Gold mobilizing fluids in the Witwatersrand Basin: composition and possible sources

Abstract

Crush-leach data were obtained, using High Performance Gradient Ion-Chromatography and Capillary Electrophoresis, on individual generations of aqueous fluid inclusions in hydrothermal quartz from three different auriferous conglomerate horizons (reefs) in the late Archaean Witwatersrand Basin, South Africa. These data, supplemented by oxygen isotope analyses of hydrothermal quartz and in combination with microthermometric analyses, help to constrain the chemical composition, pH, temperature of formation and the possible source of the mineralizing fluid which, in places, was capable of mobilizing some of the primarily detrital gold in the fluvial Witwatersrand sediments. The dominant cations in the aqueous fluid inclusions are Na+ and Ca2+, with Cl− or HCO 3 − being the dominant anion, whereas K+, Mg2+, and SO 4 2− are subordinate. Most fluid inclusions have elevated NH 4 + concentrations which are directly correlated with those of N03. In a number of samples small amounts of organic acids (formate, propionate, and acetate) were also detected. A largely meteoric source is inferred for the gold-mobilizing fluids in the Witwatersrand reefs because of a lack of Br in the fluid, a composition distinctly different from that of seawater, the presence of organic acids, and δ18Ofluid values around O%o. The fluids are ascribed to hydrothermal infiltration triggered by the 2020 Ma Vredefort impact which also created a secondary permeability in the form of a dense network of micro-fractures preferentially in the conglomerate beds of the already metamorphosed Witwatersrand rock sequence. This fluid differs from the regional metamorphic fluid in the basin by having a considerably higher pH (5.7–7.2). The difference in pH might explain why the older, fairly acidic metamorphic fluid was apparently less capable of mobilizing the gold as gold solubility reaches its peak at the pH calculated for the fluid ascribed to the impact.