K-Ar dating of white micas from the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa: timing of post-depositional alteration

White micas from metamorphosed and hydrothermally-altered basaltic lavas, c onglomerates, quartzites and shales in and around the Ventersdorp Contact R eef of the Witwatersrand Basin, South Africa, were dated by the K-Ar method to constrain post-depositional thermal and mineralization processes. The m inerals were separated into various grain sizes between <0.4 and 10 mu m, a nd characterized in terms of composition, paragenetic sequence and texture, by XRD, SEM and TEM techniques. The K-Ar isochron of all white micas in th e basaltic lavas suggests an age of 1994 +/- 60 Ma, and that of the smaller mica particles (<2 mu m) in the quartzites defines a younger age of 1917 /- 66 Ma. This range is considered to reflect the timing of long lived hydr othermal alteration in the Ventersdorp Contact Reef. The older age is sligh tly younger than the intrusion of the Bushveld Complex (2060-2054 Ma) and t he Vredefort catastrophism (2020 Ma), which are well-documented events that were superimposed onto the Witwatersrand Basin. The younger age may be ass ociated with the Eburnian orogenesis along the western edge of the Kaapvaal Craton resulting in continental-scale fluid migration and hydrothermal act ivity that extended throughout the Griqualand Basin and even into the Witwa tersrand Basin. The K-Ar ages obtained here for the white mica fractions of the Ventersdorp Contact Reef in the Witwatersrand Basin confirm that the p eriod between 2.0 and 1.9 Ga was significant, as far as alteration, and pos sibly also gold mobilization, was concerned.