Abstracts 67 - 16th AGC Geoscience 2002 Expanding Horizons Adelaide 2002

16th Australian Geological Convention 1-5 July 2 0 0 2 Adelaide Convention Centre Adelaide, South Australia

GEOSCIENCE 2002: Expanding Horizons

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 67 16'^ Australian Geological Convention, Adelaide, July 2002

ISSN 0729-011X © Geological Society of Australia Incorporated, 2002-06-14 Copies of this publication may be obtained from the Geological Society of Australia Incorporated, 301 George Street, Sydney, NSW, Australia, 2002-06-14

This volume should be cited as: Preiss, V.P. (editor), 2002. Geoscience 2002: Expanding Horizons. Abstracts of the 16*^ Australian Geological Convention, Adelaide Convention Centre, Adelaide, SA, Australia.

July 1-5 2002, No. 67,457p.

Example citation for papers in this volume Mason, Douglas R., Edmonds, Warrick., and McLean, Douglas S., 2002. Thermodynamic modelling of altered and mineralised cataclastic granitoids at the Nyabirama Archaean Lodegold Deposit, Tanzania. In Preiss, V.P. (editor), 2002. Geoscience 2002: Expanding Horizons.

Abstracts of the 16"* Australian Geological Convention, Adelaide Convention Centre, Adelaide, SA, Australia. July 1-5 2002, No. 67, p.293

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 67

Australian Geological Convention, Adelaide, July 2002

Contents of Abstract Volume Listed Alphabetically by submitting author under Convention Themes MAWSON LECTURE White, AJ.R.

Bowens reaction principle and the evolution of granite mineralogy.

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Precambrian supercontinents verses supercratons.

Hydronamics of mineralising systems: active and ancient.

The upper temperature limits of microbial life.

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KEYNOTE SPEAKERS Bleaker, W. Sibson, R.H. Stetter, K.

THIRD SPRIGG SYMPOSIUM Blowing in the wind: Quaternary environments of Australia Bowler, J. (Keynote) Bestland, E.A. Lester, J., Brooke, B. and Cox, M. Brugger, J. et al.

Cameron, R.G. et al.

Cann, J.H. et al

Chivas, A.R. (Keynote) Clark, D. et al Clarke, J.D.A. Findlay, R.H., Hill, K. et al Findlay, R.H. and Haig, D.W. Forbes, M.S. et al Greene, R. S.B. et al Habermehl, M.A. et al Hearty, P.J. Hellstrom, J.

Late Cenozoic sea level changes, climatic evolution and Antarctic ice: evidence from the Murray Basin.

Age and origin of Terra Rossa soils in the NaracoorteCoonawarra area of South Australia.

Identifying factors influencing erosion of the spit on Bribie Island, SE Queensland.

Mineralogy and mobility of uranium around Mt Painter, northern Flinders Ranges, South Australia.

Mapping aeolian features in the eastem Murray Basin, southwestern N.S.W. - new insights using radiometric imagery.

A high resolution foraminiferal record of the postglacial (Holocene) marine transgression, St Vincent Gulf, South Australia.

Quaternary atmospheric processes: What came down in the last shower?

Multi-discipHnary neotectonics research in Australia: developing an improved basis for seismic hazard assessment.

Antiquity of aridity in the Atacama Desert.

Geomorphology and "^C and fission-track dating define rapid sporadic Pleistocene-present uplift from the Finisterre and Sarawaget Mountains, P.N.G. New palaeontological data from the Gasmata District, west New Britain, Papua New Guinea, and implications for the Oligocene-Recent tectonics of west New Britain.

Stable isotope investigation of cave and surface sediments in the Naracoorte region: evidence for a local aeolian source Stability of clay microaggregates in aeolian materials: USA and Australian perspectives.

Artesian springs of the Great Artesian Basin and their ages.

Stratigraphy and age of the carbonate dunes of Rottnest Island, Western Australia.

Application of multi-collector ICP-MS uranium series geochronology to very small samples: implications for Australian Quatemary studies.

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Hill, R.S. and Paull, R. Krull, E.S. and Skjemstad, J.O. McKirdy, D.M. et al Murray-Wallace, C.V. et al O'Leary, MJ. et al

Prescott, J.R. et al Sandiford, M. Tumey, C.S.M. et al

Warren, J.K. Williams M. et al

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Climatic forcing and floral succession: aridification of a continent.

Decomposition of soil organic matter: a question of age versus chemistry.

Lacustrine sapropels as proxies for late Quaternary environmental change in southeastern Australia.

Quaternary "whole rock" aminostratigraphy of southern Australian aeolianites.

Amino acid racemization geochronology of the planktonic foraminifera Pulleniatina obliquiloculata, from Queensland Trough piston cores, Australia.

Luminescence ages for the relict dune system in the SouthEast of South Australia.

Neotectonic framework of Australia.

Late Quaternary environmental changes in Queensland: a multi-proxy record from Lynch's Crater through the last 50,000 years.

Evaporites past and present: hydrological character reconciles modem and ancient depositional settings.

A re-appraisal of late Quaternary environments in the Flinders Ranges, South Australia.

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THEME 1: EARLY EARTH Symposium LI Early Life Rasmussen, B., Fletcher, LR. et al Foriel, J. et al Grey, K. Harrison, T.M. et al.

Jenkins, R.J.F. Brock, J., Logan, G. et al Norman, M. and Bennett, V. Runnegar, B. Van Kranendonk, M.J..

Walter, M. et al Winsor, C.N.

Isotopic dating of fossiliferous Precambrian sedimentary sequences: new developments, new results and an old problem - "how old are motile, megascopic organisms?"

High-resolution chemical imaging of fossilised hyperthermophile bacteria.

A sixth great extinction and recovery event? The Ediacarian Acraman bolide impact.

A mission to really early Earth: when did the Earth become suitable for habitation?

Early metazoan evolution.

Biogeochemical evidence for life in the Archaean and Proterozoic.

Siderophile elements in lunar impact melts: implications for the bombardment history of the early Earth.

Sulfur cycling during the Archean.

The role of hydrothermal systems in the flourishing of early life on Earth: evidence from the 3.49-3.43 Ga Warrawoona Group, North Pole Dome, Pilbara Craton.

Paleobiology of a 1640 Ma hydrothermal system: the McArthur River Pb/Zn deposit.

Examination of alternative mechanisms (nonbiological) to explain the development of bulbous-conical forms, with examples from the frregully Formation, Bangemall Basin, WA.

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Symposium L2 Archaean Tectonics Barley, M.E. et al Bennett, V.C. et al

Tectonic evolution of the Kalgoorlie Terrane: gold mineralisation in a late Archaean orogen.

Constraints on early Earth processes from geochemical investigations of the oldest (>3800 ma) abyssal peridotites.

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Bodorkos, S.

Cas, R.A.F. et al

Nesbitt, R.W. et al.

Nutman, h.?. et al Nutman, A.P. and Friend, C.R.L Pawley, M. et al

Rey, P. Kisters, A.F.M., Stevens, G. et al Stone, W.E. et al

Talusani, R.V.R. Thebaud, N. and Philippot, P. Van Kranendonk, M J .

Weinberg, R.F. Zang, W.

Conductive incubation and its role in the formation of dome-and-keel structure in the Archaean East Pilbara granite-greenstone terrane.

The oldest volcanics and sediments from the 3.7-3.8 Ga Isua Greenstone Belt, Greenland: implications for the earliest known palaeoenvironments on Earth.

Zircon U-Pb geochronology of the Zimbabwean Craton; "untangling the stratigraphy of greenstones belts".

The Geological setting of the oldest (>3.85Ga) possible life environments in West Greenland.

An Archaean Alps in Greenland: Archaean sutures, peridotites and an ancient continent-continent collision.

The development of Archaean granitoid domes by magma generation, transfer and emplacement: an example from the Shaw Granitoid Complex of the Pilbara Craton, W.A. Crustal scale strike slip faults in the Archaean: the result of buoyant subcontinental mantle.

3.2 Ga exhumation tectonics of a deep-crustal granitegneiss terrane on the southern margin of the Barberton Greenstone Belt, South Africa.

Lithophile element and isotope composition of hydromagmatic amphibole in high-Mg rocks of the Abitibi Belt, Canada: evidence for Archaean wet ultramafic melts.

Chemistry of late Archaean komatiites from the Mahakoshal Greenstone Belt, Central India Fluid-rock interaction within an Archaean triple junction: an example from the East Pilbara Craton, Western Australia.

Construction and internal re-organization of Earth's oldest, thickest volcanic plateau: the Archaean East Pilbara granite-greenstone terrane, Pilbara Craton, W.A. Character and timing of deformation in the NorsemanWiluna Belt, Yilgam Craton, W.A. Depositional setting of late Archaean greenstones and metasediments in the Harris Greenstone Domain, central Gawler Craton, South Australia.

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Symposium 13 Gawler Craton Allen, S.R. et al.

Betts, P. et al.

Davies, M. Direen, N.G. et al

Fairclough, M.C. et al

Ferris, G. et al Fraser, G. et al

Mavrogenes, J., Tomkins, AG. etal

Stratigraphy, distribution and geochemistry of three widespread felsic volcanic units in the Mesoproterozoic Gawler Range Volcanics, South Australia.

Orogenic evolution of the northern Gawler Craton.

Ironstone deposit models in South Australia.

A first-generation 3D model of the crustal architecture of the north-eastern Gawler Craton and implications for Olympic Dam-style mineral systems.

Three-dimensional structural and geophysical basement architecture of the southern margin of the Gawler Craton, South Australia.

Putting the Hiltaba Suite into a tectonic context.

Mesoproterozoic tectonism in the northwest Gawler Craton: "^^Ar/^^Ar geochronology, geophysical interpretations and extrapolations.

Geochronology of the Challenger gold deposit. northwest Gawler Craton, South Australia.

III

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Poster

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McLean, M.A. and Betts, P.G. McPhie, J. et al Milligan, P.R. et al

Raymond, O. et al Schwarz, M.P. et al

Simpson, C. et al Skirrow, R.G., Bastrakov, E.N. etal Skirrow, R.G. et al Woodhouse, A. et al

A three dimensional investigation of the Hiltaba Granitoid Suite geometry: implications for pluton emplacement mechanisms.

Voluminous felsic volcanism in the Mesoproterozoic Gawler Range Volcanics, South Australia.

Analysis and 3D visualisation of new compilations of potential field data to help constrain 3D geological models.

Copper-gold mineral systems in the south-eastern Gawler Craton - another Mt Isa Eastern Succession.

A plate margin setting for the evolution of the southern Gawler Craton: evidence fi-om detrital zircon and SmNd isotopic data of the Hutchison Group.

Evaluation of possible emplacement mechanisms for the widespread Yardea Dacite, South Australia.

Chemistry of high-temperature fluids in Fe-oxide CuAu systems of the Stuart Shelf basement.

Alteration and mineralisation settings in the Olympic Cu-Au province, Gawler Craton, South Australia.

Establishing a fi-amework for the Archaean development of the Gawler Craton.

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Poster

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THEME 2: PLATE MARGINS AND SUPERCONTINENTS Symposium 2A Defining processes in modern arcs.

Arculus, R.J. et al Berly, T. et al Campbell, I.H. et al

Caprarelli, G.

Crawford, A.J. et al Davidson, P. et al

Dijkstra, A.H. Elburg, M.A. et al

Finlayson, D. et al

Foden,J. et al Green, D.H. et al Green, T.H. and Adam, J.

Hergt, J.M. et al

Trachybasalt in the southern Coriolis troughs; a new global backarc basin end-member magma type.

The origins of pyroxenites in the Solomon Islands (Santa Isabel and San Jorge).

The Los Picos-Fortuna/Pajonal-El Abra Complex, a giant long-lived magma chamber associated with porphyry copper mineralisation.

Implications for Re/Os systematics of intra-plate magmas fi"om high pressure liquid metal / liquid silicate partitioning of moderately siderophile elements.

Melt inclusion evidence for the origin of Taupo Andesites.

Volatile phase exsolution - how it happens in nature?

An insight fi-om rhyolitic magmatic inclusions, Taupo Volcanic Zone, New Zealand.

Structure and petrology of fore-arc ophiolites: a late Proterozoic example from western Mongolia.

Geochemical trends across an arc-continent collision zone: magma sources and slab-wedge transfer processes below the Pantar Strait volcanoes (Indonesia).

Rabaul Volcano, Papua New Guinea: seismic tomographic imaging the magma reservoir of a restless volcano.

Sangeang Api volcano, Indonesia: relationship between alkaline arc magmas and pyroxene-rich xenoliths.

Island-arc ankaramites: primary magmas from refractory Iherzolite fluxed by CO2+H2O. Experimentally-determined trace element characteristics of aqueous fluid from partially dehydrated mafic oceanic crust at 3.0gpa, 650-700°C. Hafiiium isotope constraints on magmatism in the Lau Islands, SW Pacific.

IV

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Mclnnes, B.I.A.

Nicholls, 1. and Reubi, O. Paraschivoiu, V. and Foden, J. Rohrlach, B.D.et al

Schellart, W.P. et al Smith, LE.M. et al Talusani, R.V.R. and Sivell, WJ.

Turner, S. Woodhead, J.D. Yanagi, T.

Yaxley, G. and Brey, G.

The role of subduction in alkaline arc magmatism and Cu-Au metallogenesis: the Rosetta Stones of Lihir Island.

Evolution of mafic magmas at Slamet Volcano, Java , Indonesia.

Magma chamber processes at Tambora volcano. Eastern Sunda Arc, a geochemical approach.

Intra-arc compressive stress promotes development of slow-cooling, multiply-replenished lower crustal magma chambers parental to porphyry copper deposits.

Arc-perpendicular spreading ridges in the North Fiji Basin.

Geochemical fingerprinting of magmatic systems: examples from arc-type volcanoes.

Nd-Sr-O isotopic and trace element characteristics of the Wateranga mafic layered intrusion, southeast Queensland, Australia: implications for open system magmatic processes in a post-collisional setting.

Ultra-fast source-to-surface movement of melt at island arcs from Ra-Th systematics.

Isotopic constraints on element mobility within subduction zones.

Characteristics of arc magmatism as exemplified by the Cenozoic volcanism and plutonism in the Japanese islands.

The link between subduction and carbonatites.

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Symposium 2,2 Chris Powell Symposium: Understanding the Earth Avouac, J-P. Bennett, V.C. et al

Berry, R.F.and Burrett, C.F. Borissova, I. et al Buchan, C. et al

Cawood, P.A. Close, D et al Collins, W.J. et al Condie, K.C. Crawford, A.}, et al

Cutten, H.

Dalziel, I.W.D. Evans, D.A.D.

Active mountain building in the Himalaya of Nepal.

Combined osmium isotopic and seismic evidence for orphaned early Proterozoic mantle beneath Phanerozoic crust in the New England Fold Belt, eastern Australia.

Mexico - Tasmania correlations.

Age and Plate Tectonic Setting of the Labuan Basin.

Tectonic evolution of the Bayankhongor Ophiolite, Central Mongolia: implications for the Palaeozoic crustal growth of Central Asia.

Terra Australis Orogen: Rodinian breakup and subduction initiation in the Pacific Ocean.

Identification of new terrains in the Southern Arunta Province, central Australia.

Tectonic model for widespread granite magmatism and high-t metamorphism in the Tasmanides.

Global Changes: possible effects of two supercontinents.

120 - 0 Ma Tectonic Evolution of the SW Pacific, and Analogous Geological Evolution of the 600 to 220 Ma Tasman Fold Belt System.

The Mozambique Belt, eastern Afiica - tectonic evolution of Gondwanaland amalgamation and (?)

Rodinia breakup.

Chris Powell, Rodinia and Pannotia: Hobart '91 to Adelaide '02 and beyond.

Quantitative reconstruction of ArcheanPaleoproterozoic supercontinent Kenorland.

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Fergusson, C.L.

Findlay, R.H.

Findlay, R.H.

Findlay, R.H. Arumba, J. et al Foster, D.A. et al

Gaina, C. et al Giles, D. and Betts, P. Glen, R. A. et al Goodge, J.W.

Gray, D.R. et al Halilovic, J. et al.

Hand, M. and Goscombe, B. Hand, M., Lee, M. and Foden, J. Harrison, T.M. Hillis, R.R. Hulscher, B. et al.

Keep, M. et al Korsch, R.J. et al

Kroner, A. Lennox, P.G. et al

Li, Z.X.

Link, P.K.

Ophiolite generation, sedimentation and accretion in an early Palaeozoic remnant ocean basin, southeastern Australia.

The Collision Tectonics of Northern Papua New Guinea: Key Field Relationships in the Finisterre, Sarawaget and Adelbert Mountains and New Britain Demand a New Model.

Palaeostress analyses from the Mt Divide Fault Zone at Mt Cook, South Island, New Zealand and implication for kinematic history along the New Zealand sector of the Australian-Pacific plate boundary.

Palaeostress analyses suggest synchronous Pliocene to present regional strain partitioning across eastern central Papua New Guinea.

Dating metamorphism and exhumation of Forth Metamorphic Complex, Tasmania with implications for Paleozoic tectonics of the East Gondwana margin.

A new geodynamic model for Mesozoic-Cenozoic NeoTethys/Indian and Pacific Oceans.

Paleoproterozoic to Mesoproterozoic assembly of Australia: implications for Rodinia reconstructions.

The Narooma Terrane: implications for the construction of the outboard part of the Lachlan Orogen.

Neoproterozoic to early Paleozoic record of Rodinia breakup and Gondwana-margin convergence in East Antarctica.

Understanding the nature and cause of complex deformation patterns in the Tasmanides.

Results of shrimp analysis of detrital zircons from Palaeoproterozoic Earaheedy Group: implications for the assembly of the West Australian Craton.

What is the main central thrust of the Himalayan Orogen?

Contrasting sources of Palaeozoic mafic dykes emplaced during early Palaeozoic rifling in Central Australia.

Seventy-eight years of Himalayan-Tibetan tectonic models.

Coupled changes in pore pressure and stress in oil fields and sedimentary basins: implications for deformation.

Granitoids marking final Gondwanaland amalgamation: Preliminary U-Pb zircon ages from central Madagascar The Sumba collision and its affects on the northern Australian margin.

Relationships between tectonic elements in the Tasmanides, eastern Australia, based on deep seismic reflection profiles.

What does a former accretionary orogen look like in the lower crust?

Structural and gravity modelling along two transects of the Southern Wyangala Batholith, Molong Zone, Eastern Lachlan Fold Belt, NSW. Synchronous Neoproterozoic magmatic events in Australia and South China: two plumes, or a superplume?

Tracking recycled Australian Proterozoic zircon grains through the Belt Supergroup into modem streams, Idaho, U.S.A.

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Lister, G.S. et al Maas, R. et al

Mills, KJ. Milsom, J. Noll,C. et al Offler, R. et al.

Pirajno, F. et al

Pirajno, F. et al.

Pisarevsky, S. Roberts, J. and Fanning, M.

Rosenbaum, G. et a\.

Sandiford, M. Schellart, W.P. et al.

Sircombe, K. et al

Spaggiari, C. et al

Taylor, D.H. et al Tyler, LM. et al V. Penglis and Foden, J.

Willey, E.G. Windley, B. et al Zheng, H. et al

The formation and exhumation of high-pressure metamorphic terranes during SW-Pacific-style orogeny Nd-Sr isotopic evidence for the origin of Devonian granitic and felsic volcanic rocks of the w/estem Lachlan Fold Beh. Relationship between the Delamerian and Lachlan Fold Belts in western New South Wales.

Ophiolite problems in northern Papua New Guinea : the ins and outs.

Insights into the tectonostratigraphic development of the Owen Basin, West Coast Range, Western Tasmania.

Subduction of continental arc basaltic andesiteimplications for the tectonic history of the Southern New England Fold Belt.

A possible large igneous province (LIP) in cental Western Australia: Implications for mineralisation models.

Geology, tectonic evolution and mineralisation of palaeoproterozoic basins of the Eastern Capricorn Orogen, Western Australia.

The assembly and breakup of Rodinia: animated history.

Proximal Carboniferous volcanogenic successions, north-western Tamworth Belt: correlation and SHRIMP dating.

Constraints on the motion of the Adriatic Plate.

Tectonics and stratigraphy in Neoproterozoic basins, Australia.

Back-arc deformation in the Kuril Basin and Sea of Okhotsk: far field effect of India-Eurasia collision or the result of rollback of the pacific slab?

Reconnaissance detrital zircon geochronology provenance of the palaeoproterozoic Ashburton formation: Implications for Pilbara and Yilgam amalgamation.

Late Ordovician -Ssilurian ophiolite emplacement in the western and central Lachlan Orogen, southeast Australia.

The Delamerian Orogeny: arc-continent collision starts the accretionary growth of eastern Australia.

Palaeoproterozoic plate collision in the Kimberley Region of northern Australia: a connection to Laurentia.

Mafic magmatism in time and space - a key to correlating dispersed fi-agments of the palaeo-active margin (Late Proterozoic to Early Palaeozoic) of southeastern Gondwana.

Petrography of the Marongi Creek Beds, Yarraman Block, South Eastern Queensland.

Accretionary orogens and continental growth.

The impact of Tibetan uplift on global change.

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Symposium 2.3 Thermal and structural evolution of continental collision zones Barovich, K.M.

al

Barovich, K.M. and Foden, J.

Geochemical and Nd isotope constraints on provenance of the Willyama Supergroup, South Australia, and comparisons to the Mt. Isa Inlier Nd isotope constraints on the origin of 1580 Ma Cumamona Province granitoid magmatism.

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Bell, T.H. et al

Early formed regional antiforms and synforms that fold younger matrix schistosities: their effect on sites of mineral growth.

Buick,LS. etal U-Pb, Nd- and C-Isotope evidence for the occurrence of Palaeoproterozoic crust in the central zone of the Limpopo Belt, South Africa.

Buick, LS., Huang, M. et al Polyphase metamorphism in the Songpan-Garze Orogenic Belt (China): P-T-t-d paths and constraints from U-Pb, Sm-Nd and Rb-Sr geochronology.

SHRIMP zircon provenance studies of protoliths to the Buick, LS. et al Harts Range Metamorphic Complex (Arunta Inlier).

Late-Tertiary exhumation of core complexes in southCarter, T J. et al western USA: an application of (U-Th)/He dating.

The pitfalls of sectioning rocks perpendicular to Cihan, M. foliations and lineations in the matrix.

New geochronology from the Strangways Metamorphic Cobb, M. et al Complex, Arunta Block - central Australia.

Lithostratigraphy and depositional architecture of the Conor, C.H.H. Willyama Supergroup, Olary Domain, Cumamona Province, South Australia.

Crispe, AJ.et al Tectonic setting and magmatism in the Tanami Region.

Crowhurst, P.V. et al Using the single grain apatite (U-Th)/He thermochronology laser extraction technique to improve precision of thermal history analyses: examples from the Otway Basin, South-eastern Australia.

Elburg, M. et al Palaeozoic events around Arkaroola, northern Flinders Ranges, South Australia.

Evans, T.P. Using inclusion trail geometries to constrain pressuretemperature-time path.

Forbes, C. et al Regional scale variations in fold interference patterns: implications for the Allendale area, Broken Hill Block, New South Wales, Australia.

Gessner, K. et al Lower crustal flow in continental extension — a process for mechanical recycling of the crust during orogenic collapse.

Ghisetti, F. and Vezzani, L. Rapid transition from active crustal contraction to extensional collapse in the Apennines (Italy).

Gibson, G.M. et al Structure of mineralised Palaeoproterozoic rocks in the Outalpa Inlier, South Australia.

Hand, M. and Rubatto, D. The scale of the thermal problem in the Mt Isa Inlier.

Hand, M. et al Widespread Neoproterozoic metamorphism in Northern Scotland and constraints on Caledonian metamorphism.

Heinisch, E.R. and Foden, J. The field relations within the Tanunda Creek Gneiss and the Kanmantoo Group and its evolution.

Holm, O.H.and Berry, R.F. Structural and metamorphic evolution of the Arthur Lineament, northwest Tasmania.

Huddlestone-Holmes, C. and Analysis of curved inclusion trails in garnet Ketcham, R. porphyroblasts using computed x-ray tomography.

Leitch, E.C., Och, DJ. et al Evolution of high-pressure metamorphic rocks from the Rocky Beach Metamorphic Melange, Port Macquarie, northeast New South Wales.

Lorencak, M. et al Application of combined apatite (U-Th)/He thermochronometry and fission track analysis on cratons: possibilities and limitations.

Maidment, D.W. The extent of Cambrian metasediments and the Ordovician Larapinta Event, eastern Arunta Province, central Australia.

VIII

p.157 p.l58 Poster Poster

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McGrew, AJ. McLaren, S. et al Mendis, P J. (Lai) et al Phillips, D. and Harris, J. Raetz, M. et al Reynolds, S.D. et al Rosenbaum, G. et al Schellart, W.P. and Nieuwland, D.A. ScrimgeoLir, L et al Shaw, S.E. and Flood, R.H. Sivell, WJ. and Passmore, MJ. Soinmacal, S. et al Spandler, C. et al Wade, B.P. et al Webb, G. et al Weber, U.D. et al Welch, P.W. Winsor, C.N.

Spreading the welt or melting to spread? Late Cretaceous to Eocene tectonothermal evolution of the hinterland of the Sevier orogenic belt. Western USA. The thermal evolution of the Mount Painter province: granite genesis during thermal sag?

Geological structure of the Flinders Ranges, South Australia.

Putting the squeeze on thermobarometry : the effect of differential mineral compressibility on pressure estimates.

Stratigraphic omission explained by discovery of a regional folded ductile shear zone in the Euriowie Block, NSW. The Australian stress field: implications for plate boundary forces.

Coaxial flattening at deep levels of orogenic belts Poster (Syros and Sifiios, Cyclades, Greece).

4d modelling of pop-up structures during strike-slip Poster faulting: some insightsfi-omanalogue modelling.

The Liebig Event - 1640-1630 Ma deformation, magmatism and high grade metamorphism in the southern Arunta Province.

A reconnaissance ^"^^Hf^^^^Hf study of the New England Poster Batholith using LAM-MC-ICPMS microanalysis of zircons.

Widespread mafic and felsic post-collisional PermoPoster Triassic magmatism in eastern Australia: transform links.

Computational petrology and pyroxene thermodynamics.

Whole-rock and mineral chemistry of blueschists and eclogitesfi-omnorthern New Caledonia and some implications for subduction-zone processes.

Neodymium isotopic and geochemical constraints on provenance of sedimentary rocks in the eastern Officer Basin, Australia: implications for the duration of the intracratonic Petermann Orogeny.

A patchy orogen? Allochronous anti-clockwise P-T-t paths across the southern Delamerian Fold Belt, South Australia.

Implications of post-Palaeoproterozoic thermochronological datafi-omthe Precambrian northern Western Australian Shield.

Poster Electron microprobe monazite agesfi-ommultiply deformed schists, SE Vermont, USA: new constraints for the timing of metamorphism and deformation.

Si(2) cleavage reactivation, evidencefi-omD3 quartz veins in the Kanmantoo Group metasediments during the Delamerian Orogeny.

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Symposium 2,4

Fluids, metals and melts: their extraction, transport and emplacement in convergent place settings.

Barovich, K.M. and Ashley, P.M. Brown, M.

Peraluminous trondhjemites in the Cumamona Province, Olary Domain, SA: magmatic or metasomatic?

Melt extraction, ascent and emplacement during orogeny: feedback relations and self organization.

IX

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Buckman, S Buick, LS. et al Clarke, G.L. et al

Collins, WJ. et al Davidson, P. et al Elburg, M.A., Bons, P.D. et al Elliston, J. Fu, B. et al Hack, A. et al Honda, M. et al

Knaalc, M.L. et al Oliver, N.H.S.er al

Richards, S.W. et al

Ridley, J. Satish-Kumar, M. et al Sivell, W J. Spicer, E. et al

Stevens, G. and Clemens, J.D. Stevens, G., Preston, R.F. et al Storkey, A. et al Wing, B.A. et al

Symposium

Tectonics and metallogenesis of the Paleo-Asiatic Orogenic Belt (PAOB), NW China.

High-pressure melting and fluid flow during the Petermann Orogeny, central Australia.

High-P partial melting of dioritic gneiss: LA-ICPMS analysis of Pembroke Granulite mineralogy, Fiordland, New^ Zealand.

The granite-migmatite connection: application to Lachlan Fold Belt S- and I-type granites.

Magmatic fluids coexisting with felsic melts: an example from Rio Blanco, Chile.

A numerical model for self-organised criticality in melt accumulation and ascent in partially molten rocks.

Natural sediment properties by which mud crystallises to schists, gneisses, and igneous-looking rocks.

Fluid regimes in eclogites and granulites from the Dabie-Sulu terranes, eastern China.

Copper solubility in mineral-buffered, near-magmatic, supercritical fluids: insights from LA-ICP-MS, PIXE and EXAFS of synthetic fluid inclusion experiments.

Unusual noble gas compositions in polycrystalline diamonds: preliminary results from Jwaneng, Botswana.

Contrasting patterns of granitoid emplacement and deformation, southwest Cumamona Province Un-mixing and re-mixing: chemical extraction, transport and deposition paths accompanying regional alteration and ore deposition in the Cloncurry Fe-oxideCu-Au district, Mt Isa Block, NW Queensland.

Magma transfer, emplacement and construction: the role of migmatites and compressive deformation in controlling the shape and style of pluton emplacement.

Pathways and modes of fluid release from granites in the cores of active orogenic belts.

Origin and evolution of high temperature skam fluids at Fuka, Okayama, Japan.

Sr-Nd isotopic constraints on the origin of the Stanthorpe Granite Group: role of a depleted mantlederived component.

Experimental constraints on the role of boron in the low-pressure fluid-absent partial melting of metapelites from the Mt Stafford area, central Australia.

Melt compositions from partial melting experiments on metapelites: implications for a restite component in Stype granites, and decompression textures in granulites.

Fluids in equilibrium with silica-undersaturated magmas in the system Na20-Al203-Si02-H20: clues to the composition of fenitizing solutions.

SHRIMP U-Pb geochronology of pegmatites from the Harts Range, central Australia The magnitude and three-dimensional geometry of regional-scale fluid flow during Barrovian metamorphism, south-eastern Vermont, USA.

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2,5

Antarctic Symposium: special session in memory of Robin Oliver, 1921 - 2001 Boger, S.D. et al

Is the final suture between east and west Gondwana in East Antarctica?

X

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Curtis, M. and Millar, I.

Fanning, C.M. et al

Fanning, C.M., Pelletier, A. et al Fanning, C.M., Jacobs, J. et al Fanning, C.M., Peucat, J J . et al Fitzsimons, I.C.W.

Goodge, J. Ireland, T.R. et al Kelly, N.M. et al

Sajeev, K. and Osanai, Y.

Shabeer, K.P. et al

Shiraishi, K. et al Simpson, A.L. Talarico, F. and Sandroni, S.

Talarico, F. and Kleinschmidt, G. Williams, 1. et al Zulbati-Petrillo, F. et al

Rapid deformation, exhumation and tectonic switching during the Ross Orogeny, Pensacola Mountains, Antarctica.

A closer examination of the direct links between southern Australia and Terre Adelie and George V Land, Antarctica.

Paleoproterozoic tectonics of the Terre Adelie Craton, (East Antarctica).

Extensional collapse of the Late Neoproterozoic/Early Paleozoic East African/Antarctic orogen in central Dronning Maud Land, East Antarctica.

Southern Australia 1.6 Ga Felsic Volcanic-type recognized in the moraines of the Terre Adelie Craton in Antarctica.

Comparison of detrital zircon ages in the Pinjarra Orogen (WA) and Maud Province (Antarctica): evidence for collision of Western Australia with southern Africa at 1100 Ma. Nature of the East Antarctic Shield along the Pacific margin of Antarctica.

Geochronology of basement rocks from Marie Byrd Land, western Antarctica.

High-temperature decompression in orthopyroxenecorundum-sapphirine metapelites from the Rayner Complex, East Antarctica.

Evidence for counter-clockwise evolution of spr-qtz & opx-sil-qtz-bearing granulites from Highland Complex, Sri Lanka.

Two-stage spinel generation in the high-grade metapelites of the central Kerala Khondalite Belt: implication for prograde P-T path.

Pan-African events in the eastern Dronning Maud Land, East Antarctica.

Ross Orogeny magmatism in the Darwin Glacier region, southern Victoria Land, Antarctica.

Migmatitic granulites from the Transantarctic Mountains and central Dronning Maud Land: new constraints on the petrological evolution of the East Antarctic Shield.

The Mertz Shear Zone: new evidence of the "Mawson Continenf from the East Antarctic Shield in George V Land.

Large-scale sediment dispersal associated with the late Neoproterozoic assembly of Gondwana.

The P-T-fluid conditions of end-Archaean granulite facies metamorphism in the Vestfold Hills, East Antarctica .

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Symposium 2,6 Lava Symposium Bann, G. Batiste, C. and Ramsay, W.R.H.

Explosive volcanism in the early Gerringong Volcanics, Permian south-eastern Sydney Basin, Australia.

Facies analyses and palaeovolcanology of a multiple scoria cone complex - Mount Buninyong, Central Victoria.

XI

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Bryan, S.E. et al

Ebsworth, G.

Meakin, S. et al Mortimer, C. et al

Patia, H. et al

Pittari, A. et al

Purvis, A. and Sun, X. Reubi, O. Roach, I.e. Gonzalez, O.L. et al Sutherland, V.L.et al Tait. M.A et al

Wallace, P. et al Willey, E.G. Zaw, K. and Delia Pasqua, F.N.

Revised stratigraphy and facies architecture for the Late Devonian to Early Carboniferous Campwyn Volcanics of central Queensland: implications for the tectonic framework of the northern New England Fold Belt.

Submarine record of widespread andesitic volcanism: magnetic sandstones in the Tyndall Group, Mount Read Volcanics, Tasmania.

The Ural Volcanics: a Siluro-Devonian felsic volcanic succession in central NSW. Geomorphological and volcanic features of the Adelbert Mountains - Ramu Valley area of Papua New Guinea: implications for regional neotectonic activity.

The 1994 to 2001 eruption at Rabaul, Papua New Guinea: Evidence of repeated basaltic magma influx into a sub-caldera dacite magma reservoir.

Internal stratigraphy of the climactic Abrigo Ignimbrite, Tenerife, Canary Islands: implications for eruption and emplacement processes.

Geochemistry and facies of Cambrian volcanic rocks, eastern Warburton Basin, South Australia.

Caldera forming eruptions at Batur Volcano, Bali, Indonesia.

Volcanology of Leucitite vents at El Capitan, Central NSW. Bassian basalts: dating, Cenozoic biogeohistory and a new model for Tasmanian volcanism.

Explosive Plinian activity and dispersal in the Andes: facies and internal stratigraphy of the Late Miocene Corte Blanco Tuff; Puna Plateau, Salta Province, NW Argentina.

A further instalment in the Rabaul story: The plots thicken.

Lava pools in the Main Range Volcanics, Toowoomba, southeast Queensland.

Chemical affmities of Loei Belt volcanic-intrusive rocks, central Thailand.

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THEME 3: METALLOGENESIS AND ORE DISCOVERY Symposium 3A World-class Australian ore deposits Bateman, R. and Groves, D. Belperio, A.P. Crook, D. Daly, S.J and Conor, C.H.H. Gow, P. et al Groves, T>l.etal Hill, R.E.T. and Perring, C.S. Hodgkison, J.P. Jacques, A.L. and Smith, C.B. Jeffrey, S.

The Golden Mile, Kalgoorlie: character and controversy.

The Prominent Hill copper-gold discovery: exploration strategies for world-class iron oxide Cu-Au deposits.

Ore genesis of a nickel laterite deposit.

South Australian exploration analogues.

The Mount Isa-Lawn Hill Zn-Pb-Ag and Cu province.

Methodology for computer-assisted integrated targeting for world-class ore deposits in Australia, with emphasis on orogenic gold deposits.

Kambalda nickel deposits and their volcanic environment.

Olympic Dam: in a worldly class of its own.

The Argyle (AKl) diamond deposit. Western Australia.

The Cannington Ag-Pb-Zn Broken Hill Type deposit: a world class discovery with a silver lining.

XII

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Large, R. et al 0'Loughlm,N.T. et al Plimer, LR. Stone, M.S. et al Tedder, IJ. and Wilson, A J .

Townsend, IJ. Walshe, J.L.

What is a world-class ore deposit?

The Bondi Heavy Mineral Sands Deposits, Western Victorian Murray Basin.

The Broken Hill Zn-Ag-Pb deposit, NSW. Towards a model for the genesis of the Tertiary Yandi channel iron deposit. East Pilbara, Western Australia.

The Cadia gold-copper porphyry district, NSW. Coober Pedy Opal Field.

The building blocks of conceptual exploration models for world-class ore deposits.

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Symposium 3,2 Gold-rich mineral systems in the Tasmanides of Eastern Australia Andrew, A.S. et al Bierlein, F.P. et al

Bierlein, F.P. et al

Bottrill, R.S. et al Kreuzer, O.P.

Krokowski de Vickerod, J. et al

Mustard, R.

Squire, R.J. et al

Withnall, I.W. and Hutton, L.J. Wood, D.G. et al

Source of gold in Victorian deposits: lead and stable isotopic evidence.

Timing of orogenic gold mineralisation in NE Tasmania in NE Tasmania in the context of a metallogenic and tectonic framework for the Lachlan Orogen.

'Atypical' gold mineralisation at Malmsbury - the case for intrusion-related ore systems in the western Lachlan Orogen.

The Forster Au-Zn-Ni Prospect, Glovers Bluff area.

South-eastern Tasmania.

Kinematics, structural architecture and textures of mesothermal gold veins in the Charters Towers area, northeast Queensland, Australia.

Lineation and stress/strain analysis as an exploration technique for en echelon fault/quartz vein-hosted gold mineralisation: examples from the Tamagulla and Central Maldon goldfields, Victoria, Australia.

Textural, mineralogical and geochemical variation within the zoned Timbarra Tablelands Pluton, northern New South Wales, Australia.

The Peak Hill high sulfidation Au-Cu deposit, NSW: spatial and temporal relationships between alteration zoning, deformation and mineralisation.

Permo-Carboniferous gold deposits of the ConnorsAuburn Arch: epithermal mineralisation associated with the transition from arc to extensional volcanism.

Structural controls on the timing of gold emplacement in the Bendigo Gold Field.

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Symposium 33 Metals in hydrothermal brines and vapours Bierlein, F.P., Wilde, A. et al Binu-Lal, S.S. et al

Brugger, J. et al Carew, M.J. et al Cromie, P.W. and Zaw, K.

Occurrences of platinum-group elements with sedimenthosted orogenic gold deposits: proposed model and potential in Australia Ore fluids in Wyanad gold mineralization. South India: evidence from fluid inclusion microthermometry and gas analysis.

Formation of willemite in hydrothermal environments.

Hydrothermal alteration of the Mount Fort Constantine area, NW Queensland.

Formation of the Fu Ning Carlin-type gold deposits, Yunnan Province, China: constraints from geological setting, mineral paragenesis and ore fluid chemistry.

XIII

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Davidson, G J . et al Evans, N. et al

Liu, W. et al

Mark, G. et al.

Marshall, L. and Oliver, N. Mason, D.R. et al

McLellan, J.G.and Oliver, N.H.S. McPhail, C.D. et al Norman, M. et al

Philippot, P. et al Ryan, C.G. et al Schmidt-Mumm, A. and Penhall, J. Tonelli, M.

Ulrich, T. et al

Zhang, Y. et al

A decompression model of formation for some oxide Cu-Au deposit-forming fluids — a hypothesis.

PreHminary results of fluorite (U-Th)/He thermochronology: application to Yucca Mountain, Nevada.

UV spectrophotometric study of copper(i)-chloride complexing and application to copper transport in porphyry hydrothermal solutions.

Predicting alteration patterns in the outflow zones of hydrothermal ore systems: a case study using the 'spent' fluids from the Ernest Henry Fe oxide-(Cu-Au) deposit.

From source to sink: Evolving fluid characteristics in Mt Isa Inlier Fe-oxide-Cu-Au mineralisation.

Thermodynamic modelling of altered and mineralised cataclastic granitoids at the Nyabirama Archaean lodegold deposit, Tanzania.

Numerical modelling of deformation and fluid flow in the Hamersley Province, W.A. with implications forgenesis of large microplaty hematite ores.

Metal transport in hypersaline brines.

A new method for measuring osmium isotopic compositions using multi-collector ICPMS: application to ore deposits.

Geochemistry of individual fluid inclusions: application to mining and oil industry.

High resolution nuclear microprobe imaging and analysis of minerals and inclusions.

Epigenetic copper mineralisation in the Bimbowrie area, Olary Domain, South Australia.

Pb-Pb stepwise leaching (PbSL): a new geochronological tool for high-grade polymetamorphic terranes.

Fluid composition and its origin of the Mt Morgan CuAu deposit, revealed by quantitative LA ICP-MS analyses and isotopic signatures.

Understanding thermal structures relevant to gold mineralisation in the Witwatersrand basin: a numerical modelling approach.

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Symposium 3.4 3.4a Geochemical exploration Baulch, J.C

Carey, M.L. and McPhail, D.C.

Carr, G.R. et al

Gray, D.J. et al

Exploring for orebodies under deep cover - the surface geochemical signature of the Wallaby gold deposit, Laverton, Western Australia.

The influence of playa lake related convective flow on groundwater element distributions around gold orebodies: planning effective hydrogeochemical surveys.

Terrain-specific Pb isotope model curves for Mount Isa and McArthur - the age of the Mount Isa Zn-Pb and Cu deposits.

Karari Au deposit, Western Australia: furthering our understanding of supergene gold dispersion and calcrete Au.

XIV

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Jaques, A.L. and Huleatt, M.B. Keeling, J.L. et al Lawrance, L.M. Memagh, l.V.et

al

Rutherford, L. et al Sheard, MJ. and Lintem, MJ. Stanley, C. et al

Stone, W.E. and Crocket, J.H.

Whitbread, MA.

Whitford, D. J. et al

Australian Gold Discovery Costs - Getting Better?

Hydrothermal alteration in deeply weathered terrains.

In situ laterite: fact or fallacy? Implications for geochemical exploration.

Characterisation of hydrothermal zircons in porphyry. epithermal and lode gold deposits.

The Billeroo North ijolite-syenite-carbonatite breccia complex, Cumamona Province, South Australia.

Exploration through transported cover and deeply weathered regolith, ET Gold Prospect, Gawler Craton, S.A. Lithogeochemical exploration and hydrothermal alteration in intermediate and felsic volcanosedimentary rocks hosting the Golden Grove Massive Sulphide Camp, Yilgam Craton, Western Australia.

Platinum-Group element contents of Chromite from layered ultramafic units, Abitibi Belt, Ontario: Implications for geochemical behaviour and mineral exploration.

Ratio analysis of lithogeochemical data: providing the tools to track cryptic alteration around sediment hosted ore deposits.

Barite in the Mount Isa Inlier and McArthur Basin: sulfur and strontium isotopic composition.

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Symposium 3,4b Geophysical exploration Calandro, D. Carey, H. et al Drummond, B J., Jones, L.E.A. etal Duffett, M. Guo, B. and Lackie, M. Hodge, 1. and Nichols, W.

Lackie, M. and Edmiston, M. Lilley, F.E.M. et al Mauger, A J . and McConachy, G.W.

Data mining - reducing the risk.

Innovative electrical geophysical methods for exploration undercover.

Imaging steep dips using the seismic reflection technique in basement areas.

Relationships between geophysics and geochemistry in the Isa Superbasin.

A gravity investigation of the Hunter-Mooki and Peel faults.

Geochemical analysis of petrophysical properties of intrusions in the Callide Basin for geophysical interpretation.

Investigation of the gravity signature of the Yeoval Batholith.

The Carpentaria structure of high electrical conductivity in Western Queensland.

Mineral mapping technology applied in the Tarcoola District, South Australia.

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Symposium 3,5 Ore deposit case studies Barratt, R.M. Beresford, S.W. et al Both, R. A. et al

Potential for sediment-hosted Pb-Zn in the Paragon Group, Broken Hill Block, NSW. Kambalda revisited: re-evaluation of the world class komatiite-hosted Ni-Cu-(PGE) ore province Fluid inclusion and stable isotope evidence for the origin of the Moonta copper-gold deposits. South Australia.

XV

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Corbett, G. and Hunt, S. Giles, C. Groves, I. and Carman, C. Hell, A. J. et al Hooper, B. Jami, M. and Dunlop, A. Keays, R.R. Marshall, L. Mendis, P.J. (Lai) et al Memagh, T.P. et al Sims, D. Skwamecki, M. et al Skwamecld, M. et al Spry, P.G. and Scherbarth, N.L. Stevens, B. Stone, W.E. et al Sweetapple, M.T. et al

Teale, G.S. Webb, A D .

Zang, W. Zaw, K.

Ladolam gold deposit, Lihir Island, Papua New Guinea Discovery history of the North Mara gold deposits, Tanzania.

The Reliance zinc deposit. Flinders Ranges, South Australia.

The stratigraphy and structural framework of the Merlin diamond field. Northern Territory, Australia The Nifty copper deposit: current understanding and future potential.

The geological setting and characteristics of the Esfordi apatite-iron deposit, Bafq District, central Iran.

The role of the mantle in the formation of giant ore deposits.

Cloncurry District fold breccias: fiiend to alteration, foe to mineralisation.

Structural and geochemical control of barite veins in the Flinders Ranges, South Australia.

Gold mineral systems in the Tanami region.

Controls on high-grade gold distribution in the Vera Nancy deposits, Queensland.

Secondary lead minerals in acid sulfate soils, Mount Torrens prospect: implications for mineral exploration.

Mineralogy of the Mt Torrens gossan. South Australia.

The vanadium-tellurium-gold association at the epithermal Tuvatu gold deposit, Fiji: implications for ore deposition.

Magnetite-bearing zones in Broken Hill metasediments - signatures of black smokers?

Towards a holistic geological model for Kambalda nickel sulphide deposits.

Internal evolution and paragenesis of tantalum mineralization in the Wodgina main lode pegmatite, Wodgina pegmatite district, Pilbara Craton, Western Australia.

Lateritic nickel-cobalt mineralisation from the Weda Bay deposit, Halmahera Island, eastern Indonesia.

Chemical and mineral paragenesis of Dales Gorge Member BIF across the eastern Hamersley province.

Western Australia.

Interpretation of stratigraphy and Portia mineralisation.

Olary Domain, South Australia.

Sediment-hosted gold mineralisation at Kyaukpahto, Kawlin-Wuntho District, northern Myanmar.

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THEME 4: SEDIMENTS Symposium 4.1 Continental Shelves Adabi, M.

Bone, Y. and Schmidt, R. Bone, Y. and James, N.P.

Bone, Y . , Daniel, R. et al

Petrographic and geochemical criteria for recognition of unaltered cold water and diagenetically altered Neoproterozoic dolomite, western Tasmania, Australia.

Eocene bryozoan palaeoenvironments and assemblages.

St Vincent Basin, South Australia.

Bryozoans from Pleistocene high-energy, cool-water, continental -slope mounds. Great Australian Bight, southern Australia.

Origin and composition of shallow cool water carbonate mud. Streaky Bay, South Australia.

XVI

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Bostock, H.C. and Opdyke, B.N. Boyd, R. et al Carter, R.M. and Larcombe, P. Collins, L.B. Fellows, M.E. et al Gibson-Poole, C. and Lang, S.C. Haig, D.W.

Hocking, R. and Playford, P.E. Holdgate, G.R. et al.

Kopi, G. et al

Lang, S.C. et al

Lewis, S. et al.

Liaghati, T. et al.

Lockhart, D.A. and Lang, S.C. O'Brien, G.W.O. and Glenn, K.C. Page, M.C. and Dickens, G.R.

Paraschivoiu, E. et al.

Polloclc, R. et al.

Read, J.F. and Coffey, B.

Reid, C. Roberts, J. and Boyd, J. Ruxton,B.P.

Sedimentation rates within the Capricorn Channel, Southern Great Barrier Reef, Australia.

Synthesis of continental margin sedimentation on the southeast coast of Australia.

Cyclone pumping and sediment partitioning in the development of the Great Barrier Reef shelf system.

Tertiary foundations and Quaternary evolution of coral reef systems of the Northwest Shelf, Australia.

Acoustic seafloor mapping of south east Australia.

Poster Palaeogeographic evolution of the Jurassic to Cretaceous succession in the Petrel Sub-basin.

Stratigraphic events recorded in Upper Cretaceous chalk along 1000 km of the Western Australian continental shelf.

Siliciclastic conglomerates associated with Devonian reef complexes. Canning Basin, Western Australia.

Quaternary history and tectonics of the Gippsland Shelf - Victoria.

Cretaceous macrofossils from the Ramu Valley and the Snake River, Papua New Guinea place the northern terranes of PNG and the Owen Stanley Metamorphics in Gondwana.

Sedimentology and sequence architecture of Quaternary sediments on the south-east Queensland continental margin.

Sedimentology and geochemistry of the Julia Creek vanadium deposit, Toolebuc Formation (Albian), Great Artesian Basin.

Determination of Quaternary sediment sources using mineralogy and geochemistry in Bells Creek catchment, Pumicestone Passage, southeast Queensland.

High resolution sequence stratigraphy of a Quaternary incised valley: from coastal plain to continental shelf Surficial sediments of the Browse and Bonaparte Basin transition zone.

Massive siliciclastic flux to the Queensland Trough during transgression: a mixed siliciclastic/carbonate margin's response to late Quaternary sea-level change.

Central Great Barrier Reef Province, northeast Australia.

Three-dimensional computer modelling of sediment Poster deposition on the central-southern Enderby Terrace, Carnarvon Basin, Western Australia.

Oligo-Miocene submarine canyons in the Gambier Subbasin, southern Australia.

Sequence stratigraphic model of a Paleogene temperate/subtropical carbonate-siliciclastic continental shelf succession in a boundary current setting. North Carolina, USA. Tasmania Basin review and bryozoan fauna application in biostratigraphy.

Waves, currents and continental shelves - northern New South Wales.

Reconstructing the life of a quartz grain.

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Symposium 4.2 High resolution sequence stratigraphy Adldns, R.

Carter, R.M. et al

Cowley, W.M. Dyson, LA. Dyson, LA. Gammon, P.R. and Carter, R.M. Hough, M. Lever, H.

Mendis, P.J. (Lai) et al Nakanishi, T. and Lang, S.C.

Payenberg, T.H.B. and Lang, S.C.

Reilly, M.R.W. et al.

Root, R. et al

Regressive systems tract cycles recorded within shallow-marine and non-marine facies from the onshore Canning Basin, Western Australia.

Are slope cyclothems delimited by sequence boundaries or correlative conformities? ODP Site 1119, 395 m water depth, Canterbury Bight, eastern New Zealand.

Bundaleer: Demonstrating the value of detailed mapping in the Adelaide Geosyncline.

Evolution of the Oladdie Diapir near Carrieton, southern Flinders Ranges.

Adelaidean sedimentation and the timing of salt tectonics in the east Willouran Ranges.

When highstands are lowstands, and vice versa.

Depositional sequences and facies associations of a Middle Cambrian marine sedimentary phosphorite deposit. Duchess District, western Queensland.

Cycling through the Kennedy Group (Late Permian, Carnarvon Basin WA): Milankovitch periodicities in a Permian seaway.

Origin diapers in the Adelaide Fold Belt: Synsedimentary or syntectonic.

Visualisation of fluvial systems with integration of high resolution sequence stratigraphy and 3D seismic data in the Cooper-Eromanga Basin.

Improving understanding of conventional and unconventional reservoirs through high-resolution regional correlations: an example from the Eagle Sandstone shallow biogenic gas play, Montana, U.S.A. A basin floor fan outcrop analogue: reservoir heterogeneity and sequence stratigraphy.

The application of high-resolution sequence stratigraphy to reservoir characterisation and development: an example from Tarbat-Ipundu Field, Eromanga Basin, southwest Queensland.

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Symposium 43 Cenozoic biogeohistory of Australia DeDeckker, P.

Ding, X. et al

Fabris, A. Gammon, P. Haines, P.W. Haines, P.W. et al Li, Q. and McGowran, B.

Deep-sea records of land-sea interactions along the western margin of the Indo-Pacific warm pool since the last glacial maximum.

Last 18,000 years paleoceanography in the IndonesianNW Australian region: patterns of heat transport and sea level changes.

Northern Murray Basin, South Australia: stratigraphy, sedimentology and geomorphology.

Southern Australian Eocene sponges: indicators of deep continental weathering?

Chinaman Gully Formation, South Australia: sea level fall or event deposit?

Chinaman Gully Formation, South Australia: spherules and PGE enrichment suggest impact association.

Sediment packages and sequence biostratigraphy: the southern Australian Cenozoic record.

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McGowran, B. and Li, Q. Opdyke, B.N. and Buddemeier, R.W. Stoian, L.

Wame, M.T.

White, S. and Webb, J.

The Cenozoic record in southern Australia: twelve assertions.

Anthropogenic CO2 addition and future tropical sea surface water chemistry: a comparison to the past 70 million years.

Evolution of the environment, vegetation and climate during Oligocene-Pliocene in the western Murray Basin: palynological evidence.

Palaeosalinity transformations of the Miocene Pliocene boundary in southeastern Australia: evidence from fossil Ostracoda.

Karst development at Naracoorte, South Australia: why there?

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Symposium 4.4 State of Regolith research and future directions Agar, B. et al

Anand, R.R. Barrett, B. et al Bewert, K. and Moore, C.L. Brown, A. et al

Brown, M.C. Clarke, J.D.A. Clarke, J.D.A. and Anderson, I. Clarke, J.D.A. and Harrison, A. Duk-Rodkin, A. Duk-Rodkin, A. Fitzpatrick, R. et al.

Gibson, D. et al Gray, D.J. and Sergeev, N.B. Haddrill, P. and Moore, C.L.

Hill, L.J. et al

Holzapfel, M. and Moore, C.L. Hou, B. et al

Regolith-landform mapping at Cudgell Creek, Central West, NSW: dryland salinity hazard mitigation in granitic landscapes.

Regolith and geology or regolith geology?

Geophysical methods in saline groundwater studies: locating perched water tables and fresh-water lenses.

Regolith distribution in the Canowindra North area: implications for dryland salinity hazard mitigation.

Preliminary results from geochemical dispersion investigations, Luxemburg workings, Cumamona Province, South Australia.

Cenozoic diversions of the Shoalhaven River: the Tallong Bend revisited.

Biogenic opal in the regolith: an idea whose time has come?

Regolith mapping and the search for groundwater in a karst terrain, Balladonia, Western Australia.

Extraction of biogenic silica from soils at Barracks Flat, Queanbeyan, NSW. Preglacial drainage and placer diamond potential in the Northwest Territories of Canada.

The relationship of Klondyke placer deposits to Tertiary-Quaternary drainage evolution of Yukon River in northwest Canada.

Contribution of scapolite and pyrite weathering to salinity in the Mount Torrens area. South Australia.

Sedimentary systems in the 'Bland Basin', an upstream extension of the Murray Basin.

Supergene gold dispersal at the Federal Deposit, Western Australia.

Regolith-landform mapping at Top Creek, Central West NSW: Dryland salinity hazard mitigation in felsic volcanic landscapes.

The influence of rainfall chemistry and seasonality on the distribution of regolith carbonate accumulations (calcretes).

Dryland salinity and regolith-landform distribution in the Booberoi to Quandialla transect. Central West New South Wales.

Palaeochaimel study models and their significance: example from the Gawler Craton.

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James, P.R. Fostering research and wider community understanding of the regolith.

Joyce, E.B. and Kotsonis, A. RegoHth mapping of the Bendigo region of central Victoria and the occurrence of gold.

Kirste, D. and de Caritat, P. Geochemical modelling of processes affecting the chemical composition of ground-waters in the Broken Hill region: effects of regolith properties on mineralised zone chemical signatures.

Lau, I.e. et al Mineral mapping of regolith-covered basement terrains in the White Dam region, Olary Domain, South Australia.

Lawrie, K.C. et al Mapping salinity processes and assessing and predicting dryland salinity hazards.

Lintem, M.J. and Sheard, Regolith stratigraphy and geochemical dispersion at the M.J. Challenger Gold Deposit, Gawler Craton, South Australia.

Mahizhnan, A. et al Red-brown hardpan: distribution, origin and exploration implications in the Yilgam Craton of Western Australia.

McQueen, K.G. et al Constraining the weathering history of the Cobar region, western NSW. Moore, C.L. and Southwell, Regolith-landform mapping for dryland salinity hazard P. mitigation. Upper Tyagong Creek, Central West NSW. Pillans, B.J. Climate-driven weathering episodes during the last 200 million years in southern Australia.

Radford, N.W. Reading geochemical signals in the regolith - one key to finding deposits under cover.

Ratchford, A. and Moore, Regolith-landform mapping at Hovell's Creek, Central C.L. West NSW: dryland salinity hazard mitigation in highrelief granitic landscapes.

Schmidt, P.W. and Williams, Palaeomagnetic dating of the Hamersley Surface and G.E. deep weathering in the Pilbara-northem Yilgam region.

Western Australia.

Smith, R.E. and Wilkes, A vision for regolith research in Australia.

P.G. Wilford, J. Customised regolith maps for delineating salt stores and predicting near surface saline ground water flow, NSW.

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THEME 5: EDUCATION AND ENVIRONMENT Symposium 5A E-learning geoscience Heinson, G., Smith, A. et al DigiTAL (Digital Teaching And Learning) web resources for fieldwork, laboratory specimens and student research projects in the geosciences.

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Symposium 5,2 Coastal and Estuarine Enviroiiments Brooke, B., Lester, J. et al Murray, E. and Heggie, D. Ryan, D.A. and Heggie, D.T.

Identifying factors influencing erosion of the spit on Bribie Island, SE Queensland.

H2S Production in Lake Wollumboola, an Intermittently Closed/Open Lake (ICOLL) on the NSW South Coast.

Conceptual understanding of Australian estuaries and coastal waterways.

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Symposium 53 Teaching about the Earth system Clarke, J.D.A. et al Gostin, V. McClean, S. McMahon, A.P. et al.

Moore, C.L. Roach, I.e. and Hill, S.M.

Selecting Australian Mars analogue sites: methodology Poster and criteria.

Australian environmental geoscience: an educational sourcebook.

Changes in stage 6 Earth Science teaching in NSW schools.

The Australian Stratigraphic Names Database - updates to an Australian icon.

The learning and teaching benefits of field-based, student centred, client-related projects: regolithlandform mapping for dryland salinity hazard mitigation.

Taking technology to field-based learning: teaching regolith mapping in Broken Hill, N.S.W.

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Symposium 5,4 Public and informal education Clark, I. Twidale, C.R. and Bourne, J.A.

A Corridor Through Time: a South Australian example of a cooperative approach to Geoscience Education.

Flawed tourist signage.

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Symposium 5.5 Mining and environment Esterio, H.

The role of surface water chemistry in heavy metal transport at Drake Mining Area, NSW, Australia.

Evans, K. and Banwart, S. A. Comparison of weathering rates for mine spoil obtained fi-om unsaturated column, field and batch studies.

Ogle, G., Grady, M. et al Access denied? Prospects for mining and conservation in National Parks.

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BOWENS REACTION PRINCIPLE AND THE EVOLUTION OF GRANITE MINERALOGY A.J.R. White

VIEPS Department of Earth Sciences, La Trobe University, Vic 3083 Reaction relationships in various granites are described and their implications discussed.

High temperature granites are members of intricate suites that include mafic low Si02 members.

An economically important example is monzonite - quartz monzonite - granite. In monzonites, pyroxenes are seen to be rimmed by amphibole then by biotite, resulting from Bowen's discontinuous reaction series. Many monzonites show a more complete series olivine-pyroxenesamphiboles-biotite but not muscovite. The continuous plagioclase reaction series is not so prevalent presumably because at high temperatures plagioclase equilibrium is more likely attained.

Associated volcanics are pyroxene rhyolites consistent with high temperatures. Rocks of high temperature suites are products of fractional crystallization but the mafic parent magma is difficult to define because, as expected, many rocks are accumulates. Indeed, monzonites, in which olivine is rimmed by pyroxene, typically show chemical evidence for a cumulative origin.

Low temperature granites are members of simple suites having inherited zircon. These include moderately low and very low temperature granites. Moderately low temperature cordierite biotite granites may have rare orthopyroxene (opx) surrounded by biotite, or more commonly opx pseudomorphs seen as rectangular aggregates of quartz + biotite. Cordierite rhyolites of the same chemical composition contain abundant opx and rare biotite. These data show that opx crystallised early in granite, and this was made over to biotite by reaction with melt. Some suites include muscovite granites with rare pseudomorphs after cordierite. However, once cordierite begins to crystallize it cannot stop unless there is a reaction relationship (peritectic) in the system. When biotite + cordierite granite fractionates to biotite + muscovite granite, the system goes through a peritectic; as H2O increases in the melt phase, cordierite + melt react to muscovite. Relict clinopyroxene (cpx) within the hornblende of moderately low temperature hornblende biotite granites is common, and some hornblende may be surrounded by biotite. Rarely there are opx pseudomorphs. Chemically matching volcanics contain opx and cpx and these must have been the original minerals to crystallise from the granite: with continuing crystallisation, pyroxenes react with the hydrous melt phase to produce hornblende and biotite. Reaction relations indicate that the moderately low temperature granites were produced by melt reactions involving the dehydration of biotite. On cooling, back reactions occur at peritectics in the complex system. Since biotite and hornblende are late reaction products, modelling fractionation processes in granites, must take into account whether fractionation occurred before or after any reactions occurred; this was implied by Bowen when he made the point that the diversity of igneous rocks required a series of reaction relations (peritectics) along the path of fractionation.

Very low temperature granites include those low pressure granites with the mafic assemblage biotite + cordierite ± aluminosilicate (e.g. Cooma type). These granites have no equivalent volcanic rocks, and no evidence of early opx. But there are aggregates of andalusite and biotite which are considered to be the result of back reaction of the original melt forming reaction of biotite + aluminosilicate + quartz. Very low temperature muscovite granites (Harrow type) formed by a melt-forming reaction involving breakdown of muscovite at pressures above about 0.4 Gpa, are seen in granite bodies within low temperature migmatite complexes. These also lack equivalent volcanic rocks or any evidence for the early presence of opx. Crystallisation of very low temperature granites involves very early back reaction of the melt forming process so that their magmas rapidly crystallise and cannot evolve.

PRECAMBRIAN SUPERCONTINENTS VERSUS SUPERCRATONS Wouter Bleeker Continental Geoscience Division, Geological Survey of Canada 601 Booth Street, Ottawa, ON, Canada, KIA 0E8 Email: WBleeker@NRCan.gc.ca

Global tectonics provides the first-order conceptual framew^ork for our understanding and prediction of why and w^here mineral deposits form. Plate tectonics as the global tectonics paradigm has revolutionized this conceptual framework and provided remarkable insights into the interplay between mineralizing processes and a variety of tectonic settings, e.g. rifts and convergent margins. Substantial debate remains, however, whether plate tectonics was an equally dominant process in shaping Archean continental lithosphere, particularly prior to 3 Ga. One particularly important concept in understanding ore-forming processes through time is the supercontinent cycle.

As we can now reconstruct the amalgamation and subsequent break-up of Pangaea in great detail and have serious contenders for earlier supercontinents, e.g. Neoproterozoic Rodinia, and a more nebulous Nuna of Mesoproterozoic age, the concept of an even earlier supercontinent, Kenorland, of late Archean age, has become widely accepted (e.g., Williams et al., 1991; Hoffman, 1997). But how strong is the evidence for this general idea, which is largely based on projecting an idealized supercontinent cycle back into time?

The global record preserves ca. 35 large fragments of Archean crust/1 ithosphere ("cratons"), and a less well defined number of smaller Archean slivers. Many of these cratons are characterized by Proterozoic rifted margins and, therefore, must indeed have originated from larger ancestral "supercratons" which are here defined as: larger ancestral landmasses of Archean age with a stabilized core that on break-up spawned several independently drifting cratons. Fundamental differences between several of the better known cratons (e.g., Slave, Superior, Pilbara) suggest that the preserved cratons trace their ancestry to several independent supercratons (e.g., Sclavia, Superia, Vaalbara) with distinct amalgamation and break-up histories (Bleeker, submitted). Perhaps some of these supercratons could have occupied distant parts of a sprawling supercontinental Kenorland, but there is little hard evidence for this at present.

In the search for Archean supercratons, a craton with 3-4 rifted margins has a ca. 10% maximum probability of correlating with any of the other cratons around the globe, assuming wholesale recycling of Archean cratons has been limited since ca. 1.8 Ga. Due to repeated cycles of break-up and plate tectonic dispersal since ca. 1.8 Ga, the probability of correlation is probably independent of present-day proximity, unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean. However, most previously proposed craton correlations rely (erroneously?) on present-day proximity, implicitly extrapolating relatively recent paleogeography back to the Archean. If the original number of independently drifting cratons was much higher than ca. 35, as would be implied by Armstrong-type models of crustal growth, it is possible that due to progressive fragmentation, erosion, and efficient Proterozoic recycling of Archean crust, only a fairly unique set of ca. 35 non-matching pieces of above average stability has been preserved. This "survival of the fittest" solution may limit the prospects for successful craton correlations. Present contenders for Archean craton correlations will be reviewed.

References Bleeker, W., submitted. The late Archean record: a puzzle in 35 pieces. Lithos, Special Volume on the Slave and Kaapvaal cratons.

Hoffman, P.P., 1997. Tectonic geneology of North America; in Earth structure and introduction to structural geology and tectonics, edited by B.A. van der Pluijm and S. Marshak, McGraw Hill, New York, p. 459464.

Williams, H., Hof&nan, P.F., Lewry, J.F., Monger, J.W.H., and Rivers, T., 1991. Anatomy of North America: thematic geologic portrayals of the continent; Tectonophysics, vol. 187 (1-3), p. 117-134.

HYDRODYNAMICS OF MINERALIZING SYSTEMS: ACTIVE AND ANCIENT Richard H. Sibson Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand Brittle failure or reshear within the upper crust may be induced either by rising deviatoric stress or by weakening from increased fluid-pressure, giving rise to a continuum of loading conditions ranging from purely stress-driven under constant, usually hydrostatic fluid-pressure, to fluidpressure-driven under near-constant deviatoric stress. The stress state and the architecture of existing fault systems within this upper crustal carapace play crucial roles in determining the maximum fluid overpressure sustainable in different tectonic settings, and the conditions favouring pulsed high-flux flow at depth where high overpressures are needed to create voidspace. These factors are considered in relation to contrasting active flow systems within two subparallel seismic belts associated with the obliquely convergent Pacific/Australia plate boundary in the North Island of New Zealand; the extensional back-arc Taupo Volcanic Zone (TVZ), and the predominantly contractional Hikurangi Subduction Margin (HSM).

Within the central rhyolitic segment of the TVZ, near-surface heat transfer occurs predominantly through - 2 0 long-lived hydrothermal plumes (locally Au-bearing) spaced at ~10-16 km. Convective circulation of mostly meteoric fluids under hot/cold hydrostatic pressures is thus inferred to extend throughout a stress-driven shallow^ seismogenic zone to depths of 6-8 km, with densely distributed active normal faults and associated fractures contributing to high vertical permeability and helping to localize flow. Summed hydrothermal flow-rate along this 100 km long central segment of the TVZ to account for the total power output of--4,200 MW is tonnes/s = 1.2 x 10^ m^/yr of 250°C fluid. In contrast, along the subparallel Hikurangi Subduction Margin (HSM), seismic thrust rupturing along the subduction interface and a combination of reverse and strike-slip faulting in its hanging wall extend to - 2 5 km depth. Continual fluid replenishment from the underthrusting Pacific plate coupled vs^ith compaction and compression of the low-permeability accretionary prism have led to strong overpressuring to near-lithostatic levels in the forearc. Summed fluid loss per 100 km strikelength of the HSM is estimated at >10^ m^/yr, about two orders of magnitude less than for the TVZ, but discharge in the seismically active portions of the forearc onshore (where initially low-angle thrusts have steepened towards lock-up) is likely to be episodic, with fault-valve action triggered by the accumulation of both shear stress and fluid-pressure.

Continual high-flux flow in the hydrothermal convection systems of the magmatically active TVZ is possible because of high intrinsic and structural permeability coupled with low levels of sustainable overpressure in an extensional stress regime. In contrast, because of the low overall discharge rate in the amagmatic, compressional regime of the HSM, high-flux flow at depth can occur intermittently only when the tensile overpressure condition (Pf > a3) is locally attained to allow activation of high-permeability fault-fracture meshes. A critical requirement is the absence of throughgoing cohesionless faults that are favourably oriented for reactivation in the prevailing stress field. Thus, high-flux flow of the kind needed for mesozonal mineralization is likely only where initially low-angle thrusts in the accretionary wedge have domino-steepened to frictional lock-up and before new, favourably oriented thrusts have developed (cf Au-quartz vein systems in the Mother Lode belt, California). Similar conditions for high-flux flow may occur in regions of compressional inversion where inherited normal faults are severely misoriented for reverse reactivation, and also for systems of wrench faults rotated to frictional lock-up (e.g. Mt Isa Inlier).

The special tectonic conditions needed for concentrated flow and precipitation through 'burping' valve action in the mid-crust of convergent orogens are inherently short-lived.

Moreover, because of the physico-chemical weakening effects of hydrothermal fluid there is a strong likelihood of structural overprinting in both the brittle and the ductile regimes, making it difficult to decipher the original configuration of such deposits within a polyphase deformation sequence, unless they formed late in the orogenic cycle.

THE UPPER TEMPERATURE LIMITS OF MICROBIAL LIFE Karl Stetter Department of Microbiology and centre for Archaea, University of Regensburg, Germany During the last few years, novel groups of 'hyperthermophilic' (extremely heat-loving) Bacteria and Archaea have been isolated from terrestrial subterranean and submarine hydrothermal regions.

Hyperthermophiles grovs^ optimally between 80 and 113°C, above pasteurisation temperatures, and some species were found to survive even one hour of autoclaving at 12 P C . They exhibit an unexpected metabolic flexibility which is based on the use of sulphur, iron and nitrogen compounds, hydrogen, carbon dioxide and organic materials as energy sources present in their hot environments.

Surprisingly, within the 16S rRNA-based phylogenetic tree of life, hyperthermophiles occupy all the shortest and deepest branches close to the root. Therefore, they appear as the most primitive organisms known so far. Due to their kinds of substrates and their independence of sunlight, hyperthermophiles could thrive in any water-containing volcanic and geothermal environment, even on other planets. By their outstanding phylogeny, heat resistance and physiology, hyperthermophiles are interesting objects of study for both basic research and biotechnology.

LATE CENOZOIC SEA LEVEL CHANGES, CLIMATIC EVOLUTION AND ANTARCTIC ICE: EVIDENCE FROM THE MURRAY BASIN J.M. Bowler School of Earth Sciences, University of Melbourne bowler@earthsci.unimelb.edu.au

For many years the Murray Basin remained one of Reg Sprigg's favourite hunting grounds. From his earliest work (in the late 50s) in the Naracoorte-Robe Pleistocene shorelines, with their neotectonic signatures and palaeoclimatic implications, his dedication to Murray Basin exploration never wavered. His pioneering work in Cenozoic environmental evolution with concepts of Ice Age aridity persistently challenged the orthodox views of his day. After many frustrating years, often involving manuscript rejection by peer group referees, his later synthesis (Sprigg, 1979) remains a cornerstone of Australia's environmental evolution. Ironically it took almost 30 years for his theories of Milankovitch insolation changes to be accepted as the controlling factors in ice age climatic rhythms. His theories, largely rejected in the 1960s, have become part of today's orthodoxy in Quaternary climatic change.

Inland from Naracoorte, the entire sequence of Murray Basin Late Cenozoic shorelines stands as arguably the world's best example of Miocene to Pleistocene sea level changes. Within that sequence of oscillatory eustatic regression, the signatures of neotectonic deformation can be clearly discriminated from eustatic controls. After filtering the tectonic signal, it requires a sea level rise of more than 50 metres to explain the landward extent of stranded shorelines. Coming immediately after the Late Miocene Messinian regression, this high sea level signature, repeated in many other parts of the world, implies near total destruction of the Antarctic ice cap. Moreover the oscillatory nature of subsequent sea level changes necessary to produce more than 100 strandlines between about 5.5 and 3.5 Ma, strongly reflects the 20,000-year orbital periodicity in excellent agreement with marine oxygen isotope evidence. Throughout Early Pliocene time, Antarctic ice was oscillating more or less like the later Northern Hemisphere ice caps to which global cooling gave birth in Pleistocene time.

By 3.5 Ma, the shoreline stood near Mannum in SA, with an equivalent strandline near Edenhope in southwestern Victoria. Regolith signatures changed dramatically soon after with a major geochemical transition from silcrete-ferricrete weathering to later calcrete profiles. This change correlates with major drying associated with cooling consequent on the complete growth of Antarctic ice as sea level approached its present position about 2 million years ago.

While this evidence stands in marked contrast to the widely held views of Antarctic ice cap stability since early Miocene time, it is a view that Reg Sprigg would have welcomed; it is one to which his own Murray Basin data lend support.

Reference: Sprigg, R.C., 1979. Stranded and submerged sea-beach systems of the southeast South Australia and the aeolian desert cycle. Sedimentary Geology, 22, 53-96

AGE AND ORIGIN OF TERRA ROSSA SOILS IN THE NARACOORTECOONAWARRA AREA OF SOUTH AUSTRALIA Erick A. Bestland^ Aija C. Mee\ and Nigel A. Spooner^ 'Earth Sciences (SoCPES), Flinders University, GPO Box 2100 Adelaide 5001, South Australia ^Research School of Earth Science, Australian National University, Canberra 0200, ACT The famous Terra Rossa soil in the Coonawarra-Naracoorte area, recently involved in legal wrangling, is dominated by locally derived aeolian detritus which probably accumulated over the last 120 000-130 000 years. Two soil profiles and associated limestone and lunette deposits were investigated using the following methods: mass-balance geochemistry of bulk soil samples (major and trace elements), quantitative XRD mineralogy, strontium isotopes (^^Sr/^^Sr), as well as grainsize analysis and cation exchange capacity. These data show that the Terra Rossa soil from the Coonawarra has a thick, clayey B horizon which is geochemically homogenous and dominated by smectite. Mass-balance calculations show unrealistic weathering scenarios when plotted using silicate residuum from the underlying limestone as parent. Realistic weathering scenarios are produced with fine-grained silicate material from local lunette deposits as parent. Strontium isotopes of silicate residuum from Gambier Limestone (0.78) contrasts strongly with the clayey B horizon (0.726). Strontium isotope ratio of silicate material from a local lunette (0.725) is similar to the soil values. It is speculated that the strontium signature in the lunette and soil B horizon is dominated by weathering products of the local Palaeozoic granitic rocks and not from far travelled dust, which would have much higher, Precambrian signature.

Optical (exposure) dating of 61 individual quartz grains (single aliquot) from three samples in the Coonawarra soil profile (one from the A horizon and two from the B horizon) show that most of the quartz sand grains have been buried for only a few thousand years. Many of the grains, however, have been buried for tens of thousands of years, with three grains having exposure ages of between 105 and 109 Ka. The large population of young exposure dates represent quartz sand recently exposed in the A horizon and which have been translocated down to the B horizon. The older exposure dates are interpreted as representing grains that were buried during or soon after the accumulation of wind blown silt and clay. Our current model concerning the timing and conditions of aeolian deposition of the Coonawarra soil is that much of it accumulated during the relatively wet, last interglacial period centred around 120-130 Ka. During this time span it is thought that the playa-lunette systems in the low-lying areas to the west were particularly active and generated a significant local dust flux.

IDENTIFYING FACTORS INFLUENCING EROSION OF THE SPIT ON BRIBIE ISLAND, SE QUEENSLAND Jared Lester\ Brendan Brooke^ and Malcolm Cox^ ^Queensland University of Technology, ^Geoscience Australia email: Brendan.Brooke@ga.gov.au

Bribie Island is a large sand island at the northern margin of Moreton Bay, southeastern Queensland. A spit at the northern end of Bribie Island forms a barrier betw^een the open ocean and the estuarine foreshore of the city of Caloundra. Severe shoreline erosion in recent years has considerably narrowed the spit, raising fears of a breakthrough and subsequent erosion of the Caloundra foreshore. This study examines erosion rates and sediment transport pathways of the spit system, long-term changes in its geomorphology and the factors influencing these processes.

Most of Bribie Island comprises a series of beach-ridge plains, contrasting with the other large sand islands of Moreton Bay that formed with a coalescence of transgressive dunes. The spit consists of beach ridges and low dunes predominantly covered by coastal heath and woodland.

Bribie Island is separated from the mainland by an estuary, Pumicestone Passage, the inlet to the estuary situated at the northern end of the spit.

Rates of erosion were derived from time-series aerial photographs, bathymetric charts and field surveys and were compared with erosion rates obtained by a similar earlier study of the spit (Jones, 1992). Insights into the longer-term evolution of the spit were obtained by analysing vibrocores and sections exposed in erosional scarps cut into the eastern side of the spit by storm waves.

A sediment budget for the period 1978 - 1993 indicates the spit is eroding at a rate of approximately 144,000 m^ y r A p p r o x i m a t e l y 80% of the sand eroded from the spit is transported into the inlet of Pumicestone Passage, the rest is lost to the spit system, moving southwards with the dominant longshore drift. This loss of approximately 30,000 m^ yr"^ is a significant increase in the rate of erosion calculated for the period 1958 - 1972, 15,000 m^yr \

by Jones (1992). Aerial photographs also indicate that since 1940 the spit has been retreating by approximately 1.5 myr Exposures of estuarine mud along the eastern shore of the spit and offshore exposures of indurated muddy sand indicate the spit has been undergoing erosion and landward translation.

An articulated Anadara shell recovered from an intertidal exposure of mud has a radiocarbon age of 3330 ±70 yr BP, suggesting sea level at that time was similar to the present and that there has been significant long-term erosion of the spit.

Although storm frequency has increased during the last century, the frequency of storms is not significantly different between the two periods for which erosion rates were calculated, indicating other factors may be contributing to the increasing rate of erosion of the spit. These factors appear to be a reduction in the rate of sediment supply from offshore, an increase in the rate of longshore drift, dredging in the inlet of Pumicestone Passage and offshore and, possibly, a rise in sea level.

Reference Jones, M.R. (1992). Quaternary evolution of the Woorim-Point Carthwright Coastline, Volume 1. Project Report MA49/2, Queensland Department of Minerals and Energy.

MINERALOGY AND MOBILITY OF URANIUM AROUND MOUNT PAINTER, NORTHERN FLINDERS RANGES, SOUTH AUSTRALIA Joel Brugger^ Yann Lahaye^ Sergej Krivovichev^ Nicolas Meisser^ Stefan Ansermet^ and Peter Berlepsch^ ^ South Australian Museum and University of Adelaide, North Terrace, 5005 Adelaide Email: Brugger.Joel@saugov.sa.gov.au; ^Institut fiir Mineralogie, Frankftirt, Germany; ^Department of Crystallography, St. Petersburg State University, Russia; ^Musee Geologique Cantonal & Laboratoire des Rayons-X, Institut de Mineralogie, Lausanne-Dorigny, Switzerland; ^Laboratorium fur Chemische und Mineralogische Kristallographie, Universitat Bern, Switzerland.

The (?Paleo-) Mesoproterozoic basement of the Mount Painter Inlier contains granites and gneisses highly enriched in U and Th (several tens of ppm). The radiogenic heat released by these granites was responsible for a long-lasting thermal anomaly^ which drove and still drives hydrothermal activity. The products of this activity include many small hematite-UCu-Nb-REE deposits, a large epithermal system characterized by complex quartz ±fluorite veins and breccias, and active hot springs. Economic concentrations of uranium (e.g.

Beverley mine) occurring in Tertiary sediments surrounding the Mount Painter Inlier result from the leaching of uranium from the Inlier. Modern groundwaters in the inlier carrying up to 600 ppb U may represent analogues for the Beverley oreforming fluids^.

We present mineralogical and geochronological data aimed at understanding the last stages of U-mobility around Mount Painter.

The primary U-mineral at the N° 2 workings on Radium Ridge is ishikawaite, a uranium-rich samarskite. The hematite-U-Cu-NbREE mineralisation is crosscut by the epithermal mineralisation.

Secondary uranium minerals occur in the vugs of the epithermal quartz and in cavities resulting from the dissolution of primary Fig. 1: Uranyl layers in "spriggite" U-minerals. Beside beta-uranophane, soddyite, kasolite, metatorbemite and billietite, a new orange prismatic monoclinic Pb-U-oxide-hydroxide, with the chemical formula Pb3[(U02)608(0H)2](H20),, x ~ 3, has been identified. We propose to name this mineral "spriggite" in recognition of the contribution of Reg Sprigg to the geological understanding and conservation of the Mount Painter area. The crystal structure of spriggite contains layers formed by U^^ ions in pentagonal dipyramidal and octahedral coordination (Fig. 1). The sites containing Pb atoms and H2O molecules are disordered, and are located between the uranyl-layers. Uranyl layers with similar topology occur in ianthinite, U2'^(U02)406(0H)4(H20)4](H20)5 However, the octahedral uranium sites in ianthinite are occupied mainly by U"^^ ions, whereas they host only U^^ ions in spriggite.

The age of the secondary U-minerals found in a variety of environments in and around the Mount Painter Inlier remains poorly constrained. A method based on UV-laser ablation multi-collector inductively coupled plasma mass spectrometry has been developed to obtain in-situ ages from minerals of the meta-autunite group (M'^(U02)2(P04)2-8H20, CU, Ca, Mg, ...). Preliminaiy data suggest that it is possible to obtain ages with errors similar to those obtained by TIMS (0.2-0.5 %) from craters measuring about 100 |im in diameter. Three different varieties of meta-torbemite (large magnificent crystals; coating along schistosity; micro-crystals in vugs resulting from near-surface pyrite oxidation) consistently furnished surprisingly young ages of about 200,000 years.

References JSandiford, M., Hand, M., and McLaren, S. (1998) Earth & Planet. Sc. Lett., 163, 149-165.

Long, N. (2001) Honours Thesis, School of Earth Sciences, Monash University, Melbourne, Australia.

'Bums, P.C. et al. (1997) J. Nucl Mat., 249, 199-206.

MAPPING AEOLIAN FEATURES IN THE EASTERN MURRAY BASIN, SOUTHWESTERN N.S.W. - NEW INSIGHTS USING RADIOMETRIC IMAGERY R.G. Cameron^ S.N. Meakin\ G.P. Colquhoun\ P.A. Ruszkowski\ and S.R. Cattle^ 'Geological Survey of N.S.W., PO Box 536, St Leonards NSW 1590 ^Department of Agricultural Chemistry & Soil Science, University of Sydney, NSW 2006 Methods of mapping Quaternary morphostratigraphic features in the eastern Murray Basin have been evolving over the last 15 years, as is highlighted by a comparison betw^een the Pooncarie map sheet (1987 to 1989) and the Cargelligo map sheet (a work in progress). Geological mapping has been completed over a large area of aeolian-dominated Quaternary morphostratigraphic features across the NSW portion of the Cainozoic Murray Basin.

Recent developments in the processing of high resolution airborne radiometric data have significantly improved feature discrimination within these predominantly aeolian systems. These radiometric data show the abundance of radioelements in the top 30 cm of the earth's surface. In a ternary RGB (red, green blue) image, values of potassium (K), thorium (Th) and uranium (U) respectively are combined into an image of additive primary colours. When combined with DTM (digital terrain model) data, the combined image is an invaluable tool for interpretation.

Mapped in the 1980s, the flat, semi-arid Pooncarie map sheet area is dominated by interacting Quaternary morphostratigraphic units of aeolian, lacustrine and fluvial depositional systems that are now largely inactive. Aeolian systems dominate the area and show four geomorphological forms: sandplains, open-spaced linear dunefields (Woorinen Formation), closely spaced linear to sub-parabolic dunefields, and parabolic mallee-type dunefields (Lowan Sand equivalents).

Morphostratigraphic units were initially mapped using black and white aerial photos and selective subsequent integration with Landsat TM imagery.

In contrast, radiometric imagery was available for the Cargelligo mapping project and has had significant impact: •

Areas of low erosional remnants and near-surface 'subcrop' can be identified, located in the registered imagery in the GIS, and inspected. These features can be distinguished by neither photo-geological means, nor usually by Landsat TM images as the mallee vegetation commonly dominates the reflectance. Sites may be verified with a handheld spectrometer measuring total count, K, Th and U concentrations.

•

Radiometric imagery can also distinguish transported and in-situ material.

•

Radiometric imagery can reveal mineralogical differences in otherwise seemingly monotonous dunefields. This is supported by X-ray diffraction work on polygenetic soils near Hillston. In the western zone of the Cargelligo area, radiometric data dramatically illustrates differing source material within a continuous dunefield and sandplain (Woorinen-style) system.

•

Radiometrics can distinguish differences in composition of colluvial footslope deposits (e.g. either side of linear N-S striking ridges of Devonian rock) - usually Lowan Sands equivalents on the western side and colluvial scree on the eastern side. The quartzite ridges are inferred to have acted as a barrier to easterly migration of airborne sediments.

Acknowledgement: Cameron, Meakin, Colquhoun and Ruszkowski publish with the permission of the Director-General, NSW Department of Mineral Resources.

A HIGH RESOLUTION FORAMINIFERAL RECORD OF THE POSTGLACIAL (HOLOCENE) MARINE TRANSGRESSION, ST VINCENT GULF, SOUTH AUSTRALIA. John H. Cann^ Colin V. Murray-Wallace^ and Antonio P. Belperio^ ^School of Geoscience, Minerals and Civil Engineering, University of South Australia, Mawson Lakes, SA 5095 ^School of Geosciences, University of Wollongong, NSW 2522 ^Minotaur Resources Ltd, 3 Boskenna Avenue, Norwood, SA 5067 Vibrocore SV23 recovered nearly 4 metres of Quaternary sediments from St Vincent Gulf, about 20 km northwest of Myponga Jetty on the southeastern coast. The core w^as taken from a site where present-day water depth is c. 40 m, within a shallow, basin-like depression, topographically lower than the surrounding gulf floor. Earlier studies of this core revealed that the recovered sediments consisted of a lower Pleistocene interval of 2.5 m, overlain by L5 m of postglacial (Holocene) marine deposits, and that well preserved benthic foraminifera were abundant (Cann et al, 1993). In that study, interpretation of the in-core fossil foraminifera, based on a the distribution of foraminiferal species in modern surficial sediments of South Australian gulf waters, revealed several distinctive phases in the onset and development of the postglacial marine transgression into St Vincent Gulf. The details of that interpretation were limited by the coarse sampling interval (20 cm) and by the number of fossil marine shells subjected to radiocarbon age analysis (4). In the present study the postglacial interval was sampled continuously in 2 cm slices for microfossil analysis and an additional four radiocarbon ages were obtained from analyses of fossil bivalves.

The disconformity between the Pleistocene and postglacial sediments is lithologically marked by pyritised fragments, lithified carbonate sediment and the occurrence of some carbonate encrusted foraminiferal tests. Although there is some mixing of the fossil micobiota across the diconformity, the earliest phase of postglacial sedimentation is signified by large numbers of charophyte oogonia.

Charophytes are aquatic plants that are known to inhabit saline, but never holomarine, environments and at c. 140 cm oogonia outnumber foraminiferal tests two to one. Coincidentally, also at 140 cm, there is a marked peak (c. 40%) of Miliolinellci lubioscL^ a species noted for its ability to inhabit the saline-carbonate lacustrine environments of southeastern South Australia.

The earliest holomarine environment is marked by the onset of Nubecularia lucifuga, a species that is noted for its local dominance of the coastal Posidonia seagrass meadows. N. lucifuga makes up c. 65% of the assemblage at 130 cm, accompanied by the acme of another seagrass species, Discorbis dimidiatus, c. 8%. Above 130 cm decreased numbers of these species signify their response to the rising sea, giving way to miliolid species such as Triloculina tricarinata and T. trigonula, and to the rotaliid Elphidium macelliforme.

By 110 cm. Ammonia beccarii is a significant component of the core assemblage, expanding upcore to 46% at 96 cm, and maintaining a significant but declining presence up-core to c.45 cm. A. beccarii is well known for its ability to inhabit euiyhaline waters and is ubiquitous in the Coorong coastal lagoon of southeastern South Australia. Thus the presence of this species throughout the interval 110-45 cm is interpreted as evidence of euryhaline waters which occupied a basin-like depression, of restricted circulation, with somewhat limited access to the waters of the Southern Ocean. Increasing holomarine influence and increasing water depth is shown by incoming Massilina milletti, which reaches its acme of c. 50% at 50 cm.

Above 50 cm, the fossil assemblage is progressively dominated by two agglutinated species, Ammobaculites reophaciformis (textulariid) and Flintina triquetra (miliolid), reflecting their relative occurrence in the modem surficial sediment. These species therefore record the final part of the transgression.

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QUATERNARY ATMOSPHERIC PROCESSES: WHAT CAME DOWN IN THE LAST SHOWER? Allan R. Chivas School of Geosciences, University of Wollongong, NSW 2522 It is becoming clear that a large proportion of the chemical residua and solutes within the soils, groundwaters and partly consolidated regolith of Australia are not related to underlying bedrock. A variety of chemical and isotopic tracers show that atmospheric processes have delivered solutes, dusts and aerosols throughout the Quaternary (and before) to Australia's surface.

There are now sufficient data to construct a map of all of southern mainland Australia that shows the contribution of airborne sulphate of marine origin. Aerosolic marine sulphate contains both inorganic sulphate (with of +21%o vs CDT^) and sulphate formed by the oxidation of gaseous DMS (dimethylsulphide) and other organic sulphur-containing precursors related to plankton. The Australian map, based on S^'^S values in surficial sulphate from soils, playas, regolith and shallow groundwaters shows a remarkable pattern indicative of inorganic marine sulphate near coastlines with increasing contribution of DMS further inland. Bedrock sulphur contributes little to the surficial sulphate budget except locally over sulphur-rich rock types such as pyritic shales or near sulphur-rich ore deposits. It is possible to estimate the delivery rate and residence of such aerosolic solutes in the Australian landscape by turning to another element, chlorine, and using the ^^Cl/total chloride ratios of halite, brines and groundwaters on or in the regolith.

A west-east traverse of ^^Cl/Cl measurements from playa halite across Western Australia reveals values that increase monotonously away from the Indian Ocean coastline and mimic, but at lower values, measured ^^Cl/Cl values in contemporary rainfall. Again the relationship with relatively modern processes suggests a marine-aerosol source for continental surficial chloride. Based on a half-life of 301,000 years for ^^Cl, a mean residence time of about 750,000 years can be suggested for Western Australian terrestrial chloride. Clearly, there is a long-term salinity problem!

There are also clear isotopic and chemical indications that some principal soluble chemical components of the Australian surface are derived from sources other than marine aerosols. At least part of the calcium can be shown to be bedrock-derived, at least on Archean terranes, and carbon in calcretes has a dominant origin related to past vegetation. Nevertheless, much of the strontium and calcium within calcretes is of atmospheric origin.

The overall conclusions of this work establish that drier continents preserve a record of solute transport from the oceans to the continents that is more difficult to detect in humid continents, and which is, in the latter case, rapidly returned to the ocean. Fundamental questions also arise concerning the origin of soil salinisation particularly in semi-arid zones and the strategies to be adopted for agricultural management in salt-affected soils. Also the identification of marine versus terrestrial evaporites through geological time now becomes blurred given another class of terrestrially-hosted evaporites with "marine" chemical and isotopic signatures. ^CDT = Canyon Diablo troilite standard.

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MULTI-DISCIPLINARY NEOTECTONICS RESEARCH IN AUSTRALIA: DEVELOPING AN IMPROVED BASIS FOR SEISMIC HAZARD ASSESSMENT Dan Clark. Mark Leonard and Clive Collins Geoscience Australia, GPO Box 378, Canberra, ACT Email: dan.clark@ga.gov.au

In most intraplate regions, such as Australia, there is a general lack of understanding of why earthquakes occur where they do, or even why they occur at all. Estimates of seismic hazard are therefore necessarily based upon the assumption that the past distribution of epicentres is indicative of where future seismic activity will occur. However, this approach has been demonstrated to be less than ideal as calculations based on instrumental recordings of seismicity cannot predict the location of the larger, less frequent earthquakes with any accuracy.

An improved basis for earthquake hazard assessment could be gained by enhancing probabilistic models for seismicity with geologic data that include stress and strain conditions, the number and distribution of potentially seismogenic faults, and a better knowledge of patterns in the long- and short-term behaviour of intraplate faults.

We present our methodology and preliminary results from ongoing multi-disciplinary and multiorganisational research in the Southwest Seismic Zone (SWSZ) of Western Australia aimed at addressing this challenge. Techniques employed to date in this test-area include stress tensor reconstruction using earthquake focal mechanisms, strain field characterisation using repeat campaign GPS surveying, earthquake epicentre mapping, failure surface mapping using highprecision epicentral determinations from earthquake aftershock sequences and swarms, fault mapping/imaging by geophysical and trenching investigations of fault scarps, and dating of materials recovered from fault scarp colluvial wedges.

This study in the SWSZ is complemented by similar collaborative research in other seismically active regions of Australia, including the Flinders Rangers and the eastern highlands. The data will be compiled into Geoscience Australia's Neotectonics database, which is a dynamic and evolving research resource obtainable on request from the authors. The database is currently populated with over 60 potential and actual seismic sources from around Australia. The bulk of the data comprises faults on which Quaternary movement has been demonstrated or is suspected. We are currently building in the facility to include other categories of entry such as instrumentally defined fault planes (seismic fault plane mapping), and geophysically defined potential seismic sources.

Acknowledgements Geoscience Australia acknowledges the contribution of our collaborative partners working towards bettering our understanding of the intraplate seismicity in Western Australia: the University of Westem Australia, Curtin University of Technology, DOLA, and IGNS of New Zealand.

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ANTIQUITY OF ARIDITY IN THE ATACAMA DESERT Jonathan D. A. Clarke CRC LEME, Department of Geology, Australian National University, Canberra, ACT 0200 The Atacama Desert is the driest desert in the world. Arica and Iquique have annual rainfalls of only 0.5 and 0.6 mm, w^hile Antofagasta, Calama, and Copiopo receive 1.7, 5.7, and 12 mm respectively. It is also the only desert that extends into the tropics. This extreme aridity has resulted in a number of unique features. These include the very low^ rates of erosion and accumulation of a range of unusual salts, including perchlorates, iodates, and nitrates in the soils, as well as the more common halite, gypsum, and anhydrite.

The oldest sediments indicative of arid conditions are Late Triassic-Early Jurassic. Subsequently there are evaporitic continental and marginal marine sediments of Early Jurassic, Early Cretaceous, and Oligo-Miocene age. Deposition of salts in some of these modern lake basins has been continuous since the Miocene, as shown by Salar de Atacama where 975m of halite has accumulated. The gaps in the evaporite record are due to tectonics, and do not reflect periods of non-evaporitic sedimentation.

There are three palaeogeographic reasons why aridity has persisted for the past 200 My. Firstly, the Atacama region has been at approximately the same palaeolatitude during this period.

Secondly, the South American continent has maintained the same north-south orientation during this period. Thirdly, the desert has always been near the western margin of the continent. This has had three consequences. First, the Atacama region has always been in the subtropical high pressure (and low rainfall) zone. Second, the prevailing winds have always been dry through passage over the continental interior. Third, the continental configuration makes it likely that a cool-water current has always been active off the coast, reducing evaporation. Pliocene uplift merely intensified the aridity by placing the Atacama in the rain shadow of the Andes.

The area hosts many of the world's most significant porphyry copper-gold deposits, whose economic viability is determined by supergene enrichment zones. The presence of these zones is a reflection of the history of groundwater, tectonics, landscape, and climate of the Atacama. They date mostly between the Early Oligocene and mid-Miocene, apparently defining a "supergene time window". Either supergene mineralisation formed under different climatic conditions and was terminated by the development of aridity in the Miocene, or hydrologic changes during the Miocene uplift and incision of the Atacama were especially conducive to development of supergene mineralisation. The continuous history of aridity in the Atacama since the Late Triassic suggests the second explanation agrees better with the available evidence.

There is evidence that supergene deposits are still forming today in northern Chile. Some authors argue that supergene profiles developed during periods of dissection. Mine dewatering can trigger renewed oxidation of mineralisation. Supergene processes are therefore quiescent in the regolith unless the stability is perturbed.

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GEOMORPHOLOGY AND C'' AND FISSION-TRACK DATING DEFINE RAPID SPORADIC PLEISTOCENE PRESENT UPLIFT FROM THE FINISTERRE AND SARAWAGET MOUNTAINS, PNG Findlav: ^K.Hill; A. Raza; ^J. Keetley; ^G. Kopi; ^ P. Kia ^Geological Survey of PNG, Dept. of Mining, Private Mail Bag, Port Moresby, PNG ^Department of Earth Sciences, Latrobe University, Melbourne, Victoria, Australia

Geomorphological studies indicate multiple erosion surfaces throughout the Finisterre and Sarawaget Mountains (FSM), the latest of which are deep V-shaped incisions in an earlier rugged landscape. Some valleys contain a 170-200 m thick and extensive but incised lacustrine sedimentary fill which, on C^"^ dating at Wantoat, has a maximum age of 2280 years BP. These deposits surely indicate the infill of large landslide dams produced in a major earthquake at that time. These valley fills appear tilted and the height differentials from south to north could imply extremely rapid differential northerly uplift. ^"^C dating of a relict fluvial or lacustrine deposit at 640 m altitude in the region of the Erap River indicates a down-cutting rate of about 7-8 mm yr^ through bedrock. Considerations of the age of the Pleistocene molasse, the Leron Formation which was derived from the uplifting FSM, when allied to fission-track chronology, indicate an uplift rate of 5-9 mm yf\ This rate matches an estimated pre-320 000 year rate of 6 mm yr"^ from the northern flank of the FSM, after which the northern uplift slowed to 1.7 mm yr"^ whilst the southern flank continued to rise at 6-9 mm yr"^ to produce the present tilted and heavily incised peneplain forming the northern slope of the FSM. These rates are considerably greater than those proposed earlier, of between 0.8 and 2.1mm yr"^ and are more in keeping with the exceptionally rugged topography of these mountains.

Finally, examination of the range of all available ^"^C dates from Lae, the Markham Valley and the FSM, points to an 11,000 year hiatus in uplift between 20,000 yr BP and 9000 yr BP.

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NEW PALAEONTOLOGICAL DATA FROM THE GASMATA DISTRICT, WEST NEW BRITAIN, PAPUA NEW GUINEA, AND IMPLICATIONS FOR THE OLIGOCENE-RECENT TECTONICS OF WEST NEW BRITAIN R.H. Findlav^ and D.W. Haig^ ^Geological Survey of Papua New Guinea Department of Mining, Private Mail Bag Port Moresby, Papua New Guinea ^Dept. of Geology, University of Western Australia, Nedlands, WA 6009 Australia

New palaeontological data confirm an Early Miocene-Early Pliocene age for the Yalam Limestone at Gasmata, West New Britain. The Yalam Limestone therefore forms an outlier amidst the transgressive Pliocene Johanna beds, which wedge out north against the ?Eocene-01igocene Baining Formation, which forms the basement rocks of the area.

The new data also indicate deepening from neritic to a bathyal environment during deposition of the Yalam Limestone. This, the unconformably transgressive relationships of the Pliocene Johanna beds to the Baining Formation and Yalam Limestone, and the uplifted and tilted Pleistocene-Recent coralline beds at Gasmata indicate a progressive Miocene-Pleistocene southward bending of the Bismarck Sea plate's southern margin down towards the New Britain Trench.

15

STABLE ISOTOPE INVESTIGATION OF CAVE AND SURFACE SEDIMENTS IN THE NARACOORTE REGION: EVIDENCE FOR A LOCAL AEOLIAN SOURCE. Forbes.

Bestland, E.A^, and Wells, R..T\ ^School of Chemistry, Physics and Earth Sciences, Flinders University, Bedford Park, SA 5042, Australia. ^School of Biological Sciences, Flinders University, Bedford Park, SA 5042, Australia.

Preliminary analysis of carbon, nitrogen and strontium stable isotope data obtained from sedimentary strata located in Robertson and Wet caves, as well as surficial deposits in the Naracoorte region, have shed light on palaeoclimatic changes across the Pleistocene/Holocene boundary. Located some 10 kilometres south of Naracoorte, Robertson Cave contains a four-metre pit excavated into sediments that can be divided into three distinct stratigraphic units, which have been dated by The pit is located on the edge of the main sediment cone, which is in the middle of the cave and covers the entire floor of the cave. values of organic matter in the Robertson Cave sediments change markedly from -24 and -25 %o at 11340 ± 80 years B.P to -25.8 to -27 %o at 10090 ± 80 years B.P. This shift can be attributed to a change from a colder and drier to a warmer and wetter environment. results vary from +12 to +15 %o in unit one of Robertson Cave to +2 to +4 %o in units two and three, defining two distinct modes of sedimentation.

Heavy S'^N values for the 31000 years B.P. unit one are interpreted as a faecal signature implying a slow depositional rate in warmer and wetter conditions when local sediment filtered into the cave in minor amounts rather than being blown in. Units two and three dated between 8080 ± 100 B.P. to approximately 31000 years B.P. both display a higher sand fraction and a faster sedimentation rate than the silty unit one.

The lighter values indicate plant-derived organic matter with minor decomposition. When combined with a high rate of sedimentation these data argues for local aeolian activity, landscape destabilisation and therefore drier, cooler times. isotope ratios have been determined with the aim of tracing sediment sources in the southeast region. A marked decrease in strontium ratios from 0.7255 at 13650 ± 70 B.P. to 0.7225 at 8080 ± 100 B.P. is evident in the Robertson Cave strata. This change, combined with uniform grain size distribution between these dates, suggests that as the interglacial was reached and passed the source of the aeolian input changed, possibly from a regional to a more localised source.

Strontium isotopic analysis of surface sediments such as the Terra Rossa soil also suggest a localised aeolian source. ^^Sr/^^Sr values for Terra Rossa (0.7162-0.7297) are compatible with the local wind-derived lunette deposits (0.7257). The close relationship between the Terra Rossa values and those in the sediments in both the Robertson (0.7236) and Wet (0.7238) caves suggests that these sites most likely had inputs from the same aeolian source.

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STABILITY OF CLAY MICROAGGREGATES IN AEOLIAN MATERIALS: USA AND AUSTRALIAN PERSPECTIVE'S R. S.B. Greene\ W.D. Nettleton^ J. A. Mason^ R.A. Eggleton^ and R. Gatehouse^ ^CRCLEME, School of Resources, Environment and Society, Australian National University, Canberra, 0200, ACT, Australia. ^USDA/NRCS, National Soil Survey Centre, 100 Centennial Mall North, Lincoln, NE, 68508, USA. ^Conservation and Survey Division and Department of Geosciences, University of Nebraska- Lincoln, 113 Nebraska Hall, Lincoln, NE, 68588-0517, USA. CRCLEME, Department of Geology, Australian National University, Canberra, 0200, ACT, Australia. ^CRCLEME, Research School of Earth Sciences, Australian National University, Canberra, 0200, ACT, Australia.

Loess and pama, aeolian sediments consisting largely of silt-and sand-size clay micro-aggregates as well as silt-size quartz grains, form significant components of the regolith in both the USA and Australia. These sediments can occur in the contemporary landscape as either (i) discrete deposits e.g. dunes, or (ii) widespread sheets of material of varying thickness. The properties of these sediments, in particular the stability of their clay micro-aggregates, can have significant effects on a range of landscape processes and hence have major implications for management. For example, if the clay micro-aggregates in these sediments are highly unstable and disperse into < 2|Lim particles, the soil profiles containing these materials will be highly prone to land degradation such as soil erosion (gullying, piping and rilling), poor air quality, and surface sealing, crusting and hardsetting problems.

In this paper we discuss how a number of techniques are used to investigate the nature and the stability of the clay micro-aggregates in soil profiles of loess and pama materials. The techniques include: (i) measurement of the ratio of the 15 bar water content to clay content, (ii) the effects of different dispersion treatments, such as ultrasonics, and/or chemical dispersants, on the particle size distribution, (iii) the role of the exchangeable cation/soluble cation balance of clay particles on their physico-chemical dispersion, and (iv) micromorphological and scanning electron microscope studies.

Results indicate that clay micro-aggregates occurring in loess (or pama) materials that originated from desert or hot environments are more stable, i.e. resist breakdown in water, compared with microaggregates from glacial, or cold environments. Possible explanations, based on how the conditions in the source areas for these materials affect the nature of the particle-to-particle bonding in microaggregates, are discussed.

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ARTESIAN SPRINGS OF THE GREAT ARTESIAN BASIN AND THEIR AGES ^M.A. Habermehl and ^J.R. Prescott ^Bureau of Rural Sciences, Water Sciences Program, Canberra A.C.T. ^Physics Department, Adelaide University, Adelaide, S.A.

Springs and seepages are abundant in the marginal discharge areas of the Great Artesian Basin, which extends across 22 percent of Australia. More than 600 springs in the Basin are concentrated in 12 main groups. These permanent springs are the natural outlets of artesian groundwater from the Basin in its southern, southwestern, western and northern margins in arid and semi-arid regions.

Flowing artesian springs occur along faults, where the groundwater flows upwards, at the abutment of aquifers against impervious bedrock and where the pressure groundwater breaks through thin confining beds near the discharge margins. Diffuse discharge occurs from the artesian aquifers near the margins where the overlying confining beds are thin and waterlevels high. Springs are also present in the recharge areas along the eastern basin margin. Most of these springs comprise "overflow" or "rejection" of recharge from the aquifers, or result from the intersection of the local topography and aquifers.

Many artesian springs have built up conical mounds consisting of clayey and/or sandy sediments and carbonates, several metres to several tens of metres in diameter and up to several metres high.

The mounds comprise material derived from confined aquifers and confining beds, accumulated aeolian material and carbonate deposits. Some mounds consist of mud, but many mounds are built up of carbonates, which are dominated by tufa, travertine and very fine-grained limestone, and originated from the combined chemical precipitation of calcium carbonate out of the artesian groundwater, and precipitation by algae and bacteria. Terraced mounds and waterfall or cascade deposits produced by algae are common, though most accumulations consist of steeply sloping mounds.

Rates of discharge from individual springs are low, ranging from less than 1 L/s to about 150 L/s (the latterfi-oma spring at Dalhousie Springs, northem South Australia). Temperatures of the springwater rangefi-omabout 20° to 45° C. Hydrogeological, hydrochemical, isotope hydrological and age dating studies have been carried out on springs and spring deposits in the Great Artesian Basin since the early-1970s to determine their characteristics, mode of formation, past and present discharges of the Basin, and the changes in the hydrologic regime during geologic and recent times. Groundwater from the springs reflects the regional hydrochemistry of the Basin. The deuterium and ^^O stable isotopic signatures of spring groundwater confirm its meteoric origin and source. ^"^Carbon, uranium-thorium and thermoluminescence dating of quartzose sands and carbonate spring deposits have produced a range of ages from recent to more than 700 ka, and show the complex nature of the spring deposits. Large spring complexes with extensive deposits encapsulate aspects of groundwater discharge variations, chmate changes and local climatic effects. These include evaporation, and influences from the groundwater chemistry, changes in carbonate precipitation, dissolution, erosion, re-cementation, subsidence and micro-tectonics. The spring deposits reflect geological and hydrological changes in eastern and central Austraha during the late Quaternary, and provide an understanding of long-term changes prior to human intervention.

Thermoluminescence dating of spring deposits along the southwestern basin margin assist in the delineation of its palaeohydrology, and has been successful in identifying ages from less than 100 years, through 10-15 ka for the more active of the recent flowing artesian springs to 710 ka for part of the large mound complex of the Elizabeth Springs.

Reference Habermehl M.A. 2001. Hydrogeology and Environmental Geology of the Great Artesian Basin, Australia.

(Chapter 11). In\ Gostin, V.A. ed. Gondwana to Greenhouse - Australian Envtonmental Geoscience.

Geological Society of Australia Special Publication 21, p. 127-143,344-346.

18

STRATIGRAPHY AND AGE OF THE CARBONATE DUNES OF ROTTNEST ISLAND, WESTERN AUSTRALIA Paul J. Hearty School of Earth Sciences, James Cook University, Townsville, QLD 4811, Australia Email: paul.hearty@jcu.edu.au Rottnest Island contains a succession of late Pleistocene and Holocene bioclastic aeolianites, bounded by soils of varying degrees of development. Subtidal and intertidal marine facies are present, but only at a few localities.

Over 100 whole-rock amino acid racemisation (AAR) ratios from about 30 randomly distributed localities around the island point to distinct pulses of aeolian deposition. In a histogram, wholerock alloisoleucine/isoleucine (A/I) ratios can be divided into three broad groups. The oldest group, Aminozone E, averages 0.33 ± 0.04 (n = 16), and contains deposits radiometrically associated with MIS5e. Red palaeosol and thick calcrete generally cap the Aminozone E deposits.

Subsequently, a younger Aminozone C A/I group occurs at 0.23 ± 0.02 (n = 42); the average of two minor peaks at 0.26 ± 0.01 (n = 12) and 0.21 ± 0.01 (n = 30). Dune sets during this interval are interrupted by weak, brown to tan "protosols" indicating short intervals of dune stabilisation and vegetation. A dense, dark brown rendzina palaeosol and a 1-2 cm calcrete finalise the Aminozone C sequence. Holocene deposits ("Aminozone A"), yield an average of 0.12 ± 0.02 (n = 16).

Calibration of A/I ratios from Aminozones E and A is provided by previously published independent radiometric ages on highstand deposits at c. 125 ka (Marine Isotope Stage [MIS] 5e) and 2-6 ka (MIS 1), respectively. The whole-rock A/I results point to nearly continuous deposition during and following MIS 5e, reaching a climax several tens of thousands of years later, most likely until the end of MIS 5a, about 80 ka. The dark brown rendzina palaeosol represents a major island-wide haitus in carbonate dune deposition, which is paralleled by a significant break in A/I ratios between Aminozones C and A. This interval may correspond with the period represented by MIS 4-2. Dune formation resumed in the early-mid Holocene up to the present. However, much of the current blowout activity is taking place primarily in Pleistocene dune material. This recent activity could be attributed to deforestation, animal trampling, fires, and ecological disruption brought about by humans.

The kinetic A/I pathway from Rottnest Island (latitude 32° S) shows remarkable similarity to that of Bermuda (latitude 32°) where the same aminozones are firmly dated with U/Th ages. These findings suggest that aeolian activity on Rottnest is highly correlated with interglacial highstands when the shoreline is near the present island, rather than during glacial lowstands when the coastline would be situated 10-20 km distant.

19

APPLICATION OF MULTI-COLLECTOR ICP-MS URANIUM-SERIES GEOCHRONOLOGY TO VERY SMALL SAMPLES: IMPLICATIONS FOR AUSTRALIAN QUATERNARY STUDIES John Hellstrom School of Earth Sciences, The University of Melbourne Email: j.hellstrom@unimelb.edu.au Uranium-series disequilibrium dating using the system '^^U-J^^'^U-^^^Th has become of particular importance in establishing accurate late Quaternary chronologies. It has seen its greatest application in dating speleothems and corals but has also been applied with varying degrees of success to bone, teeth, molluscs, tufa, peat, calcrete, evaporites, marine sediments and Antarctic ice. The method requires closed system behaviour since the time of deposition, and can be greatly complicated by the initial incorporation of detrital material.

Initially uranium-series dating relied on alpha-spectrometry, w^hich limits minimum sample size to many hundreds of nanograms of uranium, corresponding to tens of grams of speleothem or grams of coral. As well as having such large sample requirements alpha-spectrometry produces imprecise ages and requires long counting times. Over the last ten years thermal ionisation mass spectrometry (TIMS) has become the standard uranium-series analytical technique, leading to reduced sample sizes and greatly improved precision. Analyses are conducted on samples containing as little as 100 nanograms of uranium, meaning a few grams of speleothem or less than 100 milligrams of coral are required. Particularly in the case of corals, most workers have opted for maximum analytical precision and where possible have used larger sample sizes.

In recent years a number of research groups around the world have applied multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to uranium-series analysis. Two potential benefits of this method are improved sample throughput due to considerably shorter analysis times, and a further significant improvement in sensitivity over TIMS due to greater ionisation efficiency. The greater sample throughput allows more analyses for a given effort, increasing both the resolution and scope of any given geochronological study. A second and possibly more important benefit of this increase in throughput is that routine use of multi-sample isochron techniques becomes viable, allowing accurate dating of samples suffering from high levels of detrital contamination such as calcrete and evaporites.

The greater sensitivity of MC-ICP-MS can lead to greater precision or smaller sample sizes, or both. Whilst improvements in analytical precision have so far led to more accurate uranium-series ages, accuracy is ultimately limited by uncertainty in decay constants and spike calibrations, and by unquantifiable breakdowns in the assumptions of closed system behaviour and zero initial thorium inclusion. The area in which MC-ICP-MS analysis can make potentially enormous gains is in reducing minimum sample size, with acceptable precision currently being obtained on samples containing less than 10 nanograms of uranium. In the case of speleothems this can greatly reduce the impact of removing samples from caves by allowing unobtrusive in situ core sampling.

Uranium-series dating of molluscs becomes more viable in that multiple age determinations can be made on a single shell, allowing careflil evaluation of the degree of open system behaviour since deposition even after aggressive cleaning of the sample. Reduced sample size demands also potentially benefit the dating of difficult systems such as calcrete and evaporites in that the difficulty of obtaining sufficient clean sample is greatly reduced.

20

CLIMATIC FORCING AND FLORAL SUCCESSION: ARIDIFICATION OF A CONTINENT Robert S. Hill and Rosemary Paull Centre for Evolutionary Biology and Biodiversity, South Australian Museum, North Terrace, Adelaide, SA 5000 and Department of Environmental Biology, University of Adelaide, South Australia 5005 Eocene central to southeastern Australia appears to have experienced uniformly high and aseasonal rainfall, as evidenced by the complex rainforest vegetation at such geographically remote places as Nelly Creek (central Australia), Maslin Bay (Gulf St Vincent), Anglesea (southern Victoria) and Regatta Point (western Tasmania). The epiphytic fungal populations on the leaves in these fossil deposits also confirm extremely w^et conditions and, more significantly, the absence of any prolonged dry season.

As the Australian climate dried, and seasonality increased, this extensive rainforest fragmented and plants with xeromorphic adaptations became more prominent. There are three possible origins for such xeromorphic plants: 1. They predate the general Cainozoic aridification. They may have evolved during an earlier arid phase and survived in localised dry niches (e.g. sand dunes, ridgetops) within Australia, or they may have evolved in these localised niches during wet phases in the Palaeogene. As the climate dried after the Eocene these arid-adapted plants expanded out from these niches.

2. They evolved from rainforest precursors as the climate dried, and therefore are of more recent origin.

3. They evolved elsewhere in the world and arrived in Australia by long-distance dispersal after the climate had dried enough to provide suitable habitats.

These hypotheses are not mutually exclusive, and there are examples of all three origins in the extant Australian vegetation.

The macrofossil record of conifers in southeastern Australia certainly supports hypothesis 2, because these rainforest taxa were able to evolve to better survive in drier conditions between the Late Eocene and the Miocene. This is particularly evident within the prominent Southern Hemisphere family Podocarpaceae.

Many taxa with primary scleromorphic adaptations (a response to low soil phosphorus levels) were exapted to developing aridity and became more prominent in the vegetation and diversified later in the Cainozoic. The macrofossil record for this is particularly clear among the Proteaceae and Casuarinaceae, where mechanisms for stomatal protection are sometimes very well developed and offer clear evidence of a xeromorphic response. Paleocene and Eocene taxa within these families usually have no sign of xeromorphic adaptations, even though a scleromorphic response is well developed. However, while this group of fossil taxa preclude hypothesis 3, they do not differentiate between the other two hypotheses and ftirther testing of the phylogenetic position of key fossil taxa is required to resolve this uncertainty.

To date there is no meaningful macrofossil record of true arid zone vegetation in Australia.

Nevertheless, an analysis of the extant taxa and the distribution of their living relatives strongly suggests that at least some arrived by crossing significant ocean barriers while Australia was an isolated land mass, providing support for hypothesis 3.

21

DECOMPOSITION OF SOIL ORGANIC MATTER: A QUESTION OF AGE VERSUS CHEMISTRY? Evelyn S. Krull and Jan O. Skjemstad CSIRO Land & Water PMB 2, Glen Osmond SA 5064 Email: Evelyn.Krull@csiro.au; Jan.Skjemstad@csiro.au Decomposition of soil organic matter (SOM) is generally assumed to be associated with changes in the soil organic chemistry as degradation proceeds. Customarily, a progression from SOM rich in relatively labile components (e.g. cellulose, hemicellulose) tovs^ards a greater fraction of more stable and refractory organic compounds (e.g. lignin, charcoal) with increasing time and degree of decomposition is presumed. These changes in soil chemistry are based on the relative turnover times (degree of bioavailability to micro-organisms) of the different organic fractions and are a function of their molecular structure (e.g. chain versus ring structure). However, the intrinsic characteristics of the soil environment (mineralogy, pH), soil biota, type of vegetation, and landscape history (e.g. frequency of fires, agriculture) can significantly modify the turnover time of organic chemical fractions.

To assess how these factors can influence the traditional view of increasing age and degree of decomposition with depth, we chose 12 undisturbed profiles that differed in soil type, mineralogy, vegetation, and climate for detailed analyses. All these soils were sampled at defined depth intervals for radiogenic (^^C) and solid-state ^^C-NMR analysis. Selected profiles were also characterised by stable carbon isotopic analysis. Furthermore, we employed high-energy UV photo oxidation to estimate the SOM fraction that was either chemically inert (e.g. charcoal) or protected (shielded) from decomposition by the mineral matrix (aggregates, adsorption on clay minerals) or chemical complexation (Al- or Fe-complex).

Our results show that • the mean residence time (^^C age) of SOM does oftentimes not consistently increase with depth but may show a zigzag pattern, abrupt jumps or no significant change in ^"^C age at all; • soil chemistry did not always progress from presumed labile to increasingly stable/recalcitrant fractions; • the amount of the protected fraction varied significantly with soil type and in many cases could be attributed to occurrence of charcoal.

These results indicate that various soil types, each with a distinctive mineralogy, pH, and landscape history, can have significantly different turnover times of SOM. The difference in turnover times is not due to the soil organic chemical make-up alone, but can be significantly modified by the capacity of soil to protect organic matter. Protection of SOM can stabilize otherwise chemically labile components and therefore render them unavailable for a longer period of time than suggested by their chemical structure. This explains why ^^C-NMR spectra often show chemically labile Oalkyl C not only at the soil surface but also at depth. Furthermore, frequent fires can produce significant amounts of charcoal, which has an aromatic structure and is therefore chemically and biologically very stable, resulting in a high mean residence time. However, this also implies that recent fires produce a very young but chemically highly stabile (aromatic) organic fraction. This results in the seemingly paradoxical situation of a relatively young soil that is characterized by a high amount of aromatic C, which is usually interpreted as an indicator of highly decomposed, old organic material.

22

LACUSTRINE SAPROPELS AS PROXIES FOR LATE QUATERNARY ENVIRONMENTAL CHANGE IN SOUTHEASTERN AUSTRALIA David M. McKirdv^ Alan J. Brenchley^ and Sally Edwards' 'Organic Geochemistry in Basin Analysis Group, Department of Geology and Geophysics, Adelaide University, SA 5005, Australia ^Geoscience Department, Onkaparinga Institute of TAPE, Majors Road, O'Halloran Hill, SA 5158, Australia

Scattered along the Coorong coastal plain of South Australia between the Murray Mouth and Millicent are many small ephemeral or semi-permanent alkaline lakes fed by seaward-flowing meteoric groundwater. The Holocene upward-shoaling carbonate-evaporite sedimentology of the lakes near Salt Creek (Warren, 1990) and in the corridor between the Robe and Woakwine Ranges (Cann et al,, 1999) is well documented. A striking feature of these lacustrine lithostratigraphic records is a dark brown to black sapropelic mudstone ( 6 - 2 0 % TOC, 0 . 1 - 1 m thick) comprising largely low-Mg calcite and aragonite. Old Man Lake, located within the Robe Range, is unusual in recording three periods of sapropel deposition and preservation. ^^C-dating of sapropels from eight different lakes hints at millennial-scale cyclicity in their formation over the interval 8 - 2.5 Ka BP. Diatoms and cyanobacteria are the main known primary sources of organic matter. Less significant contributors were Botryococcus sp. and cuticle and pollen from terrestrial and aquatic metaphytes. The hydrogen indices (HI = 3 0 0 - 9 0 0 ) and C-isotopic compositions (D^^Corg = - 2 2 to - 1 7 % o ) of the sapropels confirm their predominantly algal-bacterial affinity. Highly-branched isoprenoid alkane (C20) and alkene (C25) biomarkers signal major diatom inputs, even where siliceous frustules are not preserved due to their dissolution in alkaline pore waters (McKirdy et al, 1 9 9 5 ) . Mixed salinity-range (brackish, saline & metasaline) diatom assemblages indicate that the host sapropel formed beneath a stratified water column. However, the co-occurrence of benthic ostracods, dwarf molluscs and gastropods precludes anoxia in the hypolimnion. Ostracod C and O-isotopic data from North Stromatolite Lake suggest that its salinity increased during sapropel formation.

The foregoing observations raise some important questions: What are the ages, durations and periodicity of the sapropel events recorded in the Coorong coastal lakes? What are the key palaeolimnological triggers of the algal blooms necessary for sapropel formation? Why do sapropels of comparable age not occur in the nearby crater lakes of South Australia and western Victoria? To what extent can lacustrine sapropels in southeastern Australia be used as proxies for mid-latitude environmental change? And finally, in what way (if at all) are these sapropels similar to the Late Quaternary marine sapropels of the eastern Mediterranean and Black Seas?

References Cann, J.H., Murray-Wallace, C.V., Belperio, A.P. & Brenchley, A.J., 1999. Evolution of Holocene coastal environments near Robe, southeastern South Australia. Quaternary International 56, 81-97.

McKirdy, D.M., Hepplewhite, C.J., Michaelsen, B.H., Mazzoleni, A. & Bone, Y., 1995. Origin of sapropels in Holocene lakes of the Coorong region. South Australia. In: Grimalt, J.O. & Dorronsoro, C. (Eds), Organic Geochemistry: Developments and Applications to Energy, Climate, Environment and Human History. Selected papers from the 17th Intemational Meeting on Organic Geochemistry, Donostia-San Sebastian, The Basque Country, Spain, pp. 183-185.

Warren, J.K., 1990. Sedimentology and mineralogy of dolomitic Coorong lakes. South Australia. Journal of Sedimentary Petrology 60, 843-858.

23

QUATERNARY "WHOLE-ROCK" AMINOSTRATIGRAPHY OF SOUTHERN AUSTRALIAN AEOLIANITES Colin V. Murrav-Wallace\ Robert P. Bourman^ Brendan P. Brooke^ Matthew Hales' ^School of Geosciences, University of Wollongong, NSW, 2522 School of Environment and Recreation Management, University of South Australia, SA, 5095 Marine and Petroleum Division, Geoscience Australia, Canberra, ACT, 2609

Studies of Quaternary coastal evolution have advanced considerably in recent years because of important advances in geochronology. Many sedimentary successions and landforms, however, remain difficult to date directly. For example, the paucity of corals in many Pleistocene coastal successions has restricted the application of uranium-series disequilibrium dating, resulting in a greater reliance on other geochronological methods. Similarly, the absence of quartz sand in many ocean island settings has prevented the use of luminescence-based methods of dating. In response to these restrictions some have applied amino acid racemization (AAR) reactions directly to the dating of sediments, particularly v^here entire fossils are absent.

Amino acid racemization is a chemical reaction that involves the change in the configuration of amino acid isomers follow^ing the death of organisms. During life, amino acids are bound w^ithin peptides exclusively as left-handed (L-) molecules. Following death, the enzymic reactions that formerly maintained the disequilibrium condition cease, and a gradual, reversible reaction commences, such that L-amino acids progressively interconvert to a right-handed counterpart (Damino acids), a process termed amino acid racemization. The reaction continues until an equilibrium state is attained (50:50 mixture of D- and L-amino acid isomers). The slow reaction rate permits its application as a geochronological tool. Although dependent on diagenetic temperature and other geochemical parameters, the technique offers the potential to assign ages to marine carbonate fossils for much of Quaternary time in southern Australia.

Recently, we have applied the AAR technique to the dating of "whole-rock" sediments (bioclastic skeletal carbonate sand) derived from aeolian facies of Pleistocene coastal barriers in southern Australia. The area of investigation extends from Encounter Bay - Hindmarsh Island across to Naracoorte and down to Bridgewater Bay in western Victoria.

In a transect from Robe to Naracoorte, the extent of leucine racemization (in the total acid hydrolysate and free amino acid fractions) in the Pleistocene skeletal carbonate sand (63-250 Dm) increases monotonically with age and is consistently higher than for entire fossils from the same allostratigraphic units, reflecting the lengthy residence time for bioclasts in this high wave energy environment, and sediment recycling from the erosion of older barriers. The extent of racemization in the whole-rock samples conforms with a model of apparent parabolic kinetics and the calculated ages largely agree with previously determined luminescence ages (e.g. Reedy Creek TL age of 258 ± 25 ka, AAR 251 ± 48 ka; West Avenue TL age 342 ± 32 ka, AAR 382 ± 73 ka; Baker TL age 456 ± 37 ka, AAR 438 ± 83 ka). In addition, a preliminary AAR age of 935 ± 178 ka is assigned to the East Naracoorte Range.

24

AMINO ACID RACEMIZATION GEOCHRONOLOGY OF THE PLANKTONIC FORAMINIFERA, PULLENIATINA OBLIQUILOCULATA, FROM QUEENSLAND TROUGH PISTON CORES, AUSTRALIA Michael J. Q'Leary and Paul J. Hearty School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia Email: michael.oleary@jcu.edu.au The Queensland Trough preserves a fairly complete record of sea-level change and sediment flux in its stratigraphy. How^ever, research on sedimentary processes has been hampered by geochronological limitations of ^"^C and U-series dating. A possible resolution of this problem is provided through the application of reverse phase chromatography (RPC) on single foraminiferal tests. The large planktonic foraminifer Pulleniatina obliquiloculata is ubiquitous in deeper waters in the Queensland Trough. It resists diagenesis and is thus an ideal sample material for this first systematic application of its kind.

The extent of racemization (D/L) of aspartic acid (Asp), glutamic acid (Glu) and serine (Ser) in Pulleniatina v^as measured from core horizons having existing ^"^C and data. D/L results follow the kinetic and chemical rules outlined in previous Amino Acid Racemization (AAR) studies. The reliability of the approach is demonstrated by: 1) the conformity of D/L ratios with stratigraphic order; 2) the high level of covariance of Asp, Glu, and Ser in geological samples; 3) the consistency of amino acid concentration with age; 4) the concordance with established AAR kinetic models; and 5) the ability to correlate D/L data in cores regionally.

A by-product of measuring D/L ratios of single foraminiferal tests is a quantitative assessment of sediment mixing rates. The degree of sediment mixing due to bioturbation is reflected in the standard deviation from each core horizon. In some cases, the magnitude of mixing is great enough to call into question the significance and accuracy of existing AMS ^^C dates. RPC can be used for pre- ^'^C and prescreening of core levels to determine those with minimal mixing rates for optimal dating.

The kinetic pathway for Pulleniatina is characterized by two linear components separated by a narrow transitional zone. This trend can be modeled in terms of apparent parabolic kinetics (APK).

Over 92% of the data can be accommodated in terms of APK. Thus, reliable age estimates can be determined well beyond the range of ^"^C, and up to 500 ka at this time.

25

LUMINESCENCE AGES FOR THE RELICT DUNE SYSTEM IN THE SOUTH-EAST OF SOUTH AUSTRALIA. J.R. Prescott^*vD.J. Huntley^, G.B. Robertson^ and F.M. Williams^ ^ Physics Department, University of Adelaide, South Australia ^ Physics Department, Simon Fraser University, Canada. * corresponding author.

Reg Sprigg was one of the first to identify the relict dune system in the south-east of South Australia (SESA) with interglacial high stands of the sea on a tectonically rising land surface. This series of dunes extends back to beyond the Brunhes-Matuyama geomagnetic field reversal at 780 ka. Luminescence dating, in one or other of its various protocols, can now be relied on to give credible ages from a few tens of years to 500 ka when judged against independent geological evidence. In the case of the SESA dune system, such 18 evidence is provided by the matching of the dunes with high sea levels deduced from 5 O isotope ratios in deep sea cores. For some years an Adelaide University/Simon Fraser University collaboration has been applying luminescence dating (thermoluminescence dating and optical dating) to these dunes and has successfully matched the ages of the dunes back to about 500 ka. So far we have not obtained consistent answers when pressed beyond this, for reasons that we are begimiing to understand. In this paper we shall discuss the current state of progress of luminescence dating in the SESA system and speculate on the possibility of using what we have learned to date an older dune system in western Victoria. In later life, Sprigg suggested that this system of dunes was closely tied to that in South Australia.

26

NEOTECTONIC FRAMEWORK OF AUSTRALIA Mike Sandiford School of Earth Sciences, University of Melbourne, Victoria, 3010.

The neotectonic evolution of Australia can be understood in terms of two main processes: 1. a regional, compressive stress field apparently established in the Eocene, but greatly amplified in the late Neogene (10-5 Ma), related to plate tectonic setting.

2. continent-wide, north-down tilting related to dynamic topography generated by the northward transit of Australia "up" the flanks of a major geoid ramp.

Along with low-level seismicity, local tectonic topography, best exemplified by the fault-related landscapes of the Adelaide region, provides the most obvious manifestation of the regional compression, while the contrasting Neogene stratigraphic record of the southern and northern margins provide the main evidence for dynamic topography.

Regional compression in the Australian continent relates to its palaeogeographic plate-tectonic setting. Extensional stress regimes associated with fragmentation of Gondwana seem to have prevailed up until 55-50 Ma, at least in southeast Australia. Until this time, the Australian continent was slow moving and, to large extent, surrounded by active spreading centres. The onset of compression in the early Eocene is associated with the amalgamation of the Indian and Australian plates that accompanied the termination of spreading in the north-Central Indian Ocean (and the Tasman Sea), related to collision of India and Asia. The post-early Eocene tectonic record is one of increasing tectonic stress levels, related largely to plate margin activity. The late Neogene response of the southeast part of the continent probably reflects the Australian-Pacific plate interactions responsible for building the south Alps of New Zealand over the last 6.4 m.y. Late Neogene tectonic compression has been sufficient for failure of the central Indian Ocean, heralding the impending "divorce" of the Indian and Australian plates.

The contrasting stratigraphic records of the northern and southern margins of Australia suggest that the dynamic topography associated with Australia's transit up a major geoid "high" centred to the north of New Guinea is responsible for ~ 60 m of relative north-down, south-up tilting since the earliest Pliocene. This is most dramatically realised in the preservation of the remarkable regressive Pliocene sequences of the Murray Basin. Recognition of this dynamic topographic component should caution against using southern margin stratigraphy as a reference frame for global eustatic sea-level changes.

27

LATE QUATERNARY ENVIRONMENTAL CHANGES IN QUEENSLAND: A MULTI-PROXY RECORD FROM LYNCHES CRATER THROUGH THE LAST 50,000 YEARS Chris S.M. Turnev^ A. Peter Kershaw^ Michael I. Bird^ Keith Fifield^ and Patrick Moss^ and Nick Branch^ ^ School of Archaeology & Palaeoecology, Queen's University, Belfast, BT7 INN, UK ^Department of Geography and Environmental Science, Monash University, Clayton, Melbourne, Victoria, 3168, Australia. ^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia '^Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia '^Department of Geography, 316 Jessup Hall, University of Iowa, Iowa City, lA, 52246 USA ^Department of Geography, Royal Holloway, University of London, Egham, Surrey, TW20 OEX, UK Lynch's Crater on the Atherton Tableland, north Queensland, is one of the fevs^ sites in the world which has demonstrated a continuous record of vegetation and environmental change through the late Quaternary. The record suggests a marked increase in burning during Marine Isotope Stage 3 and is accompanied, or closely followed, by the virtually complete replacement of rainforest by sclerophyll vegetation. Here we have applied a rigorous pretreatment and graphitisation procedure for radiocarbon dating samples from the Lynch's Crater sequence. These new ages suggest that the increase in fire frequency occurred at 45,000 ^^C years B.P. compared to the previously published age of approximately 38,000 ^"^C years B.P. To test whether this change in the fire regime could be anthropgenic or natural in origin, a multi-proxy study of the sediments has been undertaken, including fine-resolution pollen analysis, bulk sediment geochemistry, and degree of humification. Preliminary results on environmental changes in the region will be presented.

28

EVAPORITES PAST AND PRESENT: HYDROLOGICAL CHARACTER RECONCILES MODERN AND ANCIENT DEPOSITIONAL SETTINGS John Warren Email: jwarren@brunet.bn Department Petroleum Geoscience, University Brunei Darussalam, Tungku Link, Bandar Seri Begawan, Brunei

Today, and throughout the Quaternary, widespread salt beds up to a few hundred metres thick accumulate only in continental settings. This always takes place within a stable brine or reflux curtain in an endorheic setting, which is fed by hybrid or hydrothermal groundwaters (e.g. Salar de Uyuni, Bolivia). There are some ancient lacustrine counterparts similar in scale and groundwater feed to those of today, such as the trona deposits of the Eocene Green River Formation. But the majority of ancient evaporite deposits, ranging in age from the Proterozoic to the Neogene, are made up of much larger and thicker volumes of salts. These are deposits that accumulated in marine-fed basinwide and platform settings, environments that have no modern counterpart.

Once we realize that these thick evaporite accumulations require a stable long-term brine curtain (-lO^-lO^ years) to accumulate to substantial thicknesses and lateral extents, we reach an understanding of why the present is not a good time to study the scale and diversity of possible evaporite settings. High amplitude, high frequency 4th-order sea level oscillations of our current "icehouse" climate do not allow the set up of stable brine curtains in present-day continental platforms and so there are no Quaternary examples of platform evaporites. Nor are there suitable rift or collision belts in arid marine-fed settings where conditions are suitable for the creation of marine basinwide drawdown deposits. The hydrological position of the active top of the brine curtain, with respect to the evaporite depositional surface, in both platform and basin-wide settings, defines the dominant textural signature of the resulting salt sequence (saline-pan, evaporitic mudflat, saltern, deeper slope and basin).

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A RE-APPRAISAL OF LATE QUATERNARY ENVIRONMENTS IN THE FLINDERS RANGES, SOUTH AUSTRALIA Martin Williams^ John R. Prescott^, John Chappell^ and Donald Adamson'^ ^Geographical & Environmental Studies, University of Adelaide, SA 5005, Australia.

2Physics & Mathematical Physics, University of Adelaide, SA 5005, Australia.

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.

4 Department of Biology, Macquarie University, NSW 2109, Australia.

Streams within the semi-arid Flinders Ranges are active today only during rare downpours, when they transport boulders, gravel and coarse sand. The late Quaternary valley-fill deposits within the ranges consist primarily of clay, silt and very fme sand, and have been incised by present streams to form terraces and terrace remnants. Such clay-rich deposits are not accumulating today. In and upstream of Brachina Gorge, in the central ranges, these remnant valley-fills are exposed in bank sections up to 18 m high. Some exposures show horizontal to gently undulating beds of fine sand and clayey silt a few centimetres thick that can be traced for several hundred metres. Gastropods, diatoms and phytoliths in these clays point to sluggish, shallow water flow under fresh to brackish conditions. AMS ^^c and OSL dating shows that these valley-fill deposits accumulated between - 3 3 and 17 ka, an interval spanning the Last Glacial Maximum (LGM), during which snowlines and temperatures were lower in south-eastern Australia. Once considered lake beds (Cock et al, 1999; Preiss,1999), the deposits accumulated in a fluvial wetland that extended westward from the middle reaches of Brachina Creek, through Brachina Gorge and joined with aggraded fan deposits beyond the ranges (Williams et al, 2001). Aggradation of this wetland requires a substantial reduction of both rainfall variability and evaporation, which would be favoured by the low temperatures and reduced incursions of summer rainfall. This is consistent with independent climatic reconstructions for the Lake Eyre region to the north of the Flinders Ranges.

Demise of the wetland was heralded by a major influx of coarse alluvium followed by channelling and dissection. Erosion was interrupted by an episode of aggradation and floodplain widening, represented by remnants of a wide terrace inset below the primary wetland surface. This episode, which is interpreted as a return to lower climatic variability, ceased with establishment of the present climatic regime, which has resulted in stripping of the Late Pleistocene deposits from much of the Brachina valley. Unresolved issues concern the hydrologic impact of aeolian dust and changes in plant cover on valley-fill accumulation.

References Cock, B.J., Williams, M.A.J, and Adamson, D.A., 1999. Pleistocene Lake Brachina: a preliminary stratigraphy and chronology of lacustrine sediments from the central Flinders Ranges, South Australia. Australian Journal of Earth Sciences 46, 61-69.

Preiss, W.V., 1999. Explanatory Notes, Parachilna, South Australia, 1: 250 000 Geological Series, Sheet SH5413. Geological Survey of South Australia, Adelaide.

Williams, M., Prescott, J.R., Chappell, J., Adamson, D., Cock, B., Walker, K. and Gell, P., 2001. The enigma of a late Pleistocene wetland in the Flinders Ranges, South Australia. Quaternary International 83-85, 129-144.

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ISOTOPIC DATING OF FOSSILIFEROUS PRECAMBRIAN SEDIMENTARY SEQUENCES: NEW DEVELOPMENTS, NEW RESULTS AND AN OLD PROBLEM - "HOW OLD ARE MOTILE, MEGASCOPIC ORGANISMS?'' Birger Rasmussen', Ian R. Fletcher^ Neal J. McNaughton^ and Stefan Bengtson^ 'Centre for Global Metallogeny, University of Western Australia, 35 Stirling Hwy., Crawley, 6009, ^Department of Palaeozoology, Swedish Museum of Natural History, Stockholm, Sweden

This presentation uses data from the Stirling Range Formation, Western Australia, but demonstrates three widely applicable points: 1. Motile megascopic organisms existed >1200 million years ago; 2. Precambrian fossils are poor indicators of the age of their host sediments; and 3. U-Pb geochronology using diagenetic xenotime and metamorphic monazite can provide valuable minimum age constraints for Precambrian (meta)sedimentary sequences.

The Stirling Range Formation is a siliceous sedimentary sequence, variably metamorphosed up to greenschist facies, at the southern margin of the Yilgarn Craton. After Ediacaran-type fossils were identified in the formation a decade ago (Cruse et al, 1993), they were used to assign an age of 540-590 Ma for sedimentation, in preference to the -1340 Ma suggested by Rb-Sr data (Turek & Stephenson, 1966). Recent work on the phosphate minerals xenotime (McNaughton et al, 1999; Fletcher et al, 2000) and monazite (Rasmussen et a/., 2001) has opened new avenues for dating Precambrian sediments that lack intercalated volcanic units, through SHRIMP U-Pb analyses of authigenic and metamorphic grains.

Analyses of detrital minerals in the Stirling Range Formation (>150 zircons, and several monazites and xenotimes) indicate sedimentation at <2000 Ma, with one monazite suggesting a maximum age o f - 1 9 0 0 Ma. This is consistent with all earlier estimates. Metamorphic monazite in the formation has low Th and very low U, together with significant common Pb. High-precision U-Pb dating is therefore not possible, but Th-Pb gives an age of 1200 Ma with total uncertainties of - 2 5 Ma. This is a minimum age for the Stirling Range Formation, obviously in conflict with the age inferred from the fossils, which therefore cannot be Ediacaran. Authigenic xenotime, probably including diagenetic crystals, has been identified and is being analysed to provide closer constraints.

These data require a complete re-evaluation of the fossils in the Stirling Range Formation, which include trace fossils suggestive of motile, worm-like organisms. They also require reconsideration of tectonic models that have used the Stirling Range Formation as an early Phanerozoic marker in continental reconstructions. In a broader context, they demonstrate that a record of motile megascopic life is preserved in sedimentary rocks that are >1200 Ma. Consequently, there is a need to search for more examples of fossils in rocks of this age, without past inhibitions. Furthermore, the ages of sedimentary sequences with known primitive fossil fauna should not be assumed from the apparent fossil associations alone.

References Cruse, T., Harris, L.B. & Rasmussen, B. 1993. The discovery of Ediacaran trace and body fossils in the Stirling Range Formation, Western Australia: Implications for sedimentation and deformation during the 'Pan-African' orogenic cycle. Australian Journal of Earth Sciences 40, 293-296.

Turek, A. & Stehenson, N.C.N. 1966. The radiometric age of the Albany Granite and the Stirling Range Beds, southwest Australia. Journal of the Geological Society of Australia 13, 449^56.

McNaughton, N.J., Rasmussen, B. & Fletcher, I.R. 1999. SHRIMP uranium-lead dating of diagenetic xenotime in siliciclastic sedimentary rocks. Science 285, 78-80.

Fletcher, I.R., Rasmussen, B. & McNaughton, N.J. 2000. SHRIMP U-Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins. Australian Journal of Earth Sciences 47, 845-859.

Rasmussen, B., Fletcher, I.R. & McNaughton, N.J. 2001. Dating low-grade metamorphic events by SHRIMP U-Pb analysis of monazite in shales. Geology 29, 963-966.

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HIGH-RESOLUTION CHEMICAL IMAGING OF FOSSILISED HYPERTHERMOPHILE BACTERL\ ^Julien Foriel ^Pascal Philippot, ^Jean Susini, Hicham ^Khodja and "^Yves Fouquet 'Laboratoire de Geosciences Marines, CNRS - Institut de Physique du Globe de Paris, T26-00/E3, case 89, 4 place Jussieu, 75252 Paris cedex 05, France ^ID-21, European Synchrotron Research Facility, BP 220 Grenoble38043 Cedex ^Laboratoire Pierre Sue, Commissariat a I'Energie Atomique, Gif sur Yvette, 91191, France ^Departement de Geochimie-M^allogenie, IFREMER, Plouzane, 29280, France In order to provide new tools for the identification and observation of possible bacterial fossils, we present a high-resolution chemical imaging protocol of micrometric scale. This protocol is based on Proton Induced X-ray Emission (PIXE) experiments performed with a nuclear microprobe at Commissariat a I'Energie Atomique, Gif sur Yvette, France and micro-XANES (X-ray Absorption Near Edge Structure) experiments performed on beamline ID-2] at the European Synchrotron Research Facility, Grenoble, France. We used these two analytical techniques in order to map chemical characteristics relevant to the understanding of bacterial processes: transition metal concentrations by PIXE and sulfur species (sulphide, sulphate) distribution by |LI-XANES. In both cases we achieved a spatial resolution of c. 1 |Lim. Presented here are the results of a first series of experiments on modem fossilised bacteria from a hydrothermal field on the East Pacific Rise.

These samples were chosen as possible analogues for early Archaean microfossils.

PIXE mapping showed an elemental distribution (S, Fe, Cu, Zn) consistent with visible features and with a metabolism using or producing metal sulphide. Micro-XANES images display different distributions of sulphates and sulphides. Such micrometric contrast in S state, in an optically homogeneous grain, hardly seems to be a consequence of abiotic mineralogical processes, but rather of metabolic activities of different kinds of bacteria. Our hypothesis is that the analysed sample is a fossilised piece of microbial mat including different biota. Micro-XANES images can therefore be seen as distribution maps of the different metabolic processes taking place in the biofilm when it was fossilised. Higher concentrations of sulfates can be attributed to the activity of sulfide-oxidising bacteria and higher sulphides counts to sulfate-reducing bacteria.

Similar experiments on Archaean microfossils could help determine what kinds of metabolism bacteria used then. Furthermore, as bacterial processes are thought to be a dominant factor in the sulphur cycle, these conclusions could be added as new constraints for the understanding of the Archaean Earth sulphur cycle.

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A SIXTH GREAT EXTINCTION AND RECOVERY EVENT? THE EDIACARIAN ACRAMAN BOLIDE IMPACT Kathleen Grey Geological Survey of Western Australia, 100 Plain Street, East Perth, 6004, Western Australia

The 'Snowball Earth Hypothesis' (Hoffman et aL, 1998) postulates an early Marinoan (c.600 Ma), glacially induced global crisis. Sea-level fall and low temperatures depleted the biota. '[C]reatures that survived glacial episodes may have taken refuge at hot springs both on the seafloor and near the surface of the ice where photosynthesis could be maintained'...'the steep and variable temperature and chemical gradients endemic to ephemeral hot springs would preselect for survival in the hellish aftermath to come' and 'the climate recovery following a huge Neoproterozoic glaciation paved the way for the explosive radiation of multicellular animal life soon thereafter' (Hoffman & Schrag, 2000).

Shelfal-marine phytoplankton (especially sensitive to environmental change) can be used to test predictions about post-Snowball biota. The Australian Neoproterozoic palynological record provides stratigraphically constrained palynological data, and is based on c. 2000 samples from 30 drillholes in the Adelaide Rift Complex and Centralian Superbasin. Post-glacial samples are barren, but eventually an impoverished biota of simple leiospheres (single-celled green algae) and cyanobacterially-dominated benthic mats recolonised and proliferated as sea level rose. However, species are the same before and after the glaciation. There is no evidence of colonisation by new, rapidly diversifying taxa, such as extremophiles from hot spring or ice-dominated refugia. Thus post-glacial diversity patterns do not match predictions. There is, however, a major change in later phytoplankton assemblages. Leiospheres decline and >50 species of large, complex acanthomorph (spiny) acritarchs, resembling modern dinoflagellate resting cysts, make their first appearance. The change from a leiosphere-dominated to an acanthomorph-dominated palynoflora is abrupt and apparently synchronous. Based on estimates of c. 595-600 Ma for the glaciation, 578 Ma for the Acraman impact event, 549 Ma for the midpoint of the Ediacara-fauna range, and 544 Ma for the Neoproterozoic-Cambrian boundary (Walter et al, 2000), acanthomorph diversification did not begin until c.580 Ma, and terminated shortly after a canyon cutting event at c.565 Ma. Thus, phytoplankton radiation took place in <15 m.y., but not until c. 20 m.y. years after glaciation. It did not take place until the second post-glacial major marine incursion and appears unrelated to sedimentology or sequence stratigraphy. The biotic evidence does not support the 'Snowball Earth' model, and extreme glaciation does not seem to have been the principal cause of biotic change.

There is, however, a remarkable coincidence between the appearance of the acanthomorphs, a organic carbon excursion (Calver and Lindsay, 1998), and the Acraman impact ejecta layer.

Diversification patterns are consistent with those for major Phanerozoic extinction and recovery events, and may indicate a sixth biotic crisis that had significant evolutionary repercussions.

Acanthomorph diversification may represent a recovery event following a bolide impact.

References Calver C. R. & Lindsay J. F. 1998. Ediacarian sequence and isotope stratigraphy of the Officer Basin, South Australia. Australian Journal of Earth Sciences 45, 513-532.

Hoftman P. F. & Schrag D. P. 2000. Snowball Earth. Scientific American, January, 68-75.

Hoffman P. F., Kaufman A. J., Halverson G. P. & Schrag, D. P. 1998. A Neoproterozoic Snowball Earth.

Science 1342-1346.

Walter M. R., Veevers J. J, Calver C. R., Gorjan P. & Hill A. C. 2000. Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon, and sulfiir in seawater, and some interpretative models. Precambrian Research 100, 371-433.

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A MISSION TO REALLY EARLY EARTH: WHEN DID THE EARTH BECOME SUITABLE FOR HABITATION? ^T.M. Harrison, ^T.R. Ireland, V . Bennett ^M. Honda, ^A. Berry, ^SJ. Mojzsis, ^R.T. Pidgeon, ^B.P. Bourdon, -pj. Ryerson, ^Y Anielin and ^J.L. Kirschvink ^Research School of Earth Sciences, The Australian National University "Department of Geological Sciences, University of Colorado ^Department of Geology, Curtain University ^Laboratoire de Geochimie et Cosmochimie, Institut de Physique du Globe ^Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory ^Geological Survey of Canada ^Division of Geological and Planetary Sciences, California Institute of Teclmology

Wlien did conditions suitable for life emerge on Earth? Since the necessary energy sources and molecular buildmg blocks for biopoesis were available during the formative stages of planetary evolution, our question reduces to: When did suitably quiescent conditions and liquid water first appear at the Earth's surface? The earliest direct evidence of a hydrosphere is in the form of >3.82 Ga marme sediments from Greenland. That these rocks contam putative C isotopic evidence of the existence of relatively sophisticated biologic activity raises the possibility that life emerged during the Hadean Eon (4.5-4.0 Ga), a period for which there is no knovm rock record. This being the case, how can we determine whether life, or an environment conducive to life, was extant during this era? The record that is preserved is in the form of Hadean detritus deposited in younger sediments. The discovery of 4.3-4.4 Ga detrital zircons from Jack Hills, Western Australia, offers the prospect of gaining unprecedented insights into surface environmental conditions during the earliest phase of Earth evolution. For example, O isotopes from these ancient zircons suggest the presence of a hydrosphere and stable continents only 200 Ma after accretion - both conditions favorable for the emergence of life. These results challenge the view that continental formation and hydrosphere development were fiiistrated by meteorite bombardment and basaltic igneous activity until -4.0 Ga. In addition to assessing the timing and mechanism of hydrosphere origin, we have embarked upon age characterization of up to 10" Jack Hills zircons in order to gain an understanding of: 1) The age and origin of the atmosphere. >4.3 Ga zircons are sufficiently old to retain a memory of the terrestrial Pu/U ratio, an unknown but key parameter in interpreting mantle-derived Xe isotopes in terms of the age of the atmosphere. Peraluminous inclusions within these zircons, mdicative of formation of the melt protolith at the Earth's surface, suggest the existence of a mature sedimentary cycling environment during the Hadean. These inclusions contain a record of intrinsic /O2 (via jiiXANES and zircon saturation systematics) that is at least in part surface related and thus may bear on the presence of life (i.e., relate to the presence/absence of reduced C at the Earth's surface).

2) A paradigm for early criistal evolution. Evidence of short-lived ^"^Sm (decays to ^"^^Nd with 103 Ma) should be preserved in continental crust if differentiation began early and produced high Sm/Nd reservoirs. While ^^^Sm-^^^Nd data have been interpreted as indicating slow, early crustal growth, early depletions may have been subsequently re-homogenized. Lu-Hf isotope data of Hadean zircons, which suggest the origin of the crust from a somewhat depleted mantle at 4.1-4.3 Ga, transcend this ambiguity as they represent crustal material that has been sequestered for up to 4.4 Ga 3) The initiation age of the terrestrial geodynamo. Hadean zircons preserved in the Jack Hills quartzites could retain the oldest record of the Earth's magnetic field. Ultra-sensitive analysis methods demonstrate that a Jack Hills zircon carries an intrinsic remanent magnetism raising the possibility of constraining the time of geodynamo activation.

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EARLY METAZOAN EVOLUTION Richard J. F. Jenkins Department of Geology and Geophysics, University of Adelaide Email: richard.jenkins@adelaide.edu.au Notions on the origins of the Metazoa have traditionally focused on protistan-fungal-animalian analogies, and disjunct lines of evidence from fossil material. Ideas concerning their rapid diversification over the Ediacaran (c. 600-540 Ma)/Cambrian interval commonly involve palaeoenvironmental hypotheses, such as the presumed role of changes in free oxygen, climatic variation (Neoproterozoic glaciations), or even palaeobiogeographic influences. Advances in protein and DNA/RNA sequencing in modem material, coupled with extraordinary new insights concerning the genetic control of ontogenetic development, pose a quantum leap in understanding.

Direct evidence from fossils is fragmentary and persistently controversial. Erect, eukaryotic, multicellular, red algae showing evidence of sexual reproduction date back 1.2 Ga. Neoproterozoic chert or dolomite preservations provide putative evidence of fungi as well as vase-shaped, testate amoebans. Perhaps the oldest real evidence of metazoans is seen in blastocoel embryos replaced by phosphate in the later Neoproterozoic of China. Widely distributed biotas of trace fossil and softbodied remains characterize the terminal Neoproterozoic, or Ediacaran. However, many still question supposed evolutionary links between these and the unqestionably more derived Cambrian metazoans.

Ideas of geological triggers for the timing of metazoan radiations suffer from both a lack of temporal control on possible global events, such as widespread glaciation (Snowball Earth ?), and the evident dearth of fossil remains in close proximity to signature sequences marking particular events.

Amino acid sequencing of short proteins during the late 1970s and 80s led to the fashionable construction of cladograms linked to the known divergence of clades as shown by the fossil record, or the so-called "molecular clocks". However, these parsimonious clocks have internal errors due to "hidden"- and "silent-substitutions". By the early nineties, routine sequencing of suitably chosen long RNA sequences analysed by bootstrapping probability models had convincingly shaped the relationships between the five biological Kingdoms, and the three divisions of higher life. The divergences of the latter were sufficiently deep in the Proterozoic as to make any estimates of the timing of their origins controversial.

The recent discovery of closely comparable, sequentially arranged, developmental Hox genes which control body-polarity, segmentation, and eye/limb/wing growth in both protostomes and deuterostomes indicates that the ontogenetic patterns of all higher metazoans are interlinked. The origins of such genes from the highly conserved DNA homeobox (the base-coded record for the Hox genes in the same order as expressed in segmentation), and the presence of this in lower metazoans (? other than for sponges), establishes the general unity of both the protostome and deuterostome sections of this Kingdom. Moreover, the remakable similarity of sequenced bases in different Hox genes poses the likelyhood that their origin represents mutational repetition of a single advantageous DNA binding regulatory protein controlling the ontogenetic development of an individual "segmenf.

Fortunately for palaeobiologists, the imprimatur of the Hox homeobox on animal form is so great that one can pronounce that the appearance of peletal faeces in the upper Wonoka Formation denotes the occurrence of worm-like lower protostomes in the Ediacaran. Worm- and arthropod-like segmented coelomates with evident tagmata (localised special adaptations of segments) show that Hox gene replication had occurred at the time of the Ediacara assemblage, and this is also made clear by the concomitant appearance of deuterostomes. The biomineralization forming the characteristic signature of the Cambrian may now be construed as simply an extension of the adaptive potential of segmentation.

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BIOGEOCHEMICAL EVIDENCE FOR LIFE IN THE ARCHEAN AND PROTERZOIC Jochen Brock\ Graham Logan^ Roger Summons^ and Roger Buick^ 'Harvard University Boston; ^Geoscience Australia, Canberra; ^MIT Boston; ^Washington University Seattle.

Molecular fossils preserved in Archean and Proterozoic rocks can provide insight into ancient biological communities (Brocks et aL, 1999). The presence of hopanes in rocks as old as 2.7 billion years indicates that bacteria were an important component of the biota. Abundant 2 methylhopane show^s that cyanobacteria were significant primary producers and that oxygenic photosythesis had evolved (Summons et al, 1999).

3 methyl-hopanes, probably derived from microaerophilic heterotrophic bacteria (methanotrophs and/or methylotrophs), were also a significant component of the biota. Importantly, the identification of a wide range of steranes indicates that eukaroyotes had evolved by this stage and their biosynthesis sets a lower limit for dissolved oxygen in the upper water column of at least PAL. By 1.7 billion years ago there is molecular fossil evidence for sulfide oxidising bacteria around sulfide ore deposits, such as McArthur River (Logan et al, 2001). Microbial mats formed in various Proterozoic environments provide further evidence for bacterial life and molecular fossils preserved in these environments aid in palaeo-environmental reconstruction. Combining organic geochemical techniques with stable isotope analysis generates an even better understanding of the sources of organic matter and the biochemical processes that were significant within the water column and sediments (Logan et al, 1999).

References Brocks J.J., Logan G.A., Buick R. and Summons R.E. (1999). Archean molecular fossils and the early rise of eukaryotes. Science 285, 1033-1036.

Logan G.A., Calver C.R., Gorjan P., Summons R.E., Hayes J.M. and Walter M.R. (1999). Terminal Proterozoic mid-shelf benthic microbial mats in the Centralian Superbasin and their environmental significance. Geochim. Cosmochim. Acta 63, 1345-1358.

Logan G.A., Hinman M.C., Walter M.R. and Summons (2001). Biogeochemistry of the 1640 Ma McArthur River (HYC) lead-zinc ore and host sediments, Northern Territory, Australia. Geochim.Cosmochim.

Acta 65, 2317-2336.

Summons R.E., Jahnke L.L., Hope J.M. and Logan G.A. (1999). 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400, 544-557.

36

SIDEROPHILE ELEMENTS IN LUNAR IMPACT MELTS: IMPLICATIONS FOR THE BOMBARDMENT HISTORY OF THE EARLY EARTH Marc Norman' and Vickie Bennett' ' Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Siderophile element compositions of lunar impact melt breccias provide a primary record of the cratering history of the early Solar System which is unavailable from terrestrial rocks. To improve our understanding of the impact histories of the Earth and Moon, we measured the abundances of highly siderophile elements (Re, Ir, Pt, Pd, Ru) in a suite of impact melt breccias collected from the rim of the 3.89 Ga Serenitatis basin by the Apollo 17 expedition. In contrast to the wet and geologically active Earth, ancient impact basins on the dry Moon have not been reworked by weathering or subsequent tectonic events, but preserve a detailed record of major geological events in the early Solar System. Lunar impact breccias are, therefore, keystones for understanding such issues as the role of large meteorite impacts in the geologic evolution of terrestrial planets, and the environment in which biology and crustal structure originated on the Earth.

Impactor Discrimination Highly siderophile elements are sensitive tracers of meteoritic contamination in impact melt rocks.

Cl-normalized HSE compositions of the Serenitatis impact melt breccias form W-shaped patterns that are enriched in Re, Ru and Pd relative to Ir and Pt, with absolute abundances ranging from -0.5 to 4% of the CI reference values. The high Pd/Pt and Re/Ir ratios of these breccias closely match the characteristics of EH-type enstatite chondrites, providing compelling evidence for the type of impactor responsible for creating the Serenitatis basin. As EH chondrites are thought to have formed in the inner Solar System, this precludes involvement of a comet or other outer Solar System body in this basin-forming impact event.

Was there a 3.9 Ga Cataclysmic Bombardment?

On a heavily cratered body such as the Moon, cumulative meteoritic infall might lead to significant siderophile element contamination in the crust prior to formation of the late basins. The fact that projectile signatures can still be recognised in the lunar breccias shows either that the pre-impact crust was relatively free of meteoritic siderophiles, or that the impact which created these melt breccias was significantly larger than the aggregate meteorite flux prior to the Serenitatis basinforming event. This argues against formation of the nearside lunar basins during a continuously declining accretionary bombardment as this would tend to obscure impactor signatures. Formation of the large lunar basins during a short-lived spike in the cratering flux at 3.9-4.0 Ga appears to be more consistent with the siderophile element compositions and age distributions of lunar impact melt breccias.

Implications for the Early Earth The large nearside lunar basins are coincident in age with the oldest terrestrial rocks, and are therefore relevant for considering the role of impacts in shaping the evolution of terrestrial continents and early life environments. Owing to its larger size and greater gravitational focusing, the Earth would have experienced a 20x greater cratering rate and a -lOOx larger mass accretion rate compared to the Moon. This implies that the Earth must have been hit by several large impacts during the crucial period in which the oldest preserved continental crust was forming and early life was evolving. If enstatite chondrites are found to be an important population contributing to the 3.9-4.0 Ga cratering cataclysm, their fractionated siderophile element pattern may have contributed to mantle heterogeneity on Earth. The dry, reduced nature of enstatite chondrites would, however, preclude a significant contribution of these planetesimals to the volatile budget of the Earth and oxidation of the terrestrial mantle.

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SULFUR CYCLING DURING THE ARCHEAN B. Runnegar Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567 Email: runnegar@ucla.edu.

The recent discovery by Farquhar and others of mass-independent fractionation of sulfur isotopes in Archean sedimentary sulfides and sulfates has implicated atmospheric (gas-phase) processes in sulfur cycling prior to the rise of oxygen about 2.2 Ga ago. However, many aspects of this discovery remain to be explored including the isotopic properties of sulfur in a range of Archean sedimentary and hydrothermal environments, the effects of photochemical and other atmospheric reactions on sulfur isotope fractionation, and whether or not the biosphere was involved in any detectable way in sulfur cycling.

We have measured sulfur isotope (^^S, ^^S, ^^S) compositions of (1) sedimentary and vein barites; (2) pyrite intimately associated with barite; and (3) pyrite disseminated in black cherts, all from the 3.5 Ga-old Warrawoona Group, North Pole area, Western Australia, using a multi-collector ion microprobe as well as more conventional methods . Microscopic pyrites from growth laminae within vein barite have the same mass-independent isotopic depletion as the barite (delta'^^S = -l%o) but are about 15%o lighter in delta^'^S compared to sulfates. This suggests in situ secondary mass-fractionation by hydrothermal processes. Disseminated pyrites in black cherts, on the other hand, commonly exhibit large positive mass-independent effects (delta^^S ~ +4%o) and very little depletion in ^^S. These results support the suggestion that the oxidized and reduced sulfur cycles were decoupled during the Archean.

They also argue against the notion that disseminated pyrite in Archean black shales resulted from the bacterial reduction of seawater sulfate. Instead, the pyrite seems to have been derived from the reduction of elemental sulfur formed, at least in part, by photochemical processes in the anoxic Archean atmosphere.

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THE ROLE OF HYDROTHERMAL SYSTEMS IN THE FLOURISHING OF EARLY LIFE ON EARTH: EVIDENCE FROM THE 3.49-3.43 GA WARRAWOONA GROUP, NORTH POLE DOME, PILBARA CRATON Martin J. Van Kranendonk Geological Survey of Western Australia, 100 Plain St., E. Perth, WA 6004 Australia e-mail: martin.vankranendonk@mpr.wa.gov.au

Many studies contend that early life on Earth probably developed at, or at least flourished near, hydrothermal vents (e.g. Russell & Hall 1997^ However, until recently, geological evidence in support of this has been lacking. Rather, existing models of the setting for Earth's oldest fossils from the North Pole Dome area of the Pilbara Craton, Western Australia, suggest that host rock cherts were deposited originally as evaporative precipitates of carbonate and sulphate in a shallow marine, or sabkha-like environment and silicified by later, unrelated events (e.g. Groves et al.

1981). An unresolved conundrum with these models is the prevalence of oxygen-rich components (sulphates, carbonates, and hematite) in cherts that were deposited in an Archaean atmosphere/hydrosphere widely regarded as being anoxic.

Results of recent mapping of the Warrawoona Group in and around the North Pole Dome has shown that fossil stromatolites occur in two distinct stratigraphic horizons, including chert-baritecarbonate beds of the c. 3.49 Ga Dresser Formation, and chert-carbonate laminites of the c. 3.43 Ga Strelley Pool Chert (Van Kranendonk, 2000). Previously described microfossils occur in chert at an intermediate level in the stratigraphy, at Chinaman Creek. All of these, and bedded cherts generally, are fed by individual swarms of weakly radiating chert veins, up to 750 m long by 3 m wide, that cut up through footwall volcanics to the bedded chert, but not above it. The veins contain ubiquitous phreatomagmatic breccia textures indicative of fluid boiling and caused wide zones of propylitic and argillic alteration in adjacent host rocks. These features indicate that the chert veins formed from hydrothermal fluids erupted from white smoker epithermal systems. Sulphate, carbonate, and Fe-oxides components of bedded cherts also occur in the hydrothermal chert veins.

Sites of hydrothermal venting have been recognized in both stromatolitic horizons, with stromatolites developed at, and downflow from, the vents, changing size and morphology away from the vents in a similar way to modem stromatolites at Yellowstone hotsprings where component microbial communities vary in response to changing water temperature (Farmer, 2000).

The Chinaman Creek microfossil sample was found to come from within a cross cutting hydrothermal breccia vein, 25 m below bedded chert, rather than in a silicified, bedded sandstone as previously stated. These observations provide the first geological evidence for a direct link between hydrothermal vents and the flourishing of early life on Earth.

Chert vein swarms were emplaced into, and above, sets of listric, normal growth faults developed over syn-volcanic laccoliths. Chert deposition occurred in restricted, shallow basins, as brine pools, probably in volcanic calderas, or nested caldera complexes. Hydrothermal circulation leached Si, Ba, and Fe from host volcanics and combined with S, C, and O from degassing magma chambers that were transported up the hydrothermal veins, providing chemical energy to early microbial communities, although photosynthesis may also have occurred.

This abstract published with permission of the director. Geological Survey of Western Australia References Farmer J. D. 2000. Hydrothermal systems: Doorways to Early Biosphere Evolution. GSA Today 10 (7), 1-9.

Groves D.I., Dunlop J.S.R., & Buick R. 1981. An early habitat of life. Scientific American 245, 64-73.

Russell, M.J. & Hall, A.J. 1997. The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front. Journal of the Geological Society, London 54, 377-402.

Van Kranendonk M.J. 2000. Geology of the North Shaw 1:100 000 sheet. Western Australia Geological Survey, 1:100 000 series explanatory notes, 86p.

39

PALAEOBIOLOGY OF A 1640 MA HYDROTHERMAL SYSTEM: THE MCARTHUR RIVER Pb/Zn DEPOSIT 'Malcolm Walter, 'Junhong Chen, 'Stuart Graham, Villiam Griffin, ^Graham Logan and ^Roger E. Summons 'Australian Centre for Astrobiology and GEMOC Key Centre Macquarie University ^Geoscience Australia ^Massachusetts Institute of Technology The 1640 Ma McArthur River lead-zinc-silver deposit of northern Australia formed in a hydrothermal system. It has been intersected by more than 100 drill holes, and is exposed in an underground mine. It represents a superb opportunity to examine this class of ore deposit, and has been intensively studied.

Our w^ork and most previous studies support the interpretation that the ore formed 10-20 m below the sediment-water interface in a marine environment below wave-base. It is possible to reconstruct the original fluid flow patterns and to determine thermal gradients, using relative abundances of Cu, Pb and Zn. Absolute temperatures are likely to range from ambient seafloor temperatures up to 250^C or more.

Polycyclic aromatic hydrocarbons (PAHs) in ore and mudstone within the ore deposit show compound distribution patterns similar to those of hydrothermally-generated petroleum in the Guaymas Basin, Gulf of California, and different from those found in conventional oil. PAH abundances and isomer distributions confirm the interpreted temperature gradient between the source of mineralising fluids and the sediments fringing the ore system. In-situ LAM-ICPMS analyses of Cu and Fe isotopes in chalcopyrite and pyrite also show systematic variations along the flow path of the mineralising fluid, becoming isotopically heavier with distance and declining T. This metal isotope pattern is likely to record both biotic and chemical effects but more work is required before these results can be fully interpreted.

Microfossils occur in chert that formed at the sediment-water interface, and in the ore and interbedded sediments. Previous studies have proposed that there are two populations in the chert: allochthonous cyanobacteria and possible microalgae, and benthic non-photosynthetic bacteria.

The ore and interbedded sediments contain filamentous microfossils up to 60 |Lim wide, and spheroidal microfossils 7-14 |Lim wide with a single aperture.

We have analysed hydrocarbon biomarkers from the ore and associated sediments sampled on a centimetre to millimetre scale, and have also analysed the composition of selected hydrocarbons. The interpretation of some of the microfossils as cyanobacteria is supported by the presence of characteristic 2 methyl-hopanes. C30 24-n-propyl steranes occur in all samples, and their parent sterols are known only from marine Chrysophyte algae. All samples contain a C25 isoprenoid derived from Archaea. An unusual biomarker distribution found in some samples is similar to that of bitumen described from Neoproterozoic sediments in a previous study that combined sedimentological, micropalaeontological, biomarker, and and S^'^S isotopic analyses to suggest that the hydrocarbon signal was derived from mats that contained sulfideoxidising bacteria. 5

S analyses indicate bacterial sulfate reduction.

This deposit is a rich source of palaeobiological information, which in turn constrains interpretations of ore genesis.

40

EXAMINATION OF ALTERNATIVE MECHANISMS (NONBIOLOGICAL) TO EXPLAIN THE DEVELOPMENT OF BULBOUSCONICAL FORMS, WITH EXAMPLES FROM THE IRREGULLY FORMATION, BANGEMALL BASIN, WA. C.N. Winsor Department of Geology and Geophysics, Adelaide Unversity

Mesoscale bulbous and conical forms are a common feature in many Precambrian carbonate sequences. They are often identified as stromatolites or originating during diagensis (Walter 1972), and if organic they are described as having characteristics closely linked to present-day organic forms. Whilst the ancient life form of many stromatolites and the benefits of correlation between areas cannot be disputed, a number of other possible processes can give rise to bulbous-conical structures in carbonates.

Although specialists are usually av^are of subtle distinguishing features, geologists in general or new professionals may not be able to easily identify these differences or know criteria that enable a post depositional event history to be constructed. The following postdepositional processes are described which may give rise to small scale bulbous or conical forms: (1) Silicification, chert nodules, (2) Clastic dykes, pre-, syn- and posttectonic, (3) Fluidized breccia, pre-, syn- or post-tectonic (4) Diapirism - mud volcanoes (5) Tepees - diagenetic, (6) Air - water escape structures, (7) Deformation of fluids plastic flow. (8) Noncylindrical folding.

Whether or not any ancient bulbous form is biogenic, it is quite evident that postdepositional processes such as folding, dissolution, overburden pressure, fracturing and silicification, can significantly modify any form, so that it may not be easily recognised. Any modification that has occurred will make identification and correlation difficult and necessitate caution in using morphology to interpret ancient life on earth.

Examples here illustrate bulbous-conical mesoscale structures in the weakly deformed Irregully Formation, which lies at the base of the Proterozoic Bangemall Basin (Winsor 1987). Some conical bulbous forms in the Formation could well be stromatolites and represent an ancient shallow marine environment. However they have been deformed to various degrees, so that their present morphology does not necessarily represent that of their original development. Other forms represent evidence of noncylindrical (folding about a cone), F1 and F2 folding. Silicification is seen as influencing the occurrence of some forms, as is the mixed clastic - carbonate nature of the rock unit. A complex history of dissolution along subvertical and subhorizontal stylolite surfaces has occurred.

The first identified surface of dissolution contains peaks that change direction over at least some bulbous forms, consistent with the associated change in bedding orientation, indicating that a biogenic origin is questionable for at least some of the forms.

Acknowledgements The assistance provided by Brian Logan formerly of the University of Western Australia during the initial stages of this research is acknowledge, as is the advise, criticism and encouragement that Dave McKirdy and Vic Gostin of Adelaide University and Wolfgang Preiss of PIRSA gave in preparation of this abstract.

References Walter, M.R., 1972. Stromatolites and the biostratigraphy and the biostratigraphy of the Australian Precambrian and Cambrian.

Palaeontological Assoc. London, Spec. Pap. in Palaeontology no. 11 Winsor, C.N. 1987. Using veins to establish a cover fold history - Irregully Formation, Western Australia. J. struct. Geol. 9, 429-440.

41

TECTONIC EVOLUTION OF THE KALGOORLIE TERRANE: GOLD MINERALISATION IN A LATE ARCHAEAN OROGEN M.E. Barley^ S.J.A. Brown\ L.A. Bucci\ B. Krapez\ A.A. Ross^ J. Hand^ R.A.F. Cas^ 1 CGM, The University of Western Australia, Crawley 6009, Western Australia 2 Normandy Exploration Ltd., 10 Richardson St East Perth 3 Department of Earth Sciences, Monash University Clayton 3168, Victoria

The Kalgoorlie Terrane in the Eastern Goldfields Province of Western Australia is one of the most intensely mineralised late Archaean (2.8 to 2.6 Ga) granite-greenstone terranes. It contains several world-class lode gold deposits that although dated at 2640 to 2630 Ma are similar in style to Phanerozoic gold deposits that form in convergent-margin orogens. Phanerozoic orogenic gold deposits are derived from metamorphic (± magmatic) fluids generated either by plate subduction below^ or terrane collision onto continental crust. They represent focused fluid-flovs^ that is an inherent consequence of terrane accretion and orogenesis. Although Barley et al. (1989) considered that gold mineralisation in the Kalgoorlie Terrane was related to convergent-margin orogenic processes, most subsequent tectonic syntheses indicate that gold mineralisation followed a 50 million year period of magmatism, metamorphism and deformation with little evidence for orogeny at the time of mineralisation. However, combination of the results of a recent tectonostratigraphic study and new dating from gold deposits links basin evolution in the Kalgoorlie Terrane to magmatism, orogeny and gold mineralisation. Rather than spanning 50 million years, there is mounting evidence that the main period of regional deformation and metamorphism (orogeny) followed terrane accretion, with peak metamorphism at <2650 and >2630 Ma linked directly to gold mineralisation.

The most likely post-Kambalda Group (<2700 Ma) tectonic history of the Kalgoorlie Terrane involves multiple submarine extensional basins (the Spargoville and Kalgoorlie Sequences) that were probably sited behind a strike-slip (i.e. oblique) convergent plate margin, and that were coeval with the emplacement of high-Ca granitoids in an adjacent magmatic belt or arc. The extensional basins were actively subsiding until at least -2660 Ma. At or after that time, the Kalgoorlie Terrane was amalgamated (collided) with the Gindalbie and Kumalpi Terranes by strike-slip tectonics. The orogenic phase of terrane evolution, followed with deposition of the Merougil and Kurrawang Sequences, as a distant collision resulted in exhumation of basement terranes. Continued compressive deformation resulted in crustal thickening in the Kalgoorlie Terrane with peak metamorphic conditions reached by -2645 Ma at mid crustal levels followed by melting of high-Ca granitoids in the lower crust to produce the syn- to late tectonic low-Ca monzogranites from -2640 to 2630 Ma. Throughout the orogeny, mantle-derived calc-alkalic lamprophyres and syenites, and related TTD dacite to rhyolite porphyries, were emplaced adjacent to major strike-slip shear zones.

As in modem terranes, gold mineralisation was associated with episodes of fluid flux and magmatism that accompanied terrane accretion, peak metamorphism and exhumation of the orogen in response to a thermal anomaly that resulted from a combination of crustal thickening and mantle derived magmatism.

The closest modem analogues of the Yilgam Craton (which at the scale of modem tectonic environments is comparable to the island of Bomeo) are within SE Asia where, since the Permian, cmst from marine basins has been juxtaposed with adjacent arcs and granite belts along thmsts and strike-slip faults to form orogenic belts. The overriding control on structural events has been the northwards movement of fragments of Gondwanaland, most of which are now sited within China and Indochina but include the Indian Sub-Continent.

References Barley M.E., Eisenlohr B.N., Groves D.I., Perring C.S. & Veamcombe J.R., 1989, Late Archaean convergent margin tectonics and gold mineralisation - a new look at the Norseman Wiluna Belt, Westem Australia.

Geology, 17, 826-829.

42

CONSTRAINTS ON EARLY EARTH PROCESSES FROM GEOCHEMICAL INVESTIGATIONS OF THE OLDEST (>3800 Ma) ABYSSAL PERIDOTITES V. C. Bennett^ A.P. Nutman', and C.R.L. Friend^ ' Research School of Earth Sciences, The Australian National University, Canberra ACT 0200 Australia Dept. of Geology, Oxford Brookes University, Oxford Brookes University, Oxford, UK

The oldest terrestrial mantle Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to -500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite-harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of c. 3650 Ma, but with an age of c. 3800 Ma most likely (Friend et al, 2002). The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at c. 3800 Ma. This is interpreted to have occurred during the development of a 3790-3810 Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. These Greenland dunites and harzburgites are currently the best characterised "sample" of the early Archaean upper mantle.

Osmium isotonic compositions Initial '^'O^nO^ isotopic compositions for these geochronologically and geologically well constrained 3.8 Ga spinel peridotites have been determined to investigate the isotopic evolution of the early terrestrial mantle (Bennett et aL, in press). The new data extend direct measurement of Os isotopic compositions to much earlier periods of Earth history than previously documented, and provide the best constraints on the Os isotopic composition of the early Archean terrestrial mantle.

Combined with published initial Os isotopic compositions from late Archean and early Proterozoic samples, and from primitive meteorites, these data indicate that the mantle, or at least portions of it, have maintained chondritic ^^^Os/'^^Os compositions for >3.8 billion years. This further requires that chondritic Re/Os ratios (and by inference chondritic platinum group element ratios) were a characteristic of the very early terrestrial mantle. In contrast, non-chondritic initial compositions of some Archean komatiites demonstrate that Os isotopic heterogeneity is an ancient feature of plume materials, reflecting the development of variable Re/Os mantle sources early in Earth history most probably related to large-scale basalt storage in the mantle.

Implications for the timing of a late, highly siderophile element rich veneer The existence of 3.8 Ga mantle peridotites with chondritic ^^^Os/^^^Os compositions and with Os concentrations similar to the mean abundances measured in modem peridotites places an upper limit on the timing of the proposed late accretionary veneer. These observations require that any highly siderophile element rich component must have been added to the Earth, transported into and grossly homogenised within the upper mantle by 3.8 Ga. Thus, either large-scale mixing of impact materials occurred on very short (O-lOOmyr) timescales in the early Earth, or, the required late (post core-formation) addition of highly siderophile elements occurred much earlier in Earth history and is unrelated to the lunar terminal cataclysm dated at -3.8-3.9 Ga. References Friend, C.R.L. Bennett, V.C. Nutman, A.P., Abyssal peridotites >3,800 Ma from southern West Greenland: field relationships, petrography, geochronology, whole-rock and mineral chemistry of dunite and harzburgite inclusions in the Itsaq Gneiss Complex. Contrib Mineral Petrol (2002) 143: 71-92, Bennett, V.C., Nutman, A.P. and Esat., T.M., Constraints on Mantle Evolution from ^^^Os/^^^Os Isotopic Compositions of Archean Ultramafic rocks from southern West Greenland (3.8 Ga) and Western Australia (3.46 Ga) (in press) Geochim. Cosmochim. Acta.

43

CONDUCTIVE INCUBATION AND ITS ROLE IN THE FORMATION OF DOME-AND-KEEL STRUCTURE IN THE ARCHAEAN EAST PILBARA GRANITE-GREENSTONE TERRANE Simon Bodorkos^ Mike Sandiford^ and Martin Van Kranendonk^ 'School of Earth Sciences, University of Melbourne VIC 3010 (email: bodorkos@unimelb.eduMu) ^Geological Survey of Western Australia, 100 Plain St, East Perth WA 6004 Crustal-scale dome-and-keel structure is largely restricted to pre-3.0 Ga granite-greenstone terranes, fuelling debate regarding the role of vertical versus horizontal tectonics in the Archaean.

In this respect, the key strato-tectonic feature of dome-and-keel terranes is the structural superposition of a dense, mafic-dominated edifice up to 10-15 km thick, atop a less dense granitic crust. The large-scale gravitational instability generated is essential but not necessarily sufficient for crustal overturn, in vievs^ of the absence of dome-and-keel architecture in Phanerozoic orogenic belts w^here density inversions of similar magnitude have been generated by ophiolite obduction. It is thus likely that dome-and-keel development requires a large-scale density inversion, coupled with a very unusual viscosity regime in the low^er crust. The effective viscosity of rocks is composition-, stress- and temperature-dependent, and in turn, crustal thermal regimes depend in equal measure on the abundance {q) and vertical distribution (/i) of radiogenic heat producing elements (HPEs). The radiogenic contribution to the crustal geotherm below^ the HPE-bearing section of the crust T^^ = qhIK where k is thermal conductivity. Consequently, low^er crustal temperatures may be increased (and its effective viscosity correspondingly decreased) by burying HPE-rich Archaean granitic crust beneath a thick greenstone edifice. For example, the burial of felsic crust with q^ = 60 mWm"^ beneath a 10 km thickness of greenstone leads to a temperature increase of 150-250°C at 25 km depth. Importantly, such temperature increases take place on timescales comparable to characteristic conductive thermal response times of the lithosphere, implying an "incubation" period of the order of tens of m.y. between greenstone emplacement and subsequent doming of the underlying granite triggered by the effective viscosity decrease of the lower crust.

This scenario is particularly relevant to the East Pilbara Granite-Greenstone Terrane (EPGGT), where three cycles of ultramafic-mafic-felsic volcanism over the interval 3515-3430 Ma were followed by eruption of a thick (4-9 km) sequence of ultramafic and mafic rocks of the Euro Basalt. Greenstone thicknesses regionally were in the range 12-15 km at c. 3345 Ma; however, large-scale dome amplification in the ovoid Mount Edgar and Corunna Downs granitic complexes was not initiated until c. 3325 Ma, although the present-day crustal configuration was largely attained by c. 3300 Ma. The concentrations of HPEs in East Pilbara granites at 3300 Ma were estimated at q^ = 60-85 mWm"^ for /i ~ 6 km. Numerical models simulating sequential burial of this felsic layer beneath a total greenstone thickness of 14 km yield minimum temperature increases (averaged over the 20-50 km depth-section) of 167''C {q^ = 60 mWm'^) to 22TC (q^ = 85 mWm"^) by the time doming was initiated at c. 3325 Ma. In terms of the viscosity structure of the mid-lower crust, the impact of such lower-crustal heating may be estimated by considering the stress regimes driving buoyancy of a spherical, granite-cored dome into an overlying greenstone sequence. For material constants appropriate to granitic rocks, temperature increases of 150-250''C at 20-30 km depth result in effective viscosity decreases of 2-3 orders of magnitude, depending on the initial thermal structure.

However, if conductive incubation drives dome-and-keel development, it is important to realise that structures in the vicinity of well-developed domes will primarily reflect incubation-related "vertical reorganisation" of the crust into a more thermally stable configuration. The rocks therefore retain little memory of the "primary" crustal assembly processes responsible for the initial greenstone-over-granite strato-tectonic architecture, in stark comparison with modem-day orogenic systems that are strongly forced by plate tectonic interactions.

Acknowledgement: Richard Blewett is thanked for providing access to Geoscience Australia's high-quality, calibrated airborne radiometric dataset for the East Pilbara region.

44

THE OLDEST VOLCANICS AND SEDIMENTS FROM THE 3.7 - 3.8 GAISUA GREENSTONE BELT, GREENLAND: IMPLICATIONS FOR THE EARLIEST KNOWN PALAEOENVIRONMENTS ON EARTH R.A.F. Cas^ , S. Beresford' and P. AppeP, 'Department of Earth Sciences, Monash University, Clayton, Victoria, 3800. ^Geological Survey of Denmark and Greenland, Copenhagen, Denmark.

The 3.7-3.8Ga metavolcanic and metasedimentary rocks of the Isua Greenstone Belt (IGB) of southwest Greenland represent the oldest known supracrustal rocks on Earth. The IGB has been subjected to at least two phases of isoclinal defomiation, at least one more open phase of folding, prograde metamorphism up to amphibolite facies, and retrograde metamorphism, to greenschist facies in some areas. The 1GB is dominated by amphibolite-facies schists. Most contacts and original bedding features are faulted, transposed or at best sheared, and all Archaean rocks of the IGB are foliated to varying degrees; some have multiple foliations. Original protolith features are rarely preserved, but occur in lower strain domains up to several hundred square metres in area that grade into high strain domains in which protolith features are unrecognisable. Protolith domains represent less than 1% of the area of the IGB. Preserved protolith features include metabasalt pillow lavas, probable banded iron formation and rare metaturbidite psammites and pelites, consistent with quiet, deep-water, ambient environments.

Abundant featureless amphibolite schists were probably basaltic pillow and massive sheet lavas and intrusions, but original volumes, and stratigraphic thicknesses are unknown. The one occurrence of a possible basaltic rudite contains highly attenuated clasts of porphyritic to amygdaloidal basalt and it could have originated as a debris flow deposit. The absence of any evidence for pyroclastic basaltic rocks indicates deep water conditions (?hundreds of metres+).

Previously interpreted eutaxitically textured felsic ignimbrite is interpreted here as a mylonitised tonalite. The dominant coherent basalts represent either a sea-floor lava field or shield volcano system. There is no physical evidence for the basalts being part of oceanic crust or an island arc volcano.

BIF's suggest outer shelf depths to bathyal depths, and represent periods of volcanic quiescence, hydrothermal venting, and varying abundance of oxygen, either due to oxygen "flushes" (early Archaean photosynthesis?) or reduction pulses (was oxygen more abundant than thought?) in the Earth's early environments and oceans.

Some "conglomerates" occur, and their origins are much debated. Most "clasts" are quartzite; some BIF clasts occur and basalt clasts are extremely rare. All clasts are intraformational. Most examples of "conglomerate" appear to be products of tectonic transposition of banded quartzpelite or BIF rocks. True conglomerates would indicate the existence of fluvial or shoreline tractional environments, indicating the existence of a large island or landmass, for which there is no other evidence. The rounding of the quartz-rich clasts is probably due to pressure solution effects during deformation and metamorphism.

45

ZIRCON U-PB GEOCHRONOLOGY OF THE ZIMBABWEAN CRATON; "UNTANGLING THE STRATIGRAPHY OF GREENSTONES BELTS ^^ R.W. Nesbitt^ C.M. Fanning^ J.F. Wilson^ H.A. Jelsma^ and M.S.A. Horstwood^ ^School of Ocean and Earth Science, SOC., Empress Dock, Southampton S014 3ZH ^Research School of Earth Sciences, ANU, Canberra, ACT 0200, Australia ^Department of Geology, University of Zimbabwe, PO Box MP 167, Harare, Zimbabwe.

Greenstone belts of Zimbabwe are amongst the most studied pieces of Archaean crust and represent prime sites for understanding crustal evolution. Research was pioneered by the country's Geological Survey, culminating in the important papers by Macgregor (1947, 1951). Macgregor proposed a time subdivision for Archaean events in Zimbabwe which he named (from oldest), the Sebakwian, Bulawayan and Shamvaian Systems.

Although the Macgregor time divisions were later changed into lithostratigraphic Groups, the concept that individual formations can be correlated across individual greenstone belts separated by several hundred kilometres of granite is still a major goal for many workers on Zimbabwean geology. This view is challenged by interpretations of Archaean plate tectonics This contribution reviews geochronological work in Zimbabwe and presents SHRIMP data on felsic volcanics taken from several of the greenstone belts. A case history is presented comparing U-Pb data on bulk zircons with SHRIMP data on individual zircons from the same assemblage.

The question of whether Macgregor was correct in his view that there are correlatable systems across the greenstone belts is now within reach and will be reviewed.

References Macgregor, A.M., 1947. An Outline of the Geological History of Southern Rhodesia. Southern Rhodesia Geological Survey Bulletin, 38.

Macgregor, A.M., 1951. Some Milestones in the Precambrian of Southern Rhodesia. Proceedings of the geological Society of Africa, 54, 27-71.

46

THE GEOLOGICAL SETTING OF THE OLDEST (>3.85 Ga) POSSIBLE LIFE ENVIRONMENTS IN WEST GREENLAND Nutman. A.P.\ Bennett, V.C.^ and Friend, C.R.L.^ ^RSES, ANU, Canberra, A.C.T. ^Department of Geology, Oxford Brookes University, Oxford, U.K. The world's oldest-known sediments are impure BIFs from Akilia, Greenland (Nutman, 1990; Nutman et al., 1997a, 2000). The BIF forms layers within amphibolites, probably derived from basalts, which together form an inclusion within polyphase orthogneisses. The inclusion's age comes from a 3.84d=0.01 Ga SHRIMP U/Pb zircon date obtained on a slightly discordant tonalite sheet cutting It (Nutman et al., 1997a, 2000; Mojzsis and Harrison, 2000). Via the >3.85 Ga BIFs, this Akilia locality clearly shows evidence of a hydrosphere, 50 million years before sediments in the oldest (3.8 Ga) tectonic panel of the Isua supracrustal belt (Nutman et a/., 1997b), ~ 150 km northeast of Akilia.

The >3.85 Ga Akilia island water-lain sediments suffered upper amphibolite (-2.7 Ga) and granulite facies (3.65 Ga) metamorphisms and are strongly deformed. Therefore the evidence for life at 3.85 Ga is still controversial. These tectonothermal processes are expected to have destroyed any delicate microfossils and sedimentary structures. In the absence of fossil morphology being a viable method, evidence for life m the earliest Archaean is sought by chemical means. Negative for reduced carbon in ancient sediments is the prime chemical tool to seek biological activity. Negative material is detected in an Akilia island > 3.85 Ga BIF by in situ ion microprobe microanalysis of carbonaceous inclusions in apatite, and has been interpreted as biological in origin (Mojzsis et ai, 1996). As summarised by those authors, microorganisms precipitate phosphate to give rise to authigenic phosphate minerals. Because these minerals are nucleated by microorganisms, they commonly include organic matter. Upon metamorphism the authigenic phosphates are supposed to recrystallise to apatite and the kerogen converts into graphite. If this association of negative graphite as inclusions within apatite is exclusively biogenic, it would be a chemofossil to recognise biological activity, even in the high grade, deformed oldest >3.85 Ga sediments (Mojzsis et aL, 1996).

We will explore further the evidence of a sedimentary system with life (Mojzsis et al., 1996) as early as 3.85 Ga. This will be by fieldwork, U/Pb zircon dating, apatite petrography/geochemistry and carbon isotope chemistry on other selected localities with earliest Archaean BIF/chert on the islands and Greenland coast within 20 km of Akilia. These localities are known to Nutman from his regional mapping for the Greenland Survey, and have not yet been investigated from the perspective of age of sedimentation and signs of early life.

References Mojzsis, S.J. and Harrison, T.M. (2000), Vestiges of a beginning: Clues to the emergent biosphere recorded in the oldest-known sedimentary rocks. GSA Today, 10, 2-6.

Mojzsis, S.J., Arrhenius, G., McKeegan, K.D., Harrison, T.M., Nutman, A.P. and Friend, C.R.L. (1996), Evidence for life on Earth before 3800 million years ago. Nature, 384, 55-59.

Nutman, A.P. (1990), New old rocks from Greenland.

International Conference on Geochronology, Cosmochemistry and Isotope Geology. Geological Society ofAustralia Abstracts, 27, page 72.

Nutman, A.P., Mojzsis, S.J., and Friend, C.R.L. (1997a), Recognition of 3850 Ma water-lain sediments in West Greenland and their significance for the Early Archaean Earth. Geochimica Cosmochimica Acta, 61,2475-2484.

Nutman, A.P., Bennett, V.C., Friend, C.R.L., and Rosing, M.T. (1997b), -3710 and >3790 Ma volcanic sequences in the Isua (Greenland) supracrustal belt; structural and Nd isotope implications.

Chemical Geology, 141, 271-287.

Nutman, A.P., Friend, C.R.L., Bennett, V.C., and McGregor, V.R. (2000), The early Archaean Itsaq Gneiss Complex of southern West Greenland: The importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution. Geochimica Cosmochimica Acta, 64, 3035-3060.

47

AN ARCHAEAN ALPS IN GREENLAND: ARCHAEAN SUTURES, PERIDOTITES AND AN ANCIENT CONTINENT-CONTINENT COLLISION Nutman, A.P.^ and Friend, C.R.L.^ ' RSES, ANU, Canberra, A.C.T. ^Department of Geology, Oxford Brookes University, Oxford, U.K.

Archaean high-grade gneiss complexes, with upper amphibolite to granulite facies metamorphism at medium to high pressures (6 to 10 kbar) contain a record of mid to deep levels of Archaean crust. The high-grade gneiss complex in the GodthabsQord district, West Greenland is superbly exposed with 1.5 km of topography, in a 200 x 100 km tract between the Inland Ice and Davis Strait. This district shows juxtaposition of unrelated amphibolite-granulite facies gneiss complexes (terranes) along 1-10 m wide amphibolite facies meta-mylonites - some of which have been followed for >100 km (Friend et al., 1987; Nutman et al., 1989). Geochronological constraints, such as SHRIMP U/Pb zircon dating of syn-kinematic pegmatites intruded along mylonites indicates that assembly of these terranes with disparate early histories took place at -2.7 Ga. These discoveries make the Godthabsflord district the world's best candidate for a well-exposed, deep section through an Archaean (-2.7 Ga) continent-continent collision zone (McGregor et al., 1991).

The recognition of terranes bounded by -2.7 Ga mylonites is strong evidence supporting a plate tectonic style collisional/accretionary explanation for the 2.7 Ga tectonothermal events in the Godthabs^ord district. However this explanation would be strengthened by stronger evidence that one or more of the recognised -2.7 Ga mylonites unequivocally represents a suture. From Alpine analogies, most diagnostic would be finding that unrelated terranes of crustal rocks are separated by a tectonic panel with lenses of upper mantle, ± exhumed crustal rocks, ± relicts of high pressure metamorphism. In the Alpine setting, panels of such rocks represent vestiges of oceanic lithosphere and sediments, trapped between colliding blocks of continental crust.

In Godthabs^ord, we have discovered of <200 ni thick tectonic panel, containing late Archaean metaperidotites, lenses of metagabbro, metabasalt and metasediment. This panel is bounded on its top and bottom by folded, metamorphosed mylonites. The mylonites separate the panel from extensive blocks of gneiss structurally above and below of different ages, and which did not have a common metamorphic and structural history until the late Archaean. A deformed, but nonmylonitic pegmatite intruded along one of the bounding mylonites has yielded a SHRIMP U/Pb zircon date of 2.7 Ga. From our reconnaissance studies, we have recognised this panel over - 2 0 km. However, from interpretation of 1:100,000 Greenland Survey maps, we predict it will extend throughout the district. So far, the panel seems to form an extensive sheet no more than a couple of hundred metres thick, affected by 2 episodes of folding, between two different gneiss terranes.

Preliminary geochemical data (Mg/Si versus Al/Si) suggest the ultramafic rocks are candidates for abyssal peridotites. This panel will be the focus of fiirther planned research, including geochemistry of the peridotites and a search for relicts of early high pressure assemblages in the associated lenses of metagabbros and metasediments. This panel is the best candidate yet for the suture zone in the GodthabsQord district, making it more promising that the district contains a late Archaean collisional orogen, formed like the European Alps.

References Friend, C.R.L., Nutman, A.P. and McGregor, V.R. (1987), Late Archaean tectonics in the Faeringehavn - Tre Brodre area, BudseQorden, southem West Greenland. Journal Geological Society London, 144, 369-

316.

McGregor, V.R., Friend, C.R.L. and Nutman, A.P., (1991), The late Archaean mobile belt through Godthabsfjord, southem West Greenland: a continent-continent collision zone? Bulletin Geological Society Denmark, 39, 179-197.

Nutman, A.P., Friend, C.R.L., Baadsgaard, H. and McGregor, V.R. (1989), Evolution and assembly of Archaean gneiss terranes in the Godthab region, southem West Greenland: Structural, metamorphic and isotopic evidence. Tectonics, 8, 573-589.

48

THE DEVELOPMENT OF ARCHAEAN GRANITOID DOMES BY MAGMA TRANSFER AND EMPLACEMENT: AN EXAMPLE FROM THE SHAW GRANITOID COMPLEX OF THE PILBARA CRATON W.A. ^Mark Pawley, ^MJ. Van Kranendonk and ^W.J. Collins ^Discipline of Geology, University of Newcastle, Callaghan, NSW, 2308 ^Geological Survey of Western Australia, Mineral House, 100 Plain St, East Perth, W.A., 6004 The origin and tectonic significance of ovoid granitoid domes of the Pilbara Craton, W.A. is controversial with models ranging from core complex formation (i.e. horizontal tectonics) to diapirism or doming models (i.e. vertical tectonics). Using field observations from the northern part of the Shavs^ Granitoid Complex, this study will attempt to discriminate between these two tectonic models.

Mapping has revealed that the -3445-3410 Ma leucogranite component of the North Shaw Suite is a complex of metre-scale leucogranite sheets that have concentrated and locally coalesced at the contact between tonalitic orthogneisses and the foliated, but non-migmatised Cooiyia Creek Granodiorite.

The sheets are undeformed and strike SE-NW, whereas the overall shape of the leucogranite complex is folded about SE-NW-trending axial planes that can be traced south into the orthogneiss, but not to the north. The orthogneiss is migmatised, with leucogranitic melt derived by in situ diatexis generally forming concordant leucosome veins and local pervasive melt networks that are cut by straight leucogranite dykes. The leucosome veins are texturally similar to the leucogranite sheets and dykes, with the proportion of leucosome and dykes increasing up structural succession towards the southern contact of the leucogranite complex. Leucogranite sheets in the main complex possess a variablydeveloped solid-state foliation that also strikes SE-NW, with steep to moderate NE dips. The concordance of the associated structures and the variable (often absent) strain preserved within the leucogranite dykes and sheets, suggests that the magma was subsequently transferred by dyking (with local pervasive melt migration) and emplaced as a series of sheets at a high angle to a NE-SW shortening direction.

The SE-NW-trending folds and solid-state foliations in the leucogranite contrast with the dominantly EW-striking foliation in the Cooiyia Creek Granodiorite. This is a solid-state foliation, which is defined by flattened mafic aggregates that are sub-parallel to the northern margin of the dome, and accompanied by a shallowly east-plunging mineral lineation. The foliation is cut by a sharp-walled leucogranite dyke that also contains the foliation, indicating that foliation development was coeval with leucogranite emplacement.

It is proposed that the transfer and emplacement of leucogranite represents the vertical reorganisation of felsic material in the Shaw Granitoid Complex during a component of doming. During this event, melt emplaced within the leucogranite complex experienced NE-SW shortening, whereas the structurally higher Cooiyia Creek Granodiorite was subjected to vertical flattening. This pattern of contemporaneous compression and extension in different parts of the dome cannot be reconciled with a core complex model, particularly as no detachment zone is observed between the two stress regimes.

Instead, this geometry can only be explained by vertical doming, whereby sinking of greenstone synclines adjacent to the dome causes compression across the lower levels, while the vertical rise of the dome causes flattening of the overlying carapace.

49

CRUSTAL SCALE STRIKE SLIP FAULTS IN THE ARCHAEAN: THE RESULT OF BUOYANT SUBCONTINENTAL MANTLE ^Patrice Rev and ^Greg Houseman School of Geosciences, Division of Geology and Geophysics, Edgeworth Building F05, The University of Sydney, NSW 2006 Australia, prey@usyd.edu.au Greg Houseman: School of Earth Sciences, The University of Leeds, Leeds LS2 9JT, U.K. Following continental collision, the convergence of landmasses in modern orogens is accommodated in part by crustal scale thrust faults responsible for thickening, and in part by crustal scale strike-slip faults that guide the lateral escape of rigid blocks aw^ay from the collision zone. In the Himalayas for instance crustal scale thrust faults such as the Main Central Thrust, the Main Boundary Thrust, and the Nan Shan Thrust coexist w^ith crustal scale strike slip faults such as the Altyn Tagh, the Kunlun and the Karakorum faults. Crustal scale strike-slip faults up to 15 km in width and extending over many hundreds of kilometre also exist in many Archaean cratons, however, crustal-scale thrust equivalent of the MBT seems to be missing. One possible explanation is that transcurrent tectonic regime accommodated most of continental convergence in the Archaean. To test this hypothesis we performed a numerical modelling that illustrates the sensitivity of tectonic regime to the density structure of the continental lithosphere. The graphs show the evolution of (azz-a^x) versus (a^z-ayy) during homogeneous thickening (thick black lines) in two slightly different continental lithosphere. Both axis define four quadrants: one extensional (where az^-axx and azz-ayy>0, therefore one compressional (where a^z-cixx and azz-ayy<0, therefore and two for transcurrent tectonic regime (where Gzz-axx and G^^-Gyy are of opposite sign, therefore azz=a2). The modelling uses the thin sheet approximation approach coupled with a triaxial state of stress. The shaded area define the stable domain (differential stresses are lower that the yield strength), it is defined by the yield strength envelop defined for a strain rate of 6.10"^^ s T h e yield strength is averaged over the thickness of the lithosphere and is therefore independent from depth. Both modelled continental lithospheres are in thermal, isostatic and near mechanical equilibrium before deformation, and both are submitted to the same boundary conditions. They are similar in all (crust=35km, SCLM-82km, TMoho-560°C, density of asthenosphere 3395 kg.m"^) but one aspect: the density of the SCLM differs from 10 kg.m \ This small density difference leads to two different tectonic regimes to unfold during convergence: compressive for the lithosphere with the heavier SCLM (SCLM:3370 k g . m f i g u r e on the right), transcurrent for the lighter one (SCLM: 3370 k g . m f i g u r e on the left). In the Archaean the density of the SCLM could have been as low as 3310 kg.m""^ hence convergence in Archaean time was most likely accommodated by lateral escape tectonics rather than thrusting and thickening. This can explain the prominence of crustal scale strike-slip faults in Archaean cratons.

Oyy=Oz2 ^xx A: Compression

50

^

O^^. O,^ B: Transpression

^

O^^ O^ C: Strike-Slip

3.2 Ga EXHUMATION TECTONICS OF A DEEP-CRUSTAL GRANITE-GNEISS TERRANE ON THE SOUTHERN MARGIN OF THE BARBERTON GREENSTONE BELT, SOUTH AFRICA Kisters, AFM\ Stevens. G\ Dziggel, A^ and Armstrong, RA^ ^Dept. of Geology, University of Stellenbosch, Private Bag XI, 7602 Matieland, South Africa ^Economic Geology Research Institute, Private Bag 3, PO Wits 2050, South Africa ^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia The tectonostratigraphic evolution of the Barberton greenstone belt (RGB) and its adjoining granite-gneiss terrain is contentious: a) Uniformitarian models emphasize the role of horizontal, accretionary tectonics during w^hich the greenstone sequences were thrusted over synkinematically emplaced granitoids; b) Non-unifomitarian models stress the significance of diapirically induced vertical tectonics to account for the typical dome-and-keel configuration of the granite-greenstone terrain. Recent studies (e.g. Dziggel et al, 2001), on the evolution of metasedimentary portions of greenstone remnants occurring in the ca. 3445 Ma granitoids to the immediate south of the BGB, show that this granite-gneiss terrain represents a mid- to lower crustal section recording pressures of up to 11 kbar and temperatures close to 700° C. Moreover, the peak metamorphic assemblages have equilibrated at ca. 3230 Ma, some 200 Ma later than the intrusion of the trondhjemitic plutons, but coinciding with the main phase of accretionary tectonics recorded in the BGB. Along the southern margin of the belt, this gneissic, terrane is now juxtaposed against the lower to midgreenschist facies rocks that make up much of the interior of the BGB. This study has documented the presence of a several hundred meter wide extensional shear zone in the southern parts of the BGB along which the high-grade metamorphic granite-gneisses, and the lower portions of the greenstone belt stratigraphy (in this area the Theespruit Formation), were exhumed and juxtaposed against the low-grade metamorphic BGB. The extensional detachment is located at the contact between the greenstone units and the Stolzburg pluton. In the Theespruit Fromation, ductile shearing initiated close to the peak of metamorphism under conditions of 5 kbar and 540° C, and was progressively overprinted by ductile-brittle greenschistfacies shear zones which reflects deformation during uplift of the basement complexes. Kinematic indicators and unidirectional lineation patterns record an initial phase of subhorizontal, NE-SW directed extensional tectonics, parallel to the trend of the Barberton greenstone belt. Synkinematic granitic dykes that intrude the extensional shear zone constrain the timing of deformation to about 3230 Ma. Subhorizontal extension and crustal thinning was followed by vertical uplift of basement gneisses and fabric steepening. The circa 3230-3225 Ma old TTG plutons in the northern part of the belt most likely intrude synkinematically with the solid-state emplacement of the Stolzburg gneiss terrane. Pervasive solid-state gneissosities along the margins of these plutons testify to the fact that their final emplacement was also a solid-state process.

The timing of the NE-SW directed extensional deformation coincides with the main phase of NW-SE directed collisional tectonics at ca. 3225 Ma. We interpret this relationship to represent the extensional collapse of the orogen. Our results indicate that both current models for the evolution of the Barberton granite-greenstone terrain are oversimplified. Basement uplift and solid-state diapirism of ca 3.2 Ga TTG plutons are a response to tectonically driven high-grade deep crustal metamorphism, as well as the intrusion of a significant volume TTG magma. Thus, the typical dome-and-keel configuration of the granite-greenstone terrain is a product of lithospheric scale processes.

Reference Dziggel A., Stevens G., Poujol M., Anhaeusser C.R. & Armstrong, R.A. 2001. Metamorphism of the granitegreenstone terrane south of the Barberton greenstone belt, South Africa: an insight into the tectono-thermal evolution of the lower portions of the Onverwacht Group. Precambrian Research, in press.

51

LITHOPHILE ELEMENT AND ISOTOPE COMPOSITION OF HYDROMAGMATIC AMPHIBOLE IN HIGH-Mg ROCKS OF THE ABITIBI BELT, CANADA: EVIDENCE FOR ARCHAEAN WET ULTRAMAFIC MELTS William E. Stoned Etienne. Deloule^ and Michelle S. Stone^ ^Centre for Global Metallogeny, University of Western Australia, Crawley Western Australia 6009 ^Centre de Recherches Petrographiques et Geochimiques-Centre National de la Recherches Scientifique, Vandoeuvre les Nancy Cedex, France ^Department of Geology and Geophysics, University of Western Australia, Crawley, WA, 6009 Detailed petrographic, electron microprobe and ion probe studies of Archaean hydromagmatic amphiboles from the Abitibi greenstone belt yield new^ insights into the origin of komatiitic, ferropicritic and tholeiitic magmas. The amphiboles occur in minor to major amounts in peridotite layers and basal chill zones of thick ultramafic sills and flows. They can be grouped into two main petrographic types: (1) amphibole in the groundmass; and (2) amphibole in melt inclusions. The groundmass amphiboles are up to mm-size intercumulus, rim and poikilitic grains of titanian pargasite-hastingsite composition. The intercumulus grains host cumulus olivines (Fo83-84 in komatiitic rocks and F079 in tholeiitic rocks), which are rounded in shape, embayed and smaller in size than those outside amphibole. The amphibole-bearing melt inclusions are hosted in cumulus olivines, spherical to ovoid in shape, 5 0 - 5 0 0 jLim in size, and modally dominated by amphibole. Insitu ion probe analyses indicate the amphibole is enriched in Nb-LREE and Zr and depleted in Sr and HREE relative to primitive mantle, contains up to 3 wtVo H2O, and overall displays 8D values from - 1 4 0 % o to 5 0 % o , including values in the accepted magmatic and mantle range of - 9 0 % o to 60%o. Such large heterogeneity cannot be attributed solely to low temperature alteration. The melt inclusions have high bulk H20/alkali ratios and low K2O content.

The petrographic and geochemical features of the hydromagmatic amphiboles provide constraints on the role of water in Archaean ultramafic magmatism. The relationships with olivine (liquidus phase) indicate amphibole formation by subsolidus reaction of residual hydrous silicate melt with olivineipyroxene. Some of the hydrous melt intruded the olivine and was entrapped as melt inclusions. Bulk compositions of the melt inclusions, comparison to experimental amphibolebasalt phase equilibria, and presence of magmatic water suggest the residual melts contained at least 2-3 wt% H2O. Adjustment for olivine crystallization suggests the initial melts contained 1-2 wtVo H2O. Such high H2O contents and the magmatic 5D compositions coupled to the high bulk H20/alkali ratio and low K2O contents of the melt inclusions implicate melting in amphibolebearing peridotite. Furthermore, positive correlation of Nb-LREE enrichment and high 5D suggests a significant role for hydrous metasomatism of the peridotite. However, more data are required to fiilly understand the meaning of the Archaean water compositions. Nevertheless, most Archaean ultramafic units lack hydromagmatic minerals, making it difficult to attribute all ultramafic melts to melting in hydrous mantle. The favoured model is that some Archaean ultramafic melts were wet and some were dry, as well as Al-depleted or Al-undepleted.

52

CHEMISTRY OF LATE-ARCHAEAN KOMATHTES FROM THE MAHAKOSHAL GREENSTONE BELT, CENTRAL INDIA Reddy V. R. Talusani Division of Earth Sciences, The University of New England, Armidale NSW 2351 The late Archaean Mahakoshal greenstone belt is a - 5 0 0 km-long, ENE-trending mid-continent rift system located in central India. It is the most striking geologic feature observed within the Archaean basement rocks of central India. The belt consists of a thick volcanic-sedimentary sequence, metamorphosed to greenschist facies and is bounded to the north by Archaean gneisses and the overlying Vindhyan Supergroup (upper Proterozoic), and to the south by Archaean gneisses and the Gondw^ana Supergroup (Permian-Jurassic). The rocks of the belt have been subjected to polyphase deformation and are traversed by ENE-trending thrust faults and minor north striking, high-angle normal faults. Magmatic activity in the greenstone belt is represented by komatiites, basalts, alkaline rocks and rhyolites (Talusani, 2001) and provides an excellent opportunity to study the tectonomagmatic evolution of the late Archaean crust and mantle. Furthermore, komatiites are of critical importance in deciphering the Earth's thermal evolution.

This contribution reports a new occurrence of komatiite flow from the Mahakoshal greenstone belt about 10 km SE of Katni in Madhya Prahesh. These komatiites represent a least altered Archaean magmatic product. Katni komatiites are cumulate rocks and are composed of olivine cumulates (70-80%), clinopyroxene, orthopyroxene and minor chrome spinel. Olivine grains are close-packed and equidimensional, and range in composition from Fogs - F 0 9 5 .

The Katni komatiites are characterized by elevated MgO (>35%), Ni (>2306 ppm) and Cr (>4091 ppm) contents typical of komatiites (cf. Amdt and Nisbet, 1982). These are low-Ti (0.24 - 0.38%) komatiites, depleted in light-REE. Nb is co-depleted with LREE. Al203/Ti02 ratios vary between 5 and 10, consistent with these rocks being interpreted as Al-depleted komatiites derived from a depleted mantle source. On diagrams of MgO vs. trace elements, Ni correlates with MgO content whereas Sc, Ce, Zr, V and Yb show negative correlations suggesting that primary MgO contents were largely controlled by olivine fractionation or accumulation and are not significantly disturbed by secondary alteration processes (Amdt, 1994). Convex-up REE patterns, co-depletion of Nb with LREE, and variably negative Zr anomalies of the Al-depleted Katni komatiites reflect melt segregations in a plume that included an extremely incompatible element depleted component, with majorite garnet in the residue, at depths of greater than 400 km.

Variably negative Zr anomalies appear to be a characteristic of Al-depleted komatiites, whether LREE enriched or depleted. These anomalies were first identified by Xie et al. (1993) in Aldepleted komatiites with LREE enrichment at Boston Township in the Abitibi belt; they were interpreted in terms of deep melting with residual majorite garnet.

References Amdt N. T. and Nisbet E. G. 1982. What is a komatiite? In: Amdt N. T. Nisbet E. G. (Eds.), Komatiites.

Allen and Unwin, London, pp. 19-28.

Amdt N. T. 1994. Archaean komatiites. In Archaean Crustal Evolution (ed. K. C. Condie); Develop.

Precambrian Geol. 11, 11-44. Elsevier.

Talusani R.V.R. 2001. Felsic volcanism from the Mahakoshal Greenstone Belt near Barhi, Madhya Pradesh, India.

International Archaean Symposium, Perth, Australia, Sept. 24-28, 2001. AGSO - Geoscience Australia Record 2001/37, p. 202-204.

Xie Q., Kerrich R. and Fan J. 1993. HFSE/REE fractionations recorded in three komatiite-basalt sequences, Archaean Abitibi greenstone belt; implications for multiple plume sources and depths. Geochim.

Cosmochim. Acta 57, 4111 -4118.

53

FLUID-ROCK INTERACTION WITHIN AN ARCHAEAN TRIPLE JUNCTION: AN EXAMPLE FROM THE EAST PILBARA CRATON, WESTERN AUSTRALIA ^Thebaud. Nicolas, ^Philippot, Pascal and ^Rey, Patrice ^Laboratoire de Geosciences Marines, CNRS - Institut de Physique du Globe de Paris, T26-00/E3, case 89, 4 place Jussieu, 75252 Paris cedex 05, France ^School of Geosciences, Division of Geology and Geophysics F05 - Edgeworth David, The University of Sydney, NSW 2006 Australia In the primitive Earth, a wide range of phenomena including the formation of gold deposits at depth, the formation of near-surface metal-sulphide deposits (Volcanogenic Massive Sulphide Deposits), and the initiation of biological activity at oceanic hydrothermal vents, were related to the mobilisation of mineralised fluids through the crust and their channelling toward the surface Therefore identifying and characterising crustal-scale Archaean plumbing systems is one of the most fundamental problems in Archaean geology and biology, one with far reaching implications.

In the mining district of the Warrawoona Syncline of the East Pilbara craton (WA), we have identified a new class of Archaean plumbing system which has channelled a massive volume of mineralised hydrothermal solutions. This plumbing system, the backbone element of which is called "foliation triple junction", most likely results from a geodynamic process involving crustal-scale gravitational instabilities.

In the triple junction of the Warrawoona Syncline quartz veins, hydraulic breccias, and micabearing retrogressive assemblages, testify to massive fluid-rock interaction. Quartz veins up to ten meters thick are ubiquitous in the triple junction. These veins formed by the infilling of tensile fractures. One family is systematically oriented perpendicular to \ the direction of maximum elongation (the direction of stretching lineation). The stretching lineation is vertical in the centre of the triple junction, and becomes progressively sub-horizontal when one move westward away from the centre ( Collins & Teyssier, 1990; Collins, Kranendonk et al. 1998).

The quartz veins rotate as well. They are sub-horizontal in the centre of the triple junction and progressively rotate to become vertical where the stretching lineation is horizontal. These observations are consistent with the hypothesis that one stage of veining was formed during the sagduction of the Warrawoona greenstone belt, in response to transient increases of pore fluids pressure (hydraulic fracturing).

Fluid inclusions analysis (Microthermometry, Raman sectroscopy) are in process and will provide data that can be used to characterise the different generations of fluids that occurs during the hydrothermal history of the Warrawoona Syncline.

References Collins, W. J., M. J. V. Kranendonk, et al. (1998). "Partial convective overturn of Archaean crust in the east Pilbara Craton, Western Australia: driving mechanisms and tectonic implications." Journal of Structural Geology, 20 (9/10): 1405-1424.

Collins W., & Teyssier, C., 1990. In: Ho S.E., Glover J.E., Myers J.S., Muhling J.R., (Eds), Third International Archaean Symposium, Perth, Geol. Dept. & Univ. Extensions, Univervisty of Westem Australia, v. 21, 36-45.

54

CONSTRUCTION AND INTERNAL RE-ORGANIZATION OF EARTH'S OLDEST, THICKEST VOLCANIC PLATEAU: THE ARCHAEAN EAST PILBARA GRANITE-GREENSTONE TERRANE, PILBARA CRATON, WA Martin J. Van Kranendonk Geological Survey of Western Australia, 100 Plain St., E. Perth, WA 6004 Australia Email: martin.vankranendonk@mpr.wa.gov.au The East Pilbara Granite-Greenstone Terrane (EPGGT) of the Pilbara Craton represents one of Earth's oldest crustal nuclei with a protracted crustal history from 3.72-2.85 Ga. Results from detailed mapping, geochronology and geochemistry show^ that the EPGGT vs^as constructed through c. 280 m.y. of successive ultramafic-mafic-felsic volcanic cycles capped by hydrothermal chert beds, from 3.515 Ga to 3.235 Ga, erupted onto 3.53-3.72 Ga sialic basement. Estimates of autochthonous stratigraphic thickness are 9-18 km, which must be considered minima as basal contacts are intruded by granitoids, upper contacts are unconformities with younger groups, and the rocks have experienced flattening. The full crustal thickness may have been 55-60 km or more.

Four main cycles of volcanism are recognized: 1) 3.515-3.463 Ga Coonterunah Group and Taiga Taiga Subgroup of the Warrawoona Group.; comprises four well-constrained sub-cycles, 14-19 m.y. in duration, capped by the <8 km thick Duffer Formation and Marble Bar Chert. 2) 3.4633.426 Ga Salgash Subgroup: a single cycle that includes a long period (32 m.y.) of dominantly rhyolitic volcanism (Panorama Formation) capped by the widespread Strelley Pool Chert during a volcanic hiatus of unknown duration. 3) <3.426-3.315 Ga Kelley Subgroup; includes the thick (4-9 km), undated Euro Basalt and capped by the high-K rhyolitic Wyman Formation (3.325-3.315 Ma) and Police Creek Chert. 4) c. 3.255-3.235 Ga Sulphur Springs Group; capped by the dacitic to rhyolitic Kangaroo Caves Formation and marker chert with VHMS deposits, and coeval with the Strelley Granite.

Basaltic rocks have remarkably consistent geochemistry up-section, becoming only slightly LREE enriched in the Euro Basalt. In contrast, felsic volcanics and associated granitoids show a secular evolution in composition: Duffer Formation derived from fractional crystallization of basaltic magma and contemporaneous with widespread TTG; Panorama Formation derived from melting of basaltic eclogite; Wyman and Kangaroo Caves Formations and their contemporaneous monzogranites derived from melting of pre-existing granitoids. Interestingly, basal komatiitic rocks become more voluminous with each cycle.

The cyclical and autochthonous nature of the volcanism is used to suggest that the early crust was constructed and thickened by successive mantle plume events. In the first two stages (3.515-3.43 Ga), plumes were essentially continuous, reflecting rapid convective heat loss in the Early Archaean mantle, and resulting in rapid crustal thickening. The dominantly tholeiitic basalt eruptives in the lower stratigraphy suggests melting in plume heads in relatively shallow mantle, consistent with thin early crust. This stage closed with a long period of felsic magmatism, possibly reflecting melting of the base of a crust that had already become significantly thick (40-50 km).

Subsequent volcanism became more episodic, possibly reflecting a slowdown in mantle convection rates. The increasing volume of komatiite in younger cycles may reflect melting of plume tails beneath thicker crust.

Eruption of the thick Euro Basalt on a previously stable crustal configuration of greenstones over syn-volcanic TTG caused an inverted density profile. Thermal incubation by the greenstone blanket caused a temperature rise and partial melting in the mid-crustal TTG, which facilitated the onset of partial convective overturn of the upper and mid crust, forming the characteristic dome-and-keel structure, which was amplified by subsequent thermo-tectonic events.

This abstract published with permission of the director. Geological Survey of Western Australia

55

CHARACTER AND TIMING OF DEFORMATION IN THE NORSEMAN-WILUNA BELT, YILGARN CRATON, WA Roberto F. Weinberg and Peter van der Borgh rweinber@geol.uwa.edu.au Centre for Global Metallogeny, University of Western Australia, WA 6009, Australia Deformation of the 2.71-2.63 Ga Norseman-Wiluna Belt has been the focus of much attention in Yilgam Craton studies because it controls the distribution of large komatiite-hosted Ni deposits, and the deposition of some of the largest orogenic lode gold concentrations on earth. Four deformation phases have been described in the literature (D1-D4). D1 developed before the end of deposition of the c. 2.65 Ga sediments (e.g. Kurrawang and Merougil sedimentary sequences). This w^as a northdirected thrusting and folding event, which was followed by a period of subsidence, and possibly extension, and sedimentation. These early structures were refolded, sheared and faulted during a later deformation cycle, defining an orogen marked by D2-D4 structures. This presentation will describe new detailed structural work and interpretation of aeromagnetic data sets, to conclude that the later deformation cycle resulted from pure shear possibly as a result of head-on collision, or accretion of other terranes. This conclusion contrasts with previous interpretations which considered this cycle to be a result of sinistral transpression. During deformation, previously emplaced granite domes behaved like competent bodies controlling local deformation by impinging on, and driving lateral and vertical escape of surrounding ductile greenstone sequences. Gold deposition took place very late in the deformation cycle, during brittle overprint of ductile structures. A new timing of deformation events is derived here from a review of early assumptions behind deformation dating in the literature, combined with recent dating of sedimentary rocks. D2-D4 took place during the period between 2.65 and 2.63 Ga, later than previously thought, broadly contemporaneous with late magmatism and gold mineralization.

56

DEPOSITIONAL SETTING OF LATE ARCHAEAN GREENSTONES AND METASEDIMENTS IN THE HARRIS GREENSTONE DOMAIN, CENTRAL GAWLER CRATON, SOUTH AUSTRALIA Zang, Wen-long^ Davies, M.\ Purvis, A.^Daly, S.^ and Fanning C M \ ^Geological Survey, Department of Primary Industry and Resources, South Australia ^Pontifex and Associates Ltd., 26 Kensington Road, Rose Park, SA 5067 ^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 The Harris Greenstone Domain of the central Gaw^ler Craton contains a complex of late Archaean ultramafics, mafics and metasediments of the Kenella/Christie Gneiss. The gneiss comprises migmatite, gamet-pyroxene-amphibole paragneiss, carbonate, banded iron formation (BIF) and volcaniclastics, metamorphosed to granulite facies. Peak-metamorphism of the Sleafordian Orogeny in the region is dated at -2440 Ma (Daly et aL, 1998). Recent drilling of the komatiitebearing sequence has intersected intervals of metasediments and volcanics interbedded v^ith komatiite flow^s and pyroclastics. Their metamorphic grade has been identified as low^er to middle amphibolite facies. The komatiite has been intruded by the -2500 Ma Glenloth Granite.

These metasediments reveal an internal cyclic nature, typically consisting of a low^er breccia unit of predominantly komatiite clasts grading into an upper unit of layered metasediments which include siliciclastics, carbonate, carbonaceous rocks and rare iron formation. Occasionally a scour base can be found between these units. The siliciclastics occur as thin-bedded to laminated metasiltstone and fine-grained metasandstone. Graded bedding is common, scour structures and cross bedding are also present. Carbonate commonly occurs as recrystallised marble. There is a 5 m interval of thinly layered to laminated limestone/marble/mudstone in TARDD 88 with rhythmic layers of stylolitic carbonaceous mudstone and pale-coloured carbonate rock. Generally the sedimentary features suggest deposition below fair weather wave base, perhaps in inner to middle shelf settings. The interbedded komatiite is generally massive, but distinctive cycles are recognisable, from a basal, Mg-rich, black to dark grey flow grading to an upper, lower-Mg, paler grey flow. Chilled margins and contact metamorphism may be recognised at the contacts between flow-cycles. Light-grey Mgpoor rip-up fragments occur in some basal flow units; altered spinifex textures are sometimes present in flow-tops, where occasional carbonate may also occur.

In drill hole TARDD 92 the 5 m thick bottom hole interval consists of volcanic/volcaniclastic rocks, chert and marble, intruded by a banded rhyodacite (dated at -2500 Ma) and overlain by a thick sequence of komatiite flows. A similar banded rhyolite in TARDD 91 intrudes a foliated granite which, in turn, intrudes the komatiite. U-Pb geochronology on the intrusive rhyodacite and host epiclastics suggests that they were deposited contemporaneously.

The stratigraphy and distribution of late Archaean rocks are poorly known in the Gawler Craton.

Metavolcanics intersected in drill core are dated at -2560 Ma north of Kingoonya and -2520 Ma on south-western Eyre Peninsula. This may suggest much more widespread volcanic activity during a period of crustal movement and deposition towards the end of the late Archaean. The southern margin of the Gawler Craton was joined to Antarctica during the Archaean and Proterozoic. The central Gawler Craton region might be interpreted as a continental shelf, marginal to an older, southern Antarctica terrain. The discovery of microfossils of probably planktonic origin in volcanic sediment suggests the conditions suitable for the survival and preservation of microorganisms.

References Daly, S. J., Fanning, C. M. and Fairclough, M. C., 1998. Tectonic evolution and exploration potential of the Gawler Craton, South Australia. AGSO Journal of Australian Geology and Geophysics, 17, 145-168.

57

STRATIGRAPHY, DISTRIBUTION AND GEOCHEMISTRY OF THREE WIDESPREAD FELSIC VOLCANIC UNITS IN THE MESOPROTEROZOIC GAWLER RANGE VOLCANICS, SOUTH AUSTRALIA S.R. Allen\ C.J. Simpson^'^ and J. McPhie^ ^Centre for ore Deposit Research, University of Tasmania, Hobart, Tasmania 7001 ^Discipline of Geology, School of Environment and Life Sciences, University of Newcastle, 2308 Three widespread felsic volcanic units, the Eucarro Rhyolite, Pondanna Dacite and Moonaree Dacite, have been distinguished in the Mesoproterozoic Gawler Range Volcanics (GRV). These three units are the largest in the GRV, each in excess of 500 km^. Each unit is -300 m thick and includes a black, formerly glassy base, a granophyric columnar-jointed interior, and an amygdaloidal outer part. The units are very gently dipping and locally separated by thin (<20 m) lenses of either ignimbrite (Mt Double Ignimbrite), tuffaceous sandstone or faults. The youngest unit, the Moonaree Dacite, covers a central area w^ith a diameter greater than 80 km. The southern two units have east-west extents in the order of 180 km, but are much less extensive from south to north (5-60 km). Erosional windows through the Moonaree Dacite reveal basement suggesting that the three units are spatially distinct.

All three units are dominated by euhedral phenocrysts and are relatively crystal rich. Both the Eucarro Rhyolite and Moonaree Dacite contain granitoid and other basement clasts and are locally mingled with compositionally different magmas. Some granitoid clasts have disintegrated, liberating feldspar and quartz crystals into the surrounding host. These liberated crystals cause textural variations, but can be identified on the basis of shape (amoeboid or skeletal) and size (megacrysts). Textural and lithofacies characteristics are consistent with the interpretation that these units are lavas; the strongly elongate distribution and wide extent could indicate that vents were aligned along an extensive east-west-trending fissure system.

Stratigraphic nomenclature has been revised to better reflect the presence of the three emplacement units. The oldest unit is the Eucarro Rhyolite, which is dominated by a plagioclase-phyric rhyolite but includes a granitoid-and-megacryst-bearing facies. It has an amygdaloidal outer part, and has mingled with a quartz-rich rhyolite (Paney Rhyolite Member). These replace the formerly defined Eucarro Dacite, Nonning Rhyodacite, Yannabie Rhyodacite and Paney Rhyolite formations. The two younger units, Pondanna Dacite and Moonaree Dacite, are chemically and spatially distinct members of the crystal-rich Yardea Dacite.

58

OROGENIC EVOLUTION OF THE NORTHERN GAWLER CRATON Peter Betts and David Giles Australian Crustal Research Centre, School of Geosciences, Monash University Wellington Road, Clayton, VIC 3800

The northern Gawler Craton preserves evidence for two Palaeo- to Mesoproterozoic orogenic cycles between c. 1.74 Ga and 1.54 Ga. The first cycle, interpreted to be part of the Kimban Orogen, is preserved within numerous discontinuous outcrops of Mount Woods Inlier. This region records a complex evolution involving poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism that occurred between c. 1740 Ma and 1580 Ma. The earliest deformation involved isoclinal folding and the development of bedding-parallel gneissic foliation (Si). Di was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism dated at 1736±14 Ma (Daly et al, 1998). Open to isoclinal east-west-oriented F2 folds formed during subsequent north-south shortening. The Engenina Adamellite (1692=t25 Ma: Daly et al, 1998) was emplaced before or during the earliest stages of D2. The tectonic history thereafter involved a period of post-D2 shear zone development and localised folding, exhumation, sedimentation, and crustal shortening. This occurred before the emplacement of the Balta Granite Suite (c. 1584 Ma: Daly era/., 1998).

The evolution of the second cycle (late Kararan Orogeny) has been inferred from interpretation of aeromagnetic and gravity data from the Goober Pedy Ridge and Mabel Creek Ridge geophysical domains. The Coober Pedy Ridge underwent a complex evolution that involved the development of several thrust and nappe generations during north-south shortening. Deformation is inferred to have occurred contemporaneously with high-temperature metamorphism at c. 1565 Ma (Daly et aL, 1998). A second high temperature metamorphic event occurred at c. 1540 Ma in the Mabel Creek Ridge (Daly et aL, 1998). Deformation in the Mabel Creek Ridge is characterised by the development of east-west trending upright folds that are overprinted by north-south folds. The late Kararan Orogen is interpreted to form the southern continuation of the c. 1.60-1.50 Ga Isan Orogen. Post-orogenic apparent sinistral strike-slip fault activity (Karari Fault Zone) has dismembered the structures developed during the late Kararan Orogeny. This activity is inferred to have occurred during the c. 1.2 Ga Musgravian Orogeny, and records the collision between the South and North Australian cratons.

Reference Daly, S.J., Fanning, C.M., Fairclough, M.C., 1998. Tectonic evolution and exploration potential of the Gawler Craton. Australian Geological Survey Organisation Journal of Geology and Geophysics 17, 145-168.

59

IRONSTONE DEPOSIT STYLES IN SOUTH AUSTRALIA

Marc Davies, Mineral Resources Group, PIRSA Ironstone is, by definition, a rock with a minimal 15-weight percent total iron (equivalent to ~ 2025% by volume iron oxide). South Australian ironstones are dominated by magnetite, haematite and limonite-goethite, and are prominent in the Gawler Craton, the Nackara Arc region of the Adelaide Geosyncline, less so in the Cumamona Province, w^ith minor occurrences in the Musgrave Block and the Adelaide Geosyncline. Ironstone has contributed greatly to unravelling the geological history of the State thanks to its often bold expression in regions of poor outcrop and to its prominent magnetic signature. Ironstone is of major economic significance to South Australia, with massive haematite currently being mined in the Middleback Range. Significant resources of lower-grade magnetite-rich rocks remain unexploited as a potential source of iron ore.

Deposition of iron-rich rocks of chemical sedimentary origin took place during the Archaean (2500-2400 Ma), Palaeoproterozoic (2000-1850 Ma) and Neoproterozoic (850-540 Ma) with the major depositional episode occurring during the Palaeoproterozoic, within which period there may have been more than one depositional cycle. Many 100s kilometres of well-laminated Palaeoproterozoic ironstone, commonly referred to as banded iron formation (BIF) are preserved, particularly in the eastern Eyre Peninsula and the Mount Woods Inlier, less so in the Olary region.

In the Nackara Arc there are -170 kilometres of strike length preserved as the Neoproterozoic Braemar ironstone facies. Examples of prospects with potential for significant resources of lower grade magnetite-rich ironstone include the Middleback Range on Eyre Peninsula, Hawks Nest deposit in the northern Gawler Craton and Razorback Ridge in the Nackara Arc region.

There were two periods of formation of ironstone, which involve secondary enrichment by nearsurface solutions. The major episode, estimated at 1800-1650 Ma, is a secondary enrichment of host Palaeoproterozoic BIF to form residual deposits of massive haematite. Best examples are the iron ore mines in the Middleback Range, e.g. Iron Duke, Iron Duchess, Iron Knight, Iron BaronIron Prince and the Iron Monarch. Deposit models include enrichment by downward percolating groundwater and enrichment by sideways/upwards moving, hydrothermal fluids of meteoric/ magmatic origin. Small, near-surface, Cainozoic residual deposits of haematite/goethitelimonite/manganese oxide occur throughout the State. Host rocks include BIF, tectonite, carbonate and ultramafic rock. Claude Hills is a nickeliferous goethite-limonite deposit hosted by a layered ultramafic rock of the Giles Complex. The Ooldea prospect is a shear-hosted, magnetite-rich ironstone associated with the major Karari Fault and formed -1700 Ma, probably from an original iron-rich sediment.

The -1590 Ma Hiltaba Suite magmatic event is spatially characterised along the eastern margin of the Gawler Craton by the formation of large masses of haematite and/or magnetite-rich metasomatic rocks variably associated with anomalous Cu, Au, Ag and REE. Best examples include Olympic Dam, Acropolis, Emmie Bluff located in basement to the Stuart Shelf, and Manxman, Peculiar Knob and Prominent Hill in the Mount Woods Inlier. Widespread ironenrichment recorded in the Cumamona region is associated with a magmatic event of Hiltaba age.

Younger hydrothermal deposits include rare micaceous haematite, e.g. Early Cambrian, faulthosted Mount Mystery micaceous haematite deposit located in the eastern Musgrave Block. Small magmatic segregations of massive, crystalline magnetite are associated with layered ultramafic rock of the Giles Complex (-1200 Ma) in the Hinckley Range, western Musgrave Block.

Minor clastic deposits of detrital micaceous haematite occur in the basal Torrensian Aldgate Sandstone at Kersbrook in the Adelaide Hills and small deposits of colluvial haematite pebbles were shed from the massive haematite deposits in the Middleback Range.

Reference PIRSA, 2000. Iron ore in South Australia. South Australia. Department of Primary Industries and Resources.

Commodity Review, 8.

60

A FIRST-GENERATION 3D-M0DEL OF THE CRUSTAL ARCHITECTURE OF THE NORTH-EASTERN GAWLER CRATON AND IMPLICATIONS FOR OLYMPIC DAM-STYLE MINERAL SYSTEMS Nicholas G. Direen, Patrick Lyons, Elizabeth A. Jagodzinski, Peter R. Milligan and Roger G. Skirrow Geoscience Australia, GPO Box 378, Canberra, ACT, 2601

We report new and reinterpreted geological and geophysical results for the basement to the Stuart Shelf, in the north-eastern Gawler Craton. Regridding of gravity and magnetic datasets at optimal cell sizes allows resolution of basement structures with subtle geophysical expression. New processing techniques applied to these data, such as multi-scale edge detection (worming), and reassessment of available drill cores permit a reinterpretation of the stratigraphy and structure of units underlying the Pandurra Formation and Neoproterozoic cover sequences. In particular, we describe a three-fold Palaeoproterozoic basement sequence, analogous to that exposed in the southern Gawler Craton on the Eyre and Yorke Peninsulas. From west to east, we identify deformed BIF, schists, and gneisses equivalent to the Hutchison Group, orthogneisses equivalent to the Donington Granitoid Suite, and deformed and preserved metasedimentary rocks equivalent to the Wallaroo Group. Intruded into the basement are structurally-controlled, high-level plutons of the -1590 Ma Hiltaba Suite. These magmas fed extensive, flat-lying felsic sheets of the Gawler Range Volcanics (GRV), as well as more localised mafic centres equivalent to the Roopena Volcanics.

Forward modelling of potential-field data and worming reveal that basement appears to have formed in a thick-skinned, transpressive regime. Structures suggestive of duplexes, megaboudinage, positive flower structures, and thrust stacks with non-ramp-flat geometry are consistent with modelled solutions. A similar structure to (or extension of) the Kalinjala Shear Zone is inferred to lie beneath the Stuart Shelf and GRV. In contrast, the -1590 Ma volcanic-plutonic province appears to have formed in an overall extensional regime, with plutons elongated NE-SW in inferred dilational jogs within a conjugate dextral transtensional fault system. Thicker depocentres of GRV also appear to have formed in graben and half-graben nested above reactivated basement faults. To the south-west, four major sheets of GRV are inferred to rest on a basement of Archaean paragneisses. There is no geophysical requirement for a massive, sub-horizontal, mafic underplate. All geophysical anomalies can be explained with reference to realistic petrophysical properties of basement rocks found elsewhere in the Gawler Craton.

Mass-balance calculations for a deposit such as Olympic Dam show that the source-rock volume for Cu is in the order of 10^-10^ km^ Under the hypothesis that the mineral system is controlled by faulting related to 590 Ma extension, faults of about 50-100 km by 10 km are required to create a large enough strain-envelope to ensure that fluids have access to the required volume of sourcerock; and that those fluids may be mobilised and transported (Cox et al, 2001). Further, faults of this size are capable of tapping fluids from a variety of rock-types. Assuming that the regional NNW-to NW-trending transtensional structures penetrate to 10 km, their interpreted lengths are sufficient for them to have imposed a first-order control on the mineral system. The loci of mineralisation may be controlled by the second-order, NE- to ENE-trending, normal faults that connect the first-order regional structures and define the margins of the dilational jogs. The limiting factor to the size and spacing of deposits may be the quantity of metal available to the system, particularly Cu. All authors publish with the permission of the Chief Executive Officer, Geoscience Australia Cox, S.F., Knackstedt, M.A. & Braun, J., 2001. Principles of structural control on permeability and fluid flow in hydrothermal systems. Society of Economic Geologists, Reviews 14, 1-24

61

THREE-DIMENSIONAL STRUCTURAL AND GEOPHYSICAL BASEMENT ARCHITECTURE OF THE SOUTHERN MARGIN OF THE GAWLER CRATON, SOUTH AUSTRALIA M.C. Fairclough^ and A. P. Belperio^ ^Geological Survey Branch, PIRSA, Floor, 101 Grenfell Street, Adelaide, SA 5001 ^ Minotaur Resources Ltd, Boskenna Avenue, Norwood, SA 5067.

Ongoing research over the last decade within the Geological Survey of South Australia has resulted in improved know^ledge of the tectonic evolution of the Gavs^ler Craton. Recent completion of three contiguous geological maps covering the southern Gaw^ler Craton, in conjunction with initiation of investigations focusing on structural and geophysical modelling, is lending new insights into the nature of the margins of the craton. Archaean to Mesoproterozoic cystalline basement of the Gawler Craton is concealed by younger cover sequences to the northwest (Officer Basin), southwest (Eucla Basin) and east (Stuart Shelf), obscuring the relationships with adjacent geological provinces and creating significant ambiguities as to their geometry and location.

Completion of the KINGSCOTE (Belperio et aL, in press), LINCOLN (Schwarz, in press) and MAITLAND (Zang, in press) 1:250 000 map sheets covering the southern Gawler Craton has provided opportunities for integration of the resultant geological data into new tectonic syntheses and provided a platform for new research. Innovative approaches currently being initiated include systematic generation of depth-to-basement digital data via use of drillhole data, automated potential-field techniques and outcrop information, within Arclnfo. Additional investigations include basement interpretations with a strong emphasis on temporal and spatial structural information, as well as geophysical modelling of structures through the use of NODDY software.

In contrast to earlier investigations, the present emphasis is on 3D and 4D studies, and a large proportion of data integration and modelling is to be done utilising gOcad modelling software, with thought given to output formats suitable for current commercially available GIS software packages.

A preliminary depth to basement coverage for the area has been compiled from various sources, along with aeromagnetics incorporating recently released offshore data compiled by Geoscience Australia (reprocessed by PIRSA) are presented. First pass tectonic and structural interpretations show a complex structural history, in part reflecting the nature of cratonic margins. For example, structures initiated during Palaeproterozoic orogenesis can be traced into Palaeozoic sediments within Investigator Strait, define Palaeozoic basins and are also reflected in Tertiary faults scarps, as shown by depth to basement data. Such complex structural histories within individual structures require considerable thought on methods of modelling and displaying such features in GIS datasets.

Careful consideration must be given to delineating stable basement vs mobile margins, as exemplified by inferred deep basement flooring the Cambrian Kanmantoo Trough which passes cratonward into shallow downwarped basement below moderately deformed Cambrian sediments on the north coast of Kangaroo Island (Belperio and Flint, 1993). This in turn is juxtaposed against block-faulted equivalent sequences overlying "stable" basement of Yorke Peninsula and Investigator Strait.

References Belperio, A. P., and Flint, R. B., 1993. Geological note: the southeastem margin of the Gawler Craton.

Australian Journal of Earth Sciences, 40, 423-426 Belperio, A. P., Fairclough, M. C. and Randabel, J. (compilers), in press. KINGSCOTE map sheet. South Australia, Geological Survey. Geological Atlas 1:250 000 Series, sheet SI53-16.

Schwarz, M. P., in press. LINCOLN map sheet. South Australia. Geological Survey. Geological Atlas 1:250 000 Series, sheet SI53-11.

Zang, W., in press. MAITLAND map sheet. South Australia. Geological Survey. Geological Atlas 1:250 000 Series, sheet SI-5316.

62

PUTTING THE HILTABA SUITE INTO A TECTONIC CONTEXT G. Ferris^ K. Barovich^; M. Hand^ ^Minerals Resource Group, PIRSA; ^Dept. of Geology & Geophysics, Adelaide University

Widespread Mesoproterozoic A-type Hiltaba Suite magmatism across the Precambrian Gawler Craton, South Australia, has historically been attributed to an intracratonic anorogenic large-scale mantle upwelling event. Greaser (1995) proposed that the felsic component of the magmatism w^as derived by intracrustal melting only, w^hile Stewart et al (1999) argued that the Nd isotope signature is best explained by partial melting of mantle-derived material, with a range of crustal mixing (up to 30%). In contrast to well-developed models for the petrogenetic evolution of the Hiltaba Suite, the tectonic regime in which magma generation occurred is not understood. The term anorogenic has been accorded largely due to lack of recognition of contemporaneous deformation in the region, and the geochemical characteristics of the rocks. However several key features of the Hiltaba magmatic event warrant a reappraisal of its tectonic setting. Firstly, recent studies by PIRSA have shown that Hiltaba-aged granites were emplaced during the development of crustal-scale shear zones that were active during compressional deformation. Secondly, the Hiltaba equivalent Gawler Range Volcanics and associated clastic sediments represent a significant basinal accumulation that attests to important extensional deformation during the Hiltaba magmatic event.

Both the convergent and extensional deformation are consistent with bulk tectonic transport in a NE-SW direction. Thirdly, the Hiltaba Suite does not plot solely in designated within-plate anorogenic granitoid fields, which is consistent with structural evidence that magmatism occurred in an active tectonic regime.

Any proposed tectonic model for the origin of the Hiltaba Suite event must incorporate the timing of magmatism relative to other Gawler Craton events, the apparently protracted interval over which Hiltaba magmatism occurred 20 Ma), a high intensity heat source to account for the high-T nature of the felsic magmatism, and the mantle signature of the Suite. Hiltaba Suite magmatism apparently began -10-15 m.y. after intrusion of the voluminous and deformed 1620 Ma St Peters Suite, which forms a NE-convex magmatic province in the SW Gawler Craton. Limited data suggest the St Peters Suite has an arc-like geochemical signature. We suggest that it represents the younger of two NE-directed accretionary events along the SW margin of the Gawler Craton, the earliest represented by the inboard c. 1670 Ma Tunkillia Suite, for which preliminary geochemistry also suggests an arc-like signature. Thus, the Hiltaba Suite should be viewed in the context of an evolving Mesoproterozoic margin to the southwestern Gawler Craton. Mantle melting to account for the high intensity heat source could be readily achieved during lithospheric thinning caused either by delamination (e.g. Piatt and England, 1994) or slab break-off (Blanckenburg and Davies, 1995). The overall geometry, and the convergent and extensional syn-Hiltaba deformation are consistent with continued outboard growth of a NE-dipping subduction zone. Variations in subduction rate may have led to cycles of extension and compression in the thermally perturbed continental region into which the Hiltaba Suite was being emplaced. Emplacement of mantle melts high in the crust would have occurred in regions of greatest extension, providing a plausible explanation for the existence of a regional gravity high beneath the Gawler Range Volcanics and the presence of basaltic flows. The recognition that the Hiltaba Suite was emplaced into an active tectonic regime associated with significant deformation means that there may be important and identifiable structural controls on fluid flow and associated mineralisation events.

References Greaser, R.A., 1995. Neodymium isotopic constraints for the origin of Mesoproterozoic felsic magmatism, Gawler Craton, South Australia. Canadian Journal of Earth Science, 32:460-471.

Piatt, J.P. and England, P.C., 1994. Convective removal of the lithosphere beneath mountain belts: thermal and mechanical consequences. American Journal of Science, 293:307-336.

Stewart, K., Schaefer, B. and Foden, J., 1999. Proterozoic magmatism and crustal growth on the Gawler Craton, South Australia. In Barbarin, B. (Ed). The Origin of Granites and Related Rocks. Fourth Hutton Symposium, Abstracts, Clermont-Ferrand France, Sept 20-25, pi91. von Blanckenburg, F. and Davies, H.J. 1995. Slab break off: a model for syn-collisional magmatism and tectonics in the Alps. Tectonics, 14:121-131.

63

MESOPROTEROZOIC TECTONISM IN THE NORTHWEST GAWLER CRATON: ' W A r GEOCHRONOLOGY, GEOPHYSICAL INTERPRETATIONS AND EXTRAPOLATIONS Geoff Eraser, Patrick Lyons and Nicholas G. Direen Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia

The Gawler Craton represents one of the largest blocks of Archaean-Proterozoic crystalline basement in Australia but its tectonic history and mineral resource potential remain relatively unknown. We present new reconnaissance "^^Ar/^^Av geochronological results which better define the thermal history of the northwest Gawler Craton. In particular, we constrain timing of tectonic activity along major structures identified in potential field data, and contrast thermal histories of crustal blocks separated by such structures. Our results are from the Fowler Subdomain (Teasdale, 1997) and the Christie and Wilgena Subdomains (Daly et al, 1998) and their bounding-structures.

Sheared granite from the Coorabie Shear Zone (SZ), separating the Christie and Wilgena Subdomains, yields muscovite and biotite "^^Ar/^^Ar ages of -1540 Ma. Potassium feldspar from the same sample yields a gently rising age spectrum, with ages spanning -1450 Ma to 540 Ma, most likely reflecting slow cooling over this time interval. This contrasts with K-feldspar collected from the Wilgena Subdomain, southeast of the Coorabie SZ, which exhibits a flat age spectrum with an age of -1545 Ma. Farther east in the Wilgena Subdomain, at the western edge of the Harris Greenstone Belt, hornblende from a metadolerite dyke yields an age o f - 1 7 0 0 Ma. Biotite from the Mt Christie area, west of the Coorabie SZ, preserves an age of -1650 Ma. Two samples from the Tallacootra SZ, which separates the Fowler and Christie Subdomains, yield "^^Ar/^^Ar mica (muscovite and biotite) ages of -1450 Ma, and a K-feldspar age spectrum which rises monotonically from -1360 Ma to -1450 Ma. Muscovite from Lake Ifould, northwest of the main trace of the Tallacootra SZ but possibly not in the Christie Subdomain, preserves slightly older ages of -1480 Ma. Taken together these results show distinct differences in thermal and tectonic history across the northwest Gawler Craton. Both the Wilgena and Christie Subdomains preserve Paleoproterozoic or early Mesoproterozoic cooling ages, while the Fowler Subdomain is distinctly younger but also contains internal diachroneity.

Previous workers proposed that deformation in the Fowler Subdomain was a consequence of continental collision between the "Mawson Continent", including the Gawler Craton, and the Yilgam Block (Daly et aL, 1998). Speculative links have also been made to "Grenvillian" tectonic activity in the Albany-Fraser and Musgrave provinces to the west and north of the Gawler Craton (Teasdale, 1997; Foster & Ehlers, 1998). The results presented here provide no evidence for Grenvillian-age tectonism in the Fowler Subdomain, although it remains possible that the Karari Shear Zone, which cuts the Coorabie and Tallacootra SZ, could have been active in Grenvillian times. Cross-cutting structural relationships apparent in magnetic images suggest that the broadly E-W-trending Yerda SZ is cut by the Coorabie SZ, thus the Yerda structure is older than the "^^Ar/^^Ar ages o f - 1 5 4 5 Ma obtained from the Coorabie SZ. All authors publish with the permission of the CEO, Geoscience Australia.

References Foster, D. A. & Ehlers, K., 1998. ^^Ar/^^Ar thermochronology of the southem Gawler Craton, Australia: Implications for Mesoproterozoic and Neoproterozoic tectonics of East Gondwana and Rodinia. Journal of Geophysical Research, 103, B5, 10,177-10,193.

Daly, S. J., Fanning, C. M. & Fairclough, M. C., 1998. Tectonic evolution and exploration potential of the Gawler Craton, Sonih AmtxdAidi. AGSO Jourrial of Geology and Geophysics, 17, 145-148.

Teasdale, J., 1997. Methods for understanding poorly exposed terranes: The interpretive geology and tectonothermal evolution of the Western Gawler Craton. Unpubl. PhD Thesis, Univ. Adelaide

64

GEOCHRONOLOGY OF THE CHALLENGER GOLD DEPOSIT, NORTHWEST GAWLER CRATON, SOUTH AUSTRALIA A. G. Tomkins\ W.J. Dunlap^ and J. A. Mavrogenes^'^ ^ Dept. of Geology, Australian National University, Canberra, ACT, Australia 0200, ^ Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia 0200.

New geochronological studies at the pelitic granulite-hosted Challenger gold deposit in South Australia have been combined with data from an earlier study to develop a temperature-time history. The new data have been derived using several isotopic decay systems including Sm-Nd, Rb-Sr and "^^Ar/^^Ar and a number of minerals that can be directly related to metamorphic events, including garnet, biotite, perthitic K-feldspar and sericite. Dating carried out previously by Fanning {in prep) identified one period of zircon growth (-2446 Ma) and two stages of monazite growth (cores of -2440 Ma and rims o f - 1 7 6 0 Ma). While little is known about the metamorphic equilibria of these minerals, zircon is known to most commonly form in pelitic rocks during granulite facies metamorphism, particularly during partial melting (Rubatto et aL, 2001), and does not form readily from hydrothermal fluids. Monazite also forms readily during granulite facies metamorphism (Rubatto et aL, 2001). Thus, there has been some suggestion of two periods of granulite facies metamorphism at Challenger.

While Nd model ages are generally considered an ambiguous indication of protolith age, the Nd model age of-2900 Ma obtained by this study is in general agreement with the previously interpreted protolith age for the gneisses of this region. Gold mineralisation was introduced prior to granulite facies metamorphism (Tomkins & Mavrogenes, in press), probably under greenschist/amphibolite facies conditions at between 2800 and 2550 Ma. However, evidence for the exact age and P-T conditions of this event was almost completely removed by granulite facies metamorphism during the Sleafordian Orogeny. This orogeny is estimated to have peaked at -2446 Ma, generating both zircon and monazite as well as garnet and perthitic K-feldspar during vapour-absent melting. Although some biotite was destroyed by melting reactions at this time, a significant amount survived peak metamorphism. The Sm-Nd garnet age of 2394 ±8 Ma is in general agreement with the estimated age of peak metamorphism in that it could take a significant length of time for the rocks to cool from the peak temperature through the closure temperature of garnet. This temperature is thought to be in the order of 550-750°C depending on size, morphology and a number of other factors.

Rb-Sr and dating of biotite produced a spread of ages ranging from 2060 ± 10.3 Ma to 1860 ± 8.8 Ma. Dating of K-feldspar gives a similar spread of ages. Given the low closure temperature of biotite in both of these isotopic systems (-300-350°C), these results indicate that the gneiss at Challenger cooled through 350°C by 2060 Ma. The 200 million year spread of ages from several closely spaced samples suggests that a second low temperature thermal event partially reset the biotite to younger ages. This can only happen if temperatures after 2060 Ma were raised into the closure temperature window for biotite (~300-350°C) or exceeded the closure temperature of biotite for a short period: a second granulite or mid-upper amphibolite facies event would completely reset the biotite ages. Thus, the monazite rim age of-1760 Ma must relate to a relatively low temperature period of new mineral growth. Monazite has been shown previously to have formed at low temperature via fluid-related processes. Further evidence of a younger metamorphic event is provided by ^^Ar/^^Ar dating of sericite, which can only have formed by fluid-induced retrograde metamorphism, giving an average age of 1624 ± 9 Ma. The mineral assemblage produced by this event is consistent with the greenschist facies.

This study has defined the timing of two major metamorphic cycles at the Challenger deposit: a granulite facies event at -2446 Ma and a greenschist facies event between 1760 Ma and 1624 Ma. These results have implications for the tectonic evolution of this part of the Gawler Craton, which was previously thought to have experienced two high temperature metamorphic cycles.

References Rubatto, D., Williams, I.S. & Buick, I.S. 2001. Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia. Contributions to Mineralogy and Petrology, v. 140, p. 458468.

Tomkins, A.G. and Mavrogenes, J. \n Press. Mobilization of gold as a polymetallic melt during pehte anatexis at the Challenger deposit. South Australia: a metamorphosed Archean gold deposit.

Economic Geology.

65

A THREE DIMENSIONAL INVESTIGATION OF THE HILTABA GRANITOID SUITE GEOMETRY: IMPLICATIONS FOR PLUTON EMPLACEMENT MECHANISMS McLean M.A and Betts, P.G. Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton Vic, 3800, Australia.

At -1595-1585 Ma, a large thermal event within the Gawler Craton resulted in the extrusion of the Gawler Range Volcanics, and widespread emplacement of the anorogenic Hiltaba Suite granitoids.

In the central - western Gawler Craton, major Archaean-aged east-west trending shear zones appear to be spatially and structurally associated with the Mesoproterozoic Hiltaba Suite. It is hypothesised that these structures were important conduits during pluton emplacement.

The geometry of a pluton can provide inferences about the emplacement mechanism. However, determining the three-dimensional geometry of a pluton is complex because the subsurface distribution of plutons is difficult to constrain. Modelling of aeromagnetic and gravity data enables assessment of the subsurface geometries of these plutons allowing inferences on the emplacement mechanism.

Two and three quarter dimensional modelling of gravity and aeromagnetic data reveals that shallow level Hiltaba Suite granitoids typically display a flat sill-like geometry. These granites are approximately 25 km in diameter and are less than 6 km deep. We propose that the Hiltaba Suite granitoids were emplaced along the major east - west trending shear zones and then propagated out laterally to be shallowly emplaced in the form of a thick sill. This emplacement mechanism is typical of a laccolith intrusion (Corry, 1988).

Reference Corry, C.E., 1988. Laccoliths; Mechanics of emplacement and Growth. Geological society of America Special Paper 220, 1-110.

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VOLUMINOUS FELSIC VOLCANISM IN THE MESOPROTEROZOIC GAWLER RANGE VOLCANICS, SOUTH AUSTRALIA J. McPhie, S.R. Allen and C. Simpson Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001

The Gawler Range Volcanics (GRV), South Australia, exemplify the scale and style of volcanism characteristic of intraplate, intracontinental volcanic provinces in being voluminous and dominated by lavas. The volcanic succession is widespread, covering some 25000 km^, and thick, locally more than 1 km. The magma volume represented by the volcanic units exceeded 4000 km^ More than half of that volume was erupted effusively, producing widespread lavas. However, the GRV are remarkable in that some 95% of the magma erupted was rhyolitic or dacitic. In this respect, the GRV differ markedly from intraplate flood basalt provinces but resemble some Proterozoic (e.g.

Mid-Continent Rift, Minnesota, USA) and Tertiary (e.g. Trans-Pecos, Texas and Snake River Plain, Idaho) intraplate volcanic successions.

The GRV consist of two parts that contrast in composition and in the kinds of volcanic units present. The lower GRV amount to -1000 m in thickness and are gently dipping. This older part appears to have been produced by perhaps four widely separated volcanic centres (Kokatha, Glyde Hill, Menninnie Dam, Tarcoola; Blissett et al. 1993), each characterised by diverse volcanic facies and complex facies geometry. All, however, were subaerial and all were mainly felsic although minor basaltic and basaltic andesite units are present. They comprise lavas, welded ignimbrites, accretionary lapilli tuffs and minor volcanogenic sedimentary units. At Kokatha, parts of the succession are particularly distinctive in consisting of interbedded andesitic lavas and felsic pyroclastic units (probably pyroclastic fall deposits). Textures in the lavas suggest this location was in the proximal to medial parts of the lava flow field whereas the felsic pyroclastic units are probably distal, illustrating the interleaving of products from different sources. At Menninnie Dam, an association of rhyolitic lava, welded fall deposits and breccia-filled dykes marks the position of a small volcanic centre. At Glyde Hill, proximal associations mainly comprise rhyolitic lavas that include large volumes of autobreccia and resedimented autobreccia.

The upper GRV consist of at least three widespread (>150 km), thick (200-300 m), rhyolitic and dacitic units. Each one has a volume in excess of 500 km\ They occupy separate areas and are texturally and compositionally distinct although all three are dominated by euhedral phenocrysts and lack vitroclastic textures. In addition, all three have similar internal textural variations that show each unit cooled independently, generating a simple cooling profile. Their textural and lithofacies characteristics strongly suggest that they are lavas. Identification of source areas for these widespread lavas is particularly difficult because they show only very subtle lateral textural variations. However, for two units, the strongly elongate distribution and location of compositional variants suggest that multiple vents were involved and probably aligned along a fissure.

The felsic lavas in the upper GRV were fed by compositionally heterogeneous magmas.

Heterogeneity was produced by fractionation within the main magma source, by the introduction of small volumes of different magmas that mingled but did not mix, and by assimilation of wallrock.

The last process produced local concentrations of partially melted granitoid clasts in the lavas. The lavas also contain small mafic inclusions that imply an important role for mafic magmas during the upper GRV volcanism, even though no mafic volcanic units were erupted.

Reference Blissett, A.H., Creaser R.A., Daly S.J., Flint, R.B. and Parker, A.J. 1993. Gawler Range Volcanics. In: Drexel J.F., Preiss W.V. & Parker A.J. eds. The geology of South Australia, vol. 1, The Precambrian. South Australian Geological Survey Bulletin 54, 107-124.

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ANALYSIS AND 3D VISUALISATION OF NEW COMPILATIONS OF POTENTIAL FIELD DATA TO HELP CONSTRAIN 3D GEOLOGICAL MODELS Peter R. Milligan, Nicholas G. Direen, Patrick Lyons and Matti Peljo Geoscience Australia, GPO Box 378, Canberra, ACT, 2601

The accuracy of broad-scale geological models and interpretations derived from gravity and airborne magnetic data is dependent upon long wavelength components of the data, which are in turn dependent upon the accuracy with which separate datasets are merged into single compilations. For magnetic data a newly developed method (Minty, 2000) reduces this complex procedure to a single least-squares problem involving adjustments to all grids. The method has been tested using independent datasets for several regions across Australia (Milligan et al, 2001).

For gravity data Murray (1998) developed optimal gridding. Examples of derived potential field products and enhanced images are presented here for new compilations of airborne magnetic and gravity data for the north-eastern Gawler Craton (NEGC) in South Australia. In one example, directional filtering removes anomalies of the Gairdner Dyke Swarm, considerably enhancing features in the Gawler Range Volcanics.

Application of automatic trend analysis and edge detection to merged compilations of potential field data are useful techniques for producing unbiased estimates of sharp lateral changes in physical properties of rocks (e.g. Blakely & Simpson, 1986; Phillips, 1997). A new method of edge detection (Archibald et al., 1999) for the analysis of magnetic and gravity data uses wavelet theory to identify points lying on the horizontal gradient maxima. Points calculated from these methods are aligned along the gradients, and have the appearance of continuous "worms". When the points generated for many levels of upward continuation of the original data are analysed in 3D, the worms provide information about the strength of the gradients, the localities of source-body edges, their depths, and their dip directions. For automatic analysis, it is more convenient if the worm points are converted into sets of vectors, or, further, into 3D surfaces (sheets) for each level of continuation. Examples are shown for the NEGC and for the Arunta area of the Northern Territory, where, in some cases, the orientation of discrete physical property contrasts are mapped.

A useful analytical technique is to compute the best-fitting straight line for those worm vectors that have a reasonable linearity, and to plot rose diagrams of the orientation of the vectors. The results may be displayed either by their lengths and directions or as histograms of their directions. Discrete areas can be windowed separately to show how dominant trend directions change in different geological settings. Rose diagrams calculated for many levels of upward continuation may show how fracture sets vary vertically, both with changing dip and strike directions. Visualisation of this information provides useful constraints on 3D geological models. An example is the mapping of structural geology of basement for the NEGC. All authors publish with the permission of the Chief Executive Officer, Geoscience Australia.

References Archibald, N., Gow, P. & Boschetti, F. 1999. Multiscale egde analysis of potential field data. Exploration Geophysics, 30, 38-44.

Blakely, R. and Simpson, R.W. 1986. Approximating edges of source bodies from magnetic or gravity anomalies. Geophysics, 51, 1494-1498.

Milligan, P.R., Minty, B.R.S., Luyendyk, T. & Lewis, A. 2001. Comparisons of total magnetic intensity grids, combined using GRIDMERGE, with two independent datasets. Geoscience Australia Record 2001/43.

Minty, B. 2000. Automatic merging of gridded airborne gamma-ray spectrometric surveys. Exploration Geophysics 3\,Al-5\.

Murray, A.S. 1998. High precision gridding of gravity data. Preview, 76, 107.

Phillips, J.D. 1997. Potential-field geophysical software for the PC, version 2.2: United States Geological Survey Open-File Report 97-725.

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COPPER-GOLD MINERAL SYSTEMS IN THE SOUTH-EASTERN GAWLER CRATON - ANOTHER MT ISA EASTERN SUCCESSION? Ollie Raymond ^ Ian Fletcher^ and Neal McNaughton^ ' Geoscience Australia, Canberra, ACT, 2601 ^ Centre of Excellence in Mass Spectrometry, Curtin University of Technology, Perth, WA, 6845

The Moonta Domain forms the southern part of the Olympic Cu-Au province on the eastern margin of the Gavs^ler Craton. Historical production comprises over 330,000 tonnes of Cu from vein and shear-hosted mineralisation in the Moonta-Wallaroo district. The domain basement comprises metasediments and metavolcanics of the Palaeoproterozoic Wallaroo Group (-1760—1740 Ma) which were deformed and metamorphosed to upper greenschist-amphibolite facies during the Kimban Orogeny (-1720 Ma). These rocks were further deformed and intruded by granitoids and minor mafic intrusions of the Hiltaba Suite between about 1600 Ma and 1575 Ma. There is a close spatial association of high temperature Fe-Na-Ca-K metasomatism of the Wallaroo Group and Hiltaba Suite intrusions. Conor (1995) termed the most strongly altered rocks the Oorlano Metasomatite, although metasomatic mineral assemblages within this rock association vary widely. Intense albite-actinolite-magnetite ± carbonate ± epidote ± pyrite alteration of metasediments is strongly associated with the contact zones of Hiltaba Suite granites, particularly the Tickera Granite. More distal albitisation of the Wallaroo Group is common but is not generally associated with significant sulphides. Biotite ± albite ± magnetite ± quartz ± apatite ± monazite ± tourmaline alteration is commonly associated with pyrite ± minor chalcopyrite, and is particularly widespread south of Moonta where numerous magnetic and non-magnetic Hiltaba Suite granitoids (previously grouped as Arthurton Granite) intrude the Wallaroo Group. Late chlorite and K-feldspar alteration is typically of restricted extent, but may also be associated with sulphides.

Biotite-rich alteration typically forms irregular magnetic anomalies, including a major 5 x 15 km alteration zone near Weetulta, and possibly a large area (-30 km x40 km) of strongly magnetic rock beneath Spencer Gulf. Fluid inclusion data indicate that highly saline, multi-cation fluids are associated with the alteration. Preliminary U—Pb SHRIMP dating of hydrothermal monazite from biotite-rich alteration in the Weetulta and Wallaroo areas yields ages of approximately 1585 Ma and 1620 Ma respectively. The Weetulta district data indicate a close temporal relationship of the biotite alteration and Hiltaba Suite magmatism. However, the older Wallaroo district age suggests hydrothermal activity may have commenced prior to intrusion of Hiltaba Suite granites.

Regional metamorphic and alteration characteristics of the Moonta Domain are similar to those of the Fe-Cu-Au mineral province of the Mt Isa Inlier Eastern Succession, where there are strong links between magmatism, regional albitisation, and Fe-Cu-Au mineralisation (eg., Oliver et al., 2001).

Biotite-magnetite metasomatism commonly occurs proximal to major Fe-Cu-Au ore deposits in the Mt Isa Eastern Succession. The shear-hosted Cu lodes and associated alteration at Wallaroo may be an analogue in the Moonta Domain. However, apart from some very minor drill intersections in prospects in the Weetulta district, no other significant Cu-Au mineralisation associated with biotitemagnetite alteration has yet been discovered in the Moonta Domain. Given that most of the Proterozoic basement of the Moonta Domain is concealed by up to 100 metres of Neoproterozoic to Cainozoic sediments and remains largely untested by drilling, the potential for discovery of Ernest Henry-style Fe-Cu-Au deposits in the Moonta Domain remains high.

References Conor C. H. H. 1995. Moonta-Wallaroo region - An interpretation of the geology of the Maitland and Wallaroo 1:100 000 sheet areas. Mines and Energy South Australia Open File Envelope 8886.

Oliver N. H. S., Mark G., Rubenach M. J., Pollard P. J., Williams P. J. & Marshall L. J. 2001. Intrusionrelated albitisation as a chemical precursor to ironstone-Cu-Au mineralisation in the Cloncurry district geochemical and isotopic evidence. Geological Society of Australia, Specialist Group in Economic Geology Special Publication 5, 64-84.

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A PLATE MARGIN SETTING FOR THE EVOLUTION OF THE SOUTHERN GAWLER CRATON: EVIDENCE FROM DETRITAL ZIRCON AND Sm-Nd ISOTOPIC DATA OF THE HUTCHISON GROUP M. Schwarz^ K. Barovich^ and M. Hand^ ^Minerals Resource Group, PIRSA; ^Dept. of Geology & Geophysics, Univ. of Adelaide The eastern Gawler Craton contains an arcuate belt of Palaeoproterozoic metasedimentary rocks (the Hutchison Group), which were deposited on the eastern margin of a dominantly Archaean continental domain somewhere in the interval 2000-1850 Ma. The Hutchison Group shows an important internal strato-isotopic organisation. Initial SNd values range between -9.4 to -8.4 for the basal Warrow Quartzite to between -3.8 and 2.9 for the overlying deeper water Middleback Subgroup. This suggests a dramatic change in the average source for Hutchison Group sediments, from a dominantly Archaean continental source during deposition of the Warrow Quartzite, to derivation from a younger, juvenile source during deposition of the Middleback Subgroup. Detrital zircon analyses support a mixed Archaean and Palaeoproterozoic source for the Warrow Quartzite with dominant populations at 2000, 2440, 2520 and 2720 Ma, consistent with provenance from local basement rocks in the eastern Gawler Craton.

The identification of a juvenile source for the Middleback Subgroup has important implications for the tectonic evolution of the eastern Gawler Craton, and provides a plausible framework within which to consider the generation of the voluminous c. 1850 Ma Donington Granitoid Suite (DGS). This linear to arcuate magmatic belt extends from southern Eyre Peninsula and Yorke Peninsula, north to the Olympic Dam region (a minimum distance of 500 km). Previous interpretations for the emplacement of the DGS have proposed generation within a continental rift environment (Schaefer, 1999; unpublished PhD, Uni. Adelaide). This was largely based on: (1) overall chemical and isotopic homogeneity (8Nd = -4.2 to -2.1) and (2), the suite was emplaced during a relatively restricted time frame (-10 Ma). However, tectonic discrimination diagrams for the DGS and associated mafic rocks are somewhat equivocal, and do not preclude magmatic emplacement in an active continental margin or alternatively derivation from partial melting of arc-type underplates or intrusions. Furthermore, the emplacement of the DGS was coincident with regional, medium-pressure, granulite-grade metamorphism and the development of steep structural fabrics (Zang & Fanning, 2001) We suggest that the Donnington Granitoid Suite is a collision or subduction-derived suite associated with NE-directed subduction of Late-Archaean-Palaeoproterozoic lithosphere beneath a younger continental region to the east and northeast, possibly represented by the basement to the currently outcropping Cumamona Province, or an unrecognised smaller continental fragment. Erosion of this juvenile continental domain is recorded by the isotopic composition of the upper parts of the Hutchison Group. Collision at c. 1850 Ma resulted in deformation of the Hutchison Group and parts of the DGS, producing features previously ascribed to the early stages of the Kimban Orogeny (Parker, 1993).

Convergence at around 1850 Ma was followed by a long-lived extensional and trans-tensional regime that produced regional-scale mafic dyke swarms and magmatically involved sub-basins over the interval -1850 - 1730 Ma that developed in a continental interior setting. The transcurrent regime evolved into dextral transpression during the interval -1730 and 1700 Ma, resulting in the development of the Kalinjala Shear Zone during the Kimban Orogeny between -1730 and 1700 Ma. This period of convergence was coincident with A-type magmatism associated with initiation of the Willyama basin in the Cumamona Province, a style of coupled continental interior basin developmentmagmatism that characterises the eastern Gawler Craton.

References Parker, A.J. (Compiler), 1993. Palaeoproterozoic. In: The geology of South Australia. S.A. Geol Surv. Bull 54, 51-105.

Zang, W. and Fanning, C.M., 2001. Age of the Kimban Orogeny revealed - U-Pb dating on the Corny Point Paragneiss, Yorke Peninsula. MESA Journal, 23, 28-33.

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EVALUATION OF POSSIBLE EMPLACEMENT MECHANISMS FOR THE WIDESPREAD YARDEA DACITE, SOUTH AUSTRALL\ Carol Simpson^'^, Jocelyn McPhie^ and Sharon Allen^ ^Centre for ore Deposit Research, University of Tasmania, Hobart, Tasmania 7001 ^Discipline of Geology, School of Environment and Life Sciences, University of Newcastle, 2308 The Yardea Dacite is the youngest exposed volcanic unit within the Mesoproterozoic Gawler Range Volcanics in the Gawler Craton of South Australia. It has an area of at least 13,400 km^ and an east-west extent of 190 km and has traditionally been considered as a single homogeneous unit.

It is conspicuously porphyritic, containing abundant predominantly euhedral phenocrysts of plagioclase, alkali feldspar, augite and pigeonite. The majority of the unit is red/brown in colour, however, narrow discontinuous intervals of black, originally glassy dacite occur sporadically at the base and within the outcrop area of the Yardea Dacite.

Mapping for this study has revealed the existence of two spatially and compositionally distinct emplacement units within the Yardea Dacite; the Pondanna Dacite Member in the south and the overlying Moonaree Dacite Member in the north. Each dacite has a narrow black base characterised by flow banding and a very fine-grained groundmass, but the bulk of each member is massive to columnar jointed red dacite with a variably fine-grained to spherulitic or granophyric groundmass. Gradational relationships involving mingling at a flow banding-scale between the basal black and interior red dacites has been observed in both dacites. In places where the top of the Pondanna Dacite Member is not exposed due to faulting, it is amygdaloidal. The Moonaree Dacite Member consists of two facies, a red granophyric dacite and a brown quartz-bearing dacite which occur in distinct areas but are also locally intermingled. Scattered lithic inclusions, predominantly granitic in composition and ranging from cm-scale clasts to megablocks up to 50 m diameter, have been found within the red dacite facies of the Moonaree Dacite Member. These display partial melting textures and evidence of in situ disintegration. Both members of the Yardea Dacite contain abundant feldspar and rare quartz megacrysts, although these are also more prominent within the Moonaree Dacite Member.

Chemically, the various components of the Yardea Dacite lie on the same fractionation trend but form subtly distinct groups. The Pondanna Dacite Member displays a small range in SiOi values (68.6-70.3 wt.% Si02 recalculated anhydrous), however samples from the interior of the main part of the unit are slightly more mafic than those near the base, possibly indicating eruption from a compositionally weakly zoned magma chamber. The Moonaree Dacite Member is more complex, containing at least two compositionally distinct, but in places mingled, black dacites in addition to the red facies (66.8-68.3 wt.% SiOs) and quartz-bearing facies (68.3-69.2 wt.% SiOs).

Previously, the Yardea Dacite has been interpreted as a welded ignimbrite sheet, principally because of its enormous size and the identification of possible fiamme textures within the black basal dacite. Welding textures were not found anywhere within either member of the Yardea Dacite during this study and other textural features including the even distribution of predominantly euhedral phenocrysts, flow banding at the base and partially amygdaloidal tops are more consistent with interpretation of these dacites as extensive lava flows. Further strong evidence for a lava origin is provided by the mingling between compositionally distinct batches of magma within both dacite members. The overall paucity, yet localised strong concentrations, of megablocks within the Moonaree Dacite Member and their in situ disintegration is inconsistent with a pyroclastic origin but is compatible with emplacement as a lava. The eruption of such vast volumes of magma as lavas, which are normally characterised by high viscosities, may be best explained by higher than normal eruption temperatures and their extrusion from multiple vents along extensive fissures.

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CHEMISTRY OF HIGH-TEMPERATURE FLUIDS IN FE-OXIDE CU-AU SYSTEMS OF THE STUART SHELF BASEMENT Evgeniy N Bastrakov\ Roger G Skirrow\ Terry P Memagh\ Chris G Ryan^ and Esme VanAchterbergh ^ ^Geoscience Australia, GPO Box 378, Canberra, ACT 2601 ^CSIRO Exploration an Mining, PO Box 136, North Ryde NSW 2113 Skirrow et al. (2002, this volume) identified three major types of alteration assemblages associated with Cu-Au mineral systems along the eastern margin of the Gawler Craton: (a) calcsilicate - alkali feldspar ± magnetite ± pyrite ± pyrrhotite ± chalcopyrite (CAM); (b) magnetite - biotite ± pyrite ± chalcopyrite (MB), and (c) hematite - sericite - chlorite - carbonate ± pyrite ± Cu-Fe sulphides ± U, REE minerals (HSCC). The Stuart Shelf basement is dominated by the CAM and HSCC assemblages, with extensive Cu-U-Au mineralisation generally associated with the relatively oxidised and lower temperature HSCC assemblage.

Currently, there are two competing hypotheses explaining the relationship between the CAM and HSCC assemblages: (1) mixing of high-temperature fluids with low-temperature fluids of surficial origin (sedimentary brines or meteoric waters), and (2) interaction of surficial fluids with preexisting CAM assemblages. The source of Cu and Au, and their precipitation mechanisms remain controversial, and the role of the fluids responsible for the CAM alteration is not clear.

To address these issues, we completed a pilot study of fluid inclusions associated with the CAM alteration at two prospects in the Stuart Shelf basement: quartz-magnetite-sulphide veins at Bopeechee and unmineralised high-temperature amphibole-K-feldspar-magnetite breccia at Emmie Bluff. Overall, the CAM alteration stage from both prospects is characterised by three petrographically distinct fluid inclusion types: multiphase brine inclusions (V+L+nS), liquiddominated inclusions (L+V), and vapour-dominated inclusions (V+L). The focus of the present study is the early multiphase brine inclusions (MBI).

MBI contain up to seven solid phases, including ubiquitous halite. Solid phases identified by the Raman microprobe include ferropyrosmalite ((Fe,Mn)8Si60i5(OH,Cl)io), calcite, trioctahedral K-Na mica, and chalcopyrite. Based on halite dissolution temperatures, salinity of the aqueous phase in MBI is in the order of 40 wt% NaCl equiv. Upon heating, MBI tend to decrepitate at temperatures above 400°C, prior to complete homogenisation. This behaviour is consistent with the quartzmagnetite isotopic temperatures reported for Emmie Bluff "skams" by Gow (1996) (5]0-550°C).

The metal-transport capacity of the high-temperature brines from MBI was examined using the CSIRO-GEMOC Nuclear Microprobe (FIXE). The analytical results suggest that chalcopyritebearing MBI contain up to 3-5% of Cu, and chalcopyrite-free MBI up to 0.05% Cu. Assuming that chalcopyrite crystals could have been accidentally trapped, the latter data characterise the metaltransport capacity of fluids prior to chalcopyrite saturation; comparable Cu concentrations (up to 0.1%) were reported for the fluids of the "ironstone stage" for the Starra Fe-oxide Cu-Au deposit in the Cloncurry District (Williams et al, 2001). The measured Br/Cl ratios of the Stuart Shelf MBI (0.003-0.1) lie beyond the range of typical magmatic values, allowing for the possibility of fluid origin as sedimentary basin or crystalline basement brines. Whether the MBI fluids were the only source of Cu, or were complemented by Cu introduced by later surficial fluids, is a subject of further research.

GA authors publish with the permission of the Chief Executive Officer, Geoscience Australia References Gow P. 1996. Geological evolution of the Stuart Shelf and Proterozoic iron oxide-associated mineralization: insights from regional geophysical data. Unpublished PhD thesis, Monash University.

Williams, P. J., Dong, G., Ryan, C. G, Pollard, P. J., Rotherham, J. F., Memagh, T. P., & Chapman, L. H. 2001. Geochemistry of hypersaline fluid inclusions from the Starra (Fe oxide)-Au-Cu deposit, Cloncurry District, Queensland. Economic Geology, 96\ 875-883.

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ALTERATION AND MINERALISATION SETTINGS IN THE OLYMPIC CU-AU PROVINCE, GAWLER CRATON, SOUTH AUSTRALIA Roger G. Skirrow, Evgeniy Bastrakov, and Oliver L. Raymond Geoscience Australia, Minerals Division, GPO Box 378, Canberra , A.C.T. 2601 The early Mesoproterozoic Olympic Cu-Au province extends over 500 km along the eastern margin of the Gawler Craton. Although the boundaries are not yet well defined, and much of the province is concealed beneath Neoproterozic to Cainozoic cover, the metallogenic belt is inferred to transgress several tectonic domains of mainly Palaeoproterozoic meta-sedimentary and metaigneous basement. Iron oxide - rich hydrothermal systems of the Peake and Denison Inlier, Mabel Creek Ridge, and south-central Gawler Craton may prove to be extensions of the Cu-Au province.

We have identified three major regions of early Mesoproterozoic hydrothermal and magmatic activity: in the Mount Woods Inlier in the north, in basement to the Stuart Shelf (which hosts the Olympic Dam Cu-U-Au deposit), and in the Moonta-Wallaroo-Roopena region in the south of the metallogenic province. Each of the regions contains high- to low-temperature Fe-oxide-bearing alteration, Cu-Au±U mineralisation, and felsic to mafic magmatism of the -1590 Ma Hiltaba Suite intrusions, with or without Gawler Range Volcanics. The three regions are inferred to represent the imprint of separate crustal-scale thermal anomalies.

Re-logging of drill core, and petrological studies reveal important similarities and systematic variations in hydrothermal mineral assemblages along the length of the metallogenic belt. The key assemblages are: (a) calcsilicate - alkali feldspar ± magnetite ± pyrite ± pyrrhotite ± chalcopyrite (CAM); (b) magnetite - biotite ± pyrite ± chalcopyrite (MB), and (c) hematite - sericite - chlorite carbonate ± pyrite ± Cu-Fe sulfides ± U, REE minerals (HSCC). Higher grade and more extensive Cu-Au±U mineralisation is generally associated with the relatively oxidised and lower temperature HSCC assemblage (250-300°C, or less, based on fluid inclusion data). In most cases, this assemblage overprints the CAM and MB assemblages, which represent the products of high- to moderate-temperature (-500° - 350°C) hydrothermal fluids of intermediate, or in places, reduced oxidation state (i.e. magnetite-pyrite or magnetite-pyrrhotite stability). Based on drill holes examined to date, albite and biotite are the dominant alkali alteration products in the MoontaWallaroo district, whereas K-feldspar or sericite are the main alkali silicates in basement to the Stuart Shelf. All three assemblages (CAM, MB and HSCC) including albite and K-feldspar are well represented in the Mount Woods Inlier (e.g., CAM & MB at the Manxman and Joes Dam prospects; HSCC at the Prominent Hill prospect). Sodium- and chlorine-bearing varieties of scapolite, or pseudomorphs of scapolite, have been identified in all three regions of hydrothermal activity. The sources of halides and metals in the hydrothermal fluids are currently under investigation using fluid inclusion microanalytical techniques.

The crustal levels of the hydrothermal systems as they are currently exposed at the base of cover rocks are inferred to vary dramatically, even within the three regions of hydrothermal activity.

Brittle-ductile shear-hosted CAM and MB assemblages, such as those in the Moonta-Wallaroo district, developed at deeper crustal levels than breccia-hosted HSCC assemblages (e.g., Olympic Dam). We suggest that uplift or unroofing of some of the hydrothermal systems occurred during and/or after their development, resulting in 'telescoping' of deeper and shallower alteration patterns. CAM, MB, HSCC alteration and associated Cu-Au±U mineralisation represent a possible spectrum of settings from deeper, higher-temperature environments to near-surface, lowtemperature settings where there was greater involvement of surficial fluids in the hydrothermal systems .

Acknowledgements: All authors publish with permission of the CEO, Geoscience Australia

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ESTABLISHING A FRAMEWORK FOR THE ARCHAEAN DEVELOPMENT OF THE GAWLER CRATON Ailsa Woodhouse\ Greg Swain^ Mike Schwarz% Martin Hand^ and Mark Fanning^ ^Dept. of Geology and Geophysics, Adelaide University, ^Minerals Resource Group, PIRSA ^Research School of Earth Sciences, Australian National University

Recent models for tectonic development of the Gawler Craton have emphasized a role for accretionar>^-style processes in shaping the Proterozoic evolution of the Craton. Collision at -1850 Ma IS interpreted to have built the Donington Granitoid Suite along the eastern margin of the Craton (Schwarz et ai, 2002), and convergence over the interval ~ 1700-1600 Ma is interpreted to have produced at least two magmatic arc systems represented by the 1690-1670 Ma Ifould Suite and the 1630-1610 Ma St Peters Suite (Ferris et ai, 2002). A common feature of these models is that the Archaean domain of the craton is essentially interpreted to have acted as a cratonic nucleus onto which younger material was accreted. However at present there is comparatively little known about the broad tectonic development of the Archaean part of the craton. Developing such a framework is crucially important for placing rocks systems such as the highly prospective Harris Greenstone Belt mto a broader context.

The Archaean portion of the Gawler Craton consists of a complex assemblage of medium and highgrade domains of metasedimentary complexes, volcanics, granitoids and mafic and ultramafic greenstone belts which comprise much of the southern central and northern parts of the craton.

One of the key questions to be addressed in developing framework models for the evolution of the Gawler Craton, is to investigate whether the Archaean was a single "cratonic" nucleus, or itself represents an amalgam of terranes that were assembled during Archaean or Palaeoproterozoic convergence.

Existing data point to similarities in the geological development of the Archaean in different parts of the craton. Regional high-temperature low-pressure granulite facies metamorphism affected (meta-) sedimentary sequences at around 2440-2400 Ma in both the Sleaford and Mulgathing Complexes, and was associated with the generation of partly crustally derived granites at ~ 2500 Ma (Daly et al., 1998). However limited isotopic data from 2500 Ma granites points to a much older crustal component (Iwaniw^, 2000), hinting at the nature of the basement to the metasedimentary sequences in the Sleaford and Mulgathing Complexes.

While there are similarities in the style and timing of metamorphism in different parts of the Archaean, it is not known whether the tectonism affected a single basinal system.

In this contribution w^e will present detrital zircon and Nd isotopic data from three widely separated Archaean regions on the Gawler Craton: (1) Wangary and Camot Gneisses in the southern Gawler Craton, (2) metasediments in the Kimba region and (3) the Christie and Kenella Gneisses in the Mulgathing Complex in the central northern part of the Craton. The data will provide an important baseline within which to consider the Archaean development of the Gawler Craton.

References Daly, S.J., Fanning, C.M. and Fairclough, M.C. 1998. Tectonic evolution and imphcations for exploration potential of the western Gawler Craton. AGSO Journal of Australian Geology and Geophysics, 17:145-168.

Ferris, G.M., Barovich, K. and Hand, M. (this volume). Putting the Hiltaba Suite into a tectonic context.

Geological Society of Australia. Abstracts.

Iwaniw A, 2000; unpub. Evidence of recycling of Archaean continental crust; a geochemical and Nd-Sr isotope study of Gawler Craton Granitoids, South Australia. Honours thesis, Adelaide University.

Schwarz, M P., Barovich, K. and Hand, M. (this volume). A plate tectonic setting for evolution of the southern Gawler Craton, from detrital zircon and Sm-Nd isotopic data of the Hutchison Group. Geological Society ofAustralia. Abstracts.

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TRACHYBASALT IN THE SOUTHERN CORIOLIS TROUGHS; A NEW GLOBAL BACKARC BASIN END-MEMBER MAGMA TYPE. R.J. Arculus\ R.A. Binns^ T.F. McConachy^ B.I.A. Mclnnes^ and C.J. Yeats^ \ Department of Geology, Australian national University, Canberra, ACT 0200.

I CSIRO Exploration and Mining, North Ryde, NSW 2113.

A major discovery of the Vanuatu-Australia Vents Expedition {Franklin 08/2001) w^as that the supposedly non-magmatic and sediment-filled Coriolis Troughs (see e.g., Recy et al., 1990; Pelletier et al, 1993), some 50 km behind the active volcanic front of the southern Vanuatu island arc, are in fact hydrothermally active, and floored by young trachybasalt lava fields (as predicted by Price et al, 1993). From north to south, the Troughs are the Vate, Erromango, and Futuna Basins.

Preliminary study of the major and trace element composition of glasses from the newly discovered Nifonea Ridge in the Vate Trough shows these are unique globally in terms of backarc basin types of rock, even among nascent rifts like the Sumisu Rifts (Izu-Bonin) and Okinawa Trough (Ryukyu arc) (see e.g. Hochstaedter et al, 1990; Shinjo & Kato, 2000).

Firstly, they are trachybasalts with > 5 wt% Na20 + K2O at 50 Si02. Secondly, abundances of diagnostic trace elements, normalised to the "primitive upper mantle" reference [see: http://earthref org/GERM/main.htm]) are remarkable on several grounds: firstly, a strong light vs heavy rare earth element enrichment is indicative of relatively low % of partial melting of the mantle source; more significant are the relatively high Nb (-17 ppm) and low (relative to Ce) Pb (2.5 ppm) abundances which are completely antithetic to those of arc lavas being erupted merely ~ 50 km to the west, and are characteristic of a distinctive mantle source being tapped by some mantle plumes termed " h i g h - | L i " , requiring the partial melting of long term, recycled, and previously hydrothermally altered oceanic lithosphere (Hofmann, 1997).

The upper mantle sources of mid-ocean ridge lavas in the Pacific and Indian Oceans are isotopically distinct (Hamelin et al, 1986). The boundary between these mantle domains lies along the length of the active arc systems in the western Pacific, from the Izu-Bonin-Mariana in the north to the Lau-Tonga system in the south (Hickey-Vargas et al, 1995). There is evidence that the Manus and North Fiji Backarc basins are both underlain by Indian-type mantle (Woodhead et al, 1998, 2001; Peate et al, 1997). The Vanuatu arc is unusual in that in the vicinity of the collision zone with the D'Entrecasteaux Ridge, the arc magmas seem to be tapping an Indian source in the mantle wedge overlying the subducted Australian Plate, whereas the arc volcanoes to the north and south of this collision zone are tapping Pacific-type mantle wedge sources (Crawford et al, 1995). Globally, it appears the most refractory (i.e., prior meltdepleted) arcs are those with accompanying and active backarc basins - the hypothesis is that a potential mantle source is advected from a melting event beneath a backarc ridge towards the arc, and a zone of fluid ingress from the subducting lithosphere triggers another melting stage (e.g., Woodhead et al, 1993). In the Vanuatu case, we now observe the most actively spreading region on Earth (Lagabrielle et al, 1997) forming the North Fiji Basin presumably establishing a highly depleted mantle residue. But the Coriolis Troughs are not only tapping relatively fertile mantle but also possibly a geochemical type other than the Indian-type mantle upwelling beneath the North Fiji Basin.

75

THE ORIGINS OF PYROXENITES IN THE SOLOMON ISLANDS (SANTA ISABEL AND SAN JORGE).

Thomas Berly Jorg Hermann^ and Richard J. Arculus ^. ^Geology Department, and ^Research School of Earth Sciences, Australian National University, Canberra.

The Solomon Islands are a NW to SE-trending double chain of islands, the older basement of which was formed by S W-directed subduction of the Pacific Plate beneath the Indo-Australian Plate, between the Eocene and Early Miocene. At 10 Ma, the Ontong Java Plateau (OJP) collided with the Solomon arc. In response to this collision, polarity of subduction was reversed; NE-directed subduction beneath the Solomon arc began. Thin fault slices of peridotites, pyroxenites, gabbros, and basalts are now exposed in SE Santa Isabel and neighbouring San Jorge. Some pillow lavas have been identified as OJP basalts but the origins of the ultramafic rocks remain uncertain.

The pyroxenites are associated in the field with harzburgites, dunites and sometimes incorporated into serpentine massifs (NW San Jorge). These coarse-grained rocks contain mostly (but in variable proportions) orthopyroxene (opx) (70<En<85) and clinopyroxene (cpx) (Wo=47.5; En=49.5). Olivine (OL) (85<FO<93) only occurs rarely as well as Cr-spinel (sp) (60<Cr#<73). Secondary phases include magnesio-homblende, tremolitic-hornblende, tremolite, pyrite and serpentine. The hornblende is related intimately to fluid inclusions (trails) and forms blebs within cpx.

Two-pyroxene thermometry gives equilibration temperatures between 800 and 1000°C. The assemblage cpx-opx-sp-ol requires 7<equilibration pressures<17 kbar. Pyroxenites are characterised by high and relatively constant Mg-numbers (87-92) comparable with the peridotites (-90). However CaO content is variable (2-20%) reflecting varied mineralogy; AI2O3 content is very low (<5%).

Trace element abundances (determined by LA-ICP-MS) of both whole rock and cpx shows that: (1) cpx dominates the whole-rock budget for rare earth (RE) and other lithophile elements (E); (2) there is variable depletion in light relative to medium REE (0.1<(La/Nd)N<0.5) and flat to convex medium to heavy REE (0.5<(SmA'b)N<1.5); (3) overall REE abundances ranging from 0.4 to 1.3 times chondritic. Primitive mantle-normalized multi-element diagrams of cpx are marked by positive anomalies in Cs, Ba, Pb and Sr. Similar selective enrichments in pyroxenites from Ronda have been attributed by Garrido et al (1998) to chromatographic effects of melt percolation through peridotite.

But according to Downes (2001), enrichments in these elements in ultramafic rocks requires metasomatism by hydrous fluids from subducting slabs.

In arc settings, at low pressures (-15 kbar), aqueous fluids, which start to be released from the slab, can interact with the mantle wedge. At these T-P conditions, hydrous melt can interact with mantle wedge and crystallise pyroxene-rich rocks with features found in Solomon Islands. But Schmidt et al.

(1998) suggest that the dehydration of the descending slab is continuous involving hydrous phases chlorite, lawsonite, phengite, epidote, and amphibole. During exhumation, secondary fluids result in formation of hornblende and affect the pyroxenites; fluid inclusion trails are recorded mainly in cpx.

Exhumation of those px-rich rocks can result from backthrusting of arc portions during collision of the OJP (Petterson et al, 1998), or they may be carried by buoyant serpentinites (Hermann et al, 2000).

References Schmidt, M. W. and S. Poli (1998). "Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation." Earth and Planetary Science letters 163: 361-379.

Garrido, C. J. and J. L. Bodinier (1999). "Diversity of mafic rocks in the Ronda peridotite: evidence for pervasive melt-rock reaction during heating of subcontinental lithosphere by up welling asthenosphere." Journal of Petrology 40(5): 729,754.

Petterson, M. G., T. Babbs, et al. (1999). "Geological-tectonic framework of Solomon Islands, SW Pacific: crustal accretion and growth within an intra-oceanic setting." Tectonophysics 301(1-2): 35-60.

Hermann, J., O. Muntener, et al. (2000). "The importance of serpentinite mylonites for subduction and exhumation of oceanic crust." Tectonophysics 327: 225-238.

Downes, H. (2001). "Formation and modification of the shallow Sub-continental lithospheric mantle:a review of geochemical evidence from ultramafic xenolith suites and tectonically emplaced ultramafic massifs of western and Central Europe." Journal of Petrology 42(1): 233-250

76

THE LOS PICOS-FORTUNA/PAJONAL-EL ABRA COMPLEX, A GIANT LONGLIVED MAGMA CHAMBER ASSOCL\TED WITH PORPHYRY COPPER MINERALISATION Ian H. Campbell and Julian R. Ballard Research School of Earth Sciences, Australian National University, Canberra, ACT, 0220, Australia.

The Los Picos-Fortuna/Pajonal-El Abra complex is an Eocene system of calc-alkaline intrusions that are associated with porphyry copper mineralisation at El Abra, in northern Chile. The individual stocks and intrusions, vs^hich make up the complex, are grouped into units that have distinctive petrological characteristics that can be mapped in the field. Laser ICP-MS U-Pb dating of zircons of a representative intrusion from each of these units, has shown that they their chemistry correlates with age, with the youngest intrusions being the most chemically evolved. The total age range for the complex is 5.5 Ma. Although the average of parameters such as MgO or FeO, which decrease with fractional crystallisation, change systematically with time, if all of the data from each of the mappable units are considered, there is considerable overlap between the fields. This overlap results in the most evolved sample from the older units being more evolved than the most primitive sample of the next youngest unit. However the average and most primitive magma from each of the units shows a steady evolution with time. These observations can be explained as the intrusions being fed by a larger magma chamber at depth that was being periodically replenished by new pulses of magma and if the new pulses mixed with the fractionated magma in the chamber. The steady progression in the chemistry of the unit with time requires the parent chamber to retain a memory of earlier fractional crystallisation and this is only possible if the intrusion remained melt over its full 5.5 m.y. history.

That is that the parent magma took at least 5.5 m.y. to solidify. This result is unexpected. Numerical modelling of the Bushveld complex, the largest exposed magma chamber, shows that this intrusion crystallised in about 200,000 years, a factor of 30 to 50 times faster than the parent intrusion postulated to have fed the intrusions of the Los Picos-Fortuna/Pajonal-El Abra complex. What characteristics of this system allowed it to survive for 5.5 Ma.?

The crystallisation time for a laterally extensive sill-like body increases with its heat content and decreases with the heat flux through its roof The heat content of a magma chamber is proportional to its height, h and its heat flux, F, which is proportional to AT/D, where AT is the temperature difference between the liquidus temperature of the magma and the roof rocks, and D is the thickness of the roof heated by the cooling intrusion. By the time a sill cools to its solidus temperature, D ~ h, so that the crystallisation time for the sill increases with the square of its thickness and decreases with the temperature difference between the magma and its roof rocks.

Irvine (1970) has calculated the crystallisation time for sills with thicknesses of 0.5, 2.0 and 8.0 km and at AT's of 1200, 800 and 400''C, assuming a difference of 200"" C between the liquidus and solidus temperature for the magma. For a 8 km thick sill at AT's of 1200, 800 and 400" C these are 0.33, 0.9 and 5.5 m.y. and for a 2 km sill they are 20,000, 55,000 and 330,000 years respectively. It is apparent that a magma chamber can survive for 5.5 m.y. or longer provided it is at least 8 km thick and if its AT < 400° C, or less than 200 ""C if the solidus temperature is used for the magma [ATs]. Although Irvine's results show that a 2 km thick sill cannot remain molten for more than 330,000 years for ATs 200"" C, extrapolation of these results suggests that a 2 km thick sill can probably survive for 5.5 m.y. if ATs is less than -lO"" C. We conclude that a laterally extensive sill can survive for 5.5 m.y. if it is thick and if the formed in the lower crust where AT is small.

Reference Irvine, 1970. Can. J. Earth Sci., v.7, 1031-1061.

77

IMPLICATIONS FOR Re/Os SYSTEMATICS OF INTRA-PLATE MAGMAS FROM HIGH PRESSURE LIQUID METAL / LIQUID SILICATE PARTITIONING OF MODERATELY SIDEROPHILE ELEMENTS Graziella Caprarelli'"^ ^Department of Environmental Sciences, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007, Australia. ^Mail Code SA 13, NASA/JSC, 2101 NASA Road 1, Houston, Texas 77058-3969, USA.

Recycling of the oceanic crust into the mantle plays a major role in creating mantle heterogeneities that are reflected in the geochemical compositions of intra-plate basalts. Os is a compatible element, while Re is moderately incompatible and its ^^^Re isotope P-decays to ^^^Os. Therefore, high ^^^Os/^^^Os (and ^^^Os/'^^Os) ratios of intra-plate basalts are interpreted as a signature of recycled ancient (> 2Ga) oceanic crust (e.g., Hauri and Hart 1993). However, Re and Os are also strongly siderophile elements, which means that their partitioning may be substantially influenced by the possible presence of metal phases in the melt sources and by their physical conditions. High ratios in some OIB are explained as signatures of an outer core-derived liquid metal component (Widom & Shirey 1996; Brandon et al 1998). Hence, Re and Os decouple in the presence of metal, as the development of highly radiogenic Os requires a reservoir with high Re/Os ratios. Unfortunately, experimental partitioning data of Re are still scarce (and vary across a very large range of values), and partitioning models of Os rely upon the thermodynamic behaviour of Ir because of total lack of Os partition coefficient data. This situation contributes to the difficulty of unequivocally interpreting Re and Os compositions of basalts.

A positive correlation between Co (compatible element) and V (moderately incompatible) abundances in basalts was explained by a similarity of behaviour involving liquid metal/liquid silicate partitioning (Caprarelli 2002). Correlation between Co and V abundances, and ^^^Os/^^^Os ratios in Mauna Loa and Koolau basalts, for which increasing proportions of a liquid core component were inferred (Brandon et al., op. cit.\ suggests that the siderophile behaviour of Co and V can be used as a model for the Re-Os system. The higher concentrations of Co and V (relative to Re and Os) in natural silicate melts makes these elements suitable for experimental and analytical work over a broad range of basaltic compositions, bypassing the difficulties inherent in direct experimental work with Re and Os. Multianvil experiments were conducted at pressures > 5 GPa, using graphite capsules loaded with KLB-1 peridotite and Fe+Ni±Co±V mixtures, at temperatures > 2373 K (Caprarelli et al., in prep.). Total melting of both metal and silicate was achieved, and experimental charges were analysed by EPMA and LAM. Preliminary results indicate that, when no mineral phases are present, Co-V distribution coefficients (Kd = D^''met/sii/D^met/sii) dccrcase with increasing pressure. This confirms that the behaviour of siderophile elements during magma genesis cannot be unequivocally explained by considering only plate tectonics scenarios, irrespective of how strongly incompatible or compatible the elements are during petrogenesis: depth of provenance and presence of liquid metal in the source are critical parameters. These affect elemental and isotopic Re-Os ratios of intra-plate basalts.

References Brandon A.D., Walker R.J., Morgan J.W., Norman M.D. & Prichard H.M. (1998) Coupled ^^^Os and '^^Os evidence for core-mantle interaction. Science 280, 1570-1573.

Caprarelli G. (2002) Variation of ScA^ ratios in the mantle: a geochemical link to the lower mantle percolation of core melts? LP5C XXXIII, #1325, Houston TX (USA), 11-15/3/2002.

Caprarelli G., Draper D.S., Xirouchakis D. & Agee C.B. {in prep.) Liquid metal / liquid silicate partitioning of Co and V at high pressures and temperature and implications for core - mantle segregation processes.

Hauri E.H. & Hart S.R. (1993) Re-Os isotope systematics of HIMU and EMU oceanic island basalts from the south Pacific Ocean. Earth and Planetary Science Letters 114, 353-371.

Widom E. & Shirey S.B. (1996) Os isotope systematics in the Azores: implications for mantle plume sources.

Earth and Planetary Science Letters 142, 451-465.

78

MELT INCLUSION EVIDENCE FOR THE ORIGIN OF TAUPO ANDESITES A.J. Crawford\ V.S. Kamenetsky\ RC Priced JA Gamble^ 1: Centre for Ore Deposit Research, University of Tasmania, 2: School of Earth Sciences, Waikato University, 3: School of Earth Sciences, Victoria University of Wellington The Kermadec Arc - Havre Trough intra-oceanic arc-backarc basin system continues southward into the continental crust of New^ Zealand to form the Taupe Volcanic Zone (TVZ). High heat flov^ and abundant felsic magmatism characterise the last 1-2 million years of TVZ magmatism, but andesitic and basaltic centres also occur. TVZ andesites (56-64% Si02) are systematically shifted from the arc andesite field defined by large data sets for lavas from the Vanuatu and Sunda arcs, and indicate either additional source component(s), transit-storage-mixing processes, or parental magmas, to those andesites erupting from intra-oceanic arc stratovolcanoes. Given the presence of continental crust and thick turbidite-rich sedimentary pile beneath the TVZ, this is not surprising.

However, the key compositional shifts for the TVZ andesites relative to the arc andesites are in the direction of increased MgO, CaO, and decreased FeO, Na20, K2O and P2O5 at any Si02 level.

These pronounced differences might suggest either mixing between basaltic magmas and rhyolites, or broad boninitic affinities for at least some TVZ parental magmas. We have carried out a detailed petrological study of basalts, olivine-bearing andesites and quartz tholeiites from Waimarino, Red Crater, Ohakune, Pukeonake, Tarawera, and Mahuia (Ruapehu). The Pukeonake, Waimarino and Mahuia lavas contain abundant olivine phenocrysts to F094 g, and have common boninitic chromites with Cr# ~ 0.80. Available evidence, therefore, indicates that a range of highMg parental magmas have been emplaced into the Taupo zone, ranging from more BABB-like basalts such as the Kakuki basalt, to more boninitic compositions such as Pukeonake and Waimarino. We will present results of a detailed melt inclusion study of these lavas aimed at evaluating the nature of the parental magmas of the TVZ andesites, the importance of mixing between mafic and felsic magmas to generate the andesites, and the mechanism responsible for generating boninitic magmatism in this setting.

79

VOLATILE PHASE EXSOLUTION - HOW IT HAPPENS IN NATURE? AN INSIGHT FROM RHYOLITIC MAGMATIC INCLUSIONS, TAUPO VOLCANIC ZONE, NEW ZEALAND. Paul Davidson, Vadim S. Kamenetsky, Sharon Allen Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania.

The exsolution of an immiscible volatile-rich phase during the cooling of a magma is taken for granted in such diverse fields as volcanic degassing and the source of metals and ligands in the formation of orebodies. Theoretical and experimental research has suggested hov^ exsolution occurs but, given the complexity of the process, there appeared little hope of observing the process in natural systems. However, one example has been discovered in magmatic inclusions in phenocrysts of quartz from recent (< 65 Ka) rhyolitic lavas from the Okataina Volcanic Centre.

Okataina is one of two potentially active rhyolitic centres in the Taupo Volcanic Zone, New Zealand. The most common type of inclusions are clear glass with abundant amphibole daughter crystals, and one or more shrinkage bubbles with constant bubble/inclusion volume ratio (Fig. la).

Almost as common are completely crystalline volatile-bearing melt inclusions, and clear or brown glass inclusions with no daughter crystals. In many cases bubbles show a liquid-vapour boundary (Fig. lb) and contain one or several cubic minerals (Fig. 1 c, d). The crystal-bearing bubbles (globules) are typically smaller than shrinkage bubbles, and may be present in great numbers. The amount of solid phases (chlorides, carbonates, sulphates) they contain testifies to significant sequestration of volatile and other elements from their parental melt (fluid). A separate type of inclusions is recorded around the completely crystalline melt inclusions as a halo of aqueous twophase fluid, filling healed decrepitation cracks (Fig. le).

Fluid-enriched phases found in the Okataina rhyolitic quartz represent post-trapping exsolution of a magmatic fluid from a melt, rather than the accidental trapping of a phase coexisting with the melt.

This is well supported by constant bubble / inclusion volume ratios. The absence of fluid-only primary magmatic inclusions implies that at the time of trapping the system was undersaturated in volatiles, and degassing had not begun. The melt inclusions exsolved the original magmatic volatiles into globules and/or expelled them into decrepitation cracks (now healed).

The discovery of a snapshot of exsolution, frozen within magmatic inclusions, provides an opportunity to examine the composition of these unevolved, uncontaminated aqueous primary magmatic fluids. As this exsolution may happen in a similar way in a larger scale (in natural magmas), the data from inclusions can provide insight into the sequestration of metals by such fluids, and help to delimit the physical and chemical parameters of this process in natural systems.

Fig. 1. Primary inclusions in quartz phenocrysts in rhyolitic lavas from the Okataina Volcanic Centre, showing, a: glass with amphibole daughter crystals and a shrinkage bubble, b: two-phase liquid/vapour bubble in glass, c: glass with a globule of aqueous liquid and crystals, d: glass with cubic crystals nucleated on the shrinkage bubble, e\ decrepitation haloes of secondary two-phase aqueous inclusions. Scale bars are all 50 |Lim.

80

STRUCTURE AND PETROLOGY OF FORE-ARC OPHIOLITES: A LATE PROTEROZOIC EXAMPLE FROM WESTERN MONGOLLV ^ Arian H. Diikstra, ^Fraukje M. Brouwer, ^W. Dickson Cunningham and ^Gombosuren Badarch ^Department, of Geology, Cuitin Universit}^ of Technology, Perth, WA (Until -1/6/2002: Dept. of Geology, University of Leicester, UK, ahd3@le.ac.uk) ^Geophysical Laborator>^ Carnegie Institution of Washington, Washington DC, USA. ^Department of Geology, University of Leicester, Leicester, UK ^Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, Ulanbaatar, Mongolia Ever since the recognition that ophiolites are on-land exposed fragments of oceanic lithosphere, it has been debated whether ophiolites represent 'normal' ocean floor created at mid-ocean ridges, or the basement of fore- or back arc basins. In many cases, geochemical studies of ophiolitic lavas and dykes have revealed supra-subduction zone (SSZ) characteristics (e.g., in Oman, Troodos, Bay of Islands, Vourmos). However, this SSZ geochemical signature is often only clearly expressed in late-stage dykes and in high-level lavas. In general, the plutonic sections of these so-called "SSZ ophiolites" are not any different from plutonic rocks from modem-day mid-ocean ridges.

Interestmgly, a subtle geochemical SSZ signature has also been found in volcanics from the modem-day Chile Ridge, suggestmg that it is not restricted to fore- and back-arc environments.

The rocks from the late Neoproterozoic (± 570 Ma) Dariv Ophiolite in Westem Mongolia are quite unusual compared to those of typical "SSZ ophiolites" and of modem mid-ocean ridges. In Dariv, the upper part of the ophiolitic section comprises altemating basaltic and rhyolitic lava flows and mutually intmsive mafic and intermediate-to-felsic sheeted dykes. The plutonic section in Dariv consists of a well-developed igneous layered complex consisting of cumulate dunites, chromites, harzburgites, orthopyroxenites and pyroxene-bearing modally layered gabbros. The igneous layered complex is characterised by the early fractional crystallisation of orthopyroxene (before plagioclase), suggestmg formation from a high-Si02, boninitic magma. The mantle section consists of strongly depleted (serpentinised) harzburgites.

In ever>^ aspect, the Dariv rocks show a remarkable petrological resemblance to volcanic, plutonic and uhramafic rocks recovered from the Eocene Bonin-Mariana fore-arc system. We propose that, together with some other unusual ophiolites (e.g., the Papua New Guinea Ophiolite, the Betts Cove Ophiolite in Newfoundland, the Khan Taishir Ophiolite in Mongolia and the Agardagh Tes-Chem Ophiolite in Tuva), the Dariv Ophiolite defines a class of 'tme' fore-arc ophiolites. We fiirther argue that the lack of convincmg fore-arc ophiolite characteristics in the bulk part of sections of socalled "SSZ ophiolites" suggests that they formed by a two-stage scenario of spreading at a midocean ridge followed by only minimal cmst formation in a subduction zone environment.

81

GEOCHEMICAL TRENDS ACROSS AN ARC-CONTINENT COLLISION ZONE: MAGMA SOURCES AND SLAB-WEDGE TRANSFER PROCESSES BELOW THE PANTAR STRAIT VOLCANOES aNDONESL\).

Marlina A. Elburg^'\ Manfred van Bergen^, Jurian Hoogewerff^ John Foden\ Pieter Vroon^'^, Iskandar Zulkamain^ Asnawir Nasution^ ^ Department of Geology and Geophysics, Adelaide University, Adelaide SA 5005, Australia. ^ Faculty of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands ^ Faculty of Earth Sciences, Free University, Amsterdam, The Netherlands ^ Department of Geology, Royal Holloway University of London, Egham, Surrey, United Kingdom. ^ RDCG-LIPI, Bandung, Indonesia ^ Volcanological Survey of Indonesia, Jl. Diponegoro 57, Bandung, Indonesia * Now at: Max Planck Institute for Chemistry, P.O.Box 3060, D-55020 Mainz, Germany The four Pantar Strait Islands form the westernmost part of the extinct sector of the east Sunda arc. They show remarkable across-arc variations in elemental abundances (K2O: 1.2-4.3%), trace element ratios (Pb/Ce: 0.4-0.18; Ce/Yb: 20-55) and isotope ratios (^^^Nd/^^^Nd: 0.51263-0.51245; ^^Sr/^^Sr: 0.7053-0.7068; ^ ^ W ^ ^ P b : 19.29-19.15) with limited variation in fractionation stage. Pb isotopes are decoupled from Sr and Nd isotopes, with the frontal volcanoes showing the higher Nd and Pb, and lower Sr isotopic ratios.

The isotopic and trace element ratios of the volcanic samples are best explained by modification of a MORB-type source by a fluid and a partial melt of subducted continental material (SCM). The Pb isotopic array indicates that the mantle source has a Pb isotopic compositions similar to OIB, and that the SCM is more similar to North Australian sediments than to dredge sediments from the continental shelf. The frontal volcano contains the highest proportion of the fluid component, with a small contribution of partial melt. The source of the rear-arc volcano is strongly influenced by a partial melt of SCM that had undergone a previous dehydration event, by which it lost most of its fluid-mobile elements such as Pb. High Th/Nb ratios indicate that the SCM partial melt was in equilibrium with rutile.

Although a strong increase in Ce/Yb ratios across the arc suggests a role for garnet during SCM melting, Tb/Yb ratios do not show any significant increase. Rb/Ba ratios are similar in all four volcanoes and are lower than for MORB or SCM, indicating that mantle melting took place in the presence of residual mica.

The relatively large across-arc increase in incompatible elements cannot solely be explained by an increasing addition of SCM partial melt. Other processes, such as changing mantle wedge fertility and smaller degrees of partial melting towards the rear of the arc, are likely to have played a role too.

Comparison with a more westerly across-arc transect shows that the relatively low ratios of the frontal volcano, and the decoupling of Pb from Sr and Nd isotopes, are unique to the Pantar Strait volcanoes. This is likely to reflect magma generation in a collisional environment, where the leading edge of the Australian continent, rather than subducted sediment, contributes to the magma source.

82

RABAUL VOLCANO, PAPUA NEW GUINEA: SEISMIC TOMOGRAPHIC IMAGING THE MAGMA RESERVIOR OF A RESTLESS VOLCANO Doug Finlayson^ Oli Gudmundsson^ Ima Itikarai^ Yuichi Nishimura^, Hideki Shimamura'^ and Wally Johnson^ ' 6 Neilson St., Garran ACT 2605, Australia ' Danish Lithosphere Centre, Oster Voldgade 10, L, DK 1350 Copenhagen K, Denmark ' Rabaul Volcanological Observatory, PO Box 386, Rabaul, PNG ^ Laboratory for Ocean Bottom Seismology, Hokkaido University, Sapporo 060, Japan ' Geoscience Australia, GPO Box 378, Canberra ACT 2601, Australia The Rabaul volcano is associated with a subduction zone near the triple junction formed by the Pacific, South Bismarck and Solomon lithospheric plates. It has an eruption history extending back at least 0.5 Myr (Nairn et al., 1995; Wood et al., 1995) that includes mainly basaltic and andesitic cone building events and dacitic (and rarely rhyolitic) explosive eruptions. The currently active caldera formed since 3500 years BP with a major ignimbrite eruption and caldera collapse at 1400 years BP forming Rabaul harbour. This event was followed by the build up of a shallow magma reservoir under Rabaul harbour. Melt inclusions in eruption material suggest that mixing of mafic magma from a mantle source with the silicic magma chamber is the trigger for recent eruptions (McKee et al., 1984; Roggensack et al., 1996). Volcano-related earthquake activity is shallow (< 10 km) and associated with magma movement within the upper part of the caldera.

A 1997 seismic tomography survey of the Rabaul caldera defined the physical nature of the magma reservoir under Rabaul harbour. The survey determined the P-wave velocity structure to a depth of about 12 km using both explosive and earthquake seismic sources and identified a 30-35 km^ low velocity region (?magma reservoir) at 3 to 6 km depth beneath the central Rabaul caldera. The imaging highlighted the heterogeneity in P-wave velocity both laterally and vertically within the Rabaul caldera, indicating significant complexity within quite a small area, consistent with that observed in outcrop of Palaeozoic eroded calderas in other parts of the world. The low-velocity region at Rabaul (< 5.0 km/s) is identified with the part of the caldera under Rabaul harbour thought to be associated with the current magma injection system. There are also high-velocity (>6.0 km/s) rock units around the caldera rims that are interpreted to indicate large volumes of mafic intrusive rock at shallow (<4 km) depths.

Acknowledgements This project was funded by the Australian Agency for International Development (AusAID) and the Japan International Cooperation Agency (JICA) as a contribution to a volcano monitoring improvement program for the Rabaul following the 1994 eruption.

References McKee, C. O., Lowenstein, P. L., De Saint Ours, P., Talai, B., Itikarai, L, Mori, J. J., 1984. Seismic and ground deformation crises at Rabaul caldera: prelude to an eruption? Bull. Vole., 47, 397-411.

Naim, I. A., McKee, C. O., Talai, B., Wood, C. P., 1995. Geology and eruptive history of the Rabaul Caldera area, Papua New Guinea. J. Vole. & Geother. Res., 69, 255-284.

Roggensack, K., Williams, S. N., Schaefer, S. J., Pamell, R. A., 1996. Volatiles from the 1994 eruptions of Rabaul: understanding large caldera systems. Science, 273, 490-493.

Wood, C. P., Naim, I. A., McKee, C. O., & Talai, B., 1995. Petrology of the Rabaul Caldera area, Papua New Guinea. J. Vole. & Geother. Res., 69, 285-302.

83

SANGEANG API VOLCANO, INDONESIA: RELATIONSHIP BETWEEN ALKALINE ARC MAGMAS AND PYROXENE-RICH XENOLITHS. John Foden\ Rick Vame^, Marlina Elburg^'"^ Simon Turner^ ^Department of Geology and Geophysics, University of Adelaide, Adelaide, South Australia, 5005. john.foden@adelaide.edu.au ^School of Earth Sciences, University of Tasmania^ ^Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 IRJ, U.K. Vax-Planck-Institut fur Chemie, Postfach 3060, 55020 Mainz, Germany

Sangeang Api is an active alkaline volcano in the Indonesian east Sunda Arc with an eruptive history of less than 1 m.y. duration. Its products comprise potassic, volatile-enriched and silica undersaturated lavas and an abundant population of entrained clinopyroxene-rich mafic and ultramafic xenoliths. Sangeang Api has much in common with magmatic systems which host porphyry Cu-Au mineralisation elsewhere in Indonesia and the western Pacific. Its oxidised, H2O and Cl-rich lavas are very like those from the Feni-Tabar islands which host the Lihir Au- deposit (Mclnnes et al., 2001).

There is very good evidence that suites of lavas and xenoliths are co-magmatic and that the xenoliths are cumulate rocks whose crystallisation has driven the differentiation of the Sangeang Api melts. Our data suggest that the xenoliths sample a magma tract feeding the Sangeang volcano. In this flow rates vary depending on the flux rate of magma from the mantle source regions. Waning flux rates lead to amphibole and phlogopite growth and lithospheric enrichment with restricted percolation flow. Enhanced input leads to channelised flow and the production of discrete magma chambers.

Observed alkaline glass resulting from decompressive incongruent melting of amphibole in some xenoliths is evidence that these metasomatic zones are sources of a high-K alkalic end member to the arc magmatic spectrum. Melting of clinopyroxene-rich assemblages in the base of this tract will yield Ca-rich primitive arc ankaramite. The mixing of melts generated in the arc lithosphere with primary mantle melts will generate a range of hybrid magmas including K-rich suites.

Reference Mclnnes B. etal (2001). Earth and Planet. ScL Letts, 188, 169-183.

^ Professor Rick Vame sadly passed away on Sunday July 8^^ 2001 after a prolonged illness.

84

ISLAND-ARC ANKARAMITES: PRIMARY MAGMAS FROM REFRACTORY LHERZOLITE FLUXED BY CO.+Rfi. D.H.Green, M.W.Schmidt and W.O.Hibberson Research School of Earth Sciences, Australian National University, Canberra, ACT

Among the primitive magmas of some island arcs are distinctive magmas known as islandarc ankaramites. Modem examples include lavas from the volcanoes of Vanu Atu, such as Western Epi, Ambrym, Tanna, Aoba, Merelava, and Gaua. The magmas are characterized by phenocrysts of olivine (Mg# >91), refractory chrome spinel (Cr# >70) and diopside ( Mg#.92 ) and by chemical compositions with Ca0/Al203 >1.3. Previous experimental studies at high pressures have found olivine and chrome spinel as low-pressure liquidus, followed or accompanied by diopside, and diopside as the liquidus phase at higher pressures.

Compositions do not approach orthopyroxene saturation at any pressure under dry conditions or with H2O. These studies led to hypotheses of melting of wehrlite or clinopyroxenite sources, rather than Iherzolite compositions believed to dominate the mantle wedge above subduction zones.

We have experimented with primitive ankaramite magma from Western Epi,Vanu Atu, exploring the effects of variable H2O+CO2 contents on liquidus phases at 1.5 and 2.0 GPa. The method used was the 'sandwich technique' in which the Western Epi ankaramite glass was equilibrated with refractory 4-phase Iherzolite (01 + Opx + Cpx + Cr-Sp) at 1250MOO^'C, 1.5-2.0 GPa with variable CO2 and H2O contents dissolved in the melt-layer. We have established that the presence of dissolved CO2 (carbonate) in the melt increases the CaO/AbOs and normative diopside contents of melt coexisting with 4-phase Iherzolite to values of 1.4 to 1.5.

We infer that island-arc ankaramites are products of partial melting of refractory Iherzolite in the mantle wedge above a subducted slab. Melting is fluxed either by (C-H-O) fluids or by carbonatite melt that also provide incompatible elements with slab- or mantle wedgesignature to the refractory Iherzolite source. In movement from source to surface, ankaramitic magmas of this type become fluid-saturated at around 1.0 GPa (due to rapidly decreasing CO2 solubility). Loss of (C-H-O) fluid is accompanied by crystallization of phenocryst phases and by oxidation (Fe^^ to Fe^^) observed in spinel compositions and Mg# variations.

85

EXPERIMENTALLY DETERMINED TRACE ELEMENT CHARACTERISTICS OF AQUEOUS FLUID FROM PARTIALLY DEHYDRATED MAFIC OCEANIC CRUST AT 3.0GPa, 650-700 C. Trevor H. Green and John Adam ARC National Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC), Dept.

Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia.

Altered oceanic crust undergoes dehydration as it is subducted. The released aqueous vapour has the potential to remove important amounts of trace elements from the altered crust, and to modify trace element ratios in the overlying mantle wedge and in the residual eclogitic crust. To experimentally assess this process a mid-ocean ridge tholeiitic basalt (MORB) has been enriched (lO-lOOppm) in 25 trace elements (Li, B, Cu, As, Rb, Sr, Y, Zr, Nb, Mo, Sn, Sb, Cs, Ba, La, Ce, Sm, Ho, Lu, Hf, Ta, Pb, Bi, Th, U)and crystallized at 3.0 GPa, 650° and TOO'' C for 2 weeks, with 30-50vv1:.% added water. Single and double capsule configurations were used, with the latter facilitating capture of discrete quenched solute areas, accompanied by large (50-100|i) crystals. At 650 C the fluid coexists with omphacite, lawsonite, chloritoid, coesite and accessory phengite, rutile, Ti-magnetite and talc. At 700 C garnet is conspicuous, along with omphacite, coesite and accessory phengite, rutile and rare lawsonite, staurolite and talc; chloritoid is absent. Pronounced mineralogical zoning occurred in all capsule types, linked with unavoidable thermal gradients.

LAM ICP-MS analyses establish patterns of trace element partitioning between minerals (data for omphacite, lawsonite, phengite, chloritoid and garnet) and fluid, but not absolute values. An estimate of the latter is made, assuming about 5wt% of solute dissolved in the aqueous fluid at these temperatures and pressure. However, ratios of trace element partitioning can be well constrained. The fluid shows strong enrichment of LIL relative to HFSE, and moderate enrichment of LIL relative to REE. It also shows decrease in Rb/Sr, Cs/Sr and Ba/Sr from 650° to 700° C, apparently controlled by residual lawsonite, and increase in La/Lu, clearly controlled by garnet. In addition, omphacite fractionates both Zr/Nb and La/Lu, contributing to a decrease in Zr/Nb and an increase in La/Lu in the fluid. Rutile (analysed with an electron microprobe) with Nb/Ta ~ bulk starting composition strongly takes up Nb and Ta, but cannot cause the almost 4-fold increase in the Nb/Ta ratio observed in the fluid relative to the bulk. Omphacite, garnet and chloritoid all have Nb/Ta less than and lawsonite greater than the starting composition. Omphacite and garnet strongly reject Sr, Ba, Rb and Cs; lawsonite favours all REE (HREE slightly more so), Sr, U and Th, whereas phengite favours Rb, Ba and Cu. Chloritoid most readily accommodates V, Cr, Ni and Zn. If these results are applied to the trace element characteristics of subduction zone magmatism, then involvement of a fluid derived by dehydration processes may increase LIL/HFSE and LIL/REE and decrease Zr/Nb in subduction-linked magmas, relative to MORB. Lowest Zr/Nb ratios point to the greatest relative involvement of clinopyroxene in the residue. Absence of major change to La/Lu restricts the role of garnet. Higher Rb/Sr, Cs/Sr and Ba/Sr may point to a low geothermal gradient in the subduction zone such that lawsonite remains stable during the dehydration process that produced the fluid. Residual rutile may explain Nb and Ta depletion without markedly affecting Nb/Ta ratios. At this pressure there is no evidence that the derived fluid will have a low Nb/Ta ratio.

86

HAFNIUM ISOTOPE CONSTRAINTS ON MAGMATISM IN THE LAU ISLANDS, SW PACIFIC. Janet M Hergt and Jon D Woodhead.

School of Earth Sciences, The University of Melbourne, VIEPS, Victoria 3010, Australia

Subduction related to the presently active Tofua Arc was initiated in the Eocene along the so-called 'Vitiaz arc' and, less than 6 Ma ago, this old arc began to split—^the extension associated with the opening of the Lau Basin. Remnants of the early arc were abandoned to become the now-dormant Lau Islands, with the locus of magmatism migrating to the east (the Tofua Arc). Thus in the Lau Islands we are afforded a rare subaerial record of early arc magmatism, including events predating back-arc basin formation. Magmas preserved in the Lau Islands represent volcanism (i) within the early arc (Lau Volcanic Group—LVG), (ii) during the earliest stages of rifting and back-arc basin formation (Korobasaga Volcanic Group—KVG), and (iii) continuing well after the Lau back-arc basin was established (Mago Volcanic Group—MVG). The temporal progression of magmatism recorded in the Lau Islands is particularly significant in view of the evidence for mantle heterogeneity and possible geodynamic processes recorded in basalts from the adjacent Lau Basin.

Pb isotope compositions of basalts from back-arc sites drilled during ODP Leg 135 led Hergt & Hawkesworth (1994) to propose a model involving variations in the composition of the sub-arc mantle wedge, and advection within the wedge during back-arc rifting ('corner flow'). The data define two linear trends which converge at high values of ^^^Pb/^^'^Pb and extend, at lower values, into either a field for Pacific MORB or Indian MORB. If their hypothesis is correct, and the original 'Vitiaz'system in this region was underlain by Pacific MORB mantle, then at least the LVG should preserve evidence for a Pacific MORB source component in its Pb isotope composition. Furthermore, there should be evidence to support the temporal and/or geographic migration of mantle domains beneath the Lau Islands during back-arc basin formation in the KVG and MVG lavas.

Unfortunately, Pb isotope results for Lau Island rocks define only scattered arrays, and do not extend to sufficiently low ^^^Pb/^^^Pb to be confident of any distinction between the two.

Importantly, Pearce et al, (1999) proposed that Hf isotope compositions of magmas, when combined with Nd, may also distinguish the Indian vs Pacific character of the mantle source. Hf is less prone to alteration compared with Pb and has enabled us to investigate the interplay between mantle sources in this region.

A suite of representative back-arc samples of Indian and Pacific MORB affinities (based on Pb isotope data) were analysed for their Hf isotope compositions and confirm these can be distinguished in this region. Following this, samples from the LVG, KVG and MVG were selected in order to investigate the temporal evolution of magmatism, and within these subgroups, samples were selected across a wide geographic distribution. Although the distinction is not as clear as that for the back-arc magmas, the three groups show only minimal overlap in Hf-Nd isotope space. The mantle sources required for the LVG and KVG appear to record a change from Pacific to Indian MORB with time, possibly confirming the observations of Hergt & Hawkesworth (1989) for the Lau Basin magmas. The MVG however, is quite distinct, with OIB affinities, and cannot easily be related to either the LVG or KVG. Reference Hergt, J.M. & Hawkesworth, C.J., 1994. Pb, Sr, and Pb isotopic evolution of the Lau Basin: implications for mantle dynamics during backarc opening. Proceedings of the Ocean Drilling Program 135, 505-517 Pearce, J.A., Kempton, P.D., Nowell, G.M. & Noble, S.R., 1999. Hf-Nd element and isotope perspective on the nature and provenance of mantle and subduction components in western Pacific arc-basin systems.

Journal of Petrology 40, 1579-1611

87

THE ROLE OF SUBDUCTION IN ALKALINE ARC MAGMATISM AND Cu-Au METALLOGENESIS: THE ROSETTA STONES OF LIHIR ISLAND Brent LA. Mclnnes CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 1670 Fluid-metasomatised peridotite xenoliths from the Tubaf volcano xenolith locality in the Tabar-LihirTanga-Feni arc, Papua New Guinea provide a direct view of the fundamental processes associated w^ith mass transfer in subduction zones. Although suprasubduction xenolith localities are extremely rare, the Tubaf site is indeed unique in that it is located adjacent • • • o n e of the w^orld's largest and youngest volcano-hosted gold deposits (Ladolam, Lihir island). The character of the xenoliths and the proximity of the xenolith locality to the Ladolam ore deposit makes them petrological and geochemical "Rosetta stones", allowing linkages to be made between mantle wedge processes, alkaline arc magmatism and Cu-Au metallogenesis.

Investigations of the xenolith suite reveal that the mantle underlying the Tabar-Lihir-Tanga-Feni island arc is composed of a harzburgitic residue from an early partial melting event in a mid-ocean ridge environment. This chemically depleted mantle was later enriched during fluid-peridotite metasomatism in an arc environment.

Strongly metasomatised peridotite xenoliths have planar, interconnected vein networks containing oxidised and alkali-enriched mineral assemblages (phlogopite-magnetite-sulfide othopyroxenite) generated at 800"^ to 1000°C and AFMQ« +2. The veins are formed in dilational fractures, indicating that metasomatism of peridotite occurred via hydraulic fracturing as a result of the influx of slab-derived hydrous fluids into the mantle. The hydrous fluids introduced fluid-soluble major elements (Si, Al, Na, K, and S) and trace elements (Sr, Ba, Rb, Th, U and Pb and LREE). The HFSE are neither enriched nor depleted during hydrous metasomatism, indicating that the so-called "arc signature" of high LILE/HFSE and LREE/HFSE is related to the solubility of these components in hydrous fluids under mantle conditions. Oxidation of the sub-arc peridotite is linked to the introduction of dissolved SO2 from the upper portion of subducted, altered oceanic crust. Enrichment of chalcophile elements (Pd, Cu, Au, Re and Os) is extreme in veined peridotite samples, but Os isotope investigations reveal that the mantle is the main source of ore metals in the veins (<10% crustal osmium).

In a metallogenic context, the essential role of subduction appears to be the generation of a flux of oxidising fluids capable of redistributing fluid-soluble metals (Au, Cu and Pd) within the mantle wedge under subsolidus conditions. Fluid saturation ultimately results in the formation of oxidized alkali- and metal-enriched metasomatic assemblages. Preferential partial melting of these assemblages occurs as the buoyant, metasomatized mantle ascends into the inverted thermal gradient of the mantle wedge generates alkaline arc magmas with elevated Au, Pd and Cu contents. The transition from diffusive to advective fluid flux through the Lihir mantle wedge is linked to post-collisional slab kinematics.

Because plate reorganization and terrane accretion processes affect slab dehydration rates and generate anomalous fluid:mantle ratios, it is contended that the formation of prolific Cu-Au deposits in orogenic belts is enhanced as a result of dramatic changes in plate motion.

88

EVOLUTION OF MAFIC MAGMAS AT SLAMET VOLCANO, JAVA, INDONESIA Olivier Reubi and Ian Nicholls School of Geosciences, Monash University, Vic 3800, Australia Slamet volcano, in Central Java, Indonesia is an active calc-alkaline stratovolcano composed largely of basalts and basaltic andesites. Despite emplacement within an apparently simple tectonic setting, Slamet basalts contain a wide range of phenocryst compositions which indicates that they are the products of a variety of magmatic processes. On the basis of their mineralogy, petrography and geochemistry, Slamet basalts may be subdivided in two groups. The first group contains a single population of olivine phenocrysts with mean composition ~ Fogi, close to the composition expected to be in equilibrium with a magma of the whole-rock composition. However, distinct high- and low-Ni sub-populations may be distinguished at any given Fo content. Chromian spinel inclusions within the high-NiO olivines have higher Cr# and Fe^^/Fe^^ ratio and lower Ti02 contents than those within low-NiO olivines. Plagioclase and pyroxene phenocrysts display dominantly oscillatory zoning, with cores close to equilibrium with whole-rock compositions.

The second group of basalts contains two populations of olivine phenocrysts: the first is ~Fo9o-78, close to expected equilibrium compositions; the second is ~ F070-62, clearly too Fe-rich to be in equilibrium with whole-rock compositions and composed mainly of reversely zoned crystals. In terms of NiO content, a single (low-Ni) population is observed. Chromian spinel inclusions with high Cr# and Fe^VFe^^ ratio and low TiOi are not observed within the olivines of this group.

Plagioclase and pyroxene phenocrysts are typically reversely zoned and display ubiquitous disequilibrium textures. The cores of these crystals are not in equilibrium with host basalt compositions.

The mineralogy of these two groups of basalts suggests that the first resulted from mixing between two parental magmas produced from contrasted sources, probably a relatively depleted and H2Opoor harzburgite and a less depleted and more hydrated harzburgite to Iherzolite. Subsequent evolution occurred in a magmatic system in which variations in volatile contents played a more important role than magma mixing. The second group of basalts was produced from the less depleted source only, but its parental magmas interacted extensively with remnants of earlier crystal mush/magma batches. Open-system processes operated early in the life of all these magmas, and strongly influenced their geochemistry. Magma mixing was a ubiquitous process and together with fractional crystallisation controlled the evolution of the basaltic magmas of Slamet volcano.

89

MAGMA CHAMBER PROCESSES AT TAMBORA VOLCANO, EASTERN SUNDA ARC: A GEOCHEMICAL APPROACH Viorel Paraschivoiu, John Foden Geology Department, Adelaide University, Adelaide SA, 5005

Best known for its catastrophic eruption of 1815, Tambora is an active calcalkaline volcano w^ith unusual high-K, undersaturated lavas, yielding Aze-normative trachybasalts and trachyandesites. In recent years new^ geochemical data has become available which allows a better understanding of magmatic processes governing the eruptive behavior of Tambora.

We present new geochemical data in an attempt to build a model for the magmatic processes which led to the 1815 eruption. New data support ideas put forward by previous workers (Foden, 1986, Grall-Johnson, 1997) who agree that a shallow magma chamber has cooled slowly, crystallising under equilibrium and exsolving a high pressure aqueous liquid which triggered the 1815 eruption by failure of the magma chamber roof. The 1815 eruption produced ignimbritic flows covering large areas, especially on the southeastern flanks of the volcano but also of significant extent in the northeast part. These deposits can be divided into several horizons of dark grey pumice and light yellow pumice, all of which contain xenoliths. These xenoliths are gabbros and pyroxenites and are interpreted as cumulates formed by equilibrium crystallisation of plagioclase, clinopyroxene and magnetite in the magma chamber during a dormant stage which lasted for over 5000 years (Foden, 1986, Grall-Johnson, 1997, Yamamoto et al, 2000). Foden (1986) explained the transition from the trachybasalt parent magmas to trachyandesite by crystallisation of a mineral assemblage similar to that of the xenoliths. New trace element and isotopic data are used to test this idea and is presented here in an attempt to better constrain the processes proposed by previous studies. The shift to trachyandesite is inferred here to be due to mineralogical control as suggested by the oxide-oxide plots correlated with trace-element diagrams, which show a marked decrease of CaO and Sr against MgO at 3% MgO. The idea suggested by Foden (1986) of increase in the amount of plagioclase crystallised at this point is re-examined here.

In contrast with the basalt-andesite-dacite volcanic association rich in plagioclase and hy- and Qnormative produced by Rindjani on neighboring Lombok Island, with a S i 0 2 - K 2 0 relation in reasonable agreement with the theoretical norms for a volcano in an island arc setting, the trachybasalts and trachyandesites erupted by Tambora are undersaturated. They also have a high K content coupled with relatively primitive isotopic compositions. This was explained by GrallJohnson (1997) by a mixture of three sources for Tambora magmas: a depleted mantle component, an enriched mantle component (Rb/Nb>10), and a high U/Pb component. The high K content of the lavas is explained by small melting fractions in the source in the presence of phases with large distribution coefficients for REE like clinopyroxene and garnet to allow for the observed LREE enrichment and HREE depletion. LILE enrichment is accounted for by fluid addition from the downgoing slab to the lower part of the mantle wedge, while HFSE depletion is considered to be a feature of the upper part of the mantle wedge due to previous melt extraction. The mantle wedge is thus considered to be layered, with a MORB-type depleted upper layer and lower layer enriched by metasomatism with fluids from sediment and slab dehydration.

References Foden, J.D., 1986. Journal of Volcanology and Geothermal Research, 27/1986.

Grall-Johnson, 1997. PhD Thesis, 1997, University of Rhode Island.

Takada, A., et aL, Report of International research and Development Cooperation ITIT Projects - Research On Volcanic Hazard Assessment in Asia, (Indonesia) March 2000 Yamamoto et al 2000

90

INTRA-ARC COMPRESSIVE STRESS PROMOTES DEVELOPMENT OF SLOWCOOLING, MULTIPLY-REPLENISHED LOWER CRUSTAL MAGMA CHAMBERS PARENTAL TO PORPHYRY COPPER DEPOSITS Bruce D.Rohrlach^ Robert R.Loucks^ and J. Michael Palin/'^ 1- Research School of Earth Sciences, ANU, Canberra. 2- Geology Department, University of Otago, NZ.

Porphyry Cu-Au districts in Cenozoic continent-margin and intra-oceanic arcs around the Pacific Rim are spatially and temporally linked with increases of compressive stress that are usually related to attempted subduction of relatively buoyant (thicker or warmer) lithosphere. Episodes of ore formation cluster in time and space. This metallogenic clustering is a product of shared regional compressive stress which inhibits magma ascent by sub-vertical dyke propagation and promotes development of sub-horizontal magma chambers in the lower crust, where the trapped magma proceeds to crystallise cumulates until the residual melt evolves to sufficient buoyancy to propagate sub-vertical dykes. Volcanics and epizonal plutons related to porphyry-Cu ore invariably display trace-element evidence that the melts segregated from high pressure (lower-crustal) cumulates consisting largely of Al-rich augite and hornblende ± garnet, but little or no plagioclase. Our numerical modelling of cooling rates of lower-crustal and shallow sub-volcanic chambers in arc settings with well characterised geothermal gradients shows that magma chambers in hot lower crust cool very slowly and tend to live long enough to undergo multiple million-year-scale cycles of magma replenishment and fractional crystallisation and tapping, over the course of which concentrations of "incompatible components" such as H2O, CI and SO3 accumulate to exceptional concentrations relative to major elements (Si02, AI2O3, Na20 etc). This widely applicable general model is well illustrated by our detailed studies of the Tampakan volcanic-intrusive complex and decoupled porphyry Cu and high-sulphidation Cu-Au deposits in Mindanao, southern Philippines.

In the case study of the Tampakan district, we integrate tectonic history with geological, geochronological, petrochemical, crustal stress and thermal modelling constraints to formulate a model for arc-scale porphyry Cu-Au metallogenesis in compressional arc segments. Synthesis of tectonic reconstructions, and modem plate motions from GPS data, reveal that compression commenced at Ma, peaked at Ma during subduction reversal, and is presently relaxing.

Compression was induced by transpressional docking of NE and SW Mindanao along the Cotabato fault zone and accretion of the segmented northern Halmahera arc against SE Mindanao. The peak compression spans porphyry Cu mineralisation at 4.25-4.29 Ma C^Ar-^'^Ar) and high-sulphidation Cu-Au mineralisation at 3.29-3.22 Ma ("^^Ar-^^Ar; K/Ar). We used ^^^U-^^^Pb geochronology on 471 zircon samples from throughout the Tampakan volcanic succession to parameterise time series in chemical compositions of volcanic rocks and phenocrysts, and time-series in temperature, oxygen fugacity and wt % H2O in the pre-eruptive magma over the past 7 m.y. U/Ti, U/Ge and Th/Ti ratios in dated detrital zircon grains resolve multiple million-year-scale magma rechargeand-crystallisation cycles within a long-lived lower crustal chamber. This deep chamber resides at 18-22 km depth (~5-6 kbars; Al-in-homblende geobarometry). The cyclic ramp-up and drop of those element ratios coincides with a 7 Myr-long "sawtooth" cyclic ramp-up in concentrations of volatiles and incompatible trace elements in erupted andesites and dacites. Water contents climbed from 2.9 wt % to 7.3 wt % as Si02 evolved from 57 to 67 wt %, because the accumulation of volatiles in residual melt was passed down through multiple replenishment and crystallisation cycles. The lower crustal chamber was periodically tapped to form overlying sub-volcanic chambers and 4 overprinting stratovolcanoes within the Miocene to Recent Tampakan polygenetic volcanic complex. Successive batches of increasingly H20-rich melt leaving the lower crustal chamber should begin to exsolve a hydrothermal fluid at successively greater depths. Hydrothermal fluid exsolving at greater depths is denser and more efficient in scavenging Cu from the melt, because the fluid-melt partition coefficient of Cu is extremely pressure sensitive. We conclude that metallogenic fertility at convergent margins is ultimately modulated by compressional stress which induces deep entrapment, build-up to anomalously high water contents and fluid exsolution at midcrustal depths in ascending magmas.

Acknowledgement: These results are an outcome of the first author's PhD dissertation. WMC Resources Ltd's sponsorship is gratefully acknowledged.

91

ARC-PERPENDICULAR SPREADING RIDGES IN THE NORTH FIJI BASIN W.P. Schellart, G.S. Lister and M.W. Jessell Australian Crustal Research Centre, School of Geosciences P.O. Box 28Ey Monash University, Melbourne, VIC 3800, Australia

Back-arc basins are continental or oceanic basins up to several kilometres deep and have horizontal dimensions of hundreds to several thousands of kilometres. They occur on the concave side of arcshaped topographic features. Back-arc basins form in overall convergent tectonic settings in the overriding plate during rollback of the hinge-line of the subducting plate. In most intra-oceanic back-arc basins the structural pattern is rather simple, w^ith individual spreading ridges and rift segments that strike approximately parallel to the arc. Examples include the Mariana Arc, the IzuBonin Arc, the Tonga Arc, the Kermadec Arc and the Lesser Antilles Arc. In the North Fiji Basin, however, the structural pattern is complex, with several ancient and active spreading ridges striking 70°-90° to the strike of the New Hebrides Arc (Pelletier et al 1993, 1998). These high-angle spreading ridges relocated southward during the asymmetrical opening of the North Fiji Basin. To better understand these remarkable features we have simulated the structural development of the North Fiji Basin and the New Hebrides Arc with small-scale laboratory models (analogue models), where the results have inspired us to come to several tentative conclusions.

We propose that the orientation of the high-angle spreading ridges are related to the asymmetrical opening of the back-arc basin around a hinge point, where they form close to the hinge point.

Relocation of these spreading ridges is most likely related to subduction of a buoyant topographic feature on the subducting plate (the West Torres Plateau) along the New Hebrides Trench. The initiation of subduction of the plateau started at 0.9-0.7 Ma (Meffre & Crawford 2000, 2001). This resulted in localised collision between the New Hebrides Arc and the plateau, retarded rollback of the subducting slab along the northwest comer of the trench and reduced extension and shearing in the northwest comer of the North Fiji Basin. Back-arc extension continued in the rest of the North Fiji Basin owing to continued rollback of the southem part of the subducting slab. In the basin, active extension was separated from the slightly or non-extending northwest corner by a zone striking at high angle to the New Hebrides Arc, i.e. the Hazel Holme extensional zone. This extensional zone formed after 1 Ma (Pelletier et al 1993), which closely coincides with the initiation of subduction of the West Torres Plateau. The impingement of a small scale topographic feature on the subducting plate (the d'Entrecasteaux Ridge) into the overriding plate at ~ 3 Ma only led to local deformation and fragmentation of the arc, but did not have a significant influence on the spreading pattem in the back-arc basin.

References Meffre S. & Crawford T. 2000. Ridge subduction in the New Hebrides intra-oceanic island arc: implications for the recognition of collision events in ancient arc-related sequences. Geological Society of Australia, Abstracts 59, 341.

Meffre S. & Crawford A.J. 2001. Collision tectonics in the New Hebrides Arc (Vanuatu). The Island Arc 10, 33-50.

Pelletier B., Lafoy Y. & Missegue F. 1993. Morphostructure and magnetic fabric of the northwestem North Fiji Basin. Geophysical Research Letters 20, 1151-1154.

Pelletier B., Calmant S. & Pillet R. 1998. Current tectonics of the Tonga-New Hebrides region. Earth and Planetary Science Letters 164, 263-276.

92

GEOCHEMICAL FINGERPRINTING OF MAGMATIC SYSTEMS: EXAMPLES FROM ARC-TYPE VOLCANOES. ^lan E. M. Smith, ^Susan L. Donoghue and ^Richard C. Price ^Department of Geology, University of Auckland, Auckland, New Zealand ^Department of Earth Sciences, University of Hong Kong, Hong Kong, SAR, China ^School of Science and Technology, Waikato University, Hamilton, New Zealand In arc-type volcanoes the record of magmatic evolution resides in two main components, a constructional cone facies that is composed mainly of the primary eruption products (lava flows, pyroclastic flows, pyroclastic falls) and a ring plain facies that is made up mainly of redistributed material (debris flow deposits, fluvial deposits) and distal primary tephras. In combination these two facies represent the magmatic history of the system that fed the volcano but in each the time scale and the part of the history that may be represented is different. A basic problem in unravelling the magmatic system feeding an arc-type volcano is reconciling these two facies to produce an integrated eruption history.

We have looked at data sets representing parts of the magmatic record of four major arc-type strato-volcanoes, Ruapehu and Egmont in New Zealand, Merapi in Indonesia and Rainier in western North America. These volcanoes are each located on their respective volcanic fronts with the exception of Egmont which lies at the rear of the northern New Zealand arc. Detailed analysis of the variations through time of the composition of magma erupted from these volcanoes shows clear and consistent patterns on a variety of different time scales. On a shorter time scale (of the order of 10^-10^ years) variations in composition follow patterns that are consistent with AFC processes that are logically intracrustal processes. On longer time scales (lO'^-lO^ years) compositional patterns reflect deep-seated processes that drive the evolution of the magmatic system as a whole. There are contrasts in chemical variation patterns that appear to be related to the fundamental parameters that define magmatic systems and these appear to be independent of obvious factors such as position within the arc.

For example, Egmont and Merapi volcanoes show strikingly similar patterns of geochemical evolution despite the fact that they have different positions in their respective arcs. In the Merapi system the rate of operation of the processes is an order of magnitude faster but the compositional trends are the same. Merapi and Egmont also have comparable mafic phenocryst assemblages that are most commonly augite-hornblende +olivine (+biotite). Ruapehu and Rainier volcanoes also show similar patterns of geochemical variation but these are distinct from those shown by Merapi and Egmont. Ruapehu and Rainier have mafic phenocryst assemblages that are typically dominated by hypersthene+augite and only rarely include olivine or hornblende.

Our conclusion is that it may be possible to identify characteristic geochemical behaviour that is a 'fingerprint' of a particular type of arc-type magmatic system. These geochemical 'fingerprints' are determined by a particular set of parameters that define the nature of the magmatic system but they are not specifically linked to position in the arc. In other words they are independent of their position relative to the underlying Wadati-Benioff Zone. Different time scales of the same geochemical fingerprint are a reflection of geodynamic setting, particularly of geothermal environment. Ultimately the determinant for a particular pattern of geochemical behaviour may be something as simple as the role that amphibole plays in the system. For example for an Egmont/Merapi type 'fingerprint', evolution of the system probably involved recycling of early underplated parts of the system by amphibole melting at the base of the crust whereas Ruapehu/Ranier type systems may reflect a pyroxene-dominated system higher in the crust.

93

Nd-Sr-0 ISOTOPIC AND TRACE ELEMENT CHARACTERISTICS OF THE WATERANGA MAFIC LAYERED INTRUSION, SOUTHEAST QUEENSLAND, AUSTRALIA: IMPLICATIONS FOR OPEN SYSTEM MAGMATIC PROCESSES IN A POST-COLLISIONAL SETTING Reddy V. R. Talusani and Warwick J. Si veil Division of Earth Sciences, The University of New England, Armidale NSW 2351

The origin of layered intrusions is a key problem in petrology. They are the exposed examples of solidified magma chambers and have long served as a testing ground for a wide range of petrologic problems (Wager and Brown, 1968; McBimey, 1975, 1995). The Wateranga intrusion (~243 Ma) is one of several Permo-Triassic mafic intrusions that were intruded during the evolution of the northern New England Fold Belt. Gabbro is the major component of the intrusion. The intrusion is free from metamorphic recrystallization and deformation, revealing a post-collisional character.

Based on detailed mineral chemistry, whole-rock geochemical and Sr-Nd isotopic data, the igneous stratigraphy of the Wateranga intrusion can be divided into three zones: (1) the Lower Zone, (2) the Middle Zone and (3) the Upper Zone. The Lower and Upper Zones are mainly composed of gabbro, with lesser norite, troctolite, anorthosite, orthopyroxenite and rare picrite. The Middle Zone is composed largely of norite, with lesser gabbro, troctolite, anorthosite, orthopyroxenite and rare picrite. Microprobe analyses of coexisting clinopyroxene and orthopyroxene in different rocks provide consistent P-T data defining magmatic crystallization conditions as 1120 - 1010 The intrusion represents cumulates produced from a magma chamber which is periodically refilled and subsequently fractionated. Isotopic ratios of Nd and Sr have been measured in samples spanning most of the available stratigraphy of the Wateranga intrusion in order to detect and quantify input into the magma chamber during crystallization and examine the striking facies relationship between the gabbro and norite zones. The whole-rock initial (Nd values, initial 87Sr/86Sr ratios, Mg#s of clinopyroxene, orthopyroxene and olivine, An% of plagioclase, and the whole-rock incompatible trace element abundances and ratios vary with depth and display broadly parallel trends through Zones 1 and 2, and into Zone 3. Abrupt reversals occur across the zone boundaries, indicating open-system addition of new mafic magma. Nd and Sr isotopic data indicate that the intrusion formed from at least three geochemically distinct magmas. Initial (Nd values in the analyzed whole-rock samples range from +3.26 to +6.44, initial 87Sr/86Sr ranges from 0.70262 to 0.70491, and (180 ranges from +6.21 to +9.65%o. The Nd-Sr-O isotopic variations observed in the Wateranga intrusion are best explained by a model involving variable amounts of lithospheric mantle involvement in addition to asthenospheric mantle components.

Chondrite-normalized REE patterns of the intrusion show strong similarities with continental tholeiites. The rocks of the intrusion are characterized by depletion of Nb, P and Ti. These anomalies do not appear to be the result of HFSE fractionation during melting in the source. This is best explained by the enrichment of the lithospheric mantle by subduction-derived fluids and its subsequent involvement in extensional magmatism, where asthenospheric melts had interacted with the subduction-modified lithospheric mantle during ascent. The geodynamic model to account for the Wateranga tholeiitic magmatism involves a post-subduction, Permo-Triassic asthenospheric rise due to a lithospheric thinning associated with extension. This model has important bearing for our understanding of post-collisional basic magmatism in the northern New England Fold Belt.

References McBimey A. R. 1975. Differentiation of the Skaergaard Intrusion. Nature 253, 691-694.

McBimey A. R. 1995. Mechanisms of differentiation of layered intrusions: Evidence from the Skaergaard Intrusion. Journal of Geological Society of London 152, 421-435.

Wager L. R. and Brown G. M. 1968. Layered Igenous Rocks. Edinburgh: Oliver and Boyd, 588 pp.

94

ULTRA-FAST SOURCE-TO-SURFACE MOVEMENT OF MELT AT ISLAND ARCS FROM Ra-Th SYSTEMATICS Simon Turner Department of Earth Sciences, Wills Memorial Building, University of Bristol, Bristol, BS8 IRJ, U.K. E-mail: simon.turner@bris.ac.uk

The velocity of melt ascent from its source through Earth's mantle and crust to the surface is extremely hard to determine, even though such measurements would place important constraints on the mechanisms of melt transport and the physical behavior of the mantle during partial melting.

Island arc lavas have Ra-excesses that extend to higher values than those observed in mid-ocean ridge or ocean island basalts. The Ra-excesses are largest in the most primitive lavas and positively correlated w^ith the fluid-sensitive index Ba/Th and therefore appear to have been introduced into the base of the mantle melting column by fluids released from the subducting plate.

Preservation of the Ra signal requires transport to the surface arguably in only a Uw hundreds of years and directly constrains the average melt velocity to the order of 1000 metres per year. Thus, melt segregation and channel formation can occur rapidly in the mantle and island arc melting models need to incorporate high melt ascent velocities. Such conditions probably reflect the vigor of fluid-induced melting compared with mid-ocean ridges and ocean islands where melting rates are dictated by upwelling rates on the order of a few centimetres per year. However, it may be that these velocities are generally applicable, in which case the widely held notion of basalts migrating slowly through the mantle in interconnected but vanishingly small pore spaces will have to be abandoned. The island arc Ra-excesses also decrease with increasing magmatic differentiation. If these decreases reflect the time taken for crystal-liquid differentiation, then this must take < 8,000 years and might be almost instantaneous if such decreases are due to mixing processes. Thus, if melts do stall in the crust it is generally for short periods of time (< 8000 years) and it appears that the majority of island arc basalts and andesites spend less than a few thousand years traversing the crust. This leaves very little time for crustal interaction (e.g. MASH) which must either occur very quickly or else may be restricted to continental areas with very thick lithosphere.

95

ISOTOPIC CONSTRAINTS ON ELEMENT MOBILITY WITHIN SUBDUCTION ZONES Jon D Woodhead School of Earth Sciences, The University of Melbourne, VIEPS, VIC 3010, Australia The high field strength elements, Nb, Ta, Zr, Hf, and Ti have long held a special fascination for arc geochemists. Empirical observations, coupled with experimental studies, have led to these elements being considered the type example of a larger group of so-called 'conservative' elements w^hich are assumed to remain largely immobile during slab dehydration reactions. As a result the HFSE are often invoked as the key to seeing through chemical variations caused by the subducting slab providing, in essence, a 'window' into the pre-subduction nature of the mantle wedge.

The element Hf is unique among this group in combining high 'field strength' (charge relative to ionic radius) character with considerable potential as an isotopic tracer of both source and process in subduction zone systems. Until very recently, however, the arduous nature of Hf isotope analysis by thermal ionisation mass spectrometry (TIMS) has hindered progress in this field, so that there have been comparatively few published Hf isotope data for arc lavas. A detailed Hf isotope study of oceanic arc lavas and paired arc/back-arc settings has been conducted using a Nu Plasma Multicollector ICPMS and hafnium separation procedures developed at the University of Melbourne.

Here it is demonstrated, contrary to expectations, that the Hf isotopic compositions of arc lavas are always displaced significantly from their co-existing back-arc spreading centres which can be considered to sample the local mantle. This is true not only of those arcs in which direct sediment melting or AFC-like processes within the crust are implicated, but also in low-K tholeiitic arcs where hydrous fluids are believed to be the dominant medium of slab-to-mantle transport. This observation calls into question the concept of 'conservative' or 'immobile' elements and suggests that some transfer of material from the subducting slab into the sub-arc mantle wedge probably occurs for almost all elements. These conclusions have significant implications for models of arc geochemistry and it is apparent that considerable shifts in currently accepted thought may well be required to accommodate the emerging data.

96

CHARACTERISTICS OF ARC MAGMATISM AS EXEMPLIFIED BY THE CENOZOIC VOLCANISM AND PLUTONISM IN THE JAPANESE ISLANDS Takeru Yanagi Department of Earth and Planetary Sciences, Kyushu University 812-8581 JAPAN An arc-trench system is a site of intense igneous activity and also a present-day site of active continental grovv1;h. The igneous activity there has two modes of occurrence. One is volcanism, which produces a large amount of volcanics on the volcanic arc. Rocks of the calc-alkaline series predominate on mature arcs. The other is the emplacement of granitic plutons, which are found in both eroded mature volcanic and non-volcanic frontal arcs. Igneous activity there can transform magma composition from that of basaltic primary magma formed in the mantle through that of calc-alkaline magmas and finally to the specific composition of the average upper continental crust.

The most probable differentiation scheme of this volcanic activity is open-system crystallization differentiation in a refilled chamber, which is periodically fed with primary magma from the mantle and from which cumulate is continuously removed.

A ridge of the arc-trench system generally has double configuration: a volcanic arc lying on the inner margin and a frontal arc lying on the outer margin of the ridge. This configuration does not depend on the maturity of island arcs. Physiographic configuration of island arcs indicates how continental grow1:h occurs in the island arc. Growth alternates between the volcanic and frontal arcs. Miocene igneous activity in Japan indicates that the emplacement of granitic plutons occurs alternately in the volcanic and frontal arcs. The volcanic arc is geologically recorded as a belt of intimate association of volcanic rocks and subsequent granitic plutons, and the outer arc as a belt of intimate association of low-P/high-T metamorphic rocks and granitic plutons with few or no contemporaneous volcanic rocks. The convective flow induced by slab subduction seems to indispensable for the alternate growth of the volcanic and frontal arcs. The alternate growth seems to be due to the rate of magma supply. A high rate may result in the growth of the volcanic arc and a low rate may result in the grow1;h of the frontal arc.

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THE LINK BETWEEN SUBDUCTION AND CARBONATITES Greg Yaxley^ and Gerhard Brey^ ^Research School of Earth Sciences, The Australian National University, Canberra ACT 0200 ^Institut fiir Mineralogie, Universitat Frankfurt, Senckenberganlage 28, Frankfurt/M 60054, Germany.

Carbonate-bearing eclogite may be an upper mantle source for some carbonatites because of the similarity of their radiogenic isotope compositions to HIMU ocean island basalts (OIBs). The HIMU signature has been linked to the emplacement of previously subducted oceanic lithosphere into the source regions of OIBs, after long term storage (1-2 Ga) in the mantle.

Calcitic carbonate is a common constituent of altered oceanic basalt. Earlier experimental studies demonstrated that during subduction of carbonate-bearing mafic oceanic crust to eclogite facies conditions, calcite will recrystallise as calcite-dolomite solid solution, and will form a residual, refractory phase in an eclogitic residue in equilibrium with hydrous, siliceous partial melts (rhyodacites to dacites). It will therefore survive subduction into the upper mantle without melting or decarbonation. Subduction is therefore a viable means of recycling crustal carbonate back into the mantle, where it may be emplaced into carbonatite source regions.

We have experimentally determined the high pressure (2.5-5.5 GPa) phase and melting relations of a garnet + clinopyroxene + carbonate assemblage with the aim of assessing the feasibility of carbonated eclogite as a source for some crustally emplaced carbonatites. The solidus of our composition was at 1100°C at 2.5 GPa, 1220°C at 3.5 GPa and 1330X at 5.0 GPa. Carbonate melts were sodic calcio-dolomitic in composition, and were markedly more calcic than the sodic dolomitic carbonate melts produced by partial melting of carbonated peridotite.

Our solidus temperature represents an upper limit for the solidi of natural, carbonate-bearing eclogites in the upper mantle. Increasing whole rock alkali contents will decrease the carbonate solidus temperature at a given pressure. Altered MORBs typically have more Na20 than our experimental composition (2-3 wt% cf 0.9 wt%). Eclogite xenoliths, brought from the lithosphere to the surface by kimberlites and possibly representing recycled oceanic crust, have similar Na abundances to MORE. Also, natural upper mantle carbonate-bearing eclogite may contain volatiles such as F" and H2O, which lower carbonate melting temperatures.

The potential temperature of ambient convecting upper mantle is generally assumed to be 1280°C. Our carbonated eclogite solidus is about 70°C below this adiabat at 5.0 GPa, conditions well above the peridotite-C02 solidus. The solidi of putative natural carbonated eclogites are likely to be further below the adiabat. Therefore, if discrete bodies of carbonated eclogite are present in the peridotite-dominated convecting upper mantle, the carbonate would be molten in the upper 250 or more kilometres. Carbonate melt is expected to segregate from its source eclogite at very low melt fractions and infiltrate surrounding peridotitic wall rock. If PT conditions are above the peridotite-C02 solidus, the carbonate liquid will remain molten as it passes into the peridotite, but its composition will change. Depending on PT conditions, liquids in equilibrium with wall rock peridotite may be sodic dolomitic carbonatite, or carbonated undersaturated silicate melts such as olivine melilitites, olivine nephelinites or kimberlites.

These liquids will carry the isotopic signature of their source, i.e. oceanic crust recycled through subduction.

These results support the involvement of ancient, carbonated, recycled oceanic crust in the petrogenesis of some crustally emplaced carbonatites and associated undersaturated silicate magmas, and possibly some kimberlites.

98

ACTIVE MOUNTAIN BUILDING IN THE HIMALAYA OF NEPAL J-P Avouac LDG, CEA, Bruyeres-le-Chatel, France Over the last decade a number of investigations have been carried on in the Himalaya of Nepal that shed some light on on-going orogenic processes. Seismological monitoring has revealed a belt of intense microseismic activity which follow^s the front of the high range Interseismic straining was documented from geodetic measurements. Geological and geophysical investigations have highlighted the geometry of crustal structures and deformation cumulated over several seismic cycle was quantified from a geomorphologic approach. We show that these data support the view that most of the deformation is localized along a major thrust fault that is probably responsible for the recurring M>8 earthquakes, such as the 1934 Bihar-Nepal event which partly destroyed Kathmandu. In the sub-Himalaya, abandoned Holocene terraces have recorded active folding with uplift rates up to 1.5 cm/yr. It shows that as much 21.5+/-1.5 mm/yr of horizontal shortening is accommodated by localized slip along the MFT. Structural geology suggests that this fault emerges from a decollement at the top of the Indian basement, at 5-6 km depth, extends nothwards beneath the Lesser Himalaya and roots into a mid-crustal sub-horizontal shear zone beneath the Higher Himalaya and southern Tibet that could be imaged from seismic experiments in southern Tibet. Incision rates along the major rivers across the Himalaya of Nepal are in keeping with this geometry : little incision in the Lesser Himalaya and Southern Tibet is observed and up to 5lOmm/yr in the Higher Himalaya, a pattern consistent consistent with 20-23mm/yr of slip along the ramp-and-flat geometry of the MHT. By contrast, geodetic data show that, during the interseismic period, horizontal shortening is mainly absorbed within a 100 km wide zone that spans over the Higher Himalaya, while deformation in the sub-Himalaya and southern part of the Lesser Himalaya seems negligible. It indicates that the fault is fully locked with horizontal shortening being mainly absorbed within a 100 km wide zone that spans over the Higher Himalaya. This information is reconciled by a mechanical FEM model in which we consider a lithospheric section with a realistic rheology, submitted to horizontal shortening. The model accounts for surface processes and the thermal structure of the lithosphere. In this model, on the long term, provided that friction on the MHT is low (less than 0.2-0.3) shortening across the Himalaya is essentially accommodated by localized frictional slip along the MHT in the brittle upper crust and by ductile flow in the lower crust beneath the high range and southern Tibet. During the interseismic period, the MHT is fully locked from the sub-Himalaya to beneath the Higher Himalaya. Microseismic activity is enhanced in the zone of increasing Coulomb stress. Elastic strain and stress accumulate until it is released by the M-8 earthquakes during which deformation is transferred up to the foothills.

99

COMBINED OSMIUM ISOTOPIC AND SEISMIC EVIDENCE FOR ORPHANED EARLY PROTEROZOIC MANTLE BENEATH PHANEROZOIC CRUST IN THE NEW ENGLAND FOLD BELT, EASTERN AUSTRALIA V. C. BennettV B.L.N. Kennett\ and R.W. Carlson^ ^Research School of Earth Sciences, The Australian National University, Canberra ACT 0200 Australia Email: vickie.bennett@anu.edu.au) ^Department of Terrestrial Magnetism, Carnegie Institution, Washington, DC; We are combining advances in geophysical and isotopic imaging techniques to reveal age and structural features in the deep lithosphere, with the goal of understanding the complex processes leading to the origin, modification and preservation of continents. In Eastern Australia the New England fold belt (NEFB), comprises a >1300 km long by >200 km wide composite terrane, assumed to represent sequential accretion of arc fragments along the eastern margin of Gondwana during the Late Paleozoic to Early Mesozoic. The oldest crystallisation ages of c. 530 Ma are from ophiolite associated plagiogranites. Tomographic images of the velocity structure of the mantle in this region have been produced as part of the SKIPPY portable seismic array project. These data reveal that the lithospheric mantle beneath portions of the NEFB is anomalous compared with the surrounding eastern third of the Australian continent and adjacent ocean regions and is characterised by faster S wave speeds. This signature may originate from several effects including colder temperatures related to the presence of older lithospheric mantle.

To constrain the origin of these seismic signatures and the relationship between crust and underlying lithospheric mantle age structures, we have determined Re and Os concentrations and Os isotopic compositions from 25 spinel peridotite xenoliths hosted by Tertiary basalts within the NEFB and adjacent terranes. Olivine plus spinel mineral separates were analysed, rather than whole rocks, to avoid ambiguities associated with secondary sulphide mobility or metasomatism.

Isotopic compositions and concentrations were determined using high precision isotope dilution combined with carius tube digestion and TIMS methods. Xenoliths from localities within the NEFB exhibit a range of model ages; the most tightly constrained ages from the lowest Re/Os samples with some samples indicating melt extraction ages >2.4 Ga. This is in contrast to xenoliths with similar low Re/Os ratios but with Phanerozoic model ages from terranes to the north and south of the NEFB. Although the xenoliths provide direct samples of only very limited volumes of lithospheric mantle and may represent only localised preservation of ancient lithosphere, the coincidence of old Os model ages combined with the regional anomalous fast S wave signature suggests that very large portions of eastern Australian may be underlain by ancient lithospheric mantle. These ages are not recorded in the overlying crust. Thus, the widespread presence of Proterozoic mantle beneath Phanerozoic crust on the eastern margin of Australia would reflect large scale decoupling of crust from upper mantle lithosphere. Proterozoic lithospheric mantle in the NEFB could represent remnants of lithosphere displaced from older terranes now present to the south or west, or it may represent the roots of allochthonous continental fragments amalgamated to eastern Australia.

Evidence for large scale decoupling of crust and lithospheric mantle raises questions on the meaning of continental growth histories based only on upper crustal samples, as well as posing new complexities, and opportunities, for continental reconstructions.

100

MEXICO - TASMANIA CORRELATIONS. R.F. Berry and C.F. Burrett School of Earth Sciences, University of Tasmania, Hobart, Tas 7001, Australia

Tasmania forms an enigmatic province within the Neoproterozoic to Cambrian history of Australia. It lies at the boundary between Australia and North America in most Rodinia reconstructions. The AUSWUS reconstruction places Tasmania against Mexico while the SWEAT reconstruction places Tasmania near western Canada.

The Proterozoic stratigraphy of Tasmania has some common features with South Australia but there are major differences. Recent re-evaluation of the geology of King Island has removed one major problem in the correlation of the Adelaide Fold Belt with NW Tasmania. The Wickham Orogeny (760 Ma) is now regarded as a minor event related to granitoid emplacement and not a substantial regional defomiation. Low angle unconformities in the Neoproterozoic of Tasmania may be related to this event.

Both Tasmania and western USA have a range of shallow water sequences and multiple basalt events that scatter through the Neoproterozoic with minor unconformities. The new dating on King Island indicates a metamorphic event at 1270 Ma slightly younger than the East Kootenay Orogeny in Canada but similar to the Elzeverian Orogeny in Texas. The Wickham Orogeny has features similar to the Goat River Orogeny in Canada. Two orogenic events are recognised in Mesoproterozoic Australia, an earlier 1300 Ma event and a later 1100-1000 Ma event. The metamorphism on King Island may correlate with the earlier orogenic events. The nearest known examples, to Tasmania, of this event are in the eastern Albany Fraser Province and the Musgrave Ranges.

Detrital zircon age spectra are available from a range of Neoproterozoic and Cambrian sandstones across these provinces. There is published data available from the Adelaide Fold Belt, Amadeus Basin, Tasmania and western USA. Despite probable bias in the sample selection and differences in analytical method, these data sets provide a measure of provenance similarity between the areas.

The spectra were compared using the Kolmogorov-Smimov test (cf. Berry et al 2001). Many detrital age spectra from Tasmania are extremely similar to the USA samples. In particular, in sandstones deposited before the breakup of Rodinia, the samples from Mexico are extremely similar to Tasmanian sandstones. For example, the El Alamo Formation has an 86% chance of containing the same zircon population as the Bowry Formation, Tasmania. The samples from the Adelaide Fold Belt are similar to the western USA but not to the same extent. The Marino Arkose has a 62% probability of being the same zircon population as the Aguila Unit in Mexico. Despite this, the Tasmanian samples do not match the South Australian samples very well although three samples from Tasmania are very similar to the Heavitree Quartzite.

Detrital zircon inheritance spectra from western Canada are very different from the South Australia and Tasmania spectra. The similarity of zircon province data between USA, South Australia and Tasmania is not explained by the SWEAT hypothesis where Tasmania is adjacent to NW Canada.

References Berry R. P., Jenner G. A., Meffre S. and Tubrett M. N. 2001. A North American provenance for Neoproterozoic to Cambrian sandstones in Tasmania? Earth and Planetary Science Letters 192, 207-222.

101

AGE AND PLATE TECTONIC SETTING OF THE LABUAN BASIN. ^I. Borissova. ^M.F. Coffin, 'P.A. Symonds and ^J. Sayers 'Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Australia ^Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan

The Labuan Basin lies in deep water adjacent to the eastern Kerguelen Plateau. The basin is about 800 km long and 300 km wide and contains up to 4.5 km of sediment. A general lack of geophysical data and geological samples in this remote basin has inhibited understanding of its stratigraphy and crustal origin. Our new seismic stratigraphic interpretation of the Labuan Basin is based on deep multichannel seismic data collected by Geoscience Australia in 1997 during Rig Seismic surveys 179 and 180 integrated with results of Ocean Drilling Program (ODP) Leg 183 (1998-1999).

The new data reveal that the character of Labuan Basin basement differs significantly in its eastern, western and southern parts. The western Labuan Basin is extensively faulted and its seismic stratigraphy is similar to that of the adjacent Kerguelen Plateau. The eastern Labuan Basin is characterised by large NW-SE trending basement ridges of probable intrusive origin, separated by deep lows. The thickest sediments are present in the easternmost part of the basin. The basal sequences here are older than those in the western part and may be partly volcanogenic. The southern part of the Labuan Basin is characterised by smooth flat-lying basement at 7.5-8.0 s TWT, which is similar to basement proximal to the Antarctic margin to the west of Bruce Rise.

Good correlation between Cretaceous sedimentary successions on the Kerguelen Plateau and in the Labuan Basin suggests that the Labuan Basin basement is likely to be as old as that of the adjacent Kerguelen Plateau, where it is dated as Aptian to Albian. The inferred presence of Lower Cretaceous sediments in the Labuan Basin contradicts most plate tectonic reconstructions, which consider the Labuan Basin to have formed in Late Cretaceous or even Cenozoic time.

Prior to breakup between Australia and Antarctica the Labuan Basin was conjugate to the Diamantina Zone - an area of extremely rough topography lying to the south and west of the Naturaliste Plateau, southwest of the Australian mainland. Comparison of the Labuan Basin and Diamantina Zone shows distinct similarities in their structural style. Recent sampling of the Diamantina Zone by scientists aboard N / 0 Marion Dufresne (cruises 80 and 110) recovered peridotites in a narrow zone of basement ridges restricted to its southern part. Pre-breakup plate tectonic reconstructions show that prominent basement ridges in the conjugate (northeast) part of the Labuan Basin are likely to be part of the same peridotite zone. A peridotite composition for the easternmost basement ridges in the Labuan Basin is consistent with the absence of magnetic anomalies over these features. ^^Av/^^Av dating of peridotites in the Diamantina Zone (Beslier et al., 2001) suggests a Cenomanian age. Exhumation of peridotites corresponds to the period of maximum extension in the Diamantina Zone/Labuan Basin preceding Campanian breakup, which marked the start of ultra-slow spreading in the Great Australian Bight. Interpretation of the seismic data indicates that the extensional terrain formed earlier, probably in Aptian to Albian time.

The Labuan Basin/Diamantina Zone were surrounded by continental fragments, such as Naturaliste Plateau, Bruce Rise, Elan Bank, and parts of the southern Kerguelen Plateau. Our analysis of different plate tectonic scenarios suggests that portions of the Labuan Basin may be underlain by continental or transitional crust.

102

TECTONIC EVOLUTION OF THE BAYANKHONGOR OPHIOLITE, CENTRAL MONGOLIA: IMPLICATIONS FOR THE PALAEOZOIC CRUSTAL GROWTH OF CENTRAL ASIA Craig Buchan''\ Jorg Pfander^ Dickson Cunningham^ Tim Brewer^ and Brian F. Windley^ ^Tectonic Special Research Centre, Curtin University of Technology, Perth, WA. ^Max-Planck-lnstitut fur Chemie, Mainz, Germany. ^University of Leicester, Leicester, UK.

The mechanism of continental growth of Central Asia is currently debated between models invoking continuous subduction-accretion, or punctuated accretion due to closure of multiple ocean basins. Ophiolites in Central Asia may represent offscraped fragments in an accretionary complex or true collisional sutures. The Bayankhongor ophiolite, a NW-SE striking sublinear belt 300 km long and 20 km wide, is the largest ophiolite in Mongolia and possibly Central Asia. The Bayankhongor area is divided into four major lithotectonic units based on interpretation of structural and lithological data from three cross-strike transects: Baidrag complex, Burd Gol, Bayankhongor Ophiolite, and Dzag zones. The Archaean Baidrag complex comprises tonalitic granulites and metasediments. The Burd Gol zone is a metamorphosed sedimentary and igneous melange. The Bayankhongor zone contains the dismembered ophiolite forming a serpentinite melange. The Dzag zone consists of asymmetrically folded chlorite-mica schists resembling metaturbidites. The structure is dominated by steeply dipping, NE directed thrusts and NE-vergent folds.

The data suggest the Bayankhongor ophiolite marks the closure of an ocean separating two microcontinents: the Baidrag complex with the Burd Gol accretionary complex to the south, and a northern continent which forms the basement for the Hangai region. Subduction was towards the SW with NE-directed ophiolite obduction onto a passive margin represented by the Dzag zone.

Geochemical and Nd isotope studies of the ophiolitic rocks suggest that they were derived from a heterogeneous mantle source composed of a depleted N-MORB and enriched E-MORB component. A model for the tectonic setting of the ophiolitic rocks is presented in which the NMORB rocks represent melts produced at a mid-ocean ridge, whilst the E-MORB rocks represent off-axis flows or melts produced at transform ridge intersections. Basalts from the Delb Khairkhan melange have island arc-like chemistry and provide evidence that the ophiolite may have been trapped within a supra-subduction setting prior to obduction.

New ^^^Pb/^^^Pb zircon evaporation ages for granites and rhyolite dykes that intrude the ophiolite and its neighbouring lithotectonic units, suggest that the ophiolite was obducted at c. 540 Ma at the beginning of a collisional event that lasted until c. 450 Ma. The new data combined with that of previous studies indicate regional correlation of isotopic ages north-westward from Bayankhongor to southern Tuva. These data record oceanic crust formation at c. 570 Ma, followed by approximately 30 million years of subduction-accretion that culminated in obduction of ophiolites, collision related metamorphism, and magmatism in the period c. 540-450 Ma. Correlation of isotopic-age data for the ophiolites of western Mongolia and southern Tuva suggest that the ophiolites define a major collisional suture in the Central Asian Orogenic Belt which helps define the southern and western margins of the Hangai continental block.

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TERRA AUSTRALIS OROGEN: RODINIAN BREAKUP AND SUBDUCTION INITIATION IN THE PACIFIC OCEAN Peter A. Cawood Tectonics Special Research Centre, Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6001, Australia Email: p.cawood@info.curtin.edu.au

The Pacific Ocean formed through Neoproterozoic rifting of Rodinia and despite a long history of plate convergence this ocean has never subsequently closed. The record of ocean opening through continental rifting and the initiation of subduction are preserved in the Neoproterozoic to late Palaeozoic Terra Australis Orogen. This orogen extends from the northeast coast of Australia south through Tasmania, New Zealand and the Trans Antarctic Mountains across southern Africa and into South America. It has an along strike, pre-dispersal length of approximately 10,000 km and an across strike width of up to 1500 km. It incorporates the Adelaide fold belt and its along strike equivalent the Ross fold belt, the Lachlan, Thompson and Tuhua fold belts and the New England fold belt, the Cape Basin of Southern Africa, and the Cordillera Frontal, Precordillera, and Pampeanas of South America. The termination of the Terra Australis Orogen at 300 ± 20 Ma was associated with merging of Gondwana and Pangea and is represented by a pan-Pacific Gondwana margin orogenic event (e.g. Hunter-Bowen orogeny). This marked a stepping out in the position of the plate boundary and commencement of the classic Late Paleozoic to Mesozoic Gondwanide Orogen. The Terra Australis Orogen can be divided into a series of basement blocks of either continental or oceanic character that can be further subdivided on the basis of geographic affinity (e.g. Laurentian vs Gondwanan) and proximity to inferred continental margin sequences (e.g. periGondwanan vs intra-oceanic). These divisions reflect initial tectonic setting and provide an insight into the character of the orogen through time. The orogen incorporates elements that are inferred to have lain outboard of both West and East Laurentia within Rodinia. Timing of the rift to drift transition for the South American elements of the orogen range from 570 to 530 Ma whereas estimates of the timing of this event for the East Australian/Antarctic elements range from 760 to 580 Ma. The younger ages from the East Australian segment are based on geochemical and age data for rift-related igneous activity. However, what is uncertain is whether this represents opening of the Pacific Ocean or rifting of a continental ribbon from the East Gondwana margin into a Pacific ocean that had already opened in the mid-Neoproterozoic. Subduction was established at or close to the Gondwana margin by 530 Ma but an earlier and perhaps ephemeral phase may have commenced at around 570-590 Ma. If evidence for this latter phase were substantiated then it would require a wide Pacific ocean to be already established at this time.

Disruption and deformation of the continental margin successions and the initiation of subduction and convergent plate margin activity within the Terra Australis Orogen corresponds with a period of global plate reorganization associated with final breakup of Rodinia and assembly of Gondwana.

This involved opening of the lapetus ocean, final assembly of Gondwana through closure of the Mozambique ocean, and rifting of Siberia off northern Laurentia. The temporal equivalence of this series of end-Neoproterozoic Wilson cycle stages suggests they may be interdependent and that their far-field effects could have influenced subduction initiation in the Pacific. The margins of the Mozambique Ocean were aligned approximately orthogonal to the trend of the East Gondwana margin preserved in the Terra Australis Orogen. With collision of East and West Gondwana along the Mozambique Belt, continued plate motion may have been transferred to the East Gondwana Pacific Ocean margin resulting in strike slip deformation and margin decoupling. The initiation of subduction in the late Neoproterozoic to Early Cambrian marks the inception of the Pacific 'ring of fire', yet throughout the Phanerozoic the Pacific has remained a major ocean basin. Although the Pacific has been cited as an example of the declining stage of the Wilson cycle of ocean basins, its protracted history of ongoing subduction contrasts with the clear evidence for opening and closing of oceans preserved in the lapetus/Atlantic and Tethyan realms. The Terra Australis and other orogens that bound the Pacific are accretionary orogens and did not form through the classic Wilson cycle of ocean closure and continental collision.

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IDENTIFICATION OF NEW TERRAINS IN THE SOUTHERN ARUNTA PROVINCE, CENTRAL AUSTRALIA Dorothy Closed Ian Scrimgeour^ Christine Edgoose\ Andrew Cross^ Jon Claoue-Long^ and Tony Meixner^ 'Northern Territory Geological Survey, PO Box 2655, Alice Springs, NT 0871, Australia ^Geoscience Australia, PO Box 378, Canberra, ACT 2601, Australia The 1690-1600 Ma Southern Arunta Province lies on the southern margin of the North Australia Craton. Combined mapping, targeted geochronology and geophysical interpretation by the NTGS in conjunction with Geoscience Australia in the western section of the Southern Arunta Province has resulted in further subdivision into two east-west trending terrains, the boundary of which is defined by a marked change in regional gravity signature.

The Haast Bluff Terrain in the south forms a westerly continuation of the 1690-1660 Ma Maddems Yard Metamorphic Complex of the central Southern Arunta Province. In the east, the terrain is dominated by migmatitic orthogneisses, yielding SHRIMP U-Pb zircon ages of 1690-1660 Ma and includes minor metasediments. To the west, a felsic volcanic complex including extrusives, subvolcanic intrusives and chemical sediments has a SHRIMP U-Pb zircon age of 1679 ± 3 Ma. The Yaya Terrain in the north comprises packages of psammites and pelites, calcsilicates, quartzites, mafic granulites, biotite-sillimanite-garnet pelites and massive cordierite pelites.

Detrital zircon dating indicates a maximum depositional age of 1660 Ma with age spectra indicating the Haast Bluff Terrain as the likely source. Voluminous intrusion of chamockites, mafic plutons and high K calc-alkaline granites and granodiorites into the sediment package occurred at 1640-1630 Ma. Metamorphism during the 1640-1630 Ma Liebig Event varied across the two terrains with the Yaya Terrain reaching conditions of 900°C and 10 kbars (Scrimgeour et al, this volume). The Haast Bluff Terrain shows an abrupt change in metamorphic grade suggesting further division into an upper amphibolite facies domain in the east (the Glen Helen domain) and a greenschist facies domain in the west (the Kuta Kuta domain). Following the Liebig Event, the Haast Bluff Terrain was exhumed allowing the deposition of a sequence of interbedded sandstones, mudstones and arenites (the Ikuntji Metamorphics). Detrital zircon data from the uppermost beds indicates a maximum depositional age for the sequence of 1640 Ma. Both terrains were pervasively reworked during the Chewings Orogeny under amphibolite conditions, producing a strongly non-coaxial regional fabric. SHRIMP U-Pb zircon rims provide the first well-constrained age of 1589 ± 4 Ma for this event. Limited vergence indicators suggest tectonic transport to the south during the Chewings Orogeny.

Contrasting Nd model ages and protolith ages indicate the Southern Arunta Province does not form the northern extension of the Musgrave Block. Nd model ages of 2.1-2.3 Ga and protolith ages of 1690-1600 Ma have been obtained for the Southern Arunta Province whilst the Musgrave Block is characterised by Nd model ages of 1.7-1.9 Ga and protolith ages of 1600-1500 Ma. The dynamic geological history of the Southern Arunta Province from 1690-1590 Ma is characterised by cyclic deposition, magmatism and metamorphism suggesting an active tectonic setting consistent with a convergent plate margin. The plate margin event is interpreted to represent the Cordilleran style accretion of the Southern Arunta Province onto the North Australian Craton, during compressional events at 1640 and 1590 Ma. Synchronous extension and deposition in the North Australian platform cover sequences suggest a continental back arc environment in response to these events.

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TECTONIC MODEL FOR WIDESPREAD GRANITE MAGMATISM AND HIGH-r METAMORPHISM IN THE TASMANIDES W. J. Collins Department of Geology, University of Newcastle, Callaghan, NSW, 2308, Australia

The Tasmanides of eastern Australia represent a period of rapid creation of continental crust, when almost 30% of the Australian continent grew in 300 Ma. Accounting for this creation mechanism requires a tectonic model that not only accommodates the structural and tectonostratigraphic constraints, but also the thermal aspects, including the widespread granite magmatism and high-J metamorphism. Even the typical regional greenschist metamorphism requires anomalous average geotherms of 35-40°C/km.

Consensus tectonic models for the Tasmanides are gradually emerging. Most workers now agree that the Tasmanides formed in a convergent margin setting and that the Lachlan Fold Belt (LFB) formed by closure of a large backarc system behind a long-lived subduction system, the New England Fold Belt (NEFB), but the location and significance of subduction zones remains contentious. Some consider a single outboard zone existed throughout the orogenic history, requiring that widespread deformation and granite generation be mostly intraplate, but such models suffer greatly from an adequate mechanism for the widespread granites and mafic rocks, and the migrating magmatic waves across the Lachlan. Realisation that many granites are associated with

primitive mafic rocks of arc or backarc affinity, variably contaminated by deeply buried Ordovician turbidites, favours models of multiple subduction zones in the Tasmanides.

Another major conundrum is the broad stratigraphic correlation of Ordovician turbidites over the east and central LFB, within a region of inferred multiple subduction zones. This does not support "Turkic-type" continental growth models of migrating sedimentary (accretionary) wedges prograding ahead of migrating arcs. In contrast, a Bengal Fan environment has been advocated to explain the widespread continuity of LFB turbidites, but the sediments interfinger with the welldeveloped, intra-oceanic Macquarie arc in the eastern LFB. This can be resolved if one considers the LFB as a vast Early Paleozoic oceanic arc-backarc system, similar to the Philippine Plate or SW Pacific, which was subsequently flooded by turbidites, but then segmented into subducting plates. In this model, the NEFB remains as the leading-edge, subduction-accretion complex.

An initial oceanic backarc system developed during the Early Cambrian, in response to slab rollback in the Paleo-Pacific (Panthalassan) Ocean, creating the Kanmantoo Trough and outboard oceanic arc, but was partially closed at -500 Ma (Delamerian Orogeny) as the Selwyn block moved inboard, perhaps induced by ocean plateau accretion. Following plateau accretion(?), Panthalassan slab roll-back re-initiated and produced the remainder of the LFB during Early Ordovician times.

The rising Delamerian highlands flooded this vast Cambrian-Ordovician oceanic arc/backarc system with turbidites, producing buoyant oceanic crust. This crust resisted subduction following segmentation into two microplates during intense Early Benambran deformation and crustal thickening at -460-450 Ma. East-directed subduction of the inboard plate (west LFB) continued orogenic contraction in that region from Early Silurian until Late Devonian times. On the outboard microplate (east LFB and NEFB), repeated Silurian-Permian extension-contraction cycles (tectonic switching), probably associated with intermittent arrival and subduction of relatively buoyant ocean plateaus, progressively converted the remaining arc/backarc crust, then the long-lived accretionary prism, into a series of overprinting, but outboard-stepping, narrow, overlapping orogenic belts.

Widespread extensive magmatism and high-7 metamorphism in the east LFB and NEFB is related to tectonic switching, when ongoing extension associated with protracted slab retreat decompressed and semicontinuously melted the asthenosphere, producing an enormous, protracted advective heat flux that persisted from Cambrian (530 Ma) to Triassic (230 Ma) times. Periodic ocean plateau subduction induced transient contraction and thickened the crust during successive "deformation events" (Benambran, etc). Early Silurian (430 Ma), E-directed subduction initiation of the inboard slab (west LFB) produced the enigmatic high-J Omeo Metamorphic Complex and associated S-type plutons. As this slab peeled away westward, posttectonic magmatism migrated in that direction, terminating with the Late Devonian (360 Ma) granites in the Bendigo zone of the west LFB.

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GLOBAL CHANGES: POSSIBLE EFFECTS OF TWO SUPERCONTINENTS Kent C. Condie Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801 USA

Growth of the Neoproterozoic supercontinent Rodinia between 1300 and 900 Ma is not recorded in seawater Sr isotopes until the tenninal collisions at 1000-900 Ma, when ^^Sr/^^Sr ratios increase from about 0.7055 to 0.707. This suggests that earlier craton collisions did not result in recycling of large volumes of old continental Sr into seawater. Continental crust production rate remained low for the entire 400-My period as Rodinia fomied. Other than a small increase at 1250 Ma, remains low during supercontinent grow1;h, mostly 2-4 %o. Black shale and phosphate deposition rates were also minimal. A gradual increase in diversity and numbers of both acritarchs and stromatolites during this time interval may reflect increased nutrients delivered to seawater from erosion of the growing supercontinent.

Rodinia began to fragment in East Africa about 850 Ma and this initial breakup is recorded by a minimum in the seawater ^^Sr/^^Sr ratios 0.7055) between 850 and 750 Ma, and a very slight increase in juvenile crust production rate. Three short-lived excursions occur between 900 and 800 Ma, reflecting perhaps, mixing of deep and shallow marine waters or release of methane and CO2 from gas hydrates. Black shale and phosphate deposition rates remain very low during this time. Although the number of species of acritarchs continues to increase, stromatolites reach a peak in the number of forms around 900-800 Ma. Continued input of nutrients delivered to the oceans by erosion, together with increased amounts of CO2 introduced into the atmosphere/ocean system during the early stages of supercontinent breakup, may be responsible for a peak in stromatolite diversity.

Although seawater ^^Sr/^^Sr ratios remained rather low during the breakup of Rodinia, an increase to about 0.707 occurred at about 700 Ma reflecting early Pan-African collisions in the eastern Africa. Despite the opening of new ocean basins, juvenile continental crust production rates remain low. Seawater increased to 6-8%o, with two short-lived negative excursions during global glaciations. An increase in the deposition rate of black shale and sedimentary phosphate also occurred at this time. A broad high reflects increased burial of organic carbon in rift basins accompanying supercontinent breakup. A decline in and in the rate of black shale deposition 650-550 Ma may reflect the growth of Gondwana, which resulted in a decrease in the number of suitable basins for carbon burial. Paleoclimates were cooler from 800-600 Ma due to CO2 draw-down caused by carbon burial and enhanced weathering. During breakup of Rodinia, stromatolites decreased in diversity as did the number of species of acritarchs. These changes may reflect a decrease in carbonate oversaturation of seawater as more carbon was buried; a decrease in the rate of carbon fixation by photosynthesis; or/and increasing levels of P in seawater become toxic to microorganisms.

As Gondwana began to form by Pan-African collisions 600-500 Ma, the ^^Sr/^^Sr ratio of seawater increased rapidly from 0.707 to about 0.710, reflecting extensive uplift and erosion of older continental crust. In addition, the growth rate of juvenile continental crust increased, caused in part by the trapping of juvenile oceanic crustal terranes within Pan-African orogens. A dramatic decrease in to nearly -4%o 600 Ma reflects the Varanger glaciation. Spikes in phosphate abundance during the 600-500-Ma interval correlate with events and suggest enhanced ocean upwelling. Increasing sea level between 600 and 550 Ma may be related to the breakup of Pannotia, which continued to provide basins for carbon burial and phosphate deposition. A large peak in acritarch speciation at 590 Ma may be related to increased nutrients associated with the formation of Gondwana.

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120 - 0 MA TECTONIC EVOLUTION OF THE SW PACIFIC, AND ANALOGOUS GEOLOGICAL EVOLUTION OF THE 600 TO 220 MA TASMAN FOLD BELT SYSTEM Anthony J. Crawford^ S. Meffre', P. A. Symonds^ ^Centre for Ore Deposit Research, University of Tasmania, Hobart, TAS 7001 ^Geoscience Australia, GPO Box 378, Canberra, ACT 2600

We review the tectonic evolution of the SW Pacific east of Australia from -120 Ma until the present. A key factor that developed early in this interval and played a major role in the subsequent geodynamic history of this region was the calving off from eastern Australia of several elongate microcontinental ribbons, including the Lord Howe Rise and Norfolk-New Caledonia Ridge. These were isolated from Australia and from each other during a protracted extension episode from -120 Ma to 52 Ma, with oceanic crust accretion occurring from 85 - 52 Ma and producing the Tasman Sea and the South Loyalty Basin. Generation of these microcontinental ribbons and intervening basins was assisted by emplacement of a major mantle plume at 100 Ma beneath the southern part of the Lord Howe Rise, which in turn contributed to rapid and efficient eastward trench rollback.

A major change in Pacific plate motion at ~ 55 Ma initiated east-directed subduction along the recently extinct spreading centre in the South Loyalty Basin, generating boninitic lithosphere along probably more than 1000 km of plate boundary in this region, and growth of the Loyalty D'Entrecasteaux arc. Continued subduction of South Loyalty Basin crust led to the arrival at about 38 Ma of the 80-60 m.y. old western volcanic passive margin of the Norfolk Ridge at the trench, and west-directed emplacement of the New Caledonia ophiolite. Lowermost allochthons of this ophiolite are Maastrichtian and Palaeocene rift tholeiites derived from the underthrusting passive margin. Higher allochthonous sheets include a boninitic lava slice, which itself was over-ridden by the massive ultramafic sheets that cover large parts of New Caledonia and are derived from the colliding forearc of the Loyalty-D'Entrecasteaux arc. Post-collisional extensional tectonism exhumed the underthrust passive margin, parts of which have blueschist-eclogite assemblages.

Following locking of this subduction zone at 38-34 Ma, subduction jumped eastward, to form a new west-dipping subduction zone above which formed the Vitiaz arc, that contained elements which today are located in the Tongan, Fijian, Vanuatu and Solomons arcs. Several episodes of arc splitting fragmented the Vitiaz arc and produced first the South Fiji Basin (33-25 Ma) and later (10 Ma to present) the North Fiji Basin. Collision of the Ontong Java Plateau, a large igneous province, with the Solomons resulted in a reversal of subduction polarity, and growth of the Vanuatu arc on clockwise-rotating, older Vitiaz arc and South Fiji Basin crust. Continued rollback of the trench fronting the Tongan arc since 6 Ma has split this arc, and produced the Lau Basin -Havre Trough.

This SW Pacific style of crustal growth above a rolling back slab is applied to the 600-220 Ma tectonic development of the Tasman Fold Belt System, and explains key aspects of the geological evolution of eastern Australia. In particular, collision between a plume-triggered 600 Ma volcanic passive margin and a 510-515 Ma boninitic forearc of an intra-oceanic arc had the same relative orientation and geological effects as that which produced New Caledonia. A new subduction system formed probably at least several hundred km east of the collision zone and produced the Macquarie arc, in which the oldest lavas were erupted --480 Ma. Continued slab rollback induced regional extension and the growth of narrow linear troughs in the Macquarie arc, which persisted until terminal deformation of this fold belt in the Late Middle to Late Devonian.

A similar pattern of tectonic development generated the New England Fold Belt between the Late Devonian and Late Triassic. Parts of the New England Fold Belt have been broken from Australia and moved oceanward to locations in New Zealand, and on the Lord Howe Rise and Norfolk-New Caledonia Rise, during the post-120 Ma breakup. Given that the Tasman Fold Belt System grew between 600 and 220 Ma by crustal accretion like the SW Pacific since 120 Ma, facing the open Pacific Ocean, we question whether the eastern (Australia-Antarctica) part of the Neoproterozoic Rodinian supercontinent was joined to Laurentia.

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THE MOZAMBIQUE BELT, EASTERN AFRICA - TECTONIC EVOLUTION OF GONDWANALAND AMALGAMATION AND (?) RODINL\ BREAKUP Huntly Cutten Tectonics Special Research Centre, University of Western Australia, 35 Stirling Highway, Crawley WA 6009

The Mozambique Belt of eastern Africa is a major suture located in Kenya, Tanzania and Mozambique and extends north into the Arabian Nubian Shield and south into East Antarctica. The suture was previously considered a result of the collision of East and West Gondwanaland but it is now recognized that Gondwanaland formed from the accretion of a series of smaller independent terranes over a period of c. ICQ million years. The Arabian Nubian Shield is dominated by upper crustal, juvenile Neoproterozoic rocks of low metamorphic grade formed from the assembly of several arc and back arc basin systems. It contains ophiolites occurring as dismembered nappes having a supra-subduction zone chemistry. Further south, metamorphic grade increases (amphibolite to granulite facies), juvenile material decreases and reworked older crustal material predominates. In Kenya and Tanzania, the Mozambique Belt flanks the Archaean Tanzanian (Congo) craton (predominantly 2600 Ma granitic rocks). In Kenya, the belt comprises supracrustal rocks including paragneisses, quartzites, schists, amphibolites, migmatites, ultramafic intrusives, post-kinematic granites and pegmatites, largely metamorphosed to amphibolite grade. The original sediments have not been precisely dated but are age constrained between 1200 Ma and 800 Ma (Mosley 1993). Evidence for rifting is scarce. Bimodal volcanics and coarse clastic units are an essential part of a rift succession. Bimodal volcanics have not been recognized, but may have been overlooked in the quartzo-feldspathic gneisses. Low-grade coarse clastic volcano-sedimentary sequences (the Embu and Ablun Formations) occur as high-level thrust sheets over the passive margin sequence and may be part of the rift-related sedimentary succession, but have not been dated. Whether the Congo craton was part of Rodinia is debated. The identity of the craton which rifted to the east is unknown but might be revealed by a study of detrital zircons in the passive margin succession. In Tanzania, the western part of the Mozambique Belt comprises mid-crustal amphibolite facies orthogneisses with extensive reworking of the Archaean basement indicated by Sm-Nd TDM ages >= 2500 Ma, similar to those of the Tanzanian craton. The eastern part of the belt, the Eastern Granulites, comprises lower crustal rocks, granulites and anorthosites recognized as klippe remnants having TDM ages of 1100 Ma - 800 Ma and U-Pb ages of -^640 Ma (Muhongo et al. 2001). Appel et al. (1998) determined peak metamorphic conditions of 9.5 - 11 kb and ~ 800^ C. Granulite metamorphism was previously considered to date the continent-continent collision of East and West Gondwanaland, but more recently has been recognized as resulting from underplating due to magmatic arc collision. Appel et al. (1998) identified a composite anticlockwise P/T path for granulite metamorphism consistent with magmatic underplating, residence in the mid-lower crust, subsequent cooling (640 Ma - 620 Ma) and exhumation prior to c. 500 Ma. Cross sections through Kenya, Tanzania and Mozambique demonstrate the tectonic settings of the Mozambique Belt as outlined above.

References Appel, P., Moeller, A, and Schenk, V. 1998. high-pressure granulite facies metamorphism in the Pan-African belt of eastern Tanzania; P-T-t evidence against granulite formation by continent collision. Journal of Metamorphic Geology, 16, 491-509.

Mosley, P.N. 1993. Geological evolution of the late Proterozoic "Mozambique Belt" of Kenya.

Tectonophysics ,221, 223-250.

Muhongo, S., Kroner, A. and Nemchin, A.A. 2001. Single zircon evaporation and shrimp ages for granulitefacies rocks in the Mozambique Belt of Tanzania. Journal of Geology 109, 171-189.

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CHRIS POWELL, RODINIA AND PANNOTIA: HOBART '91 TO ADELAIDE '02 AND BEYOND Ian W. D. Dalziel Institute for Geophysics, University of Texas at Austin, Austin, Texas 78759 USA, and TSRC University of Western Australia, Nedlands It was at the Gondwana meeting in Hobart in 1991 that Chris Powell first heard of the SWEAT (Southwest United States-East Antarctica) hypothesis in the course of a session that he had organized on the assembly of Gondwanaland. In quintessential 'Punchy Powell' style, he not only embraced the idea and moved on to test it with available data, but also took the next step, a successful proposal for an ARC-funded centre for studies of the supercontinents of which Australia had been a part over 3.0 billion years of Earth history. Through many meetings and field excursions on almost every continent, he assembled an enthusiastic team to assemble Rodinia, recognizing it as a testable focus for the tectonically unknown, and previously unknowable, majority of Earth's tectonic history. With the late Raphael Unrug he initiated a successful proposal for an IGCP Project on the topic IGCP#440). He also breathed life into the hypothesis of another end-Precambrian (Gondwana plus Laurentia) supercontinent following opening of the Pacific Ocean basin, with his suggestion of the name 'Pannotia.' As the great Aussie he was, he loved this idea of an 'all southern' supercontinent.

Though founded in field geology, Chris personally concentrated on the paleomagnetic approach in the Tectonics Special Research Centre at the University of Western Australia. At the time of his death this was leading to a dichotomy of views regarding the reconstruction of Rodinia. In typical fashion, he relished this debate as a sign of healthy scientific interaction. I will attempt in this presentation to focus on the role that Chris Powell and other Australian scientists have played in elucidating the Rodinia concept. As Chris would have preferred however, I will concentrate on new ideas from young scientists that may lead us to a workable, testable solution to this great problem of planetary history.

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QUANTITATIVE RECONSTRUCTION OF ARCHEAN-PALEOPROTEROZOIC SUPERCONTINENT KENORLAND David A.D. Evans Tectonics Special Research Centre, Department of Geology and Geophysics, Yale University, New Haven CT 06520-8109, USA Email: dai.evans@yale.edu Globally widespread early Paleoproterozoic (c. 2500-2200 Ma) large igneous provinces manifest the breakup of a supercontinent amid an environment of low-latitude glaciations and progressive buildup of atmospheric oxygen. Previous speculations on the configuration of this supercontinent, Kenorland, have been based purely on non-unique stratigraphic comparisons. Recently obtained palaeomagnetic and geochronological constraints on rift-related Paleoproterozoic rocks from many of the best preserved cratons lead to quantitative, albeit speculative, reconstructions of Kenorland during the protracted interval of its fragmentation. The best constrained palaeogeographic element is the Superior craton, with a series of precisely dated and reliably determined palaeomagnetic poles throughout the interval 2470-1880 Ma. The Kaapvaal craton has yielded reliable poles between 2220-1860 Ma, and a sustained Superior-Kaapvaal connection is not possible throughout that entire interval. However, a direct or indirect connection is permitted at 2220 Ma, if the large igneous provinces on both cratons, precisely of that age, represent the onset of their separation. A new, primary palaeomagnetic remanence retained by the 2450-Ma Woongarra Rhyolite, covering the Pilbara craton, yields the first reliable and well dated, early Paleoproterozoic pole for that block. As with the previous example, a direct or indirect connection with the Superior craton is permitted by this result, and the 2450-Ma large igneous province may have extended from Superior, through Karelia and onto Pilbara. Other blocks, such as the Hearne, Slave, Yilgam, and Tanzania cratons may be joined to the assembly, although their palaeomagnetic constraints are more forgiving. An intriguing outcome of this analysis, using the Superior craton as a common frame of reference, is that Kaapvaal and Pilbara lay on the same side of Kenorland, facing a common ocean. Thus the classic Vaalbara comparison of Archean-Paleoproterozoic geological features between the two blocks is confirmed to first order. A second implication of the reconstruction is that low-latitude glaciation accompanied breakup of the long-lived Kenorland supercontinent; this is reminiscent of the low-latitude ice ages accompanying fragmentation of Neoproterozoic Rodinia, and may divulge general processes relating supercontinents and longterm palaeoclimate.

Ill

OPHIOLITE GENERATION, SEDIMENTATION AND ACCRETION IN AN EARLY PALAEOZOIC REMNANT OCEAN BASIN, SOUTHEASTERN AUSTRALL\ Christopher L. Fergusson School of Geosciences, University of Wollongong, NSW 2522 A remnant ocean basin is defined as "a shrinking ocean basin, which is flanked by at least one convergent margin and whose floor is typically covered by turbidites derived predominantly from associated suture zone(s)" (Ingersoll et al 1995). An example of a remnant ocean basin is the Cambrian-Ordovician succession of the Lachlan Fold Belt. Other remnant ocean basins in the Tasmanides include turbidites of the Broken River-Hodgkinson Fold Belt (north Queensland) and the Shoalwater terrane of the northern New England Fold Belt. Modem examples of remnant ocean basins include the Bengal Fan, the Orinoco Fan and the Indus Fan. All these turbidite successions are being accreted in active subduction complexes (Sunda Forearc, Barbados Ridge complex, Makran subduction complex respectively). Another example of an ancient remnant ocean basin is the Triassic Songpan Garzi complex in China (Ingersoll et al 1995).

The basement of the Ordovician turbidite succession is exposed in the Heathcote Greenstone Belt and in several fault slices east of the Governor Fault in the Tabberabbera Zone of eastern Victoria. These successions contain abundant ultramafic-intemiediate rocks of boninitic composition that elsewhere are recognised as having formed in the youthful phase of the development of supra-subduction zone ophiolites (Shervais 2001). These rocks are consistent with a tectonic setting involving collision and subduction associated with development of the Ross-Delamerian orogenic belt. The setting is broadly analogous to the present-day western Pacific Ocean with island arcs, continental margin volcanic arcs and passive margin-^island arc collisions (as in Taiwan and Papua New Guinea). Deposition of the vast Ordovician turbidite fan(s) that were derived from the Ross-Delamerian mountain ranges occurred in the oceanic realm east of the mountain belt. Detrital zircon ages indicate that additional sediment must have also been sourced from the mountainous terranes of Gondwana formed as a result of Pan-African collisions in the late Neoproterozoic. The Middle Ordovician setting of the remnant ocean basin is the most cryptic with little evidence of on-going subduction anywhere in the system. During this time the volume of detritus entering the remnant ocean basin remained impressive and is consistent with a wider source in Gondwana than just the Ross-Delamerian mountain chain which by this time must have been considerably eroded. In the latest Middle Ordovician closure of the remnant ocean basin is indicated by the initiation of voluminous arc volcanism in the Molong volcanic province and development of accretionary wedges. Sedimentary successions of remnant ocean basins are usually preserved in subduction complexes that form when one or more subduction zones closes the basin. Foster and Gray (2000) advocated this process; they recognised three subduction complexes in the Lachlan Fold Belt that formed from the accretion of the Ordovician turbidites. The modem analogues given above provide support for accretionary wedge models for the Lachlan Fold Belt.

References Foster D. A. and Gray D. R. 2000. Evolution and structure of the Lachlan Fold Belt (orogen) of eastern Australia. Annual Reviews ofEarth and Planetary Sciences 28, 47-80.

Ingersoll R. V., Graham S. A. and Dickinson W. R. 1995. Remnant ocean basins. In: Busby C. J. & Ingersoll R. V. eds. Tectonics of Sedimentary Basins, pp. 363-391. Blackwell Science, Cambridge, Massachusetts.

Shervais J. W. 2001. Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites.

Geochemistry, Geophysics, Geosystems 2(article), paper number 2000G000080[20,925 words].

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THE COLLISION TECTONICS OF NORTHERN PAPUA NEW GUINEA: KEY FIELD RELATIONSHIPS IN THE FINISTERRE, SARAWAGET AND ADELBERT MOUNTAINS AND NEW BRITAIN DEMAND A NEW MODEL R.H. Findlay Geological Survey of Papua New Guinea Department of Mining Private Mail Bag PORT MORESBY Papua New Guinea

A revised lithostratigraphy is presented for the Finisterre, Sarawaget and Adelbert mountains of northern Papua New Guinea, which form a thrust complex within the modem, broad, transpressive transform-fault system between the New Guinea Trench and the New Britain Trench. The region is underlain by the ?Eocene to Middle Miocene Wantoat Group (Finisterre Volcanics and Mebu Formation) and the conformably overlying reef carbonates and related sedimentary rocks of the Middle Miocene to Pleistocene Gowop Group. In the Finisterre and Sarawaget mountains these units overthrust the continentally derived ?Eocene?Pliocene Mena Formation, the Pliocene Nariawang Formation and the Pliocene Ouba Formation, which was derived from uplift of the PNG Highlands.

There is a gradational relationship between the Ouba Formation and the upper part of the Gowop Group and a probable gradational relationship between the Ouba Formation and the Mena and Nariawang formations. The Ouba Formation overlies the Wantoat Group unconformably, and the Mena Formation and the Finisterre Volcanics interdigitate.

These lithostratigraphic relations demand a re-appraisal of the interpretation of the Finisterre Volcanics as an allochthonous terrane, which collided with the Australo-PNG craton in Pliocene times. The Finisterre Volcanics are most readily interpreted as having formed as an autochthonous plateau in the back-arc basin or intra-arc rift-basin of the Sepik Arc to the south which collided with Australia during the Oligocene.

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PALAEOSTRESS ANALYSES FROM THE MT DIVIDE FAULT ZONE AT MT COOK, SOUTH ISLAND, NEW ZEALAND AND IMPLICATIONS FOR KINEMATIC HISTORY ALONG THE NEW ZEALAND SECTOR OF THE AUSTRALIAN-PACIFIC PLATE BOUNDARY R. H. Findlay Geological Survey of Papua New Guinea Dept. of Mining, Private Bag, Port Moresby, Papua New Guinea

The Main Divide Fault Zone (MDFZ) of the Mt Cook region, New Zealand, is alleged to be the backthrust of the Alpine Fault, which fornis the Australian-Pacific plate boundary in South Island, New Zealand.

The palaeostress system for the Alpine Fault is well defined and may be compared with the palaeostress fingerprint of the MDFZ. According to the geometry of lineations in the mylonitic schist of the Alpine Fault, ai should trend about 110°-290°, thus matching predictions from faults known related to the Alpine Fault and predictions possible from the known present motion of the Pacific Plate relative to the Australian Plate 472 small-scale faults were measured in and adjacent to the MDFZ and its splays in the Mt Cook area. The poles of these faults plot in a clear pattern defining a best-fit girdle (M-plane) dipping 45"" to 105°. This does not match the M-plane for the Alpine Fault, as determined from the geometries within the 8-to-4 my-old mylonitic schist of the Alpine Fault. Thus the MDFZ is incongruent to the Alpine Fault and therefore the MDFZ did not form an original part of the kinematic system of the Alpine Fault, although it may have been reactivated to produce movement and later structures compatible with motion on the Alpine Fault.

Analyses of small faults in the hanging wall and footwall of the MDFZ confirm three trends for Oi 045", 070^-090^ and 110-120".

The 110-120" trend of ai matches that predicted from the geometry of the Alpine Fault and the trend of the convergence vector of the Pacific Plate in South Island, New Zealand. It is similar to the results from active splays of the Alpine Fault through the Marlborough region of South Island.

The other two trends would be consistent with systematic rotation of the convergence vector of the Pacific Plate from a southwesterly trend, through a westerly trend to the present trend, as suggested by recent paleomagnetic studies.

The 045" trend is totally incompatible with either strike-slip along or reverse motion across the Alpine Fault but is compatible with dextral strike-slip on the north to northnortheast segments of the MDFZ and the dextral megakinks in the Mt Cook region. The 070"-090" trend is consistent with production of the westsouthwest trending folds and high-strain zones in the Alpine schist in a dextral slip regime. The 110"-120" trend is consistent with formation of the Alpine Fault as a linear discontinuity between 8 and 4my ago.

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PALAEOSTRESS ANALYSES SUGGEST SYNCHRONOUS PLIOCENE TO PRESENT REGIONAL STRAIN PARTITIONING ACROSS EASTERN CENTRAL PAPUA NEW GUINEA. R.H.Findlav, J.Arumba, I.Abiari, P.Kia, T.W. Kilya, G.Kopi, C.Mortimer Geological Sun^ey of Papua New Guinea Dept. of Mining, Port Moresby, Papua New Guinea

Palaeostress analyses of faults between the Fmisterre Mountains and Port Moresby, PNG, are beginning to reveal two major, synchronous, probably Pliocene to Present palaeostress systems which mmiic modem extension-compression directions derived from the fault-plane solutions of the earthquakes of the last 40 years. The respective palaeostress systems also mimic the present northwards motion of the Australian plate and the eastwards motion of the Pacific Plate.

In the Finisterre Mountains, at the Maniape Prospect (Bismarck Mountains), Okapa, and the Aure Scarp the predominant al plunges shallowly and trends north to northeast (Finisterre System).

This matches both the convergence vector of the Australian Plate and the thrust-fault compression directions derived from earthquakes below the Finisterre Mountains and in western New Britain.

In contrast, at the Kora Creek Mine 3km west of the Maniape Prospect, and at the Kathnell Mine 20km southwest, mineralised fault systems display evidence for a a l plunging shallowly southeast.

This system (Kora Creek System) is mimicked by the palaeostress derived from unequivocal reverse motion on the Sunshine Fault (alleged to be the northern boundary fault of the mineraliferous Wau-Bulolo Graben), by a palaeostress system reworking faults in the Cretaceous Chim Formation at Menyamya, by the palaeostress system found in the Mt Wilhehn-Bundi region of the Bismarck Mountains and by the palaeostress sytem derived from the near-flat Koki Thrust at Konedobu m Port Moresby. The orientation of this al is consistent with the WNW motion of the Pacific Plate in the PNG region and matches the extension-compression directions of the Port Moresby earthquake of 1979.

As both the Finisterre and Kora Creek systems match the results of modem fault plane solutions then they are seen as geologically contemporaneous. Thus the different orientations of the two als is taken as indicating that regional strain-partitioning is occurring. As the al of Kora Creek System mimics the westwards convergence vector of the Pacific Plate, then this may be a response to the motion of this plate rather than the northwards motion of the Australian Plate. This is surprising as the influence of Pacific Plate motion would appear of necessity to be buffered by the microplates intervening between the Pacific Plate and PNG.

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DATING METAMORPHISM AND EXHUMATION OF FORTH METAMORPHIC COMPLEX, TASMANIA WITH IMPLICATIONS FOR PALEOZOIC TECTONICS OF THE EAST GONDWANA MARGIN David A. Foster^ David R. Gray^ Michael Hartley^ and Catherine Spaggiari^ ^Department of Geological Sciences, Box 112120, University of Florida, Gainesville, FL 32611, USA ^School of Earth Sciences, University of Melbourne, Melbourne, Victoria, 3010 ^School of Geoscience, Monash University, Melbourne, Victoria 3800

The Neoproterozoic-Cambrian passive margin of Gondwana underwent a series of major orogenic events starting in Cambrian time with the Delamerian - Ross - Tyennan Orogeny. In Tasmania the Tyennan Orogeny was initiated by partial subduction of the former passive margin and obduction of an ophiolitic sheet. Metamorphism associated with this event ranges from greenschist to ecologite facies with faulted contacts between packages of different metamorphic grade. Published U-Pb zircon analyses from eclogite facies rocks in the Franklin metamorphic complex and amphibolite facies rocks in the Forth complex indicate peak metamorphic conditions were reached at 511 ± 8 Ma and 514 ± 5 Ma, respectively. "^^Ar-^^Ar analyses of muscovite from greenschist facies fault zones associated with the Forth metamorphic complex give ages of 520 ± 0.6 and 518 ± 0.8 Ma indicating that deformation predating obduction was underway by -520 Ma. Muscovite from amphibolite or upper greenschist facies rocks in five locations of the Forth-Ulverstone complex give tightly clustered ^Vr-^^Ar ages of 505.4 ± 0.7 to 504.9 ± 0.9 Ma. These data indicate rapid cooling of the Forth complex at about 505 Ma. This rapid cooling was probably due to exhumation as a response to regional extension that eventually led to the formation of the Mount Read volcanic complex and associated graben in Tasmania. Mainland Australia (Mount Stavely complex) and Antarctica (Bowers Terrane) also experienced extension and calc-alkaline volcanism at -505-500 Ma perhaps resulting from renewed subduction under Gondwana following Delamerian-Tyennan collision. Roll-back of subduction from about 505 Ma produced a backarc basin =1000 km wide by early Ordovician time that became the basement for the western Lachlan turbidites. Tasmania may have remained attached to Gondwana during this extensional event, with the back arc basin opening to the east, or it could have been separated from the mainland by part of the basin. Another Middle Cambrian oceanic volcanic arc existed to the east of the western Lachlan basin, that may have been an independent arc system or an extension of the Mount Read volcanic arc onto mainly oceanic crust. Based on data from the western Lachlan Orogen in Victoria, the marginal oceanic basin closed from -450 Ma to -420 Ma. In this interpretation, Tasmania was a key part of two major orogenic intervals along the Gondwana margin: (1) The Delamerian-Ross Orogeny, when Tasmania collided with an oceanic island arc and was thrust back onto the Gondwana passive margin (2) The Lachlan Orogeny (450-380 Ma), when extended parts of the Tyennen Orogen in Tasmania were contracted back onto Australia-Antarctica, closing the back-arc basin in front of (east) and north of Tasmania, and reactivating Delamerian-Tyennan-Ross structures on the craton and within Tasmania.

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A NEW GEODYNAMIC MODEL FOR MESOZOIC-CENOZOIC NEO-TETHYS/INDIAN AND PACIFIC OCEANS 'Carmen Gaina, 'Dietmar Muller, ^Christian Heine and ^Gilles Borel School of Geosciences, The University of Sydney, N.S.W 2006, Australia ^Institut fur Geologie, Mineralogie & Geophysik, Ruhr-Universitaet Bochum, Germany ^Institute of Geology and Paeontology, University of Lausanne, CH-1015 Lausanne, Switzerland

The dispersion of Gondwanaland together with the expansion of the Pacific Ocean led to several episodes of subduction that changed the margins of Eurasia and eastern Australia. The relationship between the two major oceanic domains (Neo-Tethys and Pacific) are very poorly understood due to the destruction of these paleo-plate boundaries. We attempt to tackle this problem by reconstructing the two oceanic domains since Mesozoic times.

We use a combination of methods and datasets in order to reconstruct subducted oceanic regions older than 150 million years. The Paleo-oceans are modelled by creating "synthetic plates" whose locations and geometry is established on the basis of preserved M-sequence magnetic lineations, paleogeography, regional geological data and the rules of plate tectonics. Plate boundaries, which are introduced and modified in time and space, give only little room for alternative plate model solutions. These plate limits are governed by rheological laws, which provide stable constraints for reconstructions when geological information is scarce. Based on the new set of paleo-isochrons, we construct paleo-age grids that will provide an accurate estimation of seafloor spreading/subduction rates, as well as the global distribution of oceanic crust ages for the last 160 million years.

The Jurassic and Cretaceous Neo-Tethyan ocean floor north and northeast of Australia, of which the Argo abyssal plain is the only remnant, has been modelled based on available geological and geophysical data. We test the hypothesis of multiple seafloor spreading episodes, that implies that the opening of the Argo oceanic basin continued north of Greater India joining a triple junction NE of Arabian plate, and connecting the Argo spreading ridge with the Somali Basin between Africa and Madagascar. Around 132 Ma seafloor spreading north of Greater India stopped (possibly due to the Neo-Tethyan ridge subduction under the Eurasian plate) and triggered the opening of Enderby basin between India and Antarctica. This event corresponded with breakup along the entire western Australian margin.

The Pacific, Farallon/Phoenix and Izanagi plate geometries are also reconstructed based on preserved oceanic floor and the history of subduction along the Asian and Australian margins. In addition, we reconstructed the controversial back-arc spreading history east of Australia and the complicated record of spreading and subduction north of Australia. The latter includes the opening of the Philippine and Caroline basins, and the gradual destruction of the Solomon Sea. The paleoage grids illustrate where subduction zones were located and the age of subducted slabs during the past 160 million years. These estimates are useful for analysing the history of active margins through time.

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PALEOPROTEROZOIC TO MESOPROTEROZOIC ASSEMBLY OF AUSTRALIA: IMPLICATIONS FOR RODINL\ RECONSTRUCTIONS David Giles and Peter Betts Australian Crustal Research Centre, School of Geosciences, Monash University Wellington Road, Clayton, VIC 3800

Intraplate settings and processes have dominated previous interpretations of the Palaeo- to Mesoproterozoic geology of the Australian plate. With increasing evidence for plate tectonic processes at craton margins at this time, it is now possible to reconcile the Palaeo- to Mesoproterozoic evolution of the Australian plate in a plate tectonic context. This interval was a major period of continental growth with approximately 60% of the Australian continent amalgamating between c.1.80 Ga and 1.50 Ga. This continental growth occurred along two Australian plate margins between c. 1.80 and 1.50 Ga and the resulting continent formed a fundamental building block for the supercontinent Rodinia.

The first margin was located along the southern edge of the continent and records a protracted history of arc magmatism, as well as accretionary and collisional orogenesis associated with a southward migrating north-dipping subduction between c. 1.80 and 1.60 Ga which may have extended into the southern margin of Laurentia. The amalgamation of the West and North Australian cratons (c. 1.80-1.78 Ga) and the North Australian and Gawler cratons (c. 1.74-1.69 Ga) defines the major episodes of continental growth. Coeval with subduction was the development of extensive far-field continental back-arc basins over a large area of the continent interior. The architecture and evolution of these intracontinental basins were controlled by roll-back of the northdipping subduction zone and break-up of Australia and Laurentia at c. 1.66-1.65 Ga. This resulted in the fonnation of an ocean basin to the east of the presently exposed Proterozoic Australian terranes. Closure of this basin between c. 1.60 and 1.50 Ga resulted in the development of a westdipping subduction zone beneath eastern Australia before collision between Laurentia and Australia.

This collision is recorded by the development of an extensive orogenic belt preserved in the Mount Isa and Georgetown inliers, the Cumamona Province, and the northern Gawler Craton. Arc magmatism occurred in the Georgetown Inlier at c. 1.55 Ga contemporaneous with the orogenesis.

Between c. 1.45 and 1.1 Ga, the South Australian Craton, consisting of the Curnamona Province and the Gawler Craton, rifted from the remainder of the North Australian Craton and was re-attached in its present configuration during episodic collisional orogenesis at c. 1.33-1.10 Ga. During this event Laurentia and Australia also rifted and the Belt Purcell Group was deposited.

The Australia-Laurentia connection lies in the geographic heart of Rodinia reconstructions.

Although there are several hypothesised configurations for an Australia-Laurentia connection, their precise connection is speculative. Intriguingly, many of the proposed piercing points between the two continents pre-date the final amalgamation of Rodinia (c. L3-L1 Ga). It is possible, given the complex tectonic interactions between the two continents, that the configuration of the continents changed throughout the Palaeo- and Mesoproterozoic, and that the hypothesised SWEAT, AUSWUS, and AUSMEX configurations represent different stages of the Australia-Laurentia interactions.

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THE NAROOMA TERRANE: IMPLICATIONS FOR THE CONSTRUCTION OF THE OUTBOARD PART OF THE LACHLAN OROGEN R. A. Glen', I. Stewart^ and L G. Percival' ' Geological Survey of New South Wales, Department of Mineral Resources, Box 536 St Leonards, New South Wales 1590. ^ School of Biological Sciences, Monash University, Clayton, Victoria 3168 In its type area around Narooma, the Narooma Terrane comprises the Wagonga Group, which consists of the Narooma Chert overlain by the Bogolo Formation. The Narooma Chert is divided into two parts. Conodont age control shows that the lower, largely massive part ranges in age from Late Cambrian at the base into well-dated Darriwilian-Gisbornian (late Middle to early Late Ordovician) chert at the top. The upper part comprises interbeds of chert and argillite that contain Eastonian (Late Ordovician) graptolites. The overlying Bogolo Formation is dominated by argillite.

Local conglomeratic facies are either basalt-rich (no separate basal 'Kianga Basalt' is now recognised), or else contain clasts of sandstone, chert and basalt that predate deformation and which are olistostromal in origin. Where not deformed by later faulting, the boundary between the Narooma Chert and Bogolo Formation is gradational. We found no evidence of a regional early decollement fault between the two units.

At outcrop scale, the Narooma Chert contains evidence for at least two sets of structures with early structures folded around later folds that trend NW and which generally plunge to the NW. The argillaceous part of the Bogolo Formation is dominated by one (to two) generally continuous cleavages. In the conglomeratic part, clasts have been folded, cleaved, extended, quartz-veined and faulted.

At map scale, the Narooma Terrane does not define an antiform. Instead, it consists of a stack of imbricate thrust slices in a regional syncline caught between two thrust faults that juxtapose the terrane against the adjacent, coeval Adaminaby Terrane. These faults are best defined where Narooma Chert is thrust over Bogolo Formation. The most complete slices pass up from Late Cambrian chert through Darriwilian-Gisbornian chert into the upper Late Ordovician chertargillite, and then into the overlying Bogolo Formation. The soles of these thrusts contain multiply foliated chert. Late shear bands indicate a (?minor) strike-slip component to the faulting.

The Narooma Terrane, with chert overlain by muddy ooze, is interpreted as an oceanic terrane that accumulated remote from land for -50 million years. The localised presence of blocks of chert, basalt, basalt breccia and sandstone in the argillite records the uplift of seafioor, containing a Cambrian seamount, as the terrane drifted westwards towards the Australian landmass.

Although the Narooma Terrane lay outboard of the intraoceanic Ordovician Macquarie Arc, we find no evidence for it or its surrounding Adaminaby Group having formed in an accretionary prism/subduction complex. Indeed, the Adaminaby Group is stratigraphically and lithologically similar to, and is probably a fault repetition of backarc, craton-derived turbidites. The presence of metamorphic layering in Ordovician turbidites is not indicative of offscraping or underplating in a subduction zone. We suggest that the Narooma Terrane is part of buoyant oceanic terrane containing a seamount that collided with the outboard part of the Lachlan Orogen. This terrane was partially subducted in the Late Ordovician, to be subsequently thrust back over the Adaminaby Terrane, in the Early Silurian.

Glen and Percival publish with permission of the Director-General, New South Wales Department of Mineral Resources.

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NEOPROTEROZOIC TO EARLY PALEOZOIC RECORD OF RODINIA BREAKUP AND GONDWANA-MARGIN CONVERGENCE IN EAST ANTARCTICA John W. Goodge Dept. of Geological Sciences, Southern Methodist Univ., Dallas, TX 75275, USA Email: jgoodge@mail.smu.edu Neoproterozoic to Ordovician rocks from the Transantarctic Mountains (TAM) record the tectonic transformation from a rifted passive margin to an active orogenic margin along the paleo-Pacific edge of East Antarctica. This transition is particularly w^ell-preserved in siliciclastic and carbonate successions of the central TAM, w^here integration of stratigraphic, structural and provenance data provides a detailed record of the timing and changing tectonic regime. Major tectonic stages are recognized as follows.

Rifting and passive margin. East Antarctica should no longer be view^ed as a single coherent entity through the Proterozoic (Fitzsimons 2000), and its role in the assembly and disassembly of Rodinia remains uncertain. This is apparent along the Pacific margin of Antarctica, where various cratons are suggested as Neoproterozoic conjugates and where there is disagreement on the age of breakup.

Evidence of rifting at -750 Ma is revised by age data from the TAM indicating that crustal extension, magmatism, and sedimentation are <680 Ma. Late Neoproterozoic sandstones deposited across this narrow rifted margin in a near-shore to shallow-marine setting represent mature, multicycle sediments derived primarily from adjacent 2.8, 1.9-1.4, and 1.3-1.0 Ga cratonic sources.

Platform. Incipient Ross-orogen tectonic activity is recorded in crystalline basement rocks by 540 Ma, yet a thick clastic-carbonate succession shows that a discontinuous inner-shelf platform was developed by the middle Early Cambrian. Sandstones beneath the carbonates reflect a cratonicshield provenance similar to the earlier stage.

Synorogenic. The platform succession was terminated abruptly in Botomian time by interruption of carbonate production, influx of mud and silt deposits, and onlap of coarse alluvial-fluvial debris upon high-relief erosion surfaces. This initial sedimentary change occurred in response to flexure caused by thrust loading or negative buoyancy related to subduction. The provenance of late Early Cambrian to Ordovician deposits changes from a composite shield signature to a younger activemargin igneous source. In the TAM, fresh first-cycle detritus was eroded from proximal igneous and metamorphic rocks of the emerging Ross Orogen (mostly 580-520 Ma). Large volumes of molassic sediment were shed into forearc marginal basins. Sandstones in the Pensacola Mountains contain Grenville- and Pan-African age detritus, suggesting an East African Orogen source.

Late orogenic. Synorogenic deposits were in turn invaded by late-orogenic plutons as magmatism shifted outward during trench retreat. Profound denudation, followed by pre-Devonian peneplanation, removed the entire volcanic carapace and exposed the plutonic roots of the arc.

Comparison of detrital mineral ages in these deposits yields minimum cooling rates o f - 1 2 °C/m.y. in the source area, suggesting denudation rates of -0.2 mm/a; this is comparable with rates in convergent belts and, along with evidence of left-oblique slip, indicates both erosional and tectonic control on denudation. Convergence along the Pacific margin accommodated inter-cratonic suturing elsewhere in Gondwana.

Reference Fitzsimons I.C.W. 2000. Geology 28, 879-882.

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UNDERSTANDING THE NATURE AND CAUSE OF COMPLEX DEFORMATION PATTERNS IN THE TASMANIDES David R. G r a y ' , David A Foster^ and Frank P. Bierlein^ ' VIEPS School of Earth Sciences, University of Melbourne, Melbourne, Vic 3010 ^Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA ^ VIEPS School of Geosciences, Monash University, Melbourne, Vic 3800

There is general acceptance that cratonisation of eastern Australia has involved stepwise accretion of the Delamerian, Lachlan and New England Orogens respectively. Controversy remains however, as to what constitutes an orogeny in the rock record and as to what drives orogeny in the Tasmanides. The Lachlan part perhaps remains the most enigmatic and most widely disputed. Questions remaining for the Lachlan include: the mechanism for closing the marginal basin, an explanation for the variable thermal pattern expressed by metamorphism and magmatism, and how the tectonic framework evolves through time? Structural thickening took place during plate convergence in an oceanic setting along the eastern margin of Gondwana from -520 Ma through 340 Ma, with accretion of structurally thickened submarine fans, accretionary complexes, former volcanic arcs and oceanic crust, and micro-continents such as Tasmania. The nature, causes and tectonic setting of this period of cratonisation are still hotly debated with arguments based on geochemistry, thermal regimes, regional structural architecture and metallogenic associations. Many current workers argue that construction of the Lachlan Orogen is intraplate and inboard of a single, long-lived subduction system off of Gondwana at this time. But how does the system work? Chris Powell in his 1983 Geol. Soc. Aust. article provided an insightful solution by changing the Tasmanide margin through time, from the hanging wall of Marianas-type subduction, to wrenching along a transform plate boundary, to final Andean-type subduction. There is no doubt that the final stages in the eastern Lachlan involve marked magmatism typical of the Basin and Range (extensional environment). Chris Powell, again in 1983, first argued for E-migration of the thermal front due to rollback of the subduction system, a scenario now strongly promoted. As to whether Lachlan construction can be simply viewed as overall extension punctuated by brief compressional episodes, the Royden "retreating orogen" or Carparthian-type, depends on the tectonic reference frame and the relative durations of extension versus compression as expressed in the rock record. Clearly subduction is involved. The Lachlan deformation pattern involves three "thrust-systems" that constitute the western, central and eastern parts respectively. These were first established by looking at vergence (dips of faults and regional fold axial surfaces), and were later supported by Ar-Ar geochronology. The faults in the western Lachlan show a generalised E-younging (450-420Ma). This eastward younging probably relates to imbrication and rock uplift of the sediment wedge, because detailed analyses show that the decollement system is as old in the east as it is in the west. Overall deformation in the eastern Lachlan is younger (400-3 80Ma), apart from the Narooma Accretionary Complex (~445Ma). Preservation of extensional basins and evidence for basin inversion are largely restricted to the central and eastern parts of the Lachlan Orogen.

Metallogenic associations, including turbidite-hosted orogenic lode Au (±As-Sb) (western Victoria), sediment-hosted (epigenetic) Cu-Au and Pb-Zn (e.g. western NSW), granite-hosted Sn-W (+skam-greisen) (e.g. central NSW), porphyry Cu-Au (+Au-skam, mantos Pb-Zn and epithermal Au-Ag) (central northern NSW) and volcanogenic-hosted massive sulphides (eastern NSW) with overprinting minor "post-tectonic" granite-related magmatic Au (±Mo-SbCu-Te) mineralisation (Victoria), can help to assess the geodynamic setting through time. The overall different geotherm in the western Lachlan, coupled with the presence of serpentinite matrix- melange incorporating blueschist knockers like the Franciscan of California, must also be considered. The presence of continental ribbons (like the Lord Howe Rise/Campbell Plateau)- (e.g. Tasmania/Selw^n block extension underneath Melbourne Zone, Victoria) have implications for the deformation patterns. Detailing cratonisation of eastern Australia requires "fitting" of the pieces of a tectonic "jigsaw" puzzle, but then a requirement of how it works.

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RESULTS OF SHRIMP ANALYSIS OF DETRITAL ZIRCONS FROM THE PALAEOPROTEROZOIC EARAHEEDY GROUP: IMPLICATIONS FOR THE ASSEMBLY OF THE WEST AUSTRALIAN CRATON J. Halilovic^ P.A. Cawood', A. Jones^ and F. Pirajno^ ^Curtin University of Technology, Dept. of Applied Geology, Perth 6845, Australia ^Geological Survey of Western Australia, 100 Plain Street, East Perth 6004, Australia

The Palaeoproterozoic Earaheedy Basin in Western Australia, which contains the Earaheedy Group, lies at the eastern end of the Capricorn Orogen. The Earaheedy Group is a 5 km thick succession of shallow marine clastic and chemical sedimentary rocks, which is divided into two subgroups. The Tooloo Subgroup, at the base, consists of the Yelma (oldest), Frere and Windidda (younger) formations. The overlying Miningarra Subgroup consists from base to top of Chiall (older) and Wongawol formations, Kulele Limestone and Mulgarra Sandstone. Regional stratigraphic relationships indicate that the Earaheedy Basin is younger than 1840 Ma (Rasmussen and Fletcher, in press, EPSL) and older than 1650 Ma (Pirajno et al, 1999; GSA, Abstract Series, 59).

U-Pb SHRIMP dating of 285 detrital zircon grains obtained from six siliciclastic samples from the Yelma Formation, the Wandiwarra and Princess Ranges members of the Chiall Formation, and the Mulgarra Sandstone yielded a range of both Archaean (3.4 to 2.5 Ga) and Palaeoproterozoic (2.4 to 1.8 Ga) ages. Grains of Archaean age are dominantly in the range 2.7-2.6 Ga, similar to that of the Yilgam Craton, which unconformably underlies the southern margin of the Earaheedy Basin. Age peaks in the range 2.3-2.2 Ga, 2.0 Ga and 1.8 Ga, characterize the Palaeoproterozoic detritus, with the two younger age peaks overlapping with those of the adjoining Gascoyne Complex of the Capricorn Orogen (Occhipinti et al, GSWA Record 2001/8), which lies west of the basin. Source components with ages in the range 2.3 to 2.2 Ga are extremely rare in Western Australia.

Undefomied granite in the Mullingarra Complex within the Pinjarra Orogen to the west of the Darling Fault yielded an age of around 2.2 Ga (Cobb et al, 2001, GSA Abstract Series, 54). In addition, rare rims on zircons from gneisses within basement to the Glenburgh Terrane of the Gascoyne Complex have yielded ages of around 2.4 Ga (P. Kinny pers. comm., 2002), suggesting some early Palaeoproterozoic activity in this region.

The U-Pb detrital age data indicate that in addition to the conventionally invoked Yilgarn source for the Earaheedy Basin, Paleoproterozoic detritus from west of the basin including the Gascoyne complex and possibly the Mullingarra complex also constituted a significant detrital source and require an easterly to north easterly flowing distributary system. Facies, thickness and paleocurrent data within the basin are consistent with this interpretation and indicate that not only had the Gascoyne complex been accreted to the Yilgarn by the time of sediment accumulation, but a major early Palaeoproterozoic terrane, possibly incorporating components of the Mullingarra Complex, now west of the Yilgarn, was also present.

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WHAT IS THE MAIN CENTRAL THRUST OF THE HIMALAYAN OROGEN? Ben Goscombe and Martin Hand Department of Geology and Geophysics, Adelaide University, Adelaide, 5005, S.A.

The Main Central Thrust of the Himalayas has been assumed by a large number of previous workers to be the dominant crustal-scale thrust controlling the structural architecture of the Himalayan orogen, v^ith the corollary that it is the effective boundary betv^een the upper- and low^er-plates w^ithin the orogen. This view of the Main Central Thrust is largely based on its location between domains with contrasting average metamorphic character.

However in reality the Main Central Thrust is an unconformity between two basinal sequences; the Palaeoproterozoic Lesser Himalayan Sequence and the overlying Neoproterozoic Greater Himalayan Sequence. Both the sequences were deposited on the northern margin of the Indian continent prior to the Himalayan collision. Structural mapping along nine profiles across the Main Central Thrust and into the upper-most Greater Himalayan Sequences in eastern Nepal reveal that the Main Central Thrust is not a significant deformational structure in the sense that it represents a zone of significantly concentrated shear strain. The main structure controlling the metamorphic and structural architecture of the eastern Himalayan metamorphic front occurs at higher structural levels than the traditionally referred to Main Central Thrust. We have named this controlling structure the High Himalayan Thrust.

The High Himalayan Thrust is the boundary between the upper- and lower-plates and is marked by a zone of intense deformation that separates highly metamorphosed hanging wall rocks from the lower grade footwall. In contrast there is no structural or metamorphic discontinuity or zone of high strain at the Main Central Thrust and no evidence for large-scale lateral movement along it. Strain is equally partitioned across the Main Central Thrust and metamorphic grade varies continuously throughout the entire metamorphic front below the High Himalayan Thrust. The Main Central Thrust is a sheared unconformity and should be referred to, at best, as the "Main Central Unconformity", or at worst, to keep with the inertia of past nomenclature, the "Stratigraphic Main Central Thrust".

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CONTRASTING SOURCES OF PALAEOZOIC MAFIC DYKES EMPLACED DURING EARLY PALAEOZOIC RIFTING IN CENTRAL AUSTRALIA Martin Lee, Martin Hand and John Foden Department of Geology and Geophysics, Adelaide University, Adelaide SA 2002 The rift-related Cambro-Ordovician Harts Range Metamorphic Complex, in the eastern Arunta Block, central Australia, contains a suite of early Ordovician meta-basaltic rocks (the Harts Range Dykes) that were emplaced syn-kinematically into low-angle regional-scale ductile extensional detachment zones that accommodated N-S extension. The early Ordovician extension was coeval with the deposition of the Larapinta Group in the Amadeus Basin, and was associated with development of the Larapintine Seaway, which linked Australia's eastern margin to the Canning Basin.

Major element variation within the dykes is consistent with an evolutionary path dominated by olivine, and to a lesser extent clinopyroxene and plagioclase fractionation. Based on trace element and isotopic criteria, the dykes have been subdivided into two suites: Group 1 and 2.

Although the tectonic setting of dyke emplacement is well-constrained, geochemically-based tectonic discrimination criteria suggest that the Group 1 dykes have N-type MORB affinities, while Group 2 dykes were apparently derived from E-type MORB or volcanic/island arc sources.

Group 1 dykes are characterised by higher Zr and Y but lower Nb, Ba, La, Ni, Cr and Ce values than those of Group 2. High Zr/Nb ratio values of 62 for the Group I dykes suggest they were derived from a depleted source, as does the near flat REE pattern, (LaA'b)N = I.21, which is N-type MORB like. Group 1 dykes have positive SNd(480) values associated with less contaminated, depleted signatures. Group 2 dykes have lower Zr and Y, but higher Nb, Ba, La, Ni, Cr and Ce compositions than Group 1 dykes, and have Zr/Nb ratio values of around 14, indicating an arc-like source. They also display a slightly LREE enriched pattern (LaArb)N = 5.12, similar to that of E-type MORB. The low and variable 8Nd(480) values II.5, -2.1 and 0 and extremely high values of the Group 2 dykes suggests varying degrees of enrichment.

The SNd(480) values and variations in Nd and Sr concentrations in the Harts Range Dykes provide evidence for multiple magma sources. Geochemical and isotopic data suggest that Group 1 dykes have upper asthenospheric, depleted mantle characteristics similar to other meta-tholeiites in the Harts Range region. The enriched geochemical signature of Group 2 dykes may reflect contamination by a metasomatised mafic crust. However, a subductionmodified and chemically isolated continental lithospheric mantle source is preferred, because of the close-fit comparison in trace element data with a similarly interpreted adjacent suite of dykes. The bulk average continental lithospheric mantle (CLM) is ruled out as a dominant source for the enriched chemical signature of the Group 2 dykes, because it does not exhibit the HFS-element depleted and LIL-element enriched character of a subduction-modified continental lithospheric mantle. Additionally, the Group 2 dykes have higher incompatible element compositions than the bulk average CLM.

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SEVENTY-EIGHT YEARS OF HIMALAYAN-TIBETAN TECTONIC MODELS T.M. Harrison Research School of Earth Sciences, The Australian National University, Canberra, A.C.T. 2601 Even prior to w^estern geologists gaining access to the great expanse of Tibet, Chris Powell recognized that the Himalayan-Tibetan orogenic system was the ideal modem analogue with which to understand the tectonic history of ancient orogens. His early speculations on the evolution of the Indo-Asian collision zone challenged mainstream thought and influenced many geologists, including myself, to become interested in understanding what is often referred to as the great natural laboratory of tectonics.

Since the publication of Argand's treatise in 1924, numerous models have been advanced to explain the present distribution of crust within the Indo-Asian orogen, including those that predict wholesale uplift (e.g., mantle delamination and Powell's delayed under-plating model), progressive growth (e.g., under-thrusting of India, under-thrusting of Tarim, continental injection), lateral responses (e.g., orogenic collapse, horizontal extrusion), and inheritance of an elevated terrane (e.g., multiple collision, intra-arc thickening). However, it is only within the last 20 years that investigations of the timing and magnitude of deformation in Tibet and surrounding regions have yielded constraints on the crustal thickening and elevation histories that can help us select among these models.

We now know that >1400 km of N-S shortening was absorbed within the Himalayan-Tibetan orogen since the onset of collision at - 7 0 Ma and that it is broadly manifested as: 1) discrete thrust belts with relatively narrow zones of contraction or regional decollement (e.g., MCT, Qimen TaghNorth Kunlun thrust system), and 2) distributed shortening over a wide region involving basement rocks (e.g.. Nan Shan and western Kunlun thrust belts). Crustal shortening appears to have begun synchronously in the early Paleogene in both the Tethyan Himalaya and Nan Shan, some 1400 km to the north, suggesting that the plateau began to be constructed between the northern margin of India and the Qilian suture zone simultaneously and not through sequential propagation from south to north. Paleozoic and Mesozoic tectonic histories appear to have exerted strong control on the Cenozoic strain distribution and history during development of the plateau (e.g., Cenozoic thrust belts developed along pre-existing sutures; the Triassic flysch complex is spatially correlated with Cenozoic volcanism and thrusting; basement-involved thrusts in the Nan Shan and Kunlun Shan follow pre-Cenozoic tectonic belts). Also, the Early Cenozoic tectonic history of the orogen guided the Late Cenozoic strain distribution and petrogenesis in the Himalaya. The main strike-slip faults in the orogen are transfer faults linking either major thrust belts (e.g., Altyn Tagh system) or extensional systems (e.g., Karakorum fault).

The pattern of crustal displacement described above is inconsistent with most permutations of the aforementioned end-member tectonic models but instead requires a specific, time-dependent transfer among several of these processes, often with multiple mechanisms operating simultaneously. The parameters that appear most important in dictating which mechanisms are dominant at any one time are: the location and geometry of pre-existing lithospheric weakness, the distribution of topography before and during the collision, the geometry of the indenter and extruded blocks, the magnitude of boundary stresses, and the age of the lithosphere. The discrete changes we infer in the manner in which continuous Indo-Asian convergence was accommodated provides a cautionary tale against interpreting episodic phenomena in the geological record in terms of discontinuous processes, particularly in light of the growing appreciation that complex physical systems driven by structureless inputs can exhibit highly intermittent dynamics. Perhaps the clearest lesson emerging from our study of the Indo-Asian collision is that the continental lithosphere's complex history and geometry exerts a powerful control on continuous plate convergence being manifested in the geological record as episodic phenomena.

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COUPLED CHANGES IN PORE PRESSURE AND STRESS IN OIL FIELDS AND SEDIMENTARY BASINS: IMPLICATIONS FOR DEFORMATION Richard R. Hillis National Centre for Petroleum Geology and Geophysics, Adelaide University, SA 5005, Australia Repeated pressure measurements undertaken throughout the depletion of oil fields demonstrate that reduction in pore pressure (Pp) is accompanied by a reduction in total minimum horizontal stress (Gh), a phenomenon described as oil field-scale pore pressure/stress (Pp/ah) coupling. Virgin pressure measurements (i.e. those unaffected by depletion) through normally- and over-pressured sequences in sedimentary basins demonstrate that overpressure development is accompanied by an increase in GH, described as sedimentary basin-scale Pp/ah coupling. Virgin pressures in overpressured sedimentary basins suggest that an increases at approximately 70-80% of the rate of increase in pore pressure in the Canadian Scotian Shelf, the Australian North West Shelf and the North Sea. Oil field-scale Pp/ah coupling is clearly driven by anthropogenic changes in pore pressure due to depletion. However, it is not clear whether, at the sedimentary basin-scale, increasing pore pressure drives the increase in ah or increasing ah generates the overpressure. The former may occur due to poro-elastic behavior or due to stresses being controlled by the frictional limits to slip/faulting and the latter due to horizontal stress-driven disequilibrium compaction or tectonic shearing.

Oil field- and sedimentary basin-scale ah coupling operate over different time and spatial scales.

Pp/ah data for the same region have not been previously explicitly compared. With depletion of the Ekofisk Field, North Sea, minimum horizontal stress decreased at approximately 80% of the rate of reduction of reservoir pore pressure (ie. Aah/APp-0.8). Virgin pressures measured in exploration wells surrounding the Ekofisk Field (Norwegian quadrants 1 and 2) indicate that with overpressure development Aah/APp~0.73 (assuming shallow, normally pressured sequences are representative of overpressured sequences prior to overpressure development). Hence, despite the different temporal and spatial scales, and possibly different causal mechanisms, the rate of decrease of minimum horizontal stress with pore pressure due to depletion of the Ekofisk Field is similar to the rate of increase of minimum horizontal stress with pore pressure due to overpressure development in the surrounding region. Basin-scale exploration pressure data in the Ekofisk region may thus provide an indication of the reservoir stress changes associated with depletion.

The total vertical stress (G^) is given by the weight of the overburden and is unaffected by changes in pore pressure. Hence, contra to simple, uncoupled models of the effect of pore pressure on rock failure, differential stress in normal fault regime basins (ay -ah) increases as pore pressure decreases, and decreases as pore pressure increases. Decreased differential stress with increased pore pressure implies that a greater increase in pore pressure can be withstood prior to failure than would otherwise be predicted, and increases the propensity of tensile, rather than shear failure, occurring with overpressure development. Knowledge of the coupled nature of oil field-scale pore pressure/stress changes is critical because they can lead to the collapse of uncased wellbores, sand production and to faulting/fracturing and seismicity with field development.

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GRANITOIDS MARKING FINAL GONDWANALAND AMALGAMATION: PRELIMINARY U-Pb ZIRCON AGES FROM CENTRAL MADAGASCAR B. Hulscher^ I.C.W. Fitzsimons^ and C.McA. P o w e l l 'TSRC (Tectonics Special Research Centre), Department of Geology and Geophysics, The University of Western Australia, 35 Stirling Hw^, Crawley, WA 6009, Australia e-mail: bhulscher@tsrc.uwa.edu.au ^TSRC, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845 * Deceased

During the final closure of the Mozambique Ocean, continental fragments collided and amalgamated into Gondwanaland, thus producing the East African Orogen. In many parts of this collisional belt, the exact timing and nature of tectonic events is not clear. These events are largely thought to have occurred between 650 and 500 Ma. Here, the focus will lie on Madagascar because of its prime location in the suture zone between West and East Gondwanaland, and because of its excellent exposures of Pan-African structures and plutons in older basement and supracrustal rocks.

Central Madagascar contains a basement/cover series of Archaean gneisses (Antananarivo Block), in tectonic contact with a Proterozoic cover sequence (the Itremo Group). The latter comprises greenschist to amphibolite facies quartzites, pelites and marbles, deposited some time between 1855 and 800 Ma. Both basement and cover were extensively reworked during Gondwanaland amalgamation, with voluminous syn-tectonic to post-tectonic granitoid intrusions.

New, preliminary U-Pb SHRIMP zircon ages are reported for three samples taken near the southeastern tectonic margin between the Itremo Group and the basement gneisses. The granitoid intrusions yielded zircon crystallization ages of mainly 510-580 Ma and a basement sample yielded --2.5 Ga. The latter, a homblende-biotite bearing gneiss (5 km north of Fenoarivo) contains magmatic zircons of-2.5 Ga and the magmatic precursor to this gneiss is assumed to have crystallized at this time. The 'stratoid-like' Talata Vohimena granite, which intrudes the basement and is syn-tectonic, has no xenocryst cores and the ages fall within a -510-555 Ma range, which is again interpreted as the magmatic crystallization age. The post-tectonic Vohitrakanga (or 'South Antoetra') Granite intrudes the Itremo Group and has xenocryst cores of -2.5 Ga, probably derived from the underlying Archaean basement, but sector-zoned rims and (new) euhedral grains without cores yield ages from -550 to -580 Ma and are interpreted as reflecting the granite crystallisation age.

Our data indicate that the (SE) basement to the Itremo Group comprises -2.5 Ga magmatic rocks, consistent with -2.5 Ga basement ages found elsewhere in Madagascar (Tucker et al 1999). They also reinforce the existence of an extensive, late Pan-African granitoid suite in Central Madagascar (cf. 520-580 Ma ages, Handke et al 1999; Tucker et al 1999) which has intruded both basement and cover rocks and is most likely related to the amalgamation of Gondwanaland. Although these granites occur both in the basement and in the Itremo Group, this event appears to have been higher grade in the basement, where it was associated with pervasive deformation, amphibolite- to granulite-facies metamorphism and partial melting of older lithologies. The tectonic significance of the 510-580 Ma granitoids is still debated. The youngest granitoids may signify extensional collapse of the EAO, or perhaps they merely intruded into local transtensional regimes during regional transpression. In any case, they structurally mark the final stage of this continent-continent collision in Madagascar.

References Handke M.J., Tucker R.D. and Ashwal L.D. 1999. Neoproterozoic continental arc magmatism in west-central Madagascar. Geology 27, 351-354.

Tucker R.D., Ashwal L.D., Handke M.J., Hamilton M.A., Le Grange M. and Rambeloson R.A. 1999. U-Pb geochronology and isotope geochemistry of the Archaean and Proterozoic rocks of north-central Madagascar.

Journal of Geology 107, 135-153.

127

THE SUMBA COLLISION AND ITS AFFECTS ON THE NORTHERN AUSTRALIAN MARGIN Myra Keep^ and Ian Longley^ ^Tectonics Special Research Centre, Geology Department, University of Western Australia, Nedlands, 6009. ^Woodside Energy, 1 Adelaide Terrace, Perth 6001.

Sumba Island, Indonesia, occupies a forearc position within the Australian/Indonesian convergent margin. It represents a sliver of arc crust that has moved into its current position from elsew^here.

Tectonically it lies at a junction between different lithospheric types. To the west, an intra-oceanic volcanic arc system occurs at which Australian oceanic lithosphere converges on Indonesian arc lithosphere. To the east, a volcanic arc-continental collision zone involves Australian continental lithosphere in collision with the Indonesia arc. Sumba lies at the transition between these different regimes, and represents the focus of a number of structural and tectonic features. We suggest that collision of a promontory of Australian continental lithosphere with Sumba at Ma created many of the structural features on the Australian side of the plate boundary, partitioned the lithosphere into structural domains that reflect different deformation histories since the Neogene (22 Ma), and contributed to the uplift of Sumba at Ma. The shape of the collided promontory controls the shape of the uplifted Sumba Block. The proposed 8 Ma age for collision correlates well with deformation events in Sumba, the Timor Sea and the Browse Basin. Structures created or reactivated by the collision bound deformational provinces within the North West Shelf of Australia.

128

RELATIONSHIPS BETWEEN TECTONIC ELEMENTS IN THE TASMANIDES, EASTERN AUSTRALIA, BASED ON DEEP SEISMIC REFLECTION PROFILES R.J. Korsch,^'^ B.J. Drummond,^ M.G. Nicoll,^ T.J. Barton,^ T. Fomin,^ B.R. Goleby,^ D.W. Johnstone,^ L.E.A. Jones^ and R.A. Glen^ ^ Predictive Mineral Discovery Cooperative Research Centre ^ Geoscience Australia, GPO Box 378, Canberra ACT 2601 Australia ^ Geological Survey of New South Wales, Department of Mineral Resources, PO Box 536, St Leonards NSW 1590 Australia Deep seismic reflection profiling over the last 20+ years has provided images across many parts of the Tasmanides in eastern Australia. Here we examine the relationships between some of the major tectonic/structural elements in the Tasmanides, such as the boundaries between the continental craton and the Delamerian Orogen, the Delamerian and Lachlan orogens, and the Lachlan and New England orogens, plus the nature of the boundaries between some of the major units within the Lachlan and New England orogens.

The Tasman Line is defined as the boundary between outcrops of Precambrian crustal elements to the west and Palaeozoic crustal elements to the east. It separates the older Mt Isa and Broken Hill blocks from the younger Delamerian and Thomson orogens. SE of Broken Hill in western New South Wales, it coincides with NE-trending gravity and magnetic anomalies and a topographic feature bounded by the course of the Darling River. Predominantly SE dipping shear zones in the craton truncate against strong, NW dipping reflections in the middle to lower crust beneath the Phanerozoic terranes and define a triangular shaped region in the crust at least 80 km wide. This underpins the boundary between the craton and younger terranes which, therefore, is not a "line" but rather a broad deformation zone.

In western Victoria, the boundary between the Lachlan and the Delamerian orogens is now accepted as the Moyston Fault. Very strong reflections at about 2 to 5 sec TWT associated with the Moyston and Pleasant Creek faults show that they are parallel, planar, east-dipping thrusts. These two faults define a deformed zone a few kilometres wide between the two orogens. To the east in the Lachlan Orogen, a series of west dipping thrust slices appear to be truncated by the Pleasant Creek Fault. In the Delamerian Orogen, to the west, the upper part of the crust is not as reflective as the middle crust, but it does contain significant reflections, the strongest of which are east-dipping, and are interpreted as a series of thrust faults.

As shown by several deep seismic profiles, the crustal architecture along the suture between the New England and the Lachlan-Thomson orogens varies considerably. In the north, the seismic lines are dominated by east-dipping structures, with the New England Orogen being thrust over the Thomson Orogen. Seismic lines in southernmost Queensland are dominated by gently west-dipping structures in the lower crust, but east-dipping thrusts in the upper crust. The accretionary wedge part of the New England Orogen was underthrust below the easternmost Lachlan Orogen, but the forearc basin component (Tamworth Belt) of the New England Orogen was thrust westwards over the Lachlan Orogen. A seismic profile in the southern New England Orogen contains both east- and west-dipping structures, and shows similar relationships between the New England and Lachlan orogens as in the lines in southernmost Queensland. Thus, the New England Orogen, at least in its southern half, is a doubly vergent orogen that developed in the Late Palaeozoic to Triassic.

In summary, in western Victoria, the Lachlan Orogen being thrust over the Delamerian Orogen confirmed previous ideas. However, the Lachlan-New England lines were a surprise in showing major changes along strike, with the New England Orogen having a single vergence in the north and being a doubly vergent orogen in the south. Thus, deep seismic reflection profiling in eastern Australia has been instrumental in providing constraints on the geometry, in the third dimension, of some of the major boundaries between tectonic elements within the Tasmanides.

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WHAT DOES A FORMER ACCRETIONARY OROGEN LOOK LIKE IN THE LOWER CRUST? Alfi-ed Kroner Institut fur Geowissenschaften, Universitat Mainz, 55099 Mainz, Germany Accretionary orogens in the upper crust are dominated by trench deposits, obducted ophiolites, exotic terranes and well defined structural boundaries such as shear zones. The Neoproterozoic Arabian-Nubian shield (ANS) of western Arabia and NE Africa, the huge terrain of the Neoproterozoic to Palaeozoic Central Asian mobile belt (CAMB) and the present Indonesian Archipelago are prime examples of such orogens. In the ANS and CAMB, field relationships, rock associations, differences in structural style and metamorphic grade, and geochronology have led to the recognition of terrane assemblages that can be related to processes of lateral accretion as now observed in the southwest Pacific and lasting for several hundred million years. In the ANS, ocean crust and arc formation began about 900 Ma ago, and terrane accretion was completed by -600 Ma, whereas in the CAMB the oldest oceanic crust formed some 1000 Ma ago, and terrane accretion continued into the late Palaeozoic.

Typical rock associations in the above upper crustal accretionary orogens are trench sediments, island-arc volcanics, calc-alkaline granitoids, dismembered ophiolite suites and gneissic rocks (microcontinents?) constituting exotic terranes and mostly of distinctly older age and more complex tectono-metamorphic history than the surrounding lower grade rocks. Shear zones frequently separate the terranes and in the ANS also constitue seismic discontinuities extending to the lower crust.

The middle to lower crustal high grade assemblages constituting the Neoproterozoic Mozambique belt (MB) of East Africa, Madagascar, southernmost India, Sri Lanka and East Antarctica is considered to be a deep crustal analogue to the upper crustal accretionary belts described above.

Typical characteristics are (1) voluminous calc-alkaline granitoid suites, now layered gneisses, and interpreted as root zones of arc terranes, (2) tectonic interdigitation of Archaean to Palaeoproterozoic gneisses with Neoproterozoic rocks, probably brought about during accretion of arcs and microcontinents, (3) extensive ductile shear zones, locally decorated by mafic granulites believed to be remnants of ophiolite suites, (4) marked differences in rock formation age (based on dating of magmatic zircons) across structural boundaries, (5) significant differences in metamorphic age (based on dating of metamorphic zircons) in different terranes of the MB, suggesting that collision producing the high-grade assemblages was not contemporaneous across the belt.

The heterogeneity in rock type, structural style, metamorphic evolution and age across the MB makes it unlikely that this wide orogenic domain resulted from simple collision of two large continental masses, namely West and East Gondwana, but was produced by continuous accretion of oceanic and continental terranes over a period of -^300 Ma, from about 900 Ma to about 600 Ma. The upper crustal rocks of this orogen were probably eroded shortly after final amalgamation of Gondwana some 550 Ma ago and with it the typical hallmarks of accretion such as trench sediments and ophiolite assemblages. In some regions of the MB such as Madagascar and Sri Lanka, terrane boundaries were recognized by structural mapping in combination with metamorphic petrology, zircon dating and analyses of Nd isotopes. A scenario is now emerging for the MB suggesting that it is the lower crustal equivalent of the ANS, and if this interpretation is correct the MB is one of the best natural laboratories to study accretionary processes in the middle to lower continental crust.

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STRUCTURAL AND GRAVITY MODELLING ALONG TWO TRANSECTS OF THE SOUTHERN WYANGALA BATHOLITH, MOLONG ZONE, EASTERN LACHLAN FOLD BELT, NSW Paul G. Lennox^ Robert Trzebski', Anthony J. Johnston^ & Martin M. Scott^ ^ School of Biological, Earth & Environmental Sciences, UNSW, Sydney 2052 ^Geological Survey of New South Wales, Department of Mineral Resources, PO Box 53, Orange NSW 2800 The Wyangala Batholith is a north-south elongated batholith (13-50 km x 130 km) consisting of over 30 individual plutons, which are mainly S-type granites and monzonites, rare I-type granodiorites and tonalites, and minor diorites and gabbros. This batholith intrudes Lower Palaeozoic volcanics, volcaniclastics and turbidites of the Molong Zone, which is thought to be a remnant of the former Macquarie Arc. Research on the Carcoar, Barry and Sunset Hills granites ("three granites") which are considered to be part of the Wyangala Batholith shows that ascent and emplacement was assisted by pre-existing faults and possibly occurred during the opening of the adjacent Hill End Zone in the Early Silurian.

Studies of the quartz texture, cathodoluminescence and Al-in-hornblende barometry of the three granites indicates that the Carcoar and Barry granodiorites experienced a continuous and rapid ascent to depths of 4-6 km with development of syn- and post-magmatic foliations, whereas the Sunset Hills Granite intruded at 10-12 km depth and exhibits a post-magmatic foliation and evidence of uplift and transpression.

Previous studies on the main plutons of the northern Wyangala Batholith have suggested passive emplacement by roof uplift, upward displacement of fault blocks or (local) stoping and then uplift and deformation of the plutons and country rocks along regional shear zones generally bounding the eastern margin of these plutons.

Gravity and structural modelling along two east-west transects across the northern Wyangala Batholith revealed that the plutons have bowl-shapes and discreet root zones up to 7 km depth under their eastern sides for the northern transect and similar root zones on their western sides for the southern transect. These root zones appear to be near major faults and these outcrop at the contact between the pluton and country rocks. Structural analysis indicates these major faults and/or shear zones occasionally change dip along their length, generally show a west-over-east sense of movement and both dextral and sinistral strike-slip histories. Generally, it appears that the movement on the shear zones and faults reflect east-west contraction. Thus north-south faults show mainly dip-slip movement, whereas northeast-southwest faults show dextral transpression and northwest-southeast faults show sinistral transpression.

New structural and gravity analyses along two east-west transects across the southern Wyangala Batholith, one through Crookwell and the other through Gunning, will be compared and contrasted with the two east-west transects in the northern Wyangala Batholith. The results from all transects will facilitate reconstruction of the overall 3D shape of the Wyangala Batholith and improve our understanding of the emplacement history of plutons within this batholith.

Acknowledgements One Small ARC Grant in 2000 (RMS3203), two University Research Support Program Grants in 2001 (PS00165) and 2002 (PS01596) and the Deutsche Forschungsgemeinschaft have supported our research on the Wyangala Batholith. Sarah Bailey & Dave Kitching (Cambridge University, UK) and Tom Southwell & Karol Czamota (UNSW) are thanked for mapping along different transects. The School of Surveying and Spatial Information Systems, UNSW kindly provided the digital GPS system and Mike Moore assisted us with its operation. Anthony Johnston and Martin Scott publish with permission of the Director General, NSW Department of Mineral Resources.

131

SYNCHRONOUS NEOPROTEROZOIC MAGMATIC EVENTS IN AUSTRALIA AND SOUTH CHINA: TWO PLUMES, OR A SUPERPLUME? Z.X. Li Tectonic Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Crawley, WA 6009, Australia Email: zli@tsrc.uwa.edu.au Neoproterozoic bimodal magmatic activities spread from c. 850 Ma to c. 750 Ma in Australia (Fig. la). Apart from the c. 825 Ma rocks previously recognised as of mantle plume origin, there are also c.

850 Ma lamprophyric dykes in the southern Yilgam, 810-800 Ma kimberlite pipes and lamproites in northern Kimberley, 802 ± 10 and 777 d= 7 Ma rift volcanics in the Adelaide Rift Complex, 111 ± 1 Ma and 760 ± 12 Ma granites in western Tasmania and King Island that were previously regarded as of orogenic origin but with little supporting evidence, and the 755 ± 3 Ma Mundine Well dyke swarm in Western Australia.

Recent SHRIMP geochronological analysis in South China revealed comparable magnetic pulses during the Neoproterozoic in South China (Fig. lb), with rock types ranging from granitoids, gabbros, dyke swarms to rift volcanics. There was a week pulse at around 850 Ma (phase 1'), followed by two major phases of bimodal magmatic activity, one at between 830 Ma and 800 Ma (phase 1), and the other between 780 Ma and 750 Ma. Phase 1 started before continental rifting began, and lasted well into the rifting stage. Phase 2 was entirely syn-rift. Both phases were accompanied by rapid doming and crustal unroofing across the continent.

Magmatic rocks during the early stage of phase 1 in South China, as well as coeval magmatic rocks in Australia, have previously been interpreted as of plume origin. As shown in Figure lb, this phase of magmatism apparently lasted well into the rifting stage on both continents. Apart from geochemical characteristics, their plume origin is also supported by the syn-magmatic rapid doming and un-roofing.

Such interpretation is consistent with having South China sitting against eastern Australia in Rodinia.

C. 780 Ma radiating mafic dyke swarms in western Laurentia point to a plume centre west of Laurentia. If the Australia-South China-Laurentia configuration in Rodinia is correct, the plume centre would be again located beneath South China and eastern Australia. A mantle plume would provide the heat source for the 780-750 Ma bimodal magmatism in both South China and Australia, including the "problematic" 780-760 Ma granites in western Tasmania and the King Island, and the arcuate-shaped 755 ± 3 Ma Mundine Well dyke swarm (possible ring dykes). REE and trace element analysis of c. 760 Ma mafic dykes in western South China indeed revealed characteristics similar to known plume-induced mafic rocks.

The question then arises as to why would there be two successive plume events at the same region, c. 100 Ma apart, yet both were related to the same continental rifting event that led to the breakup of Rodinia? Why similar events are also seen in other Rodinia fragments such as Laurentia, India, Madagascar, and southern Africa? We suggest here that, like the breakup of the supercontinent Pangaea, the breakup of Rodinia may also have been caused by a mantle superplume (the Rodinia superplume), which caused widespread anorogenic bimodal magmatism, rapid crustal unroofing, and continental rifting.

(a) Australia

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Fig. 1 Two major episodes of magmatic activities in Australia and South China during the Neoproterozoic

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TRACKING RECYCLED AUSTRALIAN PROTEROZOIC ZIRCON GRAINS THROUGH THE BELT SUPERGROUP INTO MODERN STREAMS, IDAHO, U.S.A. Paul Karl Link^ and C. Mark Fanning^ ^Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA ^Research School of Earth Sciences, The Australian National University, ACT 0200, Australia Proposed supercontinent reconstructions are often reliant on the comparison of the stratigraphy and age of related or linked sequences. The age of detrital zircons within sedimentary sequences is one of the more popular tools used to investigate possible links. In the proposed juxtaposition of North America with Australia within the western part of the supercontinent that would become Rodinia after the Grenville orogeny, the Mesoproterozoic Belt Supergroup has been cited as providing key evidence for a former link.

The Mesoproterozoic Belt Supergroup (1470 to 1370 Ma) crops out extensively in the mountains of east-central Idaho and along the continental divide in adjacent Montana. Several lines of evidence (paleocurrents, facies change, Nd isotopes, and ages of detrital zircons) suggest that the intracratonic fluvial and shallow-water strata of the Belt Supergroup were derived from a southwestern provenance, possibly central and southern Australia.

We have obtained reconnaissance SHRIMP U-Pb ages on detrital zircons from Pleistocene to Recent sediments and modem stream sands derived from areas underlain by the Belt Supergroup.

We can thus track the recycling of zircons from the Belt Supergroup into young fluvial systems.

From the 220 concordant analyses of detrital zircons that yield Meso- and Paleoproterozoic ages and are likely derived from recycled Belt strata, 16% have ages in the range 1510 to 1610 Ma. The prominent -1580-1600 Ma age peak corresponds to the crystallisation age for the extensive Gawler Range Volcanic event within the Gawler Craton of South Australia. Similar age magmatic rocks occur within the North Australia Craton (Mt Isa and Georgetown Inliers), and to a lesser extent, within smaller inliers in North-east Queensland. Thus there are a number of Proterozoic blocks that could provide zircons of this distinctly Australian -1580-1600 Ma age range. This is significant since this age source has not been identified within North America.

Another eleven percent of the detrital zircons have ages between -1350 and -1450 Ma, i.e. magmatic sources that were synchronous with Belt deposition. This age range is not prominent within the Australian continental crust and one must seek a source from within the southwestern part of the Belt Supergroup Basin.

Twenty-eight percent of the detrital zircons have ages in the range -1650 to -1750 Ma. These zircons could have been derived from the southwest extensions of the Yavapai and Mazatzal provinces of the southwest US. However, it is equally probable that zircon of this age could be derived from the extensive Proterozoic magmatic crust within the North Australian and Gawler Cratons, and also Cumamona Province (for example the Broken Hill Inlier). Twenty nine percent have ages from -1760 to -1900 Ma, and could have been derived from the southwest extension of the Great Falls tectonic zone in Idaho, that is now intruded by Cretaceous Idaho Batholith. Both the North Australian and Gawler Cratons may also be considered likely sources for this age range.

The detrital zircon age patterns of the Belt Supergroup, and (herein) sediments derived directly from that group, have been one of the key criteria for the linking of North America and Australia.

However, given incomplete knowledge of Asian Mesoproterozoic cratons, only the 1510 to 1610 Ma zircons are definitively non-North American, with the 1580 to 1600 Ma zircons uniquely Australian.

133

THE FORMATION AND EXHUMATION OF HIGH PRESSURE METAMORPHIC TERRANES DURING SW-PACIFIC-STYLE OROGENY G.S. Lister and M.A. Forster School of Geosciences Australian Crustal Research Centre, Monash University, Melbourne 3800 Victoria Email: gordon@mail.earth.monash.edu.au

The formation and exhumation of coherent high-pressure metamorphic terranes has been a topic of debate for several decades. The role of the down-going slab has been the focus of all models that attempt to explain this phenomenon, essentially using the subducting slab to drag the terranes to be metamorphosed down to great depths in the Earth (i.e. >80-150 km beneath the surface). This fascination with the down-going slab extends even to the low geothermal gradients associated with eclogite-blueschist terranes, since these are generally ascribed to 'refrigeration' effects associated with subduction.

The role of the down-going slab may be little more than that of a bystander however. When it comes to analyse the processes involved in eclogite-blueschist metamorphism such terranes are invariably distal from the active subduction zone, and they are found in the over-riding plate. In many cases there is no reason to assume these terranes were ever attached to the subducting slab.

They occur in the back-arc and they often are found beneath obducted ophiolites, with which they have long been assumed to have a close genetic association.

This contribution proposes a mechanism explaining the association between obducted ophiolites and eclogite-blueschist terranes, and an alternative hypothesis is set forward for the origin of coherent high-pressure terranes formed during SW Pacific-style tectonism.

Conventional models for the genesis of coherent high-pressure terranes propose either buoyant return of subducted terranes (aided and/or abetted by serpentine diapirs), or variants of a 'comer flow' mechanism (e.g. the "paddle-board" analogy where it is imagined that the buoyant terrane is dragged down until it breaks free and bobs towards the surface). These mechanisms take little heed of field observations, in particular the overall shallow-dipping or sub-horizontal attitude of ductile shear zones and faults associated with exhumation. These data point to the role of "in plane" deviatoric stress, and the effects of large-scale continental extension.

Renewed roll-back of subducting oceanic lithosphere in front of an orogenic zone after an accretion event has the capacity to exhume very deep levels of the Earth's crust or lithosphere, because the crust and lithosphere can be severely extended in such environments. Since this e^umation mechanism does not directly depend on the relative buoyancy of individual tectonic slices, it is possible to readily exhume even ultrahigh-pressure (UHP) metamorphic rocks, or even deeper (microdiamond bearing) mantle rocks. We discuss how coherent slices of the over-riding plate above a major subduction zone may be overthrust and sent to great depths in vastly overthickened crust, during the preceding accretion event, and why roll-back of the subducting lithosphere will recommence immediately subsequent to such an accretion event. We illustrate how the over-riding plate will then be subject to extreme extension, and why, as a result, episodes of high-pressure metamorphism will be closely followed by periods of rapid exhumation and uplift.

The model is illustrated for the evolution of the high-pressure terranes of northern New Caledonia, and a possible application of the model to the evolution of classic Alpine cross-sections will be presented. In terranes where this mechanism has been at work we expect a geometry that is typical of lithospheric dislocations that have reversed their shear sense, and several candidate structures, in different orogenic belts, will be identified. These structures may be marked by the presence of chains of boudins of relatively high-pressure rocks, bounded above and below by different age shear zones, with distinct kinematic significance, and/or by the occurrence of Fransciscan-type boulder melanges. We suggest that major dislocations that have reversed their shear sense areof a relatively common occurrence, implying that the propsoed mechanism for the exhumation and exposure of high-pressure terranes may eventually find more general application.

134

Nd-Sr ISOTOPIC EVmENCE FOR THE ORIGIN OF DEVONIAN GRANITIC AND FELSIC VOLCANIC ROCKS OF THE WESTERN LACHLAN FOLD BELT Roland Maas^ and Ian A. Nicholls" ^ School of Earth Sciences, University of Melbourne, VIC 3010 ^ School of Geosciences, Monash University, VIC 3800 Granites and associated volcanics are important components of the Paleozoic western Lachlan Fold Belt (LFB). Their ages and compositions can constrain mid/lower crustal compositions, crustmantle interactions and the timing and causes of thermal perturbations that lead to crustal melting.

Here we compare Nd-Sr isotopic data (mostly new, but some literature data) for 400 and 370 Ma felsic complexes in western (Stawell and Bendigo zones, Beaufort to Charlton) and central (mostly Melbourne Zone, Strathbogie to Wilsons Promontory) Victoria, respectively.

I and S type granites occur in both study areas; central Victoria also has well-preserved subaerial volcanic sequences of both I- and S-type character (e.g., Dandenongs Complex, Acheron and Cerberean Cauldrons, Violet Town Volcanics). In western Victoria, c. 400 Ma granites have the following isotopic ranges: I-types 8Nd(i) from -7 to +4, ^^Sr/^^Sr(i) from 0.7105 to 0.7045; S-types 8 to -3, 0.718 to 0.709. Equivalent ranges for the c. 370 Ma granites of central Victoria are: I-types -6 to +2, 0.7125 - 0.7045; S-types -7.5 to -4, 0.712 - 0.707. Central Victorian volcanic complexes tend to mirror the gramtes: I-types -6 to +3, 0.708 - 0.704; S-types: -5.5 to -1, 0.7114 - 0.7085. S type complexes tend to have lower and less variable 8Nd (although considerable overlap exists) than the I-types but both compositional types have comparable respective ranges for 8Nd in the two study areas. By contrast, S-type complexes in western Victoria have a much larger range in ^^Sr/^^Sr than the cental Victorian S-type complexes.

Overall, isotopic variations for west-LFB I-t>^es resemble those for I-types in the east-LFB. However, central Victorian S-types differ markedly from their eastern cousins, showing considerable clustering near the primitive (high 8Nd, low ^^Sr/^^Sr) end of the Kosciusko-BerridaleBatholith S-type trend. The western Victorian S-types show a more typical pronounced hyperbolical trend extending to ^^Sr/^^Sr near 0.720, but neither of the S-type associations from the western LFB reaches 8Nd <-8, compositions commonly found in the widespread Bullenbalong Suite S-type granites of the eastern LFB. Likewise, the lowest 8Nd found in the I-type granites in both eastern and western LFB are similar (-8 vs -7). We draw the following tentative conclusions: (a) Lower crustal sources lack major fertile components with 8Nd <-8. Alternatively, if Itypes form by syn-magmatic mixing/crustal assimilation, the most evolved I-ypes would reflect broadly constant mixing proportions of primitive and evolved end-members across the LFB. (b) Likewise, 400 and 370 Ma S-type sources appear to lack significant fertile components with 8Nd <-8, despite the presence of topical Ordovician-Silurian metaturbidite as high-grade enclaves. (c) Results for both I- and S-types rule out an origin of any felsic magmas in central Victoria from a source involving Proterozoic basement (Selwyn Block), if such basement is comparable to known Neoproterozoic crust in SE Australia (Adelaide FB, NW Tasmania). Reactivated Mt Read-type lithologies would be a more suitable crustal magma source material. (d) Trends for the various data fields appear to broadly converge on the field for central Victorian S-type granites. If this is not coincidental, it may indicate that a crustal component of this composition is important in both I and S types in the western and eastern LFB. (e) I-type trends towards high 8Nd reflect increasing mantle involvement.

135

RELATIONSHIP BETWEEN THE DELAMERIAN AND LACHLAN FOLD BELTS IN WESTERN NEW SOUTH WALES Kingsley J. Mills Geological Survey of New South Wales

Regional mapping of the Koonenberry district by the Geological Survey of New South Wales has revealed the extent of a major unconformity betw^een rock sequences that were strongly deformed during the Delamerian Orogeny and an overlying sedimentary Late Cambrian to Ordovician fossiliferous siliciclastic sequence. The structure of the Koonenberry district is dominated by the northeast to northwest trending Koonenberry Fold Belt that wraps around the eastern margin of the Curnamona Province. The Koonenberry Fold Belt is made up of tightly folded, vertically cleaved and thrusted packages of low-grade metasediments and volcanic rocks (Kara beds, Teltawongee beds, Ponto Group and Gnalta Group) and is a well-established expression of the exposed Late Cambrian Delamerian Fold Belt in western New South Wales. The Kara beds make up the oldest package and consist of a thick sequence of non-fossiliferous slates, siliciclastic rocks and dolomitic units that appear to be lithologically equivalent to the upper part of the Late Proterozoic Adelaidean sequence exposed to the west of the Bancannia Trough. The Kara beds contain the Mt Arrowsmith Volcanics ranging from alkali basaltic pillow lavas to evolved dacitic and rhyolitic units and associated intrusives. The Teltawongee beds form a sequence of monotonous, dirty, graded turbiditic sandstones with occasional slaty marker horizons. Very rare fauna and trace fossils suggest a Cambrian age and a possible correlation with the Kanmantoo Group of South Australia.

The Teltawongee beds are exposed on the eastern edge of the Bancannia Trough, in a narrow central belt and as a wide expanse to the east of the Koonenberry Fault. The unfossiliferous Ponto Group package, consisting dominantly of fine-grained turbiditic sandstones and slaty phyllites and minor tholeiitic volcanics, is everywhere fault bounded against other packages. Early SHRIMP dates on zircons from thin felsic tuff units have yielded dates of 516Ma or younger. The fossiliferous Early to Middle Cambrian Gnalta Group is exposed as a less deformed sequence at the eastern edge of the Bancannia Trough west of Cymbric Vale and west of Mt Arrowsmith.

Subsequent to the tight folding, thrusting and burial of these older units during the Delamerian Orogeny in the Late Cambrian, the region was uplifted and planed off by marine and near shore fluvial erosion. This erosion resulted in an extensive exposure of a high angle unconformity that was then overlain by a fossiliferous Upper Cambrian to Ordovician siliciclastic sequence. This sequence, variously named in different places Scopes Range Beds, Mootwingee Group, Cupala Creek Formation, Kayrunnera Group and Funeral Creek beds appears to have been deposited close to a northwest trending shore line. In the more marine Kayrunnera Group to the northeast sedimentation began in the Mindyallan stage (Late Cambrian) and continued into the Early Ordovician. There may be some slight diachroneity towards younger basal units in the west, near Mt Arrowsmith, Mutawintji and Scrope Range, although lack of suitable fossil preservation in near-shore siliciclastic sediments limits proof of this. This post-Delamerian sequence is interpreted here to be the near-shore equivalent of the Lachlan Fold Belt sequences to the east. Low homoclinal dips and weak folding is characteristic of the Cambro-Ordovician over much of the Koonenberry district. However, deformation increases to the northeast where older Delamerian folds show refolding of units in the basement and the younger Kayrunnera Group shows a welldefined slaty cleavage with a more easterly trend than the dominant northwest trend of Delamerian folds to the west. This younger deformation appears to be associated with an ESE trending fold belt well-defined in the aeromagnetic image to the north near Yancannia. Recent mapping near Yancannia has revealed the presence of slaty rocks of unknown age forming fossil islands at the southern edge of the Mesozoic Eromanga Basin. The slaty cleavage in the Cambro-Ordovician Kayrunnera Group and the folded rocks near Yancannia, and their on-strike continuation in the Tibooburra region may well represent the westernmost expression of the Lachlan Fold Belt.

Published with permission of the Director-General, NSW Department of Mineral Resources.

136

OPHIOLITE PROBLEMS IN NORTHER PAPUA NEW GUINEA: THE INS AND OUTS

J. Milsom Department of Geological Sciences, University College London, London WCIE 6BT Large ophiolite masses are now frequently interpreted as remnants of oceanic forearcs emplaced on continental margins in the course of arc-continent collision. This interpretation implies a fomier association between the ophiolite and a calc-alkaline magmatic arc at a distance of 100 - 200 km.

A relationship of this type between mafic/ultramafic and calc-alkaline igneous belts can be identified in central New Guinea, where ophiolites emplaced along the northern flank of the main mountain spine lie some 100 km south of exposures of arc-volcanic basement in the north coast ranges. There is, however, no obvious comparable relationship in the case of the largest of the New Guinea ophiolites, the Papuan Ultramafic Belt of the eastern peninsula, since this is backed to the north and east by the oceanic Solomon Sea. A similar problem exists in the case of the coeval but much smaller Marum Complex ophiolite further west, the magmatic arc to the north being widely assumed to have arrived only within the last 10 million years.

Associated calc-alkaline magmatic arcs can be recognised for these ophiolites only by assuming a complicated history of collision, post collision arc-forearc separation and sea floor spreading, followed by renewed contraction and a very recent and ongoing second collision. The case for such a sequence of events can, however, be made on the basis of quite independent geological evidence. If the processes responsible are general in their nature, they could explain the apparent absence of arc-magmatic belts in association with many other supposedly forearc ophiolites.

137

INSIGHTS INTO THE TECTONOSTRATIGRAPHIC DEVELOPMENT OF THE OWEN BASIN, WEST COAST RANGE, WESTERN TASMANIA. C. Noll and M. Hall School of Geosciences, Monash University, Melbourne, Victoria 3800.

The Owen Basin of the West Coast Range, Western Tasmania, has been revisited in order to better constrain the development of this Late Cambrian - Early Ordovician basin. Key localities have been examined between Mt Jukes and Arnold Peak in order to define both the stratigraphic and structural architecture of the basin.

The Owen Basin contains an overall upward fining, transgressive stratigraphy comprising coarse grained siliciclastics. Early basin fill is dominated by pebble to boulder Lower Owen Conglomerate (LOC), deposited by a proximal coarse grained braided river system that was operating with distinctly unidirectional palaeofiow from eastern highlands. This fluvial system was replaced by a sandy braided river system in the south and a temporally equivalent marine environment in the north of the basin. Marine conditions are characterised by pro-delta and thickly bedded proximal turbidite deposits of the Middle Owen Sandstone (MOS). The basin was subsequently inundated by prograding fluvial deposits of coarse sandstone and pebble to boulder conglomerate of the Middle Owen Conglomerate (MOC). Along the basin's length the MOC gives way to the transgressive Upper Owen Sandstone (UOS), which is characterised by a heavily bioturbated siltstone, sandstone and conglomerate braid delta succession. The regionally transgressive Pioneer Sandstone gradationally overlies UOS across the basin and thickens basinward, while basin margin deposits of Pioneer Sandstone are marked by a major angular unconformity.

Basin fill thickens substantially towards the western basin margin fault system, and progressively onlaps basement to the east. Structural control of the basin is evident with a characteristic wedgeshaped basin geometry, although the basin margin fault system, known as the Great Lyell Fault, remains elusive and enigmatic. However, the key to understanding Late Cambrian basin architecture lies in clearly identifying post-depositional deformation episodes. Two distinct Devonian episodes are identified within the Owen Basin, including north-south oriented reverse faults and associated folds (Di), and a pronounced west-northwest oriented fault and fold trend (D2). The west-northwest trend overprints all other structures and is the only episode with a well developed penetrative cleavage (S2), although locally developed axial planar cleavage (Si) associated with Di folding has been mapped in some areas. D2 fold patterns are largely dependent on earlier fold geometry, with variable plunges associated with pre-existing fold limbs, an interaction which leads to the spectacular local development of Type 1 interference patterns within the Owen Basin.

The most revealing structures, however, are associated with the reactivation of east dipping faults that appear to have also controlled deposition. Folding of beds against faults across which there are significant stratigraphic thickness changes is interpreted as evidence for basin inversion and provides crucial insights into the tectonostratigraphic evolution of the Owen Basin.

138

SUBDUCTION OF CONTINENTAL ARC BASALTIC ANDESITE IMPLICATIONS FOR THE TECTONIC HISTORY OF THE SOUTHERN NEW ENGLAND FOLD BELT R.Offler\ R.Glen^ and H. Hyodo^ ^Discipline of Geology, School of Environmental and Life Sciences, University of Newcastle, NSW 2308. ^Geological Survey of New South Wales, Department of Mineral Resources, PO Box 536, St Leonards, NSW 1590. ^Research Institute of Natural Sciences, Okayama ayama IUniversity of Science, Okayama, 700-0005, Japan.

Exotic blocks of varying composition commonly occur within the serpentinite melange of the PeelManning Fault System (PMFS), southern New England Fold Belt. At Woodsreef, an exotic block approximately 70 m in length and 42 m in width occurs in serpentinites. It shows evidence for at least two phases of ductile deformation and one phase of brittle deformation. Riebeckite, albite and quartz, and less commonly neptunite, actinolite and aenigmatite occur in varying proportions; aegerine, aegerine augite (up to 38%Jd) and sphene are relict phases.

Zr/Ti02 and NbAf ratios suggest that the assemblages in the exotic block formed from protoliths of basaltic to basaltic andesite composition. Further, major element chemistry indicate that the basaltic to basaltic andesites have undergone extensive alteration prior to subduction resulting in substantial loss of Ca and in some samples P, and enrichment in Na and Si producing a rock of approximately peralkaline composition. Chondrite normalized REE patterns ((LaA^b)N = 6.1-12.8), ThA^b (2.014.06), Ta/Yb (0.14-0.74) ratios and Ti-Zr and Rock/MORB plots suggest that this rock was derived from protoliths of calc-alkaline, continental arc composition. The presence of relict magmatic magnesio-hastingsite in one sample confirms the calc-alkaline affinity of the protolith. Major, trace and REE data obtained from blueschist blocks located -1km north of the study area indicate that they have been derived from a protolith similar to that determined for the exotic block.

Ar^^/Ar^^ dating of felsic and mafic fractions from one sample from the exotic block reveals ages of 1200-1 lOOMa (albite-in vein), 326±2.0Ma and 291±3.7Ma (riebeckite-rich matrix). These ages suggest the presence of excess Ar as all minerals have been subjected to the same thermal history.

Thus the maximum age for cooling through the closure temperature for riebeckite is considered to be 291 Ma. We interpret this cooling to be related to an uplift event that occurred after subduction.

Our results indicate that the exotic block at Woodsreef was derived from a continental arc source to the east of the Gondwana margin. In transit to the margin or during subduction it underwent alteration and subsequently recrystallisation and deformation in the deeper levels of the subduction zone. Replacement of the high P assemblages in some parts of the exotic block by lower P-T riebeckite-bearing assemblages then took place during uplift at 291 Ma. The data obtained in this study and from exotic blocks elsewhere in the PMFS (Offler & Sano; unpubl.) reveal for the first time that fragments of a continental volcanic arc collided with the eastern margin of Gondwana during the -300 million year history of convergence.

139

A POSSIBLE LARGE IGNEOUS PROVINCE (LIP) IN CENTRAL WESTERN AUSTRALIA: IMPLICATIONS FOR MINERALISATION MODELS Franco Piraino^ Paul A. Morris^ and Michael T. D. Wingate^ 'Geological Survey of Western Australia, 100 Plain Street, East Perth 6004, Australia ^Tectonics Special Research Centre, Dept. of Geology & Geophysics, The University of Western Australia, Crawley 6009, Australia The Palaeo- and Mesoproterozoic Earaheedy, Edmund and Collier sedimentary basins of central Western Australia are intruded by large volumes of mafic sills and dykes, across an area that extends for about 1000 km along an east-southeast trend. The mineralogy of these mafic rocks is dominated by plagioclase-clinopyroxene-orthopyroxene, v^ith interstitial granophyre (orthoclase-quartz intergrowths). Olivine is absent. Sills range in thickness from a few metres to more than 100 metres.

Contact metamorphic effects are poorly developed, because the country rocks are dominantly siliciclastic, but local melt patches are observed along the contact with some sills. The few available age determinations include: U-Pb baddeleyite and zircon ages of 1070+6 Ma and 1465+3 Ma (Wingate, 2002; GSWA Record 2002/4) in the western parts of the Edmund Basin; a whole rock KAr age of 968±19 Ma (Nelson, 2002; GSWA Record 2002/2) from the Glenayle Dolerite, which intrudes the <1200 Ma sedimentary rocks of the Collier Basin; a whole rock K-Ar age of 1058+13 from basaltic rocks at 1600 m in drill hole GSWA Empress lA in the Officer Basin (Stevens and Apak., 1999; GSWA Record 4); and a whole rock Rb-Sr age of 1050 Ma (Compston, 1974; J. Geol.

Soc. Aus., 21, 403-411) from a dolerite sill in the easternmost part of the Earaheedy Basin. These ages indicate two magmatic events, one at 1465 Ma, the other at around 1070-1000 Ma. The 1465 Ma event appears to be confined to the western Edmund Basin, whereas the 1070-1000 Ma is far more widespread. Mafic intrusive rocks are variably fractionated tholeiites,, with most samples having Mg# (i.e. 100Mg/(Mg + Fe^^)) < 60, and Ni and Cr < 100 ppm. Samples with Mg# > 55 show weak light rare earth element (LREE) enrichment with (La/Yb)cN 3-7, and (La)cN 30 - 70 (where CN is chondrite normalised). Variations in large ion lithophile/high field strength element (i.e. LIL/HFS) ratios, such as Th/Nb, at high Mg# indicate local variations in source chemistry The overall uniform petrographic and geochemical character of these rocks and their large volume indicates that they are likely to be from a single source and were emplaced in an extensional tectonic setting. The 1070-1000 Ma time-frame appears to be associated with a global thermal event also recorded in North America (Mid-continent rift system flood basalts) and southern Africa (Umkondo mafic igneous province). The mafic sills that intrude the Edmund and Collier basins in the west, the Earaheedy and Collier Basins in the east and the mafic lavas intersected in Empress lA are coeval with the 1050-1080 Ma volcanic rocks and dykes of the Musgrave Complex (Glikson et al., 1996; AGSO Bulletin 239). It is therefore possible that in central Western Australia these igneous mafic rocks constitute a large igneous province (LIP), trending east-southeast for at least 1200 km, and termed Bangemall LIP (Morris and Pirajno, GSWA Record 2002/5).

The recognition of a Mesoproterozoic LIP in central Western Australia has important implications for mineral potential, including Noril'sk type Ni sulphides and PGE, and base and precious metal hydrothermal mineralisation. Structurally-controlled base and precious metal mineralisation is widespread in the Edmund and Collier basins. This hydrothermal mineralisation could have been emplaced into pre-existing structures as a result of the high geothermal gradients induced by the intrusion of the LIP mafic melts.

140

GEOLOGY, TECTONIC EVOLUTION AND MINERALISATION OF PALAEOPROTEROZOIC BASINS OF THE EASTERN CAPRICORN OROGEN, WESTERN AUSTRALIA Franco Pirajno', J. A. Jones\ R. M. Hocking^ and J. Halilovic^ 'Geological Survey of Western Australia, 100 Plain Street, East Perth 6004, Australia ^Curtin University of Technology, Dept. of Applied Geology, GPO Box U 1987, Perth, Australia The Palaeoproterozoic Yerrida (Windplain and Mooloogool Groups), Bryah (Bryah Group), Padbury (Padbury Group) and Earaheedy (Earaheedy Group) basins, form a volcano-sedimentary belt extending for about 700 km along the northern margin of the Archaean Yilgarn Craton, cover a total area of approximately 70 000 km^ and are part of the eastern portion of the Capricorn Orogen.

The development of these basins, began at about 2.2 Ga and continued for almost 400 million years, to about 1.8 Ga, recording a complex history of sedimentation and volcanism. The Windplain Group (c. 2.17 Ga) of the Yerrida Basin was initiated as an intracratonic sag, within which siliciclastics, originating from low-relief areas, and evaporites accumulated. To the west, the volcano-sedimentary succession of the c. 2.0 Ga Bryah Basin had a more complex history. An initial phase of sea-floor spreading generated oceanic crust at the northwest margin of the Yilgarn Craton. This was followed by the closure of the sea arm and accretion of this oceanic crust onto the Yilgarn Craton margin; with deposition of clastic and chemical sedimentary rocks in the waning stages of this accretion episode. The dominantly clastic and chemical sedimentary rocks of the Padbury Basin (c. 1.96 Ga) developed as a foreland basin on top of the Bryah Basin, during convergence and subsequent westward collision, which thrust the southern part of the Gascoyne Complex onto the Yilgarn Craton (Glenburgh Orogeny). During the c. 1.83- 1.78 Ga Capricorn Orogeny, the Bryah and Padbury Groups, behaved as a coherent tectonic unit, that was thrust towards the southeast, over the Windplain Group. At this stage, the rejuvenated Yerrida Basin was filled with turbidites, other mass-flow deposits and continental flood basalts. These rocks have been assigned to the Mooloogool Group. The Earaheedy Basin (c. 1.84-1.8 Ga) lies at the easternmost end of the Capricorn Orogen and unconformably overlies rocks of the Yilgarn Craton, Yerrida Basin and possibly the Bryah Basin. The shallow marine clastic and chemical sedimentary rocks of the Earaheedy Group are divided into the Tooloo (lower) and Miningarra (upper) Subgroups. The Earaheedy Basin developed in a passive margin setting, and is interpreted to have been much larger than its present-day exposure.

The Bryah and Padbury basins contain economically important orogenic-mesothermal gold lodes, copper-gold volcanogenic massive sulfides, sedimentary manganese, and iron ore. The Yerrida and Earaheedy basins appear to be less endowed with mineral deposits, but this may be due to lack of sustained mineral exploration in the region. In the Yerrida Basin, a giant Pb-carbonate deposit (Magellan) is sited at the unconformity between the upper Juderina Formation, and the lower unit of the Earaheedy Group. The Frere Formation of the Earaheedy Group contains large resources of iron and manganese.

141

THE ASSEMBLY AND BREAKUP OF RODINIA: ANIMATED HISTORY S.A. Pisarevsky Tectonics Special Research Centre, The University of Western Australia, 35 Stirling HW, Crawley WA 6009, Australia

There is general agreement that the Earth's continental crust could have been assembled in a supercontinent, Rodinia, in the late Mesoproterozoic and early Neoproterozoic. Rodinia is thought to have been produced by collisional events of broadly Grenvillian (late Mesoproterozoic) age, and to have been relatively long-lived (c. 1000-750 Ma)(Pisarevsky et al, 2002 and references therein).

Laurentia is thought to lie at the core of Rodinia, because it is surrounded by late Neoproterozoic passive margins formed during the breakup of the supposed supercontinent. Australia, part of Antarctica and possibly South China are considered to have lain along Laurentia's western margin, and Baltica, Amazonia and Rio de la Plata cratons along its eastern margin. The position of Siberia is disputed, but it is generally shown as lying along the northern margin of Laurentia. The position of the Congo and Kalahari cratons is uncertain, with at least four positions having been shown for Kalahari in the last few years.

Combining palaeomagnetic, geochronological, and geological data, Powell and Pisarevsky (2002) proposed a new model for the Neoproterozoic tectonic history of East Gondwanaland, in which India (together with the Rayner block of Antarctica) was not a part of Rodinia, but collided obliquely with the rest of East Gondwanaland (West Australia and the Mawson craton of Australia - Antarctica) in late Neoproterozoic. Wingate et al (2002) proposed a new AUSMEX Australia Laurentia fit based on new high-quality palaeomagnetic and geochronological data from West Australia. All these data have been incorporated into our previous animated model of the assembly and breakup of Rodinia (Powell et al, 2001). Since it has been issued, new palaeomagnetic and geochronological data from Siberia, Amazonia, and Congo became available. It caused some changes in the suggested configuration of Rodinia, and, consequently, in its animated history.

The breakup of Rodinia probably started at about 820 - 800 Ma by rifting between Australia Mawson - Kalahari and South China - Laurentia - Rio de La Plata. At 780 - 770 Ma, the spreading center jumped to a position between Laurentia and South China, and the initial branch was aborted. Kalahari detached from Australia at 760 - 750 Ma, and the Rodinia fragments began the slow journey toward their reassembly in Gondwanaland and, ultimately, Pangaea. A possible analogy for such a set of events is the opening of the North Atlantic Ocean.

References Pisarevsky S. A., Wingate M. T. D., Powell C. McA., Johnson S. & Evans D. A. D. 2002. Models of Rodinia assembly and fragmentation. In: Yoshida M., Windley B. & Dasgupta S. eds. Proterozoic East Gondwana: supercontinent assembly and breakup. Geological Society of London Special Publication (in press).

Powell C. McA. & Pisarevsky S.A. 2002. Late Neoproterozoic assembly of East Gondwana. Geology, 30, 36.

Powell C. McA., Pisarevsky S. A. & Wingate M. T. D. 2001. A new shape of Rodinia. Gondwana Research 4,136-131.

Wingate M. T. D., Pisarevsky S.A. & Evans D. A. D. 2002. A revised Rodinia supercontinent: no SWEAT, no AUSWUS. Terra Nova, 14 (in press).

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PROXIMAL CARBONIFEROUS VOLCANOGENIC SUCCESSIONS, NORTHWESTERN TAMWORTH BELT: CORRELATION AND SHRIMP DATING.

John Roberts^ and Mark Fanning^ 'School of Biological, Earth & Environmental Sciences, University of New South Wales, Sydney NSW 2052 ^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200

A proximal volcanogenic succession in the northern Tamworth Belt, the Willuri Volcanics, extends from the southern margin of the Nandew^ar Range to Carroll; it is bordered to the west by the Mooki Thrust and to the east by the Plagyan Fault. SHRIMP U-Pb ages (AS3 standard) indicate that the bulk of the sequence ranges in age from 324 to 310 Ma, equivalent to the Clifden Formation, Rocky Creek Conglomerate and Lark Hill Formation in the Rocky Creek Syncline and Currabubula Formation in the Werrie Syncline. These structures lie east of the Trevallyn-Plagyan Fault system and contain comparatively distal volcanic assemblages. The Willuri Volcanics have been attributed to all of the above-mentioned Carboniferous formations (Brown et al., 1990). The youngest rocks within the Willuri Volcanics are earliest Permian in age (290 Ma).

Igneous units within the Willuri Volcanics are predominantly rhyolitic ignimbrites, but include rhyolite flows, crystal-rich ignimbrites, a dacite dome, dacitic ignimbrite and andesite.

Intertonguing packages of these units suggest derivation from at least three volcanic sources. The northernmost, probably centred around Mount Kaputar, produced a package (Kaputar) containing the Plagyan Ignimbrite and three other ignimbrites which extends southwards to Dripping Rock. A more extensive package (Piney Range-Tulcumba) to the south ranges in age from 324-310 Ma and is dominated by rhyolitic ignimbrites and a dacite dome (4x1.5 km). Ignimbrites from near the base of this sequence extend southwards across a structurally complex area at Wean Gap into the base of the sequence in the northern part of Tulcumba Ridge. Rhyolitic ignimbrites in the upper part of the succession north of Wean Gap may be equivalent to the extensive Birken Head Rhyolite Member.

The latter unit expands in the mid-section of the Piney Range-Tulcumba package and extends southwards to the central part of Gunnan Ridge. A third major package is defined by units mappable throughout the eastern slopes of central to southern Tulcumba Ridge and Gunnan Ridge; it includes andesite and a characteristic dacitic ignimbrite at the base, followed by rhyolitic ignimbrites and a crystal-rich ignimbrite.

Sandstone and conglomerate above the Birken Head Rhyolite is followed by a crystal-rich ignimbrite (305 Ma, with Pb loss), further sedimentary rocks and rhyolitic ignimbrite. A rhyolitic ignimbrite from near the top of the section at the northern end of Tulcumba Ridge has an age of 290 Ma, which is clearly Early Permian. Although no disconformity has been detected between the crystal-rich ignimbrite dated at 305 Ma and the 290 Ma rhyolite, it is difficult to reconcile the 15 Ma time difference with the small stratigraphic separation of the two units without invoking an hiatus.

The Darthula Block (Opdyke et al. 2000), north of the Nandewar Range and west of the Trevallyn Fault, in a comparable structural position to the block containing the Willuri Formation, contains the Clifden Formation, Rocky Creek Conglomerate and Lark Hill Formation. SHRIMP dates and ignimbrite stratigraphies indicate that the bases of these fomiations are diachronous relative to the same units in the Rocky Creek Syncline. Apart from the ignimbrite-rich Rocky Creek Conglomerate, formations in the Darthula block are more distal than the Willuri Volcanics.

References Brovm, R.E., Krynen, J.P. and Brovmlow, J.W. 1990. Manilla-Narrabri 1:250,000 Metallogenic Map. Geological Survey of New South WaleSy Sydney.

Opdyke, N.D., Roberts, J., Claoue-Long, J., Irving, E. and Jones, P.J. 2000. Base of the Kiaman: its definition and global stratigraphic significance. GSA Bulletin 112, 1315-1341.

143

CONSTRAINTS ON THE MOTION OF THE ADRIATIC PLATE Gideon Rosenbaum, Gordon Lister and Cecile Duboz School of Geosciences Australian Crustal Research Centre, Monash University, Victoria 3800, Australia

The Adriatic plate consists of the Adriatic Sea and surrounding regions and is considered to be of continental origin. It is a continental mass that was actively involved in the Late Cretaceous to Cenozoic collision that gave rise to Alpine orogeny. Therefore, reconstructions of the evolution of the Alpine orogen are directly dependent on kinematic constraints of the Adriatic plate. These constraints, however, are a matter of controversy in the geological literature. The first school of thought advocates attachment of Adria to the African plate based on a substantial coherence of palaeomagnetic data from Africa and Adria since the Early Mesozoic (Channell 1996, Muttoni et al 2001). Other studies, particularly those dealing with large-scale tectonic reconstructions (e.g.

Dercourt et al 1986), have encountered geometric problems when attaching Adria to Africa and permitted independent motion of Adria during the Cenozoic.

In this study, we have established revised kinematic constraints for the motions of Africa, Iberia and Europe using a comprehensive list of kinematic parameters (Euler poles of rotation and rotation angles), which are predominantly derived from magnetic isochrons in the Atlantic Ocean.

The calculated kinematic constraints have been applied in a reconstruction model for the tectonic evolution of western Tethys since the Middle Jurassic using an interactive software package (PLATYPLUS). In this reconstruction, the Adriatic plate has been assumed to be an African promontory attached to the motion of Africa. The reconstruction shows a relatively good fit of the continents prior to the opening of the central Atlantic, with the Adriatic plate located in the westernmost Tethys Ocean. The effect of syn-rift stretching has been considered in our kinematic constraints, resulting in a more westerly position of Iberia at the Middle Jurassic. This configuration eliminated a considerable overlap of Adria and Iberia recognised in earlier reconstructions. However, there are still minor geometric problems with terranes of ambiguous origin and poor kinematic constraints (e.g., Corsica, Sardinia and Kabylies). The reconstruction also shows the formation of a new ocean (the Liguride Ocean) during Middle-Late Jurassic, whose remnants are found in ophiolitic complexes throughout the western Mediterranean and the Alps.

However, the tectonic evolution of the Alps as inferred from geological observations cannot be adequately explained. We therefore conclude that a model of Adria as an African promontory is plausible but cannot solely account for Alpine orogeny, which possibly involved independent motions of additional allochthonous terranes.

References Channell, J. E. T., 1996. Palaeomagnetism and palaeogeography of Adria. In: Morris, A. & Tarling, D. H. (Eds.), Palaeomagnetism and tectonics of the Mediterranean region 105, Geological Society, London, Special Publications, pp. 119-132.

Dercourt, J., Zonenshain, L. P., Ricou, L. E., Kazmin, V. G., Le Pichon, X., Knipper, A. L., Grandjacquet, C., Sbortshikov, I. M., Geyssant, J., Lepvrier, C., Pechersky, D. H., Boulin, J., Sibuet, J.-C., Savostin, L. A., Sorokhtin, O., Westphal, M., Bazhenov, M. L., Lauer, J. P. & Biju-Duval, B., 1986.

Geological evolution of the Tethys belt from the Atlantic to the Pamirs since the Lias.

Tectonophysics 123, 241 -315.

Muttoni, G., Garzanti, E., Alfonsi, L., Cirilli, S., Germani, D. & Lowrie, W., 2001. Motion of Africa and Adria since the Permian: paleomagnetic and paleoclimatic constraints from northern Libya. Earth and Planetary Science Letters 192, 159-174.

144

TECTONICS AND STRATIGRAPHY IN NEOPROTEROZOIC BASINS, AUSTRALIA Mike Sandiford School of Earth Sciences, University of Melbourne, Victoria, 3010.

The various Neoproterozoic Basins of central and southern Australia provided yet another focus that concentrated the mind of Chris Powell, in no small part because of the fascinating insights they provide into the links between stratigraphy and tectonics. These basins (including the Georgina, Amadeus, Ngalia, Wiso, Officer Basins and the Adelaide "geosyncline") represent the fragmented remains of the formerly more continuous Centralian Superbasin. This talk explores some of the tectonic lessons provided by the construction and fragmentation of the Centralian Superbasin, focusing on two examples relating to: 1. the controls on the locus of successive rift events associated with deposition of the Burra and Umberatana Groups in the northern Flinders Ranges (Adelaide "geosyncline"), and 2. the relationship between stratigraphic and tectonic development of the Alice Springs Orogen in central Australia.

In both cases, the evolving distribution of deformation can be understood in large part in terms of the thermal consequences of prior tectonic activity. This provides important geological evidence pertaining to the temperature sensitivity of lithospheric rheology, and highlights the role that intraplate tectonic activity plays in organizing the thermal and mechanical character of the lithosphere. Indeed, in true "Powell-esque" fashion, it will be suggested that this subtle record of tectonic and stratigraphic feedback in these basins points to a profound process of geochemical self-organization at the scale of the continents!

145

BACK-ARC DEFORMATION IN THE KURIL BASIN AND SEA OF OKHOTSK: FAR FIELD EFFECT OF INDIA-EURASIA COLLISION OR THE RESULT OF ROLLBACK OF THE PACIFIC SLAB? W.P. Schellart, M.W. Jessell and G.S. Lister Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, VIC 3800, Australia Geological and geophysical data of the Sea of Okhotsk indicate that it extended during the Eocene - Early Miocene (Worrall et al 1996) and that the Kuril Basin formed during the Middle Miocene (Maeda 1990). These periods of extension are coeval with formation and shearing along the Saklialin-Hokkaido dextral shear zone (Fournier et al 1994). This implies that these structures share a similar origin. Furthermore, the geometrical shape of the Kuril Basin and paleomagnetic data of the arc imply an anticlockwise rotation of the Kuril Arc during extension of the Sea of Okhotsk and opening of the Kuril Basin. This suggests that deformation in the overriding plate is the result of asymmetrical anticlockwise hinge-line retreat of the Pacific slab subducting beneath the Kuril Arc. In this work we present results of analogue experiments to simulate the tectonic evolution of the Kuril Arc and back-arc region. The experiments simulate the overriding plate (Okhotsk plate), which has been extended by asymmetric spreading due to anticlockwise retreat of a door, simulating rollback of the subducting Pacific plate. The model results show the formation of a N-S to NE-SW oriented dextral shear zone near the far edge of the retreating boundary (e.g. Kuril Arc Japan Arc junction) analogous to the Sakhalin Hokkaido dextral shear zone. Contemporaneously, normal faults and grabens form, striking parallel to the retreating boundary near the far edge and more oblique near the door hinge. This is similar to extensional structures observed in the Kuril Basin and the Sea of Okhotsk. Furthermore, the model shows that the amount of extension progressively decreases away from the retreating boundary. This appears to have also happened in the Kuril - Okhotsk region, where the bathymetry progressively decreases from the Kuril Basin towards the north, most likely reflecting the thickness of the underlying thinned continental crust. It is also shown that with progressive deformation, extension is increasingly accommodated by the region close to the retreating boundary. This can account for Eocene - Early Miocene extension in the Sea of Okhotsk followed by Middle Miocene spreading in the Kuril Basin.

Based on the experimental results, it is suggested that the structures observed in the Kuril region are not necessarily the far field effect of the India - Eurasia collision, as proposed by other authors (e.g. Worrall et al 1996). This model does not explain the extent of extension in the Sea of Okhotsk and the Kuril Basin, the wedge shaped geometry of the Kuril Basin, anticlockwise rotation of the Kuril Arc, as well as the north-south oriented strain gradient in the Kuril Basin and Sea of Okhotsk. From comparison of the geological and geophysical data with the results of the analogue models, we conclude that structures observed in the Kuril - Okhotsk region and the Sakhalin Hokkaido dextral shear zone are best explained by southeastward anticlockwise rollback of the Pacific slab and collapse of the bounding overriding plate towards the retreating hinge-line.

References Fournier, M., Jolivet, L., Huchon, P., Sergeyev, K. F. and Oscorbin, L. S. 1994. Neogene strike-slip faulting in Sakhalin and the Japan Sea opening. Journal of Geophysical Research 99, 2701-2725.

Maeda, J.I. 1990. Opening of the Kuril Basin deduced from the magmatic history of central Hokkaido, North Japan. Tectonophysics 174, 235-255.

Worrall, D. M., Kruglyak, V., Kunst, F. and Kuznetsov, V. 1996. Tertiary tectonics of the Sea of Okhotsk, Russia: Far-field effects of the India-Eurasia collision. Tectonics 15, 813-826.

146

RECONNAISSANCE DETRITAL ZIRCON GEOCHRONOLOGY PROVENANCE OF THE PALAEOPROTEROZOIC ASHBURTON FORMATION: IMPLICATIONS FOR PILBARA AND YILGARN AMALGAMATION. Keith Sircombe Tectonics Special Research Centre, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, ph. 08-9389-7871 Email: ksircombe@uwa.edu.au

The deformed basement and sedimentary basins of the Capricorn Orogen record the Proterozoic amalgamation of the Pilbara and Yilgarn cratons in Western Australia. However, the interpretation of this geological record remains uncertain with differing models postulating a range of tectonic regimes and timing.

In order to test some of these models, this project is investigating the detrital zircon geochronology of a key section of the Capricorn Orogen: the Ashburton Basin and in particular the Ashburton Formation. The Ashburton Fomiation is a 5-12 km thick turbiditic succession of mudstone and immature sandstone with minor amounts of conglomerate and volcanics. Based on field, modal and palaeodirection data, Thome and Seymour (1990) interpreted the succession as a longitudinal deep-marine foreland basin that formed as collision with the Yilgarn Craton began in the east.

During early phases detritus was fed from the southeast, presumably from the uplifted Sylvania Inlier. Upper successions were joined by submarine fan systems prograding northwards as the continuation of the collision ultimately culminated in the fluvial-dominated sediments preserved in overlying basins.

Seven samples of the Ashburton Formation were collected to represent a spatial and temporal transect across the longitudinal basin. Detrital zircon was extracted using standard methods and 70 grains in each sample were analysed for U/Pb isotopic age using excimer laser ablation-inductively coupled plasma-mass spectrometry at the RSES, Australian National University, Canberra.

Samples from lower in the succession tend to have polymodal and wide-ranging age components, whereas higher succession samples tend toward younger and unimodal components. While the polymodal samples contain individual grain ages that potentially match the younger range of known ages in the Pilbara and Yilgarn cratons, there are no prominent clusters in this age range and Archaean ages are uncommon. All samples have prominent late Palaeoproterozoic components that are interpreted as having a provenance in similarly aged granites known from the southern margin of the Capricorn Orogen (Occhipinti et al., 1998).

The results confirm the two-stage evolution of the basin. Older successions were derived from a variety of sources, possibly including upper successions of the Hamersley Basin. The lack of Archaean ages grains suggests that the northern Pilbara, related inliers or even northwestern Yilgarn terranes were not prominent sources.

All samples reveal exposure to late Palaeoproterozoic events related to the formation of the Gascoyne Complex during the PilbaraYilgam collision. This exposure becomes prominent in upper successions, thereby confirming the shift in provenance to southerly sources. Along with recently revised stratigraphic constraints these data suggest that the Ashburton Basin was a tectonically complex geological feature and the relationships with adjoining basins require careful reconsideration.

References Occhipinti S.A., Sheppard S., Nelson D.R., Myers J.S. & Tyler I.M. 1998. Syntectonic granite in the southern margin of the Palaeoproterozoic Capricorn Orogen, Western Australia. Australian Journal of Earth Sciences 509-512.

Thome A.M. & Seymour D.B. 1991. Geology of the Ashburton Basin, Western Austalia. Western Australian Geological Survey Bulletin 139.

147

LATE ORDOVICIAN - SILURIAN OPHIOLITE EMPLACEMENT IN THE WESTERN AND CENTRAL LACHLAN OROGEN, SOUTHEAST AUSTRALL\ Catherine Spaggiari^ David R. Gray^ and David A. Foster^ ^School of Geosciences, Monash University, Melbourne, Victoria 3800 ^School of Earth Sciences, University of Melbourne, Melbourne, Victoria, 3010 ^Department of Geological Sciences, Box 112120, University of Florida, Gainesville, FL 32611, USA Dismembered, Middle to Late Cambrian ophiolites preserved in the Heathcote Fault Zone and Governor Fault Zone of the w^estern and central Lachlan Orogen, Victoria, were emplaced as fault slivers during orogenesis in Late Ordovician to Silurian times. Geometrical, chronological, structural and metamorphic constraints indicate that these ophiolites did not undergo Tethyan-style obduction onto a continental margin, but w^ere emplaced during intra-oceanic thrusting by a combination of processes including accretion during offscraping, accretion as blocks in melange during underplating, and as duplexes or imbricate fault slivers. This is in contrast to Cambrian ophiolites in Tasmania that are interpreted to have sheet-like form, and were obducted onto the passive continental margin of eastern Gondwana at 15 Ma (Meffre et al., 2000). Radiometric and palaeontological constraints indicate that the Cambrian oceanic basement in the western and central Lachlan Orogen formed shortly after the main shortening phase of the Delamerian-Tyennan-Ross Orogeny, between 505 and 495 Ma. This crust probably formed in a supra-subduction zone (SSZ) environment during roll-back of a west-dipping subduction zone that formed after ophiolite emplacement onto the Gondwana margin and during widespread extension in Tasmania and western Victoria (see Foster et al., this volume). SSZ magmatism most likely produced both backarc and forearc assemblages that evolved to include calc-alkaline rocks now preserved in the Mount Wellington Fault Zone (Jamieson-Licola / Darkly River greenstones). Age constraints for Cambrian rocks from Dookie, Howqua, and Licola all overlap at around 500 Ma, which supports the interpretation that they formed in the same SSZ. Lithological associations and sedimentation patterns indicate that topographic highs formed part of this Cambrian oceanic floor, in part due to the SSZ environment (e.g. seamounts) but possibly including fragments of slightly older, remnant transitional oceanic or oceanic arc crust that may have rifted away from mainland Gondwana following Cambrian obduction. This led to the development of W-Pacific style microplates and two major basins, now grossly preserved as the western and central Lachlan Orogen structural zones.

This configuration affected deformation patterns in these basins, with the central Lachlan Orogen converging obliquely upon the western Lachlan Orogen, during which time the ophiolites were emplaced. Disruption of the SSZ crust in the Heathcote Fault Zone and Governor Fault Zone at Howqua produced distinctive fault sequences formed during underplating and accretion of melange beneath duplexed oceanic crust and turbidites. Rocks deformed at shallower crustal levels are preserved in the Governor Fault Zone at Dolodrook, and are interpreted as the remnant of an offscraped topographic high that perhaps contributed to initiation of lock-up of subduction along this margin, just prior to collision with "arc-like" SSZ rocks in the Mount Wellington Fault Zone (Barkly River greenstones). The formation of the Heathcote and Governor Fault Zones, and ophiolite emplacement, took place within a backarc setting inboard of a major subduction zone to the east from approximately 450 to 420 Ma, followed by periods of reactivation in Devonian and Carboniferous times.

Reference Meffre S., Berry R.F., and Hall M. 2000. Cambrian metamorphic complexes in Tasmania: tectonic implications. Australian Journal of Earth Sciences. 47, 971-985.

148

THE DELAMERIAN OROGENY: ARC-CONTINENT COLLISION STARTS THE ACCRETIONARY GROWTH OF EASTERN AUSTRALIA David H. Taylor, Ross A. Cayley, Vincent J. Morand and Kylie E. Wohlt Geological Survey of Victoria, P.O. Box 500 East Melbourne, Victoria 3002

The Delamerian Orogen is the oldest most inboard part of the Tasmanides, recording initial convergence as the Pacific Ocean began to close after enormous expansion during Rodinia breakup. During break-up the thick passive margin sequence of the Adelaide Geosyncline developed and geophysical data suggests that rift tholeiites related to the break-up may lie at its base on attenuated continental crust, as better documented in Tasmania and western NSW. The Delamerian Orogeny drove the Adelaide Geosyncline back across its continental basement as a west-verging fold and thrust belt. Geophysical and drilling data from western Victoria suggest that, like Tasmania, this deformation was caused by arc-continent collision that resulted in westward obduction of forearcarc ophiolites onto the continental margin. Boninitic chemistry of the ophiolites demands formation in an intraoceanic arc above a subduction zone. Botomian age trilobites (-520 Ma) at Heathcote in Victoria and dating of the Heazlewood MUC in Tasmania (-514 Ma) show that this oceanic crust formed just before the arc-continent collision (-514-500 Ma). This tight timing is no coincidence— it is the old, cold and dense oceanic crust near the continental margin that is likely to founder to initiate the intraoceanic subduction. An east dipping subduction geometry then shortly thereafter drew in the continental margin on the subducting plate beneath the young, hot and buoyant intraoceanic arc forming on the over-riding plate. The Hummocks Serpentinite may be the most westerly remnant of the obducted ophiolites (Dimboola Igneous Complex) since largely dismembered by post-collisional collapse and erosion. The original collisional suture (Yanac Suture) has been modified by later tectonics and is obscured by the Grampians-Stavely Zone that includes post-collisional calc-alkaline volcanics (Mount Stavely Volcanics). There is a belt of high T-low P metamorphics with pre- to post-collisional mantle-derived intrusives in the deformed continental margin rocks. This belt runs obliquely across the structural trends of the entire orogen, marking what must have been a zone of extension/high-heat-flow in the subducting plate margin.

700 Ma Rodinia breakup

S20 Ma subduction

510 Ma arc-continent collision

This scenario of Delamerian arc-continent collision provides some constraints on models for the later evolution of the western Lachlan Fold Belt. It accounts for the oceanic basement that floors the western Lachlan having been created before the collision rather than afterwards. A steep easterly dipping lithospheric scale boundary visible in deep seismic and teleseismic data, with overlying subduction-enriched lithosphere, can be interpreted as the fossil Cambrian subduction zone—something at odds with models for the western Lachlan that require a later shallow westdipping subduction zone here. The Tyennan Orogeny in Tasmania has also been interpreted as resulting from an arc-continent. Compelling parallels in the crustal geometries, rock sequences and timing of events for both Western Victoria and Tasmania during the Cambrian suggest they were part of the same convergent margin during an arc-continent collision. If Tasmania was part of this margin it is perhaps surprising that its position is next pinned in the early Ordovician to be outboard of this margin (as the Selw^n Block) to the east of the western Lacihlan. Complex plate rearrangements to accommodate the collision (rather than simple subduction flipping) are, however, in evidence, with the western Lachlan showing that parts of the pre-collisional oceanic crust remained undeformed in a passive deep marine setting to accumulate the Lachlan turbidite pile. Such a complex plate rearrangement might account for the rapid outboard movement of Tasmania/the Selwyn Block (or perhaps Tasmania was never part of the Cambrian margin despite the compelling parallels?).

149

PALAEOPROTEROZOIC PLATE COLLISION IN THE KIMBERLEY REGION OF NORTHERN AUSTRALIA: A CONNECTION TO LAURENTIA? I. M. Tvler^ S. Sheppard' and K. M. AnsdelP 'Geological Survey of Western Australia, 100 Plain Street, East Perth 6004, Western Australia ^Dept. of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada.

The assembly of Laurentia during the Palaeoproterozoic has been interpreted as taking place through plate tectonic processes involving terrane accretion and plate collision. The AUSMEX reconstruction of Rodinia, based on new palaeomagnetic data, places the southwestern margin of Laurentia against the eastern margin of Proterozoic Australia. Models for the Australian Palaeoproterozoic had assumed that tectonic and magmatic activity occurred within a late Archaean craton. However, remapping, combined with new geochronological, geochemical and geophysical data suggests that Proterozoic Australia was also assembled by plate-tectonic processes, although these may have differed from those operating at the present.

As an example the Halls Creek Orogen in the Kimberley region of northern Australia preserves a record of terrane accretion between 2500 and 1850 Ma that was followed by collision between the Kimberley Craton and the North Australian Craton at 1820 Ma. Geophysical data show differences in the buried basement on either side of the orogen. Detrital zircon populations suggest that the eastern basement is Archaean, whereas the western basement is quite different, consisting of a series of accreted late Archaean to early Palaeoproterozoic terranes.

Three tectonostratigraphic terranes are exposed in the orogen, each with histories that reflect disparate tectonic settings between 1910 and 1820 Ma. Turbiditic metasedimentary rocks that had previously been correlated across the orogen, were deposited, and deformed and metamorphosed at different times in each terrane. The eastern terrane was a passive continental margin between 1910 and 1840 Ma. Within the central terrane an oceanic island arc or ensialic marginal basin developed at 1865 Ma, which was the locus of rifting at 1840 Ma. The western terrane formed between 1870 and 1850 Ma as a rift within older terranes accreted to the Kimberley Craton prior to 1900 Ma. Granites and gabbros intruded the western terrane between 1865 and 1850 Ma. The central terrane was intruded by tonalite sheets at 1850 Ma, with the eruption of felsic volcanic rocks and the formation of associated VMS deposits at 1840 Ma. Further granites and gabbros intruded mainly the central terrane between 1835 and 1805 Ma. Each magmatic event has contrasting chemical and Nd isotope compositions and was derived from separate sources in different tectonic settings.

The Halls Creek Orogen does lack accretionary complexes, blueschists and ophiolites. Some of the granitic rocks are comparable to adakites in Phanerozoic orogens, and to Archaean high-Al TTG suites. However the granitic rocks are overall more silicic than igneous rocks from Phanerozoic Andean-type margins, and have almost exclusively negative initial values. Therefore, a greater degree of crustal recycling took place than is seen in Phanerozoic convergent margins. A model involving some form of plate-tectonics is preferred to best explain the juxtaposition of different cratons and terranes formed in disparate tectonic settings. However, the absence of some key characteristics of modern-style plate tectonics suggests that the processes have changed or evolved since the Palaeoproterozoic.

The Palaeoproterozoic evolution of the Halls Creek Orogen is broadly similar to that of the TransHudson Orogen. A connection between the two around northeastern Australia would produce a Palaeoproterozoic collisional belt on a scale and diversity similar to the present Alpine-Himalayan Orogen.

150

MAFIC MAGMATISM IN TIME AND SPACE - A KEY TO CORRELATING DISPERSED FRAGMENTS OF THE PALAEO-ACTIVE MARGIN (LATE PROTEROZOIC TO EARLY PALAEOZOIC) OF SOUTH-EASTERN GONDWANA. Van Penglis and John Foden Department of Geology and Geophysics, University of Adelaide, S.A., Australia

The Adelaide Foldbelt in South Australia and the Transantarctic Mountains in Antarctica are components of the formerly contiguous Neoproterozoic to Early Palaeozoic continental margin orogen along the south-eastern fringe of the palaeo-supercontinent of Gondwana. The margin orogen extended for over 5000 km and dispersed fragments from this region are also seen in Tasmania, New Zealand and Victoria.

The pre-, syn- and post-magmatic activity of the orogens (Delamerian in the Adelaide Foldbelt and Ross in the Transantarctic Mountains) provide an account of the genesis and evolution of the plate reorganisation. In the Adelaide Foldbelt and Victoria, several mafic suites and their potential sources have been identified.

(1) Overlying Archaean to mid Proterozoic orogenic basement sequences of the Adelaide Geosyncline are Late Proterozoic tholeiitic basalts marking an early rifting stage (ca. 1000 Ma to ca. 900 Ma). They are suspected of having an asthenospheric source. This early rifting and later renewed cycles resulted in thick Late Proterozoic to Mid-Cambrian sedimentary sequences being deposited in shelf to near shelf environments (Turner and Foden 1990).

(2) Alkali basalts in the Adelaide Geosyncline are associated with renewed rifting in the Early Cambrian 545 Ma) and suspected of being sourced from enriched sub-continental lithospheric mantle (SCLM) - (Foden et al 2001).

(3) The Cambro-Ordovician Delamerian Orogeny (--514 Ma to -485 Ma) brought sedimentation to an end. During the deformation event, tholeiitic basalt activity recommenced in association with Iand S-type granites. The basalts are believed to be similarly sourced as the Late Proterozoic tholeiites (Foden et al. 2001).

(4) Deformation of the Ross and Delamerian orogenies was terminated by a phase of Late Cambrian to Early Ordovician uplift 485 Ma). This was followed by genesis of undeformed gabbros and norites and related A-type granites. It has been suggested that a tholeiitic basalt (from asthenospheric decompression), contaminated firstly by enriched SCLM and secondly by crustal lithosphere, is the source of the gabbros and norites (Turner 1991).

(5) In Victoria, subduction zone related calc-alkaline suites and boninites (both Middle to Late Cambrian) have been identified. The calc-alkaline suites have been recognised in the Stavely and Mt. Wellington Greenstone Belts, whereas the boninites are identified in the Heathcote and Mt. Wellington Greenstone Belts (Crawford 1988).

Similar suites of mafic rocks have been discovered in the dispersed fragments of the palaeo-active margin of the south-eastern Gondwana. Compilation of a data base from research carried out within all the dispersed fragments (geochemistry and geochronology of mafic rocks, coupled with sedimentation, deformation, cleavage, metamorphic, cooling and uplift histories) provides an opportunity for the construction of fence diagrams (with time and space coordinates). These analyses allow us to track the onset of subduction beneath the S. E. Gondwana margin and to map its western-most migration prior to easterly retreat in the Ordovician.

References Crawford, A.J., 1988. Geology of Victoria (2"^ ed): 37-62.

Foden, J. etal., 2001. Chem. Geol.,V. 182: 663-695.

Turner, S.P. and Foden, J.D., 1990. Mafic Dykes and Emplacement Mechanisms: 431-434.

Turner, S.P., 1991. Unpublished Ph.D. thesis, Uni. of Adelaide: 1-482.

151

PETROGRAPHY OF THE MARONGHI CREEK BEDS, YARRAMAN BLOCK, SE QLD E. C. Willey University of Southern Queensland (and 35 Mina Street)y Toowoomba, Qld

Petrography indicates a tight provenance for the Maronghi Creek beds (MCB) and perva-sive development of microstructural features in the MCB and associated hornblende and augite microdiorites (Willey 1999). The study area - the largest outcrop of MCB {ss) of pre-latest Carboniferous-earliest Permian age (Willey 1998) - covers roughly 20 km x 20 Ian and lies in the southern Yarraman Block which also comprises the Sugarloaf Metamorphics and associated intrusions. The MCB is a broken formation dominated by poorly sorted, massive, subangular arenites and associated lutites, with very rare granule conglomerates, representing deposition from turbidites, with thin interbedded acid shard tuff. Also present are rare cherts and three small outcrops of altered pil-lowed felsic volcanics. Structures suggest emplacement in an accretionary wedge (Willey 1998) most probably of Devonian-Carboniferous age.

The petrography indicates two provenances: (a) a rhyodacitic volcanic source - monominerallic, unstrained and occasionally embayed quartz; oligoclase, orthoclase (sometimes microperthitic) and microcline; no mafics or opaques, very rare tourmaline and zircon; volcanic clasts - eutaxitic, devitrified glass and porphyritic (in feldspars and quartz, but no mafics or opaques); very rare granophyric clasts; and (b)an intraformational sediment source - clasts mainly mud/mudstone and silt/siltstone, some sand/arenite (also very rare chert), found chiefly in coarser-grained samples.

Quartzes have (a) an essentially volcanic origin and (b) a maximum size of 0.4-0.6mm irrespective of size of other components or sample mean grainsize, suggesting a restricted ultimate source for all the material. The arenites show a continuum with four end members: viz, (i) a feldspar-rich suite; (ii) a volcanolithic-rich suite; (iii) a coarser-grained suite rich in re-worked sediments; and (iv) a suite rich in subrounded to rounded suggesting prior residence in high energy environments.

Inter-bedded tuffs indicate the same volcanic source as the sediments. Distribution of the suites does not suggest a ghost stratigraphy and could reflect tectonic mixing. Comparison with regional Devonian-Carboniferous accretionary wedge masses indicates close similarity to petrofacies A and B of the Coramba beds (Korsch 1981) because of absence of mafics, although andesite clasts present in these petrofacies have not been observed in the MCB. Fractures and (often repeated and diverse) veins to cataclastic and mylonitic deformation characterises all but 23 of over 330 thin sections of MCB and associated microdiorites. That so many sections show these features appears significant since field samples were not generally selected to study structural features, and because of the small size (20 mm x 20 mm) of actual rock samples studied. Vein margins are generally sharp, but some show diffuse margins. Vein fills include: drusy, eutaxial, and crack-seal quartz; drusy, eutaxial and felty feldspar; rare drusy and felty calcite; diffuse- and sharp-edged 'cataclasites', and brown fine-grained cataclasites or pseudotachylites. Vein character varies dependent on material cut. Cataclasites and mylonites show scaleindependent self-similarity in thin section, hand specimen and outcrop. Not all structures can be attributable to accretionary wedge emplacement (Willey 2000, p. 148).

Acknowledgement, Chris Fergusson.

References Korsch R. J. 1981. Some tectonic implications of sandstone petrofacies in the Coffs Harbour Association, NEO, New South Wales. Journal of the Geological Society of Australia, 28, 261-269.

Willey E. C. 1998. The Maronghi Creek Beds: A preliminary appraisal. Queensland Government Mining Journal 99 (No. 1161), 49-56.

Willey E. C. 1999. Intrusives in the southern outcrop of the Maronghi Creek beds (SE Qld). In Flood, P. G. (Editor) Regional geology, tectonics and metallogenesis - New England Orogen,pp. 267-273. Published by Earth Sciences, University of New England, Armidale, NSW, Australia.

Willey E. C. 2000. Esk Trough-Yarraman Block contact, an unconformity: its nature and implications for regional tectonics. Journal of the Geological Society of Australia, 47, 139-152.

152

ACCRETIONARY OROGENS AND CONTINENTAL GROWTH Brian Windley and Shigenori Maruyama Department of Earth & Planetary Sciences, Tokyo Institute of Technology, 0-okayama2-12-l, Meguro, Tokyo 152-8551, Japan. smaruyam@geo.titech.ac.jp Email: bfsv@leicester.ac.uk

In contrast to collisional orogens, accretionary orogens form by the accretion of island arcs, accretionary wedges, ophiolites, and plume-generated oceanic plateaus, seamounts and oceanic islands; they may also contain slices of small continental blocks, passive margin sediments, and continental margin magmatic arcs. Island arcs are typically the most common component. Thus accretionary orogens are characterised by much crustal growth of juvenile material derived directly or indirectly from the mantle; they have been the principle means of continental growth through Earth history.

The processes of formation of modern accretionary orogens can be best observed in the western Pacific, within which the Mesozoic-Cenozoic Japanese islands are the best-studied example. Older orogens include Archaean greenstone belts worldwide; Birimian, W. Africa; Yavapai, SW. U.S.A; Arabian-Nubian Shield; Cadomian, N W Europe; Cordillera of W. North America; and the Altaids of Central Asia. However, recent advances in understanding trench-accretionary geology in Japan have yet to be applied to most older accretionary orogens.

The nature of accreted ocean floor material in accretionary orogens is poorly understood, largely because the role of ophiolites (Penrose definition) has been over-emphasised, because they were defined in 1972 only as components of collisional orogens, because accretionary orogens were not recognised at that time. Accretionary orogens contain far more slices of ocean floor than collisional orogens. For example, in southern Japan there are over 1000 slices of ocean floor, but only about 4 ophiolites. On approaching the trench, sheeted dykes, gabbros and ultramaflc rocks are normally subducted; only the basalts, cherts, hemipelagics and sandstones are accreted to the accretionary wedge and growing orogen. As a consequence of this imbalance of understanding, the cherthemipelagic-sandstone sequences in accretionary orogens have been poorly studied, whereas in Japan their ridge-trench migration history at trace element level, and their bedding-parallel, thrust imbrication are well understood.

In this paper we aim to draw upon current knowledge of the modem ocean floor and trenches in order to help understand their contribution to the Earth's accretionary orogens. This will include consideration of: the role of ridge subduction in creating orogeny, the differences between modem and Archean ocean-ridge metamorphism, tomography and crust-mantle re-cycling, the recognition of plateaus in ophiolites with implications for plume-superplume origin, the presence or absence of associated high-pressure and ultrahigh-pressure rocks, the style of accretionary processes during the Archean crustal growth stage, and changes in stratigraphy and crustal thickness of oceanic crust and ophiolites with time.

Mantle overtum at 2.7-2.6 Ga and 2.1-1.9 Ga resulted from a change from two-layer mantle convection to whole mantle convection and was responsible for the introduction of voluminous juvenile material to the crust and the formation of many accretionary orogens. Plumes generated flood basalts where continents existed, and oceanic plateaus and islands in oceanic lithosphere.

Ridge accretion rates and subduction rates were enhanced, leading to formation of many arcs and massive generation of accretionary orogens. These mantle overtums were responsible for the largest periods of continental growth in Earth history.

153

THE IMPACT OF TIBETAN UPLIFT ON GLOBAL CHANGE H. Zheng^ K. Butcher^ and C. Lawrence^ ^Department of Marine Geology and Geophysics, Tongji University, 200092, Shanghai, PR China ^Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, Australia The northw^estem margin of the Tibetan Plateau is a step-like feature where elevation rises from 1 to 1.5 km along the southern margin of the Tarim Basin to c. 5+ km in less than 30 km.

Streams draining from the plateau carry coarse boulders well out into the Taklamakan Desert, and debris flows form on alluvial fans between the streams. The Taklamakan Desert has shifting dunes of fine-grained sand, and wind-blown silt decorates the piedmont and northfacing mountain slopes up to elevations of 5 km. Finer dust particles are exported from the region in episodic dust storms that carry the dust eastward into the more humid cental-eastern parts of China. The piedmont is the zone of interaction between transport of a great variety of clast sizes into the Tarim Basin and the redistribution and sorting of the finer particles by aeolian processes.

Our analysis of the 7 to 10 km clastic wedge of Cretaceous to Cenozoic sediments deposited in the piedmont of the northwestern edge of the Tibetan Plateau shows that 3 to 4 km accumulated in the last 4.5 m.y. Eocene to Early Oligocene limestone on the northwestern Tibetan Plateau demonstrates that it was at sealevel at 35 Ma. The succeeding Oligocene and Miocene redbeds are fine to very fine-grained sandstone, siltstone and shale deposited in lakes, fan deltas and low-energy westward-draining streams. The sandstone is quartzose, and bedded gypsum in the Miocene redbeds indicates evaporitic conditions. A marked change in grainsize and composition of the clastics occurred at 4.6 Ma, with deposition of coarse, lithic detritus in an east-draining fluvial system. The grain-size and sedimentation increase upward, with 1.3 km of mainly boulder conglomerate being deposited in the Yecheng section between 3.6 and 2.5 Ma. Pale yellow siltstone bands intercalated with the conglomerate are aeolian dust, similar to the loess currently coating the north-facing alluvial fans and mountain slopes in Tibet. This marked change in facies in the early Pliocene marks the beginning of the main phase of the uplift of northern Tibet, and contemporaneous with the change around 3.5 Ma from aeolian red clay deposition on the northern Chinese Loess Plateau to the coarser silty yellow loess.

The onset of these changes precedes the 2.6 Ma onset of the northern ice age by less than 1 million years, suggesting that the uplift of Tibet intensified the northwestern winter monsoon and triggered the Northern Hemisphere ice age.

154

GEOCHEMICAL AND Nd ISOTOPE CONSTRAINTS ON PROVENANCE OF THE WILLYAMA SUPERGROUP, SOUTH AUSTRALIA, AND COMPARISONS TO THE MT ISA INLIER Barovich, Karin Conor, Colin^ and Foden, John^ ^Adelaide University; ^Geological Survey Branch, Mineral Resources Group, PIRSA

The geochemical and Nd isotope compositions of siliciclastic sedimentary rocks from the 1720 1640 Ma Willyama Supergroup in the Olary Domain (OD) of the Curnamona Province allow us to: 1) evaluate correlations with the Broken Hill Domain (BHD); 2) constrain possible source terranes for basin fill; 3) draw comparisons to possible Mt. Isa Inlier basin sedimentary sources. Initial ENCI values for the OD Willyama Supergroup range from about -2.8 in the lowermost part of the sequence to values of -4 to -6.8 throughout the rest. The lower part includes mantle-derived ca 1710 Ma rhyolitic volcanic rocks (initial 8Nd from -2 to +2). These may have contributed to the sedimentary detritus, resulting in elevation in initial values in the lower parts of the basin.

Age frequency patterns from detrital zircons in the OD Willyama Supergroup include some early Archaean zircons and a range of Palaeoproterozoic ages, dominated by a 1780 Ma component (Page et al., 1998; 2000). The age patterns are not strictly compatible with a Gawler Craton source, and in addition, the initial 8Nd isotope values are far too positive to allow sole derivation from the Archean to Palaeoproterozoic rocks of the Gawler Craton, which at 1700 Ma averaged an initial ENCI of -12. A component more isotopically juvenile than the Gawler Craton is required.

Comparison of geochemical analyses of Willyama Supergroup from the OD with the BHD Broken Hill Group (Slack and Stevens, 1994) offers corroboration of the Province-wide correlations, based most recently on geochronological studies. As in the BHD, the trace element abundances of OD Willyama Supergroup samples require a provenance dominated by enriched felsic igneous material.

A firm case for the derivation of the Willyama sediments from an Arunta source with its abundant K-REE-U-Th -rich 1780-1750 Ma granites is provided by a combination of detrital zircon ages and geochemical correlations. Based on U-Pb zircon ages, correlations for basin development have been drawn between the Curnamona Province and the Mt. Isa Inlier (Page and Sun, 1998) and spatial correlations have been proposed in palaeogeographic reconstructions of Proterozoic Australia (Betts and Giles, 2001). However, geochemical data from Superbasin sequences in the Leichhardt River Fault Trough of the Mt Isa Inlier (Erikkson et al., 1992) reflect a mixed mafic and felsic provenance unlike that for the Willyama Supergroup. Those workers proposed the eastern Kalkadoon-Leichhardt Belt as a likely sediment source. Sparse Nd isotope data from Eastern Fold Belt rocks (Page and Sun, 1998) are less negative than the bulk of the Willyama Supergroup, and also support the Kalkadoon-Leichhardt Belt as a source terrain. Models that spatially link the Willyama and Mt Isa Inlier will have to explain the provenance differences that are suggested by the geochemical and Nd isotope data.

While the Curnamona Province is famous for its massive Pb-Zn-Ag deposits, the LREE and HFSE enriched nature of the Willyama Supergroup suggests that granite and pegmatite-related U-Th-REE deposits are a viable exploration target throughout the region, particularly as recent work by Barovich and Foden has confirmed that late syn- to post-orogenic OD granites are clearly derived from the sediments.

References Betts, P.G. and Giles, D. (2001) GeoL Soc, Aust. Abstr. 64, 7-8.

Eriksson, K.A., Taylor, S.R. and Korsch, R.J. (1992) Geochim. Cosmochim. Acta, 56, 899-909.

Page, R.W. and Sun, S-S. {199S) AJES 45, 343-361.

Page, R.W., Conor, CH.H. and Sun, S-S. {199S) BHEI1998, 89-93.

Page, R.W., Stevens, B.P.J., Gibson, G.M. and Conor, C.H.H. (imO) BHEI2000, 72-75.

Slack, J.F. and Stevens, B.P.J. (1994). Geochim, Cosmochim. Acta, 58, 3633-3653.

155

Nd ISOTOPE CONSTRAINTS ON THE ORIGIN OF 1580 MA CURNAMONA PROVINCE GRANITOID MAGMATISM Barovich, KM and Foden, J Department of Geology and Geophysics, University of Adelaide, Adelaide, SA

The Palaeoproterozoic Olarian Orogeny in the Curnamona Province's Olary Domain (CD) w^as accompanied in its latter stages by spatially extensive syn- to post-orogenic granitoid magmatism, commonly termed the 'regional granitoids'. Although often discussed as a single S-type suite, very limited U-Pb zircon analysis provide dates ranging from 1616 + 9 Ma (Fanning et al., 1995) for an unfoliated granite in the eastern OD, to 1579 + 2 Ma (Ludwig and Cooper, 1984) for an also unfoliated granite in the western OD. Given some significant petrological, geochemical and isotopic differences between the eastern and western granites we propose that the apparent 37 ± 9 Ma age gap is geologically significant, perhaps recording two temporally and spatially discrete tectonomagmatic events in the evolution of the Olary Domain.

The eastern OD suite are classic two-mica, S-type granites with uniformly high Si02 (71-74%), and Na20/K20 ratios about 1. They are strongly peraluminous (ASI > 1.1) and commonly entrain metasedimentary enclaves. Initial 8Nd values are uniformly negative (-4 to -8) and imply a source dominated by anatexis of their 1720-1640 Ma Willyama Supergroup supracrustal host rocks.

The western OD suite are more compositionally diverse, falling into three groups: monzogranites, trondhjemites and a more mafic granodiorite to diorite series. These three groups show comagmatic relationships in the field. The mildly peraluminous muscovite-free monzogranites range in Si02 from 69 to 75%. Where not Na-metasomatised, the trondhjemites differ little geochemically from the monzogranites, except for extreme Na20/K20 ratios of up to 8. The initial 8Nd values of the western OD granitoids range to less negative values than for the eastern S-type granitoids, and exhibit a trend of increasing 8Nd with decreasing Si02. Significantly, the highest initial 8Nd values of the mafic granitoid suite (-2.9) are far too high to be derived solely from the supracrustal host rocks, and their genesis requires at least a limited amount of an isotopically juvenile (mantle) component.

Geochemical and isotopic evidence suggest the western OD mafic granitoids may reflect a significant mantle input not seen at the exposed crustal level for the eastern OD granitoids. The western OD granitoids are near age-equivalents of the Hiltaba Suite on the Gawler Craton, allowing that the Olary Domain granitoid series may be a distal, more crustally dominated expression of this voluminous mantle-based A- to I-type Hiltaba thermal event.

References Fanning, C.M. (1995). PRISE report to South Australian Dept. of Mines and Energy.

Ludwig, K.R. and Cooper, J.A. (1984). Cont Mineral Petrol, 86, 298-308.

156

EARLY FORMED REGIONAL ANTIFORMS AND SYNFORMS THAT FOLD YOUNGER MATRIX SCHISTOSITIES: THEIR EFFECT ON SITES OF MINERAL GROWTH T.H. Bell*. A.P. Ham And K A. Hickey School of Earth Sciences, James Cook University, Townsville, O. 4811, Australia *Corresponding author: tim.bell@jcu.edu.au In the multideformed and metamorphosed cores of orogens regional folds commonly fold pervasive matrix schistosit}^ suggesting that they formed quite late in the tectonic history. However, quantitative microstructural investigation of four such folds has revealed that this is not the case and that they formed early in the orogenic history. The schistosit>' folded around their hinges is the consequence of overprintmg an upnght fold with a steep axial plane by a younger event with a subhonzontal axial plane. This subhonzontal foliation has then been folded by further tightening of the macroscopic fold during subsequent horizontally directed shortening, and/or rotated into subparallelism with the compositional layering due to reactivation of the folded foliation. Such a deformation path would explain why refolds at a regional scale are uncommon in plan view in most orogenic belts with complex deformation histories. The relative simplicity of regional folds in plan view does not necessarily carr}^ over to their shape in cross-section. Folds with steep axial planes may be rotated to form nappe folds of high amplitude in cross-section by the development of zones of sub-horizontally dipping foliation with consistent shear senses.

Multiple periods of porphyroblast growth accompanied the multiple phases of deformation that postdated the initial development of these folds. Some of these phases of deformation were attended by the development of large numbers of same asymmetry spiral-shaped inclusion trails in porph\Toblasts on one limb of the fold and not the other, or larger numbers of opposite asymmetr>^ spirals on the other limb, or similar numbers of the same asymmetry spirals on both limbs.

Significantly, the largest disparity in numbers from limb to limb occurred for the first of these cases.

For all four regional folds examined, the structural relationships that accompanied these large disparities were identical. In each case the shear sense operating on steeply dipping foliations was opposite to that required to originally develop the fold. Reactivation of the folded compositional layermg was not possible for this shear sense. This favoured the development of sites of relatively coaxial shortening early during the deformation history, enhancing microfracture and promoting the growth of porphyroblasts. These distributions of inclusion trail geometries from lunb to limb cannot be explained by porph}Toblast rotation, or folding of pre-existing rotated porphyroblasts within a shear zone, but can be explained by development of the inclusion trails s>iichronous with successive sub-vertical and sub-horizontal foliations.

157

U-Pb, Nd- AND C-ISOTOPE EVIDENCE FOR THE OCCURRENCE OF PALAEOPROTEROZOIC CRUST IN THE CENTRAL ZONE OF THE LIMPOPO BELT, SOUTH AFRICA I S. Buick^ I.S. Williams^ R.L. Gibson^ LCamvright^ and R.Maas\ ^Department of Earth Sciences, La Trobe University, Bundoora, Vic. 3086, Australia "Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia ^ Department of Geology, University of the Witw^atersrand, Johannesburg, South Africa "^School of Geosciences, Monash University, Clayton, Vic. 3800, Australia The 250 km long, E-W trending, predominantly granulite-grade Limpopo Belt is situated between the Archaean Kaapvaal and Zimbabwe Cratons in southern Africa. It comprises a Central Zone (CZ) flanked by Northern and Southern Marginal Zones (NMZ and SMZ), all of which are shear zone bounded. Both the NMZ and SMZ contain dominant TTG intrusive suites and subordinate metasedimentar}' rocks that are thought to be the granulite-facies equivalents of granite-greenstone sequences of the adjacent cratons. In contrast, the Central Zone contains a sequence of granulitegrade supracrustals (Beit Bridge Group: metapelites, metapsammites, marbles, quartzites and calcsilicate rocks) that is lithologically markedly different from that found on the adjacent cratons and whose provenance remains unclear. The CZ was metamorphosed to granulite grade at - 2 . 6 Ga, -2.0 Ga and possibly at -3.2 Ga (Kroner et al, 1999). CZ metasediments are intruded by the - 3 .2 Ga Messina Layered Mafic Complex and extensive suites of now migmatitic granitic gneisses with mferred intrusive ages of -3.2 Ga, -2.7-2.5 Ga (Kroner et al., 1999). Metasediments of the Beit Bridge Group are therefore inferred to have been deposited prior to -3.3 Ga. However, several new lines of evidence suggest that the CZ contains metasediments of both Archaean and Palaeoproterozoic provenance.

Firstly, while new SHRIMP U/Pb zircon age determinations obtained from several CZ metasediments (metapelites and metapsammites) show near-concordant detrital zircon populations no younger than c.3.3 Ga, two granulite-facies samples contain near-concordant detrital (high Th/U oscillatory-zoned) zircon cores in the age range - 2 . 7 - 2 . 3 Ga, suggesting that the protoliths were deposited in the Palaeoproterozoic. Secondly, recently published and newly obtained Nd-isotope data show that CZ metasediments have Nd model (TDM) ages in the range - 2 . 4 5 - 3 . 6 Ga, with the majorit}' of samples having Nd-model ages of less than 3.3 Ga. The anomalously young ages are best explamed by the variable recycling of Archaean crust in Palaeoproterozoic sediments. Thirdly, granulite-grade CZ metacarbonates have anomalously high values (-+5 to +7%o PDB) that are likely to represent minimum values for the pre-metamorphic isotopic composition of their protoliths. Archaean carbonates with such elevated values are unknown anywhere in the world. By contrast, high carbonates are well documented worldwide from the Palaeoproterozoic at 2 . 1 - 2 . 4 Ga, and high carbonates of this age range occur on the adjacent Kaapvaal Craton and on the margm of the Zimbabwe Craton (Magondi Fold Belt). Taken together, these data imply that the Central Zone contains a previously unrecogmsed Palaeoproterozoic cover sequence, contrary to the prevailing view that all sedimentary- protoliths deposited at -3.3 Ga. The recognition of this younger crustal component requires a major revision of the tectonic evolution of the Limpopo Belt.

References Kroner, A., Jaekel, P., Brandl G., Nemcliin, A.A. & Pidgeon, R.T. 1999. Single zircon ages for granitoid gneisses in the Central zone of the Limpopo Beh, Southern Africa and geodynamic implications.

Precambhan Research 93, 299-337.

158

POLYPHASE METAMORPHISM IN THE SONGPAN-GARZE OROGENIC BELT (CHINA): P-T-T-D PATHS AND CONSTRAINTS FROM U-PB, SM-ND AND RBSR GEOCHRONOLOGY M. Huang\ I S. Buick\ R. Maas^ and I S. Williams^ Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3086, Australia "Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

The Danba Domal Metamorphic Terrane (DDMT) occurs within the Songpan-Garze Orogenic Belt of western China. It comprises a series of structural domes cored by Mesoproterozoic orthogneisses m sheared contact with an overlying cover sequence of Neoproterozoic-Triassic metasedimentar}^ rocks. Barrovian-type metamorphism occurred in the cover sequence within a large decollement zone. Three tectonometamorphic events can be identified. An early event (Di-Mi) was divided into two progressive episodes. The first episode (Mia) is related to an incipient crustal thickening (Du) immediately before the emplacement of late Triassic granitoids. The second caused prograde metamorphism with conditions from biotite to kyanite grade (Mib), during which the crust was greatly thickened due to southward decollement and shortenmg (Dib). A subsequent highertemperature and slightly lower-pressure sillimanite- and migmatite-grade overprint (M2) developed durmg predominantly E-W compression (D2), and locally truncates Mi isograds. Lastly, a late stage greenschist-facies retrogression (M3) is related to the development of NW-SE oriented ductile thrusts and strike-slip zones (D3). P-T conditions for metapelites and amphibolites vary from c.3-5 kbar and c.410°-530X (biotite zone) to 570^-600^C (staurolite and kyanite zones; Mi), and from c. 5kbar and 640 (sillimamte zone)-725^C (migmatite zone; M2). Clockwise P-T-t paths were inferred for the staurolite and kyanite zones.

SHRIMP U-Pb datmg of monazite and Sm-Nd dating of growth zoned garnets indicates that Mi occurred between c. 204 and c.l90 Ma. In contrast, Sm-Nd dating of compositionally homogeneous garnets and U-Pb dating of titanite suggests that M2 occurred between c. 165 and c.l58 Ma. Throughout the DDMT, Rb-Sr muscovite ages range from c. 138 to 100 Ma, whereas Rb-Sr biotite ages cluster at c.35-26 Ma. Age determinations, combined with the peak temperature and closure temperature estimates, suggest initially slow cooling rates of c.4-7''C/Ma from c.l60 to c.l40 Ma, followed by very slow cooling at rates of c.2-3''C/Ma between c.l40 Ma and c.30 Ma. Initial slow cooling is thought to be the result of limited post-tectonic isostatic uplift subsequent to extensive crustal thickening. The regionally consistent biotite Rb/Sr ages indicate that accelerated cooling across the entire terrane commenced at - 3 0 Ma. This late segment of the cooling history, similar to that proposed for the other regions of the Tibet-Qinghai Plateau (Copeland et al, 1995; Mock et ai, 1999), suggests that uplift of the Qinghai-Tibet Plateau, including the SGOB, has taken place predominantly in the last - 3 0 Ma in response to the contmuing northwards suturmg of India and Eurasia.

The DDMT is a compound structural dome largely produced by large-scale interference of Di and D2 folds. The tectonometamorphic evolution corresponds to regional crustal thickening and top-tothe-south decollement caused by the subduction of the Yangtze Block under the North China Block during the Indosinian Orogeny (Mi), followed by shortening and uplift due to E-W collision between the Tibet and Yangtze Blocks (M2) during the Yanshanian Orogeny. Further uplift or exhumation and coolmg occurred during the Himalayan Orogeny, and was associated with the development of a major NW-SE oriented transpressional fault zone system.

References Copeland, P., Harrison, T. M., Pan, Y., Kidd, W. S. F., Roden, M. & Zhang, Y. Q. 1995. Thermal evolution of the Gangdese batholith, southern Tibet: A liistor}^ of episodic unroofing. Tectonics 14, 223-236.

Mock, C., Amaud, N. O. and Cantagrel, J. M. (1999) An early unroofing in northern Tibet? Contraints from "''^Ar-^^Ar thermochronolog}^ on granitoids from the eastern Kunlun range (Qianghai, NW Cliina).

Earth and Planetary Science Letters 171, 107-122.

159

SHRIMP ZIRCON PROVENANCE STUDIES OF PROTOLITHS TO THE HARTS RANGE METAMORPHIC COMPLEX (ARUNTA INLIER).

I.S. Buick^ LS. Williams^ M. Hand', J. Mawby', J. Miller' ^Department of Earth Sciences, La Trobe University, Bundoora, Vic. 3086, Australia 'Research School of Earth Sciences, Australian National Universit\\ Canberra, ACT 0200, Australia ~ Department of Geology and Geophysics, Universit>^ of Adelaide, SA 5005, Australia 'Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa The amphibolite- to granulite-facies Irindina Supracrustal Assemblage (ISA; Harts Range Metamorphic Complex, eastern Arunta Inlier) has a structural thickness of around 7 km, and consists of structurally lowermost marble and quartzite (Naringa Calcareous Member), intermediate-level intercalations of metapelitic (Irindina Gneiss) and metabasic units (Harts Range Meta-Igneous Complex), and a structurally uppermost metapelite- and calcsilicatedommated unit (Brady Gneiss). Its vertical lithological succession, and the chemistry of mafic rocks, is consistent with deposition in a continental rift environment. Until recently it has been universally accepted that deposition of the ISA occurred prior to ~ 1.74 Ga and that it therefore constitutes part of the Arunta Inlier basement to the Neoproterozoic to Palaeozoic Centralian Superbasin. However, here we present new SHRIMP U-Pb zircon analyses of detrital zircons from granulite-facies quartzite (Naringa Calcareous Member), amphibolite-facies metapelite (Brady Gneiss), and early Cambrian sediments fi-om the Amadeus Basin. Detrital age distributions are dominated by populations of concordant grains at ~ 1.3-1.0 Ga and -0.7-0.5 Ga; zircons older than -1.5 Ga are not common except in locally-derived sediments whose zircon populations were obtained fi-om -1.74 Ga granitoids that occur at high structural levels m the local basement (Entia Gneiss Complex). These data, together with recently published zircon provenance data from the Irindina Gneiss and Harts Range Meta-Igneous Complex, shovc^ that the depositional age of the Harts Range Metamorphic Complex is much younger than previously thought. Specifically, the distribution of detrital zircon ages is very similar to that of both the -520 Ma Goyder Formation, and the -500 Ma Pacoota Sandstone of the Amadeus Basin (Centralian Superbasin), but markedly different to the -570-540 Ma Arumbera Sandstone, suggesting that the Harts Range Metamorphic Complex was deposited no earlier than 540 Ma, and more probably at -520-500 Ma. The data imply that the high-grade Harts Range Metmorphic Compex does not constitute part of the Arunta Inlier, but instead is actually part of the Centralian Superbasin succession.

Precursors to the ISA were deposited in a Cambrian rift sequence (termed the Irindina subbasin) that was located between the presently preserved remnants of the Centralian Superbasin (Amadeus and Georgina basins). The development of the Irindina sub-basin appears to represent part of a widespread and long-lived episode of north-south directed extension in parts of central Australia associated with eruption of the continental Antrim Plateau (Ord and Bonaparte Basins), Mooracoochie (Warburton Basin) and Table Hill (Oficer Basin) Basalts, and the development of a system of sub-basins along the northern margin of the Amadeus Basin. The protoliths to the Harts Range Metamorphic Complex represents the basal sequences of an exceptionally deep sub-basin within m the broader extent of the Centralian Superbasin.

Detrital zircon age distributions in the Harts Range Metamorphic Complex are also similar to those in sediments of similar age in the Adelaide Rift Complex and the Anakie Inlier (NE Qld) that were deposited during east-west directed rifting along the passive margin of Gondwana.

160

LATE-TERTIARY EXHUMATION OF CORE COMPLEXES IN SOUTHWESTERN USA: AN APPLICATION OF (U-Th)/He DATING T. J. Carter^ Kohn, B. Gleadow, A. J. Foster, D. A.l Belton, D. X.' and Woodhead, J. D.' ^School of Earth Sciences, The Universit}^ of Melbourne, Park\ille, VIC 3010, Australia.

"Dept. of Geological Sciences, University of Florida, Gainesville, FL 32611, USA. It is well established that extended continental crust, under certain conditions, does not fail along high-angle normal faults as predicted by fundamental rock mechanics theor\^, but rather along lowangle normal faults (< 30^). Both modes of faulting are seen in the Basin and Range Province of western North America, superimposed on the same terrane. While the high angle faulting is relatively well-understood in this province, the low-angle episodes are more enigmatic. As lowangle normal faulting is an integral part of the overall extension process, a clear understanding of how the low-angle faults evolve, their timing, geometr}^ and rates of movement will establish a better understanding of how continents react to different stress regimes.

Two metamorphic core complexes within the highly extended Colorado River Extensional Corridor of Arizona, California and Nevada have been studied using apatite (U-Th)/He thermochronometr>^ These data allow the determmation of thermal histories dowTi to -70°C in these rapidly cooled rocks, whereas under the same geological conditions apatite fission track (AFT) data are generally applicable down to ~100-110°C. In addition, previous AFT study was hampered by relatively low uranium concentrations and young ages, giving rise to large uncertainties in the age determinations.

Despite these potential limitations, previous first-order estimates of fault slip rate, dip of fault while active, tuning of extension and the paleogeothermal gradient were reported. The higher resolution of the apatite (U-Th)/He data provides a means to test these different aspects of late-stage cooling in core complexes.

Chemehuevi Mountains Interpretation of previous AFT data resulted in an estimate of 3.3 ± 0.9 km/m.y. (± 2 sigma) for the average rate of slip on the Chemehuevi detachment fault. To date, all the (U-Th)/He analyses have yielded ages within error of the corresponding AFT ages, but their improved resolution is beginning to provide a more detailed picture of the core complex's exhumation history. Rather than a single slip rate for movement along the detachment, the (U-Th)/He data suggest a tw^o-stage process, with rates of up to 1 km/m.y in the late stages and a slower rate in the earlier stages.

Harcuvar Mountains Previous AFT data from the Harcuvar Mountains yielded a detachment fault slip rate estimate of 7.7 ± 3.1 km/m.y. Howwer, the He data suggest a significantly faster slip rate for the last 30 km of exhumation. Increase in extension rate for both this core complex and the Chemehuevi Mountains appears to have occurred after the formation of a secondary break-away fault which merged with the origmal detachment.

This project shows that in combining two low-temperature thermochronometers, AFT and apatite (U-Th)/He, new and usefol insights into the waning stages of extensional tectonism can be gleaned from even the most well-studied terranes.

Acknowledgements This work was supported by the Australian Research Council and the Australian Institute of Nuclear Sciences and Engineering.

161

THE PITFALLS OF SECTIONING ROCKS PERPENDICULAR TO FOLIATIONS AND LEVEATIONS IN THE MATRIX Mustafa Cihan School of Earth Sciences James Cook University Studies of inclusion trails in porphyroblasts using thin sections cut perpendicular to the lineation and another cut parallel to the lineation but perpendicular to the foliation commonly reveal very little information about porphyroblast timing relative to the matrix foliation. Inclusion trails from such section orientations can appear continuous with the matrix foliation where the multiple vertical sections with different strikes cut from same rock reveal they are truncated. The reason for this is the fact that sections perpendicular and parallel to the lineation, but perpendicular to the foliation in the matrix, contain strain shadows for that foliation where the inclusion trails exiting the porphyroblast can be continuous v^ith matrix foliation relics in the strain shadow. This problem is illustrated by the Robertson River Metamorphics where early studies using sections perpendicular and parallel to the lineation but perpendicular to the foliation revealed a relatively simple history of porphyroblast growth with inclusion trails generally continuous with the matrix foliation. However, multiple vertical thin sections with different strikes has revealed a far more complex history than could have been expected from the relative continuit\^ of inclusion trails with the matrix foliations initially observed. The thin sectioning of these rocks in multiple orientations independent of the orientation of the foliations in the matrix reveals truncations and/or intersection of successively formed foliations trapped in porph>Toblasts and truncated by the matrix. This has given rise to better understanding of complex deformation histor}^ The Robertson River Metamorphics contain evidence for up to six deformations in the matrix recognized from both outcrop and thin section observations. However, a much more extensive deformation and metamorphic history is preserved within numerous porph^Toblasts present in these rocks. The detailed study of inclusion trails, especially in garnet porphyroblasts, has revealed that there are four phases of garnet growth associated with different deformation events, several of which predate the matrix foliations. Multiple phases of growth are also clearly recognisable from the compositional maps of those samples in the zoning patterns.

Porphyroblast - matrix relationships in garnet porphyroblast bearing samples containing spiral shaped inclusion trails reveal at least eight deformation events have occurred in the Robertson River Metamorphics. The succession of the foliation intersection axes (FIAs) formed during those events in porphyroblasts as a whole indicate that there are three sets of FIA. This involved fundamental changes in the direction of bulk shortening with time from NNW-SSE to N-S to W-E somewhat similar to that determined for the Mount Isa Province.

Key words: FIA(s), inclusion trails, porphyroblast, Robertson River Metamorphics.

162

NEW GEOCHRONOLOGY FROM THE STRANGWAYS METAMORPHIC COMPLEX, ARUNTA BLOCK - CENTRAL AUSTRALIA Matthew Cobb^ Peter Kiimy^ and Martin Hand^. ^Tectonics Special Research Centre, Curtin University of Technology. ^Department of Geology and Geophysics, University of Adelaide.

The Strangways Metamorphic Complex (SMC) is a complexly deformed package of granulites, gneisses, schists and associated granitoids, situated approximately 150km NE of Alice Springs in Central Australia. The lithologies of the SMC are thought to represent the metamorphosed products of supracrustal protoliths (Warren, 1979; Stewart et al., 1984). Recent attempts at constraining the tectonic, temporal and spatial evolution of the SMC are all marked against the initial work of Shaw et al. (1984) and Stewart et al. (1984). This landmark work spurred on subsequent research and m the last 15 years, has seen the production of two comprehensive structural and metamorphic syntheses within the SMC (Goscombe, 1989; Norman, 1991). While these models are detailed in their account of the structural and related metamorphic development of the complex, the absolute geochronology of both the pre-metamorphic and orogenic history of the SMC are still largely undefined, as is the tectonic setting of both. Previous geochronological studies of the SMC have been generally focussed on a much larger scale, as part of Arunta-wide studies (for a review, see Collins and Shaw, 1995), and have resulted in contention regarding the timing of two major metamorphic episodes within the complex. The recent work of Ballevre et al. (1999) for the first time dated two major metamorphic episodes, using zircon extracted from well constrained leucosomes, within the NW region of the SMC. Their results indicate Uvo metamorphic episodes occurring temporally very close, in disagreement with much of the previously published geochronology for these events. As part of ongoing PhD research, new samples taken from the Erontonga Metamorphics in the southwestern SMC, have yielded SHRIMP ages in accordance with the recent results of Ballevre et al. (1999). Zircons extracted fi-om a cordiente gneiss, record three distinguishable populations of ages. CL imagmg of analysed zircons reveals grams that exhibit thick (30-40 |Lim) rims of weakly to unzoned zircon, surrounding either cores of a similar nature or truncated zoned fi-agments. The latter cores are typical of zircon of detrital origin, and yield ages that span a range between 1800 and 2000 Ma. The former cores define a well constrained smgle age population at 1728 ±5 Ma. Rim analyses yield a single age population at 1706 ±7 Ma. Interpretation of these initial results notes the very strong correlation between the ages of weakly zoned cores and rims of the Erontonga sample, with the results of Ballevre et al. (1999) indicating the latter were accurate m their assessment of the timing of metamorphism, and that this IS consistent across the western SMC at least as far as the Erontonga Metamorphics, situated roughly 45km southeast of Edwards Creek and Woolanga Bore. Additionally, core analyses from Erontonga zircons that resemble detrital material may be used to indicate a preliminary maximum deposition age for the protolith sedimentar}^ material of the complex, at 1763 ±8 Ma, based upon the youngest analysis in this group. Further research on this topic is continuing, with other metasedimentar}' rocks from units across the entire SMC targeted for accessory^ phase analysis. It is anticipated that results will provide data on provenance, depositional record and metamorpliism, leading to fiirther constraints on the pre-metamorphic history and metamorphic record of the SMC. References Ballevre, M., Hensen, B.J., and Moller, A., 1999, SGGMP Field Guide 4, Geol. Soc. Aust.

Collins, W.J., and Shaw, R.D., 1995, Precambrian Research, v. 71, p. 315-346.

Goscombe, B., 1989, PliD thesis, Melbourne Universit}.

Norman, A.R., 1991, PhD thesis, Macquarie University.

Shaw, R.D., Stewart, A.J., and Black, L.P., 1984, AJES, v. 31, p. 457-484.

Stewart, A.J., Shaw, R.D., and Black, L.P., 1984, AJES, v. 31, p. 445-455.

Warren, RG., 1979, Nature, v. 278, p. 159-161.

163

LITHOSTRATIGRAPHY AND DEPOSITIONAL ARCHITECTURE OF THE WH.LYAMA SUPERGROUP, OLARY DOMAIN, CURNAMONA PROVINCE SOUTH AUSTRALIA. Colin H.H. Conor Geological Survey, PIRSA, 101 Grenfell Street, Adelaide, SA. 5000, Australia.

A formal, useable lithostratigraphic scheme has been developed for the Olary Domain, Cumamona Provmce, utilising where possible previous work which is mostly unpublished or informal (Conor m press). In addition the scheme has benefited from new and critical geochronology (Page et al. in press). The Cumamona Province is a largely buried circular Palaeo-Mesoproterozoic entity, which mcludes a basm fragment that contains sediments and volcanics of the Willyama Supergroup. The upper Willyama Supergroup is Pb-Zn prospective, not only because it hosts the Broken^Hill deposit but also because it is tied chronostratigraphically to Mount Isa and northern Australia. The region was overprinted by Cu-Au mineralisation late during the -1.63 Ga Olarian Orogeny, a period which encompasses the formation of the 01>Tnpic Dam Cu-Au-U-REE deposit.

Deposition of the Willyama Supergroup is constrained by recent U-Pb chronology from -1.71 Ga to after -1.64 Ga; however neither the base nor top of sediment fill have as yet been recognised.

The distribution of the Saltbush and Mount Howden subgroups and the Pb-Zn-rich Broken Hill Group suggests that over a period exceeding 50 million years, that is during upper Willyama Supergroup times, the basin filled with sediment from a southeasterly direction. This interval in the Olary Domain is represented by the Stratheam Group, near the base of which the tuffaceous Plumbago Formation is interpreted to record a major flooding event at 1.694 Ga. It IS suggested that an erosional surface exists below the base of the Stratheam Group (Bimba Formation), and this break therefore marks the top of the Cumamona Group. The Cumamona Group IS defined by the presence of volcanics of the 1.71 Ga Basso Suite, which is not registered above the tuffaceous Plumbago Formation. The upper part of the Cumamona Group is the Cu-Au anomalous, evaporite-prone Ethiudna Subgroup and this is underlain by the psammopeliticpsammitic Wiperaminga Subgroup. Subdivison of the Wiperaminga Subgroup is made difficult by locally intense migmatisation and metasomatic alteration of the Olarian Orogeny.

DEPOSITIONAL MODEL FOR T H E WILLYAMA SUPERGROUP Schematic section NVv/ SE

Mount Howden

Ameroo Hili Mulga Creek

<1672 ±7 Ma Group

Shallov/ Y/Qtef to e m e f g e n t (evaporiric)

~ 2

P s CO

Reqre-ssive event Transgrssive event 1694 Ma t i m e !tne, rnaxhrium flooding

.

Mount Howden ^ S'jbgroup

Saltbush Subgroup

65^] ±7 Ma

. ::: Run^bago r-OiniaKon ; i-i-BimiJo" Formotior I..,.. i • [ I I I ' Ethiudna Subgroup P e r v h u m u c k calcoibitite' Wiperaminga Subgroup

Broken Hill 1641 jt 5 tVta Paragon

l693±2Ma:: I

I

I

BJoken Hil Group

I

I

1710

1715±5Ma

164

I

^

+ 3

kW4+4Ma :: iVl'i'i'i'i'i'l 'WneyWhey schist' 1712+

3Ma

1693 ± 4 Ma::::;:::::::::: I I. I I. I r. tI. U 'lower Allendale' Himolaya Forrriotion

TECTONIC SETTING AND MAGMATISM IN THE TANAMI REGION ^Crispe, A.J.. Vandenberg, L.C., ^Hendrickx, M.A., ^Dean, A.A., ^Cross, AJ. and -Smith, J.B. ^Northern Territory Geological Survey, PO Box 2655, Alice Springs. ^Geoscience Australia, PO Box 378, Canberra.

The tectonostratigraphy of the Tanami region and its implications for the development of the Northern Australian Craton continue to develop with the incorporation of recent SHRIMP U-Pb zircon ages.

Our tectonostratigraphy supports a rift-sag-turbidite model ("Barramundi" association) on attenuated Archaean basement (Hendrickx et al 2000). The MacFarlane Peak Group (MPG), Dead Bullock Formation (DBF) and Killi Killi Formation (KKF) are proposed to represent these three phases, respectively. MPG arenite in the type area yielded a youngest detrital zircon age of 1877±21 Ma (2c7) although some grains may have been reset by the nearby intrusion of 1809±3 Ma granite. Widespread samples of the turbiditic KKF have consistent detrital zircon age spectra, with the youngest populations -1840 Ma. Thus the age data support the succession from MPG to DBF and KKF; the contact between DBF and KKF is structurally concordant, but may represent a hiatus.

The Tanami Orogemc Event (TOE) is the earliest deformation affectmg the MPG, DBF and KKF. This resulted in tight folding and regional low-grade metamorphism increasing from greenschist facies in the west to amphibolite facies toward the south and east. The Mount Winnecke Group (MWG) and by inference the Mount Charles Formation (MCF) bimodal volcanic and sedimentary rocks were deposited between 1825 and 1815 Ma and were unaffected by the TOE. The period 1815-1790 Ma is characterised by volummous granite intrusions, which are predominantly l-tynpQ monzogranite and granodiorite. These granitoids invariably contain inherited zircon, which range in age from comagmatic to mid-Archaean. Approximate 2500 Ma age populations are common in both igneous and sedimentary rocks of the Tanami region, indicating widespread recycling of Archaean basement. Small Archaean gneiss outcrops have been identified to the north and east (Browns Range Dome and Billabong complex).

The stratigraphy indicates there is evidence for an early rift package (MPG) contemperaneous with initial riftmg in the Tennant Inlier, and with felsic volcanism in the Halls Creek Orogen attributed to hot-spot st}4e magmatism (Sheppard et al. 1999). Deposition of DBF is consistent with thermal sag after a tectonic event, with the KKF as flysch, deposited in response to the onset of the TOE. Within a few million years of the TOE, bimodal volcanic rocks (MWG and MCF) were deposited in an incipient rift. Our earlier work recognised that the Tanami region was a long-lived transpressional zone that began with the TOE-Halls Creek Orogeny prior to -1825 Ma. The large volumes of 1815-1790 Ma granitoid in the Tanami region were emplaced after the TOE and cannot be directly attributed to slab melting dunng subduction. This supports the findings of Sheppard et al. (1999) who attnbuted the Halls Creek Orogeny to the collision of the NAC with the Kimberley Craton following northwest-directed subduction. Therefore in the Tanami region, over-thickening of the crust involvmg melting of Archaean basement in response to continent-contment collision is most likely to have generated the 1815-1790 Ma gramtiods.

References Hendrickx, M.A., Slater, K.R., Cnspe, A.J., Dean, A.A., Vandenberg, L.C. and Smith, J.B. 2000.

Palaeoproterozoic stratigraphy of the Tanami Region: regional correlations and relation to mineralisationpreliminar>^ results. Northern Territory Geological Sun^ey Record GS2000-13.

Sheppard, S., Tyler, I. M., Griffin, T. J. and Taylor, W.R. 1999. Palaeoptroterozoic subduction-related and passive margin basalts in the Halls Creek Orogen, northwest Australia. Australian Journal of Earth Sciences 46, 679-690.

165

USING THE SINGLE GRAIN APATITE (U-Th)/He THERMOCHRONOLOGY LASER EXTRACTION TECHNIQUE TO IMPROVE PRECISION OF THERMAL HISTORY ANALYSES: EXAMPLES FROM THE OTWAY BASIN, SOUTHEASTERN AUSTRALLV P.V.Crowhurst\ P.F. Green^ and LR. Duddy" ^CSIRO Petroleum Resources, PO Box 136, North Ryde, NSW 1670 "Geotrack International Pty Ltd., 37 Melville Rd., Brunswick West, Victoria 3055 The themiochronological technique of (U-Th)/He dating is based on the accumulation and diffusive loss of Helium produced by alpha decay of Uranium and Thorium impurities within apatite grains.

Apatite (U-Th)/He ages are progressively reset by heating, due to the diffusive loss of the radiogemc helium, with total loss occurring at temperatures around 80°C (for timescales involving millions of years). When integrated with information from AFTA (Apatite Fission Track Analysis) and other thermal mdicators (e.g. vitrinite reflectance - VR), this technique allows more precise thermal histor>^ constraints to be established at relatively low temperatures (50 to 80°C).

Data are compared between the furnace heating technique to extract helium from multiple grains and the new single gram laser extraction system. The furnace techmque can require greater than 10 individual apatite grains to be analysed together, especially if they are young and have low U and Th contents, because of the low He >ield relative to the significant blank background that is created by this type of heating. The advent of the laser technique has reduced the background values by up to two orders of magnitude, thus allowing analysis of single grains.

Samples that have cooled slowly or resided in the He partial retention zone are particularly sensitive to grain size variation. The single grain analysis allows for greater control and ease of testing of various grain sizes within an mdividual sample.

In the Otway Basin (eg. Lindon-1), the He apatite data obtained via the furnace technique produced a profile of age reduction with increasing depth except near the bottom where ages started to increase. These samples were re-analysed via single grain laser extraction and obtained reduced ages as expected. This possibly highlights that there can be down-hole contamination, where one spurious grain can have a significant effect on age determinations.

Here, we illustrate how^ integration of (U-Th)/He dating of apatite with AFTA and VR data in a number of Otway Basin wells provides much tighter constraints on the lower temperature history, and provides improved precision on the timing of basin inversion during Tertiary times.

Integration of (U-Th)/He dating of apatite with existing thermal history^ reconstruction techniques in this way provides more a reliable basis for constraining thermal histories in sedimentary basins.

166

PALAEOZOIC EVENTS AROUND ARKAROOLA, NORTHERN FLINDERS RANGES, SOUTH AUSTRALIA Marlina Elburg^'^, Paul Bons^'^, Joel Brugger^"'' ^Department of Geology and Geophysics, Adelaide University, Adelaide SA 5005 -Present address: Max Planck Institute for Chemistry, P.O.Box 3060, D-55020 Mainz, Germany ^Institut fiir Geowissenschaften, Universitat Mainz, Germany ^Present address: Institut fiir Geowissenschaften, Universitat Tubingen, Sigwartstr 10, D-72076 Tubingen, Germany. "^The South Australian Museum, North Terrace, Adelaide, SA 5000 The Mt Painter Inlier forms an isolated outcrop of (?Palaeo-) Mesoproterozoic basement rocks that are surrounded by a <850 Ma Adelaidean cover sequence. All units were affected by the CambroOrdovician Delamerian Orogeny. We present a detailed structural, geochemical, and geochronological study of the area between Arkaroola Village and Mt Painter, which includes lower Adelaidean cover rocks and Mesoproterozoic basement granites, gneisses and schists.

Basement gneisses and A-type granites were dated at 1576 ± 5 Ma (Pb-Pb zircon evaporation, Elburg et al 2001). The dominant foliation (which overprints at least one older foliation) is truncated by the Adelaidean unconformit\^ (Elburg et al 2001). The lower Adelaidean units were deposited m active horst-graben structures with developing growth-faults (Drexel et a/. 1993).

Feeder dykes of the - 8 0 0 Ma Wooltana Volcamcs preferentially followed these faults.

The Delamerian Orogeny folded Adelaidean rocks and reactivated the synsedimentary faults. A biotite foliation developed in the Adelaidean cover, syn-kinematically with andalusite and cordierite. It also folded and crenulated the basement foliation(s) and locally developed a new penetrative crenulation cleavage. Deformation in the basement was localised in shear zones and faults, which continued into the cover and formed pathways for pegmatites and A-type leucogramtes, which were dated at 496 ± 8 Ma (Sm-Nd garnet-whole rock isochron; 8Ndi -13).

Shear zones in the basement were dated at 485 ± 2 Ma (U-Pb on monazite in biotite shear bands).

Activity of faults and shear zones continued in a progressively brittle fashion, associated with extensive metasomatic activity. This caused retrogression of Delamerian porphyroblasts, and postkinematic gro\\th of K-felspar, actinolite-tremolite and scapolite. Massive quartz and coarse diopside-sphene vems, which were dated at 449 ± 4 Ma ("^^U-^^^Pb isochron), were deposited along faults. This is broadly coeval with the generation of the primary^ uranium deposits in the area (-460 Ma, on monazite) and the large British Empire Granite (449 ± 2 Ma, I-type; 455 ± 4 Ma, S-t>T3e; U-Pb on monazite). Hydrothermal activity and fault movement continued with massive Mt Gee type quartz and haematite sinter deposits (-450 Ma, U-Pb discordia upper intercept) and even present-day hot springs and earthquakes along the Paralana Fault. Post-Delamerian tectonic activity was characterised by block-tectonics m which 100-1000 m scale blocks were shifted and tilted, with relatively little internal deformation. Supergene alteration is recorded by secondary uranium minerals, that consistently give young ages (<4Ma, U/Pb; -200 ka, U/Th disequilibrium).

A major step forward in the understanding of the (post-) Delamerian history of the area is the recognition that the -490 Ma metamorphic peak, associated with folding, cleavage formation, andalusite, cordierite and biotite growth and pegmatite/granite intrusion, is separated from the -450 Ma hydrothermal/magmatic event that caused retrogression of the earlier metamorphic minerals and extensive alteration. A complicating feature of the area is the continued re-activation of shear/fault-zones, which repeatedly served as pathways for magmas (-800 Ma mafic dykes, -495 Ma pegmatites) and hydrothermal fluids (-450 Ma quartz, sphene-diopside veins, Mt Gee sinters).

References Drexel J.F., Preiss W.V.& Parker A.J. (Editors) 1993 The geology of South Australia. GSSA-Bull 54.

Elburg M.A., Bons P.D, Dougherty-Page J., Janka C.E., Neumann N. & Schaefer B. 2001. Age and metasomatic alteration of the Mt Neill Granites at Nooldoonooldoona Waterhole, Mt Painter Inlier, South Australia. Australian Journal of Earth Sciences 48, 721-730.

167

USING INCLUSION TRAIL GEOMETRIES TO CONSTRAIN PRESSURETEMPERATURE-TIME PATH CALCULATIONS Thomas P. Evans School of Earth Sciences, James Cook University of North Queensland Email: thomas.evans@jcu.edu.au Calculating P-T-t paths for metamorphic terrains is a common approach to elucidating the orogenic processes responsible for metamorphism. One of the most widely used methods for generating a PT-t path is to model the P-T stability fields of the mineral assemblages occurring within a rock sample, and the composition of the minerals that comprise those assemblages, and then to correlate the model calculations with observed assemblages and compositions. This modelling is typically done by setting up and solving simultaneous equations of mineral equilibria utilising experimentally derived thermodynamic data on the minerals involved. The principal difficulty with this techmque is interpreting the mineral growth history that is preserved within the rock. It is crucial to have a precise account of the mineral paragenesis within a metamorphic rock before comparisons with theoretical mineral compositions and stability fields can be made.

Microstructural studies of the orientation of tectono-metamorphic fabrics included within porphyroblasts provide an excellent means of critically assessing the relative timing of mineral growth, both within an individual sample, and across a metamorphic terrain.

Multiple episodes of porphyroblast growth in amphibolite-facies pelitic schist can commonly be distinguished from each other on the basis of the orientation of developing crenulation hinges that have been preserved within growing porphyroblasts. These hinges are the product of the partial overprinting of a pre-existing foliation by a newly forming foliation, and are thus termed Foliation Intersection Axes or FIAs. FIAs generally occur as distinct populations that are regionally consistent in orientation and succession and thus can be used to correlate deformation and mineral growth events on a regional scale. The relative timing of the development of different FIAs is determined from the superposition and cross-cutting relationships in microstructure preserved from the core to the rims of porphyroblasts and in the matrix. On this basis, the relative timing of the growth of porphyroblastic phases can be placed in a temporal hierarchy determined by the FIA(s) that they preserve. This allows individual FIA forming events to be characterised in terms of a stable mineral assemblage in any given sample, and for that mineral assemblage to be correlated across a region with other mineral assemblages containing the same FIA. A series of P-T-t paths calculated for a suite of pelitic schist from the Littleton Formation New Hampshire is presented and the succession of stable mineral assemblages, and the composition of the minerals in each sample is predicted accurately by the model system employed for each sample.

P-T-t paths constrained by FIA-controlled timmg relationships between minerals are compared to paths calculated from garnet zoning profiles for the same area. The use of microstructure to constram and correlate mineral equilibria yielded a more detailed and extensive P-T-t path than did approaches based solely on growth zoning in garnet. More importantly, this technique provided a means of temporally correlating mineral grov^h between samples across the field area, hence describing the evolution of the P-T gradient across the region durmg the course of metamorphism.

168

REGIONAL SCALE VARLVTIONS IN FOLD INTERFERENCE PATTERNS: IMPLICATIONS FOR THE ALLENDALE AREA, BROKEN HILL BLOCK, NEW SOUTH WALES, AUSTRALIA ^Caroline Forbes, ^Pete Betts, ^Gordon Lister and ^Maarten Krabbendam ^School of Geosciences Australian Cmstal Research Centre, Monash Universit>\ Clayton VIC 3800, Australia "British Geological Survey, Murcliison House, West Mains Road, Edinbrugh EH93LA, UK

Fold interference patterns are a common phenomenon in polydeformed terranes and are a useful guide for establishing the geological history and kinematic controls on the deformation. Ramsay (1962) defined 'classic' end-member examples of fold interference patterns: Type 1 (dome and basm); T>q3e 2 (mushroom - crescent); and 3 (convergent - divergent). From structural mapping in the Allendale area in the northern Broken Hill Block, New South Wales, unusual fold interference patterns between F3 and F4 fold generations were identified. These structure did not completely comply with any of the 'classic' fold interference patterns.

The Allendale area is a complex, polydeformed terrane that has undergone at least three deformation events associated with high-grade metamorphism (up to amphibolite facies). The structural geometry of the area is the result of fold interference of earlier recumbent F3 folds superimposed by later ~N-S trending, upright F4 folds. Throughout the entire region, T>pe 2 and Type 3 fold interference patterns suggest F3 folds were originally recumbent with variably oriented axial traces.

Overpnnting relationships mdicate modified Type 3 fold interference pattern in the southern Allendale area m which the eastern limb of the F4 Paps Synform was originally the hinge zone of a macro-scale F3 recumbent fold. Dominantly Type 2 fold interference patterns are preserved near the Allendale Mine.

The identification of a previously unidentified macro-scale recumbent fold hinge, and deformation producing varying fold interference patterns throughout the Allendale area raises the question of how^ this structural geometr}^ could be developed. The opening of an early recumbent fold hinge that IS now incorporated into the lunb of a later upright fold can be interpreted as the result of a) 'squashing' and opening the recumbent hinge zone, or b) shearing and unfolding of the recumbent hinge zone.

The identification of unusual structural relationships within the Allendale area shows that the 3D geometr>^ is complex. The variation in axial trace orientation to produce 2 and T>Tpe 3 fold interference patterns may be explamed by a) twisting and then foldmg the recumbent F3 fold axial trace, or b) a pre-existing sheath-geometr}^ of the F3 recumbent fold.

Reference Ramsay, J. G. 1962. Interference patterns produced by the superposition of folds of similar type. Journal of Geology 466-481.

169

LOWER CRUSTAL FLOW IN CONTINENTAL EXTENSION — A PROCESS FOR MECHANICAL RECYCLING OF THE CRUST DURING OROGENIC COLLAPSE? K. Gessner*^ C.Wijns^ ^ L. Moresi\ A. Ord^ and R. Weinberg ^ CSIRO Exploration and Mining, PO Box 1130, Bentley WA 6102, Australia "Department of Geology and Geophysics, University of Western Australia, Crawley WA 6009, Australia *email: klaus.gessner@csiro.au The mode of continental extension is likely to be determined by the mechanical strength of the lithosphere, which is largely dependent on its thermal structure. In the case of a high geotherm the lower crust will be hot and of very low effective viscosity, resulting in a weak layer between the bnttle-elastic upper crust and the highly viscous upper mantle. Stretching of this system is likely to be accommodated by strong partitioning of deformation between ductile flow of the lower crust and brittle faulting of the upper crust. Brittle failure of the upper crust may be expressed by distributed normal faulting over a large area (e.g. North Sea basin). Alternatively the deformation may be localised onto relatively few normal fault systems in the brittle upper crust, each accommodating large displacements, and resulting in local exhumation of the lower crust (e. g. metamorphic core complexes of the Basin and Range province or the Aegean Region). We explore the sensitivity of brittle faulting and lower crustal flow to material property distributions using the two-dimensional Lagrangian Integration Point finite element code Ellipsis (Moresi et al., 1999a,b).

This approach is umque in its capability to allow simulation of ver>' large strains, which is an essential requirement when extreme localisation, such as detachment faults, need to be accurately modeled on the crustal scale. Our results indicate that the spacing of brittle faults is controlled by lateral strength heterogeneities and vertical contrasts in rheology. In the absence of lateral heterogeneities, the ratio of upper to lower crustal strength determines whether the upper crust is completely dissected by few detachment faults (leading to the formation of metamorphic core complexes) or whether many, densely spaced faults with limited displacement occur. The effect on lower crustal flow is significant, since only in the former case does large-scale flow of the lower crust occur. This leads to a re-organisation of the thermal structure and also means that considerable volumes of lower crustal material may experience a change in P and T conditions.

The low effective viscosity and the resulting scale of flow in the lower crust during core complex formation also suggest that new upper crust may be generated by exhumation, and the cooling of mid- and lower crustal material which has been squeezed out beneath the dissected 'rafts' of the upper crust. Redistribution of crustal material on such a scale may be a viable mechanism by which thickened continental crust can spread after collision.

References L.Moresi, H.-B. Miihlhaus, and F.Dufour. Particle-in-cell solutions for creeping viscous flows with internal interfaces. In H.-B. Miihlhaus, A. Dyskin, and E. Pasternak, editors, In Bifurcation and Localization in Soils and Rocks. Rotterdam, 1999a. Balkema.

L. Moresi, H.-B. Miihlhaus, and F. Dufour. Viscoelastic formulation for modelling of plate tectonics. In H.B. Miililhaus, A. Dyskin, and E. Pasternak, editors, In Bifurcation and Localization in Soils and Rocks^ Rotterdam, 1999b. Balkema.

170

RAPID TRANSITION FROM ACTIVE CRUSTAL CONTRACTION TO EXTENSIONAL COLLAPSE IN THE APENNINES (ITALY) ^Francesca Ghisetti and ^Livio Vezzani ^Dipartimento di Scienze Geologiche, Universita di Catania, Italy "Dipartimento di Scienze della Terra, Universita di Torino, Italy Tectonic evolution of the Apennines during late Miocene-Present times has been driven by roll-back of the Adriatic plate, and opening of the Tyrrhenian extensional basin in a back-arc position. As a consequence, extensional structures of progressively younger age from west (hinterland) to east (foreland) have been superposed on the Apenninic thrust belt. Migration of the extensional front at up to 20-25 mm/y is recorded by progressive opemng of downfaulted syntectonic basins, inversion of pre-existing structures, and related changes in crustal thickness and topography. Close association of shortening and extension in space and time, leading to the present juxtaposition of seismically active normal faulting and thrusting in adjacent belts, make the Apennines an excellent site for analysing progressive stages of superposed extension, and for addressing key problems such as: 1. the geometry of new-formed faults versus the reactivation and inversion of pre-existing thrust faults; and, 2. the time needed for the transition from contraction to extension to be completed.

Based on stratigraphic and structural investigations, and on stable isotope geochemical sampling of fault rock assemblages on thrust and normal faults, we present a SW-NE transect of the central Apennines, from the Tyrrhenian to the Adriatic margin.

Deformation of the upper crust is tied to available seismological and geophysical information. The section illustrates that less than 7 Ma of extensional deformation (from the late Miocene in the hinterland to early Pleistocene in the foreland) has resulted m a strong contrast between thin (<10 km), hot crust of the Tyrrhenian domain and thick (>30 km), cold crust of the Adriatic domain. These differences are reflected by: 1. eastward-decreasing amounts of extensional deformation superposed onto the thrust belt; 2. mismatching topographic gradients between the inner, collapsed regions, and the intensely uplifted outer regions; and, 3. different regimes of fluid circulation associated with decreased transcrustal permeability from the extended hinterland to the actively contracting foreland.

The evolutionary trend of the Apennines fits a model where incipient extension in the east of the thrust belt is accommodated initially by shallow, high-angle faults that contribute negligible extensional strain. At present, these faults cross the external Adriatic areas where seismicity^ is moderate and still dominated by thrusting and strike-slip faulting.

Mature stages of the progressive transition to extension are recorded along the western Tyrrhenian margin. Here, larger amounts of extensional strain and crustal permeability are accommodated by listric faults detached in the mid-lower crust. Seismic normal faultmg m this area is diffuse, but moderate (M < 5). Between these end-members lie the topographically elevated regions of the thrust belt, currently undergoing the transition towards mature extension. These regions are in a state of critical equilibrium relative to the collapsed Tyrrhenian margin and release the most seismic energy from moderate-tolarge normal fault ruptures (5 < M < 7). These relations suggest a strong control on the generation of large earthquakes at depths of 10-15 km by the amount of crustal stretchmg, the depth of detachment of normal faults, and fluid circulation. Eastwards migration of the extensional front with time is thus accompanied by a parallel migration of the belt of maximum seismic energy release.

171

STRUCTURE OF MINERALISED PALAEOPROTEROZOIC ROCKS IN THE OUTALPA INLBER, SOUTH AUSTRALIA George M. Gibson^ Alistair Crooks" & Michael Szpunar" ^Geoscience Australia, GPO Box 378, Canberra, ACT 2601 ^Primaiy Industries and Resources SA, GPO Box 1671, Adelaide, SA 5001 Exploration targets m Palaeoproterozoic rocks (Willyama Supergroup) of the Outalpa Inlier, South Australia include the regionally extensive Bimba Formation, a marble and calc-silicate dominated umt known for its high base-metal content (Pb, Zn, Mn, Cu, Co). This unit is a correlative of the Ettlewood Calc-silicate in the Broken Hill Group (NSW) and is spatially associated with an important redox boundary that elsewhere in the Olary area served as the locus for several recently discovered mineral deposits (Kalkaroo, White Dam, Portia). This boundar}^ is visible in aeromagnetic images of the Olary region and because of its obvious potential as both a marker horizon and guide to mineralisation, it was selected, along with the adjacent units, for detailed structural investigations as part of the Broken Hill Exploration Initiative. These investigations focussed on Ameroo Hill with the goal of better understanding the three dimensional geometr>^ of the area as well as clarifying the origin of this important boundary and the extent to which it is structurally and/or stratigraphically controlled.

Stratigraphy in the Ameroo Hill area incorporates one or more unconformities and has been deformed by at least four phases of pre-Adelaidean deformation (D1-D4). The Bimba Formation immediately overlies one such unconformity and typically coincides with the transition of a variably oxidised succession of migmatised psammopelitic gneisses, quartz-albitites, calc-albitite, and minor calc-silicate rock (Cumamona Group) into an overlying sequence of psammopelitic schist that is increasingly graphitic towards its base (Stratheam Group) (Conor, 2000). Tight to isoclinal macroscopic folds identified in both sequences are of D2 and D3 age. They produced widespread structural repetition of lithological units and fold interference patterns that conform to the modified arrow-head type (non-coaxial deformation). D2 was also associated with northeastdirected thrust faulting that locally emplaced high-grade metamorphic rocks over lower-grade metamorphic facies. Microfabrics and metamorphic mineral assemblages in the Ameroo Hill area further indicate that D2 was accompanied by crustal thickening and deforms an even earlier highgrade layer-parallel fabric. The origin of this fabric remains unclear although formation in an extensional tectonic environment cannot be ruled out. A later episode of more upright W-E folding and associated shearing (D4) further complicates the regional structure, making for a complex outcrop pattern m which lateral and vertical continuity of the Bimba Formation is neither assured nor predictable without a thorough understanding of the three-dimensional structural geometr}^ Pb-Zn-Cu mineralisation in the Outalpa Inlier is not confined to a single horizon. The most obvious gossans m some areas are associated with a 5-20 m thick calc-silicate unit lying some tens of metres below the Bimba Formation and separated from it by a sequence of thin-bedded psammitic schist. This calc-silicate unit forms part of the underlying Cumamona Group (Ethiudna subgroup; Conor, 2000) and constitutes a secondary^ target for mineral exploration. It is manganiferous as well as sulphide rich, and is distinguished from the overlying Bunba Formation by its occurrence within a sequence of composite biotite gneisses (meta-sandstone) that are also host to a thin but regionally persistent quartzite layer. The most effective drilling programs and exploration strategies are likely to be those that target mineralisation at more than one stratigraphic level, including sub-Bimba depths, and which acknowledge that the original stratigraphy may have been substantially modified by deformation.

Reference Conor, C.H.H., 2000. Definition of major sedimentary and igneous units of the 01ar\^ Domain, Cumamona Province. Mesa Journal, 19,51-56.

172

THE SCALE OF THE THERMAL PROBLEM IN THE MT ISA INLIER Martin Hand^ and Daniela Rubatto^ ^ Department of Geolog>' and Geophysics, Adelaide University, Adelaide SA 5005 " Research School of Earth Sciences, Australian National Universit>^ Canberra ACT 0200 Understanding the thermal evolution of the high geothermal gradient Mt Isa Inlier in northern Australia has been hampered to a large extent by a paucity of thermochronological data. While this issue has been addressed to some extent by the generation of a large "^Ar-^^Ar dataset (Spikings et al, 2001) there is still comparatively little data constraining the timing of peak or near peak metamorphic conditions across the Inlier. We present U/Pb and Sm/Nd isotopic constraints on the tnnmg of peak metamorphism m the western and central parts of the Mt Isa Inlier.

In the Hazeldene region of the western part of the Inlier, cordierite-orthoamphibole gneiss contains monazite aligned in the regional peak metamorphic foliation (c. 600''C). The monazite yields a concordant age of around 1575 Ma, which is interpreted to record the timing of peak metamorphic conditions in that area. Rutile from a nearby chlorite-orthoamphibole-bearing retrograde shear zone gives an age ~ 450 Ma. In the Rosebud S>iicline in the central Mt Isa Inlier, monazite from a peak metamorphic (sillimanite zone) cordierite-orthoamphibole schist contains cores rich in quartz inclusions that yielded ages around 1570 Ma. Overgrowths and new crystals record a younger event at -1540 Ma. The sample also contains a few monazite crystals with oscillatory zoning that give ages around 1505-1510 Ma. Rutile from the same sample gives an age of around 1465 Ma. Cordierite-andalusite-bearing schist collected down grade of the sillimanite-in isograd contams monazites with cores givmg -1570 Ma and overgrowths -1515 Ma. A garnet SmyWd age from close to the sillimanite-in isograd gives 1570 Ma. Further east, Sm/Nd ages from two gametstaurolite-bearing samples in the Tommy Creek Block give -1585 Ma and 1575 Ma. The garnets are growth zoned and record an up-pressure evolution during the development of the regional foliation.

In combination with existing U/Pb data from the eastern succession (Page and Sun, 1998), our data mdicate that regional high geothermal gradient metamorphism (HGGM) occurred approximately synchronously across the Mt Isa Inlier at around 1575 Ma. The data identify a compelling and large-scale thermal problem. There is no known regional magmatism in the Mt Isa Inlier in the interval 1600-1510 Ma. Furthermore there appears to be insufficient enriched granitic rocks to provide regional radiogenic-dnven heating in the manner envisaged by McLaren et al. (1999).

However the length scales of regional HGGM in the Mt Isa Inlier are of similar magnitude to the extent of the Mt Isa Superbasin, and we suggest that burial of heat producing stratigraphic sequences m the Mt Isa Superbasin generated the primary high geothermal gradient regime. This may have been locally supplemented by contributions from enriched granites and secondary processes leading to localised thermal events such as those recorded at -1540 Ma and 1505-1520 Ma in the Rosebud Syncline. Furthermore our rutile ages (which may record cooling through 500-550®C) are essentially identical to the bulk of hornblende "^^Ar-^^Ar ages from the eastern and central eastern parts of the Inlier (Spikings et al., 2001). Although this suggests rather than proves slow ( - 1 ""C/Ma) cooling of the inlier, it highlights that the Mt Isa Inlier may be a world class exception to the general paradigm descnbmg the generation of high geothermal gradient conditions in convergent settings.

References Spikings, R.A., Foster, D.A., Kohn and Lister, G.S., 2001. Post-orogenic (<15 Ma) thermal histor>^ of the Proterozoic Eastern Fpld Beh, Mt Isa Inlier, Australia. Precambrian Research, 109, 103-144.

Page, R.W. and Sun, S.-S., 1998. Aspects of geoclironology and crustal evolution in the Eastern Fold Belt, Mt Isa Inlier. Australian Journal of Earth Sciences, 45, 343-361 McLaren, S. Sandiford, M. and Hand, M., 1999. High radiogenic heat-producing granites and metamorphism - an example from the western Mt Isa Inlier, Australia. Geology, 27, 679-682.

173

WIDESPREAD NEOPROTEROZOIC METAMORPHISM IN NORTHERN SCOTLAND AND CONSTRAINTS ON CALEDONIAN METAMORPHISM Martin Hand, Nick Lisowiec and Jo Mawby Department of Geolog}^ and Geophysics, Adelaide University, Adelaide SA 2002 The crustal architecture of northwestern Scotland was largely shaped by the Ordovician-Silurian (470-430 Ma) Caledonian-Appalachian collision, which produced a series of nappes that occur in the hanging wall of the Mome Thrust Zone. The nappes contain interleaved Neoproterozoic cover (the Mome Series) and reworked Lewisian basement and were thrust westward for more 100km across the basement in the Laurentian foreland.

In north Sutherland m northernmost Scotland, the structurally lowest Moine Nappe contams garnethornblende and gamet-staurolite ± kyanite-bearmg assemblages that formed at around 640'^C and 10 kbar and are assumed to be Caledoman m age (e.g. Dalbneyer et al, 2001, Geological Society London, 158, 501-512). However Sm/Nd data from the garnet-bearing assemblages give ages that range between 830-800 Ma indicating that the principal metamorphic character of the Moine Nappe reflects Knoydartian rather than Caledonian metamorphism. The garnets preserve uppressure prograde zomng and formed during the development of a pervasive layer parallel Si foliation that was associated with north-directed transport (Holdsworth, 1989, Geological Society London 146, 809-823). The garnet Sm/Nd ages from north Sutherland are similar to U/Pb gramte and Sm/Nd garnet ages from the SW highlands ~ 200km to the south (Vance et aL, 1998., Geology, 26, 927-930; Rogers et a/., 1998, Geological Society London, 146, 789-798; Fnend et al, 1991T Contnbutions to Mmeralogy & Petrology 128, 101-113), miplying that the entire Moine Senes underwent medium to high-grade metamorphism at around 800 Ma. At this stage there is no direct evidence of whether the c. 800 Ma event was extensional or compressional, however at face value, up-pressure metamorphism of Supracrustal rocks to depths of around 35 km implies significant convergent deformation. We speculate that this event was transpressional in nature with bulk transport along the axis of the north-trending Moine Superbasin.

Sm/Nd data mdicates that garnet-bearing assemblages (725''C and lOkbar) in the migmatitic Kirtomy Nappe, which overlies the Moine Nappe, formed at 430 Ma and therefore record highgrade metamorphism associated with Scandian nappe stacking late in the Caledoman. The highgrade Silurian metamorphism is -- 35 Ma younger than zircon ages from the Kirtomy migmatites (Kinny et al., 1999, Geological Society London, 156, 1143-1152), indicating that the structurally highest Mome Zone Nappes record two Caledonian events (c. 465 Ma and c. 430 Ma). The c. 800 Ma garnet-bearing metapelitic assemblages in the underlymg Mome Nappe are partially replaced by syn D2 staurolite-chlorite assemblages. These mterpreted Caledoman-aged assemblages formed at around 4-5 kbar, 550^C. The large difference m Caledoman bunal between the Mome and Kirtomy Nappes implies that the largest Caledonian (Scandian) displacements m northern Scotland occurred along the Naver Thrust, which separates Caledonian migmatites from the comparatively low-grade Caledonian assemblages in the underlying Moine Nappe.

174

THE FIELD RELATIONS WITHIN THE TANUNDA CREEK GNEISS AND THE KANMANTOO GROUP AND ITS EVOLUTION. Eduard Reginald Heinisch and Jolin Foden Department of Geology and Geophysics, University of Adelaide, Adelaide SA 5000 The Tanunda Creek Gneiss is a NNW/SSE elongated strongly deformed gneissic granite intrusion (10km by 2km) near Tanunda in the Barossa Valley, South Australia. It is composed of a medium gramed quartz- K-feldspar- plagioclase granodiorite with some feldspar megacrysts up to 5cm. The granite has intruded the originally turbiditic Backstairs Passage Formation of the Kanmantoo Group (KG). This sequence is recrystallised to a porphyroblastic psammo-pelitic gneiss that preserves 1 cm layering in the pelitic lithologies and prominent (~0.5m) sandy units. At its west margin the Gneiss has a 400 m transition contact zone with the KG where it is intruded on a metre spacing by metagramtoid dykes ranging in thickness from 5 cm to 2 m. The evolution of the Gneiss started early in the Delamerian Orogeny with its intrusion at 513±5 Ma (U-Pb zircon age: Burtt, pers. comm., 2002) as an I-t}T3e s>ii-tectonic granite. The body is very like the Rathjen Gneiss (Foden et al, 1999) 50 km to the SE, suggesting multiple intrusions of similar magmatic bodies in this early Delamerian stage.

The Gneiss is intruded and mterleaved with metadolerite dykes ranging m width from 4 m to 100 m. In common with the host gneiss, these show a pervasive, steep, south-plunging lineation.

The mafic dyke intrusions exhibit either sharp or merged contacts with the granodiorite, providing good evidence for contemporary mingling of mafic and granodioritic magmas.

The Gneiss and the host metasedunents are dominated by a strongly developed, often steeply dipping Si foliation with a NNW/SSE strike. A strong Li lineation in the host metasedimentary gneiss, the metadolerite and in the metagranitoid has a general SE strike, plunging at between 70° and 80°. This lineation is like the prominent Li lineation in the Rathjen gneiss further south where it has a N-S strike and is subhorizontal. There is good evidence that the Tanunda Creek Gneiss is in a steeply south-plunging major fold, possibly recording transpressional motion on major N-S shear zones in the middle to late stages of the Delamerian Orogeny. The close association of granite and mafic magmas indicated by the metadolerites in the Tanaunda suite implies a role for mantle heat and mass transfer, perhaps contributing to crustal weakening and the development of the early stages of inversion of the Kanmantoo basin to form the Delamerian Fold Belt.

References Foden J. et al., 2002. J. Geol Soc. Lond., vol 159.

Foden, J., Sandiford, M, Doughert\'-Page, J., and Williams, I. (1999) The geocheniistr>^ and geochronology of the Rathjen Gneiss: implications for the early tectonic evolution of the Delamerian Orogen.

Australian Journal of Earth Sciences, 46, 377-389.

Burtt A.C., 2002. Pers Comm., Geological Surx^ey, Mineral Resources, Primary Industries and Resources South Australia.

175

STRUCTURAL AND METAMORPHIC EVOLUTION OF THE ARTHUR LINEAMENT, NORTHWEST TASMANIA Oliver H. Holm^ * and Ron F. Berry^ Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001 *current address Geoscience Australia, GPO Box 378, Canberra, ACT 2601 The Arthur Lineament is a Cambrian age, high strain metamorphic belt in northwest Tasmania, Australia. It is east-dipping, NE-trending, 5 to 10 km wide and approximately 110 km long. The lineament consists of phyllitic to schistose lithologies that have been subject to medium- to high strain, and variably metamorphosed to blueschist or greenschist grade. The lineament comprises several different units. The major lithological unit in the southern Arthur Lineament is the late Neoproterozoic ''eastern" Ahrberg Group, which comprises metamorphosed psammite and psammopelite units, transitional to tholeiitic metabasalts and mafic volcanogenic metasediments. The southern Arthur Lmeament also contains the Bowry Formation, which contains basalt/doleritederived tholeiitic amphibolites that are intruded by minor deformed 111±1 Ma granitoids, a unit correlated with the Ahrberg Group (AGC) and Neoproterozoic highly strained turbidite sequences (Burme and Oonah Formations).

The ''eastern'' and "western" Ahrberg Group sequences, and the AGC, are interpreted to be lithostratigraphic correlates. The units have sequences of transitional tholeiites that are enriched relative to MORB, and are increasingly tholeiitic towards the top of the stratigraphic pile. The Bowry Formation also comprises transitional tholeiites, however they are more evolved than the other units, and have REE patterns only slightly enriched relative to MORB. The main orogeny to affect northwestern Tasmania was in the Early to Middle Cambrian. This event (the Tyennan Orogeny) consisted of three deformational episodes. The first two episodes (CaDi and CaD2) were high strain events, related to arc-continent collision. These involved the emplacement of allochthonous slices, and the structural repetition of units within the lineament. CaDi produced isoclinal folding, a schistose axial planar fabric and shearing parallel to the CaFi axial plane. CaDi was more pervasive than CaDi, and produced tight to isoclinal folds with a schistose axial planar fabric. The CaSi and CaS2 axial planar foliations are commonly sub-parallel, and the fold axes are close to coaxial. The orientation of CaVx and CaV2 folds changes fi-om the low to high strain domains. This is consistent in both the north and the south of the lineament. Structural analysis suggests that the change in orientation is due to a strongly rotational shear component during the CaDn event, and this is interpreted as evidence for south-directed transport. The subsequent CaDs event, in the latest Cambrian, involved east-directed thrusting. It produced as>inmetric folds with gently west-dipping long limbs and steeply east-dipping short limbs.

In summary, in northwestern Tasmania, following the Cambrian arc-continent collision, the blueschist grade allochthonous slices were obducted and transported to the south of the subduction zone (CaDi). Other parautochthonous rocks were also transported to the south during this high stram event, with associated shearing and folding occurring at greenschist grade metamorphic conditions.

Following this, a near coaxial high strain deformational event took place, detaching and overthrusting other parautochthonous slices to the south, as they were variably metamorphosed to greenschist and upper greenschist-amphibohte facies (early CaD^)- During these events, strain-related rotation of the developmg folds took place, resulting in a change in orientation of the fold axes from east-west to north south, and foliations from south-dipping to east-dipping, from the low^ strain zones into the high stram zones. The revised metamorphic geochronology of the Arthur Lineament indicates the early CaT>2 event occurred around 505-510 Ma. During CaD\ and early CaD2, the parautochthonous lithologies were structurally emplaced on top of the less deformed, autochthonous units. In the final stage of assembling the tectonostratigraphic 'pile', the various slices were stacked together, and lower greenschist facies metamorphic conditions were developed uniformly in all allochthonous, parautochthonous and autochthonous blocks (late CaDi). Subsequent to this, the CaDs event caused the tilting of the tectonostratigraphy, to complete the development of the Arthur Lineament.

176

ANALYSIS OF CURVED INCLUSION TRAILS IN GARNET PORPHYROBLASTS USING COMPUTED X-RAY TOMOGRAPHY Cameron Huddlestone-Holines\ and Richard Ketcham^. ^School of Earth Sciences, James Cook University; cameron.hholmes@jcu.edu.au "Department of Geological Sciences, University of Texas at Austin; ketcham@mail.utexas.edu A new technique is presented that allows the orientation of the axes of curvature of curved inclusion trails in individual garnet porphyroblasts to be determined. Previously this has only been possible through the laborious process of serial sectioning. This new technique utilizes data generated by X-ray computed tomography (X-ray CT) and volume rendering. It is demonstrated on a sample of garnet schist from the Ordovician Cram Hill Formation in southeastern Vermont.

X-ray CT is an exciting technique whose potential for geological applications is still being explored. It enables the internal structure of intact rock samples to be visualized in three dimensions. The images generated show differences in the attenuation of X-rays as they pass through the sample. At the X-ray energies used (100 - 200keV), the amount of attenuation is a function of the density and atomic number of the material being imaged. The densit}^ and mean atomic number differences between garnet porph>Toblasts and inclusion and matrix material such as calcite, quartz, ihnenite, muscovite, biotite, feldspar and graphite allows these features to be resolved using this technique. The spatial resolution of X-ray CT is dependent on many factors including the source-detector arrangement and sample size. In this case an 11mm diameter cylindrical core of the sample was analysed with the resulting resolution 20 x 20 x 29|Lim per voxel.

Figure 1 shows a vertical slice taken through the imaged volume.

A 3-D rendering of the data was constructed in OpenDX, an open source data analysis package freely available at http://wv\vv.opendx.org. To determine the orientation of the curvature axis, a series of virtual vertical slices were made through an mdividual porphyroblast at var>dng orientations. The orientation at which the as>Tnmetry of curvature in the inclusion trail flips represents the strike of the axis of curvature. Once the strike had been determined, the plunge was then determined similarly using sections inclined within the strike plane. The accuracy to which the strike and plunge of the axis are determined is ±2.5^

Figure 1. Slice through 3-D volume rendering of garnet schist from the Littleton Formation, VT. Grey scale image with white representing maximum attenuation of X-rays. Matrix material has been rendered transparent. The 3-D volume rendering on the left shows the location of the shce.

In recent literature, such axes of curvature in porphyroblasts have been referred to as Foliation Intersection/Inflection Axes (FIAs). The FLA technique allows the overall trend of the FIA(s) in a sample to be determined from a series of oriented thin sections. As a result, the porphyroblast to porphyroblast variation m FIAs within a sample can only be estimated. To allow FIAs to be used to their full potential, questions about the statistical confidence that can be assigned to determined trends need to be addressed. This new technique will allow study into the intra-sample variation in the orientation of FIAs. This may also lead to a contribution to the debate as to whether or not these curved inclusion trails form as a result of rotation of porphyroblasts relative to an external reference frame.

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E V O L U T I O N OF H I G H - P R E S S U R E M E T A M O R P H I C R O C K S F R O M T H E R O C K Y B E A C H METAMORPHIC M E L A N G E , PORT MACQUARIE, NORTHEAST NEW SOUTH W A L E S .

David J. Och\ Evan C. Leitch\ Graziella Caprarelli^ and Teruo Watanabe^ Environmental Sciences, University of Teclinology, Sydney, NSW 2007 "Earth and Planetaiy Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan, 060 A range of high-pressure metamorphic rocks including eclogite, glaucophane schist, and omphacitite, as well as epidote amphibolite and tremolite marble, form phacoids in the Rock>^ Beach Metamorphic Melange. The high-pressure phacoids retain a record of up to four metamorphic episodes (M1-M4) and structures mdicative of six deformational events (Di-De). The Ml assemblage, of blueschist facies and developed during Di, includes apatite, titanite, actinolite, magnesioriebeckite and glaucophane, and is preserved as sigmoidal inclusion trails in later (M2) porphyroblasts of ahnandine and lawsonite. M2, the peak metamorphic episode, produced eclogitic rocks (aknandine-omphacite-lawsonite-±quartz), which based on data in Holland (1983) and Krogh (2000) cr>^stallised under static conditions at a pressure of at least 1.5GPa and a temperature of about 550°C, possibly m the late Neoproterozoic (Watanabe et al 1998). Subsequent to peak M2 conditions, omphacite developed undulose extinction and subgrain formation while temperatures still exceeded 470±50°C (cf. Piepenbreier and Stockhert 2001). Extensional micro-veins oriented normal to compositional layering in the eclogite are filled by slender prisms of omphacite that have grown normal to the vein walls and are lower in jadeite content than the omphacite in the host rock.

The vein omphacite probably formed under falling temperature and pressures of < 1.2GPa (cf.

Holland 1983). Subsequently the eclogites were variably retrogressed to rocks sharing the stable M3 blueschist facies assemblage (glaucophane-phengite-ahnandine-K-feldspar-quartz-titanite) in the Ordovician (470Ma) (Fukui et al 1995). M3 conditions (pressure c. 0.9 - l.lGPa, temperature c. 400°C) were maintained during the formation and subsequent folding of a prominent schistosity (D2 and D3) and the development of later small kinks (D4). Late replacement of glaucophane by actinolite and accompan>dng crystallisation of chlorite, stilpnomelane, quartz, calcite, talc and titanite marks a greenschist facies (M4) episode at pressures < 0.5GPa and temperatures < 350''C that also affected the chlorite-actinolite melange matrix in which accompanying deformation (Ds05) was concentrated.

The evolution of the Rocky Beach Metamorphic Melange is strikmgly smiilar to that of the Varvara Boudin on the island of los in the Aegean Sea and adjacent rocks as described by Forster and Lister (1999) although in the present case the phacoids are embedded in greenschist rather than blueschist facies rocks and have probably been brought to their present structural level by entrainment m a serpentinite protrusion.

References Forster, M. A. and Lister, G. S., 1999. Separate episodes of eclogite and blueschist facies metamorphism in the Aegean metamorphic core complex of los, Cyclades, Greece. Iir. Mac Niocaill, C. & Ryan, P. D. eds. Continental tectonics, pp. 157-177. Geological Society Special Publications London 164.

Fukui, S., Watanabe, T., Itaya, T. and Leitch, E. C., 1995. Middle Ordovician high PT metamorpliic rocks in eastern AustraUa: E\idence from K-Ar ages. Tectonics 14, 1014-1020.

Holland, T. J. B., 1983. The experimental detennination of activities in disordered and short-range ordered jadeitic pyroxenes. Contribution to Mineralogy and Petrology 82, 214-220.

Krogh, E. R., 2000. The gamet-clinopyroxene Fe~^-Mg geothermometer: an updated calibration. Journal of Metamorphic Geology 18, 211-219.

Piepenbreier, D. and Stockliert, B., 2001. Plastic flow of omphacite in eclogites at temperatures below 500°C - implications for interplate coupling in subduction zones. International Journal of Earth Sciences 89, 197-210.

Watanabe, T., Fanning, M. and Leitch, E. C., 1998. Neoproterozoic Attunga Eclogite in the New England Fold Beh. Geological Society ofAustralia, Abstracts 49, 458.

178

APPLICATION OF COMBINED APATITE (U-Th)/He THERMOCHRONOMETRY AND FISSION TRACK ANALYSIS ON CRATONS: POSSIBILITIES AND LIMITATIONS ^M. Lorencak, ^B.P. Kohn, ^K.G. Osadetz and ^A.J.W. Gleadow ^School of Earth Sciences, The University of Melbourne, VIC 3010, Australia ^Geological Survey of Canada, 3303 33'^ St. N.W., Calgary, Alberta T2L 2A7, Canada In cratomc settings, where Phanerozoic stratigraphic or tectonic information is commonly scarce or missing, (U-Th)/He apatite thermochronometr}^, when used in combination with apatite fission track (AFT) analysis, can potentially be a powerful tool for elucidating low temperature thermal histones. Such an approach could provide thermal histories independent of the AFT system and allow further constraints on modelling of time-temperature cooling paths. However, the difficulty m reproducmg (U-Th)/He ages from rocks w^hich have experienced slow cooling and prolonged residence in the He partial retention zone, presents a potential problem (e.g. Reiners 2002). An earlier study by some of us on the Fennoscandian Shield in Finland has suggested the potential for replicatmg (U-Th)/He apatite data in cratonic environments when taking into account certain factors such as the absence of U zoning in apatites and excess He produced by U and Th-rich micro-inclusions.

Here, we report the results of a combined (U-Th)/He and AFT study on the southern Canadian Shield to further illustrate the potential of this application. A 3440 m deep exploration borehole, drilled by INCO Limited, in the relatively undisturbed north range of the Paleoproterozoic Sudbury structure in southern Ontario, has been sampled at -200 m intervals. Apparent AFT ages from the 19 samples collected range from -360 Ma at the surface to - 1 4 0 Ma at the total depth and these define a sinuous trend with an amplitude of -100 m.y. Mean horizontal confined track lengths (HCTL) var}^ between -11.0 and 13.6 microns, and the standard deviations of the track length distributions fall between -1.7 and 3.5. Chemical composition of apatite ranges from 0.1 to 2 wt% CI. The horizontal confined track length (HCTL) pattern is also sinuous, but in a more subdued manner and forms a mirror image of the AFT age pattern in that increases in mean HCTL broadly correlate with decreasing apparent AFT age. Apatite aliquots from 10 of the samples were chosen for (U-Th)/He thermochronometry. Sample suitability was assessed on the basis of homogeneous distribution of U based on prints made on mica solid state track detectors used for AFT dating.

These samples, mainly of gabbroic composition, were also relatively free of major inclusions. (UTh)/He ages range from -250 Ma at the surface to 0 Ma at -3000 m at a bottom hole temperature of -60°C. Replication of the ages was possible within error limits of ±lsigma for most samples.

Ages are consistently younger than their coexisting AFT ages and closely mimic the sinuous pattern which characterises the AFT age versus depth profile. Interpretation of this enigmatic pattern has to consider processes which affect both the AFT and (U-Th)/He apatite systems independently. The results suggest a burial of the southern Canadian Shield under 2-3 km of foreland sediments following the Alleghanian Orogeny in Permian to early Triasssic time. They further confirm the suitability of the (U-Th)/He apatite method in slowly cooled cratonic environments with ngorous sample selection procedures.

Acknowledgments We thank INCO Limited for providing sample material. Tliis work was supported by the Australian Research Council and the Australian Institute of Nuclear Science and Engineering.

References Reiners, P.W. 2002. (U-Th)/He chronometry experiences a renaissance. EOS, Transactions, American Geophysical Union 83 (3;, 21-27

179

THE EXTENT OF CAMBRIAN METASEDEMENTS AND THE ORDOVICIAN LARAPmTA EVENT, EASTERN ARUNTA PROVINCE, CENTRAL AUSTRALIA. David W. Maidment^ l Martin Hand^ and Ian S. Williams^ ^ Geoscience Australia, GPO Box 378, Canberra ACT 2601 ' Research School of Earth Sciences, Australian National University, Canberra ACT 0200 ^ Department of Geolog>^ and Geophysics, Adelaide University, Adelaide SA 2002 The recent identification of granulite grade early Cambrian metasediments in the eastern Arunta Provmce has highlighted the lack of understanding of Palaeozoic tectonics in central Australia and the links between the evolution of the eastern continental margin and the interior. Existing data suggest that Cambrian sediments of the Irmdina Supracrustal Assemblage (ISA) were deposited in a localised intracratonic rift associated with voluminous basaltic magmatism. Burial to >35 km occurred by -470 Ma within a largely marine palaeogeographic environment. However, while this basic framework is known, little is known about the extent of the rift sequence and the high-grade Ordovician metamorphism or the processes by which sediments were buried.

SHRIMP analysis of detrital zircons fi'om metasediments and metavolcanics belonging to the ISA confirms earlier work which indicated a maximum depositional age of 540-560 Ma for much of the sequence. Detrital zircons in ISA metasediments are dominantly 1.0-1.3 Ga and/or 1.6-1.9 Ga old, consistent with sources in the Musgrave and Arunta Inliers. Quartzites from the Stanovos Gneiss and near Badens Bore east of the Harts Range have no detrital zircon grains younger than early to mid-Neoproterozoic, which might indicate deposition at this time or reflect a lack of younger source regions, similar to Early Cambrian sediments of the Amadeus Basin. Felsic megacrystic gneiss from the Upper Stanovos Gneiss has an igneous protolith age of --520 Ma, the first granite of this age identified in central Australia. Field relationships and inherited zircon ages suggest it is derived from partial meltmg of the ISA, and thus defines a minimum age for deposition of the associated metasediments. Neoproterozoic to Cambrian metasediments have been found up to 100 km east of the Harts Range near Atula homestead, increasing the known E-W extent of the ISA to -200 km. The Early Ordovician Larapinta Event has been identified west of the Harts Range, near Mt. Pfitzner and as far east as Atula Homestead, defining a minimum E-W extent of -250 km.

Palaeoproterozoic basement has been identified at several locations previously considered part of the ISA. Megacr>^stic gneisses 2.5 km N of Rockhole Dam and at Watsons Creek as well as felsic gneisses 7 km SE of Rockhole Dam and at Mt. Bird have protolith ages between 1735 Ma and 1760 Ma. The general absence of 460-480 Ma zircon overgrowths in these basement rocks is consistent with the interpretation that the ISA was thrust over basement during convergent deformation that commenced at -450 Ma. The preserved ISA in the Harts Range appears to be a relatively thin veneer above Palaeoproterozoic gneisses of the Strangways Metamorphic Complex.

The complex interleaving of basement and Cambrian sequences is largely controlled by deformation at -450 Ma and during the Alice Springs Orogeny between 300-400 Ma. The ISA was thus deposited in the Neoproterozoic to Early Cambrian, possibly within a localised rift within the Amadeus and/or Georgina Basins. These sediments must have been rapidly buried since partial melts denved from the sediments formed and crystallised by -520 Ma. This burial might have been a result of rapid sediment accumulation within the developing rift or convergent deformation in the Cambrian, possibly equivalent to the Petermann or Delamerian Orogenies. A regional unconformity and change in sedimentation patterns in the Amadeus Basin in the Late Cambrian might be a consequence of such deformation. Further extension resulted in widespread upper amphibolite to granulite facies metamorphism at 480-460 Ma. The ISA and Larapinta Event coincide with a prominent WNW-trending low magnetisation corridor in regional aeromagnetic data. This corridor is likely caused by a combination of demagnetised basement and lowsusceptibility ISA metasediments. It extends SE to the Cambro-Ordovician Warburton Basin and NW towards the Canning Basin, which was initiated in the Early Ordovician, suggesting that rifting may have been continental in scale.

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SPREADING THE WELT OR MELTING TO SPREAD? LATE CRETACEOUS TO EOCENE TECTONOTHERMAL EVOLUTION OF THE HINTERLAND OF THE SEVIER OROGENIC BELT, WESTERN USA Allen J. McGrew Department of Geolog>^, The Universit}^ of Dayton, Dayton, OH 45469-2364 Email: Allen.McGrew@notes. uda^lon. edu The Sevier orogenic belt developed in a back-arc setting behind the Late Cretaceous Sierra Nevada Idaho arc batholith belt. The history of arc magmatism was interrupted for a period of >20 m.y. in Early Tertiary tmie commonly attributed to "flat slab" subduction. This period ended abruptly but time-transgressively with a flare-up of magmatism beginning in Early Tertiary time in the north and youngmg progressively southward toward southern Nevada in Miocene time.

The renewed magmatism is thought to flag the cessation of flat slab subduction and approximately coincides with the earliest clear records of hinterland tectonic extension at any given latitude. Did asthenospheric upwelling and/or delamination of the mantle lithosphere at the end of the flat-slab period simultaneously trigger volcanism, crustal weakening and the onset of extension? Or conversely, did crustal gravitational collapse trigger extension and possibly also the volcanism? Which was more important in driving the onset of extension, buoyancy forces in the mantle or in the crust? In either case, what role did the thermal and rheological evolution of the lithosphere play during this transition?

Two terrains from contrasting structural levels in northeastern Nevada yield complementary^ insights into early Tertiary crustal structure and magmatic and tectonic evolution of the Sevier hinterland. The East Humboldt Range metamorphic core complex exposes a terrain exhumed from depths >30 km durmg a protracted Tertiary extensional history. In contrast, the Copper Mountains exposes a terrain exhumed from upper mid-crustal depths of -10 km during moderate Late Eocene extension.

Thermobarometric and chronologic constraints indicate that peak PT conditions in the East Humboldt Range of -8.5 kb, 750°C were achieved synchronously with intense migmatization and emplacement of a large recumbent fold-nappe at approximately 85 Ma. This timing also coincides with maximum rates of arc magmatism to the west and peak thrust rates in the Sevier belt to the east, thus establishing an important kinematic link between the key tectonic elements of the Late Cretaceous Cordillera.

Subsequently, this terrain may have expenenced 2-3 kb of decompression by - 5 0 Ma, but at present no surface-breaking normal faults can be definitively associated with this deep crustal decompression.

Intrusion of a thick quartz dioritic sill at depth and eruption of widespread ignimbrites at - 4 0 Ma appear to flag a new phase in hinterland tectonic history. A kilometer-scale mylonitic shear zone and regionally extensive detachment fault system initiated at least by 30 Ma and probably significantly earlier. This extensional system exhumed the East Humboldt core complex from mid-crustal depths of 20-25 km and temperatures >500°C to near-surface conditions by - 2 2 Ma. Like the East Humboldt Range, the Copper Mountains records a history of pre-Late Cretaceous (-110 Ma) foldmg and thrust-faulting, arc magmatism and modest tectonic burial but, unlike the East Humboldt Range, the Copper Mountains show little sign of Late Cretaceous deformation, suggesting that hinterland tectonism during this crucial penod was confined primarily to deep-crustal flow, with the upper crust behaving relatively passively. In the Copper Mountains the onset of Tertiar}^ extensional tectonism is narrowly backeted between 41 and 37.4 Ma by a syntectomc sedimentary sequence deposited m an extensional basin floored by Late Eocene volcanics similar to those observed m the East Humboldt Range and throughout northeastern Nevada. Palinspastic restoration requires at least 10 km displacement on this Late Eocene fault system. The Late Eocene Copper Basin flora suggests that the paleoelevation of this terrain was not significantly higher in Early Eocene time than it is today, an observation difficult to reconcile with the crustal gravitational collapse hypothesis. Taken together, the results from northeastern Nevada suggest that the seminal event triggering the onset of extension was not collapse of overthickened continental crust but positive mantle buoyancy accompanied by widespread magmatism and thermal weakening of the lithosphere at the end of the flat-slab tectonic regime.

181

THE THERMAL EVOLUTION OF THE MOUNT PAINTER PROVINCE: GRANITE GENESIS DURING THERMAL SAG? ^Sandra McLaren. ^Mike Sandiford, "Richard White and "Roger Powell ^present address, Research School of Earth Sciences, Australian National University, ACT, 0200 "School of Earth Sciences, University of Melbourne, Vic 3010 Email: sandra.mclaren@anu.edu.au; niikes@unimelb.edu.au The Mount Painter Province (MPP) consists of dominantly Mesoproterozoic basement rocks overlain by the Neoproterozoic Adelaidean cover sequence. The region is the northernmost basement block of the Adelaide Fold Belt and is charcterized by high surface heat flow (126 mWm" Cull, 1982). At least 70 mWm"^ of this measured heat flow, twice the accepted global average, resuhs from radiogenic heat production in the basement rocks, making the MPP crust one of the most thermally anomalous regions in Australia.

This extraordinary heat producmg element enrichment has played a fundamental role in the thermal and geological evolution of the MPP crust. Here we focus on its role in modulating the history of the region during its Delamerian and post-Delamerian history.

One particular problem is the origin of the British Empire Granite (BEG). The BEG is a large (23 km^), c. 500-480 Ma, strongly peraluminous granite which intrudes Proterozoic rocks near the structurally deepest part of the exposed basement. It contains coarse muscovite, K-feldspar, plagioclase, quartz and some garnet (Neumann, 1996). The granite exhibits complex interfingering and gradational contacts with the enclosing partly migmatitic metasediments suggesting that it was derived locally. However, the absence of similar aged melts and the lack of co-magmatic volcanics or mafic intrusions raise questions about the mechanism of granite generation.

Sandiford et al (1998) advocate a model for Delamenan metamorphism in the MPP in which the observed metamorphic signature is explicable in terms of the thermal effect of burial of the radiogenic heat producing basement beneath the Adelaidean cover sequence. This model predicts no additional heat input from external sources. If this model is correct, then the thermal perturbation leading to melting and generation of the BEG must also be a result of burial; that is, granite generation as a result of thermal sag.

Recent work by White et al. (2001) on partial melting equilibria in metamorphic systems enables problems of granitic magmatism to be addressed quantitatively. This approach, together with sunple thermal models, has been adopted to evaluate the validity of the "thermal sag" hypothesis.

Results suggest that although temperatures at 4 kbar (the metamorphic pressure of the surrounding rocks) were sufficient to begm meltmg, the volume of granite observed could not have been produced in a closed system under these conditions. Whereas open system processes involving water addition could account for the melt volume, this seems geologically unreasonable. More likely is open system processes involving melt addition. This implies that the modem BEG represents a melt "pooling" site and is actually a mix of melt sourced from the range of rock types m the mid crust, together with some small amount of m-situ melt.

References Cull J.P. 1982. An appraisal of Australian heat-flow data. BMR Journal of Australian Geology^ and Geophyhsics, 8, p. 329-337.

Neumann, N, 1996. Isotopic and geochemical characteristice of the British Empire Granite as indicators of magma provenance and processes of melt generation in the Mount Painter Inlier, SA. BSc Hons thesis.

University of Adelaide.

Sandiford, M., Hand, M. and McLaren, S. 1998. High geothermal gradient metamorphism during thermal subsidence:Earth and Planetary Science Letters, 163, 149-165.

Wliite, R.W., Powell, R. and Holland, T.J.B. 2001. Calculation of partial melting equilibria in the s>^stem NCKFMASK Journal of Metamorphic Geology, 19, 139-153.

182

GEOLOGICAL STRUCTURE OF THE FLINDERS RANGES, SOUTH AUSTRALIA P. J. (Lai) Mendis. P. R. James and R. A. Both Department of Geolog>^ and Geophysics, University of Adelaide, South Australia The Flinders Ranges consist of Neoproterozoic to Cambrian sequences of sediments, which were deposited in the Adelaide Geosyncline. The sediments have been folded into two series of upright, shallowly plunging folds, which trend NNW (Fi) and ENE (F2), where the earlier folding is more prominent than the latter.

A balanced cross-section across the (Fi) Enorama Anticline in the central Flinders Ranges, and its similarities with other (Fi) anticlines, indicates a west verging fault-propagation fold geometry. Minor parasitic structures studied on both limbs of the anticline support this mechanism of folding. The study suggests that the sequences were possibly thrust along a decollement over the basement, and then folded to form a series of imbricate fans, resulted in fault-propagation fold geometry. The folds contain gently dipping forelimbs and forelimb synclines, steeply to gently dipping back limbs and back limb anticlines, thickening or thinning of sequences and the occurrence of stratigraphically lower sequences on back limbs than on the forelimbs. The longest Fi axial trace of the central Flinders Ranges continues through the Enorama - Oraparinna Anticlines and extends for hundreds of kilometers from Beltana to the Worumba Anticline and further to the southern Adelaide Fold Belt.

A senes of notheastely oriented fractures and faults originated during the Di deformation, thus forming grabens across the Fi anticlines. Graben structures appear to have also moved on decollement surfaces over the basement. The decollement surfaces of both the Fi anticlines and the graben structures appear to have continued as listric thrust faults into the upper sequences, resulting steep to moderate dips adjacent the axial traces of anticlines. Breccia deposits commonly known as diapirs possibly formed during these thrusting events and were emplaced in the cores of folds as well as adjacent to the apices of the grabens.

A study of minor and micro-scale F2 folds also suggests fault-propagation fold geometr>^ However, those on the eastern limb of the major Fi axial trace from the Beltana to the Worumba Anticline tend to show SE vergence while those on the western limb show NW vergence. This is in agreement with the occurrence of major dextral kinks along the Beltana to Worumba (Fi) Anticlines, suggesting a dextral rotation of Fi structures. The largest dextral kink of the Fold Belt, the Nackara Arc and a series of parasitic arcs between anticlines were possibly formed by the dextral rotation of Fi axial traces between anticlines, during D2 deformation.

Superimposition of folds has formed complex egg-cartoon structures both at major and minor scales.

The dextral rotation of Fi axial traces into an F2 orientation and overprinting of bedding-slip lineations suggest Di structures predated D2.

Thus, the study suggests the Flinders Ranges of the Adelaide Fold Belt suffered two consecutive deformations durmg the Delamenan Orogeny, as a result of ENE-WSW and NNW-SSE compressions, respectively to produce Di and D2 structures.

183

PUTTING THE SQUEEZE ON THERMOBAROMETRY : THE EFFECT OF DIFFERENTIAL MINERAL COMPRESSIBILITY ON PRESSURE ESTIMATES ^David Phillips and ^Jeff Harris ^School of Earth Sciences, The University of Melbourne, Melbourne, Victoria 3010 ^Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK

The determination of equilibrium temperatures and pressures from experimental or empirical thermobarometers is critical to the interpretation of the thermal and tectonic histories of metamorphic rocks in the Earth's crust and upper mantle. In some instances, thermobarometric determmations are carried out on minerals that are included in one another. This study considers the potential for maccurate pressure estimates, due to differences in thermal compressibility between the inclusion and host phase. Diamond is an ideal candidate for investigating this problem, as it exhibits a very low thermal compressibility and provides a 'closed system' repositor}^ for silicate inclusions such as garnet and pyroxene. The effects of differential thermal compressibility are evaluated by considering thermobarometric results for silicate inclusions in diamonds from the Kimberley Mines, South Africa.

The inclusion population of the Kimberley diamonds is dominantly of peridotitic paragenesis and comprises an unusually large number of co-existing inclusions, such as garnet + orthopyroxene mineral pairs, which are usefiil for establishing both temperatures and pressures of equilibration.

Thermobarometry determinations on pairs of touching mineral pairs yielded average temperatures of 1070T and pressures of 53kb, while non-touching inclusions yielded average values of 1240T and 61kb. The discrepancy in temperature estimates is readily reconciled in terms of mantle cooling subsequent to diamond crystallisation. The pressure difference could be an artefact of the thermobarometers used or it may be due to uplift of part of the mantle, subsequent to diamond crystallisation. Alternatively, the pressure discrepancy may result from differences in the thermal compressibility of diamond and silicate inclusions.

The latter alternative was modelled usmg the thermodjuamic dataset of Holland and Powell (1998) and the THERMOCALC software package (e.g. Powell et aL, 1998). Model input parameters assumed diamond crystalhsation at 1250T and 60kb, followed by cooling to 1050T at 60kb external pressure. As diamond has a much lower thermal compressibility than silicate inclusions, the volume of the inclusion cavit>^ remains ahnost constant and the internal pressure experienced by the silicate inclusions is proportional to changes in external mantle temperatures. Cooling of the mantle by 2 0 0 T from 1250T causes a reduction in internal pressure of ~4kb. While this value is less than the thermobarometr\^ estimate for co-existing inclusions, it should be stressed that the inclusion pressures are average values and that thermobarometnc estimates are subject to relatively large uncertamties, typically ± 5 0 T and ± 2kb.

In conclusion, thermodynamic modellmg suggests that differential compressibility between encapsulating minerals and their mclusions is likely to have a measurable effect on 'intemaF mclusion pressures, which will be controlled by changes in temperature as well as external pressure.

These results have implications not only for the interpretation of thermobarometric results from diamond inclusions, but also for pressure/temperature calculations of metamorphic rocks in which minerals are included in one another.

References Holland and Powell 1998. J. Met. Geol., 16, 309-343 Powell etal. 1998; J. Met. Geol., 16, 577-588.

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STRATIGRAPHIC OMISSION EXPLAINED BY DISCOVERY OF A REGIONAL FOLDED DUCTILE SHEAR ZONE IN THE EURIOWIE BLOCK, NSW Michael Raetz, Caroline Forbes and Gordon Lister School of Geosciences Australian Cmstal Research Centre, Monash University, Cla>ton VIC 3800,

We report the discover^' of a folded ductile high-temperature shear zone in the Euriowie Block north of Broken Hill in New South Wales. The shear zone has been traced around F3 fold hinges (Parmga anticline, Brown et al. 1992) for over 16 km. Similar lithologies mapped over a strike length of 40 km suggest the possibility that the shear zone may also be folded around F2 folds.

In the Paringa anticline the shear zone is a mylonitised granite approximately 200 metres wide and defines the boundary of the Alma Gneiss (a granite gneiss) where it is structurally overlain by folded metasediments of the Broken Hill group (Brown et al 1992). Previous mapping has described the rocks in the shear zone as leucocratic, quartz feldspar gneiss with sericitised sillunamte wisps and lenticules of quartz (unit "If, Brown, 1995). Amphibolite sills are onentated sub-parallel to the shear zone boundaries and are concentrated adjacent to the shear zone.

The shear zone in the Paringa anticline coincides with an area of missing stratigraphy (equivalents of the Himalaya Formation). The discovery of this shear zone can explain why strata are missing {cf. earlier explanations of an F1 fold or facies changes, Brown et al. 1992). The Himalaya Formation is also locally absent between Pumamoota and Yanco Glen and near Stephens Creek and adjacent to large area of Alma Gneiss at Broken Hill. Such omissions could be explained: i) as excisions at the base of thrust-nappes; ii), as caused by extensional faults or shear zones (excision being one of the keys to extension); and iii), as unconformities. Extension (prior to D2) is considered the most plausible cause of the shear zone in the Paringa anticline.

References Brown, R.E., Stevens. B. J. and Stroud, W.J. 1992. The Early Proterozoic Willyama Supergroup in the Euriowie Block, New South Wales. Geological Survey of New South Wales, Ouaterly Geological Notes, 88, 1-37.

Brown, R. E. 1995. Gairdner 1:25 000 Geolog}^ Sheet, 7234-IV-N. New South Wales Geological Survey, Sydney.

185

THE AUSTRALIAN STRESS FIELD: IMPLICATIONS FOR PLATE BOUNDARY FORCES Scott D. Reynolds^ Richard R. Hillis^ and David D. Coblentz' ^ National Centre for Petroleum Geolog}^ and Geophysics, University of Adelaide, South Australia 5005 ^ Sandia National Laboratories, Geophysical Technology Department, Albuquerque The Australian stress map is one of the fundamental geophysical databases for the Australian continent. Additional in situ stress data are continually being added to improve our limited understandmg of the intraplate stress field of the Australasian region under the auspices of the new Australasian Stress Map Project. Currently the Australian stress map comprises 331 reliable mdicators of the onentation of horizontal, tectonic stresses in the Australian crust. Stress province definition and stress trajector>^ mapping have clarified the regional stress trend across the Australian continent. This regional stress trend is unique when compared to other continental areas m that stress orientations as a whole are variable and do not parallel the N-NNE absolute motion direction of the Indo-Australian plate. Consequently, the Australian continent provides an ideal setting m which to study the interaction betw^een tectonic forces and the intraplate stress field.

New finite element modellmg of the Indo-Australian plate has been conducted usmg a 'basis-set' approach to compute the predicted stress field. This approach enables the evaluation of a very large number (several million) of boundary and potential energy force combinations acting on the plate.

Constraint for the modelling is provided by the 'observed' regional stress field based on the 12 stress provmces defined from the Australian Stress Map database. The stress provinces provide significantly improved constraints compared to those used in previous modelling studies.

Modelling results indicate that the Australian intraplate stress field is inherently non-umque m that a large number of different boundary force combmations can produce similar predicted stress fields. Nevertheless a number of fimdamental conclusions may be drawn about the tectonic forces acting along the principal plate boundary segments. (1) The Himalayan and New Guinean boundaries exert a compressional force on the LAP producing a stress focussing normal to the boundanes and rotating between them. (2) Fitting the stress field in the Bowen Basin requires compressional boundary forces along the Solomon and New Hebrides subduction zones directed towards the interior of the LAP. (3) East-west compression in eastern Australia requires only a small compressional force along the Tonga-Kermadec subduction zone. (4) Fitting the stress field in southeastern Australia (Otway Basin and Gippsland Basin stress provinces) requires compressional forces along the New Zealand, Puysegur Trench, and Macquarie Ridge boundary segments. (5) Significant tensional slab-pull forces exist only along the Java subduction zone.

The orientation of the modelled stress field over most of Australia is robust for a number of the best fitting models, with predicted stresses in northeastern and southern Australia the most sensitive to variations in the plate boundar>^ force combination. Furthermore, the modelling suggests that large sections of eastern Australia exhibit a relatively isotropic stress field compared to the rest of Australia. Additional data in northeastern Australia has the greatest potential to further constrain the plate boundar>' forces acting along the northeastern plate boundar>^

186

COAXIAL FLATTENING AT DEEP LEVELS OF OROGENIC BELTS (SYROS AND SIFNOS, CYCLADES, GREECE) Gideon Rosenbaum^'^, Dov Avigad^ and Mario S^chez-Gomez^ ^ ^ Institute of Earth Sciences, Hebrew Universit>^ of Jerusalem, Israel " School of Geosciences Australian Crustal Research Centre, Monash University, Victoria 3800, Australia ^ Departamento de Geologia, Universidad de Jaen, Spain The Cycladic Islands in the Aegean Sea are a natural laboratory for orogenic processes exposing high-pressure rocks in the footwall of extensional detachments. These rocks belong to a branch of the Alpine orogeny where high-pressure metamorphism took place in Late Cretaceous to Eocene times, whereas later deformation was predominantly governed by extensional tectonism associated with the formation of the Aegean Sea as a back-arc basin. Earlier studies showed that non-coaxial deformation played a major role during exhumation of the Cycladic high-pressure terrane (e.g.

Lister et al. 1984, Jolivet & Patnat 1999). However, non-coaxial structures do not represent the deformation m deep crustal levels because they are usually associated with greenschist-facies mylonites superimposed on earlier high-pressure fabrics. In this study, we attempted to elucidate how strain was accommodated at deep crustal levels during the formation and exhumation of the high-pressure terrane.

We studied the structure and the metamorphism of a relatively coherent high-pressure rock section exposed on the islands of Syros and Sifiios. At least three deformation phases associated with eclogite- and blueschist-facies conditions (P = 8-15 kbar; T = 400-550''C) were recognised. The earliest deformation fabric (SI), preserved as inclusion trails within garnet porphyroblasts, is aligned to define a sub-vertical schistosity (at present orientation) and may indicate that deep crustal thickening involved upright folding. The currently dominant fabric in the high-pressure rock section, S2, is a usually moderately dipping schistosity orthogonal to the earlier fabric (SI). It locally contains NW trending glaucophane lineations, symmetric pressure-shadows and eclogitic boudins. The symmetric structures associated with this fabric seem to indicate coaxial vertical thinnmg, Glaucophane-beanng shear bands (S3), with top-to-NW sense of shearing, locally crosscut the earlier structures. The latest recognised fabric (D4) is scarce and often absent within the HP rocks. It is associated with top-to-NE kinematic criteria that formed at greenschist-facies conditions (P = 4-7 kbar; T = 400-450°C).

Based on these observations, it is suggested that partitioning of strain occurred at different crustal levels and at different times. Deep crustal deformation was governed by thickening via upright folding (Dl) followed by coaxial vertical thinning (D2 and D3), whereas non-coaxial shearing occurred when the rocks were already exhumed to relatively shallow crustal levels (D4). The earliest fabrics (Dl to D3) pertain to Alpine orogenesis and possibly to syn-orogenic extension, while the latest fabric (D4) corresponds to whole-crust back-arc extension.

References Jolivet, L. & Patriat, M., 1999. Ductile extension and the formation of the Aegean Sea. In: Durand, B., Jolivet, L., HorvMi, F. & Seranne, M. (Eds.), The Mediterranean Basins: Tertiary Extension within the Alpine Orogen 156, Geological Society, London, Special Publications, pp. 427-456.

Lister, G. S., Banga, G. & Feenstra, A., 1984. Metamorphic core complexes of the Cordilleran type in the Cyclades, Aegean Sea. Geology 12, 221-225.

187

4D MODELLING OF POP-UP STRUCTURES DURING STRIKE-SLIP FAULTING: SOME INSIGHTS FROM ANALOGUE MODELLING W.P. Schellart* and D. A. Nieuwland Strukturele Geologie en Tektoniek groep, Faculteit der Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands *Present address: Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, VIC 3800, Australia.

In analogue modelling, pop-up structures have previously only been described in models of basement mvolved strike-slip deformation with a stepover or restraining bend configuration (Richard et al 1995; Dooley et al 1999; McClay and Bonora 2001). Although the experimental design of these models is elegant, some of the boundar}^ conditions of these models are not very realistic. Furthermore, the structures observed in these models show pop-up structures with a relatively large surface extent compared to basement fault separation and are bounded by shallowdipping faults 30""), while these features are not necessarily general characteristics pop-up structures We describe the results of an analogue experiment with an alternative model configuration to investigate the structural development of pop-up structures. This model consisted of a cover sequence of sand overlying two parallel running basement faults, which experienced the same sense and the same amount of pure strike-slip during deformation. The ratio of basement fault separation (1 cm) to overburden thickness (4 cm) was small. The experiment has been recorded in an X-ray tomograph, by which it was possible to investigate the 3D structural evolution of the model. Dunng deformation, an elongated, rhombic pop-up structure developed centrally above the basement faults. The faults, which bounded the structure, displayed convex upward as well as straight to concave upward shapes, with most faults dipping between 60"" and 90°. During deformation the topography above the basement faults increased to a maximum of ~ 4.8 mm, compared to a maximum subsidence of ~ 1.5 mm on both sides of the basement faults. In the core of the pop-up structure, an elongated low-density zone developed, parallel to the longer axis of the pop-up structure and confined in 3D by the pop-up structure. This structure developed s}Tichronously with the pop-up structure and could act as a hydrocarbon reservoir in nature. Also, local maximum horizontal principal stresses displayed a considerable amount of rotation during progressive deformation, from 45 to the strike of the basement faults in an early stage, to (sub)parallel in a more advanced stage and finally back to --- 45"" in a late stage of the experiment.

The results of this analogue experiment show structures similar to those found in natural strike-slip systems and provide a 3D kinematic analogue for their evolution.

References Dooley T., McClay K. & Bonora M. 1999. 4D evolution of segmented strike-slip fault s\ stems; applications to NW Europe. In: Petroleum geology of Northwest Europe: proceedings of the 5th conference (edited by Fleet A. J. & Boldy S. A. R.) 5, London, United Kingdom, 215-225.

McClay K. R. & Bonora M. 2001. Analog models of restraining stepovers in strike-slip fault s}^stems.

American Association of Petroleum Geologists Bulletin 85, 233-260.

Richard P. D. Naylor M. A. & Koopman A. 1995. Experimental models of strike-slip tectonics. Petroleum Geoscience, 1, 71-80.

188

THE LIEBIG EVENT - 1640-1630 Ma DEFORMATION, MAGMATISM AND HIGH GRADE METAMORPHISM IN THE SOUTHERN ARUNTA PROVINCE Ian Scrimgeour\ Pete Kinny", Christine Edgoose^ and Dorothy Close^ ^Northern Territory Geological Survey, P.O. Box 2655, Alice Springs, N.T. 0871 "Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Teclmolog>^ GPO Box U1987, Perth, W.A. 6845 Recent NTGS studies and SHRIMP U-Pb geochronology in the western part of the southern Arunta Province have resulted in the recognition of tv\^o distinct terrains: the 1660-1630 Ma Yaya Terrain in the north, and the 1690-1660 Ma Haasts Bluff Terrain in the south (Close et al, this volume).

Furthermore, a new metasedimentary sequence in the Yaya Terrain, the Yaya Metamorphic Complex, and a major tectonothermal and magmatic event at 1640-1630 Ma (the Liebig Event) have been identified.

The east-trending Yaya Terrain is up to 50 km wide and extends >300 km along strike, and contains the Yaya Metamorphic Complex, which has undergone bimodal magmatism and granulite facies metamorphism. This package is dominated by pelitic and psammitic metasediments grading into a more heterogneous sequence containing calc-silicate, massive cordierite pelite, mafic rock and quartzite. Detrital zircon populations indicate a maximum deposition age of 1661 ± 10 Ma, with subsequent intrusion of voluminous chamockite, granite and gabbro at 1640-1630 Ma (Close et al, this volume).

The metamorphic evolution of the Yaya Complex during the Liebig Event is best preserved at a locality knovm as Hill 830, where metasediments of the Yaya Complex occur in a 3 x 1 km megaboudin that is surrounded by lower grade c.1590 Ma Chewings Orogeny fabrics. These metasediments include massive cordierite-rich rocks wirth orthopyroxene-sillimanite and sapphirinebearmg assemblages consistent with deep crustal ultra-high temperature metamorphism, of >900''C and 10 kbar. Adjacent large syn-tectonic gabbro and chamockite bodies suggest that the high temperatures can be attributed to mafic and chamockitic intrusions into the deep crust. Breakdown of orthopyroxene, sillimanite and garnet to sapphirine and cordierite suggests near-isothermal decompression at temperatures greater than 850"C. Metamorphic zircon rims from an Opx-Sil-Crd granulite from Hill 830 have an age of 1638 ± 8 Ma, whilst two Grt-Bt-Sil metapelites have metamorphic ages of 1641 ± 14 and 1639 ± 20 Ma. To the south, in the Glen Helen Metamorphics of the Haasts Bluff Terrain, amphibolite facies migmatites have 1688 ± 16 Ma zircon cores and isotopically disturbed metamorphic rims, the oldest of which gives an age of 1640 ± 12 Ma (2a). No evidence has been found for a metamorphic event relating to the 1680-1660 Ma 'Argilke Event', which was proposed by previous workers in the southern Arunta Province.

Our evidence implies that a major tectonothermal event affected the southern Arunta at 1640-1630 Ma. Zircon evidence suggests only 10-20 Ma between deposition and deep crustal metamorphism of the Yaya Complex, and high-T decompression textures suggest rapid exhumation, consistent with overthickened crust. The timing of the Liebig Event corresponds to a major hairpin bend m the apparent polar wander path for northern Australia, implying that it is event of regional significance consistent with a collisional event at a plate margin. We propose that the Yaya Complex was deposited on the margin of the Haasts Bluff Terrain at 1660-1640 Ma, and that the Liebig Event represents collision between the Southern Arunta Province and the North Australian Craton at 1640 Ma.

189

A RECONNAISSANCE ^^W^^Hf STUDY OF THE NEW ENGLAND BATHOLITH USING LAM-MC-ICPMS MICROANALYSIS OF ZIRCONS S. E. Shaw andR. H. Flood GEMOC, Department of Earth and Planetary Sciences, Macquarie Universit}^, NSW 2109 Following the procedures of Griffin et a l , (2001) we have examined the ^^^Hfi^^^^Hf ratios of single zircons from the supersuites of the New England Batholith. A nmnber of studies of the U/Pb ages of the zircons of the New England Batholith indicate little if any of the zircon is inherited. Zircons contain ca. 1.5% HfOi and this abundance allows ratio determinations to a precision of ± 0.00005 (2 sigma), more than sufficient to calculate meaningful mantle model ages of crustal source components. Youngest model ages (ie most mantle-like) are from the Clarence River Supersuite which have model ages close to emplacement ages. The S-type Bundarra Supersuite, the Moonbi Supersuite and the Uralla Supersuite all have some zircons with model ages that are Neoproterozoic. Zircons of the I-t\^e Moonbi Supersuite from Tamworth and Tenterfield have a significant range of ^^^Hfi^^^^Hf ratios that suggests source mixing of a mantle component around the cr}^stallisation age of 250 Ma and a crustal component around 600-700 Ma. The Triassic plutons of the Hastings Block in the south east have model ages that are more crustal than the Clarence River Supersuite but approximatly the same as the youngest model ages from the Moonbi Supersuite. The Neoproterozoic to Early Palaeozoic model ages of the Moonbi Supersuite are not in accord with earlier suggestions that these granites might have been derived from much younger crustal protoliths or are even fractionated mantle magmas.

Detailed analyses of zircon of the zoned Walcha Road pluton, Moonbi Supersuite, indicates that the most felsic (fractionated) central zone of the pluton has ^^^HC^^^te ratios more mantle-like than the intermediate and marginal zones of the pluton, counter intuative to conventional wisdom. Flood and Shaw (2001) suggested that this might indicate mixing of the most felsic fractionates of the crustal melt with the most felsic fractionates of the mantle heat source magma. Most zircons from three microgranitoid enclaves from the Moonbi Supersuite have similar model ages to the plutons.

References Flood R.H. and Shaw S.E. 2001. The S-type granite source-rock debate: possible additional felsic component formed by fractionation of mantle derived mafic heat-source magmas. In: Chappell B. W. and Fleming P.D eds. 2001 S-type granites and related rocks. Australian Geological Survey Organisation, Record 2001/02 104pp, p'41-42.

Griffm W.L., Xiag Wang, Jackson S.E., Pearson N.J., O'Reilly S.Y., Xisheng Xu and Xinmin Zhow., 2002.

Zircon chemistr}^ and magma genesis, SE China: In situ analysis of Hf isotopes, Tonglu and Pingtan Igneous Complexes. Lithos, in press.

190

WIDESPREAD MAFIC AND FELSIC POST-COLLISIONAL PERMO-TRIASSIC MAGMATISM IN EASTERN AUSTRALIA: TRANSFORM LINKS W. J. Sivell and M. J. Passmore Division of Earth Sciences, The University of New England, Armidale 2351.

Permo-Triassic mafic layered intrusions and dykes in Queensland, together with voluminous 245235 Ma granitoids of the New England Batholith (NEB), are emplaced within accretionary pnsm complexes of the NEO, m an offset belt from Tamworth to Mt Morgan. Orientations, isotopic compositions, mferred emplacement mechanisms and restricted age ranges of granitoids and dykes imply an extensive (>1600 km) N-S trending, short-lived, partly transform-related, post-collisional domain in eastern Australia during the Permo-Triassic. The voluminous Permo-Triassic granitoids are relatively undeformed, high-level, post-tectonic intrusions, emplaced in tensional domains associated with contemporary volcanic rocks. Most of these high-K intrusions postdate isotopically depleted, more mafic magmas at the roots of an extensive Cordilleran margin magmatic arc (e.g. tonalites, low-K granodiorites of Clarence River Supersuite; Br} ant et ai, 1997). A small total range of ages and some cominon chemical and isotopic features, suggest a single widespread postcollisional regime, possibly reflecting slowed and shallowing subduction, with regional extension, at a time when the subduction hinge had migrated well east of the present Australian coastline.

They include highly fractionated granites of the Stanthorpe Granite Group (SGG), part of the Moonbi Supersuite.

Detailed mapping has revealed the composite nature of the SGG, distinguishing numerous discrete mappable (km-scale) granitoid stocks, with distinct contact relations, mineralogy and geochemistry. Granitoid and dyke orientations indicate magma emplacement within a major N-S trending dextral transcurrent fault regime, most likely related to nascent development of the regionally important Demon Fault (NW-oriented SGG granitoid stocks are emplaced parallel to a 3 (between secondary^ shear zones) while NE-oriented cross-cutting dyke suites trend perpendicular to a3. Ascent and fractionation of magma, leading to assembly of SGG plutons comprising numerous stocks with wide compositional diversity, was driven by alternating dilation and compression within a crustal-scale anastomosing strike-slip shear zone/fault system.

Granitoid geochemistr}^ is dominated by fractionation and filter-pressing in active shear zones that produced local efficient separation of crystals and evolved residual melts. Local mingling and mixing of mafic and felsic melts formed hybrid diorites.

On isotopic grounds, the enclave-forming mafic melts, and "shoshonitic" underplated source rocks inferred for SGG granitoids, incorporated components derived from young (isotopically slightly depleted) subcontinental lithospheric mantle (SCLM). Gabbros and (hybrid) diorites associated with the granitoids involved additional strongly depleted (MORB-source like) mantle end-members. Close Nd-Sr isotopic similarities with SGG gabbros, MME, diorites, etc., are shown by analysed Queensland Penno-Triassic mafic intrusions, as well as by several post-collisional dyke suites in Gympie Province. The data suggest similar source components within a closely related tectonomagmatic regime. The layered intrusions include the (1) Somerset Dam (2) Goomboorian (3) Wigton (4) Wateranga (5) Goondicum (6) Eulogie Park (7) Boogargan (8) Windah (9) Bucknella and (10) Fred Creek complexes. The -243 Ma multiphase Wateranga intrusion has initial-Sr isotopic values (0.7028 to 0.7049) neatly encompassing Srinit for the SGG granitoids, gabbros and related rocks (0.7034-0.7047), and s^d values (+3.5 to +6.5) closely similar to SGG gabbros and diorites (+3.7 to +4), as well as post-collisional dykes at Gympie (+4.3 to +6.7). Like continental tholeiites, isotopic/geochemical signatures reflect melting with variable proportions of depleted mantle and subcontinental lithospheric mantle components. For >1600 km along strike, Permo-Triassic magmatism occurred in response to extension that followed cmstal tliickening (most pronounced in the south), lithospheric delamination and consequent hot mantle influx which initiated melting in the asthenosphere, SCLM and underplated crustal sources. Dextral trans-tensional tectonics in the Gulf of California- San Andreas-Sierra Nevada oblique subduction system in the SW United States may represent a tectonomagmatic regime analogous to the Permo-Triassic southern Queensland-Gympie-NEB setting, with plutonic components of the central Sierra Nevadan magmatic arc emplaced via dilation/transcurrent motion induced by oblique subduction during the Late Cretaceous.

References Bryaiit, C. J., Arculus, R. J. and Chappell, B. W. 1997. Clarence River Supersuite: 250 Ma Cordilleran tonalitic I-t>pe intrusions in eastern Australia. Journal of Petrology 38, 975-1001.

Sivell W. J. and McCulloch M. T. 2001. Geochemical and Nd-isotopic systematics of the Permo-Triassic Gympie Group, southeast Queensland. Australian Journal of Earth Sciences 48, 377-393.

191

COMPUTATIONAL PETROLOGY AND PYROXENE THERMODYNAMICS Silvano Sommacal^ Malcolm Sambridge^ and Hugh StC. O'Neill^ ^Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200.

Phase assemblage relationships and melting processes taking place in the Earth's mantle are not accessible to direct observation. One powerful approach to this problem is deciphering the physicochemical conditions in which they formed. Thermod>Tiamic models to investigate lower crust-upper mantle phase assemblages in complex systems do not exist and available thermodynamic data either refer to individual phases or have been derived to study phase assemblages under moderate pressure and temperature.

A new^ thermodynamic model to describe phase equilibria betw^een coexisting clinopyroxene and orthopyroxene as a function of composition, temperature and pressure has been developed. The chemical composition of phases has been unambiguously expressed through their number of cations per formula unit {'constituents \ s}Tnbol ' I f ) , which are subject to constraints by both stoichiometry and charge balance. Directly related to the constituents are the site occupancies {Xf. ) for each element z in phase (j).

For each phase, the molar free energy (G^ is given by the sum of contributions from: 1) the Gibbs free energy of the end-members, 2) ideal mixing on sites, and 3) excess mixing terms. The principle underlying the formulation of the term due to the Gibbs free energy of the end-members has been elucidated. As an example, the expression of this term for a general (Na-Ca-Mg-Fe^'^-AlCr-Fe^^-Si-Ti) pyroxene system (32 end-members) has been derived.

At any given temperature and pressure a closed multi-phase system is at its equilibrium condition when the chemical composition of the phases present in the system and the number of moles of each are such that the Gibbs free energy of the system reaches its minimum value.

From a mathematical point of view, the determmation of equilibrium phase assemblages can, in short, be defined as a constrained minimization problem. Numerical techniques for constrained optimization is an active field of research in which major advances have been occurred m the past twent}' years. To solve the Gibbs free energy minimization problem two different approaches {"inverse" and "forward") have been undertaken. For the 'inverse' problem, a chi-sqiiare measure method of data-fitting is used while in the 'forward' the minimization is carried out with a feasible Iterate Sequential Quadratic Programming' method (FFSQP). The system's Gibbs free energy is minimized under mass balance, stoichiometr}^, charge balance and positivity constraints.

Initial application of the programs is to assemblages of coexisting pyroxenes (orthopyroxene, and low Ca- and high Ca clmopyroxene) m CaO-MgO-FeO-SiO. (i.e. CMFS), CaO-MgO-Al.Os-SiOz (i.e. CMAS) and CMFAS systems. Phase relations within such systems and their subsystems are calculated over a wide range of temperatures and pressures and compare favorably with experimental constraints.

192

WHOLE-ROCK AND MINERAL CHEMISTRY OF BLUESCHISTS AND ECLOGITES FROM NORTHERN NEW CALEDONIA AND SOME IMPLICATIONS FOR SUBDUCTION-ZONE PROCESSES. 1,2

Carl Spandler^ Joerg Hennann", Richard Arculus^ and John Mavrogenes^'

^ Department of Geology, Australian National University, Canberra ^ Research School of Earth Sciences, Australian National University, Canberra It IS now widely accepted that a flux of mass and energy released from subducting oceamc crust at depth plays a crucial role in the development of magmatic arcs (and continental crust) and hence, is a vital process affecting the chemical differentiation of the earth. However, our understanding of the transfer of elements from the subducting slab to the overlying mantle wedge (the site of generation of most arc magmatism) is very poor. Verj^ few geochemical studies have focussed on the previously subducted crust in order to better constrain the composition of high pressure phases (minerals, fluids or melts) and the partitioning of elements between them.

In north-eastern New Caledonia (NC) lies one of the world's largest and most contmuous sequences of blueschist and eclogite facies rocks. These rocks originally comprised part of an oceanic crust that was subducted to depths of up to 70km, and subsequently exhumed (Clarke et al 1997). The sequences contain a large diversity of protolith rock-types and abundant evidence of high-pressure fluid flow and alteration. Utilising state-of-the-art microanalytical techniques (Laser Ablation ICP-MS), we have determined the major and trace-element composition of a range of blueschist and eclogite-facies samples and their component minerals. Results indicate that there is a diverse range of mafic protoliths, including MORB (both E and N types), back-arc basin basalts and oceanic island basalts. The fact that all of these mafic lithologies have been subducted beneath arcs makes the diversity of metamorphosed equivalents particularly important. Comparison of the chemical composition of the NC blueschist and eclogites with unmetamorphosed equivalents found to the along the west coast of NC (Cluzel et al. 2001) indicate that ver>^ little change in traceelement composition has occurred during prograde metamorphism. This has unportant consequences for models of slab devolatisation in the fore-arc region.

For all samples examined, the most important host minerals for trace elements include zoisite/epidote (>50% of whole-rock REE, Th, U, Pb, Sr), phengite (>90% of Cs, Rb, Ba; >30% of Sr and Pb), rutile, zircon and titamte (>95% of HFSE) and garnet (^50% of HREE). In contrast, amphibole and omphacite account for ver>^ little of the trace element budget of these rocks.

References Clarke G. L., Aitcliison J. C. & Cluzel D. 1997. Eclogites and blueschists of the Pam Peninsula : a reappraisal. Journal of Petrology 38, 843-876.

Cluzel D. Aitchison J. C. & Picard C. 2001. Tectonic accretion and underplating of mafic terranes in the Late Eocene intra-oceanic for-arc of New Caledonia (Southwest Pacific): geodynamic implications.

Tectonophysics 340, 23-59.

193

NEODYMIUM ISOTOPIC AND GEOCHEMICAL CONSTRAINTS ON PROVENANCE OF SEDIMENTARY ROCKS IN THE EASTERN OFFICER BASIN, AUSTRALIA: IMPLICATIONS FOR THE DURATION OF THE INTRACRATONIC PETERMANN OROGENY. B P. Wade\ M. Hand, K M. Barovich, J. Foden Department of Geology and Geophysics, Adelaide University, Adelaide, SA 5005, Australia ^Corresponding author. Fax: +61-8-8338-0055.

E-mail: wade.ben@saugov.sa.gov.au Neodyinium isotopic data from Neoproterozoic to Ordovician sedimentary^ rocks in the eastern Officer Basin in southern Australia highlight the evolving provenance roles of the Gawler Craton and Musgrave Block basement complexes that border the Officer Basin. hiitial 8Nd values of around -13 for the early Neoproterozoic basal sequences (Pindyin Sandstone and Alinya Formation) indicate they were largely derived from the Archaean to Palaeoproterozoic Gawler Craton, w^hich bounds the Officer Basin to the south. At around 645 Ma, a major excursion of initial 8Nd values in the metasediments to around - 9 indicates significant unroofing of the Mesoproterozoic Musgrave Block, which forms the northern margin of the basin. Uplift of the Musgrave Block at around 645 Ma is interpreted to mark the onset of the intracratonic Petermann Orogeny, which was a major orogenic event that shaped the lithospheric architecture of southern central Australia. Combined with existing isotopic and stratigraphic data, the isotope data from the eastern Officer Basin suggests that the Petermann Orogeny was either relatively long-lived, or a series of events, spanning more than 100 Ma. The change in initial 8Nd is accompanied by geochemical data that indicate a slight increase in mafic trace element indicators in the mid to late Neoproterozoic successions (Mumaroo Formation and Dey Dey Mudstone, Mena Mudstone Member), and earliest Cambrian succession (Arcoeillinna Sandstone). This rise is interpreted as an increasing sediment contribution to the basin fill from the Musgrave Block, which is as a whole more mafic than the Gawler Craton.

Subsequent to 645 Ma, SNd values of syn-Petermann Orogeny sediments diverge from the Musgrave Block trend, suggesting that there was an ongoing contribution from the Gawler Craton, despite the deposition of the sequences in the geographic foreland of the Petermann Orogen. The return to a mixture of Gawler Craton and Musgrave Block provenance suggests that, to a large extent, sediment derived from the Petermann Orogen bypassed the eastern Officer Basin for much of the Petermann Orogeny.

194

A PATCHY OROGEN? ALLOCHRONOUS ANTI-CLOCKWISE P-T-T PATHS ACROSS THE SOUTHERN DELAMERIAN FOLD BELT, SOUTH AUSTRALLV. ^ ^ Gordon Webb. ^Martin Hand and 'Wolfgang Preiss 'Department of Geology and Geophysics, Adelaide University, South Australia 'Office of Minerals and Energy Resources, PIRS A, South Australia Shear zones within metapelitic sequences along eastern Kangaroo Island in the Southern Delamerian Fold Belt, South Australia, preserve a petrologic record of successive mineral paragenesis during progressive deformation. A P-T pseudosection in KFMASH shows that the sequence of mineral growth and reaction records heating to ~ 550 (at pressures below 2 kbar) resulting in the early growth of cordierite (along with muscovite and quartz). A pressure increase of around 1-2 kbar is implied by the subsequent growth of andalusite followed by fibrolite and staurolite. Post-kinematic staurolite, chlorite, and muscovite are interpreted to record both the cessation of penetrative deformation and the waning of thermal metamorphism. This sequence of mineral paragenesis indicates an "anti-clockwise" P-T path. The up-pressure P-T evolution in the eastern Kangaroo Island shear zones implies that the shear system was tectonically buried during and subsequent to its development. A logical conclusion is that the shear zones were incorporated mto the footwall of thrusts that developed in the hinterland. The petrologic record on eastern Kangaroo Island is essentially identical to the metamorphic evolution recorded in the apparent thermal aureole of a syn-tectonic granite at Petrel Cove, on southern Fleurieu Peninsula (which lies approximately 40 km eastward across the orogenic strike). Sm-Nd data from garnet-bearing assemblages from the Kangaroo Island shear zones indicate that the up-pressure segment of the anti-clockwise P-T-time path occurred somewhere in the interval 509 Ma and 512 Ma. However, Sm-Nd data from metapelites at Victor Harbour indicates that metamorphism occurred at 498 ± 5 Ma. On both eastern Kangaroo Island and at Petrel Cove, metamorphism and deformation appears to have been triggered by magmatic heating. This data suggests a tectonic model for the evolution of the Southern Delamerian Fold Belt where anti-clockwise P-T loops occur on the local scale, dependent on the patterns of magmatism during convergent deformation.

195

IMPLICATIONS OF POST-PALAEOPROTEROZOIC THERMOCHRONOLOGICAL DATA FROM THE PRECAMBRIAN NORTHERN WESTERN AUSTRALIAN SHIELD ^U.D.Weber, ^B.P.Kohn, 'D.A.Foster, ^D.RNelson and ^AJ.W.Gleadow ^School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia "Department of Geological Sciences, University of Florida, Gainesville, PL 32611, USA ^Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia

In this reconnaissance study we present new thermochronological data from the northern part of the Western Australian Shield, including the Pilbara Craton, the northern part of the Yilgam Craton and the intervening Proterozoic terranes. Techniques employed include "^Ar/^^Ar dating of Kfeldspars and muscovites, apatite fission track and (U-Th)/He thermochronometry. Combining numerical models of the K-feldspar "^Ar/^^Ar and fission track data provides constraints on possible time-temperature cooling paths for the Shield from temperatures below -450° C. Previous geochronological data from the Pilbara Craton and the northern Yilgam Craton using •^^Ar/^^Ar analysis of hornblende, muscovite and biotite and U/Pb zircon SHRIMP data yield middle to late Archaean ages, while Mesoproterozoic sphene fission track ages have also been reported. These data and the fact that exposed rocks of the Western Shield are mostly Archaean and Palaeo- to Mesoproterozoic in age with a fairly subdued geomorphology led to the notion that the West Australian Shield has been tectonically stable over a long period of geological time.

K-feldspar data mdicate slow coolmg from 2300-2200 Ma, 1800-1600 Ma or 1200-1000 Ma depending on sample location. Some samples yield minimum ages of - 5 0 0 Ma, which suggests either a small amount of reheating and minor argon loss at that time, or a minor pulse of increased cooling. Apparent apatite fission track ages range between 290-180 Ma with confined horizontal mean track lengths between 11.5-14.3 jam. Preliminary apatite (U-Th)/He results yield ages - 2 4 1 Ma or older, although most samples do not duplicate well due to heterogeneous U distribution and the presence of micro-inclusions.

Numerical modellmg of the K-feldspar ^Ar/'^Ar data reveal accelerated cooling -1800-1600 Ma, 1100-900 Ma and - 5 0 0 Ma and modelled apatite fission track data indicate a cooling episode between 320-280 Ma from temperatures > -120^ C. Coolmg inferred by the ^Ai/^^Ai K-feldspar data, probably reflect tectonic events related to the waning stages of the Capricorn, Paterson and Pan-African Orogenies respectively. Permo/Carboniferous cooling, inferred by the apatite fission track data, is probably related to the development of the passive continental margin to the west and north of the shield, which subsequently led to the break-up of Greater India from the Australian continent. This last event is also partly reflected in the development of the Phanerozoic basins surrounding the Shield, which acted as depocentres for at least some of the erosional clastic sediments denuded off the Western Shield. Assuming that an average present day geothermal gradient of -18±2''C.km'^ was prevalent during the late Palaeozoic and Mesozoic, then the mmimum of -50''C of cooling inferred by the fission track modelling suggests an overall denudation of at least -2.5-3.1 km of section from the Western Shield. In an independent study the volume of clastic sediments deposited m the marginal Phanerozoic basins was calculated, suggestmg that - 4 km of basement section has been removed from the Shield since the Palaeozoic.

Acknowledgments Tliis work was supported by the Australian Research Council and the Australian Institute of Nuclear Sciences and Engineering

196

ELECTRON MICROPROBE MONAZITE AGES FROM MULTIPLY DEFORMED SCHISTS, SE VERMONT, USA: NEW CONSTRAINTS FOR THE TIMING OF METAMORPHISM AND DEFORMATION. Peter W. Welch School of Earth Sciences, James Cook University, Townsville, Qld. 4811, Australia.

Email: peter.welch@jcu.edu.au Numerous phases of garnet growth have been revealed by detailed studies of foliation inflection/intersection axes preserved in porphyroblasts (FIAs) in Acadian metamorphics in the Chester Dome region of Vermont, U.S.A. A regionally consistent succession of four differently trendmg FIAs in garnet porphyroblasts have been used to determine relative timing of deformation and mineral growth. Matrix microstructures postdate all but the youngest garnet growth, that is only the youngest garnet growth have inclusion trails that are continuous with matrix microstructures. Samples containing monazites trapped as inclusions in garnets as well as in the matrix were identified and monazites were dated using the U-Th total Pb method. WDS compositional maps were collected for all of the monazites that were analysed to check compositional domains before point analyses were collected. Monazites were then analysed for U, Th, Pb and Y at the electron microprobe facilit}^ at the University of Massachusetts.

Ages were then calculated for monazite grains determined from a number of analyses per grain.

Weighted averages were then determined for garnet core, rim or matrix in each sample. Monazites ages show a progression that ranges from 430 to 350 Ma within porphyroblasts and as young as 310 Ma in the matrix. Single samples reveal ages ranging through 70 Ma from porphyroblast cores, rims and matrix. Ages determined from monazite grains within garnet porphyroblasts link directly to penods of multiple deformation and episodic garnet growth. Four distinct garnet growth events formed successively around 424, 405, 386 and 366 Ma. These monazite inclusions, which lie within the various inclusion trails that define the FIAs, provide absolute ages for multiple periods of deformation and episodic phases of garnet growth.

Thus multiple garnet growth occurred throughout Acadian deformation and metamorphism in Vermont and orogenesis was far more prolonged than previously has been thought to be the case.

Microprobe dating of monazite inclusions without carefiil separation of phases of garnet growth by microstructural studies will lead to a spread of ages that will confuse rather than elucidate the metamorphic and structural histor}^ Analysis of monazite grains in the matrix alone will likely only present ages for the youngest of the events. These ages can be amalgamated from grain to grain to yield apparently precise ages, but such ages reveal nothing about the deformation and metamorphic processes operating during orogenesis, or the overall continuity of the deformation and metamorphism that accompanies.

197

Si(2) CLEAVAGE REACTIVATION: EVIDENCE FROM D3 QUARTZ VEINS IN THE KANMANTOO GROUP METASEDEMENTS DURING THE DELAMERIAN OROGENY C.N. Winsor Department of Geolog}^ and Geophysics, Adelaide University The dilational history and regional relationship between quartz veins and macro to mesoscale folds in the Kanmantoo Group metasediments, have been established through an examination of structural relations along selected traverses. These immature turbiditic clastics and minor carbonates were deposited in the Kanmantoo Trough, locally intruded by granite, regionally metamorphosed and shortened during the Cambrian to early Ordovician Delamerian Orogeny.

Three shortening events are regionally recogmsed, of which it is concluded that the penetrative foliation generally is Si, but in localised intense strain zones is transposed into S2. Di and D3 are related to two prominent macrofolding events: Di resulted in tight folds and a local slaty cleavage (Si) which follow the belt's arcuate trend. F3 folds trend mainly NW to NNW transecting the orogen and Fi folds.

Early discussion by the author and others (Winsor et aL 1999) focussed on the economic significance of quartz veins in the trough. The veins are variably oriented, some subparallel to bedding, predating Fi^ 2 & 3 folds, however many strike NW or NNW, dip steeply W, transect Si, Fi folds and are subparallel to F3 axial planes. Veins with this orientation are commonly transected by a fracture cleavage, which is geometrically consistent with Si in adjacent metasediments, suggesting that Si cleavage was reactivated after D3. Alternatively some veins trending NW could predate Di (Si) and be coincidently parallel to F3 axial planes, establishing a preferred weakness normal to the later D3 compressional axes. Although cleavage reactivation has been documented (e.g. Bell 1986), where vein/cleavage relationships suggest reactivation, the exact relationships may be difficult to determine.

The regionally consistent vein geometry and the relationship to tectonic indicators, is used in association with dilational offsets to establish a dilation history, involving recurrent dilation along all directions of the principle strain axes during Di to D3. Veins comprising the mam NW striking set and the other sets recognised by offsets, maintain their orientation across the curvature of the belt (through the Fleurieu Arc); suggesting that either these veins: 1) predate the curvature, 2) follow a structural weakness inherited due to stress accommodation or 3) follow a stress inhomogenit}^ produced as a result of the curvature. The three prominent sets, those trending NW and the two believed to comprise a conjugate set, which is symmetrical about the main set, are on average wider and exhibit greater width variation in the central portion of the orogen, impljdng that although the Fleurieu Arc may have originated early in the structural histor}^ its arcuate nature affected the character of later responses.

Acknowledgements. Collaborative assistance by Bob Wiltshire (University of SA) and field assistance by Joanne Hough (PIRSA) are acknowledged.

References Bell T.H., 1986. Foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning. J. met. pet., 4, 421-444 Winsor, C.N., Wiltsliire, R.G., Gatehouse, C.G., 1999. The potential economic significance of Delamerian. MESA Journal 12, 37-42.

198

PERALUMINOUS TRONDHJEMITES IN THE CURNAMONA PROVINCE, OLARY DOMAIN, SA: MAGMATIC OR METASOMATIC? Barovich. KM^ and Ashley, PM' ^Adelaide University, Adelaide, South Australia; ^University of New England, Armidale, NSW A suite of 1580 Ma trondhjemites in the Cumamona Province 01ar>^ Domain (OD), SA, is host to U-Th-REE mineralisation in veins, stockworks and breccia-hosted masses (Ashley, 1984). While the feldspar-normative classification scheme identifies these as trondhjemites (high in Ab), they are not characteristic of classic Precambrian trondhjemites (high AI2O3, >15 wt%), metaluminosity, or steep HREE depleted patterns, suggestive of mantle derivation. Instead, they are peraluminous with only slightly depleted HREE patterns, and field relationships and Nd isotope data (Barovich et aL, this volume) indicate derivation directly from surrounding supracrustal country rocks.

Extensive albitisation, in which both K feldspars and plagioclases are variably replaced by albite, and micas are destroyed, is well-documented in both the supracrustals and earlier granitoids throughout OD evolution. The trondhjemites display co-intrusive contacts with a monzogranite suite with sub-equal Na20 and K2O. It could be argued that the trondhjemites are a Nametasomatic alteration product of the monzogramtes, as Elburg et al (2001) demonstrated for the Mesoproterozoic Nooldoonooldoona trondhjemite and the Mt. Neill Granite m the Mt. Painter Inlier, in the northwestern Cumamona Province. It is important to establish a magmatic versus post-crystallisation origin for the trondhjemites, to assist exploration modelling for the origin of the U-Th-REE mineral deposits hosted by the trondhjemites.

Evidence for the magmatic origins of the trondhjemites is provided by: 1) Evaluation of the petrography of the trondhjemites; 2) Comparison of w^hole rock major and trace element abundance patterns (esp. Na20, CaO, K2O, Rb, Ba) between the monzogranites and the trondhjemites, and also with patterns for known variably Na-metasomatised older OD granites; 3) Comparison of plagioclase compositions between the trondhjemites and altered older granites.

Texturally, the plagioclase in the trondhjemites shows no evidence of albitisation of igneous Kfeldspar, and there is no visible development of albite rims on plagioclase cr}^stals. Biotite appears fresh and is a low-Fe, F-bearing type. Na20 vs K2O contents illustrate two distmct fields for the monzogranites and the trondhjemites, while the altered 1700 Ma granites show mixing between these oxides. Rb, Ba and Sr abundances for the trondhjemites relative to the monzogranites do not show large depletions in normalised spidergram plots, as is documented for the Na-metasomatised older granites. Microprobe analyses of plagioclase from the trondhjemite, while albitic (Ab94), are not of the extreme compositions seen in the altered older granites (Ab99).

Trondhjemite melts have been generated expenmentally from low-temperature H2O fluxed melting of K-rich sedimentary source rocks (Patino Douce and Harris, 1998). Fluid addition promotes melting by lowering the solidus. The OD trondhjemite compositions are slightly more albitic than compositions experimentally produced, but the protolith material in the OD (pervasively and variably albitised metasedimentar}^ rocks) is far more Na-nch than any of the experimental source material used. The trondhjemites are also co-intrusive with a more mafic granitoid suite. Nd isotope data (Barovich et al., this volume) for the more mafic suite suggest a magmatic component more juvenile than the supracrustal rocks. Kent et al. (2000) has documented a 1575 + 26 Ma CaNa-Fe metasomatic alteration event m the OD, roughly coincident with the trondhjemite, monzogranite and mafic granitoid suite emplacement. The mafic granitoids may reflect a thermal input into the OD, supplying a mafic component (the less negative 8Nd) and the heat necessar}^ to mobilise fluids and assist in generation of the trondhjemitic melts from the albite-rich sedimentar>^ pile.

References AsWey, P.M., 19M. Mweraliwn Deposita. 19, 7-18.

Elburg et al., 1001. Australian Journal of Earth Sciences , 48, 721-730.

Patino Douce and Harris 1998. Journal of Petrology, 39, 689-710.

Kent et al. 2000. Lithos. 54, 33-62.

199

MELT EXTRACTION, ASCENT AND EMPLACEMENT DURING OROGENY: FEEDBACK RELATIONS AND SELF ORGANIZATION Michael Brown Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, USA (mbrown@geoLumd.edu) Collisional orogens are characterized by clockwise P-T-t evolution, which means that in the orogemc core, where temperatures exceed the solidus for common crustal rocks, melt may be present throughout a significant portion of the burial to exhumation cycle. Field observations in exhumed orogens show that deep crust is migmatitic, and geophysical data have been interpreted to suggest the presence of melt at depth in active orogens. A consequence of these results is that orogemc collapse in mature orogens may be controlled by a partially-molten layer that decouples crust from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Experiments show that melting begins at multigrain contacts among the reactant phases.

Analogue modeling suggests that grain boundar>^ flow and intra- and inter-grain fracture flow are the principal grain-scale melt-flow mechanisms, whereas deformation of melt-bearing rocks is accommodated by melt-enhanced granular flow. The link from grain-scale melt flow to crustalscale melt transfer cannot be observed directly and must be investigated through field and laboratory studies of exhumed anatectic crust or using analogue models. Migmatites provide a record of where melt was located in melt-bearing crust. At the grain scale, mineral pseudomorphs of gram-boundary-melt fibns and intra-gram melt pockets record a minimum melt distribution at the this scale. In studies of contact aureole migmatites, localization of grain boundary melt is controlled by pre- or syn-anatectic fabric (strain) and/or differential stress. At the outcrop scale, migmatites m exhumed orogens have leucosomes that are oriented in the metamorphic fabrics or located in dilational sites. These leucosomes commonly connect with centimetric to metric scale concordant or discordant bodies of granite that have filled structures that once transfered melt to shallower crustal levels. Thus, melt migration pathways relate to rock fabrics and these are linked to melt transfer structures, such as magma fractures and viscous flow in channels and cylinders, by apparent dilational structures, including interboudin fractures and extensional shear surfaces.

Leucosome in structures at all scales appears to be continuous. These observations suggest that structures at different scales hosted melt at the same time, and that melt segregation and extraction are s>Titectonic processes. Thus, pseudomorphs of melt pockets, leucosomes in depleted migmatites and larger bodies of granite record the remnant grain to orogen scale permeability and transfer network. Evolution of such networks and amplification of anomalies are poorly understood. Melt segregation and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure buildup. Based on geochemical data, we can use statistical methods and modeling to evaluate whether migmatites are sources or feeder zones for gramtes, or simply segregated but stagnant melt in residue. Volumetric strain is accommodated by melt loss. Granites emplaced at shallower crustal levels are a necessar>^ complement to meltdepleted lower crust. Plutons are emplaced at various depths in the crust and have a variety of 3-D shapes that vary systematically with depth. The switch from ascent to emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength or state of stress) the ascent column, or by the ascending magma being trapped just below the bnttleviscous transition zone or by intersecting some discontinuity in the crust that enables horizontal magma emplacement and pluton inflation. Feedback relations between rates of pluton filling, magma ascent and melt extraction maintain compatibility among these processes. The switch from ascent to emplacement is illustrated with examples from the Acadian orogen, USA and the Variscan orogen, western France. At shallow crustal levels in the Acadian orogen lateral expansion occurred by displacement along locally developed asymmetric detachments induced in the contact zone above bodies of melt trapped just below the brittle-viscous transition zone by developing recumbent structures. In contrast, in the Variscan orogen, melt trapped at tectonic discontinuities enabled reactivation as extensional detachments to facilitate orogenic collapse.

200

TECTONICS AND METALLOGENESIS OF THE PALEO-ASIATIC OROGENIC BELT (PAOB), NW CHINA Solomon Buckman, School of Geoscience, Minerals and Civil Engineering, University of South Australia.

Email: solomon.buckman@unisa.edu.au The growth of continents is a topic of global significance and has profound implications on the formation of large mineral deposits. Eurasia is a landmass containing numerous mountain belts of different ages. The PAOB is a major tectonic feature that extends for more than 2000 km from the Ural Mountains of Russia and Kazakhstan through the Chinese Tian Shan and Junggar Mountain Ranges, across Mongolia and NE China. It separates the Siberian Craton from the Tarim/Northem China/Korean Craton to the south. It records the history of several Paleo-Asiatic oceans that were consumed at ancient subduction zones. Remnants of this oceanic crustal material (ophiolites) are preserved along sutures between continental or island arc fragments and represent key marker units m unravelling the tectonic evolution of the region.

Contemporary' models for continental collision and deformation are largely based on observations of the Cenozoic tectonic evolution of the Tibetan Plateau and Himalaya. Similar but older collisions have occurred further to the north in Central Asia. These have received little detailed documentation or study. The PAOB represents a massive amount of continental growth during the Paleozoic and is one of the last relatively unstudied/unresolved tectonic collages on Earth. It is now a composite of continental, island-arc and oceanic terranes, which formed throughout the Phanerozoic and were accreted to continental nuclei such as the Kazakhastan, Siberian and Tarim blocks. These three continental blocks collided in the Late Carboniferous.

The PAOB is the largest and most economically important of the crustal blocks that comprise Eurasia, hosting world-class Au, Cu/Mo, Pb/Zn, Cr and Ni deposits. The concentration of metal deposits umque to the PAOB is a consequence of complex tectonic activity during the Paleozoic involving the closure of several oceans and the collision of large continental fragments. There is a distinct concentration of orogenic gold deposits formed during the Carboniferous - notably Muruntau, Kumtor, Bakyrchik, Chinese Tian Shan and West Junggar. This corresponds with the final closure of an ocean separating the Siberian + Kazakhstan Block in the north from the Tarim Block to the south. Field investigations by the author identified a continental convergent margin terrane (Toli terrane) and accompanying accretionary complex (Kulumudi terrane) ranging in age from Devonian to Late Carboniferous. These two terranes may represent the final subductionrelated event before the collision of the Tarim Block with Siberian + Kazakhstan Block. The Toli and Kulumudi terranes contain abundant epithermal and porphyry related gold mineralisation, whereas the older (Cambrian-Silurian) terranes that collided with the Toli + Kulumudi terrane in the Late Carboniferous, contain little or no mineralisation. These older terranes have not been subject to convergent margin I-type magmatism and instead, formed a passive margin with the interveining oceanic lithosphere that was subducting beneath the Toli + Kulumudi terranes.

The Paleozoic was an important period of crustal growth in Central Asia. The positive continental crust budget is largely due to the accretion of island-arc, accretionary wedge and ophiolite complexes to continental nuclei and may be analogous to the numerous arcs, ocean basins and subduction complexes being formed in SE Asia today. Previous models which propose a single long-lived, north dipping subduction zone cannot account for this as an exclusively Andean/continental type margin would be generated.

SE Asia is host to numerous world-class ore deposits, as is Central Asia. Historically, much of Central Asia has been closed to foreign investment. This is changing rapidly and these areas are now being viewed as a potential source of world-class ore deposits. An accurate understanding of the tectonic evolution of a region can significantly improve exploration success.

201

HIGH-PRESSURE MELTING AND FLUID FLOW DURING THE PETERMANN OROGENY, CENTRAL AUSTRALIA. I S. Buick\ D. Closed, 1. Scrimgeour , C. Edgoose% J. Miller ^ C. Harris^ and I. Cartwright'^ ^Department of Earth Sciences, La Trobe University, Bundoora, Vic. 3086, Australia ^ 'Northern Territory Geological Survey, Alice Springs, N.T. 0871, Australia Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa ^Department of Earth Sciences, Monash University, Clayton, Vic. 3168, Australia

Durmg the -0.56 Ga Petermami Orogeny, basement rocks of the Musgrave Inlier 1.3 Ga granuhtes (Mi), -^1.2 Ga granites, and -^1.1 Ga and --0.8 Ga mafic dykes) were thrust northwards over the southern margm of the Amadeus Basin durmg intraplate transpression.

At these crustal levels rocks contain a variably developed, but generally pervasive protomylonitic fabric developed under transitional garnet granulite- to eclogite-facies conditions (M2a: --11-13 kbar, -750 X), and related to north-vergent Petermann-age deformation (D2a). At higher crustal levels the D2a foliation in the same rock types can be traced into discrete shear zones (D2b). These shear zones show evidence for extensive partial melting to form coarse-grained leucosomes containing cm-diameter hornblende and garnet m both meta-granite and mafic dykes (now garnet amphibolites). D2b shear zones developed at P-T conditions ranging from 11-13 kbar and -^700 C, to kbar and -^650T. Migmatites are found only within the D2b shear zones and can be traced outside into the deformed (D2a), unmigmatised and less hydrous metagranites and mafic dykes, suggesting that melting was water-saturated. Migmatitic leucosomes within the D2b shear zones are themselves variably overprinted by mylonitic fabrics believed to have developed during continued D2b deformation.

Metamorphosed granites and mafic dykes show a wide range of whole rock values (metagramtes: -16.1 to + 8.0 %o SMOW; recrystallised mafic dykes: +4.4 to + 8.6 %o). The highest values (granites: +7.3 + 8.0 %o; mafic dykes: +5.9 to + 8.6 %o) generally occur m the structurally deepest, unmelted levels. The values of the deepest-level metabasites and metagranites are similar to the range expected for igneous precursors (contmental mtraplate basalts and I-type granites, respectively). The lowest values occur at structurally higher cmstal levels that underwent extensive partial melting in discrete D2b shear zones. High-temperature oxygen isotope fractionations between minerals in these low-^^0 rocks require interaction with low-^^0 fluids during, or prior to, the Petermann Orogeny. The lowest-^metagranites (whole rock values generally as low as -2 to -3%o, locally as low as -16.1 %o) occur within the D2b shear zones. However, non-migmatitic metagranites away from shear zones may have values as low as +2 to +3%o..

Therefore, in detail the patterns of isotopic resetting do not closely relate to the extent of shearing and partial melting. One possible explanation for this lack of a simple relationship is that the basement rocks were hydrothermally altered, and variably isotopically reset, at shallow crustal levels prior to the Petermann Orogeny. Hydrothermal alteration may have been most intense along normal fault systems m the granites. These hydrothermally altered igneous rocks were then metamorphosed during the intra-plate Petermann Orogeny, with reactivation of normal faults as thrusts, and localisation of melting in these already preferentially hydrous deformation zones. Such a model explains the apparent decoupling of isotopic resetting, deformation and partial melting. The occurrence of low-^^0 amphibolites in the shear zones is consistent with hydrothermal alteration occurring in an extensional setting during emplacement of the latest dyke swarm i.e. as late as 0.8 Ga.

202

HIGH-P PARTIAL MELTING OF DIORITIC GNEISS: LA-ICPMS ANALYSIS OF PEMBROKE GRANULITE MINERALOGY, FIORDLAND, NEW ZEALAND G. L. Clarke^ J. A. Stevenson^ ^ N. R. Daczko^N. J. Pearson^ and K.A. Klepeis^ ^ School of Geosciences, F05, University of Sydney, NSW 2006 Australia ^Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109 Austraha ^Department of Geology, University of Vermont, Burlington, VT 05404 USA ^Present address: Department of Geoiog}^ and Geophysics, Yale University, New Haven, CT 06520, USA ^Present address: Department of Geological Sciences, University of Texas at Austin, Austin, TX, USA Granulite facies gabbroic and dioritic gneisses m the Pembroke Valley, Milford Sound, New Zealand, are cut by vertical and planar garnet reaction zones (GRZ) in rectilinear patterns. In gabbroic gneiss, narrow dykes of trondhjemitic leucosome are surrounded by fine-grained garnet granulite that variably recrystallized the host two-pyroxene hornblende granulite at conditions of 7>750°C and kbar. The garnet reaction zones cut contacts between gabbroic gneiss and diontic gneiss, but change m morphology at the contacts to zones with a septum of coarse-grained garnet surrounded by trondhjemitic leucosome. The dioritic gneiss additionally contains isolated garnet grains enclosed by leucosome, and short planar trains of garnet grains linked by spatially restricted leucosome. Partial melting of the dioritic gneiss, mostly controlled by hornblende breakdown at water-undersaturated 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 recr>^stallization to garnet granulite. New trace element data acquired for minerals in various textures using LA-ICPMS, geochemically links trondhjemitic veins in the gabbroic gneiss to sites of partial melting in the dioritic gneiss. Peritectic garnet in the dioritic gneiss and trondhjemitic veins has a REE pattern inherited from hornblende in the meh-producing reaction. Garnet in the GRZs has a distinct REE pattern, inherited from the sub-solidus replacement of hornblende in the gabbroic gneiss. The trace element data indicate that formation of the garnet reaction zones was not isochemical, and there was extensive geochemical interaction of wall rock (gabbro) and the passing melt. The partial melting of dioritic gneiss similar to that exposed in the Pembroke Valley is proposed as a possible source for the higher level alkali-calcic, Na-rich granitoids of the Separation Point suite.

203

THE GRANITE-MIGMATITE CONNECTION: APPLICATION TO LACHLAN FOLD BELT S- AND I-TYPE GRANITES W. J. Collins. B. Healy and S.W. Richards Department of Geology, University of Newcastle, Callaghan, NSW, 2308, Australia The tantalising link between granites and migmatites was observed in the field back m the 19'^ century, and it fuelled the granite controversy between H.H. Read and NX. Bowen in the middle part of the 20^ century. Although we now think of granites and most leucosomes in migmatites as products of melting, rather than as granitized products of country-rock, we still find the petrological link between the two is quite elusive. In overthickened crust, a strong case can sometimes be made for a middle crustal migmatite source and upper crustal granite sink, as shown by Sawyer (1998) for the Late Archean Opatica overthrust belt of the Superior Province, Canada. Solar and Brown (2001) have provided a Paleozoic example from the Appalachians, USA. Patino-Douce and Harris (1998) have experimentally reproduced the Neogene Himalayan granites by melting adjacent crustal rocks at 6-8 kbar pressure (--mid-crustal depth) and temperatures 750-770°C. In all these cases, the product was leucogranite, with compositions similar to some component, or a combination of components, of associated migmatites.

However, the great majority of granitic batholiths from around the world are more mafic than leucogranite, with an average composition of granodiorite, and were much hotter (>800°C).

T>T3ical Sr and Nd isotopic signatures of these batholithic rocks indicates an additional juvenile crustal and/or mantle component in the source. This applies equally well to the Lachlan S- and It}npQ granite batholiths. Such batholiths usually contain mafic enclaves, syn-plutonic mafic dykes and minor gabbroic complexes, so the field evidence also implies a multi-component input. In general, based on major element compositions, batholithic granites can be subdivided into similar broad groups as basalts (Frost et al., 2001), implying significant heat and material contribution from the mantle during granite generation.

So where does this leave us with the contrasting Lachlan S- and I-type granites and their link with migmatites? We have been investigating inigmatitic aureoles from the base of the I-type Bega Batholith (Kameruka pluton) and S-type Murrumbidgee Batholith (Clear Range and Murrumbucka plutons) in southeastern Australia. At the Kameruka pluton base, granitic sheets have intruded subconcordantly into leucosome-rich, schlieric migmatites (diatexites), producing hybrid granitic rocks. These rocks contain the typical megacrystic K-feldspar of the pluton and abundant small cordierite-biotite enclaves of migmatite paleosome. All hybrid variations between pristine granite and migmatite exist. Rare dykes of hybrid material have intruded the pluton. Chemically and isotopically, the hybrids define a difiuse cluster between granite and low-grade country-rock sediment (migmatite protolith), but they do not overlap the well-defined linear chemical trend, nor the isotopic compositions of the pluton. This implies that the I-type granite Kameruka pluton was not strongly contaminated by migmatite.

The metamorphic aureole of the Murrumbidgee Batholith widens southward with its southern extension being the Cooma Complex. In effect, this is the region below the batholith base (Richards and Collins, 2001). The diatexitic migmatites (Cooma granite) that form the liighest-grade core of the Complex can be traced northward into the batholith, initially as abundant km-scale lenses between stromatic migmatite apophyses and Murrumbucka tonalite sheets, but farther north in the central part of the batholith they become smaller, less common rafts in a sea of granite. There, the tonalite is locally associated with gabbros, with transitional margins illustrating hybridisation between gabbro and granite. Across-strike traverses show a complete gradation from Cooma granite to Clear Range granodiorite, to Murrumbucka tonalite. Chemically, the Murrumbucka tonalite can be modelled as a 50:50 mix of Cooma diatexite and gabbro. Isotopically, the same result is obtained. Thus, in this case, migmatite appears to have been incorporated eji masse with hot mafic magmas to produce this large S-type batholith.

References Frost, R.B., Arculus, R.J., Bames, C.G., Collins, W.J., Ellis, D.J., 2001. J. Petrology. 42, 2033-2048.

Patino-Douce, A.E. and Harris, N., 1998. J. Petrol, 39, 689-710.

Solar, G.S. and Brown, M., 2001. J. Pet., 42, 789-823.

Richards, S.W., Collins, W.J., 2001. J. Metamorphic Geology, 20, 119-134.

Sawyer, E.W., 1998. J. Pet, 39, 1147-1167.

204

MAGMATIC FLUIDS COEXISTING WITH FELSIC MELTS: AN EXAMPLE FROM RIO BLANCO, CHILE. ^Paul Davidson. ^ Vadim Kamenetsky, ¥ete Hollings, ^Dave Cooke and ^ Peter Frikken ^Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania. ^Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada Aqueous fluids exsolved from felsic melts are usually assumed to be a major vehicle for transporting materials in and out of magmatic systems, and the primary source of metals for many types of mineralisation. The existence and composition of such fluids, although not well documented at present, is a matter of wide geological interest. Examples of primary magmatic fluids can be found in magmatic inclusions in quartz and feldspar phenocrysts from intrusive and extrusive rhyolites spatially associated with the Rio Blanco Cu-Mo deposit, Chile. The host rocks belong to the La Copa Volcanic complex (4.9-3.9 Ma), which was the final phase of volcanism at Rio Blanco. Several types of coexisting inclusions in phenocrysts represent a range of magmatic phases present at the time of crystallisation. Type-1 are glass melt inclusions with shrinkage bubble(s) (6.2±1.9 vol%) ± silicate daughter crystals. Type-2 are inhomogeneous crystalline melt inclusions, consisting of felted masses of silicate crystals and interstitial aqueous fluid (see details in Davidson and Kamenetsky, 2001). Composite inclusions may contain glass, crystals and fluids in variable proportions. Fluid-rich phases occur as fluid-filled bubbles in composite inclusions (Fig. 1, 3), or as individual primary fluid inclusions with negative crystal shapes (Fig. 2). Bubbles are usually single phase and may be dark or clear in appearance with or without translucent crystals, and often coexist in the same inclusion (Fig. 1). Thermometric experiments show that clear bubbles can be completely frozen, and have final ice melting temperatures between -0.7 & -ll.O^'C, indicating salinities up to at least 15 wt% NaCl equivalent. Dark bubbles either show no freezing behaviour, or only the formation of thin ice fihns.

The contention that fluid-rich bubbles in glass were the result of inhomogeneous trapping of aqueous phases, coexisting with silicate melt, is supported by the observation that: • fluid-only magmatic inclusions imply an independent aqueous fluid phase in the magma • phase ratios within composite inclusions containing fluid bubbles are variable • composite inclusions contain too much aqueous fluid for it to have exsolved post-trapping • the elevated salinities of fluid bubbles imply magmatic origin.

The droplets of primary magmatic aqueous liquid preserved in studied magmatic inclusions provide strong evidence for the existence of exsolved aqueous phases in felsic melts, and the opportunity to constrain the parameters of exsolution and compositions of immiscible phases.

Fig.l. Dark (vapour) and clear (aqueous liquid) bubbles in glass inclusion in quartz.

Fig. 2. Vapour-dominated magmatic inclusion in quartz.

Fig. 3 Clear bubble in glass inclusion quartz, frozen, at -47.5°C. Scalebars are all 20 ^m Reference Davidson, P. and Kamenetsky, V. S. 2001. Immiscibility and continuous felsic melt-fluid evolution within the Rio Blanco porphyry system, Chile: evidence from inclusions in magmatic quartz. Economic Geology, 96, 8.

205

A NUMERICAL MODEL FOR SELF-ORGANISED CRITICALITY IN MELT ACCUMULATION AND ASCENT IN PARTLVLLY MOLTEN ROCKS Paul D. Bons^'l Marlina A. Elburg^ ^ Boudewijn P. van Milligen^ Alvar Soesoo^ and Cees W. Passchier^ ^Tektonophysik, Institut fiir Geowissenschaften, Universitat Mainz, D-55099 Mainz, Gennany. bons@mail. uni-mainz. de 'Present address: Institut fur Geowissenschaften, Universitat Tubingen, Sigwartstr 10, D-72076 Tubingen, Gennany. ^Department of Geology and Geophysics, Adelaide University, Adelaide SA 5005, Australia ^Present addressstitute for Chemistry, P.O.Box 3060, D-55020 Mainz, Germany ^ -'Asociacion EURATOM-CIEMAT, Avenida Complutense 22, 28040 Madrid, Spain ^Institute of Geology, Tallinn Teclinical University, 7 Estonia Ave, Tallimi 10143, Estonia Melt segregation, accumulation and ascent cause profound and irreversible changes to the rocks that contain the melt. This occurs from the small scale where melt-filled pore space and fractures are created upon melting and destroyed when melt is extracted, to the large scale where space is created for the emplacement of plutons. Most Darcian continuum models for melt and fluid transport describe transport properties of the rock with a history-independent permeability parameter, which does not properly describe the changes in the properties of the rock matrix.

Darcian contmuum models for porous or fracture flow are therefore insufficient. We propose that transport is discontinuous and in batches, and that accumulation occurs by stepwise merging of batches (Bons et al 2001). A simple numerical model for stepwise accumulation and transport of batches by propagation of hydrofractures is presented that shows how such a system may quickly develop mto a self-organised critical (SOC) state (Bons & van Milligen 2001). In this SOC state, the distribution of melt batch volumes is self-similar and can be described by a power law, with an exponent M that lies between 2/3 and 1, irrespective of details such as shape of batches. M=2/3 represents maximum melt concentration efficiency. AtM=l the system is on the boundary between dispersion (M>1) and concentration (M<1) of melt. Once a SOC state is established, the system is capable of discharging any additional melt without fiirther change to the system itself. Deformation aids melt extraction efficiency, as it increases the mobility of hydrofractures, enhances accumulation, and hence lowers the exponent M. In the model presented here, full connectivity of melt needs never to be reached in the system and melt transport and extraction can occur at melt fractions as low as <2%. We predict that the chemical evolution of melt from source to emplacement level may be governed by the discontinuous mixing of batches, each with different histories. If so, and provided no subsequent homogenisation occurs in a higher-level magma chamber, a pluton may retain information about its SOC accumulation and ascent history in its chemical heterogeneity.

References Bons P.D., Dougherty-Page J. and Elburg M.A. 2001. Stepwise accumulation and ascent of magmas. Journal ofMetamorphic Geology 19, 627-633.

Bons P.D. and van Milligen B P. 2001. A new experiment to model self-organized critical transport and accumulation of melt and hydrocarbons from their source rocks. Geology 29, 919-929.

206

NATURAL SEDIMENT PROPERTIES BY WHICH MUD CRYSTALLISES TO SCHISTS, GNEISSES, AND IGNEOUS-LOOKING ROCKS" John Elliston, Elliston Research Associates Pty Ltd., 10 (B) The Bulwark, Castlecrag.N.S.W. 2068.

Natural basin sediments are high-energy particle systems created from crystalline silicates by the enormous energy inputs of grinding to fine-grained sediments and the imposed decay of weathering processes (mainly hydration). "Weathering" does not stop when sediment particles are dumped in the sea. Hydration (chemical reaction of seawater with particle surfaces) continues during the long soakage times. This produces much finer, highly hydrated marine clays, hydroxy-carbonates, siliceous and ferromagnesian slimes and oozes. Sediments buried down to 7 km remain as particulate (finely divided) material. They behave as stiff plastic mud rather than as shale.

Current developments in colloid and surface chemistry now define the intrinsic properties of highenergy particle systems. Behaviour of all particulate materials such as plastic sediments is governed by surface energy and charge on the particles. Tlie smaller particles in ordinary clay and amorphous silica that make up marine muds and oozes have the greatest surface energy and charge. These particles interact with each other and to equilibrium with water, ions, other charged particles and available surfaces. The 'colloidal processes' and particle interactions that give rise to features commonly preserved in the rocks are plasticity, cohesion and fracture, diffusion, thixotropy, rheopexy, adsorption, desorption, syneresis, accretion, concretion, shear thinning, and enhanced crystallisation.

There is abundant evidence in the rocks to show that the behaviour of clays and other charged sediment particles in present sedimentar}^ basins is the same as their behaviour in ancient sediments.

They had the same physical and chemical properties. Enormous volumes of hydrolysed sediments reduced essentially to clay, amorphous silica, hydroxy carbonates, or banded iron formations, have been disturbed by earthquake shocks. Turbidity currents, massive slides as "mud glaciers", shelf failures, and immense volumes of over saturated mud updoming as diapirs have been triggered by earthquakes.

Subaqueous sediment movements on a large scale provide the opportunity for charged particles to interact dynamically. Pasty flow or shear in concentrated sediment pastes allows the high-energy particles to re-arrange themselves so that surface energy is reduced. Colloidal particles aggregate in accord with the properties of the respective particles. These are dictated by the basic physical properties that relate to the particles and the system such as particle size, particle shape, James Hutton. 1785 particle charge, charge distribution on DATE 1800 surfaces, and van der Waal's attraction. The GROWTH OF atomic geometry of the particles, of water MAINSTREAM EARTH SCIENCE and the configuration of surface adsorption THEORY sites determine the static and dynamic (COnlin&dlO theory of solulions and behaviour 1850 particle interactions that change the fabric ot silicate malts) INDEPENDENT and texture of the sediments.

GROWTH OF COLLOID AND SURFACE CHEMISTRY

Vine and

Dietr and (piar® tectonics) 'Seismic reflection profiles /Dc^anogiaphic surveys

dawdlopmer^ts in physical chemistry still to be appRe<J 1900 in the earth sclertc^s) 1910 CHFMS7S—i 1920 Peterfi 1930 Freundlich •erjagoirt 1940 Latenrvgw myulr 1950 V O veexander rbM k i960 Al Boswel L et>edev 1970 Eit^l Heal Iter y' 1980 Stumm .Healy 2000

^ [wider theorelical base resolves problematic!^ ^ rock textures and the oriqin of ore deoositsr

Ordinary shales and sedimentary rocks are produced by simple lithification without shear. Plastic deformation of unconsolidated sediments results in fissility and schistosity.

Pasty flow produces uneven "metamorphic coarsening" and a variety of metamorphic and gneissose textures develop after crystallisation.

Liquefaction results in igneous-looking intrusive or formerly fluid rocks.

Keywords: particle s}^stem properties, colloidal processes, particle interactions.

207

FLUID REGIMES IN ECLOGUES AND GRANULITES FROM THE DABIE-SULU TERRANES, EASTERN CHINA Bin Jacques L. R. Touret^ and Yong-Fei Zheng" ^ Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, NL-1081 HV Amsterdam " Department of Earth and Space Sciences, University of Science and technology of China, Hefei 230036, P R. China * Present address. Economic Geology Research Unit, School of Earth Sciences, James Cook University A combmed oxygen-isotope and fluid-mclusion study has been done on high- and ultrahigh-pressure (HP and UHP) eclogites and granulites from the Dabie-Sulu terranes in eastern China, believed to represent recycled, deepest crustal rocks during contmental collision. Both rock-types, enclosed within either garnet peridotite or paragneiss, marble and metagranite, have preserved remnants of early, prograde- and/or peak-metamorphic fluids, modified durmg uplift. Recorded P-T conditions (mineral assemblage) indicate a maximum depth of burial at 100 - 200 km (P = 30 - 60 kb; T = 600 - 800 T ) m Early Tnassic (220 - 240 Ma), and then exhumation at around 180 Ma. The HP-UHP rocks show a wide range m from -10 to 10 %o, relative to SMOW. High-temperature oxygenisotope fi-actionations preserved between quartz and garnet preclude significant retrograde isotope exchange during exhumation. Hence, the variable oxygen-isotope composition is a pre-metamorphic signature of the precursors: ^^0-depleted rocks correspond to surface precursors having interacted with surface meteoric water prior to subduction. As far as the fluid inclusions are concerned, striking differences exist beUveen eclogites and granulites; H2O (variable salinity) with minor gases (N2, CO2, CH4) m HP/UHP eclogites, CO2 (± H.O ± CH4 ± carbonate) m HP granulites.

Aqueous inclusions in eclogites cover a wide range of salinity, from halite-bearing brines to low-salmity fluids. A few pure N2 inclusions occur in undeformed eclogite. The primary character of N2 and high-salinity (and some low-salinity) aqueous inclusions indicates that they are remnants of UHP metamorphic fluids, uhimately derived from sediment pore fluids, hydrothermal fluids or meteoric waters. Methane, coexisting with high-salinity brines, probably formed during serpentmisation of mantle-derived peridotites under the influence of C02-nch aqueous fluids. The chlormity of the aqueous fluid is positively correlated to the Cl-content in amphiboles and the oxygen-isotope composition of eclogites: Highly saline brines occur in high-5^^0 (> 3 %o) eclogites with high-Cl content amphibole (up to 4 wt.%), whereas low-salinity fluids, believed to be remnants of ancient meteoric waters, occur in low-5^^0 (< -4 %o) eclogites with very-low-Cl content amphibole (< 0.2 wt.%).

In conclusion, the fluid evolution during continental collision can be summarised as follows: (1) Protolith-formmg stage (pre-metamorphic): Initial fluids in the protoliths include sedimentary pore waters (e.g. evaporites), hydrothermal fluids (high-salinity), and/or ancient meteoric waters (low-salinit\0. (2) Prograde-metamorphic stage (burial): Fluid compositions are modified by selective water incorporation into the mineral host (e.g., omphacite), leadmg to salinity increase. (3) Peak-metamorphic stage: These fluids are preserved within crystal-size domains in a number of rock-formmg mmerals (garnet, omphacite, etc.). N2, formed at this stage, possibly occurs through the breakdown of feldspar/mica. (4) Retrograde-metamorphic stage (exhumation): Continuous, significant compositional changes lead to a reorganisation and local redistribution of peak fluids durmg decompression. Fluid mixing is due to a limited influx of external CO2 and low-salinity fluids.

The major result of this study is to show that surface fluid remnants, preserved in inclusions, can survive a burial of > 100 km during continental collision and subduction and subsequent exhumation. Durmg the whole metamorphic evolution, free fluids in eclogites and neighbouring granulites occurred only in limited amounts and were strictly internally buffered, with only a little external fluid introduced during the latest stages of decompression, near the present-day surface.

208

COPPER SOLUBILITY IN MINERAL-BUFFERED, NEAR-MAGMATIC, SUPERCRITICAL FLUIDS: INSIGHTS FROM LA-ICP-MS, PEXE AND EXAFS OF SYNTHETIC FLUID INCLUSION EXPERIMENTS Alistair Hack'. John Mavrogenes'' aiid Andrew Berry' 'Rese^ch School of Eardi Sciences, Australian National University, Canberra, ACT, 0200 'Deparunent of Geology, Australian National University, Canberra, ACT, 0200 Hydrothermal fluxes of metals in the crust and mantle are important geological phenomena of interest to both economic mineral explorers and other Earth scientists. Unfortunately, hydrothermal processes remain poorly understood because of the scarcity of experimental hightemperature and -pressure mineral solubility data. Simply, this paucity of relevant information is due to the limitations of traditional experimental methods. However, by using a non-conventional experimental and analytical approach to investigate the hydrothermal behaviour of copper v^e have been able to access new high temperature and pressure regions relevant to the near-magmatic hydrothermal environment. This has been achieved by utilising synthetic fluid inclusions coupled with analysis of individual inclusions by a number of complimentary micro-analytical techniques, including, excimer laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), particle induced X-ray emission (FIXE) and synchrotron extended X-ray absorption fine structure spectroscopies (EXAFS).

The present study investigates copper solubility in supercritical fluids under a variety of mineralbuffered conditions by using LA-ICP-MS and PIXE to measure the copper concentration of individual quartz-hosted fluid inclusions trapped at the experimental conditions. Effects studied include, temperature, pressure, salinity, pH and oxygen fugacity. The experimental design chosen allows the solubility data to be interpreted thermodynamically thereby allowing information on copper complexation and its speciation to be derived. However, at this stage the preliminary solubility data are not detailed enough to determine the copper chloride speciation by this method.

To help constrain copper speciation preliminary experiments using synchrotron EXAFS at elevated temperature and pressure have been undertaken. Initial results show, as expected, copper(I) to be the dominant oxidation state and suggest that [CuClJ"^ maybe the dominant species even at extreme salinities. Further processing of the data is necessary before more details of the copperchlorine complexation are available.

The experiments cover a wide range of salinities, from dilute to concentrated brine-like compositions. Notably, the solubility data indicate a significant 'salting out' (or saturation) effect for copper as chloride concentration increases in the experiments. That is, copper solubility is lower than expected in concentrated saline fluids. This result may be important for understanding why copper, in subcritical two-phase (liquid-Hvapor) hydrothermal environments, appears to partition preferentially into the lower-salinity vapor relative to the coexisting concentrated brine in many natural porphyry-epithermal systems.

209

UNUSUAL NOBLE GAS COMPOSITIONS IN POLYCRYSTALLINE DL\MONDS: PRELIMINARY RESULTS FROM JWANENG, BOTSWANA Masahiko Honda ^ David Phillips^ and Jeffrey W. Harris^ ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 "School of Earth Sciences, The University of Melbourne, Melbourne, Victoria 3010 ^Division of Earth Sciences, Universit>^ of Glasgow, Glasgow G12 8QQ, UK Research goals in noble gas geochemistry include understanding the structure of the Earth's mantle and the creation of a coherent model of its evolution. In this regard, noble gas compositions m mid-ocean-ndge basalts (MORBs) and in ocean island basalts (OIBs) such as from Loihi Seamount, Hawaii, and Iceland, have provided very useful information on the mantle. However, virtually all these data are from samples that are effectively of zero-age, and therefore, they only give information about the present composition of mantle noble gases. It is critically important to determine if there is any systematic variation of mantle noble gas composition with time. In particular, in relation to the current debate about mantle structure and evolution (e.g. two-layered convection vs. whole mantle convection), information on noble gas compositions in ancient mantle can provide very strong constraints. If noble gas measurements are made on mantle-derived samples of different ages, these can be used to evaluate as to what degree, if any, the upper mantle had interacted with the lower mantle, and allow further refinement of models concerning mass transport, including volatiles, in the mantle. However, attempts to determine the noble gas composition of ancient mantle by analysis of older geological samples have, with few exceptions, been unsuccessful, in part owing to the lack of suitable samples.

Diamonds have unique characteristics which make them potentially very useful as sources of noble gases from the mantle. This is because: (1) diamonds have been demonstrated to retain significant amounts of mantle-derived noble gases, (2) most diamonds appear to be derived from 150 km to 200 km depth m the Earth, (3) diamonds cover a wide range of crystallization ages of between 0.6 and 3.5 billion years, (4) diamonds have been shown to have low diffusivities for noble gases so that they are highly retentive of noble gases, and (5) diamonds typically have suffered little interaction with crust or atmosphere, owing to their great crystallization depths and extremely rapid emplacement to shallow crustal levels in kimberlite and lamproite pipes. Thus, diamonds provide a direct window into the ancient mantle. As such they are unique as sources of mantlederived noble gases.

As a pilot study, we have undertaken noble gas analysis of four black bort diamonds from the Jwaneng kimberlite pipe, Botswana, by stepwise heating and vacuum crushing. All diamond samples released large quantities of atmospheric noble gases from lower temperature step-heating and initial vacuum crushing experiments.

Graphitisation steps and subsequent crushing experiments produced small fractions of MORB-like Ar and Xe, but no evidence for MORB-like Ne. Xe isotopic anomalies are the largest recorded from mantle samples. The above observations indicate that the MORB-like noble gas component in the diamond source region of the mantle was overwhelmed by atmospheric noble gases, possibly introduced into the mantle source during ancient subduction-related processes. If correct, the mantle may contain significant amounts of atmospheric noble gases as a secondary major noble gas component. The separation of atmospheric and MORB components during step-heating and crushing suggests that the former component may be hosted by later diamond overgrowths, although release from altered mineral mtergrowths cannot be totally discounted.

210

CONTRASTING PATTERNS OF GRANITOID EMPLACEMENT AND DEFORMATION, SOUTHWEST CURNAMONA PROVINCE Mathias L. Knaak, A. Jolin Parker and Richard B. Flint Geosurveys Australia Pty Ltd, 334 Glen Osmond Rd, Myrtle Bank SA 5064, Australia Introduction The Cumamona Province, a near-circular entity consisting of Palaeoproterozoic metasedimentary and metavolcanic rocks (Willyama Supergroup) and Palaeoproterozoic to Mesoproterozoic plutonic rocks, IS well delineated in airborne magnetic and gravity data. The southwest portion of the province coincides with a significant regional N-S trending gravity anomaly of low amplitude and dimensions of -lOOxBOkm. Semi-detailed geological mapping and geophysical interpretation were undertaken in the Billeroo area of the Plumbago Inlier, which encompasses the southern limits of the gravity anomaly and coincides with the northwest limits of exposure for the Olary Domain.

Regional geology The --1590 Ma old 'regional granites' and related intrusives that form -30% of basement exposures m the region coincide with the southern limit of the prominent gravity anomaly. The intrusives fonn northeast-trending elongate domal structures that are arranged in an almost en echelon array flanked by highly migmatised, polyphase-deformed psammopelitic metasedimentary rocks. For much of the mapped area, trends for Olarian Orogeny OSi, OS2 and OS3 fabrics within metasedimentary units are all similar and broadly to the east-northeast.

The degree of migmatisation and proportion of melt component within metasedimentary rocks increase towards the granitoid plutons. The trend of migmatisation fronts defined both in the field and in K-U-Th ternary gamma-ray spectral miagery are irregularly shaped but outline a broad northwards-trending corridor at a high angle to deformational trends.

Granitoid geology Irregularly shaped and zoned composite plutons ranging in diameter from --1-5 km comprise coeval mafic and felsic phases.

Monzogranite is the dominant composition.

K-feldspar phenocrysts up to 5 cm long are commonly aligned, in part defining an igneous fabric trending north to north-northeast. Small volumes of more mafic phases range in composition from granodiorite to quartz diorite to diorite and commonly occur as dykes, small intrusive bodies up to 50 m wide and mafic enclave swarms at the margins of the larger monzogranite bodies. At the contacts of larger mafic bodies, complex zones of magma mingling comprise hybrid granitoids of various compositions containing abundant oval to round enclaves (2-10 cm across) of fine-grained microdiorite. Magnetite is a common accessory within most granitoid phases. However, airborne magnetic imagery defines either mottled very weakly magnetic domains or linear to semi-circular anomalies of moderate amplitude. Muscovite is totally absent from all phases. Only OD3 structural elements are present within the granitoids and these are superimposed upon the earlier igneous alignment of K-feldspar phenocrysts. Granitoid textures and fabrics range from massive to strongly foliated, especially within localised zones of ductile-brittle deformation. High-grade metamorphic fabrics typical for ODi and OD2 are absent.

Discussion Structural relationships clearly demonstrate that emplacement of the composite zoned plutons is post-OD2 but pre- to syn-ODs of the Olarian Orogeny. The trends for igneous fabrics and migmatisation fronts support the view that the significant N-S trending regional gravity low reflects the presence of a much larger granite body than suggested from the exposures. Individual plutons are relatively small at km and their shape and size possibly reflect the influence of OD3 However, they are an integral part of a regional batholith that extends northwards for -100 km, perhaps aligned along a major crustal lineament. The concealed regional granitoid batholith and envelopmg metasedimentary units, particularly along structures close to the contact, are very prospective targets for uranium and copper-gold mineralisation.

211

UNMIXING AND RE-MIXING: CHEMICAL EXTRACTION, TRANSPORT AND DEPOSITION PATHS ACCOMPANYING REGIONAL ALTERATION AND ORE DEPOSITION IN THE CLONCURRY FE-OXIDE-CU-AU DISTRICT, MT ISA BLOCK, NW QUEENSLAND Nicholas H.S. Oliver. Geordie M. Mark*, Peter J. Pollard, Lucas J. Marshall Patrick J. Williams, Michael Carew, and Timothy Baker EGRU, School of Earth Sciences, James Cook University * present address: School of Geosciences, Monash University Mass balance equations, and stable and radiogenic isotope data for host rocks, regional metasomatic albitites, intrusions, Cu-Au ores and their immediate host alteration have been used to determine the 10 m to 10 km-scale mass transfer paths leading to deposition of this ore type. The scale of transfer of each element provides the fundamental clues to the source, transport and deposition processes, but has been difficult to constrain because of the variability of host rock chemistry and of the distribution and intensity of alteration. The ultimate source for copper and gold is most likely to have been volatile release from two or three important phases of the Williams Batholith (1527, 1515 Ma ± 15 Ma), because a) these elements are apparently not consistently scavenged from country rocks during albitisation also induced by magmatic volatile release, and b) fluid inclusion data from previous work around the granite-hosted Lightning Creek prospect shows high Cu concentrations in PIXE analyses of fluid inclusions, despite the paucity of chalcopyrite (Perrmg et al., 2000).

Immiscible phase separation near the tops of the crystallising plutons, triggered by the depth of emplacement and the high CO2 and NaCl content of the magmas, released albite-stable brines and C02-rich vapour (Pollard 2001), facilitatmg brecciation, metal transfer (Cu and Au) and varying degrees of consequent exchange of other components via fluid-rock reaction. Whole rock geochemical data for transformation of a variety of initial rocks (calc-silicate, diorite, pelite, gabbro) towards albitite indicates that Na addition to the rock from a hypersaline brine accompanied the loss of Fe, K, Ba, Rb ± Sr, Co, V, Mn from altered rocks to the brine. The modified fluid, still containing Cu and Au but now enriched in Fe and K in particular, caused local biotite-magnetite alteration (metasomatic ironstones) in pelitic, semi-pelitic and volcanic host rocks in breccias nested in shear zones, and at fault bends and intersections. Stable isotope data (Marshall and Oliver, this volume), as well as the enrichment of some pelitic rocks during ironstone formation in small amounts of Ca as well as the elements above, suggests that the originally igneous-dominant brine must have interacted with the calcareous Corella Fm prior to ore deposition in the overlying rocks, but did so without dropping out the Cu and Au. Hydrothermal geochemical modelling and chemical reasoning suggest this was facilitated by high temperatures and very low sulphur concentrations in this fluid. This entire process produced many barren ironstones, but only locally were Cu-Au deposits formed. In some Cu-Au deposits, prior sulphide concentrations may have played a role, reflected by anomalous S isotope values, but the clear igneous signature of the bulk of the sulphur, and the requirement that the metal-bearing brine was sulphur poor, requires another magmatic sulphur source independent of the brme. Based on some recent literature from volcanic fields (Hattori & Keith 2001), and also on the observation of widespread mafic-felsic magma mingling in the Mt Isa Block, we speculate that a S-beanng fluid was released by reaction of S-bearing vapour in mafic magmas with water from felsic magma, upon magma mingling. Mixing of H2S(±S02)-C02 bearmg vapour with chemically evolved brines was the most efficient mechanism for ore deposition in the largest of the deposits, Ernest Henry.

References Hattori K.H. and Keith J.D. 2001. Contribution of mafic melt to porphyry copper mineralization: evidence from Mount Pinatubo, Philippines, and Bingham Canyon, Utah, USA: Mineralium Deposita, 36 99806.

Perring C.S., Pollard P.J., Dong G., Nunn A.J. & Blake K.L. 2000. The Lightning Creek sill complex, Cloncurry District, northwest Queensland: a source of fluids for Fe oxide Cu-Au mineralization and sodic-calcic alteration. Economic Geology, 95, 1067-1089.

Pollard P.J. 2001. Sodic(-calcic) alteration associated with Fe-oxide-Cu-Au deposits: an origin via unmixing of magmatic-derived H20-C02-salt fluids. Mineralhmr Deposita, 36, 93-100.

212

MAGMA TRANSFER, EMPLACEMEMENT AND CONSTRUCTION: THE ROLE OF MIGMATITES AND COMPRESSTVE DEFORMATION IN CONTROLLING THE SHAPE AND STYLE OF PLUTON EMPLACEMENT S.W. Richards. B. Healy and W.J. Collins Department of Geology, Newcastle University, Callaghan, N.S.W., Australia The Tuross Tonalite, Kameruka Granodiorite and the southern Murrumbidgee Batholith, located within the southeastern Lachlan Fold Belt (LFB), NSW, Australia, are surrounded by 50-500m wide niigmatitic aureoles characterized by leucosome-rich, schlieric diatexite. The migniatites are schlienc diatexites that contain distinct cordierite-rich (+/- sillimanite), preferentially aligned fragments or xenoliths suspended within a coarse, equigranular, strongly flow foliated (schlieric) quartz + plagioclase-rich leucosome matrix. Asymmetric tails and imbrication exhibited by the xenoliths and magmatic S-C fabrics defined by the schlieren indicate s>Ti-migmatite asymmetric flow and suggest that the migmatites represent leucosome-rich shear zones. At the migmatitegranite margins, the migmatites have been locally and variably hybridized with the granite indicating early coeval migmatite generation and pluton emplacement.

Combined geophysical mvestigations (gravity and magnetics) and detailed mapping of internal magmatic and pluton margm characteristics have shown that the migmatites are always located along the basal margin of the pluton. The Tuross Tonalite is a sub-rounded, basin shaped pluton with inward dipping margins. The migmatites surrounding the pluton mimic the inward dipping contact, therefore, the migmatites are located directly below the pluton's margin. Similarly, the Kameruka Granodiorite exhibits a moderate to shallow, east-dipping western contact. The schlieric foliation exhibited by the migmatites, the sheeted contact, internal magmatic mineral alignment and mafic enclave orientation also dips to the east, therefore, the migmatites also developed below the east-dipping western contact of the Kameruka pluton. Finally, the sheeted intrusions of the southeastern Murrumbidgee Batholith exhibit schlieric migmatites as both sheets within the granite and adjacent to (below) the east dipping western margin.

In each of these examples the migmatites developed immediately before and early during pluton emplacement. This is supported by the limited degree of contamination between migmatite and gramte and also implies that migmatite generation, shearing and hybridization are restricted to the pluton's margin during magma transfer. We suggest that the migmatites are shear zones that focused and facilitated INITIAL granite magma migration, i.e. transfer and emplacement. Final pluton emplacement and construction occurred ahnost independently of the shear zone. We would also suggest that the style of pluton (chamber) construction is controlled primarily by the conditions of regional stress.

The sub-rounded, shallow basin shape of the Tuross Tonalite, the lack of an internal axial planar magmatic/tectonic foliation and preservation of delicate magma mingling structures reflects a relative paucity of regional compressive deformation at the time of emplacement. The elongate shape of the Kameruka Granodiorite combined with the strong parallelism between pluton margins, internal magmatic features and the inter-sheeted pluton margin suggest a structural control on emplacement. Here, shearing within adjacent migmatities facilitated an initial sheeted style of pluton emplacement and created a zone of hybridization along the pluton margin. However, the lack of shear fabric within the pluton indicates that pluton construction was unrelated to shearing.

Numerous feeder dykes and a well-developed, coarse-grained, cumulate texture indicate construction under relatively 'passive' conditions, in contrast with the Murrumbidgee Batholith, which is highly elongate, thoroughly hybridized with migmatites and, together with magmatic lineations and shear indicators, suggests emplacement and construction during transpression.

The transition from Tuross Tonalite to Kameruka Granodiorite and to Murrumbidgee Batholith represents a series of plutons all of which were initially localized within a migmatite shear.

However, different stress regimes during emplacement of the different plutons, (low to high, respectively) are reflected in the shape and style of emplacement. Chamber shape is not controlled by the migmatites but the conditions of regional stress at the time of emplacement.

213

PATHWAYS AND MODES OF FLUID RELEASE FROM GRANITES IN THE CORES OF ACTIVE OROGENIC BELTS John Ridley Department of Earth and Planetar>^ Sciences, Macquarie University, NSW 2109 Many metamorphic belts, in particular low-pressure belts, are associated with voluminous synorogenic granite plutons and batholiths that mtruded to and crystallised at moderate depth in the cnist. The large volume of these intrusions means that they may have made a significant contribution to the crustal fluid budget during the metamorpliism of surrounding and overlying rocks. Additionally, the granites are a potential source for mineralising hydrothermal fluids.

The Southwest Gneiss Terrain and adjacent parts of the Southern Cross Province of the Archean Yilgam Craton of Western Australia is dominated at the present level of exposure by multiple suites of granites that have been deformed to variable degrees, and that crystallised over a 60 m.y. period from 2690 to 2630 Ma (Nemchin & Pidgeon, 1997; Qiu & Groves, 1999). Granite cr>/stallisation depths were equivalent to 4 to 6 kbars pressure. The geology of adjacent areas, and of rare screens of supracrustal rocks between granites, show that at the time of intrusion, a mixed granite-greenstone terrain of supracrustal rocks and isolated granite plutons that was undergoing active orogenesis with low-pressure metamorphism covered the granites.

Three t>T3es of structures within the granites potentially mark sites of fluid movement or release: - Rare planar and laterally continuous quartz veins; - Localised swarms of planar sheeted to stockworked quartz-feldspar pegmatites; - Swarms and isolated pegmatitic quartz-feldspar pipes.

The pegmatitic pipes are of particular interest as textures and relations suggest that these are structures marking fluid segregation fi-om and movement through the host granites while the gramtes were partially molten. The pipes are near-vertically aligned, generally subcircular, 5 cm to 1 m in diameter, most frequently 1 0 - 2 0 cm, and worm-like and discontinuous along their length.

Euhedral quartz terminations are outlined by planes of fluid inclusions within the pipes showing that they were fluid filled during development. Fluid inclusion densities are consistent with entrapment at near liquidus temperatures given the likely pressure of crystallisation. The primary fluids of the inclusions are low- to moderate-salinity H2O-CO2 mixtures in all examples investigated.

With respect to fluid types, it is clear that, contrary to what is often assumed, low to moderate salinity C02-bearing fluids can be introduced magmatically into active metamorphic belts, and may be the dommant magmatically denved fluid in these belts. The ratios of the major fluid components of the lower salmity fluids in these granites are indistinguishable from those of orogenic gold deposit fluid. The distribution of the pipes shows that fluid release into a metamorphic terrain is most likely heterogeneous on a one to ten kilometre scale, witli the siting of pipe swarms apparently controlled by the shape and structural heterogeneity of the granite, presumably through stress fields.

References Nemchin A. A. & Pidgeon R. T. 1997. Evolution of the Darling Range batholitli, Yilgarn craton. Western Australia: A SHRIMP zircon study. Journal of Petrology 25, 625-649.

Qiu Y. & Groves D. I. 1999. Late Archean collision and delamination in the southwest Yilgam Craton: The driving force for Archean orogenic lode gold mineralization. Economic Geology 94, 115-122.

214

ORIGIN AND EVOLUTION OF HIGH TEMPERATURE SKARN FLUIDS AT FUKA, OKAYAMA, JAPAN M. Satish-Kumar, Y. Yoshida, H. Wada and 1. Kusaclii Dept of Biology & Geosciences, Shizuoka University, Sliizuoka 422-8529, Japan Dept of Earth Science, Faculty of Education, Okayama University, Okayama 700-8530, Japan Skam fomiation in the igneous-limestone contact metamorphic environment is most suited to studies of metasomatism, fluid flow and diffusion transport of elements. Most studies to date have concentrated on low to medium temperature skam fomiation resulting from hydrothermal fluids emanating from granitic intmsions. However, there is still ambiguity in the mechanism of skam formation and related processes at high temperature lOOO'^C) if accompanied by large-scale fluid flow in contact aureoles. The contact aureole at Fuka, Okayama, Japan is famous for its peculiar occurrence of extensive high-temperature skam resulting from the intmsion of Mesozoic quartz monzonite into Paleozoic marine carbonate rocks. The common high-temperature minerals found in this skam are spurrite [Ca5Si208(C03)] and gehlenite [Ca2Al2Si07]. Other associated minerals such as foshagite and hillebrandite form by hydration reactions at a later stage. The occurrence is also notable for the finding of eight new calcium-boron-bearing minerals and tens of other rare minerals. Although earlier studies were focussed on the mineralogical aspects and geochemical mass transport during the formation of skam, not much is known about the physical conditions, fluid composition and distance and quantity of fluid migration. Here we present petrologic and systematic stable-isotope results of the high-temperature spurrite-bearing skam formation at Fuka, and consider the transport of material, formation conditions and origin of fluids.

The spurrite skam examined in the present study is from a limestone mine at Fuka, where several mine faces expose the contact of igneous rock and limestone. The skam is divided in to two zones, the wollastonite zone and the spurrite zone. The wollastonite zone is very thin (few cms) and comprises a mineral assemblage of wollastonite + vesuvianite + hydrogrossular. In contrast, the spurrite zone is about 1 m in width and has a mineral assemblage of spurrite + tilleyite + hillebrandite. Petrogenetic grids were constmcted for the observed mineral assemblage in the simple system Ca0-Si02-C02 considering metasomatic aqueous silica input. Stable isotope alteration within the marble from the contact with the spurrite zone and along the skam were investigated. Millimeter-scale calcite samples were then carefully drilled out from slab samples perpendicular to the contact. Spurrite from the contact zone was reacted with phosphoric acid in vacuum to release CO2 which was measured for carbon and oxygen isotopes. The carbon isotope values at the contact of spurrite and marble show a smooth diflfusion profile, whereas the oxygen isotopes show more scattered values. The values become constant after a distance of 30 mm away from the contact with the spurrite zone. The isotope values in the spurrite zone shows a constant pattem, with a scatter controlled by the pure calcite and pure spurrite end member values.

The spurrite skam at Fuka formed during high-temperature contact metamorphism with considerable amounts of material transport from the intmsive quartz monzonite. Average aqueous silica concentration of the fluid was about 2.5 x 10^ mol/litre. Temperature conditions during spurrite formation were between 980''C and 1080''C and Xco2 was between 0.25 and 0.42. Largescale carbon and oxygen isotope shifts in the spurrite zone were caused by the combined effect of decarbonation and massive fluid flow. Earlier geochemical studies also suggested extensive mass transport during skam formation.. The carbon and oxygen isotope profile within marble from the spurrite-marble contact indicates that carbon has moved by lattice diffusion, whereas oxygen moved by both lattice and grain boundary diffusion. Also, it is found that during high temperature (>1000®C) skam formation the diffusion constants of carbon and oxygen species are similar, in contrast to low temperature hydrothermal skam-forming fluids, where oxygen diffuses several times faster than carbon.

215

Sr-Nd ISOTOPIC CONSTRAINTS ON ORIGIN OF STANTHORPE GRANITE GROUP: ROLE OF A DEPLETED MANTLE-DERIVED COMPONENT W. J. Sivell Division of Eai th Sciences, The University of New England, Aimidale 2351.

Systematic isotopic trends for Penno-Triassic post-tectonic granitoids, mafic microgranular enclaves (MME) and crosscutting mafic-felsic dykes, as well as intimately associated gabbros and (hybrid) diorites from the Stanthorpe Granite Group (SGG), part of the Moonbi Supersuite, imply a range of interactions among discrete source components. The bulk of SGG granitoids and MME possess unifonn initial Sr and Nd isotope values (Srinit~0.7045±.0002; sNd-+2). Like the Moonbi to the south, the SGG are isotopically primitive, despite their felsic nature, indicating that they were derived from only slightly older source rocks.

The SGG (high-K) granitoids (chiefly granodiorite, monzogranite and syenogranite, along with alkali granite and rare monzodiorite) are chemically, mineralogically and isotopically distinct from tonalites and granodiorites of the Clarence River Supersuite (CRS) to the east, wliich possess more marked Cordilleran affinities. Whereas the CRS represents partial melting of basahic underplate, the Moonbi-SGG are enriched to ver>^ high levels in Sr, Ba, Pb and Th like potassic rocks of the Sierra Nevada Batholith, suggesting distinctly shoshonitic source rocks. Initial Sr-Nd isotopic ratios of these rocks are similar to those of other igneous rocks sourced in sub-continental lithospheric mantle (SCLM) (e.g. NSW leucitites). Thus the granites may have formed by melting of (shoshonitic) material only a short time previously extracted from a slightly depleted SCLM source and underplated in the post-collisional setting proposed for these granitoids.

Young SCLM may only recently have formed by slab modification of a supra-subduction zone mantle wedge. Many SGG granitoids are charged with MME incorporated as mafic magma globules into host felsic magmas (Sivell & Passmore 1999). The MME have isotopically unevolved compositions (Srinit=0.7045) closely similar to those of SGG granitoids and inferred source "shoshonitic" underplate.

While potentially reflecting equilibration due to (rapid) liquid state isotope difiusion with host granite magma, MME isotopic compositions may directly reflect partial melting of mafic underplate or shallow SCLM. Isotopic values of gabbroic rocks (eNd-^3.7; Srinir-0.7040-0.7044), essentially coeval with SGG granitoids, lie on the join between the isotopic composition of the MME/(inferred SCLM source), and that calculated for Penno-Triassic MORB-source depleted manfle (DM) (BNd-^8.5; Srinit'-0.7026). The gabbros reflect involvement of a source which is more depleted than the young Tithospheric' source for the bulk of the granitoids and MME, and most likely represent mixing between asthenospheric and lithospheric mantle components. They imply an ongoing role for asthenospheric manfle at shallow depths during development of the SGG, compatible with its occult presence identified by slightly depleted Nd-isotopic compositions for the granites. Other rocks with compositions straddling tliis DM-SCLM trend include the earliest post-collisional mafic dikes in Gympie Province, SE Queensland (i.e. Langton Dolerite, sNd = +6.2 to +6.7). Furthermore, isotopic compositions of rocks from layered mafic intrusions (sNd = +3.5 to +6.5; Srinit = 0.7028-0.7049) further north in Queensland also neatly encompass isotopic values for SGG gabbros and MME. Mafic rocks from oriented dike suites at Stanthorpe are likewise isotopically similar to the gabbros. Significantly, the occurrence of some zoned dykes with mafic rims and felsic cores, isotopically akin to SGG gabbros and granites respectively, attest to coexistence of (depleted mantle derived) mafic and (crustal) granitoid melts. Importantly, numerous mafic to (hybrid) dioritic SGG intrusions have lower Srinit (--0.7034-0.7038) and slightly higher eNd values (+4.0) than the gabbros, despite their more felsic compositions, and define a discrete trend between compositions of SGG granodiorite and that inferred for DM or compositionally primitive crust derived from an isotopically depleted mantle source (e.g. young subducted lithosphere, island arc basement or andesitic lower crust (LC)). Similarly, GEMOC zircon 176H£/177Hf isotopic data for Stanthorpe Granite at Bald Rock indicate the presence of one magmatic endmember with a distinctively crustal Hf isotopic signature, and a more felsic magma derived from either the mantle or a veiy young crustal source (young manfle-derived mafic intrusives into the LC?). A felsic yet isotopically depleted component derived from a young, possibly eclogitic source is also required in the genesis of Permo-Triassic post-collisional microdiorite dykes associated with Au in Gympie Province (eNd = +4.3 to +5.7), and possessing shoshonitic affinities (Sivell and McCulloch, 2001). The post-collisional transform setting is intrinsic to the involvement of these source components. A relatively mafic SGG quartz monzodiorite with numerous migmatized sedimentary inclusions shows isotopic compositions modified by an additional eastern Australian Palaeozoic greywacke end-member.

References Sivell, W. J. and McCulloch, M. T. 2001. Geochemical and Nd-isotopic systematics of the Penno-Triassic Gympie Group, southeast Queensland. Australian Journal of Earth Sciences 48, 3 77-393.

Sivell W. J. and Passmore, M. J. 1999. Stanthorpe Granite Group -1: Petrologic diversity and the results of recent 1:10000 mapping. In: Flood P. G. ed. New England Orogen: Regional Geology, Tectonics and Metallogenesis. University of New England, Armidale, pp.

216

EXPERIMENTAL CONSTRAINTS ON THE ROLE OF BORON IN THE LOWPRESSURE FLUID-ABSENT PARTIAL MELTING OF METAPELITES FROM THE MT STAFFORD AREA, CENTRAL AUSTRALLV Esme Spicer\ Gary Stevens\ Ian Buick^ and John Terlet^ ^Department of Geology, University of Stellenbosch, Private Bag XL Matieland 7602, South Africa "Department Of Earth Sciences, LaTrobe University, Bundoora, Victoria 3086, Australia ^CEMMSA, University of Adelaide, South Australia 5005, Australia The Mt Stafford terrane in the Arunta Inlier of central Australia exposes a sequence of greenshist to granulite facies metapelitic and metapsammitic rocks that are unusually in having undergone partial anatexis and migmatization beginning in the andalusite stability field. Field evidence mdicates substantial partial melting in the metapelites, although there is little evidence for melt segregation.

In contrast, the metapsammites are interpreted to have melted little or not at all. Bulk chemical variations have been used to account for the difference in melting behaviour between the two metasedimentary rock types, and the potential melt fluxing influence of boron has been discussed as possibly accounting for muscovite melting at temperatures low enough to allow anatexis in the andalusite stabilit>^ field. In order to better constrain the latter variable, this study has investigated the fluid absent partial melting behaviour of 6 different metapelitic compositions over the PT interval of 2.9 to 3 .2 kbar and 1000 to 750°C. Four natural samples were selected, two aluminous metapelites and two relateively I0W-AI2O3 metapelites. In each group one sample was chosen with low H2O (0.50-0.54 wt%) in samples 7 and 8 respectively) and one with relatively high H2O (0.91.71 wt%) in samples 70 and 71 respectively. Two additional compositions were generated by adding tourmaline to sample 8, resulting in boron concentrations of approximately 1 and 5 wt% respectively. The results of the experiments between 800 and 1000° C on the B-poor samples are detailed in the table below.

High Al metapelites Low Al metapelites Run Products IVIelts Run Products T IVIeltVo. New Residual Mg# A/CNK T Melt%. New Residual 1000 84 Spl 32 1.84 1000 69 Grt,Spl 950 57 Spl, Crd Fsp, Bt 29 1.37 950 54 Spl, Crd 8 900 39 Spl, Crd Fsp, Bt 56 1.44 7 900 31 Spl, Crd Fsp, Bt 850 19 Spl, Crd Fsp, Bt 37 1.83 850 22 Crd Fsp, Bt 800 18 Spl, Crd Fsp, Bt 45 1.98 800 16 Fsp, Bt, And 1000 81 Spl 35 1.69 1000 54 Spl,Grt 950 79 Spl 36 1.65 950 48 Spl, Crd 70 900 72 Spl 36 1.61 71 850 37 Spl, Crd Fsp 850 58 Spl, Crd Bt 26 1.62 800 46 Spl, Crd Bt.And 31 1.55

IVIg# A/CNK 38 1.87 28 2.10 36 1.34 44 1.55 37 1.46 33 1.47 17 1.78 18 1.80

All the expermients produced peraluminous granitic melt compositions. The highest melt fractions occurred in the most water-rich, high-Al metapelites. At 750® C melt fractions in all samples were low (< 5%). The tw^o boron-enriched samples appear to have melted more at this condition than the equivalent boron-poor samples, but the effect is minor and subtle. To account for andalusite as a reaction product of wet melting of muscovite, the wet muscovite solidus would have to be depressed by at least 70° C. These experiments in the fluid-absent system suggest this is unlikely.

Water is the primary control on melt proportion in the high temperature runs Boron concentration, AI2O3 concentration and Na/Ca ratios all exert a secondary control on melt proportion at temperatures well above the solidus. The presence of tourmaline and the Na/Ca ratio exert a primary but minor control on the temperature of the dry muscovite solidus. These findings, extrapolated to the wet melting system, do not appear to support the hypothesis that boron allows muscovite melting in the andalusite stability field 600'' C at 3 kbar).

217

MELT COMPOSITIONS FROM PARTIAL MELTING EXPERIMENTS ON METAPELITES: IMPLICATIONS FOR A RESTITE COMPONENT IN S-TYPE GRANITES, AND DECOMPRESSION TEXTURES IN GRANULITES Gary Stevens^ and Jolin D. Clemens^ ^Dept of Geolog}^ University of Stellenbosch, Private Bag XI, 7602 Matieland, South Africa -School of Earth Sci. & Geog., Kingston University, Penrhyn Rd, Kingston-u-Thames, Surrey KTl 2EE, UK

This study examined the major-element melt compositions produced m 17 studies of fluid-absent partial melting of alummous metasediments, over the range 750 to 1000 X and 0.3 to 1.0 GPa. The experiments employed a range of natural and synthetic starting compositions, and were conducted over a broad range of P-T conditions. However, the melt compositions produced are strikingly similar and can all be classified as peraluminous leucogranites. Peraluminous granites, particularly those representative of hot, relatively HsO-undersaturated magmas capable of intruding the upper crust, have been shown to form through fluid-absent melting reactions that involve the breakdown of muscovite and biotite in metasedimentary rocks. Thus, in comparison with data on the compositions of natural S-type granites, the compositions of the experimental melts can be used to constrain aspects of granite petrogenesis. In many of the experimental studies, melt proportions were constrained by either rniage analysis of SEM backscattered electron images, or mass balance calculations. In conjunction with the melt and starting mixture compositions, these data can be used to model the bulk compositional shifts (fi-om protoliths to residua) that must occur during melt segregation episodes in granulite-facies rocks.

The experimental melt compositions have been compared with the compositions of 769 Australian Stype granites, and 211 S-type granites from the Cape Granite Suite in South Africa. The data include both volcanic and plutonic rocks. In general, on mol proportion ternary projections from SiOa and H.O, plots of A (AI2O3) vs FM (FeO + MgO) vs CNK (CaO + Na20 + K2O) show a poor correlation between the natural granite compositions and those of the experimental melts. Only about one third of granites fi-om either database plot within the field defined by the experimental melts. In addition, about half the experimental melts plot in a field that coincides with no natural granite compositions. Typically, experimental glasses are more leucocratic and aluminous than natural granites. The natural granite compositions can be modelled as mixtures of the experimental glass compositions, plus garnet or cordierite - common restitic phases left after partial melting reactions that mvolve biotite breakdown in metasedunents. The interpretation of such chemical features is, however, complicated by the fact that many natural granitic rocks represent melts with accumulated early magmatic Grt, Opx, Crd, Bt and PI. Also, there is isotopic and petrological evidence for the presence of a minor, juvenile, mantle-derived component in some S-ty^e rocks.

Extraction of the experimental melt compositions from their protoliths would results in no systematic shift in A values of the bulk compositions on an AFM projection from K-feldspar, quartz and H2O. However, the bulk rock Mg#s (molecular MgO/MgO + FeO) of residua are always shifted to higher values as a result of melt extraction. Quantitative extraction of the melt fraction formed at 900 T would typically shift the residuum Mg# to values 2 to 6% higher than that of the protolith. In a broad range of relatively magnesian bulk compositions, where assemblages of either Grt + Crd + Sil, or Grt + Crd + Opx coexist at granulite-facies P-T conditions, this shift would cause the proportion of garnet to decrease via the reactions Grt + Sil + Qtz = Crd or Grt + Qtz = Crd + Opx. Textural evidence for these reactions in granulites is generally regarded as indicating decompression.

However, in restitic or migmatitic granulites, where melt segregation has resulted in bulk compositional shifts, this is not necessarily the case.

218

FLUIDS IN EQUILIBRIUM WITH SILICA-UNDERSATURATED MAGMAS IN THE SYSTEM NA2O-AL2O3-SIO2-H2O: CLUES TO THE COMPOSITION OF FENITIZING SOLUTIONS Robin F. Preston\ Gary Stevens^, Terence S. McCarthy^ ^Economic Geology Research Institute, University of the Witwatersrand, Private Bag 3, PO Wits, 2050, South Africa "Department of Geolog}^ University of Stellenbosch, Private Bag XL 7602 Matieland, South Africa ^Department of Geology, University of the Witwatersrand, Private Bag 3, PO Wits, 2050, South Africa Fenites result from alkali metasomatism associated with silica-undersaturated alkaline magmatism, and are characterized by addition of alkalis, iron and magnesium, albitization, nephelinization, removal of silica and the formation of alkali pyroxenes and amphiboles. In an attempt to constrain the fluid compositions involved, we have investigated the compositions of the aqueous fluid phases m equilibnum with various silica-undersaturated alkaline magmas, in the simplified system AI2O3Na20-Si02-H20 at SSC^C and 1 kilobar. Experiments were run for 120 hours in cold-seal pressure vessels, housing a platinum capsule containing an Al203-Na20-Si02 mixture and up to 50 wt% deionised water. Starting compositions straddle the nepheline-albite join, and include peralkaline and alkali-granitoid compositions.

The quenched run products all contained a homogeneous glass and an aqueous fluid, and, in most cases, a radial crystalline phase which often occurs as spherical beads. These never occur as mclusions in the glass and are interpreted to be a fluid quench phase. Several glasses also contained albite, nepheline or quartz crystals. Starting compositions plotting to the peralkaline side of the nephelme-albite join in Al203-Na20-Si02 space produced fluids that were highly enriched in dissolved solids (Si02, AI2O3 and Na20, in the range 30 - 50 wt%) resulting in substantial; fractionation of the quenched glass composition away from that of the starting mixture. Fluid compositions in crystal-free runs were calculated using a mass balance approach that incorporated the composition of the glass (determined by EMPA), composition of starting materials and carefiilly determined masses of the run products and starting materials. A typical calculated fluid composition, produced in conjunction with a melt consisting of Si02 = 33.91, AI2O3 = 30.23, Na20 = 19.98 andH20 = 15.88 (all m wt%), is: 19.31 wt% Si02, 6.11 wt%Al203, 15.88 wt% Na20 and 58.40 wt% H2O. Estimates of melt-H20 contents, based on the difference between the weight percent totals of the carefiilly controlled electron microprobe analysis and 100% of quenched hydrous glasses show systematic variation with respect to melt composition, decreasing with increasing Si02 content.

Model calculations indicate that the experimentally produced fluids are capable of converting a granite to a nepheline syenite composition at low fluid/rock ratios. Albitization and removal of quartz (in the form of sodium metasilicate), formation of sodic pyroxenes (acmite) and ultimately nepheline are characteristic of the process modeled here, and are analogous to the general features and processes observed in some natural fenites. In addition, model calculations indicate that the alteration is accompanied by significant volume increase (up to 33%), which could be responsible for granulation textures commonly observed in fenite aureoles.

219

SHRIMP U-Pb GEOCHRONOLOGY OF PEGMATITES FROM THE HARTS RANGE, CENTRAL AUSTRALIA J Angela Storkev'. Ian Williams" & Ian Buick' ^ Department of Earth Sciences, La Trobe University, Bundoora 3086, VIC, Australia "Research School of Earth Sciences, Australian National University, Canberra 0200, ACT, Australia

The Harts Range forms part of the southeastern Arunta Inlier; it has a complex Palaeoproterozoic and Palaeozoic metamorphic and deformation histor>'. The timing of high-grade metamorphism in the Harts Range is now well established as bemg Ordovician (c. 470 Ma). However the age of emplacement of extensive pegmatite swanns (Harts Range pegmatites), which were mined extensively for muscovite up until the 1940s, is largely unstudied. These pegmatites fonn sills and dykes up to 2 km long and locally 50 m wide, and intrude all three major lithological associations in the Harts Range: the Harts Range Metamorphic Complex (HRMC), the stnicturally underlying Entia Gneiss Complex (EGC), and the Bruna Granitic Gneiss, which separates the HRMC from the EGC. The pegmatites are particularly abundant in the upper metasedimentary unit of the HRMC (Brady Gneiss) compared to the mafic unit of the HRMC, suggesting that they may represent partial melts fonned during peak metamorphism and therefore may be locally derived (Hand, Mawby, Miller et al., 1999). The pegmatites are coarse-grained and contain a common mineral assemblage of qtzplagiJcsp±grt±bt+ms±tur±zr±mon ±ttn±ap±cc±hbl, and they vary in orientation. The pegmatites contain weakly to strongly developed foliations and display discordant to concordant relationships with the surroundmg rocks, implying that the timing of pegmatite emplacement is broadly late synkinematic (Hand, Mawby, Miller et al, 1999). However, it is unclear whether pegmatites were emplaced during single or multiple events. Only two pegmatties have been previously dated: a folded pegmatite in the EGC gave a SHRIMP U-Pb zircon age of c. 330 Ma (Hand, Mawby, Kinny and Foden, 1999), and a weakly deformed biotite granite from the Mallee Bore region gave a SHRIMP UPb zircon age of c. 387 Ma (Buick et al., 2001). This study presents prelimmary SHRIMP U-Pb zircon and monazite data to fiirther constrain the age of emplacement of the Harts Range pegmatites.

Initial SHRIMP U-Pb analyses of monazite or zircon from five pegmatites from the HRMC gave a range of Palaeozoic ages. Monazite from pegmatites at three different locations gave ages of c. 360 Ma, c. 415 Ma and c. 440 Ma. Zircon from a pegmatite gave an age of c. 466 Ma, and zircon from a pegmatitic vein within a metabasic rock gave an age of c. 474 Ma. These ages are similar to those from previously dated Harts Range pegmatites and may indicate that pegmatite emplacement did not occur during a single event, but during several events over a period of about 150 million years.

References Buick, I. S., Miller, J. A., Williams, I. S. and Cartwright, I. 2001. Ordovician high-grade metamorpliism of a newly recognised late Neoproterozoic terrane in the northern Harts Range, central Australia. Journal of Metamorphic Geology 19, 373-394.

Hand, M., Mawby, J., Kinny, P. and Foden, J. 1999. U-Pb ages from the Harts Range, central Australia: evidence for early Ordovician extension and constraints on Carboniferous metamorphism. Journal of the Geological Society, London 156, 715-730.

Hand, M., Mawby, J., Miller, J., Ballevre, M., Hensen, B., Moller, A. and Buick, I. S. 1999. Tectonothermal evolution of the Harts and Strangways Range Region, eastern Arunta Inlier, central Australia. Geological Society ofAustralia, Specialist Group in Geochemistry, Mineralogy and Petrology Field Guide 4, 73.

220

THE MAGNITUDE AND THREE-DIMENSIONAL GEOMETRY OF REGIONALSCALE FLUID FLOW DURING HARROVIAN METAMORPHISM, SOUTHEASTERN VERMONT, USA B. A. Wing^ and J.M. Ferry' ^Department of Geology and Earth System Science Interdisciplinary Center, University of Mar>dand, College Park, MD 20742 USA, wing@essic.umd.edu 'Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218 USA Introduction The stable isotopic and mineralogic effects of regional metaniorphic fluid flow are well documented but its magnitude and geometry remain the subject of active debate. We addressed the magnitude and three-dimensional geometry^ of metamorphic fluid flow on the regional-scale through an areally-integrated inverse approach. After validating the inverse method with the results of simple models of stable isotope alteration and fluid flow, we applied it to petrologic and stable isotopic data to solve for the full, three-dimensional metamorphic fluid flow field over an area of -120 km" in a Barrovian terrain in southeastern Vermont, USA. Inverse technique and validation Isotopic and petrologic studies of metamorphic fluid flow during metamorphism typically employ a locally enforced, time-integrated tracer mass balance equation that relates changes in the amount of tracer component / in the fluid during metamorphism, Ri, (recorded by prograde mineral reactions and shifts in whole-rock isotopic compositions) to the time-integrated fluid flux vector, q , the tracer concentration in the fluid, C,, and any volatile gain or loss during metamorphism, Rfj.

(1) Here the n term recognizes the effects of processes other than advection or reaction (e.g., kinetics, diffusion, hydrodynamic dispersion). A finite-element analogue of (1) was constructed from the predictions of an analytical model of alteration due to density-driven fluid flow. The predicted distribution of compositions was inverted for the original flow field by minimizing a combination of the non-advective misfit in (1) and the smoothness of the resulting flow field.

Except for the most coarsely resolved distributions, the inversion returned a flow field that matched the original flow field over greater than 95% of the inverse domain. Tracer data (Xco2, ^^O, ^^C) from metacarbonate rocks in a Barrovian metamorphic terrain in southeastern Vermont provided a field test of the inverse method.

Field inversion and implications In southeastern Vermont, metamorphism occurred at a peak pressure of - 7 kbars and at peak temperatures of -500 to -575 Metacarbonate rocks of the Waits River Formation developed the following metamorphic zones with increasing grade of metamorphism: oligoclase, biotite, amphibole, and diopside. Diopside-zone rocks form a thin aureole around a small granite pluton in the terrain. For fluid in equilibrium with 38 samples of the Waits River Formation at the peak of metamorphism, Xco2 values, ^^O compositions, and ^^C compositions range from 0.02 to 0.26 mol CO./mol fluid, 11.2 to 19.3 %o, and -4.0 to 4.0 %o. Changes m whole-rock CO2, ''O, and ^^C were calculated from the estimated modal abundances and isotopic compositions of each sample's immediate lower grade equivalent. Calculated whole-rock CO2, and ^^C changes vary from 0 to 3.6x10 ' mol C02/cm' rock, 1.20x10 ' to 2.29x10"^ %o mol O W rock, and -4x10-^ to 6x10"^ %o mol C/cm^ rock. A least-squares inversion of this data returned the magnitude and full threedimensional geometry of regional metamorphic fluid flow at the peak of metamorphism. Locally, flow was non-uniform and concentrated near exposed and subsurface granitic plutons. Timeintegrated magnitudes range from - 0 to >10"^ mol fluid/cm^ rock and directions vary from vertical to horizontal. Over the entire -120 km^ of the terrain, an average of -10^ moles of fluid passed through each square centimeter of rock. The inversion predicts that the regional-scale orientation of lithologic layering controls the gross geometr>^ of fluid flow during metamorphism.

221

IS THE FINAL SUTURE BETWEEN EAST AND WEST GONDWANA IN EAST ANTARCTICA? ^ S.D. Boger\ C J.L. Wilson^ & C.M. Fanning^ Australian Crustal Research Centre, Monash University, Victoria 3800, Australia ^ ^School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia -Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia U-Pb SHRIMP dating from east Antarctica point to the existence of a laterally contmuous belt of late Pan-African age (-550-500 Ma) metamorphics that stretch from the Antarctic coast at Prydz Bay at least 700 km inland to the southern Prince Charles Mountams. This orogemc belt is interpreted to represent the youngest suture between the components of Gondwana. This scenario is consistent with: (1) the abrupt termmation of -990-900 Ma tectomsm recognised in the Rayner Province-Eastern Ghats regions of Antarctica and India against Pan-African age orogemc belts to both the east and the west, (2) the lack of equivalent-aged terranes (-990-900 Ma) found elsewhere in Antarctica, or other previously adjacent continents, and (3) by the distinct detrital-zircon populations obtamed from either side of this proposed suture. If this belt indeed represents a suture, it defines the eastern margin of a separate lithosphenc block that consists of a large section of east Antarctica and India that did not form part of east Gondwana or Rodima as they are currently reconstaicted. This has implications for the construction of both supercontinents.

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RAPID DEFORMATION, EXHUMATION AND TECTONIC SWITCHING DURING THE ROSS OROGENY, PENSACOLA MOUNTAINS, ANTARCTICA. Mike Curtis and Ian Millar British Antarctic Survey, High Cross, Madingley Road, Cambridge, UK.

During end-Neoproterozoic to Early Palaeozoic times, the southern margin of Gondwana was affected by widespread subduction related orogenesis. In East Antarctica this event is known as the Ross Orogeny, the product of which is exposed along the 3,000km length of the Transantarctic Mountains. In the Pensacola Mountains, the Ross Orogeny is considered to represent a prolonged period of deformation composed of two distinct contractional deformation events, punctuated by a period of Early to Middle Cambrian extension and sedimentation.

The oldest and most intense of the Ross deformation events in the Pensacola Mountains (Early Ross orogeny) affected a sequence of metagreywackes (Hannah Ridge Formation), producing tight to isoclinal, shallowly plunging, Fi folds that possess a slaty, axial planar cleavage. Steeply plunging asymmetric F2 folds and their associated cleavage are superimposed on the NNE-SSW Di structural grain, forming a consistent clockwise oblique angle. Together with several shear sense indicators, the angular relationship of D2 and Di suggests they formed as a result of sinistral reactivation of Di structures. At Serpan Peak, new meso and micro-structural observations of the Serpan Peak granite indicates that the earliest, granodiorite phase was intruded prior to, and subsequently deformed by the regional Di deformation event. These foliated granodiorites were in turn crosscut by biotite granite sheets, microstructural analysis of which indicates they were emplaced syntectonically during D2 deformation.

New U-Pb SFIRIMP ages for these granite phases constrain the emplacement ages, and by structural relationship, the early Ross Di and D2 deformation events as being 505±3 Ma. Given the within error ages for these deformation events, we interpret them as being the product of a single progressive sinistral transpressional deformation event. Unconformably overlying the deformed Hannah Ridge Formation are late Middle Cambrian limestones. Accepting 500 Ma as the Middle / Late Cambrian boundary, our new structural and geochronological constraints imply that early Ross deformation occurred rapidly, in conjunction with relatively rapid exhumation from lower greenschist facies (~3 mm/a). Exhumation was closely followed by an extensional event at 500 Ma, characterized by bimodal volcanism and interpreted as a backarc basin (Rowell et al. 2001).

By the latest Cambrian to Early Ordovician times the Pensacola Mountains region experienced a second, less intense contractional deformation event, the Late Ross orogeny.

Our new data reveal that contrary to existing models the Ross Orogeny, in the Pensacola Mountains at least, was a condensed sequence of rapidly changing tectonic events. Such tectonic switching is seen at retreating subducting margins, where the intermittent arrival of buoyant oceanic plateaus result in flat slab subduction and transient periods of contractional deformation and inversion in the backarc region. Given the limited length of plate margin likely to be affected by such flat slab subduction events, correlation of stratigraphic and tectonic events along the entire length of the Ross orogen, and its contiguous orogens, e.g. the Delamerian fold belt, are unlikely.

Reference Rowell A.J., Van Schmus, W.R., Storey, B.C., Fetter, W.R. & Evans, K.R., 2001. Latest Neoproterozoic to Mid-Cambrian age for the main deformation phases of the Transantarctic Mountains: new stratigrapliic and isotopic constraints from the Pensacola Mountains, Antarctica. Journal of the Geological Society, 158, 295308.

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A CLOSER EXAMINATION OF THE DIRECT LINKS BETWEEN SOUTHERN AUSTRALLV AND TERRE ADELIE AND GEORGE V LAND, ANTARCTICA ^

C. MarkFaiming\ Rene-Pierre Menot\ Jean Jacques Peucat^ and Anne Pelletier^ Research School of Earth Sciences, The Austrahan National University, ACT 0200, Australia ^Umversite J. Monnet, UMR CNRS 6524, 42023 - Saint Etienne cedex 2, France. ^ Geosciences Rennes, UMR CNRS 6118, 35042 - Rennes cedex, France

The Gondwana configuration of Australia and Antarctica places the southern margin of Australia, notably Eyre, Yorke and Fleurieu Peninsulas in contact with the coast of Terre Adelie (TA) and George V Land (GVL). This link across the Southern Ocean is precisely constrained by the presence of relatively low grade, c. 1710-1765 Ma garnet-magnetite bearing phyllites that crop out at Cape Hunter m Commonwealth Bay, GVL and on Price and Golden Islands, and in drill core from Coffin Bay Peninsula, South Australia. With this piercing point firmly established, one can examine further the comparisons between the Gawler Craton (GC) of South Australia and the Terre Adehe Craton (TAC) of TA and GVL within the Mawson Continent. For example, in detail the strong magnetic anomaly associated with the phyllites is composite and more recent work in the Mt Hope area of southern Eyre Peninsula (SEP) reveals an Archaean volcanic sequence that is yet to be identified within TA or GVL. The oldest sequence in SEP is a centrally located, older supracrustal sequence (Camot Gneisses CG) that underwent granulite facies metamorphism at c. 2420 Ma. To the west, a major terrane boundary exists between the lower crustal CG and high crustal level granitoids of the Button Suite; the latter seen to intrude amphibolite facies, grey layered gneisses (Wangary Gneiss) interpreted as a low grade equivalent of the CG. A similar relationship can be inferred for TA and GVL. The granodiorite at Cape Denison intrudes grey layered gneisses, though as within SEP, tectonic overprinting by the c. 1710 Ma Kimban Orogeny gives rise to complex shear zones that mask many original relationships. In both SEP and TAC, the high level granitoid suite was emplaced between c. 2520-2560 Ma. Nd systematics show that it is derived from a relatively primitive source and not from melting of a CG type source.

The Cape Gray to Stillwell Islands area of GVL comprises gametiferous paragneisses correlated with the CG of the SEP. U-Pb zircon ages and Sm-Nd systematics similarly document the presence of sigmficantly older crust at c. 3100-3200 Ma. New SHRIMP zircon and monazite data for the GVL paragneisses place the high granulite event between c. 2420-2440 Ma. The presence of a 1710 Ma overprint is also recorded in GVL and so the Kimban Orogeny can be traced to the TAC. Thus far, there is no record of the c.2000 Ma Miltalie event in the TAC. However, more detailed studies may identify the Miltalie event, as m the SEP it is interpreted to juxtapose the upper (granitoid) and lower (paragneiss) crustal blocks, enabling deposition of the Hutchison Group across this whole basement.

The 1850 Ma Donnington Suite forms a major batholith along the eastern margin of the SEP and across to western Yorke Peninsula. To the east of the Mertz Glacier in GVL, a porphyritic granite at Pengum Point was thought to be a correlative. However new SHRIMP zircon ages for this granite, and a red granite at Cape Webb document c. 515 Ma and c. 500 Ma magmatism respectively; with abundant older inheritance to c. 3000 Ma. Clearly these are part of the Ross Orogen and, as also supported by Nd systematics, the granitoids cropping out between the Mertz and Ninnis Glaciers are now correlated with the Cape Willoughby Granite of Kangaroo Island and others in the Adelaide Fold Belt.

The presence of Ross-aged magmatism immediately east of the late Archaean granitoids and paragneisses of the TAC is the focus of ongomg studies. Significantly, the Mertz Galcier may correspond to the boundary between the TAC of the Mawson Continent and the NeoproterozoicOrdovician Fold belt of the Ross-Delamerian Orogen at the Pacific margin of Gondwana.

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PALEOPROTEROZOIC TECTONICS OF THE TERRE ADELIE CRATON (EAST ANTARCTICA) Pelletier, Gapais, D.^ Menot, R.P.\ Fanning C.M.I Peucat, J.J.' ^ Universite J. Monnet, UMR CNRS 6524, 42023 - Saint Etienne cedex 2, France. " Geosciences Rennes, UMR CNRS 6118, 35042 - Rennes cedex, France ^Research School of Earth Sciences, The Australian National University, ACT 0200, Australia The Terre Adelie Craton (TAC) consists of Paleoproterozoic (1.7 Ga) metasedimentar}^ sequences (Pointe Geologie-Cap Bienvenue area) in tectonic contact with Archean blocks (Port Martin Commonwealth Bay area). The Paleoproterozoic rocks of the TAC show two types of structural domams: domes preserving early horizontal structures and vertical shear zones striking 340-360°.

Domes show a flat-lying to gently dipping composite foliation. Bulk flattening strains are revealed by double boudinage of leucosomes and mafic dykes. A NNW striking stretching lineation and cm-scale shear bands attest to dominant NNW sub-horizontal flow. Vertical shear zones, up to km-scale in width, contain a vertical composite foliation. Associated stretching lineations have variable plunges and become stronger toward the Archean eastern block. Strain indicators (deformed dykes and leucosomes, mafic boudins) and shear band patterns indicate large flattening strains due to vertical flow, combined with an increasing component of dextral strike-slip toward the Archean block.

Vertical shear zones reflect dextral transpressive motions whereas the origin of the flat-lying fabrics remains ambiguous. In detail, flat-lying and vertical zones display complex time-relationships. At regional scale, vertical structures are dominant and post-date the horizontal ones. This suggests that domains with flat-lying foliations could have been wider during early deformation stages, and became increasingly transposed into vertical zones during progressive deformation. At a smaller scale, however, relationships can be ambiguous and the vertical foliation may be locally transposed mto sub-horizontal fabrics with stretching lineations parallel to the vertical shear zones. Such relationships could reflect possible local successions in space and/or time between vertical and horizontal flows.

Deformation-metamorphism relationships indicate that all the ductile structures, from earliest synfoliation subhorizontal folds in domes to later vertical foliations in shear zones, are coeval with migmatization, in HT- LP to MP conditions and intrusion of mantle derived-magmas. In addition, geochronological data do not provide evidence for successive and distinct tectonic and metamorphic events. Furthermore, some structural characters of domes, such as the attitude of dome long axes and stretching lineations at low angle to the strike of shear zones, are compatible with bulk transpression.

Sub-horizontal flow at a high angle to the shortening direction could account for at least part of the structure of dome-shaped domains. The HT-LP synkinematic conditions suggest a rather hot and therefore weak and a buoyant crust. Observed deformations could thus reflect combinations of, and partitioning between, transpression-induced thickening and longitudinal flow, possibly gravitydriven, at low angle to the convergence zone.

It has been proposed that the TAC and the Gawler Craton (GC) in South Australia were two parts of a continuous continental Archean-Paleoproterozoic block before the opening of the Austral Ocean.

This may be corroborated by the strain patterns observed in the TAC. Indeed, the steeply-dipping shear zones show similarities with those located at the western and eastern boundaries of the GC. They display similar NS orientations, reflect dextral transpression, and show early flat-lying fabrics marked by a principal stretch at low angle to the boundaries of the craton. Moreover, the GC and the TAC show similar lithologies, a HT-LP metamorphic event (around 1.7 Ga), and late granitic intaisions (~1.6Ga).

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EXTENSIONAL COLLAPSE OF THE LATE NEOPROTEROZOIC/EARLY PALEOZOIC EAST AFRICAN/ANTARCTIC OROGEN IN CENTRAL DRONNING MAUD LAND, EAST ANTARCTICA ^

^

J. Jacobs', R. Klemd\ C M. Fanning^ W. Bauer'' Fachbereich Geowissenschaften, Universitat Bremen, PF 330440, 28334 Bremen, Germany - Institut fur Mineralogie, Universitat Wiirzburg, Am Hubland, 97074 Wiirzburg, Germany Research School of Earth Sciences, The Austrahan National University, Canberra, ACT 02000, Australia Geologisches Institat der RWTH Aachen, WiUlnerstr. 2, 52056 Aachen, Germany

The East African/Antarctic Orogen resulted from the continent-continent collision of East and West Gondwana during the Pan-African event at c. 600-510 Ma. The collision overprinted large areas of older, mamly Mesoproterozoic crust with the metamorphism reaching granulite facies grade. The collision history is well documented by folding and thrusting, a clockwise PT-path and metamorphic zircon growth at c. 580-560 Ma (Pan-African I). The convergence was succeeded by an extensional phase, probably representing orogenic collapse of the East African/Antarctic Orogen. This Pan-African II event has been dated at c. 530-510 Ma and is characterised by largescale extensional structures, finally resulting in the post-tectonic intrusion of voluminous A2-t>T3e granitoids.

We have investigated a gabbro complex that intruded early during the Pan-African II event. Two gabbros give new SHRIMP U-Pb zircons ages of c. 525 Ma and c. 530 Ma. These ages are mterpreted as crystallisation ages and confirm the interpretation that the gabbro was emplaced during the Pan-African II event. The gabbro was subsequently intruded by a network of granite dykes and veins. Whereas the gabbro appears entirely undeformed, the granite dykes are strongly mylonitised, along extensional shear zones indicating pronounced strain partitioning of the gabbro complex. Withm the granite mylonites large tension gashes have developed during mylonitization, indicating very high stram rates. Quartz c-axis onentations from quartz of the tension gashes show a distmct cross-girdle, which formed during pure shear deformation. Fluid inclusion data from the gramte mylonites and the associated tension gashes mainly reveal recrystallization-related mtracrystallme C02-dommant mclusions. The highest densities are relatively low (< 1 g/cm^), typical for low pressure granulites. The fluid inclusion data are interpreted to represent the last stages of a retrograde PT-path that is characterised by simultaneous cooling and decompression during extensional exhumation.

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SOUTHERN AUSTRALIA 1.6 Ga FELSIC VOLCANIC-TYPE RECOGNIZED IN THE MORAINES OF THE TERRE ADELIE CRATON IN ANTARCTICA J.J. Peucat\ R. Capdevila\ C.M. Fanning\ R.P. Menot^ and L. PecoraV ^Geosciences Reimes, UMR CNRS 6118, 35042 - Reimes cedex, France ^Universite J. Monnet, UMR CNRS 6524, 42023 - Saint Etienne cedex 2, France ^Research School of Earth Sciences, ANU, Mills Road, ACT0200, Australia Terre Adelie (TA) and George V Land (GVL) constitute the Terre Adelie Craton (TAG) in East Antarctica. Outcrops are scarce and restricted to sporadic coastal exposures with some small nunataks on the continent. An alternative mechanism of gathering information about the sub-glacial geology of Antarctica is provided by the more extensive moraines that also occur along the coastal ice edge. A broad range of rock types can be observed, some are totally exotic compared to the local geological setting yet can be used to expand our knowledge of the TAG. This is particularly the case of prominent brick-red, undeformed felsic volcanic erratics which can be found along c. ICQ km of coast line of TA and GVL. All of these felsic volcanic erratics exhibit porphyritic texture with various amount of phenocrysts, ranging from 1 mm to more than 1 cm in size, surrounded by an aphanitic groundmass. Plagioclase, K-feldspar, quartz, biotite and small magnetite phenocrysts are observed in all samples. Some have small phenocrysts of clinopyroxene, clino-amphibole and ilmenite. Hydrothermal alteration which provides brick-red colour occurs in all samples.

These volcanics range in composition from rhyodacites to alkali-feldspar rhyolites. It is a hypovolcanic, pyroclastic Is. and ignimbritic magmatism which is chemically compatible with anorogenic and post-orogenic settings. It corresponds to a high temperature ferropotassic calcalkaline magmatism emplaced at c. 1.6 Ga (SHRIMP and Pb evaporation zircon ages). The initial eNd values (close to 0 at 1.6) Ga are interpreted to result from mixing of 1.6 Ga mantle-derived magmatism with crustal sources or, alternatively, derived from an old mafic depleted lower crust mixing with old felsic component to produce a source of intermediate eNd composition.

It is similar in age, in petrographical and chemical compositions to the Gawler Range Volcanics (GRV) described in the Gawler Craton (GC) of South Australia, and more specifically similar to the Lower Gawler Range Volcanics. Such similarities strengthen correlations previously established between the GC and TAC that together formed a Rodinia nucleus, the so-called Mawson Continent (MC). Moreover, the present petrological, geochemical and radiometric data give new insights on the major latest thermal event that the MC underwent before cratonization. The presence in TAC of 1.6 Ga felsic volcanics in all respects eqivalent to the GRV of the GC signifies that there is a similar large volcanic province sub-glacial in the east Antarctic craton.

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COMPARISON OF DETRITAL ZIRCON AGES IN THE PINJARRA OROGEN (WA) AND MAUD PROVINCE (ANTARCTICA): EVIDENCE FOR COLLISION OF WESTERN AUSTRALLV WITH SOUTHERN AFRICA AT 1100 MA L C. W. Fitzsimons Tectonics SRC, Dept of Applied Geology, Curtin University, GPO Box U1987, Perth WA 6845, Australia

Grenville-age rocks in East Antarctica and formerly contiguous belts in southwest Australia, India and southern Africa preserve evidence for multiple tectonic events between 1350 and 900 Ma, and although previously regarded as a single continuous collisional orogen, they are now known to comprise a number of distinct provinces juxtaposed by 550 Ma tectonism. Three major Grenvilleage domains exist in Antarctica, each with a different age for high-grade collisional tectonism: namely the Maud (1090-1030 Ma), Rayner (990-900 Ma) and Wilkes (1330-1130 Ma) provinces, which are closely related to similar rocks in southern Afnca, the eastern Ghats of India and the Albany-Fraser Orogen of Western Australia, respectively. A fourth group of Grenville-age rocks occurs as displaced blocks in the Neoproterozoic Pinjarra Orogen of Western Australia, which preserve evidence for high-grade tectonism at 1090-1030 Ma. This age range corresponds'closely with that of the Maud Province, raismg the intrigumg possibility that the two are related.

Grenville-age rocks m the Pmjarra Orogen comprise 1090 Ma granitic orthogneiss in the Leeuwin Complex and psammitic to pelitic paragneiss in the Northampton and Mullingarra complexes, deformed and metamorphosed to amphibolite or granulite facies at 1080-1030 Ma. This tectonism is traditionally believed to reflect collision of Australia with India at c. 1100 Ma, but this need not be the case given that East Gondwana is now known to have assembled at 550 Ma. The Maud Province is interpreted as an 1150-1100 Ma magmatic arc and back-arc basm, developed at the margin of an unexposed craton that collided with the eastern margin (present-day coordinates) of the Kalahari Craton of southern Africa at c. 1100 Ma, resulting in pervasive deformation, granulitefacies metamorphism, and magmatism at 1090-1030 Ma. The unidentified craton 'is widely assumed to be the East Antarctic Shield, but again this need not be the case given the widespread evidence that East Antarctica did not assemble until 550 Ma. Evidence for the identity of the colliding cratons in both cases is provided by detrital zircon.

Comparison of SHRIMP U-Pb zircon age data for three paragneiss samples from the Pinjarra Orogen (Bruguier et al. 1999; Cobb 2000) and two samples from the Maud Province (Amdt et al.

1991; Harris 1999) reveals a number of striking similarities. 2100-1110 Ma detrital grains dominate all samples, with a marked lack of c. 1500 Ma grains. Although different samples are dominated by different populations within this range, significant detrital populations at 1100-1115, 1160-1220, 1280-1310, 1350-1390, and 1420-1440 Ma occur m samples fi-om both regions. Age spectra from the Pinjarra Orogen and Maud Province are indistinguishable within the uncertainties of the data, and imply that paragneisses in both areas were part of the same sedimentary sequence eroded from the same source rocks; i.e. they are fragments of the same collisional orogen. Pre1130 Ma detrital populations in both areas correspond to the ages of basement rocks in the AlbanyFraser Orogen and Wilkes Province, whereas 1130-1100 Ma grains were eroded from the magmatic arc exposed m the Maud Province, consistent with deposition at an active margin of the West Australian Craton. The sedimentary rocks were then deformed and metamorphosed as the Kalahari Craton collided with this convergent margin at 1100 Ma. References Amdt N. T., Todt W., Chauvel C., Tapfer M. & Weber K. 1991. Geologische Rundschau 80, 759-777.

Bruguier O., Bosch D., Pidgeon R. T., Byrne D. I. & Harris L. B. 1999. Contributions to Mineralogy and Petrology 136, 258-272.

Cobb, M. M. 2000. BSc (Hons) thesis, Curtin University of Teclinology, Perth (unpubl.).

Harris P. D. 1999. PhD thesis, Rand Afrikaans University, Johannesburg (unpubl ).

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NATURE OF THE EAST ANTARCTIC SHIELD ALONG THE PACIFIC MARGIN OF ANTARCTICA John W. Goodge Dept. of Geological Sciences, Southern Methodist Univ., Dallas, TX 75275, USA Email: jgoodge@mail.smu.edu The East Antarctic shield (EAS) is a key piece in the Proterozoic to early Paleozoic configuration of supercontinents during the transformation of Rodinia and Gondwana. Underlying most of the Transantarctic Mountains (TAM), however, are rocks that record the early Paleozoic active margin of Gondwana and its subsequent breakup beginning in Jurassic time. Thus, there are few geologic clues to Proterozoic paleogeography along fully one third of the EAS perimeter. However, a small wmdow into the Pacific margin of the EAS is provided by limited exposures of high-grade metamorphic and igneous basement in the central TAM, and the provenance signatures from Neoproterozoic to lower Paleozoic siliciclastic cover successions provide proxy samples of the EAS interior. Together, these rocks provide important constraints on the nature of the adjacent icecovered EAS. U-Pb and Sm-Nd data from the Nimrod Group document multiple geologic events spanning 2.5 b y. of Archean to early Paleozoic history, culminating in thermomechanical reworking and magmatism during the Ross Orogeny. The oldest gneisses represent Archean magmatic crust generated from juvenile mantle melts between -3.15-3.00 Ga. They are similar in age to rocks in the Denman Glacier area of Wilkes Land, suggesting an incipient period of crust formation in the EAS. Later episodes of high-temperature metamorphism, anatexis and magmatism ensued at -2.96-2.90 and -2.5 Ga. Some gneissic, eclogitic, and meta-igneous rocks record deep-crustal metamorphism and magmatism between -1.73-1.72 Ga (Nimrod Orogeny). Although cryptically preserved, these rocks indicate crustal thickening in the late Paleoproterozoic and are temporally correlated with -1.7 Ga events in Terre Adelie and the Gawler Craton of Australia. They are also similar in age and character to deep-seated events (Ivanpah Orogeny) in the Mojave province of the southwestern U.S.. The similar pattern of -1.7 Ga deep-crustal overprinting on Archean protoliths suggests that a collisional belt of this age transected the Antarctic-Laurentian region of Rodinia and may extend from the central TAM beneath the ice sheet to the Gamburtsev Subglacial Mountains or Vostok Subglacial Highlands.

Siliciclastic rocks of Neoproterozoic to early Paleozoic age along the TAM margin include narrow rift-margin deposits (<680 Ma) and a younger (-515-475 Ma) syn- to post-orogenic molasse succession. An EAS source is inferred for both assemblages based on paleocurrent indicators, depositional facies, and ages of detrital zircons. Zircons from mature passive-margin sandstones indicate distal input from the adjacent Archean and Proterozoic shield, with dominant ages of 2.8, 2.5, 1.8-1.6, and 1.5 Ga. The detrital suites also include distinguishable 1.3, 1.1, and 1.0 Ga populations that correlate with individual Grenville-age belts in the EAS and adjacent East Gondwana cratons. The detritus includes significant -1.4 Ga first-cycle zircon, suggesting erosion of a Mesoproterozoic magmatic province like the trans-Laurentian granite belt. Persistence in the younger succession, and an absence of large cratonic terrains of this age in present-day Antarctica or Australia, suggests that the trans-Laurentian granite province may extend into the ice-covered EAS. Abundant -1.8-1.6 Ga detritus is consistent with this interpretation, indicating extension of Mojave-Yavapai-Mazatzal crust into the EAS. Together, these data suggest paleogeographic linkage between East Antarctica and western Laurentia prior to -1.0 Ga.

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GEOCHRONOLOGY OF BASEMENT ROCKS FROM MARIE BYRD LAND, WESTERN ANTARCTICA ^ T. R. Ireland\ S. D. Weaver^ J.D. Bradshaw^ R.J. Pankhurst^ ^ Research School of Earth Sciences, The Austrahan National University, Canberra 0200 ^ Dept. of Geological Sciences, Canterbury University, Christchurch, NZ ^ British Antarctic Survey, High Cross, Madingley Road, Cambridge, UK

The geological history of Western Antarctica is an essential component of the proto-Pacific Gondwana margin. The record from this margin extends from the west in New Zealand to the east m South America. Marie Byrd Land (MBL) constitutes the largest physiographic region of Western Antarctica yet is characterized by poor basement exposure and poor geological control. The nature of the pre-Jurassic margin has been reviewed by Pankhurst et al. (1998).

MBL can be divided into interior (Ross) and exterior (Amundsen) provinces, with Ross Province havmg affinities to the Paleozoic record of the Gondwana margin in northern Victoria Land, western New Zealand and SE Australia, while the Amundsen Province has affinities to the Median Tectonic Zone (Triassic-Cretaceous) in New Zealand. To further the understandmg of the geochronology of the basement of MBL, we have determined the U-Pb ages of metamorphic monazites. Monazite is formed under upper-amphibolite conditions (garnet zone) and is commonly found in metapelites and paragneisses of this grade.

Alexandra and Fosdick Metamorphic Complexes, Edward VII Peninsula and Ford Ranges region, mclude high-grade gneisses as well as plutonic rocks and Cambrian-Ordovician metasedimentary rocks (Swanson Formation). The Swanson Formation has a zircon age spectrum with the Gondwana greywacke signature. The zircon protolith of the paragneisses is dominated by Paleozoic sources (ca. 320-400 Ma). Large euhedral monazites yield ages of 100 Ma with common older mheritance reaching back to 330 Ma and reinforce the Paleozoic protolith and Cretaceous metamorphism indicated by zircon and other geochronology. The metamorphism correlates with the rift-related magmatism that heralded the separation of the New Zealand block from MBL (Weaver et al. 1994).

Fatten Bluff (Hobbs Coast) includes A paragneiss enclave shows a broad, possibly bimodal, peak close to 330 Ma, with a low-age tail corresponding to post-150 Ma Pb-loss . Monazites show a bimodality with a dominant age at less than 100 Ma, and a slightly older indication at 110 Ma. Mount Petras is an isolated mountain SE of Dumas Range. It consists mainly of a Cretaceous plutonic-h>pabyssal igneous complex emplaced into deformed schistose basement. Zircons show a marked peak in the age distribution at 340-380 Ma with some older grains as well. No monazites were recovered from the Mt Petras sample.

Mount Murphy, m eastern MBL, is a dissected Cenozoic stratovolcano, built over a basement exposed on its northern flank. Zircons from orthogneiss indicate a single event at 505 Ma. Paragneiss shows a zircon age spectra related to Gondwana greywacke. Monazites indicate two ages with the dominant peak at 370 Ma and lower peak at 450 Ma. Some Pb loss is indicated.

References Pankliurst R.J., Weaver S.D., Bradshaw J.D., Storey B.C., & Ireland T.R. 1998. The pre-Jurassic margin of Gondwana in Marie Byrd Land Antarctica. Geoclironology and geochemistry of pre-Jurassic superterranes in Marie Byrd Land, Antarctica. Journal of Geophysical Research 103B, 2529-2947.

Weaver S.D, Storey, B.C., Pankhurst, R.J., Mukasa, S B., Divenere, V.J. & Bradshaw, J.D. 1994.

Antarctica-New Zealand rifting and Marie Byrd Land lithospheric magmatism linked to ridge subduction and mantle plume activity. Geology 22, 811-814.

230

HIGH-TEMPERATURE DECOMPRESSION IN ORTHOPYROXENECORUNDUM-SAPPHIRINE METAPELITES FROM THE RAYNER COMPLEX, EAST ANTARCTICA N.M. KellV'l S.L. H a r V and G.L. Clarke^ ^Department of Geology & Geophysics, University of Edinburgh, Edinburgh, Scotland, EH9 3JW "Division of Geology & Geophysics, School of Geosciences, University of Sydney, NSW, 2006

Silica-undersatiirated, high Mg-Al metapelites from the Oygarden Group of islands preserve reaction textures that suggest metamorphism during the Rayner Structural Episode (RSE) in Kemp Land was followed by apparent isothennal decompression (ITD). The Oygarden Group, which lies m western Kemp Land near the boundar>^ between the Archaean Napier Complex and Neoproterozoic Rayner Complex, is predominantly composed of Archaean crust reworked during the RSE at c. 930 Ma. Deformation during this event in the Oygarden Group is characterised by ductile thrusting at middle to lower levels (D3), followed by the development of a crustal scale, high-grade extensional shear zone (D4).

High Mg-Al metapelite occurs as a 30-100 cm diameter boudins that are enveloped by intense S4 layering. S4 is only developed in schistose rinds that occur on the boudin margins; cores preserve an S3 foliation that is oblique to S4. The boudins preserve a compositional zoning that is inferred to be metasomatic in origin and is overprinted by variations in mineral assemblage. Three types of metapelite occur: 1) garnet-poor (orthopyroxene and sapphirine, with minor corundum and sillimanite); 2) garnet-bearing (orthopyroxene, sapphirine, sillimanite, cordierite and garnet); and 3) komerupine-bearing (orthopyroxene, sapphirine and komerupine, with lesser sillimanite and garnet). In all types, phlogopite is abundant in schistose rinds. Phase relations in these high Mg-Al metapelites indicate peak metamorphic conditions of 1 kbar and 7>820''C. Based on equilibria modelled in FMAS, garnet inclusions in syn-Ss sapphirine in the core areas of all metapelite boudins suggest that peak pressures during Rayner metamorphism were attained early in D3, followed by garnet breakdown to form orthopyroxene, sapphirine and corundum through decreasing pressure during D3. In Type I metapelite, sapphirine and sillimanite coronas separating orthopyroxene and corundum reflect decompression following peak conditions. Similar textures in Type 3 metapelite indicate that initial decompression was accompanied by heating, followed by near ITD. Sapphirine and corundum inclusions to syn-S4 komerupine in these assemblages also suggest that decompression continued during D4. The geometry of sapphirine and sillimanite coronas between orthopyroxene and corundum provide clear evidence of initial equilibrium between these minerals and the first unequivocal evidence for orthopyroxenecorundum stability in natural rocks. In Type 2 metapelite, garnet porphyroblasts are embayed by s>Tnplectic intergrowths of orthopyroxene and sapphirine, with or without plagioclase and cordierite; orthopyroxene, sapphirine and sillimanite; and orthopyroxene and sillimanite, with or without plagioclase and cordierite. In Type 3 metapelite garnet is also embayed by orthopyroxene and komerupine. These gamet breakdown textures, which overgrow S4 fabrics, are interpreted to have formed as a result of near-ITD late in, and following D4, describing the final phase of a clockwise PT trajectory for the Oygarden Group. This trajectory is consistent with independent textural and barometric indicators in mafic granulites and is in contrast to an apparent anticlockwise PT path reported for other parts of the Rayner Complex.

In addition to the major minerals described above, the textural context of accessory minerals that are intergrown with orthopyroxene-sapphirine symplectites and matrix, suggest that monazite, xenotime, apatite, mtile and minor zircon grew contemporaneously with symplectite formation.

Preliminary chemical dating of monazite and xenotime has produced ages equivalent to, and slightly younger than other ages for the RSE, indicating that the symplectites formed during decompression during this orogenic event.

231

EVIDENCE FOR COUNTER CLOCKWISE EVOLUTION OF SPR-QTZ & OFXSIL-QTZ-BEARING GRANULITES FROM HIGHLAND COMPLEX, SRI LANKA. K. Saieev and Y. Osanai Department of Geolog}^ Okayama University, Tsusliima Naka 3-1-1, Japan 700-8530

The Highland Complex of Sri Lanka is known for its Spr-bearmg granulite and ultrahightemperature (UHT) metamorphism (Osanai, 1989; Osanai et ai, 2000). Raase & Schenk (1994) and Kriegsman & Schumacher (1999) also reported similar assemblages from Highland Complex.

The evidence of prograde clockwise evolution of pelitic granulites was reported by Hiroi et al (1994). In the present study we report Spr-Qtz and Opx-Sil-Qtz assemblages, which preserves evidence for counter clockwise evolution. These rock types are exposed in some rock quarries towards the south of Ganipola, central Sri Lanka. The Spr-Opx-Sil-Qtz granulites form thm layers intercalated with other pelitic as well as and mafic granulites. The general foliation trends 355^ with a S-W dip. The mclusions of Spr-Qtz in Grt porphyroblasts are considered to be the previous highest-grade assemblage preserved. Other than Spr and Qtz, Sil, Bt and Opx are also seen as inclusions in Grt. The Opx-Sil-Qtz assemblage present in the matrix associated with Grt was formed during the post-peak evolution. Crd and Spl presented similarly are considered to form during the decompression. Crd always forms moat or symplectites, coexisting with mafic phases in all samples. Opx-Crd±Spl symplectite present in the matrix is formed after reaction between Grt and Spr. Bt is considered to be the final product formed during low P-T conditions by the hydration reaction. The above textural features represent a series of reactions like, Grt+Spr+Qtz->Opx+Sil; Spr+Qtz-^Crd+Opx+Sil; Opx+Sil+Qtz-^Grt+Crd; Grt->Opx+Crd±Spl. Some samples m which Spr IS completely absent, and Grt-Opx-Sil±Qtz is stable, present as intergrowths is considered to be form at a high-/02 condition compared to the earlier. This equilibrium assemblage could have been formed by the reaction SpH-Qtz->Grt+Opx+Sil. In the same sample Opx-Sil±Qtz intergrowths are present around Fe-Ti oxides and Crd represents Fe-Ti 0xide+Crd->0px+SiH-02, is interpreted as further evidence for high-/02 conditions during metamorphism. The chemical compositions of each mmeral also support the textural interpretations. Grt is pyrope-rich m the cores (50-59 %) compared to the rim (45-50 %). Opx shows varying composition according to the textures.

Porphyroblastic Opx cores are having highest AI2O3 content, up to 12.88 wt% while Opx-Crd symplectites and Opx-Sil intergrowths have 8 to 9 wt % of AI2O3. Using the Opx AI2O3 composition isopleth of Harley & Motoyoshi (2000) on the FMAS system resulted a temperature more than 1 HO'^C at 11-12 kbar which could be the near peak temperature which follows isobaric cooling. Thus Opx-Sil-Qtz assemblage stabilized after Spr-Qtz above the Spl-free invariant point of the FMAS system, formed at I0W-/O2 condition. The Opx-Crd±Spl symplectite formed during FMAS contmuous reaction indicate isothermal decompression. In higher-/02 conditions similar exhumation took place around the Spr-free invariant point possibly from the Spl-Qtz field to GrtOpx-Sil stability. Both Spr-bearing and -absent assemblages evolved in the counter clockwise direction under same P-T conditions while the mineral phase equlibria vary due to the variation in the /O2 conditions durmg the metamorphic process. Similar variation in /O2 conditions has been reported from the Spr-Qtz granulite blocks evolved in clockwise direction (Osanai et al, 2000).

This IS the first findmg of Spr-Qtz & Opx-Sil-Qtz-bearing granulites which represents a counter clockwise evolution and highest P-T condition from the Highland Complex, Sri Lanka, so that the time of metamorphism is of significance in understanding tectonic processes, which will be discussed during presentation.

Reference Harley, S.L & Motoyoslii, Y. 2000. Contributions to mineralogy and petrology 138, 293-307.

Hiroi, Y., et al. 1994. Precambrian Research 66,245-263.

Kriegsman, L. M. & Schumacher, J. C. Journal of Petrology 40, 1211-1241.

Osanai, Y. 1989. Seminar on recent advantages in Precambrian Geology of Sri Lanka, IFS Kandy.

Osanai, Y., et al 2000. Journal of Geosciences, Osaka City University 43, 227-247.

Raase. P & Schenk, V. 1994. Precambrian Research 66, 265-294.

232

TWO STAGE SPINEL GENERATION IN THE HIGH-GRADE METAPELITES OF THE CENTRAL KERALA KHONDALITE BELT: IMPLICATION FOR PROGRADE P-T PATH K.P. Shabeer^ T. Okudaira^ and K. Sajeev" ^ ^Department of Geosciences, Osaka City University, Osaka 558-85 85, Japan ^Department of Earth Sciences, Okayama University, Okayama 700-8530, Japan Two generations of spinels are identified fi-om the granuhte facies metapehtes of the central Kerala Khondalite Belt, southern India. Spmels of the first generation show brownish color and were formed during prograde metamorphism. They are formed by the consumption of Zn-, Cr- bearing biotite and garnet and sillimanite following the prograde reaction: Bt + Grt + Sil = Spl + Crd + Kfs + Melt. The formation of second-generation greenish spinels occurred during a subsequent retrograde stage by following the reaction: Grt + Sil = Spl + Crd + Qtz. The textural features indicate a close association of the first generation brown spinels and biotite. Biotite contains up to 0.34 wt% of ZnO and 0.30 wt% of Cr203. Each grain of the brownish spinel shows continuous compositional zoning owing to increasing Zn and Cr contents towards the grain margins. This chemical and textural evidence suggests that the brown spinels formed by the breakdown of biotite containing Zn and Cr. It is proposed that the zinc-saturation limit of biotite depends on the metamorphic conditions and that decreasing pressure with increasing temperature led to the release of Zn from biotites and formation of brownish spinels.

233

PAN-AFRICAN EVENTS IN THE EASTERN DRONNING MAUD LAND, EAST ANTARCTICA K. Shiraishi', C.M. Fanning\ T. Hokada^ K. Misawa' and H. Kagami' 1 National Institute of Polar Research, 1-9-10, Kaga, Itabashi-ku. Tokyo 173-8515 Japan 2 Res^rch School of Earth Sciences, The Australian National University, ACT 0200, Australia 3 Dqjartment of Geology, National Science Museum. Shinjuku, Tokyo 169-0073, Japan 4 Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan

The Indian Ocean sector of East Antarctica has been considered to be the southern continuation of the Mozambique belt in continental plate reconstructions. In eastern Dronning Maud Land (DML) r^^'Aofi.'^® discrete, isolated high-grade plutono-metamorphic terranes. These are, from west -20 E) to east (~50°E), the Sor Rondane Mountains (SRM), Yamato-Belgica Complex (YBC) Lutzow-Holm Complex (LHC), Rayner Complex (RAC) and Napier Complex (NAC). As a key to understanding this area in a Gondwana context, one must necessarily distinguish between Pan African and Grenville-aged plutonic and metamorphic events. So far we have obtained SHRIMP U-Pb zircon age determinations for thirty metamorphic and plutonic rocks from eastern DML and these are examined m conjunction with the geological and petrological studies of the respective terranes.

The basement of the SRM consists of Grenvillian juvenile crust (-1100-1300 Ma) with minor amounts of an Archean component; as supported by Nd isotopic data. The granulite facies regional metamorphism (Ml) occurred between 900-1000 Ma associated with felsic magmatism of tonalite m the southwest and chamockite in the north. The terrane was subsequently metamorphosed at -630-590 Ma under granulite to amphibolite metamorphic conditions (M2). An M3 event is associated with extensive A-type granitoid activity from 560 Ma and the youngest granite intrusions occurred at -515 Ma. The YBC is characterised by widespread granite and syenite intrusions at -535 Ma. Metamorphic rocks are mainly amphibolite facies quartzofeldspathic gneiss that yields a -540 Ma metamorphic age and subordinate granulite facies rock suggests an older metamorphism occurred at -620 Ma. Thus two stages of metamorphic zircon growth are recognised during the Pan-Afiican events in SRM and YBC regions.

In the LHC to the east, the peak metamorphic age is 530-550 Ma with no indication of extensive older metamorphic events, although there are inherited (?detrital) ages within the metasedimentary rocks. Note however that the -1000 Ma meta-trondhjemite from Cape Hinode and the -1000 Ma meta-igneous rock from the westernmost part of the LHC are exceptions. Petrochemical studies of the meta-trondhjemite indicate that it was formed by the partial melting of oceanic crust The 1000 Ma Rayner event is not found in the coastal region of the eastern "RAC" (west of NAC) but unique ages of-780 Ma have been obtained. Only the inland region of the RAC records -900-1000 Ma metamorphic and magmatic activity. In contrast to SRM and YBC, numerous inherited Paleoproterozoic to Archean ages from LHC and RAC suggest the provision of continental material from NAC and unknown Paleoproterozoic terranes in the east or elsewhere.

These SHRIMP U-Pb zircon data, together with previous petrological, geochemical and Sm-Nd isotopic studies allow us to summarise the following constraints for the tectonic history in the eastern DML: 1) The protolith of SRM is an accretionary complex to an as yet unidentified older continent that existed further west (south?) of SRM during pre-Grenvillian times.

2) The lack of -1000 Ma inherited zircon in the metasedimentary rocks from LHC suggests that the Rayner crust was not exposed (or did not contribute detritus) during the deposition of the protolith of LHC along the margin of the Napier continent.

3) The Cape Hinode meta-trondhjemite is considered to have formed as a magmatic arc accreted to the Napier continent at -1000 Ma. 4) Taking into account the contrasting geological and petrological features between the LHC and YBC, the boundary between YBC and LHC is a major continent-continent collision suture at 550-520 Ma. 5) Prior to this collision, at -590-630 Ma the SRM and YBC were metamorphosed during an inferred period of westward subduction of the oceanic crust.

6) The younger Pan-African peak metamorphic event of the LHC and surrounding regions thus represents the last stage of amalgamation of Gondwana in this section of East Antarctica.

234

ROSS OROGENY MAGMATISM IN THE DARWIN GLACIER REGION, SOUTHERN VICTORIA LAND, ANTARCTICA A.L. Simpson Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand* The Darwm calcic suite, named for the Darwin Glacier region of southern Victoria Land, Antarctica, consists of calcic plutons emplaced during the Cambrian Ross Orogeny. The Darwin calcic suite includes the batholith-size Carlyon Granitoid and two smaller intrusions (< 1 km^), the Blanks Granite and the Cheney Bluff Granodiorite. The Carlyon Granitoid is variably foliated, biotite-homblende granodiorite that makes up most of the Brown Hills. The Blanks Granite is a moderately foliated, biotite granite pluton that crops out at Blanks Peninsula. The foliated, biotite Cheney Bluff Granodiorite intrudes the A-type Foggy Dog Granite (FDG) suite at Cheney Bluff.

In all three plutons, plagioclase is oligoclase to andesine, biotite is annite-rich and hornblende, when present, ranges from ferrohomblende to ferrotschemiakite.

The emplacement age of the Darwin calcic suite, constrained by the earlier alkaline FDG suite and the later adakitic Cooper Granodiorite, is between 536 and 515 Ma. The FDG suite, intruded by the Cheney Bluff Granite, has been dated, using U-Pb isotope systematics of titanite and zircon, at 536 ± 8 Ma. The Cooper Granodiorite is unfoliated and therefore is interpreted to post-date the deformation associated with the Ross Orogeny. The Cooper Granodiorite has a re-interpreted U-Pb titanite crj^stallisation age of 515 Ma. The geochemistry of the three plutons is broadly consistent and the suite ranges from 66 to 76 weight percent (v^%) silica. Trends on major and trace element Barker plots are broadly consistent with the fractionation of plagioclase, hornblende and biotite. The suite is classified as an I-t>Tpe volcanic arc granitoid that is calcic sensu stricto (Peacock index) and calc-alkaline sensu latu (AFM diagram). The Darwin calcic suite is enriched in Large Ion Lithophiles (LILE) and Light Rare Earth Elements (LREE) relative to MORB, and displays negative Eu/Eu* anomalies. The Cheney Bluff Granodiorite is enriched in Na20 and Sr, relative to the other plutons.

The Darwin calcic suite has been compared to the Dry Valleys calc-alkaline (DVla) suite and the Beardmore Glacier region calc-alkaline suite. Both these suites are calc-alkaline sensu stricto and are interpreted to have formed during the Ross Orogeny. The DVla suite, which is dominated by the emplacement of the ~ 800 km^ Bonney Pluton, has an inferred emplacement age of ~ 505 Ma. The Beardmore calc-alkaline suite is interpreted to have been emplaced between 540 and 500 Ma. The Darwin calcic suite has initial ^^Sr/^^Sr ratios that are broadly similar to the Beardmore calcalkaline suite, rather than the DVla suite.

The Darwin calcic suite is anomalously calcic and iron-enriched compared to both the Beardmore calcic suite and the DVla suite. The Darwin calcic suite has a Fe* (FeOt/FeOt + MgO) ratio of 0.8 at 68 wt% Si02, compared to the Beardmore calc-alkaline suite and the DVla suite which have Fe* ratios of 0.75 and 0.72, respectively. The Peacock index for the Darwin calcic suite is 65, whereas the DVla suite and the Beardmore suites have indices of 57 and 60 respectively.

* Current Address: Department of Earth Sciences, The University of Queensland, Brisbane, QLD 4072, Australia

235

MIGMATITIC GRANULITES FROM THE TRANSANTARCTIC MOUNTAINS AND CENTRAL DRONNING MAUD LAND: NEW CONSTRAINS ON THE PETROLOGICAL EVOLUTION OF THE EAST ANTARCTIC SHIELD Sonia Sandroni and Franco M. Talarico Dipartimento di Scienze della Terra, Universita di Siena, via del Laterino 8, 53100 Siena - Italy

Graniilite metamorphic complexes showing migmatitic structures are widespread in several areas of the East Antarctic Shield (e.g. Ravich & Kamenev, 1975). New occurrences are reported and petrologically described from four distinct high-grade terrains: 1) the Mesoproterozoic basement of central Dronning Maud Land (cDML), strongly reactivated during the Pan-African event; 2) a high-grade tectonic unit overlying ?Neoproterozoic/Cambrian (0.9-0.5 Ga) ophiolites in the northern Shackleton Range (Herbert Mountams) (SR-HM); 3) the Palaeoproterozoic Read Group in the southern SR (SR-RG); and 4) the ?Mesoproterozoic Campbell Glacier high-grade complex occurrmg as structural relics within the Ross Orogen in northern Victoria Land (NVL).

Leucosome-bearing granulites are mainly pelitic-psammitic, basic compositions being restricted to SR-RG and cDML. Leucocratic segregations typically show porphyroblasts of garnet and/or orthopyroxene, and they range from usually undeformed small isolated ovoid patches to vein-like arrays, set m a finer-grained, foliated matrix. Although fluid-dominated metamorphism with external buffering (infiltration of channelled COj-rich fluid) can account for the chamockitic leucosomes in some of the cDML samples, petrological and microstmctural data, (including evidence of igneous microstructures), combined with experimental and theoretical models, indicate that: 1) vapour-absent dehydration-melting of biotite (metapelites) or of biotite + hornblende (metabasites) is the most likely dominant process assisting the formation of leucocratic segregations in all other metamorphic complexes; and 2) local compositional heterogeneities (in terms of both mineral or bulk-rock composition) are apparently significant in promoting the activation of incongruent-melting reactions, particularly in basic granulites.

Integration of new petrological data with existing geological, petrological and geochronological datasets from cDML, SR and NVL provides further evidence that all four granulite complexes are charactenzed by clockwise P-T paths, with an initial isothermic decompression (ITD) followed by isobaric cooling (IBC). In detail, P-T conditions, inferred ages, and correlation of the leucosomeformmg event with the pol>phase P-T-t trajectory (and related geotectonic implications) for the four migmatitic granulite occurrences can be summarized as follow: region

P-T conditions

P-T path/inferred tectonic regime

cDML

leucosome formation age t c. 512 Ma

5 kb, 750-900°C

prograde isobaric heating/ thermal perturbation due to syenite/ chamockite emplacement heating and P increase/crustal thickening

SR-HM

490<t<514Ma

>10 kb, >800°C

SR-RG

1.65<t<1.76Ga

6-10 kb, 800°C

ITD/exhumation

NVL

0.5<t<1.8-1.9Ga or c. 500 Ma

4.8-7 kb, 850-950°C

ITD/exhumation

Reference Ravich M. G. and Kamenev E. N. 1975. Crystalline basement of the Antarctic Platform. John Wiley & Sons, New York.

236

THE MERTZ SHEAR ZONE: NEW EVIDENCE OF THE "MAWSON CONTINENT" FROM THE EAST ANTARCTIC SHIELD IN GEORGE V LAND GeorgKleinschmidt^ and Franco Talarico" ^Geol.-Palaont. d. Universitat Frankfurt, Senckenberganlage 32, D-60054 Frankfurt/Main - Germany ^Dip. Scienze della Terra, Universita di Siena, Via del Laterino 8,1-53100 Siena - Italy

Three major structural domains, with distinct structural evolutions, lithological assemblages and partly different metamorphic evolutions have been distinguished in the East Antarctic Shield in the area between Commonwealth Bay and Ninnis Glacier in George V Land (Kleinschmidt & Talarico, 2001).

The 'Western domain" is composite, including the low grade Cape Hunter Phyllite, the amphibolite facies Cape Denison granitic orthogneiss and amphibolites, and mylonitic partially retrogressed felsic and mafic granulites from Madigan Ntk and the Watt Bay area (Garnet Point, Pigeon Rocks). Watt Bay paragneiss/metabasite unit records a polyphase metamorphic evolution, including at least 3 stages, from medium P - granulite grade conditions (Ml), through low P upper amphibolite/granulite grade (M2), to low T amphibolite and greenschist facies conditions (M3).

Enderbitic orthogneiss and minor sillimanite-gamet felsic granulites dominate the "Mertz Glacier domain" (Correll Ntk, Mt Aurora and Mt Murchison) which is characterized by a prominent N-S trending shear zone, the Mertz Shear Zone (MSZ), at least 200 m to c. 5 km m width. The MSZ comprises a series of progressive and overprinting shear structures, which developed during changed metamorphic conditions (from early low-P granulite to lower amphibolite and greenschist facies) but all with similar kinematics (dextral shearing). This protracted deformational evolution is punctuated by two generations of mafic dykes which emplaced either before Ml (Cape Pigeon Rocks) or during/after M2 (Aurora Pk, Mt Murchison).

The ''Eastern domain" (including the coastal exposures between Mertz and Nmnis glaciers) mainly consists of different varieties of granitoids, in places slightly foliated. Metamorphic rocks are here represented by 1) up to several metre thick rafts of upper ampliibolite facies paragneiss, possibly equilibrated at similar conditions as M2 in the Western Domain (Penguin Point area) and 2) distinctly lower grade, contact metamorphosed schists occuring as small (metre to decimetre in size) xenoliths (eastern Ainsworth Bay).

Comparison with the coastal region of southern Australia (Eyre Peninsula) indicates that the ductile MSZ (polyphase evolution and structural position) closely matches the main features of the pre1610 Ma Kalinjala Mylonite Zone within the Gawler Craton (Foster & Ehlers, 1998). This shear zone is a major dextral tectonic and lithological boundary up to 300 km in length and up to 4 km in width, separating Archaean and Palaeoproterozoic metasediments to the west, from late Palaeoproterozoic orthogneisses and granites (Lincohi Complex, 1850-1750 Ma) to the east. The Antarctica/Australia bridging by means of the Mertz and Kalinjala Shear Zones fits perfectly with the correlation given by Oliver & Fanning (1997) and can contribute to the reconstruction of Gondwana, Rodinia and Farming's (1997) Mawson Continent.

References Fanning C M., 1997. The "Mawson Continent: Archaean to Proterozoic crust in the East Antarctic Shield and Gawler Craton, Australia. lUG 9 Strasbourg 23-27March 1997, Terra Nova, 9, Abstr. Suppl. 1, 164.

Foster D A. & Ehlers K., 1998. ^^Ar-^^Ar thermochronology of the southern Gawler Craton, Australia: Implications for Mesoproterozoic and Neoproterozoic tectonics of East Gondwana and Rodinia. J. geophys. Res., 103 (B5), 10177-10193.

Kleinschmidt G. & Talarico F., 2001. The Mertz Shear Zone. Terra Antartica Reports, 5, 109-115.

Oliver R.L. & Fanning C.M., 1997. Australia and Antarctica: Precise correlation of Paleoproterozoic terrains.

In: Ricci, C.A. (Ed.): The Antarctic Region: Geological Evolution and Processes, Terra Antartica Publications, 163-172.

237

LARGE SCALE SEDIMENT DISPERSAL ASSOCIATED WITH THE LATE NEOPROTEROZOIC ASSEMBLY OF GONDWANA ^ Ian Williams^, John Goodge^ Paul Myrow^ Kevin Burke^ and Jeff Kraus' ^ Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200 '^Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA Department of Geology, Colorado College, Colorado Springs, CO 80903, USA Department of Geosciences, University of Houston, Houston, TX 77204, USA 'Tethys Geoscience, L.L.C., 120 Nova Lane, Pine, CO 80470, USA

Volummous early Paleozoic sediments that dommate the Lachlan Fold Belt (LFB), eastern Australia, are just a small part of the so-called Gondwanan 'mud pile', quartz-rich sand- and mudstones of similar age found m most of the former Gondwana continents: Australia, New Zealand, Antarctica, Africa, South America and India. Although broad similarities in age and lithology do not necessarily imply a common origin, they do raise the question of whether the production of such large volumes of sediment over such a wide area in a relatively short period of time might have a common cause.

Sedmientar>^ structures in the LFB indicate paleocurrent flow towards the north, northeast and east, leadmg to a general belief that the sedmients were derived from the nearby margin of the Gondwana continent. That margin was dominated by the mountains of the Ross-Delamerian Orogen, which d^eloped diachronously from southern Australia to southern Antarctica in the late Neoproterozoic. Ar/ Ar studies of detrital muscovite support this concept, showing that the sediments originated from a terrane rapidly uplifted and eroded in the late Neoproterozoic to early Paleozoic (Turner et a I., 1996). Detrital zircon provides another perspective, however. Throughout the southern LFB, New Zealand and northern Antarctica, the early Paleozoic sediments are dominated by two main composite detrital zircon populations, 500-650 Ma and 0.9-1.2 Ga. This is not the age mix to be expected from erosion of the exposed Proterozoic rocks of east-central Australia. Further, the same age pattern is also found in sediments of similar age from South Africa (Armstrong et ai, 1998) and the Transantarctic Mountains. Over a distance of several thousand kilometres along the former eastern Gondwana margin the sediments appear to have been derived from the same composite cratonic region. In contrast, various older Proterozoic sedmients have a range of detrital zircon age patterns, apparently of local derivation.

The most likely source of the remote sediments is the Mozambique Belt, the youngest and largest of the Pan-African orogenic belts formed during the last stages of the assembly of Gondwana in the late Neoproterozoic. At its southern end, this belt follows the line of a major Grenville orogen that once extended along the future India-Africa-Antarctica rift. The combination of Grenville and Pan-African aged rocks in the belt, with a Pan-African metamorphic overprint and contributions from Ross-Delamerian magmatism, would account well for the dominant zircon and muscovite ages m the eastern Gondwana 'mud pile'. But the 'mud pile' also extends into northern Africa and possibly also India. Does that portion also have the same derivation? Our initial reconnaissance studies of Cambro-Ordovician sediments in Algeria and India suggest the origin is similar, but not the same. Samples from north eastern Algeria, one Cambrian and one Ordovician, are both dommated by 600-700 Ma zircon with significant amounts of Paleoproterozoic zircon, but little of Grenville age. In contrast, samples of early Paleozoic sediments from the Greater and Lesser Himalayas m northern India have yielded abundant detrital zircon of Grenville and Pan-African ages, but variously mixed with other zircon, presumably of more local derivation. As in Australia and Antarctica, sediment predating the 'mud pile' has a very different provenance. These new data show that if the Mozambique Belt was the principal source of the 'mud pile' sediments, then sediments derived from different parts of the belt sampled different mixes of source rocks.' This provides the opportunity to identify specific sedimentary packages with specific parts of the belt or related belts, and to use the remote sediment sequences to study the progressive unroofing of the Pan-African orogens.

References Annstrong R.A., de Wit M.J., Reid D., York D. & Zartman R. 1998. Cape Town's Table Mountain reveals rapid Pan African uplift of its basement rocks. Journal of African Earth Sciences 27, 10-lL Turner S.P., Kelley S.P., VandenBerg A.H.M., Foden J.D., Sandiford M. & Flottmann T. 1996. Source of the Laclilan fold belt flysch linked to convective removal of the lithospheric mantle and rapid exhumation of the Delamerian-Ross fold belt. Geology 24, 941-944.

238

THE P-T-FLUID CONDITIONS OF END-ARCHAEAN GRANULITE FACIES METAMORPHISM IN THE VESTFOLD HILLS, EAST ANTARCTICA F. Zulbati-Petrillo and S.L. Harley Geology and Geophysics, University of Edinburgh, Edinburgh, Scotland, EH9 3JW Granulite metamorphism m the Vestfold Hills block of East Antarctica occurred at 2501-2496 Ma ( D l - M l ) and 2481-2475 Ma (D2-M2). These tectonothermal episodes were separated by the emplacement of a varied suite of intrusives, the Crooked Lake Gneiss, mto older tonalitic orthogneisses, mafic gneisses and two suites of supracrustal rocks, the Taynaya Paragneiss and the Chelnok Paragneiss (Black et al, 1991; Snape et al, 1997). Mineral assemblages in these older rocks have been used previously to estimate the P-T conditions of metamorphism for M l and M2, with conflicting results. Whereas Collerson & Sheraton (1986) estimated P-T conditions in the range 8-10 kb and 900-1050^C for D l - M l , Harley (1993) obtained peak M l conditions of only 830880''C at P<8.5 bar from the Taynaya Paragneiss. Hence, the Chehiok Paragneisses have been investigated here in order to resolve and constrain the P-T conditions of peak metamorphism in the Vestfold Hills, and document its post-peak fluid-rock interaction history.

Peak temperatures of M l are constrained to be >820''C by the stability of meionitic scapolite (EqAn 82) in Chelnok calc-silicates. Reintegrated compositions of exsolved subcalcic clinopyroxenes (W03234Eni2-i3Fs54) and pigeonites (Woio-iiEn2oFs69-7o) in a Chehiok metaironstone yield peak-T in the range 880-910°C, whilst reintegrated compositions of perthitic feldspars give minimum estimates of 850±50''C. These results support those of Harley (1993) but rule out the extreme T conditions proposed by Collerson & Sheraton (1983): the Vestfold Hills is not an example of UHT metamorphism. Pressures of metamorphism during D l - M l are estimated as 7.8-9. Ikb at 8 5 0 + 5 0 T from the equilibrium hercynite + sillimanite = ahnandine + corundum. Pressures of 7.7+lkb are calculated usmg retrieval methods based on Fe-Mg-Al relations in orthopyroxene coexisting with garnet when the phase Fe/Mg ratios are readjusted to yield T=850°C. In all. M l metamorphism of the Chelnok Paragneiss occurred at peak P-T conditions of 8±1.0 kb and 850±50^C. Calc-silicate granulites that occur as rare decimetre-scale lenses preserve a complex history of postpeak grandite garnet (and clinozoisite) formation at the expense of assemblages involving scapolite, plagioclase, clinopyroxene, calcite and grossular-rich garnet. Late andraditic garnets occur as coronas and symplectites, in fracture networks and veins, and as metasomatic rinds on scapoliteplagioclase-pyroxene assemblages, consistent with their production through fluid infiltration. Initial coronas resulted from the infiltration of fluids with XC02<0.6 at 8 2 0 T , whilst later infiltration of fluids with XC02<0.1 at 700''C led to the successive development of clinozoisite and garnet. Mineral compositional zoning demonstrates that: 1) garnet formation requires consumption of the esseneite component in pyroxene; and 2) the reactions are open-system, requiring the addition or removal of Si02, probably dissolved in the infiltrating fluid. Fluid infiltration into the Chehiok calc-silicates, most likely associated with the emplacement of the Crooked Lake Gneisses and subsequent D2 deformation, does not require high terrain-integrated fluid fluxes as the calc-silicates constitute less than 0.1% of the exposed granulites.

References Black L.P., Kinny P.D., Sheraton J.W. & Delor C.P., 1991. Precambrian Research, 50, 283-310.

Collerson K.D. & Sheraton J.W., 1986. In: (Pickard J., ed) The Antarctic Oasis: Terrestrial Environments and History of the Vestfold Hills. Sydney, Academic Press, 21-62.

Harley S.L., 1993. Antarctic Science, 5, 389-402.

Snape, I., Black, L.P. & Harley, S.L. 1997. The Antarctic Region: Geological Evolution and Processes, Terra Antartica Publication, Siena, 139-148.

239

EXPLOSIVE VOLCANISM IN THE EARLY GERRINGONG VOLCANICS, PERMLVN SOUTH-EASTERN SYDNEY BASIN, AUSTRALL\ Glen Bann Department of Geolog}^ Australian National University, ACT, Australia The mid-Permian, early Gerringong Volcanics of the Broughton Formation, Shoalhaven Group, southern Sydney Basin, consist of explosively erupted, andesitic to basaltic, subaerial products of island arc volcanoes. Evidence mdicates a relatively proximal source from eruptions ranging from mild Strombolian to violently explosive Vulcanian or Plinian phreatomagmatic/Surtseyian eruptions.

The volcanic arc consisted of a number of large volcanoes, the largest of which was situated 40km south-southeast from present Jervis Bay. Jervis Bay itself was the site of an active volcano which erupted into a shallow sea depositing a 2-3m laterally extensive 'bedded' tuff, composed predominantly of glass shards with rare marine fossils throughout, named the KooLee Tuff Member (Bann and Jones, 2001). Evidence for this eruption is also found in the University of Wollongong No. 1 Drillcore, drilled in the University grounds ~60km to the north of Jervis Bay, and in Upper Kangaroo River ^30km to the north-west. There are four hypotheses for the eruption and depositional mechanisms for this tuff: (1) An ash fall from a significant distal explosive eruption; (2) A debris flow or lahar shed from the slopes (subaerial or marine?) of a (proximal?) volcano following deposition after an explosive eruption; (3) Deposition through water (i.e., gravity) as a pyroclastic surge that travelled across the sea surface or; (4) Deposition beneath the sea, deposited as a pyroclastic surge displacing the water, with the resultant total density of the surge front greater than that of the water (as discussed by previous authors).

A dyke/sill at Kinghom Point, 5 km north of Jervis Bay, intruded wet, unconsolidated shallow marine sediments of the lower Berry Formation and is probably associated with this volcanism.

The Koo-Lee Tuff Member, the Coolangatta Latite Member, a 30 m thick basaltic flow, and the Black Forest Tuff Bed, a thin cm tuff bed also containing shards and pumice fragments located at the base of the Broughton Formation, indicates that non-explosive and explosive volcanism were penecontemporaneous with deposition during this period (i.e., mid-Permian).

The volcanoes shed significant amounts of volcanic detritus as lahars, debris flows, turbidites and tempestites, into the cold, northerly-flowing, tidal, shallow sea-way lying between the volcanic chain to the east and the mainland to the west. This detntus swamped cratonic, quartzose sediment derived from the Lachlan Fold Belt to the west, and resulted in high rates of volcanic-derived sediment accumulation in the eastern half of the basin. A sequence of conformable, coarsening-up strata developed from the shelf sediments of the Berry Siltstone to the volcanic-derived Broughton Formation. This volcanic episode accounts for the abundant volcanic detritus in the Berry Siltstone and the lowermost Westley Park Sandstone Member in the Broughton Formation. Glacio-eustatic sea level transgressions and regressions were overprinted by the influence of active volcanism and tectonism in this part of the Sydney Basin.

240

FACIES ANALYSIS AND PALAEOVOLCANOLOGY OF A MULTIPLE SCORIA CONE COMPLEX - MOUNT BUNINYONG, CENTRAL VICTORIA. Cliristie Batiste^ and W.R.H Ramsay^ ^School of Ecology and Environment, Deakin University, Melbourne Campus, Burwood, Vic. Located approximately 11 kilometres southeast of Ballarat in Central Victoria, Mount Buninyong belongs to the Newer Volcanics phase of volcanism, which was widespread throughout westerncentral Victoria from the Pliocene to Recent. Mt. Buninyong is a composite complex comprising two prominent cones that were formed during independent phases of eruption. Standing at 740 metres above sea level and covering an area of just under four square kilometres, the complex overlies folded Ordovician basement. Late Devonian granites are inferred to underlie the north-east margin of Mt Buninyong.

The initial episode of cone building saw the development of a large scoria cone with a basal diameter of approximately 2 km. Pyroclastic deposits comprising scoriaceous lapilli-sized to very coarse fragments including dense ballistic blocks and pyroclastic fall eruptions were derived from a crater now partially obscured by the younger cone building event. Two lava flows emanate from the older cone. The first, from the eastern side of the complex, travelled a distance of six kilometres to the south and southeast. The second lava flow followed pyroclastic fall eruptions and was released from the northwest flank from a lava lake developed within the crater. The northwest flank was subsequently breached as a result of the lava emission, the flow moving southwest towards the present-day township of Buninyong before intersecting and continuing south along the Yarrowee River.

Resurgence in volcanic activity resulted in the opening of a second vent on the site of the existing cone, causing partial destruction of the pre-existing cone. Renewed pyroclastic eruptions resumed cone building and the establishment of a new^ cone. Pyroclastic air-fall deposits erupted through Strombolian fire-fountaining are similar to the massive and poorly sorted deposits of the older cone, however, crude bedding is evident in some exposures. Near-vent bombs are up to one metre in diameter with several retaining their spindle and ellipsoidal shapes.

The new cone forms a steep-sided asymmetrical structure with a basal diameter of 1.12 km. The height varies from 180 metres above the surrounding plain on the western flank to 240 metres on its eastern side, the latter being considerably higher and steeper suggesting a westerly palaeo-wind direction during eruption and a rapid cone-building duration. The crater is preserved on the western flank and a linear feature extending 100 m south-west from the crater is a possible remnant fissure.

The lavas are an olivine hawaiite {ne ~ 12, m/ab + ~ 43; Irving and Green, 1976) and comprise olivine phenocrysts set in a fine-grained groundmass of feldspar, clinopyroxene, opaque oxides • glass. Xenoliths and xenocrysts of assumed mantle origin occur both in the flows and within pyroclastic fragmentals. These inclusions are predominantly Cr-diopside Iherzolite nodules with pale green Cr-diopside, and Cr-Al spinels. Nodules of magnesian olivine formed during early fractionation in the mantle or through disaggregation of the Cr-diopside series xenoliths (Price et al, 1988), are up to ten centimetres in diameter and often form the core of volcanic bombs. Xenocr>^sts of quartz displaying corona textures and fluid inclusions are of shallow crustal origin and were most likely derived from the underlying Palaeozoic sedimentary rocks.

References Irving, A. J. and Green, D. H., 1976. Geochemistry and petrogenesis of the Newer Basalts of Victoria and South Australia. Jour. GeoL Soc. ofAust, 23: 45-66.

Price, R. C. et al, 1988. Cainozoic volcanic rocks. In J. A. Douglas and J. G. Ferguson (Eds), Geology of Victoria, Victorian Division Geological Society ofAustralia, Melbourne, pp. 439-452.

241

REVISED STRATIGRAPHY AND FACIES ARCHITECTURE FOR THE LATE DEVONIAN TO EARLY CARBONIFEROUS CAMPWYN VOLCANICS OF CENTRAL QUEENSLAND: IMPLICATIONS FOR THE TECTONIC FRAMEWORK OF THE NORTHERN NEW ENGLAND FOLD BELT. Scott E Brvan. R.J. Holcombe, and C.R. Fielding Department of Earth Sciences, The University of Queensland, St Lucia, Brisbane, Queensland 4072 Upper Devonian to Lower Carboniferous strata of the Campwyn Volcanics of east central Queensland preserve a substantial sequence of first cycle volcaniclastic sedimentary and coeval volcanic rocks that record prolonged volcanic activity along the northern New England fold belt at this time. Previous work has described the formation as bemg dommated by a recurring assemblage of mafic volcanic-dominated volcano-sedimentary facies that formed part of the fore-arc and arc assemblages of a Late Devonian-Early Carboniferous age supra-subduction zone volcanic arc. The conclusions of our research, includmg many of our direct observations (rock types, palaeoflow data), contradict this previous work.

The style and scale of volcanism varied systematically with time, and a well-defined succession of Late Devonian, mafic volcano-sedimentary rocks underlying a silicic volcanic sequence that passes upward mto Early Carboniferous limestone-bearing sedimentary rocks is recognised. We define two facies associations for the Campwyn Volcanics. A Lower Facies Association is dominated by mafic volcaiuc-derived sedimentary breccias with subordinate primary mafic volcanic rocks that have undergone predominantly quench fragmentation processes producing hyaloclastite and peperite. The sedimentary breccias record episodic and high energy, subaqueous depositional events sourced fi-om a mafic lava-dominated terrain. Some breccias contain a high proportion of attenuated dense, glassy mafic juvenile clasts suggesting a syn-eruptive origin. The Lower Facies Association coarsens upwards overall fi-om a lithic, sand-dominated sequence through a thick interval of pebble- to boulder-grade polymict volcanic breccias, cuhnmating in facies that show evidence for subaerial exposure (e.g., palaeosols and up to 100 m thick basaltic lava and aa lava breccia sequence).

The sihcic Upper Facies Association marks a significant change in eruptive style, magma composition and eruptive sources, and the widespread development of subaerial depositional conditions. Crystal-rich, high-grade, low to high-silica rhyolite ignimbrites dominate the base of the Upper Facies Association, which biostratigraphic and zircon age control indicate are early-mid Toumaisian in age or older (-^363-350 Ma). The ignimbrites represent extra-caldera facies with individual units up to 40 m thick, mostly lacking coarse lithic breccias (maximum clast size - 4 0 cm diameter). Thick deposits of pyroclastic matenal interbedded with fine-grained siliceous sandstone and mudstone (locally radiolarian-bearing) were emplaced from pyroclastic flows that crossed palaeoshorelines, or represent large volumes of pyroclastic material redeposited soon after eruption.

However, some matrix-supported, block-bearing lithic-pumice-crystal breccias may also reflect more proximal subaqueous silicic explosive eruptions. The ignimbrite-dominated sequence passes upwards into pnmarily ignimbrite-derived sedimentary facies (crystal-lithic sandstones), with abundant detrital volcamc quartz and feldspar. Limestone is common in the upper part of the Upper Facies Association, and several beds are oolitic (cf Rockhampton Group of the Yarrol terrane).

Overall, the Upper Facies Association fiines upward and is transgressive, recording a return to shallow marine conditions.

Palaeocurrent data from all stratigraphic levels within the Campwyn Volcanics indicate the regional sediment dispersal direction was to the northwest, and opposed to the generally accepted notion of easterly sediment dispersal from a volcanic arc source. The silicic Upper Facies Association correlates in age and lithology to Early Carboniferous silicic volcanism in the Drummond (Cycle 1) and Burdekm Basins, Connors, and Yarrol terranes of eastern Queensland. The widespread development of silicic volcanism in the Early Carboniferous indicates that silicic (rift-related) magmatism was not restricted to the Drummond Basin but a rather more substantial silicic igneous province is indicated.

242

SUBMARmE RECORD OF WIDESPREAD ANDESITIC VOLCANISM: MAGNETIC SANDSTONES IN THE TYNDALL GROUP, MOUNT READ VOLCANICS, TASMANIA. Greg Ebsworth CODES SRC, University of Tasmania.

The Cambrian Mount Read Voicanics (MRV) in western Tasmania consist of volcanic and lesser sedimentar}^ rocks v^hich host several world-class massive sulfide deposits. The late Middle Cambrian Tyndall Group forms the uppermost part of the MRV. It comprises the Comstock Formation and overlying Zig Zag Hill Formation. The Comstock Fomiation is further divided into the Lyncliford Member and overlying Mt Julia Member (White and McPhie, 1996).

The Lynchford Member includes units of andesitic crystal-rich volcanic sandstone (CRVS) which are t>qDically magnetic. These and other units of similar andesitic CRVS are regionally widespread in the MRV, occurring intermittently over its entire 120 km strike length. Correlation of the separate CRVS occurrences with the Lynchford Member is supported by their stratigraphic relationships, biostratigraphy, petrography, and clinopyroxene chemistry.

Petrographically, andesitic CRVS units from throughout the MRV are remarkably similar. They are cr}^stal-rich (30-80 modal %) with abundant feldspar and minor quartz, clinopyroxene and titanomagnetite, plus rare zircon and apatite. Lower crystal contents occur in finer grained lithologies characterised by abundant bubble-wall shards. Lithic clasts are dominantly subangular felsic to intemiediate volcanic clasts and less common subrounded granitoid and metasedimentary clasts. The sandstone units are typically tens to hundreds of metres thick, massive to weakly stratified, and may be normally graded. They are commonly interbedded with black mudstone.

Microprobe analyses for clinopyroxenes from andesitic CRVS throughout the MRV are tightly grouped on the CaO vs MgO and AI2O3 vs FeO plots and generally allow discrimination of andesitic CRVS from other intermediate to mafic units in the MRV. The CRVS clinopyroxene analyses overlap with those from basaltic andesite units from near the top of andesitic volcanic complexes. The latter form several major intrusive to extrusive intrabasinal volcanic centres in the MRV White and McPhie (1996) interpreted the CRVS units of the Comstock Formation to be syneruptive mass-flow deposits, emplaced in a submarine, below wave-base setting. Depositional units were interpreted to be sourced indirectly from pyroclastic flows produced by extrabasinal subaerial or shallow marine explosive volcanic eruptions. Sequences conformably underlying the andesitic CRVS units, such as the Southwell Subgroup and Yolande River Sequence (Corbett, 1992), contain dacitic to rhyolitic volcaniclastic units interpreted to have a similar extrabasinal, syn-eruptive genesis (e.g. McPhie and Allen, 1992). These relationships suggest the andesitic CRVS units are a volumetrically minor but distinctively mafic phase within a larger felsic volcaniclastic succession. Being distinctive and widely distributed in the MRV, they have great potential in stratigraphic correlation.

References Corbett, K. D. 1992. Stratigrapliic-Volcanic Setting of Massive Sulphides in the Cambrian Mount Read Voicanics, Tasmania. Economic Geology 87, 564-586.

McPhie, J. and Allen, R.L. 1992. Facies Architecture of Mineralized Submarine Volcanic Sequences; Cambrian Mount Read Voicanics, Western Tasmania. Economic Geology 87, 587-596.

White, M.J. and McPliie, J. 1996. Stratigraphy and palaeovolcanology of the Cambrian Tyndall Group, Mt Read Voicanics, western Tasmania. Australian Journal of Earth Sciences 43, 147-159.

243

THE URAL VOLCANICS: A SILURO-DEVONIAN FELSIC VOLCANIC SUCCESSION IN CENTRAL NSW S. Meakin\ R.G. Cameron^ G.P. Colquhoun\ C. Simpson^ L.M. Barron^ and S.A.Wilde^ ^ Geological Survey of NSW, PO Box 536, St Leonards NSW 1590 ' Discipline of Geology, School of Environmental and Life Sciences, University of Newcastle NSW 2308 Isotope Studies Group, Curtin University of Technology, GPO Box U1987, Perth, WA 6845 The Lake Cargelligo region contains excellent exposures of silicic volcanic, volcaniclastic and clastic sedimentary rocks of the Ural Volcanics deposited in the Siluro-Devonian Rast Trough of the central Lachlan Orogen. The Ural Volcanics lie at the top of a broadly transgressive-regressive sequence comprismg basal coarse, basement-derived, alluvial fan clastics (Boothumble Formation), overlain by quartz-rich turbidites (Crossleys Tank Formation), which are in turn conformably overlain by the Mt Boorithumble Formation and Ural Volcanics. The sequence is interpreted as syn-rift to rift fill. Overlying sag deposits were presumably removed by erosion, possibly now preserved only on the adjacent shelves as the shallow marine Walters Range Group.

Isotopic and fossil evidence indicates that volcamsni probably spanned the latest Silurian to Early Devonian, with the upper limit poorly constrained. A marine fossil assemblage found in volcaniclastic breccia horizons mdicates an Early Devonian age (most likely Pragian to Emsian).

This is generally consistent with new SHRIMP U/Pb zircon dates which have been obtained for coherent volcanic facies within the sequence: 410±8, 414±13, 413±5 and 424±6 Ma, the last age probably reflecting inheritance of zircons.

A diverse range of lithologies has been mapped m the Ural Volcanics, including rhyolitic to dacitic lavas (and/or shallow intrusions), porphyritic megacrystic intrusions, volcaniclastic rocks composed of angular volcanic detritus (probably of pyroclastic ongin), turbidites and monzodiorite intrusions. Measured sections through the Ural Volcanics contain volcaniclastic rocks and mterbedded facies with textures indicating that they were originally glassy rocks. The presence of euhedral phenocrysts in low to moderate abundance, columnar jointing, vesicles and spectacular flow banding suggest that these rocks are lavas or shallow intrusive porphyries. Apparent vitriclastic textures at some sites may be largely the product of secondary alteration, which has produced a pseudobreccia texture. However some of these textures may be due to quench fragmentation, or autobrecciation during emplacement. Interbedded mass-flow volcaniclastic rocks contain mixed coherent volcanic and pyroclastic detritus (K. Bull and J.McPhie, pers. comm.) and have graded tops due to settlmg of suspended detritus. Rare shard-rich sandstone with vitriclastic textures may represent waterlain airfall tuff. Sedimentary features suggest that deposition was mamly below wave base. Rare volcaniclastic mass-flow breccias containing stromatoporoid {Atelodictyon sp.), coral {Squameofavosites sp.), echinoderm and brachiopod detritus provide evidence of marine conditions, with debris sourced fi-om a shallow marine shelf of limited extent.

Preliminary geochemical data analysis reveals that the Ural Volcanics are dominantly rhyolitic to dacitic in composition but range to andesitic and basaltic in small volumes. The volcanics have Sand I- type affinities and the inferred tectonic settmg of eruption is a continental intra- or back-arc extensional basin. Petrographic evidence suggests that the rhyolitic rocks have an S-type magmatic affinity, indicated by the presence of garnet and cordierite phenocrysts. Locally, there are strong concentrations of fluorite in lavas and tufifaceous sediments, perhaps indicating an A-type influence. Late stage I-type intermediate dykes and sills contain disseminated sulfides. The Ural Volcanics also contain minor gold and base metal occurrences. The Brovms Reef deposit (stratabound Zn-Pb sulphides with lesser Cu, Au, Ag) lies in steeply-dipping interbedded volcaniclastic and fossiliferous rocks of the underiying Mt Boorithumble Formation.

Acknowledgement: Meakin, Cameron, Colquhoun and Barron publish with the permission of the DirectorGeneral, NSW Department of Mineral Resources.

244

GEOMORPHOLOGICAL AND VOLCANIC FEATURES OF THE ADELBERT MOUNTAINS-RAMU VALLEY AREA OF PAPUA NEW GUINEA: IMPLICATIONS FOR REGIONAL NEOTECTONIC ACTIVITY C. Mortimer, L.K. Wooller. R.H. Findlay, G. Kopi, T.W. Kilya.

Geological Survey of Papua New Guinea.

Department of Mining, Private Bag, Port Moresby, Papua New Guinea.

Recent investigations in the Adelbert Mountains and Ramu Valley region have confirmed the presence of Pliocene high-Ti02, I0W-K2O lavas (Uvo Volcanics). These rocks occur in a remnant suite of large calderas along the coast north of the Adelbert Mountains. Geochemically, the Uvo Volcanics are affined to volcanics found in extensional regimes and confirm that the Ramu Valley and Adelbert Mountains, and probably the Finisterre Mountains and Markham Valley region, formed an active extensional basin (Ramu Infrabasin) in Pliocene times, when rapid uplift was occurring in the PNG Highlands to the south. This basin was filled with the clastic derivatives of the uplifting Highlands to form the Late Miocene to Late Pliocene Ouba Formation.

The drainage patterns and river profiles of rivers draining north and south from the Adelbert Mountains confirm a complex history of past Pliocene uplift and subsequent profound erosion withm a regional picture of general uplift. Presently-active high-angle reverse faults and strike-slip faults have affected the drainage progressively and we estimate coastal uplift to be as great as 4.5 mm yr .

The uplift history of the Adelbert Mountains, and their geomorphology, contrasts with that of the Finisterre Mountains where large scale overthrusting is well established.

In the Adelbert Mountains, the latest phase of uplift appears to have commenced recently by reversal of the normal faults that produced the extensional Pliocene grabens. This is attributed to present underthrusting by the northeastward-moving Australian Plate.

Finally, the stratigraphic relationships of the Late Miocene to Pliocene Ouba Formation to the underlying Finisterre Volcanics confirms that these latter rocks do not form an autochthonous terrane which docked with the Australia Plate in Pliocene times.

245

THE 1994 TO 2001 ERUPTION AT RABAUL, PAPUA NEW GUINEA: EVIDENCE OF REPEATED BASALTIC MAGMA INFLUX INTO A SUB-CALDERA DACITE MAGMA RESERVOIR. H Patia\, S.M. Eggins% C.O. McKee^ RJ. Arculus\ and R.W. Johnson^ ^Department of Geology, Australian National University, ACT 0200, Australia "Research School of Earth Science, Australian National University, ACT 0200, Australia ^Geophysical Observatoiy, P O Box 323, Port Moresby, Papua New Guinea ^Geoscience Australia, Canberra, ACT 2601, Australia

The latest eruption at Rabaul Caldera consisted of two phases: Phase 1 began on 19 September 1994 when Vulcan and Tavurvur volcanoes erupted simultaneously from opposite sides of the caldera. Phase 1 activity ended on 16 April 1995. The total volume of pyroclastic material erupted was 300 X 10 m . Phase 2 commenced in November 1995 when eruptive activity resumed at Tavurvur and continued intermittently until mid-2001. Seven Strombolian eruptions, of which four produced lava flows, occurred between May 1996 and July 1997. The total volume of pyroclastic deposits erupted during Phase 2 was about 25 x lO^m^ Magma mixing and mingling arising from probable new influxes of basaltic magma into a dacite magma reservoir is a feature in both Phases 1 and 2. The nature of the basaltic and dacitic magma interactions has been assessed using petrological and geochemical methods. Magma interaction and mixing between dacite magma and basaltic magma in Phase 1 and 2 is revealed by XRF wholerock data. The dacite/basalt mixing trajector>^ projects towards a composition that intersects the estimated liquid line of descent at >7 wt% MgO. The dacite magmas erupted from Vulcan and Tavurvur during Phase 1 and from Tavurvur during Phase 2 are indistinguishable (about 1.9 wt% MgO, 63 wt% Si02), and similar in composition and mineralogy to those rocks erupted during the 1878 and 1937 eruptions at Rabaul. They are not as evolved as the Rabaul Ignimbrite (65-67 wt% Si02), erupted during the latest major calderaforming eruption about 1400 years BP (Wood etai, 1995).

The Phase 1 and 2 dacites are characterized by a dominant phenocryst assemblage comprising plagioclase (An5o-6o) and lesser clinopyroxene (Mg# 70-75), orthopyroxene, titanomagnetite, sulfide, and apatite. Mixed magmas of andesitic composition have strongly bimodal phenocryst assemblages and well-developed sub-populations cr>^stallized from basaltic and dacitic parent magmas. The basaltic phenocryst sub-populations are dominated by relatively magnesian clinopyroxene (Mg# strongly centered upon Mg#s4), magnesian olivine (F084), calcic plagioclase (An85-95), and titanomagnetite. There is limited or no development of inverse compositional zoning in the dacite phenocryst populations, as indicated by core and rim microprobe analyses and detailed compositional profiles. In contrast, the basaltic plagioclase phenocry^st population comprises both unzoned (no sodic rim development) and normally zoned An9o cores and Anso-eo rims) subpopulations that are attributable to fresh and previous basaltic magma inputs to the dacite magma reservoir.

Reference Wood, CP., Nairn, LA., McKee, C.O. and Talai, B., 1995. Petrology of the Rabaul Caldera area, Papua New Guinea. J. VolcanoL Geotherm. Res., 69: 285-302

246

INTERNAL STRATIGRAPHY OF THE CLIMACTIC ABRIGO IGNIMBRITE, TENERIFE, CANARY ISLANDS: IMPLICATIONS FOR ERUPTION AND EMPLACEMENT PROCESSES. A. Pittari\ R.A.F. Cas\ C. J. Edgar\ H. J. Nichols% J. A. Wolff' and J. Maiti^ ^School of Geosciences, PO Box 28E, Monash University, Victoria, 3800, Australia '^Department of Geology, Washington State. University, Pullman, WA, 99164, USA 'Institute of Eartli Sciences "Jamne Almera", CSIC, Lluis Sole Sabaris s/n, 08028, Barcelona, Spain The basaltic to phonolitic Las Canadas volcanic edifice and its summit caldera complex is the dommant feature on the oceanic island of Tenerife and has been active for the past 3 million years.

Since 1.57 Ma , the Las Canadas edifice has undergone three major constructive to destructive cycles of explosive volcanism and caldera collapse (Marti et. al, 1994). The last cycle, represented by the Diego Hernandez Formation, cuhninated with the eruption and deposition of the 0.196 Ma (Nichols et. al^ 2001) Abrigo Ignimbrite and was associated with the final major caldera collapse event. The Abrigo Ignimbrite is probably the most widespread and voluminous volcanic unit on Tenerife and consists of a light-grey, lithic-rich, phonolitic ignimbrite up to 25 m thick. Its internal stratigraphy is best developed on the southern and southeastern slopes of the island and is made complex by lateral facies variations.

The lowermost depositional unit is generally a massive, lithic-rich ignimbrite with an upper pumice concentration zone. Pebble-cobble sized lithic concentration zones are common and occur as two types: localised zones near the base consisting of locally derived lithics and a regionally extensive zone of vent-derived lithics towards the top of the depositional unit. A thin accretionar}^ lapilli-rich ash layer commonly overlies the lower unit. Above this is a laterally variable zone of relatively lithic-poor, massive to stratified ignimbrite, which envelops high relief pumice-rich Ignimbrite lobes. This zone grades upwards into the upper massive, lithic-rich depositional unit.

The stratigraphy is fiarther complicated by the presence of additional localised depositional units associated with the local topography.

On the northern and western slopes, the stratigraphy consists of multiple depositional units of massive, lithic-rich ignimbrite.

The Abrigo ignimbrite developed as a series of pyroclastic flow pulses which interacted strongly with the topography during transport and deposition , including entrainment of local lithics. The accretionary lapilli-rich ash layer represents the co-ignimbrite ash cloud of the first flow pulse.

Massive, lithic-rich ignimbrite units represent periods of high energy steady flow whereas lithicpoor massive to stratified zones were deposited by lower energy pulsating flow conditions. During the latter, lobes of pumice-rich ignimbrite detached from the main flow and advanced ahead of it.

The lithic-rich nature of the ignimbrite, as well as the coarse vent-derived lithic concentration zone, suggests that the eruption was intimately related to a period of caldera collapse.

References Marti J, Mitjavila J. and Arafia V. 1994. Stratigraphy, structure and geochronology of the Las Canadas caldera (Tenerife, Canary Islands). Geological Magazine 131(6), 115-121.

Nichols H. J., Wolff J. A., Larson P. B., Pittari A., Edgar C. J., Cas R. A. F. and Marti, J. 2001. The complex liistor\' of a caldera-forming magma: the El Abrigo Ignimbrite, Tenerife, Canary Islands. Eos Trans. AGU, 82 (47), Fall Meeting. Supplement, Abstract V42D-1059, 2001.

247

GEOCHEMISTRY AND FACIES OF CAMBRIAN VOLCANIC ROCKS, EASTERN WARBURTON BASIN, SOUTH AUSTRALIA ^

Xiaowen Suii' aiid Alaii Purvis ^ Petroleum and Marine Division, Geoscience Australia, GPO Box 378, ACT 2601; ^ Pontifex and Associates, PO Box 91 Kent Town, S.A. 5071

The Warburton Basin is an Early Palaeozoic petroleum exploration frontier basin. The eastern Warburton Basin contains at least 3500 m of flat-lying to folded volcanics, carbonates and sihciclastics. Evolution of the Warburton Basm was influenced by volcamc activity. The volcanics can be differentiated into two major phases, which can be divided into four major genetic groups of volcanics in nine facies.

The widely distributed early Mooracoochie Volcanics consist of rhyolite and rhyodacite lava flows, igmmbrites and volcaniclastic rocks. These were emplaced rapidly and in large volume during the Late Early to Early Middle Cambrian (517 ± 9 Ma). Analysed core samples have immobile elements characteristic of rhyolites and dacites. Facies analysis suggests a subaerial caldera in the Gidgealpa area; and a rhyodacitic cryptodome or synsedimentary sill in the vicinity of Kalladeina 1 Comparison with acid volcanics in the Meandarrah Rift in eastern New South Wales indicates a rifl settmg for these volcanics, similar to that of the slightly older (530-527 Ma) Mt Wright Volcanics in northwestern New South Wales.

The overlying late phase volcanics consist of basalts and picrites in Gidgealpa and Jena wells.

These comprise porphyritic and amygdaloidal basalt and hyaloclastite, episodically emplaced only in deep-water environments. These resulted in hyaloclastite intercalated within deep-water carbonates m several Gidgealpa wells and in submarine basalt facies within deep-water mudstone and shale in the Mudlalee to Kobari areas. Geochemical data on ten core-samples show that they are within-plate alkali-basahs, indicating renewed intra-plate extension. Trilobites in the Gidgealpa wells indicate that the age of the episodic basalt ranges from prior to Late Templetonian-Floran (512-514 Ma) to Mindyallan (505-509 Ma) Stages. These basalts may be coeval with the widespread Antrim Plateau Volcanics, with a dyke in the Kimberley region of Western Australia recently dated at 513 ± 12 Ma. These volcanics and associated sedimentary deposits indicate an initial and extensive continental rift setting that was subjected to marine incursion, with continued rifting maintaining deep water in areas with basalt flows. This interpretation will lead to a better understanding of tectonic histor>^ of basin architecture, and petroleum systems m the Warburton Basin.

The Mooracoochie Volcanics are similar to Early Cambrian (Atdabanian-Tommotan) acid and calcalkaline volcanics m northwestern New South Wales (Cymbric Vale Formation, Mt Wright Volcanics), tuff in the Heatherdale Shale in the Stansbury Basin, and vitric tuff in the Billy Creek Formation, in the Arrowie Basin. They are also similar to rather older alkali basalts in the Truro Volcanics in the Stansbury Basin (> 526 Ma), and the --586 Ma Mt Arrowsmith Volcanics in the Wonominta area of northwestern New South Wales. This suggests episodic within-plate alkaline volcanic activity from ^580 to -505 Ma, inboard of the Tasman Orogenic Zone, with more calcalkaline volcanics at 526-514 Ma, possibly with a "basin and range" setting.

248

CALDERA FORMING ERUPTIONS AT BATUR VOLCANO, BALI, INDONESIA Olivier Reubi School of Geosciences, Monash University, Vic 3800, Australia Batur IS an active stratovolcano on the island of Bali, Indonesia, with two well developed concentric calderas. Its volcanic activity may be divided into five main periods: (1) Building stage dommated by basaltic to andesitic lava flows.

(2) First caldera collapse associated with the eruption of dacitic ignimbrites.

(3) Formation of an andesitic to dacitic lava-dome complex and an associated tuff ring within the caldera.

(4) Second caldera collapse episode with eruption of a dacitic ignimbrite.

(5) Buildmg of a basaltic andesite stratovolcano within the second caldera.

Reconstruction of the pre-caldera morphology of Batur volcano indicates that it consisted of a main basaltic to andesitic cone in the NW sector flanked by a basaltic parasitic cone on its SE slope. The first caldera, which is centred in between these two cones, is 10 x 14 km in size and at least 700 m deep. Its floor is irregular, comprising a deeper zone in the NE sector and a NE-SW-oriented depression in the SW sector. The subsequent volcanic activity occurred mainly within these depressions and partially filled them. The second caldera collapsed within the first caldera. Its exact shape is unknown. However the NW rim suggests an elliptical shape with a 7 km long main NESW-oriented axis. The recent volcanic activity is located within the second caldera. The principal cone of the historically active Batur volcano is situated at the centre of the second caldera.

The eruption associated with the collapse of the first caldera, resulted in the deposition of a 100 m thick sequence of partially to densely welded ignimbrites and minor pumiceous fallout deposits within the caldera. The ignimbrites are typically 4-6 m thick, lithic-rich (20-30%) and comprise a wide range of lithic types. Three cooling units separated by non welded fallout deposits and pyroclastic flows are recognized. The deposits associated with the second caldera collapse consist, within the caldera, of a 40 m thick sequence of intercalated phreatomagmatic and magmatic surge and fallout deposits overlain by a 20-30 m thick, massive and lithics-poor (<10%) ignimbrite, which locally comprises a basal coarse lithic lag breccia. The lithics within these deposits consist essentially of black glassy dacite.

These observations suggest that the two calderas resulted from contrasting eruption d}iiamics. The first eruption is believed to be characterized by moderate to high discharge rate, possibly from multiple vents, over a prolonged period of time, which formed a collapsing fountain feeding pyroclastic flows. The second eruption occurred from a single vent and started with low discharge rate that resulted in the development of an unstable Plinian column. The discharge rate drastically mcreased during caldera collapse leading to the collapse of the column and deposition of the ignimbrite.

249

VOLCANOLOGY OF LEUCITITE VENTS AT EL CAPITAN, CENTRAL N.S.W.

"-ynm-hlinerals

O.L. Gonzalez\ I.C. Roach^ and K.G. McQueen^ ^ CRC LEME, University of Canberra, ACT 2601 Education Australia, CRC LEME, University of Canberra, ACT 2601 and CRC LEME, Australian National University, ACT 0200

Lavas of the eastern Australian Leucitite Suite are perhaps the most enigmatic of all the Cainozoic intraplate volcanics. A new investigation of the regolith and leucitite lava outcrops at El Capitan (Gonzalez 2000, 2001), approximately 56 km NE of Cobar, NSW, reveals much previously unreported detail and adds to research into the volcanology of these rocks. Lava flows at El Capitan now stand up to 50 m above the surrounding plain. The actual amount of relief inversion since midMiocene eruption is not known but is at least 17 m. Lavas are underlain by bleached pre-basaltic alluvial sediments which contam silcreted plant fossils. A number of small mineshafts indicate previous mineral exploration of these deep leads.

Leucitite outcrops stand out as bright curvilinear features amongst shorter wavelength anastomosing maghemite-filled channels and palaeochannels, draining to the northeast, on 400m line spacing 1st vertical derivative reduced-to-pole aeromagnetics. Two small circular dark spots on the aeromagnetics coincide with leucitite outcrops at Wilga Tank (on the southwest of the field) and Sunrise Tank (on the southeast of the field). On closer investigation the dark spots are revealed as small negative magnetic anomalies of 125 nT (Wilga Tank) and 225 nT (Sunrise Tank) below the regional background of c. 57,000 nT and are centred over the two outcrops. Further investigation of samples from these two outcrops confirms that they are reverse-magnetised (Brad Pillans pers. comm. 2001). We now believe that these two outcrops are vents that supplied the leucitite lava to the El Capitan field.

The Wilga Tank outcrop is semicircular, consisting of c. 300m' of radially-jointed basalt with abundant small mantle and crustal xenoliths. The outcrop has a small bench of scoria on its western side lying between the main outcrop and a smaller, perched lava flow capping stream sediments in a palaeochannel underlain by a palaeo-weathering profile. The Sunrise Tank outcrop is much larger and consists of a low, rounded hill. The edges of the lava pile here are covered by alluvium derived from the nearby Tooram Hills, consisting of a sequence of Devonian conglomerates. There are no obvious volcaniclastic rocks here, however the rocks also contain mantle xenoliths and are radially jomted. The Sunrise Tank outcrop is located over the Eagle Lineament, believed to be a major fault and the northern extension of the Gilmore Suture, and an ideal magma conduit. The Sunrise Tank locality appears to have been the major vent, from whence the majority of the El Capitan lavas erupted, and the Wilga Tank locality a satellite vent.

The remainder of the outcrops consist of thick pahoehoe lava flows with ropy-textured tops much like more modem weathered examples, for instance in the McBride and Atherton provinces of northern Queensland. Flows may have undergone considerable inflation. Flow edges are generally well exposed revealing numerous vertically layered sub-horizontal megavesicles oriented parallel to the flow tops, well formed columns and a layer of settled mantle and crustal xenoliths towards the base. A number of v-shaped micropegmatite veins are visible striking parallel to the flow edges.

These dip inwards towards the flow core and appear to behave similariy to inflation clefts, indicating expansion in the outer skin of the inflating lava flow and the core-ward propagation of a crack, later filled by micropegmatite as the flow core slowly crystallised.

References Gonzalez O.L. 2000. Volcanic vents at 'El Capitan', Cobar N.S.W. LA VA News 3, 18-20.

Gonzalez O.L. 2001. Landscape evolution of the El Capitan area, Cobar, NSW. B.App.Sc. Honours thesis, Universit}^ of Canberra, unpublished.

250

BASSIAN BASALTS: DATING, CENOZOIC BIOGEOHISTORY AND A NEW MODEL FOR TASMANIAN VOLCANISM FX. Sutherland\ S.M. Forsytlr and H. Zwingmann^ ^Geodiversity Research Centre, Australian Museum, Sydney, NSW 2010 -Mineral Resources Tasmania, PO Box 56, Rosney Park , Tas 7018 ^Petroleum Exploration Teclmologies, CSIRO Petroleum, Perth, WA 6102 S. M. Fors>th publishes with permission of the Director, Mineral Resources Tasmania Cenozoic basalts extend across Tasmanian Bassian shores and islands and appear within Bass Basin sedimentary sequences. Along northern Tasmania, substantial valley-fill flows interleave with palynological zones in terrestrial sediments. Valley thalwegs mostly descend north to Bass Strait and extend below sea-level. On the northwestern coast, aquagene basalt ages are constrained in age by foraminiferal biozones in high-stand marine beds. Within the Bass Basin, basalts intersect either terrestrial or marine biozone. New basalt K-Ar dating from Bassian islands and northern Tasmania are combined here in a review of Bassian volcanic/stratigraphic relationships. This leads to a radically modified model for Bassian/Tasmanian volcanism.

A basamte plug m central King Island (61.7±1.3 Ma) is the oldest known Tasmanian Paleogene basalt, whereas an alkali basalt flow at sea-level on northern Flinders Island has a Neogene age of 20.0±2.7 Ma. Thick alkali basalt flows within the Tamar Trough have Paleogene ages 29.4±0.9 Ma and overlie Nothofagidites asperus Zone beds, whereas other basalts overlie Proteacidites tiiberculatus Zone beds. This and other evidence suggests several extrusive ages. The main lava field m northwestern Tasmania (Seymour 1989) yields basalts within Upper N. asperus to lower P.tiibercidatiis Zones that correlate with a 40-24 Ma age range, compatible with basalts dated from these sequences (38-26 Ma). Northwestern aquagene basalts overlap marine beds within forammiferal zones N4/5-N9 that correlate with a 23-16 Ma age range (Quilt}^ & Telfer, 1994).

Later, restricted alkaline lavas and centres have been dated at 16-8 Ma. Bass Basin includes latest Cretaceous/Paleogene basalts (minimum age 54 Ma), Eocene-Oligocene basalts (Tasmanian Devil1) and Lake Oligocene-Early Miocene aquagene basalts and intrusives (28-20 Ma).

In Tasmania, the most voluminous lava fields and aquagene basalts occupy the northwesterncentral area and include extensive tholeiitic and transitional basalts in the petrological spectrum (Seymour 1989). This distribution indicates that a significant thermal anomaly was present to generate these basalt magmas. A recent tectonic model for Bass Basin evolution advocated a Late Cretaceous (?) elongated NNW-SSE embryonic seafioor spreadmg rift, that formed mafic doming under the Cenozoic sedimentary sequence (Gunn et al. 1996). A deep mantle thermal anomaly below Bass Basin, initiated around 65 Ma, could provide long term melting under overriding, northward migrating Tasmanian lithosphere. Using post-65 Ma absolute motions for SE Australia (Sutherland, in press), a Bassian mantle plume combined with more eastern thermal anomalies from earlier Tasman rifting gives a more complete model for Tasmanian volcanism than yet provided.

References Seymour, D.B.(compiler) 1989. Geological Atlas 1: 50,000 Series Sheet 36 (8015N). St Valentines.

Explanatory Report Geological Sun^ey of Tasmania, Hobart.

Quilt}^ P.G. and Telfer, A. 1994. Marine Neogene samples from around Tasmania: an extension to the Miocene/Miocene marine record in Tasmania. Papers and Proceedings of the Royal Society of Tasmania^ 128, 41-56.

Gunn, P.J., Mitchell, J.N. and Meixner, T.J. 1996. The structure and evolution of the Bass and Durroon Basins as delineated by aeromagnetic data. AGSO Record 1996/4 (unpublished).

Sutherland, F.L. in press. /;?. Muller R.D. and Hillis R.R. eds. 'Boomerang' migratory intraplate Cenozoic volcanism, eastern Australian rift margins and Indo-Pacific mantle boundary. The Evolution and Dynamics of the Australian Plate. Joint Special Publication of the Geological Societies of Australia and America.

251

EXPLOSIVE PLINIAN ACTIVITY AND DISPERSAL IN THE ANDES: FACIES AND INTERNAL STRATIGRAPHY OF THE LATE MIOCENE CORTE BLANCO TUFF; PUNA PLATEAU, SALTA PROVINCE, NW ARGENTINA M.A. R.A.F. Cas^ and J.G. Viramonte' ^ School of Geosciences, Monash University Clayton, PO Box 28E Victoria Australia, 3800 ^ Institutio Geonorte, Universidad Nacional de Salta, Buenos Aires 177, (4000) Salta, Argentina ' Email: tait@maiLearth.monash.edu.au; Phone: +61 3 9905 4879; Fax: + 61 3 9905 4903 Explosive silicic volcanism on the Puna Plateau of the Central Andes has been extremely widespread smce the Middle to Late Miocene and continues today. Typically, this type of activity has produced thick pyroclastic flow (ignimbnte) deposits, emanating from large collapse calderas (e.g. Cerro Galan, La Pacana). Very few examples of large volume plinian fall deposits are recogmsed in the region, which may be a direct result of the extreme altitude at which these eruptions occur and the lower density atmosphere into which explosive eruption columns are propagated.

When compared with other explosive silicic eruptive products on the Andean Puna, the Late Miocene Corte Blanco Tuff (CBT) is unique. The source for the CBT is defined as the Ramadas Volcanic Centre (RVC) located in NW Argentma's Salta Provmce at an altitude o f - 3 7 0 0 m. The RVC is described as a caldera-like structure, geomorphologically defined by a 4 x 5 km "amphitheatre", set within hills of uplifted Precambrian basement. Presently, thick Quaternary conglomerates cover much of the local landscape, hence the RVC and its associated deposits are largely obscured.

Broadly, the CBT is descnbed as a cry^stal-poor, white, gametiferous rhyolitic pyroclastic sequence, with a juvenile population overwhelmmgly dominated by spectacular tube pumice, and a highly variable lithic content. The nature of the CBT's internal stratigraphy is poorly understood, and the mechanisms of the eruption remain controversial, a problem complicated by a lack of continuous outcrop. This work considers the individual internal facies of the CBT and presents new interpretations for their emplacement, based largely on detailed field geology.

The CBT comprises a wide range of facies types including a coarse lithic lag breccia and a thick sequence of perlitised pyroclastic surge and intercalated fallout deposits, located within and surrounding the vent margins. The preserved sequence is up to 20 m thick, 20 km from source and includes basal surge deposits and subordinate massive, ash-rich units, interpreted as intra-plinian igmmbrites. The dominant facies in the sequence is a stratified, well sorted, clast-supported pumiceous lapillistone, which has previously been described as a fines-depleted ignimbrite.

However, the extreme angularit}^ of both juvenile and lithic clasts and the diffusely stratified nature of the deposits are consistent with aerial transport and deposition from a sustained, plinian-style eruption column and downwmd plume. All known deposits of the CBT are located to the east of the Ramadas centre, with geochemically correlatable outcrops recognised over 300km to the east of the RVC. These distal tephra comprise well-sorted, graded, fine-grained ash units that are still >lm thick and are consistent with widespread pyroclastic dispersal and deposition. The complete lack of outcrop to the west of the RVC is consistent with instantaneous deflection of the column by strong upper atmospheric w^inds.

The CBT sequence differs from other large volume silicic volcanic deposits on the Puna, with the dominant transportational and depositional mechanism involving plinian fall-out from a sustained and widely dispersed eruption column and strongly deflected downwind plume. The new facies interpretations presented here underline the need for reconsideration of the style of eruption in terms of Andean Volcanology.

252

A FURTHER INSTALMENT IN THE RABAUL STORY: THE PLOTS THICKEN Patricia Wallace\ Richard Arculus^ and Stephen Eggins" ^Department of Geology, Australian National Universit>, Canberra "Research School of Earth Sciences, Australian National University, Canberra The 1994 volcanic eruption at Rabaul highlighted an awareness of the inadequacy of our understanding of the local processes leading to such events. In the ensuing years the work of the Rabaul Earthquake Location and Caldera Structure (RELACS) program (1999), Nairn et al (1995), Wood et al. (1995), Tregonmg et al. ( 2000), and Smton et al. (2002) amongst others, have contributed greatly to our knowledge of the local Rabaul system, the regional tectonics and the influence of back-arc spreading in the Manus Basin.

The present study is an investigation of the petrology and geochemistry of the currently dormant volcanic centres, including Watom Island, in the immediate environs of Rabaul. This serves to supplement the investigation by Herman Patia on the recent volcanics and provide a more complete context to formulate a model for the petrology and geochemistry of the Rabaul system.

Geochemical analyses (XRF, ICP-MS, and laser ICP-MS) completed to date, indicate both differences and similarities between the volcanic centres. The lavas from Turunguna (all <50% Si02) are significantly more HREE depleted than other Rabaul lavas. Most of the erupted magmas have a progressive K and other LILE enrichment with increasing distance from the trench i.e. from SE to NW, but this also correlates with relative age.

The mclusion of samples dredged from Raluan caldera (Sonne 68 - OLGA II, 1990) confirms the existence of low-K magma at Rabaul in contrast to medium to high K compositions from the other volcanic centres.

References Gudmundsson, O., Johnson, R.W., Finlayson, D.M., Nishimura, Y., Shimamura, H., Terashima, A., Itikarai, L, Thurber, C., 1999. Multinational seismic investigation focuses on Rabaul Volcano. EOS Transactions, American Geophysical Union, Vol. 80, No 24:269.

Nairn, I. A., McKee, C.O., Talai, B., Wood, C.P., 1995. Geology and eruptive history of the Rabaul Caldera area, Papua New Guinea. Journal ofVolcanology and Geothermal Research, 69:255-284.

Sinton, J.M., Ford, L.L., Chappell, B., McCulloch, M.T., 2002. Magma genesis and mantle heterogeneity in the Manus back-arc basin, Papua New Guinea. Journal of Petrology (in press).

Tregoning, P., McQueen, H., Lambeck, K., Jackson, R., Little, R., Saunders, S., Rosa, R , 2000. Present-day cmstal motion in Papua New Guinea. Earth Planets Space. 52: 727-730.

Wood, C.P., Nairn, I.A., McKee, C.O., Talai, B., 1995. Petrology of the Rabaul Caldera area, Papua New Guinea. Journal ofVolcanology and Geothermal Research, 69: 285-302.

253

LAVA POOLS IN THE MAIN RANGE VOLCANICS, TOOWOOMBA, SE QLD E.G. Willey University of Southern Queensland (and 35, Mina Street), Toowoomba,

Queensland

The numerous small eruptive centres, characteristic of the Tertiary Main Range Volcanics (MRV) around Toowoomba, contrast with the large Tertiary central type volcanos described elsewhere in the region (Kean 1993; Ashley et al 1995 and references therem). In Toowoomba, the MRV comprises basalts and interbedded tuffs, with rare boles, sediments and brown coals. It rests on a land surface with thin, generally immature palaeosols developed on freshwater Mesozoic sedimentary rocks.

Post-MRV laterites form hilltop-capping outhers. Another laterite interrupts the MRV sequence, separating basalt dated at 27.3Ma from basalts above this profile dated at 21.4 to 18.2Ma. A reference section (Warrego Highway, Main Range crossing) shows a 330m sequence of 1620 horizontal flows; elsewhere tuffs are more common. For more details see Willey (in press). Eruptive centres typically display an explosive crater-forming phreatic stage followed by an effusive magmatic stage with lava filling the phreatic crater; an explosive magmatic stage may occur between these two stages. Before eruption, heat from rising magma has altered undisturbed country rock; previously this was attributed to weathering.

The phreatic stage generates accidental tuffs containing segments of fresh basalt columns up to 1 m in diameter, smaller fresh to slightly altered basaltic fragments, quartz grains, pebbles and blocks from the underlying Mesozoic strata, and laterite fragments; recycling of earlier tuffs also occurred.

Near eruptive centres, tuffs are poorly to very poorly sorted, with 20-200 mm diameter fragments, but ranging up to lOm^ layering is steep and poorly defined. Away from the eruptive centre, the flatter-lying, moderately to poorly sorted ash to medium-grained lapilli tuffs show well-defined layering, typically parallel (mantling) or rarely cross-bedded (tuff-flow), with occasional ballistic blocks.

The magmatic stage fills the crater with lava, which may show differentiation of xenoliths/ xenocrysts and volatile segregation to form pegmatites. Overflow from the crater generates the more t\^ical horizontal lava flows. When an intervening explosive magmatic stage occurs, typically flattened/sheared bombs up to 1.50 m long line the crater interior which, being warm, inhibits development of a vesicular base to the crater-fill basalt. With no explosive magmatic stage, bases of the crater-fill basalts have zones of stretched vesicles. Rootless fumarolic conduits (due to dewatering of undisturbed underlying tuffs) cut basalts. Conduits often rise from depressions in underlying tuffs, are common in flows, and also disrupt crater-fill basalts.

Cooling jointing in the crater-fill lava results in vertical columns at the top of the mass; they curve downwards and outwardly meeting colimins perpendicular to the underlying sloping crater walls.

This pattern is modified around dewatering fumarolic conduits. The erosion of tuffs and underlying altered country rocks (basalts, etc.) of these eruptive centres results in prominent rocky hills surrounded by screes. Similar features also lie buried within the MRV. Many fresh basalt crater fills are cut by single anomalous rock-floored streams, not attributable to erosion. These streams may be localised along surface cracks resulting from post-solidification 'intumescence' beneath the centre.

Within 300 km' around Toowoomba 30 eruptive centres have been recognised; most are isolated, but one linear arrays suggests possible feeder dykes. The nature of these eruptive centres suggests a lava field (Johnson 1989).

References Ashley P.M., Duncan R.A. & Feebrey C.A. 1995. Ebor Volcano and Crescent Complex, northeastern New South Wales: age and geological development. Australian Journal of Earth Sciences 42, 471-480.

Johnson R.W. (Ed) 1989. Intraplate volcanism in eastern Australia and New Zealand. Cambridge University

Press, Cambridge.

Kean D. 1993. The Geology and geochemistry of the Mt Alford Intrusive Complex of Southeast

Queensland.

Honours Thesis, University of Queensland.

Willey, E.C. (in press) Urban geology of the Toowoomba conurbation, SEQld, Australia, in Bobrowski P.T. and Catto, N. (Eds) Urban Geology: Australia and New Zealand. Quaternary International.

254

CHEMICAL AFFmiTIES OF LOEI BELT VOLCANIC-INTRUSIVE ROCKS, CENTRAL THAILAND Della-Pasqua F.N.\ Khin Zaw" ^ School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Austraha.

Email: femando. dellapasqua@utas. edu. au ^Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Australia Email: Khin.Zaw@utas.edu.au

Introduction The Loei belt is a mineralized volcano-plutonic belt hosting major Cu-Au and stratabound base metal deposits. It forms an arcuate N-S trending belt, sandwiched between the Shan-Thai and Indochina terranes, and extends across central Thailand into Laos. Major and trace element characteristics of 38 samples collected along the Loei belt, include rocks of alkaline and subalkaline affinities. Sub-alkaline rocks vary in composition from basalt, andesite, dacite and rhyolite, whereas those of alkaline affinities are basalts only.

Geochemistry The sub-alkalme basaltic rocks have Mg# <65, with <8.5 wt % MgO, 15-19 wt % AI2O3, 5-11 wt % CaO, 3-5 wt % NasO, <2 wt % K2O, <30 ppm Ni and <250 ppm Cr. Ni and Cr contents are also lower relative to MORB values. The alkali basalts instead are characterized by higher Ti02 (L3-2 wt %), Nb (20-30 ppm), Ni (70-140 ppm) and Cr (150-900 ppm) contents. Intermediate rocks include andesite, dacite and trachyandesite. Overall, the intermediate rocks preserve similar features as those of more mafic rocks, with strong to weakly developed MORB-normalised Nb and Ti anomalies, variable enrichment of mobile elements, and increasing depletion of compatible elements Ni and Cr. The felsic (rhyolite) rocks constitute the more evolved end-member compositions for the suite. Compositionally, the felsic rock samples are characterized by higher S1O2, K2O, Nb, Rb and lower AI2O3, MgO, CaO, P2O5, Sr and Sc. MORB normalized values of felsic rocks also show strong LILE enrichment. Ba, P, Ti and Cr anomalies are also well developed, but a Nb anomaly absent; and there is a marked depletion in compatible elements Cr and Ni. Because the samples were collected from a large area along the Loei belt and there is very little chronological control, it is difficult to develop a well-constrained model for the origin of these rocks. Nonetheless, the composition of least fractionated (basaltic) samples clearly indicates alkaline and sub-alkaline affinities. Compositions of alkali rocks fall within trace element fields of within-plate basalts and are transitional betw^een alkali and tholeiitic affinities. Sub-alkaline basalts fall within the continental volcanic arc fields. With the exception of the alkaline basalts, all mafic rocks have similar trace element features including Ti depletion and variable LILE enrichment. The alkali basalts have distinct trace element patterns compared to other Loei mafic rocks, and are therefore regarded as a compositionally distinct group.

Conclusions Major and trace element composition of the mafic rocks indicate that the mantle source to these rocks IS relatively enriched LIL elements with respect to a MORB source. Chemical characteristics of these rocks are consistent with a mantle source that has been modified by LILE enrichment process. Subduction is one likely process and thus the transitional affinities of some of the alkali basalts could be reconciled with a partially enriched mantle source, possibly by early subduction.

Further accurate geochronological data is required to determine the timing relationships between volcano-plutonic units and mineralisation events in the Loei belt.

255

THE GOLDEN MILE, KALGOORLIE: C H A R A C T E R A N D C O N T R O V E R S Y Roger Bateman^ and David Groves^ ^Instimte ofEarth and Music Sciences, 2/579 Haiman St Kalgoorlie, WA 6430, Australia Email: rogerbateman@bigpond.com ^Centre for Global Metallogeny DepartmentofGeologyandGeophysics,Universit}^ of Western Australia, Crawley WA 6009 Australia A giant Archaean gold deposit The Fimiston lodes in the Golden Mile (Kalgoorlie, Western Australia) consist of about 1000 named lodes, including Oroya Shoot, and have been mined nearly continuously since their discovery in 1893.

Up to 2001, over 45 M oz had been mined, and there remain reserves of 12 M oz. In 1903 alone, 1.226 M oz of gold were recovered at an average grade of 41 g/t. It is a giant in both absolute terms of ounces of gold, and in relative terms when compared with other deposits in the Yilgam craton.

Character: the gold-pyrite-telluride lodes of the Golden Mile Fimiston gold deposits, m the Eastern Goldfields Provmce (Yilgam Craton, Western Australia), occur m mafic and ultramafic lavas erupted at 2710-2690 Ma. These are overlain by felsic volcanogenic sandstones, conglomerates and shale (2660-2680 Ma). These rocks were then intruded by the Golden Mile Dolerite (2675 Ma) and other tholeiitic gabbro sills. Four deformation events are recognized: Di thrusting, followed by D2 east-west reverse faulting, cuhninating in right-lateral D3-D4 strike slip movement. Mineralization at Fimiston consists of gold-sulphide-telluride breccia lodes with crustiform-open cavity fill textures. Mesothermal and epithermal styles have been proposed.

Mmeralization of Mt Charlotte style is represented by quartz veins that overprint Fimiston lodes. The two mineralization styles are quite distinct in timmg and inferred fluid composition.

Controversy: early- or late-orogenic timing Early-orogenic timing is based on the fact that all fault generations cross-cut the lodes. While some lodes do cross from Golden Mile Dolerite into the so-called Kalgoorhe Syncline, no lode crosses the Di Golden Mile Fault, some lodes terminate against it, and deformed remnants of lode occur within the fault zone. D2 reverse faults offset the lodes and the Golden Mile Fault by 1-10 metres in the Western Lodes, with no evidence that the Golden Mile Fault has been reactivated after D2. The Oroya Shoot was formed as a dilatant zone during D2. In D3, lodes were again deformed: some deformed lodes lie along east-dipping D3 shears, but many lodes are undeformed and dip steeply west, and terminate against D3 faults. Quartz-stockwork deposits such as Mt Charlotte and Mt Percy formed during D4 movements, and these stockworks cross-cut Fimiston-style lodes. Fimiston formed late in late Di. A late-orogenic timing is based on less direct geological evidence and recent geochronology. The Golden Mile is identical in terms of structures and lithologies that control other gold deposits in the Kalgoorlie Terrane, which are demonstrably late-D2 to D3 in age. It is prominent in all prospectivity maps that assume that the present geometry on maps is essentially identical to that at the time of gold mineralization, as are all other major gold camps. Stress mapping demonstrates that the Golden Mile is a low mean-stress site only if the D3 Golden Pike Fault was present at the time of mineralization, and one of the reasons for the giant size of the Golden Mile is the local duplication of the host Golden Mile Dolerite by a Di structure. Additionally, a U-Pb m zircon SHRIMP age for a lamprophyre dyke (s>aiOroya Shoot) is 2638 ± 6 Ma. This age is late-D2 or D3, and is equivalent to some robust ages of several other gold deposits in the Yilgam Craton. The detailed geological observations at the mine scale that suggest an early-orogenic timing are attributed, in this interpretation, to the non-propagation of brittle-ductile stress across Di ductile shear zones and to minor reactivation of suitably-oriented D2 structures during D2 to D4 events in which oi was in a broadly similar orientation to D2.

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THE PROMINENT HELL COPPER-GOLD DISCOVERY: EXPLORATION STRATEGIES FOR WORLD-CLASS IRON OXIDE CU-AU DEPOSITS Antonio P. Belperio Minotaur Resources Ltd 3 Boskenna Avenue NORWOOD SA 5067 Australia

The blmd Cu-Au-Ag-U-REE discovery at Prominent Hill, 800 km north of Adelaide, by Minotaur Resources in November 2001 has dramatically re-ignited interest in exploration for world-class Olympic Dam-style iron oxide-hosted deposits. It has also forced a re-think of exploration paradigms and models that have long placed magnetite-hosted and haematite-hosted Cu-Au systems at opposite ends of hydrothermal genetic pathways. The discover}^ has also vindicated the junior/major exploration alliance strategy developed between Billiton Exploration (now BHP Billiton) and Minotaur Resources.

Durmg the 1980s and 1990s, CRA Exploration, Burmine and more recently Normandy Exploration demonstrated that the Mt Woods area was one of significant and widespread iron-oxide alteration reflected in the occurrence of large hydrothermal skams and magnetite and haematite breccias. The alteration zones, contain varying amounts of base metal, with early drill holes intercepting intervals such as 287 metres at 0.23% copper and 186 metres at 0.13% copper in magnetite breccias. Much of the exploration was focussed on the Joes Dam and Manxman prospects that showed great similarities to the Ernest Henry or Cloncurry-style of mineralisation.

Exploration interest by Minotaur and Billiton focused on a large number of higher crustal level targets that had not been adequately drill tested. From 3D inversion modelling of gravity and magnetic data, six targets (Armstrong North, Neptune, Uranus, Peculiar Knob North, Manxmen B, and Blaze), were selected for testing in the first year of the joint venture. Prominent Hill, the fourth of the SIX targets to be tested, was drilled in November 2001 and deepened to 720 m in December.

The vertical hole passed through 108 metres of younger sediments before intersecting a massive haematite-supported breccia. Original rock textures are almost totally destroyed with siliceous, sericitic rock fragments varying fi'om a rock-flour to clasts several centimetres in diameter.

Haematite dominates both the matrix and breccia clasts. Gold is concentrated in the top twenty metres of the drill hole, but is anomalous throughout. Copper mineralisation as chalcocite dommates the upper part of the drill hole and occurs as disseminations and thin veins within haematite matrix breccia. Copper mineralisation as bomite and chalcopyrite is variably developed from 450 to 581 m, together with the appearance of high grades of rare earth elements (cerium + lanthanum) and uramum. From 581 m, the drill hole entered a zone of rocks dominated by volcanic lavas and apparently unmineralised silica-haematite breccia. The presence of volcanic lavas intercalated with the breccia suggests a volcanic orifice setting, similar to that at Olympic Dam. The extensively milled nature of the main haematite breccia, the zonation of mineral types (gold, silver, chalcocite, bomite, chalcopyrite, uramum, cerium and lanthanum) withm it, and a central core of barren silica-haematite breccia all present a remarkably similar scenario to the zonation of rock types and minerals seen at Ohmpic Dam. Step out drilling of the Prominent Hill Prospect commenced in February 2002, guided by a number of geophysical surveys that better outline the host iron body and the likely trend of mineralisation.

Results from this drilling will allow^ the structural trends and controls, and the extent of mineralisation to be more accurately defined. The Mount Woods Joint Venture involves Minotaur (earning 19%), as operator and BHP Billiton (earning 51%), and Normandy Exploration Pty Ltd (dilutmg to 23.94%), Sons of Gwalia Ltd (diluting to 3.78%), Sabatica Pty Ltd (dilutmg to 2.28%).

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ORE GENESIS OF A NICKEL LATERITE DEPOSIT: THE GEOLOGIST'S PERSPECTIVE David Crook BSc, MAIMM Exploration Manager Heron Resources Limited.

Nickel laterite mineralisation is hosted in the weathered mantle of the Walter Williams Formation, the lowermost, predominantly adcumulate facies of the Linger and Die Group (GSWA, 1993). This stratigraphic position hosts the Goongarrie, Cawse, Sibena and other lateritic nickel deposits.

The Linger and Die Group can be traced over an area of at least 35 x 150 km, which makes it one of the most extensive komatiite bodies m the world. The Walter Williams is estimated to be 600-900 thick and forms the lower part of the overall komatiite unit. This lower sequence consists of a basal orthocumulate, succeeded by mesocumulate, adcumulate, olivine harrisite and a thin olivine orthocumulate.

The olivine adcumulate sequence is thought to have formed as accumulations of olivine crystals formed by crystal sedimentation from a continuously erupting komatiite magma passing through insulated lava tubes. (Hill et al 1988). ''Nickel Laterite'' mineralisation is used to describe accumulations of nickeliferous clays or iron oxides formed as a result of the extreme weathering of susceptible komatiite rocks.

Nickeliferous laterite mineralisation has formed by weathering processes since the Tertiary, and is generally restricted to olivme adcumulate - mesocumulate komatiites. The genesis of nickel laterite ores mvolves a combination of removal of soluble minerals and elements, resulting in the concentration of the more resistant elements within the extremely leached upper weathering profile, and concentration by supergene processes, forming watertable-related enrichment horizons within the upper weathering profile.

The clay upper regolith position is characterised by massive, earthy clays, sometimes described as ''plasmic", comprised predominantly of goethite with varying proportions of haematite and magnetite (or maghemite) and kaolinite or halloysite (both "kandites") and gibbsite.

There is a very strong positive correlation between Ni and FeO, and Co with Mn in the clay upper profile, which clearly suggests that Ni substitutes mto the goethite lattice while Co is included in the manganese oxide/hydroxide lattice.

S.G. decreases as nickel grade increases due to progressive weathering and removal of soluble minerals (MgO, Si02 from weathering antigorite and chlorite) resulting in a clay upper profile that is rich in resistant minerals such as goethite and kaolin.

Once all soluble minerals have been removed the profile will have condensed by a ratio of about 4:1 by volume, which alone will result in a grade of about 1% Ni within an ore that now consists predominantly of goethite and kaolin or gibbsite.

Additional Ni and Co is introduced into the ore profile by supergene processes, including the deposition of manganese oxides and hydroxides, raising the grade of the ore beyond 1% Ni.

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SOME SOUTH AUSTRALIAN EXPLORATION ANALOGUES Sue Daly and Colin Conor Minerals Resources Group, PIRSA South Australia is significantly under-explored because of predominantly thin but persistent sedimentary cover. Exploration activity is essentially model driven because of poor outcrop. A greater understanding of local mineralised terrains, as well as carefully selected examples fi-om elsewhere in Australia and internationally, will encourage focussed, successful exploration in South Australia. Three important analogues are Olympic Dam, Broken Hill and Mount Isa. Olympic Dam Cu/U/Au/Ag/REE Deposit (1620 Mt 1.1% Cu, 0.4 kg/t U O 2.4 g/t Au, 0.4 g/t Au) analogues most likely to be found in the central and eastern margin of the Gawler Craton and the Cumamona Province.

IS hosted by polyphase brecciated and sericite/hematite altered Hiltaba Suite granite, contains breccia bodies onentated parallel to major strike-slip faults. host breccia preserves restricted layering produced by hydrothermal streaming and hydraulic fracturmg. comprises ore deposited from hot (150-350°C), reduced, iron rich fluid in contact with cooler oxygenated surface water. Sulphide zoning occurs with pyrite at depth, followed by chalcopyrite, bomite and chalcocite.

Sm/Nd values indicate mantle sources for metal. contains Gawler Range Volcanics, ultramafic and mafic intrusives that show hyaloclastic and explosive contacts with wet breccia. deposit age is indistmguishable from the 1592 ± 2 Ma Gawler Range Volcanics.

Broken Hill Pb/Zn/Ag Deposit (280 Mt 10% Pb, 8.5% Zn 148g/t Ag) analogues are likely to be found in the Cumamona Province. is essentially stratiform, where characteristic zinc and manganese anomalous host sediments, can be traced for tens of kilometres.

IS hosted by a siliclastic-dominated basin that contains interlayered 1690 Ma acid and basic volcanics/subvolcanics. Mineralisation appears to be coincident the cessation of volcanism. A Pb/ Pb model age of 1675 Ma is interpreted to reflect metamorphic modification, is zoned, pyrite/pyrrhotite poor, high in iron, manganese, lead, zinc, silver, carbonate, phosphorus, fluorine and gold, but relatively low in copper, with a characteristic ore halo rich in manganese and silica. contains metal deposited from warm (100 - 200°C) saline, dense, reduced fluid (distal from volcanic sources) contaimng low levels of reduced sulphide (H2S) interacting with cold oxidised seawater. Sulphur isotopes indicate a predommantly magmatic source either directly or leached from volcanics in underlying sedimentar}^ pile, was multiply deformed by the high grade c. 1600 Ma Olarian Orogeny.

Mount Isa Pb/ Zn Deposit (150 Mt 7% Zn, 6% Pb, 150 g/t Ag) analogues are most likely to be found within the, carbonaceous northern Cumamona Province and the central and eastem Gawler Craton. host sediments are dolomitic, pyritic, 1653 ± 7 Ma, carbonaceous siltstones (TOC < 7%). ore was deposited from cool (<200°C) oxidised, near neutral, saline sulphate bearing fluid at the sediment seawater interface. Sheet like mmeralised lenses preserve primary, often delicate, sedimentary textures. base metal sulphides were produced by biogenic sulphate reduction where heavy S values indicate a closed or partly closed basin. ore fluids were focussed by major faults as confirmed by geochemical vectoring. Fe and Mn abundances in dolomite increases toward ore.

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THE MOUNT ISA~LAWN HILL ZN-PB-AG & CU PROVINCE. ^Paul Gow, ^lan Garsed, Alice Clark^ ^doMIM ExT)loration Pty Ltd, GPO Box 1042, BRISBANE QLD 4001 ^CDEK, 5 Spruce Si Mount Isa 4825 The Mount Isa-Lawn Hill Zn-Pb-Ag and Cu province hosts the world class Proterozoic sedimenthosted Zn-Pb-Ag deposits at Mount Isa, Hilton/George Fisher, and Century, and the metasedimenthosted Cu system at Mount Isa. Second tier deposits include the Lady Loretta Zn-Pb-Ag and Mount Gordon Cu systems. The area records a protracted Proterozoic geological evolution (O'Dea et al., 1997), spanmng the Barramundi Orogeny (--1870 Ma), at least three extensional events resulting in deposition of several volcano-sedimentar>^ 'cover sequences', and compression, mversion and wrenchmg of the Isan Orogeny (1590-1500 Ma).

Various ore genesis models have been proposed. The Zn-Pb-Ag deposit models range from syngenetic or diagenetic/early burial (basin-stage) to syn-orogenic replacement (orogenic-stage).

Current thinking on models for Cu mineralisation is more uniform, with a s}ai-Isan Orogeny, late metamorphic model generally proposed. Despite the variety of ore genesis models, there are key geological elements common to most of the models which need to be understood to effectively explore for these deposit t>T3es. These include: •

•

•

the structural architecture of both the basement arid the Proterozoic basins. There is a strong spatial relationship betw^een crustal-scale structures (Mt Isa, Termite Range, and Mount Gordon Faults) and the key deposits. Large-scale structures that are exposed or under thin Phanerozoic cover have generally been mapped and/or recognised. The challenge exists to delineate the reactivated deeper-seated structures which are only expressed at surface by subtle structural indicators in the host cover sequences. the basin settings in terms of litho- and sequence-stratigraphy. With the exception of the Mammoth deposit, there is a distinctive set of host rocks that is common to all deposit types (carbonaceous, pyritic or dolomitic siltstones and shales). These host rocks form both mechanical and chemical traps for diagenetic-epigenetic mineralisation, as well as being indicators to active growth fault environments where Zn-Pb-Ag accumulations may form.

Deposition of favourable host lithologies occurred in localised basins adjacent to the crustalscale faults during a number of discrete tectonic events (Southgate et al, 2000). the thermal and deformation history. Recent ore genesis models highlight the link between the establishment of fluid flow systems and the tectonothermal history. These include the Century deposit model (Broadbent et al, 1998) which emphasises the role of tectomcally driven migration of basin fluids, and the convection-driven model for the Isa-Cu system (Heinrich et a l , 2001). In terms of targeting syn-orogenic Cu mineralisation, interaction between the preexistmg architecture and late-Isan Orogeny deformation is crucial. Reactivation of both the early extensional architecture and the early-Isan Orogeny fault systems has produced complex surface map patterns, and played a role in creating favourable sites, particularly fault-fold systems, for s>ii-orogenic Cu mineralisation.

References Broadbent et al 1998. Geology and origin of the shale-hosted Zn-Pb-Ag mineralization at the Centuiy deposit. Northwest Queensland, Australia, Economic Geology, 93, 1264-1294.

O'Dea et al 1997. Geodynamic evolution of the Proterozoic Mount Isa terrain, In: Burg, J.-P. & Ford, M. (eds.) Orogeny Through Time, Geol. Soc. Spec. Publ. No. 121, 99-122.

Heinrich et al 2001. Large-scale advection and small-scale fluid mixing: A geochemically constrained hydrod}Tiamic model for Cu mineralisation at Mount Isa, EGRU Contrib. 59, 78-79.

Southgate et al 2000, Chronostratigraphic basin framework for Palaeoproterozoic rocks (1730-1575 Ma) in northern Australia and implications for base-metal mineralisation, Aust Jour Earth Sci, 47, 461-483.

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METHODOLOGY FOR COMPUTER-ASSISTED INTEGRATED TARGETING FOR WORLD-CLASS ORE DEPOSITS IN AUSTRALIA, WITH EMPHASIS ON OROGENIC GOLD DEPOSITS. David I. Groves^'Carl Knox-Robinson^ and Warick M. Brown^ ^ Centre for Global Metallogeny, University of Western Australia, Crawley, Western Australia 6009 ^ Predictive Mineral Discovery Co-operative Research Centre ^ Spatial Analysis Services Over the last decade, the discovery rate of world-class to giant mineral deposits has been low, and the cost per unit metal high, despite globalisation of the mining industry and increased access to new, potentially well-endowed terranes. If discovery rate of superior ore bodies is to increase as mmmg companies are forced to explore increasingly more covered terranes, there needs to be a paradigm shift in the way exploration is conducted, with greater emphasis on geological concepts and data mtegration.

For the fixture, it will be important to develop a quantitative, integrated, tectonic- through districtto deposit-scale, fiill-coupled physical-chemical modelling capacity capable of predicting generic deposit styles not yet identified. This is the aim of the recently established Predictive Mineral Discover}' CRC. However, there is already the under-utilised geological potential to predict the potential metal endowment of terranes, from province- to district-scale, and hence focus intensive geochemical, geophysical and drilling campaigns into small, highly-favourable geological environments. To do this, superior geological information is required through sophisticated mterrogation of continuous, high-qualit}^, airborne geophysical and other remotely-sensed data sets.

For predictive geological targeting of known mineral-deposit styles, there are a number of requirements: 1) a sound, across-scale knowledge of those deposit styles, 2) a clear perception of controlling geological parameters that can be identified in regional exploration databases, 3) quantification of those parameters in known well-endowed terranes, provinces and districts, and 4) integration of those critical parameters into a metal endowment, or prospectivity, map. Orogenic gold deposits are particularly suited to such analysis because, in general, they formed late in terrane evolution and hence their geological features, are not significantly modified. Research in the Yilgam Block, Western Australia show^s that larger gold deposits are controlled by the conjunction of critical parameters which include: 1) nature of host rocks, 2) complexit>^ of lithological associations, 3) rheological contrasts betw^een component rock types, 4) geometry and orientation of rock units and pre-existing structures, 5) availability of crustal- to regional-scale shear zones as hydrothermal plumbing systems, and/or 6) proximity to rigid bodies, such as granitoids, with complex geometries. These can be quantified using appropriate measures of parameters such as composition, rheology, proximity, abundance, complexity and orientation, among other parameters.

It is shown that endowment maps produced using Vectorial Fuzzy Logic or Artificial Neural NetAvorks as integration media can define small areas within Yilgam greenstone belts that have ver\^ high endowment potential, or prospectivit}^, for orogenic gold deposits, and that many overlap with areas defined by other independent techniques. More reconnaissance studies of other greenstone belts suggest that the methodology has global application. The potential to use this methodology for prediction of the endowment potential of terranes for other hydrothermal mineraldeposit st>des is discussed.

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KAMBALDA NICKEL DEPOSITS AND THEIR VOLCANIC ENVIRONMENT R.E.T. Hill and C.S. Perring CSIRO Exploration and Mining, Kensington, WA, 6151 On January 28th, 1966, Western Mming Corporation's diamond drill hole KDl at Kambalda intersected 2.7 m of massive Ni-Cu sulfide, which assayed 8.3 wt% Ni, hosted by komatiite. Thus began the WA Nickel Boom of the 1960's and early 1970's and the start of a world-class mtegrated Kambalda mckel industry which has produced 25 million tonnes of ore containing -805,000 tonnes of mckel (average grade 3.2 wt%). Nineteen nickel ore accumulations (rangmg from 0.2-9 Mt) have been located m the Kambalda mmmg field, fifteen of which have been exploited through seven separate mmmg complexes since 1967.

The sulfide mineralisation forms semi-continuous ribbons (<2 km long, rarely >300 m wide, and <5 m thick), or discrete pods, confined to four sub-parallel lava pathways, 600-1200 m apart and 15 km long, that fed the stacked flows of the Silver Lake Member (SLM) of the Kambalda Komatiite flow field. The sulfides occur at or near the base of the lowermost flow unit and at the base of the next one or two overlying flow units (hangingwall ore). Basal contact ores typically occupy an embayment structure in the underlying Lunnon Basalt and there is generally an antithetic relationship between the presence of sediments and the presence of sulfide at the basal contacts, although there are significant exceptions. The sulfide shoots are variably stratified comprising massive, net-textured and disseminated components.

The Lava Pathways (-500 m wide and 15-100 m thick), have well-developed spinifex-textured and aphamtic crusts (up to 3 .5 m) and are choked with olivine ortho- to mesocumulates. Individual lava pathways grade laterally mto thinner (5-30 m, tj^ically 20 m) stacked sheet flow lobes separated by sedimentar}^ units (the non-ore environment). These flow lobes are dominated by layered olivine orthocumulates, and their crusts are generally thinner with poorly developed spinifex textures. It is now generally believed that this volcanic architecture reflects a flow-field emplacement process similar to that proposed for Hawaiian lavas and contmental flood basalts. The essential features of this process are: persistent, relatively quiescent, and long-lived lava flow; lobe-by-lobe advance; mflation of single and coalesced sheet flow lobes under thickening, composite, visco-elastic and brittle crust; and the progressive formation of thermally insulated preferred lava pathways or tubes.

These pathways focus lava flow, provide efficient transfer of lava to continuously advancing flow fronts, act as loci for thermo-mechanical substrate erosion, and feed lateral lava breakouts through fractured and uplifted crust. Thick umts of the SLM exhibit small-scale igneous layering in olivine cumulates, contain multiple layers of vesicles and exhibit spinifex-textured upper crusts - features which suggest xenocrystic olivine sedmientation, episodic lava injection and episodic growth.

Thermo-mechanical erosion is believed to occur by the progressive removal and re-formation of an omni-present, decoupled, hybrid melt, boundar>^ layer at the base of active lava pathways. The removal and partial assimilation of such a xenomelt layer at sites of erosion above sulfidic interflow sediments, followed by transport of immiscible sulfide blobs which scavenge chalcophile elements from their flowmg olivine-bearing host, and subsequent accumulation by gravitational settling at a site of deposition during changes in flow dynamics, is the most likely process of ore formation. This process is consistent with the inflationary flow-field model and it provides an elegant explanation for the broad range m volcanic architecture, lithological associations, and geochemical data expressed by the ore environment within, and distal to, the Kambalda Dome.

The ore-confinmg emba}Tnents in the Lunnon Basalt probably served to focus initial lava flow and onent lava pathways, however the possibility that the sub-parallel orientation of the lava pathways resulted from a steepening of the substrate slope cannot be dismissed. The crude across-pathway alignment of ore accumulations may reflect repetitive depositional environments caused by kinks in slope.

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OLYMPIC DAM: IN A WORLDLY CLASS OF ITS OWN J.P. Hodgkison O.D Operations, WMC Resources Limited, PO Box 150, Roxby Downs, SA 5725 The Olympic Dam Cu-U-Au-Ag-Fe Oxide deposit near Roxby Downs, South Australia is located near the northeastern margin of the Gawler Craton. The Olympic Dam Breccia Complex has a footprint 5 kilometres in length, is oriented in a west-northwest direction and sits entirely within the 1.6 Ma Roxby Downs Granite host.

The breccia complex is unconformably overlain by 350 metres of barren, flat-lying, Neoproterozoic and Cambrian sediments of the Stuart Shelf Sequence. This package reaches a thickness in excess of 1 kilometre in areas distal to the deposit. Mineralisation is syngenetic and is interpreted to have occurred in a volcanic crater environment. Iron and metal-rich, magmatic-derived hydrothermal fluids migrated along major lineaments, possibly caused during intracontinental rifting, and mvaded the Roxby Downs Granite. Phreatic and phreatomagmatic eruptions led to several generations of brecciation and ore deposition events due to the build up of volatiles and interaction with surficial water. Iron metasomatism of the granite has led to hematite replacing most of the granite components and early hydrothermal magnetite. This hydrothermal alteration process has resulted in a continuum of breccia ty^pos from the weakly brecciated Roxby Downs Granite with less than 5% iron near the margin of the deposit, to a hematite-quartz end member with 60% iron in the centre of the deposit. Between the two barren end members, the breccias comprise varying proportions of granite and hematite as well as some exotic Gawler Range Volcanic and maficultramafic clasts.

The breccia complex has a general alteration and metal zonation pattern with a siderite-magnetitechlorite-chalcopyrite-pyrite association at depth and laterally distal to the deposit, while a goldchalcocite-bomite-silica-hematite-sericite-chlorite association occurs in areas that are shallow or laterally proximal to the deposit. The copper grade tends to correlate with an increase in hematite content, until the barren quartz-hematite breccia is encountered and there is also a good correlation between uranium and copper.

WMC commenced their search for copper in Australia during the late 1960's and based their exploration upon a sediment-hosted stratabound model. WMC was attracted to South Australia by its known copper occurrences, which are numerous though small in size. The Stuart Shelf region was identified as an area which could provide a suitable sedimentary host rock, and exposures of mafic volcanic rocks to the south provided encouragement that a suitable copper source rock could exist at depth further north. Regional gravit\^ and magnetic data from the Bureau of Mineral Resources highlighted coincident anomalies adjacent to major tectonic lineaments. During 197576, nine surface diamond drill holes tested the anomalies at Olympic Dam while one hole tested the anomalies at Acropolis to the southwest. During the drill program half the holes were barren and the program could have easily been terminated. However, the persistence of the exploration team paid off as RDIO encountered 170 metres of 2.1% Cu on the western margin of the deposit and provided the mcentive to move to a large-scale delineation drill program.

Production commenced in mid-1988 at 1.5 million tonnes to yield 45,000 tonnes of copper.

Production increased through two optimisation phases betw^een 1992-1995 to 84,000 tonnes of copper and the most recent expansion has seen production lifted to 200,000 tonnes of copper in 2000. Olympic Dam is currently ranked as the 15^^ largest copper producer in the world. The reserve, which is in excess of 700 million tonnes, will have a mine life of 70 years at current mining rates. The resource, which is approximately 2.5 billion tonnes, indicates that the mine life could be much greater. The ability to sustain an operation for such a period of time reflects the world-class status of the deposit.

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THE ARGYLE (AKl) DIAMOND DEPOSIT, WESTERN AUSTRALIA ^L. Jaques and ^ C. B. Smith ^Geoscience Australia, Canberra, ^Consultant to Rio Tinto Mining and Exploration Ltd The Argyle (AKl) diamond pipe (with its associated alluvial diamond deposits) lies --100 km south of Kununurra in the East Kimberley region of Western Australia. The Argyle (AKl) deposit is one the world's great diamond deposits with a total diamond resource >1 billion ct established since evaluation and mining commenced. Currently estimated resources are 168 Mt @ 2.8 ct/t (Rio Tinto, 2001). The deposit owes its significance to its size and especially to its extremely high grade of 100-680 ct/t, higher than any other primary kimberlite deposit.

The Argyle (AKl) pipe was discovered in 1979 by the Rio Tinto-managed Ashton Joint Venture after diamonds were recovered in two 40 kg reconnaissance samples collected from the Smoke Creek drainage, 20 km downstream from the pipe, during systematic stream sediment sampling of the Kimberley region. Commercial production commenced in 1985 and, since 1986, Argyle has produced - 3 0 - 4 0 milhon ct p.a., about 30% of the world's supply of natural diamonds by weight, 5% by value. Argyle diamonds are mostly small (mean <0.1 ct/stone), brown, frosted, strongly resorbed dodecahedral stones of mdustrial and cheap gem quality with an average valuation of -^US$10/ct. About 5% are of gem quality and these mclude rare highly prized pink diamonds.

The Argyle (AKl) pipe, dated at 1147 ± 47 Ma, intrudes Palaeo-Mesoproterozoic clastic sediments overlying the 1.89-1.8 Ga crystalline basement of the Halls Creek Orogen. The pipe covers 47 ha, extends 1500 m north-south, and tapers in width from 600 m in the north to less than 50 m in the south. The pipe is a diatreme formed by multiple phreatomagmatic eruptions when olivine lamproite magma encountered groundwater. Recent company drilling mdicates the pipe comprises three coalescing vents, all tilted 30"" north, developed on a steeply-dippmg fissure that forms the southern tail. The diatreme is infilled by diamondiferous olivine lamproite pyroclastic and reworked pyroclastic rocks (lahars, pyroclastics and minor crater lake sediments) and intruded by rare olivine lamproite dykes. The dominant volcaniclastic type - polygenetic, quartz-rich pyroclastics with sparse juvenile fragments ("sandy tuffs") - formed by disaggregation of poorly cemented sedmients into which the diatreme was emplaced. Juvenile material is comprised of finegramed to altered glassy olivine lamproite characterised by high MgO, Ni and Cr contents, high abundances of K, Rb, Ba, Th, U, Ta, Nb, LREE, Sr, P, F, Hf, Zr and Ti, and high and low ^'^^Nd/^'^Nd ratios. The magma is inferred to result from small degrees of partial melting of ancient (~2 Ga) metasomatised mantle peridotite near the base of the sub-continental lithosphere. Rare xenoliths of mantle harzburgite and Iherzolite, some canying diamonds at high grades, represent sub-Kimberley mantle from 160-190 km depth. Argyle diamonds are dommantly of eclogitic paragenesis, strongly depleted in ^^C, and formed at temperatures of --1100-1500^ at depths of -145-230 km, possibly in a late Palaeoproterozoic subduction system.

Key elements in the formation of the world-class Argyle diamond deposit are suggested to be: • Formation of eclogite-rich diamondiferous mantle source rocks in sub-continental lithosphere beneath the Kimberley Craton by incorporation of subducted lithosphere resulting from a Paleoproterozoic orogeny; • Emplacement of lamproite magma at -1200 Ma along reactivated trans-lithospheric faults formed durmg the Halls Creek Orogeny; • Phreatomagmatic eruption of magma into unconsolidated sediments forming 3 main vents that were filled largely by crater-filling reworked pyroclastics; and • Preservation by burial until unroofing and formation of associated alluvial deposits in river terraces in the Tertiary.

Reference Rio Tinto, 2001. http://w^TV.riotinto.com/investors/searchPresentations.asp.

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THE CANNINGTON AG-PB-ZN BHT DEPOSIT; A WORLD CLASS DISCOVERY WITH A SILVER LINING Stuart Jeffrey Principal Geologist, Cannington Mine, BHP Billiton , PO Box 5874 Townsville MC, Qld 4810 The Cannington Ag-Pb-Zn BHT deposit, located 135 km SE of Cloncurry m NW Queensland, is within the Eastern succession of the Proterozoic Mount Isa inlier. Discovered under 10-60 m of cover in 1990 through the development and sustained application of an exploration model (Walters et al., 2002), Cannington has become the world's largest single mine producer of silver and lead and also supplies a significant proportion of the worlds zinc concentrate. At 30^ June 2001 the total Cannmgton resource was estunated to be 40.7Mt at 545g/t Ag, 12.2% Pb, and 4.3% Zn. The geology and mmeralisation styles have been well documented in the literature (Walters and Bailey, 1998; Bailey 1998). Essentially the deposit consists of five main economic lode horizons that can be further subdivided mto 10 mineralisation types based on Pb/Zn ratios, Fe-rich versus silica rich lithologies, and relative position (footwall or hangingwall). These lodes are hosted within a sequence of gametiferous psammites that are folded around a core amphibolite body, all of which are contained within a migmatitic quartzo-feldspathic gneiss terrain.

While Cannington has many geological similarities with other BHT deposits, the deposit represents an unusual but different example of this style of mineralisation. While the resource may not be as large as other BHT's, the enrichment of Ag through a process of late metasomatic zone refining has ensured that the Cannington deposit is of economic significance on the world stage.

References Bailey, A. 1998. Cannington silver-lead-zinc deposit, in Geology of Australian and Papua New Guinean Mineral Deposits, (Eds: D A Berkman and D H Mackenzie), p. 783-792.

Walters, S. and Bailey, A. 1998. Geology and Mineralisation of the Cannington Ag-Pb-Zn Deposit: An Example of Broken Hill-Type Mineralisation in the Eastern Succession, Mount Isa Inlier, Australia.

Economic Geology 93, p. 1307-1329.

Walters, S., Skrzeczynski, B., Whiting, T., Bunting, F. and Arnold, G. 2002. Discoveiy and Geology of the Cannington Ag-Pb-Zn Deposit, Mount Isa Eastern Succession, Australia: Development and Application of an Exploration Model for Broken Hill-t>pe Deposits. Economic Geology Special Publication, in press.

265

WHAT IS A WORLD CLASS ORE DEPOSIT? Ross Large. David Cooke and Garry Davidson Centre for Ore Deposit Research, University of Tasmania

Every explorer wants to find a world class deposit; but what are they and how do we define them?

Smger (1995) groups wwld class deposits into two categories; giants and super giants. Giant deposits of a particular metal are the largest 10% and supergiants are the largest 1% of deposits.

If we consider zinc deposits, Smger (1995) proposes that giant deposits contain greater than 1.7 Mt of Zn; super giants contain greater than 12 Mt of Zn. On this basis, there are thirty giant stratiform zinc deposits, and eight super giant deposits in the world. Five of the super giants (62%), are located m Australia. These are; Broken Hill (#2), McArthur River (#4), Century (#6), George Fisher (#7) and Hilton (#8). All the Australian super giants are of Palaeoproterozoic age. If we consider combined Zn+Pb+Ag metal content then Mt Isa Pb-Zn and Cannington join the super giant category. The top super giant stratiform Zn deposit is Red Dog, Alaska, of Devonian age.

Of a total of over 700 VHMS deposits in the world, thirty two are giant deposits, but none reach the super giant category in copper, zmc or lead content alone. However based on a combination of Cu, Pb and Zn it can be argued that Brunswick No. 12 (New Brunswick), and Kidd Creek (Ontario) have super giant status.. The largest number of giant VHMS deposits are m the Iberian Pyrite Belt m Spam and Portugal (including Neves Corvo, Aljustrel, Aznalcollar, La Zarza). Five of the 32 giants are m Australia, making up 16% of the VHMS giant pool. The five Australian VHMS giants are; Rosebery (#4), Mount Morgan (#10), Hellyer (#18), Mount Lyell (#25) and Woodlawn (#32).

Three of these giant VHMS deposits are in the Cambrian Mount Read Volcanics, Tasmania.

From Kirkham and Dunne's (2000) database of 783 porphyry and related epithermal deposits, a total of 93 are classified as giants in terms of contained copper ( > 2Mt Cu), and seven are super giants ( > 24 Mt Cu), including five deposits m the Chilean Andes. None of the giant copper porphyr}^ deposits are in Australia. In terms of contained gold, a total of 71 porphyry and epithermal deposits are classified as giants (> 1001 Au), and only two are super giants (Grasberg and Lihir; > 1200 t Au). When considered as a single system, the porphyry Au-Cu deposits of the Cadia district NSW (586 t contained Au) are in the world's top 10 porphyr}^ and epithermal gold deposits. The Grasberg deposit is the only porphyry system that comes close to being a super giant with respect to both copper and gold (23.4 Mt Cu and 2340 t Au).

Iron oxide Cu-Au deposits form a much smaller class than porphyry Cu-Au, with only about forty deposits world wide, although this depends on how the lOCG class is defined. There is only one super giant deposit (Olympic Dam), and seven giant deposits, all of which occur in South America (Brazil and Chile). Ernest Hemy^, the only other significant deposit m Australia, falls just short of giant status m both Cu and Au. References Singer. D. A.. 1995, World Class Base and Precious Metal Deposits - A Quantitative Analysis: Economic Geolog}^ V. 90, p. 88-104.

Kirkliam, R.V. and Dunne, K. P. E., 2000, World distribution of porphyry, porphyry-associated skam, and bulk tonnage epithermal deposits and occurrences: Geological Survey of Canada Open File 3792a, 26p.

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THE BONDI HEAVY MINERAL SANDS DEPOSITS, WESTERN VICTORIAN MURRAY BASIN. N T. OXoughlin, B.L. Farrell, D. Judkins, T. McGuire, P. Slyth, and S. Hart Basin Minerals Limited, P.O.Box 1786 West Perth, WA 6872, Australia.

Basin Minerals Ltd is developing a major Mineral Sands Field in the Douglas Project area SW of Horsham, western Victoria. The field comprises shoreline systems in the West Wimmera Strand Plain that have economic concentrations of medium to coarse-grained ihnenite, leucoxene, rutile and zircon. The most important mineralisation discovered is the Bondi Strandline System which hosts Douglas Stage One Mining Area with the Bondi Main, Bondi East and Bondi West Deposits.

The Bondi Deposits were discovered in 1999 by RC aircore drilling of Pliocene beach and foredunes identified through regional photo-geological interpretation. Basin Minerals had pegged the Douglas Project area in 1998 after recognising strong geological similarities with an Eneabba-style geological setting. CRA Exploration Pty Ltd carried out extensive drilling in the region during their exploration for fine-grained 'WIM'-style minerahsation in the 1980's and early 1990's. Upon CRA/RTZ's surrendermg of leases m the Douglas area, Basm Minerals established an extensive database of CRAE drill data. Re-evaluation of these data, together with photo-geological mterpretation and digital elevation modelling, gave evidence of local coarse-grained mineralisation associated with strandlme settmgs and demonstrated the close inter-relationship between the finegramed deposits sought by CRAE and beach settings likely to host coarse-grained strandlines.

Within weeks of commencing drilling. Basin Minerals had discovered several significant deposits and rapidly carried out a regional drilling program targeting analogous geological settings along the southern palaeo-coastline of the Murray Sea. Several other strandline deposits were discovered in this program before attention was turned to the economic development of these discoveries in the Douglas Project area. A positive pre-feasibilit>^ study was completed in 2000-2001. Grid drilling of the southern part of the Bondi and Bondi East deposits in early 2001 identified extensive zones of high-grade mmeralisation adequate to support the development of the first stage of a major mineral sands project (Douglas Stage One). The subsequent feasibility- study was completed in 2002 with production scheduled to commence in late 2003.

Economic mineralisation has been identified over a 20 km x 5 km composite barrier system in the southern palaeo-coastline of the Pliocene Murray Sea. Regional longshore drift is interpreted to have carried large quantities of heavy mineral into this trap site with coarser grained (100-200 high grade, strand-style mineralisation concentrated at the southern termination of NW-trending beach ridges and dunes where they meet Palaeozoic basement forming the rugged palaeo-coastline, with several extensive, fine-grained 'WIM'- style bodies within local embayments. The Bondi Mineralised system is unique m the Murray Basin because of the multiple stacked lenses, wide (>1 km), thick (often > 20 m) composite mineralised bodies with high heavy mineral grades and low overburden. Valuable heavy mmeral typically comprises >90% of the bulk heavy mineral concentrate; mineralogy vanes distinctly across the deposits. High zircon grades are localised adjacent to the ancient coastline within individual trap sites and within the main strandlines.

The Bondi deposits were formed in a variety of environments within a major beach barrier system.

Bondi East is interpreted as a high-stand beach deposit, formed in a series of arcuate bays by extensive reworking of offshore marine sediments. Approximately two kilometres to the west, the classic, high grade strandline at Bondi Main is flanked to the east by an extensive zone of silty sands in which ihnenite has been ahnost entirely altered to leucoxene. This 'Eastern Zone' mineralisation is interpreted to have formed within a back-beach lagoonal environment. The deposits show evidence of significant modification with a component of wash-over and reworked dune material and evidence of a break in the main barrier ridge, possibly created by a river system transectmg the main deposit fi-om NE to SW. With resources containmg ihnenite, leucoxene, rutile and zircon, the Douglas Stage One Project is expected to produce in excess of 400,000 tonnes p a. for well over ten years. Extensions to the Bondi Main and Bondi East strandlmes ,along with adjacent mineralised strands and other conventional strandline resources identified by Basin Minerals m the Murray Basin, mdicate the potential to support a significant mineral sands project for well over 20 years.

267

THE BROKEN HILL Zn-Ag-Pb DEPOSIT, NSW Ian R. Plimer, The University of Melbourne The Broken Hill orebody is hosted by the 6-7 km thick metasediment-dominated Willyama Supergroup of the Broken Hill Block. The Willyama Supergroup comprises a regionally mappable sequence of migmatite and composite gneiss of uncertain parentage; felsic meta-igneous rocks of volcamc and granitic parentage; mafic meta-igneous rocks which were once flows, tuffs, sills, dykes and lopoliths; metasediments of pelitic and psammopelitic composition; calc-silicate and sodic rocks of evaporitic parentage; and metamorphosed submarine hydrothermal precipitates, including the Zn-Ag-Pb orebodies, quartz-gahnite rocks, garnet rocks and iron formations. The Willyama Supergroup was deposited in a rift setting, similar to the modem Red Sea, at 1690 ± 10 Ma following initial separation of the crust from the mantle at 2300 to 2100 Ma. The Willyama Supergroup has undergone multiphase regional amphibolite to granulite facies metamorphism coeval with deformation in the Olarian Orgeny. Three major episodes of folding have been recognised. Large-scale nappe-like Fi structures produced recumbent isoclinal folds and the Broken Hill orebody lies m the downward limb of one such Fi structure. Tight upright F2 folds are the most visible structure in the area. F3 folds are also commonly upright but smaller in size. Fi/Mi and F2/M2 took place at 1650 Ma and 1600 Ma respectively with M3/D3 at 1570 Ma. Mylonite and retrograde schist zones, probably late Olarian, were reactivated at 1200 Ma and again at 520 Ma, and have probably sporadically moved during the Phanerozoic.

Mineral phase relationships of post-tectonic granitic dykes dated at 1490 ± 20 Ma suggest intrusion of hot rocks at depths of greater than 13 km thereby reflecting a long history of isobaric cooling after the Olarian Orogeny. Rifting at 830 Ma resulted in the intrusion of tholeiitic mafic dykes, the opening of the Adelaidean sea and the deposition of Neoproterozoic mafic volcanics, marine clastic and carbonate sediments and terrestrial and marine glacial sediments. At Broken Hill, these Neoproterozoic sediments unconformably overlie the Willyama Supergroup and, because these sediments contain clasts of the Willyama Supergroup, at least 13 km were eroded during the uplift and cooling of the Willyama Supergroup rocks between 1490 and 830 Ma. Alkaline mafic dykes were emplaced at 520 Ma. The mdividual sulphide rocks at Broken Hill have a distinct metal tenor, gangue mineralogy and trace element signature. In addition, within each sulphide mass there is enormous heterogeneity with large local variations in mineralogy, grainsize, texture, chemistry and metal content. Most of the heterogeneities result from the long and poorly understood histor>^ of tectonism. The Broken Hill orebody is cut by numerous mylonite zones which have affected the structure, texture, mineralogy and chemistry of the sulphide rocks as a result of fluid-rock interactions, re-dissolution and re-precipitation of selected components and differential plastic flow of sulphide and silicate rocks.

There are a number of Zn-Ag-Pb deposits and provinces of Palaeoproterozoic age (e.g. Cannington, Qld.; Sullivan, B.C., Canada; Bergslagen Province, Sweden; NW Cape Province, South Africa; Wilkes Land, Antarctica) suggesting that there was a global event of ore formation at around 1700 Ma. The geological setting and the geochemical similarity with modem submarine hydrothermal precipitates and the isotopic signatures suggest a mixed source for components. Some of the Broken Hill sulphide rocks (e.g. No 3 lens) appear to have no clastic input whereas other sulphide rocks (e.g. C Lode, Western Mineralisation) appear to have a clastic component thereby suggesting competing hydrothermal and clastic deposition or stratal replacement of clastic sediments. The high grade and tonnage of the Broken Hill sulphide rocks probably derives from hydrothermal fluid focus, rapid exhalation and localised precipitation in numerous short bursts during Thackaringa and Broken Hill Group times. Many sulphide masses along the main line of lode at Broken Hill remain unmined, unexplored and unstudied (e.g. Western Mineralisation; Eastern Mineralisation; Brisbane Blocks Mineralisation) hence, despite 119 years of mining, the morphology, mineralogy, petrology and geochemistry of Broken Hill sulphide rocks are poorly known. Any models at Broken Hill are therefore constrained by the paucity of reproducible data.

268

TOWARDS A NEW MODEL FOR THE GENESIS OF THE TERTIARY YANDI CHANNEL IRON DEPOSIT, EAST PILBARA, WESTERN AUSTRALIA Stone, M.S\ George, A.D.\ Barley, M.E. \ and Kneeshaw, M. ~ ^Department of Geology & Geophysics, University of Western Australia, Crawley 6009 -BHPBilliton Iron Ore Pty Ltd, 200 St George's Terrace, Perth 6000 Detailed pit and outcrop mapping of BHP Billiton's Yandi channel iron deposit is providing insight into fluvial processes that operated during deposition of this world-class deposit in the early-mid Tertiary. The deposit is traceable for >80 km m the Marillana Creek catchment, and is well exposed m four operating open pit mines. Yandi is a high grade, low^ phosphorous/alumina "pisolitic" iron deposit with a global resource estunated to be >3500 Mt along the entire catchment. Five promment rock units are traceable along strike and permit correlation between open pits and unmined deposits.

These units are locally known as the Eastern Clay (EC), granule-pebble conglomerate. Main Ore Zone (MOZ), Basal Denatured (ore) Zone (BDZ), and Basal Conglomerate (BCong).

The distribution of rock types and sedimentar>^ features at Yandi indicate that deposition in a meandering fluvial system, and that primary ore compositions and textures are overprinted by multiple phases of groundwater alteration. Key evidence for this interpretation is that sedimentary features such as lateral accretion surfaces, channel scours and fills, graded bedding, cross-bedding, reworked horizons, and slumping are common at Yandi, but have not been previously well described.

Bedding is well preserved throughout the deposit despite modification of original grain shapes and textures and secondar>' overprint by silica and/or alumina. More specifically, solution pipes and cavities filled with a variety of clays are well developed in the upper part and margins of the deposit.

Secondary porosity is partially to wholly in-filled by multiple phases of silica.

The granule-pebble conglomerate and the EC differ from the other units and possibly reflect changes in the source area. The MOZ and the BDZ may be the same depositional unit, with the BDZ recording alteration by groundwater and destruction of primary textures and associated sedimentary features. Examination of the BCong suggests that it is a lag deposit reworked during MOZ deposition. This interpretation is supported by the dominance of basement-derived clasts and the presence of 'ripped up' BCong clasts, and the variable thickness of the unit along strike. Pit and outcrop mappmg is continuing to be integrated with sedimentary and geochemical analysis, and potentially biostratigraphic analysis, to develop a holistic ore genesis model for this world-class iron deposit.

269

THE CADIA GOLD-COPPER PORPHYRY DISTRICT, NSW I.J.Tedder\ A. J Wilson^ ^Newcrest Mining Limited, ^CODES,University of Tasmania Cadia IS a major, Late Ordovician gold-copper porphyry district, set in the Molong Volcanic Belt of the eastern Lachlan Fold Belt, NSW. Four gold-copper porphyry deposits, with a combined resource in excess of 18 Moz gold and 2.4 Mt copper, have been defined since the initial discovery of the Cadia Hill deposit by Newcrest Mining Limited in 1992. The Cadia Hill open pit mine (proven reserve of 170 Mt grading 0.78 g/t Au, 0.18% Cu) is producing around 300,000 ounces of gold per annum from 17 Mt of ore. The underground Ridgeway gold mine (probable reserves of 32 Mt grading 2.7 g/t Au, 0.76% Cu), commenced ramping up to full production in April this year and will produce up to 350,000 ounces per year from 4 Mt of ore. This makes the Cadia district the largest gold system (and producer) in eastern Australia with good potential for additional development at Cadia Far East (inferred resource of 63 Mt with L7 g/t Au and 0.48% Cu).

The Cadia district comprises a thick sequence of intermediate volcanic conglomerates and breccias (Forest Reefs Volcanics, FRV) that conformably overlie planar-laminated siltstones of the Weemalla Formation. Pre-mineral dykes and sills of basic to intermediate composition have intruded the comagmatic FRV. Gold-copper mineralisation is spatially and genetically related to composite shoshonitic monzodioritic to quartz monzonitic dykes and stocks of the Late Ordovician Cadia Intrusive Complex (CIC). Silurian siltstones and Miocene basalts unconformably overlie much of the FRV and CIC, and partially conceal porphyry-related hydrothermal alteration and sulphide mineralisation.

The porphyry gold-copper deposits of Ridgeway, Cadia Quarry, Cadia Hill and Cadia East - Cadia Far East are aligned NW-SE over a six kilometre strike length, parallel to one of the main regional structural trends. Mineralisation is associated with both sheeted (subparallel to the mineralised corridor) and stockwork quartz-sulphide veins that are centred on multistage porphyritic monzonite to quartz monzonite intrusions of the CIC. Broadly stratabound, disseminated copper-gold mineralisation in the upper part of Cadia East is hosted entirely within volcaniclastic units.

Mineralisation at each deposit is closely associated with potassic to calc-potassic alteration assemblages (biotite-actinolite-orthoclase-magnetite-bomite-gold) that are overprinted by pervasive propylitic alteration (chlorite-epidote-hematite-calcite). An outer zone of sodic alteration (silicaalbite±pyrite) is common in the peripheries of the hydrothermal systems. In the upper levels of Cadia East-Cadia Far East, there is an intense phyllic alteration assemblage (silica-albite-sericitepyrite± tourmaline) that overprints earlier potassic alteration. Cadia Hill and Cadia Quarr>^ have the weakest hydrothermal alteration of the Cadia porphyr>^ deposits, with potassic alteration confined to thin selvages around sheeted quartz-chalcopyrite veins.

Zonation of sulphide minerals is well developed at Ridgeway and Cadia Far East, where a bomiterich core (closely associated with highest gold grade) passes outwards through chalcopyrite to pyrite-rich assemblages. The reverse is true for Cadia Hill however, with the upper parts of the sheeted vein envelope dominated by bomite, which grades into chalcopyrite- and pyrite-rich assemblages with increasing depth. Gold correlates most closely with chalcopyrite at Cadia Hill.

The Cadia porphyry deposits show many of the characteristics of porphjTy-related deposits from around the world. What makes the Cadia district exceptional is the high gold-grade of the Ridgeway and Cadia Far East deposits. The possible cause of this gold-enrichment is still being researched, but magnetite-rich assemblages in the early stages of alteration and mineralisation, and the close correlation between bomite and gold in the high-grade systems suggest that highly oxidised alkaline intrusions are favourable for the formation of high grade, gold-rich deposits.

270

C O O B E R P E D Y O P A L FIELD Jack Townsend, Consulting Geologist 8 Milton Avenue, Plympton Park, SA 5038, Australia World class opal deposit Coober Pedy was discovered in 1915 and worked by individual and syndicate miners since, producmg much of South Australia's opal. It is part of the vast Great Artesian Basin (GAB) which hosts and produces precious opal from weathered marine and terrestrial sediments.

Theories range from a contemporaneous early Cretaceous age for the formation of opal to a late Tertiary emplacement throughout much of the Cretaceous sediments in Queensland, New South Wales and South Australia. The GAB at one stage covered approximately a fifth of the continent providmg tremendous potential for Australian opal.

Production history Coober Pedy has been producing opal for 87 years in a haphazard yet continuous fashion.

Estimates exceed $625 M during that time and for more than half that period it was Australia's and the world's largest producer. Production increased sigmficantly after the Second World War, a second time with improved technology in the sixties and again in the seventies when this state's opal production first exceeded $10 M per annum. South Australia has produced more than an estmiated $1 billion total to the end of 2000.

Geological setting, Most commercially produced opal in Australia is hosted by Cretaceous sediments of the GAB and some adjacent sediments such as the Palaeozoic at Mmtabie. Small occurrences have been found in granites and sediments containing feldspar or clay which weather to kaolin and silica providing silica for the groundwater and eventual opal deposition. Minor opal occurrences are associated with volcanic rocks which provided silica rich fluids for opal formation.

Genesis and exploration models The formation of opal has long been attributed to silica-rich groundwater and a falling water table held up by clay layers or some form of aquitard, allowing the concentration of silica and formation of silica spheres. Once the spheres are formed the process continued to deposit closepacked layers of uniform spheres. These spheres were cemented by silica later as the gel dried out to form precious opal. Many deposits of opal are associated with major and micro faults which have broken up the ground allowing weathering to occur and provide suitable sites for the deposition of opal. Palaeochannels also appear to play an important role in opal deposition.

Contemporaneous theorists have suggested that opal formed in early to late Cretaceous times and one worker suggests that microbes may be responsible for initiating the process. Many workers suggest that an alkaline environment changing to acidic conditions are required chemically for opal spheres to be formed. One worker proposes marginal basin alkaline solutions changmg to more acidic conditions with the influx of acidic fluids and sediments during the regression of the GAB. Opal was found in most areas as lag pieces of precious opal weathered out of the host material and strewn over the ground, incorporated in soils, and in drainage channels at the edge of silicified scarps. Since most of these "floaters" have been found, miners are now looking for any other available tools to assist m locating ftirther precious opal.

State departments of Minerals and Energy have assisted in mapping and drillmg programs in the past and more recently have turned to remote sensing, airborne geophysics as well as the earlier tools of geology and topography utilising a more scientific approach as applied to mineral exploration. Combinmg all of these data into a GIS system has allowed opal explorers and miners to focus on areas of greater potential.

271

THE BUILDING BLOCKS OF CONCEPTUAL EXPLORATION MODELS FOR WORLD CLASS ORE DEPOSITS John L. Walshe CSIRO Exploration and Mining There is a sense of irreversible change in the resource industr}^ A much smaller fraternity of exploration geoscientists will find tomorrow's resources. Their challenge will be to find high grade, large tonnage deposits, with specified metallurgical qualities and low levels of toxic metals, in a fraction of the time taken by earlier generations, and at a fraction of the cost of traditional exploration methodologies.

New exploration concepts and models are required to sustain this effort. The models must be valid at all scales if they are to impact on area selection. The models must permit deposit properties to be understood from data gathered at regional to crustal scales. Such demands will refine and reshape the unresolved metallogenic questions that thread the history of the discipline of economic geology. Why are some terranes of the earth's crust highly mineralized and yet others apparently are not? Why did some epochs of earth history produce giant deposits but most did not? What processes determine the distribution of deposit sizes within mineralized provinces and districts?

That such issues have remained unresolved for so long simply serves to highlight the limitations of our current understandmg of ore forming processes.

Current ore deposit models are captive to the empirical classification schemes of the 19^ and 20^ century. The best of the process models are only valid within the confines of these empirical schemes. It will be necessary to transcend the empirical barriers between deposit classes such as porphyr}^ Cu deposits, lode Au deposits, sediment hosted Pb-Zn deposits and Fe deposits in order to progress the development of scale-integrated process models with genuine predictive capacity.

The architectural and geodynamic elements of scale-integrated process models are apparent at regional to crustal scales: trans-crustal faults, regional seals, packages of oxidized or reduced rocks, uplift histories, plate-scale deformation events that coincide with specific metallogenic epochs.

These elements may be correlated with deposit scale information. In contrast, most information about the nature and fimction of fluids within mineral systems is derived at deposit- to micro-scale.

The location and nature of fluid reservoirs and fluid flow paths in the larger picture is less clear.

The architectural constraints on major mineral systems hint at extensive lateral and/or vertical fluid flow. But the nature and roles of possible far-field fluid reservoirs, e.g. reservoirs in the mantle, remain a matter of conjecture.

All mineral systems involving transport of metals in solution require sustained chemical gradients at the site of mineral deposition in order to build large, high-grade deposits. The most effective processes to sustain these gradients involve multiple fluids of contrastmg chemistry.

Understanding the interplay between architecture, dynamics and reservoirs of fluids of contrasting chemistry, at all scales, is arguably the key task m developing robust, scale-integrated, process based exploration models. And the clues to the extent of the interplay: Kambalda Ni juxtaposed with St Ives Au, Bingham Canyon juxtaposed with Great Basin Au, juxtaposed lode Au and VHMS deposits in the Abitibi Belt, Tintina Belt Au in Yukon/Alaska juxtaposed with the Pb-Zn deposits of the Selwyn Basin.

272

SOURCE OF GOLD IN VICTORIAN DEPOSITS: LEAD AND STABLE ISOTOPIC EVIDENCE ^Anita S. Andrew. ^Brian L. Gulson, ^Graham R. Carr and 'Reid R. Keays, 'CSIRO Exploration & Mining, P.O. Box 136, North Ryde NSW 1670 'VIEPS, School of Geosciences, Monash Universit>^ Clayton VIC 3800 Vein deposits hosted by turbidite sequences have historically been a major source of gold. The Victorian deposits are the richest example of this type, producing nearly 2,500 tonnes of gold in the last 150 years. Despite decades of scientific investigation, the source of gold remains controversial.

Isotopic data allow fingerprinting of the metals and fluids and can be used to infer sources and precipitation mechanisms. This paper presents a summary of lead and stable isotope data for veinrelated minerals from gold mineralisation, host greyw^acke, greenstones and mafic dykes, and regional sulfides. New data are presented from Stawell and Fiddlers Creek in the Stawell Zone, Bendigo, Ballarat, Clunes, Wattle Gully and Tooleen in the Bendigo Zone and Great Rand and within the Woods Point Dyke Swarm (WPDS) in the Melbourne Zone. These are interpreted in the context of recent geochronology and tectonic models and are compared with mineral deposits and source rocks from the Lachlan Fold Belt (LFB).

Lead and stable isotope studies of the ores show only small variations within deposits and a generally narrow range for the gold province. Lead isotope ratios are similar for both high Pb sulfides and gold within the same deposit and are interpreted to indicate common factors in their source and deposition. The Pb isotope data identifies two source types of Pb for the gold ores. The Fiddlers Creek, Bendigo, Ballarat, Clunes and the Great Rand deposits are dominated by a crustal Pb isotope signature whereas the deposits at Stawell, Tooleen and deposits associated with the WPDS are dominated by mantle Pb isotope signatures. Possible crustal sources are granites, host turbidites or older sediments. Examples of mantle sources include Cambrian greenstones, and younger mafic rocks. The model age for the sources for Pb also varies between deposits. The oldest model ages occur in the Stawell Zone with the Magdala and Fiddlers Creek ores containing Pb as old as Cambrian. In the Bendigo Zone the oldest model ages are about 450Ma, distinctly older than the Ordovician VMS deposits of the LFB. The model age for the ores hosted in the WPDS is Devonian and identical with the age of the magmatic host. In interpreting the Pb isotope data, its significance as an indicator of the source of gold or as an indicator of a depositional mechanism resulting from local mixing of host rock Pb with gold bearing fluids, needs to be assessed. The sulfur isotope data for all deposits cluster around zero. Deviations from this are towards the immediate host rock signatures so that no clear distinction can be made between events based on sulfiir isotope data. A sulfiir isotope signature around zero is charactenstic of most magmatic sources, mantle or crustal. Sedunents generally have highly variable 5^'^S values.

The results of the Pb and stable isotope study of gold deposits from the LFB of Victonan highlight the complexity of ore formation even in terrains with many geological similarities.

1. The mineralization in the Stawell, Bendigo and Melbourne Zones spans a 100 million year period between 500 and 400 Ma with mineralization in the Stawell Zone generally older than in the Bendigo zone which is in turn older than in the Melbourne Zone. The spread of ages within a zone indicates prolonged hydrothermal activit}^ with individual hydrothermal events sampling Pb from a heterogeneous source.

2. There is Pb and S isotope evidence of crustal and magmatic components to all the ores, with the Bendigo zone mineralization predominantly crustal and the Stawell and Melbourne zones having a sigmficant (up to 50% of Pb) component derived from mantle sources.

3. Alteration haloes around mineralization are complex with possible overprinting of hydrothermal signatures.

273

TIMING OF OROGENIC GOLD MINERALISATION IN NE TASMANIA IN THE CONTEXT OF A METALLOGENIC AND TECTONIC FRAMEWORK FOR THE LACHLAN OROGEN Frank P. Bierlein\ David A. Foster, David R. Gray^ and Gan^^ J. Davidson'^ Victorian Institute of Earth and Planetary Sciences, Monash University, PO Box 28E, Melbourne Vic 3800 ^Department of Geological Sciences, University of Florida, Gaines\dlle, FL 32611, USA ^School of Earth Sciences, University of Melbourne, Parkville Vic 3052 ^CODES, University of Tasmania, GPO Box 252-79, Hobart Tas 7001 The Palaeozoic in NE Tasmania hosts 100s of lode- and disseminated-style gold deposits. The large majority of these have produced insignificant amounts of gold, but several areas such as Beaconsfield, Lisle, Lefi-oy and Mathinna have proven to host significant resources. Total production fi-om the goldfields of NE Tasmama is estimated at around 53 tonnes. Although this figure pales in comparison with the total production of -2,500 tonnes from the goldfields in the Stawell, Bendigo and Melbourne zones of central Victoria, deposits in both regions share many similarities and it is logical to assume that they formed under analogous circumstances during the development of the Lachlan Orogen. Similarities include the nature of the host rocks (predominantly Ordovician to Devonian turbidites), metamorphic grade (sub- to mid-greenschist facies), close spatial association with felsic intrusives (some deposits are hosted by Devonian granites), features of hydrothermal alteration (sericitisation and carbonatisation of greenschist assemblage, ubiquitous carbonate spots, pyrite and arsenopyrite porphyroblasts), and the style of mineralisation (free gold, largely in laminated to massive quartz veins, low sulphide-to-quartz ratio). Thus, comparisons between the timing of orogenic gold in NE Tasmania and gold emplacement elsewhere in the Lachlan Orogen may provide important clues for correlating the Palaeozoic metallogenesis of NE Tasmania with mainland Australia.

New "^Ar/^^Ar data from 10 sediment-hosted orogenic gold deposits in NE Tasmania constrain most ore formation in these occurrences to between 380 and 400 Ma. Data from alteration in some deposits (West Volunteer Reef, Lefroy, and Tasmania Reef, Beaconsfield) give dates of --420 Ma. The 380 - 400 Ma ages for the formation of orogenic gold agree well with an inferred Early to Middle Devonian timmg for peak deformation and folding across much of NE Tasmania (e.g. Reed 2001). Interestingly, a -385 Ma age has also been recorded from isolated vein gold mineralisation at King River, within the Mount Read Volcanics (western Tasmania). The ca. 420 Ma dates suggest that some of the mineralisation was generated during an earlier phase of deformation in Silurian tnne, which has been proposed recently for NE Tasmania. Gold mineralisation hosted by some of the Middle Devonian post-tectonic granites in NE Tasmania may be genetically related to magmatism following orogeny, but these deposits formed virtually s>iichronously with deformationrelated systems. Based on geological, structural, tectonic and metallogenic similarities with central Victoria, NE Tasmania could be interpreted as a lateral equivalent of the turbidite-dominated foldthrust belt of the western Lachlan Orogen. The timing of initial gold formation in NE Tasmania (420-400 Ma) overlaps with only one of two phases of 'metamorphism-related' orogenic gold mineralisation in the Stawell and Bendigo zones (455 - 440 Ma & 420 - 400 Ma). The timing of polymetallic, 'intrusive-related' gold mineralisation in central Victoria (420 - 400 Ma & 380 - 365 Ma), and emplacement of 'epizonal' Au-Sb mineralisation in the Melbourne Zone between 380 and 365 Ma have some chronological similarities with NE Tasmania but the differences in style suggest some differences between the two regions during the evolution of the Lachlan Orogen. The presentation will assess these aspects and, using miplications from metallogenic constraints, attempt to place NE Tasmania into a tectonic-metallogenic model for the Lachlan Orogen.

Acknowledgements: P.B. Hill and G. MacDonald (Allstate Explorations), R. Fulton (UTas).

274

ATYPICAL' GOLD MEVERALISATION AT MALMSBURY - THE CASE FOR INTRUSION-RELATED ORE SYSTEMS IN THE WESTERN LACHLAN OROGEN Frank P. Bierlein\ Stafford McKnight^, Andrew Radojko\dc^ Robert Witham^ and Helen Waldron^ Victorian Institute of Earth and Planetary Sciences, Monash University, PO Box 28E, Melbourne Vic 3800 ^School of Sciences, University of Ballarat, PO Box 663, Ballarat Vic 3353 ^Geological Survey of Victoria, 240 Victoria Parade, East Melbourne Vic 3002 ^Becquerel Laboratories, Lucas Heights Science and Technology Centre, PMB 1, Menai NSW 2234 The western sub-province of the Lachlan Orogen is host to copious gold mineralisation, having produced in excess of 2,500 t of gold since 1851 (Ramsay et al., 1998). The vast majority^ of production from pnmar}^ mineralisation (i.e., other than placers) has come from quartz vein-hosted, 'gold-only' deposits (e.g. at Stawell, Bendigo, Ballarat) that formed synchronously with regional metamorphism and thrusting between 455 and 440 Ma, with remobilisation and secondary phases of mineralisation occurring between 420 and 400 Ma. These so-called 'orogenic lode gold' occurrences can be shown to pre-date the onset of post-tectonic magmatism in the Stawell and Bendigo structural zones by up to 80 m y. Recent studies have shown that mafic to felsic magmatism which occurred at 420 - 400 Ma, and again at 380 - 365 Ma is closely associated with a geochemically distinct style of polymetallic gold mineralisation. Deposits of this association are commonly characterised by elevated Sb, W, Mo and Cu, and are spatially (if not genetically) related to crustal melting processes.

Mineralisation at Mahnsbur>^, situated in the easternmost part of the Bendigo Zone and flanked by two major Devonian granites, displays features that are atypical of orogenic lode gold deposits in Victoria. These include a biotite-cordierite thermal overprint of the host turbidites, presence of a complex gold-scheelite-fluorite-cassiterite-tetradymite-stibnite-[Cr-Ni-As-Bi]- sulphide assemblage, and the occurrence of skam-like assemblages. Furthermore, the mineralised system is hosted by a linear stockwork breccia with a north-easterly trend, in contrast to nearby gold-bearing, laminated to massive quartz vems that trend NNW. New ^Ar/^^Ar data constrain the age of the polymetallic mineralisation to -370 Ma which is indistinguishable from the ages of nearby granite intrusions.

Silicate-sericite-carbonate alteration associated with ore formation clearly post-dates the effects of thermal metamorphism, indicative of a late S>TI- to post-granite emplacement of the mineralisation.

Although the nearest outcrop of granite is 10km to the east of Mahnsbury, high-resolution geophysical data point to the presence of one or several sub-surface igneous bodies within close proximity of the deposit.

Intrusion-related gold systems constitute an important class of mineral deposits, with a large number of deposits containing at least 30t of gold known to exist in orogenic belts in Alaska, the United States, central Europe, the Andes, China and the Phanerozoic of eastern Australia (e.g. Kidston).

The magmatic-hydrothermal deposits are invariably associated with convergent plate margin settings and commonly co-exist with orogenic lode gold deposits (e.g. Lang & Baker 2001). Although the deposit at Mahnsbur>^ might fall short of a world-class resource, the recognition of intrusion-related gold mineralisation in the western sub-province of the Lachlan Orogen has implications for exploration in central Victoria, and also provides important clues regarding the tectonic framework and evolution of the Palaeozoic in eastern Australia.

Acknowledgements: Australian Institute of Nuclear Science and Engineering (Grant 01-008 to FPB); Bas van Kiel (Consultant Geologist); Peter Reynolds (Dalhousie University).

Reference Lang, J.R. and Baker, T. 2001. Intrusion-related gold systems: The present level of understanding.

Mineralium Deposita, 36: 563-582.

275

THE FORSTER AU-ZN-NI PROSPECT, GLOVERS BLUFF AREA, SOUTHEASTERN TASMANL^.

R.S. Bottrill J. Taheri, and C.R. Calver, Mineral Resources Tasmania Alluvial gold was first recorded near Glovers Bluff in the upper Huon district in 1877-1881 but until recently the area was generally considered to be relatively unprospective for metallic mineralisation. It was only seriously explored geologically from the 1980's, and a small gold resource, estimated at 1.14 Mt @ 0.45 g/t Au, has been delineated from 20 drillholes (Sedimentary Holdings, 1996).

The gold deposit is hosted by an altered assemblage of skams and siliceous rocks associated with Neoproterozoic dolostones. Associated rocks include Neoproterozoic quartzites and Early Cambrian ultramafic-derived conglomerates, sandstones and shales. These rocks form an inlier within Permian to Carboniferous mudstones, intruded and underlain by Jurassic dolerite. Other Igneous bodies in the area include some small mafic to lamprophyric dykes, an inferred Devonian gramte underlying the deposit and a Cretaceous alkaline intrusive complex lying 25 km to the SE. The rocks are generally all poor in sulphides and carbonaceous matter.

Disseminated, sulfide-poor, low-grade gold mineralisation (< 0.2g/t Au) occurs with minor Ni-FeCo-arsenides and Pb-Zn-Cu sulfides in magnesian skams, associated siliceous zones and silica-clay zones. The skams include diopside skams, ophicalcites (serpentinised forsterite marbles and bmcite marbles) and bmcite marbles (some replacing periclase marbles), all indicating high-temperature formation (>600®C). Higher grade gold (>lg/t) is virtually restricted to argillaceous silica-clay zones overlying these skams.

Silicified rocks are a prominent alteration type in both the skams and dolostones, and are found in other areas of Tasmania. They include vuggy quartz, chalcedony and opal, and locally contain complex retrograde calc-silicate assemblages plus some minor gold (< 0.2g/t Au), Cu-Pb-Zn-Fe sulphides and Co-Ni-Fe arsenides. They are probably post-Permian in age, as they are associated with silicified Permian rocks, and are probably mostly pre-Jurassic in age, being intmded and enveloped by dolerite, although some quartz-prehnite veinlets do occur in the dolerite. Fluid inclusions in the quartz breccias are of high temperature (300-400''C), low salinit}^ and are CO2bearing.

Talc-amphibole-chromite rocks apparently formed sedimentar>' rocks, and are locally Ni-rich (as silicates).

from

alteration

of

ultrabasic-derived

The high temperature skam assemblages are mferred to be related to the dolerite, the only sizable mtmsions confirmed in the area. The siliceous rocks largely appear to predate the dolerite. The bulk of the dolerite is mferred to be a large cone sheet that has forced the Proterozoic and Cambrian rocks upward through the Permian sequences.

Some features of the deposit superficially resemble epithermal or Carlin-style mineralisation, but the bulk of the data do not support these models. The primar}^ Au-base metal mineralisation is considered to be related to the Jurassic dolerite cone sheet intmsion into silicified dolostones and ultrabasic-derived conglomerates, forming magnesian skams and other altered rocks. This intmsion also mobilised some gold and base metals, partly into pre-existing, silicified karst stmctures. Gold was probably residually concentrated during later (Tertiary?) weathering which produced the silicaclay zones, which may also occupy more recent karst-fill stmctures.

276

KINEMATICS, STRUCTURAL ARCHITECTURE AND TEXTURES OF MESOTHERMAL GOLD VEINS IN THE CHARTERS TOWERS AREA, NORTHEAST QUEENSLAND, AUSTRALIA Oliver P. Kreuzer Economic Geology Research Unit, James Cook University, Towns\ille, Qld 4811 Email: 01iver.Kreuzer@jcu.edu.au The Charters Towers Goldfield is the fourth largest in Australia (Solomon & Groves 2000) and produced in excess of 6.5 million ounces Au (Levingston 1972). Charters Towers is located in the Ravenswood Batholith of the eastern Lolworth-Ravenswood Block. The block is part of the northern Tasman Fold Belt System and its E-W striking structural grain is in contrast to the dominant N-S trend of the Tasmanides.

Siluro-Devonian lode gold mineralisation in the Charters Towers area is hosted by Ordovician to Silurian I-type granitoids. Metamorphic basement units contain relatively few deposits. Gold mineralisation occurs as brittle fault and shear zone hosted quartz-sulphide veins. Two conjugate vein sets are recognised: 1) approximately E-W striking and 2) roughly N-S striking lode sets. Both are shallow to moderately dipping. E-W striking veins are larger and historically the most productive. Wallrock alteration is controlled by the orientation of fractures that host gold mineralisation, and consists of proximal sericitic and distal propylitic alteration types.

Structural architecture and kinematics of the Siluro-Devonian lodes are poorly constrained because previous work was restricted to historic mining plans and a limited amount of drillcore. The model proposed here is based on mapping of open pit and underground workings in the Charters Towers area. In this model gold mineralisation occurred during a compressional, roughly NE-SW directed event that produced reverse movement on host fractures. The concept is supported by: 1) slickenlines on host fractures, 2) flat to shallow dipping splay terminations suggesting horizontal fracture propagation, 3) preferential mineralisation of shallower dipping vein segments in undulatmg lodes, and 4) the abundance of shallow to moderately dipping veins and the absence of mineralised subvertical to vertical structures. Reverse and normal faulting postdating the gold mineralisation caused some distortion of the auriferous veins.

Analysis of vein textures indicates that ore mineral deposition is strongly controlled by vuggy or brecciated vein sections. Microscopic gold grains, blebs and veinlets are preferably located in fractures and defects of paragenetically early sulphides. Coarser gold grains are found in quartz and along embayed quartz/sulphide gram boundaries.

References Levingston, K. R. 1972. Ore deposits and mines of the Charters Towers 1:250.000 sheet area, north Queensland. Queensland Department of Mines Report 57.

Solomon, M. and Groves, D. I. 2000. The geology and origins of Australia's mineral deposits. Centre for Ore Deposit Research (University of Tasmania) and Centre for Global Metallogeny (University of Western Australia) Publication 32.

277

LmEATION AND STRESS/STRAIN ANALYSIS AS AN EXPLORATION TECHNIQUE FOR EN ECHELON FAULT/QUARTZ VEIN HOSTED GOLD MINERALISATION: EXAMPLES FROM THE TARNAGULLA AND CENTRAL MALDON GOLDFDELDS, VICTORIA, AUSTRALIA ^ J Krokowski de Vickerod and ^G.B. Ebsworth ^ Geological Survey of Victoria, ^CODES SRC, University of Tasmania This paper presents a new structural model for fault/quartz reef hosted gold mineralisation with en echelon to anastomosing geometries, and a new analytical technique based on lineation distribution and stress/strain analysis of folding and faulting. The model and technique have been developed during work on the Tamagulla (Krokowski de Vickerod et a l , 2001) and Maldon (Ebsworth and Krokowski de Vickerod, 1999; in press) goldfields in the Bendigo zone of the Lachlan Fold Belt, Central Victoria.

Apart from describing and classifying the geometry, characteristics and genesis of fold and fault deformations and associated mineralisation, the key problem the model tries to address is the occurrence of high grade shoots associated with abundant, coarse, erratically distributed gold withm brittle-ductile reef systems. The model explains the geometry of deformations, especially those with en echelon^ spiral geometries and common Riedel splays, the distribution of stress and strain during their genesis, and the consequences for fluid flow patterns and associated gold mineralisation.

Economically important ore shoots at Tamagulla and Maldon occur in left- or right-lateral pull-apart structures at the margins of en echelon reef segments. The reef segments consistently exhibit west side-up transport, whereas the sense of the strike-slip component in the pull-apart structures is smistral at the northern ends of reef segments and dextral at their southern ends due to opposite signs of an associated vorticit>^ within them. High grade mineralisation only occurs where the pull-apart structures are well developed. Turbidite sequences t>^ically lack recognisable and persistent marker horizons. Instead, Li lineation of the primary deformation (folding) may be used to describe the geometry' of overprinting deformations and is useful in the analysis of stress/strain in en echelon fault-reef systems, particularly in determining the margins of fault-related quartz reef segments where ore shoots commonly occur.

In a more regional sense, the model involves a strike-slip component of deformation which has been documented both at Tamagulla and Maldon. The component makes it possible to mterpret stmctural highs around major ore shoots as brachy-anticline-s>^cline (domal) stmctures with approximate NNW and/or NNE trends caused by the strike-slip and/or torsional components of left- and/or rightlateral transpression, respectively. The axes of such brachyfolds are oriented at a small angle (around 30^) to the direction of strike-slip movement. Relatively late, regional sinistral transpression transformed into dextral transpression and formed brachyfolds that may explain the diagonal pattem of domes in the Bendigo zone. Experimental studies indicate that transpressional stmctures like brachyfolds, en echelon faults, cleavage etc. typically exhibit changes in geometry and position durmg the different stages of deformation. This means, for example, that the position of stmctural highs within the folds may have moved during regional deformation. The movement of the host stmctures during deformation caused the locus of associated gold deposition to move as well. At the local scale gold mineralisation was controlled by the strike-slip and/or torsional components of faulting. Field and experimental studies indicate it may be possible to predict the history of these movements and as a consequence, predict the position of associated gold shoots. The model and techmque may improve efficiency in defining exploration targets, especially for deposits with abundant, coarse, erratically distributed gold.

References Ebsworth, G.B. & Krokowski de Vickerod, J. The Central Maldon Goldfield, 1:5 000 map and geological report. Geological Survey of Victoria, VIMP Report (in press).

Krokowski de Vickerod, J., Cuffley B. and T. Evans. 2001. Tamagulla Goldfield, Central Victoria, 1:10 000 map area. Geological Survey of Victoria. VIMP Report 71, pp. 160.

278

TEXTURAL, MINERALOGICAL AND GEOCHEMICAL VARIATION WITHIN THE ZONED TIMBARRA TABLELANDS PLUTON, NORTHERN NEW SOUTH WALES, AUSTRALIA. R. Mustard Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia.

The Timbarra Tablelands pluton is one of many predominantly monzogranite I-type plutons which constitute the Moonbi Supersuite in northern New South Wales, Australia. The Timbarra Tablelands pluton is an areally extensive (-550 km^) complexly zoned intrusion. The pluton comprises an outer rim of Rocky River monzogranite (Zones 1 to 3), an intermediate zone of Sandy Creek syenogranite (Zone 4A to 4C), surrounding a core of Surface Hill syenogranite (Zones 5 to 7). All seven zones are recognizable in the field based on spatial distribution, variation in grain size, texture, and mafic mineral, megacry^st and enclave content.

The granites forming the Timbarra Tablelands pluton belong to the Timbarra Tablelands suite, and represent a continum of co-magmatic intrusions. The suite is calc-alkaline, high-K, and varies from mildly metaluminous to weakly peraluminous with increasing fractionation. The suite has undergone a high degree of fractionation, with the average Rb/Sr ratio ranging from 0.4 in the least evolved ver>^ coarse-grained monzogranite (Zone 3) to 46 in the most evolved very fine-grained biotite microgramte (Zone 6). Trace element modelling indicates that the range in composition exhibited by the pluton can be produced by crystal fractionation from a single parent melt with a startmg composition equivalent to the least evolved (enclave poor) samples from Zone 3. Zones 4, 7 and 6 were formed from an estimated 40 to 55%, 55 to 65%, and 65 to 90% fractional crystallization respectively.

The Timbarra Tablelands pluton ranges m composition from granodionte-monzogranite (Zone 3) to alkali-feldspar granite (Zone 6). Compositional variation within the suite is continuous from 63 to 78 wt % SiOi. Increasing Si02 is accompanied by decreasing AI2O3, Ti02, CaO, FeO, Fe203, MgO, MnO, P2O5, Sr, Ba, Cr, Cu, V, Zn, Zr, Ni, Pr, Sm, and Eu. Na20, trace elements Rb, U, Pb, Nb, Y, Ta, Cs, Nd, Ga, Hf, Sc, As, Sn, Ag, Be, and heavy rare earth elements Gd, Tb, Dy, Ho, Er, Yb, Lu all display a steep increase in concentration above 75.25 wt % Si02. In contrast K2O, trace elements Th, F, and light rare earth elements La, Ce, Nd exhibit a rapid decrease in concentration above 75.25 wt % Si02. Geochemical data on variation diagrams reveal a continuous single liquid line of descent, however adjacent zones along the trend are not spatially contiguous. The complex zonation patterns in the Timbarra Tablelands pluton are considered to be the result of the intrusion of three pulses of magma, all of which are sourced from a single underlying, vertically stratified magma chamber. Upon emplacement each magma pulse underwent fractional crystallization. The initial pulse of magma forming the outer Rock}^ River monzogranite had a composition equivalent to Zone 3. This underwent outward directed crystal fractionation forming Zone 2. Sidewall crystal accumulation of k-feldspar megacr>^sts produced the "cumulate" crowded porphyritic monzogranite (Zone 1) along the magma chamber walls. The intermediate Sandy Creek syenogranite was formed from the second magma pulse which fractionated inwards from Zone 4A to Zone 4C. The third pulse of magma formmg the Surface Hill syenogranite initially formed a chilled carapace (Zone 5) upon emplacement. A bouyant highly evolved felsic liquid produced from crj^stallisation of the medium to coarse-grained syenogranite core (Zone7) ascended and ponded in the roof zone and pluton margins, where it crystallised to form a very fine-grained aplite carapace (Zone 6).

The pluton was interpreted to be emplaced at mesozonal levels (-1.8 ± 0.6 kbars, 5 to 10 km depth) and crystallized at temperatures between 820''C to 620°C under moderately oxidizmg conditions (log /D2 = -17 to -24). The association of gold-molybdenite mineralisation at Timbarra with moderately oxidized l-ty^Q magmas is consistent with fractionation-redox controls on ore element behaviour in magmatic systems.

279

THE PEAK HELL HIGH SULFH)ATION AU-CU DEPOSIT, NSW: SPATIAL AND TEMPORAL RELATIONSHIPS BETWEEN ALTERATION ZONING, DEFORMATION AND MINERALISATION Richard J. Squired Walter Herrmann" and Glenton J. Masterman^ ^Department of Earth Sciences, The University of Melbourne, Victoria, 3010 "Centre for Ore Deposit Research, University of Tasmania, Hobart, TAS 7005 The Peak Hill Au-Cu deposit, located east of the Peak Hill township in central-western NSW, is hosted by strongly deformed and hydrothermally altered late Ordovician to early Early Silurian (early Llandovery) volcanic rocks of the Goonumbla Volcanics (formerly Mingelo Volcanics).

Detailed short wavelength infrared spectral analyses have identified distinctive hydrothermal alteration zones that are sub vertical, with a vuggy silica and pyrophyllite core (about 350 m wide and at least 550 m long) grading out through paragonite-muscovite, kaolinite to chlorite-epidote altered facies. This zonation reflects decreasing temperature and neutralisation of acid fluids with increasing distance from the core.

Siliceous zones, bladed quartz-pyrite veins, magmatic/hydrothermal breccias and pyrite veinlets all overprmt the phyllosilicate altered zones, but predate the auriferous quartz-pyrite-barite veins. The mineralised veins occur predominantly in the paragonite-muscovite zone generally less than 50 m from the boundary with the pyrophyllite zone (cf. Allibone, 1998). Alunite and kaolinite occur in fractures and cavities and thus post-date the mineralised veins and the phyllosilicate and siliceous altered zones.

The zonation of ore minerals and alteration assemblages are clearly analogous to those of volcanicrelated high sulfrdation epithermal deposits. Although no evidence exists to support a strong primar}^ lithological control on the localisation of the core of the alteration system (cf., Degeling et a l , 1995), the (west-) northwest and northeast-striking boundaries between the pyrophyllite and paragonite alteration zones may reflect the orientations of early syn-volcanic faults that acted as fluid conduits. The comcidence of high grade (>5 g/t) drillhole intercepts with zones of silicic alteration and quartz-pyrite-bante veins suggests that fluid chemistry and rheology of the host rocks were also important controls on the distribution of mineralisation at Peak Hill. Alteration and mineralisation at Peak Hill was broadly co-eval with formation of the Late Ordovician to early Llandovery (440-437 Ma) porphyry-related deposits elsewhere in the Junee-Narromine and Molong Volcamc Belts. Deposition of the late Llandover}^ Cotton Formation and development of the mtense north-south (Si) foliation both post-date formation of the Peak Hill deposit (cf.

Allibone, 1998). The Early Devonian Ar/Ar isotopic age of a K-mica sample from Peak Hill (Perkins et al, 1995), does not represent the age of phyllosilicate alteration associated with mmeralisation (cf. Allibone, 1998), but probably represents the age of muscovite recry^stallisation associated with intense deformation (Di) at Peak Hill during early movement along the Parkes Thrust.

References Allibone, A., 1998. Synchronous defonnation and hydrothermal acti\dt>^ in the shear zone hosted highsulphidation Au-Cu deposit at Peak Hill, NSW, Australia. Mineralium Deposita , 33, 495-512 Degeling, P R., Corbett, G.J. AND Leach, T.M., 1995. The Peak Hill high sulphidation gold deposit, NSW. hr. Mauk, J.L., St George, J.D. (Eds) Proceedings, Pacrim Congress 1995, Auckland, New Zealand. The Australasian Institute of Mining and Metallurgy^ 175-180 Perkins, C., Walshe, J.L., and Morrison, G., 1995, Metallogenic episodes of the Tasman Fold Belt System, eastern Austraha. Economic Geology, 90, 1443-1466.

280

PERMO-CARBONIFEROUS GOLD DEPOSITS OF THE CONNORS-AUBURN ARCH: EPITHERMAL MINERALISATION ASSOCIATED WITH THE TRANSITION FROM ARC TO EXTENSIONAL VOLCANISM L.J. Hutton and I.W.Withnall Department of Natural Resources and Mines, Queensland, P.O. Box 194, Brisbane, Queensland Gold in the Connors-Auburn Arch is closely associated with Permo-Carboniferous volcanism.

Major deposits include Cracow, Mount MacKenzie, and Mount Britton.

The Cracow deposits and small prospects near Rannes occur in basalt and basaltic andesite of the -308-291 Ma Camboon Volcanics. They have an epithermal setting associated with regional argillic and locally K-feldspar alteration in dilatant fault zones and have colloform silica gangue. A rhyolite dyke that cuts the mineralisation at Cracow is dated at -291 Ma. Mount Mackenzie is a high-sulphidation advanced argillic Au system in basalt and basaltic andesite of the Lizzie Creek Volcanic Group. The age is constrained at -300 Ma with the pervasive alteration being overlain by andesite dated at -296 Ma. Farther north, Au at Mount Britton also occurs in basalt and andesite of the Lizzie Creek Volcanic Group. A rhyolitic ignimbrite near the top of the sequence in this area gave an age of -291 Ma. The tectonic setting of the Permo-Carboniferous volcanics is currently the subject of debate. They have long been accepted as continental margin arc volcanics, but some recent interpretations have favoured an extensional rather than subduction-related origin.

All compositions from basalt to rhyolite are represented in the Permo-Carboniferous volcanics, with a tendency towards bimodality towards the top. Mafic volcanics have ver>^ strong subductionrelated geochemical signatures, and show an excellent match with modem lavas from the southern Andes and the High Cascades of Oregon (see Figure), agreeing with previous comparisons of preCretaceous intrusives of the Connors Arch with the coastal batholiths of the American Cordillera.

However, the geochemistry of the Connors-Auburn Arch lavas partly overlaps with that of volcamcs in extensional continental settings like the Basin and Range Province of the western USA, which have higher contents of most trace elements in spidergrams. The data are therefore consistent with an interpretation that the volcanism records the transition from a continental margin arc that persisted throughout the Carboniferous to an Early Permian extensional phase.

This interpretation is supported by the fact that other evidence for extension is confined to the Early Permian. Any topographic high that existed along the site of the Connors-Auburn Arch until then was removed by extension and subsidence that allowed widespread marine sediments of the Bowen Basin to flood across the Arch. Dates from the top of the Lizzie Creek and Camboon Volcanics suggest that extension was initiated at -290 Ma. A felsic dyke swarm indicating regional extension of the Connors Arch has been dated at 284 Ma. The Artinskian Rookwood Volcanics and Berserker Group that both host VHMS base metal deposits were related to this extensional event A - Connors - Auburn Late Carboniferous - Early Pemiian

L JL

B - Andean & Cascades Volcanic Arcs

0 - Basin and Range Province

Comparison of igneous rocks, with Si02 value between 50 - 60% from the Connors - Auburn Arch, Andes, Cascades and the Basin and Range Province Sr

Rb K

Th Ba

Ce Nb

Zr P

Sm Hf

Ti

281

STRUCTURAL CONTROLS ON THE TIMING OF GOLD EMPLACEMENT IN THE BENDIGO GOLD FIELD D.G. Wood F.P. Bierlein, G.S. Lister, L. Ailleres Australian Crustal Researcli Centre, School of Geosciences, PO Box 28E, Monash University, Vic 3800 The Bendigo Gold Field is situated in the central Victorian gold province of southeastern Australia, and IS the site of concentrated orogenic lode-gold mineralisation within the Palaeozoic Lachlan Fold Belt. Historically Bendigo is Victoria's largest gold producer with -600t gold produced from hard rock mining. Compilation of historic mining data, along with a comprehensive drilling and exploration program undertaken by Bendigo Mining N.L. (BMNL) has led to the recent Swan Decline construction. This has allowed a unique opportunity to study excellent underground exposure of an auriferous system in conjunction with detailed cross sections and data sets produced from extensive drill core. Knowledge of overprinting relationships and relative timing of structural deformation and gold precipitation are essential to the understanding of any mineralised system.

Extensive analysis carried out by BMNL geologists on drill core and exposed auriferous structures highlight the complexities associated with gold mineralisation at Bendigo. These complexities include the 'nugget effect', with gold occurring as free milling coarse grains up to 2mm in size in quartz veins.

Gold is precipitated from hydrothermal fluids introduced along fault-controlled fluid pathways, however the exact source(s) of both the fluids and the gold are not well understood. Available geochronological data constrain the major mineralising event to 440Ma; subsequent later fluid events have only mmor associated gold precipitation. Pervasive alteration is associated with the mmeralising fluid events, and includes silicification, carbonatisation, sericitisation and a strong arsenopyrite correlation within metres of the quartz-gold reefs. Mineralised quartz vein arrays are closely associated with steeply dipping reverse fault zones that transect the hinges of NNW trending chevron-folded anticlines. The reverse fault zones have extremely variable gold grades between anticlines. Grades in historically mined ore shoots range from <lg/t to >300g/t Au. The geometry controlling the higher gold content of these structures is not well understood. Gold is also concentrated in vein shoots that can persist along strike for tens of metres. Clearly, these shoots carry particular importance in exploration, however their relatively narrow diameter and commonly discontinous formation mean that consistent targetmg is often difficult.

Re-interpretation of existing drill core data and examination of new core show that arrays of eastdipping reverse faults transect the larger west dipping fault zones. The areas of offset along the westdipping zones betw^een east-dipping faults form vertical zones of dilation. These dilational sites host concentrated gold mineralisation. Although these observations are indicated by the drilling of only a small portion of the New Bendigo gold project, this correlation between structural development and ore formation is also seen in a three-dimensional model of the Sheepshead anticline built in gOcad™, where a vertical zone of elevated gold grades occurs metres below the intersection of two such faults.

A scale-integrated approach that clarifies how micro- and macro-structural data relate to ore genesis will aid in understanding the formation and post-ore history^ of the Bendigo gold deposit. Our approach utilises a combination of microstructural analysis, detailed two-dimensional interpretation and three-dimensional modelling of ore-bearing structures. A better understanding of the structural controls on mineralisation at Bendigo has implications for analogous deposits elsewhere in Victoria, as well as orogenic lode-gold deposits in fold belt terrains worldwide.

Acknowledgements We are grateful for support and input provided by Bendigo Mining N.L. (especially P.Quigley and D. Tumbull).

282

OCCURRENCES OF PLATINUM-GROUP ELEMENTS WITH SEDIMENTHOSTED OROGENIC GOLD DEPOSITS: PROPOSED MODEL & POTENTLVL IN AUSTRALLV Andy Wilde\ Frank P. Bierlein\ Andy Wilde\ Alaster Edwards^ Kevin Ansdell^ David Gamett^ and Alexander Yakubchuk^^ Victorian Institute of Earth and Planetary Sciences, Monash University, PO Box 28E, Melbourne Vic 3800 ^Edwards & Associates, 658 Riversdale Road, Camberwell Vic 3124 ^Department of Geological Sciences, Universit>^ of Saskatchewan, Saskatoon SK S7N 5E2, Canada ^Becquerel Laboratories, Lucas Heights Science and Technology Centre, PMB 1, Menai NSW 2234 ^Natural History Museum, London SW7 5BD, United Kindom The vast majority of the world's supply of platinum-group elements (PGE) comes from mineral deposits hosted in mafic and ultramafic mtrusive rocks, such as the Bushveld intrusions in South Africa. Although several sediment-hosted deposits are known to contain unusual concentrations of PGE (e.g. Kupferschiefer, Coronation Hill), there has been no concerted attempt to research the possibility of low temperature hydrothermal platinum-group element mineralisation and to explore for such deposits. Elevated levels of Pt and lesser Pd (and rarely Os and Rh) have also been reported from several mesothermal, or 'orogenic' gold deposits that are hosted by carbonaceous metasedimentary rocks (Goncharov et al 1995). In the case of the Sukhoi Log giant gold deposit (Siberia) Pt and Pd are present as a variet}^ of minerals including native metals, alloys, arsenides and telluro-bismuthinides. There have been some substantial intersections of economic-grade PGE at Sukhoi Log, including 1.45 g/t Pt over 102.3 m and 2.42 g/t over 40.5 m. A study of 83 samples across quartz vein lodes at Natalka (Kolyma) revealed PGE up to 40 g/t. An important finding of this study was that, unlike Sukhoi Log, high platinum values do not necessarily correlate with high gold. Highest PGE values (averaging 2 g/t total PGE) were found in altered host-rocks adjacent to quartz veins. Similarly, levels of Pt and Pd of the order of 0.5 - 4 g/t and variable Pt/Pd ratios have been documented in the Nezhdaninsk gold deposit. It has been suggested that much of the PGE in these deposits may be located in carbonaceous material.

It is unlikely that elevated PGE concentrations in these sediment-hosted Au-PGE deposits formed via introduction in an oxidised brine. In cases where elevated PGE have been noted, these accumulations are associated with high levels of sulphide and carbonaceous matter, implying presence of anoxic conditions. Abundant fluid inclusion data suggest low salinity fluids and the alteration assemblages are not particularly oxidised. Sulphate minerals are rare if present at all. Hydrological processes acting at redox boundaries and the catalytic activity of organic mattermay have provided the controlling mineralising mechanisms. Few thermodynamic data exist for aqueous bisulphide complexes of Pt and Pd but assuming transport of Pt and Pd as bisulphide complexes, oxidation is a possible depositional mechanism. This would be hard to envisage in a metamorphic dewatering model, but is more likely with an oxidised C02-rich fluid derived, for example, from a cooling mtrusion or meteoric sources. Such a scenario gains support from the common association of carbonate with the orebodies.

New evidence presented here from a number of orogenic gold deposits located in central and eastern Asia (e.g. Muruntau, Kumtor, Besopantau, Natalka), and central Victoria (e.g. Fosterville, Ballarat) suggests that PGE are not unusual in orogenic sediment-hosted gold deposits and may even be a ubiquitous component. The geology of central Victoria is similar to that of the Kolyma region of the Russian Far East, and both regions are well endowed vvdth gold. Exploration criteria for sedimenthosted PGE deposits could therefore be considered to be essentially identical to those for orogenic sediment-hosted gold deposits.

Acknowledgements: Australian Institute of Nuclear Science and Engineering (Grant 02-006 to FPB); Greg Hall (Placer Dome Asia Pacific); S. McKnight (University of Ballarat), H. Waldron (Becquerel Labs).

Reference Goncharov et al. 1995. Geol. Soc. Am. Abstracts with Programs, Vol.27, 5, p. 21.

283

ORE FLUIDS IN WYANAD GOLD MINERALIZATION, SOUTH INDIA: EVIDENCE FROM FLUID INCLUSION MICROTHERMOMETRY AND GAS ANALYSIS S. S. Binu-Lal\ Takayuki Sawaki^ Hideki Wada^ and S. Biju-Sekhar^ ^Department of Biolog\^ and Geosciences, Shizuoka Universitv, 836 Ohya Shizuoka, Japan.

"Asia geothennal research group, Institute of Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8567, Japan. ^Department of Geosciences, Osaka Cit>^ University, Sugimoto 3-3-138, Sumiyoshi-ku Osaka, Japan.

Archaean gold deposits consists a major source of gold in the world and are a resuh of large, complex mineralizing systems that have developed within many terrains. In southern India, major gold deposits occur in Archaean granite-greenstone terrains in the north (Kolar, Hutti, Ramagiri) and within Proterozoic granulite terrains in the south (Wyanad). The Wyanad Gold Field (WGF) is located within the Moyar-Bhavani lineament - a Proterozoic transcrustal shear zone, which has been reactivated during the Pan-African. Even though direct age data of gold is not available from WGF, the localization of gold within this Proterozoic shear zone is suggestive of a late Proterozoic age for the deposit. Gold in WGF is mainly concentrated within the quartz vein systems, which are emplaced within the mesoscopic faults and shears in the region. They cut across the regional fabric of the country rocks, suggesting that the gold quartz veins were emplaced in a post tectonic and post metamorphic episode of distentional crustal shearing. The quartz veins do not show any preference to any particular lithology, but show strong structural control. These veins occur in a variety of lithologies metamorphosed over upper amphibolite to granulite grade. Primary gold as visible specs or interstitial grains and veinlets are seen within quartz as well as within the principal sulphide mineral pyrite, suggesting that there might have been at least two episodes of mineralization. In some cases they do occur with the less frequent sulphide minerals like pyrrhotite and chalcopyrite also. The ubiquitous occurrence of fluid inclusions within quartz, which host gold and suphides, is suggestive of a definite correlation between gold and fluid inclusions. These fluid inclusions might have also contained the major ore formmg fluid in WGF. Fluid inclusion studies thus forms as an indispensable prerequisite for understanding the chemical processes of ore formation in WGF. Systematic fluid inclusion studies carried out on gold bearing quartz veins from WGF, showed the frequent occurrence of Type I (primary) biphase inclusions, having a composition of H2O-CO2, Type II (pseudo-secondary) H2O-CO2 inclusions, and a late. Type III (secondary) aqueous inclusions.

Microthermometric studies carried out on different samples revealed a near pure CO2 composition, associated with the aqueous phase, with melting temperatures close to -56.6. Clathrate melting temperatures obtained from the primary and pseudo-secondary inclusions show^ that the fluids w^ere low to moderately saline (2 to 14 wt% NaCl equiv.). The homogenization temperatures of type I (primary) inclusions ranged betw^een 250 and 330°C w^hile that of the pseudo-secondary between 230 and 300°C. The secondar}' aqueous inclusions showed much lower homogenization temperatures ranging between 130 and 220''C. Fluid mclusion microthermometric data permit densities of the fluids to be calculated and the calculated fluid densit}^ ranged between 0.8 and 1.0g/cm^ The isochores pertaining to these fluid densities intercept, the trapping temperatures, within pressure ranges of 1.3 and 3. Ikbars.

Fluid inclusion gas analysis carried out on selected samples using a quadrupole mass spectrometer included individual as well as bulk inclusion analysis. The analysis shows that the fluids present were mainly H2O and CO2 with minor quantities of N2 and CH4, reconfirming our visual analytical data. We infer that the fluids responsible for the Wyanad gold mineralization evolved from a low to moderately saline H2O-CO2 fluid, precipitated gold at temperatures of ca. 300. The origin of the fluids is thought to have been from ultimate magmatic sources.

284

FORMATION OF WILLEMITE IN HYDROTHERMAL ENVIRONMENTS JOQI Brugger\ D C. "Bear" McPhail' and John Waters^ ^ South Australian Museum and Universit>^ of Adelaide, North Terrace, 5000 Adelaide Email: Brugger. Joel@saugov. sa.gov.au ^Australian National Universit>\ Canberra, Australia ^Monash Universit>^ Victoria 3800, Australia.

Willemite (zinc silicate) forms in a variety of geological environments and by processes ranging from low-temperature alteration of zinc-sulphide ores in arid environments to magmatic-hydrothermal transport and deposition related to highly evolved alkaline magmas. Economic willemite is found solely in carbonate-hosted deposits (e.g., Franklin, New Jersey; Vazante, Brazil; Beltana, South Australia; Kabwe, Zambia). Recent interest in these unconventional Zn-deposits results from high zinc grades that can exceed 40 wt%, relatively low environmental impact due to the lack of acid-generating sulphides in the waste, and advances m ore processing technologies.

It is controversial whether carbonate-hosted willemite deposits form by supergene or hypogene alteration of pre-existing sulphide deposits, or by primary formation from hydrothermal fluids. Recent data on Vazante, Beltana, and Kabwe indicate formation at temperatures in excess of 150°C and under oxidizing (hematite stable) conditions.

In order to understand the conditions that result in primary willemite formation, especially mstead of sphalerite, we have calculated the solubilities of sphalerite and willemite over wide ranges of temperature (25 - SOOT), chloride concentration ( 0 - 5 molal), dissolved sulphur concentration (0 - 0.1 m), pH ( 3 - 1 0 ) and oxidation potential. The figure below shows the solubilities of sphalerite (dotted surface) and willemite (meshed surface) as a function of temperature and log ao2(g) (referenced to acH4(g/aco2(g) = 10,000) at constant pH, I S , I C l and quartz saturation. The magnetitehematite buffer is shown for reference by thick lines on the solubility surfaces. In the presence of sulphur, willemite is predicted to form instead of sphalerite under more oxidising (e.g., magnetite-hematite, sulphate predominant) or alkaline (high pH) conditions, especially at temperatures greater than approximately 150''C. •'^^uref'^ These results have been applied to understand the formation of the Beltana willemite deposit in the Adelaide Geosyncline, South Australia. This small but high-grade deposit (1 Mt @ 37% Zn) is hosted in dolomites of the Cambrian Ajax Limestone, next to an ^^mtrusive" contact with the diapiric, halite-bearing, siliciclastic Callanna Group. The ore body is associated with hematite alteration, and is characterised by the absence of sulphides: willemite is the only Zn ore mineral, and the arsenate hed\^hane (Ca2Pb3(As04)3Cl) is the main Pb-mmeral. Preliminary dynamic geochemical modelling shows that v^llemite will precipitate in response to water-rock interaction and/or fluid mixing processes. The modelling often predicts the precipitation of late sulphides, as a result to the reduction of sulphate in the fluid that parallels the oxidation of Fe"^ in the fluid to form hematite. We suggest that the presence of arsenate in the hydrothermal fluid may have prevented sulphate reduction at Beltana, as arsenate has a higher oxidation potential than sulphate.

285

HYDROTHERMAL ALTERATION OF THE MOUNT FORT CONSTANTEVE AREA, NW QUEENSLAND Michael J. Carew\ Geordie Mark^, Nick Oliver^ ^EGRU, School of Earth Sciences, James Cook University "School of Geosciences, Monash University, Clayton, VIC 3168 Mount Fort Constantine (MFC) occupies an area approximately 10 km southwest of the Ernest Henry Fe oxide-(Cu-Au) deposit, and is dominated by a 150 xlOOm outcrop composed of weakly brecciated porphyritic igneous rocks that have undergone var>^ing degrees of Na-Ca alteration. Along with a region around Mount Margaret (MM), these areas are the only exposed Proterozoic rocks near the Ernest Henr\^ deposit, and therefore provide the most useful laboratory for studying the spatial and temporal characteristics of alteration suites observed in proximity to Ernest Henr}^ Ernest Henr}-style Fe oxide-Cu-Au deposits have become a prime exploration target within the last decade., A genetic model for distinguishing between barren/sub economic and economic systems, however, has yet to be fully developed. With most deposits located in regions of little to no outcrop, understanding the spatial and temporal characteristics of hydrothermal activity at MFC may be directly applied to the evolution of the Ernest Henry hydrothermal system.

The plagioclase-phyric igneous rocks in the MFC area are similar to those that host Cu-Au mineralisation at the Ernest Henry mine, and are typically pale pink where unaltered. Most commonly, however, they are altered by albite and subsequent Na-Ca alteration. Albitisation produces fine-grained pale coloured rocks that lack relict protolith texture. Albitised rocks are overprinted by Na-Ca alteration associated with ductile-brittle deformation, and NW-SE trending breccia pipes (<1.5 m wide and >20 m in length). Na-Ca alteration produced actinolite, magnetite, albite, quartz, apatite, titanite and diopside. The relationship between Na-Ca alteration and brecciation is well-documented, where field studies show clear evidence of zonation in alteration intensity: from extensively altered rocks in the core of milled breccias to lower degrees of alteration where the rocks are less fractured. Both calc-silicate and plagioclase-phyric igneous rocks show evidence of intense brecciation, which is commonly focussed along contacts between rock units.

Of particular interest are the barren ironstone formations located 150 m SE of Mt Fort Constantine.

The barren ironstone formed by magnetite/hematite alteration of previously albitised plagioclase phync Igneous rocks, and is composed of predominantly of magnetite with lesser hematite, actinolite, quartz and apatite. Despite the contact between the albitised igneous rocks and ironstone being quite sharp, close inspection of small magnetite veins suggest the albitised igneous rocks were the precursor to the ironstone. Consequently, the magnetite-rich ironstones are considered to post-date albitisation, but their relationship to other phases of hydrothermal alteration is as yet unconstrained.

K-feldspar alteration is the youngest significant episode of hydrothermal alteration that effected rocks within the MFC area, and preferentially replaces previously albitised rocks. K-feldspar alteration is often associated with thin <1.5 cm quartz veins.

The paragenesis and mineralogy of hydrothermal alteration observed at MFC strongly resembles that observed cutting the granitoids at MM, which suggests that fluids responsible for the alteration in the two areas may be derived from a similar source. MM is largely composed of a multi-phase sodic intrusive body (1528±6 Ma) of similar age to the K-rich intrusions in the Williams Batholith. The post-intrusive timing of alteration in the MM area, and its similarit>^ to the alteration in the MFC area suggests both hydrothermal systems were formed at the same time (post ca 1530 Ma). The hydrothermal alteration observed at both MFC and MM may reflect similar alteration suites observed at Ernest Henry, providing implications for the timing and spatial characteristics of Cu-Au mineralisation in the Eastern Mount Isa Inlier and elsewhere.

286

FORMATION OF THE FU NEVG CARLm-TYPE GOLD DEPOSITS, YUNNAN PROVINCE, CHINA: CONSTRAINTS FROM GEOLOGICAL SETTING, MINERAL PARAGENESIS AND ORE FLUID CHEMISTRY P.W. Cromie^ and Khin Zaw^ CODES, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, 7001, Australia Email: ^pwcromie@vic.australis.com.au; "Khin.Zaw@utas.edu.au Introduction Palaeozoic to Mesozoic aged carbonate and siliciclastic sequences in southern China host Carlintype gold deposits in a graben setting along the southwestern margin of the Precambrian South China Block in Fu Ning county, Yunnan province, China. The Fu Ning gold deposits contain epigenetic micron-sized disseminated gold and are hosted by: 1) Devonian carbonaceous mudstone units at the Kuzhubao and Bashishan deposits, and 2) fault breccia zones at the contact between Triassic gabbro and Devonian mudstone units at Bashishan. Gold mineralisation in the Fu Ning deposits generally occurs along zones with strong deformation and enhanced porosity^, especially where earlier lowangle N-trending thrust faults are cut by NW-trending strike-slip and/or NE- trending normal faults, enabling hydrothermal fluids to be focused into host-rocks.

Mineralogy Sulphide minerals occurring in the Fu Ning gold deposits are; pyrite, arsenopyrite, stibnite and minor iron-poor sphalerite. Gangue minerals are quartz, sericite, calcite, ankerite and chlorite. Hypogene ore grades range from 1 to 7 g/t Au and up to 18 g/t Au at Kuzhubao and are generally less than 3 g/t Au at Bashishan. Sub-microscopic gold mineralisation appears to be closely associated with finely dissemmated arsenic-rich pyrite and arsenopjTite in the Stage HI mineral assemblages at both Kuzhubao and Bashishan. Electron microprobe traversmg techniques established that the rims of individual Stage III arsenopyrite and pyrite crystals associated with gold mineralised samples from the Kuzhubao and Bashishan deposits contain elevated levels of arsenic.

Ore fluid chemistry Data collected from primary two-phase (liquid-vapour) fluid mclusions in gold-ore Stage III quartz from the Fu Ning gold deposits yielded homogenisation temperatures (Th) ranging from 183°C to 274'C at Kuzhubao and 210 T to 327°C at Bashishan. Sahnity data from the Fu Nmg gold deposits mdicate that there were possibly two fluids present during gold deposition, involvmg; 1) an early low to moderately salme fluid with 0.8-6.5 wt% NaCl equiv., that is similar to the salimt>^ occurring in shearzone hosted gold deposits with metamorphic derived fluids and; 2) a late stage moderately high saUnit>^ fluid with 11.8-13.4 wt% NaCl equiv., mdicating possible derivation from connate waters and/or brine sources. Laser Raman spectrometry of gold-ore stage primary fluid inclusions detected abundant CO2 and trace CH4 gases, suggesting that the Fu Ning gold deposits were formed under reduced ore fluid conditions. Sulphur isotope studies of the Fu Ning gold deposits showed that S^'^S values for pyrite and arsenopyrite associated with gold-ore mineralisation during Stage III at Kuzhubao and Bashishan are isotopically similar and moderately heav>', with a range from +9 to +15 per mil and most likely indicate an origin from connate waters and/or brines. The Kuzhubao and Bashishan sulphur isotope results also fall mto the range of values reported for other Carlin-t>T)e gold deposits.

Conclusions Fluid chemistry data from the Fu Ning gold deposits suggests a Carlin-t}T)e genetic model involving fluid mixing bet\\^een: 1) CO2 rich (deep) metamorphic fluids and 2) moderately saline, reduced connate waters and/or basinal brmes, especially along permeable zones such as extensional faults cutting thrust faults, where sudden changes in fluid pressures can occur and are prospective sites to enable ore deposition.

287

A DECOMPRESSION MODEL OF FORMATION FOR SOME OXIDE CU-AU DEPOSIT-FORMING FLUIDS — A HYPOTHESIS. Garry J. Davidson land Brett Davis2 ^ CODES SRC, University of Tasmania GPO Box 252-79, Australia 7001.' c/- Delta Gold PO Box 152, Kalgoorlie, WA 6430.

While the magmatically driven evaporite-derived brine model of Barton & Johnson (1996) very satisfactonly explams the geological features and geochemistry of some oxide Cu-Au deposits and their host sodic-calcic alteration envelopes, the model does not yet account well for all examples.

The features of the most economically significant examples are in particular not well explained.

Broadly, features that are not well-explained in some districts include (1) severe brecciation and formation of diatremes; (2) absence of evidence of evaporites; (3) oxide Cu-Au characteristics of some hydrothermal ores associated with carbonatite magmatism (Groves 2000); and (4) sulfur isotopic characteristics.

Rye (1993) has advanced the view that oxidation of hydrothermal fluids as a result of decompression durmg rapid ascent can account for the sulfur isotope systematics of some high sulfidation epithermal deposits. We propose that this mechanism also may explain the defining features of some large oxide Cu-Au deposits, and at one stroke explain why such systems are so oxidised, brecciated, and iron-rich.

The oxidation state of hydrothermal fluids is predommantly controlled by redox reactions amongst the sulfur species (Giggenbach, 1997). Studies of the oxidation state of decompressing magmatic systems shows that at high temperatures, rapid decompression of melts leads to oxidation of H2S to SO2, thereby increasing the overall oxidation state of the melt. This is also applicable to H2S-SO2bearmg fluids at > 400"" C. Rye (1993) hypothesised that rapid ascent of such fluids prevents buffering of their oxidation state by reduced wallrocks, but under slower ascent conditions, equilibrium can be expected between all sulfur species in the fluid and wallrock.

Characterisation of the high temperature end-members of many oxide Cu-Au fluids is showing them to be distinctive in having high CO2 and high salinity components. Some authors have proposed this distmctive fluid to be magmatically derived (e.g., Pollard 2000). Whatever it's source, subjecting such a fluid to rapid ascent and decompression would generate large volume increases that would likely result in major brecciation of hostrock. Decompression is also an extremely efficient mechanism of Fe scavenging (Hemley et al. 1986), which together with the high salinities can account for the extreme Fe enrichments observed.

Sulfur isotope systematics of oxide Cu-Au systems have to date been strongly biased towards sulfide minerals, ie have provided an inadequate incomplete record. However, detailed analysis of sulfate-sulfide pairs at the Monakoff deposit (Mt Isa Inlier; barite-carbonate-fluorite-magnetitecpy), and reconnaissance level sulfate data from Starra (Rotherham et al. 1998), indicates that m both systems high temperature sulfate and sulfide had similar compositions, and in these cases, a magmatic rather than seawater S source was mdicated. This is consistent with the predictions of the decompression model for the origin of oxidised fluids: these deposits have sulfur isotope features that are well explained by the decompression of a C02-rich magmatic fluid, rather than heating of an evaporitically derived brine.

References Barton & Johnson (1996) Geology 24, 259-262; Giggenbach (1997) In Geochemistry of hydrothermal ore deposits, Barnes H.L. (ed.) 737-796; Groves (2000) GeoL Soc. Am Abstracts, Boston 2001, A-2 Heinley ^ra/. (1986) Geology, 14, 377-379 Rotherham er fl/. {\9n) Econ. GeoL, 93, 1435-1449 Rye (1993) Econ. GeoL 88, 733-753.

288

PRELIMINARY RESULTS OF FLUORITE (U-TH)/HE THERMOCHRONOLOGY: APPLICATION TO YUCCA MOUNTAIN, NEVADA Noreen Evans\ Nick Wilson", Jean Cline^, Brent Mclnnes^ and J. Byme^ ^CSIRO, Exploration and Mining, PO Box 136, North Ryde, NSW 1670 Australia -Geological Survey of Canada, 3303-33rd St. N.W., Calgaiy AB, Canada T2L 2A7 ^Dept. of Geoscience, University of Nevada, Las Vegas, Box 454010 Las Vegas, NV 89154-4010 USA ^ Dept. of Chemistry, Materials and Forensic Sciences, University of Technology, Sydney, PO Box 123, Ultimo, NSW 2007 Australia Fluorite (CaF2) is a mineral commonly found in association with pegmatites, metasomatized limestone and low-temperature hydrothermal deposits. The need to determine the age of fluoritebearing ore deposits has led to the previous development of isotopic techniques for fluorite geochronology usmg Sm-Nd (e.g., Chesley et al., 1991) and U-Th-Pb (Hofstra et al., 2000) methods. Fluorite (U-Th)/He isotope systematics may potentially be used to provide thermal histor}^ information, as has previously been demonstrated for apatite, titanite and zircon. In this paper we report the results of (U-Th)/He dating trials on fluorite deposited as a secondary hydrothermal mineral withm Miocene tuff at Yucca Mountain, Nevada.

Yucca Mountain is located 144 km northwest of Las Vegas and is composed of a 1 - 3 km thick sequence of shallowly eastward dipping felsic welded and non-welded volcanic tuffs of the 12.8 Ma Paintbrush Group C^Ar/^^Ar; Saw>^er et al, 1994). Post-emplacement fluid flow deposited secondary minerals including calcite, fluorite, opal and chalcedony in fractures and joints within the tuff. By integrating fluid inclusion and opal/chalcedony U-PbAJ-series geochronology data, Wilson et al. (submitted) found that the secondary mmerals precipitated from meteoric fluids at temperatures above 35°C prior to 5.3 million years ago, and at ambient temperatures after that time.

Petrographic study of 155 samples located uniform layers of fluonte that do not contain mineral inclusions, but do contam 2-phase fluid inclusions. This allows comparison of the (U-Th/He) ages to temperatures and age constraints posed by U-Pb dating. Fluorite crystals were separated from specimens containing 1 - 3 mm thick layers of precipitated fluorite. Several milligrams of > 100 300|im diameter cr>^stals were then hand-picked to ensure that the analysed fluorite was free of mmeral inclusions.

He-diffusion experiments were carried out at Caltech on a handpicked sample of colourless fluorite. All grams were between 200 and 350 microns in diameter. Assuming a cooling rate of 10 ""C / Ma, the helium closure temperature for fluorite appears to be 60 ± 5 ""C. A (U-Th)/He age of 12.8 ± 0.8 Ma was determined for a paragenetically early fluorite associated with deposition of the tuffs and is identical to "^Ar/^^Ar ages for tuff formation. A second sample from paragenetically younger fluonte, m a different area of the site, gave a (U-Th)/He age of 6.9 ± 0.4 Ma, consistent with paragenetic relationships and constraints posed by U-Pb ages on opal and chalcedony.

References Chesley, J. T., Halliday, A. N.. and Scrivener. R. C. 1991. Samarium-neod>inium direct dating of fluorite mineralization: Science 252, 949-951.

Hofstra, A.H., Premo, W.R., Emsbo, P, Cline, J.S. and Aleinikoff, J.N. 2000. U-Th-Pb dating of hydrothermal minerals from Carlin-t>pe gold deposits: Results and Evaluation: In Geology and Ore Deposits 2000: The Great Basin and Beyond, (eds). J.K. Cluer, J.G. Price, E.M. Struhsacker, R.F. Hardyman and C.L. Morris. 61-65.

Sawyer, D.A., Fleck, R.J., Lanphere, M.A., Warren, R G., Broxton, D.E. and Hudson, M R. 1994. Episodic caldera volcanism in the Miocene southwestern Nevada volcanic field: Revised stratigraphic framework, ^"^Ar/^^Ar geoclironology and implications for magmatism and extension. Geological Society of America Bulletin 106, 1304-1318.

Wilson, N. S. F., Cline, J. S. and Amelin, Y. V. Temperature and Timing of Secondary Mineral Precipitation at Yucca Mountain, NV: Fluid Inclusion Analyses, U-Pb, and U-series Dating: Submitted to Geochimica.

Cosmochimica Acta.

289

UV SPECTROPHOTOMETRIC STUDY OF COPPER(I)-CHLORn)E COMPLEXEVG AND APPLICATION TO COPPER TRANSPORT IN PORPHYRY HYDROTHERMAL SOLUTIONS ^ Weihua Liu, 'D. C. 'Bear' McPhail and ^Joel Brugger School of Geosciences, Monash University, Vic 3800, Australia ^Present address: CSIRO ExT)loration and Mining, PO Box 136, North Ryde, Sydney, NSW, 1670 "Present address: Department of Geology, Australian National University, Canberra, 0200 Present address: The South Australian Museum, S.A. 5000

Copper transport and deposition in hyper-saline hydrothermal fluids can result in the formation of hydrothermal copper deposits and are mainly controlled by the stability of copper(I) complexes with ligands such as chloride and hydrosulphide. However, our understanding of the behavior of copper(I) chloride complexes at elevated temperatures and in hyper-saline brines is limited by the conditions of existing experimental studies where the maximum chloride concentration is 2 m. This study was designed to investigate copper(I) chloride complexes at much higher chloride concentrations, up to 9.1 m, using a UV spectrophotometric method.

The UV spectra of copper(I) bearing LiCl solutions were measured at temperatures between lOO'^C and 250''C at vapor-saturated pressures. Quantitative interpretation of the spectra shows that CUCI2' , CUCI3" and CuCU" were present in the experimental solutions. The fitted logarithms of formation constants (log K) for CuCl2' are in good agreement with the previous results of solubility experunents reported by Xiao et al. (1998) and Liu et al (2001). The log K values for CuCls^' also agree with those of Liu et al (2001) and theoretical estimates of Sverjensky et al (1997).

How^ever, this study presents the first experimental formation constants for CuCU^', a complex that predominated at chloride concentrations greater than 5 m.

Standard state thermodynamic properties and parameters of copper(I) chloride complexes were successfully regressed from the experimental data of this study and selected previous studies, which allowed the extrapolation of the thermodynamic properties of copper(I) complexes over wide temperature and pressure ranges (up to 1000® C and 5 kb).

Based on the new log K values generated from this study, the calculated chalcopynte solubility at 250-400''C and 500 bars, and at different pH and NaCl concentrations, indicates that a h>^ersaline, neutral to weakly acidic solution can dissolve thousands of ppm of copper at 400''C. Chalcopynte solubility decreases with decreasing temperature, decreasing chloride concentration and increasing pH. It is suggested that in addition to cooling, fluid mixing (dilution of saline fluids), boiling and fluid-water interaction can all affect the deposition of copper from hydrothermal solutions.

290

PREDICTING ALTERATION PATTERNS IN THE OUTFLOW ZONES OF HYDROTHERMAL ORE SYSTEMS: A CASE STUDY USING THE ^SPENT^ FLUIDS FROM THE ERNEST HENRY Fe OXIDE-(Cu-Au) DEPOSIT. Geordie Mark\ Andy Wilde\ Nick Oliver and Patrick Williams" ^School of Geosciences, Monash University, Clayton VIC 3168, Australia "School of Earth Sciences, James Cook Universit>^ Towns\dlle QLD 4811, Australia.

Determmmg the likely outflow zones to hydrothermal systems associated with Fe oxide-(Cu-Au) mineralisation provides a new method for the exploration of these systems. Fe oxide-(Cu-Au) deposits provide an attractive target for exploration companies because of their large size (100lOOO's Mt) and diverse metal association. Conventional approaches to defining genetic models for various st}des of Fe oxide-(Cu-Au) mineralisation generally consider the pre- and syn-ore processes to generate targeting strategies for exploration, but post-ore hydrothermal processes are largely Ignored when constructing mineralogical, geochemical and geophysical criteria for ore-related hydrothermal systems. This data can be used to help define which pathways were utilised by hydrothermal fluids that deposited mineralisation. Geochemical modelling using measured compositions of depleted ore-stage fluids provides a powerful tool to not only constrain the down stream alteration patterns from ore deposits, but has the potential to identify ore-forming processes for new styles of mineralisation, or establish links between known mineralisation styles. An excellent example of the latter is the Ernest Henry system, where 'spenf ore fluids contain high Zn (0.5-1.0 v.t%) and Pb (0.3 wt%) and exhibit the potential to form sigmficant Zn-Pb mineralisation.

This study employed Geochemist's Workbench to model 'spenf fluids from the Ernest Henry Fe oxide-(Cu-Au) deposit. Microthermometric and Proton Induced X-Ray Emission (PIXE) analysis of primary^ fluid inclusions in ore-stage quartz identified a significant population of high temperature (450-35OT), high salmit}^ multisolid inclusions. The inclusions contain relatively depleted K, Fe and Cu compared to other elements within the fluid (e.g. Ba, Ca and Mn), and represent the residual after ore deposition. As such they are ideal candidates to predict the alteration patterns associated with post-ore outflow. Modelling of alteration patterns was undertaken at 300X usmg both Titration and FLUSH methods, and was undertaken to determine the relationship between the mmeralogy of alteration and host rock composition at variable time-integrated fluid:rock ratios.

Rock Type Felsic Igneous rocks Mafic Igneous rocks

Fluid:Rock <20:1 >20:1 <20:1 >20:1

Main Mineral Association Albite, hematite, magnetite, epidote & muscovite Magnetite, muscovite, quartz & minor fluorite Albite, muscovite, hematite, barite, epidote & sphalerite Muscovite, hematite, quartz & bante

The results of modelling fluid flow through igneous rocks shows that significant differences in the mmeralogy of alteration along outflow zones are predicted with infiltration betw^een rock tynpQS and also within each rock t^^^e with progressive fluid infiltration (see table above). These differences effect both the chemical and geophysical characteristics of the rocks, and include significant early albitisation followed by formation of a muscovite-, quartz- and magnetite- or hematite-rich rock.

Barite and mmor early sphalerite are deposited only in rocks containing primary sulphide. These results suggest that the S content of the rock plays a significant role in the deposition of S-bearing minerals, but also requires /D2 and pH to be buffered keep the mmeral phase stable through continued fluid infiltration. The observed evolution m the mineralogy of alteration with progressive infiltration suggests the potential for the formation of locally complex alteration paragenesises, and raises the question of how different mineral associations in ore systems relate. Further work will be undertaken to constrain this behaviour.

291

FROM SOURCE TO SINK: EVOLVING FLUID CHARACTERISTICS IN MT ISA INLBER FE-OXBDE-CU-AU MINERALISATION Lucas J. Marshall and Nicholas H.S. Oliver James Cook University Fe-oxide-Cu-Au deposits of the Eastern Succession of the Mt Isa Inlier are associated with regionally extensive metasomatic systems of different affinities. If alteration assemblages are to be used as exploration criteria for these deposits, an understanding of how different alteration systems relate to one another and to mineralisation is critical. Here we present new and previously published stable isotope data from calcite mineral separates from both regional rocks and ore deposits, in an attempt to better characterise metasomatic fluids.

Marbles and calc-silicate rocks of the Eastern Succession show progressive shifts in stable isotope values from unaltered marine carbonate values ~20%o, -1 to 3 % o ) towards magmatic ~ll%o, ~ - 7 % o ) and meteoric ~ 4 % o ) fluid values. Isotopic variations in leastaltered marbles from different regions within the Eastern Succession can be largely explained in terms of varying degrees of decarbonation that corresponds to varying peak metamorphic conditions. Isotopic data from alteration and breccia- and vein-infill correlates well with variations in calcite trace element chemistry, reflecting magmatic and later meteoric fluid sources.

Isotopic data from the Dugald River Pb-Zn-Ag deposit lies along a well-defined two-phase mixing curve between marine carbonate and graphitic-metasediment end-members. No magmatic signature is seen, precluding a genetic relationship with Fe-oxide-Cu-Au mineralisation. Analyses from the Ernest Henry Fe-oxide-Cu-Au deposit, hosted in the marble and calc-silicate rock dominated Mary Kathleen Group, reveal interaction between dominantly magmatic fluids (with a minor meteoric input) and marine carbonates. Data from Fe-oxide-Cu-Au deposits hosted in the Maronan Supergroup illustrates a three phase mixing field between values in equilibrium with magmatic fluids, marine carbonates, and graphitic metasediments, the last of which are common in the Maronan Supergroup. The shift in data towards marine carbonate values suggests that fluids associated with these deposits were magmatic, but interacted with the thick carbonate sequences of the Mary Kathleen Group prior to ore deposition in the Maronan Supergroup. We postulate that changes in fluid chemistry as a result of extensive metasomatism of Corella Formation rocks may have been a necessary precursor to Fe-oxide-Cu-Au mineralisation in the Cloncurry District.

Marine Carbonates

0 ^^^ / o^- ^ Q

-5 Magmatic -10

O o

-15 -j -20

-25

o

§8c9o

O

O

' O (P ,0 ocP

C33

^ ^

Graphitic metasediments

« Mary Kathleen Group: Cu-Au o Maronan Supergroup: Cu-Au + Dugald River: Pb-Zn-Ag

-30 10

15

20

25

Figure 1. C and O isotopic data from Cloncurry District ore deposits largely defines a three-phase mixing field between marine carbonates, graphitic metasediments and calcite in equilibrium with magmatic fluids.

292

THERMODYNAMIC MODELLING OF ALTERED AND MINERALISED CATACLASTIC GRANITOIDS AT THE NYABIRAMA ARCHAEAN LODEGOLD DEPOSIT, TANZANIA Douglas R. Mason\ Warrick Edmonds" and Douglas S. McLean^ ^Mason Geoscience Pt>^ Ltd, Adelaide, Soutli Australia <masongeo@ozemail.com.au> ^Afrika Mashariki Gold Mines Limited, Tarime, Tanzania <amgmsite@africaonline.co.tz> ^Afhka Mashariki Gold Mines Limited, Stirling, South Australia <doug@eagm.com.au> The Nyabirama lode-gold deposit lies on the crustal-scale Mara-Nyabirama Shear Zone (North Mara region, Tanzania), and is hosted by cataclastically deformed Archaean granitoids. Brittle deformation under lower greenschist facies conditions allowed invasion by mineralising hydrothermal fluids, which generated pervasive and fracture-controlled alteration assemblages dominated by albite, quartz, sericite and pyrite, with or without minor arsenopyrite, chlorite, dolomite, and native gold. Thin quartz-pyrite-carbonate(-gold) vems host a minor proportion of the gold, most of which occurs disseminated through altered granitoid. Zonation of alteration and mineralisation is centred on the cataclastic deformation zones, with proximal ore zones characterised by high abundance of native gold, low sericite, and low arsenopyrite. The relatively uniform composition of the gramtoid host rocks provides an opportunity to investigate the controls on alteration and gold deposition using thermodynamic modelling techniques.

Constraints on P-T conditions provided by fluid inclusion studies suggest that mineralisation occurred at C and P--2 kb, from hydrothermal fluid of moderate XCO2 and low salinity.

Thermodynamic modelling based on minimisation of Gibbs free energies and internally consistent thermodynamic database demonstrates the following: 1) Reaction between granitoid host rock and mineralising fluid at increasing fluid/rock mass ratios (F/R) accounts for the principal alteration assemblage. Ore zone abundances of gold are deposited for F/R -10-100, together with appropnate abundances of pyrite and arsenopyrite. Undersaturation of the mineralismg fluid with respect to Au produces lower, but nevertheless ore grade, gold abundances.

2) The zoned occurrence of arsenop>Tite at margins of ore zones is modelled by a fluid that is slightly oxidised rather than fully reduced with respect to aqueous sulphur species.

3) Temperature differences of a few tens of degrees C, such as might have existed over the depth range of the deposit, fail to produce any significant difference in alteration assemblage or amount of gold deposited in host granitoid, but result in increased deposition of native gold in veins if the fluid was initially close to saturation with respect to Au. 4) Boiling (unmixing) of initially Au-undersaturated hydrothermal fluid in veins results in deposition of significant amounts of native gold (-10-12 ppm). Varied temperature of boiling produces little effect on the amount of native gold deposited, but rapid decrease in load pressure (e.g. transient seismic activit>0 causes increasing amounts of gold to be deposited with increasing pressure differential (gold -0-12 ppm for pressure differential 500-1000 bar).

293

NUMERICAL MODELLING OF DEFORMATION AND FLUID FLOW IN THE HAMERSLEY PROVINCE, W.A. WITH IMPLICATIONS FOR GENESIS OF LARGE MICROPLATY HEMATITE ORES J.G. McLellan and N.H.S. Oliver Economic Geology Research Unit, James Cook University, North Queensland, Australia.

Numerical modelling of deformation and fluid flow on a regional scale was utilised to test recent models for deep (> 5 km) penetration of surface fluids involved in genesis of Whaleback st}4e microplaty hematite ores in the Pilbara. Current models suggest they formed in the waning stages of the c. 2300 Ma Opthalmian Orogeny.

This study uses a finite difference continuum modelling code, FLAG (Fast Lagrangian Analysis of Continua), to test whether deep penetration of surface fluids is mechanically feasible in relation to ore formation. Two "conceptual models" with varying degrees of complexity included a fully saturated Mohr Coulomb constitutive model, hydrostatic gradients and permeability of faults two orders of magnitude greater than surrounding rock types. Model boundary conditions were conducive to an extensional collapse of the mountam range. During extensional deformation surface fluids are applied to the top of the mountain range (rainfall) which consequently produces downwards fluid flow along sub-vertical faults for reasonable strain rates and topographic elevation. Throughout deformation, fluids move progressively deeper into the model. Deeper seated fluid is forced upwards from the base of the model due to fluid overpressure induced by the load of the overlying mountain range. Both fluids mix at intermediate levels and this mixing process becomes progressively deeper within the model as extension takes place. Fluid mixing is apparent at the BIF and fault boundaries, as is lateral fluid migration along the BIF layers. This study supports the hypothesis that dov^nward flow of meteoric fluid may have played a role in the evolution of the microplat}^ hematite ores. However, unlike the model of Morris (1985) in which downw^ard penetration is essentially superficial, the association of fluid flow^ with extension may have allowed deep fluid penetration (> 5 km) and potential fluid mixing, as proposed by Powell et al. (1999) and Taylor et al (2001).

294

METAL TRANSPORT IN HYPERSALEVE BRINES D.C. ^Bear^ McPhail^ and Weihua Liu^ School of Geosciences, Monash University, Vic. 3800, Australia ^Present address: Department of Geology, Australian National University, Canberra, 0200 -Present address: CSIRO Exploration and Mining, PO Box 136, North Ryde, Sydney, NSW, 1670

The transport of many metals (e.g., Pb, Zn, Cu, Fe, Au) in chloride-rich brines is important in many geological environments, e.g., from saline lakes and groundwater to ore-forming fluids in hydrothermal ore deposits, e.g., epithermal, SEDEX, VHMS and porphyry. In order to understand metal transport in geological and other systems we need to understand the behaviour of the metals.

I.e., how they exist in brines and how they react with minerals, rocks and gases. We need to know the nature of the metal complexes in brines and their thermodynamic properties over wide ranges of temperature (e.g., to greater than 600''C), chloride concentration (near 0 to >50 wt.% NaCl equivalent; seawater - 3.2 NaCl equivalent) and to a lesser extent pressure (1 bar to > 2 kbar). The nature of the metal chloride complexes affects the transport of the metal, in particular the solubility of metal-bearing minerals. For example, in the case of lead, the complexes change from PbCr to PbCl2(aq) to PbCls" and possibly PbCU^' with increasing chloride concentration. This results in galena becoming increasingly soluble with increasing chloride concentration and that effect becomes more pronounced at high chloride concentrations. In addition, we need to know the activity-composition relationships for individual complexes in hypersaline brines, because they can make orders of magnitude difference in the calculated solubilities of minerals.

Our group's recent experimental and thermodynamic modelling studies on Cu chloride complexing demonstrate the need for knowledge about the nature of metal chloride complexes and the magnitudes of activity coefficients (activit>^ = activity coefficient x concentration) that are necessar}^ in thermodynamic modelling of copper transport. At temperatures below approximately 250°C, CuCU^' becomes the predominant form of copper at chloride concentrations greater than 5 or 6 molal (seawater - 0.6 molal). This means that the solubility of copper minerals becomes ver}^ sensitive to changes in chloride concentration in hypersaline brines. At higher temperature, e.g., 400°C, CuCV" becomes predominant at chlonde concentration greater than approximately 2 molal, meaning that changes in chloride concentration (e.g., fluid mixing) become even more important at temperatures typical of porphyry and other ore-forming environments. The magnitude of activity coefficients is uncertain but our experimental data and modeling studies indicate that they could be lower than 10"^ for highly charged complexes (e.g., CuCU^ ) in saline to hypersaline chloride brines at temperatures greater than SOO'^C. This means 6 orders of magnitude difference betv^^een the activit}^ used in thermodynamic models and the concentration that might be expected in ore fluids!

Our predictions of chalcopyrite solubility^ show that thousands or even tens of thousands of ppm copper could be transported in hyerpsaline brines at temperatures greater than approximately 350°C, in agreement with recent measurements of copper concentrations in individual fluid inclusions from porphyry deposits. At lower temperatures chalcopyrite solubility^ is much less, on the order of tens of ppm or lower levels.

In similar, but preliminary^ experiments on oxidized Fe(III) chloride complexing we have found that FeCU' predommates at chloride concentrations above approximately 8 molal at 25°C, but at 200°C it predominates at chloride concentrations greater than only 1.5 molal. Preliminary interpretation of reduced Fe(II) chloride spectra at 25''C suggests a change in chloride complexation at chloride concentrations greater than approximately 5 molal and the effect is even stronger at chloride concentrations up to 15 molal. The effect is similar at temperatures up to 85°C. Our results show that it is now possible to tackle the difficult problems of measuring the properties necessary to understand metal transport in hypersaline brines, at least at temperatures up to approximately 300°C.

295

A NEW METHOD FOR MEASURING OSMroM ISOTOPIC COMPOSITIONS USING MULTI-COLLECTOR ICPMS: APPLICATION TO ORE DEPOSITS Marc Norman^ Vickie Bennett^', Malcolm McCulloch\ Les Kinsley^ ^ Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 The Re-Os isotope system can provide useful information about the age and source characteristics of a variety of magmatic, hydrothermal and sedimentary deposits. It is especially applicable to economic geochemistry as both Re and Os are chalcophile, permittmg direct investigation of the timing of sulfide deposition and metal sources. However, complex anal>tical requirements have Imuted the broader application of Re-Os isotopic studies. Here we describe a sunplified method for high-precision Os isotopic analysis using a new generation multi-collector ICPMS, and pilot investigations of two examples representing diverse styles of Australian ore deposits.

A simplified method for Os isotopic analysis Currently the preferred method for Os isotopic analysis is thermal ionisation mass spectrometry using negatively charged OsOs^"^^ ions. This technique is capable of analysing ver>^ small amounts of Os to high precision, but it requires highly specialised mass spectrometiy^ and demanding chemistr}^ to isolate and purify the Os prior to analysis. We have developed a simplified method of Os isotopic analysis using the Neptune multi-collector, magnetic sector ICPMS (MC-ICPMS) which provides high-precision data without the need for complex chemical separations. This approach takes advantage of the volatile nature of oxidised Os (as OSO4) by sparging an Os-bearing gas directly into the mass spectrometer for isotopic analysis. The method is applicable to a variety of ore deposits, and provides a significant competitive advantage through increased productivity.

Powdered samples are dissolved and equilibrated with an enriched isotope spike (^^^Os) usmg concentrated HNO3+HCL in a sealed, high-pressure carius tube at 230 ""C. This ensures complete oxidation and equilibration between spike and sample, and allows the concentration of Os to be precisely determined by isotope dilution. The sample is then transferred to a teflon vial equipped with a transfer cap assembly plumbed directly into the ICPMS torch. Volatile 0 s 0 4 is entrained by argon gas bubbled through the liquid sample, and introduced directly into the ICPMS plasma for real time isotopic analysis using static collection into multiple faraday cups, or for small signal intensities combined SEM/Faraday collection. Replicate measurements of isotope ratio standards demonstrates excellent agreement between data obtained by thermal ionisation, solution aspiration MC-ICPMS, and vapour phase MC-ICPMS analysis. A t>pical analysis consisting of 3 blocks of 10 cycles each (30 replicates) can be completed in about 10 mmutes. Following the Os analysis, rhenium can be extracted from the residual solution and analysed separately by isotope dilution.

Application to Ore Deposits For this pilot study, we measured the Os isotopic composition of a pyxite separate from the Mt. Lyell deposit (western Tasmania), and a bulk ore sample from a komatiite-hosted NiS deposit in Western Australia. The NiS ore has a measured ^"Os/^^^Os of 0.14986 ± 0.00006 (0.4 per mil 2SE) and an Os concentration of 182.6 ppb. This indicates a predomniant mantle contribution to this komatiite-hosted deposit (compare present day mantle value of ^^^Os/^^^Os = 0.128). In contrast, the Mt. Lyell pyrite has much less Os (0.39 ppb) and a considerably more radiogenic measured ^^^Os/^^^Os of 7.346 + 0.013 (1.8 per mil 2SE), indicating a greater crustal component in this granite-modified VHMS type deposit.

The new vapour phase MC-ICPMS technique is suitable for Os isotopic studies of a variet}^ of sulfide ore deposits. Applications currently under development include source tracing of ore m.etals, and Re-Os dating of molybdenites, black shales, and other sulfides.

296

G E O C H E M I S T R Y OF I N D I V I D U A L F L U I D I N C L U S I O N S : A P P L I C A T I O N TO M I N I N G A N D OIL I N D U S T R Y ^Pascal Philippot, ^ Julien Foriel, ^Benedicte Menez, "Alexandre Simionovici and ^Sylvain Bohic ^Institut de Physique du Globe de Paris, Laboratoire de Geosciences Marines, 4 place Jussieu, case 89, 75252 Paris cedex 05, France.

Email: pliilippo@ipgpjussieu.fr "European Synchrotron Research Facility, BP 220, 38043 Grenoble, France We have developed an experimental protocol aimed at analyzing single fluid inclusions using the X-ray microfluorescence set up installed on line ID22 of the European Synchrotron Research Facility. (ESRF; Menez et al., 1999; 2001; 2002; Philippot et aL, 1998, 2000; 2001).

Concentration and spatial distribution of major (CI, K, Ca, Mn, Fe) and trace elements (Ti, Cu, Ni, Zn, V, As, Ba, Br, Rb, Sr, Au, Ce, Hf, Nb, Zr, Th, Pb, U) were determmed m mdividual fluid inclusions from from the Dunbar oil reservoir. North Sea, and a vanety of ore deposits worldwide.

These include: Chivar emerald deposit (Columbia), Au-bearing quartz veins from Brusson (Switzerland), Cevennes (France), Getchell (USA) minning districts, and quartz veins of the Streltsov (Russia), Oklo (Gabon) and Calamar (Australia) uranium deposits. High-spatial resolution fluorescence X-ray maps and micro-tomography were collected for several fluid inclusions. The crux of the approach was to investigate the complexit}^ and diversity of natural matenal in order to tackle first order geodynamic features. These include: Tracking Au and U in single fluid inclusions.

These two elements are of major economic and/or environmental interest and a knowledge of the fluid phase responsible for their transport and concentrations is crucial for the mining industry and nuclear waste programs. Uranium concentration and distribution in a single inclusion was determined in fluid inclusions from the Streltsov uranium deposit, Russia. We were also able to image mfra-micrometric gold grain in single fluid inclusions from quartz vein of the Cevennes Audeposit (France).

Predicting oil reservoir compartmentalization.

Recognition of oil reservoir heterogeneities responsible for field compartmentalization is a key issue to reach the production targets and to prevent or reduce expenses involved by unrecognized barriers to fluid flow. During exploitation of Dunbar field, North Sea, a connectivity discontinuity was found, preventing extraction from the lower part of the reservoir. Sr residual salt profiling showed a major break in ^^Sr/^^Sr ratio corresponding to a coal layer that behaved as a first order permeabilit}^ barrier. In order to test if individual fluid inclusion analysis can be used to characterize impermeable barriers in oil reservoirs, primary fluid inclusions lining overgrowth zones of sandstone quartz were investigated using SXRF technique in samples collected on both sides of the barrier. Our results clearly show the occurrence of two different fluid populations on both sides of the barrier, thus illustrating the great potentiality of the technique for characterizing petroleum generation, expulsion and migration dunng hydrothermal activity.

References Menez, B., Philippot, P ., Mosbah, M., Gibert, F., 1999, NIMB, 158, 533-537.

Menez, B., Simionovici, A., Philippot, P.., Bohic, S., Gibert, A., Chukalina, M. 2001. NIMB 181, 749-754 Menez, B., Philippot P., Bonnin-Mosbah, M., Simionovici, A., Gibert, F., 2002. Geochim. Cosmochim. Acta, in press.

Pliilippot P., Menez, B., Chevallier P., Gibert, F., Legrand F., Populus, P. 1998 chem. geol., 144, 121-136.

Philippot, P., Menez, B., Simiono\dci, A., Cuney, M., Chabiron, A., Snigirev A., 2000. Terra Nova, 12, 84 89 Pliilippot, P., Menez, B., Drakopoulos, M., Simionovici, A., Snigirev, A. 2001. Chem. GeoL, 173, 151-158.

297

HIGH RESOLUTION NUCLEAR MICROPROBE IMAGING AND ANALYSIS OF MINERALS AND INCLUSIONS Chris G. Ryan^'l Esme van Achterbergh^ ^ Tin Tin Win^ and Chris Yeats^ ^ CSIRO Exploration and Mining, PO Box 136, North Ryde NSW 2113, Australia.

- GEMOC National Key Centre, Macquarie University, NSW 2109, Australia.

NMP web site: http://wvvw.nnip.csiro.au/ Nuclear microprobe analysis using proton mduced X-ray emission (PIXE) and y-ray emission (PIGE) provide rapid, non-destructive tools for quantitative trace and major element imaging of mineral assemblages and quantitative analysis of individual fluid and melt inclusions in minerals and the nnaging of their contents. The new CSIRO-GEMOC Nuclear Microprobe can focus a 3 MeV proton beam into a 1.3 \xm beam spot for fluid inclusion analysis and intense beams for mineral imaging at 1.8 |Lim resolution. Quantitative elemental images are projected using the recently improved Dynamic Analysis algorithm, and detection limits down to --0.2 ppm have been achieved m minerals and melt inclusions, 40 ppb in diamond, and - 2 0 ppm in - 2 0 |Lim fluid mclusions (Ryan et aL, 2001a). PIXE from fluid mclusions is modeled to provide quantitative analysis of fluid composition without the use of standards (Ryan et al, 2001b).

Examples: (Fig.l) elemental images of coexisting brine and vapor inclusions from the Batu Hijau porphyry copper deposit, Indonesia, showing Cu partitioning into the vapor phase; (Fig. 2) selected unages of a melt inclusion in clinopyroxene from the A154 kimberlite, Slave Craton Canada, showing the average concentration for the melt and below it the detection limits in ppm; (Fig. 3) images of As and trace Au in pyrite from the Emperor epithermal Au-Ag-Te deposit, Fiji. halite b) Fe

a) CI

Fia 1 A

Brine

heri^^te ' S Ca e) Fe Vapor t^ Cu yclaughter

0.15%

1 chalco^ynte 10 urn

d) Ca

bubble

2.5%

:: external

References

Ryan, C.G., Jamieson, D.N., Griffin, W.L., Cripps, G., Szymanski, R., 2001a. The New CSIRO-GEMOC Nuclear Microprobe: First Results, Performance and Recent Applications, Nucl Instr. Meth. B181; 12-19.

Ryan, C.G., Mclnnes, B.M., Williams, P.J., Dong, G., Win, T.T., Yeats, C.Y., 2001b, Imaging Fluid Inclusion Content using the New CSIRO-GEMOC Nuclear Microprobe, Nucl. Instr. Meth. B181: 570577.

298

EPIGENETIC COPPER MINERALISATION IN THE BIMBOWRIE AREA, OLARY DOMAIN, SOUTH AUSTRALIA Andreas Schmidt Mumm, Jolin Penhall Department of Geology and Geophysics, Adelaide University, Adelaide, S. A. 5005 Post-peak metamorphic vein type copper mineralisation has been fomid in a wide variety of geological settings across the Olary Domain of the Cumamona Province. Three mineralised units were investigated aromid the Bimbowrie HS: the former Maijorie and Doughboy Well mines, and a Cu-sulphide showing in the East Doughboy breccia body. Fluid histories were investigated by comparing and contrasting the epigenetic vein deposits situated in ferruginous schist in a gneissic terram, m a sequence of metapelite, and within stratabound, sodic altered breccia.

Ore mineral paragenses in all three deposits contain primary euhedral pyrite and subhedral to anhedral chalcopyrite. The mineralisation in the East Doughboy Breccia contains bomite with fine exsolution lamaellae of chalcopyrite suggesting true paragenetic formation. At all three location, primary^ ferric sulphides have been altered to haematite and goethite, Cu-sulphides have been altered to covellite and digenite. Malachite and azurite are found as products of surficial weathering of the Cu mmerals. Microprobe analyses of the primary ore minerals revealed increased Co content 0.73.5Wt.%) in the pyrites of the Marjorie Mine mineralisation, whereas in the Doughboy mmeralisation Co content was distinctly below 500ppm.

Complex and varied history^ of fluid mobilisation and migration resulted in poly-stage mineral deposition and redeposition in vein systems. Fluid inclusion generations, trapped during mineral growths and in secondar\^ fracture systems can be classified in the general terms of CO2 rich, aqueous with complex electrolyte composition, hypersaline CaCl2 rich, NaCl-CaCb rich and NaCl rich. Most inclusion generations show complex genetic relationships with overlapping compositional and physical properties. However a gradational trend from high to moderate salimty and high temperature CO2 -rich inclusions (Th: 300-345 C) to moderate temperature inclusions (Th: 210255 C and 130-195 C) can be inferred. Variation and complexity of the fluid inclusion forming events in small copper deposits situated in geographically close but lithologically different settings, indicate fluid systems controlled by local lithologies and structural settings rather than regional scale crustal fluid mobihsation.

Sulfur isotope signatures of quartz vein hosted chalcopyrite and pyrite are largely uniform within a narrow range of d^^^S values of -0.3 to +2.67ooCDT, bomites revealed values of -9.5 to -8.87ooCDT. This is compatible with an input of sulfur from a magmatic or mixed magmatic/meteoric source. Sulfur isotope equilibrium geothermometry suggests that sulphides at Marjorie were deposited from ore formmg fluids with temperatures of 296 ± 40''C. This corresponded to the minimum trapping temperatures of complex, saline fluid inclusions of 315 ± 30°C found as both primary and secondar}^ inclusions in quartz.

While the sulphur in the Cu-Fe-sulphide mineralisations in the Bimbowrie area appears to be derived from similar, possibly magmatic sources, the metal composition as well as the dissolved electrolytes m fluid inclusions point to variable sources and distinct fluid-wallrock chemical exchange on a local scale.

299

Pb-Pb STEPWISE LEACHING (PBSL): A NEW GEOCHRONOLOGICAL TOOL FOR HIGH-GRADE POLYMETAMORPHIC TERRANES. Maurizio Tonelli, Jon Woodhead and Janet Hergt School of Earth Sciences, pmd*CRC, The University of Melbourne, Vic. 3010, Australia

In this contribution we present new direct-dating metamorphic ages for garnet porphyroblasts and associated retrograde staurolite from the vicinity of the Southern Cross mine in the Broken Hill inlier. Garnet and staurolite from different structural settings have been dated by a new sequential dissolution method. The procedure is based on stepwise leaching of metamorphic phases by weak to intermediate strength acids. Our results substantiate existing Proterozoic metamorphic ages obtained on minerals of less certain significance.

Staurolite and garnet are important phases in areas of medium to high-grade metamorphism.

Temperatures experienced by metamorphic rocks in such conditions commonly exceed 600 leaving U-Pb, Pb-Pb and Th-Pb as the only widely employed geochronometers. Pb-Pb dating of metamorphic minerals associated with distinctive metamorphic grades ('mineral isochrons') have proven to be a usefiil tool for deciphering the metamorphic history within polymetamorphic terranes. Unfortunately, this method is subject to a number of potential problems: 1. Low parent/daughter ratios, which limit the isotopic spread obtained on ^^^Pb/^^Pb vs 206p^/204p^ diagrams 2. Sensitivity of the (U-Th)-Pb systems to post-intrusive disturbance (i.e. hydrothermal alteration, thermal overprinting) 3. Uncertainties in the closure temperatures of minerals 4. Frequent presence of U-and/or Th-rich inclusions (zircon, monazite) in the host phase, which can greatly influence its radiogenic Pb budget and apparent age.

PbSL is a new silicate digestion technique based on sequential acid treatment, which allows the selective recovery of radiogenic and common Pb components from a mineral, making single-phase Pb-Pb datmg possible. After leaching, Pb-isotope ratios were measured using a Nu plasma multicollector ICP-MS with thallium internal standard normalization. Results of successive leaching steps are plotted m conventional uranogemc (^^^Pb/'^Pb vs "^^Pb/'^Pb) and thorogemc ('^^Pb/'^Pb vs Pb diagrams. PbSL derived '''PbP'Pb trends, together with petrographic observations, reveal that staurolite hosts |i-size inclusions of an opaque phase while the garnets are essentially inclusion-free The pol>TDhase nature of metamorphism in the Broken Hill inlier is confirmed by our garnet Pb-Pb ages: 1599 ± 2.5 Ma; 1594 ± 7 Ma; 1585 ± 19 Ma; 1523 ± 55 Ma. Importantly, two of our ages are identical to the recent zircon dates (1597 ± 3 and 1594 ± 7) of Page et al. (2000) but, unlike zircon, have the advantage of being linked petrographically to distinct periods in the structural/metamorphic histor\\ In contrast, the staurolite data yield apparent ^"^^Pb/^^^Pb isochron ages of 1323 ± 11 Ma and 874 ± 34 Ma i.e. two age components.

The sequential dissolution method (PbSL) is a new tool that allows direct dating of metamorphic phases with relatively unradiogenic Pb ratios; fiirthermore, no correction for common lead is needed. Ongoing research and development seeks to extend the PbSL technique to other mineral phases such tourmaline, kyanite and ore minerals, illustrating the exciting potential of this field.

The special symposium dedicated to PbSL in the coming 2002 12th Goldschmidt conference is a reflection of the growing acceptance of this technique by the scientific communit}^ Reference Page, R.W., Stevens, B.P.J., Gibson, G.M. and Conor, C.H.H., 2000. Geochronology of Willyama Supergroup rocks between Olary and Broken Hill, and comparison to northern Australia. Australian Geological Sun^ey Organisation, Record 2000/10, 72-75

300

FLUID COMPOSITION AND ITS ORIGIN OF THE MT MORGAN CU-AU DEPOSIT, REVEALED BY QUANTITATIVE LA ICP-MS ANALYSES AND ISOTOPIC SIGNATURES Thomas Ulrich^*. S.D. Golding\ B. S. Kamber^ Kliin Zaw^ and A. Taube^ ^Department of Earth Sciences, The University of Queensland *pesent address: Department of Geolog>^ National University of Australia, 0200 Canberra "Radiogenic Isotope Laboratory, The University of Queensland, Brisbane 4072 Australia ^Centre for Ore Deposit Research, Universit>^ of Tasmania, GPO Box 272-79, Hobart 7001 Australia ^47 Wentworth Terrace, Rockhampton, Queensland 4700, Australia

The Mt Morgan massive sulfide Au-Cu deposit is characterized by two contrasting minerahzation styles. The first conforms to sub-seafloor replacement processes and is composed of massive pyrite and stringer veinlets. This metal accumulation most likely formed by channeling fluids along faults and then by more diffuse fluid flow into unconsolidated volcanoclastic sediments in a submarine environment. Only minor chalcopyrite and gold mineralization is related to this style. The second stage of mineralization is clearly fracture controlled and is characterized by quartz and quartzchalcopyrite veins crosscutting the massive pyrite. The areas with the stockwork veins contain high gold and copper grades, with the highest grades associated to minor gold-silver tellurides that commonly occur together with chalcopyrite. The deposit is hosted by Devonian felsic volcanic rocks and surrounded by the Mt Morgan Tonalite, a low-K, I0W-AI2O3 tonalite-trondhjemite-dacite (TTD) complex. Small hydrothermal breccia pipes containing fragments of tonalite/trondhjemite and volcanoclastic rocks indicate hydro-fracturing caused by fluid pressure, most likely induced by magmatic activity.

Lead isotope analyses show a common source for the lead, and possibly other metals, in the sulfides and the intrusions of the Mt Morgan Tonalite complex. Generally, the data lie on a mixing line between two sources (depleted mantle and erosion mix) and the narrow cluster of data points indicates a similar, well-mixed, but composite reservoir for the intrusions and sulfides of Mt Morgan. There is an apparent trend of somewhat higher ^^^Pb/'^'^Pb and ^^^Pb/^^^Pb ratios for the plagioclase separates from the intrusions relative to the sulfides. In contrast, most Pb isotope signatures for unaltered equivalents of the hosting volcanic rocks are different (except for two samples) from the mineralization. Consequently, Pb isotopes give no unequivocal answer as to the source of the metals, because the two samples from the volcanic rocks with similar lead signatures as the sulfides contain the highest Cu concentrations of the volcanic rocks and could therefore be a fertile source of metals.

The fluid inclusion record shows variable salinities from the intrusions and the inclusions associated to the mineralization. The latter inclusions show a conspicuous low average salinity of 40 to 80 % of seawater salinit>^ The homogenization temperatures are confined to an interval of 210'' to 270°C. The composition of the fluid has been determined by Raman spectroscopy and laser ablation ICP-MS. A consistent and uniform array of COi-bearing fluids has been found and quantitative LA-ICP-MS fluid inclusion analyses reveal high As and Sb for the later vein-related Au-Cu mineralization and five to ten times higher Cu concentration than in the fluids associated with the massive pyrite mineralization. Sulfur isotopes can be interpreted either as oxidized magmatic fluid or fractionated Devonian seawater.

The proposed model for Mt Morgan infers that the massive sulfide mineralization been mainly derived from seawater with minor or no magmatic contributions. In contrast, based on the ore textures, composition and salinity values of the fluid we conclude that magmatic activity, namely a magmatic vapor phase, has condensed and mixed with seawater and caused the high grade Cu-Au minerahzation.

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UNDERSTANDING THERMAL STRUCTURES RELEVANT TO GOLD MINERALISATION IN THE WITWATERSRAND BASIN: A NUMERICAL MODELING APPROACH Zhang. Y., Hobbs, B E., Ord, A, Walshe, J.L. and Zhao, C. CSIRO Exploration & Mining, PO Box 1130, Bentley, WA 6102, Australia

Temperature gradients are miportant factors controlling the precipitation of minerals and metals in a mineralizing system. We explore here the thermal structures proposed to be relevant for the Witwatersrand gold mineralizing system. Mineralizing temperatures can often be estimated, based on the observation of mineral assemblages and alteration patterns. In the Witwatersrand basin, the work of Bamicoat et al. (1997) indicates that the maximum temperature for the Central Rand Group IS 370°C at peak Libanon thrust loading period and 290^C at Syn-Klipriviersberg compression period. These temperature data suggest that anomalously high geothermal gradients (>100''C/km) were present in the region during both events. A question then arises as to what factor was responsible for such large gradients. Possible candidates include, for example, high mantle thermal flux, high radioactive heat production, low thermal conductivity in some stratigraphic units, the influence of eruption of a volcanic lava cover and also the advection of heat through a highly permeable fault. This study explores the influences of these factors through scenario testing using a numerical modeling method.

We first adopted a conductive heat transport theory in the models. The basic heat transfer equation IS Fourier's Law, Q, = -K dT/dx,, where Q, is heat flux, K is thermal conductivity, T is temperature and X is distance. The construction of a thermal model requires the specification of geometrical structures based on data fi-om regional structural analyses. The specification of densities, specific heat, thermal conductivities, radioactive heat production at the time of mineralisation and mantle thermal flux is also required for the model. This was based on the results of previous studies and a literature review. The modeling results indicate that it is generally very difficult to have such high geothemi gradients through conductive heat transfer. However, the models show that for the peak Libanon thrust loading event, thick thrust slab loading in combination with a high mantle heat flux and low erosion rates can result in consistent high temperatures at the Central Rand Group levels as reported (Bamicoat et al. 1997) and with a more realistic geotherm. For the Syn-Klipriviersberg compression period when the Central Rand Group was situated at approximately 2.5 km depth level, conductive heat transport models with realistic mantle flux and thermal conductivity values cannot explain the estimated high temperature, 290°C, for the Central Rand Group (Bamicoat et al.

1997). However, a model simulating the influence of extmsion temperature of Ventersdorp lava (on the top of the section) shows that the resultant peak temperature at the Central Rand Group can be greater than 290°C but only for every limited time scales. Our conclusion is that hydrothermal fluid flow at the time of mineralisation influenced the regional geotherm and led to anomalously high temperatures. We discuss this issue at length in this study.

Acknowledgments We would like to thank Nick Fox, Ian Hutchinson, Andy Bamicoat, Abhen Father and Keith Kenyon for their help and constructive discussion on the work. Tliis study was funded by AngloGold.

References Bamicoat, A C., Henderson, L, Knipe, R.J., Yardley, B.W.D., Napier, R.W., Fox, N.P.C., Kenyon, A.K., Muntingh, D.J., Strydom, D, Winkler, K.S., Lawrence, S.R. and Cornford, C. 1997. Hydrothermal gold mineralization in the Witwatersrand basin. Nature, v. 386, pp. 820-824.

302

EXPLORING FOR OREBODIES UNDER DEEP COVER - THE SURFACE GEOCHEMICAL SIGNATURE OF THE WALLABY GOLD DEPOSIT, LAVERTON, WESTERN AUSTRALIA Justin C. 1. Baulch Placer Dome Asia Pacific, Level 1, 34 Colin St, West Perth, WA 6005 The Wallaby gold deposit is located approximately 11 km WSW of the Granny Smith Mine, about 26km SW of Laverton, in the Northeastern Goldfields of Western Australia. The deposit is situated on the northern edge of Lake Carey, an ephemeral salt lake, and is covered by between 30m and 100m of transported lacustrine clays, alluvial channel sands and consolidated aeolian sands.

The Wallaby mineralisation consists of a series of stacked, en echelon, shallowly dipping continuous lodes w^ithin a moderately plunging (SO"" 190°) pervasive magnetite-actinolite alteration "pipe", which is annular in cross-section. A total of 8 E to NE dipping main lodes and 2 sets of NE dipping "linking" lode systems have been identified. Mineralisation has been identified to a vertical depth of over 1000m. The mineralisation is hosted within a monotonous, matrix supported, mafic-dominant polymict conglomerate, intruded by a series of pre- to synmineralisation, highly fractionated monzonitic, to alkali syenitic, to carbonatitic dykes.

At 31 December 2001, the Indicated and Inferred Resource at Wallaby was 67.1 Mt @ 3.30g/t for 7.7 Moz, of which 18.2 Mt @ 3.44g/t for 2.0 Moz was in Reserve status.

A programme of orientation surface geochemistry was carried out in 1998 and 1999, using a suite of conventional and partial leach/digest analytical methods. Orientation sampling was initially conducted at 50m spacings on a 1.8km east-west line and a 0.8 km north-south line, with the lines crossing over the centre of the deposit. The programme was constrained to the north by an exploration lease boundary and to the west and south by Lake Carey.

Three samples were taken at each sample site: a bulk soil sample and a sample sieved to -80#, both submitted for analysis by aqua regia/AAS, and a third sample sieved to -20#, submitted for analysis

by Bulk Leach Extractable Gold (BLEG).

Five partial extraction/digestion analytical methods (Enzyme Leach^^, MMI-B, Genalysis Terraleach PL1/PL3 and AMDEL Deepleach 11) were trialled on the east-west line. Samples were sieved to the various mesh sizes recommended by the proprietors of each method.

Gold peaks were geographically consistent in most methods, with the highest values occurring a few hundred metres outside the eastern and western margins of the known mineralisation. The -80# aqua regia/AAS technique returned the highest response (maximum of 29ppb). In most methods, a pronounced low occurred over the mineralisation, with most samples returning values below or close to the relative lower detection limits of the various techniques. These peaks are interpreted to coincide with the margins of the annular alteration "pipe". Many of the elements enriched in the mineralisation and associated alteration, such as W, Mo, Sb, As and the light REE also exhibited strong, distinct surface signatures. The Genalysis PLl leachant revealed strong Mo, W and As anomalies directly over the mineralisation with distinct lower peaks outside the mineralisation, on one or both sides.

The orientation work showed that subtle, yet significant, surface geochemical anomalies can form over or around deeply buried mineralisation, which can provide a powerful exploration tool in the generally geochemically hostile hypersaline lake and lake margin country of the Yilgam Craton, particularly when used in conjunction with magnetics and other geophysical methods.

303

THE INFLUENCE OF PLAYA LAKE RELATED CONVECTIVE FLOW ON GROUNDWATER ELEMENT DISTRIBUTIONS AROUND GOLD OREBODIES: PLANNING EFFECTIVE HYDROGEOCHEMICAL SURVEYS M.L. Carev^-^ and D.C. McPhail' ^School of Geosciences, Monash University, Melbourne Australia, ^WMC Resources, Peth Western Australia Numerical modelling using the SUTRA code and physical sandbox modelling are used to study density-driven convective groundwater flow adjacent to playa lakes. Convective flow associated with Lake Lefroy, a playa lake in the St Ives gold camp of the Eastern Goldfields of Western Australia, strongly influences the distribution of anomalous groundwaters around gold deposits in the area. Much of Australia's mineral wealth is situated in its semi-arid interior where playa lakes are common and continued exploration success is reliant on identifying strategies effective in these terrains.

Hydrological and geochemical data from 78 groundwater-sampling wells were used to develop a groundwater flow model for the area around Lake Lefroy. Groundwater flows into the playa lake where it evaporates and then due to its increased density sinks until it reaches a permeability barrier beneath the playa lake at which point it flows back beneath the regional groundwaters forming a convective mixing zone up to 4 km wide. The Argo deposit is located within this mixing zone; shallow groundwaters overlying mineralisation have anomalous gold values, but samples that would have been interpreted to be down-flow from Argo in a simple flow model did not show anomalous gold values. In fact these waters were hypersaline brines flowing back from the playa and had not interacted with Argo mineralisation.

The sandbox model is a scaled two-dimensional representation of the playa lake and adjacent groundwater aquifer system and is designed to illustrate how the location of mineralisation relative to the convective mixing zone can influence the distribution of anomalous groundwaters. Dye representing mineralisation is injected into the sandbox model in a series of locations relative to the convective groundwater flow regime, which is established in the tank prior to introduction of the dye. A series of time-lapse images illustrates how the location of the mineralisation within the flow regime can result in very different groundwater anomaly patterns.

From this information sampling strategies targeting groundwaters most likely to have interacted with mineralisation can be designed.

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TERRAIN-SPECIFIC Pb ISOTOPE MODEL CURVES FOR MOUNT ISA AND McARTHUR - THE AGE OF THE MOUNT ISA Zn-Pb AND Cu DEPOSITS ^G. R. Cam ^G. J. Denton, ^M.J. Korsch, ^B. L Gardner, ^J. M. Parr, ^A. S. Andrew, ^D. J. Whitford, ^L. A. I. Wybom and ^S-S. Sun ^CSIRO Exploration & Mining P.O. Box 136, North Ryde NSW 1670 ^CSIRO Petroleum Resources P.O. Box 136, North Ryde NSW 1670 ^AGSO - Geoscience Australia GPO Box 378, Canberra ACT 2601

The CSIRO - AGSO Pb isotope model for the Proterozoic of Northern Australia was based on the observation of a systematic relationship between the U-Pb zircon ages of the host sequences and the Pb isotope ratios of major mineralisation in the regions (Sun et al., 1994). It had an internal "control" to relate the model ages to true geological ages unlike the general models such as Gumming and Richards or Stacey and Kramers. The HYC mineralisation in the McArthur Basin was used as the "control" because there was a precise isotopic signature and a precise age for the host sequence. In addition, detailed stratigraphic and structural analysis of the mineralisation and its host sequence had convinced Hinman (1996) that the mineralising event occurred at the same time as, or soon after, sedimentation. Thus the Pb isotope signature of HYC was set at a model age of 1640 and ages of 1653 Ma and 1575 Ma were obtained for Mount Isa Pb-Zn and Century Zn-Pb respectively. The fundamental assumption in the model of a syn-sedimentary to syn-diagenetic origin for HYC, although a widely accepted concept, was clearly a weakness in the light of alternative epigenetic models (e.g. Perkins and Bell, 1998).

To provide a far more objective control, we have determined the initial Pb isotope ratios of magmatic units for which unambiguous U-Pb zircon ages are available. These new Pb isotope data for granitic rocks indicate that the CSIRO-AGSO Model is accurate to ±10 Ma in the terrains studied, where data plot close to separate growth curves defined for the Eastern and Western Fold Belts. The errors result from our inability to precisely determine initial Pb isotope ratios of the magmatic rocks as well as probably from real isotopic variability in rocks of the same age.

Based on this, we can confine the age of the HYC mineralisation to within 10 Ma of the age of sedimentation. In addition there is no Pb isotope imprint of this event in rocks stratigraphically younger than 1636±6 Ma. The Mount Isa, George Fisher and Hilton deposits have indistinguishable Pb isotope Population 1 data distributions that are very homogeneous, with model age ranges for of only 3 Ma. This population has a model age of 1655±10 Ma, the same as the host metasedimentary rocks (1652±7 Ma) within the analytical precision of the U-Pb dating and the accuracy of the model. The Mount Isa Cu mineralisation has significantly lower ^^^Pb/^^'^Pb ratios (based on high precision double-spike analyses) than the Zn-Pb mineralisation and plots below the Western Fold Belt Grov^h curve. The initial ratio of this mineralisation was very similar to the Eastern Creek Volcanics and distinctively different from the younger sedimentary and felsic volcanic units. We conclude that the Cu mineralisation was sourced from a different fluid to the Zn-Pb mineralisation, and in a different tectonic regime.

References Hinman M (1996) Constraints, timing and processes of stratiform mineralization at the HYC Ag-Pb-Zn deposit, McArthur River. In: Baker, T., Rotherham, J., Richmond, J. et al (Eds). MIC '96: New Developments in Metallogenic Research: The McArthur - Mount Isa-Cloncurry Minerals Province. James Cook University EGRU Contribution 55, 56-59.

Sun, S.-S., Page, R. and Carr, G. 1994. Lead-isotope-based stratigraphic correlations and ages of Proterozoic sediment-hosted Pb-Zn deposits in the Mount Isa Inlier. Australian Geological Survey Organisation.

Research Newsletter 20, 1-2.

Perkins, W. G. and Bell, T. H. 1998. Stratiform replacement lead-zinc deposits: a comparison between Mount Isa, Hilton, and McArthur River. Economic Geology 93, 1190-1212

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KARARIAU DEPOSIT, WESTERN AUSTRALIA: FURTHERING OUR UNDERSTANDING OF SUPERGENE GOLD DISPERSION AND CALCRETE AU David J. Gray J Nikita B. Sergeev' and Claudio G. Porto^ 'CRC LEME CSIRO Exploration and Mining, PO Box 1130, Bentley, WA 6102.

Email: david.gray@csiro.au ^Departamento de Geologia - Instituto de Geociencias - Federal University Rio de Janeiro.

Email: portoc@uol.com.br

The Karari (formerly Carosue Dam) Au deposit is located approximately 110 km north east of Kalgoorlie at 30°10 S, 122°22 E. Karari differs from many other Au deposits in the Yilgarn Craton in that Au mineralization occurs in felsic rocks (volcanoclastic sandstones and trachyte volcanics).

The w^hole area is generally flat and deeply vs^eathered, with a calcareous surface layer (generally 1 - 6 m), within 5 - 20 m colluvium-alluvium, lacustrine clays and quartz sands. The in situ regolith is composed of 0-30 m of mottled clays and clay saprolite, generally ferruginized, commonly truncated to mottled zone and 10-96 m of saprolite designated as strongly, moderately or weakly oxidized. The saprolite over mineralization is strongly depleted in Au (< 30 ppb cf > 300 ppb) to 330 m RL (i.e., about 30 m below surface, approximating the base of the strongly oxidised zone), i.e., the strongly oxidised zone, ferruginous zone and non-calcareous transported cover are all Aupoor. A number of observations indicate that the base of the depletion zone closely matches the saprock-saprolite boundary, rather than a specific elevation. There is little indication of Au dispersion at the base of weathering.

There is a clear Au enrichment at surface, associated with calcrete, apparently translocated several hundred metres south and upslope, relative to the underlying mineralization. The northern part of the mineralization has strongly truncated laterite and a thick transported zone and therefore reduced Au transport to the surface. The central part of the mineralization is the only part of the study area with conditions conducive to a Au anomaly at the surface, with mineralization, moderately intact residuum and less than 10 m of transported cover. In addition, the centre of the mineralized area shows a clear area of regolith that has not been Au-depleted, possibly due to silicification and occlusion of the Au. This is probably the main source of the surface Au. The surface Au distribution is statistically less skewed than at depth, indicating significant redistribution.

However, gold grain studies showing a significant primaiy component to this Au indicate that much of this surface mobility is physical transport of primary Au, presumably towards the south.

The Carosue Dam groundwaters are saline to hypersaline (2.5 - 11.6% TDS), and acid (pH 3.2 6.0), similar to groundwaters in the Kalgoorlie region, though 2 - 3 times less saline.

Concentrations of many elements are similar to the acid and saline Kalgoorlie groundwaters. The Carosue Dam groundwaters show significant K-depletion, probably due to alunite precipitation under acid conditions. Base metals and REE have greater dissolved concentrations than the neutral central and northern groundwaters, whereas anionic chalcophile elements (e.g.. As, Sb) have low concentrations.

The high salinity and acidity of the shallow groundwaters at this site means that the dominant mechanisms for the mobilization of Au, typical of the southern Yilgarn, would be the chloride (AUCI2 ) or iodide (AUI2') complexes, though this would be less important for deeper, more reduced groundwaters. The groundwaters range from moderately to highly oxidising: two of the groundwater samples have Eh values high enough to allow dissolution of > 2 |ng/L Au, with another three groundwaters able to dissolve > 0.2 iig/L Au. As a consequence, Au concentrations are, unlike other minor elements, as high if not higher than other Yilgarn sites. Because of the strong control by salinity and acidity, very few elements give hydrogeochemical signatures useful for exploration. The best (though patchy) correlation with mineralization was for dissolved Au. Acknowledgements.

AMIRA Project 504 Sponsors, Aberfoyle Resources Ltd and Pacmin Mining Corporation are thanked for financial and other support. CRC LEME is supported by the Australian Cooperative Research Centres Program.

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AUSTRALIAN GOLD DISCOVERY COSTS - GETTING BETTER? M.B. Huleatt and A.L Jaques Geoscience Australia, Canberra The Australian gold industry has grown enormously over the past 25 years. Australia is the world's third largest gold producer (after South Africa and the USA) and gold is now Australia's third largest mineral (excluding petroleum) export, worth $4887 million in 2000-2001. Australian gold production in 2001 was around 290 t, down by just under 7% on the peak of 311 t produced in 1997-1998.

The dramatic growth of Australian gold production (from 16t in 1976) has been facilitated by the development in the 1980s of the carbon-based gold extraction processes that made mining and treatment of low grade 1 g/t) gold deposits commercially feasible. This, coupled with improved gold prices, advances in understanding of gold deposits, and new exploration tools has seen substantial grow1:h in gold exploration expenditure which peaked in 1987-88 at $890 million and in 1996-97 at $800 (in 2000-01 dollars).

Exploration has resulted in the substantial growth of Australia's gold resources in this period and the discovery of at least 25 world-class (>100t contained gold) deposits including Boddington (>750 t including Wandoo), Cadia Hill (179 t), Cadia Ridgeway (118 t), Jundee-Nimary (241 t), Sunrise Dam (205 t), Wallaby (156 t), Kanowna Belle (176 t) and Callie (134 t). A total of 12 337 t Au were added to the resource base and $9.8 billion (in 2000-01 dollars) was spent on gold exploration between 1977 and 2000. Of the 12 337 t Au added to the resource base 66% (8121 t) was from new deposits.

Discovery costs for total resources (including both demonstrated and inferred) have fluctuated from year to year and vary between decades but remained between $20-30 per oz (in 2000-01 dollars).

However, discovery costs vary greatly between new discoveries and addition of resources at existing deposits. Discovery costs in the 1980s ($29.22 per oz) were higher than those of the 1970s (1977-79) ($23.08) and the 1990s ($22.11 per oz) reflecting the much greater contribution of resources from new discoveries in that period. The grovv1;h in the resource base in the 1990s was dominated by addition of resources at deposits discovered in the 1980s. Additions to the resource base in the period since 2000 have to date also been dominated by extensions of resources existing deposits.

Australia has a particularly good record of gold discovery at costs that are very competitive globally but the challenge facing the industry is to continue that record of success. Significant parts of Australia remain under-explored but much of the prospective area is under cover. The challenge for exploration is cost effective discovery of major deposits under cover. The conjunction of high prospectivity in these areas, the record of past success at very attractive costs and the advances in exploration technologies suggest that continued exploration success is likely.

Early indications that continued success will be forthcoming are seen in the 2001 discovery of copper-gold-uranium mineralisation at Prominent Hill in the Gawler Craton, SA, and high quality drill intersections reported from numerous gold prospects around the country in both proven and greenfields provinces.

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HYDROTHERMAL ALTERATION IN DEEPLY WEATHERED TERRAINS J.L. Keeling^ A.J. Mauger\ M.D. Raven^ and W.P. Gates^ ^CRC LEME, c/- Mineral Resources Group, PIRSA, PO Box 1671, ADELAIDE, SA 5001. ^CSIRO Land and Water, PMB2 Glen Osmond, SA 5064 Field portable and airborne instruments which record the reflectance spectra of geological samples provide a rapid, cost effective means of capturing large spectral data sets that can be used to recognise and map, at various scales, the alteration mineralogy of hydrothermal systems. The application of spectral techniques in weathered terrains is complicated by overprinting or destruction of the alteration mineralogy by the products of weathering. In a preliminary study of the effect of weathering on the recognition and mapping of hydrothermal alteration minerals, four sites in South Australia were reviewed . These included the Poona Mine in the Moonta copper district, Birdwood gold and Williamstown sillimanite/kyanite deposits in the Mount Lofty Ranges, and Uley Mine near Port Lincoln on southern Eyre Peninsula.

Spectral logging of drill core from the Poona Mine shows a shift to higher wavelengths in white mica Al-OH absorption corresponding to a change to phengitic (Mg, Fe) composition in wallrock immediately adjacent to veins of sulphide ore. The narrow zone of mica alteration is within a broader alteration envelope showing patchy Mg substitution for Fe in metamorphic chlorite and smectitic alteration of chlorite. This alteration suite is not recognised in the spectra of weathered porphyry host which is dominated by kaolin polymorphs, kaolinite and halloysite. Halloysite is broadly distributed as patches and thin coatings on joints and fractures about the vein copper mineralisation and persists to depths of around 80 m, some 40 m below the inferred base of weathering. The distribution is interpreted as resulting from cyclic water table fluctuations with precipitation of halloysite from Al-rich, acidic solutions produced by oxidation of sulphide in the zone of weathering.

Extensive kaolinisation, >50 m depth, at the Birdwood gold deposit is also attributed to weathering involving oxidation of sulphide in pyritic, biotite schist. The widespread kaolinisation possibly masks earlier hydrothermal alteration associated with gabbro intrusives. Sericite and kaolinite alteration are inferred to accompany gold deposition in anticlinal hinge zones, later the sites of most intense weathering. Several generations of kaolin are indicated from electron microscope studies. Deconvolution of detailed x-ray diffraction traces of kaolinite taken from anticlinal hinges (Frost et al, 2002) suggest the presence of remnant dickite, a high-temperature kaolin polymorph.

At the Williamstown deposits, 10 km north of Birdwood, massive pods of metasomatic sillimanite and kyanite cut by muscovite shears are extensively altered to kaolin by hydrothermal fluids. The well-crystalline kaolinite, which often pseudomorphs replaced sillimanite, contrasts with kaolinite and halloysite alteration from later weathering.

Vein networks of nontronite, an iron-rich smectite, in amphibolite and graphitic gneiss at the Uley Mine near Port Lincoln are a product of dissolution and mobilisation of iron in silicates by hydrothermal fluids which post-date peak metamorphism (Keeling et al, 2000). Two nontronites are observed with distinctive colour and spectra. Nontronite veins are destroyed in the upper 1015m of the weathered profile which is dominated by kaolinite, iron oxides and groundwater calcite.

References Frost, R.L., Van der Gaast S.J., Zbik, M., Kloprogge, J.T. and Paroz, G.N. (2002). Birdwood kaolinite: a highly ordered kaolinite that is difficult to intercalate- an XRD, SEM and Raman spectroscopic study. Applied Clay Science, 20, 177-187.

Keeling, J.L., Raven, M.D. and Gates, W.P., 2000. Geology and characterisation of two hydrothermal nontronites from weathered metamorphic rocks at the Uley Graphite Mine, South Australia. Clays and Clay Minerals: 537-548.

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IN SITU LATERITE: FACT OR FALLACY? IMPLICATIONS FOR GEOCHEMICAL EXPLORATION Louisa M. Lawrance Adjunct Assoc. Prof, and Lecturer in Geochemistry, Centre for Global Metallogeny, Department of Geology and Geophysics, The University of Western Australia, 35 Stirling Highway, Crawley, W.A. 6009 Introduction Deep lateritic weathered profiles are widespread in equatorial regions. Indurated sub-horizontal laterite peneplains that typify these landscapes are thought to be remnant in situ old land surfaces.

The laterite is an iron-rich, porous argillaceous material in which resistant mineral contents are elevated compared to other regolith horizons. Various forms contain discrete fragments, nodules, and pisoliths, some with multiple concentric ferruginous coatings, in a gritty clay matrix characterised by a network of solution pipes and vermiform textures. The indurated nature of lateritic duricrust led early researchers to conclude that it was formed in situ from intensive weathering and fractionation of residual saprolitic horizons. However, some laterites show features that unequivocally indicate a transported origin. Such variation in their possible genesis has caused confusion, and called into question the effectiveness of surface geochemical exploration programs in lateritic terrain. This presentation shows that laterite can form by the same process, (a) as a residual product of in situ weathering, and (b) as a cemented valley-accumulated lateritic detritus, and assesses the implications for mineral exploration.

Features that indicate a residual product of in situ weathering • Primary rock fabric and textures are preserved within some nodules and pisoliths.

• Structures, such as quartz veins, can be traced through the profile.

• Intact concretionary coatings. These are fragile and can not survive rigorous transport.

• Contacts between saprolite, mottled, and laterite horizons commonly appear gradational.

• Laterite is generally best developed over ferruginous basement rocks.

Features that indicate a ferricreted transported lateritic detritus • Laterite is heterogeneous in composition. Mixed rounded and angular fragments, buckshot gravel, and rotated blocks of ferricrete are common.

• Heterogeneous multi-layered concretionary coatings on fragments and cavity surfaces.

• Laterite that unconformably overlies upper or lower saprolite, or saprock residuum.

• Alluvial and channel-fill sediments, and trough bedding in the laterite.

• Saprolite, mottled clay, and laterite horizons that are not in this sequential order.

• Multi-layered laterite, frequently separated by clay-rich materials.

• Shallow sloping or concave laterite surfaces, and horseshoe-shaped surface distributions.

• Laterite peneplains have a linear distribution that broadly defines regional palaeo-drainage.

Model and exploration significance Ferruginous surface coatings, on nodules and pisoliths, characterise all laterite types and make them appear outwardly similar. These coatings comprise mostly colloidal clay and iron oxides deposited from groundwater during seasonal near-surface drainage. Materials most subject to repeated pervasive overprinting and coating are residual weathered basement outcrops and lateritic detritus exposed in topographic low environments in the landscape.

Thus, laterite with variable compositions and origins may form at the same time, in the same drainage system.

Seasonal drying of the lateritic surface results in silica cementation, and induration to form ferricrete duricrust. With ongoing landscape evolution, this resistant unit may become elevated in the landscape through erosion of the surrounding saprolitic surfaces and relief inversion. Thus, several multi-generational remnant laterite duricrusts, derived from different palaeo-catchments of various ages, may coexist in local proximity to each other. Each one contains components with potential trace element anomalism that may be sourced from anywhere in the palaeo-catchment area, including distal to the laterite or directly underneath. Colloidal iron oxides are strong adsorbers of trace elements, and make laterite an ideal surface sampling medium. But the potential for incorporation of in situ and transported materials in laterites, makes careful mapping and profile interpretation vital prerequisites to geochemical exploration in lateritic terrains.

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CHARACTERISATION OF HYDROTHERMAL ZIRCONS IN PORPHYRY, EPITHERMAL, AND LODE GOLD DEPOSITS. Terrence P. Memagh^ Kenneth C. Lawrie\ Esme van Achterbergh^ and Christopher G. Ryan^ 'Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia ^CSIRO Division of Exploration and Mining, North Ryde, NSW, Australia

Hydrothermal zircons have been previously reported from F- and/or C02-rich systems (Rubin et al., 1993), quartz-tourmaline mesothermal gold veins (Claoue-Long et al., 1990; Kerrich and King, 1993), and at the Olympic Dam Cu-uranium deposit (Oreskes and Einaudi, 1990). Zircon mobility has also been inferred from the volatile-induced transport of immobile-element-enriched, magmatic-hydrothermal fluids at shallow levels in high-K calc-alkaline volcanic systems in acidsulphate hydrothermal alteration of rhyolitic volcanics. However, it has previously been difficult to prove that the zircons were truly hydrothermal in origin. This paper outlines the methods we have used to characterise and chemically fingerprint hydrothermal zircons from porphyry Cu-Au, epithermal and lode gold deposits.

Hydrothermal zircons are characterised by the following multi-techniques: (1) their distinct morphology and their paragenetic relationships to the ore and mineral assemblages within the veins, (2) the higher concentration of zircon within the quartz veins relative to that of the host rocks, (3) the abundance of solid (e.g. pyrite, rutile, arsenopyrite, alunite) and/or fluid inclusions within the zircons, and (4) their trace element zonation as measured by the CSIRO-GEMOC nuclear microprobe. The following table give some examples of elemental enrichment/zonation in hydrothermal zircons examined in our studies: DEPOSIT Enterprise Bottle Creek Gidginbung Nena The Dam

TYPE Lode Gold Deposit Lode Gold Deposit Epithermal Deposit Epithermal Deposit Porphyry Deposit

ELEMENTAL ZONATION Cu, As, Ag, Sn, Sb, Ba Fe, As, Sr, Sn ,Sb, Ba As, Sb, Th, Yb, U, Y, Hf Fe, Cu, Yb, Th, Sn, Sb, Ba Fe, Cu, Sn ,Sb, Ba

Many mineral deposits cannot be precisely dated due to the lack of suitable zircon-bearing host rocks or due to the resetting of other isotopic systems. Mineral deposits are particularly vulnerable to subsequent modification due to their formation in near surface and/or dynamic tectono-magmatic environments. In these instances, U-Pb dating of hydrothermal zircons directly associated with mineralisation may have a particular advantage over other isotopic systems that rely on less stable minerals.

References Claoue-Long, J.C., King, R.W. and Kerrich, R., 1990. Archaean hydrothermal zircon in the Abitibi greenstone belt: constraints on the timing of gold mineralisation. Earth and Planetary Science Letters, 98, 109-128.

Kerrich, R. and King, R., 1993. Hydrothermal zircon and baddeleyite in Val-d'Or Archaean mesothermal gold deposits: characteristics, composition and fluid inclusion properties with implication for timing of primary gold mineralisation. Canadian Journal of Earth Sciences, 30, 2334-2351.

Oreskes, N. and Einaudi, M.T., 1990. Origin of rare earth element-enriched hematite breccias at the Olympic Dam Cu-U-Au-Ag deposit, Roxby Downs, S. Australia. Economic Geology, 85, 1-28.

Rubin, J.N., Henry, C.D. and Price, J.G. 1993 The mobility of zirconium and other "immobile" elements during hydrothermal alteration. Chemical Geology, 110, 29-47.

Acknowledgements TPM and KCL publish with permission of the CEOs of Geoscience Australia and CRC LEME.

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THE BILLEROO NORTH IJOLITE-SYENITE-CARBONATITE BRECCIA COMPLEX, CURNAMONA PROVINCE, SOUTH AUSTRALIA ^Lachlan Rutherford. ^Andrew Burtt, ^Martin Hand, ^Karin Barovich, ^John Foden ^Department of Geology and Geophysics, University of Adelaide, South Australia, 5005 ^Primary Industries and Resources of South Australia, PO Box 2355, South Australia, 5001 The Billeroo North alkaline igneous and breccia complex is situated approximately sixty kilometres northwest of Olary, Cumamona Province, South Australia. Geochemical and field observations indicate that an evolving alkaline melt system (ijolite-syenite-brecciated carbonatite series) was emplaced into a syn-magmatic breccia system.

Relatively silica-undersaturated and Na20-rich ijolite is interpreted to have fractionated from a mantle-derived magma. Further fractionation produced relatively minor amounts of nepheline syenite and more abundant porphyritic syenite that are less silica-undersaturated, REE-enriched, and relatively depleted in Na20 and enriched in K2O. Dykes of syenite and sub-alkaline tholeiitetype melt crosscut the ijolite. Emplacement of the alkaline rocks was facilitated by brecciation, with the magmatic rocks both pre- and post-dating breccia formation. Within the breccia, blocks of Fe-rich carbonatite may represent an early derivative of the parental melt and carbonatite also locally forms the breccia groundmass. The generation of the REE-enriched Billeroo North alkaline igneous complex has important implications about enrichment and melting processes in the mantle (Bailey, 1987). Plausibly the widespread, intensive and pervasive alkaline alteration characteristic of the Willyama Supergroup in the Olary Domain may be related to alkaline magmatism.

The nepheline syenite, syenite, ijolite and breccia are anomalous in zinc and copper, and contain visible primary Cu mineralisation. Therefore the Billeroo North alkaline igneous complex shows potential for being an extreme REE-enriched end-member of the Proterozoic Cu-REE-U-Au mineralisation style, and supports the suggestion that this style of mineralisation may have involved mantle-derived sources (e.g. Johnson and McCulloch, 1995).

A pervasive east-northeast trending tectonic foliation is developed in the ijolite, breccia and parts of the syenites that we correlate with the regional -1600 Ma S3 fabric in the Olary Domain.

Metasedimentary enclaves in the breccia have a bedding parallel foliation (Si?) that is crenulated

by S3. Future geochronological investigations will determine a crystallisation age for the alkaline complex and provide a minimum age constraint on the regional bedding parallel foliation in the Olary Domain.

References Bailey, O.K. 1987. Mantle metasomatism - perspective and prospect. In: Fitton, J.G. & Upton, B.G.J, eds.

Alkaline Igneous Rocks, pp. 1-13. Geological Society Special Publication 30.

Johnson, J.P. & McCulloch, M.T. 1995. Sources of mineralising fluids for the Olympic Dam deposit (South Australia): Sm-Nd isotopic constraints. Chemical Geology 121, 177-199.

311

EXPLORATION THROUGH TRANSPORTED COVER AND DEEPLY WEATHERED REGOLITH, ET GOLD PROSPECT, GAWLER CRATON, S.A. MJ. Sheard^ and M.J. Lintem^ 1. Geological Survey Br., Mineral Resources Group, PIRSA, GPO Box 1671, ADELAIDE SA 5001 2. CRC LEME, c/- CSIRO Exploration & Mining, PO Box 1130, BENTLEY WA 6102 Background and setting The ET gold prospect lies some 730 km north west of Adelaide within the Western Gawler Craton - Christie Domain on the BARTON map sheet (centre: 53J, 6636500N, 339200E). It was discovered using gold-in-calcrete sampling in 1995. A surficial Au-anomalous area of--3 x 1 km has been delineated, within an appreciable cover of orange siliceous dunes, colluvium, alluvium overlying silcreted and weathered crystalline metasediment (Christie Gneiss). Some 200 RAB holes have been drilled in this area to saprock or protolith. Cover units have posed a challenge to integrating surface chemistry with any deeper regolith and rock chemistry. A source to the surface anomaly has not been located by drilling.

Geomorphology and stratigraphy ET prospect consists of undulating desert dune terrain (-30 m relief with a moderate vegetative cover where Late Quaternary dunes overlie a subdued silcrete pediment on basement. Dunes align ~E-W and may attain a thickness of 7-10 m. These overlie colluvial to alluvial red-brown hardpan (<1—3 m, late Cainozoic); underlying both is a poorly outcropping, massive pedogenic silcrete horizon (<1-2 m to m. Tertiary) containing an unconformity. Total transported cover can reach ~11 m. Two lithofacies end members of the underlying weathered Archaean Christie Gneiss are recognised: a felsic form and a basic-mafic form. Both are deeply weathered (>40->75 m) and are easily distinguished visually in the weathered zone (except in the silcrete) by their distinct colours.

PIMA spectra have delineated mineral and weathering variation across the prospect and can detect the unconformity via a kaolinite crystallinity index. Data modelling in 3D has helped in interpreting chemical dispersion patterns and palaeo-landscape development.

Geochemistry A principal objective was to examine the geochemical implications for exploration in transported overburden. The main Au anomaly is associated with a thinly covered topographic high of in situ regolith flanked by lower lying deeper sand. Drilling data suggest that neither the upper regolith nor the calcrete Au anomalies are fully explained by the known extent of mineralisation. Multielement geochemistry has limited value by itself in delineating additional anomalies. Most of the surficial Au is found associated with calcrete in transported regolith, suggesting at least some mobilisation of Au in chemical form through the sand. There were some 'false' anomalies located i.e. anomalies without underlying mineralisation, which imply that Au has been mobilised well away from its source or that the drilling has been inadequate. False backgrounds were also detected, i.e. no anomaly over mineralised ground at two sites implies that surface samples cannot always be entirely relied upon to detect all mineralisation at prospect scale. The use of partial extraction procedures using surficial soils is unlikely to bring additional benefits over more conventional analyses at ET. Conclusions Calcrete is the best sample medium and better than the sandy soil at ET. Calcrete appears to 'see through' limited thicknesses of transported cover. Evidence for lateral down slope movement of Au in transported overburden was detected, implying that calcrete can potentially provide a larger target anomaly for broad-based sampling programs.

Acknowledgments: In-kind support provided by the Gawler Joint Venture (Resolute Ltd and Dominion Mining Ltd) plus funding support via the "Targeted Exploration Initiative of South Australia" were gratefully received. CRC LEME is supported by the Australian Cooperative Research Centres Program.

312

LITHOGEOCHEMICAL EXPLORATION AND HYDROTHERMAL ALTERATION IN INTERMEDIATE AND FELSIC VOLCANO-SEDIMENTARY ROCKS HOSTING THE GOLDEN GROVE MASSIVE SULPHIDE CAMP, YILGARN CRATON, WESTERN AUSTRALIA. CliiYStanlev^ Nigel Radford^ John Martyn^ Stefan Gawlinski\ Dept. of Geology, ^ Acadia University, Wolfv^ille, Nova Scotia, Canada [cliff.stanley@acadiau.ca] ^ Dept of Applied Geology, Curtin University of Technology, c/o CRC LEME, PO Box 1130, Bentley, WA, 6102, Australia Email: nigel.radford@csiro.au U 9 Monteith Street, Turramurra, NSW2074 Australia Email: martyn@hutch.com.au ^ Newmont Golden Grove Operations Pty Ltd, PMB 7, Geraldton, WA 6530, Australia Email: stefan.gawlinski@newmont.com.au

The Golden Grove mining camp in Western Australia, host to the Scuddles and Gossan Hill volcanic-hosted massive sulphide (VHMS) deposits, contains a mineral inventory of 33.89 Mt at 7.5% Zn, 2.0 %, Cu 1.1, g/t Au and 67.8g/t Ag. Recent lithostratigraphic mapping of the camp surface and logging of drillcore has refined geological knowledge of the volcano-sedimentary stratigraphy and structure within the camp. A concurrent and ongoing, major and trace element lithogeochemical exploration program using drillcore from the camp has similarly provided detailed knowledge about the chemostratigraphy of units both stratigraphically above and below VHMS mineralization. However, comparison of lithostratigraphic and chemostratigraphic information has allowed a synergistic synthesis of the stratigraphic relationships within the camp, and has facilitated structural interpretation, volcanic and sedimentary facies reconstructions, recognition of intrusive rocks, and navigation of drillcore to specific VHMS targets during exploration. This has also lead to an understanding of why zones of mineralization within the camp occur where they do.

The lithogeochemical data have been used to gain an understanding of the primary, igneous compositional variations within the volcano-sedimentary units. Ti/Zr ratios have been used to circumvent hydrothermal alteration effects that impede compositional identification, and accurately discriminate between andesite, dacite, rhyodacite and rhyolite igneous rocks. Furthermore, the extent and composition of fractionating of quartz and feldspar within the different units have been determined using Pearce element ratio analysis and illustrate that different igneous units underwent fundamentally different fractionation paths. Finally, the intensities of various forms of hydrothermal alteration (sericite, chlorite, ankerite) have been mapped across the camp in three dimensions.

Compositional variations associated with these three alteration styles allow determination of the specific net water-rock reactions responsible for the hydrothermal alteration. The water-rock reactions allow determination of the volume changes associated with different alteration products, and these explain textural characteristics associated with different styles of alteration (massive and pervasive sericite, versus fracture- or void-controlled chlorite). These reactions have also been used to identify exploration parameters specific to different styles of hydrothermal alteration. The spatial patterns expressed by these lithogeochemical parameters have contributed significantly to exploration for VHMS mineralization in deep drilling programs within the camp. In short, the ongoing lithogeochemical exploration program has significantly contributed to exploration successes at Golden Grove and continues to be used as a valuable exploration tool.

313

PLATINUM-GROUP ELEMENT CONTENTS OF CHROMITE FROM LAYERED ULTRAMAFIC UNITS, ABITIBI BELT, ONTARIO: IMPLICATIONS FOR GEOCHEMICAL BEHAVIOUR AND MINERAL EXPLORATION William E. Stone^ and James H. Crocket^ 'Centre for Global Metallogeny, University of Western Australia, Crawley, Western Australia 6009 ^School of Geography and Geology, McMaster University, Hamilton, Ontario, L8S 4M1 The contents of Os, Ir, Ru, Pt, Pd and Au in chromites separated from peridotite samples of Archaean layered komatiitic, ferropicritic, and tholeiitic units from the Abitibi greenstone belt w^ere determined by radiochemical neutron activation analysis. The w^ork w^as undertaken to investigate the mechanisms of fractionation and concentration of platinum-group elements (PGE) in layered units, and to evaluate the implications of these parameters for PGE exploration. The chromite separates were subject to acid leaching as a final purification step. Evidence of sulphide and platinum-group mineral (PGM) inclusions was sought but none found. PGM phases upsection in sub-economic Fe-Ti oxide-related PGE mineralisation within pyroxenite and gabbro layers were discovered by electron microprobe and scanning electron microscopy examination. However, subsequent analytical work suggests that one of the five chromite separates might contain submicrometric inclusions of platinum-group minerals.

The ZPGE-Au values of the chromites are 50-600 ppb and the general abundance trend is Ru>Ir«Os>Pt>Au>Pd. Osmium-Ir-Ru are higher in chromites than in peridotites and initial liquids, whereas Pd and Au are lower and Pt is variable. Chondrite-normalised PGE profiles for the chromites are characterised by negative slopes from the Os-Ir-Ru region (highest) to the Pd-Au region (lowest) with Pd/Ir ratios ranging from 0.01 to 0.10. Platinum is the most variable of the PGE, as indicated by the large variations in ratios such as Pt/Ru (0.02-2.76) or Pt/Pd (5-400). The overall enrichment of Os-Ir-Ru relative to Pd-Au is similar to the PGE proportions found in chromitites from ophiolites.

Comparison of the chromites to bulk rock peridotites indicate that no more than 40% of the Ir (OsRu) is held in chromite, although in one sample 80% of the Pt is in chromite. The majority of the PGE are apparently in another phase, possibly olivine. The chromites enriched in Pt are regarded as representing a spinel composition transitional from the low Pt chromitites in ophiolites to the high Pt chromitites in layered intrusions. The PGE compositions confirm that Os-Ir-Ru partition into chromite, whereas Pd-Au partition into residual liquid in sulphur-undersaturate melt. The behaviour of Pt is more complex. Enrichment of Pt in chromite is proposed to be related to Pt spinel structure compounds in oxidised ferropicrite and tholeiitic residual melts, and to be indicative of Fe-Ti spinel-related PGE mineralisation in overlying pyroxenite and gabbro layers.

Hence, the Pt signature of chromite in these layered units is considered to be a significant indicator of PGE mineralisation potential.

314

RATIO ANALYSIS OF LITHOGEOCHEMICAL DATA: PROVIDING THE TOOLS TO TRACK CRYPTIC ALTERATION AROUND SEDIMENT HOSTED ORE DEPOSITS Whitbread, M. A. Cooperative Research Centre for Landscape Environments and Mineral Exploration.

University of Canberra, Canberra, ACT 2601, Australia Many economic base metal deposits are the product of metal sulphide precipitation from hydrothermal fluids. The chemical reactions between these mineralising fluids and the rocks hosting the deposits commonly produce "haloes" of distinctive mineralogical and geochemical character, zoned about the deposits. The extent of the alteration depends on how extensive the fluid pathway is and the importance of diffiisive processes, but is generally much larger than the deposits themselves. Exploration efforts commonly use geochemistry to identify these alteration haloes.

Geochemical changes due to alteration can be obscured by closure and pre-existing variations in the host rocks. Closure effects can be removed by calculating a ratio of two or more elements, and this has been used in many 'empirical alteration indices'. Unfortunately, background variations such as mineral assemblage differences between fine and coarse sediments, can severely impair the applicability of such empirical measures. Changes unrelated to mineralisation can cause a sample's index score to be highly anomalous, and the score gives no clue as to what processes have caused the chemical variation.

Pearce Element Ratios (PERs) are molar ratios formulated using a conserved element as the denominator. The PER approach is focussed on predictable major element variations that result from mineral changes occurring during formative or later processes. Scatterplots are constructed such that such that background variation is represented by a linear mineralogical trend. Deviations from the linear trend are considered to be later, potentially ore-related, processes. In this way PERs can be used to filter out unwanted geochemical 'noise' and allow observation of alteration geochemistry associated with the ore-forming event. The intensity of mineral 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 "parent" and "daughter" rocks, as is required in other mass balance techniques. This allows PER diagrams to accommodate large numbers of samples onto individual plots.

General Element Ratios (GERs) are molar ratios, but unlike PERs do not use a conserved element.

GERs rely on covariation of elements to pinpoint mineral controls on chemical variation and can be used in situations where no evidence of conserved elements exists. Because minerals commonly plot as points on GER plots, different minerals can be plotted at different locations on the diagram.

A tie line can be established between altered and background mineral suites and samples can then be quickly assessed for their exploration significance.

PERs and GERs have been applied to the Elura orebody, located 50 km north of Cobar, in centralwest 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 siltstone-sandstone turbidites of the Cobar Basin.

Weathered and fresh rocks located laterally and vertically from the deposit were sampled.

Cryptic alteration is successfully identified in the Elura case study. In fresh rocks it is possible to use PER diagrams to navigate within the alteration zone towards ore. Cryptic alteration can be partially detected in weathered samples, but major element alteration indicators for Elura are often destroyed by the weathering process. Such destruction may not be the case for other deposits.

315

BARITE IN THE MOUNT ISA INLIER AND MCARTHUR BASIN: SULFUR AND STRONTIUM ISOTOPIC COMPOSITION ^David J. Whitford, ^Anita S. Andrew and ^Graham R. Carr, ^CSIRO Petroleum, P.O. Box 136, North Ryde NSW 1670 ^CSIRO Exploration & Mining, P.O. Box 136, North Ryde NSW 1670 Barite is a common accessory mineral associated with mineralized systems including VHMS, SEDEX, vein-style, MSV-type and porphyry deposits. Barite is readily recognized in the field and, because of its association with mineralisation combined with its resistance to weathering, is potentially a useful exploration indicator.

Isotopically, barite is of interest because of its potential to yield information on sulfur source and/or the redox state of hydrothermal fluids (S isotopes). Furthermore, the Sr isotopic composition of barite can yield information about the source of Ba and potentially, associated ore metals. The S and Sr isotopic composition of barite from drill-core, surface and outcrop samples from both the Mount Isa Inlier and the McArthur Basin have been analysed to test use of barite in discriminating hydrothermal fluids and to constrain their origin.

The isotopic composition of both Sr and S in barite is highly variable. ^^Sr/^^Sr ratios range from 0 . 7 0 4 to > 0 . 7 2 . Sulfur isotope values range from 0 to > 5 0 % o (CDT). The S and Sr isotopic compositions of barite represent the isotopic compositions of the fluid at the time of barite precipitation. Potential source components that can be recognized include seawater and bacterially modified seawater as well as mantle-derived and crust-derived components. There is no good correlation between Sr and S isotope measured on barite probably because the principal source of Ba (indicated by Sr) is likely to different from the principal source of S. In the Mount Isa - McArthur region, several mineralizing events recognized from Pb isotope metallogenic studies, have characteristic Sr and S isotope signatures preserved in barite.

Metallogenic events in the region that can be recognized include: Eastern Fold Belt (Isan Event), Lawn Hill Platform (Lady Loretta Event, Isan Event), and McArthur Basin (Nathan Group Event, Johnston's Cu Event, Late Roper Event).

In the Eastern Fold Belt, stratabound, vein and float barite has a distinctive hydrothermal component and is distinctly different to Lawn Hill and McArthur barite types. The overall similarity within the Eastern Fold Belt may reflect the regional metamorphic/granite emplacement processes that dominate the mineralization styles. In the Lawn Hill Platform, a hydrothermal component can be recognized from the Sr isotope signature of barite, but S isotope values do not unambiguously indicate a hydrothermal input. Modelled S/Sr characteristics of hydrothermal fluids in the Lawn Hill Platform and the Eastern Fold Belt are different. Strontium and S in barite from the Eastern Fold Belt are dominated by a hydrothermal source derived from crustal rocks whereas S and Sr in barite from the Lawn Hill Platform are dominated by bacterially modified seawater.

Veins from the Lawn Hill Platform show significant addition of a crustal hydrothermal fluid similar to that postulated for the Eastern Fold Belt.

316

DATA MINING - REDUCING THE RISK Domenic Calandro Minerals and Energy Resources South Australia, PIRSA Exploration expenditures in Australia have dropped by almost half in the last four years.

Competition for risk investment dollars has become intense and the mineral exploration industry is often perceived by investors to be high risk, for little or no return.

Increasing exploration costs impact significantly on investor perception. "World class deposits" are difficult to find, with the main opportunities under cover or at significant depth, requiring increased drilling expenditures. Decisions are based on a combination of mineral endow^ment, cost structure and the business risk.

Intelligent identification and processing of information and data are critical in the early stage of exploration. It guides business decision making and shapes the way exploration programs are designed, particularly in the critical early stages. It is the quality and accessibility of data that contributes to the reduction of risk.

Data are being acquired by both Government and the private sector at an astonishing rate. Data management, accessibility and distribution have become one of the industry's hottest issues. There has been, quite literally, a data "explosion" with vast amounts of data being acquired in a variety of formats and standards.

The pre-competitive datasets acquired by Minerals and Energy Resources South Australia (MER) often in collaboration with other agencies such as AGSO and CSIRO, has dramatically reduced the risk for companies venturing to discover new prospects/deposits. The risk factor then largely becomes a factor of time.

Data Mining is an information extraction activity, the primary goal of which is to discover hidden facts and relationships contained in databases. Using a combination of machine learning, statistical analysis, modelling techniques and database technology, data mining finds patterns and subtle relationships in data.

Access to these valuable databases by industry through conventional methods is becoming dated and inefficient. Reducing the amount of time taken to access, compile and analyse data is vital in risk reduction. MER is utilising the World Wide Web to promote and distribute data and information. The South Australian Resources Information Geoserver (SARIG) provides the mineral exploration industry with the ability to electronically • access and interact with data; • acquire data from the vast pool of geospatial information; and • lodge licence applications.

SARIG now incorporates the well known and well populated bibliographic database (SAMREF).

SAMREF summarises open-file envelopes and other reports, publications and documents and contains over 100,000 records. Searches for information can be undertaken at both a broad and detailed level. MER is aggressively assessing technology that will enable all geoscientific data held within its databases to be accessible through a single interface and enable the transfer of large volume, spatially aware datasets. Within two years all of MER's databases and their contents will be accessible in this manner by anyone, anywhere in the world.

317

INNOVATIVE ELECTRICAL GEOPHYSICAL METHODS FOR EXPLORATION UNDERCOVER ^Hashim Carey. ^Brendan Coleman. ^Michael Sexton, ^Graham Heinson and ^Stewart Greenhalgh ^Department of Geology and Geophysics, Adelaide University, Adelaide SA 5005 E-mail: Hashim.Carey@adelaide.edu.au and Brendan.Coleman@student.adelaide.edu.au ^Newmont Australia Ltd, 27 Greenhill Road, 27 Greenhill Rd, Wayville SA 5034

Electrical geophysical methods have been one of the principal tools for exploration of mineral deposits beneath cover for many decades. Such methods include (a) naturally occurring selfpotential measurements (mineral and electrokinetic potentials), (b) applied galvanic potential measurements (surface and downhole DC resistivity, Mise-a-la-Masse and tomography), and (c) induced polarisation measurements. Developments in multi-channel digital-data collection and computer modelling, inversion and visualisation have enhanced the speed of acquisition, model resolution and interpretation. This paper reports on two innovative developments in electrical geophysics for exploration beneath the regolith.

1. Mise-a-la-Masse measurements, in which a single down-hole current electrode is used to determine conductive mineralisation by mapping the surface distribution of resulting electrical potentials, have been conducted in a VMS deposit in Western Australia. The Mise-a-la-Masse method adds value to near-miss drill holes that do not intersect mineralisation but pass close by, as the injected electric current are deviated towards the conductive ore. A combination of three-dimensional modelling, inversion and image-source reconstruction, with visualisation techniques, provide significant new information at relatively low cost.

2. Two and three-dimensional DC resistivity and induced polarisation measurements have been made above a small low-grade metallic sulphide deposits in South Australia, and a resistive epithermal gold-hosting quartz vein system in Queensland. The combination of methods is particularly effective at defining the location of conductive ore bodies and the structural geology of the more resistive hosts, even beneath tens of metres of conductive cover. The three-dimensional resistivity tensor method, in which pairs of orthogonal current and potential dipoles can be used to define both the magnitude and orientation of the resulting electric current density vector J, was found to be useful at mapping non-continuous conductive regions.

Two and three-dimensional modelling and inversion algorithms are in good agreement, and provide structural constraints at a fraction of the cost of drilling.

318

IMAGING STEEP DIPS USING THE SEISMIC REFLECTION TECHNIQUE IN BASEMENT AREAS L.E.A. Jones, B.J. Drummond and T.J. Barton ANSIR, Geoscience Australia, GPO Box 378, Canberra City, ACT, 2601

Deep seismic reflection images of several basement areas in Australia often lack reflections with dips as steep as those observed in outcrop. Four factors determine the ability of the seismic reflection method to image steeply dipping surfaces, whether: 1) the steeply dipping surface is within the window of the crust being imaged; 2) the seismic source and receiver configurations apply a dip filter to the data; 3) the data processing sequence discriminates against reflections from steeply dipping structures; 4) the structures are large enough and thick enough to be imaged.

The window of crust able to be imaged is determined by the length of the seismic line and the recording time. Shallowly dipping surfaces will be imaged when the recording system is vertically above. Steeply dipping surfaces will be imaged when the recording system is to the side. For the long regional seismic profiles typically recorded in Australia, surfaces with steep dips of 80-90"^ would be imaged in the top 5-10 km; shallowly dipping surfaces should be imaged to depths of 5060 km. Dips as steep as 40-50° should be imaged throughout most of the lower crust.

Seismic source and receiver arrays are employed to enhance near vertically travelling energy and to discriminate against source-generated noise such as surface waves and air waves that travel subhorizontally. The array configuration is a compromise between attenuating noise and passing through reflected energy from the side of the source point, for example from steeply dipping surfaces. In recent surveys, the arrays used passed more than half of the reflected energy from reflectors to the side of the source, and therefore did not limit the dip resolution of the method.

In modem seismic imaging experiments, each point of the subsurface is sampled many times by different source-receiver pairs and the data combined during processing to produce the final seismic section (Common Depth Point, or CDP imaging). This stack of the data requires an accurate estimate of the speed at which seismic energy travels in the subsurface. Estimates of the speed are affected by the dips of the reflectors - the stacking velocity is the true speed divided by the cosine of the dip. This means that reflections from steeply dipping surfaces have to be stacked using speeds much higher than those used for reflections from horizontal surfaces. Fortunately, modem data processing methods such as Dip Moveout and Prestack Migration account for this.

Therefore, there is no technical reason why the seismic imaging technique as applied to date on regional deep seismic profiles in Australia should not have imaged steeply dipping structures.

Examples from the Lachlan Fold Belt and the Northem Yilgam demonstrate that dips up to 60° can be imaged. The reason why reflections from steeply dipping reflections are not apparent in some seismic images must lie elsewhere. There are a number of possibilities.

A contrast in seismic impedance (product of density and seismic wave speed) is necessary between two pieces of rock to produce a reflection. Therefore, in normal circumstances seismic imaging will not image boundaries between two pieces of similar rock, or rock fabric such as foliation. The volume of the impedance contrast must be large enough to be imaged. For near surface rocks in the basement areas studied, the layers have to be more than 40 m thick and planar surfaces more than several hundred metres across to be within the limits of resolution. Therefore the detailed structure observed at outcrop scale will be below the limit of resolution.

The lack of steeply dipping reflections in some seismic images of basement areas is therefore probably a function of the scale of the stmctures, and not of the intrinsic ability of the seismic method to image steeply dipping structures. The implication is that seismic images provide a robust view of the large scale structure of the subsurface.

Published with the permission of the Chief Executive Officer of Geoscience Australia.

319

RELATIONSHIPS BETWEEN GEOPHYSICS AND GEOCHEMISTRY IN THE ISA SUPERBASIN M. Duffett' Centre for Ore Deposit Research, University of Tasmania ^Present address: Northern Territory Geological Survey, P.O. Box 2655, Alice Springs, NT 0871 Email: Mark.Duffett@nt.gov.au

Generalised assumptions of the relationship between rock physical properties and broad lithology are the basis of geophysical interpretation, and as such are well known. However, direct relationships between lithogeochemistry and rock physics are seldom quantified outside the mine or petroleum environment. The purpose of this work was to examine opportunistically, and through review of published databases, the nature and strength of some correlations between rock composition and petrophysics in the context of mineral exploration.

The density of pre-Leichhardt Rift felsic igneous rocks (e.g. Leichhardt Volcanics) is very strongly controlled by their chemical composition, such that their Si02 content may be confidently estimated to within a few per cent from their density, and vice versa. A complementary, strong positive correlation between density and mafic elements (Fe203 tot, MgO, CaO) is also present.

Magnetic susceptibility varies over several orders of magnitude in the pre-Haslingden felsics, but no simple relationship could be discerned between geochemical abundances and magnetic susceptibility in these rocks.

There are few significant correlations between density and elemental concentrations in the Eastern Creek Volcanics. The lack of correlation between density and Si02 is a particularly notable contrast to the pre-Leichhardt Rift felsics. In contrast to the felsic igneous units, there is strong positive correlation between magnetic susceptibility and Fe concentration. The Eastern Creek Volcanics are generally dense and highly magnetic, but geochemically- and petrologicallyindicated alteration spatially associated with base metal mineralisation has resulted in significant reductions in both density and magnetic susceptibility.

Bulk density, sonic velocity and resistivity of the Proterozoic sedimentary units investigated, particularly the lower McNamara Group, are very strongly dependent on the relative proportions of carbonate and siliciclastic material present. Dolostones have high density, velocity and resistivity (strongly correlated with MgO+CaO), while a predominance of siliciclastic material results in low values for these parameters (strong negative correlation with Si02).

There is a definite positive relationship between McNamara Group magnetic susceptibility (very low range) and geochemical indices incorporating Fe and Mn, presumably due to weakly ferromagnetic and paramagnetic Fe- and Mn-bearing minerals. Positive correlation between the 'SEDEX alteration index' (Large and McGoldrick, 1998) and magnetic susceptibility indicates magnetic susceptibility logging and ultra-high resolution magnetic surveys as potential tools for rapid reconnaissance detection of alteration spatially associated with stratiform Zn-Pb-Ag deposits in dolomitic sedimentary basins. Elucidation of the relationships between geochemical and geophysical parameters improve the prospects for mapping lithological and alteration patterns critical to mineralisation rapidly and efficiently.

Reference Large, R.R. and McGoldrick, P.J., 1998. Lithogeochemical hales and geochemical vectors to stratiform ZnPb-Ag deposits, Part 1. Lady Loretta Deposit, Queensland. Journal of Geochemical Exploration 63, 37-56.

320

A GRAVITY INVESTIGATION OF THE HUNTER-MOOKI AND PEEL FAULTS Bin Guo and Mark Lackie Department of Earth and Planetary Sciences, Macquarie University, 2109 Australia The Tamworth Zone thrusts and folds have been mapped and described since the 1930s. They form the western margin of the New England Orogen in New South Wales. The Mooki and Peel Faults bound this north-northwest-trending belt of folded middle to upper Paleozoic shelf and terrestrial sedimentary rocks on the west and east, respectively. The Mooki thrust fault is a boundary of the shelf and terrestrial sediments, where the Tamworth belt is thrust against flat-lying Permian strata, whereas the Peel thrust fault separates shelf sedimentary rocks from Palaeozoic cherts, argillites, and metabasites. Both regional gravity and magnetic data reveal significant anomalies over the Tamworth belt, and two large linear gravity and magnetic anomalies occur along the Mooki and Peel Faults. Several local structural models in the Tamworth Zone have been proposed to constrain the thrust geometries based on interpretations of Magnetic data (Ramsay and Stanley, 1976) and a seismic line BMR91.G01 (e.g. Korsch et al, 1997). However the orientation of the Peel Fault at depth is still uncertain. Ramsay and Stanley (1976) suggested a dip on the Peel Fault of 65° to the east, and Korsch et al. (1997) proposed that the Peel Fault has had a complex movement history, a west-dipping structure probably truncating the east-dipping Peel Fault.

Two detailed gravity surveys across the Tamworth belt of 100 km length were conducted, one passing through Quirindi, Wallabadah and Nundle, and the second through Gunnedah and Manilla to north. These data give new evidence that the Tamworth belt has a high value of gravity compared to the bounding rocks, and that a prominent positive gravity anomaly occurs in the Gunnedah Basin. The value of gravity increases by 14-15 mgal over the west boundary of the Tamworth Belt, whereas to east it decreases by 18 mgal in the south and up to 70 mgal in the north toward the margin of the Tamworth Belt (the southern profile is displayed in Figure 1). It is important to note that the gravity begins to drop quickly west of the Peel fault, indicating that the gravity signature is related to rock units within the Tamworth belt rather than the belt as a whole.

At this stage, modelling of gravity anomalies across the southern part of Tamworth belt based on the two surveyed profiles, has not constrained the geometry of the Peel fault, therefore further investigation combining detailed magnetic data is necessary.

ro 4640 ' 4635 4630 ^

4625

O

4620 4615 4610 100

120

Distance (km) Fig 1 The variation of the gravity value across the Tamworth Belt

References Ramsay W. R. H. and Stanley J. M. 1976. Magnetic anomalies over the western margin of the New England foldbelt, northeast New South Wales. Geological Society of America Bulletin 87, 1421-1428.

Korsch, R. J., Johnstone, D. W. and Wake-Dyster K. D. 1997. Crustal architecture of the New England Orogen based on deep seismic reflection profilling. Tectonic and Metallogenesis of the New England Orogen, Geological Society of Australia Special Publication 19, 29-51.

321

GEOCHEMICAL ANALYSIS OF PETROPHYSICAL PROPERTIES OF INTRUSIONS IN THE CALLIDE BASIN FOR GEOPHYSICAL INTERPRETATION Iain Hodge and Wes Nichols Callide Coalfields Pty Ltd, P. O. Box 144, Biloela Queensland 4715 Callide Coalfields produces a sub-bituminous, very sub-hydrous, low rank, steaming coal with good combustion properties, primarily for domestic power generation. Geological input into the planning of coal extraction requires expedient production of detailed structural, lithological, analytical, geophysical and geotechnical data. Within certain mine areas in the Callide Basin, intrusions have displaced significant tonnages of coal. This creates problems in predicting the resource/reserve base for future supply and, where mined, has caused dilution of the product coal. In 1998, a diapir consisting of greater than 75% clay minerals was found in the coal seam at Trap Gully 8A pit ramp.

This material is generically known as "white trap" and the origin of this intrusion was unknown. In all, including a sample of the "white trap", 10 samples from intrusion outcrops were collected from across the Callide Basin. Full ash oxide and petrophysical analysis on samples from intrusions within the Callide Basin has identified the genetic links between these intrusions and allowed for improved identification and location of intrusions across the coalfields. These samples were analysed for their ash oxide constituents, immobile elements and petrophysical characteristics. All of the samples were found to have originated from the same magmatic origin and the fresher basalt samples proved to be similar petrophysically.

The geophysical implications of these results suggest that it is possible to apply those petrophysical properties for the basalt samples to all igneous intrusions in the Callide Basin. This is particularly important when considering the interpretation of both airborne and ground-borne magnetic data.

Location of igneous samples from the Callide Basin for geochemical analyses

/

/' A / / -\

Diagram for differentiation of igneous rocks on a TAS chart Sample No

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Unaltered A

BIIOI

B

BH02

iI3

HTEHOI

®

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^

01-103 ocoz

i

Least Altered

Most Altered

S 6

10

f

O

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Discrimination of basalt fields on a ThHf-Ta ternary plot

^

TAS plot for Callide Basin samples

322

INVESTIGATION OF THE GRAVITY SIGNATURE OF THE YEOVAL BATHOLITH Mark Edmiston and Mark Lackie Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, 2109.

Email: mlackie@laurel.ocs.mq.edu.au The Yeoval Batholith is a large intrusive complex of Early Devonian Rocks situated around the township of Yeoval in central New South Wales. The eastern phase of the batholith, the Yeoval complex, has been mapped (Wyborn et a. I, 1999) into four different rock units, although more than 95% of the surface of the complex is covered by just two of these units. Unit Dyg of the Yeoval Complex is a relatively low density, felsic granite that occupies the northwestern side of the complex while the southeastern area is covered by the more mafic, intermediate-density granodiorites and gabbros of the Naringla Granodiorite. The Nallawa Complex occupies the western side of the batholith. It has been proposed by Wyborn et al (2000, unpublished) that large compositional variations exist within the batholith due to convective fractionation. This proposal is supported by regional gravity data where the batholith produces a gravity high, which suggest larger volumes of dense, mafic material than is not seen on the surface. A detailed gravity survey was conducted over the Yeoval Complex of the batholith to examine the density structure and subsurface shapes associated with the proposed compositional variations. The survey consisted of two detailed profiles, which were used for modelling of the rocks at depth, as well as infilling of the regional gravity data. Samples of intrusive and host rocks were taken to provide information about surface density contrasts that were needed for modelling.

Extensive computational modelling revealed several important facts about the sub-surface of the Yeoval Batholith. The models reveal that densities found on the surface of the felsic phase of the Yeoval Complex cannot extend to depth. A rock with a higher density is required beneath this unit to produce the gravity anomaly measured. The near-surface shape and the density distribution within the felsic phase have been determined, indicating the felsic phase does have a density increase with depth. It is denser around its contact with the host rocks at depth and typically has steep sided contacts near the surface. However, there are ambiguities regarding the thickness of this unit caused by the decreasing resolution of gravity data with depth. A similar density distribution and shape has been modelled for the main unit of the Nallawa Complex. The depth to the base of the Naringla Granodiorite can be estimated with more confidence and modelling of the thinner, mafic phase of the Yeoval Complex has revealed a flat basal contact and a depth estimate of 850 m has been made.

References Wyborn, D., et al, 1999. Yeoval Batholith, pp 211-223. In Meakin, N. S. and Morgan, E. J. (compilers).

Dubbo 1:250 000 Geological Sheet SI/55-4, Edition, Explanatory Notes. Geological Survey of New South Wales, Sydney, xvi + 504 pp.

Wyborn, D., Chappell, B. W. and James, M., 2000. Examples of convective fractionation in high temperature granites from the Lachlan Fold Belt, (unpublished).

323

THE CARPENTARIA STRUCTURE OF HIGH ELECTRICAL CONDUCTIVITY IN WESTERN QUEENSLAND 'F.E.M. Lilley. 'LJ. Wang, 'F. H. Chamalaun and 'IJ. Ferguson 'Australian National University, ^Geoscience Australia, ^Flinders University, ^University of Manitoba Email: Ted.Lilley@anu.edu.au The Carpentaria structure of high electrical conductivity in w^estern Queensland is a major element in the electrical conductivity composition of the Australian continent. Investigation of it is significant for its own sake, and is also important as a case history in the general understanding of continental conductivity structure. Follow^ing its discovery in 1995 as a conductivity anomaly by reconnaissance magnetometers arrays (Chamalaun et al 1999), detailed magnetotelluric observations were carried out in 1997 along a transect crossing the anomaly, between Cloncurry and Julia Creek (Wang 1998). The magnetotelluric results define a good conductor within the crust beneath the sediments of the Eromanga Basin, and give form to the conductivity anomaly as first detected. The conductor extends over a depth range of tens of kilometres. This structure, evidently shown also by aeromagnetic and gravity data, is interpreted as the eastern boundary of the Mt Isa Block at a plate suture, which was later covered by the sediments of the Eromanga Basin. Seismic tomographic results show a major gradient in seismic wave-speed in the region. It appears the potential field, electromagnetic and seismic methods have detected different characteristics of the same geologic structure, with complementary results. The electromagnetic results define horizontal position well, and give evidence of highly-conducting material from the crust to a depth of tens of km. The seismic results extend the depth of the boundary into the upper mantle. The case history supports the hypothesis that the major conductivity anomalies of the geomagnetic deep-sounding method mark continental sutures, of fundamental significance in recording the creation of continents (Lilley et al 2002). As the general area has known mineralised shales of economic importance, the cause of the high electrical conductivity has particular significance, and will be discussed.

References Chamalaun, F.H., Lilley, F.E.M. and Wang, L.J. 1999. Mapping the Carpentaria Conductivity Anomaly in northem Australia. Physics of the Earth and Planetary Interiors 116, 105-115.

Wang, L.J. 1998. Electrical Conductivity Structure of the Australian Continent. PhD thesis, The Australian National University.

Lilley, F.E.M., Wang, L.J., Chamalaun, F.H. and Ferguson, I.J. 2002. The Carpentaria Electrical Conductivity Anomaly, Queensland, as a major structure in the Australian Plate. In: Hillis R. and Muller D. eds. Evolution and Dynamics of the Australian Plate, Special Publication, Geological Society of Australia, in press.

324

MINERAL MAPPING TECHNOLOGY APPLIED IN THE TARCOOLA DISTRICT, SOUTH AUSTRALIA ^Maugen A J and ^McConachy, G.W. ^Primary Industries and Resources SA, ^Euro Exploration Services, Adelaide, SA

Tarcoola, situated on the Trans Australia Railway 600 km northwest of Adelaide in the heart of the Gawler Craton, has been the focus of gold exploration since 1893. Two kilometres northwest of the town site of Tarcoola, the workings of the abandoned underground Tarcoola Gold Mine are situated on a ridge of exposed Fabian Quartzite Member of the Tarcoola Formation. Along the ridge numerous workings ranging from scrapings to shafts and open vertical stopes a few metres wide have provided an anthropogenic overprint to the distribution of surface minerals.

The Tarcoola goldfield contains at least three different styles of gold mineralisation within the most explored 1 km^ of the goldfield. Although historic production has been dominated by exploitation of narrow (<2 m) quartz veins crosscutting quartzites of the Tarcoola Formation, recent exploration has defined resources associated with "pug clay" zones and polymetallic reef systems hosted in both sediments and granite. Limited exploration of nearby brecciated sediment/ironstone located at the basal portion of the Tarcoola Formation has also intersected high-grade gold mineralisation in epithermal style quartz veins.

A typical profile through these mineralised zones commences with approximately 25-30 m of highly crystalline kaolinite together with variable proportions of illite/muscovite overlying a strongly smectitic horizon hosting much of the gold mineralisation. Supergene Au-enrichment is frequently intersected at this redox boundary associated with kaolinite and alunite.

Long held interpretation on the genesis of this goldfield relates the emplacement of quartz reefs to basement fault movement, probably associated with granitic intrusion. Later compression produced strike slip movement along the quartz reefs and reverse faults parallel to the strike of the sediments.

New data suggest that the genesis of this multistage ore-forming hydrothermal system appears to be more spatially and temporally associated with granites and fractionated derivatives of the Hiltaba intrusive complex. Gold resources and significant occurrences occur adjacent to small, oxidised, monzonitic stocks that intrude both the metasediments and Tarcoola granite Recent hyperspectral airborne survey data combined with surface use of a shortwave infrared mineral analysing spectrometer has demonstrated that surface mineralogy can be used to determine new models for mineralisation.

The hyperspectral survey recorded reflected solar radiation in the wavelength range of 450 nm to 2500 nm. After processing for atmospheric effects the data were converted to surface reflectance to enable comparison with library spectra measured under laboratory conditions. Preliminary processing highlighted variations in the image related to the absorption features near 2200 nm which are largely influenced by the presence of AlOH minerals such as kaolin and muscovite.

Other processing examined the 2300-2350 nm features associated with MgOH and FeOH minerals such as chlorite and amphibole. Both images revealed geographically coherent regions, which were later examined with the portable mineral analyser and found to reflect subtle mineralogical changes across defined geological boundaries.

Across one such boundary the variation in the wavelength of the 2200 nm absorption feature associated with illite indicated a change in the proportion of various cations in the lattice structure of the phyllosilicate. Such a change can be used to infer different temperatures of formation which in turn can be linked to recharge-discharge hydrothermal circulation models for ore deposition.

325

POTENTIAL FOR SEDIMENT HOSTED PB-ZN IN THE PARAGON GROUP, BROKEN HILL BLOCK, NSW R. M. Barratt Geological Survey of New South Wales, 32 Sulphide St. Broken Hill 2880

Graphitic meta-sediments of the Paragon Group are exposed in the northern Broken Hill Block (BHB) of far-western NSW. Precise geochronology by Page et al. (2000) ascribes the same age to tuffaceous units near the base of the Bijerkemo Metasediment of the Paragon Group, and in the Urquhart Shale, host of the giant Mt Isa Pb-Zn (Cu) deposits, 1200 km to the north. Proterozoic geological domains in southeastern, central and northern Australia have been implicated in late Paleoproterozoic intra-plate processes (Southgate et al., 2000). The same favourable conditions may have occurred in the Cumamona Province (CP), as occurred in late Paleoproterozoic subbasins in northern Australia. Broken Hill deposit models led to the discovery of Cannington in the Mt Isa Block, can a Mt Isa model lead to discovery in the Broken Hill Block?

Recent chronostratigraphic studies of the Isa Superbasin (ISB; Southgate et al., 2000) and Cumamona Province (Page et al., 2000), permit direct comparison. In both, bimodal volcanism and quartzo-feldspathic sedimentation in extensional, continental (evaporitic in CP) environments is interpreted pre 1695Ma. Deposition of coastal, nearshore marine to shelf sand to silt is recorded in the ISB and psammites, pelites and minor calc-silicates were deposited, also in a probable shallow^ marine shelf environment in the BHB (w^ith a volcanic component), to ca. 1670 Ma. Felsic volcanic activity between 1690 and 1680Ma is the last proximal volcanic input to both basins and precedes a change to the deeper shelf and then anoxic, restricted deposition of the Urquhart Shale and the Paragon Group. These reduced units host multiple, large stratiform / stratabound iron and basemetal sulphide deposits in the ISB. In 1986, CRAE drilled concealed targets beneath the Mundi Mundi plain, w^est of Broken Hill and encountered graphitic, pyritic, meta-sedimentary rocks w^ith minor visible sphalerite and galena.

Re-logging in 1998 led to correlation of the graphitic rocks with the Paragon Group and definition of a 160m, mineralised zone with maxima of 1.38% Zn and 1750ppm Pb in DDH P016 (Leyh, 2000). Coarse-grained sulphide veins and ellipsoids have syn-post metamorphic textures w^hereas finer grained, stratiform disseminated to massive pyrite bands and isolated, concentrically zoned sulphide blebs may be syngenetic or diagenetic. The hosts are novs^ identified as upper Cartwrights Ck Metasediments (including King Gunnia Calc-Silicate) and basal Bijerkemo Metasediments.

Past stream sediment surveys covering exposed Paragon Group units define coherent anomalous zones (Zn>100 ppm, Pb>60 ppm) near Bijerkemo, Mundi Mundi Ck and Cartw^rights Ck. Infill sampling constrains sources to the upper Cartw^rights Ck and/or lower Bijerkemo Metasediments, the anomalous hosts in DDH P016. Pb-Zn targets are, therefore, associated with the same units of the Paragon Group in at least four separate areas. These prospective units and their equivalents are also believed be extensive beneath shallow cover in the mid-northem Cumamona Province. This, the potential for grade and grain-size enhancement of giant deposits by metamorphism and the established mining infrastmcture in the area make them very desirable, under-explored targets in this domain, and worthy of serious consideration.

Published with permission of the Director-General, NSW Department of Mineral Resources.

References Leyh, W. and Conor, C. H. H. 2000. Stratigraphically controlled metallogenic zonation associated with the regional redox boundary of the Willyama Supergroup. MESA Journal 16, 39-47.

Page, R. W., Stevens, B. P. J., Gibson, G. M. and Conor, C. H. H. 2000. Geochronology of Willyama Supergroup rocks and comparison to northern Australia. In: Peljo, M. ed. Broken Hill Exploration Initiative: Abstracts from May 2000 conference, Broken Hill. AGSO Record 2000/10, 72-75.

Southgate, P. N., Bradshaw, B. E., Domagala, J. et al. 2000. Chronostratigraphic basin framework for Paleoproterozoic rocks (1730-1575 Ma) in Northern Australia and implications for base-metal mineralisation. Australian Journal of Earth Sciences 47, 461-483.

326

KAMBALDA REVISITED: RE-EVALUATION OF THE WORLD CLASS KOMATIITE-HOSTED Ni-Cu-(PGE) ORE PROVINCE ^S.W. Beresford ^R.A.F. Cas, V. Lahaye, ^M. Jane and ^W.E. Stone School of Geosciences, Monash University, P.O. Box 28E, Clayton 3800, Victoria ^Centre for Global Metallogeny, University of Western Australia Nickel sulphide mineralisation is located in structural embayments or troughs at the base of the basal komatiite lava flow at Kambalda, Western Australia. Kambalda is the type locality for komatiite-hosted NiS ore deposits. The ore-confining trough structures are present at the basal contact between the Kambalda Komatiite Formation and footwall metabasalt. A spectrum of trough types exists between strongly structurally modified volcanic channels and troughs of clearly structural origin in which no primary volcanic features are preserved and the present geometry is largely a function of the differing phases of post-emplacement deformation. The troughs in no way reflect thermally eroded channels.

Primary contact relations, geochemistry, and textural and vesicle distribution of komatiites is consistent with emplacement of komatiite lava flows under laminar flow conditions. However, the absence of sedimentary units beneath komatiites with coherent flow tops in ore environments seems contradictory and suggests both turbulent and passive laminar emplacement respectively.

The absence of platinum group element (PGE) depletion in the host komatiite crust (and core) and presence of erosive basal contacts, suggests the komatiite lava was initially turbulent and probably open channel fed. It is during this initial stage that deposition of the NiS deposits occurred. As the flow evolved, widened and thickened, laminar flow conditions prevailed and the komatiites are inferred to have flowed through the development of interior magma tubes. The constant lava flowthrough or recharge is consistent with the ore-lava geochemical disequilibrium. Associated with the master flow are lateral breakouts, most of which are barren and define what is called the flanking facies. These breakouts exhibit characteristics typical of inflated laminar flowing, non-erosive passive lava flows.

Primary whole rock and grain scale trace element and isotopic geochemistry (when screened through significant alteration affects) indicates limited (<3%) to no silicate assimilation and is at odds with prevailing ground melting models. However the absence of sedimentaiy units in the ore environment indicates syn-emplacement erosion. We propose that wet unconsolidated sediments were lost to the overlying water column during physical erosion by the early turbulent channelised komatiite pulses. The origin of S is attributed to devolatisation (S and water) of the sedimentary substrate and explains the striking association of sulphide-infilled vesicles and ore in proximal ore shoots. Further isotopic work is in progress to test the hypothesis of the origin of the volatiles (S and/or water).

Revised models for Kambalda ore genesis have implications for komatiite sequences worldwide including the assessment of greenfields terrains such as the Lake Harris komatiites in the Gawler Craton, South Australia.

327

FLUID INCLUSION AND STABLE ISOTOPE EVIDENCE FOR THE ORIGIN OF THE MOONTA COPPER-GOLD DEPOSITS, SOUTH AUSTRALIA R. A. Both^ S. Morales Ruano^ and S. D. Golding^ ^Department of Geology and Geophysics, University of Adelaide, Adelaide, South Australia 5005 ^Departamento de Mineralogia y Petrologia, Universidad de Granada, 18002 Granada, Spain ^Department of Earth Sciences, University of Queensland, St. Lucia, Queensland 4072

The adjacent Moonta and Wallaroo mineral fields in the eastern part of the Gawler Craton, South Australia, were worked continuously from 1860 until 1923. New discoveries at Poona and Wheal Hughes in the Moonta field led to a further period of production from 1986 to 1994. Total production from the two fields was more than 355,000 t of copper and 21 of gold. The Moonta ores are vein deposits occupying fractures and shear zones within the Palaeoproterozoic Moonta Porphyry but show a spatial relationship with granite of the Mesoproterozoic Hiltaba Suite. The paragenetic sequence consisted of four stages separated by episodes of fracturing; the first two stages were dominated by Fe oxides, the third by Fe sulfides and the final stage by a Cu-Fe-CoAu-Zn-Pb-S assemblage.

Three types of fluid inclusions are present in quartz associated with the oxide and sulfide minerals.

Type 1 fluid inclusions are two-phase vapour-rich, with homogenization temperatures up to 473^C and low to moderate salinity (2 to 22 wt.% NaCl equivalent). Type 2 are two-phase liquid-rich with homogenization temperatures up to 269^C and low to moderate salinities (0.7 to 22 wt.% NaCl equivalent). Type 3 are liquid-rich, solid(s)-bearing with homogenization temperatures up to 467^C and high salinities (28 to 55 wt.% NaCl equivalent). The coexistence of types 1 and 3 suggests that these fluid inclusions resulted either from trapping of boiling fluids or represent two immiscible fluids, most likely derived from a magma. Salinity-homogenization temperature relationships indicate mixing of the hypersaline fluid and the vapour-rich fluid with a surfacederived fluid of low temperature and low to moderate salinity.

8DH2O values (-22 to -31%o) and 5I80H2O values (2.9 to 5.1%o) calculated from analyses of chlorite and 8 1 ^ 0 h 2 0 values calculated from analyses of quartz (0.6 to 8.8%o) suggest mixing of magmatic water and surface-derived water. values of ore sulfide minerals have a narrow range (-2.3 to 6.4%o) and their similarity with values of disseminated sulfides from the Moonta Porphyry and interdigitated Doora Schist (-1.5 to 4.6%o) suggests assimilation of crustal sulfur by the magma.

The combined fluid inclusion and stable isotope data support previous proposals, based on field and mineralogical evidence, for a genetic relationship between the Moonta ores and the Mesoproterozoic granitoids.

328

LADOLAM GOLD DEPOSIT, LIHIR ISLAND, PAPUA NEW GUINEA ^Greg Corbett and ^Steve Hunt ^Corbett Geological Services, 29 Carr St, North Sydney, NSW, 2060 Australia Email: cgs@speednet.com.au ^Lihir Management Company PO Box 789 Port Moresby, Papua New Guinea Email: sjh@lihir.con.pg

Exposures in the Minifie open pit and continued drilling at Lienetz and elsewhere have contributed towards new interpretations for the origin of this huge ore system (18 million ounce Au resource).

The removal of about 1 km from the top of the volcanic edifice in an Mt St Helens sideways collapse formed the Luise caldera, and facilitated the rapid transition from porphyry to epithermal style alteration and mineralisation. This sector collapse created regional scale seaward trending spoon shaped or listric faults within the remaining portion of the caldera, and these structures channelled continued heat and fluid flow, with potassic followed by epithermal alteration and mineralisation. Prior to sector collapse (at about 0.336 Ma), monzonite porphyry intrusions were associated with many events of potassic alteration (0.342-0.917 Ma) and complex anhydritebiotite-magnetite-orthoclase-pyrite breccias intersected below about 200m below current surface in drilling. These rocks contain porphyry gold mineralisation grading to several g/t and associated overprinting silica-sericite-pyrite alteration.

Rapid depressurisation during sector collapse promoted pre-mineral phreatomagmatic (diatreme) breccia formation (Corbett et al., 2001). Structural ore controls are apparent as the spoon shaped faults contribute towards the moderate north dipping ore geometry in the Minifie pit, and at Lienetz, an overall flat geometry, commonly floored by the anhydrite breccias, and transacted by steeper feeder structures. Intersections of vertical NS Letomazien and NW dipping Minifie structures also exhibit high grades. Lithological controls are characterised by secondary permeability developed by leaching and intense low temperature K-feldspar flooding, typically of lithologies with primary permeability (agglomerates), early in the mineralisation process, while later ore-related sulphide flooding may display breccia controls.

Generally refractory gold mineralisation is encapsulated within fine-grained arsenical pyrite, pyrite and marcasite deposited by rapid cooling promoted by rock reaction and fluid mixing which displays gold grades to several g/t Au rising in feeder structures. Fluid evolution is evidenced by overprinting quartz (opal-chalcedony)-pyrite-marcasite vein breccias which contain elevated gold grades, and possible improved metallurgical characteristics. Free gold is locally recognised in these and other structures with barite-chlorite-comb quartz gangue. The porphyry gold, K-feldspar-pyrite flooding, rare carbonate-base gold mineralisation and free gold, represent a full spectrum of magmatic arc intrusion-related low sulphidation epithermal gold mineralisation (Corbett, 2002).

Near surficial warm acid sulphate condensate waters reacted with the host rocks to produce late stage advanced argillic (alunite-silica-kaolin-pyrite) grading peripherally to argillic (kaolin-illitechlorite-pyrite) alteration in the upper part of the hydrothermal system and local eruption breccias.

Current geothermal activity within the Luise caldera is progressively cooled prior to mining by deep injection of cool water ahead of mining.

References Corbett, G., Hunt, S., Cook, A., Tamaduk, P., and Leach T., 2001, Geology of the Ladolam Gold Deposit, Lihir Island, from Exposures in the Minifie Open Pit. In: Hancock, G., ed.. Geology, exploration and mining conference, July 2001, Port Moresby, Papua New^ Guinea, Proceedings: Parkville, The Australasian Institute of Mining and Metallurgy, p. 69-77.

Corbett, G.J., 2002, Epithermal gold for Explorationists: Presidents Lecture - Australian Institute of Geoscientists web site: www.aig.asn.au

329

DISCOVERY HISTORY OF THE NORTH MARA GOLD DEPOSITS, TANZANIA Chris Giles Joint founder, East African Gold Mines Limited

Current published resources for the North Mara gold deposits total 49.7 Mt @ 2.6 g/t containing 4.1 million ounces of gold. Production at a rate of 200,000 ounces per annum is scheduled to commence in September 2002.

Initial selection of the North Mara project area followed several months field assessment of the gold potential of the Archaean greenstone belts of the Lake Victoria goldfields region of northwestern Tanzania by Colin Coxon and the author in 1993. Based on the observed association of gold mineralisation with zones of intense silica-pyrite (± sericite) alteration lying along major brittle-ductile shear zones, it was perceived that the North Mara region held good potential for large low grade gold deposits, by analogy with other prolific Archaean gold-bearing terrains. At the time, rich, thin quartz reef systems in the area were the subject of intensive local mining activity.

At the outset the field operations in Tanzania were faced with numerous challenges, mainly resulting from almost non-existent infrastructure and some reservation amongst the local miners concerning the possible threat to their simple gouging operations posed by a foreign mining company. These challenges often required unorthodox creative solutions. For example, the key person in the early days was an experienced Australian driller, who managed to locate, rebuild and operate an aging ex-UN diamond drilling rig. Despite numerous breakdowns, within twelve months of the initial field assessment, this drilling rig had established the shallow dimensions of two large low grade gold deposits. Ensuring representative sampling of the drill-core was a problem that could only be overcome by setting up sample preparation facilities on site, given that there were no assay labs in the country and overseas airfreight was prohibitively expensive. Initial geological work was extremely cost-effective, comprising basic geological mapping, trenching and channel sampling. It was trenching, for example, that led to discovery of the thick mylonite lode system of the Nyabirama deposit, that returned an initial drill intersection of 51m @ 4 g/t gold. Confidence and trust of the local people was only achieved through living and working amongst them and responding positively to their pressing community need for schools and medical assistance.

Ultimately, it was attention to this aspect that carried the project through several crises that could have resulted in a premature end and led eventually to granting of mining licences with the cooperation of the local miners.

The bulk of the gold resource in the Nyabirama gold deposit occurs in quartz cataclasite that occupies mylonitic shears especially towards the footwall contact of the host foliated granodiorite.

Superimposed brittle fracture quartz veining contains abundant free gold, and has been extensively worked by the local miners. At the Nyabigena deposit, gold is hosted in massive silica-pyrite zones formed as the result of near total destruction of silicate minerals in a subvolcanic dacite porphyry by sulphur-rich hydrothermal solutions. In both cases the better gold intersections correlate closely with the most intense silica - pyrite alteration of the host rocks, which in turn is generally found in the central and most intensely deformed portions of the brittle-ductile shear systems.

The key factors leading to the discovery of the economic North Mara gold deposits were : 1. Initial recognition of favourable geological indicators for large disseminated style gold deposits marked by zones of invasive silica-pyrite alteration coincident with major regional structures.

2. A loyal team comprising many local people and a few key multi-skilled expatriates capable of devising innovative solutions to the daily obstacles faced by the exploration work.

3. Development of good community relationships through a policy of local employment and practical assistance with worthwhile social projects.

4. Basic geological data gathering including mapping, trenching and simple drilling methods.

5. Investors prepared to take high risk in backing their judgement of the mineralisation potential.

330

THE RELIANCE ZINC DEPOSIT, FLINDERS RANGES, SOUTH AUSTRALIA. ^lain Groves and ^Cris Carman, ^Perilya Limited ^Colorado School of Mines Reliance is a high-grade willemite deposit hosted within Lower Cambrian carbonate rocks of the Arrowie Basin in the Northern Flinders Ranges of South Australia. Perilya Limited discovered the deposit in October of 2001 while conducting RAB drilling of structural targets between the Beltana and Aroona Zn mines. It is structurally controlled, located within a 15 km long, NW-trending corridor of zinc mineralization and is bounded by the Norwest and Aroona fault systems. The Norwest Fault is a regional crustal structure, and the Aroona Fault is a SW verging thrust fault with uneven throw. Reliance is concealed beneath 5 to 50 m of transported Quaternary alluvium.

The Beltana, Reliance and Aroona ore bodies are among the highest-grade zinc deposits in the world. Their combined pre-mining resource approximates L5Mt @ 33%Zn and 2%Pb. Controversy over the genesis of these deposits focuses on two opposing theories: supergene willemite formed by enrichment of sulphide mineralization and hypogene willemite. While the latter model is preferred, supergene processes also clearly affect the hypogene ore. The hydrothermal Vazante willemite deposit in Brazil (>13 MT @ 22% Zn) is considered analogous.

The Reliance host rocks are stromatolitic and oolitic dolomite and Archaeocyathid limestone of the Woodendinna Dolomite and Wilkawillina Limestone (-530 Ma). Hematitic dolomite alteration and brecciation of the host limestone occurs around the mineralization. The red colouration of the dolomite is caused by fine-grained hematite within and around dolomite crystals. Zinc occurs in the alteration envelope with hematite (up to 5%), in the dolomite lattice and within the matrix of clast-supported breccias. Post-ore extensive manganoan-calcite alteration and weak veining is intimately associated with mineralisation.

The ore consists of willemite and smithsonite, with minor coronadite and smectite clays. The elemental signature of mineralization comprises enhanced Zn-Cd and lesser Pb-As-Mn-Sb. Both lateral and vertical zonation is evident, with Pb and As more abundant in the footwall and at depth.

Unlike Beltana and Aroona, Reliance contains up to 2g/t Ag. All willemite deposits in the area are sulphur poor, containing no sulphides. Cadmium is restricted to the upper 100 m with values peaking in the near surface directly above the ore body due to supergene enrichment.

The heterogeneous willemite texture indicates varied mineralising mechanisms: partial to massive replacement of host rock, open space filling and brecciation. Fluid inclusion studies failed to identify two-phase inclusions within willemite from Beltana: studies are currently under way for Reliance. Results from late-stage calcite veins and vugh fill in the Beltana ore indicate deposition from fluids at 70 to 170°C, regarded as a minimum depositional temperature of the ore fluid. Based on mineral assemblages and textures, the ore fluid was a low-sulphur acidic oxidising solution.

Willemite has weathered variably to smithsonite in the near surface and along the deposit margins to 120m below surface. The most striking feature of Reliance is that ore occurs on either side of a deep circular quartz sand-filled karst over 250 m deep and 60 m in diameter. Sulphide minerals are present throughout the karstic sand as swirls of fine-grained pyrite. Some quartzite pebbles within the sand are also sulphide rich. Significant sulphides were intersected in the Parachilna Formation, which stratigraphically underlies the host units, in the wall of the circular karst.

The footwall of the eastern Reliance ore body is a sand-filled linear cavity with a high proportion of clay. Karsting was post-mineralisation as the ore body has been partially stoped out and redeposited as angular detrital willemite grains in layers within the clayey sand. Whether the karsting is related to Pb-Zn sulphide mineralisation at depth remains to be determined.

331

THE STRATIGRAPHY AND STRUCTURAL FRAMEWORK OF THE MERLIN DIAMOND FIELD, NORTHERN TERRITORY, AUSTRALIA ^A.J.Hell ^G.Rheinberger, ^W.H.R. Ramsay, ^S.Pooley and 'N.W.Archbold ^Deakin University ^Exploration Geologist-Rio Tinto ^Mine Geologist -Merlin Diamonds

The Merlin diamond field is located in the Batten Fault Zone, southern McArthur Basin. Twelve recognised intrusive kimberlite pipes occur throughout the Bukalara Plateau, distributed within a 10 X 5 km area. These kimberlites, predominantly of the diatreme facies, intrude Proterozoic shelf sediments, the youngest being the Bukalara Sandstone, which comprises a relatively flat-lying sequence of sandstone, siltstone, mudstone and minor conglomerate. Cretaceous sediments occur as thin remnant cover intermittently and unconformably overlying the Bukalara Sandstone. Two formations have been recognised throughout the plateau including a Neocomian terrestrialestuarine conglomerate-sandstone sequence unconformably overlain by an Aptian-Cenomanian transgressive marine sandstone sequence. The kimberlite pipes, except for E.Mul and E.Mu2, are covered and 'corked' by a locally unique infill sequence comprising marine Cretaceous sediments and a thickened Tertiary weathering profile.

Three major types of infill sequences have been observed including two sandstone profiles and a sandstone-claystone sequence. A generalised infill profile overlying these kimberlites comprises a basal conglomerate overlain by sandstone, fossiliferous sandstone, and a claystone unit. Smaller diameter kimberlites have a thicker and more complete infill sequence. Slickenside features near the periphery of the pipe, an increase in jointing, and the absence of kimberlitic components within these sediments, suggests the sequence is not part of the crater facies, and was preserved as a result of post-emplacement weathering and subsidence.

The Merlin kimberlites occur within a NNW (340°)-trending structure, locally termed the Merlin Fault, which lies to the east and runs sub-parallel to the regional Emu Fault. Dextral strike-slip displacement of at least 8 m across this fault trend is observed from the Sacramore section of Palomides-Sacramore kimberlite. Vertical displacement of up to 20 m within the upper levels of the Proterozoic sediments occurs near these intrusives. Marginal precursor kimberlite dykes, associated with the main intrusions are controlled by a localised 15® fracture trend. Some kimberlites also intrude along the BOS'" regional Calvert Fault trend. These three fracture trends appear to be the major structural components for kimberlite emplacement throughout the Bukalara Plateau.

332

THE NIFTY COPPER DEPOSIT: CURRENT UNDERSTANDING AND FURTHER POTENTIAL Bruce Hooper Straits Resources, 35 Ventnor Avenue, West Perth WA 6005, Australia Email: bhooper@straits.com.au The Nifty Copper Deposit lies approximately 450 km southeast of Port Hedland, Western Australia in the Neoproterozoic Paterson Orogen. The mine is owned by Straits Resources and is producing 25,000 tpa of high quality copper from a heap leach and SX-EW copper operation. A resource of 148 Mt @ 1.3 % Cu has been defined that includes a chalcopyrite resource of 78 Mt @ 1.8% Cu and a leachable Reserve of 27.6 Mt @ 1.1% Cu. at 0.4% Cu cut-off.

The primary ore is a coarse chalcopyrite with minor pyrite that is oxidized near surface, especially in the northern limb.

Secondary mineralisation grades upwards gradationally to rotted chalcopyrite - chalcocite - cuprite - native copper through to malachite. The malachite also forms a horizontal blanket at the palaeo-water table into surrounding unaltered shales.

Mineralisation is hosted in a sequence of carbonaceous shales and siltstones associated with an intense silica and silica-dolomite alteration system. The alteration is broadly controlled along bedding-parallel horizons and forms a synformal structure with the thickest, highest grade portions in the keel of the synform that grades outwards to a silica-dolomite alteration. The copper deposit has an overlying thin lead-zinc zone associated with strong pyrite and silica alteration in the Pyrite Marker bed. The footwall contains a strong pyrite-silica-dolomite alteration with anomalous zinc and cobalt. The northern limb shows a strong shearing and bedding-parallel fabric on the margins of the deposit in the footwall.

Recent work has defined the fluids to be hot, mildly reduced, saline and moderately acidic that are responsible for the early silicification and associated copper deposition (Anderson, 2001). The high grade intense silica-altered pods form lozenge-shaped bodies within the alteration envelope and grade outwards laterally to silica-dolomite-pyrite zones.

Genetic models proposed are for formation of the deposit during thrusting in the Miles Orogeny, driving hydrothermal fluids from deep in the Yeneena Basin. Mineralisation formed in preferred structural and chemical settings controlled by bedding close to the basin margin. Debate centres on the timing of mineralisation and formation of the synform.

Potential for ore repetitions both along strike and beneath the orebody are considered high. The basin contains potential for similar style orebodies as at Maroochydore (51 Mt @ 1 % Cu) as well as copper-gold systems like Telfer and Magnum and zinc deposits such as Warrabarty.

Reference B. Anderson 2001 (in press). The Geology of the Nifty Copper Deposit, Throssell Group, Western Australia: Implications for Ore Genesis. Economic Geology.

333

THE GEOLOGICAL SETTING AND CHARACTERISTICS OF THE ESFORDI APATITE IRON DEPOSIT, BAFQ DISTRICT, CENTRAL IRANMorteza Jami and Alistair Dunlop School of Geology, University of NSW Email: m.jami@student.iinsw.edu.au The Esfordi apatite-iron deposit is the most phosphorus rich of a series of iron oxide deposits in Bafq district of central Iran. During the consolidation of the Pan African Arabo-Iranian platform, tectonic activity ended with an episode of tensional intracratonic rifting. The volcano sedimentary rocks of the infra Cambrian Rizu series, and Tashk Formation, together with basic lavas are developed on the high-grade metamorphic Boneh-Shorou complex. The Tashk Formation lies between Boneh-Shorou complex and Rizu series and shows a gradually upward decrease in grade of metamorphism from amphibolite facies to slightly metamorphosed rocks. Greenschist-grade metamorphism and granite emplacement extend through the cover sequence with local alkaline metasomatism along narrow tectonic zones.

The main ore body at Esfordi is hosted within calc-alkaline to alkaline volcanic rocks of the Rizu series. Rhyolitic tuffs are prominent with intercalated epiclastics. Locally the associated host rocks are cut by prominent suite of dolerite dykes. The mineralised sequence is unconformably overlain by a series of massive dolomites.

The main iron oxide body has a lenticular shape, 400 m long and up to 60 m thick. These iron oxides are typically massive or brecciated with local banding or veining. The mineralogy is dominated by hematite and magnetite with minor ilmenite, apatite, calcite, actinolite and pyrite. In most cases magnetite is partially replaced by fine hematite. Brecciation is most common along the margins of the body while the core has a more massive structure. Breccia clasts are more massive hematite and magnetite or altered felsic volcanic rock set in a matrix of bladed hematite and apatite.

The phosphate-rich zone is situated above or interfingers with the iron oxide body, with which it has both sharp and gradational contacts. It forms irregular lenses, up to 350 m long and 40 m thick, of apatitic zone with an average phosphorous grade of 22 %. The mineralogy of the phosphate zone is dominated by apatite and calcite, with minor amount of hematite and actinolite. K-feldspar, quartz, sericite, garnet and sphene are accessory minerals in both zones. The margins of the phosphate body are also brecciated. The core of the body is massive white apatite with minor hematite disseminations. The upper brecciated part often has prominent purple colouration.

Microscopic examination indicates this is due to a mosaic of fine hematite between granular apatite. Electron microprobe analysis suggests the apatites are highly enriched in REEs, containing up to 1.3%, though phases such as monazite are also present.

The tuffaceous rhyolitic host rock is subject to variable alteration which extends up to 100 m from top of iron and apatite zones. Alteration produces a green colouration in the rocks, which is associated with the development of actinolite, chlorite, epidote in veins and disseminations. A coarse-grained selvage of actinolite is present along the upper contact of the phosphate zone.

Variable amounts of magnetite and apatite extend into the alteration zone.

The characteristics of Esfordi apatite-iron oxide deposit initiate comparison with the Kiruna deposits, the origin of which is still controversial. The similarities extend to apatite, iron oxide and whole rock REE patterns.

334

THE ROLE OF THE MANTLE IN THE FORMATION OF GIANT ORE DEPOSITS Reid R. Keays, Victorian Institute of Earth and Planetary Sciences, PO Box 28E, Monash University, Victoria 3800 The location of giant ore deposits in selected portions of the earth's crust has long been an enigma.

It is generally recognized that many ore deposits are the products of coincident geological processes and factors. For example, major Ni-Cu-PGE sulfide deposits are the products of a combination of a number of factors including high volume, highly energetic and high MgO magmatism, crustal sutures, sulfide-undersaturated magmas, a crustal source of S, and appropriate structural traps. This paper vs^ill demonstrate that the very large PGE reserves of the Bushveld Complex are the products of two stages of enrichment, w^ith the first stage being deposition of PGE-rich sulfides in the crust at depth below^ the Bushveld magma chamber, and the second stage being the dissolution of these sulfides in a second magma.

The main mineralized horizons in the Bushveld Complex, the UG-2 and the Merensky Reef occur at the bases of cyclic units of rocks. The Merensky Cyclic unit consists of the chromitite-bearing pegmatoidal pyroxenite (the Merensky Reef) at its base overlain by a pyroxenite, follow^ed by norite, followed by anorthosite. All of the PGE drop off systematically from the base of the cycles to its top; in the case of the UG-2 cycle, Pd decreases from 5000 ppb at its base (the UG-2) to 0.8 ppb at the top of the cycle. The upwards decrease in PGE in each of the cycles is consistent with all of the PGE in each cycle having been introduced dissolved in the magmas that formed the cycles. As these magmas cooled, they became sulfide-saturated and the first sulfides to segregate, forming the ore zones, were the most PGE-rich. Sulfides which segregated from the magmas higher up in the cycles had progressively less PGE available to them. Mass balance calculations indicate that the magmas that formed the Merensky Reef at Rustenberg Mine contained ~ 250 ppb Pd and 00 ppb Pt. These Pd and Pt contents are much higher than those of any known magma.

It is suggested that prior to formation of the platiniferous horizons in the Bushveld Complex, mantle-derived magmas interacted with S-bearing rocks in the lower crust where these magmas became sulfide saturated and as a result deposited PGE-rich sulfides. Evidence to support interaction with the crust includes highly radiogenic Os isotope signatures in both the UG-2 and the Merensky Reef. These sulfides were later dissolved by highly S-undersaturated magmas such as boninitic or siliceous high magnesium basaltic magmas which then were emplaced into the Bushveld Complex chamber. Evidence to support the involvement of these magma types is supported by the composition of dykes and sills which occur in the footwall of the Bushveld Complex and are believed to be co-magmatic with the Bushveld magmas. In other Ni-Cu-PGEsulfide deposits, such as Voisey's Bay or Noril'sk, it is probable that this second and more dynamic magma entrained the sulfides and introduced them into their present locations.

Such a model may also be applicable to a wide range of other ore deposit types such as Archaean Au deposits. In these cases, instead of a second magma being the carrier for the ore-forming metals, the metals were transported by a hydrothermal fluid.

335

CLONCURRY DISTRICT FOLD BRECCIAS: FRIEND TO ALTERATION, FOE TO MINERALISATION Lucas J. Marshall James Cook University

Introduction The Cloncurry District of the Mt Isa Inlier has recently been interpreted as a west-vergent fold and thrust belt, largely deformed during the Isan Orogeny ('-1590Ma - 1500Ma). Several aspects of the structural evolution of the district remain poorly understood, in particular, the setting and structural controls on the genesis of extensive breccias in the Mary Kathleen Group. Extensive field mapping w^as undertaken around Cloncurry in order to address this issue, and to relate regional breccias to Fe-oxide-Cu-Au mineralisation.

While fault-related and intrusivehydrothermal breccias exist in the district, the most voluminous Cloncurry District breccias were the result of folding under retrograde conditions.

Isan-aged fold breccias Outcrop patterns in the Cloncurry District are dominated by tight to isoclinal D2 folds, commonly coincident w^ith the peak of metamorphism. In all rock units, D2 folds closely approximate Class 2 (similar) folds (Ramsay, 1967), and reflect formation under high-temperature peak metamorphic conditions. The morphology of retrograde D3 folds is highly variable. Within metasediments and metabasites of the Maronan Supergroup, D3 folds exhibit significant hinge thickening, and approach Class 2 fold shapes. Within quartz- and feldspar-rich calc-silicate rocks of the Mary Kathleen Group, layer thickness is maintained throughout D3 folds, resulting in Class lb (parallel) fold forms. Space problems associated with Class lb folding are locally accommodated through Class 3 folding of adjacent incompetent marble layers, hinge zone dilation, decollement structures and limb faults, all of w^hich are widely recognised space accommodation structures (Ramsay, 1967; p. 416.) Also noted are fracture sets that exhibit systematic relationships to D3 folds in calcsilicate rock layers. These commonly grade into mosaic- to milled-breccias that have resulted in hinge-zone thickening and accommodation of space problems developed during D3 folding.

Discussion Large competency contrasts between marbles and calc-silicate rocks of the Mary Kathleen Group, and the development fold breccias during retrograde folding provide an explanation for the abundance of brecciation in this formation as opposed to rocks of the Maronan Supergroup. This brecciation has allowed metasomatic fluid access, and consequent Na-Ca and K-Fe alteration in large volumes of rock, and a potentially large source for dissolution of ore and gangue components within the Mary Kathleen Group. In contrast, fluid flow and metasomatism were restricted to more discrete conduits in the Maronan Supergroup. The widespread nature of brecciation in the Mary Kathleen Group has minimised the occurrence of discrete structural ore deposition sites in these rocks. From this it is interpreted that marbles and calc-silicate rocks of the Mary Kathleen Group represent less prospective epigenetic ore hosts than other rock sequences where a smaller number of discrete conduits dominate the fluid flow regime. This is supported by the distribution of large Isan-aged ore deposits in the district, which are far more abundant within the Maronan Supergroup.

The Ernest Henry Cu-Au deposit is the only large epigenetic ore deposit found in Mary Kathleen Group rocks of the Cloncurry District, and it is hosted in a thick sequence of the relatively competent Mt Fort Constantine felsic volcanics, rather than in typical brecciated Mary Kathleen Group marbles and calc-silicates.

References Ramsay, John G. 1967. Folding and Fracturing of Rocks. McGraw-Hill, Inc., New York.

336

STRUCTURAL AND GEOCHEMICAL CONTROL OF BARITE VEINS IN THE FLINDERS RANGES, SOUTH AUSTRALIA P. J. (Lai) Mendis, R. A. Both and P. R. James Department of Geology and Geophysics, University of Adelaide, South Australia

Barite veins are widely distributed in the Adelaide Fold Belt (AFB), with the main occurrences in the central Flinders Ranges. The veins have lengths up to 300m and widths generally up 3m. The most important deposit is the Oraparinna Barite Mine, which has been the major producer of industrial grade barite in Australia since 1905.

A study of data from all known barite veins in the Flinders Ranges shows that three types of near vertical veins dominate, with orientations of N26^E, N45°E and N80^E, and near-vertical mutual vein intersection axis. At the Oraparinna Barite Mine in Bunkers Graben the veins have been rotated anticlockwise by 45° around a horizontal NW axis, plunging the vein intersection axis 45°NE. This rotation can be explained by SW verging, layer parallel tectonic transportation of the rock sequence in the graben, similar to that on the eastern limb of the Enorama Anticline.

Dilational fractures that developed in association with diapirs and grabens during the the NE-SW compression of Delamerian Orogeny provided sites for barite vein deposition, thus the veins commonly occur in the vicinity of these structures. As the tension fractures dilated, the veins grew from micro to major scale following the antitaxial mechanism of vein formation. Studies of major and micro veins demonstrate that barite grew as fibrous and elongate grains, continuing across the median line structure, which is defined by fluid inclusions and/or host rock inclusions. Veins also contain border parallel layers of host rock inclusions. The veins grew in essentially isolated dilational fractures, although some veins cross cut each other, as seen in the Oraparinna Mine.

The veins show an increase in Ba and decrease in Sr contents from the median line towards the border. Production + reserves data for barite in the AFB show siltstones are the most favorable host lithology and the Brachina Formation, dominated by siltstones, is the most favorable stratigraphic unit. Whole rock analyses show that the Brachina Formation has the highest background Ba content of all stratigraphic units in the AFB. However, the lowest Ba values for the Brachina Formation are recorded within the Oraparinna Mine. Electron microprobe analyses reveal K-feldspars in the Brachina Formation contain up to 1.2 wl; % Ba in non-mineralised areas, compared with up to 0.6 wt % Ba in samples from the mine^ Microprobe scan maps suggest Ba migrated through the K-feldspar grains towards barite-forming sites.

Analyses of Sr isotopes show that ^^Sr/^^Sr ratios increase from the median layer to border of the veins. Comparison of ^^Sr/^^Sr ratios of veins in several areas with those of their adjacent host rocks demonstrates that the Sr was derived from the host rocks. Calculations suggest that approximately 3.4 Ma would have been required to form a 2m thick vein.

Sulphur isotope data suggest mixing of formation waters at vein-forming sites during folding.

However, both Sr and S isotope data indicate that those veins formed close to diapirs (e.g. within 500m from the Oraparinna Diapir) received ingredients from both the host rocks and the diapir.

337

GOLD MINERAL SYSTEMS IN THE TANAMI REGION Terrence P. Memagh\ Andrew S. Wygralak^ Geoffery Fraser^ and David L. Huston^ ^Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia ^Northern Territory Geological Survey, GPO Box 2901, Darwin, NT 0801, Australia

The Tanami region has become one of Australia's premier Proterozoic gold provinces, having already produced 122 t of gold, and still has high exploration potential. This region contains more than 60 gold occurrences including three established gold fields (Dead Bullock Soak, The Granites and Tanami) as well as several significant gold prospects (Groundrush, Titania, Crusade, Coyote and Kookaburra). The Callie deposit (>5 Moz total resource) located in the Dead Bullock Soak goldfield is currently the largest mine in this region.

Fluid inclusion studies indicate that the ore fluids in the Dead Bullock Soak and The Granites goldfields were low to moderate salinity (4-10 % NaCl eq), moderate to high temperature (260460 °C) and gas rich. In contrast, ore fluids in the Tanami goldfield were low temperature 0 2 0 220 °C) with only minor CO2. O and H isotopic data are consistent with either a metamorphic or magmatic origin for the ore fluids with some mixing with meteoric fluids evident in the Tanami district.

The fluid inclusion data also indicate that the deposits have formed over a range of physicochemical conditions and depths. Groundrush appears to have formed at the greatest depths and has the most reduced (CH4-rich) fluids. The Granites goldfield and the Callie deposit formed at shallower depths. The Granites fluids were C02-rich but also had variable N2 and CH4. The Callie fluids show only small variations in temperature and salinity and are more oxidised with only CO2 and N2 being detected. The Tanami deposits appear to have formed at the shallowest levels and are dominated by low-salinity aqueous fluids, although some C02-bearing fluids have also been detected. Thus it appears that a regional gold mineral system operated in the Tanami with fluid compositions being modified by interaction with the host rocks or by fluid mixing.

Considerable uncertainty surrounds the age of gold mineralisation. The spatial relationship between many of the gold deposits and granitoids has led to proposed genetic links between granite intrusion and mineralisation. These granites have since been dated at between 1825 and 1795 Ma. Preliminary ^^Ar/^^Ar results from sericite from the Carbine pit in the Tanami goldfield yields a total gas age of 1810 Ma, i.e. similar to the age of the granites. However, "^^Ar/^^Ar analysis of biotite in mineralised veins from Dead Bullock Soak indicate an age of 1720-1700 Ma. This suggests that mineralisation at Callie may be related to fluid migration driven by the Late Strangways Orogeny that was responsible for widespread deformation and metamorphism in the Arunta Province to the south-east.

Results of this study indicate that gold deposits of the Tanami region are surprisingly diverse, with some being basalt and dolerite hosted, some in banded iron formations and some being sediment hosted. Moreover, fluid inclusion data shows that the deposits formed over a range of depths and "^^Ar/^^Ar dating may indicate the occurrence of more than one mineralising event. Consideration of this diversity from a mineral systems perspective, highlights the likelihood that undiscovered gold deposits in the Tanami region will have a range of characteristics and may occur in a number of different lithologies.

Acknowledgements This work forms part of the North Australia Project, a collaborative project between Geoscience Australia and the Northern Territory Geological Survey. TPM, GF, and DLH publish with permission from the CEO of Geoscience Australia.

338

CONTROLS ON HIGH-GRADE GOLD DISTRIBUTION IN THE VERA NANCY DEPOSITS, QUEENSLAND Dale Sims Newmont Australia, 100 Hutt St, Adelaide 5000.

The Vera Nancy deposits are located 55 km South East of Charters Towers in North Queensland and are wholly owned and operated by Newmont Mining Corporation. Past production and current Reserves total over 1.5 Moz Au. Mining by underground methods and processing via C.I.P. is producing gold at a rate of 0.23 Moz/year at a total cost of around $162/oz. Four main deposits are in production along the same steeply dipping structure. They occur over a 2.2 km strike length and extend to a maximum depth of 600 m. These low-sulphidation quartz-carbonate epithermal deposits have been dated around 330+10 Ma and are hosted in a sequence of andesitic volcanics on the northern margins of the Late Devonian to Carboniferous Drummond Basin (Evans and Jones, 1997; Richards et al, 1998). Vein width ranges from 0.5-16.0 m in these deposits.

Low-sulphidation quartz vein hosted epithermal mineralisation (White and Hedenquist, 1995; Corbett and Leach, 1998) produces deposits which are closely constrained within and adjacent to the controlling structures hosting them. The structures act as conduits for mineralising fluids which undergo pressure / temperature changes leading to boiling, compositional change and fluid mixing in the critical zone. Consequent mineral deposition results in the structure being infilled with predominantly quartz-carbonate, +/- sulphides, adularia and precious metals. Relatively narrow alteration envelopes form adjacent to and above the structure. Ore shoots in these deposits typically have plunging elongate shapes with anisotropic grade distributions. Epithermal deposits form at 'shallow levels' with precious metals generally occurring over a vertical extent of a few hundred metres.

Using Conolly diagrams to examine vein geometry in conjunction with vein width and metal distribution (Sims, 2000) it can be inferred that high ore grades at Vera Nancy are controlled by both the primary geometry of the mineralised structure and the intersections of the main mineralised veins with subordinate splay veins. The main mineralised structure is thought to have dilated as a consequence of extensional tectonics while the splays acted as conduits bringing oxidising surficial fluids into mixing zones precipitating high gold grades.

References Corbett, G J and Leach, T M. 1998. Southwest pacific rim gold-copper systems: structure, alteration and mineralisation. Society of Economic Geologists, Special publication number 6.

Evans, R C. and Jones, B H. 1997. The Discovery and evaluation of the Vera-Nancy deposit. North Queensland, in World Gold '97 Conference proceedings, pp 233 - 237 (The Australasian Institute of Mining and Metallurgy: Melbourne) Richards, D R, Elliott, G J and Jones B H, 1998. Vera North and Nancy gold deposits, Pajingo, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 685-690 (The Australasian Institute of Mining and Metallurgy: Melbourne) Suns, D A, 2000. Controls on High Grade gold distribution at Vera Nancy Mine, in 4th International Mining Geology Conference proceedings (The Australasian Institute of Mining and Metallurgy: Melbourne) White, N C and Hedenquist, J W, 1995. Epithermal gold deposits: styles, characteristics and exploration.

Society of Economic Geologists, SEG Newsletter, Number 23, pp. 9-13.

339

SECONDARY LEAD MINERALS IN ACID SULFATE SOILS, MOUNT TORRENS PROSPECT: IMPLICATIONS FOR MINERAL EXPLORATION Marian Skwamecki. Robert Fitzpatrick and Mark Raven Co-operative Research Centre for Landscape Environment and Mineral Exploration (LEME)/ CSIRO Land & Water, PMB N"2, Glen Osmond, South Australia, Australia, 5064 Detailed geochemical and mineralogical investigations w^ere conducted on inland acid sulfate soils (ASS) at the Mount Torrens prospect (45 km east of Adelaide) where minor Pb-Zn-Ag mineralisation occurs in calc-silicate rocks at the base of the Talisker Calc-siltstone. The weathering of the sulfides and calc-silicate rocks has produced sulfate-rich groundwaters, which have led to the development of a range of acid sulfate soil materials, such as sulfidic materials (secondary sulfides containing mainly pyrite) and sulfuric horizons (oxidised sulfidic materials containing a variety of oxyhydroxysulfate and oxide minerals). The aims of the study were to investigate the nature of the secondary minerals occurring in ASS and determine the hydrogeochemical processes that produce the ASS. This information is critical in understanding the geochemical dispersion processes in regolith in the Mount Lofty Ranges and their implications for mineral exploration.

An acid sulfate soil profile (MT056) overlying the mineralised zone was described and sampled in an acidic saline seep in the upper Herrmanns catchment. The soil profile comprises five horizons: (i) 0-3 cm, iron oxide crust (ferrihydrite) - A1 horizon; (ii) 3-13 cm, grey sandy loam matrix with few bright yellow mottles and some black sulfidic material (pH <3.5) - A2/sulfiiric horizon; (iii) 13-15.5 cm, grey sandy loam matrix with many bright yellow mottles and some black sulfidic material (pH <3.5) - E/sulfuric horizon; (iv) 15.5 -20 cm, sandy clay loam, Btgl horizon/sulfidic material; (v) 20-25 cm, grey to black light clay, Btg2 horizon. The acid sulfate soil classifies as a Hydraquentic Sulfaquept.

The black sulfidic materials contain secondary framboidal pyrite.

Plumbojarosite (PbFe6(S04)4(OH)i2) and plumbogummite (PbAl3(P04)2(0H)5.H20) occur in the sulfuric horizons, which have developed from the oxidation of the black sulfidic materials. These minerals are absent in similar materials lateral to the mineralised zone. Plumbojarosite and plumbogummite are associated with goethite, jarosite, quartz, clays, mica and detrital monazite.

The sulfuric horizons with secondary Pb minerals are characterised by greater Bi, Cd, Cu, In, Mo, Pb, T1 and Zn concentrations than those lateral to the mineralised zone and reflect the dispersion halo around the mineralised zone. At the prospect scale, a geochemical dispersion halo up to 750 m in width occurs around the mineralisation and is defined by anomalous concentrations of As, Ba, Bi, Cd, Cu, P, Pb, Sn, T1 and Zn in Fe oxide gels and black sulfidic materials.

The model for the formation of these sulfidic materials involves saline groundwaters enriched in sulfate (with Pb, Zn, etc. sourced from the mineralised zone) seeping up through soils, concentrating by evaporation and forming various precipitates. The combination of rising sulfaterich groundwaters, anaerobic conditions associated with saturated soils, and organic carbon in soils yielded pyrite-enriched material through anaerobic bacterial reduction of sulfate. Weak and/or incipient oxidation of the sulfidic materials has produced minerals such as jarosite and plumbojarosite in sulfuric horizons overlying mineralisation.

These sulfidic materials and sulfiiric horizons constitute a potentially new sampling medium for mineral exploration and provide broader dispersion haloes around mineralisation than conventional sample types. In addition, there is significant potential using sulfidic materials and sulfuric horizons in seeps to locate blind mineralisation in areas of cover.

340

MINERALOGY OF THE MT TORRENS GOSSAN, SOUTH AUSTRALIA Marian Skwarnecki and Robert Fitzpatrick Co-operative Research Centre for Landscape Environment and Mineral Exploration (LEME)/ CSIRO Land and Water, PMB Glen Osmond, South Australia, Australia, 5064 Detailed mineralogical investigations w^ere conducted on gossans at the Mount Torrens prospect (45 km east of Adelaide) v^here minor Pb-Zn-Ag mineralisation occurs in calc-silicate rocks at the base of the Talisker Calc-siltstone. The sulfides and calc-silicate rocks have weathered to a variety of sulfates, phosphates, halides, oxides, native elements and layer silicates. This study forms part of a broader project investigating geochemical dispersion in regolith in the Mount Lofty Ranges.

The most abundant sulfates are barite, jarosite group minerals and anglesite. Barite occurs in the following associations: (i) as rims to relict hyalophane grains, associated with boxworks after pyrite; (ii) in goethite, either within the colloform banding, suggesting co-precipitation with the goethite, or in cavities in boxworks after pyrite, locally associated with iodargyrite; (iii) disseminated, in saprolite or gossan, associated with silicates (quartz, kaolinite, goethite-kaolinite pseudomorphs after biotite) and plumbogummite. Plumbojarosite is abundant in surface gossan exposures, but is uncommon at depth. Jarosite is disseminated in gossan and saprolite, as a weathering product of pyrite, and may occur associated with boxworks after pyrite associated with plumbogummite. Less commonly, jarosite pseudomorphs after pyrite occur. Anglesite is rare and occurs along quartz and kaolinite grain boundaries associated with goethite and anatase.

Plumbogummite is the dominant phosphate; gorceixite is rare. Plumbogummite occurs as subhedral grains, aggregates of subhedral grains or grains with colloform structure in the following associations: (i) disseminated, in saprolite, associated with quartz, Fe oxides, kaolinite and barite; (ii) associated with jarosite; (iii) rarely, in composite grains with gorceixite, as rims (?replacement) on gorceixite; (iv) disseminated in Fe oxides; (v) rarely, replacing plumbojarosite; (vi) in boxworks after pyrite.

Carbonates are generally rare or absent in the regolith. Cerussite occurs as relatively coarse laths associated with quartz, microcline, plumbian kaolinite, plumbian Fe oxides (in boxworks after sulfides), and minor plumbogummite and plumbojarosite. Fine-grained relict galena grains are commonly disseminated through the cerussite. Small grains (<1 Dm diameter) of native gold were found: (a) occluded in a quartz grain, associated with Fe oxides and disseminated plumbogummite; and (b) as irregular grains in cavities in Fe oxides. Iodargyrite (Agl) is widely distributed in small amounts. It occurs in cavities in Fe oxides, locally associated with barite or jarosite. Less commonly, iodargyrite forms relatively coarse grains along grain boundaries of Fe oxides and/or kaolinite, associated with disseminated barite, or in goethite-kaolinite veinlets.

Anatase occurs as: (i) complex intergrowths with goethite, typically in weathered metasiltstones, associated with silicates (kaolinite, quartz, weathered biotite) and rarely associated with disseminated plumbogummite and gorceixite; (ii) coatings on quartz grains in cavities, associated with disseminated plumbogummite and opaline silica suggesting that Ti was mobile during weathering; (iii) disseminated subhedral grains, in weathered metasiltstones. Microprobe analyses indicate some Fe is present, together with Zr02 (up 1 %), PbO (up to 2.48 %) and V2O5 (up to 0.73 %). The minor amounts of Zr substituted in the anatase structure suggest some mobility of Zr during weathering.

Goethite and hematite are abundant in gossan and saprolite. These minerals occur as: (i) concentrically zoned or banded boxworks after sulfides (dominantly pyrite), comprising alternating bands of Fe oxide with minor kaolinite, and kaolinite-rich bands with less Fe oxide; (ii) boxworks after sulfides, locally filled in by barite, iodargyrite and mixtures of goethite and kaolinite; (iii) weathering products of mafic minerals (e.g., biotite); (iv) complex intergrowths of goethite with anatase; (v) goethite-kaolinite mixtures, typically in weathered metasiltstones, or filling boxworks in gossan.

341

THE VANADIUM-TELLURIUM-GOLD ASSOCIATION AT THE EPITHERMAL TUVATU GOLD DEPOSIT, FIJI: IMPLICATIONS FOR ORE DEPOSITION Paul G. Spry and Nancy L. Scherbarth Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, U.S.A. 500113212

The intimate spatial relationship between vanadium and gold minerals, particularly in epithermal and mesothermal gold-bearing telluride ores, has long been knov^n. In large mesothermal telluridebearing gold deposits such as the Golden Mile (Western Australia) and the Hemlo deposit (Ontario), at least 27 V-bearing silicates and oxides have been documented (Gatehouse et al., 1983; Pan and Fleet, 1992). In contrast to these tw^o mesothermal gold deposits where a wide variety of vanadium minerals were identified, roscoelite (K(V^^,Al,Mg)2AlSi30io(OH)2) and/or vanadian muscovite are generally the only vanadium-bearing minerals present in epithermal gold telluride deposits. One or both of these minerals have been observed in the Cripple Creek (Colorado), Boulder County District (Colorado), Emperor (Fiji), Spotted Horse, Maginnis, Mayflower, and Gies (Montana), and Porgera (Papua New Guinea) gold telluride deposits. However, an unusual exception is the epithermal Tuvatu gold-telluride deposit, Fiji, where calaverite is intimately associated with (in approximate order of abundance): roscoelite (up to 32.7 wt. % V 2 O 3 ) , karelianite ( V 2 O 3 ) , vanadian muscovite, Ti-free nolanite ((V,Fe,Ti,Al)ioOi4(OH)2, with between 65.2 and 87.3 wt. % V 2 O 3 ) , rutile (with up to 5.2 wt. % V 2 O 3 ) , schreyerite (V2Ti309), and a possible new umiamed mineral, VSi03.

The Tuvatu gold-telluride deposit is one of several low-sulfidation epithermal gold systems localized along the >250 km northeast trending Viti Levu lineament, Fiji, which are genetically associated with alkaline magmatism. The Tuvatu deposit (300,000 oz Au), second in size in Fiji to the Emperor gold-silver telluride deposit (11.5 Moz Au), is generally hosted in sub-vertical, N-S and NNE-SSW trending veins as well as shallow S-dipping veins, and appears to be intimately related to porphyry Cu-style mineralization and to the emplacement of the alkaline 4.85 Ma Navilawa Monzonite and Sabeto Volcanics. Vein structures contain pyrite, marcasite, chalcopyrite, sphalerite, Se-rich galena, tennantite, native gold, bismuth, tellurium, and the following tellurides: calaverite, petzite, hessite, stuetzite, sylvanite, krennerite, coloradoite, tellurobismuthite, bismuthinite, and altaite. The telluride-native gold stage of hydrothermal mineralization overprints the spatially related porphyry Cu mineralization. Fluid inclusions in sphalerite, quartz, and adularia from the telluride-native gold stage show a range of homogenization temperatures from 200"" to 340''C, with a prominent peak between 290"" to 3 0 0 T , and salinities of 0.7 to 15.6 eq. vA.% NaCl. An estimated pressure correction of < lO'^C suggests trapping temperatures at around 300''C. Thermodynamic calculations for the systems V-Al-K-Si-O-H (Cameron, 1998) and Au-Te-Cl-S-OH at estimated conditions of formation of the telluride-native gold stage of mineralization at Tuvatu (e.g., 300"C, SAu - 1 ppb, I T e = 1 ppb, SS = 0.001 m, EV = 0.0001 m, and aK = 0.01), using the isocoulombic method, show that the stability fields of calaverite, roscoelite, and karelianite overlap in pH-y02 space near the hematite-magnetite buffer and at neutral to slightly acid conditions.

These calculations, in addition to textural evidence, suggest that these minerals were deposited together at Tuvatu and likely explain the common coexistence of roscoelite and calaverite in epithermal gold telluride deposits elsewhere. The presence of magnetite with up to 0.7 wt. % V2O3 in the Navilawa Monzonite is consistent with the derivation of V from the alkalic intrusives, which are also considered to be the source of Au and Te in the Tuvatu deposit.

References Cameron, G.H. 1998. The hydrothermal evolution and genesis of the Porgera gold deposit, Papua New Guinea.

Unpublished Ph.D. thesis, Australian National University.

Gatehouse, B.M., Grey, I.E., and Nickel, E.H. 1983. The crystal chemistiy of nolanite (V,Fe,Ti,Al)ioOi4(OH)2, from Kalgoorlie, Western Australia. American Mineralogist 68, 833-839.

Pan, Y. and Fleet, M.E.1992. Mineral chemistry and geochemistry of vanadian silicates in the Hemlo gold deposit, Ontario, Canada. Contributions to Mineralogy and Petrology 109, 511-525.

342

MAGNETITE-BEARING ZONES IN BROKEN HILL METASEDIMENTS SIGNATURES OF BLACK SMOKERS? B.P.J. Stevens Geological Survey of New South Wales, 32 Sulphide St. Broken Hill 2880

The subject of this investigation is tabular magnetic zones in othenvise non-magnetic metasediments of the Broken Hill and Sundown Groups, in the Palaeoproterozoic Broken Hill Block. The zones extend for up to 20 km along strike, and typically several tens of metres across strike. It is suggested here that these zones resulted from oxidation of iron sulphide particles raining down from black smoker plumes. Evidence shows that these ferric iron-bearing zones formed early in the geological history. Some have associations with oxide facies BIFs, typically associated with deposition of massive sulphides, and some occur at a stratigraphic level close to that of the Broken Hill orebody. Others, however, occur higher up in the Sundown Group, suggesting that this previously uninviting unit may have potential for Broken Hill type mineralisation.

Evidence that the ferric iron-bearing zones formed early, are as follows: 1. The tabular zones are clearly folded and faulted in places.

2. Detailed mapping of outcrops in the "Monuments" area, with a susceptibility meter, showed that magnetite concentrations are parallel to bedding, and are not parallel to schistosity, where schistosity diverges from bedding.

3. In an area southeast of Yanco Glen, geological mapping, ground magnetometer surveying and magnetic susceptibility mapping showed that an aeromagnetic anomaly was produced by a unit of massive magnetic pelite, sliced into a series of en echelon blocks by northerly trending D3 shears.

4. In the Sundown area, the pattern of lithological data and sedimentary younging data, permit a complex series of magnetic anomalies to be interpreted as a single interval in mid to upper Sundown Group, repeated by folding and retrograded shearing.

5. In thin section it can be seen that the magnetite is disseminated, unlike detrital magnetite, which occurs as heavy mineral concentrations. Magnetite occurs as inclusions in high grade metamorphic quartz, and interstitially between quartz grains. It is overgrown by coarse, retrograde muscovite, and in places magnetite is flattened in the high grade schistosity. The magnetite grains are typically independent of ferrous iron-bearing minerals: biotite and garnet. Hence magnetite is unlikely to have formed by interaction of normal reduced metasediment, with an oxidising solution introduced into the rock during or after high grade metamorphism.

The evidence shows that the magnetite or its precursor was pre-tectonic, deposited during sedimentation or diagenesis. Diagenetic deposition would most likely be controlled by porosity/permeability. In the Monuments area, some psammite beds show greater magnetic susceptibility than the adjacent pelites, but some massive pelites give the highest magnetic response. In the area SE of Yanco Glen, all of the high magnetic response is generated by massive pelite. Magnetite was not preferentially deposited in the most porous/permeable rocks. Magnetite can be produced biogenically, but the concentrations are typically too low to produce the observed magnetic highs.

The most likely mode of deposition of the magnetite or its precursor, is one where chemically precipitated magnetite rained down as sedimentation occurred. The association of some magnetic metasediments with lenticular BIFs, and with the top of Hores Gneiss, points to black smoker plumes as a likely source of ferric iron precipitation, from the oxidation of iron sulphide particles.

Published with permission of the Director-General, NSW Department of Mineral Resources.

343

TOWARDS A HOLISTIC GEOLOGICAL MODEL FOR KAMBALDA NICKEL SULPHIDE DEPOSITS William E. Stoned Stephen W. Beresford^ and Nicholas J. Archibald^ ^Centre for Global Metallogeny, University of Western Australia, Crawley, Western Australia 6009 ^School of Geoscience, Monash University, Clayton, Victoria 3800 ^Fractal Graphics Pty Ltd, West Perth, Western Australia 6010

Integrated structural and volcanic studies in the Kambalda Dome district are providing m w insights into the geologic controls on the geometry and configuration of komatiite peridotite-associated NiS deposits. Ore genesis models emphasise lava channels in deep physical embayments (troughs) formed by turbulent flow and thermal erosion of substrate, as the basis of exploration models.

However, although originally volcanic in origin, the concepts of deep thermal erosion troughs and turbulent lava channels are questioned in recent structural and volcanic studies. Analysis of a new three-dimensional model of the Kambalda exploration database reveals ore shoot-scale controls consistent with the superimposed effects of post-volcanic deformation. The troughs are generally asymmetric, with most having re-entrant down-dip margins and upright up-dip margins, consistent with an origin by fold-related thrusting. Moreover, some troughs confine volcanic channels, but others transgress and truncate channels. Troughs <5m deep contain serpentine-altered komatiitic peridotite with relict cumulate textures and matrix-disseminated sulphide ore shoots. Deeper troughs (up to 100 m) are up to 3 km long, strongly linear, bounded by discrete tectonic faults, and contain talc-carbonate altered serpentinite with or without significant massive sulphide. Primary contact relationships and textural variations in the host komatiites, particularly coherent or quenchfragmented margins, are consistent with lava emplacement under laminar flow conditions and endogenous grow1;h. However, the absence of sedimentary units underlying komatiites with coherent flow tops in the ore environment implicates turbulent and laminar flow, respectively.

The results of these multi-disciplinary studies confirm the distinction of volcanic channels and trough structures and demonstrate that the ore shoots are associated with channels, but not necessarily with deep troughs. The paucity of sedimentary units in the ore environment suggests the komatiite lava was initially turbulent and open channel fed. During this initial stage, physical erosion of sediment and deposition of the NiS occurred. As the flow evolved, widened and thickened, laminar flow conditions dominated through the development of interior lava tubes beneath surface crusts and growth was endogenous. The asymmetric trough geometries, truncation of channels and talc-carbonate alteration indicate the possibility that the deep troughs are tectonic in origin, formed by D1-D2 thrust-folding and D3 doming and faulting during the tectonometamorphic evolution of Kambalda Dome.

In considering the spectrum of volcanic-depositional and structural-metamorphic controls, a continuum of Kambalda ore shoots is proposed, from a mainly volcanic controlled end-member (3 of 15 shoots; shallow troughs, serpentinised hanging wall) through structurally modified (11 of 15 shoots; deep elongate troughs, talc-carbonated hanging wall) to a structurally controlled endmember (1 shoot; tectonically emplaced in hanging wall). The strong structural controls on ore shoot geometry and lack of deep thermal erosion troughs at Kambalda means that global exploration models should be revised. A complete, unified holistic model would ultimately explain why and what types of terranes and structural highs are most prospective for Kambalda-style NiS mining camps.

344

INTERNAL EVOLUTION AND PARAGENESIS OF TANTALUM MINERALIZATION IN THE WODGINA MAIN LODE PEGMATITE, WODGINA PEGMATITE DISTRICT, PILBARA CRATON, WESTERN AUSTRALIA Marcus T. Sweetapple^ Peter L. F. Collins^ and Gregory R. Lumpkin^ 'Dept. of Applied Geology, Curtin University of Technology, GPO Box U 1987, Perth, W.A. 6845 ^Materials Division, Australian Nuclear Science and Technology Organization, PMB 1, Menai, N.S.W. 2234 The Wodgina main lode pegmatite is an albite-type pegmatite of the LCT (Li-Cs-Ta) petrogenetic pegmatite family. It has the form of a 1 km long, east dipping sheet terminating in a bulbous to saddle-shaped form, hosted in metakomatiite. Albite-type pegmatites are poorly documented world-wide and the Wodgina main lode is the only known albite-type pegmatite identified in Australia.

The Wodgina main lode pegmatite displays an extreme degree of fractionation typical of tantalum mineralised rare metal pegmatites. It lacks the classical concentric zoned structure about a quartz core typically ascribed to such bodies, but has a layered structure best developed where the pegmatite body has a dyke form. This layered structure comprises hanging wall and footwall albite units, dominated by radial aggregates of the 'cleavelandite' variety of albite that show varying degrees of replacement by fine-grained sugary albite. The central portion of the dyke is composed of aplitic to granitic textured banded layers comprising varying proportions of albite, muscovite and quartz.

There is a continuum in paragenesis, geochemistry and mineralogy between the sugary albite and 'cleavelandite', which is suggestive of an initial event of crystal grow1:h rate much greater than nucleation, which is then succeeded by nucleation rate exceeding crystal growth rates, with decreasing temperature. Rapid crystallization of 'cleavelandite' and sugary albite is likely to have taken place under strongly undercooled conditions, probably during the transition from a highly fluxed hydrous magma to a hydrothermal fluid.

The bulk of the primary tantalum mineralization is within the 'cleavelandite' unit as manganotantalite (ideally MnTa206), with minor wodginite (Mn4Sn4Ta8032). Whole rock geochemical data and a mineralogical association with apatite suggest that phosphorus was the dominant volatile component involved in tantalum crystallization in this unit and in the banded aplitic layers, probably in conjunction with fluorine. Textural evidence suggests that tantalum mineralization in the form of manganotantalite crystallized early in the petrogenesis of this unit, with at least some of the manganotantalite forming seed nuclei for 'cleavelandite' crystallization.

Additionally, some manganotantalite is present at the contact between the footwall 'cleavelandite' unit with the central banded aplitic textured unit. It is suggested that this mineralization is due to a boundary layer crystallization front that excluded incompatible elements such as tantalum, rather than resulting from gravitational settling.

A late, minor stage of tantalum crystallization is present as fine microlite (general formula (Na,Ca)2m(Ta>Nb,Ti)206(F,0H,0)i.n./7H20) which crystallized with lithium-bearing micas. This microlite mineralization is associated with elevated lithium, tin and fluorine, and is likely to be the result of a late-stage hydrothermal fluid moving into the central portion of the pegmatite at the same time as the alkali and volatile elements were lost into the host metakomatiite.

K/Cs and K/Rb ratios in muscovite and in whole rock geochemistry indicate that high levels of fractionation did not exert a direct control on tantalum crystallization. Rather they are a primary signature of the magma from which the pegmatite crystallized. Local variations in these ratios are controlled by the ability of Cs and Rb to substitute for other alkali elements in minerals, most notably potassium, and are particularly enriched in the lithium-bearing micas.

345

LATERITIC NICKEL-COBALT MINERALISATION FROM THE WEDA BAY DEPOSIT, HALMAHERA ISLAND, EASTERN INDONESIA. Graham S. Teale Teale and Associates Pty. Ltd., PO Box 740, North Adelaide, SA 5082

The Weda Bay Ni-Co laterite deposits occur in the central section of Halmahera Island in the northeastern part of the Indonesian Archipelago. The deposits developed over a major ophiolite complex and were first drilled in May, 1996. Subsequent exploration has outlined a resource of 204 million dry tonnes of 1.37% nickel and 0.11% cobalt. The potential for a continued increase in the resource is high as areas within the contract of work (e.g. Pintu) contain known nickel laterites.

The Ni-Co laterites outcrop poorly and are underlain by a variety of ultramafic rock-types with the most dominant being serpentinised (partially to totally) harzburgite and lesser dunite. In addition, troctolite, Iherzolite, clinopyroxenites, anorthosite, various gabbros, wehrlite, dolerite and mafic volcanics have also been recognised. The dunites and harzburgites are composed predominantly of olivine, enstatite and chromite. Nickel content in olivines and enstatites ranges from 0.2% to 0.6% and 0.05% to 0.15% respectively. The average nickel grade of the ultramafic basement is 0.28%.

Olivine Mg/(Mg + Fe) are in the range 0.91-0.94, with a similar range (0.91-0.93) for enstatite.

During serpentinisation nickel-rich olivine breaks down to form antigorite and clinochrysotile. The fresh serpentine generally contains low NiO values (below 0.15%) although some higher concentrations (to 1.05%) were noted. Nickel tends to exit olivine forming a variety of alloys and sulphur-poor species that occur as minute grains (l-5|u) throughout the serpentine groundmass. The highly reducing nature of the serpentinisation process allows awaruite (NisFe) to form as well as a host of other phases such as heazlewoodite, pentlandite, bomite, millerite, etc. These phases, upon weathering, provide a significant proportion of Ni^^ to smectitic clays that occur interlayered with serpentine.

The Ni-Co laterites are on average 10 m thick and can be broadly divided into limonitic, transitional and saprolitic. Faults and fractures in the basement allow for deeper weathering and thicker laterite development. X-ray diffraction and electron-microprobe studies of the limonitic zone indicate that the dominant phases are goethite, smectite (including nontronite) and maghemite.

Residual chromite, often rimmed by secondary magnetite is common. Nickel content in the smectites (l-4%NiO) are lower than those in the transitional and saprolite zones. Electronmicroprobe investigations of Mn±Co±Ni±Fe-rich oxides indicate that nickel-rich asbolane, cobaltrich asbolane-lithiophorite intermediate mineral, lithiophorite and asbolane-lithiophorite intergrowths are present. A number of Mn-oxides have also been identified. Nickel-rich asbolane can contain up to 30% combined NiO and CoO and high scandium (-0.15%) phases are present.

The saprolite zone is dominated by "weathered" serpentine-smectite intergrowths. Serpentine in this zone is more hydrous indicating that the serpentine, sensu stricto, is changing to interlayered smectite-serpentine (or "gamierite"). The nickel contents rise dramatically in the "serpentines" being in the range of l%-6% NiO. Iron increases significantly and MgO is depleted with the MgO moving down to the base of the saprolite zone and depositing as brucite or ferrobrucite.(~0.1% NiO). Olivines that survived the serpentinisation process are broken down in the saprolite zone to smectite-goethite aggregates. These brown coloured mixtures can contain up to 1.5% NiO and therefore must scavenge nickel moving down or concentrating in the profile. Weathered "serpentines" immediately adjacent to the "brown coloured", weathered and replaced olivine is further enriched in NiO containing up to 5.5%NiO. Acknowledgement The author would like to thank Weda Bay Minerals for permission to publish this information.

346

CHEMICAL AND MINERAL PARAGENESIS OF DALES GORGE MEMBER BIF ACROSS THE EASTERN HAMERSLEY PROVINCE, WESTERN AUSTRALIA A. D. Webb^ N. H. S. Oliver^ and G. R. Dickens^'^ ^ Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Queensland, 4811, Australia ^Department of Earth Sciences, Rice University, Houston, Texas, 77005-1892, USA The widely cited supergene model for the giant microplaty hematite deposits of the southern Hamersley Province, Western Australia, suggests that original banded iron formation (BIF) vs^as enriched to iron ore through the simultaneous oxidation of magnetite to martite and replacement of chert with goethite, followed by burial metamorphism (Morris, 1985). However, systematic work on the chemical and mineralogical changes of the economically important Dales Gorge Member across the province has discovered rocks in the south that are significantly different from equivalent horizons in the north and nearby ore horizons in Mt. Whaleback (Webb et aL, submitted ms). These rocks are informally called 'altered BIF' and their presence suggests a more complex mineralization process. Either altered BIF represents ore precursor at Mt. Whaleback, or it represents the distal alteration halo developed around the orebody during an oxidation phase.

During alteration, magnetite in unaltered BIF oxidizes to martite, while carbonates, talc and stilpnomelane are dissolved. Although the oxidation of magnetite results in minimal change to major element oxide contents, the dissolution of gangue releases HCO3", Mg^^, K^ and H4Si04.

Consequently, Fe203T contents increase in altered BIF through the loss of MgO, CaO, LOI and minor Si02. Even though a number of phases are lost during this conversion, there are only subtle textural differences between unaltered and altered BIF. From a chemical and mineralogical perspective, quartz represents the only difference between altered BIF and microplaty hematite ore. However, hematite in altered BIF mainly occurs as martite and not microplaty hematite. Hence, in addition to silica removal, iron recrystallization or dissolution and reprecipitation must have also occurred to produce ore. Textural evidence suggests that this iron was most likely derived from adjacent martite layers, as supported by the geochemistry.

The reactions implied by these paragenetic steps can all be driven by oxidizing fluids and are therefore not model dependant. However, recently described magnetite/carbonate-rich, silica-poor rocks at Mt Tom Price (Taylor et aL, 2001) suggest that this alteration sequence is even more complex and that silica removal is possible without oxidizing fluids. Nonetheless, there are three important conclusions that stem from this work: (1) no single process can produce all of the altered rocks and ore at Mt. Whaleback, (2) oxidation of magnetite to martite can occur independently of silica removal or replacement and (3) the iron needed for microplaty hematite formation is locally derived.

References Morris, R.C., 1985. Genesis of iron ore in banded iron formation by supergene and supergene-metamorphic processes, A conceptual model, in Wolf, K.H. (Ed.), Handbook of strata-bound and stratiform ore deposits. Volume 13, Elsevier Amsterdam, pp. 73-235.

Taylor, D., Dalstra, H.J., Harding, A.E., Broadbent, G.C. and Barley, M.E., 2001. Genesis of high-grade hematite orebodies of the Hamersley Province, Western Australia. Econ. GeoL, 96: 837-873.

Webb, A.D., Dickens, G.R. and Oliver N.H.S., submitted ms. From BIF to iron ore: Major element chemistry and mineralogy of the Proterozoic Dales Gorge Member and surrounding shales at Wittenoom and Mount Whaleback, Hamersley Province, Western Australia.

347

INTERPRETATION OF STRATIGRAPHY AND PORTIA MINERALISATION, OLARY DOMAIN, SOUTH AUSTRALIA Zang, Wen-long Geological Survey, Department of Primary Industry and Resources, South Australia

Stratigraphy of the Willyama Supergroup (1720-1640 Ma) in the Olary Domain has been poorly defined until Conor (2000) divided it into two groups and four subgroups. A study of the sequence stratigraphy across the region recognises four Olary sequence (OSQ) sets in the Supergroup. OSQ set 1 contains psammite, migmatite, albitite and gneissic schist, including the Outalpa and George Mine Formations. OSQ set 2 contains the Ethiudna Subgroup, deposited from lowstand fluvial deposits of the Cathedral Rock metasandstone, transgressive deltaic/shelfal Mooleugore Formation and Peryhumuck Formation to highstand, shelfal Bimba Formation. This sequence is characterised by tvs^o unconformities bounded at the base and top. OSQ set 3 contains the low^er part of the pelite sediments of the Stratheam Group and OSQ set 4 the upper pelite - metasandstone complex. The two younger sequence sets are separated by a trough cross-bedded, fluvial/estuarine sandstone. The Portia Prospect w^as probably situated in middle to outer shelf settings during deposition of Ethiudna Subgroup, where the Bimba Formation hosts many Cu-Au deposits.

Olarian deformation comprises at least three events. Primary bedding or So in the Willyama Supergroup is largely transposed. Tectono-stratigraphic study suggests that the initial deformation might have started as early as -1640 Ma, evidenced by the youngest date (-1650Ma) on a tuffaceous metasediment in the Stratheam Group and a gossan layer (~1610-1630Ma) w^hich cuts across the Si layering. In the Portia Prospect, the metasediments contain distinct Si foliation and layering, which are folded in F2 and F3 structures.

The Portia Prospect contains Cu, Au and multi-element mineralisation, mainly by carbonate (Bimba Formation). The prospect is an anticlinal structure which is interpreted to comprise N-S trending F2 and NE-SW trending F3 folds. The primary mineralisation occurs mainly in F3 fold axes or fault planes, as layering or fault breccia fills. The carbonate, bracketed by impermeable pelite and metasiltstone, form a stratigraphic trap for hydrothermal fluids. Generally three major alteration events can be recognised. The first is involved with initial deformation and early albitisation or sodic - argillic alteration in siliciclastics; in carbonate, recrystallisation form primary layering and scapolite-calcite-biotite ±epidote ±albite alteration is common in silty carbonate rocks.

The second alteration is a multiple-staged, regional mineralisation event and occurred during 16301605Ma (Teale and Fanning, 2000). The Bimba Formation in the Portia Prospect underwent extensive sodic-calcic alteration and Cu-Au mineralisation, forming pyrite-chalcopyrite ±magnetite ±haematite deposits. The third event in the area might be prompted by faulting and intrusion of a granite to the south, enriching or upgrading the existing Cu-Au mineralisation along F3 fold axis and NE trending fault fractures, which also form the major magnetic anomalies in the prospect.

Similar structural settings and mineralisation are also recognised in the Kalkaroo Prospect, where the mineralisation is hosted mainly by the Bimba Formation and controlled by a NE-SW trending F3 anticlinal structure. Understanding of the stratigraphy and structural development may provide vital information for further exploration in the region.

Acknowledgements. The author thanks Colin Conor for valuable discussions and Pasminco Exploration, Werrie Gold Limited and MIM Exploration for permission to publish.

References Conor, C. H. H., 2000. Definition of major sedimentary and igneous units of the Olary Domain, Cumamona Province. MESA Journal, 19, 51-56.

Teale, G. S. and Fanning, C. M., 2000. The Portia - North Portia Cu-Au(-Mo) Prospect, South Australia: timing of mineralisation, albitisation and origin of ore fluid. In: Porter, T. M. (ed.), Hydrothermal iron oxide copper-gold and related deposits: a global perspective. Australian Mineral Foundation, Adelaide, 137-147.

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SEDIMENT HOSTED GOLD MINERALISATION AT KYAUKPAHTO, KAWLINWUNTHO DISTRICT, NORTHERN MYANMAR Khin Zaw^ Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, 7001, Australia Email: Khin.Zaw@utas.edu.au

Introduction The Kyaukpahto gold deposit is located at Latitude 23° 47' 55" N and Longitude 95° 56' 35" E in the Kawlin-Wuntho district, Sagaing Division, Myanmar. Gold mineralisation is hosted in sandstone of Male Formation (Lower to Middle Eocene) and occurs as stockwork and disseminations. The high-grade gold mineralisation is locally concentrated in breccia zone. The gold deposit is closely associated with a NNE-SSW trending fracture system that forms as tensional open fractures. These tensional zones are thought to be directly related to the Sagiang transfer fault system that has several hundred kilometres of northward dextral movement during post-Upper Miocene (Khin Zaw, 1989, 1990).

Mineralogy Major sulfide minerals are pyrite and arsenopyrite with minor galena, chalcopyrite and sphalerite.

Gold mineralisation is associated with intense silicification, sericitisation and argillic alteration.

Trace element geochemical studies indicate that silver, copper, arsenic and antimony are associated with the gold mineralisation at Kyaukpahto. Gold occurs predominantly as free gold grains (electrum) in stringer quartz veins. Native gold is also a dominant mineral in the oxidised zones. The gold grains vary in size from 1 |im to 250 |Lim and electron microprobe analysis of the grains yields a range of fineness from 844 to 866.

Ore fluid chemistry Fluid inclusion studies reveal three major fluid inclusion types in quartz from the mineralised zone, based on phases observable in the inclusions at room temperature and paragenetic relationships: (1) Type I, two-phase, H2O liquid and vapour inclusions, (2) Type 11, three-phase with H2O liquid, vapour and CO2 liquid inclusions and (3) Type III, two-phase, liquid-rich inclusions with variable liquid and vapour ratios. Both Type I and II inclusions are primary as they can be texturally related to the growth zones of the host quartz. Type EI inclusions are secondary and cross-cut the grain boundaries of host minerals. Primary Type I fluid inclusions in the quartz are 5-15 jiim across and yield homogenisation temperatures of 239°-310°C and salinities of 1.210.9 wt % NaCl equivalent. Laser Raman spectroscopic analysis indicates the presence of CO2 and CH4 in Type II fluid inclusions. Sulphur isotope analysis of disseminated pyrite in the ore zone by laser ablation method indicates

values from - 5 . 4 8 %o to + 5 . 5 3 %o.

Conclusions The Kyaukpahto deposit lies along the major SE Asia/SW Pacific porphyry Cu±Au and epithermal gold belt, and the geology, alteration, ore mineralogy, fluid inclusion microthermometry and sulphur isotope data are compatible with epithermal ore deposit model for the deposit. However, the presence of C02-liquid bearing fluids and the detectable CH4 in the gaseous phase of the fluid inclusions conflict with a typical epithermal origin for the Kyaukpahto deposit. The host sandstone sequence, trace element geochemistry, and the disseminated nature of sulphide and gold mineralisation are also common characteristics of sediment-hosted gold deposit, and these features may indicate a Carlin-like affinity.

References Khin Zaw (1989). Geology, v. 17, p. 93-95.

Khin Zaw (1990). Journal of SE Asian Earth Sciences, v. 4, no. 4, p 293-335.

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PETROGRAPHIC AND GEOCHEMICAL CRITERIA FOR RECOGNITION OF UNALTERED COLD WATER AND DIAGENETICALLY ALTERED NEOPROTEROZOIC DOLOMITE, WESTERN, TASMANIA, AUSTRALIA. Mohammad H. Adabi School of Earth Sciences, Shahid Beheshti University, Tehran, Iran Petrography, elemental and isotopic composition of Neoproterozoic dolomites at Renison in western Tasmania have been investigated to determine the characteristic features of unaltered cold and diagenetically altered warm water dolomites. These deposits occur within the central region of the Dundas Trough, consisting of Proterozoic to early Paleozoic sedimentary rocks. The oldest sediments in the Dundas Trough are the shallow shelf carbonates and siliciclastics of the Success Creek Group (-1000 m) which unconformably overlie the Oonah Formation. The Success Creek Group is followed by a thick succession of marine sediments of the Crimson Creek Formation (-5000 m). Carbon isotope chemostratigraphy, suggests a Neoproterozoic age of about 570-820 Ma for the Crimson Creek Formation and Success Creek Group respectively (Adabi, 1997).

Extensive glaciogenic deposits of Neoproterozoic age occur across central Australia (Brookfield, 1994) and also in Tasmania. Neoproterozoic marine sedimentation in western Tasmania occurred between two major global glaciations (i.e., Sturtian, - 7 3 0 Ma and Varanger glacials - 6 0 0 Ma) and three different diamictite horisons have been reported in these sequences. The association of dolomite with diamictites, and particularly the presence of glacial erratics in these sediments, indicates a glacio-marine origin.

Four major types of dolomite occur in the Neoproterozoic sequence studied outside the Renison mine area. These dolomites have been classified according to increasing crystal size (i.e., dolomicrite, dolomicrosparite, dolosparite and vein dolomite). These dolomite types are also distinguished and recognised on the basis of Mg, Mn and Fe content and and values.

Petrographically, dolomicrites are interpreted as least-altered dolomites, as original depositional textures are well preserved and show little evidence of recrystallization and diagenetic reorganisation. Least-altered dolomites show high values of Mg, heavy and values, and low Fe and Mn content, compared to coarsely crystalline dolomites. The marked increases in Mn and Fe and decreases in Mg and and values from finely to coarsely crystalline dolomites are due to an increased amount of diagenetic alteration. The most likely explanation for and depletion in dolomicrosparite and dolosparite appears to be temperature-related fractionation.

Dolomicrites probably formed at or near the surface by either direct precipitation or during very early diagenesis, with Mg^^ being supplied by seawater. The calculated palaeotemperature of seawater during the Neoproterozoic, considering 8^= -6%o and the least-altered dolomite value of -l±l%o, indicates that the seawater temperature was around ± 4° C (Irwin equation) or IX" ± C (Land equation). Similarly, determination of palaeotemperature of warm water Proterozoic dolomite, not associated with glacial sediments value of -6±l%o), yielded a valid temperature result corresponding to C (Irwin equation) or 35"" ± 5° C (Land equation). The coarsely crystalline dolomites are epigenetic in origin. They probably began to form at shallow levels of burial and continued to form over a range of depth and temperature. This is supported by their textures, elemental compositions, and positive correlation between and values.

References Adabi, M.H. 1997. Application of carbon isotope chemostraigraphy to the Renison dolomites, Tasmania: a Neoproterozoic age. Australian Journal of Earth Sciences, 44, 767-775.

Brookfield, M.E. 1994. Problems in applying preservation, facies and sequence models to Sinian (Neoproterozoic) glacial sequences in Australia and Asia. Precambrian Research, 70, 113-143.

350

EOCENE BRYOZOAN PALAEOENVIRONMENTS AND ASSEMBLAGES, ST VINCENT BASIN, SOUTH AUSTRALIA Rolf Schmidt^'^ and Yvonne Bone^ ^Programs & Research, Museum Victoria, Melbourne, Victoria 3001 2"Dept. Geology & Geophysics, University of Adelaide, South Australia 5005 The first stratigraphically-detailed taxonomic study of Eocene Bryozoa in the St Vincent Basin has revealed faunas ranging from high abundance and diversity to almost monotypic assemblages.

Some formations contain over 200 species of cheilostome bryozoans and over 50 species of cyclostome bryozoans. This makes them more diverse than all other macrofauna present combined.

By far the highest diversity occurs within the sediments of the first fiilly marine conditions, namely the Tortachilla Limestone on the eastern (Fleurieu Peninsula) side of the basin and the basal section of the Mullovvurtie Formation on the western (Yorke Peninsula) side of the basin.

The presence of a basement high centred on what is now Kangaroo Island produced a barrier-like shallow carbonate sand-bar and keys situation at the mouth of Gulf St Vincent, resulting in restricted circulation within the St Vincent Basin during much of the Tertiary. This gave rise to a range of predominantly shallow-water environments not encountered in other basins around Australia at this time. The paleoenvironments varied from open marine sea grass beds to highly restricted and eutrophic estuarine facies. Although this resulted in varied bryozoan assemblages, several species and genera occur in all facies and are therefore probably opportunistic taxa.

Some genera that are common in the shallow paleoenvironments are only found living in water deeper than 100m to-day. It is probable that the St Vincent Basin was always less than 50 m deep.

These bryozoan genera may either originally have lived in shallower waters and later adapted to deeper conditions or the restricted character of the basin created environments analogous to deeper water. The high ratio of cheilostomes to cyclostomes is typical of modem (sub)tropical environments. The paucity of azoozanthellate corals, however, makes this interpretation tenuous.

Cyclostomes are also more susceptible to 'pollution' and may indicate unfavourable conditions.

The relative abundances of articulated branching forms within the beds of the Tortachilla Limestone document strong facies variations which are not apparent at the outcrop level. Freeliving lunulitiform colonies are displayed by the disparate families Otionellidae, Calloporidae, Lepraliellidae and Didymosellidae. To-day this growth form is typical of shifting sands but in the St Vincent Basin it is only found in the Tortachilla Limestone, and often with bryozoan forms that are not indicative of a mobile substrate.

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BRYOZOANS FROM PLEISTOCENE HIGH-ENERGY, COOL-WATER, CONTINENTAL-SLOPE MOUNDS, GREAT AUSTRALIAN BIGHT, SOUTHERN AUSTRALIA. ^Yvonne Bone and ^Noel P. James ^Dept of Geology and Geophysics, Adelaide University, South Australia 5005, Australia ^Dept of Geological Sciences, Queen's University, Kingston, Ontario K71 3N6 Canada The Great Australian Bight (GAB) is an enormous latitude-parallel shelf that has been the site of heterozoan carbonate deposition since mid-Eocene time. This shelf faces the Southern Ocean, so that it is a high-energy, storm-dominated, mid-latitude carbonate system, with wave a abrasion depth of -70 metres and sw^ell wave-base of -120mwd. Seismic images show mound-like structures in the near-surface continental shelf slope region. These structures were drilled during ODP Leg 182, at -200 to - 350mwd.

The mounds are muddy and characterised by prolific bryozoan growth, with minor to rare occurrences of foraminifera, echinoid fragments, small molluscs, serpulids, brachiopods, ostracods, sponge and tunicate spicules and peloids. Sea-floor relief is usually only a few metres, but the seismic images suggest that when active, they were occasionally up to 40 m high and persisted hundreds of metres laterally. Currently, it appears that no mounds are actively growing.

The youngest mounds probably grew during glacial low-stands when the nutrients needed for high productivity were available.

The bryozoans present in the mounds show high diversity as well as high density, with 74 genera identified from two holes. All grovs1:h forms are present, as are both cheilostome and cyclostome groups, with the former dominating the sediments volumetrically and the latter dominating the sediments in terms of individual grains. Fine sand/mud fractions comprise the same genera as those found in the coarser fractions, but with delicate branching, articulated branching and articulated zooidal forms more numerous. Similarly, species differences are found in the two size fractions, e.g. the articulated branching Cellaria rigida is common, with C. australis occasionally common in the coarse fraction in contrast with the mud to fine sand fraction where C. tenuirostris is common and C. bicomis is occasionally common.

Adeona, one of the most abundant bryozoans in the GAB to-day, is conspicuous by its absence.

However, this may be a function of the water depth in which the mounds were growing, as Adeonas preferred environment for growth today ranges from approx. 60mwd to approx.

120mwd, with rare fragments found at greater depths, due to post-mortem down-slope transport.

352

ORIGIN AND COMPOSITION OF SHALLOW COOL WATER CARBONATE MUD, STREAKY BAY, SOUTH AUSTRALIA. Richard Daniel\ Yvonne Bone^ Noel James^ and Chris von der Borchl ^Dept of Geology and Geophysics, University of Adelaide, South Australia, 5005. ^Dept of Geological Sciences, Queen's University, Kingston, Ontario, K7L3N6, Canada. ^School of Earth Sciences, Flinders University, Bedford Park, South Australia, 5042.

Research and literature on shallow^ cool-w^ater, open marine carbonate muds are not abundant.

Deeper water (>40 m), temperate, continental shelf, carbonate muds from the southern Bass Basin, New Zealand, western Europe and North America have been reported. Deposition of shallow cool-water carbonate muds in a semi-protected, open circulation marine environment with water depths from 8 m to 15 m have not been recorded.

Temperate carbonate muds at Streaky Bay, South Australia, were sampled from a range of these semi-protected marine environments. Streaky Bay is typical of the unusual bay morphology along western Eyre Peninsula, i.e. relatively deep-water inner basins, representing a significant shallow marine marginal lithofacies present along the largest cool-water carbonate province on Earth.

Carbonate mud (0-60%) and medium-very fine sand occur in inner areas protected from the effects of sea swells by sand-shoals and tidal deltas. The mud contains >95% CaCOs, consisting of low, medium and high-magnesium calcite (LMC, IMC, HMC) and aragonite, with aragonite»LMC=IMC>HMC. Aragonite»LMC>IMC in the bulk sample whereas the reverse is true of the mud fraction. Disintegrating bryozoan particles provide the IMC. Comparisons between bulk sediment compositions and the resident calcareous biota show a strong correlation. The susceptibility of this biota to disintegration (physical/biological diagenesis) determines which organisms constitute the mud.

Significant contributors are molluscs>bryozoans>coralline algae>foraminifers>echinoderms>sponge spicules>serpulids with minor contributions from brachiopods, tunicates, corals and coccoliths.

The disintegration of skeletal material is produced through maceration coupled with abrasion fi-om medium to high-energy sea movement. Possible dissolution of mud-sized skeletal material appears to be occurring, similar to that studied in Drake Passage, South America.

The significant grain-size of the mud is from 20-63 jxm, with LMC or aragonitic prismatic particles and tabular particles dominant. The dominant morphologies of these particles are prismatic and tabular. The prismatic particles are composed of LMC or aragonite and the tabular particles of LMC, with minor aragonite, HMC and IMC. Other significant particle morphologies are porous polygonal, blocky and composite particles. Polygonal particles are LMC, HMC and IMC and consist of sub-|Lim platelets and crystallites. Blocky particles are usually composed of LMC or aragonite with a homogenous structure consisting of sub-jiim crystallites. Composite particles are composed of a range of calcite mineralogies. They consist of disintegrated particles from calcareous skeletal biota e.g. bryozoans (platelets), coralline algae (crystallites), molluscan nacreous layers (platelets), foraminifera (crystallites and larger platelets) ascidians and coccoliths, and in some instances, may be of faecal origin.

No evidence of skeletal overgrowth was observed (under SEM) on any disintegrated particles in the mud fraction, which verifies the unsaturated candition (with respect to carbonate) of the seawater in the marginal cool-water environment.

353

SEDIMENTATION RATES WITHIN THE CAPRICORN CHANNEL, SOUTHERN GREAT BARRIER REEF, AUSTRALIA Bostock H.C. and Opdyke B.N. Department of Geology, Australian National University, Canberra, ACT, Australia 0200 The Capricorn Channel forms a large embayment between the Capricorn Bunker Group and the Swain Reefs in the Southern Great Barrier Reef. The Channel slopes down at a very shallow gradient to the Southeast until it eventually merges with the Tasman abyssal plain. There is a slight increase in gradient between 600 and 1000m but there is no well defined shelf break in this area and hence no well defined continental slope. The floor of the Channel is covered by predominantly fine grained carbonate mud and sand.

A series of gravity cores and water samples were taken at a range of depths in the Capricorn Channel on the Franklin Cruise 01/97. Age models have been constructed for 2 of the cores (FR97GC-11, depth 502m, 23°23'07S, 153°22'00E, and FR97GC-12, depth 990.5m, 23°34'37S, 153°46'94E,) based on oxygen isotope data and comparison with SPECMAP record of Martinson et al (1987). Several ^^C AMS dates have also been determined for GC-12. Both of these cores are 5m in length but show a significant difference in sedimentation rates. GC-11 core record extends back as far as MIS 12/13, ~500,000yrs and has an average sedimentation rate of approximately Icm/kyr, whilst GC-12 core gives a record back to MIS 5a, ~80,000yrs giving an average of approximately lcm/150yrs.

One of the most interesting aspects of these cores are that the trend in sedimentation rates is the reverse of that found on other marine slopes. Dunbar et al (2000) found that cores from the northern GBR - Queensland trough showed a decrease in sedimentation rates with depth and distance from shore for the last transgression 15ka-6.5ka and high stand 0-6.5ka. There is no evidence for any slumping, turbidity currents, or exposure surfaces affecting the integrity of the cores as they are situated away from topographic highs. They are also located well above the lysocline, which, from the water data, has been calculated to begin at approximately 3000m. GC11 shows a good correlation between the S^^Opdb Globigerinoides ruber and the CaC03% with carbonate production high during the interglacials and low during the glacials. 5^^Cpdb G. ruber also shows a correlation with but with a slight lag at the glacial terminations. GC-12 also shows a correlation between and CaC03%, with CaC03% higher in the interglacials. The higher CaC03% in both the cores is the result of increased carbonate production with the rising temperatures and sea level during the interglacial. The cores both show increased sedimentation rates during the interglacials, but is it solely related to production levels or is the sediment accumulation rate affected by scouring, or other processes?

Other cores taken on cruise FRO 1/97 recovered much less than 5m, with FR97GC-15, (2892m at the end of the Capricorn Channel, 23^49'15S, 154°37'34E) retrieving less than 50cm in the core barrel. This suggests that different water masses, which form the bottom currents at different water depths, are winnowing the ocean floor at certain depths and not at others. So sediments can accumulate at one depth, whilst at another they are scoured away. How do these water masses change through glacial/interglacial time intervals and affect the variations in sedimentation rates seen throughout cores? Understanding these currents will allow prediction of better sites for high resolution, high sedimentation cores in the future.

References Martinson et al (1987) Age dating and orbital theory of the Ice Age. Development of a high resolution 0300kyr chronostratigraphy. Quatemary Research 27, pi-29.

Dunbar et al (2000) Sediment flux across the Great Barrier Reef Shelf to the Queensland Trough over the lastSOOkyr. Sedimentary Geology 133, p49-92.

354

SYNTHESIS OF CONTINENTAL MARGIN SEDIMENTATION ON THE SOUTHEAST COAST OF AUSTRALIA Ron Boyd, Jennifer Wadsworth, Jason Roberts, Kevin Ruming University of Newcastle, NSW, 2308, Email: Ron.Boyd@nev^castle.edu.au

The south east Australian continental margin has a unique sediment dispersal system. A series of coastal rivers have supplied sand to the shoreline and continental shelf throughout the Tertiary and Quaternary. Low^ pressure systems in the Tasman Sea have maintained a southeasterly w^ind and w^ave regime onto the SE Australian margin. The result has been the formation of a 1200 km longshore transport system from southern NSW to Fraser Island in Queensland. Large volumes of clastic sediments have accumulated at the northern end of the system forming sand islands up to 120 km long and 40 m high. Due to a NW change of coastal orientation, this shoreline sediment dispersal system is now supplying coastal sand beyond the sand islands, directly to the shelf edge and down the continental slope into deeper water. This region thus provides a new process for supplying sand to the deep ocean.

The northward wave-driven longshore transport system is confined to the shoreline, shoreface and inner shelf. Wave energy is high and prevents the accumulation of muddy sediments above 50-60 m water depth and also likely inhibits the formation of large bedforms such shelf shoals seen on lower energy margins. The outer shelf and upper slope is a temperate carbonate province that is impacted by the East Australia Current (EAC) and contains widespread but subdued temperate carbonate mounds. The EAC geostrophic permanent ocean current flows south at velocities of up to 4 knots along the eastern Australian margin and transports carbonate sediments southward and seaward over the shelf edge. The eastern Australian margin on average is less than 50 km wide, but thus contains two major sediment dispersal systems heading in opposite directions.

Unlike other well-escribed continental margins such as the Gulf of Mexico, or established sequence stratigraphic models, the south east Australian margin has deeply incised Quaternary valleys only landward of the modem highstand shoreline and not across the shelf. In addition, the lowstand shoreline position south of the Manning River was located on the outer shelf while north of here the lowstand position was located either at the shelf break or on the upper continental slope. In all cases the lowstand shoreline was not a site of major sediment accumulation. The thickest Quaternary sediments are located landward of the modem shoreline in coastal river valleys, and in the midshelf highstand shorelines of oxygen isotope stage 3. These midshelf shorelines are developed along the entire east coast and commonly consist of a division into three transgressive-regressive packages deposited under an overall falling sea level toward the stage 2 lowstand.

The eastem Australian margin is a narrow passive margin with a history that extends back to the Mesozoic, but asymmetric rifting and restricted sediment supply has resulted in a sedimentdeficient margin. The total sediment thickness above basement is usually less than 500-700 m with the majority of this made up of Miocene and older sediment wedges. No rift-related faulting or syn-rift basins appear to be present on the margin. The southeast Australian margin thus provides a well-documented example of modem coast and shelf processes but presents a strong contrast with most other well-known examples of margin development. It therefore provides an important case study to generate a broad spectrum of sources for comprehensive continental margin sedimentation models.

355

CYCLONE PUMPING AND SEDIMENT PARTITIONING IN THE DEVELOPMENT OF THE GREAT BARRIER REEF SHELF SYSTEM Larcombe, P. and Carter. R.M. Marine Geophysical Laboratory, James Cook University, Townsville 4811 Australia (piers.larcombe@jcu.edu.au) Understanding the relative impact on shelf sediments of daily low^-energy versus episodic highenergy phenomena (e.g. cyclones, tsunami) is crucial to our understanding of how shallow^ w^ater sedimentary systems function. Around 30-40% of today's continental margins lie in the tropics and sub-tropics, where cyclones are major mechanisms of sediment supply to the shelf, and sediment transport upon it. Understanding the sedimentary dynamics of tropical shelves at various stages of sea level is therefore a fundamentally important issue. However, current sedimentation models for tropical shelves are strongly influenced by studies of ocean plateaux such as the Bahamas, and often do not fit well with the characteristics displayed by mixed terrigenous-carbonate systems, which are geologically common and important in petroleum exploration.

The modem Great Barrier Reef (GBR) is part of the world's largest and best known mixed terrigenous-carbonate continental margin. The GBR shelf contains three shore-parallel sedimentary belts. An inner shelf zone of terrigenous sedimentation at depths of 0-22 m; a middle shelf zone of sediment starvation at depths of 22-40 m; and an outer shelf reef tract with its inner edge at c. 3540 m depth. These zones are controlled by the dynamics of northward, fair-weather, alongshelf drift, driven by southeasterly trade winds, and by the regular passage of tropical cyclones. Cyclones cause wind-driven north-directed middle shelf flows in excess of 130 cm/s, which erode the seabed, concentrate the sparse mobile sediment into sand ribbons, and advect suspended load onto the outer part of the nearshore terrigenous sediment prism and into inter-reef depocentres within the reef complex. Cyclones largely control the input of new sediment into the Great Barrier Reef system, via river flooding, seabed erosion or reef breakage. They also help to control the partitioning and dispersion of the main shore-parallel belts of terrigenous inner shelf, sediment-starved middle shelf, and outer shelf carbonate reef tract sediment. Acting as a sediment pump, especially during interglacial highstands, cyclones have exerted great control on the development of the modem GBR province and its sediments by maintaining a broad shelf-parallel zone of episodically mobilised sediment and scoured seabed, upon which coral reefs have been unable to form.

Contrary to current models, (i) GBR storm beds are most likely to be preserved intact close to the shoreline, and they also become coarser-grained away from the shoreline; and (ii) for the central GBR, "highstand shedding" only applies to carbonate sediment at the scale of local reefs; systemwide, oceanographic controls cause high rates of carbonate sedimentation on the slope during both sea-level rise and highstand; concomitantly, terrigenous sediment accumulates fastest on the slope during sea-level rise, and slowest during sea-level lowstand and highstand.

356

TERTIARY FOUNDATIONS AND QUATERNARY EVOLUTION OF CORAL REEF SYSTEMS OF THE NORTH WEST SHELF, AUSTRALIA Lindsay B. Collins Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA, 6845, head@lithos.curtin.edu.au The North West Shelf is a modem tropical ramp which is underlain by Cretaceous-Tertiary carbonates, with clastic reservoirs at depth. Coral reef systems, discontinuously developed during the Quaternary, vary from fringing reefs to isolated oceanic atolls, some of which resemble the downslope buildups of the geological record. Quaternary evolution of the reef systems is being documented using regional mapping, seismic imaging, coring and U-series dating. The well constrained sea level data from the Abrolhos reefs and carbonate platforms (at 28-29.S^'S) have also been applied to the North West Shelf reefs.

The Tertiary sediment pile in North West Shelf basins occurs as a prograding wedge, thin at the coast and thickening to 2,500 m at the shelf edge. Unconformity-bounded sedimentation cycles usually correlate between the basins, and where documented consist of bryozoan-dominated grainstones, packstones and wackestones, Nummulitid packstones and wackstones, and mixed glauconitic/phosphatic sands (some with condensed sequences) interbedded with chalky skeletal packstones/wackestones in which bryozoans, echinoids and foraminifers are common.

During the Late Tertiary -Quaternary coral reefs developed discontinuously along the North West Shelf, in a range of latitudinal, oceanographic and topographic settings. Their geological record is preserved on a number of timescales, from inter-annual isotopic records of sea surface temperature change, with continuous records up to 200 years in length, to glacio-eustatic sea level records through the Quaternary, preserved as fourth and fifth order cycles. Coral reefs of the North West Shelf include isolated oceanic reefs (Ashmore Reef, Seringapatam and Scott Reefs, Rowley Shoals), island-associated shelf reefs of the Kimberley coast and Dampier Archipelago; Pilbara reefs (Barrow and Montebello Islands) and Ningaloo Reef, adjacent to North West Cape.

The Ningaloo fringing reef at 20-22®S, Australia's longest fringing reef, records Holocene and Last Interglacial phases of reef growth in a tectonically stable environment, overlying Tertiary carbonates of the Cape Range, which is flanked by uplifted Plio-Pleistocene terraces and reefs.

Scott Reef (at 14°S) is a macrotidal, isolated oceanic reef which overlies a carbonate platform and a major gas discovery. Seismic profiles reveal a Last Interglacial (c. 125,000 year) reef system, with a well developed, partly karstified lagoon, and circular reef rim, but reefs which apparently grew to sea level are now 30 m below present sea level, indicating significant subsidence in the Late Quaternary. Contemporary reefs grew during the Holocene in the accommodation space provided by subsidence and are up to 35 m thick. The Rowley Shoals (15-17''S) comprise one of the most perfect morphological series of reefs known, and these emergent, annular reefs rise from depths of 200-400 m. Seismic profiles suggest that Late Quaternary subsidence has been an important control on Holocene reef growth, and differential subsidence has influenced reef morphology in different reef systems.

Coral reefs are known as repositories of biodiversity which are susceptible to environmental change and events such as coral bleaching. Less well known, however, is the potential spatial association between coral reefs and active and palaeo-hydrocarbon seeps. As further exploration and development occur in and around coral reefs, and management intensity increases, there is a need for better understanding of human and natural impacts (cyclones and coral bleaching), biological processes, and the geological controls on reef growth and development, as part of management plans.

357

ACOUSTIC SEAFLOOR MAPPING OF SOUTH EAST AUSTRALIA Melissa E. Fellows^ James J .Daniell\ Peter T. Harris ^ and Andrew D. Heap^. ^Geoscience Australia, GPO Box 378, Canberra ACT 2601,^GPO Box 252-80, Hobart Tas 7001.

Acoustic facies mapping of south east Australia was undertaken, as one of several map layers, to understand the relationship between seabed geology and benthic habitat. The acoustic map layer generated was used in conjunction with other spatial data to determine seafloor morphology and sediment process as part of regional marine planning by the National Oceans Office. The data were generated by high frequency echo-sounders that record the acoustic response of surface sediments on continuous, paper-based echograms. These were interpreted for echo-character, based on Damuth's (1980) classification scheme.

Acoustic facies mapping was the ideal method for regionally mapping a large area in a short period of time, using existing data. Damuth (1980) classified echo-character into three main categories; Type I (distinct), Type II (indistinct, prolonged) and Type III (indistinct, hyperbolae). The echocharacters recorded in south east Australia were extremely variable, including all of those recognised by Damuth fi'om other areas. Variations were a direct result of the underlying geological features, including sediment type, layering, structure and topography, which directly control benthic habitat.

Additional data sets such as sediment samples, swath imagery, and high-resolution bathymetry are needed to define the echo-types in terms of regional sedimentary process. While ground-truthing with seafloor samples in this area is yet to occur, comparisons were made with high-resolution bathymetry, swath imagery and the ground-truthing results from Whitmore & Belton (1997), Rollet et al (2001) and Damuth (1980). Types I and II are associated with sediment covered areas.

Sediments were predominantly oozes, with Type lis having more sand content. Type Ills are associated with areas of rugged topography, with thin veneers of ooze. Bass Lake, a bathymetric low, was defined by prolonged Type II echo-characters, surrounded by less prolonged Type I echocharacters. Canyons incising the continental slope were recognised by hyperbolic Type III echocharacters. Bass Canyon showed Type III, in incised valleys, Type I on the canyon floor with areas of Type n, possible debris flows. The Cascade Seamount similarly displayed steep slopes of Type III, the plateau of Type I and more prolonged Type II to the east, a possible debris flow.

Echo-characters were mapped at a higher resolution than other data sets available for marine regionalisation. Thus the acoustic facies were much higher in detail than the map of geomorphic units, interpreted from the bathymetry map. This has important implications when producing a bioregionalisation. For each scale of regionalisation, data are required to at least the next level lower; so for example to define provinces at scales of 100s to 1000 km, data are needed on a smaller spatial scale, of lOs to 100s km to ensure the boundaries are properly defined.

References Damuth, J.E, 1980. Use of high frequency (3.5 - 12 kHz) echograms in the study of near bottom sedimentation processes in the deep sea. Marine Geology, 38, 51-75.

Rollet, N., Fellows, M. E., Struckmeyer H. I. M., Bradshaw B. E., 2001. Seabed character mapping in the Great Australian Bight. Geoscience Australia Record 2001/42, Geoscience Australia, Canberra ACT 22p Whitmore, G.P. and Belton, D.X., 1997. Sedimentology of the South Tasman Rise, south of Tasmania, from 'groundtruthed' acoustic facies mapping. Australian Journal of Earth Sciences 44, 677-6SS.

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PALAEOGEOGRAPHIC EVOLUTION OF THE JURASSIC TO CRETACEOUS SUCCESSION IN THE PETREL SUB-BASIN C.M. Gibson-Poole^ and S.C. Lang^ ^National Centre for Petroleum Geology and Geophysics, Thebarton Campus, University of Adelaide, SA 5005

In the Petrel Sub-basin during the Jurassic and Cretaceous (Troughton, Flamingo and Bathurst Island Groups), relative sea-level changed a number of times, exposing and flooding areas of the continental shelf. Thus, a range of depositional environments are preserved in the sedimentary record. The integration of 2D seismic stratigraphic interpretations with wireline log correlations, core lithofacies associations and biostratigraphy enabled a sequence stratigraphic framework and palaeogeographic evolution to be determined. Eight unconformity-bounded sequences were identified in the Petrel Sub-basin from the base of the Plover Formation to the Bathurst Island Group, with depositional environments ranging from fluvial to marine.

During Sequence 1 (C. torosa - D. complex. Plover Formation) and the lowstand systems tract (LST) of Sequence 2 (C cooksoniae - W. indotata, Plover Formation), the rocks are dominantly massive or occasionally cross-bedded medium- to coarse-grained sandstones with terrestrial organic-walled microfossils and a blocky low gamma ray (GR) log motif. These are interpreted to represent a laterally extensive, amalgamated complex of braided fluvial channels, deposited over a widespread area during a period of low accommodation relative to sediment supply. The subsequent transgressive systems tract (TST)-highstand systems tract (HST) cycles of Sequence 2 (W. digitata - W. spectabilis, Elang Formation) indicate a rise in relative sea level that resulted in a shallow marine influence and finer grained delta plain, interdistributary bay and shoreface sediments being deposited. Relative sea level continued to rise during the deposition of Sequence 3 (D. swanense - early D. jurassicum. Frigate Formation), whereby the deposition of shales, high gamma ray (GR) log motifs and the presence of marine microplankton indicate a marine shelf palaeogeography.

There was a significant drop in relative sea level during the LST of Sequence 4 (£). jurassicum.

Frigate Formation), resulting in sediment bypass and subaerial exposure of the shelf. LST forced regressive shorelines are seen on the seismic between the Petrel Field and Bougainville-1, and LST fans have also been identified in the Malita Graben (at the NW end of the Petrel sub-basin) by Robinson et al (1994). Relative sea level then rose again during the subsequent TST-HST cycles of Sequence 4 (D. jurassicum - P. iehiense. Frigate Formation), with the deposition of lower shoreface to shelf very fine-grained sandstones at Petrel-1.

Relative sea-level fell again during the deposition of Sequence 5 (P. iehiense - K. wisemaniae.

Sandpiper Sandstone) and the LST of Sequence 6 (K. wisemaniae - C. delicata. Sandpiper Sandstone), with the development of a forced regressive shoreface NW of the Petrel Field and another LST fan in the Malita Graben identified by Robinson et al (1994). Relative sea level then rose again during the subsequent TST-HST cycles of Sequence 6 {K. wisemaniae - E. torynum.

Sandpiper Sandstone), depositing fine-grained, glauconitic middle to lower shoreface sandstones.

These are interpreted to represent linear shorelines striking in a crescent shape across the Petrel Sub-basin, representing infilling from both the SW and SE. Sequences 7 (5. tabulata-A. cinctum, Echuca Shoals Formation) and 8 {D. davidii - D. multispinum/P. infusorioides, Bathurst Island Group) represent a drowning of the previous coastal system with the deposition of marine shelf shales across the whole of the Petrel Sub-basin.

Reference Robinson P. H., Stead H. S., O'Reilly J. B. & Guppy N.K. 1994. Meanders to fans: a sequence stratigraphic approach to Upper Jurassic-Lower Cretaceous sedimentation in the Sahul Syncline, North Bonaparte Basin. In: Purcell P. G. & Purcell R. R. eds. The Sedimentary Basins of Western Australia, pp. 223242. Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth 1994.

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STRATIGRAPHIC EVENTS RECORDED IN UPPER CRETACEOUS CHALK ALONG 1000 KM OF THE WESTERN AUSTRALIAN CONTINENTAL SHELF David W. Haig Department of Geology and Geophysics, The University of Western Australia Chalk deposits of late Coniacian to early Campanian age are present throughout the western sector of the on-shore Southern Carnarvon Platform in the Carnarvon Basin (Toolonga Calcilutite) and in the Gingin-Dandaragan region in the onshore Perth Basin (Gingin Chalk). These units are remnants of fine-grained carbonate deposits that covered a broad continental shelf from about 45°S to 55°S palaeolatitude. The rocks remain flat-lying, and have undergone very little burial diagenesis. The Toolonga Calcilutite is 19.5 m thick in its type section located in the southern part of the Southern Carnarvon Platform. Variation in thickness (to about 200 m) on the Southern Carnarvon Platform may be related to contemporaneous faulting and sedimentation keeping pace with subsidence. Three lithological units are recognized in the Toolonga Calcilutite at its type section: (1) a basal, 0.5 m thick, friable glauconitic chalk with a layer of nodules at base; (2) friable white chalk, 8 m thick, intensely bioturbated, with common Inocemmus shell fragments; and (3) friable greenish marly calcilutite with a thin layer of phosphatic nodules at base, 11.5 m thick in type section (covered by calcrete). Further north, unit 3 becomes thicker and forms the entire Toolonga Calcilutite in the Giralia Anticline. The Gingin Chalk in its type section in the Perth Basin consists of two units: (1) a basal friable unit, 2.5 m thick, characterised by clasts of chalk (most about 1-2 cm) set in a glauconite-quartz-chalk matrix; overlain by (2) a friable unit of intensely bioturbated "chalk", 18 m thick, containing abundant medium to coarse glauconite and coarse to very coarse quartz. The thickness of Gingin Chalk varies considerably in the type area (from 0 m, 3 km northwest of the type section, to 20.5 m at the type section), and the contents of glauconite and quartz are also variable.

The Gingin Chalk overlies the Molecap Greensand (a medium to coarse glauconitic, quartz-rich, sand). The contact is topographically very irregular over short distances. The glauconite and quartz populations in the chalk correspond to those found in the Molecap Greensand, and were probably reworked from topographic highs of this unit. Where there is evidence of chalk being deposited on a topographic high (e.g. where the base is younger than at the type section, and the overall thickness is less), clean white chalk is present. The Gingin Chalk is overlain abruptly (probably disconformably) by the Poison Hill Greensand (a medium-grained glauconitic sand of late Campanian age). The Toolonga Calcilutite disconformably overlies either Albian, middle Cenomanian, or lower to mid-Turonian units. In the southern region of the Southern Carnarvon Platform, the Toolonga Calcilutite is capped by calcrete. Further north, the upper Campanian Korojon Calcarenite lies disconformably above the Toolonga Calcilutite.

Lithologically the chalk is remarkably uniform both laterally and vertically, and this is accentuated by intense bioturbation. No hardgrounds have been identified. The chalk is composed mainly of foraminifera in the sand fraction and nannofossils in the mud fraction. A remarkably uniform pattern of frequent appearances and disappearances of foraminiferal species (both benthic and planktonic types) is present. Over 20 foraminiferal datum levels can be used to correlate stratigraphic sections of the chalk. Are these stratigraphic patterns related to environmental (possibly bathymetric) changes and were the changes synchronous over this 1000 km stretch of low-gradient continental shelf? Can any major erosional breaks in the succession be identified that may mark sequence boundaries? Do these breaks coincide with lithological changes? Does the very irregular base of the Gingin Chalk (and possibly the erosional surface on which the Toolonga Calcilutite sits) relate to the Yallalie meteorite impact recognized by M.C. Dentith and co-workers in the Dandaragan region of the Perth Basin?

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SILICICLASTIC CONGLOMERATES ASSOCIATED WITH DEVONIAN REEF COMPLEXES, CANNING BASIN, WESTERN AUSTRALIA. Roger Hocking and Philip E. Playford Geological Survey of Western Australia, 100 Plain Street, East Perth WA 6004 Email: roger.hocking@mpr.wa.gov.au Carbonate-dominated reef complexes of Givetian to Famennian (Middle and Late Devonian) age extend for 350 km along the northern margin of the onshore Canning Basin in Western Australia, adjoining Precambrian rocks of the King Leopold Orogen and Kimberley Basin ('Kimberley Block'). Two major phases of reefal development are recognized, the retreating Givetian and Frasnian Pillara Sequence, and the advancing Famennian Nullara Sequence. Siliciclastic conglomerates are associated with both sequences, and interfmger with reefal carbonates in discrete platform, marginal-slope, and basin settings to form a complex tapestry of carbonate and siliciclastic deposition.

Conglomerate depocentres were active at different times and in different settings along the shelf, with pronounced pulses in the late Frasnian and early Famennian. The conglomerates are interpreted as the product of major tectonic pulses rather than eustacy for two related reasons.

Firstly, individual conglomerate bodies span two to three million years or possibly more, because some extend over at least forty metre-scale (?Milankovich-duration) cycles in associated platform carbonates. Secondly, in a traditional sequence-stratigraphic model, clastic progradation should occur during lowstands when accommodation is less, forming lowstand fans in basinal settings. In contrast, siliciclastic conglomerate bodies on the Lennard Shelf are found in platform, marginalslope and basin settings, and span a much longer time. Progradation of both reef platforms and associated conglomerates appears to have been greatest during highstands.

Conglomerate bodies are mostly located at major rectilinear notches in the basin margin, reflecting major drainage outlets from the Precambrian hinterland. A greater abundance of conglomerate bodies on the southeast Lennard Shelf may reflect greater tectonism near the intersection of the southeast-trending King Leopold Orogen and the southwest-trending Halls Creek Orogen. The provenance of all conglomerates is the Precambrian Kimberley Basin to the northeast with lesser input from the adjacent, but less resistant. King Leopold Orogen, and even some contribution from the reef complexes themselves. Stylolites in carbonate clasts derived from the reef complexes indicate substantial rapid burial followed by uplift and reworking.

Where associated with platform facies, conglomerate deposition was in alluvial fans to alluvial slope aprons, grading to fan deltas. Conglomerate bodies commonly grade laterally through sandstone into carbonate platform facies. Metre-scale shallowing-upward cycles of sandstone above carbonate are common, and become more calcareous away from the conglomerate depocentres. In places, laterally continuous platform-carbonate intervals extend through conglomerates, marking periods of decreased terrigenous influx. Conglomerates in basin settings were deposited as submarine fans and slope aprons. They interfmger with tongues of distal marginal-slope carbonates, and a more terrigenous, silty marginal-slope and basin facies of the reef complex, the Virgin Hills Formation, coincides with the height of conglomerate deposition in the late Frasnian and early Famennian. There are eight significant conglomerate units. From southeast to northwest these units are the Sparke Conglomerate (late Frasnian, platform and basin), Bobs Bore Conglomerate (early to late Frasnian, basin), Elma Conglomerate (late Frasnian and Famennian, basin and platform), Mueller Conglomerate (no age control, platform to basin hinterland), Barramundi Conglomerate (Famennian, primarily platform). Stony Creek Conglomerate (Frasnian, platform and basin). Big Spring Formation (Givetian to Frasnian, platform), Behn Conglomerate (late Frasnian and Famennian, primarily platform to marginal slope), and Van Emmerick Conglomerate (Givetian - Frasnian and Famennian, platform, marginal slope, and basin).

361

QUATERNARY HISTORY AND TECTONICS OF THE GIPPSLAND SHELF VICTORIA G.R. Holdgate^ SJ. Gallagher\ J.B. Keene^ D. Moore^ S. Shafik^ A.J. Smith\ M.W. Wallace^ 'Earth Sciences, Uni. Melbourne, VICT 3010. ^Earth Sciences, Uni. Sydney, NSW Geological Survey, DNRE, 250 Victoria Pde, VICT 3002.' Strat. Solutions, Geology, ANU, Canberra.

The subsurface Quaternary geology of the Gippsland Shelf has been examined from piston cores, foundation bores and onshore stratigraphic drilling. Shallow sparker seismic and airborne magnetic coverage supplements this data.

Fourteen piston cores obtained on the RV Franklin FR12/98 cruise indicate low^-carbonate medium to coarse sands dominate the upper 1.5-2.5 m of the inner shelf, with a coarse-gravel shell hash at the base. On the middle to outer shelf areas, the carbonate content increases to 70-80% CaCOs. The basal shell hash disconformably overlies cemented limestone containing evidence of aragonite dissolution and weakly developed meniscus cements of micritic carbonate, indicating previous exposure to the vadose zone and meteoric water. Two nannofossil dates from cemented limestone cores indicate ages within Subzone CN14b (dated between --0.26 and 0.47 Ka), and that the upper -1.5-2.5 m is Holocene.

Eight engineering foundation bores for each of the major oil and gas field platforms have been logged to about 150m below seabed. The three principle facies encountered are: a) Facies A (fine grained limestones and limey marls) below 50 m contain 60-80% CaCOs. Clear equant and micritic cements and meniscus textures suggesting subaerial exposure are found between -50-70 m. Benthonic foraminifera below 50 m suggest outer shelf water depths. Nannofossil dating indicates Late Pliocene ages within Subzones CN11-12. b) Facies B (fine-coarse-pebble quartzcarbonate sand) between 10 and 50 m in the inner shelf contain 40-80% CaCOa, grades to facies A in the outer shelf Benthonic foraminifera in Facies B suggest middle to inner shelf water depths.

Nannofossil dates indicate Early-Middle Quaternary ages within Subzones CN13a-14b (~1.95-0.26 Ma), c) Facies C (carbonate poor carbonaceous and micaceous fine quartz sand) occurs as discontinuous lenses from 10 to 50 m below seabed. The sparse benthonic foraminifera are innershelf or Gippsland (euryhaline) Lake conditions.

DNRE magnetic imaging across the Gippsland Shelf and onshore provide details of buried magnetic palaeo river channels and barrier systems. The river systems trend south-southeast from the Snowy, Tambo, Mitchell, Avon, Macalister and Latrobe Rivers across the shelf Sparker seismic profiles recorded during the RV Franklin FR12/98 cruise identifies the magnetic palaeochannels as seismic "smudges" from 20-40 m below seabed, and can be correlated with Facies C lenses, (i.e. are probably Early-Middle Quaternary features). Magnetic palaeo-barrier systems trending south-southwest in the inner shelf and onshore beneath the Gippsland Lakes are orientated at an angle of 20° to the modem 90-Mile Beach trend. Offshore, they stratigraphically equate to the top of Facies A (Middle-Late Pliocene). They extend inland as far as the facies limit for the Pliocene Jemmys Point Formation.

Bore-hole stratigraphy in the adjacent onshore Gippsland Lakes at Giffard, Nuntin, Bengworden and Spermwhale Head reveals the Pliocene barrier sequence occurs from -100 to 120 m below land surface, overlain by a succession of fluvial sand-gravel and lacustrine mud facies. Two muddy facies contain the estuarine bivalve Anadara trapezia, are separated by fluvial gravel-barrier sands facies, representing earlier developments of the Gippsland Lakes - 90-Mile Beach-type barriers.

The bivalve suggests they represent deposition over the last -0.40 Kyrs. Underlying ferruginous cemented pebbly sandstones probably correlate with the magnetic palaeo-river channels and have an Early-Middle Quaternary age, similar to offshore Facies C. The magnetic river channels cut across present day uplifted structures such as the Baragwanath Anticline, suggesting some uplift history post-dates the Middle Quaternary.

362

CRETACEOUS MACROFOSSILS FROM THE RAMU VALLEY AND THE SNAKE RIVER, PAPUA NEW GUINEA PLACE THE NORTHERN TERRANES OF PNG AND THE OWEN STANLEY METAMORPHICS IN GONDWANA. G. Kopi^ I. Abiari\ P.G. Quilty^, T.W. Kilya\ S.Nekitel\ R.H.Findlay\ C.Mortimer\ P.Kia^ ^Geological Survey of Papua New Guinea, Dept. of Mining, Port Moresby, Papua New Guinea ^Dept. Geology, University of Tasmania, Hobart, Tasmania, Australia

Cretaceous macrofossils found at tw^o separate localities in the northern part of mainland Papua New Guinea provide important constraints on terrane analysis in Papua New Guinea as they provide limits to the potential provenance of Papua New Guinea's basement terranes.

Trigoniid species were collected from the zeolite facies Walumer Metamorphics on the northern flanks of the Schrader Range, immediately south of the Ramu Valley (82083E, 94537N, 1:100 000 Sheet 7788, RAIN).

A faunal assemblage containing trigoniids and Inoceramus sp. in turbiditic sandstone beds was collected from the Owen Stanley Metamorphics adjacent to the Snake River (04554E, 922078N, 1:100 000 Sheet 8283, WAU) in Morobe Province.

The Wulamer Metamorhics lie on the northernmost side (outboard) of the numerous terranes which in Cenozoic times accreted to Australia to form the basement complex of the Papua New Guinea Orogen. These inboard terranes include possibly Devonian beds, Permo-Triassic metamorphics and intrusives, and a Jurassic-Cretacous sedimentary series. Should the trigoniids in the Wulamer Metamorphics be confirmed as similar to those in New Zealand and New Caledonia, then the Wulamer Metamorphics and these inboard terranes must have flaked off Gondwana before their Cenozoic accretion to Australia.

The presence of Inoceramus in the Owen Stanley Metamorphics, which extend from the Snake River eastward throughout the Papuan Peninsula, confirms that this large terrane of predominantly medium- to high-grade metagreywacke was derived from Gondwana. Similar metagreywacke sequences are known in New Zealand, New Caledonia and the Antarctic Peninsula. Although Cretaceous shelf deposits, such as may have formed the host substrate for the molluscan fossils at the Snake River, are known in northern Queensland, there are no obvious Cretaceous lithological correlates here that match the Owen Stanley Metamorphics.

363

SEDIMENTOLOGY AND SEQUENCE ARCHITECTURE OF QUATERNARY SEDIMENTS ON THE SOUTH EAST QUEENSLAND CONTINENTAL MARGIN Simon C. Lang\ Duncan A. Lockhart^ and Sherrilea Ramsay^ ^National Centre for Petroleum Geology and Geophysics (NCPGG) and Australian Petroleum Cooperative Research Centre (APCRC), 30-32 Stirling St., Thebarton, Adelaide, 5005. slang@ncpgg.adelaide.edu.au ^Woodside Energy, Perth ^School of Natural Resource Sciences, Queensland University of Technology Quaternary sedimentation on the south east Queensland continental margin from the Gold Coast to Caloundra including Moreton Bay and the adjacent coastal plain has been investigated using a program of high resolution seismic profiling, coring, grab sampling and C^"^ dating. The results presented here are based on the RV Franklin FR15/98 cruise supplemented by numerous small craft based surveys over the last decade. Betw^een Tw^eed Heads and Caloundra, the continental shelf broadens to nearly 25 km wide east of the sweeping siliciclastic barrier shorefaces of the Southport spit and South Stradbroke Island, North Stradbroke Island and Moreton Island duneisland barriers, and the Bribie Island strandplain. The inner shelf coastal prism (to m) is dominantly quartzose siliciclastic shoreface sand deposited by mainly south-to-north longshore drift, whereas the outer shelf is a carbonate hardground and post-glacial transgressive lag scoured by the strong north-to-south East Australian Current (comprising loosely cemented coralline algae, benthic and planktonic forams and glaucony). The mid shelf (between -40 and - 9 0 m AHD) comprises a series of down-stepping, shore-parallel shelf ridges comprising a mixture of quartzose and relict carbonate sand interpreted as falling stage lowstand paleoshorelines (>18 ka). Moreton Bay forms a large back-barrier lagoon fringed by tidal flats, strandplains, estuaries and bay-head deltas along the coastal plain, and large sandy tidal delta complexes entering the bay from the south, east and north. Fringing reefs occur around some of the bay islands. The centre of the bay is a zone of low sedimentation.

The continental margin can be described as a sediment-deficient, wave-dominated continental margin outside Moreton Bay, and a mixed wave-tide or fluvial-tide dominated lagoon and estuary within Moreton Bay. All the major rivers entering Moreton Bay were incised during the last glacial maximum in the late Pleistocene (>18 ka) and there is evidence of older compound fluvial valley fill (gravel and medium-coarse sand) produced by numerous lowstand cycles during the Pleistocene. Subsurface mapping of the incised valleys show that they do not extend out to the shelf edge, with most of the incision occurring into bedrock and the inner shelf coastal prism (<45m of incision). The Nerang, Pimpama, Pine, and Caboolture Rivers as well as the smaller creeks that enter the bay in the south and within the Pumicestone Passage behind Bribie Island, are estuarine due to the relative dominance of tidal processes, and low sediment supply. These classic underfilled incised valley estuaries comprise sediments recording mainly the transgressive systems tract (<9 ka). The Brisbane River and to a lesser extent the Logan River comprise a full sequence, including late Holocene, highstand bay-head deltas in the central and southern parts of Moreton Bay due to locally high sediment supply. However, because of the overall low sediment supply, most of the coast still lies within the late transgressive systems tract. Progradation on the coastal and delta plain by fluvial sediments represent a thin veneer of a highstand systems tract (<6.5 kyBP).

The region forms a useful analogue for petroleum reservoir exploration and development in comparable ancient settings. Key outcomes include the lack of incised valleys on the shelf mainly because sea level did fall below the shelf edge, and the erosion of falling stage shorefaces by shelf currents leading to palimpsest mid-shelf shore-parallel shelf ridges. Tidal- and bay-head deltas represent vast potential reservoirs, but the main problem will be development of regional seal.

Incised valleys may contain fluvial reservoirs at their base sealed by estuarine muds, but lateral seals represent a significant hydrocarbon seal risk.

364

SEDIMENTOLOGY AND GEOCHEMISTRY OF THE JULIA CREEK VANADIUM DEPOSIT, TOOLEBUC FORMATION (ALBIAN), GREAT ARTESIAN BASIN Stephen Lewis^ Gerald Dickens^ Robert Henderson^ Simon Coxhell^ ^James Cook University, Townsville, QLD ^Rice University, Houston, Texas ^Xcell Diagnostics, Perth, WA The mid Cretaceous (Albian) Toolebuc Formation of the Eromanga Basin contains two major facies: Inocemmus coquina and organic-rich shale. Both facies contain significant vanadium concentrations(fresh coquina 0.15%; shale 0.32% V2O5). The formation crops out at the surface along the St. Elmo Structure east of Julia Creek. Weathering processes at this location have removed the organic matter and enhanced the vanadium concentrations (coquina 0.24%; shale 0.42% V2O5). Xcell Diagnostics (formally Fimiston Mining N.L.) has drilled the region and produced a large geochemical dataset for evaluation of the V resource. This dataset was collected for bulk resource evaluation using samples homogenised over drill intervals of 1 m. The "Fimiston dataset" has been extended and re-evaluated employing a suite of new, more comprehensive analyses, to better characterise the geochemical attributes of the Toolebuc Formation and its enclosing stratigraphy.

The data show that vanadium is strongly enriched in the Toolebuc black shale and coquina relative to the underlying and overlying shale units. Although there are lateral variations in vanadium contents within the black shale and coquina units along the St. Elmo Structure, bulk vanadium concentrations for full intersections of the Toolebuc Formation remain surprisingly constant.

Toolebuc Formation intersected by GSQ Manuka 1 drillcore, about 400 kms south of Julia Creek, shows similar vanadium concentrations indicating that the formation, at a regional scale, is a major repository for vanadium. Covariation between total organic content (TOC) and vanadium indicates that this metal was originally concentrated in organic matter, probably bound in porphyrins. A range of other heavy metals, including Cd, Cu, Ni, Mo, Ni, and Zn, show similar systematics.

Extractions from fresh black shale and coquina using hydrochloric and hydrofluoric acids indicate that the vanadium is now predominantly associated with the non-organic components and it is considered to be absorbed onto clay species. Goethite-rich horizons in weathered intervals (thought to be equivalent to pyritic rich bands in the fresh material) show exceptionally high vanadium contents. This relationship indicates that vanadium was remobilised during weathering to become bound in iron oxide/hydroxide phases.

Vanadium concentration by the Toolebuc Formation was facilitated by its accumulation on an anoxic seafloor, with the preservation of organic carbon as a major sedimentary component.

However this circumstance alone is insufficient to account for the observed concentration of this trace metal. The Toolebuc Formation has an extensive distribution and its bulk vanadium content may be calculated from its volume and concentration levels as some 4.7 x 10^^ tonnes, exceeding by an order of magnitude the vanadium contents of all the modem oceans. Unusual levels of vanadium must have applied in the epeiric sea to which the Toolebuc Formation related. This may have been generated by the hydrothermal flux to the global ocean induced by enhanced seafloor spreading activity in mid Cretaceous time or it may relate to a weathering flux from an active volcanic terrane inferred to have existed on the eastern borderland of the Toolebuc Sea.

365

DETERMINATION OF QUATERNARY SEDIMENT SOURCES USING MINERALOGY AND GEOCHEMISTRY IN BELLS CREEK CATCHMENT, PUMICESTONE PASSAGE, SOUTHEAST QUEENSLAND Tania Liaghati, Micaela Preda and Malcolm Cox School of Natural Resource Sciences, Queensland University of Technology, Brisbane, QLD 4001 Bells Creek catchment is located 80 km north of Brisbane in the very northern part of Pumicestone Passage. The catchment area is around 25 km^ and comprises three sub-catchments (Lamerough, Bells and Halls). The area is of low^ relief and is bounded to the west by outcropping sandstone bedrock and to the east by the tidal mangrove-lined shore of Pumicestone Passage.

This study determines the source of sediments within both freshwater and tidal sections of the catchment, using their geochemistry and mineralogy. Also considered are the various processes that have modified the sediments during transportation and deposition, as well as post-depositional processes. Of note are the chemical weathering of primary minerals and the subsequent formation of secondary minerals, such as clays. Due to their physico-chemical properties (e.g. surface area, and internal structure), clays may act as geochemical traps for heavy metals. For example, smectite has high exchange capacity and can adsorb larger quantities of metals than does kaolinite.

Three categories of sedimentary material were analysed: a) recent estuarine sediments, b) estuarine/fluvial floodplain soils, and c) bedrock (Landsborough Sandstone). New samples of sediment were collected manually using PVC tubes driven into the creek bank. The soil and bedrock samples were dried powder from previous drilling and were provided by Queensland Acid Sulfate Soils Investigation Team (QASSIT). In order to obtain geochemical and mineralogical comparisons between estuarine sediments, soils and bedrock material within the catchment, a total of 24 samples were analysed for extractable cations, using aqua regia digestion (IHNOs: 3HC1); the metals analysed for are: V, Cr, Cu, Zn, Pb, As, Fe and Mn (a summary is presented in the table). Mineralogical analysis using X-ray diffraction (XRD) was also carried out.

Samples Recent estuarine sediments (n=6) Soils (n=12) Bedrock (n=6)

V 11-33 1-45 3-41

Cr 4-26 4-47 3-16

Cu <1-6 <1-6 <1-15

Zn 22-68 22-63 23-43

Pb 4-7 3-12 3-19

Fe 4194-40917 392-35105 755-27001

Mn 5-38 2-23 2-21

The most abundant primary minerals identified were quartz (50-80%), and feldspars (1-20%).

Clay minerals such as kaolinite and smectite were also found in significant percentages, while hematite was the only iron oxide present. Smectite was mainly concentrated in upper fresher sections of catchment within alluvial material. However, in downstream sections, smectite has been weathered to kaolinite. Overall, soils of estuarine origin contained higher amounts of kaolinite and smectite comparing to the other two categories. The mineralogy of most samples displayed positive correlation with metal concentrations. Samples with high proportion of secondary minerals (e.g. kaolinite and smectite) contained higher concentration of metals such as V, Cr and Zn, as well as Fe demonstrating the greater capacity of weathered sediments for metal adsorption.

The feldspathic nature of the sandstone is preserved in unconsolidated sediments. In terms of metal occurrence Zn is the dominant trace metal followed by V and Cr. In unconsolidated sediments this overall pattern is preserved, with the exception of Cr, which in some areas is more abundant than V. This occurrence may be due to the overall immobility of Cr. Considering the mineralogical and geochemical data from this study, the Landsborough Sandstone is the primaiy source of unconsolidated sediments.

366

HIGH RESOLUTION SEQUENCE STRATIGRAPHY OF A QUATERNARY INCISED VALLEY: FROM COASTAL PLAIN TO CONTINENTAL SHELF Duncan A. Lockhart and Simon C. Lang

Southern Moreton Bay and the adjacent continental shelf have provided a natural laboratory in which to describe the sedimentology, sequence stratigraphy and coastal evolutionary processes of a wave-dominated coastline. The incision of the Logan/Albert fluvial system has been mapped across the coastal plain, through the lagoon and under the inner to mid-continental shelf The valley system is absent from the outer continental shelf due to a combination of extremely low gradient, a hard substrate and reduced stream power during the relatively arid glacial periods.

The Logan/Albert River valley was incised as a result of multiple low sea level events associated with glacial maxima in the Quaternary. In the Logan/Albert River valley three sequences have been identified on the basis of core logging and C^'^-AMS dating. Sequence boundaries within the upstream-incised valley appear to be coincident with the development of force regressive, shelfperched, early lowstand wedges on the continental shelf adjacent to the southern Moreton bay area.

Each of these shelf-perched sequences represents coastal plain deposits forming at a stage of sediment by-pass in the upper reaches of the fluvial valleys.

A continuous record of valley fill from late lowstand to highstand during the last post-glacial transgression has been recovered by deep coring within the Logan/Albert River incised valley.

Within the incised valley polymictic gravels and quartz-lithic sands interbedded with overbank deposits of mud and silt were deposited within a late lowstand to early transgressive setting as fluvial point-bars confined to the valley thalweg. These deposits have been dated at 13,650 ±60 yBP (Beta-104824). The transgressive surface overlying these deposits has been dated at 9,130 ±60 yBP (Beta-104825). It is recognised by the first presence of estuarine faunal assemblages while the quartz-lithic and poorly sorted character of the sediments remains unchanged. High amplitude, concordant reflectors confined to the greater valley walls overlie the estuarine sands. These reflectors represent laminated estuarine mud deposited during the early to mid-transgressive phase of sedimentation. The thickness of these mud-dominated sediments in conjunction with a rapid sediment accumulation rate (approximately 11 mm/year) is a result of the creation of extensive accommodation space associated with the rapid transgression experienced on the east Australian coast during the period (up to 2.5 m/100 years).

A highly erosive tidal ravinement surface represents the onset of deposition of quartzose marine sands within a large flood tide delta complex associated with late transgressive to early highstand conditions. The last post-glacial transgression reached its peak at 6.5ka approximately 1.5 m higher than present sea level. Intact Anadara trapezia shells recovered fi-om a depth of 3 m adjacent to the +2.5 m topographic contour on the coastal plain gave Amino Acid Ratio values consistent with an age of approximately 6.5 ka indicating that the entire Pimpama Coastal Plain was inundated at the peak of the transgression. Shells recovered from core at a depth of 6.1 m on the coastal plain were dated at 4,780±60 yBP (WK-7663). Given the thickness of the overlying section it is apparent that the area was still part of an active flood tidal delta setting well beyond the peak of sea level at 6.5 kyBP. The current highstand is recognised seismically in the southern Moreton Bay area as a downlapping unit in the vicinity of the current bayhead delta of the Logan River. Sediments associated with the bayhead delta of the Logan River are muddy, quartz-lithic sands and estuarine mud. Massive marine quartzose sands from the present-day flood tide delta associated with the Jumpinpin tidal inlet also contribute to highstand sedimentation.

367

SURFICIAL SEDIMENTS OF THE BROWSE AND BONAPARTE BASIN TRANSITION ZONE. Kriton C. Glenn^ and G.W. O^Brien^ ^Geology and Geophysics Department Adelaide University Adelaide SA 5001 ^National Centre for Petroleum Geology and Geophysics, Adelaide University Adelaide SA 5001 The transition zone of the Brow^se and Bonaparte Basins is an established commercial hydrocarbon province. It is also located at the confluence of the Indian and Pacific Oceans on one of the worlds largest continental shelves. Depths are typically 70 - 100m with several reefs and/or islets rising to 4m above mean sea level. One of the striking bathymetric features of the transition zone is The Penguin Deep, a structurally controlled channel system running east-west from the Bonaparte Basin to the Browse Basin at the northern end of the Yampi Shelf. This terminates in a submarine delta sequence which has three distinct prograding sediment packets located in deeper water in front of the submerged wave-dominated delta front. Thirty two sediment samples from a series of nine gravity cores and a suite of fifty-seven sea floor samples from the Penguin Deep and associated delta feature have been assessed for grain size, TOC TIC and biogenic make-up. Initial observations of the sediments show that they comprise mainly reworked carbonates together with minor terrigenous sediments and a high biodiversity/low populations of modem benthic foraminifera.

368

MASSIVE SILICICLASTIC FLUX TO THE QUEENSLAND TROUGH DURING TRANSGRESSION: A MIXED SILICICLASTIC/CARBONATE MARGIN'S RESPONSE TO LATE QUATERNARY SEA-LEVEL CHANGE, CENTRAL GREAT BARRIER REEF PROVINCE, NORTHEAST AUSTRALIA M.C. Page^ and G. R. Dickens^'^ ^School of Earth Science, James Cook University, Townsville, Qld Australia 4811 ^School of Earth Science, Rice University, Houston, TX USA 77005

The northeast Australian margin is the largest extant example of a tropical mixed siliciclastic/carbonate depositional system. Substantial fluvial inputs from the Australian landmass discharge onto a broad shelf along v^ith major carbonate production associated w^ith the Great Barrier Reef Complex. Previous work has identified 0.5 to 2 m thick siliciclastic-rich intervals in cores from the slope of this margin. In line w^ith generic depositional models for mixed siliciclastic/carbonate systems, these intervals have been interpreted by several authors as reflecting high off-shelf fluxes of terrigenous sediment during sea-level low^stand or, alternatively, as intervals of carbonate starvation (e.g. Harris et al., 1990; Kronen and Glenn, 2000). However, two studies (e.g. Peerderman and Davies, 1993; Dunbar et al., 2000) have questioned these interpretations, instead suggesting these intervals represent massive siliciclastic input to the slope during transgression, a view wholly inconsistent with genetic models for the evolution of continental margins.

Here we present new results from five widely spaced cores on the slope of the northeast Australian margin. Consistent with previous investigations, we find ca. 0.25 to 1 m thick siliciclastic-rich intervals in all cores. Utilising high-resolution records of bulk carbonate concentration and AMS radiocarbon dating of planktonic foraminifera, we also clearly show that the siliciclastic-rich intervals in all five cores were deposited between ca. 11 and 7 ka and represent up to a six-fold increase in off-shelf siliciclastic fluxes during late transgression. In complete contrast to model expectations, massive quantities of siliciclastic material were discharged from the shelf to the slope and basin after sea level transgressed the shelf.

Widely accepted models for sediment deposition are fundamentally flawed on this margin because either (1) siliciclastic and carbonate components interact on a broad shelf so that subaerial carbonate (karst) hills on the outer shelf cause river avulsion and sediment storage during lowstand, or (2) a prominent sea level stillstand(s), perhaps coincident with the Younger Dryas, led to extensive shoreface erosion and removal of sediment on the outer shelf.

References Dunbar, G.B., Dickens, G.R. & Carter, R.M. 2000. Sediment flux across the Great Barrier Reef Shelf to the Queensland Trough over the last 300 ky. Sedimentary Geology 133, 49-92.

Harris, P.T., Davies, P.J. & Marshall, J.F. 1990. Late Quatemary sedimentation on the Great Barrier Reef continental shelf and slope east of Townsville, Australia. Marine Geology 94, 55-78.

Kronen, J.D.Jr. & Glenn, C.R. 2000. Pristine to reworked verdine: keys to sequence stratigraphy in mixed carbonate-siliciclastic forereef sediments (Great Barrier Reef) Marine Authigenesis: From Global to Microbial SEPM Special Publication No. 66, 387-403 Peerdeman, P.M. & Davies, P.J. 1993. Sedimentological response of an outer-shelf, upper-slope sequence to rapid changes in Pleistocene eustatic sea level: Hole 820A, Northeastem Australian Margin. In: McKenzie, J. A., Davies, P. J., Palmer-Julson, A. et al. eds. Proceedings of the Ocean Drilling Program Scientific Results 133 pp. 303-313. Ocean Drilling Program, College Station, Texas.

369

THREE-DIMENSIONAL COMPUTER MODELLING OF SEDIMENT DEPOSITION ON THE CENTRAL-SOUTHERN ENDERBY TERRACE, CARNARVON BASIN, WESTERN AUSTRALIA Evelina Paraschivoiu^ Cedric Griffiths^, Chris Dyt^ and Simon Lang^ 1 National Centre for Petroleum Geology and Geophysics, The University of Adelaide, SA 5005 2 CSIRO Division of Petroleum Resources, PO Box 1130, Bentley WA 6102 Fonvard stratigraphic modelling is being increasingly used for reconstructing recent and ancient sedimentation at various scales, in depositional environments ranging from fluvial and nearshore to shelfal and deep marine. The 3D depositional modelling program 'Sedsim' that has been used in this study simulates the transport, deposition and erosion of uncompacted sediment on a bathymetric surface. The algorithm employed is based on numerical approximations to fluid flow^ equations, while the hydraulic regime governing the fluid flow is controlled by a range of geomorphologic, climatic, tectonic and oceanographic factors. The simulation output, at each grid node, consists of a succession of equal time layers each characterized by the thickness of each of the sediment grain size classes used as input.

An area of 50x70 km on the central-southern Enderby Terrace, Carnarvon Basin, Western Australia, provided the setting for a modelling study that re-played sedimentation and erosion over a geological time interval spanning 2.5 million years in the Early Cretaceous. The modelled sedimentary volume represents a marine siliciclastic succession, formed in depositional environments ranging from proximal nearshore to basin-floor settings, over a period of overall marine transgression during the post-rift phase in the evolution of the continental passive margin.

The Sedsim input was based on geological interpretation of 2D and 3D seismic profiles, well data from around 30 wells, high-resolution sea-level curves, tectonic activation history, modem analogues, and palaeo-climatic reconstructions. A range of assumptions and scenarios regarding the palaeo-geographical conditions during the simulated time were assessed, as well as different factors and parameters influencing sedimentation, in order to better understand the causes of rock heterogeneity. The outputs from the simulation include 3D interactive display of sediment grain size, facies and distance from shore, 3D chronostratigraphy and 2D maps of sediment thickness, sand percentage and palaeo-topography. The results were matched against seismic stratal geometries and architecture, and compared to lithological profiles available in the control wells.

The Sedsim model reproduced the known geometry of the sediment bodies and the location of stratigraphically significant surfaces within the sedimentary succession. Practical questions relevant to oil exploration could be answered: Where are the sands located? What are the relationships between various sand bodies? How extensive and interconnected are the various facies? What are the location and extent of sealing intervals in relation to possible reservoirs? Where and how are the flow barriers distributed? By quantifying the conceptual depositional model we were able to generate a predictive model of facies distribution for the inter-well volume, with implications for the petroleum exploration in the Enderby area. These results are being further used as the basis of a reservoir-scale depositional model over the Stag oil field - to predict and quantify stratigraphically controlled facies heterogeneity below seismic resolution.

370

OLIGO-MIOCENE SUBMARINE CANYONS IN THE GAMBIER SUB-BASIN, SOUTHERN AUSTRALIA Rosalie Pollock

Qianyu Li^ Brian McGowran^ Simon Lang^

^ Geoscience Australia, Petroleum & Marine Division, GPO Box 378, Canberra, ACT 2601 ^ National Centre for Petroleum Geology and Geophysics, Adelaide University, Adelaide, SA 5005 ^ Department of Geology and Geophysics, Adelaide University, Adelaide, SA 5005 The Gambler Sub-basin of the western Otway Basin lies on the passive continental margin of southern Australia, w^hich was shaped during the breakup of eastern Gondwana between the Early Cretaceous and Palaeogene. We have known for decades that the modem margin is incised by submarine canyons arrayed from the Perth to Gippsland and Sorell Basins that formed by some combination of tectonically generated morphology and late Neogene glacio-eustasy. Also known for some time is the existence of palaeo-canyons in the Gippsland Basin, which seem to be strongly clustered chronologically within the Eocene siliciclastics and Middle Miocene carbonates (McGowran, 1979).

In addition to the numerous modem submarine canyons reported on the southem Australian margin, three palaeo-canyon systems have been identified within the Gambier Limestone of the South Australian Gambier Sub-basin. These canyon systems were initiated near the end of the Early Oligocene, and filled with Late Oligocene or younger sediments. Compared to the Early Miocene canyons identified in the eastem Otway Basin by Leach and Wallace (2001), the Gambier canyons are new as to both their locale and their time of formation. Their interpretation is based on 2D seismic data and well data including biostratigraphy.

Favourable environmental conditions during the Oligocene and Early Miocene led to deposition of the Gambier Limestone, a widespread, prograding extra-tropical carbonate platform. A world-wide glacio-eustatic sea level fall in the Early Oligocene exposed the shelf in the Gambier Sub-basin causing widespread erosion and minor fluvial incision on the shelf and subsequent formation of "nick points" at the shelf edge. During the following marine transgression later in the Oligocene, the shelf was inundated and the "nick points" provided conduits for erosive turbidity currents to enlarge the canyons to the spectacular dimensions (up to 615 m deep and 5 km wide) observed on seismic data. No less than 20 successive, laterally migrating canyon cut and fill events within the same location as the initial canyon and ranging from Late Oligocene to Middle Miocene have been observed and mapped on seismic data across the shelf in the Gambier Sub-basin.

The thick, dominantly fine-grained carbonate sheet logically represents a potential regional seal to underlying clastic reservoirs. However, the possibility exists for carbonate reservoir sands to be present within the palaeo-canyons, sealed by surrounding fine-grained carbonates. Although no hydrocarbons have yet been identified in the carbonates of the Gambier Sub-basin, the canyons provide an analogue useful for establishing the scale, intemal architecture and geometry of canyon fill systems.

References Leach, A.S. and Wallace, M.W. 2001. Cenozoic submarine canyon systems in cool water carbonates from the Otway Basin, Victoria, Australia Eastern Australasian

Basin Symposium, November 2001,

Melbourne,

Victoria, 465-473.

McGowran, B. 1979. The Australian Tertiary: foraminiferal overview. Marine Micropalaeontology, 4, 235264.

371

SEQUENCE STRATIGRAPHIC MODEL OF A PALEOGENE TEMPERATE/SUBTROPICAL CARBONATE-SILICICLASTIC CONTINENTAL SHELF SUCCESSION IN A BOUNDARY CURRENT SETTING, NORTH CAROLINA, USA ^Brian Coffey and ^J. Fred Read ^Exxon-Mobil, Houston, Texas ^Department of Geological Sciences, Virginia Tech, Blacksburg, Virginia 24061 Thin sectioned well cuttings through the 0 to 450 m thick Paleogene succession of North Carolina was integrated with available seismic data, to generate a sequence stratigraphic model for these mixed carbonate-siliciclastic continental shelf units. Paleogene deposition was initiated by Late Cretaceous deep submergence of the shelf, and generated a distinctive profile consisting of a shallow inner shelf, inner shelf break, deep shelf and shelf-continental slope break. The Paleogene succession overall consists of deep shelf marls overlain by bryozoan carbonates, that are overlain in turn by siliciclastic units. The succession contains several supersequences made up of smaller scale sequences.

During supersequence lowstands, the inner shelf may have been exposed, and lowstand systems tracts evident on the seismic are lobe-shaped, possibly siliciclastic units. Extensive hardgrounds formed at supersequence boundaries, whereas on the upper continental slope, erosion by the ancestral Gulf Stream occurred.

Transgressive systems tracts of sequences on the inner shelf are bryozoan-echinoderm-facies that backstep onto shallow shelf quartzose molluscan facies. On the deep shelf, the ancestral Gulf Stream remolded fine-grained sediment units.

Highstand systems tracts on the inner shelf consist of fine wackestone-mudstone and downdip planktonic and spiculitic marls, that are overlain by progradational bryozoan-echinoderm grainstone and packstone units. In near-shore locations, sea level fall caused these to be overlain by prograding back-barrier silts and shell beds, coastal sands, and sandy molluscan shoreface deposits. On the deep shelf during highstand, there was intense remolding of fine-grained sediment bodies into elongate shelf-parallel marl lobes by the ancestral Gulf Stream and local complete removal.

The warm water biotas (mollusks, bryozoan species, larger forams) suggest that these bryzoan shelf carbonates were warm temperate to subtropical rather than cool water, which probably reflects their more southerly position relative to present day, the warm ancestral Gulf Stream, and overall warmer climates of the Eocene in the region.

372

TASMANIA BASIN REVIEW AND BRYOZOAN FAUNA - APPLICATION IN BIOSTRATIGRAPHY Catherine Reid School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001.

Permian marine rocks in the Tasmanian Basin contain rich bryozoan faunas that can provide a useful biostratigraphy for the basin. Despite a varied lithostratigraphic nomenclature through the basin, in most areas the same general lithostratigraphic sequence is found. Basal, usually nonfossiliferous tillite (Wynyard/Truro Tillite) and glendonitic mudstone (Woody Island/Quamby Formation) are overlain by fossiliferous marine mudstone (Bundella Mudstone/Golden Valley Group), with minor thin limestone. A non-marine sequence (Faulkner/Liffey Formation) follow^s and is overlain by fossiliferous marine rocks. In southern Tasmania these beds develop into thick fossiliferous limestone and siltstone (Cascades Group). Above these limestones are variously fossiliferous sandstone and siltstone (Malbina Formation). Northwards a depositional hiatus precedes the onset of calcareous marine beds, with the overlying fossiliferous sandstones much thinner than southern correlates. Throughout the basin the uppermost marine beds are poorly fossiliferous bioturbated mudstone and siltstone (Femtree Mudstone/Bogan Gap Group) with conglomerate horizons.

Permian Bryozoa from the glaciomarine Tasmania and southern Sydney Basins, Australia, are reassessed following the taxonomy of Morozova (1974) and Morozova and Lisitsyn (1996) and are described using the descriptive parameters of Snyder (1991). Previously the fenestrate faunas of these regions have not been examined internally. The Permian bryozoan faunas of the Tasmania Basin are abundant, but of low diversity, with cosmopolitan genera and many endemic species.

Taxa are mostly from the Fenestrata and Trepostomata, with rare Cryptostomata and Cystoporata.

Thirty-four species (twenty-two new) are recorded in the genera Levifenestella, Rectifenestella, Mackinneyella, Parapolypora, Paucipora, Polypom, Polyporella, Pseudopolypora, and Shulgapora (Fenestrata); Dyscritella, Dyscritellina, Paralioclema, and Stenopora (Trepostomata); Streblotrypa (Cryptostomata). The fauna shares cosmopolitan species and genera with the Sydney Basin. The occurrence of Cryptostomata and Cystoporata is limited in comparison to the faunas from Queensland, Western Australia and Productus Creek, New Zealand.

Within fenestrate faunas of the Tasmania Basin, internal examination has revealed a number of species and genera that were previously grouped together as long-ranging species of variable mesh dimensions. In the Tasmania Basin a useful bryozoan biostratigraphy has been developed from the Sakmarian to the Kazanian. Five Bryozoan Faunizones (A-E) are proposed, and are correlated with the macro invertebrate Faunizones of Clarke and Banks (1975). Each bryozoan faunizone is defined by a group of species, and their abundance, rather than first and last appearances of single species. \ ^ i l e preparation time for bryozoan study is significant, the ability to use small fragments allows for their application in biostratigraphic studies using drill-core material, where macroinvertebrate groups cannot provide reliable and accurate biostratigraphic information.

References Clarke M. J. and Banks M.R. 1975. The stratigraphy of the lower (Permo-Carboniferous) parts of the Parmeener Super-Group, Tasmania. In: Campbell K.S.W. ed. Gondwana Geology. Proceedings of

the

International Gondwana Symposium, pp. 453-467.

Morozova I. P. 1974. Revision of the bryozoan genus Fenestella. PaleontologicalJoumal 8, 167-180.

Morozova I. P. and Lisitsyn D.V. 1996. Revision of the Genus Polypora M'Coy, 1844. Paleontologicheskii

Zhurnal 4,38-47.

Snyder E. M. 1991. Revised taxonomic procedures and paleoecological applications for some North American Mississippian Fenestellidae and Polyporidae (Bryozoa). Palaeontographica Americana 58, 275 p.

373

WAVES, CURRENTS AND CONTINENTAL SHELVES ~ NORTHERN NEW SOUTH WALES Jason Roberts and Ron Boyd School of Geoscience, University of Newcastle, NSW 2308 Northern NSW marks the beginning of uninterrupted, longshore sand transport culminating in the large offshore sand islands of southern Queensland, e.g., Moreton and Fraser Islands. Shelf morphology since sea level stabilised approximately 6500 yrs ago has been largely determined from the interactions of high wave energy, the southvs^ard flow^ing East Australia current, antecedent geology and longshore sediment transport, processes inferred from Quaternary sedimentology.

The integrated data set for this study comprises 12 shallow^ penetrating vibracores, analogue sidescan sonar and 800 km line length of high-resolution seismic reflection profiles. Incorporation of existing vibracore descriptions (50) and 20 carbon-^"^ ages within a GIS framework has allowed an accurate facies model to be constructed, reflecting the scale dependant factors of formation.

Highstand sedimentation during oxygen isotope stage 1 is best understood within a hierarchal spatial framework. At the first-order scale (10-20 km^), the margin consists of a divided innershelf clastic and outer-shelf carbonate-rich facies, until the shelf break is reached between 90 and 125 m. Sediment below this depth is a distinct calcite mud-rich upper slope facies. Second-order (1-10 km^) classification includes storm and current-produced graded bed facies. Surficial bedforms within these facies include: featureless uniform coarse sand, <1 m amplitude fine to coarse sand waves, gravel-filled scours and 1-2 m diameter crescentic scours in the nearshore, shoreface and outer shelf plain. Bedforms and facies succession produced from wave and current energy is developed on a mobile, shallow, sediment-deficient substrate with intermittently exposed bedrock.

These highstand surficial shelf sediments are derived from two distinct provenances. Firstly, inner shelf sediments are rich in coarse quartz and feldspar sands with lesser carbonate gravel. A variable lithic component is sourced from the adjacent Paleozoic New England Fold Belt and Mesozoic Clarence-Moreton basin, as are the locally abundant inner shelf heavy mineral deposits.

In contrast the clay-sized fraction is dominantly carbonate (low and high-Mg calcite and aragonite). Secondly, outer shelf sediments are carbonate-dominant, fine to coarse sand and gravel, derived from a diverse assemblage of molluscs and bryozoans. A strong biological-facies association is noted in these surficial shelf sediments, where the inner- and mid-shelf contain disarticulated bivalves sourced from nearshore and estuarine environments. With increasing water depth sediments contain a higher proportion of gastropods and diverse zoarial (family) forms of bryozoans (25wt%) atypical of other Australian shelves. The outer shelf sediments also contain a mixed assemblage of abraded shallow water species and deeper water organisms, which may reflect current across shelf transport processes and/or relict biotic detritus from times of previously lower sea level. Upper slope sediments further comprise a distinct facies of mud-rich fine-medium calcite sand (45-55 w1;%), sourced mainly from bryozoans, and sponge spicules. Concurrent with this change in composition is a significant decrease in mean grain size.

The Late Quaternary continental margin of northern NSW is a high energy, sediment-deficient low accommodation setting. Preserved on the shelf are stage 2 nearshore and estuarine transgressive deposits with overlying stage 1 continental shelf sediments. These estuarine and nearshore deposits consist of strongly bioturbated interbedded sands and muds. Separating these is a sharp-based wave ravinement surface produced during transgression of the entire shelf Vertical shelf facies successions comprise featureless graded beds and uniform sands, reflecting both first and second order processes. The low accommodation character of the margin is reflected in the minimal vertical extent (typically under 15 m) of transgressive and highstand deposits. Further, surficial sediments overlie carbonate-cemented clastic hardgrounds or Paleozoic basement. Thus stratigraphic development reflects the primary control of sea level, first order wave and current processes and the antecedent geology. The high-energy wave ravinement surface produced during the post-glacial marine transgression has traversed the entire shelf, forming a sharp-based storm bed or gravel lag.

374

RECONSTRUCTING THE LIFE OF A QUARTZ GRAIN Bryan P. Ruxton Division of Science and Design University of Canberra ACT 2601 The etching or encrusting of mineral grains may reflect their provenance and their age. Here a sequence of several stages is proposed in the life of an etched, embayed, otherwise euhedral quartz grain in a partly aeolian seam occurring between colluvial units on the lower slopes of a mountain in Hong Kong.

An original quartz phenocryst in an acid volcanic tuff was the high temperature beta form which on cooling reverted to the alpha form with a volume shrinkage of 0.86 per cent. The tuff was exposed, weathered and the quartz grain released. It was transported and deposited in a marine mud during a high sea level stand in the Hong Kong Harbour area. While in the alkaline environment some of the quartz was dissolved from an enlarging embayment and redeposited as the lower temperature alpha form around the rim. Later the marine mud was exposed to the atmosphere and the content of pyrite oxidised giving rise to an acid sulphate soil. On further weathering the always alpha quartz was more resistant than the former beta quartz and jutted out from the terminal face. The dissolution constant k for the alpha quartz is believed to be around mole m"^ s"^ compared with ' mole m" s" for the former beta quartz.

After desiccation of the soil during falling sea levels the tiny (0.1 mm) quartz grain was air lifted onto the lower slopes of Victoria Peak and deposited on the surface of a debris flow. An associated glass grain with a hydrated rim would suggest an age of around 52,000 years for this event. Since then a shallow indent has been etched on the embayed surface of the quartz grain. Some 830 |Lig Si02 (13.8 jumole) has been removed from a cross sectional area of 70 giving a dissolution rate constant for this former beta quartz as mole m"^ s'\ This dissolution rate is similar to that of the quartz in a quartz diorite saprolite in Puerto Rico where the mean annual temperature is the same as that in Hong Kong (22''C). Both sites are, or have been, covered with tropical rainforest.

375

REGRESSIVE SYSTEMS TRACT CYCLES RECORDED WITHIN SHALLOWMARINE AND NON-MARINE FACIES FROM THE ONSHORE CANNING BASIN, WESTERN AUSTRALIA Rhonda Adkins School of Earth Sciences, James Cook University, Townsville, QLD 4810 Email: Rhonda.Adkins@jcu.edu.au The mid-Permian Tuckfield Member of the Poole Sandstone is exposed around the periphery of the St. George, Poole, and Grant Ranges of the onshore Canning Basin, Western Australia. In the St. George and Poole Ranges, the Tuckfield Member outcrops as a 50 to 100 metre thick package of coarsening- and thickening-upward cycles. These cycles consist upward of siltstone, sandstone, and conglomerate, with minor amounts of silty mudstone at the base of some cycles. In the Grant Range, the Tuckfield Member outcrops as a less than 50 metre thick package of fining- and thinning-upward cycles. These cycles consist upward of sandstone, siltstone, and silty mudstone.

In the Grant Range, abundant mud pellets and mud-pellet conglomerates occur throughout the interval. Primaiy sedimentary structures, trace fossils, and the vertical succession of facies suggest that the Tuckfield Member was predominantly deposited in a foreshore environment within the St. George and Poole Ranges, and on a sand-dominated coastal plain within the Grant Range.

At the outcrop scale, cycles in all three ranges vary in thickness from 1 to 10 metres and dictate the geomorphological features of the area. Laterally continuous benches form at the tops of cycles in the St. George and Poole Ranges. In the Grant Range, benches are also present. However, they are not laterally continuous and, unlike in the St. George and Poole Ranges, they occur mid-cycle instead of at the tops of cycles.

Gamma-ray and grain-size data, collected from outcrops in all ranges, were analysed to better define cyclicity within the Tuckfield Member. Using both of these data-sets, several orders of metre- to decametre-scale cycles can be identified in both shallow-marine and non-marine (coastal plain) facies. Cycles from both depositional environments correlate strongly with the known midPermian Milankovitch periodicities of eccentricity, obliquity, and precession. This indicates a glacio-eustatic driving mechanism as the probable cause of both the shallow-marine and nonmarine cycles.

376

ARE SLOPE CYCLOTHEMS DELIMITED BY SEQUENCE BOUNDARIES OR CORRELATIVE CONFORMITIES? ODP SITE 1119,395 M WATER DEPTH, CANTERBURY BIGHT, EASTERN NEW ZEALAND: ^ Carter. R. M., ^ Gammon, P., ^ Millwood, L. and Graham, I. ' James Cook University, Townsville, ^ University of Adelaide, Adelaide, ^ University of Texas, Arlington, ^ New Zealand Institute of Geological & Nuclear Sciences, Lower Hutt

Site 1119 is located on the upper slope of the Canterbury Bight, 5 km seawards of the modem 150m-deep shelf edge, in a vs^ater depth of 396 m, and almost 100 km from shore. The geographic situation of the site during glaciations w^as profoundly different, w^hen it was located in a palaeowater-depth of -266 m and only 15 km seawards of the lowstand shoreline. Sediment supply to the site therefore differs strongly between lowstand and highstand. Today, during the Holocene highstand, detritus from the mountainous Southern Alps is entrained in a northward-moving, coastal sediment system, and the seabed at site 1119 is relatively starved of terrigenous input. In contrast, during lowstands terrigenous sediment was supplied directly to the upper slope by glacially-fed rivers. Site 1119 was drilled in part to establish the stratigraphic signature of this sediment duopoly.

Site 1119 penetrated 495 m of mud punctuated by 0.5-2 m thick intervals of well-sorted very fine sand. The hole terminated in sediment of late Pliocene (-3.0 Ma) age, which, allowing for unconformities, equates to a long-term average sedimentation rate - 2 8 cm/ky. Seismic profiles show that the upper 84 m of site 1119 represents deposition as part of the shelf-edge foreslope sediment wedge, whereas the deeper parts of the succession were deposited as AAIW sediment drifts. The foreslope wedge comprises two sedimentary cycles (sequences) which correspond to marine oxygen isotope stages (MIS) 7-1. Interglacial MIS 7, 5 and 1 are represented by thin intervals of sharp-based (with Chondrites burrows), shelly {Tawera, Neothyris), olive-brown sand, which grade up into MIS 6 and 4-2 glacial sediment which is massive, bioturbated, micaceous, grey mud with rare bathyal molluscs {Zygochlamys delicatula). Using a depositional time-scale based on measurements of nannoplankton zonation, grey-scale intensity (proxy-carbonate %), oxygen and carbon isotope ratios, and ®Be abundance, sedimentation rates reached a maximum of >150 cm/ky during the MIS 3 stadial (-60-30 ka). A sharp pulse of enhanced sedimentation accompanied the last post-glacial ice-melt between -17-15 ka. In contrast, interglacial sands and silts accumulated an order of magnitude slower, commonly at rates <10 cm/ky. These sedimentation rate differentials are consistent with ^^Be enrichments of up to two orders of magnitude during the slow deposition of interglacial sands.

The two foreslope sedimentary rhythms at site 1119 (MIS 7-6 and 5-1) comprise a diastemdelimited, bipartite, sand-silt sequence motif that is already known from the onland PlioPleistocene Mangaopari Basin, New Zealand (Gammon, 1997). That the Mangaopari cycles are on average only 3-7 m thick relates to a lower sedimentation rate rather than marking a fundamental difference in type. The site 1119 results show (i) that the burrowed "base" of each package of interglacial sand corresponds to times of sharp sea-level rise, i.e. equates with transgression on the shelf rather than being a sequence-boundary-equivalent; (ii) that the correlative conformity to the MIS 2-1 sequence boundary is not expressed as a diastem, but corresponds to a 2.5 m thick interval of mud (deposited slowly, at - 2 0 cm/ky) which immediately precedes the high sedimentation rate pulse (-150 cm/ky) caused zby post-glacial melting; (iii) that sedimentation rates change drastically, often by more than an order of magnitude, between warmer (higher sea-level) and colder (lower sea-level) times, and (iv) during the MIS 5-1 cycle, the highest sustained sedimentation rates occurred at the intermediate sea-level of the MIS 3 stadial.

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BUNDALEER: DEMONSTRATING THE VALUE OF DETAILED MAPPING IN THE ADELAIDE GEOSYNCLINE Wayne M. Cowley Office of Minerals and Energy Resources, PIRSA The recently completed Bundaleer 1:50 000 digital geological map is the last in the revision of the BURRA 1:250 000 sheet in the Adelaide Geosyncline north of Adelaide. On much of BURRA, Neoproterozoic sediments are extensively concealed below soil cover; thus painstaking, detailed mapping is required in order to substantially improve on the pre-existing BURRA mapping. There are two main reasons for this: firstly, with the increasing emphasis on genetic stratigraphy principles in the understanding and subdivision of these sediments, sequence boundaries assume greater importance, but they can be located, and their true nature gained, only from close field inspection. In any case, poor exposure limits the mapping of such horizons away from a few key localities and air photos frequently display few geological features. Secondly, geophysical techniques are of limited use in these non-magnetic sediments; and spectral remote sensing images are dominated by the pattern of paddocks in this agricultural area. Radiometric surveys, with response being largely independent of the human imprint, cover the region but have neither the spatial resolution nor the careful processing required to extract a lithological signal at the detail required for correlation.

In the broad syncline northeast of Spalding, mapped only as Sturtian Tapley Hill Formation (Umberatana Group) on BURRA, new mapping has revealed overlying Marinoan sediments of the Upalinna and Yerelina Subgroups and Wilpena Group up to Ulupa Siltstone in the synclinal core.

The basal Marinoan unit here, the Cox Sandstone Member, hosts stratabound gold deposits at Mongalata, 45 km southeast (11 000 oz) and at Waukaringa, 140 km northeast (60 000 oz), but is totally unexplored on Bundaleer as well as in other parts of BURRA where the Cox was previously unrecognised. As most gold deposits in the region are found in structural positions in sandy units at fold closures or flexures, the folded Yerelina Subgroup (Gumbowie Arkose and Grampus Quartzite) in the hinge of this syncline is also worthy of investigation; the Pitcaim and Dustholes Mines, about 85 km to the northeast, are examples at this stratigraphic level.

The Spalding Inlier on southwestern Bundaleer comprises complexly folded and faulted Willouran rocks (River Broughton Beds) which have been elucidated only through detailed field mapping.

The inlier comprises limestone, dolomite, carbonaceous siltstone and lesser sandstone, with rare possible mafic and acid tuffaceous sediment. Prominent are bodies of dolerite of uncertain age, some as irregular stocks and some as sills, and with hydrothermal alteration; finer-grained mafic bodies with interpreted pillow structures may be extrusive.

Two small occurrences of ?lamprophyric rocks were discovered and may be part of a hitherto unknown diamond-prospective province which has yielded the indicator minerals found by Dampier Mining near Spalding.

Copper mineralisation is represented by the historic Broughton Mine and Wheal Sarah, but modem exploration techniques have not been applied across the Inlier.

Compared to inliers of Willouran rocks in the Flinders Ranges, carbonate breccia is uncommon in the Spalding Inlier. Thus, diapirism probably played little part in its origin, and its margins with the enclosing Torrensian sediments are largely faulted and not intrusive or convoluted. Substantial facies contrast between the early Torrensian sediments west and east of the Inlier indicate that the Inlier was an upfaulted block during sedimentation, and acted as a barrier to the influx of clastic sediment from the Gawler Craton to the west. Later Torrensian sedimentation is more uniform either side of the Inlier, and suggests that the fault block was overstepped.

This mapping on Bundaleer, presented here, as well as the mapping on the remainder of BURRA by the author and Wolfgang Preiss, will also allow more reliable exploration for commodities such as dimension stone (marble, sandstone, slate) and industrial and construction minerals.

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EVOLUTION OF THE OLADDIE DIAPIR NEAR CARRIETON, SOUTHERN FLINDERS RANGES Ian A. Dyson Salt Tectonics Australia Research Email: dysoniana@aol.com Sedimentation in the Adelaide Geosyncline occurred largely as a result of salt withdrawal associated with diapirism. Salt withdrawal structures are represented in part by dominantly northwest and northeast trending synclines that are well exposed in the Flinders Ranges. Excellent examples are found on the ORROROO 4-mile geological sheet. Some 10 km east of Carrieton, mapping of the Oladdie Diapir provides a cross-section detailing the evolution that culminated in the development of a number of en-echelon salt withdrawal minibasins during the Torrensian, Sturtian and Marinoan. The diapir displays a triangular-shaped base that passes upward into a trunk some 5 km high and 10-100m wide. Three beds of allochthonous diapiric breccia are found within the Burra Group. The best developed is a tongue c. 50 m thick that was extruded as a salt glacier during deposition of a thin, transgressive unit assigned to the Skillogalee Dolomite. The breccia can be traced uniquely into a Callanna source bed that crops out in a small anticline further to the east.

The trunk of the diapir resembles a Christmas tree diapir and is interpreted as an apparent salt wall that separates 2 Sturtian minibasins, development of which was contemporaneous with deposition of the Tindelpina Shale and Tapley Hill Formation. Each minibasin was host to vastly different rates of sediment deposition. Interesting facies variations include a 30 m-thick shoreface sandstone facies at the top of the Caernarvon Greywacke Member with no overlying Wockerawirra Dolomite Member in the northern minibasin. An equivalent but very thick (>100 m) algal-laminated dolomite in the southern minibasin overlies a roll-over anticline developed in the basal Tapley Hill Formation, and is in turn overlain by 3 bands of Wockerawirra-type dolomite. The uppermost band overlies the sequence boundar>^ at the top of the Tapley Hill Formation instead of a Cox Sandstone facies. This suggests that the Wockerawirra dolomites were deposited in areas of relatively high accommodation. The gossanous swaley cross-stratified sandstone facies was deposited during forced regression and may represent an Eudunda Arkose Member equivalent that hosts copper mineralisation at the Kapunda Mine. Downbuilding associated with development of the Christmas tree is illustrated by the adjacent upturning of Umberatana sediments on the north flank. An angular discordance between the Nuccaleena and Brachina Formations, also observed in the Beltana minibasin and Umberatana Syncline, suggests another phase of salt withdrawal followed deposition of the Marinoan cap dolostone. The salt withdrawal provided accommodation for deposition of abnormally thick transgressive units, eg. Bunyeroo Formation. At Oladdie Diapir, downbuilding accompanied by salt withdrawal and coupled with loading effects resulted in development of a syndepositional syncline directly above the minibasin. The Pamatta Pass canyon near the base of the Wonoka Formation was cut during development of this syncline. Palaeocurrents of the canyon fill are parallel to the axis of the syncline. This is typical of many Wonoka canyons that are almost without exception confined to synclines (eg. Angepena, Umberatana), and suggests a connection between salt withdrawal and canyon formation. A head of diapiric breccia is found in the nose of the syncline at Oladdie, with the lower part enveloped in Ulupa Siltstone and Tarcowie Siltstone on the south and north flanks respectively. Vertical beds of Tindelpina Shale separate the Ulupa Siltstone from the diapiric breccia. Below this level, only vertically-bedded rafts of insoluble material remain and consist of 5-10 m thick Callanna Group sandstone. This part of the salt wall is a vertical weld where the stratigraphy on the southern side is down-turned and offset by km, suggesting that asymmetrical salt withdrawal was operative during minibasin development at Oladdie Diapir. Such minibasins quite possibly contain the thickest depositional sequences in the Adelaide Geosyncline and may be separated by Christmas tree diapirs (eg. Pinda, Wirrealpa), suggesting that the sequences are dominantly tectono-eustatic in origin. Cycles within individual sequences resulted from the interaction between glacio-eustacy and salt withdrawal. The nature of this interaction is important when identifying salt withdrawal structures as potential sites for the accumulation of base metals and hydrocarbons in those basins that were affected by salt tectonics.

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ADELAIDEAN SEDIMENTATION AND THE TIMING OF SALT TECTONICS IN THE EAST WILLOURAN RANGES Ian A. Dyson Salt Tectonics Australia Research. Email: dysoniana@aol.com Field evidence from the East Willouran Ranges suggests that Burra Group sediments were deposited in a series of salt withdrawal minibasins. The formation of such minibasins was initiated by a major phase of Torrensian extensional tectonics in the Adelaide Geosyncline. A classic example is the minibasin bounded by the Breaden Hill and Witchelina diapirs, previously interpreted as piercement and thrust structures respectively. The minibasin was formed by massive evacuation of Callanna Group sediments where type sections for each of the constituent formations of the Curdimurka Subgroup are defined. The sediments crop out in sedimentary sequence and 3 types of breccia are found. The first type is found near the base, in the middle and at the top of the succession that corresponds to the Rook Tuff, the boundary between the Dunns Mine Limestone and Recovery Formation, and the top of the Boorloo Siltstone respectively. They are typically polymict, comprising both angular and well-rounded clasts and display flow banding, imbrication and slumping. In 1920, Mawson observed a very thick (100-500 m) breccia at Breaden Hill and suggested a partly tectonic and partly fluvial origin. The breccias were referred to as the Breaden Megabreccia by Murrell (1977) and interpreted as olistostromes associated with graben foundering

by Rowlands et al. (1980), but they are interpreted here as salt glaciers that were extruded during periods of transgressive sedimentation. The discordant base of each glacier is not a major unconformity, but instead represents a high-frequency halokinetic sequence boundary. Thus the Callanna Group comprises at least 3 halokinetic sequences. The thickness of each sequence thins dramatically southward and was caused by possible dissolution of evaporite-rich component formations. One of these, the Recovery Formation, thins southward from 2200 m in its type-section to 100 m at the base of the minibasin where reverse faulting defines zones of contraction caused by salt dissolution. The Hogan Dolomite and Cooranna Formation also suffered a major reduction in thickness. This mechanism was possibly focused in an area influenced by renewed rifting at the end of Callanna Group deposition. Salt withdrawal and subsequent collapse of the Callanna sediment pile was the cause of minibasin formation. Copper at the Rook and Dunns mines is found within fractured Dunns Mine Limestone adjacent to dissolution breccia at the base of the Recovery Formation. The base of the Curdimurka Subgroup overlies a brecciated shear zone that includes remnants of Dome Sandstone. It is interpreted as a salt weld at the base of a possible minibasin that acted as a sink for upper Callanna Group sediments. The Burra Group sediments in the minibasin are bounded by what was previously mapped as the West and East Willouran faults. The base of the Burra Group overlies a brecciated shear zone that formed when dolomite of the Boorloo Siltstone underwent subversion associated with the initiation of salt withdrawal. A 500 m long shear zone at Douglas Gully contains copper and minor gold. In Wattle Creek, the base of the Burra Group onlaps brecciated remnants of Callanna Group sediments. Burra Group sediments within the minibasin (e.g. Witchelina Quartzite) are quite thick because of pronounced accommodation. Further salt evacuation and downbuilding of the minibasin resulted in extrusion of a salt glacier at the top of a thin, dark coloured shale assigned to the Myrtle Springs Formation and marked the cessation of Burra Group deposition. At Breaden Hill, a salt glacier was deposited at the base of the Tindelpina Shale and thins further to the south. Here, either the Wilyerpa Formation or Serle Conglomerate of the Umberatana Group directly overlies the Burra Group with an angular discordance previously mapped as the East Willouran Fault. It is interpreted as a salt weld and represents the floor of a Sturtian minibasin. North of Boorloo Mine, basal Tapley Hill Formation west of a small triangular diapir overlies the Boorloo Siltstone. Here, the tectonically disturbed boundary is also interpreted as a salt weld. Thus, Adelaidean sedimentation in the East Willouran Ranges was contemporaneous with minibasin development, commencing with deposition of the Dome Sandstone and culminating in large-scale slumping of the Amberoona Formation (Coats and Dalgamo, 1983) in salt withdrawal synclines southwest of the Breaden Hill-Witchelina minibasin.

Reference: Coats, R.P. and Dalgamo, C.R., 1983. Large-scale slumping in the Umberatana Group, Willouran Ranges.

Abstracts Geol Soc. Aus. 10:63-64.

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WHEN HIGHSTANDS ARE LOWSTANDS, AND VICE VERSA. ^Gammon, P.R. and ^Carter, R.M. ^University of Adelaide, Adelaide, Australia, 5005 Email: paul.gammon@adelaide .edu.au ^James Cook University, Townsville, Queensland, Australia, 4811.

The Sequence Stratigraphic Model uses two variables, sediment architecture and relative sea-level change. In many outcrop instances neither of these variables is demonstrable since architecture is usually on scales too large, and relative sea-level is not known independently of the sediments themselves. In such situations facies models are generally used to determine shelf sediment distribution. Oucrop facies interpretation, from whatever data, therefore takes on special sequence stratigraphic significance, because any changes in depositional environment and therefore shifts in the facies model are then potentially interpreted as corresponding relative sea level changes. Even more critical are facies boundaries, which form the sequence stratigraphic surfaces that distinguish systems tracts. Outcrop often has another serious limitation, in that the geographic distribution of sedimentary sections does not produce a definitive shoreline-basin transect, and the construction of the facies model therefore falls back on an interpretative correlation to a modem shelf analogue.

One of the common analogues used in siliciclastic facies is the graded shelf model, which correlates the onshore increase in wave energy with increasing grainsize, i.e. grainsize decreases offshore. There are known to be many exceptions to this general rule, but in outcrop study the recognition of these differences is often highly difficult.

Mangaopari Basin in the southeastern comer of New Zealand is a Pliocene-Pleistocene basin that falls into the above category in that outcrop sections do not delineate architecture nor confirm a shoreface to basin transect. The Mangaopari Basin facies model constmction therefore follows the graded shelf siliciclastic model that fits the modem New Zealand east coast shelf. Overall the Mangaopari Basin fill was controlled by regressive tectonic uplift with superimposed highfrequency sediment cycles that isotopic analysis confirms are Milankovitch frequency glacioeustatic in origin. The composite section considered here starts in slope deposits and ends in shallow marine sediments deposited seaward of the lowstand shoreline. This eliminates potential problems with erosion surfaces in the analysis. Apart from one very thick cycle, each Milankovitch cycle is - 1 2 m thick, and is in general a simple coarser-grained - finer-grained altemation. At the section base Milankovitch cycles are "out of phase" with the graded shelf model, in that the sediments are coarser grained when oxygen isotopes indicate it is a glacio-eustatic highstand. At the top of the section Milankovitch cycles are "in phase" with the graded shelf model, in that sediments are coarser grained when oxygen isotopes indicate lowstands. Fluctuations in mollusc and foraminifer assemblages between coarser and finer-grained intervals appear to be predominantly substrate controlled and do not resolve the dilemma that either the isotopes or the sedimentology/biofacies are misleading.

Ocean Drilling Project Leg 182, hole 1119 was drilled in 350 m of water off east coast South Island and also shows an "out of phase" relationship with the graded shelf model, having coarser grained sediments during known highstands and vice versa. The out-of-phase relationship is driven by high rates of sediment supply during glacial periods, with outer shelf and slope starvation during interglacial periods. This core is in many ways identical to Mangaopari Milankovitch cycles. The graded shelf model is frequently invoked in sequence stratigraphy, and its limitations were resolved at Mangaopari due to modem analogues combined with a known sea-level history. Without such high-resolution control, gross misinterpretation of sediment architecture, systems tracts, and system tract boundaries would result. The resultant Mangaopari-modem sequence model indicates rates of sediment supply can combine with sea-level changes as a first-order control sequence control, with substantial probable changes in sediment architecture.

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DEPOSITIONAL SEQUENCES AND FACIES ASSOCIATIONS OF A MIDDLE CAMBRIAN MARINE SEDIMENTARY PHOSPHORITE DEPOSIT, DUCHESS DISTRICT, WESTERN QUEENSLAND. Michelle Hough School of Earth Sciences, James Cook University, Townsville, Queensland, Australia, 4811.

The lower Middle Cambrian, Monastery Creek Phosphorite Member comprises a sizeable sequence of high-grade, marine, sedimentary phosphate rock. Located on the western margin of the Burke River Outlier of the Georgina Basin, approximately 150km southeast of Mt. Isa, northwestern Queensland, the member occurs within the lower portions of the basinal stratigraphy, and is currently the only phosphorite deposit, of many known from the Cambro-Ordovician Georgina Basin, to be exploited. Previous regional investigations of the Georgina Basin have established that phosphatic deposition accompanied a marine transgression which inundated the older craton, forming part of a facies mosaic of epeiric association. The epeiric context of the deposit is quite distinct from the continental margin setting of contemporary sites of phosphogenesis. Detailed microfacies analysis has focused on establishing the palaeoenvironmental conditions that sponsored Middle Cambrian phosphogenesis within the Georgina Basin.

Phosphorite horizons occur predominantly as sand-sized, peloidal, fluorapatite framework grains within a siliceous or calcareous matrix. Previously, the siliceous and calcareous phosphorite have been regarded as separate primary facies. Petrographic analysis shows that the 'siliceous facies' is the result of a siliceous diagenetic overprint that has selectively altered particular beds within the sequence. Phosphatic limestone is commonly seen in various stages of siliceous replacement throughout the deposit and it is considered as the primary lithology prior to diagenetic alteration.

Silicification is more marked in certain areas of the deposit than in others, suggesting both lithological and geometrical controls on diagenetic alteration. Weathering and oxidation has removed the calcareous matrix in many areas and has generally induced the loss of organic material.

A number of discrete phosphatic and closely associated non-phosphatic microfacies can be recognised within the phosphorite interval. A peloidal phosphatic facies association comprises horizons that are typically well-sorted, contain fragmented bioclasts and exhibit planar and crosslamination as well as lenticular bedding, all features of which are indicative of reworking by lowmoderate energy traction currents. A bioclastic facies association reflects a well-developed benthic invertebrate community. It exhibits biogenic lamination particularly in thin interbeds of collophane mudstone, considered to be of algal mat association. The occurrence of algal mat accumulations and the dominance of bioclastic material imply deposition in a low-energy environment experiencing low rates of sedimentation. A phosphorite-poor siliciclastic, organic-rich facies association forms the bounding stratigraphy to the high-grade phosphatic interval. It is characterised by finely laminated bituminous, pyritic, organic-rich shale with a reduced bioclastic component. Suspended load deposition and anoxic conditions typify the depositional conditions of this association. The suite of microfacies is reminiscent of a shallow marine carbonate succession, with decreasing energy and oxygen concentrations up sequence, suggestive of a transgressive systems tract. In terms of phosphogenesis, the intimate interlayering of microfacies is indicative of complex cycles of phosphatisation, reworking and traction current transportation, winnowing, reburial and possibly renewed phosphatisation. The Monastery Creek Phosphorite Member represents a condensed sequence where the products of early diagenetic, synsedimentary phosphogenesis were accumulated over a substantial level of slow sea-level rise.

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CYCLING THROUGH THE KENNEDY GROUP (LATE PERMIAN, CARNARVON BASIN WA): MILANKOVITCH PERIODICITIES IN A PERMIAN SEAWAY Helen Lever School of Earth Sciences, James Cook University This paper aims to study the cycles within the onshore Kennedy Group in detail, in order to determine the processes causing the cyclicity. The onshore Kennedy Group occurs at the top of a Late Carboniferous to Late Permian post-rift succession deposited in the Merlinleigh Sub-basin of the Carnarvon Basin. The Merlinleigh Sub-basin, and other basins in Western Australia, including the Perth and Canning Basins, initiated during the Devonian as failed rift arms during the separation of parts of the Gondwanan continental margin. Extension-driven subsidence of the thinned crust increased in the Late Carboniferous and Early Permian, and initiated the deposition of non-marine glacial sediments, followed successively by marine glacially influenced sediments, and fully marine sediments. Much of this succession displays some degree of cyclicity.

The onshore Kennedy Group is divided into three formations, and this study has enabled further subdivision into 12 informal members based on changes in depositional environments.

All of the Kennedy Group is made up of cycles on varying scales. Cycles are coarsening upwards, and interpreted as having been deposited during falling sea-level. Transgressive and high-stand sediments are thin to absent in most cycles. There are several scales of cyclicity, ranging ft-om a pervasive 1-2 m scale cyclicity, the 5-9 m scale that dominates outcrops, to the 40-50 m scale cycles that can be seen in distant outcrops. Spectral analysis of grainsize and outcrop gamma confirms the presence of cycles, and comparison of ratios of detected cycles to known Milankovitch periodicities suggests that Milankovitch-induced climate and/or sea-level change was the driving force for much of the cyclicity observed.

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ORIGIN OF DIAPIRS IN THE ADELAIDE FOLD BELT: SYNSEDIMENTARY OR SYNTECTONIC? PJ. (Lai) Mendis, P.R. James and R.A. Both Department of Geology and Geophysics, University of Adelaide Many diapirically emplaced piercement structures have been recognized, and a variety of scales, particularly in the central and northern Flinders Ranges of the Adelaide Fold Belt. These structures typically are composed of breccias including a polymict variety of clasts in a mixed mud and carbonate matrix. The clasts are commonly derived from the low^est sequence of Adelaidean stratigraphy, i.e. Neoproterozoic Callanna Group, but may also include Palaeo- to Mesoproterozoic granitic and/or mafic basement material, as well as overlying sediments up to and including Cambrian formations (e.g. in the Beltana Diapir). Diapirs characteristically contain (a) large (up to kilometre-scale) rafts of sedimentary units, (b) dolomite mineralization including dolomite caps and (c) tongues of breccia which emanate from the diapir into the hosts, with grain size generally decreasing away from the diapir. Host sequences show (a) significant local stratigraphic thickening or thinning, (b) local marginal folding of host sediments and (c) temporal changes in composition and texture. Previous investigations and modelling, including laboratory sand-box analogs, have so far favoured a synsedimentary origin for these diapiric structures.

The dipairs of the Flinders Ranges show three types of associations, viz. (a) axial traces of regional anticlines, (b) apices of graben structures and (c) major through-going faults. The anticlines occur in two major orientations, which are recognized as superimposed upright folds, with Fi traces NNW-SSE and F2 traces ENE-WSW. The diapirs are much more elongate in the Fi direction than the F2. The Enorama Diapir, for example, occurs along the axial trace of the Enorama Fi Anticline.

A balanced cross-section across this structure indicates that it comprises a SW-verging faultpropagation fold with 26% overall shortening. The eastern limb of the fold is thrust over the western limb, forming a forelimb syncline and back limb anticline pair, resulting in the occurrence of lower sequences on the eastern limb. The cross-section supports the interpretation of intial thrusting of the Adelaidean sequence over a sub-horizontal hidden decollement surface, on or closely overlying the basement, that probably formed a major breccia zone. The decollement has caused further thrust contraction along an upwardly steepening listric fault surface which penetrated into the upper sequences, brecciating those sequences as well, and causing the emplacement of the apparent diapiric structure. Calculations show at least 5.3 km depth to the breccia zone over the basement.

As well as the diapirs, a range of minor to micro-detachment/fault propagation folds have been recognized from sedimentary sequences adjacent to the major diapirs of the central Flinders Ranges. These minor fault/fold structures consist of decollement surfaces along bedding, which propagate and cut up and across the layering as listric thrusts to penetrate into upper sequences, producing gouge breccia up to the tip-line of the thrust. Breccias are dominated by clasts from the lower sequences gouged from adjacent to the decollement surface, but also include clasts from the competent basal layer as well. These breccia deposits are uniformly concentrated in the cores of small-scale anticlines. Thickening and thinning of limbs, diapir-like piercement injections into limb sequences causing change of composition of limbs, and parasitic folding on limbs can be observed. These minor structures reveal shortening of between 17 and 29% and provide compelling structural analogs to those relationships demonstrated from the Enorama Anticline and its cross-section.

The minor folds which contain diapiric breccia are tectonic structures belonging to the CambroOrdovician Delamerian Orogeny. They display bedding-slip lineations and a structural geometry, and are parasitic to those of the major structure. Therefore, comparison of these minor folds with major structures such as the Enorama Anticline/Diapir suggests a syn-tectonic origin for the structures associated with diapirs.

384

VISUALISATION OF FLUVIAL SYSTEMS WITH INTEGRATION OF HIGH RESOLUTION SEQUENCE STRATIGRAPHY AND 3D SEISMIC DATA IN THE COOPER-EROMANGA BASIN Takeshi Nakanishi and Simon C. Lang National Centre for Petroleum Geology and Geophysics (NCPGG) and Australian Petroleum Cooperative Research Centre (APCRC), Adelaide University Email: nakanish@ncpgg.adelaide.edu.au, Email: slang@ncpgg.adelaide.edu.au An integration of sequence stratigraphic concepts and advanced 3D seismic data visualisation is a powerful tool to identify sedimentological features in the subsurface. This tool has been applied to the non-marine successions of the Cooper-Eromanga Basin, South Australia, and a variety of fluvial systems were visualised using open file well and 3D seismic data. In each case, fluvial facies and key surfaces (eg. sequence boundary, flooding surface) were recognised to identify systems tracts within unconformably bounded successions, and these were linked to key seismic reflections in the seismic data.

Isolatedfluvial channel belts of the Toolachee Formation in the Pondrinie Field area The laterally discontinuous fluvial deposits of the late Permian Toolachee Formation lie above a sequence boundary resulting from the late Permian Daralingie uplift, and was interpreted as an alluvial transgressive systems tract in the Pondrinie Field area. The distribution of non-coal intervals derived from seismic negative amplitude, is sinuous and elongate, and the orientation is mfluenced by a fault bounding the Pondrinie structural high. On the side of the elongated negative amplitude pattern, smaller scale sinuous patterns are represented by variable amplitude. The seismic amplitude patterns suggest the existence of sinuous fluvial channel belts surrounded by splay complexes and floodplain in the downthrown fault block.

Incised valley fdl and crevasse splay complexes of the Poolowanna Formation in the Pondrinie and Moorari Field areas The basal Jurassic Poolowanna Formation lies unconformably on the Permian-Triassic successions.

The laterally discontinuous fluvial sediments of the middle part of the Poolowanna Formation were assigned to an alluvial transgressive systems tract. In the Pondrinie area, the seismic high amplitude which is a proxy for the distribution of a coaly interval, is characterised as an elongated pattern with wedge-shaped tributary patterns on both sides, filling the paleo-topographic low area. The spatial thickness variation of the interval and the amplitude distribution suggests an incised valley filled with transgressive fluvial and coal-prone sediments. In the Moorari Field area, the seismic amplitude variation of the Poolowanna Formation represents sheet-like and meandering channel distributions differing from the Pondrinie Field area. These amplitude patterns suggest the existence of sheet-like crevasse splay deltas and sinuous crevasse splay channels on a floodplain beside a major fluvial channel belt. The differences in sedimentary environment of the Pondrinie and Moorari area reflect the paleo-topography during Poolowanna time.

Meandering channel belts of the Birkhead Formation in the Merrimelia Field area The lateral change of fluvial deposits of the late Jurassic Birkhead Formation in the Merrimelia Field area suggests the interval was deposited as an alluvial transgressive systems tract overlying the sequence boundary at the top of the Hutton Sandstone. The seismic amplitude variation in the interval demonstrates meandering channel patterns. Tying well log motifs to the seismic amplitude, the charmel patterns were interpreted as abandoned channels or oxbows within fluvial channel belts, and the spatial distributions of point bar sands, shales plugging abandoned channels, crevasse splay sandstones and floodplain shales were demonstrated.

The integration of sequence stratigraphy and 3D seismic data visualisation can produce a major impact to reduce uncertainty in sedimentological interpretations and also be valuable for predicting detailed distributions of reservoir and seal rocks for hydrocarbon accumulations in this basin.

385

IMPROVING UNDERSTANDING OF CONVENTIONAL AND UNCONVENTIONAL RESERVOIRS THROUGH HIGH-RESOLUTION REGIONAL CORRELATIONS: AN EXAMPLE FROM THE EAGLE SANDSTONE SHALLOW BIOGENIC GAS PLAY, MONTANA, U.S.A. Tobias H.D. Payenberg and Simon C. Lang Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, Canada The Eagle sandstone in northern Montana and equivalent rocks in southern Alberta produce biogenic gas from a depth between 300 and 800m. The economics of an inexpensive drilling campaign have so far outstripped the need for detailed stratigraphic work. However, rising gas prices and the depletion of many older reservoirs increase the need for a better stratigraphic understanding of the reservoirs, in order to push existing field boundaries and improve exploration success.

Although regional geophysical data should provide a good stratigraphic understanding, 3D seismic is uneconomical because of dense receiver spacing (and thus high costs), while 2D seismic has too poor resolution to image the internal stratigraphic architecture of the Eagle Sandstone. In a study area of over 37,000 km^ a total of 2760 wells were integrated with 7 outcrop sections and 5 cores to create a new stratigraphic framework for the interval. By using an allostratigraphic approach, six progradational and one retrogradational allomembers were defined. Each progradational allomember comprises a full set of shelfal, shoreline and coastal/delta/alluvial plain depositional environments. The sand-rich shoreline deposits are the primary exploration target.

Existing lithostratigraphic correlations and earlier sequence-based stratigraphic frameworks were used to predicting reservoir-quality sands, but were insufficient to predict prolific intervals, because of limited reservoir understanding. The new allostratigraphic framework, however, provides a detail of reservoir complexity not previously described by correlations. It also predicts gas occurrence in only two of the six sandstone-rich intervals, increasing predictability.

386

A BASIN FLOOR FAN OUTCROP ANALOGUE: RESERVOIR HETEROGENEITY AND SEQUENCE STRATIGRAPHY M.R.W. Reillv\ S.C. Lang', and N.M. Lemon.'

'National Centre for Petroleum Geology & Geophysics, Thebarton Campus, University of Adelaide, 5005.

Abstract Deepwater sand-prone reservoirs represent attractive exploration targets, especially in frontier basins. Of key interest are the reservoir quality, geometry, distribution and connectivity of sandprone facies. Outcrop analogues can provide detailed insights into the geometries and internal architecture of sand-prone turbidites and related deepwater sediments. The Donkey Bore Syncline in the Northern Flinders Ranges of South Australia contains a generally fine-grained turbidite succession (Bunkers Sandstone) of Early Cambrian age that outcrops on three sides of a triangular syncline covering 22 km^. At the base of the turbidite succession is a sand-prone interval, which is interpreted as a basin floor fan. This paper outlines the sedimentology and depositional architecture of the basin floor fan, influenced in part by salt diapirism, and suggests how this knowledge can be applied to reservoir models.

The Bunkers Sandstone basin floor fan comprises over 30 m of section and flanks an active salt diapir to the east (Wirrealpa Diapir) that shows signs of being tectonically active during initial deposition of the turbidite facies (e.g. reworking of grains from the diapir). The basin floor fan deposits are dominantly massive clean sandstones that are stacked or interbedded with siltstone.

The basin floor fan is interpreted to be a lowstand systems tract siliciclastic deposit that overlies micritic limestone deposited during the previous highstand. The basin floor fan is in turn overlain by a fine-grained turbidite facies deposited during the following transgression and comprises thin turbidite units, thin massive sandy debris flows that grade into hemipelagic siltstone.

The geometries of depositional elements were resolved by mapping key stratigraphic surfaces, linked via eight measured sections and accompanying spectral gamma logs, through the basin floor fan. Four alternative correlation panels were constructed using different aspects of the spectral gamma ray logs combined with the stratigraphic logs. The Thorium count of the spectral gamma provided the optimum discrimination between sandstone and siltstone beds. However, correlating individual beds with any certainty became increasingly difficult for distances greater than 500m from each measured section.

Derived from the correlation panels, calculated vertical and horizontal connectivity values indicate a high degree of heterogeneity within the basin floor fan. Cross-plots of sand percent verses connectivity values produced meaningful results concerning vertical connectivity within the basin floor fan. The results indicate that a greater than 20% sand content is required within any particular section before interconnectivity between individual beds can occur. Heterogeneity is poorly resolved using conventional wireline log suites (e.g. gamma ray), but is greatly improved if spectral gamma logs are utilised. Further results from this study indicate that the apparent sheet-like amalgamated sand-body (basin floor fan) is actually a series of stacked depositional lobes. The data set provides a high-resolution analogue for understanding the internal architecture of hydrocarbon reservoirs that were deposited in deepwater depositional environments.

Acknowledgements ExxonMobil for providing financial support and Or John McPherson of ExxonMobil for his interest and guidance.

Primary Industries and Resources South Australia (PIRSA) for the loan of their portable spectral gamma ray scintillometer.

387

THE APPLICATION OF lUGH-RESOLUTION SEQUENCE STRATIGRAPHY TO RESERVOIR CHARACTERISATION AND DEVELOPMENT: AN EXAMPLE FROM TARBAT-IPUNDU FIELD, EROMANGA BASIN, SOUTHWEST QUEENSLAND Robert Root^ Simon Lang^ and Dennis Harrison^ ^National Centre for Petroleum Geology and Geophysics, University of Adelaide, SA, 5005, Australia ^Santos House, Level 14, 60 Edward Street, Brisbane, QLD, 4000, Australia The value of sequence stratigraphy to petroleum exploration stems from understanding spatial stratigraphic relationships and developing new play concepts. In contrast, the value of sequence stratigraphy to reservoir development stems from providing stratigraphic divisions that facilitate geologically meaningful models of reservoir heterogeneity. Understanding the relationship between reservoir properties and depositional facies allows modem analogues to be used as tools for selecting meaningful geological models that guide reservoir development.

This study uses an oil field data set from Tarbat-Ipundu Field in Southwest Queensland, Australia to demonstrate how sequence stratigraphy can be applied at the reservoir scale. Sequence stratigraphic surfaces were mapped across the study area using fiillhole core, wireline log suites, FMI/FMS image logs and palynological data. Indications of reservoir flow properties from conventional core analysis, petrography and wireline logs were then used to evaluate the utility of the sequence stratigraphic framework for improving recovery efficiency and reservoir management.

The Wyandra Sandstone Member of the Cadna-owie Formation is a thin (~20 m), fluvial to marginal marine volcaniclastic sandstone reservoir. Reservoir heterogeneity results almost exclusively from diagenetic processes, but the spatial distribution of medium- to very coarsegrained sandstone within the reservoir is a significant influence on the occurrence and intensity of diagenesis.

Sequence stratigraphic surfaces mapped across the field define a lower fluvio-lacustrine highstand systems tract, a middle sheet-like, fluvial lowstand systems tract, and an upper estuarine to marginal marine transgressive systems tract. The majority of reservoir quality sandstone (permeability >10 mD, porosity >20%) is contained within the lowstand systems tract. However, to understand the internal architecture of the reservoir, modem fluvial depositional analogues are employed to explain plausible distribution of medium- to very coarse-grained sandstone.

Comparison of reservoir stratigraphy and sedimentology at Tarbat-Ipundu Field with similar modem fluvial systems suggests that medium- to very coarse-grained sandstone is partitioned to the fan apex of lower gradient fan delta systems, where relatively high-energy channelised facies amalgamate. Based on the likely orientation of depositional systems, interpreted from paleocurrents and basin structure, uncertainty in locating reservoir quality rock at Tarbat-Ipundu Field may be reduced. The implication for reservoir development is that a genetically meaningful sequence stratigraphic framework is essential to understand reservoir heterogeneity.

388

DEEP-SEA RECORDS OF LAND-SEA INTERACTIONS ALONG THE WESTERN MARGIN OF THE INDO-PACIFIC WARM POOL SINCE THE LAST GLACIAL MAXIMUM Patrick De Deckker Department of Geology, The Australian National University, Canberra ACT 0200

There are three significant climatic modes that operated over the Indo-Pacific Warm Pool since the LGM. These are based on numerous proxies (faunal analysis of planktic foraminifers and calcareous nannoplankton, clays, pollen, carbonate and trace metal percentages, and stable isotopes) studied on several cores from the eastern Indian Ocean. The first mode, characteristic of the glacial period, saw^ a reduced extent of the Warm Pool compared to today w^ith increased seasurface salinity (SSS), the absence of a barrier layer and reduced sea-surface temperatures (SST).

On land, precipitation was reduced by 30 to 40%, and monsoonal climatic conditions were absent.

The thermal structure of the upper ocean was different, with a steeper thermocline and a very deep ACD compared to today.

The second mode commenced at approximately 14 ka with the onset of monsoonal rains, a change in alkalinity near the surface of the ocean and a progressive increase in SST. The latter reaches a maximum at ~ 11 ka. The thermal structure of the upper ocean is altered and this is paralleled by a change in the nutricline.

The third mode is characteristic of the Holocene which saw monsoonal activity peaking at around 8 ka, seen by maximum river discharge, a resultant barrier layer with a low salinity cap, a substantial shallowing of the ACD and a significant vegetational change on land. Since that period, climate changed with decreased SST since 6 ka. The last 3 000 years saw more contrasting climatic conditions, dune reactivation in northwestern Western Australia, such as seen today with typical ENSO signals (with wet and dry years), an enhanced Leeuwin Current and broad salinity fluctuations.

Acknowledgements Contributors to the data obtained from several of the cores are: Barrows, T. T., ANU, Australia; Gingele, F., Wamemunde, Germany; Okada, H., Hokkaido Univ., Japan; Martinez, I. J., Medellin, Colombia; Sirocko, F., Univ. Mainz, Germany; Takahashi, K., Hokkaido Univ., Japan; and van der Kaars, S., Monash Univ., Australia.

389

LAST 18,000 YEARS PALEOCEANOGRAPHY IN THE INDONESIAN-NW AUSTRALIAN REGION: PATTERNS OF HEAT TRANSPORT AND SEA LEVEL CHANGES Ding X.\ Guichard F. ^ Bassinot F. ^ Labeyrie L. ^ ^ China University of Geosciences, Beijing 100083, China. ^ Laboratoire des Sciences du Climat et de TEnvironnement, Domaine du CNRS, France.

Climatoceanographically, the Indonesian Archipelago belongs to the Western Pacific Warm Pool (WPWP), and is strongly affected by monsoon wind action from the northern Indian Ocean and western Pacific Ocean. Because an apparent sea level gradient develops due to different wind regimes between these two ocean basins, the warm, relatively fresh Pacific water moves through the Indonesian archipelago into the Indian Ocean. This Indonesian Throughflow is the only low-latitude conduit connecting major world oceans.

We studied three piston cores from the Indonesian Archipelago collected during a SHIVE cruise under a joint French-Indonesian marine geological research program: SHI9006 (4.33223S, 117.59584E, 1999 m), SHI9034 (9.09764° S, 111.00721° E, water depth 3330 m), and SHI9022 (11.35444° S, 122.03799° E, 2313 m). High-resolution stable isotope and planktonic foraminiferal analyses were supplemented by AMS-^'^C dating and sedimentary rate calculation, as well as the estimate of sea surface temperature (SST) using transfer function FP-12E on isotopic results. The following generalizations can be made: 1) At 14-12.5 ka, during the last deglaciation Termination IA, SST increased rapidly by 3-4°C in summer and 1-2°C in winter, sea level rose by tens of metres, and the cold-water planktonic population that was common in the Last Glacial Maximum decreased sharply from 12-15% to almost zero within the Archipelago. This rapid rise of sea level in a period of 1.5 ka could have led to sudden widening of both the Makassar and Lombok Straits for warm Pacific water pouring into the Java Sea and subsequently the eastern Indian Ocean.

2) At - 9 . 5 ka, in Termination IB, sea level rose rapidly by further tens of metres, resulting in the connection between the South China Sea (SCS) and the Java Sea. Due to a high precipitation and stronger summer monsoons, a large amount of warm and less saline water flowed probably from the SCS shelf region through Indonesian pathways, as indicated by the notably lightened 5 ^^O values and low SST. 3) At ka, the upwelling faunal population increased significantly in areas close to SHI9034, likely due to stronger southeast monsoons.

390

NORTHERN MURRAY BASIN, SOUTH AUSTRALIA: STRATIGRAPHY, SEDIMENTOLOGY AND GEOMORPHOLOGY Adrian Fabris Office of Minerals and Energy Resources, PIRSA

Introduction A recent TEISA project by PIRSA has enhanced the understanding and prospectivity of the northern Murray Basin. Detailed analysis of a series of strand plain and barrier island features has identified key processes in their formation related to concentration mechanisms of heavy mineral sands (HMS). The Loxton-Parilla Sands (host to HMS deposits) can be understood in terms of littoral depositional environments. This gives meaning to anomalous intersections of HMS and aids navigation of future drilling.

Generalised stratigraphy The Tertiary succession accumulated over three major depositional events: 1.

2.

3.

Floodplain and swamp environments dominated, with deposition of carbonaceous sands, clays and silts (Warina Sand and Olney Formation). A minor marine incursion in the late Eocene was marked by the deposition of fossiliferous clays and marls (Buccleuch Formation).

A major marine incursion in the Late Oligocene to Mid-Miocene resulted in fluvial environments being replaced by lagoonal and marginal marine muddy facies (Geera Clay), and in deeper water areas, by marls such as the Ettrick (Winnambool) Formation and limestones of the Murray Group. A global sea-level fall in the Late Miocene brought an end to this depositional cycle and led to local erosion and weathering.

A rapid marine transgression in the Latest Miocene led to the drowning of fluvial tracts and the occurrence of shallow marine depositional environments. This is represented by clays with occasional sandy, silty, carbonaceous and calcareous beds (Bookpumong Formation). As rising sea levels steadied, a move towards highstand deposition saw a change to prograding beach strandplains and barrier islands (Loxton-Parilla Sands). Between these barriers and within a large freshwater lake known as Lake Bungunnia, variegated silty clays were deposited in fluvio-lacustrine environments (Blanchetown Clay).

Loxton-Parilla Sands Sediments of the Loxton-Parilla Sands were studied using the analysis of geophysical logs and sedimentological studies of grainsize, rounding, sorting, skewness, kurtosis and colour. The combination of these parameters allowed the identification of a number of zones associated with varying depositional environments within the Loxton-Parilla Sands. These included; lower shoreface, upper shoreface, foreshore to backshore (berm, beach, immature dunal and washover fan), backshore (dunal), fluvial and backbarrier lagoonal.

Formation of barrier island features The strandline features within the Murray Basin have previously been recognised as prograding barrier complexes of the Loxton-Parilla Sands (Roy et aL, 2000). Recent drilling by PIRSA has shown that the largest of these features are a result of still-stand deposition and the formation of stacked barrier island complexes. Sedimentological analysis of the Loxton-Parilla Sands has been used to show that these features contain multiple stacked beach deposits and back barrier lagoonal sediments associated with barrier island complexes rather than strand plain deposition. This implies that that these features are prospective for HMS accumulations, not only into reworked foreshore sediments, but also in washover fans, associated with storm wave deposition.

Reference Roy, P.S., Whitehouse, J., Cowell, P.J. and Oakes, G., 2000. Mineral sands occurrence in the Murray Basin, Southeastern Australia. Economic Geology, 95:1107-1128.

391

SOUTHERN AUSTRALIAN EOCENE SPONGES: INDICATORS OF DEEP CONTINENTAL WEATHERING? Gammon, P.R. University of Adelaide, Adelaide, Australia, 5005 Email: paul.gammon@adelaide.edu.au Neritic biosiliceous sponge sedimentation occurs across 2000 km of the Late Eocene southern coastline. The palaeogeography of well-preserved Western Australian deposits indicates these sediments formed in protected embayment and drow^ned estuarine environments during the last highstand of the greenhouse world. Palynology of coeval non-marine lignites indicates a humid temperate environment with low seasonality despite the 55°S palaeolatitude. Biosiliceous sediments grade laterally outboard to normal open marine bryozoan chalks relatively typical of southern Australian Tertiary basins. The sponge fauna and an impoverished diatom flora indicate that protected embayments and estuaries were of normal marine salinity and relatively oligotrophic.

Benthic competition from photoautotrophic or calcareous biota is not apparent, yet there is nothing to suggest these environments were hostile. Indeed, the sponge faunal assemblage is characteristic of deep-water, slow-growing tropical environments. Everything suggests that these were calm, stable marine environments, but unusually and highly advantageous to biosiliceous organisms.

Such conditions were even maintained up to 200 km inland along the drowned estuaries, indicating minimal surface run-off, despite the humid climate, from the peneplained Australian hinterland.

Sponges are limited in the modem ocean by a lack of dissolved silica which they utilise for spicule production (Maldonado et al., 1999). Biosiliceous sedimentation is characteristic of areas with high dissolved silica fluxes that are usually attributed to upwelling. Sponges tend to dominate in areas of high dissolved silica concentrations, with environments too deep and/or cold for carbonate secreting organisms and benthic photoautotrophs (e.g. Conway et al, 1991). Dissolved silica is the key to sponge competition. In Late Eocene southern Australian sediments, verdine (a form of glaucony) and clinoptilolite (a high-silica zeolite) replace sponge spicules and rigid skeletons. This replacement requires high silica activity, which was supplied by dissolving biosiliceous skeletal parts/fragments. These minerals therefore also indicate a relatively enriched silica flux was the driving constant in these environments. Combined with a calm, stable, environment, this flux enabled the usually sub-dominant sponges to completely overwhelm all other competitors.

The facies distribution of biosiliceous inboard to calcareous outboard indicates that the silica flux could only have come from the hinterland, not upwelling. However, the sedimentology clearly indicates that surface run-off, and therefore organic nutrient input, into these environments was minimal. The only remaining option for humid precipitation to exit the hinterland is via enhanced groundwater flow, as could be expected of a peneplained hinterland. Due to leaching, groundwaters are usually enriched in silica relative to surface waters, but poor in other organic nutrients, and are ideal for promoting slow-growing sponges whilst not promoting more competitive benthic organisms. Fixation by biosiliceous organisms drove diagenesis that recycled silica within the calm, protected environments, further enhancing the advantages conferred by the groundwater silica input. It is suggested that the Late Eocene biosiliceous sediments are a record of enhanced silica movement and crustal weathering within the Australian landmass, which correlates well with and perhaps helps explain the extensive Colville silcrete throughout inland southern Australia.

References Conway, K. W., J. V. Barrie, W. C. Austin, and J. L. Lutemauer, 1991, Holocene sponge bioherms on the westem Canadian continental shelf. Continental Shelf Research, 11, 771-790.

Maldonado, M., M. C. Carmona, M. Uriz, and A. Cruzado, 1999, Decline in Mesozoic reef-building sponges explained by silicon limitation. Nature, 401, 785-788.

392

CHINAMAN GULLY FORMATION, SOUTH AUSTRALIA: SEA LEVEL FALL OR EVENT DEPOSIT? Peter W. Haines School of Earth Sciences, University of Tasmania, Hobart TAS

The Chinaman Gully Formation (CGF), of Late Eocene to Eocene/Oligocene boundary age, is an important marker horizon within the St Vincent Basin, South Australia. This thin m at type section) sandy horizon separates two argillaceous marine units, the Blanche Point and Port Willunga Formations, and contains a mixed marine and terrestrial biota. The biota, lithology and oxidised surface outcrop are generally accepted as evidence that deposition was in response to sea level fall (the Chinaman Gully Regression'). Here it will be argued that the sedimentological character of the CGF is more consistent with deposition as an energetic event deposit requiring no extrinsic change in sea level.

The CGF comprises a series of graded sand beds, mudstone and diamictitic sand-mud mixtures.

Texturally the sand is likely to be of beach, dune and possibly fluvial derivation. In drill core the formation is black, organic rich and locally pyritic. It is burrowed down from the top, with bioturbation decreasing downwards and typically ceasing m below the top. Note that the sedimentologically defined top of the event deposit is not necessarily the same as the formation top chosen by previous workers. The CGF has only been recognised close to the palaeo-shoreline in the Willunga, Noarlunga and Adelaide Plains sub-basins. From available sub-surface information, the formation thickens rapidly towards the palaeo-shoreline (up to 10+m), but disappears basinward.

There is no evidence of significant basal incision. The CGF has not previously been recognised along the western margin of the St Vincent Basin, but it is here suggested that the unit may be represented by a 70 cm thick graded event deposit 1.3 m below the Port Julia Greensand Member of the Rogue Formation at Port Julia.

The CGF is interpreted as a single (although pulsed) event deposit. It has many features in common with mega-tsunami deposits such as those proximal to the K-T boundary Chicxulub crater in Mexico. The sediments are considered to have been derived from the foreshore and near-shore terrestrial environments during repeated wave resurges that washed wedges of sediment seaward.

Rapid deposition lead to preservation of abundant terrestrial plant matter and precipitation of pyrite in anoxic conditions. Associated acidic fluids were unconducive to the preservation of incorporated calcareous marine fauna. Post-tsunami burrowers bioturbated the surface layers; those more tolerant to low O2 conditions were capable of extending to greater depths. The presence of abundant siliceous spherules (comprising zeolites and palagonite) with characteristics similar to altered K-T boundary spherules, and weak PGE enrichment, suggests impact as the generative energy source (Haines et al. 2002). No extrinsic sea level change is required. Locally there is evidence that the basal Port Willunga Formation was deposited in shallower or more energetic water than the upper Blanche Point Formation. This is expected under the event deposit scenario; it should have been shallower by an amount equal to the pre-compaction thickness of the CGF at any specific location. Associated evidence for more open marine conditions suggests that the event was accompanied by some palaeogeographic changes. The location of the putative impact site remains speculative (see discussions in Haines, 2000).

References Haines P.W. 2000. Tsunami(?) deposit of terminal Eocene age, South Australia: The inferred effects of a low-angle multiple impact event. In: Catastrophic Events and Mass Extinctions: Impacts and Beyond, Vienna, July 2000, Abstracts p. 63-64.

Haines P.W., Norman M., Della-Pasqua F. & Schaefer B.F. 2002. Chinaman Gully Formation, South Australia: Spherules and PGE enrichment suggest impact association. Geological Society of Australia Abstracts - this volume.

393

CHINAMAN GULLY FORMATION, SOUTH AUSTRALIA: SPHERULES AND PGE ENRICHMENT SUGGEST IMPACT ASSOCIATION Peter W. Haines\ Marc Norman^'^, Fernando Della-Pasqua\ Bruce F. Schaefer^ ^School of Earth Sciences, University of Tasmania, Hobart, TAS; ^Present address: RSES, Australian National University, Canberra, ACT, ^Department of Earth Sciences, The Open University, Milton Keynes, UK.

The Chinaman Gully Formation (CGF), of Late Eocene to Eocene/Oligocene boundary age, is a thin unique stratigraphic unit within the St Vincent Basin of South Australia. The CGF is generally considered the product of a brief sea level fall, but Haines (2000a,b, 2002) argues that its sedimentological character is more consistent with deposition as an event deposit, specifically a mega-tsunami deposit. The presence of abundant siliceous spherules and a weak PGE enrichment, as detailed below, suggest that tsunami generation may have been caused by an impact event.

Spherules

Siliceous spherules -0.1-2 mm in diameter are present in all outcrops of the CGF and in the two drill cores studied, being concentrated in discrete horizons in the upper part of the formation.

Combined SEM and XRD studies indicate that the spherules have an outer crystalline shell composed of heulandite/clinoptilolite (zeolite) and are either hollow or have a soft waxy amorphous core interpreted as palagonite (a common breakdown product of low silica glasses). In some cases the outer shell is poorly formed or absent. Original shapes are interpreted as spheres, ovoids and rarely dumbbells, but compaction has often led to distortion. The spherules closely resemble altered K-T boundary impact spherules, which, in rare instances are known to preserve relic glass. By analogy, we propose that the CGF spherules were originally glass, and potentially of impact origin, ie. microtektites.

Platinum Group Element (PGE) abundances

Five samples from spherule-bearing portions of the CGF and one of underlying mudstone (background) were analysed for PGE abundances using isotope dilution ICP-MS at the University of Tasmania. All samples were from drill core. Ir, Pt and Re are slightly elevated (up to -2.6 times) in the spherule layers compared to background. In one sample Ir reaches 0.21 ppb compared to 0.08 ppb background. This enrichment is surprising as the spherule-bearing layers contain high terrigenous sand content expected to be poor in PGEs, and the considerable thickness of the CGF (4 and 6.5 m in the drill holes sampled) will tend to dilute any extraneous PGE components, unless concentrated largely in the spherules themselves. Thus the PGE enrichment, although weak, is tentative evidence for an extraterrestrial contribution. Os abundances and Re-Os isotopes were measured at The Open University, UK. In contrast to other PGEs, Os is slightly depleted in the CGF relative to background levels. ^^^Os/^^^Os ratios in the spherule layers are higher than the surrounding sediments, which have ratios consistent with Eocene seawater. This is consistent with a distinctive source for the spherules comprising ancient continental crust, with minimal extraterrestrial contribution to the Os budget.

References Haines P.W. 2000a. Cainozoic multiple impact event in South Australia and its inferred signature within terminal Eocene stratigraphy of the St Vincent Basin. Geological Society ofAustralia Abstracts 59, 204.

Haines P.W. 2000b. Tsunami(?) deposit of terminal Eocene age. South Australia: The inferred effects of a low-angle multiple impact event. In: Catastrophic Events and Mass Extinctions: Impacts and Beyond^ Vienna, July 2000, Abstracts p. 63-64.

Haines P.W. 2002. Chinaman Gully Formation, South Australia: Sea level fall or event deposit? Geological Society ofAustralia Abstracts - this volume.

394

SEDIMENT PACKAGES AND SEQUENCE BIOSTRATIGRAPHY: THE SOUTHERN AUSTRALIAN CENOZOIC RECORD Oianyu Li and Brian McGowran Department of Geology & Geophysics, The University of Adelaide, Adelaide SA 5005

Sequence biostratigraphy begins with calibrating recognized biozones and species occurrence and disappearance datums against sequence surfaces that might reflect changes in sea level affected by glacials or tectonics. A systematic approach utilizes assemblage data especially the abundance variations of species and groups, and adds three aspects to the endeavour. The first is to seek and test whether an observed fossil event or horizon is controlled by a climatic event or transgression/regression, thus being potentially reversible or recurring, unlike speciation and extinction. The second is to emphasize the recurring nature of biofacies, giving facies the status of a proxy for geohistorical and environmental change including changes in sea level. The third aspect is to scrutinize the evidence for ongoing changes in fossil communities, in order to decipher macroevolutionary patterns on a longer time scale.

Our results from foraminiferal studies show that the physical packaging of Cenozoic neritic strata in southern Australia into unconformity-bounded units is chronologically consistent with putative global sequences and sea level. Species abundances used as proxies for various environments display recurring and progressive fluctuations in close harmony with sequence packaging. These third-order patterns of recurrence and sequential change are sustained also at higher frequencies or cycles of sediment packages.

Biofacies characterizations of sequence stratigraphy include: (1) abundance variations of the prominent species coinciding with changes in lithofacies or lithostratigraphy because of a facies effect; (2) assemblages responding to sea level change mainly by species coming and going as well as their abundance; and (3) distinct and strongly successional clustered groups reflecting both sequence packaging and assemblage evolution.

Sequence boundaries inferred from biofacies are proxies to actual sequence boundaries, which should be drawn on erosional or other physical surfaces. There is evidence that the inferred boundaries fit better with physical boundaries at deeper water localities where faunal and physical signals appear to be more consonant. Also emerging is evidence that some of the third-order sequences may have been packaged by periodic subsidence in basins along the southern Australian margin due to a stepwise seafloor spreading over the last 50 million years.

395

THE CENOZOIC RECORD IN SOUTHERN AUSTRALIA: 12 ASSERTIONS Brian McGowran and Qianyu Li Geology and Geophysics, The University of Adelaide, Adelaide SA 5005 L The stratigraphic record falls naturally into three parts or second-order allostratigraphic packages: (i) Early Palaeogene siliciclastics, late Paleocene and early Eocene; (ii) late Palaeogene to early Neogene (Eocene-Miocene) including the most extensive marine transgressions, carbonates and coals; (iii) late Neogene (Miocene-present) including carbonates.

2. The stratigraphic record reflects its sensitive mid-latitude situation during Cainozoic time.

Examples are: extratropical excursions by large, benthic, photosymbiotic foraminifera; switch-on and switch-off behaviour by the Leeuwin Current; biostratigraphic windows supplied by planktonic foraminiferal incursions.

3. Australia's motion northwards (equatorwards) has been exaggerated as a control on its biogeohistory.

Influential palaeoceanographic shifts and reversals across tens of degrees' latitude are much faster than plate motion.

4. The modern neritic environment is an heuristic model for the late Neogene, less so for the older neritic environments.

The modern is "high-energy lagoonal" in a contracted trophic resource continuum whereas antecedent environments were frequently 'iow-energy estuarine" in an expanded TRC. Modem biotic associations are icehouse-robust not greenhouse-fragile.

5. The Cainozoic record overall follows the trend of global cooling including reversals.

Early Palaeogene siliciclastic wedges are consonant with invigorated spreading and marginal subsidence; the "prograding cap" better samples the maturing Southern Ocean; warming reversals are reflected faithfully in the regional patterns.

6.The second'order allostratigraphic packages comprise well-punctuated third-order packages.

Local unconformities are seen to be regional and local marine transgressions are seen to be regional and rapid (at third-order scales).

7. Biofacies packages reflect the third-order allostratigraphic packages.

Biofacies based on foraminifera cluster strongly according to the distribution of unconformities and transgressions; biofacies record an overall trend through Cainozoic time of neritic ventilation consistent with cooling and more vigorous circulation.

8. Regional configurations are consistent with the putatively global third-order patterns of sequence stratigraphy.

Physical unconformities tracked also by biofacies are consistent with sequence boundaries (in marginal basins) and third-order glacials (inferred from oceanic oxygen isotopes).

9. Regional stages have numerous parallels with other chronostratigraphic systems.

Chronological parallels with New Zealand, Paratethys and standard stages are powerful evidence for some exogenic (including eustatic) effect penetrating regional and tectonic influences on stratigraphic patterns.

10. Regional chemofacies are not diachronous.

Neritic chemofacies (chert-carbonates, opaline silicas, primary dolomites, dark sediments) are synchronous and packaged.

11. Regional chronofaunas have parallels between biogeographic and environmental realms.

Biofacies demonstrate oceanic-neritic links bridging to the terrestrial realm, and TethyanIndoPacific links with coeval turnovers at end-Maastrichtian, late Bartonian, end-Priabonian, and end-Langhian ages implying environmental forcing not self-organizing.

12. Benthic communities are ''Gleasonian'' not ''Eltonian'' in their responses to environmental perturbation.

Benthic foraminiferal communities tend to be haphazard samples of a regional species pool ("Gleasonian" ecological state) rather than associations of interdependent and co-evolving species each in its own special niche ("Eltonian").

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ANTHROPOGENIC CO, ADDITION AND FUTURE TROPICAL SEA SURFACE WATER CHEMISTRY: A COMPARISON TO THE PAST 70 MILLION YEARS. Bradley N. Opdvke and Robert W. Buddemeier^ The Australian National University, Department of Geology, Canberra ACT 0200 ^ Kansas Geological Survey, University of Kansas, 1930 Constant Ave., Lawrence, Kansas 66047 Rising atmospheric CO2 concentrations will reduce calcification rates within coral reef ecosystems around the world over the next century. This will probably have a profound effect on those ecosystems. As geologists we can look to the past to predict how these ecosystems will respond to changes in atmospheric chemistry. To find ancient analogues to what the ocean chemistry will be like in the 21'^ century, we must look to the Paleogene. So we ask the question: What were tropical calcium carbonate saturation states like throughout the Cenozoic? And what was the biotic response within the neritic carbonate communities to these changes?

It appears obvious that pCOi concentrations in the atmosphere and ocean were higher during the Paleogene. Data and modelling also indicate that the Mg/Ca of sea water would have been much lower than we measure today. Both these indicators would lower the relative ease with which aragonite precipitates in the modem ocean and favour calcite precipitation. In this talk we will discuss the implications of these changes on the neritic biota and the morphology of shallow water carbonate deposits on a global scale. We can also demonstrate that predicted near-fixture conditions in ocean chemistry may be without evolutionary precedent.

397

EVOLUTION OF THE ENVIRONMENT, VEGETATION AND CLIMATE DURING OLIGOCENE-PLIOCENE IN THE WESTERN MURRAY BASIN: PALYNOLOGICAL EVIDENCE Lilliana Stoian Office of Minerals and Energy Resources, PIRSA Palynological analyses of samples from several drillholes in the western Murray Basin show distinctive microfloristic assemblages. Dinoflagellate cysts offer a reliable method for distinguishing the Late Miocene-Early Pliocene Bookpumong Beds from similar marginal marine sediments deposited during Oligocene-Middle Miocene transgressions. The following palynological zones has been recognised: Upper Nothofagidites asperus Zone (lowermost Oligocene), Proteacidites tuberculatus Zone (Late Oligocene to Late Early Miocene), Canthiumidites hellus Zone (Late Early to Middle Miocene) and Monotocidites galeatus Zone (Late Miocene to Early Pliocene).

Based on the interpretation of lithology and fossil content, the environment of deposition of the Olney Formation (Early Oligocene into Early Miocene) is interpreted as generally non-marine in the eastern part of the basin, and marginal marine to coastal-plain, fluvio-deltaic in the western part. The presence in PIVA 10 of abundant Nothofagus spp. with common Phyllocladidites mawsonii indicates a rich evergreen Nothofagus dominated rainforest in the vicinity of the drillhole, and a coastal swamp and marsh environment developed nearby the site.

The deposition of Geera Clay (Late Oligocene-Early Miocene) is associated with a shallow to marginal marine environment. The presence of grass and sedge pollen, freshwater dinoflagellate cysts and freshwater algae Botryococcus in these sediments suggest a fluvio-deltaic and fluviolacustrine environment. The fossil assemblage at the top of the unit indicates the presence of a swampy environment.

The Winnambool Formation (Early Miocene-Middle Miocene) was deposited in a shallow-marine environment adjacent to an estuarine and fluvio-lacustrine environment, based on highest abundance of marine Dinoflagellate cysts and fresh water algae Botryocccous. The Bookpumong Beds were deposited close to an estuarine environment, which graded into a shallow marine to brackish assemblages. The top of the unit indicates a fluvio-deltaic environment.

Reconstruction of vegetation types in the western Murray Basin is based on fossil pollen and spores and samples from PIVA 10 and Oakvale-1 drillholes have been taken into account to define the major pollen groups and their vertical distribution.

During the Oligocene the floras were dominated by Nothofagidites Podocarpaceae, Casuarinaceae and Araucariaceae and rare Myrtaceae.

spp. with

common

From Early Miocene into Middle Miocene both Nothofagidites spp. and Myrtaceae dominate the palynofloras. There is a decline of cool-temperate Nothofagus evergreen rainforest, the climate became drier and Araucariaceae, Casuarinaceae and Myrtaceae occupied the coastal habitat, with minor mangrove vegetation developed along coastline.

From Late Miocene into Early Pliocene a more open forest developed with Myrtaceae, Araucariaceae and Casuarinaceae replacing the Nothofagus-dommditQd rainforest. In coastal areas Casuarinaceae and non-Eucalyptus Myrtaceae were present and some mangrove vegetation developed along the shoreline. Forest communities became more open, allowing low shrubs and herbs to become prominent.

398

PALAEOSALINITY TRANSFORMATIONS OF THE MIOCENE - PLIOCENE BOUNDARY IN SOUTHEASTERN AUSTRALIA: EVIDENCE FROM FOSSIL OSTRACODA Mark T. Wame School of Ecology and Environment, Deakin University (Melbourne Campus) 221 Burwood Highway, Burwood, Vic., 3125

Marginal marine ostracods One of the most conspicuous palaeoenvironmental changes to occur during the Miocene - Pliocene transition in southeastern Australia was the emergence of broadly preserved marginal marine palaeoenvironments typical of coastal barrier and estuarine depositional settings. This is evidenced by the common occurrence of euryhaline Ostracoda (i.e. Osticythere sp.) in sediments of latest Late Miocene and Pliocene age deposited along the northern bounding coastline of Bass Strait (i.e.

Black Rock Sandstone, Moorabool Viaduct Sands, Wannaeue Formation). Euryhaline Ostracoda are those adapted to habitats subject to fluctuating salinity conditions.

Major palaeoenvironmental changes around this time are also suggested by the appearance of marginal marine Ostracoda in some offshore oil well sections of the Gippsland Basin. For instance fossils of brackish water ostracods (i.e. Cytheridea sp) occur as rare presumed allochthonous bioclasts in open marine neritic facies within the Kingfish 8 well at 858.5m depth (Wame and Whatley, 1994). This particular microfossil occurrence probably records down-shelf current transportation of brackish water forms into offshore palaeoenvironments via submarine channels.

It may also point to the presence of late Neogene fresh water plumes extending out into Bass Strait from river mouths along the southeastern coast of mainland Australia.

Neogene tectonics, volcanism and palaeoclimates Within offshore Bass Strait and onshore hinterland stratigraphic sections in which later Neogene marine faunas have been recorded, there is always present a major shift in ostracod biofacies across marked unconformity surfaces of the Miocene - Pliocene transition (i.e. Nepean 1 borehole at 178.3m depth; Wame, 1993). These biofacies shifts reflect changes in depositional conditions that were probably caused by a combination of regional hinterland uplift, associated increased rates of basin sedimentation and latest Miocene to earliest Pliocene transgressive and high global (eustatic) sea levels.

The sudden appearance of conspicuous euryhaline Ostracoda around the Miocene - Pliocene boundary in southeastern Australia broadly corresponds in time with the appearance of fresh water (lacustrine) ostracod faunas within the Cenozoic stratigraphic record of southeastern Australia.

The later event is partly a consequence of the common damming of stream and river courses by basaltic lava flows, which were initiated around this time (Newer Volcanics). However, the combination of euryhaline and fresh water ostracod appearances around the Miocene - Pliocene boundary also suggests a change to wetter palaeoclimatic conditions and associated increased fresh water influxes into marginal marine coastal realms of the Bass Strait seaway.

References Wame, M.T., 1993. Micropalaeontological evaluation of eustatic and tectonic influences on Late Tertiary marine sedimentation within the Port Phillip at Western Port Basins, Victoria, Australia. In\ McKenzie K.G. and Jones P.J. eds. Ostracoda in the Earth and Life Sciences, pp. 259-275. A.A. Balkema, Rotterdam, Netherlands.

Wame, M.T. and Whatley, R.C., 1994. Palaeo-oceanographical significance of Miocene deep sea Ostracoda from the Kingfish-8 well, Gippsland Basin, S.E. Australia. Australian Journal of Earth Sciences 41, 525531.

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KARST DEVELOPMENT AT NARACOORTE, SOUTH AUSTRALIA: WHY THERE? Susan White^ and John Webb^ 1 Dept of Earth Sciences, Latrobe University Bundoora, Victoria 3083 The Lower Southeast Region of South Australia and a substantial part of southwestern Victoria form a limestone province with discontinuous areas of intensive karst development such as Naracoorte, where the relatively high density of caves is atypical for the karst province as a whole.

The Naracoorte caves have attracted a great deal of interest and research since the discovery of extensive Pleistocene vertebrate fossil deposits in 1969. The solution pipe entrances to some chambers acted as pit traps, and in Victoria Fossil Cave, large, now extinct animals like the thylacine, marsupial lion (Thylacoleo), giant kangaroos and diprotodontids fell into the cave, and are preserved in the extensive bone deposits. The fossils accumulated in several episodes during the Middle Pleistocene (100,000-400,000 years ago).

The entrances to the Naracoorte caves are either vertical solution pipes less than 1 m across and up to 20 m deep or collapse windows. Both open into horizontal systems of collapse chambers floored by rubble cones and connected by low, wide, solutionally sculptured passages often partially filled with sediment. Sand from dunes on the surface sometimes falls into the caves through such solution pipes to form distinctive sand cones. The sand plugging the pipes may collapse from time to time to form a surface sinkhole. Phreatic passages, sometimes with spongework, are present, particularly in the lower levels near the water table, where they may be partially filled with clay. Caves that reach the water table have still, shallow pools with calcite rafts on the surface; the level of these lakes has fluctuated over time in response to changes in the regional water table. The longest cave, Victoria Fossil Cave, has over 3000m of an extensive rambling network of large collapse chambers and smaller connecting passages, some very well decorated.

Naracoorte cave passages show a preferential northwest-southeast alignment, more or less parallel to the Kanawinka Fault which cuts through the host Gambier Limestone as a cliff-line formed originally by fault movement, but later modified by wave erosion. The fault has moved in the last few million years; the uplifted northeastern side is topographically higher.

Karstification at Naracoorte may have started when the sea retreated and first exposed the limestones to weathering around 15 million years ago. Cave formation probably slowed when the sea returned million years ago and covered the area for 2-3 million years. Around 900,000 years ago the sea advanced to the Naracoorte area for the last time, forming the sea cliff and sand dunes there. The caves may have been flooded by this sea level advance. Blanche and Alexandra Caves contain columns and flowstone (not yet dated) that have been extensively dissolved, exposing their internal layering, and suggesting that the caves were flooded after the speleothems had formed. The Naracoorte caves have remained dry since then. In Victoria Fossil Cave the oldest flowstones, growing directly on the floor of a chamber, are over 500,000 years old. Calcite deposition in the caves occurred during times when the climate was relatively wet, most recently about 20,000 years ago; there is only limited speleothem deposition at present.

It is uncertain why cave development at Naracoorte is so extensive, but it may be reflect a greater input of aggressive water at this location, perhaps from the nearby Mosquito Creek, and possibly accentuated by uplift along the Kanawinka Fault. Alternatively, there may be structural control of groundwater flow, so that it has been focussed by a greater concentration of joints and fractures in this area.

400

REGOLITH-LANDFORM MAPPING AT CUDGELL CREEK, CENTRAL WEST NSW: DRYLAND SALINITY HAZARD MITIGATION IN GRANITIC LANDSCAPES. Bree Agar, Kathleen Harvey and C. Leah Moore Dryland Salinity Hazard Mitigation Program, University of Canberra, ACT 2601 An emerging dryland salinity problem has been recognised in regolith developed on the Young Granodiorite, in Cudgell's Creek Catchment, central west New South Wales. Indicators of rising water tables and introduced salt include: dead trees, clear standing water, increased presence of salt and water tolerant vegetation, new areas of salt scalded ground, reduction in crop yield, extensive road degradation, and presence of waterlogged soils and hummocky ground. The regolith developed on this rock type has the ability to buffer saline solutions. When first introduced it takes time for the indicators of dryland salinity to manifest in the landscape. Hence, by the time a significant dryland salinity hazard is recognised in this area, the processes causing the salinity are well developed. The original source of salt in the Young district is believed to be cyclic salt from marine aerosols and in windblown dust (parna) introduced to the area over an extended period of time. It appears that near surface fluid flow mobilises salt from the regolith zone.

Nineteen regolith-landform units were defined during compilation of a 1:15 000 scale map.

Alluvial channels range from narrow, incised (up to 3 m deep and to 5 m wide) ephemeral channels, to deeper alluvial (10 m to 20 m wide and greater than 5 m deep) channels with permanent water. Alluvial plains are mainly situated at the lower reaches of the catchment where Cudgell's Creek enters Burrengong Creek. Bedrock crops out as tors and has been subdivided into bedrock on hills (90 m - 300 m), low hills (30 m - 90 m ) and rises (9 m - 30 m), and sub-cropping granodiorite with less than 1 m of colluvial cover. The remaining units describe the erosional and depositional colluvial landforms that dominate the catchment.

Regolith-landform mapping has assisted development of a landscape evolution model for the Cudgell's Creek catchment. In the Early Silurian, the Young Granodiorite intruded. Subsequent uplift and erosion has exposed the batholith. In the Quaternary topographic highs were dusted with aeolian material (pama) originating from inland Australia. Weathering, erosion and anthropogenic modification have shaped the present landscape.

A ground electromagnetic (EM) induction survey will be conducted to assess the magnitude of the dryland salinity problem. Electrically conductive materials within the regolith include pore water (fresh and saline) and charged clay minerals. Analysis of regolith profiles has allowed clearer understanding of the porosity and primary permeability of regolith materials, factors that control shallow fluid flow. Mapping provides evidence of spatial distribution of regolith-landform units and this will facilitate interpretation of the EM survey results.

Regolith mapping is an essential part of a multi-disciplinary approach to dryland salinity hazard mitigation in upland areas. Tools that allow effective interpretation of EM surveys in the Young district directly contribute to modification of land management practices.

Acknowledgements The authors acknowledge support of this project by the Department of Land and Water Conservation, Central West NSW, and the Cudgell's Creek Landcare Group.

401

REGOLITH AND GEOLOGY OR REGOLITH GEOLOGY? R. R. Anand CRC for Landscape Environments and Mineral Exploration, CSIRO Exploration and Mining, ARRC, 26 Dick Perry Avenue, Kensington, WA 6151.

Introduction This presentation review^s the needs of the exploration and environmental industries and points to some important trends in current and future regolith geoscience research. Until recently geologists and regolith researchers have tended to ignore each other despite the controls of the shape of the hills and valleys and the regolith on them lie in the underlying lithology, its strength, structure, local tectonic and volcanic features, the regional tectonic situation, climate and palaeoclimate.

Which of these is dominant depends on the size of the area and on the time-scale of development.

Similarly, the regolith can be used to interpret past geological and climatic conditions.

The Australian regolith is the product of the processes of weathering, erosion, deposition and physical and chemical transformation. It reflects a complex interplay of the forces of tectonism, changes in sea level and climate that have occurred since the Late Palaeozoic. These forces have and are still moulding the continent. They have left a landscape underlain by residual and sedimentary regolith of various ages that is the focus of mineral exploration in Australia. Regions of outcrop, shallow subcrop and, in many provinces, in situ regolith have been well explored so that the greatest potential for further discoveries now lies beneath areas of substantial cover. These covered areas are complex in their sub-surface relationships, regolith type, stratigraphy, facies variation, ages and types of weathering. Many inter-relationships and formation processes are poorly understood, so exploration is difficult, expensive and successes are few.

As with mineral exploration, understanding environmental issues requires knowledge of the characteristics, origins and evolution of regolith materials. Properties of regolith influence surface and subsurface patterns of groundwater recharge and discharge, the amount and composition of groundwater and the directions of flow. Knowledge of these is vital in understanding and managing the landscape.

The need There is a need to determine: • Changes in tectonism, eustasy and climate and their effects on the landscape and the regolith through time.

• The distribution, stratigraphy and sedimentology of the sedimentary successions.

• A three dimensional understanding of the regolith.

• Dating of the principal events in the evolution of the regolith.

• The understanding of chemical, biological, mineralogical and hydrological processes in the formation of regolith and consequently geochemical anomalies.

• The role of biota and organic materials, and their impact on regolith formation.

• The relationships between weathering events, weathering processes and geochemical dispersion from mineralisation.

• The influence of regolith properties on geophysical signals from concealed deposits.

• The full potential of geophysical techniques for 3-D mapping of the regolith.

402

GEOPHYSICAL METHODS IN SALINE GROUNDWATER STUDIES: LOCATING PERCHED WATER TABLES AND FRESH-WATER LENSES ^Barrett. B., ^Heinson, G., ^Hatch, M. and ^Telfer, A. ^Department of Geology and Geophysics, Adelaide University, Adelaide SA 5005. ^Zonge Engineering and Research Organisation, 98 Frederick Street. Welland SA 5007 ^Australian Water Environments, Level 1 - 68 The Parade, Norwood SA 5067 Ph. +61-8-8303-5493 Fax +61-8-8303-4347 Email: Brian.Barrett@adelaide.edu.au The effectiveness of ground-based, non-invasive geophysical tools for the delineation of stratigraphy and hydrology in saline ground-water environments has been tested in an area covered by a salt interception scheme, near the tov^n of Waikerie, South Australia. A fast and informative tool is required for monitoring salinisation effects around the Stockyard Plains Disposal Basin (SPDB) that is used as a storage lake for salt water pumped from aquifers close to the River Murray. Such salinisation effects include the formation of a perched saline water-table close to the SPDB and consequent threats to local fresh-water lenses.

Hydrological observations from boreholes provide accurate in-situ information, but are expensive to drill and maintain and require significant time commitments. Additionally, point-source information from these boreholes must be interpolated between widely-spaced samples. An alternative approach is to use geophysical techniques that have traditionally been applied in the mineral exploration industry. Ground penetrating radar (GPR), direct current (DC) resistivity, transient EM (TEM) and low induction number (LIN) frequency domain EM (FEM) were compared in terms of efficiency and quality at two survey locations near the SPDB. LESf FEM measurements were fast to implement but suffered from a non-linear response with ground conductivity in high conductivity areas. GPR methods were less effective in this project, due to high signal attenuation. Fast time-sampling TEM successfully depicted a perched saline watertable related to leakage from the SPDB, and was the most useful technique for delineation of hydrogeology due to a high vertical resolution. DC resistivity was the slowest technique, but was successful in imaging a fresh-water lens. Results from this study suggest that TEM and DC Resistivity methods are useful tools for both fresh-water detection and hydrogeology monitoring in saline groundwater environments.

403

REGOLITH DISTRIBUTION IN THE CANOWINDRA NORTH AREA: IMPLICATIONS FOR DRYLAND SALINITY HAZARD MITIGATION. Kristy Bewert and C. Leah Moore Dryland Salinity Hazard Mitigation Program, University of Canberra, ACT 2601 A relatively deep (up to 30 m thick) regolith veneer in a belt from Canowindra to Cudal in central WQst NSW, has provided the foundation for crop-based agriculture for several decades. Soil studies in this area in the early to mid-1990's indicated the presence of salts in the subsurface, but declared that their was "no salting evident" at the land surface. A little over half a decade later there is now widespread indication of land saturation and salinisation including reduced crop yield, presence of salt and water tolerant plant species in increasingly higher parts of the landscape, saline scalds in paddocks and deterioration of roads. The average watertable depth in the area is 1.5m. In some areas, it comes within 0.5m-Im of the land surface. Areas with elevated water tables show gullying (moderate to extensive) and moderate to high structural degradation hazard. The susceptibility to gully erosion, particularly in drainage lines, and structural instability leading to heightened erosion are characteristic of salinity-affected areas.

This study identifies that the spatial physiographical, geological, structural, hydrogeological, pedalogical and vegetation information available in the Canowindra-Cudal region is at a scale (typically 1:250,000) that cannot be readily utilised for land management planning. A systematic program of focussed regolith-landform, structural geology and hydrogeologic maps need to be developed for the area, to identify the local and regional influences on groundwater systems, to evaluate the salt store in this region and to better understand the shallow and deep fluid flow through the landscape in this area.

The Canowindra-Cudal area is structurally complex and groundwater is able to move through the an extensive network of major faults, splay faults and joint and cleavage planes in the fractured rock aquifers that underlie the regolith materials. Land managers are charged with mitigating against the hazard associated with salt movement through the landscape via regional and local groundwater systems while attempting to maintain productivity of their land.

Mapping the distribution of regolith materials in the landscape is fruitful where the regolith veneer is relatively thin as the surface expression reflects what is happening in the shallow subsurface.

However, in areas of thick regolith cover, regolith-landform mapping should be supplemented with drilling programs and geophysical surveys in order to constrain the three-dimensional configuration of regolith units. This allows more sound identification of shallow fluid migration pathways.

Detailed structural analysis is required to define the deep fluid migration pathways that influence regional groundwater systems. This work complements ground electromagnetic (EM) induction survey work. An EM survey would help define the magnitude and distribution of the saturation and salinisation problem. A multidisciplinary program of this nature would facilitate identification of the causes of land saturation and salinisation problems and would help land managers to strategically and cost-effectively manage these problems.

Acknowledgement The authors acknowledge support of this project by the Department of Land and Water Conservation, Central West NSW.

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PRELIMINARY RESULTS FROM GEOCHEMICAL DISPERSION INVESTIGATIONS, LUXEMBURG WORKINGS, CURNAMONA PROVINCE, SOUTH AUSTRALIA. Aaron Brown, Ian Lau, Amy Kemich, Andreas Schmidt-Mumm, Pat James, Graham Heinson and Martin Williams Adelaide University and CRC LEME Email: aaron.brown@adelaide.edu.au The Proterozoic Cumamona Province lies across the border of South Australia and New South Wales, with outcrop in the Willyama, Mt Painter and Mount Babbage inliers. Based on lithological and geophysical differences the southern part of the province has been divided into two domains, the Olary Domain (OD), and the Broken Hill Domain (BHD). The Cumamona Province has widespread occurrences of Au and basemetal mineralisation, and due to the location of the world's largest Pb-Zn-Ag deposit within BHD, the area has been of interest for explorationists. However due to the extensive cover of Mesozoic and Cainozoic sediments and regolith across much of the Province, identification of mineralisation has proved difficult. The potential for another large deposit is considered high, and a greater understanding of the landscape evolution, regolith development and the chemical dispersion pathways will assist in the identification of mineralisation under cover.

An integrated approach using geochemistry, regolith mapping and remote sensing has been applied to the problem of the three dimensional chemical dispersion at the Luxemburg Cu-Au workings.

The w^orkings occupy a shallow valley, 25km east of the Olary township, with lithologies around the main workings dominated by a paragneissic sequence (layered gneiss, quartzite and migmatitic Gneiss), various granites (A-type, foliated and S-type), and an amphibolite body. This locality has been chosen for study due to the high chemical contrast between the amphibolite, and the remaining lithologies, thus providing a strong signal for tracing the geochemical dispersion from the amphibolite into the abundant regolith cover in the area. Geochemical samples have been collected from the regolith profile using percussion coring. Samples from the cores have been taken for analysis by icp-ms, xrd and xrf Preliminary analysis and interpretation is currently underway.

A regolith map of the valley containing the main workings has been produced, by the interpretation of orthoimagery, a digital elevation model (DEM), Advanced Spacebome Thermal Emission and Reflection Radiometer (ASTER) satellite data, and field mapping using a modified RED scheme.

ASTER satellite remotely sensed data vs^as used to map the Luxemburg Prospect in a number of ways. Thermal Infrared (TIR) and Short-wave infrared (SWIR) spectra were analysed and edgemembers identified, allowing for mapping of the amphibolite, quartz and kaolinite as well as the discrimination of the mine workings and lithologies in the area. Orthoimagery has been used in the production of a digital elevation model (DEM) and to georectify and verify the results from the ASTER data. The DEM was used for landform mapping, drainage analysis, and in the three dimensional integration of geochemistry, field mapping and remotely sensed data.

This integrated approach allovs^s for the production of three dimensional elemental dispersion maps, and the evaluation of the effects of different parts of the regolith profile on geochemical dispersion patterns.

405

CENOZOIC DIVERSIONS OF THE SHOALHAVEN RIVER THE TALLONG BEND REVISITED M. C. Brown 51 Debenham St., Mawson, A.C.T., 2607 The upper Shoalhaven River in southeast New South Wales flows north for 110 km in a broad highland valley 500-700 m a.s.I., in which there are Oligocene and Eocene terrestrial sediments and mid-Eocene basalts. It abruptly turns east into the lower Shoalhaven at the Tallong Bend near Tallong and flows into the sea 50 km to the east. The river is deeply incised into Palaeozoic bedrock near Tallong in the Shoalhaven Gorge, about 500 m deep and 2 km wide. The lower Shoalhaven and tributaries have a barbed pattern, with several tributaries entering from the northeast or southeast. Between the lower Shoalhaven and the east-flowing Tallowa Gully 3-6 km to the north is a 10 km long palaeovalley, the Caoura Palaeovalley. The palaeovalley bed is about 520 m a.s.I. at its western end and 500 m a.s.I. at its eastern end, and it is filled with up to 130 m of Oligocene basalt and sediment. It is truncated by the Shoalhaven Gorge at its western end 2.5 km downstream from Tallong Bend. At Iron Pot Clearing on the south side of the gorge 6 km downstream of Tallong Bend is another palaeovalley 1.5 km long and trending WNW. It is filled with about 90 m of Oligocene basalt and sediment, and its bed is about 540 m a.s.I. It is truncated at its west end by the Shoalhaven Gorge and at its east end by a tributary gorge.

About 6 km north of the Tallong Bend is the low divide on Palaeozoic rocks between the Shoalhaven and the Wollondilly River which drains north. There are 4 gaps in the divide at altitudes between 620 and 650 m. W. G. Woolnough and Griffith Taylor proposed in 1906 that the Shoalhaven formerly continued north across this divide to join the Wollondilly; and was diverted east in the late Cenozoic along a reversed former tributary. The Tallong Bend and the barbed drainage downstream strongly suggest this; but the river was apparently diverted east no later than mid-Eocene, because the bed of an Eocene tributary palaeovalley 35 km upstream at Nerriga is at least 110 m lower than gaps in the Shoalhaven-Wollondilly divide.

F. A. Craft in 1931 mapped the basalt and the Caoura Palaeovalley. He showed that Shoalhaven drainage was to the east before basalt filled the palaeovalley, and suggested that basalt formerly dammed the Shoalhaven, causing the alluviation upstream. These findings were confirmed by J. F. Nott in 1992 who showed that Oligocene sediments upstream, up to 120 m thick and the same age as the basalt, were mainly lacustrine. Nott concluded that the basalt dam was in the present valley, which was almost as wide as now but only 120 m deep; and that the basalt flowed into it from shallow tributary valleys.

Truncation of the Oligocene palaeovalleys by the Shoalhaven Gorge, the shape and size of the Caoura Palaeovalley, and a lack of evidence that the lower Shoalhaven was in its present position draining east before eruption of the basalts, indicate that Oligocene easterly drainage was along the Caoura Palaeovalley. Tallowa Gully appears to be a lateral stream; and I interpret the lower Shoalhaven south of the palaeovalley as a reversed former tributary which originally flowed west through the Iron Pot Palaeovalley and joined the Caoura Palaeovalley at its western end. The Oligocene lake level may have been high enough to overflow the Shoalhaven-Wollondilly divide, temporarily reinstating the ancestral northerly drainage before it was drained by headward erosion of the lower Shoalhaven.

Oligocene easterly drainage via the Caoura Palaeovalley implies later minor upwarp of the Tallong area relative to the upstream valley, since the base of Oligocene lacustrine sediments 20 km upstream is up to 30 m lower than the bed of the palaeovalley. Deep incision of the Shoalhaven Gorge is best explained as the result of 400-500 m of mid to late Cenozoic relative downwarp of the coast. The probable early Cenozoic or earlier easterly diversion of the Shoalhaven may have resulted from an earlier phase of this deformation.

406

BIOGENIC OPAL IN THE REGOLITH: AN IDEA WHOSE TIME HAS COME? Jonathan D. A. Clarke CRC LEME, Department of Geology, Australian National University, Canberra, ACT 0200 The role of organisms in the behaviour of silica in marine and lacustrine environments has long been recognised. They play a key role in the formation of siliceous oozes and chert deposits, in phytoplankton nutrition, and in the global cycling of silicon. Less well understood is the role played by silica-depositing organisms in the regolith, even though the fact they do play a role has been knov^n for at least one hundred and sixty years. In recent decades there has been an increased aw^areness of the importance of such organisms in the terrestrial silica cycle.

Biogenic opal produced by vascular plants, diatoms, and siliceous sponges has been found in soils and terrestrial sediments of all continents except Antarctica since the middle of the 19 century.

The opal particles range in size from fine silt to fine sand. Almost all soils contain detectable opal up to levels of 2-3%, and a significant number contain values in excess of 5%. Even higher values have been found from soils and sediments of all continents in a wide range of soil types. The most important factor is poor soil drainage and seasonal to permanent water logging. This encourages the proliferation of silica- producing organisms. Such conditions have been found in the soils and aquatic sediments of the monsoonal tropics, tropical rain forests, temperate forests, tropical savanna, tropical islands, semi-arid grasslands and savanna, and temperate woodland and grassland. The presence of a volcanic substrate also appears favourable in some cases, but is not necessary in all. Biogenic opal preferentially collects in the A horizon of soils and, to a lesser extent in the B horizon. This preferential distribution facilitates identification of palaeosols in stacked sediment sequences. Biogenic opal is also a component of windblown dust, even in arid environments.

Biogenic opal is significant to regolith processes in a number of ways. Firstly, as is the case in marine environments, it is likely to be important in silica cycling and storage because of its greater lability compared to quartz. Secondly, dissolution and reprecipitation of opal A as opal CT or micro-quartz may play a role in cementation and silicification of regolith to form silica hardpans and silcrete.

Thirdly, the organisms that form biogenic opal have can considerable palaeoenvironmental significance and be valuable in reconstructing regolith evolution. Finally, some forms of biogenic silica, in particular sponge spicules, can present a health hazard. Their high abundance in some soils and sediments needs to be considered when assessing the health implications of airborne dust.

Further research is needed to more accurately understand the source of biogenic opal. The distribution of biogenic opal in a wide range of landscapes from the tropics to the deserts and alpine regions must be clarified. The factors that control the accumulation and diagenesis of biogenic opal in the regolith, especially with respect to the formation of hard setting soils, hardpans, and silcretes, is another potentially fruitful avenue for research. There is a need for a better understanding of the distribution of terrestrial biogenic opal with time. Finally, more work is needed on the geochemistry of terrestrial biogenic opal, especially whether it has a distinct trace element of isotopic signature and the role it may play in the establishment of geochemical anomalies.

407

REGOLITH MAPPING AND THE SEARCH FOR GROUNDWATER IN A KARST TERRAIN, BALLADONIA, WESTERN AUSTRALIA Ian Anderson and Jonathan D. A. Clarke CRC LEME, Department of Geology, Australian National University, Canberra, ACT 0200 Balladonia lies on the western margin of the Eucla Basin. The area is under pastoral lease and numerous exploration licences have been issued in the past, especially for lignite, gold, uranium, and mineral sands.

Potable groundv^ater is a major limiting factor in the region, though hypersaline w^ater can be obtained from the palaeovalleys peripheral to the Eucla Basin. At present potable w^ater can only be obtained from diverting runoff from areas of granite outcrop. The search for potable groundwater in the Balladonia region has been ongoing for over 130 years, without success.

The stratigraphy of the western margins of the Eucla Basin extends consists largely of Eocene and Miocene sediments. Stratigraphic architecture is modified by proximity to the granitic AlbanyEraser Range, with more clastic sedimentation close to the margins of the basin and around granite inselbergs. In this western embayment we find white chalky bryozoan limestone Eocene Wilson Bluff Limestone on the seaward side of a line of granitic highs grading into the coeval biosiliceous clays, silts, and sands of the Pallinup Formation behind the barrier. These overlie carbonaceous silts and sands of the Pidinga Formation along the extreme western margin, and are overlain through much of the area by the Miocene Nullarbor Limestone.

The Balladonia region is made up of several landforms. The eastern region consists of a karst plain with a series of indurated or calcreted rises. Intervening depositional regions are large karst depressions filled with a mixture of limestone floaters, colluvium from the flanks of residual rises and stratified alluvial and aeolian sands, silts and clays. Calcrete in this region is slabby and massive, with pedogenic carbonate pisoliths accumulating in the generally thin soil horizons.

The western margin is dominated by a low relief scarp, approximated by the 170 m contour on the DEM, cut into Eocene sediments. This elevation is equivalent to the Miocene highstand, with calcreted Eocene sediments having thick accumulations of pisolitic calcrete. The region is punctuated by a series of inliers of Precambrian crystalline basement or granite inselbergs. These outcropping granites are generally surrounded by benches of indurated limestone, and many were probably islands or shallow reefs during the last major transgression in the Miocene.

Unconsolidated cover materials appear to have a more complex mineralogy than in the eastern regions; the sources include granitic materials from the Eraser complex and amorphous silica, possibly from spicular sediments.

Vegetation associations are a key to mapping the regolith landforms in this region, as on the karst plain stands of trees are found exclusively on the topographic highs of the calcreted residual limestone rises, with depositional depressions giving way to mainly low relief salt bush and blue bush. The terrace of Eocene sediments also well covered by trees. Large native trees are also absent around outcropping granites, and in regions where crystalline basement is close to the surface.

408

EXTRACTION OF BIOGENIC SILICA FROM SOILS AT BARRACKS FLAT, QUEANBEYAN, NSW Angela Harrison and Jonathan D. A. Clarke CRC LEME, Department of Geology, Australian National University, Canberra, ACT 0200 Biogenic silica is a common component of Australian soil, although little investigation into this aspect of Australian soil has been undertaken. Three major types of biogenic silica commonly occur in soil. These include sponge spicules, diatom frustules but, most importantly, phytoliths.

Barracks Flat was selected as a suitable study area because the samples had been collected for a previous and unrelated study.

Barracks Flat is located on the southeastern fringe of Queanbeyan, in the southern tablelands of New^ South Wales. This small flood plain (about 100 m by 300 m) is formed on Barracks Creek, a tributary of the Queanbeyan River. Quaternary alluvial deposits infill the flood plain and can be divided into three distinct horizons. A yellow to brown coloured sandy clay loam forms the upper horizon. A dark brown medium to medium heavy clay layer lies about a metre below the surface in the south of Barracks Flat and is exposed in the north. Below these clays is a second sandy clay loam to loamy sand horizon.

The initial aim of this investigation was to determine the total weight percentage of biogenic silica, as well as the grain proportions of spicule, diatom and phytolith present in soil samples from Barracks Flat, Queanbeyan, NSW. The samples investigated in this experiment were acquired from two auger core holes with depth intervals averaging 15 cm. The method used was physical separation of the opal fraction by size and density and then determining proportion in the soil by mass. The weight percentages of biogenic silica range from less than 1% to above 7%. The accuracy to which these were determined is quoted to two decimal places. The bulk of the identifiable opal was as phytoliths, with less than 5% of the opal as diatoms and sponge spicules combined. The bulk of the opal (up to 65%) was non-identifiable.

An obvious increase in the proportion of biogenic silica is seen at the transition from the upper sandy layer to the dark clay layer. The upper horizon generally has less than I % biogenic silica, whereas the dark clay horizon averages about 3% biogenic silica. The clay horizon shows a decrease in biogenic silica from the top downwards. Hole 1 has nearly 6% biogenic silica near the top of the clay horizon and hole 2 over 7% at this level. In holes 1 and 2, biogenic silica in the dark clay layer decreases to levels higher than those in the upper sandy horizon.

This study confirms that biogenic opal is a significant component of soils. The fact that much of it occurs as unidentifiable fragments suggests that its abundance has been significantly under estimated in previous soil studies. Opal may therefore play a significant, but previously under appreciated role in soil and regolith processes. Further research is planned to compare the results obtained from physical separation with those obtained by chemical analysis.

409

PREGLACIAL DRAINAGE AND PLACER DIAMOND POTENTIAL IN THE NORTHWEST TERRITORIES OF CANADA Alejandra Duk-Rodkin Geological Survey of Canada, 3303-33rd Street, N.W., Calgary, Alberta, Canada 2L 2A7 A preliminary reconstruction of paleo-drainage at the Lac de Gras area in the Slave Province, Northw^est Territories of Canada indicates that in preglacial times, this region was drained by an east-flow^ing river system. Reconstruction of paleo-topography permitted the placement of the main river axes onto their paleo-thalwegs. Paleo-topographic reconstruction was carried out by eliminating linear erosional features, and by reconstructing surfaces to their former levels based on erosional remnants preserved throughout the Cenozoic. One of these features is a meander scar (of about 8 km radius) that crosses the Lac de Gras area from west to east.

An east-flowing drainage system drained the region from early Tertiary to the end of the Pleistocene. The retreat of the late Pleistocene (Late Wisconsinan) Laurentide ice was responsible for the establishment of a north-flowing Mackenzie River system, thereby establishing an Arctic drainage for this region. This was the only continental ice sheet to reach the eastern slopes of the northern Cordillera. However, it is not known how many continental glaciations may have affected the Northern Interior Plains of Canada without reaching the northern Cordillera, and without drastically changing the drainages of this area.

Kimberlite pipes bearing diamonds are abundant at Lac de Gras and some pipes are being mined in this area. The emplacement time of kimberlite pipes ranges from late Cretaceous to early Tertiary (Eocene). Kimberlite pipes are topped by small lakes of Pleistocene age. Some of the pipes have been extensively eroded (>60%) and it is highly likely that placer diamond deposits occur in the region. Placer potential is suspected because of the east-flowing paleo-drainage systems, of which the meander scar is a remnant. A preliminary classification of the paleo-drainage using the Strahler classification (1:7 000 000 scale), indicates this portion of the drainage system to be a third order stream, and a tributary to a fourth order stream that flowed into Hudson Bay. A comparison of the drainage classification and geologic setting of the Kimberlite pipes of the Lac Des Gras region with those of the Orange River in South Africa, suggests some striking similarities. Both drainage systems have a comparable setting and both have the same stream order. Most interesting however, is that placer diamond occurrences in the Orange River are contained in fourth order streams, suggesting that the same hydraulic conditions which preserved placer diamonds in South Africa, may have been present in the Lac de Gras region. Higher resolution topographic analysis in both areas will need to be carried out.

Placer potential in northern Canada is perhaps higher to the east and southeast of Lac de Gras, possibly as far east as Hudson Bay and the Atlantic Ocean, considering that rivers drained in that direction for approximately 62 million years. It is also possible that some other minor placers might be related to glacial deposits resulting from reworking of surficial materials from the Lac de Gras area, by Pleistocene continental ice that moved in a northwesterly direction.

410

THE RELATIONSHIP OF KLONDIKE PLACER DEPOSITS TO TERTIARYQUATERNARY DRAINAGE EVOLUTION OF YUKON RIVER IN NORTHWEST CANADA Alejandra Duk-Rodkin Geological Survey of Canada, 3303-33rd Street, N. W., Calgary, Alberta, Canada 2L 2A7 The greatest placer potential in northwest Canada lies in pre-glacial drainage systems. The largest placer gold deposits are associated with Pliocene pre-glacial fluvial gravels of the Dawson Range (in particular the Klondike Plateau) of west-central Yukon. The abundance and/or lack of gold concentration in this area was controlled by the ability of the streams to aggrade as a response to changes caused by regional tectonic (formation of the Tintina Trench), differential uplift and regional denudation. Gold-bearing gravels in the northern and eastern slopes of the Dawson Range are found in low order tributaries of the pre-glacial south-flowing Yukon River. Glaciation played an important role in masking and changing the direction of drainage and in creating new channels in this region. Therefore establishing pre-glacial drainage patterns is an important tool in placer exploration in this region.

The earliest known drainage changes relate to late Miocene formation of the Tintina Trench, and early Pliocene uplift of the St. Elias Mountains. Prior to these events the Yukon River drained south into the Gulf of Alaska. Adjustment to the tectonic controls was manifest in cycles of incision and aggradation in both the trunk and tributary streams, and was responsible for the accumulation of thick gold-bearing Pliocene gravels in the west-central Yukon. Initial regional glaciation (c. 3 Ma) diverted southward drainage towards the northwest, becoming part of the Kwikhpak River in Alaska. Minor drainage changes occurred during the mid-Pleistocene Cordilleran Glaciation (ca. 200 ka), and lastly during Late Pleistocene Continental Glaciation (ca 30 ka) when the Laurentide Ice Sheet diverted part of the Arctic drainage (Porcupine River) westward into the Yukon River basin. The present Yukon River drainage basin is about 30% larger than the ancestral Kwikhpak River basin. Retreat of the Late Pleistocene Laurentide Ice Sheet integrated the Arctic and Atlantic drainages on the eastern slopes and adjacent plains, forming the Mackenzie River. Overall, the drainage has been changed by glaciation(s) more than 95% in northwest Canada.

During early Tertiary time the northern Canadian Cordillera formed the headwaters of drainages entering the Atlantic, Arctic and Pacific oceans. Rivers draining most of the eastern slopes flowed to the Labrador Sea via Hudson Bay and the Labrador Sea (paleo-Mackenzie River), while Arctic Ocean drainage discharged into the Beaufort Sea (Porcupine and Peel Rivers) and Pacific drainage discharged into the Gulf of Alaska (Yukon River) and Bering Strait (Kwikhpak River). Disruption of these drainage systems has occurred as a result of tectonic evolution and Tertiary / Quaternary glaciation.

411

CONTRIBUTION OF SCAPOLITE AND PYRITE WEATHERING TO SALINITY IN THE MOUNT TORRENS AREA, SOUTH AUSTRALIA Robert Fitzpatrick, Marian Skwamecki and Mark Raven Co-operative Research Centre for Landscape Environment and Mineral Exploration (LEME)/ CSIRO Land & Water, PMB Glen Osmond, South Australia, Australia, 5064 Dryland salinity in the high rainfall (>550 mm p.a.) catchments in the Mount Torrens region is a growing concern to property holders because of the rapid increase in waterlogged saline scalds. In the lower slopes and flats extremely saline soils (ECse 8- >16 dS/m) comprise 3% and slightly saline soils (ECse 1-4 dS/m) comprise 10%. Saline acid sulfate soils occur in eroded "iron ochre" scalds and exhibit a whitish surface crust when dry with salts comprising chlorides (halite) and sulfates (e.g. sideronatrite, natrojarosite, gypsum and barite). These saline soils are formed because of land clearing, erosion and excess discharge of saline-sulfatic groundwaters. They occur mostly in catchments underlain by metasiltstones of the Naime Pyrite Member (Talisker Calc-siltstone, Kanmantoo Group) that contain high amounts of stored salt. Similarly, subsoil horizons in mid slopes underlain by these parent materials are slightly saline and sodic.

Up until now, it has been believed that salt stored in this regolith was derived over many thousands of years, from salt being accumulated in this old landscape from the large (i.e. usually 20 to 200 kg/ha/yr) quantities of salt blown in from ocean by wind and rain. The aim of this investigation was to determine whether accumulated ions like chloride, sulfate and sodium are associated with rock weathering during regolith formation. This information is critical in understanding the geochemical dispersion processes in regolith in the Mount Lofty Ranges and their implications for dryland salinity and steam water salinity.

Detailed geochemical and mineralogical investigations were conducted on samples from a 26 m profile through weathered (15 m thick pallid zone) to fresh metasiltstones of the Naime Pyrite Member (Talisker Calc-siltstone, Kanmantoo Group) in a diamond drill hole (MTG2) from the Mount Torrens prospect. Samples were examined by optical microscopy, scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and X-ray fluorescence (XRF).

In the fresh state, the rock comprises quartz, biotite, plagioclase, microcline, scapolite and pyrite. Kaolin minerals dominate the altered host rocks in the pallid zone. Scapolite refers to a family of complex tetragonal alumino-silicate minerals containing SO4, CI, Na, CO3 and other anions. The anion composition of the various scapolite minerals commonly is described as a complete solid solution in the ternary system Cr-C03^""S04^'. Marialite (e.g. Na4Al3Si9024Cl) and mizzonite are the only two scapolite end member minerals identified in samples. At the weathering front, pyrite and scapolite become unstable and kaolinite, Fe oxides and jarosite are the stable phases. These mineralogical changes are mirrored by major depletions in Ca, CI, Co, HREE and Y, Mg, Na, Ni, S at the interface. Mass balance calculations indicate that for every 1 kg of rock weathered, 14 g CI, 30 g Na, 10 g Mg and 40 g S are lost to groundwaters.

A schematic process model has been constructed to illustrate and quantify the water-rock interactions. The mass balance modelling shows that the most important reactions involve oxidation of pyrite and the weathering and dissolution of scapolites containing CI, Na, SO4 and other anions. The dissolution of scapolite and pyrite and formation of kaolinite, jarosite, goethite and hematite is indicative of rapidly changing local environments and variations in pH and rates of S, CI, Fe, and Na mineralisation. These biogeochemical processes are responsible for the development of the deeply weathered regolith profile in upland positions (containing CI and Na) and saline acid sulfate soils (Hydraquentic Sulfaquepts) in adjacent low-lying parts of the catchment. Information on the mineralogy and geochemistry of the regolith has potential to be used to identify and predict conditions of soil, stream water and groundwater salinity.

412

SEDIMENTARY SYSTEMS IN THE ^BLAND BASIN', AN UPSTREAM EXTENSION OF THE MURRAY BASIN David Gibson^ Kok Tan^ and John Wilford^ ^Cooperative Research Centre for Landscape Environments and Mmeral Exploration (CRC LEME), Geoscience Australia, PO Box 378, Canberra, ACT, 2601 ^CRC LEME, University of Canberra, C/- Geoscience Australia, PO Box 378, Canberra, ACT, 2601 The 'Bland basin' is an alluviated palaeovalley tributary of the Lachlan River located in the Temora-Wyalong area of the Lachlan Fold Belt, NSW. Sediment is up to 120 m thick, covering about 6000 km^ across a broad valley floor 125 km by up to 80 km w^ide. It is undated, but must be of a similar age to Miocene to Holocene sediment in the Lachlan palaeovalley. Ephemeral Bland Creek and its tributaries, which flow to Lake Cowal at the southern margin of the Lachlan floodplain, drain the basin. Modem sediment transport and deposition appear to be minimal, at least partly due to the small catchment area of the basin, and thus small sediment supply and runoff.

Coalesced alluvial/sheetwash fans with slopes of 2-4 m/km form most of the basin surface, with local steeper fans around steep erosional areas, and narrow inset floodplains along major watercourses. Drilling and airborne EM show that the landscape prior to sedimentation was complex, with strike ridges of relatively resistant sedimentary rocks separated by broad valleys eroded on thick, highly weathered saprolite of andesitic volcanics and associated intrusives, and ultramafics. We have studied the sediments through logging of materials from aircore and continuously cored diamond, XRD, visual grainsize estimate (mud, sand, gravel), laser diffraction grainsize, geochemistry (XRF - ICP-MS), PIMA, 3 channel gamma response of samples, wireline gamma and magnetic logs. Some of these were drilled as part of the GILMORE project, but materials from hundreds of mineral exploration industry holes were also examined.

The complex palaeotopography and small catchment areas have resulted in a complex interplay of sediment types, with source areas for sediment available in certain parts of the 'basin' changing as the basin filled, and as local palaeodivides were buried. However, five main informal stratigraphic units have been identified on the basis of grainsize and stratigraphic position.

1. Basal sands and gravels at the base of the sequence along palaeodrainage lines. Gravels are polymict, reflecting local resistant rock types. Maghemite absent.

2. Massive, highly weathered mud with pronounced hematite mottling in a grey to white matrix, containing up to 20% floating quartz sand grains. Kaolinite and quartz predominate, rare detrital maghemite grains and granules.

3. A sandier interval with maghemite gravel beds. Quartz and kaolinite predominate 4. A clay-rich unit (kaolinite and smectite, minor quartz) 5. An upper unit with mixed sand, silt and clay, largely with a very high coarse silt component.

Local highly discontinuous maghemite gravel beds.

Bedding and sorting throughout the sequence is mostly poor. These features and the small catchment area for the basin suggest that deposition has been mainly by low energy colluvial systems (debris flow, sheet flow), rather than large scale alluvial systems. However, the clay-rich unit appears to be lacustrine; a large lake could have formed at times when local sediment supply could not match the rising sediment level in the Lachlan palaeovalley. The high silt content of the upper part of the sequence suggests a reworked aeolian dust component to the sediment. Realisation of the variability of the sediment has only been possible through the examination of hundreds of drill holes. Consideration of palaeogeography has helped to explain some of the local variations.

Detailed gamma analysis shows that Th emissions dominate the response, with intermediate U and low K, reflecting the highly leached nature of the sediments. Integration with detailed grainsize analysis suggests that K, U and Th reside mostly within different clay/silt size fractions. Thus subtle textural differences within apparently homogeneous mud can cause significant changes in total gamma response. Anomalous U is locally associated with detrital maghemite granules.

413

SUPERGENE GOLD DISPERSION AT THE FEDERAL DEPOSIT, WESTERN AUSTRALIA Nikita B. Sergeev and David J.Grav CRC LEME c/o CSIRO Exploration & Mining.

Email: Nikita.Sergeev@csiro.au

The dispersion of Au in the regolith and groundwater has been studied at the Federal deposit, located about 40 km N of Kalgoorlie, Western Australia at 30^24'S and 121^24'E. Gold mineralization occurs primarily as Au-quartz-pyrite veins, as well as Au and sulphide veinlets along a fracture system within granodiorites and monzogranites of the Scotia pluton.

The granitoids of the Scotia pluton are commonly weathered to 60 m, with a further deepening of 10-15 m over the Federal mineralized zone due to the presence of sulphide weathering and shearing. The granitoids are hornblende-rich, which has resulted in development of smectite in the saprolite, replaced by kaolinite in the clay saprolite. A continuous 2-16 m thick cover of transported overburden, consisting of ferruginous duricrust and sandy colluvium, overlies the residual bleached clay saprolite. The regolith modification under semi-arid to arid conditions since the Miocene included silicification of the basal facies of the sediments and the top of the residual profile, hardpanization of the colluvium and development of pedogenic carbonates close to the surface.

Gold enrichment in the regolith occurs at two levels: near the weathering front and at the transition from clay saprolite to saprolite. The lower enrichment zone is up to 27 m thick, with Au maxima 6-12 m above the weathering front. Within the orebody, Au concentrations are 2-3-fold greater, compared to the primary rock (250-290 ppb Au). Assuming, that the residual Au concentration in the saprolite is commonly less than 1.5 times, the data indicate chemogenic Au redistribution. The enrichment extends patchily to the east, up to approximately 400 m at a 60 ppb cut-off. However, the majority of the enrichment is located above the slightly mineralized (up to 30 ppb Au) primary rocks, suggesting that this concentration is due to a combination of residual and chemogenic Au accumulations, with transport of Au possibly following fault structures. The upper enrichment is several metres below the clay saprolite to saprolite boundary. Within the orebody, supergene enrichment occurs as a chain of local Au-rich (up to 1 ppm) spots along the mineralized structure.

The overlying clay saprolite is depleted in Au (30 ppb).

At the base of transported overburden, Au is concentrated in colluvial duricrust (75 ppb), mostly associated with ferruginous nodules and quartz, indicating mechanical transport of Au. Near the surface, in calcareous soil and colluvium, the majority of Au (83-97%) is associated with carbonate nodules indicating chemogenic Au mobilization in the calcrete environment. The morphology of supergene Au grains in carbonate nodules suggests a bacterial origin of the Au. Groundwaters in the Federal area are acid (pH 5.8) and saline becoming less acid to the north.

However, they are less corrosive than in the Kalgoorlie region, and none of the groundwaters have Eh values high enough to allow dissolution of appreciable Au, consistent with the low concentrations of Au dissolved in groundwaters. No clear hydrogeochemical signature could be derived that mirrored the presence of mineralization in the Woodcutters area. The best candidate as a hydrogeochemical pathfinder for Au in this area is Mo, though results are equivocal, due to a lack of background samples.

Acknowledgements AMIRA Project 504 Sponsors, Aberfoyle Resources Ltd and Pacmin Mining Corporation are thanked for financial and other support. CRC LEME is supported by the Australian Cooperative Research Centres Program.

414

REGOLITH-LANDFORM MAPPING AT TOP CREEK, CENTRAL WEST NSW: DRYLAND SALINITY HAZARD MITIGATION IN FELSIC VOLCANIC LANDSCAPES. Peter Haddrill & C. Leah Moore Dryland Salinity Hazard Mitigation Program, CRC LEME, University of Canberra, ACT 2601 Top Creek Catchment, approximately 35 km south of Cowra in central west NSW, is wholly developed on regolith over Silurian felsic volcanic lithologies. This project places Top Creek catchment in its regional geological and regolith setting, and regolith-landform mapping will facilitate interpretation of shallow fluid flow patterns in the Top Creek area. Indicators of dryland salinity have been recognised in this area for a number of years and mitigation strategies applied in the adjacent Breakfast Creek catchment to the north have been used as a case study for dryland salinity hazard management in the past. This Landcare Group is now keen to evaluate the extent of the land salinisation problem in the Top Creek Catchment.

Another aim of the study is to provide a description of the features that typify moderate to high relief Silurian felsic volcanic ground-water-flow systems (GFS) using the Top Creek Catchment on the Douro Volcanics as a case study. This is particularly important as other felsic volcanic successions of similar age, including the Illunie Volcanics, have been segregated into a different part of the GFS classification. This study intends to establish whether there is scientific basis for the difference in classification or whether the classification should be modified.

Regolith-landform mapping has commenced and will assist with evaluation of where salt is stored in the landscape surrounding Top Creek and how this salt can be mobilised into the hydrologic system at Top Creek. A comparison will be made between the geological and regolith landform characteristics of the Douro and Illunie Volcanics in the Boorowa North and Upper Tyagong areas respectively, in an attempt to clarify whether these units should be differentiated in the GFS classification for the Lachlan region. Links have been established with local land managers and government advisers to facilitate the comparison of products of this research with existing resources, with a view to assisting with land management decision making in the Top Creek area.

A comprehensive review of the regional geology, regolith geology, landscape analysis, dryland salinity, groundwater flow systems (GFS) and interpretation of remotely sensed imagery provides the foundation for this study. Regolith-landform mapping of the Top Creek area is being carried out at 1:15,000 scale. This area is on the Goulbum 1:250,000 scale and Boorowa 1:100,000 scale map sheets. An overview of the local geology and extant native vegetation will take place at the same time as regolith-landform mapping. Aerial photographs, geophysical data sets and other available remotely sensed imagery are being used to aid mapping and detailed landform analysis. Geographic Information Systems (GIS) techniques will be used to compare a range of data including satellite imagery, geophysical data sets, digital elevation information, and aerial photography over the Top Creek area, to enhance interpretation of geological and regolith-landform information, and characterisation of the Siluro-Devonian Felsic Volcanic groundwater flow systems. An attempt will be made to develop a conceptual model of salt flux, including a preliminary salt budget, for the Top Creek field area. Interaction with the NSW DLWC Central West Salt Group and the local Landcare group forms an important focus for this work.

415

THE INFLUENCE OF RAINFALL CHEMISTRY AND SEASONALITY ON THE DISTRIBUTION OF REGOLITH CARBONATE ACCUMULATIONS (CALCRETES) Leanne J. Hill\ Patrice de Caritat^ & Richard A. Eggleton^ ^ CRC LEME, Department of Geology, Australian National University, Canberra A.C.T. 0200 ^CRC LEME, Geoscience Australia, GPO Box 378, Canberra, A.C.T. 2601 ^Department of Geology, Australian National University, Canberra A.C.T. 0200 Climate has been suggested as a significant factor in controlling the distribution of Regolith Carbonate Accumulations (RCAs) in the pedogenic zone. Aridity is most w^idely asserted as the primary control on RCA development. There are, however, significant changes in the morphologies and landscape associations of RCAs between the widespread 'pedogenic' RCAs of semi-arid southern Australia to the more limited (mostly restricted to low-lying landscape settings) 'groundwater' RCAs of arid central Australia. Similarly, there is a decrease in vegetation cover between these regions, with mallee communities dominating to the south and mulga communities to the north. This significant change in regolith, climate and vegetation has a dramatic effect on chemical cycling and, therefore, on residence and dispersion of elements.

More specifically, the distribution of pedogenic RCAs recently has been suggested to strongly correspond with areas that experience a winter rainfall regime (Hill et al. 1999). In this case seasonality is a significant factor in the distribution of RCAs. This may be due to its influence on the retention of water in soils and influence on vegetation growth, but more significantly the environmental chemical setting of areas with different rainfall regimes may also be a limiting control on distribution.

In western NSW, where the change from predominantly pedogenic to groundwater carbonates is marked (Hill et al. 1999), there is a significant difference in the chemical composition of atmospheric deposition throughout the year. In particular, the supply of Ca and Mg from the atmosphere decreases from south (West Wyalong, Lake Cargelligo, Cobar) to north (Tibooburra, Kayrunnera).

The enrichment of Ca and Mg to winter-dominant rainfall may be due to a stronger marine influence, with predominant wind directions being from the Southern Ocean. In contrast, summer rainfall is mainly derived from off the north coast of Australia and becomes more dilute the further south it falls. As a result, areas receiving predominantly Ca- and Mg-rich winter rainfall and also characterised by relatively high evaporation to rainfall ratios will experience a widespread addition of carbonate materials in the pedogenic zone.

Linkage between rainfall seasonality and chemical sources has widespread implications for variations in biogeochemical and geochemical dispersion pathways across the Australian continent. This is of major importance for determining the presence and utilisation of pedogenic calcretes as geochemical sampling media in regional exploration programs.

Acknowledgements This research is supported with an Australian Postgraduate Award Scholarship and the Australian Government's Cooperative Research Centres Program.

Reference Hill, S.M., McQueen, K.G. and Foster, K. 1999. Regolith carbonates in eastern Australia: characteristics and potential as an exploration sampling medium. In: Regolith '98, New approaches to an old continent Program and abstracts, pp.91-208.

416

DRYLAND SALINITY AND REGOLITH-LANDFORM DISTRIBUTION IN THE BOOBEROI TO QUANDIALLA TRANSECT, CENTRAL WEST NEW SOUTH WALES. Michael Holzapfel and C. Leah Moore Dryland Salmity Hazard Mitigation Program, CRC LEME, University of Canberra, ACT 2601 Landholders in the Booberoi to Quandialla (B-Q) Transect area, located in central west NSW, have been concerned about an emerging dryland salinity problem since the late 1990's. Borehole information and electromagnetic induction investigations support anecdotal observations. The presence of indicator vegetation, waterlogging of soils and salinisation of land are becoming increasingly prevalent, with two well-documented sites including 'Strathairlie' near Quandialla, and 'Back Creek' near West Wyalong. The B-Q Transect area lies within the Bland Creek Catchment, a broad open plain of subdued topography receiving sediments from elevated rises located to the west, south and east. Stream flow across the alluvial plains and low angle alluvial fans is intermittent with most of the flow being diverted into groundwater storage or lost to evaporation. Rarely do streams flow into Lake Cowal to the north.

A partial electromagnetic (EM) induction survey has been conducted by the Department of Land and Water Conservation (DLWC) Central West NSW Salt Group. This allows evaluation of the magnitude and spatial distribution of the salinity problem. DLWC are also monitoring a number of piezometers in the area. As part of an approach to assist with hazard mitigation and land management, two regolith-landform maps are being compiled at 1:10,000 scale in the Back Creek and Quandialla areas. A third, more regional regolith-landform map (1:100,000 scale) provides a regolith-landform transect over the Bland Creek area and provides context for the more detailed mapping areas.

The western quarter of the B-Q Transect area partially overlaps with the recently completed Gilmore Project, a broad multi-disciplinary study, coordinated by AGSO (now Geoscience Australia), and involving eight other organisations including CRC AMET, the Bureau of Rural Sciences (BRS), the CRC for Landscape Evolution and Mineral Exploration (CRC LEME), the NSW Department of Land and Water Conservation (DLWC), the NSW Department of Mineral Resources (DMR), the Australian National University (ANU) and the University of Canberra (UC). Regolith-landform information provided as part of the Gilmore Project was incorporated into the 1:100,000 scale regolith-landform mapping over the B-Q Transect. Associated Gilmore Project datasets including radiometrics, magnetics and a digital elevation model have also been used in the compilation. Use of existing Gilmore regolith-landform information will help to provide consistent extended regolith-landform coverage for the broader Bland Creek Catchment.

The new regolith-landform maps will aid in interpretation of the existing EM survey images and together with other geophysical techniques, help piece together the three-dimensional characteristics of the Bland Creek Catchment and help with the development of a model for shallow fluid migration. The three-dimensional integration of regolith-landform mapping, electromagnetic studies, bore information and other geophysical methods is critical in determining the interaction, distribution and movement of groundwater in the Bland Creek Catchment as buried palaeochannels are the preferred fluid pathways. The distribution of these palaeochannels has implications for future dryland salinity outbreaks, the remediation of current outbreaks and mineral exploration closer to the well-known Wyalong Goldfield.

417

PALAEOCHANNEL STUDY MODELS AND THEIR SIGNIFICANCE: EXAMPLE FROM THE GAWLER CRATON, SOUTH AUSTRALIA B. Hou^' L.A. Frakes' and N.F. Alley^ ^The University of Adelaide ^Primary Industries and Resources South Australia, and CRC LEME The precise geometric definition of palaeochannels on the NW Gawler Craton of South Australia is important in the exploration for placers (e.g, gold and heavy minerals), secondary geochemical deposits (e.g. uranium) and even for groundwater resources which probably occur in the palaeochannel sediments. Knowledge of palaeochannel architecture and any concentration of minerals in the palaeochannels is also of interest as guides to the location of bedrock lode deposits and in the planning and interpretation of exploration drilling programs.

Integrated geoscientific datasets have contributed to an understanding of palaeochannels which drained the Gawler Craton during the Tertiary and their significance for mineral exploration in the region. Our study was aimed at development of a comprehensive model to assist exploration in palaeodrainage terrains. This was achieved through the integration of models from multiple geological and geophysical methods. These included interpretations from field exposures, a compendium of geological and drilling data, computer modelling of ancient landscapes, topographic and digital elevation models, remotely sensed imagery, magnetics, seismic, gravity, airborne and ground transient electromagnetics, and radiometrics, all of which contributed to a systematic investigation of both shape and depth of the channels. Physical property contrasts that exist between the channel sediments and the underlying bedrock, for instance, can be differentiated by geophysical methods to locate the palaeochannel thalweg.

Evidence from sedimentology was combined with that from other geological and geophysical methods to arrive at a general reconstruction of palaeochannel architectures and depositional environments. The palaeochannels were originally incised into the pre-Middle Eocene landscape, mostly weathered basement, and Tertiary fluvial, lacustrine, estuarine and marine sediments accumulated within them during the Eocene and Miocene. Detailed sequence stratigraphy and facies analysis over the palaeodrainage network establish the changes in sedimentation as sea level and sediment supply varied.

Significant refinements in remote sensing and geophysical techniques, data processing, sedimentology and computer-aided interpretations are today providing a quick, economic and efficient means of modelling palaeodrainage in this highly prospective terrain. The most successful procedure for defining the palaeochannels is to combine imagery and geological and geophysical methods to yield architectural and evolutional models of palaeochannel development. Though no palaeochannel deposits in the region have yet been mined, on-going research is focussed on predictive models for placer and other deposits, including controls on dispersion of bedrock mineralisation cut by palaeodrainage.

418

FOSTERING RESEARCH AND WIDER COMMUNITY UNDERSTANDING OF THE REGOLITH Patrick R. James Education and Training Program Leader, CRC Landscape Environments and Mineral Exploration (LEME), Department of Geology and Geophysics, University of Adelaide, SA 5005 Over the five previous years of the CRC LEME, the Education and Training programme demonstrated an enviable record of success and achievement in activities ranging from undergraduate and postgraduate support, to organising and presenting conferences, workshops and short courses on regolith studies. This was with the objective to markedly strengthen the teaching and research training in landscape evolution and its relation to mineral exploration in Australia.

The newly formulated and named CRC LEME has a much wider and more challenging brief, with the inclusion of new State Government and University partners across Australia, many new individual members and students, plus the new significant environmental research strand. There are many new projects and programmes, and many new collaborations and synergies developing, which are rapidly expanding the value and business of the organisation.

To take advantage of this new level of research association and support, and to capitalise on the potential of the new LEME, the Education and Training Programme will need to look to many new areas of activity for it to thrive and grow.

These must include: • an internationally recognised and respected progamme of undergraduate, postgraduate and industry, face-to face, field-based and online LEME courses (including master classes), with a quality assurance component to support its aspiration of national and international success, • a large, vibrant and mobile but identifiably collaborating community of LEME researchers and students, • significant industry sponsorships of many LEME activities • an excellent and modem LEME promotional publication strategy, • the application of eLeaming and mLeaming strategies underpinned by a strong LEME worldwide web presence • an ability to deliver LEME courseware electronically and asynchronously • a capacity to host online conferences and workshops, • an understanding of, and ability to apply, the best practice currently already residing within its core partners, including their Managed Learning Environments (MLE's) and Virtual Learning Environments (VLE's), which must be seamlessly integrated to enable LEME to overcome the tyranny of distance between these partners and their widely scattered members.

419

REGOLITH MAPPING OF THE BENDIGO REGION OF CENTRAL VICTORIA AND THE OCCURRENCE OF GOLD Joyce\ E. B and Kotsonis^, A. ^Associate, CRC LEME, School of Earth Sciences, The University of Melbourne, Victoria 3010. ^Minerals Industry Research Institute, University of Ballarat, PO Box 663, Ballarat, Victoria 3353.

The Geological Survey of Victoria, in collaboration w^ith Melbourne and Ballarat universities, has conducted a number of regolith mapping projects in central and western Victoria over the past five years, including the Bendigo 1:100 000 sheet w^hich vs^as mapped by the authors in 2001, and will be published later by the GSV. The geological structure of the Bendigo sheet area is dominated by the Ordovician Castlemaine Group, deformed during the Silurian Benambran Orogeny into north-south trending regional folds and faults, and intruded by the post-orogenic Upper Devonian Harcourt Batholith. There is significant evidence for post-Palaeozoic (predominantly Cainozoic) movement along some of these major bedrock faults, displacing deep leads at Sebastian, Guildford and other areas. These metasediments form subdued ranges with undulating topography. A dissected plateau sloping to the north is highly weathered to a saprolitic clay, with ferricrete and residual reef quartz. Bedrock ridges tend to follow the northerly strike of bedding, modified by faulting and jointing. Radiometric imagery has revealed variations in the character of the regolith, often related to boundaries along major bedrock faults, suggesting that the composition of the regolith may be influenced by neotectonism. The regolith may also be reflecting bedrock variations associated with hydrothermal gold mineralisation.

Remnants of early Cainozoic drainage (White Hills Gravel) made up of well rounded quartz sand, coarse to very coarse gravel and conglomerate are preserved on hill tops and the upper slopes of bedrock ridges. These deposits were formed by the earliest known drainage systems in the region, and overlie highly weathered Palaeozoic bedrock. The surface of these deposits is moderately to strongly cemented with silcrete, clay argillans, and minor ferricrete, and forms a duricrust and resistant cap several metres thick overlying weathered quartz cobble conglomerate. The White Hills Gravel was extensively mined for gold in the 19^ century.

Mid-Cainozoic gold-rich deep lead deposits include the Huntly and Telegraph deep lead (and the minor leads of Forest deep lead and Ironstone Hill deep lead), and the leads at Marong and Raywood, and the Elysian deep lead at Neilborough (Calivil Formation). Possible outcrop of this unit occurs at Fosterville and at Axedale, where both have gradational soil profiles with weak mottling of the saprolite.

The only documented occurrence of secondary gold in the Bendigo area is at Fosterville. The nature of this gold occurrence is poorly understood, with only limited work having been conducted. Secondary gold occurs within oxidised quartz-gold-stibnite veins in the oxide zone of mineralisation at Robbins Hill pit. These veins host primary crystalline nuggetty gold that has undergone refinement and remobilisation, with precipitation of colloform and small cauliflower encrustations of extreme fineness. The extent of the remobilised gold is limited to the colloform iron oxides precipitated within the oxidised veins and does not occur in the surrounding host rock. The primary crystalline nuggetty gold tends to have uniform silver content of 2.6 to 2.8 %wt Ag, whereas the refined gold has variable silver contents from <1 %wt to 35 Vowt Ag, and the remobilised gold is extremely fine (<0.1 %wt Ag).

The timing and extent of secondary gold mineralisation in the Bendigo area, and its relationship to weathering is still unclear, but evidence from Fosterville indicates gold movement within the regolith, and in particular, within in situ weathering profiles developed on the Palaeozoic bedrock.

420

GEOCHEMICAL MODELLING OF PROCESSES AFFECTING THE CHEMICAL COMPOSITION OF GROUNDWATERS IN THE BROKEN HILL REGION: EFFECTS OF REGOLITH PROPERTIES ON MINERALISED ZONE CHEMICAL SIGNATURES Dirk Kirste and Patrice de Caritat Cooperative Research Centre for Landscape Environments & Mineral Exploration (CRC LEME), c/- GeoScience Australia, GPO Box 378, ACT 2601, Australia.

Geochemical modelling is used to investigate the potential to determine chemical tracers for recognising mineralized areas w^ithin and beneath regolith cover in the Broken Hill region of N.S.W. and adjacent S.A. The models simulate the natural physical and chemical processes affecting the composition of groundwaters before and after coming in contact w^ith different types of mineralization observed in the region. Constraints on the models come from studies of the regional variations in groundw^ater, bedrock and regolith composition, as well as simple hydrological models. Physical processes in the regolith comprise evaporation, transpiration, and mixing. Chemical processes primarily involve water-rock interactions which include dissolution, precipitation, alteration or weathering, adsorption, and ion exchange. These processes are applied to rainwater compositions both prior to and after contact with mineralized areas and the resultant fluids are compared to groundwater compositions from the region. A number of tracers are recognised from the modelling experiments; these include stable isotopic and trace metal composition, though the results are very dependent on the composition of the regolith and mineralization (including gangue minerals and alteration products), and extent of weathering encountered. Indications are that groundwater chemical composition can be used to recognise mineralization in areas of cover.

421

MINERAL MAPPING OF REGOLITH-COVERED BASEMENT TERRAINS IN THE WHITE DAM REGION, OLARY DOMAIN, SOUTH AUSTRALIA. Lau, I.e., James, P.R. and Heinson, G.S. Cooperative Research Centre for Landscape Environment and Mineral Exploration, Department of Geology and Geophysics, University of Adelaide, North Terrace, South Australia, Australia. 5005. Phone +61 8303 5376. Fax +61 8303 4347, ian.lau@adelaide.edu.au The White Dam copper-gold-molybendenite prospect, together with the adjacent Wilkins and Green & Gold mineral workings offer a promising location for investigating the spectral characteristics of regolith overlying basement and altered mineralised rocks. The White Dam prospect is situated in Proterozoic basement of the Cumamona Province and located approximately 25 kilometres north east of the township of Olary, South Australia. The host rock to the mineralisation is a bedded quartzo-feldspathic biotite leucocratic gneiss of upper amphibolite grade. Other outcrops of high grade basement gneiss, granite, albitite, calc-silicate and overlying metasedimentary rocks, as well as retrograde shear zones, occur in the region. Colluvial and alluvial cover sequences dominate much of the area, while quartz lag deposits are abundant throughout the study region and create breaks in the saltbush that occur in typical banded vegetation patterns. Geological remote sensing and mineral mapping has predominantly been performed in regions of prominent outcrop with very little vegetation. Considerably less research has been attempted in weathered terrains where much of the bedrock is obscured by regolith or vegetation. Five HYMAP airborne hyperspectral strips, flown in December 1998 have been processed and basic mineral maps were produced for the selected regions of interest. The data cover a total area of 350 Km^ with a spatial resolution of approximately eight metres.

Similar techniques were used with the longer wavelength (SWIR and TIR) bands of the ASTER satellite imagery to discriminate minerals and cover types. The 90 metre spatial resolution thermal infrared bands were able to map quartz lag deposits where they were large enough to produce a signature.

Orthoimagery was used as an underlay and a digital elevation model generated to allow a terrain perspective to be incorporated into remotely sensed regolith and landform mapping of the regions of interest. Drill core and rock chips from the White Dam prospect have been analysed with a field spectrometer and a three-dimensional model of the regolith was constructed which will be integrated with a three-dimensional geochemical dispersion model for the mineralisation in the region. Ground truthing and follow up surveys with a portable field spectrometer have been performed to verily the results of the mineral mapping and to gain surface regolith measurements.

422

MAPPING SALINITY PROCESSES AND ASSESSING AND PREDICTING DRYLAND SALINITY HAZARDS Lawrie, K.C.,^ Pain, C.F./ Gibson, D.L.,^ Munday, TJ.,^ Wilford, j / & Jones, G.^ ^ CRC Landscape Environments and Mineral Exploration, d - Geoscience Australia, ACT 2601 ^ CRC LEME, CSIRO Exploration and Mining, c/- Geoscience Australia, ACT 2601 ^ Bureau of Rural Sciences, Department of Agriculture, Forestry and Fisheries, ACT, 2601 Regolith studies over the last decade have provided new^ insights into the evolution of the Australian continent. These studies have show^n the regolith to be more variable than predicted, and that regolith, geomorphology and landscape evolution are important inputs into understanding groundw^ater flow and the processes that lead to dryland salinity. For buried landscapes, sediment distribution and composition, and saprolith character are more complex than anticipated, and are often unrelated to present-day materials. Groundwater flow in these environments is still poorly understood. However, at a catchment and particularly at a sub-catchment scale, the effects of complex regolith on local groundwater flow systems are likely to be particularly significant.

Differences between surface and buried materials have been correlated with palaeo-environmental change, and/or changes in sediment provenance that cannot be interpreted from surface distribution of lithologies.

Mineral systems analysis of bedrock materials has led to the recognition that most solid geology maps usually record only the primary character and mapped boundaries of the materials at deposition or crystallisation. In many terranes, geological maps do not record the distribution of significant primary hydrothermal alteration mineral assemblages, and/or zones of metamorphism and deformation. Significant correlations between these secondary overprints and landscapes can be mapped up to catchment scale in terranes such as the Lachlan Fold Belt.

Many studies in the last decade have also shown that terrestrial sediments in Australia's inland basins have undergone significant weathering subsequent to deposition. A limited number of studies have shown that weathering and low temperature diagenesis can significantly affect the porosity and permeability characteristics of regolith materials. Thus, an understanding of the primary facies distribution may only partly give an answer to the transmissivity of materials.

Regolith is also a significant store of salt, and much of the salt stored over the last few hundred thousand years is now being mobilised by groundwater that is no longer in hydrogeological balance.

In the Salinity Mapping and Support Program, part of the government's National Salinity and Water Quality Action Plan, a holistic systems approach has been adopted to map and predict salinity hazards at a catchment scale. A multi-disciplinary systems approach that utilises remotely sensed data and geophysical technologies to map salt stores, regolith architecture and groundwater flow systems provides a framework for mapping salinity processes. It also forms the basis for conceptual and predictive models, and in some instances has the potential to assist in the designtargeted intervention strategies.

Acknowledgements This abstract is published with the permission of the CEOs of CRC LEME, Geoscience Australia and the Bureau of Rural Sciences.

423

REGOLITH STRATIGRAPHY AND GEOCHEMICAL DISPERSION AT THE CHALLENGER GOLD DEPOSIT, GAWLER CRATON, SOUTH AUSTRALIA M. J. Lintem^ and M. J. Sheard^ ^CRC LEME, CSIRO Exploration and Mining, PO Box 1130, Bentley, WA 6102. ^Geological Survey Branch, Mineral Resources Group, PIRSA, PO Box 1671, Adelaide, SA 5001.

The Challenger Gold Deposit lies in the northern Gawler Craton, South Australia, 750 km NW of Adelaide. A 1.5 km NW-SE section (termed a "regolith line") was chosen for the study of geochemical dispersion and regolith stratigraphy across three mineralised zones (Zones 1, 2 and 3).

The principal mineralisation (Zone 1) outcrops on the flank of a low rise. Zone 2 occurs about 400 m to the SE of Zone 1. Zone 3 is located beneath about 20 m of sediments about 800 m to the SE of Zone 1. Mineralisation is associated with silica and arsenopyrite alteration in the Archaean Christie Gneiss, a garnet-rich paragneiss consisting of plagioclase, perthitic K-feldspar, quartz, cordierite, garnet, biotite, and a trace of graphite.

The Christie Gneiss has weathered to a saprolite of variable thickness (averaging 30 m) that contains abundant primary quartz. The upper regolith (0-6 m) is cemented by silcrete and calcrete.

Small quantities of ferruginous material occur on the surface as lag. The area is arid (-200 mm of rainfall), of low relief and dominated by bluebush shrubland (Maireana sedifolia), with pockets of mulga {Acacia sp.) in thin (< 2 m) aeolian sands.

Sampling was mostly along the regolith line and comprised cuttings from special regolith drilling to 6 m, cuttings from exploration drilling to 60 m, profile samples from eight 3 m deep pits, soil, calcrete, silcrete, lag and vegetation. Samples were analysed for fifty elements and their mineralogy.

Results and recommendations are: 1. There are two principal regolith units, one in situ and the other transported. The in situ unit is deeply weathered Archaean basement (saprolite). The transported unit is mainly fluvial.

2. Distinguishing between transported and in situ regolith is important for geochemical sampling as the units have dissimilar geneses, mineralogies and geochemical signatures. The light REE discriminate between the two units.

3. The elements in the in situ regolith associated with mineralisation fall into two broad groups: sulphide-related (Ag, As, Bi, Cd, Cr, Cu, Fe, Mo, S, Se and Zn) with Cr and possibly W, and alteration-related (Ba, Cs, K, Rb and Tl). The use of these elements as pathfinders is dependent on the sample medium and the regolith unit.

4. Calcrete is ubiquitous and is recommended as the best exploration sample medium for Au in the in situ unit providing broad, high-contrast anomalies, but Cu and As are also useful. Use of near-surface calcrete for exploration in transported overburden is less clear, as it may be ineffective when the transported overburden is thick (>10 m), except, possibly, on a regional basis.

5. Silcrete lag is a potential sample medium for Au. Where calcrete is absent, silcrete or soil on in situ regolith are recommended as sampling media.

6. Non-precious opal (potch) was identified in siliceous palaeochannel materials. Although precious opal was not encountered, there is a potential for it to occur in this environment and could be an additional exploration focus in strongly silicified saprolite developed on felsic rocks.

Acknowledgments The Gawler Joint Venture (Resolute Ltd and Dominion Mining Ltd) provided in-kind and financial support.

CRC LEME is supported by the Australian Government's Cooperative Research Centres Program.

424

RED-BROWN HARDPAN: DISTRIBUTION, ORIGIN AND EXPLORATION IMPLICATIONS IN THE YILGARN CRATON OF WESTERN AUSTRALIA. ^Anna. Mahizhnan, ^R. R. Anand and ^S. A. Wilde ^CRCLEME-CSIRO Exploration and Mining ^Curtin University of Technology

Red-brown hardpans occur extensively in Western Australia in the arid and semi-arid regions of the Murchison, Pilbara and Eastern Goldfields drainage divisions, between longitudes 115°E and 124°E and latitudes 23°S and 30°S. They occupy an area of about 360,000 sq. km and two thirds of which occurs in the Yilgam Craton, wherein exploration for gold and base metals is now focused.

This paper presents the characteristics and processes of red-brown hardpans formation and their implications for exploration.

In the Yilgam Craton red-brown hardpans are believed to occur mainly north of the 'Menzies Line'. However, regional and local investigations reveal the presence of red-brown hardpans in the Kalgoorlie region, well south of the Menzies Line. In addition, calcretes and red-brown hardpans occur together in many places, south of the Menzies line. This distribution suggests that red-brown hardpans were once more extensive and have been subsequently replaced by carbonates to form calcretes. Red-brown hardpans predominantly occur in regions with <250 mm annual rainfall. In present-day higher rainfall (400 to 500 mm) regions, red-brown hardpans are undergoing weathering. The age of the red-brown hardpans, estimated by paleomagnetic dating of iron oxides is from 780,000 years to the present (Brad Pillans, written communication, 2002).

Red-brown hardpans occur at or near the surface and may vary from less than one metre to more than 10 m in thickness. They are developed in a variety of regolith materials, showing varying developmental stages ranging from incipient through intermediate to mature. The mineralogy of the cement is complex. Data from XRD, SEM, TEM, EFTEM, FTIR and SWIR investigations show poorly-ordered kaolinite and opal-A in the cement. Illuvial multilayered argillaceous cutans with a composition of silica and alumina in a ratio of 2:1 are responsible for the cementation. Secondary silica (Si 95%) coatings are common, mainly as opal-A, on ped surfaces and on the inner walls of voids and vughs. Etch pits and dissolution pits are developed in these coatings and some of them are filled by kaolinitic clays. Selective dissolution experiments in acid ammonium oxalate show that oxalate-soluble amorphous and poorly ordered alumina and silica in red-brown hardpans are in the order of 1.6 to 2 Al203/Si02 molar ratio. These results suggest that red-brown hardpans were formed where there was sufficient water during the wet season to dissolve alumina and silica, but insufficient enough to leach them. During the subsequent dry season, the dissolved alumina and silica suspended in solution precipitated as poorly-ordered kaolinite and opal-A. Successive dissolution and precipitation leads to the fusion of poorly-ordered kaolinites and opal-A at a nanometre scale to become the cement that progressively cemented the regolith materials together.

Geochemical investigations carried out at the Federal open pit. Broad Arrow, north of Kalgoorlie indicates that there are Au anomalies in red-brown hardpans. Transported overburden at the Federal open pit is 2-10 m thick and derived from adjacent greenstone and granitic terrains. Red-brown hardpans are 3-5 m thick and developed in alluvium and colluvium that overlies the primary mineralisation of gold hosted in granodiorite. The processes leading to the development of Au haloes in red-brown hardpans were examined. Gold concentration in red-brown hardpans is 3 to 50 ppb against the back ground anomaly of 10 ppb. Sequential and partial extraction analyses show significant correlation of Au with Ag, Ca, Ce, Co, Mg, Mn and Ni. These results suggest that the gold concentration in red-brown hardpans is due to: (a) mechanical dispersion (b) hydromorphic dispersion in the matrix from the underlying mineralisation.

425

CONSTRAINING THE WEATHERING HISTORY OF THE COBAR REGION, WESTERN NSW K.G. McQueen^ B J. Pillans^ and M.L. Smith^ ^CRC LEME, University of Canberra, ACT 2601 ^CRC LEME, Australian National University, ACT 0200 ^School of Applied Geology, University of New South Wales, 2052 The Cobar region is an elevated palaeoplain straddling the divide betw^een the Lachlan and Darling Rivers. It lies on the margins of the Mesozoic-Cainozoic Eromanga and Murray Basins and preserves a history of w^eathering and landscape evolution extending back to the Mesozoic.

Establishing the w^eathering history of this block has implications for refining the regional, postPalaeozoic landscape history, interpreting regolith materials, understanding geochemical dispersion processes and confirming palaeoclimatic reconstructions for southeastern Australia.

Preserved weathering profiles vary considerably across the region (<3m to >100m thick). This variability has resulted from a combination through time of variable depth of w^eathering, controlled by the interplay of lithological variation, permeability and availability of vs^ater, and variable erosion, reflecting differential tectonic uplift and incision. Older profiles are preserved beneath resistant duricrusts, palaeochannel sediments and leucitite lava flow^s. Some of these older profiles are inverted in the present landscape.

In the El Capitan-Wilga Tank area northeast of Cobar, Miocene leucitite lavas are preserved as remnant caps and plugs in a palaeovalley. At Wilga Tank, a deep w^eathering profile characterised by a ferruginous mottled zone and underlying bleached saprolite is preserved beneath a dissected flow. This buried profile is similar to those common throughout the region. An adjacent volcanic plug and lava flows at El Capitan, 8 km to the north, have been radiometrically dated at c. 15 Ma. Weathering profiles on the leucitites are thin without significant ferruginisation. Palaeomagnetic dating of the upper part of the sub-flow weathering profile yields a Middle Miocene age, similar to that of the leucitites.

Palaeomagnetic dating has also been conducted on ferruginous weathering profiles just east of Cobar (5 km) and at Elura (50km NW of Cobar), McKinnons (35 km SW of Cobar) and New Cobar (2.5 km S of Cobar). At the first three sites, results indicate two periods of stable iron oxide fixation in the Latest Cretaceous to Early Palaeocene (60 ± 1 0 Ma) and in the Middle Miocene (12 + 3 Ma). At the New Cobar open pit, dating of oxidised saprolite (after Early Devonian shales and siltstones) has yielded a Jurassic age (c. 180 Ma).

These results show that intense chemical weathering and ferruginisation extended into the MidMiocene and that an older. Early Palaeocene ferruginisation has been preserved at some sites. The period just prior to the Middle Miocene (16 Ma) was characterised by hot climatic conditions, high global sea levels and a major marine incursion in the adjacent Murray Basin. This would have promoted profile development, particularly as conditions were characterised by high perennial rainfall and high watertable levels. From the Middle Miocene, conditions became drier and falling watertables allowed oxidation and dehydration of the profiles, precipitating stable iron oxides. This is consistent with the Middle Miocene age for ferruginous materials in most of the profiles. Deep weathering was restricted after the Middle Miocene, as indicated by the limited profiles on the leucitites of this age. The Early Palaeocene ferruginisation reflects oxidation of deep weathering profiles developed through the Late Cretaceous. Cretaceous to Early Tertiary fluvial sediments are preserved in topographically inverted palaeochannel remnants in the Cobar area, consistent with stable exposure at this time. The Jurassic age preserved in the weathering profile at New Cobar suggests that this profile has either been preserved close to the surface since this time without much detectable modification or buried and re-exposed much later. The other sites have only been exposed since the latest Cretaceous or had any older profiles removed by significant erosional stripping prior to this time.

426

REGOLITH-LANDFORM MAPPING FOR DRYLAND SALINITY HAZARD MITIGATION, UPPER TYAGONG CREEK, CENTRAL WEST NSW. Paul Southwell and C. Leah Moore Dryland Salinity Hazard Mitigation Program, CRC LEME, University of Canberra, ACT 2601 The Upper Tyagong Creek catchment is located north of the small township Greenethorpe, in central west New South Wales. For the past century the principal land use has been intensive cereal cropping and sheep and cattle grazing. The area has been extensively cleared except in the northwest and northeast of the catchment. As a result, there is water table rise and growing dryland salinity outbreak, adversely affecting the economic viability of land.

Because it is a headwater catchment of the Lachlan River, salt leaving the Upper Tyagong Creek catchment contributes to increased salinisation of lowland areas. An increased understanding of the processes effecting upland dryland salinity are critical for the assessment and rehabilitation of directly affected areas, as well as those downstream. The production of catchment scale regolithlandform and geology maps and the detailed study of factors affecting salinity (mineral weathering pathways, groundwater chemistry, salt and water in the regolith) provide landholders and managers with resources to be used at a paddock scale. This is something that more regional studies lack.

Regolith-landform features in the field area are diverse, with weathered bedrock hills dominating the landscape in the northwest (Conimbla Range) and east (Broula Range) of the catchment.

Scattered areas of weathered bedrock and colluvial material on erosional low hills characterise the north and central parts of the field area. Extensive colluvial erosional low hill and rise regolithlandforms occur in the southwest and southeast where deep weathering of granitic bedrock has occurred. Major alluvial units are associated with Tyagong Creek and Kellys Creek, the major drainages in the area.

The majority of dryland salinity outbreak in the area occurs on two lithological units, the Illunie Volcanics and Young Granodiorite. Groundwater flow through fractured rock aquifers occurs in both. The regolith mantle is thicker on the Young Granodiorite so the penetration of shallow groundwater is deeper than on the Illunie Volcanics. The hydrogeochemistry of groundwater in these rock types is affected by mineral weathering, marine accession, aeolian accession, rainfall and ion exchange. In the Illunie Volcaincs, localised severe scalding and salt crusts form where groundwater is ponded behind barriers to flow such as farm dams or geological constrictions.

Impeding groundwater flow close to the land surface allows capillary rise and evaporation of initially quite fresh groundwater (1000 |is/cm). On the Young Granodiorite, groundwater is more saline (2500-3300 |Lis/cm), and the watertable is close to the land surface over a larger area. Salinity outbreak is not as severe here because of regolith thickness and distribution, vegetation cover and type, and solution buffering.

A ground electromagnetic induction (EM) survey was undertaken over the majority of the field area by the Department of Land and Water Conservation (DLWC) to distinguish areas of highly conductive material. The regolith-landform map correlates well with the EM survey in areas underlain by Young Granodiorite, high readings typically correlating with alluvial regolith landforms. However, this widespread correlation was not reflected in areas underlain by Illunie Volcanics, where high conductivities correlated instead with features such as farm dams and bedrock constrictions.

Bedrock type and regolith character have a large impact on dryland salinity. The reasons for manifestation of dryland salinity can vary greatly over short distances. Factors affecting dryland salinity at a particular location must be assessed in detail before the most effective mitigation strategies can be successfully implemented.

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CLIMATE-DRIVEN WEATHERING EPISODES DURING THE LAST 200 MA IN SOUTHERN AUSTRALIA Brad Pillans CRC LEME, Australian National University, Canberra, ACT, 0200 Abundant geological evidence indicates that there were major changes in Australian climate, on timescales ranging from glacial-interglacial fluctuations in the Pleistocene, through to longer-term changes such as the Late Tertiary arid shift and the Late Cretaceous-Early Tertiary "hothouse earth". Given that many parts of the Australian continent have been subaerially exposed for hundreds of millions of years, it is important to knov^ hov^ such past climate changes are reflected in regolith development. For example, the location and interpretation of geochemical anomalies in deeply w^eathered, complex terrains w^ould be better understood by mineral explorers if the times and environmental conditions under which they formed were known.

Modem, broad-scale relationships between soil characteristics, weathering processes, and climate suggest that past climate changes may be expressed as recognisable weathering imprints in regolith.

An extension of this idea is that the formation of certain regolith materials (e.g. bauxites) may be episodic, depending on the nature and duration of past climate changes. An alternative view is that since weathering processes are continuously operating, regolith character is an integration of all those processes over time, and climatic effects will be blurred or eliminated.

The survival of ancient weathering products in Australian regolith is well documented by Ar^^Ar^^ dating of alunite, jarosite and manganese oxides, oxygen isotope dating of kaolinite and paleomagnetic dating of iron-oxides. These studies demonstrate that, once formed, many secondary minerals remain stable to subsequent modification over tens or even hundreds of millions of years.

Furthermore, their ages can be used to distinguish between the episodic versus continuous weathering models.

Paleomagnetic results from weathering profiles across southern Australia do not produce a continuum of ages. Rather, there is a marked tendency for ages to cluster. One particularly remarkable episode at 60±10 Ma, is widely represented in weathering profiles developed on rocks which range in age from Archean to Cretaceous, at sites from the eastern Yilgam through northern South Australia, southern Queensland and northern NSW. At around 60 Ma southern Australia lay at high latitude, straddling the Antarctic Circle. With no Antarctic ice cap, climate was wet, cool temperate, and vegetation was dominated by conifer forests and woodlands. In contrast, during late Tertiary time (10 ± 5 Ma), which is another interval represented by many paleomagnetic ages, climates had become seasonally drier (though rainfall was still higher than present) and warmer, as southern Australia drifted to lower latitudes, and sclerophyll elements in vegetation communities were becoming more dominant at the expense of rainforest taxa. Thus the two major episodes of iron oxide formation in the Tertiary occurred under differing bio-climatic regimes. A third cluster of paleomagnetic ages, though represented by fewer sites, is Jurassic (180±10 Ma).

428

READING GEOCHEMICAL SIGNALS IN THE REGOLITH - ONE KEY TO FINDING DEPOSITS UNDER COVER. Nigel W. Radford CRC LEME, Curtin University of Technology.

Mineral exploration in Australia is in rapid decline, annual expenditure having fallen from A$1250 M in 1996-97 to A$650 M in 2000-01. This crisis is partly due to low metal prices and perceived problems w^ith land access. A serious but less publicised issue is that it is simply getting harder to explore in Australia. Areas of "easy" exploration, where cover is thin or absent, amount to less than 25% of the land surface. These areas have been largely explored, many of them more than once.

Exploration companies, eager for rapid success, are channelling exploration budgets into those overseas countries where exploration is perceived to be both "easy" and the country to be "underexplored". Companies are often prepared to exchange low levels of political risk in Australia for these perceived advantages overseas, in higher risk environments.

In areas of cover, direct geological prospecting methods are largely ineffective. Explorers resort to indirect methods such as geophysics and geochemical sampling in holes drilled into the lower parts of the regolith profile. But even in situ regolith is widely perceived as an impediment to both geophysics and geochemistry.

In recent years numerous case histories have been presented showing results from soil sampling in areas of cover. Some of these have demonstrated that, under as yet poorly defined conditions, subtle geochemical signatures exist on the surface above mineralisation covered by sometimes tens of metres of post-mineralisation cover. Detection of such anomalies is sometimes enhanced by the use of selective or partial extractions, linked to highly sensitive ICP MS analytical techniques.

However, some case histories fail to demonstrate a cause-and-effect link between the weak surface expression and buried mineralisation.

Soil sampling in areas of cover is attractive to exploration companies due to low costs and speed of coverage relative to more definitive methods. However, in the absence of nearby empirical evidence, such as an orientation study over known mineralisation, it is impossible for the explorer to discriminate, in a routine survey, between a negative result (nothing there) and a null result (wrong technique).

The challenge for modem geochemists, when trying to "see through" post-mineralisation cover, is to do it with enough certainty to resolve the negative from the null result. The "acid test" is, can the ground be relinquished on the basis of "no anomalies"? In order to achieve such definitive sampling, the geochemist needs to be able to asses his results in the full knowledge of what has been done to the samples (which has implications when proprietary digestions are used) and with an understanding of the mechanisms capable of moving metals into the near surface through post mineralisation cover. Without these understandings, such surveys will remain empirical, not predictive. The challenge for exploration geochemical research is to understand the mechanisms that carry metals to surface, and to place these mechanisms into the 4D regolith history of the study area.

429

REGOLITH-LANDFORM MAPPING AT HOVELL'S CREEK, CENTRAL WEST NSW: DRYLAND SALINITY HAZARD MITIGATION IN HIGH-RELIEF GRANITIC LANDSCAPES Angela Ratchford & C. Leah Moore Dryland Salinity Hazard Mitigation Program (DSHMP), CRC LEME, University of Canberra, ACT 2601

The Hovell's Creek Catchment, north of Frogmore in central west NSW, has its headwaters on regolith developed over Ordovician metasedimentary lithologies but largely flows through highrelief granite of the Wyangala Batholith. Indicators of dryland salinity including tree death, salt scalds, presence of salt and water tolerant plant species, clear standing water, reduced crop yield in low-lying areas, puffiness of soils and gullying are manifest in the extreme upper reaches of the catchment, but are almost non-existent in the main reaches of Hovell's Creek, where it flows through granite country. The influence of regolith substrate on salt stores and mobilisation of salt through Hovell's Creek Catchment forms the focus of this study.

Regolith-landform mapping in the Hovell's Creek Catchment will facilitate interpretation of shallow fluid flow and assist with description of the features that typify high-relief granitic groundwater-flow-systems (GFS). The physicochemical properties of surface water entering and leaving the section of Hovell's Creek the flows over a granitic substrate will help establish the origin of components of the salt flux through the Hovell's Creek system, and quantify seasonal variations in water quality. This work will also enable evaluation of where salt is stored in the landscape surrounding Hovell's Creek and how this salt can be mobilised into the hydrologic system at Hovell's Creek. Links have been established with local land managers and government advisers to facilitate the comparison of products of this research with existing resources, including the local ground Electromagnetic Induction (EM) survey, with a view to assisting with land management decision making in the Hovell's Creek area.

A comprehensive review of the regional geology, regolith geology, landscape analysis, dryland salinity, hydrogeochemistry, groundwater flow systems (GFS) and interpretation of remotely sensed imagery forms the basis for the study. Regolith-landform mapping of the Hovell's Creek area at 1:15,000 scale has commenced. This area is on the Goulbum 1:250,000 scale and Boorowa 1:100,000 scale map sheets. An overview of the local geology and extant native vegetation is being conducted at the same time as regolith-landform mapping. Aerial photographs, geophysical data sets and other available remotely sensed imagery are being used to aid mapping and detailed landform analysis.

A year-long water sampling program commenced in January 2002, to constrain physicochemical characteristics of the water of Hovell's and Oakey Creeks. This sampling program is now 50% complete and preliminary results indicate that there may be a net dilution through the main reaches of Hovell's Creek. Although land salinisation is less common in this part of the catchment, here are issues of post-low-rainfall-period saline fluxes in Hovell's Creek and, when the river is flowing, the low-concentration salt load to areas further downstream may be an issue of concern.

Upon completion of field mapping a description will be prepared characterising the features typical of high relief granitic ground water flow systems (GFS) using the Hovell's Creek catchment on the Wyangala batholith as a case study. An attempt will be made to develop a conceptual model of salt flux, including a preliminary salt budget, for the Hovell's Creek field area. A comparison of the local EM survey with the regolith-landform map will facilitate an evaluation of salt distribution in Hovell's Creek. Interaction with the NSW DLWC Central West Salt Group and the local Landcare Group forms an important focus for this work.

430

PALAEOMAGNETIC DATING OF THE HAMERSLEY SURFACE AND DEEP WEATHERING IN THE PILBARA-NORTHERN YILGARN REGION, WA Phillip W. Schmidt^ and George E. Williams^ ^CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 1670 ^Department of Geology and Geophysics, University of Adelaide, SA 5005

Late Mesozoic and Tertiary deep weathering and peneplanation in Western Australia produced extensive landsurfaces that now are undergoing dissection. These ancient landsurfaces include the spectacular Hamersley Surface in the Pilbara region and the lateritised and deeply w^eathered 'Older Plateau' surface in the northern Yilgam region. An early episode of dissection stripped the Hamersley Surface of its uppermost lateritic mantle, producing extensive valley-fill that includes important pisolitic iron-ore deposits. Despite the economic relevance of these palaeolandsurfaces, the timing of their episodic weathering and dissection is poorly known.

Magnetisations of weathered profiles are acquired during crystallisation of hematite and are called Chemical Remanent Magnetisations (CRM). Following palaeomagnetic dating of Tertiary weathering in the Perth Basin [1], palaeomagnetic studies in several continents have established the ages of additional weathering episodes ranging from Cenozoic to Proterozoic [2-4]. Here we report on a preliminary palaeomagnetic study of oriented deep drill cores and associated block samples spanning 40-65-m-thick lateritised and weathered profiles below the Hamersley Surface south of Munjina and the Older Plateau surface east of Wiluna that aims to date weathering episodes in the Pilbara-northem Yilgam region.

Intense weathering was encountered to depths of 40 m and 65 m in two diamond drill holes 200 km east of Wiluna. The palaeomagnetic directions of the weathered rock are about an axis with a mean direction of D = 183.5°, I = 55.3° (n = 23, k -33.9, ags = 5.3°). The number of normal directions and reverse directions is almost equal. The pole position is located at 80.0°S, 105.3°E (dp = 5.3°, dm == 7.5°). This pole agrees with the palaeomagnetic pole from lateritic profiles in the Perth Basin [1] and is consistent with a mid-Miocene age of deep weathering.

Natural remanent magnetisation of samples from 20 sites in the weathering profile below the Hamersley Surface is distributed bimodally. Although samples have yet to be treated to isolate the weathering component, the steepest reverse directions have a D ~ 180°, I = 60°. Assuming these steepest reverse directions are the least contaminated by recent field components, this finding would place the pole position related to weathering at about 70°S, 120°E. Depending on error, this pole position corresponds to Late Eocene or Oligocene. Thermal cleaning will be conducted to verify the pole position and determine the error.

Our results confirm the usefulness of palaeomagnetism for constraining the age of CRM, weathering and mineralising events associated with the crystallisation of magnetic minerals.

References 1. Schmidt, P.W. and Embleton, B.J.J., 1976. Palaeomagnetic results from sediments of the Perth Basin, Western Australia, and their bearing on the timing of regional lateritisation. Palaeogeogr., Palaeoclimatol., Palaeoecol., 19, 257-273.

2. Idnurm, M. and Schmidt, P.W., 1986. Palaeomagnetic dating of weathered profiles. Geol. Surv. India Mem., 120, 79-88.

3. Williams, G.E. and Schmidt, P.W., 1996. Palaeomagnetic dating of sub-Torridon Group weathering profiles, NW Scotland: verification of Neoproterozoic palaeosols. J. Geol. Soc. Lond., 154, 987-997.

4. Schmidt, P.W. and Williams, G.E., 1999. Paleomagnetism of the Paleoproterozoic hematitic breccia and paleosol at Ville-Marie, Quebec: further evidence for the low paleolatitude of Huronian glaciation. Earth Planet. Sci. Lett., 172, 273-285.

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A VISION FOR REGOLITH RESEARCH IN AUSTRALIA R.E.Smith and P.G.Wilkes Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME)

Regolith geoscience in Australia is in the middle of an unprecedented grov^h in knowledge. This has come about through recognition by the mineral industry of the importance of understanding and dealing with the regolith in exploration of deeply weathered terrains and areas of cover, and the initiatives of Geological Surveys, Universities and Cooperative Research Centres focussing their attention on these issues. Furthermore, over the last two or three years we have seen the application of multi-disciplinary geoscience approaches bringing new insights to the regolith setting of areas affected by environmental problems such as dryland salinity.

The biggest need is clearly recognised : The capability to map and understand the regolith in 3D, or 4D with time. Important advances have been made. Examples include the Yandal Greenstone Belt in WA (Phillips and Anand, 2000), the Lawlers district in WA (Anand et al, 1991), and the Gilmore project in NSW (Lawrie et al. 2000).

Despite important studies such as these, we are still lacking, as far as the authors are aware, a single demonstration district study that has all the datasets and ingredients required to make the most of existing technologies.

A number of geophysical developments have important contributions to make in regolith research and these need careful integration with geological and geochemical information. These developments include : ABM systems, gravity gradiometry, distributed arrays of geophysical sensors, and improved inversion techniques. Airborne surveys are particularly useful because of their rapid areal coverage but need some ground surveys and geological control, particularly of the regolith, to ensure maximum effectiveness. This is an iterative process.

In order to make substantial inroads we need initially several, and then many, district studies in well chosen landscape positions and regolith settings for which the most comprehensive range of datasets (geology, geophysics and geochemistry) are generated and supported by extensive drilling.

In addition, age dating of the regolith will increasingly constrain ideas on regolith processes.

This can be done but it won't just happen. We need to mobilise our resources and work collaboratively to do this. With areas carefully selected, one senses that the results in terms of advancing science would be stunning.

References Anand, R.R., Churchward, H.M., Smith, R.E. and Grunsky, E.C. (1991). Regolith-landform development and consequences on the characteristics of regolith units, Lawlers District, Western Australia. CSIRO Division of Exploration Geoscience Restricted Report 166R, 160pp. Reissued as LEME Open File Report, 62, 1998, Perth.

Lawrie, K.C., Dent, D.L., Gibson, D.L., Brodie, R.C., Wilford, J., Reilly, N.S., Chan, R.A. and Baker, P., 2000: A geological systems approach to understanding the processes involved in land and water salinisation, AGSO Research Newsletter, 32: 13-15, 26-32.

Phillips, G.N. and Anand, R.R. (Editors). 2000 Yandal Greenstone Belt: Regolith, Geology and Mineralisation. AIG Bulletin No 32, 400pp

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CUSTOMISED REGOLITH MAPS FOR DELINEATING SALT STORES AND PREDICTING NEAR SURFACE SALINE GROUND WATER FLOW, NSW. J. Wilford CRCLEME, cA AGSO, GPO Box 378, Canberra ACT 2601, Australia Australia is threatened by increasing loss of agricultural production and water quality, and damage to infrastructure and biodiversity due to salinity. Approximately 5 million ha of farmland is affected by saline seepage or rising saline groundwater, and this area is expected to triple in the next 50 years. Many areas of dryland salinity occur within a diverse regolith cover that may be up to several hundred metres thick. The ability to rapidly map regolith materials and predict likely salt stores and saline ground water flow is therefore essential in developing and implementing remedial management strategies.

An integrated catchment-based approach has been developed that involves interpretation and modelling of high-resolution airborne gamma-ray spectrometry and magnetics and, digital elevation models to generate a series customised regolith maps for environmental assessment. This approach has been applied to the Harden and Boorowa catchments within largely erosional landscapes of the Southern Tablelands of New South Wales.

Regolith materials are mapped using airborne gamma-ray spectrometry imagery. Gamma-rays emitted from the surface will relate principally to the primary mineralogy and geochemistry of the bedrock, and the nature of secondary weathering (regolith materials). Geomorphic thresholds within different geochemical bedrock groups are then used the separate these primary and secondary gamma-ray responses. Soil maps including depth, texture and mineralogical properties are derived from this technique together with ground soil/regolith measurements.

The DEM was processed to derive a series of terrain indices (e.g. slope, surface curvature, slope length, flow direction, roughness, wetness index) to characterise catchment geometry, geomorphic process and hydrological gradients. The DEM and an enhanced vertical derivative of the airborne magnetics were used to generate a detailed structural lineament map for each catchment.

Integration of the modelled regolith maps with stream and soil ECs showed a good correlation between high salt concentrations and relatively thick clay soils and highly weathered saprolite.

These soils are mainly confined to smooth hills with low relief and lower concave slopes and broad valley floors. Thin soils and moderately to slightly weathered saprolite generally contained little salt. Combining these gamma-ray-predicted salt stores with areas of local wetness (potential discharge zones) derived from the DEM shows where salt outbreaks might occur, or where the highest salt loads are discharging into the streams. The prediction of near surface saline groundwater flow is further refined with the inclusion of structural lineaments (faults and fractures) that act has conduits or barriers to groundwater movement.

Regolith and salinity maps generated using this approach can contribute greatly in whole-ofcatchment environmental management. Salt store maps can be used to prioritise remedial activities and soil maps, together with predicted drainage characteristics, could be used in planning revegetation intervention strategies.

433

DigiTAL (Digital Teaching And Learning) WEB RESOURCES FOR FIELD WORK, LABORATORY SPECIMENS AND STUDENT RESEARCH PROJECTS IN THE GEOSCIENCES Andrea Smith, Andreas Schmidt Mumm, Patrick James, P. and Graham Heinson Department of Geology and Geophysics, Adelaide University, Adelaide SA 5005. Ph. +61-8-8303 Email: Graham.Heinson@adelaide.edu.au The principal aim of this study, funded by a Faculty of Science Learning and Teaching Grant in 2001, has been to make significant and useful resources available on-line to add value to fieldw^ork, laboratory studies and research within the University and at remote locations.

Three programs, developed through the integrated DigiTAL w^eb site, are: 1. DigiTATE, a virtual mineral collection sourced from specimens displayed in the Department's Tate Museum. The site has been constructed to assist the student in developing mineral identification skills in hand specimen and in thin and polished section. In addition to key mineral identification criteria, these pages also contain information regarding the history, uses and occurrences of each mineral. Minerals can be searched using the A-Z index or through a floorplan and legend of the Tate Museum 2. DigiTRIP, virtual field trips in the 1st to 4th year of the Department's programs. Field trips are essential to the learning experience of geoscience students, and the w^eb cannot replace such activities. However, the web can play a role by making resources connected with the fieldcamp available to students, such as photographs, video, maps and post-fieldtrip quizzes on key observations and understanding.

3. DiglTERM, an on-line glossary of geological terms that are linked through hyperlink activated pop-up windows in DigiTATE and DigiTERM, or as a searchable archive.

A prototype DigiTAL web site was made available on the Department's web server in March 2002, and is currently under development and evaluation. It is anticipated that content will be added during the year in student projects and field trips to form an integrated teaching resource for all years. The DigiTAL web site will also be useful for schools and in the community to promote the study of geosciences.

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IDENTIFYING FACTORS INFLUENCING EROSION OF THE SPIT ON BRIBIE ISLAND, SE QUEENSLAND Jared Lester^ Brendan Brooke^ and Malcolm Cox^ ^Queensland University of Technology, ^Geoscience Australia Email: Brendan.Brooke@ga.gov.au

Bribie Island is a large sand island at the northern margin of Moreton Bay, southeastern Queensland. A spit at the northern end of Bribie Island forms a barrier between the open ocean and the estuarine foreshore of the city of Caloundra. Severe shoreline erosion in recent years has considerably narrowed the spit, raising fears of a breakthrough and subsequent erosion of the Caloundra foreshore. This study examines erosion rates and sediment transport pathways of the spit system, long-term changes in its geomorphology and the factors influencing these processes.

Most of Bribie Island comprises a series of beach-ridge plains, contrasting with the other large sand islands of Moreton Bay that formed with a coalescence of transgressive dunes. The spit consists of beach ridges and low dunes predominantly covered by coastal heath and woodland. Bribie Island is separated from the mainland by an estuary, Pumicestone Passage, the inlet to the estuary situated at the northern end of the spit.

Rates of erosion were derived from time-series aerial photographs, bathymetric charts and field surveys and were compared with erosion rates obtained by a similar earlier study of the spit (Jones, 1992). Insights into the longer-term evolution of the spit were obtained by analysing vibrocores and sections exposed in erosional scarps cut into the eastern side of the spit by storm waves.

A sediment budget for the period 1978 - 1993 indicates the spit is eroding at a rate of approximately 144,000 m^ yr"\ Approximately 80% of the sand eroded from the spit is transported into the inlet of Pumicestone Passage, the rest is lost to the spit system, moving southwards with the dominant longshore drift. This loss of approximately 30,000 m^yr^ is a significant increase on the rate of erosion calculated for the period 1958 - 1972, 15,000 m ^ r ^ by Jones (1992). Aerial photographs also indicate that since 1940 the spit has been retreating by approximately 1.5 myr"\ Exposures of estuarine mud along the eastern shore of the spit and offshore exposures of indurated muddy sand indicate the spit has been undergoing erosion and landward translation. An articulated Anadara shell recovered from an intertidal exposure of mud has a radiocarbon age of 3330 ±70 yr BP, suggesting sea level at that time was similar to the present and that there has been significant long-term erosion of the spit.

Although storm frequency has increased during the last century, the frequency of storms is not significantly different between the two periods for which erosion rates were calculated, indicating other factors may be contributing to the increasing rate of erosion of the spit. These factors appear to be a reduction in the rate of sediment supply from offshore, an increase in the rate of longshore drift, dredging in the inlet of Pumicestone Passage and offshore and possibly, a rise in sea level.

Reference Jones, M.R. (1992). Quaternary evolution of the Woorim-Point Carthwright Coastline, Volume 1. Project Report MA49/2, Queensland Department of Minerals and Energy.

435

H,S PRODUCTION IN LAKE WOLLUMBOOLA, AN INTERMITTENTLY CLOSED/OPEN LAKE (ICOLL) ON THE NSW SOUTH COAST Emma Murray, and David Heggie Geoscience Australia, GPO Box 378, Canberra, ACT, Email: emma.murray@ga.gov.au Of increasing concern in south-eastern Australia is the declining health of estuaries and coastal watenvays. In NSW, 51% of the 133 estuaries are either modified or extensively modified (from OzEstuaries database http://vmw.ozestuaries.org). The impact of humans on these estuaries is set to intensify, if the current rate in population grovv1;h along the NSW coast continues. This highlights the increasing importance of estuarine management. If w^e are to maintain the recreational, commercial and natural values of estuaries, then v^e need careful and informed management underpinned by a good understanding of key estuarine process. One such process is the breakdown of organic matter in estuarine sediments and the onset of anoxia, a condition usually considered to indicate an unhealthy waterway.

We present the findings of an investigation into hydrogen sulfide (H2S) production in Lake Wollumboola, an intermittently Closed/Open Lake (ICOLL) on the NSW South Coast. H2S is a product of sulfate reduction. Sulfate reduction is a process of organic matter breakdown, which occurs in the anoxic sediments of many estuaries and is a significant component of the carbon cycle in shallow-water ecosystems. Sulfate reduction occurs to such an extent in Lake Wollumboola that the smell of H2S, or rotten egg gas, frequently causes considerable discomfort to local residents.

We measured the porewater and bottom water concentrations of H2S, S04^", CI", and Fe^^ from sediment cores taken from 12 sites spanning the different sediment facies (central basin, fluvial delta, and marine delta). The grain size, major elements and TOC (total organic carbon) of the solid phase were also measured. From this data, we concluded that sulfate reduction occurs extensively in Lake Wollumboola, with porewater concentrations of H2S up to lOmM. We revealed the spatial distribution and depth distributions of H2S, S04^", CI", Fe^^, grain size, major elements and TOC. H2S concentrations were generally highest in the central basin and fluvial delta (average about 3000|LIM), and lowest in the marine delta (average about 158|AM). TOC, the fuel for sulfate reduction, was similarly, highest in the fluvial delta and central basin (~6.0Wt%) and lowest in the marine delta (~1.3Wt%). Suprisingly, two sites in the central basin had low levels of H2S (<346|LIM) but high rates of sulfate reduction, as indicated by the porewater sulfate to chloride ratios in the top 10cm of sediment. We also found that the sediment of these two central basin sites had the highest concentrations of TS (total sulfide) in the lake. We suggest that at these sites, the availability of reactive iron has resulted in extensive precipitation of H2S as iron sulfides.

In Lake Wollumboola, high levels of TOC, a lack of oxygen and the availability of porewater sulfate, result in the bacterially mediated production of H2S. However, the availability of reactive iron controls the amount of free H2S in sediment porewaters.

436

CONCEPTUAL UNDERSTANDING OF AUSTRALIAN ESTUARIES AND COASTAL WATERWAYS D.A. Ryan and D.T. Heggie Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Estuaries are extensively used by Australians - they are popular sites for development, industry, urbanisation and recreation.

Estuaries are essentially dynamic and contain ephemeral accumulations of sediment; these sediments provide important habitats for plants and animals.

Recent estimates suggest that there are about 1000 estuaries and coastal waterways in Australia, almost half of which are considered to be near pristine. Globally, estuaries have been recognised as the critical interface between catchment practices and the marine environment, however they have often fallen in the gap between marine and catchment management policies.

The Australian coastline is almost 70 000 km in length (at a 0.1 km scale) and spans 34 degrees of latitude, from tropical to subtropical and temperate environments. Nationally, estuaries occur over a wide range of geological, climatic and topographic conditions and consequently display a great variety of morphology. Australia is a dry continent - our estuaries differ from classic Northern Hemisphere systems. Geoscience Australia, and the CRC for Coastal Zone, Estuary and Waterway Management are developing a suite of models to illustrate the variability of Australian systems.

These conceptual models will provide a framework in which the physical form (classification) and ecological function of estuaries and coastal waterways may be compared and visualised. They explain natural processes, and communicate complex ideas, knowledge and data into a form appropriate for use by all stakeholders. These processes include the effects of wave, tide and river energy, natural and human-induced sedimentation, nutrient dynamics, flushing characteristics, and ecosystem support.

Conceptual models have been constructed through our own research, reviews of the literature, collaboration with project partners, and feedback from State agencies and other stakeholders in the coastal zone. These models are based upon Geoscience Australia's recent geomorphological classification of Australian estuaries, which identified seven main waterway types. The models may also be used as planning and management tools to demonstrate the links between the form (geomorphology) and the function (hydrodynamic, sediment transport, geochemical and biological processes) in coastal waterways. Due to inherent differences, each of the seven key waterway types may respond differently to various kinds of pressures. For example, wave-dominated estuaries are more susceptible than tide-dominated estuaries to increased suspended sediment loads from the catchment, due to the fact that high suspended sediment concentrations are a typical feature in pristine tide-dominated estuaries. Similarly, nutrients, toxicants and sediments released into estuaries are more likely to be retained, whereas in deltas most material is simply exported to the marine environment. Thus it can be seen that an understanding of the processes operating in various coatal waterways is important for the effective management of competing stakeholder needs (such as commercial and recreational fishing, aquaculture, ports and navigation, environmental protection, urban and rural development, scientific research, and human amenities.

The estuarine environment is an example of an area where geoscience, ecology, hydrology and geochemistry all play critical roles in supporting a wide range of natural processes and human activities. As a member of the CRC for Coastal Zone, Estuary and Waterway Management, Geoscience Australia is expanding upon the work of the NLWRA to integrate the perspectives of a wide range of coastal stakeholders. From this first comprehensive and National series of conceptual models of Australia's estuaries and coastal waterways will be produced. Use of webbased technologies (and links to the OzEstuaries database - http://www.ozestuaries.org) will allow efficient distribution of products and effective feedback from all stakeholders in the coastal zone.

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SELECTING AUSTRALIAN MARS ANALOGUE SITES: METHODOLOGY AND CRITERIA Jonathan D. A. Clarke.^ Deckert, J.,^ Gostin, V.,^ Hoogland, J./ Laing, J.,^ Lemke, Leyden, J./ Mann, G.,^ Murphy, G.,^ Stoker, C.,^ Thomas, M.,^^ Waldie, J.,^^ Walter, M.,^^and West, M.^^ 1 Department of Geology, Australian National University, Canberra, ACT 0200, 2 Westprint Maps Nhill, VIC, 3 Department of Geology, University of Adelaide, SA, 4 Centre for Hypersonics, University of Queensland, QLD 4072, 5 School of Tourism & Hospitality, La Trobe University, Bundoora,, VIC, 6 NASA Ames Research Center, CA, United States, 7 School of Medicine, Flinders University, Bedford Park, SA, 8 School of Information Technology, Murdoch University, Murdoch, WA, 9 6/75 Field Street Clifton Hill, VIC 3068, 10 Geoscience Australia, ACT, 11 Department of Aerospace Engineering, RMIT, Fishermans Bend, VIC, 12 School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, NSW 13 10 Department of Geology, Macquarie University, North Ryde, NSW

The Mars Society is a privately funded organisation whosQ aim is to further the goal of the exploration and settlement of Mars, by broad public outreach to instill the vision of pioneering Mars, supporting government funded Mars exploration programs round the world, and conducting Mars exploration on a private basis. Part of its mission is to select areas w^orld-w^ide w^hich can serve as Martian analogues. These areas must meet three criteria. First, they must have evidence of past or present processes that provide analogues to past or present Martian processes. Second, they must provide a range of surfaces and materials similar to that expected on Mars which allow testing of technology. Third, they must bear some visual similarity to Martian landscapes. Two areas have already been selected for Mars analogue research: Devon Island in the Canadian Arctic, and southern Utah. Further areas are under review.

The Australian chapter of the Mars Society (MSA) has reviewed a number of regions in Australia for Mars analogue potential. These offer a number of features not found in the other analogue areas, including the ability to simulate long range surveys and traverses, ancient landscapes and regolith, aridity, an extremely dusty and saline environment, groundwater dominated landscapes, and both past and present geothermal systems.

The regions were defined as 200-km diameter circles on the basis of the preliminary endurance of the Human Operations Prototype (HOP) pressurised rover under development by the MSA. They contained a range of features of interest, including different Mars analogue processes, diverse surface conditions for engineering tests, and favourable security, logistics, and land access. Six regions in South Australia were highlighted: Dalhousie, Moon Plain, Woomera, Arkaroola, and Sturts Stony Desert. Moon Plain, Woomera, and Arkaroola were the leading contenders, with Arkaroola finally selected as the prime site because of established Mars analogue research at the modem hydrothermal system at Paralana Hot Spring and the Carboniferous hydrothermal system at Mt. Gee. The other sites may be studied at a later date.The MSA expects to complete the HOP and habitat module in 2002 and commence field operations in 2003. The planned test program includes robotics, teleoperations, communications, operations research, regolith terrain mapping, microbiology, and micropaleontology. The MSA is looking for sponsorship and donations towards this goal.

For further information see the MSA web page http://www.marssociety.org.au/

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AUSTRALIAN ENVIRONMENTAL GEOSCIENCE: AN EDUCATIONAL SOURCEBOOK Vic Gostin Department of Geology & Geophysics, University of Adelaide, Adelaide, South Australia 5005 Email: victor.gostin@adelaide.edu.au Many environmental issues involve interactions betw^een rock, soil and v^ater, and Australian geoscience has much to offer towards their better management. The new book "GONDWANA to GREENHOUSE: Australian Environmental Geoscience'' aims to supplement international texts with a compendium of key articles by AustraUan experts, emphasising the centrality of geoscience in many environmental problems. The book title conveys the concept of an ancient land that has been subject to milleimia of environmental change, which currently incorporates human-generated cUmatic instability. This book brings modem geoscientific knowledge to the attention of a wider audience, and should form part of the education of environmental scientists. The editor's Introduction leads on to 28 chapters arranged under six themes: Theme 1 - Ancient Australia and environmental changes. Australia's Gondwanan inheritance has resulted in large areas with deep regolith of weathered rock, poor soils, ancient drainage and limited water supplies. Recognising this ancient profile is crucial to understanding the present soil degradation and salinisation. The first chapter examines Quaternary environmental fluctuations in order to better appreciate the long-term evolution of the Australian landscape. The next four chapters deal with problem soils, and with the environmental repercussions of the very ancient topography and drainage systems of SW Australia, including Kambalda.

Theme 2 - Geohazards in urban communities. Australia's highly urbanised population is subject to various geological hazards including widespread soil heave, landslides and the occasional earthquake or tsunami. Cairns, in tropical Queensland is used to demonstrate the geohazard risk assessment methods under the CITIES project of Geoscience Australia, and the Illawarra region is used to illustrate landslides and mudflows.

Theme 3 - Mining and radioactivity. While mining disturbs less than 0.02% of Australia's land area, its effects are more widespread and land rehabilitation remains a lasting problem. One large chapter deals with these issues, and a separate chapter discusses the geochemistry and hydrogeology of mine wastes from Mt Lyell, Tasmania. An important chapter focuses on natural radioactivity and uranium mining in Australia, and reports on the identification of geologically safe repositories for nuclear wastes.

Theme 4 - Water and sedimentary basins. Four chapters deal with the Great Artesian, MurrayDarling, and Gippsland basins with issues of water use, subsidence, and the scourge of increasing salinity. Clean and dependable supplies of water for Aboriginal communities in central Australia is addressed in one chapter.

Theme 5 - Coastal and nearshore environments. Since about 80% of Australia's population lives near the coast, eleven chapters are devoted to these environments begirming with the Swan coastal plain in W. Aust., and its geoheritage features. Local sea-level history reflects both changing global sea levels and earth movements, as discussed in a chapter on S. Aust. Other chapters cover the highly populated coastal zone of NSW and SE QsL, and discuss the geological history of estuaries, pollution, water management, remediation of quarry sites, and conflicting land use.

Sydney's four major estuaries and the adjacent continental shelf are the focus of detailed pollution studies. The Great Barrier Reef of Australia features in two chapters that describe the geological and biological aspects of its grow1:h, and various threats to its health. Studies of several estuarine environments in tropical northern Australia show how their geological history is relevant to their conservation and better management.

Theme 6 - Marine geoscience. The last two chapters discuss the importance of marine geoscience to almost every sector of the marine economy and improved seabed management. It includes the submarine environments across Torres Strait that would affect the laying of vulnerable gas pipelines.

'GONDWANA to GREENHOUSE' - V.A. Gostin (Editor) GSA Special Publication #21

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CHANGES IN STAGE 6 EARTH SCIENCE TEACHING IN NSW SCHOOLS Steven McClean Knox Grammar School

During the 1990's there was a 65% reduction in the number of students studying Geology in New South Wales schools. While the numbers in 1992 reached a high of 588, by 2000 that number had decreased to just 203. The course was very similar to many first year university Geology courses and was taught by a core of school teachers with geological training or at least a strong interest in the subject. As these people retired or moved into other areas of teaching the number of schools offering Geology steadily shrank.

In 2000, a new subject called Earth and Environmental Science was introduced which had the most radical changes of any of the subject changes to the various Science disciplines taught at school in New South Wales. The response by schools across the state to take up the new subject was overwhelming, with more than 2000 students taking the course in their Preliminary Year (first three terms of Year 11). The initial Higher School Certificate candidature for Earth and Environmental Science was 1250, a six-fold increase on the last year that Geology was offered. This year at Knox Grammar there has been a further 35% increase in numbers and if this is applied elsewhere, the future of the subject appears sound. This has important ramifications for the teaching of Earth Science at university level in that a more diversified approach to Earth Science teaching could well lead to improved enrolments, but more immediately, there are a large number of school teachers with little exposure to Earth Science teaching the subject. This conclusion can be drawn from the number of students who attempted an elective that was basically Biology-based (60% of the candidates) compared to the number that attempted the two Geology-based electives (35%of the candidates). This large group of teachers, and their students, are a ready market for any tertiary institution that is prepared to be proactive in the supply of teaching materials for schools.

440

THE AUSTRALIAN STRATIGRAPHIC NAMES DATABASE - UPDATES TO AN AUSTRALIAN ICON Andrew P. McMahon, Cathy E. Brown and Donna M. Cooper Geoscience Australia, GPO Box 378, Canberra, ACT, 2601, Australia

The Australian Stratigraphic Names Database (ASND) is an essential component of Australian geology. The responsibility for maintaining this lexicon rests with Geoscience Australia.

In 1949, the Bureau of Mineral Resources (BMR), now Geoscience Australia (GA), established the National Register of Stratigraphic Names, to assist geoscientists in working with the new Australian Code of Stratigraphic Nomenclature. Until 1979, this was maintained as a card file index (Lenz et al. 1996). Since then various upgrades, including an Oracle database, have continued to refine the level of information collected and amount made available for each stratigraphic unit. The ASND was one of the first GA databases online, with simple forms to query the database "live" (see http://www.ga.gov.au/oracle/stratnames.html).

The major focus of changes to the ASND has been to enable greater online access for all clients.

Apart from the existing stratigraphic name and bibliographic queries already available online, GA have recently added a number of simple online forms allowing: viewing of the indexing backlog; viewing of complete unit definitions; article submission; reservations of names; definition form completion; and general enquiries to ASND staff. These updates will be demonstrated in the presentation, and at the GA stand.

Information collected for each stratigraphic name includes name; parent; overlying and underlying units; province; age; thickness; and location. GA is also currently working towards a solution for capturing "unnamed units". This has required new fields, including lithological description; these fields may also be used for named units.

Each stratigraphic name also has a list of references from which the stratigraphic information is obtained. Due to the large volume of references that include stratigraphic information, a priority schema has been developed, ensuring that references with the most relevant information are indexed first and that this data is available promptly. A recent modification allows clients to access these priority listings. There is also a form for clients to provide bibliographic details for new references.

Unit definitions, previously in hard copy form and inaccessible to clients, have been converted to digital format, and made available online.

In addition to querying stratigraphic information, GA now provides facilities for clients to reserve stratigraphic names and to submit unit definitions directly into the database.

GA will continue to develop ways to provide access to all the information available for Australian geological units, including sequence stratigraphic units, and unnamed units where required, thus enabling access to a thorough and comprehensive lexicon, for all Australian geoscientists.

Reference Lenz, S.L., Brown, C.E., Bond, L.D., Rybum, R.J., 1996. Guide to the Australian Stratigraphic Names Database, Australian Geological Survey Organisation, Record, 1996/16.

441

THE LEARNING AND TEACHING BENEFITS OF FIELD-BASED, STUDENT CENTRED, CLIENT-RELATED PROJECTS: REGOLITH-LANDFORM MAPPING FOR DRYLAND SALINITY HAZARD MITIGATION. C. Leah Moore Dryland Salinity Hazard Mitigation Program, CRC LEME, University of Canberra, ACT 2601 In July 2001 the Dryland Salinity Hazard Mitigation Program (DSHMP - affectionately known as DiSHMoP) was established at the University of Canberra. This program is a collaborative interaction between the Department of Land and Water Conservation (DLWC) Central West NSW Salt Group, the University of Canberra (UC), and more recently the Cooperative Research Centre for Landscape, Environments and Mineral Exploration (CRC LEME). It is a student-centred learning group that focuses on preparation of detailed local-catchment-scale (e.g. 1:10,000 scale) regolith-landform maps that are used together with other layers of information (e.g. remotely sensed imagery, geology and soil landscape maps, electromagnetic induction surveys) to constrain regolith distribution, shallow hydrogeology and landscape characteristics.

More recently this work has also been used to characterise hydrogeomorphic units (HGU) in the groundwater-flow-system (GPS) schema, and within DLWC central west NSW provides a basis for land management unit (LMU) discrimination. All of the DSHMP work is carried out cooperatively with local Landcare groups, who have been very supportive of the student projects. As end-users the Landcare members not only foster community goodwill by allowing student access to their properties, but also benefit from the production of maps and reports that facilitate their land management planning.

Students prepare a tangible product (a published map) and prepare a report for clients, in addition to a thesis including extension projects for Honours and Masters students. This mechanism allows students to generate publications, sometimes prior to graduation, and provides a transparency to employment as a professional. Students are responsible for all field coordination and liaison with land managers and industry colleagues.

From a pedagogical perspective the student "ownership" of the projects has typically led to an extremely high standard of project coordination and map production. The benefits of networking with land managers and staff of government agencies, and the development of students' professional integrity are positive aspects to the program that can only be measured indirectly. In less than 12 months student members of DSHMP have received 9 awards or grants from professional bodies, including the GSA. This reflects on the quality of the work generated by the students in DSHMP, and the calibre of the students attracted to the program.

Students can enter the program at Masters level. Honours level or at senior undergraduate level via a Research Project in Applied Science, Professional Practice, or the field mapping component of a third year subject. Land Appraisal. There is a strong emphasis on regolith-landform mapping, and extension into discipline areas of particular interest to the student is encouraged. To date DSHMP has relied heavily on the sound undergraduate foundation provided by the Earth and Land Science course, and more recently the Ecology and Environmental Science course at UC, and it is a credit to the staff contributing to these courses. The ongoing support of DLWC central west NSW and CRC LEME has been instrumental in this student-centred-leaming opportunity being extended to Honours and Masters students.

442

TAKING TECHNOLOGY TO FIELD-BASED LEARNING: TEACHING REGOLITH MAPPING IN BROKEN HILL, N.S.W. Ian C. Roach^ and Steven M. HilP ^MTEC-Minerals Education Australia, CRC LEME, University of Canbena, ACT 2601 ^CRC LEME, University of Canberra, ACT 2601 The Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME) offers a range of laboratory- and field-based Honours- and Masters-level shortcourses as part of the National Geoscience Teaching Network (NGTN) of the Minerals Tertiary Education Council (MTEC). Courses are open to academia and industry and are of one week (Honours) or two weeks (Masters) duration, based in Canberra and Melbourne or various mining centres around Australia including Kalgoorlie, Charters Towers and Broken Hill. Courses are designed to give participants the skills and knowledge primarily to map and explore for minerals within the regolith but also to recognise and possibly ameliorate hazards within the regolith relating to dryland salinity, chemical pollution and civil engineering.

During Easter 2002 CRC LEME ran an intense week-long Honours-level course entitled "Regolith Mapping and Field Techniques" based in Silverton, west of Broken Hill. This course was designed to introduce participants to regolith mapping and GIS in a field-based setting and also incorporated a field Occupational Health and Safety component. Participants were encouraged to bring IBMcompatible notebook computers and Global Positioning System (GPS) receivers, although the University of Canberra furnished this equipment for those who were unable to do so themselves.

The curriculum covered basic regolith mapping, introductory Maplnfo GIS and interpretation of aerial photography and remotely-sensed imagery including airborne radiometrics and magnetics, gravity, Landsat TM satellite imagery, ASTER multispectral satellite imagery and HyMap™ hyperspectral imagery. Data sets were provided on CD-ROM and also included several Digital Elevation Models (DEM) at various scales including a 30 m (ASTER), 100 m (airborne radar altimeter) and 250 m (AUSLIG 9-second) DEM. Participants were given an overview of the regolith and landscape evolution of the Broken Hill region in a one day east-west transect across the region, from the Murray Basin through the Broken Hill Block to the Mundi Mundi Plain. During this introduction participants also became familiar with aspects of the neotectonic evolution of the Broken Hill landscape, an important feature to recognise for mineral explorers and geomorphologists alike. On the second day participants were treated to an overview of the Cockbum region by one of the local landowners who described landcare efforts and gave an insight into field etiquette. Participants were then set loose in groups to map a 4 km^ area near Cockbum (the Mundi Mundi Prospect), along the NSW-South Australia border. For the next two to three days participants mapped during the day and worked on map production in the evening within a portable GIS computer laboratory set up at the Silverton base.

Assessment was based on the completed map and accompanying report, which included field site data point information, submitted after the course finished as both hard copy and CD-ROM. As a result of this course students learned to develop a regolith map from start to finish, interpret aerial photography and remotely-sensed imagery, operate a GPS and GIS and set up an off-campus computer laboratory during an intense five day period.

443

A CORRIDOR THROUGH TIME: A SOUTH AUSTRALIAN EXAMPLE OF A COOPERATIVE APPROACH TO GEOSCIENCE EDUCATION Ian Clark University of South Australia Mawson Lakes, South Australia, 5095 Brachina Gorge in the Central Flinders Ranges of South Australia provides a unique section across the geological record. In a distance of less than 20 km the rocks present a record of the evolution of the Earth System during the period from about 700 Mya to 500 Mya. The combination of a semi-arid climate and a deeply eroded narrow^ gorge allows an almost continuous outcrop of moderately- to steeply-dipping sedimentary rocks to be viewed. These rocks contain evidence for the early evolution of the atmospheric system, climate change (icehouse and greenhouse), and the evolution of the biosphere from cyanobacteria (stromatolites) to complex life forms including the Ediacaran soft-bodied fossils and the subsequent Cambrian fauna .

It is possible for visitors to stand with one foot on either side of the Cambrian -Precambrian boundary or to see a thin layer of rock produced 600 Mya when a huge meteorite hit the Earth several hundred kilometres to the west. As well as these unique features there are all of the more regularly encountered features of a sequence of sedimentary rocks that have undergone deformation and have been exposed on the Earth's surface for more than 400 million years.

All of these features have made this an area of interest to geoscientists which has been visited for many years by researchers, undergraduate classes, K-12 classes and most recently by the general public. It is now enclosed in a National Park formed by the acquisition of a number of former sheep grazing properties.

The incorporation of the gorge into the national park and a general increase in nature-based tourism has meant that the area is being visited by many people who are unable to interpret the geology of the area and who, without help, remain unaware of its particular significance.

This has resulted in a partnership between the National Parks Service, the Geological Survey and the University of South Australia to provide information suitable for interpretation by visitors.

Signs, a brochure, a coloured book, a geological map and information in an Interpretive Centre have been prepared and as well a training program for local Park Rangers and tour operators has been undertaken.

The outcome of this initiative, the dilemmas and the processes involved in its undertaking will be described in this presentation.

444

FLAWED TOURIST SIGNAGE C. R. Twidale & J. A. Bourne Geology and Geophysics Department, The University of Adelaide, Adelaide, S.A. 5005 Email: rowLtwidale@adelaide.edu.au Information offered patrons in the Botanical and Zoological gardens is accurate as is signage at some other tourist sites. But not all. In signage, popular publications and spoken presentations, geological and geomorphological information which is inaccurate, internally inconsistent, and/or irrational, is commonplace. Sources are not acknowledged, and interesting features and facts are overlooked. Some language employed is ungrammatical, confusing and misleading. This is of considerable concern for overseas visitors, or for Australians whose first language is not English.

Specific examples are cited, as are reactions and responses (if any) from the responsible authorities when such shortcomings are drawn to their attention.

The authors consider that the public has a right to the most accurate and up-to-date information, presented in a logical, clear and comprehensible manner. This approach does not preclude scientific integrity. Sources of data ought to be acknowledged. Those in positions of responsibility ought to be satisfied with nothing but the best available at the time.

445

THE ROLE OF SURFACE WATER CHEMISTRY IN HEAVY METAL TRANSPORT AT DRAKE MINING AREA, NSW, AUSTRALIA Harry Esterio Southern Cross University, Australia - Universidad de Concepcion, Chile PO Box 157 Lismore NSW 2480, Australia Mining activities in the Drake Mining area, NSW, Australia, have affected surface water trace element chemistry along Saw^pit Creek. This work seeks to evaluate the effect of past mining practices in five small open pits. Determine physicochemical parameters; characterise metal movement and speciation; and geochemical modelling.

Surface water samples were collected between 1999 and 2001 in 33 site locations. Concentrations of Al, Cr, Mn, Zn, Cd, Fe, Cu, Ni, and Pb were measured in total and filtered phases. Waste rocks carrying pyrite, chalcopyrite, sphalerite, and galena had been exposed to weathering generating acid and releasing metals thus contributing to high trace metal concentration in the site. However, there are only few minerals (e.g. calcite and ankerite) or rocks that can consume acid, in addition, alteration, mineralisation, and weathering have resulted in a deposit with a high potential for generating acid mine drainage, metal release and minimal capacity to neutralise this acid.

Water has been investigated using speciation techniques that include chemical analysis in unfiltered and filtered samples, using a filter pore size of 0.2 |um. Metal concentrations were determined by ICP-MS. Sulphate and chloride were determined using a LACHAT QuickChem 8000.

The results showed that outside the mine, in Plumbago Creek, the pH is neutral and the trace metals are found in the total phase. Downstream the junction with Sawpit Creek, SO4 and CI concentrations decreased due to dilution that came from the uncontaminated waters of Plumbago Creek. Cu, Pb y Zn is adsorbed for colloidal Fe and Al. The increase in the pH and the formation of Fe and Al colloidal in the junction of the rivers, allows the condition for the coprecipitation and adsorption of trace metals.

In Sawpit Creek trace metals are found in the filtered phase. The pH is 4 and concentrations are above background and ANZECC Guidelines for Drinking Water (1992). The water quality is degraded due to old mining operations, sulphide minerals and high concentrations of trace metals.

These acidic and contaminated waters are mixed with the neutral and uncontaminated waters of Plumbago Creek; thus neutralizing and precipitating metals such as Ferrihydrite and Gibbsite.

The geochemical modelling identified Gibbsite and amorphous aluminium that control the aluminium concentration. Iron is controlled by the precipitation of iron oxides and oxyhydroxide of iron. Manganese is controlled by Manganite, Hausmanita, Pirolusite and Picroite.

The speciation techniques used in this study have shown to be useful for discriminating between the contribution of different possible sources and total contaminant loading in natural waterways.

This technique can contribute to detect elevated trace metal concentrations that are a source of possible contamination and must be considered in water quality management of the Drake mining area.

Keywords

Surface waters, AMD, water speciation.

446

COMPARISON OF WEATHERING RATES FOR MINE SPOIL OBTAINED FROM UNSATURATED COLUMN, FIELD AND BATCH STUDIES Evans, K.A., Banwart, S.A. Department of Civil and Structural Engineering, University of Sheffield, Mappin St, Sheffield, SI 3JD, England. Email: k.evans@sheffield.ac.uk Mining activities frequently expose large volumes of fresh rock to atmospheric conditions.

Subsequent rapid weathering and release of rock constituents may then lead to elevated concentrations of polluting elements and extreme pH solutions. Cost-effective prediction of the potential severity and timescale of contamination is essential both for pre-mining environmental assessments and for remediation of resource-limited abandoned sites. Laboratory assessments are the most convenient method of measuring the contamination potential of spoil. Rudimentary acidbase accounting tests are popular, because of the relative ease with which they can be performed and interpreted. However, these tests measure only the time-integrated potential for acid or base production, with no incorporation of rate considerations. Interpretation of more sophisticated tests is also problematic. Experiments on individual minerals frequently show a poor agreement with experiments on polymineralic materials, and weathering rates obtained from batch experiments are generally one to three orders of magnitude faster than those found in the field. This has been attributed to a number of factors, including differences in temperature, surface area, grainsize, hydrology and pH between laboratory and field.

Here we present results from weathering studies of material performed at three different scales, under differing reaction conditions. Measurements of 1) field weathering element fluxes are compared to 2) fluxes from laboratory unsaturated column and 3) saturated batch experiments for material from a coal spoil heap, a deep coal mine, a limestone-hosted lead spoil, and a mudstonehosted tin/copper bearing spoil.

Significantly different behaviour was observed for the different lithological types. Equilibrium with secondary phases controlled release of contaminants from the limestone material under all three circumstances, implying that field fluxes can be calculated directly from laboratory results if field hydrological conditions are sufficiently well quantified. Ion exchange and secondary mineral formation controlled release of elements from the coal spoil heap material; however, dissolving and exchanging assemblages were substantially different in the laboratory to those inferred from field results, where oxygen exhaustion and a greater depth of spoil affected the release of contaminating elements. Kinetically controlled dissolution of minerals was observed for less weathered material from the deep coal mine, although secondary mineral formation was significant. The expected one to three orders of magnitude difference between field and batch release rates was observed, but release rates from the unsaturated columns was surprisingly low. A linear relationship between water content and the reactive/physical surface area ratio (RSA/PSA) could not adequately account for this observation. Therefore grainsize distributions and soil-water retention curve data were used to successfully calculate ratios between active and inactive surface areas, although solutions are likely to be non-unique.

Geometric and hydrological constraints on RSA/PSA are probably the most important factor to be considered when transferring laboratory results to field situations. Extrapolation of results from batch monomineralic experiments is likely to be inappropriate as interactions between reacting minerals in the form of ion exchange and secondary mineral formation control the release of many elements. Care should also be taken as substantially different processes may be taking place in portions of the field site for which physical and chemical conditions have not been replicated in the laboratory.

447

ACCESS DENIED? PROSPECTS FOR MINING AND CONSERVATION IN NATIONAL PARKS

Greg Ogle\ Michelle Grady^ and Declan Andrews^ ^Campaigns Coordinator, The Wilderness Society (SA) ^Executive Officer, Conservation Council of SA ^Campaigner, The Wilderness Society (SA)

The South Australian Labor government will have to find a new balance betw^een conservation and mining development needs. In recent years the national park system has been dominated by a multiple use ideology which has seen mining in a number of National Parks and Conservation Parks. This ideology, and the regulatory regime it spawned, have failed politically and have not delivered good conservation outcomes. This paper takes a case study approach to argue that, even with best practice, mining is simply not compatible with conservation in some areas. The national park system needs to be restructured and a new mining-conservation dialogue is needed to create the map within which sustainable mining can take place.

448

Abiari, I. Adabi, M. Adam, J. Adamson, D. Adkins, R. Agar, B. Ailleres, L. Allen, S.R. Alley, N.F. Amelin, Y. Anand, R.R. Anderson, I. Andrew, A.S. Andrews, D. Ansdell, K.M. Ansermet, S. Appel, P. Archbold, N.W. Archibald, N.J. Arculus, R.J. Armstrong, R.A. Arumba, J. Ashley, P.M. Avigad, D. Avouac, J.P. Badarch, G. Baker, T. Ballard, J.R. Bann, G. Banwart, S.A. Barley, M.E. Barovich, K.M. Barratt, R.M. Barrett, B. Barron, L.M. Barton, T.J. Bassinot, F. Bastrakov, E. Bateman, R. Batiste, C. Bauer, W. Baulch, J.C.I. Bell, T.H. Belperio, A.P. Belton, D. X. Bengtson, S. Bennett, V Beresford, S.W.

p8 Berlepsch, P. p76 Berly, T. pp 34,209 Berry, A. pp 101,176 Berry, R.F. pp 6, 16 Bestland, E.A. pp59, 66,118,169 Betts, P. p404 Bewert, K. pp 122,274,275,282,283 Bierlein, F.P. p284 Biju-Sekhar, S. p75 Binns, R.A. p284 Binu-Lal, S.S. p28 Bird, M.I. p2 Bleeker, W. p44 Bodorkos, S. p222 Boger, S.D. p297 Bohic, S. pp 351, 352,353 Bone, Y. pp 167,206 Bons, P.D. pll9 Borel, G. plOl Borrisova, I. p354 Bostock, H.C. pp 183,328,337,384 Both, R.A p276 Bottrill, R.S. p34 Bourdon, B.P. p24 Bourman, R.P. p445 Bourne, J. A. p5 Bowler, J.M. pp 357,374 Boyd, R. p230 Bradshaw, J.D. p28 Branch, N. p23 Brenchley, A.J. p 103 Brewer, T. p98 Brey, G. p36 Brock, J. pp 7,24,405,435 Brooke, B. p81 Brouwer, F.M. p441 Brown, C.E. pp 200,406 Brown, M. p42 Brown, S.J.A. p263 Brown, W.M. pp 8,167,285,290 Brugger, J. p244 Bryan, S.E. p42 Bucci, L.A. p 102 Buchan, C. p201 Buckman, S. Buddemeier, R.W. p 397 pp 158,159, 160,202 Buick, I.S. 217,220 p36 Buick, R.

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Conor, C.H.H. Cooke, D. Cooper, D.M. Corbett, G. Cowley, W.M. Cox, M. Coxhell, S. Crapez, B. Crawford, A.J. Crispe, A.J. Crocket, J.H. Cromie, P.W. Crook, D. Crooks, A. Cross, A.J. Crowhurst, P.V. Cunningham, D. Cunningham, W.D. Curtis, M. Cutten, H. Daczko, N.R. Daly, S.J. Dalziel, I.W.D. Daniel, R. Daniell, J.J. Davidson, G. Davidson, P. Davies, M. Davis, B. de Caritat, P.R. Dean, A.A. Deckert, J. DeDeckker, P. Della-Pasqua, F.N. Deloule, E. Denton, G.J. Dickens, G.R. Dijkstra, A.H. Ding, X. Direen, N.G. Donoghue, S.L. Drummond, B.J. Duboz, C. Duddy, I.R. Duffett, M. Duk-Rodkin, A. Dunlap, W.J. Dunlop, A. Dyson, I. A.

p238 Burke, K. plOl Burrett, C.F. p311 Burtt, A. p289 Byrne, J. p317 Calandro, D. p276 Calver, C.R. pp 9,244 Cameron, R.G. p77 Campbell, I.H. p 10 Cann, J.H. p227 Capdevila, R. pp 78,178 Caprarelli, G. pp212, 286 Carew, M.J. p318 Carey, H. p304 Carey, M.L. p 100 Carlson, R.W. p331 Carman, C. Carr, G.R. pp 273,305,316 Carter, R.M. pp 356, 377, 381 Carter, T.J. p 161 Cartwright, I. pp 158, 202 Cas, R.A.F. pp 42,45,247,252, 327 Cattle, S.R. p9 Cawood, P. A. pp 104, 122 Cayley, R.A. p 149 Chamalaun, F.H. p324 Chappell, J. p30 Chen, J. p40 Chivas, A.R. pll Cihan, M. p 162 Claoue-Long, J. pl05 Clark, A. p260 Clark, D. pl2 Clark, I. p444 Clarke, G.L. pp 203,231 Clarke, J.D.A. pp 13,407,408,409,438 Clemens, J.D. p218 Cline, J. p289 Close, D. pp 105,189,202 Cobb, M. p 163 Coblentz, D.D. p 186 Coffey, B. p372 Coffin, M.F. p 102 Coleman, B. p318 Collins, C. pl2 Collins, L.B. p357 Collins, P.L.F. p345 Collins, W.J. pp49, 106,204,213 Colquhoun, G.P. pp 9,244 Condie, K.C. p 107 450

pp 155, 163, 259 pp 205,266 p441 p329 p378 pp 7,366,435 p365 p42 pp 79,108 p 165 p314 p287 p258 p 172 pp 105,165 p 166 p 103 p81 p223 p 109 p203 p259 p 110 p353 p358 pp 266,274,288 pp 80,205 pp 57, 60 p288

pp416,421 p 165 p438 p389 pp 255, 394 p52 p305 pp 347, 365, 369 p81 p390 pp61,64, 68 p93 pp 129,319 p 144 p 166 p320 pp 410,411 p65 p334 pp 379, 380

Frankes, L.A. Fraser, G. Friend, C.R.L. Frikken, P. Fu,B. Gaina, C. Gallagher, S.J. Gamble, J.A. Gammon, P.R. Gapais, D. Gardner, B.L. Gamett, D. Garsed, I. Gates, W.P. Gawlinski, S. George, A.D. Gessner, K. Ghisetti, F. Gibson, D.L. Gibson, G.M. Gibson, R.L. Gibson-Poole, C. Giles, C. Giles, D. Gleadow, A.J. W. Glen, R. Glen, R.A. Glenn, K.C. Golding, S.D. Goleby, B.R. Gonzalez, O.L. Goodge, J.W. Goscombe, B. Gostin, V. Gow, P. Grady, M. Graham, I. Graham, S. Gray, D.J. Gray, D.R. Green, D.H. Green, P.F. Green, T.H. Greenhalgh, S. Grey, K. Griffin, W. Griffiths, C. Groves, D.I. Groves, I.

p370 p51 pp 243,278 p247 pp 106,189,202 p323 p293 p283 p23 pp 246,253 p416 pp 82, 84, 167,206 p207 p446 p 111 p447 p289 pl68 p391 p62 pp 46, 74,133,143 222,224,225,226,227,236 p267 Farrell, B.L. p358 Fellows, M.E. p324 Ferguson, I.J. pll2 Fergusson, C.L. p63 Ferris, G. p221 Ferry, J.M. p242 Fielding, C.R. p28 Filfield, K. pp 14,15,113, 114,115 Findlay, R.H. 245,363 p83 Finlayson, D. pp 340, 341,442 Fitzpatrick, R. Fitzsimons, I.C.W. pp 127,228 pp31,69 Fletcher, I.R. p211 Flint, R.B. p 190 Flood, R.H. pp 82, 83,90,125,151,155 Foden J. 156,175,194,311 p 129 Fomin, T. pp 169,185 Forbes, C. pl6 Forbes, M.S. pp 32,297 Foriel, J. p 134 Forster, M.A. p251 Forsyth, S.M. pp 116,121,148,161 Foster, D. A. 196, 274 p32 Fouquet, Y.

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451

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pp 141, 361 Hocking, R.N. p322 Hodge, I. p263 Hodgkison, J.P. p234 Hokada, T. p242 Holcombe, R.J. p362 Holdgate, G.R. p205 Hollings, P. pl76 Holm, O.H. p417 Holzapfel, M. pp34,210 Honda, M. p82 Hoogewerff, J. p438 Hoogland, J. p333 Hooper, B. p46 Horstwood, M.S.A. p418 Hou, B. p382 Hough, M. p50 Houseman, G. p 159 Huang, M. p 177 Huddlestone-Holmes, C. p307 Huleatt, M.B. p 127 Hulscher, B. p329 Hunt, S. p26 Huntley, D.J. p338 Huston, D.L. p281 Hutton, L.J. p 139 Hyodo, H. pp 34, 230 Ireland, T.R. p82 Itikarai, I. p226 Jacobs, J. pp 264, 307 Jacques, A.L. p61 Jagodzinski, E.A. pp 352,353 James, N. pp 183,337,384,405 James, P.R. 419,422 p334 Jami, M. p327 Jane, M. p265 Jeffrey, S. p46 Jelsma, H.A. p35 Jenkins, R.J.F. pp 92, 146 Jessell, M.W. p83 Johnson, W. p 131 Johnston, A.J. p246 Johnston, R.W. p 129 Johnstone, D.W. pl22 Jones, A. p423 Jones, G. p 141 Jones, J. A. pp 129,319 Jones, L.E.A. p420 Joyce, E.B.

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p267 Judkins,D. p234 Kagami, H. p301 Kamber, B.S. pp 79, 80,205 Kamenetsky, V.S. pp 273,335 Keays, R.R. p308 Keeling, J.L. p362 Keene, J.B. pl28 Keep, M. p 14 Keetley, J. p231 Kelly, N.M. p 100 Kennett, B.L.N. p405 Kemich, A. p28 Kershaw, A.P. pl77 Ketcham, R. p32 Khodja, H. pp 15,115,363 Kia, P. pp 115,245,363 Kilya, T.W. pp 163, 189 Kinny, P. p296 Kinsley, L. p34 Kirschvink, J.L. p421 Kirste, D. p51 Kisters, A.F.M. p237 Kleinschmidt, G. p226 Klemd, R. p203 Klepeis, K.A. p211 Knaak, M.L. p269 Kneeshaw, M. p261 Knox-Robinson, C. pp 161,179,196 Kohn, B. P. ppl4,115,245,363 Kopi, G. p305 Korsch, M.J. pl29 Korsch, R.J. p420 Kotsonis, A. p 169 Krabbendam, M. p42 Krapez, B. p238 Kraus, J. p277 Kreuzer, O.P. p8 Krivovichev, S. Krokowski de Vickerod, J. p 278 p 131 KrSner, A. p22 Krull, E.S. p215 Kusachi, L p390 Labeyrie, L. pp 321, 323 Lackie, M. pp 8, 327 Lahaye, Y. p438 Laing, J. Lang,S.C. pp 359, 364, 367,370, 371 385, 388 p356 Larcombe, P. 453

pp 10,24 Murray-Wallace, C.V. p279 Mustard, R. p238 Myrow, P. p385 Nakanishi, T. p82 Nasution, A. p363 Nekitel, S. p 196 Nelson, D.R. p46 Nesbitt, R.W. p89 Nicholls, I. p 135 Nicholls, LA. p247 Nichols, HJ. p322 Nichols, W. p 129 Nicoll, M.G. p 188 Nieuwland, D.A. p83 Nishimura, Y. p 138 Noll, C. pp 37, 296, 394 Norman, M. pp 43, 47, 48 Nutman, A.P. p368 O'Brien, G.W.O. p 192 O'Neill, H.StC. p 178 Och, D.J. p 139 Offler, R. p448 Ogle, G. p233 Okudaira, T. p25 O'Leary, M.J. pp212,286,291,292 Oliver, N.H.S. 294,347 p267 0'Loughlin,N.T. pp 354, 397 Opdyke, B.N. pp 170,302 Ord, A. p 179 Osadetz, K.G. p232 Osanai, Y. p369 Page, M.C. p423 Pain, C.F. p91 Palin, J. M. p230 Pankhurst, R.J. p370 Paraschivoiu, E. p90 Paraschivoiu, V. p211 Parker, A.J. p305 Parr, J.M. p206 Passchier, C. p 191 Passmore, M.J. p246 Patia, H. p434 Patrick, J.P. p21 Paull, R. p49 Pawley, M. p386 Payenburg, T.H.B. p203 Pearson, N.J. p227 Pecora, L.

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Rey, P. Reynolds, S.D. Rheinberger, G. Richards, S.W. Ridley, J. Roach, I.e.

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p 182 p316 p 170 pp 283,291 pp 244,400,425 pp 413,423,433 p432 pp 152,254 p26 p431 pp 158, 159, 160, 180 220, 238 p30 pp212, 291 p270 p222 p46 p289 p298 pp 103,153 p221 p 140 pp41,198 p275 p281 p 149 p247 p282 pp 87, 96, 161, 300 p74 p245 p338 p285 p97 p98 pp 75,298 p215 p57 pp 255, 287, 301,349 p302 p302 p 154 p208 p239 p82 p251

457

GEOSCIENCE AUSTRALIA

PRIMARY INDUSTRIES AND RESOURCES SA

Government of South Australia

^CRCLEME

BEACH PETROLEUM

Cooperative Research Centre for

Landscape Environments and Mineral Exploration

P E R I

L

Y A

Dominion Mining Limited

16th A u s t r a l i a n G e o l o g i c a l C o n v e n t i o n 1-5 July 2 0 0 2 A d e l a i d e C o n v e n t i o n Centre Adelaide, S o u t h Australia