GOLD IN SULFIDES MINERALS IN THE HIGH TEMPERATURE PORPHYRY ENVIRONMENT

Au in magmatic-hydrothermal systems may often co-precipitate with common Cu-Fe sulfide minerals. The importance of sulfur activity in magmatic-hydrothermal systems is widely known, but its effect on the Au concentrations within Cu-Fe sulfides has not been studied extensively. The equilibrium Au concentrations at a variety of sulfur activities and temperatures have been measured in bornite (Bn), high temperature chalcopyrite (ISS), and pyrrhotite (Po). Experiments were conducted with cold-seal pressure vessels at 500-700°C, 100 MPa, with oxygen fugacity buffered by Ni-NiO, and sulfur activity buffered by select Cu-Fe-S mineral assemblages. Au capsules were loaded with the select mineral assemblage, a 5 wt.% NaCl (eq.) aqueous solution composed of NaCl+KCl+HCl+H₂O, and a pre-fractured quartz chip to trap fluid inclusions during the run. Sulfide mineral run products were analyzed by an Electron Microprobe (EMPA) to determine the concentration of Au and document the textures of the sulfides minerals after quench. Au exsolution textures were observed in Bn and ISS as fine stringers throughout the mineral grains, whereas no exsolution textures were observed in Po. Maximum concentration of Au within Bn and ISS was reported as two numbers, a total concentration which incorporated the Au stringers and a matrix concentration which did not. At 500°C, 600°C, and 700°C with a log sulfur activity of -9.75, -7.77, and -5.8, respectively, the total concentration of Au in Bn is 800±330 ppm (1σ), 2600±2400 ppm (1σ), and 3300±2300 ppm (1σ), with Au concentrations remaining in the matrix of 400±100 ppm (1σ), 600±310 ppm (1σ) and 500±140 ppm (1σ), respectively. At those same conditions, the total concentration of Au in ISS is 370±80 ppm (1σ), 830±130 ppm (1σ), and 1200±350 ppm (1σ), with Au concentrations remaining in the matrix of 400±100 ppm (1σ), 600±310 ppm (1σ) and 500±140 ppm (1σ), respectively. Thus, Au concentrations in Bn and ISS were found to increase with increasing temperature and increasing sulfur activity. These results provided better constraints on Au precipitation conditions and resource modeling for the mining industry. A complete model describing the relationship between sulfur activity, temperature, and Au concentrations in Bn, ISS, and Po has yet to be established and is undergoing further research.