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The oldest known occurrence of magmatic anhydrite in a shallow sub-volcanic rock is at the Oligocene andesitic Eagle Mountain volcanic center (Huerto Formation, San Juan volcanic field), which immediately postdates the pyrrhotite-bearing Fish Canyon Tuff. Estimates of intensive parameters derived from the compositions of coexisting minerals (hornblende+plagioclase+anhydrite+pyrrhotite+S-rich apatite+augite+hypersthene+FeTi-oxides) indicate that the Eagle Mountain magma was wet, oxidized, and S-rich. It was much like K-rich andesitic and dacitic anhydrite-bearing magmas recently erupted from the subduction-related volcanoes El Chichón, Mt. Pinatubo, and Lascar. Although the Oligocene geodynamic setting of the San Juan volcanic field is poorly constrained, and its enormous volume is not easily reconciled with an arc setting, the Eagle Mountain occurrence emphasizes how closely some of its magmas resemble those from active continental margin arcs. S-rich magmas similar to the Eagle Mountain andesite also may have contributed to the formation of some hydrothermal ore deposits of the San Juan volcanic field.

Anhydrite, pyrrhotite, and sulfur-rich apatite: tracing the sulfur evolution of an Oligocene andesite (Eagle Mountain, CO, USA)