CRC LEME
Open File Report 23
ABSTRACT
Morphology and geochemistry of particulate gold in the lateritic
regolith, Mystery Zone, Mt Percy, Kalgoorlie, WA
Gedeon, A.Z. and Butt, C.R.M.
The characteristics of grains of free gold in different horizons
of the lateritic regolith have been studied at the Mystery Zone
of the Mt Percy mine, Kalgoorlie. Primary mineralization consists
of Ag-rich (to 50% Ag) free gold and Au and Ag tellurides in pyritic
carbonate alteration zones and quartz veins. The gold distribution
in the regolith is typical for the region, with minor enrichment
and wide lateral dispersion in surficial lateritic gravels and duricrust,
leaching and depletion in the underlying clay-rich horizons (5-15
m depth) and some secondary concentration and possible minor dispersion
within the saprolite.
Gold grains have been mechanically panned and examined by scanning
electron microscopy, observing particularly their size, shape, morphology,
condition and composition. Primary gold grains persist only into
the lower saprolite, where they are associated with Ag-poor secondary
gold and Ag halide crystals. Secondary gold was recovered from all
horizons of the regolith except from the depleted zone. Several
different morphologies have been observed, namely subhedral to euhedral
crystals, anhedral (including xenomorphic) forms and irregular,
hackly aggregates having the form of three-dimensional dendritic
growths. The majority of grains are less than 50 µm in diameter
and most are nearly equant in shape. Most grains are corroded, some
severely so, whereas others are pristine, and it is assumed that
these represent several generations of gold mobilization and precipitation.
Only very few gold grains were recovered from the lateritic duricrusts
and gravels or soils and it is assumed that most of the gold in
these horizons is present as very fine particles in secondary iron
oxides or pedogenic carbonates.
The observed gold distribution is probably the result of two or
three mechanisms of chemical dispersion. Firstly, mobilization occurred
during lateritization, when most, if not all, primary gold in the
duricrust was dissolved and presumably reprecipitated as very fine
particles with iron oxides. The second phase has been during later
arid periods, when primary gold remaining in the saprolite has been
dissolved by saline groundwaters and, in particular, leached from
the upper clay-rich horizons, and reprecipitated as Ag-poor grains,
again in the saprolite. Finally, remobilization via vegetation has
led to the accumulation of gold in the pedogenic carbonates.
Last updated: Tuesday, January 04, 2000 03:15 PM
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