| The Pine Grove porphyry molybdenum deposit in southwestern Utah occurs in the eroded vent of a lower Miocene ((TURN)24 m.y.) rhyolitic ash flow/dome volcano complex. Initial eruptions from the Pine Grove system began with volatile-rich rhyolitic magma that contained quartz (9.1% (delta)('18)O), sanidine (Ab(,36-34)), plagioclase (Ab(,84-80)), Fe-rich biotite, garnet (Al(,62) Sp(,33) Gr(,4)), titanomagnetite (8-11% Usp.), ilmenorutile (?), zircon, monazite, and xenotime. Late ash flow eruptions simultaneously vented rhyolitic magma as well as dacitic magma that contained quartz ((TURN)8.2% (delta)('18)O), sanidine (Ab(,29-28)), plagioclase (Ab(,73-52)), Mg-rich biotite, hornblende, augite, titanomagnetite (11-18% Usp.), ilmenite, zircon, apatite, and sphene. Geothermometry (using two-feldspar, (delta)('18)O for quartz-garnet, and magnetite-ilmenite methods) suggests a temperature range of (TURN) 650-670(DEGREES)C for the rhyolitic magma and 670-860(DEGREES)C for the dacitic magma. Although no discontinuity in temperature existed between the rhyolitic and dacitic portions of the magma chamber, mineral compositions indicate that a compositional gap was present between the two magmas. Eruption volumes suggest a chamber volume of approximately 100 km('3).;The dominant ore-related intrusion was emplaced immediately following concurrent intrusion and rapid mixing of rhyolitic and dacitic magma. Underplated mafic magma may have initiated convection, surging of dacitic magma, and exsolution of volatiles that resulted first in the ash flow eruptions and later in mineralization. Magma characteristics that may have been important in generating a Mo-rich ore fluid from a magma with near average concentration of Mo are: (1) high fO(,2); (2) large temperature gradient; (3) high alkali feldspar/plagioclase ratio; (4) post-eruptive polymerization increase; and, (5) convection.;Rhyolitic and ash-flow tuffs, domes, and ore-related intrusions are high-silica (78-76% SiO(,2)), metaluminous units that exhibit enrichment in K, Sc, Mn, Zn, Rb, Y, Nb, HREE, Ta, and U compared to dacitic units showing converse enrichment in Mg, P, Ca, Ti, Fe, Sr, Zr, Ba, LREE, Eu, Hf, and Th. Crystallization of the phases present in the dacitic portion of the chamber coupled with thermodiffusive processes may have generated a buoyant rhyolitic liquid that accumulated in the roof zone of the chamber. |
