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, ![Formula][1] , was buffered by ISS (Cu = Fe), ISS + bornite, or ISS + pyrrhotite mineral assemblages, which induced a ![Formula][2] between −11 ± 1 and 0.4 ± 0.8 (1σ). The solubility of Au in bornite increased from 1,000 μ g/g at 500°C to 1,800 μ g/g at 700°C and with an increase in the ![Formula][3] of −11 to −6.0. The solubility of Au in ISS was evaluated as both a function of temperature and the ![Formula][4] and increased from 300 μ g/g at 500°C and low ![Formula][5] to 4,000 μ g/g at 700°C and high ![Formula][6] . The ![Formula][7] was found to exert a control on Au solubility as, at 700°C, Au solubility in ISS increased from 1,100 to 4,000 μ g/g as the ![Formula][8] 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 μ g/g as the ![Formula][9] increased from −8.3 ± 0.7 to −2 ± 1 (1σ). Gold in pyrrhotite ranged from 300–500 μ g/g and did not vary appreciably over the entire range of temperature and ![Formula][10] studied. Nernst-style partition coefficients for gold between ISS and pyrrhotite, ![Formula][11] , were calculated at 600°C to be 3.6 ± 1.9 (1σ) for a ![Formula][12] of −2 ± 1 (1σ) and at 700°C to be 7.3 ± 4.7, 7.8 ± 2.9, and 10 ± 4 (1σ) at ![Formula][13] of −0.2 ± 0.4, 0.3 ± 0.4, and 0.4 ± 0.8 (1σ), respectively. Partition coefficients for gold between bornite and ISS, ![Formula][14] , 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 ![Formula][15] 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 ![Formula][16] , whereas Au in pyrrhotite did not vary over the range of temperature and ![Formula][17] 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 ![Formula][18] in controlling the solubility of Au in sulfide minerals and provide important constraints for exploration. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /embed/mml-math-4.gif [5]: /embed/mml-math-5.gif [6]: /embed/mml-math-6.gif [7]: /embed/mml-math-7.gif [8]: /embed/mml-math-8.gif [9]: /embed/mml-math-9.gif [10]: /embed/mml-math-10.gif [11]: /embed/mml-math-11.gif [12]: /embed/mml-math-12.gif [13]: /embed/mml-math-13.gif [14]: /embed/mml-math-14.gif [15]: /embed/mml-math-15.gif [16]: /embed/mml-math-16.gif [17]: /embed/mml-math-17.gif [18]: /embed/mml-math-18.gif