Abstract

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.

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