DISSOLVED VOLATILE CONCENTRATIONS IN AN ORE-FORMING MAGMA

Infrared spectroscopic measurements of glass inclusions within quartz phenocrysts from the Plinian fallout of the 22 Ma tuff of Pine Grove ( southwestern Utah) show that the trapped silicate melt contained high concentrations of H2O (6-8 wt%) and CO2 (60-960 ppm). Inclusion compos itions are consistent with either open- or closed-system degassing of this high-silica rhyolite during ascent from 16 km (430 MPa) to 9 km ( 250 MPa) depth, prior to eruption. Intrusive porphyries from the Pine Grove system are nearly identical in age, composition, and mineralogy to the tephra, and some contain high-grade Mo mineralization. Assuming that the porphyry magmas originally contained similar abundances of v olatile components as the erupted rocks, they would have been saturate d with fluid at pressures far greater than those at which the porphyri es were emplaced and mineralized. Even if no initial exsolved fluid wa s present when the magma began to ascend, it would have contained 19 t o 39 vol% bubbles at the approximately 3 km depth (80 MPa) of emplacem ent of many porphyries. The decrease in magma density and increase in porosity would have facilitated magma convection as well as advection of magmatic aqueous fluid through an interconnected network of bubbles . The data are consistent with formation of Climax-type Mo porphyry de posits by prolonged fluid flux from a large volume (> 20 km3) of relat ively Mo-poor (1-5 ppm) magma.