Variation in Copper Isotope Ratios and Controls on Fractionation in Hypogene Skarn Mineralization at Coroccohuayco and Tintaya, Peru

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.