Abstract

Present models of the derivation of the gold in the Witwatersrand conglomerates of South Africa as detrital grains directly from primary deposits in a source area consisting mainly of Archaean schist belts meets with a volume problem. Witwatersrand deposits have yielded about 923 kg/km2 Au compared to about 65 kg/km2 Au in the richest known schist belts. Differences in the extent of mining activities cannot account for these differences in yield. Gold fineness, grain size, and morphology of the gold particles are also difficult to reconcile with a purely detrital derivation. Data on the mobility of gold in the hydrosphere and in the weathering cycle are used to erect a model which predicts that most of the gold in the Witwatersrand conglomerates was derived from a source area in trace amounts in a 'dissolved' form and precipitated at or close to the edge of the depositional basin, possibly under organic influences. Subsequent sedimentary reworking and metamorphic/diagenetic remobilization led to the formation of the complex association of minerals observed now in the conglomerates.

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