Evolution of hydrothermal alteration facies at the Cerro Corona Cu-Au porphyry deposit, Northern Peru

Abstract

The Cerro Corona deposit is an active copper gold porphyry deposit, located in the Hualgayoc District, Northern Peru. Its magmatic evolution is conformable with the current regional model of crustal thickening, flat slab subduction and uplift. The deposit comprises of multiple mineralogically and chemically indistinguishable biotite quartz diorite porphyry intrusions, followed by intrusions of hornblende granodiorite and andesite dykes. Igneous amphibole chemistry of these later intrusions suggests an increasing input of deeper, mafic hydrous melts into a shallower magma chamber, similar to the paragenesis at the nearby Yanacocha district. Hydrothermal alteration is similar to other porphyry-type deposits, however the argillic alteration telescopes onto the entire deposit, with pervasive argillic alteration extending to a depth of 400m from the current surface. This suggests a significant uplift during hydrothermal alteration. Error is propagated through the mass transfer calculations, allowing for a more confident interpretation of mass transfer models. These models show large net transfer of major and minor components during potassic, propylitic and sericitic alteration, but less drastic changes during argillic alteration. A 3D net mass transfer models is done for the first time on a porphyry deposit, which reveals a previously unidentified zone of potassic alteration in the north. Infrared spectroscopy revealed the dominant clay mineral is smectite, with mixture of illite- and/or kaolinite. Kaolinite-dominant argillic alteration appears to be related to sulphide oxidation near the surface and telescopes down along preferentially permeable zones, whereas smectite-dominant argillic alteration occurs at near neutral conditions, deeper and peripheral to zones of sulphide mineralisation. Intracrystalline oxygen isotope fractionation indicates illite formed at temperatures of ~300°C and kaolinite >150°C. The calculated fluid compositions suggests illite-formation fluids are composed of >70% magmatic-derived fluids; kaolinite-formational fluids being an equal mix of meteoric and magmatic fluids and smectite-formational fluid being predominantly heated meteoric fluid.