Skarn Alteration and Mineralization at Coroccohuayco, Tintaya District, Peru

Abstract

Coroccohuayco is a porphyry-related copper skarn deposit in the Andahuaylas-Yauri batholith of southeast Peru with little evidence for the presence of a significantly mineralized hydrothermal system at surface. The deposit occupies the hinge zone of a synform, hosted by the Cretaceous Ferrobamba Formation. Hydrothermal alteration of the carbonate protolith produced skarn between the underlying hornfels-altered Mara Formation and an overlying preskarn sill-like diorite intrusion, with the main zone of mineralization generally below 200 m from the surface. The spatial distribution of skarn was dominantly controlled by permeability variations in the carbonate protolith. The deposit consists of prograde garnet-dominant calc-silicate alteration with mineralization of chalcopyrite-bornite ± chalcocite disseminated as grains in calc-silicates. Precious metal mineralization occurs as gold and silver minerals mostly associated with bornite-chalcocite mineralization disseminated within more distal granular garnet alteration. Fluids were derived from the crystallization of several porphyry phases that intruded into an upper crustal level, possibly as a cupola above an underlying batholith. Fluid inclusion data from garnet and pyroxene indicate that the prograde skarn alteration formed between 400° to >600°C from highly saline fluids. Data from quartz indicate that copper mineralization formed between 250° to 400°C, with magnetite deposition over the upper part of this range. Retrograde alteration is generally magnetite (+carbonate-silica) replacement of calc-silicates and is locally associated with copper mineralization. Hydrous retrograde alteration, commonly observed in other copper skarn systems, is of lesser importance at Coroccohuayco. This is likely related to the geochemical effects of the overlying diorite intrusion which could have impeded the influx of oxidized meteoric waters into the system or buffered the composition of late-stage fluids, as well as the oxidized Fe-rich character of the prograde skarn.