VIMP Report 77 The regolith of the Bendigo 1:100 000 map area A. Kotsonis and E.B. Joyce March 2003 Bibliographic reference: Disclaimer: KOTSONIS, A. AND JOYCE, E.B., 2003. The regolith of This publication may be of assistance to you but the the Bendigo 1:100 000 map area. Victorian Initiative State of Victoria and its employees do not guarantee for Minerals and Petroleum Report 77. Department that the publication is without flaw of any kind or is of Primary Industries. wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or © Crown (State of Victoria) Copyright 2003 other consequences which may arise from you relying Geological Survey of Victoria on any information in this publication. ISSN 1323 4536 ISBN 07306 9499 2 Keywords: Bendigo, 7724, regolith, regolithlandform mapping, geomorphology, gold, landform, landscape evolution, weathering. This report may be purchased from: Minerals Business Centre, Department of Primary Industries, 8th floor, 240 Victoria Parade, East Melbourne, Victoria 3002 For further technical information contact: Manager, Geological Survey of Victoria, Department of Primary Industries PO Box 500, East Melbourne, Victoria 3002 Authorship and acknowledgements The authors wish to acknowledge the Geological Survey of Victoria for their support of regolith studies in Victoria including this report. Much of the work here is sourced from ongoing Ph.D. research conducted at the University of Ballarat by Kotsonis. Stephen Carey, Martin Hughes and Stafford McKnight are gratefully acknowledged for their support of the post-graduate project. Neville Rosengren from La Trobe University provided expert knowledge on the geology and regolith of the Bendigo area. This report includes airborne geophysical data from the Geological Survey of Victoria and Geoscience Australia. The report was edited by Linda Bibby and David Taylor and formatted by Gayle Ellis. David Higgins drafted the figures. Tavis Beer and John Dunleavy produced the accompanying map. Author contacts Andrew Kotsonis School of Science The University of Ballarat Victoria 3353 Bernie Joyce Senior Research Fellow School of Earth Sciences The University of Melbourne Victoria 3010 Email: [email protected] BENDIGO REGOLITH 1 Contents Abstract 3 1 Introduction 5 2 Geology 8 2.1 Geological evolution 8 OrdovicianJurassic 8 Jurassicmiddle Cainozoic 8 Middle CainozoicQuaternary 8 2.2 Geological structure 8 Neotectonics 10 3 Geomorphology 13 3.1 Landscape subdivisions 13 Western Uplands 13 Riverine Plain 13 Lava flows 15 3.2 Drainage 15 Drainage development 15 4 Regolithlandform units 18 4.1 Transported units 18 Alluvial 18 Colluvial 24 Marine 24 Glacial 25 Human activity 25 4.2 Volcanic units 26 4.3 In situ units 28 Deep marine turbidites (Castlemaine Group) 28 Contact metamorphosed turbidites (Castlemaine Group) 30 Granite (Harcourt Batholith) 30 5 Regolith resources 32 5.1 Gold 32 5.2 Clay 32 5.3 Sand and gravel 33 5.4 Other resources 34 Crushed rock 34 Dimension stone 35 6 Synthesis 36 6.1 Discussion 36 In situ units 36 Transported units 36 6.2 Landscape evolution 38 7 Conclusions 41 References 42 Appendix 1 46 Full list of RTMAP codes for regolithlandform units Appendix 2 48 Regolithlandform unit summary data Appendix 3 57 Regolith excursion guide List of figures 1. Regolithlandform map of BENDIGO 6 2. Regional geological map of Victoria showing the location of BENDIGO 7 3. Simplified geology of BENDIGO 9 4. Simplified geological structure of BENDIGO 11 2 BENDIGO REGOLITH 5. Digital terrain model of BENDIGO with major geomorphic subdivisions 15 6. Drainage map of BENDIGO 16 7. A: radiometric ternary ratio image of BENDIGO. B: single channel radiometric images 2021 8. Radiometric ternary ratio image of northwestern BENDIGO 22 9. Aap. Thin layer of mining residue preserved above floodplain alluvium 22 10. Aep. Gravels flanking mineralised bedrock rise 23 11. ACer. Joints in silicified quartz conglomerate 24 12. OMep. Typical profile developed on marine sands 25 13. Faf. Mining residue over an old soil profile 25 14. VFvl, VFep. Soil profiles. A: old flow with little soil. B: young flow with thick soil 27 15. SMel1. Moderately weathered bedrock exposed in the Sugarloaf Range 29 16. SSeh3. Typical profile of slightly weathered saprock and thin lithic soil profile 30 17. SHel2. Typical profile showing highly weathered granite saprolite 31 18. Secondary gold. Scanning electron microscope images 33 19. Sand and gravel resources. TMI image of part of BENDIGO showing potential resources 34 20. Crushed rock resources 34 21. Landscape evolution diagram for BENDIGO 39 22. Simplified relationships between the transported regolith units in BENDIGO 47 23. Aam. Typical profile 47 24. Aam. Aerial photograph showing Spring and Bullock creeks 47 25. Aap. Typical profile 48 26. Aep. Typical profile 48 27. Aep. Regolith profile at Mackenzies pit 49 28. ACer. Typical profile 49 29. ACer. Duricrust developed in quartz conglomerate 49 30. Cfc. Typical profile 50 31. Cfc. Colluvium exposed near Nuggetty mine 50 32. OMep. Typical profile 51 33. Faf. Typical profile 51 34. VFep. Idealised cross section of the area around the Coliban River 52 35. VFep. Typical profile 52 36. VFep. Boundary between a flat-topped lava residual and the slope down to the lateral stream 52 37. Idealised cross section for the Bald Hill volcano and surrounds, showing regolith relationships 53 38. VFvl. Typical profile 53 39. Idealised cross section for BENDIGO 54 40. SMel1, SHel1, SVer1. Typical profile over the bedrock units 54 41. SSeh3. Typical profile 55 42. SSeh3. Soil profile over slightly weathered, contact metamorphosed Castlemaine Group 55 43. Idealised cross section through the Harcourt Batholith and its contact aureole 55 44. SHel2. Typical profile 56 45. Simplified regolithlandform map of BENDIGO showing excursion sites 58 List of enclosures Bendigo 1:100 000 regolithlandform map BENDIGO REGOLITH 3 Abstract largely due to time since formation and position in the landscape. Over 697 t of gold has been won from the area around The landscape history of BENDIGO prior to the early and under Bendigo and until recently, mining at Palaeogene is poorly understood. Minor occurrences Fosterville produced 30 00040 000 ounces annually. of Permian glacial deposits, which are subsurface Both of these goldfields occur in areas of exposed beneath Cainozoic sediments near the Leichardt bedrock and are surrounded by a generally thin cover Fault, indicate Permian glaciation has affected the of transported and volcanic material that effectively landscape. Mesozoic deep weathering is not shields a large area of highly prospective bedrock preserved in BENDIGO; the deep weathering on the from historical exploration. Understanding the bedrock RLUs is probably post-Mesozoic and distribution and origin of regolith materials is of associated with an eroded early Cainozoic fundamental importance when designing and, palaeoplain that developed prior to late Eocene uplift importantly, interpreting the results of, geochemical of the Western Uplands. The soil cover now exposed sampling programs. on the bedrock RLUs is young and not connected with This report summarises the results of this deep weathering. However, a ferruginous regolithlandform mapping of the Bendigo 1:100 000 duricrust developed on the bedrock (Castlemaine map area (BENDIGO) in central Victoria, and Group) is preserved on the margins of the Uplands provides a synthesis for the development of the beneath a thin veneer of transported regolith, and regolith and the evolution of the landscape. may represent part of the original soil cover at the time of deep weathering. The regolith in BENDIGO can be divided into three groups of regolithlandform units (RLUs): Palaeozoic The transported units are not extensive in the bedrock units, Cainozoic transported units, including Western Highlands, but their distribution, landscape transported mining residue that has accumulated position and regolith development provide important from the 1850s onwards, and Cainozoic basaltic lava clues for reconstructing the drainage development flows. and landscape evolution of the area. The broad architecture of the drainage has been little changed Bedrock units with predominantly in situ regolith since the Paleocene in much of BENDIGO. occur over metasedimentary rocks of the Castlemaine Group, contact metamorphic aureole The oldest preserved transported unit is the White rocks and granitoids of the Harcourt Batholith. Hills Gravel, which delineates the earliest known Weathering profiles are dominated by saprolite of drainage system in the region and is now preserved variable thickness mantled by lithic gradational and on hill tops and on the upper slopes of bedrock ridges. duplex soils. The nature of the regolith now These deposits are cemented with a duricrust of preserved on the bedrock RLUs has been controlled silcrete, argillans, and minor ferricrete, and were by Cainozoic erosion and landscape evolution, with formed as the end product of erosion of the Mesozoic deep weathering interpreted to have begun in the palaeoplain after extensive dissection, and Cretaceous to early Palaeogene. Evidence exists for weathering continuing until the middle Cainozoic late Cainozoic movement along all the major bedrock (upper age limit of late Miocene). structures within BENDIGO. Movement along these Erosion and weathering continued throughout the structures has affected weathered profiles and soils early and middle Cainozoic. All the buried leads