VIMP Report 81 - The regolith of the Ararat 1:100 000 map area
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Product description:AbstractRegolith mapping in the Ararat area of western Victoria was completed to define the major regolith–landform units and to assess the effect of regolith development on the bedrock geochemistry of known mineralisation at the Mount Ararat copper deposit and Big Hill gold deposit. Regional regolith mapping across the Ararat 1:100 000 map sheet (ARARAT) was supplemented by a detailed 3.5 x 6 km inset map centred on the Mount Ararat copper deposit and extending north and south across the Mount Ararat range. ARARAT is prospective for orogenic gold mineralisation and volcanic hosted massive sulphide deposits within Cambrian mafic volcanic and pelitic schists in the hangingwall of the Moyston Fault. It straddles the boundary of the Lachlan and Delamerian Fold Belts and the southern margin of the Murray Basin, incorporating a wide range of elements in the landscape, a variety of differing surface and bedrock geological units and a centre of significant gold production. Mapping identified thirty-three regional regolith–landform units that broadly fall within either saprolite-dominated hills and rises or transported cover. A statistical examination of the geochemistry across both the copper and gold mineralisation highlighted several trends that may act as vectors in exploring for similar mineralising styles. Across the Mount Ararat copper deposit, copper is associated with Zn–Ag–Au–Fe–Co–Mo–Cr–S in weathered and fresh rocks. This represents massive sulphide mineralisation hosted within quartz–actinolite schist. In the soils, a similar geochemical trend to the bedrock is observed, complicated by differing soil horizons rather than regolith development. In general, the most reliable soil horizon for detecting the copper mineralisation was found to be the C horizon, where the dispersion halo ranged from 50 to 100 m and mineralised copper values were typically greater than 1000 ppm. An examination of copper values in soils across differing regolith landform units highlighted no significant difference in population means between colluvium and fresh to highly weathered bedrock. Transported cover and duricrust development both adversely effect copper chemistry and need to be considered when evaluating soil geochemistry. Across the Big Hill gold deposit, multivariate rock geochemistry and clay mineralogy was evaluated with respect to mineralisation, mine stratigraphy (mine schist or volcanogenics) and degree of weathering (weathered versus fresh). The geochemistry of weathered mineralised rock indicated that gold is associated with As–Fe–Cu–S–Pd in the mine schist and As–Fe–Pb–W–Ca in the volcanogenics. Weak carbonate alteration signatures are also associated with and adjacent to mineralisation. Clay mineralogy indicated the widespread presence of kaolinite and white mica (illite). Muscovitic illite appears to be more closely associated with well mineralised samples than does phengitic illite, particularly in the volcanogenics. The combination of geochemistry, favourable rock types, structural setting and the unusually high landscape position of the Big Hill gold deposit in the Stawell goldfield are important factors in exploring for similar gold mineralisation at surface and under cover. Palaeotopographic highs, a reflection of intense quartz veining associated with the mineralisation, coincident with carbonate alteration, kaolinite and more muscovitic illite and are all indicative of a Big Hill (and Magdala) style of mineralising system. Bibliographic reference Williams, B. & Radojkovic, A.M., 2004. The regolith of the Ararat 1:100 000 map area. Victorian Initiative for Minerals and Petroleum Report 81. Department of Primary Industries. Download The downloadable version of this report is supplied in PDF format: 34310 VIMP81 Report (PDF 72.7MB) Please be patient when downloading large files as this might take some time depending on your internet connection. Related products:
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