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Open File Report 16

Mineralogy and geochemistry of the Glasson Gold Deposit, Callion, Yilgarn Block, WA

Llorca, S.M.

This report describes a mineralogical and geochemical study of the weathered Glasson Au deposit and its country-rocks, in the Callion area, Yilgarn Block, Western Australia. The fresh country rocks are amphibole- and plagioclase-rich metabasalts. On weathering, the amphibole and plagioclase are initially replaced by smectites, opaline silica and kaolinite, and then, higher in the profile, by kaolinite, opaline silica and goethite. As the amphibole and plagioclase are weathered, Ca, Mg, Mn and some Si are leached; as, in turn, the smectites are consumed, the remaining Mg, Co and some more Si are leached away. This results in an increased relative concentration of elements such as Fe, Al, Ti, Cr, Cu, Zr, Ga and Sc. Nickel and Zn seem to accumulate in this part of the profile where smectites disappear.

The fresh mineralization consists of quartz veins within a shear zone containing additional amphibole, chlorite and mica, plus small amounts of sulphides. Gold is present both as free Au in the quartz and in the sulphides. In the overlying weathered zone, the sulphides are replaced by mixtures of goethite and rutile. Amphibole and chlorite are gradually replaced by smectites and kaolinite, passing upwards to kaolinite and goethite. Mica is residual, though it is altered to hydromica. Some halite has precipitated from groundwaters permeating the shear zone.

Lateritization occurred under humid conditions during the Cretaceous to mid Miocene. As the sulphides weathered, Cu, Zn, As, Au, Pb and W were mobilized. These elements precipitated with the goethite, both within the weathered sulphide zone and in the country-rock (as far as 10-20 m away for As, Pb, Au). Although the Au included in the sulphides was redistributed, Au in the quartz vein remained in situ. Some of this Au was still protected by the quartz and the Au located along the fractures which was attacked (its Ag leached), and the Au was re-cemented virtually in situ by the goethite precipitated in these fractures. Previously deposited secondary Au was also partially re-cemented in the fractures within goethite. Chloride-rich solutions that circulated along the quartz vein under the post-lateritisation arid conditions mobilized some of the Au that had been so far protected by the quartz. This Au was redeposited within the quartz vein and within 20 cm of it.

During the Cretaceous to mid Miocene humid period, Au close to the surface was remobilized and dispersed for up to 100 m from the quartz vein, probably by complexing with humic acids derived from soil organic matter. In the drier climates that followed, calcrete formed close to the surface and Au was fixed within it. Finally, mechanical dispersion at the surface caused As, Au, Pb and Cr enrichments in the pisolitic cover as far as 100 m away.

In a context such as the one of the Glasson deposit, the best indicators for Au mineralization are Au, As and, to a lesser extent, Pb and W. Chromium seems a good indicator of the shear zones.

Last updated: Tuesday, January 04, 2000 12:53 PM


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