Equilibrium mineral–fluid calculations and their application to the solid solution between alunite and natroalunite in the El Indio–Pascua belt of Chile and Argentina

The effects of temperature and bulk-fluid K/Na on the alunite−natroalunite solid solution have been determined at 200 bar and 100–300 °C. These are conditions typical of magmatic-hydrothermal alteration and precious-metal mineral deposition in high-sulfidation environments, which are characterized by the presence of alunite-bearing alteration zones. Our model is based on published experimental parameters, which indicate that both alunite and natroalunite are stable over this temperature range, and that increased Na substitution is favored at higher temperature. Our calculation results predict that, at all temperatures, fluids in equilibrium with natroalunite are significantly more Na-rich than those typical of modern volcanic-hydrothermal environments. The results also indicate that a large variation in fluid K/Na is required to precipitate both K- and Na-rich alunite at high temperature (assuming near-equilibrium conditions). At lower temperature, much less variation in fluid composition can yield compositions near those of the end-members. ls on alunite–natroalunite solid solution in natural systems were evaluated using electron-microprobe data from high-sulfidation gold deposits of the El Indio–Pascua belt. The results indicate that higher temperatures of deposition correlate with a greater range of Na substitution in alunite. Elevated Na content in alunite is also locally attributed to higher concentrations of Na in the source fluid as a result of the leaching and alteration of andesitic host rocks. Dacitic to granitic host lithologies in other regions of the belt are correlative with more K-rich alunite compositions. The results from this study suggest that the use of alunite K–Na composition as a guide to mineral exploration should be treated with caution.