EVIDENCE FOR THE ORIGIN OF HYDROTHERMAL ALTERATION IN GRANITOIDS AFTER WITWATERSRAND BASIN DEPOSITION, SOUTH-AFRICA

In order to help resolve the controversy of whether hydrothermal alter ation in granitoids associated geographically with the gold-bearing me tasediments of the Witwatersrand and Ventersdorp Supergroups is deuter ic or the result of later hydrothermal activity, a study was made of t he carbon isotopic and fluid inclusion compositions of alteration prod ucts in two typical examples of altered granitoid and in the gold-bear ing Kimberley Reef of the Central Rand Group (Witwatersrand Supergroup ) and the unconformably overlying Ventersdorp Contact Reef. The deltaC -13 values of hydrothermal carbon nodules in altered granitoids (-25.7 to -36.0 parts per thousand) and of nodular carbon (fly-speck) in the Ventersdorp Contact Reef (-20.6 to -32.4 parts per thousand) are ligh t, consistent with an organic origin and inconsistent with a magmatic origin. Fluid inclusions in quartz of the altered granitoids and in qu artz of shear zones in the gold-bearing reefs display almost identical populations of at least two types. One type, which occurs in all alte red granitoid bodies and in the shear zones of the gold-bearing strata , contains H2O, CH4, no daughter minerals, high salinities (up to 30 e quivalent wt% NaCl) and low homogenization temperatures (normally betw een 90 and 250-degrees-C), with Na and Ca as the dominant cations, cha racteristics of diagenetic formation brines and/or retrograde metamorp hic fluids. The fluid and stable isotope characteristics in the hydrot hermally altered granitoids and the gold-bearing strata are inconsiste nt with the hydrothermal alteration of the granitoids being exclusive of a deuteric origin. These data are consistent with major parts of th e alteration being a result of sediment dewatering affecting both the granitoids and gold-bearing horizons. Therefore, the hydrothermally al tered granitoids do not support their role as a possible source for th e sulphides, gold and carbon nodules of the Witwatersrand Basin as sug gested by Klemd and Hallbauer (1987) and Robb and Meyer (1987, 1990).