Abstract

The intergrowths and compositions of supergene copper sulfide minerals from drill hole MOR-4511 in the Western Copper area of the Morenci mine, Greenlee County, Arizona, have been examined by reflected light microscopy and electron probe microanalysis (EPMA) to better understand the formation of supergene sulfides with implications for hydrometallurgical processing. The supergene copper sulfides occur in three main textures: partial to complete replacement of chalcopyrite, partial replacement of pyrite, and partial to complete replacement of one another. Compositions of copper sulfides vary widely, but (Cu+Fe):S ratios of 1.80 ± 0.05, 1.92 ± 0.03, and 1.10 ± 0.10 are dominant. No stoichiometric Cu2S was found. At shallower depths in the supergene blanket and near/within faults, high (Cu+Fe):S phases (with ratios of 1.80 ± 0.05 and 1.92 ± 0.03) replacing primary chalcopyrite and pyrite or lower (Cu+Fe):S supergene sulfides are dominant, and near the base of the blanket low (Cu+Fe):S phases (with ratios of 1.10 ± 0.10) replacing primary chalcopyrite or higher (Cu+Fe):S supergene sulfides gradually become more dominant. This indicates high concentration of Fe3+, Fe2+, and Cu2+, necessary to form high (Cu+Fe):S phases, at shallower depths and near sources of unreacted fluid, such as faults. Formation of low (Cu+Fe):S phases directly from chalcopyrite or from high (Cu+Fe):S phases could be controlled by decreased concentrations of iron species and Cu2+ due to reaction with primary chalcopyrite and pyrite as fluids descend or migrate away from faults, reduced access to supergene fluids, and/or lower pyrite-chalcopyrite ratios. The compositional patterns of supergene copper sulfide minerals observed at Morenci are similar to those observed in other supergene enrichment blankets of porphyry copper systems worldwide and are even more similar to compositions seen in leaching experiments of synthetic copper and copper-iron sulfides.

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