Archean lode-gold deposits: fluid flow and chemical evolution in vertically extensive hydrothermal systems

The range of conditions of formation of lode-gold deposits from the sub-greenschist to the lower-granulite facies in Archean greenstone belts, and the generally steeply plunging, vertically continuous pipe-like or tabular geometries of individual deposits, indicate long-distance hydrothermal fluid advection along well-defined channelways in the upper and middle crust. From presently available gold solubility data, destabilisation of gold-bisulphide complexes through H2S loss from the fluid to the wallrock was the dominant gold precipitation mechanism within these hydrothermal systems as a whole. This inference is supported by the S:Au ratios of ores. Sulphur and Au precipitation in the hydrothermal system is estimated to be relatively inefficient, with only 10-50% of S or Au contained in the fluid precipitated over any kilometre length of fluid channelway. The relative inefficiency of gold precipitation allowed mineralisation over a significant depth range in a crustal profile.