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Geological Society of Australia

ABSTRACTS Numhpr 29

CONFERENCE ON

TECTONICS AND MINERALIZATION OF THE LACHLAN FOLD BELT

CANBERRA 4-6 FEBRUARY 1991

GEOLOGICAL SOCIETY OF AUSTRALIA INC. ABSTRACTS AND FIELD TRIP GUIDE NUMBER 29

LFB 91

Abstracts of a conference on "Tectonics and Metallogenesis of the Lachlan Fold Belt" Australian Academy of Science, Canberra, February 4-6 1991

Printed by Union Offset Co. Pty Ltd, Canberra

LIST OF CONTENTS

ORGANISING COMMITTEE

SPONSORS

ALPHABETICAL LIST OF AUTHORS

iii-iv

PROGRAM

v-ix

ABSTRACTS

1-66

FIELD TRIP GUIDE: Thrusting in the Eastern Lachlan Fold Belt: Goulburn-Bungonia region (C.L. Fergusson)

67-94

ORGANISING COMMITTEE CHRIS FERGUSSON - Wollongong University ) ) Convenors DICK GLEN - Geological Survey of NSW ) JOHN WALSHE - Australian National University BRUCE CHAPPELL - Australian National University MIKE RICKARD - Australian National University ERWIN SCHEIBNER - Geological Survey of NSW

CONFERENCE SECRETARIAT Australian Convention and Travel Services (ACTS) GPO Box 2200 Canberra ACT 2601 (Phone 06.257.3299)

LIST OF SPONSORS The Geological Survey of Australia (Inc.) Specialist Group in Tectonics & Structural Geology IGCP Project 267 Paleozoic Terranes in the Circum Pacific Orogens New South Wales Department of Minerals and Energy

Financial Support was also given by: Australian Academy of Science - 25th IGC Fund Geopeko Logistical Support was also provided by: Geological Survey of New South Wales; Department of Minerals & Energy University of Wollongong Australian National University

iii

ALPHABETICAL LIST OF AUTHORS Allibone Baillie and Powell Barron et al Blevin and Chappell Brewer Carr, Dean, Gulson and Suppel Chappell Collins and Vernon Coney Cooper and Tulloch Crawford and Berry Crook et al Crowe and Rickard Dadd Elliott, Gray and Woodward Fergusson and Coney Fowler and Lennox Gee Gibson Gibson and Nihill Glen Glen, Dallmeyer and Black Gray Heithersay and Walshe Hinman Hopf Jenkins and Sandiford Johnson Lackie and Rood Lennox, Hawley and Fowler Leven, Stuart-Smith, Musgrave, Rickard and Crook Liu and Fleming McQueen and Perkin Mandyczewsky, Gray and Hine Mills Morand Nicholls O'Reilly and Griffin Pemberton et al Perkins, McDougall, Claoue-Long, Walshe and Chivas Paterson, Fowler, Vernon and Flood Phillips Powell

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 33 34 35 36 37 38 39 40 41 42 43 44

Rankin, Clough and Gatehouse Scheibner Seccombe Seccombe, Lu, Gulson and Mizon Smith Stevens Stuart-Smith, Hill, Rickard and Etheridge Sun Suppel and Gilligan Surman and Kjolle VandenBerg Wellman White, Wyborn and Chappell Williams, Chappell, McCulloch and Crook Willott and Ashley Wilson, Gray and Barton Wilson, Will, Cayley and Chen Windh and Zentilli Wormald and Price Wyborn Yadong Zhanlin, Kwak and Merry

45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66

LFB-91 DAY 1 - Monday 4 February 1991

Opening ceremony

9.00 a.m. - 9.15 a.m.

Official opening by J.N. Cramsie, Director, Geological Survey of New South Wales, Department of Minerals and Energy

1. S.Y. O'Reilly and W.L. Griffin - Beneath the LFB: evidence from integrated petrologic and geophysical studies 9.15 a.m. - 9.45 a.m. 2. I.S. Williams, B.W. Chappell, M.T. McCulloch and K.A.W. Crook - Inherited and detrital zircons - clues to the early growth of crust in the LFB 9.45 a.m. - 10.15 a.m. 3. G. Gibson - Medium-high pressure metamorphic rocks of the Tuhua Orogen, western New Zealand as lower crustal analogues of the LFB, SE Australia 10.15 a.m. - 10.45 a.m. Morning Tea

10.45 a.m. - 11.15 a.m.

4. B.W. Chappell - Tectonic evolution of the LFB: petrological constraints 11.15 a.m. - 11.45 a.m. 5. R.J. Wormald and R.C. Price - Mid to Late Palaeozoic magmatic and crustal evolution in the Temora region, central southern NSW 11.45 a.m. - 12.05 p.m. 6. P. Wellman - Division of the Tasman Orogenic System into geophysical domains using the pattern of gravity and magnetic anomaly trends 12.05 p.m. - 12.25 p.m. Lunch 12.25 p.m. - 1.40 p.m. 7. R.J.F. Jenkins and M. Sandiford - Tectonic interpretation of the southern Adelaide Fold Belt 1.40 p.m. - 2.10 p.m. 8. S.F. Liu and P.D. Fleming - The Cambrian passive margin in South Australia 2.10 p.m. - 2.30 p.m. 9. K. Mills - Tectonic evolution of the Wonominta Block 2.30 p.m. - 3.00 p.m. 10. A.J. Crawford and R.F. Berry - Early tectonic evolution of the LFB: a Tasmanian perspective 3.00 p.m. - 3.30 p.m. Afternoon Tea

3.30 p.m. - 4.00 p.m.

11. A.H.M. VandenBerg - Some aspects of Cambrian and Ordovician tectonics in the LFB in Victoria and SE NSW 4.00 p.m. - 4.30 p.m. 12. R.A. Cooper and A.J. Tulloch - The LFB in New Zealand? 4.30 p.m. - 5.00 p.m. 13. D. Wyborn - The tectonic significance of Ordovician magmatism in the eastern LFB 5.00 p.m. - 5.30 p.m.

DAY 2 Tuesday 5 February 1991 14. R.A. Glen - A thrust and strike-slip model for the Lachlan Orogenic Belt and implications for tectonic development 8.30 a.m. - 9.00 a.m. 15. D.R. Gray - LFB crustal architecture and tectonic evolution: a Victorian perspective 9.00 a.m. - 9.20 a.m. 16. C.J.L. Wilson, T.M. Will, R. Cayley and S. Chen - Structural evolution of the Ararat-Stawell region 9.20 a.m. - 9.40 a.m. 17. C.G. Elliot, D.R. Gray and N.B. Woodward - Relating Tasmania to the LFB 9.40 a.m. - 10.00 a.m. 18. P.W. Baillie and C.McA. Powell - Tectonic affinity of the LFB in Tasmania 10.00 a.m. - 10.20 a.m. Morning Tea

10.20 a.m. - 10.50 a.m.

19. C.L. Fergusson and P.J. Coney - LFB of southeastern Australia: an atypical convergent margin orogenic belt 10.50 a.m. - 11.20 a.m. 20. C.McA. Powell - Silurian collisional underplating in the LFB 11.20 a.m. - 11.50 a.m. 21. E. Scheibner - Assessment of Siluro-Devonian tectonic models for the LFB 11.50 a.m. - 12.10 p.m. 22. P.G. Stuart-Smith, R.L. Hill, M.J. Rickard and M.A. Etheridge - The stratigraphy and deformation history of the Tumut region: implications for the development of the LFB 12.10 p.m. - 12.30 p.m. 23. J.W. Pemberton, A.J. Wright and others - Evolution of the northern Capertee High 12.30 p.m. - 12.50 p.m. Lunch

12.50 p.m. - 2.00 p.m.

24. W.J. Collins and R.H. Vernon - Orogeny associated with low-pressure metamorphism - evidence from the LFB and the Arunta Inlier, Australia 2.00 p.m. - 2.20 p.m. 25. S.R. Patterson, T.J. Fowler, R.H. Vernon and R.H. Flood - Contrasting styles and timing of emplacement of Murrurabidgee- and Bathurst-type batholiths in the LFB, Australia 2.20 p.m. - 2.40 p.m. 26. S.E. Johnson - Structural/metamorphic history of the Cooma Complex, southeastern Australia 2.40 p.m. - 3.00 p.m. 27. P.J. Coney - The Lachlan Belt in the context of Circum-Pacific tectonics 3.00 p.m. - 3.30 p.m. Afternoon Tea

3.30 p.m. - 4.00 p.m.

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DAY 2 Tuesday 5 February 1991 Poster Session

4.00 p.m. - 5.00 p.m.

28. B.P.J. Stevens - Northwestern NSW and its relationship to the LFB 29. Yadong Chen - Importance of mafic inclusions in the Palaeozoic I-type granites of the LFB 30. P.G. Lennox, S.P. Hawley and T.J. Fowler - Intraorogenic Barry Granite two granites in one! 31. T.J. Fowler and P.G. Lennox - Characteristics of granite deformation in the epizonal environment: the Davys Creek Granite, south of Bathurst, NSW 32. M.A. Lackie and R.H. Flood - The elongate Braidwood Granodiorite: a tilted subrounded pluton 33. R.A. Glen, R.D. Dallmeyer and L.P. Black - Problems of isotopically dating deformations in low-grade rocks - the Cobar (Australia) example 34. G. Gibson and D.N. Nihill - Glenelg Metamorphic Complex: western margin of the LFB or extension of the Delamerian Orogen into western Victoria? 35. C.J.L. Wilson, D.R. Gray and T.J. Barton - Deep crustal seismic imaging of the crust in the western LFB, central Victoria 36. V.J. Morand - Siluro-Devonian mafic dykes in the eastern LFB: tectonic and metamorphic indicators 37. E.I. Nicolls - Evidence for Early Carboniferous shearing in the Wagonga beds, NSW 38. J. Leven, P.G. Stuart-Smith, R. Musgrave, M.R. Rickard and K.A.W. Crook - A geophysical survey across the Tumut Synclinorial Zone, NSW 39. K. Dadd - The Eden-Comerong-Yalwal volcanic zone: a continental rift zone or not? 40. L.R. Rankin, B.J. Clough and C.G. Gatehouse - Mafic suites in basement beneath the Murray Basin: new data for the Early Palaeozoic history of the Tasman Orogenic Province 41. J.V. Smith - Structural analysis in convergent and divergent wrench zones with special reference to the LFB 42. W. Crowe and M.J. Rickard - The Murrumbidgee mylonite zone

43. Sun Yan - The tectonic development of Palaeozoic fold belts in eastern China 5.00 p.m. - 5.20 p.m.

viii

DAY 3 - Wednesday 6 February 1991 METALLOGENESIS 44. G.N. Phillips - Gold in the southern LFB, Victoria 8.30 a.m. - 9.00 p.m. 45. L.M. Barron, D.W. Suppel, E. Slansky, Z. Johan, M. Ohnenstetter and R. Spencer - The Fifield Platinum Province (FPP) 9.00 a.m. - 9.30 a.m. 46. P.S. Heithersay and J.L. Walshe - The geochemical and tectonic ancestry of the Goonumbla porphyry copper gold deposits 9.30 a.m. - 10.00 a.m. 47. C. Perkins, I. MacDougall, J. Claoue-Long, J.L. Walshe and A.R. Chivas Age of gold mineralization, LFB, NSW 10.00 a.m. - 10.30 a.m. Morning Tea

10.30 a.m. - 11.00 a.m.

48. K.G. McQueen and D.J. Perkin - Siluro-Devonian sediment-hosted sulphide deposits in the southeastern LFB 11.00 a.m. - 11.20 a.m. 49. B.R. Willott and P.M. Ashley - Stratabound skarn or volcanogenic massive sulphide? Solution to the dilemma at Leadville, northern LFB, NSW 11.20 a.m. - 11.40 a.m. 50. K.A.W. Crook, J.L. Walshe and F. VanderHor - Siluro-Devonian extensional tectonism and metallogenesis in the LFB: insights from a modern Manus Basin analogue 11.40 a.m. - 12.00 a.m. 51. A.H. Allibone, S.K. Ren and P.S. Heithersay - Coeval deformation and alteration in a shear zone hosted gold deposit, Peak Hill, NSW 12.00 p.m. - 12.20 p.m. Lunch

12.20 p.m. - 1.30 p.m.

Poster Session

1.30 p.m. - 2.30 p.m.

52. A.M. Brewer - The Lucky Draw gold deposit, Bathurst district, NSW 53. A. Mandyczewsky, N. Gray and R. Hine - Junction Reefs gold deposits 54. J.A. Surman and I. Kjolle - Geology of the Browns Creek copper-gold skarn deposit, Blayney, NSW 55. D.W. Suppel and L.B. Gilligan - Metallogenesis of the LFB in NSW 56. J. Lindhorst, G. Cordery, A.H. Allibone and C. Doon - The Gidginbung Au deposit, NSW 57. Zhanlin Gao, T.A.P. Kwak and N. Merry - The sediment-hosted Nagambie gold deposit, Victoria, Australia 58. P.K. Seccombe - Sulphur isotope and fluid inclusion studies of turbidite-hosted Cu-Au vein deposits at Cobar, NSW - source of ore components in a low-grade metamorphic environment 59. P.K. Seccombe, J. Lu, B.L. Gulson and K.J. Mizon - Source of gold in a slate belt - geochemical modelling of the Hill End Goldfield, NSW 60. M. Hinman - Structural setting of mineralization at Peak, Cobar, NSW. Diamond drillcore structure using a structural manipulation spreadsheet 61. S. Hopf - The Mount Aubrey epithermal gold deposit 62. J. Windh and M. Zentilli - Early syntectonic origin for bedding-parallel (gold)-quartz veins in Palaeozoic turbidite successions

DAY 3 - Wednesday 6 February 1991 63. G.R. Carr, J. A. Dean, B.L. Gulson and D.W. Suppel - Lead isotope signatures of Ordovician-to-Permian mineralization in the LFB - genetic and exploration implications 2.30 p.m. - 2.50 p.m. 64. A.J.R. White, D. Wyborn and B.W. Chappell - A granite classification for the economic geologist 2.50 p.m. - 3.10 p.m. 65. P.L. Blevin and B.W. Chappell - Relationships between granites and mineral deposits in the LFB 3.10 p.m. - 3.30 p.m. Afternoon Tea

3.30 p.m. - 4.00 p.m.

Summation Keynote Address 66. D.G. Gee - Palaeozoic plate movements and the interplay of compression and extension in the Scandes 4.00 p.m. - 5.00 p.m.

COEVAL DEFORMATION AND ALTERATION IN A SHEAR ZONE HOSTED GOLD DEPOSIT, PEAK HILL, N.S.W. Allibone, A.H. Department of Geology, James Cook University, Townsville, Q4811 Ren, S.K. , and Heithersay, P.S. Geopeko, cnr Clarke and Alluvial Strs., Parkes, N.S.W. 2870 1

Au mineralisation and associated intense hydrothermal alteration at Peak Hill is tightly constrained within a north-south striking shear zone 600-800m wide. The dominant feature of this zone is the pervasive north-south foliation, which is cut and overprinted by northwest to west .striking shears, resulting in development of intense subvertical rodded fabrics, parallel to the stretching lineation. Hypogene alteration at Peak Hill can be divided into sericite-pyrite, silica-pyrite and pyrophyllitealunite types. Along the western margin of the shear zone foliation development and sericite-pyrite alteration are coincident with destruction of illite-chlorite-smectite assemblages in the adjacent, undeformed Mingelo Volcanics, indicating coeval sericite-pyrite alteration and north-south foliation development. Fine-grained silica-pyrite alteration forms several elongate, concordant, steeply clipping zones up to 50m wide in the shear zone. Timing of silicification in relation to sericite-pyrite alteration is not clear, textural relationships having been obliterated by continuing deformation. Silica-pyrite alteration corresponds to anomalous Au grades < 0.5ppm, indicating incipient Au mineralisation associated with this phase of alteration. Significant Au mineralisation is associated with younger vein-like zones of coarse grained sulphide±barite that replace the silica-pyrite alteration. Sulphide minerals are dominated by pyrite with accessory enargite, chalcocite and tennantite. Pyrophyllite-alunite alteration occurs as syn-northwest to west shearing, pressure shadow style overgrowths on sericite-pyrite and silica pyrite. Pyrophyllite and Au mineralisation exhibit a broad spatial correlation. Textures are complex and it is not clear whether Au-sulphide mineralisation pre-dates or post-dates pyrophyllite growth. Textural relationships indicate that alteration and mineralisation were caused by fluids focussed into an active shear zone at Peak Hill. This has important implications for exploration aimed at finding similar deposits.

TECTONIC AFFINITY OF THE LACHLAN FOLD BELT IN TASMANIA P. W. Baillie1 and C. McA. Powell2 1 Department of Resources & Energy, Division of Mines and Mineral Resources, PO Box 56, Rosny Park, Tasmania 7018 2 Department of Geology, The University of Western Australia, Nedlands, WA 6009 The Lachlan Fold Belt is represented in Tasmania by turbidite successions of Ordovician and Early Devonian age, which crop out predominantly in northeastern and eastern Tasmania. These rocks, known as the Mathinna Beds, have previously been regarded (Williams 1978) as a single conformable unit deposited without interruption, and deformed only during the mid-Devonian "Tabberabberan" Orogeny. Recent work has revealed that the Mathinna Beds comprise two distinct and disparate rock successions, and raises questions about how and when the successions came to be juxtaposed next to the Precambrian-cored West Tasmania Terrane. The older turbidite unit consists dominantly of lutite with subordinate arenite. Slaty cleavage pervades the lutite, and the arenite contains a spaced, stripy cleavage indistiguishable from that reported by Powell (1983) from Ordovician turbidites in the Eastern Lachlan Fold Belt. The lutites contain Early Ordovician fossils, and the slaty and stripy cleavages have been folded to recumbent orientations. A K/Ar date of 415±6 Ma (Adams et al. 1985) and a Rb/Sr date of 423±22 Ma (Cocker 1982) indicate that the main slaty-cleavage deformation was Silurian. The younger and more extensive turbidite unit is a quartzose succession derived from the southwest and deposited in a NNW-elongated trough. The only known fossils are Early Devonian, and the rocks are deformed regionally into tight, upright to E-overturned folds trending NNW. The axial surfaces of these folds have the same orientation as the younger folds in the older* succession, and are overprinted by the Late Devonian granitoids. Tectonic comparisons with coeval successions elsewhere in Tasmania and in southeastern Australia suggest that the East Tasmania Terrane was separate from the West Tasmania Terrane prior to the Silurian, but could have been juxtaposed by the beginning of the Devonian. The younger turbidite succession appears to be an extension of the Melbourne Trough in the Devonian, with sediments derived from uplifted quartzose areas to the southwest. Amalgamation of the terranes could have occurred during the end-Silurian deformation which affected many parts of the Lachlan Fold Belt in, and immediately to the east of, the Wagga Metamorphic Belt. The final consolidation of the northeastern Tasmanian flyschoid terranes onto the main Precambrian-cored part of Tasmania probably occurred during the mid-Devonian, and could have involved overthrusting from the east. References: Adams C.J., Black L.P., Corbett K.D. & Green G.R. 1985. Reconnaissance isotopic studies bearing on the tectonothermal history of Early Palaeozoic and Late Proterozoic sequences in Western Tasmania. Aust. J. Earth. Sci. 32:7-36. Cocker J.D. 1982. Rb-Sr geochronology and Sr isotopic composition of Devonian granitoids, eastern Tasmania. J. Geol. Soc. Aust. 29:139-157. Powell C. McA. 1983. Geology of the NSW South Coast. Specialist Group in Tectonics and Structural Geology Field Guide No. 1. Geol. Soc. Aust., Sydney, 118pp. Williams E. 1978. Tasman Fold Belt System in Tasmania. Tectonophysics 48:159-206.

THE FIFIELD PLATINUM PROVINCE (FPP)* L.M. Barron , D.W. Suppel , E. Slansky , Z. Johan , M Ohnenstetter , R. Spencer 1

1. 2.

Geological Survey of N.S.W., Department of Minerals and Energy, PO Box 536, St Leonards NSW 2065 GIS, BRGM-CNRS, Orleans, France

The FPP occurs as a 70 km wide band within the Lachlan Fold Belt, extending from Derriwong in the south and at least to Nyngan in the north. It is characterized by: 1) a well defined zone of gravity highs (Parkes Terrace) within the low specific gravity rocks of the Girilambone Anticlinorial Zone. 2) multiple occurrences of thin irregular bodies of serpentinized ultramafics. 3) a line of zoned felsic/ultramafic complexes, identified as Alaskan Complexes, which have many features in common including inherent mineralization of Platinum Group Elements (PGE), and these are the source for the deep lead alluvial deposits in the Fifield region which have produced some 650 kg of platinum nuggets and 325 kg of gold since the 1880's. A regional synthesis of the FPP and comparison with similar provinces in other countries leads to the following model for a platinum province. The essential tectonic components are crustal thinning of a failed back arc setting, with geosutures marked by thin bodies of Alpine peridotite. There may be several such belts configured in parallel if the tectonic regime underwent rapid lateral accretion. At depth and due to a raised geotherm in the extensional regime, high temperature felsic magma is produced which can be tholeiitic and/or shoshonitic. The extensional region in general, and specifically the geosutures, are invaded by the felsic magma, producing both volcanic and intrusive complexes. Zoned alkali ultramafic complexes occur as a result of assimilation of Alpine peridotite by the felsic magma at depths of 12-18 km, producing large volumes of very uniform ultramafic reaction cumulates and hybridized cortlanditic ultramafics set within an intrusive complex of dominant felsic material. This process is exothermic, favouring dominant conversion of the Alpine peridotite, but the hybridized ultrabasic rocks retain certain characteristics of the Alpine peridotite such as the presence or absence of orthopyroxene in the ultramafics and the geochemical signature of the PGE associated with the resultant complex. Depth controls the relative abundance of felsic versus ultramafic rock types, and the internal relative abundance of dunite within the intrusive column of the zoned complex. Crystallization of the felsic magma at pressures of 2-6Kb, resulted in the formation of a supercritical fluid enriched in halogens and felsic components. This fluid migrated through the ultramafic mass causing local development of coarse pegmatoidal veinlets to 2m and formation of 5-10m pegmatoidal bodies, both features surrounded by haloes enriched in accessory minerals cleared from the original rock volumes. The minerals involved in these pegmatoidal processes are dominantly those of the ultramafic suite, but locally include felsic minerals such as Kfeldspar. The presence of uniform levels of residual grains of refractory chromite in the ultramafic pegmatoids indicate that the overall process is dominantly metasomatic. A hydromagmatic phase occurs as well, causing some dunites to carry technically aligned veinlets of chromitite with mineral chemistries which are unusually enriched in Mg and Ti. The supercritical fluids leached PGEs from the ultramafic rocks and possibly also from shoshonitic felsic rocks (which have significant background values), and then selectively redeposited them in the ultramafic (and felsic??) rocks, especially in the newly forming pegmatoidal bodies. Subsequent further removal of material can also occur, resulting in local re-concentration of residual components and PGEs and causing Fe-enrichment of the host mineralogy. Temperatures of 850-650°C are indicated, and PGM grainsizes vary enormously from a few microns up to single crystal nuggets of 1-2 mm, with the coarser crystals occurring in the higher temperature settings, for instance within chromitite veinlets in dunites. Although the overall PGE geochemical signature is dictated by the nature of the original Alpine peridotite, the local PGE mineralogy varies according to temperature and sulphur fugacity in this essentially sulphur-depleted system. Solidification led to loss of the supercritical fluid and a drop in temperature. The resultant low partial pressures of the volatile components encouraged local differential overprinting of the finer grained PGM assemblages by minerals enriched in lower temperature volatile components: As, Sb, Se. The present depth of erosion and degree of tectonic emplacement varies from complex to complex, and this causes the principally depth-dependent pegmatoidal and mineralization patterns to also differ. Mineralization can be targeted by rocktype, pegmatoidal texture, local structure or by the chemistry of associated phases. However, exploration by companies has shown some zones of mineralization are irregular and non-continuous, and that only some pegmatoidal bodies and haloed regions are mineralized. Unique to the FPP, up to fivefold enrichment has occurred during the development of thick laterite profiles especially over the serpentinized (olivine-rich) lithologies. Growth of nuggets in the lateritic profile does not seem to have occurred, but there are rare nuggets which are zoned in a way suggestive of a very low temperature growth history. Erosion and alluvial concentration of the PGM has produced high grade deep lead placer deposits but also much lower grade precursors, namely eluvial and floodplain deposits. * Published with permission of Director-General, N.S.W. Department of Minerals and Energy.

RELATIONSHIPS BETWEEN GRANITES AND MINERAL DEPOSITS IN THE LACHLAN FOLD BELT P. L. Blevin and B. W. Chappell Department of Geology ANU P.O. Box 4 Canberra ACT 2601 Granites within the LFB can be grouped into suites and supersuites using textural, modal, chemical and isotopic criteria. Variation within suites may be controlled by fractional crystallization or restite unmixing processes. Suites can be classified as being either supracrustal (S) or infracrustal (I) depending on their source rock compositions. The correspondence of granite compositions with their source rocks allows for the recognition of basement terranes present within the deep crust beneath the present cover of the LFB. Mineral deposits associated with the LFB granites can be grouped according to metal associations which include: Sn, W, Mo, Cu, Au and Ag-Pb-Zn. Deposit types within each metal association vary from relatively few (eg. veins and skarns for the Au and Ag-Pb-Zn associations) to a complex array of deposit types for the Sn association (veins, stockworks, skarns, carbonate replacements, breccias, disseminations, pegmatites and dykes). Overall relationships of mineral deposits and granites within the LFB are similar to broad patterns recognised elsewhere (Sn with S types, Mo and Cu with I types etc). Significant tin mineralization is restricted to fractionated S-type suites of the Wagga and Bassian Basement Terranes and is virtually absent from restite controlled S-type suites such as those within the Kosciusko Basement Terrane. Mo and to a lesser extent W are associated with l-type suites that may be either dominantly restite (Bathurst and Bega Basement Terrane) or fractionation controlled (the Boggy Plain Supersuite of the KBT). Similar correlations between metal association and degree of granite fractionation are less evident for other metal associations and suggest that other factors and processes are important in generating mineralization. These factors may include magma volatile content, composition and timing of exsolution, oxygen fugacity, sulphur content! and the depth of magma emplacement. Some of these factors are interdependent and ultimately relate back to granite source type.

THE LUCKY DRAW GOLD DEPOSIT, BATHURST DISTRICT, N.S.W. A.M. Brewer1 1 RGC Exploration Pty. Ltd., Canberra. A.C.T., 2609.

P.O. Box 590, Canberra,

The Lucky Draw gold deposit was located as a greenfields discovery in 1985, and is located approximately 8 kilometres north east of Burraga, N.S.W. Recoverable ore reserves of 1.48 million tonnes of 3.53g/t gold are contained in an in situ resource of 2.08 million tonnes of 2.68g/t gold. A smaller deposit of similar style is located at Hackneys Creek 800 metres to the north with estimated recoverable reserves of 190,000 tonnes of 2.12g/t gold. The Ordovician terrain which hosts the Lucky Draw deposit consists of a sequence of low titanium shoshonites, ankaramites and related cumulates which contain minor sedimentary lenses (Rockley Volcanics) which overlie biotite quartzites, chlorite quartzites, meta-pelites and potassic meta-pelites of the Triangle Group. Within the Triangle Group lithologies there is a unit of laminated quartz-albite rocks and aluminous iron rich schists which are host to the gold mineralisation and which has not been previously recognised. These schists contain numerous assemblages (chlorite ± gedrite ± garnet ± hercynite ± staurolite; biotite ± quartz ± andalusite ± cordierite ± staurolite) and are characterised by Si02 concentrations in the range 20% to 40%, total Fe as Fe203 of up to 42% in Fe sulphide bearing schists or up to 34% in Fe-sulphide absent schists, MgO contents in the range 4%-18% and Na20, CaO and K20 concentrations that are (excluding the biotite rich rocks) negligible or absent. The sequence in the vicinity of the deposit has been contact metamorphosed by the intrusion of the Carboniferous Burraga granite. Away from the main mine area these rock types occur as chloritic schists and pelites and contain anomalous levels of gold, bismuth and occasionally tellurium. Mineralisation may be broadly classed as a gold-bismuth-tellurium association and is generally sulphur poor. Gold occurs principally as the native element with up to 11% silver; it is also present in maldonite (Au2Bi) , Bismuth occurs bismuthinite, native bismuth and a number of bismuth tellurides. Molybdenite occurs as a common associate in the mineralisation and is often intergrown with the gold and bismuth bearing phases. Minor amounts of pyrite, pyrrhotite, chalcopyrite, arsenopyrite and wittichenite have also been observed within the ore zones. Precise geological relationships remain difficult to ascertain, but the deposit is broadly stratabound and is considered to represent an overprinting by gold-bismuth-tellurium mineralisation on pre-existing hydrothermally altered mafic tuffs and greywackes by an hydrothermal cell emanating from the adjacent intrusive granite body.

LEAD ISOTOPE SIGNATURES OF ORDOVICIAN-TO-PERMIAN MINERALIZATION IN THE LACHLAN FOLD BELT - GENETIC AND EXPLORATION IMPLICATIONS Graham R. Carr 1 , Judith A. Dean 1 , Brian L. Gulson 1 and David W. Suppel 2 * 1.

CSIRO Division of Exploration Geoscience, PO Box 136, North Ryde NSW 2113

Geological Survey of NSW, Department of Minerals and Energy, PO Box 536, St Leonards NSW 2065

In the Lachlan Fold Belt, determining the many associations of mineral deposits and their genesis is difficult because of the complex tectonic and metallogenic history (Suppel et al, 1990). Lead isotopes provide both objective data to aid deposit classification and information as to their origin. In addition, the Pb isotopic signature, or fingerprint, of each style of mineralization can be used in exploration programs for recognition of further occurrences. The initial Pb isotope ratios of a wide range of styles of mineralization encountered in the LFB are summarized below: 1. Ordovician crustal signature. Gold and base metal deposits in metamorphosed Ordovician turbidites of the Girilambone Group have homogeneous Pb isotopic compositions ( 2 0 6 Pb/ 2 0 4 Pb ratios 18.00-18.08) which conform to orogene (i.e. "average crust") models such as that of Cumming and Richards (1975). Thus the model ages of such mineralization are consistent with the Lower Palaeozoic. Mineralization with a very similar signature is also found adjacent to the Ordovician basement in the Cobar Supergroup and the Mineral Hill Synclinorial Zone. 2. Ordovician mantle signature. Ordovician basaltic to andesitic volcanic sequences occur in a number of belts across the LFB (collectively termed the Molong Volcanic Arc) and host an important group of Au deposits which formed at high levels in the crust as porphyry (e.g. Goonumbla) to epithermal styles (e.g. Gidginbung). The initial ratios of these deposits plot in linear arrays ( 206 Pb/ 204 Pb ratios 17.65-18.35) well below growth curves of crustal rocks, indicating the Pb was extracted from rocks derived from at least two discrete mantle reservoirs. It is unlikely that there was any contribution to the mantle signatures from upper crustal Pb. 3. Silurian crustal signature. Widespread Silurian submarine acid volcanism in the Tumut, Cowra-Yass, Hill End and Captains Flat-Goulburn Synclinorial Zones was associated with deposition of massive sulfide mineralization (e.g. Woodlawn, Captains Flat). This mineralization has very homogeneous isotopic compositions ( 20 6 Pb/ 204 Pb ratios 18.06-18.12) which plot close to the orogene model curve and with slightly higher 2 0 6 Pb/ 2 0 4 Pb ratios than the Ordovician crustal signature. 4. Devonian crustal signature -1. Mineralization associated with Early Devonian turbidites in the Cobar Basin and with felsic volcanics in the Mt Hope and Rast Troughs to the south has homogeneous Pb isotopic compositions ( 2 0 6 Pb/ 2 0 4 Pb ratios 18.07-18.16) which also plot close to the orogene model curve and can be distinguished from both the Ordovician and Silurian crustal signatures based on their 206pb/204pb r a tios. 5. Devonian crustal signatures - 2. Mineralization associated with granitoid intrusion principally in the Devonian, has higher 2 0 6 Pb/ 2 0 4 Pb ratios (18.12-18.22) and is more heterogeneous than the Silurian crustal signature and the Devonian crustal signature -1. 6. Carboniferous signatures. Mineralization associated with the emplacement of granitoids during the Kanimblan orogeny has a wide range of isotopic compositions ( 206 Pb/ 204 Pb ratios 18.15-18.40) which overlap the Devonian signatures, and which reflects both mantle and crustal sources of Pb. References: Cumming, G.L., and Richards, J.R., 1975. Ore lead isotope ratios in a continuously changing Earth Planet. Sci. Lett., 28, 155-171.

Earth and

Suppel, D.W., Lewis, P.C., Carr, G.R., Dean, J.A. and Glen, R.A., 1990. Gold deposit associations of the Lachlan Fold Belt in New South Wales. In: Proceedings Aus. IMM Pacific Rim Congress 90, Gold Coast Old 6-12 Mav 79 1990, pp. 523-532. ' '

D.W. Suppel publishes with the permission of the Director-General, N.S.W. Department of Minerals and Energy

TECTONIC EVOLUTION OF THE LACHLAN FOLD BELT: PETROLOGICAL CONSTRAINTS B.W. Chappell Department of Geology, The Australian National University, GPO Box 4, Canberra City, ACT 2601 The area of exposure of Palaeozoic rocks in the Lachlan Fold Belt (LFB) is approximately 300,000 km2. Granites comprise 63,000 km2 of that area and hence make up about 30% of the belt. Related volcanic rocks are also abundant in some parts of the LFB. These rocks all resulted from the partial melting of the deep crust. Because of that source and because of their abundance in the LFB, granites and associated volcanic rocks provide significant information about the basement rocks of that region. They provide important penological constraints on the crustal structure of the LFB and its tectonic evolution. Many granites, particularly the more mafic ones, have chemical compositions that reflect those of their source rocks. This imaging property of granites frequently results from the granite magma carrying unmelted source material, or restite. In the other case of higher temperature and completely liquid magmas, uncommon in the LFB, the chemical correspondence between source rocks and granites is less marked, but is still present. An immediate consequence of the correspondence between source rock and granite compositions is that those granites derived from older igneous rocks are generally metaluminous, and those that came from metasedimentary sources are peraluminous. These are the I-type (igneous or infracrustal) and S-type (sedimentary or supracrustal) groups. Within these broad groups further more detailed subdivisions can be made, into suites and supersuites, corresponding to more subde differences in source-rock compositions. Within the LFB there is a sharp eastern limit to the occurrence of S-type granites of deep crustal origin, the IS-line of White et al. (1976), who suggested that that line represents the eastern limit of thick crystalline crust, possibly of Precambrian age. This line is now recognized as one of several in the LFB that separate granite provinces of different character. Those differences in the exposed granites imply similar differences in the deep crust and the separate areas delineated by those lines were termed basement terranes by Chappell et al. (1988). Those separate terranes were covered by extensive and rather monotonous Ordovician flysch deposits and are now evident only from the provincial characteristics of the granites. Chappell et al. (1988) suggested that those terranes correspond to separate Proterozoic microplates which accreted at the eastern edge of the Australian continent prior to the period of tectonism reflected in the exposed surface rocks. It is now thought that the timing of that accretion was related to the dominant 600-500 Ma inheritance found in zircons in the granites and Ordovician sediments of the eastern part of the LFB (Williams et al., 1991). There is no penological evidence for subduction at the time of granite magmatism in the LFB. The widespread, but not universal, occurrence of S-type granites argues strongly for the presence of thick crust through much of the LFB, containing a substantial metasedimetary component. Among the I-type granites, examples low in K and of quartz-dioritic composition, the plutonic rocks of the island arcs, are not present. Mafic tonalites, so characteristic of the Cordillera, are rare, and where they occur they probably reflect the composition, and also the tectonic regime, of an earlier event that could correspond to the assembly of the basement microplates. The I-type granites of the LFB are dominantly granodioritic with an average K2O content of 3.4%. Such compositions, in large volumes, cannot have been derived directly from the products of a subduction zone. Chappell & Stephens (1988) suggested that two prior episodes of partial melting and fractionation occurred to produce the I-type granite source rocks. The first of these would have corresponded to the production of the primitive crust of island arcs, producing a basalt to andesite or quartz-diorite composition. The second stage involved the reprocessing of that material to produce tonalitic compositions analogous to many of the plutonic rocks of the Cordillera, which subsequently acted as source rocks for production of the I-type granites widely distributed throughout the LFB. The two dominant ages of inheritance in zircons found by Williams et al. (1991) at - 1000 Ma and 600 to 500 Ma, could correspond to those earlier events. Under that scenario, the LFB represents a Caledonian-age fold belt whose precursor materials were fashioned during major Grenville-age and Pan African-age tectonic events. Reference s * Chappell, B.W. & Stephens, W.E., 1988. Trans. R. Soc. Edinburgh: Earth Sci., 79, 71-86. Chappell, B.W., White, A.J.R. & Hine, R.,1988. Aust. Jour. Earth Science, 35, 505-521. White, A.J.R., Williams, I.S. & Chappell, B.W., 1976. / . Geol. Soc. Aust., 23, 105-112. Williams, I.S., Chappell, I.S., McCulloch, M.T. & Crook, K.A.W., 1991. This volume.

OROGENY ASSOCIATED WITH LOW-PRESSURE METAMORPHISM evidence from the Lachlan Fold Belt and the Arunta Inller, Australia W.J. Collins, Department of Geology, The University of Newcastle, Newcastle, NSW 2308, Australia R.H. Vernon, School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia. ABSTRACT One of the most significant features of the Lachlan Fold Belt (LFB) is that upper crustal sections are preserved, in contrast to major collision belts. Low-grade Ordovician turbidites and Siluro-Devonian silicic volcanic cover sequences dominate the 1000 km-wide, low-P metamorphic belt of the LFB, which differs from major zones of convergence, such as the Cordillera of western North America, where substantial crustal thickening, consequent isostatic rebound and formation of core complexes (e.g. Coney & Harms 1984) has exposed moderate-P terranes; they are characterised by clockwise P-T-t paths. Low-P metamorphic belts, characterised by anticlockwise P-T-t paths, are typical of the Australian continent, and include the vast Proterozoic fold belts of central and northern Australia (e.g. Etheridge et al. 1987). Much of the LFB is too low grade to determine P-T-t paths, but porphyroblast/matrix relationships in the highest-grade metamorphic zone within the Omeo Complex, indicate that the thermal peak was reached before or was synchronous with the earliest deformation (Morand 1990), similar to that in the high-grade metamorphic zones of the northern Arunta Inlier, central Australia (e.g. Clarke et al. 1990). In both areas, the regional metamorphism is low-P, high-T (LPHT) type, is centred on major granite intrusions as aureoles, and is characterised by anticlockwise P-T-t paths, but in the outer (lower-grade) parts of the aureoles, peak metamorphic conditions post-date the earliest deformation. This diachronous relationship indicates outward migration of a thermal front from an "orogenic core" centred on granitoid intrusions. Although deformation, metamorphism and magmatism are simultaneous, deformation began only after heating of the terrane, indicating that the controlling factor in this type of orogeny, was heat-focussing in the mid-crust by granitoid intrusion. A possible tectonic environment is the internal zone of a magmatic arc at an active continental margin. The cumulative effect of granite intrusion, with heat contributions from mafic magma, generates regional metamorphic aureoles and thermally weakens the mid-crust, inducing "magma-enhanced deformation". Shortening is accommodated by ductile shear in the midcrust and by passive upright folding in the upper crust. Compression occurs during cooling of the terrane, but only modest overthickening results, as indicated by near-isobaric cooling paths and the preservation of upper crustal sections. Production of normal-thickness crust suggests that the original crust was thin. The final result is the production of discrete LPHT terranes that are spatially restricted around abundant granitoids, show thin-skinned deformation, and have anticlockwise P-T-t paths. Continued granitoid intrusion produces "post-tectonic", discordant plutons at high crustal levels within regionally extensive low-P terranes, such as the LFB. REFERENCES Clarke G.L., Collins W.J. & Vernon R.H. 1990. J. Metamorphic Geol. 8, 65-88 Coney P.J. & Harms T.A. 1984. Geology 12, 550-554. Etheridge M.E., Rutland R.W.R. & Wyborn L.A.I. 1987. Amer. Geophvs H y Geodynamic Ser. 17, 131-147. Morand V.J. 1990. J. Metamorphic Geol. 8, 1-12.

Union '

umon

The Lachlan Belt in the context of Circum-Pacific Tectonics P. J. Coney Department of Geosciences, University of Arizona, Tucson, Az. 85721 There is considerable evidence that the Pacific Ocean basin has had a remarkable permanency at least throughout Phanerozoic time. The orogenic systems that have evolved around its margins are accretionary continental margin orogens and show little or no evidence of continental collisions in their evolutionary history. This is in dramatic contrast to the Circum-Atlantic and Tethyan realms which have experienced repeated openings and closures of, or successive transfer of continental fragments across, ocean areas that were relatively never large. In other words, the Wilson Cycle has dominated tectonic evolution of Atlantic and Tethyan realms, but has not been important in the Circum-Pacific. The northeastern margin of the Pacific Ocean is the North American Cordillera which is a "classic" continental margin-accretionary system dominated by mainly passive to extensional tectonics from Late Precambrian to early Mesozoic. A well developed complex miogeoclinal terrace and significant fringing or "exotic" arc-trench systems and some "oceanic" accretions are its make-up, all progressively consolidated into North America from mid-Mesozoic time to the present. The northwestern margin of the Pacific Ocean is the collage of Asia which was produced by Tethyan tectonics, not Pacific tectonics — ie., the progressive transfer of Gondwanaland fragments across Tethys to Baltica-Siberia. Only since early Mesozoic time have minor Pacific accretions, such as Japan, produced the present margin. The southeastern, southern, and southwestern margin of the Pacific Ocean is South America, Antarctica, and Australia. Through Paleozoic-early Mesozoic time they were joined and a very enigmatic Pacific margin orogenic system extended for 20,000 km. from northwestern South America to northeastern Australia. The Lachlan fold belt in particular, and the Tasman belt in general, are important windows into that enigma. Lack of a well developed throughgoing miogeocline,. extensive lower Paleozoic fairly deep-marine turbiditic and occasionally submarine volcanic facies are common along the margin, often directly juxtaposed against the cratonic interior. The tectonic evolution is dominated by prolonged histories of first Late Cambrian then Early Silurian-Early Mesozoic convergent to transpressive and accretionary tectonics, often accompanied by extraordinary magmatism, which progressively consolidated a considerable "oceanic" to "quasi-continental" real estate into the Gondwanaland craton. Since the fragmentation of Gondwanaland in the mid-Mesozoic, only the Andean margin has continued convergent consolidation. Large-scale tectogenesis and consolidation in the Circum-Pacific seem to correspond to periods when the "absolute motions" of the main continental blocks caused their margins to advance over the adjacent Pacific Ocean floor crust.

THE LACHLAN FOLDBELT IN NEW ZEALAND? R. A. Cooper, A.J. Tulloch DSIR Geology and Geophysics, PO Box 30368, Lower Hutt The Western Province of New Zealand contains many features in common with the Lachlan Fold Belt but in detail there are some significant differences. Two tectonostratigraphic terranes are recognised, separated by the north trending Anatoki Thrust. The Buller (western) terrane is largely composed of a relatively uniform sedimentary suite of quart-rich clastics (manly turbiditic sandstone), siltstone and black shale, and ranges in age from basal Ordovician to late Ordovician - a clear analogue of the Ballarat-Bendigo belt of Victoria. In contrast, the Takaka (eastern) terrane contains a varied assemblage of Middle and Late Cambrian Volcanics, carbonate and turbidites, Ordovician carbonate, shale and sandstone, and Silurian quartite. Equivalents in Australia are uncertain. In the Buller terrane the main deformation (Greenland event - latest Ordovician or Silurian; ? = Benambran) produced the regional, generally N-trending upright folds. In the Takaka terrane the main tectonic event (Pre-Baton event - latest Silurian or earliest Devonian; ? = Bowning) produced the major N-trending, generally overturned, folds and associated E-dipping thrusts. Both events were accompanied by regional greenschist facies metamorphism. Time of suturing of the two terranes is poorly constrained; structural evidence suggests end of Silurian whereas magmatic evidence suggests post-Devonian. The earliest magmatic event is represented by Middle Cambrian island arc volcanism and I-type granite clasts in Cambrian conglomerate; both are confined to Takaka terrane. Paleozoic granitic plutonism (Karamea Suite) is otherwise restricted to Buller terrane and is spread over a belt at least 300-450 km wide. Sparse radiometric data indicate most plutonism occurred in the range 370-310 Ma, with the bulk in the early Carboniferous. Initial Sr isotope ratios reported range from .710 to .727. Overprinting by Cretaceous plutonism/tectonism has reset isotopic systems in many rocks. Paleozoic granitoids are dominated by the S-type Karamea Suite. Two A-types are recognised and the sole I-type candidate has a 286 Ma age (K-Ar). The age and S-type character of these Paleozoic granites does not allow direct correlation with any granite terranes in the LFB or Antarctica. Shear zone hosted quartz -Au lodes in low grade rocks dominate the metallogenesis, similar to Victoria. Minor, uneconomic, W-Sn mineralisation characterised by sheelite, is associated with a few Karamea Suite plutons 10

EARLY TECTONIC EVOLUTION OF THE LACHLAN FOLDBELT: A TASMANIAN PERSPECTIVE

A.J. Crawford, R.F. Berry Geology Department, Univ. of Tasmania GPO Box 252C, Hobart, TAS 7001 In contrast to the mainland Lachlan Foldbelt, Late Proterozoic basement(?) and Cambrian rocks are exposed over large areas of western Tasmania. They offer the possibility of solving the paradox regarding the continental versus oceanic nature of the basement beneath southern Australia, and better understanding the Late Proterozoic and Cambrian tectonic history of this southern part of the Lachlan Foldbelt. Around 600Ma, attenuation and eventual rifting of Proterozoic continental crust resulted in formation of a thinned passive continental margin transected by small rift basins in which basalts transitional to MORB accumulated. Eastward-directed intra-oceanic subduction commenced to the east of the passive margin at some time before Middle Cambrian, forming an oceanic arc with boninites and low-Ti magnesian basalts in the forearc region. Continued subduction of intervening oceanic crust between the arc and the passive margin resulted in arrival of the leading edge of the passive margin at the trench, and an arc-continent collision. The timing of this collision is broadly age-bracketed within the Middle Cambrian (-530 - 510 Ma). Arc-continent collision emplaced one or more extensive sheets of forearc crust, dominated by low-Ti basalts and boninites and their cumulate complements, on to underthrusting thinned continental crust. We argue that emplacement of these ophiolitic allochthonous sheets terminated at the end of the Middle Cambrian, briefly 'ocking the plate boundary. Continued compression at the suture initiated what was probably a short-lived episode of westward-directed subduction beneath the newly-emplaced allochthon and passive margin basement, and generated the high-K andesites of the Central Volcanic Complex of the Mount Read Volcanics in relaxation rifts parallel to the suture. Rebound backthrusting along the eastern side of the Central Volcanic Complex of the Mount Read Volcanics belt exhumed underthrust Proterozoic crystalline crust which now forms the Tyennan Region, and a foreland basin half-graben, the Dundas Trough, formed along the western edge of this basement inlier. Post-collisional shoshonitic volcanics of the Western Volcanic Sequence of the Mount Read Volcanics accumulated in this graben, having been generated by delayed partial melting of subduction-modified subcontinental mantle. Continued extension in this post-collision collage of crustal elements led to limited rifting and emplacement of the tholeiitic Henty Dyke Swarm through at least the Central Volcanic Complex. Lower crustal generation of extensive sheet-like intrusive quartzfeldspar porphyrite bodies and associated Tyndall Group felsic explosive volcanism occurred within the graben zone. Continued uplift and excavation of Tyennan Region Proterozoic crystalline crust provided abundant coarse siliciclastic detritus of the Owen Conglomerate and correlates which flooded into and gradually filled the Dundas Trough . It is interesting to note that the major events invoked in the above model to have generated continental crust (allochthon emplacement, reversal of subduction, post-collisional volcanism and graben formation, rebounding and excavation of underthrust crystalline basement, graben filling) all occurred probably within a 20 Ma period. This sequence of events, which spanned the Middle and Upper Cambrian, constitute, an orogeny in the traditional sense. Yet is is generally considered that the only Palaeozoic orogeny which affected Tasmania was the Late Devonian Tabberabberan Orogeny. Late Cambrian deformation is well known in the Adelaide Foldbelt (Delamerian Orogeny) and in the New Zealand and Transantarctic Mountains segments of the oncecontinuous foldbelt along the eastern margin of early Palaeozoic Gondwanaland. It is not recorded from the mainland section of the Lachlan Foldbelt in Victoria, although given the nature of Cambrian outcrops along major Devonian faults in Victoria, this is hardly surprising.

SILURO-DEVONIAN EXTENSIONAL TECTONISM AND METALLOGENESIS IN LACHLAN FOLD BELT: INSIGHTS FROM A MODERN MANUS BASIN ANALOGUE

THE

Keith A.W. Crook1, John L. Walshe1 and Fop VanderHor1 and Shipboard Parties, R/V "Moana Wave" Cruise MW8517/18 and R/V "Akademik Mstislav Keldysh" Cruise 21 1

Department of Geology, Australian National University, Canberra

Extensional tectonism during the Silurian to Early Devonian is a notable feature of the Lachlan Fold Be t. In the north, in central western NSW, extension has dismembered an Ordovician island arc. Further south an extensive body of deformed Ordovician quartz-rich flysch, which is convergent-margin related, has been partly dismembered. In addition to mineralisation associated with the Ordovician volcanic arc, there is widespread mineralisation in and adjacent to the Middle Paleozoic extensional features. Stratiform copper-zinc mineralisation is typical. Copper-lead-zinc mineralisation characterises extensional features that disrupt the Ordovician flysch. This mineralisation includes major deposits at Captains Flat and Woodlawn. Felsic or felsic and mafic volcamcs and volcaniclastics are intimately associated with the mineralisation. The eastern Manus Basin provides a partial modern analogue for this type of tectonism and its associated metallogenesis. In this area, the Oligocene volcanic arc of New Britain and New Ireland, together with its Neogene post-subduction platform limestones, is being dismembered by a series of rifts and transform faults that form part of the northern boundary of the South Bismarck plate. The Oligocene volcanics, like those in the LFB Ordovician, are in part shoshonitic. The eastern Manus Basin and adjacent areas lacks any analogue for the Ordovician quartz-rich flysch of the LFB. Water-column methane, manganese, and turbidity anomalies show that hydrothermal venting is an important feature of the eastern Manus Basin. This is associated with recent seafloor calcalkalic volcanism, varying from basalt to dacite. Structurally, the eastern Manus Basin is markedly asymmetric, with the locus of modern volcanism lying much closer to the southern side of the basin than to the northern side. Structural, topographic and magnetic data suggest that jumps in the locus of extension may have occurred. We postulate that two styles of seafloor mineralisation may be present in the eastern Manus Basin. In area£ where crustal extension is greatest, in which basaltic volcanism is typical, zinc-copper mineralisation associated with black smokers is likely. Further east, where extension is less and the island arc crust is still comparatively thick, hydrothermal systems associated with deep-seated felsic magma chambers are likely. Here, hydrothermal plumbing systems would extend to greater depths and be constrained by the more limited extension, so that boiling and phase separation may occur. In this situation, a brine phase could remain separate, being vented at the surface as mineralised brine pools precipitating stratiform lead-zinccopper deposits, analogous to that at Woodlawn (McKay, 1989; McKay & Walshe, 1990). The association of active hydrothermal systems, plumes in the water column which can be mapped on the basis of gas, metal, turbidity and temperature anomalies, and metal enrichment in surrounding sediments, offers the prospect of understanding the relationship between seafloor hydrothermal systems and metal anomalies in sediments. Submersibles would be used for insitu process studies. Detailed biostratigraphic subdivision of mid-Paleozoic sedimentary sequences may provide a basis for undertaking similar studies in fossil systems. The pattern of metal anomalies in these sediments would then be used as a basis for predicting the locations of the parent hydrothermal systems. These potential sites of mineralisation, located at variable depths below the present surface, would become exploration targets. McKay, W.J. 1989 A study of the geological setting, nature and genesis of the Woodlawn base metal deposit, NSW Australia. PhD thesis Australian National University. McKay, W.J. & Walshe, J.L. 1990 Environment and mechanism of formation of the Woodlawn massive sulphide deposit, NSW. Abs. 10th Aust. Geol. Conv., p.5.

THE MURRUMBIDGEE MYLONITE ZONE W Crowe* and MJ Rickard* *

Department of Geology, Australian National University

The contact between Late Silurian volcanics and the Murrumbidgee batholith is a wide mylonitic shear zone 3 km to 5 km wide extending from near Canberra southwards to Cooma. Much of the granite is weakly to moderately foliated; an early magmatic foliation marked by oriented feldspars and elongated xenoliths is overprinted by a tectonic fabric. An eastward increase in shearing is marked by segregation and concentration of biotite into anastomosing layers wrapping around resistant feldspar, and quartz is recrystallized into elongated ribbons. Plagioclase is progressively altered, especially in core zones, to sericite and epidote and grain size is reduced. Kfeldspar, both orthoclase and microcline, occurs throughout, but XRD analysis shows a progressive increase in triclinicity with increasing strain. A secondary C-foliation was developed concurrently with mylonization. Initially developed at 30° to S planes, the angle between S and C surfaces is progressively reduced with increasing deformation; in places discrete ultramylonite bands develop parallel to C. The sheared volcanics are lower grade with quartz, feldspar and chlorite phenocryst set in a segregated and layered matrix of sericite and quartz ± plagioclase. Foliation increases with shearing westward towards a faulted contact with the granite. Kinematic indicators, principally S-C fabrics and biotite 'fish', indicate a sinistral movement sense. The generally steep W dipping S-fabric and moderately N-plunging stretching lineation suggest west over east thrusting. The mylonite zone is probably associated with the emplacement of the batholith under upper greenschist-facies conditions. Juxtaposition of the mylonite zone against both sheared and weakly deformed volcanics is due to later brittle faulting. This major structure lies close to the S-I line but the relationship of the mylonite to this granite terrane boundary is not clear. Possibly the edge of a buried ancient craton locates the zone of strong compressional shearing.

The Eden - Comerong - Yalwal Volcanic Zone: A continental rift zone or not ? Dr. Kelsie Dadd, Department of Applied Geology, University of Technology, Sydney, P.O. Box 123, Broadway, N.S.W. 2007. The mid to Late Devonian Yalwal Volcanics, Comerong Volcanics, Boyd Volcanic Complex and associated gabbroic and A-type gramtic plutons torm part of a continental volcano-tectonic belt, located parallel to the coast ot southeastern Australia; the Eden - Comerong - Yalwal Volcanic: Zone (tVAj. The EVZ is characterised by an elongate outcrop pattern, bimodal, maftc-ieisic volcanism, and A-type granitic plutons. Sedimentary rocks are rare in tfie northern and central parts of the EVZ and form only a minor component in tne south. Within the Comerong Volcanics, felsic volcanic rocks include ash tlow and ash fall deposits as well as extensive lava flows. Mafic volcanic rocks were deposited primarily as lava flows with minor mafic tuff and lapilli tuff. Previous interpretations that suggest the volcanic zone was a fault bounded rift are rejected in favour of an elongate volcano-tectonic belt similar to the Yellowstone-Snake River Plain region (Y-SRP) in the United States of America. Both regions have (1) mafic lavas ranging in composition from olivine tholeute to evolved ferrobasalt, (2) alkaline, typically high-Si rhyolitic rocks with enriched Y, Zr and Th, (3) extensive rhyolite lava flows, (4) plains-type basalt lava flows, and (5) a paucity of sedimentary rocks. The Y-SRP may have developed by migration of the American plate over a fixed hot spot leading to the northeastward spatial-temporal progression of the focus of volcanic activity. Hypothetically extending this hot spot model to the EVZ suggests that the Australian plate was moving southward at a rate of between 3 and 6 cm/yr. The EVZ was both much wider (up to 70 km) and much longer in the Devonian than the belt defined by outcrop, and it is a compatible size analogue for the YSRP. The main difference between the two volcanic belts is the lack of large ignimbrites and recognised caldera complexes in the EVZ.

RELATING TASMANIA TO THE LACHLAN FOLD BELT C.G. Elliott , D.R. Gray and N.B. Woodward 1

1 2

Department of Earth Sciences, Monash University, Melbourne, Victoria, 3168 Australia Department of Geology, University of Maryland, College Park, Maryland, USA, 20872.

There has never been general agreement about how Tasmania relates to the LFB, and in the past twenty years the two have been correlated in at least four different ways. Our work in both areas shows that even though some rock types are similar, the lithologic associations and structural styles are very different and do not correspond. We believe that Tasmania shared orogenic events with the rest of the Tasmanides but was not contiguous with the exposed LFB until Mesozoic times. Our reasons for believing that Tasmania and the LFB cannot be correlated include: 1) In East Tasmania, Ordovician and Silurian meta-turbidites are deformed into east-facing recumbent folds overprinted by upright folds. Rocks of similar ages and depositional environment occur in the Melbourne zone of the LFB, but have no equivalent recumbent folds; 2) In West Tasmania, there are more than 10 exposed Precambrian blocks. No Precambrian rocks are exposed in the LFB. The Precambrian rocks in Tasmania are now juxtaposed with Cambrian active margin sequences and Ordovician shallow marine and subareal siliciclastic carbonate rocks. In the LFB, the only Cambrian rocks are mafic volcanic slivers, and most Ordovician rocks are deep water turbidites; 3) Silurian and Devonian strata in West Tasmania are shallow marine sedimentary rocks. In the LFB they include deep water deposits and, in the east, unstable shelf, sedimentary and volcanic rocks; 4) The early and mid-Paleozoic structural style of northern West Tasmania is dominated by ?west-directed Cambrian thrusts and west-directed Devonian thrusts. Faults rather than folds are the dominant structural element. In the LFB, tight, angular folds dominate surface exposures and overlie east-directed Devonian thrusts; 5) Devono-Carboniferous granitoids and upper Paleozoic glacigenic rocks are common to both Tasmania and the LFB. Similar rocks of similar ages, though, are common to Lower Paleozoic fold belts around the world, and do not aid accurate correlations between tectonic terranes. There is published evidence, though, that the Wilsons Promontory granite belongs to the same igneous suite as the East Tasmania granites (Stump et al. 1986). This relates Tasmania to the mainland in Devonian times, but since the Wilsons Promontory Granite is not correlated with other LFB granites it does not tie Tasmania to the LFB at this time; 6) At least half of Tasmania is overlain by a voluminous sheet of Jurassic dolerite. There is no equivalent in the LFB. If Tasmania was not in its present position until the Mesozoic Era, there must be a Mesozoic movement zone between Tasmania and the LFB. There are no faults of the expected orientation or magnitude in Tasmania. Up to 155km of right lateral motion can be explained if, in Mesozoic times, Tasmania separated from the mainland on a trend normal to the axis of the Gippsland and Bass basins rather than the NNE trend that some have proposed. The remaining discordance might be the product of transcurrent faults beneath Cenozoic cover in southeast Victoria, as was suggested by Stump et al (1986). Stump, E., White, A.J.R., & Borg, S.G. 1986. Reconstruction of Australia and Antarctica: Evidence from granites and recent mapping. Earth & Planetary Science Letters 79: 384-360.

Lachlan Fold Belt of southeastern convergent margin orogenic belt

Australia:

atypical

Christopher L. Fergusson1 and Peter J. Coney •'"Department of Geology, University of Wollongong, P.O. Box 1144, Wollongong, N.S.W. 2500, Australia department of Geosciences, University of Arizona, Tucson, Arizona, U.S.A. Abstract

The Lachlan Fold Belt of southeastern Australia records an enigmatic history related to convergent margin processes in the Cambrian to Carboniferous. The orogen contains four lithotectonic associations: (1) a Cambrian submarine mafic volcanic succession, locally ophiolitic, with thick black shales in the east, (2) a widespread Ordovician quartz-rich turbidite and black shale succession with scattered centres of andesitic volcanics and associated clastics and limestones, (3) a complex and highly variable Silurian to Middle Devonian succession of silicic to mafic volcanics, clastics and limestone with abundant plutonic rocks, and (4) an Upper Devonian succession dominated by shallow marine and fluviatile rocks and some silicic-mafic volcanics and related intrusives low in the succession. Major deformation with an overall shortening of 60% (1000 km), achieved by thrusting, folding and cleavage development, affected the fold belt during the Early-Middle Silurian (Benambran Orogeny), Late SilurianMiddle Devonian (Bowning-Bindian-Tabberabberan Orogenies) and Early Carboniferous (Kanimblan Orogeny). The tectonic setting of the fold belt is cryptic but widespread silicic magmatism and major contraction indicate that the belt developed at a convergent margin at least for the Silurian to Carboniferous. The width of magmatism and zones of deformation cannot be simply related to an Andean-type continental margin. Instead the Lachlan Fold Belt is regarded as an atypical convergent margin orogenic belt that possibly owes its unusual features to rapid plate convergence affecting a pre-existing crust of unusual composition.

CHARACTERISTICS OF GRANITE DEFORMATION IN THE EPIZONAL ENVIRONMENT: THE DAVYS CREEK GRANITE, SOUTH OF BATHURST, NSW. T. J. Fowler 1 and P. G. Lennox 2 i. Geology Department, La N o r t h e r n V i c t o r i a ; 2. G e o l o g y

Trobe University College of D e p a r t m e n t , U n i v e r s i t y o± NSW .

The Davys Creek Granite lying south of the Bathurst Granite is a late-tectonic epizonal granophyre earlier grouped with the gneissic/mylonitic Wyangala and Wologorong Batholiths (Figure). Like these plutons the Davys Creek Granite has Stype chemistry and is transected by a cleavage (S) equivalent to the regional cleavage (S2). However in the Davys Creek Granite mylonites are rare and appear restricted to "soft" lithologies with fine micrographic textures. S is mainly a composite fabric of anastomosing crack-like mica folia which follow grain boundaries; and plastically deformed groundmass. The Davys Creek Granite behaved both brittly and plastically due to high temperatures but low confining pressure accompanying its deformation. In the low pressure environment of emplacement the cooling magma experienced a pressure quench leading to rapid spherulitic growth of quartz and alkali feldspar in a supercooled magma which coexisted with an exsolved aqueous phase. The aqueous phase strongly influenced the mechanical properties of the incompletely consolidated granite, enhancing and occupying tectonically induced cracks along weak grain boundaries. Micas, chlorite, epidote and kaolinite have precipitated along these cracks. Emplacement during tectonic compression was via multiple dyke injection along beds and cleavages forming roughly cleavage concordant masses with strong S (eg the western part of the larger granite mass). As tectonism faded, magma buoyancy overwhelmed the waning tectonic compressive stresses distorting dyke walls and aureole and forming bulbous granite masses with poor S cleavage as in the enclosed central part of the plutons where S2 and S show marked discordance.

SUMMATION KEYNOTE PAPER Palaeozoic Plate Movements and the Interplay of Compression and Extension in the Scandes David G. Gee, Department of Geology, University of Lund, Sweden There is abundant evidence to suggest that the processes going on in the lithosphere today are similar to those that went on in the early Phanerozoic. Nevertheless, we are still far from being able to reconstruct the movements of the Palaeozoic continents and integrate the fragmentary evidence mto a coherent picture ol Palaeozoic plate tectonics. Deciphering the latter requires a global strategy that the recent years of glasnost arc now making possible (e.g. via the ILP Palaeomap Project). Central to both regional and global reconstruction is the role of palaeomagnetism, closely integrated with palaeontology, palaeoclunatology and structural geology, to define the independent terranes and track their paths from initial rifting to final accretion. In the context of the North Atlantic Caledonides, we have moved far from the early concepts of ocean opening and closing, often referred to as the Wilson Cycle. The importance, indeed dominance of oblique movements during both rifting and accretion, together with the complexities of collision along irregular plate margins is increasingly recognized. Palaeomagnetism provides not only control of terrane latitude in time, but also of terrane rotation. The latest evidence from Russo-Baltica indicates that this major continent, during migration from southerly latitudes, rotated counter-clockwise through more than ninety degrees during the Ordovician, prior to collision with equatorial Laurentia. The rotation hypothesis needs closer analysis; if correct, the implications are profound - the ocean(s?) that opened, during development of the Baltoscandian and Laurentian passive margins, was not the ocean (Iapetus) that closed. And existing reconstructions of prePhanerozoic tectonic elements will require fundamental reassessment. After the postulated Ordovician rotation, the Scandian collision of Russo-Baltica and Laurentia appears to have been orthogonal and comparable to that of the Himalayan collision between India and Asia. Further to the southwest, Avalonia crossed Iapetus during the Ordovician to dock with Laurentia in the Silurian. This microcontinent was probably sutured to Russo-Baltica (along the Tornquist lineament) prior to collision with Laurentia. Oblique (sinistral) docking of Avalonia with Laurentia has been recognized in the British Isles, but the relationships of this collision to that along the Tornquist zone and Scandinavia remains enigmatic. As in the case of India and the Himilayas, the Baltoscandian margin of Russo-Baltica functioned as a major indenter during Scandian collision with Laurentia; much of the fragmentation of the Arctic Caledonides (e.g. on Svalbard), during and after collisional thickening, may have resulted from this indentation. Transcurrent movement along the "southwest margin of Russo-Baltica (Tornquist lineament) during Scandan indentation readily finds a modern analogue in the Chaman Fault, along the Afghan-Pakistan border. Within the Scandinavian Caledonides the first tectonothermal activity (Finnmarkian) influenced the Baltoscandian margin at c. 520-480 Ma B.P., prior to the postulated rotation of Russo-Baltica referred to above. The Iapetus Ocean had closed by the Early Silurian and Scandian Collision thereafter involved vast underthrusting of the Laurentian margin. Collision more than doubled the thickness of the crust resulting in the crystallization of the classical eclogites in the southwest Norwegian basement at c. 430-420 Ma. B.P. Underthrusting by Baltica continued through the Silurian and probably well into the Devonian, accompanying the very rapid uplift of the hinterland during the Early Devonian (as demonstrated by U / P b zircon and sphene and Ar/Ar hornblende and muscovite ages of 405-395 Ma. B.P.). Mid to Late Devonian Old Red Sandstone molasses overlies the eclogite facies basement in the hinterland being separated from the latter by major detachments. Thus, the Scandes provide unique opportunities for studying the interplay of extension and compression within a collisional orogen. Upper crustal extension, accompanying collision and underthrusting, apparently was succeeded by full lithosphere extension towards the end of the Devonian. Analysing the interrelationships of extension and compression within the Scandes is involving a variety of structural metamorphic and isotopic studies. Controlling the relationships between the thrust geometry and timing of movement in the mountain front and PTt history of the high grade hinterland is an important part of this analysis. Fundamental, however, to these interpretations is the application of integrated geophysical investigations, with reflection seismic profiling playing an essential role, for controlling deep structure from the mountain front to the hinterland. Recent studies in the central part of the Scandes have shown that the frontal, decollement can be traced from the foreland at least one hundred and fifty kilometres into the hinterland, cuttmg deeper into the basement and incorporating increasing thicknesses of these crystalline rocks in the allochthon. This evidence suggests that the eclogite facies units of the hinterland were uplifted bv a combination of underthrusting and upper crustal extension, prior to the final phase of lithosphere extension in

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MEDIUM-HIGH PRESSURE METAMORPHIC ROCKS OF THE TUHUA OROGEN, WESTERN NEW ZEALAND AS LOWER CRUSTAL ANALOGUES OF THE LACHLAN FOLD BELT, SE AUSTRALIA

G.M. Gibson University College of Southern Queensland, Toowoomba QLD 4350 Lower Palaeozoic rocks are widely distributed in the western province of New Zealand and range from Ordovician greywackes, slates and minor paragneiss in the Buller terrane to weakly metamorphosed (subgreenschist to greenschist facies) Cambrian mafic volcanics, Ordovician limestones and Silurian siliciclastic rocks in the Takaka terrane. Higher grade rocks occur mainly in Fiordland; they include 350-380 Ma amphibolite facies metasediments held to be the more highly metamorphosed equivalents of the Buller and Takaka terranes although a late Precambrian protolith age for these rocks cannot be ruled out. Together with the Buller and Takaka terranes these rocks were regionally metamorphosed and deformed during the mid-Palaeozoic Tuhua orogeny, and prior to late Mesozoic continental rifting and breakup of the Pacific margin of Gondwana were part of a much more extensive Palaeozoic orogen which included the Lachlan Fold Belt and extended from southern New South Wales through Victoria and Tasmania into New Zealand and Antarctica (Victoria Land). In common with the Lachlan Fold Belt, regional metamorphism in the western province occurred largely under low P:T conditions (andalusite - sillimanite facies series) such that in Fiordland and the higher grade parts of the western province low pressure (3-5 kb) sillimanite and sillimanite + potash feldspar + cordierite assemblages developed in rocks of appropriate bulk chemical composition. Higher pressure (7-9 kb) rocks in which kyanite is the dominant aluminosilicate polymorph are almost entirely confined to Fiordland and represent the deepest exposed crustal levels within the Tuhua orogen. At these levels migmatized pelitic schists containing kyanite and garnet are interstratified with calc-silicate and quartzofeldspathic rocks, tonalitic gneisses, metamorphosed anorthosites and granite orthogneisses. These rocks were exhumed from depths of 25-30 km and offer useful insights into the nature and composition of the continental crust beneath what was once part of eastern Gondwana; exhumation itself was brought about by late Mesozoic crustal extension combined with more recent transpression along the mid-Tertiary Alpine Fault. Assuming plate reconstructions for Gondwana during the Palaeozoic are valid, the lower to middle crustal rocks of Fiordland may give some indication as to what lies at depth beneath the Lachlan Fold Belt.

6LENELG METAMORPHIC COMPLEX: WESTERN MARGIN OP THE LACHLAN FOLD BELT OR EXTENSION OF THE DELAMERIAN OROGEN INTO WESTERN VICTORIA?

G.M. Gibson1, D.N. Nihill2 1 School of Applied Science, University College of Southern Queensland, Toowoomba QLD 4350 2 Geology Department, University of Melbourne, Parkville VIC 3052 The Glenelg Metamorphic Complex has been affected by at least three phases of deformation (D1-D3) and metamorphism up Qto the amphibolite facies. Peak metamorphic conditions (570-600 C; 3-4 kb) occurred during the D2 deformation and coincided with synkiriematic emplacement of the 480 Ma Wando Granodiorite thereby giving rise to a foliated thermal aureole in which low P:T metamorphismm (andalusite-sillimanite facies series) was accompanied by tight to isoclinal D2 folding and ^ the superposition of biotite, garnet, staurolite and andalusite + cordierite metamorphic zones upon an earlier and much more regionally extensive D1 deformation and associated subgreenschist facies metamorphism. Low grade, regionally metamorphosed rocks possess a slaty cleavage dipping 60 northeast arid range from virtually unmetamorphosed continentderived, quartz-rich greywackes through slates into dolomitic phyllites. Higher grade rocks include pelitic schists, psammites and calc-silicate rocks in which the dominant fabric is of D2 age and varies from a crenulation cleavage through to a thoroughly penetrative axial-plane schistosity. This foliation affects the Wando Granodiorite and aureole rocks alike and for the most part parallels the igneous contact; it is more steeply inclined than the earlier D1 slaty cleavage but maintains the same NW-SE trend. Other igneous rocks in the complex, including metabasites and serpentinite, have been affected by both the D1 and D2 deformations and probably represent shallow sills and minor intrusions emplaced into the sequence prior to regional deformation and metamorphism. Metabasites are of inferred Cambrian age and exhibit tholeiitic compositional affinities; they are increasingly common towards the inferred base of the metasedimentary sequence and may indicate that the Glenelg Metamorphic Complex is floored by Cambrian metabasaltic rocks in common with other parts of Victoria. This similarity aside, the age and tectonothermal history of the Glenelg Metamorphic Complex are probably more in keeping with that of the South Australian Kanmantoo Group. This group forms an important part of the Delamerian orogen and as such the Glenelg Metamorphic Complex was probably marginal, or at best transitional, to the Lachlan Fold Belt.

A THRUST AND STRIKE-SLIP MODEL FOR THE LACHLAN OROGENIC BELT AND IMPLICATIONS FOR TECTONIC DEVELOPMENT RA. Glen, Geological Survey of New South Wales, Department of Minerals and Energy, Box 536 PO, St Leonards, NSW, 2065* ABSTRACT The Lachlan Orogenic Belt (LOB) of this paper extends from the Torrens Hinge Zone (THZ) in South Australia to the Tasman Sea, and encompasses the region of Palaeozoic deformation in southeastern Australia. The conventional interpretation of the LOB as dominated by steep structures is challenged by recent published and unpublished work from various parts of the LOB which provide evidence for the involvement of thin-skinned tectonics in the evolution of the LOB. A synthesis of these data suggests that the LOB is partitioned into a far western overthrust belt (with minor strike-slip movement), a western thrust belt, a central strike-slip belt, and an eastern, mainly underthrust belt with minor strike-slip related thrusting and deformation. In the Far Western Belt, structures along the inboard margin are Cambro-Ordovician in age and are west vergent, basement-involved near Adelaide, but east vergent further north where the Callana beds form a decollement horizon. The magnitude of thrusting is at present contentious. Peculiarities in the Adelaide area are cessation of thrusting at the THZ (a backstop against rigid basement and the original edge of the LOB), and the absence of molasse west of the THZ. Most molasse was presumably shed east, and was mainly submarine except in the Gnalta Shelf area of western NSW. Further outboard, accretion (overthrusting) of Cambrian greenstone terranes over Precambrian basement and late Precambrian continental margin sequences occurred at the end of the Middle Cambrian in western Tasmania, and possibly also elsewhere in the belt. Accretion (overthrusting) of Cambrian arc terranes in western NSW and Victoria probably also occurred around the Cambro-Ordovician. Further east, in the Western Belt, in Victoria, east vergent thrusting and folding of Cambrian and Lower Ordovician continental margin facies turbidities accompanied Early Silurian and Middle Devonian thrusting of the Cambrian greenstone terranes eastward over similar turbidites ranging in age from Ordovician to Silurian. Minor left-lateral strike-slip faulting also accompanied these deformations. In the Central Belt of the LOB, between the Gilmore Suture (or just to the east) in the east and the Mt Wellington Fault Zone, (or just west of it) in the west, thrusts and recumbent and upright folds trend approximately east/west (at Tabberabbera, Omeo, Albury, Tumut, Girilambone), indicating a major earliest Silurian north/south shortening event accompanied by dextral strike-slip on bounding faults, which was subsequently overprinted by north/south to northwest-trending structures. Strike-slip activity in the Central Belt continued into the Silurian, with the emplacement of oblique granitoids, opening and closure of pull-apart basins around Tumut, and into the Devonian, with opening and closure of transtensional basins around Cobar. In the Eastern Belt, east/west thrusting commenced in the earliest Silurian (being preceded by formation of east/west trending folds, thrusts and some cleavage formation) and continued episodically thereafter, with documented thrusting in the earliest Silurian, mid-Silurian, and earliest Devonian. Most of these thrusts verge east, although some verge west, and south to southeast ,and were accompanied by some strike-slip activity. Earliest Silurian thrusting was accompanied by the accretion (oblique overthrusting) of one or more arc terranes of variable Ordovician age, and their juxtaposition against Ordovician terrane(s) comprising non volcanic-derived Early Ordovician turbidities and Late Ordovician starved black shales. Beginning in the mid Silurian, overthickened crust built up by thrusting underwent orogenic collapse, with formation of shallow to deep marine basins, emplacement of granitoids, and outpourings of vast amounts of felsic volcanics. Renewed east vergent thrusting occurred in the Middle Devonian and in the Carboniferous and was accompanied by buckle folding and widespread cleavage formation, which in Ordovician rocks, overprints early thrust-related structures. Granitoids were mylonitised during later stages of emplacement, and middle crustal metamorphic complexes (Omeo, Cooma, Cambalong, Jerangle) were thrust east over Late Silurian sediments and low grade Ordovician ones. Thrusting of the Cambrian chert-greenstone Wagonga beds (part of the Victorian greenstone belt terrane) over the Ordovician (and imbrication of these two units) probably occurred at this time. The mid Devonian thrusting was approximately coeval with formation of northeast and northwest-trending strike-slip faults which formed dispersal or escape structures. Carboniferous thrusting was out of sequence, and varied in intensity, with the formation of high and low strain zones. The necessity of balancing contractional deformation across the LOB demands the presence of a layered crust with several major detachments-in Late Ordovician black shales, within the Early Ordovician (in the Lancefieldian in Victoria), at the base of, or within, Cambrian greenstones and within the regional metamorphic complexes (depth of about 10-12 km). Lesser detachments also lie in the Early Silurian and in the Late Devonian. Published with permission, Director-General, New South Wales Department of Minerals and Energy

East vergent thrusting and deformation in the Eastern and Western Belts of the LOB probably developed in response to the latest Ordovician-earliest Silurian underthrusting of a previously rifted off piece of the Australian craton- which provided a source for I type granitoids east of the I-S Line- and which also triggered off accretion of the arc and turbiditic terranes. Continued east/west shortening in response to this underthrusting slab continued from end Ordovician to Carboniferous. Minor strike-slip activity and north/south shortening in both these belts also continued from the Lower Ordovician to the Carboniferous. Together with similar but more significant activity in the Central Belt, these oblique structures are due to non orthogonal plate underthrusting and arc accretion, which created deformations which were partitioned into different combinations of north/south and east/west shortening and and strike-slip activity. The inboard part of the LOB has similarities to the Cordilleran and Appalachian miogeoclinal prisms, (although deformation here is early rather than late in time), an analogy which becomes enhanced if the nowinternally thickened Kanmantoo Group (laid down on thinned continental crust to the west and oceanic crust to the east) was emplaced by large scale thrusting over the Proterozoic rift to miogeoclinal sediments Further east, substrate of the LOB probably consists of old continental crust (plus relics of old arcs and ocean floor) below a regionally extensive greenstone detachment. East of the I-S Line, substrate is underthrust crustal material. Internally thickened Ordovician strata (thickness <5km) and underlying greenstones must comprise much of the upper crust extending down to the source area for S-type granitoids.

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PROBLEMS OF ISOTOPICALLY DATING DEFORMATIONS IN LOW GRADE ROCKS - THE COBAR (AUSTRALIA) EXAMPLE.' RA. Glen . R.D. Dallmeyer^ and L.P. Black 1

1. 2. 3.

Geological Survey of N.S.W., Department of Minerals and Energy, Box 536 PO, St Leonards, N.S.W., 2065 Geology Department, University of Georgia, Athens, Ga., 30692, U.SA. Bureau of Mineral Resources, Box 378 GPO, Canberra, A.C.T., 2601.

ABSTRACT The intracratonic Cobar Basin, N.S.W., opened under left-lateral transtension and was mainly inverted under right-lateral transpression with the development of a high-strain flower zone along the eastern margin [characterised by near meridional thrusts, folds and regional cleavage (structural zone 1)] and lower strain zones 2 and 3 to the west [characterised by widely spaced WNW-trending regional F1 folds, thrusts soiling into a detachment, NE-trending F2 folds and accompanying uncommon SI and S2 cleavages]. An age dating program in the inverted Cobar basin provides an insight into problems of establishing deformation ages in low-grade sediments by isotopic age dating, and also helped pave the way for a tectonic interpretation of the area. Samples for isotopic dating were restricted to fresh rock containing penetrative SI cleavage and were obtained from exploration and mine sites. Fine-grained lithologies from the Chesney Formation, Great Cobar Slate and CSA Siltstone were sampled for whole-rock dating: SI sericitic micas could not be separated from detrital micas and dated separately. Rb-Sr isotopic dates predate deposition of these Devonian rocks, giving a regional isochron of 442± 4 Ma. Three K-Ar dates from zone 1 give indistinguishable ages of 402 + 8, 402_+ 8, 405+ 8 Ma and suggest a late Early Devonian age of cleavage formation. Four Ar/Ar spectra on whole rocks from zone 1 give intermediate temperature plateaux around 400 Ma and total gas ages ranging from 378 to 397 Ma. In zone 2, total gas ages from three samples range from 401 to 423 Ma. Approximate intermediatetemperature plateau for one sample indicates an age around 405 Ma: the other sample give a good plateau around 437 Ma. These data are interpreted as reflecting cooling after a low grade cleavage forming event which occurred at c 400 Ma, with this age indicated by the K-Ar data and the Ar/Ar plateaux. Older ages indicated by the Rb-Sr data and some of the Ar/Ar data from zone 2 are interpreted as basement provenance ages preserved within detrital micas and feldspars which did not undergo complete re-equilibration during the 400 Ma event. These ages correspond with metamorphism of the Girilambone Group (up to low amphibolite grades) and intrusion of foliated granitoids around 440 Ma, and intrusion of post-tectonic granitoids around 420 Ma. The preservation of these old ages in the Ar/Ar system from the northern part of zone 2 suggests lower deformational strain and/or temperature of metamorphism during the 400 Ma event here. Further south and east in zone 1, the isotopic data suggest (near) complete re-equilibration of detrital phases during the 400 Ma event. The presence of detrital micas in cleavage microlithons with only minimal internal deformation optically (with only limited crinkling and kinking), the relative little amount of wholesale grain rotation (with early foliations still preserved in some places) all suggest that this homogenisation of mica occurred with minimum optically visible disruption to the lattice structure. The presence of a 400 Ma late Early Devonian inversion event in zones 1 and 2 of the former Cobar Basin largely bypassed the southwestern part of the basin and the adjacent shelf to the west which only underwent minor block faulting and uplift at this time. Establishment of the main inversion event at around 400 Ma required rethinking of previous tectonic models on the evolution of the Cobar Basin which envisaged one period of Carboniferous inversion after formation of the successor fluviatile basin on the west. Coupled with new structural mapping, the isotopic dating program led to the erection of a model of basin formation and inversion controlled by strike-slip and thrust tectonics, whereby different margins of the basin evolved somewhat independently in time.

Publication authorised by Director-General, N.S.W. Department of Minerals and Energy and Executive Director, B.M.R.

LFB CRUSTAL ARCHITECTURE AND TECTONIC EVOLUTION A Victorian Perspective D. R. Gray, Department of Earth Sciences, Monash University, Melbourne. Victoria. 3168. The Lachlan fold belt deep-crustal structure is enigmatic, with various proposed petrogenetic, structural/tectonic and thermo-mechanical models of the crust. In the western part, exposed m Victoria, the LFB is characterised by a 35 km thick composite crust. At the present level of exposure the allochthonous, imbricated teiranes of the surface geology are marked by a commonality ot iitholacies (quartz-rich turbidites), structural style (chevron folding and fault-imbrication) and metamorphic grade (low greenschist facies), apart from the fault-bounded, northwest-trending, wedge-shaped low r / high T Wagga metamorphic belt (Gray, 1988; Coney et al.,1990). Other key aspects include the voluminous high level felsic magmatic intrusive phases (evidenced by 20-30% areal extent ot granites presently exposed) which appear temporally and spatially tied to the regional deformation and metamorphism, and the relative stability of the LFB since the Late Devonian; uplift and erosion have been minimal with preservation of shallow-level (< 2 km) Late Devonian cauldron complexes. Petrogenetic horizontally layered crustal models (e.g. Clemens, 1988) and thermo-mechanical models involving crustal thickening/lithospheric thinning (Sandiford, 1989; Loosveld & Etheridge, 1990) may explain crustal and thermal budgets and granite genesis, but do not consider the nature and geometry of the surface structure. Deep crustal seismic reflection profiling suggests that geometrically complex mid- to lower crustal detachments are possibly associated with crustal-scale duplexing (Gray et al., 1991). Low angle (<20° on nonmigrated sections) detachments either intersect or sole into the crust-mantle boundary, forming a crustal-stacking wedge typical of lithospheric- or A-type subduction. Upper crustal reflections show that major, moderately to steeply dipping reverse faults exposed at the surface flatten with depth and have listric form. The laterally-extensive chevron-folded quartz-rich turbidite sequence of the Lachlan fold belt is therefore allochthonous with respect to the lower crust. The latter may consist of diverse lower crustal blocks up to 20 km thick, 200-300 km in length and width, composed of both ensimatic and ensialic fragments (Chappell et al., 1988). Juxtaposed fold-thrust zones of different age and vergence, as well as strike-slip movement components along reactivated faults, provide major constraints for tectonic reconstruction of the LFB in the Early to Middle Palaeozoic. In the western part of Victoria the Glenelg and Stawell zones show structural concordance with Kanmantoo belt trends and are conformable with the ancient continental margin (e.g. Tasman line). In the eastern part structural zones (Tabberabbera, Buchan and Mallacoota zones) display gross symmetry about the Wagga metamorphic belt (WMB) with the WMB separating a stratigraphically and structurally distinct eastern LFB (Qrdovician arc and fore-arc sequences?) from the western LFB (Ordovician back-arc sequences?). The central part between the Avoca fault and Mount Wellington fault zones (Ballarat-Bendigo and Melbourne zones) has acted as an isolated wedge that was deformed and subsequently moved southwards with respect to the Stawell zone and northwards with respect to the Tabberabera zone. Displacements on faults are considered to be on the order of hundreds of kilometres and not thousands of kilometres. Geological and geophysical evidence from ancient orogenic belts, such as the LFB, suggest that long-formed (over 100 Ma) composite-crust can evolve by crustal stacking (tectonic underplating) of micro-continental blocks (oceanic plateaux?) beneath thin oceanic (back-arc) crust during lithospheric subduction. In the LFB east-directed (away from the craton) thrusting of the sedimentary coversequence, requires some form of underthrusting at least in the Silurian (Fergusson & VandenBerg, 1990) and Early Devonian periods (Fergusson et al. 1986), and possibly during the Middle-Cambrian. This does not preclude the effects of mantle heat input which may be necessary to cause melt generation in the mid- to lower-crust. References Coney, P. et. al. 1990. J. Struct. Geol., 12, 519-543. Chappell, B.W., White, A.J.R. & Hine, R. 1988. Aust. Jour. Earth Sci., 35, 505-524 Fergusson, C.L., Gray, D.R. & Cas, R.A.F. 1986. Geology, 14, 519-522.' Fergusson, C.L. & VandenBerg, A.H.M. 1990. J.Struct.Geol., 12, 577-589. Gray, D.R. 1988. in 'Geology of Victoria' (eds.J.G. Douglas & J.A.Ferguson} 1-36 Gray, D.R., Wilson, C.J.L. & Barton, TJ. 1991. Geology Loosveld, R. & Etheridge, M.A. 1990. J.Met.Geol., 8, 257-267. Sandiford, M.J. 1989. Earth & Planetary Science Letters, 95, 85-96.

The Geochemical and Tectonic Ancestry of the Goonumbla Porphyry Copper Gold Deposits P.S.Heithersay1 and J.L. Walshe2 1#Geopeko# Parkes 2# Australian National University, Canberra

The Ordovician Goonumbla Porphyry Au Cu deposits are representative of a small number of porphyry copper deposits with significant gold credits. They are also amongst the oldest deposits of their type in the world. The deposits themselves are characterised by quartz, bornite, gold stockwork zones associated with multiphase dikes and stocks of quartz-monzonite porphyry. The mineralised systems are spatially associated with larger diorite-monzodiorite to quartz-monzonite complexes which are themselves weakly mineralised. The host shoshonitic volcanics are associated with a collapsed caldera with a marginal ring dike complex of monzonites and diorites. Whole rock and trace element analyses reveal linear trends in K20 vs S ^ and Zr vs S ^ plots. The bulk of the volcanics and intrusives which make up the ring dike complex fall on the steeper potassic (alkalic) trend in contrast to intrusives related to the diorite-monzodiorite to quartz monzonite complexes, which fall on a flatter, more "calcalkaline" trend. A range of trends in K 2 0 vs diagrams is common in volcanics and intrusives of the shoshonite association. The Goonumbla volcanic complex ranges from high K calcalkaline rocks to dominantly shoshonitic rocks. A review of shoshonitic rocks reveals a range of tectonic settings in which shoshonitic rocks form. However their trace element geochemistry reveals a typically "arc-like" signature in all cases. Shoshonites are more commonly minor components of a larger tholeitic and/or calcalkaline pile. This variation is absent in the Goonumbla area and also appears to be missing in other Ordovician volcanic sequences in NSW. This lack of variation is hard to reconcile against complex arc models proposed for Ordovician volcanism. Shoshonites are commonly found in the early stages of arc development (eg Marianas) or the end stages of arc development (eg Fiji) . The structural setting can be a rift environment or related to faulting and uplift (eg SE New Guinea). Recent dating of zircons from the oldest part of the Ordovician succession revealed relict, euhedral zircons which returned an age of 511 my (Late Cambrian), (Perkins et al, in press). These are possibly sourced from interaction of Ordovician magmas and Cambrian volcanics beneath the Giralambone Beds, which underlie the Goonumbla succession. Cambrian Volcanics are known to have definite arc affinities in Victoria and Tasmania, hence subduction in the Cambrian between the Wagga and Kosciusko microplates (Chappell et al, 1988) would have enriched the mantle wedge in potassium group elements.

If the microplates had accreted by the Cambro-Ordovician it is possible that a wedge of Cambrian volcanics together with underplated gabbros and ultramafics partly define the prominent gravity ridge known as the Parkes Terrace. Reactivation and incipient rifting in the Early to Late Ordovician would have allowed small amounts of partial melting of the enriched mantle wedge giving rise to potassic shoshonitic volcanics. The thickened crust at this time would have allowed moderate to high pressure fractionation to occur giving rise to potassic magmas with small silica enrichment. Batches of magma possibly ponded at higher levels giving rise to caldera resurgence. Low pressure fractionation generated more silicic magmas and importantly concentration of volatiles. Breaching of these high level chambers focussed fluid flow and provided a pathway for initially volatile rich quartz monzonite porphyries followed by volatile depleted monzonite porphyries. A complex sequence of initially prograding magmatic hydrothermal fluids formed the porphyry copper gold systems in the Goonumbla area. The model outlined provides the links between tectonic, magmatic and hydrothermal processes which need to work in concert in order to form significant Porphyry Cu Au deposits. References Perkins, C. , McDougall, I.., Claue-Long, J., and Heithersay, P. , (in press). 40Ar/39Ar and U-Pb Geochronology of the Goonumbla Porphyry Cu-Au Deposits, N.S.W. Econ.Geol. Chappell, B.W., White, A.J.R., and Hine, R. , (1988). Granite provinces and basement terranes in the Lachlan Fold Belt., Southeastern Australia. Australian Journal of Earth Sciences, v35. p.4. ~ ~

STRUCTURAL SETTING OF MINERALIZATION AT PEAK, COBAR. NSW. Diamond Drillcore Structure using a Structural Manipulation Spreadsheet Mark Hinman Structural-Economic Research Group. Department of Geology, James Cook University. Townsville. Q. 4812 At Peak base and precious metal mineralization is situated within a distinctively structurally anomalous volume of rock that presents a target to the explorationist 2 to 3 times the size of any alteration or mineralization. The 3-dimensional structural setting of the base and precious metal ore bodies was resolved entirely from a structural analysis of early exploration holes before any underground access was available. This analysis was undertaken when detailed surface structural mapping revealed an extensive structurally anomalous zone 250 meters above the Peak ore bodies indicative of strains higher than the regional norm. The drillcore analysis showed not only that this structural anomalism occupied a volume of rock 2 to 3 times that which displayed signs of alteration and mineralization but that regionally homogeneous macroscopic structural elements showed extreme modification in the region of mineralization. Unless the regional or semiregional picture was known the anomalism of the mine environment may never have been revealed and valuable pointers not discovered. This danger is indicated by the fact that the present and planned underground development at Peak is entirely within the structurally anomalous volume. This poster presents some detail about the STRUCTURAL MANIPULATION SPREADSHEET written by the author that allows easy, rapid and accurate generation of structural data from drillcore. It also details the structural setting of the Peak ore bodies derived from such a drillcore structural analysis in the form of cross sections and level plans and indicates the particular structural anomalism associated with the mineralization. The boldness of this structural expression and the relative ease with which structural data can be won from drillcore would indicate the value of detailed structural analysis in the Cobar field.

THE MOUNT AUBREY EPITHERMAL GOLD DEPOSIT S. Hopf Department of Geology, University of Newcastle, NSW 2308 The Mt. Aubrey gold deposit, 30 km NE of Parkes in the Lachlan Fold Belt, is hosted by a Palaeozoic hot-water dominated epithermal system of illite-adularia type. The deposit occurs in a Devonian (?) sequence of intercalated intermediate to basaltic lavas with minor volcaniclastics. These rocks are overlain by the Devonian Dulladerry Rhyolite which is dominated by ignimbrites and shows no evidence of hydrothermal alteration. The Mt. Aubrey basalts are vesicular, display interstitial textures with augite occupying the spaces between labradoritic plagioclase microphenocrysts, and are relatively unaltered except in strongly fractured zones. In contrast, the dacitic to andesitic seriate-textured volcanics contain thin layers of volcaniclastics of similar composition and tend to be more intensely altered. The intermediate volcanics are plagioclase-phyric and contain minor chloritised mafics (pyroxene?, biotite?). Hydrothermal alteration by near neutral chloride fluids resulted in the formation of secondary quartz (crystalline quartz, jasper, chalcedony), calcite (commonly bladed), chlorite, illite, albite, adularia, epidote, pyrite and prehnite. All of these phases, with the exception of prehnite and amorphous silica, occur as a secondary replacement of igneous phases. Chlorite formed at temperatures of 200-300°C based on the six component solid solution model of Walshe (1986). This is in agreement with the occurrence of illite and epidote which implies a minimum temperature of about 250°C (Browne, 1978). At this temperature coexisting calcite, prehnite and epidote indicate a log (fC02)fluld of 0.5-0.6. In addition, low to moderate fluid C02 contents are suggested by vein assemblages comprising one or more of; quartz (commonly as lattice quartz), calcite (bladed), epidcte, chlorite, illite, pyrite and albite. Lattice quartz formed by the pseudcmcrphous replacement of bladed calcite during boiling. A low fluid NaCI content (up to 6 wt% NaCI) was determined by fluid inclusion studies (carried out by BHP-Utah Minerals). From research conducted in modern geothermal systems in New Zealand, the type of igneous feldspar replacement can assist in identifying marginal and upflow zones (Christenson, 1987). In the Mt. Aubrey system feldspars in highly permeable (upflow) zones are replaced by adularia and albite (+ calcite, + epidote), whereas in marginal areas feldspars are replaced by illite + calcite, or albite. In addition, marginal areas are characterised by the occurrence of silicified horizons containing amorphous silica. Fluid flow in the Mt. Aubrey basalts was controlled by fractures (small faults, joints). High fluid/rock ratios caused intense alteration adjacent to the fractures, whereas the relatively impermeable host rocks were only weakly altered. In contrast, fluid flow in the strongly altered intermediate lavas and volcaniclastics was more pervasive, and hence equilibrium with the hydrothermal fluids was achieved. Gold occurs in the upper portions of the system in fractured basalt where it is associated with quartz veins. Gold was probably present as a Au(HS)2 complex and H2S loss caused by boiling resulted in gold precipitation. The presence of bladed calcite, lattice quartz and adularia is evidence that boiling has taken place. References: Browne, P.R.L. (1978). Hydrothermal alteration in active geothermal fields. Annual Reviews of Earth and Planetary Science, 6, 229-250. Christenson, B.W. (1987) Fluid-mineral equilibria in the Kawerau hydrothermal system Taupo Volcanic Zone, New Zealand. Ph.D. thesis, Auckland, 452pp. Walshe, J.L. (1986). A six-component chlorite solid solution model and the conditions of chlorite formation in hydrothermal and geothermal systems. Economic Geology, 81, 681-703

TECTONIC INTERPRETATION OF THE SOUTHERN ADELAIDE FOLD BELT R.J.F. Jenkins and M. Sandiford

Department Adelaide,

Geology

and

Geophysics,

southern

Adelaide

Fold

University

The

Belt

comprises

inner

carrying

a little deformed para-antochthonous

cover

probable

allochthon

or younger

sheet

Adelaide,

GPO

Box

498,

crystalline

core

complexes

S.A.

broadly

allochthonous,

deformed mid

folded

Sturtian

and

higher

metamorphic

and

Complex.

Low

angle

allochthon

above

anticlinorium

highly

still

dislocation relatively

embracing

the

Marinoan

between

'Burra

deposits

stack

The

'upper'

translation

"Burra

sediments,

higher

also

inferred

and

the

the more the

and

contact

the

Kanmantoo

between

Group"

Group* rocks, a

comprising

the para-autochthon

obvious.

'mid'

of older Burra Group

deformed

thrust-faulted with

the

more openly deformed Umberatana Group is especially puzzling and enigmatic, and it is an

open

question

compressive Of

particular

changes

the

contrasting Peninsula

facies

Early

Cambrian

substantial

the

tectonic

post

"Burra

mechanism

sedimentation

during

the

Normanville

and

Group"

the

responsible

Early

pre

'Sturtian'

the

dramatic

Kanmantoo

for

Cambrian,

reflected

Groups.

Fleurieu

Kanmantoo

New

Complex

than

thermal or flexural bulge may have generated

condensed

Ranges.

older

the

the rapid subsidence leading to Kanmantoo deposition was underway - 5 2 6 Ma

A concommitant

whether occurred.

interest

style

ago.

Flinders

deformation

interval

evidence

on northern

previously

disconformity

shows

that

Kangaroo

suspected,

Yorke

possible

northern

Island, the 'lower'

posing

the

intriguing

a widespread

Peninsula

mid

and

the

equivalent

the

"Kangaroo

possibility

Island

that

Group';

'Kanmantoo'

deposition began in that region at an older time than in its type area (east of Adelaide). The

Lower

Kangaroo

southeasterly

transport

records

abrupt

changc

suggesting

Convergent

heat

strain

felsic,

and

probably

and

while

in the

deformation

sedimentary

Group

sediment,

a m a j o r change

Kanmantoo and

Island

Kanmantoo

the

current

directions

depositional

and

time

mafic,

magmas

both

show

east-

'upper'

sediments

Kangaroo

Island

Group

towards

the

west-northwest,

regimen.

metamorphism

wedge was underway

space

molassic

already

thermally

reflecting

intimately

the

within

thickening

lithosphere.

primed

by - 5 1 6 Ma with both the distribution

the

advective

movement

of of

STRUCTURAL/METAMORPHIC HISTORY OF THE COOMA COMPLEX, SOUTHEASTERN AUSTRALIA. Scott E. Johnson Department of Geology, James Cook University, Townsville, Queensland, 4811, Austalia. Preliminary examination of microstructural relationships in the lower grade schists and higher grade gneisses of the Cooma Complex indicates three stages of prograde porphyroblast growth. In the schists, these growths appear to have been controlled by the following reactions: 1) chlorite + muscovite + quartz = cordierite + biotite + H 0, 2) chlorite + muscovite + quartz = cordierite + biotite + andalusite + H 0 and 3) cordierite + muscovite = biotite + andalusite + quartz. In the gneisses, the situation is less clear, but the growths appear to have been controlled by the following reactions: 1) biotite + muscovite + quartz = cordierite + K-feldspar + H 0, 2) muscovite + quartz = andalusite + K-feldspar + H 0 and 3) cordierite + K-feldspar + H 0 = biotite + sillimanite + quartz. The retrograde history is less clear and still being investigated. The porphyroblastic phases resulting from the above reactions overgrow and preserve progressive stages in the structural development of these rocks, and each reaction appears to have occurred during the development of a different foliation. Thus, four foliations (the earliest one possibly bedding) are preserved. Measurements of inclusion-trail orientations indicate that the porphyroblasts overgrew alternately subvertical and subhorizontal foliations, which appears to be a common occurrence in regionally deformed and metamorphosed porphyroblastic rocks (Bell & Johnson, 1989; Johnson, 1990), and the implications of this for the Lachlan Fold Belt are discussed. These relationships are much clearer in the lower grade pelitic rocks than in the gneisses, due to the finer matrix-grain size and partitioning of deformation during porphyroblast growth. As is generally the case, subvertical and subhorizontal foliations in porphyroblasts contrast with the variable orientations of the same foliations measured in the field. In the first instance, this may lead to problems when trying to reconcile the different-scale observations. However, carefulfieldwork commonly leads to the same conclusion reached through microstructural work, that is, matrix-foliation rotation relative to geographic coordinates, rather than porphyroblast rotation relative to geographic coordinates, is responsible for the contrasting observations. The Cooma schists provide a rare, and excellent example of an area where porphyroblast/matrix rotation relationships can be directly observed in the field. Subhorizontal foliations and folds containing porphyroblasts with subvertical inclusion trails can be observed rotating through 90° into alignment with an overprinting subvertical foliation, yet the porphyroblast inclusiontrail orientations, which are occasionally visible on the outcrop, remain unchanged. Foliation rotation due to progressive deformation is likely to be a major cause of problems with deformation correlationfromone place to another in the Lachlan Fold Belt. For example, much of the schist at Cooma can only be convincingly said from field observations to contain one, and occasionally two, foliations. Yet, detailed microstructural observations involving several hundred thin sections confirm that all the schists experienced the same deformation history, which is only fully recognised in the field at a few key outcrops. There are certainly many localities in the Lachlan Fold Belt where such key outcrops are not available. In these localities, microstructural analysis may provide the only tool for determining the full deformation history, and therefore allowing confident regional correlation of deformation fabrics. 2

REFERENCES Bell, T.H. & Johnson, S ^ 1989. Porphyroblast inclusion trails: the key to orogenesis. Journal of Metamorphic Geology. 7, 279-310. v

Johnson, SX^ 1990. Deformation history of the Otago schists, New Zealand, from progressively developed TOW P - P orogenesis and its implications. Journal of S t r u c k m i C r O S t r U C t U r e S : u

Uft

coUa

THE ELONGATE BRAIDWOOD GRANODIORITE: A TILTED SUBROUNDED PLUTON. Lackie M.A,1 and Flood R.H. 2 1...CSIRO, Div of Exploration Geoscience, Nth Ryde. 2...Macquarie University, Nth Ryde. The individual Siluro-Devonian plutons in the north of the Bega Batholith in the Lachlan Fold Belt (LFB) , commonly have an elongate shape, rather than the more circular shape which is observed in post-deformational Carboniferous plutons of the LFB and Late Permian-Triassic plutons of the New England Batholith. This study looks at the subsurface shape of one of these plutons, the meridionally elongate Braidwood Granodiorite (~60km ESE of Canberra) . The structure of the pluton was determined from the magnetic fabric of the pluton and modelling of the associated aeromagnetic anomaly. Samples from the pluton show a weak but distinct magnetic foliation which is commonly parallel to the long axis of the pluton (Fig. 1). The magnetic foliation of samples from the western side of the pluton dip shallowly (<30°) to the west, while the magnetic foliation of samples from the eastern side have moderate to steep (-65° - -85 ) dips to the east. Modelling of the magnetic anomaly associated with the pluton shows that its eastern contact dips moderately-steeply to the east, while the western contact dips shallowly to the west. The modelling also shows that the pluton extends -25km to the west beneath its coeval volcanics that also dip shallowly to the west. The associated aeromagnetic anomaly and documented magnetic fabric are consistent with the Braidwood Granodiorite being an initially steep sided/flat topped subrounded pluton that has been tilted to the west by an angle of about 20° (Fig. 2). These data suggest that the elongate plutons of the Bega Batholith have been tilted. Preliminary studies of the associated aeromagnetic anomalies of the Shannons Flat Granodiorite (-70km W of Braidwood) and unnamed S-Type granites to its immediate east suggest that those plutons may have been tilted to the east.

(a) v

r * " "

SHF

10km

v Volcanics — Foliations

Figure 2. (a) Cross-section of undeformed Braidwood Granodiorite. Western phase (W), Eastern phase (E). (b) Tilted version of (a). SHF, Shoalhaven Fault.

Figure 1. Magnetic foliations for the Braidwood Granodiorite. Dips in degrees.

INTRAOROGENIC BARRY GRANITE - TWO GRANITES IN ONE! 1

P. G. Lennox , S. P. Hawley2 and T. J. Fowler3 1 Department of Applied Geology, University of N.S.W., P.O. Box 1, Kensington, N.S.W. 2033 2 Coal Geology Branch, Joint Coal Board Building, Singleton, N.S.W. 2330 3 Geology Department, La Trobe University College of Northern Victoria, Bendigo, Vic. 3550 The syntectonic Barry Granite is a meriodionally elongated body consisting of an almost I-type northern body (NB) and an S-type southern body (SB). Whereas the NB is apparently undeformed and has abundant mafic xenoliths, the SB is variably deformed and hydrothermally altered. The NB was intruded into the Upper Ordovician Angullong Tuff, while the SB was intruded into the multiply deformed Ordovician Abercrombie Beds. There is only limited exposures around the SB showing altered cordierite (?) spotting due to contact metamorphism. The geochemistry of both bodies does not rule out the possibility that both were derived from the one magma source with the NB containing greater restite, while the SB has less restite and more silica and alkalis. Contact relationships suggest the Barry Granite intruded along almost meriodionally trending foliation planes within the Abercrombie Beds/Angullong Tuff. Cross-cutting foliations within the Abercrombie Beds/SB suggest a possible anticlockwise rotation of the maximum principal stress axis from NW-SE to WSW-ENE between intrusion and deformation/metamorphism i.e. late Middle Devonian Tabberabberan Orogeny and early Carboniferous Kanimblan Orogeny. Rb/Sr dating of biotites in the SB indicates a 363 ± 2 Ma age for the deformation of this granite, more or less consistent with the Kanimblan Orogeny. The lack of a tectonic foliation in the NB is consistent with the presence of abundant feldspar and hornblende (brittle minerals), compared with the often excellent fabrics developed in the SB which is rich in quartz and mica (ductile minerals). Both bodies of the epizonal Barry Granite were probably intruded during the Tabberabberan Orogeny causing limited hornblende-hornfels contact metamorphism and were subsequently deformed and regionally metamorphosed to greenschist facies grade during the Kanimblan Orogeny.

A GEOPHYSICAL SURVEY ACROSS THE TUMUT SYNCLINORIAL ZONE, NSW Jim Leven1, Peter Stuart-Smith1,2, and Bob Musgrave2 Mike Rickard2, Keith Crook2, 1

Bureau of Mineral Resources, Canberra Geology Department, Australian National University

An integrated geophysical study across the Tumut Synclinorial Zone, NSW, using seismic, magnetic and gravity techniques has investigated the underlying structure of this feature. A major aim of this survey was to elucidate the structure of the faults which bound the trough: the Mooney Mooney Fault Zone to the east and the Gilmore Fault Zone to the west. Reflections in the seismic section generally show poor lateral continuity. For interpretation, the seismic data has been divided it into domains which display a similar reflection character or "seismic facies". The resulting model has been used as the basis of both the gravity and the magnetic interpretation. The seismic and the gravity interpretations suggest that the Mooney Mooney Fault Zone has a near vertical attitude to a depth of around 10 km. Below this depth, the seismic reflections suggest that the structure of the middle crust is gently westward dipping. Interpretation of the magnetic data shows the Coolac Serpentinite to be nearvertical above 5 km depth, soling westward into a more horizontal attitude below a depth of 5 km. There is no evidence in the seismic data for the Gilmore Fault Zone, although the seismic profile finished approximately 3 km east of this western bounding fault. The gravity profile is dominated by the positive anomaly associated with the dense Bullawyarra Schist in the Jindalee Block near where this unit crops out adjacent to the Mooney Mooney Fault Zone. Although the Coolac Serpentinite dominates the magnetic profiles, this ultramafic body has almost no expression on the gravity profile. A negative Bouguer anomaly is associated with the Gocup Granite, and to a lesser extent with the Ordovician to Early Silurian flysch sediments in the Tumut Block. Within the Tumut Synclinorial Zone, we can identify two domains of higher reflectivity on the seismic section. Beneath the Jindalee Block, the top of this more reflective domain is shallow, whereas in the Tumut Block, farther to the west, it commences beneath the Gocup Granite at a depth of greater than 7 km. The tops of these zones of increased reflectivity are interpreted to be the base of the Ordovician to Early Silurian strata, but whether the top of these two domains can be correlated remains unresolved. The reflection seismic data indicates no major structural differences between the crust underlying the Tumut Synclinorial Zone, and that beneath the Goobarragrandra Block to the east However, the seismic character of the lower crust of the Lachlan Fold Belt beneath the Tumut region is relatively non-reflective and, in this respect, is significantly different from the lower crust beneath the Thomson Fold Belt farther north.

THE CAMBRIAN PASSIVE MARGIN IN SOUTH AUSTRALIA S. F. Liu & P. D. Fleming Dept of Geology, La Trobe University, Bundoora, VIC 3083 Cambrian tectonics in southeastern South Australia and western Victoria has not been well understood because the large area in the Murray Basin is covered by Tertiary and younger sediments. New and recently available geochemical data on mafic and granitic rocks and metasediments, and also the recently published geophysical data in this region allow a re-assessment of the Cambrian tectonics in this region. The authors present a synthesis of the tectonic aspects of available geological and geophysical data and favor a passive margin model at Cambrian time in South Australia. The Early-Middle Cambrian Kanmantoo Group comprises a thick 4 km) mainly turbidite-like sequence. This Group has been suggested to have been deposited in a continental slope and rise environment (von der Borch 1980). However, some shallow water facies are also present in the Group (Gatehouse et al. 1990 and Preiss 1990) so that the model of von der Borch may be a broad generalisation. Nevertheless, the sedimentary environment of this Group is significantly different from those of the underlying Early Cambrian Normanville Group and the Late Proterozoic Adelaidean. The thick (~ 20 km) Adelaidean sequence is commonly regarded as deposited mainly in intracontinental rift troughs. As noted by von der Borch (1980), the regional unconformity between the Late Proterozoic Adelaidean and the Early Cambrian Normanville Group was probably a "breakup unconformity". The Normanville Group consists of mainly shallow marine sandstones, shales and limestone. The speculated continental breakup probably occurred in the earliest Cambrian. Our current geological and geochemical work demonstrates that MORB-like dykes/sills/plugs intruded at various stages of the deformation history in the southern Adelaide Foldbelt at 500±10 Ma. Some intruded pre- to syn-Dl while others intruded post-Dl and D2. The MORB-like chemistry of the dykes indicates that the lithosphere underwent well advanced extension (stretching and thinning) prior to and/or during the time of magmatism, thus consistent with a Cambrian passive margin and marginal sea in this region. Recent work by Anderson (1990) suggests that some MORB-like dykes/sills intruded pre- to syn-Dl and post D1 in the Glenelg River complex at about the same time as those in the southern Adelaide Foldbelt. This supports the suggestion that the Cambrian marginal sea probably extended to the Glenelg River complex in western Victoria. Recent studies on the 500 Ma granites in the Kanmantoo domain suggest to Foden et al. (1990) that the granites might be derivedfrommelting of an underplated mafic layer, but to Gray (1990) that they were probably derived from melting of the Kanmantoo Group metasediments. New Nd isotope data of a granitic dyke from Kangaroo Island and two Kanmantoo Group metasediments (Daly, Liu & Gray [in prep.]) and a set of whole rock major and trace element data of this Group (Gray & Liu, unpubl.) support the suggestion of Gray (1990) that at least some granitic magmas such as those in the Encounter Bay area and Kangaroo Island were probably generatedfrommelting of the Kanmantoo Group metasediments. The lack of geochemical indication of involvement of Precambrian material and the probability that the Kanmantoo Group metasediments were the source mean that either the granitic rocks derivedfromPrecambrian rocks were not emplaced at the present exposed crustal level, or most likely that there was little Precambrian crustal material beneath much of the Kanmantoo Group, especially beneath the region where granite is exposed. If the older (mainly Precambrian) continental crust beneath the Kanmantoo Group was very thin, this may explain the fact that the Kanmantoo Group (the youngest sedimentary unit in the southern Adelaide Foldbelt) was most affected by deformation and metamorphism. Tight to isoclinal D1 and D2 folds are common in this group (up to four deformations recognised in it) in the eastern Mt Lofty Ranges while fold structures in the Adelaidean are mostly simple and open style. The highest metamorphic grades (sillimanite and migmatite grades) were centered in the Kanmantoo Group. Summarizing the available geological and geophysical data, it is suggested that the Early Cambrian Normanville Group was probably deposited in a setting of the early stage of passive margin development, and the overlying Early-Middle Cambrian Kanmantoo Group was probably deposited in a well developed passive margin environment with the continent immediately to the west.

SILURO—DEVONIAN, SEDIMENT-HOSTED SULPHIDE DEPOSITS IN THE SOUTHEASTERN LACHLAN FOLD BELT K.G. McQueen

and D.J. Perkin

1 Faculty of Applied Science, University of Canberra, PO Box 1 Belconnen ACT 2616

2 Bureau of Mineral Resources Geology and Geophysics, PO Box 378 Canberra ACT 2601 During the Late Silurian-Early Devonian the tectonic setting of the southeastern region of the Lachlan fold belt was characterised by high heat flow, felsic magmatism, volcanism and crustal extension. Most ore deposits in the region are related to this prolonged thermo-tectonic episode and its various crustal manifestations. Extensive sequences of subaerial volcanics, minor fluviatile sediments, carbonates and marine epiclastic sediments were deposited in several magmatic arches and associated shallow marine embayments in regions now overlying major batholiths. Submarine volcanics, volcanoclastics, turbiditic shales and sandstones were deposited in one or several large, deeper rifted basins. The magmatic arch settings contain small, epigenetic vein, stockwork and disseminated sulphide deposits. These are mainly copper, lead-zinc and gold deposits of epithermal-mesothermal style. Other types of vein and minor skarn deposits containing base metal, gold, bismuth, tin and tungsten mineralisation are associated with the syn- and post-volcanic plutons in these areas. The deeper basin sequences host important volcanogenic massive sulphide deposits, particularly near volcanic centres. Small stratiform and stratabound sulphide deposits occur in the shallow marine sequences in areas distal from major volcanic centres. The latter are not well known but appear to represent a class of deposit distinct from the massive volcanic-hosted deposits. They may also have a greater exploration potential than previously realised. Sediment-hosted sulphide deposits are best known from the southern N.S.W.Victorian section of the region. They occur in shallow marine, generally carbonate-rich, sequences now preserved in scattered thrust slices, small downfaulted blocks and some larger structural basins. Mineralised areas commonly show signs of exhalative hydrothermal activity as well as later fluid movement and sulphide deposition. Several styles of sulphide mineralisation occur including: (1) stratiform, pyritic mineralisation (with minor Cu-Pb-Zn) generally developed in shales and siltstones near the base of the sequences; (2) stratabound, massive and vein—like sulphide bodies with Pb—Zn and variable Cu, hosted by similar rocks; (3) irregular veins, pods and disseminations of galena-rich mineralisation in associated limestones. Some of the galena-rich mineralisation contains significant Ag, while minor Au has been reported in the pyritic stratabound deposits. All styles of mineralisation appear to have formed during hydrothermal activity which occurred during and after host rock deposition. This activity was volcano-magmatic related, although there may also have been some sulphide remobilisation during later deformation and low grade metamorphism. Available Pb isotope data indicate a homogeneous lead source, similar to that for the volcanogenic massive sulphide deposits in the same region. Limited S isotope studies show low positive $ S J 4 values (+3 to +10%.) consistent with partial, inorganic reduction of seawater sulphate or an oxidised, magmatic S source.

JUNCTION REEFS GOLD DEPOSITS A.Mandyczewsky, N.Gray and R.Hine - Climax Management P/L Replacement style gold ore at Junction Reefs is hosted by sulphiderich, calc-silicate skarns in 3 deposits known as SheahanGrants, Frenchmans and Cornishmens. The calc-silicates are developed in a calcareous stratigraphic unit, approx. 40m thick, in the upper part of a marine sequence formally called the Malongulli Fm. This unit is locally known as the "orebed" sequence and has been mapped over a strike length of 4km. Currently the gold resource at Junction Reefs is as follows: Sheahan-Grants Mine Frenchmans Deposit Cornishmens Deposit

1.8 mt 0.46 mt 0.46 mt

@ @ @

2.8 g/t 3.7 g/t 2.1 g/t

The beds of the ore horizon dip gently at 20° to the NW. Local structures include faults, joints and latite dykes and generally follow preferred NW or northerly orientations. All intrusives in the area have shoshonitic affinities. A stock, centrally located between the 3 deposits and locally known as the Junction Reefs Monzodiorite is believed to be the source of mineralisation. It has been dated at 440 Ma, while a dyke from Sheahan-Grants was dated at 470-480 Ma, suggesting that the host sediments are at least lower Ordovician in age (C.Perkins, pers. comm.). Evidence of hydrothermal alteration exists over a wide area at Junction Reefs with low but anomalous values of Cu,As,S, Pb,Zn,Co,Au and Bi present. A close association between Au and skarn mineralogy exists centred around the monzodiorite stock. Economic concentrations of Au appear some distance from the stock in a halo dominated by a retrogressed mineral assemblage consisting of calcite, quartz and chlorite. Closer in, the mineralogy changes to ferrohastingsite and clinopyroxene while immediately adjacent to the stock garnet is dominant. At Sheahan-Grants the outer mineralisation boundary is characterised by a sharp skarn contact or "marble front". Gold values are highest near this contact and gradually tail off towards the stock. There is a general association between Au and sulphides. The main sulphides are pyrrhotite, arsenopyrite, pyrite and minor chalcopyrite. Gold ore mineralisation is envisaged to have formed in 3 stages. The emplacement of the Junction Reefs Monzodiorite caused metamorphism of the calcareous orebeds to form garnetwollastonite skarns. This was followed by a period of hydrothermal activity which commenced with metasomatism of the skarns by Fe-rich fluids. During this period the sulphides and Au were deposited. Further activity during falling temperatures induced retrogression of the mineral assemblage. Local structures were important in directing fluid movement.

TECTONIC EVOLUTION OF THE WONOMINTA BLOCK K.J.Mills Department of Geology and Geophysics, The University of Sydney, NSW 2006 The Wonominta Block in western New South Wales is an area of over 20 000 square kilometres of poorly to moderately exposed Palaeozoic and older rocks. The block is surrounded by Mesozoic or younger sediments and is separated from the Broken Hill Block by the Bancannia Trough. The block has been the focus of some recent interest because it records transitional events between the development of the Adelaide-Kanmantoo Fold Belt and the Lachlan Fold Belt, and because it has exploration potential in the search for diamonds, gold and base metals. Field mapping has revealed that the Wonominta Block can be divided into a number of stratotectonic units. The oldest unit is a fault bounded sequence of multiply deformed low to medium grade metasediments, basic volcanics and acidic tuffs, that host some sulfide deposits. This unit may be compared with the middle Proterozoic Willyama Supergroup of the Broken Hill Block, or with other Proterozoic sequences in Tasmania or South Australia. The second unit is an extensive sequence of carbonaceous slates containing rare clean quartzite beds, with some dolomitic limestones and rare polymictic conglomerates in its upper section. This unit is equated with the upper Proterozoic Adelaidean sequence of the Broken Hill Block, but differs in having a sequence of alkalic basaltic pillow lavas and tuffs interbedded with quartz-rich rocks at one level within the unit. A thin slate bed at the top of this unit marks a transitional stratigraphic contact with a thick overlying sequence of graded lithic and quartzofeldspathic sandstones with thin slate interbeds. This thick turbidite sequence, forming the third unit, is widespread within the Wonominta Block, becomes less oxidised to the east, contains rare trace fossils and sponge spicule shale horizons and is isoclinally deformed along with the two older units. This unit is thought to be analogous with the Kanmantoo Group in South Australia. The more fossiliferous lower to middle Cambrian Gnalta Group, exposed near Mount Arrowsmith and near Mount Wright on the western edge of the Wonominta Block, may represent a shallow water near shore sequence equivalent to this deeper water turbidite unit to the east. The fossiliferous upper Cambrian to lower Ordovician Mootwingee Group in the west and the Kayrunnera Group in the east overlie the previous units with a pronounced unconformity. This unconformity represents the Delamerian Orogeny in western New South Wales and the overlying units define the near shore facies at the western edge of an extensive sedimentary depository that was destined to become the Lachlan Fold Belt.

sILURO-DEVONIAN MAFIC DYKES IN THE EASTERN LACHLAN FOLD BELT: TECTONIC AND METAMORPHIC INDICATORS. Vincent J. Morand, Department of Geology, James Cook University,Townsville, Qld. 4811. Late Silurian to Early Devonian mafic to intermediate dykes are common in the eastern Lachlan Fold Belt, where they intrude mainly granites of about the same age (Bowning Deformation) and Ordovician metasediments. They are metamorphosed, but some igneous phases remain in many dykes. Volcanic equivalents of the dykes have not been positively identified. In the vicinity of the Wyangala Batholith in N.S.W., relic clinopyroxene compositions (titaniferous augite) indicate within plate alkalic basalt, as does the presence of igneous kaersutite (Morand 1984). In the southern part of the Omeo Metamorphic Complex in eastern Victoria several types of mafic to intermediate dykes are present, including low to medium-K basalts, Mg-rich andesites, basaltic andesites and alkali olivine basalt (Eberz 1987). At least some are coeval with the 405 Ma Swifts Creek Pluton. The alkalic magmas in both areas are not directly related to subduction, and an extensional, ensialic back-arc environment is envisaged for their formation. Other magma types intruding the Omeo Metamorphic Complex are similar to magmas of Andean or evolved island arc tectonic settings, and Eberz (1987) suggested that these required a contribution from the remnant subducted oceanic plate or mantle modified by Ordovician subduction. In a north trending belt extending from the Wyangala Batholith to the Cooma region, the strike of mafic dykes is generally N to NE, with steep dips (Morand 1984, Hayden 1980), consistent with E-W extension during emplacement, or possibly dextral transtension along a NW trend. In the Omeo Complex, strikes are more variable, but NW trends follow earlier formed foliations and dextral strike-slip faults, while NNE trending dykes near the Ensay Fault follow pinnatetype fractures developed during dextral shear along the fault (Eberz 1987) . Dykes intruding metamorphics of low to high grade in the Omeo Complex have all been metamorphosed to greenschist facies, with assemblages of actinolite + chlorite + albite + epidote + sphene + muscovite. They are not deformed even where they intrude foliated Late Silurian granites, and hence post-date the main part of the Bowning Deformation, while recording regional metamorphism associated with this deformation. Those further east in the Wyangala Batholith area (Morand 1984) and the Jerangle area near Cooma (Hayden 1980) were metamorphosed in the Siluro-Devonian, along with the enclosing metasediments and granites, to greenschist and amphibolite facies, with highest grade assemblages of blue-green hornblende + plagioclase + ilmenite ± brown biotite (T = 550° C) . Thus an eastward increase in grade is indicated for Siluro-Devonian regional metamorphism. The dykes here are commonly deformed, with a N striking, steep foliation that is also present in granite and Ordovician rocks, resulting from east-west compression that must have occured soon after the extensional period that gave rise to the dykes. REFERENCES: Eberz G. 1987. Ph.D. thesis, M o n a s h University, unpubl. Hayden P. 1980. Ph.D. thesis, A u s t r a l i a n National University, unpubl. Morand V.J. 1984. Ph.D. thesis, University of Sydney, unpubl.

EVIDENCE FOR EARLY CARBONIFEROUS SHEARING IN THE WAGONGA BEDS, NSW. Elaine I. Nicholls School of Earth Sciences, Macquarie University, N S W

2109

The Mid- to Late Cambrian/Early Ordovician Wagonga Beds are the easternmost

rocks of

the LFB near Batemans Bay, on the south

coast of NSW. Four generations of deformation (Di-Eu) have been mapped

features

these

complexly

associated

with

folded

and

faulted

provide evidence

rocks. for

Several

considerable

shear deformation and displacement of rocks along shear faults. Inclined lineation

folds with axes plunging to the NW (defined

flattened

elongate

and a stretching

clasts

pelitie

units), that is sub-parallel to the fold axes, provides evidence for

local high shear

parallel

the

strain. The clasts are parallel to sub-

cleavage

surface.

pervasive, moderately

inclined

(55°),

This

cleavage

westerly dipping surface

that defines the N-S structural grain in the WGB. The orientation of the clasts infers a NW to SE direction of transport or shear direction. Other local features such as folds initiated along D 3 faults support a west over east sense of movement.

Supportive

evidence for shear deformation is the relative location of Late Ordovician rocks that appear to be a slice enclosed between older Mid-Cambrian

and

relationship

is consistent with

the presence of

major

fault

considerable

shear

Late

Cambrian/Early

along

which

Ordovician

rocks.

This

at least one

displacement

has

occurred. D 3 in the Wagonga Beds has been mapped to the west where it is equivalent to the Early Carboniferous deformation-of the

Late

Devonian

Merrimbula

Synclinorium.

Group

rocks

the

Budawang

BENEATH THE LACHLAN FOLD BELT: EVIDENCE FROM INTEGRATED PETROLOGIC AND GEOPHYSICAL STUDIES Suzanne.Y. O'Reillyi and W.L. G r i f f i n 2 1 School of Earth Sciences, Macquarie University, Sydney, NSW 2109 2 Division of Exploration Geoscience, CSIRO, North Ryde, 2113 The integration of petrologic and geophysical data sets provides a powerful, synergistic methodology for geologically realistic modelling of the composition, structure and tectonic evolution of the crust and upper mantle. Petrologic data from inaccessible deep crust and mantle regions are provided by fragments (xenoliths) of deep-seated rock types entrained in ascending magmas. These xenoliths are transported to the surface in 10-30 hours, too fast for alteration or significant re-equilibration. They yield the locations of specific rock types in the underlying crustmantle section, and large specimens can be used to determine the petrophysical characteristics (density, acoustic velocity, magnetic properties, electrical and thermal conductivity, heat production) of specific rock types existing at given depths. Such xenoliths of lower crust and upper mantle material are abundant in basaltic rocks (ca 200-12Ma) which occur across a significant area of the Lachlan Fold Belt (LFB) and also across the Sydney Basin sequences which obscure direct observation of the basement rocks which may be an extension of the LFB. These provide unique information about the LFB including: (i) The characterization of the lower crust beneath the LFB as dominantly basaltic. Repetitive volcanic underplating by basaltic magmas has played a major role in its tectonic evolution. (ii) Recognition of mantle domains with different geochemical and deformation histories beneath the LFB and its extensions. High 87Sr/86Sr values for some mantlederived rock types indicate the influence of a subduction component. Rare sheared xenoliths define regions of deformation in the mantle. (iii) Estimation of the depth to the crust-mantle boundary. This varies from about 28-38 km beneath different parts of the LFB. (iv) Construction of realistic stratigraphies for crust-mantle sections so that seismic models can be constrained using actual rock types, their depth of occurrence and their calculated or measured acoustic velocities (Vp analogues). (v) Characterization of basement beneath the Sydney Basin as being an extension of the LFB sequences. (vi) Construction of empirical geothermal gradients to constrain thermal modelling and heat production estimates; the combination of these geotherms with magnetic measurements on xenoliths and MAGSAT data allows production of 3-dimensional heatflow contour maps. Paleogeotherms calculated from xenoliths represent the thermal state of a given lithospheric column at the time of entrainment in the host basaltic magma. All such paleogeotherms so far calculated lie on a single pressure/temperature path and represent a very high geothermal gradient to at least about 30 km. (vii) Modelling of underplating rates using xenolith-derived parameters. Such modelling indicates that for the Tertiary volcanism intrusion took place in a layer 4 km thick at a rate of about 900 m/Ma. This model is consistent with uplift rates for the Eastern Highlands and with seismic reflection profiles showing a layered lower crust.

EVOLUTION OF THE NORTHERN CAPERTEE HIGH J. W. Pemberton, A. J. Wright, A. N. Booth, Colquhoun, A. G. Cook and B. D. Millsteed Department of Geology, Wollongong, NSW 2500

University

Wollongong,

Campbell, G.

Box

1144,

In the Cudgegong district, Late Ordovician basalt and andesite were deposited as submarine lava and debris flows; geochemical data are typical of calc-alkaline lavas erupted in island arc settings. To the west and northwest of Rylstone, probable Late Ordovician chert and volcaniclastics represent pelagic deposits and turbidites. No Llandoverian (Early Silurian) strata' are known in the district. A thick, shallow marine to emergent, Wenlockian to Ludlovian (Middle to Late Silurian) sequence unconformably overlies Ordovician rocks in the Mudgee-Cudgegong area. The lowermost strata were deposited in a southeast-facing coastal zone and include fluvial channel, subtidal to supratidal flat, and marine shelf deposits. The subsequent eruptive episode produced dacite and rhyolite lava emplaced as a lava dome and a thick volcaniclastic ash and mud flow succession. Near Rylstone, lithologies and fauna are similar to the Cudgegong succession and represent deposition in a northwest-facing coastal zone. Contacts between Late Silurian and Early Devonian strata are either faulted or unconformable, in the latter case representing the Bowning deformational event. In the Cudgegong-Rylstone district, the Lochkovian to Zlichovian Kandos Group (fine- to coarse-grained clastics, limestone, dacitic ignimbrite, volcaniclastics and lava) were deposited in shallow marine to subaerial environments. Equivalents of the Group can be traced as far as the Capertee Valley in the south and Mount Knowles in the north. In the Queens Pinch Belt, the bulk of the Early Devonian sequence is redeposited and the closely associated shelf and turbidite facies results from deposition adjacent to the western margin of the Capertee High- Middle Devonian strata are apparently developed only in the Mount Frome Limestone. In the Mudgee-Cudgegong district, a thick Late Devonian sequence overlies older rocks mostly with low angle discordance (Lambian Unconformity). The strata result from a transgression producing initially fluviatile, then shallow marine conditions. The tectonism which deformed the Late Devonian strata predates intrusion of the Middle Carboniferous I-type Aarons Pass Granite (327 Ma), and extrusion of the latest Carboniferous (292 Ma) Rylstone Volcanics which consists of rhyolitic ignimbrite, lava and volcaniclastics. Most of the region was affected by sub-Permian peneplanation; however, some Late Devonian erosional remnants protrude above the unconformity surface. Thin flat-lying remnants of the Early Permian Snapper Point Formation represent a sandy transgressive sequence near the western margin of the Sydney Basin.

AGE OF GOLD MINERALIZATION, LACHLAN FOLD BELT, NSW C Perkins1, L McDougall2, J. Claou6-Long2, J- L. Walshe 3 and A.R. Chivas 2 1 Geology Department and Research School of Earth Sciences, The Australian National University, G.P.O. Box 4, Canberra, ACT 2601 2 Research School of Earth Sciences, The Australian National University, G.P.O. Box 4, Canberra, ACT 2601 3 Geology Department, The Australian National University, G.P.O. Box 4, Canberra, ACT 2601 The Ordovician to Early Silurian succession in NSW is dominated by mafic (high-K or shoshonitic) submarine to subaerial volcanic sequences, intercalated with limestone beds (Cambrian/Ordovician and Ordovician/Silurian boundaries are about 510 and 440 Ma respectively). The Ordovician to Early Silurian magmatic rocks host gold and copper mineralization in systems such as the Gooonumbla porphyry copper-gold deposits, the Copper Hill porphyry copper deposit, the Sheahan-Grants skarn gold and Browns Creek skarn copper-gold deposits, the Glendale monzonite-hosted gold deposit and the Gidginbung epithermal gold deposit. Isotopic dating provides a means by which the age of these systems may be determined, and a combination of K-Ar and U-Pb dating has been used to date alteration minerals and host rocks to deposits. Hydrothermal sericites from Goonumbla and Glendale have been dated by the step heating technique and yield ages of approximately 440 Ma, interpreted to be the time of mineralization. A minimum age of about 427 Ma has been obtained by dating of hydrothermal amphibole from Browns Creek, a comparable age to that yielded by magmatic amphibole from the Long Hill Diorite which outcrops near the deposit. Hydrothermal alunite from Gidginbung yields 4 0 Ar/ 3 9 Ar ages ranging from approximately 400 to 420 Ma, and the latter date is interpreted as a minimum age for mineralization. U-Pb dating by ion microprobe on magmatic zircons from volcanic rocks which occur near the Goonumbla and Gidginbung deposits gives ages of about 440 Ma. Magmatic hornblende and zircon from subvolcanic monzonite intrusives near Sheahan-Grants and Glendale yield a range of ages from about 480 to 430 Ma. K-Ar dating of magmatic hornblende from pre- and post- mineralization diorite intrusives at the Copper Hill deposit gives dates of about 440 Ma. Isotopic dating illustrates a temporal association between the Ordovician to Early Silurian shoshonitic magmatism and gold and copper mineralization in the case of the Goonumbla, Copper Hill, Glendale and probably the S.heahan-Grants deposits, and illustrates that the Ordovician rocks in the LFB, NSW, are highly prospective for gold and copper mineralization. In addition, other episodes of gold and copper emplacement occur in the region, and mineralization at Browns Creek may be related to the Middle Silurian Long Hill Diorite.

CONTRASTING STYLES AND TIMING OF EMPLACEMENT OF MURRUMBIDGEE- AND BATHURST-TYPE BATHOLITHS IN THE LACHLAN FOLD BELT. AUSTRALIA Scott Paterson , Tom Fowler^, Ron Vernon and Dick Flood3 1 Department of Geological Sciences, University of Southern California, Los Angeles, CA 90089-0740, 2 Geology Department, University College of Northern Victoria, Bendigo, Victoria, Australia 3550, 3 School of Earth Sciences, Macquarie University, Sydney, New South Wales, Australia 2109 Vallance (1969) grouped granitoids in the Lachlan fold belt into three types: Cooma, Murumbidgee (foliated, lack static contact aureoles, syntectonic), and Bathurst (massive, static, high-temperature aureoles, post-tectonic). We have previously discussed several Murumbidgee-type batholiths and will compare these to two nearby Bathurst-type granitoids, the Bathurst and Bruinbin plutons. The Carboniferous Bathurst batholith and satellitic Bruinbin pluton are unusual in the Lachlan fold belt in that they cut across regional structures. Both are massive, lacking pervasive solid-state or strong magmatic foliations, and are generally quartz monzonites, although the Bathurst batholith is a composite body with mafic phases present. Wall rock contacts are usually very sharp and discordant even at the scale of centimeters. Stratigraphic units, bedding, and regional cleavage are all truncated by pluton margins. Wall rock structures are only rarely deflected near the margins, and regional total strains, which range up to 70% horizontal shortening and 225% vertical extension, do not change in intensity or type near the plutons. Hornfelsic contact aureoles are well developed, although some deformation did occur during and after porphyroblast growth. We argue that these plutons are not associated with regional faults and that emplacement mechanisms must involve stoping and/or uplift of overlying rocks by a crystal-poor magma, although we find the lack of xenoliths in the plutons puzzling. Timing of emplacement postdates regional deformation, although there is evidence near the Bathurst batholith that the Hill End basin-bounding faults were still active. Finally, we note that our studies largely support Vallence's suggestions: Murumbidgee-type batholiths are emplaced as concordant sheet-like bodies, are often associated with faulting, and in a general sense are syntectonic. Bathurst-type batholiths are not associated with faults, are emplaced by stoping or roof uplift, and at the scale of the pluton are posttectonic. 1

GOLD IN THE SOUTHERN LACHLAN FOLD BELT, VICTORIA G Neil Phillips Geology Department and Key Centre In Economic Geology James Cook University Townsville Qld 4811 Palaeozoic gold deposits are widespread throughout the Tasman Orogen of eastern Australia but are particularly concentrated in the Victorian section of the Lachlan Fold Belt. The latter, in fact, accounts for 2 percent of the world's gold (i.e. 2000 tonnes Au) although major mining ceased early this century and the area has not seen the scale of revival that has affected other major goldfields through the 1980's. The Victorian goldfields are characterized by a number of very large deposits (Bendigo, 700 tonnes; Ballarat, 300 tonnes; Walhalla - Woods Point, 100 tonnes) and many hundred small to medium deposits. Of all the deposits, Stawell has had the most significant production over the last decade (around 1 tonne of Au annually). Gold in Victoria is virtually confined to the areas of Palaeozoic rocks and their placer products. Significant gold deposits have been mined in virtually all parts of Palaeozoic Victoria; however, the greatest production has come from a small area of central-western Victoria bounded by Ballarat, Bendigo and Wedderburn. All the rock sequences in the interval from Cambrian to Middle Devonian are gold-mineralized somewhere within the state. Furthermore, the actual host rock lithology varies widely and again includes almost all types represented in the pre-Carboniferous succession; e.g. shale, greywacke, granite, lamprophyre, granodiorite, basalt and conglomerate. In certain areas, host compositions have been chemically favourable for gold deposition (e.g. carbonaceous units) but no favourable chemical characteristic is universal. The metamorphic grade surrounding the gold deposits is predominantly greenschist facies but examples in higher metamorphic grades are well-established. Structural control of the deposits has been documented for many decades (saddle reefs at Bendigo, ladder veins in Morning Star mine at Woods Point) to the extent that there appears to be no well-documented examples that do not exhibit some structural control and a common structural timing is possible. Notwithstanding, the nature of the structural control varies widely and it seems most unlikely that any particular structural settings have universal or even province-wide applicability: only on the scale of individual deposits do one or more structural settings have particular significance and favourability. On the basis of spatial relationships, a close association with intrusive rocks has been suggested at many deposits with an implication of some magmatic source for fluids and/or gold. However, when viewed regionally, an essential link to a magmatic source is less clear. The central Victorian goldfield represents a part of the Earth's surface particularly favoured for gold deposits in the same league as the rich sections of the Mother Lode, Canadian Shield and Nevada, and only overshadowed by the Witwatersrand. The underlying reason for Victoria's favourability for gold is far from resolved by modern studies but several individual reasons can be ruled out The rock packages and the individual host lithologies in the Victorian goldfields do not appear vastly difference from other parts of the Lachlan Fold Belt. Similarly, the hosting structures do not appear unique, and the inferred fluid types broadly approximate those in many other metamorphic belts. The problem of pinpointing what factors "went right ior gold in Victoria are also problems in the world's other major gold provinces, and resolution ol the dilemma is likely to require broad scale overviews integrated with local geological detail.

SILURIAN COLLISIONAL UNDERPLATING IN THE LACHLAN FOLD BELT C. McA. Powell Department of Geology, The University of Western Australia, Nedlands, WA 6009 The history of the eastern Lachlan Fold Belt can be divided into two major episodes. The first, from the beginning of the Cambrian until the earliest Silurian relates to the formation of greenstones and widespread quartzose turbidite aprons in generally deep-marine conditions, and is of oceanic affinity. The second, from Early Silurian until terminal deformation in the mid-Carboniferous, involves formation of narrow, linear grabens and the intrusion and effusion of vast volumes of silicic magmatic rocks, and has a continental signature. The changeover from the first regime to the second occurs abruptly in an interval of 5 million years and suggests that the basement to the foldbelt changed character from oceanic to continental. Isotopic and petrologic information about the great silicic outpouring in the eastern Lachlan Fold Belt appears to indicate that the melts came from continental crustal sources beneath the widespread quartzose Ordovician flysch, and some evidence indicates that this basement might be Precambrian. These observations seem to be incompatible with the evidence of the Ordovician and older sedimentary facies, which suggests an oceanic basement then. A possible solution to the dilemma is that the Cambrian and Ordovician rocks accumulated on oceanic basement adjacent to Gondwanaland up until the beginning of the Silurian, and that at that time subduction of an oceanic plateau or some previously-rifted thinned continental crust led to collisional underplating of the eastern half of the Lachlan Fold Belt. If subduction of the Palaeo-Pacific plate was able to proceed until the pull of the cold downgoing oceanic plate was balanced, and then exceeded, by the upward buoyancy of the entrained oceanic plateau/thinned continental crust, we have a mechanism to explain the sudden rise of base-level and the source of the silicic magmatic outburst in the mid-Silurian to mid-Devonian interval. The abrupt change in surface level of the eastern Lachlan Fold Belt in the Early Silurian would have occurred by isostatic adjustment as soon as the buoyant subducted material came in contact with the overlying crust, and the appearance of massive amounts of silicic melt from 5 to 50 million years later would have resulted from partial melting of the underplated crust. The S-l line, the S-l-A sequence of first appearance of granitoids, isotopic evidence for Precambrian crust, and the existence of basement terranes not necessarily matched with surficial tectonostratigraphic terranes, are all explicable by this model. The western extent of the collisional underplating would have been roughly along the Cobar-Sale line ( Powell et al. 1990). The uplift of the region to the east in the Early Silurian defined the Cobar and Melbourne Troughs as linear basins between the underplated region and the older Gondwanaland continent to the west. Final consolidation of the Lachlan Fold Belt by renewed convergence was accomplished in the mid-Devonian to mid-Carboniferous interval. Modern analogues for the kind of material which could have been underplated in the Early Silurian are the Lord Howe Rise and the Ontong Java Plateau, and an analogue for a region which appears to have been underplated in the late Cainozoic is Tibet. It is instructive to note that while there has been enormous underthrusting along the Himalayas in the last 20 Ma, the surface of Tibet has been under extension. Parallels can be drawn with the Eastern Lachlan Fold Belt. Reference: Powell C.McA., Li Z.X., Thrupp G.A. & Schmidt P.W. 1990. Australian Palaeozoic palaeomagnetism and tectonics - 1. Tectonostratigraphic terrane constraints from the Tasman Fold Belt. J. Struct. Geol. 12:553-565.

MAFIC SUITES IN BASEMENT BENEATH THE MURRAY BASIN: NEW DATA FOR THE EARLY PALAEOZOIC HISTORY OF THE TASMAN OROGENIC PROVINCE L. R. Rankin, B. J. Clough and C. G. Gatehouse, South Australia.

Department of Mines and Energy.

Results of recent stratigraphic drilling and interpretation of geochemical and image-processed aeromagnetic data indicate the presence of 3 major suites of mafic/bimodal igneous rocks (and associated metasediments) in the Early Palaeozoic basement to the Murray Basin. • Suite 1 - ?Early Cambrian basalt plus interbedded carbonate platform-facies limestone and pyritic shale recovered in drillcore from the northwestern termination of the Stavely Belt. • Suite 2 - ?Mid-Late Cambrian deformed metadolerite, metabasalt and albitite which occurs in a discontinuous arcuate zone from west of the eastern Mt Lofty Ranges to south of the Willyama Inliers and Broken Hill Block. Geochemically similar to metabasics intruded during late stage Kanmantoo Group sedimentation. • Suite 3 Late Cambrian-Early Ordovician post-Delamerian Orogeny granitoids, porphyritic rhyolite, ultramafic and mafic intrusives in the Padthaway Ridge-Black Hill area. A sequence of basalt-andesite in the Yumali-Coonalpyn region is structurally juxtaposed with this suite, but is geochemically similar to Suite 1. Tentative correlation of Suite 1 basalt-sediment assemblage with the platformal Stansbury Basin sequence poses some interesting implications for the timing and nature of the Stavely Belt. Two major scenarios can be argued. The first is that the Stavely Belt, including the Mount Stavely Volcanic Complex, is Early Cambrian, and equivalent to the Normanville Group. A problem with this model is the change along the belt from calc-alkaline volcanics in the southeast to within-plate basalt in the northwest. The second model interprets the Suite 1 sequence as an allocthonous block of Stansbury Basin volcanics and sediments incorporated into a calc-alkaline island arc Stavely Belt during Mid-Late Cambrian convergence of the Australian Craton and a (?) Victorian cratonic block. The MORB-like characteristics of the Suite 2 metabasics suggest extensional thinning of Precambrian continental crust during deposition of the Kanmantoo Group. Whether this produced a continental-oceanic crustal margin is debatable. Deformed granitoid xenoliths within Suite 2 metabasics m several drillholes may represent segments of Precambrian crust. The arcuate zone of Suite 2 may represent the deepest section of the Kanmantoo Trough, with possible shallowing to a cratonic platform to the east. This work provides some new data and highlights some questions r e l e v a n t to the evolution of appropriate tectonic models for the Palaeozoic history of the entire Tasman orogenic system. 45

ASSESSMENT OF SILURO-DEVONIAN TECTONIC MODELS FOR THE LFB* E. Scheibner, NSW Geological Survey, Department of Minerals and Energy, Box 536, St Leonards NSW 2065 The LFB as defined by Scheibner (1972, 1978, 1987) (earlier synonym "Central and Southern Highland Fold Beir of Packham 1969) (not in sense of Cas 1983) is a composite orogenic belt. There are no known outcrops of Precambrian basement. The various tectonic models proposed for the Siluro-Devonian (S-D) development of the LFB depend to large degree on character of the basement. This has variously been postulated to be oceanic (Crook 1969,1980), continental (Hills 1956; Rutland 1973, 1976) or some combination of the two (Scheibner 1974; Compston and Chappell 1979). The evidence favours the last postulation, but the extent of Precambrian continental basement, its age (Palaeo-Mesoproterozoic versus Neoproterozoic, i.e. MPan-AfricanM Penguin Fold Belt and its equivalents, or both, cf. Scheibner 1990), its structural form (i.e. little reworked separated microcontinents, or strongly reworked and extended continental crust forming microcontinents or technically underplated external terranes, for example volcanic arcs, Scheibner 1985) remains unresolved. The microcontinental blocks acted as structural backstops during deformation and can explain the anomalous east vergency of thrusts (Cox et al. 1983; Glen and VandenBerg 1987; Fergusson and VandenBerg 1990) away from the old foreland. It is however misleading to equate the most easterly occurrence of Precambrian basement rocks within the LFB with the eastern edge of the Australian Precambrian Shield or Craton. It is similar to extending the Palaeozoic Australian Craton to New Caledonia today. Powell (1984) recognized three categories of Palaeozoic tectonic models: 1) intra-oceanic arc evolution (Crook 1980a, b); 2) early intra-oceanic arc evolution followed by arc-continent collision and tectonic underplating (Crawford 1983); 3) continental margin-back arc basin-volcanic arc systems with many specific models (Oversby 1971; Packham and Falvey 1971; Packham 1973; Solomon and Griffiths 1972, 1974; Scheibner 1972,1974; Cas et al. 1980). While certain consensus has been reached in respect of the Cambrian and Ordovician scenarios, including localized microcontinental basement, a diversity of opinions remains in respect of S-D development of LFB. These opinions range from "fixist" e.g. Proterozoic basement terranes assembled prior to Ordovician (Chappell et al. 1988), through mobilistic settings involving a wide region of back-arc extension and subsequent complicated inversion, including transtension and transpression (Scheibner 1974, 1987; Glen 1990), through basin-and-range setting (Cas 1983; Powell 1983, 1984), through a complex volcanic arc (Fergusson and VandenBerg 1990) to multiple arc-fore-arc basins (Crook 1980a, b). The model of progressive sinistral strike-slip tripling up of the Ordovician arc-back-arc basin (Packham 1987) is unique. The fact that the Boggy Plain Supersuite (Wyborn et al. 1987) traverses two basement terranes is an argument in favour of the Molong-Monaro Superterrane underthrusting the Wagga Terrane along the Gilmore Suture (Scheibner 1985) and indicating post-Ordovician displacement of basement terranes. The fact that the stitching igneous rocks of the Central Victorian Magmatic Province diagonally traverse three basement terranes requires explanation. This Late Devonian igneous province perhaps could be related to extensional collapse of the Tabberabberan Highlands. As yet there is no consensus on the origin of S-D igneous activity in the LFB. Its large width is supposed to preclude it being subduction related (Chappell et al. 1988). The Silurian anatectic granites (Fagan 1979) were related to Benambran crustal pile-up (Pogson 1982) and emplaced during extension, detachments perhaps facilitating their rise. Other S-D igneous rocks were related to rifting in pull-apart and transtensional zones. Some M-type intrusions were also related to extension and a shallow asthenosphere. However, there is still a large bulk of rocks related to areally extensive crustal melting which needs explanation. Crustal delamination and consequent asthenospheric upwelling (Wyborn 1988) and an eastwards migrating subcrustal heat source (Powell 1983) have been proposed as a cause, but these fail to explain anomalous age distribution of plutons. The basin-and-range models rely on apparent similarity with the North American Cordillera (Cas 1983; Powell 1983). There the main extension was caused by gravitational collapse of the Laramide orogenic belt and only subsequently was extension caused by boundary forces (Coney 1987). So this model can be at best applied to the mam Benambran collisional belt, with the other extensional regions affected by boundary forces. There is also controversy m respect of the sense of transtension, being dextral (Powell 1983), while nearly all other authors suggesting sinistral sense. Critical is the mechanism of extension, rifting and basin formation (i.e. McKenzie 1978 versus Wernicke 1985). On balance both simple and pure shear must have been involved, as pure shear would enable the widespread rift related igneous activity, with some M-type intrusions associated with deep detachments and asthenospheric upwellings . Deep seismic studies should contribute to elucidation of basin formation processes m LFB during the S-D. •Published with the permission of the Director General, NSW Department of Minerals and Energy

SULPHUR ISOTOPE AND FLUID INCLUSION STUDIES OF TURBIDITE-HOSTED Cu-Au VEIN DEPOSITS AT COBAR, NSW - SOURCE OF ORE COMPONENTS IN A LOW-GRADE METAMORPHIC ENVIRONMENT P.K. Seccombe Department of Geology, University of Newcastle, NSW 2308 Structurally controlled Cu-Au deposits form quartz-sulphide vein sets and occupy shear zones in turbidites of Early Devonian age, close to the E margin of the Cobar Basin. A range in sulphur isotopic composition (S^S) from 3.8-11.2 %o (average 7.9 %<>) is obtained for 103 measurements from deposits in the central Cobar area. With the exception of the New Cobar mine (mean = 7.9 %o), all deposits indicate similar distributions. The sulphur isotopic composition appears to be independent of the sulphide mineralogy and is characterised by isotopic disequilibria among coexisting sulphides. Sulphur is likely to have been derived from rock sulphide in the trough-fill sequences of the Cobar Supergroup and underlying Girilambone Group basement rocks, but may have an additional connate sulphur component. Homogenisation temperatures (Th) for H20-C02-3wt% NaCI (equivalent) inclusion fluids from early- to syn-tectonic quartz veins range from near 150°C to 370°C. Th for C02bearing fluids represent true trapping temperatures of about 350°C. Th near 200°C for aqueous inclusions, require temperature corrections due to lithostatic load of possibly +150°C. The aqueous inclusions apparently represent a fluid that was trapped later in the deformation event than the C02bearing fluids, but at similar temperatures. The geochemical data demonstrate that differences in metal content of individual deposits in the Cobar area cannot be attributed to major differences in temperature or salinity of the transporting fluids. Rather, control of metal transport by variations in pH of the fluids may be important. Sulphur isotope data suggest that higher pH conditions in ore fluids were associated with magnetitedominated Au-Cu deposits (e.g. Occidental, Great Cobar), in contrast with those where pyrrhotite is the major iron sulphide. Differences in the metal content of the Cobar ores may also be due to the type of source rock and the structural pathways along which ore fluids moved during low-grade metamorphism. An interpretation of the sub-surface structure at the eastern margin of the Cobar Basin (Glen, 1990) is used to discuss the role of regional and local structure in focusing ore-forming fluids during deformation. Access provided to Girilambone Group basement rocks by the E-dipping Great Chesney Fault may explain the high Au and Cu content of the group of ore deposits from East Cobar to Occidental localised along this structure. Mixed Girilambone Group basement and Cobar Supergroup sources, tapped by fluids focused along the W-dipping Cobar and Myrt Faults, may be inferred for Au, Ag and Cu mineralisation at the Great Cobar, Dapville and Gladstone mines. Golddeficient base metal mineralisation distant from the central area (e.g. CSA mine) appears to be related to W-dipping structures that penetrated Cobar Supergroup sequences exclusively. The formation of Cobar ore deposits is considered analogous to models proposed for the origin of turbidite-hosted gold deposits, whereby metals and sulphur, derived from crustal rocks are scavenged by metamorphic fluids, focused along major structures and deposited in response to transient pressure reduction in structural traps during deformation. Reference Glen, R.A., 1990, J. Structural Geol., 12, 601-620.

SOURCE OF GOLD IN A SLATE BELT - GEOCHEMICAL MODELLING OF THE HILL END GOLDFIELD, NSW P.K. Seccombe1, J. Lu\ B.L. Gulson2 and K.J. Mizon2 1 Department of Geology, University of Newcastle, NSW 2308 2 CSIRO Division of Exploration Geoscience, PO Box 136, North Ryde, NSW 2113 The Hill End goldfield produced 56t of lode gold from bedding-parallel vein sets and lesser cleavageparallel and fault-controlled veins hosted by a multiply-deformed Late Silurian slate-metagreywacke turbidite sequence. Veins consist of quartz, carbonates, muscovite and chlorite, accompanied by lesser pyrite, gold, pyrrhotite, chalcopyrite, sphalerite and galena. Hostrock alteration is characterised by the development of chlorite and carbonate. The presence of cleavage-parallel veins, differing degrees of shortening and folding in veins oblique to cleavage, dynamic recrystallisation textures preserved in quartz, shear veins and non-deformed planar gold-bearing veins in fault zones all indicate that veining has spanned the entire Early Carboniferous (and midDevonian ?) deformational event. Gold (and sulphides) appears to have been introduced late in the paragenesis. Preliminary sulphur isotope data indicate that the underlying turbidite sequence has acted as a source of sulphur (S^S range = -2.8 to 17.8 %©). Fluid inclusions in the quartz-carbonate veins reveal a complex association involving (i) low salinity, aqueous, (ii) low salinity, C02-bearing and (iii) multi-phase types containing one or two solid phases. Homogenisation temperatures (Th) from quartz and calcite from a wide variety of veins range from 120°C to 370°C. Results of a major lead isotope study demonstrate that coarsely laminated quartz veins are characterised by two isotope populations in pyrite, arsenopyrite and galena, with 207Pb/206Pb ratios of -0.862 and -0.860. The same populations are encountered in gold-rich, thin (1-5mm wide) cleavage-discordant quartz leaders that intersect the principal bedding-parallel veins. The five samples of gold analysed from the leaders so far, belong to the population with lower ^Pb/^Pb. The differences in Pb isotopic composition for the vein systems suggest that they were derived from different sources, perhaps at slightly different times. ^Pb/^Pb values extending down to -17.0 for disseminated pyrite and arsenopyrite in the host metasediments are much lower than in the sulphides from the quartz veins. What was the source of gold? The Hill End trough-fill sequence is currently favoured as the main source of mineralisation and this is consistent with mass-balance calculations and sulphur isotope and fluid inclusion data. Although the Pb isotope data for syngenetic pyrite and arsenopyrite lie on a well-defined array with the ores and hostrocks on a ^ P b / ^ P b vs 206Pb/204Pb diagram, corresponding to an apparent age of -1400 Ma, we consider this to be an artefact of mixing and not a real source age. Pyrite and arsenopyrite from one of the background samples have high Pb contents of -1160 ppm and the ^Pb/^Pb of 17.81 is quite different from pyrite in the quartz veins. Thus local remobilisation is not the main source of metals in the quartz veins and we favour remobilisation and mixing of syngenetic sulphide and other background rock lead as the viable alternative. Variable mixing sources at slightly different times could account for the two isotope populations in the quartz veins. The very low ^Pb/^Pb and ^ P b / ^ P b values in some of the background pyrites may reflect derivation from an igneous source with a significant mantle Pb component, such as the widespread Ordovician andesitic/basaltic sequences of shoshonitic affinity known from the adjacent volcanic arcs. Compared with the turbidite-hosted gold deposits of the Victorian Ballarat slate belt, the Hill End goldfields have a lead isotope signature characterised by slightly higher *>7PbTPb and **PbTPb values, consistent with their derivation from a source with a higher crustal Pb component.

STRUCTURAL ANALYSIS IN CONVERGENT AND DIVERGENT WRENCH ZONES WITH REFERENCE TO THE LACHLAN FOLD BELT J.V. Smith Department of Applied Geology, University of Technology, Sydney. PO Box 123 Broadway NSW

On the plate tectonic scale deformation of rocks is generally controlled by plate displacements. Consequently, stress-based terminology is inappropriate and stress-based analysis is ineffective. Biddle and Christie-Blick (1985) expressed preference for kinematic terms over stress terms, for example, convergent wrenching rather than transpression. Experimental work on clay shows this to be not merely a question of preference but accuracy. The shearing of clay under compressive normal stress can result in divergent kinematics. There is also experimental and field evidence of shortening parallel to rifts concurrent with tensile stress parallel to the rift. Both these phenomena can be simply explained by structural analysis which incorporates the ability of displacements to cause stresses. As stresses can be caused by displacement imposed on rocks the concept of a "remote stress" controlling deformation is inappropriate. Mount and Suppe (1987) demonstrated that in the vicinity of the San Andreas Fault the compressive stress is at a high angle to the fault (84°) consistent with Pliocene and younger folds and thrusts which trend at 6°. The conventional model (Wilcox et al, 1971) predicts compressive stress at an angle of 45°. The difference occurs because the stresses which exist during the shearing of intact rock are local and transient and can be quite different in orientation to background or remote stresses. McCoss (1986) proposed a boundary between wrenching and extension and between wrenching and contraction where displacement is at an angle of ±19.5° to the zone. This was based on a theoretical analysis which assumed constant volume. Divergent wrench experiments on clay show a boundary at a displacement angle of 55° to the zone which separates dominant strike-slip faulting from dominant normal faulting. The orientation of displacement can be determined from the orientations of structures (Sanderson and Marchini, 1984). In addition, it is demonstrated in experimental work on clay that the angle between conjugate shear structures is also dependent on the displacement orientation and changes in a systematic way from 60° typical in wrenching (Wilcox et al, 1971) to about 30° between zigzag fault structures in rifts (eg. Freund and Merzer, 1976). The implications of selected aspects of these findings on divergent and convergent wrench models of parts of the Lachlan Fold Belt are shown.

Biddle, K.T. and Christie-Blick, N., 1985. Glossary - Strike-slip deformation, basin formation and sedimentation. In: Biddle and Christie-Blick (eds). Strike-slip deformation, basin formation and sedimentation. Soc. Ec. Pal. Min. Spec. Pub. 37, 375-386. Freund, R. and Merzer, A.M., 1976. The formation of rift valleys and their zigzag fault patterns. ' Geol. Mag., 113, 561-568. . McCoss, A.M., 1986. Simple construction for deformation in transpression/transtension zones. J. Struct. Geol., 8, 715-718. Mount, V.S. and Suppe, J., 1987. State of stress near the San Andreas fault: Implications lor ' wrench tectonics. Geology, 15,1143-1146. Sanderson, D.J. and Marchini, W.R.D., 1984. Transpression. J Struct. Geol 6, *49-458_ Wilcox, R.E., Harding, T.P. and Seely, D.R., 1971. Basic wrench tectonics. Am. Assoc. Pet. Geol. Bull., 57, 74-96.

N O R T H W E S T E R N NEW S O U T H W A L E S A N D ITS R E L A T I O N S H I P T O T H E L A C H L A N FOLD B E L T B P 0 Stevens Geological Survey of NSW 32 S u l p h i d e St B r o k e n Hill N S W 2880 D e p a r t m e n t of M i n e r a l s and E n e r g y Geological data and i n t e r p r e t a t i o n of r e g i o n a l geophysics permits r e - d e f i n i t i o n of the limits of fold b e l t s in w e s t e r n NSW. M a j o r d i s c o r d a n c e s in t r e n d s at fold b e l t m a r g i n s m a y delineate sutures and r e f l e c t relative ages of initial d e v e l o p m e n t of the fold belts; i.e. K a n m a n t o o and C o b a r O m e o * Fold Belts (KFB and C O F B ) p r e - d a t e d T h o m s o n F o l d Belt (TFB) which p r e - d a t e d Lachlan Fold Belt (LFB). M o s t of the NW NSW P a l a e o z o i c is w i t h i n the KFB and T F B . In NW NSW the KFB includes a p o s s i b l y Late Proterozoic d e f o r m e d flysch wedge, Early C a m b r i a n s h a l l o w s e d i m e n t s and rift v o l c a n i c s from a d i v e r g e n t c o n t i n e n t a l m a r g i n , Late Cambrian d e f o r m a t i o n and c o a r s e c l a s t i c s , O r d o v i c i a n shelf sediments, E-M Silurian hiatus, Late Si 1 u r i a n - E a r l y D e v o n i a n rifting and e x t e n s i v e M i d - L a t e D e v o n i a n fluvial s e d i m e n t s . The TFB contains gravity and m a g n e t i c a n o m a l i e s c o m p a r a b l e to those produced by a n d e s i t i c arcs in the LFB. T h e s e may represent an Early O r d o v i c i a n a n d e s i t i c arc w h i c h c o l l i d e d with the COFB and KFB in the M i d d l e O r d o v i c i a n . Granitic intrusions from 410 to 368 Ma show c o n t i n u e d instability, similar to that in the LFB, but r e m o t e from any s u b a u c t i o n zone east of the LFB. The Alice S p r i n g s - K a n i m b l a n O r o g e n y the KFB, TFB, COFB, and LFB.

deformed

and

* Newly defined as that part of the LFB w e s t of T u l l a m o r e Suture.

the

QM.

Ac£tla>tife FoU Belt

Published with permission of the D i r e c t o r - G e n e r a l D e p a r t m e n t of Minerals and Energy.

stabilised

THE STRATIGRAPHY AND DEFORMATION HISTORY OF THE TUMUT REGION: IMPLICATIONS FOR THE DEVELOPMENT OF THE LACHLAN FOLD BELT. P.G. Stuart-Smith , R.I. Hill , M.J. Rickard , and M.A. Etheridge 1 Bureau of Mineral Resources, Australia, PO Box 378 Canberra, ACT 2601, Australia Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601 Geology Department, Australian National University, GPO Box 4, Canberra, ACT 2601 1

The Tumut region forms the southern part of the Tumut Synclinorial Zone in the southeastern part of the Lachlan Fold Belt (LFB). The area contains two major tectonostratigraphic sequences: Ordovician to Early Silurian quartz-rich to quartz-intermediate flysch and volcanics; and an overlying fossiliferous Early/Late Silurian volcanic sequence. Rhyolite within the older sequence yields a U-Pb zircon age of about 425 Ma. Previously both of these sequences were regarded as forming part of the Tumut Trough , an Early Silurian palaeogeographic feature. However, arenites from the two sequences are compositionally distinct and differences in composition of clinopyroxene phenocrysts from mafic volcanics in both sequences reflect differing tectonic environments. Both tectonostratigraphic sequences were meridionally folded during the Siluro-Devonian Bowning Orogeny following intrusion of the Gocup Granite at about 415 Ma. An earlier deformation, characterised by thrust faulting, E-W recumbent folding and later local coaxial upright folding, is present only in the older flysch. This earlier deformation is compared to the Benambran Orogeny that affects the Ordovician metamorphics of the Wagga Metamorphic Belt and is tightly constrained to about 425 Ma. Fold characteristics of this deformation are indicative of thin-skinned intraplate transpressional deformation rather than classical collisional tectonics as envisaged by some workers for the Benambran Orogeny here and elsewhere in the LFB. The composition and deformational history of the older flysch indicates it is part of the Ordovician to Early Silurian Molong Volcanic Arc. Consequently, the concept of an Early Silurian Tumut Trough* incorporating these rocks is rejected. Instead, it is proposed that up to 2500 m of Early/Late Silurian rocks formed a pull-apart basin (the Tumut Basin). Thus the Silurian history of the Tumut region, previously considered unique in the LFB, is little different from other basins of similar age throughout the LFB. 1

The Tectonic Development of Palaeozoic Fold Belts in Eastern China SUN Yan (Department of Earth Sciences, Nanjing University, Nanjing 210008, P.R. of China) Abstract The tectonic development of Palaeozoic fold belts in eastern China is viewed in terms of plate tectonic and flake tectonic concepts. All Palaeozoic fold belts, whether they belong at active or passive continental margins, indicate obvious characteristics of collisional orogenic belts. There are three Palaeozoic fold belts trending NE to ENE at the margins of continental plates in east China; they are from north to south the Daxianganling, Qinling and southern Huanan (southeastern China) fold belts. Taking the latter as an example the Wuyi-Yunkai early Palaeozoic island-arc fold system is situated southeast of the Jiangnan Proterozoic composite terranes and to the northwest of the Zhenhe-Dapu deep fault zone, which represents the site of ancient subduction and collision. The rock assemblages consist of calc-alkaline volcanics, lightly metamorphosed clastics and cabonates. Major decollement or shear systems including interbed slip, bed-cutting overthrusts and gliding nappes are clearly developed in the Huanan Palaeozoic fold belts, separating upper Proterozoic, lower Cambrian, Silurian, lower Carboniferous and upper Permian rock sequences. The rock mechanical and seismic-wave properties of these sequences have been systematically determined.

According to the tectonic layering, material layering and energy layering in the crust the cataclastic, ductile-slip and viscous rheological deformation characters can be recognized from shallow to deep levels.

The thin-skined

cover of lower Cambrian etc. is characterised by the cataclasites the mobilized basement of the upper Proterozoic manifests ductile-slip mylonites.

In fact the Huanan early Palaeozoic fold belts were formed by

folding in the cover slipping on a mobilized basement.

The viscous

rheological mylonite of the deep structural level is seen in the subduction zone at the margins of the belts (Yifeng-Waizai and Zhenhe-Dapu deep-fault zones).

The collisional amalgamation of the Palaeozoic orogens in southeastern China developed stack-mountain systems, and three kinds of stack structures — the single, the opposite and the deviate type have been distinguished.

Most of

the Huanan Palaeozoic fold belts belong to the opposite dip-slip type.

The Palaeozoic fold belts developed in southeastern China have undergone several broad orogenic cycles; the Caledonian and Indosinian cycles, are very important for the tectonic development of the slip systems and stack structures.

The Caledonian cycle (405±5 Ma) showed an intense

compressive, orogenesis and the early Indonesian cycle continually indicated a mountain building event.

The late Indosinian cycle became tensile and

facilitated a transformation of the mechanical nature of the slip systems. Thus the basic tectonic frameworks of the Palaeozoic fold belts developed in southeastern and eastern China were established through repeated tectonic cycles.

METALLOGENESIS OF THE LACHLAN FOLD BELT IN NEW SOUTH WALES* D.W. Suppel and L.B. Gilligan Geological Survey of NSW, Department of Minerals and Energy, Box 536, St Leonards NSW 2065 The metallogenesis of the Lachlan Fold Belt (LFB) reflects a more complex history than most orogenic belts. This probably is due to the role of two Precambrian microcontinental blocks (Victorian and Molong) which became cores of separate tectonostratigraphic terranes during the Benambran Orogeny (collision) in Early Silurian time and a postulated oceanic plateau which technically underplated basement east of the S-I line also during the Benambran Orogeny (Scheibner 1985). The pre-Benambran metallogenesis is dominated by the development of the Molong Volcanic Arc which, on the basis of its shoshonitic character, is thought to have developed on continental-type crust, perhaps the Molong microcontinent. The shoshonitic sequences host large porphyry (to epithermal) copper-gold deposits, medium to large epithermal gold deposits, and minor copper deposits. The post-Benambran tectonic history is dominated by either terrane dispersal or complicated back-arc development prior to the Kanimblan Orogeny in the Early Carboniferous which converted the LFB into a neocraton. The main metallogenic associations and some examples are: Volcanogenic: medium to large Kuroko-type massive sulphide deposits in Middle to Late Silurian rift or basin sequences; small to medium epithermal gold deposits in Mid to Late Devonian felsic rocks (although some deposits may be structural and related to Early Carboniferous deformation). Granitic: medium to large tin (-tungsten) deposits associated with Silurian to Early Devonian S-type and leuco-granitoids in the west of the LFB, modest molybdenum and copper-lead-zinc-silver deposits associated with Early Devonian I-type granitoids in the east and Early Carboniferous I-type granitoids in the northeast of the LFB. Mafic-ultramafic: primary, lateritic and alluvial platinum deposits associated with Alaskan-type mafic to ultramafic composite intrusions in a well defined belt in the Fifield region (the age of the intrusions is uncertain and they could be pre-Banambran); small podiform chromite cumulate deposits in serpentinite tectonically emplaced along the eastern margin of the Tumut Trough. Structural: large gold and base metal deposits formed by metahydrothermal processes during closure of the Cobar Basin in Early Devonian time in the west of the LFB; gold deposits possibly associated with deep crustal structures (eg the Gilmore Suture) and imbricate thrusts formed in thin skinned tectonic environments (eg Budawang Thrust System, (?) Parkes Thrust (Glen and Lewis 1990)); small to large metahydrothermal quartzvein and saddle reef deposits in folded sedimentary rocks (eg Hill End); medium to large gold deposits associated with fault bounded serpentinite (eg Lucknow). References Scheibner, E., 1985 Suspect terranes in the Tasman Fold Belt System, Eastern Australia. In: D.G. Howell (Editor), Tectonostratigraphic Terranes in the Circum-Pacific Region. Circum-Pacific Council for Energy & Mineral Resources. Earth Science Series. 1. 493-514. Glen, R A , and Lewis, P.C., 1980 The Budawang Synclinorium: a mineralised thrust system with implications for the Carboniferous (and younger) tectonics of New South Wales. New South Wales Geological Survey - Quarterly Notes 81.1-16. Published with the permission of the Director General, NSW Department of Minerals and Energy.

GEOLOGY OF THE BROWNS CREEK COPPER-GOLD SKARN DEPOSIT, BLAYNEY N.S.W. J. A. Surman1 and I. Kjolle2 1 BHP Gold Mines LTD, Browns Creek, P.O.Box 57, Blayney, NSW 2799. 2 Department of geology, ANU, GPO Box 4, Canberra, ACT 2601. The Browns Creek copper-gold skarn deposit is situated 8 km west of Blayney N.S.W., and is estimated to have produced about 60,000 oz of gold in the mining period 1870-1914. The current phase of mining began in 1980 and known reserves will allow open cut production until the end of 1992, by which time total mine production will have been approximately 2 million tonnes at 4 g/t gold and 0.3 % copper. The deposit is located within the northern part of the MolongSouth Anticlinorial Zone, an Ordovician-Silurian tectonic unit within the Lachlan Fold Belt. The host rocks include the Blayney basalt (a new name replacing the old term "Angullong Tuff (Doone Wyborn, pers. comm.)); the Cowriga Limestone which occurs as a localized unit within the Blayney Basalt; and the Long Hill Granodiorite intruding the volcanics and limestone. Gold and copper mineralization occurs in skarn and clay zones developed at volcanic-limestone or granodiorite-limestone contacts, or in structurally favourable zones of shearing and faulting within these lithologies. The character of the skarn varies widely laterally, but less vertically. Which of the major skarn minerals wollastonite, grossular-andradite and diopside-hedenbergite dominate, and their textural relationship, seems mainly to depend on which host rock has been replaced. Gold is intimately associated with the major sulphides, bornite and chalcopyrite, and less commonly in microfractures through the skarn without sulphides. The clay ore comprised nontronite clay and clay breccias containing weathered fragments of all lithologies. It occured within 70 metres of the surface, clearly associated with major shear zones and maybe related to karst weathering of the limestone. It has recently been mined out. Petrography and geochemistry currently being undertaken by Idunn Kjolle, as part of a Ph.D.-study at ANU on the genesis of the Browns Creek deposit, will be presented during the poster session in the context of detailed mapping carried out by mine staff and will comprise the following: Petrography of host rocks and ore; paragenetic sequence of various stages of skarn development and destruction; changing composition in time and space of prograde and retrograde skarn and opaque minerals; integration of petrographic study with C. Perkins' dating work on the deposit; geochemical analysis by XRF of ore, altered and relatively unaltered intrusive phases in the pit, and of the Long Hill and Carcoar Granodiorites, both of which from mapping appear to belong to the same intrusive phase.

SOME ASPECTS OF C A M B R I A N AND O R D O V I C I A N T E C T O N I C S IN T H E LACHLAN FOLD B E L T IN V I C T O R I A AND S E NEW S O U T H W A L E S A.H.M.

VandenBerg

Geological Survey of Victoria, PO Box 173, East Melbourne, Vic.

3002

Whilst the early history (Cambrian-Ordovician) of the Lachlan Fold Belt has received much recent attention, there is still very little agreement on the plate-tectonic setting of various stratotectonic units. On the mainland the information on the Cambrian especially is sparse. Age control is completely lacking from the supposedly Cambrian rocks in W Victoria and W NSW. In W Victoria, the small proportion of greenstone that is exposed belongs to several associations, including boninitic and calc-alkaline andesitic rocks, but not M O R B type basalts (Crawford, 1988). These are overlain by a richly unfossiliferous sequence of continent-derived quartz-mica turbidites (St Arnaud beds). These are inferred to be Late Cambrian. The undoubtedly Cambrian greenstone belts in central Victoria contain a melange of volcanics that include M O R B type basalts, boninites and their associates, and a calc-alkaline andesitic suite. These are overlain by reworked, greenstone-derived sediments, pelagites and minor limestone (Crawford, 1 9 8 8 ) - - a l l consistent with a setting well away from a continent. Some at least appear to be fragments of true oceanic crust. They occupy duplexes, which structurally overlie younger rocks the whole forming a large imbricate fan system (Fergusson et al., 1986). In the Heathcote belt, the Cambrian pelagites are overlain by several km of very monotonous (both laterally and vertically) but richly graptolitic quartz-mica turbidites, which span the whole of the Ordovician (Cas & VandenBerg, 1988). Farther east in central Victoria, however, in the Mount Easton and Wellington River belts, the Ordovician consists of two different portions: a thick, poorly fossiliferous Lower Ordovician quartz-mica turbidite pile, and a thin, richly graptolitic pelagite (mostly black shale) spanning almost the entire Upper Ordovician (Cas & VandenBerg, 1988). These two contrasting units have an extraordinary distribution, eastwards to the east coast, and northwards to at least as far as Goulburn (Fergusson & VandenBerg, 1990; Lewis, Glen, VandenBerg, Stewart unpubl.). They have suffered considerable shortening (with transport towards the SW, S and E ) (Glen & VandenBerg, 1987; Fergusson & VandenBerg, 1990). When removed, it gives a minimum extent of 500 km along strike, and 800 km across strike (assuming 5 0 % shortening)--several times the present extent of the entire Adelaide Fold Belt! The extraordinary extent of the turbidite sheet and its lithological uniformity suggests that it is part of a single terrane, with its source lying far to the west and southwest, in the newly folded^ and uplifted Adelaide Fold Belt, the Tyennan block (Tasmania) and Northern Victoria Land (Antarctica). The Early Ordovician shoreline lay close to the shallow marine sequences of Scopes Range and Bynguano Range in western NSW. In NSW this turbidite sheet is broken by the 'Molong Volcanic A r c ' but this appears to be exotic: it has contributed nothing to the turbidites, which are essentially quartz-rich and contain only a few per cent of igneous grains. It is suggested that this exotic terrane came from the north, and was emplaced into its present position during the Silurian. This emplacement was part of the Silurian deformations (Benambran-Quidongan 'orogenies') and gave rise to the anomalous belt of arcuate structures that have their focus in the southern 'bow' of the exotic terrane, which is located in Limestone Creek east of Benambra. The

transport direction on thrust faults and asymmetric folds is consistently away from the 'bow', rather like bow-waves: towards the SW at Tabberabbera in the SW (Fergusson et al., 1987); towards the S at Buchan and Reedy Creek in the S (Orth et al., in prep.) and towards the SE in the eastern part of the Yalmy Fold and Thrust Belt (Glen & VandenBerg, 1987). References: Cas, R.A.F., & VandenBerg, A.H.M., 1988: Ordovician, in Geology of Victoria. Geol. Soc. Aust. Victorian Div., pp. 63-102. Crawford, A.J., 1988: Cambrian, in Geology of Victoria.

Ibid., pp. 37-62.

Fergusson, C.L., Gray, D.R. & Morand, V., 1987: Tectonostratigraphic terranes, fold-thrust zones and regional metamorphics in the Palaeozoic of central-eastern Victoria. Geol. Soc. Aust. Specialist Group in Tectonics and Structural Geology field guide, 50 p. Fergusson, C.L., & VandenBerg, A.H.M., 1990: Middle Palaeozoic thrusting in the eastern Lachlan Fold Belt, southeastern Australia. J. Struc. Geol. 12, pp. 577-589. Glen, R.A., & VandenBerg, A.H.M., 1987: Thin-skinned tectonics in part of the Lachlan Fold Belt near Delegate, southeastern Australia. Geology 15, pp. 1070-1073. Orth, K., VandenBerg, A.H.M., Nott, R.J. & Simons, B.A., in prep. Geology of the Murrindal 1:100,000 geological map. Geological Survey of Victoria Report.

DIVISION OF THE TASMAN OROGENIC SYSTEM INTO GEOPHYSICAL DOMAINS, USING THE PATTERN OF GRAVITY AND MAGNETIC ANOMALY TRENDS

Peter Wellman

Bureau of Mineral Resources, P.O. Box 378, Canberra ACT 2601

The Tasman Orogenic Zone is divided into twelve geophysical domains. The domains are discrete regions with a consistent dominant trend in both gravity and magnetic anomalies. The relative age of the structures in adjacent domains is shown by the trends in the younger domain cutting across the ends of trends of the older domain. With few exceptions the domains become younger eastwards, consistent with an eastward growth of the Australian continent by successive cratonization or accretion of cratonized fragments. Most domains can be divided lengthways into three bands; a reworked band formed by partial reworking of the edge of the adjacent older domain; a band with strong long-wavelength anomalies that are due in part to change in crustal structure and to bands of igneous and metamorphic rocks associated with the boundary between domains; and a distal band with weak long-wavelength anomalies. This band structure is best developed where adjacent domains have trends at high angles, and part of the older domain has been removed or reworked. The older part of the Tasman Orogenic System has this band structure poorly developed, while the younger eastern part it is well developed, so there is likely to be some difference in the mode of crustal development in the two parts. Parts of the Tasman Orogenic System have previously been divided using the composition of granites, using structure, and using the character of the seismic reflection profile of the non-sedimentary part of the crust. The position of these boundaries agree well with the boundaries of the domain subdivisions discussed here.

A granite classification for the economic geologist, by A.J.R. White1, D. Wyborn2, & B.W. Chappell3. 1 Apothecary's Hall, Cox Street Port Fairy Victoria 3284 2 BMR Geology & Geophysics, PO Box 378 Canberra City ACT 2601 3 Department of Geology, ANU GPO Box 4 Canberra ACT 2601 Granites can be divided into eight different types depending on (i)their source-rocks, (ii) whether or not they contain restite, and (iii) whether or not they are capable of undergoing fractional crystallization. Only three of these types are important progenitors of metallic mineral deposits, and those three types all owe most of their compositional variation to fractional crystallization. Granite liquids which contain abundant restite cannot evolve until almost all restite is removed by various crystal-liquid separation processes. They therefore cannot concentrate metals, and more particularly volatiles, which we regard as fundamental to the formation of granite-related ore deposits. Granites which have a large proportion of their liquid crystallizing over a relatively narrow temperature range are also restricted in their ability to undergo fractional crystallization. Hiqh temperature, K-rich granite liquids, such as those of the Boggy Plain Supersuite of the Lachlan Fold Belt, and liquids rich in volatiles such as F, B, Li are capable of undergoing fractional crystallization, because (i) they have a large range of crystallization temperature, and (ii) there are large density differences between primary liquid and differentiated liquid. Fractionation of the liquid is probably achieved by the process known as convective fractionation, where the physical properties of primary and derivative liquids differ to the extent that they cannot easily mix. Thus convective fractionation produces liquids that are progressively more enriched in volatiles and in important trace metals such as Li, Be, Sn, Mo, Sb, Ta, W, Bi, Pb, U. Examples of all granite types can be found in the Lachlan Fold Belt, but the relative proportions of each type is quite different from that of younger continental margin batholith terranes which are supposed to be related to subduction at consuming plate margins. The implication is that LFB granites are not the products of subduction processes. The collection of enough information on a granite to assign it to one of the eight types will allow considerably focusing of exploration activity and reduced exploration expenditure.

INHERITED AND DETRITAL ZIRCONS—CLUES TO THE EARLY GROWTH OF CRUST IN THE LACHLAN FOLD BELT I.S. Williams , B.W. Chappell , M.T. McCulloch and K.A.W. Crook 1 Research School of Earth Sciences, The Australian National University, GPO Box 4, Canberra, 2 Department of Geology, The Australian National University, GPO Box 4, Canberra, ACT 2601 The chemical and isotopic compositions of the mid-Palaeozoic granites of southeastern Australia reflect those of their source rocks, partly because most of the granite magmas, particularly the more mafic ones, carried with them unmelted source material, or restite (Chappell et al., 1987). From chemical and isotopic analyses of the granites it is therefore possible to infer not only what rock types might have formed their protoliths, but also the protoliths' age. Isotopic data from the Kosciusko, Berridale and Bega batholiths, near the southern end of the Lachlan Fold Belt, consistently point to the granites' source-rocks being dominated by isotopic systems that are as old as mid to late Proterozoic. The S-type granites are rich in inherited zircon having mean U-Pb ages between 1950 and 1350 Ma, a source-rock Rb-Sr isochron calculated from some of the I-type granites suggests a mean source age of about 1100 Ma (Compston and Chappell, 1979) and the NdoM model ages for granites from the Berridale and Kosciusko batholiths are in the ranges 1450 to 750 Ma and 1550 to 1300 Ma for the I- and S-type granites respectively (McCulloch and Chappell, 1982). New Nd analyses from the I-type Bega Batholith give Nd M model ages in the range 1900 to 750 Ma and, together with new Sr isotopic analyses, show those granites to become progressively more isotopically evolved westwards. A limitation of all these isotopic techniques is that they cannot resolve the ages of individual components within the source rocks. That can only be done by dating a restite component, such as the inherited zircon, directly. Inherited zircon is present in most of the granites as small, commonly indistinguishable, cores within zircon crystals precipitated from the magma. Using the SHRIMP ion microprobe it has been possible to measure the Pb-U-Th isotopic composition of more than 1000 zircons from 23 plutons and enclaves and thereby begin to establish the ages of the principal components in the granites' sources and how those components, and their relative abundances, might change geographically. . The granites range in age between 440 and 380 Ma. Zircons with inherited cores are rare in the I-type granites, but in the S-types virtually every zircon contains an older core. Assumptions must be made about the Pb-loss history of each core before its age can be inferred, but nevertheless a self-consistent pattern of ages has emerged. In most granites most inherited cores are 650 to 450 Ma old, there is a lesser group with ages between 1075 and 800 Ma, and a lesser group again that is up to 3350 Ma old but with few obvious modes in the age distribution. Not all zircon age groups are represented in each granite, and there is a marked lack of the two older groups in the I-type granite enclaves. There is little difference between the I-and S-type granites in the relative abundances of the various inherited zircon age groups, except in the modes in the oldest group. Zircons extracted from four Ordovician sandy mudstones, country rock to the Bega Batholith, also have been dated. They show the same age groups as the inherited zircons in the granites. The relative proportions of the groups differ widely between the mudstones, but on average the 650-450 Ma group is less abundant in the sediments than in the granites. The inherited zircons in the granites are unlikely to be derived directly from the Ordovician country rock. Rather, it appears that 1075-800 and again 650-450 Ma ago there was considerable igneous activity in southeastern Australia that both formed the sources for the I-type granites, probably by underplating, and generated much of the detritus which formed the sources for the Stype granites and, after recycling, much of the early Palaeozoic flysch. The presence of the 11003350 Ma inherited zircon in many of the I-type granites is evidence for a small component of sediment in their source. The chemical and isotopic trends across the Bega Batholith suggest that the amount of sediment in the source increases westwards. References Chappell, B.W., White, A.J.R. & Wyborn, D., 1987. J. Petrology, 28,1111-1138 Compston, W. & Chappell, B.W., 1979. in McElhinny (ed.) The Earth: Its Origin, Structure and Evolution, Academic Press, London, 377-426. McCulloch, M.T. & Chappell, B.W., 1982. Earth Planet. Sci. Letts, 58, 51-64. 1

STRATABOUND SKARN OR VOLCANOGENIC MASSIVE SULPHIDE? SOLUTION TO THE DILEMMA AT LEADVILLE, NORTHERN LACHLAN FOLD BELT, N.S.W. B. R. Willott1 and P.M. Ashley 2 1. Plutonic Resources Ltd, 12 Kings Park Road, West Perth, W.A. 6005 2. Department of Geology & Geophysics, University of New England, Armidale, N.S.W. 2351 Extensive exploration of Zn-rich sulphide deposits at Leadville in the northern part of the Lachlan Fold Belt, N.S.W. over two decades has been largely on the premise that the mineralisation represents felsic volcanic-associated massive sulphides. The major deposits are hosted in a sequence of Silurian felsic volcanics, calcareous and psammopelitic sediments. The rocks have been regionally metamorphosed to lower greenschist grade and subsequently overprinted by a contact metamorphic event related to the emplacement of the nearby Carboniferous l-type Gulgong Granite. Field, geochemical, mineralogical and isotopic evidence indicates that the mineralisation is of metasomatic skarn type derived from limestone replacement and genetically related to the Gulgong Granite. We consider that a stratotectonic setting suggestive of a contact metamorphosed volcanogenic massive sulphide system is merely coincidental. Compelling evidence for an epigenetic replacement origin of the Leadville deposits includes: 1 ) Proximity to evolved granitic source rocks and reactive carbonate horizons. A prominent aeromagnetic anomaly interpreted to indicate a buried intrusive lies immediately northwest of the major mineralisation and hydrothermally altered microgranite dykes occur to the southeast. 2 ) A typical skarn mineralogy dominated by prograde grandite garnet, magnetite, quartz and minor clinopyroxene, overprinted by retrograde epidote, chlorite, calcite, clay, fluorite, magnetite, hematite and sulphides. The latter are coarse grained, essentially non-banded and dominated by Fe-rich sphalerite and pyrite, with minor galena, chalcopyrite and ?cosalite. 3 ) Lack of textural or lithological indications of an exhalative origin, e.g. no fine laminations, no complimentary siliceous or baritic exhalites. 4 ) Primary geochemical characteristics (e.g. Zn, Pb, Cu, Ag, Au, Ba, Mn, As, Sb relationships) are consistent with Zn-Pb skarns and atypical of Lachlan Fold Belt VMS deposits. 5 ) Distinctive gossan geochemistry and mineralogy, unlike Lachlan Fold Belt VMS gossans. 6 ) Lead isotope ratios ( 2 0 6 P b / 2 0 4 P b = 18.37 - 18.40) are significantly more radiogenic than signatures for Palaeozoic Tasman Fold Belt VMS deposits ( 2 0 6 Pb/ 2 0 4 Pb = 18.03 - 18.17). 7 ) Sulphur isotope values (3 34 S = 1.4 - 6.0 per mil) suggestive of an uncontaminated, single , magmatic source of hydrothermal sulphur. These are generally lighter than values from Lachlan Fold Belt VMS deposits. When viewed initially, aspects of the Leadville mineralisation appear contradictory. However careful application of field and laboratory criteria lead to the conclusion that a replacement origin akin to the Zn-Pb skarn model of Einaudi et al. (1981) is entirely consistent with the data. An origin by submarine exhalative processes appears implausible. Reference: Einaudi, M.T., Meinert, L.D. & Newberry, R.J. (1981). Skarn deposits. ESQIL GPOI. 75th Anniv. Vol.. 317-391.

DEEP CRUSTAL SEISMIC IMAGING OF THE CRUST IN THE WESTERN LACHLAN FOLD BELT, CENTRAL VICTORIA C J. Wilson , D. R. Gray and T J. Barton 1 School of Earth Sciences, The University of Melbourne, Melbourne, 3052, Australia. 2 Department of Earth Sciences, Monash University, Melbourne. Victoria. 3168. 3 Bureau of Mineral Resources, Canberra, A.C.T., 2601, Australia. Seismic reflection profiling in the southern Lachlan fold belt Australia indicates a composite and structurally complex crust with mid- to lower crustal detachments possibly associated with crustal-scale duplexing. Low angle (<20° on nonmigrated sections) detachments either intersect or sole into the crust-mantle boundary, forming a crustal-stacking wedge typical of lithospheric- or A-type subduction. Upper crustal reflections show that major, moderately to steeply dipping reverse faults exposed at the surfaceflattenwith depth and have listric form. The chevron-folded quartz-rich turbidite sequence of the Lachlan fold belt is therefore allochthonous with respect to the lower crust In accord with granite isotopic data and geochemistry the imbricated lower crust is probably thinned Proterozoic? continental crust. Like the northern Appalachian maritimes, the Lachlan fold belt has a composite crust with diverse lower crustal blocks (up to 20 km thick, 200-300 km in length and width) overlain by allochthonous, imbricated terranes of the surface geology. Such belts appear to show seismic evidence for deformation of the entire crust, involving crustal-scale delamination of the type proposed by Oxburgh (1972). The survey consisted of two east-trending traverses, 6 and 8 km in length, located in the immediate hanging wall of the Heathcote fault zone some 15 km north of the township of Heathcote in central Victoria. Geological Setting: The Heathcote fault zone (HFZ) defines the eastern margin of the chevron-folded Bendigo-Ballarat structural zone (Gray, 1988). It has a width of up to 4 km, a strike length of 100 kms and represents the leading edge of an imbricate fault system in the Bendigo-Ballarat zone. The HFZ juxtaposes Cambrian against Devonian quartz-rich turbidites and shows varying structural style and complexity along its length. It is bounded on the east by the steeply west-dipping Mount IdaMclvor and Mount William reverse fault system, and has a west-over-east sense of movement and a throw of about 14 km. Within the zone a sequence of Cambrian andesites, boninites, tholeiitic dolerites and basalts, siliceous shales, and volcanogenic sedimentary rocks is variably deformed. The metavolcanic rocks are foliated adjacent to these faults and are altered to foliated talc rock, actinoltic rock, quartz-magnesite rock and minor serpentinite. Results: The test survey produced a 20 s two-way traveltime (TWTT) reflection profile of the crust. Four groups of reflections are apparent on the stacked section. An upper level west-dipping reflection extending from 1.5 to 4 s TWTT, is the subsurface continuation of the Heathcote fault zone which flattens with depth. This reflective zone cuts at a low angle numerous weak, laterally short W dipping reflections between 1.5 and 4.5 s TWTT, which characterise the chevron-folded sequence in the hanging wall and footwall of the Heathcote fault. The middle section of the profile consists of a zone of E dipping reflectorsfrom4.5 s to 8 s TWTT followed by a zone of W dipping reflectorsfrom8 s to l i s TWTT. These appear to be a series of fault-bounded imbricate slices linked above and below by thrusts (i.e. a lower crustal duplex). The overlapping nature of groups of these reflections and the slight discordances between them suggest that they are more likely to be structurally controlled rather than representing petrologic variations in the lower crust. The nature of this crust is unknown, although granite geochemistry suggests sialic to ensimatic compositions (Chappell et al., 1988). The base of reflections in the lower crust (8.5 s TWTT on the east and 11 s TWTT on the west) appears to correlate with the seismic Moho at about 36 km (Gibson et al., 1981). A quiet region immediately beneath the Heathcote fault zonefrom2 to 9 s TWTT on line 89-09 is most likely due to a low signal to noise ratio caused by windy conditions when these shots were fired. References Chappell, B.W., White, A.J.R. & Hine, R. 1988. Aust. Jour. Earth Sci., 35, 505-524. Gibson, G., Wesson, V., and Cuthbetson, R., 1981. Jour. Geol. Soc. Aust., 28, 341-356 Gray, D.R. 1988. in 'Geology of Victoria' (eds.J.G. Douglas & J.A.Ferguson), 1-36. Oxburgh, R.E., 1972. Nature, 239, 202-204. 1

STRUCTURAL EVOLUTION OF THE ARARAT-STAWELL REGION C.J.L. Wilson , T.M. Will , R. Cayley and S. Chen School of Earth Sciences, The University of Melbourne, Parkville, Vic. 3052, Australia. Geological Survey of Victoria, 115 Victoria Parade, Fitzroy, 3065, Australia. 1

The SW margin of the Lachlan Fold Belt contains evidence of deformation events (DiD3) that predate the emplacement of Devonian granitic intrusions. Synchronous with regional chevron shaped folds are movements along discrete decollement surfaces and the development of discrete fault-bounded high strain zones. These are associated with emplacement of slivers of Cambrian meta-volcanic units at higher stratigraphic levels within the Cambro-Ordovician turbidite cover. A strain analysis, using deformed amygdales preserved in the meta-volcanics, reveals strong flattening and extension (<9:1) parallel to a prominent south-west dipping lineation which defines the transport direction of the thrusting. Further deformation that is confined to steep dipping shear zones (D4) is associated with the emplacement of the gold mineralisation in the Cambro-Ordovician sequence and the deformation of the Silurian Grampians Group. Faulting in the region is predominately reverse but contains a strike-slip (D5) component. Some late stage normal faults (D6) are present. Using the robust regression technique of Will and Powell (1990) palaeostress tensors have been calculated from field observations. Robust regression has the advantage over the commonly used least-squares estimate that it is able to identify influential outlier data which would, if not detected and removed from any data set, generally spoil any palaeostress analysis. The palaeostress field responsible for the different faulting events changed gradually from a NE-SW towards a N-S directed regional compressional stress field. The style of faulting and consistent eastward transport is consistent with the development of an allochthonous crust developed by tectonic underplating of lower crustal blocks. The entire sequence has been affected by a mid-greenschist facies regional metamorphic event. Peak-metamorphic conditions have been inferred from meta-volcanic rocks and mafic units; derived from a strongly differentiated tholeiitic suite. These have been calculated to be 1.7±0.7 (2a) kbar and 450±20 (2a)°C for mafic volcanic rocks from Stawell. Reference Will, T.M. and Powell, R. (1990). A robust approach to the calculation of palaeostress directions from fault plane data. Submitted to J. Struct. Geol.

EARLY SYN-TECTONIC ORIGIN FOR BEDDING-PARALLEL (GOLD)-QUARTZ VEINS IN PALAEOZOIC TURBIDITE SUCCESSIONS Jacqueline Windh1 and Marcos Zentilli2 1 Key Centre for Strategic Mineral Deposits, The Geology Department, The University of Western Australia, Nedlands, Western Australia, Australia 6009

department of Geology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5 Bedding-parallel quartz-vein systems are common in deformed Palaeozoic turbidite successions; in some cases they host significant economic gold deposits. The common, and presumably genetically most significant, factors of such vein systems (which include both bedding-parallel (including "saddle reef") and co-genetic cross-cutting veins) can be derived by comparison of a number of examples, from (a) the Bendigo-Ballarat goldfield (production ca. 7501 Au) and (b) the Hill End goldfield (production ca. 561 Au), both within the Lachlan Fold Belt, eastern Australia; (c) the Meguma goldfield, eastern Canada (production ca. 601 Au); and (d) the Ogofau Mine in the Welsh Basin, Wales (production 1.21 Au). The vein systems show striking similarities in the following aspects: 1. The veins are hosted by Palaeozoic (Cambrian to Devonian) turbidite successions, 2. The host rocks have been affected by only one major tectonic event, which produced tight to isoclinal upright folds and metamorphosed the sedimentary rocks to greenschist grade, 3. Bedding-parallel veins occur most commonly within, or at the margins of, mudstone (now slate) beds, 4. Bedding-parallel veins have been folded and boudinaged during the main deformation event, and therefore must have been emplaced relatively early in the tectonic history, 5. Some bedding-parallel veins and some of the co-genetic discordant veins (in particular at Bendigo-Ballarat) were emplaced relatively late during folding, indicating that vein formation continued throughout the deformation, 6. Granitoids and lamprophyric rocks, where present, cut structural trends and produce contact-metamorphism of veins, and thus clearly post-date both deformation and vein emplacement, 7. Mineralogy of veins (outside of contact aureoles of granitoids) is relatively restricted, consisting generally of quartz, carbonate (calcite, ankerite and/or dolomite) chlorite, sericite, and sulphides (mainly pyrite, arsenopyrite, pyrrhotite with lesser base metal sulphides)±native gold, 8. Wall-rock alteration around veins is generally minor, and consists of weak carbonatization, and local sericite and arsenopyrite alteration immediately adjacent to veins, 9. The dominant type of primary fluid inclusions within the veins is low salinity (up to 5 to 10% NaCI equivalent), two phase H 2 0 -C0 2 (±CH 4 ) inclusions, 10. Temperatures of vein formation determined from fluid inclusion entrapment temperature calculations and from mineral composition geothermometry range from 260° to 430°C. The common features described above indicate that vein formation commenced early in the tectonic history, probably as a result of fracturing induced by supralithostatically pressured fluids evolved during diagenesis and/or metamorphism. Vein emplacement continued throughout the deformation history. Although some gold appears to have been deposited in the early stages of vein formation, in the richest deposits (Bendigo-Ballarat) much of the gold was clearly deposited during the latest phases of fold tightening. The source of the gold is still equivocal; however, field relations clearly indicate that granitoid and lamprophyre emplacement post-dates formation of the mineralized veins, so that spatially related intrusive rocks cannot be the source of the gold. Gold may have been scavenged from sedimentary and volcanic rocks both below and laterally displaced from the gold deposits and emplaced in favourable structural sites during metamorphism/deformation.

MID TO LATE PALAEOZOIC MAGMATISM AND CRUSTAL EVOLUTION IN THE TEMORA REGION, CENTRAL SOUTHERN NSW. R.J. Wormald, R.C.Price Victorian Institute of Earth and Planetary Sciences, Department of Geology, LaTrobe University, Bundoora, Vic. 3083 The igneous rocks of the Temora region, on the western margin of the Bogan Gate Synclinorium (BGS), define three magmato-tectonic events; 1/ Early Silurian monzodiorites, quartz monzodiorites and associated shoshonitic volcanics flank the western margin of the BGS near Temora. Geochemically the rocks have high abundances of IC2O, Ba, Sr and LREE and are depleted in Nb. The magmas from which these rocks crystallised are believed to have evolved by pyroxene crystal fractionation from a basaltic magma ascending through a thickened crust (30-40km) during the initial stages of rifting that formed the BGS. An age of 431 ± 2 Ma has been obtained by the 40Ar/39Ar method on a hornblende from a quartz monzodiorite north of Temora ( C Perkins pers. comm). The intrusion of these mantle derived magmas marked the beginning of an extensive Basin and Range style extensional / strike slip magmato-tectonic event across the LFB. 2/ The early Silurian Wallundry Suite, within the BGS north of Temora, is composed of plagioclase rich cumulate gabbros, diorites, quartz monzodiorites and fractionated granites. Mineral chemistry and whole rock geochemical trends suggest that the suite evoved by fractionation of plagioclase and mafic phases from a primary quartz monzonite magma. The high Na20, K2O, Zr and REE abundances of the primary magma are typical of A-type granites. A total rock Rb-Sr isochron age of 431 ± 4 Ma and an initial ratio of 0.7039 ± 1 are obtained from fresh granitic members of the suite and an ill defined age of 421 ± 13 Ma is obtained when data from all members of the suite is included on the isochron plot. Biotite (Rb-Sr) and amphibole (K-Ar) ages of 404 ± 4 Ma and 403 ± Ma respectively represent early Devonian deformation or an uplift event. The chemical and isotopic characteristics of the primary magma are best explained by partial melting of primitive CambroOrdovician tonalitic basement. The intrusion of tholeiitic magmas during rifting and high heat flow associated with crustal thinning and the opening of the BGS ware responsible for the high temperatures that led to partial melting. Unlike the majority of the LFB granites, magma mixing and restite unmixing were not important processes producing the range of rock types. 3/ Late Devonian biotite and hornblende-biotite (I-type) granites and aegirine-arfvedsonite peralkaline (A-type) granites, which crop out north of Temora, post date deformation of the Silurian granites and sediments. They are associated with rare dolerite and quartz syenite intrusions. The I-type granites, some with rapakivi textures, are potassic and have trace element characterisics transitional between Iand A-type granites. The peralkaline granites are high in Nb, Zr, and REE, similar to A-type granites, and have low Sr and Ba abundances and low Eu/Eu* indicating an origin by feldsparfractionation.A total rock Rb-Sr isochron from the I-type granites and least evolved of the peralkaline granites yields an age of 364 ± 2 Ma and an initial ratio of 0.7042 ± 2. The more evolved peralkaline granites were isotopically disturbed by post- magmatic sodium metasomatism. The observed range of rock types were formed by partial melting of a heterogeneous crustal source of tonalite, monzodiorite and quartz monzodiorite during injection of basaltic magmas into the crust, with the peralkaline granites coming from a source with a higher monzonitic component. The melts, being relatively dry, were able to move to high levels in the crust andfractionateduring transit This late Devonian magmatic event has not been previously recognised from the Temora region and has previously been recognised only in the Eden-Comerong-Yalwal Rift and the central Victorian magmatic provinces of the LFB.

The tectonic significance of Ordovician magmatism in the eastern Lachlan Fold Belt by D. Wyborn 1 1 BMR Geology & Geophysics, PO Box 378 Canberra City ACT 2601 The shoshonitic rock series is a suite of igneous rocks derived from the mantle and containing high K/Na plus low abundances of high field strength elements. It is commonly associated with calkalkaline igneous rocks, but in most modern examples a contemporary active subduction zone is not present. Mantle-derived shoshonitic magmatism in the Lachlan Fold Belt commenced in the Early Ordovician and abruptly terminated at the end of the Ordovician, when it was replaced by tholeiitic magmatism and the onset of wholesale melting of the lower to middle crust. Associated calkalkaline igneous rocks are insignificant. The shoshonites are widely distributed across the eastern part of the Fold Belt in a series of discontinuous belts and volcanic centres, the largest of which is the Molong belt extending from east of Cowra to Dubbo. The Molong and Parkes-Peak Hill belts can be traced on magnetic maps under the Eromanga Basin. The broad geographical distribution is not typical of the curvilinear belts of volcanics associated with subduction zones, and would be even greater if the intervening basins (or troughs) were unfolded. All rocks that have been analyzed to date are either clearly shoshonitic or show chemical features t h a t indicate shoshonitic affinities prior to alteration and metamorphism. In some areas lavas rich in olivine and clinopyroxene (absarokites) dominate, in others the m o r e evolved plagioclase-rich shoshonites are dominant. There is no evidence yet of either a temporal or geographical change in magma series. It is suggested that the Ordovician magmatism was not accompanied by coeval subduction as has been generally assumed. More likely, the mantle beneath the Lachlan Fold Belt was modified during an earlier subduction event, perhaps associated with the assembling of the b a s e m e n t blocks recognized from granite studies. The cause of melting in the Ordovician may be related to mantle overturning prior to the upwelling of unmodified asthenosphere. The hot (>1200°C) asthenosphere was the source for the Silurian tholeiites, and provided conductive heat for the melting of the lower to middle crust.

IMPORTANCE OF MAFIC INCLUSIONS IN THE PALAEOZOIC I-TYPE GRANITES OF THE LACHLAN FOLD BELT Yadong Chen School of Earth Sciences, Macquarie University, Sydney, 2109 To date, the geochemical data set on the mafic inclusions in the Palaeozoic I-type granites of the Lachlan Fold Belt (LFB) is the largest compared to the data available on comparable inclusions found in I-type granites of other fold belts around the world. Although many hypotheses have been put forwarded to explain the origin of these inclusions, these can be essentially placed into three lines of thought in terms of chemical compositions: direct upper mantle magma input, crustal derivation and mixture of mantle- and crustal-derived components. The geochemical data strongly suggest that most mafic inclusions (if not all) in the LFB are not directly derived from the upper mantle, nor can they be mixture of a simple two-component system. Instead the available data are more consisent with a crustal derivation. These data include low abundances of Ni (mostly <10 ppm) and Cr (mostly <30 ppm), high abundances of total REE (even higher than in the host granites), Rb, Pb, Th, U and Nb (these elements all have abundances similar to those in the respective host granites), high initial Sr/ Sr ratios (mostly between 0.706 and 0.710), negative eNd (between -3 and - 8) and inherited zircons. Different mechanisms in the crust may have been operative in the formation of the mafic inclusions in distinct granite suites. For example, mafic inclusions in some suites may represent cumulates or different batches of crustal partial melt. However, in many LFB suites (e.g., the Glenbog Suite of Bega Batholith), the data are more consistent with the inclusions representing the modified, refractory, source rock fragments. The modifications include both compositional and microstructural aspects. 87

The Sediment Hosted Nagambie Gold Deposit, Victoria, Australia ZHANLIN GAO, TEUNIS A . P . KWAK

Department of Geology, La Trobe University, Bundoora, Vic. 3083, Australia AND NICHOLAS MERRY

Perseverance Corporation Limited, Nagambie, Victoria, Australia The Nagambie gold deposit, Central Victoria, Australia, consists of approximately 7 million tons of ore averaging 1.2 grams/ton gold. Mineralization is largely confined to E-W, WNW NNE oriented shears and faults contained in a doubly plunging anticlinal dome of Devonian siltstones and mudstones. Approximately North - South trending fractures and faults, generally unmineralized and a few minor N-S trending folds represent a N-S trending anticline which was subsequently refolded along an approximately E-W axis. Known ore grades occur almost exclusively in limonitized fractures up to 60 meters below the surface and not in primary quartz-stibnite veins present. Varying degrees of silicification and mild argilic alteration accompanies ore as well as anomalous Sb and As values. Sulfides contents in primary ore are less than 1 vol. %. Five primary quartz veining stages occur, of which the first two (A,B) are barren crenulated to massive quartz, followed by stibnite-bearing veins hosting primary gold (C), stibnite+gold-bearing quartz stringers (D) and finally unmineralized veins (E). The earliest veins (A,B) give higher temperature (220-300°C) while later gold-bearing veins are lower temperatures (130-180°C). Salinities are also different: higher in the earlier veins (4.8 to 7:8 versus 1.0 to 8.8 NaCl equivalent). Stages A and B probably represent pre-ore veins largely oriented in the axial plane while the others are related to mineralizations. Ore grade area may simply represent weathered primary concentrations but limited deep drilling (to only a depth of 120 meters) has failed to identify ore in unweathered rock. The characters of high purity of gold (99.4%Au), its botryoidal nature and its close association to limonite suggest concentration was by surface leaching and concentration near the water table.

THRUSTING IN THE EASTERN LACHLAN FOLD BELT: GOULBURN-BUNGONIA REGION

Christopher L. Fergusson Department of Geology, University of Wollongong, P.O. Box 1144, Wollongong, NSW 2500, Australia

A field guide prepared for the 2-day pre-conference field trip of the LFB-91 - Tectonics and Metallogenesis of the Lachlan Fold Belt meeting, Canberra, February 1991.

INTRODUCTION Southeastern Australia contains the Lachlan Fold Belt, an enigmatic feature with a unique orogenic history related to complex interactions between Gondwana and the adjoining palaeo-Pacific Ocean (Scheibner 1985, 1987). The main problem encountered in understanding the tectonics of the Lachlan Fold Belt is to explain the conversion of the extensive Ordovician continental-margin deep-marine turbidite wedge into a craton by Late Carboniferous time, and how the widespread Middle Silurian to Middle Devonian silicic magmatism and convergence relates to this process. Cas (1983) and Powell (1984) have adopted a classic palaeogeographic approach to solve the problem and have argued that the Middle Silurian to Middle Devonian was a period of basin-and-range tectonics which Powell related to dextral transtension. The Middle Silurian to Middle Devonian history of the Lachlan Fold Belt, however, included widespread compressive deformation and a basin-andrange style of tectonics is therefore inappropriate. Recently, thrusting and detachment have been recognized as important processes in the deformation history of the Lachlan Fold Belt (Fergusson et al. 1986, Glen & VandenBerg 1987, Fergusson & VandenBerg 1990). This field guide examines the evidence for Middle Palaeozoic thrusting in the Goulburn-Bungonia region of the eastern Lachlan Fold Belt (Figs. 1 & 2). These structures are part of a zone of folds and imbricate thrusts (Bungonia-Delegate fold-thrust zone, Fig. 1) that occupies the eastern part of the Lachlan Fold Belt and whose development in the Silurian to Middle Devonian enabled crustal thickening from the Ordovician passive-margin turbidite wedge to a continental region by the Middle Devonian. STRATIGRAPHY The pre-Permian stratigraphy of the Goulburn-Bungonia region is dominated by three major successions (Fig. 3) : Ordovician turbidites and black shale (with rare Early Silurian shale), Late Silurian to Early Devonian mixed sedimentary and volcanic units with associated plutons, and Late Devonian quartzose clastics (Creaser 1973, Carr , et al. 1981). The Ordovician unit, incorrectly called the Triangle Group by Fergusson & VandenBerg (1990), is subdivided into three units. (1) A thick basal unit containing monotonous predominantly thick-bedded (50 cm - 2 m) turbidites with less abundant thin-bedded (< 50 cm) turbidites, amalgamated sandstones, mudstones and thin-bedded cherts. Recently Ian Stewart has discovered conodonts of Darriwilian to early Gisbornian age in cherts on the Goulburn-Bungonia road. (2) A relatively thin interval (probably only 200-300 m) with a monotonous succession of thin-bedded turbidites (stripy facies) with abundant thick-bedded turbidites

E23 S-D

Post—Carboniferous

cover

Mainly

A A i i A A

Silurian-Devonian successions O r d o v l c l a n v o l c a n i c s within the turbidite w e d g e

£ 3

Ordovlcian island

Hotham domain (with e a s t - w e s t t r e n d s ) 1 I

arc

O r d o v l c i a n turbidite

wedge

I (with Dceovmo nmi ao nn cSoi vl ue rr )i a n -

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Pre-Ordovlclan basement (oceanic) Gllmore Fault

Long Plain Fault S-D

l-S line

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F i g . 1. M a p of the eastern Lachlan Fold Belt in A u s t r a l i a and the location of F i g . 2 .

southeastern

Sydney Basin Upper Devonian quartzose clastics lLamoie Group) Lower Devonian conglomerate, silicic volcanics (Tarago Conglomerate, Murrays Flat beds) Lower Devonian clastics, volcanics (Tangerang Formation, Bindook Complex) Mainly Early Devonian granites (Some Carboniferous in east) Upper Silurian quartzose strata Upper Silurian silicic volcanics, clastics Ordovician quartzose turbidite units

Faults

Stratigraphic contacts (some faulted)

Road - stop in text

Fig. 2. Regional geology of the Canberra-Goulburn district with Stops for Day 1. Areas shown in more detail in Figs. 5 and 7.

(a) West (WWVWVWwWi Lambie o ~ ° O ° o Group Ni

Lambie Group

—"i —t

Bindook Volcanic ComplexTangerang Formation Bungonia Limestone

Murray s Flats beds

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Wollondilly bedsTowrang beds Lower Silurian shale

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- (3) Ordovician unit

Volcanics, volcaniclastics -r1000m

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Shale Quartz turbidites Unconformity ^ F — F a u l t e d contact

Fia 3 stratigraphic columns for the structural domains to the west (a) and east (b) of the Yarralaw Fault in the Goulburn-Bungonia region.

near the top of the unit. This unit has only been mapped in the Shoalhaven River area east of Bungonia. In areas of intense deformation, with strong cleavage development, units 1 and 2 are usually not separated. (3) The highest unit consists of approximately 300 m of black shale, siliceous mudstone and some turbiditic sandstone beds. Interbedded sandstone and mudstone, mainly thick-bedded turbidites, form a 80 m thick interval near the base of some sections bracketed between late Gisbornian and Eastonian black shales. A complete graptolite succession exists from the late Gisbornian (Gi) , through Eastonian zones Eal-Ea4, into the early Bolindian (Bo). This black shale is very distinctive and is mapped at many places throughout the Lachlan Fold Belt, although its stratigraphic significance is commonly not appreciated. East of the Yarralaw Fault the Ordovician succession is overlain with a low-angle unconformity by the Late Silurian to Early Devonian succession (Fig. 3) . This consists of basal limestone and shale of the Bungonia Limestone overlain conformably by volcaniclastics, quartzose clastics and primary igneous rocks of the Tangerang Formation and Bindook Volcanic Complex. The latter are extensively intruded by plutons of the Early Devonian Marulan Batholith (Carr et al. 1980). West of the Yarralaw Fault Early Silurian shale has an unknown but probably faulted relationship with Late Silurian quartz-rich turbidites and some limestone of the Wollondilly beds (Naylor 1950, Creaser 1973). Farther west again the Ordovician succession is faulted against a lithologically similar Late Silurian succession called the Towrang beds by Creaser (1973) . These are faulted against the Emsian Murrays Flats beds, containing intermediate to silicic volcaniclastics and volcanics (Creaser 1973), to the west. Early Devonian volcanic and volcaniclastic units are angularly unconformably overlain by shallowly dipping Late Devonian shallow marine strata of the Lambie Group. The poor exposure along the contact and the scarcity of bedding in the underlying units has prevented resolution of the angularity of this unconformity. STRUCTURE The structure of the region has been described in detail by Fergusson & VandenBerg (1990) and only a relatively brief, but updated, summary is given here. The Yarralaw Fault (Fig. 2) separates the Goulburn-Bungonia region into two structural domains. The domain to the west includes strata with a welldeveloped slaty cleavage whereas the domain to the east has less well-developed cleavage, especially in the Ordovician unit. Thrusting is considered important in both domains but is best displayed in the eastern domain. The Yarralaw Fault dips to the west and Late Silurian rocks

are thrust over Early and Late Devonian successions indicating $ L ; r ? S t S° m f m ? v e m e n t i n the Carboniferous deformation. The Yarralaw Fault is a member of a family of thrust faults that eastern Lachlan Fold Belt. To the south of Windellama the Yarralaw Fault is lost amongst poorly understood Ordovician strata and the thrust motion is probably taken up along other faults. Domain west of the Yarralaw Fault The structure of this domain consists of an anticlinorium (Naylor 1950) containing well-cleaved Ordovician and Silurian rocks with abundant tight to close mesoscopic folds. Thrust faults are exposed at several localities (e.g. Stop 10) and are westerly dipping as is the slaty cleavage. High strains occur and are indicated by rotation of fold axes to reclined attitudes and local intense cleavage development. On the macroscopic scale thrusts probably occur along the west side of Late Ordovician black shale slices and one Early Silurian slice that have been mapped by Creaser (1973) amongst what are now known to be Early to early Late Ordovician turbidites. The Late Devonian Lambie Group at Goulburn (Fig. 2) is a relatively undeformed shallowly dipping to flat-lying succession that forms the eroded keel of the Cookbundoon Syncline. This contrasts with the Syncline farther north where the Late Devonian rocks are tightly folded with a steep west-dipping overturned limb (Powell & Fergusson 1979). Domain east of the Yarralaw Fault East of the Yarralaw Fault in the Shoalhaven River area the distribution of the late Gisbornian to early Bolindian black shale (unit 3) indicates substantial fault repetition in the Ordovician unit. The overall structure is an E-verging imbricate fan. Strata are typically steeply dipping (>70 ) as are the axial planes of most minor folds, all major folds and all the main strike-parallel faults. All structuresotrend to the NNE and the folds all plunge less than about 20 . Folds have a chevron to flattened chevron style. An axial planar cleavage is rarely developed, and where found is a crenulation cleavage in mudstones superimposed on a common beddingparallel cleavage. The timing of the deformation in the Shoalhaven River area is poorly constrained as it has no associated major break in sedimentation. The minimum age is provided by the Early Devonian Marulan Batholith (Carr et al. 1980), which clearly cross-cuts and post-dates all steeply dipping structures in the Bungonia Limestone and the Ordovician unit. The contact between these latter two units is a low-angle unconformity probably marking the onset of deformation. In the Shoalhaven River area lack of a major break in the depositional record coincident with the deformation creates a paradox. Deformation and sedimentation have been synchronous. Magmatic activity in the region appears to have had a retarding affect

on the deformation which ceased with intrusion of the Marulan Batholith whereas it continued until the Middle Devonian to the west. Moderately deformed Late Devonian strata occur in a narrow belt to the east of the Yarralaw Fault. These rocks have a distinctive deformation style (see Stop 13) that postdates the main regional deformation. DISCUSSION Implications The discovery of widespread thrusting in the Bungonia-Goulburn region has the following implications for the structural and tectonic development of the Lachlan Fold Belt. (1) Upright buckle-controlled folding, usually involving flattening of chevron-type folds, has been regarded as the principal mechanism of deformation in the Ordovician turbidite successions of the Lachlan Fold Belt. Areas, such as Yalmy and the Shoalhaven River area (Glen & VandenBerg 1987), where distinctive well-dated black shales are repeated in many fault slices, demonstrate that thrusting is probably considerably more important in the deformation of the Ordovician turbidite unit than previously considered. In the absence of such welldated units geologists have assumed that they are dealing with a dominantly buckling-mode of deformation and that the stratigraphic succession is not repeated by thrusting. In poorly dated monotonous units it is very difficult to establish the stratigraphic succession and complications t produced by thrusting are not recognized. (2) The structure in the Shoalhaven River area is an easterly verging imbricate fan. Where detailed structural work has been undertaken in the Bungonia-Delegate fold-thrust zone an easterly vergence has been found (Powell 1984, figs. 192 and 208, Fergusson et al. 1986, Glen & VandenBerg 1987, Fergusson & VandenBerg 1990). This indicates a west-over-east sense of tectonic transport which is peculiar as in most orogenic belts the sense of transport is usually towards the craton except that locally developed in subduction complexes. (3) An understanding of the overall structure of the eastern Lachlan Fold Belt is hindered by the lack of fossiliferous horizons in the bulk of the Early and early Late Ordovician succession. The distribution of fault slices is complex and a progression of younger to older thrust slices away from the thrust front, as commonly encountered in foreland fold-thrst belts and accretionary subduction complexes, is absent. (4) Deformation in the Bungonia-Delegate fold-thrust zone must have commenced either in the period of disruption in the Early Silurian following the demise of the Ordovician continental margin sediment wedge or soon after in the Middle Silurian. The low-angle unconformity in the Shoalhaven River area

between the Ordovician unit and the Bungonia Limestone and l h e ^ n a Mudstone and Tombong beds at Delegate (Glen 987} records Z J * 9 ongoing deformation at this time. With the generation of crustally-derived granites in the Late Silurian and most abundantly in the early part of the Early Devonian (Powell 1984) it is clear that the deformation had caused substantial crustal thickening considering that at the v. Ordovician the crust must have consisted of prooacle oceanic crust overlain by several kilometres of quartz-rich turbidites (Fergusson 1987). Thus the timing of formation of the Bungonia-Delegate fold thrust zone is Middle Silurian to Middle Devonian. Crustal Evolution Fergusson & VandenBerg (1990) presented a crustal section depicting the crustal structure of the eastern Lachlan Fold Belt (Fig. 4). From cross-section UV (Bungonia Creek, Fergusson & VandenBerg 1990 fig. 8) shortening of 76% has been calculated. Taking this estimate as a representative approximation of shortening in the region, and an initial sedimentary package of about 5 km, then the major deformation accounts for a 20 km thick upper layer of the present crust. The lower crust probably consists of a Proterozoic maficsedimentary layer that provided a source for the granitic rocks (Chappell et al. 1988). The crustal evolution of the Lachlan Fold Belt is controversial with two main views: (1) that the Orogen developed on in situ Proterozoic crust that provided a source layer for the granites (Chappell et al. 1988) ; and (2) that the Orogen developed on oceanic crust and has subsequently been structurally emplaced above a suitable granitic source (Crawford et al. 1984). The structural style implies major detachment probably within the Cambrian succession and the lower part of the Ordovician unit and considerable tectonic thickening of rocks above the detachment, by thrusting and intra-thrust sheet shortening (mainly by chevron folding, e.g. buckling in the western Main Ranges of the Canadian Cordillera, Price 1981). The crustal section shown in Fig. 4 shows that the Cambrian-Ordovician complexes and overlying rocks were detached from their underlying lithosphere, which was presumeably subducted, and underthrust by a suitable granitic source. Tectonics Tectonic development of the Lachlan Fold Belt is poorly understood owing mainly to the unique history of the orogen romDared to other Phanerozoic orogenic belts (see recent review by Coney et al. 1990). Only a brief summary is given here with emphasis on the eastern Lachlan Fold Belt. Th^ Cambrian to Ordovician rocks are regarded as having formed ^ a finale tectonic regime, when much of the Lachlan Fold Belt was in an oceanic setting, that during at least the

Bungonia-Delegate West Gilmore Wagga-Omeo Fault Metamorphic Belt

fold-thrust

zone

Budawang Synclinorium

Fig. 4. Crustal section of the Bungonia-Delegate fold-thrust zone and regions further west (see Fergusson & VandenBerg 1990, fig. 1 for location). Lower crust in the eastern half of the section represents an oceanic plateau underthrust from the east.

South Coast Melange

mar r^2V1Cian 9inal to the Delamerian mountain chain of Gondwana. The tectonic development of this oceanic realm is uncertain, but may have included convergent plate boundaries J? Cambrian, which formed arc-related volcanism in Victoria (Crawford 1988) . m the Ordovician the only conceivable evidence for plate boundary activity is the Molong-Capertee arc andesitic volcanic pile, commonly regarded as an island arc (Powell 1984) . The relationship of these rocks to any Ordovician convergent margin is obscure and it is possible that they are an allochthon thrust over the Ordovician craton-derived turbidite wedge (Fergusson & VandenBerg 1990, Powell et al. 1990, p. 558).

The Early Ordovician quartzose turbidite succession and overlying Late Ordovician black shale were deposited in an intraplate oceanic environment adjacent to the Delamerian mountains which provided a source for the detritus. A relatively stable intraplate setting must have existed for at least 70 million years (probably longer) and was subsequently destroyed during the Silurian to Middle Devonian in a time of continuous orogeny in the Lachlan Fold Belt. This began with the Benambran Orogeny, during which the Ordovician turbidite succession was strongly deformed and metamorphosed in the Wagga-Omeo Metamorphic Belt. Subsequently, in the Late Silurian to Middle Devonian, the Bungonia-Delegate fold-thrust zone formed with widespread sedimentation, volcanic and Plutonic activity. One possible scenario is that subduction commenced east of the region in the Early Silurian and a major Proterozoic oceanic plateau choked the subduction zone and was underthrust throughout this period (Fig. 4). Atypical subduction developed with delamination of the Proterozoic crust from its underlying lithosphere which was subducted. The underthrust oceanic plateau provided a source for the granites and subduction of the delaminated lower lithosphere provided a heat source for melting of the lower crust. In the Late Devonian the region was covered by a widespread sheet of molasse derived from the craton which was deformed in the Early Carboniferous in an intraplate backarc setting west of a volcanic arc along the western side of the New England Orogen (Powell 1984, Murray et al. 1987). The deformation extended across the whole Lachlan Fold Belt and is similar to the Late Cretaceous-Early Tertiary Laramide Orogeny in the Rocky Mountains of western North America. The latter formed due to high rates of plate convergence which caused affects extending up to 1500 km inland behind the active plate boundary (Coney 1978) .

DAY 1 - CANBERRA TO BUNGONIA GORGE The field trip starts with a road traverse from Canberra to Bungonia before arriving at our major destination of the day, Bungonia Gorge. The geology of the Canberra region is dominated by a relatively undeformed Silurian volcanic and sedimentary succession, which is bounded to the east by a much more strongly deformed Ordovician unit. As for most of the Lachlan Fold Belt the Ordovician unit has a twofold subdivision into a lower turbidite succession and an upper black shale succession. STOP 1 - Capital Hill Unconformity on State Circle This stop occurs on State Circle to the east of Commonwealth Avenue and is Stop 5-6 of Crook & Powell (1976, p. 52). The late Wenlockian Camp Hill Sandstone unconformably overlies the late Llandoverian State Circle Shale. According to Crook et al. (1973) this unconformity marks the Quidongan Orogeny. Bolger (1982) , however, has questioned the validity of this event because the break at Benambra (i.e the time for formation of the Wagga-Omeo Metamorphic Belt) encompasses the whole Early to Middle Silurian interval which covers the time for deposition of both the rock units at this locality. Thus the Benambran Orogeny incorporates the Quidongan event which therefore is redundant. One feature of interest at this locality is the relatively weak deformation of both units, which contrasts to the much more deformed younger strata that are seen to the northeast. From here continue along State Circle and follow the signs to Queanbeyan and take the Braidwood-Batemans Bay road (14 km' from Stop 1). STOP 2 - Inverted Folds in Ordovician unit - Queanbeyan This stop occurs 2 km east of the intersection of the Yass and Bungendore roads on the Queanbeyan-Braidwood road and somewhere near Stop 5-7 of Crook & Powell (1976, p. 52). According to Crook & Powell the strata in these roadcuts are Late Ordovician but they probably are Early or early Late Ordovician. Two main deformations are evident: (1) a cleavage-forming deformation with abundant mesoscopic folds that are northerly trending with axial planes dipping steeply to the east; and (2) an earlier deformation that is responsible for the inversion of all strata in these roadcuts. The early folds are part of a recumbent fold system that lacks axial planar cleavage and has been mapped throughout the Cullarin Horst which extends about 10 km east of here to the Lake George Fault (Stauffer & Rickard 1967). The significance of these early structures is a mystery, although P.G. StuartSmith has recently identified similar structures in the Tumut region which he interprets as flower structures developed along strike-slip faults. The second deformation is part of the main regional deformation that formed the BungoniaDelegate fold-thrust zone.

here we follow the road east across the Cullarin Horst r^l uplifted ^ring Cenozoic reactivation of the Lake George Fault. The Lake George Fault (14.5 km east of Stop 2) rorms a prominent scarp that farther north has disrupted drainage patterns and formed Lake George.

STOP 3 - Mulwaree Fault - Queanbeyan-Braidwood Road This stop (39.4 km east of Stop 2) occurs on the BungendoreBraidwood road about 20 km east of Bungendore (GR 372971 Braidwood 1:100 000 Topographic Map). According to the Braidwood 1:100 000 Geological Map the Ordovician unit occurs west of the Mulwaree Fault and to the east is the Devonian Boro Granite, overlain by a veneer of Quaternary sediments (Felton & Huleatt 1975). In this developing road cutting the Mulwaree Fault (Fig. 2) consists of highly fractured and sheared fine-grained igneous rocks (probably Silurian and not Ordovician) with a moderate W-dipping shear fabric (see Fergusson & VandenBerg 1990, fig. 6b). Continue along the Braidwood road east 5.4 km and turn north at the T-intersection and drive to Tarago (21 km) . The route crosses rolling country on the Boro Granite east of the Mulwaree Fault which has also been reactivated in the Cenozoic era. In the Tarago district (Fig. 2) Henry (1975) has mapped a regionally extensive W-dipping slaty cleavage which has been traced to the Goulburn-Bungonia region. Henry also attributed this structure to thrusting and identified low-angle thrusts. From Tarago drive north through Lake Bathurst (6 km) and continue on for another 18.6 km to a T-intersection and turn right into Painters Lane. Drive east for 6.5 km to the main road junction and then turn southwards onto the GoulburnWindellama road and drive 5.1 km to the road cutting at Stop 4. These roads traverse the Gundary Plains which are underlain by the Early Devonian Murrays Flats beds (Creaser 197 3) , also known as the Gundary beds (Felton 1974). STOP 4 - Murrays Flats beds - Sweeneys Hill Stop 4 is a series of road cuttings on the Goulburn-Windellama road just east of Sweeneys Hill at GR 503322 (Goulburn 1:100 000 Topographic Map). These cuttings expose cobble conglomerate and quartzo-feldspathic sandstone of the Early Devonian Murrays Flats beds. The conglomerates consist of clasts of quartzite, lithic sandstone, mudstone and silicic volcanics Some of the clasts have a strong cleavageschistosity and were probably deformed prior to deposition of the conglomerate. These lithologies are similar to the Tarago Conglomerate which was deposited in a terrestrial environment fHpnrv 1978) The Murrays Flats beds consist of quartz-rich ian^tones * feldspathic sandstones, shales, conglomerates, limestonesanlesi?es, dacites, and volcaniclastics. Creaser M 973) found an early Emsian coral in this unit in a road cutting on the Hume Highway east of Goulburn Thus the Murrays Flats beds are also a similar age to the Emsian Tarago

Conglomerate. In both the Goulburn-Bungonia region and the Tarago district (Fig. 2) rock units as young as Emsian (Creaser 1973, Henry 1975) are involved in the thrust-related deformation and this shows that the deformation extended into the Middle Devonian. From Stop 4 drive north towards Goulburn for 16.5 km where the Goulburn-Bungonia road is intersected, turn right at the Yintersection and head east towards Bungonia. Stop 5 is 6.4 km along the road and is a road cutting opposite the property "Lark Hill". Stops 5 to 11 occur along the Goulburn-Bungonia road and their locations are shown on Fig. 5. STOP 5 - Murrays Flats beds - Goulburn-Bungonia road A t this stop (GR 552436 Bungonia 1:25 000 Topographic Map) more lithologies, including possible ignimbrites, of the Murrays Flats beds are seen in the road cuttings. Some weathered fine-grained igneous rocks and volcaniclastics occur in the cuttings at the Lark Hill turnoff with steeply dipping lithic sandstones and mudstones 300 m east of the Lark Hill turnoff. From here drive 2.1 km east to a junction with a property entrance on the north side of the road. The road cuttings on the south side of the road are Stop 6. STOP 6 - Towrang beds Folded strata of the Late Silurian Towrang beds, a quartzrich turbidite succession, occur at this stop (GR 569435 Bungonia 1:25 000 Topographic Map). Flutes are visible on the base of one bed indicating that the folds are upward facing. The folds are close with a steeply dipping axial planar cleavage and are part of the regional deformation. Generally, folds in the Towrang beds are either m-type or they verge to the west, in exposures with homoclinal strata the bedding-cleavage relationships indicate vergence to the west (Fig. 6) . These relationships are anomalous as they indicate that the Towrang beds underlie the older Ordovician unit. They could result from either compression of a pre-existing system of W-dipping listric normal faults or from backthrusting along an E-dipping fault. The contact between the Murrays Flats beds and the Towrang beds also appears faulted and with a similar geometry to the Towrang beds-Ordovician unit contact. From Stop 6 drive 1.5 km east up the hill and along the flat to the start of a long descent, Stop 7 is the road cutting at this point. STOP 7 - F 1 Folds in Towrang beds This cutting (GR 579425 Bungonia 1:25 000 Topographic Map)

Jerrara

Road Late Devonian

}

Lambie Group

Unconformity Silicic Early Devonian volcanics J Bindook Volcanic Complex Andesitic-silicic "I Early Devonian volcanics,clastics J" Murrays Flat beds Quartz sandstone,! Late Silurian mudstone J Towrang beds Quartz sandstone, mudstone,limestone Black shale, chert Quartz sandstone, mudstone,chert

Late Silurian Wollondilly beds Ordovician unit

Bedding

Younging determined Overturned

Early Silurian graptolites Bolindian graptolites

Eastonian graptolites Thrust fault

Fold axis

m-shaped folds s-shaped folds z-shaped folds

Anticlinorium CLF/DMM/GD1322

Fig. 5. Map of part of the domain to the west of the Yarralaw Fault (see Fig. 2 for location) . The major structure is an anticlinorium cored by the Ordovician unit (Ot) and containing a major fault marked by a zone of tectonic melange on its eastern limb (Stop 10) . The core of a synclinorium, which in Fig. 2 occurs in the Lambie Group, is sheared off by the Yarralaw Fault.

Anticlinorium

Fig. 6. Cross section AB from the domain west of the Yarralaw Fault. Dashed lines drawn on the cross section represent the S^ cleavage. Note that the Yarralaw Fault thrusts the cleaved Silurian Wollondilly beds (Slw) above the poorly cleaved Late Devonian Lambie Group (Dll) and the Early Devonian Bindook Complex (Deb) . The M marks the location of melange at Stop 10.

, 1- £ couples in the Towrang beds. The folds This upright structures with a vergence to the west, qtiiir^r Zi opposite to that expected for the location of this structure with respect to the anticlinorium that occurs to the ov ra] 1 ZZZZ'i. ® ; structure implies that the Towrang beds =Hnf Y u n d , e r l l e the Ordovician unit to the east (note * relationships are found in road sections north «?,? f I hGre)' ** exPlanation for these relationships was suggested at Stop 6. From Stop 7 drive 2 km east to the large road cutting on the south side of the road (Stop 8). STOP 8 - Cherts in Ordovician unit At this stop (GR 597425 Bungonia 1:25 000 Topographic Map) thin-bedded cherts form a thick horizon below thin-bedded turbidites (Bouma CDE beds). All strata are steeply dipping and truncations in Bouma C micro-crosslaminated layers indicate that the chert unit forms the core of an anticline. This same chert horizon occurs in several cuttings along the road and has been tightly folded (Fig. 5). The chert is intercalated with the Ordovician turbidite succession and presumably represents a period of sediment starvation and radiolarian ooze accumulation (perhaps resedimented by turbidites) on the Ordovician submarine fan. Ian Stewart (pers. comm.) has discovered conodonts of Darriwilian to early Gisbornian age in the cherts and Hiro Ishiga has extracted radiolaria from these rocks. From here continue east 1.9 km to the roadcut near the top of hill (Stop 9) . Note the prominent strike ridges north and south of the road formed by the bedded chert. STOP 9 - Reclined F-j^ folds in Ordovician unit The road cutting (GR 614424 Bungonia 1:25 000 Topographic Map) on the north side of the road has a steep W-dipping cleavage that is axial planar to some tight reclined F^ folds that are locally refolded by weak F~ folds. The F^ folds were probably rotated to steep plunges during the imposition of high strain on the rocks (Ramsay 1979). Continue another 1.5 km east to the major road cutting (Stop 10) .

STOP 10 - Melange in Ordovician unit A major fault zone is exposed in this roadcut (GR 628419 Bungonia 1:25 000 Topographic Map). Bedding has been obliterated throughout the exposure and the rock is a tectonic melange with pods of quartz sandstone contained in a matrix of scaly mudstone (Fergusson & VandenBerg 1990, fig. 6a). The scalv fabric in the matrix consists of polished fragments (1-5 mm! of cleaved mudstone with quartz slickenfibres. The pods of sandstone have pinched terminations indicative of ductile

extension and formation of pinch-and-swell structure prior to their disruption into separate pods. The rock has formed by brittle shearing of previously cleaved mudstone accompanied by ductile deformation of folded sandstone beds in a steep W-dipping fault zone that occurs on the eastern side of the anticlinorium. From here continue another 1.75 km east to the quarry on the south side of the road. STOP 11 - Bolindian black shale This quarry (GR 643414 Bungonia Topographic Map) contains Bolindian black shales, with abundant graptolites in fresh rock. Naylor (1950) found Early Silurian graptolites in shales exposed in low roadcuts several hundred metres east along the road. Continue 550 m east along the road to the prominent bend with a cutting on the north side of the road (Stop 12). STOP 12 - Overturned strata in Late Silurian Wollondilly beds In this cutting (GR 647412 Bungonia 1:25 000 Topographic Map) quartz-rich turbidites of the Late Silurian Wollondilly beds (equivalent of the Towrang beds) dip steeply to the west and truncations in Bouma C micro-crosslaminated layers indicate that the beds young east, i.e. they are overturned. The location of these strata on the overturned limb of the anticlinorium is also indicated by cleavage dipping shallower to the west than bedding. Throughout this area most strata are overturned to the east on the eastern limb of the main anticlinorium. The limb is truncated by the Yarralaw Fault and the Wollondilly beds are thrust over the Late Devonian Lambie Group. Continue 2 km to the roadcuts that occur along the rise beyond the creek crossing (Stop 13). STOP 13 - Folds in Late Devonian strata A strip of shallowly W-dipping* Late Devonian Strata occur east of the Yarralaw Fault and are usually weakly deformed with spaced cleavage found in mudstones in the cores of several mesoscopic folds on the Bungonia-Goulburn road (Fig. 5) . At Stop 13 (GR 663407 Bungonia 1:25 000 Topographic Map) more deformation is encountered than usually occurs in the normally shallowly west-dipping strata. As for other folds in this unit these are broad angular disharmonic structures with steep SW to WSW trending axial planes and shallow southwesterly plunges (Fig. 5). There is local oveturning of strata on some of the fold limbs and some bed disruption in thin-bedded mudstone. The extensive deformation occurring at this locality is attributed to a WSW trending narrow zone of deformation oblique to the Yarralaw Fault. These structures have a style consistent with their origin at a shallow crustal

c ^ L < ? S S O C l a ! ; e 2 W i t h 1 i^ited NNE-SSW shortening. The « V ^ ° U S T d t ^ o r m a t i o n w as a relatively mild event in this F o l d B e l t an P ^ L L f, <* anomalous in orientation as ^Powell 1984)1S d e f o r m a t i o n typically reflects E-W compression ™ H i ] ? U e D 3 ' 6 ^ e a s t t o Bun 9onia village and turn right on the road to Bungonia Caves, Stop 14 is 2.8 km along this road at a ma3 or cutting. STOP 14 - Tangerang Formation - Bungonia Caves road Massive quartz-rich sandstones and siliceous rocks form a prominent strike ridge in the Early Devonian Tangerang Formation. This formation consists of deep-marine, shallowmarine and subaerial volcaniclastics, volcanics and quartzrich clastics that conformably overlie the Late Silurian Bungonia Limestone. Folds occur in the cutting (GR 713400 Bungonia 1:25 000 Topographic Map). Continue 6 km northeast along the road to the boom gate, near the visitors centre (Bungonia State Recreation Area) and continue 1.7 km to the car park at Bungonia Lookdown (Fig. 7). Here, the route of Day 2 is observed in the creek below. The Lookdown is on the massive lower limestone of the Late Silurian Bungonia Limestone. East of the limestone strata of the Ordovician unit dip steeply to the east, apparently truncated at an unconformity surface, that is almost certainly faulted, beneath the Bungonia Limestone. To the east the forested hill, with much bare ground, is Mt Ayre and this consists of Late Ordovician black shale (unit 3) . To the east in the distance the flat-lying Early Permian Shoalhaven Group forms an escarpment at the top of the Shoalhaven River Gorge. From the signpost find and follow the track several hundred metres north to Cardinal View (Stop 15). STOP 15 - Thrusting in the Bungonia Limestone - Cardinal View Cardinal View (GR 271451 Caoura 1:25 000 Topographic Map) is a vantage point on the southern side of Bungonia Creek that provides a spectacular panorama of the local geology. In spite of the abundant cliff exposures structural relationships within the Bungonia Limestone in this area are controversial. Cardinal View is located on the massive lower limestone of the Bungonia Limestone and this unit continues north across the creek to the large South Marulan limestone quarry. The black features visible in the limestone in the quarry are dolerite sills and indicate that bedding in the limestone is near vertical East of the quarry steep E-dipping strata of the Ordovician unit are visible, and the Ordovician unit forms the weathered mass on the eastern abutment of the quarry. To the west up Bungonia Creek are brown steep cliff dotted hill slopes in the Tangerang Formation formed mainly by

V vv VVV V V V/V V V V V V v y-v / v v v v v v yv vvvvvvvv Vvvvvvvvvv vvvvvvvvvv VVVVVVVVVVJ vvvvvvvvv\f vvvvvvvvvv vvvvvvvvvv vvvvvvvvv vvvvvvvvv vvvvvvvvv/ vvvvvvvvv/

V Adams vy ^Lookout v

v v v Vv v v v/ vvvyvvvy v v v// v v v / v v vv v v v/ vvy vvv v V V/V v v v v V V\V V V V ' vv vy vvvVv/ vvv vy vvvv/v vvvy V V V/ V V / L I O UOil V V/ -

® Stop 15

[ Bungonia Lookdown

F i g . 7 . M a p of the S h o a l h a v e n River area e a s t of the Y a r r a l a w F a u l t (see F i g . 2 for location) . F . H . F . = Frome H i l l F a u l t . N o t e the r e p e t i t i o n of the b l a c k shale (unit 3 = shaded unit) and u n i t 2 (dashed unit) of the O r d o v i c i a n u n i t (Ou) along steep c o n t r a c t i o n f a u l t s . The V - p a t t e r n is the T a n g e r a n g F o r m a t i o n , and the u n d e r l y i n g B u n g o n i a L i m e s t o n e is d i v i d e d into a w e s t e r n limestone (Lsu - the m i d d l e and u p p e r l i m e s t o n e of Carr et a l . 1981), a c e n t r a l shale (Sh - the lower shale of Carr et a l . 1981) and an e a s t e r n limestone (Lsl - lower limestone of Carr et a l . 1981) . See F i g . 5 for a key to symbols.

Frome Hill ami •t?e the hill in the foreground is at l u t Rn° 5 t m i d d l e b e d d e d limestone dips to the west acroTs thp r r f l 2k S^ s t hsouth side. This limestone is traced s to the ^w ?hf K® ° ^ t hwiasr dlim a prominent limestone bluff to Fault?' estone is a fault (the Frome Hill S l n t h e fo which t°twall truncated at the fault, f Cardi a * l View and in the north wall of Bunaon^ r ° t h e S WS t ° sandstorms * , r a t a i n the footwall are shales, lithic w l f conglomerates of the lower shale and they dip tn W i ! ! i n • °t hl t o c3o0r e a on fd la o w e r down in the canyon flatten rho f syncline that is truncated by 11 Fault T h e l o w e r s h a l e wa SSJl ' s deposited in a deeper environment than the limestone with the sandstones and conglomerates occurring in graded beds and deposited by turbidity currents. The Frome Hill Fault is marked by a narrow crush zone with several metres of strongly recrystallized limestone above the fault. The nature of the fault movement has two published accounts. Carr et al. (1981) recognized a subdivision in the Bungonia Limestone with the following order: a lower limestone (at Cardinal View and in the quarry) , a lower shale (in the footwall of the Frome Hill Fault) , a middle limestone (the bedded limestone on Frome Hill) , an upper shale and an upper limestone. The bedded limestone on Frome Hill is the middle limestone that was interpreted by Carr et al. (1981) as in fault contact with the underlying and stratigraphically underlying lower shale. In the depths of Bungonia Canyon the middle limestone rests on the lower limestone and the lower shale is cut out by the fault, which was therefore regarded as a normal fault (Fig. 8). In my interpretation (see Fergusson & VandenBerg 1990) the stratigraphic order of units in the Bungonia Limestone proposed by Carr et al. (1981) is not accepted, a test of this stratigraphy is not possible due to the lack of age-diagnostic fossil assemblages. Instead the lower and middle limestones are regarded as equivalents, with considerable facies variation, and the Frome Hill Fault is a thrust that has duplicated units in the Bungonia Limestone. Strong support for this hypothesis was gained from the recognition of widespread thrusting in the adjoining Ordovician unit. There are some difficulties with the geometry of the Frome Hill Fault. Firstly, the fault in the the middle limestone is a hangingwall flat, this is most unusual lithology for a hangingwall flat. For example, in the Canadian Rocky Mountains flats occur in weak lithologies such as shales (Price 1981). Secondly, the cutoff in the lower shale is at an angle of 20-30°, somewhat higher than that normally encountered in weak lithologies. In the lower limestone the cutoff angle in the footwall is a high-angle, much greater than that normally encountered. These observations are difficult to reconcile on a balanced cross-section (Fig. 8).

(a) EAST

WEST

ORDOVICIAN UNIT

LATE SILURIAN |c BUNGONIA LIMESTONE r 200m

(b)

F.H.F

400 -

0 -

H.S.=V.S.

Fig. 8. (a) Sketch section by Carr et al. (1981) for Bungonia Canyon. (b) Cross -section of Bungonia Canyon — complete and submit for perusal by CLF.

DAY 2 - BUNGONIA CREEK - SHOALHAVEN RIVER TRAVERSE a ^ a r e a ^ ^ i e X ^ i r l e s the geology of the Ordovician unit in al on Shoalhaven R i ^ F ^ a ^ ^ R ^ Bungonia Creek and the foiinwQ i ^ Tm raCk?*e a s t' ' F r o m Bungonia Lookdown the route luA llf^i . towards Mt Ayre. After 750 m and Sh rt c l i m b U off t ^ L ° P t o M t Ayre the Red Track turns and ?nTinw descends towards Bungonia Creek. left flnnr r^ l :hand track a descent of about 400 m to the Ir^r ^ L Bungon J- a Cr®ek bearing in mind that what goes down must come sup. Many features of interest occur along the route everal of notes' these will be covered in the following At creek level are thick-bedded turbidites dipping moderately to steeply to the east, that also young in this direction. Continue downstream for 250 m until the black shale unit is reached, then cross to the north bank and walk upstream beyond the black shale to exposures of melange (Stop 16). STOP 16 - Fault Zone in Bungonia Creek Most of the faults in the Ordovician unit are marked by relatively narrow zones of gouge and sheared rock. The most spectacular example occurs in the hangingwall of the fault at this stop (Fig. 9, GR 281456 Caoura 1:25 000 Topographic Map) where a zone of tectonic melange up to 100 m in width is present. The melange structure is best observed in the mudstone-rich part of the exposure where there are many fault-terminated fragments of quartz sandstone beds with well-preserved sedimentary structures (see Fergusson & VandenBerg 1990, fig. 11a). The matrix consists of scaly fabric superimposed on the bedding-parallel cleavage with steeply pitching striations. Boudins show a common sense of asymmetry with clockwise rotated fragments along shears that

Fig. 9. Cross-section UV from the Shoalhaven River area (see Fig. 7 for location).

have a sinistral offset and have the correct orientation for Riedel shears at a low-angle to the shear fabric. Some relatively smaller boudins show anticlockwise rotation along dextral shears at high-angle to the shear fabric and mayrepresent conjugate Riedel shears. The overall motion is sinistral oblique-slip, although the inferred slip direction from the orientation of shears is shallower than the striations in the scaly mudstone. The fault zone is one of the three major thrusts shown in cross-section UV (Fig. 9) and places thick-bedded turbidites of unit 1 against black shales of unit 3. The black shales and underlying strata are repeated by these three major thrust faults (Fig. 9) . From this stop return downstream, note the abundant graptolites in the black shale, which locally is rich in pyrite. The next stop is the exposure of stripy rocks 300 m downstream from Stop 16. STOP 17 - Stripy beds At this stop (GR 283454 Caoura 1:25 000 Topographic Map) distinctive stripy thin-bedded alternating mudstones and micro-crosslaminated sandstones of unit 2 are well exposed. This unit is always found below the black shales and is common in the Shoalhaven River area. Jones, Fergusson & Zambelli (manuscript in preparation) describe these rocks in detail and interpret them as contourites. Continue downstream about 600-700 m to the Shoalhaven River and then walk upstream on the rocky west bank (Stop 18). STOP 18 - Strike-slip Faults and Melanges in the Shoalhaven River A late cross-cutting fault system clearly postdates the main E-verging imbricate fault system and these are best exposed at Stop 18 (GR 291456 Caoura 1:25 000 Topographic Map). Here, are three main sinistral strike-slip faults with displacements of the order of 50 m. Both sinistral and dextral faults occur, all typically with subhorizontal quartz-fibre lineations indicating strike-slip displacements. Sense of shear is indicated along these faults by: offset distinctive beds, fault-related flexures and asymmetric boudinage structures (as above these are formed by offset along Riedel shears) found within melange zones along the faults. Orientations of these faults are consistent with a crude conjugate system with the a- principal stress direction orientated WNW-ESE, that is similar to the compression direction for the main deformation. These late faults only affect the Ordovician succession. Melanges also occur in fault zones parallel to bedding and contain excellent shear sense criteria, such as rotated stepped beds, asymmetric boudins and shear bands, that

dextral strike-slip motion along the shear (see Fergusson & VandenBerg 1990, figs, lib and c) .

fabric

P s t r e a m crossing excellent exposures of stripy beds Uprig lt f o l d i s (see Fig. 9 for structure) . About upstream from the Bungonia Creek-Shoalhaven River 3unction a large anticline is exposed in the northeast bank, ? ' Note the shaped folds in black shale 200 m downstream from stop 19.

T k™

STOP 19 - F 1 Anticline o o ^ r U C t U r e i s t h e eastern-most anticline shown on Fig. 9 (GR 296449 Caoura 1:25 000 Topographic Map) and has a near vertical western limb and a moderately dipping eastern limb with an axial plane dipping 50 to the east. From here continue upstream about 500 m to the bluff of black shale and cross the river here if safe to do so. Across the river walk upstream 200 m to the big M fold exposed on the north bank (Stop 20) . STOP 20 - Big M Fold and section This is the best exposed section in the area (Fig. 10, GR 292444 Caoura 1:25 000 Topographic Map). The major thrust in the western part of the section has unit 2 with several z-shaped fold couples in the hangingwall and intensely folded late Gisbornian black shale in the footwall. East of the contorted black shale is a syncline and the big M anticline. In the core of the syncline is a thin interval of upper Gisbornian black shale, and it appears likely that this structure is backthrust to the west over the contorted black shale. The thick-bedded turbidites forming the big M fold are clearly late Gisbornian or older and are therefore thrust over the thick slice of Eastonian black shales to the east. This faulted contact is difficult to see despite the cliff exposure, some evidence of shearing is evident in the disrupted sandstone beds that occur in the black shale adjacent to the contact. All the faults in this section are difficult to discern, and it is clear that without the aid of the fossiliferous horizons they would probably have been overlooked. Displacement along most of these faults is probably relatively small with the exception of the western thrust. This section serves to illustrate the difficulties encountered in determining structure in the Ordovician successions of the Lachlan Fold Belt. From here return downstream to Bungonia Creek and follow the steep track up to the top of Mt Ayre (a climb vertically of 450 m) and back to the Bungonia Lookdown.

Fig. 10. Cross section WX from the Shoalhaven River area (see Fig. 7 for location). ACKNOWLEDGEMENTS Funded by grants from the Australian Research Council (ARC A38715525) and the University of Wollongong. The New South Wales National Parks and Wildlife Service allowed access to the Morton National Park and Bungonia State Recreation Area. Dave Martin drafted the diagrams. Paul Carr kindly reviewed and improved the clarity of this guide. REFERENCES Bolger, P.F. 1982. Ordovician and Silurian stratigraphy and structure in the Wombat Creek-Benambra area, northeast Victoria. Proceedings of the Royal Society of Victoria, 94, 35-47. Carr, P.F., Jones, J.G. & Wright, A.J. 1980. Dating of rocks from the Bungonia district, New South Wales. Proceedings of the Linnean Society of New South Wales, 104, 113-117. Carr, P.F., Jones, J.G., Kantsler, A.J., Moore, P.S. & Cook, A. C. 1981. The geology of the Bungonia district, New South Wales. Proceedings of the Linnean Society of New South Wales, 104, 229-244. Cas, R.A.F. 1983. A review of the palaeogeographic and tectonic development of the Palaeozoic Lachlan Fold Belt of southeastern Australia. Geological Society of Australia, Special Publication 10. Chappell, B.W., White, A.J.R. & Hine, R. 1988. Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences, 35, 505-521. Coney, P.J. 1978. Mesozoic-Cenozoic Cordilleran plate

tectonics. Geological Society of America Memoir, 152, •53 — 50 . Coney P j Edwards, A., Hine, R., Morrison, F. , & Windrim, v. 1990. The regional tectonics of the Tasman orogenic system, eastern Australia. Journal of Structural Geology, 12, 519-543. Crawford, A.J. 1988. Cambrian. In: Geology of Victoria (edited by Douglas, J.G. & Ferguson, J.A.). Geological Society of Australia, Melbourne, 37-62. Crawford, A.J., Cameron, W.E. & Keays, R.R. 1984. The association of boninite low-Ti andesite-tholeiite in the Heathcote Greenstone Belt, Victoria; ensimatic setting for the early Lachlan Fold Belt. Australian Journal of Earth Sciences, 31, 161-175. The geology of the Goulburn-BraytonCreaser, P.H. 197 3. Bungonia area. Unpublished B.Sc.(Hons.) thesis, Australian National University. Crook, K.A.W., Bein, J., Hughes, R.J. & Scott, P.A. 1973. Ordovician and Silurian history of the southeastern part of the Lachlan Geosyncline. Journal of the Geological Society of Australia, 20, 113-138. Crook, K.A.W. & Powell, C.McA. 1975. The evolution of the southeastern part of the Tasman Geosyncline. Field Guide for Excursion 17A, 25th International Geological Congress Australia, 122p. Felton, E.A. 1974. Goulburn 1:250 000 Metallogenic Map. Geological Survey of New South Wales. Felton, E.A. & Huleatt, M.B. 1975. Braidwood 1:100 000 Geological Map. Geological Survey of New South Wales. Fergusson, C.L. 1987. Early Paleozoic back-arc deformation in the Lachlan Fold Belt, southeastern Australia: implications for terrane translations in eastern Gondwanaland. In: Terrane Accretion and Orogenic Belts (edited by Leitch E.C. & Scheibner E.). American Geophysical Union, Geodynamic Series 19, 39-56. Fergusson, C.L., Gray, D.R. & Cas, R.A.F. 1986. Overthrust terranes in the Lachlan Fold Belt, southeastern Australia. Geology, 14, 519-522. Fergusson, C.L. & VandenBerg, A.H.M. 1990. Middle Palaeozoic thrusting in the eastern Lachlan Fold Belt, southeastern Australia. Journal of Structural Geology, 12, 577-589. Glen, R.A. & VandenBerg, A.H.M. 1987. Thin-skinned tectonics in part of the Lachlan Fold Belt near Delegate, southeastern Australia. Geology, 15, 1070-1073. Henry, R.L. 197 5. The stratigraphy and structure of the Tarago area, New South Wales. Unpublished M.Sc. thesis, Australian National University. Murray, C.G., Fergusson, C.L., Flood, P.G., Whitaker, W.G. & Korsch, R.J. 1987. Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Australian Journal of Earth Sciences, 34, 213-236. Naylor, G.F.K. 1950. A further contribution to the geology of the Goulburn district. Journal and Proceedings of the Royal Society of New South Wales, 83, 279-287. Powell, C.McA., 1983. Geology of the N.S.W. South Coast and adjacent Victoria with emphasis on the pre-Permian

structural history. Geological Society of Australia, SGTSG Field Guide 1, 118p. Powell, C.McA., 1984. Ordovician to earliest Silurian: marginal sea and island arc; Silurian to mid-Devonian dextral transtensional margin; Late Devonian and Early Carboniferous: continental magmatic arc along the eastern edge of the Lachlan Fold Belt. In: Phanerozoic earth history of Australia (edited by Veevers, J.J.). Oxford University Press, Oxford, 290-340. Powell, C.McA. & Fergusson, C.L. 1979. The relationship of structures across the Lambian Unconformity near Taralga, New South Wales. Journal of the Geological Society of Australia, 26, 202-219. Powell, C.McA. , Li, Z.I., Thrupp, G.A. & Schmidt, P.W. 1990. Australian Palaeozoic palaeomagnetism and tectonics—I. Tectonostratigraphic terrane constraints from the Tasman Fold Belt. Journal of Structural Geology, 12,553-565. Price, R.A. 1981. The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky Mountains. In: Thrust and Nappe Tectonics (edited by Price, N. J. & McClay, K.). Special Publication Geological Society of London, 9, 427-448. Ramsay, D.M. 1979. Analysis of rotation of folds during progressive deformation. Bulletin of the Geological Society of /onerica, 90, 732-738. Scheibner, E.C. 1985. Suspect terranes in the Tasman Fold Belt System of eastern Australia. In: Tectonostratigraphic Terranes of the Circum-Pacific Region (edited by Howell, D.G.). Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series 1, 493-514. Scheibner, E.C. 1987. Paleozoic tectonic development of eastern Australia in relation to the Pacific region. In: Circum-Pacific Orogenic Belts and the Evolution of the Pacific Ocean Basin (edited by Monger, J.W.H. & Francheteau, J.). American Geophysical Union, Geodynamic Series 18, 133-165. Stauffer, M.R. & Rickard, M.J. 1967. The establishment of recumbent folds in the Lower Palaeozoic near Queanbeyan, New South Wales. Journal of the Geological Society of Australia, 13, 419-438.