Ore Breccias in the Rio Blanco-Los Bronces Porphyry Copper Deposit, Chile
Vargas R. Ricardo, Lewis B. Gustafson, Monica Vukasovic, Tidy F. Enrique, M. Alexandra Skewes, 1999. "Ore Breccias in the Rio Blanco-Los Bronces Porphyry Copper Deposit, Chile", Geology and Ore Deposits of the Central Andes, Brian J. Skinner
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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, fo2, 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.
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Geology and Ore Deposits of the Central Andes
Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages:
1. Normal subduction extended into Eocene time.
2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera.
3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle.
4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab.
5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner