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Open File Report 106

Geochemical and spatial characteristics of regolith and groundwater around the Golden Delicious Prospect, Western Australia

Bristow, A.P.J., Gray, D.J. and Butt, C.R.M.

The dispersion and solubility of Au and other elements in the regolith, and the nature and distribution of regolith materials has been studied at and around the Golden Delicious deposit near Laverton, Western Australia. The deposit has a resource of 6.1 Mt ~ 1.3 g/t Au hosted by a suite of Archaean granitoids that intrude mafic volcanic and volcaniclastic greenschist host rocks in the southern part of the Laverton tectonic zone. The mineralised and country rocks are deeply weathered, and subsequently partly eroded to the clay zone and upper saprolite. They are now covered by up to 20 m of colluvium - alluvium derived from eroding mafic regolith several km to the ESE. Late-stage hematite mega-mottling has overprinted the upper residual horizons. The entire regolith is saturated with saline, weakly alkaline groundwater below approximately 10 m. Much of the regolith, including the transported overburden, has been extensive modified during arid conditions. Samples of regolith and groundwater from a drill traverse across the deposit have been examined in detail to determine element distributions, their relationship to regolith evolution and their significance in exploration. Elements associated with Au mineralisation were W, Sb, K and REE, though none displayed a very direct correlation with Au. Only Au showed evidence of significant remobilization, the other elements approximating their distribution in the Archaean prior to weathering.

The distribution, solubility and accessibility of Au in the regolith, its concentration in groundwater, and the chemistry and depth of groundwater, suggest that Au has been dissolved from mineralisation, and re-precipitated in the base of the transported overburden to give a Au anomaly (12-100 ppb), 500 m across strike, offset slightly down-slope of underlying mineralisation. No evidence, other than the Au contents of the groundwater and the slightly soluble nature of Au in the weathered Archaean, was found to suggest significant Au depletion or mobilization and it is suggested, based on the distribution and solubility of other elements associated with Au mineralisation, that the present Au distribution in the residual regolith is similar to that prior to weathering. Despite the lack of evidence for significant Au mobility, there is some enrichment (12-80 ppb) in the top few metres of the residuum for 300 m across strike, directly overlying mineralisation, that may be associated with late stage mottling or residual concentration. Gold in groundwater is anomalous, (0.046-0.18 1µg/L), though slightly offset to the west, for 200 m across strike of mineralisation. Tungsten is anomalous (8->50 ppm) below the top few metres of the residuum for 400 m across strike, and indicates a broader primary halo than Au in the primary mineralisation. The distribution of Sb concentrations when normalised to Fe is similar to that of W. Neither Sb nor W are anomalous in the transported overburden overlying mineralisation and are relatively insoluble in the weathered Archaean.

Last updated: Sunday, August 05, 2001 13:59:33


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