GOLD MICRO-INCLUSIONS IN SULFIDE MINERALS

Gold in hydrothermal systems has been documented with bornite and chalcopyrite in porphyry ore deposits such as at Bingham Canyon and as “invisible” Au in pyrite and arsenopyrite in the Carlin trend. The Au associate with these sulfide minerals can occur as separate minerals or as micro-inclusions within the sulfides. The micro-inclusions may be the result of Au co-precipitating with the sulfide minerals and being incorporated as inclusions or because it was contained within the sulfide mineral structure during its formation at elevated temperatures and later exsolved during cooling. Whereas the solubility of gold within select Cu-Fe and Fe sulfide minerals has been explored previously as a function of temperature, no study has also varied the fugacity of S₂. Experiments were performed 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. Gold capsules were loaded with the select mineral assemblage and a 5 wt.% NaCl (eq.) aqueous solution composed of NaCl+KCl+HCl+H₂O. The experiments were run for a minimum of 12 days before being rapidly quenched. Gold stringers and blebs were observed throughout the quenched bornite and ISS, whereas none were observed in pyrrhotite. We interpret these blebs and stringers to be exsolution products formed during quench. Gold in pyrrhotite ranged from 300-500 μg/g and did not vary appreciably. 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 S₂ fugacity of -11.0±1 to -5.6±0.1 bars (1σ). The solubility of Au in ISS increased from 300 μg/g at 500 °C to 4000 μg/g at 700 °C with the fugacity of S₂ exerting the principal control on Au solubility as, at 700 °C, Au increased from 1100 to 4000 μg/g as it increased from log -5.6±0.1 to 0.7±0.8 bars (1σ). These data suggest that Au may exist as AuS_0.5 within ISS and possibly bornite. Gold in pyrrhotite is hypothesized to be contained interstitially and was not found to be impacted by changes in S₂ fugacity. Additionally, our results demonstrate that Au will partition preferentially into ISS relative to pyrrhotite in systems with an ISS + pyrrhotite assemblage and into bornite for the bornite + ISS assemblage.