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Abstract

The porphyry copper mineralization at the Zaldívar deposit is confined to a NE-striking corridor of early- and late-intermineral granodioritic and dacitic porphyry intrusions and associated magmatic-hydrothermal breccia bodies. Country rocks comprise Early Permian rhyolite and andesite of La Tabla Formation plus comagmatic granitoids and Late Triassic andesite dikes. Middle Eocene andesitic rocks are common but of ill-defined distribution. Hydrothermal alteration consists of centrally located, magnetite-bearing potassic assemblages that are partially to completely overprinted by chlorite-epidote and sericitic alteration zones. The bulk of the hypogene metal resource was introduced synchronously with potassic alteration and A- and B-type veinlets during emplacement and evolution of multiple centers of biotite-bearing, early-intermineral porphyry and breccia bodies. Late-intermineral, hornblende-bearing dacite porphyry phases and associated breccia centers were emplaced later than the A- and B-veinlets but prior to multiple D-type veinlet generations and contributed additional, although lower grade, mineralization. Late-mineral dacite dikes are barren. Extensions to the east and northeast connect Zaldívar with Escondida Norte, and both can be considered as separate, coalescing porphyry copper deposits.

Two discrete porphyry copper systems coexist at Zaldívar: Early Permian and late Eocene. The minor, copper-only Early Permian event (~290–285 Ma) was associated with an evolved, end-stage rhyolite porphyry phase of the La Tabla magmatism. The major late Eocene event (38.6–36.1 Ma) produced copper in addition to gold, molybdenum, and silver. Protracted Eocene porphyry copper alteration and mineralization, over ~2.5 m.y. as constrained by numerous U-Pb (zircon) and Re-Os (molybdenite) ages, was coincident with the high rates of uplift and denudation synchronous with contractional Incaic deformation. Earliest-stage porphyry intrusions at 39–38 Ma were probably associated with the terminal stages of a volcanic edifice, likely a dome complex, whose erosion products were deposited in contiguous, synorogenic basins. District-wide precursor magmatism of intermediate composition was active between 45 and 41 Ma. Oxidation and enrichment were active between ~17 and 15 Ma (supergene alunite), consistent with the chronology of supergene activity throughout the district and wider region.

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