Hypogene Evolution of the Escondida Porphyry Copper Deposit, Chile
Rubén A. Padilla-Garza, Spencer R. Titley, Christopher J. Eastoe, 2005. "Hypogene Evolution of the Escondida Porphyry Copper Deposit, Chile", Andean Metallogeny: New Discoveries, Concepts, and Updates, Richard H. Sillitoe, José Perelló, César E. Vidal
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At Escondida, the propylitic, potassic, and quartz-sericitic hydrothermal mineral assemblages are centered on a 38 Ma granodioritic stock composed of at least four phases. These early intrusive and related hydrothermal events were followed by rhyolitic intrusive and extrusive rocks and sulfide-rich veins of two advanced argillic alteration events at ~ 36 and ~34 Ma. The final hypogene copper grade of the deposit varies between 0.2 and 1.0 percent, reflecting varying degrees of overprinting by the successive copper mineralization stages.
The homogenization temperatures of primary fluid inclusions from quartz-orthoclase- (stage A), quartzsericite- (stage B), and quartz-alunite-bearing veins (stage C) vary between 500° and 560°, 280° and 380°, and 200° and 340°C, respectively, with estimated depths of trapping ranging from 1.5 to 3.0 km.
The δ34S values of sulfide minerals from the potassic assemblage range from −3.2 to −2.0 per mil, whereas values from the quartz-sericite assemblage vary between −1.1 and +0.6 per mil. These values suggest a common source of magmatic sulfur and indicate that sulfide minerals of stage B were not formed by leaching sulfide from stage A. The δ34S values of stage C sulfide minerals range from −2.5 to +2.7 per mil, which suggests that sulfide for the advanced argillic event may have been derived, in part, by leaching sulfide from the two earlier hydrothermal stages. Calculated values of δD and δ18O for water coexisting with igneous and hydrothermal minerals indicate a dominantly magmatic component in stages A and B and a mixture with meteoric water at the lower temperatures of stage B and during the advanced argillic event (stage C). Stage C may have formed by circulatión of surficial meteoric water heated by the 34 to 36 Ma subvolcanic rhyolitic rocks. It is also possible that some 2 m.y. after the emplacement, uplift, and denudatión of the main Escondida porphyry system, a new porphyry copper stock was emplaced in the same structural weakness zone, and that the subvolcanic rhyolite and advanced argillic alteration and associated mineralization represent the upper levels of this younger porphyry system.
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A variety of metals and deposit types define the metallogeny of the Andes from Colombia through Ecuador, Peru, and Bolivia to Argentina and Chile, although porphyry copper and epithermal gold deposits undoubtedly predominate and will continue to do so. Discoveries over the last 30 yrs or so, predominantly in the central Andes and especially Chile, have been made using routine, field-based geologic and complementary geochemical methods, a situation that is considered unlikely to change radically in the foreseeable future. The only clearcut evolutionary change is the increased number of deposits being discovered beneath pre- and postmineral cover. The predictive capacity of conceptual geology has had minimal impact on the Andean discovery record but is thought to offer much promise for the future. This introductory article selects mineralization styles and relationships as well as some broader metallogenic parameters as simple examples of geologic concepts that may assist exploration. Emphasis is placed on porphyry copper ± molybdenum ± gold and high-, intermediate-, and lowsulfidation epithermal gold ± silver deposits, although reference is also made to several carbonate rock-hosted precious and base metal deposit types and styles as well as subvolcanic tin, volcanogenic massive sulfide, and slate-belt and intrusion-related gold deposits. Particular emphasis is placed on the potential for exceptionally high grade porphyry copper, porphyry gold, epithermal gold, and subvolcanic tin deposits. Deposits resulting from the oxidation, enrichment, and chemical transport of copper and zinc and mechanical transport of gold and silver during supergene weathering are also briefly highlighted.
Si bien la metalogenia de los Andes de Colombia, Ecuador, Perú, Bolivia y Chile se encuentra definida por una gama de metales y estilos de mineralización, son los depósitos tipo pórfido de cobre y epitermal de oro los que dominan en el presente y continuarán prevaleciendo en el futuro. Los descubrimientos de los últimos 30 años, predominantemente en los Andes centrales y especialmente en Chile, han sido realizados mediante métodos geológicos rutinarios de campo, generalmente complementados satisfactoriamente por métodos geoquímicos. Se estima que esta situación difícilmente experimentará variaciones radicales en un futuro cercano. El único cambio destacable en esta historia evolutiva está dado por el aumento apreciable de descubrimientos de depósitos cubiertos, bajo cobertura pre o postmineral. A nivel andino, la capacidad predictiva de la geología conceptual ha tenido un impacto mínimo en el número total de descubrimientos, aunque se piensa que su uso debiera garantizar buenas perspectivas futuras. El presente artículo