Magmatic sulfides and Au : Cu ratios in porphyry deposits: an experimentalstudy of copper and gold partitioning at 850 degrees C, 100 MPa in a haplogranitic melt pyrrhotite intermediate solid solution gold metal assemblage,at gas saturation

The behavior of copper and gold in the phase assemblage: pyrrhotite (Po)-in termediate solid solution (Iss)-haplogranitic melt-gold metal (capsule) at 850 degrees C, 100 MPa and gas saturation (a(H2O) approximate to 0.9) has b een investigated. The results are presented as Nernst partition coefficient s. The partition coefficient, D-Cu(Po/Melt) = 2.6 +/- (0.3) x 10(3) (1 sigm a), was determined for Po and melt in equilibrium with an Iss of compositio n Cu(19 +/- 1)Fe(30 +/- 1)S(51 +/- 1) (2 sigma). For gold, the partition co efficients are: D-Au(Iss/Melt) = 5.7 +/- (2.2) x 10(3) (1 sigma) between Is s and melt, and D-Au(Po/Melt) = 140 +/- 40 (1 sigma) between Po and melt; t hese imply D-Au(Iss/Po) = 42 +/- 9 (1 sigma) between Iss and Po. The oxygen fugacity for the experiments, log f(O2) = - 13.2 +/- 0.3 (1 sigma), was de termined by using the hydrogen sensor technique. The sulfur fugacity, log f (S2) = - 1.0 +/- 0.7 (1 sigma), was calculated from the composition of Po. In our experiments, melts at Iss saturation have Cu/Fe by weight of ca. 1/1 00. The results indicate that crystallization of Iss may deplete a melt in gold, and crystallization of Po may deplete a melt in copper. However, a me lt saturated with Iss (at a given bulk composition, temperature, pressure, oxygen and sulfur fugacity and FeO concentration) will have a fixed concent ration of Cu, preventing strong depletion of the melt in Cu until Iss is fr actionated from the melt or changes in environmental conditions destabilize Iss. A thermodynamic model for the partitioning of Cu and Au between Po an d melt in terms of oxygen and sulfur fugacity indicates that D-Cu(Po/Melt) and D-Cu(Po/Melt) may increase greatly with increasing sulfur fugacity and decreasing oxygen fugacity within the Po stability field. The model also su ggests that destabilization of both Iss and Po relative to magnetite due to changes in oxygen and sulfur fugacities can result in the release of Cu an d Au from the sulfides to the melt. (C) 1999 Elsevier Science B. V. All rig hts reserved.