Metals in Arc Magmas: The Role of Cu-Rich Sulfide Phases

Based on experiments performed on hydrous andesitic melts at 1000°C, 150 MPa, fO2 from the Co-CoO to Ni-NiO buffer, and log fS2 equal to -0.5 to -1.5 (bar), greater than 32 ± 4 ppm copper (all uncertainties = 1 sigma, standard deviation of the mean) in the silicate melt favors the formation of a Cu-Fe sulfide liquid (CFSL) relative to pyrrhotite at sulfide saturation. This concentration is well within the range encountered in intrusive and extrusive rocks suggesting that saturation by sulfide liquids is a common occurrence in magmatic arc systems consistent with observations in naturally occurring andesites. Nernst-type partition coefficients determined from these experiments highlight the importance of accurately modeling the composition of the sulfide phase present during partial melting or fractional crystallization: Dpyrrhotite/melt = 1320 ± 220 for Cu, 1.73 ± 0.37 for Mo, 90 ± 19 for Ag, and 500 ± 87 for Au, whereas DCFSL/melt = 7,800 ± 1,400 for Cu, 0.45 ± 0.14 for Mo, 6,800 ± 1,300 for Ag, and 84,000 ± 19,000 for Au. Data from these experiments support a direct correlation between the solubility of gold and the concentration of sulfur in the silicate melt at low fO2, as well as a dependence of the solubility of gold on fS20.25 in pyrrhotite and CFSL. As a part of this research, pyrrhotite of variable copper concentration was equilibrated at 1000°C in sealed evacuated silica tubes to determine a method that allows the equation of Toulmin and Barton (1964) to be used to calculate fS2 for Cu-bearing pyrrhotite. This method is consistent for pyrrhotite with up to 6 wt % Cu by using N=2*[(XCu+XFe)/(1.5XCu+XFe+XS)]. These data suggest that separation of CFSL from the magma along with crystalline phases during fractional crystallization can reduce the likelihood of magmatic hydrothermal ore formation. For example, modeling 30 % Rayleigh fractional crystallization (F=1.0 to F=0.7), with 0.1% sulfide among the separating phases, and an initial 65 ppm Cu in the silicate melt, would result in the sequestration of up to 50% of the initial Ag, 60 % Cu, and > 99 % Au

Experimental study of the partitioning of Cu, Ag, Au, Mo and W among pyrrhotite and immiscible Fe-S-O and silicate melts

Partition coefficients have been determined for several transition metals amongst a rhyolitic silicate melt, pyrrhotite, and an immiscible Fe-S-O melt. Two sets of experiments were performed in sealed silica tubes at 1035°C-1045°C, FMQ-NNO O2, and logS2 ~1 bar, and analyzed by EMPA and LA-ICP-MS. Experiments yielded DFe-S-O/melt ±1σ(mean) for Au =300±100, Ag =120±20, Cu >200, Mo =90±10 and W =9±3, and Dpo/melt for Au =120±50, Ag =58±8, Cu >200, Mo =35±3 and W =1.2±0.6x10-3. Some preliminary data are also presented on the partitioning of Si, Ti, V, Co, Zn, Re, Th and U. Modeling predicts the loss to a daughter melt of up to 50% of the initial Au, >37% Cu, 24% Ag, 19% Mo, and 2% W when Fe-S-O melts are removed with other cumulate phases during fractionation. This would reduce the metal available to potential intrusion-related hydrothermal ore deposits