Abstract The porphyry copper mineralization at the Zaldívar deposit is confined to a NE-striking corridor of early- and late-intermineral granodioritic and dacitic porphyry intrusions and associated magmatic-hydrothermal breccia bodies. Country rocks comprise Early Permian rhyolite and andesite of La Tabla Formation plus comagmatic granitoids and Late Triassic andesite dikes. Middle Eocene andesitic rocks are common but of ill-defined distribution. Hydrothermal alteration consists of centrally located, magnetite-bearing potassic assemblages that are partially to completely overprinted by chlorite-epidote and sericitic alteration zones. The bulk of the hypogene metal resource was introduced synchronously with potassic alteration and A- and B-type veinlets during emplacement and evolution of multiple centers of biotite-bearing, early-intermineral porphyry and breccia bodies. Late-intermineral, hornblende-bearing dacite porphyry phases and associated breccia centers were emplaced later than the A- and B-veinlets but prior to multiple D-type veinlet generations and contributed additional, although lower grade, mineralization. Late-mineral dacite dikes are barren. Extensions to the east and northeast connect Zaldívar with Escondida Norte, and both can be considered as separate, coalescing porphyry copper deposits. Two discrete porphyry copper systems coexist at Zaldívar: Early Permian and late Eocene. The minor, copper-only Early Permian event (~290–285 Ma) was associated with an evolved, end-stage rhyolite porphyry phase of the La Tabla magmatism. The major late Eocene event (38.6–36.1 Ma) produced copper in addition to gold, molybdenum, and silver. Protracted Eocene porphyry copper alteration and mineralization, over ~2.5 m.y. as constrained by numerous U-Pb (zircon) and Re-Os (molybdenite) ages, was coincident with the high rates of uplift and denudation synchronous with contractional Incaic deformation. Earliest-stage porphyry intrusions at 39–38 Ma were probably associated with the terminal stages of a volcanic edifice, likely a dome complex, whose erosion products were deposited in contiguous, synorogenic basins. District-wide precursor magmatism of intermediate composition was active between 45 and 41 Ma. Oxidation and enrichment were active between ~17 and 15 Ma (supergene alunite), consistent with the chronology of supergene activity throughout the district and wider region.

Abstract The porphyry copper mineralization at Los Pelambres is contained in two contiguous deposits, Los Pelambres (Cu-Mo) and Frontera (Cu-Au), which together constitute the third largest copper concentration (∼36 million metric tons (Mt) Cu) in the Miocene to early Pliocene belt of central Chile. Los Pelambres is centered on a composite, N-oriented, ∼ 4.5- × 2.5-km precursor quartz diorite stock emplaced within the regional, NNW-striking, E-vergent Los Pelambres reverse fault. The fault places intensely deformed Late Cretaceous volcanic and late Oligocene to early Miocene volcanic and volcanosedimentary rocks of the Los Pelambres Formation over gently folded early Miocene volcanic rocks of the Pachón Formation. Copper-gold mineralization at Frontera is hosted mainly by andesite of the Pachón Formation. Hydrothermal alteration at Los Pelambres-Frontera conforms to the classic zonal pattern in which a potassic center grades laterally to an annular sericitic zone surrounded by a propylitic halo. The bulk of the hypo-gene metal resource is hosted by multiple veinlet generations within potassic alteration, of which type 4 (quartz ± K-feldspar ± biotite ± sericite ± phengite ± andalusite ± corundum), A, and B types are volumetrically and economically the most important. The type 4 veinlets are regularly distributed throughout Los Pelambres and Frontera, whereas highest intensities of A and B veinlets display a spatial correlation with at least 20 small (∼ 200-m diam), SE-plunging magmatic-hydrothermal centers. These centers comprise one or more intermineral porphyry intrusions of dacitic (porphyry B) and andesitic (porphyry A) compositions along with igneous and hydrothermal breccias, the apical parts of which contain aplite and pegmatite pods. These centers acted as a series of miniature porphyry copper deposits whose coalescence generated the Los Pelambres-Frontera ore-body. This coalescence also led to deposit-scale sulfide zoning, from internal chalcopyrite-bornite through chal-copyrite-pyrite to external pyrite. Abundant hydrothermal magnetite accompanies the gold-bearing copper mineralization in biotitized andesite at Frontera. The sericitic alteration is largely pyritic, but a NE-striking, SE-dipping corridor of D-type veinlets that overprints the potassic alteration in the northwestern quadrant of Los Pelambres contains copper sulfosalts. The internal portions of this corridor are characterized by advanced argillic assemblages, defining the roots of a once more extensive lithocap. On the basis of detailed U-Pb zircon dating, the intrusive magmatism at Los Pelambres-Frontera lasted ∼ 3.8 m.y., from emplacement of the precursor Los Pelambres stock between ∼ 14 and 12.5 Ma, through generation of numerous porphyry B and A phases and associated magmatic-hydrothermal centers between ∼ 12.3 and 10.5 Ma, to intrusion of late mineral porphyry at Frontera at ∼ 10.2 Ma. Similarly, the copper, molybdenum, and gold mineralization was introduced during a protracted interval of ∼ 1.7 m.y., between 11.8 and 10.1 Ma, as constrained by Re-Os molybdenite geochronology. The entire system cooled to nearly ambient temperatures by ∼ 8 Ma, as supported by temporally overlapping K-Ar, Ar/Ar, and (U-Th)/He ages, and was exposed to the effects of supergene oxidation and immature enrichment by ∼ 5 Ma. Plio-Pleistocene glaciation partially eroded a former, more widespread supergene chalcocite blanket, the remnants of which accounted for the bulk of the ore mined during the first 10 years of the Los Pelambres open-pit operation. The southeast-inclined geometry of the entire Los Pelambres-Frontera system, including the porphyry centers and northeast structural corridor defined by sericitic and advanced argillic alteration, are ascribed to synmineral tilting. The tilting accompanied regional tectonic uplift during crustal shortening and thickening, which were controlled by thick-skinned reverse faults active ∼ 60 km farther east in Argentina.

Abstract A variety of metals and deposit types define the metallogeny of the Andes from Colombia through Ecuador, Peru, and Bolivia to Argentina and Chile, although porphyry copper and epithermal gold deposits undoubtedly predominate and will continue to do so. Discoveries over the last 30 yrs or so, predominantly in the central Andes and especially Chile, have been made using routine, field-based geologic and complementary geochemical methods, a situation that is considered unlikely to change radically in the foreseeable future. The only clearcut evolutionary change is the increased number of deposits being discovered beneath pre- and postmineral cover. The predictive capacity of conceptual geology has had minimal impact on the Andean discovery record but is thought to offer much promise for the future. This introductory article selects mineralization styles and relationships as well as some broader metallogenic parameters as simple examples of geologic concepts that may assist exploration. Emphasis is placed on porphyry copper ± molybdenum ± gold and high-, intermediate-, and lowsulfidation epithermal gold ± silver deposits, although reference is also made to several carbonate rock-hosted precious and base metal deposit types and styles as well as subvolcanic tin, volcanogenic massive sulfide, and slate-belt and intrusion-related gold deposits. Particular emphasis is placed on the potential for exceptionally high grade porphyry copper, porphyry gold, epithermal gold, and subvolcanic tin deposits. Deposits resulting from the oxidation, enrichment, and chemical transport of copper and zinc and mechanical transport of gold and silver during supergene weathering are also briefly highlighted. Si bien la metalogenia de los Andes de Colombia, Ecuador, Perú, Bolivia y Chile se encuentra definida por una gama de metales y estilos de mineralización, son los depósitos tipo pórfido de cobre y epitermal de oro los que dominan en el presente y continuarán prevaleciendo en el futuro. Los descubrimientos de los últimos 30 años, predominantemente en los Andes centrales y especialmente en Chile, han sido realizados mediante métodos geológicos rutinarios de campo, generalmente complementados satisfactoriamente por métodos geoquímicos. Se estima que esta situación difícilmente experimentará variaciones radicales en un futuro cercano. El único cambio destacable en esta historia evolutiva está dado por el aumento apreciable de descubrimientos de depósitos cubiertos, bajo cobertura pre o postmineral. A nivel andino, la capacidad predictiva de la geología conceptual ha tenido un impacto mínimo en el número total de descubrimientos, aunque se piensa que su uso debiera garantizar buenas perspectivas futuras. El presente artículo

Abstract A variety of metals and deposit types define the metallogeny of the Andes from Colombia through Ecuador, Peru, and Bolivia to Argentina and Chile, although porphyry copper and epithermal gold deposits undoubtedly predominate and will continue to do so. Discoveries over the last 30 yrs or so, predominantly in the central Andes and especially Chile, have been made using routine, field-based geologic and complementary geochemical methods, a situation that is considered unlikely to change radically in the foreseeable future. The only clearcut evolutionary change is the increased number of deposits being discovered beneath pre- and postmineral cover. The predictive capacity of conceptual geology has had minimal impact on the Andean discovery record but is thought to offer much promise for the future. This introductory article selects mineralization styles and relationships as well as some broader metallogenic parameters as simple examples of geologic concepts that may assist exploration. Emphasis is placed on porphyry copper ± molybdenum ± gold and high-, intermediate-, and lowsulfidation epithermal gold ± silver deposits, although reference is also made to several carbonate rock-hosted precious and base metal deposit types and styles as well as subvolcanic tin, volcanogenic massive sulfide, and slate-belt and intrusion-related gold deposits. Particular emphasis is placed on the potential for exceptionally high grade porphyry copper, porphyry gold, epithermal gold, and subvolcanic tin deposits. Deposits resulting from the oxidation, enrichment, and chemical transport of copper and zinc and mechanical transport of gold and silver during supergene weathering are also briefly highlighted.

Abstract Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebody: (1) Sewell stock and other quartz diorite-tonalite intrusions of the eastern part crystallized from 6.46 ± 0.11 to 6.11 ± 0.13 Ma (zircon U-Pb); (2) quartz diorite-tonalite, immediately southeast of the orebody, with biotite 40 Ar/ 39 Ar plateau ages of 5.63 ± 0.12 and 5.47 ± 0.12 Ma—these ages agree with a hydrothermal overprint on zircons from the intrusions of the previous episode at 5.67 ± 0.19 to 5.48 ± 0.19 Ma (U-Pb); (3) Teniente dacite porphyry crystallized at 5.28 ± 0.10 Ma (zircon U-Pb); (4) a dacite ring dike encircling the Braden pipe crystallized at 4.82 ± 0.09 Ma (zircon U-Pb); and (5) minor dacite intrusions and dikes yielded a biotite 40 Ar/ 39 Ar plateau age of 4.58 ± 0.10 Ma, and sericite 40 Ar/ 39 Ar plateau ages of 4.56 ± 0.12 to 4.46 ± 0.10 Ma. All these felsic intrusions were emplaced within country rocks of late Miocene according to an apatite fission-track age of 8.9 ± 2.8 Ma for a mafic sill, in accord with previous K-Ar ages of 12.0 ± 0.7 to 6.6 ± 0.4 Ma for volcanic rocks from the district. Molybdenite Re-Os dating at El Teniente revealed ore deposition at 6.30 ± 0.03, 5.60 ± 0.02, 5.01 to 4.96, 4.89 ± 0.08 to 4.78 ± 0.03, and 4.42 ± 0.02 Ma, concurrent with the five intrusive episodes. The Re-Os system for molybdenite was unaffected by the various hydrothermal episodes. In contrast, the 40 Ar/ 39 Ar system of micas was reset by high-temperature (>350°C) fluid circulation and provides only a partial record of the latest history of development of this supergiant ore-forming system; biotite, sericite, and altered whole-rock samples collected throughout the orebody yielded 40 40 Ar/ 39 Ar plateau ages ranging from 5.06 ± 0.12 to 4.37 ± 0.10 Ma. These ages reveal a period of hydrothermal activity, which extended either continuously or episodically, for at least 0.69 ± 0.22 m.y. (±2σ) and that comprises a succession of three episodes of ore deposition. Separate hydrothermal episodes are thus interpreted to have lasted <0.69 ± 0.22 m.y. The Braden breccia pipe in the center of the deposit was formed as a single synmineralization event, probably related in time to the injection of the dacite ring dikes at 4.82 ± 0.09 Ma (zircon U-Pb). It was followed by quartzsericite alteration within and peripheral to, the pipe from 4.81 ± 0.12 to 4.37 ± 0.10 Ma (sericite 40 Ar/ 39 Ar). The successive intrusions of felsic bodies and their respective crystallization processes were immediately followed by genetically related, short-lived episodes of ore deposition, each associated with hydrothermal alteration. This multistage evolution, inferred from systematic dating, was not apparent from previous geochronologic data and is inferred to have contributed to the enormous volume and richness of the El Teniente. Thermal modeling of apatite fission-track data suggests that the porphyry system cooled very rapidly to temperatures below 105° ± 20°C, most likely before the intrusion of a postore hornblende-rich andesitic dike at 3.85 ± 0.18 Ma (hornblende 40 Ar/ 39 Ar). This dike cuts the southern part of the El Teniente deposit and marks the end of igneous activity in the orebody.

Abstract Epithermal mineralization in the El Indio-Pascua belt, a world-class Au-Ag-Cu district located along the Chile-Argentina frontier, formed in the late Miocene within an actively uplifting tectonic block of the Cordillera Principal. Previous studies have demonstrated a spatial and temporal relationship between epithermal processes and a series of regional erosional surfaces (pediplains) for all the major deposits in the belt. Ore deposition occurred beneath the 12.5 to 14 Ma Azufreras-Torta pediplain but near the head of actively incising 6 to 10 Ma Los Ríos pediment valleys. New isotopic and paragenetic data for several major deposits (Pascua-Lama, Tambo, El Indio) suggest that this geomorphologic setting had a significant impact on the processes that produced preore alteration, ore deposition, and postore processes. Barren, preore alteration of late Oligocene to early Miocene age occurs throughout the belt and is well-developed in the vicinity of the major ore deposits. This alteration is of a hypogene advanced argillic type, and isotopic data for alunite and associated sulfide minerals are characteristic of a magmatic-hydrothermal origin. Alunite-pyrite pairs, where available, suggest depositional temperatures of 190° to 350°C. All economic mineralization in the belt is of an epithermal type and is dominated by high-sulfidation—state mineral assemblages. At both Pascua-Lama and Tambo, alunite is intergrown with, or host to, much of the precious metal mineralization. Isotopic data are consistent with a magmatic-hydrothermal origin for much of this alunite, and ore deposition resulted from the boiling of magmatic-dominated fluid at temperatures of ca. 200° to 300°C. A transition from magmatic-hydrothermal to magmatic steam-dominated processes is evident in both systems, and at Tambo this transition is associated with significant Au deposition. Large steam-heated alteration blankets developed above these mineralized centers during this time, although the isotopic signature of this alteration is not typical of most steam-heated zones and is characterized by magmatic-dominated fluid. In contrast to Pascua-Lama and Tambo, alunite at El Indio is restricted to banded alunite sulfide veins that postdate most other mineralized veins in the district. Constraints on mineralization are more difficult to determine for this deposit due to its wide age range (<6.2–7.8 Ma) and different ore styles. A lithologic contact between the Tilito Formation volcanic rocks and overlying andesites of the Escabroso Group most likely enhanced fluid mixing and suppressed widespread boiling of magmatic fluid, although evidence for local boiling in the late, banded alunite sulfide veins is recognized. Overall, isotopic studies and other features of these deposits suggest that magmatic-hydrothermal processes and epithermal ore deposition were strongly influenced by pediment erosion and semiarid climatic conditions. The dominance of magmatic condensates for the duration of the major mineralizing systems, even in near-surface alteration zones, is consistent with an arid climate and limited availability of meteoric water. Telescoping of alteration assemblages and the occurrence of supergene alteration to appreciable depths below the surface suggest drops in the paleowater tables during late stages of the hydrothermal systems. At both Tambo and Pascua-Lama, fluid inclusion and mineralogic evidence for fluid boiling during the main stages of mineralization is recognized. Similarly, in both these systems, a transition to magmatic steam-dominated processes also argues for rapid changes in hydrostatic pressures. In the absence of large volcanic edifices and the potential for topographic collapse, erosion at the head of the Los Ríos pediment valleys, where most deposits are located, is thought to have had a major influence on high-level fluid-flow regimes. Rapid draining of the outflow zones by local catastrophic erosion may have promoted boiling in the upflow zones and allowed lateral outflow at lower topographic levels, thereby significantly modifying the nature of these richly mineralized systems.

Abstract The Jerónimo sedimentary rock-hosted disseminated Au deposit is located within the Potrerillos district of the Atacama region of northern Chile, east of the Potrerillos porphyry Cu-Mo and El Hueso high-sulfidation Au deposits. Prior to development, the Jerónimo deposit contained a resource of approximately 16.5 million metric tons (Mt) at 6.0 g/t Au. Production began in the oxidized, nonrefractory portion of the deposit in 1997 and terminated in 2002. During that time, approximately 1.5 Mt at 6.8 g/t Au was mined by underground room-and-pillar methods, from which a total of approximately 220,000 oz of Au was recovered by heap-leach cyanidation. Jerónimo mineralization occurs as irregular strata-bound lenses within particular bioclastic limestone units of the Jurassic Asientos Formation. The manto-shaped mineralized zone extends over an area of approximately 2.0 by 1.3 km and averages 6 m in thickness. Mineralization and alteration are focused along subvertical fractures and joints within the bioclastic units. Alteration involved decarbonatization followed by the formation of the following assemblages: (1) intense, pervasive, replacement-style silicification; (2) carbonate, mainly restricted to vugs, consisting of Mn carbonate (rhodochrosite and kutnohorite) in the center of the orebody and calcite-dolomite on the margins; and (3) argillization, consisting of illite as widespread disseminations and veinlets and kaolinite as vug fillings in the center of the deposit. Other common alteration minerals include apatite, rutile, monazite, and barite. The ore mineral suite consists of pyrite, arsenopyrite, sphalerite, lead sulfosalts, orpiment, and realgar, with minor coloradoite, altaite, cinnabar, and cassiterite. Gold is present dominantly as submicron-sized grains, ranging from 140 nm to 1.13 μ m, that are encapsulated in pyrite, arsenopyrite, quartz, and realgar and also occur within vugs in the silicified matrix. Lead isotope results of the main-stage sulfide and sulfosalt minerals ( 206 Pb/ 204 Pb: 18.564–18.644; 207 Pb/ 204 Pb: 15.592–15.662; 208 Pb/ 204 Pb: 38.536–38.638) indicate that lead in the ore fluids was dominantly from a Tertiary magmatic source, with input from a more radiogenic source—igneous Carboniferous to Triassic basement rocks and/or the overlying Jurassic limestone and sandstone. Carbon and oxygen isotope compositions of ore zone rhodochrosite and kutnohorite, ranging from δ 18 O of 16.65 to 22.52 per mil (VSMOW) and δ 13 C of −2.84 to −1.3 per mil (PDB), suggest contributions from both magmatic and Jurassic limestone wall-rock sources. The critical features that define the style of mineralization at Jerónimo include lithological and structural control, enrichment in Au-As-Mn-Zn-Pb-Ag-Hg, silicification and carbonate alteration, the presence of native Au grains, the close spatial association with porphyry and related styles of mineralization, and isotopic evidence for a magmatic contribution to metals and hydrothermal fluids. These characteristics are more similar to carbonate-replacement deposits than to typical Carlin-type sediment-hosted Au deposits. Structural and isotopic data suggest that Jerónimo is late Eocene-early Oligocene in age, but the precise temporal and genetic relationships of Jerónimo to other magmatic-hydrothermal systems in the district are unknown.

Abstract The porphyry copper systems of the El Salvador region, represented mainly by El Salvador, Potrerillos, Exploradora, Sierra Jardín, and Coya, are located in the southern part of the middle Eocene to early Oligocene porphyry copper belt of northern Chile. They have distinctively higher gold grades (between 0.1 and 0.5 g/t) and slightly lower molybdenum grades (<100 ppm) than the systems located in the northern parts of the belt. The El Salvador and Potrerillos deposits each contains resources of >600 million metric tons (Mt) at >0.6 percent Cu and 0.1 to 0.2 g/t Au. The Exploradora porphyry copper-gold prospect contains geologic resources of ~100 Mt at 0.3 percent Cu and 0.2 g/t Au, with an overlying leached capping enriched in gold, averaging ~0.5 g/t. The Sierra Jardín and Coya prospects show the lowest Cu contents (0.1–0.2 %), but the Au tenor is in the 0.1- to 0.5-g/t range. The deposits are related to discrete magmatic pulses emplaced in different lithotectonic environments, including the borders of Paleocene volcanic structures (El Salvador, Sierra Jardín; 45–40 Ma) and dilational jogs and reverse and transfer faults (Exploradora, Potrerillos, and Coya; 37–31 Ma). Host rocks are mainly volcanic and marine sedimentary rocks of Jurassic to Cretaceous age. The most remarkable feature of the deposits is their relationship to multiphase, syntectonic tonalite to granodiorite intrusions. There is strong superimposition (telescoping) of intrusion and alteration-mineralization phases, including early, magnetite-rich, potassic alteration-mineralization, moderately to weakly developed sericitic alteration, and late-stage overprinted advanced argillic alteration. Postmineral phreatomagmatic activity is characterized by pebble dikes and/or diatreme breccias. The appreciable number of gold-rich porphyries (as opposed to their gold-poor counterparts) discovered during the past ten years in the Chilean Andes (e.g., La Fortuna, Cerro Casale, Esperanza) implies that geologic conditions that favor their development are more widespread than previously considered. Several key characteristics of gold-rich porphyry copper deposits in the El Salvador region can be used for exploration purposes and open new ground to prospecting.

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.

Abstract At Escondida, the propylitic, potassic, and quartz-sericitic hydrothermal mineral assemblages are centered on a 38 Ma granodioritic stock composed of at least four phases. These early intrusive and related hydrothermal events were followed by rhyolitic intrusive and extrusive rocks and sulfide-rich veins of two advanced argillic alteration events at ~ 36 and ~34 Ma. The final hypogene copper grade of the deposit varies between 0.2 and 1.0 percent, reflecting varying degrees of overprinting by the successive copper mineralization stages. The homogenization temperatures of primary fluid inclusions from quartz-orthoclase- (stage A), quartzsericite- (stage B), and quartz-alunite-bearing veins (stage C) vary between 500° and 560°, 280° and 380°, and 200° and 340°C, respectively, with estimated depths of trapping ranging from 1.5 to 3.0 km. The δ 34 S values of sulfide minerals from the potassic assemblage range from −3.2 to −2.0 per mil, whereas values from the quartz-sericite assemblage vary between −1.1 and +0.6 per mil. These values suggest a common source of magmatic sulfur and indicate that sulfide minerals of stage B were not formed by leaching sulfide from stage A. The δ 34 S values of stage C sulfide minerals range from −2.5 to +2.7 per mil, which suggests that sulfide for the advanced argillic event may have been derived, in part, by leaching sulfide from the two earlier hydrothermal stages. Calculated values of δ D and δ 18 O for water coexisting with igneous and hydrothermal minerals indicate a dominantly magmatic component in stages A and B and a mixture with meteoric water at the lower temperatures of stage B and during the advanced argillic event (stage C). Stage C may have formed by circulatión of surficial meteoric water heated by the 34 to 36 Ma subvolcanic rhyolitic rocks. It is also possible that some 2 m.y. after the emplacement, uplift, and denudatión of the main Escondida porphyry system, a new porphyry copper stock was emplaced in the same structural weakness zone, and that the subvolcanic rhyolite and advanced argillic alteration and associated mineralization represent the upper levels of this younger porphyry system.

Abstract The Esperanza porphyry copper-gold deposit is located approximately 60 km south of Calama, in the porphyry copper province of northern Chile. Although partly exposed, historically mined from small-scale pits, and intermittently explored over many years, its true size and potential were appreciated only in 1999. Discovery was the direct result of detailed geologic mapping of key rock types and hydrothermal alteration assemblages and zoning and was partly underpinned by a property-wide ground-magnetic survey. The geology of the region is typical of the Cordillera de Domeyko and includes several fault-controlled basement blocks of late Paleozoic age and a number of sedimentary and volcano-sedimentary sequences of Mesozoic and Cenozoic age. Of these, the Late Cretaceous Quebrada Mala Formation and the middle Eocene domes of the Estratos de Cerro Casado are widely distributed in the area. Much of the region is mantled by moderately consolidated gravels of middle Eocene to middle Miocene age, collectively grouped as the Calama and Tambores Formations. The regional structure is dominated by several north-northeast-trending splays of the Domeyko fault system, which display evidence for both strike-slip and reverse movements and exert a strong control on the location of Esperanza. The deposit is part of a northeast-trending corridor of middle Eocene porphyry deposits that includes Telégrafo, Centinela, and Polo Sur. At Esperanza, a series of structurally controlled, medium-grained granodiorite porphyry dikes intrude a sequence of massive andesite flows and interbedded pyroclastic and calcareous volcano-sedimentary horizons of the Quebrada Mala Formation. Hydrothermal alteration consists of a core of potassic alteration partly overprinted, but mainly surrounded by, intermediate argillic, quartz-sericitic, and propylitic assemblages. Early biotite-bearing alteration from the central potassic zone yields a 40 Ar- 39 Ar age of 41.3 ± 0.3 Ma. Hypogene copper-gold mineralization occurs dominantly as chalcopyrite and bornite in multiple stockworks of pyrite-poor, A- and B-type veinlets with quartz, K-feldspar, biotite, magnetite, apatite, and anhydrite, which are spatially and genetically associated with the potassic assemblages. Primary fluid inclusions in these veinlets possess homogenization temperatures (T h ) of between 435° and 592°C and salinities in the 41 to 60 wt percent NaCl equiv range. Minor molybdenite accompanying these veinlets yields an Re-Os age of 41.80 ± 0.13 Ma. Overprinted intermediate argillic alteration is characterized by chlorite, illite, smectite, and greenish sericite, with chalcopyrite and pyrite, whereas quartz-sericitic assemblages are barren of copper and dominated by disseminated and veinlet pyrite in classic D-type veinlets. Primary fluid inclusions in quartz veinlets from these assemblages show lower T h (217°–330°C), although still retaining a magmatic component to generate salinities of 40 to 53 wt percent NaCl equiv. Within the potassic core, anhydrite becomes increasingly abundant with depth and, locally, forms a large structurally controlled massive body with interbedded proximal skarn rich in garnet and diopside. Supergene copper mineralization is developed in the upper 150 m of the deposit where it is characterized by atacamite and chrysocolla with subordinate brochantite, copper wad, and copper-rich clays. Minor amounts of chalcocite, covellite, native copper, and cuprite occur near the redox interface. From a regional standpoint, Esperanza confirms that copper-gold and copper-molybdenum deposits coexist in continental arcs within the same metallogenic belt, and porphyry copper and copper-gold mineralization in the northern Chile porphyry copper province was, at least in part, intimately associated with contractional deformation during the middle to late Eocene Incaic orogeny.