CU ISOTOPE FRACTIONATION USED TO DETERMINE SUPERGENE PROCESSES AT YANACOCHA MINE IN NORTHERN PERU

The analysis of copper isotope ratios in Cu-sulfide minerals at Yanacocha mine in northern Peru aids in the understanding of weathering activity and resultant supergene alteration within the Au- Cu porphyry system. The ores originated from shallow epithermal activity related to a deeper hydrothermal porphyry system. In this study, we investigated Cu isotope ratios in order to ascertain the degree of weathering and fractionation of Cu near the surface and thus pinpoint more precise locations for both the leached zone and enrichment blanket. Fractionation of Cu isotope ratios within low-temperature hydrothermal ore-forming environments has been documented extensively; however, documentation of fractionation and stoichiometric enrichment processes of Cu from low-temperature fluids in epithermal environments has been scant by comparison. The covellite-chalcopyrite-enargite phases in the weathering-vulnerable area near the surface yielded n = 50 65Cu values of -2.52‰ ± 1.88 (1σ), and chalcocite-chalcopyrite- enargite zones lying beneath produced n = 25 values of 0.81‰ ± 1.94. The unweathered epithermal zone below yielded n = 36 values of -1.11‰ ± 1.61. These values reveal clear Cu isotope enrichment of minerals at progressively greater depths in addition to significant leaching of near-surface residual Cu-sulfides due to oxidation and dissolution. Isotopically heavier secondary Cu-sulfides are seen in the enrichment zone resulting from the precipitation (through reduction) of low pH weathering solutions. Previous studies have shown that weathered high- temperature hydrothermal ore forming environments show a similar pattern of Cu isotope signatures in leached, enriched, and unweathered areas as presented in this weathered epithermal zone.