La Caridad Vieja: Vestiges of a removed HS-epithermal system in the La caridad porphyry Cu–Mo system, northeastern sonora, Mexico, based on XRD, SEM, SWIR spectral analysis and U–Pb zircon dating

Abstract

The present work contributes with new data to better understand the geology and spatial distribution of the alteration and mineralization events, as well as new U–Pb zircon ages, that allow elucidating the magmatic history of the La Caridad Vieja mineral deposit, which is part of the porphyry Cu–Mo system from La Caridad mine, northwest Mexico. Mineralization is hosted in a sequence of volcanic rocks from the Late Cretaceous-Paleocene Tarahumara Formation and is associated with the emplacement of quartz monzonite stocks, which intruded to volcanic rocks. Detailed studies based on petrography, XRD, SEM and SWIR, indicate that the La Caridad Vieja is characterized by a zone of phyllic alteration dominated by qz-ser in the deepest parts, and it is associated with molybdenite + chalcopyrite + pyrite + galena + sphalerite, with values ranging from 0.1 to 0.9% Cu and <0.02% Mo. Above the phyllic alteration zone, there is an intermediate argillic alteration zone (quartz + kaolinite + halloysite + smectite + illite), mainly with pyrite. In specific areas, advanced argillic alteration is observed (alunite + pyrophyllite + dickite + kaolinite + halloysite) accompanied by pyrite + enargite + famantinite + barite. The new U–Pb radiometric ages in zircons indicate an age of 54.71 ± 0.39 Ma for the upper part of the Tarahumara Formation, while a sample of the mineralizing stock yielded 51.95 ± 0.48 Ma. The results of this investigation suggest that La Caridad Vieja corresponds to a portion of the upper part of the Cu–Mo porphyry of La Caridad, which was downthrown by the La Caridad fault, during the Basin and Range extensional event in the Miocene. The reconstruction of the geological model suggests that La Caridad Vieja corresponds to the transitional zone between the Cu–Mo porphyry and an epithermal high sulfidation mineralization zone, which was eroded. Prior to the dislocation caused by the La Caridad fault, the eroded area was covered by a ferruginous conglomerate overlain by rhyolite flows dated at ~24 Ma. On the other hand, geochronological data suggests that the mineralizing event at La Caridad Vieja, although it is contemporaneous with the porphyry system of the La Caridad mine, it continued in this area at least until ~52 Ma.