Abstract The Witwatersrand basin represents a 5- to 8-km-thick succession of dominantly clastic sediments deposited on continental basement prior to 2.7 Ma. Regional greenschist facies metamorphism affected every gold field, and higher grades were reached in the basin center and margins. Fluid activity during deformation enhanced strain accommodation and led to extensive alteration within the 1- to 2-m-thick reef packages of conglomerate, carbon seam, arenite, and shale. Important associations of gold within the Witwatersrand basin are a widespread Au-Fe-S and/or Au-C chemical association and a strong relationship of economic gold mineralization to unconformity surfaces. Host-rock types for gold mineralization appear less critical since both conglomerates and carbon seams host much gold, and all other rock types have high gold grades locally and anomalous values regionally. No exclusive sedimentologic setting appears to have universal importance across the basin, but on a mine scale sedimentology is extremely important in its direct and/or indirect influence on Fe (pyrite), carbon, and gold distribution. The critical structural relationships that influence gold distribution are only starting to be related to mineralization systematically; faults, folds, and dikes all appear to relate to grade changes in particular areas, although no clear genetic relationship has yet been demonstrated.

Abstract An epigenetic replacement model is proposed for Witwatersrand gold mineralization. In this model, unconformities that developed during deposition of the Upper Witwatersrand sediments were sites of selective accumulation of iron pisolites-ferricretes and carbon seams. These Fe-C-bearing intervals are part of 1- to 2-m-thick reef packages that were preferred channelways for Au-S-bearing metamorphic fluids because of their lithologic heterogeneity and preferential accommodation of strain during deformation. It is suggested that the metamorphic fluids were associated with a high-strain, cleavage-forming event recorded in the Witwatersrand, Venters-dorp, and Transvaal successions. Selective replacement of Fe-C-bearing phases by Au-S-bearing metamorphic fluids accounts for the secondary nature of the Witwatersrand mineralogy, apparent structural control on gold mineralization, the secondary texture of gold, and the presence of replacement textures involving pyrite and uraninite. Most importantly, the model also accounts for the close relationship of gold distribution to sedimentary features. If the present distribution of Fe and C approximates original sedimentary patterns, any epigenetic gold precipitated by interaction with Fe and/or C must also reflect sedimentary features. This replacement model obviates the requirements imposed by the placer model for Witwatersrand gold for a more reducing Precambrian atmosphere, for a particularly enriched source terrane, or for an unusually efficient sorting mechanism to concentrate gold in a diverse range of rock types.