Abstract

Four main stages of alteration and mineralization are present at La Caridad, Sonora, Mexico. In stage I, quartz veins are associated with orthoclase-anhydrite-biotite hydrothermal alteration within the La Caridad intrusive complex and with pervasive biotitization in andesites and diorites. A zone of propylitic alteration surrounds this biotitic zone. Weak molybdenite ± chalcopyrite ± magnetite ± pyrite ± sphalerite mineralization formed in stage I. stage II, the main hydrothermal mineralization event, is represented by quartz veins with pyrite-sericite + chlorite and is associated with chalcopyrite + pyrite ± molybdenite. In stage III, lead-zinc-silver mineralization formed in veins that were emplaced peripherally to the main system, locally overprinting the earlier events. In stage IV, intermediate-sulfidation mineralization, represented by quartz-tennantite-chalcopyrite-pyrite-sericite veinlets, formed in the core of the porphyry deposit system. The La Caridad Antigua mine, located ~3 km east of the La Caridad orebody, has a high-sulfidation epithermal assemblage that includes pyrophillite, kaolinite, alunite, quartz, and barite as gangue, and a variety of sulfides including, chalcopyrite, pyrite, enargite, tetrahedrite-tennantite, and chalcocite. A new 206Pb/238U age of zircons from an associated porphyry stock at La Caridad Antigua is 55.0 ± 1.7 Ma, identical to previous U/Pb ages determined in the porphyry Cu deposit and consistent with a spatial and temporal link between La Caridad and La Caridad Antigua deposits. Fluid inclusion data indicate that stages I and II at La Caridad were deposited by saline hydrothermal fluid at 360° to 460° and 330° to 410°C, respectively. stages III and IV were deposited by low-salinity hydrothermal fluid at 330° to 370° and 260° to 320°C, respectively. Oxygen and hydrogen isotopes indicate deposition of stages I, II, and IV either from magmatic vapor or from mixtures of magmatic and highly evaporated (lacustrine?) formation water and mixing with meteoric water in stages II and III. Sulfur and oxygen isotopes in stage 1 anhydrite indicate external derivation of sulfate and mixing of igneous and external water sources.

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