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

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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.