Abstract

Primary Au mineralization in the Mystery Zone, Mt Percy, occurs in fuchsite–carbonate alteration zones at the contact with felsic porphyries intruding the Hannan’s Lake Serpentinite. The mineralized sequence is concealed beneath an almost complete lateritic regolith over 60 m thick. The regolith consists of 50 m-thick saprolite, which is clay-rich in the top 10 m, plasmic and mottled clays, lateritic residuum (duricrusts and gravels) and pisolitic soils; the surficial horizons contain pedogenic carbonates. Elements occurring with primary Au mineralization are S, Ag, W, As, Sb, Te, K, Rb and, possibly, Ba and Pb, but except for Ag and Te, none has a mineralogical association with Au.

The Au distribution in the regolith is typical for the region, with minor enrichment and lateral dispersion in lateritic residuum, a strong association with pedogenic carbonates, leaching and depletion in the underlying clay-rich horizons and some secondary concentration in the saprolite. Primary and saprolitic Au mineralization is indicated by a broad Au anomaly (100 to >1000 ppb) in the soils and lateritic residuum, and by high concentrations of W (5 to >40 ppm), Sb (7 to >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 duricrusts overlying the talc–chlorite rocks display abnormal geochemical signatures. They have low Cr contents (<1000 ppm), because primary Cr is present in weatherable chlorite rather than resistant chromite, but are enriched in ‘immobile’ elements (e.g. Zr) derived from the porphyries. The element distribution patterns are products of deep weathering under warm humid conditions in the early Tertiary or earlier, followed by semi-arid conditions that continue to the present. Chemical and physical mobilization of gold was restricted to the surficial lateritic residuum in the humid phases, but depletion and supergene enrichment in the saprolite, and concentration with pedogenic carbonates, are the result of chemical mobility under semi-arid conditions.

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