Zircon ion microprobe studies bearing on the age and evolution of the Witwatersrand triad

Abstract The Ventersdorp and Witwatersrand Supergroups and the Dominion Group (collectively referred to as the Witwatersrand triad) represent some of the oldest and economically most important volcano-sedimentary basins within the Kaapvaal Craton of southern Africa. Precise age constraints which are vital to models aimed at understanding the formation, mineralisation and evolution of these basins have been hampered by analytical and geological problems. These problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins. Felsic and mafic volcanics from the Ventersdorp Supergroup yield ages of 2709 ± 4 Ma and 2714 ± 8 Ma, respectively constraining the timing of the closure of the Witwatersrand basin. A maximum age for the Witwatersrand basin is indicated by the age of 3074 ± 6 Ma for volcanics from the underlying Dominion Group. Thus a period of c. 350 Ma was available for the accumulation of the full Witwatersrand basin succession. Unfortunately, dating within the Witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin. These new data show that previous age determinations on the Ventersdorp, Witwatersrand and Dominion basins were inaccurate, and that the economically important Witwatersrand basin must now be recognised as Archaean.