Solubility and partitioning behavior of Au, Cu, Ag and reduced S in magmas

Abstract

Experiments have been conducted at 200MPa, 800–1030°C, and fO2 0.8 log units below the Ni–NiO buffer to determine the solubility of Au, Cu and Ag in silicate melts and pyrrhotite. The metal activities were imposed by using Au965Ag020Cu015 alloy capsules. Sulfur-free and sulfur-bearing systems were studied with otherwise identical melt compositions to assess the relative effect of S on the solubility of these metals. The data show that the major element composition of the silicate melt only moderately affects the solubility of Au, Ag and Cu between basalt and dacite, yielding solubilities identical within 50% relative. In comparison, solubilities in the rhyolite melts are lower by a factor of 1.5–2.5 for Cu and higher by up to a factor of 5 for Ag, depending on the aluminum saturation index of the melt. The solubilitiy of Ag significantly increases with increasing peraluminousity above an aluminum saturation index of 1. The effect of melt composition is significant on the solubility of Au in S-bearing melts, in part due to its effect on the sulfur concentration at sulfide saturation. The effect of S is the most pronounced in peralkaline rhyolites and mafic melts, and minimal in peraluminous rhyolites. The solubilities of all three metals significantly decrease with decreasing temperature. The concentration of sulfur in the melt at sulfide saturation and its volatile/melt partition coefficient are primarily determined by the FeO activity in the melt and the activity coefficients of dissolved FeS species, which appear to correlate with the degree of melt polymerization. The volatile/melt partition coefficient of reduced S increases from 79±4 (1σ) to 635±80 as the melt composition changes from basalt to slightly peraluminous rhyolite, and from 225±13 to 776±148 as the aluminium saturation index increases from 0.7 to 1.1 in rhyolites.At 1000°C, pyrrhotite/silicate melt partition coefficients for Cu increase from 540±30 (1σ) to 1140±110 from basalt to dacite, whereas the partition coefficients for Ag are nearly constant at 50±10. The partition coefficients of Au increase from 180±20 to 900±210 from basalt to dacite. Pyrrhotite/rhyolite melt partition coefficients for Cu, Ag and Au increase by about an order of magnitude as temperature drops from 1000°C to 800°C.At typical S concentrations for arc magmas (500–2000μg/g), the budget of Ag in the magma will not be controlled by pyrrhotite, whereas primary control on the Au and Cu budget by pyrrhotite may only be relevant in intermediate to felsic magmas.