GU Report 16 - Lithogeochemical haloes surrounding central Victorian gold deposits: Part 2 – Secondary dispersion
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Product description:Executive summary This report follows on from a previous investigation by Arne et al. (2008) of primary lithogeochemical haloes in fresh rock surrounding central Victorian gold deposits. New whole-rock lithogeochemical data have been generated from 652 samples of weathered bedrock from the Bendigo, Ballarat East, Fosterville, Costerfield and Castlemaine goldfields, as well as from near the Wildwood deposit in the Stawell corridor. Following a four-acid digestion, outcrop, diamond drill core and pulped auger or aircore samples were analysed for a suite of 27 elements by inductively coupled plasma mass and atomic emission spectrometry (ICP-MS and ICP-AES). Gold was analysed either by fire assay with an atomic absorption finish, or by aqua regia digestion followed by ICP-MS. The elements Zr, Ba and Ti were analysed by pressed pellet X-ray fluorescence (XRF). Crusher splits from each sample were analysed by CSIRO using HyChipsTM for hyperspectral data in the visible to near infrared and shortwave infrared spectrum. In addition, multi-element data were compiled from a variety of sources for an additional 6884 weathered rock and residual soil samples from the above areas and from the Stawell and Kewell Au deposits. The data have been used primarily to define goldfield-scale threshold values for Au, As, Sb and, for the Stawell area, Cu/Fe and As/Fe ratios. Lower threshold values, which separate background from anomalous samples, are around 10 ppb for Au, between 20 and 40 ppm for As, and generally 3 ppm for Sb in sandstone. Upper threshold values, which distinguish anomalous samples from strongly anomalous samples, are typically between 60 and 100 ppb for Au and between 50 and 100 ppm for As. These threshold values delineate secondary geochemical footprints of the various goldfields that are characterised by the presence of anomalous metal contents in rock and residual soil over a broad area. These footprints are elongate in the north–south direction, giving aspect ratios of between 1.4 and 10. The size of secondary geochemical footprints varies: they increase in size from Stawell (1.8 km by 0.3 km) to Costerfield-style (3 km by 0.5 km) to Fosterville-style (12 km by 3 to 5 km) to Bendigo/Ballarat-style (10 km by 7 km at Bendigo and 5 km by 2 km at Ballarat East). Multivariate anomalies were also identified within these footprints by using Au, As and Sb levels, and, in some instances, Cu/Fe ratios, and Bi or W levels. Elevated metal contents are not pervasive within these zones of element enrichment, so it is possible to obtain false negative responses from individual samples within a goldfield. It is therefore important that multiple samples over a wide area are analysed. Individual deposits have much smaller secondary dispersion haloes than the goldfields themselves, but their haloes have similar aspect ratios. These regional-scale footprints and secondary dispersion haloes provide a guide for gold exploration in areas of central Victoria where the Palaeozoic basement is covered by sedimentary and volcanic deposits. Download The downloadable version of this report is supplied in PDF format. Appendices are also supplied (ZIP). Bibliographic reference Arne D.C. & House, E., 2009. Lithogeochemical haloes surrounding central Victorian gold deposits: Part 2 – Secondary dispersion. Geoscience Victoria Gold Undercover Report 16. Department of Primary Industries. Related products:
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