THE PETROLOGY AND GEOCHEMISTRY OF THE YERINGTON BATHOLITH AND THE ANN-MASON PORPHYRY COPPER DEPOSIT, WESTERN NEVADA (JURASSIC, GEOCHRONOLOGY, ALTERATION)

Field, petrologic, and geochemical data presented herein constrain the origin of porphyry copper mineralization in the Jurassic Yerington batholith. The batholith, with 58-68% SiO(,2), Peacock index (TURNEQ) 56, high K(,2)O (3.0 wt% at 60% SiO(,2)), high strontium ((TURNEQ)1100 ppm), and low initial strontium isotopic composition (0.7040), is similar to high-K orogenic andesites. Three major units were emplaced, from oldest to youngest: granodiorite, quartz monzonite, and porphyritic quartz monzonite (PQM), including porphyry. They show progressive decrease in volume, increase in depth of roof emplacement ( 3 km, respectively), increase in grain size, and increase in silica content ((TURNEQ)60, (TURNEQ)66, (TURNEQ)68 wt% SiO(,2), respectively). Two concordant U-Pb zircon dates of 169 m.y.b.p. on granodiorite and 168 m.y.b.p. on mineralized quartz monzonite porphyry (QMP) indicate the batholith was emplaced within one million years. Phase petrology, stable isotopes, and both iron-titanium oxide and biotite compositional data indicate each plutonic unit crystallized while water-saturated at relatively high oxygen fugacities to the solidus at 700 (+OR-) 25(DEGREES)C, and that minerals reequilibrated to 500 (+OR-) 50(DEGREES)C during cooling in the presence of a magmatic water phase.;Hydrothermal alteration affected (TURNEQ)25 km('3) of rock at the Ann-Mason porphyry copper deposit. Main stage biotite (potassic) alteration and copper mineralization was contemporaneous with a central QMP dike swarm and was accompanied by flanking/deeper oligoclase-actinolite (sodic-calcic) alteration. Both alterations contain high salinity fluid inclusions trapped at 300-480+(DEGREES)C, but field relations, petrology, and stable isotope data suggest that magmatic fluids caused potassic alteration, whereas largely non-magmatic fluids, flowing up a temperature gradient, caused sodic-calcic alteration. Late stage albite-chlorite and lesser sericitic alterations cut main stage alteration, have pyritic mineralization, and contain low salinity fluid inclusions that homogenize at 150-250+(DEGREES)C. Stable isotope data and phase petrology suggest that late stage fluids were either ocean water or isotopically heavy meteoric water. I propose that most of the copper in the deposit came from a high salinity, water-rich magmatic fluid that separated from the crystallizing PQM/QMP magma, but that up to 16% may have been derived from leaching during sodic-calcic alteration.