THE RED HILLS INTRUSIVE SYSTEM, PRESIDIO COUNTY, TEXAS: THE EASTERNMOST LARAMIDE PORPHYRY COPPER-MOLYBDENUM DEPOSIT IN SOUTHWESTERN NORTH AMERICA

New zircon U/Pb and molybdenite Re/Os analyses for the Red Hills porphyry copper-molybdenum deposit yield ages of 64.2 ± 0.2 Ma and 60.2 ± 0.3 Ma, respectively, indicating that the intrusion and mineralization are distinctly older than all other Tertiary magmatism (48 to 17 Ma) in Trans-Pecos Texas. The Red Hills deposit in Presidio County, Texas, is the easternmost porphyry copper-molybdenum system in southwestern North America. The Red Hills intrusion previously had been postulated to be genetically related to the 32-Ma Chinati Mountains caldera, located approximately 1 km to the north. The Laramide age for the Red Hills porphyry makes it contemporaneous with the majority of the porphyry copper systems (75 to 54 Ma) in Arizona, New Mexico, and northern Mexico.

Geologic mapping reveals that the quartz-monzonite porphyry intruded the Permian Ross Mine and Mina Grande Formations, as well as the Cretaceous Presidio Formation. Four distinct alteration types are recognized within the intrusion: potassic, phyllic, propylitic, and argillic. 40Ar/39Ar analyses of sericite indicate an early Tertiary age of alteration which is compatible with the other geochronology. Mineralization within the sericitized stockwork porphyry consists of chalcopyrite, chalcocite, and molybdenite within quartz-pyrite veins. Sphalerite-bearing garnet skarn occurs as small replacement pods in sedimentary rocks along the periphery of the intrusion.

Fluid inclusion studies of quartz veins indicate that the fluids associated with phyllic alteration had a mean temperature of 399°C ± 58°C based on measured homogenization temperatures. SEM combined with EDA confirmed the presence of halite, sylvite, chalcopyrite, pyrite, hematite, and bixbyite daughter minerals within fluid inclusions. Estimated salinities for the fluid inclusions range from 33 to 47 wt. % NaCl equiv. Pressure estimates based on contemporaneous trapping of immiscible fluids indicate a range of formation pressures from 20 to 30 MPa, corresponding to depths of formation of 2 to 3 km.