ALTERATION, HEAT, AND WITWATERSRAND GOLD - 111 YEARS AFTER HARRISON AND LANGLAAGTE

The Witwatersrand Basin records evidence of widespread alteration with in the Central Rand Group in all goldfields and well into the central part of the basin, and also within mineralized portions of the West Ra nd Group. This alteration has affected all rock types and is character ized by assemblages of hlorite-muscovite-pyrite-rutile-tourmaline-quar tz, or combinations of these minerals, plus variable albite, paragonit e, margarite, kaolinite, kyanite, sudoite, and pyrrhotite. Alteration assemblages are spatially related to mineralization, with both being b est-developed in the Central Rand Group, and within the goldfields. An approximate estimate of the volume of rock in the Witwatersrand Basin affected by this alteration is 5000 - 50 000 km(3), and the temperatu re of the implicated fluid exceeded 300 degrees C. Based on alteration assemblages, fluid inclusions, and metal ratios, the composition of t his fluid had many similarities to the fluids inferred to be responsib le for the transport of gold in many slate-belt and Archaean greenston e gold deposits (e.g. similarities in temperature, pressure, salinity, redox state, H2O-CO2-H2S composition, and associated elements such as As, Sb, and Hg). The distinctive K-alteration assemblage in the Witwa tersrand goldfields of muscovite plus pyrophyllite is more likely to r eflect the pool buffering capacity of the Central Rand Group rocks fol lowing diagenesis rather than unusual fluid acidity. Acidity is poorly constrained, and K/H was probably not constant throughout the alterat ion system. Regional metamorphism alteration of greenschist facies gra de which stabilized pyrophyllite-chloritoid has affected all the goldf ields around the basin margin. This metamorphic pattern in the goldfie lds has been linked to the amphibolite-granulite facies metamorphism i n the Vredefort dome area in the basin centre, and both metamorphic pr ogressions are inferred to be part of one high-temperature-low-pressur e metamorphic event. Although the most obvious major thermal event to explain metamorphism around the Witwatersrand Basin is the Bushveld Ev ent at 2050 Ma, the geological evidence better supports an age of meta morphism, generation, and migration of substantial fluid and alteratio n of the Witwatersrand goldfields around 2700 - 2600 Ma. The latter is also the likely age of gold mineralization. The coincidences of goldf ields and alteration spatially, and Witwatersrand alteration fluid and greenstone gold fluid compositionally, give credence to the concept t hat a major hydrothermal fluid event after burial of the Witwatersrand Basin was a critical element in the formation of this enormous goldfi eld. This hydrothermal scenario casts serious doubts on the placer mod el for Witwatersrand gold, but importantly continues, and possibly enh ances, the role of sedimentology, stratigraphy, and structure in the l ocalization of gold. In particular, iron and carbon played critical ro les in precipitation of gold from solution, and the distribution of bo th Fe and C was influenced by sedimentary processes.