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Open File Report 45

Dispersion of gold and associated elements in the lateritic regolith, Mystery Zone, Mt Percy, Kalgoorlie, Western Australia

Butt, C.R.M.

The dispersion of gold and over 40 other elements in the lateritic regolith has been studied at the Mt Percy gold mine, near Kalgoorlie. Primary Au mineralization in the Mystery Zone occurs in fuchsite-carbonate alteration zones at the contact with porphyries intruding the Hannan's Lake serpentinite. The mineralized sequence has been deeply weathered and is concealed beneath an almost complete lateritic regolith over 60 m thick. The regolith consists of saprolite (50 m), which is clay-rich in the top 10 m, plasmic and mottled clays, and surficial horizons of lateritic gravels, lateritic duricrusts and pisolitic soils. The surficial horizons contain pedogenic carbonates.

Duplicates of grade-control composite samples from two sections across the Mystery Zone were collected at 1 or 2 m intervals at each 2.5 m level throughout the regolith as mining progressed. Samples of fresh rocks were obtained from diamond drill core. A selection of these samples, including a complete section through the primary mineralization, was analysed to illustrate element distributions in the fresh and weathered rocks. The elements associated with primary Au mineralization appear to be S, Ag, W, As, Sb, Te and, possibly, Ba, K and Pb, but except for Ag and Te, none has a very direct correlation with Au.

The Au distribution in the regolith is typical for the region, with minor enrichment and wide lateral dispersion in surficial gravels and duricrust (particularly associated with the presence of pedogenic carbonates), leaching and depletion in the underlying clay-rich horizons and some secondary concentration and minor dispersion in the saprolite. Primary and saprolitic Au mineralization is indicated by a broad superjacent Au anomaly (100->1000 ppb) in the soils and lateritic horizons, and by high concentrations of W (5->40 ppm), Sb (7->16 ppm) and As (10-200 ppm). High K contents, corresponding to resistant muscovite, give surface expression to the alteration zone. Although Au contents are <100 ppb in the underlying clay-rich horizons, Sb, W and, to a lesser extent, As remain anomalous; similarly, Ba and K contents remain high, indicating the porphyries and alteration zones respectively.

The porphyries and ultramafic rocks can be discriminated geochemically throughout much of the regolith by relative abundances and ratios of Ti, Zr, Ba and K. However, the lateritic horizons, particularly the duricrusts overlying the talc chlorite rocks of the Hannan's Lake serpentinite, have abnormal geochemical signatures. These duricrusts have low Cr contents (<1000 ppm), because primary Cr is present in weatherable chlorite rather than resistant chromite, but are enriched in "immobile" elements derived from the porphyries. The distribution patterns of the elements are discussed in terms of landform evolution and their significance to exploration.

Last updated: Thursday, January 06, 2000 08:40 AM



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