Magmatism and Related Au-Cu Mineralization in the Hualgayoc Mining District, Northern Peru

Abstract

Abstract The Hualgayoc district in the Cajamarca region of northern Peru experienced middle Miocene andesitic to rhyolitic magmatism and magmatic hydrothermal activity that produced Au and Cu mineralization, including the Cerro Corona porphyry Au-Cu, Tantahuatay high-sulfidation epithermal Au, and the AntaKori skarn Cu-Au-Ag deposits. We examined 32 samples from 22 units that encompass the entire igneous rock record in the district. Our new U-Pb dating of 454 zircon grains from these 22 igneous units yielded Concordia ages showing continuous magmatic activity in the district, from 14.8 to 9.7 Ma. Igneous activity in the eastern part of the district took place between 14.8 and 14.0 Ma, including the Cerro Corona intrusive complex that hosts a porphyry Au-Cu deposit. Magmatism in the western part of the district occurred between 13.7 and 11.5 Ma and included the Tantahuatay Volcanic Complex that hosts a high-sulfidation Au deposit. The different styles of mineralization and ages of igneous rocks in the eastern and western parts are accounted for by deeper erosion to the east. Bulk-rock compositions indicate that parental magmas originated from amphibole-rich juvenile lithospheric mantle or lower crust and evolved through amphibole fractional crystallization. Amphibole and zircon compositions indicate that parental magmas of all igneous units in the Hualgayoc district were water-rich, >3 wt % H2O, and oxidized above the fayalite-magnetite-quartz (FMQ) redox buffer. Our observations in the Hualgayoc district suggest that oxidized conditions and high-water contents are necessary to produce porphyry-type mineralization, but that other factors were also critical to form mineralization, such as a shallow depth of magma emplacement, with near-vertical shape of intrusions. These findings are likely applicable elsewhere.