattening of a downgoing lithospheric slab due to the buoyancy of a plume rising beneath a continental margin subduction zone. If so, the --180 Ma Karoo and Ferrar LIPS associated with the opening of the ocean basin between East and West Gondwanaland at -165 Ma resulted from impingement of this plume and are unrelated to the thermal insulation of the shallow mantle beneath Gondwanaland. It would then follow that the plume itself played an active, possibly critical, role in the initial breakup of the supercontinent. The Late Paleozoic "Ancestral Rockies" deformation in the southwestern United States could be yet another example of orogeny driven by a plume that initiated the break-up of Pangea approximately 15 my earlier in the Central Atlantic region.

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A NEW LOOK AT THE ELURA DEPOSIT, NSW. Vladimir David, Paul Leevers and Angela Lorrigan Pasminco Elura Mine, Cobar, NSW Introduction The Elura deposit is located in western NSW, Australia, 43km NNW of the town of Cobar. The pre-mining resource was 45Mt of ore at 8.5%Zn, 5.3%Pb, and 69ppm Ag, nearly half of which has been extracted since 1983. The ore system consists of steep, pipe-like accumulations of massive sulphide occurring in 6 pods aligned on a NNW trend. The main pipe extends 900m from surface. The five others, to the north west, extend to the same depth from 450m below surface. In recent years, information available from deep drilling has seen an advancement in the understanding of the ore body. This renaissance of Elura geology has naturally been accompanied by a measure of controversy. The current theories will be discussed in this paper. Regional Setting Elura is close to the north west boundary of the Cobar Basin, one of several intra-continental basins in the central part of the Lachlan Fold Belt. The basin architecture is thought to be controlled by north west - trending, extensional listric faults and internal north east - trending transfer faults developed in a transtensional regime during the late Silurian.(Glen 1990). Stratigraphy In the northern part of the basin (Cobar Trough) sedimentation progressively advanced from basal conglomerates through sandstone and siltstone into deep water turbidite sequences. The stratigraphy in the vicinity of the Elura deposit is as follows: KOPYJE GROUP (Brookong Formation) Elura Limestone Interpreted as reef limestone, this unit directly underlies the deposit .It is comprised of interbedded calcarenite and packstone containing crinoids and bryzoan (Carolan, 1999) .Laterally it passes into fine black muds, containing blocks of limestone, these are interpreted as fore and back reef deposits. Massive Mudstone Massive, dark grey mudstone with occasional crinoid fragments. This unit has been intersected in 2 holes and occurs to the west of the Elura deposit and the Elura Limestone. Sand/Siltstone This unit is the lateral equivalent of the Elura Limestone and occurs on the eastern side of the deposit, becoming more sand-rich towards the east. It consists of heavily bioturbated, fossiliferous siltstones and sandstones, interpreted as having been deposited in a shallow-water shelf environment. NURRI GROUP (CSA Siltstone) The CSA Siltstone of Schmidt(1980). hosts the Elura ore body. In the mine area it can be sub-divided into three units. Transitional Siltstone Overlying the units of the Kopyje Group to the east of the Elura Mine, this unit is predominantly massive to poorly bedded siltstone containing flecks of chlorite, sub-angular and sub-rounded mudstone clasts and scattered fossil fragments near the base of the unit. Siliceous Sandstone This unit occurs at the base of the ore body, overlying the limestone reef formation. It is a massive, fine-medium grained, quartz-rich sandstone which is usually silicified. Classic CSA Siltstone This is a finely-bedded turbidite sequence, typically 70% siltstone and 30% sandstone (Schmidt, 1980). Overall the sequence fines upward from the coarser siliceous sandstones below. Structure The deposit occurs vertically above the NNW-trending Elura Limestone, on the northern side of a NE-trending first order transform structure (the Buckwaroon Fault). The limestone is thought to have accumulated on a topographic high on the rotated, footwall block of a NNW-striking, syn-sedimentary fault (Coller, 1998). Re-activation of basin-parallel(NNW-trending) syn-sedimentary structures during Glen's D1 and D2 appears to have resulted in the development of splayed, fan structures, deflected against the limestone barrier and terminating in anticlines at the fault leading edges. Elura occurs within one of these anticlines. Sinistral movement on the Buckwaroon structure also appears to have occurred at this time, resulting in additional dilatancy at Elura. The deposit itself can be divided into 3 strain fields: • The contraction field extends into limestone beneath the Elura deposit and locally into turbidite units. The neutral withdilatancy no volume change is distributed immediately beneath the deposit and represents the roots of mmeralisation. •• The zone of field positive hosts the massive sulphides. In the upper areas of the mine, bedding within the CSA Siltstone wraps around the ore pods and folds plunge outwards (radially) from the ore. The intensity of folding decreases away from the ore body. There is evidence from drilling for a set of NNE-trending structures that displace the limestone and exert control on the spatial distnbution of the mineralisation at depth. Genesis The two different schools of thought that have emerged with regard to the genesis of the deposit, are summarised below: Model 1 • The limestone reef complex developed on a growth fault and extended laterally into shallow water sediments (Kopyje Group) • Extension on the growth fault and within the basin caused slumping and deposition of the CSA Siltstone over the Kopyje Group • Diagenesis caused migration of carbonates into overlying sediments. This secondary carbonate was concentrated above the growth fault. • Basin inversion and activation of the Buckwaroon fault lead to generation of the NNE trending faults in the limestone. The intersection of these faults with the growth fault acted as focal points for mineralising fluids and enabled replacement of secondary carbonate with mineralising fluids. • Dilation within the mineralising envelope caused migration of the more ductile sulphides upwards into the zone of greatest dilation. This penetration of the ore mass into the turbidites formed the drape folds at the top of the deposit. Model 2 • Dilatancy was developed at the leading edge of the fault fan during basin inversion. • Dilatancy was enhanced by increasing sinistral movement on the Buckwaroon Fault. • A tension field developed at the base of the ore body with an estimated 60% shortening. • In the neutral strain field the passage of fluids as a result of seismic pumping resulted in cyclic fluid over-pressuring and hydrothermal vein development. • In the dilatancy field a conical-shaped fracture developed as a result of strike-slip displacement on reverse faults(after Ramsay and Hubert, 1987). • • •

Operation of fluid valves (after Sibson, 1999) enabled cyclical entry of fluids into the zone. Fluids deposited the sulphide assemblages, resulting in concentrically zoned, antitaxial replacement and cavity-infill mineralisation. Post-mineralisation movement on the Buckwaroon structure rotated the ore and mechanically lifted the ore body up a relatively small distance, making an apparent spiral penetrative dome. References

CAROLAN, P, 1999 Geology of shelf strata and carbonate mineralisation at the Elura mine Cobar, NSW. BSc(Hons)thesis, University of Woolongong (unpubl). COLLER, D. 1999. Elura Ore Body Controls and Regional Exploration Review Final Report, April 1999. Prepared: by Maptec for Pasminco Exploration. GLEN, R.A. 1985. Basement control on the deformation of cover basins: an example from the Cobar district in the Lachlan Fold Belt, Australia. Journal of Structural Geology, 7, .301 to 515. RAMSAY, J.G. & HUBERT, M. I. 1987. The Techniques of Modem Structural Geology, Vol.2 Folding and Fracturing, 309-700 p (Academic Press: London). SCHMIDT, B.L. 1980. A geology of the Elura Ag-Pb-Zn deposit, Cobar district, N. S. W. MSc thesis, Australian National University, Canberra (unpubl.). SIBSON R.H.1995. Selective fauh reactivation during basin inversion: potential for fluid redistribution through fault-valve action. From B U C H A N A N , J G. &. B U C H A N A N , P. G (eds), 1995, Basin Inversion, Geological Society Publication, 88, 3-19

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Geological Convention, Sydney, July 2000

THE OCEAN CRUST DYKE/BASALT HYDROTHERMAL SULFUR ANOMALY—INSIGHTS INTO ITS GROWTH AND GEOMETRY DURING CRUSTAL EXTENSION ON MACQUARIE ISLAND, SOUTHERN OCEAN. Garry J. Davidson', Rick Vame\ and Anthony V. Brown^ 'School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001 ^Mineral Resources Tasmania, PO Box 56, Rosny park, Tasmania 7018

ODP hole 504B in the eastern Pacific intersected a thick zone of 1-5% sulfide sulfur at the contact between basalts and underlying sheeted dykes. Sulfur isotopic enrichment above typical igneous values in this zone has been cited as evidence of the transfer of seawater sulfate into ocean crust during off-axis hydrothermal alteration. The process is likely to be common to any aquifer contact between high-temperature reduced fluids in low permeability rocks, and lower temperature sulfate-bearing fluids in adjacent more porous rocks. Assuming that the process operates uniformily everywhere, it has been calculated to remove 2.1xl012g/yr of S from the oceans. However, the homogeneity of the process is far from established and is evaluated here. At Macquarie Island (Southern Ocean), extensive cross-sections through typical ocean crust lithologies have been studied, providing an insight into the mechanisms of hydrothermal alteration during ocean spreading. This crust formed between 8 and 12 Ma on the Troto-Macquarie' Spreading Ridge during slow spreading, prior to the margin evolving to its current transpressive state. The island is now -5.5 km east of the main plate boundary within the Pacific plate. This study has focussed on a dyke-basalt transition in a 7.5 km long, paleo-ridge-parallel section, where the initiating edge of a sheeted dike swarm is exposed, as well as its upper basalt contact. This area is here termed the Sandell Bay Sheeted Dykes (SBSD). The area was mapped in detail, and sampled along two E-W transects 1.5 km apart. The crust consists of a western strongly rotated basaltic sequence, that is intruded by subvertical SBSD. These are in turn overlain by a second shallowly east-dipping basah sequence. The site was subject to at least two major episodes of magmatism and extension prior to modem uplifting transpression. The first produced and rotated the western basalts around an axis to the east, and predated the SBSD. The second post-dated the SBSD, and produced (1) large, ridge-parallel, hydrothermally altered (Cu-bearing), east-dipping normal faults; (2) ridge-normal prehnite±anhydrite veins and faults; (3) reactivation of the upper dyke-basalt transition. The eastern basalts are severely depleted in sulfur, particularly where iron oxyhydroxide alteration is observed, whereas the sheeted dykes are enriched to locally depleted. There is a 5-50 m wide zone of severe sulfur enrichment on the upper dyke/basalt contact. Structurally controlled contacts (3, above) are enriched compared to intrusive contacts, and are also visibly severely altered to muscovite-quartz ± epidote. Both laser-ablation and bulk sulfur chemical extraction was used to determine the isotopic composition of reduced and oxidised sulfur along the profiles. There is isotopic evidence for oxidised fluid circulation throughout the upper basalts and also locally beneath the dyke/basalt contact. This is the first support of the original observation of a ubiquitous off-axis reduced sulfur zone in oceanic crust. However, estimates of the sulfur mass involved must consider that the zone is not homogeneous, but is strongly enhanced by structural permeability development. 117

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MELT INCLUSION STUDIES OF IMMISCIBLE HYDROSALINE MAGMATIC FLUIDS: THE FIRST STAGE OF HYDROTHERMAL FLUIDS? Paul Davidson and V. S. Kamenetsky Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania.

Central to most orthomagmatic models is the idea that a cooling and crystallising granitoid intrusion can exsolve H20-rich fluids, which can then sequester ore-forming elements and transport them to suitable trap sites. To investigate this proposition we have undertaken a magmatic inclusion study of several Chilean Pliocene rhyolites. The chosen rhyolites mark the end of a 4 My. period of magmatism, early in which the copper-tourmaline breccias of the Los Bronces - Rio Blanco porphyry-Cu deposit were emplaced. We have used magmatic inclusions in quartz phenocrysts from feldspar+quartz+biotite-phyric rhyolites of the La Copa Volcanic Complex, and the quartz+feldspar-phyric Don Luis Porphyry to examine magmatic and fluid evolution in the late- and post-magmatic phases of these units. Several major types of magmatic inclusions in phenocrysts have been noted. Type-S inclusions are characteristically clear glassy inclusions containing a large shrinkage bubble. The high silica glass (average 74 wt % SiOi) commonly contains quartz and feathery green amphibole? daughter crystals, whereas the bubble contains only lowpressure vapour with small crystals occasionally attached to their walls. Type-S inclusions have an almost constant bubble:inclusion ratio 20 vol %), even in examples with multiple bubbles. Type-H inclusions characteristically consist of white crystalline masses separated by brine-filled interstitial cavities that appear dark in transmitted light, due to micron and sub-micron scale crystalline coatings (primarily silicates). LA-ICPMS and PIXE microprobe data shows concentrations of Cu, Pb, Zn, As, and Sb as micron scale crystals in the linings of the interstitial cavities, implying that the fluids were relatively enriched in metal. Type-H inclusions are of similar size and shape to type-S inclusions, and coexist with them in growth planes. Type-C inclusions are composites, consisting of a bubble of hydrosaline silicate melt (type-H) inside an inclusion of glass (type-S). The ratio of bubble to glass is very variable, unlike shrinkage bubbles in type-S inclusions. Microphenocryst inclusions are euhedral crystals of feldspar, biotite, apatite, zircon, and titanomagnetite trapped in type-S and type-H inclusions, as well as occurring as individual inclusions in the host quartz phenocrysts. In addition to melt inclusions, the Don Luis Porphyry (but not the La Copa Rhyolite) contains a rich and varied population of fluid inclusions, ranging from brine + vapour + 6 crystalline phases, to simple 2-phase brine + bubble or brine + halite inclusions. The inclusions described provide convincing evidence of conditions in the magma during crystallisation of quartz phenocrysts. The constant bubble inclusion volume ratio in type-S inclusions implies trapping of a homogeneous silicate melt. The microphenocryst inclusions are consistent with a sampling of the phenocryst population coexisting with that melt. The textures of type-H inclusions suggest an essentially homogeneous substance (at the time of trapping) which subsequently unmixed on cooling, crystallising various aluminosilicates with abundant interstitial brine. The coexistence of types-S & -H inclusions plus the very variable bubble to glass ratio in type-C inclusions provide proof of a coexisting immiscible fluid phase at the time of trapping. Post-cooling, the fluid inclusions suggest fluid evolution continued, extending from primary magmatic fluids down to typical hydrothermal fluids recorded in porphyry-Cu deposits.

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OVERLAPS BETWEEN PHREATIC AND PHREATOMAGMATIC BRECCIATION - IMPLICATIONS FOR THE DEVELOPMENT OF HYDROTHERMAL SYSTEMS. AN EXAMPLE FROM THE KELIAN GOLD DEPOSIT, EAST KALIMANTAN, INDONESIA Andrew G.S. Davies, David R. Cooke and J. Bruce Gemmell Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, 7001 Many low and high sulfidation epithermal and porphyry systems are spatially and temporally related to a group of polymict breccia bodies, loosely referred to as diatremes (Sillitoe, 1997; Sillitoe and Bonham, 1984). Neither the characteristics nor the mode(s) of formation of these breccias are well documented. Their interpretation is hampered by inadequate and genetic terminology, and a lack of understanding as to how they relate to ore genesis. As these breccias are associated with some of the largest magmatic-hydrothermal ore deposits in the circum-Pacific (e.g. Grasberg and Kelian, Indonesia; Acupan, Philippines; Cripple Creek, USA: Pueblo Viejo, Dominican Republic; Yanacocha, Peru; El Teniente, Chile) it is important to understand their formation. Diatreme breccias are a product of a type of phreatomagmatic explosion. Phreatomagmatic explosions are those in which a magma contacts external water resulting in explosive fragmentation when the water is flashed to steam. A juvenile magmatic component is involved in the explosion and the transfer of heat from magma to water drives steam production. The contribution of magmatic volatiles to the gas phase is usually minor. Phreatic explosions occur when water is heated to the point at which the fluid pressure exceeds the confining pressure, and the water is flashed to steam. The source of heat in phreatic explosions may also be magmatic, but there is no direct contact between magma and water at the explosion site. No juvenile magmatic material is incorporated in these explosions. The products of phreatic explosions are common in hydrothermal ore deposits, and are loosely referred to as hydrothermal breccias. Phreatomagmatic and phreatic explosions represent the end members of complex fragmentation processes. There is evidence that phreatomagmatic and phreatic processes can be intimately linked and overlap. Historic explosions and eruptions at the Rotomahana-Waimungu geothermal system in New Zealand demonstrated that intrusion of magma into an active geothermal system could result in linked phreatomagmatic and phreatic explosions. These eruptions changed the hydrology of the geothermal system rapidly and irreversibly (Simmons et al, 1993). Recent work at the Kelian epithermal gold deposit. East Kalimantan, Indonesia indicates that intrusion of felsic magma into an active hydrothermal system in the early Miocene initiated a complex and overlapping series of phreatomagmatic and phreatic explosions. Prior to these explosions the hydrothermal system was not depositing gold. The products of the phreatic explosions are spatially and temporally related to the early stages of gold deposition. It is suggested that the phreatomagmatic explosions catastrophically altered the hydrology of the hydrothermal system, triggered widespread hydrothermal explosions and contributed to the development of the auriferous hydrothermal system. The well-exposed and preserved phreatomagmatic and phreatic breccias at Kelian provide an ideal opportunity to examine the relationships between phreatomagmatic (diatreme) breccias and mineralisation in a magmatic-hydrothermal system. The catastrophic physical and chemical changes produced by phreatomagmatic explosion in active hydrothermal systems may help to explain the spatial and temporal relationships between diatreme breccias and many of the largest porphyry and epithermal deposits in the circum-Pacific. References SILLITOE, R.H., 1997. Characteristics and controls of the largest porphyry copper-gold and epithermal god deposits in the circum-Pacific region, Australian Journal of Earth Sciences, 44:373-388 SILLITOE, R H & BONHAM, H F Jr, 1984. Volcanic landforms and ore deposits, Economic Geology, 79:1286-1298. SIMMONS, S.F., KEYWOOD, M., SCOTT, B.J. and KEAM, R.F., 1993. Irreversible change of the Rotomahana-Waimangu hydrothermal system (New Zealand) as a consequence of a volcanic eruption, G^o/ogv, 21: 643-646.

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TSUNAMI AT AIT APE - "IS IT SAFE TO GO BACK?" Hujsji L Davies^ Jocelyn M Davies^, Wilfred Y Lus\ Russell C B Perembo', Nelson Joku', Harrison Gedikile^ and Michael Nongkas^ ^Geology Department, University of Papua New Guinea, University PO, NCD ^Western Earthquake Hazards Team, United States Geological Survey, 525 S. Wilson Ave, Pasadena CA, 91106 USA

The maximum energy of the tsunami that struck the Aitape coast, Papua New Guinea, on 17 July 1998 was focussed on a 14-km sector of coastline centred on the villages of Arop, Warapu and Nimas. Here the wave height was 10 m or more and all structures within 400500 m of the shoreline were destroyed, leaving only some concrete foundation blocks. The total length of coastline where houses were damaged or destroyed was 40 km. More than 1600 people were killed and 10,000 survivors were forced to relocate to new village sites some distance inland. The event has been investigated by overseas tsunami experts and others, intrigued by the narrow focus and unusual severity of the tsunami, and the modest energy (M7) of the associated earthquake. A consensus has emerged that the tsunami was the result of an earthquake-induced submarine landslide and that a newly-mapped submarine canyon served to focus the energy of the tsunami on to the 14-km sector of coastline. Our role has been to collect eye-witness accounts; map the pattern of destruction; map the distribution and character of tsunami sediments; contribute to marine investigations (WYL, MN); and provide information about the tsunami to the survivors and the general pubHc. From survivor accounts we built up a picture of the timing of the arrival of the wave and the shape of the wave. Mapping and sampling of underwater rock exposures revealed a history of periodic, presumably co-seismic, subsidence locally through at least the last 1000 years. In addition, on behalf of the National Disaster Management Organisation (NDMO) we convened a conference of scientists, survivors and managers in September 1999 to review the response to, and management of, the disaster. The end product of the conference, a report that describes and analyses the disaster, is in press. The questions most frequently asked by the survivors were why was no warning given, and was it safe to return to the former village sites? Currently there is no cost-effective way to provide early warning of near-source tsunamis for all of the PNG coastline. The only effective safety measure is to ensure that all people are aware of the warning signs of a tsunami and know what to do. To this end we are assisting the NDMO in a nation-wide campaign to promote tsunami awareness and preparedness. The former village sites at Warapu and Arop are considered unsafe for re-occupation because, when a tsunami threatens, there is no escape route.

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MULTIPLE ORIGINS OF THE EASTERN AUSTRALIAN ALLUVIAL DIAMONDS R.M. Davies\ Suzanne Y. O'Reilly' and W.L. Griffin ^ ' GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW, 2109 2 CSIRO Exploration and Mining, P. O. Box 136, North Ryde, NSW, 1670, Australia Diamonds in eastern Australia occur in alluvial deposits overlying Phanerozoic basement. The primary source of the diamonds is not known. No feeder pipes of kimberlite or lamproite are known, and none of the usual indicator minerals occurs in association with the diamonds. Alluvial diamonds from Wellington, Bingara, Copeton and Airly Mountain (NSW) form two distinct groups, here termed A and B (Davies et al., 1999a). Group A diamonds are similar to those found in kimberlites and lamproites worldwide and are inferred to have formed in Precambrian lithospheric mantle. Group B diamonds have unusual characteristics that indicate they formed in a subduction environment. Surface features of diamonds of both groups indicate that all were emplaced at the surface by magmas. Group A diamonds are similar to diamonds found in kimberlitic and lamproitic hosts in Archean and Proterozoic cratons worldwide in their primary crystal form, internal structure, mineral inclusion composition (mainly peridotitic) and carbon isotopes. Re-Os ages (3.4 and 2.1 Ga; Pearson et al., 1998) determined in situ for sulfide inclusions in two Group A diamonds constrain the origin of these diamonds to ancient mantle sources. This age information and the extensive surface abrasion structures and radiation damage suggest that the Group A diamonds represent an older group of diamonds that have been in secondary collectors for a significant time. If this is so, it is feasible that the Group A diamonds may be derived from a variety of sources, including possible (but unidentified) sources in Antarctica. Group B diamonds are unlike any other diamond suites worldwide in their combination of shape, surface features, strained and irregular internal structures (Davies et al., 1999b), enriched carbon isotopes, and Carich eclogitic mineral inclusions. These features are best explained as a product of diamond growth in a highP/T dynamic environment such as a subducting slab. The diamonds may have formed during arc-continent collision at the time of the development of the New England Fold Belt. This would explain why the major known concentration of Group B diamonds is within the New England Fold Belt at Copeton and Bingara, and is consistent with the Phanerozoic ages for diamond emplacement determined from mineral inclusions (326±43 Ma; 39Ar-40Ar for multiple clinopyroxene inclusions from a single diamond; D. Phillips, pers. comm., 1998; 218±6 Ma; U-Pb for an in situ sphene inclusion; Davies and Kinny unpubl. data). All Group B diamonds have resorption textures that indicate emplacement by magma, and all show sorting and some degree of abrasion. Group B diamonds that make up a small proportion of the alluvial diamond population at Wellington, in the Lachlan Fold Belt, are typically more abraded than Group B diamonds from Copeton and Bingara. References

PEARSON, D. G., DAVIES, R.M., SHIREY, S.B., CARLSON, R.W. AND GRIFFIN, W.L., 1998, The age and origin of eastern Australian diamonds: Re-Os isotope evidence from sulfide inclusions in two diamonds from Wellington, New South Wales: Extended Abstract 7th International Kimberlite Conference, South Africa, p. 664-666. DAVIES, R. M., O'REILLY, S. Y., AND GRIFFIN, W. L., 1999a, Diamonds from Wellington, NSW: new insights into the origin of eastern Australian diamonds: Mineralogical Magazine, v. 63, p. 447-471. DAVIES, R. M., O'REILLY, S. Y., AND GRIFFIN, W. L., 1999b, Growth structures and nitrogen characteristics of Group B diamonds from Wellington and Bingara, eastern Australia: Proceedings of the Seventh International Kimberlite Conference, p. 156-163.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

METAL CONCENTRATION AND SPECIATION TRENDS IN COCKLE CREEK, LAKE MACQUARIE, N.S.W. AUSTRALIA. F. Davies-McConchie\ D. McConchie^ G. Birch^ and T. RyffeP ^ Environmental Geology Group, School of Geosciences, University of Sydney, Sydney, Australia. ^ Centre for Coastal Management, Southern Cross University, Sydney, Australia.

Cockle Creek receives inputs from several major industrial sources and from sewage treatment plants, and metal concentrations at many sites in the creek are in the very high range (e.g., Pb >5,000 mg/kg; Zn >5,000 mg/kg; Cd >500 mg/kg; and Cu >300 mg/kg)) of values recorded for other similar waterbodies in Australia. Metal (Ag, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Sb, and Zn) concentration and speciation trends in sediment cores suggest that metals undergo a series of early diagenetic reactions in the sediment after deposition. At or near the surface, metals are primarily present as adsorbed and oxide/oxyhydroxide bound species, however, what happens below the surface varies with distance up the creek. Sulphides are sparse in the upper reaches of the creek although the transition from oxidised surficial sediment to more reduced sediment is within a few mm of the sediment surface. Organic matter is abundant in the sediment and below the redox transition zone most metals are present as adsorbed species or organometallic complexes. Despite Eh conditions below -200 mV and an adequate supply of organic matter, sulphide-bound metals are rare, possibly because periodic freshwater influxes are sufficient to prevent the development of large populations of sulphate-reducing bacteria. The sediment also smells strongly of hydrocarbons in many places and the presence of these compounds may also inhibit the activity of sulphate-reducing bacteria. In the lower reaches of the creek, and in the lake, the proportion of the trace metals bound as sulphides increases rapidly below the redox transition zone; this increase begins at about 2 cm in the lower creek sediment and about 10 cm depth in the lake sediment. The difference in depth to the transition zone appears to reflect the influence of bioturbation which pushes the redox boundary lower in the sediment profile in and near the lake. The highest concentrations of metals in sediment in the Cockle Creek area are found in the midsection of the creek near and downstream from the lead-zinc smelter and a coal processing facility. In this section of the creek, assessment of metal speciation is complicated by the presence of abundant coarse metal-rich slag particles that are removed in the >62.5 \xm grainsize fraction. Had whole sediment samples been analysed (rather than the <62.5 |im fraction only) the total metal concentration for some metals would certainly have been much higher, but trace metal speciation trends associated with transformations may have been masked. Cockle Creek sediments are clearly highly contaminated, with some metals being present at very high levels. During sample collection, some boys who were fishing in the creek advised us not to eat any fish we might catch because 'they taste metallic and we're only catching them to feed to the cat'. Even though most metals are bound as sulphides where sulphate concentrations in pore waters are sufficient for production of biogenic sulphides, or oxides where sulphate levels are lower, the bioavailable metal load in Cockle Creek is very high, and no doubt has an adverse impact on the local ecosystem and 'the cat'.

122

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

KANOWNA BELLE GOLD MINE - ANATOMY AND HISTORY OF A PLUMBING SYSTEM Brett K. Davis', Nick Archibald^ and Alex Aaltonen' ' Delta Gold Ltd, PO Box 152, Kalgoorlie, WA, 6430 ^ Fractal Graphics, 39 Fairway, Nedlands, WA 6009 The world-class Kanowna Belle Gold Mine is located 18 km NE of Kalgoorlie within the greenstonedominated Boorara Domain, a subdivision of the Eastern Goldfields of the Yilgam Craton, Western Australia. The deposit is hosted by sedimentary volcaniclastic and conglomeratic rocks, which are separated into hangingwall and footwall sequences by a major, steeply SSE dipping zone of structural disruption. This structure represents the product of at least three temporally distinct stages of deformation, comprising the Fitzroy Mylonite, the Fitzroy Shear Zone and the Fitzroy Fault, which have produced clear structural overprinting relations. Importantly, this structure has localised emplacement of a porphyritic granodiorite body, the Kanowna Belle porphyry, which hosts at least 70% of known mineralisation. Localisation of highest grade mineralisation and most intense alteration around the composite structure emphasises its importance for acting as the major plumbing system for auriferous fluids. Formation of the Fitzroy Mylonite and Fitzroy Shear Zone are interpreted to have occurred during regional south-to-north D1 thrusting. A switch in far-field stress axes to the approximately ENE-WSW D2 orientation caused reactivation of the Fitzroy Shear Zone, resulting in sigmoidal folding of pre-existing structures and formation of a shallow lineation associated with sinistral transcurrent shearing. The Kanowna Belle porphyry cross-cuts fabrics associated with the D1 Fitzroy Mylonite and Fitzroy Shear Zone and is in turn overprinted by 82. Strain accumulation during D2 sinistral shearing caused the competent Kanowna Belle porphyry to undergo brittle-ductile failure, resulting in emplacement of carbonate breccias, carbonate vein stockworks, siliceous breccias, silica-carbonate-pyrite-sericite veinlets (stringers) and sheeted vein arrays, all of which host mineralisation. Fitzroy Shear Zone reactivation, brecciation of the Kanowna Belle porphyry, and emplacement of mineralisation are all interpreted to have occurred synchronous with formation of S2 and are products of regional D2. A vein paragenetic history comprising eight temporally distinct stages has been resolved. Two temporally, mineralogically and spatially distinct styles of mineralisation have been recognised, corresponding to stages 3 and 4 of the vein emplacement history. The first, volumetrically minor, stage of mineralisation is represented by a telluride-associated mineralogy and is restricted to crustiform carbonate (±quartz) veins and breccias in the vicinity of the Fitzroy Shear Zone. Veins show structural overprinting relationships consistent with emplacement synchronous with regional D2. Telluride-gold mineralisation occurs as microfracture and microvug infill that overprints these. Free gold occurs in association with altaite, coloradoite and melonite with rare hessite. No gold tellurides have been noted. The second stage of gold emplacement comprises the bulk of economic mineralisation and overprints the telluride-associated stage. It represents a telluride-absent mineralisation phase that displays a strong D2 control on lode geometry. Gold occurs within silica-carbonate-pyrite-sericite 'stringers', which comprise stockwork geometries in close proximity to the Fitzroy Shear Zone, but become more regularly aligned with the regional S2 away from the shear zone. Pyrite varies in inclusion content from inclusion-rich to inclusion poor. Generally, inclusion poor pyrite occurs as intensely growth-zoned rims on inclusion-rich cores, with growth zoning tentatively correlated to arsenic content of the fluid from which pyrite was derived. Microtextural relationships indicate emplacement of both inclusion-poor and inclusion-rich pyrite forms to be controlled by, and to have been emplaced during, regional D2. Gold occupies D2 extensional sites adjacent to pyrite crystals and occurs as fracture infill of deformed D2 pyrite. Consequently, deposit-scale, development-scale and microscale relationships are all consistent with an interpretation of Kanowna Belle as a major gold deposit formed during regional D2 as a result of reactivation of a major D1 structure.

123

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ABANDONED ALLUVIAL TIN MINES IN NE TASMANIA: CHARACTERISATION FOR SUCCESSFUL REMEDIATION. J.M. de Jong^ R. Bennett', D.C. 'Bear' McPhail' and D.L. Dunkerley' ' Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University. ^ School of Geography and Environmental Science, Monash University. Contact: Jacinta, Centre for Mined Land Rehabilitation, Uni of Qld, St Lucia, 4072 The Ringarooma Basin in north-eastern Tasmania has been the major source of alluvial tin in Tasmania, with tin production in excess of 40,000 tonnes during the mining period (1875-early 1980s). Derelict mine sites now litter the basin. Mineral Resources Tasmania (MRT) have prioritised the remediation of three mine sites around Mt Cameron: Endurance, Star Hill and Monarch. To date, all efforts at remediating and revegetating the disturbed land (mine sites and waste dumps) have had limited success. In order to remediate the sites effectively, a detailed understanding is required of the physical, chemical and biological processes occurring in the waste dumps and surrounding areas. The main deposit at the Endurance mine site formed in a former tributary of the Ringarooma River and was - 3 0 m below the land surface. Shallow surface deposits were more characteristic of the Monarch and Star Hill mines. During mining of the deposits, sulphide minerals present in the overburden sediment, were disturbed and disseminated in the waste dumps. Subsequently, an acid mine drainage (AMD) problem has developed at the sites. In particular, the AMD problem at the Endurance site is of concern, given the extent of area affected, the potential for contamination of adjacent pasture land and other adverse impacts offsite. In 1998 and 1999, two studies were undertaken to understand the processes at work on these abandoned alluvial tin mine sites: one investigating the poor reestablishment of vegetation across the three sites, the other characterising the impact of the overburden sediment area on groundwater and surface waters on the Endurance site. The local Gladstone region supports a mosiac of sclerophyll forest and sedge-heathland communities, dominated by species leptospermum scoparium, Kunzia ambigua, Banksia marginate and Allocasurina monilifera. Although species diversity and richness are approximately the same between disturbed and nondisturbed areas, vegetative cover is as low as 1-2% within the mine sites compared to >90% within established locations. The sediment, originally derived from weathering of the local granite, is quartz rich (up to 95% vol) with minor feldspar altered to sericite and clays, iron oxides and sulphides (<1% pyrite). The sediment is naturally acidic (pH 4.6-5.4). They are poorly structured and have high infiltration rates (low water holding capacity), providing a poor medium for plant establishment and growth. Despite the high infiltration rate, wind and erosional processes are evident on all mine sites and adversely affect seedling survival. In addition, the sediment is infertile, with lower concentrations of macronutrients (N, P, Ca, Mg and K) than established sites and a low cation exchange capacity, inhibiting plant access to the already limited macronutrients. The groundwater at Endurance and in areas at Monarch and Star Hill is acidic (pH 2.4 to 4.5) and contains high concentrations of metals: Fe (up to 268 ppm), A1 (up to 83 ppm), Pb (1-341 ppb), Zn (10-452 ppb) and Cu (Below Detection Limit - 8 ppb) in excess of the Australian and New Zealand Environment and Conservation Council (ANZECC) guidelines for aquatic ecosystems. Thus there is the potential for plant dieback after establishment once the plants begin utilising the groundwater. Apart from detrimental effects on established plants, acidic groundwater also adversely affects surface waters on site. This is illustrated particularly well along Conundrum Creek on the Endurance mine site. After receiving an acidic groundwater seep. Conundrum Creek becomes more acidic (dropping from a pH of 4.7 to 3.6) and contains greater concentrations of Fe and Al. The creek then drains into the local Ruby Lagoon . . . Given the low hydaulic conductivity of the waste sediment and the presence of pyrite, acid is predicted to be generated for many years to come, mobilising major and trace elements and contaminating groundwater and surface waters at least at the Endurance site. Remediation options for the sites include capping the waste dumps (to minimise further AMD production and provide a suitable medium for vegetation establishment and growth) and passive treatments of contaminated waterways. In order for remediation to be successful, however, controlling erosional processes must also be considered. Acknowledgements: Financial and logistical support from Mineral Resources Tasmania (MRT) and Scientists, Engineers, Managers and Facilitators (SEMF).

124

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DISTRIBUTION OF GOLD AND BASE METALS IN REGOLITH MATERIALS AT MANDAMAH PROSPECT, CENTRAL WESTERN NEW SOUTH WALES N. De Souza Kovacs\ C. L. Moore^ and J. Wilford^ ^CRC LEME, University of Canberra, ACT 2601 ^CRC LEME, Australian Geological Survey Organisation

Semi-quantitative observations have been used to delimit the regoHth stratigraphy above the concealed Mandamah porphyry-related Au-Cu deposit in Central West NSW, and to constrain the distribution of secondary mineralisation more precisely. Parameters used include down hole distribution and nature of mottled zones, sediment grain size, presence and morphology of quartz, presence and nature of iron concretions, magnetic susceptibility, and recognised retention of primary texture. A Portable Infrared Mineral Analyzer (PIMA) and X-ray diffractometer were used to identify minerals and to detect changes in degree of disorder of kaolinite. PIMA analysis was used to delimit the down hole relative proportion of bound water in regolith materials. The transported unit lies on an irregular erosional unconformity in pallid saprolite, about 45 m below the land surface. Sandy sediment dominates from 0 m to 18 m, gravel from 18 m to 25 m, sandy material from 25 m to 31.8 m, and clay-rich sediment below 31.8 m. The transported unit is characterised by two mottled zones, one from 3.7 m to 18 m, and a second 31.8 m to 45 m. Magnetic susceptibility measurements were taken on very small samples of drillcore, and were used for comparative analysis downhole only. Susceptibility readings are variable, but generally very low, from 0.0002 to 0.0012 SI units down to 31.8 m, zero to 0.0001 SI units in the region of the contact, and increase from 0.0001 to 0.0012 SI units down hole from 52.1 m. Iron concretions are concentrated in a horizon 31.8 m to 36.1 m below surface, above the base of transported regolith, and along fractures in the interval 79.9 m to 89.2 m. The degree of disorder of the kaolinite is variable, but is low at the base of the transported material and high in in situ pallid saprolite. Primary porphyritic rock texture is preserved in lower saprolite. Copper anomalies (up to 6400 ppm) increase with depth, and are higher in the saprock (>85 m depth), than in the lower saprolite (56.3 m to 84.5 m). Gold anomalies are irregularly distributed down hole. Gold anomalies up to 5 ppm occur at the base of transported regolith, at the top of the pallid saprolite zone, in the saprock and in the bedrock. In places the lower saprolite contains Au values of 5 to 10 ppm and Cu values of 1700-2000 ppm. Anomalous gold and base metal values are associated with both dispersion along the base of a transported unit, and with residual metal concentration in saprolite and supergene enrichment in the interface saprolite and saprock. Concentration of copper, lead and zinc in the saprolite and saprock appears to result from weathering of previously primary mineralisation. Concentration of gold in the lower saprolite and saprock are associated with primary mineralisation. The mechanisms for concentrating gold in the top of the pallid saprolite and in the base of the transported unit are not yet identified. Acknowledgments: This work is supported by the Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), and is published with the permission of the Director of CRC LEME.

125

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOCHEMICAL CONTRAST BETWEEN IGNEOUS PHASES WITHIN THE CROJINGOLONG PLUTONIC COMPLEX, SOUTHEASTERN AUSTRALIA: IMPLICATIONS FOR MAGMA GENESIS A. A. Dean Department of Earth Sciences, Monash University, Melbourne, Australia

The spatial relationship between mafic, intermediate and felsic intrusives is a common feature of most granitic complexes. However, the role of mafic (mantle-derived) magmas in the generation, evolution and emplacement of intermediate to felsic magmas is uncertain.The Late Silurian/Early Devonian(~400Ma) Bega Batholith in southeastern Australia comprises more than 130 separate plutons, many of which include mafic/felsic dyke swarms, small parasitic mafic/intermediate/felsic intrusions and mafic microgranitoid enclaves thus providing an ideal locality to investigate such relationships. The more primitive isotopic (Sr,Nd,Pb) character for some intrusives along the eastern margin of this batholith, has been used as evidence for limited mixing associated with underplating and late stage intrusion into the crust of mantle derived magmas. (McCulloch and Woodhead, 1993, Geochim. Cosmochim. Acta, v57, p659). The Croaj ingolong Complex, southeastern Victoria, comprises predominantly monzogranite to granodiorite outcrop with zones of microtonalitic to microdioritic enclaves which are clearly genetically related to common metre scale coeval dykes, most of which are composite containing mingled tonalitic and leucogranitic phases. Petrographic evidence supports an initial assumption from field evidence, that the enclaves represent encapsulated portions of more mafic magma that has mingled physically with the host granite, as has been concluded elsewhere. Homogeneity between the mafic/felsic rocks indicates assimilation and equilibration of chemistry during the slow cooling of plutons. Trace element and isotopic values infer that mafic microgranitoid enclaves and syn-intrusive tonalitic/dioritic dykes and parasitic plutons represent intermediate hybrids (between mantle and mean continental crust values). The chemistry of these rocks is modified according to the degree of interaction with meta-igneous or meta-sedimentary crustal material, and they retain variable primary chemical characteristics of parental (basaltic/andesitic) magmas fundamental to the initiation of Lachlan Fold Belt felsic magmatism. Significant Sr and Nd isotopic variation indicates the magmas have mixed (initial 87Sr/86Srgranodiorite ~ 0.704, enclaves ~ 0.705, dykes/sheets -0.706 and leucogranite/aplite -0.711; ENd between +6 and -6).0verlap between values is minimal and most common between the enclave/hybrid/dykes-sheets indicating mixing at various stages throughout the history of the complex. Oxygen isotopes give typical 'V type values (9.0 - 10.2%o). When plotted against Sr values minor crustal contamination is inferred.

126

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological

Convention,

Sydney, July

2000

ZIRCON FORMATION DURING GARNET BREAKDOWN IN ROGALAND, SOUTH-WESTERN NORWAY. Helen Degeling, Stephen Eggins and David Ellis Department of Geology, Australian National University, ACT, 0200 Zircon U-Pb geochronology is widely used to date metamorphic events. However, the formation of zircon during metamorphism is poorly understood. As a consequence, the interpretation of zircon ages from metamorphic rocks can be ambiguous. This ambiguity can be removed if specific periods of zircon growth can be related to well-constrained metamorphic reactions. Garnet breakdown is already recognised as a zircon-forming process (Eraser et al. 1997), and can readily be related to a specific part of an orogenic cycle by employing any one of a number of geothermobarometers. In this study, we present results of a detailed examination of two Zr-liberating, garnet breakdown reactions preserved in high-grade rocks in Rogaland, southwestern Norway. Laser Ablation ICPMS has been used to quantify the distribution of Zr in both reactant and product phases. The rocks in southwestern Norway record regional (Mi) metamorphism (~700°C, 0.8 GPa) related to the -1200 Ma Sveconorwegian Orogeny (Wielens et at. 1981), which has been overprinted by high-grade contact metamorphism (M2). M2 developed during intrusion of the Rogaland anorthosite complex between 930 and 920 Ma (Duschesne et al 1993). Temperatures ranged from >1000°C proximal to the intrusion, to ~700T at the limits of the aureole, at a constant pressure of 0.4 GPa (Jansen et al 1985). Where M2 temperatures exceeded 800°C, Mi garnet has broken down to osumilite and orthopyroxene (Reaction 1; e.g. garnet + biotite + quartz ^ osumilite + orthopyroxene). In lower-grade parts of the aureole (~710°C) garnet breakdown to cordierite is common (Reaction 2; garnet + sillimanite + quartz ^ cordierite), as a result of decompression from M] (0.8 GPa) to M2 (0.4 GPa). The cordierite coronas around garnet are noteworthy in that they contain abundant grains of metamorphic zircon. Although we attribute this zircon growth to the release of Zr during garnet breakdown, osumilite coronas around relic garnet do not contain such zircons. Furthermore, LA-ICPMS analysis of several samples revealed that the osumilite-producing garnet contained over twice the amount of Zr than the garnet precursor to cordierite (50ppm versus 20ppm, respectively), yet only the latter resulted in zircon formation. No other reactant phases, in either Reaction 1 or 2, contain significant Zr. It is clear that the absolute abundance of Zr in garnet is not the limiting factor in zircon formation, and it is therefore important to consider factors such as the significance of other phases on the product side of the reaction. Cordierite produced by Reaction 1 contains <0.01ppm Zr, implying that Zr released from garnet was excluded from the cordierite lattice and was thus free to form zircon. Osumilite and orthopyroxene, on the other hand, contain similar amounts of Zr as the reacting garnet (40ppm and 25ppm, respectively), thus inhibiting zircon formation. Understanding the processes by which metamorphic zircon may form, or be inhibited from forming, is an essential tool for constraining the rates and time scales for orogenesis. This study demonstrates that breakdown of a Zr-bearing phase during metamorphism will only result in new zircon growth if Zr is incompatible with respect to the product phases. In Rogaland, new zircon growth has been texturally constrained to a period of decompression from Mi to M2. In situ isotopic dating of these zircons will confirm the viability and practicality of relating zircon formation to specific stages of an orogenic cycle. Acknowledgements: This project is supported by an ARC research grant to D. Ellis, and GEMOC. References DUSCHESNE J., SCHARER U. & WILMART E.

1993. A 10 M a period of emplacement for the Rogaland anorthosites, N o r w a y : evidence

from U-Pb ages. Terra Abstracts 58, 64. FRASER G., ELLIS D. & EGGINS S. 1997. Z i r c o n i u m abundance in granulite-facies minerals, with implications for zircon geochronology

in high-grade rocks. Geology 25, 607-610. Jansen J. B. H., Blok R. J. P., Bos A. & Scheelings M.

1985.

PRELIMINARY RESULTS FROM THE BAMBLE AREA, S NORWAY. CRUST IN THE NORTH ATIANTIC

PROVINCES,

GEOTHERMOMETRY AND GEOBAROMETRY IN ROGALAND AND IN: A. C . TOBI & J. L. R. TOURET EDS. THE DEEP

PROTEROZOIC

PP. 4 7 7 - 4 9 7 .

Wielens J. B. W., Andriessen P. A. M., Boelrijk N. A. I. M., Hebeda E. H., Priem H. N. A., Verdurmen E. A. Th. & Verschure R. H. 1981. ISOTOPE GEOCHRONOLOGY IN THE HIGH-GRADE METAMORPHIC PRECAMBRIAN OF SOUTHWESTERN NORWAY: NEW DATA AND REINTERPRETATIONS. NORSK GEOLOGISK

UNDERSOKELSE 359,

1-30.

127

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

FIELD AND IMAGE-DERIVED SPECTRAL ENDMEMBERS AS INDICATORS OF SOIL SALINTY Remv L. Dehaan and Geoffrey R. Taylor School of Geology, University of New South Wales Sydney, NSW 2052

Irrigation induced salinity has become a major problem in many parts of the world. In areas where rising saline ground waters interact with the upper parts of the soil profile the destruction of primary clay minerals and the direct precipitation of evaporite minerals occurs in areas known as discharge zones. In such zones vegetation is often destroyed and soil degradation is rampant. Strategies for the early detection of salinity using HyMap have been investigated. HyMap is a commercially available airborne hyperspectral imaging spectrometer that can be configured to give a variety of spectral and spatial resolutions. The test site for this research is located within the Murray Darling Basin, near the town of Pyramid Hill and close to the New South Wales/Victorian border. Vegetation cover, soil conductivity and depth to water table are routinely monitored making this site an excellent location for the testing of salinity mapping techniques. Strategies for mapping spectral endmembers as indicators of insipient salinity using both field-derived and image-derived spectra are assessed. Spectral feature fitting and mixturetuned matched filtering approaches are adopted. Soil and vegetation indicators of salinity are identified. Disturbed bare ground due to ploughing operations can be differentiated from bare ground due to salinity. Several categories of saline soil can be mapped using both field and image spectra and these relate well to the surface expressions of soil salinity. Increased salinity is shown in image-derived endmembers by the appearance of an infrared reflectance plateau between 800nm and llOOnm, hydrate-related absorption features at lOlOnm, llSOnm, 1780nm, 1950nm and 2210nm, the decrease in depth of the hydroxyl absorption feature at 2200nm and the widening of the water absorption features at 1950nm and 1400nm. Field spectra show similar characteristics with the hydroxyl and water absorption features being slightly more asymmetrical. Vegetation indicators of salinity include halophytic vegetation comprising the "succulent" species of Samphire and Sea Elite and several species of native grasses comprising Sea Barley Grass, Windmill Grass and Tall Wheat Grass. Image-derived and field-derived spectra are used to classify pixels and provide an accurate record of the groundcover conditions prevailing at the time of image acquisition. The spatial distribution of soil endmembers derived from the HyMap imagery show a similar distribution to the salinity distribution determined by ground-based geophysical surveys. Acknowledgements: Financial and logistical support from Terry Cocks of Intergrated Spectronics Pty. Ltd., North Ryde is gratefully acknowledged. The assistance provided by staff from the Victorian Department of Agriculture at the R & D Block, Pyramid Hill is also gratefully acknowledged.

128

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

FIELD SPECTROMETRY AND HYPERSPECTRAL IMAGERY FOR REGOLITH MAPPING AT MRANGELLI, NEAR COBAR, NSW. Remv L. Dehaan and Geoffrey R. Taylor School of Geology, University of New South Wales Sydney N.S.W., 2052, Australia The Mrangelli test site is in the Cobar Basin, 25km west of Cobar. The test area has low topographic relief, however local zones of silicification, associated with mineralised structures or more resistant lithologies (sandstones and conglomerates of the Biddabirra Formation) form topographic highs. The lower slopes are dominated by variably weathered siltstone and mudstone of the CSA Formation. Colluvium and alluvium rest on truncated weathering profiles in vegetated drainage lines. Pisoid lags are abundant, whereas lithic lags increase in abundance towards elevated outcrops. A FieldSpec FR hand-held spectrometer was used to acquire reflectance spectra in the range 350nm to 2500nm of in situ rocks, soils and vegetation. The spectra of soils and lags can be characterised into three types based on their absorption features. They include iron-rich soils, soils with a significant lithic component (lithic lag) and soils dominated by pisolites (pisoid lag). The 2200nm feature in all soil and lag types is indicative of the hydroxyl ion in clays and hence it is likely that clay minerals form a dominant component in these surface materials. Large absorption features at 1400nm and 1900nm are due to uncombined water and occur in all samples. The iron-rich soils show distinctive absorption features at 680nm, 920nm and around 2200nm, which indicate a mixture clay, hematite and goethite dominated by goethite. The lithic lag shows feature at 885nm and 2200nm, which indicates a mixture of clay, goethite and hematite, dominated by hematite. The pisoid lag shows absorption features at 860nm and 2200nm, which indicates a mixture of clay, hematite and maghemite dominated by maghemite A hyperspectral image was acquired by the HyMap airborne imaging spectrometer over the Mrangelli region in May 1999. HyMap can be configured to give a variety of spectral and spatial resolutions and is built by Integrated Spectonics Pty. Ltd. of North Ryde. Spectral data was acquired in 128, approximately 20nm bands, covering the spectral range 400-2500nm. Calibrated imagery and spectra viewed in ENVI show that the spectra of various objects closely resemble those expected from field and library spectra. Lag compositions can be identified empirically in principal component imagery by colour variations in the multicomponent images. The poster shows a three-band RGB image of principal components 2, 4 and 7. In this image dark purple areas relate to lags dominated by pisolites, light blue areas relate to silicified sandstones and siltstones, red and pink areas relate to iron-rich soils with a significant lithic component, while green areas relate to massive red, iron-rich soils. A synthetic true colour composite comprised of bands 3, 9 and 15 (455, 545 and 638nm) displayed as blue, green and red is presented in the poster. An abundance map created using spectral feature fitting techniques for the maghemite-dominated pisoid lag field spectra is overlaid on this image. Spectral feature fitting is an absorption feature based method that matches image spectra to reference spectra using a least squares technique and continuum-removed data. The derived maghemite abundance map encompasses all know areas of pisoid lag but needs refining as it also shows some areas that are known from field work to have red, hematitic, soils. On the basis of spectra derived from the image data it is suggested that more accurate maps of the various lag types will be achieved. It is proposed that maps of maghemite lag distribution created from HyMap imagery could well be a useful precursor to a soil geochemistry program. Silicified meta-sediments have distinctive spectral signatures due to a kaolinite component and these can be reliably mapped in the field by their field spectra and from the imagery by their distinctive image-derived spectral signatures. Such maps will provide a useftil additional tool to the exploration geologist working in the Cobar area. Acknowledgements: Financial and logistical support from Terry Cocks at Intergrated Spectronics Pty Ltd is gratefully acknowledged. The provision of unpublished data by David Cohen and A1 Dunlop of the LTNSW is also greatly appreciated.

129

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59

!5"" Australian Geological Convention, Sydney, July 2000

CEMETERY DECAY PRODUCT PROFILES: TWO CASES IN AUSTRALIAN, UNCONSOLIDATED, SANDY AQUIFERS Boyd B. Dent Dept. Environmental Sciences, University of Technology, Sydney, PO Box 123, Broadway, NSW, 2007. When a human body is interred (buried) in a cemetery its decay is influenced by a complex spectrum of factors including:- the features of the remains; the funereal aspects of the interment; whether or not the body is coffmated; the hydrogeological setting; soil characteristics; the cemetery's burial management practices. In sandy soils, which because of their high porosity and intrinsic permeability are likely to represent the greatest case for aerobic decay processes, the most rapid development of decay products is expected. But this is not instantaneous and varies, in as yet uncorrelated ways, with the factors aforementioned: the variations are spatial and temporal. This paper profiles data gathered from two sites studied during the recent National Study of Cemetery Groundwaters, at Botany Cemetery, Sydney, NSW - sited on Holocene aeolian sand dunes (representative K = 0.8 m/day), and at Guildford Cemetery, Perth, WA - sited on Holocene shallow marine, clayey and silty sands and fine sands (representative K = 0.035 m/day). The data show that when cemeteries are considered on a whole-site basis the products reaching the cemetery boundaries are in low, but recognisable concentrations. The primary indicators of the decay processes are the electrical conductivity, pH, total forms of inorganic and organic nitrogen, orthophosphate, chloride, total organic carbon, and sulfate; however, caution must be exercised to eliminate other anthropogenic and natural sources. In the immediate vicinity of the remains, short-term microbiological indicators like faecal coliforms/E. coli, faecal streptococci, and Pseudomonas aeruginosa are sometimes present; although the decay plume is better characterised inorganically. As travel distances increase the presence and incidence of these organisms is rapidly attenuated. Some analytes of representative hydrogeochemical profiles for piezometer monitoring sections developed along groundwater gradients are tabulated. The data represent arithmetic means from up to 6 sampling rounds over the period 11/1996 - 9/1998. The field difficulties of establishing piezometers in cemeteries has dictated that data at times has to be spatially averaged and considered conceptually rather than adhering to analysis of a profile continuum. {rainfall mm} Piezometer Position & # EC nS/cm PH units Eh mV tot inorg N * total N TOC total N/ TOC P04 totP CI S04 sample dist $

Guildford Cemetery, WA {798}

Botany Cemetery, NSW {1103} background

(11)

559 6.4 + 142 0.6 0.55 7.0 0.08 0.7 0.01 81 105

down gradient

boundary

(8)

(9)

(2)

middle cemetery (7)

downgradient boundary (8)

520 6.4 + 142

603 5.9 +123 11.7 10.5 16.0 0.66 2.3

499 6.3 + 148 12.9 16.7

0.42 0.5

670 6.4 +149 4.9 4.9 8.0 0.61 1.9

265 5.8 +211 8.2 9.4 4.5 2.09 1.8

0.02

0.01

0.02

0

0

0

113 84 220

67 35 170

64 39 50

30 10

94 44 270

50 28 95

adjacent recent burials (3)

1092 6.5 +1 4.6 10.9

26.0

6.6 6.4 8.0 0.80 1.9

background

20.2 0.83 1.3

# samples

~ median; * analytes in mg/L if not specified; $ downgradient distance from previous sample point (m) The hydrogeochemical profile appears to be influenced by stored nitrogen in the soils and this is demonsfrated by "high" and "out-of-character" nitrogen contents in background samples. Data exist to show that groundwaters within cemeteries can be effectively cleaner than background groundwater (Woronora, Sydney, NSW; Launceston, Tas; Botany; Guildford). In the sandier soils of cemeteries which get re-worked by burial practices, the soil-stored nitrogen is likely to be significantly lost.

130

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 IAustralian Geological Convention, Sydney, July 2000

SIGNIFICANT SURFACES AND ACCOMMODATION TRENDS IN PARALIC COAL SEAMS C. Diessel, R. Boyd, J. Wadsworth, and G. Chalmers Department of Geosciences, The University of Newcastle, Callaghan, N.S. W. 2308, Australia

Paralic coal measures usually occupy ancient coastal regions in which they are part of marine to terrestrial parasequences. Landwards, the marine intercalations wedge out so that formerly separated coal seams amalgamate before they become replaced by terrestrial deposits. This means that successive coal plies in an amalgamated seam are likely to represent the stacked remnants of parasequences. It also means that coal composition should reflect the accommodation trends that were operative in the more fully developed parasequences further basinward. We have used petrographic methods to study accommodation trends in paralic coal seams in Canada, Germany and Australia and found that ancient mires in which well-balanced accommodation / peat-accumulation ratios existed for long periods of time tend to produce thick coal seams with minimum detrital mineral content and maximum plant-tissue preservation, the latter in the form of structured vitrinite (telovitrinite). Indicators of oxidation such as inertinite and inertodetrinite are rare, and the optical properties of telovitrinite, i.e. mean reflectance and fluorescence are a normal reflexion of the thermal history of the coal with little statistical variation about the arithmetic mean. A high accommodation / peat-accumulation ratio causes frequent flooding of the peat. The plants that grow under such conditions in the mire struggle for survival, and if the rate of accommodation increase exceeds the rate of plant growth and peat accumulation, the mire will drown. This either terminates peat accumulation altogether or leads to a seam split if there is a lateral gradient in the rate of accommodation increase. Coal formed up-dip of the split axis, i.e. where peat still managed to accumulate under a high accommodation rate, commonly contains a high proportion of detrital minerals due to the frequency of flooding. The proportion of structured vitrinite is reduced because cellulose-destroying bacteria thrive under the low-acidity and often eutrophic conditions in the limnotelmatic environment. The proportion of structured inertinite, i.e. fusinite and semifusinite, is low but its fragmented form, i.e. inertodetrinite, is quite common, together with concentrations of other dispersed organics, particularly spores and pollen. The high level of bacterial activity in the mire causes the vitrinite precursors to be enriched in lipid-derived hydrogen which causes the telovitrinite reflectance to be lower than normal while its fluorescence is higher. Because of the high watertable and frequency of flooding, there is a great deal of mixing of autochthonous and allochthonous vitrinite precursors. This results in a greater than normal statistical variation of the optical properties of telovitrinite. A low accommodation / peat-accumulation ratio causes oxidation and partial combustion of the organic matter. Although the low watertable would reduce the influx of detrital minerals into the mire, the oxidative loss of biomass tends the enrich all inorganic matter. The vitrinite content is generally low but the proportion of structured inertinite as well as its fragmented form may be quite high. The sporinite content is often low, and most spores show signs of corrosion. Telovitrinite reflectance is higher and its fluorescence is lower than normal due to the depletion of hydrogen in the source material. Because most of the vitrinite precursors have been derived from in-situ material, the variation of the optical properties about their respective mean values is usually quite low. The application of these parameters to the vertical profile analysis of coal seams revealed a variety of accommodation trends and made it desirable to define five new surfaces of varying significance in onshore sequence stratigraphy.

131

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

"ANNELIDICAL GEOMETRY": A 3-D GEOLOGICAL AND GEOPHYSICAL APPRAISAL OF MAJOR STRUCTURES IN THE EASTERN LACHLAN OROGEN USING MULTI-SCALE EDGE ANALYSIS Nicholas G. Direen', Nicholas J. Archibald^ E. June HilP, Timothy E. Mackey\ Ollie L. Raymond', Patrick Lyons', Russell J. Korsch' and Darren J. Holden^ ' Australian Geodynamics Cooperative Research Centre, AGSO, GPO Box 378, Canberra ACT 2601 ^ Fractal Graphics Pty Ltd, 39 Fairway Nedlands WA 6009

A combination of multiscale edge analysis ("worming") and forward modelling of potential field data, combined with new geological mapping and deep seismic reflection transects acquired by the Australian Geodynamics Cooperative Research Centre, has led to advances in the definition of crustal architecture in the eastern Lachlan Orogen of New South Wales. Worming identifies the location of the edges of geological bodies based on gradient information in potential field data. These edges can be viewed at varying levels of upward continuation, allowing the identification of geological boundaries from the shallow through to whole-of-crust scales. Bouguer gravity and total magnetic intensity data for 8 X 1:250 000 scale map sheets covering the eastern Lachlan Orogen have been processed in this way, with striking results. The new data cover the area between the Wagga Group, in the west to the Hill End Trough, in the east, and from the southern Surat Basin to the Lake George extensional basin. Potential field analysis, integrated with newly available seismic reflection interpretations, reveals new complexity to previously identified structures such as the Gilmore Fault Zone, the Coolac-Narromine Fault Zone, the Jugiong Shear Zone, and the Parkes Fault Zone. For example, the linkage of the Gilmore Fault Zone south into Victoria, as suggested by some workers, is not well supported by these data. The postulated Lachlan Transverse Zone appears problematic, with no recognisable, coherent potential field signature in any dataset. As well as adding significant third dimension information to these crustal boundaries, the deep crustal architecture of Ordovician volcanic belts has also been clarified. For instance, the Parkes-Narromine belt shows distinct mid- and deep-crustal linkages to the Lake Cowal belt, and both are characterised by large diameter circular features which persist to the highest levels of upward continuation, suggesting deep crustal or even mantle roots. The likely extension of the Molong belt under shallow cover is identified in the north of the region in the Gilgandra map sheet area. Acknowledgements: AGCRC authors at AGSO publish with the permission of the Chief Executive Officer, Australian Geological Survey Organisation, and the Director, Australian Geodynamics Cooperative Research Centre.

132

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE TURN OF THE WORMS: MINERAL SYSTEMS IMPLICATIONS OF CRUSTAL STRUCTURES IN THE EASTERN LACHLAN OROGEN DERIVED FROM MULTI-SCALE EDGE ANALYSIS Nicholas G. Direen^ Nicholas J. Archibald^ Timothy E. M a c k e y \ Ollie L. Raymond^ Patrick Lyons^ Russell J. Korsch and E. June HilP 'Australian Geodynamics Cooperative Research Centre, AGSO, GPO Box 378, Canberra, ACT, 2601 ^ Fractal Graphics Pty Ltd, 39 Fairway Nedlands WA 6009

The Australian Geodynamics Cooperative Research Centre has been developing exploration models for several mineral provinces in Australia. In the eastern Lachlan Orogen of New South Wales, development of the exploration models has included an integrated analysis of new potential fields and deep crustal seismic data, together with geological data. Interpretations of the bulk crustal architecture, age and composition are constrained by matching field-based petrophysical and structural observations with crustal blocks defined by multi-scale edge analysis, or "worms", of magnetic intensity and Bouguer gravity data. These results have been analysed using a mineral systems approach (Wybom et a/., 1994) for several types of systems including volcanic hosted massive sulphides, porphyry related mineralisation including skarns, Cobar-type basin inversion systems, and structurally controlled epigenetic gold systems. Particular attention has been paid to the identification of major crustal structures capable of tapping the deep crust and or upper mantle, delineation of the geometry of fertile Ordovician volcanic packages, and several phases of felsic intrusions. Mineral systems interpretations have been constrained with relationships to known mineral occurrences from the Mineral Resources NSW METMIN database. Fertile spatial associations have then been mapped under more recent cover of the Sydney, Gunnedah and Great Artesian Basins. This approach has identified several areas of high prospectivity for the target mineral systems in areas of shallow cover, as defined by depth to basement analysis of potential fields. In addition, the integrated analysis of multiple datasets has clarified genetic models for some specific deposits in the eastern Lachlan Orogen. Acknowledgement: AGCRC authors at AGSO publish with the permission of the Chief Executive Officer, Australian Geological Survey Organisation, and the Director, Australian Geodynamics Cooperative Research Centre. Reference WYBORN L. A. I., HEINRICH C. A. & JAQUES A. L. 1994. Australian Proterozoic mineral systems: essential ingredients and mappable criteria. Australian Institute of Mining and Metallurgy Publication Series 5/94, 109-115.

133

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

QUANTIFYING ENHANCED SOIL COHESION DUE TO THE ROOTS OF Casuarina cunninghamiana. B. B. Docker and T.C.T. Hubble School o f Geosciences, University o f Sydney, N S W 2 0 0 6

Several studies have demonstrated that the clearance of riparian vegetation reduces the stability of alluvial river banks and can promote bank collapse during or just after flooding. A major factor implicated in lowering the stability of devegetated river banks is the absence of tree roots. Modelling slope stability in this situation requires quantification of the vegetative component of soil cohesion and establishing the depth to which tree roots penetrate and reinforce the soil. It is generally believed that tree roots contribute to soil strength through an increase in apparent cohesion and have little if any affect on the frictional component of that strength. An assessment of the vegetative component of cohesion provided to soils by roots of the tree Casuarina curminghamiana (river she-oak) is reported. A small plantation of these trees has been grown on soils considered representative of alluvial riverbanks of the upper Nepean River. In situ shear-box tests have been performed on soils reinforced by tree-roots and on the same soil without tree-roots. These tests indicate that these tree roots provide a significant increase in soil strength. The trees tested were one and half years old and measured an average of three metres in height. The root systems of some juveniles were excavated and exposed to reveal their subsurface architecture and biomass. These Casuarina cunninghamiana specimens exhibit a single large tap root from which extrudes a dense network of lateral roots. A tentative relationship between tree-stem height and rooting depth is reported which indicates that the central taproot can penetrate to a depth at least one-quarter to one-third of the height of the tree in an alluvial soil. Field examination of the roots of a large mature tree exposed in a slump scar elsewhere along the upper Nepean indicate that it is not unreasonable to infer that this relationship can hold for mature trees rooted in deep soils as well.

134

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE LIQUID BISMUTH COLLECTOR MODEL: AN ALTERNATIVE GOLD DEPOSITION MECHANISM Naomi Douglas ^ J o h n Mavrogenes'^ Alistair Hack^ and Richard England^ ^ Department of Geology, Australian National University, Canberra, ACT, 0200. ^ Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200. ^ Consulting Geologist, Townsville, Qld, 4810. Bismuth is often found as a significant component in hydrothermal gold deposits (for example Tennant Creek, NT; Mount Todd, NT; Lucky Draw, NSW; Maldon, Vic). However, it is commonly disregarded because of its low economic value. This study outlines the potential genetic importance of bismuth in hydrothermal gold deposits. Bismuth is distinguished by its low melting point of 271.4°C, which decreases with increasing pressure. Native bismuth is stable compared to its oxide and sulfide compounds over a large range of f 0 2 and fS2. Therefore, where soluble bismuth complexes are destabilised above 271.4°C, and at fS2 below the pyritepyrrhotite buffer, bismuth will precipitate as a liquid rather than a solid phase. As a liquid, bismuth will remain mobile and reactive, long after its initial precipitation fi-om solution. A close association between bismuth and gold is noted in bismuth-bearing gold deposits, either as discrete phases or as the intermetallic mineral maldonite (Au2Bi). Such an association suggests that liquid bismuth is intimately involved in the process of gold deposition. As most geothermal fluids are undersaturated in gold by several orders of magnitude at high temperature (Brown, 1986), unusual processes, such as boiling, are necessary to trigger gold deposition. We propose the collection of gold out of solution into liquid bismuth as an alternative depositional mechanism. To test this hypothesis, we carried out a series of experiments on the partitioning of gold between a hydrothermal solution and liquid bismuth. Our experiments showed that the presence of liquid bismuth removed gold from an undersaturated solution. The binary phase diagram for gold-bismuth shows that a bismuth-rich liquid will dissolve ~20wt% gold at SOOjC (Okamoto and Massalski, 1983). This value is far higher than the capacity of any hydrothermal fluid at this temperature. Therefore, the partition coefficient for gold between liquid bismuth and the hydrothermal fluid will be very high. This represents a powerful mechanism to concentrate gold from an undersaturated solution. Further, a bismuth-rich liquid containing gold will persist as a discrete phase down to very low temperatures (below 250°C). Crystallisation of gold and bismuth will occur at later stage than most silicates, hence the common intimate association between the two metals. We believe that this process is analogous to natural systems, where liquid bismuth collects gold from undersaturated solutions. Therefore, deposits containing significant amounts of bismuth may have formed without the requirement of gold saturation in the fluid. References BROWN, K.L. 1986. Gold deposition from geothermal discharges in New Zealand. Economic Geology 81, 979-983. OKAMOTO H. & MASSALSKI T.B. 1983. Au-Bi (Gold-Bismuth). Bulletin of Alloy Phase Diagrams 4, 238-240.

135

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A REVIEW OF SULPHUR ISOTOPE RESULTS FROM LATE SILURIAN VHMS MINERALISATION, HILL END TROUGH, NSW Peter M Downes\ Phil Seccombe^ and Steve Brown^ ^Geological Survey of New South Wales, PO Box 536, St. Leonards NSW 1590 ^School of Geosciences, University of Newcastle, Callaghan N S W 2308 The north-eastern Lachlan Fold Belt contains significant volcanic-hosted massive sulphides in Late Silurian felsic volcanics and associated sediments of the Hill End Trough. On the western side of the Trough, the Mumbil Group hosts the Lewis Ponds, Mt Bulga, Calula, Commonwealth, Kempfield (barite), and Peelwood deposits. Other significant units include the Chesleigh Group which hosts the Belara and Sunny Comer mineralisation, and the Tannabutta Group which hosts the Lue base-metal mineralisation (Accost prospect). The signature for the Lewis Ponds, Mt Bulga (Chisholm 1976), Belara and Accost mineralisation are all very similar and vary from -1.7 to 5.9 per mil. The results suggest that sulphur in these deposits was derived largely from magmatic sources, although with some contribution from seawater sulphate. The values for pyrite from the Calula mine area lie in a narrow range from 4.0 to 7.6 per mil (av. 6.1 per mil; Seccombe and Skirrow, unpublished data). At Kempfield, Bums and Smith (1976) reported values for galena (3.4 to 6 per mil), sphalerite (4.2 to 8.4 per mil), pyrite (8 to 10 per mil) and barite (29 per mil). Sulphides from the Commonwealth Mine have values ranging from 3.1 to 10.1 per mil (av. 7.6 per mil) with the majority of analyses clustered between 7.0 to 10.1 per mil (James 1984). The S^'^S values for Sunny Comer range from 1.7 to 10.7 (av. 7.4) per mil, with pyrite and galena from the massive sulphides tightly clustered (5.7 to 8.8 per mil). Multiple sources of sulphur are inferred for the John Fardy deposit at Peelwood. values for pyrite, from black shales (range - 2 . 1 to 1.3 per mil), are lower than chalcopyrite and pyrite from cherty exhalites (range 4.6 to 11.4; av. 8.2 per mil). The massive sulphides have higher values (11.9 to 13.7; av. 13.0 per mil), similar to a silica-rich tuffaceous unit (12.3 to 13.3; av. 12.8 per mil). Sulphides associated with later syntectonic vein quartz range from 1.9 to 4.8 per mil. The 8^'^S results from John Fardy are the highest in our study, suggesting a greater contribution from seawater sulphate, compared to other VHMS deposits in the Hill End Trough. The 8^'^S values appear to be independent of the host rock units, however a relationship is proposed between mineralisation and proximity to felsic volcanic centres. The Mt Bulga, Lewis Ponds and Accost occurrences are close to felsic volcanic centres and have more magmatic signatures. By contrast, the Sunny Comer, Peelwood, Calula, Kempfield and Commonwealth areas are more distal and are characterised by ^'^Senrichment and weaker magmatic signatures. An exception to this is the Belara deposit, which is hosted by reworked felsic volcanics and has a magmatic 8^'^S signature. The data indicates a link between water depth, potential for boiling of hydrothermal fluids and resulting mineralisation style. Massive sulphide occurrences, such as Belara (deeper-marine) and Lewis Ponds and Mt Bulga (basin margin), contrast with veinstyle/disseminated mineralisation at Accost (shallow marine). However, all of these deposits have a similar isotope distribution. Acknowledgment: Publication is with the permission of the Director-General, N S W Department of Mineral Resources. References BURNS M.S., & SMITH J.W., 1976. ^"^S/^^S Ratios in some sulphides of the Lachlan Fold Belt.

Australasian

Society of Exploration Geophysicists, Bulletin 7 no 1, pp 43- 44. CHISHOLM J.M., 1976. Geochemical zoning in a volcanogenic Cu-Pb-Zn-Ag sulphide deposit at Mt Bulga, NSW, Australia. PhD Thesis, University of New South Wales, Sydney (unpubl.). JAMES A.C., 1984. The geology and genesis of base metal mineralisation in the Yarragal area. New South Wales. BSc (Hons), Thesis University of Newcastle, Newcastle (unpubl.).

136

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney. July 2000

WEATHERING MECHANISMS AND RATES OF DECAY OF SYDNEY DIMENSION SANDSTONE Deirdre Dragovich School o f Geosciences, University of Sydney

Hawkesbury Sandstone, a medium to coarse-grained quartzose sandstone, has been widely used as dimension stone in the Sydney region due to its attractive appearance, ready availability, ease of working and relative durability. The stone is relatively permeable, has variable grain size and matrix (clay) content, and widely differing durability. Weathering processes involving solution, hydration, organisms, pollutants and salts have altered the physical and chemical properties of the stone, resulting in a loss of cohesion, a reduction in bulk density, an increase in volume, an increase in surface area, an increase in permeability, a reduction in strength, and the formation of new minerals and solutions. Weathering outcomes include micro-pitting or surface roughening, scaling and honeycombing. Stone deterioration proceeds more slowly where shallow 'case-hardened' (iron-rich) surfaces have developed, although breaching of this layer results in accelerated rates of decay. Rates of weathering are variable, depending on lithological and environmental (weathering and site) factors, but usually exceed 1 mm annually where honeycomb pits are formed. In the Sydney region, salt weathering results in a pattern of decreasing weathering rates with (i) increasing height above the spray zone and (ii) increasing distance from oceanic salt inputs. The water absorption capacity and clay content of a stone provide useful indicators of its durability. Weathering of dimension stone is generally most important when it proceeds rapidly and non-uniformly and where this deterioration affects load-bearing capacity or adversely impacts on aesthetic values.

137

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PROBLEMS OF DEFORMATION STYLE IN THE NORTHERN LACHLAN FOLD BELT D.W. Dumey Department of Earth and Planetary Sciences, Macquarie University, NSW 2109

The Lachlan Fold Belt (LFB) of southeastern Australia displays many remarkable and enigmatic features, not least in the post-Benambran/Quidongan deformation effects on Upper Silurian to Devonian rocks of the northern LFB, the subject of this discussion. Continuing debate concerns such questions as (a) whether the structure of these rocks is best explained by thrust-dominated models, (b) the extent to which convergent deformation preceded or postdated Late Devonian sedimentation, and (c) whether some of the movements were convergent or divergent. It is suggested that 'rheo-logic', or how rocks might reasonably be expected to deform under given conditions, should form a basis for interpreting the significance of observed structures, rather than merely an assumption that the amount of deformation reflects how many events a rock has experienced. Competency of major lithotectonic units is fundamental for governing intensity and distribution of deformation. In an upper crustal regime, competency will depend mainly on resistance to diffusion-flow: weakest in turbidites and other muddy units, moderate in coarse volcaniclastics and shelf sandstones and limestones, moderate to strong in volcanics, and strongest in quartzite and felsic to mafic plutonic rocks. The following are some scenarios where heterogeneous competency distributions may affect deformation intensity and distribution: 1) Differentiation into submeridional paleogeographic highs and lows (the 'custard-tart' model). The troughs, comprising mainly incompetent strata on a thin crust, should be more deformed and thickened than contemporary rocks in the adjoining highs. Depth of erosion and degree of metamorphism should thus be greater in the troughs (which could explain why Upper Devonian rocks are often absent from these zones). Trough margins should be reverse-faulted outwards where deformed thickness exceeds that of the highs and reversefaulted inwards where it does not. 2) Plutons versus slates (the 'fruit-cake' model). Granitoid plutons ('nuts' and 'cherries') should be less deformed than the commonly Ordovician turbidite enclosing country rocks ('cake mix'), and then mainly by plastic flow (shear zones) on their margins. Intrusions of batholith dimensions may stiffen a zone so that it behaves semi-competently as a whole. 3) Competent units in major folds (the 'hamburger' model). The greatest deformation should occur in less competent rocks in the inner arcs of folded more competent layers: e.g., in synclines stratigraphically above thick quartzite or volcanic rocks and in anticlines stratigraphically below such units (except in granite).

Given the longitudinal stiffening effect of major batholithic and volcanic belts in paleogeographic highs, and weak intervening troughs, the most likely option for orientation and kinematics of deformation appears to be - variable normal to oblique convergence across the belts, including possible strike-slip partitioning at belt margins, depending on displacements at the boundaries of the LFB as a whole (the upright, and thick, 'banana-skin' model).

138

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A THRUST-RAMP MODEL FOR GOLD MINERALISATION AT THE ARCHAEAN TRONDHJEMITE-HOSTED TARMOOLA DEPOSIT, LEONORA CAMP, WESTERN AUSTRALIA Paul Duuring and Steffen G. Hagemann Centre for Teaching and Research in Strategic Mineral Deposits, Department of Geology and Geophysics, The University of Western Australia, Nedlands, 6907, Western Australia;

Most Archaean gold deposits in the Yilgam Craton of Western AustraHa have a strong structural control whereby ore zones are localised within shear and fault zones. Tarmoola is atypical in that ore zones are not hosted by any discrete structure but are sited in a pressure shadow that developed along steeply dipping margins of a trondhjemite pluton due to ramp-thrusting of supracrustal rock over the pluton during gold mineralisation. The trondhjemite-hosted, Tarmoola deposit has resource estimates of at least 3.1 Moz gold and is situated in the Leonora Camp about 30 km NW of Leonora in the Norseman-Wiluna Belt of the Yilgam Craton in Western Australia. The mine lithostratigraphy comprises a domed, up to 100 m wide, massive trondhjemite pluton that is surrounded by ENE-striking, supracrustal rocks that dip moderately (50°) to the NE and include pillowed tholeiitic basalt, basaltic komatiite, chloritic siltstones, and a volcaniclastic unit. The pluton and supracrustal rocks are cut by several andesitic feldsparporphyry intrusions. The chlorite-actinolite-epidote metamorphic mineral equilibrium assemblage in tholeiitic basalt indicates regional greenschist facies metamorphism. Goldbearing extensional and crack-seal veins post-date metamorphism, occur in all rock types, and comprise mainly quartz and carbonate minerals with minor pyrite, chalcopyrite, sphalerite, galena, gold, and rare scheelite. Major gold zones (> Ig/t Au) in the supracrustal rocks are located above the pluton and along the steeply dipping, eastern and northeastern margins of the pluton. Ore zones in trondhjemite are controlled by conjugate auriferous quartz-carbonate veins that strike WNW and dip steeply to the SW. Northwest, subhorizontal compression during the gold event formed conjugate goldbearing veins in the trondhjemite. A < 1 m wide, shear zone located along the shallowly west-dipping trondhjemite margin has S-C shear fabrics that indicate oblique reversesinistral movement. The dominantly thrust movement of supracrustal rock over the pluton caused the reactivation of the steeply east-dipping trondhjemite margin and a pre-existing subparallel foliation in the supracrustal rock. Failure along the pluton margin and foliation caused the development of zones of low deviatoric stress. Ore fluids drained into these dilatant areas to formation ore zones. The Granny Smith gold deposit near Laverton in the Yilgam Craton is analogous to Tarmoola in that the granitoid acted as a competent body in ductile country rock. Ore zones formed in areas of low deviatoric stress along steeply dipping pluton margins. Acknowledgements: This abstract is part of a doctoral study funded by an Australian Postgraduate Award scholarship and PacMin Corp. Ltd. Wayne Spilsbury and Peter Langworthy are thanked for their permission to publish this abstract.

139

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE HYDRATION OF SILICA AND ITS ROLE IN THE FORMATION OF QUARTZ VEINS John Elliston Elliston Research Associates Pty Ltd., lOB The Bulwark, Castlecrag, N.S.W. 2068 Simplistic views of silica solubility in some geological literature should be updated to include more recent developments in chemical research. The aqueous chemistry of silica involves not only the dissolution of mono-silicic acid, but a sensitive equilibrium over a wide range of parameters to the oligomers, polymerisation to colloidal silica and development of disordered solvated layers or adsorption on solid surfaces. The 'resident species' of natural silica in basin sediments is mainly a series of small particles of amorphous polymeric silica adsorbed on surfaces of other sediment substrates. These particles can be aggregated by simple shear (viscous flow) or released from surfaces by changes in salinity or pH. Current consideration of hydrated of silica species and their role in the formation of quartz veins therefore: • Indicates the source of silica that forms common quartz veins and the way it is desorbed from other sediment substrates to form suspensoids in ore forming brines. • Provides an explanation for the fluid injection of quartz veins and hydraulic fracturing. • Provides an explanation for plastically distorted forms of quartz. • Provides an explanation for colloform textures in quartz veins. • Provides an explanation for the suspension of heavy fragments in quartz veins and quartz-magnetite lodes. • Provides an explanation for episodic quartz vein re-mobilisation and re-injection. • Provides an explanation for the occurrence siliceous concretions. • Provides an explanation for the formation siliceous accretions. • Provides an explanation for ptygmatic quartz veins. • Provides an explanation for replacement by silica. It is still sometimes assumed that thermodynamic equilibria, correctly established for chemical reactions in solution, would apply under similar conditions within the pore spaces of natural sediments. 'Modelling' of diagenetic reactions, ore forming solutions, "leaching" by brines, convective re-cycling of concentrated saline solutions, and precipitation of ore minerals has been attempted on this basis. Diffusion rates and adsorption equilibria control chemical reactions in sediment pore fluids. Solubility constants change markedly in the immediate vicinity of solid surfaces. These 'catalytic' or surface effects must be considered in any studies of the release, movement, and concentration of mineral species within natural particle systemsost natural quartz appears to crystallise from various aggregates of polymeric silica. Fluids included in these crystals could therefore be those rejected during syneresis and crystallisation of the pre-crystalline polymers. The salinity of such entrapped fluids would have no relation to that of the fluids by which the original polymeric precursors were introduced. The assumption that homogenisation temperatures measured for fluid inclusions enclosed in such quartz crystals reflect those of the depositing suspensoids could be false. Banded iron formations are recognised as having been deposited as silica gel (siliceous ooze) and ferric hydroxides. If these interlayered 'chemical sediments' had been subjected to mechanical disturbance during diagenesis, separation to form the massive primary iron orebodies would have occurred by differences in Bingham yield point and rheology. There is clear evidence that some massive haematite ores have been mobile and intrusive. In many places they are associated with quartz and chert breccias. The thixotropic and rheological properties of silica gel and ferric hydroxide indicate that large haematite bodies originate by simple mechanical disturbance of the original sedimentary materials during diagenesis. Natural concretionary structures which occur in orebodies and a variety of rock types derived from basin sediments can develop only in aqueous particle systems. The development of polyrimmed concretions or multi-layered colloform textures depends on the polar nature of water molecules and the basic properties of the particles involved. Particle size, particle shape, particle charge, van der Waal's attraction, and charge distribution on particle surfaces govern the behaviour of particulate species. The formation of polyrimmed concretions or oolites involves at least six of these basic behavioural characteristics (adsorption, diffusion, van der Waal's attraction, 'close packing', syneresis, and desorption). Recognition that diffusible polymeric amorphous silica is mobilised into veins and lodes, silicifles wall rocks, forms opal, replaces shells and tree trunks, etc. clearly provides a better basis for understanding how diffusible hydroxy-sulphide particlulate species might similarly be mobilised into veins and lodes, permeate shales, form framboids, replace fossils and plant fragments or the fine shale bands in layered orebodies, etc.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE PETROPHYSICS OF THE HAWKESBURY SANDSTONE D.W. Emerson Systems Exploration (NSW) Pty Ltd Honorary Research Associate, School of Geosciences, University of Sydney 2006 NSW A laboratory mesoscale petrophysical study was carried out on 100 Hawkesbury Sandstone core samples from the central Sydney Basin; a few mudstones were also tested. Properties measured were mass (dry bulk density, effective porosity, water saturated bulk density, apparent grain density, mass % water absorption), magnetic (volume susceptibility), galvanic electrical (saturated state resistivity @ 1 kHz, and phase lag [IP effect] @ 0.1 Hz), and ultrasonic P wave velocity under uniaxial loading. Most of the core samples were quite fresh and came from a 265 m thick sequence, the top of which was intersected at 25 m depth under Wianamatta Shale by Pacific Power's Eveleigh borehole (RL - 2 5 m amsl) near Redfem, ~3 km south of the Sydney CBD. Geophysical logs run in this hole were scanned for natural gamma count readings, and also for sonic velocities, which agreed well with the laboratory determinations. Solutions of 10 ohm m (550 ppm) and 40 ohm m (150 ppm) resistivity [ 2 5 T ] were used to vacuum-saturate the fresh and weathered cores, respectively, for the mass, electrical and velocity tests. The results, summarised below (Table of mean values), reflect, in physical terms, the quartz (60%) - clay (20%, 12:8:2 approx proportions of kaolinite/illite/ mixed layers) - secondary silica (10%) - miscellaneous detritus and carbonate cement (10%) - nominal composition of porous Hawkesbury Sandstone. Electrical anisotropy was found to be significant and po 1 : po// averaged 1.60 for the Eveleigh samples. Velocity anisotropy was minor with Vp / / : Vp 1 averaging 1.04. The velocity and other data may be used to make empirical estimates of rock strength. Crossplots of data show inverse relationships between resistivity and porosity, and velocity and porosity. Significant IP effects observed in the sandstones have been ascribed to their sparse graphite content. These and other relationships between properties are fairly broad and are explicable in terms of texture, heterogeneity, and compositional variation (as has been found in other studies). In transposing the resistivity and velocity data to the macroscale (at shallow depths of geotechnical interest) difficulties arise owing to the dearth of documented calibrated data on rock discontinuity physical characteristics at various exposed levels in the Hawkesbury Sandstone. However if certain assumptions are made then estimates might be made of in situ properties in jointed/fractured zones.

DBD g/cm^

lithology

porosity Pa %

WBD g/cm^

IP ^ galv. elec mag k j • resistivity phase lag mrad Slx10'° ohm m (Pp, normal to bed)

• water SGGA abs. mass g/cm^ %

gammaj u'sonic count ; i Pwave vel. Vp m/s (Po, normal to bed)

API

" (Pw= 10) . Eveleigh-Central Sydney fresh (weath.) Wian. Gp mudstorii

2.25

16.3

2.42

fresh Hawkesbury Ss

2.36 (0.09) '

12.5 (3.02)

2.49 2.69 (0.07) • (0.04) '

2.58

7.5

2.65

: 2.79 1

2.52

11.8

2.64

i 2.86 1

(sdev) mudstone in H Ss sequence Nan-abeen Gp claystone 7 Sydney localities shallow-sl. weath

2.69 1.03 (0.37)

(sdev)

48

9

97

(7)

(48)

30

100

7

1667

19

9'

I

---4

slightly weath H Ss

I

15

- I

14.9 (3.9)

1.42

4"

(0.10)1 j (ao4) j

(0^) i

(6)

2.21 (0.12) :

18.7 (3.5)

2.66 2.39 (0.09) 4 (0.14)

3.0 (2,^

(3)

2.68 ^

10 (s dev)

3755

(28)

(218) 3660

60

3600

262 _ (276)

3370 (399)

29 (6)

(Pw = 40_).

Bush Rock - Sackville North weathered H Ss

66

JPw = 40)

2.28 (0.13)

2.43

76 21 (5)

107

141

216

28

J1221 T

(9)

i !

2618 (482) In=6]

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TIDAL RHYTHMITES AS HIGH-RESOLUTION TIME CLOCKS IN BASIN ANALYSIS: EXAMPLES FROM THE APPALACHIAN BASIN, USA Kenneth Eriksson', Daniel Miller^ and Rhonda Adkins^ ' Department of Geological Sciences, Virginia Tech, Blacksburg, Virginia 24061, USA ^Marathon Oil Company, 1501 Stampede Ave., Cody, Wyoming 82414, USA ^Department of Geology, James Cook University, Townsville, Queensland

Tidal rhythmites consist of submillimetre- to centimetre-scale interbedded sandstone and mudstone laminations that display systematic variations in laminae thickness. In the absence of fossils, rhythmites provide evidence for marine depositional environments and serve as high-resolution depositional records from which it is possible to evaluate the completeness of the stratigraphic record. Tidal rhythmite successions are found within highstand facies of stacked, unconformity-bounded cyclothems that comprise the middle-Pennsylvanian Breathitt Group in Kentucky. Specifically, progradational deltaic deposits contain several 15-metre thick, delta-front rhythmite intervals in which individual clay-draped sandstone beds commonly occur as thick-thin pairs. The sandstone beds range in thickness from 0.5 to 20 cm, and are interpreted as tidal event laminae that reflect semi-diurnal ebb tidal flows. Cycles of up 28 sandstones are interpreted as neap-spring cycles. Where rhythmite deposits preserve several months of continuous sedimentation, successive neap-spring cycles display a thick-thin relationship reflecting unequal lunar perigean and apogean tides. Estimated accumulation rates that persisted for a number of months ranged in general between 20 and 100 cm per neap-spring cycle, but locally reached 20 cm per day in the most proximal deltaic environments. Another rhythmite succession is preserved as the the upper-Mississippian Pride Shale in southern West Virginia. This unit attains a thickness of up to 60 meters where it filled a NE-SW oriented foreland trough (300 km long by 50 km wide) that extends southward into northeastern Tennessee. The Pride Shale, together with the underlying incised fluvio-estuarine facies and overlying delta-plain facies, comprise an unconformity-bounded, fourth-order depositional sequence of ca. 400 ka duration. Detailed analysis of laminae bundling in the Pride Shale reveals a hierarchy of tidal cycles (semi-diurnal, semi-monthly neap-spring and monthly perigee-apogee) and a nontidal cycle of seasonal, fluvial discharge. Decompacted annual cycle thicknesses indicate that accumulation rates ranged between 3 and 20 cm per year, and locally reached rates of up to 60 cm per year. Because the annual cycles average 10 cm in thickness, the bulk of the Pride Shale in any one vertical section is estimated to have accumulated rapidly in ca. 600 years. Simultaneous progradation of coalescing clinoforms, each with an inclination of 2^ and a width of ca. 10 km, is inferred to have infilled the Pride Basin in the relatively short time period of 100 ka. Most of the time represented by the entire depositional sequence thus is recorded in the bounding unconformities and in a condensed section at the base of the prodeltaic beds. In both the Breathitt Formation and the Pride Shale, deltaic rhythmite deposition rapidly infilled the available accommodation space that developed between the longer term sequence boundaries.

142

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AREA SELECTION IN AUSTRALIA - RISK REDUCTION AND VALUE CREATION IN MINERAL EXPLORATION Michael A Etheridge and Phillip Uttley, SRK Consulting Level 2, 118 Alfred St, Milson's Point, NSW 2061

Mineral deposit discovery is a simple two-step process. First, acquire secure title over the ground that contains a deposit of the type and size required, and, second, apply the appropriate techniques to locate and delineate it! This is, of course, a gross simplification of what is a complex and highly risky process. However, it does have an important corollary - no matter how good one's exploration techniques, it is not possible to make a discovery on ground which does not contain a deposit. In other words, selecting the area(s) most likely to host a mineral deposit is critical to managing risk and creating value in mineral exploration. This paper will discuss two general aspects of area selection, with illustration from Australian examples. 1. The importance of being in a well-endowed mineral province, armed with a well formulated geological model for the key deposit style(s). Singer (1995) starkly demonstrated the very high level of concentration of historic production plus reserves of several base and precious metals in a few very well endowed countries / mineral belts. Australia ranks in the top 10 producers of a wide range of commodities. The Eastern Goldfields of the Archaean Yilgam Craton ranks as a world-class gold belt by any criteria, and we will examine how this impacts on area selection strategies for gold companies. The other truly world-class belts in Australia are the Proterozoic basins of northern Australia for Zn-Pb-Ag, the Hamersley Basin for iron ore and the Sydney-Bowen Basin system for coal. We will discuss area selection for stratiform Zn-Pb-Ag under cover in the Mt Isa Macarthur region to illustrate the importance of having a robust geological model in such a well-endowed terrane. 2. Managing risk, progress and value at the area selection target generation and drill testing stages of exploration. The business performance of mineral exploration is currently very much under the microscope, both within mining companies and by investors in general. It is difficult to demonstrate that exploration has created value for investors, either in many individual companies or across the market as a whole, over the last 10-20 years. We will examine the typical risk / progress / value profile of exploration to illustrate the importance of both selecting the area(s) most likely to contain the deposit, and progressing quickly and at minimum cost to defining and drill-testing geologically robust targets. We conclude that, despite (or partly because of) substantial advances in exploration technology, explorers have become increasingly risk-averse. The proportion of budgets spent on defining geochemical anomalism has dramatically increased, but that devoted to drill-testing of robust geological targets has decreased. As a result, we have become better and better at defining "mineralisation" and less effective at finding mineable deposits. As Singer and Kouda (1999", p.l 12) succinctly stated, "Exploration is an economic activity, so the definition of failure must also be economic; that is, a technical success of locating a mineral deposit must be considered a failure if the deposit is not economic to mine at the time of discovery." References Singer, D.A., 1995. World class base and precious metal deposits - a quantitative analysis. Economic Geology, v. 90, pp. 88-104. Singer, D.A. and Kouda R., 1999. Examining risk in mineral exploration. Natural Resources Research, V. 8, pp. 111-122.

143

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TOWARD A QUANTITATIVE RECONSTRUCTION OF KENORLAND, THE ARCHEAN-PALEOPROTEROZOIC SUPERCONTINENT David A.D. Evans and Chris McA. Powell Tectonics Special Research Centre, The University of Western Australia, Nedlands, WA 6907, Australia Although a daunting task, the quantitative reconstruction of Proterozoic supercontinents, and their assembly and dispersal histories, can carry important implications for geodynamics, long-term paleoclimatology, and biological evolution. Much recent work has focussed upon Rodinia, the late Mesoproterozoic to early Neoproterozoic immediate predecessor of Pangea. The existence of earlier supercontinents has long been suggested by global peaks in orogenesis and isotopic ages at 2.7 and 1.8 Ga; respectively, these hypothesised supercontinents have been named "Kenorland" and "Nuna." Globally widespread large igneous provinces are common at 2.45 and 2.2 Ga, perhaps indicative of Kenorland's breakup during a protracted interval of ca. 250 Myr. Which cratons did Kenorland comprise? Its geological basis lies within the Superior craton of southern Canada, and commonly depicted bedfellows are Baltica and the Wyoming Province. Some authors have espoused the existence of a counterpart Archean-Proterozoic supercontinent, "Zimvaalbara," containing the Zimbabwe, Kaapvaal, and Pilbara cratons, linked by widespread bimodal igneous activity at 2.7-2.8 Ga and extensive cratonic sedimentation at 2.6-2.4 Ga, in marked contrast with Superior's NeoarcheanPaleoproterozoic tectonic record. Nonetheless, the bimodal volcanic episodes at 2.45 and 2.2 Ga, which define Kenorland's fi-agmentation, are represented on the Pilbara (both episodes) and the Kaapvaal (2.2 Ga only). We suggest that the Kenorland supercontinent could have included these cratons. Paleomagnetic data are reasonably complete for Superior at 2.45 Ga and 2.2-2.0 Ga, for Kaapvaal at 2.2-1.9 Ga, and for Pilbara and Baltica at 1.8 Ga. If the 2.45-2.2 Ga igneous activity on these cratons represents supercontinental fragmentation, then we should compare the older available paleomagnetic data to test possible juxtapositions. The most direct test in this regard is between the Kaapvaal and Superior cratons at 2.2-2.0 Ga. Direct juxtaposition of southern Kaapvaal against eastern Superior is permitted at 2.22 Ga, according to primary data from the Ongeluk (Kaapvaal) and Nipissing (Superior) igneous events. Such a "Supevaal" connection cannot have persisted to 2.0 Ga, according to the respective apparent polar wander paths that show post 2.2-Ga divergence. The "Supevaal" connection, if valid, would juxtapose blocks with admittedly substantially distinct geological histories between 2.7 and 2.4 Ga. Nonetheless, we may consider Phanerozoic cratonic North America as a relevant example of indisputably joined regions with largely contrasting histor>': the mainly Archean Canadian shield that was exposed throughout most of the Phanerozoic, and the mainly Paleoproterozoic centt-al United States that was repeatedly transgressed with cratonic sedimentation during the last 500 Myr. Contrasting geological histories, unfortunately, do not serve as critical falsifying tests of Precambrian continental reconstructions. Quantitative reconstruction of Kenorland, perhaps building upon the "Supevaal" connection, will require more precisely dated, reliable paleomagnetic data from Archean-Paleoproterozoic cratons. Meantime, several generally related issues may be considered. Widespread glaciations, including low-latitude occurrences, appear to be associated with supercontinental fragmentation, perhaps akin to the proposed Neoproterozoic "Snowball Earth" episodes. At immediately higher stratigraphic levels, a dramatic rise of atmospheric free oxygen appears to accompany high rates of organic carbon burial indicated by the "Lomagundi" carbon-isotope excursion. These factors could be related by a biological productivity bloom, centred on newly rifted continental margins, in the Paleoproterozoic "Snowball" aftermath.

144

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NEOPROTEROZOIC BASALTS FROM NORTHWEST TASMANIA: MULTIPLE MANTLE SOURCES DURING CONTINENTAL RIFTING John L. Everard Tasmanian Geological Survey, Mineral Resources Tasmania In western Tasmania, Neoproterozoic shelf sequences comprising variable proportions of carbonates, clastics and mafic volcanics commonly rest unconformably on (?)Mesoproterozoic siliciclastic sequences. In the Smithton Synclinorium (SS) in the far northwest, the Togari Group (TG) contains a middle volcanosedimentary unit, the Kanunnah Subgroup, the age of which is constrained at 650-580 Ma by isotope chemostratigraphic data on enclosing carbonate units (Calver, 1998). The volcanics consist mainly of massive to locally pillowed, sparsely plagioclase+-clinopyroxene+-olivine-phyric, dom.inantly tholeiitic basalt (Spinks Creek Volcanics, SCV), which has undergone prehnite-pumpellyite to greenschist facies metamorphism. These lavas and associated subvolcanic intrusives can be subdivided into at least seven geochemically tightly defined members, each with a distinct regional distribution and stratigraphic position. East of the syndepositional Roger River Fault (RRF), fractionated (Mg# 54-59) low- TiOs (-0.7%) tholeiites (Group A) occur at the base of the SCV. These lavas have nearly flat, slightly concave REE patterns and moderately high Nb/Zr (-0.23). Elsewhere in the SS, the near-primitive (Mg# 66-71) but otherwise similar Group P occurs at least partly as subvolcanic intrusions. The uncommon and widely dispersed Group E lavas are strongly fractionated (Mg# 28-37) but have similar Nb/Zr. Higher in the sequence, groups B, C and D are fractionated tholeiites (Mg# 46-62) with low Nb/Zr (0.030.12), successively higher Ti02 (-1.1%, - 1 . 6 % , - 2 . 3 % ) and linear, successively more LREE- enriched REE patterns. Group B is most common in the south of the SS, Group C forms the bulk of the thick pile east of the RRF against which it may have ponded, and Group D predominates in the west. Group F, which occurs west of the RRF, mainly as subvolcanic intrusions but also as rare lavas at the top of the SCV, has distinctly alkalic affinities, with higher Nb/Zr (0.45), P2O5, K2O and strongly LREE-enriched, concave REE patterns. It is suggested that Groups B, C and D were produced by successively smaller degrees of batch partial melting of a depleted, shallow asthenospheric or lithospheric source, whilst the older Groups A, P and E contain a minor component derived from a LREE- and Nb- enriched, possibly plume-derived source. In the youngest Group F this enriched component is dominant. Impingement of an upwelling mantle plume on the thin Neoproterozoic lithosphere may have induced continental rifting and associated magmatism. East of the SS, the NNE-trending mainly tholeiitic dolerite dykes of the Rocky Cape Dyke Swarm may belong to the same magmatic episode, but have higher Si02, K2O and Rb, suggesting crustal contamination. Other western Tasmanian Neoproterozoic sequences seem likely broad correlates of the TG, but most lie east of the Arthur Lineament, a zone of high strain and Cambrian, locally high pressure metamorphism (Arthur Metamorphic Complex, AMC). This feature may have also been initiated by Neoproterozoic continental rifting, leading here to ocean opening (c.580- ?550Ma). It is suggested that subsequent closure (?550-500Ma) was initially by W- to NW-directed intra-oceanic subduction, until an arc-continent collision at about 515Ma resulted in the southeastward obduction of both arc (including boninites, low Ti-tholeiites and ultramafic cumulates) and oceanic components (MORB and pelagic sediments) over the passive margin. A brief episode of E- to SE-directed subduction and associated continental margin volcanism (early Mt Read Volcanics, MRV) was terminated by a continental collision at about 500Ma, with the AMC forming along the suture, followed by late post-collisional volcanism in the MRV. This model may explain the distribution of early Cambrian allochthonous oceanic sequences (which are unknown west of the AMC), the origin of the AMC, and the similarity of its metamorphic age with the igneous age of the MRV, whilst permitting broad correlation of both the Neoproterozoic shelf and older siliciclastic sequences. Reference

CALVER, C. R. 1998. Isotope stratigraphy of the Neoproterozoic Togari Group. Australian Journal of Earth Sciences 45, 865-874.

145

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EVOLUTIONARY AND CLIMATIC CONSEQUENCES OF OPENING THE TASMANIAN SEAWAY: RESULTS FROM OCEAN DRILLING N.F. Exon\ J.P. Kennett^ M.J. Malone^ and Shipboard Scientific Party 'Australian Geological Survey Organisation, GPO Box 378, Canberra 2601 ^Marine Science Institute, University of California, Santa Barbara, CA 93106, USA ' Ocean Drilling Program, Texas A&M University, College Station, TX 77845-9547, USA In early 2000, the Ocean Drilling Program's vessel, the JOIDES Resolution, will core five sites off Tasmania during Leg 189. The research plan is described below, but initial post-cruise results will be given at the conference. Leg 189 is designed to study changes in sedimentation, oceanography, and climate caused by Australia's movement northward from Antarctica in the last 40 million years. The Tasmanian Seaway between Australia and Antarctica played a key role in the development of ice-age conditions, rarities in Earth's history. In the Cainozoic, major ice sheets developed in both polar regions, initially on Antarctica and later in the Northern Hemisphere. After drilling DSDP Leg 29 in 1973, it was proposed that climatic cooling and ice sheet development resulted from plate-tectonic changes. These changes progressively thermally isolated the Antarctic continent, as the Circum-Antarctic Current developed, following the opening of the "gateway" of the Tasmanian Seaway at about 30 Ma, and the Drake Passage south of South America at about 20 Ma. The northward migration of Australia and South America strengthened the thermal separation of Antarctica from the tropical ocean, eventually resulting in the development of the Antarctic cryosphere (ice-sheet). The world's ocean changed to one that cooled toward the poles and with depth. These paleoceanographic changes apparently played a fundamental role in the development of Cainozoic climate evolution, paleoenvironmental changes such as sea-level fluctuations, and in terrestrial and marine biotic evolution. Thus, the opening of the Tasmanian Seaway appears to have been vital to the Cainozoic global evolution of the Earth. Coring on ODP Leg 189 should be of the continuity, quality, and resolution to frilly test the hypothesis of potential relationships among plate tectonics, circum-Antarctic circulation, and global climate, and to time the key events. The five drill sites, located in water depths from 2475 to 3580 m, will be fully cored. Changes in sedimentation should have resulted from Australia-Antarctica rifting in the Eocene (40 Ma), the onset of Circum-Antarctic surface water circulation (area at 70°S), and the development of deep-water circulation (60°S). In addition, sediments recovered from drilling should document the changes as the area moved steadily northward to its present latitude on 45°S. These will include variations in geochemistry and water mass temperatures, and environmental and biotic changes as the Indian and Pacific Oceans became linked. Three sites are located at different latitudes in the Indian Ocean, one site in the Pacific Ocean, and one site between the two oceans. The average penetration is planned as 600-900 m. Most sites are designed to penetrate to strata 40 million years old, recording all the climatic changes preserved in calcareous sediments since the middle Eocene. The relatively shallow region off Tasmania, including the sunken South Tasman Rise to the south, is one of the few places on the globe where wellpreserved and almost complete marine middle Eocene to Holocene carbonate-rich sequences can be drilled in present-day latitudes of 40-50°S, and past latitudes of up to 70°S. Marine deltaic sequences characterise the Eocene, and carbonates that were deposited in ever-increasing water depths characterise the younger sequences.

146

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ARCHAEAN OBDUCTION TECTONICS R.K. Pagan Department of Mining Geology, Western Australian School of Mines, Kalgoorlie, W.A. Recent insights into the deep crustal character beneath Archaean greenstone belts in the Eastern Goldfields of W.A., revealed in seismic reflection profiles, and the regional metamorphic zonal geometry, highlight a number of important constraints on any tectonic model for the region that are not adequately explained in existing tectonic models. Principal among these are the accumulation of a thick, largely medium to deep marine sequence developing above a very thick continental crust that should have been a substantial emergent landmass of significant topography at this time. Add to this the unexplained rapid onset and short duration over an enormous area of a very high geothermal gradient, generating regional metamorphic conditions that should have resulted in an entirely molten lower crust that cooled both upwards to the surface and downwards against the mantle and that should have precluded any mafic and ultramafic magmas fi-om reaching the surface. To try and account for these and other irregularities in the known geology, a new tectonic model is presented for the Archaean that has its beginning in the Hadean and has more in keeping with lunar and planetary evolution than more recent plate tectonic processes on Earth. Arguments are presented for rigid plate motions and convergence resulting in continental scale obduction and the stacking of entire ocean basins and their characteristic Archaean granite-greenstone assemblages, floored on a thin Hadean sialic oceanic substrate, along regional scale detatchment surfaces. These Archaean ocean basins formed originally as a thin sialic crust separating much more substantial sialic continental blocks that accumulated above down-welling vorticies in the solidifying mantle ocean of the Hadean. The thin sialic aprons adjacent to these emergent "proto-continents/granite-gneiss terranes" formed crustal depressions that were to form the first oceans. Deep fractures developed on this thin sialic veneer allowed vast outpourings of basic and intermittent ultramafic lavas to flood the ocean basins in much the same way as the broadly temporal and possibly structural equivalent lunar mare basins formed and filled. In contrast to their lunar equivalents these basins on Earth where submarine and once filled with several kilometres of mantle-derived volcanics eventually depressed the underlying thin sialic substrate beneath the granite melting liquidus, suddenly triggering substantial melting, granite diapirism and submarine felsic volcanism at the surface. The lower reaches of the adjacent continental areas at this time were largely molten, fiielled by substantial radiogenic heating, and the entire crustal section was turning itself over as granitic melts rose towards the surface, erupted as felsic ash, progressively became more deeply buried to eventually remelt. This cycle would continue until sufficient cooling allowed the lower crustal sections of this evolving granite-gneiss terrane to become substantially solid. Lateral driving forces beneath the ocean basins of the day, in the absence of sufficient density contrasts to allow plate subduction instead triggered large scale obduction. Horizontally stratified Archaean ocean crust was shunted up and over adjacent emergent continental granite-gneiss terranes along subhorizontal shear, thrusts and detachment surfaces as stacked thrust sheets. Once obducted, regional scale metamorphism powered by crustal heatflow from the overridden continental substrate heated the overthrust greenstone slabs. This metamorphic event was short-lived as uplift unroofed the greenstone slabs, contributing significant greenstone detritus onto adjacent emergent foreland aprons and progressively shallowing marine shelves. The moon was going through a parallel but somewhat different evolution not involving marine basins, the melting of felsic subcrust, lateral tectonic forces, obduction or acid volcanism. The difference may have been in part due to the smaller size and the absence of oceans and sufficient water to efficiently transport the heat energy and trigger many of these processes.

147

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MICROPROBE TECHNIQUES FOR THE ASSESSMENT OF PETROLEUM SOURCE ROCKS - A CASE STUDY FROM THE TIMOR SEA M. Faiz, R. Wilkins, N. Sherwood and N. Russell CSIRO Petroleum, PO Box 136 North Ryde, NSW 2113, Australia The Timor Sea area is one of the most prolific petroleum producing regions in Australia. The evaluation of thermal maturity and oil generative potential of the region by traditional techniques such as vitrinite reflectance (VR) is often complicated in this region because of marine depositional environments and complicated organic matter chemistr)/. The use microprobe techniques such as FAMM (fluorescence alteration of multiple macerals) analyses and EPMA (electron probe microanalysis) as well as conventional organic petrological analyses together yield important insights into the evaluation of thermal maturity and petroleum generation potential of source rocks. The FAMM technique employs a laser fluorescence microprobe developed at CSIRO Petroleum, to solve problems related to maturity assessment, most notably that of vitrinite reflectance (VR) suppression (Wilkins, . In this study of Timor Sea source rocks, EPMA has been used for the first time to determine C,N,0,S and H (by difference) contents of dispersed organic matter. FAMM analyses of the samples indicate that the Jurassic and Cretaceous sections contain vitrinites (organic matter derived from the woody tissues of plants) that have anomalously low reflectances. These measured reflectances therefore cause an underestimation of maturity by - 1 0 % to 50% (relative) and therefore the use of VR values for petroleum generation modelling significantly underestimates prospectivity of the basin. FAMM-derived maturity assessments provide a more reliable maturity indicator than VR for these source rocks. EPMA analyses indicate that the H content of the vitrinites are generally high, ranging from 6.5 to 8.5 wt%. The atomic H/C ratios of these vitrinites are similar to that of organic matter derived from lipid-rich, resinous and waxy tissues that have an enhanced petroleum generation potential as compared with vitrinite. The perhydrous nature of vitrinites in the Vulcan Sub-basin is mainly related to the precursor plant type and the depositional environment. Vitrinite in the Jurassic sections studied is most perhydrous where it has been deposited in shallow marine, shelfal environments possibly under anoxic conditions. The vitrinites in the Triassic sediments which were deposited in fluvial environments are mostly orthohydrous and therefore the VR gives a reasonable measure of the thermal maturity. Acknowledgments: We acknowledge BHPP and Mobil (Exxon) Exploration and Producing for funding a large part of the study. We also appreciate the assistance of Joseph Kurusingal of CSIRO Petroleum in maintaining and developing instrumentation for FAMM studies .Barry Searle of the University of NSW gave valuable technical assistance in carrying out the EPMA and Leila Gurba provided high quality standard materials for EPMA. Reference WILKINS R.W.T., WILMSHURST, J.R., RUSSELL, N.J., HLADKY, G. ELLACOTT, M.V. & BUCKINGHAM, C.P. 1992. Fluorescence alteration and the suppression of vitrinite reflectance. Organic Geochemistry 18, 629-640.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE FINISTERRE VOLCANICS AND TECTONICS OF NORTHERN PAPUA NEW GUINEA - THE TRUE STORY R.H. Findlay^ and R.A. Binns^ ' Geological Survey o f PNG, Private Mail Bag, Port Moresby, Papua N e w Guinea ^ CSIRO Exploration and Mining, PO Box 136, North Ryde N S W 1670

The Finisterre Volcanics of northern Papua New Guinea are regarded traditionally as part of the Melanesian island-arc, which is supposed variously to have begun collision with the Australian plate in either the Early Miocene, Middle Miocene or Pliocene times. New stratigraphic and geochemical data show that the Finisterre Volcanics are not part of the Melanesian island-arc but rather that they are formed of back-arc material related to subduction beneath the Sepik arc, which collided with Australia in Oligocene to earliest Miocene times. The Finisterre Volcanics interdigitate with an ?Eocene-01igocene-Miocene proximal fan series (Mena Formation; formerly Mena Series, Mena Beds, Sukurum unit and Sukurum Formation) derived from a microcontinent/island-arc system to the south (Sepik arc). The Mena Formation contains Oligocene calc-alkaline lavas typical of an island-arc and is one of many similar units found across Papua New Guinea and which constitute part of the Wogamush Group. Although the Wogamush Group formed initially during the Oligocene collision between the Sepik arc and the Australian plate it includes also deposits formed in an extensive Middle and Late Miocene event (Maramuni Event). In the Finisterre Mountains, the clearly visible interdigitating stratigraphic relationship between the Finisterre Volcanics and Mena Formation demands that neither were separated by a subduction zone nor a major ocean basin in Oligocene times. The unusual geochemistry of the nepheline-normative, high-K, low-Ti, high-Mg Finisterre Volcanics, which contrasts markedly with the calc-alkaline volcanics of New Britain and New Ireland, is best explained as indicating a back-of-arc setting, in which the Finisterre Volcanics form the back-arc basin floor and a plateau or plateaux north of the contemporaneous and contiguous Sepik arc. The Middle and Late Miocene Maramuni Event, which produced short-lived voluminous volcanics and granitoid intrusions, was the result of northward jumping of the Sepik arc's subduction zone to produce a short-lived Middle and Late Miocene, southward-dipping subduction zone along the line of the Ramu and Markham Valleys. The stratigraphic record confirms uplift of the Papua New Guinea Highlands in Pliocene times. In the Ramu Valley and Adelbert Mountains the sediments (Ouba Formation) derived from this event rest unconformably on but are overthrust by the Finisterre Volcanics. Obduction of the Finisterre Volcanics and associated rocks commenced in Pleistocene times in response to continued northward motion of the Australian plate. This obduction event does not represent a Pliocene collision between an island-arc "Finisterre terrane" and the Australian plate, as has been proposed in the past decade.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MONITORING PLATE MARGIN EARTHQUAKE AND VOLCANIC EVENTS: A MAJOR UPGRADE OF THE PHILIPPINES SEISMIC RECORDING NETWORK D. M. Finlayson', V. Wesson^ G. Gibson^ and T. Kiddle^ 'Australian Geological Survey Organisation (AGSO), Canberra. ^Seismology Research Centre (SRC), Mindata Pty. Ltd. Melbourne. ^Bureau of Meteorology (BoM), Melbourne. During the period January - March 2000 the seismic recording network in the Republic of the Philippines was upgraded with the installation of digital seismic recording equipment at thirty-four stations throughout the country. This national seismic recording network is operated by Philippines Institute of Volcanology and Seismology (PHIVOLCS) based in Quezon City, Manila. The upgrade was undertaken as a Japanese Government grant aid project (Managers - Japan Weather Association, JWA) by AGSO, BoM and SRC in partnership with the Mitsubishi Corp, Tokyo.

SOUTH

CHINA

The seismic recording system at each station was installed by staff from SRC, AGSO and BoM and built around SRC Kelunji-D digital seismic data loggers with associated SRC KD2 ink-on-paper drum recording systems. At all stations there were both three-component short period sensors (Geotech S13-J) and three-component accelerometer sensors (Geotech PA22). At two stations (Tagaytay and Baguio) there were also three-component broadband sensors (Guralp CMG3T). All recorders were tied to the Global Positioning System (GPS) time standard.

SC

^^

SULAWESI

Data from the data loggers are buffered continuously to PCs for analysis of events at each site, but in addition, auto-picking of the onset of seismic events triggers an alarm function and data are sent automatically to PHIVOLCS Head Office data receiving room if phone line connections are available. If no phone line is available, P and S-wave onset time data are sent by the observer using an established two-way radio system. Semiautomatic data analysis at Head Office

SCA

Earthquakes and volcanic eruptions on the western Pacific active margin threaten the social and economic development of 70 million Filippino people. Manila alone, with a population of over 10 million people, is one of the world's mega-cities at risk from earthquake damage. The introduction of routine digital three-component recording represents a significant improvement in the capacity of the PHIVOLCS to provide timely information on events. The ability to analyse P and S-waves digitally at recording stations across the whole country on a uniform basis for the first time, provides a sound basis for the introduction of greatly improved geohazard mapping and impact reduction strategies in the Philippines.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE CIRCUM EAST ANTARCTIC MOBILE BELT—THREE DIFFERENT GRENVILLE-AGE OROGENS JUXTAPOSED DURING GONDWANA ASSEMBLY I. C. W. Fitzsimons Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia East Antarctica has long been recognized as the key piece of Gondwana, abandoned at the South Pole as the other southern continents drifted northwards after the late Mesozoic break-up of the supercontinent. East Antarctica also has a critical location at the nucleus of Rodinia, the early Neoproterozoic supercontinent assumed to have amalgamated along late Mesoproterozoic to early Neoproterozoic "Grenville-age" mobile belts that form an important component of many Precambrian shields. One of the major pieces of evidence used to support the Rodinia reconstruction is that it links the Late Mesoproterozoic Grenville Province of eastern North America with the Circum East Antarctic Mobile Belt, a mobile belt of similar age that crops out around much of the East Antarctic coastline. This combined Grenville-Antarctic orogen separates North America—East Antarctica from various marginal fragments including cratons now located in South America, Africa, India and Australia. The Circum East Antarctic Mobile Belt is thus part of the principal suture in the Rodinia reconstruction, but a critical evaluation of age data from East Antarctica indicates that it is not a single continuous orogen and should not be used to constrain the configuration of the Rodinia supercontinent. Isotopic data collected over the last ten years from East Antarctica have identified two regions of late Neoproterozoic to early Cambrian (Pan-African) tectonism, which divide the Circum East Antarctic Mobile Belt into three Grenville-age segments, the Maud, Rayner and Wilkes Provinces. The relationship between these segments depends upon the nature of any displacements along the truncating Pan-African mobile belts: if Pan-African tectonism involved only limited transport, then the concept of a single Grenville-age Circum East Antarctic Mobile Belt is valid and this belt must have been an original feature of Rodinia, but if PanAfrican tectonism involved closure of a major ocean basin or continental-scale transcurrent tectonics, then the present-day alignment of the three Grenville segments around the Antarctic coastline is an artefact of Gondwana assembly and cannot be used to reconstruct pre-Gondwana supercontinents. One test of these models is to compare the timing of tectonism in the three Grenville-age segments, and attempt to correlate these collisional events across the Pan-African mobile belts. U - P b zircon data indicate that the Maud, Rayner and Wilkes Provinces each have a distinctive age signature for late Mesoproterozoic—early Neoproterozoic collisional tectonics, and can be correlated with similar rocks in the Namaqua-Natal, Eastern Ghats, and Albany-Fraser/Musgrave Provinces respectively of Africa, India and Australia. Two pulses of high-grade tectonism can be distinguished in the WiIkes-Albany-FraserMusgrave Province at 1330-1280 Ma and 1200-1130 Ma, whereas tectonism in the Maud-Namaqua-Natal and Rayner-Eastem Ghats Provinces occurred at 1100-1030 Ma and 990-900 Ma respectively. Terminal ocean closure and continental collision must have occurred at different times in each fragment. The consistency of ages within each segment and the correspondence of the segment boundaries with Pan-African mobile belts is compelling evidence for East Antarctica comprising three separate Grenville-age convergent margins that were juxtaposed by Pan-African tectonism. This evidence for three spatially and temporally discrete episodes of ocean closure and continental collision between 1350 and 900 Ma indicates that Rodinia assembly occurred in several stages over a period of at least 450 Myr. This time interval is sufficient for considerable complexities in the amalgamation history, with truncation of older collisional belts by younger sutures, and models that treat all Grenville-age mobile belts as part of a simple network of equivalent collisional orogens should be discarded.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15'^ Australian Geological Convention, Sydney, July 2000

THE WALCHA ROAD ADAMELLITE: A LARGE ZONED PLUTON IN THE NEW ENGLAND BATHOLITH AUSTRALIA. R. H. Flood and S. E. Shaw GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW, 2109, Australia.

The Walcha Road Adamellite, a large (260 sq.km) zoned pluton of Late Permian age, grades from mafic homblende-biotite adamellite along the margin (colour index 25) into a porphyritic adamellite with sub-equal amounts of phenocrystal and groundmass K-feldspar and then to a leuco-adamellite (colour index 7) central zone that is finer grained and has less than 1 % K-feldspar phenocrysts and little or no hornblende. The compositional variation is from 63-74% silica. Laser ablation ICPMS zircon U/Pb age determinations are about 250 Ma and are comparable with conventional Rb/Sr biotite/bulk rock ages. The pluton has a fabric best defined by the preferred orientation of elongate feldspars. Even near the margins where this foliation is most strongly developed the foliation and is inferred to be a magmatic flow foliation. The strike of the foliation generally parallels the compositional and textural zonation contours and is steeply dipping except in the centre. Microgranitoid enclaves that commonly form local clusters are an obvious but very minor part of the pluton. These enclaves include micromonzonites, microsyenites and microgranodiorites and microadamellites compositionally not very different from the host pluton. The mineralogical zonation involves a changes in plagioclase (An25-An8), biotite (mg 5646), hornblende (mg 65-60), titanite (more LREE depleted and HREE enriched in the pluton centre) and zircon (REE enrichment by factor of 10 from pluton margin to pluton core). The systematic change in zircon composition between margin and centre of the pluton suggests that the zonation is the result of crystallisation of magmas of progressively changing composition. Three adjacent small stocks of pyroxene micro-monzonite of the similar age indicate that largely liquid magmas of intermediate composition were being emplaced during the same general period. Differentiation of such magmas might provide an explanation for the range of compositions within the Walcha Road pluton although we see as it unlikely, that at any one time, the amount of magma present approached the total volume of the final pluton, preferring to envisage the pluton developing its full size by the gradual addition of more magma as crystallisation proceeds. The bulk rock chemical and isotopic composition are used to evaluate the degree to which the range of rock compositions can be derived from a single magma or whether some change in the magma composition being delivered from the source region is also required.

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Geological Convention, Sydney, July 2000

CRUST-MANTLE INTERACTION AND THE FORMATION OF GRANITIC TERRANES IN EASTERN GONDWANALAND John Foden Department of Geology and Geophysics, University of Adelaide, Adelaide , South Australia. Australia 5005. Since the Early Palaeozoic, eastern Gondwanaland had a broadly convergent Pacific margin subject to enigmatic periodic transitions from extension to contraction, and is quite unlike the Andean margins of the eastern Pacific. Prominent magmatic provinces in this zone include the granites of the Cambro-Ordovician Delamerian and Ross Orogens and of the Siluro-Devonian Lachlan Fold Belt. The magmas produced in this trailing convergent margin setting are not well explained by application of any single one of the five standard paradigms of igneous petrogenesis (plumes, decompressional melting, hydrous fluxing, crustal thickening or radiogenic heating). Evidence for the direct association of granite production with subduction is often lacking. Felsic magmatism occurs during convergence and continues during the post-tectonic stages. Mafic magmatism occurs in the preconvergent extensional phase, and continues weakly during the convergent stage. In the Ross-Delamerian Orogenic Belt, the cessation of deformation is accompanied by abrupt uplift with bimodal magmatism and molassic sedimentation, suggesting sudden change in crustal buoyancy. The convergent stage appears to localise initial deformation at the thermally weakened axes of prior rifts and following early recumbent fold-thrust dominated thickening, shortening proceeds by periodic phases of localised upright folding. Granite magmatism is explained as a mantle melt-crust mixing AFC -type process with assimilation (Sandiford et al., 1992, Foden et al., 1999) increased during crustal thickening and convergence. The ongoing role of the mantle during peak crustal thickening is clear from its occult presence, identified by Nd-isotope compositions, in the granite magmas and as upper crustal mafic intrusions. As the lithosphere should logically be thickened under conditions of convergent deformation, the persistence of mafic magmas after the cessation of the initial rift indicates that some parts of the asthenosphere have not been depressed by a thickened lithospheric keel. The on-going presence of mantle of asthenospheric temperatures at shallow depths during peak crustal thickening implies that initial convergent deformation tightened an anticlinal fold of that hot mantle which was originally beneath the pre-convergent rift, forming a mantle involution within the core of the crustal fold belt. This provided a continuing source of mafic magma, and was a heat source for crustal melting. Its conductive cooling over 10 - 30 Ma weakened the crust to allow ftirther localised shortening. Finally the crust became sufficiently weakened to allow the gravitational segregation (sinking) of the mantle involution and the elimination of Moho topography with consequent rapid crustal uplift. The expelled mantle was geochemically modified during its contact with the crust and became a ftiture anomalous enriched lithospheric mantle source. In the Delamerian-Ross orogen this model is supported by several direct lines of evidence. These include: •

•

The granite chemistry shows temporal variation which results from varying mixing proportions of crust and mantle melt controlled by the evolving strain history of the belt. These granites do not appear to be subduction related. The mafic magmatic rocks also show temporal geochemical shifts coordinated with the orogenic strain history. Compared with the pre-tectonic basalts, the younger suites have isotopic and trace element characteristics considerably shifted from those of the depleted mantle, consistent with melting of contaminated mantle sources. The implication is that these mantle sources became enriched during the orogenic process. This type of mantle -crust interaction may be peculiar to continental margins at the early stages of transition from passive to convergent states when the ridge-push stresses are transmitted by the oceanic lithosphere to generate deformation at thermally weakened intra-continental rift zones.

The Ross - Delamerian Orogen is the site of voluminous dolerite magmatism and this has geochemical characteristics, including highly anomalous Sr-, Nd-, Pb- and Oxygen isotope compositions, which are exactly those of the early Ordovician, post-Delamerian mafic rocks and suggest re-sampling of the anomalous mantle created during the Cambrian orogenic event. References SANDIFORD,M., FODEN, J. , SHAOHUA ZHOU & TURNER,S. (1992) Transactions of the Royal Society of Edinburgh: Earth Sciences, 83, 83-93. FODEN, J., SANDIFORD, M., DOUGHERTY-PAGE, J., & WILLIAMS, L , 1999): Australian Journal of Earth Sciences 46, 250 -263.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EFFECTS OF SEAWATER AND MAGMATIC FLUIDS IN THE PANORAMA VMS DISTRICT, PILBARA CRATON: A STABLE ISOTOPE STUDY Susan L. Folkert and Steffen G. Hagemann Centre for Strategic Mineral Deposits, The University of Western Australia, Nedlands WA 6009, Australia The 3.24 Ga Panorama district presents an excellent opportunity to examine the P-T-X evolution of a VMS system and to investigate the origin of hydrothermal fluids and metals. The district has excellent exposure, has undergone sub-greenschist facies metamorphism, and is revealed as a cross-section through sub-volcanic granite intrusions, a coeval submarine volcanic pile, Cu-Zn massive sulfide ore, and an exhalite unit. The regional alteration zones define seawater-dominated hydrothermal convection cells through the volcanic pile and the top of the subvolcanic intrusions (Brauhart et al, 1998). Oxygen and hydrogen isotopes have been measured on four distinct regional vein systems related to hydrothermal alteration associated with VMS mineralization. Oxygen isotope fluid values (5^^0h2o, VSMOW) were calculated from 5^^0qtz using fluid inclusion homogenization temperatures and the quartz-water fractionation equation of Matsuhisa et al (1979). Hydrogen isotope fluid data (5Dh2o, VSMOW) were determined directly from fluid inclusions. The isotopic signatures of granite-hosted chalcopyrite-sphalerite-cassiterite-quartz veins 5.0 to 9.3%o, [av. 7.1±1.9%o(la), n=8, T=305°C]; 5Dh2o= -48 to -27%o, [av. -37±8%o, n=10]) and quartz-topaz-muscovite greisen (6'^Oh2o= 4.8 to 7.9%o [av. 5.6±0.8%o, n=13, T=450°C]; 5Dh2o= -36 to -21%o [av.-29±6%o, n=6]) are interpreted to represent fluids partitioned off the crystallizing magma and correlate with the "evolved" magmatic water field defined by Giggenbach (1992). Panorama unaltered granite has a 5'^Oh2o range from 7.9 to 8.3%o (av. 8.1±0.2%o, n=4) at T=650°C, the approximate minimum melt temperature of granite. The average 5'^Oqtz (9.2±1.0%o, n=9) measured for quartz-pyrite veins associated with quartz-sericite alteration zones at the top of the granite intrusion are weakly enriched relative to unaltered granite (5'^0qtz= 8.7±0.2%o). Similar '^O enrichments for whole-rock analyses within the Flavrian Pluton, Noranda, are probably related to hydrothermal recharge into the central parts of the pluton (Cathles, 1993). As there is a large temperature dependence on the quartz-water fractionation curve, the calculated 5^^0h2o (-6.1 to -2.5%o. av. -4.5±1.0%o, n=9, T=170°C) for the quartz-pyrite veins is anomalously light and the 5Dh2o (-30 to -18%o, av. -25±4%o, n=6) is depleted relative to VSMOW. The 5'^Oh2o range (-3.7 to 3.4%o, av. -0.5±2.7%o, n=10, T=152°C) and 5Dh2o range (-48 to -26%o, av.-35±10, n=4) for quartz-sericite veins associated with feldsparsericite-quartz-ankerite alteration at the top of the volcanic pile (ie, the paleoseafloor) are consistent with low temperature isotopic exchange reactions between volcanic rocks at the seafloor and recharging seawater. The 5'^Oh2o values (4.5 to 6.3%o, av. 5.6±1.0%o, n=3, T=250°C) and 5Dh2o values (-49 to -30%o, n-2) for quartz interstitial to Cu-Zn massive sulfide ore at Panorama occupy a field intermediate between the isotopic composition of seawater and magmatic water. These values are '^O-enriched (~3%o) and D-depleted (~10%o) relative to values reported for the Kuroko hydrothermal fluids (de Ronde, 1995). The Kuroko data has been interpreted as indicating - 1 0 to 25% magmatic water mixed with - 7 5 to 90% seawater. The range in isotope data for vent fluids at Panorama could, therefore, represent a larger contribution of magmatic water to the hydrothermal fluids. Alternatively, the range in 8^^0h2o and 5Dh2o for the vent fluid at Panorama could be indicative of seawater which has undergone isotopic exchange with volcanic rocks before being discharged. References

BRAUHART C.W., GROVES D.I. & MORANT P. 1998. Regional alteration systems associated with volcanogenic massive sulfide mineralization at Panorama, Pilbara, Western Australia. Economic Geology 93, 292-302. CATHLES L.M. 1993. Oxygen isotope alteration in the Noranda mining district, Abitibi greenstone belt, Quebec. Economic Geology 86, 1483-1511. DE RONDE C.E.J. 1995. Fluid chemistry and isotopic characteristics of seafloor hydrothermal systems and associated VMS deposits: potential for magmatic contributions. Mineralogical Association of Canada Short Course Volume 23, 479-509. GIGGENBACH W.F. 1992. Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin. Earth and Planetary Science Letters 113, 495-510. MATSUHISA Y., GOLDSMITH J.R. & CLAYTON R.N. 1979. Oxygen isotope fractionation in the system quartzalbite-anorthite-water. Geochemica et Cosmochimica Acta 43, 1131-1140.

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THE ENIGMA OF THE BROKEN HILL QUARTZ-ALBITITES Caroline Forbes, Maarten Krabbendam and Pete Betts Australian Crustal Research Centre, Monash University, Clayton VIC 3168 In this contribution, the protolith of the quartz-albitite rocks in the Broken Hill Block, New South Wales, will be discussed. Quartz-albitites are of general interest to mineral exploration because they are associated with various metal deposits in several terranes; in the Broken Hill Block they have been suggested as a potential source of lead for the Broken Hill orebody. The Broken Hill quartz-albitites occur within the Thackaringa Group below the Broken Hill Main Lode. Psammo-pelitic units occur above and below the quartz-albitites. The rocks are light coloured and comprise uniformly grained quartz + plagioclase (Ang-is) + minor biotite and pyrite locally interlayered with micaceous paragneiss. They have unusually high Si02, Na20 content, and very low CaO, MgO, K2O (all <1%), Pb and Zn (<20 ppm) contents compared to surrounding units. Complex zircon populations occur in the quartz-albitites (Love, 1992; Donaghy, 1997; Nutman and Ehlers, 1998). The precursor of these rocks is subject to debate. Suggested protoliths include: rhyolite, tuff, clastic sediments or evaporites. Metasomatism and/or analcimitization have been suggested to have altered the rock. Cook and Ashley (1992) proposed an evaporitic protolith for the Broken Hill quartz-albitites in the nearby Olary Block on the basis of hypersaline fluid inclusions, occasional occurrence of scapolite, pseudomorphic voids after common evaporite minerals and boron isotope evidence. The mechanism of evaporite to quartzalbitite transformation, however, is unclear: it would imply significant enrichment in Si and Al, concomitant with a depletion of CI (and Ca and S, if anhydrite was present). It is also unclear how detrital zircons (Donaghy, 1997) could have become incorporated into the original evaporite. Altered rhyolite or rhyolitic tuffs is also a possible protolith, but they would have had a very special composition (to justify the unusually high Si and Na content of the quartz-albitites) and do not account for the detrital zircon populations. An arkosic protolith would account for the zircons, but would require an unusually albite-rich source region or extensive post-depositional alteration. Analcimization of tuffs was suggested by Plimer (1977). An analcime-rich precursor is attractive as it can explain the high Na content and the occurrence of scapolite and saline fluid inclusions, as analcimization is commonly thought to occur in saline lakes. Prograde metamorphism (at T>200°C) of a quartz-analcime rock would readily produce a quartz-albitite. However, analcime-rich rocks can be produced fi-om tuffs, argillic or arkosic sediments (rhyolite sensu sthcto probably being to massive to react). A mix of unanalcimized tuffs and arkosic sediments would explain many of the characteristics of the quartz-albitites. The available evidence points to the Broken Hill quartz-albitites containing a sedimentary component that was altered at some time in the history of the Broken Hill Block. If this is the case, the low metal content of the quartz-albitites is potentially due to the same process that altered the rock. References COOK, N. D. J. & ASHLEY, P. M. 1992. Meta-evaporite sequence, exhalative chemical sediments and associated rocks in the Proterozoic Willyama Supergroup, South Australia: implications for metallogenesis. Precambhan Research 56, 211-226. DONAGHY, A. G. 1997. Pre-1690 Ma tectonic and thermal activity in Broken Hill, NSW, Australia? Results from structural mapping and SHRIMP U/Pb geochronology. In: Geodynamics and Ore Deposits Conference, Conference Abstracts, February 19-21, pp. 109-110. Australian Geodynamics Cooperative Research Centre.

S. 1992. Possible ages and origins of some rocks of the Willyama Supergroup, New South Wales. BSc. Honours thesis, Australian National University.

LOVE,

NUTMAN, A. P. & EHLERS, K. 1998. Archaean crust near Broken Hill. Australian Journal of Earth Sciences 45, 687-694.

PLIMER, 1. R. 1977. The origin of the albite-rich rocks enclosing the cobaltian pyrite deposit at Thackaringa, N. S. W., Australia. Mineralium Deposita 12, 175-187.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PHYSICAL PROPERTY CHANGES AS A MONITOR OF EAST ANTARCTIC SEDIMENTATION HISTORY: FIRST RESULTS FROM ODP LEG 188 (PRYDZ BAY) C.F. Forsberg\ Jens Gruetzner^ B. Taylor^ M.Rebesco^ A.K. Cooper^ P.E.O^Brien^ and Leg 188 Shipboard Scientific Party. 'Norwegian Polar Institute, Polarmilj senteret N - 9 2 9 6 Troms, Norway. ^GEOMAR-Forschungszentrum, 24148 Kiel, Germany. ^ Jacques Whitford and Assoc., Dartmouth, NS B3B 1W8, Canada. ^ Osservatorio Geofisico Sperimentale, Borgo Grotta Gigante 42/C Sgonico, Trieste 34010, Italy. ^ Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA. ^ Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601.

Three sites were drilled during ODP Leg 188 to Prydz Bay, East Antarctica. Site 1166 is located on the outer continental shelf, Site 1167 recovered sediments from the Prydz Charmel trough-mouth fan on the continental slope, and Site 1165 was drilled on the Wild Drift near the base of the continental slope. Examples of how changes in physical properties can help to unravel both the depositional history and the depositional processes characteristic for the three settings are given here. On the continental shelf (Site 1166), sediments have been subjected to glacial erosion and compaction. The shear strengths in the upper 150 mbsf at Site 1166 show a depositional history that includes at least one or two periods during which the sediments were compacted either by a thicker sediment column that has been removed by erosion, or by a glacier. The loads needed are equivalent to about 2950 kPa at 20.5 mbsf (which corresponds to a sediment column 250 m thick or 330 m of non-buoyant ice) and 3700 kPa at 100 mbsf, (which corresponds to a sediment column 300 m thick or 420 m of nonbuoyant ice). On the continental slope (Site 1167), physical properties display a marked step-like change to higher velocities, higher bulk densities, lower grain densities and lower porosities at about 210 mbsf within a thick unit of Pleistocene debrites, probably indicating a shift in sediment provenance from a relatively high proportion of sedimentary rocks in the early Pleistocene to igneous and metamorphic rocks after this time, which suggests a major change in the Lambert Glacier/Amery Ice-Shelf drainage system. On the continental rise (Site 1165), decimetre- to meter-scale alterations of dark grey laminated terrigenous clays and biogenic-rich greenish grey sediments have been described and are reflected in lightness measurements. The lightness variations show a strong correlation with changes in bulk density and magnetic susceptibility. Spectral analyses in the depth domain for intervals with good magnetostratigraphic age control reveal a pattern of spectral peaks for the Late Miocene/early Pliocene that most likely displays Milankovitch periodicities and indicates that the physical properties reflect orbital-driven changes in the marine depositional environment.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SKARN MINERALISATION AT THE WORLD-CLASS CADIA PORPHYRY Au-Cu DEPOSITS, NSW. David B. Forster and Philip K. Seccombe School of Geosciences, The University of Newcastle, Callaghan NSW, Australia, 2308. The Cadia deposits located near Orange NSW, are among several examples of Ordovician, shoshonite-related porphyry Au-Cu systems in the eastern Lachlan Fold Belt. Current mining operations are focused on quartzmonzonite intrusive-hosted propylitic, sericitic alteration and usually higher-grade "sheeted" quartz - gold veins which form a resource of greater than 9.4 million ounces Au and 1.2 million tonnes Cu [1]. The nearby Big and Little Cadia skam deposits also have potential as economic ore bodies. They occur in more calcareous units of the volcanic host sequence. A number of smaller skam bodies have also been identified in drill core. Both the skarn and porphyry deposits have been dislocated by multiple faulting events. The broader aims of the current study include resolution of several critical and previously poorly understood aspects; i) to establish the physical distribution of skam mineralisation relative to the porphyries, major structures and host lithologies, ii) paragenetic relationships between skams and porphyries, and iii) the nature and composition of the mineralising fluids. A six stage vein and mineral-structural paragenesis has been defined for the overall development of the porphyry-skam system. It includes at least three major fluid cycles. Stage-1 is divided into two sub-stages based on the spatial distribution of alteration. Stage-la mineralogy consists of a relatively broad distribution of unzoned, red-brown high Ti gamet (-Xgrossular 0.50.3. in volcanics peripheral (up to 30-60m) to the eastern intrusive contact of the Cadia Hill Monzonite. This mineralogy is considered to represent an isochemical homfels formed as a result of the emplacement of the "barren" portion of the pluton [2]. Stage-lb: 3D mapping of lithology and alteration zones, suggests that fluid infiltration within I km of the Cadia Hill Monzonite was controlled by i) permeable, calcareous lithologies and ii) proximity to major, initially extensional, mineralising, steeply dipping, ~ESE-WNW trending faults. This caused mobilisation of carbonate and subsequent growth at high f 0 2 , of bladed hematite and calcite which form the bulk of the Cadia skam bodies. Stage-2 is characterised by diverse anhydrous mineralogy. Magnetite forms overgrowths on bladed hematite. Yellowish andraditic gamet (-Xandradite >0.95) pyrite and fine-grained bluish quartz form aggregates up to 5cm thick along some permeable, calcareous lithological contacts, faults and vein margins. Quartz flooding of permeable volcanic units is also present. The stage may reflect infiltration of magmatic-dominated (prograde) hydrothermal fiuids. This hypothesis is to be tested by fluid inclusion and stable isotopic studies. Stage-3 was associated with extensive and pervasive propylitic alteration and deposition of some early sulphide. Most Stage-2 gamet was retrogressed to hydrous minerals (mainly epidote) at this stage. Stage-3 propylitic alteration may be post-dated by phyllic (quartz-sericiteipyrite associated with significant Au) alteration which also occupy broader ~WNW to N W trending zones within the monzonite. This corresponds to the trend of major intmsions, as defined by aeromagnetic imaging [1]. Stage-4 represents the main period of Au-Cu mineralisation at Cadia and appears to coincide with major structural activity. Chalcopyrite and lesser bomite replace hematite blades, reflecting a reduction in fluid f 0 2 . Controlling structures include WNW and NNW-trending faults and joints. Quartz veining in the skams is likely to be contemporaneous with porphyry-style, ~NNW-trending quartz - Kfeldspar (selvaged) sheeted vein systems at Cadia Hill and Cadia Quarry. Larger sericite and chalcopyrite (sometimes in pressure shadows) developed late in deformed zones. Other mineralised faults include ~N and minor, late-stage NE - trending joints and faults. Stage-5 represents the waning stages of hydrothermal activity with voluminous deposition of calcite and hematite. Stage-6: Low-grade regional burial metamorphism, which probably occurred in the Late Ordovician [3] produced an assemblage consistent with lower greenschist facies. Late, post-Silurian thrusting has further disrupted the geology and formed new, high and low angle faults. At Little Cadia however, identifiable marker horizons have enabled reconstruction of the original skam morphology which has been displaced by - W N W - E S E faults. The Cadia deposits are an excellent example of association between porphyry and Fe-rich skam deposits. They emphasise the importance of improving understanding of fluid processes and fluid dynamics for LFB deposits thereby improving exploration strategies for skam systems in a region where potassic, mineralising intrusives and Ca or Mg-rich sediments are commonly juxtaposed. References 1. Newcrest Mining Staff, 1998. Australasian Inst. Min. Metall., Monogr. 22:641-646. 2. Jamtveit, B., Wogelius, R. A. & Eraser, D. G., 1993. Geology. 21:113-116. 3. Perkins, G. R., Walshe, J. L. & Morrison, G., 1995. Econ. Geol. 90:1433-1466.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EXTENSIONAL SHEAR ZONES FORMED DURING GONDWANA BREAKUP IN THE OTAGO SCHIST, NEW ZEALAND M.A. Forster and G.S. Lister Australian Research Centre, Department of Earth Sciences, Monash University, Melbourne, 3800 Australia A structural study in the Otago Schist has shown that multiple generations of shear zones exist throughout the region. The "nappe" model advocated by previous workers does not seem to apply, and the size and geometry of the recumbent folds is open to debate. Younger shear zones transect the older recumbent folds, and contrary to the results of previously published work, they are not the attenuated limbs of these folds. Older shear zones are themselves folded by younger generations of recumbent folds. Elongate domal structures are found throughout the Central Otago region. New Zealand. These have a geomorphology that is reminiscent of core complex geometry, although the current view is that these structures formed solely as the result of Quaternary deformation. One of the hallmarks of extensional terranes is the development of unconformities subparallel to the outcrop of regionally extensive detachment faults and/or the underlying sub-horizontal ductile shear zones. Parallelism of the Cretaceous-Miocene unconformity with the intensely deformed fabrics that are characteristic of the dome flanks is a distinctive feature in the geomorphology of the Central Otago Schist, in particular the Dunstan Range. This is what is to be expected if shear zones and low-angle faults throughout the Otago Schist are extensional in character. One valid interpretation of the geometries observed is that the recumbent folds formed during periods of crustal shortening, and the transecting shear zones formed during periods of crustal extension. In this case there have been several switches in tectonic mode, with oscillations from crustal shortening to crustal extension, and back again to crustal shortening. To test this hypothesis a detailed fabric and microstructural study has been combined with an 40Ar/39Ar thermochronology study in the Central Otago Schist through the Dunstan Range (along the cross-section shown in the figure above). Mineral growth events in these rocks took place during ductile deformation under middle- to low-greenschist conditions, at a level in the Earths crust in which 40Ar/39Ar ages in white mica are only partially reset during metamorphic events (in the Argon Partial Retention Zone). Under these conditions new grown metamorphic white mica is capable of retaining enough of its radiogenic argon to allow the timing of particular deformation/recrystallization events to be determined, although under such conditions considerable heterogeneity in 40Ar/39Ar apparent ages can be obtained during step-heating experiments. The 40Ar/39Ar data obtained could not be interpreted using conventional techniques based on the recognition of plateaux in the apparent age spectra. The spectra obtained appear to be the result of mixing between gas populations with distinct ages, a feature which appears to characterise apparent age spectra produced from rocks in the Argon Partial Retention Zone. Therefore a new method of interpretation has been introduced, relying on the definition of asymptotes and limits in the apparent age spectra. The technique works best for step-heating experiments that have produced a multitude of steps in the apparent age spectra, as was the case for the experiments reported. Application of the method of asymptotes and limits to the data so far collected in this study sheds new light on the timing of deformation/mineral growth events in the Otago Schist. The oldest fabrics occur in the northern part of the Dunstan Range, in the Bendigo fold hinge. Although there is no clear correlation of apparent age variation with microstructure, the older mica in the folded fabrics around the Bendigo fold hinge may have grown at - 1 2 0 Ma (a time supposedly at the termination of Jurassic collision in the Rangitata event). Recrystallization associated with the second generation of shear zones (e.g. the E-W directed Devil's Creek shear zone) appears to have taken place at - 1 0 0 Ma, based on asymptotes and limits in apparent age spectra from these structures. Variation in apparent age spectra produced along a transect through the third generation Northbum Shear Zone shows that younger apparent ages are evident at lower structural levels (where the mylonite fabric is best-defined). The apparent age spectrum from intensely deformed and recrystallized white mica separated from this shear zone has a well-defined asymptote at - 8 5 Ma. These are interesting ages to have obtained for they coincide with the onset of continental extension associated with separation from Antarctica at - 1 0 0 Ma, and the opening of the Tasman Sea at - 8 5 Ma. This suggests that some of the ductile structures in the Otago Schist may be extensional in origin, and related to Gondwana breakup. More detailed work (including additional geochronology) is necessary to provide further constraint.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney, July 2000

SYDNEY DIMENSION SANDSTONE: THE VALUE OF PETROGRAPHY IN STONE SELECTION AND ASSESSING DURABILITY Brenda J. Franklin University of Technology, Sydney Many buildings and other structures in the Sydney area are composed of sandstone, mostly from the Triassic Hawkesbury Sandstone. On the basis of broad composition, colour and colour change, dimension sandstone can be classified into six groups. However, most Sydney sandstone buildings are composed of either/or both of Sydney yellow block sandstone and quartz-rich sandstone. The well-known Sydney yellow block sandstone is a fine- to medium-grained 'self-colouring' sandstone which is usually grey when quarried, but oxidises and darkens on exposure to the atmosphere to a warm yellow-brown colour. Quartz-rich sandstone is white, grey or pale yellow in colour when freshly quarried and does not darken or change colour significantly on exposure to the atmosphere. It was used extensively in the early days of the Colony, particularly before the 1850s when the yellow block quarries at Pyrmont were first opened. Sandstone is a natural product whose physical properties, including colour, texture and pattern, and strength, can vary widely, even within the same quarry. Dimension sandstone has to meet certain requirements relating to the quality of the stone before it can be considered suitable for use in either restoration work or on any modem-day structure. Test procedures to determine quality include an investigation of the petrography of the stone by means of microscopic examination. Petrography identifies the minerals and the texture of a rock, and can be specially tailored toward identifying features, which are important to dimension sandstone, which is to be used in the built environment. It has a number of advantages and disadvantages and the importance of comparative analyses cannot be overemphasised. A number of distinct petrographic factors can be identified which affect the durability and performance of the stone. These include the amount of secondary quartz, the nature of the binder, the porosity, the freshness of the stone and the presence of any discontinuities or zones of structural weakness. The petrography of a number of Sydney yellow block and quartz-rich sandstones is summarised, together with a description of the three main mineral components - quartz, clay and siderite. Examples of the use of petrography in elucidating various problems relating to durability are given. The characteristic failure of yellow block stone containing the defect colloquially known as 'black line' has long been observed by stonemasons. However, because its cause had not been scientifically established, the rejection of stone containing this defect on either aesthetic or durability grounds was often challenged. The evidence presented here identifies the mineralogical causes of the problem and validates the rejection of stone containing this defect. Petrography is also able to detail the style of sandstone decay with depth. This aids architects and engineers to make decisions with respect to the amount of stone replacement required during restoration.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE MODULUS OF RUPTURE TEST AND ITS SIGNIFICANCE FOR DURABILITY OF DIMENSION SANDSTONE Brenda J. Franklin^ and John Young^ 'Faculty of Science, University of Technology, Sydney ^NSW Department of Public Works and Services

The modulus of rupture test is one of the routine laboratory engineering tests carried out on dimension sandstone. It is most applicable in predicting the stone's capabilities under tensional stress. These stresses are caused by strong wind, thermal and moisture changes, vibration and warping. Any of these stresses may be concentrated by physical discontinuities in the stone. Wet modulus of rupture values and the wet/dry ratio are particularly critical in assessing performance. A graph, which plots wet/dry modulus of rupture ratios and gives a guide to dimension sandstone quality has been devised by Winkler (1994).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TOWARD A QUANTITATIVE, HOLISTIC UNDERSTANDING OF ROCK ALTERATION PATTERNS R Freij-Ayoub, J.L. Walshe, H-B Muhlhaus, B. Hobbs and A. Ord CSIRO, Division of Exploration and Mining, PO Box 437 Nedlands WA 6009

All mineral deposits are associated with wall rock alteration although there is commonly considerable debate regarding the relative timing of the alteration and mineralisation. Nevertheless, the spatial association of rock alteration and mineralisation presents a clear targeting halo for drilling. Hence, a precise understanding of the chemical, mineralogical, spatial and temporal relationships between rock alteration and mineralisation can provide guidance to the mineral exploration industry and if this knowledge is incorporated as a technique that supplements and even guides mapping procedures, it can result in great savings through increasing the efficiency of mineral discovery. In this paper, hydrothermal alteration of a quartz-K-feldspar-pyrite rock is simulated numerically by coupling fluid flow and chemical reactions. The infiltration and mixing of two metal bearing fluids, one which is highly acidic and oxidised, through bearing CO2, and the other which is highly reduced, through bearing CH4, creates the necessary environment to precipitate secondary silicate minerals: quartz, muscovite and/or pyrophyllite and iron minerals. It also provides the chemical gradients that allows the gold saturated fluids to deposit gold. The experiment is conducted at constant temperature and pressure of 350°C and 2kb respectively. The precipitation and/or dissolution of the secondary minerals is controlled by mass action relations. In our simulations we solve transport equations for the primary elements and satisfy mass action relations in a sequential manner using an implicit scheme in a finite element code (Fastflo). The pore fluid velocity is assumed to be constant. The change of rock volume due to the dissolution or precipitation of the minerals, which is directly related to their molar volume, is taken into account. Feedback into the rock porosity and the reaction rates is included in the model. We discuss the development of mineral zonation patterns and their relationship to gold mineralisation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RE-OS ISOTOPE SYSTEMATICS OF THE RADIO HILL NI-CU-PGE COMPLEX, WEST PILBARA CRATON, WESTERN AUSTRALIA Louise R. Frick', David D. Lambert^, Dean M. Hoatson^ and Jannene S. McBride' 'Australian Crustal Research Centre, Monash University, PO Box 28E, Victoria 3800, Australia ^Department of Earth Sciences, Monash University, PO Box 28E, Victoria 3800, Australia ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia The Radio Hill Complex is one of several layered mafic-ultramafic intrusions that were emplaced into the west Pilbara Craton at ca. 2.9 Ga. It is believed to be genetically related to other similar intrusions in the area which include the Munni Munni, Andover, Dingo and Maitland Complexes (Hoatson et al. 1992). The Radio Hill Complex is the only one of these intrusions to contain economic quantities of Ni and Cu at the present time. This relatively small intrusion ( - 1 2 0 0 m stratigraphic thickness) is divided into two zones, an upper gabbroic zone and a lower ultramafic zone, the base of which possesses massive magmatic sulfide mineralisation (pyrrhotite, pentlandite, chalcopyrite, magnetite). Re-Os isotopic data have been obtained for a suite of samples from underground, outcrop (gabbroic zone samples) and drill core. Os concentrations in the mineralised zone are very high, ranging from 19 to 191 ppb Os, with low Re/Os ratios (<2.3). These characteristics are not typical of most low tenor, basalt-hosted magmatic sulfides (Lambert et al. 1999). The isotopic data for the massive sulfide samples yield a precise Re-Os isochron with an age of 2892 ± 34 Ma (MSWD = 1.06) and an initial 1870s/1880s = 0.1265 ± 0.0028. This age is in agreement with the Sm-Nd age of 2927 ± 13 Ma of the nearby Munni Munni Complex that is believed to be genetically related to the Radio Hill Complex (Hoatson et al. 1992). The Radio Hill massive ores yield a relatively unradiogenic initial Os isotopic composition (gOs = +17.6), compared with the ores from other Ni-Cu-PGE deposits (e.g., Voisey's Bay gOs = +1100, Duluth gOs = +800, Sudbury gOs = +500; Lambert et al. 1999). The Radio Hill data are more similar to Re-Os isotopic data from Noril'sk-Talnakh where gOs is also low (Walker et al. 1994). Many models for the petrogenesis of magmatic sulfide deposits have proposed that crustal contamination is an important mechanism for driving the parental silicate magma to sulfide saturation. However, it has been demonstrated by Lambert et al. (1999) that turbulent ore forming/deposition processes are capable of masking the effects of crustal contamination, if the immiscible sulfide ore magma experiences a high R-factor (eg. >10,000), where R-factor is defined as the mass ratio of silicate magma to sulfide magma that it has equilibrated with. However, modelling of the Radio Hill ore system using Re, Os and PGE (Hoatson et al. 1992) concentrations suggest that the R-factor in the Radio Hill ore was low (100-200). Therefore, the initial Os isotopic composition of the ore should be radiogenic (high gOs >+500) if local crustal contamination triggered sulfide saturation. The slightly radiogenic initial Os isotopic composition suggests that the bulk parental magma was a crustally-contaminated magma. R-factor modelling further suggests that the parental magma to this intrusion was neither a basalt nor a pure komatiite, but more likely a contaminated komatiite (with a relatively high Os concentration). This agrees with the previous findings of Hoatson et al. (1992) who proposed a parental magma of siliceous high-magnesium basalt (SHMB) composition. Our Re-Os isotope modelling agrees with the trace element, Nd and Sr isotope modelling of Hoatson et al. (1992), that contamination of a komatiite (Re = 0.48 ppb, Os = 1.3 ppb, gOs = 0) with 15% 3.5 Ga crust of granodioritic composition (Re = 0.1 ppb, Os = 0.005 ppb, gOs = +890) can yield a SHMB magma (Re = 0.16 ppb, Os = 0.20 ppb, gOs = +18.5) which could be parental to the Radio Hill Complex ore system. Os concentrations in the silicate samples are much lower, from 0.039 to 1.1 ppb Os, with relatively higher Re/Os ratios, up to 18. These samples have experienced post-intrusion disturbance of the Re-Os isotope systematics and yield a poorly constrained isochron of ca 2.0 Ga age which coincides with the development of the Capricorn Orogen to the south. The massive ores were apparently not disturbed by this event, potentially due to the very high concentration of Os in the ores relative to metamorphic fluids. Acknowledgements: Thanks to Agip Australia Pty Ltd for access to the mine and drill core, and for Australian Nickel Mines NL for continuing cooperation. Financial support for this project was provided by the Australian Research Council and the Australian Crustal Research Centre. References HOATSON D. M., WALLACE D. A., SUN S.-S., MACIAS L. F., SIMPSON C. J. & KEAYS R. R. 1992, Petrology and platinum-group element geochemistry of Archaean layered mafic-ultramafic intrusions, west Pilbara Block, Western Australia. Australian Geological Survey Organisation. Bulletin 242. LAMBERT D. D., FOSTER J. G., FRICK L. R. & RIPLEY E. M. 1999, Re-Os isotope geochemistry of magmatic sulfide ore systems, In Lambert D. D. & Ruiz J., eds. Application of Radiogenic Isotopes to Ore Deposit Research and Exploration, pp. 29-58. Reviews in Economic Geology 12. WALKER R. J., MORGAN J. W., HORAN M. F., CZAMANSKE G. K., KROGSTAD E. J., FEDORENKO V. A. & KUNILOV V. E. 1994, Re-Os isotopic evidence for an enriched-mantle source for the Noril'sk-type, ore-bearing intrusions, Siberia. Geochimica et Cosmochimica Acta 58, 4179-4197.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EARLY PALAEOZOIC ARC BASIN SEQUENCE IN THE TAMWORTH BELT: CONSTRAINTS FOR EAST GONDWANA TECTONICS ^Terry G. Furey-Greig, ^Evan C. Leitch and ¥ e t e r A. Cawood 'Environmental Sciences, University of Technology, Sydney, New South Wales, 2007. ^Tectonics Special Research Centre, Applied Geology, Curtin University, Perth, Western Australia, 6845. Assemblages of convergent margin rocks formed in Early Palaeozoic time close to the East Gondwana margin occur in the Ross Fold Belt of the Transantarctic Mountains, the Adelaide and Lachlan fold belts of southeast Australia, the Tuhua Orogen of New Zealand, the New England Fold Belt in northern New South Wales and the Mt Windsor Subprovince of northeast Queensland. However in recent tectonic syntheses the last two have commonly been ignored. Here we outline the nature and tectonic significance of the Early Palaeozoic sequence in the Tamworth Belt of the New England Fold Belt. An Early Palaeozoic sequence, overlain unconformably although with no obvious structural disparity, by Early Devonian strata, has been mapped in the Tamworth Belt near Woolomin. The sequence has been divided into three units, the Murrawong Creek, Pipeclay Creek and Haedon formations. The Murrawong Creek Formation comprises at least 450 m of pebble to boulder debris flow conglomerate, turbidite sandstone and siltstone, siliceous mudstone and ash-fall tuff Limestone boulders characterise a 35 m thick conglomerate stratigraphically low in the formation which also contains cobbles of volcanic rocks ranging from basalt to rhyolite. The limestone clasts have yielded a diverse Middle Cambrian (FloranUndillan) fauna; they are believed to have been derived from shallow marine deposits that accumulated on the margin of the depositional basin penecontemporaneous with mass-flow transport of clastic debris into deeper water. The fauna shows strong biostratigraphic affinities with those in similar age rocks from the Georgina Basin (Dr G.A. Brock and Mr M.J. Engelbretsen pers comm). The Murrawong Creek Formation fines up into the Pipeclay Creek Formation which consists of up to at least 1600 m of granule conglomerate, sandstone and siltstone turbidites, mudstone and ash-fall tuff Paraconodonts recovered from a cherty bed within the Pipeclay Creek Formation are 'consistent with a Middle Cambrian to early Late Cambrian age'. Both Cambrian formations were derived from the west relative to present day co-ordinates. The Haedon Formation consists of about 100 m of strata that belong to two major facies, a bedded bioclastic calcarenite to calcilutite facies representative of autochthonous limestone accumulation unconformably above the Pipeclay Creek Formation, and an ill-stratified conglomerate-coarse sandstone facies containing allochthonous limestone masses up to at least 4 m in longest dimension. The latter facies appears to occupy a broad channel the margins of which are made up of the bedded limestone. Conodonts from both the bedded limestone and the allochthonous blocks are of late Arenig to early Llanvim age. All Early Palaeozoic formations accumulated close to a major source of arc-derived detritus with the presence of tuff in the Cambrian rocks indicating contemporaneous volcanism. No tuffs have been identified hi the Haedon Formation but abundant coarse lithic volcanic detritus suggests derivation from a continually rejuvenated source. Volcanic clasts from the Murrawong Creek Formation were derived from a low-K orogenic suite and sandstones from all three units show a similar provenance. Clearly magmatic arc activity was as significant a feature of Early Palaeozoic New England as it was of the Lachlan Fold Belt and the Ross Orogen, and the Tamworth Belt rocks preserve amongst the earliest records of arc volcanism close to the eastern margin of Gondwana. The Early Cambrian age of ophiolitic rocks of probable supra-subduction zone origin found along the Peel Fault just east of the Early Palaeozoic sequence is consistent with an ophiolitic basement and forearc setting for accumulation of the Early Palaeozoic sequence. However the recent report of c.570 Ma eclogite also associated with this suture suggests that plate convergence probably started in the Neoproterozoic. The history of the New England Fold Belt from this time until the Triassic is dominated by convergent margin activity and provides an example of the longevity of such margins once established, and demonstrates the fecundity of the spreading system within the Pacific Ocean and its precursors.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MICROCONTINENT FORMATION AROUND AUSTRALIA Carmen Gaina School of Geosciences, University of Sydney, NSW, 2006.

Microcontinents are common in the accreted continental geological record, but relatively rare at present. Many of today's microcontinents are found in the Tasman Sea and in the Indian Ocean. They include the East Tasman Rise, the Gilbert Seamounts, the Seychelles, Elan Bank (Kerguelen Plateau), and possibly the Wallaby Plateau off West Australia. We review their history of formation, and investigate the mechanisms that led to their isolation. We identified an association of plume-related microcontinent isolation and subsequent long-term asymmetries in oceanic crustal accretion in the Tasman Sea and in the central Indian Ocean. Tasman Sea continental fragments formed by ridge jumps onto adjacent continental margins after seafloor spreading in the southern Tasman Sea had commenced. The East Tasman Plateau was separated from the Lord Howe Rise at about chron 34 (83 Ma) and the Gilbert Seamount Complex rifted off the South Tasman Rise at roughly 77 Ma, by ridge jumps in opposing directions. A thermal anomaly in the Ross Sea area in the Late Cretaceous would have been located south of the South Tasman Rise, and could have given rise to ridge jumps towards the southeast in the southernmost Tasman Sea, creating both the Gilbert Seamount complex continental fragment, as well as subsequent excess accretion on the southern Lord Howe Rise. A hotspot which would have been located under the central Lord Howe Rise at Tasman Sea breakup time (-95 Ma) might have triggered the formation of the east Tasman Plateau due to ridge jumps toward the northeast. In the central Indian Ocean, spectacular exposures of granite make the Seychelles a type example for a microcontinent. As in the Tasman Sea, ridge-plume interaction has been responsible for separating a thinned continental sliver from a large continent (India). Elan Bank as part of the Kerguelen Plateau represents another example for a continental fragment in the Indian Ocean. OOP Leg 183 drilling on Elan Bank recovered gamet-biotite gneiss clasts, providing unequivocal evidence for the presence of continental crust (Frey et al., EPSL, 2000, 176, 73-89). A likely mechanism for its separation from India is given by the interaction of the Kerguelen Plume with the spreading center, leading to a northward ridge jump onto the eastern Indian continental margin in the Late Cretaceous, thereby isolating Elan Bank. The Wallaby Plateau off West Australia, located between the Perth and Cuvier abyssal plains, may also include slivers of continental crust, but unequivocal evidence is not available as it has never been drilled. A plume-ridge interaction model has also been proposed for its formation. In summary we conclude that many microcontinents form by re-rifting of a young continental margin in the vicinity of mantle plumes. A yield-strength minimum along the landward edge of a rifted margin, thermally enhanced by heating from a mantle plume, may cause a ridge jump onto this zone of weakness. The model is consistent with the observation that many accreted terranes exhibit shallow continental margin sequences of carbonates or siliciclastic rocks in association with metavolcanics. At the same time, the model does not apply to all microcontinents, such as those formed in association with transforms or active margins (e.g. Lomonosov Ridge, Arctic Ocean).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

REPETITIVE MARINE TO LACUSTRINE FAUNAE CHANGES IN THE GULF OF CARPENTARIA ^ A. Garcia. ^ A.R. Chivas, ^ J.M. Reeves, ' S. Holt,' M.JJ. Couapel and ^ P. De Deckker ^ School of Geosciences, University of Wollongong, NSW 2522. ^ Department of Geology, The Australian National University, Canberra ACT 0200

The Gulf of Carpentaria is a shallow epicontinental sea between Australia and Papua New Guinea. Its maximum depth is ~ 70 m, and the sill connecting to the west with the Arafura Sea (Indian Ocean) is approximately 53 m below sea level, while to the east, Torres Strait that connects to the Pacific Ocean, is about 12 m below sea level. In 1997, six cores were obtained using a giant piston-corer on board the Marion Dufresne, as part of the International Marine Global Changes Study (IMAGES). These cores reached depths from 6 to 15 m, encompassing the history of the Gulf of Carpentaria for the last 130 ka. At present, only the two longest cores (13.50 and ~ 15 m), have been prepared for micropalaeontological analysis, and our palaeoenvironmental reconstruction is based on them. Because the area has been subaerially exposed in periods of sea-level lowstands, the two cores collected in the northern shallower part of the gulf are shorter, and highly affected by pedogenesis. The other cores, collected in an area closer to the depocentre of the basin, show better preservation of fauna representing each non-marine/marine event, despite each unit's containing a high number of reworked micro fossil specimens. The analysis of calcareous microfossils, has provided an invaluable tool for the reconstruction of the different palaeoenvironments. This study includes foraminifers, ostracods, charophytes and coccoliths, as major taxonomic groups indicative of different ecological conditions. This multidisciplinary approach is needed in order to reconstruct the complex succession of non-marine to marine environments found in the Gulf. At least two major non-marine/marine cycles are recognised with probably some shorter marine intervals, as minor incursions of sea water during a non-marine sequence. The marine assemblages indicate open ocean to shallow-water conditions, while the nonmarine taxa allowed the identification of mixed environments connected with the ocean (deltaic, estuaries or lagoons), or continental water-bodies without any connection with the ocean, and varying from saline lakes with evaporites to fresh-water lakes.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A NEW VIEW OF CONTINENTAL MARGINS James V. Gardner U.S. Geological Survey, Menlo Park, CA 9 4 0 2 5

A new perspective on continental margins has evolved with a revolution of seafloormapping technology. Studies of continental margins blossomed in the post-war era of ocean exploration beginning in the 1950's but were based on hand-drawn interpretations of fathometer tracings, such as the famous Heezen and Tharp physiographic maps of the 1960s. These were our first views of the oceans at a global scale but the details of the continental margins were lacking because of sparse data. The 1970s were devoted to unraveling the architecture of continental margins, the third dimension, through advances in geophysics and data processing. The 1980s could be thought of as the decade that concentrated on margin processes. A lot of energy was expended in trying to understand shelf and slope deposition and the mechanisms of mass failure. And in the last decade of the century, morphology and process studies are merging into a focus on quantitative geomorphology guided by the use of high-resolution multibeam seafloor-mapping systems. The new comprehensive views of continental margins provided by high-resolution multibeam seafloor-mapping systems reveal regional-scale segments of the margin at meter resolution. These systems resolve small-scale features that typically are not seen in the more traditional types of geophysical data. Examples of small-scale features include outcrops, fields of subtle bedforms, glacial groves, and linear sand ridges on the continental shelf Somewhat larger, but still subtle, features are linear gullys, submeter in depth and only a few meters in width, that are commonly found on the U.S. Pacific upper slope. Zones that appear to be fresh-water sapping are also common features on the upper and lower slope. Major debris-flows features are delimited, some having rafted decimetersized blocks kilometers out onto the basin floors. Major failures of the U.S. Pacific slope appear to be rare, but are found and it is interesting to note that margin failure appears to be localized rather than pervasive. Many submarine canyons, once depicted as rather unique, uncomplicated, submarine conduits, turn out to be very complex, multistage features that have many geomorphic characteristics in common with subaerial fluvial systems. With the best of the new generation of high-resolution multibeam systems, coregistered backscatter is simultaneously collected with bathymetry. Backscatter, similar in appearance to a sidescan-sonar image, provides a geological perspective of the surface of the margin. With the combination of backscatter accurately draped over the bathymetry, the surficial geology of the margins is more clearly apparent. Areas of outcrop can be identified and correlated to the related bathymetry. GIS and remote-sensing techniques are now being applied to high-resolution multibeam data and are providing quantitative descriptions of the gradients and accurate geometry of features. Remote-sensing techniques such as supervised classifications can now be performed on the backscatter and bathymetry data, together with ground-truth sediment data, to generate maps of predicted surficial-sediment facies. 3D visualization is now advanced enough to allow the viewing of these vast data sets in their entirety as perspective views and flybys to better interrogate the data and grasp the actual geomorphic complexities of continental margins.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE LAKE TAHOE DEBRIS AVALANCHE James V. Gardner U.S. Geological Survey, Menlo Park, CA 9 4 0 2 5

McKinney Bay is a large reentrant in the western margin of Lake Tahoe that was created by a mass failure about 300 ky ago. The event generated a major debris avalanche that carried very large blocks across the basin. Measurements of block length, width, height and distance from an arbitrary point immediately east of the headwall scarp were made on all 223 blocks on the floor of the lake. A distinct break in trend of block length, width, and height with distance corresponds to the location of the toe of a debris apron. The dimensions of the blocks in the basin have more variability than those west of the toe. Blocks in the eastern part of the basin floor have a wider variability of sizes than those in the western part of the basin floor. The Tahoe debris avalanche apparently traversed across the lake floor, reflected against the eastern margin, and was deflected to the north, south, and back toward the west, effectively flowing back onto itself Subparallel, acoustically-stratified seismic reflectors occur beneath one of the large blocks but appear to be undisturbed by the emplacement of the block. A 30-m-thick zone of disturbed reflectors overlies the stratified-reflector packet, which is intum overlain by a 50m-thick zone of undisturbed reflectors. These observations suggest the disturbed-reflector zone represents matrix material of the debris avalanche and the upper acoustically stratified zone represents post-failure sedimentation. The average sedimentation rate of 15 cm/ky, based on published conventional bulk ^"^C dates, although not necessarily representative for the entire sediment sequence, nevertheless suggests an age in the range of about 300 ka for the occurrence of the debris avalanche. One of the effects of the Tahoe debris avalanche should have been a considerable lake seiche that would have generated large run-up waves onto the land. The run-up waves might have produced the equivalent of tsunami deposits at various locations around the lake. There are published descriptions of scattered glacial till and erratics mapped at elevations at least 30-m higher than the present lake level that are explained as a result of higher lake levels caused by glacial tongues that blocked the outflow to the lake. Once the lake level reached a critical height, it is thought the ice tongues became buoyant, causing a catastrophic collapse of ice dams (jokulhlaup). The new multibeam images and discovery of major failure deposits provides an alternate hypothesis. The scattered till and erratics may be the equivalent of a tsunami deposit above lake level, formed by the lake seiches created by the Tahoe debris avalanche. The deposits are described as boulders overlain by soil with abnormally high clay content. This description is similar to those of interpreted tsunami deposits.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AREA SELECTION AND GOVERNMENT REGIONAL SURVEYS R Dennis Gee Northern Territory Geological Survey, PO Box 2901, Darwin NT 0801 Area selection involves the systematic gathering and interpretation of data sets in order to identify which geological region the explorer will search for a specified suite of commodities. The primary drivers are intrinsic geological potential, and position in the contemporary exploration cycle - the assessment of which requires access to quality databases. Australia is well endowed in these, however they are far from optimal. This presentation focuses on who compiles the datasets, how they are accessed and used, and what they should contain. Australia's strength goes back to the foundation charters of the State Geological Surveys at the turn of the last century, which were overtly promotional of the mining industry, and which continues through the traditional period of data gathering up to the 1980s. That continental scale geology and geophysics of that era directly contributed to important discoveries, but today most explorers would regard them as obsolete. The 1980s gold boom saw the first high-resolution airborne magnetic surveys of mineral fields, CAD systems and mining software for project management, and the emergence of university centres dedicated to orebody models specific to our terraines. Although there were some notable discoveries due to geological perspicacity, area selection was largely empiric, which is not surprising because hydrothermal gold systems have vertical polarity. At this time many of the geological surveys were marching into remapping programs, and constructing microfilm/microfiche databases of company reports. To some extent the Australian Geological Surveys got left behind in this developing technological era. The first of the Australian Exploration Initiatives involving state-funded regional geophysical surveys aimed at the exploration industry kept SA and NSW in touch. WA entered into advanced-purchase delayed-delivery arrangements of multiclient surveys. NT had quietly been undertaking regional surveys on a piecemeal basis commencing in 1982. Apart from some geophysical surveys, not a lot of data was contributed to these modem datasets by the State Geological Surveys or BMR / AGSO. In the 1990s, the emergence of GIS platforms, exploration databases, image processing software, and LAN and WAN systems run from central servers, fostered the growth of large corporate databases, and in-house area-selection and project-generation teams. The systems were populated with datasets that were both primitive and expensive. Capital and operating costs for areaselection units were high. Service bureaux grew into a commercial niche, and developed innovative interpretation products. These included prototype attributed prospectivity maps, and multi-client aeromagnetic and bedrock interpretation maps of the Eastern Goldfields. Here for the first time we see the successful marriage of geology and geophysics. But the area-selection bubble burst in the late 1990s, with the collapse of the commodity markets and the. shift to growth by acquisition rather than discovery. Today there are very few companies supporting project-generation think-tanks, and the corporate datasets presumably lie dormant. In this time of irreversible change, some generative work will shift to the service bureaux. In this new environment the Geological Surveys have two specific roles. Firstly they have to develop truly functional datasets and then make them freely available. The core datasets would include: • • • • • • • • •

Seamless geological maps at various nominal scales, in industry standard GIS format; Bedrock geological maps based on the integration of geological and geophysical information; Line data and grids of high resolution fully DGPS controlled airmag and radiometrics, over all terrains Seamless stitches of images of high-resolution airmag on various algorithms; Exploration geochemistry spatial datasets on industry-standard format, Exploration company reports in PDF with spatial interface Regolith spatial datasets of geochemical significance; Digital topobases over the entire country to - AUSLIG data is still inadequate; Current tenure and cadastre spatial datasets.

Other datasets such as in-fill gravity, airborne gradiometery and AEM (as a mapping tool) will be required, but these are currently out of the range of the best funded surveys. We are still a long way from achieving the core datasets. All these datasets need to be web enabled - a major challenge still not solved by any of the Geological Surveys. Secondly, Geological Surveys have to venture down the path of area identification and project generation. It is not sufficient to simply provide information and data. Certainly NTGS intends to develop a portfolio of exploration leads and plays (to borrow terms from petroleum exploration) as part of the promotion under the current Exploration Initiative. This demands the continuance of studies of mineral systems and ore-deposit models by other organisations. The focus of these portfolios will clearly be directed to blind and covered plays. Geological Surveys must resist the temptation to dissecting known mineral belts, and primarily focus on new resource plays, and in so doing provide information and ideas in advance of the next wave of explorers. Three such regions are emerging in the under-explored terrains of Northern Territory. New airmags in the TennantTanami link area show that the basement structure can be clearly deciphered under thin cover of Cambrian Wiso Basin. Secondly the extensive correlation of the 1.8-1.6 Ga MacArthur basin strata demands that these platform covers extended south to the proto-continental margin, and would be found in the metamorphic terrains of the Arunta Province. Finally new data on the depocentre of the Cambro-Ordivician Georgina Basin indicate the presence of marginal growth faults on each margin, sedimentational influences from previously unrecognised basement structures, barite-lead occurrences in a trough-to-shelf transition, and elevated thermal regimes related to adjacent high-grade Ordovician metamorphism.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SULFUR ISOTOPE CHARACTERISTICS OF SUBAQUEOUS EPITHERMAL MINERALISATION, CONICAL SEAMOUNT, PAPUA NEW GUINEA J. Bruce Gemmell and Robina Sharpe Centre for Ore Deposit Research, University of Tasmania, Box 2 5 2 - 7 9 , Hobart, Tasmania

A major occurrence of polymetallic epithermal-style vein mineralisation and associated pyritic stockwork was mapped and sampled on the summit of Conical Seamount, a newly discovered subaqueous alkaline volcano south of Lihir Island, Papua New Guinea in 1998 (Herzig et al, 1999). Mineralized rock consists of stockwork and disseminated sulfides similar to ore currently being mined at the Ladolam deposit on the island of Lihir. This discovery represents a new type of seafloor mineralisation and has important implications for understanding the VHMS - epithermal transition in the subaqueous environment. Conventional and laser ablation techniques were used to determine the sulfur isotope signature of the mineralisation, compare it to mineralisation at Lihir and to investigate the sources of sulfur (magmaticiseawater) in subaqueous epithermal mineralisation. Sixty two (16 conventional and 46 laser) sulfur isotope analyses from over the lateral extent of the summit region of Conical Seamount have S^'^S values ranging from - 2 0 . 2 4 to +3.98%o. The two sulfur isotope analytical methods give similar results, although conventional measurements tend to have narrower variation (range - 1 1 . 5 3 to O.9396o; mean - 2 . 4 l % o ) than laser measurements (range - 2 0 . 2 4 to 3.98%o; mean -1.83%o). The Conical Seamount sulfides contain some of the lightest sulfur isotope values yet measured on the modem seafloor. Two sulfur isotope values for sulfate in alunite o f + 7 . 5 and +6.4%o were reported by Herzig et al ( 1 9 9 9 ) . In general the Conical Seamount mineralisation has very light values over a large range, but there is a shift from heavy values in the inner zone (semi-massive pyrite + sphalerite) varying to progressively lighter values in the intermediate zone (pyrite, As-sulfides, sphalterite-galena, electrum) and outer zone (pyrite stockwork). Porphyry and transitional (porphyry to epithermal) mineralisation types at Lihir have similar sulfur isotope values as the intermediate zone at Conical. The Lihir epithermal and Conical outer zone mineralisation have comparable sulfur isotope values, however the Conical inner zone has heavier sulfur isotope values compared to the Lihir data. In the absence of biogenic sulftir and/or evidence of boiling at Conical Seamount, these light sulfur isotope ratios are likely related to the disproportionation of magmatic SO2 into H2S and H2S04^". This is accompanied by kinetic effects enriching ^^S in the sulfide and ^^S in the sulfate resulting in values lower than the starting composition 0%o) for sulfide (avg. -1.94 %o and values higher than the starting composition (but lower than modem seawater: +21%o) for associated alunite. The sulfur isotopic ratios of sulfide/sulfate and the presence of platy alunite strongly supports the conclusion that metal-rich magmatic fluids and gases have contributed to the hydrothermal system at Conical Seamount. References Herzig, P.M., 1999, Epithermal-type gold mineralization at Conical Seamount: a shallow submarine volcano south of Lihir Island, Papua New Guinea, Mineral Deposits: Processes to Processing, Stanley et al. (eds), Balkama Rotterdam, Volume 1, p. 527-530.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MID-MIOCENE COOLING IN THE NORTHERN QILIAN SHAN, NORTHEASTERN MARGIN OF THE TIBETAN PLATEAU, REVEALED BY APATITE FISSION-TRACK AND VR ANALYSIS Annette D. George', Susan J. Marshallsea^ Karl-Heinz Wyrwoll\ Chen Jie^ and Lu Yanchou^ Tectonics Special Research Centre, University of Western Australia, Nedlands WA 6907 Australia 2. Geotrack International, Melbourne, West Brunswick VIC 3070 Australia 3. Institute of Geology, China Seismological Bureau, P.O. Box 9803, 100029, China Growth of the Tibetan Plateau along its northeastern margin by crustal shortening and thickening is linked to the Altyn Tagh Fault, a - 1 5 0 0 km long sinistral strike-slip fault which has played a significant role in accommodating the convergence of India and Eurasia. The NW-trending Qilian Shan (mountains) that mark the northeastern edge of the plateau rise nearly 5000 m above the adjacent Juice Basin of the Hexi Corridor. The Qilian Shan is a mid-Paleozoic orogenic suture belt composed of numerous thrust slices of deformed Lower Paleozoic (Cambrian-Silurian) metasedimentary and metavolcanic strata. The older Precambrian basement is also exposed in some thrust sheets, as are remnants of Upper Paleozoic-Jurassic strata. Cretaceous fluvial sandstones and conglomerates are locally preserved. The neighbouring Juice Basin contains a thick succession of Neogene-Quatemary fluvial strata (-3.5 km thick) which unconformably overlies Lower Cretaceous rocks. Adjacent to the Qilian Shan, the oldest Neogene strata in the Juice Basin are Miocene in age overlain by more extensive Pliocene and Quaternary sediments, whereas basal Oligocene strata are exposed on the northern side of the basin. Neogene and Quaternary strata are being actively deformed by thrust-related folding along the margins and within the Jiuxi Basin. Apatite fission-track and vitrinite reflectance data were obtained from Precambrian to Tertiary outcrop samples from the northern Qilian Shan and Juice Basin to provide constraints on timing of exhumation and cooling. All the samples record multiple cooling episodes. Tertiary cooling from peak paleotemperatures (40-120®C) beginning sometime between 20 and 10 Ma (mid-Miocene) is common to all samples. This age is significantly older than previous models which proposed initiation of uplift in the Pliocene-Pleistocene, i.e., no older than 5-6 Ma. For any reasonable paleo-geothermal gradient, the degree of mid-Miocene cooling requires km-scale exhumation to explain the data; e.g. 30®C/km requires removal of at least 1 and up to nearly 4 km of section across the region. The thermal history of the region prior to the mid-Miocene is complex with multiple episodes of cooling revealed by the AFTA and VR analysis. Cretaceous samples indicate maximum paleotemperatures of 105120®C were reached soon after deposition, with cooling beginning prior to - 1 2 0 Ma. Similarly, Jurassic samples also reached maximum paleotemperatures soon after deposition (indicated by the VR data), overprinted by subsequent Cretaceous cooling episodes. In most of the Paleozoic samples, evidence for preMesozoic cooling has been masked by subsequent high Cretaceous paleotemperatures, however, one sample records a Permo-Triassic event with cooling beginning between 270 and 230 Ma. The most significant AFTA result from this study is that deformation in the mid-Miocene was more extensive than indicated in previously proposed models for the tectonic development of the northern Qilian Shan and Juice Basin. The results are more consistent with a broadly distributed deformation zone rather than supporting discrete deformation zones migrating systematically to the northeast. Furthermore, a midMiocene age is broadly coincident with Cenozoic uplift and exhumation along other margins of the Tibetan Plateau.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

OIL-BEARING FLUID INCLUSIONS: GEOCHEMICAL ANALYSIS TECHNIQUES AND GEOLOGICAL APPLICATIONS Simon C. George, Tim E. Ruble, Robinson A. Quezada, Manzur Ahmed and Paul F. Greenwood CSIRO Petroleum, PO Box 136, N. Ryde, NSW 1670, Australia Oil-bearing fluid inclusions (oil inclusions) are small samples, often <10 ^m in diameter, of pore fluid trapped in petroleum reservoirs during crystallisation of minerals such as quartz, feldspar and calcite. Oil inclusions typically occur in diagenetic cements, such as quartz overgrowths, and in healed microfractures which cross-cut both detrital grains and diagenetic cements. Oil inclusions are most commonly recognised by their fluorescence under ultraviolet (UV) light. The presence of oil inclusions in water-wet reservoir sandstones provides a record of palaeo-oil migration. Establishing the geochemical composition of oil trapped in fluid inclusions is an analytical challenge due to their small size. For example, a 10 ^im diameter, spherical inclusion contains <0.4 ng of oil. A prerequisite for successftil oil inclusion analysis is an effective separation of the host mineral grains and cements, followed by an extremely thorough clean of the exterior mineral surfaces to remove formation fluids and drilling contaminants. A combination of physical separation techniques including magnetic separation, sieving and heavy liquids with chemical treatments including solvents, hydrogen peroxide and chromic acid has been found to be effective, Australian petroleum reservoirs are mostly sandstones, with oil inclusions trapped in quartz, so relatively aggressive chemicals can be utilised. It is particularly important to remove diagenetic clays which have a very high surface area and act a strong adsorbents for oil. It is possible to use outcrop samples, historically-stored core and cuttings samples if these clean-up protocols are rigorously observed. Geochemical analysis of the oil trapped in fluid inclusions is achieved using two main techniques. Low molecular hydrocarbons (CI to CI2) are best analysed using an on-line crushing technique directly onto the capillary column of a gas chromatograph-mass spectrometer (GC-MS). About 50 mg of cleaned quartz concentrate is manually crushed in a heated (350°C) inlet. Higher molecular weight hydrocarbons (C12 to C36) including polycyclic biomarkers are best analysed using an off-line crushing techniques, during which 1 to 10 g of cleaned quartz concentrate is crushed under solvent. The oil released from the inclusions partitions into the solvent, which can then be worked up for analysis on a variety of G C - M S and other instruments. A third method being developed is laser micropyrolysis GC-MS, which has potential for the geochemical analysis of individual oil inclusions. A laser beam is focussed down a microscope and used to decrepitate inclusions, the oil from which is swept onto a GC-MS system. So far only groups of inclusions have been successfully analysed using this technique, due to sensitivity considerations. Particular useful geological applications of the molecular composition of oil inclusions (MCI) technique are when reservoired oil has been altered or lost by biodegradation, later gas charge or trap failure. In these instances, oil inclusions provide one of the best methods for obtaining a pristine geochemical signature of the palaeo-oil, which is useflil for oil-source and oil-oil correlations. In current oil zones, inclusion oils are commonly observed to have a lower thermal maturity than the reservoired oil, implying that inclusion oils dominantly trap early charge into a reservoir. This observation is useflil because it enables first charge oil, which may have been generated from a different source rock, to be separately analysed from the currently reservoired oil, which may be the mixed product of several charges from different source rocks at different maturities. Lastly, offshore oil wells are commonly drilled with oil-based drilling muds or may be contaminated by diesel or other drilling additives, leading to difficulty in interpreting indigenous hydrocarbon distributions. Inclusion oils offer the possibility of analysis of a pristine oil avoiding drilling mud contamination.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LOW-ANGLE STRUCTURES IN THE BROKEN HILL BLOCK, NSW: LEGACY OF THRUST TECTONICS OR EXTENSIONAL COLLAPSE? George Gibson Minerals Division, Australian Geological Survey Organisation Deep seismic reflection imaging along a NW-SE transect orthogonal to regional strike in the Paleoproterozoic Willyama Supergroup shows that the crust in the Broken Hill block is subdivided into different structural domains by SE-dipping shear zones (Gibson et al., 1998), many of which penetrate to deep crustal depths and truncate shallower-dipping reflectors representing older shear zones or regions of "normal" layered crust. This structural geometry has been interpreted in terms of a northwest-vergent fold and thrust belt superimposed upon an older orogenic fabric (Gibson, 1998). A similar geometry has been revealed in seismic images of other orogenic belts, including the Grenville front (tectonic zone) where thruststacking of micro-terranes was followed by crustal penetrating ductile imbrication under high-pressure-high temperature conditions leading to ramping of formerly deeply buried rocks to the near surface (Green et al., 1988). Support for the operation of an analogous two-stage process to explain uplift and the present distribution of high-grade rocks in the Broken Hill block comes from the fact that the bulk of crustal thickening and high-grade metamorphism in the Broken Hill block predates formation of the SE-dipping shear zones. These shear zones are of D3 age and dip much too steeply for any attendant thrust stacking to have produced more than a fraction of the 60-65 km crustal thickening estimated to have occurred in the Broken Hill region based on published barometric calculations (0.5-0.6 GPa) for the observed mineral assemblages (Phillips, 1980). Crustal thickening and high-grade metamorphism more likely occurred during an earlier episode of recumbent folding and associated thrust stacking although whether the shallow-dipping fabric imaged in the seismic profiles originated during D1 deformation or in response to an additional and previously unrecognised phase of (D2) recumbent folding (Gibson, 1998) in the Broken Hill region has yet to be frilly resolved. The D1 and D2 deformations reported here both occurred under amphibolite-granulite facies conditions and both gave rise to sub-horizontal or shallow-dipping fabrics (SI and S2). However, peak metamorphic conditions involving the formation of sillimanite - K feldspar - biotite ± garnet ± cordierite mineral assemblages in pelitic rocks post-date the formation of SI as evidenced by straight sillimanite inclusion trails in post-Dl garnet porphyroblasts. These same garnets are wrapped by the D2 fabric indicating peak metamorphic conditions were probably reached immediately following the D1 deformation and prior to the cessation of D2 deformation. This interpretation is consistent with a structural model of early crustal thickening through the formation of D1 nappes followed by extensional collapse of a thermally weakened orogenic belt undergoing high grade metamorphism and contemporaneous intrusion by granitic magmas. Extensional collapse is consistent with the shallow attitude of the D2 structures and the observation that S2 commonly contains a pronounced stretching lineation defined by sillimanite. Alternatively, the D2 event may also be thrust-related in which case extensional collapse may have occurred late during D l . A pre- or early syn-D2 age for extension is consistent with the observation that D2 folds deform low-angle shear zones which juxtapose younger, lower grade rocks against older, higher-grade ones. Acknowledgement: Published with permission of Chief Executive Officer, AGSO References GIBSON, G. M. (1998): AGSO Record 1998/2: 75-77; GIBSON, G. M. et al. (1998): AGSO Record 1998/11, 55pp; GREEN, A. G. et al. (1988): Geology 16: 788-792; PHILLIPS, G. N. (1980): Contrib. Mineral. Petrol. 75: 377-386.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MULTIPLE MECHANISMS OF MELTING IN GRANULITES IN THE VREDEFORT DOME, SOUTH AFRICA R.L. Gibson and A.J. Ashley Department of Geology, University of the Witwatersrand, WITS, Johannesburg 2050, South Africa Mid-crustal Archaean granulite-grade gneisses in the central parts of the Kaapvaal Craton of southern Africa were uplifted in the 70-km-wide Vredefort Dome as a result of a large meteorite impact strike at 2.02 Ga ago. The gneisses comprise a predominantly tonalite-trondhjemite-granodiorite suite with subsidiary pelitic, ironstone and mafic xenoliths that experienced partial melting concomitant with deformation at ca. 3.1 Ga ago. Beyond a radial distance o f - 1 0 km from the centre of the dome, the gneisses retain their Archaean metamorphic and structural features but are, in addition, characterized by shock-diagnostic microdeformation features such as planar lamellae in quartz, and by abundant development of pseudotachylitic breccias. Within 10 km of the centre of the dome, however, these Archaean features have been variably destroyed by the development of diffuse patches and networks of fme-grained feldspar. These features have been variously interpreted as recrystallized pseudotachylitic breccias, recrystallized shock melts or homfelses developed against a hidden mafic intrusion. Petrographic analysis of samples from several localities, including a borehole core from near the centre of the dome, suggests that these features formed in two ways: •

•

Irregular patches of fine-grained monomineralic plagioclase are attributed to shock melting of large Archaean plagioclase grains at shock pressures in excess of - 3 5 GPa during the impact event. It is unclear whether the fme-grained plagioclase aggregates crystallized directly from the shock melt or developed by recrystallization of a glass. Diffuse vein networks comprising fine-grained plagioclase, K-feldspar, quartz and minor amounts of opaque oxides, biotite, amphibole and/or pyroxene are linked to pseudomorphous replacement of Archaean hornblende ± biotite in the host gneisses by fine-grained plagioclase + pyroxene + opaques. These textures suggest that these veins represent partial melts of more hydrous Archaean assemblages. Mineral textures such as oikocrystic biotite, K-feldspar and quartz, and granophyric quartz-plagioclase intergrowths suggest direct crystallization from a melt. The variation in composition of the melts is attributed to the heterogeneous nature of the Archaean source rocks.

Support for the dual origin of the melts is provided from pelitic granulites which crop out around the periphery of the central zone. These rocks were metamorphosed to gamet-cordierite-biotite±orthopyroxene migmatites at - 3 . 1 Ga ago. Following the impact event, they experienced temperatures sufficient to enable partial breakdown of garnet to cordierite-orthopyroxene symplectites. In addition to pseudotachylitic breccias, however, these rocks also contain partially recrystallized plagioclase grains as well as thin reaction rims between plagioclase and quartz and orthopyroxene mantling biotite. We conclude that farther from the centre of the impact structure, lower shock pressures led to only partial shock melting of plagioclase, and small amounts of partial melting of biotite + quartz + plagioclase. The delicate melt textures preserved in the rocks testify to the static nature of the metamorphism following the shock event. Sporadic outcrops of a coarse biotite granite near the centre of the dome are spatially associated with restitic cordierite-K-feldsparrutile homfels. Previous workers suggested that the restite was produced during the pre-impact (Archaean) granulite event. However, we attribute it and the granite to complete dehydration melting of biotite following the impact event. This is supported by a U-Pb SHRIMP zircon age of 2017 ± 5 Ma for the granite. Independent geobarometric constraints constrain the post-impact metamorphic pressure at between 0.2 and 0.3 GPa. The metamorphic parageneses suggest a strong lateral temperature gradient during this event, from -900-950 °C in the centre to - 5 0 0 °C some 20 km from the centre. The unusual textures in the central 10 km of the dome indicate that the post-impact temperatures here exceeded the solidus for a wide range of rock types that had already experienced a prior granulite facies partial melting event. The fine grain size of the melt rocks and the symplectites testifies to rapid cooling. The heat for this metamorphism is attributed largely to the combined effects of structural uplift of rocks pre-heated along an existing crustal geotherm and conversion of elastic strain energy in mineral lattices to thermal energy during the decay of the impact shock wave.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE EARLY TO MIDDLE PROTEROZOIC CONFIGURATION OF AUSTRALIA AND ITS IMPLICATIONS FOR AUSTRALIAN-US RELATIONS David Giles and Peter Betts Australian Crustal Research Centre, AGCRC, Department of Earth Science, Monash University In a recent paper Karlstrom et al. (1999) proposed a configuration of the supercontinent Rodinia (which they termed the AUSWUS reconstruction) that places the eastern margin of Australia adjacent to the southwest margin of North America during the Middle and Late Proterozoic. We believe that this configuration is essentially correct but can be improved by a more detailed consideration of the geologic evolution and internal configuration of eastern Australia during the Proterozoic. Geochronological data collected in the last 5 to 10 years provides evidence that the eastern Mount Isa Inlier, the Georgetown Inlier and the Willyama Inliers were once part of a coherent Early to Middle Proterozoic geologic terrane. The three inliers can be correlated based on: (1) the nature and age of supracrustal sequences (c. 1.7-1.6 Ga), the nature and age of the extended continental crust (c. 2.7-2.3 Ga) onto which these sequences were deposited and the nature and age of orogenesis (c. 1.6-1.5 Ga). A simple reconstruction of the North and South Australian cratons based on this correlation requires an approximately 55o rotation and eastward translation of the South Australian Craton relative to its present position. This reconstruction aligns the Early to Middle Proterozoic mobile belts of the Arunta Inlier and the Gawler Craton into a continuous belt that appears to have been a long-lived accretionary plate boundary (c. 1.88-1.67 Ga) along the southern margin of the Australian continent. This belt continued to the south and west, through Antarctica, linking with the 1.8-1.6 Ga accretionary terranes of southwest North America (the Yavapai and Mazatzal terranes, Hoffman, 1988; Karlstrom et al., 1999). During this interval both Australia and North America lay to the north of a long-lived accretionary plate boundary that stretched from western Australia to Baltica. This is not to say that the two continents were connected in the AUSWUS configuration during this entire interval. In contrast, there is evidence for the consumption of oceanic crust on both the western margin of North America and the eastern margin of Australia (Georgetown Inlier) between - 1 . 8 and 1.55 Ga. Gradual consumption of oceanic crust resulted first in the accretion of the Wopmay Orogen (then part of the c. 1.901.84 Ga Barramundi terranes of northern Australia) onto northwest North America along the Rimbey Suture (c. 1.85-1.78 Ga; Eaton and Cassidy, 1996). Oceanic crust may also have formed east of the Georgetown Inlier as a result of the intracratonic extension (c. 1.71-1.66 Ga) that is preserved throughout northern and eastern Australia. Final amalgamation of the continents occurred during the 1.60-1.50 Ga period of orogeny that affected eastern Australia (Isan Orogeny, Olarian Orogeny). The present configuration of the Australian Precambrian terranes, used in most Proterozoic reconstructions resulted from an extra cycle of extension and break up (c. 1.45 Ga - Roper Group, Pandurra Formation, Belt Supergroup) and subsequent collision (c. 1.3-1.1 Ga - Albany-Fraser and Musgravian orogenies). This configuration is not relevant to pre-1.3 Ga reconstructions. References Hoffman, P.F. 1998. United Plates of America, the Birth of a Craton: Early Proterozoic Assembly and Growth of Laurentia. Annual Reviews in Earth and Planetary Sciencel6, 543-603. Eaton, D.W., Cassidy, J.F. 1996. A relic Proterozoic subduction zone in western Canada; new evidence from seismic reflection and receiver function data. Geophysical Research Letters 23, 3791-3794. Karlstrom, K.E., Harlan, S.S., Williams, M.L., McLelland, J., Geissman, J.W and Ahall, K. 1999. Refining Rodinia; geological evidence for the Australia-Western U.S. connection in the Proterozoic. GSA Today 9, 1-7.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PETROLOGICAL AND GEOCHEMICAL SIGNIFICANCE OF FELSIC VEINS WITHIN THE GABBROIC LOWER OCEAN CRUST DRILLED DURING ODP LEG 176 AT THE SOUTHWEST INDIAN RIDGE Trinity Gilmore. Yaoling Niu, Ken Collerson and Sue Golding Department of Earth Sciences, The University of Queensland, Brisbane, Qld 4072 Ocean Drilling Program (ODP) Hole 735B, Southwest Indian Ridge, drilled during Leg 118 (1987) and Leg 176 (1997), revealed the first in situ section 1.5 km) of lower ocean crust. The drill core rocks are dominated by gabbro and olivine gabbro with variable amounts of troctolite, Fe-Ti gabbro, and Fe-Ti gabbronorite. Within these gabbroic lithologies are numerous (> 200) small (a few mm to several cm thick) felsic veins. Many of these veins show primary igneous textures and sharp intrusive contacts. Some preserve igneous textures, but have undergone subsequent hydrothermal alterations. Most veins are leucodiorite dominated by plagioclase plus some green amphiboles. Other lithologies include diorite, trodhjemite, and tonalite with variable amounts of dark amphibole, quartz, and biotite. Granitic veins were also recovered with up to 28% quartz and 24% alkali feldspars plus hornblende, biotite, pyroxene, Fe-Ti oxide, apatite, and zircon. Superimposed on the primary phases in some of the veins are metamorphic mineral assemblages: actinolite, secondary plagioclase, epidote, titanite, chlorite, quartz, clays, sulfides, and oxides. Despite the small volume of these veins (~ 0.5% of the entire gabbroic section recovered), their occurrence as an integral part of the gabbroic sequence has important implications for the physical and chemical processes taking place in the course of magma evolution and ocean crust accretion at slow-spreading ocean ridges. Their close spatial association with Fe-Ti oxides plus their primary igneous textures indicate that the felsic veins originate fi-om extreme degrees of in situ differentiation; Fe-Ti oxide crystallization at very late stages of basaltic melt evolution will naturally lead to silica-rich residual melt - the vein material. This extreme differentiation seen in numerous highly localised melt pockets scattered throughout gabbroic sequence argue strongly against current models of magma chamber processes, in which extreme differentiation would only occur in a magma lens atop the crystal-mush zone represented by the gabbroic sequence. The felsic veins may serve as "conduits" for subsequent fluid migration, which explains why many of these veins are hydrothermally altered. A fluid inclusion study on secondary quartz of some of the alteration assemblages gives homogenisation temperatures varying from 270°C to 420°C. While the ultimate source of the fluid awaits confirmation from O and H isotope data (being collected), Sr isotope data on secondary plagioclase (^^Sr/^^Sr = 0.7028 - 0.7036 with the average of 0.7031, significantly greater than 0.7029, the highest value of Southwest Indian Ridge basalts) suggests an important role of seawater. This suggests that seawater may penetrate deep into the lower crust level, and has important implications for models of the origin and evolution of hydrothermal fluids, mineralisation and biological development at ocean ridges. Significantly, the felsic veins with elevated abundances of incompatible elements (Fig. 1) relative to the host gabbros and mid-ocean ridges basalts (MORB) are important geochemical "sinks" in the ocean crust. Given the distinct geochemical (Fig. 1) and physical (lower solidus temperatures) properties of these "sinks" from the host gabbros, they are thus expected to have important geochemical consequences during ocean crust subduction, subduction-zone magmatism and crust-mantle recycling. Multi-incompatible elements

Rare earth elements

100

100

• with ZX'Wpeaks"

{N = 7)

•v^iihZr-Hf"troughs"

{N = 14)

10

10

I {N-MORB I) Average Host Gabbros 0.1

0.1 Ce La

Nd Pr

Sm

Gd Eu

Dy Tb

Er Ho

Yb Tm

Lu

Ba T h U La Pr Sr Z r Sm Eu T b V Er Y b R b N b Ta Ce Pb N d H f Ti G d D y H o T m L u

Figure 1. N-MORB (Sun and McDonough, 1989) normalised rare-earth element (left) and multiincompatible element (right) plots of felsic vein lithologies within the ODP Leg 176 gabbros.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

FISSION TRACK IMAGING OF THE AUSTRALIAN CONTINENT Andrew Gleadow', Barry Kohn', Paul O'Sullivan', Roderick Brown' and Kerry Gallagher^ 'Australian Geodynamics CRC, School of Earth Sciences, University of Melbourne ^T.H. Huxley School of Environment, Earth Science and Engineering Imperial College, London

Techniques to reconstruct the thermal histories of rocks in the low-temperature environment of the upper crust have previously found important applications in such areas as sedimentary basin analysis, the evolution of convergent orogenic belts and studies of extensional tectonic environments. Increasingly large sets of thermochronology data are now becoming available on an even larger scale from the apatite fission-track system (for temperatures <~110°C). Additional information is also now starting to emerge at even lower temperatures (<~75°C) from the apatite (U-Th)/He system. The results from such systems can be used in novel ways to examine and visualise the evolution of the upper part of the continental crust. Quantitative modelling has brought a new dimension to this work and forward-modelling procedures are now well established which give "best fit" thermal histories for temperatures less than about 110°C. In many cases the modelled thermal histories provide a proxy for rock transport towards the surface as overlying material is removed by denudation. Such information allows a reconstruction of the denudation history of the land surface. Applying these modelling procedures to large regional arrays of fission-track data means that the thermal history information can now be integrated with other regional data sets to provide an important new perspective on crustal and surface evolution. Apatite fission track analysis has now been completed on nearly 3000 surface outcrop samples across Australia under an AGCRC project to image the thermotectonic evolution of the exposed basement rocks of the continent. The samples analysed are mostly from rocks of broadly granitic composition and the apatites separated from these are overwhelmingly fluorapatites, which are consistent in their fission-track annealing properties with those on which the numerical annealing models are based. By interpolating the calculated thermal histories a series of images can be generated showing the palaeotemperature of the present surface at various times in the past. A second group of images can then be derived from these which describe the amount of surface denudation, based on estimates of thermal gradients. In turn, estimates of the amount of removed section can be 'backstacked' onto a present-day digital topographic model, and isostatically adjusted, to provide a first-order view of the evolution of palaeotopography through time. These fission-track derived images have the potential to provide a quantitative understanding of upper crustal movements and surface processes, over time scales up to hundreds of millions of years. Further calculations can then be made on the virtual landscapes constructed. For example, the denudation models can be used to predict sediment volumes and to trace the evolution of drainage basins, at least on a broad scale. This opens up a new range of mass-balance calculations on the amounts of eroded material and sediment accumulation in appropriate depocentres. The acquisition of (U-Th)/He data on a similar regional scale should provide more robust information on the lowest temperature (<60°C) part of the thermal history, still poorly constrained by the fission track data.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SEISMIC EVIDENCE FOR A MAJOR ARC-BACK ARC COLLISION IN THE LACHLAN OROGEN R.A. Glen' l R.J. Korsch^'^ L.E.A. Jones^'^ and D.W. Johnstone''^ ' Australian Geodynamics Cooperative Research Centre ^Geological Survey of New South Wales, Department of Mineral Resources, PO Box 536 St Leonards NSW 2065 ^Australian Geological Survey Organisation, PO Box 378 Canberra ACT 2601 A key tectonic question in the Lachlan Orogen is the relationship between the two coeval yet contrasting Ordovician units - the intermediate to mafic volcanics, volcaniclastics and intrusives of the Ordovician Macquarie arc (dismembered largely as a result of mid Palaeozoic rifting into several structural belts) and the coeval craton-derived, quartz-rich turbidites and cherts of the backarc basin and ?forearc basin. In order to help answer this question, the Australian Geodynamics Cooperative Research Centre and the Department of Mineral Resources (through the Geological Survey of New South Wales) recently carried out deep seismic reflection profiling along 2 offset east-west lines with total length of nearly 100 km north of West Wyalong in central New South Wales. This transect passes immediately south of the Lake Cowal deposit and was designed to investigate the crustal architecture of the very poorly exposed Junee-Narromine Volcanic Belt, which represents the core of the Macquarie arc (Glen et al. 1998), flanking coeval quartz-rich turbidites and overlying younger Silurian and Devonian units. Preliminary interpretation of the seismic profiles suggests the presence of major crustal scale antiforms and contractional shear zones in inferred Ordovician (and ?Cambrian) volcanics, as well as faulted contacts between the volcanics and turbidites. We consider that these structures reflect major crustal-scale shortening (with inferred volcanics underthrusting the backarc basin fill) that occurred largely when the Macquarie arc and its oceanic substrate collided with the backarc basin around the beginning of the Silurian. In the west in line 99AGSL2, the top 2 to 3 sec TWT (~6-9 km) of crust below outcrops of Ordovician Wagga Group turbidites and intrusive Silurian granitoids is poorly reflective. It does, however, contain several east-dipping strong narrow bands of reflections that are interpreted to be east-dipping thrusts that extend to a depth of 2 sec TWT (~6 km). The middle crust below this, down to ~8 sec TWT at the western end of the line, is more strongly reflective, and we interpret it as correlating with Ordovician and ?Cambrian volcanics. This zone contains several variably persistent ( - 4 5 ° west-dipping) strong bands of reflections up to 0.3 sec (~1 km) wide that are interpreted to be contractional shear zones that cut through crust marked by more gently dipping reflections. The uppermost shear zone forms the floor thrust to the east dipping thrusts in the Wagga belt. To the east, this zone defines a large crustal antiform (amplitude of 4 sec TWT, ~12km) that lies in the hanging wall of a major west-dipping reflective zone that extends down to 8 sec TWT or deeper off the western edge of the line. The upper part of the hanging wall antiform is cut by a steeper (-60°) westdipping fault that correlates with a major fault inferred from regional aeromagnetic data just east of the Lake Cowal deposit. We suggest that this fault formed the fluid pathway to the deposit. The upper part of the antiform is difficult to resolve and we are not sure as yet how the inferred slices of Ordovician volcanics around and west of the Booberoi Fault link into the more reflective volcanics defining the antiform. In the central part of the traverse (western part of line 99AGSL1), the upper 1 sec of the crust shows good reflections that correlate with Late Devonian strata, occupying the core of the Carboniferous Tullamore Syncline, and also with folded Early Devonian rocks in the Currowong Syncline to the east. Elsewhere, the upper 1 sec is poorly reflective, both in the Ordovician Lake Cowal Complex west of the Tullamore Syncline (separated from it by linked thrust and strike-slip faults of the Marsden fault system) and also in the Silurian rocks that pass down into multiply deformed quartz-rich turbidites of the Ordovician Kirribilli beds east of the Currowong Syncline. More reflective crust below 1 sec is correlated with Ordovician and ?Cambrian volcanics at depth. These show kilometre scale antiforms and synforms that, in some cases, are hanging wall antiforms above a zone of strong west-dipping reflections that extend off the section to the east. Reference Glen, R. A., Walshe, J. L., Barron, L. M. &. Watkins, J. J., 1998. Ordovician convergent-margin volcanism and tectonism in the Lachlan sector of east Gondwana. Geology, 26, 751-754.

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GEOLOGICAL SOCIETY OF AUSTRALIA, .ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE DEVELOPMENT OF A METHOD FOR ASSESSING EFFLUENT TOXICITY USING THE DOUGHBOY SCALLOP. Christopher Golding^ Elaine Baker', Peter Schneider^ Rick Krassoi^ and Fleur Pablo^ ^University of Sydney, School of Geosciences, Sydney NSW, Australia 2006 ^Sinclair Knight Merz Ecotoxicology Laboratory, 100 Christie Street, St Leonards, NSW, Australia 2065 ^Centre for Ecotoxicology, University of Technology, Sydney, Gore Hill, NSW, Australia 2065

A marine Toxicity Identification Evaluation (TIE) protocol was developed for use with larvae of the Doughboy scallop, Chlamys asperrimus, a bivalve mollusc distributed along the south-eastern Australian coastline. The scallop was found to be a more reliable test species than another local bivalve, the Sydney rock oyster, Saccostrea commercialis. This procedure allows for the identification of toxic species within complex effluents and natural waters. The TIE protocol was successful in characterising the toxicity of three prepared effluents containing a metal (copper), organophosphate pesticides (diazinon and chlorpyrifos) and a pH-dependent volatile chemical (ammonia). The newly developed TIE protocol was subsequently applied to dry weather water samples collected from two sites along the lower Lane Cove River. Toxicity at the first site, located directly downstream of a wet-weather sewage overflow outlet, was most probably related to the presence of copper (at a concentration of 17 g/L). Toxicity in the water collected from the second site, located further upstream adjacent to industrial land, was most probably due to the presence of a combination of 7 g/L copper and 3.26 mg/L total ammonia (0.21 mg/L NH3).

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Geological Convention, Sydney, July 2000

TOWARDS THE THIRD DIMENSION IN THE EASTERN GOLDFIELDS, WA FROM COMBINED SEISMIC REFLECTION AND POTENTIAL FIELD STUDIES B.R. Golebv. B. BelK R.J. Korsch, T. Fomin, A.J. Owen, M.G. Nicoll and B.J. Drummond Australian Geodynamics Cooperative Research Centre (AGCRC), Australian Geological Survey Organisation (AGSO), GPO Box 378, Canberra, ACT, 2601, Australia. With the recent acquisition of approximately 190 km of deep seismic reflection data and coincident 280 mspaced gravity data in the Kalgoorlie region of the Eastern Goldfields, the AGCRC's Yilgam Seismic project has been able to enhance its three-dimensional knowledge of this economically significant region. The deep seismic survey builds upon results from an earlier transect that had imaged the granite-greenstone successions and the underlying crust. The new deep seismic reflection data were collected on a grid of lines within a 50 km x 50 km area over the mineralised Kalgoorlie - Mt Pleasant region. The grid was tied to existing deep seismic surveys, including AGSO's 1991 regional Eastern Goldfields Transect and the 1997 AGCRC/KCGM (Kalgoorlie Consolidated Gold Mines) Kalgoorlie Survey. Three of the major gold producing areas: the Golden Mile region, the Scotia-Kanowna region and the Mount Pleasant region were covered by several EW and NS traverses. In addition the Bardoc Shear, Zuleika Shear and several of the regional faults were crossed by several seismic lines to study along-strike variations of these structures. The new seismic data image regional variation of the 3D geometry of the greenstone successions and its basal detachment surface. The seismic has also imaged the regions shear zones soling into the basal detachment. Despite the significant NS variation in the geometry of several shear zones identified in the seismic image, the detachment model for the base of the greenstones is still valid. The new gravity data, combined with additional geological outcrop information collected by the Geological Survey of Western Australia along each traverse, have been used to further constrain the 3D geometry. When integrated with structural information from the interpreted seismic sections, the gravity has proved invaluable in defining depths limits to the granite and greenstone sequences. As a result, possible orientations of several mapped lithological units and the shape of various granite bodies crossed have been reinvestigated. Magnetic modelling has further developed the geometry of the granite-greenstone bodies by limiting the orientation and thickness of the magnetic units. These constraints have improved the geological interpretation of seismic data and produced a surface and depth consistent model for the Kalgoorlie - Mt Pleasant region. This geologically consistent model is now used for understanding of the tectonic evolution and the mineral system history in the region.

Acknowledgments: The Authors pubHsh with the permission of the Chief Executive Officer, AGSO and the Director, AGCRC.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CRUSTAL STRUCTURE - THRUST STACKS AND GOLD PATHWAYS WITHIN THE ARCHAEAN GRANITE-GREENSTONES OF THE EASTERN GOLDFIELDS, WESTERN AUSTRALIA: AS REVEALED BY DEEP SEISMIC REFLECTION PROFILING B.R. Golebv, B.J. Drummond, R.J. Korsch, T. Fomin, A.J. Owen and B. Bell Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601, Australia. Understanding the three-dimensional structure of a region has a profound impact on ones understanding of how the region evolved and in particular, where mineralised fluids might have flowed. The A G C R C ' s Yilgam Seismic Project was established to develop a 3D model of the Archaean, Eastern Goldfields using a combination of deep seismic reflection profiling, gravity data and new geological information. This region is one of the largest producers of gold in the world and a knowledge of the 3D evolution of the region and its mineral systems is fundamental to understanding exploration models. Deep seismic reflection data from the Eastern Goldfields, WA, collected in 1991 provided information on the crustal structure of both the granite-greenstone successions and the middle and upper crust. The results indicated that the total crustal thickness is about 33 km, and that the supracrustal granite-greenstone succession is about 4-7 km thick. Angular relationships between the overlying greenstones and a widespread subhorizontal reflective zone suggest that the greenstone base is a regional detachment surface. This geometry, together with inferred duplex structures within the overlying greenstones, implied movements in the order of tens of kilometres, although there are few direct constraints on movement history or direction. The seismic image from the middle crust showed structures similar to those imaged within the greenstone succession. These similarities between the middle crust and the greenstones implied the entire crust suffered tectonic events in which both the order and direction of deformation were similar. Thrusting and folding were predominant in the greenstones while thrust stacking was important in the middle crust. Gold in the region is mostly in second or higher order shears off the major fault systems. The resulting crustal model implied a deformation history summarised by 1) a D, thrusting and recumbent folding event, 2) a regional D2 ENEWSW shortening event, followed by 3) subsequent overprinting and disruption by structures consistent with regional strike-slip D3 faulting. D3 shear zones imaged by the seismic method mostly soled onto the regional (D2) detachment surface. However, two shears, the west-dipping Bardoc and Avoca, appear to cut through the detachment surface and extend into the mid-crust, with the Bardoc linking at depth with the east-dipping, crustal scaled Ida Fault. This geometry led to a fluid flow model in which gold-rich fluids moved up surfaces defined by the mid-crustal thrust duplexes until they linked with the west-dipping Bardoc Shear, where they were focussed into discrete zones to be later deposited in later stage structures. In 1999, additional data were collected to extend our understanding to 3D. A grid of deep seismic reflection data within a 50 km by 50 km area were collected over the mineralised Kalgoorlie - Mt Pleasant region. These new data show the three-dimensional geometry for the greenstone succession and its basal detachment surface. Importantly, the detachment model for the base of the greenstones still holds, though the original simple west-dipping nature of the Bardoc Shear has been complicated with the seismic images. In the new data, the Bardoc Shear dips west, as in the earlier section, but the dip varies along strike. The thickness and orientation of the deformation zone also varies along strike, possibly associated with topography on the fault surface. Other shear zones sole onto the detachment surface. One key economic questions being addressed includes identifying the presence of 'buried' felsic bodies within the seismic data and their along-strike extent. These imaged felsic bodies, if located beneath the greenstone succession, might play a significant role in the precipitating of gold from the mineralizing fluids, as does the 3D extent of the mid-crustal 'zone-of-noreflections' beneath the main mineralised region. One structural question being investigated is whether the emplacement of the granites into the greenstone succession is magmatic or structural in origin. New gravity data and additional geological information along the traverses have ftirther constrained the 3D geometry, resulting in a geologically consistent model that can be used as a predictive tool to understand the tectonic evolution and the mineral system history of the region. Acknowledgments: The Authors publish with the permission of the Chief Executive Officer, AGSO and the Director, AGCRC.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TOWARDS IMPROVED UNDERSTANDING OF THE DEEP STRUCTURE OF THE AUSTRALIAN NORTH WEST MARGIN FROM COMBINED REFRACTION/REFLECTION SEISMIC STUDIES Alexey Goncharov\ John Kennard\ Jim Colwell\ Tanya Fomin\ Alexander Kritski^ Peter Petkovic\ Philip Symonds^ & Jacques Sayers^ ^ AGSO, GPO Box 378, Canberra, ACT, 2601, Australia ^ University of Sydney, School of Geosciences, NSW 2006, Sydney, Australia Australia's North West Margin (NWAM) is segmented into four discrete margin-scale compartments (Carnarvon, Canning, Browse and Bonaparte) which have distinct rift and reactivation histories (O'Brien, et al., 1999). A large amount of research has been carried out dealing with the geological and tectonic evolution and hydrocarbon distribution within these individual segments. Relatively few papers, however, have dealt with the development of the region as a whole. This work is a contribution towards developing an integral picture of the deep crustal structure of the NWAM on the basis of:- consistent interpretation of AGSO's network of deep seismic reflection profiles; accurate velocity information derived from the AGSO's unique refraction/wide-angle reflection data set recorded by ocean-bottom seismographs (OBS). The OBS survey was undertaken along 5 profiles (Carnarvon, Canning, Browse, Vulcan and Petrel) of total length 2764 km. Data in this experiment were recorded to maximum offsets of 300 km. All OBS transects coincided with previously recorded deep crustal reflection profiles. The distinctive observation from the co-interpretation of the OBS and conventional reflection data is that prominent seismic reflectors and changes in reflectivity patterns in conventional reflection data do not necessarily correspond to signiflcant bulk velocity discontinuities imaged by refraction/wide-angle data. For example, the 6.0 km/s refractor imaged along the Petrel line shows rather poor correlation with the reflectivity of the crust imaged by the conventional reflection data. Similarly, the velocity model boundaries and iso-velocity lines do not coincide with the events identified in the reflection section along the Vulcan line, and the OBS data show no significant velocity increase at the base of the upper crust as imaged by prominent reflections at 6 s two way time. A significant local velocityincrease imaged by the OBS data in the depth range 15-23 km within the lens-shaped lower crustal body detected on the Carnarvon line does not produce any noticeable response in the reflection section. An example of good correlation between the reflectivity pattern and the velocity model, is that the bottom of the middle crust identified on the Carnarvon line is very close to the boundary of the velocity increase from -5.0 to - 5 . 7 km/s documented by the OBS data. We conclude that conventional reflection technology tends to highlight fine seismic stratification of the crust while refraction/wide-angle methods image better the bulk velocity changes in the crust that may not correspond to reflectivity boundaries. Only a combination of the two techniques provides a clue to a consistent geological interpretation of seismic data, allowing to map the structure of both upper and lower crust and to define the amount of crustal extension. Crustal scale OBS-derived models of the NWAM show a very significant degree of variation, particularly in the lower crust. Velocity scans of these models on the basis of velocity values predicted by our petrophysical modelling technique do not reveal significant volumes of rocks with gabbro-type bulk geochemistry. Therefore, underplating in the region appears to have been restricted only to the Canning compartment and the outer, western part of the Carnarvon compartment where such material is present. Reference O'BRIEN, G.W., MORSE, M., WILSON, D., QUAIFE, P., COLWELL, J., HIGGINS, R., AND FOSTER, C.B., 1999. Margin-scale, basement involved compartmentalisation of Australia's North West Shelf: a primary control on basin-scale rift, depositional and reactivation histories. APPEA Journal, 39, 40-63.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

COMPETENCY CONTRAST, STRAIN LOCALIZATION, AND MICROSTRUCTURAL EVOLUTION AT A VARIETY OF CRUSTAL LEVELS Laurel Goodwin ^ and Basil Tikoff ^ ^Earth and Environmental Sciences, New Mexico Tech, Socorro, NM USA ^Geology and Geophysics, University of Wisconsin, Madison, WI USA

Observations of fine-grained, shallow crustal, fault cores and greenschist to granulite facies mylonite zones suggest basic similarities in the character of foliations and lineations expressed as compositional variations in polymineralic rocks in these very different environments. In particular, we note segregation of rheologically distinct phases and evidence of localization of strain in rheologically weak domains. These similarities are somewhat surprising given the differences in the processes that accommodated deformation. We propose that they reflect the fundamental control of competency contrast on millimeter-scale strain localization. Competency contrast can be effected by mineralogical heterogeneity, variations in crystallographic orientation, grain-size variations, and/or pre-existing mechanical anisotropy, all of which can change over time with deformation and/or metamorphism or alteration. Thus, deformation history is inherent in this approach to understanding fabric development. Compositional bands initiate as local instabilities created by rheologic contrasts between different phases or monomineralic domains. A given instability is growth-limited by the availability of the weakest phase. Continued deformation partitioned largely into the foliation planes will facilitate mechanical as well as chemical segregation of incompetent phases, as shown by phyllosilicate-rich laminae in many mylonites and clay-rich laminae in fine-grained fault materials. The spacing of foliation planes should reflect the grain size of the strongest phase or polygranular domain, which will be affected by the deformation processes, strain versus recrystallization rate, finite strain, and deformation history. It will also be influenced by the relative percentage of strong versus weak grains. The orientation of a foliation or lineation is dictated by the deformation path, addressed in a companion abstract. The microstructural evolution of a rock is thus history-dependent and reflects deformation processes, mineralogical changes (e.g., metamorphism), and the orientations and distribution of phases, which are strongly affected by competency contrast and kinematics/strain. Elucidating how competency contrast, the kinematic history, and deformation processes interact during alteration or metamorphism is critical to understanding the microstructures of naturally deformed rocks.

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1Australian

Geological Convention, Sydney, July 2000

DEFORMATION GRADIENTS AND RATE OF MET AMORPHIC TRANSFORMATION:A KEY TO IDENTIFY TECTONOMETAMORPHIC UNITS (AUSTROALPINE UNITS, CENTRAL ALPS, ITALY) Guido Gosso, Maria lole Spalla, Michele Zucali, Davide Gazzola and Francesca Salvi Dipartimento di Scienze della Terra, Universita di Milano, and CNR-Centro di Studio per la Geodinamica Alpina e Quatemaria, Via Mangiagalli 34,1-20133 Milano, Italy Tectonic events consequent to ancient changes of crustal thickness and to thermal perturbations imprinted signatures on metamorphic tectonites, that may be deciphered to infer the geodynamics of the past. At the scale of a mountain belt, thickness variations are envisaged to result from mechanically discontinuous translations of discrete crustal elements (i.e. tectono-metamorphic units) accompanied by large scale partitioning of deformation. Therefore analytical individuation of tectonically distinct and thermally characterised crustal units are crucial and the equivalence of mineralogical, structural and chronological signatures within each unit are indispensible to assess their size. However, in many metamorphic belts, extent, degree and timing of metamorphic re-equilibrations and fabric changes are only sporadically known and consequently the understanding of the thermo-mechanical record and the tectonic subdivisions persist highly interpretative, and mainly based on litho! stratigraphic grounds or on "metamorphic field gradients" (England and Richardson 1977; Spear et al. 1984). It has been shown recently that the regional distribution of dominant metamorphic imprints does not necessarily correspond to the "metamorphic field gradient" and therefore this concept cannot be used to distinguish tectono-metamorphic units in terrains that underwent polyphase deformation and metamorphism without considering the effect of strain heterogeneities (Spalla et al., 2000). Actually, within a single tectonometamorphic unit, different metamorphic imprints can areally dominate, or the same dominant metamorphic imprint can occur in tectono-metamorphic units characterised by different P-T-d-t evolutions. The Italian Consiglio Nazionale delle Ricerche is concluding a project that stimulated the development of new mapping techniques, to support the activities of the Servizio Geologico Nazionale. For this project we have applied to Alpine polymetamorphic tectonites the structural correlation criteria developed by (Connors and Lister, 1995; Johnson and Duncan, 1992; Williams, 1985), supported by microstructural analysis devoted to reconstruction of P-T-d-t paths (Johnson and Vernon, 1995); this approach has been integrated with observations on the degree of granular scale reorganisation. The resulting map is structurally and petrologically oriented and includes graphical representation of: i) rotation, dislocation, and superposition of structures, ii) incompatibilities of metamorphic assemblages related to planar fabrics, iii) strain variations (coronites, normal S/L tectonites, mylonites) imposed under the same metamorphic stage. The application of this method to the central Austroalpine demonstrates that, during Alpine metamorphism, this basement was a single tectono-metamorphic unit, in which a deformation gradient controlled the distribution of metamorphic transformations. This single tectono-metamorphic unit includes two stratigraphic units of the literature (Formazione di Pietra Rossa and Formazione di Cima Rovaia), which corespond respectively to two diachronous dominant metamorphic imprints. References CONNORS, K.A. AND LISTER, G.S. (1995): Polyphase deformation in the western Mount Isa Inlier, Australia; episodic or continuous deformation? Joum. Struct. Geol.l7, 305-328. England, P.C. and Richardson, S.W. (1977): The influence of erosion upon the mineral facies of rocks from different metamorphic environments. Joum. geol. Soc. London 134, 201-213. JOHNSON, S.E. AND DUNCAN, A.C. (1992): Fault identification in complexly deformed schist terrains: examples from the USA and Australia. Tectonophysics216, 291-308. JOHNSON, S.E. AND VERNON, R.H. (1995): Inferring the timing of porphyroblast growth in the absence of continuity between inclusion trails and matrix foliations: can it be reliably done? J. Struct. Geol. 17, 1203-1206. SPALLA, M.I., DI PAOLA, S., SILETTO, G.B. AND GOSSO, G. (2000): The role of structural and metamorphic memory in the individuation of tectono-metamorphic units: the Southalpine basement of the Como Lake. J. Geodynamics, 30, SPEAR, F.S., SELVERSTONE, J., HICKMOTT, D., CROWLEY, P. AND HODGES, K.V. (1984): P-T paths from garnet zoning. A new technique for deciphering tectonic processes in metamorphic terrains. Geology 12, 87-90. WILLIAMS, P.F. (1985): Multiply deformed terrains - problems of correlation. J. Struct. Geol.7, 269-280.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CONTROLS ON PORPHYRY-RELATED COPPER-GOLD MINERALISATION IN THE NEW GUINEA FOLD BELT - INSIGHTS FROM GEODYNAMIC MODELLING Paul G o w \ Phaedra Upton^ Chongbin Zhao^ and Kevin Hill^ ^AGCRC, CSIRO Exploration and Mining, PO Box 437, Nedlands, WA 6009 ^Geology Department, University of Otago, PO Box 56, Dunedin, New Zealand ^AGCRC, VIEPS Department of Earth Sciences, La Trobe University, Melbourne, Australia 3083

Two- and three-dimensional numerical modelling techniques, constrained by key geodynamic data, provide insights to the controls on development of porphyry-related CuAu mineralisation in the Tertiary collision zone of New Guinea. A variety of modelling processes have been used, including coupled mechanical-fluid flow and coupled fluid flow-thermal-chemical modelling. The exercise crosses several scales, from the collisionalmargin scale to that of an individual intrusive system. The modelling shows that creation of local dilational zones to facilitate magma emplacement within this broadly compressional setting can be accomplished by reactivation of arc-normal transfer faults where they cut the weakened fold belt. Additionally, dilation occurs at the northern end of the fold belt where fluid overpressuring is caused by collision-related south-directed fluid flow being localized into the more permeable units of the Mesozoic passive margin sedimentary sequence. Rapid uplift and erosion, which may be a mechanism for magmatic fluid release in these systems, is shown to be greatest to the west in Irian Jaya where the stronger Australian crust acts as a buttress. Within the fold belt, uplift is greatest near the margins, where increased orographically-induced precipitation and erosion is coupled with marked strength contrasts producing broad domes that expose the lower parts of the stratigraphy. At the scale of an individual magmatic complex, two-dimensional coupled fluid flowthermal-chemical modelling uses a scenario of fluid mixing to calculate the distribution and magnitude of metal precipitation within a convecting system. The modelling highlights the interdependence of the spatial permeability structure, the regional temperature gradient, and the geometry of the convection cells, and how this impacts on the distribution of metal precipitation. The most critical factor in terms of spatial distribution of mineralisation is the permeability structure, whilst grade development is most strongly controlled by the relative magnitudes of the species concentrations in the two mixing fluids. The results from these models provide an understanding of some of the processes that occurred along the complex northern Australian collisional margin, and how these processes impact on deposit formation. They also provide some broad guidelines as to where and when the Cu and Au deposits are more likely to form, not only within this terrane, but also more generally within compressive and epizonal magmatic environments.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE THREE-DIMENSIONAL UPPER MANTLE P-WAVE VELOCITY STRUCTURE OF WESTERN VICTORIA Frank M. Graeber \ Gregory A. Houseman \ and Stewart A. Greenhalgh ^ ' Department of Earth Sciences, Monash University, PO Box 28E, Clayton, VIC 3800, ^ Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005

The Lachlan Foldbelt teleseismic survey 1998 (LF98) provided the teleseismic arrival-time data that was used to carry out a non-linear tomographic inversion for P-wave velocity {Vp) heterogeneity in the upper -300 km of the mantle under western Victoria. LF98 forms the first phase of the ongoing Murray Basin, Adelaide and Lachlan Foldbelt Teleseismic Survey (MALT). It's major aim is to investigate whether lateral variations in Vp in the upper mantle are associated with the major surface structural zones. Models obtained from the application of surface wave tomography lack the spatial resolution for resolving those features, and little a priori information from seismic profiling is available. We can identify three zones of distinctly differing lithospheric background velocities that correlate well with the surface geology. The most prominent negative anomaly (-4%) in Vp is found at a depth of 45-60 km underneath the eastern part of the Newer Volcanic Province. It is situated below the area of highest density of Pliocene and Pleistocene eruption centres north-west of Melbourne. Taking into account the high heatflow values measured in that area, this anomaly is interpreted as a hotspot-related high temperature anomaly reducing the mantle velocities. Although compositional effects and seismic anisotropy could explain part of the anomaly, the maximum temperature anomaly inferred from our results could be as great as 400®C. The related coherent area of significantly lower than average velocities extends westward along the Newer Volcanic Province to the western end of the array, and in some areas down to depth greater than 150 km. There is no evidence for a continuation of that anomalous zone to greater depths, which, considering the subdued volcanic activity, suggests that mantle-driven hotspot activity has ceased. A strong velocity contrast, which separates close to average Vp to the east, and higher than average Vp (2-3%) to the west, is found down to about 100 km across the Moyston Fault Zone (MFZ). The MFZ is recognised as the major structural boundary between the earlyPaleozoic Delamerian Orogen in the west and the Lachlan Orogen in the east. In the tomographic models the transition zone appears to be east-dipping throughout the lithosphere, which is in agreement with the overall vergence of surface structures in the region and with the seismic image of the MFZ at depth. Moreover, the higher overall Bouguer gravity anomaly of the Delamerian extends east across the MFZ and has been interpreted as the signature of the lower Delamerian crust underlying the western margin of the Lachlan. In the absence of evidence of thermal activity, the velocity contrast across the MFZ could thus be explained by a compositional contrast attributed to the distinct histories of these two lithospheric regions. In the south this boundary could continue beneath the Newer Volcanic Province, as is suggested by a distinct discontinuity in Sr-isotopic ratios (the so-called Mortlake discontinuity) and by a change in the geochemistry of plutons of similar age. Here, however, any velocity contrast associated with the southward continuation of the MFZ has apparently been overprinted by the thermally reduced mantle velocities along the Newer Volcanic Province. Our model and evidence from geochemical data suggest that the MFZ not only forms a crustal suture, but also has to be seen as a major lithospheric boundary.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEM CORUNDUMS FROM BASALTIC ERUPTIVES TUMBARUMBA FIELD, NSW I.T. Graham. F. Lin Sutherland, R.E. Pogson, G.B. Webb, and R.R. Coenraads Geodiversity Research Centre, Australian Museum, 6 College Street, Sydney, NSW, 2010

The Tumbarumba alluvial gemfield, within the Snowy Mountain basalt province, is prospective for sapphires, zircons and rare rubies. The gemstones show magmatic corrosion and represent xenocrysts derived from local basaltic eruptives. Rare examples occur in basalt matrix, but most are reworked crystals shed from sub-basaltic leads. Gem mineralogy Blue, green to yellow colour-zoned sapphires dominate and include pale-rimmed crystals. Patchy vari-coloured, pastel hued sapphires also appear and pink sapphire and ruby form a minor component. The typical blue, green, yellow sapphires exhibit trace element contents and colour absorption spectra typical of magmatic corundums {CviO^IG^iiO^ <1) and contain Nb-rich columbite as a prominent syngenetic inclusion. Some sapphires show marked radial, crystal-controlled spokes of concentrated exsolved Ti and Fe oxides. Other sapphires and ruby have trace element contents and colour absorption spectra of a metamorphic character (Cr203/Ga203>l) but range into features transitional towards magmatic sapphires. Zircons form pale to dark red crystals and contain 35-1435 ppm U. Fission track dating suggests three age groups, 26.6±3, 21.6±2, and 15.6±3 Ma. The youngest zircons are bright polished grains with low U contents (37-161 ppm) and suggest only minor alluvial reworking. U-Pb isotope dating of a zircon at 23.3±1.7 Ma, indicates relatively close crystallisation and volcanic discharge ages. Small corroded almandine garnet grains (al 6975, py 17-20, sp 4-9, an 3-4) are common in the gem concentrates. Their composition and intergrown quartz suggest a metamorphic crustal origin. Gem origin The gem field is associated with primitive to mildly evolved basanites, alkali and transitional basalts (Mg# 0.77 to 0.57) Basah K-Ar dates range between 18-24 Ma, but suggest separated episodes. Mantle-nomalised incompatible multi-element plots indicate most basaltic magmas were generated in amphibole (+ apatite) enriched mantle sources. Transitional basalt shows a simpler incompatible element pattern, lacking an amphibolerich source signature and represents a greater degree of partial mantle melting. Comparisons of zircon and basalt ages suggest episodes of zircon-bearing gemstone eruption were interspersed with basaltic lava effusions, as follows: Gem eruption 27-26 Ma, basalt effusion 24-21 Ma, gem eruption 22-21 Ma, basalt effusion 19-18 Ma, gem eruption 16-15 Ma. The magmatic sapphires (and zircons) are thought to crystallise from minor felsic melts generated in amphibole-rich-mantle. Expulsion followed in subsequent later basaltic activity arising from small plume upwellings. The metamorphic sapphire and ruby suites may be derived from metasomatic bodies associated with a local serpentinite belt.

186

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney, July 2000

THE LATE MIOCENE - EARLY PLIOCENE BIOGENIC BLOOM AT DSDP SITE 590 AND ODP SITE 1125, SOUTHWEST PACIFIC Katharine M. Grant, and Gerald R. Dickens Department of Earth Sciences, James Cook University, Townsville, QLD 4811

Many sites drilled beneath frontal regions of the Indian and Pacific Oceans contain records with marked changes in sediment deposited during the latest Miocene and earliest Pliocene (ca. 7 - 4 Ma). In particular, there are significant increases in mass accumulation rates (MARs) of biogenic components (e.g., P, Ba, CaCOs, opal), decreases in sedimentary redox potential, and compositional changes in benthic foraminiferan assemblages. An hypothesis called the "biogenic bloom" links these observations by stating that there was a substantial increase in primary productivity at Indo-Pacific divergence zones during the latest Miocene - earliest Pliocene. The cause(s) of the biogenic bloom remain poorly understood, although the event clearly involves profound changes in global nutrient cycling. Large-scale changes in global nutrient cycling should be recorded in the stable carbon isotope composition (5'^C) of marine carbonates. Such records exist for the latest Miocene and earliest Pliocene, yet most of this data is not calibrated to a common timescale. Furthermore, there is a discrepancy between 5'^C records of foraminifera and bulk sediment. Both benthic and planktic foraminifera record a negative 6'^C excursion of 0.5 1.0 %o in the latest Miocene (the "Epoch 6" or "Chron 3" carbon shift). By contrast, bulk sediment 5'^C records a 1 - 2 %o depletion in the Pliocene. Significantly, no studies of upper Miocene and lower pliocene sediment have analysed 5'^C in all phases at the same location or independent proxies of palaeoproductivity. A multi-proxy approach is used in this study to examine productivity and 5'^C variations over the past 10 my at frontal zones in the southwest Pacific. DSDP site 590 underlies the Tasman Front in the central Tasman Sea, and ODP site 1125 underlies the subtropical convergence east of New Zealand. Bulk samples of nannofossil ooze at both sites were analysed for CaCOs content, Ba and major element concentrations, and 6'^C. Two size fractions and single foraminiferan species from different water depth habitats were also analysed for 6'^C. These latter analyses included bulk fines (< 63 i^m), bulk "nannofossils" (5 - 25 nm), the surface-dwelling planktic foraminifera Orbulina universa (>350 ^m) and Globigerinoides sacculifer (300-400 ^m), and the deeper-dwelling planktic foraminifera Globigerina bulloides (300-350 i^m;, Neogloboquadrina pachyderma (300-350 (xm) and the Neogloboquadrina lineage acostaensis-humerosa-dutertrei (300-350 jxm). All of our records, as well as those from earlier S'^C studies at site 590, have now been calibrated to a common and current timescale. Between 5.5 - 4.0 Ma there are coeval peaks in the Ba:Al ratio and CaCOs MARs, suggesting an interval of elevated surficial productivity. At this time there is also a substantial downward trend in bulk sediment 5 C. This decline in 6'^C is not reflected in the foraminiferan records. However, 5'^C values of surface and subsurface-dwelling foraminifera converge, suggesting a breakdown of the thermocline. Acknowledgment: K. Grant was a shipboard participant on ODP leg 181.

187

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOLOGY OF THE GLENELG RIVER COMPLEX SEGMENT OF THE DELAMERIAN OROGENIC BELT C.M. Gray\ A.I.S Kemp^ J.A.C. Anderson^ D.J. Bushell', D.J. Ferguson' and J. Fitzherbert^ 1 Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 2Department of Geology, Australian National University, Canberra, ACT 0200 3Deparrtment of Geology and Geophysics, University of Sydney, NSW 2006

The Cambro-Ordovician Glenelg River Complex of western Victoria is a metamorphicigneous segment of the Delamerian Orogenic Belt moulded during the terminal orogeny at --^500 Ma. While poorly exposed, comprehensive mapping of narrow exposures in incised drainages reveals two northwest-southeast striking regional metamorphic zonations dominated by quartzofeldspathic metasedimentary rocks, which prograde towards an axial granitic batholith. Five deformations are recognised with the first four essentially coaxial and having a northwest-southeast trend. D1 is rarely expressed as remnant foliation in D2 crenulation cleavage. D2 was the most significant phase producing isoclinal folds, transposition, and a pervasive regional foliation. D3-4 developed V-shaped upright folds with little axial planar fabric. In contrast, D5 produced northeast-southwest trending open folds. The southwestern metamorphic zonation extends from biotite to migmatite grade. At low grade the lithologies are metagreywacke, metasiltstone and slate. Very rarely, repeated interbedding of metagreywacke and slate suggests an origin as proximal turbidites. Serpentinite sheets and pods were tectonically interleaved in low grade areas during D2. Higher grade rocks are dominantly quartzofeldspathic schists grading to migmatites with lesser porphyroblastic schists, calc-silicates and metaquartzites. There are three phases of metabasites, originally metagabbros and metadolerites. The northeastern metamorphic zonation is exclusively quartzofeldspathic at sillimanite and migmatite grade with a regional concentration of pegmatites adjacent to the sillimanitemigmatite zone boundary. Mesoscopic interleaving of quartzofeldspathic schist, migmatite, pegmatite and muscovitic granite is characteristic; this culminates in striking gradation from migmatite into individual granitic plutons and also into the central batholith. Two peaks of granitic magmatism match the timing of leucosomes in migmatites. Pre- to syn-D2 activity is concentrated as variously foliated plutons embedded in the southwestern metamorphic zonation, with an extensive compositional range from tonalite-granodiorite to local diorite, gabbronorite and pyroxenite. The andalusite-sillimanite-style metamorphic zonation was established complex-wide by D2. Subsequently, the thermal high contracted to the northeast and the more extensive syn-, post-D4 to pre-D5 magmatism produced a host of muscovite-bearing granitic rocks in the northeastern zonation and developed the central batholith. The batholith is ~15km wide, expressed over a strike length of 40km, and contains numerous plutons, dominantly unfoliated, grey granodiorite with mafic igneous enclaves and frequently minor muscovite. The final event was the emplacement of distinctive, buff to pink, post-tectonic granite unrelated to the above igneous rocks. Several poorly-exposed plutons in the west of the complex are part of an extensive concealed batholith.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EASTERN AUSTRALIA CRUSTAL EVOLUTION: TIMING AND TEMPORAL LINKS BETWEEN DELAMERIAN AND LACHLAN OROGENS, INCLUDING TASMANIA David R. G r a y \ David A Foster^ Robin Offler^ Catherine V. Spaggiari^ and Michael Hartley^ ' VIEPS Department of Earth Sciences, Monash University, Melbourne, Vic 3800 ^Department of Geological Sciences,University of Florida, Gainesville, FA 75275-0395, U S A ^Department o f Geology, The University of Newcastle, Callaghan, N S W ^ VIEPS Department of Earth Sciences, La Trobe University, Melbourne, Vic 3083 Much of the cratonisation of eastern Australia has involved accretion of structurally thickened submarine fans and accretionary complexes to form crust of continental thickness and character. This took place during plate convergence in an oceanic setting along the eastern margin of Gondwana from - 5 0 0 Ma through 340 Ma. Problems relate to the nature and extent of orogenesis and the role of Tasmania in, as well as the links to, the larger orogen. Recent geochronology, in particular Ar-Ar dating of mica growth and cooling in slates and schists respectively has shown anomalous history in Tasmania as well as significant reactivation and activity in the Delamerian Orogen (DO) during Lachlan Orogen (LO) evolution. Post Rodinia-breakup at ~820Ma, the cratonic margin underwent rifting attenuation and Late Proterozoic continental margin sedimentation with: 550-540 Ma shortening event recorded by Jura-style folding and cleavage development (southern Flinders Ranges of proto-DO). 532-526 Ma partial rifting and deposition of Kanmantoo Group (proto-DO) 520-490 Ma (MAJOR DEFORMATION) • high-T metamorphism/ deformation in the southern Adelaide Fold Belt (Mt Lofty Ranges) and Mt Pamter superimposed on the earlier fold structures (DO). • ophiolite obduction of oceanic crust in Tasmania along with significant deformation and metamorphism (Ulverstone Metamorphics), exhumation of metamorphic complexes (Forth MC), high P metamorphism in rocks within the Arthur Lineament and Mt Read volcanism in Dundas "Trough" period of oceanic crustal growth in complex SW Pacific style oceanic setting in the proto-Lachlan Orogen (Dookie, Howqua, Dolodrook MORB, boninites, ultramafics; Licola and Jamieson intra-oceanic island arc andesites; Mt Stavely andesites) due to subduction roll-back and back-arc spreading/ accompanied by turbidite fan deposition in the westernmost part (Stawell Zone) • reactivation of shear zones in the Precambrian basement massifs (Broken Hill block) 490-460 Ma (COMPLETION OF CRATONISATION DELAMERIAN OROGEN) • post-orogenic magmatism, cooling, and erosional exhumation in the Delamerian Orogen extensive turbidite fan deposition in the proto-Lachlan Orogen coupled with initiation of E-directed deformation of the sediment wedge (western Stawell Zone) 450-400 Ma (MAJOR DEFORMATION) • convergence and complex micro-plate interactions (oceanic setting) in developing Lachlan Orogen, both E- (central LO) and W- (western LO) directed subduction, coupled with high T metamorphism and magmatism; erosional exhumation in west with sediment output into prograding marginal basin (Melbourne "trough") W-directed collisional shortening in NE Tasmania (Mathinna Beds) and deformation of western Tasmania. • Reactivation of shear zones in the Delamerian Orogen with continued exhumation of the belt 400-380 Ma closure of marginal basin (Melbourne "trough") accompanied by localised strong to intense N-S folding (Mitchell syncline) and regional crenulation cleavage development in central LO • post-orogenic magmatism in western LO syn-deformational magmatism and high T metamorphic belts in E-directed collisional setting coupled with extension-related volcanism (Snowy River and Boyd Volcanics) in eastern LO. • reactivation of shear zones in the Delamerian Orogen and Precambrian basement massifs (Mt Painter, Broken Hill and Tyennan of Tasmania) 370-340 Ma (COMPLETION OF CRATONISATION LACHLAN OROGEN) post-orogenic magmatism in the eastern part of the western LO (central Victorian magmatic province) E-directed deformation in eastern LO (Hill End "Trough" and Capertee "High") followed by post-orogenic magmatism (Bathurst granite)

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GEOLOGICAL SOCIET Y OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PRESSURE EFFECT ON ACCESSORY MINERAL SATURATION IN EVOLVED GRANITIC MELTS WITH DIFFERING K/NA RATIOS Trevor H. Green Department of Earth and Planetary Sciences, Macquarie University NSW 2109 Australia

Granitic magma series commonly show^ variation in K/Na ratio between different suites, possibly affecting the saturation level, at a given silica content, of accessory minerals such as apatite (phosphorus-saturation) or titanite or ilmenite (Ti-saturation). In turn, this will have important impact on trace element distribution in further fractionated products because of the ability of these minerals to take up rare earth elements (REE) (apatite, titanite) or high-field-strength elements (HFSE) (rutile, ilmenite). Direct determination of apatite and Ti-rich mineral solubility in silicic melts as a function of K content and pressure should allow assessment of the potential role of these accessory minerals in controlling trace element distribution in the crust, as governed by intra-crustal melting and granite evolution at different crustal depths. Accordingly, the solubility of Ti and P-rich accessory minerals has been examined as a function of pressure and K/Na ratio in two series of highly evolved silicate systems. These systems correspond to (a) alkaline, varying from metaluminous to peralkaline with increasing K/Na ratio (b) strongly metaluminous (essentially trondhjemitic at the lowest K/Na ratio), and remaining metaluminous with increasing K/Na ratio (to 3).The experiments were conducted as far as possible at a fixed temperature of 1000°C, with water contents varying from 3-5% wt. at low pressure (O.SGPa), increasing through 5-10% wt. at 1-2.5 GPa to 10-15%wt. at 3.5 GPa. Pressure was extended outside the normal crustal range so that results would also apply to derivation of hydrous silicic melts from subducted oceanic crust. For the alkaline composition series the Ti02 content of the melt at Ti-rich mineral saturation decreases with increasing pressure, and is little affected by K content until the melts are peralkaline, when Ti-saturation levels at fixed pressure increase as K content increases. The P205 content of the alkaline melts at apatite saturation increase with increasing pressure, but decrease with increasing K content (and peralkalinity). These contrasting results for P and Ti saturation levels point to contrasting behaviour of Ti and P in the structure of silicate melts. For the trondhjemite composition series the Ti02 content of the melt at Ti-rich mineral saturation also decreases with increasing pressure, but contrasts with the alkaline compositions, by showing decreased Ti-solubility with increasing K content, at fixed pressure. The P205 content of the trondhjemitic series melts at apatite saturation decrease with increasing K content. No consistent variation in apatite solubility with varying pressure was observed. Thus P and Ti do not show contrasting behaviour in the metaluminous trondhjemitic melts, differing from their behaviour in the alkaline melts, and reflecting different structural attributes in the different melt series. The results have application to A-type granite suites that are typically peralkaline, and to contrasting I-type metaluminous suites that frequently exhibit differing K/Na ratios from one suite to another.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RELATIVE AGE OF FELSIC MAGMATISM AND DEFORMATION IN THE CENTRAL AND EASTERN YILGARN CRATON, WESTERN AUSTRALIA J. E. Greenfield, S. F. Chen, S. Wyche, A. Riganti, and D. R. Nelson Geological Survey of Western Australia, Kalgoorlie Regional Office, P C Box 1664, Kalgoorlie W.A. 6430 Recent mapping, structural studies, interpretation of newly acquired aeromagnetic data, and new SHRIMP U - P b geochronology have identified a sequence of geological events in the central Southern Cross Province of the central Yilgam Craton that allows direct comparison with geological models for the Eastern Goldfields Province in the eastern Yilgam Craton. In the central Southern Cross Province, a c. 3.0 Ga age for deposition of the mafic-dominated lower greenstone sequence is inferred from an intrusive porphyry dated at 3023±10 Ma. The first recognizable deformation event ( D l ) in the greenstones produced east-trending recumbent folds. Reversals of shear sense during D2 suggest that at least some of the regional shear zones may have been active during D l . Following D l , local extension in the Marda-Diemals area allowed the deposition of clastic sediments of the Diemals Formation, eruption of calc-alkaline volcanic rocks of the Marda Complex (2732±3 Ma), and the diapiric intrusion of the Pigeon Rocks Monzogranite (2729±4 Ma). Upright, northtrending folds, which overprint Dl structures, formed during D2 east-west compression. Late during D2, westward-directed inhomogeneous shortening reoriented earlier D2 structural trends and reactivated major shear zones. Pre- to syn-D2 magmatism is represented in the Marda-Diemals area by granites aged between 2711±4 and 2693±7 Ma. Conjugate north-northeasterly trending dextral and east-south easterly trending sinistral brittle faults, and Proterozoic, mainly east-west, fractures traverse the entire craton. The greenstones of the Eastern Goldfields Province include mafic, ultramafic, sedimentary, and felsic volcanic rocks that were deposited between c. 2720 and c. 2675 Ma (Nelson, 1997). Recumbent folds and thrusts with evidence of northwards movement ( D l ) preceded a D2/D3 event involving east-west shortening and widespread granite magmatism dated between c. 2675 Ma and c. 2640 Ma (Swager, 1997). The data obtained from this study provide important constraints on crustal evolution models for the evolution of the Eastern Goldfields and central Southern Cross Provinces: • •

•

•

•

&#61623; Although similar in style, Dl is older in the Southern Cross Province than in the Eastern Goldfields Province, &#61623; The c. 2.7 Ga felsic volcanism in the Southern Cross Province is older than felsic volcanism in the Eastern Goldfields Province, and was not associated with contemporaneous deposition of mafic, ultramafic, and sedimentary rocks, &#61623; Limited geochronological data suggest that rocks of the c. 2.7 Ga felsic volcanic event become progressively younger from west to east across the Yilgam Craton. Felsic volcanic rocks in the Murchison Province to the west are mainly c. 2.75 Ga (Pidgeon and Hallberg, 2000), 10-30 Ma older than the Marda Complex felsic volcanics, and 30-75 Ma older than those in the Eastern Goldfields, &#61623; The geometry and structural style of the regional-scale shear zones in the central Southern Cross Province are similar to structures in the Eastern Goldfields Province. However, constraints on the age of these structures provided by granitoid intrusions suggest that, in the Southern Cross Province, this deformation (D2) may have begun 10-30 Ma before the same style of deformation in the Eastern Goldfields, &#61623; Late, cross-cutting structures in the Southern Cross Province can be traced into the Eastern Goldfields Province and postdate cratonisation.

These data indicate that the tectonic processes involved in the development of late Archaean granitegreenstones in the Yilgam Craton were active in different areas at different times over a period of up to 75 Ma. References

NELSON D. R. 1997. Evolution of the Archaean granite-greenstone terranes of the Eastem Goldfields, Westem Australia. Precambrian Research 83, 57-81. SWAGER C. P. 1997. Tectono-stratigraphy of late Archaean greenstone terranes in the southern Eastem Goldfields, Westem Australia. Precambrian Research 83, 1 1 ^ 2 . PIDGEON, R. T., and HALLBERG, J. A., 2000. Age relationships in supracmstal sequences in the northem part of the Murchison Terrane, Archaean Yilgam Craton, Westem Australia: a combined field and zircon U - P b study. Australian Joumal of Earth Sciences 47, 153-165.

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EARLY PALEOGENE WARM CLIMATES AND VEGETATION IN SOUTH-EASTERN AUSTRALIA. David R. Greenwood', Patrick T. Moss^ and Anthony J. Vadala' ' School of Life Sciences & Technology, Victoria University of Technology, PO Box 14428, Melbourne City MC, VIC 8001; ^ Geography & Environmental Science, Monash University, Clayton, VIC 3168.

The Late Paleocene and Early Eocene (i.e. early Paleogene) represent periods of substantial globally warm climate, with numerous data indicating ice-free continents and poles, and shallow meridional thermal gradients. Within this interval a significant transient warming occurred, the Late Paleocene Thermal Maximum or LPTM; the deep sea and high-latitude oceans warmed from -ll'^C to virtually eradicating meridional gradients. Increased levels of atmospheric C02, the main greenhouse gas, have also been linked to the LPTM and Early Eocene warmth. Climate modelling sensitivity studies have shown that high latitude areas and continental interiors show the greatest sensitivity to baseline assumptions (e.g. polar stratospheric clouds, varying levels of greenhouse gases, vegetation, orbital forcing and continental responses). In prior studies, the greatest discrepancies between model output and the palaeontological proxy evidence of climate has been for high latitudes and continental interiors, highlighting gaps in our understanding of the mechanisms acting to either transfer heat to higher latitudes, or retain heat there. One possible factor is specifying realistic vegetation types. South-eastern Australia in the early Paleogene lay close to the Antarctic circle. In this study, data from fossil floras in south-eastern Australia are used to reconstruct vegetation and climates for the Late Paleocene and Early Eocene. Our data show that for both the Late Paleocene and Early Eocene, complex, multistratal species-rich forests were predominant in south-eastern Australia, under mesothermal humid climates (MAT 16-18°C, CMMT >3°C, MAP >2000 mm/yr). While minor seasonal freezing temperatures (sufficient to produce occasional light snow cover) cannot be excluded for interior and upland areas, coastal sites must have remained frost-free. These forests were dominated by taxa characteristic of present-day mesothermal-megathermal high-rainfall forests; e.g. Cunoniaceae, Elaeocarpaceae, Gymnostoma (Casuarinaceae), Lauraceae (e.g. Beilschmiedia, Cryptocarya & Endiandra), and Proteaceae (Helicia). The megathermal mangrove palm, Nypa, was present in coastal sites. Elements characteristic of modem-day microthermal to mesothermal forests were also present; e.g. Nothofagus (Nothofagaceae), Eucryphia (Eucryphiaceae) and Podocarpaceae (Acmopyle & Dacrycarpus). It is likely based on the light regime at high latitudes, and based on limited physiognomic evidence that these forests contained at least some deciduous members in their canopy, including taxa from families that today in Australian forests are evergreen. Modelling of climate for the early Paleogene should therefore incorporate deciduous angiosperm-dominated forest cover for southern Australia, which potentially will alter output of winter conditions due to differences in albedo and the hydrological balance of the lower atmosphere. Acknowledgement: Research supported by a grant to DG by the Australian Research Council.

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DIAMONDOID HYDROCARBON RATIOS AS INDICATORS OF BIODEGRADATION IN AUSTRALIAN CRUDE OILS Kliti Grice, Robert Alexander and Robert 1. Kagi Centre for Petroleum and Environmental Organic Geochemistry/ APCRC/ Centre of Excellence in Mass Spectrometry, School of Applied Chemistry, Curtin University of Technology, GPO Box U1987, Perth, W. A. 6001, Australia.

Adamantane (I) and diamantane (II) which occur naturally in petroleum, belong to a class of compounds generally known as diamondoids. This is because they are comprised of rigid fused carbon structures from which diamond is built. Diamondoids are significantly more resistant to microbial degradation than most other petroleum components. In the present study the effect of biodegradation on diamondoid distributions in petroleum has been investigated on a series of crude oils reservoired in two Australian sedimentary basins, the Gippsland Basin and the Carnarvon Basin. The ratio of methyladamantanes to adamantane rises with increasing biodegradation. Significant changes in the ratio occur at extreme levels, indicating that diamondoids can be indicators of petroleum biodegradation especially when most other hydrocarbons have been removed. Similarly the methyldiamantane/ diamantane ratio also increases with biodegradation and is highest in the most biodegraded oil studied indicating that the first change in this ratio only occurs at extreme levels of biodegradation. Adamantane and diamantane are affected by biodegradation, however, the proposed diamondoid biodegradation parameters are not significantly affected by differences in maturity. The methyladamantane/ adamantane ratio can also be used to assess mixed crude oils where different contributions have been biodegraded to different extents.

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TRACKING MAGMA MIXING: IN-SITU HF-ISOTOPE ANALYSIS OF ZIRCONS W.L. G r i f f i n ' X . W a n g ' ^ S.E.Jackson', N.J.Pearson', S.Y. O ' R e i l l y ' a n d X . X u l , ^ ' GEMOC ARC National Key Centre, Department, of Earth and Planetary Sciences, Macquarie University, Sydney 2109, Australia ^CSIRO Exploration and Mining, 51 Delhi Road, North Ryde 1670, Australia ^ Deparment of Geology, Nanjing University, Nanjing, China Field relationships in the 115 Ma Pingtan complex in coastal Fujian Province, SE China show extensive mingling of mafic and felsic magmas, producing pillow-like enclaves, and suggest formation of at least some intermediate members of the suite (granodiorite, quartz diorite) through hybridisation (Xu et al., 1999). Similar Sr (initial 87Sr/86Sr =0.7065) and Nd (eNd = -1.5) isotopic compositions imply that both the felsic and the mafic magmas represent contain a significant crustal component, but Nd model ages (TDM =lGa), significantly younger than known crust in the region (1.4-2.0 Ga), suggest a juvenile component as well. To investigate the processes of crust-mantle interaction, and specifically the timing of any mixing of crustal and mantle components, we have carried out a detailed study of the Pingtan rocks, using LAM-MC-ICPMS techniques to analyse Hf isotopes in individual zircon grains. Zircons (* 150x50 | L i m ) were separated from a granite, a granodiorite enclave (pillow) in the granite, a quartz diorite enclave, a granodiorite and a gabbro. The zircons in each rock show several growth stages, recognised by studies of external and internal morphology in cathodoluminescence images. Detailed studies by EMP (Hf, Y, U, Th) and LAM-ICPMS (REE, U, Th, other trace elements) show that changes in internal morphology or zoning style are accompanied by changes in trace-element patterns, reflecting changes in magma composition and/or physical conditions. It is clear that the zircons have acted as recorders of the evolution of the magmas, and that this evolution included abrupt changes in composition. Hf isotope measurements were done using a Merchantek LUV266 NdiYAG laser probe, attached to a Nu Plasma multi-collector ICPMS. Typical spot sizes were 30-40 microns; typical internal precision on 1-2 minute ablations was 176Hf/177Hf of ±0.00002-6 (2SE). Interferences of 176Lu and 176Yb on 176Hf were corrected by monitoring 175Lu and 172Yb; the accuracy and precision to which these overlaps can be corrected are comparable to the internal precision given above (Griffin et al., 2000). Time-resolved data collection commonly showed that distinct "stratigraphy" in Yb/Hf, Hf intensity and 176Hf/177Hf was intersected by the beam during drilling, making it possible to select parts of the signal representing major zones. Zircons from all samples show a major peak at 176Hf/177Hf-0.2828±0.00005 ( eHf =+4, TDM model ages - 6 0 0 Ma). This is a minimum age for the source and may reflect the remelting of Proterozoic lower crust with low Lu/Hf . The zircons of the granodiorites and gabbro are mainly of this type, but some have more radiogenic Hf The quartz diorite and the granite also contain this type, as well as others with less radiogenic Hf (176Hf/177Hf =0.2826-27; TDM = 0.8-1.0 Ga). Both rocks also contain zircons with more radiogenic Hf (76Hf/177Hf =0.28290.2830). Individual grains are typically zoned from less to more radiogenic Hf compositions, but some are reversely zoned. Late-stage zircons generally have Hf isotope compositions intermediate between the extremes. The isotopic zoning patterns, and the presence of distinct isotopic populations corresponding to recognised growth stages, cannot be explained by fractional crystallisation or restite resorption, but require the mixing of magmas with disparate Hf-isotope compositions, derived from different sources. Correlations between trace element patterns and Hf isotope compositions indicate that these magmas also were different in composition. The trace element and Hf isotope data suggest that mixing of crustally-derived magmas with a juvenile component, followed by crystallisation of zircon from the mixed magma, has taken place several times during the evolution of the Pingtan complex. The repeated input of mantle-derived magmas probably has been important in providing heat for the production of the granitic complexes, but the isotopic data suggest that the mantle-derived magmas also have contributed new material to the crust. The use of zircon as a process recorder recovers information on the evolution of magmas that is lost during the analysis of wholerock samples. The availability of rapid, low-cost in-situ microanalysis of trace-element patterns and Hfisotope compositions of zircons, coupled with the detailed study of internal morphology, is about to bring major advances in our understanding of magma genesis in crustal settings. References GRIFFIN, W.L, PEARSON, N.J., BELOUSOVA, E., JACKSON, S.E., O'REILLY, S.Y., van ACHTERBERG, E., AND SHEE, S R. 2000. Geochim. Cosmochim. Acta 64, 133-147. XU, X., DONG, C., LI, W. AND ZHOU, X. 1999. Lithos 46, 299-315.

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LATE TIMING OF YILGARN GOLD MINERALIZATION AND SIGNIFICANCE TO AREA SELECTION David 1. Groves Centre for Global Metallogeny, Department of Geology and Geophysics, U W A , Nedlands, WA 6907

Within the Yilgam Block, most gold deposits are orogenic deposits, formed during accretionary or collisional tectonics pre-stabilization of the Yilgam Craton. The gold deposits formed over a large depth range from deep-seated, low-salinity, H2O-CO2-CH4 fluids, channeled initially along crustal-scale deformation zones, including those along diapiric granitoid margins at depth, but focussed into lower order structures adjacent to gold depositional sites. At these sites, supralithostatic fluid pressures, competency contrasts between rocks and chemically reactive rocks all play a key role in gold localization. Structural studies indicate that hosting or controlling structures are commonly earlier-formed structures that were reactivated during D3-D4 in a D1-D4 deformational sequence. Available geochronology indicates that most gold deposits formed at 2.64-2.63 Ga, but that some may have formed at ca. 2.66 Ga and others as late as ca. 2.61 Ga. Thus, deposits formed 40-90 m.y. (mostly 60-70 m.y.) after the major basic-ultrabasic volcanic event in the greenstone belts, in agreement with independent structural timing. The late timing of the orogenic gold deposits is pivotal to geologically-based area selection because their structural geometries, and those of enclosing goldfields and terranes, on geological maps and sections, are essentially similar to those during gold mineralization. This is supported by the fact that the gold deposits commonly have predictable and repetitive geometries, such as structural heterogeneities within or adjacent to shear zones or rigid granitoids, or specific "lock-up" fold-thrust structures: for example, the giant Kalgoorlie goldfield shows remarkable geometrical similarity to the giant Timmins goldfield, Canada. Further, most controlling structures are steeply dipping, although there are some important exceptions. The late gold timing, generally steep geometries and knowledge of gold-related regional stress-fields allow the application of computer-based stress mapping and GIS-based prospectivity mapping, provided there are high-quality geological maps available, normally produced by integration of aeromagnetic map interpretation and ground mapping. The best-constrained map of a significant gold province, the GSWA 1:250,000 solid-geology map of the Kalgoorlie Terrane, has been used as a test of the applicability of the methodologies. Low minimum-stress anomalies (dilation zones) correlate well with known goldfields, as do high-prospectivity zones defined by GIS-based prospectivity maps based on several critical-parameter combination methods, particularly fuzzy logic. There is good overlap (almost 80%) between anomalous areas defined by the two methodologies. There is evidence that the prospectivity mapping discriminates larger deposits, with the most prospective category, which occupies 0.3% of the greenstone belts, hosting about 15% of the deposits but about 80% of gold production. Other computer-based methodologies, such as artificial neural networks and quantitative shape analysis, show potential as additional tools to produce an integrated area-selection map for Yilgam orogenic gold deposits. Acknowledgements: The contributions to the research on which this abstract is based, by Carl KnoxRobinson, Juhani Ojala, Grace Yun, Stephen Gardoll, Warick Brown and Peter Holyland are gratefully acknowledged.

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AEROMAGNETIC EVIDENCE OF POSSIBLE PALEAOHOTSPOT ACTIVITY IN AUSTRALIA Peter Gunn Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 Magnetic data images magnetic rocks down to the Curie point geotherm, which depending, on local geothermal gradients, may be deeper than 40 kilometres. Magnetic data is thus likely to be capable mapping large magnetic bodies throughout much of the crustal section and, in some localities, below the Moho. The Third Edition of the Magnetic Anomaly Map of Australia (Milligan and Tarlowski, 1999), which presents a complete aeromagnetic coverage of Australia, images various large amorphous magnetic anomalies. Detailed inspection and modelling leads to the conclusion that many of these anomalies are due to magnetic sheets several kilometres thick. Several of these magnetic sheets have been modelled to occur at mid-crustal levels. It is proposed that the magnetic sheets are the result of massive sill emplacement and flood basalt flows above ancient mantle plumes. As described by Campbell (1998), current ideas on mantle plumes invoke the viscous ascent of hot material originating in the vicinity of the core mantle boundary. During its rise towards the base of the lithosphere the material develops a flattened head fed by a feeder tail. While the bulk of the plume material ponds at the base of the lithosphere, adiabatic decompression and melting occurs, which results in the eruption of flood basalts. Although not specifically discussed by Campbell, this process is also likely to result in the emplacement of sills. The Proterozoic Carson Volcanics and the contemporaneous underlying Hart Dolerite sills (up to three kilometres thick), of the Kimberley Basin of north western Australia, which are generally accepted to have a hotspot origin (Griffin and Tyler, 1997), would appear to be a example of such a situation. The distribution of these units is reflected by a broad magnetic high. Meixner and Gunn (1997) have modelled the subsurface geometry of these magnetic units and have identified a central feeder to the magma system. While central feeders do not appear to be commonly recognised in literature on flood basalt provinces it is possible that they are an important feature allowing magma ascent in the crust. Magnetic data in the south eastern comer of Western Australia shows a line of seven regularly spaced magnetic lows, each surrounded by an amorphous magnetic high and with many of the magnetic lows being the focus of major dyke swarms extending radially for hundreds of kilometres. It is proposed that the magnetic lows mark the passage of a line of mantle plumes with the magnetic low mapping the feeder pipe from the underlying plume head at the base of the lithosphere. The amorphous magnetic highs around the central feeders are thought to be due to massive sills. If it is accepted that the above examples demonstrate types of responses due to magma emplaced at upper crustal and mid-crustal levels above mantle plumes it is possible to use the Magnetic Map of Australia to identify numerous magnetic anomalies which may owe their origin to hotspot activity. The identification of the location of ancient plume heads can be significant for locating depocentres and predicting the facies distribution in sedimentary basins because doming associated with mantle plumes can cause uplift and erosion followed by cooling induced subsidence. Tectonic activity and erosion of magma systems due to hotspot activity could expose assemblages of mafic and ultramafic differentiates that could host chrome, platinum, nickel, cobalt and titanium. Acknowledgement: This abstract is published with the permission of the Chief Executive Officer of the Australian Geological Survey Organisation. References CAMPBELL, I.H. 1998 The mantle's chemical structure: insights from the melting products of mantle plumes. In Dynamics and Evolution of the Earth's mantle, Cambridge, 259-309. GRIFFIN, T.J. & TYLER, I.M. 1997 Geology of the King Leopold Orogen. Geological Survey of Western Australia, Bulletin 143. MEIXNER, A.J. & GUNN, P.J., 1997 Three dimensional kinematic modelling of the magnetic field of the southern Joseph Bonaparte Gulf Exploration Geophysics, 28, 260-264. MILLIGAN, P.R., & TARLOWSKI, C., 1999 Magnetic anomaly map of Australia (Third Ed.) scale 1:5 000 000, Australian Geological Survey Organisation, Canberra.

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DETERMINATION OF FLUID ORIGINS FROM TRACE ELEMENT ANALYSIS OF MOLYBDENITE AT ERNEST HENRY, NW QUEENSLAND Chris Gunton\ John Mavrogenes^and Phil Blevin^ ^Enterprise Mine, Mount Isa Mines Limited, Mount Isa QLD 4825 ^Geology Department, The Australian National University, Canberra ACT 0200 ^Research School of Earth Sciences, The Australian National University, Canberra ACT 0200 Ernest Henry, located in the Eastern Fold Belt of the Mount Isa Inlier, is an Fe-Oxide hosted Cu-Au deposit containing an average molybdenum concentration of ISOppm. Molybdenite occurs as an early forming sulfide mineral in a magnetite dominated breccia matrix as fme-grained, euhedral to subhedral crystals. Elemental correlations reveal a strong spatial correlation of Mo with Cu, Au, As, Co and U. This is unusual as Molybdenite typically forms from highly fractionated magmatic fluids, while Cu + Au mineralisation is typical of more mafic, less evolved sources (Pollard et al. 1998). Three possible sources for metals in the Ernest Henry deposit were proposed: i) ii) iii)

Fluids originated from sediments associated with Cover Sequences 1 and 2; Fluids of magmatic origin possibly associated with the Williams and Naraku Batholiths; A fluid mixing model combining fluids from i) and ii).

Trace element analyses (by LA-ICP-MS at the RSES, ANU) of Ernest Henry molybdenites detected anomalous concentrations of Co, Cu, Se, Zr, Ag, Te, W, Re, Pb, Bi and U. This suite of elements highlights the 'garbage can' nature of molybdenite at Ernest Henry. Re, W, Te and Se substitute for Mo in the crystal lattice of molybdenite due to their similar ionic size and charge. Ernest Henry molybdenites average 275 ppm W, and 200 ppm Re. These relatively high concentrations suggest a magmatic source component in the deposit. Furthermore, the Re/W ratio, an indicator of granite evolution (Johnston et al, 2000) is near 1 at Ernest Henry. This value records a magmatic input sourced from a moderately to highly evolved granite, which corresponds to the surrounding Williams and Naraku Batholiths. As Cu + Au mineralisation is not normally associated with these more evolved magmatic sources, it is unlikely that a single fluid source is responsible for the formation of Ernest Henry. Therefore a fluid mixing model whereby descending, relatively oxidised Cu + U ±Au ±Co ± As bearing fluids mixed with an ascending, relatively reduced Mo + Re + W + Se + Te ± Au bearing magmatic fluid is proposed. Ore precipitation was triggered by the mixing of these two fluids thereby reducing the Cu + U ± Au ± Co ± As fluid and oxidising the Mo + Re + W + Se + Te ± Au. References POLLARD, P.J. MARK G . AND MITCHEL, L.

1998. Geochemistry of Post 1540 Ma granites in the Cloncurry district. Northwest Queensland. Economic Geology. 93, 1330-1344.

JOHNSTON A., MAVROGENES J. AND BLEVIN P. 2000. Molybdenite trace elements as an indicator of granitic

source. This volume

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DEPOSITIONAL EFFECTS ON COAL RANK INDICATORS IN THE SAN JUAN BASIN (USA). COMPARISON TO AUSTRALIAN COAL BASINS Lila W. Gurba^ and James E. Fassett^ ' School of Geology, University of New South Wales, Sydney 2052, NSW, Australia ^ U.S. Geological Survey, Box 25046, MS 939, DEC, Denver, Colorado 80225, USA

The San Juan Basin of New Mexico and Colorado (USA) is the most productive coal-bed methane basin in the world, producing fi-om coal beds in the Upper Cretaceous Eruitland Formation. A detailed study is in progress to establish key petrographic characteristics of Eruitland coals as a basis for comparison with Australian deposits in support of coal-bed methane exploration. This paper is based on a study of samples taken from a core of the Eruitland Formation and associated rocks intersected in the Gasbuggy No. 1 test hole drilled in the eastern San Juan Basin. The core was sampled at the USGS Core Storage facility at the Denver Federal Center, Colorado. In addition to coal petrology (coal type, vitrinite reflectance and fluorescence, micro fracture pattern, and mineral matter) the chemical composition of individual macerals and minerals in selected samples was analysed using an electron microprobe, a technique recently applied at UNSW to regional, basin-analysis studies. Results of these analyses are related to the depositional environments of the Eruitland Formation and associated rock units, as determined by independent (USGS) stratigraphic and sedimentologic studies (Fassett, in press). Upper Cretaceous rocks in the San Juan Basin were deposited on the west side of the Western Interior Seaway as the northwest-trending shoreline transgressed and regressed repeatedly. The final regression of the sea is represented by the shoreface-marine Pictured Cliffs Sandstone. The Pictured Cliffs is underlain by the marine Lewis Shale and overlain by the continental Eruitland Formation. Coal beds are present in the Gasbuggy core in the main body of the Eruitland Formation between depths of 3,680-3918 ft. and in a thin Eruitland tongue in the Pictured Cliffs Sandstone at 4055-4072 ft. Eruitland coals represent back-shore, coastal swamp deposits (Fassett, in press). Coal samples from the Eruitland Formation tongue in the marine Pictured Cliffs Sandstone display anomalously low vitrinite reflectance values. The mean maximum vitrinite (telocollinite) reflectance for coals from the main body of the Eruitland Formation is 0.8-0.9%. Coal-bed samples from the lower tongue of the Eruitland have vitrinite reflectance values of 0.6-0.7% (Rymax), similar to values for vitrinite (telocollinite) from coal fragments dispersed in the Pictured Cliffs Sandstone. The lowest vitrinite-reflectance values from the Gasbuggy core samples are from coals intimately associated with a marine-influenced depositional environment. The carbon content of telocollinite determined by electron microprobe is 85-86% in the main body of the Eruitland Formation and is 86-87% in coals from the lower Eruitland tongue and the Pictured Cliffs Sandstone. The oxygen content of telocollinite decreases evenly from 10% to 7.5% from the top of the Eruitland Formation to the lower Eruitland tongue. The marine depositional environment does not seem to have influenced the carbon content of the telocollinite. Reflectance variations, however, appear to be influenced by the original depositional environment and by source-vegetation type (maceral type), in addition to burial depth. A similar study of Permian coals in the Gunnedah Basin, NSW, has shown a strong influence of depositional environment on vitrinite reflectance of coals in the high-volatile bituminous rank range (Gurba and Ward, 1998). Reflectance suppression of up to 0.2% has been noted in some marine-influenced vitrinite (telocollinite). Abnormally low vitrinite reflectance values resulting from environmental influences (proximity to marine environments) may at first seem to confiise the normal coal-rank-increase with depth, however, abnormally low vitrinite-reflectance zones could be potentially valuable markers for identifying flooding-surface sequence boundaries in detailed stratigraphic and sedimentologic investigations. REFERENCES Fassett, J.E., in press. Geology and coal resources of the Upper Cretaceous Eruitland Formation, San Juan Basin, New Mexico and Colorado: in Kirschbaum, M.A., Roberts, L.N.R., and Biewick, L.R.H., Geologic assessment of coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah: Chapter Q of U.S. Geological Survey Professional Paper 1625-B. Gurba, L.W. and Ward, C.R., 1998. Vitrinite reflectance anomalies in high-volatile bituminous coals of the Gunnedah Basin, New South Wales, Australia. International Journal of Coal Geology, v. 36, p. 111140.

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COALBED METHANE GAS GENERATION IN THE GLOUCESTER BASIN, NSW Lila W. Gurba^ and Carl R. Weber^ ' The University of N e w South Wales, School of Geology, Sydney, N S W , 2052 ^ Pacific Power, Box 5257, Sydney, 2001

The Gloucester Basin is a Permian infrabasin, 60 km long and up to 12 km wide. It has been filled with up to 2,000 m of coal-bearing fluvio-deltaic sediments. At the Stratford prospect in the northern part of the basin, the sequence contains 6 major coal seams and more than 20 minor seams. The seams are gas-saturated and a methane resource in excess of 50 PJ has been identified in an area of 5 km^. A detailed coal petrology investigation of 200 coal core samples, some of them oriented, has been used to prepare a 3-D model of thermal maturity in the basin in conjunction with coal type and coal microstructure. The model has been compared with the C B M parameters, including methane content, methane purity and coal seam permeability. The results indicate that there is a strong genetic relationship between the coalification pattern in the basin and CBM generation. There is also a relationship between coal microstructure and permeability. At the Stratford prospect, the coal rank is within the zone of the main stage of thermogenic gas generation. Mean maximum vitrinite (telocollinite) reflectance (Rvmax) ranges from 0.85 to 1.5%, and the vertical coalification gradient is quite high, at 0.07 Rvmax /100 m. The gas content, with some notable exceptions, follows the rank pattern and also increases with depth. Abnormal changes in paleothermal gradient, which coincide with changes in gas content, have been ascribed to the presence of fault blocks. The coal rank model has been used to confirm the presence of such faults and to assist in seam correlation. From the oriented coal samples, the orientation and magnitudes of the principal reflectance axes (reflectance anisotropy) were determined. The results indicate that the vitrinite (telocollinite) in Gloucester coals has biaxial optical properties. The biaxial nature of the vitrinite is thought to result from asymmetrical growth of its molecular structure, and to be related to stress fields that developed contemporaneously with coalification. The coals were found, under the microscope, to contain a characteristic microstructure. This consists of slit-like openings, which collectively have served to enhance gas adsorption capacity, permeability, and the high gas desorption rates obtained in the basin. The slit-like openings, which are preferentially developed within telocollinite layers, are probably caused by shrinkage as devolatilisation has taken place during coalification. They are arranged in patterns that vary markedly in vertical sequence. The orientation of the slits changes from parallel to bedding to inclined to bedding at differing angles. It is postulated that the orientation of the microstructure in the coal is the result of tectonic stresses.

Acknowledgements: This work was undertaken as part of Australian Research Council Collaborative Research Grant No. ARC SPIRT C39917621.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEODYNAMIC HISTORY OF THE AUSTRALIAN REGION Michael Gumis Seismological Laboratory 252-21, California Institute of Technology, Pasadena, California 91125 U.S.A.

The geological history of the Australian region from the Jurassic to the present as well as the present day state of the region are placed within a mantle dynamics perspective. The area of interest (from New Guinea in the North, Antarctic in the South, and the active margin between New Zealand and Tonga in the east) provides us with a set of tectonic elements which have changed considerably since the Jurassic: Cessation of subduction and rifting on the eastern continental margin of Australia, the initiation of subduction along the Norfolk ridge during the Eocene, the subsequent migration of a subduction zone toward the east, and the overriding of an old subduction zone by a new oceanic ridge to the south of Australia. There are many similarities compared to the rest of the world, but there are unique features related to regional tectonic evolution. Foremost among these are: The out of phase platform marine inundation of Australia during the Cretaceous, the deepest segment of the mid-oceanic ridge (Australia Antarctic Discordance, AAD) south of Australia, and the largest accumulation of subducted slabs in the transition zone beneath the Fiji plateau. The tectonic evolution of this region is placed within the context of geodynamics using mantle seismic tomography and three-dimensional models of mantle flow with plate tectonics. We propose that the anomalous Cretaceous vertical motion of Australia and distinctive geochemistry and geophysics of the AAD were caused by a subducted slab which migrated beneath the continent during the Cretaceous, stalled within the mantle transition zone, and is presently being drawn up by the Southeast Indian Ridge (Gumis et al. 1998). During the Early Cretaceous the eastern interior of the Australian continent rapidly subsided, but must have later uplifted on a regional scale. The mantle beneath the AAD has been interpreted as being cooler than normal, but it could also result from the sampling of an old back-arc basin. Beneath the AAD, seismic tomography shows an oblong, slab-like structure orientated N-S in the transition zone and lower mantle, consistent with an old subducted slab. Using three-dimensional models of mantle convection with imposed plate tectonics, we show that these well documented features are related to mantle processes. Our models of mantle flow couple plate tectonic evolution with mantle convection. We focus on models in which Australia is initially assembled with Gondwanaland with subduction beneath its restored eastern margin and then follow the evolution of the region up to the present day. We also show how the reinitiation of subduction along the Norfolk ridge leads to the present structure of the Tonga subduction zone. Reference Gumis M., Muller R. D., & Moresi L. 1998. Dynamics of Cretaceous vertical motion of Australia and the Australian-Antarctic discordance. Science, 279, 1499-1504.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

HYDROGEOLOGY AND ENVIRONMENTAL GEOLOGY OF THE GREAT ARTESIAN BASIN, AUSTRALIA M.A. Habermehl Bureau of Rural Sciences, Land and Water Sciences Division, Canberra, A.C.T., Australia The Great Artesian Basin is a confined groundwater basin, which underlies arid and semi-arid regions across 1.7 million km2 or one-fifth of Australia. The basin's groundwater resources were discovered around 1880, and their development allowed the establishment of an important pastoral industry. Pastoral activity, town water supplies, petroleum and mining ventures in (and a major mine outside) the Basin area are all totally dependent on artesian groundwater. The Great Artesian Basin is a multi-layered confined aquifer system, with aquifers in Triassic, Jurassic and Cretaceous continental quartzose sandstones. Intervening confining beds consist of siltstone and mudstone; Cretaceous marine sediments form the main confining unit. The basin is up to 3000 m thick, and is a large synclinal structure, uplifted and exposed along its eastern margin and tilted southwest. Recharge occurs in the eastern margin, an area of relative high rainfall, and the western margin in the arid centre of the continent receives minor recharge. Regional groundwater movement is towards the southern, southwestern, western and northern margins, where springs discharge and produce carbonate mounds. Potentiometric surfaces of the Triassic, Jurassic and Early Cretaceous aquifers are still above groundlevel, but pressure drawdowns of up to 100 m were recorded from 1880 to the 1990s in developed areas. Some waterbores ceased to flow, necessitating groundwater to be pumped. About 4700 flowing artesian waterbores were drilled in the main Lower Cretaceous-Jurassic aquifers at depths of up to 2000 m, but average 500 m. About 3100 controlled and uncontrolled artesian waterbores remain flowing with an accumulated discharge of 1500 ML/day. About 35000 non-flowing artesian waterbores, generally using windmill operated pumps, tap shallower Cretaceous aquifers. Groundwater quality of the Lower Cretaceous-Jurassic aquifers is good at 500 to 1500 mg/L total dissolved solids. Groundwater is suitable for domestic, town water supply and stock use, though unsuitable for irrigation in most areas. Groundwater temperatures at the boreheads range from 300 to 1000 C, and are a potential geothermal energy source, but artesian spring temperatures are 200 to 450 C. The basin comprises abundant hydrocarbon reservoir (and some source) rocks, and commercial and sub-commercial oil and gas is produced from Jurassic and Cretaceous sandstones, contradicting earlier beliefs that the basin-wide groundwater throughflow had flushed out hydrocarbons. Environmental geology issues relate to the development of the artesian groundwater resources of the Great Artesian Basin and include aspects of sustainable groundwater use, groundwater and rangelands management. Extraction of artesian groundwater by the pastoral industry during the last 120 years has affected the groundwater conditions in the Basin through large scale regional drawdowns, which caused reduced artesian pressures and reduced discharges from flowing artesian waterbores. Recent assistance programs funded by Federal and State governments aim to rehabilitate waterbores in poor condition and equip bores with control valves. The replacement of the inefficient open earth drain distribution system, which causes up to 95 percent wastage of the water produced, with a piping system is encouraged. These measures will benefit groundwater and rangeland management and assist to alleviate land degradation and plant and animal pest problems. Groundwater use by the petroleum and mining industries during the last 20 - 35 years affect some parts of the Basin. The South Australian part of the Basin is shown as an example where the combination of groundwater exploitation for the pastoral industry, town and homestead water supplies and extraction by the petroleum and mining industries impact on the Basin's groundwater conditions and on the naturally flowing artesian springs. In Situ Leach (ISL) uranium mining techniques have been introduced in this part of the Basin area. However, the ISL is carried out in a confined Tertiary aquifer on the surface of the Great Artesian Basin, which is hydraulically separated from the Basin's aquifer. Other environmental geology issues include the changes to the eastern recharge areas iii Queensland and New South Wales caused by land use changes, and the possible effects on groundwater recharge and groundwater contamination. Reference HABERMEHL M.A. 2000. Hydrogeology and Environmental Geology of the Great Artesian Basin, Australia. In: Gostin V.A. ed. Gondwana to Greenhouse - Environmental Geoscience: an Australian perspective Geological Society of Australia Special Publication.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOLOGY IS ALIVE AND WELL IN NSW POST-COMPULSORY EDUCATION Rosemary Hafiier The N e w South Wales Board of Studies, G P O Box 5300, Sydney, N S W , 2001.

The new HSC course, Earth and Environmental Science, was introduced into New South Wales schools this year and replaces a course in Geology. The new course aims to provide students with an understanding of the physical systems and processes that operate in both aquatic and terrestrial environments. It also explores the origin of the Earth, plate tectonics, and some of the major events in the Earth's history - these include changes in the lithosphere, atmosphere, hydrosphere, cryosphere and biosphere as well as the evolution of organisms since the origin of life on Earth. The course also focuses on human interactions with the earth's natural environments. Extensive investigation and consultation preceded the preparation of the Stage 6 Earth and Environmental Science Syllabus. The first step was the identification of key issues through the Stage 6 Science Symposium with a report being submitted to the Board of Studies in June 1998. The second step was to undertake a literature review and prepare a writing brief for consultation. The Board of Studies subsequently endorsed the writing brief in November 1998. A draft syllabus was prepared in 1999 and sent for further consultation. Many teachers, professional associations, academics and parents across New South Wales took the opportunity to consider and respond to the draft writing briefs and syllabus. The response to the Earth and Environmental Science course has been very positive, with strong support for the scope and direction of the content which is seen as relevant, interesting and focussing on current ideas. Anecdotal evidence indicates that the uptake of this course has been substantial compared to that of the former Geology course. A much greater number of post-compulsory students are now being exposed to Geology and the Earth Sciences.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HEMATITE IN OROGENIC ARCHEAN LODE-GOLD DEPOSITS: VECTOR TO BONANZA TYPE GOLD MINERALIZATION? ' Steffen G. Hagemann, V a s m i n e A. Gardner, ^ and Edward Mikucki ' Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, WA 6009; ^ E .& J. Mikukci Geological Consultants, 2285, Coolgardie Street, Mundaring 6073 Hypogene hematite is observed in some Archean lode-gold systems in the Yilgam Craton of Western Australia and the Abitibi greenstone belt in the Superior Province of Canada. Significantly, it has been described in giant and large Archean lode-gold deposits such as the Golden Mile (in parts of Fimiston style lodes), and Granny Smith, respectively. There is, however, little information about the significance of hematite alteration to the formation of high-grade gold shoots and the implications it may have for fluid redox conditions, fluid sources and gold precipitation mechanisms. A review of hematite alteration in 18 orogenic lode-gold deposits in the Yilgam Craton and Abitibi greenstone belt in Canada revealed that hematite is observed mostly in granitoid, felsic porphyry and mafic rocks that are metamorphosed to sub-amphibolite facies. Hematite occurs in two types: (1) brick-red hematite coating in hydrothermally altered wallrocks, and (2) specular hematite in quartz veins and breccias. Brick-red hematite occurs pre- (Wiluna), early syn- (Granny Smith) and late syn- (Lawlers) gold mineralization, whereas specular hematite occurs late-syn- (North Kalgurli-Fimiston lode), and post- (Jupiter) gold mineralization. Early hematite alteration may relate to seafloor alteration and is overprinted by gold related hydrothermal alteration (Wiluna). Hematite alteration that postdates gold mineralization occurs in barren ankerite-chlorite-quartz veins (Jupiter) or in irregular chlorite-hematite filled fractures (Granny Smith). Hematite that occurs synchronous with main-stage gold mineralization is observed in quartz-hematite-pyritesericite-ankeriteigold (Golden Mile) or pyrite-muscovite-hematite±gold (Porphyry) proximal alteration zones. Hematite in late-syn-gold mineralization occurs in equilibrium with pyrite±gold (Nimary stage 2) in quartz veins and breccia matrices. Hematite alteration indicates relatively oxidising conditions that are generally above those of the aqueous sulfate-sulfide buffer. Such ore-fluid conditions could result from multiple processes and/or fluid sources. Age constraints on Yilgam and Abitibi granitoids, and the low salinities, high X C 0 2 and weak alkaline pH of most gold ore fluids, argues against an oxidized magmaticfluid source for hematite and gold. In contrast, phase separation and/or fluid mixing, possibly with oxidized surface waters, could provide effective mechanisms for both oxidation of deep-sourced ore fluids and gold precipitation, and are consistent with available field evidence. The preliminary investigation of hematite in gold mineralization reveals that hematite alone does not constitute a significant vector towards gold, but instead may provide important constraints on gold precipitation mechanisms in certain cases. In particular, the hematite-gold association, and the oxidised ore-fluid conditions it indicates, are probably common indicators of fluid mixing or phase separation processes.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CAINOZOIC MULTIPLE IMPACT EVENT IN SOUTH AUSTRALIA AND ITS INFERRED SIGNATURE WITHIN TERMINAL EOCENE STRATIGRAPHY OF THE ST VINCENT BASIN Peter W. Haines School of Earth Sciences, University of Tasmania, GPO Box 252-79 Hobart, Tasmania 7001 A series of closely aligned elongate structural features lie within a 230 km long NNE-trending corridor stretching across southern South Australia from Kangaroo Island to the edge of the Murray Basin. These features are mostly expressed as valleys or basins marked by a halo of intense brittle deformation and deep weathering. Although some are subtle and largely buried, others are topographically prominent such as the 25x8 km Barossa Valley. The features cut obliquely across structural, stratigraphic and geophysical trends and are hosted by rocks of varying age. Geophysical evidence indicates that they are superficial and unrelated to the geology at depth. Definitive evidence of shock metamorphism, including multiple sets of planar deformation features aligned to crystallographic indices in quartz, has been located around two of the structures named Crawford and Flaxman (Haines et al. 1999). Shock metamorphism provides clear evidence for the impact genesis of these two sites, and it is speculated that the entire corridor is of related origin. The elongate shapes suggest a very low-angle impact trajectory (probably <5°). During such 'grazing' impacts the projectile may become decapitated such that its upper part ricochets down range, as indicated by an examination of such crater fields on other planets and by laboratory experiments. The only precedent known on Earth is the much smaller Rio Cuarto crater field in Argentina. Crawford is a -8.5x3.5 km elongate crater, while Flaxman is a -10x1 km long pseudotachylyte-bearing and shock metamorphosed breccia zone considered to represent a secondary down range ricochet feature related to Crawford. The trajectory was towards the NNE. Flaxman is unlike any other impact structure on Earth and may effectively represent a 'skid mark' produced by essentially tangential impact of a ricocheting fragment. It is speculated that the entire inferred crater field may represent secondary and tertiary impacts generated from a single large grazing impact into the Southern Ocean south of Australia. The best age constraints, assuming that all of the features share a common origin, come from the sediments infilling the Barossa Valley, which probably date from the Early Oligocene (Haines et al. 1999). The structural feature defining the southern end of the aforementioned corridor is referred to as Pelican (after Pelican Lagoon) and is a -18x8 km largely buried basin crossing eastern Kangaroo Island from the Southern Ocean to Gulf St Vincent (Haines 2000). St Vincent Basin has been undergoing marine sedimentation since the Middle Eocene. If Pelican is of impact origin, a tsunami and ejecta deposit should be preserved within the mostly quiet water basin succession. The thin Chinaman Gully Formation (CGF), long recognised as a unique local marker horizon within the basin stratigraphy, appears to ftilfil all predictions. This unit is of terminal Eocene or Eocene-Oligocene boundary age (Haines 2000). It has previously been considered as the signature of a brief regressive event, but detailed sedimentological examination does not support this interpretation. The CGF comprises a series of graded sand and fine gravel beds intercalated with mudstones. Much of the sand displays textural evidence of beach derivation. The formation is carbonaceous and pyritic at depth and locally contains much terrestrial plant matter, while micropalaeontological evidence suggests a mixture of marine, marginal marine and terrestrial sources. The unit is burrowed down from a single horizon at the top suggesting that it is a single event deposit, the individual graded beds probably resulting from the reflection of the tsunami back and forth across the confined gulf, emplacing successive wedges of shore derived material into the basin. The CGF is 2 m thick at its type section, but thickens rapidly toward the palaeo-shoreline. In drill core the top of the formation contains an abundance of -0.1-1 mm palagonitic spherules (spheres, ovoids, rare dumbbells) closely resembling those found at many K-T boundary sites (Haines 2000). These are considered to be the alteration product of low-silica impact glass droplets. The spherule layer is an ideal site to search for shocked quartz and a geochemical signature of the projectile. This work is in progress. REFERENCES Haines P.W., Therriault A.M. & Kelley S.P. 1999. Evidence for mid-Cenozoic(?), low-angle multiple impacts in South Australia. Meteoritics & Planetary Science, 34, A49-A50. Haines P.W. 2000. Tsunami(?) deposit of terminal Eocene age. South Australia: the inferred effects of a lowangle multiple impact event. Impact 2000 Conference, Vienna, abstracts.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AREA SELECTION IN THE YILGARN Greg C Hall Placer Dome, 25 Barker Street, Perth WA 6104 'Area Selection' is defined as that work you may complete in the office before going to the field or pegging tenements. The Yilgam is prospective for large gold deposits and the selected targets are gold deposits greater than one million ounces of contained gold. After 20 years of exploration in the Yilgam, my own approach to area selection has evolved from a purely pragmatic empirical approach, to a mineral systems approach, which focuses on the underlying causative processes. A mineral systems approach involves geodynamic modelling of representative gold deposit systems. Geodynamic modelling of Yilgam gold deposits has been described by Ord et al, 1998 and the exploration model derived from that modelling, by Hall, 1998. Area Selection in the Yilgam is a three-step process.

Define the mineral system Metamorphic grade maps define the basic parameters of a gold mineral system, by recording the conductive heating pattern (metamorphism), which is related to the convective heating pattern (hydrothermal fluid flow). Conductive heating patterns are characteristically modified by convective heating patterns into pairs of high grade/low grade metamorphism, separated by a regional gradient. Outlines of these paired zones define possible mineral systems. Systems that are likely to be gold-rich commonly already contain significant gold accumulations. Select those systems that have demonstrated gold potential. Selected systems appear to correspond with 'back arc basin' type regional settings. Define internal elements Within the selected regions define the intemal major fault geometry and define the major rheological units. Focus on major shear zone development. Note the distribution of volcanic and sediment packages. Note the presence of buried granite bodies intemal to the greenstones. Note the presence of conglomerate basins (Timiskaming type) which suggest linked faults.

Define target corridor and targets Identify the major outflow fault (i.e. fully penetrating the crust, and dipping in the opposite direction to all the other major faults and shear zones) and the dip direction. The hanging wall zone, within 10km of the fault trace defines the target corridor. Determine the critical spacing of major gold deposits, e.g. at Kalgoorlie the critical distance is 30km. Define low mean stress positions, especially above buried plutons. Low mean stress regions at critical spacing intervals constitute primary targets. References Hall, G.C., 1998. Autochthenous Model for gold Metallogenesis and Exploration in the Yilgam. In Geodynamics and Gold Exploration in the Yilgam Conference, Perth WA, August 1998, Australian Geodynamics Cooperative Research Centre Conference Volume, Ord, A., Hobbs, B.E., Walshe, J.L., Zhao, C., 1998. Developments in the Simulation of Geodynamic Processes with direct application to Yilgam Gold Mineralisation. In Geodynamics and Gold Exploration in the Yilgam Conference, Perth WA, August 1998, Australian Geodynamics Cooperative Research Centre Conference Volume.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CENOZOIC HISTORY OF SE ASIA AND THE SW PACIFIC: ORIGIN AND EVOLUTION OF ISLAND ARCS Robert Hall SE Asia Research Group, Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK

The Cenozoic was a period of major tectonic events which influenced life and climate in SE Asia and the SW Pacific. Subduction maintained volcanic arcs which formed discontinuously emergent island chains crossing the region. Early in the Cenozoic the major collision of India with Eurasia enlarged the area of land in SE Asia. Later, the continuing collision with Australia led to connections between Australia, Eurasia, and the Pacific, accompanied by the disappearance of some arcs and initiation of others. GPS measurements and seismicity record the complexity and speed of change of recent tectonics but provide little help in longer-term reconstruction of the region. Plate boundaries shifted rapidly in the Cenozoic. During convergence of the major plates there have been important episodes of extension, forming ocean basins and causing subsidence within continental regions, probably driven by subduction. It is clear that arcs can be geologically ephemeral features. Within eastern Indonesia, New Guinea and the Melanesian arcs there are multiple Cenozoic sutures with very short histories compared to most well-known older orogenic belts. They preserve a record of major changes in tectonics including subduction polarity reversals, elimination of volcanic arcs, changing plate boundaries, and extension within an overall contractional setting. Rapid tectonic changes have occurred within periods of less than 5 Ma. Many of these events, although important, would be overlooked in older orogenic belts because the dating resolution required to identify them, even when the evidence is preserved, is not possible. Strike-slip faulting is observably very important within the Pacific-Australia-Eurasia convergent setting yet appears in few tectonic models. Long-term strike-slip deformation can explain many of the complexities of the Neogene history of New Guinea such as magmatism and its absence, and thermo-chronological data showing very young and rapid cooling of the New Guinea mobile belt and fold belt. Here too, convergence has been accompanied by opening of marginal basins, and extension related to strike-slip faults resulting from partitioning of oblique convergence at plate boundaries. Arc magmatism is intimately connected to tectonics. The changing boundary of the Pacific and Indian mantle domains can be mapped from reconstructions of the region. Plume activity may have initiated some arcs and marginal basins but most new Cenozoic subduction systems appear to have been initiated close to boundaries between thick and thin crust following collision or plate reorganisations. Since 25 Ma subduction hinge rollback has been accompanied by significant arc volcanism and in most cases by marginal basin formation as the mantle wedge was constantly replenished by an inflow of hot undepleted mantle. In contrast, periods of hinge advance seem to be marked by reduction or cessation of volcanic activity. The changes in tectonics and magmatic activity caused equally rapid changes in land-sea distribution, topography and climate, which have in turn influenced formation and preservation of natural resources, and distribution and evolution of biota. The challenge for this century is to understand better the mechanisms by which all these are interlinked.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EARLY PALAEOGENE PLANKTIC FORAMINIFERAL AND CARBON ISOTOPE STRATIGRAPHY, ODP HOLE 762C, EXMOUTH PLATEAU, NORTHWEST AUSTRALIAN MARGIN Haidi J. L. Hancock \ George C. Chaproniere Gerald R. Dickens and Robert R. Henderson ^ ^ School of Earth Sciences, James Cook University, Townsville, QLD 4811, Australia ^ Department of Geology, Australian National University, Canberra, ACT 0200, Australia.

Although the North West Shelf of Australia is an important region for petroleum exploration and palaeoceanographic investigations the stratigraphy for the Palaeogene is poorly documented, especially for foraminifera. Ocean Drilling Program (ODP) Site 762 on the Exmouth Plateau contains an expanded Palaeogene sequence with abundant, calcareous microfossils. Early Palaeogene cores from this location were examined in this study for their planktic foraminiferal assemblages and carbon isotope compositions (Subbotina spp.). The sequence from 502.96 to 307.8 mbsf was deposited between the early Paleocene and Middle Eocene, and contains all planktic foraminiferal Zones Pic through PIO of the current global scheme, except Subzone P4b. Planktic foraminifera are generally very well preserved and 75 species belonging to 17 genera were identified. Despite a relatively high latitude Palaeogene location for Site 762, planktic foraminiferal biozones are generally in phase with those of the currently used global scheme for subtropical locations (Berggren et al., 1995). However, rare, patchy or non-occurrences of the zonal marker species Globanomalina pseudomenardii, Morozovella velascoensis, M. formosa, Planorotalites palmerae and Hantkenina nuttalli, make some correlations difficult. Planktic foraminiferal assemblages show three increasing diversity trends: at the P3a/P3b boundary with the first appearance of muricate Acarinina species; in the middle of P4a/b with the first appearances of many warm water Morozovella and Acarinina species; and at the P8/P9 zonal boundary with the arrival of late early Eocene Acarinina species. Overall, trends in &#61540;13C of planktic foraminifera are similar in shape to global isotope curves spanning the early Palaeogene, although the prominent short term negative excursion across the latest Paleocene thermal maximum (LPTM) is partly missing, probably because of a gap in core recovery. Combined with nannofossil biostratigraphy and a recalibrated magnetostratigraphy, the planktic foraminiferal record at Site 762 provides an integrated and detailed early Palaeogene stratigraphy for the North West Shelf Importantly a complete, expanded and shallow buried early Palaeogene sediment record exists on the Exmouth Plateau. The location is ideal for future scientific drilling to understand profound palaeoceanographic variations in the Palaeogene.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE NORTH AUSTRALIAN LOW-Ti ANTRIM PLATEAU FLOOD BASALT PROVINCE Linda M Hanley Research School of Earth Science, Australian National University, Canberra, ACT, 0200 The Antrim Plateau Volcanics (APV) and stratigraphic equivalents (Nutwood Downs, Helen Springs Peaker Piker, and Colless Volcanics) comprise a little known continental flood basalt province of early Cambrian age that cover an extensive area (1700km by >900km) of northern Australia from the West Kimberley to western Queensland. Although the present outcrop extent is some 35,000 km^ (Bultitude, 1976), Antrim Plateau basalts underlie the sedimentary strata of a number of Palaeozoic basins, so that the original extent may have been -400,000 km^ with an estimated volume of at least 0.1 million k m l Previous radiometric dating studies of APV basalts by K-Ar and Rb-Sr methods produced inconsistent results with ages ranging from latest Neoproterozoic to Ordovician. However, an early Cambrian age is confirmed by a SHRIMP zircon age of 513± 12 Ma (Hanley & Wingate, 2000) obtained for the - 2 5 0 km long Milliwindi dolerite in the West Kimberley. The dolerite is geochemically identical to the APV basalts and was probably a feeder dyke to basalt that once covered the Kimberley region. The APV consist of a series of - 2 0 - 6 0 m thick lava flows of mostly fme-grained massive basalt, with less common plagioclase-phyric porphyritic basalt, and conspicuous vesicular flow tops. The flows are intercalated with thin beds of aeolian sandstone and stromatolitic chert. The basalts attain their greatest overall thickness (~ 1000m) in the East Kimberley region of Western Australia and have the greatest outcrop extent in the Victoria River region of the Northern Territory. Early Cambrian basaltic volcanism was associated with the initiation of widespread basin development across northern Australia, possibly linked to breakup of microcontinental blocks along the margin of East Gondwanaland. The APV basalts are low-Ti tholeiites with MgO values ranging from ~4 to 9 wt%. The basalts display well defined fractionation trends on plots of major and trace elements against MgO. They are characterised by low (FeO)MgO=8.0, (TiO2)MgO=8.0, (CaO/A^Os) M g 0 = 8 . 0 values and trace element abundances similar to low-Ti tholeiites of the Mesozoic Karoo, Tasmania, Transantarctic and Parana provinces. The APV show remarkably constant geochemical features across the province and are characterised by elevated Th/Nb and Rb/Ba ratios that resemble Post-Archaean Terrigenous Shale (PATS). Moreover, the APV in comparison to other continental flood basalts, have one of the strongest sediment-like signatures yet recognised. Although low-Ti tholeiitic flood basalt provinces have been associated with Mesozoic Gondwana breakup, it is apparent that basalts with sediment-like geochemical signatures were also produced in the early Cambrian at a time of rifting of the northern margin of East Gondwanaland. How the basalts obtained this strong sedimentary-like geochemical signature is controversal, and the extent to which the lithosphere or crust is involved remains unresolved at present. References Bultitude R.J. 1976. Flood basalts of probable early Cambrian age in Northern Australia. In: Volcanism in Australasia. Elsevier. Amsterdam. 1-20. Hanley L.M. & Wingate M.T.D. in press. An Early Cambrian Dolerite Dyke from the West Kimberley - an intrusive phase of the Antrim Plateau Flood Basalts of northern Australia. Austrralian Journal of Earth Sciences, (submitted).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CORDIERITE AS A SENSOR OF HIGH-GRADE METAMORPHIC PROCESSES Simon L Harley Deparment of Geology and Geophysics, University of Edinburgh, Edinburgh, EH9 3JW, UK

New experimental data on the uptake of H2O and CO2 by cordierite at high pressures and temperatures enable its use as an effective monitor of fluid activities and fluid- and meltrelated processes that occur in the deep crust during and after metamorphism. Dehydration-melting of biotite-bearing mineral assemblages through reactions such as Bt + Sil + Qz = Crd + Kfs + L (A) produces cordierite-bearing migmatite complexes and leucogranites at conditions of 0.25-0.7 GPa and 725-900°C. Cordierites that coexist with melts in experiments related to reaction (A) contain 0.4-1.6 wt % H2O, depending on the pressure-temperature (P-T) conditions, and calculated aH20 along (A) varies from 0.3 at low pressure (0.4 GPa) to 0.17 at 0.8 GPa. These results are consistent with cordierites from several granulite facies terrains that yield aH20 conditions in the range 0.15-0.4 (e.g. Prydz Bay, Antarctica; Reynolds Range, Central Australia; Madagascar; Kerala; Broken Hill, Australia). Calculated aco2 in these migmatites are variable (0.15-0.50); in most cases total fluid activity is less than unity and consistent with fluid-undersaturation. Experiments also constrain Dw, the distribution of H2O between granitic meh and coexisting cordierite [Dw "" H20^eit / H20crd ], to be in the range 2.5-4.5 for the P-T-aH20 conditions calculated for anatexis. Hence, the H2O contents of melts that formerly equilibrated with the I0W-H2O (0.6-1.2 wt %) migmatite cordierites are 2.8-4.4 wt%. This production of hydrous cordierite in dehydration-melting also limits melt productivity, such that Dw of 2-5-4.5 cause a 25-35 % relative decrease in the amount of meh produced from reaction (A) compared with analogous reactions involving anhydrous minerals. Cordierite can also be used to constrain the compositions of fluids infiltrated into highgrade rocks during their exhumation. Cordierites formed with anthophyllite in 1590-1580 Ma metasomatic zones in the Reynolds Range contain 0.8-1.1 wt% H2O and negligible CO2, consistent with post-peak infiltration of a C02-poor (aco2 < 0.25), hydrous (aH2o > 0.40) and probably saline fluid at 0.3 GPa and 650°C. Similarly hydrous cordierites (H2O = 1.3-1.5 wt%; CO2 = 0.05-0.30 wt%) that typify the orthoamphibole-in zone of the Limpopo suggest infiltration of moderate aH2o fluids (0.23-0.29 at 0.6 GPa, 625°C) to rehydrate the terrane. These C02-poor (aco2 < 0 . 1 ) fluids, potentially liberated from crystallising melts, may have been significantly saline in order to reduce water activities to those calculated. Cordierite also is a key phase in symplectite and corona textures formed through reactions operating in granulites on their post-peak P-T paths. The cordierite-producing reactions, which also have the potential to consume fluid, include Grt + Sil + Qz = Crd and Grt + Qz = Opx -f Crd. In the Rauer Group of East Antarctica and Kerala Khondalite Belt of India such coronal cordierites are low in H2O (0.2-0.4 wt%), high in CO2 (0.8-0.9 wt%) and have elevated channel Xcoi (0.55). These examples suggest that garnet-breakdown reactions conventionally ascribed to near isothermal decompression in many granulites must also involve the scavenging of low-aH20 (0.05-0.2), moderate-high aco2 (>0.4) fluids available subsequent to the metamorphic peak.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DISCOVERY OF A HOLOCENE DRIFT DEPOSIT ON THE GEORGE VTH CONTINENTAL SHELF, EAST ANTARCTICA ~ AN EXPANDED RECORD OF REGIONAL SEA ICE AND ANTARCTIC BOTTOM WATER PRODUCTION? Peter Harris', Giuliano Brancolini^ Leanne Armand^ Belinda Brown'*, Martina Busetti^ Tara Deen"^, Giovanna Giorgetti^ Alix King^ Massimo Presti^ and Fabio Trincardi^ ' Antarctic CRC and Australian Geological Survey Organisation, GPO Box 252-80, Hobart, Tasmania 7001, Australia ^ National Institute for Oceanography and Experimental Geophysics (OGS), PO Box 2011, Trieste, 34016, Italy ^Antarctic CRC and lASOS, GPO Box 252-80, Hobart, Tasmania 7001, Australia "^School of Earth Sciences, University of Sydney, Sydney NSW 2006, Australia ^Department of Earth Science, University of Siena, Via Laterina 8, 53100, Siena, Italy ^Department of Geological, Environmental and Marine Sciences, University of Trieste, Via E. Weiss 2, 34127 Trieste, Italy ^Institute of Marine Geology, CNR, Via Gobetti 101, 40129 Bologna, Italy.

A joint Italian/Australian expedition to the George V coast was conducted in FebruaryMarch 2000, and collected 39 sediment cores up to 5.5 m in length, seabed photographs from 14 stations and over 1,800 km of multi-channel seismic and Chirp seafloor imagery. The expedition discovered and mapped a shelf sediment drift deposit covering about 400 km^ lying in an >800m deep section of the George V basin west of the Mertz Glacier. It is a true "drift" deposit, since these sediments exhibit bedforms, contain foreset bedding and display a depositional architecture indicative of sediment transport. A significant observation is that the drift appears to be sourced from the outer continental shelf, with sediment being transported landwards, across the shelf and into an 850m deep inner shelf basin. The "Mertz Drift" is over 35 m thick and core samples demonstrate that it is composed of laminated, anoxic, gelatinous olive green, silicious mud and diatom ooze (SMO). Preliminary shipboard counts of the laminae suggest a thickness of from 4 to 20mm, with a mean of about 7mm. While the lower sediments are laminated, there is a 20 to 50cm thick sandy drape at the surface over the whole of the drift. This suggests that a recent (late Holocene) change in the depositional environment has occurred, possibly related to changes in the extent of the nearby Mertz Glacier tongue and/or to the persistence of sea ice over the shelf area.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

INVESTIGATION OF THE SPATIAL AND TEMPORAL POLLUTION IN THE TONALLI RIVER (NSW): PRELIMINARY GEOCHEMICAL DATA Jennifer Harrison\ Henk Heijnis^ Graziella Caprarelli^ Robert Chisari^ and Henri Wong^ ^ Department of Environmental Sciences, UTS, Broadway, NSW, 2007 ^ Environment Division, ANSTO, Lucas Heights, NSW 2234 Investigation of geochemical and isotopic characteristics of stream sediments from the Tonalli River, a tributary at the southern end of Lake Burragorang, in the Blue Mountains National Park (NSW), was undertaken in order to evaluate the effect and dispersal of pollutants from the abandoned Yerranderie silverlead-zinc mine site. A comparative study of samples from Lacys Creek, located about 15 km north of the Tonalli River, was also conducted to determine base levels for the concentrations of trace elements in the sediments. The Burragorang valley was flooded in the 1950s as a result of the construction of Warragamba dam. Since this time mining activities have ceased, however acid mine drainage and pollutant distribution continue to be of interest with respect to human impact studies, mine site rehabilitation studies and water quality studies. As a result of the mining activity at Yerranderie, an increased contribution of toxic metals to the catchment system may have occurred. Both the temporal and the spatial distribution of these metals are determined from core samples and surface sediment samples respectively. Core samples and surface sediment samples were collected from various locations in the Tonalli River and the Lacys Creek. The cores had variable lengths, determined by the nature of the material collected. Water samples were also collected from the sediment sampling sites, both at the water surface and near the sediment/water interface. The sediment cores were cut at regular intervals of 0.5 cm. Both the core sediment and the surface sediment samples were dried at room temperature to prevent sublimation of Hg and As. All of the dried samples were then crushed in a mortar. On the core fractions thus treated, dating by the 210Pb method was carried out. Dating was not carried out on the samples of surface sediments, for which a present age is inferred. All sediment samples were digested and diluted and ICP-MS and ICP-AES techniques were used to determine the concentrations of Pb, As, Zn, Cu, Cd, Hg and Ag in the sediments. Grain size and organic content of the sediments were also determined and utilised as correction factors for the elemental concentrations and the 210Pb dating. Water samples were filtered and acidified to a pH of land digested and diluted for ICP-MS and ICP-AES measurements. Grain size analysis revealed that the clay content is constant throughout the length of cores, with the silt and sand content varying complementarily. The upper portions of the cores are characterised by about 50 to 70% silt. The lower portions of the cores contain up to 30% of silt. The sedimentation rate of the Tonalli River/Lake Burragorang junction was determined from the 210Pb dating and a significant decrease in sedimentation as well as the shift to finer sediments correlate with the construction of the dam downstream. The sedimentation and grain size distribution data indicate a sedimentary environment within which deposition changes from fluvial deposition to lake deposition. Uncorrected elemental concentrations ranged from 0.3 ppm (Cd, sample #9150) to 242 ppm (Zn, sample #9132). Grain size corrected data show that concentrations of the pollutants are higher in samples containing a greater percentage of silt. As expected, the pollutants have a greater affinity for smaller sediment particles, which result in lower pollutant concentration in the water column. Preliminary interpretation of the data obtained indicates that the grain size distribution within each sample affects the observed elemental concentration trends. This suggests that the geochemical behaviour of the elements and the statistical methods used to analyse the data obtained have a significant impact on the usefulness of the analytes to indicate the effect and sources of pollution at the studied sites.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SEDIMENTOLOGY OF THE SHOALHAVEN GROUP ALONG THE WESTERN MARGIN OF THE SOUTHERN COALFIELD Chris Harwood and Adrian Hutton University of Wollongong, Northfields Avenue, Wollongong NSW 2522

The Sydney Basin is one of two major coal-producing basins in the eastern half of the Australian continent. Correspondingly, most sedimentological studies relating to the Permian sequence of the Sydney Basin have concentrated on the Illawarra Coal Measures and little attention has been given to the older Shoalhaven Group. It is generally recognised that the Permian sequence of the Sydney Basin represents cyclic deposition corresponding to transgressive- regressive sea level changes. In the southern part of the Sydney Basin, recent studies have shown that the Shoalhaven Group can be divided into five depositional systems (with two systems containing coal-forming stages, Clyde Coal Measures and Yarrunga Coal Measures) representing transgressive and regressive stages. Along the western margin of the Southern Coalfield, the sequence is much thinner and the five depositional systems cannot be clearly defined. Facies variations along the western margin of the Southern Coalfield indicate a major marine transgression with basal conglomerates grading upwards through near shore sandstone and finally bioturbated shelf siltstones containing dropstones indicative of a glacial environment. Basement topography influenced the depositional styles and consequently the facies types. The deposition of a Marrangaroo Conglomerate, the lowermost unit of the Illawarra Coal Measures in this area, indicates the marine transgression was abruptly halted.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOLOGICAL SIGNATURES OF EXPLOSIVE ERUPTIONS IN GRANITIC PLUTONS: EXAMPLES FROM COASTAL MAINE, USA David P. Hawkins^ and Robert A. Wiebe^ 'Department of Geology and Geography, Denison University, Granville OH 43023, ^Department of Geosciences, Franklin and Marshall College, Lancaster PA 17604 Large portions of many granitic plutons consist of cumulate material that was deposited on the floor of a crystal-poor magma chamber. These parts of a pluton are a stratigraphic record of processes and events in the magma chamber; for example, episodic influx of mafic magmas is commonly recorded by zones of mafic enclaves or mafic sheets that ponded on the chamber floor. The mafic sheets provide 'way-up' indicators and a potential link to volcanic eruptions from the chamber. The link hinges on the recognition from theoretical studies and observations in both ancient and modem intermediate to silicic volcanic systems that the influx of mafic magma into a more silicic crustal magma chamber can trigger explosive eruptions. Here, we consider what type of geological evidence for eruption might be preserved in the layered portions of granite plutons and we describe some examples of those geological features from granite plutons in the USA. The 1991 eruptions of Mt Pinatubo provide a well-documented model for predicting what type of evidence might be preserved in a layered granite pluton. The seismic and geologic record of the eruptions indicate that multiple injections of basaltic magma into a shallow (6-8 km) crustal magma chamber triggered the explosive eruptions. The final stages of the eruption sequence were characterized by numerous seismic explosions in the country rock surrounding the upper portions of the magma chamber. The energy of these explosions was sufficient to shatter the country rocks and thereby clear a free pathway for the magma that eventually escaped from the chamber. This 'throat-clearing' stage of the eruptive sequence, which lasted for several days, was characterized at the surface by episodic explosive eruptions. Thus the eruptive sequence of Mt. Pinatubo indicates that the influx of mafic magma was followed closely (weeks to days) by explosive shattering of the magma chamber roof and walls. Much of that material is likely to have collapsed onto the floor of the chamber either then or later during the main phase of the eruption. In either event the resulting magmatic breccia would be in close association with the recently ponded mafic magma. Two granite plutons in Maine, the Vinalhaven pluton and the Cadillac Mountain pluton, are spatially associated with magmatic breccias that formed in the active magma chambers. Within the layered sequence of the Vinalhaven granite, angular blocks of the country rocks, ranging in diameter from centimeters to hundreds of meters, occur at several stratigraphic intervals, extend for several kilometers along strike, and are consistently in close association with major zones of mafic sheets and pillows which represent mafic influx into the chamber. The lithology of blocks within the stratabound breccia layers mirrors the lithologic variation of the country rocks along the northern margin (tilted roof) of the pluton: deformed early Paleozoic metamorphic blocks occur in the east, and relatively undeformed younger volcaniclastic/epiclastic blocks occur in the west. These relations indicate that several influxes of mafic magma into the chamber appear to have been followed closely in time by vertical collapse of the shattered roof The dominantly pyroclastic rhyolitic rocks exposed on the north margin (tilted roof) of the pluton are similar in age and composition to the granite and suggest that eruptions from the Vinalhaven magma chamber were explosive in nature. The association of mafic influx with zones of country rock blocks appear to represent repeated collapse of the shattered roof resulting from explosive eruptions triggered by the influx of mafic magma. The northeastern margin of the Cadillac Mountain pluton, which appears to represent a moderately inwarddipping wall of the original magma chamber, varies along strike from a zone of shattered country rock to a magmatic breccia of large, rotated country rock blocks. It was initially injected by low-T biotite granite into the breccia zone, followed immediately by higher-T fayalite+clinopyroxene hypersolvus granite that appears to represent a flux of magma from the interior of the chamber. The coincidence of the country rock brecciation with rapid upward movement of deeper and hotter magma suggests that the brecciation was related to a major eruption out of the chamber.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AREA SELECTION IN MINERALS EXPLORATION: A GLOBAL PERSPECTIVE Douglas Haynes BHP Minerals Discovery, Milton, Queensland An analysis of 361 base metals, gold, diamonds, iron ore and manganese mineral discoveries shows that since 1945, most discoveries outside the former Soviet Union have been made in Australia, Botswana, Canada, Chile, Republic of South Africa, and south western USA. Discovery rates peaked in the late 1960's and early 1970's, but since the early 1990's, a rapid decline in the success rate occurred, despite rising

exploration expenditure culminating in a record global exploration spend of $A4.5 billion in 1997. The expenditure peak and decrease in discovery rate indicates that in areas with a mature exploration history, most deposits with which exploration geoscientists are familiar have been found, and that exploration

geoscientists have either lost or never have had the ability to specifically target and find the large ore deposits. The relative success rates of technologies in exploration defined in the analysis are illustrated in the Table.

Ore Deposit Type

Mesoth. Au Porphyry Cu Epith. Au Sed. Cu Sed Pb-Zn-Ag VHMS Ni-Cu-PGE FeOx Cu-Au Diamonds Unconf. U

Known (All Discoveries) 47% 44 35 35 33 18 17 17 0 0

Prospecting (New)

10% 10 9 13 4 5 23 0

6

7

Geology (New)

36% 39 47 43 38 26 23 29 13 57

Geochemistry (New)

41% 38 35 13 35 21 20 35 56 0

Geophysics (New)

13% 13 9 31 23 48 34 36 25 36

Success in ore discovery can be optimised by (a) focusing minerals exploration on provinces or "terrains" with an immature exploration history, (b) selecting the new provinces which show the largest negative differences in known mineral deposit endowment in comparison with their well-explored geological analogues, and (c) in the selected provinces, placing an emphasis on follow up of previously known mineralisation, and on the use of geology and geochemistry rather than geophysics in exploration. If success is to be optimised in areas with a history of modem minerals exploration, selectivity in targeting large ore deposits must be enhanced by: (a) erecting ore deposit "conceptual models" that define the differences between the large and small ore deposits in terms of signals that can be readily visualised in standard data sets used by exploration geoscientists, and then using such models to target these large deposits only, and (b) constructing "generalised"' conceptual models that define and logically link the geological features that are common to a range of large ore types and using these to target ore deposits which do not necessarily fit conventional ore deposit classification schemes. Successful, selective targeting will require collection of data which are best for definition of the key geological features in the regions of concealed bedrock, namely, magnetic and gravity potential field data, and multispectral scanning data, and use of sensitive detection tools, particularly geochemical techniques, that enable detection of subtle signals to mineralisation through transported and lithified overburden. The targeting using the conceptual models and the mapping tools must be focussed on areas which have escaped effective prior blanket geochemical prospecting. Generation of conceptual models specific to targeting large deposits requires a concentrated research effort aimed at: (a) systematic comparison of geological features between large and small ore deposits followed by definition of features which differentiate the large and the small; (b) documentation and definition of all of "common" features, for example, structural architecture, gross upper crustal lithological successions, and gross upper crustal oxidation states associated with large, but apparently disparate ore deposits; and (c) simulation of metal source-transport-deposition scenarios that realistically provide for simultaneous mass transfer (with provision for local disequilibrium and equilibrium conditions), rock deformation, fluid flow, and metal precipitation in hydrothermal and magmatic systems, so that theory assists the empirically-based research to generate a robust definition of the common features associated with the large ore deposits. 214

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

STUDY OF ANOMALOUS LOW VITRINITE REFLECTANCE PROFILES IN THE BARROW SUB-BASIN, WESTERN AUSTRALIA: IMPLICATIONS OF TWO EFFECTS RELATED TO PERHYDROUS SUPPRESSION AND OVERPRESSURE RETARDATION Sheng He and Mike Middleton Centre of Excellence in Petroleum Geology, School of Applied Geology, Curtin University of Technology, GPO Box U 1987, Perth, WA 6845, Australia

The anomalous low vitrinite reflectance (Ro) profiles within the Middle-Upper Jurassic to Lower Cretaceous from four exploration wells in the Barrow Sub-basin have been investigated. The compaction and pressure data show that the shallow normal compaction sequence, and the deep undercompaction sequence, is associated with a normal pressure system and an overpressure system, respectively. Most of the measured Ro data in both normal pressure and overpressure systems of four wells with similar rift thermal histories are lower than predicted Ro data from basin modeling, using a current basin model that is controlled by temperature and time. In the shallow normal pressure system, the organicmatter maturation is significantly distinct from that in the deep overpressure system. In the normal pressure system, the Rock-Eval Tmax profiles exhibit normal trends, corresponding to predicted Ro and equivalent Ro (by FAMM). This is indicates that the normal pressure has no influence on hydrocarbon generation. Therefore, the low Ro profiles in the normal pressure system are believed to be suppressed due to perhydrous vitrinite. Moreover, the suppression of reflectance also occurs in the upper part of the deep overpressure system. In the overpressure system, the measured low Ro profiles are associated with abnormally low Tmax and equivalent Ro trends, and the production index (PI) displays abnormal change in Bambra-2. The true low maturation is confirmed to be retarded by the overpressure. In two wells, the low Ro values in the upper part of the overpressure system show that they are caused by two effects related to the suppression of reflectance due to perhydrous vitrinite and retardation of overpressure. The overpressure retardation results in true low maturation, but the influence of hydrogen-rich vitrinite maceral may further enhance Ro low abnormality. This study of anomalous low Ro profiles in the sub-basin is important for understanding organic-matter maturation and petroleum generation in the overpressure system.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CHEMICAL EFFECTS OF SQUEEZING A GRANITIC CRYSTAL MUSH DURING COMPRESSIVE DEFORMATION: THE KAMERUKA PLUTON, LACHLAN FOLD BELT B. Healv^ W.J. Collins', S.W. Richards', and R. A. Wiebe^ 'Department of Geology, School of Geosciences, University of Newcastle, Newcastle, NSW, 2308. ^Department of Earth Sciences, Franklin and Marshall College, Lancaster, PA 17604-3003, USA. The Kameruka Granodiorite is a large (570km^) I-type pluton in the central Bega Batholith, eastern Lachlan Fold belt. At the southern extremity of the pluton, a basal km-thick sheet of Kameruka granodiorite is overlain by Pericoe adamellite. Structural analysis of the underlying migmatitic aureole indicates that the pluton has been folded into a macroscopic basinal structure. The axial plane of the fold is steep and Ntrending, defined by euhedral plagioclase and K-feldspar grains in the granodiorite. Microstructural relations show that this foliation varies from magmatic to solid-state throughout the sheet, indicating syn-plutonic deformation: it is not observed in the overlying, younger Pericoe adamellite. Small-scale folds within the migmatitic aureole also have steep, N-trending axial planes, subconcordant with the foliation in the overlying sheet. This foliation contains leucosomes, indicating that folding occurred during peak metamorphic conditions, consistent with syn-plutonic compressive deformation. A number of mafic injections within the pluton have been interpreted as mafic and silicic layered intrusions (MASLI) systems. These injections display features comparable with sedimentary structures along their lower contacts, such as load-cast and flame structures, with a sharp chilled base moulded around compacted cumulate crystals. These contacts are interpreted as representing deposition of mafic melt on an interface between an underlying, crystal-rich zone and overlying crystal-poor zone within the granitic magma chamber. Therefore, the vertically-stacked mafic injections require deposition on an aggrading chamber floor, so that plutons provide a stratigraphic and temporal record of crystal accumulation throughout the construction and cooling history of a granitic magma chamber. The granitic texture is a close-packed framework of large (>lcm) euhedral grains of feldspars and quartz enclosed in a finer-grained interstitial phase comprising a quartzofeldspathic mineral assemblage and biotite. At cm-scale, the mafic layers rest on a bed of quartz-feldspar crystals, which typically comprise >80 % of the contact interface. Limited evidence exists for interaction between mafic melt and felsic crystals, such as new rims on cumulate plagioclase, but in the interstitial zones, mixing has produced diffuse hybrid magmas. We interpret the texture to reflect a grain supporting medium of cumulate crystals, with interstitial pockets of granitic melt that have locally hybridised with mafic liquid. These contacts provide strong evidence that the granites were crystal-rich mushes at the time of MASLI deposition. The Kameruka suite defines linear geochemical trends for most elements between 67-72 wt% Si02. The Kameruka sheet described above is most felsic at the base, systematically becoming more mafic upwards. Therefore, it became more mafic with time. Despite the chemical variation, plagioclase and biotite compositions show no variation from bottom to top. It implies that the melt composition does not change throughout the crystallisation history of the sheet. This variation cannot be attributed to crystal fractionation nor restite unmixing, both of which should produce felsic magmas at the top. Rather, chemical variation appears to simply result from differing crystal-melt proportions at different places in the sheet, which is reflected in sympathetic modal abundances. The only feasible way to achieve this variation in such a crystalrich mush is to remove (filter-press) felsic melt from the magma chamber. At any instance in a magma chamber that is aggrading upward, the lower portion is more crystallised than the upper. Therefore, the upper portion, with less crystals, has a greater potential to lose melt. If melt is expelled, the granite will be dominated by the cumulate crystals and be mafic. Thus, when subjected to compressive deformation, progressively greater amounts of melt were expelled from the upper portion of the Kameruka sheet. Mass balance modelling suggests that the lowermost rocks contain 53% frozen melt and 47% crystals (70% SiOs), whereas the uppermost rocks contain 15% frozen melt and 85% crystals (67% Si02). We conclude that the Kameruka sheet was a crystal-rich mush during much of its history as a magma chamber, and that geochemical variation was generated by "filter-pressing" interstitial melt out of the mush during syn-plutonic squeezing associated with compressive deformation

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

Fe-Si-Mn OXIDES OF THE PACMANUS SEAFLOOR MASSIVE SULFIDE FIELD, EASTERN MANUS BASIN, PAPUA NEW GUINEA Sonja Heath^ Christopher J. Yeats^ and Raymond A. Binns^ 'Department of Geology and Geophysics, The University of Sydney, N S W 2006 ^ CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 1670 Actively forming, felsic-hosted seafloor massive sulfide mineralisation in the back-arc basin environment is considered to be the best modem analogue for ancient volcanic-hosted massive sulfide (VHMS) deposits. Modem hydrothermal sulfide fields are universally associated with exhalative Fe-Si-Mn oxide accumulations and these are thought to be the protoliths for the Fe +/- Mn-rich exhalites that are often spatially and temporally related to ancient VHMS mineralisation. Fe-Si-Mn oxides associated with polymetallic sulfide mineralisation at the felsic-hosted PACMANUS seafloor hydrothermal field in the Manus back-arc basin are deposited rapidly from low temperature (<100oC) hydrothermal fluids, forming fields of mounds and chimneylets, as well as cmsts and stains on inactive chimneys and volcanic talus. The oxides are deposited across a redox gradient as the reduced, warm hydrothermal fluid rises and mixes with cold, relatively oxidised seawater. Yellow, amorphous Fe oxyhydroxide with opaline silica and minor green nontronite is initially deposited and this is overgrown by black Mn oxides, predominantly todorokite and bimessite, forming an outer cmst. The development of filamentous silica microstmctures in the oxides indicates there has been a contribution from microbial biocatalysis to the deposition of the oxides. Oxides from PACMANUS fall into two chemical groups: Fe203-Si02 rich material, which corresponds to the yellow-orange Fe oxyhydroxides, and MnO-rich material, corresponding to the black-grey Mn oxides. Ti02 and AI2O3, which are generally present in low abundance in both chemical groups, show a strong positive correlation, indicating they are sourced from physical contamination of the oxides by dacitic volcanic fragments. The oxide samples also contain up to 1% P2O5, 2% CaO and 2-10% Na20, which are partially hydrothermal in origin. Chemical partitioning between Fe- and Mn-rich oxides is the dominant factor controlling the trace element distribution of the oxides. At PACMANUS, the Fe-Si oxides have a much stronger hydrothermally derived trace element signature than the Mn oxides, which predominantly show enrichment in elements that are typically scavenged from seawater and incorporated into hydrogenetic oxides. Only Sb appears to be consistently enriched in both hydrothermal Fe-Si and Mn oxides, relative to hydrogenetic samples collected from elsewhere in the Eastem Manus Basin. Fe oxyhydroxides from PACMANUS have a slightly LREE-enriched chondrite-normalised pattem with a positive Eu anomaly. This pattem is typical of modem seafloor hydrothermal systems and is readily explained by feldspar destructive alteration in the footwall volcanics. Mn oxides show a similar LREEenriched pattem with a positive Eu anomaly and a negative Ce anomaly, indicating that a seawater signature overprints a hydrothermal one. When considered in conjunction with the trace element geochemistry of the PACMANUS oxides, this suggests that hydrothermal signatures in exhalites that may act as vectors to ore for ancient VHMS deposits are likely to be best preserved in Fe-rich exhalite horizons. Like ancient exhalite horizons, the PACMANUS Fe-Si-Mn oxides are thin (<1 m), discontinuous, Fe +/Mn-rich layers that have a close stratigraphic association with massive sulfide mineralisation. Filamentous microstmctures similar to those observed in the PACMANUS oxides are present in ancient exhalites. The major element geochemistry of the PACMANUS Fe-Si-Mn oxides is similar to that of ancient exhalites, as are the REE abundances and chrondrite-normalised REE pattems. However, with the exception of the relatively young Kuroko exhalite, the trace element chemistry of ancient exhalites differs considerably from that of the PACMANUS oxides. This suggests that diagenetic and metamorphic processes readily modify the original trace element geochemistry of Fe-Si-Mn oxides, making it difficult to use exhalite geochemistry as a targeting tool for mineral exploration.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TESTING OF SYDNEY SANDSTONE J.L.Heiman CSIRO Building, Construction and Engineering, Sydney

Sydney sandstone is prone to deteriorate and decay in the city's polluted atmosphere, and these processes have been exacerbated by lack of maintenance over many decades. Because the Eastern Suburbs and Pyrmont quarries which supplied the prized yellow block sandstone have all closed, restoration stone has to be obtained from quarries with a shorter record of in-service performance. The new stone must therefore be carefully assessed for strength, durability and weathering characteristics. The laboratory tests used in making this assessment are briefly described. However these tests must be supplemented by onsite building monitoring, since the processes of stone decay are poorly understood. In many case the location and drainage of the masonry elements have more effect on the long-term deterioration than the properties of the stone itself

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SALT ATTACK AND TREATMENT IN SANDSTONE AND BRICK STRUCTURES J.L. Heiman^ and K. W. Riley^ ^CSIRO Building, Construction and Engineering, North Ryde NSW ^CSIRO Energy Technology, Lucas Heights NSW

This paper summarizes investigations into sandstone and brick buildings damaged by salt attack. Of the 23 structures examined in this survey, 18 were built with sandstone, the remainder of brick; 19 of the buildings were in the Sydney district and constructed of Hawkesbury Sandstone. The data obtained over two decades are reviewed. The salt contamination (chorides, nitrates and sulfates of calcium, magnesium, potassium and sodium) found in Sydney buildings is deposited by windblown aerosols and rain or from rising damp. Deposits of sulfates are mainly found on the upper levels of buildings and isotopic analysis indicates that these are from fossil fuel combustion. Nitrates are more likely to be present 1 -2 m above the ground, where rising damp has carried contamination from leaking sewage pipes or fertilised gardens. The distribution of salts is related to their relative solubilities. It is shown that the effectiveness of the desalination techniques, water flushing, poulticing and the use of sacrificial renders is dependent upon a number of factors. These include the solubility, concentrations and depths of the salts in the sandstone. Desalination of Sydney sandstones can be difficult, since the stone often has a low permeability because of the presence of clay, and salts at depths greater than 50 mm are not easily removed.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HUNTER-BOWEN OROGENESIS IN THE HODGKINSON PROVINCE OF THE NORTHERN TASMANIDES Robert A. Henderson and Brett K. Davis School of Earth Sciences, James Cook University, Townsville, Queensland 4411 Delta Gold, 293-297 Hay Street, Kalgoolie WA6433 Penetrative deformation induced by the Permian-Triassic Hunter Bowen Orogeny is well established for the New England Fold Belt but its perpetuation westward into rocks of older convergent systems is not characteristic of the Tasman Orogenic Zone. For the Hodgkinson Province at the northern extremity of the zone, however, penetrative fabrics of Hunter Bowen age are developed at regional scale in low grade Early to mid Palaeozoic metasediments of flysch association. These rocks had previously been pervasively tectonised by Late Devonian-Early Carboniferous orogenesis. Hunter Bowen fabrics overprint those of earlier generation and are intimately associated with widespread granitoid plutonism. The timing of fabric development in orogenic systems is difficult to substantiate, especially where metamorphism is of low grade. The approach we have taken is to examine microstructures developed within the structural/metamorphic aureoles developed adjacent to granitoids of known age as established by isotope geochronology. The record of fabrics preserved within andalusite and cordierite porphyroblasts which grew synchronously with granitoid emplacement provides clear evidence of fabric age. In addition, fabrics overprint some dated granitoids and are variously developed in scattered cover sequences which contain Permian fossils. The earliest Hunter Bowen fabric generation (S3) was gently inclined and considered to reflect crustal extension. It is very widely represented, both at micro- and meso-scale and was spatially and temporally associated with early-phase plutonism dated as Early Permian. The development of this structural phase is attributed to upper crustal collapse following crustal thickening due to plutonism and associated heat flux. The development of Hunter Bowen contractional (S4) fabrics in the Hodgkinson Province is strongly influenced by a broad WNW-ESE trending structural corridor, the Desailly Structure which bisects the terrane. The corridor is up to 30 km across and is characterised by a population of veins, discrete faults and granitoid apophyses with a common orientation. To the north of the Desailly Structure, S4 is variously developed in nine separable structural zones which extend across the province. It is commonly represented as a pervasive crenulation cleavage but in some tracts its development is coaxial with, and inseparable from, pre-existing Late Devonian-Early Carboniferous (S2) foliations. Structural/metamorphic aureoles around several granitoid plutons are largely of D4 generation and a number of plutons are overprinted by either zonal or pervasive S4 fabrics. To the south of the Desailly structure, D4 fabrics are restricted to the eastern sector of the province and Late Carboniferous and Permian cover sequences to the west are unaffected. The profound partitioning of D4 expression indicates the structure represents a translational zone, separating crustal sectors which accommodated different amounts of Hunter Bowen shortening strain. The age range of granites contemporaneous with, or overprinted by, S4 indicates that the Hunter Bowen Orogeny persisted for perhaps 40 Ma in the northern Tasmanides, embracing most of the Permian Period and possibly extending into the Early Triassic. Granite - country rock relationships are remarkably heterogeneous across the province and are unrelated to pluton size, geochemical characteristics, age or location. Steeply inclined pre-existing (D2) structures are considered to have guided the ascent of magma into the upper crust where gently inclined D3 fabrics sponsored the formation of laccolithic magma chambers. The imposition of D4 strain caused the reshaping of magma chambers. Over-pressuring induced by shortening strain of the country rocks caused some to be mobilised upwards as diapirs which developed concordant relationships and substantial structural/metamorphic aureoles. For others, over-pressuring caused the roof to lift by hydraulic means with wall contacts acting as shear surfaces of displacement and discordant country rock relationships prevail. The variable development of S4 fabrics within plutons suggests complex interactions between pluton size and heat budget, and the spatial and temporal partitioning of strain and strain rate. Expression of the Hunter Bowen Orogeny in the Hodgkinson Province suggests that the pattern of plate convergence which characterised this orogenic episode in N S W and southern and central Queensland continued to the north. It follows that elements of the New England Fold Belt are likely to be represented on the submerged crustal sector of north Queensland represented by the Queensland Plateau.

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THE TANAMI REGION: A POSSIBLE PALAEOPROTEROZOIC TRANSPRESSIONAL PASSIVE MARGIN IN NORTHERN AUSTRALIA. M.A. Hendrickx, L.C. Vandenberg, K.R. Slater, A. Crispe, A. Dean, J. Smith and A. Wygralak Northern Territory Geological Survey ( N T G S ) , Alice Springs. Palaeoproterozoic rocks in the Tanami Region occur as widely separated, discontinuous, deeply weathered or silicified outcrops which complicate the task of understanding the geology of the region. Preliminary results of integrated geological studies by NTGS indicate the geological history is similar to the Pine Creek and Halls Creek orogens of northern Australia. In the Palaeoproterozoic we have identified a possible early rift stage (MacFarlane Peak Group) dominated by mafic volcanic and volcaniclastic rocks with minor clastic sediments and calcsilicates. MacFarlane Peak Group is overlain by a thick succession of clastic sediments (Tanami Group) representative of a passive margin sequence. The lower Tanami Group comprises carbonaceous siltstone with minor banded iron formation and calcsilicate (Dead Bullock Formation), characteristic of deposition in an oxygen starved basin. The upper Tanami Group is a thick monotonous sequence of turbiditic sediments (Killi Killi Formation). Dolerite sills intrude both MacFarlane Peak Group and Tanami Group. They predate the first deformation event in the Tanami Region. MacFarlane Peak and Tanami groups were deformed, metamorphosed and intruded by granite during a major orogeny around 1845-1840 Ma. This event is associated with four deformation events (Dl-4) together with greenschist to amphibolite facies regional metamorphism (Ml). D1 structures are variably orientated due to later deformation and are characterised by asymmetric, disharmonic F1 fold couples. The morphology of the SI axial planar fabric is dependent on lithology and ranges from a discontinuous, anastomosing dissolutionstyle cleavage to a slaty cleavage. Fabric-porphyroblast overprinting relationships indicate that Ml occurred late syn-to post-Dl. D2 fold structures are oriented mainly north. Within pelites and lower grade equivalents the S2 fabric is a spaced crenulation and/or crenulation cleavage. Rare oblique F1 and F2 fold interference structures are also observed. D3 faults cut D1 and D2 structures, however the kinematic history of D3 remains uncertain. D4 structures are characterised by northeast to east striking chevron folds and kinkband structures. S4 fabrics are generally fracture style cleavages within siliceous units. These observations suggest that basement rocks formerly at mid-crustal depths during D1 and D2 were by D3 and D4 times at significantly higher crustal levels. Major deformation during D l - 4 was followed by localised zones of extension (D5), forming small narrow basins which were filled with shallow marine sediments in the west (Pargee Sandstone) or pillow basalt and turbiditic sediments in the east (Mount Charles Formation). Widespread granite intrusion and volcanism occurred between 1830-1810 Ma. This is represented in the Tanami Region by three granite suites (Coomarie Suite, Winnecke Suite and Frederick Suite) and two volcanic complexes (Mount Winnecke Formation and Nanny Goat Volcanics). D6 structures formed after the emplacement of the Coomarie Suite (-1810-1800 Ma) and are spatially coincident with several significant gold deposits. In the Tanami mine area numerous small scale brittle D6 faults form a network of structures mineralised at upper crustal levels. Gold mineralisation at The Granites and Dead Bullock Soak is coincident with localised post D l - 4 shear zones within suitable lithologies and fold structures, at a deeper crustal than the Tanami corridor. A series of post orogenic I-type granites (Granites Suite) were emplaced post D6 between 1800-1795 Ma. The Birrindudu Group, a 2 km thick platform sequence dominated by quartz arenite with minor carbonate, was deposited onto a subdued landscape sometime after intrusion of the Granites Suite (-1790-1700 Ma?). D7+ thrust faults and oblique slip thrusts cut all earlier structures and have caused folding within the overlying Birrindudu Group cover sequence and hence may be younger than - 1 7 0 0 Ma. Offset in the distribution of Cambrian Antrim Plateau Volcanics indicates significant reactivation of structures during the King Leopold and/or Alice Springs orogenies. It is suggested that the change in regional stress conditions (from east-west directed shortening during D1 and D2, to more northwest-southeast to north-south directed shortening during D3-D7+), reflects a shift away from the influence of collisional orogenic processes to the north west (eg Hall Creek Orogenic zone), to the influence of a long-lived crustal scale transpressional shear zone in the southern Tanami Region.

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EXHUMATION AS FAST AS SUBDUCTION? Joerg Hermann Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 The calculation of exhumation rates requires accurate determination of age and pressure in metamorphic rocks. The classical approach to this complicated problem has been by dating a series of minerals, which have different, presumably known closing temperatures. Through an inferred temperature-pressure relationship, time could then be linked to pressure. While this approach might be successful in slowly exhumed, slowly cooled granulite facies rocks, it completely fails when applied to fast exhumation of highpressure rocks. The main reason is that these rocks underwent near isothermal decompression preventing a reliable temperature-pressure correlation. We present here a combined isotopic-petrologic study of metamorphic titanite in order to determine exhumation rates of eclogite facies rocks. Titanite has been dated in-situ with SHRIMP, which has the main advantage that the growth zones of this metamorphic mineral can be easily linked to the paragenesis of the host rock and thus to metamorphic conditions. We dated three titanite generations from calcsilicates of the Dora Maira (DM), an ultra-high-pressure (UHP) unit in the Western Alps that underwent Variscan high temperature metamorphism and Alpine UHP metamorphism followed by retrogression to greenschist-facies conditions. The first titanite generation was found in a sample preserving the UHP paragenesis. Titanite is in textural equilibrium with omphacite and garnet and contains inclusion of omphacite providing evidence that it equilibrated at peak metamorphic conditions ( - 3 7 kbar, 750*C). The second and the third generations of titanite were found in a retrogressed sample with symplectites replacing omphacite. The second generation is situated in titanite cores and coexists with sodic augite and oligoclase. The third titanite generation is in equilibrium with diopside and albite. The calculation of the equilibrium albite -> jadeite and quartz indicates pressures of about 10-12 and 4-5 kbar for the second and third titanite generation, respectively. Dating was preceded by back scattered electron imaging of the titanite crystals in order to detect chemical zoning and mineral inclusions. The three titanite generations have distinct Ti-Al contents making it possible to relate age to titanite generation. Titanite was analysed in-situ directly in thin sections using the SHRIMP ion microprobe, which allowed dating of crystals displaying textural equilibrium with major minerals. The age of the peak metamorphic titanite generation is 35.1±0.9 Ma whereas the second and the third titanite generations yield slightly younger ages of 32.9±0.9 Ma and 31.8±0.5 Ma, respectively. These ages are formation ages and not cooling ages because inherited Variscan titanite cores were found in both samples, indicating that opening of the U-Pb system in titanite was never reached. The combined petrologic-isotopic study on titanite reveals that the UHP rocks of the DM unit were exhumed from 115 to 35 km depth with a mean exhumation rate of 3.6 cm/y. This velocity is significantly faster than conversion between Europe and Africa in the Eocene-Oligocen times (1 cm/y) and compares to fast plate motions only. It is nearly an order of magnitude faster than assumed erosion rates at that times. We suggest that fast travel from 115 to 35 km depth resulted from tectonic exhumation, which was driven by buoyancy and possibly enhanced by slab break off Subsequently, the exhumation of the DM slowed down and proceeded at rates of 1.6 cm/y up to ~17km depth. This still high exhumation rate indicates that exhumation was probably associated with normal faulting and tectonic unroofing. The onset of conglomerate sedimentation in the Alps postdates these tectonic processes and the rise of the Alps concomitant with intensive erosion might be responsible for the final exhumation of the Dora-Maira UHP unit to the earth's surface.

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MESOZOIC-TERTIARY EVOLUTION OF THE NORTHERN AUSTRALIAN MARGIN Kevin C. Hill Australian Geodynamics Cooperative Research Centre, Earth Sciences, La Trobe University, Melbourne 3083 The 1500 km long New Guinea (NG) margin of the Australian continent has been convergent since the Eocene, but has its roots in Triassic and Jurassic rifting. As on the 2000 km long N W Shelf, the rifting phase left a series of inboard Jurassic grabens and a 300-500 km wide outboard belt of extended continental crust, with a highly irregular outer margin. Originally, Cenderawasih Bay and the present day Sepik area were embayments, probably floored by Jurassic oceanic crust like the Argo Abyssal Plain. These embayments separated the Birds Head, Border Mountains and Kubor promontories of extended continental crust, akin to the Exmouth Plateau. At the end of the Jurassic, the continent-ocean boundary in NG was characterised by 200 or 500 km wide orthogonal steps, controlled by the NNE and W N W basement fabric. Triassic-Early Jurassic, rift-related volcanism in NG died during Middle Jurassic to Neocomian thermal subsidence. Aptian-Campanian volcanism in NG and Eastern Australia, indicates subduction beneath the extended NG margin which was terminated in the Late Cretaceous, associated with accretion of the Sepik terrane subduction complex. Late Cretaceous uplift and denudation of the platform area in PNG may have resulted fi-om the cessation of subduction. The Paleocene saw voluminous magmatism in NG associated with oceanic spreading in the Coral Sea and the formation of marginal oceanic basins along the northern margin, perhaps splitting the Cretaceous arc. At this stage the northern edge of the Australian craton was bound by Jurassic rifts, 300-500 km of extended continental crust, 100-300 km of Paleocene oceanic crust and probably the remnants of a Cretaceous arc adjacent to Mesozoic oceanic crust. In the Middle Eocene, Australia commenced its rapid movement northwards converging with the Pacific Plate. Subduction to the north developed within the Pacific Plate, remote ft-om Australia, forming the Philippine and Caroline plates in a back-arc setting. The Philippine-Caroline arc was irregular and offset by transforms, as in the western Pacific arcs today. Thus, as Australia moved north it collided with different parts of the arc at different times. In the Oligocene, the Bewani Mountains, which was part of the Philippine arc, collided with NG, reactivating the Sepik complex. The collision probably created a small orogenic belt as in Timor today, but without interrupting carbonate sedimentation along the inshore NG margin. In the Late Oligocene to Early Miocene the bulk of the Philippine-Caroline arc impinged on the extended NG margin and the Ontong-Java Plateau collided with the Solomons arc causing loss of two subduction zones and a significant plate realignment. This resulted in Early to Middle Miocene oblique subduction of the Solomon Sea Plate to the west beneath NG and wrench tectonics in northern NG, but with continuing carbonate deposition on the inshore areas. In the Early Miocene, extension of the outboard NG margin resulted in the formation of metamorphic core complexes from - 2 0 - 1 7 Ma, subsequently intruded by the Middle Miocene Maramuni Arc. In Cenderawasih Bay, the Jurassic oceanic crust continued subducting to the north beneath a sliver of continental crust sliced off the Border Mountains promontory. A change in plate motion towards the end of the Middle Miocene saw rapid convergence between NG and the Philippine-Caroline plates, terminating Solomons subduction and causing orogenesis throughout NG. In the Late Miocene and Pliocene, the deformation propagated southward building the Fold Belt in NG. The structural style in the mountains was fundamentally controlled by the plate margin offsets and by the orogenesis impinging on the old, cold, strong Australian lithosphere south of the Jurassic rifts. In Cenderawasih Bay, the Jurassic oceanic crust was consumed as the continental sliver collided with the Bird's Neck building the Lengguru Mountains. However, new Plio-Pleistocene oceanic crust developed north of the sliver, flooring the present Cenderawasih Bay. In the Pliocene, the plate convergence became more oblique, reducing compression and creating strike-slip faults in the Mobile Belt. At the western and eastern ends of the orogenic belt, adjacent to new oceanic crust, the mountains collapsed and Pleistocene metamorphic core complexes developed.

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PREDICTING COPPER-GOLD MINERALISATION IN NEW GUINEA Kevin C. Hill ^ R. Dan Kendrick\ Peter V. Crowhurst\ and Paul Cow ' ' Australian Geodynamics CRC, Earth Sciences, La Trobe University, Melbourne, Australia 3083. ^CSIRO Exploration and Mining, PO Box 437, Nedlands, WA 6009, Australia.

It has long been known that some of the world's largest mineral deposits occur along major crustal lineaments and it has been proposed that mineralisation occurred during reactivation of these lineaments. However, predicting the likely locations of mineralisation along a lineament and identifying the prospective lineaments is more difficult. In New Guinea, the upper crust is rich in Pliocene and Late Miocene copper-gold deposits, yet the host intrusives are emplaced mainly in a fold and thrust belt and are of mantle origin, not directly subduction-related. Study of the structure, thermochronology and geodynamic setting of four of the biggest deposits, Grasberg, Porgera, Ok Tedi and Frieda River shows some common patterns that may help the search for future deposits. The New Guinea Fold Belt was formed in the Late Miocene to Pliocene by a compressive event associated with 100-200 kms of plate convergence. NNE-trending, crustal-scale lineaments, orthogonal to the strike of the fold belt, are well defined geologically and are roughly parallel to the structural grain in basement as recorded by aeromagnetic data. If the continent-oceanic suture is identified by the southern limit of the central New Guinea ophiolites, then all the deposits lie along major NNE-trending lineaments defined by >50 km offsets of the suture. These lineaments may reflect original offsets in the continental margin of New Guinea that were reactivated during the compression. Balanced and restored cross sections through three of the deposits show that they are directly associated with crustal-scale Mesozoic extensional fauhs, parallel to the fold belt, which were inverted during the compression. The Frieda deposit, in the Mobile Belt, is more complex, being associated with medium to high grade metamorphic rocks, but ^^Ar/^^Ar cooling ages on fault rocks show that the faults there were also crustal in scale and active at the time of mineralisation. Fission track analyses indicate that the extensional faults underlying Grasberg and Porgera were inverted at the time of mineralisation and that the hangingwall was being uplifted and, at Porgera, denuded at that time. The same may also be true at Frieda River, but the rocks have had a subsequent burial and uplift history. At Ok Tedi, the event is so young that it is difficuh to record associated denudation. When the age of mineralisation is plotted against distance from the basement thrust nearest the mountain front, a correlation is found, with the youngest deposit at the mountain front and the oldest furthest away. Thus, during Late Miocene-Pliocene compression, the crust was thickened, accompanying melting of the underlying mantle, and crustal scale faults were reactivated. Where the crustal faults intersected orthogonal lineaments, local zones of dilation occurred, allowing emplacement of mantle magmas and associated mineralisation. When the deformation propagated southwards, so did the crustal thickening and reactivation of major faults, causing emplacement of younger magmas and minerals.

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IN SEARCH OF THE WOONONA SEAM: A REAPPRAISAL OF THE GEOLOGY OF THE SOUTH AND WEST OF THE SOUTHERN COALFIELD M.B.L. Hill, B.C. Kirby, and S. Cozens N S W Department of Mineral Resources The NSW and the Commonwealth Governments commenced the Southern Comprehensive

Regional

Assessment in mid 1999. The CRA, part of the Regional Forest Agreement process, included six State Forests and a large number of Crown Land blocks on the Illawarra Plateau within the Southern Coalfield. The NSW Geological Survey was called on to provide detailed and accurate resource assessments of these areas and to advise Government as to whether they should be transferred to the National Park Estate. Initial examination of relevant Departmental resource models indicated that the models were inadequate for the intended purpose. Field checking quickly showed that part of the existing stratigraphy published by the Coalfield Geology Council of NSW in 1999 was incorrect and required modification. A project was initiated to remap the southern and western escarpments including the valleys of the Wingecarribee and Nattai rivers and their tributaries, and to correlate across to the east using available borehole data. A fundamental premise of the Southern Coalfield stratigraphy, first suggested in 1961 and accepted by later authors, is that the conglomerate overlying the Shoalhaven Group in the west of the Coalfield is the Marangaroo Conglomerate of the Western Coalfield. The coal seam directly overlying the Conglomerate is currently identified as the Woonona Coal Member, the lateral equivalent of the Lithgow Coal of the Western Coalfield and the Bayswater Coal of the Hunter Coalfield. As a result of the remapping, it can now be shown that the coal seam has been misidentified. The coal seam overlying the conglomerate, from the southwestern part of the Coalfield near Belanglo State Forest and as far north as Bonnum Pic Mountain, is in fact the Tongarra Coal. The sandstone unit overlying the Coal, now named the Wanganderry Sandstone Member, which is currently defined as being laterally equivalent to the Wilton Formation, actually erodes into the Tongarra Coal and is laterally equivalent to the top of the Coal and the lower part of the Bargo Claystone. This unit is hereby redefined as the Bunnigalore Formation. In this paper, the above conclusions are supported by measured sections at key locations and by cross sections across the Coalfield. Plans are presented showing the lateral extent of the major stratigraphic units. Significant changes are suggested to the extent of the American Creek Coal Member, the Kembla Sandstone and the Burragorang Claystone. The nature, characteristics and environments of deposition of the key units are reviewed and reassessed. Implications for the evolution of the Sydney Basin are discussed. Acknowledgement: Publication approved by the Director General, NSW Department of Mineral Resources.

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SWATH-MAPPING AUSTRALIA'S SOUTHEAST CONTINENTAL FOR GEOSCIENCE AND ENVIRONMENTAL MANAGEMENT: AUSTREA SURVEY 2000

MARGIN

P.J. H\\\\ N. Rollet\ G. Bemardel', C. R. Calver^ and M. Alcock' ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania 7018

In February 2000, the Australian Geological Survey Organisation (AGSO) completed a major 2-month seabed swath-mapping and geophysical survey program off southeast Australia for the National Oceans Office (NOO) and Envirormient Australia (EA). The program was completed over two cruise legs, AUSTREA-1 & -2, using the 85-m French oceanographic and geoscience research vessel L 'Atalante. The southeast Australia survey covered 15,000 km and mapped about 170,000 km^ of seabed - an area more than twice the size of Tasmania. The work was done for marine zone planning and management, for Law of the Sea purposes, for assessment of seabed living and non-living (petroleum and mineral) resources, and geological and biological research, as a major step towards implementation of Australia's Oceans Policy and Australia's Marine Science and Technology Plan, and in particular, the development of the Southeast Regional Marine Plan by the National Oceans Office. Data collected included Simrad EMI 2D multibeam swath-bathymetry and backscatter imagery, 6-channel Gl-gun seismic, digital 3.5 kHz sub-bottom profiles, gravity and total field magnetics. Also collected was oceanographic information - XBTs to 1800 m depth and underway ADCP (current), sea surface temperature and salinity measurements. The survey mapped the volcanic slopes of Lord Howe Island and Ball's Pyramid to the 12 nautical mile outer limits of a proposed Marine Protected Area, revealing a rugged terrain of volcanic cones, flows and canyons likely to harbour diverse benthic communities. The steep and narrow rifted continental margin off the NSW South Coast was shown to be deeply dissected by canyons and to contain gigantic continental fault blocks and ?synrift volcanic seamounts and ridges. The survey completed mapping of the huge Bass Canyon complex off southeast Victoria, revealing detailed morphology of tributary canyons up to 1000 m deep adjacent to the Gippsland oil fields. Important fishing grounds of the Southeast Trawl Fishery were mapped off Tasmania, including volcanic and carbonate pinnacle terrain off St Helens, Cascade Seamount on the East Tasman Plateau, volcanic seamounts of the Southern Hills and on the South Tasman Rise, and the heads of canyon systems incised into the sedimented upper slope off west Tasmania. Mapping of the Tasmanian Seamounts Marine Protected Area, south of Hobart, was completed, with thirty additional volcanic seamounts found just east and north of the MP A. The seismic profiles confirmed the existence of potential frontier petroleum basins off the east, south and west coasts of Tasmania. Parts of the deeply-canyoned upper and mid slope of the Otway Basin were mapped off northwest Tasmania, Victoria and South Australia. The Great Australian Bight Benthic Protected Area of the GAB Marine Park was fully surveyed below the 500 m isobath and was shown to be generally a uniform slope, with the gigantic Nullarbor Canyon crossing its southeastern comer, gouged into deformed Late Cretaceous sediments. The vast amount of new information gathered on this survey, plus data from earlier geoscience cruises, will form the basis for further collaborative studies between NOO, AGSO, CSIRO Marine Research and other Australian marine research agencies.

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THE AUSTRALIAN STRESS MAP Richard R. Hillis and Scott D. Reynolds National Centre for Petroleum Geology and Geophysics, University of Adelaide, South Australia 5005

Knowledge of the in situ stress field of the Australian Continent has greatly increased since compilation of the World Stress Map in 1992. Since compilation of the World Stress Map, the number of reliable (A-C quality ranked) in situ stress orientation data for the Australian Continent has increased from 95 to 319, principally by analysis of borehole breakouts and drilling-induced tensile fractures in petroleum wells. The new data reveal that stress orientations are variable across the Australian Continent as a whole. However, within individual provinces (one hundred to a few hundred kilometres), stress orientations are generally broadly consistent. Two techniques have been used to elucidate regional in situ stress trends: the definition of stress provinces and stress trajectory mapping. Both techniques reveal consistent patterns. The western part of the Australian Continent is characterised by broadly east-west oriented maximum horizontal stress (Carnarvon Basin and Perth provinces). Along the northern Australian margin the east-west trend rotates to NE-SW (Canning Basin, Northern and Southern Bonaparte Basin, Irian Jaya and New Guinea provinces). This swing in stress trajectories along the northern Australian margin is broadly paralleled to the south where east-west oriented maximum horizontal stress in the Perth province rotates to NNE-SSW in the Amadeus Basin (central Australia). The Bowen Basin also exhibits NNE-SSW oriented maximum horizontal stress. Moving west to east in the southern part of the continent, maximum horizontal stress rotates from east-west in the Perth Basin to NW-SE in southeastern Australia (Otway Basin and Gippsland Basin provinces). The area of divergence between NNE-SSW and NW-SE maximum horizontal stress orientations in central eastern Australia is characterised by east-west or poorly defined (low horizontal stress anisotropy) regional stress trends (Cooper Basin, Flinders Ranges and Sydney provinces). In most continental areas such as midplate North America, South America and western Europe, regional stress orientations parallel the direction of absolute plate velocity. From this it is inferred that the forces driving and/or resisting plate motion are responsible for regional stress orientations in those continental areas. The new data confirm that, unlike these areas, stress orientations in the Australian Continent do not parallel the direction of absolute plate velocity. Nonetheless, the regional pattern of stress orientation within the Australian Continent is consistent with control by plate boundary forces, if the complex nature of the northeastern boundary of the Indo-Australian Plate is recognised. Considering the Indo-Australian Plate as a whole, there is an anticlockwise rotation of stresses from broadly north-south in India to NW-SE in the vicinity of the Ninety East Ridge to east-west in the Carnarvon and Perth provinces, and finally to NE-SW along the northern Australian margin and in New Guinea. This broad rotation can be accounted for by focusing of stresses orthogonal to the Himalayan and New Guinea continental collision segments of the northeastern boundary of the Indo-Australian Plate.

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RELATIVE TIMING OF DEFORMATION, METAMORPHISM AND MINERALISATION WITHIN THE WILLYAMA COMPLEX. Quinton Hills and Gordon S. Lister Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton VIC 3800 Drillcore samples from two prospects situated north of the Willyama Inlier, which intersect highly prospective Olary and Broken Hill Domain metasediments of the Willyama Supergroup were investigated to establish the relative timing of the deformation, metamorphism and mineralisation of these metasediments. The rocks in these prospects are less deformed and metamorphosed (mid-amphibolite facies) than most rocks in the exposed Broken Hill & Olary Blocks and are therefore more suitable for studying the early tectonic history of the Willyama Complex. Willyama Supergroup metasediments preserved within these prospects have comparable deposition, deformation and metamorphism histories. Microstructural and petrographical analyses show a first static metamorphism ( M l ) that is defined by the presence of a quartz foam texture and randomly oriented biotite and muscovite. This quartz-biotite-muscovite assemblage appears to have been overgrown by the prograde growth of garnet and then andalusite (M2). It is unclear from the microstructure whether M l & M2 are separate thermal events or represent a single prograde event. Many of the M2 andalusite grains that preserve inclusions of the quartz foam texture and randomly oriented biotite and muscovite also preserve inclusions of a quartz-biotite-muscovite foliation (SI) in their outer rim, indicating that the peak of M2 was synchronous with deformation ( D l ) . A second quartz-biotite-muscovite foliation (S2) crenulates SI. This S2/S1 overprinting relationship is commonly included within AlSi205 porphyroblasts (M3- defined by andalusite at Polygonum and sillimanite at Thunderdome) indicating that the Dl event was then overprinted by another high-grade metamorphism (M3) and synchronous deformation (D2). The S2 foliation is locally overprinted by a strongly elongated, fine-grained muscovite fabric (S3) most commonly preserved in discrete shear zones, suggesting that this deformation (D3) occurred as metamorphic conditions were retrograding from the peak-metamorphic M3 (associated with D2) conditions. Zinc, lead and silver mineralisation, characteristic of Broken Hill Type deposits, that is preserved within these prospects was present prior to D2-M3, suggesting a pre- to syn-tectonic relative timing. Copper and gold mineralisation overprints all deformation microstructures and metamorphic assemblages indicating that it formed either during the retrograding of the pressure and temperature conditions of M3 or as the result of a later hydrothermal event suggesting a late syn- to post-tectonic relative timing. Donaghy et al. (1998) recognised a previously unidentified deformational fabric of the same metamorphic grade and relative timing as the quartz foam texture within the lower Willyama Supergroup stratigraphy. The foam texture may have been associated with a deformation that occurred before the SI fabric identified in this investigation. We suggest that the foam texture and gamet-andalusite growth are the result of two separate metamorphic events (Ml and M2 respectively) associated with two deformational events. The three deformational events and associated metamorphic assemblages identified are interpreted to represent the tectonism associated with the Olary Orogeny. M l and the earlier fabric identified by Donaghy et al. (1998) is interpreted to represent earlier metamorphism and deformation, maybe associated with the crustal extension. References Donaghy, A. G., Hall, M. & Gibson, G., 1998. The Palaeoproterozoic Thackaringa Group: Deposition, Deformation and Stratigraphy. Abstract: Broken Hill Exploration Initiative (BHEI) Conference, October 19-21, 1998. Broken Hill, New South Wales, Australia.

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Geological Convention, Sydney, July 2000

MAGMA MINGLING, MIXING, AND FILTER-PRESS FRACTIONATION WITHIN AN ACTIVE SHEAR ZONE, RAVENSWOOD BATHOLITH, NORTHEAST QUEENSLAND. Elizabeth Hoadley and Michael Rubenach Department of Earth Sciences, James Cook University, Townsville, Queensland, Australia. Magma mingling of acid, intermediate and basic plutonic rocks within the Early Ordovician to Early Devonian Ravenswood Batholith was identified as a minor variant within the plutons by regional mapping (Wyatt et al 1970, Hutton & Crouch 1993). A small area near Macrossan, representing the contact zone between Macrossan Gabbro and the Lavery Creek Granite, exhibits the compositional range and intrusive relationships, the occurrence of enclave-rich units, and heterogeneous development of tectonic and magmatic foliations. Interaction of comagmatic mafic and felsic magmas producing mingled and mixed intrusions, although extensive mixing to produce a homogenous hybrid is not present in outcrop. The mingled units were emplaced into a zone of active deformation and shearing, the transition from a net-veined unit to an enclave-rich unit being the result of continuing intrusion and a combination of magmatic and tectonic flow. The combination of textures (the heterogeneous distribution of phases, minerals and grainsizes, rapakivi texture, quartz and feldspar ocelli, zonation and mafic clots), within the diorite units (part of the Macrossan Gabbro) and the enclaves, constitute a "textural assemblage" which is compatible with a magma mixing model (Hibbard, 1991). The strong alignment of enclaves and mineral foliation within the units is primarily magmatic although there is considerable overprinting deformation that indicates intrusion of the magma into an actively deforming area. The matrix of the enclave-rich unit exemplifies the transition from magmatic flow to deformational flow as the units became progressively more crystalline with localization of deformation in the shear zone into which the magmas intruded. The variation in the geochemical composition of the mafic units is attributed to a combination of fractionation and mixing with a felsic unit. The partially crystallised hybridised dioritic unit was intruded by a felsic magma to produce a predominantly net-veined unit. Two groups of enclaves are recognized, the angular 'dioritic' enclaves (which resulted from stoping and assimilation of partially crystallized dioritic wallrock) and mafic microgranular enclaves which have rounded and cuspate boundaries. The diorite enclaves show a variation from uncontaminated diorite to a hybrid intermediate between the felsic matrix and the diorite. The variation in the hybridization of the enclaves reflects the degree of crystallization at the time of inclusion. Multiple synplutonic mafic dykes intruded into the hybrid unit at varying stages during the development of the enclave-rich unit and were disrupted, producing microgranular mafic enclave swarms with chilled and crenulate margins, and dykes which terminated in trains of enclaves, adding to the complexity of the enclave rich unit. Despite the spectacular outcrop patterns of the matrix and the enclave-rich unit, the geochemistry of the felsic units appears dominated by fractionation rather than mixing processes. The variation in felsic composition is interpreted as a product of filter-pressing in the shear zone that produced geochemical trends similar to traditional mineral fractionation. Filter-pressing in an active shear zone (e.g. Wiebe 1980), produced localized efficient separation of crystals and magmas. Small quantities of increasingly fractionated magmas squeezed off during such processes mingled with the enclave-rich unit which produced its characteristically heterogenous nature.

References HIBBARD, M. J. 1991. Textural anatomy of twelve magma-mixed granitoid systems. In: J. Didier and B. Barbarin (eds) Enclaves and granite petrology. Developments in Petrology 13, 431-444. HUTTON, L. J. & CROUCH, S. B. S. 1993. New and revised igneous units in the Charters Towers 1:100 000 sheet areas. Queensland Government Mining Journal 94, 32-47. WIEBE, R. A. 1980. Commingling of contrasted magmas in the plutonic environment: examples from the Nain Anorthositic Complex. Journal of Geology 88, 197-209. WYATT, D. H., PAINE, A. G. L., CLARKE, D. E. & HARDING, R. R. 1970. Geology of the Townsville 1:250

000 sheet area, Queensland. Bureau of Mineral Resources, Australia, Report 127.

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GEODYNAMIC MODELLING OF THE BROKEN HILL MINERALISING SYSTEM Bruce Hobbs, John Walshe, Alison Ord, Yanhua Zhang, and Chongbin Zhao AGCRC, CSIRO Exploration and Mining, PO Box 437, Nedlands, WA 6009 The origin of the Broken Hill ore body is the topic of considerable controversy and there are at least seven ore genesis models in vogue (Parr and Plimer, 1993). Unfortunately none of the data are definitive with respect to one model or another and progress in the area is hindered by protagonists adhering to their pet model by inventing processes that enable a problematic data set to fit their particular model. We attempt here to overcome some of these difficulties by adopting a very simplistic approach and assume that each data set is to be interpreted at face value with no attempt to distort the data so that it ?fits? some predetermined philosophy. The data sets to be taken at face value are: (i) (ii) (iii) (iv)

The carbon isotopic data which are ambiguous and can be taken to indicate either a source of carbon fi-om organic sources and/or from deep crustal/mantle sources. The sulphur isotopic data which indicate a plutonic/hydrothermal source for the sulphur. The lead isotopic data which indicate a crustal source for the lead but perhaps with some mixing with a mantle source. The lead model age which indicates an age for the mineralisation (1675 Ma), 15 million years younger than the SHRIMP U-Pb ages for the host sediments (1690 Ma).

However the errors are such that an origin synchronous with sedimentation is still possible. In the form of model constraints, we also assume that Broken Hill Type mineralisation is a true ?type?, namely, Pb/Zn mineralisation that occurs in what are now amphibolite to granulite grades of metamorphism. This observation implies that the mineralisation is spatially and temporally associated with high grade metamorphism or that the site of mineralisation is associated with diagenesis or low grade metamorphism but is such that late in the geotectonic history that site is predestined to undergo high grade metamorphism. Since the only experimental data available on Pb/Zn solubilities involves oxidised fluids we are forced to develop scenarios that involve these data sets. These data indicate that it is unlikely that circulating fluids would be saturated in both Pb and Zn at temperatures above approximately 270^C. Accordingly, the temperature of the mineralising environment cannot exceed ca. 270^C in these models. These data sets constrain us to models involving mineralisation environments that are post depositional but early in the development of a thrust terrain with the thrusts ultimately over-riding the mineralisation with high grade metamorphism ensuing due to thickening of the crust; perhaps these thrusts are inverted normal faults inherited from an earlier period of extension. The two scenarios we explore in detail involve: (i) supply of H2S and CO2 from a deep crustal source along an early thrust and mixing of this fluid with metal and S04^" bearing crustal fluids; fluid flow is driven solely by topographic loading arising from early thrusting; and (ii) the same source of fluids as in scenario (i) except now an extra driving force involves fluid convection in a lithostatically pressurised fluid system. Both scenarios are capable of producing Broken Hill type orebodies within 1-10 million years. We develop exploration criteria for each scenario involving geographical siting with respect to the thrust systems, alteration patterns in the host rocks, metal depletion halos and host rock permeabilities. References PARR, J.M. AND PLIMER, I.R. 1993. Models for Broken Hill-type lead-zinc-silver deposits. In Kirkham, R.V., Sinclair, W.E., Thorpe, R.I. and Duke, J.M. (Eds.): Mineral Deposit Modelling. Geological Association of Canada Special Paper 40, 253-288.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

STACKED PROTEROZOIC BASINS IN CENTRAL WESTERN AUSTRALIA. R. M. Hocking and K. Grey Geological Survey of Western Australia, 100 Plain Street, East Perth, W. A. 6004 Four Proterozoic basinal successions are present adjacent to the northeast Yilgam Craton, in central Western Australia: the Earaheedy, Scorpion, Collier, and Sunbeam Groups. The Palaeoproterozoic Earaheedy Basin is a 5 km-thick succession that rests directly on Archaean granites and greenstones of the craton. The presently exposed portion is now interpreted as the landward rim of a basin that extended much further north, southeast and east of the Pilbara Craton, and was an active depocentre after c. 1790 Ma. The basin was deformed along the Stanley Fold Belt, probably at c. 1760 Ma (Pirajno et al., 2000) during the second phase of the Yapungku Orogeny of the Paterson Orogen (Bagas and Smithies, 1998). The Scorpion Group is faulted against the Earaheedy Basin, and contains (as clasts) rocks reworked fi-om the Earaheedy Group. The group is a mixed siliciclastic-carbonate-evaporite succession possibly more than 10 km thick, which was deposited in a northwards-deepening coastal setting, sourced from adjacent areas to the south. It is a probable correlative (based on tentative stromatolite biostratigraphy) of the Mesoproterozoic Edmund Group, the lowermost component of the Bangemall Basin that is exposed about 200 km to the west (Martin et al., 1999). U-Pb SHRIMP dating suggests deposition of the Edmund Group began at about 1640 Ma. The Collier Group, a younger and less-deformed succession in the Bangemall Basin, crops out immediately west of the Scorpion Group and may onlap it. In this area, the Collier Group is a ?5 km-thick, sand-rich succession sourced ft-om the Yilgam Craton to the south and southwest. Deposition was again in high-energy coastal environments. The Collier Group predates the Edmundian Orogeny (<1020 Ma) in the western Bangemall Basin, and probably predates the Miles Orogeny of the Paterson Orogen (1132-800 Ma, Bagas and Smithies, 1998) to the northeast, but its age is otherwise unconstrained. The Neoproterozoic Sunbeam Group (northwest Officer Basin, basal Centralian Superbasin) overlies the Collier, Scorpion, and Earaheedy Groups. Its age is between c. 825 and 800 Ma, based on radiometric age determinations of rocks bracketing the correlative Townsend Quartzite and the Tarcunyah Group (Bagas et al, 1995, 1999). The Paterson Orogeny (550 Ma) deformed the Tarcunyah Group and the Sunbeam CrL.jp adjacent to the Pilbara Craton, but had virtually no effect to the south, adjacent to the northeast Yilgarn Craton. The lower part of the Sunbeam Group is a 1 to 2 km thick, sand-rich, coarsening-upward deltaic to fluvial succession, with a source to the east and southeast. The provenance is thus within or east of the present area of the Officer Basin, rather than from the Yilgam Craton. The Musgrave Complex, 400 km to the east, is the nearest area currently east of the Officer Basin but may not have been emergent at that time. The Coompana Block (W. A. - S. A. border) was probably emergent, but is considerably further away. A sediment-transport path this long is excessive for a texturally immature sequence, so a closer source is probable, within the area now overlain by the Officer Basin. If so, the basal deposits in the northwest Officer basin are older than those in the central Officer Basin, and deposition of the basal succession (lower Sunbeam Group, Townsend Quartzite and correlatives) moved progressively eastwards. Limited palaeocurrent data from the base of the Centralian Superbasin succession elsewhere agree with the concept of subsidence starting in the west and progressing eastwards. Regional evaporitic and carbonate intervals above the basal siliciclastics indicate subsequent basin-wide, layer-cake deposition after the initial progressive subsidence. References B A G A S L., G R E Y K . & W I L L I A M S I. R. 1995. Reappraisal of the Paterson Orogen and Savory Basin.

Geological Survey of Western Australia Annual Review 1994-95, 55-63. 1:100 000 sheet. Geological Survey of Western Australia 1:100 000 geological series explanatory notes, 38p. B A G A S L . , G R E Y K . , HOCKING R . M . & W I L L I A M S I. R . 1 9 9 9 . Neoproterozoic successions of the northwestern Officer Basin: a reappraisal. West. Geological Survey of Western Australia Annual Review 1998-99, 39^4. M A R T I N D . M C B . , T H O R N E A . M . & C O P P , I. A . 1 9 9 9 . A provisional revised stratigraphy for the Bangemall Group on the E D M U N D 1 : 2 5 0 0 0 0 sheet. Geological Survey of Western Australia Annual Review 1998-99, B A G A S L. & SMITHIES R . H. 1998. Geology of the Connaughton

51-55. PIRAJNO F , JONES J . A. & HOCKING R . M . 2 0 0 0 . Revised stratigraphy of the Palaeoproterozoic Earaheedy

Group: implications for the tectonic evolution of the Earaheedy Basin, Western Australia. This volume.

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GEOMORPHER:DIGITAL SEISMIC RECONSTRUCTION AS AN INTEGRAL PART OF THE MODERN INTERPRETATION PROCESS. Nick Hoffman Australian Geodynamics CRC: Earth Sciences, La Trobe University, Melbourne 3083.

The goal of any seismic interpretation is to identify and quantify both the structural and stratigraphic components of the signal and to separate their influence on the final observed section. Structure is interpreted in terms of the location and geometry of faults and any associated folding, while stratigraphy is interpreted as a succession of genetically related seismic sequences within an overall chronostratigraphic framework and evolving basin geometry. There is an element of circularity in this process of disentangling the components of the geometric signal. Fault displacements are generally defined by offsets of key reflectors, which requires unique identification of those reflectors within a complex and laterally varying system. Facies changes and growth across faults can dramatically change the nature of reflections and confuse the correlation. Conversely, the internal geometry of a seismic sequence and its contact relations with over- and underlying sequences can be confused by faulting and folding. In extreme cases, the deformation may totally mask the sequence stratigraphy. Although Interpreters have been trained to allow for and correct many of the artefacts and points of confusion, the final product is often subjective and non-unique. In some ways, the progress of digital workstation-based interpretation technology has made things more difficult since sophisticated digital comparison tools are not available. Before workstations, paper seismic data was routinely cut along fault planes and respliced to correct for fault movements. What is required is a quick and simple tool that permits reassembly of the digital seismic data in the same way. A number of tools are available that allow digital reassembly of seismic data but none is yet totally integrated into the interpretation process and the workstation software. Most of the tools are slow, expensive, or unavailable outside of specialist consortia. In order to fill this void, a simple desktop tool is presented here that permits rapid and robust restoration of extracts from seismic sections. The product can be viewed as "before and after" images, or as a digital movie simulating the deformation process. Unlike paper restorations, many horizons can be restored simultaneously to produce a chronostratigraphic-like dataset. Examples are shown from Australia and worldwide demonstrating dramatic improvements in the interpretation of severely deformed strata. Detailed seismic sequence analysis can be carried confidently through salt domes and compressional folds. Other examples illustrate improved accuracy of correlation through extensional fault systems and across major facies belts. Acknowledgments: This work originated while the author was employed by BHP Petroleum, and some of the data examples are courtesy of BHPP. Continued support by the Australian Geodynamics CRC and further data examples from AGSO are also acknowledged.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CRYSTALLIZATION OF ORBICULAR GRANITE John P. Hogan' and Tom Dewers^ ^ Department of Geology and Geophysics, University of Missouri-Rolla, Rolla, Missouri 65409-0410, USA ^ School of Geology and Geophysics, University of Oklahoma Norman, Oklahoma 73019-0628, USA

Orbicular granite occurs locally along the margin of the coarse-grained A-type Reformatory Granite of the Wichita Mountains Igneous Province, southwestern Oklahoma, USA. Orbicular granite is well developed adjacent to stoped blocks of finer-grained granite and of rhyolite that are penecontemporaneous with Reformatory Granite. Orbicular granite crops out in an area <10 m^ in which orbicules with radii <1 mm to >40 mm are abundant. The orbicules are spherical domains comprised of multiple alternating concentric shells. Individual shells consist of intergrowths of radial arranged feldspar and quartz grains, and minor equant oxides. Core to rim traverses across each shell are marked by systematic changes in size and in aspect ratio of quartz and feldspar grains. Grains adjacent to the interior boundary are extremely fine-grained and "equant". Traversing the interior of the shell grains increasingly become thicker, elongate rods. In addition, the concentration of fluid-inclusions(?) increases abruptly and dramatically in the quartz rods, and to a lesser extent in feldspar grains. The outer portion of each shell is marked by a gradual return to equant grain shapes, yielding a microgranite texture, and a gradual reduction in inclusion concentration to essentially nil. Variation in inclusion density produces a macroscopic color pattern with darker zones being richer in inclusions, and presumably implies local and transient saturation of the boundary layer. Changes in grain morphology consistently correspond with changes in inclusion density within the shells. Unraveling formation of, volumetrically insignificant but texturally striking, orbicular granite has potential to yield insight into the conditions leading to formation of typical granite textures common to the interior of plutons. We argue that textural variations characteristic of these orbicules develop from oscillatory changes in growth rates and in nucleation rates arising from an internal reaction-diffusion instability that induces cyclic changes in compositional undercooling, or in super-saturation, at the crystal-meh interface. This contrast with some models of orbicule development which call upon externally driven changes in intrinsic variables to induce the alternating textures, such as episodic eruption induced pressure changes. A kinetic model accounting for nucleation, growth, and species diffusion developed for the albite-H20 system demonstrates this instability. Rate laws for nucleation, crystal growth, and species diffusion are parameterized using experimental data from the litterature. A linear stability analysis and numerical simulations of the model show that oscillatory crystallization can develop in such systems with increasing undercooling, by a supersaturation-nucleation-depletion cycle. An examination of parameter interdependencies suggests that the instability is most pronounced at undercoolings of 100 to 200''C and intermediate H2O contents, will dampen with increasing H2O contents up to saturation, and will be nonexistent with little to no H2O is present. This is consistent with the common occurrence of rare orbicular textures along the upper contacts of plutons, a locality subject to rapid cooling rates and accumulation of escaping volatiles.

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G E O L O G I C A L S O C I E T Y OF A U S T R A L I A , A B S T R A C T S N o . 5 9 1Australian Geological Convention, Sydney, July 2000

A STRUCTURAL AND MET AMORPHIC PROFILE OF THE ARTHUR LINEAMENT AND SURROUNDS, NORTHWEST TASMANIA Oliver Holm Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001, Australia. The Arthur Lineament is a NE-trending Early Palaeozoic high strain metamorphic belt in NW Tasmania. It separates shallow marine siliciclastic sediments and dolerite sills/dykes of the Neoproterozoic Rocky Cape Group and unconformably overlying carbonates, shelf sediments, metabasalts and mafic volcanogenic metasediments of the Late Neoproterozoic Ahrberg Group (to the west) from Late Neoproterozoic turbidites of the Bumie/Oonah Formation (to the east). The major lithological unit in the Arthur Lineament is the Timbs Group, which is dominated by metamorphosed psammite and psammopelite units. Some units of the Timbs Group contain basalt/dolerite-derived tholeiitic amphibolites and are intruded by minor deformed 777±7 Ma granitoids. Northwestern Tasmania underwent several changes in tectonic setting in the Neoproterozoic and Early Palaeozoic, being subjected to the Wickham Orogeny {WO) at 760-780 Ma, and the Tyennan Orogeny (7D) at 510±10 Ma (Turner et al., 1995, Turner et aL, 1998). Several subsequent deformational episodes occurred, resulting in a series of complex faulting and folding patterns. The WO featured uplift, generation of tholeiitic basalts and sediment shedding and minor granitoid emplacement. This occurred to passive continental margin setting. There is no recognised mesoscopic structural deformation attributed to the WO in Tasmania. The lithologies generated during the WO were later deformed during the TO, resulting in the Arthur Lineament. The TO is regionally significant in western Tasmania. It has been correlated with the Ross Orogeny in Antarctica, the Delamerian Orogeny in South Australia and episodes of the Pan-African Orogeny. The TO has been interpreted to represent the aborted subduction of the Late Neoproterozoic passive margin in an arccontinent collision, followed by exhumation during the post-collisional re-equilibration (Berry & Crawford 1988, Crawford & Berry 1992). In the Arthur Lineament the TO has produced two deformations (TD, 2), both of which are high strain, and schistose textures which formed at a blueschist- and lower amphibolitefacies metamorphic peaks respectively. The highest grade assemblages are found in the easternmost units of the Timbs Group. Structural analysis of the Arthur Lineament indicates a north over south transport direction. rS, is axial planar, inclined (50/150) and is associated with moderately plunging (60/150) TP, isoclinal folds. Pre-7^2 thrusting is interpreted to be synchronous with the 7Di event. In areas of intensely developed TD2, TPj is unrecognisable. TD2 produced an axial planar fabric (7^2) and tight to isoclinal folds (rF2)- On the north coast TS2 and TF2 undergo rotation with increasing strain. In areas of moderate strain, several hundred metres to the east of the lineament, 7^2 is recumbent to moderately inclined, dipping south (30/200) with fold axes gently to moderately plunging west (15/280). In the high strain areas proximal to, and within the lineament, TS2 is gently to moderately inclined, dipping southeast (30/120) with fold axes gently plunging south (15/200). Widespread albitisation is a feature of the Arthur Lineament and is synchronous with TD2. West of the Arthur Lineament, the Rocky Cape and Ahrberg Groups are characterised by a west over east folding and thrusting event, possibly of Cambrian age. This has resulted in close folds with a gentle to moderate plunge to the south (40/190) and a moderately inclined slaty axial planar cleavage (40/250). A mid-Devonian event is correlated with the Tabberabberan Orogeny of Victoria, and overprints the Rocky Cape and Ahrberg Groups, parts of the Timbs Group, and the Bumie/Oonah Formation. It is responsible for two relatively weak deformational events that result in dome-and-basin interference patterns. Structural, metamorphic, whole rock- and mineral-chemistry analysis to assess the validity of the model proposed by Berry & Crawford (1988) and Crawford & Berry (1992) is ongoing. REFERENCES BERRY R.F. & CRAWFORD A.J. 1988. The tectonic significance of Cambrian allochthonous mafic-ultramafic complexes in Tasmania. Australian Journal of Earth Sciences, 35, 523-533. CRAWFORD A.J., & BERRY R.F. 1992. Tectonic implication of Late Proterozoic-Early Palaeozoic igneous rock associations in western Tasmania. Tectonophysics, 214, 37-56. TURNER, N.J., BLACK, L.P., & KAMPERMAN, M. 1995. Pre-Middle Cambrian stratigraphy, orogenesis and geochronology in western Tasmania. Geological Society of Australia Abstract Series, 39, 51 -56. TURNER, N.J., BLACK, L.P., & KAMPERMAN, M. 1998. Dating of Neoproterozoic and Cambrian orogenies in Tasmania. Australian Journal of Earth Sciences, 45, 789-806.

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SEQUENCE AND KINEMATICS OF MULTIPLE DEFORMATION IN THE TAEMAS BRIDGE AREA, NSW: A PRELIMINARY OUTLINE David A.I. Hood and David W. Dumey Dept. of Earth and Planetary Sciences, Macquarie University, NSW 2109

Detailed mapping and meso-structural study in deformed shelf carbonate sediments of the Lower Devonian Murrumbidgee Group in the Gilgandra-Cowra-Yass Zone of the Eastern Lachlan Fold Belt near Taemas Bridge, Yass, New South Wales, has revealed up to four distinct folding events with both thrust- and wrench-kinematic deformation styles (Hood, 1996; location to be visited in Post-Convention Field Trip FA2). A post-Late Devonian or Early Carboniferous (Kanimblan) age for the events can be inferred from an earliest (Fl) fold influence on semi-concordant Middle to Upper Devonian strata in adjoining parts of the Black Range Synclinorium. Fold sequence at Taemas Bridge, on the eastern margin of the Synclinorium, was determined from map-scale Type 1 (dome-basin) and Type 2 ('banana') interference relations between the folds. Kinematic (incremental X-Y-Z strain axis) history of the main (F2 and F3) events was deduced from sequences of stylolites, extension-veins, vein-arrays, minor faults and bedding slip-fibres in limestone beds. Fl folds are mostly broad, gentle to open, upright folds, in places close and overturned, with a partially obscured overall NE-SW trend. They are poor in meso-structures but account major plunge reversals of the later F2 and F3 folds. F2 folds are close upright N-S to NNE-SSW-trending folds which may show early thrustkinematic deformation but are mainly of wrench character (with NNE extension), suggesting sinistral transpression or shear motion parallel to the bounding NNW-trending Deakin Fault. NNW-SSE to NW-SE-trending open to close, upright to E-overtumed, F3 folds may alternate with domains of F2 folds, merge with them, or overprint them by 'banana' folding. F3 folding was thrust-kinematic for most of its history, including early layerparallel contraction and later steep-limb attenuation, but may also display late-stage wrench deformation (with NNW extension). This system corresponds to the NNWtrending Black Range Synclinorium major structure and its associated reverse faulting in this region and indicates a mainly simple contraction normal to the Synclinorium and to the Deakin Fault. F4 folds are gently to steeply WNW- to W-plunging dome-basin and 'banana'-style gentle warps of F2 and F3 folds and fold-limbs, with possible associated wrench deformation and E-W extension, attributable to weak submeridional shortening. These observations suggest that further information about the history of a tectonic episode can be gained from examination of rocks that are sensitive to development of incremental strain indicators. Such information may be important for constraining models of tectonic evolution. Thus we show that this region was subject to a complex multi-phase contraction and wrench history rather than a single 'transpressive' event. Reference Hood D. 1. A. 1996. Sructural history of the Early Devonian Cavan area. New South Wales. Unpubl. BSc. Hons, thesis, Macquarie University, Sydney.

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DETERMINING THE EFFECTS OF LAND USE ON WATER QUALITY IN THE FITZROY RIVER BASIN. OPPORTUNITIES FOR GEOSCIENCE BASED TECHNOLOGY. Anthony M. Horn Department of Natural Resources, 80 Meiers Road, Indooroopilly, Brisbane, Queensland, 4068 Identifying and managing the effect of anthropogenic (human) activities on natural waterways is becoming increasingly important from an ecosystem protection and an economic point of view. As well as increasing community pressure to preserve aquatic communities, an emerging issue is the International Standard Organisation's standard ISO 14 000, which is developing the requirement that many local exports are produced in an Environmentally Sustainable Development (ESD) way. However, there are considerable challenges in determining what effect anthropogenic activities are having on natural water systems, particularly in the absence of long term pre-development datasets. In regards to stream water quality, these challenges include the identification of natural or unimpacted water quality conditions and determining whether any excursions fi-om these levels are significant, are a long term trend or are merely a function of climatic oscillations. Technologies from the geosciences are well placed to help isolate land use related signals from those associated with natural catchment processes. Several approaches are being used in the Fitzroy River Basin as part of a broader study looking at the effects of land use on water quality. Sediment transport rates are being investigated in the Fitzroy River itself and in the Dawson and Comet tributary systems. Samples are taken during flood events and bed load, suspended load and nutrient (Total Nitrate and Total Phosphate) loads are calculated. Additionally, theoretical transport rates are being determined at several other locations using less precise, but still useful empirical formulae. Early indications are that nutrient concentrations can be reasonably well correlated with suspended loads in grazing lands. The provenance of the suspended load within ten tributary systems is being investigated through analysis of the constituent clays using X-ray Diffraction (XRD) and Inductively Coupled Plasma Mass Spectrometer (ICPMS) analysis. It is expected that these techniques will permit the identification of the major source areas of suspended material. Additional field investigations may be required to determine whether the source of clays is likely to be predominantly from the stream bed and banks, or from gully and sheet erosion within the catchment. Investigations of groundwater level trends and salinity concentrations have also been undertaken to determine the sustainability of abstraction rates and the possible effects of land clearing. Excessive depletion of groundwater resources can have an effect on baseflow levels in streams and increasing groundwater salinity levels may impact on stream water quality. Additionally, water types based on bulk water chemistry have been determined using cluster and principal component analysis techniques. These water types are useful for identifying the dominant natural chemical influences on water quality including the relative contribution of certain major lithologies such as basalts, granites or Mesozoic sediments These geoscience based approaches when combined with catchment land use maps, the regional development history, stream hydrographs and climate records can provide some useful insights into the potential land use effects on water quality. In turn, this will allow the development of more appropriate management strategies to maximise economic returns while maintaining high quality aquatic ecosystems.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 !5"" Australian Geological Convention, Sydney, July 2000

STABILITY OF CONTINENTAL LITHOSPHERE UNDER TRANSPRESSION: THE NORTHERN MARGIN OF THE AUSTRALIAN CONTINENT Gregory A. Houseman Department of Earth Sciences, PO Box 28E, Monash University, VIC 3800

Continental lithospheric mantle is generally denser and therefore is potentially gravitationally unstable with respect to the mantle convection system which it overlies. That instability might take the form of a buoyancy-driven overturn in which cold dense mantle lithosphere sinks into the upper mantle and hot asthenosphere rises up to underplate the residual lithosphere. Nevertheless, from observation we know that the continental lithosphere generally does not participate in the mantle convection system, but maintains stability for long periods of geological time. The simplest explanation of this apparent stability relies on our understanding of the thermal and dynamic equilibrium of a stratified fluid system which obeys a non-Newtonian viscous constitutive law. Buoyancy-driven instabilities which initially grow exponentially in a Newtonian fluid display a power-law growth if viscosity is non-Newtonian. At early stages of growth the displacement rates are vanishingly small, but the displacement rates grow as the third power (for olivine) of the displacements. With sufficient growth there is eventually a catastrophic and apparently sudden growth of the instability, leading to the lower part of the lithosphere being removed in one or more blob-like downwellings. Acting against the growth of this hydrodynamic instability, thermal diffusion can remove the driving force for instability by damping out any horizontal variation in temperatures within the lithosphere. The result of these two competing factors is referred to as a finite-amplitude instability: the lithosphere is stable provided any perturbation is sufficiently small, but if the stratification is disturbed by a sufficiently large perturbation, then it will become unstable. Crustal thickening driven by convergence in a continental environment is the most likely means of initiating a gravitational instability that results in rejuvenation of the lithosphere. In the Australian continent, this process may be occurring today on the northern margin of the plate, where the New Guinea region is subject to a transpressive stress field caused by the interaction of Australian and Pacific plates. Seismicity beneath Papua-New Guinea shows an inverted U-shaped zone with limbs dipping to the north and south (Pegler et al., G. J. Int., 122, 961-981, 1995). Part of this zone may be interpreted as subduction of the Solomon Sea plate, but intermediate-depth seismicity beneath the Papuan Peninsula has also been interpreted as representing the detachment of a lithospheric root following crustal thickening. Numerical experiments in which gravitational instability is initiated by imposed external convergence demonstrate the possibility of downwelling flow occurring on both sides of a convergent zone. The likelihood of this double-downwelling mechanism occurring is increased if the crustal layer is weak but buoyant compared to the mantle lithosphere. Such models can explain in a dynamically self-consistent manner the observed (Pegler et al. 1995) orientafion of earthquake P (compression) and T (extension) axes in the lithosphere and upper mantle beneath the New Guinea central highlands. Taxes in the downwellings are essentially down-dip. Beneath the Finisterre Ranges at depths greater than 70 km, however, extension indicated by earthquake T axes is parallel to the shortening direction at the surface, an observation that can be explained simply if the lithosphere in this region is being stretched and thinned by the lithospheric mantle flowing into the two adjacent downwelling regions.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MALT: THE MURRAY BASIN, ADELAIDE AND LACHLAN FOLDBELT TELESEISMIC SURVEY (1998-2000) Gregory A. Houseman^ Frank M. Graeber', and Stewart A. Greenhalgh^ 'Department of Earth Sciences, PO Box 28E, Monash University, VIC 3800 ^Department of Geology and Geophysics, University of Adelaide, SA, 5005.

Because of its central location relative to the active earthquake belts along the northern and eastern margins of the Australian plate, south-eastern Australia is very well placed for seismological investigations that rely on teleseismic sources. The SKIPPY project made good use of this fact to obtain large scale tomographic maps of the lithosphere and upper mantle beneath the Australian continent, based on the inversion of surface waves. Surface wave inversion is however intrinsically limited in its horizontal resolution because of long wavelength and horizontal propagation paths. In order to obtain better horizontal resolution we have designed and are carrying out the first large scale 3D seismic tomography experiment in Australia based on body wave arrivals from teleseismic events. The object is to construct new high-resolution 3D maps of the velocity structure of the lithosphere and uppermost mantle in south-east Australia. Such maps will help to constrain tectonic models and provide information about thermal and compositional variation within and beneath the lithosphere, potentially imaging major lithospheric structural features. We have targeted for this investigation a broad transect across the Lachlan Foldbelt in Victoria, the Murray Basin and the Adelaide Foldbelt in South Australia. This transect crosses several major structural boundaries including the so-called Tasman line. P-waves from distant earthquakes traversing the lithosphere on sub-vertical paths are analysed to obtain relative arrival time residuals. Given a good range of source distances and azimuths, tomographic imaging techniques can provide good horizontal resolution of the velocity structure, limited only by the Fresnel diffraction limit and the station spacing, in this case on the order of 30 km. The MALT project involves 3 separate arrays, each consisting of 40 short-period verticalcomponent seismic stations deployed (to the extent that vehicle access permits) on a 30 km by 50 km grid. The LF98 (Lachlan Foldbeh - 1998) array covered the western Lachlan Foldbelt in Victoria, from Bendigo and Geelong almost to the South Australian border, a rectangular array of approximately 270 km by 150 km. The MB99 (Murray Basin - 1999) array covered the southern part of the Murray Basin straddling the SA-Vic border, from Charlton in the east to Tailem Bend in the west. The AF2000 (Adelaide Foldbelt - 2000) array crosses the Adelaide Foldbelt from Port Pirie and Adelaide in the west to the border with New South Wales and Victoria. Each of these arrays operated (AF2000 is still operating) for approximately 4 months, recording (continuously, at 20 samples per second) on the order of 100-200 significant plate boundary earthquakes. In addition, a small subarray of 4 broadband triaxial seismographs is providing complementary information about crustal and lithospheric structure based on receiver function analysis. Preliminary results from the tomographic interpretation of the arrival time residuals from the LF98 dataset demonstrate the successful application of the teleseismic tomography technique in Southeast Australia. We have been able to image the variation of seismic wavespeed within the lithosphere beneath western Victoria, revealing features consistent with the known history of recent volcanism and interpretation of surface structures.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HELPING THEM THROUGH: THE USE OF FORMATIVE ASSESSMENT, TRIAL EXAMS AND WEBMCQ TO ASSIST STUDENTS SURVIVE AND EXCEL IN THEIR FIRST UNIVERSITY EXAMS. Thomas C.T. Hubble School of Geosciences, The University of Sydney, New South Wales, Australia, 2006.

Formative assessment is a formal name given to the method of providing students with non-compulsory questions or tests in which the resulting marks not used for assessment. Ideally, student scores for these practice questions or tests are not even recorded by the teaching staff This contrasts with more common assessment and test procedure, known as summative assessment, in which assignment or test marks do contribute to a student's course mark. Therefore formative assessment method is an ideal way for students to ascertain their progress in a course without the risk of recording a poor score. Two complimentary methods of formative assessment have been introduced into first year Geology and Engineering Geology classes at the University of Sydney. These consist of a trial exam program and sets of self-assessment questions presented over the Internet using the package WebMCQ. Trial exams were introduced into all first year Geology classes at the University of Sydney in 1997. They examine course content up to about week nine of the semester and are administered in week ten during practical class sessions. Trial papers have the same format and level of difficulty as the following end-of-semester exam but are only a third to half the length. Trials are marked and returned to students in week eleven. Those students with a mark less than 45% are informed that they are at serious risk of failing the semester one exams and are strongly encouraged to attend an exam assistance tutorial group. Students scoring between 45% and 55% are defined as being potentially at risk of failing and are also encouraged to attend the tutorials. The tutorials that follow the trial exams deal with how questions in both the trial and previous exam papers should be answered. Specific difficulties or problems that students have can also be discussed in the tutes. WebMCQ is a hypertext based self-assessment package that delivers multiple choice questions by way of a web-browser. Students receive an instant response on their answers as well as feedback on the question topic. This system was implemented for Engineering Geology One, in semester One 1999 and the student response has been very pleasing. Exit surveys conducted at the conclusion of the course exams indicate a very positive response to the both the trial exam program and the WebMCQ materials. Students indicate that they these two formative assessment methods greatly assisted their exam preparation. They also indicate that these two programs improved their exam performance.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

VEGETATION, DREDGING AND RIVER BANK STABILITY: EXAMPLES FROM THE NEPEAN RIVER, NEAR CAMDEN NSW Thomas C.T. Hubble School of Geosciences, The University of Sydney, New South Wales, Australia, 2006.

River banks bordering weir lakes on the Nepean River in the Camden Valley have been subject to wide spread waterline toe erosion and mass failure of the upper banks and levees between Theresa Park Weir and Menangle Weir. A combination of several environmental factors appears to precipitate failure rather than a single cause. Contributing factors include: the long term stability of the channel's location during the Holocene; the thinning of river-side vegetation by inappropriate riparian practices since settlement of the region in the early nineteenth century; construction of a series of small water supply weirs in the Camden Valley at the beginning of the twentieth century; changes in the flood-flow regime due to dam construction in the upper catchment between 1900 and 1930; a high frequency of floods since the late 1940s; deepening and widening of the channel by sand dredging operations since the mid-1960s; and in the last decade or so the bottom feeding activities of an introduced fish - the European Carp. Three examples of river bank collapse on the Nepean River in the Camden area are examined. The consequences of human modifications to two previously stable upland river banks are presented and are compared to a relatively unmodified and currently stable bank profile. The three cases are examined and explained using Xslope; (a computer program based on Bishop's Method of Slices). Emphasis is placed in the modelling on estimating the contribution of tree roots to the Factor of Safety by back-figuring. This slope failure modelling indicates that several critical thresholds are probably breached when upper bank mass failures occur. Firstly, the banks need to be saturated and subjected to rapid drawdown, ie. the conditions extant just after flooding. Secondly, vegetation densities must be low, which is the case when mature trees are absent from the banks. Thirdly, removal of a significant amount of the bank toe promotes collapse and mass failure of the upper banks. The consequences of both devegetating, and modifying the geometry of river banks during sand extraction is also examined.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SORTING OF HEAVY MINERAL GRAINS ON BEACHES: ASSESSMENT OF EXISTING HYPOTHESES Michael G. Hughes^ Peter Nielsen^, Jock B. Keene' and Peter J. Cowell' ^ School of Geosciences, Edgeworth David Building (F05), University of Sydney NSW 2006 ^ Department of Civil Engineering, The University of Queensland, Brisbane QLD 4072

Sorting of detrital mineral grains in the swash zone was investigated using data on sedimentology and flow kinematics obtained from Fishermans Beach, on the NSW coast. Samples were taken from a bed at the beach surface that was enriched in heavy minerals, and from the bed below. The mineralogy of the samples was determined and measurements of grain size, settling velocity and a surrogate for grain shape were obtained. The heavy-mineral grains had both a smaller intermediate diameter and smaller settling velocity than the light-mineral grains; the difference between the two mineral types was typically in the range 0.25-0.50 (j) and 2.0-2.5 cms"', respectively. On the basis of these sedimentological measurements alone, any one of three previously hypothesised mineral sorting mechanisms could provide a feasible explanation for the development of the heavymineral enriched bed: suspension, entrainment, or shear sorting. A numerical model for swash, based on the nonlinear shallow-water wave theory, is used to provide a quantitative description of flow kinematics in the swash zone at the time the heavy-mineral-enriched bed was formed. For the probable wave height responsible for producing the enriched bed, modelled maximum flow velocity and bed shear stress at the mid-swash position reached 3.5 ms"' and 79.51 dynes cm"^, respectively. Given the shallow water depths and the lack of sorting in the horizontal direction of flow, suspension sorting does not seem to be of overriding importance at Fishermans Beach. The large bed shear stresses seem to also preclude entrainment sorting, because for most of the time all mineral types are predicted to be in motion. These results conflict with recent studies of lowgradient, fine-grained beaches. Shear sorting was the only sorting mechanism feasible within the modelled flow constraints at Fishermans Beach. New measurements of flow velocities and turbulence from a number of beaches are presented, which confirm the theoretical contention that shear sorting is operable in the swash zone. Nevertheless, our understanding of this sorting mechanism, and the other sorting mechanisms (suspension and entrainment), which may be more effective on gently-sloped beaches, is presently inadequate to model the formation and preservation of heavy mineral enriched beds. We conclude by outlining our approach to developing such a model, and indicate how such a model could be integrated into existing models for large-scale coastal behaviour. These large-scale models have the potential to greatly improve on existing placer exploration methods.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ARCHAEAN EPITHERMAL DEPOSITS IN THE PILBARA CRATON, WESTERN AUSTRALIA David L. Huston', Brett Keillor^ Alan E. Marshall, Jon Standing^ Richard Blewett' and Terry Memagh' 'Minerals Division, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^Resolute Ltd., PC Box 7232 Cloisters Square, Perth, WA 6850 ^Qestore, 21 Viewway, Nedlands, WA 6009 ^Fluid Focus, 23 Milson St, South Perth, WA 6151 After the recognition of epithermal textures in quartz veins by company geologists in the central Pilbara Craton, AGSO has undertaken studies to document these veins and to their distribution in the Pilbara Craton and overlying Hamersley Basin. Epithermal veins, as characterised by textures and trace element geochemistry, formed during two periods in the history of the Pilbara Craton, a minor event at ca 3450 Ma (Miralga Creek; Groves, 1987) and a more significant event, probably at ca 2750 Ma, based on Pb isotope model ages. The deposits studied in detail by this research project formed during this latter event, and are well developed in the central Pilbara and in the Gregory Range along the eastern margin of the craton. We have documented epithermal veins at the Becher, Orange Rock, Sams Ridge and Opaline Well prospects. The hosts to these deposits include turbidite, diorite, high-Mg basalt and tourmaline granite, with ages between 2950 and 2765 Ma. The vein systems examined are generally steeply dipping with north to north-northwest strikes. The most abundant quartz type present in is chalcedony. Generally the chalcedony is massive, but it is also commonly banded and brecciated. Less common quartz types include pseudoacicular, bladed pseudomorphs after carbonate or sulphate minerals, and colloform-crustiform. All quartz types locally show multiple brecciation. At the Becher prospect, bladed limonite pseudomorphs, possibly after siderite form the latest paragenetic stage. At the Opaline Well veins, bladed tluorite pseudomorphs are also present. Whole rock analyses of surface samples from the veins indicate that they are generally anomalous in Sb and As, and some vein systems are anomalous in Bi, Te, W, Hg and base metals. Using the classification scheme of Morrison et al. (1986) for quartz textures and this trace metal assemblage, the surface expression of some of these vein systems is above the boiling zone. Preliminary fluid inclusion data fi-om the Becher system indicates the presence of two fluids, a high salinity (15 wt. % NaCl eq.), moderate temperature (150°C) fluid and a low salinity (<3 wt. % NaCl eq.), slightly higher temperature (200°C) fluid. The epithermal vein systems in the central Pilbara are inferred to be related to the opening of the Hamersley Basin at ca 2770 Ma and associated basaltic and felsic volcanism in the Fortescue Group at the base of the Hamersley Basin. The orientation and inferred age of the vein systems is most consistent with this origin. These Archaean deposits are preserved because the overlying Fortescue Group has protected them ft-om the erosion, and because much of the Pilbara Craton has been geologically quiet since the end of the Archaean. References GROVES I.M. 1987, Epithermal/porphyry style base- and precious-metal mineralization in the Miralga Creek, eastern Pilbara block. BSc Hons thesis, University of Western Australia (unpubl.). MORRISON G., DONG G. & JAIRETH, S. 1990, Textural zoning in epithermal quartz veins. AMIRA PReport (unpubl.).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HYDROTHERMAL VEINS AND ALTERATION, MARVEL LOCH GOLD MINE, WESTERN AUSTRALIA M. Huston and W.K. Witt Sons of Gwalia Ltd, 16 Parliament Place, WA 6005 The poster summarises the hydrothermal setting of lodes at the Archaean Marvel Loch gold deposit, located in amphibolite facies gabbroic and ultramafic rocks of the Southern Cross greenstone belt. Western Australia. The geological setting and structural characteristics of the deposit are described in a separate abstract (this volume). Gabbro-hosted lodes include Main lode, Sherwood lodes and Contact lode in the north pit and East lode in the south pit. The Boulder-New lode system (south pit) is also partly hosted by gabbroic rocks. In the north pit, a 50-400m wide gabbroic unit is characterised by widespread secondary biotite, which increases in abundance towards the western, mylonitised contact with mafic schist and ultramafic rock. The hydrothermal system comprises a complex alternation of various metasomatic bands that range from a few centimetres to several metres thickness. These can be generalised into a number of alteration zones, named after the dominant type of metasomatic band. Characteristics of the dominant metasomatic assemblage in each alteration zone are summarised below. Biotite alteration zone: Homblende+plagioclase+biotite; bio:hbd<l:2; weak to moderate foliation. This distal alteration zone may extend to the eastern gabbro contact and has been noted up to 2 km north of the Marvel Loch pit. Potassic/calc-silicate alteration: Homblende+plagioclase+biotite; bio:hbd=l :2-2:l; strong pervasive foliation and mineral lineation. This zone extends up to 150 m fi-om the mylonitised contact. Potassic alteration: Plagioclase+biotite+quartz, andalusite, sulfides; bio:hbd>2:l; this 10-15 m wide zone is adjacent to the mylonitised contact and is characterised by intense recrystallization, grainsize reduction and mylonitisation. The presence of andalusite suggests leaching of Na and Ca, as well as addition of K. A calcsilicate alteration zone occurs within the Kcs zone, either side of the Main and Sherwood lodes, dividing it into inner and outer parts. It is characterised by the assemblage plagioclase+diopside and is typically massive to weakly foliated but is recrystallized and finer-grained than the metagabbro and Kcs zone rocks. It has a low sulfide content and is not mineralized, but disseminated magnetite is concentrated near the inner transition into Kcs alteration. The eastern calc-silicate zone is about 25 m wide but the western zone is much thinner, possibly attenuated by movement on the mylonitised contact. The gold-bearing Contact, Main and Sherwood lodes are zones of plagioclase alteration, up to 5 metres wide, which produces a bleached rock comprising plagioclase + biotite + sulfides. The lodes are enveloped sequentially by potassic/calc-silicate alteration, calc-silicate alteration and biotite alteration but the symmetry is complicated by the intense potassic alteration associated with the mylonitised contact. The Contact lode contains a prominent quartzsulfide vein and occurs within an envelope of potassic alteration in which biotite has entirely replaced metamorphic amphibole. Veins may be absent in the plagioclase alteration zone of the Sherwood lode but additional gold occurs in association with quartz-diopside-sulfide veins in the proximal potassic/calc-silicate alteration zones either side of the Sherwood lode. Very thin (mm-scale) zones of K-feldspar + hornblende + biotite + plagioclase + sulfides occur adjacent to some of these mineralized veins. Minor disseminated pyrrhotite is widespread throughout the gabbroic host rock but mineralization is generally associated with several percent sulfides, inlcuding arsenopyrite or pyrite, in addition to pyrrhotite. The Contact lode vein contains minor galena and sphalerite, as well as native gold. Ultramafic-hosted lodes including parts of the Boulder-New lode (south pit) are sulfide-bearing diopside-rich veins (+/-quartz, calcite, forsterite, amphibole) These lie within broad zones of potassic/calc-silicate alteration (biotite+tremolitic amphibole+sulfides). In the north pit, the Undaunted lode is a quartz-sulfide vein within a similar alteration envelope. In both pits, proximal potassic/calc-silicate alteration is enveloped by up to several hundred metres of talc alteration (foliation-controlled bands of talc in tremolite+chlorite+serpentine rock).

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HIGH-TEMPERATURE MEDIUM-PRESSURE CRUSTAL ANATEXIS IN THE CAPE RIVER AREA - EVIDENCE FOR ORDOVICIAN OROGENESIS IN THE WESTERN LOLWORTH-RAVENSWOOD PROVINCE. L.J.Hutton Department of Mines and Energy, Queensland The Cape River Province (formerly part of the Lolworth-Ravenswood Province) lies in the northern Thompson Orogen, lying about 100-150km west-south west of Townsville, north Queensland. Aeromagnetic trends in the southern Lolworth-Ravenswood Province are oriented east-west, with marked discontinuity to the north-east trends in the Thompson Orogen in central and southern Queensland. The Cape River area lies close to this discordance. The Lolworth-Ravenswood Province comprises three major Early to mid Palaeozoic granite batholiths, intruding a basement comprising Neoproterozoic to Early Cambrian metasediments and meta-basalts, and Cambrian to Early Ordovician sedimentary rocks. In the Cape River area, the Fat Hen Creek Complex intrudes the Cape River Metamorphics of probable Late Neoproterozoic to Early Cambrian age, and contains a series of Ordovician I and S-type granitic gneisses interlayered with basement gneisses. The presence of S-type granitic gneiss and upper amphibolite grade metamorphism in the Cape River area is interpreted as resulting from Ordovician orogenesis. Dating of I and S-type granitic gneiss from the Fat Hen Creek Complex have yielded Late Cambrian to Early Ordovician ages indicating a major magmatic/metamorphic event at that time. Quartzo-feldspathic gneiss within the Fat Hen Creek Complex may be higher-grade equivalents of the Cape River Metamorphics or may be an older basement. S-type granitic gneisses in the Fat Hen Creek Complex comprise quartz + plagioclase + biotite ± cordierite ± garnet ± K-feldspar ± muscovite. They are generally strongly foliated, forming elongate para-autchonous bodies interlayered with the possible basement gneisses. Migmatites are present but leucosomes are sparse suggesting that melt fractions are low. Peraluminous, quartz-rich, biotite-rich, cordierite-bearing granitoids can form by fluid-absent melting of biotite-plagioclase bearing metasedimentary rocks under upper amphibolite to granulite facies metamorphism Experimental data suggest that partial melting of a pelitic or greywacke source at 5-7Kb and -SOO'^C will produce the mafic mineral assemblage in the Fat Hen Creek Complex. The temperatures and pressures are similar to those calculated using the biotite/gamet geothermometer for the Fat Hen Creek Complex and are also similar to maximum prograde temperatures and pressures determined for the Cape River Metamorphics (Strachotta, 1998). REE patterns and mineral chemistry show the similarity between peraluminous, quartzrich, biotite-rich, cordierite-bearing granitoids in the Fat Hen Creek Complex and meta-sediment from the Cape River Metamorphics suggesting a model with the Cape River Metamorphics metasediments as a source for the S-type granitoids in the Fat Hen Creek Complex. Also, the similarity of their patterns suggests that the granitoids are not formed from melts separated from their source rocks, but are more likely homogenised mixtures of melt and restite. It is probable that the Fat Hen Creek Complex comprise catazonal, anatcetic melts with low melt fractions, probably insufficient for effective separation of melt from restite. Such cordierite-bearing granitoids form in a continental collision setting, with accompanying thickening of continental crust. The origin of the hornblende-bearing granites (1-types) is problematical. It is most likely that hornblendebearing granites intruded into the Cape River Metamorphics during the Early Ordovician metamorphism (possibly providing a heat source??), and were emplaced in their current position during subsequent orogenesis. The depth estimates for crustal anatexis in the Fat Hen Creek Complex, together with similar estimates for metamorphism in the Cape River Metamorphics, imply depths of ~20km during the Ordovician, clearly a result of crustal thickening. Such crustal thickening most likely occurs during orogenesis, and possibly resulting from continental collision. The disruption of aeromagnetic trends noted earlier in this abstract may be a result of this orogenesis. References Strachotta, C., 1998: The metamorphic petrology of the Cape River Metamorphics. Unpublished honours thesis, Queensland University of Technology.

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Geological Convention, Sydney, July 2000

LATE MESOPROTEROZOIC AND NEOPROTEROZOIC TO CAMBRIAN CRUST IN NORTH QUEENSLAND- IMPLICATIONS FOR CONTINENTAL RECONSTRUCTIONS Laurie Hutton' and C. Mark Fanning^ ' Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001 ^ Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 In the Cape River area, the Cape River Metamorphics of probable Late Neoproterozoic to Early Cambrian age are intruded by the Fat Hen Creek Complex, a series of Ordovician I and S-type granitoids interlayered with basement gneisses. Quartzo-feldspathic gneiss within the Fat Hen Creek Complex may be higher grade equivalents of the Cape River Metamorphics or may be an older basement. Hornblende-bearing granite from one of the gneissic bands contains complex and simple zircons which have yielded 207Pb/206Pb ages of 1105 ± 15 Ma and 1238 ± 38 Ma. Lithic arenite in the Cape River Metamorphics is derived from an almost exclusively Late Mesoproterozoic source with 14 of the 15 zircons analysed yielding an age of 1145 ± 21 Ma. SHRIMP dating of I and S-type granitoids from the Fat Hen Creek Complex have yielded Late Cambrian to Early Ordovician ages indicating a major magmatic/metamorphic event at that time. The presence of S-type granites and upper amphibolite grade metamorphism in the Cape River area are thus interpreted as resulting from Ordovician orogenesis. The Cape River Metamorphics/Fat Hen Creek Complex are inferred to be overlain by the Late Cambrian to Early Ordovician Seventy Mile Range Group and are intruded by Late Silurian to Early Devonian granitoids of the Reedy Springs, Lolworth and Ravenswood Batholiths. A comparison of the geochemistry of Late Silurian to Early Devonian granitoids from the Georgetown Region, Reedy Springs Batholith, Lolworth Batholith and Ravenswood Batholith suggest chemically similar source rocks for the Georgetown and Reedy Springs granitoids with different source rocks for the Lolworth and Ravenswood Batholiths. Similarly, a comparison of inherited zircon populations and ind model ages from granitoids from all three batholiths suggest that, although a Precambrian component is present in all of their source rocks, there are differences especially between the Georgetown/Reedy Springs granitoids and the Ravenswood Batholith. A review of Ind Model ages from granitoids in North Queensland reveal a range from about ~0.9Ga to ~2.6Ga. Model ages in the range 0.9-1.2Ga are found in the Seventy Mile Range Group, Mount Leyshon Volcanics and in the Ravenswood Batholith, units which occur in the southern Charters Towers Region. Permian to Carboniferous granitoid rocks intruding the Georgetown Region show a range of model ages from 1.5-2.0Ga while Silurian to Devonian and Proterozoic granitoids range from 2.0-2.5Ga. The significance of these data lies in proposed continental reconstructions in eastern Australia during the Late Mesoproterozoic, Neoproterozoic and Early Cambrian. Three conclusions are drawn: •

The existence of Grenvillian age (900-1200Ma) crust in or beneath the Cape River area extends the known distribution of rock of this age from the Musgrave area of Central Australia. Continental reconstructions using the SWEAT hypothesis extend Grenvillian belts in America through Antarctica, the Albany-Fraser belt in southern Australia and in the Musgrave region of Central Australia.

•

A second proposed pre-Neoproterozoic continental reconstruction juxtaposing North America and Australia is called AUSWUS. This reconstruction places eastern Australia against southern North America. Belts based on ind model ages are traced through the southern United States and are similar to those in the Cape River area in North Queensland, perhaps supporting the AUSWUS reconstruction.

•

The presence of Mesoproterozoic and Neoproterozoic to Early Cambrian rocks in the Thomson Fold Belt raises some questions as to the nature of the Diamantina Lineament (Tasman Line) which is previously believed to mark the eastern limit of the Precambrian Australian Craton. It is possible that the Thomson Orogen is floored by thinned Proterozoic crust formed during the breakup of Laurentia from Gondwana in the Neoproterozoic, and separated from the thick craton to the west by the Diamantina Lineament. The multiply deformed nature of Neoproterozoic to Early Cambrian sequences in the Anakie Metamorphics and Cape River Metamorphics east of the Tasman Line contrasts with only mildly warped, but similar aged sequences in the Georgina Basin west of the Tasman Line. Similarly, Ordovician orogenesis recorded in the Cape River Metamorphics and Fat Hen Creek Complex appears to have had no effect on the Georgina Basin sequence.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOSTATISTICAL CHARACTERISATION OF PETROPHYSICAL PROPERTIES IN THREE-DIMENSIONAL GEOLOGICAL AND GEOPHYSICAL MODELLING Thong H. Huvnh and Laurent Ailleres Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton VIC 3800 The main objective in hydro-geological descriptions is the characterisation of those reservoir heterogeneities that influence the amount, position, accessibility and flow of fluids through the reservoir. However, descriptions at the desired resolution is generally difficult because of the sparse sampling yielded by traditional methods of data acquisition. One of the most extensively used approaches is based on geostatistical methods in which spatial variations are predicted between distribution sampling points in order to establish potential variabilities. However, geostatistical simulation methods have not, to date, been widely used in ore body modelling. The modelling schemes currently available for integrated geological and geophysical modelling all suffer fi-om the inability to simultaneously be constrained by both geological and geophysical data (Jessell, 1997). Most current attempts at geological modelling routinely build up a three-dimensional model of surfaces and/or regular cubic cells by standard interpolation between the known data. From this three-dimensional model, a synthetic potential field response can be calculated by assigning rock property values to specific regions (Ailleres & Jessell, 1997). This data-based modelling approach maximises the use of the available hard geological constraints then uses the parallel geophysical data as a test of the three-dimensional interpretation. However, the direction in which to proceed with the geological and/or geophysical model when the calculated and observed potential fields do not match remains less clear. We present a provisional examination for the multiply-constrained geostatistical modelling of petrophysical rock properties and potential field data associated with the complexly folded and highly metasomatised Cannington Ag-Pb-Zn deposit, situated in the Mount Isa Inlier region, north-west Queensland. This model provides a framework for the prediction of magnetic susceptibility and density variability in real-space. It objectively assimilates the recognition of anisotropic patterns in orientation, geometry and spatial frequency of occurrence that can be assessed for influence on mining provisions, economic extraction of resources and ultimately near-mine exploration of additional ore. The systematical evaluation of trends adopted within our model was conditioned using ordinary kriging as the basis of simulation to establish interpolation. This use of statistical information is particularly attractive because it admits the plausibility of quantifying the view of improving our interpretation by improving our interpolation. When these considerations are combined with observations of the local structure, it would be possible to construct highly spatial predictive rock property distribution and propagation trends. Preliminary results of the simulation have yielded encouraging models for characterising the rock property distribution in the local structure. This has lead to refined images of the calculated potential field anomalies which show a reasonably greater level of correlation with the target data than with initial training-models. Acknowledgements: The authors wish to thank BHP Cannington for their generous support of this study and for permission to publish. This contribution is released with the permission of the Director of the Australian Geodynamics CRC.

References Ailleres, L. & Jessell, M.W., 1997. Use of Noddy for the computation of the geophysical response of a 3D model built using gOcad. Abstracts, Geophysics Down Under : Geological Structures and their geophysical signatures. Specialist Group on Solid-Earth Geophysics, Special Issue Newsletter, No. 24, Marysville, Victoria, 1-2. Jessell, M.W., 1997. Integration of geological constraints. Abstracts, Geodynamics and Ore Deposits Conference, Australian Geodynamics Cooperative Research Centre, Ballarat, Victoria, 70-72.

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GEOCHRONOLOGICAL RE-EVALUATION OF THE NYMAGEE 1:250000 SHEET Daniel R. L. Isaacs^ Phillip Blevin' and Richard Armstrong^ ^Department of Geology, Australian National University, Canberra, ACT 0200 ^Research School of Earth Sciences, Canberra, ACT 0200 The Erimeran Granite, Tarran Volcanics and Nymagee Igneous Complex (NIC) are a group of intimately associated felsic igneous bodies located in the central northwestern Lachlan Fold Belt (LFB). This area of the LFB contacts the mineralised Mineral Hill Belt to the east as well as the old Nymagee Mine and Cobar Mineralised Zone to the north. The emplacement timing of the NIC and the Erimeran Granite therefore provide important constraints on the timing and origin of these deposits. Accurate dates also have important implications for the tectonothermal development of the LFB. In addition, mineralisation is thought to be associated with movement on the Gilmore Fault Zone (GFZ) a part of which is straddled by the Tarran Volcanics. As such, this relationship may provide a maximum age for the last movement on the fault. This study involves geochronological analysis of the Erimeran Granite, Tarran Volcanics and the NIC that will facilitate a more accurate model for the development of the region. The Erimeran Granite is a peraluminous body, with secondary muscovite. It contains phenocrysts of biotite, K-feldspar, albite and quartz, which is recrystallised into strongly sutured bands. Minor phases include apatite, pinite and titanite. The NIC comprises of peraluminous equigranular, muscovite-biotite bearing granites and porphyritic muscovite-biotite bearing adamellites. Muscovite in the NIC is subhedral, complicating the distinction between primary and secondary growth. Biotite, perthite, muscovite and quartz are all major phases with tourmaline occurring in small amounts. The Tarran Volcanics are group of intrusive and extrusive, felsic, metaluminous rocks. Large phenocrysts of euhedral pink alkali feldspar are dominant, with phenocrysts of quartz, biotite and some albite. Microphenocrysts of titanite, allanite and xenotime are present in a groundmass of quartz, K feldspar, and biotite. Pyrite and magnetite also occur. The Erimeran Granite and the NIC, which have 'S-Type' affinities have been dated using Rb-Sr and thought to be Early Silurian at 419Ma (Pogsen, 1991). The Tarran Volcanics have not previously been dated, although they are thought to be Devonian due to their unconformable relationship with the Erimeran Granite (Suppel & Gilligan, 1993). Previous U/Pb dating by Spandler (1998) found the Mineral Hill Volcanics to be 428 ± 3.9Ma and the nearby Wilmatha Granite to be 422 ± 3.5Ma. Preliminary U/Pb ages have been obtained for the NIC, Erimeran granite and Tarran Volcanics, using the SHRIMP at the Australian National University (ANU). The U/Pb dates for the NIC are ambiguous, although they trend towards older dates than previously thought. Additionally, the Erimeran Granite is older, with preliminary ages of 431 ± 15Ma (95% confidence). Interestingly, the Tarran Volcanics may have a similar age to the Erimeran Granite at 432.8 ± 6.9Ma (95% confidence); these two dates are indistinguishable within the error of the data. Locally, the age gap between the Erimeran Granite and the Tarran Volcanics was thought to be at least lOMa. However, this is obviously not the case (ie. the Erimeran granite and the Tarran Volcanics are broadly coeval). Furthermore, in conjunction with the dates from Spandler (1998) the development of the NymageeMineral Hill regions can be pushed back by at least 10-15Ma. On a larger scale, the timing and development of the eastern LFB can be refined, including movement on the GFZ, the timing of the Benambran Orogeny, and the length of the magmatic period surrounding the development of the igneous rocks along the GFZ. Additional dating will elucidate further a precise date for the NIC, subsequently allowing a more precise model of the area to be formulated.

References Pogson,P. J., 1991. Geology of the Bobadah 1:100 000 sheet 8233. New South Wales Geological Survey, ^ydn^y. Spandler, M., 1998. The geology of Mineral Hill Field, Central NSW: Igneous evolution and Cu/Au mineralisation. Unpublished Honours Thesis, Australian National University, Geology Department.

Suppel, D. W. & Gilligan, L. B., 1993. Metallogenic Study and Mineral Deposit Data Sheets, Nymagee 1:250 000 Metallogenic Map. New South Wales Geological Survey.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DETERMINATION OF COPPER ISOTOPE RATIOS BY LA-MC-ICP-MS S.E. Jackson, A. Botfield, W.L. Griffin, and N.J. Pearson GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, 2109

The stable isotope ratios of metals commonly found in sulphide ore deposits {e.g., Cu, Zn, Fe, Sb, Ag) may be fractionated significantly during ore-forming processes (redox reactions, etc,) and thus provide important information on the source, transport and depositional mechanisms of these metals. However, little work has been done on these isotopic systems because of past analytical difficulties in making precise measurements. Recently developed laser ablation (LA)-multi-collector (MC)-ICP-MS technology now allows routine rapid, in situ determination of the isotope ratios of these metals. This study reports on: (a) the development of an analytical protocol for the measurement of the Cu/^^Cu ratio of chalcopyrite, (b) preliminary data for samples from a wide range of copper-bearing mineral deposits, and (c) a preliminary study of the spatial distribution of Cu isotope ratios within the Cadia Hill porphyry Cu-Au deposit. The instrumentation used in this study was a Merchantek LUV266 laser sampler operated with either Ar or He as the sample carrier gas. The ablated material was transported into a Nu Plasma MC-ICP-MS for isotopic determination. Instrumental mass bias was corrected using the measured ^^Zn/^'^Zn ratio of a Zn-bearing aerosol added continuously to the ablated sample carrier gas flow via a T-junction. The isotopes of copper (and other metals) are fractionated significantly by volatilisation and condensation processes cmerative during the laser sampling/transport processes, both of which favour depletion of ^Cu relative to ^^Cu in the transported material. This fractionation may be reduced by using high laser pulse energies, low repetition rates and, most significantly, by ablation in He, which reduces dramatically the condensation blanket of sulphidic material around the ablation site. Using these conditions, relatively stable Cu isotope ratios may be maintained over a long ablation period (up to 4 minutes). However, absolute ratios are still significantly different to those measured by solution-MC-ICP-MS. To produce accurate ratios, therefore, all analyses were referenced to a chalcopyrite Astandard@ measured repeatedly during an analytical session under identical ablation conditions to the Asamples@. The standard chosen was an isotopically homogeneous chalcopyrite sample from the Bougainville porphyry copper deposit. All data are reported in epsilon ^^Cu/^^Cu976 units; i.e,, ^^Cu/^^Cu ratio of the sample relative to NIST SRM 976 (^^Cu/^^Cu - 0.44563) in parts per 10,000. All samples were analysed 5 times, with typical external precision (2 S.E.) of ca. 5 epsilon units. Preliminary results include: (1) Samples analysed from a wide range of deposits (magmatic, magmatic-hydrothermal, sedimentary) show a range in ^^Cu/^^Cu values of ca. 40 epsilon units. (2) All the chalcopjTite samples associated with igneous activity (magmatic segregation, porphyry Cu, VMS, epithermal) have epsilon ^^Cu/^^Cu976 values between -10 and +10 epsilon units. (3) Samples from sedimentary deposits, including the sandstone-hosted Zambian Cu belt, and the shale-hosted White Pine (Montana) deposit, have significantly lower ^^Cu/^^Cu976 values (by up to 30 epsilon units). (4) Samples from the Cadia Hill porphyry Cu-Au deposit show statistically significant variations in ^^Cu/^^Cu that correlate generally with gold grade.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE TURONDALE FORMATION, HILL END TROUGH, NSW: FACIES INTERPRETATION AND PALAEOGEOGRAPHY Elizabeth A. Jagodzinski Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601. The Turondale Formation is the oldest of two volcanogenic formations within the Early Devonian Crudine Group, the better known of which is the Merrions Formation (formerly Merrions Tuff). The host environment for the Crudine Group was a relatively deep, quiet marine basin known as the Hill End Trough. Ambient basin sedimentation within the Turondale Formation is represented by thick, hemipelagic shale intervals, which contain discrete interbedded siltstone and fine sandstone beds deposited by fine-grained, small-volume turbidity currents. Ambient sedimentation was periodically interrupted by the emplacement of various volcaniclastic mass-flow facies described below. Coherent dacite porphyries locally intrude the Turondale Formation as thick sills. Crystal-rich volcaniclastic sandstones (CRVS) composed of unabraded juvenile pyroclasts (mainly crystals and crystal fragments) were emplaced as cold, water-supported mass-flows. They are divided into two subfacies based on bedding thickness: megaturbidites (>4m) and turbidites (<4m). The abundance of juvenile pyroclasts, lack of sedimentary reworking and extreme thickness and volume of the megaturbidites indicate they are syn-eruptive deposits, derived fi-om contemporaneous subaerial to shallow marine explosive volcanic eruptions. These eruptions spawned pyroclastic flows that entered the nearby marine basin and either travelled for some distance across the water prior to sinking through the water column, or were immediately submerged at the shoreline. Significant volumes of ash and pumice, and complementary crystal/lithic enrichment in the derivative megaturbidites occurred either through gravitational and hydraulic segregation and/or elutriation in secondary eruption columns generated by phreatic explosions. Tuffaceous cherts and cherty sandstones distally associated with the megaturbidites represent suspension settling and reworking of the separated ash component. The thinner CRVS turbidites are post-eruptive deposits. Immediately following large-scale volcanic eruptions, temporary sediment over-supply to alluvial/fluvial depositional systems in the source region resulted in rapid deltaic progradation at the shoreline, and generation of small-volume, low concentration mass-flows at the unstable delta front. The pattern of sedimentation indicates two major influxes of volcaniclastic detritus, separated by a significant period of repose during which ambient sedimentation was re-established in the basin. CRVS at the base and the top of the formation have the same composition, indicating two eruptive phases of the same volcanic source, or similar magmatic sources. A syn-eruptive style of sedimentation, characterised by abundant CRVS megaturbidites, dominates the basal volcaniclastic package. Thick, syn-eruptive megaturbidites mark the onset of the second eruptive phase, and overlying thinner and finer grained turbidites reflect reworking and resedimentation of the freshly erupted debris as depositional systems in the source region returned to a more normal state. The coincidence of bedding thickness and grainsize maxima, maximum number of thick sedimentation units, maximum lithic clast size and abundance and highest sand-to-shale ratios in the Sofala district, suggests sediment derivation from a volcanic area to the south or southeast of Sofala, probably located on the adjacent Capertee High. This is supported by palaeocurrent indicators in the Turondale Formation and contiguous units, which suggest dominant sediment influx from the southeast. South of Sofala, polymictic volcaniclastic conglomerates exhibiting channel geometries suggest a local feeder-channel environment emanating from a point source on the basin margin, proximal to this area. Large allochthonous limestone blocks would have been resedimented short distances downslope from the littoral environment by gravitational processes. Chaotic mudstone/volcaniclastic sandstone slump deposits (intraformational breccias) are interpreted to be derived from local slope collapse at the basin margins. The Huntingdale Volcanics, outcropping in the Capertee Valley about 30 km east of Sofala, contain thick, extensive ignimbrite sheets that are compositionally similar to the CRVS, and may represent the source region for the volcaniclastic component of the Turondale Formation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CRYSTAL-RICH VOLCANICLASTIC MEGATURBIDITES OF THE TURONDALE AND MERRIONS FORMATIONS, HILL END TROUGH, NSW: ARGUMENTS FOR PYROCLASTIC FLOW DISINTEGRATION UPON ENTERING THE SEA. Elizabeth A. Jagodzinski^ and Ray A.F. Cas^ ' AGSO, GPO Box 378, Canberra, ACT 2601. ^ Department of Earth Science, Monash Univ., Clayton, Vic. 3168. Low temperature subaqueous mass flow deposits of pyroclastic debris have been described in both marine and lacustrine environments. Such deposits may originate from: 1) subaqueous eruptions; 2) subaerial pyroclastic flows that enter water; or 3) slumping and resedimentation of unstable slope composed of loose pyroclastic detritus. Distinguishing between deposits formed by the latter two mechanisms is generally extremely difficult. Crystal-rich volcaniclastic sandstones (CRVS) form a distinctive end-member of this facies, in which light/fme pyroclasts (pumice and fine ash) are significantly depleted relative to coarse/dense juvenile pyroclasts (crystals, crystal and lithic fragments). This pumice and ash depletion is a key factor in arguing that CRVS are generated by complex interaction between subaerial pyroclastic fiows and the sea. A comparison between basin-wide, thick, voluminous crystal-rich volcaniclastic megaturbidites (CRVS) of the Turondale and Merrions Formations, Hill End Trough, NSW, and slump-derived epiclastic (mainly carbonate) megaturbidites within other deep marine basins shows that the two types of deposits are comparable in size, but differ in frequency of occurrence and internal facies profile. Whereas epiclastic megabeds are isolated occurrences within thick packages of 'normal' basin sedimentation, the volcaniclastic megaturbidites dominate basin sedimentation over thick intervals, reflecting a rapid accumulation of detritus or supply event in the source region. The size and volume of the beds, the abundance of juvenile pyroclasts and their frequency of occurrence are consistent with the continuous replenishment of source material through periodic pyroclastic flow-forming eruptions, and are key factors in establishing that CRVS have a genetic connection with active volcanism (i.e. they are essentially syn-eruptive). However, this still does not discern whether the pyroclastic flows entered the sea and transformed in transit into water-supported mass flows, or if pyroclastic material was briefly stored at the shoreline or on the shelf prior to redeposition (by slumping). This issue may be resolved by comparing the internal organisation of CRVS and slump-derived epiclastic 'megaturbidites'. In the latter, the term 'megaturbidite' is not applied sensu stricto. The beds commonly exhibit a lateral and vertical continuum from proximal debris flows to distal megaturbidites. i.e. The initially viscous and cohesive mud-rich flows evolve hydrodynamically away from the source, through ingestion of water, fluidisation of the flow and elutriation of the flne mud component. In contrast, CRVS do not exhibit evidence of cohesive flow, even near points of sediment influx around basin margins. Similar volcaniclastic megabeds deposited in more proximal, shallow-marine settings within the Bindook Volcanic Complex, do not display any evidence of initial deposition by slumping or cohesive debris flow mechanisms. If the presumably initially ash-rich pyroclastic deposits had been stored on the shelf prior to (penecontemporaneous) slumping into the basin, a proximal to distal, debris flow to turbidite flow transformation would be expected by analogy to the slump-derived epiclastic megabeds. The absence of such flow transformation suggests much of the fine-ash component of the pyroclastic flows was removed prior to transportation into the deep-marine environment. This favours the model that CRVS are directly fed from subaerial pyroclastic flows that entered the sea, as this scenario provides several possible mechanisms for the removal of significant volumes of ash (and pumice). If the pyroclastic flows initially travelled for some distance across the water, ash and pumice would be separated from dense/coarse clasts through gravitational segregation and hydraulic sorting (and flotation of pumice). Alternatively, pyroclastic flows may have submerged immediately and interacted explosively with seawater, with ash and pumice elutriated through the secondary eruption columns. In both cases, further elutriation of fine ash from the head and body of the derivative megaturbidites would also occur.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MINERAL POTENTIAL OF AUSTRALIA Subhash Jaireth, Ian Lambert, Yanis Miezitis, and Lynton Jaques Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601

A GIS-based assessment of the mineral potential of Australia is to be compiled by AGSO. This is to be done in consultation with State and Northern Territory geological surveys and will build on the assessments conducted as part of the Regional Forest Agreement process. This paper will outline the approach being used for mineral potential assessment, summarise the initial geoprovince-scale assessments, and review proposed work on consideration of mineral systems for selected time intervals. The initial product will be a national compilation of geoprovince-scale (c. 1:5 million scale) assessments for major styles of gold, base metal and uranium mineralisation. This will be followed by a 1:1 million scale compilation, with more detailed assessments as appropriate for particular regions. Compilation of information on mineral potential is important for a number of reasons: • •

•

•

mineral potential maps can promote exploration, including by international investors, particularly in greenfield areas; the process of systematic assessment of mineral potential helps identify priorities for future studies. This will be an important element in the scoping phase of each new regional program under the National Geoscience Mapping Accord; there has not been a systematic compilation of mineral potential that can be factored into land use decisions alongside broadscale "environmental" tools such as the National Wilderness Inventory and the Interim Biogeographic Regionalisation of Australia. It is important to build on the acknowledgement achieved in the Commonwealth-State Regional Forest Agreement process that decisions or policies on conservation of land are more likely to be sustainable if they integrate mineral potential with other values; and considerations of land use plarming and infrastructure priorities for regional Australia should take account of the distribution of areas of high mineral potential. This has been a feature of the Commonwealth-State-industry Regional Minerals Program.

The main outputs of the project will be: •

•

a GIS that includes layers for mineral potential, certainty of assessment, geoprovince time-event plots, ore deposit models with regional-scale assessment criteria (critical elements of mineral systems), and grade and tonnage distribution curves for deposit styles; and thematic layers (maps) showing mineral potential for different commodities, deposit styles and geoprovinces.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SOME GEOLOGICAL FALLACIES Peter M. James, Consulting Geotechnical Engineer, Brisbane

A number of geological concepts appear to have been introduced into the literature without scrutiny and without subsequent review. These include: Isostasy. The maximum vertical loading intensity imposed by the crust on the lithosphere is probably that of a large sea mount. However, even this load is an order of magnitude less than the available bearing value, based on creep strengths at the Moho. Continental ice caps stress deeper levels of the Earth but 15 m yr of ice caps in Antarctica have not resulted in the sort of subsidence attributed to the more transient Pleistocene ice sheets. Indeed recent uplifts have occurred in Antarctica, despite the ice caps. The sediments high on fold mountains have also been uplifted to their present position and did not arrive by compensation. It is therefore proposed that the lithosphere is quite capable of supporting any known crustal loadings without distress and that isostasy is not a real phenomenon. Submarine Canyons. 90% of submarine valleys can be traced out as extensions from existing drainage systems on land, yet canyon size bears no relationship to river size nor to potential sediment loadings. This, among other hydraulic considerations, negates any suggestion of a turbidity current origin. Canyons on the rocky coast of Corsica can be explained as having formed during periods when the Mediterranean was dry. A similar origin is logical for ocean canyons, based on very large changes in sea level. Evidence of massive sea level changes also includes anhydrite at 2 - 3 km depth in the Atlantic; oxidised basalts at 2.5 km depth near the Galapagos; sand and reef detritus at 5 km depth, 500 km out into the Indian Ocean. Such massive sea level changes can be demonstrated to be a direct and necessary product of polar wander. Crustal Stresses. If Heim's Rule on the equalisation of horizontal and vertical stresses at depth were correct, there would be no earthquakes. A study of reservoir induced seismicity indicates that horizontal stresses in the crystalline basement need bear no relationship to stresses measured in near surface situations. Moreover, tensile stresses in the basement are common. The predictions of mobilism on this matter are invalid since no cognizance is taken of the stresses imposed on the crust by latitude changes, which can be in excess of 1 X 10^ kPa.. Rifts. Mechanics of materials principles require that rifts are caused by crustal tension and this has been invariably confirmed by field evidence. Bulging of the crust as a cause of rifts is not a possible mechanism. Expanding Earth. No known physical mechanism can be invoked to explain how the solid Earth body might expand. Rates of Earth rotation since the Palaeozoic dispel any consideration that the moment of inertia of the Earth might have increased to the extent required by even minor expansions. To remain relevant, modem Earth Sciences must be based on acceptable mechanics of materials principles.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention. Sydney, July 2000

SOME MYTHS OF MOBILISM Peter M. James Consulting Geotechnical Engineer, Brisbane

Subduction. Subduction can be seen to work only if frictional resistance on the upper side of the subducted tongue is ignored. The magnitude of stresses to cause recrystallisation on the leading edge of subduction must likewise be ignored, as also the fact that lighter crustal material is alleged to be pulled down into heavier lithospheric material. Undisturbed sediments of Mesozoic age on the base of oceanic rifts in the Pacific are not compatible with the concept of subduction. Ocean Spreading. Ocean spreading rates are inferred from magnetic striping, age-based on overlying sediments. No actual recovery of such stripes has yet been made for confirmation. A plot of alleged ocean spreading rates reveals a direct relationship to the size of the ocean, suggesting that rates increase with size. Since the spreading mechanism is presumably unchanged, the opposite should occur. Alleged variable rates of spreading are postulated on either side of fracture zones in the north-west Pacific and elsewhere, yet these fracture zones are aseismic. Finally, a logical case against ocean spreading is available from items such as the presence of the St Peter & Paul rocks (800 m yr old) in the equatorial mid-Atlantic and The Brothers Islands (Pre-Cambrian) in the Red Sea. Sediments on a Moving Basement. Simple analysis shows that any sediments on a moving basement would buckle/undergo thrust faulting as a preferred mechanism. On the Juan de Fuca Plate, for instance, embryo fold mountains should be being produced every million years. Yet Pliocene sediments here are only gently folded and younger sediments are horizontal. Static conditions are again implied. Plate Boundaries. A study of historical seismicity ( by Ambraseys) indicates that, 500 years ago, plate boundaries in the Middle East might have been drawn in quite differently. The assumption of rigid plates and fundamental boundary conditions is thus called into question. Brittle Crust. In the mobilist framework, the observed behaviour of the brittle crust is taken as diagnostic of very large plastic deformations in the substrata, yet the two materials have quite different properties and should be treated separately. The response to geoid changes in both materials would be entirely different, but this has so far been ignored in the mobilist model. If the modem Earth Sciences are to shed the present aegis of mysticism, they need to be based on the present-day knowledge of mechanics of materials. To suggest that future research will reveal the truth of presently untenable mobilist mechanisms is not a scientific approach but an act of faith.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

INTERACTION OF METAMORPHISM AND DEFORMATION: CRYSTAL- TO CRUSTAL-SCALE R.A. Jamieson, C. Beaumont, and the Geodynamics Group Departments of Earth Sciences and Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 3J5, For more than thirty years Ron Vernon has influenced the ways in which metamorphic geologists look at interactions between metamorphism and deformation. We now understand quite a lot about how factors such as rheological and compositional heterogeneities, fluid flow pathways, and strain gradients influence where, when, and how metamorphic mineral assemblages and structures form and evolve on the microscopic and outcrop scales. It is a logical extension of Ron's work to investigate the ways in which metamorphism and deformation influence each other on much larger scales. The controls on these crustal-scale interactions are not easy to document by direct observation or laboratory experiment; our approach involves numerical experiments based on coupled thermal-mechanical computer models of convergent orogens. The results presented here focus on interactions between metamorphism and crustal deformation that is driven by cryptic subduction of sub-orogenic lithosphere. On the orogenic scale, crustal deformation affects the distribution of metamorphic rocks in a number of different ways, of which deformation of pre-existing isograds is probably the most obvious. More fundamentally, deformation and redistribution of tectonically accreted radioactive material ("tarm") and the rapid displacement of hot or cold rock within an orogen control the thermal structure of the crust, and therefore the metamorphic process itself In turn, the evolving thermal structure of an orogen affects tectonic style by changing the rheological properties of the crust, and metamorphic reactions that change material properties or produce fluid or melt will also affect the style of deformation at least locally. The thermal structure of a model orogen reflects the competition among rates of heat production, advection, and diffusion. This can be expressed in terms of three dimensionless ratios - the thermal Peclet ratio (Pe = advection/diffusion), and two Damkohler ratios (Dm = heat production/advection; Div = heat production/diffusion; Pe = Div / Dm). At high convergence rates, where Pe » 1, rapid transport of cool material into or beneath a model orogen limits the effectiveness of diffusive heat transport. If convergence rate decreases during orogenic evolution, so that diffusion out-competes advection (Pe < 1), the orogen will heat up. Heat production, or the rate of self-heating resulting from radioactive decay, increases with time as a function of the concentration of heat-producing elements and the overall volume of "tarm". Burial of "tarm" within or beneath a model orogen can partly offset the cooling effects of subduction, erosion of heatproducing upper crust, and heat loss from the surface (both Dm and Div > 1). In our coupled thermal-mechanical models, deformation of the upper crustal heat-producing layer produces geometrically complex distributions of "tarm" that strongly influence the distribution of crustal isotherms. As the models evolve, large lateral and vertical thermal gradients develop, leading to significant lateral variations in P-T conditions and crustal strength. For some model conditions, temperatures at mid-crustal depths exceed 800°C, and lower crustal isotherms are inverted because cool material continues to flow into and beneath the model orogen. However, model crustal thermal gradients are not linked in a simple way to peak grade profiles at the model surface, which are also controlled by the rate and mechanism of exhumation. Crustal geothermal gradients at the time and place of metamorphism should not be expected to correspond to metamorphic field gradients observed by geologists at the surface millions or billions of years later. The models calculate P-T-t paths for selected points that are tracked throughout the evolution of the model. If we assume that the maximum temperature (Tmax) experienced by a rock will be recorded in its mineral assemblage, "peak grade profiles" across the model surface at selected times provide a convenient way to compare model results with observations from real metamorphic belts. Results will be presented from models with different initial values of Pe, Dm, and Div, and therefore different model metamorphic histories, and these results will be compared with first-order observations from natural examples.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CYCLICITY IN BACKREEF CARBONATES OF THE DEVONIAN REEFAL PLATFORMS OF THE LENNARD SHELF, CANNING BASIN, WESTERN AUSTRALIA: IMPLICATIONS FOR REEF DEVELOPMENT AND PALAEOBATHYMETRY John S. Jell and R. L. Scott Brownlaw. Department of Earth Sciences, The University of Queensland, QLD 4072

Metre-scale sedimentary cycles are well-developed in the Frasnian Pillara Limestone backreef carbonates of the Lennard Shelf, Canning Basin, northwestern Western Australia. They have been interpreted as shallowing-upwards successions and are defined by an ideal predictable stacking pattern of eight facies. Commonly, the basal deeper water parts consist of fine carbonate muds with scattered shelly detritus (1. mudstone facies) or with branching Disphyllum or massive Argutastrea rugose corals and in places branching Thamnopora tabulate corals (2. coral facies). The overlying shallower water beds are more resistant and are composed of stromatoporoids in carbonate muds and sands, with four facies distinguishable (in ascending order, 3. tabular stromatoporoid, 4. subspherical stromatoporoid, 5. Stachyodes, 6. Amphipora facies). Some cycles are capped by shallower 7. peloidal limestone and 8. fenestral limestone facies. Cycles are variable in development and not all facies are present in all cycles. The cycles are defined from three lower Frasnian measured sections, at Menyous Gap, Guppy Hills, and Horse Spring, where 48, 76 and 102 cycles have been recognised, respectively. The cycles average 3.47-3.94 m in thicknesses. Estimates of cycle duration are not precisely determinable (17-25 ka), but concur with the calculated Middle Devonian Milankovitch band for periodicities of precession and obliquity (17-40 ka). The cycles are interpreted to have formed due to high-frequency eustatic sea-level fluctuations. The two methods of facies coding utilizedfor defining time series for spectral analysis of the Pillara Limestone, produced similar results. The spectra highlight consistent cyclicity with spectral energies 0-0.3 cycles/m. The peaks translate to cycle durations of 17-850 ka using an estimated accumulation rate of 0.17 m/ka. Given the likelihood that the peaks correspond with known Milankovitch periodicities, they translate to accumulation rates 0.12-0.34 m/ka. Hence, high-frequency eustatic sea-level fiuctuations, primarily attributable to obliquity and eccentricity, are interpreted to have controlled the back-reef cycles within the Pillara Limestone. Fischer plots of the measured sections are used to imply the relative sea level history during the Frasnian, and they show strong correlation. The correlation is also constrained by the first occurrence of Argutastrea hullensis in each section. The plots define numerous fourth-order (256-320 ka) and three third-order (1-2.1 Ma) sequences. Cycle development is seemingly related to the interplay of the various orders of eustasy, with thinner cycles formed during highstand and shelf-margin systems tracts, and thicker cycles manifesting transgressive systems tracts.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOSCIENCES IN THE PROTECTION OF MARINE BIODIVERSITY Chris Jenkins Ocean Sciences Institute, Sydney University

Maintenance of marine biodiversity and viable marine ecosystems are major objectives of natural resource management agencies world wide. However mapping the distribution of marine biodiversity at even modest scales is near impossible given the enormous number of marine species and the difficulties in comprehensively sampling large areas underwater. It is well known that the usual quantitative biological techniques are unable to keep pace with demands for coastal development, fishing, and marine conservation management. For these reasons biologists are exploring ways in which geosciences might assist. In many large scale projects, mapping patterns in marine diversity is done through the use of physical variables assumed to be strongly correlated with patterns in species distribution. These variables are frequently based on more readily surveyed parameters such as bathymetry, seabed type, current, and temperature. Where no biological data is available physical descriptors may be used to predict habitat within certain limitations. Where biological data exists, the coverage of sparsely sampled data can be extended using physical data in predictive models of habitat dependence. In Queensland, maps generated from the auSEABED database were used to help identify habitat boundaries and areas of high substrate diversity in the Great Barrier Reef Marine Park. Expert workshops of scientists used these seabed maps together with bathymetry, oceanography and spatial models of marine biodiversity to define bioregional boundaries for planning marine protected areas. Some geoscience techniques like swath sonar are also directly applied in sampling organisms like seagrasses. Decline in estuarine seagrass abundance is a special cause of concern, as seagrasses provide nursery habitat for many commercial and other species, sediment stabilization, oxygenation and important organic nutrients. Leisure craft, ferry propellors, anchors, shoreline development and siltation can all potentially impact on seagrass distributions. In NSW legislation decrees that projects leading to >10% loss of seagrass must replace the loss. But how is a loss to be measured ? Seagrasses show clearly on sidescan and multibeam images, and research is progressing towards quantitative results.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NUMERICAL SIMULATION OF TRANSIENT AND STEADY-STATE MICROSTRUCTURES Mark W. Jessell Department of Earth Sciences, Monash University, P.O. Box 28E, Clayton, Victoria. 3800

Over the past 20 years numerous microstructural indicators have been developed that attempt to quantify the orientation and magnitude of the stress or strain tensors during deformation. With the wealth of data that can be collected using modem SEM and microprobe techniques, we are in a position to characterize microstructures and chemical distributions in great detail. In order for many of these indicators to work we have to assume that the boundary conditions during deformation and the preserved microstructures had reached a steady-state (for example when using grain-size for palaeo-stress determination). Unfortunately we are lacking indicators that tell us whether the microstructures we see preserved in rocks are actually representative of steady-state conditions, or whether instead they represent transient conditions or have been modified subsequent to the deformation. It is not even clear how long a microstructure will be preserved after a change in the boundary conditions. In this project we are using the Elle microstructural simulation system to investigate steady-state microstructures in systems involving the competing processes which control grain boundary geometries. In particular we can simulate deformation in single and polyphase shear zones in which the intra-crystalline processes involved in sub-grain formation and deformation interact with the inter-crystalline process of grain boundary migration. By using periodic boundary conditions we have carried out experiments to high shear strain which enables us to study the grain boundary geometries which develop with increasing strain, and also to observe they modification subsequent to deformation. In this talk we follow the behaviour of two shear zones which are deformed at different temperatures until steady-state conditions are reached, followed by a change in boundary conditions which leads to a re-equilibration of the microstructure, and finally we look at the post-deformation history.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

SOURCE OF ORE-FORMING COMPONENTS AT THE PEAK MINE, COBAR, NSW - EVIDENCE FROM ISOTOPE STUDIES Zhivu Jiang and Philip K. Seccombe Department of Geology, The University of Newcastle, Callaghan, NSW 2308, Australia Lead isotopes were used to trace the source of the ore-forming metallic elements at the Peak deposit. Samples used for the analyses include not only galena and pyrite in the ore, but also whole rock samples from the rhyolite, Chesney Formation host rocks (siltstone, slate and ?tuff), metasedimentary rocks from the basement and K-feldspars fi-om post-ore potassic vein alteration. All of the lead isotope ratios measured from ore sulphides at Peak are surprisingly homogeneous and lie on two short arrays on the ^^^Pb/^^^Pb V5 ^^^Pb/^^^Pb and ^^^Pb/^^^Pb V5 ^^^Pb/^^^Pb diagrams. These linear distributions mostly result from the effects of minor mass-fractionation during mass spectrometry measurements, which causes data points to extend into lines with larger slopes than the coeval isochron. Lawrie and Hinman (1998) divided their ore-lead isotope data into three groups, namely basement, early-basinal and mixed signatures. However, the data are grouped within analytical error and do not permit distinction. Lead isotope data from the Chesney Formation host rocks and basement metasedimentary rocks are characterised by distinctively low ^^^Pb/^^^Pb ratios (low |LI, 235y/204p^ ratio) and, but high levels of radiogenic lead C^^Pb and An exception involves a sample of metasiltstone host rock 16.336). The large ^ ' W ^ b and ratios are typical signatures for lead in metasediments, since the lead normally derives from multiple sources during sedimentation. Moreover, lead isotope ratios in the sediments would increase due to radioactive decay of trace uranium and thorium after deposition. These differences indicate that the lead within either metasedimentary host rocks or basement rocks cannot provide a source of lead for the mineralisation, evening considering the homogenising effects of metamorphism. The homogenised ore-lead at Peak with an age of 420Ma at intersection of the array and the lead isotope growth-curve suggests that the ore-lead was initially concentrated within basement granites, then remobilised by hydrothermal fluids into the Cobar Basin sequences. The source of ore fluids at Peak also have been traced by their hydrogen and oxygen isotope compositions. Inclusion fluids within sulphide minerals, including chalcopyrite, sphalerite and galena from paragenetic stages 2 and 4 have been extracted for hydrogen and oxygen isotope analyses. Inclusion fluids from various stages of vein quartz were also analysed for hydrogen isotope compositions. Stage-2 major Cu-Au mineralisation appears to be related to fluids of significantly low and 5D values, ranging from - 6 . 3 to 3.4%o and - 1 1 5 to -96%o, respectively. Fluids responsible for the stage-4 major Pb-Zn-Ag mineralisation at Peak are also characterised by low and 5D values between - 4 . 6 and l.6%o, and - 1 1 0 and -68%o, respectively. In contrast, fluids corresponding to formations of the vein quartz have remarkably large and 5D values (-0.5 to 9.9%o and - 7 1 to -41%o, respectively. It is inferred that depleted and 8D values of inclusion fluids within sulphide minerals from the two stages of ore mineralisation at Peak favour a model of deeply circulated meteoric fluid involvement, not metamorphic fluids. S, CI and F would be the most important anions involved with complexing and transport of ore-forming elements. Hydrothermal, syngenetic and metamorphic sulphide minerals in the ore and metasedimentary rocks have been analysed for sulphur isotope analysis. The values for all sulphide minerals from various ore bodies define a relatively narrow range between 5.4 and 8.9%o. However, the S^'^S distributions for both syngenetic and metamorphic pyrite and pyrrhotite from the country rocks are significantly different from the values obtained for ore sulphides. S^'^S values for syngenetic pyrite and pyrrhotite exhibit large positive values ranging from 16.3 to 17.2%o. In contrast, S^'^S values of metamorphic pyrite and pyrrhotite are relatively depleted in with values ranging from -4.1 to 5.9%o. Hydrothermal sulphur has an initial S^'^S^s value of 6.2%o, inferring a deep-seated, homogeneous source of sulphur. Unlike the fluids related to the formation of vein quartz, the fluids associated with mineralisation at stages 2 and 4 possess significantly large contents of F" and CI". Together with aqueous sulphur species, these anions played an important role in remobilising the ore-forming metals from depth.

258

G E O L O G I C A L S O C I E T Y OF A U S T R A L I A , A B S T R A C T S N o . 5 9

1Australian

Geological Convention, Sydney, July 2000

SIGNIFICANCE OF FLUID INCLUSIONS WITHIN SULFIDE MINERALS - AN EXAMPLE FROM THE PEAK AND ELURA DEPOSITS, COBAR, NSW Zhivu Jiang, Yanyan Sun and Philip K. Seccombe School of Geosciences, The University of Newcastle, Callaghan, NSW 2308, Australia Fluid inclusions developed in quartz and sulfide minerals from the Peak and Elura deposits were used for bulk analyses of chemical ( K ^ N a \ Ca^^ F , C\\ CO, CO2, CH4, H2 and N2) and isotopic (H and O) compositions in order to understand the evolution and origins of the ore-forming fluids at these two deposits. Ore-forming fluids trapped in the sulfide minerals are characterised by relatively high contents of F", CI" and CH4 compared with the fluids responsible for formation of quartz veins at these two deposits. These high contents of CI" and F" in the ore-forming fluids might have played an important role with sulfur in extracting and transporting the metallic elements from basement rocks through to mineralisation sites in the Cobar Basin sequence. Chemical compositions have been used to derive temperatures, salinity, pH and logyD2 of the ore-forming and quartz vein-forming fluids. The results show that ore-forming fluids at Peak and Elura have higher temperatures (ranging from 22T to 370°C and 200^ to 343^C, respectively), high salinities (from 2.5 to 31.2vd% and 5 to 41.7wt% NaCl equivalent) and are more reduced than the fluids forming quartz veins at Peak and Elura. pH values of the ore-forming fluids at Peak and Elura range from 4.66 to 6.01 and 5.72 to 7.51, respectively. Isotopic compositions of the inclusion fluids are significantly different within vein quartz and sulfide minerals. and 8D values of the ore-forming fluids range from - 6 . 3 to 1.6%o and - 1 1 5 to -68%o (Peak), and 6.5 to 0.4%o and - 7 3 and -43%o (Elura), respectively. In contrast, and 5D values for formation of vein quartz range from - 0 . 5 to 9.9%o and - 7 1 to -41%o (Peak), and 4.1 to 15.4%o and - 1 3 0 to -53%o (Elura), respectively. The isotopic arrays of the inclusion fluids within sulfide minerals suggest deeply circulated meteoric waters formed the major mineralisation at Peak and Elura, rather than metamorphic fluids (Figure 1). Due to dissolution and reprecipitation, water/rock interaction involving chemical and isotopic exchange must have taken place between the meteoric water and metamorphic rocks to form alteration minerals such as vein quartz, chlorite and white mica. As a result, fluids trapped in vein quartz lost the isotope signature of meteoric water and approached that of the metamorphic rocks. Simple analysis of the isotopic compositions of the inclusion fluids within vein quartz and altered hydrous minerals may give a different explanation for the origin of the fluids. The best samples representing oreforming fluids are inclusion fluids trapped in ore-forming minerals, such as sulfides.

+10

Metamorphic water (300° - 6 0 0 X )

initial composition of •deeply circulated ore forming fluids Inclusion fluids in vein quartz at Peak inclusion fluids in sulfides at Peak

-110

fluids associated alteration minerals at Elura inclusion fluids in sulfide at Elura I I

0

10

20

Figure 1: Plot of the isotope data for the fluids at the Peak and Elura deposits, Cobar.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PORPHYROBLAST MICROSTRUCTURES SOME CENTRAL DEBATED ISSUES Scott E. Johnson Department of Geological Sciences, University of Maine, Orono, ME 04469-5790, USA

Porphyroblast microstructures have attracted the attention and imagination of geologists for more than a century. The issue of how to interpret them has strongly polarized structural geologists over the last two decades, and has certainly led to some exciting debates! This talk will focus on some central issues that have surfaced in these debates, and will attempt to show that continued research into these microstructures may reveal additional clues about the structural and metamorphic development of orogenic belts. Porphyroblasts commonly overgrow developing crenulations resulting in sigmoidal inclusion trails. Where the trails are relatively planar they can be measured in 2-D sections. Resulting histograms or rose diagrams of measurements from individual samples commonly show a large range of orientations up to plus or minus 20-40 degrees. However, in several published studies the mean strikes of inclusion trails from individual samples maintain fairly consistent orientations from sample to sample across large study areas, even though considerable deformation occurred after porphyroblast growth. Are these data useful for reconstructing the early structural "grain" in orogens? In some rocks there appears to be a microstructural record of near-orthogonal foliation development. In some instances there are supporting mesoscale and macroscale data, but in others the evidence is entirely microstructural and so caution must be exercised when speculating about causes. When the foliations in question are alternately steeply- and gently-dipping many possible causes can be listed, but an important question is whether these geometries are local aberrations, or whether they reflect orogen-scale processes. If compatible mesoscale or macroscale evidence is exposed in some orogens but not in others, how do we decide whether and when microstructural observations are meaningful? Spiral-shaped inclusion trails are perhaps the most fascinating of all porphyroblast microstructures. Can these trails form by overgrowth of sequentially developed crenulations, or must they always form in shear zones by growth during rotation relative to a single foliation fixed to the flow plane? Documentation of a clear relationship between shear strain and porphyroblast rotation in a single shear zone may help resolve the debate, but such studies are fraught with complications, some of which will be illustrated using rocks from the Main Central Thrust Zone in central Nepal. Aside from the debate surrounding their origins, most of the recent work done with spiral trails has focussed on determining the orientations of the spiral axes - work that can be done with simpler sigmoidal trails as well. According to recent papers, regardless of how these microstructures form the orientations of spiral axes over large areas may provide information on directions of shortening during orogenesis. It has even been suggested that they may provide information about relative plate motions. How can we test these ideas?

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RING COMPLEXES - SUBVOLCANIC MAGMA PLUMBING SYSTEMS Scott E. Johnson Department of Geological Sciences, University of Maine, Orono, ME 04469-5790, USA

Ring complexes represent the frozen remains of magma transfer zones that linked subvolcanic magma chambers to overlying calderas/volcanoes. Ring complexes can contain a wide variety of intrusive phases including cone sheets, ring dikes and massive central intrusions, and the intimate timing relationships between the various phases and syn-emplacement deformational structures provide rare insights into the emplacement and evolution of upper-crustal magmatic systems. Recent work in the western Peninsular Ranges Batholith of northern Baja California, Mexico, has revealed a previously unrecognized province of ring complexes, some of which are very well preserved. Ring complexes are only rarely exposed at Earth's surface, and so this is an exciting discovery. Emplacement of the Baja California ring complexes was accompanied by ductile wall-rock deformation. For example, the Zarza Complex is surrounded by a deformation aureole with 38% bulk shortening, and all kinematic indicators show that wall rocks in the inner aureole moved down relative to those in the outer aureole. Geological relationships within the Zarza preclude diapirism or in-situ lateral expansion to form the aureole, and so the deformation resulted from downward aureole flow, possibly in conjunction with "sinking" of the entire complex. How do these ductile deformation fabrics link in 3-D with brittle fabrics around higher-level complexes and calderas? The Baja California complexes provide information on the host-rock material-transfer processes that facilitated magma ascent in the upper crust. These complexes preserve evidence for: (1) downward transport of country rocks in the deformation aureole relative to those outside the aureole; (2) stoping during ascent and emplacement of massive central intrusions; (3) collapse along kinematic zones (ductile ring faults) that bound individual intrusive centers; (4) country-rock partial melting; and (5) deformation of country-rock screens caused by cone-sheet emplacement. Although multiple processes are recognized, the majority of material transfer during emplacement of the complexes was vertical. Is this range of processes typical in plumbing systems below volcanoes/calderas? Structural and intrusive relationships in the Baja ring complexes are consistent with a generalized three-stage developmental model, which broadly corresponds to Lipman's (1984) major divisions for caldera development. (1) An initial magma chamber is formed, which was mafic in the Zarza example, but may vary in composition. At some stage pressure in the magma chamber causes fracturing and disruption of the overlying crust. These fractures fill with magma that freezes to form cone sheets. (2) Disruption of the overlying crust during cone-sheet emplacement relaxes an important energy barrier to voluminous magma transport from the chamber. The resulting network of fractures and sheet contacts provides a vast array of potential magma pathways through which the massive core intrusions later stope to form central conduits that may supply volcanic eruptions at the surface. (3) Resurgence of the chamber and/or intrusion of a broadly cogenetic nested pluton may partially or completely destroy evidence for the earlier history of the system through a range of exposure levels. How can this model be tested?

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HIGH-PRESSURE METAMORPHISM IN THE ZAMBEZI BELT, NORTHERN ZIMABAWE: EVIDENCE FOR PAN AFRICAN-AGED CONTINENTAL COLLISION BETWEEN THE ZIMBABWE AND CONGO CRATONS Simon P. Johnson Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands 6907, Western Australia. The tectonic significance of the Pan African-aged Zambezi Belt and Lufilian Arc in central, southern Africa has long been in dispute. It is thought by many that this orogen was entirely intracontinental; however, recent investigations suggest that the Zambezi Belt represents the mid-crustal levels of a Himalayan-style orogenic belt and the Lufilian Arc, the foreland basin to this orogenic system. High-pressure eclogite and whiteschist fragments occur throughout the Zambezi Belt and Domes Region of the Lufilian Arc as part of a 150 km long, 40 km wide zone. This abstract presents new data for the high-pressure Kadunguri Whiteschists of the Chewore Inliers, northern Zimbabwe. The Kadunguri Whiteschists crop out on the southern margin of the Mesoproterozoic Chewore Ophiolite as a semi-continuous block of whiteschist some 5 x 1.5 km. The whiteschists form a southeasterly dipping sequence with the basal portion being comprised of a 'typical' whiteschist assemblage of talc + kyanite without chlorite and the top of the sequence being comprised of radially arranged orthoamphibole-bearing whiteschist. All lithologies contain abundant hematite and dravite. Whole rock major element analysis of the whiteschists indicate that all lithologies can be described in the simple MFASH system with all Fe occurring as Fe^^, thus indicating the highly oxidised nature of the lithologies. A modalised S i 0 2 - M g 0 AI2O3 and metasomatic isochon plots illustrates that the lithological / geochemical variation is related to metasomatic MgO enrichment (and alkali depletion) of ocean island meta-basalts from the neighbouring Chewore Ophiolite. Mineral parageneses indicate peak P r conditions of between 1 3 - 2 1 kbar and 550 650°C under high JO2 and p H 2 0 conditions. Syn-tectonic mineral phases such as kyanite porphyroblasts have preferentially grown within the foliation plane and define a dominant lineation trend. These structural fabrics are identical to those within the underlying Chewore Ophiolite and surrounding Chewore Inliers, where they have been dated at 524 ± 16 Ma (Goscombe et al., 1998). SHRIMP dating of zoned metamorphic zircons from the whiteschists reveal a more complex history with concordant ages at 1100 - 1050 Ma; 890 880 Ma and ages <590 Ma. It is unclear to the significance of these age clusters, however, it does indicate that the whiteschist protolith is older than 1100 Ma and possibly (due to the geochemical similarity) the same as the Mesoproterozoic OIB's of the Chewore Ophiolite. The Kadunguri Whiteschists record a very-high pressure tectonothermal / metasomatic event, the peak of which requires a minimum geothermal gradient of 10°C km thus indicating subduction zone / suture zone geotherms and a depth of burial of at least 50 km. The similarity in structural fabrics between the whiteschists and those known to be Pan African; the growth of peak metamorphic porphyroblasts within, and defining this fabric and the presence of <590 Ma metamorphic rims on zircons, indicates that this tectonothermal event was related to continental collision during the Pan African event. This suggests that the Zambezi Belt represents a significant suture in West Gondwana and that the Zimbabwe / Kalahari and Congo cratons did not finally amalgamate until the construction of Gondwanaland at c. 520 Ma. Reference GOSCOMBE, B., ARMSTRONG, R., & BARTON, J. M. 1998. Journal of Petrology 39 pp 1347 - 1384.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MOLYBDENITE TRACE ELEMENTS AS AN INDICATOR OF GRANITIC SOURCE Alex Johnston^ John M a v r o g e n e s ^ a n d Phil Blevin' 'Geology Department, The Australian National University, Canberra ACT 0200 ^Research School of Earth Sciences, The Australian National University, Canberra ACT 0200

Thirty seven molybdenite samples from a wide range of granite-related hydrothermal systems were analysed for trace elements by laser ablation ICP-MS. These show a systematic variation in the concentration of Se, Sn, Te, W, Re, and Bi. A consistent correlation between these elements and molybdenum suggest that they reside in the lattice of molybdenite. Thus, these trace elements provide a characteristic signature of the granitic source. Re/W ratios in molybdenite vary systematically with granite source type. Less evolved arc Cu porphyries have extremely high Re/W ratios. More evolved continental type Cu porphyries (such as Bingham Canyon) have intemediate Re/W ratios, while extremely fractionated Mo porphyry systems (such as Climax) have extremely low Re/W ratios (Fig. 1). In addition to those trace elements that are incorporated in the molybdenite structure, there are also discrete inclusions. These Pb, Bi and Te phases may have been dissolved within the molybdenite structure at elevated temperatures and subsequently exsolved upon cooling. Therefore, Bi may also correlate with the bulk geochemistry of the deposit. 9 " Cu - Au Porphyry

7-

§

<D q:

6H

432 -

0-

Sn - Mo Porphyry

-1Degree of Evolution

Molybdenite offers good potential for constraining the geochemistry of granitic bodies associated with mineralisation. Trace elements commonly taking up lattice substitutions within molybdenite, and inclusions appear to correlate with compositional evolution. Therefore, molybdenite geochemistry may prove o o useful as a petrogenetic tracer o for magmatic processes that O O occur prior to mineralisation.

Figure 1. Log Re/W in molybdenite vs. granitic evolution.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE NEED FOR USING COLLABORATIVE AND MULTIDISCIPLINARY APPROACHES TO RESEARCH THE BIOGEOCHEMISTRY OF COASTAL ZONE ECOSYSTEMS Ron Johnstone Centre for Marine Studies, University o f Queensland, Brisbane, Queensland 4 0 7 2 Australia

In the light of the growing human pressure being placed on aquatic ecosystems in Australia, and other parts of the world, there is a clear need to better understand and manage the impact that this might have on the environment. In addition to the more obvious effects that such pressures may have, such as the total removal of habitats, it is also necessary that we have an understanding of the more subtle impacts occurring within some of the fundamental processes and functional aspects of these ecosystems. The study of nutrient dynamics and biogeochemistry encompasses many of these foundation aspects, and the research it entails generally needs to have a perspective that is multi-scalar, and also multidisciplinary in its application. With this in mind, this talk will examine some of the issues of scale within biogeochemical research within the coastal zone and consider how this might effect the conclusions we draw about a given process or system. Also, in an attempt to encourage muhidisciplinary research and collaboration, examples will be drawn from different research fields to highlight some of the issues involved, and to exemplify how our understanding of ecosystem processes, and their management, is dependent on collaborative and multidisciplinary research initiatives.

264

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LITHOSTRATIGRAPHY OF CONTINENTAL SHELF, TROUGH-MOUTH FAN AND SEDIMENT DRIFT DEPOSITS, ODP LEG 188, PRYDZ BAY, EAST ANTARCTICA A.Kaiko\ P.E. O'Brien^ and Leg 188 Shipboard Scientific Party, ^Department of Applied Geology, Curtin University of Technology, GPO Box U1987 Perth, W.A. 6845. ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601

ODP Leg 188 drilled three sites between the Prydz Bay continental shelf and the continental rise to document the onset of, and fluctuations in East Antarctic glaciation. On the shelf, Site 1166 recovered upper Pliocene-Holocene diamictons and diatomaceous claystones directly above Upper Eocene-Lower Oligocene diatomaceous claystones with interbedded sand. Below are undated, matrix-supported, fluvio-deltaic sands and carbonaceous mudstones that mark the transition from glacial to pre-glacial conditions. Site 1167, on the Prydz Channel trough-mouth fan, recovered Pleistocene debrites composed of thick homogeneous packages of gravel-rich sediments with sandy-silt matrix, separated by thin homogeneous and laminated mudstones. The debrites mark glacial phases with high sediment input from ice grounded at the shelf break, and the hemipelagic mudstones are interglacial phases with contour-current deposition. An uphole shift in the distribution of sedimentary and granite lonestones indicates a shift in the Lambert Glacier/Amery Ice-Shelf drainage system. Site 1165 cored a 999-m Lower Miocene-Holocene section of the Wild Sediment Drift on the continental rise. Sediments throughout the hole alternate between intervals of a) dark grey laminated and homogeneous terrigenous clays (i.e. muddy contourites), and b) homogeneous, biogenic-rich, greenish-grey clays with a coarse sand-to-gravel-sized fraction (i.e., hemipelagic deposits with IRD). The sediments result from cyclic changes in bottom currents, terrigenous input and biologic productivity. Sedimentation rates decrease from ca 15 cm/ka in the Early Miocene to ca 5 cm/ka in the Middle Miocene, and to ca 1.5 cm/ka in Pliocene and younger sediments, reflecting reduced fine-grained terrigenous sedimentation. The Middle Miocene is a time of increased deposition of ice-rafted debris, clay minerals and siliceous microfossils.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

POST-RIFT TECTONIC SUBSIDENCE AND PALAEO-WATER DEPTHS IN THE NORTHERN CARNARVON BASIN; WESTERN AUSTRALIA A.R. Kaiko' and A.M. Tait^ 'Centre of Excellence in Petroleum Geology, School of Applied Geology, Curtin University, W.A. Recent work in the Barrow and Dampier sub-basins suggests that, following Jurassic rifting, accommodation space was created by simple thermal sag in the Late Jurassic to Early Cretaceous. Therefore, Tertiary sedimentation was a deep-water in-fill system. In the past, palaeo-water depth estimates for Jurassic to Recent sediments were based on environmental and age data from palynology and micropalaeontology, and matching identified time breaks to changes in the relative sea-level curve. This indicated that water depths in the Barrow and Dampier sub-basins were less than approximately 250 m to 300 m from the Late Jurassic to Tertiary. Along the Rankin Trend, to increase water depths and create accommodation space for the Late Tertiary carbonate progrades. Tertiary subsidence was invoked and related to the collision of the Australian and South East Asian plates. Applying this history, tectonic subsidence curves from one dimensional modelling indicate that post-rift thermal sag ceased in the Early Cretaceous and the Tertiary subduction-related subsidence was equivalent to or exceeded that of the extensional post-rift sag phase. Recognition of vitrinite reflectance suppression effects has removed the need for a recent heating event. Regionally, therefore, the main heating event was associated with Early-mid Jurassic rifting. Tectonic subsidence curves related to the post-rift thermal decay indicate that peak subsidence rate and formation of accommodation space occurred during the Late Jurassic to Early Cretaceous. Seismic indicates that structuring related to Miocene reactivation was localised along structural features / trends and was not a regional subduction-related subsidence event. Associated uplift was generally only a few hundred metres. In the Browse Basin, micropalaeontology studies indicate that Cretaceous water depths were significantly greater than during the Tertiary. This shallowing in water depth was rapid and coincided with increases in the sedimentation rates related to the arrival of progades in the area. Tectonic subsidence curves from one dimensional models are concave up, similar to theoretical thermal subsidence curves and show no major Tertiary subduction-related subsidence. Assuming that the background tectonic subsidence for the region is related to simple thermal sag it is theoretically possible to re-evaluate palaeo-water depths by adjusting the modelled tectonic subsidence curve to fit a simple theoretical tectonic subsidence curve. This approach applied to a series of wells across the Dampier Sub-basin gives water depths (during the Late Cretaceous to Early Tertiary) of approximately 250 m at Rosemary 1, 500 m at Madeleine 1, 800 m at North Rankin 1 and 1000 m at Brigadier 1. These water depths are similar to the accommodation space indicated by the height of Tertiary foresets noted on depth converted seismic. Arrival of the Tertiary progrades in-filled the accommodation space causing a general shallowing of water depth.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AREA SELECTION IN AUSTRALIA Bruce D. Kay Normandy Mining Limited, 100 Hutt St, Adelaide, 5000

Although area selection is the vital link in the exploration chain, traditional project generation methods are becoming less useful in the mature Australian environment to assemble strong tenement positions. There are several ways in which area selection can be undertaken and they usually involve a combination of the following activities: • • • • •

basic prospecting, reconnaissance sampling and staking; long-term conceptual and scientific studies by centralised "think tanks"; regional project generation attached to exploration teams; joint ventures with tenement holders corporate activity eg acquisitions, mergers.

In the past twenty years, there has been a major shift in the method of area selection in Australia from the long term scientific studies to entrepreneurial and corporate methods based on quick empirical data analysis. Many large companies have terminated their research groups and are using alliances with junior companies to undertake exploration. Native title has further complicated the land selection issue in Australia. Companies would originally avoid aboriginal land in Central Australia, but ironically, these areas are now more accessible to exploration than pastoral land affected by the Wik decision. The Normandy approach to area selection in the past ten years has been to build dominant positions in the best Australian mineral belts by a combination of corporate acquisition, joint ventures and regional project generation studies. Acquisitions have provided operating mines with cashflow, a strong and experienced workforce and large tenement positions with excellent potential for further discoveries. In a twelve year period, ten different Normandy transactions have lead to consolidated positions in the Archaean of Western Australia (Kalgoorlie, Boddington, Yandal Belt, Golden Grove), the Proterozoic of Central Australia (Callie, Tennant Creek) and the Permo Carboniferous of North Queensland (Mt Leyshon and Vera Nancy). These positions have been strengthened by near mine and greenfields discoveries which have enabled the Group to become Australia's largest producer of gold and formed a basis for major expansions offshore. Tenement rationalisation is a key part of this strategy and many properties have been divested, farmed out or relinquished in the twelve year period. It would have been extremely difficult to build up this extensive land position if the Company had been limited solely to open ground or joint ventures.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SALT BALANCE OF THE BUCKINBAH CREEK CATCHMENT, MACQUARIE RIVER BASIN, NEW SOUTH WALES, AUSTRALIA. G. A. Kazemi^ and W. A. Milne-Home^ ^Department of Geology, Faculty of Science, Shahrood University, Shahrood, Iran ^National Centre for Groundwater Management, University of Technology, Sydney, P O BOX 123, Broadway, NSW, 2007, Australia

Buckinbah Creek Catchment (BCC) is a tributary of the Macquarie River Basin in eastern Australia with an area of 802 Km which faces the problem of secondary soil salinisation locally known as "Dryland Salinity". The land clearing and the consequent rise in the water table levels are the deriving forces behind the problem of soil salinisation. Hourly flow and salt concentration of Buckinbah Creek (BC) as well as the salinity of rainfall were measured throughout 1998 in order to calculate the salt budget of the BCC. The results showed that in 1998 the salt output from the BCC, with a conservative estimate, was 1542 tonnes whilst the salt input through rainfall was only 439 tonnes, leading to a salt output/input ratio of 3.5. The extra-salt reserve of the BCC both in the unsaturated soil zone and in the saline shallow unconfmed aquifer that need to be flushed out before the catchment regains its pristine condition is determined to be 197588 tonnes. With the current salt imbalance, i.e. salt output/input ratio of 3.5, it takes about 220 years to deplete all the extra salt that are stored in the soil profile. The rainfall trend in the future, i.e. above average or below average rainfall condition, will play a significant role in determining the length of required time. In this study, the salt production and consumption through weathering processes and biomass uptake, respectively, were considered to be negligible. If these two are taken into account and numbered, then the required time to deplete extra salt will increase. The development of a less expensive-user friendly methodology to quantify these two processes (weathering processes and biomass uptake) is considered to be overdue.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE ENIGMA OF THE METAL SOURCES FOR THE NI-CU-PGE SULPHIDE DEPOSITS OF THE SUDBURY IGNEOUS COMPLEX : IMPLICATIONS FOR MINERAL EXPLORATION Reid R. K e a v s \ Peter C. Lightfoot^ and Will Doherty^ 'Mineral Exploration Research Centre, Laurentian University, Sudbury, Ontario, P3E 2C6, Canada ^Inco Ltd., Exploration Department, Highway 17 West, Copper Cliff, Ontario POM IMO ^Geological Survey of Canada, 501 Booth Street, Ottawa, Canada, K1A 0 E 8 There is now a general consensus that the world's largest repository for economic Ni-Cu-PGE sulphide deposits, the 1.85 Ga Sudbury Igneous Complex (SIC), was generated by the impact of a large meteorite which struck Early Proterozoic (Huronian) and Archaean target rocks. However, whereas some researchers (e.g.. Grieve, 1994) propose that the entire SIC was produced from the proposed resultant 2.2 km thick impact melt sheet, other authors (e.g., Naldrett et al., 1986) have suggested that the SIC is a product of up to 75 % crustal contamination of mantle-derived magmas. A similar debate has focussed on the source of the metals in the Ni-Cu-PGE sulphide deposits, with some authors arguing that they were derived entirely from the impacted target rocks while other authors maintain that they were derived from the crustally contaminated mantle-derived magmas. In order to address the problem of the source of the metals in the SIC and its Ni-Cu-PGE sulphide deposits, we have carried out a detailed study of a drill hole (Mac 91) which was collared at the base of the Granophyre and drilled through the Transition Zone Quartz Gabbro, Felsic Norite, Mafic Norite and Sublayer (the ore-bearing fragment-rich, sulphide-rich unit at the base of the SIC).. There is a systematic downwards increase in Cu, Ni and S from the top of the Felsic Norite in Mac 91 ( where, e.g., Ni = 14 ppm) to the base of the Mafic Norite (where Ni = 1000 ppm). Furthermore, these elements exhibit strong correlations, indicating that they are all hosted by sulphides throughout the succession. In contrast, while Ni and Cu are also exhibit sulphide control in the Sublayer, they are extremely heterogeneously distributed. Nickel, Cu and S increase systematically from 10, 10, and 500 ppm, respectively, at the top of the Felsic Norite to 1000, 1000, and 10,000 ppm, respectively at the base of the Mafic Norite. The strong correlations between Ni, Cu, S and Se (an analogue of S), plus the significant upwards decrease in Cu/Y ratios (which would remain constant in a fractionating S-undersaturated magma), as well as MORB-like Pd/Cu ratios provide strong evidence that the magma was S-saturated throughout the crystallization of the Felsic and Mafic Norites as well as during the formation of the Sublayer. Hence, sulphide melt was a cumulus phase throughout the crystallization of these units and the chalcophile metals (including Ni and Cu) were extracted from the silicate magma by the sulphide melt. Nickel and Cu increase from 1% Ni and 1% Cu in 100 % sulphides at the top of the Felsic Norite to 5 % Ni and 4.5 % Cu at the base of the Mafic Norite. It is significant that the tenor of sulphides at the base of the Mafic Norite fall in the middle of the range of the Ni and Cu tenors of SIC contact ores. The distribution of Ni and Cu can be modelled using the Ralyeigh Fractionation Law and the R factor equation using an R factor of 1000, Dcu^"'^^'' = 700, DNI = 250, Dn. opx/sii ^ ^^ ^^^ ij^j^i^j ^^^ ^ ^ contents of 250 ppm and 120 ppm, respectively. References GRIEVE, R.A.F. 1994, An Impact Model of the Sudbury Structure, j n Proceedings of the Sudbury Noril'sk Symposium, Special Volume 5, edited by P.C. Lightfoot & A.J. Naldrett, p. 119-132. LIGHTFOOT, P. C., KEAYS, R. R., MORRISON, G. G., BITE, A., & FARRELL, K. P. (1997b). Geochemical relationships in the Sudbury Igneous Complex: origin of the Main Mass and Offset Dykes: Econ. Geol., 92, p. 289-307. NALDRETT, A.J., RAO, B.V. & EVENSEN, N.M. (1986). Contamination at Sudbury and its role in ore formation: in Metallogeny of Basic and Ultrabsic Rocks, London, The Institution of Mining and Metallurgy, p. 75-91. TAYLOR, S. R. & MCLENNAN, S. M., 1995, The geochemical evolution of the continental crust. Rev. Geophys. 33, p. 241-265.

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SWATH-MAPPING AS A TOOL FOR GROUND-TRUTHING STUDIES OF CONTINENTAL MARGINS, GIPPSLAND BASIN, SOUTH EAST AUSTRALIA Jock K e e n e \ M. Hughes', M. Wallace^ G. Holdgate^ S. Gallagher^ J. Daniels^ A. Smith^ N. Exon^ and P. Hill^ 'School of Geosciences, The University of Sydney, NSW 2006 ^School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052 ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 Swath-mapping has provided multibeam bathymetry of the deeper water (>2,000m) region of the offshore Gippland Basin (Hill et al 1998). This data set has enabled follow up sampling cruises, for the first time, to carry out pinpoint sampling to ground-truth sedimentary facies in this environment. The basin margin is revealed as a complex mix of erosion (canyons and mass wasting) and deposition on both the continental slope and in the canyon floor. The submarine topography is well illustrated with a video fly-through using high-end graphics work stations (VisLab, Sydney University) with visualisation software (Fledermaus). This allows perspectives of submarine topography not easily envisaged by other means. Slumps, channels, fault scarps, and debris flows are identifiable. When overlaid with an acoustic backscatter map it particularly highlights the location of sands and debris flows (high returns) in the channel floor. The modem continental slope located in the offshore Gippsland Basin is clearly of two types, reflecting bedrock trends and erosion/deposition history. The western slope a the head of the main canyon (Bass Canyon) is a unique feature of the area, with a steep slope falling from 800m to 1800m in 4km, and consisting of a uniform series of parallel ridges and runnels. There are some 40 ridges and runnels over a distance along slope of 40km. Each of these runnels is 50-150m deep and has relatively smooth sides and floor, running directly downslope. They most likely formed during sea level lowstands and are now draped with a veneer of hemipelagic sediment. To the south there is a broad uniform continental slope, gently rising from the Bass Canyon floor at 3000m to the shelf edge some 80km to the south-west. Backscatter image and isobaths define low parallel ridges, trending roughly east-west, over large areas of this slope. Three 60-70km long canyons cut this slope. The canyons are relatively narrow, 500-800m wide, and 150-200m deep with flat floors. Over short distances they are incised over 500m and are still only a kilometre wide. They are boxedshaped in cross section with rock outcrop on the sides. They meander and have levees and terraces. (Exon et al 1999). When ground-truthed by sampling and deep sea photography the facies can be identified. Samples recovered from the upper continental slope (200-1000m water depth) range from greenish grey fine silty muds to fine sands. Cores on the upper slope reveal turbidites containing medium to coarse sand. The sands are both bioclastic and epiclastic. Sediments from the canyon floors are bioclastic sands or silts. Sediments recovered from the lower continental slope (1000-4000m water depth) are dominated by brown to green-grey foraminifer-bearing silty clays and muds with occasional sand and varying amounts of fine bioclasts. On the floor of a channel at 3693m in Bass Canyon there is a clean, well sorted medium to fine sand. Between channels cores and grabs sampled debris flows. References EXON N.F., HILL P.J., KEENE J.B. & SMITH S.M. 1999. The "SOJOURN 7" Swath-Mapping Cruise of R.V. Melville off Eastern Tasmania and in the Gippsland Basin. AGSO Record 1999/7, 50p. HILL P. J., EXON N.F., KEENE J.B. & SMITH S.M. 1998. The continental margin off east Tasmania and Gippsland: structure and development using new multibeam sonar data. Exploration Geophysics 29, 410419.

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Geological Convention, Sydney, July 2000

3D STRUCTURAL MODELLING OF THE MORAN OILFIELD, PAPUA NEW GUINEA COMPARED TO CAPE LIPTRAP Jeffrey T. Keetley and Kevin C. Hill. Australian Geodynamics CRC, La Trobe University, Melbourne 3083 Oil production from anticlines in the Papuan Fold Belt is well established, yet their 3D geometry is poorly known due to karst limestone and jungle cover limiting data acquisition. The 40+ wells, surface mapping and improved seismic data in the Kutubu production area allow the construction of 3D models. The interpretations can be tested and enhanced by 3D structural restoration. The models can be further refined by comparison with well known structural analogues such as the superbly exposed mesoscopic fold structures in Devonian turbidites at Cape Liptrap, near Melbourne. Within the Kutubu production license, detailed structural modeling and restoration using the fault-parallel flow algorithm within 3DMoveTM enables detachments and imbricated zones to be restored. 3DMove restored models of the Moran Anticline have led to interpretations of lateral variations in fault geometry and the formation of imbricated forelimb thrusts. These thrusts are associated with decreasing dip of the frontal thrust as well as interference with the Agogo-Mananda trend to the southwest. This implies partially synchronous growth of the Mananda and Moran anticlines. Restoration of the 3D model suggests - 4 0 % shortening from an original length of 46 km. Initial shortening probably occurred as a detachment fold or fault-propagation fold with - 2 0 % shortening, followed by thrust faults breaking through both the forelimb and backlimb with a further 20% shortening. Although restored separately, these events were not wholly discrete and probably overlapped in time. Whilst overall shortening was - 4 0 % , base Darai shortening was 12 km more than that in the underlying Toro formation indicating that the original Toro interpretation should be revised. Similar shortening of 40% is apparent on the regional GeosecTM lagifu-Nembi section. The real power of 3D modeling will be in revising the model incorporating the Cape Liptrap analogue to test alternative concepts. At Cape Liptrap, a one cubic metre fault propagation fold, with a different profile at each end, is being dissected to determine the internal 3D structure. Key observations from Cape Liptrap are: 1. Individual sandstone layers were thrust to form ramp anticlines, duplexes, fault propagation folds and detachment folds whilst the intervening shales were commonly deformed by pure shear. A consequence of this is that whilst thrust shortening is balanced across all sandstone beds, no thrust propagates through the shales to the next sandstone. This has important consequences for the interpretation of surface folds at depth in areas with thin competent beds separated by thick shales, as in Papua New Guinea. 2. Folding and thrusting occur equally downwards as well as upwards, such that ramp anticlines are often symmetrical in cross section, given a 180° rotation about a pole parallel to the fold axis. This is interpreted to be due to the relatively soft nature of the thick pile of sediments at the initial time of deformation, such that the effect of gravity was minimal. Shales/muds may be best modeled as a fluid. 3. The folds commonly involve thickening of the mudstones in the forelimb and then thrusts breaking through the competent sandstones in the forelimb. This is similar to processes inferred in the oil-bearing anticlines of Papua New Guinea. 4. Fault propagation folds and rounded open folds developed where sandstone:shale ratios are 1:2 and 1:4. Chevron folds developed where sandstone: shale rafios are 1:1. Within the chevron fold sequence, the folds only persist vertically for 1.5 - 2 x the wavelength of the fold and horizontally for 3 - 4 x the wavelength. 5. Overall the measurable shortening is approximately 50%. The current 3D Moran model mainly assumes constant bed thickness, but varying the 3D thickness of the leru Formation in the fold hinge and limbs as observed at Cape Liptrap can test this assumption. The effects of these changes on the 3D geometry of the reservoirs will allow assessment of variation in reserve estimates and possibly develop new play concepts. Acknowlegements: This work is published with the permission of the Director of the AGCRC, who supplied a scholarship to Keetley and laboratory funding. Midland Valley generously supplied 3DMove for the structural analysis whilst Paradigm Geophysical made available GeosecTM. The Chevron JV sponsored the study and gave permission for publication.

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GOLDEN CITIES - A NEW TYPE OF GOLD DEPOSIT IN ARCHAEAN GRANITE Harjinder S. Kehal' and Julian R. Stephens^ 'AMX Resources Limited, South Perth, W.A. ^Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia. Golden Cities an exciting new gold discovery in the Yilgam Craton of Western Australia highlights the significance of a new style of Archaean granite-hosted gold mineralisation. Previous gold exploration in the Yilgam Craton has mostly focused on greenstone belts, with little interest shown in the vast areas covered by granites (sensu lato). Recent exploration in the Yilgam Craton has outlined medium to large size gold deposits hosted within granitoids adjacent to greenstone contacts (e.g. Tarmoola and Granny Smith). However, the discovery of the Golden Cities deposits, hosted in the Scotia-Kanowna Granitoid Complex (SKGC), laterally removed from greenstones, represents the first of a new style of Yilgam Craton gold deposit. AMX Resources Ltd's precursor CIM Resources Ltd acquired the tenements over the SKGC in mid 1994 and reconnaissance soil sampling over areas underlain by granite undertaken in 1995 resulted in the discovery of a number of >200 ppb gold-in-soil anomalies. Quotes from AMX's 1996 prospectus best describe the initial thinking on the anomalies -"It is possible that rafts of greenstone occur within the granitoids, with associated mineralisation, as gold deposits without greenstone and occurring well within granitoids are not common." Initial RAB drilling of the gold-in-soil anomalies in June 1996, yielded 3m @ 3.45g/t Au and 9m @ 1.2 g/t Au in weathered granite 5 km away from greenstones! Follow-up RC drilling on the Havana Suva prospect retumed 56m @ 2.2g/t Au and 2Im @ I.9g/t Au. It quickly became apparent that an extensive gold mineralisation system had been discovered, generating enormous interest during the last half of 1996. As a result, there was a rush by exploration companies to acquire any ground over similar granitoid bodies in the Yilgam Craton. Major RC and diamond drilling programs began in early 1997 and continued until late 1998 at Golden Cities. The current gold resources at the Havana Suva and Jakarta deposits are 17 Mt @ I.62g/t Au for 887,000 oz with 0.5g/t cut-off. The adjacent Federal deposit (330,000 oz) was discovered by Centaur Mining and Exploration in 1997. The Golden Cities granite-hosted gold deposits lie within the SKGC, an ellipsoidal body 60 km long and 12 km wide that occupies the core of the SSE-plunging Scotia-Kanowna Anticline. A seismic traverse across the SKGC indicates it is underlain by felsic volcanic rocks at I to 3 km depth (Goleby et al., 1993), with flat-lying granite contacts in drill-core suggesting emplacement as sills. I-type, magnetite series, mediumcoarse grained biotite-homblende granodiorite intrusions dominate the SKGC. Most deformation has been accommodated in brittle stmctures, with three dominant, regional-scale stmctural trends; 1) NNW striking and ENE dipping fracture/alteration envelopes that contain most of the economic gold mineralisation i.e. the Havana Suva and parallel Federal trend 2) WNW trending, steep fracture zones (up to 200m wide) with strong haematite alteration, conjugate to the NNW striking mineralised envelopes 3) NE striking faults that extend into the greenstone successions to the east and west of the SKGC. Gold mineralization is locally controlled by two different fracture/vein sets; 1) Minor NW striking, NE dipping fractures and veins 2) Major NE striking and NW dipping fracture/vein set, en-echelon to the NNW striking mineralized envelopes. Gold mineralisation is controlled by the intensity of fracturing and minor quartz veining, with best grades associated with crackle to mosaic breccias. The ore mineralogy is relatively simple, being dominated by pyrite with lesser chalcopyrite, and common coarse gold. Trace ore minerals include galena, bismuthinite and native bismuth, aramayoite (Ag(Sb,Bi)S2) and tellurides. Gangue alteration minerals are albite, biotite, silica and sericite, with minor hematite, chlorite, epidote, carbonate, titanite and apatite. There is a low relative abundance of quartz veins compared to most Archaean greenstone-hosted deposits, although those that do occur commonly contain coarse gold. Fluid inclusions from similar auriferous quartz veins at the nearby Federal deposit are H 2 0 - C 0 2 dominant and have low salinities (Phillips and Zhou, 1999). The discovery of gold deposits in Archaean granite, laterally removed from greenstones has 'opened up' large areas of the Yilgam Craton that were previously considered unprospective for gold. Phillips and Zhou (1999) argue a metamorphic origin for the Golden Cities deposits. This is favoured by the structural setting, where faults may propagate from greenstones upward through the overlying granite. Their dismissal of a magmatic fluid origin, however, seems somewhat premature. The carbonic fluids associated with gold mineralisation are similar to those in many greenstone-hosted deposits, but intriguingly, also similar to those associated with many magmatic-related deposits (Lang et al., in review). References GOLEBY, B.R., RATTENBURY, M.S., SWAGER, C.P., DRUMMOND, B.J., WILLIAMS, P.R., SHERATON, J.W., & HEINRICH, C.A., 1993 - Archaean cmstal stmcttire from seismic reflection profiling, Eastem Goldfields, Westem Australia. AGSO Record, 1993/15. PHILLIPS, G.N., & ZHOU, T., 1999 - Gold-only deposits and Archaean granites. Society of Economic Geologists Newsletter, number 37 Lang, J.R., Baker, T., Hart, C.J.R., and Mortensen, J.K., submitted to Society of Economic Geologists Newsletter, July, 1999 - An Exploration Model for Intmsion-Related Gold Systems

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PROTEROZOIC REWORKING OF ARCHAEAN CRUST IN THE RAYNER COMPLEX, EAST ANTARCTICA. N.M. Kellv^ G.L. Clarke' and C.M.Fanningl 'School of Geosciences, F05, University of Sydney, NSW, 2006, Australia ^Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia.

Archaean crust, tectonically reworked during the Mesoproterozoic, forms the Oygarden Group of islands in western Kemp Land, east Antarctica. The rocks are part of the Rayner Complex, and lie near the eastern boundary of the Archaean Napier Complex. The oldest rock is a layered composite orthogneiss, intruded by a felsic orthogneiss that locally cuts a high-grade Si gneissosity. A second event (D2) caused the transposition of most fabrics and lithologies into S2. The Rayner Structural Episode (D3 and D4) resulted in sub-horizontal thrusting (D3) at lower crustal levels (P~9kbars and T>800°C) and large-scale (2-3 kms wide) extensional shear zones (D4) at similar conditions. Decompression of the terrane occurred late in, or post-dated, a D4 event. Zircons from 7 samples and titanites from 1 sample were imaged by cathodoluminescence and backscattered electron techniques, and then analysed using the SHRIMP II at ANU. Data indicate a c. 3500 Ma age for igneous protoliths to the oldest rocks in the area, before widespread magmatism and metamorphism that accompanied Di at c. 2700 Ma, and D2 at c. 2400-2500 Ma. Metamorphic zircon in felsic orthogneiss and inherited magmatic zircons in later pegmatites indicate limited magmatism and metamorphism at c. 1600 Ma. These data provide the first evidence for an event of this age in this part of Antarctica. D3 occurred at c. 930 Ma and resulted in the dominant fabric through rocks the Oygarden Group. D4 occurred at c. 925 Ma, within error of D3, suggesting these two events were part of one tectonic event. A chamockite from the Stillwell Hills (c. 100 kms east of the Oygarden Group) has an age of c. 920 Ma. This chamockite is c. 40-60 Ma younger than the Mawson and northern Prince Charles Mountains chamockites, and represents the most western extent of magmatism on the Mawson-Kemp Land coast. Peak metamorphism and magmatism that accompanied the Rayner Structural Episode occurred some 40 Ma later in the Oygarden Group than similar events that affect parts of the Rayner Complex on the Mawson Coast and northern Prince Charles Mountains. 2400-2500 Ma ages for metamorphism in the Oygarden Group correlate with late metamorphism in the Napier Complex that occurred at c. 2450 Ma. These two areas therefore share a common history from at least this time, and the area of the Rayner Complex from the Oygarden Group to the Stillwell Hills represents part of the Napier Complex reworked during the Proterozoic Rayner Structural Episode. Magmatism and metamorphism at c. 1600 Ma in the Oygarden Group correlates with similar ages for high grade metamorphism in the Eastern Ghats, India, south of the Godivari Rift. This age may represent the timing of amalgamation of the Napier Craton to eastern India.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GENESIS OF PERALUMINOUS GRANITIC ROCKS: A SOURCE-BASED PERSPECTIVE Tony LS. Kemp^ D J . Ellis' and C.M. Gray^ 'GEMOC, Department of Geology, ANU, Canberra, A.C.T. ^Department of Earth Sciences, La Trobe University, Bundoora, Vic. Numerous models have been proposed to explain the occurrence of strongly peraluminous granitic rocks in orogenic belts, but their specific petrogenesis, and intrinsic petrochemical diversity, still remain controversial. Particularly problematic is the role of entrained residual source material, or 'restite' in controlling the extreme compositional variation typical of these rocks. These issues are unresolved in part because of a reliance upon inferences drawn from studies of upper crustal plutons that are displaced from the generative region and inevitably represent the end-product of many interactive petrogenetic mechanisms. However, the Cambro-Ordovician Glenelg River Complex (GRC) of western Victoria exposes a range of peraluminous granitic rocks close to their source region, and therefore provides a rare opportunity to evaluate source-based processes and test the efficacy of restite entrainment. These phases, referred to as 'Harrow type' rocks, occur as dykes, plutons and sill-like bodies with unmodified igneous textures and abundant magmatic muscovite; intimate field association with migmatites and lack of mafic enclaves suggests that all formed purely by anatexis of the host metasedimentary sequence. Collectively, a restricted range of Si02 content (71.7-76.7%) is evident, together with low Rb/Sr and poor correlation between major elements and silica on variation diagrams. Two subgroups are resolved, leucocratic adamellite to tonalite plutons and relatively mafic, biotite-rich granodiorites. The former are predominant and heterogeneous on outcrop scale, typically with sheeted structures. Chemically, a pronounced and uniform depletion in ferromagnesian elements (FeOt <1.5%) is characteristic, along with a high Sr signature (>230ppm) and striking dispersion of data along a vertical array for K2O and Ba; variation within individual plutons approaches that of the group as a whole. In contrast, mafic Harrow type granodiorites extend to lower Si02 but distinctly higher Ti02, FeOt (>2.5%) and Rb/Sr. Importantly, leucocratic Harrow type rocks are linked to leucosomes of well-segregated stromatic migmatites, which have derived by water-fluxed partial melting of quartzofeldspathic precursors. Field and chemical evidence suggests that felsic plutons comprise an amalgamation of these leucosome-derived melts, transported from the source region via sheet-like conduits and emplaced as individual magma batches. The low Ti02 and FeOt therefore reflects the remarkably effective melt segregation mechanism in the protolith, such that residual biotite was not significantly entrained. Variation in K2O, Sr and Ba within individual bodies reflects poorly-blended melt contributions from different metasedimentary horizons and further implies that leucocratic plutons do not have a single, specific protolith composition. This aspect is only evident due to the lack of homogenisation between constituent magma batches during pluton assembly, such that the chemical signature of each separate source component is preserved. In higher level plutons sourcerelated heterogeneities tend to be erased by magmatic processes, so that the evidence for multiple protoliths is masked or obliterated. K-poor leucotonalites have a similar petrogenesis, except that protoliths were semipelitic and K2O and Ba were retained in the residue by excess muscovite and refractory biotite. In contrast, mafic Harrow type granodiorites have numerous metasedimentary enclaves and exemplify the mobilised melt-rich portions of poorly-segregated diatexitic horizons, into which they are transitional. Hence, the biotite-rich character and entrained restitic material of mafic granodiorites was inherited from the precursor, though this circumstance is exceptional for magmagenesis in the GRC. Leucocratic and residue-rich granitic rocks in the GRC therefore do not represent part of a single magmatic lineage but are end-members of ftindamentally different melt segregation and transport processes in the source. Felsic plutons demonstrate that batches of granitic magma may be efficiently extracted from chemically disparate protoliths at low melt fraction without wholesale entrainment of refractory material, resulting in the formation of restite-poor peraluminous plutons close to the source region. Imperfect mixing between magma batches is therefore responsible for the marked compositional heterogeneity within plutons. These observations emphasise the profiindity of source-based processes and question the utility of restite separation for generating compositional diversity within peraluminous granitic rocks in general, except in minor localised cases.

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Geological Convention, Sydney, July 2000

THE LINK BETWEEN CRUSTAL ANATEXIS AND MAFIC MAGMATISM FOR THE PETROGENESIS OF GRANITIC ROCKS: AN EXAMPLE FROM WESTERN VICTORIA Tony I.S.Kemp^ D.J. Ellis' and C.M. Gray^ 'GEMOC, Department of Geology, ANU, Canberra, A.C.T. ^Department of Earth Sciences, La Trobe University, Bundoora, Vic. It is commonly assumed that migmatites formed by water-saturated partial melting represent 'failed granites' and bear no connection with the upper crustal batholithic T - and 'S'-type granitic plutons of many orogenic belts; in contrast, these are thought to form by higher temperature biotite- and hornblende-dehydration melting. To the contrary, we present compelling field and chemical evidence that leucocratic 'minimum melts' formed by water-fluxed anatexis have effectively segregated from their migmatitic protoliths and interacted with coeval mantle-derived mafic magmas, which also constitute the heat source, to generate a spectrum of metaluminous to peraluminous granitic rocks. Implications for the petrogenesis of granitic bodies in general are fundamental. In particular, temperatures of high level melts, extrapolated to source, may bear little relevance to regional metamorphic temperatures at depth. The field area lies within the Delamerian Glenelg River Complex (GRC), western Victoria, where diverse metaluminous to peraluminous granitic phases were emplaced synchronous with fluid-present anatexis within the host metasedimentary sequence. Distinctive and unifying petrographic and geochemical features facilitate grouping of these granitic rocks into separate magma 'types' the most important of which, in sequence of relative intrusion timing, are the Wando, Wennicott, Tuloona and Loftus Creek types, the latter post-dating most deformation. Also discerned are Harrow type granitic rocks, a variable group of muscovite-rich leucogranitic-tonalitic phases formed by near-eutectic partial melting of metasedimentary protoliths, typically without entrainment of residue (see companion poster). The early syn-tectonic Wando and Wennicott magma types comprise primitive hornblende tonalites to minor granodiorites with abundant mafic igneous-textured enclaves; they have contrasting enclave populations and different K2O variation. Post-dating these, plutons of the Tuloona magma type range from mafic biotite tonalite to more felsic and commonly muscovite-bearing granodiorite and adamellite bodies transitional to migmatite. Mafic igneous enclaves are ubiquitous but subordinate to metasedimentary enclaves in more felsic varieties. All plutons are crosscut by homogeneous bodies of texturally-unique hornblende granodiorite of the Loftus Creek magma type, of which an alkaline signature and elevated Sr contents are diagnostic. Significantly, intrusion of granitic rocks was accompanied by mantle-derived mafic magmatism, initially apparent as MORB-like gabbroic sills and high-Al gabbrogabbronorite bodies, but supplanted later in the structural history by mafic dykes with shoshonitic affinities. Mantle magmatism therefore records the transition from a convergent or collisional tectonic regime to a postcollisional extensional setting. Field and coherent geochemical relationships indicate that petrogenesis of all granitic magma types of the GRC involved mixing between mafic magma and efficiently-segregated Harrow type melts, with variation in the end-members responsible for the systematic petrographic and compositional differences between each. In the case of Wando types, the end-members are high-Al gabbro and K-rich adamellite, with differences between plutons attributed to varying proportions of these components. In contrast, Wennicott type rocks require an intermediate-K mafic end-member, which does not outcrop but is represented by swarms of doleritic enclaves. The linear trend towards lower K2O with increasing Si02 within this group results from blending with muscovite leucotonalite. Tuloona type rocks represent variable combinations of muscovite leucotonalite and high-Al gabbro-gabbronorite, though here the mixing trend is complicated by local interaction with more potassic Harrow type magmas. Post-compressional Loftus Creek type rocks are most distinctive, with the parental magmas generated by mixing between crustally-derived leucotonalites and shoshonite; the intrinsic high K, Ba and Sr of this group was clearly imparted by the latter. Hence, the chemistry of each granitic magma type mirrors changes in the mafic magmatism with time, and therefore tracks the tectonic evolution of the orogen. This study therefore exposes the folly of relying on upper crustal granitic plutons to deduce the compositional character of the source rocks, and subsequently invalidates the simplistic T and 'S' classification scheme. Clearly, there remains a dire need for careful field-based studies in lower crustal anatectic terranes if the mechanisms of granite generation, and therefore crustal differentiation, are to be properly understood.

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THE MANTLE BENEATH AUSTRALIA B.L.N. Kennett, Research School of Earth Sciences, Australian National University, Canberra 0200 The earthquakes lying in the major earthquake belt to the north and east of the continent provide useful probes of the seismological structure beneath the Australian region. Suitable events are quite frequent in the zone of seismicity which extends from Indonesia, through New Guinea to Fiji and then down through Tonga and New Zealand. The number of permanent stations with high-fidelity recording is small but provide a substantial data set over time. Coverage of the continent has been enhanced by a systematic program of deployment of portable broad-band seismographs. The Skippy project 1993-1996 provided over 60 different recording sites across the continent with a inter-station spacing of about 400 km. These stations typically been occupied for 5 months which is sufficient to get good coverage of regional seismicity. The continentwide coverage has been supplemented by more detailed studies. The Kimba project in 1997-1998 has provided good coverage of the Kimberley region (WA/NT) including the mobile belts. The Quoll project in 1999 has been designed to provide higher-resolution information in southeastern Australia and links to the delay-time work being carried out in Victoria and South Australia by a cooperative project between Monash and Adelaide Universities. The portable and permanent sites offer high-fidelity recording of the seismic wavefield across a broad range of frequencies. As a result it is possible to exploit the characteristics of the many different types of seismic waves which travel from the source to the sensors. Each of these seismic phases acquire information about the seismic structure during their passage along their path from source to receiver. P and S body waves (with frequencies above O.I Hz) provide information on mantle structure from 100 - 800 km, with a geographic coverage concentrated in the north of Australia, as a result of the source distribution. We are able to extract information on the seismic velocity distribution and also the attenuation structure and its variation with frequency. An alternative source of information comes from the large amplitude surface waves which occur late in the seismograms, these sample the mantle nearly horizontally and their characteristics provide strong constraints on the average structure along the paths from source to receiver. The techniques used to extract information from the seismograms on three-dimensional structure are similar in principle to medical tomography. The characteristics of waves which have travelled in many different directions through the region beneath Australia are combined to build up images of the threedimensional seismic structure. From the surface waves we have been able to build a relatively detailed 3-D shear wave structure for both seismic wavespeed and azimuthal anisotropy, and are now in the process of developing a more general anisotropic model exploiting both the vertical and horizontal components of seismic recording. For body waves we can provide both independent checks on the surface wave results especially for the structure of the lithosphere, and also a moderate resolution image of structure at greater depth. The net result is that we now have for the first time a fair picture of the structures in the mantle beneath the Australian region. For the lithosphere, structures are certainly resolved and consistent between different styles of analysis for with horizontal scales larger than 500 km. In the deeper mantle we can resolve structures on scales around 1000 km. There are substantial contrasts in structure in the lithosphere and upper mantle. The region beneath the central cratons is characterised by a fast 'lid' extending to at least 200 km and locally to 300 km or more. Whereas on the eastern margin of the continent, the fast seismic 'lid' does not extend to depths larger than about 100 km and there is a pronounced low-velocity zone between 100 and 200 km depth. The transition between these two regimes is complex and does not have a simple relation to the conventional Tasman line marking the separation of Precambrian and Phanerozoic outcrop. The edge of the thick, fast, wavespeeds lies approximately at 140-141uE, and further east there is a second contrast between a moderately fast zone extending to about 150 km depth and the main region of lowered velocities. Azimuthal anisotropy (1-2 percent) is oriented mainly east-west in the uppermost mantle, probably due to past deformation, but the direction changes significantly below 150 km to a more northerly direction consistent with current plate motion. This suggest that deformation is occurring within the region of fast seismic wavespeeds. The lithospheric velocities of SH waves are generally significantly higher than for SV waves and appear to require strong fabric in lithospheric structure. The features in the seismic lithosphere most likely represent the scars of past cycles of continental dissipation and amalgamation and represent important constraints on past processes. The information from body waves confirms the presence of strong variations in lithospheric S wave structure though these are not as pronounced for P waves. The zones of high SV wavespeeds have very high Q and are underlain by material with much higher attenuation with a substantial frequency dependence. There is certainly structure also in the mantle transition zone though this has no obvious link to the surface geology.

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THERMAL METAMORPHISM OF VOLCANIC ROCKS ON BARTON PENINSULA, KING GEORGE ISLAND, ANTARCTICA Hveoncheol K \ m \ Moonsup Cho^ Jong Ik Lee' and Moon Young Choe' 'Polar Research Center, Korea Ocean Research and Development Institute, Sa-dong 1270, Ansan 425-170, ^School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea

King George Island, located near the southern termination of the South Scotia Ridge, Antarctica, consists of three tectonic blocks bounded by large-scale strike-slip fauhs. Plutonic intrusions are restricted to the central block called the Barton Horst. Volcanic rocks inside the Barton Horst are commonly altered, in contrast to the prevalent occurrence of fresh volcanic rocks outside the horst. Although some features of alteration have been described, there have been few attempts to characterise the conditions under which these altered rocks were formed. We have found a widespread occurrence of low-grade metamorphic assemblages in the Barton Peninsula, and report here on the origin of these mineral parageneses. Barton Peninsula, located in the southwestern part of King George Island, mainly consists of Early Cretaceous to Tertiary volcanic breccia, basalt, basaltic andesite, tuff and granodiorite. Volcanic rocks occupy most of the peninsula and range in composition from basalt to andesite. Texturally they comprise mainly stratified lapilli tuffs, volcanic breccias and lava flows. These volcanic rocks have experienced thermal metamorphism up to the upper greenschist facies, producing calcic-amphibole (Amp), epidote (Epi^ chlorite (Chi), calcite (Cc), white mica, plagioclase and opaque minerals (hematite, ilmentite and pyrite). Spherulitic aggregates of quartz often fill amygdales, while rare grains of prehnite and laumontite occur in a few amygdales. Two metamorphic zones, Cc-Chl and Amp-Chl zones, are defined on the basis of assemblages [+ plagioclase, quartz (Qtz) and opaque minerals], characterised by epidote + chlorite [VA calcite and actinolite IVA hornblende + epidote + chlorite fA calcite, respectively. The transition between two zones is governed by the reaction: 3Chl + lOCc + 21Qtz = 3Amp + 2Epi + 8H20 + 10C02. The distribution coefficient of Mg-Fe* (total Fe) between actinolite and chlorite (0.561.03) and the coexistence of actinolite and oligoclase support a low-pressure metamorphism. Moreover, compositions of amphiboles are compatible with those of lowpressure facies series. Metamorphic temperatures estimated from the chlorite geothermometry are mostly in the range of ca. 250-300iiEC. These resuhs together with the regional distribution of two metamorphic zones suggest that metavolcanic rocks on Barton Peninsula have experienced low-pressure thermal metamorphism, probably associated with island-arc magmatism during the late Mesozoic-Tertiary subduction of the southeast Pacific plate under the Antarctic continent.

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THE LIFE OF A SUBDUCTION ZONE - MASS AND TECTONIC BALANCE IN THE SUBDUCTION ZONE Gaku Kimura Department of Earth and Planetary Science, University of Tokyo, JAPAN

Subduction zone is one of the critical places to understand the mass flux in our planet; continents are created through subduction zone magmatism and conversely surficial materials including continents are taken back into the deep interior of the mantle. Several recent calculations of mass flux suggest that influx of subduction erosion plus delamination in subduction zone seems more dominant than outflux due to the continental birth through magmatism. This first order caluculation gives a paradox that continetal mass is decreasing although the most hypotheses suggest gradual or episodic growth of continents through the earth history. Tectonic balance between the accretion and subduction erosion is one of the keys to solve this mass flux paradox. Recent investigations of subduction zone around Japan and accretionary complex on land of the Japanese Islands give us a new insight for the tectonic process and a life of the subduction zone. Eastern margin of the Japan Sea is a juvenile subduction zone where the oceanic crust of the Japan Sea is collapsing and accreting through vital earthquake process. The Nankai Subduction zone of the southwest Japan is a well-studied accretion margin. Strong coupling along the subduction interface is revealed both due to high thermal regime and seamount subduction. Ongoing underplating of the seamount is suggested in terms of high resolution studies of seismicity by JAMSTEC team. The high resolution studies of the Japan Trench (by JAMSTEG) indicate that a thick pile of low velocity layer can be traced upon the subducting Pacific Plate at least down to several tens of kilometer depth. The pile may be a subduction erosion complex of high pressure type metamorphic rocks. Recent structural study of accretionary complex on land indicates that subduction erosion, especially basal erosion process is well recorded although the subduction erosion has been thought to be an evidence-missing process so far. Vivroseismic study in Hokkaido, where is the collision juncture between the Northeast Japan and Kuril Arcs, clarified that the delamination of the lower crust of the Kuril Arc is taking place. Taking all the phenomena mentioned above together with recent progress of other subduction zones, tectonic accretion of oceanic crust is going only in a juvenile stage of subduction or the case of hot subduction zone. In the other case of subduction,, subduction erosion appears dominant.

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THE LEWISIAN COMPLEX - A COLLAGE OF DISPARATE TERRANES ASSEMBLED DURING THE PALAEOPROTEROZOIC ERA P.D. Kinny ' and C.R.L. Friend ^ ^ Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, WA 6845 ^ Department of Geology, Oxford Brookes University, Oxford 0X3 OBP, U.K. The Lewisian Complex of northwest Scotland is arguably the most well-known, well-exposed and accessible example of a Precambrian high-grade metamorphic gneiss complex. Originally mapped over 100 years ago, it has long been regarded as a 'type locality' for the study of middle to lower crustal processes, and a 'testing ground' for new geological techniques. Astonishingly, however, the history of the Lewisian and its relationships to other ancient landmasses preserved in the North Atlantic region are still poorly understood. Geological correlations across the complex are mostly anchored on the work of Sutton & Watson (1951) who regarded the complex as a contiguous piece of Archaean crust that was variably reworked along shear zones in the Proterozoic. Such ideas were established on structural and lithological criteria long before reliable geochronological techniques became available, and have become entrenched in the literature. The recent application of robust, high-precision geochronometers by ourselves and others (e.g. Corfu et al., 1994, Kinny & Friend, 1997), has provided critical constraints on the absolute ages of different components of the complex, and on the various tectonothermal episodes which have shaped and modified the Lewisian complex over time. These new data have shown that existing models need major revision. For example, whereas the subdivision of the mainland Lewisian into a central region of granulite facies rocks, flanked to the north and south by amphibolite facies rocks, is still widely regarded as due to the Proterozoic exhumation of different crustal levels within a block of single-aged crust (e.g. Park et al., 1994), mounting geochronological evidence indicates that: (1) different parts of the complex were formed at different times; and (2) blocks of differing age have experienced different metamorphic histories. Our most recent work has identified further important differences between units exposed on the mainland and across the Minch strait on the islands of the Outer Hebrides. For example, granites in the South Harris injection complex, dated at c. 1675 Ma, are 200 million years younger than granites in the Northern region of the mainland with which they have been correlated (Deamley, 1962), whilst a c. 3125 Ma protolith age of a tonalitic gneiss on Harris intruded by these granites is significantly older than any rocks yet identified on the mainland. Such evidence suggests that the Lewisian represents a collage of Archaean crustal blocks formed and evolved at different times, and whose assembly into their present configuration was not completed until the end of the Palaeoproterozoic. References CORFU, F., HEAMAN, L.M. & ROGERS, R. 1994. Contrib. Mineral. Petrol. 117, 215-228. DEARNLEY, R. 1962. Q. J. Geol. See. London 118, 143-176. KINNY, P.D. & FRIEND, C.R.L. 1997. Contrib. Mineral. Petrol. 129, 326-340. PARK, R.G., CLIFF, R.A., FETTES, D.J. & STEWART, A.D. 1994. Geol. Soc. London Spec. Report 22, 13-25. SUTTON J. & WATSON, J. 1951. Q. J. Geol. Soc. London 106, 241-296.

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LATE PALAEOZOIC EVOLUTION OF THE SOUTHERN NEW ENGLAND OROGEN: PALAEOMAGNETIC CONSTRAINTS FROM THE ROCKY CREEK BLOCK OF THE TAMWORTH BELT Chris Klootwijk Australian Geodynamics Cooperative Research Centre Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 The Carboniferous-Early Permian succession of the Tamworth Belt has considerable potential for defining the Late Palaeozoic polepath for the New England Orogen. If this polepath's first-order representation for cratonic Australia can be confirmed, it also has considerable potential to resolve the dilemma of two different interpretations for the Late Palaeozoic polepath for cratonic Australia and Gondwanaland (the SEP- and KGpolepaths, see Klootwijk 1997) with their differing implications for regional and global tectonics. This potential stems from the high suitability of the frequent felsic volcanic intercalations within the mainly volcaniclastic succession for determining primary magnetizations despite various magnetic overprints, and for SHRIMP U-Pb zircon dating. Preliminary palaeomagnetic results from reconnaissance studies in four tectonic units of the Tamworth Belt — from north to south, the Rocky Creek, Werrie, Rouchel and Gresford Blocks — have previously been compiled into a preliminary (TB-) polepath for the New England Orogen (Klootwijk 1997). This preliminary polepath resembles the KG-polepath for cratonic Australia, but not so the more widely published SLP-polepath. Extensive palaeomagnetic, rockmagnetic and magnetic fabric followup studies throughout these four blocks of the Tamworth Belt are now nearing completion, with results substantiating and refining the outline of the preliminary TB-polepath. Results and implications are reported for the Rocky Creek Block, the first of these follow-up studies. Detailed thermal demagnetization studies of Visean to Westphalian felsic volcanics and volcaniclastics from the Rocky Creek Block (64 sites, >700 samples) have established well-defined primary magnetization results for 29 sites, and evidence for four magnetic overprint phases of widespread (related to recent/mid Cainozoic weathering and to the Permo-Triassic Hunter-Bowen Orogeny) or more local occurrence (related to Late Cretaceous opening of the Tasman Sea and to latest Carboniferous-Early Permian initiation of the BowenGunnedah-Sydney Basin). The primary site-mean results have been combined into 7 mean-site poles covering 26 sites (5 positive fold tests at confidence levels of 95% to 99%) with a further 3 single-site poles. These 10 pole positions outline an extensive Visean to Westphalian polepath, refining and detailing the preliminary TB- polepath for the Tamworth Belt. The new palaeomagnetic, rockmagnetic and magnetic fabric results for the Rocky Creek Block demonstrate the following: • Consistent volcanic flow and volcaniclastic transport directions in the older succession of the Rocky Creek Block, the Caroda Formation to Clifden Formation, and more variable directions in the cataclysmic younger succession, the Rocky Creek Conglomerate and Lark Hill Formation. • No noticeable rotational deformation between the three major thrust sheets of the Rocky Creek Block: the Rocky Creek, Kathrose and Darthula Thrust Sheets. • Good agreement between contemporaneous pole positions for the Rocky Creek Block (the High Valley andesite tuff of the Caroda Formation, the Peri Rhyodacite Tuff of the Clifden Formation) and for the Rouchel and Gresford Blocks (the Native Dog Member of the Isismurra Formation, the Mirannie Volcanic Member of the Seaham Formation). This indicates absence of significant rotational deformation between the northwestern and southwestern parts of the Tamworth Belt and contrasts with recent interpretations for a 70° to 80° counterclockwise rotation of the Rouchel and Gresford Blocks versus cratonic Australia. • Further evidence for a Late Devonian-Early Carboniferous northward excursion over more than 30° of latitude, culminating in the middle Visean. If proven representative for cratonic Australia and Gondwanaland, this excursion provides a mechanism for the Early-to-middle Carboniferous Kanimblan and Alice Springs Orogenies as caused by far-field stresses originating from convergence between Greater Australia and the Central Asian Fold Belt on the southern periphery of the Siberian Craton. Reference Klootwijk C.T. 1997. Phanerozoic configurations of Greater Australia: Evolution of the North West Shelf Part two, Palaeomagnetic and geologic constraints on reconstructions. AGSO Record 1996/52, 85 pp.

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A DEEP SEISMIC REFLECTION PROFILE ACROSS THE GILMORE FAULT, EASTERN LACHLAN OROGEN R.J. Korsch,^'^ K.C.Lawrie,^ D.W. Johnstone''^ and L.E.A. Jones^'^ ^ Australian Geodynamics Cooperative Research Centre ^ Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601 Australia As a contribution towards developing an exploration model for the eastern Lachlan Orogen, the Australian Geodynamics Cooperative Research Centre (AGCRC) acquired a deep seismic reflection profile across the Gilmore Fault in 1999. The line, in the vicinity of Barmedman on the Cootamundra 1:250 000 sheet, is nearly 90 km long and recorded to 20 sec TWT (about 60 km depth) using vibroseis equipment managed by the Australian National Seismic Imaging Resource (ANSIR). The Gilmore Fault is interpreted as a major crustal structure separating the Wagga-Omeo Belt from the Junee-Narromine Volcanic Belt, and has been variably proposed as a crustal suture zone (the 'Gilmore Suture') or a strike-slip fault, or as part of an imbricate fault zone. Significant porphyry-related, structurallycontrolled Au-Cu and Au ore deposits occur in Ordovician-Silurian volcano-sedimentary host rocks within the fault zone, which may have acted as a major fluid pathway. The fault zone is, however, predominantly under cover and hence the deep seismic reflection technique was used to examine its geometry in the third dimension. The seismic profile commenced in the Wagga-Omeo Belt in the west, crossed the Junee-Narromine Volcanic Belt and continued further east across younger Palaeozoic rocks. The Wagga-Omeo Belt, consisting of Ordovician quartz-rich turbidites intruded by Silurian granites, is poorly reflective but contains weak, eastdipping reflections. It has a wedge shape, being about 5 sec TWT (15 km) thick in the west, tapering to the east. The belt is floored by a unit consisting of a series of strong west-dipping reflections, which we interpret to be Ordovician and/or Cambro-Ordovician volcanic or ocean floor material. Thus, the major boundary between the two belts appears to be a major west-dipping structure at the top of the volcanic unit which we interpret as an east-directed thrust. At the surface, however, the Gilmore Fault {sensu stricto) appears to be a high level, east-dipping, west-directed backthrust off the top of the Ordovician volcanic pile at depth, thrusting the Ordovician Gidginbung Volcanics over rocks of the Wagga-Omeo Belt. East of the Gidginbung Volcanics is the Combaning Formation, generally interpreted as being deposited in a Siluro-Devonian extensional basin. The Combaning Formation occurs in the hanging wall of an east-dipping thrust, which is sub-parallel to the Gilmore Fault. Internal reflections within both the Gidginbung Volcanics and Combaning Formation suggest a relatively open folded, flat-lying stratigraphy. This is supported by limited field observations. Further east, the Ordovician Belimebung Volcanics appears to be bounded by faults, with an east-dipping thrust defining its western margin and a subvertical fault in the east. The Barmedman Granite is relatively thin, only about 500 ms ( - 1 . 5 km) thick. The uppermost part of the crust at the eastern end of the seismic profile is dominated by two stratigraphic successions, the western one we interpret to be the Siluro-Devonian Combaning Formation and the eastern one to be the Late Devonian Hervey Group. The Combaning Formation appears to thicken to the west, providing support for its deposition in a half graben. The formation is also internally deformed with large fold structures evident. It is bounded on the east by a west-dipping thrust fault that puts this formation on top of the Hervey Group, which here is up to 3 km thick. This thrusting is thus Carboniferous or younger in age. In summary, the seismic profile is dominated by a major east-directed thrust system that puts the WaggaOmeo Belt over the Junee-Narromine Belt, and thus could be considered to be a crustal-scale suture. The Ordovician volcanics appear to underlie all of the surface rocks east of the Gilmore Fault that are younger than Ordovician, and occurs at depths below 1-2 sec (3-6 km). The Gilmore Fault appears to be a high-level, west-directed backthrust soling onto the suture at the top of the Ordovician volcanics at a depth of about 1.2 sec TWT, that is, at about 3.6 km.

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WEATHERING AND SECONDARY GOLD, BENDIGO REGION, CENTRAL VICTORIA, AUSTRALIA. A. Kotsonis\ S. McKnight', M. J. Hughes', D.C. Ame^ S.P. Carey' and E.B. Joyce^ ' Minerals Industry Research Institute, University of Ballarat, PO Box 663, Ballarat, 3353 ^ Martin Hughes and Associates, 1034 Geelong Road, Mt Clear, Ballarat, Vic, 3350 ^ School of Earth Sciences, University of Melbourne, Parkville, 3052 Western Australian School of Mines, Curtin University of Technology, Kalgoorlie WA 6430 The economic significance of weathering in the Victorian gold province has been largely ignored and is only now being addressed. This is due to the perception that deep lateritic weathering and associated supergene enrichment are not features characteristic of the Victorian gold province, although there is widespread evidence of secondary gold in the weathered zone. Within the Bendigo region, deep weathering profiles and duricrusts are discontinuous and poorly preserved. However, remnant pallid zones developed on Palaeozoic metasedimentary rocks and Devonian granites, and isolated outcrops of ferruginous and clay duricrusts indicate two distinct intervals of deep chemical weathering: (1) an earlier deep weathering event developed on metasedimentary rocks possibly associated with a ferruginous duricrust and soil, and (2) a pisolitic ferricrete profile overlying mottled clays developed on Shepparton Formation alluvial sediments and interpreted as a lateral equivalent of the Karoonda Regolith of Pliocene age. An investigation of these weathering profiles in relation to mineralisation north and east of Bendigo indicates intense weathering of the metasediments with kaolinite to depths in excess of 70 m in some places. There is a strong association of As with ferricrete, with higher levels in the early duricrust and lower levels in the pisolitic ferricrete of the Karoonda Regolith. There is a strong correlation between Au and As in fresh metasedimentary rock at depth, reflecting primary gold-arsenopyrite association in gold mineralisation. However, there is no similar correlation between Au and As in the saprolite developed on metasedimentary rock. Where the early duricrust overlies mineralised quartz veins, there appears to be some minor gold enrichment within the duricrust and the underlying 5-10 m of saprolite where extensive ferruginisation has occurred. The weathered oxide zone of mineralisation at Fosterville, approximately 22 km east of Bendigo, may be equivalent to the older of these deep weathering profiles. The Fosterville orebody consists of an upper oxide zone where gold occurs in finely particulate form (1-10 *m diameter) both as free gold and within iron oxides resulting from the breakdown of sulphides. It is underlain by a disseminated pyrite-arsenopyrite assemblage, where gold occurs in the free state in minor amounts but principally as micron sized gold within pyrite and arsenopyrite. However at two localities studied, gold interpreted as supergene occurs in the oxide zone within iron-cemented quartz veins, often as specimens of variably disseminated "sponge" gold up to several millimetres in diameter, usually in contact with the quartz, and intimately intergrown with the iron oxides. Detailed examination indicates that the gold is associated with oxidised stibnite pseudomorphs, poorly crystalline, colloform, hydrated Fe-Sb oxides and quartz, and is very different to the predominant pyritearsenopyrite mineralisation at Fosterville. Within these mineralised veins at least two generations of quartz can be identified, an initial high-fluid inclusion quartz and a second euhedral quartz phase, both of which host stibnite with only the high-inclusion phase hosting gold. This suggests stibnite and possibly gold emplacement may have occurred throughout development of these veins. Although some gold shows textural relationships suggesting a non-supergene origin, there are remarkable differences in fineness between what are inferred to be two distinct populations of 980-998 and 935-946, sometimes in single gold grains. Colloform gold of high fineness also occurs within thin iron oxide fracture veins that cross-cut Fe-Sb oxides, and these features suggest at least some supergene refinement or re-precipitation. A relative sequence of weathering can be established for the gold-stibnite veins. The most obvious feature associated with weathering is the oxidation of stibnite, which is followed by precipitation of iron oxides that show high levels of As (up to 4.27 wt %), probably due to the oxidation of the surrounding disseminated arsenopyrite. Colloform gold was precipitated at this time. This in turn was followed by infilling of voids with clay argillans which show anomalous levels of phosphorous (up to 0.73 wt %).

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REACTION-HARDENING DURING EXHUMATION OF FELSIC ECLOGITE Maarten Krabbendam Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Melbourne VIC 3800, Australia. In many continental high-pressure (HP) and ultrahigh-pressure (UHP) terranes, mafic eclogite occur as pods within gneisses or schists with lower pressure (amphibolite- or greenschist-facies) assemblages. Metamorphic and structural evidence shows that these lower pressure assemblages developed during exhumation and hence post-date peak-pressure conditions. This selective retrogression (s.l) has been partially explained by Heinrich (1982), who suggested that pelitic HP assemblages are more hydrous than LP pelites, so that decompression involves rapid dehydration reactions. In contrast, the eclogite to amphibolite transition requires hydration, which may be inhibited by lack of water. Since felsic eclogite (Phengite + Qtz (or coesite) + Cpx + Zo ± Ky) is also more hydrous than amphibolite-facies quartzo-feldspathic gneiss this model is also applicable to felsic rocks. In the Western Gneiss Region in Norway (a large Caledonian HP/UHP terrane) the following field observations were made. A) Exhumation-related deformation of the quartzo-feldspathic amphibolite-facies gneisses is strong but fairly homogenous (i.e. poorly partitioned) over vast tracts (>40 000 of km2) of the terrane. B) Felsic eclogite is preserved, but only in domains that behaved as low strain zones during exhumation, commonly adjacent to mafic eclogite bodies. C) Extreme competency contrasts occur between felsic and mafic eclogite where these rocks occur together in larger domains. Thus, in addition to the metamorphic processes described by Heinrich (1982), deformation may have played an important role during decompression metamorphism. Point c) suggests that the mechanical strength of felsic eclogite, controlled by phengite or zoisite, is lower than the strength of its amphibolite-facies equivalent, which is controlled by feldspar or quartz. Stockhert & Renner (1998) also suggest very low strength of continental UHP rocks. A model of overall reaction-hardening is proposed here to explain the observations and (if generally applicable) the general rarity of felsic eclogite. Metamorphic reactions can have both transient and permanent effects on the rheology of rocks. Grain size reduction and weakening effects caused by dehydration can be transient effects if subsequent grain growth or drainage occurs (Rubie 1990). Changes of mineralogy, however, can cause permanent effects if the bulk rheology of the reactant is markedly different from the reaction product. When the reaction product is inherently softer than the reactant, overall and permanent reaction-softening occurs. Subsequent strain increments deform preferentially the reaction product, resulting in strong localisation of strain and reaction (e.g. phyllonitic shear zones in granite). In contrast, if the reaction product is inherently harder than the reactant, overall reaction-hardening is expected, even though during the reactions transient softening may occur, for instance by temporary grain size reduction. During reaction-hardening, subsequent increments of strain will preferentially deform the reactant, rather than the reaction product. Deformation and reaction fronts propagate through the entire rock mass, away from the hardened zones. The implication of this is that every domain in the rock (except where shielded from deformation) is deformed and metamorphosed to quartzo-feldspathic gneiss. Thus, reactionhardening is expected to result in homogenisation of strain and metamorphism, especially when operating in conjunction with dehydration reactions, precisely what is observed in the Western Gneiss Region and other HP and UHP terranes. The combination of dehydration and reaction-hardening provides a credible explanation for the pervasive deformation and decompression reactions that are typical of gneisses that surround mafic eclogite-boudins. The tectonic implication of this process is that continental HP and UHP terranes may have experienced profound density changes during orogenesis. Reaction-hardening associated with dehydration may also operate during prograde granulite-facies metamorphism. References

Heinrich C.A. 1982. Kyanite-eclogite to amphibolite facies evolution of hydrous mafic and pelitic rocks, Adula Nappe, Central Alps. Contributions to Mineralogy and Petrology 81, 30-38. Rubie D.C. 1990. Mechanisms of reaction-enhanced deformability in minerals and rocks. In: Barber D.J. & Meredith P.G. eds. Deformation processes in minerals, ceramics and rocks, pp. 262-295, Unwin Hyman London. Stockhert B. & Renner J. 1998. Rheology of crustal rocks at ultrahigh pressure. In: Hacker B.R. & Liou J.G. eds. When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-Pressure Rocks, pp 57-95. Kluwer Dordrecht.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TECTONIC SIGNIFICANCE OF LATE-STAGE SILICICLASTIC SEDIMENTARY SEQUENCES IN THE ARCHAEAN EASTERN GOLDFIELDS PROVINCE OF WESTERN AUSTRALIA Bryan B. Krapez Department of Geology, University of Western Australia, Nedlands, WA, 6907

Many late-stage siliciclastic conglomerate-sandstone sequences of Archaean greenstone provinces are fluvial deposits of small fault-bound basins that immediately predated granitoid emplacement, regional uplift and peak deformation. In the Eastern Goldfields Province, the best-known late-stage siliciclastic sequences are the Kurrawang and Merougil Formations, and the Jones Creek Conglomerate but equivalent sequences include the Penny Dam, Yilgangi, Lancefield, Wallaby and Mount Lucky Conglomerates, and the Mount Belches and Yandal Sandstones. All the sequences are unconformable on greenstones or nonconformable on granitoids, indicating that they post-dated regional uplift and subaerial erosion. The sequences are preserved in structural lows, either between downthrown fault-blocks, on the downthrow-sides of strike faults or in the cores of synclines. Facies analysis reveals that the sequences record two types of depositional system: proximal submarine-fan (Kurrawang, Penny Dam, Mount Belches, Yilgangi, Lancefield, Wallaby, Mount Lucky) and proximal to medial braid-plain (Jones Creek, Yandal, Merougil). There is no preserved time-stratigraphic link between the systems. With the possible exception of the Jones Creek Conglomerate, none of the sequences preserves evidence for deposition in a fault-bound basin. On the contrary, the submarinefan sequences in particular imply large-scale depositional systems. SHRIMP detrital-zircon provenance dating indicates that detritus for submarine-fan sequences was derived from same tectonic-cycle granitoids and greenstones, relict tectonic-cycle granitoids and greenstones, and Early Archaean basement. In contrast, detritus for fluvial sequences was derived only from same tectonic-cycle granitoids and greenstones. The submarine-fan sequences are interpreted to represent remnants of one province-scale basin, whereas the fluvial sequences represent remnants of one or two intraprovince-scale basins. Similar modem siliciclastic sequences that unconformably overlie greenstone-like terrains represent proximal submarine and subaerial facies of remnant ocean basins during orogenic closure of back-arc or marginal seas.

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Geological Convention, Sydney, July 2000

FOAM TEXTURES IN QUARTZ, PLAGIOCLASE AND ANALOGUE MATERIAL: AN INVESTIGATION IN TEXTURAL EQUILIBRIUM Joem H. Kruhl Institut fuer Geologic, Geotechnik und Baubetrieb, Technische Universitaet, D-80290 Muenchen, Germany The shape, pattern and crystallographic orientation of grain boundaries represent important characteristics of crystalline material and its deformation and annealing history. On the basis of experimental annealing of analogue material and empirical data fi-om naturally annealed quartz and plagioclase the strong influence of the crystallographic orientation of grain boundaries on their development and geometry and on the development of a foam texture can be demonstrated. Foam textures represent self-similar patterns with fractal dimensions which do not indicate 'textural equilibrium' but possibly the temperature level of annealing. During annealing grain sizes stabilize at certain values. On a statistical basis, these values can be applied as a geothermometer, in case of plagioclase with an accuracy of ca. ±25°C. The grain boundaries in foam textures are frequently built by two or several stt-aight segments which are rotated against each other. These segments preferentially occupy specific crystallographic - in the case of quartz rhombohedral - positions, namely in relation to both the neighbouring crystals. Even strongly kinked boundaries and dihedral angles largely deviating from the 'equilibrium angle' of 120° may be stable over long periods of annealing. Parts of single boundaries may migrate whereas other parts are stationary. Migration may be localized and there are indications that foam textures react inhomogeneously during annealing. In general, it follows that the anisotropy of surface energy has much influence on the development of foam textures and that each modelling of texture development has to include the influence of crystallographic orientation of grain boundaries. The results of the present study suggest that the most promising methods to characterize foam textures (as well as any other types of grain boundary patterns) and their development are methods of fractal geometry and a detailed evaluation of crystallographic orientations.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PRELIMINARY GEOCHEMICAL RESULTS FROM ODP LEG 188: PRYDZ BAY, ANTARCTICA Kelly A. Kryc', Mark Lavelle^ P.E. O^Brien^ and Leg 188 Shipboard Science Party ^ Earth Sciences, Boston University, 685 Commonwealth Ave. Boston, Massachusetts 02215, USA. ^British Antarctic Survey, High Cross, Madingley Rd., Cambridge, CB3 OET. United Kingdom ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia

A transect of three drill sites on the continental rise (Site 1165), slope (Site 1167), and continental shelf (Site 1166) were drilled during ODP Leg 188 to Prydz Bay, Antarctica. Interstitial waters (IW) were analyzed for salinity, CI", pH, alkalinity, S04^', NH4^, Si02(aq), Mg^^, Ca^^, Sr^^, Li^, Na^, K^, Mn^^ and Fe^^. Cores were also analysed for hydrocarbon gases, and selected sediment samples were characterised for the amount and type of organic matter. The IW profiles at Site 1165 reflect the balance between diffusion from sea-water, sediment reactions, and diffusion from a Ca-rich lithology deeper in the section. S04^" is linearly depleted with depth (28 to 2 mM) to --150 mbsf. NH4^, P04^" and alkalinity show large increases over the same depth interval. Dissolved Si02 values increase rapidly to just below the solubility limit of opal-A (-^1100 DM) between 200 and 400 mbsf, then decrease to 120 |LIM at total depth. Headspace CH4 increases rapidly below the S04^" minimum, and reaches residual concentrations equivalent to 30 mM at total depth. Corg contents are low (avg. 0.4%) except for an organic-rich (2%) interval at 110-112 mbsf. Ca^ and Mg^^ are non-linearly (inverse) correlated, suggesting independent sources and sinks. At Site 1167, CI" and S04^" increase rapidly by >3% from the seafloor to - 2 0 mbsf, possibly indicating the presence of high salinity last-glacial sea-water. From - 2 0 to -lOOmbsf S04^' and alkalinity show minor decreases (30 to 28 mM and 2.5 to 1.5 mM, respectively), and NH4^ increases linearly (0 to 86 |LIM). Organic matter content is low (<1%), and is dominated by a component derived from the erosion of sedimentary basins on the shelf. At Site 1166, S04^" is linearly depleted from the seafloor (28 mM) to 150 mbsf (8 mM), and NH4^ shows an equivalent increase to 1277 |LIM. From 0 to 75 mbsf, alkalinity, Si02 and K^ decrease, while Ca^^ increases, suggesting diagenetic silicate reactions are occurring. From 150 to 300 mbsf, Ca^^ and Mg^^ show parallel trends, suggesting diffusional processes are dominant. Headspace CH4 did not rise above background levels, but an organic carbon-rich (9.2%) bed was present at the base of a fluvial/deltaic sand, and a deeper carbonaceous claystone unit had uniformly high Corg contents (2.8-5.2%) in the interval between 275 and 315 mbsf.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE APPLICATION OF SOIL-GAS GEOCHEMISTRY TO PRECISELY LOCATE LA VICTORIA FAULT NEAR PARACOTOS (VENEZUELA)^ John J. LaBrecque, Pedro A. Rosales and Pedro R Cordoves Centro de Quimica, Instituto Venezolano de Investigaciones CientiTicas, Caracas, Venezuela Mailing Address: Centro de Quimica, 8424 N W 56th Street, Suite CCS 00204, Miami, F1 33166, U S A

Measurements of radon (total radon, Radon-222 and Radon-220) and other soil-gases (C02, 02 and H2) were performed routinely during 1998 and 1999 across a narrow valley near Paracotos, Venezuela in an attempt to precisely locate the La Victoria fault. The transect was about 300 meters long with eleven sampling points. The soil-gas probes were inserted to a depth of 45 cm in the beginning and later on completely to a depth of 63 cm. The radon sampling and measurements were accomplished with a Pylon AB-5 radiation monitor and Lucas scintillation cells. The other soil-gases were directly determined with an Anagas, CD95 monitor and an Infra-red Gas Analyzer (MKIIC) both coupled with a Hydrogen pod. The radon values for more than twenty different sampling periods over a two year period resulted with anomalous values between 75 and 150 meters along the transect. There were three consecutive anomalous values each time. But strangely, the anomalies of the radon values were in the form of a doublet at 116 and 141 meters rather than a simple single peak in the middle and the gas flow was similar for the sampling points between 75 and 150 meters. The graph of the relative CO2 values were usually similar to the radon graphs but in some cases, the anomalous values were seen as a simple single peak and corresponded to the 141 meter sampling point. While the anomalous values of Hydrogen were usually in a form of a single peak that corresponded with 141 meter sampling point. Only a few times were values for Hydrogen higer than 100 ppm and detected at most of the sampling points, usually only one or two points resulted with small values near the 141 meter sampling point. Based on the radon values alone, we would have to conclude that the fault probably lies near or between the 116 and 141 meter sampling points, but with the additional data of the CO2 and H2 soil-gases one could say that the fault is probably near the 141 meter sampling point. Thus, we have concluded that soil-gas geochemistry studies can precisely locate active faults and in this case one, which only has about one mm of movement (displacement) per year. Acknowledgement: This work was partially supported by a research grant from the Venezuelan National Research Foundation (CONICIT), Project No. SI-95000448.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PALAEOMAGNETIC INVESTIGATION OF LATE PALAEOZOIC VOLCANIC UNITS FROM THE NORTHERN NEW ENGLAND OROGEN, QUEENSLAND. M. A. Lackie Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109,

The New England Orogen comprises a number of Palaeozoic and Mesozoic terranes. This paper presents the results of a palaeomagnetic study of volcanic units from the southern part of the Connors-Camboon Province in Queensland. The age range of the Province is 320 - 280 Ma, with this study concentrating on the Late Carboniferous Torsdale Volcanics and the Early Permian Camboon Volcanics from the Cracow area (25.3°S, 150.3°E). The Camboon Volcanics sampled were generally andesitic ignimbrites and lavas yielding a primary direction in both magnetite and haematite carriers. All directions were reversed and analysis of the data shows a southerly steep down direction with a mean direction of Dec = 171.7°, Inc - 7(5.(5° (ags = 6.1k = 52, N=12) which results in a palaeomagnetic South pole of 50.4''S, ISS.Q'^E (10.5,11.3). This is consistent with other Australian Early Permian data indicating a primary magnetisation age. The Torsdale Volcanics sampled were generally silicic ignimbrites that yielded a primary direction in both magnetite and haematite carriers. The directions are generally reversed, although some normal polarities are observed. The directions are not as steep as those observed for the Camboon Volcanics and show a more westerly character, with a mean direction of Dec = 217.8"", Inc - 68.8"" ((ags = 7.9°, k = 59, N=7) which results in a palaeomagnetic South pole of (11.4,13.4). The pole is consistent with the Late Carboniferous age of the unit.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SYNTHESIS OF PALAEOMAGNETIC RESULTS FROM SILURO-ORDOVICIAN ROCKS OF THE LACHLAN FOLD BELT M.A. L a c k i e \ P.W. Schmidt^ and D.A. Clark^ ^Department o f Earth and Planetary Sciences, Macquarie University, Sydney N S W 2109. ^CSIRO Exploration and Mining, PO Box 136, North Ryde N S W 2113.

Gondwana's early Palaeozoic apparent polar wander path remains ill-defined particularly for the Siluro-Ordovician segment. Rocks of this age in the Lachlan Fold Belt tend to be magnetically overprinted and most do not appear to contain magnetic grains of optimum Stability to retain primary remanence directions. To address this problem we have started a study of Siluro-Ordovician volcanics, intrusions, associated homfels and mineralisation/alteration systems in central-western NSW. Often haematite and/or finegrained magnetite are alteration products and it is the palaeomagnetic signature of these minerals we seek. Pilot samples were collected from fresh and altered rocks of the Late Silurian Wondalga Granodiorite, the Late Ordovician Gidginbung Volcanics at Temora and gabbroic diorites and monzodiorites of the Early Silurian Wallundry Suite. As well, samples were collected from the Late Ordovician Tettenhall Monzodiorite and Tallwood Monzonite and their associated homfels, and basalt's within the Forest Reefs Volcanics. Samples were also taken from oriented exploration core, the samples being from intrusions, homfels and haematite altered volcanics close to mineralisation systems. The palaeomagnetic components from the Adelong and Temora alteration systems were unstable. All oriented drillcore sampled from the Adelong system, was overprinted by the initial drilling, with poor palaeomagnetic data obtained from the surface samples from the Adelong system. Encouraging results were obtained from monzonite samples from oriented core from a drill hole from the Orange region but only a few samples were not strongly overprinted by the initial drilling. Samples from diorite, gabbro and norite intmsions from Wyalong, Fifield and Hylea have been studied by us and students over a number of years and generally yield directions north-east and shallow, reminiscent of (anti-podal) directions found in the Early Ordovician Black Hill Norite, SA. These units are being re-examined to yield more precise palaeomagnetic results. These older results and new results will be combined to give an overall synthesis of Siluro-Ordovician palaeomagnetic results from the Lachlan Fold Belt.

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GABBROIDS, BASALTS & LAMPROPHYRES OF THE NEW ENGLAND BATHOLITH:A LEGACY OF LATE CARBONIFEROUS TO MIDDLE TRIASSIC ARC MIGRATION. Bill Landenberger and William J. Collins School of Geosciences, The University of Newcastle, Callaghan NSW, 2308 Australia. The voluminous New England Batholith (NEB) of eastern Australia temporally spans the latest Carboniferous to the latest Triassic, and comprises a large variety of S-type, I-type and A-type granitoids. Tectonic inferences cannot be reliably drawn from the trace element signatures of these felsic rocks since they were largely generated from crustal materials, and hence such signatures are inherited from the crustal protoliths. However, mafic plutonic rocks temporally accompany most of the granitoid suites of the batholith and basalts occur in the surrounding basins. Intrusion of the oldest suite of granitoids in the batholith, the S-type Hillgrove Suite, was accompanied by the gabbros and diorites of the Bakers Creek Suite in the latest Carboniferous. Primary magmas of this suite are primitive gabbros unaffected by crustal contamination. Trace element patterns (on N-MORB normalised multielement diagrams) of these gabbros are relatively flat with moderate to strong HESE depletion and minor LILE enrichment. These patterns resemble juvenile back-arc basin basalts. Early Permian mantlederived magmas comprise basalts present in rift basins (Gloucester, Sydney, Cranky Comer, Nambucca). These basalts have still flatter trace element patterns with no trends of depletion or enrichment, and show progressive change from OIB to MORB-like patterns through this period. Most I-type granitoids of the N E B intruded during the Late Permian to Early Triassic, and most are associated with lamprophyre dykes. The lamprophyres exhibit steep patterns, with very strong LILE enrichment, and moderate to strong negative Nbanomalies. Late Triassic mafic rocks are mainly associated with A-type granitoids of the batholith, and include minor tholeiitic gabbros and moderately alkaline mafic rocks. Protracted arc magmatism along the NNW-trending >Kuttung= arc during most of the Carboniferous indicates that stable west-dipping subduction occurred throughout this period. Arc activity was terminated by rapid (eastward) arc and subduction zone retreat in the latest Carboniferous, heralded by intrusion of the Bakers Creek Suite primitive gabbros. These rocks have depleted asthenosphere signatures, accompanied by a minor, yet pronounced slab flux component. Progressive back-arc development, lithospheric thinning and crustal extension in the Early Permian, are indicated by the OIB and MORB like signatures of the syn-rift basalts. The trend of eastward arc migration and extension was reversed in the Late Permian by compressive deformation associated with the Hunter-Bowen Orogeny. Westward arc advance was monitored by increasing amounts of easterly derived volcanic material into the Sydney Basin. The mantle contribution to the New England Batholith in the Late Permian and Early Triassic, was dominated by lamprophyres with steep trace-element patterns that suggest derivation from thickened lithosphere, a scenario consistent with the re-establishment of arc magmatism in a continental margin thickened by arc collision (Hunter-Bowen Orogeny). During the mid-late Triassic, minor mantle-derived tholeiitic gabbroids, alkaline mafic rocks, and generation of A-type granitoid suites, indicates a rapid return to lithospheric thinning and associated crustal extension (development of the Clarence-Moreton and Lome basins), which eventually progressed into major rifting during the Cretaceous.

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GROUND SELECTION FOR STRATIFORM ZINC DEPOSITS IN NORTH AUSTRALIAN PROTEROZOIC BASINS: SOURCES, AQUIFERS, SEALS, FAULTS, TRAPS AND HALOS. Ross R. Large^ Stuart W. Bull', David R. Cooke', Grant Garven^ Peter J. McGoldrick' and Jianwen Yang' 'Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, Australia 7001 ^Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 North Charles Street, 301 Olin Hall, Baltimore, USA MD 21218 Recent research on basin architecture (Leaman, 1998), fluid chemistry (Cooke et al., 2000), basin hydrology (Garven and Bull, 1999) and lithogeochemistry (Large et al., 2000) has highlighted some of the key elements of area selection for stratiform zinc deposits in the Australian Palaeoproterozoic. Tectonic events that control fault architecture, development of organic-rich shale depocentres and fluid flow patterns during deposition of the mineralized carbonate successions, are critical for zinc transport from the source regions deep in the basin to the trap sites on or close to the basin floor. Sources of fluids: hydrological models suggest that free convection of modified marine (or lacustrine?) water descending into the basin along specific faults and aquifer systems will provide sufficient fluid to form large tonnage deposits in reasonable time intervals. Source of metals: felsic and possibly mafic volcanics and volcaniclastic sediments that typically comprise the initial phase of basin sedimentation are the likely source of zinc and lead. A two-stage metal transport process is envisaged. Stage 1 occurs early in the evolution of the carbonate succession and involves leaching metals from these sub-surface sources and deposition in the hematitic sandstone aquifers. Stage 2 involves transport of metals from the aquifer to the trap site during the ore-forming event. Source of salinity: marine water already modified by evaporation could increase further in salinity during fluid recharge through the shallow water carbonate succession by dissolution of evaporites, enhancing its capacity as a metal transport medium. Aquifer system: hematitic sandstones at the base of the mineralized carbonate succession represent the hydrothermal fluid aquifer. Their hematite-bearing nature is critical in maintaining oxidised fluid conditions conducive to maximum metal transport (Cooke et al., 2000) under a normal geothermal gradient (100-200°C). Fault architecture: the configuration and magnitude of active syn-sedimentary faults and their intersection geometry in relation to the aquifer system controls the fluid recharge and discharge patterns in the basin. This is the most important factor controlling the sites of stratiform zinc mineralisation. Seals and reservoirs: regionally, the mineralized carbonate and shale succession act as a leaky seal over the aquifer system. During long periods of tectonic stability associated with shallow water carbonate platform sedimentation, fluids trapped in the aquifer system undergo continual convection to produce a high salinity homogeneous metalliferous sulfate brine. Periods of tectonic activity associated with sub-basin development, cause the seal to be broken along active faults, resulting in metalliferous brine discharge from the reservoir. Recharge from surface waters replenish the reservoir during each tectonic cycle. Trap sites: metals are deposited in carbonaceous shales due to interaction of the hot oxidised metalliferous brine with organic-rich bottom waters and/or sediments. Under these conditions aqueous sulfate is reduced to H2S leading to zinc and lead sulfide precipitation. Lithogeochemical halos: extensive halos of Mn- and Fe-bearing carbonate, occur along strike and into the hangingwall of the ores. Enrichment of thallium and various alteration indices can be used to define the halos and predict potential stratigraphic horizons for zinc mineralisation (Large et al., 2000). References COOKE D. R., BULL S. W., LARGE R. R., McGOLDRICK P. J. 2000. The importance of oxidised brines for the formation of Australian Proterozoic stratiform sediment-hosted Pb-Zn (SEDEX) deposits. Economic Geology 95. GARVEN G. & BULL S. W. 1999. Fluid flow modeling of the HYC ore system, McArthur Basin, Australia. In: Stanley, C. J., et al. Mineral Deposits: Processes to Processing, Fifth biennial SGA Meeting and the Tenth Quadrennial lAGOD Symposium 2, pp. 849-852. LARGE R. R., BULL S. W.& McGOLDDRICK P. J. 2000. Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn-Pb-Ag deposits. Part 2: HYC Deposit, McArthur River, Northern Territory. Journal of Geochemical Exploration 64, 1-2, 105-126. LEAMAN D. E. 1998. Structure, contents and setting of Pb-Zn mineralisation in the McArthur Basin, northern Australia. Australian Journal of Earth Sciences 45, 1, 3-20.

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FLUID FLOW WITHIN THE MARBLES AND CALC-SILICATES OF THE HARTS RANGE GROUP, CENTRAL AUSTRALIA Patricia Lavery and Ian Buick Department o f Earth Sciences, La Trobe University, Bundoora, Victoria 3083

Enigmatic marbles and calc-silicates of the Harts Range Group (HRG), central Australia, contain mineral assemblages with oxygen isotope signatures that are incongruent with simple models for fluid infiltration. Their petrology and stable isotope geochemistry do not conform to models for equilibrium up-temperature, down-temperature, or isothermal infiltration. The HRG marbles and calc-silicates occur within the eastern Arunta Inlier which underwent regional metamorphism, from amphibolite to granulite facies, at around 475 Ma (Mawby et al, 1999). The marbles and calc-silicates of the HRG occur within the structurally lower Irindina Gneiss as well as the overlying Brady Gneiss. The Irindina Gneiss is a multiply metamorphosed high pressure granulite facies sequence that includes interlayered metabasites, metapelites, marbles and calc-silicates. The marbles and calc-silicates are deformed and have been intruded by the Palaeozoic Harts Range pegmatites. The Irindina carbonates are commonly cross-cut by anastomosing networks of quartz veins which contain grossular garnet, diopside and titanite. The Irindina marbles and calc-silicates of the Harts Ranges typically contain low variance assemblages rich in wollastonite and grossular garnet. The presence of appreciable quantities of wollastonite and grossular garnet is indicative of infiltration by a water-rich fluid. Infiltration of an externally derived water-rich fluid is also expected to cause a downward shift in oxygen isotopic signatures. The Irindina wollastonite-rich carbonates have calcite 5180 values that range from 15 to 26%o. The highest 5180 values are similar to those expected for unaltered sedimentary limestones. In addition, marbles and calc-silicates with high wollastonite contents commonly contain silicate minerals with 5180 values that are much lower than, and isotopically unequilibrated with, calcite. This oxygen isotope disequilibrium is significant as the calcite would be expected to reset more easily than the silicate phases. The disequilibrium may be indicative of rapid fluid infiltration, possibly related to regionally significant pegmatite intrusion after the metamorphic peak. The Brady Gneiss is the structurally highest unit of the Harts Range Group and has been metamorphosed to upper amphibolite grade. It contains calc-silicates that are inter-layered with gamet-biotite gneiss and are cross-cut by the Harts Range pegmatites. The Brady calc-silicates are composed of scapolite, clinozoisite, titanite, clinopyroxene, plagioclase, amphibole ± calcite. Calc-silicate whole rock oxygen isotope values range from 6 to 11.5%o where the cross-cutting pegmatite silicate whole rock oxygen isotope analyses yield values of about 8%o. Brady calc-silicates yield calcite 5180 values of approximately 15%o which are substantially lower signatures than found in the Irindina carbonates. It is possible that the Brady calc-silicates were infiltrated by a fluid associated with the pegmatite intrusion thus altering the stable isotope values. The Brady Gneiss calc-silicates do not display disequilibrium features between calcite and the silicate phases, as seen in the Irindina carbonates. Reference M A W B Y J., H A N D M., & FODEN J. 1999. Sm-Nd evidence for high-grade Ordovician metamorphism in the Arunta Block, central Australia. Journal of Metamorphic Geology 17, 653-668

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Geological Convention, Sydney, July 2000

EARTH SCIENCE MENTOR PROGRAM: A JOINT INITIATIVE OF THE GEOLOGICAL SOCIETY OF AUSTRALIA AND SCIENCE EDUCATORS ASSOCIATION OF THE AUSTRALIAN CAPITAL TERRITORY Ken Lawrie\ Niels Hider^, Sonia Cousins^ Patrick Lyons^, Vic Dobos^ and Greg McNamara^ 'Geological Society of Australia, ACT Division, C/- AGSO, GPO Box 378, Canberra ACT 2601 ^President, Science Educators Association of the Australian Capital Territory ^Promotions and Education Manager, Geological Society of Australia ^Education Sub-Committee, Geological Society of Australia, ACT Division ^Manager, Earth Science Education Centre, Australian Geological Survey Organisation The Earth Science Mentor Program is a joint project between the ACT Division of the Geological Society of Australia (GSA), and the Science Educators Association of the ACT (SEA*ACT). The Program was developed with the aim of increasing awareness of the earth science among secondary teachers and students by providing the opportunity for interaction with, and support from, professional earth scientists. The Mentor Program consists of an Earth Science Resource Folder for secondary science teachers, and a Mentor Folder for volunteer earth scientists. A brief training program and introduction to mentoring is provided for earth scientists, and mentors are assigned to teachers and schools by a central coordinator. The concept of earth science mentoring in schools is not a new one. Anecdotal evidence suggests that many geoscientists in Australia, including the ACT, have been providing an ad hoc mentoring service to teachers and students for many years. Discussions with several geoscientists indicated that a more coordinated and organised approach to assist these volunteer efforts would be welcomed. As a national membership-based organisation, the GSA is an appropriate coordinator of an Earth Science Mentor Program and supplier of "mentors". The program is supported by the ACT Division and its Education Sub-Committee, the national Specialist Group in Geological Education, and by the Executive Committee who contributed $1000 towards to cost of producing the resource folders. Local industry in the ACT sponsored the folders and SEA*ACT have provided the necessary educational support at a local level. In addition, full time staff from ESEC (AGSO's Earth Science Education Centre) will help coordinate the pilot Mentor Program in the ACT, as AGSO regards participation by its personnel as a corporate contribution. The Earth Science Resource Folders were distributed free of charge to all secondary school/college science departments in the ACT and Queanbeyan during Earth Science Week, October 1999, and contain: introduction to the folder and the Mentor Program, including contact details for the organisers; • list of geoscience education resources and events in the ACT, including relevant contact people,; • textbook index identifying geoscience topics and concepts in popular school text books; • an extensive list of internet sites containing geoscience information and/or activities; • a list of commercially available multi-media resources; • information about the GSA and SEA*ACT. The Earth Science Mentor Folders are given to professional earth scientists who volunteer to become Mentors. They contain similar information to the Resource Folders for teachers, with one major addition: • a Note to Mentors, outlining what mentoring is about, how the program will work, getting started with a school, options for types of mentoring activities, tips on giving in-school presentations and dealing with teachers and students, and contact details for the organisers. One of the major challenges facing the long term success of the Program will be ensuring that regular contact is maintained with Mentors and participating schools and teachers. We need dedicated coordinators at the local and national level who are well informed about the Program and geoscience education activities generally. Major changes in employment patterns over the last year have significantly changed the nature of the available pool of professional earth scientists willing to be Mentors. If this pilot Mentor Program in the ACT is successful, similar programs will be established in other Australian cities and major regional centres. Discussions with other professional science teachers groups, such as ASTA (Australian Science Teachers Association), have taken place with a view to expand the mentor program nationally.

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SALT DYNAMICS IN A COMPLEX LANDSCAPE : TAKING A MULTI-DISCIPLINARY APPROACH IN CENTRAL-WEST N.S.W. K. C. Lawrie', T. J. Mundav^ D. Dent^ J.Wilford\ D.L. Gibson^ R. Brodie^ N. Reilly^ and R. Chan^ ^Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601, Australia ^CRCAMET, c/- CSIRO, Private Bag, P.O. Wembley, WA, 6014, Australia \ a n d and Water Sciences Division, Bureau of Rural Sciences, AFFA, Canberra, ACT, 2601. ^CRCLEME, c/- AGSO, GPO Box 378, Canberra ACT 2601, Australia An integrated approach, involving interpretation of high-resolution airborne geophysics, together with an understanding of soil, regolith and bedrock geology, and hydrology, is being used to map and quantify dryland salinity across the Lachlan/Murrumbidgee watershed within the Murray-Darling Basin in centralwest New South Wales. This project (The 'Gilmore' Project), representing a collaborative effort between federal and state government agencies, CRCs, university researchers and the rural and mineral exploration industry, aims to delineate the distribution of salt in the landscape and place constraints on its storage and mobility. The study area was chosen to compare and contrast salt stores and delivery systems in floodplain and incised undulating hill landscapes. The project has acquired new datasets including high resolution airborne magnetic, radiometric and electromagnetic (AEM; TEMPEST system) surveys, ground geophysics, hydrological datasets (surface stream flow and sub-surface borehole data), and regolith and bedrock geological mapping from extensive industry and government-funded drilling. These datasets have enabled identification of previously unrecognised salt stores in the sub-surface. They have also been used to identify potential constraints on groundwater flow (e.g. buried basement topography, faults) and buried palaeochannels that may act as preferential sub-surface conduits for both saline and fresh groundwaters. At the same time, salt sources have been identified in the upland areas. Current work is directed to developing linkages between these salt sources, contemporary drainage, groundwater flow and the movement of salt. In the south of the study area in the Murrumbidgee catchment, valleys partially filled with up to 60 m of transported sediments are separated by rolling hills. Significant saline goundwater discharge has been noted from sediment-filled upland basins with restricted outflows and areas that are overlain by thick clay soils. Saline stream flow from the hills mostly disappears into alluvial fans or into the alluvium of the valleys of the southern area and flood plains in the north. The northern part of the study area, in the Lachlan catchment, is an area of low relief characterised by up to 120 m of transported sediments above a zone of deeply weathered bedrock (typically 50 to 100 m thick). Sediment accumulation occurred through aggradation of alluvial fans and in fluvial systems in the Bland palaeo-valley. AEM data validated by boreholes reveal a complex pattern beneath the valley and floodplain surfaces. Within the transported sediments, the AEM images displayed as interval conductivities, suggest that extensive sumps of saline (and sulphate-rich) groundwater within clay-rich sediments occur within 5 to 30 m of the surface. At depth beneath the floodplains in the northern area, buried bedrock ridges partition the deeper sediment fill into a series of filled, restricted basins. Sub-surface groundwater outflow from these basins occurs through narrow gaps in the bedrock ridges. Saline groundwaters have been sampled in these palaeochannels and in similar channel-fill sediments that are concordant with contemporary freshwater drainage in the valley bottoms of the southern area. Airborne magnetics and AEM have also identified a palaeodrainage system that is discordant with the contemporary drainage. It is suggested that sediments of these buried palaeodrainage systems act as significant conduits for the movement of salt in the groundwaters of the area. Preferential groundwater flow is interpreted as being through more transmissive sediment-fill in buried palaeochannels and in fans, by inter-layer flow (eg at the transported sediment-saprolite interface), and through fracture networks (macropores) in saprolite. These new datasets provide a basis for ftirther work to resolve how salt is stored, how it is remobilised and how it is delivered to streams and salt outbreaks. They are also of use in modelling hydromorphic dispersion of metals for mineral exploration. Acknowledgments: This paper is published with the permission of the CEO of the Australian Geological Survey Organisation, the Directors of CRC AMET and CRC LEME, and the Executive Director of BRS.

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Geological Convention, Sydney, July 2000

MAPPING PORPHYRY Au-Cu AND EPITHERMAL Au MINERAL SYSTEMS UNDER COMPLEX REGOLITH COVER, LACHLAN FOLD BELT, N.S.W. K. C. Lawrie\ T. J. Munday^ D.L. Gibson^ R. Chan^ R. Brodie^ J.Wilford^ N. Reilly^ and K. Foster^ 'Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601, Australia ^CRCAMET, c/- CSIRO, Private Bag, P.O. Wembley, WA, 6014, Australia ^CRCLEME, c/- AGSO, GPO Box 378, Canberra ACT 2601, Australia The eastern Lachlan Fold Belt is host to world class porphyry Au-Cu and epithermal Au deposits. Exploration for analogous deposits in this belt is hampered by burial of prospective bedrock beneath complex Tertiary regolith cover. This paper reports on the geophysical and regolith expressions of Au-Cu and Au mineral systems in an area of central-west NSW. In particular, attention has been paid to recognising the larger scale hydrothermal alteration footprints associated with discrete prospects, associated structures, and the surficial and buried landform expressions of the mineral systems. This multi-disciplinary study (Gilmore Project) was facilitated by the availability of substantial mineral industry datasets including drillcore materials and high-resolution magnetics and radiometrics. New datasets acquired include an airborne electromagnetic (AEM) survey (TEMPEST), a deep seismic reflection profile, high resolution aeromagnetic and radiometric data, regolith and bedrock geology including ore deposit geochemical and geochronological data, and regolith, hydrology, geochemical, geophysical, and bedrock lithology data from new governmentfunded drill holes. The project area straddles the Gilmore Fault Zone, a major east-directed thrust system that puts the WaggaOmeo Belt over the Junee-Narromine Belt (Korsch et a!., this volume). In the study area, porphyry-related, structurally-controlled Au-Cu and high sulphidation Au-Ag ore deposits are localised in Ordovician-Silurian volcano-sedimentary host rocks within the Gidginbung Volcanic Belt. The thrust-related deformation appears to post-date much of the Au-Cu and Au mineralisation. In the south of the study area the landscape consists of rolling hills and valleys partially filled with up to 60 m of transported sediments. The adjacent hills have patchy, thin veneers of sediment. The northern part of the study area has a few low hills, but is dominantly a flat plain produced by infilling of the Bland palaeo-valley. Mineralogical, grain size and textural analysis of regolith materials, in conjunction with analysis of borehole gamma and magnetic susceptibility wireline logs, suggest that the sediment infill involved deposition in low angle aggrading fans and in palaeo-river systems. The northern area contains up to 120 m of transported sediments. Significant vertical and lateral variations in clay mineralogy are mapped. Throughout the study area, a zone of weathered bedrock (saprolite), that varies from 20 m up to 100 m thick, forms the base of the regolith. The regolith cover is characterised by marked variations in electrical conductivity, with a strong lithodependence in late-time data (bedrock/saprolite influence). More conductive areas are commonly associated with Ordovician-Silurian volcanic and intrusive rocks. A combination of ground and downhole electrical geophysics and drill hole geology indicates a variable vertical conductivity structure, with more conductive zones commonly associated with the transported sediments, and less conductive materials in saprolite. Drillhole-calibration of the AEM data has revealed a complex buried palaeo-topography that is related to both primary lithologies and hydrothermal alteration. In general, alteration zones containing a high silica component, such as the Gidginbung and Dobroyde high sulphidation systems, occur as hills and /or buried ridges, while more chlorite- and clay-rich alteration zones are expressed as topographic lows. Structurally-controlled quartz vein prospects form buried topographic ridges in the area. The landform expression and preservation potential of individual deposits or alteration zones is strongly inter-dependent upon the nature, scale and proximity of adjacent zones. A number of buried sub-basins have also been delineated, with restricted outflow zones between concealed bedrock ridges. Hydromorphic dispersion (and enrichment) of gold occurs at transported sediment/saprolite interfaces, and locally this interface coincides with an active aquifer. Oxidation and supergene enrichment significantly modify primary metal signatures in saprolite. These studies provide a framework for exploration models within the bedrock mineral systems, and enable modelling of physical and hydromorphic dispersion of Au and base metals in regolith profiles. Acknowledgments: This paper is published with the permission of the CEO of the Australian Geological Survey Organisation, and the Directors of CRC AMET and CRC LEME.

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HYDROGEOLOGY OF THE HAWKESBURY SANDSTONE IN THE SOUTHERN HIGHLANDS OF NSW IN RELATION TO MESOZOIC HORST-GRABEN TECTONICS AND STRATIGRAPHY R. John Lee Hydroilex, 38 Gibbs Street, Miranda

Recent hydrogeological investigations for groundwater in the southern part of the Sydney Basin have resulted in the deHneation a complex horst-graben structural corridor that extends along a northwesterly trend of 50km transected by conjugate northeasterly trending faults. The zone with associated Jurassic intrusives and extrusive Tertiary volcanics characterise a regional uplift of 200m coincident with the Mittagong Ranges. Bounding structures are both concealed beneath Tertiary volcanics, and masked by Wianamatta Group shales within areas of elevated topography and poor outcrop. Interpretation of regional geophysical data suggests that basement faulting controls the structural corridor. Structural trends having similar northwesterly orientations form both important structural boundaries and monoclines to the south and north of Mittagong. Systematic geophysical well logging and strati graphic analysis along and adjacent to this feature have been successful in defining the primary groundwater controls having both a structural and stratigraphic contribution. Stratigraphic log analysis has resulted in the subdivision of the Hawkesbury Sandstone into three mappable lithofacies, correlable over an extensive area, and extending into the depocentral part of the basin. Upper and lower facies are dominated by clean medium to coarse quartzose sandstones having excellent porosity and permeability, separated by a sequence characterised by clayey fine-grained sandstones and shales. Facies modelling of the sandstone bodies and their depositional environments have provided evidence of significant vertical differentiation of the sequence. Alluvial depositional environments are variable, ranging from riverine braided, meandering, and distributary mouth bar deposits. Regional correlations identify both intraformational disconformities and local erosion at the top of the sequence. Excellent groundwater potential generally exists in both the upper and lower parts of the Hawkesbury Sandstone associated with increased sandstone porosity and permeability. The middle part of the sequence is considered to be an important confining layer. Groundwater recharge is both within fracture zones and extensive areas of sandstone subcrop. Water quality generally deteriorates where the overlying Wianamatta Group sequence of shales is present, and within the deeper parts of the basin.

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THE STRUTURAL GEOLOGY OF THE KINDEE DISTRICT AND THE AGE AND TIMING OF DISPLACEMENT OF THE HASTINGS BLOCK, SOUTHERN NEW ENGLAND FOLD BELT Evan C. Leitch and Pablo Lara Environmental Sciences, University of Technology, Sydney, New South Wales, 2007 During the Permian - Triassic dispersion of the Carboniferous- Devonian convergent margin assemblage that makes up much of the exposed New England Fold Belt the Hastings Block, of arc margin-forearc basin heritage, was displaced northward at least 100 km from a position along strike from the tectonically comparable Tamworth Belt to its present location outboard of the associated accretionary subduction complex (eg Roberts & Geeve 1999). However neither the timing of this displacement nor details of the movements involved are agreed. In an attempt to constrain the displacement history of the block we have carried out a structural investigation in the Kindee district adjacent to its western boundary. Stratified rocks of the Kindee district consists largely of volcaniclastic conglomerate, sandstone and siltstone of Devonian and Carboniferous age (Roberts et al., 1995) although Early Triassic intrusive and extrusive silicic igneous rocks are also present. Two episodes of folding have been identified. The first (Dj) gave rise to upright to slightly west-vergent folds of overall gentle north-north-west plunge with adjacent major hinges separated by several kilometres. These structures are tight to open and of overall parallel form. They are best developed in the older thinner bedded rocks in the western Kindee district where they are accompanied by common congruent mesoscopic folds and by a widespread imperfect slaty cleavage (Si) confined to siltstone. Faults the mapped traces of which are near-parallel to macroscopic hinge surface traces of the Di folds, and which occupy the eastern limb of two of the major anticlines, are interpreted as west-dipping thrusts that formed during the Di episode. A second episode of folding (D2) is indicated by the presence of mesoscopic folds that are oriented at a high angle to those of Di and deform Si as well as bedding. D2 also produced map-scale flexure of Di hinge traces and the trace of associated faults. The vergence of D2 folds and their common steep plunge suggests they are related to movement on widespread sinistral N W to N N W strike-slip faults. A prominent feature of the structure is the presence of a domain of steeply north dipping and younging strata in the north central section of the Kindee district, bounded by northwest and north-north-west striking faults beyond which the rocks strike fairly consistently north-north-west, dip steeply, and young generally east. The east-west domain is the result of anti-clockwise rotation between two major sinistral strike-slip faults, accompanied by reverse movement on a reactivated Di thrust. The structure of the Kindee district closely resembles that of the Tamworth Belt near the Peel Fault in the style of Di folds and their close association with thrust faults. This similarity suggests that the structures formed while the Hastings Block and the Tamworth Belt were still contiguous. Although the age of the Kindee Di structures cannot be closely constrained those in the Tamworth Belt formed in the Late Permian which is thus a maximum age for displacement of the Hastings Block. A minimum age is set by the 228 Ma Werrikimbe Volcanics which overlaps the faulted contact between the Hastings Block and the Yarrowitch Block to its west, always providing it is this contact along which movement was concentrated. The structure of the Kindee district favours anti-clockwise rotation of the Hastings Block of about 150 degrees during displacement whereas palaeomagnetic data suggest either 130 degrees clockwise or 230 degrees anticlockwise rotation (Schmidt et al. 1994). References ROBERTS J. & GEEVE R. 1999. Allochthonous forearc blocks and their influence on an orogenic timetable for the Southern New England Orogen. In Flood P.G. ed. New England Orogen, pp. 105-114.Earth Sciences, University of New England, Armidale. ROBERTS J., LEITCH E.G., LENNOX P.G. & OFFLER, R. 1995. Devonian-Carboniferous stratigraphy of the southern Hastings Block, New England Orogen. Australian Journal of Earth Sciences 42, 609-634. SCHMIDT P.W., AUBOURG C., LENNOX P.G. & ROBERTS J. 1994. Palaeomagnetism and tectonic rotation of the Hastings Terrane, eastern Australia, Australian Journal of Earth Sciences 41, 547-560.

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USING GEOPHYSICS AND GEOLOGY TO DETERMINE THE UPPER CRUSTAL ARCHITECTURE IN THE NORTHERN PART OF THE EASTERN LACHLAN FOLD BELT (ELFB). P.O. Lennox^ R. Trzebski, D. Palmer\ W. SiebeP, and R. Armstrong ^ School of Geology, University of New South Wales, Sydney 2052 ^ University of Tuebingen, Germany ^ Prise, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200

Seismic reflection and seismic refraction surveys show that the crustal architecture of the northern section of the Eastern Lachlan Fold Belt near Wyangala Dam consists of a near surface layer about 2-3 km thick underlain by a 12-13km thick layer with a base near 15 km. The latter is thought to represent a tectonically thickened package of the Adaminaby Group and granites of the Wyangala Batholith. The upper crustal unit east and adjacent to the Wyangala Batholith consists of 3 to 5 km thick, wedge to sheet-like granites reaching up to 5-6 km depth, often bounded by faults with multiple movement histories. The granites are surrounded by multiply deformed greywackes and volcaniclastics showing lower greenschist-facies metamorphism. The evidence is poor that the almost east-west oriented boundary between the greywacke-dominated Adaminaby Group to the south and the volcaniclastic-dominated Coombing Formation or basalt-dominated Blayney Volcanics to the north is a thrust fault. The lack of early east-west or northwest-southeast foliations and associated folds seems at odds with their development within the Ordovician Mumbil Shelf rocks to the north. Some of the regional faults probably extend kilometres into the upper crust and have demonstrable long and complex movement histories involving minor to significant early dip-slip followed by later minor to significant strike-slip. Some of the east-west to westnorthwest-eastsoutheast oriented dolerite dykes within the northern Wyangala Batholith may represent the surface expression of deep-seated weaknesses in the upper crust. These are but one element in a number of similarly oriented features which find their expression in the subtle Lachlan Transverse Zone. New SHRIMP and conventional dating of zircons from the granites and Ar/Ar dating of key regional events within the Molong High have provided a useful preliminary framework for building a model of the evolving upper crustal architecture in the ELFB. After deposition and reworking of the sedimentary and volcaniclastic packages and extrusion of basic volcanics within the apron of volcanoes there followed deformation related to the Benambran event with late stage granite emplacement at around 6-12 km within the upper crust. It is likely on regional grounds, that the 1-2 km thick Adaminaby Group has been thrust-repeated with the thin Warbisco Shale to form a package up to 10 kilometres thick. Granite emplacement may have been facilitated by pre-existing mainly north-south trending faults. The plutons of the northern Wyangala Batholith may have forcibly used the fault system to facilitate their emplacement. The weakly deformed I-type Carcoar and Barry Granodiorites have wedge-shaped profiles and root zones near the geophysically modelled position of faults which are often exposed at the surface. The more deformed Sunset Hills Granite appears to have been assembled around 12 km depth and moved via possibly west-over-east thrusting to within 4-6 km of the surface. The granites and the Copperhannia Thrust were deformed during the Bowning, Tabberabberan and Kanimblan events. The Barry Granodiorite has been moved kilometres eastward after its emplacement on a poorly exposed fault along its eastern margin. The Barry and Carcoar Granodiorites may have been tilted about an eastwest axis with the southern part representing a deeper part of the pluton.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1 Australian Geological Convention, Sydney, July 2000

NEOTECTONICS AND CAINOZOIC VOLCANICS: IMPLICATIONS FOR LONG-TERM LANDSCAPE EVOLUTION A.C. Lewis\ S.M. Hill' and I.C. Roach^ 'CRC LEME, University of Canberra, ACT 2600 ^CRC LEME, Department of Geology, Australian National University, ACT 0200 A dominant paradigm in models for the long-term evolution of the Australian landscape emphasises the great antiquity and relative tectonic stability during the Cainozoic. The regional significance of tectonically disrupted Cainozoic volcanics to landscape evolution has been largely overlooked, despite the fact that basalt data have been applied to various landscape studies. Radiometric dating allows relatively reliable ages to be placed on basalt remnants and, from this, a maximum age for their tectonic disruption. Neotectonic disruption can be recognised in other facets of the inland and coastal Australian landscape and, when combined with evidence of faulted volcanics, presents a new view of landscape evolution for eastern Australia. The area of the headwaters of the Shoalhaven catchment is the primary region of study for this research. Preceding landscape studies in this area have concentrated on the middle and lower portions of the catchment, with very little attention given to the river's headwater region. Previously, basalt remnants in the middle and lower Shoalhaven catchment were used to determine long-term rates of river incision, headward gorge extension and scarp retreat (e.g. Young 1983, Nott et al. 1996), or were identified as the parent material of ancient weathering profiles (e.g. Ruxton & Taylor 1982). Miocene basalt in the upper catchment is reported as being displaced by the Shoalhaven Fault (Wybom & Owen 1986), however, previous studies have found that the Shoalhaven Plain is stable and has not experienced neotectonism. A re-evaluation of the volcanics on the Shoalhaven Fault could help resolve this and contribute to the debate on the evolution of the Eastern Highlands. There are many other localities in southeastern Australia where Cainozoic basalt and associated regolith material is faulted. Sites in New South Wales include the Berridale Fault and the Towamba Lineament of the Monaro Volcanic Province. There are also several sites in the Older and Newer Volcanics of Victoria, including the Tawonga, Heath Hill, Corinella, Jones Ridge, Bridgewater, Swan Lake, Rowsley, Muckleford and Damum Faults and the Lovely Banks and Yarragon Monoclines. The lack of available research on faulted volcanics stems from confusion of what faulting and other forms of tectonic disruption look like in the field. This research aims to re-evaluate and document existing information on these locations and characterise various forms of tectonic disruption of Cainozoic volcanics. Types of tectonic disruption include faulting, monocline formation and tilting. Morphologically similar features such as internal flow degassing and cooling structures, ponding of flows, colluvial slope cover and flows that cover topographic irregularities can potentially be misinterpreted as tectonic disruptions. Faulting of Cainozoic volcanics may therefore testify to the ongoing tectonic controls on the landscape and, most importantly, to neotectonic contributions to the evolution of landscape features such as the Eastern Highlands. References NOTT, J., YOUNG, R. AND MCDOUGALL, I., (1996) Wearing Down, Wearing Back and Gorge Extension in the Long-term Denudation of a Highland Mass: Quantitative Evidence from the Shoalhaven Catchment, Southeast Australia. The Journal of Geology. Vol. 104, pp. 224-232. RUXTON, B.P. AND TAYLOR, G., (1982) The Cainozoic Geology of the Middle Shoalhaven Plain. Journal of the Geological Society of Australia. 29, pp. 239-246. WYBORN, D. AND OWEN, M., (1986) 1:100 000 Geological Map Commentary: ARALUEN, New South Wales. Bureau of Mineral Resources, Geology and Geophysics. YOUNG, R.W. (1983) The Tempo of Geomorphical Change: Evidence from Southeastern Australia. The Journal of Geology. Vol. 91, pp. 221-230.

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AGSO'S TEACHER RESOURCES KITS AND TRAINING SESSIONS Gary B. Lewis Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601, Australia

The Australian Geological Survey Organisation (AGSO) is Australia's federal government geoscience research agency. The AGSO Geoscience Education Unit has operated since 1994 with the aim of increasing the awareness of school students, through their teachers, of the important role geoscience has to play in the environmental and economic future of the country. One project operated by the unit involves the development of teacher resource kits for primary and secondary schools. These kits are developed to inspire non-geoscience teachers to introduce aspects of geoscience into their classrooms by providing simple to read background information and reproducible classroom activities. The kits are developed by staff with recent teaching experience, but not necessarily with geoscience backgrounds, which ensures the appropriateness of the materials, especially at primary school level. To date AGSO has produced over 30 different kits. These kits cover topics such as Plate tectonics, Earthquakes, Volcanoes, Geological Maps, Remote Sensing, Landslides and more. The kits are advertised and sold at almost cost price around Australia with over 18,000 having been distributed over the last four years. A recent evaluation shows that teachers estimate that they will use these kits with over 1 million times with students over the next two years. While the development of the kits has been important, the Unit also operates a series of teacher training workshops based on the resources. These workshops aim to increase the worth of the teaching materials by empowering the teachers to use them in the classroom. Sessions have been operated in Sydney, Melbourne, Perth, Brisbane, Hobart, Darwin, Newcastle, Wollongong, Wagga Wagga, Armidale, Cairns, Townsville, Geelong, Bendigo, Ballarat, Alice Springs and Tennant Creek and other major centres. These sessions have attracted 1800 teachers. These projects, as well as the other AGSO Geoscience Education projects (such as field trips for government officers) are operated on a cost recovery basis. This means that the projects are at times heavily supported by other organisations such as the Australian Surveying and Land Information Group (AUSLIG), Minerals Council of Australia and commercial organisations.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ODP DRILLING IN THE GREAT AUSTRALIAN BIGHT REVEALS THE EVOLUTION HISTORY OF THE SOUTHERN MARGIN Oianyu Li and Brian McGowran Department of Geology and Geophysics, The University of Adelaide, SA 5005

Biostratigraphic data from ODP Leg 182 indicate that the Cenozoic succession in the Great Australian Bight was truncated by five major unconformities at 30-28, 20-17, 14-12, 9-7 and 5-3 Ma, each with at least 2 million years in duration. These sediment sequences and unconformities confirm the neritic record that the evolution of the southern Australian margin during the Cenozoic was in steps and has been infiuenced mainly by regional tectonic configuration and global climatic and eustatic change. An accelerated separation between Australia and Antarctica and a warming in the IndoPacific region in the later part of the Eocene triggered the accumulation of neritic carbonates in basins along the subsiding southern margin. The occurrence of neritic sediments at ~800m in the Bight and onshore in the Nullarbor Plain suggests that a shelf as wide as 400km, or almost twice today's width, extended offshore to what is now the upper slope. While the shelf was bathed in relatively warm water, the deeper part of the Bight was poorly oxygenated with a shallow CCD (3870m or less present water depths). Chert indicating cooler bottom water first formed in the later part of the Eocene at sites from 700m or more, and became common in the Oligocene to Middle Miocene and at sites even from the upper slope (500-800m) and shelf edge (~200m). During the Early Oligocene, the CCD deepened, the seafioor became ventilated and pelagic ooze and chalk succession started to accumulate, probably as responses to the initiation of the Antarctic Circumpolar Current (ACC). The mid-Oligocene unconformity event at 30-28 Ma removed the entire Early Oligocene record from Site 1130 (488m), corresponding to a glacioeustatic lowstand and probably also a stronger flow of the ACC after the full opening of the Tasman Gateway. A temperate regime remained throughout the Oligocene and Early Miocene, temporarily interrupted in the Late Oligocene by short periods of warming or inflow of a warm water similar to the present-day Leeuwin Current. The 20-17 Ma unconformity eliminated the entire Early Miocene and part of the Late Oligocene record in two sites from 288m and 488m respectively. Sections representing the Miocene climatic optimum at 15-16 Ma are widespread, and overlain either by sediments of the Late Miocene (mainly 10-7 Ma and younger) at sites from the west or even the Late Pliocene (2-3 Ma) at sites from the east of the drilling area. This warming triggered a further stratification of the surface water that was previously powered mainly by the AAC. The unconformities at 1412 Ma and 9-7 Ma disrupted the Middle to Late Miocene sedimentation not only on the upper slope but also the continental rise, because in the deepest site (Site 1128 at 3875m) the Upper Miocene ooze unconformably overlies the Lower Oligocene green clays and packstones. The unconformity at 5-3 Ma, detected at all 9 sites, is more prominent in the east where a large part of the Pliocene and Late Miocene is missing. Although these unconformities may represent non-deposition or a large scale sediment removal during the process of basin inversion and some of them have been superimposed on one another at a particular locality, their coincidence with major falls in global sea level and second-order sequence boundaries indicates a teleconnection between the regional and global patterns. An unusual phenomenon is that the greatly expanded Pleistocene section, with an accumulation rate of over 300 m/m.y., probably resulted from a rapid subsidence, an increased productivity, and a more vigorous sediment transfer offshore by swells since 2 million years ago.

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NEOPROTEROZOIC MAGMATISM, CRUSTAL UNROOFING, AND CONTINENTAL RIFTING IN AUSTRALIA AND SOUTH CHINA: RESULTS OF A MANTLE PLUME AT THE START OF RODINIA BREAKUP Z.X. Li ^ X.H. Li p.D. Kinny ^ and H.Zhou^ ^ Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA 6907, Australia 2 Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, PO Box 1131, Guangzhou, China ^ Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, WA 6845, Australia

Zhao et al. (1994) first suggested that the Gairdner and Amata Dyke Swarms in Central and southeastern Australia, dated as 827±6 Ma (Wingate et al., 1998) and 824±4 Ma (Sun and Sheraton, 1996) respectively, and the possibly co-magmatic Bitter Springs volcanics in the Amadeus Basin and the Wooltana volcanics in the Adelaide Fold Belt (interpreted as continental flood basalts), are of mantle plume origin. They suggested a plume centre close to the Adelaide region. A same-aged (827±9 Ma) gabbro has been reported in the Broken Hill region also (Wingate et al., 1998). Furthermore, the hiatus between the Gairdner dykes and the Sturtian glacial unit (? ca. 750 Ma) could be the result of rapid uplift above the ascending plume head (Li et al., 1999). If Laurentia was connected directly to eastern Australia in Rodinia, as proposed by many workers, one would expect to find evidence of such a mantle plume (e.g., same-aged dyke swarms, rift volcanics, crustal unroofing etc.) along its western margin. However, no such evidence has yet been reported. On the other hand, there is evidence from South China that can best be explained by it having been right above such a mantle plume. This includes (1) 828±7 Ma mafic and ultramafic dykes/sills that share the same age as the mafic dykes in Australia; (2) likely coeval mafic and ultramafic dykes/sills with Al203/Ti02 ratios of --10 (Li, 1996) suggesting high-temperature mehs (Sun S.S., pers. comm., 1998); (3) widespread 840-820 Ma granitic intrusions possibly due to heat conducted from the plume head; (4) dykes of similar age which, together with the Nanhua and Kangdian rift systems, are focussed upon southwestern South China; (5) continentalscale rapid unroofing between the above-mentioned magmatic intrusions and the onset of continental rifting; and (6) ca. 810 Ma bimodal rift volcanics with geochemical and Nd isotopic signatures resembling the Hawaii OIB and the Ethiopian CFB. These observations are consistent with the Neoproterozoic plume-related events in Australia, and therefore support the model that South China was adjacent to eastern Australia in Rodinia (Li et al., 1995). The model suggests that a ca. 830 Ma mantle plume may have initiated the continental rifting in the lead-up to the breakup of Rodinia. References

LI, X.H., 1996. Scientia Geologica Sinica, 31, 218-228. LI, Z.X., LI, X.H., KINNY, P.D. & WANG, J., 1999. Earth Planet. Sci. Lett., 173, 171-181. SUN, S.-S. & SHERATON, J.W., 1996. Aust. Geol. Surv. Org. (AGSO) Bull. 239, 135-143. WINGATE, M.T.D., CAMPBELL, I.H., COMPSTON, W. & GIBSON, G.M., 1998. Precamb. Res., 87, 135-159. ZHAO, J.X., MALCOLM, M.T. & KORSCH, R.J., 1994. Earth Planet. Sci. Lett., 121, 349-367.

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THE INTRUSIVE HISTORY OF THE GOONUMBLA PORPHYRY CU-AU DEPOSITS, NSW Vanessa Lickfold CODES SRC, University of Tasmania, Hobart Eight intrusive phases have been recognised in the Endeavour 22, 26 and 27 porphyry Cu-Au deposits hosted in the Goonumbla Volcanic Complex close to Parkes in central-west NSW, Australia. The first and second stages of intrusive activity consist of a coarse-grained (2-3mm), equigranular, biotite monzodiorite intrusion (Stage 1) and a K-feldspar-rich, equigranular monzosyenite intrusion (Stage 2). These two intrusions are restricted to the basal portions of the E26 deposit. They resemble the Stage 3 brick-red biotite monzodiorite. The third stage of the intrusive history is represented by the intrusion of a brick-red biotite monzodiorite phase (Stage 3), which ranges in texture from densely porphyritic, through seriate to equigranular. Plagioclase ( - 8 0 % ) and biotite ( - 1 0 % ) are the dominant phenocrysts, while the groundmass consists of Kfeldspar ( - 9 0 % ) and quartz (-10%). These biotite monzodiorite intrusions are rarely cross-cut by quartz veins, though varying degrees of potassic alteration is typical. Disseminated Cu mineralisation and evidence for synchronous percolating magmatic fluids (presence of magmatic-hydrothermal quartz) are rare. Unequivocal cross-cutting relationships between Stages 1, 2 and 3 are absent. It is possible that they are all minor textural and compositional variants of the biotite monzodiorite. These biotite monzodiorite intrusions in E22, E26 and E27 are thought to represent the parent stocks for the Cu-Au deposits. They occur in both the basal and peripheral parts of the mineralised centres. Stage 4 of the intrusive history at Northparkes was marked by the intrusion of a dark-red, crowded, mafic quartz monzonite porphyry (QMP), which is comprised of 50-60% phenocrysts in a microcrystalline groundmass of K-feldspar + quartz. Plagioclase ( - 6 0 % ) + K-feldspar ( - 2 0 % ) + mafics (10-20%; bi + hbl + mag ± px) constitute the phenocryst population of these pipe-like intrusions, which are vertically extensive in all three deposits and can reach heights of >500m from the parent stocks. Quartz veins typically comprise <1% of the total rock and commonly occur with K-feldspar ± sulphides centres. These intrusions contain rare bomite/chalcopyrite blebs (<5mm). Evidence of the exsolution of magmatic-hydrothermal fluid is provided by the presence of intergrowths of vein-like quartz (prismatic, euhedral crystals) and aplitic materal. The fifth and sixth intrusive phases are pale pink to orange-red, crystal-crowded QMP's that contain 1 -2% primary mafic phenocrysts. A common feature of these and earlier intrusions is the dominance of plagioclase and K-feldspar phenocrysts. However, here the abundance of K-feldspar increases to - 3 0 % (rarely 40%) at the expense of plagioclase, which decrease from 60-70% to - 5 0 % . Well-zoned K-feldspar megacrysts (phenocrysts >10mm) are unique to these two QMP intrusive phases. The groundmass is typically finegrained (<0.5mm), equigranular K-feldspar and quartz with disseminated sulphides (<1% to - 5 % ) . Stage 5 and 6 intrusions are widespread in the central parts of the Endeavour systems, extending vertically as pipes for 400-700m, with diameters of 100-200m. Temporal relationships between Stages 5 and 6 are ambiguous. There is evidence that both phases have intruded each other, possibly indicating that multiple pulses of the same phase intruded over a short time interval. Some intrusions are associated with the main stockwork/mineralising event and associated exsolution of a magmatic-hydrothermal fluid. "Brain rock", a unidirectional solidification texture occurs in Stage 6 intrusions. Stage seven consists of fine-grained K-feldspar and quartz aplite dykes. They occur in all of the intrusive phases described above. It is currently unclear whether the aplite dykes were emplaced as the end-stages of each phase or as a single late-stage intrusive event. They are grouped here as Stage 7 due to the lack of obvious textural features or cross-cutting relationships that would allow them to be discriminated further. Stage eight of the Endeavour intrusive history is represented by zero porphyries (<0.1% Cu and <0.1 g/t Au grade). These are monzonite porphyry intrusions that are uncrowded in terms of phenocryst abundance and consist of K-feldspar + plagioclase, mafic ( - 1 0 % ; px + mag ± hbl) in a fine-grained groundmass. In the three deposits studied, zero porphyries are restricted to the E26 deposit. No quartz veins or aplite dykes cross-cut the zero porphyries, suggesting that they were the last intrusive event in the Endeavour porphyry systems. Acknowledgements: The author thanks North Limited for financial support and for allowing this publication.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE ELECTRICAL CONDUCTIVITY STRUCTURE OF THE AUSTRALIAN CONTINENT: A REVIEW F.E.M.(Ted) Liliey and Francois H. Chamalaun Australian National University, Canberra and Flinders University, Adelaide The physical process of electromagnetic induction takes place at the surface of Earth on a wide range of frequencies, and thus of length-scales. While for "local" problems source fields may be man-made and are sometimes termed "artificial", for regional problems the source fields required are greater than can be engineered; fortunately the Earth itself provides source fields of global scale, of sufficient power for their use to probe Earth from the surface downwards, exploiting the phenomenon of electromagnetic induction. The electric currents providing these source fields fiow external to Earth, in its ionosphere and magnetosphere. Their power comes from the Sun. The physical property controlling the electromagnetic induction process is that of electrical conductivity, with the magnetic permeability of Earth taken generally to be that of free space. Information on the electrical conductivity structure of the crust and mantle is thus obtained independent of other geophysical properties, such as density from gravity studies, seismic wave-speed from seismology, and magnetic susceptibility and remanence from aeromagnetic images. Thus electromagnetic induction studies produce an extra and independent data-set on crust and upper mantle structure. Observations of electromagnetic induction at Earth's surface are obtained following the magnetotelluric and magneto-variational techniques. The observed data indicate, in the first instance, whether the nearby conductivity structure is 1-D (so that conductivity varies with depth only) or is 2-D (for which the structure has a distinct geologic strike) or is the most complicated: 3-D. The information which may be obtained about the Earth varies in any given situation. As frequency of analysis increases (and period of disturbance analysed decreases), information becomes more local, as the scale-length reduces. Because an electromagnetic survey of regional conductivity structure requires observing stations set up for times of hours to days, depending on the objective, the coverage of a continent like Australia is necessarily slower than say for a gravity survey, and especially slower than for the rapid method of aeromagnetics. However, by now Australia has been covered in a regional sense. The conductivity structure may be characterised by quite large areas of uniform structure separated by strong "conductivity anomalies", the whole being set in a highly-conducting surrounding sheet of seawater. The oceans which surround the continent give rise to a ubiquitous geomagnetic "coast effect" around the coasts. The linear conductivity anomalies lend themselves to detailed investigation by magnetotelluric traverses, and recent results for the Flinders Anomaly in South Australia, and the Carpentaria Anomaly in Queensland, indicate both to be due to highly conducting material which may be present in the upper mantle, and which intrudes into the crust. In reviewing results for Australia it is important to mention also that several of the conductivity anomalies strike from the continent out to sea, and these are the subject of present investigation using marine instruments and techniques. In particular the Eyre Peninsula Anomaly was traced across the South Australian continental shelf in 1998; the Carpentaria Anomaly was studied in the Gulf of Carpentaria in 1999; and plans are in hand to investigate, this year, the continental shelf offshore from the (onshore) Canning Anomaly in Western Australia. Another frontier in Australia at present is determining the extent to which quite local conductivity structures influence the time-fluctuating magnetic field in which aeromagnetic surveys take place.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

FORMATION OF THE DENCHAI GEM SAPPHIRES, NORTHERN THAILAND: MINERAL CHEMISTRY AND FLUID/MELT CHARACTERISTICS P. Limtrakun', Khin Zaw^ C.G. Ryan^ T.P. Memagh^ T.J. Falloon\ D.A. Steele ^ and P. Wathanakul^ ^School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001, Australia ^Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001, Australia ^CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 2113, Australia ^Australian Geological Survey Organisation, Canberra, ACT 2601, Australia ^Central Science Laboratory, University of Tasmania, GPO Box 252-74, Hobart, Tasmania, Australia ^Department of General Science, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand The Denchai gem sapphire deposits are located in Phrae Province, Northern Thailand. They are found in close spatial association with Tertiary alkaline basaltic rocks, which unconformably overlie Permo-Triassic sedimentary and volcanic rocks. The Denchai gem sapphires are recovered from alluvial placer and palaeochannel deposits at shallow depths. Quantitative electron microprobe analyses of trace and minor elements contents (iron, titanium, chromium, gallium and vanadium) in the Denchai sapphires have been obtained. The abundances of these elements in the Denchai sapphires are as follows: Fe203 (0.49-0.57wt%), Ti02 (0.034-0.23wt%), Cr203 (<0.01wt%) Ga203 (0.012-0.028wt%) and V2O5 (<0.01wt%). The sapphires Ga203 abundances and Cr203/Ga203 values (<1), suggest that they formed as a result of igneous processes, based on the empirical classification of Sutherland et al. (1998) which discriminates between corundums of metamorphic and igneous origins. The majority of the fluid/melt inclusions present in the Denchai sapphires have either negative crystal or rounded shapes and are 10-100 |im in size. Primary inclusions occur within coloured growth bands, whereas inclusions that occur along healed fractures are either secondary or pseudosecondary. Based on optical studies, three types of primary fluid/melt inclusions can be distinguished: Type (1) C02-rich inclusions. These inclusions are two phase, CO2 bearing inclusions (confirmed by Laser Raman Spectroscopy) with the vapour phase comprising less than 10-15 vol.%. The inclusions have been studied using a FLUID INC. Adapted U.S.G.S. Gas-Flow Heating/Freezing System. The melting temperatures of CO2 solid are around -56.1 to -57.8 ""C. The homogenization temperature of the CO2 liquid into vapour ranges from 24.3-31.1 °C. Type (2) polyphase inclusions. These inclusions contain a fluid bubble, which occupies about 20-30 vol.% of the inclusion volume, an aqueous phase which occupies 10-15 vol.% of the inclusion and solid phases. The solid phases are rutile, ilmenite and the other unidentified minerals. Type (3) silicate-melt inclusions. A preliminary Proton-Induced X-Ray Emission (PIXE) analysis of a Type 3 inclusion demonstrates high concentrations of K (~5wt%) and Ca (~0.6wt%) in the silicate glass. PIXE imaging also confirmed the presence of rutile in the melt inclusions (Ti ~2wt%), as well as Mn (0.1 wt%), V (0.3wt%), Rb (200 ppm) and Zr (700 ppm). Together our preliminary results demonstrate that the Denchai gem sapphires are likely to be related to CO2rich alkaline magmatism. Reference F. L., SCHWARZ, D., JOBBINS, E. A., COENRAADS, R. R. & W E B B , G. 1998. Distinctive gem corundum suites from discrete basalt fields: a comparative study of Barrington, Australia, and West Pailin, Cambodia, gemfields. Journal of Gemmology 26(2), 65-85.

SUTHERLAND,

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

COAL AND CONGLOMERATE IN THE NEWCASTLE COAL MEASURES LATERAL FACIES OR COEVALLY UNRELATED? Grahame Lindsay and Chris Herbert Mining and Exploration Geology Services (MEGS) P.O. Box 840, Toronto, NSW The coalesced Vales Point, Wallarah and Great Northern Coals comprise the topmost unit of the Late Permian Coal Measures in the Central Coast area of the Sydney Basin. Reaching a combined thickness of 8m the coal is split by three, and possibly four, conglomerate-filled channels and two erosional surfaces. Terrestrial Early Triassic sedimentary rocks unconformably onlap and erode the top of the coal. Detailed correlation of individual coal plies using brightness profiles and gamma-ray logs has shown that the coal was deposited episodically in layers or plies averaging about 0.5m thick. Although the coal is remarkably free of clastic bands, the plies have been correlated for more than 20km across the exploration licences held by Coal Operations Australia Limited near Wyong. However, the coal is split at several stratigraphic levels by conglomerate-filled channels up to 50m thick and 5km wide. The channels splitting the Wallarah and Great Northern Coals are sharp edged and trend slightly sinuously from north to south which is the presumed sediment transport direction from the New England Fold Belt source area. Although not yet frilly defined, the sandstone/conglomerate splitting the Vales Point and Wallarah Coals trends in an east-west direction. It is generally thought that coal originated from low-lying peat mires in floodbasins adjacent to, and coeval with, active river systems. This concept is not supported here for the topmost coals in the Newcastle Coal Measures. Instead, correlations show that the conglomerates were deposited at a time that equates to the hiatus between coal plies and that each conglomerate was deposited at a different stratigraphic level in the coal. The channels erode underlying coal plies, in places removing all coal to the base of the seam. There is no systematic deterioration in coal quality in plies immediately adjacent to, and underlying, the channels. However, subsequently deposited coal plies which pass over the channels may, in places, deteriorate in quality or disappear. At each split level, the only clastic evidence in the coal for close-by, massive, conglomerate-filled channels is a grey mudstone or clay pellet layer, less than 1 cm-thick, that may extend for a distance of l-5km away from the channel. In addition to conglomerate-filled channels the upper part of the Wallarah/Great Northern Coal contains two erosional surfaces marked by distinctive gamma-ray spikes and, in places, claystone layers l-2cm thick. These horizons are conformable within the seam to the east but unconformable to the west. The upper unconformity surface is more pronounced, cutting down through the frill Wallarah/Great Northern Coal to the underlying Fassifem Coal. Low-ash, dull coal overlies the surfaces except in the west where a sandstone/shale split is present. Bioturbation, in places, indicates a possible transgressive, estuarine origin for this split. It is proposed that the coal is a composite of individual plies which originated from peat mires that grew cyclically during base-level rises and ceased active accumulation during base-level falls. During base-level falls, lowered water table caused growth of the peat mire to cease creating a hiatal surface at the top of each ply. Simultaneously fluvial channels were activated by increased gradient to carry coarse bedload gravel through the inactive peat mire which was effectively an area of sediment bypass. Later, rising base-level caused gravel to deposit in the channels, but eventually pushed the locus of deposition further upstream into the hinterland of the Sydney Basin. Continuing base-level rise reactivated adjacent peat mire growth as the water table rose, initiating the accumulation of another ply above the hiatal surface. To account for the lack of clastic dilution of the peat mire adjacent to the channels it is probable they were raised blanket mires located above flood level. Although streams were probably located within the channels during base-level rises they were not transporting and depositing coarse-grained bed-load sediments simultaneously with peat growth and accumulation. Repetition of base-level rise and fall is considered the main controlling factor in producing vertically stacked coal plies with interfingering conglomerate splits and erosional surfaces.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EPISODICITY DURING OROGENESIS G.S. Lister, M.A. Forster and T J . Rawling Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Melbourne 3800 Australia Deformation events and episodes of metamorphic mineral growth are usually regarded as relatively local phenomena. It is not expected that specific events and episodes within an orogenic sequence would exactly correlate over large distances. There is no obvious reason, for example, to assume that deformational and/or metamorphic events in the Western European Alps should directly correlate with events taking place in the Aegean continental crust, - 1 0 0 0 km distant. Yet linked episodes of defonnation and metamorphism appear to take place at the same time over large distances, even in apparently unrelated segments of the same orogenic belt. This episodic behaviour is often associated with switches in tectonic mode and may be the result of orogenic surges and/or periods of accelerated roll-back of a subducting lithospheric slab following accretion events. An "orogenic surge" involves an overall switch in tectonic mode in deforming crust overlying a basal detachment. A period of crustal thickening may have lead to an instability because the crust eventually exceeded a critical thickness, and then began to collapse. Conversion of mechanical energy to heat as the result of pervasive strain through the rock mass may produce a short-lived thermal and/or fluid fluxing event, and an episode of metamorphism takes place in consequence. Rheological parameters are thereby changed, so that the formation of ductile shear zones is favoured. Strain-softening in these movement zones then facilitates ftirther collapse of the orogen. Accretion events are asociated with similar behaviour. The arrival of a continental mass at a subduction zone does not prevent ongoing convergence. Because convergence can no longer be so readily accommodated by subduction, the over-riding plate is subject to crustal shortening. This results in the closure of marginal basins in the over-riding plate, and the emplacement of ophiolite sheets. High pressure (blueschist-eclogite facies) metamorphism may be the consequence, as coherent slices of the over-riding plate are overthrust, and sent to great depths in vastly overthickened crust. After such a collision large scale reorganization of plate kinematics takes place, but not before ongoing convergence as above results in a major epoch of orogenesis. Where continent meets continent, and convergence continues, collapse of the orogen involves "surges" (as in glaciers). After an accretion event, however, where the newly formed mountain chain faces an actively subducting oceanic slab it will be "torn apart" as the result of a major epoch of lithosphere-scale extensional tectonism. This will be induced by renewed retreat (i.e. "roll-back") of the flexure of the subducting slab, after the accretion event. As a result the most extended regions on Earth appear to form in the over-riding plate, adjacent to retreating slabs. But why would exact correlations of tectonic events appear to be evident, on a global scale? Several mechanisms have been proposed that would explain episodic behaviour involving switches in tectonic mode. Each of these mechanisms has the capability of inducing an episode of extensional tectonism subsequent to a prolonged period of crustal shortening, but the models above do not explain why tectonic and/or metamorphic events might correlate over large distances. According to these models episodic behaviour can be governed only by what takes place in single cross-sections of an evolving orogenic belt. In terms of the particular mechanism with which we are concerned, a certain period of time may need to pass before the crust can thicken to the point at which critical behaviour is once again emergent. Orogenic surges that take place while the system is still attempting to build up sufficient crustal thickness may have little impact because only large collapse events achieve runaway status. The description of the model changes once the size of a movement zone becomes significant relative to the entire orogen, and once the scale of extensional movements on the "detachments" becomes large in relation to the total displacement accumulated during a preceding period of crustal shortening. Episodic behaviour on a global-scale can be explained as the response to an accretion event in an assemblage of stiff lithospheric plates separated (in part) by orogenic zones. If the individual orogenic zones have had sufficient time to achieve a critical state, a single accretion event may trigger a number of (apparently unrelated?) orogenic surges or collapse events.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NEOCENE SEDIMENTARY BASIN EVOLUTION IN NORTHERN PAPUA NEW GUINEA: A MODEL FOR CONVERGENT MARGIN BASIN EVOLUTION Keyu Liu^ and Keith A. W. Crook^ 1 CSIRO Petroleum Resources, P.O. Box 136,NSW 1670, Australia 2 Hawaii Undersea Research Laboratory, University of Hawaii, Honolulu, HI 96822, USA

The Neogene sedimentary basins in northern Papua New Guinea, including the onshore Northern New Guinea Basin, the Markham Basin and the offshore basins in the western Solomon Sea along the northern Australian continental margin, were formed as a result of interaction between the north-moving Australian Plate and the west-moving Pacific Plate. The temporal and spatial evolution of these basins was directly controlled by the Neogene arc-continental collision events in the region. Onshore field mapping and offshore geological and geophysical survey of the RamuMarkham collision zone revealed that these sedimentary basins are associated with embayments in the margin of the under-riding Australian Plate which are separated by promontories. Seven plate promontories and six embayments have been identified along the collision zone from the Ramu Basin in the northwest (145^12' E 5^30'S) to the Deboin Spur in the western Solomon Sea in the southeast (HS^'SO'E T\5 S). The contemporary sedimentary facies within these basins change from alluvial, to shallow marine, and deep marine sediments along the collision zone from NW to SE, but the facies transitions are locally interrupted by the promontories. The spatial and temporal evolution of these basins is exemplified by the evolution history of the Neogene Markham Basin. The Markham Basin was initially developed as a remnant basin in the late Miocene between two plate promontories on the under-riding Australian Plate margin and evolved into a slope basin in the Plio-Pleistocene and an intra-montane basin at the present-day. Sedimentary facies within the Markham Basin change from deepwater submarine fan deposits sourced primarily from the Australian Plate to shallow marine and alluvial fan delta deposits exclusively sourced from the over-riding South Bismarck plate. During an oblique collision of two plates (e.g. arc-continent), the irregular geometry of the under-riding plate has significant effects on the initiation, development and preservation of sedimentary basins. Sedimentary basins will develop first as small isolated basins at embayments on the plate margin during the initial stage of the collision. Such basins will initially receive sediments primarily from the under-riding plate from both axial and transverse transportation conduits. As the collision continues, the basins will be elevated and incorporated into the slope of the overriding plate, forming slope basins. The sediments will then be primarily sourced from the uplifted upper plate. The basins will eventually elevated about the base level forming intra-montane basins within the sutured part of the collision zone during the late stage of the collision. Lateral migration of the sedimentary basins along the collision zone is likely to be interrupted by the promontories. The abrupt shift of sediments from a lower platedominated provenance to an upper plate-dominated provenance in each basin can be used as a timing indicator of the collision event.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 IS"" Australian Geological Convention, Sydney, July 2000

ORIGIN AND DIAGENESIS OF A QUATERNARY IRONSTONE IN BABANANGO AREA, NORTHERN KWAZULU-NATAL, SOUTH AFRICA K. W. Liu Department of Geology, University of Durban-Westville, Private Bag X54001, Durban 4000, South Africa

A Quaternary ironstone deposit was newly found in the Babanango area, northern KwaZulu-Natal Province, South Africa, which is presently still in the formation process. The ironstone occurs as thinly bedded layers and is hosted in the shallow subsurface by a semi- consolidated Quaternary soil profile. It occurs mainly as a single bed with a thickness between 20 and 210 cm, but sometime occurs as two separate beds with a ironpoor soil layer between. Generally, the soil profiles in this area are thick and well developed and, from the surface down, comprise a weathered unconsolidated grass-rootlet zone (25-65cm thick), semi-weathered talus zone (45-120cm), semi-cemented iron-rich zone (25-75 cm), unweathered semi-consolidated talus zone (50-165cm), semi- or fullyconsolidated iron-rich zone (20-210cm) and fresh unweathered parent rock, i.e. Late Carboniferous diamictites or Achaean quartzites. All the iron-rich zones (layers) are lenticular in shape, extending laterally for a few tens to hundreds metres, then pinching out and changing to another iron-rich layer. It is noticeable that the iron-rich layers are thicker in topographic depression (valleys), and that they are diachronous. Petrologically, iron minerals occur as concretion aggregates, percolation pipes, pisolites, pore-lining rims and fissure-filling bands, with the concretions are the dominant occurrence. The average diameter of the concretions ranges from 0.4 to 2.5 cm, rarely up to 10 cm. Some of the concretions are relatively pure, containing little detrital material, but others contain many quartz and feldspar sandy grains. Mineralogically, the ironstone comprises hematite, goethite and limonite, with gangue minerals of quartz, plagioclase, chert and clay minerals, as well as minor weathered pyroxene and biotite. It is notably that hematite is the dominant iron-rich mineral, and that goethite and limonite occur only in minor amounts. Chemical analyses show that, the ironstones are composed of Si02 13.82-71.24%, AI2O3 4.05-12.00% and FeiOa 11.51-78.91%, with L.O.I. 5.38-12.07%. Meanwhile they contain higher trace elements of Cr 322.9-514.3 ppm, V 223.7-565.6 ppm and Zr 249.5-331.7 ppm. Geological and geochemical evidence shows that the ironstone is diagenetic in origin; Mafic igneous rocks and metabasites, which are widely distributed in the surrounding area, are the source of iron. Climate and ground water circulation have played a major role in the formation process; the high temperatures of the subtropical climate, the high porosity of the talus deposits, and the dominant vertical circulation of groundwater led to iron being leached, transported and then precipitated above an impermeable layer of bedrocks.

309

GEOLOGICAL SOCIETY OF AUSTRALIA, A B S T R A C T S No. 59 1Australian Geological Convention, Sydney, July 2000

THE JINGGU STRIKE-SLIP BASIN OF TERTIARY IN SW YUNNAN, CHINA: AN EFFECT OF THE COLLISION BETWEEN INDIAN AND ASIAN CONTINENTS Shanvin Liu ^ and Chunyan Zhang ^ ^ Department of Geology, China University of Mining and Technology, Beijing, 100083, China ^ Shanxi Company of Coal Geology, Taiyuan, Shanxi, 030045, China

The Neotethyan ocean between Indian and Eurasian continents began to subduct in Late Cretaceous and closed in Middle Eocene (Bhattacharjee, 1991), which caused intracontinental thrusting and strike-slip faulting in western Yunnan and adjacent areas. In Early Miocene, continued compression between the two plates resulted in a large-scale strike-slipping in the region (Zhong et al., 1988). Along with the processes, the Jinggu strike-slip basin was formed, which can be reconstructed with two stages: late Paleogene transpressional basin and Neogene pull-apart basin. In Middle Eocene-Oligocene the collision between the Indian and the Eurasian plates led to oblique compression and dextral strike-slipping in S W Yunnan, inducing formation of a transpressional basin. The basin was filled with alluvial and fluvial sediments. On the west of the basin the boundary faults thrust successively to NEE, forming an overstep thrust sequence. On the east, dextral strike-slipping of the Wuliangshan fault dislocated deposits and their source area about 50km. In Neogene, on the setting of tectonic escape in west Yunnan and adjacent regions (Tapponnier et al., 1986), sinistral strike-slipping of the Wuliangshan fault caused the formation of a S-shaped pull-apart basin along a releasing bend of a branch of the fauh. The basin was filled with alluvial fan-fan delta, lacustrine and fluvial deposits. Its evolutionary history has been reconstructed with 4 stages: pre-pull-apart at the beginning of Miocene, pull-apart in early Miocene, depression in middle-late Miocene and shrinking in early Pliocene. The development of the basin reflects changes of regional stress field and movement of blocks in western Yunnan and adjacent areas. That was closely related to tectonic evolution of Tibet and Southeast Asia, and is a result of the collision between Indian and Eurasian plates and post-collision intracontinental deformation. References BHATTACHARJEE, C. C. 1991. The ophiolites of northeast India — a subduction zone ophiolite complex of the India-Burma orogenic belt. Tectophysics 191, 213-222. TAPPONNIER, P., PELTZER, G. & ARMIJO, R. 1 9 8 6 . O n the m e c h a n i c s o f the c o l l i s i o n b e t w e e n India and A s i a . In: COWARD, M . P. & RIES, A . C. Collision

Tectonics,

pp. 1 1 5 - 1 5 8 . B l a c k w e l l .

ZHONG, D. L., TAPPONIER, P., Wu H. W. et al. 1988, Large strike-slip fault — an important style for postcollision intracontinental deformation. Chinese Science Bulletin 7, 526-529.

310

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15""Australian Geological Convention, Sydney, July 2000

THE STABILITY OF COPPER(I) ACETATE COMPLEXES IN HYDROTHERMAL SOLUTIONS BETWEEN SOX AND ISCC Weihua Liu. D C "Bear" McPhail and Joel Brugger Victorian Institute of Eartii and Planetary Sciences (VIEPS) Department o f Earth Sciences, Monash University, Vic 3800, Australia

Acetate is considered as the predominant organic species in basin brine, and kinetic experiments showed that acetate is very stable at temperature under 200°C. Therefore, its capacity of transporting metal in geological fluid has been evaluated experimentally or theoretically. There have been many experimental studies over recent two decades on metal-acetate complexing at elevated temperatures, but so far there are no copper(I) acetate experimetnal studies available in the literature. This study uses mineral solubility experiment to determine the aqueous speciation of Cu(I) acetate complexes and their dissociation constants between 50°C and 250°C and saturated water vapor pressures. Cuprite (cuprous oxide) was chosen in order to avoid redox reactions in the experiments. Sodium acetate concentrations varied from 0.01m to 2m. The same concentration of acetic acid was added to buffer pH of solutions. The experiments were conducted in evacuated silica glass tubes, which are placed in a water bath (50°C) or laboratory oven (100°C and 150°C). Cu(II) concentrations are negligible based on the lack of color in the quenched experimental solutions and UV-Vis absorbance spectra of the quenched solutions. Measured copper concentrations vary between 0.0001m and 0.2m. Copper concentration increases with increasing temperature and increasing chloride concentration. Logarithms of the equilibrium constants (log K) of copper complexes were fitted by non-linear regression to the measured copper concentrations and acetate concentrations. Interpretation of our results shows that CuAC(aq) and CuAc2 are present in our experiments, and that copperacetate complexing is much stronger than theoretical estimation by Shock and Koretsky (1993, Geochimca et Cosmochimica Acta 57(20), 4899-4922.). Based on the logKs of copper acetate complexes, preliminary thermodynamic calculation suggests that copper acetate complexes be important for copper transport in organic-bearing solutions.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15""Australian Geological Convention, Sydney, July 2000

SEDIMENT QUALITY GUIDELINES: THEIR DEVELOPMENT, USE AND IMPLEMENTATION Edward R. Long National Centers for Coastal Ocean Science, United States National Oceanic and Atmospheric Administration, 7600 Sand Pt. Way NE.

Potentially toxic chemicals that enter lakes, estuaries, rivers, and oceans readily become sorbed to suspended particulate matter and ultimately sink to the bottom where they become incorporated into the bed sediments. Sediment-sorbed toxicants can subsequently act as a source of exposure for aquatic and marine life. To aid in the interpretation of data from chemical analyses of sediments, numerical sediment quality guidelines (SQGs) have been developed. A variety of methods have been used to develop such SQGs, all based upon either measured or modelled relationships with toxicity or other tests of biological effects. Empirical approaches have been used to develop SQGs that are most useful in estimating the probabilities that sediments would be toxic or not, but less useful in the identification of which chemical(s) might have caused the toxicity. Theoretical (modelling) approaches have been followed to devise SQGs that are best used to identify causal relationships with toxicity, but these guidelines have not proven very useful in estimating probabilities of toxicity. Recently, consensus-based SQGs have been developed by compiling different guidelines derived for the same narrative purpose. Considerable efforts have made to quantify how well SQGs perform in accurately predicting the presence and absence of toxicity in sediments. Primarily, these field validation studies have been conducted in the U.S. with statistical analyses of matching chemical and toxicity data. Similar field validation studies have been intiated in New South Wales, Western Australia, South Korea, and the United Kingdom. North American SQGs have been developed primarily as informal (non-regulatory) guidelines and to a lesser extent as regulatory standards or criteria.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

ROCK FABRIC AND MAGNETOSTRUCTURE CONSTRAINTS ON THE EMPLACEMENT OF THE RENCA BATHOLITH SIERRA DE SAN LUIS, ARGENTINA Monica G. Lopez de Luchi\ Augusto Rapalini^ Eduardo Rossello^ and Silvana Geuna^ ^ CIRGEO, Buenos Aires, Argentina ^ Laboratorio de Paleomagnetismo Daniel Valencio, FCEN, UBA, Buenos Aires, Argentina ^ Opto, de Ciencias Geologicas, FCEN, UBA, Buenos Aires, Argentina In the Eastern Pampean Ranges (EPR) two main metamorphic-igneous cycles has been defined: (640-540 Ma) Pampean and (540-330 Ma) Famatinian (Dalla Salda et al., 1998 and references therein). Late Famatinian (420-330 Ma) plutonism reflects a strong structural control, NNE for the earlier emplacement of pure crustal melts generated at different depths, and WNW, associated with extensional tectonics, for plutons that have a mixed mantle-crustal signature (Lopez de Luchi & Dalla Salda, 1997). This latter Late DevonianEarly Carboniferous magmatism is well developed in the eastern Sierra de San Luis and is represented by concentrically zoned batholiths that are associated with syenites and monzonites that are equivalents to vogesites and durbachites. The Late Devonian ellipsoidal and zoned Renca batholith, located in NE Sierra de San Luis comprises an external porphyroid biotite&#61617;homblende granodiorite/monzogranite (Unit 1) and a central equigranular biotite-muscovite monzo/leucomonzogranite (Unit 2). High Mg-K monzonitic rocks and syenitoids appear as enclaves, stocks and synplutonic dykes. In order to determine its geometry, conditions and processes involved in their emplacement, a multidisciplinary study has been performed. The steep to vertical foliation that is recognised in the porphyroid granitoids parallelizes the X-Y plane of the enclaves, the synplutonic dykes attitude and the contact between the batholith two units. These internal features are concordant with the ductile aureole that developed by meso- and micro-folding of the regional NNE S2/S3 of the medium grade metamorphic Ordovician country rocks. Foliation results from a combination of both magmatic and high temperature solid state flows; the latter as the result of the emplacement of the central monzogranite which originated a radial compression. K &#61627;0 as measured in kynematic indicators indicate flattening (Lopez de Luchi et al. 2000). As part of the geophysical characterization of the body, a systematic magnetic fabric study was carried out. Sixty sampling sites were evenly distributed on most of the batholith. Two to five oriented cores were collected at each site. Bulk susceptibility (K), anisotropy of magnetic susceptibility (AMS) and natural remanent magnetization (NRM) measurements were measured for all specimens. K values indicate a bimodal distribution that correspond, respectively, to Unit 1 (ferromagnetic) and Unit 2 (paramagnetic), which is very similar to the NRM intensity distribution. These match the magnetometric response of this body, with significantly different magnetic signatures for each unit. The same distribution is observed in the anisotropy degree, with higher values in the porphyroid granitoid (Unit 1). A correlation of anisotropy degree with K is observed in this body. The AMS has a predominantly foliated fabric, with a distribution of magnetic foliation planes generally sub-vertical and parallel to the boundaries of the ring-like Unit 1. Unit 2 shows less consistent orientation of the foliation planes, which are frequently subhorizontal. The magnetic fabrics of this body agree with other fabric parameters and are consistent with a model of radial stress produced on Unit 1 during emplacement of the central Unit 2. References DALLA SALDA, L. H.; LOPEZ DE LUCHI, M. G.; CINGOLANI, C. & VARELA, R.,1998. LaurentiaGondwana collision: the origin of the Famatinian-Appalachians Orogenic Belt.: In Pankhurst, R. J. and Rapela, C.W.(eds):The Proto-Andean Margin of Gondwana Geological Society Special Publication N°142: 219-234, London LOPEZ DE LUCHI, M.G. & DALLA SALDA, L.H. 1997. Late Famatinian Granitoids: a progressive stabilizing crust in SW South America. Terrane Dynamics 97, International Conference on Terrane Geology, Christchurch, New Zealand. Conference Abstracts: 103-106 LOPEZ DE LUCHI, M.G., ROSSELLO, E.A & LE CORRE, C., 1999. Emplacement related structures of the Renca Batholith, Sierra de San Luis, Argentina. Geophysical Research Abstracts (in press)

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TOOLOOM - A NEWLY RECOGNISED, INTRUSIVE RELATED, EPITHERMAL GOLD FIELD IN NORTHEASTERN NSW G.G. Lowden R.M.D. Meares and M.J. White Malachite Resources NL, P O Box 42 Lindfield NSW 2070

The Tooloom gold field is located in northeastern NSW, approximately 70 km northwest of Casino. Numerous old workings in hard rock and alluvials are spread over a 100 km^ area and at least 5000 people were working the field in the 1860s-70s. Published information about Tooloom is scarce and until Malachite's involvement, there had been no significant modem exploration and no drilling has taken place. Gold in the Tooloom area is characteristically coarse and nuggets are common, the largest recorded containing 140 ozs of gold. Native gold hosted by quartz vein stockworks is visible in outcrop and many of the alluvial nuggets comprise fragments of eroded gold-bearing quartz vein material. The Tooloom gold field lies within the Emu Creek Block, a small tectonic unit within the New England Fold Belt of northeastern NSW. Rocks in the area consist mainly of gently folded, fine to coarse grained clastic sediments of the Permo-Carboniferous Emu Creek Formation. Regionally these sedimentary rocks are intruded by granitoid rocks of Permian to Triassic age. Within the Tooloom area, mapping by Malachite has identified several previously unrecorded bodies of granitic and dioritic rocks, including porphyritic and equigranular types, that may relate to the regional granitic suite. Gold mineralisation accompanied by moderate to strong quartz-sericite-pyrite alteration occurs in both the dioritic intrusives and their sedimentary wall rocks. To the east and north, these rocks are overlain unconformably by Jurassic sediments of the Clarence - Moreton Basin. The region displays a strong structural grain, with dominant NNE and NW trending structures affecting drainage and commonly controlling mineralisation. The most important types of mineralisation recognised to date may be described as: (i) carbonate-base metal veins - gold and silver occur in structurally-controlled, carbonate (-silica) veins, with abundant pyrite and prominent sphalerite and galena; (ii) epithermal quartz veins - gold occurs in somewhat colloform, banded quartz veins with a claycarbonate alteration assemblage and some pyrite but only minor base metal sulphides; (iii) quartz veins in dilational zones - gold occurs in quartz veins in various host rocks where a flexure or off-set in a fault creates a local low pressure environment; (iv) breccia zones - gold occurs in strongly fractured to intensely brecciated rocks, including both sediments and intrusives, where dilational stress creates substantial low pressure zones; and (v) quartz vein stockworks and sheeted zones - gold occurs as particles of native gold, commonly quite coarse in grain size, enclosed in quartz vein stockworks and zones of sheeted quartz veins hosted by brittle fractured sandstone and conglomerate of the Emu Creek Formation, especially around the margins of dioritic intrusions. Most significant gold occurrences are strongly expressed as stream sediment geochemical anomalies and two main centres of mineralisation approximately 5 km apart have been recognised to date. Age of gold mineralisation at Tooloom appears to be similar to that at Gympie, some 260 km to the north and there are similarities in style and setting, including the common occurrence of coarse grained, primary gold. However, mineralisation at Tooloom seems to have a stronger affiliation with intermediate intrusive rocks. Furthermore, the scale of mining that has so far produced more than 3.4 million ounces of gold at Gympie is yet to take place at Tooloom.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15'^ Australian Geological Convention, Sydney, July 2000

FAULT CONTROLS ON GOLD MINERALISATION IN THE YOUNG GRANODIORITE Patrick Lyons', Oliver Raymond' and Morrie Duggan^ ' Australian Geodynamics ORG, AGSO, GPO Box 378, Canberra ACT 2601 ^ 14 Bird Place, Flynn ACT 2615 The Late Silurian S-type Young Granodiorite, in the Lachlan Orogen of NSW, was host to at least 60,000 oz of lode gold at an average grade of 21 gt-1. The gold was won mostly from steep to vertical WNW-striking quartz veins associated with pyrite, arsenopyrite, galena, chalcopyrite, and minor sphalerite. The veins generally strike about 290° to 295° and dip steeply or vertically. About 90% of production came from Medcalf s Reef, about 4 km northeast of Harden. The major regional structures are the NNW-trending Mooney Mooney Fault and the N-S-trending Jugiong Shear Zone. The Mooney Mooney Fault occurs along the western contact of the Young Granodiorite and separates it from rocks of the Ordovician to Early Silurian Tumut trough. It is characterised by S-C mylonites with steeply dipping extension lineations (Warner et al., 1992). The shear fabrics occur in the country rocks and for up to 2 km into the granodiorite. From about 10 km southeast of Wallendbeen to the Tertiary volcanic complex just south of Adjungbilly Creek, about 24 km north-northeast of Tumut, the contact is intermittently intrusive. The Jugiong Shear Zone occurs within the Young Granodiorite and splays from the Mooney Mooney Fault at the Tertiary volcanic complex. It is mappable for about 90 km to the north. Although the structures controlling the Au-bearing veins are not apparent from traditional mapping or detectable in regional TMI data and their vertical derivatives, they are clearly visible in gamma ray spectrometric data due to their K-alteration envelopes. The spectrometric data show the Jugiong Shear Zone; narrower N-S-trending faults; and a prominent set of WNW-trending faults. The Harden and McMahon's Reef deposits lie along WNW-trending faults, as does a barren, 4 km-long quartz vein, cropping out about 8 km southwest of Harden. The age of mineralisation is unknown, but mapping by Raymond et al. (2000) shows that movement on the northern continuation of the Mooney Mooney Fault steepened the limb of the Parkes Syncline, in rocks of the Late Devonian Hervey Group. This implies a last movement during the Carboniferous period and suggests that mineralisation may the same age. A number of the NNW- to NW-trending structures link the Mooney Mooney Fault to the Jugiong Shear Zone and probably allowed the transfer of strain that protected the intrusive part of the contact from deformation. References

RAYMOND, O.L., DUGGAN, M.D., LYONS, P., SCOTT, M.M., SHERWIN, L., WALLACE, D.A., KRYNEN, J.P., YOUNG, G.C., WYBORN, D., GLEN, R.A., & LEYS, M. 2000. Forbes Second Edition (1:250 000 geological map SI55-7), Australian Geological Survey Organisation, Canberra/Geological Survey of New South Wales, Orange. WARNER, P.J., MARSHALL, B., & FRANKLIN, B.J. 1992. The Mooney Mooney Fault System and Coolac ophiolite suite in the tectonics of the Tumut Trough, southeastern Australia. Australian Journal of Earth Sciences 39, 127-140.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

QUENCHED MELT INCLUSIONS IN PHENOCRYSTS OF A SILURIAN DACITE (LACHLAN FOLD BELT): THE PETROGENESIS OF MAFIC S-TYPE MAGMAS Roland Maas', Vadim Kamenetsky^, Ian A Nicholls^ and David Steele^ ' Department of Earth Sciences, La Trobe University, Melbourne ^ CODES, University of Tasmania, Hobart 3 Department, of Earth Sciences, Monash University, Melbourne 4 Central Science Laboratory, University of Tasmania, Hobart The restite model of Chappell and White ascribes most compositional variation in SE Australian granitic and volcanic suites to the unmixing of magmas composed of undissolved, relatively mafic source material (the restite) and a felsic melt. Critics of the model advocate a greater role for large-scale crustal assimilation, magma mixing and fractional crystallization (e.g. Collins, 1996, Trans. R. Soc. Edin. Earth Sci. 87, 171). One of the more contentious aspects of the restite model is the identity of restite minerals. Definitive evidence to support a restitic origin for rock-forming minerals in granites is very limited, in part due to modification of original signatures of the restite in a plutonic environment. Volcanic rocks, in particular Stype volcanics, may offer a better chance to examine potential "restite" crystals. We have studied a reasonably well-preserved phenocryst population in a crystal-rich (ca. 56% crystals) peraluminous dacitic (Stype) lava of the Silurian Hawkins Volcanics north of Canberra (Cowra excursion stop 2, Wybom et al, 1991, Hutton-2 excursion guide). The volcanics are thought to be extrusive equivalents of the widespread mafic Stype Bullenbalong Suite of granitoids. The entire phenocryst population (17% quartz, 19% plagioclase An51, 10% biotite Mg55, 4% orthopyroxene Mg48, 5% cordierite Mg 65„ trace garnet Mg25) of this dacite has been interpreted as erupted (metamorphic) restite that experienced only limited magmatic processing prior to quenching (Wybom and Chappell, 1986, Geol. Mag. 123, 619). The dacite was believed to represent a primary magma produced by wholesale mobilization of partially (e.g. 40%) melted mature greywacke. Crystal and melt inclusions are common in phenocrysts of quartz, orthopyroxene (Mg45.6-50.7), cordierite, plagioclase, and also in apatite. For example, quartz phenocrysts contain euhedral inclusions of plagioclase (An50-73), orthopyroxene, apatite and biotite. Quenched melt inclusions in quartz and orthopyroxene are composed of silicate glass and a shrinkage vapor bubble. Sulfide globules occur within silicate melt inclusions or scattered through host phenocrysts. Melt inclusions in plagioclase and cordierite are often altered but those in quartz and orthopyroxene are fresh. The silicate melt inclusions provide the only preserved derivative of the dacite's melt phase; the (once glassy ?) groundmass is altered. Compositions of melt inclusions in quartz and orthopyroxene are similar, with Si02 75-79%, AI2O3 13-15%, FeOt 0.1-1%, <0.1% MgO, CaO 0.5-1%, Na20 2.4-3.1%, K2O 5-7%, P2O5 0.03-0.2%, A/CNK 1.14-1.22. By comparison, the bulk dacite has 68% Si02, 4.4% FeOt, and 2.1% MgO. Quenching of trapped silicate melt droplets occurs during very fast cooling which explains why melt inclusions are ubiquitous in volcanics and absent in most plutonic rocks. The presence of quenched melt inclusions in the major phenocryst types of the Hawkins dacite therefore supports a shallow-level igneous rather than restitic (metamorphic) origin for these crystals. Trapping of high-Si02 melt within magmatic rims on restitic cores cannot explain why these inclusions are found even in the cores of their host crystals - a fully igneous origin is much more plausible. Subtle variations in melt inclusion composition can be explained by fractional crystallization, consistent with evidence from trapped mineral inclusions. This suggests that fractional crystallization played a significant role in the petrogenesis of these dacites. If an igneous origin is accepted for the major phenocryst types, the amount of possible restite in the dacite reduces to perhaps 5% or less. This is a remarkably low figure for a mafic S-type; such a small amount of restite would severely limit the role of any restite unmixing in producing compositional variation in granite and volcanic suites.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RE-OS ISOTOPIC EVIDENCE FOR CRUST-MANTLE INTERACTION IN THE GENESIS OF CONTINENTAL INTRAPLATE BASALTS FROM THE NEWER VOLCANICS PROVINCE, SOUTH-EASTERN AUSTRALIA Jannene McBride\ David Lambert^ Ian Nicholls^ and Richard Price^ ' Australian Crustal Research Centre and Department of Earth Sciences, Monash University, VIC 3800 ^ Department of Earth Sciences, Waikato University, Hamilton, New Zealand The origin of the enriched geochemical signatures observed within many continental intraplate basalts is controversial. The most widely proposed hypotheses are 1) that these signatures are inherited from a subcontinental lithospheric mantle source or 2) that they reflect lithospheric (crust and/or mantle) contamination of asthenospheric mantle/plume-derived magmas en-route to the surface. The Re-Os isotopic system is an excellent tool for distinguishing between these two distinctive enrichment processes, as unlike the more commonly used Sr-Nd-Pb isotopic systems, there is a strong Re-Os isotopic contrast between old subcontinental lithospheric mantle and crust. The Newer Volcanics Province in south-eastern Australia provides an excellent opportunity to investigate the relative influences of subcontinental lithospheric mantle and/or crust on the genesis of a young (< 5 Ma) continental intraplate basaltic province. The very young age of the Newer Volcanics Province basalts eliminates the need to age-correct isotopic ratios, an added source of uncertainty. In addition, the subcontinental lithospheric mantle beneath this region is relatively well characterised due to a number of geochemical studies of peridotite and pyroxenite mantle xenoliths (e.g., Frey & Green, 1974; McDonough et al. 1991, Griffm et al. 1988; McBride et al .1996) carried within the Newer Volcanics Province basalts. The Newer Volcanics Province has been subdivided into two series, the Plains and Cones, on the basis of geomorphology and composition (Price et a l . 1997). We have analysed two nepheline-hawaiites from the Cones series and six olivine-tholeiites from the Plains series basalts. The samples analysed exhibit a large diversity in their chemical and Nd, Sr and Os isotopic compositions. Plains series olivine-tholeiites and Cones series nepheline-hawaiites have distinctive isotopic compositions and are clearly not related to each other by a simple genetic process affecting a single mantle source. The Cones series ne-hawaiites have trace element abundances and Nd (eNd = +4.1 & +3.8), Pb (206Pb/204Pb = 18.62 & 18.70) and Os (gOs = +6 & +7) isotopic compositions which fall within the range of ocean island basalts (OIB). In contrast, the Plains series ol-tholeiites have lower eNd (+0.9 to +2.3), more radiogenic Sr (87Sr/86Sr = 0.7048 to 0.7054), and considerably higher gOs (+42 to +250). The radiogenic Os isotopic compositions observed within the Plains series ol-tholeiites can be generated either by melting of a radiogenic source or, alternatively, contamination of the magmas en-route to the surface. The relatively low Os concentrations observed in the Plains series ol-tholeiites (Os = 12 to 45 ppt) compared to the more "primitive" Cones series ne-hawaiites (Os = 160 & 250 ppt), render them more susceptible to contamination processes which can obscure their primary mantle signatures. Radiogenic pyroxenite layers within the subcontinental lithospheric mantle, sampled as Al-augite suite xenoliths, may be a potential source reservoir. However, trace element trends and isotopic modelling suggests that the unusual geochemical signatures observed within the Plains series ol-tholeiites are the result of assimilation of continental crust possessing variable isotopic signatures. The Cones series ne-hawaiites have trace element and isotopic compositions consistent with their derivation from melting of either a mantle plume (OIB-type source) or "veined" subcontinental lithospheric mantle. References FREY, F.A. AND GREEN, D.H. 1974. The mineralogy, geochemistry and origin of Iherzolite inclusions in Victorian basanites Geochim. Cosmochim. Acta 38, 1023-1059 GRIFFIN, W. L., O'REILLY, S. Y. AND STABEL, A., 1988. Mantle metasomatism beneath western Victoria, Australia: II. Isotopic geochemistry of Cr-diopside Iherzolites and Al-augite pyroxenites. Geochim. Cosmochim. Acta 52, 449 MCBRIDE, J.S, LAMBERT, D.D., GREIG, A. AND NICHOLLS, I.A 1996. Multistage evolution of Australian subcontinental mantle: Re-Os isotopic constraints from Victorian mantle Xenoliths. Geology 24, 7, 631-634 MCDONOUGH, W. F., RUDNICK, R. L. AND MCCULLOCH, M. T., 1991. The chemical and isotopic composition of the Lower Eastern Australian Lithosphere: A review. Geol. Soc. Aus. SP 17, 163-188 PRICE, R. C., GRAY, C. M. AND FREY, F. A., 1997. Strontium isotopic and trace element heterogeneity in the plains basalts of the Newer Volcanic Province, Victoria, Australia. Geochim. Cosmochim. Acta 61, 1, 171 - 192.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59

1Australian

Geological Convention, Sydney, July 2000

THE EVOLVING GLOBAL ENERGY MIX: IMPACTS FOR AUSTRALIA IN THE 21ST CENTURY Peter J. McCabe U.S. Geological Survey, Federal Center MS 939, Denver, CO 80225

Currently oil accounts for about 38% of the world's energy consumption. Coal accounts for 23.5% and natural gas 22.5%. The remaining 16% of our energy supply is derived from non-hydrocarbon sources, predominantly from hydro and nuclear power plants. At the beginning of the 20^ century over 90% of our energy was derived from coal. Oil substituted for coal in the transport sector of the energy market. Natural gas, and to a lesser extent oil, have largely substituted for coal in the domestic heating and in manufacturing sectors. Despite the loss of market sectors coal is still an important part of the energy mix because of its use in electrical generation. Production reached all time highs just before the collapse of the Soviet empire. At the beginning of the 21'^ century there is much speculation as to how the energy mix will evolve over the coming decades. Predictions range from a neoMalthusian perspective of an imminent peak in oil production, with subsequent dire consequences for the world economy, to a comucopian perspective of low energy prices due to technological advances. Critical to current debates are the concepts of resources and reserves that have lead to considerable misunderstanding. Estimates of resources and reserves are inventories of the amount of fossil fuel perceived to be available over some future period of time. They are nm a measure of a finite supply. As resources/reserves are depleted over time, additional amounts of fossil fuels are inventoried. Throughout most of the 20^ century crude oil reserves in the United States, for example, represented a 10-14 year supply. For the last 50 years, resource estimates have represented about a 60-70 year supply. Division of reserve or resource estimates by current or projected annual consumption therefore is circular in reasoning and can lead to highly erroneous conclusions. Likewise, predictions based on production histories of fossil fuels and an assumption of finite energy supplies can be highly inaccurate. Examination of some energy resources with well-documented histories leads to two conceptual models that relate production to price. The closed-market model assumes that there is only one source of energy available. Although the price initially may fall because of economies of scale, long-term prices rise as the energy source is depleted and it becomes progressively more expensive to extract. By contrast, the openmarket model assumes that there are a variety of available energy sources and that competition among them leads to long-term stable or falling prices. At the moment, the world approximates the open-market model and there appears little reason to suspect that long-term price trends will rise significantly over the next few decades. Changes in the future energy mix will occur as technological advances change the relative costs of different fuels in various market sectors and when policies are implemented because of perceived environmental and strategic costs of certain fuels. Concern about global warming may lead to political pressures to reduce coal use in power generation and oil use in the transportation sector. The development of affordable ftiel cells, lower costs for LNG, new gas-to-liquids technologies, and the deregulation of power industries suggest that the global demand for natural gas could rise substantially in the early decades of the 21'^ century. Renewable energy is likely to remain only a small fraction of the total energy mix for some time to come. A new assessment by the U.S. Geological Survey suggests that Australia has substantially more undiscovered oil (28% increase) and natural gas (59% increase) than had previously been estimated. Countries in Southeast Asia are also thought to have substantially more undiscovered natural gas than previously estimated, but have less undiscovered oil. Increase in global gas demand may well create a market for the extensive gas resources of the Northwest Shelf, and new discoveries of oil may allow Australia to continue be largely self-sufficient in oil during the early decades of the 21'^ century.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

FORELAND TO FOLD BELT: CONTROLS ON OROGENESIS AND PETROLEUM TRAPS IN NEW GUINEA B.A. McConachie and P.J. Stanmore Santos Asia Pacific Pty Ltd The Papuan-Akimeugah basin foreland in PNG and eastern West Papua (formerly Irian Jaya) comprises several large compartments and contains fault structure trends that control fold belt deformation in central Warim in West Papua. Understanding these relationships can also be used by analogy to better understand the oil productive fold belt anticlines in PNG. Pre-fold belt basin architecture and structure are primary controls on the development of fold belt anticlines. Deep detached thrusts that produce reservoir involved hanging wall anticlines are the lowest risk traps in the basin. The oil productive Kutubu-SE Gobe trend comprises a series of such anticlines. The 3KB trend (Fig. 1) is a major regional structure that traverses the PNG foreland (McConachie et al., 2000). This sinistral strike slip fault system has been mapped in detail over a distance of 700 km from the Gulf of Papua to the remote Baliem Valley of central West Papua. The mapping utilised the close spaced high quality seismic grid, regional SAR data, Landsat TM composites derived from cloud free images and regional RTP magnetic data. The major productive Kutubu trend is interpreted to have originated as a similar parallel trend to the 3KB foreland trend. No drilling has yet been conducted on the central Warim trend. To the northwest, Cross Catalina-1 intersected a 51 m hydrocarbon column in relatively tight clean quartzose sandstone of the Woniwogi Formation. To the southeast, Kau-2 flowed volatile oil from an LJ5 lagifu age equivalent sandstone. The results of our analyses indicate major regional structural control on the hydrocarbon fields of PNG and West Papua. Central Warim in West Papua contains a series of reservoir-involved hanging-wall anticlines that could potentially rival the highly productive PNG Kutubu to SE Gobe oil field trend.

Fold belt trends ^ . ^ H Piirteboundaries

Oligocene P( back arc collision

Australian

Fig. 1 Oilfields and structural trends in New Guinea

Reference McConachie, B.A., Burge, C., Lanzilli, E. and Kendrick, R.D., 2000. Extensions of the Papuan Basin geology into Eastern Irian Jaya and the Papuan Foldbelt. In: Fourth PNG Petroleum Convention, Port Moresby, PNG Chamber of Mines and Petroleum, (in press).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TREATMENT OF ACID MINE WATERS USING SEAWATER-NEUTRALISED BAUXITE REFINERY RESIDUES D. McConchie\ M. Clark\ F. Davies-McConchie^ and C. Hanahan^ ' Centre for Coastal Management, P.O. Box 5125, East Lismore, NSW, 2480 ^ School of Geosciences, University of Sydney, Sydney, NSW, 2006 ^ Department of Chemical Engineering, University of Queensland, St. Lucia, Qld., 4072

The management of acid mine waters that are leaking into rivers or are stored in tailings or interception dams is one of the biggest problems facing mining and refining industries. Various management options are available but all have limitations, some are accompanied by serious environmental risks, and few provide cost-effective long term solutions; options include: do nothing, retain contaminated water in an interception dam, use electrochemical metal extraction techniques, establish bioreactors or artificial wetlands, chemically treat batches of contaminated water before discharge, or discharge the water through reactive barriers that decontaminate the water as it passes through the permeable barrier. A potentially very cost-effective variation on the chemical treatment or reactive barrier strategies is provided by seawater-neutralised bauxite refinery residues (BauxsoF^), which despite having a soil reaction pH of only about 8.4, has an acid neutralising capacity of 3.65 moles per kg and the ability to bind up to 1,000 meq of trace metals per kg. Both acid neutralisation and trace metal binding are initially rapid and then proceed more slowly to their maximum capacity over about 240 hrs. TCLP tests show that it is very difficult to leach trace elements from fresh seawater-neutralised red mud (the dried material complies with requirements for classification as an inert solid in New South Wales) and even after being used to treat AMD water only a small proportion of the trapped trace metals can be removed by compulsive exchange reagents. X-ray diffraction scans indicate that the Bauxsol™ consists of a cocktail of minerals mainly including: anhydrite/gypsum, aragonite, boehmite, brucite, calcite, cancrinite, gibbsite, hematite, hydrocalumite, hydrotalcite, p-aluminohydrocalcite, portlandite, quartz, sodalite and whewellite. Following the successful completion of laboratory trials, a larger scale field trial was carried out involving the treatment of 1.6 ML of AMD water in a toe dam at the Mt Carrington mine site in New South Wales. The results of the first full scale field trial show that the pH of the water rose from 3.58 to 8.3 and that the concentrations of all trace metals decreased markedly: A1 decreased from 60,710 |Lig/L to 30.6 |ig/L; Cd decreased from 1,659 |ig/L to 0.2 ^ig/L; Cu decreased from 42,560 |ig/L to 3.7 |ag/L; Fe decreased from 7,056 |ig/L to 33.7 |Lig/L; Pb decreased from 71 |Lig/L to <0.1 |Lig/L and Zn decreased from 64,850 |ig/L to 18.7 |ig/L. The data collected during the trial show that the removal of individual metals coincides with distinct stages in the treatment as the pH slowly rises. Water quality at the end of the trial met the ANZECC (1992) requirements for the protection of aquatic ecosystems. Consequently, after allowing 24 hours for the suspended red mud particles to settle, the treated water was discharged from the dam through a sand filter to remove any residual suspended particles. Following the initial trial, which was carried out slowly over five weeks to collect kinetic data, the trial was repeated with treatment being completed in 24 hrs and followed by treatment of a 600 ML tailings dam. This paper describes the geochemistry o f the seawater-neutralised bauxite refinery residues, their use in the A M D treatment trials, and the results obtained in the initial and subsequent trials.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MODELS OF EARTHQUAKE PROCESSES FOR SEISMIC HAZARD ASSESSMENT IN AUSTRALIA Kevin McCue and Cvetan Sinadinovski Australian Geological Survey Organisation, Canberra, Australia

After one hundred years of monitoring earthquakes in Australia we know that large earthquakes of magnitude 6 or more have a return period of about 5 years and have the potential to cause enormous damage. We have superimposed the epicenters on geophysical datasets depicting crustal structure. Disappointingly these maps show no correlation of the seismicity with topography, nor with the geology, gravity, geomagnetic signature or crustal element boundaries. Two models of the seismicity of Australia have been espoused for hazard assessment and we have tested those models using the AGSO earthquake database. Analysis of the hazard using the past-earthquake and uniform hazard models results in, on the one hand, a complex contour map subject to potential change with each new unexpected earthquake and on the other, a map depicting uniform hazard at a level that is considered sufficiently low by engineering standards that it could be neglected. We have devised a third model, a physically based one, which we have called the Coulomb model, which is based on the observed pattern of epicentres of Recent and known prehistoric earthquakes and young volcanoes. The model is explained in terms of the known plate tectonic boundary stresses and in turn explains some of the mapped continentscale lineaments. The model explains 95% of the current Australian epicentres and will be tested for long term variations against future earthquakes and paleoseismicity yet to be mapped.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ABUNDANCES AND DISTRIBUTION OF TRACE ELEMENTS IN THE LOWER OCEAN CRUST: A STUDY OF SEISMIC LAYER 3 GABBROS DRILLED DURING ODP LEG 176 AT THE SOUTHWEST INDIAN RIDGE Suzanne Mackie and Yaoling Niu Department of Earth Sciences, The University of Queensland, Brisbane, Qld 4072 The ocean crust is thought to be an important geochemical reservoir, but our present knowledge of this reservoir is imperfect. Popular models of chemical geodynamics use average composition of mid-ocean ridge basalts (MORB) as proxy for ocean crust, but this use neglects the fact that MORB are only a compositional endmember and constitute no more than - 1 0 - 1 5 % of the total crustal mass. Therefore, characterisation of the actual composition of ocean crust is needed. This effort, however, has been hampered by the inaccessibility of the lower ocean crust. Gabbroic sections of ophiolites have been inferred as proxy, but there is mounting evidence that most of these ophiolites do not represent normal ocean crust, but formed by supra-subduction zone processes. Ocean Drilling Program (ODP) Hole 735B, drilled during Leg 118 (1987) and Leg 176 (1997), revealed a 1.5 km section of lower ocean crust formed at the Southwest Indian Ridge. The drill core samples (mostly gabbro and olivine gabbro with some troctolite, Fe-Ti gabbro, and FeTi gabbronorite) provide a prime opportunity for characterising the abundances and distribution of elements in the lower ocean crust. Figure 1 summarises the results of our study (a total of 55 elements analyzed using ICP-OES and ICP-MS) on representative lithologies (49 whole-rock samples and clinopyroxene and plagioclase separates in 38 samples) recovered during Leg 176. It is clear that the gabbros differ from MORB in having (1) depleted abundances of incompatible elements (except Sr), particularly the high field strength elements (Nb, Ta, Zr, Hf, Th and U); and (2) significant fractionation between elements that are considered to have nearly constant ratios in MORB (e.g., Nb/Ta « 17, Zr/Hf « 35 Ce/Pb « 25). These differences reflect the cumulate nature of the gabbros, and importantly, the previously unknown effects of constituent minerals in fractionating these elements during magma cooling and evolution. These new data demonstrate the need of a realistic model composition of bulk ocean crust, which will be presented in full at the meeting.

Figure 1. Trace element data of ODP Leg 176 gabbros and mineral separates.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

CARBONIFEROUS TECTONISM AT MOUNT PAINTER - THE ALICE SPRINGS OROGENY MOVES SOUTH Sandra McLaren\ Mike Sandiford^*, Jim Dunlap^ and Ian McDougalP ^ Department of Geology and Geophysics, University of Adelaide, SA, 5005 ^ Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200 * present address, School of Earth Sciences, University of Melbourne, Vic, 3010

Deformation and metamorphism in the Neoproterozoic sequences uncomformably overlying Mesoproterozoic basement of the Mount Painter Inlier in the northern Flinders Ranges, South Australia, has been ascribed to the Cambro-Ordovician Delamerian Orogeny, -515 - 490 Ma. The tectonic history subsequent to this time is poorly constrained. However, the occurrence of Carboniferous granitoids in the nearby Warburton Basin (Gatehouse et al., 1995), a Carboniferous-Permian hydrothermal fluid flow event in the Mount Painter Inlier (Idnurm & Heinrich, 1993) and young (-175-260 Ma) apatite fission track ages in the Northern Flinders Ranges (Foster et al., 1994) suggest a more complex Phanerozoic tectonic history. Here, we present new data which constrains the thermal history of the Mount Painter region during the post-Delamerian period. This history has been assessed through ^W/^^Ar age measurements on micas, and multiple diffusion domain thermal modelling of K-feldspar "^^Ar/^^Ar data. The new data allow two important features of the cooling history to be identified: • K-feldspar diffusion domain modelling suggests that the terrane cooled below 200°C as late as 310 Ma, and several samples demand a period of very rapid cooling between about 320 and 330 Ma. • A wide spread of mica plateau ages (from about 400 to 350 Ma) suggest that prior to this cooling event the terrane remained at temperatures ~320°C for approximately 80 Ma. This allowed the partial resetting of muscovite and biotite thermochronometers (Dunlap, 2000). An elevated present day surface heat flow measurement from Mount Painter (-120 mWm"^) suggests geothermal gradients as high as 40°Ckm"\ Assuming that the argon results reflect cooling associated with denudation, the data imply that the Mount Painter terrane has been exhumed from depths of at least 8 km since about 330 Ma, with a minimum of 3 km of this denudation occurring in the interval 330 to 320 Ma. Spatial variations in cooling ages imply differential denudation consistent with ramping on the Paralana Fault. These data imply significant Carboniferous aged tectonism in this region, which we ascribe to the Alice Springs Orogeny (ASO). Thus the signature of the ASO extends far beyond central Australia, making it a true continent-scale intraplate orogeny. References DUNLAP W.J. 2000. Natures diffusion experiment: The cooling-rate cooling-age correlation. Geology 28, p. 139-142. FOSTER D.A., MURPHY J.M. & GLEADOW A.J.W. 1994. Middle Tertiary hydrothermal activity and uplift of the northern Flinders Ranges, South Australia: Insights from apatite fission-track thermochronology. Australian Journal of Earth Sciences 41, p. 11-17. GATEHOUSE C.G., FANNING, C.M. & FLINT, R.B. 1995. Geochronology of he Big Lake Suite, Warburton Basin, northeastern South Australia. Quarterly Geological Notes, Geological Survey of SA 128, p. 8-16. IDNURM M. & HEINRICH C.A. 1993. A palaeomagnetic study of hydrothermal activity and uranium mineralization at Mount Painter, South Australia. Australian Journal of Earth Sciences 40, p. 87-101.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

QUARTZ-SILLIMANITE VEINS AND NODULES ASSOCIATED WITH LATE LEUCOGRANITES, ADIRONDACK HIGHLANDS, NEW YORK STATE, U.S.A. James McLelland^ and Jean Morrison^ 'Department of Geology, Colgate University, Hamilton, N.Y., USA ^ Department of Geology, University of Southern California, Los Angeles, California, 90089, USA

Pink leucogranites along the Moose River near Port Leyden, N.Y., western Adirondack Highlands, contain abundant veins and nodules consisting of quartz and sillimanite that exhibit strong N50E-N20E orientations. The nodules consist of severa generations and clearly represent disrupted veins. Several generations of veins are also present, the latest showing little deformation. Nodule size is of the order of 2-10 cm in length and 1-5 cm across. The veins have the same width but may be several m across. Within both veins and nodules, sillimanite tends to concentrate at the center and quartz to the outside, but the relationship is far from perfect. Both veins and nodules exhibit crosscutting relationships with eachother and with country rock schleiren in the granite. The granite has been dated by U-Pb zircon SHRIMP 11 techniques and yields an age of 1035 +/- 8 Ma thus fixing the maximum ageof the veins and nodules. A large, undeformed pegmatite related to the granite crosscuts all features, including the veins and nodules, and yields a single grain UPb TIMS age of 1034 +/- 8Ma which is the minimum age for the veins and nodules. This places vein and nodule formation within the time interval of magmatic solidification and cooling. This conclusion is supported by the observation that granite dikes crosscut the veins and nodules. Associated with the granite-vein-nodule system are large quartz veins that crosscut every thing and contain abundant scattered sillimanite. The largest of these is 30-50 m across and can be followed for 1 km. It contains 2-4 cm long needles of altered sillimanite. Also present are zircons of the same morphology and age as those in the granite and presumably relict from it. We interpret the overall system to be the result of cation leaching of granite by hydrothermal fluids during the waning stages of pluton solidification. In the still hot carapace, acidic hydrothermal fluids moved along fracture sets leaching alkalis from feldspars and leaving behind quartz and sillimanite. The source of the acidity is uncertain, but acidic solutions are also indicated by wide-spread hydrothermal magnetite and associated sodic metasomatim of the same age. Oxygen isotope investigations suggest that the fluids involved are consistent with surface derived waters that evolved in saline evaporitic conditions or by exchange with evaporitic sediments and that exchanged with the rocks at 625-675''C. This requires that the pluton intruded at relatively shallow levels of km. Note that the hydrothermal fluids notonly leached alkalis, but also must have transported alumina and silica for substantial distances in order to account for zonation in small veins and nodules and sillimanite in the large veins. Crosscutting hydrothermal magnetite-silliamnite veins also indicate alumina transport over large distances.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOTECHNICAL INVESTIGATION AND RECLAMATION OF SANDSTONE AND SHALE QUARRIES IN THE SYDNEY REGION. G.H. McNally Douglas Partners, Sydney

This paper discusses the geotechnical characteristics of abandoned sandstone and shale quarries, and how these influence their after-use as industrial, residential and recreational land. Because most of the sites have been used for waste disposal between their initial extractive phase and one of these later land uses, they may be lightly to heavily contaminated. However, this paper concentrates on the geotechnical engineering problems of fill settlement, pile seating, rock face support and slope reshaping. Scores of these former quarries, mostly already built upon, occur within the Sydney CBD and its surrounding suburbs. Sandstone dimension stone quarries were the more numerous, though Ashfield Shale pits were larger - up to 5 Mm^ at Sydney Park, St Peters; most of the sandstone workings were very small, less than 1 ha in extent. Commercial sandstone quarrying began at Pyrmont in the 1860s, and this area dominated production up to about 1930. However from the late 19th century many sandstone quarries located in the eastern suburbs, the best known being at Maroubra and Bondi. The main products were cut stone ashlars for foundation courses and retaining walls, but macadam roadbase ('road ballast') was also generated. Large-scale shale extraction also dates from the 1860s, in the Newtown - St Peters - Marrickville area. During the twentieth century large shale pits moved outwards with the advance of suburbia, mainly towards the southwest. Present day geotechnical investigations are needed because many of the overbuilt innercity extractive sites are now being converted from industrial premises to high density residential developments. Though such sites present foundation difficulties, the land is valuable enough to allow for expensive geotechnical investigations (and even more expensive environmental studies and remediation work!) The favoured exploration techniques include deep test pits dug by hydraulic excavators; auger probing and standard penetration tests; airphoto interpretation to determine excavation limits and filling history, and large-scale loading tests to assess compressibility of the filling materials. Conversely, tube sampling, standard soil mechanics testing and geophysical techniques are not usually applicable. The investigation aims are to establish the fill depth, the rockhead profile, the presence of buried obstacles and hazards, groundwater conditions, and fill settlement parameters. Examples of quarry site redevelopment from Sydney and the central coast region are presented. The main problems from the sandstone sites are long-term rock slope stability (especially where weathering is active, and faces are blast-damaged or fractured by stress relief), determining the buried rock surface, and reshaping hard and irregular quarry benches into suitable and stable landforms. Former shales quarries are most commonly used for recreational reserves - largely because their depth (up to 30m), methane emission and leachate generation, but partly because of the demand for inner-city sporting facilities.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SANDSTONE FILL MATERIALS G.H. McNally Douglas Partners, Sydney

Weathered and fragmented sandstone is the most abundant source of earthfill and rockfill embankment materials in the Sydney region. Earthfill is essentially a clayey sand mixture obtained from weathered sandstone saprolite at shallow depth. It can be roller-compacted to a dense embankment material of low permeability, which is amenable to laboratory and onsite testing (and hence to specification and quality control). Rockfill is the more stoney material obtained from fresher sandstone in deep excavations. It is free-draining, prone to post-construction settlement, but too course for routine testing. Construcdon of deep freeway cuttings, tunnels and underground caverns has obliged Sydney engineers to make more use of sandstone rockfill in recent years, rather thn the more easily worked earthfill which was previously preferred. Hawkesbury Sandstone is by far the major source of sandstone geomaterials in the Sydney region. It generates good quality earthfill when excavated, but indifferent rockfill due to its tendency to break down completely with working to sand-size quartz grains plus clay binder. The coarser end of its grading curve is made up of angular weak rock fragments, which disintegrate with on-road compaction. Oversize boulders are used for embankment erosion- protection stone. Harder beds within the Hawkesbury Sandstone are sometimes crushed for second class road sub-base, rubble for gabion filling and select embankment fill. The sandstones of the Narrabeen Group are less pervasively weathered and may be fresh within a few metres of outcrop, hence they are better sources of rockfill and rubble though still relatively soft. The output of tunnel spoil from underground excavations in Sydney has increased greatly since 1990, and is likely to become even more abundant in future years. This material is being processed for sub-base, select fill and a variety of sand products. Similar geomaterials are by- products from sandstone quarries operated primarily as landfill sites. The increasing volume of sandstone spoil, especially in basement carparks and tunnels, poses a growing disposal problem. This material is excavated by bulldozer ripping, pneumatic rockbreakers, undergound roadheaders and rock sawing - but no longer by blasting, because of objections to ground vibrations and airblast. Past disposal solutions such as 'bayhead fills on tidal mudflats are no longer environmentally acceptable, and fragmented sandstone also has to compete with abundant crushed concrete demolition waste as road sub-base.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE AGSO EARTH SCIENCE EDUCATION CENTRE Greg McNamara Earth Science Education Centre Manager Australian Geological Survey Organisation GPO Box 378 Canberra ACT 2617

The Earth Science Education Centre (ESEC) is an initiative of Australian Geological Survey Organisation (AGSO). AGSO has long recognised the need within the community for geoscience education and has provided a wide range of educational programs and materials to schoolteachers and community groups. The ESEC has been developed to meet several other community needs identified by AGSO Education Unit. They are: • Access to hands-on curriculum-linked geoscience information for school students, • Access to specialised geoscience workshops for college and other higher-level students. These needs are being met through the ESEC by providing: • structured and curriculum-linked geoscience activities, • access to geoscience professionals with the education and communication skills and experience necessary to conduct the ESEC activities, • specialised geoscience workshops for college groups within a laboratory-like environment using ESEC and other AGSO staff at AGSO, • structured and curriculum-linked geoscience ESEC activities for use in other venues. The ESEC is a large purpose-dedicated space within the AGSO building, an adjunct to the large public foyer space that houses an impressive display of rocks, minerals, fossils and geoscientific information. The ESEC provides a safe, friendly, laboratory-like environment, with access to materials and equipment not normally available to school and community groups, within which structured hands-on geoscience activities can be undertaken. The ESEC also provides an ideal space for conducting teacher training workshops and other AGSO and non-AGSO education activities. Initial ESEC offerings are aimed at Yr5-6 senior primary school students and Yr7-8-9 junior high school students. Yr 1 0 programs will follow as soon as possible with Yr 1 I12 college level activities to be developed in the near future. Activities are completed during the site visit with each student completing perhaps 6-8 activities in all. Students have a complete record of their achievements from the site visit and.have the opportunity to follow up topics of interest through the post-visit activity suggestions provided to the teacher. Teachers are also encouraged to return to AGSO as their first point of contact for their geoscience questions. Future developments include holiday programs, activities for Yrs2-3-4, activities for adult special interest groups and a travelling ESEC that visits schools in the local region.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EXTENSIVE FELSIC LAVAS IN INTRAPLATE VOLCANIC PROVINCES J McPhie, S.R Allen and C Simpson Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001

Basaltic lavas in intraplate provinces are distinctive in being the most voluminous and extensive volcanic units known. Some intraplate provinces also include voluminous and extensive felsic volcanic units. Well-preserved examples occur in the Cretaceous Etendeka volcanic province (EVP), Namibia, and the Proterozoic Gawler Range Volcanics (GRV), South Australia. The physical volcanology of the felsic units is poorly understood and yet critical for realistic interpretations of magma genesis, magma supply rates and eruption duration in these settings. Many have been interpreted to be high-temperature ignimbrites that have undergone extreme welding and rheomorphism. Such interpretations have been popular because of relationships among viscosity, discharge rate and eruption style. In particular, the typically high viscosity of felsic magmas is thought to favour explosive rather than effusive eruption styles, and to impose limits on discharge rate and hence outflow distance of lavas. Although a pyroclastic origin is consistent with these relationships and clearly correct for some extensive felsic units, others have textural and lithofacies characteristics consistent with an effusive origin. Examples in the GRV and EVP are evenly porphyritic and internally massive or flow-banded with well-developed columnar or prismatic joints and highly vesicular/amygdaloidal upper zones. Groundmasses include crystalline (spherulitic, granophyric) and glassy (perlitic) domains, and lack the vitriclastic textures typical of pyroclastic facies. Some EVP units locally have basal and top autobreccias, and lobate and ropey surfaces. These characteristics strongly imply emplacement by non-particulate, lavalike flowage, but do not uniquely constrain the eruption style. Two units in the GRV provide evidence that the eruption style was fundamentally effusive. One unit (Eucarro Dacite) is compositionally heterogeneous, comprising plagioclasephyric, plagioclase+quartz-phyric and quartz+sanidine-phyric rhyolite in laterally continuous flow bands (mm to tens of m across) and mingled domains (cm to tens of m across). Compositional flow banding and mingled domains are easily explained by effusive eruption of heterogeneous magma and lava-like outflow. Another unit (Yardea Dacite) contains lithic megablocks up to 50 m across derived from subsurface basement and granitoids. The lithic megablocks could have been entrained in the dacitic magma but not in a gas-driven explosive eruption column, again suggesting an effusive rather explosive eruption style. Felsic units in both the GRV and EVP have relatively hieh pre-eruption magma temperatures (-^lOOO^'C) and correspondingly low viscosities (10 - 10^ Pa s). If erupted effusively, extended outflow could be permitted by development of an insulating crust, as is the case for extensive basaltic lavas. Modelled cooling profiles imply that such lavas could remain active for several decades (Manley 1992), with outflow extent principally constrained by magma volume. We conclude that extensive felsic lava-like units in intraplate provinces are indeed lavas, being felsic analogues of the flood basalts with which they are associated. Reference Manley C.R. 1992. Extended cooling and viscous flow of large, hot rhyolitic lavas: implications of numerical modelling results. Journal of Volcanology and Geothermal Research 53, 27-46.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MINERALOGICAL AND GEOCHEMICAL FEATURES OF PALAEOCHANNEL SEDIMENTS AT GIDJI, W.A.: IMPLICATIONS FOR LANDSCAPE HISTORY AND GOLD EXPLORATION K.G. McQueen' and C. B. Johnson^ 'CRC LEME University of Canberra ACT 260 ^Goldfields Exploration Pty Ltd. PO Box 862, Kalgoorlie WA 6430 Remnants of an extensive palaeodrainage network are preserved on the Yilgam block of Western Australia as buried palaeochannels. In the Gidji area, 13 km north of Kalgoorlie, a major palaeochannel up to 60 m deep is infilled with sand- and clay-rich facies. These materials are overlain by younger alluvial/colluvial sands and gravels, aeolian sands and silts, playa deposits, red soils and lag materials. The palaeochannel sediments consist predominanty of low-crystallinity kaolinite, quartz and minor smectite. They contain very little muscovite, illite or chlorite and can be readily distinguished from underlying, in situ saprolite by their mineralogy and chemical characteristics (particularly K/Al and Mg/Al ratios, Fig. 1; low rare earth element contents; and low K/Rb ratios). Ferruginous pisoliths and mega mottles occur in parts of the clay-rich facies. 0.18

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Figure 1. Plot of Mg/Al vs K/Al (elemental wt% basis) for palaeochannel sands and clays and other regolith materials from the Gidji area, Kalgoorlie, Western Australia. Arrows indicate some mineralogical controls. The palaeochannel sediments were largely deposited during the Middle to Late Eocene and represent an homogenised mixture of materials derived from a mature and deeply weathered pre-Eocene landsurface underlain mainly by granites and lesser greenstones. The sediments have also undergone some in situ postdepositional weathering and associated mobilisation and redeposition of iron as oxides/hydroxyoxides. The stratigraphy of the palaeochannel sediments indicates a change from dominantly fluvial to fluvio-lacustrine conditions during deposition. In areas of gold mineralisation, the presence of palaeochannels complicates geochemical exploration. Areas of thick (>15 m) palaeochannel clays effectively mask bedrock geochemical anomalies. Placer and supergene concentrations of gold occur within and below some of the palaeochannel sediments and can be confused with bedrock anomalies unless a multi-element approach is taken. At Gidji the palaeochannel environment contains a combination of bedrock and placer/supergene gold anomalies. Placer/supergene gold anomalies typically lack associated concentrations of As, Sb and W, although As can be coincidentally associated with iron and manganese oxides in ferruginous lag, gravels and pisoliths in the palaeochannel sediments.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15'^ Australian Geological Convention, Sydney, July 2000

STRESS RELIEF EFFECTS IN SANDSTONES IN UNDERGROUND AND DEEP EXCAVATIONS IN SYDNEY L. B. McQueen Colder Associates, 88 Chandos Street, St Leonards N S W 2 0 6 5 Australia

The influence of stress relief on underground and deep basement excavations within Sydney Triassic sedimentary rocks is illustrated. Reference is made to case studies of projects in the depth range up to 200 m. The removal of vertical and lateral restraint by excavation can release stored strain energy and result in instantaneous and time-dependent rock mass movements also known as rebound. A consideration of the in situ stress field has become a routine part of investigations for these types of civil engineering projects and is necessary to assess the influence of the rebound movements. In the Sydney region the magnitude of the horizontal in situ stress is typically higher than the vertical overburden stress, and rebound movements have occurred in engineered excavations as a result of the release of in situ stresses. The magnitude of the regional horizontal stress is made up of gravity and tectonic components, with local influences from topographic features. Rebound movements have had a significant influence on civil engineering works including tunnels and deep basement excavations. In tunnels, stress induced features include fracturing through the rock substance in the roof and movements along bedding surfaces. These are generally confined to areas where interbedded rocks of variable stiffness, or low shear strength bedding features, occur in the roof (or floor) of the tunnel. In deep excavations, stress relief such as convergence of the excavation and buckling in the floor, are generally associated with the presence of low shear strength bedding features, which provide a release surface for horizontal movements. The formation of topographic features such as valleys by natural erosional processes also results in the removal of vertical and lateral restraint and, consequently, changes to in situ stress conditions. Tension fractures, which trend near parallel to the valley, may form in valley walls and produce a low rock mass stress condition. In the valley floors the removal of vertical loads results in a local increase in the ratio of horizontal to vertical stresses. In the Sydney region the associated rebound has resulted in the opening of horizontal bedding partings in massive sandstone and also caused development of anticlinal or buckling structures and compressional faults. Buckling and faulting have been referred to as valley bulging and appear to be more developed in layered rock masses of variable stiffness. Rock mass changes, such as an increase in density of defects, greater penetration of weathering, higher permeability and higher or lower in situ stress field, may occur because of these topographic effects. The case studies illustrate the impact of the in situ stress field, including high horizontal stresses and local topographic influences, on excavations. The projects referenced include the Cataract tunnel, an unlined water tunnel in Hawkesbury Sandstone constructed in the late Nineteenth Century, other major tunnelling projects and a deep basement excavation in the Sydney CBD.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DIAMOND AND CHROMITE GEOCHEMICAL CONSTRAINTS ON THE DACHINE COMPLEX, FRENCH GUIANA C. W. Magee and W. R. Taylor Research School of Earth Sciences, Australian National University The Dachine talc schist, located in French Guinea, is of interest because it contains numerous small diamonds, but it is geochemically unlike kimberlite or lamproite. Capdevila et al. (1999) suggested on the basis of the geochemical signatures of the refractory, immobile elements that it may be komatiite, a high temperature magma which has never before been associated with diamond occurrences. However, because the original rock chemistry has been affected by metamorphism and hydrothermal alteration, resulting in mobilization of many characteristic elements, there are other plausible alternatives to a komatiitic affinity. A possible candidate for the Dachine rock is picritic shoshonite, the magnesian endmember of the shoshonitic appinite-suite rocks, which are known intrusive rocks of the Guyana Shield. Appinite-suite intrusions are generally associated with post-collisional, arc-related magmatism. A study of the diamonds and chromite from the Dachine body suggests that the komatiite hypothesis is unlikely. The diamonds in the Dachine are characterised by highly variable dl3C values (-30 to -5 %o). Diamonds having a light carbon signature, which suggest an origin fi-om recycled biogenic carbon, appear to be more abundant than those with a -5 per mil mantle signature. All diamonds are all low in nitrogen (max. 400 ppm), and are classified on the basis of their IR spectra as either type II or type Ib-IaA stones. The low nitrogen aggregation state of the type Ib-IaA diamonds indicates that these stones have been at mantle temperatures for less than 1 million years, and they cannot have been at komatiite temperatures (1700-1750 °C) for more than 24 hours. Detrital chromites occur in streams directly draining the Dachine body and because there are no other identified metasedimentary and volcanic sources of chromite in the area these can all be assumed to have weathered from the Dachine rock. There are two populations of Dachine chromite. Ten percent of the chromites appear to be traditional diamond-indicator chromites derived fi-om sampling of mantle peridotite. The remaining chromites are magmatic in origin but have higher Nb concentrations than typical komatiite chromites. The chemistry of these chromites is consistent with derivation fi-om a shoshonitic magma. If the Dachine body is of shoshonitic affinity, a subducfion zone enironment seems the most likely place in which to form diamonds with a biogenic signature since it is at these zones that recycling of crustal material into the mantle occurs. The low N content of the diamonds suggests they may have formed by transformation fi-om graphite which was originally derived by metamorphism of carbonaceous matter. The low nitrogen aggregation state of the diamonds suggests that the graphite to diamond transformation was not followed by an extended diamond storage period in the mantle. This may be possible if the fluids that catalysed diamond formation were produced in the same event that led to formation of the shoshonitic magma.

Reference Capdevila, R., N. Amdt, et al. (1999). "Diamonds in volcanoclastic komatiite from French Guiana." Nature 399: 456-458.

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RETHINKING OROGENIC GOLD IN THE SOUTHERN LACHLAN FOLD BELT S. Maher, C.E. Willman, D.H. Taylor, R.A. Cayley and D.H. Moore Geological Survey of Victoria, P.O. Box 500 East Melbourne, 3002 The 'reconstruction' of the Tasman Fold Belt System by the Geological Survey of Victoria (VandenBerg et al, 2000) has forced a rethink of the gross controls on mineralisation. The result is an integrated pre-Permian history that explains many aspects of the distribution, timing and character of Victoria's mineral resources. Orogenic gold deposits—structurally controlled gold vein systems in metamorphic terranes—in Victoria's Lachlan Fold Belt are the source of roughly 2495 t of gold. Three-quarters of this, worth twenty-seven billion dollars, came from a seventy kilometre wide belt covering Bendigo and Ballarat between the Avoca and Whitelaw faults (AWE). Why? ... because deformation was protracted and chiefly coaxial; P/T were just right; and boninite-tholeiite source rocks were present at depth. AWE gold mineralisation was emplaced synkinematically over a protracted period spanning folding to latetectonic thrust faulting. This was chiefly driven by east-west shortening during convergence of the Selwyn Block (Neoproterozoic-Cambrian crust inferred to underlie central Victoria) and the Delamerian Fold Belt during the Benambran Orogeny. Dating by Ar/Ar shows that mineralisation was episodic, occurring at about 440 Ma and between 4 2 0 ^ 0 0 Ma (Foster et aL, 1998). The later phase of mineralisation correlates with the onset of magmatism due to melting of the structurally thickened crust in the west together with south-directed transport of the Benambra Terrane farther east. Deposits occur in chlorite zone rocks and record brittleductile cycles of deformation. They are concentrated in the hanging walls of regional-scale reverse faults, implying these have acted as plumbing systems for gold-bearing fluids. Hydrothermal alteration assemblages reflect open-system chemical and isotopic exchange between ore fluids and host rocks. Ore formation temperatures were about 300°C. Isotopic patterns point to large hydrothermal systems driven by metamorphism, and exotic source reservoirs. Cambrian boninite-tholiiete sequences that underlie rocks affected by these cycles of orogeny were probably the most important source of gold. Elsewhere in Victoria's Lachlan Fold Belt key ingredients in the 'orogenic gold recipe' are missing and production is modest. The Whitelaw Fault probably marks the western edge of the Selwyn Block at depth at the end of the Benambran Orogeny. East of the AWF to the Mount William Fault, the cold Selwyn Block is considered to have terminated Benambran prograde metamorphism in the overthrust rocks and blocked the devolatilisation reactions that drove orogenic gold mineralisation. Mineralisation east of the AWF to the Governor Fault—the area floored by the Selwyn Block—chiefly formed during the 385-380 Ma Tabberabberan Orogeny. In most areas it formed at shallow crustal levels and is characterised by stibnite assemblages, but is richest in the Walhalla - Woods Point belt where shortening was intense and deeper crustal levels are exposed. Here, mineralisation resembles older mineralisation in the AWF. East of the Governor Fault, while many areas resemble the AWF in terms of stratigraphy and metamorphic grade, orogenic gold deposits are poor cousins. This may be the result a history of shifting regional stress field and short-lived dilational sites. It is not clear why gold production is lower, with the exception of Stawell, west of the AWF. Chief differences west of the Avoca Fault are that the structural trend is northwest and deeper crustal levels are exposed. This raises two questions. Were the biotite zone rocks in the hangingwall of large faults too ductile for significant dilation to occur? Does the different structural grain reflect heterogeneous strain along the Delamerian buttress causing dilational sites to shift through time? The richness of Stawell is related to the contrasting competency of juxtaposed turbidite and tholeiitic volcanic rocks within the Moomambool Metamorphic Complex and protracted deformation. Strain partitioning during deformation played an important role in forming the large dilational system. References Vandenberg, A.H.M., Willman, C.E., Maher, S., Simons, B.A., Cayley, R.A., Morand, V.J., Taylor, D.H., Moore, D., & Radojkovic, A., 2000. The Tasman Fold Belt System in Victoria. Geological Survey of Victoria. Special Publication. Foster, D.A., Gray, D.R., Kwak, T.A.P. & Bucher, M., 1998. Chronology and tectonic framework of turbidite-hosted gold deposits in the western Lachlan Fold Belt, Victoria: 40Ar-39Ar results. Ore Geology Reviews 13, pp. 229-250.

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MOUNT KELLY: A MOUNT ISA-TYPE COPPER-GOLD DEPOSIT IN THE WESTERN MOUNT ISA BLOCK, QUEENSLAND Ken Maiden, David Hewitt and Matt Stephens Reefway Pty Ltd, GPO Box 3596, Sydney, N S W 2001

Copper deposits in the Western Succession of the Early to Middle Proterozoic Mount Isa Block form a distinct group of breccia-hosted and replacement style deposits. The deposits resulted from a regional hydrothermal event during deformation accompanying waning metamorphism, when metal-bearing fluids were focussed along faults. The deposits are localised in dilational sites formed at structural intersections and are hosted by sedimentary rocks (mainly dolomitic siltstones) which were strongly brecciated and altered during fault movement and fluid introduction. The style of mineralisation varies from stockworks to breccia fillings, disseminated zones and massive replacements. The textures record multiple overprinting phases of brecciation, veining and metal introduction. Although, in detail, there is considerable variation in deposit geometry and mineralisation style resulting from different host rocks, different fault geometry and different fluid movement history, the broad similarities support their categorisation as "Mount Isa-type" copper deposits. In the Mount Kelly area, located 100 km NNW of Mount Isa, mineralisation is hosted by carbonaceous and dolomitic siltstones of the lower McNamara Group, metamorphosed to lower greenschist assemblages. The strata are folded around north-trending fold axes and cut by the McNamara Fault, a major north-trending structure. Copper-gold concentrations are developed along NE- and ENE-trending cross faults which were active post-peak metamorphism. Primary chalcopyrite occurs in quartz-carbonate-pyrite vein stockworks and breccia-fillings in silicified siltstone. A number of historic copper workings occur in the Mount Kelly area. Exploration drilling, focussed along the NW-trending Mount Kelly Fault, has intersected mineralisation averaging around 2.8% Cu, with sporadic high copper and gold concentrations, over 120 m of strike of the fault and around 20 m in true thickness. The deposit remains open along strike and at depth. Although a resource has not yet been estimated, good progress is being made towards an initial target of 1 Mt of primary ore. In addition, there are known concentrations of secondary copper, contained within a sub-horizontal "blanket" at shallow depth, and other known mineralised structures in the area remain to be drilled. Ferruginous siliceous breccias, with copper geochemical anomalies, are the surface expression of mineralised fault zones. These distinctive outcrops provide targets for ongoing exploration for Mount Isa-type copper deposits in the region.

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CHARLES DARWIN - THE GEOLOGIST Bob Major PIRSA and 14 Hounslow Ave, Mile End, SA, 5031 During his life Charles Robert Darwin (1809 - 1882) became England's best known naturalist and probably still is. However, the brilliance of his pioneering work on evolution by natural selection and the enormous effect that it had on human self-perception has eclipsed his contributions to geology, which is unfortunate, because they were numerous and fundamental to the understanding of geology. Between 1831 and 1836 he spent five years on the British naval survey ship HMS Beagle sailing round the world recording geological and biological information and collecting specimens from continents and islands. In his lifetime he wrote thousands of letters, published 21 books (some with numerous editions) and 156 papers on geology, zoology, botany, paleontology, meteorology and ethology. Despite this prodigious output on such a wide range of scientific specialities he is known today almost solely for his (and A.R. Wallace's) idea of evolution by natural selection as described in his 1859 book "On the Origin of Species". However, before he leapt into public prominence in 1859 he was well known in scientific circles as a geologist, with the publication of four books and 20 papers on geology. Charles Darwin was as much an original thinker in geology as in biology. 1.

He described continental uplift and related it to earthquakes, igneous intrusions and volcanic activity, and concluded that they were all part of the same fundamental geological process (which he could not determine). 2. He described wide scale subsidence on which was based his ideas on the formation of coral atolls (this theory is still accepted as correct). 3. He attempted to relate uplift and subsidence by searching (unsuccessfully) for a theory of the earth. About 120 years later the current theory of the earth was formulated and is called plate tectonics. 4. He pointed out the enormous gaps in the geologic record which indicated that the Earth was far older than previously thought. 5. He pointed out that evolution by natural selection also required vast amounts of geologic time. 6. He observed, as did Lyell, that volcanoes and the dip of the layers on their flanks were built up in that position from the erupted material, and were not, as many believed, the results of "craters of elevation" whereby originally horizontal layers were supposed to have been uplifted by upwelling molten rock and gases to form the volcano shape. 7. In the Andes he related plutonic and volcanic rocks i.e. that plutonic rocks were the sources of volcanic rocks. 8. In South America he saw that cleavage-planes and foliation-planes in metamorphic rocks were closely parallel to the axis of the Andes. In addition, he concluded that cleavage planes were quite independent of the original bedding planes, and, that cleavage and foliation were not original but had been imposed on the rocks by pressure, which resulted in the recrystallisation of the original rock to give the metamorphic foliation. This was the origin of the "deformation" theory of metamorphic rocks.He made contributions to petrology and petrogenesis. • In the Galapagos Islands he described magmatic differentiation in lavas (but did not use that term). He observed how crystals sank or floated in the lava depending on the relative specific gravities, and "friction" (i.e. viscosities). He, and others, observed how trachytic lavas could be erupted before basaltic lavas from the same volcano. From these two observations he deduced that the "underground focus" (i.e. magma chamber) was differentiated into an upper trachytic zone and a lower basaltic zone. • Darwin, and others, observed how masses of granite were intruded by dykes of "greenstone" and "trap-rock" (i.e. dolerite). He believed that the trap-rock had separated downwards from the granite to the lower parts of a magma chamber and had then been intruded up through the granite and surrounding gneisses by "repeated movements into the axis of a mountain chain". • Darwin showed that some metamorphism could be caused by heat from adjacent molten rock, as in the Cape Verde Islands, at the Cape of Good Hope and the Andes. This is now termed contact metamorphism. • Darwin made contributions to glacial theory but although his, (and others), observations of rocks being carried in icebergs, and of uplift and subsidence of land, were accurate, he did not correctly apply them and so his models were wrong, e.g. his explanation for the origin of the Parallel Roads at Glen Roy (in Scotland), and, his explanation of how erratics could be found topographically higher than the outcrops from which they originated.

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Geological Convention, Sydney, July 2000

CAPE FOULWIND SUPERSUITE: CARBONIFEROUS, A-TYPE GRANITES OF WESTERN NEW ZEALAND AND PACIFIC MARGIN CONNECTIONS K.R. Malloch', I . E . Waight^ and S.D. Weaver^ 'Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand ^Danish Lithosphere Centre, 0ster Voldgade 10, L1350, Copenhagen K. Denmark Carboniferous granites of the Cape Foulwind Supersuite were emplaced in the Buller terrane of western New Zealand at ~325MA, some 50 million years after the magmatic peak of the Mid-Late Devonian subductionrelated Karamea Suite granitoids. The supersuite comprises 4 plutons composed of weakly peraluminous, high-K calc-alkaline monzogranites. These have high concentrations of Zr, Nb, Zn, Ga and Y, and high FeO/MgO ratios indicative of A-type granites. Isotopic variation is extreme with ^"^Sr/^^Srj of 0.7055 - 0.7128 and ZNdi of +3.2 to -8.4 but these data do not follow any typical mixing curves of increasing Sr with decreasing Nd values. Much of the compositional scatter is ascribed to disturbance of the Rb-Sr isotopic system during Mid-Cretaceous extensional deformation and metamorphism associated with core complex formation. However, in general terms, the granites represent variable mixtures of a composite crustal component involving Karamea Suite granitoids and Lower Paleozoic metasediments, and a high ZNd mantle component. Emplacement of the Cape Foulwind Supersuite immediately postdated a 330Ma kyanite grade metamorphism in Fiordland, New Zealand, which has been interpreted to be the result of structural or magmatic crustal thickening. A-type magmatism is often associated with extensional environments and extensional lineations in the Cape Foulwind rocks have been considered to be syntectonic. It is possible therefore that the Carboniferous magmatism represents thinning and melting of crust with concomitant emplacement of mantle-derived magmas in an extensional tectonic setting. In the NE Lachlan Fold Belt and the New England Fold Belt, the period 320-330 Ma appears to represent the peak of Carboniferous magmatic arc activity and the change to an extensional back-arc tectonic regime took place at 300-310 Ma. In Marie Byrd Land, West Antarctica, evidence is accumulating for a ~338Ma felsic igneous episode which post-dated the subduction-related Mid-Late Devonian magmatism. As for the Cape Foulwind Supersuite, this probably involved melting of Lower Paleozoic metasediments and Devonian granitoids. At present there is little evidence of this event in Northern Victoria Land apart from some Carboniferous mineral ages.

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DEFORMATION OF PARTIALLY MOLTEN ROCK: NUMERICAL MODELLING OF THE INTERPLAY BETWEEN STRAIN LOCALIZATION AND MELT FORMATION Neil S. Mancktelow Geologisches Institut, ETH Zentrum, CH-8092 Zurich, Switzerland In the lower crust, deformation and partial melting are closely linked both in space and time. Leucosomes, representing small volumes dominated by melt, are preferentially concentrated in shear zones or in the axial planes of folds. Although the regional metamorphic conditions (temperature, depth, water activity) determine the onset of melting on a broad scale, the smaller-scale spatial variation in melt distribution must be related to variations in stress and strain. Initiation of heterogeneous deformation requires initial irregularities and partial melting can localize strain through its marked softening effect and by melt-embrittlement. Melting may also be promoted by shear heating. However, this effect is limited by the negative feedback effect associated with melt-related strain softening and is critically dependent on boundary conditions - whether constant strain rate or constant load. The influence of initial irregularity distribution, rheology of matrix and inclusions (for elasto-power-law viscous materials), and imposed constant strain rate boundary conditions (pure or simple shear) on the development of anastomosing shear zones has been investigated using finite-element numerical models. In all cases, a similar pattern of conjugate higher strain/higher vorticity "shear zones" surrounding roughly diamond-shaped lozenges of lower strain/lower vorticity is established. The shear zones initiate at ca. 90° to one another, and this angle progressively decreases with increasing bulk deformation. The models demonstrate that this basic pattern is not markedly influenced by the vorticity of the imposed deformation. It follows that natural patterns may not always be characteristic of the regional tectonics and the equal development of both shear senses can occur even when the overall deformation approaches perfect simple shear! The rate of propagation of individual conjugate shear zones away fi-om the initiating weak inclusion is increased by increased effective viscosity contrast between matrix and inclusion and by power-law rheology. However, strain softening is even more effective in promoting propagation. Rapid propagation leads to straighter, narrower "shear zones" of higher strain. Clearly, heterogeneous deformation must be related to heterogeneous stress. Most importantly, a heterogeneous pattern to mean stress or "pressure" is maintained during formation of the anastomosing shear zones, with higher pressure in the shear zones and initial weak inclusions. Melting of gneissic or pelitic compositions is pressure dependent. With free water present, increased pressure promotes melting. The opposite is true for water-absent melting, where the water in the melt is provided by incongruent melting of hydrous phases (e.g. muscovite, biotite). The small percentage of leucosome in many migmatites (< 10-20%) may reflect the limited amount of free water present. For water-present conditions, viscous deformation would promote melting in the higher-pressure regions in the walls of existing melt pockets and along shear zones. Melt assisted softening would promote additional strain localization in these zones, leading to a positive feedback effect. This positive feedback between shearing, "tectonic overpressure" and melting is potentially much more effective in concentrating melt in shear zones than "shear heating", where melt-related softening has a negative effect on further heating. In this scenario, melting is entirely dynamic - if deformation ceases, the melt will immediately solidify as the pressure variations rapidly decay. Such dynamic migmatites will not be good sources for melt accumulation. The melt remains trapped within the shear zones, since it would immediately solidify if it left the shear zones and attempted to migrate to higher crustal levels.

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BULK ROCK FRACTIONATION DURING GARNET GROWTH AND ITS EFFECT ON THE ASSEMBLAGE OF ECLOGITES FROM PAM PENINSULA, NEW CALEDONIA. Brett A. Marmo School of Geosciences, University of Sydney, F05, Sydney, NSW, 2006

In eclogite and blueschist facies rocks, garnet grains commonly show extensive chemical zoning. Factors that influence this zonation include comparatively slow intra-granular diffusion, comparatively rapid growth and comparatively short duration of high-P/T metamorphism. One effect of this pronounced zonation is the progressive removal of elements forming garnet cores from any metamorphic reaction occurring at grain boundaries or in the matrix of the rock. If garnet is abundant, then the effect will influence the effective bulk rock composition. We have characterised the zonation patterns of garnet in eclogite facies metabasites from the Pouebo terrane of New Caledonia. The effective fractionation of the bulk rock composition into garnet core versus matrix compositions changes the mineral assemblage from that could be predicted by, for example, XRF whole rock analyses. Analyses of element maps, produced by applying the Bence-Albee correction algorithm to X-ray intensity maps, have been integrated with phase-equilibrium methods to produce a series of pseudosections in the Ca0-Na20-Fe0-Mg0-Al203-Si02H2O system. The peak assemblage of omphacite, garnet, clinozoisite and barroisite in Type 1 eclogites reflects conditions that involved P=19 kbar and T=600°C. If the chemistry of the whole rock is used to calculate a pseudosection in the CNFMASH system, then glaucophane and not clinozoisite is predicted to be stable at the metamorphic peak. However, the growth of garnet progressively removes FeO and AI2O3 from the volume of rock in equilibrium with the matrix and leads to the stability of clinozoisite at higher pressures. A series of pseudosections are presented to demonstrate the evolution of the stability of different assemblages during bulk rock fractionation.

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DEVELOPMENT OF LASER RAMAN AND X-RAY PHOTOELECTRON SPECTROSCOPIC PARAMETERS AS AN ADDITIONAL THERMAL MATURITY INDICATOR TO THE CONODONT ALTERATION INDEX 'Craig P. Marshall, ^Robert S. Nicoll and 'Micheal A. Wilson 'Department of Chemistry, Materials and Forensic Science, University of Technology, Sydney, P.O. Box 123 Broadway, 2007, Australia ^Petroleum and Marine Division, Australia Geological Survey Organization, GPO Box 378, Canberra 2601, Australia

Laser Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) was applied in an attempt to quantify and elucidate the chemical transformations of the conodont alteration index (CAI) in artificially matured conodonts. Coniform elements of the genus Drepanodus from the Ordovician Emanuel Formation and pectiniform elements of the genus Polygnathus from the Devonian Napier Formation, both from the Canning Basin were used in this investigation. Samples where specifically chosen to study the effects of genus, element morphology, geological age, lithology and depositional environment upon the CAI. The first order Raman spectra (900-1800 cm-1) of both sets of conodonts show three bands. These are assigned to apatite at 965 cm-1, D band (defects present in the structural units and disorder) at 1345 cm-1 and the G band (carbon-carbon in plane stretching vibration or structural ordered carbon) at 1600 cm-1. The Raman spectra recorded for both sample sets, CAI range of 1-7 are characteristic of poorly/ highly disordered carbon within the conodont organic matter. Further more, the D band becomes progressively narrower and more intense the higher the thermal treatment. The G band becomes narrower until CAI 4 and after that has no systematic change apart from the intensity decreasing with temperature. The most noteworthy spectral characteristic in relation to thermal maturity is the clearly defined linear trend of decreasing D band line-width with increasing rank. The carbon Is photoelectron regions acquired from both sample sets contain three peaks. These peaks are assigned to carbon bonded to sp3 and sp2 hybridized carbon (284.7 eV), alcohol (287.2 eV) and carboxyl (288.3 eV) functional groups. With increasing thermal treatment all the carbon constituents show a linear decrease in abundance. The poorly/ highly disordered carbon shows no progressive ordering with increasing thermal treatment. This is also in agreement with the XPS results, in which the aliphatic hydrocarbons do not aromatize with increasing temperature. Possible explanations are the effect of the apatite mineral matter inhibiting structural order (aromatization), or the apatite matrix in which the organic matter is imbedded. It is therefore clear that there is not a direct relationship between maturation and ordering. However this result for coals is different and is the basis of vitrinite reflectance as a rank parameter. Conodont organic matter however varies considerably from that in coal, being less aromatic and hence when heated does not form more ordered aromatic structures. Therefore any rank parameter needs to be on the basis of molecular structure rather than visual reflectance or colour.

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PARTIAL MELTING OF THE BROKEN HILL ORES: THE SMOKING GUN John Mavrogenes'Marita Sloan\ David Ellis^ and Ron Frost^ ^Dept. of Geology, Australian National University, Canberra, ACT, Australia 0200 ^Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia 0200 ^Dept. of Geology, University of Wyoming, Laramie, Wyoming, 82071 USA

It has long been speculated that the sulfides of the Broken Hill orebody were partially molten during peak (granulite facies) metamorphism (Brett and Kullerud, 1967; Lawrence, 1967). However, this suggestion was not accepted until recent experimental work showed that FeS-PbS-ZnS-Ag2S would be partially molten at 5 kbars and 800°C (Macintosh and Mavrogenes, 1998). Studies of the silicates associated with the Broken Hill ores have identified sulfide melt inclusions. Quartz-hosted inclusions are typically mono-mineralic, dominantly galena, and occur along trails in healed quartz fractures. However, garnets contain isolated sulfide inclusions. While some of these are single phased, the majority of sulfide inclusions are multiphase. We interpret those polyphase inclusions that contain pyrrhotite, galena, sphalerite, argentite (biotite) as trapped sulfide melts. Silver concentrations well above those of the bulk ores are interpreted to be further evidence for the presence of partial melts at Broken Hill. These melt inclusions are irrefutable evidence for the presence of sulfide melts during peak metamorphism at Broken Hill. Further evidence for the presence of a sulfide melt is the common occurrence of low melting assemblages containing Bi, As, Tl, Au and Sb. These phases could represent late stage, highly fractionated melts rich in incompatible elements, and these may be responsible for the Ag-rich extensions of the ore known as "droppers". Partial melting of the Broken Hill sulfides dramatically affects models for the genesis of the Broken Hill deposit. In this light, it is possible to reinterpret the Mn-silicate halo surrounding the ore package as the product of reactions between the host rock and the sulfide melt. In this model most of the silicate envelope around the ore body would have formed as the sulfide melt cooled after peak metamorphism. This explains the absence of penetrative fabric within the ore package and settles one of the major problems of Broken Hill geology - why the ore body appears to be syngenetic, and yet lacks most of the penetrative fabric seen in the country rock. References BRETT R. AND KULLERUD G. 1967. The Fe-Pb-S System. Economic Geology. 62, 354-369. LAWRENCE L.J. 1967. Sulphide neomagmas and highly metamorphosed sulphide deposits. Minerallium Deposita. 2, 5-10. MACINTOSH I.W. & MAVROGENES J.A. 1998. Experimental studies in the system PbS-FeS-ZnS (Ag2S: Implications for the Broken Hill orebody: Geological Society of Australia Abstracts. 49, 297.

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INFILTRATION MELTING IN AN ANALOG SYSTEM W. D. Means Department of Earth and Atmospheric Sciences, University at Albany, Albany NY 12222, USA

According to Vernon et al. (1990), infiltration of water can provoke the melting that generates the leucosome of some migmatites. It is of interest therefore to see what "infiltration melting" looks like when it is in progress, and to see what characterizes the product of later crystallization at a site where infiltration melting has previously occurred. It is also of interest to compare the processes and products of infiltration melting with those of ordinary, isochemical "thermal melting". Some experiments have been carried out on both kinds of melting, in a three-phase ammonium-based system (Park, 1994) with observations in thin section while melting and crystallization are in progress. In thermal melting, early melting occurs at sites scattered through the material where all three crystalline phases touch each other, or are separated by small pockets of previouslyformed melt. During subsequent thermal crystallization, the pockets close up again. In infiltration melting, early melting occurs at a planar melt front at a high angle to the presumed gradient in water concentration. As melting proceeds, the melt front divides into two melt fronts, a "first-melt front" and an "all-melt front", which migrate at different velocities away from the water source. The first-melt front migrates faster, so the thickness of the partially melted layer between the two melt fronts increases with time. Eventually mehing ceases in the experiments and crystallization begins, on account of evaporation of water at the former water source. Then bladed crystals grow into the region formerly occupied by all-melt, from seeds near the all-melt front. Crystallization at the all-melt front starts before melting ceases at the first-melt front, so for a transition interval, melting and crystallization are proceeding together in the same (half millimeter wide) field of view. Movies of these processes will be shown and the possible relevance to migmatites discussed.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RIDGE SUBDUCTION IN THE NEW HEBRIDES INTRA-OCEANIC ISLAND ARC: IMPLICATIONS FOR THE RECOGNITION OF COLLISION EVENTS IN ANCIENT ARC-RELATED SEQUENCES. Sebastien Meffre and Tony Crawford School of Earth Sciences, University of Tasmania, GPO Box 252-79 Hobart 7001

The collision of the D'Entrecasteaux Ridge and the West Torres Massif with the New Hebrides island arc has significantly modified the morphology of the arc, affected sedimentation within the arc and changed the chemistry of the arc. Bathymetric sections taken at intervals along the arc, perpendicular to the trench, show that prior to collision the morphology was typical of modem intra-oceanic island arcs. Collision has caused uplift of the trench and forearc (up to 6000 m), subsidence around the arc volcanic edifices (up to 2500 m) forming a large intra-arc basin, and uplift of the arcbackarc transition (up to 2000 m). These characteristics are typical of collisions in other Western Pacific island arcs such as the Tonga-Kermadec and Izu-Bonin arcs. The pattern of uplift and subsidence has important implications for the tectonic history of the New Hebrides system. The morphology of the arc shows that collision of the West Torres Massif probably accounts for at least half the uplift as well as being responsible for the slowing of subduction at 1 Ma in the northern half of the arc. Re-equilibration of the arc following collision could mask any evidence of collision prior to 3 Ma and is probably responsible for the formation of the large indentations in the arc. The pattern of uplift and subsidence, together with data from Ocean Drilling Program holes and on-shore geology shows that collision was responsible for the development throughout the arc of a major unconformity surface, the deposition of shallow water limestone and coarse volcaniclastics. This provides information which can be used to facilitate recognition of these events in ancient arc-related sequences.

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EXPLORATION MODELS FOR CU-AU SKARNS Lawrence D. Meinert Department of Geology, Washington State University, Pullman, WA 99164-2812 USA Continued exponential world population growth combined with rising standards of living (or at least consumption) has led to greatly increased demand for the building materials of modem civilization. Economic geology is, at least in part, the science of finding those building materials and in modem exploration programs this involves descriptive/genetic models for how particular ores can be found. Punctuating this ever rising demand are price cycles that determine the economic attractiveness of exploring for particular commodities. Two commodities that well-illustrate these swings of "economic attractiveness" are copper and gold. In considering exploration models for Cu-Au deposits, it is appropriate to start with "the big ones". The world's largest gold mine is the Grasberg porphyry/skam complex in the Ertsberg district, which in total is the world's largest Cu-Au resource (Meinert et al., 1997). The nearby Ok Tedi porphyry/skam complex also rates high on the list of world Cu-Au suppliers and both the Ertsberg and Ok Tedi districts lie just north of the Australian craton on the island of New Guinea. On the other side of the Pacific some of the world's most intense exploration activity is focussed in Chile and Pern, home of many of the world's largest deposits. One of the most exciting new discoveries is the Antamina skam deposit, which at full production will be the world's third largest producer of concentrates, will rank and in the world, respectively, for Cu and Zn, and will be well within the lowest-cost quartile of global production (Redwood, 1999). Thus, it seems appropriate to consider the characteristics of skam deposits in general, with particular emphasis on exploration models for Cu-Au skams in light of recent advances in understanding these sometimes complex deposits. Skam deposits have been mined for a variety of metals, including Fe, W, Cu, Pb, Zn, Mo, Ag, Au, U, REE, F, B, and Sn. Skams occur on all continents and in rocks of almost all ages. Although the majority are found in lithologies containing at least some limestone, they can form in almost any rock type including shale, sandstone, granite, basalt, BIF, and komatiite. Skams can form during regional or contact metamorphism and from a variety of metasomatic processes involving fiuids of magmatic, metamorphic, meteoric, and/or marine origin. They are found adjacent to plutons, along faults and major shear zones, in shallow geothermal systems, on the bottom of the seafioor, and at lower crustal depths in deeply buried metamorphic terrains. What links these diverse environments, and what defines a rock as skam, is the mineralogy which includes a wide variety of calc-silicate and associated minerals but is usually dominated by gamet and pyroxene. Just as mineralogy is the key to recognizing and defining skams, it is also critical in understanding their origin and in distinguishing economically important deposits from interesting but uneconomic mineral localities. Skam mineralogy is mappable in the field and serves as the broader "alteration envelope" around a potential ore body. Because most skam deposits are zoned, recognition of distal alteration features can be critically important in early exploration stages. Details of skam mineralogy and zonation can be used to construct deposit-specific exploration models as well as more general models usefiil in developing grass roots exploration programs or regional syntheses. These concepts will be illustrated with case histories from important skam districts of the world, ranging from the type locality in Sweden to the large and economically important deposits located on the Pacific Rim.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE ROLE OF MAGMATIC FLUIDS IN SKARN AND PORPHYRY SYSTEMS Lawrence D. Meinert Department of Geology, Washington State University, Pullman, WA 99164-2812 USA

Most skam deposits are characterized by two distinct styles of alteration: 1) an early prograde stage with anhydrous minerals such as garnet and pyroxene that form from relatively high-temperature, high-salinity brines and 2) a later retrograde stage with hydrous minerals such as epidote, amphibole, and chlorite that form from lower temperature, lower salinity fluids. These two alteration stages commonly are thought to reflect a dominance of magmatic and meteoric water, respectively. However, data from two different Cu-Au skam systems indicate magmatic stable isotope values (O and H) for BOTH prograde and retrograde alteration minerals. Instead of a meteoric water source for retrograde alteration differences in fluid chemistry are interpreted to result dominantly from phase separation of a common parent magmatic fluid. The Big Gossan Cu-Au skam deposit in the Ertsberg district, Irian Jaya formed from Eocene carbonate rocks, contains prograde gamet-pyroxene, and is associated with a series of 3-4 Ma granodiorite porphyry dikes intmded along a near-vertical faulted stratigraphic contact. Fluids associated with prograde skam are high-temperature NaCl-KCl brines with a low CO2 content, <0.05 mole %. Homogenization temperatures for fluid inclusions in pyroxene range from 320 to 485°C, average 410°C, and formed at a pressure of 50 MPa, equivalent to a depth of 2 km, under lithostatic conditions. Most fluid inclusions in pyroxene contain multiple daughter minerals including halite, sylvite, chalcopyrite, hematite, and anhydrite, indicating that a complex brine was present. Total salinity ranges from 38-65 wt. % NaCl+KCl, and mean salinities are 22 wt. % KCl and 35 wt. % NaCl. Fluid inclusions in quartz and anhydrite associated with retrograde alteration do not contain daughter minerals, average 7.1 eq. wt. % NaCl, and spatially associated fluid inclusions locally homogenize to liquid and vapor at 370-380°C. 5018 (VSMOW) laser fluorination analyses of 3 mg samples average 6.1%o (range, 5.57.2) for gamet, 7.4%o (7.0-8.2) for pyroxene, and 9.6%o (8.6-11.1) for amphibole. Coexisting gamet and pyroxene yield an equilibrium fractionation temperature of 370°C ± 70°C, in agreement with measured fluid inclusion temperatures. Conventional 5D (VSMOW) analyses of 100-200 mg samples average -77.5%o (range, -77 to -78) for amphibole in skam; no lighter values were detected. At Mines Gaspe, Quebec quartz monzonite porphyry stocks ( 3 8 5 + / - 2 . 8 Ma) intmded Lower Devonian sedimentary rocks producing gamet>pyroxene skam in the more calcareous units. Gamet and pyroxene skams are cut and replaced by retrograde alteration containing amphibole and sulfides. Fluid inclusions in skam samples indicate high temperatures ( T H = 3 3 4 - 5 0 6 ° C ) and high salinities ( 1 5 - 5 6 eq. wt. % NaCl), similar to Big Gossan. 5 0 1 8 (VSMOW) averages 4 . 2 % o ( 3 . 6 - 5 . 6 ) for gamet, 5 . 3 % o ( 4 . 3 - 6 . 0 ) for pyroxene, and 4 . 9 % o ( 4 . 9 - 5 . 2 ) for amphibole. Coexisting gamet and pyroxene yield an equilibrium fractionation temperature of 4 5 0 ° C ± 1 0 0 ° C , slightly higher than that calculated for Big Gossan, but again in agreement with measured fluid inclusion temperatures. 5D (VSMOW) of magmatic amphibole is - 8 7 % o , whereas amphibole in skam includes both magmatic values of - 7 8 % o and lighter values of - 1 1 5 to - 1 4 3 % o that could be interpreted to result from either progressive degassing of an underlying magma or equilibrium fluid-rock reaction with meteoric water. 343

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 JAustralian Geological Convention, Sydney, July 2000

THE SOURCE OF GOLD IN THE WESTERN LACHLAN FOLD BELT: CONSTRAINTS FROM MASS TRANSFER MODELLING Terrence P. Memagh^'^ and Evgeniy Bastrakov^ ' Australian Geodynamics Cooperative Research Centre ^ Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia. Until recently, the development of better constrained models for gold mineralisation in the western Lachlan Fold Belt has been hindered by the paucity of data on the chemistry of the ore-bearing fluids, the temperatures and pressures at the time of mineralisation, and the extent and nature of wallrock alteration. Newly available data, however, has allowed us to use theimodynamic mass transfer modelling to test some of the models proposed for gold mineralisation in the western Lachlan Fold Belt and to investigate whether the results of the modelling correlate with the styles of mineralisation and alteration assemblages observed in these turbidite-hosted gold deposits. The first model (cf. Keays, 1987) invokes initial seawater alteration of tholeiitic rocks, followed by leaching of gold from these rocks by a metamorphic fluid. This fluid then ascends into the overlying turbidite succession where fault-valve behaviour leads to phase separation and progressive water-rock interaction. Phase separation results in gold precipitation but the associated mineral assemblage of epidote, talc, quartz, and anhydrite, which occurs at high water-rock ratios, is not in accord with observed vein and alteration mineralogies. The second model (cf Bierlein et al, 1998) simulates metamorphic fluids leaching gold from interflow sediments in the mafic succession. This fluid is allowed to ascend into the overlying turbidite rocks as before. Although the fluids are not as oxidised as in the second model, the veins are predicted to be composed of mostly feldspar at temperatures of 250°C or less and to contain epidote at higher temperatures and high water-rock ratios. These predictions are not in accord with the common occurrence of veins with greater than 95 vol.% quartz and minor amounts of sericite, chlorite, pyrite and arsenopyrite. The third model (cf Gray et al, 1991; and Cox et a/., 1995) proposes that the ore-bearing fluids originated from metamorphic devolatilisation process occurring in deep level crustal rocks. This fluid then ascends to upper crustal levels as before. Phase separation leads to a relatively large decrease in the activity of sulfur in the fluid, resulting in gold precipitation and the precipitation of quartz, muscovite, arsenopyrite with chlorite, feldspar and pyrite forming at lower water-rock ratios. Therefore, this model is the one that best reproduces the vein and alteration assemblages commonly observed in the western Lachlan Fold Belt. Acknowledgements: This study was conducted as part of the Australian Geodynamics Cooperative Research Centre and is published with permission of the Director, AGCRC and the Chief Executive Officer of the Australian Geological Survey Organisation. References BIERLEIN, F. P., ARNE, D.C., BROOME, J.M.N., AND RAMSAY, W.R.H. 1998. Metatholeiites and interflow sediments from the Cambrian Heathcoate Greenstone Belt, Australia. Sources for gold mineralization in Victoria: Economic Geology 93, 84-101. COX, S. F., SUN, S. S., ETHERIDGE, M. A., WALL, V. J., AND POTTER, T. F. 1995. Structural and geochemical controls on the development of turbidite-hosted gold quartz veins deposits, Wattle Gully mine, central Victoria, Australia. Economic Geology 90, 1722-1746. GRAY, D. R., GREGORY, R.T., AND DURNEY, D.W. 1991. Rock-buffered fluid-rock interaction in deformed quartz-rich turbidite sequences, eastern Australia. Journal of Geophysical Research 96 B12, 9,681-19,704. KEAYS, R.R. 1987. Principles of mobilization (dissolution) of metals in mafic and ultramafic rocks - the role of immiscible magmatic sulphides in the generation of hydrothermal gold and volcanogenic massive sulphide deposits. Ore Geology Reviews 2, 47-63.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

NOW IS THE TIME FOR APPLYING OPTIMISATION TECHNIQUES TO GROUNDWATER RESOURCE MANAGEMENT N. P. Merrick National Centre for Groundwater Management, University of Technology, Sydney, PO Box 123 Broadway N S W 2007 Water is a scarce resource of great utility. Being scarce and vital, water deserves active policies of responsible management at local and global scales. The United Nations regards water as one of the main issues facing the world. There is general recognition that the basic needs of consumers, and the sustainability of the resource, are the main priorities for water supply. There is an urgent need to minimise water waste, maximise the efficiency of water use, optimise water allocation to competing uses, and limit access to sustainable levels. One of the impediments to more efficient use of water is the historical and cultural legacy that water is considered essentially free, with a "long history as an abundant social good" (OECD, 1998). This perception has influenced general attitudes to water and has made difficult the task of government custodians in trying to place a fair price on water as a utility. Water has never been given its full economic value. It will take some time to alter community perceptions of the fiill value of water and to alert people to the need for sustainability of water as a fragile resource. In Australia at the present time, active water reforms are in progress to reduce farmer entitlements to groundwater in over-allocated aquifers. The strategies being implemented are highly contentious, with serious social and economic ramifications. There is also a move towards full cost recovery and a growing recognition of ecosystem requirements. This is a reversal of government attitudes, as irrigation in the past was promoted and subsidised in order to facilitate regional development and to contribute to the nation's food and fibre security. Less than robust definitions and quantification of aquifer sustainable yield are under scrutiny. There is increasing pressure on groundwater managers to quantify and refine estimates and definitions of sustainable yield, and to negotiate with stakeholders a fair and equitable reallocation strategy with low environmental risk. The allocation of a limited resource between competing uses is a classical optimisation problem. Well developed mathematical techniques can be brought to bear on groundwater management problems in particular. Deterrents to this approach becoming common practice are: awareness of decision-makers, people with appropriate skills, computational demands, modelling budget constraints, and the availability of problem-specific software tools. Groundwater simulation modelling is widely practised and is now an indispensible tool for groundwater management. It is vital for determinations of sustainable yield, and for anticipating the impacts of alternative allocation scenarios. However, it does not address the real issue, and that is the equitable distribution between uses while guaranteeing sustainability. The coupling of a simulation model with optimisation software can achieve this. There are three articulated methods of solution in the scientific literature: the response matrix approach, the embedding approach, and the linked simulationoptimisation approach. Software specific to large-scale groundwater resource management problems has been developed using the response matrix approach. It features a number of practical management tools which apply over a management period (typically five years): sustainable yield limit, base allocation, viability base, carry-oyer, and borrowing. Application to the Lower Namoi Valley, the most stressed groundwater system in New South Wales, has determined optimal allocations at the farm level. This methodology has the potential to remove the anomalies in current reallocation strategies, can provide a defensible base for decision-making, and hopeftilly will ease the contention between competing uses. Apart from conventional nonlinear programming techniques, new evolutionary approaches such as genetic algorithms are starting to gain a foothold in solving problems of groundwater management. Reference OECD, 1998, Water Consumption and Sustainable Water Resources Management. Organisation for Economic Co-Operation and Development Publications, Paris, No. 50087, 64p.

345

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

COEVAL TERTIARY VOLCANIC AND PLUTONIC ROCKS, COLORADO RIVER EXTENSIONAL CORRIDOR (CREC), USA: IMPLICATIONS FOR CRUST-MANTLE INTERACTION AND MAGMA MIXING AS A PETROLOGIC PROCESS Rodney V. Metcalf\ Eugene L Smith^ and Calvin F. Miller^ 'Department of Geoscience, University of Nevada, Las Vegas NV 89154-4010 USA ^Department of Geology, Vanderbilt University, Nashville TN 37235 USA The CREC is a 50-100 km wide zone that records Tertiary lithospheric extension, volcanism, continental sedimentation, and plutonism. Compilation of published studies of CREC magmatic rocks permit an assessment of magmatic sources and processes for a well-exposed coeval plutonic and volcanic terrane. CREC volcanic rocks are divided into three groups, pre- to syn-extensional (-22-11 Ma) group 1 alkali basalt, trachyandesite, trachyte, and rhyolite and tuff, a late extensional (~10-8 Ma) group 2 tholeiite basalt, and a post-extensional (-6-4 Ma) group 3 alkali basalt. Group 1 basalts exhibit subduction-like trace element signatures, low eNd (-2.5 to -9.5) and high initial 87Sr/86Sr (Sri: 0.7065 to 0.7095). Group 2 basalts have variable isotopic compositions (eNd -3 to -8.5; Sri 0.705 to 0.708) and ocean island basalt (OIB)-like trace element patterns. Group 3 basalts have depleted isotopic compositions (eNd -1.5 to +7; Sri 0.7027 to 0.706) and OIB-like trace element patterns. Groups 1 and 2 basalts were derived from an enriched lithospheric mantle source and group 3 from an asthenospheric source. Nine Miocene plutons (-18-13 Ma) crop out in the northern CREC. Plutons are predominantly granite to granodiorite but significant mafic and intermediate rocks are present in the form of early gabbro-diorite intrusions, microgranitoid enclaves, syn-plutonic dykes and late aphanitic dykes. A general emplacement sequence for a CREC pluton is (1) gabbro-diorite, including olivine cumulates, (2) granite pluton with associated mafic microgranitoid enclaves and synplutonic dykes, and (3) bimodal mafic-felsic aphanitic dykes. Abundant field evidence of magma mingling and mixing is documented in the Wilson Ridge (WRP), Aztec Wash (AWP), and Mt Perkins (MPP) plutons. Chemical and isotopic data permits correlation of CREC mafic plutonic rocks (mafic enclaves, syn-plutonic dykes and late mafic dykes) and mantle-derived basalts (groups 1-3). Heterogeneous gabbro in MPP correlates with group 2 and 3 basalts; enclaves and dykes in MPP correlate with group 3 basalt. Enclaves and dykes in AWP and WRP correlate with group 1 basalt. Spirit Mtn pluton dykes correlate with group 2 basalt. In contrast, felsic end-members in the different plutonic suites have nearly identical chemical and isotopic (eNd -10 to -13 and Sri 0.710-0.713) compositions. Previous studies used trace element and Nd-Sr isotopic data to demonstrate that intermediate magmas in WRP, AWP and MPP formed by combined magma mixing and fractional crystallization (MFC) from mantle-derived mafic with crustal-derived felsic parental magmas. At 60-65 wt% Si02, however, hybrid compositions are significantly influenced by the felsic component such that rocks from different systems are indistinguishable despite major differences in mafic end-members. Group 1 rhyolite has chemical and isotopic compositions identical to pluton granites. Group 1 trachyandesite and trachyte show systematic chemical and isotopic variations between the felsic end-member and CREC basalts (mainly group 1) and exhibit textural evidence of open-system processes (e.g. enclaves, Ca spikes in plagioclase, mineral reaction rims). Thus group 1 intermediate volcanic rocks appear to be hybrids formed by MFC processes in magma chambers represented by CREC plutons. Specific volcanic centers have been correlated with the WRP and MPP based on age, compositional and structural data. Collectively, CREC plutonic-volcanic rocks are similar to convergent margin calc-alkaline suites in terms of mineralogy (hornblende, sphene), composition and AFM differentiation trends. Features of magma mingling and mixing are common in calc-alkaline plutonic rocks and provide a record of magma chamber processes. The plutonic-volcanic record of the CREC provides a link between these magmatic processes and magmatic products in the form of extrusive igneous rocks. Specifically, mafic and felsic mixing end-members and hybrid MFC products identified in the plutonic record can be directly related to rocks in the volcanic record that closely approximate liquid compositions.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59

1Australian

Geological Convention, Sydney, July 2000

3D THERMAL SUBSIDENCE MODELLING Mike F. Middleton Centre of Excellence in Petroleum Geology, School of Applied Geology, Curtin University, GPO Box U1987, Perth, 6845, WA.

Next year, it will be 30 years since Norman Sleep (1971) originally published his classic paper that described the thermal subsidence of the North Atlantic continental margin. Thermal subsidence is an integral part of models describing the formation of continental margins and extensional sedimentary basins, and is generally considered to drive the phase of subsidence that follows the rifting, or extensional, tectonic event. The aim of this paper is to demonstrate that the results of 3D modelling of the thermal subsidence mechanism can provide, in some situations, fully consistent explanations of subsidence patterns without the need to invoke external tectonic influences. The thermal subsidence mechanism has been proposed to cause an exponential decay of sedimentary basin subsidence with respect to time (Sleep, 1971; McKenzie, 1987). For this study, the decay of 2D and 3D thermal distributions, caused by variable lithospheric thinning, were modelled in order to compute relative tectonic (water loaded) subsidence. The results indicated that the "classical" exponential decay of subsidence occurred only above major lithospheric thermal disturbances. Away from the major lithospheric thermal anomalies, the modelling indicated that thermal-driven uplift can occur for over 50 Ma into what has been traditionally called the "thermal subsidence tectonic phase" (Figure 1). After this initial 50 Ma, the subsidence exhibits an exponential-like decay. UPLIFT

jS = McKenzie Stretching Factor

-0.3D

C

CRUST h-LITHOSPHERE 500°C

100°C

: UPPER MANTLE

20

40 60 80 A^e (Ma)

large j6

small jS ASTHENOSPHERE

Figure 1: Simple 2D rift and thermal subsidence

model

Subsidence (geohistory) curves of many wells from the North West Shelf show a more complex behaviour than a simple exponential decay, during their initial 50 Ma of thermaldriven subsidence after rifting. Deviations of the observed tectonic subsidence from a simple exponential decay, during this initial 50 Ma after rifting, can be accommodated by a 3D thermal subsidence mechanism. References SLEEP, N. H., 1971, Thermal effects of the formation of Atlantic continental margins by continental breakup, Geophys. J. Roy. Astron. Soc., 24, 325-350. MCKENZIE, D., 1978, Some remarks on the development of sedimentary basins, Earth Planet Sci. Lett., 40, 25-32.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CONTRASTING STRATIFIED PLUTONS EXPOSED IN TILT BLOCKS, ELDORADO MOUNTAINS, COLORADO RIVER RIFT, NEVADA, USA Calvin F. Miller Deptartment of Geology, Vanderbilt University, Nashville, TN 37235 USA

Roof-to-floor exposures of mid-Miocene plutons in tilt blocks south of Las Vegas reveal welldeveloped but strongly contrasting magma chamber statigraphy. The Searchlight (SLP) and Aztec Wash (AWP) plutons are well-exposed examples of the stratified intrusions that show a similar broad range in composition, from 45-75 wt% Si02. Homogeneous granites that comprise about one-third of each intrusion are virtually identical in texture and elemental and isotopic chemistry. The most mafic rocks in both plutons are cumulate gabbros that appear to be related to quenched basalt pillows which document mafic input into felsic magma chambers. Isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with varying crustal contamination, whereas more felsic rocks are hybrids that are either juvenile basalt + crustal melt mixtures or products of anatexis of ancient crust + young mafic intraplate. Despite general similarities, the two plutons differ markedly in dimensions and stratigraphy. The SLP is much thicker (-10 vs. 2.5 km) and has thick quartz monzonite zones at its roof and floor that are absent in the AWP. Isotopic and elemental compositions suggest that the upper and lower zones are comagmatic with the granite; we interpret the finer grained, slightly more felsic upper zone to represent a crystallization front and the lower zone to be cumulate. A heterogeneous, mafic-rich injection zone with distinct isotopic chemistry forms the lower two-thirds of AWP; similar rocks are relatively sparse in SLP and do not appear to have played an important role in its evolution. Thus, although both intrusions were filled by very similar magmas and both developed gravitationally-induced stratification, the two magma chambers evolved in very different ways. The distinctions may be attributable to scale and resulting longevity, and/or to subtle differences in tectonic setting.

depth, km (no v.e.) Aztec Wash pluton

^Icrystallization

front

. ,

(qtz monzonite)

H granite I mafic injection zone I cumulate (qtz monzonite) pn volcanic rocks Q

older plutonic rocks

Q

Proterozoic gneiss

348

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PLUTON CONSTRUCTION AND EMPLACEMENT IN ARCS: IMPLICATIONS FROM THE CRETACEOUS CASCADES ARC Robert. B. Miller' and Scott R. Paterson ^ ' Department of Geology, San Jose State University, San Jose, CA 95192-0102, U.S.A ^ Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089

The Cretaceous Cascades arc illustrates modes of construction and emplacement of plutons over a wide range of crustal levels 10 to 35 km). Two end-member types of large volume, dominantly tonalitic magmatic systems are preserved in this arc: discrete, large sheet-like and elliptical magma chambers that reflect regions of focused magmatism; and injection migmatite zones with variably discordant sheeted bodies of all sizes and orientations, recording poorly focused magma ascent and emplacement. Most of the larger magma chambers display early, sheet-like, mafic and heterogeneous magma batches that are gradually replaced by larger, more elliptical, felsic, and homogeneous bodies. Unfocused systems show less consistent patterns. The two magma systems intruded host rocks with gently to moderately dipping structures that were being regionally folded during emplacement. Emplacement of the focused systems was facilitated by multiple host rock transfer processes. The moderate to typically steeply dipping contacts of these large bodies are discordant to regional structures except in narrow (< 1 km wide) structural aureoles, which commonly display remarkable lateral variability. Downward deflection of host rock markers (e.g., unit contacts) and foliation typifies the aureoles, and regional subhorizontal lineation commonly steepens into down-dip orientations; vertical ductile flow thus was a major emplacement process. Sharply discordant segments of contacts of several plutons (including deep ones) and truncation of early sheeted zones by larger and more elliptical magma batches suggest that stoping was a late process, which also led to downward transport of host rock. Regional folding and ductile flow played a role in emplacement, which is difficult to evaluate, but regional faulting did not. Emplacement in the unfocused systems is more problematic, and these may represent the roots or margins of more focused systems, or issue from the tops of the latter systems. A probable link between the systems is seen around two coeval sheeted plutons. One pluton contains abundant host rock rafts along margins of internal sheets. Only sparse inclusions occur in the other pluton, but next to one of its margins a complete spectrum exists from continuous host rock between thin sheets (unfocused) to discontinuous host rock lenses, to rafts completely surrounded by tonalite. The likely transition from sheets separated by host rock, to rafts along sheet margins, to nearly inclusion-free chambers suggests the following model. As each sheet is intruded, host rock is displaced by lateral wedging and horizontal and/or vertical extension in narrow aureoles along sheet margins. This causes local folding, variable transposition of older structures parallel to sheet margins, and upwards or downwards deflection of regional structures. As this process continues, the gradual removal of host rock requires vertical transport along margins of coalescing sheets, probably by ductile flow and detachment of host rock rafts, which sink through the chamber. Early sheets themselves become detached during wedging by younger, wider sheets, and are dispersed as a larger chamber formed. With time, larger and more elliptical batches ascend through the system due to greater lower crustal melting and the presence of preheated pathways, marking the complete transition from unfocused sheets into focused systems. The common feature to both systems is the vertical transport of host rock by multiple processes in aureoles and/or inside and through the chambers.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

IDENTIFICATION OF THE MURRINGO BOTTLENECK, CENTRAL N.S.W.: IMPLICATIONS FOR DRYLAND SALINITY HAZARD MITIGATION C. L. Moore CRC LEME, University of Canberra, ACT 2601

A regional electromagnetic (EM) induction survey has identified an emerging dryland salinity problem in the south Murringo area, Douglas Ranges, Central New South Wales. Detailed landform mapping as a proxy for regolith cover, and geological mapping of the Murringo Valley, provides a framework in which the EM imagery can be interpreted. A prominent V-shaped ridgeline formed from resistant sandstone (Peak and Mandagery Formations) encloses the Murringo Valley, and reflects a north-plunging syncline in the Devonian sedimentary succession. Within the valley colluvial fans drape the ridge flanks, and alluvial fans have formed in association with streams. In the upper reaches streams have cut into these fans but farther downstream they have developed narrow fluvial plains and some terraces. There is an unusual configuration of landforms in the lower Murringo Valley. The Devonian sediments beneath the valley floor have been folded in an east-west direction, and subdued north-south trending topographic highs reveal outcrop of Devonian shales and fissile sandstones (Bumberry and Koorawatha Formations) or hide subcropping sediments. Where colluvial and alluvial fans abut against these central-valley-floor ridges there is a constriction in the modem drainage and also in subsurface fluid flow. The drainage constriction in the lower Murringo Valley, identified by mapping, has been termed the "Murringo Bottleneck". Salt affected areas lie immediately above the constriction and in the broad fluvial plain adjacent to the main watercourse downstream. The original source of salt in the Boorowa-Young area is believed to be cyclic sah introduced from marine aerosols or in windblown dust (pama) introduced to the area over an extended period of time. Near-surface fluid flow mobilises salt from the regolith zone. A constriction of shallow drainage brings saline water to the land surface. This leads to the pattern observed in the EM survey with an "hour-glass" shaped area of salt-affected ground above and below the "Murringo Bottleneck".

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CHARACTERISTICS AND ORIGIN OF MARINE ASH LAYERS FROM THE KERGUELEN PLATEAU, SOUTHERN INDIAN OCEAN (ODP LEG 183) C. L. Moore^ and P. Wallace^ 'University of Canberra, ACT 2601 ^Ocean Drilling Program, Texas A & M University, College Station, Texas 77845-9547, U S A

Fourteen vitric-rich marine ash layers, a few millimeters to two centimeters thick, were identified at three drillsites (ODP Leg 183, sitesll38, 1139 and 1140) on the Kerguelen Plateau, Southern Indian Ocean. The tephras form horizons in bioclastic sediments allowing broad biostratigraphic ages to be assigned. In general the ashes identified were crystal poor, glass-shard-rich, well-sorted and had a mean grainsize in the 75 to 150 micron range; characteristics typical of distal ash fall deposits. Felsic shards were translucent in thin section and basaltic shards were light to dark brown in colour. At Site 1138 on the Central Kerguelen Plateau, ten ash layers were recognised. Three age brackets were represented by Late Maastrichtian to Late Oligocene trachytic and basaltic fall deposits, a single Early-Late Miocene rhyolitic fall deposit and several Late Miocene to Late Pleistocene trachytic volcanic fall deposits. There is a general evolution in tephra geochemistry from older basalt dominant volcanism to younger trachytic dominant volcanism. This is consistent with the model for evolution of the Kerguelen Plateau with felsic volcanism more common in younger successions. A similar geochemical trend was observed for tephras at Site 747 (ODP Leg 120). Trachytic and rhyolitic glass shard morphologies are dominated by tube pumice (60% to 80% vesicularity) reflecting attenuation of a viscous melt at the time of eruption. Although some terminations illustrate sharp fractures that cross-cut tube vesicles, fragmentation is commonly through tube vesicles due to bubble burst (magmatic eruption). Basaltic shard morphologies are more typically blocky to platey with much lower vesicularity (-20%) reflecting a combination of magmatic and phreatomagmatic eruptive mechanisms. Site 1138 is 180 km ESE of Heard Island and this is the likely source of most of the fall deposits represented. At Skiff Bank (Site 1139), a western salient of the Northern Kerguelen Plateau, three tephras were identified. These include an Early-Late Oligocene dispersed trachybasah tephra with blocky and platey shards, and Early-Late Oligocene glass-shard-rich tube vesicular dacite fall deposit and a Early to Mid-Miocene blocky glass-shard-rich basalt fall deposit. This site is 350 km ESE of the Kerguelen Archipelago, a possible source for these tephras. The origin of the dacitic tephra in particular is not clear. At Site 1140, 270 km north of the Kerguelen Archipelago, one basaltic tephra of early Miocene age was identified. Blocky, low vesicularity glass shards with curviplanar surfaces reflect genesis in a magmatic to phreatomagmatic eruption.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PRODUCTS OF EXPLOSIVE FELSIC VOLCANISM WITHIN THE BASALT LAVA PILE OF THE KERGUELEN PLATEAU LARGE IGNEOUS PROVINCE, SOUTHERN INDIAN OCEAN C. L. Moore', P. Wallace^ and the ODP Leg 183 Shipboard Scientific Party 'University of Canberra, ACT 2601, Australia ^Ocean Drilling Program, Texas A&M University, College Station, Texas 77845-9547, USA

The eruption of highly evolved felsic magmas in the final stages of evolution of Kerguelen Plateau resulted in extensive distribution of primary pyroclastic and epiclastic materials, and some lava. The presence of felsic volcanic deposits (ODP Leg 183 drillsites 1137, 1138 and 1139) was unexpected in a large igneous province setting dominated by outpouring of mafic lava. Previous drilling (ODP Legs 119 and 120) found no evidence of felsic magmatism. At Elan Bank (Site 1137), a 15-m-thick sanidine-rich vitric tuff (-110 Ma) is intercalated between basaltic lava flows. Well-preserved cuspate and tricuspate bubble-wall glass shards in the tuff together with abundant broken crystals indicate an explosive volcanic origin. The massive texture and presence of matrix-supported angular basaltic pebbles (<5%) suggests subsequent mass-flow redeposition of the pyroclastic sediment. Higher in the stratigraphic sequence at Elan Bank, a fluvial conglomerate contains clasts of rhyolitic and trachytic lava. On the Central Kerguelen Plateau (Site 1138) a 20-m-thick volcaniclastic succession (>94 Ma) contains six trachytic pumice-lithic-breccias that were emplaced as pyroclastic flows. This sequence also includes highly altered ash fall deposits that contain accretionary lapilli implying interaction between ash and water either during or immediately post-eruption. Above this sequence rounded cobbles of flow-banded dacite were recovered. At Skiff Bank (Site 1139), part of the Northern Kerguelen Plateau, the uppermost basement (Cenozoic) contains densely-welded pyroclastic flow deposits of quartz-bearing peralkaline rhyolite, in addition to lava flows and reworked cobbles of volcanic rock ranging from sanidine-rich trachyte to rhyolite. Welding of pyroclastic deposits is possible in subaqueous environments but more typical in subaerial settings. The identification of subaerially erupted and emplaced felsic volcanic rocks compliments other evidence (e.g. deposits containing wood fragments a seed and spores; fluvial conglomerates; oxidised rinds on basaltic lava flows; presence of inflated pahoehoe lava) that suggests extensive regions of the igneous crust of the now submerged Kerguelen Plateau were erupted above water.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

PRE-RODINIAN OPHIOLITES : A PROGRESS REPORT Eldridge M. Moores Department of Geology, University of California, Davis, CA, 95616 USA c/o Tectonics Special Research Centre, Univ. Western Australia, Nedlands,WA 6907 Ophiolites-fragments of ocean crust and mantle formed at spreading centersand preserved on land-are key indicators of plate tectonic activity duringorogeny. Many well-exposed complexes display structures closely resemblingthose of modem spreading centers. Many complexes exhibit an ideal ophiolite sequence, from bottom totop, of tectonite peridotite, magmatic mafic-ultramafic plutonics, sheeteddykes, and extrusives; others display a partial sequence reminiscent ofmodem slow-spreading centers. Exhalative massive sulfide deposits, reminiscent of modem "black smokers", characterize many ophiolites. Economically important chrome spinelconcentrations are abundant in some ultramafic cumulates and in a few cases in magmatic lenses within tectonites. Realistic ophiolite interpretation involves stratigraphic andstructural/tectonic features, as well as geochemistry and petrology. Correlation of oceanic cmst with ophiolite stmcture enables a tentativeinference of spreading rates for a few complexes (e.g. Troodos, Semail). Ophiolite emplacement involves collision ofcontinental margins or island arcs with seaward-dipping subduction zones.These events produce synthetic decollement-style fold-thrust belts directed towards plate interiors that are better known, but subsidiary to, ophiolite nappes. In Alpine-type (Wilson-cycle) orogens, ophiolite emplacement predates fmalcontinentcontinent collision. In the past, confusion about the nature andtectonic significance of ophiolites led to erroneousinterpretations of several Phanerozoic-Neoproterozoic orogens as ofensialic rather than collisional (Wilson-cycle) origin. Despite geochemical evidence in many ophiolites reflective of modem"suprasubduction zone" settings, longlived mantle heterogeneities and thepresence of "suprasubduction" compositions in several modem midoceanridges suggest that chemical compositions alone are not reliable indicators of tectonic environment. Compositionaldifferences between modem mid-ocean ridges and many ophiolites may result from the tectonic process of emplacement, as well asfrom "historical contingency", i.e., magma compositions in a given tectonicsetting may have changed during the past or in different regions as afiinction of tectonic history. Integration of these factors with a new model of deep mantle structure mayprovide insight in to the relationship between pattems of global tectonicsand magmatic evolution. A global survey of pre-Rodinian orogens reveals more than 20 recognized orpossibleophiolites. These complexes characteristically display multiple deformationand amphibolite or higher metamorphic grade. Meso-Paleoproterozoicophiolites typically lack the strong contrast between cumulate (magmatic)maficultramafic rocks and tectonite lithospheric mantle present in younger examples. Because plate tectonicslikely was operating throughout the Proterozoic, this contrast suggeststhat the magmatic crust formed at spreading centers was thicker, so thatthe underlying tectonite mantle was not preserved during emplacement. Thinning of the magmatic cmst, presumablyin response to lower geothermal gradient, depletion of mantle sourceregions, or change in mantle processes, may have been abmpt. A hypothesisfor abmpt thinning about 1 Ga ofmagmatic cmst at spreading centers has not been falsified, and implies arole for "punctuated evolution" of tectonic/magmatic processes throughEarth history. Magma compositions of MesoPaleoproterozoic complexes aresimilar to Phanerozoic ones, except for a possible greater komatiitic abundance in older examples. Hotspotactivity may have been greater during Meso-Paleoproterozoic time. Thesedata imply that the present mantle compositional heterogeneity existed atleast 2.1 Ga. A few Archean ophiolites suggest a partial role, at least, for oceanic spreading similar to modemprocesses prior to 2.5 Ga.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

INTERACTIVE INVERSION IN GEOLOGY Louis Moresi and Fabio Boschetti Australian Geodynamics CRC, CSIRO Exploration and Mining, PO Box 437, Nedlands, WA 6009 Inversion is an important tool in the interpretation of geophysical data. It attempts to reconstruct rock properties distribution from measurements of their physical responses. This is achieved by a more or less structured search into a parameter (rock properties) space through the use of physical forward modeling. In recent years fast computers have allowed the development of quite sophisticated forward modeling of geological processes as well. Plate tectonics, faulting and folding, mantle convection, fluid flows could, in principle, be treated in an inverse process, much like traditional seismic/potential field problems. The potential of such application would be tremendous given that, broadly speaking, reconstructing initial geological configurations from their geological responses (determining the stress field that generated a certain folding pattern, for example), is very much what geology is about, and is an implicit inverse problem tackled on a daily basis in every geologist's brain. Basically, we dream of a final product that would allow a person to scan a geological cross section onto a computer and watch the geology unravel backward in time. This dream has to meet the reality of very hard mathematical hurdles. To the underlying non-uniqueness and instabilities we have to add a complexity prevalent in geology, that of incomplete knowledge of exactly what the model is that you want to run backwards. To overcome these many difficulties, geologists use their intuition and experience to focus only on the "geologically reasonable" models which lead to the particular formations they observe. There is, of course, a danger that their experience and intuition will filter out perfectly valid models which they have not encountered previously. In our approach we wanted to see if there is a way to combine the formal methodology of mathematical inversion with the geologist's ability to apply a highly trained brain to unravel the mechanical evolution of a geological formation. The system works by linking a geological forward model to an inverse search algorithm. A geologist sits in front of a computer where he or she is presented with a number of different models which all try to match a final state. These models can be static geological models or animations showing the history running forwards. The computer cannot tell which ones are good or bad, so the geologist ranks each of them according to his/her experience and knowledge. Once ranked, the computer can apply mathematically rigorous methods to generate a new set of models that progressively converge towards the geologist expectation. An element of mischief is also part of the approach since some of the models should be a surprise to the geologist and suggest new possibilities outside his or her experience.

In one of the tests we performed, we were provided with a simple sketch (Figure 1, top) of the distribution of faults in an extending block of crust. The exact details of the faults were not required. We were simply aiming at determining what parameters (stress and material properties) can allow the formation of such faulting pattern at the appropriate spacing to match the observations. This exercise had also been previously undertaken during a systematic sweep of the parameters space. Figure 1 (bottom) shows the model found with our approach. This is at least as good as the one produced by the systematic sweep of the model parameters, but it has been produced with much less computational and human effort.

Figure 1. Sketch of a geological section (top) and result of the inversion

The power of this approach lies in the wide variety of applications it can deal with. So far it has also been applied to mantle convection modeling, folding, and fracture formation. This inversion can help every time a problem needs visual appraisal of the results or experience and a priori knowledge. All that is required is a code that allows the user to model a process and view its result.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE DYNAMIC EVOLUTION OF THE AUSTRALIAN CONTINENT SINCE THE EARLY ARCHEAN: WHAT CAN WE LEARN FROM DYNAMIC MODELING ? Louis Moresi\ Adrian Lenardic^, Peter Sorjonen-Ward\ Roberto Weinberg^ John Walshe^and Hans Muhlhaus^ ^ Australian Geodynamics CRC,CSIRO Exploration and Mining, PO Box 437, Nedlands, WA 6009 ^ Department of Geology and Geophysics, Rice University, PO Box 1892, Houston, TX 77251-1892, USA ^ Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands WA 6907 Dynamic processes active in Archean times are extremely difficult to constrain. Vigorous convection in the mantle due to higher internal temperature and radioactive heating, and a distinctive crustal rheology due to high radiogenic heat flow are suggested directly by extrapolating back to Archean time. It is a problem of quite different magnitude to predict in detail the effects of increasing the energy available to convection. Convection, of the mantle is a fluid dynamical process which, essentially by definition, leaves no direct record of the flow geometry, stress or strain history. What evidence remains is indirect: for example, geochemical signatures of irreversible processes such as melting to produce oceanic and continental crust, strain recorded in ancient terrains, and the various geochemical/geophysical markers in the continental lithosphere which allow the recovery of pressure, temperature and age information. In order to understand the early evolution of the Australian continent it is necessary to understand the global dynamic fi-amework in which the various components found themselves. This can be done by building a mathematical model of the relevant geological processes and using the geological record to constrain this model. At present this integration of modeling and the geological record is still in its infancy. Constraints are rather loose - in part because data collection and archiving has only recently been driven by the need to provide such constraints. Models have been limited partly by the power of computers available to run simulations, but far more by the lack of algorithms which can reproduce the physics accurately. Here we will demonstrate the application of a relatively new computer code which we have developed to treat viscoelastic, brittle deformation problems at the very high strains appropriate to modeling plate tectonics and continental drift as an emergent property of the convecting mantle. We discuss a number of issues relevant to the evolution of the cratons since the Archean including: •

•

•

What was the nature of mantle convection in the early Archean ? At high mantle temperatures, plate tectonics is not feasible due to the large thickness of the oceanic crust. What form does the convection pattern take ? What was the nature of crust/mantle interaction in the early Earth — how do predicted transitions in thermochemical convective style relate to the assembly of protocontinents and would any structures be preserved in the cratons directly or indirectly ? How was the high heatflow from the Archean mantle partitioned between the continental and oceanic regions ? Is there a genuine heatflow paradox which needs to be explained, or is there a natural way for the continental heat flow to be buffered ? How can cratons survive in a hostile mantle ? Subduction supplies enough buoyancy to induce largescale engulfment of the mantle lithosphere, what are the mechanic requirements for cratonic lithosphere to survive to the present ? Are these requirements met by the secular evolution of the mantle composition clearly recorded in mantle xenoliths, or are additional differences required ? Is there any small-scale mode of convection beneath the continental lithosphere now, or in the past, and if there is, why does it not erode the geochemical signature of the ancient mantle ?

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MANTLE DYNAMICS AND THE FORMATION OF MAJOR ORE DEPOSITS Louis Moresi', Roberto Weinberg^, John Walshe^and Hans Muhlhaus^ ' Australian Geodynamics CRC,CSIRO Exploration and Mining, PO Box 437, Nedlands, WA 6009 ^ Centre for Strategic Mineral Deposits,Department of Geology and Geophysics, University of Western Australia, Nedlands WA 6907 Most active geological processes can, in one sense or another, be attributed to heat struggling to escape from the Earth's interior. The dominant manifestation of the struggle in the present day Earth is Plate Tectonics which can only be fully understood by considering the interaction of the enormous scale of the plate themselves with the much smaller scale of the active plate boundaries - in particular the brittle deformation processes at convergent margins which allow oceanic lithosphere to subduct. The minimum requirement for modeling subduction zone dynamics is the ability to solve the coupled equations which govern the balance of stresses and the evolution of temperature in the mantle, lithosphere and crust. We have developed a computational code which can simulate the evolution of the geometry, stresses and temperature in the lithosphere, mantle and crust in the subduction environment, taking into account the viscoelastic, brittle nature of the lithosphere and crust and the vigorously convecting viscous mantle. The output of these simulations includes a self-consistent slab geometry, thermal structure, Moho deflection, and stress field. We will present 2D simulations of the evolution of destructive plate margins. We are particularly interested in the dynamic relationship between subduction/collision and the stress distribution, thermal evolution and magmatic activity in the overlying crust, which may be directly applied to understanding crustal evolution and the origin of giant ore deposits. We first examine the interaction of subduction zones with continental crust in the Earth today and attempt to relate the dynamics of the model to the styles of mineralization observed. In particular we examine the origin of the large Chilean porphyry deposits and the relation of mineralization to changes in oceanic slab dips, heterogeneities and tears in the suducting slab. We next examine whether the geographical coincidence between the Barramundi "worm" (a long, sinuous line defined by an edge in the upward continued gravity field) and the large Pb-Zn deposits of Queensland (observed by Hobbs and coworkers) can be modeled as an ancient zone of convergence. Based on the secular variation of lithospheric mantle density, the latter authors have postulated that the Barramundi worm represents a Proterozoic lithospheric suture zone, with no obvious signature in the immediately overlying crust, except for the large crustal structures such as the Termite Range Fault and the Pb-Zn deposits themselves. In destructive margins, shallow-dipping slabs may displace the pre-existing lithosphere from underneath the overlying crust. We are investigating whether this process, presently occurring under Tibet, may explain the Barramundi worm, and, if so, the manner in which a reactivation of this deep suture zone may be related to the origin of Pb-Zn deposits in the upper crust. We are calibrating this model using a modem analogue: the India/Eurasia collision. A 3D code is required to explore changes in the properties of the system along the destructive margin such as changes in the subducting slab dip and the flow of material through a tear in the subducted lithosphere. Development of the 3D version of our modeling tool is in progress. References B.E.Hobbs, A. Ord, N. J. Archibald, J. L. Walshe, Y. Zhang, M. Brown, C. Zhao, Geodynamic modeling as an exploration tool. After 2000 — The fiiture of mining, Sydney, NSW, 10-12 April, 2000

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ORIGIN AND SIGNIFICANCE OF LATE- TO POST-TECTONIC LEUCOGRANITE AND ASSOCIATED QUARTZ-SILLIMANITE NODULES IN THE ADIRONDACK MTNS., NY, USA Jean Morrison^ and James McLelland^ ^ Department of Geology, University of Southern California, Los Angeles, California, 90089, USA ^Department of Geology, Colgate University, Hamilton, N.Y., USA

The Lyon Mountain (LM) leucogranite is a minimally deformed high-silica, high-alkali granite that consists mostly (-60%) of a quartz-perthite facies that commonly contains magnetite as the only mafic mineral. The LM leucogranite outcrops in the northern and western portions of the Adirondack Mountains. A quartz-albite aegerine-bearing facies accounts for -30% of the leucogranite, and a high-K facies accounts for another -10%. The LM leucogranite was emplaced at ca. 1050 to 1040 Ma, during the waning stages of the Ottawan Orogeny, following peak granulite facies metamorphism at ca. 1080 to 1050 Ma. Petrographic studies indicate that the quartz-albite aegerine-bearing facies is the result of sodic metasomatic replacement of normal leucogranite. Widespread sodic metasomatism accompanied hydrothermal alteration associated with the emplacement of the leucogranite. Somewhat younger LM-type leucogranites, emplaced at ca. 1035 Ma, occur in the Port Leyden area an are associated quartz-sillimanite nodules. Quartz (qtz), hornblende (hb), magnetite (mt) and whole rock (wr) values have been measured in 13 samples of LM leucogranite from the Lyon Mtn. mine area. Qtz values range from 12.3 to 14.7%o, hb ranges from 9.1 to 9.5%o, mt ranges from 4.0 to 7.8%o, and wr values range from 8.6 to 11.9%o. Qtz-mt fractionations range from 6.9 to 8.6, and if the individual minerals have not been reset by retrograde exchange during cooling, indicate temperatures for hydrothermal alteration associated with the emplacement of the leucogranites of 580° to 680°C. The oxygen isotope values are consistent with exchange with surface-derived waters that evolved in saline, evaporitic conditions or by exchange with evaporitic sediments. Qtz and mt ^^^O values have been measured in 9 samples from the Port Leyden area LM-type leucogranites. Qtz values range from 6.7 to 13.0%o and one mt value is 3.5%o. The qtz-mt fractionation is 6.7 which yields a temperature of-680°C. Water in equilibrium with the qtz in this sample at 680°C would be -9%o, which is consistent with an evolved surface-derived basinal brine. These waters are thought to have infiltrated the collapsing Ottawan orogen at ca. 1050 Ma and were heated by ascending leucogranitic plutons, resulting in circulation of high temperature Na, Fe, K-metasomatism. The waters were acidic resulting in alkali leaching and qtz-sillimanite veins and nodules developed in the granitic carapace. Isotopic studies are continuing on qtz-sillimanite veins and nodules from the Port Leyden area to further constrain temperatures and fluid sources.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE SYERSTON DEPOSIT - LATERITIC NICKEL COBALT PLATINUM OVER AN ALASKAN INTRUSION Geoff Motteram and Andrew Spinks Black Range Minerals N.L., Level 10, 190 St Georges Terrace, Perth, W.A. 6000

The Syerston Project is wholly owned by Black Range Minerals and is located near Fifield in Central New South Wales within the Narromine 1:250,000-map sheet of the geological survey of NSW. The Syerston Project deposit consists of lateritic enrichment developed over an Alaskan intrusion of Devonian to late Ordovician age within the Girilambone Group of sediments. The Fifield region hosts a series of Alaskan style intrusives that range in size from 2 to 50 square kilometres. The intrusions rarely outcrop, but produce a distinctive magnetic anomaly with rock types ranging from mafic to ultramafic and with best lateritic enrichment in nickel and cobalt over dunite. Early exploration identified these intrusions to host platinum mineralisation and recognition of the potential nickel and cobalt mineralisation was recognised following the exploration for platinum. The Syerston deposit is classified as a limonitic oxide type due to the mineralogy being dominated by goethite and the absence of clay minerals within the profiles. The mineralisation is hosted within four zones, which are locally referred to as the Transitional, Goethite, Silicified Goethite and Saprolite zones. These zones exhibit distinctive geological and geochemical signatures, which are unique in identifying the profile. The mineralogy of the deposit offers favourable metallurgical characteristics including the ability to upgrade the siliceous goethite zone by screening. The mineralogy being dominated by iron oxides provides for low acid consumption and low viscosities. The Fifield district is renowned for its historical platinum production of 20,000 ounce from deep leads. A one million-ounce platinum resource has recently been defined primarily coincident with the nickel and cobalt mineralisation within the Syerston Lateritic profiles. Platinum has also been identified within the overlying alluvium and significant data supports the potential for the occurrence of primary platinum mineralisation occurring within the dunite. Recent test work indicates that the platinum mineralisation and the associated chrome mineralisation are both magnetic and will respond to magnetics and gravity separation methods. Future exploration will focus on the 15 Alaskan intrusions now under control of Black Range within the Fifield district, plus the potential for recovery of platinum from regional alluvial's. The Bankable Feasibility Study for 20,000 tpa Nickel and 5,000 tpa Cobalt capacity Pressure Acid Leach development is based on a mineral resource estimation of 107.4 million tonnes at a grade of 0.66% Nickel and 0.11% Cobalt.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE BIG RIP: TECTONIC CONSEQUENCES OF EAST-WEST ANTARCTIC SEPARATION IN THE CENOZOIC R. Dietmar Muller^ and Steven C. Cande^ ^School of Geosciences, The University of Sydney, NSW 2006, Australia ^Scripps Institution of Oceanography, La Jolla, California 92093-0215, USA

Recently, major improvements in our understanding of the Australian-Pacific convergence history have been made, based on the accurate reconstruction of East Antarctic-West Antarctic relative motions in the Tertiary (Cande et aL, Nature, 2000). The new rotations show that there was roughly 180 km of extension in the western Ross Sea embayment in the Eocene and Oligocene. Tertiary extension between East and West Antarctica also provides a tectonic setting for several major Cenozoic tectonic events in the Ross Sea including the uplift of the Transantarctic Mountains and the deposition of large thicknesses of Mid-Cenozoic sediments. The revised reconstructions also remove a puzzling gap between the Lord Howe Rise and Campbell Plateau in previous early Tertiary paleogeographic maps of the New Zealand region. We investigate the relationship between volcanism in and south of the Tasman Sea with absolute plate motions based on a revised plate model. We find that the Tasmantid hotspot track is best modeled using a Pacific hotspot reference frame. This is in accordance with a relatively recent (--40 Ma) onset of subduction east of Australia, which has not acted as an efficient barrier between the mantle domains east and west of the subduction zone, partly due to a lack of slab penetration into the lower mantle. If a hotspot is now located close to the Balleny Islands, it could not have produced the Tasmanian seamount chain, which crosses the Tasman Sea, irrespective of which hotspot reference fame is chosen. This indicates that there is no relationship between these two features. We rather suggest that the hotspot that created the Tasmanian hotspot track was originally situated in the vicinity of the central Lord Howe Rise during Late Cretaceous-early Tertiary and is now located underneath the Pleiades volcanic province in northern Victoria Land in East Antarctica.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE AUSTRALIA-PACIFIC PLATE BOUNDARY AT 25 MA Robert J. Musgrave Department of Earth Sciences, La Trobe University, Bundoora, VIC 3083

The late Miocene configuration of the Pacific-Australia plate boundary from New Guinea to New Zealand is relatively well understood, and can be reconstructed by rolling back current kinematics. Restoration of the plate boundary back to the Early Miocene and latest Oligocene is more difficuU. Difficulties arise from the absence of sea-floor spreading data over the northern half of the system, a consequence of subduction after arc reversal, and from uncertainties about which features actually constituted the active boundary at this time. In New Zealand, Permian ophiolite belts at each end of the Alpine Fault mark roughly 500 kilometers of transcurrent motion. Reconstruction back to about 17 Ma removes this offset. Restoration to 25 Ma requires additional Pacific-Australia displacement. Conventional interpretations of the Stokes Magnetic Anomaly system as a continuous trace of the Permian beh through the North Island makes identification of the locus of the plate boundary over this time problematic. Limitations on the amount of Australia-Pacific motion that can be accommodated within New Zealand can be overcome by postulating a different transcurrent fault system prior to 17 Ma. The Permian ophiolite belt lies entirely on one side of this fault, freeing this constraint. The Stokes anomaly system is reinterpreted in this light. Deformation patterns in the northern part of the South Island are consistent with processes at 17 Ma when the modem Alpine Fault evolved to replace the earlier fault system, in response to a change in Pacific-Australia motion. The Early to Middle Miocene location of the northern segment of the plate boundary has also been in dispute. Most workers hold that the Solomons-New Hebrides arc was accreted from the Australian plate onto the Pacific plate following collision with the Ontong Java Plateau (OJP), and that collision halted subduction more or less instantaneously. One interpretation holds that collision halted subduction of the Pacific plate earlier, at about 2520 Ma; this requires that the Pacific-Australia boundary during the Middle Miocene lay somewhere else, presumably well south of the Solomons, in now-subducted crust. Arc-derived conglomerates are present in latest Oligocene OJP sediments accreted to the Solomons arc. Palaeomagnetic data indicate that the accreted sequence moved with the Australian plate for most or all of the Miocene. A plausible explanation requires that the former southern margin of the OJP extended far to the southwest of the current limit of the plateau, defined by the fossil trench north of the Solomons. A corollary is that subduction of the flanks of the OJP continued through much of the Miocene. Subduction of the anomalous lithosphere of the OJP may be the reason for the absence of Middle Miocene volcanics from the Solomons arc. Subduction of oceanic plateaus may be less difficult than often supposed. Collision of the OJP with the Solomons arc coincided with the end of South Fiji Basin spreading. It is likely that one arm of the South Fiji Basin (now subducted) spread behind the Solomons-New Hebrides arc; collision with the plateau at about 25 Ma may have slowed (rather than immediately stopped) subduction, ending spreading of the South Fiji Basin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DEVONIAN FLUVIAL DEPOSITION AND TECTONICS ADJACENT TO THE KOONENBERRY FAULT AT MOONA VALE STATION, FAR WESTERN NEW SOUTH WALES Gerrit Neef^ and David F. Larsen^ 'University of New South Wales, NSW 2052, Australia. ^N F M Normandy

The Wonominta Block lies west of the NNW-trending Koonenberry Fault, -180 km northeast of Broken Hill in far west New South Wales. It is largely composed of Precambrian and Cambrian deformed strata whereas east of the fault lies the small Moona Vale Trough which is largely filled with >4.8 km of Emsian-Eifelian sandstone. These Devonian strata are correlated with strata lying 40 km to the NNW at the Copper Mine Range. The Gnalta No. 1 Well in the north of the study area bottomed in indurated quartzose sandstone, inferred here to be coeval with Unit 2 of the Copper Mine Range, having passed through 700 m of coarse sandstone representing Unit 3. Overlying Unit 3 is the >3.8 km thick light-brown coloured, Menamurtee Sandstone which was sourced from the west. Near its base it is coarse grained and pebbly and there are two fish fragments of the Emsian - indicating Wuttagoonaspis fletcheri. Also occuring near its base is evidence of syndepositional activity on the Koonenberry Fault causing the emplacement of the Gum Creek Thrust (a positive flower structure related to the fault). About 2.4 km above the base of the formation lies the largely debris-flow deposited, 0-520 m thick, Moona Vale Conglomerate Member which was sourced from hills then lying west of the Koonenberry Fault. It is overlain by the 1.4 km thick upper part of the Menamurtee Sandstone. During the Tabberabberan Orogeny the Koonenberry Fault was active sinistrally causing the formation of the NE-trending Menamurtee Anticline. After the orogeny conglomerate was deposited west of the Koonenberry Fault in late Eifelian? - early Givetian? time. This was followed by a thin bedded sandstone sequence which is tentatively correlated with the mid Givetian Taghanic event - a global high sea level. Conformably above these strata is the late Givetian? - Famennian km thick largely fine-grained Peveril Peak Sandstone sourced first from the south and then from the west. Subsequently during the Kanimblan Orogeny the Peveril Peak Sandstone was deformed during a period when the Koonenberry Fault was dextral.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TRACKING THE EVOLUTION OF HEAT-PRODUCING ELEMENTS IN THE SOUTH AUSTRALIAN PROTEROZOIC Nareile L. Neumann^ Mike Sandiford^ John Foden\ Kathy Stewart' and Marlina Elburg' Department of Geology and Geophysics, University of Adelaide, South Australia, 5005 ^ Present address : School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052

Granites of the Mount Painter Block, Northern Flinders Ranges, South Australia reflect two magmatic events at 1575 and 1555 Ma. They are typically anorogenic Sr-depleted, Yundepleted granites (Wybom et al, 1992) which are enriched in incompatible elements. These granites display elevated concentrations of U, Th, Nb, Ce, La and Zr with Si02 ranging 65 to 78 %. Of particular interest is the extreme U, Th and K enrichment, with current heat production rates calculated from geochemical analyses ranging from 'typical granite' values of 4 |LiWm'^ for I-type suites, to - 6 0 |LiWm"^ in some A-type granites. The mean area-normalised surface heat production for the province is 10 |LiWm"^ over an outcrop area of approximately 790 km^. This is significantly higher than the average of 1.8 calculated for the upper continental crust by McLennan and Taylor (1996). Although the U, Th and K concentrations within Mount Painter granites are extreme, Th/U ratios for these suites are dominantly 3 to 5, suggesting that the extreme enrichment is a primary magmatic feature. Epsilon Nd isotopic values from Mount Painter granites are quite high and show very limited variation in SNd(i555) from -3.5 to -0.5. Nevertheless, the variation in LIL and HPS elements between these granites is extreme. Initial ^^^Pb/^^^Pb, ^^^Pb/^^^Pb and ^^^Pb/^^^Pb ratios from K-feldspars within these granites are much higher than the Stacey-Kramers curve of Pb evolution for the continental crust (Stacey and Kramers, 1975). This indicates that the crust in this region was enriched in U and Th for a substantial time prior to Mesoproterozoic magmatism. AFC-type processes involving mixing of mantle-derived melts with a small enriched crustal component are therefore required to reconcile high swd values with extreme U, Th and K. However, commonly perceived notions of the composition of the lower crust suggest that it is unlikely to represent a significant reservoir of radiogenic elements. Moreover, any extreme concentration of heat producing elements in the lower crust will strongly perturb the geothermal gradient, resulting in high temperature metamorphism and wholesale melting. Therefore, the magmatic processes which formed these granites must further concentrate U, Th and K already in the system to produce the extreme geochemical enrichment observed. References

MCLENNAN S.M. & TAYLOR S.R. 1996. Heat flow and the Chemical Composition of the Continental Crust. Journal of Geology, 104,369-377. STACEY J.S. & KRAMERS J.D. 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters, 26, 207-221. WYBORN L.A.L, WYBORN D., WARREN R.G. & DRUMMOND B.J. 1992. Proterozoic granite types in Australia: Implications for lower crust composition, structure and evolution. Transactions of the Royal Society of Edinburgh: Earth Sciences, 83, 201-209.

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STABLE ISOTOPE AND FLUID INCLUSION SIGNATURES OF HYDROTHERMAL FLUIDS IN TRANSCRUSTAL FAULT ZONES: SIGNIFICANCE FOR OROGENIC, ARCHEAN LODE-GOLD MINERALIZATION P. Neumayr. S.G. Hagemann and D.I. Groves Centre for Strategic Mineral Deposits, The University of Western Australia, Nedlands, W.A. 6907, Australia Large to giant (>lt) gold deposits are typically hosted in second- and third-order structures adjacent to largely barren, transcrustal fault zones. Gold-bearing hydrothermal fluids have been channelled within the transcrustal fault zones fi-om mantle and deep crustal sources into the second- and third-order structures, where gold has been deposited. Transcrustal fault zones are long-lived structures with specific deformation events relating to gold deposition in the second- and third-order structures. For example the Archaean Perseverance Fault in the Yilgam Craton of Western Australia evolved from a wide (5km) ductile shear zone during D2 to a narrow (<15m) brittle fault. During the ductile deformation, the mesozonal Matilda gold deposit formed, whereas D3 deformation along the Perseverance Fault is correlated with significant epizonal gold-stibnite mineralization along the Wiluna strike-slip fault system. In the Val-d'Or camp in Canada, only D2 reverse and D3 oblique to transcurrent movement along the Cadillac Tectonic Zone (CTZ) was related to gold mineralization in second- and third-order reverse structures. There are significant differences in the evolution of hydrothermal fluids in transcrustal and second- and thirdorder structures which can explain the different gold tenor. For example: 1) the transcrustal CTZ is a ductile shear zone, with continuous intergranular fluid flow probably open to surface, whereas the second- and thirdorder shear zones form a brittle fault network, in which fluid flow was episodic, with multiple crack and seal events in a system which was capped, 2) at the present level of exposure, preserved hydrothermal fluids in the CTZ comprise C02-CH4-dominated compositions with minor H 2 0 and H2S components, whereas there are H20-dominated H 2 0 - C 0 2 ± C H 4 fluids with a significant H2S component in the second- and third-order shear zones at the Sigma gold deposit, a major gold deposit 5km to the north of the CTZ. These differences can be explained by continuous phase separation, with C02-vapour escape into the upper portions of the ductile uncapped CTZ, contrasting with in-situ phase separation of the gold-bearing fluids in crack-seal veins in the second-order shear zones at Sigma, with trapping of both the episodic vapour and liquid components in individual sealed veins. Gold mineralization in the second- and third-order structures appears to be controlled by the high H2S activity of the aqueous hydrothermal fluids, because gold was likely carried in a bisulphide complex and was deposited during sulfidation reactions in the wallrock and phase separation in the quartz vein. In contrast, the carbonic fluids in the CTZ lacked the ability to carry significant metal ligands due to their low H2S activity. Oxygen isotopes from hydrothermal quartz within the CTZ (13.3 to 15.6 per mil, av. 14.0 per mil; VSMOW) are heavier than those from mineralized quartz veins in second- and third-order shear zones (11.8 to 12.6 per mil, av. 12.2 per mil). Calculated d l 8 0 fluid compositions for quartz in the CTZ range from 8.0 to 10.3 per mil at 350°C (based on arsenopyrite and chlorite thermometry). Hydrogen isotopes from fluid inclusion waters trapped in quartz have a large variation from -62.5 per mil to -7.2 per mil in the CTZ fluids, whereas hydrogen in fluid inclusions in quartz in the second- and third-order shear zones shows a restricted range from -67.6 to -39.8 per mil. The oxygen isotope shift of about 2 per mil from the CTZ to the second- and third-order fault zones may be explained by two competing processes: 1) the hydrothermal fluids in the CTZ equilibrated, at least partially, with metasedimentary rocks in the footwall, and/or 2) there was fractionation of oxygen isotopes during phase immiscibility of a combined H 2 0 - C 0 2 fluid. The preferential trapping of C02-rich fluids in the CTZ, and H20-rich fluids in the second- and third-order fault zones, therefore, could account for the shift in d l 8 0 . At present, the first process is preferred, because of the ubiquitous presence of the metasedimentary rocks in the footwall and the consistent d l 8 0 composition of the CTZ, even in hydrothermal quartz veins which contain significant H 2 0 . The large variation in the hydrogen isotopes in fluid inclusions in quartz in the CTZ may be explained by late Archean and post-Archean reactivation of the CTZ and the introduction of fluids related to late-fractures fills, whereas apparently minor reactivation of the second- and third-order structures resulted in a restricted range of dD.

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A COMPARISON OF CHRONOSTRATIGRAPHIC CROSS SECTIONS THROUGH THE DEEP WATER OTWAY AND BIGHT BASINS SOUTHERN AUSTRALIA Martin S. Norvick and Mark A. Smith School of Earth Sciences, La Trobe University Chronostratigraphic sections from the onshore to the deepwater Otway and Bight Basins, based on new AGSO deep seismic profiles, illustrate variations in the rifting and spreading history of Australia's southern margins. Although superficially similar, the Otway and Bight Basins had differences in their stratigraphy reflecting their location relative to different breakup events and the nature of sediment input. Initial rifting began and finished earlier in the Ceduna Subbasin because it was closer to the Greater India-Antarctica breakup. The Turonian-Santonian trans-tensional rift in the Otway Basin did not extend as far west as the Bight Basin. Perhaps the most important difference was that the Otway basin had a strike-slip margin with the western South Tasman Rise and Antarctica from the Turonian to the Middle Eocene, whereas the Bight Basin was open to oceanic crust from the Campanian onwards. The eastern Otway Basin chronostratigraphic section shows two sets of geographically separate Cretaceous rift sequences: •

•

Tithonian and Lower Cretaceous half grabens occupy onshore and inshore areas. Faulting died out upwards into post-rift geometries during the Barremian to Albian. Basin fill changed from quartzose to volcanogenic fluvial clastics in the Early Barremian. In the Cenomanian, these basins were abandoned when much of eastern Australia was uplifted. A second series of Turonian to Santonian rifts underlie the continental slope. These half grabens had their greatest growth during the deposition of the Shipwreck Group but were later replaced in by Campanian to Maastrichtian grabens (Sherbrook Group).

An unusual outer basement ridge occupies the lower slope where continental crust appears to become thinned by low angle rotational faults. The age of formation of the basement ridge and rotational faulting can be dated by onlaps, which suggest that crustal thinning occurred in the Turonian to Santonian. Near the end of the Cretaceous, graben morphology was replaced by progradation from the Australian margin (Wangerrip Group). The progradational pattern was repeated in the Middle Eocene to Miocene (Nirranda and Heytesbury Groups) but this time in a more marine setting, reflecting the first appearance of oceanic crust. The Ceduna Subbasin had a different history:• •

• •

Initial half grabens formed in the Callovian-Kimmeridgian to Barremian. The fill was much finer grained, with thick lacustrine shales. Volcanic detritus only briefly entered the basin in the Barremian. There was a change to thermal sag geometries in the Late Aptian. A significant marine transgression in the Albian was followed by the deposition of thick marine shale. A major delta became established in the Cenomanian, probably driven by drainage changes in the eastern Australian highlands and perhaps debouching through the Polda Trough. This delta prograded so rapidly that listric normal faulting occurred, with decollement in the Albian shales causing toe-thrusting in the lower continental slope. Oceanic crust began forming at c. 80 Ma (end-Santonian) but the only stratigraphic signal in the Ceduna delta was a third order sequence boundary. A dramatic stratigraphic event occurred at approximately the end of the Cretaceous, when the Ceduna delta, listric faulting and toe-thrusting abruptly stopped and was replaced by a ?Paleocene to Middle Eocene turbidite prism, located to the east, adjacent to the Duntroon Subbasin. This sedimentary change may have been caused by new end-Cretaceous breakup events along the Diamantina Fracture Zone or by changes in hinterland drainage or both.

Acknowledgment: This work was carried out as part of a Collaborative Research Project with AGSO.

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THE SEPARATION OF AUSTRALIA FROM ANTARCTICA AND ITS STRATIGRAPHIC SIGNAL IN SOUTHERN AUSTRALIAN BASINS Martin S. Norvick and Mark A. Smith School of Earth Sciences, La Trobe University The rifting history of the southern margins of Australia can be divided into at least three phases, reflecting changing stress regimes and distinct spreading events. Stratigraphic signals fi-om rift and drift events are apparent in many of the sedimentary basins, and the strength of the signal decreases with distance fi-om the event locations. Structural variations have resulted in the Bight, Otway and Gippsland Basins having dramatically different late rift-early drift characteristics. The first rifting phase began in the Callovian-Oxfordian and affected the Recherche, Bremer and Eyre Subbasins. These structures represent the right hand branch of a triple-junction rift complex, the other two arms being the Perth Basin and the India-Antarctica margin. In the Tithonian, rifting extended eastwards into the Duntroon, Otway and Gippsland Basins. By the Valanginian, first ocean crust had formed between India and Antarctica-Western Australia. Structural style in the Recherche and Eyre Subbasins changed to thermal subsidence. However, non-marine fluvial and lacustrine sedimentation continued in active half grabens in the Duntroon, Otway and Gippsland Basins, at least until the Barremian when these basins also changed to thermal subsidence. During the early Barremian, massive amounts of dacitic volcanoclastic debris began to flood the eastern basins and indeed much of eastern Australia, presumably fi-om volcanic centres in the Lord Howe Rise region. This style of volcanogenic fluvial sedimentation continued through the Aptian and Albian. The second phase began during the Cenomanian when there was widespread uplift and denudation in eastern Australia and the Trans-Antarctic Mountains, probably caused by cessation of subduction to the east. A complete reorganisation of stress directions and a divergence of individual basin development accompanied this event. Around Tasmania, sedimentation recommenced in the Turonian in a new set of rift basins that were arranged in an approximate X-shape. The Otway, Sorell and Great South Basin in New Zealand formed in a transtensional strike-slip regime and may have received part of their sediment input fi-om Antarctica. The rift basins underlying the Ross Sea and Lord Howe Rise have not been dated but may represent the other two branches of the X. Uplift of the eastern Australian highlands and the Eromanga Basin resulted in the initiation of a major river system that drained into the Ceduna Subbasin where a large delta developed in the Late Albian-Cenomanian. Rapid sedimentation rates caused syn-sedimentary listric faulting and toe-thrusting riding on Albian marine shale. The delta system abruptly stopped near the end of the Cretaceous. New spreading events along the Diamantina Fracture Zone and disruption of the onshore river system may have been responsible. During the Middle and Late Santonian, first ocean crust was generated around southern Australia, probably beginning in the southern Tasman Sea at about 85 Ma. As a result of slow extension, there was extreme attenuation of continental crust in the Bight and Otway Basins accompanied by subsidence into deeper water. New ocean crust probably started to form south of the Bight Basin by the end of the Santonian (c. 80 Ma) and also began to extend up the eastern coast of Australia and between the Campbell Plateau and Antarctica. Extension between Australia and Antarctica was so slow that not all continental connection was lost. Marine to deltaic sediments in the outer Otway and Sorell Basins (eg Sherbrook Group) continued to accumulate in a rift setting until near the end of the Cretaceous. Sedimentation styles then changed to more locally derived marine progradational wedges (eg Wangerrip Group). Further fi-agmentation of proto-New Zealand caused new rift basin complexes to form in the Late Campanian (Pakawau) and Eocene (Challenger rifts). The third stage in stratigraphic development was caused by Middle Eocene changes to fast spreading in the southern ocean, the final separation of Australia and Antarctica, and cessation of spreading in the Tasman Sea. These events caused a collapse of most of the outer continental margins into deepwater environments and widespread marine transgression. Acknowledgment: This work was carried out as part of a Collaborative Research Project with AGSO.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SUMMARY OF INITIAL RESULTS FROM ODP LEG 188 - HISTORY OF EAST ANTARCTIC GLACIATION FROM DRILLING IN PRYDZ BAY P.E. O ' B r i e n \ A.K. Cooper^ and Leg 188 Shipboard Scientific Party ' Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia ^ Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 USA.

In January and February 2000, Ocean Drilling Program (ODP) Leg 188 drilled three holes across the Antarctic continental margin to investigate the onset, buildup and fluctuations of the East Antarctic Ice Sheet. Prydz Bay was chosen because it is the downstream end of a major, long-lived ice drainage system that originates in the continent interior: the Lambert Glacier-Amery Ice Shelf system. Site 1166 on the continental shelf penetrated 381 m of pre-glacial, early glacial and glacial rocks Early Cretaceous(?) to Holocene age, with many disconformities. The oldest rocks, which are as yet undated claystone and sandstone, contain carbonaceous material and wood indicative of a braided alluvial to deltaic environment with vegetation. A disconformity separates these rocks from overlying, transgressive sandstone and mudstone and poorly sorted, shallow marine mudstones with lonestones that contain late Eocene to Early Oligocene diatoms. These deposits are glacimarine and are similar to part of the section drilled in ODP Site 742, 40 km away. A major unconformity, recovered in the core, separates these marine strata from diamicts of late Pliocene age. The diamicts are mostly subglacial deposits, and have an interbedded unit of diatom-bearing silts of likely interglacial origin. Site 1167, in 1649 m water depth, penetrated the Prydz Channel trough mouth fan, which lies seaward of the point where the Lambert Glacier reached the shelf edge. The hole sampled a 448-m thick section of Pleistocene debrites with minor interbedded, laminated mudstones and sandstones. The debrites probably mark times of glacier expansion to the shelf edge, and mudstones, phases of ice retreat. Several large-scale (up to hundreds of meters thick) cycles of systematic variation in magnetic-mineral content and other properties are observed, but are not yet fully understood, as this is the first observation of such cycles in a glacial trough mouth fan. They likely relate to cyclic changes in the icedrainage basin that feed the Lambert Glacier that built the fan. Site 1165 on the continental rise (Wild Drift) penetrated a mostly-continuous 999 m section of Early Miocene-Holocene age hemipelagic and contourite deposits. These deposits document changes in Antarctic palaeoenvironments, as follows: • Early Miocene: likely temperate glaciers with fluvial systems and large sediment supply to the rise; • Middle Miocene: more polar conditions with glacial erosion of the shelf, different clay types, increased Ice Rafted Detritus (IRD) and reduced sediment supply to the rise; and • Post-Middle Miocene: polar conditions similar to today with IRD and little sediment reaching the rise at this site. Cyclic sedimentation between terrigenous-dominated and biogenic-rich intervals is observed throughout the hole, and appears, where most evident in the upper 300 mbsf, to have Milankovitch periodicities that imply orbital forcing of the sedimentation processes.

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THE NATURE AND TECTONIC EVOLUTION OF THE PORT MACQUARIE COMPLEX, NORTHEAST NEW SOUTH WALES David Och\ Evan C Leitch', Graziella Caprarelli' and Teruo Watanabe^ 'Environmental Sciences, University of Technology, Sydney, NSW 2007 ^Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan, 060 The informally named Port Macquarie complex is exposed along the coastline for some 5 km south of Port Macquarie in northeast NSW. It is a component of the Cambrian - Triassic New England Fold Belt and has long attracted attention because of the diversity of rock types present. Serpentinite bodies include both massive and foliated phases with the latter resulting from the propagation of an anastomosing foliation through the former and leading to the formation of serpentinite melange or, where pervasive, serpentinite schist. Overprinting shear fabrics are widespread. As well as blocks of massive serpentinite phacoids of rodingite, orthopyroxenite and broken formation up to several metres in longest dimension occur within the serpentinite melange. Blueschists, retrogressed and metasomatically altered eclogite, omphacitite, and tremolite marble occur as phacoids in a 70 m wide melange zone. The melange, which has a chlorite-actinolite schist matrix, is bounded by highly sheared serpentinite. Some blueschist blocks are highly foliated with a dominant mylonitic fabric but in others irregular vein-like masses of phengite and glaucophane crystallised under near static conditions at about 465 Ma. Blueschist metamorphism may have both preceded and followed the formation of eclogite. Outcrops of pillowed and massive basalt, chert, and thin-bedded siltstone and quartz-poor volcaniclastic sandstone occur widely. In places these rocks are highly disrupted and comprise broken formation whereas elsewhere their original interstratified nature is clear. Several chert-dominated masses are continuous with chert horizons mapped for several kilometres south of the coast that have yielded rare middle Palaeozoic conodonts. The stratified rocks contain prehnite-pumpellyite facies metamorphic assemblages. Gabbroic bodies occur within the serpentinite melange and have intruded chert - basalt masses. The gabbros are extensively altered and cut by prominent shears. They range in composition and in some places retain a weak magmatic layering. Dykes of basalt, dolerite and lamprophyre occur widely. They range from unaltered and little deformed to highly altered and disrupted by discrete shear-zones. Numerous dykes occur in stratified rocks, including broken formation but have also been emplaced in gabbro and in serpentinite. Lamprophyre dykes are little deformed or altered and are possibly members of the Late Permian (c.250 Ma) lamprophyre suite widespread in northeast NSW. Variably altered earlier Permian dolerite dykes occur SW of Port Macquarie and many of the mafic dykes may belong to the same assemblage. Rare silicic dykes are minor manifestations of the I-type igneous activity widespread throughout the New England Fold Belt in Permian - Triassic time. The Port Macquarie complex indicates both Early Palaeozoic (or older) and Devonian-Carboniferous plate convergence by the presence of high pressure metamorphics and broken formation of deep marine heritage respectively. The former were exhumed from near mantle depths whereas the latter is typical of the shallow sections of accretionary subduction complexes. The basalt lavas and gabbros are either products of ridge magmatism or within plate activity, with the latter favoured by the presence of interbedded chert and basalt, and the pre-accretion invasion of these rocks by gabbro. Ordovician exhumation of the high pressure rocks may have been closely associated with the initial detachment of the ultramafics from their mantle source in an extensional forearc. Subsequent movement of the - by now serpentinised - ultramafics into the accretionary complex is indicated by the occurrence of broken formation phacoids within the serpentinite melange. This and the subsequent injection of dolerite and basalt dykes probably occurred at the onset of Early Permian extension which is widely recognised in the New England Fold Belt. Lamprophyres and silicic dykes were injected later by which time the fold belt was the site of extensive magmatic arc activity.

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CHANGE IN TECTONIC SETTING OF VOLCANICS IN THE NEW ENGLAND FOLD BELT DURING THE LATE SILURIAN TO LATE DEVONIAN. Robin Offler^ and John Gamble^ ^ Department of Geology, University of Newcastle, NSW, Australia ^ Department of Geology, Victoria University of Wellington, New Zealand Volcanic rocks in the Gamilaroi terrane and the Calliope Volcanic Assemblage in the New England Fold Belt (NEFB) are thought to have formed a once continuous intra oceanic arc east of the Gondwana margin. Data obtained in this study, are presented which show that the setting changes in space and with time. The Late Silurian to Middle Devonian meta-basalts in the Gamilaroi terrane show flat to slightly LREE depleted chondrite normalised patterns, depletion of high field strength elements (HESE), particularly Zr, Hf, Ti and Hf relative to N-MORB and high Ti/Zr and low V/Ti ratios. In addition, they have high Ni, Cr and light lithophile element (LILE) contents. These are features characteristic of intra-oceanic island arc volcanism. They are associated with low-K, less mafic volcanics, showing moderate LREE enrichment, Nb and Y contents and low Rb/Zr ratios, which supports this interpretation. The depletion of HESE in the meta-basalts indicates that the magmas were derived from a refractory source in a suprasubduction zone setting. The presence of such a zone implies that the arc was associated with a BAB, the location of which was probably to

the west where a wide back arc region is believed to have been present from the Middle Silurian (Glen et aL\99%). This spatial relationship of arc and back arc basin (BAB) implies that the subduction zone dipped to the west. In contrast to their older counterparts, the Middle-Late Devonian basalts cropping out in the most of the Gamilaroi terrane, exhibit no HESE depletion relative to MORB, have MORB-like chondrite normalised patterns and higher Ti and lower LILE contents. In particular, they have Ti/Zr ratios and MORB-like V/Ti ratios, features characteristic of back arc basins (BAB). Dolerites intruding the sequences in the Gamilaroi terrane also have BAB signatures. Such features suggest that the magmas have been emplaced in an extensional environment produced during the rifting of the intra-oceanic island arc. The progressive increase in the V/Ti ratios and Ti02 and Ee203 contents at constant MgO, of the meta-basalts, towards the NNW part of the terrane, indicates greater extension occurred in this part of the arc. The rift setting suggested by the composition of most meta-basalts in the Gamilaroi terrane, is in contrast with those in the eastern part of the terrane. The latter have moderately high Ti/Zr and low V/Ti ratios and in some samples, exhibit depletion of HESE. These are features of basalts with compositions transitional between those of island arc and BAB. The Lower to Middle Devonian mafic rocks in the Calliope Volcanic Assemblage of the northern NEFB, are in some respects chemically different to their southern counterparts, in that they show both LREE enriched and depleted chondrite normalised patterns. Further, the majority have high Ti/Zr ratios and low Zr contents as well as relatively high Th,Ta and LILE contents relative to MORB. These features are common to rocks of Middle Devonian age as well as those of Lower Devonian age and are suggestive of an arc setting existing at this time. The high within plate component reflected by the Ta contents in many of the samples is evidence for continental crust flooring this arc. These observations show that the tectonic setting for the volcanics in the Gamilaroi terrane and Calliope Volcanic Assemblage, varies both in time and space. Further studies of rocks of similar age, in other parts of the NEFB are necessary to ascertain whether the settings recognised in this study can be found elsewhere. References GLEN, R.A., WALSHE, J.L., BARRON, L.M., & WATKINS, J.J. 1998. Ordovician convergent-margin volcanism and tectonism in the Lachlan sector of east Gondwana. Geology, 26:751-754.

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MAFIC DYKES IN ANTARCTICA AND SOUTHERN EYRE PENINSUILA R.L Oliver' and C.M. Fanning^ ' Department of Geology and Geophysics, The University of Adelaide, South Australia ^ Research School of Earth Sciences, Australian National University, Canberra

Rock samples from coastal outcrops east, and inland nunataks southeast, of Commonwealth Bay, Antarctica (ECB) include mafic intrusive dykes (and flows?). A range of metamorphic conditions from initial -800 C and 6-7 kb to retrograde 500 C and 5 kb is manifested by hornblende and garnet after pyroxenes in these rocks. Such conditions are restricted to the N-S fault-bounded ECB segment of the Adelie Land-George V Land coastline, viz a fault "sliver" which can be matched to some extent with metamorphic mineralogies and chemistries in similar lithologies within a similar N-S fault-bounded segment (viz Cape Camot) at the southern end of Eyre Peninsula. From SHRIMP analyses of U-Pb in zircons contained within associated metapelite and felsic gneisses from ECB, initial metamorphic crystallisation is thought to have taken place at -2500 Ma and retrograde metamorphism at 17-1800 Ma. Gneisses at Cape Camot have an age range of 24-26 Ma.(Drexel et al, 1993, Table 3.1). Whether all the ECB mafic lithologies correspond to those at Cape Camot is uncertain. The possibility that some, at least, of the ECB mafic rocks include dykes of earlier (16-1800 Ma) emplacement, and thus do not corelate with those at Cape Camot, must be considered (see below). Average mg value of 0.548 for the ECB mafics compares with 0.492 for those at Cape Camot. A normalised incompatibility element abundance pattem for the ECB mafics shows relatively similar element variability for the majority of samples, characterised by a negative strontium anomaly, possibly due to plagioclase fractionation. The enriched composition of alkali basalt (AAE 981) is distinct. A similar strontium anomaly is apparent in the pattem for the Cape Camot (Eyre Peninsula) mafics. A uniquely high thorium value for all five Cape Camot samples may reflect the recognised mobility of this element during magmatic as well as metamorphic processes. From Sm-Nd studies, model ages of ECB mafics range from 3070 Ma to 2140 Ma. Sedimentary components of the paragneiss at Cape Camot are interpreted as having sources in the range 3000-2700 Ma.(Drexel et al, 1993, p. 36). From an imperfect 143/144 Nd- 147Sm/144Nd isochron of the ECB mafics, a possible crystallisation age is 1800 Ma. Reference DREXEL J. F., PREISS W. V. & PARKER A. J. 1993. The Geology of South Australia, Volume 1, The Precambrian.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney, July 2000

PRODUCTIVITY IN THE AUSTRALASIAN MARITIME REGION; ARE THERE DIRECT LINKS BETWEEN MILANKOVITCH FORCING AND MARINE PRODUCTIVITY IN AUSTRALASIA Bradley N. Opdyke and Anne Muller The Australian National University, Department of Geology

The atmospheric CO2 of the Last Glacial Maximum (LGM) was approximately 30% less than the preanthopogenic value of 280 ppmv. Numerous hypotheses have been proposed to explain this shift in atmospheric CO2 levels. Most scientists working in this field would agree that this change has its origins in the marine realm, though the mechanism is hotly debated. It is also agreed that the mechanism that causes the reduction in atmospheric CO2 content has something to do with lower accumulation rates of CaCOa in the shallow and the deep sea. Two of the favoured mechanisms are the Coral Reef Hypothesis which points to reduced amounts of "reefal" carbonate during the glacials compared with interglacials and simply changing the ratio of organic carbon to carbonate carbon burial in pelagic regions. The Coral Reef Hypothesis has the advantage that it is potentially fast, with a possible sea level response, but mass balance constraints indicate this can not be the whole story. The other mechanism, we will call the "Archer Hypothesis" is feasible but has no obvious mechanism to "turn on" and "turn o f f productivity. Areas such as the Southern Ocean were obvious candidates to such variation but total productivity appears to have shifted but not changed dramatically on this time scale. On the Scott Plateau we have found very clear evidence of decreasing oceanic productivity from the LGM just preceding the climate transition to Stage 1. We would like to present a new model which links these changes in productivity with the onset and strength of the South East Asian Monsoon. There is mounting evidence that the fresh water flux into the South China Sea and the Indonesian region in general was comparatively low during the LGM resulting in higher salinities in the whole Australasian region through the equatorial eastern Indian Ocean. Today a freshwater "cap" acts to reduce any potential for vertical mixing and hence diminishing the chances of higher productivity in the area . We are proposing that this "cap" is directly related to the strength of the South East Asian monsoon and as the SEA monsoon gained strength during the deglacial, productivity in the Indonesian Archipelago and eastern Indian Ocean precipitously declined to its present state at 13700 C'"^ years. AMS C"* dates from the South China Sea and the Scott Plateau show the establishment of the SEA monsoon and the final reduction in productivity correlate perfectly. The strength of the SEA monsoon has an obvious link with the warming of the Asian continent (presumably Milankovitch driven warming). This theoretical model provides an important teleconnection between the climate of the Asian continent, oceanic productivity over a wide area of the planet, and the mysterious synchronicity of atmospheric pC02 change and Milankovitch cyclicity.

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GEODYNAMIC MODELLING OF THE CENTURY ZINC MINERALISING SYSTEM: THREE SCENARIOS Alison Ord, Bruce Hobbs, John Walshe, Yanhua Zhang, Max Brown and Chongbin Zhao AGCRC, CSIRO Exploration and Mining, PC Box 437, Nedlands, WA 6009 We use the Century Zinc deposit in Northern Queensland as a specific example for discussing the concept of Geodynamic Modelling as an exploration tool. Our goal is to establish a predictive capability that enables the explorer to predict grade, tonnage, mineralogy, alteration patterns and precise location of a potential orebody given a particular exploration model. We do this through a systems approach to ore genesis which examines the complete four-dimensional mineralising system of interest, rather than individual site specific ore deposits, and which quantifies the critical mineralising processes. This paper explores three specific scenarios proposed for the formation of the Century Zinc Deposit with the aim of defining exploration criteria for the discovery of another Century style deposit. These three scenarios are: (i) That proposed in detail within Broadbent et al. (1998) namely, that the Century deposit owes its existence to the supply of metals, along with S04^", upwards along the Termite Range Fault (TRF) with in situ thermochemical reduction of sulphate to H2S in overpressured shales at the site of metal deposition. (ii) Supply of H2S and CO2 along the TRF from a deep crustal or mantle source with metals supplied from one or more of the faults which are normal to the TRF and on the NW side of the TRF. (iii) Supply of H2S along the TRF from an upper crustal aquifer such as the Lady Loretta Formation by reduction of sulphate at higher temperatures than would be true in Scenario (i); supply of metals by faults which are normal to the TRF and on the NW side of the TRF from upper crustal aquifers which are cooler than that which supplies the H2S. This scenario is discussed in Broadbent (1998, Ph.D. thesis). We numerically model each of these scenarios using coupled fluid flow-thermal transportmechanical-chemical reaction codes. The results indicate that Scenario (i) produces low grade mineralisation with no differentiation between localisation of mineralisation either to the NE or SW side of the TRF. Scenarios (ii) and (iii) both produce a Century type of deposit to the SW side of the TRF consisting of an orebody of similar grades to the Century deposit in a time period of 1-5 million years. The resultant deposit is zoned with galena at the core passing outwards to sphalerite rich material and finally to pyrite. We complete the paper by discussing the critical exploration criteria one would adopt in the discovery of another Century type deposit including geographic position with respect to the TRF, alteration patterns, metal depletion halos, host rock permeabilities and temperature of formation as indicated by host rock mineralogy and carbon reflectance data. References

BROADBENT, G.C., MYERS, R.E. & WRIGHT, J.V. 1998. Geology and origin of shale-hosted Zn-Pb-Ag mineralisation at the Century deposit. Northwest Queensland, Australia. Economic Geology, 93, 1264-1290.

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4-D LITHOSPHERE MAPPING IN AUSTRALIA AND GLOBAL COMPARISONS Suzanne Y. C R e i l l y ' , W.L. G r i f f i n ' ^ O. G a u l ' and N. Pearson' 'Key Centre for the Geochemical Evolution and Metallogeny of Continents Department of Earth and Planetary Sciences, Macquarie University, Sydney, 2109, Australia. ^CSIRO Exploration and Mining, P C Box 136, North Ryde, N S W 1670, Australia 4-D Lithosphere Mapping (O'Reilly and Griffin, 1996) combines geophysical datasets and geological, geochemical and petrophysical information from mantle-derived rocks and minerals with tectonic syntheses to track the architecture, geochemical composition, thermal state and nature of major mantle events through time in different lithosphere domains. As mantle-derived materials are not available for large regions of Australia, wellconstrained lithosphere sections from other localities globally can provide analogues. Lithosphere section interpretations in Australia can be extended laterally using geophysical modelling (including seismic, gravity, MAGSAT). Geochemical tomography sections have been constructed using 4-D Lithosphere Mapping techniques based on garnet geothermobarometry (garnet is the most common mantle mineral and contains the most pressure/temperature information) for different tectonic domains in Australia including two transects (one southern and one northern) across the Tasman Line, and sections in the Kimberley Block and surrounding mobile belts. On the regional/global-scale, these reveal thicker lithosphere and lower geothermal gradients with increasing tectonothermal age of the lithosphere domains. For example, in the southern lithosphere transect from Jugiong in Phanerozoic eastern Australia to the Eyre Peninsula in Proterozoic South Australia, the paleogeotherm decreases westward from greater than 50 mW/m2 at Jugiong to around 40 mW/m2 in South Australia and the depth to the lithosphere/asthenosphere (LAB) boundary changes from about 100km to 180 km. Furthermore, geochemical tomography sections for timeslices in the Jurassic and Permian near to and east of the Port Augusta section of this traverse indicate lithosphere thinning from about 180 to 150 km , interpreted as due to Pangean rifting in this region. These physical changes are paralleled by higher MgO and lower CaO and AI2O3 contents for progressively older lithospheric mantle, consistent with the global secular variation in mantle composition documented by Griffin et al. (1999). Olivine is the most abundant mantle mineral in all sections and the Fe/Mg ratio of olivine is important in controlling the physical properties of lithospheric regions (density, Vp, Vs). Because olivine is rarely preserved (and contains no pressure or temperature information), we have developed a technique for inverting a gametolivine Fe-Mg exchange geothermometer to calculate the Fe/Mg of olivine coexisting with each garnet grain. Application of this inversion to the southern Australian transect show high Mg# olivine in the shallow parts of the Proterozoic westerly sections with more Fe-rich olivine in the Phanerozoic east. There also is an overall trend to lower Mg with increasing depth in each section. Olivine density varies inversely with Mg# and the combination of density and geotherm differences makes thick old (Proterozoic and Archean) lithosphere buoyant relative to asthenosphere in contrast to young (Phanerozoic) Fe-rich mantle sections, which become more dense than asthenosphere at relatively low thickness (about 100 km). Olivine Mg# also affects Vp and Vs (the higher the Mg#, the lower the density, and the higher the Vp and Vs) and thus is important in seismic interpretation, especially tomography. The increase in Fe with depth in all sections may reflect exchange with asthenospheric melts near the LAB and reinforces gravitational stability of the lithospheric mantle column. At the micron-scale, GEMOC has recently developed in-situ Re-Os dating techniques for mantle sulfides: sulfide inclusions in primary mantle silicates can provide the age of the last major melting episode, commonly the lithospheric mantle formation age. Preliminary results suggest there may be relict Proterozoic or Archean lithospheric mantle domains in northern Tasmania and the New England region, the latter coinciding with the deep cold signature for SKIPPY models in this area. 4-D Lithosphere Mapping using integrated data at global, regional, outcrop and micron scales has applications to defining important present-day Australian lithosphere domains relevant to mineral exploration and to tracking the change in lithosphere architecture and composition through the geological evolution of the Australian lithosphere. References O'REILLY, S.Y. AND GRIFFIN, W.L. 1996. 4-D lithospheric mapping: a review of the methodology with examples. Tectonophysics 262, 3-18. GRIFFIN, W.L., O'REILLY, S.Y. AND RYAN, C.G. 1999. The composition and origin of subcontinental lithospheric mantle. In: Y.Fei, C.M. Bertka and B.O. Mysen (eds.) Mantle Petrology: Field observations and high-pressure experimentation : A tribute to Francis R. (Joe) Boyd, Geochemical Society Special Publication No. 6, The Geochemical Society (Houston), pp. 13-45

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PETROGRAPHY OF LITHIFIED CAVE SEDIMENTS R.A.L. Osborne School of Professional Studies, A35, University o f Sydney, N . S . W 2006.

Petrographic study of indurated cave sediments is in its infancy. Increasing international interest in palaeokarst and "caves without roofs" has drawn attention to the need for further research in this area. Lithified cave sediments at Riversliegh and Wellington Caves contain some of Australia's most important Cainozoic vertebrate fossil deposits. Workers often incorrectly identify resistant layers in ancient cave deposits as flowstone, not realising that a range of sediments deposited in caves, can, over time, become substantially indurated and that crystalline deposits, including flowstone, are changed by diagenetic processes. A great variety of clastic and non-clastic sediments is deposited in caves, in both vadose and phreatic environments. Over time, these sediments are transformed into well-indurated rocks. Lithification and diagenesis in caves takes place under generally stable temperature and pressure conditions with highly variable water tables and abundant dissolved calcium carbonate. Vadose litihifcation and diagenesis Under vadose conditions, crystalline deposits, such as speleothem, undergo significant recrystallisation involving extensive cannibalisation of small crystals and loss of depositional texture. Primary cavities are frequently filled with druse while secondary cavities may be formed and later filled with druse. Meniscus cement is deposited in coarse clastics with stable grains such as sands, frost-wedging breccias and bone breccias. Vadose water seeping through poorly sorted entrance facies deposits will both deposit spar and flush out some of the fines. This process can resuh in pelletal and calcihe-like textures and may, if continued over a significant period, replace much of the silt and clay with secondary spar. Phreatic lithification and diagenesis In phreatic conditions, acicular cements are frequently deposited in cave clastics. These often undergo neomorphic change to blocky spar. Marine carbonate sands and muds, deposited in caves flooded by the sea, frequently resemble normal limestones. Cave muds deposited in impounded karsts and then lithified, resemble carbonate-rich siltstones and mudstones. Lithified muds deposited in holokarsts are frequently difficuh to distinguish from laminated marine mudstones.

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Geological Convention, Sydney, July 2000

THERMAL MATURATION PATTERN IN THE SOUTHERN BOWEN AND NORTHERN GUNNEDAH BASINS AND THE OVERLYING SURAT BASIN SEQUENCE, NORTHERN NEW SOUTH WALES Rushdy Othman and Colin R. Ward School of Geology, The University of New South Wales, Sydney NSW 2052 Comprehensive maximum vitrinite (telocollinite) reflectance data have been integrated with stratigraphy and other maturation indicators for the southern Bowen and northern Gunnedah Basins, and part of the overlying Surat Basin succession, focussed on the area from north of Boggabri to the New South Wales - Queensland border. More than 260 polished sections from 30 exploration and stratigraphic wells were used in the study, with samples of coal and dispersed organic matter (DOM) taken from ditch cuttings and drill cores. RockEval pyrolysis, total organic carbon (TOC) content, and biomarker charecterisation using a gas chromatograph mass spectrometer (GCMS) were also investigated for selected samples. Vertical changes in vitrinite reflectance through the Permian and Mesozoic sequence have been used, in conjunction with geophysical log characteristics and other data to produce a more consistent stratigraphic subdivision throughout the study area (Othman and Ward, 1999). The vitrinite reflectance profiles show that the coals and DOM in the Permian Back Creek Group in the southern Bowen Basin have anomalously low reflectance values relative to the overlying Permian, Triassic and Jurassic sequences. Such abnormally low or suppressed vitrinite reflectance in the coals and DOM in the area could be due to the marine influence (Mukhopadhyay, 1994). Vitrinite reflectance in the Back Creek Group also seems to have a higher rate of increase in reflectance with depth. The Permian sequence in the Bellata Trough of the northern Gunnedah Basin (Bellata-1) is incomplete. The Black Jack Group, Watermark Formation and most of the Porcupine Formation are completely removed. The Maules Creek and Goonbri Formations both show lower vitrinite reflectance than the overlying Triassic sequence, due to marine influence and liptinite abundance. Suppression of vitrinite reflectance is also recorded in the Permian sequence in the Gunnedah Basin south of the study area (Gurba and Ward, 1998). The more clearly marine sediments of the Watermark and Porcupine Formations also show suppression of vitrinite reflectance. However, the vitrinite in Bellata-1 and several other wells also displays anomalously high reflectance in parts of the sequence due to igneous intrusions. Vitrinite reflectance in Wilga Park-1, in the Bohena Trough of the northern Gunnedah Basin, for example, reaches 5.51% at a depth of 737.26 m due to localised intrusion effects. Intrusions are also intersected in boreholes close to the Moree High in the southern Bowen Basin. Apart from intrusion effects, the rate of increase in vitrinite reflectance with depth, at equivalent horizons in the southern Bowen Basin, is higher towards the west, where the Permian sequence pinches out and the Triassic sequence overlies the basement. The reflectance also increases at a higher gradient with depth close to the Gil Gil High in the east and close to the Moree High in the south. The areas between the Gil Gil High and the Goondiwindi Thrust in the east, and to the west of the Gil Gil High in the western part of the Bowen Basin, appear to have lower reflectance gradients. East-west cross sections show that, except for intrusion effects, the organic matter maturation trends mainly follow the basement profile, indicating essentially a pre-deformation coalification pattern. References GURBA L.W. & WARD C.R. 1998. Vitrinite reflectance anomalies in high-volatile bituminous coals of the Gunnedah Basin, New South Wales, Australia. International Journal of Coal Geology 36, 111-140. MUKHOPADHYAY P. K. 1994. Vitrinite reflectance as maturity parameter: petrographic and molecular characterization and its applications to basin modeling. In: Mukhopadhyay P. K. & Dow W.G. eds. Vitrinite Reflectance as a Maturity Parameter: Applications and Limitations, American Chemical Society Symposium Series, 70, 1-24. OTHMAN R. & WARD C.R. 1999. Stratigraphic correlations in the southern Bowen and northern Gunnedah Basins, Northern New South Wales. In: Diessel C., Swift E. & Francis S. eds. Proceedings of the 33rd Newcastle Symposium, "Advances in the Study of the Sydney Basin", pp. 23-30. Department of Geology, University of Newcastle.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NEW SHRIMP ZIRCON RESULTS FROM BROKEN HILL: TOWARDS ROBUST STRATIGRAPHIC AND EVENT TIMING R.W. Page^ ^ B . P J . Stevens^ and G.M. G i b s o n ' 'Australian Geological Survey Organisation, G P O Box 378, Canberra, A C T 2601 ^Geological Survey, N S W Department of Mineral Resources, 32 Sulphide St., Broken Hill, N S W , 2 8 8 0 Zircon U-Pb SHRIMP geochronology is a powerful means of elucidating geological ages, providing that it is integrated with unequivocal field constraints, and providing that the fundamental assumptions which are behind any isotopic dating methods are geologically validated. In an attempt to better quantify the timing of Broken Hill's complex history and to reduce some current uncertainties, we report initial results from a new U-Pb SHRIMP investigation. This program was planned within the background of our own disparate stratigraphic and structural approaches to Broken Hill geology, and with objectives to (a) benchmark our new age results with those of previous workers as well as our own previous work in the Broken Hill Group, (b) evaluate and test the evidence for reported Archaean basement terrain, (c) date stratigraphic units in the upper parts of the Willyama Supergroup, (d) better constrain the timing of deformational events. Our U-Pb SHRIMP work on zircons from layered paragneisses in the Redan Geophysical Zone near Farmcote was catalysed by Nutman & Ehlers' (1998a) preferred interpretation that these "trondhjemitic" gneisses represent an original - 2 6 5 0 Ma protolith. Our work finds zircon provenance age signatures typical of almost all ca. 1700 Ma metasediments, whether in the Broken Hill Block or other Australian Palaeoproterozoic settings. This therefore suggests that the rocks are not Archaean basement, but are part of a Thackaringa Group package possibly deposited about 1705-1710 Ma ago. New SHRIMP work on the Alma Gneiss provides a magmatic age of 1704±3 Ma, and a minimum stratigraphic age for host Thackaringa Group. This result is within error of our ages for other granitoids (1703±3 Ma, 1704±3 Ma) in the same stratigraphic position near Farmcote. As the Thackaringa Group is no more than 1000-1500 metres thick and includes 1710-1700 Ma detrital zircons, part of the Alma Gneiss intrusion may well have been shallowly intruded, and akin to -1700-1715 Ma felsic volcaniclastic and intrusive rocks in the Olary region. Potosi Gneiss lithotypes in the Pamell Formation (middle Broken Hill Group) and Hores Gneiss (upper Broken Hill Group) were interpreted as felsic volcaniclastic rocks with depositional ages of 1693±5 and 1689±5 Ma (Page & Laing, 1992). We have now substantiated the Hores Gneiss age (1686±3 Ma) which, together with a new depositional age of 1690-1695 Ma for tuffaceous metasediments in the lower Broken Hill Group, support the conclusion that the Broken Hill Group (excluding some amphibolite intrusions) is a coherent depositional package. Suggestions by Nutman & Ehlers (1998b) that the mapped Hores Gneiss comprises later intrusions (from 1690 Ma to 1640-1660 Ma old) and underwent high-grade metamorphism at 1640-1660 Ma are at variance with our data and the conclusions we draw here. Our age for the Rasp Ridge Gneiss (1682±3 Ma) indicates that this felsic magmatism postdates middle Broken Hill Group, and was possibly coeval with Hores Gneiss. Work is in progress on Sundown Group rocks to determine maximum depositional ages and provenance using immature psammopelitic lithologies. Some sediments contain a small component of detrital zircons as young as ca. 1690-1700 Ma, but major sediment input is from terrains ca. 1790 Ma and 1820-1860 Ma old, as well as older Palaeoproterozoic and late Archaean sources. Tuffaceous siltstones in the middle Paragon Group, both in northern Broken Hill Block and Euriowie Inlier, record similar detrital zircon ages to the Sundown Group, but in addition they include pristine, unabraded zircons which date deposition at no older than 1656±5 Ma. Similar lithologies in the Dalnit Bore Metasediments (upper Paragon Group) provide a maximum depositional age of 1642±5 Ma. Progress towards dating deformational events has been focussed on obtaining zircon ages for granitoid intrusions which bracket the deformations. This enables us to possibly constrain the D2 event between 1597±3 Ma (Pumamoota road Lf gneiss) and 1596±3 Ma (Cusin Creek pluton). The D3 event can be no younger than a cross-cutting sheet of p o s ^ D j Mundi-Mundi granite (1591 ±5 Ma) whilst the pre- or syn-Ds Cusin Creek pluton provides an older age limit (1596±3 Ma). Zircon rim overgrowths have ages at - 1 6 0 0 Ma, confirming approximate synchroneity of high-grade metamorphic event(s) with deformations that were apparently closely spaced in time. Acknowledgement. Published by permission: Director-General, N S W D M R and Chief Executive Officer, AGSO. References NUTMAN A.P. & EHLERS K. 1998a. Archaean crust near Broken Hill? Aust. Journal of Earth Sciences, 45, 687694. NUTMAN A.P. & EHLERS K. 1998b. Evidence for multiple Palaeoproterozoic thermal events and magmatism adjacent to the Broken Hill Pb-Zn-Ag orebody, Australia. Precambrian Research 90, 203-238. P A G E R.W. & L A I N G W.P. 1992. Felsic metavolcanic rocks related to the Broken Hill Pb-Zn-Ag orebody, Australia. Economic Geology 87, 2138-2168.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DETAILED GEOLOGICAL MAPPING OF BEDROCK USING SEISMIC REFRACTION - NEW AND SIMPLE PROCESSING FOR AN OLD METHOD Derecke Palmer SchoolofGeology, University of New South Wales, Sydney NSW 2052 In the last two decades, there have been two major advances in the geophysical methods applied to petroleum and mineral exploration. The first has been the dramatic increase in spatial sampling with the three dimensional seismic reflection methods which are now the standard for petroleum exploration and development, and with detailed airborne magnetic and radiometric surveys are an integral part of regional mapping and mineral exploration. The second has been the extraction of greater information from these sets of spatial data with special seismic interpretation software and with image processing. Initially, these high resolution methods were considered appropriate only for the final stages of exploration programs. However, with continuing improvements in the efficiencies of data acquisition and processing, the benefits readily justify any increases in costs. Unfortunately, similar advances have not occurred with the application of seismic refraction methods to geotechnical investigations. Where refraction methods were once considered to be two decades behind reflection methods, the difference is now nearer half a century. Current accepted methodology is inefficient, excessively destructive to the environment, can provide incorrect rather than incomplete results, extracts only a fraction of the information available, and does not facilitate the input of geological expertise at early stages of the interpretation cycle. This paper demonstrates the improved depth resolution achievable with 3D refraction methods using commonly available field systems (see figure below), and the greater detail in the assessment of seismic velocity and rock fabric which is obtained with attribute processing. The results are ideally suited for incorporation into geographic information systems (GIS) and thereby facilitate greater synergy with other sets of geological data.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AREA SELECTION IN THE GAWLER AND CURNAMONA CRATONS A. John Parker Geosurveys Australia Pty Ltd, 334 Glen Osmond Road, Myrtle Bank SA 5064 Modem mineral exploration is based on conceptual models and identification of areas favourable for such mineralisation. At an entry level, traditional exploration has focused on provinces containing major mining operations, similar perceived geology (lithology, stratigraphy) to world class mineral provinces and/or regions along major crustal-scale tectonic lineaments. South Australian (SA) examples are the Willyama Inliers (Broken Hill style base metals), Adelaide Geosyncline (Zambian copper belt style mineralisation), Gawler Craton (Lake Superior or Middleback Range style iron ore) and the "G2 Corridor" (Olympic Dam). Province selection has not changed significantly but our knowledge of conceptual models is much broader and our knowledge of regional geology has been greatly enhanced by detailed mapping, geochronology, geochemistry (including isotope geochemistry) and geophysics. Consequently the focus of exploration and area selection in certain regions has changed dramatically. For example, exploration in the Willyama Inliers has shifted from mainly base metals to include Ernest Henry style copper-gold, the extent of the Willyama Complex (Cumamona Craton) has been enlarged from the Broken Hill-Olary region to Mount Painter based on geophysics, the Gawler Craton has been confirmed by detailed geochronology as a major late Archaean province and granites of the Gawler and Cumamona cratons have been geochemically classified into suites which may or may not be favourable for mineralisation. Luck may play some part, but successful mineral exploration within identified provinces hinges on careful area selection. Modem geological mapping combined with detailed interpretation of aeromagnetic data remain the principal area selection tools in SA. GIS analysis of all available geological and mineral exploration data is becoming increasingly important but, as yet, digital mineral deposit data and surface geochemical data are not readily available from traditional Govemment sources. In the late 1970's and early 1980's following discovery of Olympic Dam, area selection in SA focussed on mdimentary identification of major aeromagnetic anomalies, particularly those with coincident gravity anomalies. Since then we have seen a rapid increase in geophysical processing technology which when combined with modem low-level aeromagnetic surveys, has led to an explosion of very high quality image maps and greatly enhanced solid geology interpretation. The modem geological map is no longer a simple outcrop geology map but is a complex digital GIS including detailed integrated solid geology interpretation(s) of aeromagnetic, radiometric, gravity and Landsat etc data verified by outcrop geology and a comprehensive database of drillhole information including geochemistry. Lithological interpretation is often relatively restricted and subjective but aeromagnetic data is particularly good in identifying stmcture and areas of magnetite alteration. Integrated interpretation is therefore an excellent area selection tool for conceptual targets including Emest Henry-Olympic Dam style mineralisation in both the Gawler and Cumamona cratons. It has also been important in identifying strand lines in heavy mineral sand exploration (Murray Basin) and in identification of the gold-bearing Yarlbrinda Shear Zone and related stmctures (Gawler Craton). Over the last five years, near-surface calcrete geochemistry has played an important role in identifying potential gold mineralisation beneath relatively thin cover (eg Challenger, Gawler Craton). Delineation of areas where calcrete can be reliably used is based on existing geological maps combined with regional aerial photo and Landsat interpretation and, in particular, the distribution of Westem Myall vegetation. It enables an additional explorability parameter to be included in the area selection process Airbome multispectral scanning (HyMap) is a recent development which has significant future potential for area selection. It is an airbome equivalent of Landsat but collects data for 126 bands across the reflective solar wavelength region of 450-2500 nm. Only limited surveys have been flown to date but as part of the SA Govemment TEISA program, small regional-style surveys are being flown. Existing data over Mount Fitton talc and Witchelina magnesite deposits have given good results. Regional airbome EM surveys also have some potential for assisting area selection but in SA are restrained by interference from saline groundwater in the regolith. Data can be potentially imaged and interpreted in much the same way as aeromagnetics to map geology and stmcture and recent developments in acquisition and processing of ground EM are certainly significant for future application.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LEAD ISOTOPE CHARACTERISTICS OF VOLCANIC ROCKS AND HYDROTHERMAL SULFIDE DEPOSITS FROM THE EASTERN MANUS BASIN AND FRANKLIN SEAMOUNT, PAPUA NEW GUINEA: EVIDENCE FOR THE PACIFIC - INDIAN MANTLE BOUNDARY Joanna M. Parr, Raymond A. Binns, Christopher J. Yeats, Graham C. Carr and Barbara L. Gardner CSIRO Exploration and Mining, PO Box 136, North Ryde 1670 Submarine hydrothermal activity in the Eastern Manus Basin (EMB) is hosted by a suite of cosanguinous andesitic and dacitic to ryhodacitic lavas and associated volcaniclastic deposits in a back-arc environment to the north of the New Britain island arc. The igneous activity is related to northward subduction of the Solomon Sea Plate at the New Britain Trench. Seafloor hydrothermal activity has been observed at three major fields: PACMANUS, DESMOS and SuSu Knolls. Here we present Pb isotope data for volcanic rocks from the EMB, including the host rocks to these hydrothermal fields, for sulfides from PACMANUS and SuSu Knolls, and for barite chimneys from Franklin Seamount in the Woodlark Basin south of New Britain. Lead isotope data for volcanic rocks and hydrothermal sulfide deposits from the EMB plot at the radiogenic end of a spectrum of data for hydrothermal centres in the SW Pacific (EMB data range: ^^^Pb/'^^Pb 18.66 18.78; 15.51 - 15.56; 38.25 - 38.44, n = 35). The volcanic rocks have similar isotopic characteristics to those from other volcanic provinces nearby, including New Britain and the Tabar-LihirTanga-Feni island chain, and cluster at the edge of the Pacific MORB field. These data suggest that the lavas in all these areas are sourced from subduction-modified Pacific mantle, in contrast to the basaltic central Manus spreading zone which has Indian mantle-like affinities. Sulfides have Pb isotope values that lie within a restricted range 18.74 - 18.78; ^ ' ^ b / ^ ' V b 15.51 - 15.54; ^^^Pb/^^'^Pb 38.29 - 38.44, n = 22) and are slightly more radiogenic than values for the volcanic rocks. Between hydrothermal fields, Pb from SuSu Knolls is slightly more radiogenic than Pb from PACMANUS, which may reflect local isotopic variations between magma chambers and/or a larger crustal component at SuSu Knolls. On a regional scale, the sulfides are slightly more radiogenic than those from the Hine Hina Field in the Lau Basin. A small number of barite chimneys from Franklin Seamount yield somewhat different Pb isotope ratios 18.54 - 18.58; ^ ' W ^ ' P b 15.56 - 15.57; 38.39 - 38.44, n = 4). These data are quite unusual compared with other hydrothermal systems in the SW Pacific although they have some affinity with sulfides from the southern Lau Basin. The boundary between the Indian and Pacific Mantle reservoirs is believed to extend beneath parts of the SW Pacific (e.g. Woodhead et al., 1997). Lead isotope ratios for the EMB suggest that the Pb was derived from the Pacific Mantle rather than the Indian Mantle, whereas Pb at Franklin Seamount is consistent with an Indian Mantle source. These data, together with data from the Central Manus Basin (Agapova et al., 1994) and New Britain (Woodhead and Johnson, 1993), suggest that the boundary between the Indian and Pacific Mantles runs to the southwest of the EMB and NE New Britain, and to the northeast of the Central Manus Basin and Franklin Seamount. Pb isotope data for EMB volcanic rocks are also comparable to those for basalts from the southern Lau Basin, but are very different from those of the northern Lau Basin. Probably this too reflects Pacific versus Indian subduction-modified mantle sources for the lavas and indicates the complexity of the boundary between these two mantle components in the region. References AGAPOVA, AA., BORTNIKOV, N.S., CHERNYSHEV, I.V., VIKENT'EV, I.V., LISITSYN, A.P., TROITSKII, V.A. & SHADLUN, T.N. 1994. The lead isotope composition of the sulphide ores in the rift zones of the Manus and Lau Basins (southwestern margin of the Pacific Ocean). Geology of Ore Deposits, 36, 184-190. WOODHEAD, J.D. & JOHNSON, R.W. 1993. Isotopic and trace element profiles across the New Britain island arc, Papua New Guinea. Contributions to Mineralogy and Petrology 113, 479-491. WOODHEAD, J., EGGINS, S. & JOHNSON, W. 1997. Chemical dynamics in the New Britain volcanic arc. GSA Abstracts 104-107.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

INCLUSION TRAIL PATTERNS IN PORPHYROBLASTS FROM THE FOOTHILLS TERRANE, CALIFORNIA: A RECORD OF OROGENESIS OR LOCAL STRAIN HETEROGENEITY? S.R. Paterson' and R.H. Vernon^ ^Department of Earth Sciences, University of Southern California, 3651 University Avenue, Los Angeles, CA 90089-0740, USA ^ Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia

One controversial use of porphyroblast-matrix microstructural relationships is to infer tectonic histories, such as multiple orthogonal, orogenic events and/or changing plate motions, in orogens where other evidence for these events may be lacking. This raises a philosophical question of whether greater weight should be given to one or integration of multiple data sets in orogenic studies. We examine these issues through a comparison of regional relationships and local structures formed in and adjacent to porphyroblasts formed in contact aureoles in the Foothills Terrane, Sierra Nevada, California. Particularly informative in this study are regions where dramatic transitions in cleavage intensity and strain occur over along strike distances of < 5 km. When similar rock-types are compared in these transitions, we see no evidence of systematic changes in: (a) cleavage or strain ellipsoid orientations, (b) the relative intensity of cleavage versus lineation, and (c) the shape of calculated strain ellipsoids. A comparison of the regional data sets, the strain transitions, and local studies including porphyroblast-matrix studies, support the following conclusions: (1) except in shear zones, contact aureoles, and local zones along lithologic contacts, the Foothills terrane has a single regional cleavage, although locally formed by multiple processes; (2) the regional cleavage and locally developed porphyroblast inclusion trails have variable orientations, and neither data set supports the formation of dominantly subhorizontal and sub vertical cleavages in this orogen; (3) structural and metamorphic heterogeneities occur at all scales and can markedly affect inclusion trail patterns in porphyroblasts; (4) complex porphyroblast growth features and internal inclusion trail patterns can form in porphyroblasts that grow during short time intervals in contact aureoles, indicating that local complexity in porphyroblasts does not imply regional complexity. Because of these conclusions, we suggest that multiple data sets, rather than data acquired only from porphyroblasts, should be considered when attempting to understand the evolution of orogens. Furthermore, using microstructural information preserved only in porphyroblasts to infer orogenic processes and plate motions is generally unjustified.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE CONSTRUCTION OF MAGMATIC SYSTEMS IN ARCS: MELT MIGRATION IN DISORDERED POROUS MEDIA INDEPENDENT OF REGIONAL FAULTING Scott R. Paterson\ Keegan Schmidt', Michael Potter\ Geoff Pignotta' and Robert B. Miller^ ' Department of Earth Sciences, University of Southern California, 3651 University Avenue, Los Angeles, CA 90089-0740, USA ^Department of Geology, San Jose State University, San Jose, CA 95192-0102, USA

A popular assumption for the development of crustal-scale magmatic systems in arcs is that tectonism plays a dominant role over a range of scales and crustal depths. In contrast studies of arc systems at three spatial scales, that is at the scales of (1) orogens, (2) average fault spacing, and (3) individual plutons, indicates the magmatic systems develop relatively independently of tectonism, with the possible exception being at the site of magma generation. At the scale of orogens, statistical studies have concluded that both volcanism and magmatism are spatially and temporally random (de Bremond d'Ars et al., 1995) or clustered (Pellitier 1998) and do not show geometrical patterns reflecting nearby regional structures (Lutz and Gutmann, 1995). These results preclude significant control on magmatic pathways by tectonism and instead have been reproduced by models of fluid migration in disordered porous media. The above results agree well with the weak antithetical clustering we find at the scale of average fault spacing in 5 orogens dominated by strike-slip systems and another 5 orogens dominated by thrust systems. In every case magmatic bodies do not preferentially lie along faults in surface exposures, nor are they preferentially associated with faults in reconstructed three-dimensional block diagrams. We also find numerous examples where plutons discordantly cut across both strike-slip and thrust faults, some of which were active at the time of magma emplacement. We therefore conclude that in these orogens faults do not focus magmatism during ascent or emplacement nor are they an important material transfer process. Studies at the scale of individual plutons show that most chamber boundaries are not bounded by regional faults, that host rock is largely displaced vertically during emplacement, and that rates of magma and host rock transfer are orders of magnitude greater than rates of regional tectonism. These results indicate that the role of tectonism in the development of magmatic systems should be deemphasized at most spatial scales and that processes inherent in fluid migration in disordered porous media should be examined. Tectonism may, however, play a role in generating instabilities at magma generation sites. Furthermore in the less common examples where magmatic systems are spatially and temporally associated with faults, the coupled stress fields and rheology may influence features preserved in and around the plutons. References De BREMOND D'ARS, J., JAUPART, C., AND SPARKS, R.S.J., 1995, Distribution of volcanoes in active margins: Journal of Geophysical Research, v. 100, no. BIO, p. 20421-20432. LUTZ, T.M. AND GUTMANN, J.T., 1995, An improved method for determining and characterizing alignments of pointlike features and its implicates for the Pinacate volcanic field, Sonora, Mexico. Journal of Geophysical Research, v. 100, no. B9, p. 17,659-17,670. PELLETIER, J.D., 1999, Statistical self-similarity of magmatism and volcanism: Journal of Geophysical Research, v. 104, no. B7, p. 15425-15438.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

FIGURING OUT RON'S WORLD: METAPELITES AT LOW PRESSURE David R.M. Pattison\ and Frank S. Spear^ ^ Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, T2N 1N4, ^ Department of Earth and Environmental Sciences, Rennsselaer Polytechnic Institute, Troy NY, 12180,

A significant portion of Ron Vernon's incisive petrological work has been on low pressure (Andalusite-Sillimanite facies) metapelitic rocks. Despite the recent trend towards use of multi-equilibrium/species thermobarometric approaches to estimate peak P-T conditions and P-T-time paths of crustal metamorphism, in low pressure metapelites, petrogenetic grids are the primary tool for determining P-T(-t) information because significant mineral assemblage changes occur over P-T intervals (< ca. 50 < ca. 0.5 kbar) than are smaller than the precision of thermobarometry. Unfortunately, a priori calculation of low pressure metapelitic phase equilibria using recent versions of the most popular thermodynamic data sets has produced results that are in conflict with each other and with repeated occurrences of natural assemblages and their compositional relationships. The problem is that subtle changes to either or both of the end member and mixing data within their uncertainties can completely change the phase equilibria. Our approach to solve this dilemma is to modify one thermodynamic data set (Spear & Cheney, CMP, 1989, itself largely based on Herman, J Pet, 1988) as much as possible within its uncertainties, such that the resulting calculated phase equilibria satisfy the constraints from repeated occurrences of natural assemblages and their compositional and textural relationships (eg, Pattison & Tracy 1991, MSA Rev Mineral 26). This approach relies on a large and high quality natural data base, which we have culled from our own research and from the literature. Some of the main difficulties we have encountered in the modelling, and in reconciling the thermodynamic models with the natural assemblages, include: 1. Dealing With seemingly irreconcilable experimental data on the And=Sil equilibrium with that on cordierite equilibria 2. Deciding on mixing models for phases like the micas and chlorite that handle the major coupled exchanges in as simple manner as possible 3. Reconciling predicted Fe-Mg partitioning involving cordierite, chlorite, biotite, staurolite and garnet with the natural constraints Despite the above difficulties, the current version of our grid is in qualitative agreement with repeated natural mineral assemblage sequences in our data base. Some of the more significant features of the new grid that bear on the interpretation of low pressure metapelites will be discussed.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MULTISTAGE MAGMA MIXING: EVIDENCE FROM AN ARCHAEAN COMPOSITE LACCOLITH Mark Pawley and W.J. Collins Geology Department, University of Newcastle, Callaghan, NSW, 2308 The ca. 3.24Ga Strelley Granite is a composite laccolith in the eastern Pilbara craton of Western Australia, which was tilted during later deformation to expose a cross-section through the laccolith. The granite comprises two main phases: an outer, equigranular granite; and an inner, feldspar phenocrystic granite, intruded by a coeval mafic and hybrid intermediate magmas. Magma mixing occurred in the inner phase of the Strelley Granite, resulting in three main zones: 1) an eastern K-feldspar phenocrystic granite representing a crystal-rich felsic magma chamber; 2) a central zone of hybrid intermediate rocks, representing mixed granitic and basaltic magmas (Brauhart 1999); and 3) a western doleritic lobe. The hybrid and mafic magmas were emplaced as crystal-poor magmas into the base of the crystal-rich felsic magma chamber. Contact relations between the various magmas provide an insight into the physical mechanisms of mixing. The hybrid magmas formed during an early stage of mixing, then dyked into the overlying felsic magma (zone 1), where they were undercooled and broken into viscous metre-scale enclaves. Disaggregation of the enclaves resulted in a second stage of mixing, as mantled feldspars, ocellular quartz and enclaves were dispersed into the felsic magma. In contrast, a set of sinuous, <metre-scale, medium-grained felsic dykes radiate out fi-om zone 2. The felsic magmas ponded at a certain level and spread laterally, mixing with the crystal-rich felsic magma. Further evidence for mixing and mingling occurs at the contact between the dolerite lobe (zone 3) and the hybrid zone (zone 2). The contact is mingled on the large-scale with the felsic and hybrid magmas forming irregular entrained blobs that backvein the dolerite, indicating they were magmas and not stoped blocks. The dolerite itself is very heterogeneous, with local compositional variation indicated by the irregular distribution of resorbed feldspars and ocellular quartz. This suggests that differing degrees of mixing occurred between the dolerite and the other magmas of the Strelley Granite inner phase. In zones 2 and 3, the physical transfer of crystals between magmas has produced a range of disequilibrium textures. K-feldspar phenocrysts derived from the felsic magma chamber (zone 1) and mixed into the hybrid (zone 2) magmas possess well developed mantles of more calcic composition (Rapikivi texture), whereas Kfeldspars incorporated into the mafic magmas have undergone dissolution to produce irregular, often skeletal forms. Quartz phenocrysts that have mixed into the dolerite and hybrid magmas are ocellular, with rimming hornblende grains that are often overgrown by epitaxial quartz. Quartz phenocrysts in the dolerite are also embayed, suggesting that resorption occurred prior to ocelli development. Several stages of magma mixing can be recognised in the inner phase of the Strelley Granite. The earliest stage of mixing produced a hybrid intermediate magma and most likely occurred prior to its emplacement into the Strelley laccolith. This is suggested by thermomechanical modelling which indicates that mixing of liquids with different compositions, and therefore contrasting viscosities and densities, is unlikely to occur within magma chambers (Koyaguchi & Blake 1991). In contrast, more efficient mixing will occur in conduits where laminar, rather then turbulent flow, drives the mixing of coeval magmas (Blake & Campbell 1986). The second stage of mixing occurred when the hybrid intermediate and mafic magmas were emplaced into the magma chamber represented by the Strelley Granite. The hybrid intermediate magmas formed the segmented, undercooled dykes that disaggregated and mixed with the felsic magma. The mafic magma, which was also weakly hybridised before emplacement, mingled and only weakly mixed with the other magmas. This resulted in its mingled upper contact and the irregular distribution of disequilibrium textures. References BLAKE, S. & CAMPBELL, I.H. Contributions to Mineralogy and Petrology 94, 72-81 (1986). BRAUHART, C.W. Unpublished PhD Thesis (University of Western Australia, 1999). KOYAGUCHI, T. & BLAKE, S. in Enclaves and Granite Petrology (eds. Didier, J. & Barbarin, B.) 415-429 (Elsevier, Amsterdam, 1991).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOLOGICAL CONTROLS ON TUNNEL DESIGN - A TALE OF TWO CITIES Philip Pells Pells Sullivan Meynink Pty Ltd, Suite 1 1 1 0 East Parade, Eastwood, N S W , 2122

Sydney and Melbourne are cities of similar size and both are seeking to provide modem rail and road facilities by maximising the use of underground space. In the 1970's Melbourne developed MURLA, a new underground rail loop system. In the 1980's and early 1990's Sydney replied with the Sydney Harbour Tunnel and the Opera House Car Park. In the late 1990's Melbourne commenced the City Link project, said to be the largest infrastructure project in Australia since the Snowy Scheme. And in the same period Sydney completed the Eastern Distributor and the Airport Rail Link (ARL), and commenced the M5 East tunnel project. Tunnel design and construction, probably more than any other engineering activity, is intimately tied to geology. In Sydney most of the tunnelling has been in the subhorizontally bedded Hawkesbury Sandstone, although much of the llm-diameter ARL tunnel is through the Botany Sands. Certain characteristics of the Hawkesbury Sandstone such as: • horizontal bedding and near vertical orthogonal jointing, • horizontal field stresses greater than overburden pressure, • unconfmed compressive stress typically greater than 20MPa. have been used by Rock Mechanics engineers to develope unique wide span, flat roof excavations with support only by permanent rock bolts, i.e. no passive linings. The paper expands on these matters giving examples from the Opera House Car Park and the Eastern Distributor. An important factor in developing these designs is that in most cases in Sydney, drawdown of the groundwater table has not been a design constraint. In Melbourne most of the recent rail and road tunnels have been excavated in the Silurian Melbourne Mudstone. This geological is folded and sheared, with bedding being typically very steep. It is also quite deeply weathered, closely jointed and is frequently intruded by dykes. The net result is that the rock mechanics of these Silurian rocks is much more difficult than in the Hawkesbury Sandstone. Furthermore, the paleovalley system in the Silurian rocks contains gravel aquifers overlain by one of the most compressible geological units in Australia, the Coode Island Silt. This is not silt but a normally consolidated clay deposit. The effect of this complex Melbourne geology is to make tunnelling potentially a much more difficult process than in Sydney. The paper illustrates some of these geologicalorigin constraints by reference to some significant problems experienced in recent projects.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CONSERVING GEODIVERSITY, THE IMPORTANCE OF VALUING OUR GEOLOGICAL HERITAGE Michael Pemberton Tasmanian Parks and Wildlife Service, GPO Box 44A, Hobart 7001

Most people associate nature conservation with the protection of biodiversity. Quite clearly there is a lot more to nature, and to conservation, and this is where geoconservation or the conservation of geodiversity needs to be promoted and taken more seriously. It will assist to communicate the fascinating history of the earth and our continent to the large majority of people who find deep time truly daunting. The need for nature conservation is widely accepted by biologists and other natural scientists. However nature conservation agencies and governments across the country, and overseas, tend to emphasise the need for the conservation of biodiversity whilst virtually ignoring the geological foundation on which this is built and has evolved. In part this is because of a lack of pressure on the part of earth scientists, who are not trained or experienced in conservation theory and have consequently had little input into the development of conservation strategies and policies, particularly as they relate to geoconservation. The majority of earth scientists are trained and employed in the extractive industries. To be involved in nature conservation could be seen by some to be contrary to the goals of the profession. There is broad consensus across the community that nature conservation is legitimate and important. If we can not afford to loose biodiversity there is an equally strong case to be made for conserving geodiversity. Ecosystems depend entirely on their non-living parts be they bedrock, landforms, soils or related processes. Geo features can also have their own values irrespective of their relationships with biodiversity. However there would simply be no biodiversity without geodiversity. The story of our natural diversity, the links between geodiversity and biodiversity and how it has all evolved needs to be explained to the masses. This is a powerful way of improving community understanding of the earth sciences. There are few better places to do this than in National Parks and other natural areas. The case for conserving geodiversity may, in some respects, be more important than biodiversity given that, in a lot of instances, rare or threatened species can be propagated or bred in captivity. On the contrary many geo features have formed under conditions, climatic or geological, that are now inactive. They are essentially relict or "fossil" features which, once disturbed, will never recover or will be removed forever. If this was the case for biodiversity there would be enormous concern. The emphasis on valuing our natural environment has been dominated by biological values. The links between geodiversity and biodiversity would assist people to value the non-living environment. This would facilitate a greater appreciation of natural diversity and provide a pathway for the general public to better understand the complexities and wonders of our geological history. Concentrating on communicating our attitudes, philosophies and practices to the wider community may not be enough. We need to nurture a greater respect and appreciation of the earth's evolution and its building blocks.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

STRAIN-INSENSITIVE PREFERRED ORIENTATION OF PORPHYROCLASTS IN MONT MARY MYLONITES G. Pennacchioni \ G. Di Toro' and N. Mancktelow^ ^ Dipartimento di Geologia, Paleontologia e Geofisica, Via Giotto 1, 35137 Padova, Italy ^ Geologisches Institut, ETH-Zentrum, CH-8092 Zurich, Switzerland

The shape preferred orientation (SPO) of porphyroclasts has been determined in high temperature mylonites, which possibly mark a Permian low-angle ductile normal fault in the Austroalpine Mont Mary unit of the Italian western Alps. Sillimanite, garnet and plagioclase porphyroclasts are numerous and distributed in a fine-grained (1-5 |im) foliated matrix of biotite, quartz, muscovite, ilmenite and graphite. In total, almost 2000 different porphyroclasts were measured to obtain good statistics on the relationship between shape and orientation. The porphyroclast shapes closely approach two main regular geometric forms, namely rhomboids (sillimanite) and ellipses (garnet, plagioclase, sillimanite), and exhibit aspect ratios (R) as high as 11. Particles with R>3 are dominatly rhomboidal sillimanite and show a strong SPO, with a vector mean strength > 0.95. The long axis of the best-fit ellipse is rotated antithetically with respect to the shear sense, making an angle of 5-10° to the mylonitic foliation. The long sides of rhomboidal sillimanite are preferentially inclined at an angle varying from 10 to 20° with decreasing R, and the short sides have a nearly constant orientation at around 15-18°. Low aspect ratio (mainly elliptical) objects show low intensity SPO with a concentration of the particle long axis close to the shear plane. The two SPO's appear to be strain-insensitive, since similar SPO's are found associated with a range of microstructures in different samples, indicating a different range in shear strain. However, other than that it is "large", direct quantification of shear strain is not possible. Two main mechanisms assist the development of the SPO. In the case of R<3, the SPO is due to the slower rotation velocity of an elongate object oriented close to the shear plane combined with a continuous supply, by synkinematic fracturing and grain-size refinement, of new porphyroclast populations. In the case of R>3, and in particular for rhomboidal sillimanite, a stable position is acquired through the concomitant activity of matrix flow and extensional crenulation cleavage (ECC). Dissolution along the ECC surfaces determines the rhomboidal shape of sillimanite. It is suggested that the development of stable positions in relatively low aspect ratio particles may be related to an anisotropic non-Newtonian rheology of the matrix, inducing flow localization partitioned along the main foliation and the oblique ECC surfaces. Rhomboidal particles are then both developed (by dissolution against these surfaces) and stabilized by preferential slip on the boundaries parallel to the anisotropy surfaces in the matrix.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PORT MORESBY BASEMENT GEOLOGY: A MID-CAINOZOIC ACCRETIONARY PRISM Russell C.B. Perembo, Hugh L. Davies, Edward Neinen and Joseph Agua Geology Department, University of Papua New Guinea, University P.O. NCD, Papua New Guinea. The Waigani campus of the University of Papua New Guinea is separated from the Port Moresby business district by a block of savannah-covered hills up to 270 m high, comprising NNW-trending strike ridges and occasional cross-cutting ENE-trending valleys. Student mapping exercises in this area, conducted intermittently since 1997, have shown that the strike ridges are a stacked series of NE-dipping thrust sheets, each thrust sheet 10-60 m thick. Bedding of sediments within the thrust sheets is generally parallel to the faults and dips 20-50"" NE. The thrust sheets mostly are of Paleocene and Eocene bathyal sedimentary rocks (Paleocene Bums Peak Formation, Eocene Port Moresby Beds), but some are Late Oligocene terrestrial and shallow marine partly-volcanogenic sediments (Dokuna Tuff). Areas of lower ground towards the SW side of the block of hills comprise NNW-trending shear zones that contain blocks of the Paleocene and Eocene rock units as well as Campanian Barune Sandstone and small bodies of metagabbro and serpentinite. The shear zones were previously mapped as a single, linear "Koki Fault Zone" (Rogerson, Haig and Nion: Geological Survey of PNG Report 1981/16) but we find them to be discontinuous and not co-linear. A younger set of faults that is characterised by local development of blocky fault breccias trends ENE across the regional trend. These faults laterally displace the NNW-trending strike ridges, and provide the zones of weakness along which the ENE-trending valleys have developed. The stratigraphy of the area is essentially as described by Rogerson et al., but we have added information on the main rock units. For example, new exposures of the Bums Peak Formation in road cuts permit improved description. We found that the Eocene rocks of Port Moresby, conveniently referred to as Port Moresby Beds, comprise five main rock types that range from deep ocean siliceous siltstones and chert to turbidites and upper slope debris flow deposits. The Oligocene sediments, previously mapped as entirely volcanogenic, include some non-volcanic sandstone, siltstone and calcarenite. The Paleocene and Eocene sediments and the gabbro have been subjected to low greenschist facies metamorphism, with some development of phyllosilicate minerals including chlorite in the sediments, and development of prehnite and actinolite in the gabbro. The Oligocene sediments are not obviously metamorphosed. The stacked thrust sheets that are the dominant feature of the study area may have formed as an accretionary prism. The stacking of thmst sheets clearly was active as recently as the Late Oligocene or Early to MidMiocene and may have begun in the Oligocene or earlier.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

^^Ar/^^Ar AND K/Ar WHOLE ROCK AGE CONSTRAINTS ON THE TIMING OF REGIONAL DEFORMATION, SOUTH COAST OF NEW SOUTH WALES, LACHLAN FOLD BELT, SOUTHEASTERN AUSTRALIA: PROBLEMS AND IMPLICATIONS David Phillips ^ and Christopher L. Fergusson^ ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 ^School of Geosciences, University of Wollongong, NSW 2522 Subduction complex rocks are well exposed on the south coast of New South Wales around Batemans Bay. Farther south in the Narooma and Bermagui region, Offier et aL (1998) have determined two ^^Kxl^^kx ages of 450 ± 3 Ma and 445 ± 2 Ma. They argued that these ages constrain the timing of intense underplatingrelated deformation and we have undertaken farther work, from an area south of Batemans Bay, to test this suggestion. The 40Ar/39Ar method applied to fine-grained, low temperature metamorphic rocks, such as slates, is beset by the problem of recoil loss andlor redistribution of ^^Ar during the irradiation process. Another problem is the difficulty of distinguishing between the contributions to ^^Ar^^Ar spectra from illitelmuscovite grown during the cleavage-producing deformation and detrital muscovitelillite. In the current study, four slate samples, with variable contents of detrital white mica, were analysed by both the K-Ar and "^^Ar^^Ar step-heating methods. A separate of detrital white mica from one slate sample yields a plateau age of 500 ± 2 Ma. This result indicates that inheritance has not been eliminated by metamorphism as is commonly assumed and that the "^^Ar^^Ar ages provide only a maximum estimate for the timing of deformation. "^^Ar^^Ar analyses of state chips yield discordant, saddle-shaped age spectra, with minimum ,within-saddle' ages of ca. 420 Ma. Two slate samples give identical ^^Ar/^^Ar integrated ages of 455 ± 2 Ma. One sample contains relatively abundant detrital bedding-parallel mica flakes that are locally oblique to the regional cleavage in the rock. The "^^Ar^^Ar ages are some 20 Ma older than K-Ar ages for these same samples, suggesting that recoil loss of ^^Ar may also have affected these slates. Both recoil loss of ^^Ar and inherited white micas will yield elevated apparent ages, thus providing only maximum ages for the cleavageproducing deformation. Two other samples from slaty tectonic mellange and intensely cleaved slate have been analysed and have "^^Ar^^Ar integrated ages of 422 ± 2 Ma (K/Ar: 424 ± 5 Ma) and 415 ± 2 Ma (K/Ar: 400 ± 5 Ma). The general consistency of these results accompanied by inicrostructural observation indicating a low abundance of detrital mica, show that in these samples recoil and inheritance problems appear to be less important. Thus they provide a more reliable upper constraint on the timing the regional deformation on the south coast of New South Wales, i.e. younger than ca. 420 Ma, consistent with previously recognised regional structural constraints. Elsewhere in the Lachlan Fold Belt ^^Ar^^Ar ages on fine-grained slates have been used to provide concise constraints on the timing of deformation. The current results raise serious questions about the interpretation of these ages as representing on-going deformation and therefore tectonic models derived from these data should be treated with caution. Reference OFFLER R., MILLER J. McL., GRAY D. R., FOSTER D. A. & BALE- R. 1998. Crystallinity and bo spacing of K-white micas in a Palcozoic accretionary complex, eastern Australia: metamorphism, paleogeotherms, and structural style of an underplated sequence. Journal of Geology 106,495-509.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LASER PROBE ^^AR/^^AR STEP-HEATING ANALYSES OF SINGLE CLINOPYROXENE INCLUSIONS EXTRACTED FROM JWANENG, ORAPA AND MBUJI-MAYI DIAMONDS. D. Phillips\ J.W. Harris^ and G.B. Kiviets^ ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 ^Department of Geology and Applied Geology, University of Glasgow, Glasgow, G12 8QQ, U.K. ^De Beers GeoScience Centre, P.O. Box 82232, Southdale, 2135, South Africa.

The development of micro-analytical "^^Ar/^^Ar laser probe systems has generated a host of new applications, including the analyses of single clinopyroxene inclusions in diamonds. These inclusions may be less than 300 microns in size and contain potassium levels below 0.3 wt.%. Initial attempts to determine diamond genesis ages from analyses of syn-genetic clinopyroxene inclusions proved unsuccessful and yielded variable apparent ages, intermediate between the times of host kimberlite emplacement and diamond crystallisation. This effect was attributed to partial diffusion of pre-eruption "^^Ar to the diamond/inclusion interface during mantle residence. Laser drilling to buried inclusions was considered as a possible solution to the problem of measuring genesis ages. It was also suggested that analyses of inclusions that have been totally extracted from their host diamonds should yield the age of host kimberlite eruption. The latter possibility has important implications for constraining the sources of detrital diamond populations worldwide. To test the latter contention, clinopyroxene diamond inclusions, from the Jwaneng (Botswana), Orapa (Botswana) and Mbuji Mayi (Democratic Republic of Congo) kimberlites, were analysed in the current study. Although some inclusions yielded ages within error of the time of host kimberlite intrusion, the majority yielded apparent ages significantly older than the time of host intrusion, indicating that not all pre-eruption argon resides at the diamond/inclusion interface as originally thought. In addition, older apparent ages were obtained from lower temperature steps. Furthermore, age differences between fragments from the same inclusion indicate that the argon is heterogeneously distributed. The step-heating results suggest that the pre-eruption argon is located in low retention sites and/or at grain/domain boundaries. One possible explanation for the anomalous argon distributions involves initial diffusion of pre-eruption argon to the diamond interface region in response to mantle cooling after diamond cry^stallisation. This is followed by diffusion of some interface gas back into the inclusion in response to increased argon partial pressures caused by differential expansion during the eruption process. In cases where cracks develop around the inclusion, all pre-eruption argon may be lost from the inclusion. The current study demonstrates that the interpretation of "^^Ar/^^Ar laser probe results from extracted or partially encapsulated inclusions is complex. While some inclusions may well yield reliable host kimberlite/lamproite emplacement ages, the partial retention of preeruption argon will often lead to an over-estimation of the true result. The current data also suggest minimum genesis ages of 2.2 Ga and 2.8 Ga for two Jwaneng inclusions, which are significantly older than inferred Sm-Nd diamond genesis ages obtained for the same locality. This suggests that some portion of the pre-eruption argon is extraneous, or that the Jwaneng diamonds grew over a prolonged time period.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ENIGMATIC METAL SULPHIDE DEPOSITS OF THE WHIM CREEK BELT, PILBARA CRATON, W.A: THE GEOLOGICAL CONSTRAINTS Geoff Pike and Ray Gas Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia

Polymetallic Cu-Zn-(Pb) sulphide deposits at Salt Creek, Mons Cupri and Whim Creek form an important copper resource within the Whim Creek Belt, Pilbara Craton, Western Australia. The style of mineralisation is controversial with no deposit having definitive characteristics of any particular type of deposit (eg. VHMS, epithermal, mesothermal). In addition, there is no geological requirement to apply a single emplacement style or model to all three deposits. Mons Cupri and Whim Creek are well-exposed deposits and are described in terms of setting, timing and relationship to host rocks. They have been variously interpreted both as type-examples of VHMS deposits or late-stage, epithermal deposits. Clearly, a source for mineralising fluids is a requirement for any deposit and the inferred source for both has been km-scale dacite cryptodomes. However, geochemical comparison with Canadian felsic-dominated successions, shows that these rocks were very unlikely to have sourced any such mineralisation. In addition, new stratigraphic and geochronological evidence suggests these bodies are significantly older than the volcaniclastic/siliciclastic rocks that host all of the mineralisation. No volcanic unit of appropriate size to source the preserved volume of mineralisation is present within the mineralised succession. The source of heat/mineralising fluids must therefore be intrusive or unexposed, large volume (>lkm^) volcanic bodies adjacent to each deposit. No geological evidence exists for the latter possibility but there are potential, late-intrusive source rocks of the required volume that are spatially associated with the main resources. Whim Creek shows some evidence of syn-depositional mineralisation (eg. stratabound ore, sub-stratabound stringer zone) within an actively rifting and subsiding basin. In contrast, Mons Cupri shows mineralisation controlled by later, km-scale fault systems. Both may be related by a model involving; a) emplacement of high-level felsic intrusions into an active rift, resulting in syn-depositional mineralisation and; b) later mobilisation due to elevated heat flow and structural modification. This interpretation is consistent with new and published data on the stratigraphy, geochemistry and geochronology of the mineral deposits and host rocks. A new model is presented that integrates all available data sources into a combined model for the development of the Whim Creek Belt, its associated mineralisation and relationship to the west Pilbara Craton. Acknowledgement: GP would like to acknowledge the support of the Bicentennial Gold 88 Endowment for funding to attend the AGC

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PALEOMAGNETIC DATING OF PHANEROZOIC WEATHERING IMPRINTS, MOUNT PERCY MINE, KALGOORLIE, WESTERN AUSTRALIA Brad Pillans^ and Roger Bateman^ ^CRC for Landscape Evolution and Mineral Exploration Research School of Earth Sciences, ANU, Canberra, ACT, 0200 ^Kalgoorlie Consolidated Gold Mines, PMB 27, Kalgoorlie, WA, 6430

Archean basement rocks at Mt Percy mine have been deeply weathered in a tectonically stable, near-surface environment, possibly for much of the Phanerozoic. Secondary iron oxides in the weathered regolith acquired a magnetic remanence in the direction of the Earth's magnetic field at the time they formed. Ages of weathering-induced magnetisation are estimated by comparison with the Australian Apparent Polar Wander Path (AAPWP). Resultant ages confirm, for the first time, that Cainozoic, Mesozoic and Paleozoic weathering imprints are recorded in regolith at a single site. Oriented samples of oxidised saprolite were collected in open cut pits at Mt Percy, and subject to stepwise laboratory demagnetisation using both thermal and alternating field techniques; thermal demagnetisation generally yielded more consistent results. Remanences were measured on an ScT 2-axis cryogenic magnetometer. Characteristic Remanent Magnetisations were identified by Principal Component Analysis. Well defined, stable magnetic remanences were isolated at several sites as follows: 1) Ultramafic saprolite from two sites at depths of 15-20 m yielded a combined high temperature (>580°C) remanence direction of decl = 005.7°, incl = -73.3° (N = 20 specimens). The resultant south pole position (115.4°E; 61.3°S; Aqs^ 2.2°) lies on the late Mesozoic - Tertiary segment of the AAPWP (Idnurm 1985), with an age of 60±10 Ma. This pole is statistically indistinguishable from that of the Momey weathering profile in Eromanga Basin, southwest Queensland (Idnurm & Senior 1978). 2) Porphyry saprolite (10-15 m depth) yielded an intermediate and high temperature remanence direction of decl = 321.2°, incl = -68.0° (N = 19). The resultant south pole position (164.8°E; 54.7°S; A95 = 5.0°) lies close to both the mid-Cretaceous and Jurassic segments of the Mesozoic AAPWP (Embleton 1981). A mid-Cretaceous age is ruled out because specimens are of both normal and reversed polarities, whereas the mid-Cretaceous is characterised by normal polarity field directions only. 3) Porphyry saprolite (35-40 m depth) yielded a high temperature remanence direction of decl = 200.6°, incl = 62.2° (N = 28). The resultant south pole position (076.8°E; 69.4°S; A95 = 3.8°) lies on the early Carboniferous segment of the AAPWP (Li et al. 1991) References

EMBLETON, B.J. 1981. A review of the paleomagnetism of Australia and Antarctica. In: M.W. McElhinney & D.A. Valencio (eds). Paleoreconstruction of the continents. American Geophysical Union Geodynamics Series 2, 77-92. IDNURM, M. 1985. Late Mesozoic and Cenozoic palaeomagnetism of Australia -1. A redetermined apparent polar wander path. GeophysicalJournal of the Royal Astronomical Society 83, 399-418. IDNURM, M. & SENIOR, B.R. 1978. Palaeomagnetic ages of Late Cretaceous and Tertiary weathered profiles in the Eromanga Basin, Queensland. Palaeogeography, Palaeoclimatology, Palaeoecology 24, 263-277. LI, Z.X., MCPOWELL, C. MCA. & THRUFF, G.A. 1990. Australian Paleozoic palaeomagnetism and tectonics - II. A revised apparent polar wander path and palaeogeography. Journal of Structural Geology 12, 567-575.

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Geological Convention, Sydney, July 2000

REVISED STRATIGRAPHY OF THE PALAEOPROTEROZOIC EARAHEEDY GROUP: IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE EARAHEEDY BASIN, WESTERN AUSTRALIA F. Pirajno, J. A. Jones and R. M. Hocking Geological Survey of Western Australia, Mineral House, 100 Plain St., East Perth, Western Australia, 6004 The Palaeoproterozoic Earaheedy Basin in Western Australia, which contains the Earaheedy Group, lies at the eastern end of the Capricorn Orogen (Tyler et al., 1998). Recent geological mapping has led to a revision of the stratigraphy, the development of a new model for basin evolution, and a better understanding of the potential for mineralisation. The Earaheedy Basin is interpreted to have been much larger than its present-day exposure, extending further to the southwest and to the north, where it is masked by the overlying Bangemall and Officer Basins. Regional stratigraphic relationships indicate that the Earaheedy Basin is younger than 2200 Ma and older than 1650 Ma, and it appears to be unaffected by the 1800 Ma Capricorn Orogeny. Poor age constraints hinder more accurate placement of the basin within the regional framework. Isotopic ages for Earaheedy Group sedimentary rocks and mineralisation cluster around 1700-1800 Ma. The Earaheedy Group is a 5km thick package of shallow marine clastic and chemical sedimentary rocks that has been divided into two subgroups. The Tooloo Subgroup, at the base, consists of the Yelma Formation (older), Frere Formation and Windidda Formation (younger). The overlying Miningarra Subgroup consists of the Chiall Formation (older), Wongawol Formation, Kulele Limestone and Mulgarra Sandstone (younger). The Yelma Formation contains shale, sandstone and, in the southwest of the basin, a c.lOO m-thick carbonate unit, the Sweetwaters Well Member. The overlying Frere Formation records the onset of Fe-oxide precipitation within the basin and consists of granular iron-formation, separated by two major shale bands, and minor carbonate. The Windidda Formation consists of shale, locally stromatolitic carbonate, minor jasperoidal beds and granular iron formation. Finely laminated shale in the Windidda Formation in the north and west of the basin, is separated as the Karri Karri Member. The Chiall Formation combines, as members, the former Wandiwarra Formation and Princess Ranges Quartzite. It consists of shale, siltstone and mudstone intercalated with thick-bedded sandstone and intraclastic breccia. The Wongawol Formation consists of shale, siltstone to very fme-grained sandstone, intraclastic breccia and carbonate-glauconite breccia. The Kulele Limestone is a cyclic platform carbonate succession, consisting of carbonate units which are separated by shale and sandstone. Above it, the Mulgarra Sandstone consists of sandstone, shale and minor carbonate. The entire Earaheedy Group is characterised by a shallow-marine to coastal depositional setting, with a shoreline to the south and southeast and deepening towards the north. This is consistent with models of the granular iron-formation being the shallow water facies equivalents of deeper water banded iron-formations (Beukes and Klein, 1992). Known mineralisation in the Earaheedy Basin includes MVT Pb-Zn-Cu deposits in the Sweetwaters Well Member, near the Shoemaker Impact Structure and the large (>200 Mt) Magellan Pb deposit, which is hosted by outliers of the Yelma Formation on the Yerrida Basin. Minor stratiform Mn- and Fe-oxides are present within the shale units of the Windidda Formation. These stratiform oxides contain anomalous abundances of Cu, Ba and Pb, thus enhancing the prospectivity of the Earaheedy Basin for stratabound Cu of the Kupferschiefer type. Gold mineralisation is present in the Stanley Fold Belt (a zone of deformation along the presently exposed northern margin of the Earaheedy Group) where it is associated with mylonite and quartz veins. Any tectonic model for the inception and evolution of the Earaheedy Basin is hindered by poor age constraints. However, on the basis of current field work we postulate that the basin was part of a rifted continental margin in the northeast of the Yilgam Craton. Compressive movements, perhaps associated with the collision with the North Australian plate, formed the Stanley Fold Belt. Only the southern shelf portion of the continental margin is now exposed. The actual rift was located some distance north of the presently exposed margin and east of the Pilbara Craton, and volcanism along it may have provided the source of dissolved iron for the deposition of the Frere Formation. References BEUKES N. J. & KLEIN C., 1992. Models for iron-formation deposition, in Schopf, W. and Klein, C. (eds) The Proterozoic biosphere: a multidisciplinary study: Cambridge University Press, p. 147-151. TYLER I. M., PIRAJNO F., BAGAS L., MYERS J. S., & PRESTON W., 1998. The geology and mineral deposits of the Proterozoic in Western Australia. AGSO Journal of Geology & Geophysics 17, 223-224.

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RODINIA: BUILDING BLOCKS AND PALAEOMAGNETIC CONSTRAINTS S.A. Pisarevsky, and C.McA.Powell, Tectonics Special Research Centre, The University of Western Australia, Department of Geology and Geophysics, Nedlands 6907 WA, Australia

Although there are several versions of the configuration of the late Proterozoic supercontinent Rodinia (e.g. Hoffman, 1991; Dalziel, 1997; Weil et al., 1998), the main features are the same: Laurentia lies at the core of Rodinia and is surrounded by other blocks, with East Gondwanaland connected to its southwestern or western margin. Amazonia and the Rio de La Plata craton lie adjacent to the eastern margin, and Baltica and Siberia against the northern or northeastern margin (in present-day Laurentia orientation). The positions of other cratons - Kalahari, Congo/Sao Francisco, West Africa, North and South China - are less certain. Palaeomagnetic data provide the quantitative constraints for the Precambrian palaeoreconstructions, but are distributed very nonuniformly in time and space. Most palaeomagnetic results for Rodinia times (between ca. 1000 and 750 Ma) come from Laurentia. We have used McElhinny and McFadden's (1999) synthesis of these Laurentian data as a basis for building a testable Rodinia reconstruction. Powell et al's (2000) analysis of the Kalahari APWP for 1100-1000 Ma suggests that this craton was attached to East Gondwanaland around 1020 Ma with the Namaqua-Natal fold belt facing outboard from the Laurentia-East Antarctica core, an orientation "reversed" 180° from previous reconstructions. There are no reliable palaeomagnetic data from the Kalahari for younger Neoproterozoic times. The next most useful Neoproterozoic APWP is that of Baltica. Comparison of the Grenvillian and Sveconorwegian APWP loops leads to the conclusion that Baltica is not attached to East Greenland (Dalziel, 1997), but lies more to the south with the Rockall Plateau in between. The positions of Amazonia and Rio de La Plata are correspondingly shifted to the present south. Recent palaeomagnetic results from Siberia (Pisarevsky et al. 2000), together with new geochronological evidence, favours the Laurentia-Siberia fit suggested by Hoffman (1991) and supported by Pelechaty (1996). The position of East Gondwanaland is based mainly on palaeomagnetic results from Mount Isa and Stuart Dykes (Idnurm et al, 1995). The sparse palaeomagnetic data from the Congo/Sao Francisco craton support its proximity to Amazonia, but not in its younger Gondwanaland fit. The position of West Africa is questionable - its pre-Rodinia attachment to Amazonia suggested by some scientists conflicts with the little palaeomagnetic information available. (Note that reconstructions were made with the PLATES program from the University of Texas at Austin). References DALZIEL, I.W.D., 1997. Neoproterozoic-Paleozoic geography and tectonics: review, hypothesis, environmental speculation. Geological Society of America Bulletin 109, 16-42. HOFFMAN, P.F., 199L Did the breakout of Laurentia turn Gondwana inside out? Science 252, 1409-1412. IDNURM, M., GIDDINGS, K.A. & PLUMB, K.A., 1995. Apparent polar wander and reversal stratigraphy of the PalaeoMesoproterozoic southeastern McArthur Basin, Australia. Precambrian Research 72, 1-41. McELHINNY M.W. & McFADDEN P.L. 1999. Paleomagnetism. Continents and Oceans. Academic Press, San Diego, 386 p. PELECHATY, S.M.,1996. Stratigraphic evidence for the Siberia-Laurentia connection and Early Cambrian rifting. Geology 20, 725728. PISAREVSKY, S.A., KOMISSAROVA, R.A. & KHRAMOV, A.N., 2000. New palaeomagnetic result from Vendian red sediments in Cisbaikalia and the problem of the relationship of Siberia and Laurentia in the Vendian. Geophysical Journal International 140, 598-610. POWELL C.McA., JONES D.L., PISAREVSKY S.A. & WINGATE M.T.D. 2000. Paleomagnetic constraints on the position of Kalahari craton in Rodinia. Precambrian Research (in press). WEIL, A.B., VAN DER VOO, R., MAC NIOCAILL, C & MEERT, J.G., 1998. The Proterozoic supercontinent Rodinia: paleomagnetically derived reconstruction for 1100 to 800 Ma. Earth and Planetary Science. Letters 154, 13-24.

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THE ROLE OF CHAOTIC DYNAMICS IN THE PETROGENESIS OF GRANITOID ROCKS: MULTISCALE OBSERVATIONS AND ANALYSES G. Poli and D. Perugini Department of Earth Sciences, University of Perugia, Italy

Processes involving scale and time dependent dynamics are widespread in nature and magmatic interaction processes belong to this class of phenomena. From this point of view chaotic dynamics during magma mixing play a fundamental role in the genesis of mixed rocks at all scales. Observations and measurements are performed to assess the importance of chaotic advection in magmatic interaction systems with emphasis to the petrogenesis of granitoid rocks. In detail, this work is focused on analysis and interpretation of magma mixing products at the macro-, meso- and micro-scale considering: (i) the flow structures occurring at the outcrop scale; (ii) the occurrence of different chemical zoning in mineralogical phases within the same system; (iii) the distribution of chemical elements inside microgranular mafic enclaves. Macro-scale observations At the outcrop scale, filament-like structures of a mafic magma inside an acid host coexist with microgranular mafic enclaves evidencing the chaotic nature of magmatic interaction processes. Computer simulations of chaotic dynamical systems show good agreement between the computed structures and the structures observed on outcrops. This finding allows to study numerically systems undergoing magmatic interaction, and permits a long-term appraisal of the process evolution. Meso-scale observations The occurrence of mineralogical phases showing different patterns of chemical zoning is a common feature in granitoid rocks generated by magma interaction. Many processes can be invoked to explain their occurrence, and thermo dynamical perturbations are the most quoted. We show that chaotic flow fields coupled with thermo dynamical disequilibrium occurring inside magmas can be powerful forces to generate rocks in which crystals showing different chemical zoning coexist. Micro-scale observations Elemental X-ray distribution maps are collected within thin sections of mafic microgranular enclaves. The maps are analyzed by a computer based box-counting method to estimate their fractal dimension. Results indicate that a clear correlation between the measured fractal dimension and the geochemical evidence of interaction suffered by the enclaves exists. This method, allows to characterize enclaves as true fractal objects and produce robust and consistent results [1]. Since chaotic dynamics produce fractal structures, the occurrence of chaotic fiow fields inside magmas is strictly related to our analysis of mafic microgranular enclaves. In particular, the contemporaneous occurrence of well-mixed regions together with poorly-mixed regions in the same system is thought to be similar to the host/enclave system found in many granitoid plutons. Hence, a new origin for microgranular mafic enclaves based on the above resuUs is proposed. Chaotic dynamics can be powerful forces to produce a variety of structures at all scales in magmatic interaction systems. Digital simulations and computer graphics are of great help in deciphering what kind of dynamics are involved in such complex processes. Reference PERUGINI D. & POLI G. 2000: Earth Planetary Science Letters 175: 93-103.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SEQUENCE STRATIGRAPHY, BIOSTRATIGRAPHY AND DEPOSITIONAL SYSTEM ANALYSIS - AN INTEGRATED STUDY IN THE GAMBIER BASIN, SOUTHERN AUSTRALIA Rosalie Pollock', Simon C. Lang\ Brian McGowran^ and Qianyu Li^ ^National Centre for Petroleum Geology and Geophysics, University of Adelaide, 5005, Australia. ^Department of Geology and Geophysics, University of Adelaide, Adelaide, 5005, Australia.

The Gambier Basin represents one of the frontier basins in southern AustraHa with a few gas and carbon dioxide fields identified. It covers an area of approximately 150,000 km2 and forms part of a passive continental margin developed during the Jurassic to Tertiary resulting from the separation of Australia and Antarctica. The basin comprises CretaceousEocene deltaic clastic sediments, but since the later part of the Middle Eocene plate reconfiguration, marine transgression and introduction of a warm current into southern Australia deposition of cool-water carbonate platforms and mounds has dominated. Sequence stratigraphy based on regional seismic interpretation, detailed biostratigraphy and depositional system analysis of core material is undertaken to aid exploration in the basin. The carbonate sequence shows clinoform geometries on the outer shelf, punctuated by many incision events. Large submarine canyons are evident on the seismic probably produced by relative falls in sea level. Mounded structures, similar to those biogenic carbonate buildups observed on the Eucla Platform over 1000km to the west, are thought to have developed during transgressive events. Several high-resolution sequences have been recognised in the Middle Eocene to Middle Miocene section, as part of an overall regressive-transgressive-regressive cycle. This sequence stratigraphic framework from the carbonates will be expanded onto the underlying clastic succession, as the development of the carbonate platform is linked to the burial history of the clastics. Evidence has amounted to demonstrate that changes in regional deposition environments, especially the impacts of various faults, different rates of subsidence and sea level fluctuations, are the prime factor controlling the sediment buildup and consequently the development of the whole basin. The depositional model generated from this study will assist with development of play concepts for exploration in the Gambier and other southern Australian basins.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

INTRAPLATE SEISMISITY WITHIN SOUTH-WESTERN PART OF KANTO PLAIN, JAPAN - MANIFESTED IN HETEROGENEOUS SUBSURFACE ZONES, REVEALED THROUGH GEOMORPHOLOGICAL HYPOTHESES. Diana Polonska and Kazuoh Seo Department of Built Environment, Tokyo Institute of Technology, G5/6F, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502 JAPAN Defining tectonically controlled subsurface structures of sedimentary basins is required for a wide range of problems from technological areas, such as earthquake engineering to fundamental problems in geosciences. The geophysical limitations of seismic data influence the spatial accuracy in unveiling local subsurface patterns. Despite ever increasing sensitivity of precisely synchronized receivers, deployed in a seismic arrays and coupled with the processing power of modem workstations, a barrier of high spatial resolution exist. Expensive "serial" approach of direct drilling and making use of mining information usually does not allow to cover extensive areas to depict the whole geological complexity of a chosen territory. Such discrepancies were exhibited after processing the data of recent explosion experiments within the south-western edge of the Kanto sedimentary basin area of our case study in Japan. Calculations on seismic refi-action have shown that 2-3 km thick near- surface layers of relatively soft sediments were sequenced by heterogeneous subsurface features, resulting in overall conclusion being that measured seismic waves propagation after the explosions would best be described by block structure of the region. Situated on the southwestern, mostly hilly part of Kanto plane, the area under investigation attracts attention with its complex geologo-tectonical structure and its high socio-economical importance for Japan. Adjacent areas of the Kanto district have been devastated many times during the last centuries by severe earthquakes, usually originated from transform fault zone bounded by the Sagami Trough. Great Kanto Earthquake of 1923 with magnitude 7.9 on the Richter scale was particularly damaging for Kawasaki and Yokohama cities, situated there. It is well known that the triple plate junction between Philippine, Pacific and Eurasian plates predetermine the complicated seismotectonics. Several models of the plate configuration in this area have been proposed considering the spatial distribution of earthquakes' hypocentres, three-dimensional velocity structure, and fault model of major earthquakes. Taking into account these tectonic models, an independent analysis of the hypocent! ral distribution of recent microeathquakes has been made. Interpretation of such distribution for shallow earthquakes is given. Based on independent from seismic studies geomorphological expertise, the Earth's surface was divided onto a number of simplest morphologic structures - blocks. The sets of blocks, grouped through analysing different morphometric coefficients, forms meso-, mega- and macroblocks (with the dimensions comparable to those of the "microplates" in recently appearing multiple-microplate models). These naturally predefined polygons of various shape and dimension represent delineation of the morphostructures beyond that associated with known active faults. Consequent operations for morphotectonical reconstruction were carried out to verify the ongoing geomorphological hypothesis about geological structure and tectonics of the analyzed territory. In addition to a very good fit to explosion experiments data, we demonstrate an excellent correlation between areas of earthquake activity and the suggested block structure of the Earth's crust. Recorded epicentres, especially those of near-fie! Id zone earthquakes, are evidently showing the appropriateness of the employed methodology in clarifying subsurface heterogeneous features. Earlier studies of the relative movement of the Philippine Sea, Pacific and Eurasian plates on the basis of the distribution of hypocentres and seismic data suggest that the shallow earthquakes within the area with the hypocentres depths less than 45 km are essentially intraplate ones. Therefore, epicentres' positioning correspondence to outlined blocks structure may point only to the deep interrelationship between the former and intrinsic characteristics of the latter. Our claim is that observed patterns of the microeathquakes' distribution, including clustering in space and time, are induced by existing heterogeneous intraplate block structure, which until now is not well resolved experimentally due to instrumental and processing limitations, but is very well depicted by an independent, cameral morphometric analysis applied.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

REGIONAL LITHOSPHERIC DOMAINS IN AUSTRALIA FROM GRAVITY MODELLING Yvette H. Poudiom Diomani^ Suzanne Y. O'Reilly^ and William L. Griffin^'^ 'GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney N S W 2109 Australia ^ CSIRO Exploration and Mining, PC Box 136, North Ryde, N S W 1670 Australia Precambrian western Australia is separated from Phanerozoic eastern Australia by the Tasman Line and this line marks the boundary between a tectonically and thermally relatively young active domain to the east and a cold, thick old lithosphere to the west. The nature of this boundary has been further refmed using mantlederived xenolith and mineral data on a transect that traces the change from Phanerozoic eastern NSW across the Tasman Line to the Eyre Peninsular in Proterozoic South Australia. The paleogeotherm decreases westward from greater than 50mW/m^ to around 40mW/m^ in South Australia and it is evident that the lithosphere thickens from the eastern to the western sections. This information on the thermal state of the lithosphere is important for the interpretation of the gravity and topography dataset for Australia using the method of determining elastic plate thickness, a measure of the strength of the lithosphere. Gravity and topography data of the whole continent has now been assembled. The gravity data are provided by AGSO while the topographic data are supplied by AUSLIG. We present preliminary results of our gravity modelling in the Mount Isa domain in Northeastern Australia. Gravity data from the Mt Isa area show large central positive Bouguer anomalies which correspond to the core of Mt Isa Inlier. These large positive anomalies are interpreted as due to the presence of high density material in the crust, related to the occurrence of magmatic bodies in the lower crust. Enhancement of the gravity data shows that the anomalies are oriented N N W to NS in the north, and change to a NE-SW direction in the South. These trends correspond to the two major directions of faulting that affected the area. The central positive anomalies are surrounded by large negative anomalies on either side of the Inlier, and reflect a difference in crustal structure and densities in the area. We then use the gravity and topographic grids to estimate the strength of the lithosphere, or its effective elastic thickness (EET or Te) as a flinction of the wavelength. The results of Te analysis show lateral variations of the lithospheric strength within the area, with Te between 34 and more than 60 km. Moreover, to the west of the central part, Te is an average of 34 km, then slightly higher (40 km) in the south east, and fmaly the central part shows larger Te values (60-64 km). These results show that the lithosphere is stronger in the central part of the Mt Isa Inlier. The lithospheric domains recognised using gravity modelling techniques correspond to those defined using a combined interpretation of gravity, magnetic and geological data. Work is currently in progress to define lithospheric domains and structural differences between western and eastern Australia. An analogous study in northern Siberia (Poudjom Djomani et al., 1999) showed that such modelling could delineate lithospheric domains corresponding to major tectonic terranes mapped at the surface. Acknowledgements: We thank AGSO and AUSLIG for providing the gravity and the relief data of Australia. References Poudjom Djomani, Y.H., Griffin, W.L., Natapov, L.M., Erincheck, Y., O'Reilly, S.Y. and Morgan, P. (1999). Mapping lithosphere-scale structures on the Siberian platform. Flexural rigidity and lithospheric mantle composition: relationships to major tectonic features. Internal report for Western Mining Corporation, December 1999.

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2000 IGCP LECTURE: THE LOST MOZAMBIQUE OCEAN C. McA. Powell Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA 6907

Most reconstructions of the late Mesoproterozoic supercontinent, Rodinia, place Madagascar attached to India in its younger Gondwanaland fit on the eastern side of a wide Mozambique Ocean that separates India-Madagascar-East Antarctica from the Congo-Sao Francisco block of Africa. When Rodinia broke up during the Neoproterozoic, the PalaeoPacific Ocean formed between Australia-Antarctica and Laurentia, and the Mozambique Ocean shrank by subduction beneath either or both of the India-Madagascar and east African margins. Final closure of the Mozambique Ocean is thought to have occurred in the late Neoproterozoic during the end-Precambrian formation of Gondwanaland. The expectation from this palaeogeographic sequence is that Madagascar should contain the evidence of: • late Mesoproterozoic continental sediments formed at the edge of the Mozambique Ocean, mid- to late Neoproterozoic subduction-related magmatism and deformation related to closure of the Mozambique ocean, and continent-continent collision when east Africa collided with India, closing the Mozambique Ocean Five current or recent International Geological Correlation projects, IGCP 348, 368, 418, 419 and 440, provide some of the evidence by which this predicted succession of events can be tested. The geology of Madagascar is the key. In 1997, the international scientific community was introduced to the tectonic framework of Madagascar in a modem context during a joint field meeting of IGCP 348 and 368. In 1998, a group of five scientists from Australia, South Africa and USA completed a structural traverse by raft across the central part of the island, and in 1999 and 2000 the geological secrets of the island are being further investigated by detailed structural and geochronological studies. The success of the work has depended on international collaboration. Whether there was a late Mesoproterozoic continental margin in Madagascar is debatable. However, the present structure of Madagascar provides an almost complete structural cross-section through an end-Proterozoic continental collision that can be compared with what we might expect today 10 to 20 km below the surface in the Himalayas and southern Tibet.

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2000 MAWSON LECTURE: THE SEARCH FOR SUPERCONTINENTS C. McA. Powell Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA 6907

The existence of large areas of continental crust in a single connected land mass (a supercontinent) has been part of geological thinking for over 100 years, although ideas about the configuration and formation of supercontinents have changed markedly. In the late 1800s, Edward Suess introduced the concept of "Gondwana-land" to explain the distribution of rocks and fossils in widely separated continents, although his concept was framed in a fixist, contracting Earth in which ocean basins were foundered parts of the ancient supercontinent that had sunk below sea level. Alfred Wegener's mobilistic concept of continental drift to explain the distribution of the continents and oceans was widely accepted between 1915 and 1928, but rejected by the majority of scientists for the next 35 years. The discovery of magnetic reversals on land and in symmetrical patterns in map view on the ocean floor, coupled with the ability to date rocks with newly-developed radiometric-decay techniques, led to the discovery of seafloor spreading in the early 1960s, and thence to the theory of plate tectonics. Despite this, we know relatively little about the possible forebears of Gondwanaland and Pangea in the 88% of the Earth's history before the Cambrian. A supercontinent known as Rodinia is thought to have assembled at the end of the Mesoproterozoic, and broken up in the mid-Neoproterozoic, giving rise to the various continental fragments that assembled as Gondwanaland at the beginning of the Phanerozoic. Supercontinents before then are even more speculative. There could have been a supercontinent, Kenorland, assembled in the late Paleoproterozoic, and there is some evidence of a late Archaean and early Proterozoic supercontinent. The search for supercontinents is guided by a number of operational rules and assumptions. In the Phanerozoic, the distribution of fossils, latitudinally-sensitive sediments, and the pattern of old continental cratons and foldbelts, were very important historically in proposing and testing proposed fits. However, it was not until more quantitative physical measurements were made, such as palaeomagnetism, seafloor magnetic anomalies and gravity patterns, that continental drift was accepted and competing versions of Gondwanaland and Pangea could be adequately tested. Rules for searching for supercontinents include matching disjuncts such as fold belts and giant radiating dyke swarms, locating piercing points, considering the global budget of rifted margins, all tempered by the need to think of geometry on a sphere. The search for supercontinents in the early Proterozoic and late Archaean raises additional complications such as the need to consider secular changes in the rate of heat flow and whether the volume of continental crust has changed significantly through time.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CALCULATION OF PHASE RELATIONS INVOLVING HAPLOGRANITIC MELTS USING AN INTERNALLY-CONSISTENT THERMODYNAMIC DATA SET. Roger Powell^ Tim Holland^ and Richard White^ ^ School of Earth Sciences, University of Melbourne, Victoria 3010, Australia ^ Department o f Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK

A simple thermodynamic model is developed for silicate melts in the system Ca0-Na20-K20-Al203-Si02-H20 (CNKASH). The Holland and Powell (1998) internallyconsistent thermodynamic data set is extended via the incorporation of the experimentallydetermined melting relationships in unary and binary subsystems of CNKASH. The predictive capability of the model is evaluated via the experimental data in ternary and quaternary subsystems. This model is then extended by the addition of FeO and MgO, with the data for the additional end-members of the liquid being incorporated into the internallyconsistent thermodynamic data set. The resulting data set, with the software THERMOCALC, is then used to calculate melting relationships for metapelitic rock compositions. The main form for this is P-T and T-x pseudosections calculated for particular rock compositions and composition ranges. The relationships in these fullsystem pseudosections are constrained by the low variance equilibria in subsystems of NCKFMASH. In particular, the solidus relationships are controlled by the solidus relationships in NKASH, and the ferromagnesian mineral relationships are controlled by those in KFMASH. The calculated phase diagrams allow observations to be made about the processes involved in producing granulite facies rocks, particularly those relating to open system behaviour of rocks under such conditions.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A NEW LOOK AT THE STRUCTURAL AND TECTONIC EVOLUTION OF THE GIPPSLAND BASIN: PRELIMINARY RESULTS Michael R. Power\ Kevin C. Hill', Nick Hoffrnan' and Tom Bemecker^ 'Department of Earth Sciences, La Trobe University, Melbourne, Victoria 3083. ^Petroleum Development, Department of Natural Resources and Environment, PO Box 500, East Melbourne, Victoria 3002. The Gippsland Basin is Australia's premier hydrocarbon province (1999 reserves: 4 billion barrels crude oil and condensate, 10 TCP gas) and has been a focus of exploration since the mid- 1960's. Early success identified large anticlinal traps in the upper part of the Late Cretaceous to Mid-Tertiary Latrobe Group, that are regionally sealed by transgressive carbonate dominated sequences of the Oligocene-Recent Seaspray Group. Since most of these large, simple traps have been discovered, a new study is directed towards new structural and/or stratigraphic play fairways within the Latrobe Group, targeting deeper stratigraphic levels and deeper-water regions. The early rift-geometries of the Gippsland Basin are fundamental to understanding the later basin evolution, leading to new play concepts for petroleum exploration. In the Gippsland Basin major extension/or transtension was initiated during the Late Jurassic to Early Cretaceous by oblique rifting of Australia fi-om Antarctica. Early Cretaceous rifting failed and in the Late Cretaceous, the extension vector changed to a south-easterly direction with the opening of the Tasman Sea. This tectonic change initiated the oblique reactivation of earlier structures. Deposition of coastal plain/deltaic source and nearshore reservoir rocks occurred from the Late Cretaceous until the Early Tertiary. Mid-Tertiary inversion resulted in the formation of structural traps. Tertiary burial and subsidence of the eastern Gippsland Basin resulted in generation of hydrocarbons to charge the Upper Latrobe traps. The restoration of published regional sections highlights the issues involved with modelling a structurally complex sedimentary basin, and allows a preliminary evaluation of the basin's response to regional tectonic controls. Although the Palaeozoic basement could not be restored, the amount of brittle extension in the NESW direction was defined by measurements of restored fault displacements and decompacted sediment thicknesses were determined. There was a major change in the rate of extension, measured in the NE-SW direction, between the TithonianAlbian (60-75%) and the Late Cretaceous (1.5%). The Tithonian-Albian Strzelecki Group represents syn-rift deposition, while the Late Cretaceous sediments are part of the post-rift stage. Following extension of Paleozoic basement by approximately 75%, and filling the resulting depression with 10km of syn-rift Strzelecki Group sediments, one would expect that regional post-rift subsidence would occur shortly afterwards. One of the key findings from this work is the observed delay in basin-wide post-rift subsidence. During the Turonian-Middle Campanian subsidence was concentrated in the Central Deep depocentre, but is not recorded on the adjacent Strzelecki Terraces or Platforms until Late Eocene - Oligocene times, a delay of some 60 million years. It is clear that restorations in one orientation alone cannot accurately remove the structural deformation from a section for basins, like Gippsland, that have evolved through changes in stress directions. To account for this, a 3D basin model will be constructed. Acknowledgements:- This project is sponsored by the Victorian Department of Natural Resources and Environment (DNRE) and seismic datasets have kindly been provided by the Australian Geological Survey Organisation (AGSO), BHP Petroleum (BHPP) and Australian Seismic Brokers (ASB). Seismic interpretation and section restoration work was done using Paradigm SeisXTM and GeosecTM software.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

ROCK MICROSTRUCTURES IN THE NEW MILLENNIUM David J. Prior and John Wheeler Department of Earth Sciences University of Liverpool, L69 3BX, UK For many decades, microstructures have been used to help interpret processes in rocks. The seminal works of B.Sander, F.J. Turner, L. Weiss, H. Zwart, A.Spry, J. Christie, S.White and Ron Vernon, to name just a few, set much of the basic scientific agenda for the study of microstructures. Surges in the use and general appreciation of microstructures have often corresponded to technological developments, such as the advent of electron probe microanalysis. Recent developments in scanning electron microscope (SEM) based measurements of crystallographic orientations are at the heart of many new microstructural studies. Micro-crystallographic analysis We can measure the crystallographic orientations of many minerals with an optical microscope. Quartz and calcite c-axis data are commonplace in microstructural studies. Fuller measurements of other minerals exist, indeed, Sander was mapping variations in crystallographic orientations in the 1930s. In general, however, optical measurements are difficult and slow. X-ray and neutron goniometry are used to measure crystallographic preferred orientations (CPO), but these are bulk techniques, so that the relationship of the CPO to the microstructure has to be inferred. The selected area electron channelling pattern (SAECP) technique in the SEM and diffraction in the transmission electron microscope (TEM) can both measure the full crystallographic orientation of individual mineral grains. However, these techniques are difficult and slow to apply. Electron backscatter diffraction (EBSD) in the SEM enables crystallographic orientations of points as small as a micrometre to be measured easily, precisely and quickly. This technique, coupled to orientation contrast (OC) imaging, that provides qualitative maps of variations in crystallographic orientation, is firing the latest renaissance in microstructural studies. The purpose of this talk is to illustrate how this new technology impacts on our scientific capabilities. Two areas of research will be addressed. Quantitative microstructures? Studies of geodynamics will benefit from the development of precise, geothermometers, geobarometers, palaeopiezometers etc, based upon microstructural criteria. The precision of existing quantitative microstructural tools has been limited by the technology available to 'measure' microstructures. New EBSD based techniques allow us to define rigorous and objective microstructural parameters (domain-sizes, boundary irregularity, misorientation statistics etc). We are pursuing a programme to apply these techniques to experimentally deformed samples so that we can construct maps that show how these parameters vary with deformation conditions. Such Characteristic Microstructural Maps (CMMs) provide a better data-base for constraining the deformation conditions of natural samples. We will show how this approach is being applied to the problem of quantifying conditions in quartz mylonites. Microstructural processes. Why do we have porphyroblasts? How are inclusions included? How do cumulates develop? What is the relative contribution of dislocation creep and diffusion creep in mylonites? Existing and new models in answer to these, and other, questions can now be tested against crystallographic data sets. EBSD data give us a sample CPO. More importantly, when coupled to microstructural data, we can calculate the crystallographic misorientations. The population of misorientation angles and misorientation axes (effectively the angle and axis of rotation that map one lattice into the other) are controlled by the CPO and any grain interactions. The misorientation distribution that relates to the CPO can be calculated by looking at the misorientations between non-neighbouring grains (uncorrelated distribution). If the misorientation distribution of neighbouring grains (correlated distribution) differs from the uncorrelated distribution it shows that there is some interaction across the boundaries or that the boundary has inherited geometrical characteristics. Many grain-scale processes have predictable correlated and uncorrelated misorientation distributions. We will show new data that address the questions posed at the beginning of this section, as an illustration of the power of crystallographic data in microstructural analysis.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

BIOSTRATIGRAPHY OF ODP LEG 188 AND ITS CONTRIBUTION TO UNDERSTANDING OF THE "AMERY BASIN" Patrick G. Quilty^ and the Leg 188 shipboard scientific party ^School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, 7001

Ocean Drilling Program Leg 188 drilled three holes in the Prydz Bay Region of Antarctica in February/March 2000 - 1165 on a drift site in 3500 m depth (penetrated 999.1 m of Early Miocene-Recent): 1166 in Prydz Bay in water depth of 475.4 m (381.3 m penetration; discontinuous Early Cretaceous and younger): and 1167 drilled in 1640 m (447.5 m of a sequence that may be entirely Quaternary). Diatoms and radiolaria provided biostratigraphic control at Site 1165 in the Late Miocene and Pliocene but stratigraphic control below 650 m is poor. Foraminifera are useful mainly for palaeoenvironmental studies in the deeper parts of the core but calcareous forms generally are absent. At Site 1166, diatoms were used to identify Quaternary, Pliocene and Late Eocene-Early Oligocene intervals, but biostratigraphic control in the deeper part of the hole is, as yet, poor. Very well preserved fossil wood was recovered from several intervals but palynology is needed for dating the samples and studies on the wood should provide information on palaeobotanical links, environment and allow integration of Late Cretaceous-Early Tertiary terrestrial sequences with coeval marine sequences recently reported from Mac. Robertson Shelf to the west. Foraminifera were present throughout the section at Site 1167 but do not provide a good basis for subdivision of the sequence. Eight samples of the Neogloboquadrina pachyderma were taken for Sr dates, and most samples contained adequate specimens for oxygen isotope studies. Continuing research in this region is important because it will yield information about evolution of the environment in the bulk of Antarctica, distant from the smaller, less typical Ross Sea and Antarctic Peninsula areas where tectonics and volcanism have been major influences. ODP Leg 188 will rely very heavily on post-cruise analysis for basic results. Palynology will be critical in dating the older sequences at Site 1166 and will provide information on the palaeoenvironment and palaeobotanical affinities as Gondwana disintegrated. It will be valuable in studies of the younger section but the results are foreseeable only 'through a glass darkly'. This will be complemented by studies of fossil wood, strontium dating and oxygen and carbon isotopes. Some preliminary results are likely for the AGC.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THREE TECTONOSTRATIGRAPHIC CYCLES IN THE BROKEN HILL TERRANE BASED ON A REVIEW OF SHRIMP ^^^Pb/^^^Pb ZIRCON DATES COMPARISON WITH NORTHERN AUSTRALIA M. Raetz. M. Krabbendam and D. Giles Australian Geodynamic Cooperative Research Centre, Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Victoria 3800.

The Broken Hill Terrane as discussed herein includes both the Broken Hill and Olary Blocks. This comparative study is based on a thermal history review of the Proterozoic Willyama Supergroup in the Broken Hill Terrane and the northern Australian Proterozoic. The thermal history (assumed to reflect in zircon growth) of both regions is reviewed statistically in close to concordant ^^^Pb/^^^Pb dates obtained by SHRIMP, in several thousand zircon spot analyses from a compilation of published and some unpublished sources. Previous studies in this and other terranes have shown that SHRIMP U/Pb zircon dates, can record sediment inheritance from a source area, as well as igneous, metamorphic and hydrothermal events. We attempt to summarise the complex history of several polydeformed Proterozoic terranes into thermal history "templates" by using the full spectrum of zircon growth events that characterise the terranes. First we review the evidence for three tectonic cycles of sediment deposition in the Broken Hill Terrane (previously used stratigraphic units are given in parenthesis). Supracrustal Sequence 1 (Redan Gneiss and related migmatite units) were deposited between 1810 and 1765 Ma. These rocks contain significant Archaean inheritance. Supracrustal Sequence 2 (Thackaringa to Sundown Group) was deposited between 1765Ma and 1640Ma. Supracrustal Sequence 3 (Paragon Group) was deposited between 1640Ma and 1684Ma. Major rift-extension igneous events occurred at ca 1700-171 OMa and 1680- 690Ma. Granitoids, gabbro, and dolerite dykes dominate these events. Granitoids incorporate an inherited zircon population typical of the lower two cycles, indicating that materials similar to exposed cycle 1 and 2 supracrustal rocks are partial sources of these granitoids. In contrast, the coeval mafic dykes lack any trace of inheritance reflecting ascent through the crust along fissures without melting much wall rock. As undertaken in previous studies by other workers, a number of comparisons to the Mt Isa Eastern Succession seem valid. Striking in both regions is the apparent absence of intact supracrustals rocks older than about 1820Ma, even though the lowermost preserved strata show strong provenance from that age. Cycle 2 rocks at Broken Hill have inheritance ages very similar to Cover sequence 2 at Mt Isa. Any tectonic model proposed for the region needs to explain both the abundance of Archaean zircons in the basal strata (Archaean provenance or rifted Archaean basement?) as well as the absence of pre 1820Ma Palaeoproterozoic crust very common as Archaean cover elsewhere in the Northern Australia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE SELECTION AND DURABILITY OF HAWKESBURY SANDSTONE FOR BREAKWATER ARMORSTONE AND COASTAL PROTECTION WORKS T. M. Rannard Shirley Consulting Engineers

The large number of waterways together with the abundance of Hawkesbury Sandstone in the Sydney area has resulted in the Hawkesbury Sandstone being used historically for shoreline protection and being considered for armourstone purposes in many modem projects. However, despite its abundance and apparent strength, Hawkesbury Sandstone armourstone is rarely used in modem stmctures and there are several instances where it has performed poorly due to durability problems. There are a number of mechanical and chemical processes that act on armourstone breakwaters and contribute to the degradation of the breakwater stone. The performance of Hawkesbury Sandstone armourstone is controlled by the amount of clay material within the rock and performance can be reliably assessed by sodium sulfate and strength testing. A number of additional test methods applicable to the selection of armourstone are also discussed. A review the current literature on assessing armourstone and to provide information on the properties assessed for the selection of Hawkesbury Sandstone armourstone, including; durability, strength, and block size. In addition, methods successfully used to assess armourstone will be described. While it is common for the Hawkesbury Sandstone to be thought of as a relatively homogeneous rock mass, inspection of most large exposures indicates that the rockmass comprises a large number of interleaved lenticular beds [Packham 1969]. While some individual beds have been traced for up to 1.2km on coastal cliff faces, detailed mapping reveals that individual beds generally have a traceable extent of between 10 m to 50 m and bed thickness can vary significantly over that length. The Hawkesbury Sandstone should be considered a marginal material for armourstone use and requires good quality control practices to ensure material of sufficient durability is used. Quality control practices may include a number of complementary visual, laboratory, and large-scale test methods that in combination are able to adequately assess the likely performance of materials under most conditions. For large projects or where conditions are expected to be especially severe, the more complex techniques such as sonic testing and degradation modeling developed overseas have potential to be adapted for local conditions.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AGE AND ORIGIN OF COEVAL TTG, I- AND S-TYPE GRANITES IN THE FAMATINIAN BELT OF NW ARGENTINA. C. W. Rapela\ R. J. Pankhurst^ and C. M. Fanning^ ^Centro de Investigaciones Geologicas, Universidad Nacional de La Plata, 644 Calle No. I, 1900 La Plata, Argentina. ^British Antarctic Survey, NERC Isotope Geosciences Laboratory, Key worth, Nottingham NG12 5GG, U.K. ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia.

Located on the Palaeozoic Pacific margin of Gondwana, at the opposite extreme to the Lachlan Fold Belt, the Sierras Pampeanas of central and NW Argentina also constitute a large granitic province displaying the coeval concurrence of I and S-type magmas. The Famatinian magmatic belt consists mostly of granitoids emplaced in Early Ordovician times, after Cambrian accretion of the Pampean terrane and before the Late Ordovician/Silurian accretion of the Precordillera terrane. New SHRIMP U-Pb zircon ages, isotope and geochemical data are used to interpret the petrogenesis of this belt. Three types of granitoid are recognized in the Famatinian belt, based on lithology and geochemical data. These are (a) a minor trondhjemite-tonalite-granodiorite (TTG) group, which occurs only in the Pampean foreland, (b) a metaluminous I-type gabbromonzogranite suite, and (c) S-type granites, which occur both as small cordieritic intrusions associated with I-type granodiorites and as large batholithic masses. Twelve new SHRIMP U-Pb zircon ages establish the contemporaneity of all three types in Early Ordovician times (mainly 470-490 Ma ago). Sr- and Nd-isotopic data suggest that, apart from some TTG plutons with asthenospheric characteristics, the remaining magmas were derived from a Proterozoic crust-lithospheric mantle section (Nd model ages of 1500-1700 Ma). Granulite xenoliths in Cretaceous alkalic rocks that have been described by other authors may represent samples of this source region. Trace element modelling suggests that the TTG and I-type gabbros originated by variable melting of a lithospheric gabbroid source at 10-12 kbar and ca. 5 kbar, respectively. The voluminous intermediate and acidic I-types, which show a trend to slightly more evolved isotopic signatures than the inferred source, probably represent hybridization of the most primitive magmas with lower and middle crustal melts. The highly peraluminous S-type granites have similar isotopic and inherited zircon patterns to Cambrian supracrustal metasedimentary rocks deposited in the Pampean cycle, and were derived from them by local anatexis. Other major components of the S-type batholiths, including porphyritic biotite granites, probably involved melting of deeper crust and mixing with the I-type magmas, leading to an isotopic and geochemical continuum. This model is similar to others that have been advanced for the I- and S-type granites of the Lachlan Fold Belt of Australia and the Hercynian granites of Spain, but we argue that in the Famatinian belt at least no juvenile mantle source was involved.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TERTIARY OPHIOLITE EMPLACEMENT, MOUNTAIN BUILDING AND HIGH-PRESSURE METAMORPHISM IN NEW CALEDONIA, EASTERN AUSTRALIAN PLATE MARGIN T.J. Rawling and G.S. Lister Australian Crustal Research Centre, Monash University VIC 3800. This research considers the tectonic development of the high-pressure schist belt of New Caledonia in the context of the evolution of the Southwest Pacific region. New structural and thermochronological data from the high-pressure belt of New Caledonia is consistent with obduction, and high-pressure (eclogite facies) metamorphism in the Eocene (>40 Ma, <65 Ma). We suggest that this obduction event coincided with one of the main phases of Alpine-Himalayan orogenesis (at - 5 5 Ma) and that a mountain chain was formed that stretched from PNG towards New Zealand at this time. This orogenic belt was subsequently torn apart as the result of extensional tectonism in the over-riding plate above an east-dipping subduction zone, during rollback of the subducting slab. Mesoscopic structural analysis of the high-pressure schist belt has revealed evidence for at least three phases of shear zone development and two phases of large-scale folding. Shear zones formed during the initial orogenic event are synchronous with high-pressure low-temperature metamorphism and appear to be associated with overthrusting of an ultramafic sheet. Middle-stage shear zones are associated with large scale continental extension during which the high-pressure rocks were partially exhumed. The extended crust was subsequently folded during renewed shortening producing an orogen scale antiform-synform pair within the high-pressure beh. This event was coincident with a second episode of obduction and emplacement of the New Caledonia ophiolite. Late stage shear zones formed during renewed extension. Relatively youthful normal faults caused late block-faulting, uplifting the regional peneplain and producing the present geomorphology. "^^Ar/^^Ar thermochonometers in the high-pressure schist belt were largely reset as a result of a widespread thermal event at around 36 Ma. This event resulted in the development of small euhedral garnet and minor biotite in the highest grade rocks. It is proposed that this thermal event immediately preceded the final switch into extension and the reactivation of the many older ductile shear zones as brittle normal faults. It is likely that the timing of this thermal event was also coincident with the emplacement of the New Caledonia Ophiolite in southern and western New Caledonia. Our favoured model for the tectonic development of the high pressure schist belt of New Caledonia, and this segment of the Tertiary Australian Plate boundary, involves: 1. 2. 3. 4. 5. 6.

arrival of a sliver of rifted continental crust at an east or northeast dipping subduction zone; partial subduction of this material, inversion and closure of the back arc basin and overthrusting of several ophiolite sheets; detachment of the continental sliver from the downgoing slab via development of a basal thrust; rollback of the relatively more dense downgoing slab resulting in extension and partial exhumation of high pressure rocks in the upper plate of the system; locking of the subduction zone resulting in large scale folding in the high-pressure belt and the overthrusting of the New Caledonia Ophiolite at about 36 Ma. a switch in the locus of subduction from the New Caledonia Trench eastwards to the New Hebrides trench.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE SYDNEY SANDSTONE INDUSTRY - PAST AND PRESENT Helen Ray N e w South Wales Department o f Mineral Resources.

Sydney's inhabitants have used sandstone for thousands of years. Sandstone quarrying, as we know it today, began shortly after the arrival of European settlers and grew with the development of the city of Sydney. The use of sandstone as a load bearing building material peaked in the late 19th and early 20th centuries with the use of yellow block sandstone for many of the buildings we now regard as heritage buildings. In the 20th century sandstone continued to be an important source of construction material. Quarrying started in the centre of Sydney and has spread out, attempting to remain as close to markets as possible. The sandstone industry is divided into three sections, dimension stone, crushed and broken sandstone and friable sandstone. The dimension stone sector produces sandstone for domestic, commercial, restoration and export markets. The crushed sandstone market supplies materials for roads and fill, competing with sandstone excavated from building sites and, increasingly, recycled materials. Friable sandstone, occurring in four main areas, has been identified as the major source of construction sand for the Sydney region for the medium to long term. The amount of sandstone available for the future is limited, and land use planning has become an important part of ensuring further supply.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DEPTH-TO-BEDROCK MODELLING USING DRILLING AND THE ANUDEM TOPOGRAPHIC MODELLING PROGRAM Ollie Raymond Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601 The Forbes (NSW) 1:250 000 sheet area is about 80% covered by poorly consolidated Cainozoic sediments. These sediments have infilled a basement palaeotopography which was formed during the Eocene to Miocene by deep incision, primarily along the palaeo-Lachlan River and palaeo-Bland Creek drainage systems. The thickness and character of the Cainozoic sediments are important to hydrologists, farmers, and mineral explorers, as they are the primary source of groundwater in the region and cover highly prospective basement rocks of the Lachlan Fold Belt. The depth to slightly weathered bedrock in the Forbes sheet area was modelled using ESRI's TOPOGRID program, which is a version of the ANUDEM topographic modelling program developed by Hutchinson (1989). The ANUDEM program was designed to create hydrologically correct digital elevation models fi-om spot height, contour and stream data. The hydrologically correct method of modelling assumes that the sediment-bedrock boundary can be accurately picked from drill logs. In most cases, the transition from poorly consolidated alluvial or colluvial sediments to slightly weathered bedrock could be easily observed. However, in a few areas of deeply weathered bedrock, the transition was harder to pick. For instance, a simple driller's log may not distinguish a highly weathered or altered granite saprolite from an alluvial granitic sand or clay sediment deposited on the weathered granite bedrock. In this case, the boundary picked from the drill log would likely be the transition from highly weathered bedrock to slightly weathered bedrock, and the drainage logic of the modelling would break down. Despite this limitation in some of the drill log data, the ANUDEM model offers results vastly superior to simple non-directional gridding (e.g., IDW) of drilling data in modelling buried topography, such as in the Forbes area. Drill hole data for the model were sourced from the water bore database of the NSW Department of Land and Water Conservation (DLWC) (-2000 drill holes) and some open file mineral exploration drilling (-300 drill holes). Each drill log was examined to determine if, and where, the drill hole intersected bedrock. Drill holes that did not intersect bedrock were set aside for later analysis. The depths to bedrock were converted to elevation values by subtracting from the current digital elevation model of the Forbes area, and were treated as spot heights on the buried palaeotopography. The limits of bedrock outcrops on the Forbes sheet were obtained from the latest AGSO/NSW Geological Survey mapping, and were treated as the zero contour for the depth to bedrock. This zero contour was also converted to elevation values. The locations of known palaeovalleys, from previous DLWC surveys and from high resolution magnetic data, were digitised and treated as stream courses in the buried topography. A preliminary gridded surface of the palaeotopography was produced from these data. However, due to the wide spacing of drill holes and digitised palaeovalleys in some areas, many spurious sinks occurred in this initial model. Further palaeovalley lines were interpreted and digitised to remove obvious sinks from the model. In addition, the drill holes that did not intersect bedrock (not included in the initial modelling) were overlain on the initial model to determine where these drill holes were deeper than the modelled surface. The drill holes that were deeper than the initial model were then included in the modelling dataset. The depths of these drill holes provided only a minimum estimate of the depth to bedrock but, none-the-less, improved the model by forcing the gridded surface downwards. Finally, the elevations of the buried topography were subtracted from the current digital elevation model for the Forbes area to obtain the isopach map of depth to slightly weathered bedrock. References HUTCHINSON, M.F., 1989. A new procedure for gridding elevation and stream line data with automatic removal of spurious pits. Journal of Hydrology, 106, 211 -232.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE JEMALONG TROUGH: AN EXTENSION OF SILURIAN-DEVONIAN RIFTING IN THE EASTERN LACHLAN FOLD BELT Ollie Raymond^ Lawrence Sherwin^ Patrick Lyons^ and Martin Scott^ ' Australian Geodynamics CRC, AGSO, GPO Box 378, Canberra, ACT, 2601 ^ Geological Survey of New South Wales, PO Box 53, Orange, NSW, 2800 The Silurian to Early Devonian extensional history of the eastern Lachlan Fold Belt is well established with respect to the Hill End and Cowra Troughs. A rifting episode of similar age is described in the area to the west of these more widely known troughs. The Jemalong Trough is a north-south trending belt, over 400 km long and between 15 km and 70 km wide, extending from near Junee in the south, to east of Nyngan in the north. Much of the trough is poorly exposed, and substantial parts of it are interpreted from regional geophysics. The northern third of the trough is overlain by sediments of the Surat Basin. The Jemalong Trough contains sedimentary rocks and volcanics of the Derriwong, Ootha, and Trundle Groups. The trough lies unconformably on folded Ordovician quartz-rich turbidites of the Girilambone Group and Kirribilli Formation, and mafic arc volcanics of the Junee-Narromine belt. The southernmost margins of the trough are formed by part of the Gilmore Fault Zone, but unconformable margins are preserved for most of the length of the trough. Extension of the Ordovician arc basement, and marine incursion, commenced in the Jemalong Trough during the Pridoli (latest Silurian), later than in the Hill End and Cowra Troughs. The majority of the Jemalong Trough fill was deposited in a shallow marine environment. Sedimentation commenced with the basal conglomerates and sandstones of the Edols and Manna Conglomerates and Calarie Sandstone. Fine-grained sediments and limestones of the Derriwong Group (the Yarrabandai and Cookeys Plains Formations) were then deposited in the northern parts of the trough. The predominantly felsic Byong Volcanics were also erupted at this time. In the south of the trough, this period is represented by the Combaning Formation, which includes both high energy marine and proximal turbidite facies sedimentary rocks, as well as minor felsic volcanics. Deeper water facies of the Jemalong Trough at this time may be represented by the mudstones and fine-grained sandstones of the Ootha Group in the Condobolin-West Wyalong region (the Mulguthrie Formation and much of the Yiddah Formation). Fine grained vitric ash beds of the Yamel Volcanics, probably a distal equivalent of the Byong Volcanics, are intercalated with the Mulguthrie Formation. Following a hiatus in the late Lochkovian, sediments and volcanics of the Trundle Group were deposited throughout the Pragian in a shallowing marine to locally subaerial environment. The felsic and lesser mafic volcanics of the Carawandool and Kadungle Volcanics have very similar compositions to the older Byong Volcanics, suggesting similar source material. The volcanics are predominantly high-K rhyolites with distinctly low Th and Rb and show geochemical features that suggest derivation from melting of the underlying Ordovician arc rocks. The minor basalts have primitive compositions, suggesting some tapping of the mantle during rift-related volcanism. Minor epithermal mineralisation occurs within the Carawandool and Kadungle Volcanics. Initial inversion of the Jemalong Trough occurred in the early Emsian (-400 Ma). The Currowong Syncline, south-west of Forbes, formed at this time. Further broad open warping formed the Tullamore Syncline along the axis of the Jemalong Trough during the early Carboniferous Kanimblan Orogeny. Some shortening was also accommodated along the Marsden Thrust and probably along the Gilmore Fault Zone. However, overall, the total shortening of the Jemalong Trough was much less than in the Hill End and Cowra Troughs, probably due to the buttressing effect of the underlying Junee-Narromine volcanic belt. Regional gravity data show that the Jemalong Trough lies along a continuous, narrow gravity low within the much larger gravity high caused by the underlying Ordovician Junee-Narromine volcanic belt. The gravity anomaly of the Jemalong Trough is much less significant than the deep and broad gravity lows associated with the Hill End and Cowra Troughs. This reflects not only the smaller amount of extension and related subsidence in the Jemalong Trough, but also the smaller degree of shortening and associated thickening of the sedimentary pile.

LS and MS publish with the permission of the Director General, NSW Department of Mineral Resources.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PALAEOPROTEROZOIC KINEMATICS IN THE SOUTHERN CAPRICORN OROGEN, WESTERN AUSTRALIA Steven M. Reddy^ and Sandra A. Occhipinti^ ^ Tectonics SRC, School of Applied Geology, Curtin University of Technology, Perth, WA6845, Australia. ^ Geological Survey of Western Australia, Dept. of Minerals and Energy, 100 Plain Street, Perth, WA 6004, Australia. Developing a structural synthesis for high-strain zones that initiated during the Palaeoproterozoic is difficult because such zones are often poorly exposed and may have been re-worked during subsequent deformations. Within the Capricorn Orogen, Palaeoproterozoic high strain zones developed during the convergence, and subsequent collision of the Archaean Pilbara and Yilgam Cratons between 1830 - 1780 Ma. The Capricorn Orogen comprises a number of amalgamated terranes. These include the Yarlarweelor Gneiss Complex (YGC), originally part of the Yilgam Craton but re-worked and intruded by voluminous felsic magmatism during the Capricorn Orogeny; low-grade sedimentary and volcano-sedimentary basins (e.g. the c. 2.0 Ga Bryah and Padbury Groups); and the Gascoyne Complex, interpreted as the "high-grade core" to the Capricorn Orogen. Within the southern Capricorn Orogen, the Archaean to Palaeoproterozoic YGC lies in faulted contact with the Bryah and Padbury Groups. The faulted contact is arcuate in structure with its southern edge defined by the east-trending Kerba Fault. Detailed structural mapping of high-strain zones spatially associated with the Kerba Fault illustrates a high degree of structural complexity within the contact zone. Ductile deformation associated with foliation development is heterogeneous and more intense towards the tectonic contact between the YGC and mafic schists of the Bryah Group. Foliations lie parallel to the tectonic contact and have a strong monoclinic symmetry that defines a sub-vertical vorticity vector associated with a dextral simple shear component. Mineral lineations associated with these foliations are variably developed. Large areas of the high-strain zones record no well-developed lineation. In contrast, where mineral stretching lineations are developed they generally show a bimodal distribution in orientation with one set being parallel to the sub-vertical vorticity vector whereas a second set of sub-horizontal lineations lies orthogonal to the vorticity vector in the plane of the foliation. Foliations are folded by tight folds with axial planes that lie parallel to the foliation and hinges that are parallel to both sets of mineral lineations. Overprinting these structures are a series of brittle faults and fractures. Faults are commonly foliation-parallel and can only be recognised in areas where stepping of the detachment across the foliation leads to hanging wall or footwall cut-offs. These faults again appear to be associated with dextral shearing. Orientation of tension and shear fractures indicate a coincidence of the s2 - s3 plane with the earlier regional foliation, the axial surface of the folds and the foliation-parallel detachments. A wide variety of microstructures can be observed within the high-strain zones of the YGC. Feldspars accommodate stretching orthogonal to the vorticity vector by fracturing and subsequent rotation of feldspar fragments. Quartz preserves crystallographic preferred orientations and therefore records evidence of more ductile deformation mechanisms. In detail however, quartz microstructures show variable degrees of dynamic recrystallisation and recovery although stronger quartz fabrics and reduced grain sizes tend to be preserved towards the contact of the YCG with the mafic schists of the Bryah Group. Although foliation development on the scale of the shear zone is heterogeneous, all microstructures are characteristic of deformation at greenschist facies conditions and there is no apparent difference in metamorphic grade between the higher and lower strain zones. The high-strain zones examined in this study record a steeply dipping vorticity vector associated with dextral shear and structures that are consistent with shortening at high angles to the foliation and contact zone. This contrasts previous models for high-strain zone formation in the southern Capricorn Orogen, which are thought to be related to north over south thrusting. Instead the high-strain zones suggest formation during a progressive, dextral-transpressional deformation. Theoretical models of transpression clearly predict the switching of finite strain axes from horizontal to vertical during strongly oblique convergence and the development of mineral stretching lineations parallel to the vorticity vector. Application of this model to the southern Capricorn high-strain zones studied here requires that the variations in finite strain in different parts of the orogen reflect heterogeneous and potentially non-steady-state deformation. In addition, transpression models involving stretching of the shear zone in a vertical direction (as supported by the presence of steeply dipping lineations) may have produced uplift, and if coupled with erosion, the exhumation of metamorphic rocks. We suggest that a transpression model may be suitable to explain structural features preserved in the southern Capricorn Orogen and we aim to test these ideas with further detailed structural and geochronological studies.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE SYSTEMATIC ASSESSMENT OF DEFORMATION HISTORIES IN FRACTURE SYSTEMS Steven M. Reddy^ and Graham J. Potts^ ' Tectonics SRC, School of Applied Geology, Curtin University of Technology, Perth, WA6845, Australia. ^ Department of Earth Sciences, The University of Liverpool, Jane Herdman Laboratories, 4 Brownlow Street, Liverpool, L69 7GP, UK. The interactions between fractures in well-exposed systems provide the opportunity for detailed analysis of brittle deformations. However, the large number crosscutting and abutting relationships makes analysis laborious and time consuming and key aspects of the deformation are often missed. We have used a new approach to construct and assess relative deformation histories in order to investigate the evolution of a complex vein system. As a result of this analysis several alternative brittle deformation histories have been identified. These histories give an indication of the likely fracture propagation directions, the longevity of fracture formation and provide constraints on the mechanisms by which brittle structures develop in the upper crust. A series of veins that developed in the late-Palaeozoic Variscan deformation of Northwest Europe have been analysed systematically to establish all possible deformation histories. The veins cut shallowly dipping Carboniferous sediments in South Wales, UK and have been previously studied in some detail (Dunne and North, 1990). The veins consist of two steeply dipping sets that are orthogonal and both sets of veins display evidence of extensional opening. Dunne and North (1990) concluded that the vein system was the product of a four-stage deformation history in which the intermediate and minimum principal stresses switched orientation in the horizontal plane and that, during each stage, fractures of only one set developed. From their analysis they inferred that the scale of stress homogeneity was greater than the scale of an individual exposure. We have analysed the same series of veins using the 'younging tables' methodology of Potts and Reddy (1999). The veins are found on a well-exposed bedding surface and comprise 62 fractures that record a total of 65 relationships between various combinations of 49 fractures. Our analysis confirms that the deformation history of Dunne and North (1990) is one of the possible histories that are compatible with the relative age data. However, we have identified numerous equally valid deformation histories. These other histories are longer and more complex but all produce reconstructions that are best explained in terms of the simultaneous growth of both vein sets. Since both sets of veins are extension fractures, this pattern of growth has important implications for brittle processes. If the far-field stress is homogeneous on a scale greater than that of an exposure then either there is a small range of possible stress states or, more likely, local (material) heterogeneities may be a key factor in the production of fracture systems. In the second case these heterogeneities lead to inhomogeneous stresses on the scale of a few metres. References

DUNNE, W.M. & NORTH, C.P., 1990. Orthogonal fracture systems at the limits of thrusting an example from southwestern Wales. Journal of Structural Geology, 12, 207-215. POTTS, G.J. & REDDY, S.M., 1999. Construction and systematic assessment of relative deformation histories. Journal of Structural Geology, 21, 1245-1254.

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DYNAMIC LINKAGE BETWEEN INTERNAL ZONE EXTENSION AND EXTERNAL ZONE SHORTENING IN THE EUROPEAN ALPS Steven M. Reddy', John Wheeler^ and Robert A. Cliff^ ^Tectonics SRC, School of Applied Geology, Curtin University of Technology, P.O. Box U1987, Perth, WA6845, Australia. ^ Department of Earth Sciences, The University of Liverpool, Jane Herdman Laboratories, 4 Brownlow Street, Liverpool, L69 7GP, UK. ^ Department of Earth Sciences, The University of Leeds, Leeds, LS2 9JT, UK. Numerous studies have illustrated that within the internal zones of convergent orogens, crustal extension plays an important role in the exhumation of high-pressure metamorphic rocks. There are many geodynamic models to explain extension within convergent orogens. However, discriminating between these models is difficult and requires information on the timing of shortening and extension. Here we combine structural and geochronological data from the internal zones of the Italian Alps with sedimentological and biostratigraphical data from the external zones of the French Alps to assess the relative importance of extension and thrusting during overall plate convergence and constrain the dynamic evolution of the Alpine Orogen. In the Western Italian Alps, the Piemonte Ophiolite consists of eclogites (T» 550-600 °C and P» 18-30 kbar) structurally beneath greenschist facies rocks (T» 400 °C and P» 9 kbar). Mapping shows that the latter form a kilometrewide shear zone (the Gressoney Shear Zone - GSZ) dominated by top-SE movement related to crustal extension. Rb-Sr ages from micas within different GSZ fabrics, which dynamically recrystallised below their blocking temperature, are interpreted as deformation ages. Ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping. They show that the GSZ had an extensional deformation history over a period of c. 9 My between c. 45 - 36 Ma. This overlaps in time with the eclogite facies metamorphism. The GSZ operated over the entire period during which the footwall evolved from eclogite- to greenschist facies and was therefore responsible for eclogite exhumation. Pressure estimates associated with old and young fabrics within the GSZ are comparable, indicating that during extensional deformation there was no significant unroofing of the hangingwall. Since there are no known extensional structures younger than 36 Ma at higher levels in this part of the Alps, subsequent exhumation must have been dominated by erosion. The unroofing of high-pressure metamorphic rocks from c. 30 kbar to 10 kbar by crustal extension represents of the order of 60 km of relative vertical movement and this correlates to c.lOO km of horizontal displacement in the period of 45-36 Ma. To assess the various models that may be invoked to explain this extension we have attempted to quantify the amount of shortening that took place synchronously with this extension within the external zones of the orogen. Since the absolute ages of thrusts within the external zones are difficult to constrain, we have used the temporal evolution of the Alpine foreland basin, based on biostratigraphy, as a proxy for thrusting in the external zones. Restored sections across the external zones show c.56 km of basin migration over the period 45-36 Ma in eastern Switzerland and at least 44 km along the Annecy transect in France. These values represent minimum values of basin margin advance that are dynamically linked to the continuous advance of a thrust load and the migration of elastic flexure in the European plate. We therefore interpret foreland migration of the basin edge to foreland-directed migration of the orogenic load. Our calculations show that c.lOO km of horizontal extension in the internal zones over the period 45-36 Ma corresponds to a minimum of 44 km of shortening in the external zones. Even if our estimate of shortening is out by a factor of two (which is possible based on our assumptions), it is clear that extension was not a second order effect but was a major feature of Alpine evolution in the Eocene. Consequently we conclude that extension was contemporaneous with thrusting in the Alpine external zones and that its horizontal magnitude was broadly comparable to thrust-related shortening over the same time period. This dynamic linking of internal zone extension with external zone thrusting is consistent with motion of a wedge bounded by extensional shear above and thrusting below. Such a situation is consistent with several geodynamic models but we favour a model in which extension was driven by internal buoyancy forces. Although we have concentrated in one transect across the Alps, similar kinematic linkages around the Alpine arc could explain the broadly radial pattern of thrusting in the Western Alps.

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TIMING AND DEFORMATION OF ALLOCHTHONOUS PROTEROZOIC AND PALAEOZOIC ROCKS IN THE BADGER HEAD REGION, NORTHERN TASMANIA. Alistair Reed Mineral Resources Tasmania, PO Box 56, Rosny Park 7018. Recent mapping by Mineral Resources Tasmania in the Badger Head region on Tasmania's north coast has redefined the relationship between allochthonous sequences and identified a deformation event that may be correlated with middle Ordovician (early Benambran) deformation on the Australian mainland. The Badger Head region lies west of the Tamar River and east of the Port Sorell embayment and is characterised by a sequence of ?Proterozoic turbidites forming a topographic high. Published maps show the turbidites flanked to the east by early and middle Palaeozoic sedimentary and ultramafic rocks (Beaconsfield block) and to the west by poorly exposed Neoproterozoic or early Palaeozoic sedimentary and igneous rocks (Port Sorell complex). ?Proterozoic and early to middle Palaeozoic rocks are overlain by Permian to Triassic glaciomarine and terrestrial sedimentary rocks which are intruded by Jurassic dolerite. The history of the Badger Head and adjacent regions is complex. Mapping has demonstrated that the ?Proterozoic turbidites are faulted over ?Cambrian ultramafic and related rocks of the Beaconsfield block, consistent with interpretations of geophysical data by Leaman et al. (1973) which suggested the ?Proterozoic is allochthonous. Magnetic profiles constructed from recently collected ground geophysical data (Zengerer, 1999) across the Beaconsfield and Badger Head regions are also best explained by the presence of ultramafic rocks beneath the ?Proterozoic turbidite package. The western contact of the turbidite sequence is also a thrust, with ?Proterozoic turbidites overlying and in fault contact with younger rocks of the Port Sorell complex. Steeply NE-dipping thrusts (D4) separating repeated packages of Palaeozoic strata in the Beaconsfield block, and associated with the gold mineralising event at Beaconsfield, are present throughout the ?Proterozoic turbidite and Port Sorell complex sequences. The D4 thrusts and associated structures clearly deform fabrics that formed during emplacement of the ultramafic and related rocks and thrusting of the ?Proterozoic over younger rocks. The D4 thrusts are transgressive to the boundaries between the Beaconsfield, Badger Head and Port Sorell sequences, and do not define their boundaries as suggested by previous authors. Two main deformations pre-date D4 and both produced meso- to macroscopic recumbent folds and thrusts. Folds and quartz fibres from faults exposed along the coast show tectonic transport to the south for the earlier event (DO, but to the east for the second event (D2). Fibres show common reactivation of Di faults during D2 and it is likely that both of these events played a role in juxtaposing ?Proterozoic over younger ?Cambrian and ?Neoproterozoic strata. The timing of the D4 event is constrained to the Middle Devonian (Tabberabberan) by radiometric dating of Middle Devonian granites in NE Tasmania. By contrast, middle Palaeozoic rocks are not affected by Di, which has a transport direction consistent with Cambrian (Delamerian) deformation of rocks located west of the Badger Head region. The timing of the D2 event is more controversial. In the Beaconsfield area, D2 affects Cambrian rocks but not middle Ordovician conglomerate. A Cambrian timing for D2 is possible but would have required a change of about 90° in the tectonic transport direction (from that of DO during a single orogenic event. Furthermore, pre-middle Ordovician Mathinna Group rocks 25 km to the east show the same E-facing recumbent fold and thrust geometry as that present in turbidites in the Badger Head region. By contrast, younger Mathinna Group rocks further east show no signs of the same structure. This suggests that D2 is not Cambrian but middle Ordovician (early Benambran) in age, and that Ordovician conglomerate overlying rocks affected by D2 in the Beaconsfield block formed during tectonic uplift coincident with the D2 event. References LEAMAN D.E., SYMONDS, P. A., & SHIRLEY, J. E., (1973) Gravity Survey of the Tamar Region, northern Tasmania. Geological Survey of Tasmania Paper 1. ZENGERER, M., (1999) Gravity and Magnetics of the West Tamar District, Northern Tasmania. BSc (HonsO

thesis. University of Tasmania, Hobart (unpubl.).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

WAS THE CONTINENTAL LITHOSPHERE DESPERATELY FLAT IN THE ARCHAEAN? Patrice F. Rey\ Yvette H. Poudjom Djomani^ and Greg Houseman^ Suzanne Y. O'Reilly^ and William L. Griffin^ ' Department of Earth Sciences, Monash University, VIC 3800 ^ Gemoc National Key Centre, Department of Earth Sciences, Macquarie University, Sydney NSW 2109

Deformation of the continental lithosphere in the Archaean was likely to be fundamentally different from that of modem continental lithosphere for tw^o reasons. First, with Moho temperatures in excess of SOOT due to a higher concentration in radiogenic elements, the Archaean continental geotherm was higher than its Phanaerozoic counterpart. Such a high temperature geotherm dramatically reduces the lithospheric strength to merely that of the upper continental crust. Secondly, the Sub-Continental Lithospheric Mantle (SCLM) was buoyant and thick (120 to 150 km). For the same amount of lithospheric thickening, this implies that the gravitational force was much higher in the Archaean than in the Phanaerozoic, where a negatively buoyant SCLM balances a positively buoyant continental crust. The ability of gravitational forces to participate to lithosphere deformation is proportional to the Argand Ratio (AR): the ratio of the gravitational force (due to lateral vertical stress gradient), to the strength of the lithosphere. When AR>1 (strong buoyancy force and/or weak lithosphere), the gravitational force plays a significant role in the regional tectonic history, namely it reduces any contrast in crustal and lithospheric thicknesses. Because of the unique combination of a weak and buoyant continental lithosphere, the Argand Ratio in the Archaean was most likely » 1 . One can therefore expect that the gravitational force impeded the formation of mountain belts in Archaean time, favouring a rather homogeneous pure shear over large areas. To test this hypothesis, we have performed a ID thermo-mechanical modelling in which an Archaean lithosphere (crust:42km, SCLM: 158km, T°Moho:820T) and a Phanaerozoic lithosphere (crust:35km, SCLM:69km, T°Moho:540T) were deformed under the same conditions. Preliminary results show that, following a period of rapid thickening that double the thickness of the crust, a Phanaerozoic lithosphere evolves toward a plateau stage during which the mountain grows in width. In contrast, during the Archeaen, buoyancy-driven lateral flow prevents significant thickening, and the plateau stage is reached very early in the thickening history. Indeed, with a Moho temperature of about 820°C, the total crustal thickening an Archaean continental lithosphere can sustain is only a fraction of the initial crustal thickness. This result is broadly consistent with (1) the sub-aerial nature of volcanogenic materials in Greenstone sequences, (2) the rare occurrence of voluminous detrital sediments, (3) the relative homogeneity of the erosional level now exposed at the surface of many Archaean cratons, (4) the virtual absence of significant age gradients in the tectonic, metamorphic and magmatic activities in many cratons, and (5) the relative homogeneity of strain across large areas.

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FINITE ELEMENT MODELLING OF THE FORMATION OF KOMATIITE HOSTED NICKEL DEPOSITS AND THE NON-ROLE OF THERMAL EROSION A. Rice Departments of Geology and Physics, Rhodes University, Grahamstown, South Africa

Finite element modeling of low viscosity komatiite lava flows show that flows over ledges or across potholes are accompanied by rotors (eddies) within these depressions. These eddies are extremely efficient mechanisms for mixing and scavenging Ni into sulfides, the morphology of these eddies matching that observed in massive sulfide deposits, providing support in terms of the physics for the necessity of embayments to secure these deposits. By morphology is meant that the stagnation regime in the middle of the eddy corresponds to the barren "hole" often seen in the middle of these deposits. There is a Reynolds dependence on the amount of ore formed as well as R factor dependence. These analyses do not support thermal erosion but show instead a quenching of a crust on the substrate which amours against any chemical communication between substrate and the lava. This conforms to the observation that Hawaiian lava flows do not even melt the asphalt across the macadam which they over-run. This again because of the freezing of a low thermal conductivity crust of lava atop the macadam. The notion of thermal erosion arose from pouring hot water on waxy material which was well above the freezing point of water. This is far removed from reality as lava is injected into environments much colder than its solidus. The theoretical support for thermal erosion arose from the blunder of not allowing vertical temperature variation within the flow itself

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HOW TO FORM A RUSTENBURG LAYERED SUITE FROM A SINGLE PULSE OF MAGMA A. Rice Departments of Geology and Physics, Rhodes University, Grahamstown, South Africa

Many fluids show a marked propensity to carry suspended loads of particulates, these often separated from the rest of the flow by sharp interfaces. Rivers, fluidized bed reactors, smog and ice fogs are a few examples and there has been considerable progress in quantifying the phenomenon. If the viscosity of the fluid varies with particulate content (e.g., follows the Einstein viscosity law), the depth of the suspended load tends to stabilize once it is formed. The effective density of the suspended load includes the contribution from the particulate matter. Ice fogs form their suspended load from crystals precipitating from the air, the suspended load of ice crystals sinking to its own level of buoyancy because of the enhanced density of the air's particulate content. An early finite element analysis of cooling magma chambers showed similar behavior. A suspended load (of primocrysts) formed in the lower section of the magma chamber, which then required the upper section to be of a more evolved melt. These results prompted bringing to bear on the magma chamber environment, present practice dealing with suspended loads. The following results obtain. 1) Suspended loads can form one on top of the other, each layer more evolved in composition than the one below it. 2) The thickness (i.e., the vertical extent) of each load depends on the viscosity and convection velocity of the magma, i.e., is a function of, the Rayleigh number whose magnitude varies as the cube of the thickness. Hence the smaller the magma chamber, the thinner the suspended load. 3) This latter condition bears on the R factor and provides rationale as to why only large mafic bodies (e.g., the Bushveld Complex) possess payable reserves. 4) The application of engineering relationships for suspended load provides for a single pulse of mafic magma 10km thick 4 stratifications of thickness not far removed from those of the Bushveld Complex, e.g., the Critical Zone, the Main Zone, the Upper Zone. 5) The stratifications tend to form at about 10%-20% crystal load from which the model dictates sulfur supersaturation to occur in the Upper Critical Zone which is the case. 6) Dispersive pressure (the Bagnold Effect) dictates the accumulation of primocrysts in regions of high shear, e.g., the boundary layers. This would place chromitites and double peaks of Pt distribution in the boundary layer. 7) A single pulse magma chamber of size will secure significant assimilation of country rock to contaminate the magma. However, by the time the country rock is brought to temperature and sufficient latent heat provided to secure assimilation, the lowermost stratifications will have already formed and will contain little contamination except perhaps enhanced LREEs. 8) The middle stratifications will then possess the most contamination as by the time the last (uppermost) stratifications form, the chamber has cooled, spun down and only restites are left to attack. 9) The implication of points 7 and 8 are that there will be little contamination in the lowermost layers, most contamination in the middle layers and an intermediate amount of contamination in the uppermost layers. If the country rock is high in strontium 87, final distribution of the Sr ratio will be low at the bottom, highest in the middle and intermediate at the top which is characteristic of the Sr ratio distribution in the Rustenburg Layered Suite of the Bushveld Complex which led to the original interpretation that the Bushveld was formed by multiple injections. 11) The concentration and temperature gradients formed by the stratifications should secure the development of a multiplicity of layers through the effects of double diffusive convection.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE COOMA COMPLEX: A CRUSTAL SCALE MAGMA TRANSFER ZONE OF THE MURRUMBIDGEE BATHOLITH S.W. Richards, W.J. Collins and J.Needham Department of Geology, School of Geosciences, Newcastle University, Callaghan, N.S.W. 2308, Australia The enigmatic Cooma Complex of southeastern Australia comprises a large metamorphic aureole (~10km wide) surrounding a relatively small (3 ' 6km) granite pluton. How did such a large aureole develop around such a small pluton? The aureole is divided into a low- and high-grade zone characterised by schists + gneisses and migmatites + granite respectively. Five deformation events are recognised within the low-grade zone. D1 and D2 are preserved as inclusion trails within porphyroblasts while D3, D4 and D5 represent the three mesoscopic folding events. The dominant structures are N-trending inclined, tight, D3 folds and a well developed S3 axial planar foliation. D4 produced scattered, flat-lying folds, and a shallow, E-dipping cleavage. Steeply E-dipping, N-S trending, inclined F5 folds are associated with localised D5 reverse faults. A major change in D3 strain intensity between the low- and high-grade rocks corresponds approximately with the N-trending, D3, Spring Creek Antiform. In the high-grade zone, S3 is also the dominant foliation but it is concordant with SO, isoclinal F3 folds, and with leucosomes in the migmatites indicating high-strain during partial melting. Local F4 folds with N W trending, NE dipping axial planes, are intrafolial, isoclinal folds associated with the development of a composite SO+3+4 stromatic layering, best developed in the SW of the migmatite zone. A marked change in strain intensity also exists for D5 structures in the high-grade zone. Open to tight, N-trending, asymmetric F5 S-folds occur on the western limb of a large F5 synformal structure, but closer to the Cooma granite contact, D5 folds become isoclinal and common high-strain zones are marked by highly attenuated, sheared F5 limbs which display a dominant reverse, E-over-W sense of movement. D5 intensity increases northwards from the main granite corresponding with an increase number of S5-concordant, Cooma-type granitic lenses that have aspect ratios of up to 20:1. Microstructural relations preserved within the low-grade zone indicate that the regional metamorphic aureole at Cooma was generated during D3, consistent with metamorphic isograds that are parallel with the migmatitic D3 high-strain zone. This is in contrast with metamorphism in the high-grade zone which culminated in migmatite formation and intrusion of early-phase Cooma granite late during D3-4. This was followed by intrusion of the main granite during D5 and development of S5 retrogressive foliation in the low-grade zone, corresponding with thermal contraction during D5 in the lower-grade aureole. Nonetheless, peak metamorphism in the high-grade zone coincided with D5, particularly around the intrusions, where S5 is outlined by sillimanite wrapping around andalusite and cordierite porphyroblasts. Therefore, the regional metamorphism was associated with migmatite formation and D3 high strain, and later was focussed only around the intrusions, where D5 strain also was greatest. The partitioning of D3 and D5 intensity into regions of highest metamorphic grade, i.e. progressively inward towards the granite, mimics the thermal contraction of the complex, and highlights the effects of melt enhanced deformation. If the Cooma granite did not produce the migmatites, then what did? The Cooma Complex can be traced northward into the S-type Murrumbidgee Batholith, where the metamorphic isograds can be traced continuously for at least 50 km, ultimately contracting to <1 km around the northern most Clear Range suite of plutons. The highest-grade migmatitic aureole, which consists of Cooma type intrusions, is wedged between the batholith and Murrumbidgee Fault, and persists for some 30 km north of the main Cooma granite. Importantly, D3/D5 structures also continue northward. The Clear Range Granodiorite and related sheet-like bodies intruded as N-trending lenses concordant with S3, but they also show the S3 foliation, indicating syn-D3 emplacement. The lack of thermal overprinting of the syn-D3 regional metamorphic assemblages confirms intrusion during peak metamorphism. West of Cooma, in the low-grade aureole, the Bennetts Creek Volcanics were folded by regional D3 folds, indicating local eruption of S-type Magmas before D3. Therefore, much of the S-type magmatism associated with the intrusion of the Murrumbidgee Batholith occurred during D3 and peak metamorphism, which coincided with D3 high-strain at slightly deeper crustal levels, as represented by the Cooma Complex. We conclude that the Murrumbidgee Batholith granitic magmas intruded during regional compressive D3 deformation, with the migmatite zone at Cooma representing a D3 high-strain zone associated with S-type granite magma transfer. The low-grade metamorphic aureole was produced by conductive heating of adjacent rocks during the magma transfer process. During the waning stages of emplacement, the thermal aureole contracted and strain was partitioned into narrow D5 high-strain zones where the S-type granitic magmas were still migrating through the crust. Thus, the Cooma Granodiorite represents the product of the final stage of Murrumbidgee Batholith magmatism.

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WHY GEOCHEMICAL TRACERS HAVE FAILED US IN OUR SEARCH FOR THE SOURCE OF THE MESOTHERMAL GOLD ORE FLUID John Ridley GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109

The uniformity of many chemical characteristics of mesothermal gold deposits of Archean granite-greenstone terrains and geologically more recent low-grade metamorphic terrains implies a uniform genesis for these deposits. A specific source for the gold-carrying fluid remains, however, unresolved, despite data for a large number of geochemical and isotopic tracers from a large number of deposits. Individual tracers have been used to argue for specific fluid sources at specific deposits, however, wherever multiple tracers are available, a single source can rarely be implied. Data from multiple tracers has been used to rule out specific sources, to argue for derivation from mixed and homogenised or average crust, or for the fluid being a mixture from different source rocks. Fluid flow at the deposits is channelised along fractures and structural discontinuities in the rock mass, with chemical interchange between the fluid and wallrock during flow is indicated by alteration haloes around channelways. Several lines of evidence imply that many deposits are kilometres distant from the fluid source, in particular the lack of clear lithological associations of deposits, the mass balance requirements of the volume of source rock or fluid needed to form large deposits, and the vertical continuity of deposits. Order-of-magnitude mass balance calculations based on quartz precipitation in veins and lodes, and the average thickness of alteration haloes around lodes, suggest effective fluid : rock ratios after a few kilometres of fluid travel in these hydrological systems are of the order 10 : 1, and are thus such that the systems would be neither fluid dominated nor rock dominated. How individual tracer components will behave in such systems will be controlled by the fluid : rock partitioning of the component (Kd). We now have some multi-element composition data on the gold ore fluid from fluid inclusion analyses and phase equilibria calculations, and can hence estimate Kd's for many components. Many of the commonly used source tracers (Pb, Sr and O isotopes, K/Rb ratios) have intermediate Kd values such that they will be neither 'robust' and reflect the source composition, nor be controlled by equilibration with immediate wallrock. Rather, the source and all rock types with which the fluid has interacted with along the channelway will have influenced the fluid composition with respect to the component at a deposit. Data from multiple geochemical tracers could be inverted to determine likely source rocks, but this would require a good knowledge of the geological environment of the deposit. At present our tracer data does not distinguish between the various specific source rocks proposed (e.g. granitoid, metamorphic devolatilisation of mafic rocks, mantle) for the mesothermal gold fluid.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

APATITE FISSION TRACK THERMOCHRONOLOGY AND SOUTH EAST AUSTRALIAN LANDSCAPE EVOLUTION: CAN EXAGGERATED DENUDATION RATES BE RECONCILED? Ian C. Roach CRC LEME, Department of Geology, Australian National University, ACT 0200. Apatite fission track thermochronology (AFTT) is a double-edged sword that can be used to both identify the absolute timing of major landscape-forming events and to estimate the amount of denudation that has occurred in a landscape. This powerful tool has added much to the debate of eastern Australian landscape evolution, particularly the origin and evolution of the Eastern Highlands. However, many authors can not reconcile estimates of the amount of denudation derived from AFTT with their own knowledge of the apparent stability of eastern Australian landscapes. Thus they regard the calculated denudation rates as being exaggerated. This difference in opinion comes about principally from the insistence of applying an upper crustal palaeogeotherm of 25-30°C/km blanket-wise to all AFTT results. Recent thematic papers in the Australian Journal of Earth Sciences (Volume 46/2) related to eastern Australian landscape evolution highlight the differences of opinion. Kohn et al. (1999), in a paper relating to an AFTT study of the Kosciuszko massif, concluded that the landscape was controlled by two periods of accelerated denudation, one in the Late Permian-Early Triassic and the other in the mid-Cretaceous. They calculated that 2-2.5 km of material had been denuded from the massif since the mid-Cretaceous. Hill (1999) expressed an opposite viewpoint, describing residual landscapes of Mesozoic age existing in the same massif, indicating a much reduced denudation rate, van der Beek et al. (1999) discussed the need for more realistic models that compensate for the 'extreme temporal and spatial variability in denudation rates' possible within their own model. Particularly, they concluded that Late Mesozoic-Early Tertiary palaeogeotherms must have been higher than present. This knowledge is crucial to understanding Eastern Australian landscape evolution. A suite of mantle and lower crustal xenoliths has yielded a new palaeogeotherm for the Eocene-Oligocene Monaro Volcanic Province of southeastern New South Wales (Roach 1999), located on the tableland to the east of the Kosciuszko massif Results indicate that the upper crustal palaeogeotherm was probably > 60°C/km during the time of volcanism, similar to that extant within parts of the Newer Volcanics Province of western Victoria. This palaeogeothermal domain covered a large area, perhaps extending into the Older Volcanics of eastern Victoria and certainly continuing into the Miocene with volcanism in the Snowy Mountains Province (e.g. Kiandra, Tantangara) and Jerangle. Exaggerated denudation rates can be reconciled with the knowledge of relative landscape stability in parts of the Eastern Highlands if accurate palaeogeotherms can be constructed using xenolith suites from intraplate volcanic lava fields. Results from the Monaro Volcanic Province, applied to AFTT data, can effectively reduce the calculated 2-2.5 km of post-mid-Cretaceous denudation to far lesser levels. References HILL S.M., 1999. Mesozoic regolith and palaeo-landscape features in southeastern Australia: significance for interpretations of denudation and highland evolution. Australian Journal of Earth Sciences, 46 217-232. KOHN B.P., GLEADOW A.J.W. AND COX S.J.D., 1999. Denudation history of the Snowy Mountains: constraints from apatite fission track thermochronology. Australian Journal of Earth Sciences, 46 181-198. ROACH I.e. 1999. The setting, structural control, geochemistry and mantle source of the Monaro Volcanic Province, southeastern New South Wales, Australia. University of Canberra PhD thesis, unpublished, van der BEEK P.A., BRAUN J. AND LAMBECK K., 1999. Post-Palaeozoic uplift history of southeastern Australia revisited: results from a process-based model of landscape evolution. Australian Journal of Earth Sciences, 46 157-172. Acknowledgments: This research was conducted while the author was in receipt of a University of Canberra research scholarship at the Centre for Australian Regolith Studies, University of Canberra, ACT 2600. CRC LEME is supported by the Australian Government's Cooperative Research Centres Program.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THERMOBAROMETRY OF CLINOPYROXENE IN ANKARAMITE LAVA FROM THE MONARO VOLCANIC PROVINCE, SOUTHEASTERN N.S.W. Ian C. Roach CRC LEME, Department of Geology, Australian National University, ACT 0200. The Monaro Volcanic Province is an Eocene-Oligocene basaltic lava field lying in the southeastern comer of N S W between the towns of Cooma and Bombala. Rocks within the MVP consist of a variety of primary to weakly-evolved mafic varieties including nephelinite, basanite, alkali basalt, transitional basalt, olivine tholeiite and rare tephrite, K-trachybasalt and picrite. The stratigraphy of the MVP consists of lower subalkaline lavas (tholeiites and transitional basalts) passing upwards into alkaline lavas, all intruded through by plugs, dykes and rare maars consisting of primary to evolved alkaline rock types. The transition from subalkaline to alkaline magmatism is marked by a series of prominent ankaramite (clinopyroxene porphyritic) lavas that outcrop close to later alkaline volcanic plugs, the presumed eruption centres for the ankaramites. Ankaramite lavas are distinguished by < 2 cm diameter cpx porphyrocrysts and glomerocrysts set in a groundmass of olivine (Fo75-62), feldspar, Fe-Ti oxides, cpx microlites and zeolite. Cpx porphyrocrysts have titanian augite cores grading into Cr-poor diopside rims and commonly feature concentric compositional zoning but less commonly hourglass zoning or rare undulatory zoning, similar to that seen in partially recrystallised regionally metamorphosed quartz. Cpx has ophitic rims that commonly contain inclusions of olivine and labradorite feldspar. Cpx cores are free of inclusions. Feldspars evolve to bytownite-andesine and anorthoclase-sanidine in the interstices between porphyrocrysts. The ankaramites commonly outcrop as resistant flow units, forming distinct terraces, or as plateaux in the more eroded parts of the MVP. Some have prominent deep red to yellow bauxite caps where protected by overlying lavas. The mineral chemistry and core-to-rim compositional variation in a small suite of cpx phenocrysts was determined by electron micro-probe analysis. A sharp increase in FeO and decreases in CaO, MgO and Mg/Mg+Fe define the core/rim boundary. Rims are typically enriched in FeO, T i 0 2 , Na20 and AI2O3 and are depleted in MgO, with corresponding lower Mg/Mg+Fe, relative to cores. Pressure/temperature relationships were determined from electron micro-probe analyses using the single pyroxene thermobarometers of Mercier (1976, 1980; M76/80) and Taylor and Nimis (1998; TN98). Results according to the M76/80 thermobarometer indicate that groundmass cpx crystallised from > 30 kb to the surface at 1055-850°C and cpx porphyrocryst cores commenced crystallising at ca. 22 kb/1050°C, peaking at 15-20 kb/980-1050°C. Cpx rims crystallised through to the surface at ca. 850°C. Cpx porphyrocryst cores applied to the TN98 thermobarometer commenced crystallisation at ca. 25 kb/1200°C, peaked at ca. 18 kb/1150°C and ceased at ca. 11 kb/1055°C. Errors for the M76/80 thermobarometer are estimated at ± 4 kb/30°C and for the TN98 thermobarometer at ± 3 kb/30°C. Cpx porphyrocryst cores peaked crystallisation at 18±3 kb (TN98) or ca. 54±9 km depth, coinciding with the base of the seismic crust, which lies at ca. 50 km depth (Kennett 1997). Thus the ankaramites are interpreted to represent vestiges of underplated magmas, added to the base of the crust mid-way through the lifetime of the MVP. The ankaramites mark the sub-alkaline/alkaline magmatic transition and can also be used to determine the approximate depth of a major density contrast, here assumed to be the crust/mantle boundary. References KENNETT B.L.N., 1997. The mantle beneath eastern Australia. AGSO Journal of Australian Geology and Geophysics, 17 49-54. MERCIER J.-C., 1976. Single-pyroxene geothermometry and geobarometry. American Mineralogist, 61 603-615. MERCIER J.-C., 1980. Single-pyroxene thermobarometry. Tectonophysics, 70 1-37. TAYLOR W.R. AND NIMIS P., 1998. A single pyroxene thermobarometer for Iherzolitic Cr-diopside and its application in diamond exploration. In: 7th International Kimberlite Conference, 897-898. Acknowledgments: This research was conducted while the author was in receipt of a University of Canberra research scholarship at the Centre for Australian Regolith Studies, University of Canberra, ACT 2600. CRC LEME is supported by the Australian Government's Cooperative Research Centres Program.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

APPLICATION OF RISK ANALYSIS TO AREA SELECTION IN PASMINCO Paul Roberts and Tom Eadie Pasminco Level 7, 380 St Kilda Rd, Melbourne, Victoria 3004, A U S T R A L I A

Pasminco Exploration is organised into three principal areas - projects, project generation and support. Project generation's brief is to identify both highly zinc-prospective greenfield belts and stand alone projects anywhere in the world; projects teams are then allocated to the best of those prospective belts and/or projects. Selection of belts and ongoing rating of existing belts against one another is therefore a major challenge in managing Pasminco Exploration. Pasminco has chosen a quantitative decision analysis approach to rate belts because of the intrinsic difficulty of assessing the relative importance of different types of risk using qualitative methods. For example, what is the relative importance of country risk as against geological risk or the risk that a given geophysical technique will not find the target orebody? Many explorers would say that geological risk is more important but, crucially, cannot quantify by how much. Therefore, in the past, in many organisations, ranking the different sources of exploration risk has been made by "gut feel", albeit by highly experienced explorationists. In the current shrinking world of exploration, "gut feel" alone is no longer good enough. The purpose of decision analysis is to break down the thinking process to the point that quantitative estimates of risk can be made. In Pasminco, the first step in this process is to separate out the four principal areas of risk, which are: • • • •

how many undiscovered orebodies are likely to exist in the belt? of those, what are the chances that we can find them? what are the odds that we will get access to the ground with the target orebodies in them, before our competitors? and what is the risk that we will be unable to mine the deposit profitably because of changes in the "rules" (eg. expropriation, change in tax policy) or a breakdown in the stability of the country?

These four questions are then broken down further to the level that we can make credible quantitative assessments. Examples of the more detailed questions are: •

• • •

what grade and tonnage values are needed to meet Pasminco's minimum objectives, what is the NPV of the most likely Pasminco target discovery and how deeply buried can the target be and still be economic ("economic depth limit")? what are the controls on ore formation and how confident are we that they are right? to what extent has exploration already effectively sterilised the prospective areas (fairways), both at the surface and down to the economic depth limit? what techniques can we use to find the target orebody, what are the reliabilities, false alarm rates and unit costs of those techniques, and which combination of techniques maximises reliability at the same time as minimising the overall cost of exploration?

This approach yields a variety of quantitative measures including expected value, probability of discovering the Pasminco target and minimum cost of exploration before abandoning the belt, all of which are used to guide the decision making process.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LOW VISCOSITY SHEAR ZONES WITHIN BROADER DUCTILE TRANSPRESSION ZONES Pierre-Yves F. Robin and Alexander R. Cruden Department of Geology, 22 Russell Street, University of Toronto, Toronto, Ontario, Canada M5S 3B1

We investigate a model of ductile transpression in which a relatively thin, low-viscosity layer is sandwiched in the middle of a wider zone of higher viscosity. In nature, the high viscosity transpression zone could be a crustal-scale transpression domain, while the low viscosity zone might be a narrow, vertical, softened shear zone within that domain. In the field, the narrow central shear zone thus appears to be 'the shear zone', while the broader transpressive zone appears as its less deformed 'wall rock'. Because the thickness of 'the shear zone' is small compared to its other two dimensions, its material does not extrude from its 'wall rock': it is therefore constrained to match the in-plane strain of that 'wall rock'. The strain response of such models is examined analytically and with analogue experiments. Analytical modelling deals with small strains within zones of infinite extent and idealized geometry. Analogue models are used to confirm the analytical models as well as to study the consequences of finite strains, heterogeneities, and end-effects. The significant parameters are: convergence angle, a, and viscosity ratio, Rr^. As expected, the horizontal shear strain is concentrated in the low viscosity shear zone, but that zone is also stretched vertically, to match the vertical extension of the wall rock. Varying convergence angle and viscosity ratio leads to a number of interesting situations. E,g.\ 'lineation' (i.e. the direction of maximum principal strain) in the narrow shear zone can be horizontal, while those in the adjacent wall rock are vertical; or the strain in the walls can be dominantly vertical stretches (Flinn's A: > 1), whereas flattening strain {k^ 0) occurs in the shear zone. Such 'dual viscosity transpression' thus appears to be a viable explanation for shear zones that exhibit flattening strains rather than plane simple shear strain: in particular, it does not violate the requirement of strain compatibility or of coherence along the boundaries of the shear zone. If the low viscosity of the inner zone is related to percolation of fluids along it, the model appears to predict an intriguing oscillation of the system. The direction of minimum principal stress within the shear zone may alternate between vertical and horizontal, with the dominant fluid path alternating between the inner zone and its adjacent wall rock. If the fluid precipitates its solute content along the way, a petrographic consequence would be a repeated superimposition of veins of contrasting orientations within the zone and its wall rock.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MODELLING THE PROPAGATION OF PRESSURE SOLUTION CLEAVAGE SEAMS AND THEIR INTERACTIONS Pierre-Yves F. Robin \ Frank Fueten ^ and Michael Schweinberger ^ ' Department of Geology, 22 Russell Street, University of Toronto, Toronto, Ontario, Canada M5S 3B1 ^ Department of Geological Sciences, Brock University, St.Catharines, Ontario, Canada L2S 3A1

Cleavage seam propagation is modelled as viscous flow, with two-dimensional finite elements. The rock is modelled as a quartz-phyllosilicate mixture. The viscosity of each element depends on its composition (i.e. its proportion of quartz and phyllosilicate), and that composition is allowed to change in response to the strain history of the element. The rheological behaviour is that proposed by Robin (1979): deformation is achieved by pressure solution of quartz, with stress-induced diffusion transfer of silica, the transfer being catalyzed by the phyllosilicates. The variation of viscosity with mineral fraction correspondingly exhibits a minimum for intermediate quartz content; the viscosity rises to high values for both low mica content (no catalyst) and low quartz content (no silica to diffuse). Each element loses or gains silica at each iteration, as a direct consequence of its strain. In most runs, the total silica content of the system is constrained to remain constant. A seam is nucleated by introducing a few elements with an initial mica content that is higher than that of the rest of the rock. The propagation of a single cleavage seam can be seen as a generalization of the propagation of an 'anticrack' (Fletcher and Pollard 1981), with a stress concentration at its tip, and a consequent loss of quartz and weakening of the 'process zone'. We will follow the propagation of a single seam (after Fueten and Robin 1992), the various possible interactions between two seams (depending on their relative positions within the strain field), the development of complex and irregular segregation cleavage nucleated from scattered initial seeds, and the evolution of a rock into parallel tabular domains of alternating, but each uniform, compositions. References FUETEN, F., ROBIN, P.-Y. F. (1992) Finite element modelling of the propagation of a pressure solution cleavage seam. Journal of Structural Geology. 14, 953-962. FLETCHER, R. C., POLLARD, D. D. (1981) Anticrack model for pressure solution surfaces. Geology. 9, 419-424. ROBIN, P.-Y. F. (1979) Theory of Metamorphic segregation and related processes. Geochimica et Cosmoschimica Acta. 43, 1587-1600.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE SMECTITE-TO-CHLORITE TRANSFORMATION: A MODEL FOR CONTRASTING REACTION PATHWAYS AND REACTION PROGRESS Doug Robinson Department of Earth Sciences, University of Bristol, Bristol, UK

The transformation from smectite to chlorite is perhaps the most ubiquitous of all mineralogical transformations occurring in the Earth's crust. The transformation is interpreted as involving either a disequilibrium chlorite/smectite mixed-layering sequence or an equilibrated, discontinuous sequence of smectite - corrensite - chlorite. Here, analysis of the smectite to chlorite transformation in geothermal systems suggests instead that it proceeds via three contrasting reaction pathways involving i) a continuous mixedlayer chlorite/smectite series; ii) a discontinuous smectite - corrensite - chlorite series and iii) a direct smectite to chlorite transition. Such contrasting pathways are not in accord with that expected for an equilibrium mineral reaction series, suggesting that these pathways record kinetically-controlled reaction progress. The transformation process of smectite to chlorite is initiated over the range c. 150 200°C, but different reaction pathways then follow over broad and different temperature ranges, with chlorite not becoming dominant in some systems until c. 270°C. Thus, the progressive development of mixed-layer chlorite/smectite, corrensite or chlorite is not diagnostic of specific temperatures or even a temperature range, but is proposed to be related to the process(es) driving the reaction step. In the geothermal systems reviewed the style of reaction pathway and degree of reaction progress is closely correlated with intensity of alteration, and not to factors such as thermal gradient or clay grain size. This suggests a kinetic effect linked to the intensity of fluid/rock interaction. In settings showing incomplete reaction or low fluid/rock ratios, chlorite/smectite mixedlayering is developed. For these cases, it is proposed that a solid-state transformation process operates generating chlorite/smectite mixed-layering. Nutrient transport is probably restricted to a volume diffusive mode, and thus growth rates are slowest relative to nucleation, representing a kinetic constraint on the smectite to chlorite transformation. Areas of higher fluid-rock ratios develop the discontinuous smectite - corrensite - chlorite transformation. In these settings processes of dissolution/nucleation/precipitation are more likely and thus a higher growth rate relative to nucleation rate, leading to the discontinuous transformation. In settings with the highest porosities/permeabilities, there is the potential for advective transport that would generate the highest rate of nutrient transport. In this case, the potential is for the most rapid growth rate of all, giving rise to the development of a discrete, non-swelling end-member phase, represented by the direct smectite to chlorite transformation. This model can thus accommodate the varied reaction styles and reaction progress seen for the smectite to chlorite transformation that have been an unexplained anomaly at very low metamorphic grade. It can also account for different reaction styles and reaction progress being developed in immediate proximity due to contrasting rock morphologies.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SEABED MAPPING IN THE GREAT AUSTRALIAN BIGHT MARINE PARK: PRELIMINARY RESULTS FROM THE AUSTREA-1 CRUISE N. Rollet^ P.J. Hill\ N.F. Exon', A. Butler^ and K. Glenn^ 'Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^CSIRO Marine Research GPO Box 1538 Hobart, Tasmania 7001

In late 1999-early 2000, the Australian Geological Survey Organisation and Environment Australia undertook the AUSTREA-1 survey in the central Great Australian Bight (GAB), using the French research vessel L 'Atalante, The survey was focused on the deep-water part of the Benthic Protection Area of the GAB Marine Park (33° 20'S - 36°S, 130" 20'E 131°E), south of the Potoroo-1 petroleum exploration well. About 16,800 km^ of swath and other geophysical data were recorded in the park to localise the sites where representative examples of marine ecosystems could have been developed. These new data show that there are links between deep and shallow geological structures and provide information about some of the tectonic processes operating in this region. The data collected included multibeam swath-bathymetry and backscatter imagery, 6channel high-speed reflection seismic, digital 3.5 kHz sub-bottom profiles, gravity and magnetics. Oceanographic information was also acquired in the form of XBTs to 1800 m depth and underway ADCP (current), sea surface temperature and salinity data. The GAB Marine Park was fully surveyed between the 500 and 5000 m isobaths. The data show an upper margin sloping gently oceanwards down to the 2800 m isobath, and a continental slope that is steep between the 4000-4500 m isobaths. The most prominent physiographic feature mapped is the Nullarbor Canyon, which is incised into deformed Upper Cretaceous sediments. The canyon contains large-scale pock-marks or craters along the canyon axis. These features are up to 4-5 km across and 300-500 m deep and are also found in deep water in the canyon prolongation. This geometry has not been observed previously on this margin and their origin is unknown. The holes are possibly produced by leaking fluids and, if so, they may host chemosynthetic organic communities. Other canyons are present to the southeast of the Nullarbor Canyon with roughly the same NNE-SSW trend, but at a slight angle to the slope, which suggests that they are tectonically controlled. They are perpendicular to the normal faults observed on previous seismic lines in this region and oblique to the interpreted NW-SE transform faults which were active during the extension between Australia and Antarctica. The strong backscatter observed along the axis of the Nullarbor Canyon suggests that deep-sea currents have scoured the seafloor, leaving coarse lag deposits and exposing deeper and harder strata. As this margin is relatively sediment-starved, it is unclear whether or not the canyon is currently an active sediment conduit. Given the complex geological history of the region, and the low sedimentation rates that have persisted for the past -50 million years, it is apparent that an improved understanding of the seafloor processes that have produced the present-day physiography can best come from the integration of deep and shallow (high-resolution) seismic data with other geophysical data and geological sampling. To this end, the geological sampling program to be carried out in May 2000 on the CSIRO R/V Southern Surveyor in and near the Nullarbor Canyon will be particularly important.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE CRETACEOUS TRANS-ARGENTINE ABORTED RIFT CORRIDOR AND ITS ASSOCIATED ALKALINE VOLCANISM. Eduardo A. Rossello^'^, Monica Lopez de Luchi^ and Marcos E. Mozetic' '^ ^Departamento de Ciencias Geologicas, Universidad de Buenos Aires, ^CONICET ^Centro de Investigaciones en Recursos Geologicos, ^ Repsol.YPF S.A. The Cretaceous rans-Argentine aborted rift corridor is recognised in the central part of western Argentina by the alignment of deep and narrow N-S trending Late Cretaceous rifts (Macachin, General Levalle, Gigante, Pagancillo, Campo del Arenal, Salinas depocenters) that are associated with coeval within-plate alkaline volcanism. Basins are typically steeply dipping hemi-grabens with characteristic rift-like sedimentary sequences, which are controlled by strongly uplifted Proterozoic or Pre-Carboniferous peneplanized basement blocks that crop out at the Sierras Pampeanas. Towards the northern extreme, the belt is related with the southernmost branch of the three ones that made up the Cretaceous Salta basin rift (which evolved from a triple point and shows Pacific connections through Peru and Bolivia). Towards the south this belt evidences links, albeit segmented by the Alto Interserrano, with both the Colorado basin via the Macachin basin and the Salado basin via the Junin basin, located on the Atlantic border of the South America plate. Alkaline volcanism is developed from the Salta basin down to the south of the Sierras Pampeanas Orientales (Eastern Pampean Ranges). These rocks are either interlayered in the intracratonic basins or exposed as isolated small volcanic cones with bear no relation with Cretaceous sediments. From north to south some examples of this volcanic activity are found at Quebrada Las Conchas (Salta), Rio Belen (Catamarca), Los Condores hill at Sierras de Cordoba and in southern Sierras de Cordoba and San Luis. At the Salta basin, the Las Conchas alkaline magmatism is represented by basanites whereas at Rio Belen, Catamarca, the (131.0 ± 4.0 Ma). La Puerta de San Jose basalt is interlayered in a continental pile and constituted an isolated expression of within-plate alkaline magmatism that could either indicate a new Cretaceous depocenter or belong to an 200 km inferred extension of the present southern border of the Salta Basin. In the Sierras Pampeanas Orientales at Cordoba, Los Condores rocks are basanites to basaltic trachyandesites. The Late Cretaceous-Early Tertiary strongly alkaline volcanism (nefelinites, mellilitites and minor leucitites) that appears as isolated hills at the south of the Sierras Pampeanas of San Luis and Cordoba (La Garrapata, La Madera and Chajan hills) and as very restricted outcrops in the basement of the Sierra de San Luis, indicate low degree of partial melting of a deep mantellic sources, the ascent of those melts have been adscribed to processes related with the Atlantic ocean aperture . The intracratonic location of the corridor suggest a strong control by regional crustal weakness zones which could have allowed the ascent of mantellic-source melts like those that are represented by the basic alkaline magmas. All the depocenters depict more or less aligned troughs that integrate an almost continuous intracratonic submeridional belt that cut through the entire Argentina (from the Buenos Aires province to the Puna). This deep depocenter belt is related in both time and space with the Atlantic aperture because it could be considered as the result of a Late Cretaceous aborted intracontinental extension. Some subsurface surveys of the structures and the external architecture of the horizontal and vertical sections of the infillings indicate transcurrent features. The Andean reactivation of older discontinuities affected the entire South America, in the case under study, the listric normal faults, especially those that controlled the western flank of the depocenters were inverted. The intensity of these inversions in the basement blocks of the Andean foreland increases towards the west.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ALBITIZATION OF PELITIC AND PSAMMITIC ROCKS IN THE SNAKE CREEK ANTICLINE, MOUNT ISA INLIER STRUCTURAL CONTROLS AND P-T-t PATHS M. J. Rubenach and K. J. Lewthwaite Department o f Earth Sciences, James Cook University, Townsville

The Soldiers Cap Cap Group in the Snake Creek Anticline consists of pelitic schist, psammite, quartzite and amphibolite, and was subjected to low-P/high-T metamorphism. Porphyroblast microstructures indicate multiple growth events during the Isan Orogeny, the metamorphic peak occurring syn- to post-D3SC. Albitization is widespread, but is particularly concentrated in three belts, up to 7km long and 500m. wide. Structural controls include (1) albitite haloes adjacent to quartz veins, (2) as replacement of syn- to post-D3SC shear zones, and (3) as replacement of metasedimentary rocks adjacent to amphibolite bodies, probably as a result of ductility contrasts providing fractures for infiltrating fluids. Psammitic rocks were preferentially albitized, and pelitic rocks were also replaced where substantial albitization of psammitic rocks has occurred. The relationships between albitization, foliation development and porphyroblast growth are quite complex, but inclusion trails and porphyroblast/matrix relationships indicate that the dominant albitization event occurred broadly synchronous with the metamorphic peak. Post-peak albitites, common in the Mary Kathleen Group, are relatively rare in the Snake Creek Anticline. Although staurolite and garnet of uncertain timing in some albitized psammitic rocks contain albite inclusions, the majority of syn-D3SC porphyroblasts in albitized pelitic rocks contain quartz inclusions but are free of albite, the dominant matrix mineral and which defines S3SC partly wrapping around the porphyroblasts. However, the clearest timing criteria occur in albitite samples in which syn-D3SC andalusite contain only quartz inclusions whereas post-D3SC andalusite grains contain abundant albite inclusions. Albitization involved destruction of muscovite, and where intensely developed, replacement of quartz and biotite. Accessory minerals include apatite, rutile, tourmaline, and monazite. Porphyroblast assemblages include staurolite; garnet; staurolite + garnet ± gedrite; cordierite; andalusite ± cordierite psuedomorphs; kyanite. Late syn- to post-D3SC sillimanite occurs in some samples. Peak metamorphic conditions for albitites in the andalusite/staurolite zone were 4 kbar and SSO^'C. Cordierite grew pre-D3SC, and common replacement by syn-D3SC andalusite ± sillimanite indicates a rise in both P and T during D3SC. Reaction of early andalusite to staurolite + muscovite, and rare examples of replacement of cordierite by biotite, staurolite and kyanite, suggest a temperature drop, with or without a pressure rise, before the onset of D3SC. Isocon studies demonstrate gain of Na and loss of K, Mg and Fe during albitization. Unusual muscovite-free schists rich in biotite and staurolite occur adjacent to albitites in the eastern and western albitization belts, suggesting localized source/sink relationships for Fe and Mg. d l 8 0 values for albitites are in the range 7.6-10.4%o whereas schists and psammites are in the range 7.1-12.8%o. It is suggested that albitites formed from infiltrating magmatic fluids, which also reset most of the apparently unmetasomatized pelitic to lower values.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NEW INSIGHTS ON THE GREEN RIVER PETROLEUM SYSTEM OF THE UINTA BASIN FROM HYDROUS PYROLYSIS EXPERIMENTS Tim E. Ruble', Michael D. Lewan^ and R. Paul Philp^ 'CSIRO Petroleum, P.O. Box 136, North Ryde, NSW, Australia 1670 ^U.S. Geological Survey, Box 25046, MS 977, Denver, CO, U.S.A. 80225 ^University of Oklahoma, 100 E. Boyd St., Norman, OK, U.S.A. 73019

The Tertiary Green River petroleum system in the Uinta Basin is responsible for almost 500 million barrels of recoverable, high pour-point, paraffmic crude oil and is a classic example of hydrocarbon generation from lacustrine source rocks. Producing fields in the deep basin center have elevated pore-fluid pressures which approach 80% of lithostatic pressure. The reservoirs have no cap rock or conventional trapping mechanism and are thought to result from permeability formed via high pore pressure fractures. One explanation which has been suggested for the origin of these overpressured reservoirs is active oil generation. Basin models using Rock-Eval derived kinetic parameters have encountered difficulties in that the predicted pods of hydrocarbon generation do not accurately match the proven zones of oil production. In addition, predicted hydrocarbon generation occurs too early and too quickly in the source rocks associated with the known zone of production to account for the present overpressured reservoirs. There is a large pulse of early generation associated with rapid burial 20-30 Ma, during which time transformation ratios in the basal source pod reach > 90%. Overpressure then dissipates during a rapid decline in generation from 20 Ma to present. As a result, the geothermal gradient and/or permeability input parameters in these models must be lower than measured data to sustain overpressure. In the present study, we have used the BasinMod® program and modified various input parameters from a previously published geologic model for the Shell Brotherson 1-11-B4 well (Altamont Field). The revised model incorporates a 25°C/km geothermal gradient and 1796 m of eroded Oligocene/Miocene overburden removal. The objective was to evaluate the effects of various kinetic parameters on the location, rates and timing of oil generation in the Uinta Basin. Source rocks in both the lower black shale facies and the Parachute Creek Member of the Green River Formation were considered. Results clearly show that kinetic parameters derived from closed system hydrous pyrolysis experiments are more consistent with natural data. Rock-Eval kinetics predict significant oil generation from the rich oil-shales in the upper Green River Formation, while hydrous pyrolysis kinetics accurately predict that oil generation has been restricted to the basal source pod. Transformation ratios in the lower black shale facies range from 5-85% using hydrous pyrolysis kinetics. The remaining source potential predicted by this model suggest that active oil generation may be a viable mechanism to account for overpressuring in the overlying reservoirs.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

STRUCTURAL CONTROLS OF FLUID PATHWAYS DURING GOLD DEPOSITION AT THE VICTORY COMPLEX ST IVES GOLDFIELDS, KAMBALDA, WA Kevin Ruming\ and Stephen Cox^ ' School of Geoscience, University of Newcastle, NSW, 2308 ^ Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 The Victory Complex comprises eight main shear zones that collectively have yielded more than 1,800,000 ounces of gold. The gold mineralisation is associated with hydrothermal alteration haloes adjacent to quartz, carbonate and albite veins and breccias which are developed in parts of the shear system. Several structural controls influence the localisation of gold related veins and breccias within the shear zones: (1) lithological contacts; (2) fault bends and (3) internal shear zone structure. The Victory Complex is located on the Playa shear system, a regional NNW trending steeply east dipping shear zone. The Playa shear has a strike length of over 20 km and an inferred displacement of up to 1 kilometre due to sinistral strike slip to oblique slip movement. It is developed within metamorphosed Archaean greenstones ( 2 . 7 2 - 2.68 Ga.) composed of komatiitic and basaltic lavas, and minor sedimentary rocks. A dolerite sill and felsic and lamphrophyric dykes intrude the greenstones. Fluid inclusion data indicate the gold mineralisation was emplaced at crustal levels of 10-12 km at approximately 4 0 0 ^ after mid greenschist facies peak metamorphism. Two broad structural domains, the eastern and western, divide the Victory Complex. The eastern domain is dominated by a set of NNW trending, east dipping, listric thrust faults (Repulse, Victory, Britannia and Sirius) whereas the western domain comprises gently SE dipping reverse shear zones (30, 31, 32, 33) and a series of smaller interconnected shear zones and breccias horizons (34,35,36,37). The structures in the eastern domain formed during the second of four deformation events, as a result of SW directed thrusting. The structures in the western domain formed during W directed, D3 thrusting. Gold bearing fluids were focussed into certain parts of the shear system during late D3. Lithological Contacts Lithological contacts within the shear zones have played a significant role in localising gold bearing fluids. This control has operated on all scales from the mine to the drive scale. This type of control is important in the eastern domain where the D2 faults (Repulse and Britannia) have been reactivated during D3. During reactivation the deformation and therefore fluid flux was localised along lithological contacts within these shear zones. Fault Bends: There are two important mine scale zones of mineralisation related to fault bends on the Repulse fault. One of these occurs in a hanging-wall position, in a relatively flat section of the fault, immediately west of a major ramp in the eastern part of the Repulse fault. This position is interpreted structurally as a dilational site in terms of D3 kinematics. In the western domain, in a foot-wall position to the Repulse fault where the dip changes from shallow to steep, a series of interconnected shear zones and breccia horizons (34, 35, 36, 37) splay out from it. This zone is interpreted as analogous to a horsetail structure. Internal Shear Zone structure: Gold mineralisation within the SE dipping, D3 shear zones in the western domain appears random on the mine scale. At the scale of underground ore drives the foliation development within the shear zone is heterogeneous with distinct zones (up to 2m wide) of intense foliation development. Gold bearing veins and breccias are commonly localised along these zones. Some sections of these intensely foliated zones have boudins developed in them with veins and breccias localised within boudin necks. These zones of intense foliation development are thought to be either ramp structures or anastomosing high strain zones within the shear zones along which the gold bearing fluids flowed. These three types of structural controls on gold mineralisation at the Victory Complex have focussed high fluid fluxes through particular parts of the shear zones. This fluid localisation is interpreted as a result of the localisation of strain and consequently deformation in these parts of the shear zones. The localisation of deformation has maintained shear zone permeability which would otherwise be quickly destroyed due to the precipitation of quartz, carbonate and albite.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

FRACTURES AND FLOW: AN EXPERIMENTAL LOOK AT MELT SEGREGATION PROCESSES IN THE EARTH'S CRUST Tracy Rushmer Department of Geology, Perkins Hall, University of Vermont, Burlington, Vermont, USA Introduction Partial melt plays a significant role in crustal differentiation processes and its subsequent geochemical evolution. We can place melting, and its rheological and geochemical impact, into three main, ongoing processes that occur at different levels in the crust. At the base of the crust, or in the lower crust, melt segregation at the grain scale is the controlling factor in whether or not melt can migrate and therefore is a major influence on crustal differentiation overall. As melt collects and begins to migrate, the middle crust properties control melt transport and the scale of the processes changes from grain size to outcrop. When melt begins to crystallize and cool, emplacement occurs in the upper middle/upper crust and again the scale of observation is increased. As a result, partial melting must be viewed from a variety of scales and over a range of pressure and temperature. In addition, rheology of the crust is strongly influenced by the presence of melt. In the experiments reported here, the conditions were set to be representative of the lower-middle and lower crust and have been conducted in both static and dynamic environments. They are focused on the grain scale processes that occur during partial melting and have been used to investigate the impact of melt segregation on melt geochemistry and crustal rheology. Static vs. dynamic partial melting: The role of mineral reactions Under static conditions, melt distribution has been investigated by many studies using dihedral angles, or the surface energy control, to determine melt - solid relationships and melt segregation potential. These results have been useful in determining the nature of melt distribution, after melt is produced, under static conditions. However, the reactions which produce the partial melt itself also have a significant influence on melt extraction processes and geochemistry of those melts. For example, muscovite-bearing assemblages are important in this aspect because it has been shown that the volume change associated with muscovitedehydration melting produces a dilational strain in the source rock that induces microcracking in the matrix. The melting reaction itself therefore provides a mechanism for melt extraction by the formation of a microcrack network that is interconnected. SIMS analyses of the partially molten muscovite-bearing assemblages shows that the melt geochemical signature is as a function of time and extent of reaction and provides information on the initial trace element signatures that these early melts then carry with them from their source to middle and upper levels in the crust. The highly incompatible elements, such as Sr and Ba, most strongly reflect the extent of muscovite melting and show the influence of developing reaction products on partial melt trace element composition. REE patterns also reflect extent of reaction, showing low overall abundances for the short experiments where muscovite is not yet completely reacted. In contrast, biotite-only bearing assemblages do not produce a significant enough dilational strain during partial melting to induce fracturing. Partial melt volumes are also lower. Melt is distributed along grain boundaries and this melt distribution, in turn, may influence crustal rheology. Partial melt is not able to segregate along fractures at initial melting as with the muscovite assemblages so melt remains for a longer period of time in the source region. Significant weakening of the crust may occur during partial melting of biotite-bearing gneisses. These two contrasting melting behaviors will also control the rate at which melt pore pressure builds up in dynamic, or in deforming, environments. Deformation experiments have been conducted on the muscoviteand biotite-bearing assemblages, both while the partial melting is ongoing and after melt has been generated. The results show that in the muscovite-bearing pelite, extensive melt-induced cataclasis is produced close to the solidus suggesting rapid pore pressure development. The biotite gneiss, however, does not embrittle and melt is found parallel to foliation and perpendicular to the main compressive direction, along grain boundaries.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July, 2000

TEXTURES IN MELT-SOLID SYSTEMS: EXPERIMENTAL DEFORMATION OF PARTIALLY MOLTEN ROCKS. Tracy Rushmer and Caleb Holyoke Illb Department of Geology, Perkins Hall, University of Vermont, Burlington, Vermont, USA, In an effort to understand the fundamental processes of crustal differentiation and crustal growth, partial melting experiments have been conducted on a variety of different rock types and significant effort has been placed on determining the composition and volume percent of partial melt generated during melting. More recently, focus has included the mechanical aspects of partial melting, including determining under which physical conditions melt can migrate from its source. To this end, experimentalists have investigated partial melting under both static conditions and with deformation. Both experimental approaches have been used to determine the conditions at which melt becomes interconnected. Under static conditions, surface energy (dihedral angles) has been investigated and the role of positive volume dehydration melting reactions (melt-induced microfracture). Deformation, however, is viewed as one of the most likely processes of enhancing melt segregation as most melting occurs in active environments and deformation itself helps provide pathways. We can interpret from the microstructural and geochemical data in the experiments the important variables that control melt interconnectivity, influence its pathways and determine melt volume at the point of segregation. These include; low dihedral angles, positive volume dehydration reactions with associated high dilational strains, presence of differential stress, melt pore pressure, the strain rate and strain percent, the strength of the solid matrix and viscosity contrast. In addition, the rate of pore pressure buildup and its influence on rock rheology during partial melting can impact melt segregation rates and geochemistry. Textural data collected from these experiments are varied. Data from the partial melting deformation experiments on an amphibolite, muscovite-bearing pelite and a biotite gneiss suggest that the role melt pore pressure is important in forming zones of fracture which can enhance melt segregation rates (melt-enhanced cataclasis), but that the rate at which it develops determines the extent and temperature range under which embrittlement occurs. Even though these rock types all contain hydrous phases and undergo positivevolume dehydration melting, the rate of pore pressure development is different due to the stoichiometrics of the different melting reactions. This is particularly noticeable when comparing the muscovite-bearing pelites which have large associated dilational strains (~6 to 7% for an assemblage with 30% muscovite) with biotite gneiss (<1% for an assemblage with 30% biotite). In partial melting deformation experiments performed on a two-mica pelite (0.7 GPa, 750-920°C, 10-5/s), data show that between temperatures of 750-800°C, melt-induced cataclasis is extensive. The solidus of the pelite is determined by the reaction: muscovite + quartz + plagioclase Y melt + kspar + sillimanite + biotite. At conditions just above the solidus (750°C) cataclasis is observed and becomes most extensive at 775-800°C, but above 800°C, melt fraction is high and ductile flow begins to occur. In contrast, partial melting experiments conducted on a biotite gneiss (0.7-1.0 GPa, 850 - lOOO^C, 10-5/s) show that the rate at which pore pressure increases is such that melt can be present in the rock, but cataclasis is not induced. The solidus of the gneiss is determined by the breakdown of biotite at 900°C by the reaction: biotite + plagioclase + quartz Y opx + kspar + melt + spinel. Ductile shear zones occur within the sample at 920°C and contains reaction products. At temperatures above 950°C, ductile flow begins to occur. Cataclasis has not yet been observed suggesting that melt migration processes may be significantly different for these two rock types. Metal-silicate deformation (1.0 GPa, 925 - 1050°C, 10-5/s) Textures developed in a metal-silicate system compliment those observed in the purely silicate systems. Experiments performed on a natural chondritic meteorite that contains 20-25 vol% Fe-Ni metal and FeS show that metallic liquid can also induce local cataclasis during deformation. In addition, the lower viscosity of the Fe-Ni metal (which is not molten under these conditions) in comparison with the silicate matrix of olivine and pyroxene allows for re-mobilization of metal in response to deformation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ULTRAFINE PARTICLE MOVEMENT IN THE REGOLITH: FIELD AND EXPERIMENTAL EVIDENCE Bryan P. Ruxton Division of Science and Design, University of Canberra ACT 2601

Experiments with deionised water flowing through tubes packed with remoulded weathered granite under constant head showed very low clay eluviation after an initial flush of clay. Drying and wetting and escape of entrapped air, particularly bubbles, gave rise to severe clay shift. With debris of bulk density 1.84 tm"^ channelling commenced at eight hours. With debris of bulk density 2.06 tm'^ the tube burst at 812 ml of flow. Under steady conditions the long experiments showed final rates of clay shift of about 30 |ig g"^ day'\ Details of four runs are given in the table. PARAMETER Water passed (ml) Clay shift (gm) Density (tm"') Ca. Porosity (%) Permeability (10"^ ms"') Time (minutes) Flow rate } INITIAL (a)

RUN A 2260 1.220 1.84 32 106 590 .179

RUNB 2200 1.225 1.87 30 70 975 .085

RUNG 2220 0.921 1.89 28 3.4 10,003 .147

RUN D 812' 0.231' 2.06 23 <3.1 1283 .059

L hr"' SECONDARY (b) Flow rate ratio a/b Head (cm) 1. Tube burst at 812 ml

.239 .75 75

.168 .50 67.2

.148 1.0 137.6

.029 2.0 172.2

In two field examples the actual amounts of clay shift from ten tons of sedentary weathered granite during and after a severe rainstorm is about 250 grams. Frequency of storms is important. Rainstorms with intensifies of over 50 mm per hour occur every four years in the dry savanna of the Sudan and twelve times a year in humid tropical Hong Kong, clay shift is in proportion. Data elsewhere shows similar sediment yield per unit area in soft sedimentary rocks. Particle size distribution is also important. In dry conditions mud cracks provide initial pathways even for fine sand until wetting closes the cracks. In bimodal weathered granite fines can move through a skeletal framework. In silts saturation of free faces cause a flowing, out, working rapidly inwards. Mechanical eluviation is important in basal sapping at and below the hillfoot due to a clay content in the return flow. In the matrix of weathered rhyolitic boulder colluvium in Hong Kong a forest of halloysite struts connecting up fine silt size clay aggregates allow the movement of clay size particles (0. l|Lim to 2|Lim) between the struts. Close packed clay aggregates have microchannels 5[im in diameter also allowing fine clay movement. Macropipes are found in this material and originate from joining up of these microforms. Movement of water at 10"^ ms"^ has The drag force to dislodge particles 0.5^m in diameter. Piping can usually commence at and above this permeability. Bulk densities range from 1.2 to 1.7 tm"^ Porosities vary from 50 to 25 percent.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HIGH RESOLUTION, HIGH SENSITIVITY IMAGING AND ANALYSIS OF MINERALS AND INCLUSIONS (FLUID AND MELT) USING THE NEW CSIRO-GEMOC NUCLEAR MICROPROBE Chris G. Ryan', Esme van Achterbergh^, Brent M. Mclnnes^ Patrick J. Williams^ Guoyi Dong^' and Khin Zaw^ ' CSIRO Exploration and Mining, PC Box 136, North Ryde NSW 2113, Australia. ^ GEMOC National Key Centre, Macquarie University, NSW 2109, Australia. ^ Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville Q4811. Centre for Ore Deposit Studies, University of Tasmania, GPO Box 252-79, Hobart Tas 7001, Australia. * Present address: Great Central Mines, 580 St Kilda Road, Melbourne Vic 3004, Australia. The new CSIRO-GEMOC Nuclear Microprobe (NMP) The instrument was designed specifically for minerals analysis and imaging and to achieve ppm to sub-ppm sensitivity at a spatial resolution of 1-2 fim using X-rays and y-rays induced by MeV energy ion beams. The key feature of the design is a unique magnetic quadrupole quintuplet ion focussing system that combines high current with high spatial resolution (Ryan et at., 1999). These design goals have been achieved or exceeded. On the first day of operation, a spot-size of 1.3 |im was obtained at a beam current of 0.5 nA, suitable for fluid inclusion analysis and imaging. The spot-size grows to just 1.8 |Lim at 10 nA (3 MeV protons), ideal for mineralogical samples with detection limits down to 0.2 ppm achieved in quantitative, high resolution, trace element images. Applications of the NMP include: research into ore deposit processes through trace element geochemistry, mineralogy and fluid inclusion analysis of ancient deposits and active sea-floor environments, ore characterization, and fundamental studies of mantle processes and extraterrestrial material. Quantitative True Elemental Imaging Dynamic Analysis is a method for projecting quantitative major and trace element images from proton-induced X-ray emission (PIXE) data obtained using the NMP (Ryan et aL, 1995). The method un-mixes full elemental spectral signatures to produce quantitative images that can be directly interrogated for the concentrations of all elements in selected areas or line projections, etc. Fluid Inclusion Analysis and Imaging The analysis of fluids trapped as fluid inclusions in minerals holds the key to understanding ore metal pathways and ore formation processes. PIXE analysis using the NMP provides a direct non-destructive method to determine the composition of these trapped fluids with detection limits down to 20 ppm. However, some PIXE results have been controversial, such as the strong partitioning of Cu into the vapour phase (e.g. Yankee Lode, Mole Granite, NSW [Heinrich et aL, 1993] and Batu Hijau, Indonesia [Mclnnes et a!., 1999]), and the high concentrations of some elements in many orerelated fluid inclusions [e.g. Pb ~4 wt% at Hellyer, Tasmania (Khin Zaw et aL, 1996) and Ba ~9 wt% at Starra, Cloncurry district, Queensland (Williams et aL, 2000)]. Now, using the NMP, the internal contents of individual fluid inclusions can be imaged to show clearly that these elements reside within the fluid inclusions, and to discrimination against solid phases outside the inclusion volume. Melt Inclusion Analysis and Imaging Samples of melts and fluids, responsible for metasomatic change and evolution of the earth's upper mantle are often preserved as inclusions in xenoliths. However, their quench textures can often conceal rare minor phases that concentrate important trace elements (e.g. HFSE and REE). The penetration of MeV protons enables the detection of these contributions to - 4 0 |im depth, thus providing a tool to determine reliable melt composition, with detection sensitivities down to 0.2 ppm, and to image spatial variation in component elements at 1-2 |Lim resolution. References Heinrich, C.A., Ryan, C.G., Memaugh, T.P., and Eadington, PJ. (1993), Econ. Geol. 87, 1566-1583. Mclnnes, B.M., and Ryan, C.G., (1999), unpublished data. Ryan, C.G., Jamieson, D.N., Churms, C.L., Pilcher, J.V. (1995), Nucl. Instr. Meth. B104, 157. Ryan, C.G., Jamieson, D.N., Griffin, W.L., Cripps, G. (1999), Nucl. Instr. Meth. B158, 18-23. Williams, P.J., Dong, G., Ryan, C.G., Pollard, P.J., Rotherham, J.F., Memagh, T.P., Chapman, L.H. (2000), submitted to Econ. Geol. Zaw, K., J.B. Gemmel, R.R. Large, T. Memagh and C.G. Ryan (1996), Ore Geol. Reviews 10, 251-278.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HEAT PRODUCTION DISTRIBUTIONS, TECTONIC STYLES AND THE THERMAL EVOLUTION OF THE CONTINENTAL LITHOSPHERE Mike Sandiford School of Earth Sciences, University of Melbourne, Victoria, 3010.

"The inference, from direct observations and from heat flow measurement, that after metamorphism the heat sources are concentrated towards the surface is most readily explained as a consequence of the metamorphic process itself. If this can be shown to be the case it is a most convenient switch by which to terminate metamorphism." (Richardson, 1970). "... the problem of crustal heat production and its distribution and re-distribution by physical and chemical processes during crustal evolution is of fundamental importance and is at present little understood." Oxburgh (1980). Over the last 25 years tremendous progress has been made in our understanding the physical processes of metamorphism, and it may be apposite to ask whether we have much more to learn! Despite the manifest progress, we still do not understand the links between tectonic processes and the distribution of heat producing elements that control crustal thermal regimes alluded to by Richardson (1980). Indeed I would suggest Oxburgh's (1980) quote is still valid 20 years on! In this talk I will discuss the way in which heat production distributions may be modified by deformation of the crust, leading to long term changes in lithospheric thermal regimes. My emphasis will be to illustrate a potentially important feedback mechanism which might ultimately control the abundance and distribution of heat producing elements in the crust. Tectonic styles will be described in terms of the way in which they modify the distribution of heat producing elements. References Oxburgh, E.R., 1980, Heat flow and magma genesis: In, Physics of magmatic processes. Ed., Hargraves, R.B., Princeton University Press, p. 161 -200. Richardson, R., 1970, The relation between a petrogenetic grid, facies series and the geothermal gradient, Fortschr. Miner., 41, 65-76

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NEOTECTONICS OF SE AUSTRALIA AND THE ORIGIN OF THE INTRAPLATE STRESS FIELD. Mike Sandiford School o f Earth Sciences, University o f Melbourne, Victoria, 3010.

The intraplate stress field in the south-east part of the Australian continent, as determined by a variety of indicators such as focal mechanisms, well bore break out and neotectonic structures, shows a dominant E-W to SE-NW horizontal compression. The origin of this stress field, which is nearly perpendicular to the plate motion vector, remains obscure. In recent times two differing hypotheses have emerged to explain this stress field. Coblentz et al. (1995) attribute E-W compression to in 'plate-boundary forces' relating to comparatively recent Ma) changes in the relative plate motion between the Australian and Pacific plates which, in New Zealand, is manifest in a change from transtension to transpression along the Alpine FauU (e.g., Norris et al., 1992). In contrast, Zhang et al. (1996) attribute this stress field to gravitational stresses (i.e., an intraplate stress source) arising from the lithospheric density structure associated with the development of the east Australian passive margin during the formation of the Tasman Sea at -90 Ma. Because of the very different time-scales associated with these alternative hypotheses, their relative contributions should be reflected in the temporal record of tectonic activity in southeastern Australia. This talk focuses on the record of neotectonic activity in SE Australia including reverse fault activity responsible for the formation of the modem Mount Lofty and Flinders Ranges in South Australia and basin inversion events along the southern Australian margin. These observations are consistent with a significant rise in the magnitude of tectonic stress levels at about 5 Ma, and thus lend support to the notion that the current stress regime in SE Australia reflects, at least in part, the impact of AustralianPacific plate interactions. References

Coblentz, D., Sandiford, M., Richardson, R, Zhou, S., and Hillis, R., 1995, The origins of the Australian stress field. Earth and Planetary Science Letters, 133, 299-309. Norris, R.J., Koons, P.O., and Cooper, A.L., The obliquely converging plate boundary in the South island of New Zealand: implications for ancient collision zones, Journal of Structural Geology, 12, 715-725. Zhang, Y., Scheibner, E., Ord, A., & Hobbs, B.E., 1996, Numerical modelling of crustal stresses in the eastern Australian passive margin, Australian Journal of Earth Science, 43, 161-175.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOLOGICAL IMPLICATIONS IN REHABILITATION OF MINED VOIDS, COLLIE BASIN, WA Krishna K. Sappal, A. Qadeer Rathur and Zhong Rong Zhu School of Applied Geology, Curtin University of Technology, Perth 6001 The Collie Basin of Western Australia represents the only location where economic open cut mining of Permian Coal has been undertaken during the last forty years. The open cut mining has resulted into a number of voids infilled with acidic waters. This paper presents geological implications in a selected void of Cardiff Sub-basin of the basin, and thus outlines the source of acidity in the water. Collie Basin is intracratonic, fault-bounded, containing upto 1200 m of Early to Late Permian coal measures overlain unconformably by tertiary lacustrine sediments. The basin covers an area of approximately 230 km^ and is located 180 km southeast of Perth. The basin is subdivided into two sub-basins namely- Cardiff Sub-basin to the west and the Premier Sub-basin to the east (Le Blanc Smith (1993)). The present study was confined to the Ewington 2 Lake, an Open Cut located in the northern part of Cardiff Sub-basin. The Permian sequence consists of lithified sandstone, siltstone, shale and coal showing distinct cyclicity in its deposition. The sediments of the Ewington 2 Lake area are predominantly quartz sandstone and kaolinite clay, with small amounts of goethite and gibbsite. The lake bed sediments are predominantly kaolintic clay, indicating the main source for it from the shale in the strata. Coal seems with thickness upto 3m are distributed at a depth varying between 4 to 15m. Mining activities in the 1960's breached coal seams which form the lower walls and floor of the Ewington 2 Lake. Mining waste dumps are widespread along the edges of the Lake, (Sappal et al (2000)). Seasonal fluctuation of the local groundwater levels is in the range of Im. Two catchment areas recharge the lake from NE and SW. The lake and the adjacent swamp area form a convergent zone. High contents of the fined sediments in the strata reflect low hydraulic conductivity of the aquifer, which in turn indicates the long residence time of lake water and groundwater. This provides substantial potential for interactions between sediments (including coal) and water, pH of groundwater is acidic (4.2-5.7) and the values generally decrease towards the lake. This may reflect the influences of the dispersion of the acidic lake water. Dispersion of the acidic lake water in winter is far more widespread than in summer. Seasonal pH/TDS variations differ between bores, which appear to be controlled by the local groundwater flow pattern. The groundwater, lake water and, to some extent, rainwater are all Na-Cl type. The ionic contents in the lake water are similar to those in groundwater. Lake water has a narrow range of S04^" contents (17-24ppm), while groundwater has relatively wider range of S04^" contents (7-160 ppm, with the majority less than 50 ppm). Fe^^ (<0.05-25 ppm, but mostly >lppm), Af^ (<0.01-1.7), Ca^^ (mostly <5 ppm) contents in groundwater are low. The ionic contents in rainwater in the area are substantia^ lower (30 ppm) than those of groundwater and the lake water. The positive values of oxygen isotopic compositions in the Lake water result from strong evaporation, slow groundwater inflow and little mixing with rain and runoff The oxidation of pyrite in coal seams and carbonaceous shale, and the waste dumps surrounding the area are possible source of acidity in the water (Moncrieff (1993)). References LE BLANC SMITH, G., 1993. The geology and coal resources of the Collie Basin, Geological Survey of Western Australia, Report 38. MONCRIFF, J.S., 1993. Hydrogeology of the Collie Basin, Geological Survey of Western Australia, Report 34. SAPPAL, K.K., ZHU, Z.R., RATHUR, A.Q., 2000. Geology, hydrogeology and geochemistry of the void area. Collie Basin, WA. Curtin University of Technology Report.

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Geological Convention, Sydney, July 2000

DIATEXITE OR METATEXITE ? THE IMPORTANCE OF MELT GENERATION RATE E. W. Sawyer Sciences de la Terre, Departement des Sciences Appliquees, Universite du Quebec a Chicoutimi, Chicoutimi, Quebec G7H 2B1, Canada, Anatectic rocks are notorious for their varied and complex appearances. Despite this complexity, migmatites can be divided into two basic types. 1) Metatexite migmatites, in which the neosome consists of conspicuous leucosomes and associated melanocratic rocks, but the dominant feature of metatexite migmatites is the preservation of pre-migmatisation features such as bedding, schistosity or folds, in the part that may not yet have melted. 2) Diatexite migmatites, in which the neosome part is predominant and is coarse-grained, but does not have well-developed leucosomes or melanosomes; pre-migmatisation features are no longer preserved (except possibly in small enclaves) and the neosome has a characteristic syn-migmatisation flow (magmatic or submagmatic) foliation or flow banding. Both types could form in an open or a closed system. In closed systems the neosome bulk composition is the same as the palaeosome, but in open systems either the melt or the residual component is enriched. Many depleted granulite terranes consist of open-system, melt-depleted metatexite migmatites. There are now many estimates of the degree of partial melting from migmatites. They range from a few percent up to about 50% for metatexites, but reach 70% or more for the melt-depleted ones. Degrees of partial melting between 15 and 50% are typical for diatexite migmatites, although melt fractions may be much higher. Thus, the degree of partial melting in both types of migmatite are similar, commonly in the range 20 to 40%, so the degree of partial melting is not the primary reason why the migmatite types are so morphologically different. Based on field observations and literature descriptions, diatexite migmatites appear to form in two geological environments; 1) regional melting of muscovite-rich metapelites (e.g. St. Malo and Maine) and, 2) places where melting was very rapid, such as; a) contact melting of pelites and greywackes (Lac Kenogami, Quebec; Duluth, Minnesota) and, b) H20-fluxed melting (Opatica Subprovince, Quebec). In contrast, metatexite migmatites occur where either; 1) biotite dehydration melting occurred or, 2) where the degree of partial melting was low (i.e. low-temperature melting, or the palaeosome was not very fertile). These observations suggest that the melting reaction and rate of melting may be important factors in determining the type of migmatite that forms in a particular environment. Experimental studies show that between 6 and 8 kbars the temperature interval for muscovite dehydration melting is 15''C to 3 0 T , whereas for biotite dehydration melting (biotite+quartz = melt+orthopyroxene) it is 6 0 T t o l l 0 ° C - even wider if the biotite+aluminosilicate+quartz reaction is included. Assuming 25% muscovite in a metapelite (e.g. a St. Malo metasediment) and 25% biotite in a metagreywacke (e.g. Archaean, Quetico metasediment), then muscovite and biotite dehydration melting generate comparable melt volumes (28% cf. 40%), values typical of metatexite and diatexite migmatites. If the crustal heating rate is assumed constant between 725°C and 900''C, then muscovite dehydration melting generates melt 3 to 7 times faster than biotite dehydration melting. This difference in melt production rate may be the crucial factor in controlling which migmatite type forms during regional metamorphism. When the melting rate is rapid, melt forms faster than it can segregate and be drained away, this increases the melt fraction in the matrix sufficiently that bulk flow or magma flow occurs and a diatexite migmatite results. However, if the melt generation rate is slow, as in regional biotite dehydration melting, the melt is pervasively drained from the grain boundaries and bulk flow, or magma flow, cannot occur, consequently metatexite migmatites are formed. Metamorphic terranes where high temperatures (c. 900 °C) and large volumes of muscovite-poor, biotite-rich metagreywackes rocks coincide have low melt-generation rates which results in the almost complete extraction of the melt and the development of large regions of depleted granulite (i.e. open system metatexite migmatites), for example the Ashuanipi Subprovince of Quebec.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NATURE OF THE CONTINENT-OCEAN TRANSITION ON THE NON-VOLCANIC RIFTED MARGIN OF THE CENTRAL GREAT AUSTRALIAN BIGHT Jacques Savers, Philip A. Symonds, Nicholas G. Direen, and George Bemardel Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601

Interpretation of deep-seismic reflection data, revised interpretation of seafloor spreading magnetic anomalies and seismic refraction and potential field modelling allow us to establish the nature of the continent-ocean transition (COT), constrain the age of breakup of Australia and Antarctica, and interpret breakup processes in the Great Australian Bight. In particular, the seismic reflection data provide enhanced imaging of the crust and upper mantle under the COT and the abyssal plain. A 50-120 km-wide region, previously believed to be slow-spreading oceanic crust, is now re-interpreted as a continent-ocean transition zone composed of highly extended and intruded continental crust, underlain by a continental-lithospheric mantle. The COT is bounded to the north by a basement ridge that has an along-strike extent of at least 500 km, averages 20 km in width at its apex and broadens at depth. We interpret the ridge to be composed of altered mantle and magmatic products, rather than a metamorphic core complex, as some previous workers have suggested. Uplifted pre-Cenomanian sequences on the landward flank of the basement ridge that are onlapped by late syn-rift sediments (Cenomanian-Santonian) provide evidence for two major deformation episodes immediately prior to, and including margin breakup. Crustal blocks exhibiting high amplitude, parallel reflector sequences elsewhere under the COT are correlated with the pre-breakup. Cretaceous sequences of the Recherche and Ceduna Sub-basins. The magnetic anomaly previously identified as Chron 34 overlies a COT that is now interpreted to primarily be of continental origin, whilst Chron 33 overlies unambiguous oceanic crust. When combined with the interpretation of pre-Campanian sequences underlying the COT, this suggests that the onset of seafloor spreading should be revised from the previously accepted 95 Ma (Cenomanian) to 80 Ma (early Campanian). This would make the age of breakup roughly coeval with that in the Tasman Basin and casts doubt on the interpretation of Chron 34 on the conjugate Antarctic margin. A continental origin for the COT on the largely non-volcanic GAB margin questions the widespread interpretation that COTs on volcanic rifted margins generally overlie oceanic crust. On volcanic margins, it is easy to envisage that voluminous volcanics could mask underlying highly extended continental crust. Large (up to 6 km) offsets of the reflection Moho suggest that the upper mantle is brittle, while the style of lower crustal thinning (thinning from 15 km to km) suggests that this crust is ductile. No evidence of lithosphere-scale detachments has been found, although mid-crustal decollements are identified. Our interpretation favours a four-layer extensional model that incorporates a brittle upper crust and upper mantle, and a ductile lower crust and lower mantle, similar to the extensional model of Brun & Beslier (1996). Reference BRUN J.P. & BESLIER, M.O. 1996. Mantle exhumation at passive margins. Earth and Planetary Letters, 142, 161-173.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

MINERALOGICAL AND GEOCHEMICAL ASPECTS OF GOLD MINERALISATION ALONG THE DEBORAH ANTICLINE; BENDIGO, VICTORIA. Peter Schaubs CSIRO Exploration and Mining, P.O. Box 437, Nedlands, WA 6009 Gold mineralisation along the Deborah Anticline, Bendigo, Victoria is controlled on a scale of kilometres to metres by the location of folds, faults and quartz veins, which acted as pathways for gold-bearing fluids. The major proportion of gold is located in quartz veins that are categorised into two main groups: those associated with folds and those associated with faults. The relative chronology of the structural development and quartz-vein emplacement is characterised by the initial development of the fold and the subsequent formation of bedding- and cleavage-parallel veins. Saddle reefs may also develop at this stage while the formation of reverse faults and their associated veins occurred after folds had locked-up. Major gold-hosting structures include saddle reefs, reverse faults and composite structures, where reverse faults have disrupted saddle reefs. Some reverse faults are interpreted to have propagated along pre-existing structures, such as bedding-parallel veins. Although gold is located in fold-related structures, the majority is found in veins associated with later faults. Early sulphide assemblages in bedding- and cleavage-parallel veins are dominated by pyrite, pyrrhotite and siderite and are free of gold. Later assemblages in bedding-parallel veins and other fold-related veins are characterised by the presence of arsenopyrite, ankerite and gold. Fault-related veins are rich in sphalerite and galena with associated gold but lack pyrrhotite. Small amounts of late-stage antimony-bearing minerals occur in many vein types and post-date the precipitation of gold. Successively younger sulphide assemblages are controlled by decreasing temperature and increasing sulphur activity as well as increasing oxygen fligacity. Gold precipitation began during the middle to late stage of bedding-parallel vein development and continued until reverse faulting ceased. Some gold has been remobilised along carbonate-rich cataclasites. Stable oxygen isotope data show that quartz from all vein types is relatively homogeneous, while carbon and oxygen in carbonate have a wider range in values. Oxygen values in quartz are interpreted to have been homogenised with the host-rocks. In carbonate, carbon is enriched and oxygen depleted in progressively younger samples and is a function of decreasing temperature. Fluid inclusions in quartz veins are predominantly composed of water with carbon-dioxide gas bubbles. Nitrogen and methane make up smaller components. The sporadic occurrence of methane suggests that incoming fluids mixed with those in the host-rocks. Mineralogical data presented are indicative of one continuous phase of mineralisation and indicate that the fluid from which the sulphide minerals and gold precipitated, evolved continuously, but was sourced from one reservoir. Isotopic data suggest that fluids in the host rocks influenced this mineralising fluid and that local-scale fluid mixing occurred as suggested by Cox et al. (1995). Mixing of fluids in this situation via the process of fluid pumping has been recently simulated using numerical methods (Schaubs and Zhao, 2000) and indicates that this process is a viable means of precipitating gold. References SCHAUBS, P. & ZHAO, C. 2000. Numerical models of gold deposit formation in the Bendigo-Ballarat Zone, Central Victoria. This volume. COX S.F., SUN S.S., ETHERIDGE M.A., WALL, V.J. & POTTER T.F. 1995. Structural and geochemical controls on the development of turbidite-hosted gold quartz vein deposits. Wattle Gully Mine, Central Victoria, Australia. Economic Geology 90, 1722-1746.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NUMERICAL MODELS OF GOLD DEPOSIT FORMATION IN THE BENDIGOBALLARAT ZONE, CENTRAL VICTORIA Peter Schaubs and Chongbin Zhao AGCRC, CSIRO Exploration and Mining, P.O. Box 437, Nedlands, WA 6009 We present coupled mechanical-fluid flow and mechanical-fluid flow-chemical numerical models, to place constraints on some of the processes that formed gold deposits in the Bendigo-Ballarat Zone (BBZ) of the Western Lachlan Fold Belt of Victoria (WLFB). Regional scale models indicate that major faults did not act as fluid conduits, while small scale models simulate fluid pumping, fluid mixing and indicate that H2S has a strong control on gold precipitation. Although many gold fields in the Western Lachlan Fold Belt are located in close proximity to major faults, the faults themselves are rarely mineralised, and such gold fields are commonly located at a distance of 5-10 km into the hanging wall of the faults. Regional scale numerical models of the Bendigo gold field are aimed at answering the following questions: • Were major intrazone faults more or less permeable than the surrounding host rocks and did they act as conduits to supply fluids to gold fields? • Did the upper units of the Castlemaine Supergroup act as an impermeable cap to trap fluids and facilitate gold and quartz precipitation in the lower units? • Is the presence of a blind fault below the Bendigo gold field necessary to deliver fluids to the area? In the models, the major faults act as fluid barriers rather than fluid conduits. Shale-rich units at the top of the Castlemaine Supergroup provide an effective seal, resulting in the formation of gold deposits in the Castlemainian to Lancefieldian units below. This causes a greater proportion of gold-bearing fluids to accumulate in these deeper units, and provides a possible explanation for the difference in size between the Bendigo gold field and other gold fields in the Bendigo-Ballarat Zone such as the Ballarat gold field. The presence of west-dipping blind faults beneath major gold fields is not critical to form gold deposits in these areas, however the development of east-dipping shear zones, linking the deep, low angle portions of major intrazone faults with areas of gold mineralisation, facilitate the transport of fluids. Whereas major faults in the WLFB are not closely associated with gold deposits, small-scale faults with strike lengths of less than 1 km and small amounts of displacement are major gold bearing structures and are known to have carried large volumes of fluids because of the large amounts of quartz associated with them. Cox et al. (1995) have proposed that large volume of fluids were transported along these permeable faults and that fluids migrated into the adjacent host rocks and mixed with the host rock fluids via the mechanism of fluid-pumping. The models are aimed at determining if these faults allowed the continuous upward movement of large volumes of fluids, if fluid-pumping is a significant process in this situation and whether mixing between fluids in the faults and those in the host rock is possible. Small-scale models of a chevron fold with a permeable fault, show that flow rates are greatest within the fault and that the overall fluid flow direction is upwards. The models also simulate fluid-pumping, and suggest that fluids transported along the fault mixed with those in the host rocks. Fluids are expelled from and drawn back into the fault in a cyclical manner controlled predominantly by deformation-induced changes in volume. Intermittently high fluid pressures in the host rocks relative to the fault cause the rocks to yield in tension, simulating hydrofracturing. These zones of hydrofracturing are representative of quartz veins, which are associated with gold-bearing reverse faults. Fluids in the BBZ are known to be C02-rich with lesser amounts of CH4 and these species are interpreted to have strongly influenced gold precipitation. The effect of H2S has been interpreted to be less important on the basis of small volumes of sulphide minerals present in the deposits. Models coupling deformation, fluid flow and chemical reactions are aimed at determining the relative effects of C02, CH4 and H2S. The models demonstrate that gold is precipitated within the fault region when the concentrations of the three species increase with depth. In the models presented, gold precipitation is strongly controlled by H2S concentrations. Rates of gold mineralisation of 9 g/t over 1 million years recorded in the models are comparable to actual gold grades within the Bendigo gold field. References COX S.F., SUN S.S., ETHERIDGE M.A., WALL, V.J. & POTTER T.F. 1995. Structural and pochemical controls on the development of turbidite-hosted gold quartz vein deposits, Wattle Gully Mine, Central Victoria, Australia. Economic Geology 90, 1722-1746.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

COHESION AND FRICTION COEFFICIENTS OF DRY GRANULAR MATERIALS W. P. Schellart Epsilon Laboratory, Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Melbourne, VIC 3800, Australia Analogue modelling is useful since it offers the opportunity to study geological processes systematically where boundary conditions can be set according to the specific needs of the experimenter. Further, the structural development of a model can be investigated in two or three dimensions. The results of analogue experiments are only useful if the models are properly scaled with respect to the natural prototype. According to the scaling rules of analogue modelling, a model is representative of a natural prototype if both systems have similar distributions of stresses, densities and rheologies. When analogue experiments are executed in a normal gravity field, stresses and cohesion should be scaled down as the product of density and length vectors scale down (Horsfield 1977; Davy & Cobbold 1991). Part of my research involves the investigation of subduction roll back and related back-arc extension through analogue modelling, where a two-layered system with a brittle upper layer and a ductile lower layer has been used to model the crust or lithosphere. For the brittle layer a granular material has been used. Dry granular materials such as sand have been previously used in many analogue models of structural and tectonic processes (e.g.: Horsfield 1977; Davy & Cobbold 1991; Faccenna et ai 1999). In such models the cohesion (C) of these materials has been assumed to be negligible and the coefficient of internal friction (//) to be -- 0.58. However, Krantz (1991) showed that for sand C = 300 - 520 Pa, which is certainly not negligible. Further, Krantz (1991) found that = 0.58 - 1.00. If a scale factor of 10"^ - 10"^ (with a density factor of 1) is used (a scale factor often applied in modelling of crustal and lithospheric processes) then a cohesion of 300 - 520 Pa in the model would imply a cohesion of 300 - 520 MPa to 3000 - 5200 MPa in nature. These values for cohesion of natural rocks are much too high. Normal values range between ~ 30 - 110 MPa (Jaeger & Cook 1976). Thus, if the values of Krantz (1991) were correct, then sand is not properly scaled to model brittle behaviour of rocks (at least for these scale factors). However, Krantz (1991) did his measurements at relatively high normal stresses (600 - 3000 Pa) and extrapolated his data to smaller normal stresses (cr„), where as cr„ in the brittle layers of analogue models normally range up to a few hundred Pa. Therefore, shear tests have been executed for granular materials for smaller normal stresses. Shear tests have been made on dry granular materials (quartz sand, glass microspheres and sugar) with different grain size, rounding and sphericity. The measurements have been made with a simple shear test machine for different values of (~ 50 - 900 Pa). Shear stress has been plotted against normal stress to determine C and // for the investigated materials. The results suggest that the material behaviour for cr„ = 0 - 400 Pa is more complex than previously assumed. The fracture envelopes for all investigated materials are convex outward and converge towards a straight failure envelope with increasing cr„. Simple extensional experiments have been executed to see if this nonlinear behaviour is related to a dependence between cr„ and C or between cr„ and The results of these experiments indicate that there is no significant change in fault dip with increasing depth, thus for small cr„ values this is best described as a dependence of C on cr„. It has been found that fine grained glass microspheres are better scaled to model brittle behaviour of rocks than sand, since they have a smaller maximum cohesion {C = 160 Pa) and a value for ju (=0.65) which is closer to values for rocks, compared to sand, which has a maximum cohesion of ~ 245 Pa and a = 0.89. References Davy Ph. & Cobbold P. R. 1991. Experiments on shortening of a 4-Iayer model of the continental lithosphere. Tectonophysics 188, 1-25. Faccenna C., Giardini D., Davy P. & Argentieri A. 1999. Initiation of subduction at Atlantic-type margins: Insights from laboratory experiments. Journal of Geophysical Research 104, 2749-2766. Horsfield W. T. 1977. An experimental approach to basement-controlled faulting. Geologie en Mijnbouw 56, 363-370. Jaeger J. C. & Cook N. G. W. 1976. Fundamentals of rock mechanics. Chapman and Hall, John Wiley & Sons, New York, 585 pp. Krantz R. W. 1991. Measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures. Tectonophysics 188, 203-207.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ANALOGUE MODELLING OF BACK-ARC EXTENSION W. P. Schellart. G. S. Lister and M. W. Jessell Epsilon Laboratory, Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Melbourne, VIC 3800, Australia Back-arc extension is a geological process, related to roll back of the subducting plate. Here, subduction roll back results in the creation of space at the trench. This space is filled by the overriding plate under the release of potential energy leading to gravitational collapse and extensional structures in the back-arc region (Lonergan & White 1997). We have performed 3-D analogue laboratory experiments to model the process of back-arc extension, where we have investigated the structural and kinematic development of three basic models. All three models consist of a two-layered brittle-ductile system, situated in a rectangular box. The brittle layer is simulated by glass microspheres (90 - 180 fim), which is properly scaled to simulate brittle behaviour of rocks (Schellart in review). The viscous layer is modelled with glucose syrup, which has a viscosity of - 400 Pa s. In one of the four side-walls of the box a removable part is situated in a gap where, after removal, the layered system flows through (see figure). In the first model back-arc extension is unconstrained and the outward flow is allowed in all directions. In the second model the outward flow is allowed in one direction, perpendicular to the gate. In the third model, the outward flow is asymmetric and rotates around a hinge, where the opening of the gate is analogous to an opening door. The resulting strain pattern of these three basic models is significantly different (see figure).

Sketches of the 3 models, discussed in the text. Line drawings represent schematic deformation pattern observed strike-slip fault normal fault ^ ^

shear direction

The general patterns of deformation of these different models can be compared with natural examples of back-arc extension and suggest that these three basic models are the end members of the structural settings possible in back-arc extension. The first model can be compared with the Hellenic arc, which shows arcparallel extension and arc-perpendicular extension along the arc, as well as strike-slip faulting in the interior region. Possible other regions of comparison are the Carpathian orocline, the Calabrian arc and the Betic/Rif orocline. The second model can be compared with Mariana arc (early stage) and the Scotia arc (later stage), where extension is predominantly in one direction and strike-slip faulting is taking place at the sides. The third model can be compared with the Kermadec-Tonga arc (early stage), the Aleutian arc (more advanced stage), and the New Hebrides arc (late stage), where the back-arc regions clearly show asymmetric extension.

References Lonergan L. & White N. 1997. Origin of the Betic-Rif mountain belt. Tectonics 16, 504-522. Schellart W. P. Cohesion and friction coefficients of dry granular materials, in review.

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ACCUMULATION OF METALS AND PESTICIDES IN THE SEDIMENTS OF IRON COVE. Peter Schneider\ Elaine Baker' and Charles Essery^ 'University of Sydney, School of Geosciences, Sydney NSW, Australia 2006 ^ Department of Land and Water Conservation, 10 Valentine Av. Parramatta, Australia, 2124

Iron Cove is one of the small bay ends of the Parramatta River estuary, located approximately seven kilometres upstream of the Sydney Harbour Bridge. Water depth is generally 3-5 m, shallowing to 0.3 m between Rodd Point and Dobroyd Point. Two large stormwater canals, Hawthorn Canal and Dobroyd Canal are located on either side of Dobroyd point. Contaminant accumulation in the sediments of the bay represents both historical industrial use in the area and current stormwater sources within the catchment. Sydney Water recently completed a risk assessment that included estimation of the risks to human health and aquatic organisms from stormwater discharges in the Sydney region (SWC, 1998). In the Port Jackson geographical area the study identified risks to aquatic life from a number of chemicals including copper, silver, zinc, 2,4 dichlorophenol, alpha and beta BHC, chlordane, chlorpyrifos, dieldrin, diazinon and hexachlorobenzene. Many of these contaminants are incorporated into the local sediments where they are expected to act as a chronic source of pollution to biota and waters. Current meter data, cores and surficial sediment samples were collected from Iron Cove. Concentrations of selected trace metals and pesticides in the sediments are compared with current inputs from stormwater sources. The potential for chronic sediment toxicity is discussed in the light of current sediment contaminant concentrations and estimated future stormwater loads.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ROAD DUST AND ITS IMPACT ON ESTUARINE SEDIMENTS IN THE INTENSELY URBANISED CATCHMENT OF IRON COVE, SYDNEY, NSW, AUSTRALIA. Amy Scollen and Gavin F. Birch School of Geosciences, The University of Sydney, New South Wales, Australia, 2006.

Previous studies have indicated that non-point sources of pollution indicating stormwater runoff is one of the leading sources of heavy metal contamination in the Port Jackson estuary. Research in other parts of the world has shown the materials removed from road surfaces contribute a substantial contaminant load to catchment runoff. The primary pollutants of concern in urban road runoff are the heavy metals Cd, Cu, Pb and Zn. Atmospheric deposition, material produced by the abrasion of tyres, brake linings and road surfaces, as well as the corrosion debris of vehicle body work are the predominant source of heavy metals to urban road runoff A series of experiments were undertaken in the Iron Cove sub-catchment of Port Jackson to assess the contribution of contaminants from road surfaces entering the estuary via stormwater. Three roads were selected within the catchment on the basis of traffic volume. A series of sediment traps were placed in gutters of the three roads to determine the rates of sediment accumulation. Concurrently, samples were collected from the road surface adjacent to the sediment traps that analysed for a series of heavy metals including Cd, Cu, Pb and Zn. This material was also weighed to determine rates of accumulation. To augment these results, a further set of surface strips, consisting of five, one - square meter sections on each of the roads were cleaned at regular intervals to further investigate the rates of accumulation and heavy metal concentrations in relation to traffic density. Results reveal that the accumulation of road dust is directly related to traffic volume. Comparison between the rate of accumulation at the sediment traps and road sweepings reveal that the sweepings determine the net accumulation rate whereas the sediment traps determine the gross accumulation of sediment on the road surface. No correlation was observed between the number of days between cleaning events and heavy metal concentration. However, a strong correlation was noted between traffic volume and heavy metal concentration. Typical mean concentrations for road dust constituents (in mg kg-1) were: Cd 1.83; Cr 39; Cu 257; Mn 364; Ni 31; Pb 918; and Zn 728. Maximum concentrations observed in the road dust material (in mg kg-1) were: Cd 3.69; Cr 139; Cu 518. Mn 679; NI 59.72; Pb 2559; and Zn 2347. Concentrations of Cr, Cu, Pb and Zn in road dust were found to be up to seven times greater than residential soil guideline levels, moreover these heavy metals were up to 100 % bioavailable. Elevated concentrations were also observed in sediments from gully pots in road drains, indicating that road dust material is being supplied to the stormwater system. Hence, the ultimate destination of this highly contaminated material is the Port Jackson estuary.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

HIGH-T, LOW-P METAMORPHISM IN THE MOPUNGA RANGE, EASTERN ARUNTA INLIER: AN UPPER CRUSTAL EXPRESSION OF THE LATE STRANGWAYS EVENT Ian Scrimgeour'^ Julie B. Smith^ and Johann G. Raith' ' Institute of Geosciences, University of Leoben, A-8700 Leoben, Austria ^Northern Territory Geological Survey, P.O. Box 2655, Alice Springs, N.T. 0871, Australia ^ Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia The Proterozoic metamorphic evolution of the northern Arunta Inlier is characterised by multiple high-T, low-P metamorphic events that have been the subject of numerous studies and considerable controversy regarding their origin. These events are now widely believed to have formed by two contrasting mechanisms: 1/ 'Regional aureole' metamorphism, associated with the advection of heat by magma (e.g. Mt Stafford, Collins & Vernon 1991) and 2/ Burial of high heat-producing granites beneath an insulating sedimentary cover (e.g. Reynolds Range, Sandiford & Hand 1998). However, relatively little attention has been paid to the extensive region of high-T, low-P metamorphism that occurs to the north of the Delny-Mt Sainthill Shear Zone along the northern margin of the eastern Arunta Inlier, and its relationship to other low-pressure terrains in central Australia is not well established. In the Mopunga Range region, 80 km west of Jervois, a sequence of pelitic and psammitic metasediments underwent high-T, low-P metamorphism and partial melting during the Palaeoproterozoic. In the Deep Bore Metamorphics, pelitic migmatites underwent fluid absent partial melting at c.750-800°C and 3 kbar to produce migmatites with a high melt volume and silica-undersaturated cordierite-spinel-sillimanite restite, whilst metapsammitic rocks in the same sequence did not undergo melting. Preservation of in situ leucosomes containing corderite and spinel provide evidence of the dehydration melting reactions in the metapelites. Corderite coronas around garnet in metapsammites are locally resorbed and enclosed by a second generation of garnet, an observation which is consistent with a relatively simple near-isobaric heating-cooling path. The nearby Cackleberry Metamorphics preserve corderite-andalusite-K-feldspar assemblages and cordierite-bearing leucosomes with biotite-andalusite selvages, reflecting conditions of = 3 kbar and c.660°C. Late development of a sillimanite fabric is interpreted to reflect an up-pressure evolution following melt crystallisation, prior to near-isobaric cooling. Fine-grained andalusite-quartz symplectites after corderite in the Deep Bore and Cackleberry Metamorphics reflect late cooling into the andalusite stability field. SHRIMP U-Pb dating of metamorphic zircon rims in a cordierite-orthopyroxene migmatite fi-om the Deep Bore Metamorphics gives an age of 1730 ± 7 Ma, whilst zircon cores form a homogeneous population at 1805 ± 7 Ma. The 1730 Ma age is interpreted to reflect the timing of high-T, low-P metamorphism, synchronous with the regional Late Strangways Event. The Mopunga Range region forms part of a more extensive low-pressure metamorphic terrain that extends along the northern margin of the eastern Arunta Inlier. Metamorphic temperatures across this terrain varied between 500-800°C, at pressures of c.3 kbar, with lateral temperature gradients presumably induced by localised advection of heat by voluminous 1730-1710 Ma granites and less abundant mafic intrusions. The near-isobaric Palaeoproterozoic evolution of the Mopunga Range region is consistent with a relatively transient thermal event, due to advective processes that occurred during a regional tectonothermal event. This work forms part of FWF (Austrian Science Foundation) project PI 1879-TEC. J. B. Smith publishes with the permission of the Executive Director, Australian Geological Survey Organisation References COLLINS W.J. & VERNON, R.H. 1991. Orogeny associated with anti-clockwise P-T-t paths: Evidence from low-P, high-T terranes in central Australia. Geology, 19, 835-838. SANDIFORD M . & HAND M. 1998. Australian Proterozoic high-temperature, low-pressure metamorphism in

the conductive limit. In: Treloar P.J. & O'Brien, P.J. (eds). What drives Metamorphism and Metamorphic Reactions? pp. 23-46. Geological Society of London, Special Publication 138.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SURVIVAL OR FRAGMENTATION OF BASALTIC ENCLAVES DURING PYROCLASTIC ERUPTIONS AND THE ORIGIN OF ANDESITIC IGNIMBRITES Sheila J. Seaman' and Marshall Chapman^ ' Department of Geosciences, University of Massachusetts, Amherst, MA 10003 USA ^ Department of Physical Sciences, Morehead State University, Morehead, Kentucky 40351 USA The occurrence of chilled basaltic magmatic inclusions (enclaves) in felsic lava flows is well-documented. The preservation of enclaves in felsic pyroclastic flows is less commonly cited. Basaltic inclusions that are similar in texture and shape to some chilled magmatic enclaves in felsic lava flows and granites occur in ignimbrites in two Silurian volcanic successions in coastal Maine. On Great Cranberry Island, ignimbrites contain mostly vesicle-free basaltic inclusions. These inclusions are generally aligned and possibly flattened within the volcanic bedding plane, and have lobate and wispy boundaries. On Isle au Haut, less flattened, vesiculated basaltic inclusions, commonly hosting fragments of granite (texturally and compositionally similar to the underlying Isle au Haut granite), occur in less intensely welded ignimbrite than that of Great Cranberry Island. Both examples may represent enclaves that survived the vesiculation and fragmentation associated with explosive pyroclastic eruption. Inclusions similar in appearance, suggestive of the transport of basaltic droplets in pyroclastic flows, have been observed in a Krakatauan pyroclastic deposit (Steve Carey, pers. comm, 2000). The degree of fragmentation that a magma undergoes during an eruption depends on many factors, including: water concentration, viscosity, degree and speed of bubble growth and coalescence, degree of magma permeability, and on the shear strain applied to the magma. Rhyolitic magma typically is more viscous and has a higher gas concentration than the basaltic enclaves it hosts. Calculations relating volatile concentration to the depth of initiation of vesiculation indicate that typical rhyolitic magma with 5.5 wt.% water and log viscosity = 4.8 Pa s would begin to vesiculate at approximately 2970 m depth, while basaltic magma with 2.0 wt% water and log viscosity = 1.0 Pa s would begin to vesiculate at approximately 460 m depth. Vesiculation is typically considered to continue until the void fraction in the material is approximately 7080%, at which point fragmentation occurs, if sufficient permeability has not developed by bubble coalescence to permit degassing of the magma (Klug and Cashman, 1996). The approximate depth at which fragmentation would occur, if bubbles do not sufficiently coalesce to avoid fragmentation, is 360 m for the rhyolitic magma described above, and 160 meters for the basaltic magma. The basaltic inclusions exposed in the two sequences described above suggest that in some cases sufficient permeability develops in basaltic magmatic inclusions carried in rhyolitic magma to allow volatiles to escape the basaltic magma, effectively moving the fragmentation threshold to low enough pressures that fragmentation is avoided. Basaltic magma droplets carried in rhyolitic magma may avoid fragmentation to ash sized particles because 1) fragmentation of basaltic magma occurs at a sufficiently shallow depth that, given the time available between the onset of fragmentation and release from the vent, only disruption of very large (e.g. 1+ meter) enclaves into smaller enclaves may occur, or 2) the viscosity of basaltic magma is considerably lower than that of rhyolitic magma, permitting the easier development of connected vesicles that allow basaltic magma to efficiently degas between the vesiculation depth and the fragmentation depth. Shearing of enclaves during pyroclastic flow may enhance the ability of bubbles in the enclave magma to coalesce. Contrasts in vesicularity and shape of basaltic enclaves in ignimbrites of Isle au Haut and of Great Cranberry Island suggest that extensive vesiculation of a more hydrous basaltic magma enhances the ability of the magma to degas, but also causes a more rapid increase in enclave viscosity, compared to less extensive vesiculation of less hydrous magma, which has a shallower fragmentation depth and retains its plasticity long enough to record transport and depositional strain. A deposit of andesitic ignimbrite occurs stratigraphically directly above basaltic enclaverich rhyolitic ignimbrite and directly below basaltic tuff on Great Cranberry Island. Compositionally, the andesitic ignimbrite is a hybrid of the rhyolitic ignimbrite ( - 6 2 % ) and the basaltic enclaves (-38%). The andesitic ignimbrite hosts some small (a few mm) inclusions of basalt. The volcanic stratigraphy, the compositional relationships, and the remnants of comminuted enclaves in the andesitic ignimbrite suggest that essentially complete fragmentation of basaltic enclaves in rhyolitic ignimbrite, originating in a bimodal pluton, can produce andesitic ignimbrites that are texturally and compositionally homogeneous. Reference KLUG, C. AND CASHMAN, K.V., 1996, Permeability development in vesiculating magmas: implications for fragmentation. Bulletin of Volcanology, 58:87-100.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

BULK GEOCHEMISTRY AND MINERALOGY OF SEDIMENTS FROM THE EASTERN MANUS BASIN, PAPUA NEW GUINEA. Syriac Sebastian^ Raymond A.Binns^ and Jock B.Keene' ' School of Geosciences, University of Sydney, Sydney , N W S 2006 ^ Division of Exploration and Mining , CSIRO North Ryde, NSW 2113 The Eastern Manus Basin (EMB) is located to the north of New Britain arc and west of New Ireland, Papua New Guinea. It comprises en echelon series of mostly north-east trending neovolcanic edifices which range in composition fi-om picritic basalt to dacite-rhyodacite. They are developed as constructional features overlying older arc crust, equivalent to exposures on the islands of New Britain and New Ireland (Binns and Scott, 1993). The neovolcanic edifices contain three major sites of hydrothermal activity. These are, from west to east, PACMANUS, DESMOS and Susu Knolls (Yeats and Binns, 1998). Hemipelagic sediments with interbeds of volcanic ashes occur in the valleys of the Eastern Manus Basin. Bulk geochemistry and mineralogy of the surficial sediments in the vicinity of PACMANUS hydrothermal field have been studied. Sediments containing little or no Ca CO3 (0-11%) have been found extensively throughout the study area. The sediments contain concentrations of Fe and Mn of up to 8.0 and 4.0% respectively. Concentrations of Cu, Ba, Zn , As, V and Mo are also high relative to more typical pelagic sediments. Within 1 km of PACMANUS, Fe and Cu correlate (r-0.94), reflecting a component of hydrothermal activity. However, elsewhere there is no significant correlation between these elements. The Fe/Mn ratio varies from 1.4 to 83.4 with a mean of 13.5. This is high compared with the normal hemipelagic sediments (Average Fe/Mn ~ 1.1-6.3) of Lau and North Fiji back-arc Basins of southwest Pacific. This high Fe/Mn ratio suggests that Fe/Mn is controlled by the periodic input of volcanogenic detritus. Also, the predominance of volcanic glass in the sand fraction in the area around PACMANUS (Sebastian, et al, 1999) substantiate the volcanogenic input into the sediments. Sediments proximal to hydrothermal activity show an enrichment of silicon, arsenic, copper, vanadium and molybdenum. Strong linear relationships between S and Ba (r=+0.97), S and Cu (r=+0.92) and As and Cu (+0.91), suggest that the distribution of Cu, As and Ba is controlled by sulfide/sulfate phases which are deposited in close proximity to the vent site. Mineralogical studies on sediment samples indicate that significant amount of volcanic glass, rock fragments, opaques constitute the sediments in the Manus Basin. The preliminary conclusion is that variation in mineralogy is reflected in bulk geochemistry of the sediments. The sediments of the Manus Basin consist principally of volcanoclastic material with hydrothermal and biogenic components. The volcanoclastic material is calculated up to 76%, and hydrothermal material up to 39% and Biogenic component up to 24%. References BINNS, R.A AND SCOTT, S.D., 1993. Actively forming polymetallic sulphide deposits associated with felsic volcanic rocks in the eastern Manus back-arc Basin, Papua New Guinea, Economic Geology, 88: 2226-2236. SEBASTIAN,S., WATERS, J.C., BINNS, R.A., AND KEENE, J.B, 1999. Spatial Variation of major element geochemistry and mineralogy of sediments from the Eastern Manus Basin, AGU, V.80(46),p.F88. YEATS, C.J AND BINNS, R.A., 1998. Mafic to felsic volcanic hosts to hydrothermal activity in the Eastern Manus Basin, Papua New Guinea, GSA Abstract No.49.p.487.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LATE SILURIAN VHMS MINERALISATION AT THE JOHN FARDY MINE, PEELWOOD DISTRICT, SOUTHERN NSW Philip K. Seccombe and Stephen Brown School of Geosciences, The University of Newcastle, Callaghan, NSW, 2308 Stratiform Cu-Zn-Pb massive sulfide mineralisation at the John Fardy mine has developed close to the contact of Late Silurian slate and rhyodacitic (meta-)tuff in the Peelwood mining district of the eastern Lachlan Fold Belt (LFB), 45km N of Crookwell, southern NSW. Resource data for the deposit comprise 172 500 tonnes at 1.6% Cu, 1.2% Pb, 9.0% Zn and 25g/t Ag, with an additional 10 500 tonnes of oxide ore grading 2.65 g/t Au and 29 g/t Ag. The host rocks have been affected by at least four ductile deformation events. A strong regional N N W trending, steep W-dipping cleavage (S2) overprints an earlier foliation (SI) preserved rarely in pressure shadows adjacent to pyrite in tuff. An S3 crenulation cleavage and D4 open folds and kink bands refold S2 and are variably developed in slate and tuff Regional-scale, tight to isoclinal folding is inferred from outcrop pattern, parallelism of bedding (SO) and S2 cleavage, and sparse facing evidence. A NNW-trending highstrain zone is imaged from aeromagnetic data and roughly links all mineralised localities (Peelwood mine. Central Hill prospect, John Fardy mine, Aurora prospect, Cordillera mine). This shear zone formed post-D2 and records both sinistral and dip-slip movements. Kink bands are well developed in host rocks adjacent to this zone. Regional greenschist facies metamorphism occurred at peak temperatures of ~400oC and pressures of ~4 kb after development of the dominant mid-Devonian S2 cleavage. Colloform pyrite, galena and chalcopyrite, brittle deformation features in pyrite and arsenopyrite, folded cleavage in galena, and annealed and recrystallised sphalerite, all indicate a pre-D2 origin of the ores. By contrast, the metamorphic origin of some pyrite is clearly evident as syntectonic (D2) to post-tectonic veins and overgrowths and pyrite veins parallel to axial planes of D4 kink bands. Fluid inclusions from D2 pyritequartz veins have Th as high as 397oC and very low salinity (average 0.8 wt% NaCl equivalent), consistent with the metamorphic temperatures and a lack of associated base metals in these syntectonic veins. Strong sericite and minor chlorite alteration is encountered in footwall tuff beneath sulfide zones. Alteration zones are characterised chemically by depletion of N a 2 0 and CaO, and enrichment of MgO and K 2 0 . Sulftir isotope compositions for pyrite, sphalerite and chalcopyrite are distinctive for different lithologies. d34S data range from 11.9 to 13.7 per mil for massive sulfide, 12.3 to 13.3 per mil for silica-rich tuff, 4.6 to 11.4 per mil for cherty exhalite, -2.1 to 1.3 per mil for pyritic black shales and 1.9 to 4.8 per mil for a syn-D2 quartz vein. The preservation of distinct ranges in d34S data in adjacent rock units at John Fardy suggests that metamorphism is not responsible for the overall S-isotopic distribution, or that remobilisation has significantly affected the mineralisation. Lead isotope data for the John Fardy and Peelwood deposits suggest a crustal source for the lead and a model age of 420-425Ma; slightly older than the Cordillera mine, but within 95% confidence ellipses of the model ages of other Late Silurian LFB VHMS deposits (Woodlawn and Captains Flat). Overall the evidence favours John Fardy as a VHMS deposit similar to those of Late Silurian age in comparable lithologies throughout the eastern LFB. The deposits in the Peelwood area appear to be polygenetic, in that the bulk of the ore is syngenetic, but metamorphism has played a role in modifying the deposit and adding new mineralisation.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NEW 1:25,000 GEOLOGICAL MAP OF THE EASTERN TAMWORTH BELT, MANILLA, SOUTHERN NEW ENGLAND FOLD BELT Tim R. Sharp and Evan C. Leitch Department of Environmental Sciences, University of Technology, Sydney, Broadway, New South Wales Sydney 2007 The results of recent geological investigations of some 300km^ of Devonian-Permian rocks southeast of Manilla in northern NSW are summarised on a new 1:25,000 scale map. The map covers part of the eastern Tamworth Belt, the western of the two major stratotectonic divisions of the southern New England Fold Belt. The eastern limit of the map lies close to the Peel Fault, that here separates the Tamworth Belt from the Tablelands Complex (the second major division of the fold belt), except in the far south where the Moonbi Adamellite (250 Ma) transgresses the fault. Stratified rocks of the Tablelands Complex are of similar age to those of the Tamworth Belt but of more distal facies, of greater structural complexity, and intruded by widespread Late Palaeozoic-Triassic granites. They are not dealt with further here except to note that a thin strip of these rocks lies between the western contact of Attunga Creek Adamellite and the Peel Fauh rather than the intrusive body being bounded by this structure. The Tamworth Belt in the Manilla district comprises mainly Devonian sedimentary rocks that have been divided into seven lithostratigraphic units. The inferred oldest unit occurs in the SE where previously undescribed volcanic breccia, conglomerate, sandstone, siltstone and rare limestone occur between the converging Horse Arm and Peel faults. Articulate brachiopods from a calcareous sandstone indicate a Devonian age. Between the Horse Arm and Garthowen Thrust four structurally conformable units are exposed in a west younging sequence. The oldest unit, the Glencaim Limestone ranges up to 450 metres thick and comprises principally bedded and massive limestone and limestone breccia and an intercalated 150 m thick volcaniclastic member of coarse cross-bedded sandstone and conglomerate. Corals and stromatoporoids are present in the limestone and Lochkovian conodonts (Associate Professor R Mawson pers comm) have been obtained from below the volcaniclastic member. An erosional contact at the top of the Glencaim Limestone is overlain by a few metres of coarse cross-bedded sandstone and conglomerate that pass up into a dominantly thin-bedded sequence 750 m thick of grey and green siltstone that contains prominent coarse sandstone beds containing little-abraded mafic volcanic detritus. This unnamed unit is overlain gradationally by the distinctive thin-bedded rocks of the Frasnian Yarrimie Formation, 820 m of mudstone, siltstone, and sandstone with common thin ash-fall tuff which is in turn succeeded by the Famennian Baldwin Formation characterised by the presence of prominent horizons of coarse andesitic sandstone. Further west and north the Yarrimie and Baldwin formations are widespread, their distribution controlled by a series of N N W trending folds and associated faults. Noteworthy within the Baldwin Formation are lenses of grey-green porphyritic andesite (Browning Vale Andesite Member) that suggest contemporaneous volcanism. Older rocks, are confined to anticlinal culminations or the hanging wall of inferred thrusts and comprise the Late Emsian to Early Eifelian Sulcor Limestone, the Middle Eifelean Moore Creek Limestone and a thin intervening horizon of mudstone, siltstone and sandstone. Close to the Peel Fault the Baldwin Formation is overlain by the siltstone-dominated Mandowa Mudstone of Famennian-(?)early Carboniferous age. A prominent cobble-boulder conglomerate several tens of metres thick marks the base of the latter unit. This contains both volcanic and granitic clasts and is probably a correlative of the widespread Keepit Conglomerate of the western Tamworth Belt. The structure of this part of the Tamworth Belt is dominated by upright, gently plunging open-close macroscopic folds and subparallel faults. The latter are rarely exposed but most are inferred to be thrusts active during folding. Some thrusts have been folded and this has contributed to the complex outcrop pattern of the Emsian-Eifelian limestones. Differences in the stratigraphic sequence underlying the Yarrimie Formation across the Garthowen Thrust suggest this structure has accommodated substantial shortening. The Minderoo Thrust emplaced an internally disrupted slice of Glencaim Limestone, Early Permian clastics of the Kensington Formation and serpentinite westward above the Garthowen Thrust and was subsequently truncated by movement on the Peel Fault.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TECTONIC EVOLUTION AND MINERALIZATION OF THE EAST KIMBERLEY REGION, WESTERN AUSTRALIA Steve Sheppard and Lee Hassan Geological Survey of Western Australia, 100 Plain Street, East Perth WA 6004.

The Halls Creek Orogen in the east Kimberley region was remapped by the Geological Survey of Western Australia and the Australian Geological Survey Organisation between 1990 and 1995 (Tyler, 2000). Further work has since been undertaken in the east Kimberley to produce a data package comprising digital spatial indexes for exploration activities and all mineral occurrences, along with a report and a map (Hassan, in press). The data package enables the distribution of mineralization to be analysed within the framework of the revised stratigraphy and new tectonic model for the region. The Halls Creek Orogen formed in the Palaeoproterozoic between the Kimberley and North Australian Cratons, but it records intermittent reactivation until the end of the Palaeozoic. The oldest element of the Halls Creek Orogen is the Lamboo Complex, which comprises all the deformed and metamorphosed plutonic, volcanic, and sedimentary rocks formed between c. 1910 Ma and c. 1790 Ma. The presence of terranes, and the different geophysical nature of the crust on either side of the Lamboo Complex, indicates that it contains a Palaeoproterozoic plate margin. The youngest granite intrusions into the southern end of the Lamboo Complex may represent an extension of magmatism in the Granites-Tanami Complex. The Halls Creek Orogen also includes the deformed margins of the Palaeoproterozoic Speewah and Kimberley Basins and their correlatives, and the deformed elements of a number of Mesoproterozoic, Neoproterozoic, and Palaeozoic sedimentary basins. The east Kimberley contains a range of mineralization styles, including orthomagmatic PGE-Cr-Ni-Cu-Ti-V, volcanic-hosted massive sulphide Cu-Pb-Zn and REE-Ta, vein and hydrothermal Au, base metals, and U, porphyry Cu-Mo(-Ag), and pegmatite and stratabound W-Sn-Ta. The majority of mineral occurrences are hosted in rocks of the Lamboo Complex with particular mineralization styles showing a strong correlation with tectonostratigraphic terranes. Important deposits of lamproitic diamond, carbonatite-hosted REE, epithermal fluorite-base metals, and stratabound carbonate hosted base metals are associated with younger tectonic units. Notable vein and hydrothermal Au and U-Au mineralization is also present in the Granites-Tanami Complex. References HASSAN, L. Y. in press. Mineral occurrences and exploration potential of the east Kimberley, Western Australia. Geological Survey of Western Australia Report 74. TYLER, I. M. 2000. Geological map of the Halls Creek Orogen, east Kimberley region (1:500 000). Geological Survey of Western Australia.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

OPPORTUNITIES AND OBSTACLES FOR CAMPUS-BASED MUSEUMS IN EARTH SCIENCE OUTREACH PROGRAMS Andrew Simpson Division of Environmental and Life Sciences, Macquarie University

The natural sciences in the tertiary education sector need to provide object-based instruction in undergraduate courses. The discipline areas of geology and anthropology have traditionally developed collections most frequently in Australian universities. This has often lead to the development of a campus museum through the addition of a permanent dedicated display area. University museums have traditionally supported their host institutions in a number of ways. The display space can augment undergraduate teaching programs. It may reflect the specialist research strength of staff. It may generally enhance the quality of the campus environment for students, staff and visitors. It may also act as a "shop front window" for the University, showcasing aspects of the quality of the host organisation and thus providing a template for positive community interaction and even a potential student recruitment mechanism. Whilst many universities see stewardship of an art collection as an important part of their corporate identity, they are less likely to support stewardship of a natural history collection in financially constrained circumstances unless there is an obvious strategic advantage to do so. Often, unfortunately, lack of funding entails and/or implies an inability to understand the potential of such facilities. Some recent Australian examples are discussed. Despite the above, there are many opportunities for earth science outreach programs through campus museums. In New South Wales, the adoption of the new science syllabuses, in particular the Earth and Environmental Sciences, coincides with a restructuring of many university science disciplines allowing the development of new cross-disciplinary alliances. The new Earth and Environmental Science syllabus is an amalgam of geology, physical geography, biology and chemistry. Earth science outreach programs can therefore encompass teaching gardens, fauna reserves and other outdoor field studies centres as well as biological science museums and the chemistry laboratory. Science teachers in the secondary school sector are actively seeking new resources for implementing syllabus outcomes. Some examples from Macquarie University are discussed. Funding is a critical issue for the development of earth science outreach programs in the new science education landscape. Joint ventures through "National Science Week", the "Science and Technology Awareness Program" and "Visions of Australia" can provide opportunities for imaginative programs. Alliances with other museums and scientific conferences can also be the basis for specific programs. A healthy university museum sector can provide the additional benefits of volunteer programs and museum studies courses.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE GOONUMBLA CALDERA, PARKES, NSW: FACT OR FICTION? Carol Simpson^ Ray Cas' and Mark ArundelP ^Department of Earth Sciences, Monash University, Clayton, Victoria 3168 ^North Limited, 154 Abemethy Road, Belmont, Western Australia 6104

The Goonumbia caldera is an informal name given to a crudely circular geophysical anomaly identified within the Late Ordovician Goonumbia Volcanics during exploration for Cu-Au deposits in the Parkes region of New South Wales (Jones, 1985). The Goonumbia Volcanics, consisting of volcaniclastic rocks, coherent volcanic rocks and minor limestone, form part of the linear Junee-Narromine Volcanic Belt, one of four belts of Ordovician mafic volcanic rocks in the Lachlan Fold Belt that has attracted considerable attention for their porphyry Cu-Au mineralisation. Early exploration work in the area identified a circular Bouguer gravity low of 22 km diameter, the margins of which coincide with scattered outcrops of monzonitic and monzodioritic intrusions interpreted to represent a semi-continuous ring dyke defining the rim of a large collapsed caldera (Jones, 1985). Subsequent detailed aeromagnetic data confirmed the presence of an arcuate trend, but this reflects a prominent belt of strongly magnetic trachyandesite and small intrusions. The concept of a large caldera, filled with the upper portion of the Goonumbia Volcanics succession, became fundamental to all models discussing the geology and mineralisation in the region. In recognition of the predominance of efflisive eruption products. Hall (1993) refined the model and proposed the existence of a Hawaiian-style shield volcano that underwent block subsidence and downsagging to form a caldera depression. The present study has re-evaluated the stratigraphy and volcanological interpretation of the Goonumbia Volcanics using a facies approach and has found no convincing evidence for the existence of a caldera of this magnitude. The Goonumbia Volcanics are re-interpreted as the subaqueous volcaniclastic apron, several km thick, formed on the flanks of a shallow marine to subaerial stratovolcano. The basal part of the apron consists of crystal-rich volcanic sandstone, small limestone blocks and a more continuous horizon of fossil-poor micritic limestone. Limestone deposition was terminated by swamping of the volcano flanks by thick deposits of immature, crystal-rich volcaniclastic debris reflecting escalation of explosive volcanic activity on the adjacent edifice. The bulk of the apron is composed of thick beds of polymictic volcanic breccia and intervening turbiditic sandstone and minor siltstone but includes a probable debris avalanche megabreccia containing huge blocks of densely welded ignimbrite and trachyte lava that supports the occurrence of voluminous explosive eruptions and sector collapse events on the subaerial part of the volcano. Coherent basaltic andesite and trachyandesite horizons occur as high level sills and lava flows occur throughout the Goonumbia Volcanics and the sequence hosts a suite of younger monzonitic intrusions, some of which are associated with Cu-Au mineralisation. Acknowledgements: We would like to thank North Limited for funding this research project and for permission to present this paper. References HALL M. C. 1993. The stratigraphy and palaeovolcanology of the Late Ordovician Goonumbia Volcanics, NSW. BSc (Hons) thesis, Monash University, Melbourne (unpubl.). JONES G. J. 1985. The Goonumbia porphyry copper deposits. New South Wales. Economic Geology, 80, 591-613.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

CHARACTERISATION OF ZIRCONS IN PALAEOZOIC RED BEDS AT BOMBALA, N.S.W.: IMPLICATIONS FOR SEDIMENT AGE AND PROVENANCE K. N. Sircombe^'^ and K. G. McQueen^ ^Research School of Earth Sciences, Australian National University, Canberra ACT 0200 ^Current address Geological Survey of Canada, 601 Booth Street, Ottawa K1A 0 E 8 Canada ^CRC LEME, University of Canberra, ACT 2601, Australia Zircons from two igneous and two sedimentary units in the Bombala area of southeastern New South Wales have been examined by the Sensitive High Resolution Ion Microprobe (SHRIMP) as part of a broader characterisation study of zircon populations in Palaeozoic sedimentary rocks of eastern Australia aimed at establishing sediment source regions. The study has also established a timeframe in which to interpret these rocks. The two igneous units are the Hospital and Paradise Porphyries, occurring beneath the sedimentary units. Both give a Frasnian age that can be correlated with the Boyd Volcanic Complex. The sedimentary samples are from the basal and upper sections of the Rosemeath Formation, a fluvial 'red bed' consisting of conglomerates, coarse sandstones, and associated red siltstones and mudstones. Detrital zircons from the basal conglomeratic section at Kilbrechin indicate a dominant provenance from local Silurian granites and volcanics and a maximum depositional age that can be correlated with the Frasnian-Famennian Merrimbula Formation. However, detrital zircons from the upper coarse sandstone section of the Rosemeath Formation at Endeavour Lookout challenge the positive correlation trend with a lack of Silurian aged grains and a strong presence of Late Devonian and Early Carboniferous aged grains. Previous studies have correlated these rocks with Late Devonian units of the South Coast, solely upon the basis of stratigraphy and lithology, as palaeontological evidence was absent. These results imply either that the South Coast correlation is not valid for the upper sequences, or that the Merrimbula Formation sequences also extend upward into the Carboniferous. The presence of Early Carboniferous (347 ± 12 Ma) zircons in the upper part of the red bed sequence at Bombala implies igneous activity somewhere in the region during this time. No source material of this age is known in the immediate vicinity. The nearest recognised potential source is 220 km NNE on the far eastern edge of the Lachlan Orogen (the Wandandian pluton). Measurements on cross-bedded sandstone units in the upper Rosemeath Formation indicate northeasterly to southeasterly palaeocurrent directions. Assuming the palaeocurrent data are representative of the total transportation vector, a westerly source is indicated. The general coarseness of the Rosemeath Formation also suggests a relatively local provenance. Combined, these observations suggest a westerly source of Early Carboniferous zircons and the possibility of previously unrecognised igneous activity of this age not too far to the west of Bombala (possibly east of the Mount Howitt province in eastern Victoria, M. Brown pers. comm.). Early Carboniferous igneous activity in this region of the Lachlan Orogen may therefore have been more extensive than is currently realised. The study clearly illustrates the value of detrital geochronology in constraining source materials for sedimentary rock units. Detrital zircons provide a unique record that can be relatively easily accessed to reveal evidence of events that may not necessarily be seen otherwise, even with extensive mapping and analysis of igneous units. Outcrops of the Devonian-Carboniferous sedimentary rocks of southeastern Australia cover an extensive area from central to far eastern Victoria, northwards into the Bombala region and up to northern reaches of the Lachlan Orogen including sequences in the Darling Basin. These rocks represent the first phase of non-marine sedimentary cover on the post-orogenic Lachlan Orogen. Further work on the detrital zircons of these sedimentary rocks could yield important information about the geological history of the entire region.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GEOCHEMISTRY OF THE EBORINTRAPLATE CENTRAL-VOLCANO: IMPLICATIONS FOR PLUME-LITHOSPHERE INTERACTION W. J. Sivell & P. M. Ashley Division of Earth Sciences, The University of N e w England, Armidale N . S . W . 2351. Mafic-felsic magmas from the - 1 9 Ma Ebor intraplate central-volcano, northeastern NSW, comprise three lineages, including: (a) a suite ranging from olivine {0L-) tholeiite to icelandite and rare peralkaline quartz trachyte; (b) a suite of minor OL-tholeiites and abundant quartz {QZ-) tholeiites, evolving through dacite to rhyolite; and (c) a suite of transitional to alkaline basalts and hawaiites. Mafic lavas predominate. Comparison of the geochemistry of the Ebor shield lavas with basanite and alkali olivine basalt (AOB) from underlying (>40 Ma) lava-field eruptives of the Doughboy Province enables discrimination among diverse mantle and lithospheric contributions to the Ebor eruptives, and permits assessment of source interaction and magma dynamics during evolution of the Ebor volcano. Ol-tholeiites and voluminous 0Z-tholeiites dominate Ebor eruptives. None of the tholeiites represent primary magmas. Wide variation in incompatible element ratios for the tholeiites (eg. Y/Nh= 0.66-1.22 for OL-tholeiites; 0.89-1.78 for 0Z-tholeiites) reflect variable degrees of partial melting of an enriched garnet peridotite source, with parental OL-tholeiite magma segregating from its source at - 9 0 - 1 1 0 km depth. The chemistry of the most primitive Ebor (91-tholeiites (Mg^=62) reflects a predominant contribution from an OIB-like mantle source like the asthenospheric mantle plumes which dominate the geochemical signatures of other eastern Australian central-volcanoes. The most primitive Ol-tholeiites have higher La/Nb (0.9-1.3), and lower Mg^, Cr, Ni, T i 0 2 , P/Zr and La/Y than the earlier emplaced Doughboy lava-field basanites and AOB, which show lower Ba/Nb and La/Nb ratios than observed in eastern Australian central-volcanoes. The tholeiites lack the distinctive Rb and K depletions of the alkaline rocks, interpreted to reflect generation of the lava-field eruptives in part from a sub-continental lithospheric mantle (SCLM) source with residual phlogopite persisting during small degrees of partial melting. The most primitive OL-tholeiites also lack distinctive P, Th and La enrichments shown by the alkaline rocks, which cannot be attributed to different degrees of partial melting of either an asthenospheric or plume source, nor crustal AFC processes in these near-primary magmas. Moreover, both amphibolite and granulite facies crustal rocks have low P/Zr ratios, and high Ce/Pb and low K/P ratios in the alkaline lavas imply minimal crustal interaction. Correlation between K/Nb and Zr/Nb for the alkaline rocks suggests involvement of a subduction-modified SCLM source, but the distinctive P, Th and La enrichments, along with low K/Ba, K/Nb, Rb/Sr and Zr/Nb, are consistent with a component derived from a second metasomatised shallow SCLM endn

member (similar to some amphibole+apatite - bearing spinel peridotite xenoliths), with which high-Mg^ Barrington alkaline basalts have been proposed to have heterogeneously interacted. Importantly, these features have been imparted to varying extents to evolved Ebor (9L-tholeiites and transitional olivine basalts, suggesting contributions to their genesis from both the plume and SCLM (with its long term history of multiple episodes of enrichment and depletion). The heating effect of the plume was to induce partial melting within the SCLM to generate small volume alkaline melts (similar to those of the older Doughboy Province) which mixed with plumederived melts parental to the OL-tholeiites. In common with other eastern Australian central volcanoes, Ebor Volcano contains a high proportion of differentiated magma compositions. Ebor gZ-tholeiites show a trend of decreasing C a 0 / A l 2 0 3 (0.55-0.35) with decreasing Mg^ (56-31), indicating major CPX removal. AI2O3 contents range up to 20.5 wt%, and S i 0 2 levels are higher (relatively uniform at 51.7-53.0 wt%) than in the OL-tholeiites (Si02<51.5 wt.%). Many ^Z-tholeiites show geochemical signatures of substantial early assimilation of lower crust (LC) (eg. higher Ba/Nb, La/Nb, Ba/Th, K/P, K/Nb and Zr/Nb, and lower Ce/Pb and P/Zr than the OLtholeiites). This reflects prolonged deeper crustal residence times of parental OL-tholeiite magmas and more extensive partial melting (higher Y/Nb and lower total incompatible element contents). By contrast, OA-tholeiites (Mg^ 62-45) show little evidence of LC contamination, and less pronounced CPX fractionation ( C a 0 / A l 2 0 3 = 0 . 5 0 0.45). OL+PLAG(+CPX) removal from Ol-tholeiite magma emplaced more rapidly to shallow level gave rise to icelandite with substantially lower Sr and AI2O3. CPX-controlled fractionation occurred after these magmas attained the composition of icelandite, with ensuing rapid decrease of C a 0 / A l 2 0 3 from values identical to those in the (9Z.-tholeiites (ca. 0.45) to very low ratios (ca. 0.2). Some upper crustal contamination affected the most evolved Ol-tholeiites and icelandites. On major and trace element variation diagrams, OL-tholeiite, icelandite and peralkaline quartz trachyte show good linear correlation, but with different inter-element ratios (eg. contrasting K/P-MgO; (Na+K)/Si; P/Zr and Y/Nb) to the gZ-tholeiite-dacite-rhyolite trend. The discrete trends provide direct evidence for separate liquid lines of descent leading to peralkaline versus more oversaturated melts from parental OL-tholeiite magma, and confirm the importance of both fractional crystallisation and variable crustal involvement in initiating contrasted differentiation paths within the Ebor volcano.

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Geological Convention, Sydney, July 2000

Nd-ISOTOPIC CONSTRAINTS ON THE EVOLUTION OF GYMPIE PROVINCE: A PORTION OF AN ISLAND ARC - GONDWANA RIM ACCRETION ZONE W. J. Sivell^ & M. T. McCulloch^ 'Division of Earth Sciences, The University of New England, Armidale, NSW 2351 ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Eight subduction-related magma suites are recognized from drillcore within the Late Palaeozoic-Mesozoic volcanic-intrusive sequence of the Gympie Group, part of the tectonically anomalous Gympie Block, southeast Queensland, which lies in faulted contact to the east of deformed accretionary terranes of the New England Orogen (NEO). From a petrogenetic perspective, an important aspect of the Gympie region is that it appears to have been the site of a continent - island arc accretion event in the Early to Middle Triassic. A wide range of chemical and isotopic variations among stratigraphically well-constrained mafic-felsic pre- to post-accretion igneous suites at Gympie, as well as good correlations between trace element and isotopic ratios, reflect the nature of tectonomagmatic processes involved in the origin, dispersal, docking and eventual accretion of the Gympie Block at the Gondwana rim during Permo-Triassic continental growth. The Gympie magmas include submarine (later subaerial) island arc tholeiite basaltic tuff-breccias and lavas comprising the Highbury Volcanics, as well as andesites and dacites in the unconformably overlying Rammutt Formation, and mafic-intermediate post-accretion intrusives, including magmas with alkaline and shoshonitic affinities. Most Highbury and Rammutt lavas show uniformly high Nd-isotope ratios (? ]s[(j(270 Ma) = +7.4 to +8.1) and geochemical features (e.g., Zr/Nb>20) that reflect their depleted asthenospheric mantle source(s). These ? Nd(T) ratios are slightly lower than values inferred for attendant late Palaeozoic MORB-source mantle beneath the Gondwana rim, as represented, for example, by the adjacent Cambroon beds (? ]vf(j = +10). Rammutt andesites may have directly assimilated as much as 40% isotopically-primitive (young) terrigenous sediment, possibly akin to turbiditic greywacke of Carboniferous subduction complex origin in contiguous NEO accretionary terranes. Significantly lower initial Nd-isotopic ratios characterize Gympie post-accretion intrusive suites. These include (in order of emplacement) the transitional-alkaline multiphase Langton Dolerite sill (? = +6.2 to +6.7); geochemically depleted dolerite dikes intimately associated with gold mineralization (? ^ ^ = +4.3 to +5.7); and microdiorite dikes with some near-primary magmas of shoshonitic affinity (? Nd "" 5 to +4.7). Importantly, low ? Nd(T) values (? Nd = +4.9 to +5.4) are also shown by earlier-formed (pre-accretion) ankaramitic basalts which comprise the topmost unit of the Highbury Volcanics. An older, isotopically-enriched sub-continental lithospheric mantle component is required in these low-? magmas. Mobilization of components derived from this source as early as latest'Highbury' time suggests that the Gympie Island Arc (while probably allochthonous) was initiated proximal to the Gondwana rim at its outset. The data suggest that extension above a steepening Early Permian Benioff zone led to primitive island arc magmatism within and trenchward of Gondwana-rim subduction complex elements. Rapid ingress of depleted asthenospheric mantle into the supra-subduction zone wedge gave rise to the sequence of Highbury Volcanics, from earliest-erupted Alma member basalts with some MORB-like affinities, to Tozer and Mary basalts with more pronounced island arc tholeiite geochemical signatures. It also provided heat to melt attenuated (low-? Nd) lithospheric mantle remnants, contributing to the genesis of ankaramites. Chemical and isotopic features suggest that the nearby Early Permian Cambroon beds and Cedarton Volcanics may owe their origin to extensional tectonics within and behind the Gympie arc (Sivell & McCulloch 1997). The Gympie Block must have docked in its present position prior to granitoid intrusion of its faulted contact with contiguous terranes at 220 Ma. Further melting of lithospheric mantle ensued in a post-accretion setting from the Mid-Triassic, with asthenospheric and lithospheric mantle melts (and mobilized components of stalled slabs) involved in the genesis of refractory dolerite and shoshonite suites. The Gympie Block may have originated as one of several marine extensional basins that formed within former NEO accretionary terranes, close to the trench and subsequently displaced along the Gondwana margin by strike-slip movements. References SIVELL W. J. & McCULLOCH M. T. 1997. Geochemistry and Sm-Nd isotope systematics of Early Permian basalts from Gympie Province and fault basins in southeast Queensland: implications for mantle sources in a backarc setting at the Gondwana rim. Geological Society of Australia Special Publication 19, 148-160.

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Geological Convention, Sydney, July 2000

GEOCHEMICAL AND PETROGENETIC RELATIONS AMONG TRIASSIC STANTHORPE I-TYPE GRANITOIDS W. J. Sivell ^ and M. J. Passmore ^ ^ Division of Earth Sciences, The University of New England, Armidale, NSW 2351 ^ Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 In the Stanthorpe-Liston area, diverse Late Permian - Early Triassic post-tectonic I-type granitoids of the Stanthorpe Granite Group (SGG), part of the Moonbi Supersuite, are spatially associated with Permian volcanics. Hitherto, three major granitoid units of regional extent have been recognized: (1) the 245 Ma Undercliffe Falls Adamellite; (2) the 242 Ma Stanthorpe Adamellite; and (3) the 238 Ma Ruby Creek Granite. Recent l:10000-scale geological mapping has revealed that petrologic diversity within the SGG is far greater than previously thought, with contact relations locally delineating >12 discrete mafic and felsic intrusive suites, including hybrid magma bodies. These are: the Maryland River Quartz Monzodiorite (new unit - n.u.); Ridge Monzogranite (n.u.); Warroo Monzogranite (n.u.); Five Mile Creek Syenogranite (n.u.); Mossvale Granite (n.u.); Kia-Ora Syenogranite (n.u.); Undercliffe Falls Granodiorite (equivalent to phase 1 (Rmugl) of the Undercliffe Falls Adamellite); Undercliffe Falls Monzogranite (equivalent to phase 3 (Rmug3) of the Undercliffe Falls Adamellite, and comprising a marginal phase of the Five Mile Creek Syenogranite); Stanthorpe Monzogranite (equivalent to phase 1 (Rlsgl) of the Stanthorpe Adamellite coarse grained porphyritic adamellite); Stanthorpe Syenogranite (equivalent to phase 2 (Rlsg2) of the Stanthorpe Adamellite (coarse-grained, equigranular adamellite, dominant phase); and Ruby Creek Granite. A host of mafic-felsic (zoned) dikes (e.g. Herding Yard Creek Intrusives), layered mafic intrusions (intruding and cut by SGG), hybrid magma bodies and enclaves (Karonstadt Hybrids - n.u.) are intimately related to the granites and provide insights into processes related to SGG compositional diversity and granitoid genesis. Gabbro-monzogranite hybrid bodies result from mixing and mingling of mantle-derived mafic magma with crustal anatectic melts. Numerous mafic enclaves with quenched margins, xenocrysts of rounded quartz with coronas of hornblende, feldspars displaying rapakivi texture, and abundant net-veining textures, all imply magmatic disequilibrium conditions, due to magma mixing and hybridization. Often several different granitoid compositions occur within the one hybrid body. Zoned dikes have mafic rims and felsic cores, requiring coexistence of compositionally distinct melts spanning the range of SGG magmas. An important aspect of studies of granitic rocks is to understand how their geochemical characteristics relate to their source rocks, petrogenetic processes and tectonic environments. Geochemical data for the range of SGG granitoid suites necessitate reassessment of their origin and petrogenetic relations. Do these rocks comprise a textural and compositional continuum? The high-K, I-type SGG granitoids include some metaluminous suites (e.g. Rmugl), and numerous highly evolved peraluminous types. Mg-numbers (Mg range from 47 to as low as 10 in the extremely evolved Ruby Creek Granite and Mossvale Granite (alkali feldspar granites) with overall variation in Si02 content from 61-77.5 wt. %. Most individual plutons display relatively small ranges of Si02. In some cases, individual intrusions or groups of intrusions display continuous geochemical trends that may be interpreted in terms of restite unmixing or crystal fractionation (e.g., Rlsgl - Rlsg2). Importantly, most trends for different suites are discontinuous, sub-parallel and may not represent one continuous comagmatic sequence. Chemical signatures may be attributed to differing proportions of mantle-derived and crustally-derived components in the genesis of spatially and temporally closely-related intrusions. The post-tectonic I-type granites of the New England Batholith intruded a Carboniferous subductionaccretion assemblage during the Late Permian - Early Triassic, following a phase of subduction-related rifting in the Early Permian, and subsequent (Late Permian) deformation and uplift. Progressive magmatic episodes may have occurred in a trans-tensional regime in a subduction-related (ensialic back-arc) setting, where interaction between crustal components and mafic magmas also provided a heat source for voluminous crustal melting. Mantle components were derived from early depleted to later enriched (sub-continental) sources. Alternatively, partial melting of potentially heterogeneous (Devonian-Permian) mafic arc basement / underplate of diverse mantle parentage may have occurred, with extensive magma mingling simply coincidental. The data shed light on the nature of some fundamental processes related to granite genesis, including intimate interaction between crustal and mantle melts via hybridization, links between calc-alkaline and tholeiitic magmatism, and element budgets during continental margin arc development.

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GEOCHEMISTRY AND Nd-ISOTOPE SYSTEMATICS OF SEDIMENTS FROM THE DUN MOUNTAIN OPHIOLITE: I - CHEMICAL SEDIMENTS W. J Sivell and Waterhouse J. B. Division of Earth Sciences, The University of New England, Armidale NSW 2351. The Early Permian Dun Mountain Ophiolite Belt (DMOB), New Zealand, has been re-interpreted as having formed during a phase of infant-arc magmatism in an extensional forearc setting (Sivell & McCulloch 2000). Early-formed back-arc basin crust (including stage 1 and 2 pillow lavas and sheeted flows, respectively), preserved as remnants within the ophiolite belt, was intruded and engulfed by voluminous infant arc (stage 3) magmas and residual mantle ultramafics which comprise the bulk of the ophiolite. Refractory infant-arc mafic dikes and plagiogranites have been dated by U-Pb and Nd-isotopic methods at 270-280 Ma. Abundant sediments occur in close association with the ophiolitic volcanics and comprise two distinct associations. These are: (a) an older suite of diverse chemical sediments and turbidite argillites bearing an intimate depositional relationship to stage 1 and 2 back-arc basin eruptives; and (b) a younger suite of matrix-supported ophiolitic rudites and sandstones of (proximal) turbidite and mass flow (debris flow) origin, rich in clasts of stage 3 infant arc (forearc) rocks. (The terrigenous sediments are discussed in the companion Abstract - this volume.) There are four groups of DMOB chemical sediments, including red chert, black nodular Fe-Mn crusts, red muds and altered hyaloclastites. Red chert, mainly composed of radial-fibrous pore-fill chalcedonic quartz and hematite, forms interstices between stage 1 basalt pillows, or lenses up to 1 m thick overlying pillowed flows. Trace metal enriched Fe-Mn crusts occur as thin (<6 cm) black nodular layers along pillow lava - sediment interfaces. These are overlain in turn by red mudstones (<20 m), hyaloclastite (<20 m), and turbidite deposits, which represent an early formed deep marine assemblage. The chemical sediments, and significant metalliferous contributions to overlying turbidite argillites, provide an important link between the terrigenous sediments and the stage 1 (back-arc) basalts. Very high Fe/Ti ratios (up to 1000) in the hematitic cherts indicate a dominant hydrothermal metalliferous component. On PAAS normalised REE plots, the cherts yield overall LREE depleted patterns like seawater, with very low REE levels (10^-10 ^ PAAS), and slight negative Ce anomalies, but without the large positive Eu anomaly of high temperature hydrothermal vent fluids. Low Mn/Fe Zn/Fe, Cu/Fe, Pb/Fe and Ni/Fe ratios are less than stage 1 basalts and EPR fluids. Si and Fe supply to ocean water was promoted by halmyrolysis of proximal glassy basalt. Black nodular crusts and red muds display good negative correlations of Mn/Fe with Nd and P contents and SNdCf), and positive correlation with Ce/Ce*. Nodules show the highest Nd contents (up to 130 ppm), with IREE up to 500, as well as very high transition element contents (Z[Ni+Co+Cu] up to 2500). The hydrogenous nature of the superenrichments is indicated by Ni/Fe, Cu/Fe, Pb/Fe and Zn/Fe ratios which are higher than EPR vent fluids, consistent with a lack of detrital A1 and Ti, and confirmed by REE data. Highest Ce/Ce* ratios in some nodules reflect a predominantly seawater source for Ce, with hydrogenous adsorption to an essentially hydrothermal component at low sedimentation rates. High levels of Zn, Ba, V, Pb and Sr, slight positive Eu anomalies on PAAS normalised REE profiles, as well as intermediate - high Fe/Ti ratios, suggest a high-temperature (230-280°C) hydrothermal input. The lowest Mn/Fe, high Nd, P nodular crusts are typical of hydrogenetic Fe-Mn deposits with no diagenetic influence from underlying substrate. Importantly, the transition element enriched nodules possess SNd(T) values identical to that inferred for Permian seawater (~0). Transition element poor red muds have lower 8Nd(T^) values (1 to -2; i.e. <seawater), and Nd-isotopic model ages (T^j^=1.3-1.4 Ga) that indicate contributions from background fluvial and atmospheric particulate fallout derived from continental crust of average Proterozoic age. However, large positive Ce anomalies (Ce/Ce*=1.6) in the nodules and muds reflect a predominantly seawater source, with local bottom waters not stripped of Ce. Lower ZREE and higher Mn/Fe (up to 0.4) in the red muds than in the nodules are attributed to increased diagenetic influence. Locally, mudstones near lava-sediment interfaces have Mn/Fe, Zn/Fe, Cu/Fe, Pb/Fe and Ni/Fe ratios identical to stage 1 basalts, as well as high Ti02 (1.92.3 wt%), AI2O3 (up to 18.2 wt%), P2O5 (up to 0.63 wt%) and Nb (up to 54 ppm), unequivocally relating these rocks to the stage 1 lavas. Their REE profiles are also similar to Stage 1 basaltic glass and 8Nd(T) values (~+2) are transitional toward the higher values of the basalts. These rocks define a low-Fe/Ti 'Residual Halmyrolytic Trend' complementary to the high-Fe/Ti chert trajectory, with no hydrogenous/hydrothermal signatures. They contain material residual after low-temperature (<200^C) removal of chert components from altered hyaloclastite, variably leached of Eu by early hydrothermal fluids. REFERENCES SIVELL W. J. & McCULLOCH M. T. 2000. Reassessment of the origin of the Dun Mountain Ophiolite, New Zealand: Nd-isotopic and geochemical evolution of magma suites. New Zealand Journal oj Geology and Geophysics, 43 (2).

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GEOCHEMISTRY AND Nd-ISOTOPE SYSTEMATICS OF SEDIMENTS FROM THE DUN MOUNTAIN OPHIOLITE: II - TERRIGENOUS SEDIMENTS W. J Sivell and Waterhouse J. B. Division of Earth Sciences, The University of New England, Armidale NSW 2351. Two distinct Early Permian clastic sedimentary associations occur within the Dun Mountain Ophiolite Belt (DMOB), New Zealand. These include: (a) an early deep marine graded sandstone - turbidite argillite (TA) association intercalated with ophiolitic lower pillow lavas and upper sheeted flows (DMOB stage 1 and 2 (back-arc basin) basalts); and (b) a coarse matrix-supported, lithic-dominated sandstone + ophiolitic rudite assemblage associated with stage 3 (infant arc - forearc) magmas. The geochemistry of the sediments constrains relations between the tectonic setting of ophiolite emplacement, contiguous Palaeozoic terranes and palaeo-Pacific morphotectonic elements (e.g., Gondwana rim and island arcs). Stage 1 pillow lavas are overlain by red mudstones (20 m thick), passing upwards into green (redeposited) hyaloclastite (<20 m), and unfossiliferous graded sandstones and red and green TA (<70 m), with variable (but diminishing) metalliferous contributions that unequivocally link the TA to stage 1 (back-arc) lavas (see companion Abstract - this volume). A (<50 m) thick band of ungraded Atomodesmid-bearing grey TA is intercalated between lower and upper lavas. Following initial quiet suspension (and chemical) sedimentation, turbidite deposition took place in a shallowing deep-marine environment, with andesitic-dacitic TA compositions reflecting volcanism during pre- to syn-rift back-arc basin evolution. The TA show systematic trends of decreasing Eu/Eu* and ? Nd(T) (+2, +1 and 0.5 in red, green and grey TA, respectively) with increasing ? REE, La/Y, Th/Sc (0.1-0.8) and Si02 (53-67 wt%). Their trace element contents resemble evolved island arc rocks (e.g. Brook Streeet Volcanics (BSV) andesite-rhyodacite with similar (La/Yb)N and Eu/Eu*). But low positive ? Nd(T) values imply source components less depleted than rocks from island arcs or the ophiolite itself (? Nd(T)=+6 to +9). Th abundances and Th/Sc ratios (up to 2.0) are higher in the TA than in island arc and ophiolitic suites, similar to those of continental arcs. (Moreover, the oldest rocks in the BSV are -265 Ma, some 15 m.y. younger than the youngest (stage 3) DMOB magmas.) Antipathetic ? Nd(T) Th/Sc, Si02 correlations may reflect mixing between isotopically depleted island arc components and enriched continental crustal materials. The narrow range of positive ? Nd(T) values for the TA resembles values for calc-alkaline magmas from the Southern Volcanic Zone of the Andes (? Nd(T)=+2; Th/Sc=0.03) and suggests a predominant young differentiated continental arc source rather than old upper crust, but less dissected than the quartzofeldspathic plutonic provenance for the Torlesse (? Nd(T)<-2). The slightly higher ? Nd(T) values (and lower Zr/Nb etc.) for red and green TA suggest a minor 'hydrothermal' contribution from stage 1 basalts. The TA overlap a far wider compositional field for sediments from the Caples terrane. Exclusively terrigenous sandstones + ophiolitic rudites comprise a younger bimodal sedimentary suite of differing tectonic affinity. The coarse sandstones are rich in clinopyroxene of ophiolitic affinity, with compositions (e.g. Alz-Ti distributions) like pyroxenes from DMOB stage 3 magmas, but overlapping with pyroxenes from primitive arc basalts. They also contain significant amphibole, not present in BSV tholeiites, but abundant in ophiolitic plagiogranites. Massive poorly stratified, matrix-supported rudites (monomictpolymictic ophiolitic breccias) comprise discontinuous beds up to 10 m thick and contain abundant lithic fragments of stage 3 (infant arc - forearc) rocks, ranging from silicic plagiogranite to rare ultramafics. The sandstones possess 50-58 wt% Si02, with slight LREE enrichment ([CeN/YbN]=2.5-2.8) (less than for the TA with [CeN/YbN] = 4.1-4.9), and REE and incompatible element patterns closely resembling BSV island arc tholeiites. Th/Sc ratios are low (<0.3), implying a more mafic source than for the TA. The sandstones do not show the pronounced LREE depletion of DMOB stage 3 magmas ([CeN/YbN]=0.53-l.65). However, ophiolitic breccias, with their higher Si02 (-69 wt %), low Eu/Eu* (-0.72), [CeN/YbN]~l.56, Th/Sc (0.10.2), and higher, flatter HREE (TbN/YbN~1.0) and convex upward LREE-MREE (i.e. lower Ce/Nd, La/Sm), are transitional between the sandstones and plagiogranites. Importantly, the mafic sandstones and silicic breccias show high positive initial ratios (sNd(T)=+5 to +8), more akin to the high positive values for BSV eruptives. This implies mixing between varying amounts of young (mafic and silicic) mantle-derived source materials. CNd(T) values exceed the high positive values for Caples sandstones (sNd(T)=+3.6-+5.5). Crustal residence (mean provenance) ages are lower in the DMOB sandstones and rudites (TCR=^0.46-0.84 Ga) than for the TA (1.0-1.2 Ga). In contrast to Maitai, Caples and Torlesse sediments, little continental input is permitted by Nd isotopic data for these rocks. Predominantly continental arc sources are also precluded by the lower Si02 contents of the sandstones compared to the TA (unlike Torlesse sandstones). During nascent subduction, bimodal magmatism at infant arc volcanic centres in an extensional forearc setting, separated from continental clastic sources, shed turbidite sands and mass-flow deposits into proximal fault-bound basins.

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APPLICATION OF DATA MINING AND KNOWLEDGE DISCOVERY TECHNIQUES TO MINERAL POTENTIAL MAPPING IN A GIS ENVIRONMENT Andrew Skabar School of Electrical & Electronic Systems Engineering, Queensland University of Technology PO Box 2434, Brisbane QLD 4001, Australia

Geographic Information Systems (GIS) have emerged as an important technology in mineral exploration because they provide a powerful means of integrating, analysing and displaying large multi-source geoscientific datasets. Although the development of GIS technology and digital data manipulation techniques has enabled mineral exploration geologists to make more efficient use of resource information, many of the methods used are still inherently based on modem analogues of traditional techniques of map stacking in which layers of data are combined under the guidance of a mineral deposit model. The focus of this research is on the development of computer-based, automated techniques for processing spatially registered data sets containing a large range of correlated information for which the interdependencies are poorly understood and are in many cases purely qualitative. The research is motivated by the belief that a successful system could reduce the cost arising out of the risk and uncertainty associated with mineral exploration, and assist in the discovery of ore deposits in areas currently considered non-prospective. In the context of GIS, mineral potential mapping can be seen as a process whereby a set of input maps, each representing a distinct geoscientific variable, are combined using some function to produce a single map which ranks areas according to their potential to host deposits of a particular type. If the function is derived on the basis of measured association between mapped predictor variables and known mineralisation then the approach is said to be data-driven. All data-driven approaches are based on the assumption that the known mineral deposit occurrences in the study area constitute an adequate and unbiased sample of the true deposits in the region. Knowledge Discovery and Data Mining techniques such as Artificial Neural Networks (ANN) and Decision Tree Induction systems have recently emerged as powerful new tools for extracting implicit, previously unknown, and potentially useful information from multivariate data. Because they require neither a deposit model nor a statistical model, and because and they can usually operate at acceptable levels of accuracy when input data is incomplete or inaccurate, they are well suited to application in data-driven mineral potential mapping. These techniques have been applied to the analysis of geological, magnetic and radiometric data over the Castlemaine region of Victoria. Each 50m by 50m grid element in the study region was assigned to one of ten mineralisation favourability classes based on patterns extracted using data mining techniques. Of the 148 known primary gold deposits in the study region, 97 appear in the two highest favourability classes. The map also highlights several zones of high mineralisation potential in the vicinity of which there are no known deposits. These locations may correspond to regions in which exploration has not been extensive, and which may be highly favourable for further follow-up analysis. Andrew Skabar acknowledges the support of the Bicentennial Gold 88 Endowment Fund to this paper.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TIME CORRELATION OF MAGNETIC SUSCEPTIBILITY DATA FROM MYALL LAKES AND THE RECORD OF SEDIMENT AND CLIMATE CYCLICITY Greg Skilbeck\ Fiona Dick^ and Ed Frankel' ^ Department of Environmental Sciences, University of Technology, Sydney. NSW. 2007. Australia ^ School of Geosciences, University of Newcastle, Callaghan, 2308, Australia. Three of the thirty two cores extracted from Myall Lakes from 1997-1999 have 20 AMS '"^C ranging in age from 580 40 to 43,300 800 calibrated years before present (ybp). Two of the (ML19 and ML32) show a broad age correlation for both lithofacies and magnetic susceptibility profiles. A third core (ML28) from the centre part of Myall Lake although conforming broadly to the lithofacies patterns, differs considerably in detail, particularly in the nature of the transition from facies 3 (sapropel) to facies 2 (silty clay) (terminology of Skilbeck et al., 1999). This transition represents the change from perched swamp deposits which accumulated during the last glacial episode, to coastal lakes that were established during the Holocene marine transgression. The onset of lake sedimentation occurred rapidly at ML 19 and ML32 as indicated by the sharp facies boundaries, but was apparently not synchronous (-8370 ybp at ML19 in the west of the lake and -7740 ypb in the east at ML32). At ML28 which lies between the two, the facies 3/2 boundary is clearly gradational over 50cm and appears to be somewhat older than at either of the other two locations, presenting an interpretation problem, as ML 19 and 32 lie on any inundation routes that could be proposed from the current topography. High-resolution magnetic susceptibility data (shown below for ML 19 and 32 plot versus downhole age below) for all boreholes correlate well in depth for the non lake-margin cores suggesting a depositional influence operating at a higher frequency than that controlling lithology. Frequency analysis on raw, 1cm spaced magnetic susceptibility data reveal two dominant frequencies, one at about 2800 years (with a harmonic at 1400 years), and one at 500-550 years. Although the same frequencies are present in both curves, inspection of the graphs below shows that synchronous correlation of the profiles is not yet possible suggesting more age control will be necessary before climate cyclicity can be credibly established and interpreted. Acknowledgements: Work to date has been supported by the Australian Research Council and 14C dating has been carried out under Australian Institute of Nuclear Science and Engineering (AINSE) grant nos 97/195R and 99/124. Reference SKILBECK, C.G., FRANKEL, E., CRAMP, A.., PURSER, P. & TRIBBLE, J.S., 1999, A record of coastal change in high-stand littoral lakes from a stable passive margin (central eastern coast of Australia). In, Fletcher, C.H. and Matthews, J.V. (Eds), The non-steady state of the inner shelf and shoreline: coastal change on the time scale of decades to millennia in the Late Quaternary. Inaugural Meeting of IGCP #437 "Coastal environmental change during sea level highstands", Univ. Hawaii, Nov. 1999. pp. 195-202.

a g e (calibrated y b p xl 000)

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TIMING OF CU-AU(-MO) AND REGIONAL SODIC-CALCIC ALTERATION IN THE OLARY - BROKEN HILL REGION: MOLYBDENITE RE-OS AND TITANITE U-PB DATING CONSTRAINTS R.G. Skirrow', P.M. Ashley^ K. Suzuki^ and N.J. McNaughton^ ' Australian Geological Survey Organisation, GPO Box 378, Canberra, A.C.T. 2601 ^ University of New England, Armidale, N.S.W. 2351 ^ University of Kyoto, Noguchibaru, Beppu, Oita 874-0903, Japan University of Western Australia, Nedlands, W.A. 6907 Copper-gold (-molybdenum) mineralised systems discovered in the Paleoproterozoic Willyama Supergroup (Cumamona Province) are members of the diverse global family of iron-oxide associated Cu-Au deposits. Although regionally distributed sodic-calcic alteration is well known in many Cu-Au-Fe districts (e.g. Eastern Succession, Mt Isa Inlier; Kiruna region, Sweden), the temporal and genetic relationships between this alteration and mineralisation are not well understood, limiting its potential utility in exploration. New results from the Broken Hill Exploration Initiative offer insights on these relationships. Two principal types of regional sodic alteration occur within the Willyama Supergroup. Mainly stratabound pre- or early tectonic Na±Fe alteration, which may have commenced during diagenesis, is present in the Thackaringa Group in the Broken Hill Domain and within quartzofeldspathic to calcsilicate-bearing metasedimentary and -1715-1705 Ma felsic meta-igneous rocks of the Olary Domain. Syntectonic Na±Ca±Fe metasomatism was localised by shearing and folding attributed to mainly the D3 regional deformation event. Syntectonic alteration styles include calcsilicate-matrix breccias, vein networks, and intensely 'albitised' zones affecting diverse lithologies, with assemblages of Na-plagioclase, clinopyroxene, clinoamphibole, quartz, magnetite, hematite, garnet and titanite. Oxygen isotope geothermometry suggest formation at medium-high temperatures, consistent with fluid inclusion and mineral stability constraints. SHRIMP U-Pb dating of three titanite samples from syntectonic Na±Ca±Fe metasomatic zones and one sample from pervasively albitised granite in the Olary Domain yielded earliest Mesoproterozoic ages, all within the - 0 . 5 % errors. Consideration of titanite closure temperatures for U-Pb implies that the ages are minima for titanite-bearing alteration. Conversely, textural evidence, fabrics and field relationships indicate that syntectonic Na±Ca±Fe metasomatism postdated the upper greenschist to amphibolite facies peak of regional metamorphism, which is widely considered to have occurred at -1600-1595 Ma and sets a maximum age of the syntectonic alteration. A previously determined Sm-Nd age of 1575±26 Ma for gametepidote regional alteration is compatible with our timing constraints for Na±Ca±Fe alteration. Vein and replacement Cu-Au(-Mo) mineralisation and commonly associated local potassic and sodic alteration zones (including K-feldspar, biotite, albite, amphibole, magnetite, hematite, pyrite, carbonate) are spatially distinct from syntectonic Na±Ca±Fe regional alteration. Relative timing of Cu-Au-Mo introduction varies from pre-peak metamorphic (pre- or early-D2?) to post-peak metamorphic, late-D3, with multiple weaker deformation overprints. Molybdenite crystallisation accompanied chalcopyrite-Au deposition, and in some prospects an earlier generation is also present. Molybdenite Re-Os isotopic dating at the Portia, Kalkaroo, White Dam and Waukaloo prospects yielded 9 ages ranging over 20 m.y. in the latest Paleoproterozoic. High precision (0.04-0.5%) was obtained on individual Re-Os ages, with high reproducibility in replicate analyses (0.07-0.4%). The dating results for molybdenite imply Mo and initial Cu-Au introduction before the peak of metamorphism in at least some of the four dated systems. However, uncertainties in the decay constant for 187Re result in 0.5-1.0% errors in the absolute ages and are thus permissive of syn- or perhaps post-peak metamorphic Cu-Au-Mo mineralisation. A temporal overlap between mineralisation and syntectonic N a i C a i F e alteration cannot be ruled out, although differing spatial distributions, oxygen-hydrogen isotopic compositions of fluids and geochronological results collectively suggest that the Cu-Au-Mo systems involved fluids other than those producing regional syntectonic Na±Ca±Fe alteration.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

POSSIBLE CAUSES AND CONSEQUENCES OF PALEOGENE WARM CLIMATES Lisa Cirbus Sloan', David Greenwood^, Scott Wing^ and Matthew Huber' 'Department of Earth Sciences, University of California Santa Cruz. ^School of Life Sciences & Technology, Victoria University of Technology. ^Department of Paleobiology, Smithsonian Institution.

Geologic records for much of the Paleocene and Eocene epochs indicate globally warm climates. However, these records also suggest that the mean state of these warm climates have not been constant. Extreme warmth occurred in the latest Paleocene (the Late Paleocene Thermal Maximum, LPTM), and marine and terrestrial systems document significant cooling and warming intervals both preceding and following that event. The causes of these warm climate states and their variability are currently unknown. We explore possible causes by using climate models, and comparing the model results to paleoclimate proxy data. To address the question of early Cenozoic climate forcing, we have used global climate models to explore the impacts of atmospheric composition, polar stratospheric clouds, and orbital forcing upon Paleogene climate. Our results show that changes in these factors can generate significant variation in temperature, hydrology, wind patterns, and upwelling distributions. For example, the presence of polar stratospheric ice clouds, linked to high concentrations of tropospheric methane, produces up to 20°C warming at winter hemisphere high latitudes with little or no warming in tropical regions. In response to these clouds, mean annual temperature increases, mean armual temperature range decreases, and high latitude precipitation increases. Relatively small changes in orbital forcing produce dramatic changes in the timing and magnitude of monthly precipitation and continental runoff, mean annual temperature, and annual temperature range. In this presentation we examine the climate modeling results from three perspectives and scales: global, Australia, and western North America. The regional model results will be compared to paleoclimate conditions interpreted from paleontologic and geologic data, in an attempt to understand the causes of Paleogene warm climates.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 IS^'^Australian Geological Convention, Sydney, July 2000

THE MIOCENE PALAEOENVIRONMENTAL AND PALAEOCEANOGRAPHIC EVOLUTION OF THE SEASPRAY GROUP, GIPPSLAND BASIN, SOUTHEAST AUSTRALIA A.J. Smith', S.J. Gallagher^ K. Jonasson', D. Taylor^ M.W. Wallace', G.R. Holdgate', J. Daniels' and J.B. Keene^ ' School of Earth Sciences, University of Melbourne, Parkville, Victoria, 3010. ^ School of Geosciences, The University of Sydney, N S W 2006.

The Gippsland Basin in southeast Australia preserves up to 2.5 km of Oligocene to Recent marine carbonate sediments belonging to the Seaspray Group. During the Neogene the Seaspray Group carbonates of the Mackerel and Flounder petroleum fields lay at the shelf to bathyal palaeoenvironmental transition, an ideal palaeoceanographic setting to study the Neogene history of events in the evolving Southern Ocean. Seismic, foraminiferal and facies analyses of the Miocene Seaspray Group sediments in Mackerel-1 and Flounder-1 wells has revealed the following palaeoenvironmental history of the area: •

Relatively cool to temperate upwelling conditions prevailed in the area during the earliest Miocene. Low energy upper bathyal Globigerina ooze calcareous mudstones were deposited in the Oxygen Minimum Zone during this time.

•

Upper bathyal marls were deposited in oligotrophic conditions during the Early to Middle Miocene (c. 23-16ma). By this time the presence of abundant shallow and deep dwelling warm to cool-temperate plankton suggests that warm poorly stratified oceanic conditions prevailed in the region. This "local" event correlates with the "Miocene Climatic Optimum", when global ocean conditions were much warmer and less vertically and latitudionally stratified than present. These oceanic conditions were associated with a reduced East Antarctic Ice Cap, a relatively weak Antarctic Circumpolar Current and weak wind driven and atmospheric circulation.

•

Strong upwelling and cooler oceanic conditions initiated in the outer shelf to upper bathyal canyon facies during the upper Middle Miocene to Late Miocene (after c.l6ma). The occurrence of common shallow and deep dwelling plankton taxa in this interval suggests that the regional oceanic conditions became more vertically stratified during this time. The Middle to Late Miocene palaeoenvironmental change recorded in Seaspray sediments corresponds to global cooling and major Antarctic Ice Sheet expansion between 16 and 5 million years ago, when the world's oceans became more vertically and latitudionally stratified. The transition from a warm "Miocene Optimum" to cooler oceanic conditions from around 16 million years was associated with a strengthened Antarctic Circumpolar Current and more intense wind driven and atmospheric circulation in the Southern Ocean.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

KINEMATICS OF BRITTLE DEFORMATION IN A PLUTON ROOF ZONE, NORTHEAST NEW SOUTH WALES John V. Smith School of Resource Science and Management, Southern Cross University Geological setting Turbidites of the Neranleigh-Femvale beds of northeast NSW generally exhibit low grade regional metamorphism. However, locally in the Tweed Valley, the lack of fissility, the presence of biotite in pelites and albite spotting in psammites indicate an overprint of contact metamorphism. Whole-rock K-Ar analysis of a sample of pelitic homfels from Norries Head provided an age of 233 (-2) Ma but no similar aged intrusions are exposed in the region. This metamorphic age is similar to the 220-230 Ma age of granites of southeastern Queensland which form a colinear trend with the Tweed Coast of northeast NSW. The intrusions of southeast Queensland are also hosted by the Neranleigh-Femvale beds and comparative study shows the similarity of the contact metamorphism. In Queensland, fissility of pelites is low within approximately 1km of the granites and strong homfelsing occurs within approximately 500m of the granites. This suggests that granites occur within 500m of the surface at Norries Head and other parts of the Tweed Valley and that granites lie within 1km below surface of much of the Tweed Valley. Structural relations At Norries Head, biotite fringes on quartz veins, which occur as vein arrays in association with faults, indicates that these brittle structures occurred during the contact metamorphism and thus record deformation of the inferred pluton roof during intrusion. The faults and vein arrays have a low average conjugate angle (22®) which is typical of failure at low differential stress. The conjugate vein-fault systems record dominantly horizontal NE-SW shortening perpendicular to strike of bedding and fold axial planes. Dextral and sinistral faults (and vein arrays) are equally abundant but preliminary results indicate significantly greater average displacement occurred on dextral faults. This pattern indicates a non-coaxial defonnation involving a N-S dextral shear couple. Structural interpretation The conjugate strike-slip vein arrays and faults record plane strain in the horizontal plane. The inferred maximum principal stress (NE-SW) was compressive but the minimum principal stress (NW-SE) could have been either compressive or tensile. The sub-horizontal compressive stress trajectory is consistent with concentration of compressive stress in the rigid roof zone above a weak pluton. The plane (horizontal) strain and vorticity of the progressive deformation may have resulted from obliquity between the remote compressive stress and the trend of the roof zone as defined by the pluton shape. The observed brittle deformation is interpreted to have been triggered by stress concentration within the roof zone and high fluid pressure above the pluton. Veining and contact metamorphic effects are most pronounced in an antiformal hinge zone indicating some structural channelling of fluid flow. This style of brittle deformation may be integral to the emplacement mechanism of Permo-Triassic granites of the New England fold belt by producing a system of weak structures for advancing magma to exploit.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE GEOCHEMISTRY OF SOILS IN THE IRON COVE CATCHMENT. Rosemary Snowdon and Gavin Birch Environmental Geology Group, School of Geosciences, The University of Sydney

The current study determines the nature and concentration of heavy metals in the soils from Iron Cove catchment in the inner western suburbs of Sydney, New South Wales. The metal concentration of topsoil in the Iron Cove catchment has been analysed for Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn and Ag. Of the 374 samples analysed 56%, 34%, 33% and 17 % of them were greater than the ANZECC and NH&MRC 1992 guidelines for Zn, Cu, Pb and As samples respectively. Also 56%, 16%, 17% and 17 % of the samples had Zn, Cu, Pb and As respectively above the Schedule B (1) draft guideline investigation levels for soil and groundwater. The remainder of the metals each had ? 3% of the samples above the guidelines, where guidelines are available. Spatial distributions show that the concentration of Cu, Pb and Zn decrease with distance from the eastern edge of the catchment. The eastern area of the catchment bounded by the goods railway was found to be consistently above the guidelines for Cu, Pb and Zn. Past and present industry are believed to be major contributors to these elevated levels, as well as contamination from automobiles. Large proportions of the metal concentrations analysed were bioavailable. Bioavailability results show 80%, 87% and 43% of the EDTA samples for Cu, Pb and Zn (respectively) were found to have bioavailability above 50%, whilst 87%, 90% and 80% of HCl samples for Cu, Pb and Zn (respectively) were found to have bioavailability above 50%. Overall HCl tests returned higher bioavailability results then the EDTA tests. Areas of extreme and high soil loss were detected in areas surrounding the Hawthorne Canal and adjacent to Iron Cove, while moderate to high soil loss areas were found largely surrounding Iron Cove Canal. It is possible that soil loss in these areas is a source of contaminants to Iron Cove and Port Jackson. This study illustrates the importance of heavy metal monitoring due to the nonbiodegradable nature of these elements, and the role that past and present land use practices play on the contamination of soil.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

EXPLAINING VARIATION IN VHMS ORES (E.G. HOKUROKU, HELLYER AND MT. LYELL): THE LIKELY ROLE OF FRACTURES, DEEP FOOTWALL PERMEABILITY, AND ROCK BUFFERING CAPACITY M. Solomon and Jiangwen Yang Centre for Ore Deposit Research, University of Tasmania, Hobart

Ores in the Hokuroku Basin are relatively small because, like mid-ocean ridge deposits, they are derived from buoyant fluids with salinities, and H and O isotopic compositions, close to that of seawater (Ohmoto, 1996). Lack of feeder fauhs mean fluid flow was focused by convection physics, and we suggest that high deep-footwall permeabilities resulted in fluid flow driven by magma emplacement but dominated by convecting seawater. For the larger and richer Hellyer deposit, the ore fluids are sufficiently saline (up to 15 wt. %) to pond in a basin, thereby trapping more, perhaps all, of the metal content (Solomon and Khin Zaw, 1997). A magmatic origin is indicated for the potassic, saline fluids, which rose up syn-mineralization fractures related to basin formation (Downs, 1993). Hydrodynamic modelling of a granitic or intermediate pluton emplaced in Proterozoic basement beneath the volcanic pile shows that by lowering footwall permeability relative to assumed Hokuroku permeabilities (thus cutting the relative mass flux of seawater), and adjusting fracture permeability, yields high temperature, saline fluids at the sea floor for sufficient time to form the Hellyer orebody. The ore fluids were more reduced than in the Hokuroku Basin probably because they did not mix with ambient seawater, only connecting via molecular diffusion across the brine-seawater interface. At both Hellyer and in the Hokuroku Basin the oxidation potential of the deep ore fluids was the outcome of prolonged equilibration with igneous and sedimentary rocks in the deep footwall, regardless of the fluid source. Mt. Lyell shows similar fracture control to Hellyer, but has two dominant ore assemblages: pyrite-chalcopyrite-sericite-chlorite and pyrophyllite-quartz-bomite-chalcopyrite, and Huston and Kamprad (in press) suggest that the former is overprinted, more or less penecontemporaneously, by the latter. There are no fluid inclusion data for Mt. Lyell, but the Chester deposit, a likely analogue, has yielded salinities up to 18 % (Boda, 1991). Mt. Lyell is underlain by several km of felsic volcanic rocks, raising the possibility that after prolonged circulation (forming the early assemblage), fluid buffering became of minor significance, allowing direct access of magmatic fluids only modified by seawater dilution (i. e. high temperature, acid, oxidized solutions that formed the later assemblage). We can thus account for much of the variation in character between our three examples by varying (1) permeablity in the deep footwall, and thereby the relative degree of seawater input, (2) the role of syn-mineralization fracturing, and (3) the influence of fluid buffering by the deep footwall rocks. References Boda, S. P., 1991. The geology, structural setting and genesis of the Chester mine, northwest Tasmania. Unpublished Honours B. Sc. Thesis, Australian National University, Canberra, 111 p. Downs, R. C., 1993. Syn-depositional fault controls on the Hellyer volcanic-hosted massive sulphide deposit: Unpublished M. Sc. thesis, University of Tasmania, Hobart, 63 p. Huston, D. L. and Kamprad, J., in press. Zonation of alteration facies at Western Tharsis: implications for the genesis of Cu-Au deposits in the Mt. Lyell field, Tasmania. Economic Geology. Ohmoto, H., 1996. Formation of volcanogenic massive sulfide deposits: the Kuroko perspective. Ore Geology Reviews, 10, 135-177. Solomon, M., and Khin Zaw, 1997. Formation of the Hellyer volcanogenic massive sulfide deposit on the sea floor: Economic Geology, 92, 686-695.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NUMERICAL MODELING OF OROGENIC PROCESSES AND GOLD MINERALIZATION IN THE YILGARN Peter Sorjonen-Ward, Yanhua Zhang and Chongbin Zhao Division of Exploration and Mining, CSIRO, PO Box 437 Nedlands, WA, 6009 It is widely held that most Yilgam gold deposits of Western Australia formed at a relatively late stage in the structural and thermal evolution of the craton, yet studies of mineral parageneses also demonstrate that gold precipitation was effective over a broad range of temperatures and pressures (Groves, 1993). Where P-T conditions of mineralization are documented, late peak to retrograde conditions are indicated, particularly in domains of lower metamorphic grade. Structural studies of vein systems also favour a compressive rather than extensional regime during mineralization. These features all imply that mineralization occurred during active uplift and exhumation during ongoing compression. Spatial and temporal relationships between granitoid complexes, metamorphic grade, and mineralised high strain zones further suggest that active deformation facilitated both lateral and vertical transport of heat, fluids and magma (Witt et al., 1997). Previous numerical models of the Yilgam (Hobbs et al., 1997) explored the thermal evolution of the region within a structural framework largely derived from deep seismic data and demonstrated the potential for deep crustal convection and implications for mineralization. Here we further investigate the coupled interaction between deformation, fluid flow and thermal evolution by integrating the above boundary conditions with a more detailed crustal architecture involving opposing - though not necessarily overprinting - imbricate thrust systems (cf Drummond et al., 1997). Analogies with deformed petroleum basins suggest that this architecture has the potential for creating fault-bounded domains of differential uplift and overpressuring beneath relatively impermeable units. This "bivergent" or tectonic wedging geometry and associated backthrusting may also contribute to the formation and preservation of greenschist facies deposits, in contrast to the lower long term preservation potential for deposits formed in elevated foreland fold and thrust belts. Results of 3D FLAG models illustrate the potential for fluid focusing and mixing in shear zones, including downflow of meteoric water, lateral fluid flow driven by topographic elevation and upward flow of fluids derived from melting and metamorphism in the deep crust. They also provide limited support for the tectonic wedging concept, and suggest that "pop-up" wedges facilitate effective fluid upflow and downflow during uplift, while topographic elevation related to asymmetric thrust migration and loading tends to promote lateral fluid flow. The effect of topography appears more important however, than the precise depth or location of the site of fluid production in the deep crust. Thermal models of a simplified section across the Kalgoorlie region using the code FIDAP have effectively delineated domains of convective fluid flow, in lithostatically pressured systems, within the middle and upper crust, and identify two generic sites that are favourable for fluid mixing, notably footwall environments in major shear zones, such as the Bardoc Shear, and within broad antiforms such as the Goongarrie - Mount Pleasant Antiform. The thermal effect of small plutons emplaced at higher crustal levels ahead of a prograding metamorphic front can also have a significant impact on the pattern and intensity of fluid transport and convection, at distances considerably greater than pluton diameter. These results however, are highly dependent on the permeability structure assigned to the crust, and require that magmatic and metamorphic fluid generation is precisely timed with respect to deformation, thus reinforcing the dynamic feedback between deformation, magmatism and fluid production and migration. References DRUMMOND B. J., GOLEBY B. R. & SWAGER C. P., 1997. Crustal signature of the major tectonic episodes in the Yilgam Block, WA: Evidence from deep seismic sounding. In: Cassidy C. F., Whitaker A. J. & Liu S. F. compilers Kalgoorlie '97: An international conference on crustal evolution, metallogeny and exploration of the Yilgam craton - an update pp. 15-20. AGSO Record, 1997/41, 15-20. GROVES D. I., 1993. The crustal continuum model for late-Archaean lode-gold deposits of the Yilgam Block, Western Australia. Mineralium Deposita, 28, 366-374. HOBBS B. E., UPTON P., ORD A., ZHANG Y., ZHAO C., DRUMMOND B. & ARCHIBALD N. J., 1997. Thermal and deformation modelling of the Yilgam deep seismic transect. In: Papunen H., ed. Mineral Deposits: Research and exploration - Where do they meet? Proceedings of the Fourth Biennial SGA Meeting, Turku, Finland pp. 859-862. A. A Balkema, Rotterdam. WITT W.K., KNIGHT J. T. & MIKUCKI E., 1997. A synmetamorphic lateral fluid flow model for gold mineralization in the Archean southem Kalgoorlie and Norseman terranes, Westem Australia. Economic Geology 92,407-437.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

OCEANIC SETTING AND SUBDUCTION-RELATED TECTONICS FOR THE CENTRAL LACHLAN OROGEN, SOUTHEASTERN AUSTRALIA Catherine V. Spaggiari^ David R. Gray\ David A. Foster^ and Mark Fanning^ Australian Geodynamics Cooperative Research Centre. ^Department of Earth Sciences, Monash University, Melbourne VIC. 3168, Australia, ^Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, U.S.A. ^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.

Lithological associations, in conjunction with geochronology, palaeontology, and geochemistry demonstrate that continental crust in the Central Lachlan Orogen evolved from Cambrian oceanic crust. The western boundary of the Central Lachlan Orogen can be defined as a suture zone which records collision of an Ordovician/Silurian accretionary prism (Tabberabbera Zone) with a Cambrian, oceanic island arc (as part of the Melbourne Zone). Andesitic volcanic arc rocks have a U-Pb (zircon) age of 500 ± 8 Ma and are overlain by andesitic breccia and volcaniclastics, including olistoliths of fringing reef limestones, and slivers of shale with Lower Ordovician (La -490 Ma) graptolites. Preserved oceanic crust (ophiolite) consists of basaltic and boninitic pillow lavas, ultramafics, dolerites, and gabbros. The pillow lavas are depositionally overlain by a sequence of Lower Ordovician (La) chert and silicified shale, which is conformable with overlying turbidites. The frontal portion of the accretionary prism (Governor Fauh Zone, Howqua) consists of a fault slice of the upper portion of the oceanic crust, which structurally overlies a 3 km wide melange zone. The melange zone comprises interleaved fault slices of greenstone, phyllite, slate, scaly mudstone with sandstone/siltstone blocks, and broken formation. Metavolcanic blueschist blocks in this melange have been exhumed in a talc/serpentinite matrix. Melange rocks are polydeformed and show a pronounced fold interference pattern. Metamorphism is low grade and fabrics in the sedimentary rocks are predominantly due to pressure solution, the exception being an early mica fabric present in some of the deeper level rocks. Ar/Ar dating of this fabric indicates that it formed at around 450 Ma. This is supported by the presence of Middle to early Late Ordovician (Da-G, -467-455 Ma) graptolites in these stratally disrupted rocks, demonstrating that deformation took place not long after deposition. Preliminary U-Pb results (SHRIMP/sphene) of blueschist metavolcanics indicates that metamorphism occurred at around 445 Ma. Broken formation of Middle Ordovician (Da) age also occurs to the southeast (Governor Fault Zone, Dolodrook) in association with a large, fault-bounded serpentinite body. Overriding the melange zone are slates and pelites, associated with fault slices of the Cambrian, oceanic island arc rocks (Mount Wellington Fault Zone). Collision between the oceanic arc and the accretionary prism has produced the dominant structural trend (NW/SE), including late folding of the melange zone and overthrusting of the oceanic arc and related sedimentary rocks.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE BERMAGUI DYKES: GEOCHEMICAL AFFINITIES AND RELATIONSHIPS TO STRUCTURE IN THE SE LACHLAN FOLD BELT A.G. S p r i g g ' D . W . Dumey^ and K.A. Dadd^'^ 'Dept. of Earth & Planetary Sciences, Macquarie University, N S W 2 1 0 9 ^GEMOC Key Centre, Macquarie Uniuversity, N S W 2 1 0 9

A suite of basaltic to andesitic and rhyolitic dykes, called the "Bermagui dykes", intrudes Ordovician and Middle Devonian rocks of the New South Wales South Coast between Picnic Point and Bermagui and inland around Murrah Forest (Sprigg, 1999). The dykerocks show comagmatic intrusive relations with one another, chlorite-epidotealbited=sericite alteration and commonly solid-state deformation, suggesting a bimodal igneous association that predates Early Carboniferous, very-low grade, regional metamorphism and convergent deformation in the area. Geochemical analysis of the dykes shows within-plate affinities and fractionation and contamination trends that correspond to extrusive members of the nearby Middle-Late Devonian Eden-Comerong-Yalwal Volcanic Zone (EVZ, Dadd, 1992). The dykes thus appear to be feeders to the EVZ and are tentatively assigned to this Zone and time period. Structurally, the dykes show variable intrusive relations along both bedding and transverse wrench faults and across F1 and F2 folds in multiply deformed Ordovician host rocks, supporting the pre-Late Devonian timing for these structures previously inferred by Powell (1983). The dyke-rocks were affected by later widespread, but generally weak, semi-brittle deformation (vein-arrays, striae, minor faults and stylolitic cleavage) with a mainly ENEWSW to NE-SW thrust compression direction, as observed in overlying Middle-Upper Devonian sedimentary units (Rixon et al., 1982; Powell's F1 in Upper Devonian and F3 in Ordovician, 1983). However, NE-SW wrench compression and ESE-WNW to SE-NW and N-S thrust and wrench compression structures also exist in the dykes at several localities, showing that the Early Carboniferous deformation history was more varied than previously assumed. The dykes intrude across or along several mesokinks and one megakink band in Ordovician host rocks and show no evidence of deformation by kinking. Minor N-S compression in some of the dykes is expressed by faulting in adjacent Ordovician rocks rather than by kinking. Therefore the megakink episode (Powell's 'F4' in Ordovician, 1983) occurred prior to Late Devonian and prior to Powell's 'F3' in Ordovician (1983) in this area, instead of in the Carboniferous. The Goalen Head Gabbro is also cut by members of the Bermagui dyke suite, supporting a possible Middle Devonian rather than Jurassic age for that body and, therefore, a possible association with the EVZ. References Dadd K.A. 1992. Tectonophysics, 214, 277-291. Powell C.McA. 1983. SGTSG Field Guide 1, Geological Society of Australia, Sydney, 118 pp. Rixon L.K., Bucknell W.R. & Rickard M.J. 1982. Journal of the Geological Soc. of Australia, 30, 277-293. Sprigg A.G. 1999. Unpublished BSc. Hons. Thesis, Macquarie University, Sydney, 168 pp.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CHEMICAL CONSOLIDATION OF SANDSTONE: THE SYDNEY PROJECT Alan Spry Alan H Spry & Associates, 512 Kensington road, Wattle Park, SA 5066 One of the longest running investigations in world (23 years) has been held here in Sydney with most experimental walls still accessible for inspection. The project was sponsored by the CSIRO and carried out by Amdel Limited (Adelaide). Tests (1977-1980) were first held on 14 commercial consolidants applied to samples of 22 sandstones from Sydney to assess depth of penetration and change in modulus of rupture, ultrasonic pulse velocity, appearance, water absorption, compressive strength, resistance to salt crystallisation and Brinell hardness. This was followed by the application (1980-1982) of six of the most-promising treatments to six. Nineteenth Century sandstone walls. These Test Walls have labels affixed and are located in Sydney as follows: TW 1, 2 and 3 at the rear of the Old Mint; TW 4 outside Hyde Park Barracks; TW5 in the basement of Elizabeth Bay House, and TW 6 (and later TW 7) on the inner surface of the southeastern part of the boundary wall of Victoria Barracks. The walls differ in exposure, orientation and lithology. Each wall was studied for mineralogy, petrology and salt content. Each stone in the test panels was tested for: type and degree of decay, Schmidt Hammer Hardness and Ultrasonic Pulse Velocity, before and after treatment. The consolidants were applied to six panels about 1.5m high and Im wide at the base of each wall indicated by brass studs (still present), and designated from 1 to 6, from left to right. Treatments were: Panel 1: Dynamit-Nobel, Dynasylan 5206, 40% solution of isobutyltrimethoxysilane; (5206), Panel 2: Wacker-Chemie Stone Strengthener H, tetraethoxysilane + methyltriethoxysilane; (H), Panel 3: Dow Coming T-0419, an uncatalysed silane, (DC), Panel 4: Wacker-Chemie Stone Strengthener H, partially polymerised and catalysed tetraethoxysilane followed by a coat of oligomeric alkylalkoxysilane, 090; (H), Panel 5: Hitchen's Formrok, alkylalkoxysilane+water+solvent; (F), Panel 6: Romaroda Chemicals Polymass, methyl methacrylate and styrene polymer; (P). (Panel 7 limited application later, Brethane, methyltrimethoxysilane+water+ethanol (B). The walls were re-tested at intervals for the next 3 years and then intermittently since. The treatments varied in effectiveness from wall to wall but none was found to cause damage and most achieved some improvement. There was an absence of loose grains on all but the strongly sanding and salty surfaces (eg TW 3 & 5); an increase in Schmidt Hammer hardness of at least 10%, an increase in pulse velocity of up to nearly 50%; an increase in the strength and adhesion of exfoliating flakes; and an increase in water repellence. In general effectiveness was as follows: DC & B > 5206 > H & OH>F & PM. Inspection in 1998, at least 15 years after the last application, found that the untreated part of the walls had not deteriorated significantly but that some protection of treated parts had been achieved. This is particularly seen on TWl & TW4 several blocks of which were treated on one half but not the other and now show significantly more decay on the untreated part. The three major deficiencies in applying such treatments to large wall areas are: achieving an adequate depth of penetration in such sandstones (particularly with their case-hardening), the unpredictability of the result, and the cost of the reagent.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SYDNEY SANDSTONE: AN OVERVIEW. Alan H Spry. Alan H Spry & Associates, Consultants, 512 Kensington Road, Wattle Park, SA 5066. The term Sydney Sandstone is used here in the commercial sense to refer to those Triassic sandstones from the Sydney-Gosford region used as dimension stone in significant historic and modem buildings, principally as block or thick veneer in restoration or as thin veneer in modem curtain walls. Classification on colour behaviour and genesis suggests the following six varieties (Groups): 1, Grey (eg Wondabyne); 2, "Yellow-Block" (self-colouring or oxidising); 3, White (eg Piles Creek); 4, Quartz-rich; 5, Colour-banded (eg Somersby) and 6, Typical surface Hawkesbury. Groups 1, 4 & 5 are colour-stable. Three aspects of colour modification are of interest: the need to control self-colouring behaviour, the effects of fashion, and the need to reduce colour mismatch in replacement stone for heritage building. The theme of this presentation is that the lithology (petrology and mineralogy, particularly that of the three major components: quartz, clays, and siderite), controls the major aspects of behaviour, namely the fresh body-colour, self-colour-change, classification, physical properties, building application, decay, cleaning, and chemical treatment. Self-colour change is controlled by the abundance and degree of alteration of the siderite. In this respect, Group 2 Sydney Sandstone s are unique. The sandstones do not simply consist of quartz grains in an abundant clay matrix. The constituent minerals and textures have formed under different conditions in four stages Stage 1, Sedimentation', quartz, clay pellets (? kaolinite, illite), carbonaceous plant remains, ilmenite etc. Stage 2, Early Diagenesis: compaction, deformation, authigenesi^: recrystallisation, neocrystallisation under reducing conditions, low Eh moderate pH; forming quartz overgrowths, siderite, mixed layer illite-smectite, kaolinite. Stage 3, Late Diagenesis: change to oxidising and hydrous conditions; pH and temperature decreasing; siderite oxidised to limonite/goethite, formation of kaolinite, leucoxene; migration of Fe and formation of banding. Stage 4, Exposure: quarrying, processing, construction; further oxidation of siderite, migration of Fe and Mn to surface, browning and blackening, case-hardening. The ultimate means of evaluation of quality is the long-term performance in service. Objective and quantitative evaluation of the quality of building stone is required to determine its value, to predict the performance of a new, untried stone, to assess the possible use for a given task (including determination of Factor of Safety); to determine conformity with a Specification, to compare the properties of altematives and for Quality Assurance. Petrography is the most useful immediate, single method of evaluation but is most applicable within a limited group with well-known properties and performance and tends to be subjective. Laboratory testing of a group of physical properties (water absorption, resistance to salt crystallisation, strength, loss of strength on soaking, presence of deleterious minerals, dimensional stability) is objective but requires standard methods and techniques, experienced and careful operators, and correct interpretation. The efficient application to a project requires a detailed and meaningful Specification including designation and testing of Pre-Tender, Proof, Acceptance and Quality Assurance samples.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE REGOLITH-LANDFORM MAP AND ITS APPLICATIONS FOR MINERAL EXPLORATION, AN EXAMPLE FROM COBAR, NSW. M.J. Spry', S.M. Hill' and K.G. McQueen' 'CRC LEME, University o f Canberra, A.C.T. 2601.

The development of more effective exploration techniques is important for the future of Australia's mineral industry. Traditional methods of surface and drill sampling for geochemical analysis as well as geophysical techniques have been successful in Australia, particularly in areas of outcrop or shallow regolith cover. In areas of greater regolith thickness and complexity, new exploration techniques need to be developed, especially using low cost surface methods. An understanding of the applications of regolith materials as sampling media, their landform setting and the regional landscape history is essential to achieve this. An important development in the field of regolith geology is the application of regolith/landform maps to mineral exploration programs. These maps provide a means for improving our understanding of element dispersion pathways and processes and element hosts in different types of regolith. Regolith-landform maps also have other applications, such as in land and water management. A regolith/landform map of the Cobar area in central NSW has been prepared at 1:100,000 scale and illustrates the approach of using regolith-landform mapping in mineral exploration and land management. Regolith-landform maps can assist mineral exploration programs by: • • •

identifying the suitability and spatial application of particular exploration techniques; representing the distribution of suitable regolith sampling media; identifying regolith materials with similar geochemical characteristics such as anomaly thresholds; and,

•

representing and constraining geochemical dispersion processes and pathways.

Regolith-landform maps can assist land and water management by: • • • • •

representing soil distribution and zones of erosion/deposition; representing regolith materials important for mine site management; showing the distribution of regolith materials which are important for ecology and wildlife management; highlighting possible areas of groundwater recharge/discharge; and, showing possible pathways for nutrient cycling and distribution.

The role of landscape change is also a fundamental control on the distribution of regolith materials and will affect geochemical signatures within the regolith. Some factors to consider include the long term movement, and continual recycling of materials through the landscape, and the way that factors such as erosion and weathering have influenced the development and preserv^ation of regolith materials.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

THE GEOLOGY, MINERALOGY, AND GEOCHEMISTRY OF THE BERNERS BAY GOLD-SILVER TELLURIDE DISTRICT, JUNEAU GOLD BELT, ALASKA Paul G. Sprv\ Sally D. Casey\ Earl Redman^ and David Harvey^ ^Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, U.S.A. 50011-3212 ^Coeur Alaska, Inc., 3032 Vintage Blvd., Juneau, AK 99801 ^Coeur Rochester, P.O. Box 1057, Lovelock, NV 89419 Mesothermal gold mineralisation at Bemers Bay (>2.2 Moz Au) is unique among gold-producing districts along the Juneau Gold Belt in that tellurides are a significant constituent of the ore. The Bemers Bay district is the northernmost extension of the Belt, along which lie the large Alaska-Juneau (3.5 Moz Au) and Treadwell (3.2 Moz Au) gold districts. It consists of approximately 20 prospects and is located along a splay of the Coastal Range megalineament. Most of the prospects in the Bemers Bay district are hosted by the 105 Ma Jualin Diorite, which intmdes Triassic metabasalt of the Wrangellia Terrane and Upper and Lower Cretaceous carbonaceous phyllites of the Treadwell Formation. Gold mineralisation is located primarily along the Orval, Comet, and Lion's Head shear zones. At the large Kensington deposit (1.9 Moz Au) gold is associated with the third of four hydrothermal vein stages, which has been dated previously at 56.5 to 53.2 Ma by Miller et al. (1995). Gold-bearing minerals at Kensington, which is spatially associated with the Orval shear zone, consist of native gold (fineness of 881-979), calaverite (AuTe2) and petzite (Ag3AuTe2). These minerals occur as inclusions in pyrite, chalcopyrite, and quartz. Traces of coloradoite (HgTe), volynskite (AgBiTe2), sylvanite ((Au,Ag)2Te4), tellurobismuthite (Bi2Te3), altaite (PbTe), hessite (Ag2Te), sphalerite, bomite, pyrrhotite, galena, hematite and magnetite are also present in stage 3 veins. Electron microprobe data and cathodoluminescence studies show that calcite is present in stages 2-4 in the Kensington deposit with the addition of minor ferroan dolomite in stage 4 veins. Petzite and electmm/gold (fineness of 264-910) dominate gold-bearing minerals in veins along the Comet shear. These veins also contain greater amounts of hessite and altaite than do those along the Orval and Lion's Head shear zones. Two prospects along the Comet shear zone, Valentine and Fremming, resemble volcanogenic massive sulphide mineralisation and contain greater amounts of chalcopyrite and sphalerite than all of the other prospects. The presence of pavonite ((Ag,Cu)(Bi,Pb)3S5), bismuthinite (Bi2S3), a member of the friederichite (Pb5Cu5Bi7Si8)-aikinite (PbCuBiSs) solid solution series, and rucklidgeite ((Bi,Pb)3)Te4) also characterizes the Valentine prospect. The distribution of tellurides in the district reflect variations of physiochemical conditions of the ore-forming fluids, different levels of uplift along the Orval, Lion's Head, and Comet shear zones, and fluid compositions. Values of S^'^S of vein sulphides in the Bemers Bay district range between -10.7 and 3.2%o (n = 84), with the majority ranging from -3.0 to 1.0%o. These data, when coupled with previously published isotope values of Goldfarb et al. (1991) and Miller et al. (1995), as well as thermochemical considerations, are most consistent with an extemal, reduced source of sulphur. Values of 5'^C and of calcite and ankerite from stages 2-4 in the Kensington deposit exhibit a range from -7.57 to -4.7%o and 11.07 to 16.64%o (n = 15), respectively, whereas values of 5'^C and 5*^0 of calcite in unknown vein stages fi-om elsewhere in the district range fi-om 7.70 to -1.33%o and 11.41 to 16.58%o, respectively. Some of the isotopically light values of 5'^C of vein carbonates likely reflect local interaction of the hydrothermal fluids with the carbonaceous metasedimentary rocks of the Treadwell Formation. The overlap in stable isotope compositions of carbonates and sulfides fi-om the Kensington deposit with those from other deposits in the district suggest relatively uniform physical and chemical conditions of ore formation throughout the Bemers Bay district. Gold-forming fluids resulted from metamorphic dehydration reactions that were produced by movement along the Coastal Range megalineament.

References Goldfarb R.J., Newberry R.J., Pickthom W.J., and Gent C.A. 1991. Oxygen, hydrogen, and sulfur isotope studies in the Juneau gold belt, southeastem Alaska—constraints on the origin of the hydrothermal fluids. Economic Geology, 86, 66-80. Miller L.D., Goldfarb R.J., Snee L.W., Gent C.A., and Kirkham R.A. 1995. Stmctural geology, age, and mechanisms of gold vein formation at the Kensington and Jualin deposits, Bemers Bay district, southeast Alaska. Economic Geology, 2, 343-368.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PLIOCENE VOLCANICS IN FIJI: A WINDOW TO UNDERSTANDING THE ORDOVICIAN VOLCANICS OF THE LACHLAN FOLD BELT Rick Squire Centre for Ore Deposit Research, University of Tasmania, P.O. Box 252-79, Hobart, Tas. 7001 The Pliocene, Ba and Koroimavua Volcanic Groups are the youngest volcanic rocks exposed on the island of Viti Levu, Fiji. They represent a suitable analogue for comparison with the Ordovician volcanics of the Lachlan Fold Belt (LFB), having calc-alkaline to shoshonitic geochemical affinities and a similar diversity of volcanic facies. Viti Levu rises from the floor of the relatively shallow waters covering the Fiji Platform, generally <2000m, and has developed within a complex transform zone accommodating convergence between the Indo-Australian and Pacific plates (Hathway, 1993). The transition fi-om calc-alkaline to shoshonitic volcanism in Fiji occurred between ~5.5-3.0Ma and is interpreted to be related to the disruption of the Vitiaz arc, although the North Fiji Basin opening almost certainly commenced earlier, about 12-lOMa (Gill and Whelan, 1989). The shoshonitic volcanism is concentrated near the western rifted arc margin, furthest from the Pacific plate convergent plate boundary. This led Gill et al. (1984) to suggested the shoshonitic eruptions marked a time interval during which limited lithospheric extension led to extraction of small melt fractions from highly metasomatised, yet HFSEdepleted lithospheric mantle residual from earlier arc depletion events. The Fijian volcanics contain an abundance of submarine volcanics with only minor subaerial facies identified. They comprise basaltic lavas, including pillow lava, and texturally diverse volcanic breccia interbedded with volcanic conglomerate and sandstone (Rodda, 1976; McPhie, 1995). The facies associations are interpreted to represent a shoaling basaltic seamount deposited almost entirely in a submarine setting, below wave base. Subvertical mafic dykes are also abundant (Rodda, 1976; Seeley and Searle, 1970). The Ordovician volcanics have a similar predominance of submarine volcanic facies with few volcanic centres identified (Glen et al, 1998). They are also similarly depleted in HFSE and are relatively limited in their distribution as well as sharing similar shoshonitic affinities. It is therefore suggested that the Ordovician volcanics may have developed in a similar intra-oceanic setting to the Pliocene shoshonitic volcanics of Fiji.

References Gill, J. and Whelan, P., 1989. Early rifting of an Oceanic island arc (Fiji) produced shoshonitic to tholeiitic basalts. J. Geophys. Res., 94, 4561-4578. Glen, R.A., Walshe, J.L., Barron, L.M. and Watkins, J.J., 1998. Ordovician convergent-margin volcanism and tectonism in the Lachlan sector of east Gondwana. Geology, 26, 751-754. Hathway, B., 1993. The Nadi Basin: Neogene strike-slip faulting and sedimentation in a fragmented arc, western Viti Levu, Fiji. Geol. Soc. London, 150, 563-581. McPhie, 1995. A Pliocene shoaling basaltic seamount: Ba Volcanic Group at Rakiraki, Fiji. J. Vole. Geotherm. Res., 64, 193-210. Rodda, P., 1976. Geology of northern and central Viti Levu. Bull. 3, Miner. Resour. Div. Fiji. Seeley, J.B. and Searle, E.J., 1970. Geology of the Rakiraki district, Viti Levu, Fiji. N.Z. J. Geol. Geophys., 13:52-71.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

INTERPRETATION - THE KEY TO MAKING GEOLOGY MEANINGFUL TO EVERYONE Sharon Stacy^ and Monica Yeung^ ^Environmental Interpretations, Camelot, Tumut, N S W 2 7 2 0 ^Gondwana Dreaming, PO Box 3017, Weston Creek, ACT 2606

As geologists we have to go beyond delivering facts if we want to enthuse non-geologists about rocks, minerals, fossils, soils and landforms. We have to become interpreters and identify the significance of those facts not only on an individual basis but also to the bigger picture. We have to show how the Earth's geological history has influenced and helps us understand our total envirormient, the scenery we admire, the distribution of flora and fauna, prehistory, history and the way we live today. Our western culture, according to Charles Birch (Professor Emeritus - Zoology, Sydney University and author of "On Purpose"), has been anthropocentric in a Christian-Judeo tradition (world created for man). He points out that the future requires us to have a biocentric view (man as one element of a complex, sophisticated interrelated world). Geology sits at the very foundation of this pyramid of understanding. Lifeforms evolved in and adapted to the myriad ecological niches created on Earth through the interaction of geological events and the resulting landforms, and climate. Through interpretation we will be able to increase and enhance the public's understanding of how geology underpins our natural and cultural history. This in turn will ultimately help protect significant geological heritage sites as well as help remove some of the stigma attached to mining. Interpretation is not about who what where and when... interpretation - why and how.

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it is, in the true spirit of

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

STRUCTURAL ELEMENTS OF THE LORD HOWE RISE H.M.J. Stagg', M. Alcock', I. Borissova', A.M.G. Moore', P.A. Symonds' and S. Van de Beuque^ ' Petroleum & Marine Division, Australian Geological Survey Organisation, Symonston ACT 2609 ^ IFREMER, c/- Australian Geological Survey Organisation, Symonston ACT 2609

The Lord Howe Rise is a 400-600 km-wide ribbon of foundered continental crust that extends for about 2000 km from the Challenger Plateau, west of New Zealand, to southwest of New Caledonia. The rise lies at water depths generally deeper than 1000 m, and is flanked to the east and west by the New Caledonia and Tasman Basins. The rise detached from eastern Australia during the breakup of eastern Gondwana that preceded the formation of the Tasman Basin from 85-52 Ma. Australia's 200 nautical mile Exclusive Economic Zone (EEZ) and 'extended Continental Shelf beyond Lord Howe and Norfolk Islands takes in an area of about 1.4 million km^, comparable to that of the state of Queensland. Recent studies conducted within the Law of the Sea Project at the Australian Geological Survey Organisation have provided new insights into the tectonic framework of the region. This interpretation reveals that the rise consists of four sub-parallel provinces that extend for much of its length. From east to west, these provinces comprise: •

Shallow, planated, probable Palaeozoic basement of the Lord Howe Platform, overlain by a few hundred metres of mainly Cainozoic siliceous and carbonate oozes

•

A central rifted province, adjacent to the Lord Howe Platform, characterised by a series of poorly defined basement blocks, normally down-faulted to the west, with 24 km of Upper Cretaceous and Cainozoic syn- and post-rift section.

•

A western rift province which is separated from the central rift by a broad fault zone across which basement is down-faulted to the west. Basement and water depths are deeper than in the central rift, and the syn- and post-rift sediments are thicker.

•

A western bounding complex ridge system of known continental origin. In the north, the Dampier Ridge is separated from the western rift province by the Lord Howe and Middleton Basins, which may in part be underlain by highly extended lower continental crust. Further south, where crustal extension is less extreme, the Monawai Ridge forms an intact outer margin to the Monawai Basin.

Regional crustal lineaments have been a strong influence on the tectonic development of the region. These lineaments are strongly aligned along two trends: northeast-southwest, parallel to the Cretaceous-early Cainozoic Tasman Sea spreading direction and concentrated between the Tasman Sea and the New Caledonia Basin; and northwestsoutheast, resulting from the late Cainozoic formation of the ridge and basin complex between Norfolk Ridge and the Tonga-Kermadec Trench, and concentrated eastwards from the New Caledonia Basin. The northeast-southwest lineaments are particularly important to the structural segmentation of the Lord Howe Rise, with the most important of these being the Barcoo-Elizabeth-Fairway Lineament. This lineament extends for some 1800 km northeast from the vicinity of Jervis Bay on the Australian margin, across the Tasman Sea, Lord Howe Rise and New Caledonia Basin, to the northern end of the Norfolk Ridge complex.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

POTENTIAL GAS HYDRATES IN AUSTRALIA'S MARINE ZONES H.M.J. Stagg', P.A. Svmonds^ N.F. Exon', J-M. Auzende^ G.R. Dickens^ and S. Van de Beuque'^ ' Australian Geological Survey Organisation, Canberra, ACT ^ IFREMER, DRO/GM, c/- ORSTOM BP n"A5, Noumea, Nouvelle-Caledonie, France ^ School of Earth Sciences, James Cook University, Townsville, QLD 4811 ^ IFREMER, c/- Australian Geological Survey Organisation, Canberra, ACT

Clathrate hydrates of gas are ice-like crystalline solids formed from mixtures of water and gas. They occur in sediments and sedimentary rocks where temperatures are low, but pressures and methane concentrations are high - conditions that can occur beneath the deeper parts of continental margins and oceanic plateaus, when a suitable source is present. Gas hydrates are typically underlain by sediment containing significant quantities of free methane gas, which consequently has relatively low acoustic velocity, and this boundary can produce a strong seismic reflection. As the depth of the boundary is controlled by the sub-seabed pressure and temperature regime, its reflection tends to parallel the seabed, regardless of the geometry of reflections produced by sedimentary strata. For this reason, a 'bottom-simulating reflector' (or BSR) is now considered to be compelling evidence for the presence of gas hydrates and underlying free gas. Until recently, potential gas hydrates had not been identified within Australia's marine jurisdiction. However, recent work in frontier areas of Australia's marine jurisdiction by the Law of the Sea Project at the Australian Geological Survey Organisation has identified possible gas hydrates beneath the Lord Howe Rise, South Tasman Rise (STR) and the Labuan Basin (adjacent to the Kerguelen Plateau). The possibility exists that such deposits, if confirmed, could be a major potential energy resource, albeit in the very long term. On the Lord Howe Rise, BSRs have been identified in the Fairway Basin, the eastern flank of the Middleton Basin, on the crest of the rise above a half-graben, and possibly in the Monawai Basin to the south. The Fairway Basin occurrence is the target of ongoing investigations, mainly using sediment coring and seabed swath-mapping techniques, to try to confirm the presence of gas. Satellite synthetic aperture radar (SAR) imagery also contains some evidence of lowlevel oil slicks and films that are approximately coincident with the BSRs in the Fairway Basin and on the flank of the Middleton Basin. Some of the SAR anomalies are associated with seismic indications of fluid migration. The STR and Labuan Basin BSRs have only recently been identified and follow-up surveys are not yet scheduled. On the STR, BSRs are prominent on a number of lines in water depths of 2500-4700 m. They are particularly prominent in isolated sediment depressions within the basement of the STR, where the cross-cutting reflection character is very strong, and beneath the L'Atalante Depression, which separates the STR from the East Tasman Plateau. An unusual aspect of the BSR in the L'Atalante Depression is a second, fainter BSR, some -0.2 s deeper in the section than the prominent BSR. This second BSR is not a processing artefact, and we speculate that it may be the diagenetic imprint of a 'fossil' BSR. The Labuan Basin, which formed during the rifting of Kerguelen Plateau and Broken Ridge, contains s TWT (ca 4-5 km) of sediment which has never been sampled. A prominent BSR has been identified on a single seismic line at about 0.7 s TWT (-800 m) bsf and is prominent over a distance of about 120 km due to its strong discordance with the associated strata.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HYDROCARBON PROSPECTIVITY OF DEEP-WATER FRONTIER BASINS NORTHWEST OF NEW ZEALAND V. Stagpoole, P. King, and C. Uruski. Institute of Geological and Nuclear Sciences, P.O. Box 30-368, Lower Hutt 6009, New Zealand,

Recently acquired seismic reflection data from the deep-water (500 - 2500 m) parts of northwestern New Zealand provide critical information on the depositional history of the region. Interpretation of these data and results from hydrocarbon generation models suggest that the Reinga and New Caledonia basins, along with the deep-water parts of the adjacent Taranaki and Northland basins, offer opportunities for significant discoveries. These basins cover a combined area of more than 80,000 km2 and contain up to 8 km of Cretaceous and Cenozoic strata. Interpretation of seismic reflection data indicates the deep-water basins northwest of New Zealand have formed over the last 100 m.y. The oldest sediments were deposited in a continental margin setting, which underwent extensional faulting in the Cretaceous as the New Zealand sub-continent broke away from Gondwanaland. Syn-rift deposits probably include both thick coal-rich source-rock intervals and marine shales with source potential. Rising depositional base level through the Late Cretaceous and Early Tertiary allowed the accumulation of transgressive shoreline sandstones that form both prospective and productive reservoir units in the Taranaki Basin. Overlying seal-rock facies comprise Tertiary mudstones and hemipelagic drifts and a condensed mid-Tertiary carbonate sequence. Burial history and hydrocarbon generation models, using standard source rock parameters and lithologies based on Taranaki and Northland geology, predict that petroleum generation and expulsion has occurred over large areas. Potential reservoirs and structural plays with significantly large closure have also been tentatively identified. With modem deep-water drilling technology, the frontier basins of northwestern New Zealand must now be considered as viable petroleum exploration provinces.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CONTACT RELATIONSHIPS AND POST-INTRUSION LOADING OF THE WESTERN FIORDLAND ORTHOGNEISS, SW NEW ZEALAND J. Stevenson\ N.R. Daczko', G.L. Clarke^ and K.A. Klepeisl 1 School of Geosciences, Division o f Geology and Geophysics, University of Sydney, N S W 2006 Australia 2 Department of Geology, University of Vermont, Burlington, VT05405, USA

The Early Cretaceous Western Fiordland Orthogneiss, SW New Zealand, comprises a NNE-trending belt of amphibolite to granulite facies mafic and felsic orthogneiss that extends for more than 120 km. The Early Cretaceous emplacement depth and metamorphic P-T path experienced by this complex has been the subject of considerable debate. Various interpretations include 1) deep crustal (>45km) emplacement during extension followed by fast exhumation (Gibson and Ireland, 1995; Ireland and Gibson, 1998); 2) mid-crustal (~20km) emplacement and subsequent loading to deep crustal conditions by magma loading (Brown, 1996) or arc-continent collision (Bradshaw, 1989). An intrusive contact of the Western Fiordland Orthogneiss with the Arthur River Complex is located in the Mt Daniel area, northern Fiordland. The contact is marked by migmatisation and partial melting of the Arthur River Complex with back intrusion of trondhjemitic veins into the Western Fiordland Orthogneiss. Garnet-rich varieties of the Arthur River Complex are inferred to have experienced partial melting and appreciable melt loss. A moderate to welldeveloped Si-Li fabric in the Western Fiordland Orthogneiss is oriented sub-parallel to the intrusive contact. In low strain areas igneous assemblages of orthopyroxeneclinopyroxene-amphibole-biotite-plagioclase, with or without quartz and ilmenite are preserved. The same minerals as the igneous assemblage define the SI foliation, with the modal abundance of amphibole and biotite increased and orthopyroxene and clinopyroxene decreased. The post-intrusion history of the Western Fiordland Orthogneiss is constrained by gamet-clinopyroxene-kyanite-plagioclase-quartz-rutile-bearing symplectites that partially pseudomorph igneous and Si assemblages. References BRADSHAW, J. Y. 1989. Origin and metamorphic history of an Early Cretaceous polybaric granulite terrain, Fiordland, southwest New Zealand. Contributions to Mineral Petrology, 103, 346-360. BROWN, E. H. 1996. High-pressure metamorphism caused by magma loading in Fiordland, New Zealand. Journal of Metamorphic Geology, 14, 441-452. GIBSON, G. M. & IRELAND, T.R., 1995. Granulite formation during continental extension in Fiordland. Nature, 375, 479-482. IRELAND, T. R. AND GIBSON, G.M. 1998. SHRIMP monazite and zircon geochronology of high-grade metamorphism in New Zealand. Journal of Metamorphic Geology, 16, 149-167.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HEAVY METAL DISTRIBUTION IN RECLAIMED LANDS OF HOMEBUSH BAY, THE VENUE OF THE 2000 OLYMPIC GAMES, SYDNEY, PRIOR TO REMEDIATIONS JeongYul Suh ^ ^ G.F Birch ' and K. Hughes^ ' Environmental Geology Group, Division of Geology and Geophysics, School of Geosciences, University of Sydney, NSW 2006, Australia ^ Olympic Co-ordination Authority, 7 Figtree Drive, Homebush Bay, NSW 2127, Australia

The current study is one of numerous projects commissioned by the Olympic Coordination Authority for the development of the Ecology Data Bank, a geographic information system (GIS) that provides for an electronic record of the changing status of the Homebush Bay Olympic site over time. The primary function of the GIS is to provide site managers with an effective tool to collect data and help manage the remediated lands. In particular, the Ecology Data Bank has been designed to achieve a high degree of functionality for the information requirements associated with long-term asset management at the Homebush Bay Olympic site. The site is now home to Sydney Olympic Park and the new Millennium Parklands but in earlier times was an industrial wasteland. Past activities such as reclamation of wetlands, land clearing, shoreline remodelling and land filling all caused significant adverse environmental impacts. Most dramatic was the waste dumping. About 160 hectares of the Homebush Bay site was reclaimed through the dumping of an estimated 9 million tonnes of domestic, commercial and industrial waste. This dumping left a footprint of heavy metals in the waste matrix. Soil samples (n=4331) collected in the reclaimed lands of Homebush Bay were analysed for total sediment Cu, Pb, Zn, Cr and Hg. The relationship between heavy metal concentrations and past landuse, such as reclaimed, landfill and non-infilled areas showed marked spatial variance. Results indicate that soils in reclaimed areas of Homebush Bay were contaminated by heavy metals (Cu, Pb, Zn, and Cr) and that concentrations were high enough Cu/Pb, Pb/Zn and Cu/Zn are consistent with type of waste materials and past landuse. The OCA has implemented a comprehensive remediation strategy at the Homebush Bay Olympic site. Millions of tons of waste has been retrieved from wetlands and waterways and consolidated into four huge waste mounds which have been capped with clay and topsoil and then landscaped with native trees and grasses. Leachate drains collect ground water which flows through the landfills and pumps it to the nearby Lidcombe Liquid Waste Treatment Plant where it is treated and then released to sewer. The current investigation of the status of soils prior to remediation aims to provide GISready data sets about the surface and subsurface distribution of heavy metal concentrations in reclaimed areas of the site.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE 1997 THREDBO LANDSLIDE Tim Sullivan Pells Sullivan Meynink Pty Ltd, Suite 1 1 , 1 0 East Parade, Eastwood, NSW 2122

The Thredbo Landslide, which occurred in July 1997, was a catastrophe in human and economic terms, resulting in the total destruction of two ski lodges and the death of 18 people. There was only one survivor. It was by far Australia's worst landslide event. The Landslide has been the subject of intensive geotechnical investigations extending for over two years, culminating in the longest running and most costly Coronial Inquest in the history of the state of NSW. The Landslide was initiated along the edge of the Alpine Way above the Thredbo Village. Despite the catastrophic effects, the Landslide was quite small by most standards, comprising only 1300m3 of material. This material, a fine grained granular soil, mobilised very rapidly, resulting in a mudflow, which travelled at about 40km per hour downhill, totally destroying the ski lodges. The combined mudflow/debris tongue, formed from the landslide material and the building debris, covered an area 60m long by 30m wide. The Alpine Way has been in existence for more than 40 years. Throughout that time, the road has had a history of instability, with eight major slides and innumerable smaller local instability issues over the 20km length. The road was constructed initially as an access track for the Snowy Mountains Scheme, then upgraded a number of times, without ever significantly improving the stability. The location of the Landslide is readily understood considering the geomorphology of the area. There are three significant drainage gullies crossing the Alpine Way above the Village. One of these failed in 1964, another had a small failure in 1977 with continued cracking and movement throughout; followed by the Thredbo Landslide in 1997. The subdivision, of which the landslide is part, was planned in 1958. Throughout the 40 years there had been a large number of significant changes around the immediate area. Because of the fragile nature of the site all of these changes had the potential to affect the stability and this necessitated a huge forensic geological, scientific and geotechnical engineering study; including, history, geology, groundwater, construction activities, visual witnesses, aural witnesses, historic water observations, piecemeal excavation of the Landslide and site; and detailed scientific examination of all the infrastructure. A landslide may have many causes but only one trigger. The investigations showed the factor of safety of the site before the slide was very low, about 1.05 to 1.1, where 1.0 is marginally stable. Hence the safety margin was only 0.05 to 0.1. This low base condition was due to a number of causes, including the construction of the Alpine Way, the geotechnical character of the site, a leaking water main and the lodge foundations. However there are multiple layers of evidence from many different sources to show the Landslide was triggered by a leak from the water main. The problems with the road and the Landslide site were evident even before development. However it appears this knowledge was either ignored, forgotten or the knowledge and understanding was simply lost over time. Monitoring was installed adjacent to the site and the stability of the site was also assessed annually for a number of years prior to the Landslide. Despite this, the fragile nature of the site and the risks, together with the significance of the monitoring were not appreciated.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ERUPTIVE POTENTIAL, METROPOLITAN NEW SOUTH WALES, SYDNEY OR THE BUSH? F. Lin Sutherland Geodiversity Research Centre, Australian Museum, 6 College Street, Sydney, N S W , 2 0 1 0

Sydney Basin and its margins record repeated basaltic volcanism within a 200 km radius around Sydney since Cretaceous time. Any such volcanism today would have fallout effects in the Wollongong-Newcastle-Bathurst triangle. While future eruptive potential seems negligible, it needs consideration within this thermo-tectonic history. Volcanic outbreaks are dated within each million years between 65Ma-10Ma, except for odd gaps of 2-3 myr which may reflect incomplete sampling. After lOMa only minor eruptive events are recorded, possibly because more compressive stress fields were imposed over the region by Australian plate collision with New Guinea. This considerable drop in eruptive potential, however, does not eliminate long-term risk in the region, because: (1) trends in sporadic small scale 'eruptive events', based largely on zircon fission track dating, form an age-progressive southward trend that focuses on the Sydney Basin, between Newcastle and Wollongong (2) a seismic slow zone, probably marking a thermal anomaly, underlies this region from 80-200 km depth, based on the Skippy seismic study (3) a rhyolitic pyroclastic deposit, near Karuah, suggests an age around 1 Ma, based on detailed mapping of associated sediments. Assessments of eruptive potential in the area need to consider the eruptive periodicity along the age-progressive trail, the strength of any thermal anomaly under the region and the nature of the operative stress field. Considering the above factors, a low risk zone of potential eruption exists within Sydney Basin. Sydney, Newcastle and Wollongong lie in a region of little eruptive potential, but areas west, south and east in the Blue Mountains, Southern Highlands and offshore continental shelf may have slight risk. The long chance odds favour 'The Bush' over 'Sydney' for a volcanic event. Most likely, this would be an initial minor explosive ash shower, possibly with a rain of zircon crystals, as found in a previous Blue Mountains outbreak. Any lava flows would create further local problems. An offshore eruption would have potential for tsunami generation and could indirectly affect coastal strips. The eruptive periodicity along the age-progressive trend towards Sydney Basin is around 0.5-1.5myrs, but may be maximal due to partial sampling. The most favoured future eruptive venue is where this trail passes through eastern Sydney Basin and offshore. The seismic slow zone under Sydney Basin records mantle shear wave velocities down to Vs 4.3 from Skippy results, matching reduced velocities found under volcanic areas in western Victoria and northeastern Queensland. However, later Skippy analysis suggest a less pronounced anomaly. The Karuah pyroclastic rhyolite has features that suggest air fall into a water body and any such young, explosive felsic volcanism has implications for potentially hazardous events. The present operative stress field is largely compressional within the Sydney Basin, which would inhibit potential for eruption, but it is not fully understood. The geological setting suggests long term dormancy, rather than no eruptive potential.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

COLOURFUL CORUNDUMS FROM VOLCANIC FURNACES, EASTERN AUSTRALIA AND ELSEWHERE F. Lin Sutherland'. D. Schwarz^ R.R. Coenraads' and C.M. Fanning^ 'Geodiversity Research Centre, Australian Museum, 6 College Street, Sydney, N S W , 2 0 0 0 ^Giibelin Gemmological laboratory, Maihofstrasse 102, CH600, Lucerne 9, Switzerland ^Research School Earth Sciences, Australian National University, Canberra, ACT, 0200

Sapphire and ruby are ubiquitous accessories in some basaltic fields and feed into the coloured gemstone industry. Eastern Australia is a prime example, where gem corundums shed from 70% of basaltic lava fields (excluding central volcanoes) and are mined in large fields (New England, central Queensland). Examples elsewhere include Thailand, Cambodia, Laos, Vietnam, China, Madagascar, Kenya, Rwanda, Nigeria, Czech Republic, France, Scotland, USA. These colourful corundums represent two xenocrysts suites, one magmatic in origin, the other metamorphic, and both exhibit corrosion. Magmatic sapphires dominate, but metamorphic sapphire is prominent in some fields (Barrington, Australia; Pailin, Cambodia; Trat, Thailand). Magmatic sapphires show colour zoning and provide blue, green, yellow and particoloured cut stones. Colour absorption spectra for blue to green stones exhibit charge transfer, shared pair and Fe^"^ bands, while yellow stones give intense Fe^"^ bands. Trace element substitution characteristically show Cr203/Ga203 below 1. Syngenetic mineral inclusions encompass silicates, oxides and rarer phosphates and sulphides. Zircon, columbite and monazite assist in dating sapphire crystallisation. This approximates basaltic generation ages and in Australia extends from at least 3 to > 60 Ma. Metamorphic gem corundums show wide colour range from blue through intermediate hues to red, with subdued colour zoning. Absorption spectra lack significant Fe^'^-Fe^"^ shared pair bands, but Cr absorption bands become conspicuous in the pink, mauve, red range. Trace element substitution typically show Cr203/Ga203 above 1. Co-existing minerals include oxides, silicates, including hydrous silicates, carbonates, phosphates and sulphides, but vary from area to area. Different models exist for the magmatic sapphire crystallisation, ranging from mid-crustal melt mixing (carbonatite and silica-rich melts) to minor melting of metasomatised mantle (feldspathic, volatile-rich melts). In eastern Australia, sapphire/zircon discharge often brackets peak basaltic outpourings and may reflect peripheral mantle plume effects. Metamorphic corundum suites reflect different local basements and in Australia include: • higher Cr-level corundum suites related to ruby sapphire sapphirine spinel assemblages (metasomatic serpentinite origin?) • moderate Cr-level suites related to fassaite pyroxene-anatase assemblages (lower crust granulite origin?) • lower Cr-level suites related to sapphire, kyanite, ilmenorutile, hercynite assemblages (high-grade regional metamorphic origin?) Volcanically sampled bimodal corundum suites (magmatic and metamorphic) are more prevalent than previously considered. Some corundums show transitional trace element geochemistry between these suites. They include corundums with major postcrystallisation exsolution, but other types need study to clarify their origin.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LOCAL RECHARGE AND DISCHARGE AREAS IN THE WIMMERA REGION, SOUTHERN MURRAY BASIN, VICTORIA: THEIR ROLE IN DETERMINING GROUNDWATER QUALITY. Ian P. Swane\ Tamie R. Weaver^ Ian Cartwright^ and Charles R. Lawrence^ Hydrogeology and Environment Research Group ^ School of Earth Sciences, University of Melbourne, Victoria, 3010, Australia ^ Department of Earth Sciences, Monash University, Clayton, Victoria, 3800, Australia The Wimmera region of western Victoria forms the southern-most catchment of the Murray Basin, bordered to the south by the Dundas Plateau and the Otway Basin. The region is characterised by very little topographic relief and groundwater occurs throughout a multiple aquifer system close to the regional recharge for the southern Murray Basin. Despite the low topographic relief, significant changes in shallow groundwater chemistry are observed over a relatively short distance in both the upper unconfmed aquifer and the deepest, confined aquifer. Electrical conductivity (EC) values in the shallow, unconfined aquifer range from 1400 - 135,000 mS/cm over 27 km, or ~ 1000 km2. The objective of this research is to determine the controls on these major changes in the groundwater chemistry of the Wimmera, and the effect this may have on the transport of salts. 17 multi-level groundwater observation bores have been monitored over a 2 year period. The bores are completed within the shallow Parilla Sands water-table aquifer, the semi-confined Murray Group Limestone aquifer, and the deep, confined basal Renmark Group aquifer which unconformably overlies metamorphosed Palaeozoic basement. Although the Parilla Sands aquifer lies within the regional recharge area, the aquifer appears to be highly sensitive to local topographic controls, where local recharge and discharge have a significant effect on groundwater chemistry and the mobilisation of near-surface salts. Major local recharge occurs along the northwest striking palaeo-sand ridges in the central Wimmera. Shallow groundwater in these areas (e.g. Jungkum) has relatively low total dissolved solids (TDS) contents 650 mg/L). However, in the nearby Douglas Depression, previously argued to be a discharge area (Brownbill, 1995), groundwater in the shallow Parilla Sands aquifer is highly saline with TDS > 100,000 mg/L. In these discharge areas water quality is compromised throughout the entire system, with TDS in the basal Renmark Group aquifer ranging from 9000 to 16000 mg/L TDS. This contrasts greatly with groundwater in the Renmark Group of the Jungkum area (27 km west) and Natimuk (16 km south) where TDS contents are 700 and 1200 mg/L respectively. The overall TDS values, coupled with very high magnesium concentrations in the northern Douglas Depression indicate that deep reflux brines from salt lakes within the depression probably occur in this area. This would allow signif! icant salt loads to be transported from near-surface to deeper aquifers relatively rapidly and over short distances. Stable isotope values (52H and 5180) in groundwater across the region vary significantly both between hydrogeological formations, and over time at the same locations. However, consistent groundwater chemistry through time indicates that leakage along boreholes is unlikely. Groundwater in the Parilla aquifer near the Douglas Depression shows evaporative signatures in June 1999. By November 1999, only groundwater from the most saline bore maintained this signature indicating that H 2 0 rather than solute load in the system had changed. Clearly, local recharge and discharge areas are critically important in controlling groundwater quality throu^out the entire aquifer system in the Wimmera region. This work clarifies the causes of groundwater salinisation in the region, thereby allowing better groundwater quality management strategies to be developed. References BROWNBILL, R.J., 1995. Hydrology and chemistry of groundwater and lakes. Southern Wimmera, Victoria. Unpublished M.Sc. Thesis, University of Melbourne.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

CONTROLS ON REGIONAL DISTRIBUTION OF RARE METAL PEGMATITES IN THE ARCHAEAN PILBARA CRATON, WESTERN AUSTRALIA. Marcus T. Sweetapple\ Robert J. Hickey^ and Peter L.F. Collins' ' School of Applied Geology, Curtin University of Technology, Bentley, Perth, W.A., 6845 ^ School of Spatial Sciences, Curtin University of Technology, Bentley, Perth, W.A., 6845 The North Pilbara basement terrane of 3600-2800 Ma age consists of a series of ovoid multiphase granitoidgneiss domes bordered by sinuous synformal to monoclinal greenstone belts, composed of mafic-volcanic dominated supracrustal sequences, producing a distinctive dome and basin pattern. Pegmatites bearing rare metal mineralization are distributed throughout the terrane, frequently associated with the greenstone belts. A number of the larger pegmatite bodies host economically significant tantalum, tin, beryl and feldspar mineralization. Rare metal pegmatite occurrences across the terrane have been classified into three geochemical divisions. These divisions have been made on the basis of characteristic mineral assemblages into LCT (Li-Cs-Ta) or NYF (Nb-Y(REE)>F) family end members and an intermediate type between the two end members. These pegmatite groupings have a broad relationship with major craton-scale tectonic features. In most cases, there is substantial field and geochronological evidence for their generation fi-om the late Archaean 'younger' granite suite. The LCT pegmatite suite is contained mostly within one tectonostratigraphic domain, with all major tantalum deposits (Wodgina/Mt. Cassiterite, Tabba Tabba, Strelley) being associated with a north-east trending lineament within this domain. These pegmatites are hosted entirely within bimodal volcano-sedimentary greenstone belts. Pegmatites of this suite are characterized by the occurrence of large discrete dykes of at least five metres thick, or complexes thereof Pegmatites of the NYF suite are mostly contained within the southern portions of the Yule and Shaw Batholiths They are generally hosted within either the older gneissic granitoid complex or within the 'younger' granite suite itself Some of the pegmatites belonging to this suite may be intermediate members between the LCT and NYF suites. Pegmatites of this suite tend to occur as swarms of narrow dykes, often less than one metre thick. Most tungsten mineralization is hosted in silicic pegmatites or veins associated with the Bonney Downs Granite, in the south-eastern part of the craton. Tin mineralization is associated with both the NYF and LCT pegmatite suites. Minor beryl-bearing pegmatites are distributed throughout the craton, in association with all of batholith complexes. The major controls on the distribution of rare metal pegmatites appear to be related to: 1. Secular changes in the minor element chemistry of the 'younger' granite suite across the Pilbara Craton with time. Field evidence indicates that the LCT suite has been sourced fi-om strongly fi-actionated, high silica, peraluminous, two mica leucogranites, while the NYF suite is expected to have been sourced from high fluorine metaluminous granites with A-type characteristics. Both groups of source granites can be considered to have 'tin granite' characteristics. 2. Most rare metal pegmatite bodies are no more than 5 km way from their apparent parent granite of the 'younger' suite, and all are within 10 km. These distances are in accordance with experimentally derived models. 3. Most of the major occurrences of LCT pegmatites are hosted within mafic-ultramafic meta-volcanic lithologies. Generation of the LCT pegmatite suite appears to require interaction with cooler, volatile-rich greenstone belts sinking into the apical portions of granite domes. Contributions of H 2 0 and C 0 2 from these greenstones may have enhanced the development of a suitable environment for extraction and partition of incompatible lithophile elements. Structures within the greenstone belts have provided favourable sites for the concentration of low viscosity hydrous pegmatite magma from the cooling granites.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

CHARACTERISTICS OF TANTALUM-NIOBIUM-TIN OXIDE MINERALS FROM THE WODGINA AND MT. CASSITERITE PEGMATITES, PILBARA CRATON, WESTERN AUSTRALIA. Marcus T. Sweetapple^ Gregory R. Lumpkin^ and Peter L.F. Collins^ ^ School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, W.A., 6845 ^ Materials Division, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234. Significant tantalum mineralization is present in the Wodgina pegmatite district in the Wodgina and Mt. Cassiterite pegmatite orebodies, which have made major contributions to world tantalum production. These highly fractionated pegmatites belong to the albite and albite-spodumene classes of rare metal pegmatite respectively. While the two pegmatite orebodies have substantially different internal structure, external morphologies and bulk compositions, they display a similar degree of fractionation of rare lithophile elements, and have a similar Ta-Nb-Sn mineralogy. Both pegmatites contain a primary mineral assemblage of wodginite (Mn4Sn4Ta8032), columbite-tantalite group minerals ((Mn,Fe)(Ta,Nb)206) and cassiterite, albeit with somewhat different compositions, proportions and complexity of mineral zonation. These mineral assemblages appear to have an early paragenesis during the magmatic phase of pegmatite crystallization. Graphic textured manganotantalite is present within orbicular textured masses of pure cleavlandite (albite) in the Wodgina pegmatite, which is supportive of an early (magmatic) crystallization event. Alteration mineral assemblages in both pegmatites are dominantly comprised of the microlite member of the pyrochlore group (general formula of microlite ((Na,Ca)2-m(Ta,Nb,Ti)206(F,0H,0)l-n.pH20). Microlite in the Mt. Cassiterite pegmatite is dominantly of a conventional sodium-fluorine composition, while microlite in the Wodgina pegmatite is dominantly a calcium-rich defect microlite containing cation site vacancies. In addition, fersmite (CaNb206) and rynersonite (CaTa206) are encountered in the Wodgina pegmatite as alteration products of manganocolumbite and manganotantalite, respectively. The microlite alteration mineral assemblage shows replacement textures on grain margins and fractures in the primary grains, as well as a symplectic-type of replacement intergrowth between wodginite and microlite from the Mt. Cassiterite pegmatite. Porous textures are developed in microlite from both pegmatites, which are suggestive of an origin from the hydrothermal fluid phase of the pegmatite crystallization. The compositions of the secondary phases are controlled by the associated primary mineral phases. A minor subsolidus alteration event is also present, resulting in further Ca-rich defect microlite and unconfirmed fibrous calciotantite ((Ca,Na)Ta4011) alteration, associated with fractures in tantalum minerals. Examination of columbite-tantalite crystal structure by electron diffraction shows a frilly ordered structure shown by the arrangement of cations in the [1,0,0] crystallographic plane of this mineral. This structural arrangement is likely to have been caused by columbite-tantalite being held at high temperatures for a protracted period of time, either during or after pegmatite crystallization. Columbite-tantalite grains also show a high degree of strain and subgrain development, suggestive of either a subsolidus deformation event or deformation during pegmatite crystallization. It is suggested that the two mineral assemblages at Wodgina and Mt. Cassiterite are the result of the same primary pegmatite magma having followed different evolutionary processes. However, for both pegmatite bodies, crystallization of primary tantalum minerals proceeded from a pegmatite magma, followed by alteration under hydrothermal conditions, after the transition from a pegmatite magma to a hydrothermal fluid, followed by minor changes in alteration mineralogy under subsolidus conditions.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

PORPHYRY AND SEDIMENTARY-HOSTED GOLD DEPOSITS, CYGNET AREA, SOUTHEAST TASMANIA Jafar Taheri and Ralph Bottrill Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018 Gold and base metal mineralisation occurs at a number of sites in an ENE -trending belt, near Cygnet in southeast Tasmania. The mineralisation is spatially and temporally related to the intrusion of a Cretaceous alkaline porphyry complex and occurs both within it and the intruded Permo-Carboniferous sedimentary rocks. The Cygnet alkaline complex comprises mostly feldspar-phyric intrusive rocks which range between silica-saturated and silica-unsaturated (feldspathoidal) rocks. Silica-saturated porphyries (quartz monzodiorite to alkali feldspar syenite) are dominant and are the major hosts for gold mineralisation in the Cygnet area. The alkaline porphyries appear to have been formed during the initial stages of opening of the Antarctic-Australian rift, at about 97 Ma. The Cygnet mineralisation is a member of an unusual class of gold deposits associated with alkaline rocks intruded in a rift setting. The alkaline igneous rocks and the enclosing sedimentary rocks have been affected by a variety of hydrothermal alteration types including potassic, silicic, calc-silicate, phyllic, propylitic and argillic. Gold occurs as disseminations and in quartz, pyritic and hematitic veins, stockworks and breccias in both the porphyries and sedimentary rocks. The known alteration and associated mineralisation occur in several separate areas and do not exhibit any mineral zoning. Fluid inclusions in quartz fi-om hydrothermal breccias and quartz veins exhibit a wide range of homogenisation temperatures and salinities. Highly saline fluid inclusions appear to have been formed directly from exsolved magmatic fluids rather than by phase separation. Spasmodic brecciation, with repeated fracturing and resealing of the rocks appear to be the main reason that the fluid inclusions show a variety of salinities and homogenisation temperatures. The early-formed mineralising fluids appear to be dominantly of magmatic origin and are characterised by high oxidisation states, temperatures of 300 to >500°C and salinities up to 53 wt% NaCl equivalent. Magmatic fluids were responsible for the formation of quartz veins and hydrothermal breccias hosting or associated with base metal and gold mineralisation in and adjacent to the porphyries. Oxygen and sulfur isotope values also indicate magmatic water and sulfur sources for the early stages of hydrothermal alteration. The magmatic-dominated mineralising fluid was diluted and cooled as it mixed with convecting meteoric water. This stage of hydrothermal activity was responsible for most gold mineralisation which occurs as relatively gold-rich pyritic veinlets, disseminated mineralisation in the intruded sediments and pervasive zinc-rich clay alteration (smectite) with associated gold, lead and arsenic. The latest stage of alteration is of supergene origin and includes the formation of gold-bearing limonite and jarosite (after pyrite) in veinlets, altered clasts and fossils, and possibly the redistribution of gold in both the sedimentary rocks and porphyries. Hydrothermally altered rocks are variably anomalous in As, Pb, Cu, Au, Zn, and Mo, with each mineralisation/host-rock type association exhibiting different geochemical signatures. There are weak positive correlations among Au and Pb, Zn, Cu and As in most rock types, but the sedimentary rocks are relatively more gold enriched. Mo is anomalous only in hydrothermal breccias in porphyries and shows no correlation with Au. The occurrence of similar styles of mineralisation in other nearby areas along with the multiple, relatively shallow intrusive (<5 km) nature of the porphyries, suggests potential for the discovery of further porphyryAu type deposits in Tasmania.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MESO-LATE PROTEROZOIC IGNEOUS ACTIVITY IN THE SW PERIPHERY OF SIBERIA CRATON K. Takano \ T. Watanabe \ E. Sklyarov^ D. Gladkochub^ A. Postnikov^ K. Maehara^ and Agashev,A.^^^ ' Division of Earth & Planetary Sciences, Hokkaido University, Sapporo, Japan ^ Institute of Earth's Crust, Siberian Branch, RAS, Irkutsk, Russia ^ United Institute of Geology, Geophysics and Mineralogy, Siberian Branch, RAS, Novosibirsk, Russia The Siberia craton is one of the essential components of Rodinia and its igneous activity in the Meso-Late Proterozoic is of interest in understanding the breakup of this supercontinent. Although precies chronological data is not available, we present data relating to the igneous activity along the SW periphery of the Siberian craton and discuss the mode of breakup of Siberia. The study areas occur in the Mana River area, at the western margin of the East Sayan Mountains and the eastern part of these mountains. Eastern Sayan, near Lake Baikal. Mana River area: oceanic, pillowed basalts and arc volcanics are predominant and they occur in separate units bounded by faults. Upper Riphean limestones(800?-650Ma in Russian time scale) overly the volcanics and thus the age of the igneous rocks are believed to be Mid-Riphean(1000-800?Ma). We, however, have obtained an approximate Sm-Nd isochron age of 775Ma from a greenstone dike. Although these rocks have suffered sub-greenshist to greenschist facies metamorphism, discrimination diagrams involving immobile elements reveal that they are mostly MORB and OIT(the Beret basalts based on the locality name). In the oceanic basalts, rocks with higher Si02(52-57%) with comparatively high MgO are often found. Similar high Si02 and MgO rocks occur in igneous provinces which have experienced continental crust breakup as a result of mantle plume activity, such as the Mnanjary basalts in Madagaskar, Grande Ronde basalts of the Columbia River basalts, Faeroes basalts of the Rockall Plateau in northern Atlantic ocean. Trace element abundances in the Beret basalts are similar to the plume-related basalts found in these provinces. Because of the similarity between both the pillowed basaltic rocks in the Mana River area and plume-related basalts, we suggest that the Beret basalt may have been produced by mantle-plume activity related to the breakup of Rodinia. Eastern Sayan area: Proterozoic dike swarms are widespread in this area(Sklyarov et al.,2000, submitted). Discrimination diagram involving immobile elements suggests two distinct type of dikes, namely MORB and WPB. Three stages of dike generation have been distinguished. The oldest dikes are thought to be Lower Proterozoic in age, based on the occurrence of high-P, high-T metamorphic rocks dated at 1970Ma.(Aftalion et al.,1991). Second generation dikes have sub-alkaline or alkaline basaltic composition. The dikes in the southwestern part of the area have suffered low-P high amphibolite to granulite facies metamorphism, whereas in the southeastern, the grade of metamorphism is lower amphibolite facies or lower. These dikes were intruded between 1820-850Ma as they cut 1820-1870Ma volcanics. They are overlain by Upper Riphean terrigenouscarbonate series of the Baikalian series. Dykes of the third stage have been documanted only in restricted areas. Geochemically they belong to tholeiite-sub-alkaline series, have N-MORB affinities and an assumed age of early Vendian(600-650Ma). Preliminary synthesis suggests the dikes intruded in the second stage at about 1000-800Ma and that they were related to the earliest record of the breakup of Rodinia and opening of the Paleo-Asian ocean. Thus, if the North China Block(NCB) had been located beside the Siberia Craton, separation of the NCB from the Siberia block and opening of the Paleo-Asian ocean, occurred by 775Ma. However, to gain a greater understanding of the breakup process, accumulation of more precise chronological data are most important.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

GEOLOGY AND GEOCHEMISTRY OF THE SEDIMENT-HOSTED GOLD MINERALISATION AT BARHI AND JHAL, MADHYA PRADESH, INDIA Reddy V.R. Talusani Division of Earth Sciences, The University of New England, Armidale NSW 2351 Gold deposits occur in a wide variety of geological environments and can form through numerous geological processes. During the course of geological and geochemical investigations by the author in the central part of the Mahakoshal fold belt (MFB) an interesting occurrence of sediment-hosted, disseminated gold mineralisation was reported for the first time in dolomites near Barhi (23053'N:80048'E) and Jhal (23057'N:81004'E) villages, which are located 50 and 90 kms respectively northeast of Katni in eastern Madhya Pradesh. The Late Archean-Early Proterozoic MFB in central India is a ~500-km-long, ENE-WSW trending belt developed in a rifted basin. The exposed width of the belt varies from 5 to 40 km. The structural setting of the MFB is complex and the rocks have undergone three phases of folding and are traversed by several longitudinal high-angle thrust and transverse faults (Ameta, 1990). Periodic reactivation of the ENE-WSW trending basin-bounding faults controlled later magmatism. Exposed rocks include a strongly deformed and weakly metamorphosed supracrustal sequence dominated by metasedimentary rocks (-80%), with subordinate metavolcanic rocks of basaltic composition. Rocks near the villages of Barhi and Jhal in the central part of the MFB, consist of a volcano-sedimentary sequence metamorphosed to the greenschist facies. The main rock types include phyllite, quartzite, dolomite, metabasalt and banded hematite quartzite. Gold occurrences are hosted by dolomite, which occur as discontinuous bands interbedded with phyllites. About one hundred samples of dolomite were colleted on a grid pattern covering 400 m x 80 m and 600 m x 80 m zones in the Barhi and Jhal areas, respectively. Petrographic evidence indicates that the carbonate rocks of the MFB near Barhi and Jhal show the effects of epigenetic hydrothermal alteration. Hydrothermal effects on the host rock include decalcification, silicification and argillisation. Pyrite is the major sulphide mineral and occurs as subhedral to anhedral grains, and in clusters. Realgar and orpiment have a limited occurrence, and stibnite and cinnabar are rare. The metallic mineral assemblage consists of disseminated micron- to submicron-sized gold, realgar, orpiment, stibnite, cinnabar, and pyrite. Gold occurs with arsenic-, antimony-, and mercury-bearing sulphides as coatings on surfaces and as fracture fillings in pyrite grains that have been identified by microprobe studies. Other hydrothermal minerals include quartz, kaolinite, sericite, barite, and calcite. Base metal sulphides include chalcopyrite and sphalerite. These minerals are very rare and probably formed later in the paragenesis. Alteration of the host rocks is similar to other sedimentary rock-hosted, disseminated gold deposits but at lower alteration mineral abundances. Eighty rock samples of altered dolomite were analysed for Au, Cu, Pb, Zn, Ni, Co, As, Sb, and Hg. Of the 80 analysed samples, 52 samples contained gold contents ranging between 0.20 and 0.62 ppm. The sample with the maximum gold concentration (0.62 ppm) also contained the most significant mercury (2.20 ppm) and antimony (150 ppm) anomalies. The strong positive correlative between Au and As (with correlation coefficient, 0.88) and Au and Sb (0.94) indicates that gold is associated mainly with realgar-orpiment and stibnite. The positive correlation between As and Sb (0.79) also shows the association of As and Sb as coexisting sulphide minerals. The trace element suite of Au, As, Sb, Hg (with low concentrations of base metals) is similar to that which characterises sedimentary rock-hosted, disseminated gold deposits (Berger and Bagby, 1991). The newly reported sedimentary rock-hosted, disseminated gold mineralisation in the MFB, central India has many features in common with the Carlin-like gold deposits.

References AMETA S. S. 1990. Polyphase deformation in the Bijawan Group, Narsinghpur district, Madhya Pradesh. Geol. Surv. India Spec. Publ. 28, 211-225. BERGER B. R. AND BAGBY W. C. 1991. The geology and origin of Carlin type gold deposits. In: R.P.Foster (ed.). Gold Metallogeny and exploration. Blackie, Glasgow and London, pp 210-248

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G E O L O G I C A L S O C I E T Y OF A U S T R A L I A , A B S T R A C T S N o . 5 9

1Australian

Geological Convention, Sydney, July 2000

THE WATERANGA INTRUSION, SOUTHEAST QUEENSLAND, AUSTRALIA: A HIGH-Al AND LOW-K MAFIC, THOLEIITIC LAYERED INTRUSION Reddy V. R. Talusani, Warwick J. Sivell, and Paul M. Ashley Division of Earth Sciences, The University of New England, Armidale NSW 2351 Layered mafic intrusions have played an important role in the development of petrology, particularly in the formulation of concepts of fractional crystallisation. They are the natural laboratories for studying the insides of magma chambers. A number of layered intrusions occur in southeast Queensland. One of these is the Wateranga layered intrusion (WLI) which is located 80 km southeast of Bundaberg in southeast Queensland. The WLI (28 km2) is a tholeiitic, Permo-Triassic, undeformed, layered pluton that has been emplaced into the late Carboniferous Goodnight beds of the Goodnight Block (Cranfield, 1989). The relatively small size, excellent exposure, and uncomplicated structure of the Wateranga intrusion make it well suited for a study of its differentiation history and for quantitative modeling of the compositional changes that occurred in the Wateranga magma during crystallisation. Detailed field investigations of the WLI have led to the identification of several lithological sub-types. These have been grouped into four rock types, which are norite, troctolite, and anorthosite occurring as small elongated bodies enclosed by the more dominant gabbroic rocks. In addition, extensive drill core (600 m) examination and sampling has been carried out to fully understand the mineralogical and chemical changes within the rocks of the intrusion. Rock types change abruptly with stratigraphic height and include olivine gabbro, gabbro, norite, gabbronorite, troctolite, anorthosite, pyroxenite, and hornblendite. Layering is well developed both on a microscopic and mesoscopic scale and dips inwards towards the centre of the intrusion at angles of 10 to 45 degrees, which suggests a lopolithic shape. Layering is the result of repetition of various rock types and it appears to have been formed by crystal accumulation. Although sharp contacts between rock types are known, many of the boundaries are transitional from one rock type to another. Principal phases are plagioclase, augite, orthopyroxene, olivine, hornblende, ilmenite, and magnetite. Olivine is confined to several horizons. Plagioclase, pyroxenes and olivine are cumulate phases. Ihnenite and magnetite are ubiquitous throughout the intrusion, generally as cumulate phases. Hornblende is invariably an intercumulus phase. The general order of crystallisation was olivine, orthopyroxene, plagioclase, oxide phases, clinopyroxene, and hornblende. Orthopyroxene-magnetite symplectites occur in some samples of olivine gabbro, norite, and troctolite (Ambler and Ashley, 1977). Zoning is common in plagioclase and clinopyroxene. Plagioclase shows normal and reverse zoning whereas clinopyroxene shows sector zoning. Plagioclase compositions vary from An70 to An40 but majority of the plagioclase is within the labradorite range. The sulphide assemblage is dominated by pyrrhotite with minor pentlandite and chalcopyrite. Sulphide textures are attributed to magmatic processes followed by subsolidus exsolution with local modification by hydrothermal reworking. About one hundred twenty whole rock samples from the WLI were analysed for major oxides and trace elements. Whole rock major element profiles clearly reflect the mineralogical distribution in the various rock types. These rocks are characterised by low Ti02, K20, and P205 and high A1203 and MgO. They are enriched in Sr, Zr, Ba, V, Ni, and Cr and poorer in Rb, Th, Pb, and Nb. Fractionation followed a tholeiitic trend with iron enrichment in the liquid with local extreme enrichment of Ti, P, and Zr. Whole-rock Mg#'s range from 28 to 81. Mg#'s vary with height and display abrupt reversals, which indicate open-system addition of new mafic magma. Litho- and chemo- stratigraphic analyses of the Wateranga intrusion show that it is the product of at least three major magma pulses. The variability of chemical and mineralogical characters within the Wateranga magma may be due to the combined effects of fractional crystallisation, magma mixing and crustal contamination. Based on composition two general types of tectonic settings are proposed for WLI: a subduction related setting and an extensional, possibly rift-related setting. References Ambler E. P. and Ashley P. M. 1977. Vermicular orthopyroxene-magnetite symplectites from the Wateranga layered mafic intrusion, Queensland, Australia. Lithos, 10, 163-172. Cranfield L. C. 1989. New Palaeozoic stratigraphic units in the Maryborough 1:250 000 sheet area, southeast Queensland. Queensland Government Mining Journal, 90, 115-120.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

THE MORPHOLOGY AND CHEMISTRY OF GOLD GRAINS AT PORTIA PROSPECT, CURNAMONA, SOUTH AUSTRALIA. K. P. Tan,' R. A. Eggleton' and P. de Caritat^ Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME) 'The Australian National University, Geology Department, Canberra, ACT 0200 ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601. The Portia prospect is situated on the eastern limb of the Benagerie Ridge Magnetic Complex, Cumamona, in South Australia. Gold at the prospect is hosted in the basal Tertiary sediment- (the Portia Unit) and in the Proterozoic saprolite. Proterozoic albitite bedrock of the magnetic complex has also been found to host copper and gold. Gold grains in the sediment (Portia Unit) have irregular shapes and subrounded edges, with surficial pits and dissolution grooves of varying sizes. Gold-rich rims formed by electro-refming give rise to a porous texture with interconnected vugs. It is possible that minerals have intergrown with the gold, as indicated by flat surfaces across gold grains and the presence of elevated edges forming various euhedral casts and large rectangular voids. In contrast, some gold grains in the saprolite are highly irregular, with jagged edges, uncorroded and puckered surfaces, while others have etched surfaces with dissolution pits. A minority of grains still show euhedral prismatic shapes, albeit with rounded edges. High fineness rims are more abundant on grains in the sediment than on those in saprolite. Small (< 1 jam) gold flakes, which account for the majority of the gold (< 1 ppm) occurring in both sediment and saprolite, have been observed in panned concentrates. Such gold is mainly included in sulphides and upon weathering the gold particles would be released as particulate gold if no complexing ions were available to mobilize the gold in solution. In terms of chemistry, gold in the mineralized Proterozoic albitite has variable fineness, with Ag content ranging from 4 to 10 %, and is classified as argentiferous gold. Cu concentrations in gold could range from below detection (< 0.03 %) to high (0.13 %), while Te is mostly below detection (< 0.04 %). The gold hosted in sediment has similar chemistry to that hosted in saprolite, with Ag content varying from 2 to 10 %, except for the secondary gold rims which have a lower Ag content (0.4 - 2.7 %). The rare occurrences of sulphide and telluride inclusions are fully enclosed within the gold grains, and comprise arsenopyrite (FeAsS), galena (PbS) and altaite (PbTe). These minerals appear euhedral and unweathered, and no vug is present at the contact between the minerals and gold, which strongly suggests that the gold enclosing the minerals is primary in origin. Besides, the presence of trace to moderate amounts of Cu, Hg and Te found in the saprolite- and sediment-hosted gold, in proportions that correlate well with the primary gold in the mineralized albitite, also suggests that the gold is primary. None of the morphology typical of secondary gold such as paint-films, delicate- dendrites, or wire-like morphologies have been observed from the gold grains at Portia, nor have pseudo-hexagonal plates or secondary gold spherules adhering onto primary gold grains been found. Striation markings on a gold grain and a flattened gold grain having well-rounded edges with pitted texture indicate that mechanical transport has taken place. It is therefore interpreted that the gold hosted in sediment (Portia Unit) is placer in origin, while the gold hosted in the saprolite is in-situ, primary gold. Both types of gold have undergone varying degrees of dissolution, with the sediment-hosted gold subjected to more intense weathering, as indicated by the common presence of Agpoor rims and strongly pitted surfaces. Acknowledgements: The authors would like to thank Pasminco Exploration for both financial and logistic support for this research project. Keith Scott from CSIRO (North Ryde) has provided valuable comments on gold morphology analysis, which is greatly appreciated. This work was supported by the Commonwealth Government's Cooperative Research Centres Program.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE DELAMERIAN OROGENY IN WESTERN VICTORIA: CONSEQUENCE OF ARC-CONTINENT COLLISION David H. Taylor, Ross A. Cayley, Vincent J. Morand, Kylie E. Wohlt and David H. Moore Geological Survey of Victoria, P.O. Box 500 East Melbourne, Victoria 3002 The Middle to Late Cambrian convergent deformation of the Tasmanides was a complex and somewhat enigmatic event dismembered by Gondwana break-up with pieces now recorded in mainland Australia (Delamerian Orogeny), New Zealand, Tasmania (Tyennan Orogeny) and Antarctica (Ross Orogeny). The well exposed inboard portion in South Australia shows the platform to deep marine passive margin sequence of the Adelaide Geosyncline thrust westward back onto the older craton as a classic foreland fold and thrust belt. New mapping and geophysical data of the Geological Survey of Victoria in the more poorly exposed outboard Victorian portion suggests that arc-continent collision drove the deformation. East of the collision zone is the younger Lachlan Fold Belt which is floored by Early Cambrian oceanic crust of back-arc affinity that, importantly, remained undeformed and in a deep marine environment throughout the Delamerian Orogenyy4ready to accumulate the eroded detritus from that event to form the Lachlan turbidite pile. The new data resolves two major structural domains in western Victoria; the easterly extent of the Adelaide Geosyncline passive margin sequence (the Glenelg Zone), and a more outboard domain (the GrampiansStavely Zone) containing a mafic-ultramafic ophiolite complex (the Dimboola Igneous Complex) that is largely obscured in the area of outcrop by a late-Delamerian syn-orogenic basin (Mount Stavely Volcanics and Nargoon Group). The Glenelg Zone comprises a Cambrian deep marine terrigenous sequence that includes rift volcanics (Truro Volcanics) and mafic dykes/sills as well as fault slices of ultramafic oceanic lithosphere (the Hummocks Serpentinite) that suggest that rifting of the passive margin was continuing right up to the time of the Delamerian Orogeny. Indeed, the presence of injected mafic material has previously been interpreted as the cause of the high heat flow that may have helped initiate the high T-low P conditions which converted part of the sequence into the Glenelg River Metamorphic Complex. The presence of the large belt of highly magnetic mafic-ultramafic rocks in the north of the GrampiansStavely Zone (here-in grouped and named as the Dimboola Igneous Complex) provides the evidence for arccontinent collision. Sampled by drilling through Murray Basin cover, the complex comprises maficultramafic cumulates of gabbro and pyroxenite with sub-volcanic equivalents and basaltic lava. The rocks are all intensely altered to talc, serpentine and chlorite under static greenschist metamorphic conditions indicative of hydrothermal sea-floor alteration. In terms of petrography and alteration this large belt of rocks strongly resemble an ophiolitic fore-arc package accreted to the outboard margin of a passive margin as that margin and its rifted continental basement was progressively drawn into an east-dipping subduction zone. In Tasmania similar mafic-ultramafic rocks (the MUCs) have also been interpreted as fore-arc ophiolites being emplaced onto an older continental basement during eastward dipping subduction at this time, although a thick passive margin sequence was lacking there. The early collisional contact (the suture between the upper and lower plates) is possibly preserved as the deeply buried northwesterly trending western limit of the highly magnetic belt of the Dimboola Igneous Complex. Although poorly constrained this contact probably dips moderately east with the ophiolite in its structurally high position on the upper plate being thrust into and over the passive margin sequence. Farther south in the area of outcrop a syn-orogenic basin filled with Mount Stavely Volcanics and terrigenous sediment of the Nargoon Group overlies the contact and obscures its nature and position. The volcanics have a calc-alkaline continental affinity and have been interpreted as a post-collisional suite extruded in the final phases of the deformation. The overlying terrigenous turbiditic sediments are derived from the just deformed passive margin since they contain abundant Delamerian detrital zircons. Again there is a strong Tasmanian correlation with this entire package having a similar geological history to the Mount Read Volcanics and overlying siliciclastics of the Dundas Trough in Tasmania. The above scenario of arc-continent collision driving the Delamerian Orogeny and consequent juxtaposition of the Lachlan back-arc crust against that region ready to receive its influx of turbidites is analogous to the present day tectonic setting of Taiwan. Taiwan represents the passive margin sequence of China being thrust back westward onto the Chinese craton due to collision with the Luzon arc. The deforming passive margin sequence of Taiwan is a classic craton-directed foreland fold and thrust belt with orogenic highlands attaining 4 km elevation, shedding detritus over the subdued topography of the collided arc into the Pacific Ocean behind the arc which attains depths of 4 km only 50 km offshore.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SEMI-QUANTITATIVE MINERAL AND LITHOLOGY MAPPING OF DRILL CORE PULPS USING VISIBLE AND INFRARED SPECTROSCOPY Geoffrey R. Taylor University of New South Wales, Sydney, NSW 2052

The application of processing techniques designed for hyperspectral imagery greatly enhances the ability to map the mineralogy and lithology of drill core pulps with field spectrometry. The methodology is developed using material from a single drill hole through a mineralised sequence of rocks from central New South Wales. If some a priori knowledge of drill core mineralogy is at hand then simple unmixing of pulp spectra using library endmembers and linear unmixing techniques will provide a sensible discrimination of the drill hole mineralogy. Mineral library spectra are used in linear unmixing routines to determine the abundances of 10 minerals in drill core pulps representing between 1 and 3 m of core. Comparison with X-ray Difrraction (XRD) analyses shows that for most major constituents spectrometry provides an estimate of quantitative mineralogy that is as reliable as that provided by XRD. Confusion between the absorption features of calcite and those of chlorite causes the calcite contents determined by spectrometry to be unreliable. One proviso for this approach is that library spectra of any one mineral may differ greatly and that these differences will lead to variations in the analysed abundances. A possible strategy for dealing with this problem is to collect a local spectral library of the key minerals to be found in a project area. Convex geometry is used to recognise the spectra of those samples that are extreme and are representative of unique lithologies. Linear unmixing is used to determine the abundance of 12 lithologies in each drill hole sample and these abundances are used to interpret the geology of the drill hole. The interpreted geology agrees well with conventional drill hole logs of the visible geology and photographs of the split core. The results achieved by spectral analysis provide far more mineralogical and lithological information than that which can be derived from the visual logging of the split core. This method also maps endmembers that can be readily related to real rock types or alteration styles and thus is likely to be more useful in mapping the geology of the project area. It is strongly recommended that a spectrometer that covers the visible, as well as the shortwave infrared, be employed as many rock-forming minerals have distinctive spectral features in the visible part of the spectrum. It is concluded that the methods developed provide a quick and cost-effective way of determining the lithology and alteration mineralogy of drill core pulps. Acknowledgements: Cyprus Gold Australia assisted by the provision of logistical support and by access to unpublished geological information. Cyprus also contributed very significantly to an Australian Research Council SPIRT grant without which the research could not have proceeded. Marcus Reston is thanked for his assistance in the field while the core was being photographed, for collection of the drill core pulps and for his technical input in earlier stages of the project. Colin Ward is thanked for quantitative analysis of the XRD data.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

WEATHERING: CYCLICAL OR CONTINUOUS? A SOUTHERN PERSPECTIVE Graham Taylor' and Greg Shirtliff^ Cooperative Research Centre for Landscape Evolution and Mineral Exploration ^University of Canberra, ACT Australia 2601 ^ Australian National University, ACT Australia 0200

Extensive research was recently undertaken to find the age distribution of existing weathering profiles and related ferruginous products on the Australian continent. Information was gathered from published and unpublished sources, and resulted in some important findings. The age distribution of existing weathering profiles follows an approximate exponential increase towards the present. This corresponds to similar trends by most other geological entities on earth and can be correlated with the effect of recycling, ie destruction over time. In accordance with general theory and past assumptions a possible increase in weathering was observed during the Palaeogene, an increase much less significant than previous studies thought. There is now reasonable evidence from recent studies to suggest that such deviations from long term trends may have occurred as a consequence of increases in the potential for preservation rather than the direct influence of climate, despite the well known effect precipitation has on weathering. A broad range of ages, from Permian to Present in significant proportions supports the concept that weathering in Australia was relatively continuous and did not occur as distinct episodes.

494

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

GEOCHEMISTRY OF MANTLE XENOLITHS AND XENOCRYSTS FROM THE SKERRING KIMBERLITE, WESTERN AUSTRALIA - EVIDENCE FOR A ^800 Ma LITHOSPHERE RE-FERTILIZATION EVENT W.R.Taylor, M.Richardson and V. Bennett Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 The Skerring kimberlite is an elongate, non-diamondiferous pipe of 800 Ma age located in the North Kimberley kimberlite province of Western Australia. The pipe lies on a north-east trending lineament on which the diamondiferous Lower Bulgurri and Ashmore kimberlites also occur, some 25 km to the southwest. The Skerring pipe is noteworthy for its abundance of mantle-derived xenocrysts, principally picroilmenite and pyrope garnet. Other indicator minerals include Cr-spinel, Cr-diopside, zircon megacrysts, and graphic intergrowths of ilmenite + altered pyroxene. In this study two large silicified peridotite xenoliths o f - 2 5 X 50 cm size were collected from exploration pits. One of the xenoliths, XI, has an unusual corrosionsculpted external surface texture that suggests the xenolith may have originally been carbonatized prior to silicification. Heavy mineral extraction of the silicified peridotite xenoliths yielded Cr-spinel in one case (XI) and Cr-pyrope garnet, Cr-spinel ± olivine inclusions in the other (X2). Secondary minerals include galena and chalcopyrite. In addition to xenoliths, an array of xenocryst and megacryst minerals were sampled to constrain lithospheric composition. Major element compositions were determined for over 200 garnet grains and laser-ablation ICPMS trace element analyses were undertaken on a sub-set of 40 grains. About 40% of garnets are high-Ti, low-Cr (~3wt% Cr203) pyropes derived from fertile Iherzolite. These garnets have (EnormaP REE patterns with (Dy)cn -20-30 and (Nd)cn ~5 and nickel temperatures that fall within the range 1300 to 1500°C suggesting deep mantle sampling. About 45% of garnets are low-Ti, high-Cr (-6-10 wt% Cr203) Iherzolitic compositions with REE patterns showing variable Dy-depletion, (Dy)cn <10, and Nd enrichment, (Nd)cn ~230. The REE compositions define a trend between a strongly Dy-depleted harzburgitic garnet endmember and the fertile high-Ti pyropes. Nickel temperatures fall within the range 900 to 1500°C indicating sampling along the full mantle stratigraphic column. There are smaller populations of harzburgitic garnets (>10% Cr203), high-Ti megacryst garnets and eclogitic garnets. Chrome diopside recovered at Skerring is of more restricted composition (mg# -89, Ca/[Ca+Fe+Mg] -0.32). Application of the Tayor and Nimis (1998) Crdiopside thermobarometer yields P,T conditions of 50-55 kbar and ~1400°C compatible with the high-Ti Iherzolitic pyrope. Chrome diopside associated with other Iherzolitic garnets has not been recovered. Garnet and Cr-spinel compositions from xenoliths XI and X2 indicate they were garnet-spinel Iherzolites. Thermobarometry yields P,T conditions of ~1230°C and - 3 8 kbar, and redox conditions were relatively high at -FMQ. Combined with P,T information from Cr-diopside, a - 4 8 mWm-2 palaeogeotherm can be defined for the mantle beneath the Skerring pipe. Such a geotherm is greatly elevated over typical stable cratonic geotherms and indicates thermal disturbance. Xenocryst compositions and modelling of garnet and Crdiopside REE patterns indicate that the lithosphere beneath Skerring was also chemically modified. The most likely senario is that the lithosphere was variably refertilized by a plume-derived magma which introduced Fe, Ti, Ca and incompatible trace elements. This resulted in conversion of harzburgite and Ti-poor Iherzolite to Ti-rich Iherzolite and to the formation of megacrystic picroilmenite, garnet and zircon from crystallization of the fractionating plume magma. A Re-Os isotope study of Cr-spinel from the xenoliths indicate the timing of this event was close to the eruption age of the kimberlite at -800 Ma. It is likely that any diamond present at this time was resorbed providing an explanation for the observed negative correlation between picroilmenite and diamond content in the North Kimberley province. The effects of the inferred mantle plume were restricted to the northern CEpicroilmenite-rich zone' of the province with diamondifeous lithosphere to the south being relatively unmodified by this event. References Taylor, W. and Nimis, P. (1998) 7th International Kimberlite Conference Extended Abstracts, 897-898.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

HYDROTHERMALLY ALTERED ARCHAEAN FELSIC VOLCANICS FROM THE MT. HOPE AREA, EYRE PENINSULA, SOUTH AUSTRALIA Graham S. Teale' and C. Mark Fanning^ 'Werrie Gold Ltd, P.O. Box 740, North Adelaide, S.A. 5006 ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200

In southern Eyre Peninsula, South Australia the Archaean to Proterozoic of the Gawler Craton is comprised of the supracrustal Carnot Gneisses (Sleaford Complex) that underwent prograde granulite facies metamorphism at -2420 Ma and represent a lower crustal segment that was juxtaposed with higher crustal level granitoids (the Dutton Suite) at about 2000 Ma. The PIRSA aeromagnetic image of the southern Eyre Peninsula highlights the boundary between the magnetically intense layered Camot Gneisses and the magnetically quiet region of the Dutton Suite (-2520 to --2560 Ma). In the east, the Kalinjala Mylonite Zone, D3 of the Kimban Orogeny (--1710 Ma), separates the Palaeoproterozoic Hutchison Group (= 1860 Ma) from the -1850 Ma Donnington Granitoid Suite. In the west the Warrow Quartzite (Hutchison Group) is seen to unconformably overly the Kiana Granite (Dutton Suite). A prominent, intense north-south trending magnetic anomaly occurs between Price Island and Lake Hamilton on the western Eyre Peninsula. Oliver and Fanning (1987) describe the magnetite-rich, quartz-muscovite-spessartine garnet phyllites from Price Island and Lake Wangary DDH intersections on Coffin Bay Peninsula. Detrital zircon U-Pb age signatures indicate a major igneous component at -1765 Ma, with no younger zircons. Older zircons are present reflecting provenance from the Camot Gneisses and Dutton Suite. These phyllites are clearly not part of the Hutchison Group sensu stricto, but may be correlatives of the Wallaroo Group, Yorke Peninsula. In the Mt Hope area, the N-S magnetic anomaly is off-set and more recent and detailed drilling to the north intersected a sequence which contains "quartz-eye" felsic metavolcanic rocks that have undergone hydrothermal alteration prior to metamorphism. These meta-volcanics can now contain abundant to minor chloritoid, andalusite, muscovite, chlorite, kyanite, spessartine garnet, zincian staurolite, gahnite and disseminated chalcopyrite and sphalerite. Beds of carbonaceous phyllite can be intercalated with the meta-volcanics and are apparently underlain by mafic schists, magnetite rich meta-pelites, silicate iron formations, tremolite marbles and calc-silicates. Anomalous base metal and gold resuhs have been returned from the felsic meta-volcanics and the other altered rocktypes. SHRIMP U-Pb zircon analyses of two meta-volcanic layers reveals a dominant 2520 ± 7 Ma zoned magmatic component with inheritance to a major peak at 2720 Ma. No younger zircons have been found thus far. The presence of probable Archaean felsic volcanic rocks in the southern Gawler Craton is of great significance to both the tectonic evolution of the Craton and its mineral potential. Reference OLIVER R.L. & FANNING C.M. 1997. Australia and Antarctica: precise correlation of Palaeoproterozoic terrains. In: Ricci C.A. ed The Antarctic Region: Geological Evolution and Processes, pp 163-172. Proceedings of the VII International Conference on Antarctic Earth Sciences.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

COMPETENCY CONTRAST, KINEMATICS, AND DEVELOPMENT OF FOLIATIONS AND LINEATIONS Basil Tikoff ^ and Laurel Goodwin ^ ^ Geology and Geophysics, University o f Wisconsin, Madison, WI U S A ^ Earth and Environmental Sciences, N e w Mexico Tech, Socorro, N M U S A

Contrasts in competence are commonly observed in polyphase rocks at a variety of scales. Competency contrast is effected by mineralogical heterogeneity, variations in crystallographic orientation, grain-size variations, and/or pre-existing mechanical anisotropy. Competency contrast causes mechanical instability, and leads to strain localization. Consequently, the bulk deformation behavior of a given rock depends in a non-linear manner on the distribution and orientation of different phases and their relative competency. An understanding of the effects of competency contrast and kinematic analysis are both required to interpret the formation of foliations and lineations (fabric). Perhaps the most relevant kinematic parameters, the flow apophyses, are the orientations to which the finite strain axes are "attracted" and the locations of the slowest rotation of material lines. Knowledge of the orientations of the flow apophyses is critical to understanding rotation and elongation of material lines during flow. However, we propose that the ability of the most competent phases to attain these stable positions (attractor flow apophyses) by a given deformation process (e.g., mechanical rotation) is a function of the distribution, crystallographic preferred orientation, and volume percent of the least competent phase(s). Strain partitioning between domains of competent and incompetent phases could also result in deformation path partitioning. In these ways, both the kinematic framework and competency contrast control development of the macroscopically visible fabric and, to some extent, the microfabric. This approach to understanding fabric development provides a link between field observations and interpretations and experimental constraints on deformation processes, in order to study polyphase rocks with complex deformation histories.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

REPEATED MOBILISATION OF GOLD AS A POLYMETALLIC MELT AT THE CHALLENGER DEPOSIT, SOUTH AUSTRALIA Andrew Tomkins^ and John Mavrogenes^'^ 1) Dept. of Geology, Australian National University, Canberra, ACT, Australia 0200 2) Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia 0200 The blind Challenger gold deposit represents the first of many gold deposits which are likely to be found in Archaean-Palaeoproterozoic granulite terrains within South Australia's Gawler Craton, and possibly other similar terrains around Australia. Situated 1180km north west of Adelaide in the north west Gawler Craton, Challenger was discovered in May, 1995 by Dominion Mining Ltd and Resolute Ltd (Gawler Joint Venture) in a previously poorly explored area. The Challenger deposit is hosted by Archaean aluminous pelites metamorphosed at granulite facies conditions of ~800°C and 7.4kbar. Inclusions of gold and sulfides in garnet which formed during peak metamorphism indicate that mineralisation was introduced prior to peak metamorphism. Evidence of repeated remobilisation of gold and sulfides is easily observed and has concentrated mineralisation into a series of shallowly plunging shoots. To understand how this geometry was achieved we need to understand the processes that control remobilisation of gold-sulfide mineralisation under granulite facies conditions. At Challenger, visible gold is restricted to migmatitic leucosomes and melanosomes. Pyrrhotite, loellingite and arsenopyrite occur throughout the mineralised migmatite and are often associated with coarse native gold in leucosomes and melanosomes. Native bismuth is commonly associated with native gold in leucosomes and melanosomes. Large inclusions (up to 1mm) of gold with co-existing bismuth ± Co and Ni rich arsenopyrite [(FeCoNi)AsS] ± pyrrhotite ± maldonite (Au2Bi) are located at grain boundaries and also hosted in quartz, feldspar, cordierite and garnet grains. Propagating from these large inclusions are trails of tiny spherical inclusions (<20)Lim) of gold, bismuth, maldonite, arsenopyrite, pyrrhotite or combinations of these, along annealed fractures. Au-Bi phase relations show that it is possible for gold to coexist with bismuth as a melt at temperatures as low as 241°C (Okamoto & Massalski 1983). Experiments conducted in this study indicate that As, Fe, Co, Ni and S can exist in a polymetallic melt with gold and bismuth at temperatures consistent with the peak of metamorphism at Challenger. It is estimated that the eutectic for this complex system will exist at compositions dominated by Bi at temperatures similar to the Au-Bi eutectic. We believe that the gold-sulfide inclusions seen in migmatitic leucosomes at Challenger represent the crystallised products of a polymetallic melt. Where bismuth co-existed with gold, pyrrhotite (containing Co and Ni) and loellingite in the Challenger migmatites an initial polymetallic melt was produced, prior to peak metamorphism. This polymetallic melt initially remained immobile as the meltirock ratio was too low for melt migration to occur. When the pelitic host partially melted via vapor absent melting reactions, the melt:rock ratio increased to such an extent that melt migration became possible. Both melts (the polymetallic melt and the silicate melt) had similar rheological properties, and as such were redistributed to form a stromatic migmatite. However, the two were immiscible and remained as separate entities within individual melt accumulations. Increased mobility of the initial polymetallic melt allowed ftirther interaction with unmelted metallic components during melt migration, leading to the incorporation of more components in the melt and the development of a truly heterogeneous polymetallic melt. Crystallisation of the silicate melt left partially molten polymetallic melt as inclusions within the leucosomes, due to the polymetallic melt's much lower eutectic. Internal overpressure, developed within the polymetallic melt inclusions during relatively isobaric cooling, led to fracture propagation and fijrther melt migration out of the polymetallic melt inclusion. Annealing of these fractures resulted in inclusion trails of closer to eutectic compositions surrounding single larger polymetallic inclusions hosted in migmatitic leucosomes. Our interpretation is that the gold-sulfide inclusions observed in migmatitic leucosomes existed as polymetallic melts before, during and after granulite facies peak metamorphism. Further, we suggest that the formation of such polymetallic melts has enabled extensive remobilisation of gold and sulfides. References OKAMOTO H. and MASSALSKI T.B. 1983. Au-Bi (gold-bismuth). In: Massalski T.B., Murray J.L., Bennett L.H. & Baker H. eds. Binary Alloy Phase Diagrams, Volume 1: Ac-Au to Fe-Rh, pp.238-240.

498

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

VARIATION IN THE DISTRIBUTION OF GOLD WITHIN ARSENOPYRITE AND LOELLINGITE DURING PRO- AND RETROGRADE METAMORPHISM Andrew Tomkins' and John Mavrogenes^'^ ' Dept. of Geology, Australian National University, Canberra, ACT, Australia 0200 ^Research School of E^rth Sciences, Australian National University, Canberra, ACT, Australia 0200 In mesothermal gold deposits worldwide, the association of gold with Fe+As±S is ubiquitous. Furthermore, gold deposits displaying arsenopyrite-loellingite-pyrrhotite-gold textures in amphibolite to granulite facies host rocks have been described at a number of locations. Debate concerning timing of mineralisation relative to peak metamorphism is contentious at many of these locations (eg. Bamicoat et al, 1991; Neumayr et al, 1993). The outcome of such debate affects crustal scale genetic models for the evolution of gold producing regions. At the centre of several discussions are interpretations of textural relationships between gold, arsenopyrite, loellingite and pyrrhotite. LA-ICPMS analysis of experimental and natural samples has clarified the behaviour of gold relative to these phases during pro- and retrograde metamorphism. Experimental results show that gold is expelled from arsenopyrite during heating at elevated pressure and partitioned into loellingite during inversion of arsenopyrite to loellingite and pyrrhotite. Upon retrogression of loellingite and pyrrhotite to arsenopyrite, invisible gold is liberated from loellingite as, at high temperature, arsenopyrite is not able to significantly incorporate invisible gold. Observation and analysis of naturally occurring composite loellingite-arsenopyrite-pyrrhotite grains suggest that during initial retrogression invisible gold remains within loellingite along a difftision front as loellingite is replaced by arsenopyrite. Retrograde textures developed between gold, loellingite and arsenopyrite are controlled by the rate of loellingite destruction relative to gold diffusion. At high temperatures, during initial retrogression, gold diffuses more rapidly than loellingite is destroyed and no gold is exsolved from loellingite. Whereas, at lower temperatures, during later retrogression, loellingite destruction proceeds more rapidly than gold can diffuse, resulting in the exsolution of native gold. We have determined that it is possible to use textural observations and gold distribution analyses to constrain the timing of gold mineralisation relative to metamorphism. Summarising our work, we would expect to see the following textural relationships if gold and sulfide mineralisation were introduced under: 1.

Pre-peak metamorphic conditions -

2.

Syn-peak metamorphic conditions -

3.

Post-peak metamorphic conditions -

invisible gold in Lo but not in Asp and the formation of composite Po-Asp-Lo grains, invisible gold in Lo but not in Asp and no close association between Po and Asp-Lo grains, or invisible gold in both Asp and Lo.

However, given the high solubility of gold at upper amphibolite and granulite facies (Loucks & Mavrogenes 1999), scenario 2 is considered plausible only at extremely low sulfur fugacities and thus lower temperatures consistent with the lower amphibolite facies. Based on the work of Bamicoat et al (1991) and Neumayr et al (1993) many deposits with the textural association of scenario 1, may have been misinterpreted as synmetamorphic in origin. References BARNICOAT A.C., FARE R.J., GROVES D.I. & M C N A U G H T O N N.J. 1991. Synmetamorphic lode-gold deposits

in high-grade Archaean settings. Geology 19, 921-924.

LOUCKS R.R. and MAVROGENES J.A. 1999. Gold solubility in supercritical hydrothermal brines measured in

synthetic fluid inclusions. Science 284, 2159-2163. NEUMAYR P., CABRI L.J., GROVES D.I., MIKUCKI E.J. & JACKMAN J.A.,1993. The mineralogical distribution of gold and relative timing of gold mineralisation in two Archaean settings of high metamorphic grade in Australia. The Canadian Mineralogist 31,711-725.

499

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney, July 2000

INTRAPLATE DEFORMATION OF THE AUSTRALIAN, PACIFIC AND SOUTH BISMARCK PLATES FROM GPS OBSERVATIONS P. Tregoning Research School of Earth Sciences, The Australian National University, Canberra 0200, A.C.T.

For many years the Global Positioning System (GPS) has been used to measure presentday crustal motion - globally as well as in the Australasian region. Estimates of first-order tectonic motion have confirmed that present-day rates generally agree with plate motions averaged over a few million years. With improved data analysis procedures and a global tracking network of over 200 sites it is now possible to detect intraplate deformation of a few mm/yr within the large-scale tectonic plates as well as along plate boundaries. We show that, in general, the Australian, Pacific and South Bismarck Plates are rigid, although significant intraplate deformation occurs at the plate boundaries in Papua New Guinea where these plates collide.

500

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GOLD MINERALISED ARCHAEAN FAULT-FRACTURE NETWORKS, ORA BAND A, WESTERN AUSTRALIA Gerard Tripp ^ and Julian Veamcombe ^ 'Centaur Mining and Exploration Ltd ^ Veamcombe and Associates Pty Ltd The nature of structures controlling hydrothermal fluid focussing and Archaean lode gold deposits is controversial. Craton- and crustal-scale shear zones channeling deeply sourced fluids are central to many models, including those deduced from seismic profiling in the Ora Banda area, Western Australia. However, at Ora Banda gold mineralisation is controlled by faults with high-grade shoot development at the intersection of faults and favourable host rock. Gold mineralisation within the Ora Banda area traces out the distribution of brittle-ductile faults, indicating that the fault-fracture network was the pathway for fluid flow during mineralisation. Gold mineralisation in the greenstone-belt scale Zuleika Shear Zone (10km southwest of Ora Banda) began during the ductile deformation (D3), continued through peak metamorphism that post-dates the shearing, and finally ceased after the brittle-ductile faulting event (D4). Deposits are located where brittle-ductile fauhs and fractures (D4) cross-cut the Zuleika Shear Zone. These faults and fractures traverse the surrounding greenstones and granite and are not Riedel structures or other lower order faults genetically related to the ductile shearing. Control by craton- or crustal-scale ductile shear zones on gold fluid focussing appears equivocal. Archaean deformation in the Ora Banda area resulted in upright folds (D2), ductile shear zones (D3), and a regional-scale gold-bearing brittle-ductile fault network (D4). Early low-angle faults (Df, D l ) , documented in the surrounding Coolgardie, Kambalda and Boorara Domains are not developed in the Ora Banda Domain, and the fabrics reflect only the latest ENE-WSW shortening event. The western limb of the regional-scale ESE-plunging Kurrawang syncline (D2) is truncated by the Zuleika Shear Zone (D3). The brittle-ductile faults (D4), are developed in three principal structural orientations: N-S (dextral), NE-SW (dextral) and E-W (sinistral). These faults display mutual cross-cutting relationships and formed synchronously during a complex regional shortening event. Kinematic analyses of fault slip lineations reveal an ENE-WSW directed maximum shortening axis during faulting. Microfabrics of the faults show extensive recrystallisation with significant post-deformation recovery probably related to syn- and post-tectonic granite intrusion. Deformation mechanisms indicate that the fault-fracture event occurred at low-to-moderate temperatures typical of mid to upper-greenschist facies crustal conditions. The fault-fracture network is developed unevenly over the region, being localised in structural zones of high fracturedensity. The Ora Banda structural zone is composed of a network of interlinked faults in which cyclic ductile and brittle conditions produced cataclasite, breccia and quartz vein systems overprinting ductile fabrics. Structural zones of high fracture-density at Ora Banda, Grants Patch and Mount Pleasant are located within the NW-SE trending Ora Banda mafic sequence and are spaced approximately 10km apart. Spatial analysis of gold deposits within the Ora Banda mafic sequence using SpaDiS™ software shows clustering into groups with about 10km spacing. The largest gold deposits in the Ora Banda mafic sequence are hosted by 060°-090° trending faults sub-parallel to the regional axis of maximum shortening. At mineralisation, conditions were conducive to multiple failure episodes with fluid-pressure cycling and transient permeability as a consequence of fault reactivation.

501

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CONTROLS ON GROUNDWATER CHEMISTRY AND THE VULNERABILITY OF GROUNDWATER AND SURFACE WATER TO CONTAMINATION, YARRA CATCHMENT, VICTORIA Sarah Tweed \ Tamie R Weaver' and Ian Cartwrigh^ Hydrogeology and Environment Research Group ' School of Earth Sciences, University of Melbourne, Victoria, 3010. ^ Department of Earth Sciences, Monash University, Clayton, Victoria, 3168. The Yarra Catchment extends 120 km east of Melbourne, and is considered a significant area for Melbourne's water supply (Port Phillip Catchment and Land Protection, 1999). Due to little investigation into the groundwater quality of the Yarra Catchment an estimated baseflow of over 50 % to the Yarra River (Shugg and O'Rourke, 1995), and the diverse land use in the area, this project aims to investigate the local and regional processes influencing the groundwater chemistry via an integrated approach. Land use within the Yarra Catchment area (4045 km2) varies from agricultural (45%), to forested (35%), to urban and industrial use (15%), to forestry products (5%) (Port Phillip Catchment and Land Protection, 1999). Groundwater use within the Yarra catchment also varies, there are 549 domestic and stock users, and 71 8 irrigation, dairy and industrial users (Port Phillip Catchment and Land Protection, 1999). Both natural and human-induced processes affecting groundwater chemistry in the catchment are investigated with the aim of providing strategies for improved and sustainable land and water use. The geology of the Yarra Catchment forms an open and interconnected multi-layered fractured rock aquifer system comprising fractured Siluro-Devonian bedrock, minor granite intrusions and localised basalt flows (Shugg, 1996). Groundwater chemistry within the Yarra Catchment suggests that spatial relationships exist on both local and regional scales. Spatial heterogeneity on a local scale is probably the result of localised recharge events in areas of high surface elevations. It is also likely that changes in land use affect the amount and quality of recharge in these areas. Within the Yarra Catchment, local affects on groundwater chemistry from the predominance of recharge areas in high elevations are observed in the Dandenong Ranges. Within the Dandenong Ranges there are increasing major ion concentrations and electric conductivity (EC) values (Na: 57.8 mg/L, EC: 770 ps/cm) in areas of relatively low elevations, compared to those in areas of high elevations (Na: 8.0 mg/L, EC: 86 ps/cm). As groundwater migrates away from recharge areas (high surface elevations) there is greater opportunity for waterrock interaction to occur. This relationship is also observed on a regional scale within the Yarra Catchment, therefore reflecting the impact of high, localised recharge areas on the regional groundwater system. Groundwater samples were also analysed for 8180 and 82H to determine the origin of groundwater and processes occurring during groundwater flow. 8'80 and 82H values (-6.2 to -5.5%, and -38.1 to -32.2 9% VSMOW respectively) lie on the Melbourne Meteoric Water Line, indicating that groundwater has not been subjected to evaporation or high-temperature water rock interaction. Stable isotope values in groundwater are consistently lower than those of nearby surface water. This may clarify the timing or location of major recharge events in the system, further indicating that recharge may be focussed in higher elevations or throughout the winter months. The difference between stable isotope values in surface water and groundwater may also be used to constrain the contribution of groundwater as baseflow to surface water. The regional groundwater flow regime of the Yarra Catchment is superimposed by high, localised recharge in areas of high surface elevations as indicated by the increase in electric conductivity values and major ion concentrations in areas of relatively low elevations. Due to the high, localised recharge and high baseflow components within the Yarra Catchment, it is important to investigate local groundwater chemistry processes, and to determine the regions where groundwater, and inevitably surface water, are most vulnerable to contamination from surface processes. References Port Phillip Catchment

and

Land

auleatchmentlportphilliplyarralindex.

Protection,

1999.

Yarra Catchment

Action

Plan

website:

http:llwww.

nre.

vie.

gov.

htm

Shugg, A., 1996. Hydrogeology of the Dandenong Ranges Fractured Rock Aquifers and the comparison with similar aquifers in Victoria. MSc thesis. University of Technology, Sydney (unpubl.). Shugg, A. and M. O'Rourke, 1995. Groundwater in the Yarra Basin. Technical Document. YarraCare, Victoria.

502

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SUBDUCTION BENEATH THE NORTH ISLAND OF NEW ZEALAND: GEODYNAMICS OF COMPRESSION AND EXTENSION Phaedra Upton, and Peter O. Koons Geology Department, University of Otago, PO B o x 56, Dunedin, N e w Zealand.

Using a mechanical model consisting of an elastic slab which subducts beneath a two layer upper plate, we examine the response of the deformation patterns within the overlying plate to changes in boundary conditions and rheological properties of the upper plate and the subduction interface. The upper plate is made up of an upper brittle layer overlying a lower layer which gets weaker with depth, mimicking the temperature dependence of crustal strength. The subduction interface is modelled as brittle to a depth of 22 kms. We have run a series of models in which we change the strength of the subduction interface along strike and with depth. An extensive zone of low seismic velocity has been observed within the lower crust of the upper plate at the northern end of the on-land subduction zone. This zone is interpreted as subducted sediment and we model it as such, calculating strength parameters from the seismic velocities. We observe the effect of the weaker sediments within the lower crust of the upper plate on the partitioning and extension within the upper plate. We also vary the subduction direction from normal to oblique with the component of plate margin parallel velocity equal to the plate margin normal velocity component. For a weak subduction interface and orthogonal subduction, the deformation in the upper plate is partitioned and takes place at the trench and at a distance to it, above the ductile portion of the subduction interface. Localised deformation of the subduction interface takes place from the trench to a depth of about 40 kms. At greater depths, deformation is more diffuse. A region of extension develops within the upper plate well away from the trench. A strong subduction interface causes the upper plate to move along with the slab, deformation within the upper plate and localised deformation of the interface both occur 'landward' of the trench, above where the two plates are no longer strongly coupled. If the subduction is oblique to the margin, we see partitioning of strain between plate margin normal and plate margin parallel components if the subduction is weak. The component of margin normal convergence is accommodated within the upper crust in a similar manner to that displayed by orthogonal subduction. In contrast, the margin parallel component of subduction is accommodated away from the trench. A weak region within the lower crust of the overlying plate with an average strength subduction interface has the effect of localising most of both components of plate motion in the region of the weaker material. We make a number of predictions for partitioning of long term strain within a subduction boundary based on the strength of the plate interface and the presence of a weaker material within the upper plate. Partitioning will not occur unless the base of the upper plate, that is, the subduction interface, is considerably weaker than the material above it. Weak material within the upper plate reduces the degree of partitioning of the velocity components. The strength of the base controls the partitioning of the strain. Weak material within the overlying plate can change the details but not the basic pattern of partitioning determined by basal strength.

503

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AREA SELECTION IN THE PROTEROZOIC OF NORTHERN AUSTRALIA Rick K. Valenta MIM Exploration, Brisbane, Queensland

This abstract and talk is a summary of the written and oral communications from a group of explorers and researchers far too numerous to mention. In comparison to most of the planet, the Proterozoic of Northern Australia is an area with generally excellent and constantly improving geological mapping, datasets, metallogenic datasets, and geophysical datasets. The region has extensive areas of thin cover and strong weathering. There has been relatively intensive exploration in exposed areas, but there is significant remaining potential in both exposed and covered regions. Neglecting the giant iron ore deposits of the Hamersley basin, any explorer setting out on an area selection exercise in the region would probably be most assured of success if he or she were looking for sediment-hosted Zn-Pb-Ag, sediment-hosted Cu, Iron oxide Cu-AuREE, metasediment-hosted Au, or unconfomity U-Au-PGE. Each of these deposit styles has a unique set of: • • • • • •

Host rock ages Host rock characteristics, chemistry and rheology Important basement structures Important local structural controls Related age and chemistry of igneous suites Important empirical indicators of haloes, alteration and mineralization

Examples of identification and application of these controls for area selection for each of these deposit styles will be covered in the presentation, along with a simplified process model which justifies selection of criteria for each style.

504

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PALAEOVEGETATION CHANGES DURING JURASSIC TIMES AS DETERMINED FROM HIGHER-PLANT-DERIVED BIOMARKERS Ben O.K. Van Aarssen, Robert Alexander and Robert 1. Kagi Australian Petroleum Cooperative Research Centre / Centre for Petroleum and Environmental Organic Geochemistry, Curtin University of Technology, GPO Box U1987, Perth 6845, Western Australia

Many marine sedimentary rocks contain molecular fossils, or biomarkers, derived from higher plants. Their presence is often used as a diagnostic tool for terrestrial organic matter supply, usually in a qualitative manner, although higher plant biomarkers have also been used to quantify terrestrial vs algal and / or microbial organic matter. In principle, it should be possible to detect the effects of changes in ancient higher plant populations over time by studying variations in the assemblages of higher-plant-derived biomarkers in sediments. However, factors like differential preservation at different times, or possible diagenetic alteration of natural products might interfere with the interpretation. In this study we have tested this premise by studying higher-plant-derived biomarkers in samples from Jurassic marine sediments from three Australian locations. The relative abundances of three higher-plant-derived biomarkers, retene, cadalene and ipiHMN were measured in marine sedimentary rocks from the north west margin of Australia. It is thought that each biomarker represents input from a different plant-type. The distributions of these three compounds form a fingerprint (HPF), which reflects the composition of the input into the sediment that originated from higher plants. Variations in HPF in Oxfordian sediments were nearly identical in all three locations, with retene becoming very abundant relative to the other two compounds with decreasing age of the sediments. This finding strongly suggests that the composition of terrestrial input during deposition largely determines HPF and that the possible effects of diagenesis and catagenesis on the distribution of the three biomarkers are relatively unimportant. The marked increase in the abundance of retene relative to that of cadalene during the Oxfordian can thus be interpreted to reflect an increase in the contribution of plants that produced precursors for retene, i.e. conifers, which was brought about by a significant change in climate. This is exemplified by the distributions of retene and cadalene, as expressed in the higher plant parameter (HPP), in a suite of sediments from the Carnarvon Basin, Western Australia, which include the complete Jurassic period. The HPP profile displays three major cycles, each covering a period of at least 10 million years. This profile not only compares well with published palaeoclimate data, but also shows a remarkable similarity with second order cycles in the global sea-level curve, thus strongly supporting the proposal that variations in HPF and HPP are indications of changes in palaeoclimate. The relation with global sea-level further suggests that global factors, e.g. the atmospheric carbon dioxide concentration, may play a major role in determining the observed variations in the distributions of these higher-plant-derived biomarkers.

505

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RECONSTRUCTION OF THE GEOLOGICAL HISTORY OF AUSTRALIAN CRUDE OILS Ben O.K. van Aarssen, Robert Alexander and Robert 1. Kagi Australian Petroleum Cooperative Research Centre (APCRC), Centre for Petroleum and Environmental Organic Geochemistry,Curtin University of Technology, GPO Box U1987, Perth 6845, Australia

Distributions of methylated naphthalenes can be applied to characterise crude oils from Australia, yielding valuable information about their history of generation and accumulation, in terms of maturity, mixing, biodegradation and migration. Most crude oils contain significant amounts of aromatic hydrocarbons, among which the methylated naphthalenes are prominent. These compounds consist of a naphthalene carbon skeleton onto which one to five methyl groups are attached, thus comprising over 40 different isomers. These compounds are usually classed according to their number of methyl groups, for example trimethylnaphthalenes (TMNs), tetramethylnaphthalenes (TeMNs) etc. The relative abundances of isomers within each class display large variations caused by factors such as the source of the organic matter, the thermal stress it has experienced and reservoir biodegradation. Each crude oil thus carries in its distribution of methylated naphthalenes a geological history of generation and accumulation, whose unravelling can be of great benefit for oil explorationists. An extended suite of crude oils from several locations in Australia was analysed for their methylated naphthalene content. It was shown that consistent linear relations exist in crude oils between the isomer distributions of TMNs, TeMNs and pentamethylnaphthalenes (PMNs), which can be adequately expressed in the form of simple isomer ratios. These ratios reflect the extent of the chemical processess that control the distribution of methylated naphthalenes and are as such a measure of the amount of thermal stress the organic matter has been subjected to. This internal consistency allows for an assessment of maturity independent of source, age and location of the crude oil. When the relationships between the three ratios are not linear, secondary processes such as mixing, biodegradation or migration contamination have affected the crude oil. Close examination of the distribution of methylated naphthalenes can reveal the extent to which either of these processes has affected the oil.

506

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DIAPIRISM IN THE FAIRWAY BASIN, NORTHEAST LORD HOWE RISE: IMPLICATIONS FOR BASIN FORMATION AND GEOLOGICAL EVOLUTION S. Van de Beuque^\ J.-M. Auzende^ Y. Lafoy^, N. Exon^ and G. Dickens^ ^ Ifremer c/o Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601, Australia ^ Ifremer, BP 2059, Noumea, N e w Caledonia ^ Service des Mines et de I'Energie, BP 465, Noumea, N e w Caledonia Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601, Australia ^ Department o f Earth Sciences, James Cook University, Townsville Queensland 4811, Australia

The Fairway Basin and Fairway Ridge are located beneath the northeastern flank of the foundered continental fragment of Lord Howe Rise, adjacent to the New Caledonia Basin. In order to better understand the formation and geological evolution of this poorly understood tectonic link between the New Caledonia Basin and Lord Howe Rise, French and invited Australian scientists carried out the ZoNeCo 5 survey in the region in late 1999 on the French R.V. L 'Atalante. The survey acquired swath-mapped bathymetry and highspeed seismic data over an area of about 100,000 km^ and collected 13 piston cores. The main structures covered by the ZoNeCo 5 survey are, from west to east: • the shallow eastern flank of the planated Lord Howe Platform; • the southwards-deepening Fairway Basin, with its thick (3-5000m), faulted sediment fill, of interpreted mid-Cretaceous to Recent age; • discontinuous massifs, linked northward to the Fairway Ridge; and • the western flank of the 3600 m-deep New Caledonia Basin with its thick sedimentary fill (4-5000m) with seismic sequences similar to those in the Fairway Basin. The survey confirmed the existence of previously identified bottom-simulating reflectors (BSRs), considered to be diagnostic of the presence of gas hydrates, and showed that they occur over an area of about 70,000 km^. Previously published information suggests that the gas is of thermogenic origin and produced at depth. Nearly 100 large (5 to 15 km across) diapirs were also discovered. These are presumably sourced from mid-Cretaceous sediments in the Fairway Basin. They rise vertically through the overlying sediment, sometimes to Oligocene levels within a few hundred metres of the seafloor, and in a few cases they coalesce into elongated ridges (up to 50km in length). Whether the diapirs are salt- or mud-cored has not been resolved, but palaeofaunal indications of saline deposits in the Upper Cretaceous of New Caledonia, and recent work on inferred palaeolatitudes from hotspot traces suggesting a latitude of 45°S, do not preclude the presence of salt. The areas of the BSRs and the diapirism generally coincide, suggesting that they may be related. The diapirism has implications for the evolution of the Fairway Basin. The basin was formed by a phase of continental stretching and thinning during the early stages of the midCretaceous breakup of Eastern Gondwana. The presence of salt or organic-rich mud indicates that deposition took place in a restricted basin; salt would suggest periodic marine flooding. After oceanic spreading ceased, N-S Cainozoic compression between the Australian and Pacific Plates presumably triggered the formation of the diapirs. The end of this compression phase was marked by the obduction of the New Caledonian ophiolite, and the coeval uplift of the Lord Howe Rise and formation of the Fairway Ridge. During the Miocene, the azimuth of compression progressively changed to E-W. This continued compression further stimulated diapiric growth, and possibly opened migration pathways for the gas that is now interpreted in the shallow sediments.

507

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15""Australian Geological Convention, Sydney, July 2000

NORTHERN LORD HOWE RISE: TECTONIC SETTING AND POSSIBILITIES FOR HYDROCARBON INDICATIONS S. Vande Beuque'. P.A. Symonds^ H.M.J. Stagg^ J-M. Auzende^ and G.W. O'Brien^ ' Ifremer, c/o Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601, Australia ^ Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia ^ Ifremer, BP 2059, Noumea, New Caledonia

The Lord Howe Rise (LHR) is a major foundered continental fragment in the southwest Pacific region that is flanked by the oceanic crust of the Tasman Basin to the west, and the New Caledonia Basin and the Norfolk Ridge system (including New Caledonia) to the east. The northern end of the rise merges with a region of complex topography, which includes features such as the Lord Howe seamount (hot spot) chain, the NW-trending Fairway Ridge and Fairway Basin, and the Chesterfield Plateau. The northern LHR is a key structural component in the region which has been influenced by both Cretaceous extensional and Cainozoic compressional tectonics. The Palaeozoic margin of Eastern Gondwana was fragmented by two main phases of extension and breakup. In the mid-Cretaceous, extension led to the development of the New Caledonia Basin, which is floored by crust of uncertain affinity, and the continental component of the Norfolk Ridge system. In the Late Cretaceous, a second extensional phase culminated in the detachment of Lord Howe Rise from the eastern Australian borderland and the emplacement of oceanic crust in the Tasman Basin. General northwards convergence of the Australian and Pacific plates in the Middle Eocene led to the Late Eocene overthrusting of the New Caledonian ophiolites, reverse reactivation of former extensional faults, and uplift and erosion of the crest of the LHR and Fairway Ridge in the Late Eocene to Late Oligocene. Since the Late Oligocene, the region has been dominated by thermal subsidence accompanied by the emplacement of hotspot-related volcanics on the LHR. Initial westward subduction of the Pacific Plate under the Australian Plate was succeeded in the Early Miocene by eastward subduction of the Australian Plate beneath the Pacific. Variations in the geometry of this boundary reactivated basement faults and deformed the sedimentary section. There has been a number of important recent discoveries on the northern Lord Howe Rise that possibly reflect the generation, migration and entrapment of hydrocarbons, including: • •

•

•

Low-level oil slicks and films identified in satellite synthetic aperture radar (SAR) imagery. Indications of hydrocarbon seeps in the form of: pockmarks in the seabed; gas chimneys characterised by velocity push-down; hydrocarbon related diagenetic zones (HRDZ), emphasised by velocity pull-up; fiat spots; and buried mounds. 'Bottom-simulating reflectors' (BSRs) have been identified in seismic data in the Fairway basin and elsewhere on the LHR; BSRs are normally considered to be diagnostic of the presence of gas hydrates. A large field of diapirs (either salt- or mud-cored) beneath the Fairway Basin. These diapirs show a broad correlation with the area where the BSR is observed.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

VOLCANIC HISTORY OF THE BUCHAN RIFT, SOUTHEASTERN LACHLAN FOLD BELT A.H.M. VandenBerg Geological Survey of Victoria Geological Survey of Victoria, P.O. Box 500 East Melbourne, Victoria 3002 The Buchan Rift is a rift basin with a complex history of Early Devonian bimodal (mainly felsic) volcanism and carbonate sedimentation. The rift history can be divided into four phases (VandenBerg et al., 2000), of which the last, deposition of the marine Buchan Group, is not considered here. Early rift history. Exposure of the early units is limited. The Wombargo Subgroup along the western margin began with high-energy fluvial clastics deposited in a braided fan with increasing volcaniclastic content. A much more complex history in the south of the rift included development of a large fan delta on the western margin, a major marine phase with a shifting shoreline, and felsic volcanism producing a very thick pile of monotonous ignimbrite on the eastern margin. The Ninnie Subgroup in the extreme south records the transgression that occurred during initial rifting, when subsidence outstripped sedimentation. Volcanics include pyroclastic ignimbrites and their submarine turbiditic equivalents. The Nowa Nowa Conglomerate was deposited on the flank of a large submarine rhyolite cryptodome, the Castor Oil Lava. The Timbarra Subgroup farther north shows a strong lateral facies change, ft-om deeper water sediments in the south to fluvial in the Mount Johnson area in the north. The overall picture is of an initial, brief pre-volcanic phase of deposition in a braided stream system on a large fan delta (Wilkinson Creek Cgl and Windarra Fm), before prolonged volcanism in a rift that deepened to the south. Deposition of submarine mass flows and turbidites (Johnson Mudstone) was contemporaneous with subaerial volcanism and sedimentation farther north. The fan delta sequence fills a deep valley cut into the western rift margin. The southern part of the fan was drowned in a transgression that indicates that subsidence outpaced deposition. Mass-flow deposits and shale comprise the main marine unit on the west of the rift (Johnson Mudstone). To the north of the fan, terrestrial conditions persisted with deposition of breccia, ignimbrite, fluvial conglomerate and lacustrine turbidites. A dextral transtension component of rifting produced two discrete grabens along the eastern rift margin, both filled with mainly ponded material. The Bengal Graben is entirely filled with the very thick Tulloch Ard Ignimbrite. The Wairewa Graben in the southeast was an important source of distincive salmon-coloured quartz ignimbrites through most of the rift's southern history. The Tara Range Subgroup, which fills the graben, is entirely subaerial in the north of the graben but changes to subaqueous mass-flow deposits to the south. The middle phase of volcanism in the rift is marked by mainly ignimbrites and minor fluvial sediments. The Marroo and Berrmarr subgroups both contain rift margin breccias that reflect the high relief of the rift margins. Ignimbrite in the Marroo Subgroup may have erupted ft-om a single volcanic centre. The Mount Dawson Subgroup is an association of ignimbrite, lenticular conglomerate and breccia that suggest deposition in a structural depression. Prolonged erosion occurred after the middle phase. In the centre of the rift the Devils Den Conglomerate, mostly derived ft-om outside the rift, was deposited in a high-energy environment. The Little River Subgroup represents the final phase of volcanism. It is by far the most varied of the Snowy River Volcanics subgroups, with a complete spectrum of volcanic environments represented. Three major centres of volcanism produced the eruptions which formed the thick, widespread ignimbrites of the subgoup: the Woongulmerang Cauldron in the north, which produced the Gelantipy Ignimbrite; the Wairewa Graben in the east which erupted quartz ignimbrites, and an unidentified centre or centres in the south which erupted voluminous feldspathic ignimbrites. Numerous glassy feldspar ignimbrites were erupted fi-om an unknown source in the south, with some units containing turbidites indicating that part of the rift here was again marine. Mount McLeod was a topographic high at the southern end of the Meadow Creek Fault Zone, fi-inged by thin basal granite conglomerate fan and talus deposits. The upper part of the Little River Subgroup represents mainly non-marine sedimentation with localised eruption of bimodal and minor trachytic lavas and shows a range of shoreline facies that heralded the deposition of the fully marine Buchan Group carbonates. References VANDENBERG A.H.M., WILLMAN C.E., MAHER S., CAYLEY R.A., TAYLOR D.H., MORAND V.J., SIMONS B.A., MOORE D.H. & RADOJKOVIC A. 2000. The Tasman Fold Beh System in Victoria. Geological Survey of Victoria Special Publication.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PLUTONIC GOLD DEPOSIT N. Vickery Division of Earth Sciences, University of New England, Armidale NSW 2351 Plutonic is an Archaean greenstone hosted orogenic lode-gold deposit situated in central WA. The deposit is hosted exclusively by lower amphibolite facies metabasalts of the Plutonic Well Greenstone Belt within the Marymia Inlier. The origin of the Marymia Inlier, now surrounded and partially overlain by Palaeoproterozoic rocks of the Capricorn Orogen, is contentious. Consisting of two greenstone belts with surrounding granitic and gneissic material, the Marymia Inlier is superficially similar to terranes within the Yilgam Craton. Dating thus far indicates a Yilgarn-type age. McMillan (1996) interpreted ages of 2.72Ga and 2.69Ga of surrounding granitoids and porphyry intrusions within the greenstones in the Marymia region and Pb isotopic compositions of galena in mineralized zones at the Marymia and Triple P deposits consistent with the ca 2.63Ga mineralising event in the Yilgam Craton. Previous workers have attempted correlation of the Marymia Inlier to the West Yilgam, Southem Cross and Eastem Goldfield superterranes, however large discrepancies exist with all three, including age differences with the Eastem Goldfields and stratigraphic contrasts with the West Yilgam and Southem Cross superterranes. Locally the greenstone stratigraphy is dominated by thick altemating sequences of multiple tholeiitic metabasalt and metakomatiitic flows. Individual units are up to several hundred metres thick and contain narrow intermediate to felsic volcanic and porphyritic intmsive units and interflow sediments. A complex relationship exists between the greenstone rocks and the enveloping granitoids. Along the N W contact proximal to the Plutonic deposit, the granite is thrusted over the greenstone sequence at approximately 4050°, dipping to the NW. Elsewhere granite-greenstone contacts are more difficult to interpret and are in part tectonic and intrusive. In contrast to the Marymia deposits at the NE end of the Plutonic Well Greenstone Belt in which mineralization is hosted by mafic, ultramafic, felsic and sedimentary rocks including BIF, Plutonic and several smaller deposits in the westem end of the belt are hosted exclusively by metabasalts. Despite the multiplicity of mafic units in the area, Plutonic is hosted by one unit only, the Mine mafic, which is sandwiched between two thick metakomatiitic units and contains laterally continuous narrow sedimentary marker horizons. Mineralization at Plutonic is characterized by a series of stacked replacement-style lodes, individually up to 5m wide that are hosted within ductile shear zones oriented slightly oblique to stratigraphy. Lodes are characterized by intense banding, defined by cmde mineral segregation and mineral alignment. There is a conspicuous lack of quartz veining associated with mineralization except where early veins pre-dating mineralization, have intersected the ductile shear zones and have been subsequently deformed. Alteration of the interpreted peak metamorphic assemblage has resulted in the development of a ?lower amphibolite facies hydrothermal assemblage consisting of plagioclase-biotite-whitemica-quartz-amphibole-titanite-carbonatearsenopyrite-pyrrhotite-tourmaline-pyrite±scheelite ±gold±sphalerite. Retrogression of the alteration assemblage is evidenced by the replacement of amphibole and biotite by chlorite and the sericite iclinozoisite/epidote replacement of plagioclase. Alteration leading to the partial to complete replacement of the peak metamorphic assemblage and the obliteration of associated fabrics indicates that the Archaean gold mineralising event post-dated peak metamorphism. Gold where visible, is intimately associated with arsenopyrite and pyrrhotite. A considerable amount of gold is refractory, probably held within arsenopyrite grains. Acknowledgement: Homestake Gold of Australia is thanked for permission to publish. References McMillan N.M. 1996. Late-Archaean, syn-amphibolite facies, lode-gold deposits overprinted by Palaeoproterozoic deformation, metamorphism and hydrothermal activity at Marymia, Westem Australia. PhD thesis. University of Westem Australia Perth (unpubl.).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE JIAN CHA LING Ni-Au-DEPOSIT: LISTVENITE-HOSTED, EPIZONAL GOLD MINERALIZATION IN THE WEST QINLING MOUNTAINS, CENTRAL CHINA R.M. Vielreicher^ N.M. Vielreicher^ S.G. Hagemann' and G. Jones^ ' CGM, Dept. of Geology and Geophysics, The University of Western Australia, Nedlands, WA 6907 ^ Sino Mining Australia Pty Ltd, 16/60 Carrington Street, Sydney, NSW 2000 Introduction The Jian Cha Ling (JCL) Ni-Au deposit is located about 250 km SW of Xi'an, in the SW of Shaanxi Province, central Peoples Republic of China. It lies within the western Qinling metallogenic province, and contains in excess of 250,000 oz of gold. The JCL deposit is the first mine designed, constructed, financed and operated to international standards in China. The deposit was discovered following regional rock and soil geochemical surveys and diamond drilling in 1988, and the first gold was poured in October 1998. Regional and local geology The deposit is situated along the southern border of the E-W trending Qinling-Dabie orogenic belt which resulted from collision between the Archaean North China and South China Blocks. The Shangdan Suture Zone is the trace of N-ward subduction of the South China Block under the North China Block during the Ordovician, marking the boundary between the North and South Qinling orogen. The newly recognized Mianlue Suture Zone is the "scar" of the closure of an ocean that was formed during Mid-Silurian rifting within the South China Block and subsequent, N-directed subduction under the South Qinling orogen. Both sutures are characterized by granitoids and mafic to ultramafic complexes with ophiolitic affinities. The JCL Ni-Au mine is sited at the WNW trending Mianlue Suture Zone, within a folded and faulted, triangular-shaped, uplifted block of Archaean and Proterozoic rocks. This block is bounded by the South Qinling orogen to the N, the South China Block to the SE, and the Palaeozoic Songpan-Ganzi Basin to the W. Host rocks at JCL The dominant host rock is an ultramafic lithology for which petrographical investigations and geochemical analyses indicate a harzburgitic/dunitic parent rock. Earlier serpentinization overprinted by gold-related hydrothermal alteration processes produced rocks that show characteristics of listvenites. At JCL, these rocks are dominated by Mg-Fe-Ca carbonates, as well as lizardite and chrysotile, talc and ftichsite with minor amounts of quartz, and hypogene Cr-rich clays and carbonaceous matter; accessory mineral phases include Cr-spinel and magnetite. Substantial Ni-mineralization is also hosted within the same ultramafic body. Younger, overlying sedimentary rocks, dominated by dolomite with additional limestones and phyllitic schists, host low grade and very patchy, subeconomic gold mineralization. Structural framework Gold mineralization at JCL is controlled by a generally WNW striking and NNE dipping fault zone which shows overall reverse movement, as indicated by mapped offsets of stratigraphy and the development of extensional vein sets. Local sinistral and ±dextral oblique slip is evident where the fault varies in strike. This fault, termed Fj'^^ is expressed as a series of subparallel, brittle to brittle/ductile dislocation surfaces, which vary both in strike (NW to WSW) and dip (20° to vertical). As a result of these variations, the listvenite host rock forms rhomboid-shaped lozenges that have dimensions of up to 20m, with long axes plunging shallowly to the W, subparallel to regional fold axes. Such boudinaged bodies are restricted to within the confining fault zone which is only a few metres wide, and locally host extension vein sets that dip perpendicular to steep bounding fault surfaces. The vein sets dip shallowly to the W-SW and are locally composed of "dog-tooth" carbonate, in places with abundant realgar and orpiment. Ore assemblage and geochemical signature Reconnaissance ore microscopic studies indicate an ore assemblage of pyrite-arsenical pyrite-arsenopyrite-realgarorpiment±cinnabar±millerite±gersdorffite. No free gold has been observed, but based on comparison with other highlevel Chinese deposits, an ultra-fine grained (<l|im) distribution associated with arsenical pyrite is expected. Geochemical analyses reveal Ag-As-Hg-Sb-Se-Te-Tl as pathfinder elements which is typical for ultra-fine grained, disseminated gold deposits formed at higher crustal levels. Overall, structural and geochemical investigations suggest that JCL is an orogenic, epizonal lode-gold deposit that, although hosted in listvenites, shows an ore assemblage comparable to sedimentary rock-hosted Carl in type deposits.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention. Sydney, July 2000

CRUSTAL MELT SEGREGATION : THE NEED FOR DEFORMATION Jean Louis Vigneresse' and Jean Pierre Burg^ ' CREGU, BP 23, F-54501 Vandoeuvre Cedex, France also Department of Geology, University of Newcastle, Newcastle, NSW 23008 ^ Geologisches Institut, ETH-Zentrum, Sonneggstrasse. 5, CH-8006 Zurich, Switzerland

We present a numerical model of magma segregation from its matrix under varying deformation type (pure and/or simple shear)and rate. The model, which is Lagrangian in its description, requires an initial melt content and some thresholds that allow melt connection and melt escape. Deformation is the most important parameter, and pure shear is more effective to segregate melt. A coefficient of melt extraction, defined as the ratio of escaping melt to its initial content, is proposed. It is similar to the friction coefficient as defined in frictional experiments. The melt extraction process shows an initial rapid increase or decrease, followed by a more regular steady-state flow under ambient conditions. During this stage, the coefficient of melt extraction remains constant, though departures from its average value suggest irregular motion of melt. This is like stick-slip motion which characterizes frictional motion. The later consists in a succession of slow motion (slip) after a time of no slip (stick) that results of the frictional resistance to a steady traction. Varying the traction rate induces a transient pulse in the coefficient of friction. When parameters that control melt extraction (initial melt content, threshold for melt escape, deformation type and rate) vary, the melt extraction coefficient varies linearly, positively correlated to increasing initial melt content and to increasing pure shear, negatively with the value of the melt escape threshold. Variation with pure shear rate is non linear. A transient state is also observed when the parameters are suddenly changed. It is characterized by an exponential spike that correlates negatively with an initial melt content, and positively with the melt escape threshold. Variations of the amplitude of the exponential spike, when initial conditions are changed, also demonstrate that magma extraction presents some memory effect. Because those parameters (threshold, melt content) do not vary instantaneously by some very large value, deformation, either normal or tangential, is the major controlling parameter for melt extraction. It is required to segregate melt under crustal conditions. While implicit when observing migmatites in nature, it had never been formulated and quantified. Here, we provide an equation, with numerical values, that may quantify the respective parameters that control melt extraction. Melt is produced and extracted at any time and space in migmatites, provided chemistry and temperature conditions are adequate. In consequence, detailed chemical analysis may not be able to decipher the chronology of magma succession. Pure shear appears to be more effective than simple shear to concentrate melt. Since deformation is active during magma ascent and emplacement, it appears now to be active throughout the magmatic cycle.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

REGIONAL STRESS FIELD INTERACTION WITH MAGMA EMPLACEMENT Jean-Louis Vigneresse\ Ameglio Laurent^ and Basil Tikoff ^ ' CREGU, BP 23, 54501 Vandoeuvre Cedex, France also Dept of Geology, University of Newcastle, Newcastle, NSW 2308 ^ Dept of Geology, Rhodes University, POBox 94, Grahamstown 6140, RSA ^ Dept of Geology and Geophysics, University of Wisconsin - Madison, Madison, WI 53706.

Granitic intrusions, as observed from combined structural and geophysical data, allow us to describe two main types of plutons. Flat-floored plutons are rather thin (3-4 km) and extend in horizontal direction with a gently dipping floor toward one root zone. These contrast with the thick (up to >10 km) wedge-shaped plutons, more elongated in one direction, steep walls often associated with shear zones, and floor with no apparent deepening toward a feeder zone. We interpret this two fold discrimination as the result of a switch in the stress pattern caused by the emplacement of magma. Magma is preferentially emplaced into the plane ( a i - a2) perpendicular to the least principal stress component (a3). This plane is initially vertical, except for compressional conditions. This dilation causes a local re-organization of the stress field, by increasing the minor and intermediate principal stress components. When they overcome the lithostatic load, a drastic change in the orientation of the opening plane results, switching from vertical to horizontal. This constitutes a change from vertically-oriented, dike-shaped intrusions to sub-horizontal laccoliths. It is effective in explaining the shape of tabular (flat-floored) intrusions. We extend the model also to wedge-shaped intrusions, for which the model also applies, except it does not, or cannot reach the point of switch to compressional state. This is partly explained by the existence of crustal anisotropy which contributes to modify the shape of intrusions. We suggest that the appearance of rigidity, due to the increasing crystallization, can lead to stress reorientation. However, in the case of a longlasting crystallization interval, the appearance of rigidity is not reached sufficiently early, so magma cannot sustain sufficient stress to cause stress reorientation. The time delay to reach rigidity would explain the formation of wedge-shaped intrusions. In conclusion, we suggest that the feedback between magma intrusion and the local stress pattern controls the geometry of magma emplacement.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DETERMINATION OF TRACE ELEMENTS IN SEDIMENTARY PHOSPHORITES WITH ULTRAVIOLET LASER ABLATION ICP-MS Stephen R. Walsh Resource Science and Management, Southern Cross University PO Box 157 East Lismore, 2480 Analyses of trace elements in sedimentary phosphorites used in fertiliser production are important because many trace elements that are enriched in these rocks can be further enriched during fertiliser production and then transferred to plants and animals. Few detailed studies of trace metals in potential phosphate ores have been published and most published studies involve few samples or present data for a limited range of trace elements. This study uses UV laser ablation ICP-MS with 3mm line bums and calibration against NIST glass standards to provide rapid multi-element analyses of an extensive phosphorite specimen collection from around the world. Because there are no published reports using UV laser ablation to examine phosphorites, no limitations were placed on element combinations during initial multi-element analyses. Selection of up to 30 elements produced unreliable results for a single nodular phosphorite sample compared to data from several other analytical techniques. Results from FIXE analyses were then used for cross-calibration of several major and trace elements determined using UV laser ablation and it was found that three groups of elements need to be analysed separately. Although elements, whether as major or trace elements, within any one group can be determined in any number or combination, elements from other groups cannot be reliably determined at the same time. Group 1 includes any element not in Period 4 of the Periodic Table, the other two groups consist of particular elements from Period 4. Membership of groups is probably related to one or more of the following: 1) saturation of various mass numbers in Period 4 that coincide with argon isotopes, 2) interference from argides and doubly charged ions that form during analysis from phosphorus, calcium and other light elements in the phosphorites, and 3) the effect of the high energy/frequency UV laser. Raw data are adjusted using results from analytical techniques such as PIXE or SEM microprobe to allow for the variable amount of sample that is ablated; phosphorus is used to calibrate raw data from Group 1 and iron is used to calibrate data from Groups 2 and 3. Concentrations of trace elements differ widely between bedded, nodular, pelletal, hardground and guano-derived phosphorite types. Guano-derived phosphorites, used in Australian fertilisers for over 50 years, have high concentrations of Cd (average 90 mg/kg) and Zn (850 mg/kg) and are now largely replaced by bedded phosphorite, which has lower Cd (<15 mg/kg) but higher Pb (>200 mg/kg) concentrations, from the Georgina Basin. Detailed examination of Im profiles from Nauru Island phosphate rocks show that Cd and Zn concentrations increase with depth, whereas the concentrations of metals such as Cu, Mn, Ni, Co, Cr and Fe remain relatively constant with increasing depth. Furthermore, trace element analyses of soils from the Nauru Island RDF rehabilitation study site show that average Cd concentrations are twice that of average rock phosphate; the concentrations of other trace elements do not vary between sites. Bedded phosphorites around the world are enriched in a range of trace metals but they appear to have an original trace element signature that is low in most trace elements; however, in many areas the original signature has been over-printed by Mississippi Valley-type solutions. Nodular, pelletal and hardground phosphorites have low Cd concentrations (generally <2 mg/kg) but are enriched in U (>100 mg/kg). Preliminary data suggest that the trace element content of different phosphate ore types is sufficiently distinct that trace element composition may be useful in classifying phosphates with different genetic and diagenetic histories.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE NATURE OF THE THIRD DIMENSION IN MINERALISING SYSTEMS John L. Walshe, Bruce Hobbs\ Alison Ord ^ and Nick Archibald^ ^AGCRC, CSIRO Exploration and Mining, PO Box 437, Nedlands, WA 6009 ^ Fractal Graphics Pty Ltd, PO Box 437, Nedlands, WA 6009

Major hydrothermal ore deposits are commonly associated with faults or lineaments that may be traced for tens to thousands of kilometres on the earth's surface. Dimensional argument suggests that the third dimension of hydrothermal systems centered on such structures could be of the order of tens of kilometres. Increasingly seismic and gravity studies are identifying crustal-scale structures that transect the crust and penetrate the mantle. What is the role of these deep-seated structures? Are they simply the more fundamental elements of mid to upper-crustal architectures in which hydrothermal systems operate, utilising sources of fluids available at these crustal levels? Or are these structures conduits for fluids from the lower crust and mantle and do such fluids contribute significantly to ore forming processes in the mid- to upper crust? The lower crust is likely to contain reservoirs of water-poor fluids, some of which may be enriched in CH4 rather than CO2 with H2, N2 and H2S and/or coexist with salt melts. Such fluids could develop at times of re-hydration of the lower crust or serpentinisation of the mantle wedge by C02-bearing surface waters. Re-hydration of the lower crust or mantle, under conditions of low fluid-rock ratio, will lead to H2O depletion and CH4 enrichment relative to CO2 according to the reaction 4H2 + CO2 ^ CH4 + 2H20hydration The process will be constrained by the redox state within the mantle/lower crust and is possible in the absence of silicate melt. One likely tectonic environment for re-hydration of the lower crust and upper mantle in the absence of melting is the flat-slab architecture within convergent margin settings. Fluids dominated by CH4, CO2, N2, H2S and H2 with relatively minor amounts of water have the potential to dissolve metals as organo-metallic complexes. Some of the possibilities are Pb and Zn amide complexes, Au, Zn, Cd and Pd cyanide complexes and carbonyl complexes of Mo, W, Fe, Ni, V , Cr, Re, Ru, Os, Ir, and Rh. The metal transporting processes, in the deepest parts of the giant mineralising systems will differ from the processes that operate in the upper levels of these systems. The domain of transition from the anhydrous to the hydrous environment is potentially an oreforming environment as complexes, stable in the anhydrous domain, degrade. Gold deposits in granulite facies terrains could be examples of this process. Ore formation may also occur within the anhydrous environment providing the necessary geochemical gradients are established to promote mineral precipitation. The Broken Hill Pb-Zn deposit is possibly a product of mixing two anhydrous fluids at peak metamorphic conditions; one transporting Pb and Zn as amine complexes, the other enriched in sulfur.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney, July 2000

A LOAD OF OLD COBBLES - EVIDENCE FOR RECYCLING OF A TERRANE THE TORLESSE TERRANE, SOUTH ISLAND, NEW ZEALAND A.M. Wandres', J.D. Bradshaw', S.D. Weaver' and T.R. Ireland^ 'Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand ^Department of Geological and Environmental Sciences, University of Stanford, USA

The Torlesse superterrane is the largest of New Zealand's Paleozoic to Early Cretaceous terranes. The accretionary complex is dominated by indurated matrix-rich feldspatholithic sandstone, mudstone and subsidiary conglomerates plus minor oceanic assemblages. In its broad geological characteristics it resembles many other weakly metamorphosed sandstone dominated Mesozoic accretionary complexes around the Pacific Rim, including the Le May Group on the Antarctic Peninsula, the Complejo Duque de York in Chile and others in California, Alaska and Japan. Two terranes in the South Island are recognised, the Permian to Late Triassic Rakaia terrane and the Late Jurassic to Early Cretaceous Pahau terrane. The Torlesse superterrane is widely accepted as allochthonous and not derived from the continental crust of western New Zealand. Torlesse sources to the east of New Zealand, in Marie Byrd Land and more recently Queensland have been proposed. McKinnon (1983) proposed the Rakaia rocks as a likely source for the younger Pahau terrane, however Adams and Graham (1997) have noted that the Sr isotopic composition is incompatible with simple Rakaia recycling. We collected 350 igneous and 70 sandstone clasts from seven conglomerate locations from the two South Island terranes. First results are presented here. Geochemistry and petrography of 70 sandstone clasts from four conglomerate locations are compared with petrography and whole rock geochemistry of the detrital rocks of the Pahau and Rakaia terranes. Igneous clast U-Pb zircon ages are presented for a variety of igneous conglomerate clasts from the seven locations. Distribution patterns of igneous clast zircons indicate that the two terranes have been exposed to different sources. New sedimentary UPb zircon ages from two Rakaia and one Pahau terrane locations are presented. Depositional ages together with the sedimentary zircon distribution patterns for the two Rakaia localities indicate a contemporaneous igneous input with recycling of older material. The sedimentary zircon distribution pattern for the Pahau locality strongly suggests the recycling of Rakaia rocks and a contemporaneous input of Lower Cretaceous igneous detritus.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

SIGNIFICANCE OF IGNEOUS CLASTS - THE TORLESSE TERRANE, SOUTH ISLAND, NEW ZEALAND A.M. Wandres\ J.D. Bradshaw\ S.D. Weaver^ and T.R. Ireland^ 'Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand ^Department of Geological and Environmental Sciences, University of Stanford, USA

The New Zealand Eastern Province terranes comprise portions of a volcanic arc, arcrelated basins, slope basins and an accretionary complex, which originated along the margin of Gondwana. The Torlesse superterrane, the accretionary complex, comprises an enormous volume of quartzo-feldspathic sandstones and mudstones with subsidiary conglomerates plus minor oceanic assemblages. Two terranes in the South Island are recognised, the Permian to Late Triassic Rakaia terrane and the Late Jurassic to Early Cretaceous Pahau terrane. The Torlesse superterrane is widely accepted as allochthonous and not derived from the continental crust of western New Zealand. Studies in recent years in detrital petrology and geochemistry have established the broad type of source, and point to derivation from a continental magmatic arc or active continental margin. These methods however are incapable of establishing a specific source. Studies in petrology and geochemistry, isotope geochemistry, and dating of zircons by the U/Pb SHRIMP method of igneous clasts in conglomerates have the potential to define the character of the source more clearly. Preliminary results are presented here. The geochemistry of 350 igneous clasts from seven conglomerate locations in the South Island shows geochemical signatures typical of subduction-related magmas. Geochemistry of the igneous clasts is compared with whole rock geochemistry of the detrital rocks of the South Island Torlesse terranes. Major and trace element compositions of the igneous clasts and the detrital rocks show remarkable similarities and suggest that the igneous clasts represent the source lithologies. lon-microprobe (SHRIMP) U-Pb zircon ages are presented for a variety of igneous conglomerate clasts from seven locations in the two terranes. Plutonic and volcanic rocks from the older Rakaia terrane give ages from the Late Devonian to the Late Triassic and one clast has an upper Middle Cambrian age. The clasts range in composition from granodiorite to monzogranite and trachydacite to rhyolite. Samples from the younger Pahau terrane have an age range from the Late Jurassic to the Early Cretaceous with one clast from the early Middle Jurassic. Compositions range from granodiorite to syenogranite but are dominated by rhyolites.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney, July 2000

MINERAL MATTER IN COAL SEAMS: FOREIGN INVADER OR INDIGENOUS COMMUNITY? Colin R. Ward School o f Geology, University o f N e w South Wales, Sydney, 2 0 5 2

Mineral matter in coal embraces dissolved ions in the pore water, inorganic elements associated with the organic compounds, and crystalline or true mineral particles. Even a relatively low-ash Australian bituminous coal contains approximately 15% inherent mineral matter. This means that the New South Wales coal industry currently produces more than 15 million tonnes per year of mineral material, a fraction about which surprisingly little is known. Mineral matter represents the source of most of the problems associated with using coal, and thus has significant implications for the mining, preparation, and marketing of Australian coal deposits. Based on the processes that form them, the minerals and other inorganics in coal can be identified as either foreign invaders (introduced contaminants) or indigenous communities (inherent peat-swamp components) with respect to the sediment that makes up the coal seam. The minerals in Australian coals include quartz and a range of clay minerals (especially kaolinite), along with different types of carbonates, sulphides, phosphates and other constituents. Their mode of occurrence within the coal and their three-dimensional distribution in the seam allows the different minerals and non-mineral inorganics in individual seams to be identified as combinations of biogenic constituents (e.g. diatoms, phytoliths); organically-associated inorganics (e.g.dissolved salts, exchangeable ions); chemical precipitates (e.g. petrifactions, nodules, cleat infillings); detrital or pyroclastic contaminants. The minerals in coal can be identified by X-ray diffraction, scanning electron microscopy and related techniques. Modem developments allow these methods to be used for quantitative assessment of mineral proportions, as well as simply for mineral identification. The quantitative results have been found to be consistent with the chemical composition of the ash derived from the same coal samples. The mode of occurrence of the different nonmineral inorganics can be investigated by selective chemical leaching techniques. Especially with the capacity for quantitative assessment, mineral matter studies are being used to investigate problems in coal handling and preparation, or in marketing and utilisation for combustion and coking applications. They can also be used as an aid in seam correlation. The abundance of particular trace elements can be related to particular minerals in the coal seam, allowing mineral matter studies to play a significant role in assessing the environmental impact of coal mining, utilisation and waste disposal. Modem methods of investigation, coupled with recent advances in the understanding of coal-forming environments and sedimentary basins, allow geologists to make new and increasingly valuable contributions to coal deposit evaluation, and to the understanding of coal formation processes. These advances, many of which have been sourced from Australia, are helping to perpetuate the long and outstanding contribution of Ken Mosher as a leader in the application of geological science to the Australian coal industry.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

QUANTITATIVE MINERALOGICAL ANALYSIS OF SANDSTONES USING X-RAY DIFFRACTION TECHNIQUES Colin R. Ward' and John C. Taylor^ ' School of Geology, University ofNew South Wales, Sydney, NSW, 2052 ^ CSIRO Energy Technology, PMB 7, Menai, NSW, 2234

X-ray diffraction has long been used as a definitive technique for mineral identification, based on the measuring the internal atomic or crystal structures present in powdered rocks, soils and other mineral mixtures. Recent developments in data gathering and processing, however, have provided an improved basis for its use as a quantitative tool, determining not only the nature of the minerals but also the relative proportions of the different minerals present. The mineralogy of a series of sandstone samples from the Sydney and Bowen Basins of eastern Australia has been evaluated by X-ray diffraction (XRD) on a quantitative basis using the Australian-developed SIROQUANT data processing technique. Based on Rietveld principles, this technique generates a synthetic X-ray diffractogram by adjusting and combining full-profile patterns of minerals nominated as being present in the sample, and interactively matches the synthetic diffractogram under operator instructions to the observed diffractogram of the sample being analysed. The individual mineral patterns may be refined in the process, to allow for variations in crystal structure of individual components or for factors such as preferred orientation in the sample mount. The resulting output provides mass percentages of the different minerals in the mixture, and an estimate of the error associated with each individual percentage determination. The chemical composition of the mineral mixtures indicated by SIROQUANT for each individual sandstone studied was estimated using a spreadsheet routine, and the indicated proportion of each oxide in each sample compared to the actual chemical analysis of the same sandstone as determined independently by X-ray fluorescence spectrometry. The results show a high level of agreement for all major chemical constituents, indicating consistency between the SIROQUANT XRD data and the whole-rock chemical composition. Supplementary testing with a synthetic corundum spike further suggests that little if any non-crystalline or X-ray amorphous material is present in the sandstone samples. Mineralogical analysis of the sandstones by X-ray diffraction gave results that were generally different from those obtained by point count analysis of the same sandstone samples. This is because different types of constituents were analysed in each case. Total quartz, for example, determined by SIROQUANT, is typically higher than the proportion of visible quartz grains identified under the microscope by point counting, since quartz is also present in the rock fragments and the matrix components. Analysis of the relationships between mineralogical (XRD) and petrographic (point count) data allows the abundance of quartz, feldspar, mica and clay minerals to be estimated for the rock fragments and matrix constituents, components that are identified but not mineralogically evaluated in thin section studies. Quantitative X-ray diffraction provides data that complement the information obtained from conventional point count studies. Such XRD data have been related to the relative propensity of different sandstones to ignite methane in coal mine atmospheres by rock friction, and may well be related to abrasivity, cuttability and other geotechnical properties. 519

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE TASMAN FOLD BELT SYSTEM IN THE LATE NEOPROTEROZOIC: INSIGHTS FROM STUDY OF MT ARROWSMITH VOLCANICS

AND ATTUNGA ECLOGUE T. Watanabe^ E. Kitayama', S. Sano^ R. Offler^ E.G. Leitch^ G.M. Fanning^ Y. Orihasi ^ and T. Nishiya^ ^Division of Earth and Planetary Sciences, Hokkaido University, Sapporo, Japan. ^Geology Laboratory, Faculty of Education, Ehime University, Matsuyama, Japan ^School of Geosciences, The University of Newcastle, Callaghan, NSW, Australia. "^Environmental Sciences, University of Technology, Sydney, Broadway, NSW, Australia ^Research School of Earth Sciences, Australian National University, ACT. Australia ^ Current address: Research Institute of Earthquake, Tokyo University, Bunkyo-Ku, Tokyo Near synchronous volcanism in the Mt Arrowsmith area of western NSW and formation of eclogite near Attunga in the southern New England Fold Belt are indicated by c. 580 Ma U-Pb SHRIMP ages (Crawford et aL 1997; Watanabe etal. 1998). Sm-Nd data for the Mt Arrowsmith volcanics indicate that magmas were derived from two sources. Tholeiitic basalts have low 8 Nd (+2 to + 4.5 ) and Nb/Zr ratios(0.5 - 0.8) relatively close to values obtained from continental lithosphere in central Australia, metasomatised by Mid-Proterozoic mantle-plume activity (Zhao & McCulloch, 1993), whereas alkaline rocks show higher 8 Nd values (+4 to +5.5) and variable Nb/Zr ratios (0.12 - 0.25) indicating derivation from oceanic mantle. This suggests that the Mt Arrowsmith volcanism occurred close to the Neoproterozoic continent-ocean boundary that developed during the breakup of Rodinia. The Attunga eclogite occurs in serpentinite melange associated with the Peel Fault. Geochemical data suggest that it was derived from a MORB-like protolith which was subducted to depths of ca.35-40 km. Similar, but as yet undated rocks, occur 80 km south at Gleneden. Sodic amphibole and pyroxene formed during the early stages of subduction of these rocks are preserved as inclusions in garnet porphyroblasts. A Sm-Nd age of 535Ma has been obtained from amphibolite phacoids also found in serpentinite melange from southern New England. These rocks show 8 Nd values that are scattered and low (-7 to -12), LREE abundances that are 15 times chondrite and HREE abundances 2 times chondrite. These values resemble those for mature island arc or continental crust and the amphibolite may indicate growth of the arc system. Middle Ordovician gabbro with the composition of high-Mg basaltic andesite found intimately associated with the Attunga eclogite suggests Early Palaeozoic arc activity. On the assumption that orogen-parallel displacements have not grossly changed the relative positions of the New England and Mount Arrowsmith regions, we propose a model involving rifting, formation of an ocean basin and subsequent development of a frontal subduction system during the late Neoproterozoic. The arrangement of tectonic elements was probably similar to that of the present-day Philippine Sea region, the western part of which is essentially continental crust and the eastern part of which is oceanic crust with a frontal oceanic arc. References

CRAWFORD,A.J., STEVENS, B.P.J. & FANNING, M. 1997. Geochemistry and tectonic setting of some Neoproterozoic and Early Cambrian volcanics in western New South Wales. Australian Journal of Earth Sciences, 44, 831-852. WATANABE, T., FANNING, C. M. & LEITCH, E.C. 1998. Neoproterozoic Attunga eclogite in the New England Fold Belt. Geological Society of Australia, Abstracts, 49, 458. ZHAO, J-X. & MCCULLOCH, M.T.I 993. Melting of a subduction-modified continental lithospheric mantle : Evidence from Late Proterozoic mafic dike swarms in central Australia. Geology, 21, 463-466.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AMS FABRIC IN GRANULITES: MOUNT HAY-MOUNT CHAPPLE REGION, ARUNTA BLOCK, CENTRAL AUSTRALIA C.L. Waters. ^ B. Tikoff ^ and P. Kelso ^ ^Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, WI, 53706, USA ^Department of Geology and Physics, Lake Superior State University, 650 West Easterday Avenue, Sault Ste. Marie, MI, 49783, USA

An Anisotropy of Magnetic Susceptibility (AMS) study of granulite-facies gneisses from the southern Arunta Block, Northern Territory, was undertaken to determine the relationship of the magnetic and field fabrics in this area. Samples were taken from the Mount Chapple-Mount Hay region (northwest of Alice Springs) which is composed of Proterozoic granulites uplifted along the Paleozoic Redbank shear zone. Sampling avoided late-stage shear zones and migmatized zones. The average field foliation of the granulites strikes WNW-ESE and dips steeply SW, and average field lineation plunges steeply NE (061, 68 ±8^). The dominant magnetic mineral in these samples is relatively pure, pseudo-single to multidomain, magnetite (Kelso, 1993). The AMS analyses are compared directly with field lineation and foliation measurements at 107 sites. Magnetic susceptibility ranges from 0.00000251-0.144 (SI), with a median susceptibility of 0.018 (SI) (Kelso, 1993). Dimensionless parameters describing the magnetic anisotropy ellipsoid are Pj (anisotropy of fabric) and Tj (shape of fabric; positive = oblate, negative = prolate). Pj values range from 1.09-2.60, with an average of 1.36. Tj values range from -0.62-0.74 , with an average of 0.09. We will compare these AMS results to the observed field fabrics (i.e., N-dipping foliation of Redbank mylonites).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MET AMORPHIC PROCESSES REVISITED: ASSESSING THE EXTENT OF DISEQUILIBRIUM IN PROGRADE REACTIONS IN METAPELITES FROM THE BUSHVELD AUREOLE David J. Waters and Dan P. Lovegrove Department of Earth Sciences, University of Oxford, Parks Road, Oxford, 0 X 1 3PR, UK. Although the equilibrium behaviour of many metamorphic systems can now be predicted, the displacement from equilibrium required to drive critical reaction processes is difficult to estimate. This study of the aureole beneath the eastern lobe of the Bushveld Complex, South Africa, takes advantage of a field setting of relatively simple geometry that allows the temperature distribution and temperature-time history of the sampled material to be predicted, and a thermodynamic data set of adequate reliability for predicting the equilibrium phase relationships among metapelite minerals. The rocks display a detailed micro structural record of prograde porphyroblast growth and reactions among chloritoid, chlorite, biotite, staurolite, andalusite, cordierite and garnet, allowing an assessment of the kinetic controls on prograde reactions over a P-T range (ca. 450 - 650°C, 3 kbar) and time scales (ca. 1 Ma) comparable to those of regional metamorphism, but in the absence of significant directed stresses. The microstructures provide evidence for two significant disequilibrium controls on reaction mechanism and microstructure. Critical overstepping for porphyroblast nucleation Modelling the interplay of kinetic rate laws suggests that the crystal size distribution (CSD) of a porphyroblastic index mineral is sensitive to three principal factors: the critical overstep at which nucleation begins, the rate constant for the limiting crystal growth mechanism, and the rate of heat supply. Thus, for a given mineral species and reaction process the CSD should be largely a fiinction of heating rate. We measure the slope of the CSD in the larger size fractions, representing the acceleration of the nucleation rate in the earlier-formed crystals. This parameter shows a systematic variation with distance from the igneous contact, for both andalusite and garnet. These observations yield a series of estimates for the relationship between the critical overstep and the rate of crystal growth. Values extrapolated from experimental reaction studies suggest that the critical overstep for nucleation of andalusite may be around 5 K. Control of reaction sequence by slow dissolution of refractory phases The observed sequence of mineral growth shows marked departures from the theoretical sequence of reactions, particularly in that refractory porphyroblasts, once grown, are not consumed by the predicted later reactions, or persist as partial pseudomorphs over a significant grade interval. In these cases, the pseudomorphs approximately conserve volume, but do not conserve A1 or any other "immobile" chemical species, so that in different parts of the same thin section, chloritoid, for example, may be replaced entirely by quartz, or entirely by andalusite. These features suggest that: 1. 2.

The reaction sequence and the overall rate of critical reaction steps are controlled by the slow dissolution rates of certain phases. As a general principle, low variance (e.g. model univariant) reactions tend not to occur, and are replaced by reactions of higher variance which do not involve one or more of the more refractory phases.

In some cases, an estimate can be made of the chemical overstepping withstood by the refractory phase.

522

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 IAustralian

Geological Convention, Sydney, July 2000

MINERAL WATER COMPOSITION AND GROUNDWATER FLOW IN COLD C02-BEARING MINERAL SPRINGS, CENTRAL HIGHLANDS, VICTORIA, AUSTRALIA Tamie R. Weaver', Ian Cartwright^, Sarah Tweed', Douglas Aheame', Michelle Cooper', Kathryn Czapnik', and Joseph Tranter' ' Hydrogeology and Environment Research Group School of Earth Sciences, University of Melbourne, VIC 3010, Australia ^ Department of Earth Sciences, Monash University, Clayton, VIC 3800, Australia Mineral water from the Daylesford region of the Central Highlands, Victoria emanates from approximately 100 spring eyes, has high C 0 2 contents and is naturally effervescent. The mineral waters are also characterised by high H C 0 3 concentrations (up to 1490 mg/L) and are slightly acidic with pH values ranging from 5.83-6.89, consistent with C 0 2 contents of at least 1000-2300 mg/L. Na is the dominant cation (62-675 mg/L), and Ca and Mg concentrations range from -60-215 and - 4 0 - 1 9 0 mg/L respectively. Si, Sr, Ba, and Li are the most important minor and trace elements that characterise these waters indicating that extensive water-rock interaction has occurred during groundwater flow. The predominant bedrock in the area is Ordovician turbidite sequences containing significant amounts of quartz with only trace to minor carbonate present as ankerite spots. Graphite is also present locally. The region forms part of the Pliocene to Recent Newer Volcanic province that has produced thin basalt flows overlying the Ordovician sediments over approximately 20% of the Central Highlands mineral springs region. Si geothermometry indicates that the mineral waters were heated to temperatures of between 45 and 125 °C, and geochemical modelling indicates that the near-surface C 0 2 concentrations would correspond to acidic water with a pH of 4-5 at depth. The combination of above-ambient temperatures and acidic groundwater would promote mineral dissolution leading to the elevated concentrations of Na, Ca, Mg, Sr, Ba, and Li in solution. The chemical composition of water from individual spring outlets has been consistent over the last 20 years; however, spatial variation in the major and minor ion composition of spring water indicates that groundwater flow to individual springs occurs within relatively isolated fracture networks. Parameters that remain relatively consistent in water from different spring eyes include d l 8 0 and d2H values of water (-7.6 to &#64979;6.0%o and &#64979;40 to &#64979;35%o, respectively) and d l 3 C values of C 0 2 and dissolved inorganic carbon (DIC). The d l 8 0 and d2H values lie to the left of the local and global meteoric water lines probably as a result of C 0 2 exsolution near surface preferentially removing 180. The d2H values are lower than those of present-day precipitation in the region, and may indicate that the mineral waters currently being discharged at surface were recharged during cooler climatic conditions that occurred up to 5000 years ago. This indicates residence times of several thousand years for the mineral water flow system which would further enhance the opportunity for interaction between the warm, slightly acidic groundwater and the surrounding sedimentary bedrock. d l 3 C ( C 0 2 ) and dl3C(DIC) values indicate that the C 0 2 is associated with the volcanic activity in the region and their consistency further supports a single, regionally extensive source such as the Recent volcanism rather than more localised water-rock interaction. Acknowledgement: Funding for this research was partly provided by the Victorian Mineral Water Committee, Dept Natural Resources and Environment.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

FROM BIF TO ORE: THE CHEMISTRY OF THE DALES GORGE MEMBER AND SURROUNDING SHALES ACROSS THE HAMERSLEY PROVINCE, WESTERN AUSTRALIA Adam Webb, Gerald R. Dickens and Nicholas H. S. Oliver Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Queensland, Australia, 4811

The Hamersley Province of the southern Pilbara Craton, Western Australia contains several very large deposits of iron ore (>1 billion tonnes). The largest of these orebodies (e.g. Mt. Whaleback, Mt Tom Price, Paraburdoo) occur in the southern part of the province where original banded iron formation (BIF) of the Proterozoic Dales Gorge Member has been folded, faulted and altered to microplaty hematite. The remarkably consistent stratigraphy of the Dales Gorge Member consists of 17 BIF and 16 shale macrobands that can be correlated across the Hamersley Province to within several metres. Although others have examined the chemistry of individual layers (e.g. Trendall & Blockley, 1968, 1970; Ewers & Morris, 1981), to date there has been no systematic study documenting the chemical variation of specific units across the Hamersley Province, from undeformed BIF and shales into deformed equivalents containing iron ore. The colour, composition and structure of BIF and shale horizons in undeformed areas varies significantly from equivalent layers in deformed regions. Moreover, within the Mt. Whaleback pit itself there are clear colour transitions between black (unaltered) and red (altered) shales near ore. These observations suggest that profound chemical changes have occurred to BIFs and shales during ore formation. Using a reference frame in which Ti, Al, Y, Zr and Nb are immobile, altered BIF is consistently depleted in Si, Ba, Sr, Rb, As, Mn and enriched in Fe whereas altered shale samples are consistently depleted in all elements, including Fe. The total mass loss during alteration can be calculated from isocon plots. Using Ti and Y as immobile elements, up to 30% of the BIF macrobands have been removed, primarily through Si loss. In contrast, and non-intuitively, up to 60% of the shale macrobands have been removed. A primary difference between the mass loss of macrobands is that Fe is slightly enriched in BIFs, whereas it is depleted in shales. References EWERS, W. E. & MORRIS, R. C., 1981. Studies of the Dales Gorge Member of the Brockman Iron Formation, Western Australia. Economic Geology, 76, 1929-1953. TRENDALL, A. F. & BLOCKLEY, J.G., 1970. The iron formations of the Precambrian Hamersley Group, Western Australia, with special reference to the associated Crocidolite. Geological survey of Western Australia Bulletin 119. TRENDALL, A. F. & BLOCKLEY, J.G., 1968. Stratigraphy of the Dales Gorge Member of the Brockman Iron Formation. Geological survey of Western Australia Annual Report 1967, pp. 48-62.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

CYCLICITY AT A CARBONATE PLATFORM MARGIN, MENYOUS GAP, PILLARA RANGE, CANNING BASIN, WESTERN AUSTRALIA: IMPLICATIONS FOR REEF DEVELOPMENT AND PALAEOBATHYMETRY Gregory E. Webb and R. L. S. Brownlaw School of Natural Resource Sciences, Queensland University of Technology, Brisbane QLD 4001 and Department of Earth Sciences, University of Queensland, Brisbane, QLD 4072 Metre-scale sedimentary cycles characterise the interiors of many carbonate platforms throughout the Phanerozoic, but the nature of cyclicity has rarely been documented at platform margins. However, welldefined carbonate cyclicity occurs at a platform margin in the Frasnian Pillara Limestone at Menyous Gap, Pillara Range. The margin has a low-relief roll-over morphology wherein horizontal platform beds pass directly into dipping proximal slope beds with a pronounced break in slope. Prominent carbonate cycles in the Pillara platform interior are interpreted as shallowing-up sequences composed of: 1) recessive deep-water facies characterised by the colonial rugosans Disphyllum and Argutastrea, and by branching tabulate corals in a fine carbonate matrix; and 2) more resistant, shallow-water facies composed of tabular to domal stromatoporoids in carbonate muds and sands. Some, but not all, cycles are additionally capped by shallower facies including fenestral limestones. Restricted, possibly lagoonal mudstones occur in the shallow part of rare cycles. Cyclicity at the platform margin is recognized on the basis of alternating recessive-resistant packages. Two resistant facies associations occur. The dominant resistant facies package consists of large, tabular or domal stromatoporoids isolated in growth position in massive muddy to sandy matrix. Stromatoporoids do not form interconnected reef framework. The facies passes abruptly seaward into stromatoporoid-free mudstonesgrainstones that were deposited across the shelf break and on the sloping platform fi-ont. The facies contains plate-like tabulate corals that projected free of the substrate into the water. Neither facies shows evidence of persistent winnowing, although coarse storm deposits occur. The stromatoporoid facies forms a low mound immediately behind the platform margin (stromatoporoid bioherm) and passes into the shallow stromatoporoid facies of the platform interior. Rare cycles have a resistant facies association with massive, stromatoporoid boundstone (stromatoporoids >50% of volume) forming a true, low relief reef framework. The stromatoporoid reef facies passes seaward into the previously described platform front/reef front tabulate coral facies. Towards the platform the stromatoporoid reef facies is capped by oncoids and presumably passes into the shallowest (i.e., fenestral) facies of the platform cycles. The recessive, deep-water parts of cycles consist of tabulate coral-rich facies very similar to the deep-water coral facies of the platform interior, except that Disphyllum does not occur near the margin. The coral-rich facies was deposited across the entire platform, over the edge, and down the proximal slope, with only relatively minor lateral variation near the margin. However, the facies was, in some cases, truncated by erosion at the margin during initiation of the succeeding shallow-water phase. The occurrence of cyclicity over the platform margin and onto the slope in the Pillara Range strongly suggests that relative sea level, and not some autocyclic process, is primarily responsible for the cyclicity. The uniformity of the deep-water facies over the platform edge suggests that water depths were sufficient that margin topography caused little lateral environmental differentiation. The interpretation is supported by the very fine nature of the recessive sediments and preservation in places of delicate corals in growth position. However, strata consisting of mixed broken coral debris suggests that the facies may have been affected by at least some storms. The platform margin caused abrupt environmental zonation (i.e., stromatoporoid bioherm to tabulate coral facies) during deposition of the shallow phase, but the stromatoporoid bioherm otherwise passed monotonously into the uniform stromatoporoid biostromes of the platform interior. Water depths were still apparently below normal wave base even in the shallow phases, which lack extensive winnowing at the margin. The shallowest observed cycles had true stromatoporoid reef framework developed at the margin, but the reefs were still low relief structures forming rollover margins. The shallowest cycles were presumably well within normal wave base, but we have not observed a complete exposure. The large differentiation between deep and shallow cycle phases in all observed cases suggests that the amplitude of sea level change was relatively high (several tens of metres).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 IS"" Australian Geological Convention, Sydney, July 2000

ASSESSMENT OF LANDFILL LEACHATE CONTAMINATION OF GROUNDWATER IN A POROUS SAND AQUIFER John A. Webb Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083

In the southeastern suburbs of Melbourne there are numerous sand quarries in the Tertiary marine sand of the Brighton Group, which forms an unconfmed porous aquifer in the area. One of these was Hned and then used as a council tip for putrescible waste; after the quarry was filled it was capped and a monitoring bore put in place. Adjacent to the capped landfill is another abandoned quarry. The pool of water within the latter quarry is strongly contaminated, and the owner of the quarry asserted that leakage from the adjacent landfill was to blame. Seeps, believed to be coming from the landfill, have been observed around the edge of the pool. Analysis of the contaminated water from the pool in the abandoned quarry showed that it is very acidic (pH 2.7) and contains significant levels of dissolved metals (Fe 230 ppm, A1 78 ppm, Zn 0.6 ppm), along with a small amount of ammonia (NH3 4.3 ppm). The marine sands of the aquifer are black below the water table and contain pyrite. Where the water table has been lowered by sand quarrying, the pyrite is exposed to weathering and oxidation, releasing acidity and dissolved iron into the groundwater. Thus the contaminated water might be deriving its acidity and dissolved metals from sulphide oxidation within the aquifer, and receiving relatively little contamination from the landfill leachate. This is borne out by analysis of the leachate from the monitoring bore, showing that it is almost neutral (pH 7.9) and contains very little dissolved iron (<10 ppm). In order to quantify the relative contributions of the landfill leachate and sulphide oxidation to the contamination, chloride was used as a tracer. This ion is not involved in any reaction likely to be occurring (e.g. sulphide oxidation), and is present in much higher concentrations in the leachate (-3,000 ppm) than in the aquifer (30 ppm). Calculations show that mixing 2.5% leachate with 97.5% aquifer groundwater will give the chloride concentration of the contaminated pool. Mixing leachate and groundwater in this ratio would give 74 ppm NH3 in the contaminated pool, substantially less than the amount present, indicating that natural remediation of this contaminant is occurring. Contamination of water in the abandoned quarry represents almost entirely acid drainage formed by oxidation of sulphides in the aquifer. Although the seal on the adjacent landfill is leaking, the leachate is contributing little to the nearby contamination.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

USE OF WATER CHEMISTRY TO IDENTIFY FLOW CONDUITS IN THE POROUS GAMBIER LIMESTONE, SOUTHEAST SOUTH AUSTRALIA John A. Webb and Stanley Lithco Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 The Oligocene-Early Miocene Gambier Limestone is a largely unconfined aquifer up to 300 m thick that occupies the southeastern comer of South Australia around Mt Gambier. Bedding is horizontal or gently dipping and joints have a dominant northwest-southeast trend. The limestone is porous and has a continuous well-defmed water table that slopes at a low angle towards the coast. Nevertheless it has been postulated that the aquifer contains conduits, because of the very high transmissivity in places, the major springs along the coast, and the presence of water-filled cave systems oriented parallel to the joint direction. Within the Gambier Limestone are a number of cenotes, lying in a broad band oriented northwestsoutheast. Cenotes are collapse dolines containing water-table lakes, up to 50m wide at the surface and extending down to 95m below the land surface. Monthly sampling of the water in the cenotes was carried out over a 12 month period in 1998-1999. From this data the median concentrations of major elements and conductivity were determined. From northwest to southeast water in the cenotes shows a regional decrease in conductivity, mirrored by decreases in major ions (particularly chloride). This reflects the regional more saline groundwater being progressively diluted by direct surface input through the limestone, which is covered by thin soils or porous sands, and in places is exposed as bare pavements. Within this overall trend there are several anomalies. The two Sisters cenotes are separated by a soil-covered rubble pile only 20-40 m wide, yet have quite different water compositions, e.g. the median conductivities are 390 and 480 |LiS/cm. The lower conductivity water presumably contains a higher component of surface runoff. One or both of the cenotes must be fed by conduit flow; if both were fed by porous flow their compositions would be the same. At the southeastern end of the band of cenotes, Gouldens Hole has more saline water than the two neighbouring cenotes. Thus it too must be fed by a conduit, apparently connected to the deeper regional groundwater flow. Furthermore, the water table decreases in gradient towards the coast, and shows a gentle but distinct trough in the vicinity of the southeastern cenotes, with its axis dipping approximately southeast; both characteristics indicate substantial conduit flow in this area. Thus although the Gambier Limestone has high porosity and many of the characteristics of a porous aquifer, much of the groundwater flow through it is probably along conduits. These conduits are likely to follow the regional NW-SE joint direction rather than trend directly southwards to the coast. This shows that even porous carbonate aquifers contain high-K dissolutional networks whose orientation may not be immediately obvious from the potentiometric surface.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE PHANEROZOIC THERMOTECTONIC EVOLUTION OF THE NORTHERN WESTERN AUSTRALIAN SHIELD: EVIDENCE FROM APATITE FISSION TRACK THERMOCHRONOLOGY U.D. Weber^ , B.P. Kohn\ D.R. Nelson^ and A.J.W. Gleadow^ ^Australian Geodynamics Cooperative Research Centre, School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia ^Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia

As part of an ongoing investigation of the thermotectonic evolution of the northern part of the Precambrian Western Shield of Western Australia we have carried out a regional reconnaissance apatite fission track (AFT) study. The study area encompasses Archaean rocks of the Pilbara craton and the northern part of the Yilgam craton including the Narryer Gneiss Complex, and the intervening Proterozoic basins, eg. the Hamersley Basin. Previous U-Pb zircon SHRIMP data and ^^Ar/^^Ar analyses of hornblende, muscovite and biotite suggest that no major thermal events have occurred, and that these regions have remained relatively stable, since Mesoproterozoic time. AFT data, mostly from the Pilbara and northern Yilgam cratons (including five samples previously analysed by Ferguson, 1981), reveal cooling ages ranging between 230±8 Ma and 380±20 Ma. Mean confined horizontal track lengths fall between -^12 and 13 |im with standard deviations ranging from 1.1-2.2 |Lim. Forward modelling of time-temperature history paths for representative samples reveals a period of regional cooling of at least between 350 Ma and 280 Ma. Most paths also show a second period of cooling of =25°C from temperatures <~80-85°C. This later cooling episode occurred in the Mesozoic but its timing is less well constrained. Assuming that the average present day geothermal gradient of-^18±2°C.km-^ was prevalent since the late Palaeozoic, then the minimum of of cooling predicted by the fission track modelling suggests overall denudation of at least -3.7-4.6 km of section since that time. Phanerozoic basins (Perth, Carnarvon and Canning) adjacent to the north and west of the northern Western Shield mostly continue offshore and form complex structures containing up to '-^15 km of predominantly clastic sediments of early Ordovician to late Cretaceous age. The basins are likely to have been depocentres for much of the detritus derived from the denudation inferred from the cooling recorded by the AFT data of the crystalline terranes. Thermal history studies of sediments (based on AFT and VR data) from deep wells in the southern Carnarvon basin have identified the late Palaeozoic as the peak time for petroleum expulsion from lower Palaeozoic source rocks, in response to maximum burial. This timing is supported by our thermal history modelling. A possible causative event which could be linked to the observed late Palaeozoic cooling is tectonism related to the collision of Gondwanaland with Laurussia in Carboniferous time forming the supercontinent Pangea. Further low temperature thermochronological studies, currently in progress, using the ^^Ar/^^Ar and (U-Th)/He systems will shed further light on the post Mesoproterozoic thermal history of the northern Western Shield. Acknowledgments: This work was funded by the Australian Geodynamics Cooperative Research Centre (AGCRC) and the Australian Institute of Nuclear Science and Engineering. This paper is published with the permission of the Director, AGCRC.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PLUTONS AND SHEAR ZONES IN NE BRAZIL: A SELF-ORGANIZING SYSTEM R. F. Weinberg Departamento de Geologia, Universidade Federal de Pemambuco, Recife, Pemambuco, Brazil Now at: Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands WA 6907

Plutons closely linked to crustal-scale shear zones are common world-wide. The Borborema Province of NE Brazil, is characterized by the intrusion of 650-500 Ma Brasiliano plutons and contemporaneous shearing along strike-slip regional shear zones. Plutons in this province are characterized by a tail of sheared sheets within shear zones, linked to an arcuate magma blister intruded into low-strain rocks. Pluton shape asymmetry confirms shear sense as deduced from outcrop scale observations. Magmatic structures are parallel to regionally prevailing directions indicating syn-tectonic emplacement. Shear zones provide natural high permeability pathways for pervasive magma migration. Multiple-sheet tails are interpreted to represent the remains of magma charmels. Magma rose along shear zones and intruded into neighbouring low-strain rocks, giving rise to blisters. The growth of blisters may, most simply, be explained as an instability growth. Small, random fluctuations on the properties of the magma sheet/shear zone system (permeability, rock strength, sheet width) leads to a self-reinforcing process which resuhs in blister growth. The growing blister expands most easily into zones of tension in the surrounding low strain rock, which may more readily accommodate volume expansion. During growth, magma pressure, shearing and strain rate gradients around the shear zone mould the blister into an arcuate body. In the Borborema Province, small plutons (<300km^) have their transport and emplacement largely controlled by regional shear zones. By contrast, large batholiths have led to the nucleation of new shear zones kinematically similar to regional (pluton-independent) shear zones. Whether magma migration is controlled by shear zones or whether magma triggers shear zone nucleation, the system tends inevitably to evolve towards the same geometry. In summary, in the Borborema Province (a) the growth of arcuate magma blisters away from shear zones is a stable geometry which evolves from natural fluctuations in the system's properties; (b) whether shear zones control pluton emplacement or whether plutons localize shear zones, the end geometry is the same. Thus, the characteristic structures of the Borborema Province result from the evolution of the system towards stable geometrical relationships which dissipated input energy (self-organization).

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

COMPOSITIONAL AND THERMAL CONVECTION WITHIN THE TAVARES PLUTON, NE BRAZIL R. F. Weinberg^ A. N. SiaP, and R. R. Pessoa^ ^ Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands WA 6907, Australia ^NEG-LABISE, Departamento de Geologia, Universidade Federal de Pemambuco, Recife, Pemambuco, Brazil

Crystallisation coupled with convective removal of depleted interstitial melt has long been recognised as a mechanism of magma differentiation. Similarly, heat released by mafic magma intrusions has long been recognised as capable of driving convection in granite chambers. Direct evidence of these processes has seldom been described in granitoids. In Tavares, we mapped a number of melt extraction structures from pores of a crystal mush, and a variety of flow structures such as: a) metric tear- or mushroom-shaped blobs representing within-pluton diapirs, b) metric ellipsoids representing frozen thermal plumes of granite, driven by heat released from disrupted diorite intrusions, c) "ladder dykes" and "snail structures" representing cross sections of several superposed cylindrical magma channels (possibly feeders of diapirs and plume heads). A fundamental feature of the structures in Tavares is that they are well delineated by mafic schlieren developed at flow margins. We postulate a new model for the origin of marginal schlieren, which combines shear flow sorting and melt escape from the flowing magma into an effectively solid surrounding mush. Melt escape from melt rich magma batches into pores (schlieren) and melt extraction from pores into pockets (extraction structures) are both favoured by magmas that form an interconnected solid framework at low crystal fractions (ca. 50%), which are simultaneously fragile, ductile and permeable. These are magmas with a high wetting angle between melt-solid (close to 60 ) and a propitious crystal size and shape distribution. We propose a model of compositional and thermal convection which accounts for all described structures.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney, July 2000

BRARD'S TEST INTO THE 21ST CENTURY: SODIUM SULFATE SOUNDNESS TESTING OF DIMENSION STONE David West Hyder Consulting, Level 13/601 Pacific Hwy, St Leonards, NSW 2065

The testing of dimension stone for durability by cyclic immersion in a sodium sulfate solution and drying to allow crystallisation of the sodium sulfate salt has a history dating back nearly 200 years. This paper describes the published history of salt crystallisation tests for porous building stone in Europe, particularly the United Kingdom, and Australia. Standard test methods for sodium sulfate soundness tests have been published in Australia and Europe over the past few years. The key parameters in the two standard methods for sodium sulfate soundness testing of dimension stone are compared. Available data comparing variations in procedure using a range of Australian sandstones are presented and discussed. These data are correlated with other physical properties which may serve as predictors of durability, as well as observations of performance in service over the past century. The usefulness of the sodium sulfate soundness test as a predictor of durability of Sydney sandstone is discussed, and conclusions about the accuracy and applicability of the procedure are presented.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CONTOUR DELAMINATION AS A FORM OF DETERIORATION OF SYDNEY SANDSTONE IN BUILDINGS David West Hyder Consulting, Level 13/601 Pacific Hwy St Leonards 2065

There are many influences on the deterioration of dimension sandstone, and a number of deterioration processes. In very general terms, two principal processes can be observed on argillaceous varieties of Sydney sandstone in buildings. One of these processes is driven by the crystallisation of soluble salts in the pores of the sandstone. This leads to a variety of deterioration types, including fretting, thin layer exfoliation (usually accompanied by substantial efflorescence) and honeycombing. This process is always associated with the migration of water through the sandstone masonry, and a source of soluble salts. The second process is termed contour delamination in this paper. It is characterised by detachment of a relatively thick layer (8-15mm) of sandstone from the surface of the sandstone units. The plane of detachment follows the surface plane of the unit, without obvious regard for the original bedding of the sandstone. It is most clearly seen on projecting cornices, string courses or mouldings, where the plane of detachment follows the section of the carved stone. It also occurs on the vertical face of ashlar units. The cause of contour delamination appears to be a combination of differential stresses arising from wetting expansion and drying contraction of the sandstone unit, combined with differential thermal expansion, along with changes in clay mineral composition in the zone of evaporation which coincides with the plane of greatest stresses. This paper describes the ways in which contour delamination manifests itself on Sydney sandstone in buildings, as well as the characteristic sequence of deterioration which appears to be consistent through several varieties of Sydney sandstone. A mechanism for this form of deterioration is proposed, and areas requiring further research identified.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

BIOSTRATIGRAPHIC AND PALAEONTOLOGICAL SUMMARY: ODP LEG 188, PRYDZ BAY ANTARCTICA Patricia Whalen\ Steven Boharty^ James Pospichal^ Patrick Ouiltv\ Jason Whitehead^ and Leg 188 Shipboard Science Party ' Wolf Ridge, 968-CR-206 Eureka Springs, AR, USA. ^ Department of Geosciences,University of Nebraska, Lincoln, Nebraska 68588-0340 USA. ^ Department of Geology, Florida State University, Tallahassee, Florida 32306, USA. School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7050. ^ Department of Geosciences, University of Nebraska, Lincoln, Nebraska 68588-0340, USA.

During ODP Leg 188, three sites were cored on the Antarctic margin in and near Prydz Bay; Site 1165 on the continental rise, Site 1167 on the continental slope and 1166 on the continental shelf. An expanded Neogene section was recovered in Holes 1165B and 1165C. Thirty-two diatom and radiolarian datums were recognised through a composite section of 999 m and provide a preliminary basis for age estimates. These data indicate a relatively continuous Lower Miocene to upper Pliocene section that underlies a thin Quaternary cover. In Hole 1166A down to -153 mbsf, diatom biostratigraphy identifies three distinct intervals of Quaternary, late Pliocene, and Late Eocene-earliest Oligocene age. Predominantly sandy lithofacies were cored in Hole 1176A to a depth -447 mbsf. Calcareous nannofossil biostratigraphy places the upper -227 m of the drillcore in the Pleistocene, and, below this level, the planktonic foraminifer N. pachyderma indicates an age of <9.2Ma. Sufficient foraminiferal material was recovered from several intervals of Hole 1167A for post-cruise strontium isotope dating. With detailed post-cruise work on Leg 188 cores, microfossil-derived ecological and geochemical data should provide new palaeoclimatic and palaeoceanographic information from the Antarctic margin in several key time intervals. Initial diatom, silicoflagellate, and calcareous nannofossil assemblage data identify periods of reduced sea-ice extent (relative to present) in upper Pliocene sediments of Hole 1166 A, as well as Lower Miocene to upper Pliocene warming and cooling phases in Hole 1165B. Benthic foraminiferal assemblages in Leg 188 cores identify allochthonous versus in situ assemblages and the palaeoenvironment of original deposition of these assemblages. Planktonic foraminifers in Pleistocene sediments of Hole 1167A should also provide a detailed stable oxygen and carbon isotope stratigraphy. Additional biostratigraphic, biogeogeographic, and palaeoclimatic information should be gained from studies of dinocysts, pollen, spores, and wood material. The record of vegetation is particularly important in recognising changes in the terrestrial environment as Antarctic glaciation evolved and eradicated the terrestrial flora.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TRACE ELEMENT GEOCHEMISTRY OF THE NORTHLAND OPHIOLITE, NORTHERN NEW ZEALAND: NEW CONSTRAINTS ON TECTONIC ENVIRONMENT OF FORMATION S.A. Whattam\ J.G. Malpas\ J.R. Ali' and I.E. Smith^ 'The University of Hong Kong, Hong Kong, China; ^ The University of Auckland, Auckland, New Zealand.

The Northland Ophiolite of New Zealand is composed predominately of tholeiitic basalts with minor alkalicvolcanics. Its formation has traditionally been ascribed tomid-ocean ridge magmatism. The (younger?) homblendemodal-alkalic basalts were thought to represent seamountsbuilt on the older MORE crust, which were both subductedtogether, and subsequently thrust onto the Northland peninsula from the northeast, in the Late Oligocene. Recenttrace element analyses, however, have shown a (subtle)supra-subduction modified component for the tholeiites, and awithin-plate signature for the alkalic rocks. These newly discovered trace element signatures indicate that the tectonicenvirormient in which the Northland Ophiolite formed is not simply that of a mid-ocean ridge. An alternative model posed is that the subduction modified component of the tholeiites may reflect the onset of subduction, at the beginning of island arc formation. The association of the alkalic basalts is somewhat more enigmatic, occuring not as discrete, geographically separated fault-bound sheets seen in other ophiolites, but as interdigitated "bodies" within the tholeiites. Portions of the dismembered ophiolite (the Northland Ophiolite consists of some 25 separate massifs)are geographically associated with the older, more basic stages of the Northland-Coromandel Arc Volcanics. The Northland Ophiolite may represent early arc basement for these arc volcanics.

534

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

COMPARISON OF AN EMPIRICAL ALTERATION INDEX, PEARCE ELEMENT RATIO ANALYSIS AND THE ISOCON METHOD: AS APPLIED TO NAVIGATING LITHOGEOCHEMICAL ALTERATION SURROUNDING THE ELURA ZN-PB-AG DEPOSIT Michael A. Whitbread and C. Leah Moore Cooperative Research Centre for Landscape Evolution and Mineral Exploration, University of Canberra, ACT 2601 The Elura orebody is located 50 km north of Cobar, in mid-west New South Wales. It is a small zinc, lead, silver, deposit (33 Mt reserves at 14.1% combined Zn and Pb), hosted within Devonian aged siltstonesandstone turbidites of the Cobar Basin. Visible iron-carbonate and limited pyrite alteration and more extensive elemental anomalies are noted in the host rocks surrounding the deposit. Geochemical changes due to alteration can be obscured by closure and pre-existing variations in the host rocks. Whilst closure effects can be removed by application of 'alteration indexes', background variations can severely impair the applicability of such empirical measures. Changes unrelated to mineralisation can thus cause a sample's index to be highly anomalous, when the desired result would be the opposite. Pearce Element Ratios (PERs) can be used to filter out unwanted variations and allow observation of alteration geochemistry associated with the ore-forming event. The intensity of these changes should increase with proximity to ore. The desirable aspect of the use of PERs is that they may be applied to commercially derived lithogeochemical assays. They also do not require rigorous definition of volume or mass changes between a "parenf and "daughter" rocks, as is required in other mass balance techniques. This allows PER diagrams to accommodate large numbers of samples onto individual plots. A number of provisos exist with regards to the application of PERs. The most important are that all the rocks examined have to be cogenetic (derived from a common homogeneous parent) and that one or more conserved elements exists. Another consideration is the complexity of the controlling mineralogy, which in the case of sediments are significantly more difficult to model than the igneous rocks on which the technique was first developed. In these situations it can be difficult to determine absolute additions or losses of elements. However relative 'alteration' numbers may be determined, which can be used to rank samples of interest. If the alteration effects are inhomogeneous with distance from mineralisation, then a larger number of samples may be required to navigate towards ore. Other mass balance techniques, such as the Isocon method, can be of use in examining degrees of alteration through recognition of absolute additions or losses of elements. However, construction of Isocon diagrams requires the comparison of altered and least altered 'equivalent' samples. It is difficult to sample 'equivalent' samples when sedimentary fractionation has resulted in variable quartz, feldspar and clay proportions, even in visually uniform sandstones. Such a method is thus restricted to pairs of parent-daughter samples, and some doubt about the suitability of the choice of parent rock is unavoidable. The error induced by slight variations between parent and altered rocks is also difficult to quantify. The choice of immobile elements can also be a source of uncertainty. Use of conserved element plots (as in PER) can allow a more rigorous choice of the immobile elements, improving the value of the Isocon method. Absolute additions and losses are more easily quantified by the Isocon method than PERs, provided the above uncertainties are minimised.

535

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney. July 2000

MINERAL EQUILIBRIA CALCULATIONS IN THE SYSTEM K20-Fe0-Mg0-AI203-Si02-H20-Ti02-Fe203 AND APPLICATION TO Fe-RICH METAPELITIC GRANULITES OF THE MUSGRAVE BLOCK, CENTRAL AUSTRALIA. R. W. White', Roger Powell 1 and G. L. Clarke^ 1 School of Earth Sciences, University of Melbourne, Victoria 3010, Australia 2Departnient of Geosciences, University of Sydney, NSW, 2006, Australia

Mineral equilibria calculations in the system K20-Fe0-Mg0-Al203-Si02-H20-Ti02-Fe203 (KFMASHTO) using THERMOCALC 3.0 and the accompanying internally consistent dataset show that biotite-bearing and spinel-bearing equilibria are strongly influenced by the presence of Ti02 and Fe203 respectively. Biotite-bearing equilibria are shifted to higher temperatures and spinel-bearing equilibria to higher pressures and lower temperatures in comparison to the equivalent equilibria in K20-Fe0-Mg0-Al203-Si02H2O (KFMASH). The KFMASHTO petrogenetic grid incorperates a thermodynamic model for silicate melt allowing quantitative mineral equilibria calculations for high temperatures to be undertaken in the KFMASHTO system for the first time. The KFMASHTO grid is applied to Fe-rich metapelitic granulites of the Musgrave Block, central Australia. The metapelites contain several symplectic and coronal reaction textures that postdate a peak S2 metamorphic assemblage involving garnet, sillimanite, spinel, ilmenite, K-feldspar and quartz. The earliest reaction textures involve spinel and quartz bearing symplectites that enclose garnet and to a lesser extent sillimanite. The symplectic spinel and quartz are in places separated by later garnet and/or sillimanite coronas. The sequence of reaction textures is consistent with a post-D2 P-T path that involved a small amount of decompression followed by predominantly cooling. A later D3 event involved lower temperatures than D2 and produced a metamorphic assemblage of garnet, sillimanite, K-feldspar, magnetite, ilmenite, quartz and biotite.

536

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

FRIABLE SANDSTONE RESOURCES OF THE SYDNEY PLANNING REGION AND NEARBY AREAS John Whitehouse and Dr. Peter Roy Geological Survey, N e w South Wales Department of Mineral Resources

Large deposits of "friable" sandstone occur in the western, southern and northern parts of the Sydney planning region. Although sometimes confused with unconsolidated to partly consolidated Tertiary sand deposits deposited as valley fills on upland surfaces, friable sandstones are in situ weathered quartz sandstones of the Triassic Narrabeen Group and overlying Hawkesbury Sandstone. Friable sandstones can be extracted by ripping and crushing by bulldozers or mechanical excavators, usually without the need for drilling or blasting. The Sydney planning region uses 5-6 million tonnes (Mt) of construction sand armually. About 1.6 Mt of this sand, or some 25% of the overall consumption, comes from in situ weathered Triassic sandstones on the Newnes Plateau, Maroota, Somersby Plateau and the Southern Highlands. Secured resources in the Sydney planning region and nearby areas are about 145 Mt, and extremely large quantities of friable sandstone, estimated at well over 500 Mt on the Newnes Plateau alone, are potentially available for extraction. These deposits provide sand that is well suited for uses such as premixed concrete, concrete products and bricklaying or mortar sand. Friable sandstones could also be a future source of kaolin derived from the clay matrix of the sandstone, and high-purity colourless glass grade industrial sand at present only available in coastal areas of high envirormiental sensitivity. Traditional sources of construction sand such as alluvial deposits and coastal dunes are being depleted, and as yet undeveloped deposits are unlikely to be exploited to any significant extent in the near future due to environmental and or land use restrictions. As a result, friable sandstone could become the major source of construction sand for the Sydney planning region in the next five to ten years.

537

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CRYSTAL ACCUMULATION IN GRANITIC PLUTONS: SOLIDIFICATION BY SEDIMENTATION? R. A. Wiebe Department of Geosciences, Franklin and Marshall College, Lancaster, PA 17604-3003, USA Compositional variation occurs in most granitic plutons and has largely been attributed to three main processes: fractional crystallization, magma mixing, and restite unmixing. Arguments about their relative importance have largely focused on geochemical evidence and the interpretation of trends on Harker-type variation diagrams. Based on the assumption that granitic samples represent material (liquid or crystal mush) emplaced to form the pluton, petrologists expect that fractional crystallization will be characterized by appropriate curved trends, magma mixing by straight trends between two known end-members, and restite unmixing by straight trends between an assumed source and a silicic liquid. However, if the analyzed granitic samples generally represent the accumulation of magmatically crystallized minerals with variable amounts of trapped liquid, these standard interpretations of Harker diagrams may not be valid. Here, I consider field and petrographic observations in support of a cumulate origin for granite. Granitic plutons commonly have structures that are analogous to those found in sedimentary rocks. Layers and lenses marked by changes in mineralogy and texture occur widely. Layers and lenses are also commonly defined by concentrations of magmatic enclaves (typically finer grained and more mafic than the granite) or discrete layers of material resembling the enclaves. Several small scale features associated with these structures strongly support their interpretation as deposits on a floor of a crystal-poor magma chamber. These include load-cast structures, leucocratic flame structures and pipes, molding of enclaves around underlying crystals, and compaction of underlying crystal mush. The overall textures of many granitic rocks closely resemble the "touching framework" of early formed crystals which is characteristic of cumulate samples from mafic layered intrusions. The compositional zonation of feldspars in many granites provides much stronger evidence of their cumulate origin. Feldspars, especially plagioclase, retain a history of their growth in magmas that can be readily recognized and measured. A sequence of zones records episodes of growth and resorption that reflect changing conditions (T, P, PH20) in the adjacent magma through time. Careful studies of the zonal histories of different crystals within a single thin-section indicate that adjacent crystals commonly preserve very different crystallization histories in their cores, although their rims are essentially identical. This observation requires that the crystals initially grew in different places and from different liquids and came together only late - at the time their identical rims began to crystallize. The rocks therefore represent an accumulation of crystals from which much of the liquid (actually many different liquids) was expelled. Compositional variation in such a suite of cumulates would tend to be nearly linear in terms of elements compatible with the crystallizing minerals. Incompatible elements may also be linear or show great scatter depending on the size of the samples taken and the spacing between nucleii of accessory minerals that eventually form in the intercumulus liquid and accommodate them. These linear trends would closely resemble those thought to indicate restite unmixing. Curved trends would only be apparent if there were substantial changes in the identity or composition of the cumulus phases. The fact that different granitic suites are characterized by, for example, high or low Sr can readily be explained by the concentration of Sr in the melt and the partition coefficients of the cumulus minerals. Since plagioclase with magmatic zoning tends to be one of the earliest phases to crystallize, restite unmixing is precluded from being a significant factor in the compositional variation. The role of magma mixing is not excluded and, in fact, is required at some level to explain the contrasted crystallization histories of the feldspars. The main cause of compositional variation in granitic plutons is due to fractional crystallization, and the compositions of granitic samples largely reflect variable amounts of crystal accumulation and trapped melt.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

THE GIANT MURUNTAU GOLD DEPOSIT: WHY IS IT SO LARGE? A. R. Wilde'. P. Layer^ T. Memagh^ and J. Foster' ' GeoDiscovery Group, 21 Junction Road, Blackburn North, Victoria, Australia ^ Department of Geology & Geophysics, University of Alaska, Fairbanks, Alaska, USA ^ Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, Australia The Muruntau deposit, situated in the Kyzyl Kum Desert of Uzbekistan, is the largest known example of a sediment-hosted gold deposit outside the Witwatersrand basin of South Africa with a resource, including production, of about 170 million ounces. New "^^Ar/^^Ar analyses obtained at the University of Fairbanks (Alaska) reveal two Triassic hydrothermal sericite-forming and gold depositing events at 245 and 220 ma. Thus, gold was deposited at least 40 million years after the intrusion of nearby early Permian (286 ma) felsic plutons. Petrographic examination shows that hydrothermal alteration minerals replace contact metamorphic minerals. Published Rb-Sr data show that the gold mineralized quartz veins have not inherited the Sr isotopic characteristics of nearby intrusions. Therefore, we conclude that gold was not sourced from Permian magmas or carried by magmatic fluids. The presence of a subjacent intrusion is thought to be significant, however, in terms of its influence on the creation of dilational sites during later strike-slip faulting. Peak regional metamorphism is dated at 400 ma or older, precluding metamorphic dewatering as a source of metal, fluid and heat. Gold deposition was contemporaneous with the development of an elongate Permo-Triassic volcanic and volcaniclastic transpressional basin fill. These rocks do not outcrop in the vicinity of the Muruntau deposit but are known from petroleum wells. This basin was initiated by strike-slip movement along major regional structures such as the Karatau fault. Permo-Triassic rocks overlie Devonian and Carboniferous evaporitic carbonate and clastic rocks which form a prominent scarp to the north of Muruntau. We suggest that such Palaeozoic and Mesozoic sediments are the most plausible source of ore-forming fluids. We propose that these ore-forming fluids were channeled into the depositional site along the north-east trending MuruntauDaugyztau fault, which may be an accommodation structure related to Permo-Triassic basin formation and which was possibly active during deposition of Devonian and Carboniferous sediments. Preliminary fluid inclusion data suggest that early pre-ore hydrothermal alteration was accomplished by a two-phase fluid with a CH4-rich gas phase. Auriferous quartz veins were deposited from a two-phase fluid rich in CO2 and N2 and detectable H2S gas. The latter suggests the possibility that gold was transported into the deposit as a bisulphide complex. Fluid salinity ranged from nearly pure water ( 0 - 3 wt% NaCl equivalent - as determined by laser Raman microprobe) to quite concentrated brine (> 10 wt% NaCl equivalent). The latter is though to be responsible for late, base-metal and silver-rich and gold-poor veins and the discrete silver deposit at Cosmanachi. Current paradigms of gold transport in hydrothermal environments envisage two main scenarios. Oxidised brines can carry large amounts of gold as chloride complexes. This scenario is inappropriate here, given the absence of oxidized hydrothermal alteration assemblages. More reduced, sulphur-rich fluids carry gold as bisulphide complexes. We propose that one factor in generating a giant deposit at Muruntau was the presence of a sulphur reservoir in Devonian evaporites (mainly anhydrite). Large volumes of H2S could be liberated via thermochemical sulfate reduction, which requires temperatures of the order of 140''C (implying substantial burial). Elevated carbon content is a feature of the deposit. Preliminary analysis has shown that the carbon consists of graphite and other as yet unidentified carbon compounds. It may represent relict oil as has been proposed recently for some gold deposits of the Witwatersrand. If so, hydrocarbon fluids may have utilized the same pathways and traps as later gold-bearing hydrothermal solutions. There is no evidence that such carbon participated in gold-precipitating reactions, although it may account for CH4-rich fluid inclusions in the early stages of hydrothermal alteration.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GIS-BASED APPROACH TO MAPPING SALT MOVEMENT AND STORAGE USING CATCHMENT ANALYSIS OF TERRAIN ATTRIBUTES, AIRBORNE GAMMA-RAY SPECTROMETRY, GEOLOGY AND HYDROLOGY J.Wilford^ K. C. Lawrie^ and D. Dent^ ^CRCLEME, c/- AGSO, GPO Box 378, Canberra ACT 2601, Australia ^Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601, Australia ^Land & Water Sciences Division, Bureau of Rural Sciences, AFFA, Canberra, ACT, 2601. Multi-disciplinary studies are under way to map and quantify dryland salinity across the Lachlan/Murrumbidgee watershed within the Murray-Darling Basin in central-west New South Wales. One module of this project (The 'Gilmore' Project) is using a GIS-based approach to map salt sources and stores in incised undulating hill landscapes. The pilot study area lies largely within the Junee 1:100,000 map sheet. The project methodology utilises an integrated catchment-based approach. This involves interpretation and modelling of high-resolution airborne gamma-ray spectrometry, primary and compound topographic indices derived from a high resolution digital elevation model, and bedrock geological data. These are used to map regolith materials and quantify geomorphic and hydrological processes in individual catchments. Field calibration of the derived regolith and soil data has been carried out to validate interpretations. Examining the relationships between materials (soil and regolith) and landscape processes with salt loads in streams drainage from these catchments is providing new insights into salt movement and storage. Regolith type and estimated depth of weathering is derived from the interpretation of airborne gamma-ray spectrometry imagery. Gamma-ray spectrometry measures the abundance of potassium (K), thorium (eTh) and (eU) in rocks and weathered materials. Gamma-rays emitted from the surface will relate to the primary mineralogy and geochemistry of the bedrock, and the nature of secondary weathering (regolith materials). GIS modelling techniques have been developed which automatically separate gamma-ray responses which relate to bedrock chemistry from responses associated with regolith and soils. The degree of radioelement divergence from the bedrock response in many cases relates to deeper more highly weathered soils or wind blown pama. These relationships together with slope attributes derived from the DEM are used to predict soil properties and geomorphic activity within catchments. A suite of topographic indices (eg. slope, slope length, flow direction, roughness, wetness index) derived from a high resolution DEM are used to characterise catchment geometry, geomorphic process and hydrological gradients. This information combined with regolith thickness, porosity and permeability, and bedrock composition and structure are used to model the surface and subsurface flow of saline waters and salt storage in catchments. Surface stream flow salinity data, combined with soil and regolith mapping, demonstrate that salt exported from the hill country to the plains and major rivers comes mainly from, sediment-filled upland basins with restricted outflows, and from areas that are overlain by thick clay soils that coat the footslopes and gentlysloping upland basins. The study also found relationships between regolith type, depth of weathering, geomorphic process and hydrologic gradients with salt mobility and storage in the landscape. These relationships or rules are integrated into an expert system for land management decision support. Modelled raster maps generated from the expert system show the likely movement of saline waters and salt stores in the landscape. Catchments can be ranked according to their dry land salinity risk or potential risk to prioritise remedial management. Acknowledgments: This paper is published with the permission of the CEO of the Australian Geological Survey Organisation, the Director of CRC LEME, and the Executive Director of BRS.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MICROMAPPING, AGE MAPPING, AND CHEMICAL DATING ON THE ELECTRON MICROPROBE: POWERFUL TOOLS FOR MICROSTRUCTURAL ANALYSIS Michael L. Williams and Michael J. Jercinovic Department of Geosciences, University of Massachusetts, Amherst MA 01002 High-resolution compositional mapping and dating of monazite on the electron microprobe is a powerful tool for microstructural analysis of metamorphic rocks. High-resolution X-ray maps commonly show complex Th, U, and Pb zoning that reflects monazite growth and overgrowth events. Age maps can be constructed from the X-ray maps by applying the age equation to each pixel (after background correction and calibration). The age maps generally show less complexity than individual element maps, typically displaying older cores with one or more younger rims. The geometry of the age domains can help to link monazite growth events to deformation or metamorphic stages. Once age domains are defined by mapping, relatively precise dates are determined by averaging several spot analyses per domain (with long count time and high current). Analytical precision for a single analysis, based on propagated uncertainties, is approximately 10-20my depending on age and Th abundance. Standard errors of the mean are typically ca. 5-lOm.y. Microprobe age mapping and dating, because of its rapid and inexpensive nature, can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains in large or poorly constrained field areas. However, because of its in-situ nature and its high spatial resolution, microprobe monazite dating can be integrated into microstructural and microtextural analysis. Three specific applications will be illustrated: (1) Monazite inclusions in metamorphic porphyroblasts can be used to put specific time constraints on P-T paths and rates of metamorphic and deformational processes. For example, phase relationships in Archean/Proterozoic rocks of northern Saskatchewan may suggest a relatively typical, clockwise P-T-t path involving granulite facies metamorphism followed by exhumation. However, monazite analysis indicates that the 1.8 Ga exhumation event may have culminated a multi-stage or protracted history, involving an extended residence at deep-crustal levels. (2) Because monazite is commonly a fabric forming (and inclusion bearing) mineral, microfabrics and microtextures associated with monazite can help to constrain the age of deformation events and provide new links between metamorphism and deformation. For example, aligned, tabular monazite inclusions in staurolite and andalusite from Proterozoic rocks of northern New Mexico indicate that deformation and regional triple-point metamorphism occurred at ca. 1.4 Ga, not during the Mazatzal orogeny (1.7-1.65 Ga) as previously thought. (3) Age mapping and dating can provide insight into complex results from other geochronologic techniques. For example, previous monazite dates from the Lower Gorge of the Grand Canyon tend to spread over several tens of millions of years. Age mapping reveals that most monazite grains have a euhedral core domain that is ca. 1.69 Ga with overgrowths ranging from 1.67 to 1.64 Ga. We interpret the euhedral core to represent the time of migmatization, and the overgrowths to represent subsequent thermal or hydrothermal events during slow cooling. Single whole-monazite dates would yield an average of these domains. Monazite age mapping and dating on the microprobe allow geochronology to be an integral part of the petrological and microstructural analytical process rather than a subsequent activity. Dates can be obtained from a large number of samples rather than a small subset, and the interpretation of dates has immediate petrologic and structural context. Finally, informed decisions can be made about which samples require high-precision mass-spectroscopic analysis, and the results of those analyses can be better integrated with all other petrologic and structural data

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GARNET ROTATION AND THE INTERPRETATION OF THE TAY NAPPE Paul F.Williams', Peter Stringer' and Dazhi Jiang^ ' Geology, University of New Brunswick, Fredericton, NB, Canada E3B 5A3 ^ Geology, University of Maryland, College Park, MD, USA 20742

It has been suggested that recumbent fold nappes and associated discontinuities, with thrust geometry, develop in high grade metamorphic rocks by regional scale horizontal noncoaxial flow. The folds and discontinuities may originally have been upright structures that predated horizontal flow. Various models of this type have been suggested for the development of the Tay Nappe. We have made a detailed study of porphyroblasts from the 'Flat Belt' (D2) and the 'Highland Border Downbend' (D4) of the Tay Nappe. Inclusion trails (Si) in equant garnets may be straight to gently curved from rim to rim, or straight to gently curved in the core and strongly curved in the rims. In both the straight and curved types Si may be a penetrative foliation or, more rarely, a crenulation cleavage. In north-south, vertically oriented thin sections, cut approximately parallel to a prominent lineation that is horizontal in the Flat Belt, the sense of curvature varies little. In both the Flat Belt and the Downbend, it is persistently s-shaped, looking west, irrespective of position regionally or relative to alternate limbs of mesoscopic F2 and F4 folds. In the same thin sections, there is remarkably little variation in the orientation of Si. Surfaces defined by a preferred orientation of the approximately straight segments of Si show various relationships to the mesoscopic folds. Some define a planar surface cutting across one or more folds, commonly at a high angle to axial planar S2 and S4 foliations. Others define planar surfaces within individual fold limbs, but change orientation across axial planes. Such surfaces outline a subdued version of the host fold, having a dihedral angle that is consistently less than the interlimb angle of the host. They may be symmetrical or asymmetrical about the axial plane of the host fold and may close in the same or the opposite direction. A mechanically sound non-coaxial flow model is proposed, which is capable of explaining both the porphyroblast data and the geometry of the folds. In broad terms the model is consistent with interpretations proposed for the Tay Nappe by other workers on the basis of field observations. The porphyroblast data place limits on the relative magnitudes of the simple shear and pure shear components of the flow. They also indicate that folds generated after initiation of garnet growth are largely passive, with only a small dynamic component.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

EVOLUTION OF THE EASTERN LACHLAN FOLD BELT IN VICTORIA IMPLICATIONS FOR OBLIQUE CONVERGENCE C.E. Willman, A.H.M. VandenBerg, V.J. Morand, M.A. Hendrickx , B.A. Simons and R.A. Cayley Geological Survey of Victoria, P.O. Box 500 East Melbourne, Victoria 3002 An intensive programme of new geological and geophysical mapping has revealed that the complex eastern part of Victoria has a quite different tectonic evolution to that of western and central Victoria (VandenBerg et al., 2000). The tectonic model resulting from this work has profound implications for the evolution of the whole eastern Lachlan Fold Belt. We have adopted the term 'Benambra Terrane' (Eergusson et al., 1986) to encompass five structural zones that comprise eastern Victoria: the Tabberabbera, Omeo, Deddick (previously Buchan), Kuark (previously western half of Mallacoota) and Mallacoota zones. The Benambra Terrane was juxtaposed against the Whitelaw Terrane (Stawell, Bendigo and Melbourne zones) after a long period of orogen-parallel movement between the Early? Silurian and late Early Devonian. The Benambra Terrane differs from the Whitelaw Terrane in that it developed large low P/high T metamorphic complexes, complicated strike-slip structures, intracratonic Late Silurian and Early Devonian basins and widespread magmatism. The main difference is that the Whitelaw Terrane, after its accretion to the 'Delamerian' Australian craton in the Benambran Orogeny, remained more or less fixed in this position whereas the Benambra Terrane has been transported southward by orogen-parallel movements prior to the Middle Devonian. The Whitelaw Terrane is relatively intact with only minor strike-slip movement recorded along its western boundary and along some internal faults. By contrast, the Benambra Terrane has suffered internal fragmentation. The Benambran Orogeny in the Early Silurian was the main cratonisation event in the southeastern Lachlan Fold Belt. While it mostly produced structures formed by approximate east-west contraction, some parts of the Benambra Terrane show latitudinal structural trends suggesting a significant component of orogen-parallel tectonic transport was partitioned to these areas. The best evidence for southward tectonic transport in the terrane follows from the end of the Benambran Orogeny when large strike-slip and thrust faults (e.g. Kiewa, Tallangatta Creek and Indi faults) aided wholesale transport and in places determined the siting and deformation of subsequent intracratonic basins. The origin of these faults is uncertain y4 they may have formed late in the Benambran Orogeny or perhaps are related to a regional extensional event in the mid-Silurian. By the Late Silurian the faults were controlling (by transtension or orthogonal extension) the development of several intracratonic basins that were filled with volcanic and sedimentary rocks. In the Early Devonian the Bindian (=Bowning) Orogeny caused the whole western part of the Benambra Terrane to be fragmented into several rigid crustal blocks and transported southerly along the pre-existing strike-slip faults. Ductile fabrics associated with this movement usually only extend short distances away from the controlling faults indicating that the effects of the Bindian Orogeny were largely partitioned to the main faults. Southward movement was facilitated by a linked system of strike-slip faults and leading edge thrust faults that deformed the Late Silurian grabens by transpression along strike-slip faults (e.g. Wombat Creek Graben), or by orthogonal contraction at the leading edge of crustal fragments (e.g. Limestone Creek Graben). Renewed extension in the Early Devonian, partly controlled by the pre-existing linked fault system, led to fiirther volcanic and sedimentary deposition in several large rifts. The largest is the Buchan Rift considered to have opened by dextral transtension. The Tabberabberan Orogeny marks the end of the Benambra Terrane's southerly transport and its juxtaposition with the Whitelaw Terrane along the contractional Governor Fault. We propose that a major transform fault was active between the mid-Silurian and late Early Devonian. Named the Baragwanath Transform, this dextral strike-slip fault accommodated the displacement between the Whitelaw and Benambra terranes. The Baragwanath Transform was converted to the Governor Fault in the Middle Devonian. The transform may extend into northwestern N.S.W. between the Proterozoic-Palaeozoic rocks of the Wonominta Block to the west, and the interpreted Olepoloko Fault, the probable leading edge fault of the Thomson Fold Belt. The geometry of the Thomson Fold Belt suggests it was south-verging, consistent with southward tectonic transport of the Benambra Terrane during fold belt scale oblique convergence. References FERGUSSON, C.L., GRAY, D.R. & CAS, R.A.F., 1986. Overthrust terranes in the Lachlan fold belt, southeastern Australia. Geology 14, pp. 519-522. VANDENBERG, A.H.M., WILLMAN, C.E., MAHER, S., SIMONS, B.A., CAYLEY, R.A., MORAND, V.J., TAYLOR, D.H., MOORE, D., & RADOJKOVIC, A., 2000. The Tasman Fold Belt System in Victoria. Geological Survey of Victoria Special Publication.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MODELLING A COAL SUBCROP USING THE IMPEDANCE METHOD Glenn A. Wilson^^ David V. Thiel^ and Steven G. O'Keefe^ ^ Faculty of Engineering and Physical Systems, Central Queensland University, Rockhampton Qld 4702 ^ School of Microelectronic Engineering, Griffith University, Nathan Qld 4111

Very low frequency (VLF) electromagnetic surface impedance measurements have been used in the Bowen Basin coal deposits for over 10 years (Biggs, 1990 and Thiel, 1987). The technique has been used for identifying faults (Nichols, 1996), intrusions (Thiel, 1990) and coal subcrops (Nichols, 1995). In the past, the technique has relied on curve shape for qualitative interpretation of geological structures. Now, a new and efficient computational technique is available for the forward modelling of surface impedance data. The impedance method was derived as a low frequency eddy current modelling technique to map volume currents generated by mains power in the human body. James et. al. (1999) detailed an extension and application of the technique to enable the calculation of the surface impedance for a horizontally stratified earth and in the vicinity of lateral dislocations. Recently, the technique was extended to a self-consistent formulation (Thiel and Mittra, 2000). VLF electromagnetic waves have a nominal depth of penetration ranging from 30 metres to 100 metres, depending upon the resistivity of the local Earth. This makes VLF surface impedance measurements ideally suited to locate relatively shallow geological structures that exhibit a distinct resistivity change at their lateral boundaries. Relatively large resistivity contrasts occur where coal subcrops against the surrounding, conductive sediments of a basin margin making this feature an ideal target for VLF surface impedance mapping. In the case of coal subcrops near Middlemount, Queensland, however, the Permian coal subcrop is overlain by an unconforming, conductive Tertiary sequence of coarse argillaceous sandstone, sandy claystone and conglomerate (Day et. al., 1982). The effect of this sequence is to mask the underlying, resistive Permian sequence. The location of the coal subcrop from surface impedance data using simple two layer inversion algorithms is impossible. The impedance method has been used to model this example and the theoretical results compare favourably with the field results obtained. It is through the development of efficient, accurate, computer modelling techniques coupled with the rapid improvements in computing speed and size, that automated interpretation of electromagnetic geophysical data can be achieved. The impedance method appears to be a strong contender in this method of approach. Acknowledgements: The authors wish to acknowledge the assistance of Capricorn Coal

Management Pty Limited for granting access to their mine to conduct the field measurements presented in this paper. The authors also wish to acknowledge Shell Coal Pty Limited for granting permission to publish this paper. References Day R.W., Whitaker W.G., Murray C.G., Wilson I.H., Grimes K.G., 1982, Queensland Geology, a companion volume to the 1:2 500 000 scale geological map (1975), Geological Survey of Queensland Publication 383 James D.A., O'Keefe S.G., Thiel D.V., 1999, Eddy current modelling using the impedance method for surface impedance profiling, IEEE Transactions on Magnetics, Vol. 35, No. 3, p.p. 1107-1110 Nichols W.J.F., 1995, Location of a coal subcrop at Callide Coalfields using the TSIM geophysical method, Bowen Basin Symposium 1995 Proceedings, GSA, p.p. 257-264 Nichols W.J.F., 1996, Geophysical trials for fault location at Callide Coalfields Trap Gully Mine, Callide Basin, East Central Queensland, Mesozoic Geology of the Eastern Australian Plate (Extended Abstracts), GSA, p.p. 414-423 Thiel D.V., 1987, The utility of VLF surface impedance measurements for subsurface mapping. Pacific Rim Congress 87 Proceedings, AusIMM, p.p. 427-429 Thiel D.V., 1990, Surface impedance changes in the vicinity of an abrupt lateral boundary at the earth's surface, IEEE Transactions on Geoscience and Remote Sensing, Vol. 28, No. 4, p.p. 500-502 Thiel D.V., Mittra R., 2000, Radio Science (submitted)

544

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

INTERPRETATIONS OF BRITTLE MICROSTRUCTURES T. Wilson^ R. Offler' and S.F. Cox' \ Discipline of Geology, School of Geosciences, The University of Newcastle, Callaghan NSW 2308 Department of Geology and Research School of Earth Sciences, The Australian National University, Canberra ACT 0200 This study focuses on three separate fault systems of different ages, geometry and kinematic history in the northern Sydney Basin, N.S.W. They include: (i) the Hunter Thrust which trends NW-SE, has a minimum displacement of 50km and bounds the northeast margin of the Sydney Basin, (ii) a reverse fault at Hunter Valley No.l Mine, 90km N W of Newcastle, trending E-W with a maximum throw of 7m, and (iii) two normal faults located 6km SW of Newcastle at Burwood Beach. The aims of this paper are to demonstrate that the development of mesoscopic and microscopic brittle structures may be interpreted in terms of slip mechanisms operating during seismic, aseismic and interseismic intervals. In addition, the geometry of microscopic brittle shear-sense indicators will be described and compared with analogous structures produced in ductile regimes. At Burwood Beach, faulting has resulted in two subparallel, north trending, steeply east dipping, principal slip surfaces (PSS's). These faults are approximately 1.5m apart and define a complex fault bounded block. A total of 1 Om displacement has occurred on the western bounding fault. Stratigraphic separation between the fault bounded and hangingwall blocks indicate a vertical separation of 3m along the east bounding fault. Various damage zone structures and fault rock products have developed in association with slip along the PSS's. The deformation processes which have occurred in this particular outcrop may be classified in terms of seismic, interseismic and aseismic activity. Previous analyses have estimated the depth of burial during faulfing at 3-6km and temperatures of <100°C, well within the continental seismogenic regime. Structures associated with main shock, seismic slip events are PSS's, subsidiary slip surfaces including antithetic normal faults and reverse faults. Bedding parallel subsidiary slip surfaces, isolated within the fault bounded block, are thought to represent possible aftershock structures. This interpretation is based on the fact that a significant reorientation of principal stress directions is required for their development. Foliation development in cataclasites, gouge and ultracataclasite are attributed to aseismic creep events. It is not possible to distinguish the timing of cataclasite, gouge and ultracataclasite formation as cataclastic processes associated with their development may occur during both seismic slip and aseismic creep. Relatively undeformed baryte rosettes overprint an earlier foliated gouge and are interpreted to represent growth during an interseismic interval. Renewed slip has resulted in comminution of these rosettes. Cataclastic equivalents of ductile shear-sense indicators, such as dominoes, winged inclusions and C-S fabrics, are developed within the high strain zones of the Hunter Thrust, Hunter Valley No.l Mine and Burwood Beach faults. Dominoe type structures result from millimetre scale offsets on Riedel fractures intersecting fault fragments. Winged inclusions consist of rounded fault-rock fragments with asymmetrical tails, analogous to sigma porphyroclasts. SEM analyses of winged inclusions suggests a comminution process involved in their development. This mechanism typically results in tail compositions consisting of fragment material. Tails of different composition are occasionally observed, which suggests progressive shearing has isolated an earlier matrix. Structures similar in appearance to C-S fabrics are developed in foliated cataclasites and gouge, where Riedel fractures propagating through the zone represent the Csurfaces. In each of the above examples, shear-sense interpretations are analogous to ductile equivalents and are consistent with offsets observed on each fault. These observations suggest that brittle microstructures, developed within the gouge zones of the Hunter Thrust, Hunter Valley No.l Mine and Burwood Beach faults, are independent of the amount of slip occurring on each fault. In addition, they indicate that development of cataclastic material may result in behaviour similar to that observed in ductile shear zones.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

A SUPERCONTINENT AT THE ARCHAEAN-PROTEROZOIC BOUNDARY Brian F. Windley Department of Geology, University of Leicester, Leicester LEI 7RH, UK

Comparison with analogues in the modem Earth suggests that the Archaean-Proterozoic boundary (APB) at 2.6-2.4 Ga represents not a change in types of rocks, structures or rockformation processes, but a long-term and diachronous tectonic changeover from the Late Archaean, a time of major continental growth, to the Early Proterozoic, when stable continents had formed and then fragmented. The rock and isotopic record indicate a lack of orogenic activity in the period 2.6-2.4 Ga - no island arcs, Andean-type margins, subduction-accretion complexes or collisional orogens; in short there was most likely a supercontinent at the APB. In the period 3.2-2.6 Ga a massive worldwide flare-up of crustal growth is recorded in abundant arc/plateau-type greenstone belts and Andean-type calc-alkaline orthogneisses that led to the formation of large continental blocks by the endArchaean. In contrast, global intrusion of abundant mafic dykes into stable lithosphere after 2.4 Ga is an indication of the break-up of large continental blocks. They were followed by formation of many rifts containing clastic sediments and basaltic lavas, and then by massive carbonated-dominated platforms on the passive margins of the fragmented continents. These geological changes are reflected in geochemical changes in sediments and igneous rocks. Mantle buffering of seawater in the Archaean (seawater transfer through oceanic crust) changed to continental buffering in the early Proterozoic - increase in river flux from new, extensive continents. Arc-type greenstone belts continued to form locally across the APB, but their component basalts underwent distinctive chemical changes and komatiites decreased in amount, reflecting cooling of mantle reservoirs and later derivation from more enriched mantle sources. The Archaean-Proterozoic boundary represents the most fundamental secular turning point in deep-mantle convection patterns and processes in the geological record; inevitably it affected the development of the atmosphere and biosphere.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TERRANE ACCRETION IN CENTRAL ASIA: CONSTRAINTS FROM MONGOLIA B.F. Windlev^ G. Badarch^ and W.D. Cunningham' ' Department of Geology, The University of Leicester, Leicester, LEI 7RH; ^ Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, Ulaan Baatar, Mongolia

The Central Asian Orogenic Belt (CAOB) is one of the world's largest accretionary orogens which formed largely by the accretion of juvenile material from the Neoproterozoic through the Palaeozoic. It extends from the Pacific coast to the Urals and from Tibet to the Aldan Shield. Several very different models have been proposed to explain the development of the CAOB. Mongolia occupies a key central position within this tectonic collage. To provide a new data-bank and model which is testable, we have subdivided the geology of Mongolia into forty four terranes of island arcs, continental magmatic arcs, ophiolites, accretionary prisms, passive continental margins, microcontinents (cratons), and overlap basins. This preliminary subdivision allows much-needed, new, detailed studies of individual terranes and their boundaries. New data include: some ophiolites (Bayanhongur, NE Daribie, Khantaishir) were thrust northwards, but others (NW Daribie) southwards onto older crust, from which we infer a different sense of subduction polarity. Also the different polarity of thrusting of carbonate platforms onto cratons suggests variable directions of subduction and accretion from craton to craton. These relations point to a geometrically more complex tectonic assembly than the result of either a single or multiple, northward-dipping subduction zones, as in several recent models. It is premature and impossible at this stage to create a comprehensive and detailed tectonic model, because of the unreliability of much of the primary geological information that has been synthesized, paucity of isotopic age control, uncertainty of kinematic criteria (thrust polarity, oblique accretion), and lack of geochemical information (proportion of juvenile to old crust). New data are required for a viable model.

547

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15"" Australian Geological Convention, Sydney, July 2000

THE ORIGIN OF FOLIATION: MICROSTRUCTURAL AND MICROCHEMICAL EVIDENCE FOR THE ROLE OF PRESSURE SOLUTION AND PRESSURE SOLUTION CREEP Robert P. Wintsch and Christopher R. Amato Department of Geological Sciences, Indiana University, Bloomington, IN 47405, U.S.A.

Strong correlations between mineral composition and textural position in layered rocks provide compelling evidence for dissolution - precipitation processes in the development of foliations in slates, mylonites and layered mylonitic schists. In Martinsburg slates (Penn., USA), where mass-balance calculations show that pressure solution in a constantvolume, open system was responsible for cleavage development, BSE imaging now reveals, as predicted, that the sinks for dissolved material are local, and occur in cracks and overgrowths on detrital oligoclase and quartz grains. In mylonites from the Moine thrust and the Insubric line, overgrowths and beards on plagioclase microporphyroblasts are common, but we now find that matrix plagioclase grains (20 um long) show relatively Carich beards facing the extension direction. Thus even the smallest grains show evidence for preferred overgrowth (precipitation) parallel to extension. These examples show that precipitation (and by inference dissolution) and extension (creep) occurred in the rock simultaneously. Strongly differentiated schists from a highly strained inner contact aureole in the Santa Rosa range, Nevada, show these relationships particularly well. Here biotite and plagioclase are concentrated into pure, quartz-free biotite-plagioclase schists (p-domains) which alternate with quartz-dominated (+/- biotite +/- plagioclase) q-domains. Single grains of biotite oriented parallel to the extension direction show length-parallel bellshaped gradients in the concentrations of Fe, Mg, and Ti. Similar gradients exist across the multi-grain p-domains up to 500 microns wide where biotite displays inter-grain gradients in composition. Mg and A1 decrease while Ti and Si concentration increases toward adjacent quartz-saturated q-domains. Plagioclase shows similar trends. Single grains are universally zoned, with p-domain core compositions -An 85 and rims -An 45. Plagioclase grains in q-domain are 5 to 10 mole % richer in An content. Wide beam microprobe analyses show that the bulk An content of pdomains climbs from a low of -An 40 in the cores of p-domains to a constant (and remarkable) An 95 in the adjacent q-domains. These chemical gradients demonstrate that the differentiation processes that produced these layers involved incongruent dissolution and precipitation. Intra-grain zonation parallel to the extension direction suggests growth zoning in biotite grains in fractionating (small volume) chemical reservoirs. Inter-grain zonations cross quartz saturated q-domains to undersaturated p-domains demonstrate that standing chemical potential gradients existed during plagioclase and biotite crystallization, unmixing and layer development. Thus dissolution, mass transfer, and precipitation and extension must have been concurrent, and pressure solution assisted creep is strongly implicated.

548

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

THE MAGDALA GOLD DEPOSIT, STAWELL: TRACING THE EVOLUTION OF THE DYNAMICS OF A MESOTHERMAL GOLD-QUARTZ SYSTEM. Bronwyn Witham School of Earth Sciences, University of Melbourne, Parkville, Melbourne, Victoria, 3010

The Magdala deposit, Stawell, is currently the largest producer of gold (90,000 oz/year) in Victoria. It lies in the most western tectonic corridor, the Stawell Zone, of the Victorian part of the Lachlan Fold Belt. Gold mineralisation is controlled by shears that overprint a regional metamorphic system that contains evidence for three earlier phases of deformation. The geological/geochronological relationships indicate that this hydrothermal system was long lived and relates to a sequence of progressive deformation events. Shear zone gold mineralisation dates from 439+2 Ma (Foster et al., 1998). Gold is also associated with felsic porphyry dykes at 413+3 Ma and is contact metamorphosed by the Stawell Granite at 396+5 Ma (Arne et al., 1998). The mine stratigraphy is essentially a sequence of tholeiitic basalts with associated volcanogenic sediments overlain by a thick package of turbidites. The host rocks have been metamorphosed to lower greenschist facies, and metamorphic minerals are overprinted by a hydrothermal alteration assemblage of silica-carbonate-chlorite-sericite-stilpnomelane. The ore bodies are predominantly shear zones that are intimately associated with large quartz veins and/or zones of extensive silicification. Gold is associated with arsenopyrite-chalcopyrite-sphalerite-galenarecrystallised pyrrhotite. Gold is particularly localised where there are contrasts between the rheology and/or geochemistry of the host rocks. There are general temporal patterns that can be observed in different aspects of the Magdala deposit geology. For example, the nature of structures tend to change from being ductile to brittle, and the dominant vein types change from being a replacement style to a dilatant style. These geological relationships indicate that as the hydrothermal system evolved the nature of the fluid flow processes changed dramatically from pervasive, grain boundary flow to focussed channel flow. These changes broadly correlate with the transition from the early to late stages of the hydrothermal system. Across the mine there is a consistent paragenesis of hydrothermal minerals. Chlorite and sericite are progressively replaced by carbonate and silica. Initially, all these minerals formed pervasively in the host rocks, but the later phases increasingly .became focussed as haloes to quartz veins, or at vein-host rock contacts. The change in the distribution of minerals with time directly correlates with a localising of geochemical reactions. Focussing of reaction sites has implications for the types of gold deposition mechanisms that could operate at Stawell. Acknowledgments: Stawell Gold Mines, MPI for financial and logistical assistance to this project. References Arne, D.C., Bierlein, F.P., McNaughton, N., Wilson, CJ.L. and Morand, V.J. (1998) "Timing of gold mineralisation in western and central Victoria, Australia: New constraints from SHRIMP II analysis of zircon grains from felsic intrusive rocks." Ore Geol. Rev. 13: 251-273 Foster, D.A., Gray, D.R., Kwak, T.A.P. and Bucher, M. (1998) "Chronology and tectonic framework of turbidite-hosted gold deposits in the Western Lachlan Fold Belt, Victoria: 40Ar-39Ar results." Ore Geol. Rev. 13: 229-250

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LATE PALAEOZOIC MAGMATISM IN THE NORTHERN NEW ENGLAND OROGEN - EVIDENCE FROM U-PB SHRIMP DATING IN THE YARROL AND CONNORS PROVINCES, CENTRAL QUEENSLAND LW.Withnall\ L.J. Hutton^ C.M.Fanning^ G. Burch^ M.A. Hayward^ and P.Blake^ ^ Department of Mines & Energy, Queensland ^ PRISE, Australian National University ^ Department of Geology, Australian National University The northern part of the New England Orogen in central Queensland has been divided into three provinces, which are from east to west, the Wandilla, Yarrol and Connors Provinces. Previous workers suggested that the provinces are elements in an Early Carboniferous west-dipping subduction system with the Wandilla Province representing the accretionary wedge, the Yarrol Province a forearc basin and the Connors Province the volcanic arc. Farther west, a fourth province, the Drummond Basin, is interpreted as a back-arc basin. The Connors Province crops out in two areas, the Auburn Arch in the south and Connors Arch in the north. Prior to the present study, some workers recognised two superimposed volcanic arcs, one in the Late Devonian and a second in the Early Permian. Other workers have challenged this model suggesting that the rocks in the Connors Province were mainly Late Carboniferous to Early Permian and that they recorded a period of continental extension. U-Pb SHRIMP dating in the Connors Province has confirmed the existence of at least episodic Early Carboniferous magmatism from the Toumaisian to Namurian in both the Auburn and Connors Arches. We suggest that the Toumaisian rocks are vestiges of the Early Carboniferous volcanic arc suggested by earlier workers. Ages of ~350Ma and ~349Ma in the Connors Province are similar to ages for volcanics in Cycle 1 in the Drummond Basin and to volcanics in the lower part of the Rockhampton Group in the Yarrol Province. Magmatism in the Drummond Basin and Yarrol Province continued into the Visean although no early Visean rocks have yet been recognised in the Connors Province. The mid-Carboniferous (late Visean) may represent an important change in the evolution of the region. East of the Auburn Arch, in the Yarrol Province, this time corresponds to the boundary between the Rockhampton Group and Lorray Formation, and is marked by a sudden increase in regional radiometric response. It represents the start of a major period of intrusive and extrusive activity in the Auburn Arch. Four granites have SHRIMP ages of ~319-324Ma and the Torsdale Volcanics have ages of ~313-324Ma. In the Connors Arch, granites have SHRIMP ages of ~314-332Ma and one ignimbrite unit is -311 Ma. A second region-wide break at the beginning of the Stephanian is marked by extensive conglomerates that overlie the late Visean to Namurian volcanics and granites, particularly in the Auburn Arch (at the base of the Camboon Volcanics) and Yarrol Province (Youlambie Conglomerate). In the Connors Arch, the Leura Volcanics are also unconformable on granites. In the Auburn Arch, an ignimbrite at the base of the Camboon Volcanics which gave an age of-'308Ma, is the oldest dated rock in this episode, but an ignimbrite elsewhere in the unit gave ~298Ma. Ignimbrites in the Youlambie Conglomerate have a mean age of ~303Ma. In the Connors Arch, an extensive ignimbrite, the Lotus Creek Rhyolite, also has an age of ~303Ma. An ignimbrite from the overlying Leura Volcanics yielded zircons with a mean age of ~299Ma, but elsewhere, the unit has been intruded by the South Creek Quartz Diorite ('-304Ma) and the Iron Pot Granite (~300Ma). Magmatism continued into the Early Permian in both the Auburn (upper Camboon Volcanics) and Connors Arches (including the Lizzie Creek Volcanics, Mt Benmore Volcanics and Carmila beds). The Camboon Volcanics do not appear to have a significant break between the Stephanian and the boundary with the Artinskian Buffel Formation. However, massive conglomerate at the base of the Lizzie Creek Volcanics in the northern Connors Arch may indicate a localised break. The Coppermine Andesite (~297Ma) at the base of the Mt Benmore Volcanics unconformably overlies the South Creek Quartz Diorite and the alteration system at Mt Mackenzie, also indicating at least a local hiatus. Ages o f - 2 9 1 Ma for the Lizzie Creek Volcanics and ~293Ma for the Carmila beds help to define the span of this Early Permian volcanism. The nature of the volcanics changes from felsic ignimbrites and andesitic lavas and clastics in the Stephanian to strongly bimodal basalt lavas and rhyolitic volcanic last ics in the Early Permian suggesting a change in the tectonic setting, which may correspond with the beginning of extension that opened the Bowen Basin. We can now show that the Connors Province formed from episodic magmatism extending from ~350-290Ma, with three regional breaks that may be related to changes in tectonic setting. We support a model of west dipping subduction m the Early Carboniferous changing to continental extension in the Early Permian. The intervening period may contain elements of both settings.

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Geological Convention, Sydney, July 2000

TWO SEPARATE HYDROTHERMAL EVENTS AT MARVEL LOCH GOLD DEPOSIT, WESTERN AUSTRALIA W. K. Witt and M. Huston Sons of Gwalia Ltd, 16 Parliament Place, WA 6005 The Marvel Loch gold deposit in the Southern Cross greenstone belt lies within amphibolite facies greenstones, about 500 metres west of the southwest comer of the Ghooli granitoid dome. It is also located at the southern end of a regional-scale megaboudin, formed by mafic and ultramafic rocks, that extends 30 km northwards to Southern Cross. During synmetamorphic regional deformation, the megaboudin neck in which Marvel Loch is located was a domain of regional low mean stress (and high fluid flux). In the Marvel Loch area, the primary layering of the greenstones has been modified by shearing and faulting and most lithological contacts are zones of moderate to intense ductile strain. The deposit comprises several lodes that are hosted by steeply dipping, hydrothermally altered gabbro and ultramafic rock. Most of the northern lodes (Main, Sherwood, North Sherwood, Contact) are hosted by deformed and altered gabbro which, together with the eastern ultramafic unit form part of a layered sill. A mylonitised contact, including a thin, discontinuous metasedimentary unit, separates the mineralized gabbro fi-om a second ultramafic unit derived from komatitic volcanics to the west. At the north end of the pit, a wedge of finegrained mafic schist (metabasalt?) separates the mylonitised contact from the western ultramafic unit. The gabbro contains primary quartz and granophyre, and black hornblende, suggesting it is an Fe-rich, fractionated component of the layered sill. The Undaunted lode is hosted by the western ultramafic unit, a few metres west of the mylonitised contact. The southern lodes (Boulder-New, East) are hosted by interleaved gabbroic and ultramafic rocks within a broad zone (5-10 metres wide) of ductile shear that follows a major contact between gabbro and ultramafic rocks. Alteration zoning in the northern pit area is summarised in a poster abstract (this volume). The gabbroic unit is characterised by widespread development of secondary biotite. Secondary biotite occurs throughout the up to 400m wide gabbro and up to 2km north of the Marvel Loch pit. Within this broad alteration zone, gold is associated with metre-scale zones of plagioclase alteration and with quartz-diopside-sulfide veins in the adjacent Kcs alteration (biotite+plagioclase+homblende). The Contact and Undaunted lodes are essentially thick quartz-sulfide-native gold veins within altered mafic and ultramafic rock, respectively. The southern gabbro-hosted lodes are similar to those in the north whereas ultramafic-hosted lodes are diopside-quartzsulfide veins within a Kcs (biotite+tremolite-actinolite) alteration envelope. Foliation-controlled secondary talc forms an outer alteration halo, up to several hundred metres wide. Narrow zones of K-feldspar alteration occur locally, adjacent to mineralized veins in mafic and ultramafic rocks. Mineralized veins and alteration zones are tightly folded by a pervasive ductile, shear-related foliation. Fold limbs are attenuated leaving fold hinges that form pipe-like gold lodes plunging 70-80®S, co-linear with a mineral lineation (elongate biotite agregates in altered gabbro). The alteration system at Marvel Loch is difficult to reconcile with simple hydrothermal zoning related to a single fluid event. Firstly, biotite abundance increases towards the mylonitised contact but the contact is not the only locus of mineralization. The Contact lode occurs within the zone of maximum biotite abundance but the Main/Sherwood lodes lie further fi-om the mylonitised contact, in an envelope of Kcs alteration. An abrupt decrease in intensity of alteration at the mylonitised contact suggests late, post-alteration movement on the contact. This interpretation is supported by late segmentation of pegmatite intrusions that cut the ductile shear zone but that have been offset along later, discrete faults. A further complication concerns the distal, calc-silicate alteration zone (plagioclase+diopside) which is enveloped by Kcs alteration. This feature counters the anticipated sequential addition and removal of mineral phases in zones around the gold lodes. The complex and asymmetric zoning of alteration around the Marvel Loch lodes implies two or more superimposed hydrothermal events, or possibly two contemporaneous fluids within a single hydrothermal event. It is suggested that the secondary biotite gradient is related to ductile shear on the mylonitised contact. Gold-related alteration produced proximal plagioclase alteration, distal calc-silicate alteration. The intermediate zone of Kcs alteration is compatible with the mylonite-related secondary biotite abundance gradient. The hydrothermal system was later reactivated and offset along discrete brittle-due it le faults.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AREA SELECTION IN THE TASMANIDES Daniel G. W o o d Newcrest Mining Limited

For mineral explorers, area selection is one of the most difficult but crucial of exploration tasks. Getting it right may lead to discovery. Getting it wrong, irrespective of the quality of subsequent exploration obviously must, by definition, lead to failure. Unfortunately, there is no obvious formula for getting it right. Where it happens, it invariably results from a fortuitous combination of people, intuition, creativity, serendipity and science. The slow rate of discovery in the Tasmanides, as elsewhere, indicates the difficulty of the task. The Tasmanides occupy the eastern third of continental Australia and dominate the geology of Queensland, New South Wales, Victoria and Tasmania as an extremely complex fold belt system. This system contains four generally recognised orogenic behs of Palaeozoic age, collectively comprising a host of geological terranes and sub-terranes, and the Sydney-Bowen Basin. Intuitively, one would expect this complexity to provide a fertile environment for metal accumulation and provide opportunity for discovery. This is the case and the Tasmanides, aside from including the enormous coal inventory of the Sydney-Bowen Basin, have been host to a range of metalliferous accumulations, including major Sn-W, Cu-Au, Au and Cu-Pb-Zn deposits. If the definition of the "right ground" for deposits of these types requires the presence of a world-class representative, the areas of the Tasmanides presently satisfying this condition include the Kidston, Charters Towers, Mt Leyshon, Mt Morgan and Gympie districts of Queensland; the Cobar, Goonumbla and Cadia districts of New South Wales; the Ballarat and Bendigo districts of Victoria; and the Mt Lyell, Rosebery-Hellyer and Mt Bischoff districts of Tasmania. Within most of these, there is probably capacity for additional discoveries as deeper levels of the earth's crust are explored, or, as the boundary of the district is expanded. Consequently, the potential of these districts should not be overlooked in Tasmanides area selection if the style of deposit, which is characteristic of the district, is of interest. Future discoveries will lead to the recognition of other districts in the Tasmanides and the challenge for present explorers is to be the first to identify these districts. Previous discoveries would suggest, at least in some instances, that evidence of this potential is currently awaiting recognition in presently available data, or can be developed by relatively basic geological investigation. Invariably it would seem that in identifying the "right ground", the ability to recognise a significant mineralising system from an apparently disparate array of mineral occurrences is an important skill to have, and the importance of this and deposit models in exploring the Tasmanides cannot be overemphasised. Empirically it would also seem significant that a number of productive Tasmanides systems (deposits) lie on recognisable linear features, and the importance of northeasterly linears in Queensland and northwesterly linears in New South Wales should not be underestimated in the quest for the "right ground". Area selection in the Tasmanides, as elsewhere, does not necessarily have to rely on complex geological reasoning or the application of the latest "black box" technique to be successful. As with most scientific endeavours, simplicity is inevitably the key and the application of basic geological skills by the right person is still one of the best ways to acquire a potentially productive area in the Tasmanides.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DEFORMATION AND THE PRESERVATION OF PRESSURES: RELATIVE BAROMETRY CONSTRAINTS FROM PETERMAN AGE SHEAR ZONES IN THE WESTERN MUSGRAVE BLOCK. Brenton Worley and Richard White School of Earth Sciences, The University of Melbourne, Vic 3010.

To what extent does deformation control the preservation of metamorphic assemblages and compositions, and consequently "peak" metamorphic pressures and temperatures which are calculated using thermobarometry? High-precision relative barometry (APT; Worley and Powell, 2000) calculations on highpressure shear zones from the western Musgrave block. Western Australia, reveal that the assemblages in these shear zones equilibrated at significantly different pressure conditions. Traditional thermobarometry techniques result in large uncertainties on calculated pressures and recognition of pressure differences between individual shear zones is only possible utilising the APT relative thermobarometry approach. The shear zones were active during the Petermann orogeny, and are inferred to have accomodated internal deformation within this part of the Musgrave block, during exhumation along the Woodroffe Thrust. The significant pressurer differences, and specifically the lack of an obvious spatial pattern, indicate that deformation was the primary factor controlling recrystallisation and equilibration within these shear zones. Reference Worley, B. and Powell, R. 2000. High-precision relative thermobarometry: theory and a worked example. Journal of Metamorphic Geology. 18, 91-114.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

CRETACEOUS TECTONIC-SEDIMENTARY EVOLUTION AND RELATED PETROLEUM SYSTEMS IN BASINS OF WESTERN LIAONING, NORTHERN LIAONING AND SONGLIAO AREAS, CHINA M. Xu and P.M. Middleton School of Applied Geology, Curtin University of Technology, Perth, Australia

The basins with different structural styles, sedimentary sequences and petroleum systems developed in the Western Liaoning, northern Liaoning and Songliao areas, forming a NE trending tectonic belt bound by the Hongshan-Balihan fault to the west and the YilanYitong fault to the east. One of the representative basins in the western Liaoning area is the Fuxin basin, a NE-NNE trending fauh basin. The basin consists of an extension-related or syn-rifting supersequence that is the production of rapid tectonic subsidence. The structural style of the Songliao basin is characterized by the superimposition of the lower asymmetric fault basin and upper thermal subsidence. It is made up of syn-rifting and post-rifting supersequences with nearly the same ratio. The basins in the northern Liaoning area comprise a syn-rifting supersequence similar to that of the Fuxin basin and a post-rifting supersequence (less than 1 km) similar to the lower section of post-rifting supersequence in the Songliao basin. The petroleum systems in the area can be classified into syn- and post-rifting petroleum systems related to the syn- and post-rift supersequences. The syn-rifting petroleum systems developing in the middle of the Songliao basin are the main gas-generating systems because the deeply buried source rocks were over-mature, whereas the post-rifting petroleum systems in the Songliao basin are the most important oil-generating systems. The syn-rifting petroleum system in the Fuxin basin is a limited petroleum system for the insufficient burial depth of the possible source rocks. But the source rock of syn-rifting petroleum system in the northern Liaoning area entered the "oil window" because of the further subsidence at post-rifting stage.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

ADVANCED ARGILLIC ALTERATION ASSOCIATED WITH ACTIVELY FORMING, SUBMARINE POLYMETALLIC SULFIDE MINERALISATION IN THE EASTERN MANUS BASIN, PAPUA NEW GUINEA Christopher J. Yeats, Raymond A. Binns and Joanna M. Parr CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 1670 The Manus back-arc extensional basin is located to the northeast of the New Britain subduction trench and volcanic arc. In the eastern Manus Basin (EMB), an en echelon series of neovolcanic edifices, ranging in composition from picritic basalt to dacite-rhyodacite and referred to as the Eastern Manus Volcanic Zone (EMVZ), is developed as constructional features overlying older arc crust equivalent to exposures on the islands of New Britain and New Ireland. The geochemical and isotopic characteristics of the volcanic rocks suggest close affinities with active subaerial arc volcanoes on New Britain, rather than with backarc spreading-ridge volcanism in the central portion of the Manus Basin further west. The EMVZ contains three significant active hydrothermal sites (the PACMANUS, DESMOS and SuSu Knolls hydrothermal fields), and is considered to be an excellent modem analogue for volcanic-hosted mineralisation in ancient back-arc environments. At PACMANUS, polymetallic sulfide chimneys have been dredged from five sites, which occur over a distance of approximately 3 km along the crest of the northeast trending aphyric dacitic to rhyodacitic Pual Ridge. At one of these sites, named Snowcap, the chimney field is surrounded by a broad (200x100 m) area of diffiise venting, white bacterial mat and bleached and Fe-stained volcanic rocks. Fluid flow and consequent alteration at Snowcap is fracture-controlled by cooling cracks in the volcanics. Strongly altered volcanics are commonly cemented in massive pyrite, forming pyritic breccias. SuSu Knolls is a series of three porphyritic dacite domes, each 1.0 to 1.5 km in diameter, which form a northnorthwesterly trending edifice. Strongly altered porphyritic dacite is associated with hydrothermal venting and subhalative and exhalative Cu-rich massive sulfide mineralisation on the crest of North Su and South Su, the two largest volcanoes. Primary volcanic fracturing may have been a factor in controlling fluid flow, however there is extensive evidence of hydrofracturing. Although alteration at South Su is generally more intense than that at North Su, both fields show incipient, moderate and intense alteration and the development of hydrothermal breccias, often with cupiferous pyritic cement. Alteration at both Snowcap and SuSu is feldspar-destructive and strongly bleached material shows complete destruction of the igneous fabric. At SuSu, the alteration mineralogy consists of natroalunite, alunite (N Su only), cristobalite, tridymite and rare quartz and kaolinite. Cristobalite is the dominant silica phase at Snowcap and occurs with smectitic clays (illite-montmorillonite), natroalunite and diaspore. Some veins contain amorphous silica. Native sulfur is locally abundant in samples from SuSu and Snowcap and the dominant sulfide phase is pyrite(-ftikuchilite) with associated enargite-chameanite, covellite and chalcopyrite in pyritic breccias. Alteration is clearly coeval with volcanism, as xenoliths of strongly altered material are present in fresh unaltered dacite at both sites. Mass balance calculations comparing altered and unaltered material from SuSu and Snowcap indicate that, with the exception of Ti, A1 and Si (all near-immobile), major and trace lithophile elements are depleted in altered rocks, reflecting the destruction of the primary mineralogy and removal of primary components by acidic fluids. Sulfur and trace elements broadly corresponding to those associated with magmatic Cu-Au mineralisation are enriched in both leached and mineralised rocks. The mineralogy and geochemistry of alteration at SuSu and Snowcap is typical of the low temperature and pressure advanced argillic alteration that is associated with high sulfidation gold-copper systems in the subaerial environment. Similar alteration has previously been described for the DESMOS field, where there is no known significant sulfide mineralisation, and is also associated with submarine polymetallic massive sulfide mineralisation in the Lau Basin. It is now recognised that the geological processes that result in the formation of high and low sulfidation epithermal mineralisation in terrestrial settings are equally applicable to the formation of differing styles of volcanic-hosted mineralisation in submarine back-arc basins.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

MAGMA EMPLACEMENT IN THE LOWER CRUST AT OCEANIC SPREADING CENTERS Aaron S. Yoshinobu Department of Geosciences, Texas Tech University, Lubbock, TX, 79409, USA

Representing in excess of 50,000 kms of the Earth's plate boundaries, oceanic spreading centers (OSC), and their fossilized, interpreted ophiolitic equivalents represent an enticing and compact system in which to study a magmatic system from the inception of melting to the formation of crust. In fact, OSC's potentially offer a simpler view of a magmatic plumbing system than an arc because: 1) the physio-chemical state of the mantle can be ascertained; 2) crustal thicknesses can be constrained; 3) state of stress can be inferred; 4) rates of deformation (extension) are constrained; 5) real-time dike emplacement in the upper crust has been observed; 6) whole crustal sections are continuously exposed (e.g., Oman, Josephine); 7) normal oceanic crust lacks the compositional complexities that continental crust inflicts on migrating melts; and 8) deep ocean drilling has penetrated portions of the entire "oceanic crustal section". Observations we have made in the Josephine, Oman, Troodos, and Tortuga ophiolites, and from the Southwest Indian Ridge indicate that construction of the lower oceanic crust occurs by the addition and solidification of basaltic (± ultramafic) magmas simultaneously at various levels within the lower crust. In the Josephine ophiolite, the lower crust is composed of a mixture of gabbro, wehrlite, and dunite intrusions with mutually intrusive relationships. Gabbroic sills have been identified in the lower crust of the Oman ophiolite, whereas distinct, cross cutting gabbroic plutons are observed at Tortuga. Available data indicate that at relatively faster spreading rates (>90 mm yr-1 (?) or regions of higher melt supply), magma emplacement is concurrent with hypersolidus deformation and a complete absence of subsolidus plastic deformation. For example, in the Josephine ophiolite, hypersolidus fabric patterns show a 3-D "plume pattern" which is interpreted to reflect coupling with the underlying upwelling mantle - thus fabric geometries reflect regional strain patterns rather than internal chamber processes. At slower spreading rates (<50 mm yr-1, or regions of low melt supply), strain localization apparently occurs across hypersolidus through brittle conditions (e.g.. Southwest Indian Ridge), and possibly along low-angle normal faults/shear zones. Seismic observations and modeling suggest that small melt reservoirs exist beneath fast spreading ridges (>90 mm yr-1), and that a low velocity zone (LVZ) with a melt fraction of 5-50% (depending on melt topology) exists between the Moho and the melt lens. At slower spreading rates, continuous melt lenses have not been imaged. Based on our observations in ophiolites and the Southwest Indian Ridge, we suggest that accretion of the lower crust occurs by the injection of "sills" and small gabbroic intrusions and that in regions of low melt supply, strain may be localize at hypersolidus conditions on crystallizing intrusions. Sill formation may occur due to transient shifts in the principal stress axes during episodes of increased dike injection in the upper crust or via meh entrapment at permeability barriers in the lower crust. At inferred slower spreading rates/low melt supply ridges, episodic intrusion may result in a fluctuating temperature regime and a migrating brittle/plastic transition. Outstanding conundrums include: mechanisms of melt migration through the lower crust, topology and deformation mechanisms of LVZ, and constraining temperature conditions during sill injection.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DEFORMATION OF PARTIALLY MOLTEN GABBRO BENEATH OCEANIC SPREADING CENTERS: OLIVINE AS A RHEOLOGICAL ''ROSETTA STONE'' Aaron S. Y o s h i n o b u \ Greg Hirth^ and B e m d Leiss^ 'Department o f Geosciences, Texas Tech University, Lubbock, TX, 79409, USA, ^Department o f Geology & Geophysics, WHOI, Woods Hole, MA, USA, University of Gottingen, Germany,

The rheology of partially molten lower crust beneath intermediate to fast spreading ridges may control features such as axial bathymetry and along-axis flow, yet few constraints exist on the physical nature of the axial lower crust. We use recently calibrated flow laws for dislocation creep of olivine and diabase and diffusion creep of plagioclase aggregates to place experimental constraints on the rheology of the partially molten lower crust with an olivine gabbro composition. Motivation for this analysis comes from the observation of olivine subgrain microstructures indicative of dislocation creep in samples we have collected from the Oman, Troodos, and Tortuga ophiolites, and from ODP Hole 735B, SW Indian Ridge. Samples from these localities contain well-developed magmatic foliations, defined by aligned, euhedral laths of plagioclase. Olivine [100] axes are commonly aligned parallel to the foliation, with subgrain boundaries oriented perpendicular to the foliation. There is no other evidence for plastic deformation in the samples. Thus, we conclude that the alignment occurred in the presence of melt. Using flow laws for olivine single crystals deformed by dislocation creep along (010)[100] (the easiest slip system at these conditions; Bai et al.), dry diabase (Mackwell et al.), and diffusion creep of feldspar aggregates with melt fractions from 0-10% (Dimanov et al.), we have calculated strain rates at temperatures bracketed by the gabbro solidus and liquidus (i.e., 1100-1250 ° C) and stresses of 1-100 MPa. We use dry diabase because it is likely that the gabbroic lower crust is relatively anhydrous due to the segregation of water into MORE. At temperatures above 1100°C and stresses less than 10 MPa, strain rates for olivine deformed by dislocation creep range from 10-10 to 10-13 s-1, up to 2 orders of magnitude higher than for diabase, indicating that olivine is the weaker phase at these conditions. Varying melt fraction (0-10%) and grain size of plagioclase-melt aggregates provides further constraints on the rheological window in which olivine may deform by dislocation creep while the host remains undeformed. Together, these results are consistent with the observation of subgrain microstructures in olivine, in an otherwise plastically undeformed rock. Calculated effective viscosities (stress/strain rate) based on the olivine microstructures range from 1017-1019 Pa s. These values approach estimates of the viscosity of the upper mantle beneath ridge axes, and are significantly higher than previously suggested for the lower crust. References BAI et al., 1991, JGR, v. 96, p. 2441; DIMANOV etal., 1998, JGR, v. 103, p. 9651; MACKWELL et al., 1998, v. 103, p. 975.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

THE GEOMORPHOLOGY OF SANDSTONES IN THE SYDNEY REGION R.W. Young' and R.A.L. Wray^ 'School of Geosciences, University of Wollongong NSW ^Environmental Science Unit, University of Wollongong NSW

During the last few decades major advances have been made in the study of the geomorphology of the sandstones in the Sydney region. It is now clear that structure and lithology exert major constraints on the development of landforms at various scales. The modification of those constraints by weathering is more thoroughly understood, and long-held ideas concerning lateritization and cavernous weathering of the sandstones have been shown to be untenable. The main advance in the study of weathering has been the recognition of solutional etching of quartz and the development of 'karst' landforms over large parts of the region. Detailed studies have been made of the mechanisms and rates of the transport of debris, and of its temporary storage in upland swampy dells. More is now known about stream erosion in the sandstones, but little published material is yet available. Field observations and the application of research in geomechanics have led to a better understanding of the major clifflines in the gorges, while recognition of repeated tsunami impact has had considerable bearing on the study of coastal cliffs. Investigation of talus deposits has revealed phases of catastrophic rock fall and debris avalanching that were probably caused by a more intense seismic regime. Nonetheless, radiometric studies have shown that long-term rates of denudation in the region have been very slow.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NATURE AND CONSEQUENCES OF RAPID GLOBAL WARMING IN THE EARLY CENOZOIC James C. Zachos Department o f Earth Sciences, University of California, Santa Cruz, CA, 95064 U S A

One of the more extreme and abrupt episodes of global warming in Earth history occurred approximately 55 Mya. This event, referred to as the Late Paleocene Thermal Maximum (LPTM; Zachos et al., 1993), was transient in nature and lasted less than 150 ky. It is characterized by several significant environmental changes including 4-8°C of warming of the oceans, and globally higher rainfall. These unusual climate perturbations had a marked impact on global biota including; mass extinction of marine benthic foraminifera; temporary displacement of planktonic organisms by "exotic" taxa; and a highly unusual radiation of terrestrial vertebrates. In addition, the LPTM has been linked to a prominent perturbation in the global carbon cycle as reflected by a %o negative carbon isotope excursion (CIE) in the marine, atmosphere, and terrestrial carbon reservoirs. In this paper, I discuss how our understanding of this unique climatic event has evolved since its discovery less than a decade ago. This includes a review of the various proxy records that have been used to establish a chronology of the paleoenvironmental and paleoecological changes associated with this event. I also discuss new lines of evidence that support a popular hypothesis for the origin of this event, the methane hydrate dissociation model (Dickens et al., 1995). I close with a discussion of how investigation of this event might contribute to our understanding of the consequences of future global warming. References DICKENS, G.R., J.R. O'NEIL, D.C. REA, AND R.M. OWEN, 1995: Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene, Paleoceanography, 10, 965-971. ZACHOS, J.C., K.C. LOHMANN, J.C.G. WALKER, AND S.W. WISE, 1993: Abrupt climate change and transient climates during the Paleogene: A marine perspective, J. GeoL, 101, 191-213.

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1Australian

Geological Convention, Sydney, July 2000

TIMING OF DEFORMATIONS AND MAGMATISM OF THE SOUTHEASTERN GAWLER CRATON, YORKE PENINSULA, SOUTH AUSTRALIA Wen-long Zang^ and C.M. Fanning^ ' Geological Survey, Minerals Group, Department of Primary Industry and Resources, South Australia ^ Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia

Yorke Peninsula is located on the southeastern margin of the Gawler Craton, a craton that includes an Archaean core and Palaeoproterozoic complexes which were deformed during the Kimban (1850 - - 1 7 1 0 Ma) and early Mesoproterozoic orogenies. The craton has been divided into seven tectonic subdomains, including the Cleve and Moonta Subdomains in the southeast. The Warooka Fault Zone on southern Yorke Peninsula is a tectonic boundary separating these two southeastern-most subdomains. To the southwest of the fault zone the Gleesons Landing Granite and other Palaeoproterozoic gneisses, including the Corny Point Paragneiss, are interpreted to be basement to the late Palaeoproterozoic Wallaroo Group. The Cleve Subdomain consists of the Archaean Sleaford Complex, Palaeoproterozoic Hutchison Group and syntectonic granitoids on Eyre Peninsula. On southern Yorke Peninsula the Corny Point Paragneiss is a layered migmatite with a protolith equivalent to the middle Palaeoproterozoic Hutchison Group (2000 1860 Ma). Sedimentological evidence suggests that they were sourced from the western to northwestern Gawler Craton and deposited on a continental shelf. The Corny Point Paragneiss has been metamorphosed to granulite grade during the Kimban Deformation 1 event; SHRIMP U-Pb zircon data suggest that the metamorphism ranges between 1850 - 1835 Ma with a peak at 845 ± 7 Ma. Several plutonic suites intrude the Corny Point Paragneiss. The early phases of the Gleesons Landing Granite could be as old as - 1 8 5 7 Ma, and the later intrusives include megacrystic granite (1850 Ma) and syenogranite ( - 1 8 3 5 Ma). The Gleesons Landing Granite is considered a member of the Donington Granitoid Suite, and on southern Yorke Peninsula it was intruded by the deformed Royston Granite ( - 1 8 0 0 Ma). The Wallaroo Group ( - 1 7 7 0 - - 1 7 3 0 Ma) comprises a succession of metasediments and volcanics, which are interpreted to have been deposited in a continental rift complex, from fluvial/aeolian to shallow water environments. Both mafic and felsic intrusives and extrusives were deformed with the sediments. The felsics have U-Pb zircon ages in the range - 1 7 4 1 Ma to - 1 7 6 3 Ma. The Wallaroo Group is considered to have been deformed by the KD3 event, which has an age of - 1 7 1 0 - 1730 Ma. The group hosts a variety of mineralisation styles including the significant Moonta and Wallaroo Cu-Au deposits. The Wallaroo Group was intruded by the Tickera Granite (1598-1586 Ma), Arthurton Granite (1582 Ma) and Curramulka Gabbronorite (1583 Ma) on Yorke Peninsula. The granites and gabbronorite were deformed at least twice prior to Neoproterozoic. The granite intrusion, deformation and associated alteration are considered to be responsible for the Moonta and Wallaroo mineralisation. Early magmatic crystallisation of ore-metal-rich minerals coupled with late exsolution of the magmatic volatile phase may be promoted by deeper emplacement of cumulatic gabbronorite. Magmatic volatile phases allow for advection of hydrothermal fluids through the magma and leaching of the host rocks. Recent TEISA aeromagnetic data show complex structures across Yorke and Eyre Peninsulas, providing valuable data on the tectonic history of the southeastern Gawler Craton. Five deformation events are recognised in the basement and the Moonta - Wallaroo mineralisation is interpreted to have been associated with D4 noncoaxial faulting and hydrothermal event of the early Mesoproterozoic magmatism. T a b l e L S u m m a r y o f t h e m a j o r d e f o r m a t i o n s on Y o r k e P e n i n s u l a . Local

Regional Event

Age (million yrs)

Magmatism and major characters

D,

Kimban Deformation 1 (KDO Kimban Deformation 2 (KD2) Kimban Deformation 3 (KD3) Wartakan Deformation

-1845 1850-1835 1835-1780

Deformation of the Corny Point Paragneiss and Gleesons Landing Granite Intrusion and deformation of the Royston Granite

-1730—1710

Deformation of the Wallaroo Group, formation of the Kalinjala Mylonite zone Intrusion and deformation of the Arthurton/Tickera Granites and Curramulka Gabbronorite Intrusion of the Spilsby Suite and regional thermal resetting; K-Ar dates on micas in pegmatite.

D2 D3 D4 D5

-1580 -1500

560

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

LARGE-SCALE GLOBAL CONVECTION IN THE MANTLE BENEATH AUSTRALIA FROM 55 MA TO NOW Ming Zhang and Suzanne Y. O'Reilly GEMOC ARC National Key Centre Department of Earth and Planetary Sciences, Macquarie University, NSW, 2109, Australia The global-scale mantle convection cells in the asthenosphere are not geochemically homogeneous. The heterogeneity is most prominently reflected in the isotopic compositions (Pb-Sr-Nd) of the mid-ocean ridge basalts (MORB) that are direct partial melts from the underlying asthenosphere. Of particular relevance to Australia's geodynamic evolution from about 100 million years, are the distinctive geochemical signatures of the asthenosphere beneath the Pacific Ocean (Pacific MORB) and Indian Ocean (Indian MORB). Therefore, delineation of the boundary between the two distinct mantle reservoirs and any change in that boundary with time provide information about the patterns of global-scale asthenospheric mantle convection. This information has also allowed us to track large-scale mantle chemical reservoirs such as the distinctive Gondwana lithospheric mantle, and hence better understand the geodynamic evolution of the Australian continent from the time of Gondwana dispersal. Pb-Sr-Nd isotope data for Cenozoic basalts in eastern Australia (Zhang et al, 1999) indicate that PacificMORB type isotopic signatures characterise the lava-field basalts (55-14 Ma) in southeastern Australia, whereas Indian-MORB type isotopic signatures characterise younger basalts (6-0 Ma) from northeastern Australia. This discovery helps to constrain the changing locus of the major asthenospheric mantle convection cells represented by the Pacific and Indian MORB sources during and following the breakup of the eastern part of Gondwana, and locates, for the first time, the boundary of these convection cells beneath the Australian continent. This extends previous work in the SW Pacific back-arc basins (eg Hickey-Vargas et al., 1995) and the Southern Ocean (Lanyon et al., 1995) that indicates that the I- and P-MORB mantle convection cells have been moving in opposite directions since the early Tertiary. These new data also indicate that the Indian-MORB source is a long-term asthenospheric reservoir beneath most of the Gondwana lithosphere and that the westward migration of the Pacific MORB source may have been associated with the Tasman Sea opening (ca 85-60 Ma) along a broad front southeast of the Australian continent. Independent dynamic modelling by Gurnis et al. (1998) produced a west-pointing V-shaped boundary between 1-MORB and P-MORB in the Southern Ocean, consistent with the data from this study, and explains the present-day geochemical and geophysical characteristics of the Australia Antarctic Discordance (AAD) in the Southern Ocean. References GURNIS, M., MULLER, R.D, AND MORESI, L., 1998, Cretaceous vertical motion of Australia and the Australian-Antarctic discordance: Science, p. 1499-1504. HICKEY-VARGAS, R., HERGT, J.M., AND SPADEA, P., 1995, The Indian Ocean-type isotopic signature in western Pacific marginal basins: origin and significance, in Bebout, G.E. et al., eds.. Active Margins and Marginal Basins of the Western Pacific: Geophysical Monograph 88, p. 175-197. LANYON, R., CRAWFORD, A.J., AND EGGINS, S.M., 1995. Western migration of Pacific Ocean upper mantle into the Southern Ocean region between Australia and Antarctica: Geology, 23, 511-514. ZHANG M., O'REILLY, S.Y. AND CHEN, D.G., 1999. Location of Pacific and Indian MORB mantle in two time-slices: evidence from Pb-Sr-Nd isotopes for Cenozoic Australian basalts. Geology 27, 39-43.

561

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

REACTION-ENHANCED PERMEABILITY DURING DECARBONATION OF CALCITE + QUARTZ = WOLLASTONITE + CARBON DIOXIDE Shuqing Zhang, John D. FitzGerald and Stephen F. Cox Research School of Earth Sciences, The Australian National University Canberra, ACT 0200, Australia

Interaction between fluid flow and metamorphic reactions controls the thermal, chemical, mineralogical, and structural evolution of the Earth's crust. It could also significantly influence the nucleation and recurrence of earthquakes and may even affect the Earth's climate. Many prograde metamorphic reactions are devolatilization reactions which are driven by infiltration of fluid that is out of equilibrium with the mineral assem.blages. It has been suggested that fluid-flow and devolatilization can be mutually reinforcing processes. Clearly, permeability controls fluid flow^ and thus metamorphic reactions; the interactions between fluid flow and metamorphic reactions will in turn modify the temporal and spatial distribution of permeability. In spite of the importance of permeability during active devolatilization, its magnitude is still poorly constrained and little is known about the effects of fluid pressure, mass transfer or deformation of the Earth's crust. We have measured permeability during water-infiltration-driven decarbonation reaction of Calcite + Quartz = Wollastonite + CO2. We have examined the interactions between porosity creation from the decarbonation and porosity reduction due to plastic deformation. The aggregate used for measurement was initially composed of 90wt% calcite and 10wt% quartz and the fluid was pure water. At an effective pressure (confining pressure minus fluid pressure) of 25 MPa, the reaction proceeds to completion in less than 40 hours at temperatures of 600°-700°C. Permeability evolves from IQ-^^-^ to lO'^^ vcfi as quartz reacts to form porous wollastonite aggregates. Over the same temperature range, but at an effective pressure of 100 MPa, the permeability reduces rapidly below lO'^^ m^, and only limited volumes of dense wollastonite aggregates were produced on calcite grain surfaces. Our experiments demonstrate the importance of effective pressure in influencing reaction progress and permeability evolution during metamorphic reactions. The present experiments clearly suggest that where water-infiltration decarbonation in siliceous carbonates occurs under near-lithostatic fluid pressure conditions, reaction kinetics can be very fast and the release of carbon dioxide is continuous.

562

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

INTERACTION BETWEEN DEFORMATION, THERMAL STRUCTURE AND FLUID FLOW IN THRUSTING REGIMES AND ITS RELEVANCE TO MINERALISATION Y. Zhang, A. Ord, B.E. Hobbs and P. Sorjonen-Ward AGCRC, Division of Exploration and Mining, CSIRO, PO Box 437 Nedlands, WA, 6009 It has been widely proposed that many world-class ore deposits are spatially and structurally related to crustal-scale thrusting structures (e.g. Broken Hill, Witwatersrand and Mississippi Valley Type deposits), which are commonly associated with high topographic elevation. To understand the formation of such ore deposits and discover new ones, it is crucial to understand the geodynamics of thrusting processes, including the development of geometrical features (e.g. elevation and crustal thickening), regional stress-strain fields, the evolution of thermal structure, and fluid generation and flow patterns. In this study, we aim to explore these issues via a numerical modeling approach, with a focus on the interaction between deformation, thermal response, fluid flow and ultimately, on mineralization. We have constructed numerical models to simulate crustal columns containing large-scale thrusts. The coupling between deformation, evolving temperature and fluid flow is achieved by considering a group of mechanical-thermal-hydraulic parameters (e.g. cohesion, permeability, porosity, heat production rate, thermal conductivity and heat flux). The models are subjected to progressive shortening and displacement along thrusts. This allows us to simulate the progressive development of thrusting, the thickness change of crustal layers and geothermal response. In a separate approach, we have also built static models with variable topographic elevation, developed as a consequence of thrusting. This enables us to test more efficiently the quantitative relationship between geothermal gradients, topography and thermal condition (heat production rate and heat flux) and explore the issues related to fluid generation (including partial melting) and the presence of various fluid sources in the crust. The results show that thrusting processes influence fluid flow and mineralization in three important ways. Firstly, thrusting results in strain localizaion, the formation of shear zones and the presence of dilation zones, accompanied by a variable stress field (from intense compression to local deviatoric tension). This focuses fluids into dilated areas. Secondly, thrusting leads to the thickening of crustal layers. As crust becomes thickened, its temperature gradient is increased (a function of new crustal thickness, heat production rate, flux rate and thermal conductivity). Elevated temperature will trigger partial melting if rock compositions are appropriate. This is one practical mechanism for fluid generation and the presence of fluid sources within a thrust system. Increased temperature will also weaken crustal rocks and further affect deformation. Thirdly, thrusting structures are generally associated with high topographic elevation. High topography is the most important driving force for large-scale lateral fluid flow in the crust, mostly towards the foreland region; in contrast to this is fluid up-flow along deep cutting faults. This contrast is an important mechanism for mixing fluids from different crustal sources and levels, and the latter is critical for the formation of some ore deposits with high metal grade. We have also constructed an application model for the Broken Hill region in Australia, based on a hightemperature and high-pressure scenario (Hobbs et al., 1988) that involves high thrust-related topographic elevation. The model demonstrates that the resulting high temperature combined with the high pressure generated by thrusting is sufficient to lead to peak granulite facies metamorphism in the region and to generate partial melting post the mineralizing stage. Reference HOBBS B. E., WALSHE J. L., ORD A., ZHANG Y., and CARR G. R. 1998. The Broken Hill ore body: a high temperature, high pressure scenario. In: FINLAYSON D. M. & JONES L.E.A. eds. Proc. Symp. on Mineral Systems and the Crust-Upper Mantle of Southeast Australia, Canberra. AGSO Record 1998/2, pp. 98-103.

563

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

PREDICTIVE MODELLING OF ORE DEPOSITS IN HYDROTHERMAL/ SEDIMENTARY BASINS Chongbin Zhao, B. E. Hobbs, H. B. Muhlhaus and A. Ord CSIRO Division of Exploration and Mining, P. O. Box 437, Nedlands, WA 6009, Australia, Australian Geodynamics Cooperative Research Centre. Over the past four years, we have been making efforts to develop a practical and predictive tool to explore for giant ore deposits in hydrothermal/sedimentary basins. Towards this goal, significant progress has been made towards a better understanding of the basic physical and chemical processes behind ore body formation and mineralization in hydrothermal systems. On the scientific development side, we have developed analytical solutions to answer the following scientific questions [1-2]: (1) Can the pore-fluid pressure gradient be maintained at the value of the lithostatic pressure gradient in the upper crust of the Earth? and, (2) Can convective pore-fluid flow take place in the upper crust of the Earth if there is a fluid/mass leakage from the mantle to the upper crust of the Earth? On the modelling development side, we have developed numerical methods to model the following problems: (1) convective pore-fluid flow in hydrothermal systems [3-4]; (2) reactive pore-fluid flow in porous media [5]; (3) precipitation and dissolution of minerals in the upper crust of the Earth [6-7]; (4) double diffusion driven pore-fluid flow in hydrothermal and sedimentary systems [8]; (5) pore-fluid flow patterns near geological lenses in hydrodynamic and hydrothermal systems [9]; (6) dissipative structures for nonequilibrium chemical reactions in fluid-saturated porous media [10]; (7) various aspects of the fully coupled problem involving material deformation, pore-fluid flow, heat transfer and species transport/ chemical reactions in pore-fluid saturated porous rock masses [11-13]. The abovementioned work has significantly enriched our knowledge about the physical and chemical processes related to ore body formation and mineralization in the upper crust of the Earth. The main purpose of this paper is to summarize the fundamental scientific principles behind the predictive modeling of ore deposits in hydrothermal/sedimentary basins. In particular, from mechanical and physical points of view, we identify three main possible mechanisms, which may cause localization and redistribution of minerals in the upper crust of the Earth. These mechanisms are as follows: (1) mechanical instability of the crust material; (2) thermal instability of pore-fluid in the upper crust; (3) chemical instability of the reactive chemical species within the pore-fluid in the upper crust. Finally, some typical orebody patterns, which are generated from different mechanisms, are presented to show the formation and localization of ore deposits in the hydrothermal/sedimentary basins. References [1] Zhao C., Hobbs B. E. and Muhlhaus H. B., Analysis of pore-fluid pressure gradient and effective vertical-stress gradient distribution in layered hydrodynamic systems, Geophysical Journal Intemational, 134,519-526 (1998).

,

u

^

[2] Zhao C., Hobbs B. E. and Muhlhaus H. B., Theoretical and numerical analyses of convective instability in porous media with upward throughflow'Int. J. Num. , 23,element 629-646analysis (1999).of steady-state natural convection problems in fluid-saturated porous media [3] Zhao C., Muhlhaus H. Analy.Meth.Geomech B. and Hobbs B. E., Finite heated from' below. Int. J. Num. Analy. Meth. Geomech., 21, 863-881 (1997).

^

^

[4] Zhao C Muhlhaus H B and Hobbs B. E., Effects of geological inhomogeneity on high Rayleigh number heat and mass transfer in tluidsaturated porous media heated from below. Int. J. Computation and Methodology: Numerical Heat Transfer 33, 415-431 (1998). [5] Zhao C., Hobbs B. E. and Muhlhaus H. B., Finite element modelling of reactive mass transport problems in fluid-saturated porous media. Communications in Numerical Methods in Engineering, 1 5 , 5 0 1 - 5 1 3 ( 1 9 9 9 ) . .. , . , , ^ i . [6] Zhao C., Hobbs B. E. and Muhlhaus H. B., Finite element modelling of temperature gradient driven rock alteration and mineralization in porous rock masses, Compu. Meth. Appl. Mech. Eng., 165, 175-187 (1998). ,i ^ v, ^ .v, i [7] Zhao C., Hobbs B. E. and Muhlhaus H. B., Finite element analysis of heat transfer and mineralization in layered hydrothermal systems with upwardthroughflow,Compu. Meth. Appl. Mech. Eng., in press. ,, . , [8] Zhao C., Hobbs B. E. and Muhlhaus H. B., Numerical modellmg of double diffusive convection flow problems in porous rock masses, Proceedings'ofthe 13th Australasian Fluid Mechanics conference, 2, 1009-1012(1998). u^ ^ [9] Zhao C., Hobbs B. E., Muhlhaus H. B. and Ord, A., Finite element analysis of pore-fluid flow patterns near geological lenses in hydrodynamic and hydrothermal systems. Geophysical Joumal Intemational, 138, 146-158(1999). , i [10] Zhao C., Hobbs B. E., Muhlhaus H. B. and Ord, A.,, Finite element modelmg of dissipative structures for nonequilibrium chemical reactions in fluid-saturated porous media. Computer Methods in Applied Mechanics and Engineering, in press. • ^ . . ^ [11] Zhao C., Hobbs B. E., Baxter, K., Muhlhaus H. B. and Ord, A., A numencal study of por-fluid, thermal and mass flow in fluid-saturated porousrockbasins. Int. J. for CAE and software. Engineering Computations, 1 6 , 2 0 2 - 2 1 4 ( 1 9 9 9 ) . [12] Zhao C., Hobbs B. E., Muhlhaus H. B. and Ord, A., A consistent point-searching algorithm for solution mterpolation in unstructured meshes consisting of 4-node bilinear quadrilateral elements. Int. J.Num. Meth. Eng., 45, 1509-1526 (1999). [13] Zhao C

^

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Hobbs B E and Muhlhaus H. B., Effects of medium thermoelasticity on high Rayleigh number steady-state heat transfer and

mineralization in deformable fluid-saturated porous media heated from below, Compu. Meth. Appl. Mech. Eng., 1 7 3 , 4 1 - 5 4 (1999).

564

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

REVIEW OF 2.1-1.8 GA OROGENS AND CRATONS IN NORTH AMERICA, BALTIC A, SIBERIA, CENTRAL AUSTRALIA, ANTARCTICA, AND NORTH CHINA: A PRE-RODINIA SUPERCONTINENT? Guochun Zhao*'^, Peter A. Cawood\ Simon A. Wilde^ and Min Sun^ 'Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, W.A. 6845. ^Department of Earth Sciences, the University of Hong Kong, Hong Kong The 2.1-1.8 Ga orogenies have been recognized on nearly every cratonic block and are considered to represent a global collisional event, which resulted in the amalgamation of Archean to Paleoproterozoic blocks to form supercontinents. The available lithostratigraphic, tectonothermal, geochronological and paleomagnetic correlations of the major 2.1-1.8 Ga collisional orogens and cratonic blocks in North America, Baltica, Siberia, Antarctica, Central Australia and North China have established connections between North America, Greenland and Baltica; Siberia and North American; Central Australia, East Antarctica and North America; and North China and Baltica. These links lead to a proposal of a pre-Rodinia supercontinent that consists mostly of the major Archean-Paleoproterozoic cratons in North America, Baltica, Siberia, Antarctica, Central Australia and North China, and is referred to herein as the Hudson Supercontinent, since all its fragments were amalgamated by the Hudsonian orogeny or its equivalents. In this supercontinent, the Archean to Paleoproterozoic cratons in North America were sutured by the 1.9-1.8 Ga TransHudson, Penokean, Taltson-Thelon, Wopmay, Ungava, and Tomgat orogens; the Southern Greenland and Disko cratons in the Greenland Shield were amalgamated along the Nagssugtoqidain Orogen; the South Lapland-Karelia and Murmansk-Sorvaranger cratons in the Baltic Shield were joined by the Kola-Karelian Orogen; the eastern and western Aldan Shields in Siberian platform were connected by the Amga Belt; the East Antarctica and an unknown continental block were jointed by the Transantarctic Mountains Orogen; and the Western and Eastern Blocks in the North China Craton were welded together by the Trans-North China Orogen. Speculation of this pre-Rodinia supercontinent is also supported by late Paleoproterozoic to Mesoproterozoic sedimentary and magmatic records, including a 1.8 to 1.30 Ga subduction-related magmatic belt that extends across North America and South Greenland into the Baltic, Siberia, North China and Central Australia blocks. The occurrence of temporally and petrologically similar rocks across a distance of thousands of kilometers between these continents is impressive and supports the existence of the pre-Rodinia supercontinent. Nd isotopic studies indicate that this large magmatic belt consists of volcanic and plutonic rocks resembling those of present-day island arcs and continental margins, and they may represent a major subduction-related mantle-to-crust differentiation along the margin of the Hudson Supercontinent. Throughout much of the Hudson Supercontinent, especially in North America, Greenland, Baltica and North China, the 1.6 to 1.2 Ga age range is characterized by a wide spectrum of anorogenic igneous activity including emplacement of anorthosite massifs, chamockite intrusions, batholiths of potassium rapakivi granite, and carbonatite and alkaline intrusive bodies. A Mesoproterozoic anorthosite-chamockiterapakivi granite suite is exposed in a huge belt that trends across North America and southern Greenland into the Baltic region of northern Europe to as far east as the Ukraine Ural Mountains and the North China. The anorogenic igneous activity was probably related to an extensional underplating mechanism which preceded the dispersion of the fragments of the Hudson Supercontinent. In addition, 1.4-1.2 Ga mafic dyke swarms have been widely reported from North America, Greenland, Baltica, North China, East Antarctica and Central Australia. These mafic dyke swarms constitute a plate-wide extensional episode that may mark the youngest piercing points at which these cratonic blocks in the Hudson Supercontinent can be paleomagnetically and geologically linked. This episode of extension is regarded as having signalled the commencement of the rifting and breakup of the Hudson Supercontinent.

565

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 15""Australian Geological Convention. Sydney, July 2000

THREE-STAGE SYMPLECTITE OR CORONA GROWTH IN THE HENGSHAN HIGH-PRESSURE GRANULITES (RETROGRADED ECLOGITES), NORTH CHINA CRATON AND TECTONIC IMPLICATIONS Guochun Zhao'^ Simon A. Wilde', Peter A. Cawood' and Min Sun^ 'Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, 6845, W.A., Australia. ^Department of Earth Sciences, University of Hong Kong, Hong Kong.

High-pressure mafic granulites have been found as enclaves and boudins in tonalitictrondhjemitic-granodioritic gneisses in the Hengshan Complex, North China Craton. Petrographic evidence indicates three distinct symplectic or coronal assemblages. The Ml symplectite is represented by clinopyroxene + sodic plagioclase (An 10-20) intergrowths in which the exsolution-like sodic plagioclases make up to 3 0 ^ 0 vol.%. Similar textures have also been observed in many other high-pressure granulites and retrograded eclogites, and are considered to indicate the replacement of omphacite by plagioclase and clinopyroxene formed during the transition from eclogite facies to high-pressure granulite facies (Rubie, 1990; Smelov & Beryozkin, 1993; Moller, 1998). Also associated with the Ml symplectite are garnet porphyroblasts in a matrix of hornblende, rutile and quartz. They constitute a typical high-pressure granulite facies assemblage clinopyroxene + garnet + sodic plagioclase + quartz + rutile ± hornblende. The high-pressure granulite facies metamorphism was then followed by near-isothermal decompression, which resulted in the development of the M2 orthopyroxene + plagioclase symplectites and clinopyroxene + orthopyroxene + plagioclase coronas surrounding embayed garnet grains. Finally, the highpressure granulites underwent decompression-cooling, represented by the M3 hornblende + plagioclase symplectites on garnet. The results using the THERMOCALC program yielded P-T conditions of 13.4-15.5 kbar and 770-8407C for the clinopyroxene + sodic plagioclase symplectic assemblage (Ml); 6.5-8.0 kbar and 750-8307C for the pyroxene + plagioclase symplectite and corona (M2); and 4.5-6.0 kbar and 680-7907C for the hornblende + plagioclase symplectite (M3). The combination of petrographic textures, mineral compositions, metamorphic reaction history, petrogenetic grids and thermobarometric data defines a clockwise P-T path involving nearly isothermal decompression, suggesting that the Hengshan Complex underwent initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record a major phase of collision between two continental blocks, which resulted in the final assembly of the North China Craton at c. 1.8 Ga. References MOLLER, C. 1998. Decompressed eclogites in the Sveconorwegian (-Grenvillian) orogen of SW Sweden: petrology and tectonic implications. Journal of Metamorphic Geology 16, 641-656. RUBIE, D. C. 1990. Role of kinetics in the formation and preservation of eclogites. In: Carswell, D. A. ed. Eclogite facies rocks, pp. 111-140. Blackie, Glasgow and London. SMELOV, A. P. & BERYOZKIN, V. I. 1993. Retrograded eclogites in the Olekma granite-greenstone region, Aldan Shield, Siberia. Precambrian Research 62, 419-430.

566

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian

Geological Convention, Sydney, July 2000

K-Ar DATING OF FLUID FLOW AND FAULT MOVEMENT IN CLAY RICH GOUGE: AN EXAMPLE FROM THE NORTHERN SYDNEY BASIN H. Zwingmann'*, T. Wilson ^ and R. Offler^ ^ CSIRO - Division of Petroleum Resources, PO Box 136, North Ryde, NSW 1670 ^ Department of Geology, The University of Newcastle, Callaghan, NSW 2308 •present address : Centre of Excellence in Mass Spectrometry - School of Applied Geology, Curtin University, WA 6102 The occurrence of synkinematic and authigenic clay minerals, in particular illite, is a common feature in fault gouges. Until recently very few attempts have been made to date fault gouges. We present preliminary age data for synkinematic illite growth in two fault zones of the northern Sydney Basin. This study represents the first of its kind in Australia. Eleven samples were selected for this study from two faults situated at Burwood Beach within the northern Sydney Basin. The faults are subparallel, approximately 1.5m apart and trend N-S with steep easterly dips. A total displacement of 10m is observed on the western fault, whilst 3m of vertical displacement has occurred on the eastern fault. 1cm wide foliated gouge zones are developed along both fault surfaces that represent the regions in which principal displacement has occurred. Samples were collected from the gouge zones and host siltstones and tuffs within and outside the damage zone originating from the Lambton Subgroup of the Newcastle Coal Measures. They were disaggregated using a repetitive freeze-thaw technique to separate <2 and 2-6mm fractions. The mineralogy of the fractions was determined by X-ray diffraction (XRD) on air dried, glycolated and heated (5500C) samples. K content was determined by atomic absorption. Argon was extracted from the separated mineral fractions by ftising samples within a vacuum line and analysed for isotopic composition by mass spectrometry after addition of an 38Ar spike. XRD analyses of samples indicate that illite/smectite (I/S) and kaolinite in varying proportions are present in the fractions from the fault gouges and host rocks in the damage zone. In most samples, the I/S contains 7090% illite, suggesting temperatures <1000C during faulting. Twenty three K-Ar dates have been determined. The ages of samples in the gouge and damage zones from Burwood Beach, range from 126.8 to 164.5 (n=9; x= 148.3 Ma;sn=10.5) for the 2-6mm fraction and 122.2 to 150.9 Ma (n=10; x=137.2 Ma;sn=7.8) for the <2mm fraction. Older ages of 272 -281.8 Ma and 237-244.9 Ma for the 2-6 and <2mm fractions, have been obtained from undeformed host rock. Radiogenic 40Ar ranges for these samples range between 59.28 to 98.1% indicating negligible atmospheric Ar contamination. The reliability of the ages is also confirmed by the agreement within 2s analytical limits for the duplicate analyses of the <2mm fraction of some samples. The 272-281 Ma K-Ar dates obtained from the 2-6mm fractions of the host rocks at Burwood Beach are older than the age of the sequence in which the samples occur (245-252 Ma; Roberts et al., 1996). This suggests that detrital mica is present in these samples consistent with petrographic studies. The younger ages of 237-244.9 Ma are thought to reflect the time at which diagenesis occurred. The <2mm ages (122-150 Ma) obtained from the fault gouge is thought to reflect the last slip event occurring on the faults, which is possibly related to underplating and uplift of the Australian continent. This study highlights the potential and value of isotopic dating of synkinematic, diagenetic illite to determine upper crustal deformation events. Reference J.Roberts, J.C.Claoue-Long &C.B.Foster, 1996. SHRIMP zircon dating of the Permian System of eastern Australia. Australian Journal of Earth Sciences. 43:401-421.

567

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

AUTHOR INDEX AALTONEN, A. ABBOTT, S. ABHEARNE, D. ADABI, M.H. ADAMS, C.J. ADKINS, R. AGASHEV, A. AGNEW, M.W. AGUA, J. AHMED, M. AHN, J.H. AILLERES, L. AITCHISON, J.C. AL-BU SAIDI, S. ALCOCK, M.B. ALEXANDER, R. ALI, J.R. ALLEN, S.R. ALLEN, T.C. AMATO, C.R. AMINI, Z.Z. ANDERSON, J.A.C. ANDERSON, K. ANDREWS, S. ARCHIBALD, N. ARCULUS, R.J. ARMAND, L. ARMSTRONG, R. ARNE, D.C. ARUNDELL, M. ASHLEY, A.J. ASHLEY, P.M. AUZENDE, J-M. BADARCH, G. BAKER, D.E.L. BAKER, E. BAKER, M. BAKER, S.F. BALDWIN, S. BANN, G. BARLEY, M.E. BARNES, C.G. BARNES, R.G. BAROVICH, K.M. BARRON, B.J. BARRON, L.M. BASSETT, K.N. BASTRAKOV, E.N. BATEMAN, R. BAXTER, J.L. BEAMS, S.D. BEAUMONT, C. BEHR, H.J. BEHR, K.

BELL, B. BENM, K. BENNETT, R. BENNETT, V. BERNARDEL, G. BERNECKER, T. BERRY, R. BERTON, J.R. BETTS, P. BIANUCCI, G. BIERLEIN, P.P. BIERMEIER, C. BIGGS, M. BINNS, R.A.

123 1 523 2 3 142 488 4 386 171 80 246 6 5 32, 226, 476 193, 505, 506 6, 534 7, 328 8 548 9 188 10 101 47,123,132,133,515 11 210 247, 298 35, 282 452 173 12, 99, 454, 461,490 477, 507, 508 547 13 178, 442 14 15 16 17 3, 18, 62 19, 20 21,63 22 23 23 24 25, 344 390 26 27 254 28 28

BIRCH, G.F. BIRCH, W.D. BLACK, L.P. BLAKE, P. BLEVIN, P. BLEWETT, R.S. BOHARTY, S. BONE, Y. BONNAY, M. BORISSOVA, 1. BOSCHETTI, F. BOTFIELD, A. BOTTRILL, R. BOYD, R. BRADSHAW, J.D. BRANAGAN, D.F. BRANCOLINI, G. BRATHWAITE, R.L. BRAUN, J. BRODIE, R. BROWN, A.V. BROWN, BJ. BROWN, Belinda BROWN, K.M. BROWN, Matthew BROWN, Max, C. BROWN, Michael BROWN, R.E. BROWN, Roderick BROWN, S.J.A. BROWN, Stephen, BROWN, Steve BROWN, W.M. BROWNLAW, R.L.S. BROWNLOW, J.W. BRUEGGER, J. BUCCI, L.A. BUICK, L BULL, S.W. 568

29, 179, 180 30,31 124 495 32, 226, 438 400 66

33 34, 155, 174

68

35, 36 37 38 39, 40, 149,216,378, 447, 555 41, 122, 444, 465,480 42 43 550 197, 247, 263 44, 242 533 57 45 46, 476 47, 354 248 487 48, 49, 50, 131 51,52,516,517 53,54

210

55 65 294, 295 117 56 210

57 58 59,371 60,61 21

176

62

448 136 63 255, 525

21

311 64 292 291

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

BURBIDGE, D. BURCH, G. BURG, J.P. BURRETT, C. BUSETTI, M. BUSHELL, D.J. BUTLER, A. CALVER, C.R. CAMACHO, A. CANDE, S.C. CANTALAMESSA, G. CAPRARELLI, G. CAREY, S.P. CARMICHAEL, D. CARNEVALE, G. CARR, G.C. CARR, S.D. CARTWRIGHT, I. CAS, R.A.F. CASEY, S.D. CASSIDY, K.F. CAYLEY, R.A. CAYWOOD, P.A. CESARE, B. CHALMERS, G. CHAMALAUN, F.H. CHAN, R. CHAPMAN, M. CHAPRONIERE, G.C. CHEN, G. CHEN, S.F. CHESHIRE, A. CHISARI, R. CHIVAS, A.R. CHO, M. CHOE, M.Y. CHRISTIE, A. CLAOUE-LONG, J.C. CLARE, A.P. CLARK, D.A. CLARK, M.W. CLARK, S. CLARKE, D.B. CLARKE, G.L. CLIFF, R.A. CLOSE, D. COENRAADS, R.R. COFFIN, M.F. COHEN, D.R. COLLERSON, K. COLLINS, A.S. COLLINS, C.D. COLLINS, P.L.F. COLLINS, W.J.

65 550 512

COLMAN, R.W. COL WELL, J. CONNOLLY, J.A.D.

66 210 188 425 226 67 359 68 69, 70,211,367 282 71 68 378 72 484, 502, 523 62, 250, 389, 452 473 25 73, 332, 492, 543 163, 565, 566 74, 75 76, 131 304 294, 295 446 207 77 78, 191 57 211 79, 165 80, 277 277 35 43 88 81,289 82, 320 83 84 85, 86, 112, 273, 479, 536 412

CONOR, C. COOK, M. COOKE, D.R. COOLING, D. COOPER, A.K. COOPER, M. CORBETT, G. CORDOVES, P.R. COSTELLOE, M.T. COUAPEL, M.J.J. COUSINS, S. COUTTS-SMITH, A. COWELL, P.J. COX, Simon COX, Stephen F. COZENS, S. CRANE, M.J. CRAWFORD, A.J. CREECH, M. CRISP, A. CROOK, K.A.W. CROSS, A.J. CROWHURST, P.V. CRUDEN, A.R. CUNNINGHAM, W.D. CZAPNIK, K. DACZKO, N.R. DADD, K.A. DAIM, F.M. DALZIEL, I.W.D. DANIELS, J. DARCOVICH, K. DAVID, V. DAVIDSON, G.J. DAVIDSON, P. DAVIES, A.G.S. DAVIES, H.L. DAVIES, J.M. DAVIES, R.M. DAVIES-McCONCHIE, F. DAVIS, B.K. DAVIS, D.W. De CARITAT, P. DE DECKKER, P. De JONG, J.M. DE SOUZA KOVACS, N. DEAN, A. DEAN, A.A. DEEN, T. DEGELING, H. DEHAAN, R.L. DENT, B.

87 186, 483 32, 46 88, 89 175 90 91 485, 486 13,92,216, 290, 382, 417

569

96 181 75 93 94 95, 119, 291 96 156, 366 523 97, 98 287 99 79, 165 100, 101,293 102 241 103 104, 429, 545, 562 225 105 106, 341 107, 108 221 109, 308 110 111,224 422 547 523 85, 112, 479 469 113 114, 115 270, 463 31 116 117 118 119 120, 386 120 121 82, 122,320 123, 220 43 491 165 124 125 221 126 210 127 128, 129 130

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

DENT, D. DEWERS, T. Di CELMA, C. Di TORO, G. DICK, F. DICKENS, G.R. DICKENS, J. DIESSEL, C. DIREEN, N.G. DOBOS, V. DOCKER, B.B. DOHERTY, W. DOUGLAS, N. DOWNES, P.M. DRAGOVICH, D. DRUMMOND, B.J. DUGGAN, M. DUNKERLEY, D.M. DUNKLEY, D.D. DUNLAP, J. DUNLOP, A.C. DURNEY, D.W. DUURING, P. EADIE, T. EDGOOSE, C. EGGINS, S. EGGLETON, R.A. ELBERG, M. ELLIS, D. ELLIS, D.J. ELLISTON, J. EMERSON, D.W. ENGLAND, R. ERIKSSON, K. ESSERY, C. ETHERIDGE, M.A. EVANS, D.A.D. EVERARD, J.L. EXON, N.F. FAGAN, R.K. FAIZ, M. FALLOON, T.J. FANNING, C.M. FASSETT, J.E. FAURE, K. FELTON, E.A. FERGUSON, C.L. FERGUSON, D.J. FINDLAY, R.H. FINLAYSON, D.M. FITZGERALD, J.D. FITZHERBERT, J. FITZSIMONS, I.C.W. FLOOD, R.H.

FODEN, J. FOLKERT, S.L. FOMIN, T.

294, 540 233 68 385 460

FORBES, C. FORSBERG, C.F. FORSTER, D.B. FORSTER, M.A. FOSTER, D.A. FOSTER, J. FOSTER, K. FOUDOULIS, C.

187, 207, 477, 507, 524 58 48, 131 132, 133, 438 293 134

FOWLER, A. FRANKEL, E. FRANKEL, M. FRANKLIN, B. FRANKLIN, D. FREIJ-AYOUB, R. FRICK, L.R. FRIEND, C.R.L. FROST, R. FULTON, F. FUREY-GRIEG, T.G. FUTEN, F. GAINA, C. GALLAGHER, K. GALLAGHER, S. GALLAGHER, S.J. GAMBLE, J. GARCIA, A. GARDNER, B.L. GARDNER, J.V. GARDNER, Y.A. GARVEN, G. GAUL, 0 . GAZZOLA, D. GEDEON, T.D. GEDIKILE, H.

269 135 136 137 91, 179, 180 315 124 86 323 5, 89 33, 138, 235,469 139 421 87 127 491 362 127, 339 274 140 141 135 142 443 143 144 145

GEE, R.D. GEMMELL, J.B.

146, 425, 477, 507 147

GEORGE, A.D. GEORGE, S.C. GEUNA, S. GIBSON, D.L. GIBSON, G. GIBSON, G.M. GIBSON, R.L. GILES, D. GILMORE, T. GIORGETTI, G. GLADKOCHUB, D. GLEADOW, A. GLEN, K. GLEN, R.A. GOLDING, C. GOLDING, S.

148 305 244, 273, 368, 405, 468, 483, 496, 520, 550, 560 198 55 109 387 188 40, 149 150 562 188 151 152

570

22, 153,362 154 29, 179, 180, 181 155 156 157 158, 307 189, 468 539 295 43 52 460 100 159, 160 109 161 162 279 339 36 163 423 164 176 270 463 368 79, 165 378 166, 167 203 291 372 183 63 120 168 119, 169 170 171 313 294 150, 171 375 173 34, 174, 403 175 210 488 176, 528 425 177 178 175

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

GOLEBY GOLEBY, B.R. GONCHAROV, A. GOODWIN, L. GOSSO, G. GOW, P. GRAEBER, P.M. GRAHAM, B.P. GRAHAM, LT. GRANT, K.M. GRAY, C.M. GRAY, D.R. GREEN, P.P. GREEN, T.H. GREENPIELD, J.E. GREENHALGH, S.A. GREENWOOD, D.R. GREENWOOD, David GREENWOOD, P.P. GREY, K. GRICE, K. GRIPPIN, W.L. GROVES, D.I. GRUETZNER, J. GUNN, Peter GUNTON, C. GURBA, L.W. GURNIS, M. HABERMEHL, M.A. HACK, A. HAPNER, R. HAGEMANN, S.G. HAINES, P. HAINES, P.W. HALL, G.C. HALL, R. HANAHAN, C. HANCOCK, H.J.L. HAND, J. HANLEY, L.M. HARLEY, S.L. HARRIS, J.W. HARRIS, P. HARRISON, J. HARTLEY, M. HARVEY, D. HARVEY, K.J. HARWOOD, C. HASSAN, L. HAWKINS, D.P. HAYNES, D. HAYWARD, M.A. HE, S. HEALY, B.

HEATH, S. HEIJNIS, H.

29, 179, 180

HEIMAN, J.L. HENDERSON, R.R. HENDRICKX, M.A.

181 182, 497 183 184, 224 185, 238 12 186 187 188, 274 189, 468 111 190 191 185, 238 192 462 171 231 193

HENLEY, H.P. HERBERT, C. HERMANN, J. HEWITT, D. HICKMAN, A. HIDER, N. HILL, E.J. HILL, K. HILL, K.C. HILL, M.B.L. HILL, P.J. HILL, S.M. HILLIS, R.R. HILLS, Q. HIRTH, G. HOADLEY, E. HOATSON, D.M. HOBBS, B.E.

120, 194, 248,372, 396 63, 64, 195, 363 156 196 197 198, 199 83, 200 201 135 202 64, 139, 154, 203, 363,511 87 204 205 206 320 207 62 208 209 388 210 211

HOCKING, R.M. HOPPMAN, N. HOGAN, J.P. HOLDEN, D.J. HOLDGATE, G. HOLLAND, T. HOLM, 0 . HOLT, S. HOLYOKE III, C. HOOD, D.A.I. HORN, A.M. HORNBY, P. HOROWITZ, P. HOUSEMAN, G. HOUSEMAN, G.A. HUBBLE, T.C.T. HUBER, M. HUGHES, K. HUGHES, M.J. HUSTON, D.L. HUSTON, M. HUTTON, A. HUTTON, L.J. HUYNH, T.H. IRELAND, T.R. ISAACS, D.R.L. JACKSON, S.E. JAGODZINSKI, E. JAIRETH, S. JAMES, N. JAMES, P.

189 473 27 212 450 213 214 550 215 216

571

217 211 218,219 207,219 221,543 21 306 222 333 78 293 132, 133 184 77, 223,224, 271,400 225 226, 425 299, 472 227 228 557 229 162 92, 161,230,371,515, 563, 564 231,391 77, 232, 400 233 132, 133 270, 463 399 234 79, 165 430 235 236 47 47 414 185, 237 86, 134, 239, 240 462 480 241, 270, 282 242 243, 551 212 244, 245, 550 246 516,517 247 194, 248 249, 250 251 57 93

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

JAMES, P.M. JAMIESON, R.A. JAQUES, L.

251

JEFFRIES, S.R.

96

JELL, J.S. JENKINS, C.J. JERCINOVIC, M.J. JESSELL, M.W.

255 256 541

JIANG, D. JIANG, Z. JOHNSON, C.B. JOHNSON, K. JOHNSON, Scott E. JOHNSON, Simon JOHNSTON, A. JOHNSTONE, D.W. JOHNSTONE, R. JOKU, N. JONASSON, K. JONES, B. JONES, G. JONES, J.A. JONES, L.E.A. JONGENS, R. JOYCE, E.B. KAGI, R.I. KAIKO, A. KAIKO, A.R. KAMENETSKY, V.S. KAMO, S.L. KAMP, P.J.J. KAY, B.D. KAZEMI, G.A. KEAYS, R.R. KEENE, J.B. KEETLEY, J.T. KEHAL, H.S. KEILLOR, B. KELLY, N.M. KELSO, P. KEMP, A.I.S. KEMP, T.I.S. KENDRICK R.D. KENNARD, J. KENNEl I,B.L.N. KENNETT, J.P. KIDDLE, T. KIM, H. KIMURA, G. KING, A. KING, P. KINNY, P.D. KIRBY, B.C. KITAYAMA, E. KIVETS, G.B.

KLEPEIS, K. KLOOTWIJK, C. KOHN, B.P. KOONS, P.O. KORSCH, R.J.

252, 253 254

KOTSONIS, A. KRABBENDAM, M KRAPEZ, B. KRASSOI, R. KRITSKI, A. KRONER, A. KRUHL, J.H. KRYC, K.K. LaBRECQUE, J.J. LACKIE, M.A. LAFOY LAMBERT, D. LAMBERT, D.D. LAMBERT, I. LANDENBERGER, B. LANDINI, W. LANG, S.C. LARA, P. LARGE, R.R. LARSEN, D.F. LAURENT, A. LAURIE, J.P. LAVELLE, M. LAVERY, P. LAWRENCE, C.R. LAWRIE, K.C. LAWYER, L.A. LAYER, P. LEACH, T. LECKIE, D. LEE, J.K. LEE, R.J. LEE, S.K.Y. LEEKS, J. LEEVERS, P. LEISS, B. LEITCH, E.C.

257, 442 542 258, 259 329 20 260, 261 262 263 177, 281 264 120 463 17 511 391 177, 281 52 282 193, 505, 506 265 266 118,316 43 111 267 268 269 241,270, 447 271 272 242 273 521 188 274, 275 224 181 276 146 150 80, 277 278 210 478 279, 302 225 520 388

LENARDIC, A. LENNOX, P.G. LEVERETT, P. LEWAN, M.D. LEWIS, A.C. LEWIS, G.B. LEWTHWAITE, K.J. LI, Q. LI, X.H. LI, Z.X. LICKFOLD, V. LIGHTFOOT, P.C.

572

112, 479 280 176, 528 503 29, 43, 110, 132, 133,177, 281 282 155, 283, 403 62, 284 178 181 90 285 286 287 33,81,287, 288 507 317 162 251 289 68 394 297 291 361 513 27 286 292 484 281,293, 294, 295, 540 115 539 98 48, 49 277 296 31 101 116 557 70, 163,297,367, 449, 520 355 113, 298 105 428 299 300 427 301,394 302 302 303 269

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

323

LILLEY, F.E.M. LIMTRAKUN, P.

304

MCLAREN, S.

305

McLELLAND, J.

324, 357

LIN, N.

82

McNALLY, G.

325, 326

LINDSAY, G.

306

McNAMARA, G.

293, 327

LISHMOND, S.R.

23

McNAUGHTON, N.J.

64,461

McPHAIL, D.C.B.

124,311

LITHCO, S.

158, 228, 307, 406, 442 527

LITTLE, M.

48

MCQUEEN, K.G.

LIU, K.

308

MCQUEEN, L.B.

330

LIU, K.W.

309

MEANS, W.D.

340 314

LISTER, G.

McPHIE, J.

7, 328 329, 453, 472

LIU, S.

78,310

LIU, W.

311 80

MEARES, R.M.D. MEFFRE, S. MEINERT, L.D.

342, 343

312 313,426 116

MERNAGH, T.P. MERRICK, N.P. METCALF, R.V.

242, 305, 344, 539 345 346 215, 347, 554

LO, C.-H. LONG, E. LOPEZ DE LUCHI LORRIGAN, A.

341

99

MIDDLETON, M.

LOVEGROVE, D.P.

522

MIEZITIS, Y.

251

LOWDER, G.G.

314

MIKUCKI, E.

203

LUMPKIN, G.R.

485

MILLER, B.V.

84

LUS, W.Y.

120

LYONS, P.

110, 132, 133, 293,315, 409 316

MILLER, C.P. MILLER, D.

348 142

MILLER, R.B.

349, 380 110

LOTTERMOSER, B.G.

MAAS, R. MacKEY, T.E.

MILLS, A.W.

132, 133

MILLS, S. MILNE-HOLME, W.A.

MACKIE, S.

322

MAEHARA, K.

488

MIN, K.

MAGEE, C.W.

331

MOORE, A.M.G.

101 268 80 46, 476

MAHER, S.

36, 332

MAIDEN, K.

333

MOORE, C.L.

MAJOR, B. MAKSAEV, V.

334

MOORE, D.

84

MOORE, D.H.

332, 492

MALLOCH, K.R.

335

MOORES, E.M.

353

MALLONE, M J .

146

MORAND, V.J.

MALPAS, J.G.

534

MORESI, L.

MANCKTELOW, N.S.

MOORE, C. Leah,

336, 385

MORRISON, J.

351,535 125,350, 351 73

492, 543 354, 355, 355 324, 357

MARCHESI, C.

75

MOSS, RT.

192 358

MARMO, B.

337

MOTTERAM, G.

MARSHALL, A.E.

242

MARSHALL, C.P.

338

MOZETIC, M.E. MUHLHAUS, H-B.

MARSHALLSEA, S.J.

170

MULLER, A.

MAVROGENES, J. McBRIDE, J.

135, 197, 263, 339, 498, 499 317

McBRIDE, S.

162

McCABE, P.J.

318

McCONACHIE, B.A.

319

McCONCHIE, D.

82, 122, 320

McCUE, K.

321

McCUISH, K.L.

84

McCULLOCH, T.M.

455

McDOUGALL, I. McGOLDRICK, P.J.

301,394

McINNES, B.M.

433

MCKNIGHT, S.

36, 282

56, 83, 359

MUNDEY, T.J.

294, 295

MUNKER, C.

106

MUNROE, S.M.

104

MURPHY, J.B.

115

MUSGRAVE, R.J.

360

NEEDHAM, J.

417

NEINEN, E. NELSON, D.R. NEUMANN, N.L.

291

McGOWRAN, B.

361 386 191,528 362

NEUMAYR, P.

363

NICHOLLS, LA.

316,317

NICOLL, M.G.

573

370

MULLER, R.D.

NEEF, G.

67, 323

426 161,356, 564

179

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

NICOLL, R.S. NIELSEN, P.

338 241

NISHIYA, T.

520 322 85 120 109 364, 365 23 508

NIU, Y. NOCCOLDS, C. NONGKAS, M. NOORMETS, R. NORVICK, M.S. OAKES, G.M. O'BRIEN, G.W. O'BRIEN, P.E.O. OCCHIPINTI, S.A. OCH, D. ODONNE, F. OFFLER, R. O'KEEFE, S.G.O. OLIVER, N. OPDYKE, B.N. ORD, A. O'REILLY, S.Y. ORIHASI, Y. OSBORNE, R.A.L. O'SULLIVAN, P. OTHMAN, R. OWEN, A.J. PABLO, F. PAGE, R.W. PALMER, D. PANKHURST, R.J. PARKER, A.J. PARR, J.M. PASSMORE, M.J. PATERSON, S.R. PATTISON, D.R.M. PAWLEY, M. PEARSON, N.J. PELLS, P. PEMBERTON, M. PENNACCHIONI, G. PEREMBO, R.C.B. PERUGINI, D. PESSOA, R.R. PETKOVIC, P. PHILLIPS, D. PHILP, R.P. PICKARD, A.L. PIGNOTTA, G. PIKE, G. PILLANS, B. PIRAJNO, F. PISAREVSKY, S.A. POGSON R.E. POLI, G. POLLOCK, R.

POLONSKA, D. POSPICHAL, J. POSTNIKOV, A. POTTER, M. POTTS, G.J. POUDJOM-DJOMANI, Y.H. POWELL, C.McA. POWELL, R. POWER, M.R. PRESTI, M. PRICE, R. PRIOR, D.J. QUEZADA, R.A. QUILTY, P.G. RAETZ, M. RAGAINI, L. RAITH, J.G. RANNARD, T.M. RAPALINI, A. RAPELA, C.W. RATHUR, A.Q. RAWLING, T.J. RAYMOND, O.L. RAYMOND, Ollie RAZAKAMANANA, T.

156, 265, 286, 366 410 367 31 189, 368, 520, 545, 567 544 58, 369, 524 370 161,230,371,515, 563, 564 121, 194, 372, 396, 561 520 373 176 374 29, 179, 180 178 375 298, 376 405 377 378, 555 456 349, 379, 380 381 382

REBESCO, M. REDDY, S.M. REDMAN, E. REED, A. REEVES, J.M. REILLY, N. REY, H. REY, P.F. REYNOLDS, S.D. RICE, A. RICHARDS, S.W. RICHARDSON, M. RIDLEY, J. RIGANTI, A. RILEY, K.W. ROACH, I.e.

194, 248, 372 383 384 385 120, 386 393

ROBERTS, P. ROBIN, P-Y.F. ROBINSON, D. ROLLET, N. ROSALES, P.A. ROSSELLO, E. ROY, P. RUBENACH, M. RUBLE, T.E. RUMING, K. RUSHMER, T. RUSSELL, N. RUXTON, B.P. RYAN, C.G.

530 32, 181 387, 388 428 18 380 389 390 391 392 186 393 394

574

395 533 488 380 411 396,414 144, 392, 397, 398 399, 536 400 210 316 401 171 402, 533 403 68 445 404 313 405 436 307 110, 132, 133,315 408, 409 90 156 410,411,412 473 413 165 294, 295 407 414 227 415,416 216,417 495 418 191 219 299,419, 420 421 422, 423 424 226, 425 287 313, 426 50, 537 229, 427 171 429 430, 431 148 432 305, 433

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

RYFFEL, T.

122

SMITH, J.V.

464

SALVI, F. SANDIFORD, M.

183

SMITH, M.A.

364, 365

323, 362, 434, 435 520 436 45, 437 46, 56, 438 483 439, 440 441,442 380 289 178, 443 423 444 409 445 446 447

SMITH, P.K.

35,36 534

SANO, S. SAPPAL, K.K. SAWYER, E.W. SAYERS, J. SCHARWZ, D. SCHAUBS, P. SCHELLART, P.W. SCHMIDT, K. SCHMIDT, P.W. SCHNEIDER, P. SCHWEINBERGER, M. SCOLLEN, A. SCOTT, M. SCRIMGEOUR, I. SEAMAN, S.J. SEBASTIAN, S. SECCOMBE, P.K. SEO, K. SHADDICK, L.R. SHARP, R. SHARP, T.R. SHAW, S.E. SHEN, X.C. SHEPPARD, S. SHERWIN, L. SHERWOOD, N. SHIPBOARD PARTY SHIRTLIFF, G. SHVAROV, Y.V. SIAL, A.N. SIEBEL, W. SIMONS, B.A. SIMONY, P.S. SIMPSON, A. SIMPSON, C. SIMPSON, M.P. SINADINOVSKI, C. SIRCOMBE, K.N. SIVELL, W.J. SKABAR, A. SKILBECK, C.G. SKINNER, D.N.B. SKIRROW, R.G. SKLYAROV, E. SLATER, K.R. SLOAN, L.C. SLOAN, M. SMITH, A.J. SMITH, J. SMITH, J.B.

SMITH,. I.E. SNOWDEN, R. SOLOMON, M. SORJONEN-WARD, P.

13, 136, 157,258, 259, 448 395 105 169 449 152 89 450 409 148 146, 265, 286,352, 366, 402, 533 494 25 530 298 543 72 451 328, 452 55 321

SPAGGIARI, C.V. SPALLA, M.I. SPEAR, F. SPINKS, A. SPRIGG, A.G. SPRY, A. SPRY, M.J. SPRY, P.G. SQUIRE, R. STACY, S.

465 466 355, 467, 563 189, 468 183 381 358 469 470, 471 472 473 474 475

STAGG, H.M.J. STAGPOOLE, V. STANDING, J. STANMORE, P.J. STEELE, D. STEELE, D.A. STEPHENS, M. STEVENS, B.P.J. STEVENS, J.R. STEVENSON, J. STEWART, K. STRINGER, P. STROUD, W.J. STRUCKMEYER, H.I.M. STUWE, K. SUH, J.Y. SULLIVAN, T. SUN, M. SUN, Y.

476, 477, 508 478 242 319 316 305 333 375 272 479 362 542 21 56 37 480 481 565, 566 259

SUTHERLAND, F.L. SUZUKI, K. SWANE, LP. SWEETAPPLE, M.T. SYMONDS, P.A.

453 454, 455, 456, 457, 458, 490 459 460 55 461 488 221 462 339 463 221 445

TAHERI, J. TAIT, A.M. TAKANO, K. TALUSANI, R. TAN, K.P. TAYLOR, B. TAYLOR, D. TAYLOR, D.H. TAYLOR, Geoffrey R. TAYLOR, Graham TAYLOR, J.C. TAYLOR, S. TAYLOR, W.R.

575

186, 482, 483 461 484 485, 486 32, 46, 56, 438, 476, 477, 508 487 266 488 489, 490 491 156 73, 463 332, 492 128, 129, 493 494 519 41 331,495

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

TEALE, G.S. TELIATNIKOV, I. THIEL, D.V. TIKOFF, B. TOMKINS, A. TRANTER, J. TREGONING, P. TRINCARDI, F. TRIPP, G. TRZEBSKI, R. TWEED, S. UPTON, P. URUSKI, C. UTTLEY, P. VADALA, A.J. VALENTA, R.K. VALLERI, G. VAN AARSSEN, B.G.K. VAN ACHTERBERG, E. VAN DE BEUQUE, S. VAN DER KAARS, S. VANDENBERG, A.H.M. VANDENBERG, F. VANDENBERG, L.C. VARNE, R. VERNACOMBE, J. VERNON, R.H. VICKERY, N. VIELREICHER, N.M. VIELREICHER, R.M. VIGNERESSE, J.L. WADSWORTH, J. WAIGHT, T.E. WALL, V. WALLACE, M.W. WALLACE, P. WALSH, S.R. WALSHE, J.L. WANDRES, A.M WANG, X. WARD, C.R. WARY, R.A.L. WATANABE, T. WATERHOUSE, J.B. WATERS, C.L. WATERS, D.J. WATHANAKUL, P. WATKINS, J.J. WEAVER, S.D. WEAVER, T.R. WEBB A. WEBB, G.B. WEBB, G.E. WEBB, J.A.

WEBER, U.D. WEINBERG, R.F. WESSON, V. WEST, D. WHALEN, P. WHATTAM, S.A. WHEELER, J. WHITBREAD, M.A. WHITE, M.J.

496 46 544 182, 497,513, 521 498, 499 523 500 210 501

528 355, 356, 529, 530 150 531,532 533 534 401,412 535 314

298 502, 523

WHITE, R.J. WHITEHEAD, J.

399, 536, 553 533

503 478 143, 184 192 504 68 505, 506 433 476, 477, 507, 508 79 508, 543

WHITEHOUSE, J. WIEBE, R.A. WILDE, A.R. WILDE, S.A. WILFORD, J. WILKINS, R. WILLIAMS, I.S. WILLIAMS, M.L. WILLIAMS, P.A. WILLIAMS, P.F. WILLMAN, C.E. WILSON, G.A. WILSON, M.A. WILSON, T. WINDLEY, B.F.

537 213,216, 538 36, 539 565, 566 125, 294, 295, 540 148 43 541 105 542 332, 543 544 338 545, 567 90, 546, 547 462 548 549 550 243, 551 492 211 552 553 191 221 170 554 194 170 291,466 378, 555 217 475

73 221 117 501 84, 378 510 511 511 512,513 48, 50, 131 335 14

WING, S. WINTSCH, R.P. WITHAM, B. WITHNALL, I.W. WITT, K.W. WOHLT, K.E. WONG, H. WOOD, D. WORLEY, B. WYCHE, S. WYGRALAK, A. WYRWOLL, K-H. XU, M. XU, X. YANCHOU, L. YANG, J. YEATS, C.J. YEATS, J. YEUNG, M. YOSHINOBU, A.S. YOUNG, J. YOUNG, R.W. ZACHOS, J.C. ZAITLIN, B.

270, 463 351,352 514 161,230, 356, 371,515 516,517 194 5, 374,518,519 558 367, 488, 520 457, 458 521 522 305 28 335,516,517 484, 502, 523 524

ZANG, W-L. ZAW, K. ZHANG, C. ZHANG, M. ZHANG, S.

186 525 526, 527

576

556, 557 160 558 559 48 560 18, 305 309 561 562

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 59 1Australian Geological Convention, Sydney, July 2000

ZHANG, Y. ZHAO, C. ZHAO, G. ZHAO, H. ZHU, Z.R. ZUCALI, M. ZWINGMANN, H.

230,371,467,563 184, 230, 440, 467, 564 565, 566 302 436 183 567

577

• ORGANISING

COMMITTEE

•

Greg Skilbeck

Convention Convenor

University of Technology

Tom Hubble

Secretary

Sydney University

Misha Frankel

Treasurer

Geological Society Australia

Dick Glen

Technical Committee Chair

N S W Geological Survey

Colin Ward

Field Excursion Chair

University New South Wales

Brenda Franklin

Social Program Chair

Ian Levy

Sponsorship Joint Chair

Gympie Gold

Russell Meares

Sponsorship Joint Chair

Malachite Resources

Sonia Cousins

Public Symposium Chair

Geological Society Australia

Peter Downes

Committee

N S W Geological Survey

Chris Fergusson

Committee

University of Wollongong

• Convention Secretariat Julia & Nick Brooke - The Hotel Network

• Convention Co-Ordinator Sally Nathan - Accommodation, Conference & Event Management (ACE)

• MAJOR

SPONSORS

•

Australian Geological Survey Organisation M I M Exploration Toyota Motor Corporation A n g l o g o l d Australia • OTHER SPONSORS Gnomic Exploration Services Billiton Exploration Australia Pty Ltd North Limited Noranda Pacific Pty Ltd \/ J L L IL Vodaphone Olobalstar ^ I J n, L 1 £ Queensland Department ot

^

•

Placer Dome Asia Pacific James Cook University Anglo American Exploration (Australia) Nev/crest Mining ^ Black Range Minerals ^

Mines & Energy

Terra Search Pty Ltd

Straits Resources

Skilled Geoscience

Normandy Mining Limited

Society of Economic Geologists

•SUPPORTERS

•

University of Technology, Sydney Geological Society of Australia Australian Institute of Geoscientists N S W Department of Mineral Resources University of N S W Sydney University University of Wollongong