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

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.

Hypolithic Photosynthesis in Hydrothermal Areas and Implications for Cryptic Oxygen Oases on Archean Continental Surfaces

“…(3) Archean streams and rivers were replete with detrital pyrite grains which collected as placer deposits (Krapez, 1985). The delivery of even small amounts of dissolved oxygen to streams and rivers could have fueled microbial Fe oxidation, driving oxidation of detrital pyrite.…”

Thrust-fracture network and hydrothermal gold mineralization: Witwatersrand Basin, South Africa

“…It has a local geometry controlled by braiding and stacking of alluvial channels which commonly cut into earlier deposited VCR and incise the underlying Witwatersrand strata. Remnant terraces of older conglomerate were abandoned at higher palaeotopographic levels, such that terraces and channels of differing elevations and ages are joined by inclined slopes characterized by absent or thin VCR (Krapez 1985;Henning et al 1994;McWha 1994;Germs & Schweitzer 1994). Mafic and ultramafic lavas of the Klipriviersberg Group were extruded directly onto this topography, with occasional pebble entrainment and Fig.…”

Integrated interpretation of 3D seismic data to enhance the detection of the gold‐bearing reef: Mponeng Gold mine, Witwatersrand Basin (South Africa)

“…The average density (3.05 g/cm 3 ) for the Green Bar, in contrast, is relatively higher value than its F/W rocks. This may be attributed to the amount of iron and magnesium found in these metamorphosed chloritoid shale units (Krapež ).…”

“…The Witwatersrand gold ore bodies occur in fluvial conglomerate beds called reefs. Extensive gently dipping tabular deposits are found at several stratigraphic levels; thus, mining may take place at different levels in a mining district or mine (Krapež ; Jolley et al . ; Erismann ).…”