The giant Pascua Au-Ag-Cu high-sulphidation system is located in the El Indio belt in north-central Chile, and is hosted mainly by Triassic granitic rocks and locally by Miocene breccia bodies. Hydrothermal activity involved two distinct high-sulphidation alteration stages, the later of which was accompanied by main stage mineralization. The early hydrothermal activity consisted of extensive and pervasive advanced argillic alteration, local development of vuggy silica and distal argillic to propylitic alteration. This alteration was locally overprinted by an intermediate stage comprising argillic alteration, silicification and primary jarosite, more or less contemporaneously with the formation of Brecha Central, the main breccia body in the deposit. The superimposed second major hydrothermal event produced significant advanced argillic and vuggy silica alteration. The latter likely formed at pH near 0 owing to the limited buffering capacity of previously altered rocks. Termination of the second stage of alteration coincided with the main Au-Ag-Cu mineralizing event, in which Au dissolved in pyrite and enargite, formed inclusions in these minerals, and precipitated as isolated grains of native gold. Detailed EPMA and SIMS imaging and analyses of pyrite and enargite show that both contain Au, Ag, As, Cu, Se and Te and that their distribution is crystallographically controlled. The main trace element associations in the mineralized pyrite are As-Ag, Au-Cu and Se-Te. I suggest that the first two, As-Ag and Au-Cu, were the result of coupled substitution in the Fe site, whereas Te and Se replace S by direct anion exchange. The nature of the coupled substitutions observed in the gold-bearing pyrite underlines the fact that gold is not necessarily coupled with arsenic as previously thought and that arsenic can behave as a metal in the structure of pyrite. Approximately 55% of the gold in the deposit was scavenged from mineralizing fluids that were mainly under-saturated with respect to native gold and trapped into the structure of pyrite and enargite (50 and 5%, respectively). The remaining 45%, including - 7% as inclusions in sulphides, precipitated mainly in the form of native gold with lesser calaverite for which the gold depositional mechanism is interpreted to be an increase in pH that destabilized AuHS and AuCb". Substantial late-stage Ag-enrichment, characterized by halogen-bearing phases, is evident in the upper parts of the deposit and overprints previous alteration and mineralization. The silver enrichment event is interpreted to represent the waning stage of the hydrothermal system. Secondary leaching and weathering of pyrite, enargite and alunite created two supergene products, one dominated by secondary soluble sulphates such as voltaite, massive coquimbite, chalcanthite, and romerite, and one by jarosite. Abundant sulphides in vuggy silica altered rocks, which lacked the capacity to neutralize later oxidizing fluids, were the main locus of formation of the soluble sulphates. Elsewhere, the pH likely rose above 1, which inhibited precipitation of soluble sulphates and favoured formation of jarosite. Direct-ion SIMS images and trace element analyses of soluble sulphates indicate that these minerals likely contain gold in their structures.

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