Abstract

Mineralized rocks at the Mystery Zone gold deposit, Mt Percy, are overlain by an almost complete lateritic regolith over 60 m thick, within which there are significant variations in Au concentration. This includes an apparent Au depletion in the upper saprolite and clay horizons and enrichment in the surficial residuum and soil. Gold leaching up through the regolith has been quantified by mass balance calculations. In the lower saprolite, 55% of the Au has been leached from the porphyries, but Au is essentially immobile in ultramafic rocks. Higher in the regolith, leaching trends are similar over both lithologies, with 74–90% depletion in the clay saprolite, and 84–95% depletion in the plasmic and mottled clay horizons. Within the surficial lateritic residuum and carbonate, Au concentrations increase to significantly greater than bedrock grade, but the calculations show there is little, if any, absolute Au enrichment, compared to the fresh rock.

Silica is progressively depleted through the regolith, with more leaching over fuchsitic ultramafic rocks. Aluminium tends to remain stable in the saprolite. However, Fe shows contrasting mobility during weathering of the different lithologies. For ultramafic rocks, there is a strong absolute Fe depletion in the clay saprolite, with minor depletion in the overlying mottled and plasmic clay. In contrast, within weathered porphyries, Fe is absolutely accumulated in the most regolith horizons, possibly representing reprecipitation of Fe dissolved from adjacent ultramafic rocks.

Results from this investigation agree well with a previous Au mass balance study based on isovolumetric calculations. However, differences higher in the regolith may be due to partial collapse causing errors in earlier calculations.

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