Structural complexity and Witwatersrand gold, South Africa: a synthesis

The structural geology of the Witwatersrand Basin in South Africa is much more complex than has been generally conveyed. Some previous reviews suggest whole goldfields have avoided deformation, whereas other studies show there has been a polydeformational history of extension and contraction like that found in younger foreland basins. We show that the structural geology of all seven Witwatersrand goldfields consistently reveals deformational features on scales from thin-sections to stopes, goldfields and to the whole basin. Although there are significant structural differences between the goldfields, all record layer-parallel movement dictated by variable rock unit rheology, and this deformation is concentrated around the various gold reef groups adjacent to unconformities. Previous under-reporting of deformation has led to a distorted portrayal of the Witwatersrand that overlooks the importance of deformation, metamorphism and alteration. We find evidence for these three connected processes in all mines that we have researched and on most underground reef photographs we have studied. Gold grade is controlled by structures at all scales, and this makes the understanding of the complex deformation important economically. The direct relationship of gold to late out-of-basin thrusting, folding and fracturing is evident when studying the individual reef groups including their unconformities that extend for hundreds of kilometres laterally. The relationships between gold and structures are also evident when considering the vertical stacking of reefs through 14 km of stratigraphy and over 320 m.y. of host-rock age as illustrated near the Klerksdorp goldfield. For Witwatersrand gold, it is essential to understand and synthesise the deformation, metamorphism and alteration regardless of whether the gold-mineralising process is early or late. There is abundant evidence for structural networks and fluid connectivity at the Witwatersrand Basin scale. Major faults link to smaller-scale veins and fractures facilitated by rheological contrasts at unconformities and other boundaries. There are widespread occurrences of anomalous gold concentrations (some economic and mined) over 320 m.y. of host-rock ages. Gold grades follow folds, faults and fractures within the reef packages. Late Archean (post-Klipriviersberg Group) deformation produced both extensional deformation in the south and east, and out-of-basin thrusting to the northwest and northeast. These thrust structures correlate with gold mineralisation. The structure, metamorphism and alteration all inform on the hydrothermal processes and the scale and timing of fluids and gold. It is likely that most or all the Witwatersrand gold was introduced to the basin in the globally consistent Archean hydrothermal gold emplacement event between about 2700 and 2600 Ma and synchronous with Limpopo collision tectonics.