High-Sulfidation Deposit Types in the El Indio District, Chile
Raymond R. Jannas, Teresa S. Bowers, Ulrich Petersen, Richard E. Beane, 1999. "High-Sulfidation Deposit Types in the El Indio District, Chile", Geology and Ore Deposits of the Central Andes, Brian J. Skinner
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The El Indio district, Chile, contains two types of high-sulfidation, precious metal deposits hosted in intensely altered Tertiary rhyodacitic volcanic rocks: El Indio, with enargite-pyrite and gold-quartz mineralization in complex vein systems, and Tambo, with alunite-barite-gold, mainly in tectonic breccia pipes. This single, world-class district contains more than 10 Moz of gold, 100 Moz of silver and 1 Mt of copper. At El Indio, the banded alunite and enargite + pyrite veins, peripheral to the main copper and gold veins, suggest alternating fluid conditions prior to the spectacular high-grade gold mineralization accompanied by sericitic-argillic alteration. The δ34S, δ18O, and δD ratios indicate the 250° to 300°C, moderate to low salinity (<5 wt % NaCl equiv), weakly acidic (pH = 3.5–4.5), reduced mineralizing fluids for both El Indio ore types had a dominantly magmatic water component (>60%), with no evidence of boiling. Copper deposition is attributed to decreasing temperature while precipitation of large quantities of gold is ascribed to mixing with an acid-oxidized fluid. At Tambo, gold was deposited with early barite and again after intermediate-stage alunite, from 200° to 250°C, low-salinity, intermittently boiling fluids. The δ34S ratios at Tambo indicate that the barite fluids were mildly reduced (sulfide/sulfate ratio of 10–25) and contained disproportionated magmatic SO2. The δ18O and δD ratios indicate that alunite formed from condensed, δD-depleted magmatic vapor between gold stages. Calculations show that, within a limited range of dissolved sulfur and pH conditions, a single magmatic fluid could have evolved to produce the multiple mineral assemblages seen at both El Indio and Tambo, with the former in a deeper, more reduced, hydrothermal environment and the latter in a near-surface setting.
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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