HYDROTHERMAL GEOCHEMISTRY OF ELECTRUM - THERMODYNAMIC CONSTRAINTS

This paper illustrates the potential and also the complexity of using Au-Ag alloys (the mineral electrum) as petrogenetic indicators of ore-forming processes. Thermodynamic calculations indicate that the solubility of gold is highly dependent on the composition, or fineness, of naturally occurring Au-Ag alloys. Alloy composition, in turn, is a complex function of the temperature, a(S2), a(O2), Cl- concentration, pH, and total Au/Ag content of the system in question. Hydrothermal solutions with low salinity, near-neutral pH, and high dissolved sulfide concentration have relatively high gold solubilities and Au/Ag solubility ratios. In contrast, more saline brines have lower gold solubilities and very low Au/Ag solubility ratios. Alloys formed from either type of fluid may be Au rich or Au poor, depending on the temperature, chemical environment, and mechanism of ore deposition. The compositions of both solid solution and aqueous solution may change drastically during reaction progress, resulting in deposits which are mineralogically zoned, either on a microscopic (individual electrum grain) or mesoscopic (ore deposit) scale. Dynamic modeling calculations are presented to illustrate these effects, using either open-system (Rayleigh fractionation) or closed-system boundary conditions. Coprecipitation with other Au- or Ag-bearing minerals (e.g., argentite, chlorargyrite, sulfosalts, tellurides, or selenides) places additional thermodynamic constraints on electrum composition. For example, the composition of electrum in equilibrium with argentite may be used as a geothermometer, provided an independent estimate of as, is available. Thermodynamic analysis indicates that Au-rich argentite may be a very important primary ore mineral, especially in deposits formed at high temperature, high a(S2), and/or high ratios of dissolved Ag/Au. Textural and compositional information recorded in parageneses containing electrum and other precious metal minerals must be interpreted with caution, especially if there is reason to suspect that ore deposition was rapid, or if there is evidence of postmineralization weathering, metamorphism, or other forms of reequilibration.