PLIOCENE SUPERGENE WEATHERING AND EXHUMATION HISTORY OF THE RED HILLS PORPHYRY CU-MO SYSTEM AND THE TECTONIC HISTORY OF THE PRESIDIO BOLSON, TRANS-PECOS TEXAS

The Red Hills intrusive system hosts the easternmost Laramide porphyry copper deposit in southwestern North America. The Red Hills intrusive system (~64 Ma) is contemporaneous with and genetically related to other Laramide magmatic systems (75–54 Ma) that host porphyry copper deposits in Arizona, southwestern New Mexico, and northern Mexico. The Red Hills pluton crops out near the southern margin of the 32-Ma Chinati Mountains caldera and on the eastern flank of the alluvium-filled Presidio Bolson through which the Rio Grande flows. Existing evidence supports a temporal and genetic connection between Laramide magmatism and tectonism and the nearby Shafter polymetallic replacement deposit in Permian carbonates.

Fluid inclusion studies of the mineralized quartz stockwork in the Red Hills quartz monzonite constrain pressures from 20 to 30 MPa, corresponding to depths of formation of 2 to 3 km. ⁴⁰Ar/³⁹Ar dates for supergene alunite veinlets overprinting the mineralized stockwork range from 5.8 to 3.8 Ma. The older ages may reflect increased regional moisture related to the late Miocene opening of the Gulf of California. The apparent lack of ages younger than 3.8 Ma may relate to rapid exhumation of the Red Hills intrusive complex associated with Pliocene extension. The extension, flank erosion, and associated sedimentation created the Presidio Bolson that formed the southern terminus of the ancestral Rio Grande drainage prior to the integration of the through-going Rio Grande system in the Pleistocene. Rapid uplift and erosion also may account for the preservation of only local supergene copper enrichment at Red Hills.