Evaluating the structural character and tectonic history of the Witwatersrand Basin

Abstract

The Witwatersrand Basin in South Africa has been the focus of gold exploitation for more than 100 years, and the subject of hundreds of research investigations, principally sedimentological and metallogenic, and recently, geophysical and geochronological. Surprisingly the basin-wide structural character has not been fully investigated despite the fact that gold metallogenic models have been proposed that want structural architectures as conduits for gold mobilisation and/or sites of deposition (i.e., the modified placer and hydrothermal models). These models, which have polarised viewpoints on the source for gold (placer versus metamorphic or hydrothermal source), assume an underlying structural character to the basin and structural relationships between goldfields, but these assumptions have never been tested through evaluation of the existing structural dataset. As corollary, the published data for the structural character of the Free State (Welkom), Klerksdorp, West Wits Line, West Rand, Central Rand, East Rand and South Rand goldfields are summarised and evaluated to search for basin-wide similarities and/or correlations that would support tectonic models for the Witwatersrand basin, and explain its metallogenic character. Unfortunately, the large published historical database (>90 years) proved to be quite limited with respect to geometric, kinematic and/or relative chronological detail, but must be evaluated against an advanced understanding of Earth processes, tectonics and the structural geology approach. The dataset is largely qualitative rather than quantitative, but is useful in establishing broad tectonic trends of the formation and tectonic evolution for the Witwatersrand Basin. It suggests that two deformation events (Umzawami and Ukubambana events) mark the end of deposition in the Witwatersrand and Transvaal basins, respectively, with development of fold-thrust belts at ca. 2.7 and 2.2-2.0 Ga and these can be correlated with supercontinental cycles. The intervening period (i.e., 2.7-2.2 Ga) was characterised by at least 4 basin-forming events that culminated in the deposition of the volcanic-sedimentary sequences of the Klipriviersberg Group, and the sedimentary sequences of the Platberg, Chuniespoort and Pretoria groups. Basin formation was facilitated by the reactivation of structures formed during the Umzawami Event (or older) as feeder dykes, normal-listric faults, or growth faults. The limited structural data for the Witwatersrand basin unsurprisingly also leads to the conclusion that gold was macroscopically to mesoscopically remobilised from a primary placer source, but remobilisation was localised (goldfields-scale) and not is a basin-wide phenomena. © 2009 Elsevier B.V. All rights reserved.