COPPER ISOTOPE CHARACTERISTICS OF THE CU (+AU,AG) SKARN AT COROCCOHUAYCO, PERÚ

Copper minerals from high temperature ore deposits are analyzed using standard-sample bracketing (relative to NBS976) by MC-ICPMS in the GeoAnalytical Laboratory at Washington State University. Significant Cu isotope variations (>2‰ d⁶⁵Cu_NBS976) are observed in Cu sulfides precipitated at high temperature (³250º) in several hydrothermal systems. Precise Cu isotope analyses (2s=0.08‰) now allow us to better describe Cu behavior in ore forming processes and more directly address questions relating to deposit genesis and exploration.

We have evaluated Cu isotopic ratios in hypogene mineralization in the Cu deposit at Coroccohuayco (155 million tonnes @ 1.57% Cu, 0.16 g/t Au and 6.3 g/t Ag), located in the Tintaya district, Cusco Department, Perú. Coroccohuayco is related to late Eocene/Oligocene intrusions in the SE end of the important Andahuaylas-Yauri Cu-porphyry/skarn belt. Hydrothermal alteration (calc-silicates ± magnetite) is genetically related to monzonitic intrusives and occurs in limestone of the Cretaceous Ferrobamba Formation. The bulk of mineralization is disseminated chalcopyrite-bornite±chalcocite in skarn. Post-skarn mineralization associated with late monzonitic dikes is locally significant.

Copper isotope ratios from mine section 1400 span a range from –1.29‰ to 0.69‰ d⁶⁵Cu. Lateral and vertical isotope variations occur in the early disseminated mineralization. Copper minerals proximal to fluid sources or important fluid pathways tend to have lower d⁶⁵Cu (isotopically lighter) relative to mineralization farther from fluid sources. A specific isotopic relationship exists between co-precipitated chalcopyrite-bornite mineral pairs, with a consistent 0.4‰ increase in d⁶⁵Cu for chalcopyrite relative to bornite. In addition, Cu ratio zonation occurs across late mineralization that cuts earlier disseminated mineralization.

These results support earlier interpretations from fluid inclusion, mineral chemistry, and geologic mapping that Coroccohuayco was formed from overprinting hydrothermal events, a feature common to copper skarn and porphyry systems, and demonstrate the utility of using Cu isotopes in understanding the origin of mineralization in these complex systems.