Pleistocene recycling of copper at a porphyry system,Atacama Desert,Chile: Cu isotope evidence |
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| Authors: | Carlos Palacios Olivier Rouxel Martin Reich Eion M Cameron Matthew I Leybourne |
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| Institution: | 1.Departamento de Geologia,Universidad de Chile,Santiago,Chile;2.Marine Chemistry and Geochemistry Department,Woods Hole Oceanographic Institution,Woods Hole,USA;3.IUEM European Institute for Marine Studies,Universitée Européenne de Bretagne,Plouzane,France;4.Eion Cameron Geochemical Inc.,Carp,Canada;5.Ocean Exploration, GNS Science,Lower Hutt,New Zealand |
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| Abstract: | We present Cu isotope data of hypogene and supergene minerals from the Late Paleocene Spence Cu-Mo porphyry in the Atacama
Desert of northern Chile. Chalcopyrite displays a restricted range of δ65Cu values within the values reported for primary porphyry Cu sulfides (+ 0.28‰ to + 0.34‰, n = 6). Supergene chalcocite samples
show heavier and remarkably homogeneous δ65Cu values, between + 3.91‰ and + 3.95‰ (n = 6), consistent with previous models of Cu leaching and enrichment in porphyry
systems. Secondary Cu minerals from the oxide zone show a wider range of composition, varying from + 1.28‰ and + 1.37‰ for
chrysocolla (n = 6) to very light Cu isotope signatures reported for atacamite between -5.72‰ to -6.77‰ (n = 17). These data
suggest redox cycling of Cu during supergene enrichment of the Spence Cu deposit, characterized by a first stage of supergene
chalcocite formation from acidic, isotopically-heavy leach fluids of meteoric origin down-flowing in a semi-arid climate (44
to ~ 15-9 Ma). Reworking of the initial supergene copper assemblage, during the Pleistocene, by rising neutral and chlorine-rich
deep formation waters under well-established hyper-arid climate conditions lead to the formation of atacamite with extremely
fractionated Cu compositions. Essentially coeval chrysocolla formed by dissolution of atacamite during short episodes of wetter
climatic conditions occurring in the latest Pleistocene. |
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