The Witwatersrand Basin, South Africa: Geological framework and mineralization processes

The Witwatersrand Basin formed over a period of 360 Ma between 3074 and 2714 Ma. Pulses of sedimentation within the sequence and its precursors were episodic, occurring between 3086-3074 Ma (Dominion Group), 2970-2914 Ma (West Rand Group) and 2894-2714 Ma (Central Rand Group). Detritus was derived from a mixed granite-greenstone source of two distinct ages; the first comprises Barberton-type greenstone belts and granitoids > 3100 Ma old, and the second consists of the greenstone belt-like Kraaipan Formation and associated granitoids < 3100 Ma old. Subsequent granitoid plutonism was episodic and coincided with hiatuses in sediment deposition, but continued throughout the evolution of the basin. Many of the provenance granitoids are characterized by hydrothermal alteration, are geochemically anomalous with respect to Au and U, and may represent viable source rocks for palaeoplacer mineralization. Tectonically, the basin evolved in response to processes occurring within a Wilson cycle, associated with the encroachment and ultimate collision of the Zimbabwe and Kaapvaal cratons. Metamorphism of the Witwatersrand Basin occurred at ca. 2500, 2300 and 2000 Ma. The first two events coincided with the progressive loading of the basin by Ventersdorp and Transvaal cover sequences, whereas the last reflects intrusion of the Bushveld Complex and/or the Vredefort catastrophism. Mineralization is concentrated in the conglomerates of the Central Rand Group and is represented by a complex paragenetic sequence initiated by early accumulation of detrital heavy minerals. This was followed by three stages of remobilization caused by metamorphic fluid circulation. An early event of authigenic pyrite formation at 2500 Ma was followed at 2300 Ma by maturation of organic material, fluxing of hydrocarbon bearing fluids through the basin and the radiolytic fixation of bitumen around detrital uraninite. This was followed at around 2000 Ma by peak metamorphism which resulted in the widespread redistribution of gold and the formation of a variety of secondary sulphides. Post-depositional fluid conditions were such that metal solubilities were low and precipitation mechanisms very effective, resulting in the superimposition of both primary and secondary mineralization.