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Abstract

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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