Gold Solubilities in Bornite, Intermediate Solid Solution, and Pyrrhotite at 500° to 700°C and 100 MPa

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 over a range of sulfur activities likely to be found in the porphyry environment. The activity of sulfur in the system, $aS2sys$⁠, was buffered by ISS (Cu = Fe), ISS + bornite, or ISS + pyrrhotite mineral assemblages, which induced a $logaS2sys$ between −11 ± 1 and 0.4 ± 0.8 (1σ). The solubility of Au in bornite increased from 1,000 μg/g at 500°C to 1,800 μg/g at 700°C and with an increase in the $logaS2sys$ of −11 to −6.0. The solubility of Au in ISS was evaluated as both a function of temperature and the $aS2sys$ and increased from 300 μg/g at 500°C and low $aS2sys$ to 4,000 μg/g at 700°C and high $aS2sys$⁠. The $aS2sys$ was found to exert a control on Au solubility as, at 700°C, Au solubility in ISS increased from 1,100 to 4,000 μg/g as the $logaS2sys$ increased from −6.0 ± 0.1 to 0.4 ± 0.8 (1σ). The solubility of Au in ISS at 600°C increased from 800 to 1,800 μg/g as the $logaS2sys$ increased from −8.3 ± 0.7 to −2 ± 1 (1σ). Gold in pyrrhotite ranged from 300–500 μg/g and did not vary appreciably over the entire range of temperature and $aS2sys$ studied. Nernst-style partition coefficients for gold between ISS and pyrrhotite, $DAuISS/Po$⁠, were calculated at 600°C to be 3.6 ± 1.9 (1σ) for a $logaS2sys$ of −2 ± 1 (1σ) and at 700°C to be 7.3 ± 4.7, 7.8 ± 2.9, and 10 ± 4 (1σ) at $logaS2sys$ of −0.2 ± 0.4, 0.3 ± 0.4, and 0.4 ± 0.8 (1σ), respectively. Partition coefficients for gold between bornite and ISS, $DAuBn/ISS$⁠, were calculated to be 2.5 ± 1.8, 1.9 ± 0.7, and 1.6 ± 0.7 (1σ) for 500°, 600°, and 700°C at $logaS2sys$ of −11 ± 1, −8.3 ± 0.7, and −6.0 ± 0.1 (1σ), respectively. In summary, the solubility of Au in bornite and ISS was found to increase with increasing temperature and the $aS2sys$⁠, whereas Au in pyrrhotite did not vary over the range of temperature and $aS2sys$ studied. Further, 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. These data illustrate the importance of temperature and the $aS2sys$ in controlling the solubility of Au in sulfide minerals and provide important constraints for exploration.