AN EXPERIMENTAL STUDY OF GOLD IN SULFIDE MINERALS

Au in magmatic-hydrothermal systems may often co-precipitate with common Cu-Fe sulfide minerals. Au has been found within bornite and chalcopyrite in porphyry ore deposits such as at Bingham Canyon and as “invisible” Au in pyrite and arsenopyrite samples from the Carlin trend. The Au concentrations within these Cu-Fe and Fe sulfide minerals have been explored as a function of temperature previously, but no study has systemically varied both temperature and sulfur activity in a way that mimics the conditions of porphyry ore formation. The activity of sulfur in magmatic-hydrothermal systems controls the stable sulfide mineral assemblage and has been shown to impact the solubility and speciation of Cu and Au in a magmatic volatile phase, however, its impact on Au in sulfide minerals is unknown. Experiments were conducted at 100 MPa with an oxygen fugacity buffered by Ni-NiO, and at temperatures of 500, 600, and 700 °C, to determine the solubility of Au within bornite, high-temperature chalcopyrite (intermediate solid solution – ISS), and pyrrhotite. The activity of sulfur in the system was buffered by sulfide mineral assemblages that induced values between log -11±1 and 0.4±0.8 (1σ). Au capsules were loaded with the select mineral assemblage and a 5 wt.% NaCl (eq.) aqueous solution composed of NaCl+KCl+HCl+H₂O. Sulfide mineral run products were analyzed by an Electron Microprobe to determine the concentration of Au and their textures after quench. Au exsolution features were observed in bornite and ISS throughout the mineral grains, whereas no exsolution textures were observed in pyrrhotite. Au in pyrrhotite ranged from 300-500 μg/g and did not vary appreciably over the entire range of the experiments. The solubility of Au in bornite increased from 1000 μg/g at 500 °C to 1800 μg/g at 700 °C and with an increase in the log sulfur activity of -11.0±1 to -6.0±0.1. The solubility of Au in ISS increased from 300 μg/g at 500 °C to 4000 μg/g at 700 °C with the activity of sulfur exerting the principal control on Au solubility as, at 700 °C, Au increased from 1100 to 4000 μg/g as it increased from log -6.0±0.1 to 0.4±0.8 (1σ). Our results demonstrate that Au will partition preferentially into ISS relative to pyrrhotite in porphyry systems with an ISS + pyrrhotite assemblage and into bornite for the bornite + ISS assemblage.