Replacement and vein-type alunite deposits formed in two significantly different epithermal environments in the Marysvale volcanic field of Utah. Alunite deposits, replacing intermediate composition lava flows, formed 23 m.y. ago in near-surface, highly oxidizing conditions at the tops of hydrothermal plumes that were spaced at 3- to 4-km intervals around a monzonite stock. The deposits are horizontally zoned outward, from alunitic cores to kaolinitic and propylitic envelopes, and are vertically zoned from a lower pyrite-propylite assemblage upward through assemblages dominated by alunite, jarosite, and hematite to a flooded silica cap. The bs4S values of 11.5 to 15.4 per mil for replacement alunite along with geologic constraints indicate that sulfate sulfur was derived from underlying Mesozoic evaporites. The bs4S values of-15.3 to +5.1 per mil of underlying pyrite, however, indicate that reduced sulfur, necessary to produce low pH during oxidation, either was produced by partial reduction of the evaporite sulfate or came from another source. The transition from the pyrite-propylite zone to the alunite zone probably marks the paleoground-water surface. In some localities, deposits of natroalunite, the sodium-rich analog of alunite, are superimposed
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Cunningham et al. (1984) studied this question.