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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.

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