Low-temperature thermochronological ages of samples from the central Andes correlate with major tectonic events during Late Cretaceous and Cenozoic times. Apatite fission-track (AFT) ages show prominent clusters during the Early–Late Cretaceous in the Coastal Cordillera and the Cordillera de Domeyko; Paleocene–Oligocene ages in the western Puna Plateau and Cordillera de Domeyko; and latest Eocene–Pliocene ages in the Eastern Cordillera. These ages track the expansion of the Andean orogenic edifice, the eastern front of which migrated rapidly eastward ~200 km and ~150 km during late Eocene and Pliocene times, respectively. During the intervening time interval, ca. 35–5 Ma, the orogenic strain front migrated slowly eastward through the Eastern Cordillera. A second cluster of Cretaceous ages in the Eastern Cordillera and Santa Bárbara Ranges documents exhumation related to extension in the Salta rift. The highly unsteady pace of orogenic wedge propagation suggests that kinematics controlled local climate, rather than vice versa. The frequency of AFT ages is anticorrelated with magmatic production in the central Andean arc and the rate of convergence between the Nazca and South American plates. We propose a link between AFT bedrock cooling ages in the central Andes and exhumation related to cyclical processes of shortening, wedge propagation, magmatism, and removal of dense roots from beneath the magmatic arc and thickened hinterland region. In particular, periods of sustained exhumation associated with local crustal shortening alternate with periods of rapid eastward wedge propagation during which exhumation was more spatially diffuse across the high-elevation hinterland. Episodes of spatially confined exhumation are correlated with periods of relatively low magmatic production in the central Andean arc and relatively slow or declining plate convergence rates. We speculate that shortening in the upper crust was contemporaneous with underthrusting of lower crust and mantle lithosphere beneath the magmatic arc. Because of thermal inertia, melting of these underthrusted rocks lagged behind the shortening events themselves, thus producing the observed temporal anticorrelation between rapid shortening-induced exhumation and arc magmatism.

Abstract The Centinela district is a 40-km-long segment of the late Eocene to early Oligocene porphyry copper belt of northern Chile. The main mineralization styles in the district include porphyry copper and associated skarns with gold and molybdenum credits, oxide copper in the form of either exotic deposits or in situ oxidation zones, and supergene copper sulfide enrichment blankets. Structurally controlled, district-wide porphyry copper mineralization formed between 44 and 41 Ma, contemporaneously with transpressional tectonism along the regional-scale Centinela-Limón Verde fault zone, a splay of the major Domeyko fault system. The Domeyko fault system accommodated tectonic uplift of the Cordillera de Domeyko during early stages of the middle Eocene Incaic orogeny. Several productive porphyry copper systems in the district were emplaced syntectonically along reverse- and oblique-slip faults that controlled the shape and attitude of both the dominantly dikelike intrusions and associated alteration mineralization. All the porphyry copper deposits in the district evolved from early potassic alteration with copper mineralization present as chalcopyrite and bornite, through sericite-chlorite alteration with chalcopyrite and pyrite, to late, overprinting pyritic sericitic alteration. Some systems developed advanced argillic lithocaps in their upper parts. Much of the copper was introduced with the potassic alteration, and all the gold-rich deposits display abundant hydrothermal magnetite and a positive correlation between copper and gold. Supergene processes gave rise to oxide copper zones above pyrite-poor, chalcopyrite- and bornite-bearing protore and to chalcocite blankets over less reactive, sericitic zones containing pyrite and chalcopyrite. Coeval regional uplift, erosion, and exhumation resulted in widespread piedmont gravel deposition broadly contemporaneous with the supergene activity, with formation of exotic copper deposits alongside the shallow, actively oxidizing, pyritic parts of porphyry copper deposits. A long exploration history characterizes the Centinela district, from initial prospecting in the 1870s to formal exploration programs by major mining companies approximately 100 years later. However, initial appreciation of the district potential resulted from the discovery of the El Tesoro and Tesoro NE exotic copper deposits between 1990 and 1993 and was confirmed with the discovery of the Esperanza and Telégrafo porphyry copper-gold deposits in 1999 and 2001, respectively. Approximately 18 years of persistent exploration by Antofagasta Minerals resulted in an order-of-magnitude expansion of the district mineral inventory from an original 120,000 metric tons (t) of contained copper at El Tesoro in 1990 to the presently known resources of ~20 Mt of contained copper and ~560 t (18 Moz) gold in the several main deposits. Commercial copper production commenced at the El Tesoro SX-EW plant in 2001, with a cumulative output through 2009 of 790,000 t of cathode copper. With production start-up at Esperanza in late 2010, the district is expected to produce an additional 195,000 t Cu, 7 t (225,000 oz) gold, and 35 t (1,130,000 oz) silver per year in concentrates, whereas Mirador will contribute an additional 95,000 t of cathode copper upon attaining full production in 2012. Early stages of exploration within the district targeted exposed mineralization that had undergone historic, small-scale surface mining. The more recent exploration incorporated geologic mapping and conceptual modeling to test for both extensions to the known mineralization and new deposits as the company’s land holdings expanded. Recent efforts successfully targeted blind mineralization under extensive postmineralization gravel cover through interrogation of comprehensive data sets, use of empirical geologic models, and incorporation of conventional and relatively new geophysical tools to assist with modeling of bed-rock geology. However, the most recent discoveries employed information from nearby drill holes that were completed by previous explorers more than 23 years earlier. Discovery and wealth creation in the Centinela district are considered as products of the company’s long-term commitment to exploration and the exploration team’s view that the district had unfulfilled potential. Persistence overcame at least two cycles of economic turmoil. The most recent discoveries occurred approximately 17 years after initial discovery of El Tesoro and more than 25 years after formal exploration by major mining companies commenced in the district.

Abstract The El Peñón gold-silver deposit comprises six epithermal veins, which contain a geologic resource (measured + indicated + inferred) of 3.8 million oz (Moz) Au and 63 Moz Ag. Three of the veins are currently being mined by underground and open-pit methods. El Peñón is located in the Central Depression of northern Chile, where the geology is dominated by Paleocene and Eocene mafic to felsic volcanic rocks and minor intermediate to felsic subvolcanic rocks and intrusions. The deposit is located in the central portion of the Paleocene metallogenic belt, 165 km southeast of Antofagasta. The property covers an area of ~440 km 2 , and the ore deposit occurs within an area of approximately 15 km 2 . Late Cretaceous, Paleocene, and Eocene andesitic to rhyolitic flows, domes, tuffs, and minor intrusive rocks characterize the geology of the district. The deposit occurs within flat-lying to gently dipping, andesitic to rhyolitic pyroclastic and flow units, and volcaniclastic breccias of Paleocene and early Eocene age and is partly assignable to the Augusta Victoria Formation. El Peñón veins are partly hosted by, and spatially associated with, a 54 to 55 Ma ( 40 Ar- 39 Ar, U-Pb) rhyolite dome complex that occurs over an area at least 18 km 2 , and similar rhyolite lavas occur over an area of tens of kilometers 2 . Rocks in the district display two distinct types of hydrothermal alteration: widespread alteration associated with near-neutral pH, reduced fluid and localized alteration associated with acidic pH, oxidized fluid. Near-neutral pH, reduced fluid produced widespread replacement of phenocrysts and groundmass by quartz, adularia, albite, illite, chlorite, smectite, calcite, and pyrite; quartz-adularia flooding and cement to hydrothermal breccia intensify in the vicinity of veins. Where upflow of these fluids was focused along dominantly north- and northeast-trending structures, Au-Ag ± base metal-bearing crustiform quartz ± adularia ± carbonate veins formed, including the six veins that comprise the El Peñón deposit and several outlying prospects. Adularia from the two largest veins has been dated at 52 to 53 Ma ( 40 Ar- 39 Ar), indicating formation 1 to 3 m.y. later than the host rhyolite domes. Acidic pH, oxidized fluid produced lithocaps of massive quartz-alunite alteration, quartz-alunite cemented breccia, and, locally, weak Cu mineralization above inferred Late Cretaceous and Eocene intrusions. Isolated occurrences of quartz-alunite alteration covering hundreds of meters 2 are located at the periphery of the property, in addition to several larger areas beyond it. Locally, quartz-barite veins occur peripheral to quartz-alunite alteration and contain variable amounts of base metals and Ag with little or no Au. The veins that comprise the El Peñón deposit range from <50 cm to 22 m wide. Pervasive supergene oxidation extends to 400 m below surface. Limited drill intercepts at deeper levels consist of banded and brecciated quartz, adularia, and massive, bladed, and acicular, Ca-, Fe-, Mn-, and Mg-bearing carbonate minerals, with minor amounts of pyrite, chalcopyrite, sphalerite, and galena. Veins exhibit a wide range of crustiform textures, including comb, colloform, and lattice quartz, rhombic adularia, and abundant massive and bladed Fe and Mn oxide minerals. Recrystallization textures suggest amorphous silica and chalcedony precursors for some quartz. Coexisting liquid- and vapor-rich inclusions, lattice textures, and vein adularia are evidence for boiling conditions that were likely responsible for Au-Ag deposition. Ore minerals observed in oxidized veins consist of electrum (mostly 40–60 wt % Au), acanthite, gold, silver, silver sulfosalts, silver halides, and rarely pyrite, chalcopyrite, and galena. High ore grades are generally associated with massive bands of fine-grained quartz and adularia, breccias composed of vein material in a matrix of fine-grained quartz and adularia, and, less commonly, colloform quartz bands. Supergene processes resulted in local remobilization of Au and Ag, leaving nearly pure gold (up to 98 wt % Au) along fractures and associated with oxide masses. Fluid inclusion data from the El Peñón deposit indicate vein formation from low-salinity (<2 wt % NaCl equiv), boiling hydrothermal fluid at temperatures mostly from 230° to 260°C. Fluid inclusion data from other mineralized quartz veins in the district indicate formation from commonly boiling, dilute fluid (<3 wt % NaCl equiv) at temperatures between 180° and 330°C. Quartz-barite veins peripheral to quartz-alunite alteration formed from boiling fluid between 175° and 225°C with apparent salinities of 1 to 6 wt percent NaCl equiv. Geochemical gradients in altered rocks surrounding veins in the El Peñón district indicate that Au, Ag, As, and Sb concentrations increase toward quartz veins, and Au, Ag, As, Sb, and base metal concentrations increase toward quartz-barite veins. Geochemical analyses of altered rocks from drill holes surrounding the Quebrada Colorada vein, the highest grade vein of the El Peñón deposit, show enrichment of Au and Ag and depletion of Ca, Na, and Sr toward the vein. Comparing the geochemical data to elevation, the highest mean values for Au (205 ppb) and Ag (5.9 ppm) occur at mid levels, the highest mean values for Pb (35 ppm) and Zn (183 ppm) occur at lower levels, and the mean values for As (139 ppm) and Sb (34 ppm) increase with elevation. Arsenic (100s of ppm) and Sb (10s of ppm) anomalies occur in rocks above the Quebrada Colorada vein that are barren or contain only low levels of Au and Ag; such anomalies may be useful indicators of blind mineralization. Epithermal deposits in the Paleocene belt of northern Chile are preserved in rocks located at a long-lived convergent plate boundary with a complex history of compression and extension that has formed linear morphotectonic and metallogenic belts parallel to the plate boundary. In northern Chile, Paleocene and early Eocene epithermal deposits occur west of the uplifted Cordillera Domeyko, which contains younger porphyry Cu deposits and intrusions characteristic of deeper environments. This paradoxical situation is partly explained by a protracted history of structurally controlled basins in the Paleocene belt of northern Chile. From Late Cretaceous to Eocene time, the Paleocene belt was characterized by fault-bounded basins that experienced both subsidence and inversion; however, cumulative postmineral uplift was minimal because fluid inclusion data from El Peñón indicate erosion of only several hundreds of meters. Since the Miocene, erosion has largely ceased due to the onset of hyperaridity.