Mafic dikes displacing Witwatersrand gold reefs: Evidence against metamorphic-hydrothermal ore formation

The process responsible for the gold mineralization in the Witwatersrand Basin, South Africa, remains controversial; the debate is between a detrital placer enrichment in laterally extensive conglomerate layers, and hydrothermal introduction by metamorphic fluids long after sedimentation. While textural and geochronological data may be ambiguous, a first-order geological observation severely delimits the mechanism and timing of gold emplacement. The individual gold reefs are cut and displaced by numerous mafic dikes, which, based on geochemical data presented here, are feeders to flood basalts immediately overlying the Witwatersrand. The displacements preclude fracture-controlled fluid focusing over tens of kilometers along a single conglomerate horizon during or after regional metamorphism. Instead, basin-scale gold enrichment occurred during sedimentation or early diagenesis, well before local redistribution by metamorphic fluids.