Origin of secondary sulfate minerals on active andesitic stratovolcanoes

Sulfate minerals in altered rocks on the upper flanks and summits of active andesitic stratovolcanoes result from multiple processes. The origin of these sulfates at five active volcanoes, Citlaltépetl (Mexico), and Mount Adams, Hood, Rainier, and Shasta (Cascade Range, USA), was investigated using field observations, petrography, mineralogy, chemical modeling, and stable-isotope data. The four general groups of sulfate minerals identified are: (1) alunite group, (2) jarosite group, (3) readily soluble Fe- and Al-hydroxysulfates, and (4) simple alkaline-earth sulfates such as anhydrite, gypsum, and barite. Generalized assemblages of spatially associated secondary minerals were recognized: (1) alunite+silica±pyrite±kaolinite±gypsum±sulfur, (2) jarosite+alunite+silica; (3) jarosite+smectite+silica±pyrite, (4) Fe- and Al-hydroxysulfates+silica, and (5) simple sulfates+silica±Al-hydroxysulfates±alunite.