The Ventersdorp Contact placer: a gold-pyrite placer of stream and debris-flow origins from the Archaean Witwatersrand Basin of South Africa

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ABSTRACT

Five local varieties of the Ventersdorp Contact placer at the East Driefontein Gold Mine, Carletonville, are distinguished by clast-type assemblage, reflecting discrete provenances. The placer further comprises six lithofacies which are not restricted to specific provenance-varieties. The six lithofacies are: massive, matrix-supported conglomerate (facies Gms): massive, or crudely bedded, clast-supported conglomerate (facies Gm); channel-based conglomerate (facies Gt); single clast layers (facies Glag); horizontally stratified sandstone (facies Sh); and trough cross-stratified sandstone (facies St). Facies Gms represents debris-flow deposits, whereas the other facies are stream deposits. Lithofacies and provenance-varieties are complexly interrelated in local environments, which are themselves complexly related to geomorphic elements of a degradation surface. The maximum thickness of the placer is 7 m. The placer was deposited on a hard, i.e. non-alluvial, bedrock surface, suggesting that it is a pediment mantle. It owes its preservation to a capping of conformably overlying basalts.

Economic gold mineralization is present in all the conglomerate facies and all the provenance-varieties. Gold concentrations are high in gravel-bar deposits and channel fills but are not restricted to these environments. A major feature of gold concentration is its variability between provenance-controlled varieties. The gold concentration of a specific primary source is interpreted to be a major control on gold concentration in the final deposit. The origin of the placer as a pediment mantle, and its subsequent preservation by lava-flows, suggests that there was little potential for selective reworking of heavy minerals. Nevertheless, the Ventersdorp Contact placer on the Carletonville Goldfield is one of the richest gold placers in the Witwatersrand Basin.

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