Volcanic and intrusive history of the Pine Grove porphyry molybdenum system, southwestern Utah.

The deposits occur in the eroded vent of a lower Miocene (23-22 m.y.) ash flow, located in a tilted basin and range fault block, eroded to c 1.7 km below the pre-eruption surface. Correlation of comagmatic ash flow tuff is possible due to the presence of almandine-spessartine garnets of unique composition. It was suggested that a 100 m3 parental magma chamber zoned from rhyolite to dacite was present, initial rhyolitc eruptions being followed by dacitic ashflows, and the dominant ore related intrusions. Multiple trachyandesite injections into the base of the chamber may have caused magmatic fracturing, surging of dacite magma, and exsolution of volatiles, resulting in ash flow eruptions and later mineralisation. Magmatic processes such as late stage convection, magma fracturing and volatile fluxing may have been critical in porphyry Mo mineralisation, with around 125 000 000 tons of 0.3% MoS2.
The deposits occur in the eroded vent of a lower Miocene (23-22 m.y.) ash flow, located in a tilted basin and range fault block, eroded to c 1.7 km below the pre-eruption surface. Correlation of comagmatic ash flow tuff is possible due to the presence of almandine-spessartine garnets of unique composition. It was suggested that a 100 m3 parental magma chamber zoned from rhyolite to dacite was present, initial rhyolitc eruptions being followed by dacitic ashflows, and the dominant ore related intrusions. Multiple trachyandesite injections into the base of the chamber may have caused magmatic fracturing, surging of dacite magma, and exsolution of volatiles, resulting in ash flow eruptions and later mineralisation. Magmatic processes such as late stage convection, magma fracturing and volatile fluxing may have been critical in porphyry Mo mineralisation, with around 125 000 000 tons of 0.3% MoS2.