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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 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. [ABSTRACT FROM AUTHOR]

Hypogene chalcopyrite and bornite in copper skarn and related porphyry ore from Coroccohuayco and Tintaya, Perú, have been analyzed for copper isotope ratios. Fifty-six samples have a range in δ65Cu values of -1.29 to 2.98 per mil (relative to NIST SRM 976), using the sample-standard bracketing technique to correct for machine drift and machine isotope fractionation effects. The range of δ65Cu values of vein-hosted mineralization at Coroccohuayco overlaps with disseminated mineralization in skarn ore but also extends to lower values. The overlap of these ranges indicates that different mineralizing events in these deposits cannot be identified solely by their copper isotope compositions. Geologic and isotopic considerations of the samples analyzed in this investigation indicate that copper mineralization proximal to fluid sources (including porphyries and principal fluid pathways) tends to be isotopically lighter than more distal samples. This suggests that important copper isotope fractionation occurred during mineralization and may have been the result of equilibrium fractionation and/or indirect fractionation mechanisms related to the chemical characteristics of the hydrothermal fluid. These fractionation mechanisms have produced copper isotope zonation over tens of meters in hypogene ore during the process of ore precipitation at Coroccohuayco. [ABSTRACT FROM AUTHOR]