VIMP Report 77 - The regolith of the Bendigo 1:100 000 map area
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Product description:Download The downloadable version of this report is supplied in PDF format. Abstract Over 697 t of gold has been won from the area around and under Bendigo and until recently, mining at Fosterville produced 30 000–40 000 ounces annually. Both of these goldfields occur in areas of exposed bedrock and are surrounded by a generally thin cover of transported and volcanic material that effectively shields a large area of highly prospective bedrock from historical exploration. Understanding the distribution and origin of regolith materials is of fundamental importance when designing and, importantly, interpreting the results of, geochemical sampling programs. This report summarises the results of regolith–landform mapping of the Bendigo 1:100 000 map area (BENDIGO) in central Victoria, and provides a synthesis for the development of the regolith and the evolution of the landscape. The regolith in BENDIGO can be divided into three groups of regolith–landform units (RLUs): Palaeozoic bedrock units, Cainozoic transported units, including transported mining residue that has accumulated from the 1850s onwards, and Cainozoic basaltic lava flows. Bedrock units with predominantly in situ regolith occur over metasedimentary rocks of the Castlemaine Group, contact metamorphic aureole rocks and granitoids of the Harcourt Batholith. Weathering profiles are dominated by saprolite of variable thickness mantled by lithic gradational and duplex soils. The nature of the regolith now preserved on the bedrock RLUs has been controlled by Cainozoic erosion and landscape evolution, with deep weathering interpreted to have begun in the Cretaceous to early Palaeogene. Evidence exists for late Cainozoic movement along all the major bedrock structures within BENDIGO. Movement along these structures has affected weathered profiles and soils preserved on bedrock units, displaced buried auriferous leads, and affected modern drainage systems. Transported RLUs are poorly preserved in the Western Uplands, but form a flat-lying composite fluvio-lacustrine plain to the north, the Riverine Plain. Although these units are transported relative to the bedrock units, they also have weathering profiles on them formed during post-transport in situ weathering. The youngest RLU developed in BENDIGO is composed of residue produced by puddling machines and mine tailings, associated with historic gold mining. It is found blanketing the present floodplains along creeks draining the historic gold fields of BENDIGO. Regolith profiles over the basaltic lava and scoria of the Newer Volcanic Group can be found around Bald Hill and along the Campaspe and Coliban rivers. The differing regolith profiles in these two areas is largely due to time since formation and position in the landscape. The landscape history of BENDIGO prior to the early Palaeogene is poorly understood. Minor occurrences of Permian glacial deposits, which are subsurface beneath Cainozoic sediments near the Leichardt Fault, indicate Permian glaciation has affected the landscape. Mesozoic deep weathering is not preserved in BENDIGO; the deep weathering on the bedrock RLUs is probably post-Mesozoic and associated with an eroded early Cainozoic palaeoplain that developed prior to late Eocene uplift of the Western Uplands. The soil cover now exposed on the bedrock RLUs is young and not connected with this deep weathering. However, a ferruginous duricrust developed on the bedrock (Castlemaine Group) is preserved on the margins of the Uplands beneath a thin veneer of transported regolith, and may represent part of the original soil cover at the time of deep weathering. The transported units are not extensive in the Western Highlands, but their distribution, landscape position and regolith development provide important clues for reconstructing the drainage development and landscape evolution of the area. The broad architecture of the drainage has been little changed since the Paleocene in much of BENDIGO. The oldest preserved transported unit is the White Hills Gravel, which delineates the earliest known drainage system in the region and is now preserved on hill tops and on the upper slopes of bedrock ridges. These deposits are cemented with a duricrust of silcrete, argillans, and minor ferricrete, and were formed as the end product of erosion of the Mesozoic palaeoplain after extensive dissection, and weathering continuing until the middle Cainozoic (upper age limit of late Miocene). Erosion and weathering continued throughout the early and middle Cainozoic. All the buried leads and deep leads, and the clay rich alluvial deposits at Axedale and Fosterville, formed at this time as a response to rapid erosion and downcutting of valleys following late Eocene uplift in the Western Uplands. The source rocks (and gold) for this unit came from the erosion of the White Hills Gravel and mineralised bedrock. Intense weathering of these sediments and the surrounding bedrock resulted in kaolinisation with no ferricrete, and is associated with Miocene weathering. Rising sea levels in the late Miocene–early Pliocene in the Murray Basin backfilled and buried much of the drainage system with marine quartz sands of the Parilla Sand. The environment of deposition for this unit suggests shallow marine with estuarine embayments reaching Huntly and possibly to Epsom. Syn- and post-depositional weathering formed a pisolitic ferricrete (Karoonda Regolith) now preserved in the Parilla Sand and the exposed surface of the Shepparton Formation. During and after retreat of the sea, weathering and erosion continued, along with deposition of the fluvial and fluvio-lacustrine Shepparton Formation. Gold liberated from the bedrock and from older transported regolith was deposited as shallow leads. Extrusion of the Newer Volcanics also occurred at this time, around Bald Hill and in the Campaspe and Coliban river valleys. Today a volcanic plain surrounds the Bald Hill volcano, but greater dissection in the southeastern part of BENDIGO has reduced the lava flows along the Campaspe and Coliban rivers to flat-topped lava residuals bounded by lateral streams. Quaternary regolith development has been minimal, with incision of narrow sinuous streams and the widespread deposition of clayey and calcareous aeolian dust that mantles much of BENDIGO. These dust mantles are associated with Quaternary aridity and now form surficial calcareous soil horizons with disseminated fine earth, nodule and rhizomorph carbonate. Minor swamp deposits and aeolian dunes also formed at this time. There is increasing evidence for supergene gold enrichment in the weathering zone in Victoria, including BENDIGO. Secondary gold is documented at Fosterville where oxidised quartz–gold–stibnite veins show significant refinement of primary gold and deposition of supergene gold. The timing and extent of secondary gold in the landscape is unclear, but the evidence from Fosterville demonstrates localised gold movement within in situ regolith developed on mineralised metasedimentary bedrock. There is potential for further primary and secondary gold resources in BENDIGO. In addition to gold, the area is prospective for other regolith-related commodities, including clay suitable for a range of purposes, sand and gravel, crushed rock and dimension stone. Bibliographic reference Kotsonis, A. & Joyce, E.B., 2003. The regolith of the Bendigo 1:100 000 map area. Victorian Initiative for Minerals and Petroleum Report 77. Department of Primary Industries, Victoria. Related products:
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