CRC LEME
Open File Report 45
ABSTRACT
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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