MAGMATIC-HYDROTHERMAL FLUIDS: A SUMMARY OF TEMPORAL AND SPATIAL EVOLUTION FROM THE 6 KM VERTICAL TRANSECT OF THE PORPHYRY COPPER SYSTEMS IN THE YERINGTON BATHOLITH, NEVADA

Yerington records a relatively deep, batholith-hosted porphyry copper system where one horizontal principal stress significantly exceeded the other, leading to a single well-developed set of vertical fractures that guided both planar porphyry dikes upward >5 km from deep sources and allowed influx of non-magmatic fluids. The Jurassic subaerial to evaporitic setting of host sed-volcanic rocks trapped brines whose high density promoted convective circulation to great depth (>5 km). Magmatic fluids responsible for porphyry copper hydrothermal alteration-mineralization separated from cupolas atop the Luhr Hill granite at depths ranging from ~3 km (Yerington Mine) to ~4.5 km (Ann-Mason). Because the Luhr Hill granite became water-saturated in shallower parts first, Yerington Mine magmatic fluids were extracted earlier than the Ann-Mason fluids which produced a low Mo/Cu ratio at the former and relatively high Mo/Cu at the latter. This model is consistant with the more incompatible behavior of Mo in crystallizing melts. Accumulated magmatic fluid in Luhr Hill granite cupolas was released upward and created hydrofractured paths along which magma was emplaced and quenched to form porphyries. Initial lithostatic pressure conditions decreased to near-hydrostatic conditions, and this depressurization caused additional magma degassing and ascent of fluid along permeable porphyry dikes. K-silicate alteration and Cu-Fe sulfide deposition at >450 C characterize this stage. As magmatic fluids ceased to rise non-magmatic brines entered the dike/fracture conduits to produce sodic-calcic alteration at <450 C. This cycle of magmatic fluid release repeated numerous times. With time, the source of magmatic fluids became greater within the crystallizing cupola, so ascending fluids cutting early K-silicate zones were cooler and produced sericite-pyrite alteration at <450 C. Stable isotopic data (O,H) indicate that both K-silicate and sericitic fluids were principally magmatic. Above the porphyry Cu±Mo orebodies (at 1 to 4 km depth), sericitic and local advanced argillic alteration dominate along structural zones that funneled magmatic fluids upward from the deep cupolas, and may be time-equivalent to deeper K-silicate.