Although the Au and U deposits of the Witwatersrand basin are generally regarded as modified placer in origin, the nature of the source area and the ultimate origin of Au and U still remain a major enigma. A number of domal outcrops of the Archean basement are exposed in the Witwatersrand hinterland, revealing the presence of a diverse suite of granitoids and a minor (<20%) greenstone belt component. In addition, samples from over 130 boreholes drilled adjacent to the basin have revealed the character of subsurface Archean basement beneath early Proterozoic sedimentary and volcanic cover. The granitic subsurface basement is commonly characterized by pervasive hydrothermal alteration evident as propylitization, sericitization, carbonation, and greisenization, as well as a variety of vein-related alterations comprising quartz + or - K-feldspar + or - chlorite + or - CO 3 + or - sulfide + or - fluorite parageneses. These hydrothermally altered granites appear to have been preserved beneath early Proterozoic cover but have largely been eroded from surface outcrops. The hydrothermally altered granites, which are believed to be < or = 3,050 Ma, are significantly enriched in Au and U with respect to both surface granitoids from the Witwatersrand hinterland and the entire range of granites from the Barberton Mountain Land.The presence of enriched hydrothermally altered granites in the Witwatersrand hinterland lends support to the view that mineralization processes in the basin were dominated in the first instance by placer processes. The remarkable concentrations of Au and U that are unique to the Central Rand Group of the Witwatersrand basin indicate that the sediments in the upper portions of the sequence tapped a fertile source not present in the hinterland during earlier cycles of sedimentation. The erosional remnants of this source area are represented by the hydrothermally altered granites which are considered to have been a suite of high-level granitoids, the upper portions and roof zones of which were mineralized in Au and U and easily accessible to erosion. The emplacement of these granites, and the development of extensive zones of attendant, high-level, hydrothermal alteration, was synchronous with Witwatersrand sedimentation. The hydrothermally altered granites not only contributed to the Au- and U-rich detritus in the Witwatersrand, but their emplacement may have been linked causally to the deposition of the upper part of the sedimentary sequence.

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