There is widespread evidence for greenschist facies metamorphism in different rock types within all the Witwatersrand gold mines. The silicate mineral assemblages suggest near-uniform peak metamorphic conditions equivalent to the chlorite zone in all the major Witwatersrand gold fields. Assemblages of coexisting pyrophyllite-chloritoid have been recorded in most of the gold mines, and the critical assemblage of pyrophyllite-chloritoid-muscovite-chlorite-quartz-tourmaline-rutile-pyrite is found in most gold fields. Kaolinite, paragonite, biotite, pyrrhotite and arsenopyrite have more local distribution. Pelites (“shales”), quartzites, meta-conglomerates, metabasalts and some dykes record these metamorphic assemblages, albeit in different mineral proportions, and pyrophyllite and chloritoid are recorded from many gold-producing reefs. Andalusite, kyanite, garnet and/or cordierite-bearing assemblages indicate higher metamorphic grades in three discrete areas: along the NW margin of the Witwatersrand basin, to the NE of the Evander gold field, and around the collar of the Vredefort Dome. Concentric metamorphic facies around the Vredefort Dome culminate in granulite facies assemblages near the centre. It is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the Vredefort Dome, greenschist facies assemblages in the Vredefort collar rocks, thermal metamorphism related to intrusion of the Bushveld Complex, and the regional greenschist facies metamorphism documented in the gold fields. Metamorphic conditions of 350° ± 50°C and up to 3 kbars are inferred for the gold fields, with temperatures increasing to around 700°C in granulites of the Vredefort Dome. Other geothermometric methods, including illite crystallinity, fluid inclusions, carbon elemental ratios and vitrinite reflectance, are compatible with the above estimates, but are each associated with considerable temperature uncertainties. Fluids in equilibrium with pyrophyllite-muscovite-quartz-pyrite±paragonite are inferred during metamorphism, suggesting slightly acid conditions and elevated sulphur activities. Regional greenschist facies metamorphism documented in the Witwatersrand gold mines was accompanied by widespread deformation, large fluxes of aqueous fluid, and considerable element mobility. The low-intermediate pressure of this metamorphic terrain and the lack of large-scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern. No model for gold genesis in the Witwatersrand Basin is complete without accommodating post-burial processes including metamorphism, deformation and alteration. Any placer gold would be overprinted by these processes, and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity. It is also important to note at least two quite unusual features of the Witwatersrand metamorphic assemblages: the virtual absence of feldspar from the 3-km-thick Central Rand Group over 300 km, and the narrow range of metamorphic grade inferred for the gold mines over this same distance.