AUTHORS: Martin Reich
ABSTRACT: Weathering of the primary (hypogene) sulfide mineralization in copper deposits from Northern Chile produced several hundred meters of higher grade caps of secondary Cu-sulfides, oxides, hydroxides, carbonates, sulfates and chlorides. This supergene mineralization is a reflection of the history of groundwater flow, tectonics, landscape, and climate of the Atacama Desert. Within this context, we propose that supergene enrichment was the result of multiple cycles of Cu recycling dominated by fluids of different sources. Radiogenic (U, Th, Ar), stable (H, O, Cu) and fissiogenic/cosmogenic (36Cl, 129I) isotope data suggest reworking and redox cycling of Cu during supergene enrichment characterized by a first stage of supergene chalcocite formation from acidic leach fluids of meteoric origin down-flowing in a semi-arid climate (44 to ~9-5 Ma). Reworking of initial supergene copper assemblage, during the Pleistocene, by rising neutral and chlorine/iodine-rich deep saline waters under wellestablished hyper-arid climate conditions lead to the formation of atacamite with extremely fractionated compositions. In addition to the economic upgrading of copper, resulting from weathering and re-precipitation of copper minerals, supergene enrichment also caused strong enrichment in trace metals such as silver and gold.
KEYWORDS: Supergene, copper, Atacama, precious metals, iodine
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