Abstract

The Pine Grove porphyry molybdenum deposit in the Wah Wah Mountains, southwestern Utah, occurs in the eroded vent of a lower Miocene ash flow (23-22 m.y. old). The vent, located in a tilted basin-and-range fault block, has been eroded about 1.7 km below the pre-eruption surface; consequently, the closest outcrops of related ash-flow tuff are 10 km away from the vent. Correlation of comagmatic ash-flow tuff and intrusive porphyry is made possible by the presence of accessory almandine-spessartine garnet of unique composition in both extrusive and intrusive units.Several lines of evidence suggest the presence of a parental magma chamber roughly 100 km 3 in size and compositionally zoned from rhyolite to dacite. Each eruptive episode began with a high silica content, rhyolitic magma containing quartz, sanidine, plagioclase, biotite, garnet, titanomagnetite, monazite, xenotime, ilmenorutile (?), and zircon. Presumably in response to continued rapid eruption and deeper drawdown in the parent chamber, some ash-flow eruptions also vented dacitic magma containing quartz, plagioclase, sanidine, biotite, hornblende, augite, titanomagnetite, ilmenite, sphene, apatite, and zircon.The dominant ore-related intrusion was emplaced immediately following concurrent intrusion of rhyolitic and dacitic magma into the vent. Multiple injections of trachyandesitic magma into the base of the chamber may have initiated convection or magma fracturing, surging of dacitic magma, and exsolution of volatiles that resulted, first, in ash-flow eruptions and, later, in episodes of mineralization. Therefore, an unusual combination of eruption mechanics and chamber volume and geometry allowed sampling of possibly the entire compositional range of the chamber during eruption but prohibited the complete loss of rhyolitic magma. Magmatic processes, such as late-stage convection, magma fracturing, and volatile fluxing may have been critical in the development of porphyry Mo mineralization.

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