Abstract There are over 200 Pleistocene and Holocene Andean arc volcanoes along western South America, occurring in four distinct segments ( Fig. 5.1 ) called the Northern (NVZ; 2°N–5°S), Central (CVZ; 14°S–28°S), Southern (SVZ; 33°S–46°S) and Austral (AVZ; 49°S–55°S) volcanic zones. In the Andes of Chile alone there are more than 100 Pleistocene and Holocene stratovolcanoes, as well as a number of large volcanic fields and giant caldera complexes, of which 60 have documented Holocene eruptive activity ( Simkin & Siebert 1994 ; González-Ferrán 1995 ). These are located in the CVZ of northern Chile, the SVZ of central-south Chile, and the AVZ of southernmost Chile. Pleistocene and Holocene backarc volcanic centres, the westernmost part of the Patagonian plateau basalts, also occur in southern Chile along the border with Argentina. In addition, intraplate oceanic volcanoes form Chilean islands in the Pacific Ocean, submarine volcanism takes place along the Chile Ridge, and slab-window volcanic activity occurs above the region where the Chile Ridge is currently being subducted, both along the west coast of Chile and in the submarine environment near the trench. Pleistocene and Holocene volcanoes of the Chilean Andes provide a natural laboratory for the study of volcanism, magma genesis and volcanic hazards in the context of oceanic– continental plate collision. Andean volcanic activity results from subduction of the Nazca and Antarctic oceanic plates below the continental lithosphere of western South America ( Fig. 5.1 ). Volcanoes in the CVZ of northern Chile and the SVZ of central-south Chile occur where the angle of

Abstract The giant El Teniente copper-molybdenum deposit, located in the Andes of central Chile, is one of the world's largest known copper deposits, containing estimated resources of >75 × 10 6 t of fine copper in ore with grades greater than 0.67 percent. El Teniente has been described in the past as a porphyry deposit developed around a Pliocene dacite porphyry stock, with 80 percent of its copper mineralization hosted in Miocene andesites. However, new mapping—both regional and in underground mine workings—along with petrological studies, indicates that El Teniente, like the other giant Miocene and Pliocene copper deposits in central Chile, is actually best classified as a breccia deposit. Most of the high-grade hypogene copper at El Teniente occurs in and surrounding multiple magmatic-hydrothermal breccia pipes. Mineralized breccia complexes, with copper content >1 percent, have vertical extents of >1.5 km, and their roots are as yet unknown. These breccias are hosted in a pervasively biotite-altered and mineralized mafic intrusive complex composed of gabbros, diabases, and porhyritic basalts and basaltic andesites, and not in andesite extrusive rocks. The multiple breccias in El Teniente include copper- and sulfide-rich biotite, igneous, tourmaline, and anhydrite breccias, generated by the exsolution of magmatic fluids from cooling plutons, and also magnetite and rock-flour breccias. Surrounding biotite breccias, a dense stockwork of biotite-dominated veins has produced pervasive biotite alteration, and copper mineralization characterized by chalcopyrite >> bornite + pyrite. Later veins, with various proportions of quartz, anhydrite, sericite, chlorite, tourmaline, feldspars, and copper sulfide minerals, formed in association with the emplacement of younger breccias and felsic porphyry intrusions. These generated sericitic alteration in the upper levels of the deposit, and in some cases contributed more copper to the deposit, but in other cases eliminated or redistributed preexisting mineralization. Both the Teniente Dacite porphyry and the central rock-flour breccia of the Braden pipe, the dominant lithostructural unit in the deposit, are copper poor. Their emplacement at a late stage in the development of the deposit created a relatively barren core, surrounded by a thin (~150-m) zone of bornite > chalcopyrite, in the larger main area of chalcopyrite-rich, biotitealtered mafic rocks and mineralized breccias. The multistage development of breccia emplacement, alteration and copper mineralization at El Teniente occurred over a time span that was greater than 2 m.y., between >6.4 and 4.4 Ma, at the end of a more than 10-m.y. episode of Miocene and Pliocene magmatic activity, and just prior to the eastward migration of the Andean magmatic arc as a consequence of decreasing subduction angle. Decreasing subduction angle also caused crustal thickening, uplift and erosion, resulting in telescoping of the various breccias and felsic intrusions in the deposit. El Teniente is located at the intersection of major north-south, northwest-southeast, and northeast-southwest Andean structures, but what actually focused magmatic activity and mineralization at this one locality for so long remains an unsolved problem, the solution of which would provide an important tool for exploration of similar giant deposits.

Abstract Detailed logging of core from drill holes in the Sur-Sur and La Americana breccias in the Andina portion of the Rio Blanco-Los Bronces porphyry copper deposit, supported by petrography, geochemistry, and study of fluid inclusions, has documented zonal and temporal patterns in ore breccias over 1,600 vertical meters. Breccia textures progress from incipient crackle breccia with tourmaline veining to increased rounding of fragments and filling of open spaces by mineralization products and rock flour, reflecting many repeated pulses of brecciation. Both angular breccias and rounded clast breccias with rock-flour matrix have been mineralized primarily by the quiescent flow of hydrothermal solutions between pulses of brecciation. Sharp contacts mark boundaries between pulses separated by a period of thorough cementation, while diffuse, gradational contacts mark boundaries between more closely spaced pulses. Refracturing of the rock mass continued after consolidation of rock-flour matrix breccias, as documented by local angular rebrecciation and ubiquitous postbreccia veining. The angular, tourmaline breccia with chalcopyrite-pyrite at Sur-Sur grades downward, with decreasing tourmaline and increasing biotite in the matrix, into breccia with biotite-alkali feldspar alteration and chalcopyrite-bornite. Variably tourmalinized rock-flour breccias at La Americana extend about 400 m higher than Sur-Sur, with upward increasing ratios of specularite/tourmaline and pyrite/chalcopyrite, and decreasing grades of Cu and Mo. Minor dikes of porphyry have intruded the breccias but are themselves fragmented and appear to be contemporaneous with brecciation. Dikes that have intruded deep, high-grade Sur-Sur breccia display intense biotite alteration and disseminated chalcopyrite-bornite. A high-grade interval of La Americana breccia has been intruded by a dike with intense sericite-chalcopyrite alteration. Dikes intruding poorly mineralized La Americana breccias are barren and are not biotized. The Sur-Sur breccias were formed contemporaneously with early-stage porphyry copper mineralization at depth, and are cut by a sequence of quartz-molybdenite and sulfide veins with sericitic halos that are typical of the evolution of veining in porphyry copper systems. In both the Sur-Sur and La Americana breccia matrices, highly saline fluid inclusions, saturated with NaCl (over filling temperatures from 225° to 500°C), coexist with vapor-rich inclusions and with fluid-rich inclusions from 150° to 450°C and 2 to 30 wt percent NaCl equiv. High-salinity fluids are most abundant at depth and with higher copper grades, while liquid-rich and vapor-rich fluids are dominant near surface, particularly at La Americana. This, and prior stable isotope evidence, is compatible with the interpretation that magmatic fluids, derived from magma giving rise to porphyry dikes and the PΔV energy for brecciation, were primarily responsible for mineralization of the ore breccias. Intermixed meteoric water, however, may have been responsible for the huge volume and complex reworking of the breccias, and for apparently wide fluctuations in temperature, pressure, f o 2 , and salinity, which are suggested by fluctuations in magnetite-hematite and anhydrite saturation in the breccias. This district represents nearly an end member in the wide range of variations that are characteristic of porphyry copper mineralization. The breccias are copper ores because they were formed and mineralized by intrusions derived from a differentiated magma chamber which became saturated with typical porphyry copper-ore fluids. Location, complexity, and geochemistry offer the explorationist clues to the relatively rare tourmaline breccia, which may be ore bearing.

Abstract Large mineralized breccias are prominent features in the three giant late Miocene Andean copper deposits at Los Pelambres, Rio Blanco-Los Bronces and El Teniente in central Chile. The breccias were emplaced into Miocene igneous host rocks by the expansion of high-temperature, metal-rich fluids exsolvedfrom magmas. Minerals (tourmaline, anhydrite, biotite) precipitatedfrom these magmatic fluids in the matrices of different breccias have variable initial 87 Sr/ 86 Sr ratios, ranging from 0.7040 to 0.7049, and ∈. Nd values, ranging between +0.8 to +3.6. Although the fluids that generated the breccias may have leached some Sr from contained clasts of host rock, the ∈nd values of these breccia-matrix minerals are interpreted as the Nd-isotopic compositions of the magmas from which the breccia-forming fluids exsolved. The isotopic compositions determined for the breccia-matrix minerals differ from the host plutons. This implies that the fluids that generated the breccias were not derived from these plutons, which were already crystallized at the time of breccia formation as indicated by the angular nature of their clasts in the breccias. The fluids which generated the breccias must have exsolved from magmas crystallizing to form plutons not yet exposed at the surface, consistent with the fact that the roots of the mineralized breccias have not been encountered. Significantly, the isotopic compositions of the breccia-matrix minerals from different breccias in each deposit are variable. This indicates that the breccia-forming fluids were not derivedfrom a single magma, but from isotopically variable magmas. We suggest that the mineralized fluids that formed the late Miocene Cu-rich breccias in central Chile exsolved from multiple, compositionally variable magma batches cooling during the last stages of long-lived Andean magmatic systems. Cooling of these systems was triggered tectonically in the late Miocene as subduction angle and, as a result, subarc magma supply decreased.