Abstract

The El Bronce epithermal vein system, located in the western foothills of the Andean Cordillera of central Chile, contains 25 metric tons of gold, 105 metric tons of silver, and 16,000 metric tons of copper. The veins are hosted by volcanic rocks of the Cerro Morado (Early Cretaceous) and Las Chilcas (Early to Late Cretaceous) Formations which consist chiefly of breccias, tuffs, and lavas of andesitic composition. The subcircular Morro Hediondo caldera, with a diameter of 14 to 16 km and of Late Cretaceous age, is located immediately north of the district. Dacitic tuffs and andesitic flows and breccias associated with the caldera are assigned to the Lo Valle Formation on the basis of K-Ar ages of 83 to 80 Ma.Two groups of Early to Late Cretaceous intrusive rocks, occupying north-trending belts, are recognized in the area. The older, a quartz monzodiorite body west of Petorca, intruded the Cerro Morado Formation. The younger comprises dioritic to granodioritic stocks, dikes, and sills and includes the Petorca Porphyry (86 + or - 3 Ma) and the dioritic-tonalitic ring dike. The latter defines the margin of the Morro Hediondo caldera (80-79 Ma). Large zones of hydrothermal silicification and argillic alteration are associated with the two groups of intrusive rocks.The Morro Hediondo caldera is the most prominent geologic feature in the area. Faults and fractures related to the caldera provided structural controls for the mineralization. Several northeast- to northwest-trending faults are either radial or concentric with respect to the caldera. The most outstanding structures are the Quebrada de Castro and El Bronee structural systems.The El Bronce structural system, containing most of the mineralization in the district, consists of extensional faults, dikes, and veins exposed within a 3-km-wide and a 17-km-long, northeast-trending zone delimited by the north-northwest-south-southeast Quebrada de Castro and Petorca-E1 Durazno dextral strike-slip faults.The relative movement of this fault pair was responsible for the structural pattern at El Bronce, which involved development of a first-order dilational jog. Within this jog, two major groups of veins are recognized: the El Bronce-Guanaco-La Olla-San Lorenzo group located north of the El Bronce Creek fault and the Pedro de Valdivia-El Espino Norte group located south of this structure. Both groups of veins are vertical to subvertical and strike northwest to northeast.Detailed studies of the orebodies have shown that each ore shoot is composed of several lenses which contain as many as four ore types and a dike: (1) hydrothermal breccia ore cemented, (2) massive ore, (3) stockwork zone, and (4) disseminated zone. Orebodies may also contain barren andesite dikes. The contacts between the four ore types are generally abrupt but are locally gradational. The andesite dikes generally display sharp, locally sheared contacts. Ore minerals are dominated by coarse crystalline pyrite, sphalerite, chalcopyrite, galena, tetrahedrite-tennantite, and minor bornite. Gangue minerals consist of quartz, carbonates, barite, and chlorite. Five paragenetic stages are recognized: (1) quartz-pyrite-gold, (2) quartz-pyrite-sphalerite-chalcopyrite-gold, (3) tetrahedrite-tennantite-galena-silver, (4) barite, and (5) carbonates-chlorite-sphalerite. Hydrothermal alteration, with the formation of sericite, kaolinite, chlorite, and carbonates, mainly affected the host rocks of the stockwork and disseminated ore types. Carbonates are the principal alteration minerals in the andesite dikes.Fluid inclusions associated temporally with the precious metal mineralization show homogenization temperatures that range from 235 degrees to 344 degrees C and salinities from 4 to 10 wt percent NaCl equiv. Gold mineralization underlies a shallow zone of boiling of the hydrothermal fluids. The fluid inclusion data suggest that the precious metal zone was generated 400 to 1,200 m beneath the paleosurface. Temperature and salinity tend to decrease in the shallow levels of the veins. The calculated vertical variations in enthalpy of the hydrothermal fluids suggest that a fluid mixing mechanism was responsible for ore deposition. Hot, relatively saline, metal-rich fluids ascended and mixed with cooler meteoric fluids to cause precipitation of the iron, copper, and zinc sulfides, and subsequently the gold. Preliminary sulfur isotope studies (delta 34 S; 0.5-2.3ppm) suggest a magmatic source for the sulfur in sulfides at El Bronce.

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