Use of Cu isotopes to distinguish primary and secondary Cu mineralization in the Cañariaco Norte porphyry copper deposit, Northern Peru

A significant proportion of the copper in the Cañariaco Norte porphyry copper deposit in northern Peru occurs in chalcocite and covellite-rich veins and disseminations that exist from the surface to depths greater than 1 km. The overall range of Cu isotopic ratios of 42 mineral separates from Cañariaco varies from -8. 42 to 0. 61 ‰, with near-surface chalcocite and Fe oxides having isotopically depleted values compared to chalcocite, covellite, and chalcopyrite from deeper levels. The majority (34 of 36) of measured Cu sulfides have a typical hypogene copper isotope composition of δ ⁶⁵Cu = 0. 18 ± 0. 38 ‰, with no enriched isotopic signature existing in the Cañariaco Norte sulfide data. Thus, the copper isotope data indicate that most of the chalcocite and covellite formed from high-temperature hypogene mineralization processes and that only a minor portion of the deposit is enriched by supergene processes. The nonexistence of an enriched δ ⁶⁵Cu reservoir suggest the presence of an undiscovered lateral/exotic Cu occurrence that enriched ⁶⁵Cu that remained in solution during weathering. Regardless of the cause, the comparative analysis of the Cu isotope dataset reveals that little exploration potential for an extensive supergene enrichment blanket exists because the weathering history at Cañariaco Norte was not conducive to preservation of enriched Cu at depth beneath the leach cap.