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Open File Report 54

Geochemical background, Mt Percy, Kalgoorlie, Western Australia

Butt, C.R.M.

In an earlier study, the distribution of gold and over 40 other elements in the regolith was determined over the mineralized Mystery Zone at Mt Percy. The site is typical of supergene Au deposits in the Kalgoorlie area. However, most of the samples were from the pit, so that only those from the western margin, including some drill cuttings, were in unaltered and unmineralized rocks or their weathered equivalent. None were distant from mineralization and could provide an adequate measure of the background. A further study has been undertaken to determine the geochemical background, using samples from diamond and percussion drilling approximately 1000 mN of the Mystery pit, across the same stratigraphy. The site was not ideal because, unlike the Mystery Zone, the regolith has been partly eroded and there is no lateritic duricrust or gravel horizon. Nevertheless, it provides useful comparative data for much of the regolith over barren or weakly mineralized equivalents of the rocks of the Mystery Zone.

The unweathered rocks in the background site consist of talc-chlorite-carbonate rocks of the Hannan's Lake Serpentinite, intruded by felsic porphyries. Unlike the Mystery Zone, there is no fuchsite-quartz-carbonate alteration and the rocks are essentially barren, with a maximum Au content of 8 ppb. The weathering front is at about 60 m but the full regolith profile could not be sampled as all percussion holes were terminated in saprolite at about 34 m vertical depth. The regolith consists of strongly leached saprolite that becomes softer and increasingly clay-rich towards the surface, merging with plasmic and mottled clays at about 10 m depth. These form a transitional zone, up to 4 m thick, between the saprolite and an almost uniform cover of non-calcareous and calcareous red clay soils. The regolith samples are from a traverse about 150 m south of the barren unweathered rocks of the diamond drill section and contain minor Au mineralization, associated with now-weathered fuchsite-altered ultramafic rocks.

For the unweathered rocks and saprolites, comparisons between the background site and mineralization in Mystery Zone indicate:

  1. In both fresh and weathered ultramafic rocks, alteration is indicated by fuchsite (chromian muscovite) and by elevated concentrations of K, Ba and V. Altered porphyries can possibly be distinguished by their higher K contents.
  2. Unmineralized talc chlorite ultramafic rocks in the Mystery Zone are relatively enriched in As and Sb compared to their equivalents in the background area. This may indicate widespread weak primary dispersion from mineralization into apparently barren wallrocks. The enrichment continues into the saprolite.
  3. Unweathered mineralized porphyries have enhanced K, V, Au, As, W and Sb contents relative to unaltered background porphyries. However, although Au, As and W abundances are greater in the saprolite in the Mystery Zone, there appears to be no significant difference in Sb content compared to the minor background mineralization.
  4. Weathered fuchsitic ultramafic rocks of the Mystery Zone are enriched in Au, As, W and Sb relative to background.
  5. Overall, in addition to Au, mineralized saprolite is indicated by As (contrast x10 compared to barren talc-chlorite ultramafic rocks), Sb and W (contrasts x2, but x6 in porphyries). Regional scale thresholds are 20 ppb Au, 10 ppm As, 4 ppm Sb and 3 ppm W; local thresholds are rather greater, i.e., 90 ppb Au, 35 ppm As, 8 ppm Sb, 6 ppm W.

In the clay-saprolite and mottled clay horizons broadly similar conclusions apply, except that Au may be leached and that W and, particularly, Sb tend to be more strongly concentrated than in the saprolite, particularly over the porphyries. However, data from these horizons are too few to provide an adequate comparison. The red clay soils at the background site do not retain a clear indication of their parent lithologies, unlike immediately underlying horizons, and are, therefore, probably transported. The data indicate that Au becomes concentrated in the calcareous surface horizons, although it is not certain whether the anomaly represents a widespread enrichment (local threshold about 30 ppb) or is related directly to the concealed minor mineralization deeper in the regolith. The occurrence of Au anomaly in calcareous soils overlying buried mineralization is consistent with the findings of several other investigations in the southern Yilgarn Block.

Last updated: Thursday, January 06, 2000 09:02 AM


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