Modern and Paleofluid Pathways Revealed by Cu Isotope Compositions in Surface Waters and Ores of the Pebble Porphyry Cu-Au-Mo Deposit, Alaska

Abstract

Copper isotope ratios measured in minerals and shallow groundwater and surface waters provide insight into high-temperature mineralization and active weathering processes at the Pebble porphyry Cu-Au-Mo deposit, Alaska. The West zone of the deposit contains hypogene mineralization with a supergene overprint and a thin oxide leached capping, whereas the contiguous East zone contains only hypogene mineralization. Sulfide-rich rock powders and mineral separates have δ65Cu values that range from 0.78 to 2.28‰ (hypogene West), 0.02 to 1.55‰ (hypogene East), �3.49 to 1.88‰ (oxide West), and �5.04 to 1.27‰ (supergene West). The results from hypogene samples show that there is a systematic increase in δ65Cu values from deeper to shallower portions of the deposit. Furthermore, the δ65Cu values correlate with silicate alteration assemblages; mostly positive values correspond to quartz-illite-pyrite, sericite and quartz-pyrophyllite alteration zones which formed at relatively lower temperatures, whereas negative values characterize the higher temperature potassic and sodic-potassic domains. This empirical evidence could indicate that fractionation of Cu isotopes during hypogene alteration is controlled by pH and/or temperature variations. Shallow surface waters proximal to the deposit, and which likely interacted with underlying concealed mineralization, have heavy δ65Cu values which contrast with lighter values in waters distal from the deposit. Patterns measured in the copper isotope ratios of both solids and surface waters demonstrate the potential use of copper isotope distribution as a vectoring tool in mineral exploration and aid in understanding the sources of copper in the surface and near-surface environments.