ANDESITE MELT-VOLATILE EQUILIBRIA AND ITS IMPLICATIONS FOR THE FORMATION OF MAGMATIC-HYDROTHERMAL ORE DEPOSITS

Andesite melts (SiO₂~59 wt%) and volatiles were equilibrated at 1000 ^oC and 200 MPa in rapid quench TZM or MHC alloy pressure vessel assemblies at an fO₂ of 0.4 log units below the Ni-NiO redox buffer. The exchange coefficients of Na-K-Fe, the volatile/melt partition coefficients of S, Cl, Mo and numerous other trace elements, and the apparent solubilities of Cu and Au in the silicate melt, have been simultaneously determined as a function of melt compositional variables (K/Na/Fe ratio, Cl and S concentration) and the composition of the volatile phase (S and Cl concentration, HCl/NaCl/KCl/FeCl₂ ratios). The composition of the silicate glass was determined by EPMA and LA-ICPMS before and after the experiments and the composition of the volatile phase has been calculated by mass balance. The partition coefficient of Cl is between 1.5 and 2.2 and slightly increases with increasing total Cl concentration. The exchange coefficient of K and Na (K_K/Na=D_K^{volatile/melt}/D_Na^{volatile/melt}) is between 1.1 and 1.3, independent of the HCl/total alkali chloride ratio, and total Cl concentration. The exchange coefficient of Fe and Na [K_Fe/Na=D_Fe^{volatile/melt}/(D_Na^{volatile/melt})²] ranges from 5 to 23 and is a negative function of fluid chlorinity. The volatile/melt partition coefficient of S in Cl-free system varies between 70 and 170 and negatively correlates with the FeO concentration in the melt. The volatile/melt partition coefficient of Mo is relatively low (0.16 to 0.44) and shows a negative correlation with fH₂S, but is independent of the fluid chlorinity and HCl/alkali chloride ratio. The apparent solubility of Cu ranges from 48 to 78 µg/g at a Cu activity of 0.01 and not significantly influenced by the Cl and S concentrations in the melt. The solubility of Au is 80±20 ng/g in a Cl and S free melt, and shows no clear relation with the concentration of Cl (100±24 ng/g at 0.96 wt% Cl in the melt), but strongly increases with S addition (730±170 ng/g at fS₂=12 kPa). The determined Mo partition coefficients and comparison of Cu and Au solubility data in the melt and the volatile phase obtained at near identical conditions suggest that volatiles released from intermediate magmas will be important for the formation of Au deposits, but less significant for Mo and Cu ore formation.