A REVIEW OF THE LATE ARCHEAN VOLCANO-SEDIMENTARY DOMINION GROUP AND IMPLICATIONS FOR THE TECTONIC SETTING OF THE WITWATERSRAND SUPERGROUP, SOUTH-AFRICA

The Dominion Group is a volcano-sedimentary sequence of Archaean age underlying the Witwatersrand Supergroup in northern South Africa. New radiometric dating and recent petrologic studies make a review appropriate at this time. This review of the Group highlights its potential importance in understanding the tectonic setting and detrital source of the economically important Witwatersrand Supergroup. The sequence consists of a basal clastic sedimentary unit, with conglomerate containing placer Au and U, overlain by mafic amygdaloidal lavas and capped by massive feldspar-phyric felsic volcanics, recently dated at 3074 Ma. The mafic volcanics are amygdaloidal lavas deposited in subaerial or shallow subaqueous conditions. The felsic volcanics are mainly lavas or welded pyroclastic flows with interbedded volcaniclastic sediments. Geochemically, the Dominion volcanics are a bimodal sequence with subalkaline mafic tholeiitic basalts to andesites and low-silica rhyolite. The mafic volcanics are believed to have formed by partial melting of enriched mantle sources followed by fractionation. Relative depletions of Nb and other high field strength elements, along with Nd isotopic data, suggest a ''subduction component'' in the mantle source of the mafic magmas. The felsic volcanics were probably generated by partial melting of lower crustal sources. The Dominion volcanics, along with contemporaneous granitoid intrusions, represent a period of extensive magmatic activity on the tectonically-active northwestern flank of the Witwatersrand basin in a continental rift zone. If detrital gold in the Witwatersrand sediments was locally derived from exhalative or shallow-level lode deposits, Dominion Group magmatism could have been the driving force of the hydrothermal system producing the gold-bearing sulphide deposits along with contemporaneous hydrothermally-altered granites.