Exhumation history of the La Caridad and Suaqui Verde porphyry copper deposits in the eastern Basin and Range province of Sonora: Insights from thermobarometry and apatite thermochronology

Abstract

The Arizona-Sonora porphyry copper cluster is one of the most productive provinces of this type on Earth. There, upper-crustal deposits emplaced between Late Cretaceous and Early Eocene, subsequently exhumed and developed supergene enrichment profiles. Despite the Basin and Range regional extension, these shallow intrusions and associated mineralization were preserved in eastern Sonora in the exhumed footwalls of normal faults. In order to better constrain their exhumation history, we performed a thermobarometrical (Al-in-hornblende) and thermochronological (apatite U–Pb and fission-track) study on the La Caridad and Suaqui Verde porphyry copper deposits. On the one hand, thermobarometry allows us to determine that the Suaqui Verde porphyry copper emplaced at 4.8 ± 0.9 km-depth, which implies that Cu–Mo mineralization may occur deeper than the ~2–4 km depth classically proposed for porphyry copper. On the other hand, using thermobarometry and inverse modeling of thermochronologic data obtained for individual and drill core samples, we reconstruct the thermal history of these porphyry copper. After a very fast post-emplacement cooling, numerical models suggest that cooling of rocks was slow and mainly monotonous during exhumation. Indeed, our data do not record an acceleration of the exhumation in eastern Sonora during the Basin and Range regional extension (between ~25 and 12 Ma). However, inverse modeling of fission-track data obtained along a drill core in La Caridad allows us to reconstruct the evolution of the geothermal gradient through time, which is in agreement with the regional geodynamics. Overall, it cannot be excluded that Oligocene volcanic rocks protected the ore deposits from erosion, even if they remained relatively thin (<500 m-thick). We propose that eastern Sonora was characterized by Basin and Range extension distributed on numerous normal faults with small offsets, which allowed to preserve the upper crustal part of the Cretaceous-Paleogene magmatic arc and its numerous associated porphyry copper systems.