Insights into the magmatic and metallogeneticevolution of the Morro del Cobre porphyry copperdeposit from fluid inclusions and lead isotope signatures

Abstract

The Morro del Cobre porphyry (Cu-Mo-Au) deposit, genetically linked to the upper Miocene magmatism (∼10–9 Ma), is located over the Pampean flat-slab segment (28–33°S) in San Juan province (Argentina). The analysis of the different vein systems provided insights into the physical and chemical evolution of the hydrothermal fluids responsible for the formation of this deposit. The potassic alteration is intense to moderately penetrating in veins (A-type veins -Subtypes 1 and 2-, EB-type, and EBT-type). The fluid inclusion study determined that Type III inclusions in A1 veinlets have salinities of 40.6–56.1 wt% NaCl eq. and homogenization temperatures from 365 to 472 °C. In A2 veinlets, salinities range from 37.4 to 52.2 wt% NaCl eq., with homogenization temperatures between 343 and 472.5 °C. A transitional hypogenic mineralization stage between potassic alteration and phyllic alteration is represented by B veinlets. Type III fluid inclusions show homogenization temperatures ranging from 315.2 to 397 °C and salinities of 40.6–46.78 wt% NaCl eql. Moderate phyllic alteration is extensively distributed in the area and occurs pervasively and in D-type veinlets, overprinting the potassic alteration. A supergene alteration event is widely distributed in the area. It occurs pervasively, overprinting the potassic and phyllic alteration with an assemblage of limonite, malachite, chrysocolla, turquoise and azurite, and covellite and chalcocite replacing chalcopyrite. The Pb isotopic composition indicates a genetic relationship between the ore minerals of this deposit and the magmatism preceding the porphyry-type mineralization in the area. The mineralizing magmas of Morro del Cobre involve an older Pb from the Grenvillian basement, evidenced by the presence of medium to high-grade metamorphic xenoliths and inherited zircons. Furthermore, there is probably a Pb contribution from Gondwanic magmas, as these are the principal constituent of the Cordillera Frontal. The assimilation of fertile Gondwanan basement material by the Miocene magmatism at Morro del Cobre plays a crucial role in the formation of a productive mineral deposit.