U–Pb and Pb–Pb isotopic studies relating to the origin of gold mineralization in the Evander Goldfield, Witwatersrand Basin, South Africa

Abstract

This study presents new U–Pb and Pb–Pb isotopic data for the Central Rand Group in the Evander Goldfield, located in the eastern extremity of the Witwatersrand Basin. Single grain detrital zircon dating exhibits a more restricted range of U–Pb ages than has previously been published from elsewhere in the Witwatersrand Basin, and a shift towards younger ages. The oldest grain measured is c. 3180 Ma, while the majority of detritus falls in the range 3050–2850 Ma. Several zircon grains from the sheared and altered portions of the Kimberley Reef yield ages that are younger or similar to that of the overlying Ventersdorp lavas. These ages are attributed to partial or complete isotopic resetting of older detritus at some stages during the post-depositional history of the Basin. The existence of both detrital and authigenic pyrites is also supported by Pb–Pb isotopic data of 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 about 2370 Ma and 2020 Ma, suggesting fluid circulation in the basin at approximately these times.