LARAMIDE MAGMATISM IN TRANS-PECOS TEXAS: GEOLOGIC HISTORY OF THE RED HILLS INTRUSIVE CENTER, PRESIDIO COUNTY, TEXAS

The Red Hills intrusive system hosts the easternmost Laramide porphyry copper deposit in southwestern North America. The Red Hills and associated mineralization of the Shafter District lie along the projected western extension of the long-lived regional Chalk Draw fault. 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. Zircon U-Pb, molybdenite Re-Os, and sericite ⁴⁰Ar/³⁹Ar analyses yield ages of 64, 60, and 61 Ma, respectively, indicating that the Red Hills magmatism and mineralization are distinctly older than other Cenozoic magmatism (48–17 Ma) in Trans-Pecos Texas. The Red Hills intrusive system 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. These results significantly extend the Laramide magmatic province eastward and suggest that Laramide subduction-related magmatism and deformation are coextensive over a broad area of southwestern North America.

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 3.8 to 5.8 Ma. 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 and breaching of the Presidio Bolson enroute to integration of the Rio Grande system.