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Open File Report 99

Hydrogeochemistry in the Yilgarn Craton

Gray, D.J.

This Report is a compilation of work done to date within CRC LEME on the hydrogeochemistry of the Yilgarn Craton and its margins. The various sites can be grouped into three distinct regions: the Kalgoorlie Region Kalgoorlie region, which has acid and saline to hypersaline groundwaters; the Central region, at and immediately N of the Menzies line, which has variably saline and neutral groundwater; and the Northern region, which has neutral and mostly fresh groundwaters. These differences are reflected in trace element geochemistries, with Kalgoorlie groundwaters having high dissolved Au, base metals, REE and U but low oxy-anions (e.g., As, Sb, Mo, W, Tl, Bi), Central groundwaters having moderate dissolved Au and high oxy-anion concentrations, and Northern groundwaters having low Au, moderate oxy-anions and high V and P. Exploration strategies resulting from these observations are discussed, as are effects of transported overburden.

Gold dissolution is hypothesized to be mainly as a result of halide complexation of Au. This only occurs in saline/acid/oxidizing conditions, which do occur regularly in the Kalgoorlie region and to a lesser extent in the Central region. This is reflected in the Au concentrations observed for the three regions. The presence of dissolved Mn is hypothesized to be important for the generation of sufficiently high oxidation potentials for Au dissolution, whereas dissolved Fe appears to precipitate Au. As the primary source for dissolved Fe is from depth, this is consistent with leaching of Au in the upper part of the water table and Au precipitation at depth where the oxidized Au contacts dissolved Fe.

As well as exploration for Au, a number of pathfinder elements may be useful for lithological discrimination. This includes base metals for distinguishing various lithologies, particularly in acid groundwaters, and Cr, which appears to reliably distinguish ultramafic rocks, in all groundwater types, regardless of the degree of weathering.

Although the hydrogeochemistry of further sites should be investigated, particularly in the south-west Yilgarn, data accumulated to this point does indicate consistent hydrogeochemical behaviours within each of the three regions given here, thus allowing a consistent use of groundwater data in each region, for Au exploration, lithological discrimination or enhancing our understanding of weathering and dispersion processes.

Last updated: Sunday, August 05, 2001 11:59:14


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