Abstract
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.