Microbeam U-Pb Zircon dating of the Makwassie Formation and underlying units in the Ventersdorp Supergroup of South Africa

The Archaean Ventersdorp Supergroup is the most widespread volcanosedimentary cover sequence on the Kaapvaal Craton. it unconformably overlies the Witwatersrand Supergroup and is unconformably overlain by the Transvaal Supergroup. Due to exploration drilling for Witwatersrand gold, the Ventersdorp Supergroup is the globally most extensively drilled sequence. Early efforts to date its volcanic units have commonly been thwarted by pervasive 2.3 to 2.0 Ga alteration. In situ U-Pb zircon dating promises a panacea, but the effects of lead mobility and high common lead remain potential pitfalls. The ages of the 'type' formations of the. Ventersdorp Supergroup sensu stricto are still not well established and much of the published data is on units only considered to be correlated with the 'type' formations. Quartz porphyry rhyolites of the Makwassie Formation in the Platberg Group are reliably dated by the laser ablation ICPMS U-Pb zircon method on four samples with a combined age of 2720 +/- 2 Ma. This supercedes the first ion probe date of 2709 +/- 4 Ma which is now regarded as a minimum age due to probable ancient lead loss and associated high common lead. The published 2714 +/- 16 Ma age for the stratigraphically lower Klipriviersberg Group was affected by similar problems. The Makwassie Formation is the only formation of the Ventersdorp Supergroup sensu stricto for which an accurate and reliable date has now been established by more than one precise age determination. Two samples considered to be from the Goedgenoeg Formation, which underlies the Makwassie Formation, were investigated. The formation is best dated by a 2746 +/- 9 Ma age on a feldspar porphyry borehole sample from near Wolmaransstad. A sample from a similar rock type also assigned to the Goedgenoeg Formation, which underlies the Makwassie Formation in Wesselton Mine at Kimberley, gave an age of 2781 +/- 5 Ma. A 30 Ma period of Goedgenoeg volcanism is considered to be unlikely. The older sample may rather be a time-correlate of a stratigraphically lower unit such as the 2785 +/- 2 Ma Kanye Formation or the 2769 +/- 2 Ma Derdepoort Outlier.