FISSION XENON DATING OF WITWATERSRAND URANINITES - IMPLICATIONS FOR GEOLOGICAL ACTIVITY IN THE CENTRAL KAAPVAAL CRATON ABOUT 1 GA AGO

Four uraninite separates from different reefs from in and above the Ce ntral Rand Group (Upper Witwatersrand Supergroup) were obtained from g old mines in the East Rand, West Rand, and Welkom Goldfields of the Wi twatersrand Basin in South Africa. These samples were dated by the Xe- s-Xe-n spectrum technique. All four age spectra have a typical stepup form with plateaux for high temperature Xe release. The plateau ages a re 0.832 +/- 0.026 Ga for the Carbon Leader Reef sample from Blyvoorui tzicht Gold Mine, 1.043 +/- 0.024 Ga for the Basal Reef sample from Fr ee State Geduld Gold Mine, 1.115 +/- 0.036 Ga for the Main Reef uranin ite concentrate from Sub Nigel Gold Mine, and 1.300 +/- 0.058 Ga for t he Ventersdorp Contact Reef sample from Kloof Gold Mine. No apparent a ges in excess of 1600 Ma were observed. It is obvious from the general Witwatersrand chronological database that these fission xenon dating results do not correspond to primary crystallization ages for the uran inites. Instead, they most likely represent resetting events that affe cted the U-Xe isotopic system in these samples. Comparison with other recent chronological results obtained on rocks from the region of and around the Witwatersrand Basin indicates that the region was affected by magmatotectonic activity at 1.3 to 1 Ga ago, which, thus, was not o nly localized along the southern margin of the Kaapvaal Craton, the so -called Namaqua-Natal Thrust Front, but also affected other parts of t he craton.