Constraining the Exhumation of the Ruby Star Pluton, Pima Mining District, SE Arizona using a Multi-Thermochronometer Approach

Abstract

Arizona is host to world-class porphyry copper deposits that formed during arc magmatism related to the Laramide orogeny that were subsequently exhumed during the Cenozoic. This study focuses on the Pima Mining District 35 km southeast of Tucson, which hosts major Cu-Mo-Ag deposits associated with the Laramide Ruby Star pluton. This study uses Al-in-hornblende thermobarometry to constrain the crystallization depths of samples taken in a transect across the Ruby Star pluton, followed by a multi-thermochronometer analysis of this paleo-vertical transect to elucidate the thermal history of the pluton. We use zircon (U-Th)/He, apatite fission track, and apatite U-Th/He dating techniques to constrain the pluton’s passage through 200-45 ˚C, corresponding to ~6.5-1.5 km depth assuming a geotherm of 30˚C/km. Two distinct phases of cooling have been observed in the data, the first a rapid phase of cooling from 33-30 Ma and the second a slower phase of cooling from 28-21 Ma. We suggest that these cooling ages, along with previous structural and geochronologic work, constrain the tilting and exhumation of the Pima Mining District to a rapid phase of exhumation from 33-30 Ma, followed by a slower phase from 28-21 Ma. This first phase of exhumation predates the earliest signals of exhumation recorded in southern Arizona, while the second phase is consistent with exhumation of the proximal Catalina and Coyote metamorphic core complexes. These dates reinforce the observation that zones of major extension are optimal localities for ore deposit exploration in the American Southwest.