Origins and exploration significance of replacement and vein-type alunite deposits in the Marysvale volcanic field, west central Utah

Alunite deposits 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 delta ³⁴ S 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 delta ³⁴ S 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. Vein-type alunite deposits formed 14 m.y. ago as open-space fillings in extension fractures above a concealed stock. Crystals of alunite grew inward from the walls, forming veins of nearly pure alunite as much as 20 m thick. The delta ³⁴ S values near zero per mil indicate that the vein-type alunite sulfur probably had a magmatic source, and it appears that the sulfur may have been present as SO ₂ shortly after degassing from the magma. The probable magmatic origin for sulfur in the 14-m.y.-old vein-type alunite and its deposition from vapor-rich fluids, as well as the probable magmatic origin for sulfur in the surrounding base and precious metal deposits, suggest that the underlying stock may host porphyry-type deposits.