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Open File Report 26

Multi-element dispersion in the saprolite at the Beasley Creek Gold Mine, Laverton, Western Australia

Robertson, I.D.M.

A fence of 11 inclined percussion drill holes, which crossed from the hangingwall to the footwall of the Beasley Creek Orebody, was selected for study of geochemical dispersion in the near surface materials and in the deep saprolite. Of these, four drillholes intersected the ore, its immediate foot- and hangingwall rocks and overlying calcretes, another four intersected amphibolitic saprolites and clay-rich materials proximal to the ore (40-160 m distant) and a further three cut amphibolites and amphibolitic saprolites distal to the ore (250-400 m distant). Samples were selected from these to maximise geochemical detail near the surface and to give adequate information to bedrock or to the limit of the drilling (60-100 m). They were analysed for eight major and 28 trace elements.

The zone around the orebody has been intensely weathered to below the depth of current diamond drilling (230 m) but amphibolites 400 m to the west are only weathered to a depth of 40 m. This locally intense weathering appears to be due to acid conditions generated by the weathering of sulphides in the orebody and its sulphidic host rock. The surficial materials immediately to the east of the ore host unit (to a distance of at least 75 m) contain calcrete and some gypcrete.

Some Rb- and muscovite-rich rocks in the footwall, enriched in Au, As, Cd, Cu and Pb and sporadically enriched in Ag, Bi, Mo, Sb, Sn and W, form an ill-defined zone about 100 m wide, 150 m to the west of the orebody, at a depth of 20-50 m. This zone, which straddles the saprolite-saprock boundary, appears to be a partly weathered phyllic alteration halo which apparently has not been fully investigated by drilling.

Apart from Au, the orebody is characterised by elevated concentrations of Ag, As, Cd, Cu, Pb, Sb, W and Zn. The lateritic duricrust and mottled zone are weakly enriched in Ag, Nb and W but are generally strongly enriched in As, Bi, In, Pb, Sb, W and Sn which enhances the value of these elements as pathfinders. Cobalt, Zn and Cu tend to be depleted near the surface, reducing their effectiveness as pathfinders but they show some enrichment below the mottled zone. Bismuth, Ge and In are at low abundances and have only sporadic anomalies so, despite some surficial enrichment, they are not very effective near the surface. Both Pb and Sb are also at low abundances but they tend to be strongly concentrated a little further below the surface, so their surficial anomalies are not readily interpreted. High concentrations of Al, Fe, Ba, Ce, Cr, Ga, Mn, Ni, Rb, V and Y reflect the composition of the host lithology.

Last updated: Tuesday, January 04, 2000 03:29 PM


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