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

Controls 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.