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

Abstract This paper examines the geometry and genesis of a mineralized thrust-related fracture system, developed at a major rheological interface between Witwatersrand sediments and Ventersdorp volcanics in the Witwatersrand Basin of South Africa. At the Elandsrand Gold Mine, the conglomeratic Ventersdorp Contact Reef, which separates these successions, is imbricated by kilometre-wide zones of minor thrusts containing ultracataclastic fault rocks associated with hydrothermal fluid flow. Several sets of fractures are intimately associated with the imbricates, which link to underlying kilometre-scale thrusts. The fractures provided a linked fluid flow pathway and structural sites for precipitation of gold within the Ventersdorp Contact Reef, carrying >900f observed gold, with the remaining 100ccupying post-diagenetic matrix corrosion spaces within a few 100 µm of fractures. The relatively rigid volcanics deformed into low-amplitude kilometre-wavelength flexures, centred on the imbricate zones and underlying kilometre-scale thrusts. These major structures controlled the thrust-fracture system formation along the Ventersdorp Contact Reef-lava contact. Three progressive stages of structural evolution were identified: (1) layer-parallel shear across the contact, with stratigraphy-focused shallow fracture formation; (2) propagation of a kilometre-scale thrust towards the contact, leading to kilometre-scale flexure of the volcanics, with layer-parallel shearing in the limbs producing steep fractures and minor thrusts; (3) propagation of the major thrust along the contact, imbricating the sediments with shallow fracture formation in ramps.