Metasomatic effects have been thermodynamically computed for magma-derived sulfur gases, which may contaminate convecting meteoric fluids in the monzonite K-silicate protore, and are compared with natural assemblages in the Butte District of Montana. The advanced argillic assemblage with covellite is produced in hypogene environments at 300 degrees C by sulfur metasomatism of protore only for a limited range of net sulfur valences, essentially -1 to +2, placing limiting constraints on the possible ratios of SO 2 and H 2 S gases. A second important factor is the relative reaction rates of protore and contaminating sulfur gases. Contamination by sulfur gases is a third factor of importance. A final factor is that a minimum negative net volume of reaction causes, by wall-rock dissolution and fracture enlargement, the high permeability necessary for circulation of enormous quantities of hydrothermal fluids and vein growth characteristic of this extreme alteration facies.--Modified journal abstract.