Gold precipitation: the Big Picture from micro-chemical processes

Abstract

Samples of gold-bearing laminated veins from the Sunrise Dam Gold Mine (Western Australia) have been investigated in detail using a combination of advanced imaging and analysis techniques (electron optics, proton-induced x-ray emission, laser ablation). This analysis reveals a very complex chemical and mechanical history recorded at the micro-scale which illustrates the complexity of simple vein systems and the nature of gold precipitation. The laminations represent micro-breccias within a pre-existing quartz-carbonate vein. The gold is hosted by coarse pyrite but is intimately associated with the precipitation of apatite, dolomite, tourmaline-muscovite, rutile and zircon. The chemistry of the apatite, in-situ Sr isotope analysis, the mineralogy of the breccia and the coarse pyrite gold-host indicates that two fluids were present in the system. An oxidised, CO2- rich (water-poor), S-poor fluid with a strong mantle affinity and a water-bearing, reduced S, As, Fe-bearing fluid. The mixing of these two fluids during the cracking of a strong pre-existing quartz vein in a weak sheared host rock led to gold localisation and precipitation.