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Open File Report 23

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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