Résumé
Studies of the detrital components from the conglomerate horizons within the major goldfields of the Witwatersrand Basin, South Africa, indicate a mixed granite-greenstone source of two distinct ages, and a deposition interval between 3074 Ma and 2714 Ma. This study presents new U-Pb and Pb-Pb isotopic data for the Central Rand Group from the Evander Goldfield, located in the eastern extremity of the Witwatersrand Basin where no previous isotopic data has been derived. Single detrital zircon dating exhibits a more restricted range of ages than previously published data from the other goldfields, and a shift towards younger ages. The oldest grain measured is ca. 3180 Ma, while the majority of detritus falls in the range 3050-2850 Ma, with a mean age at ca. 2960 Ma. In addition, however, several zircon grains from shear zones and altered portions of the Kimberley Reef, yield ages that are younger or similar to that of the Ventersdorp lavas. These ages are attributed to partial or complete isotopic resetting of older detritus at some stage during the post-depositional history of the Basin. The existence of detrital as well as authigenic pyrite is also supported by the present study. A small proportion of the pyrite, mainly extracted from unaltered sediments in the Kimberley Reef footwall, yield ages that are in excess of the minimum depositional age of the Witwatersrand Basin. Authigenic pyrite, as well as detrital grains from highly altered portions of the Kimberley Reef define secondary isochrons at 2370 Ma and 2020 Ma, ages that accord with the resetting signatures from zircon in the present study and pyrite analysed in previous studies. The lead isotopic signature of the 2370 Ma event is probably associated with burial of the Witwatersrand Basin by the upper portion of the Transvaal Supergroup, and suggests circulation of highly radiogenic fluids at this time. Isotopic signatures for the 2020 Ma event are probably related to intrusion of the Bushveld Complex and/or Vredefort catastrophism, and appear to be associated with a fluid that was less radiogenic. The present study, which combines single grain U-Pb dating of the zircon with Pb-Pb studies of sulphide phases in the Evander Goldfield, has produced a number of new results which support a complex, polyphase evolution and genesis of the Au-U deposits within the Witwatersrand Basin.