Abstract The giant Escondida district in northern Chile, discovered in 1981, includes the major porphyry copper deposits at Escondida-Escondida Este, Escondida Norte-Zaldívar, Pampa Escondida, and two small deposits (the Escondida cluster), besides the Chimborazo deposit. The district contains at least 144 million metric tons (Mt) of copper. The Escondida district is part of the middle Eocene to early Oligocene porphyry copper belt, which follows the trench-parallel Domeyko fault system, a product of the Incaic transpressional tectonic phase. At the district scale, the major N-striking Portezuelo-Panadero oblique-reverse fault juxtaposes latest Carboniferous to Early Permian igneous basement with an andesitic volcanic sequence of late Paleocene to early Eocene age, both of which host the porphyry copper mineralization. Immediately before and during porphyry copper formation, a thick siliciclastic sequence with andesitic volcanic products intercalated toward the top (San Carlos strata) filled a deep basin, generated by clockwise rigid-block rotation, within the confines of the Escondida cluster. The presence of these volcanic rocks suggests that an eruptive center was still active within the confines of the Escondida cluster when deposit formation began. The deposits are all centered on multiphase biotite granodiorite porphyry stocks, which were predated by dioritic to monzodioritic precursors and closely associated with volumetrically minor, but commonly high-grade, magmatic-hydrothermal breccias. The earliest porphyry phases consistently host the highest grade mineralization. Alteration-mineralization zoning is well developed: potassic and overprinted gray sericite assemblages containing chalcopyrite and bornite at depth; more pyritic chlorite-sericite and sericitic zones at intermediate levels; and shallow advanced argillic developments, the remnants of former lithocaps that could have attained 200 km 2 in total extent. The latter are associated with high-sulfidation, copper-bearing sulfide mineralization, much of it in enargite-rich, massive sulfide veins. The Escondida and Escondida Norte-Zaldí-var deposits, formed at ∼ 38 to 36 Ma, are profoundly telescoped, whereas the earlier (∼ 41 Ma) Chimborazo and later (∼ 36–34 Ma) Escondida Este and Pampa Escondida deposits display only minor telescoping, suggesting that maximal Incaic uplift and erosion took place from 38 to 36 Ma. The Portezuelo-Panadero and subsidiary longitudinal faults in the district—inverted normal structures that formerly delimited the eastern side of a Mesozoic backarc basin—were subjected to sinistral transpression prior to deposit formation (pre-41 Ma), which gave rise to the clockwise block rotation responsible for generation and initial synorogenic filling of the San Carlos depocenter. The Escondida district was then subjected to transient dextral transpression during emplacement of the NNE- to NE-oriented porphyry copper intrusions and associated alteration and mineralization (∼ 38–34.5 Ma). This dextral regime had waned by the time that a N-trending, late mineral rhyolite porphyry was emplaced at Escondida Este and was replaced by transient sinistral transpression during end-stage formation of NW-striking, high and intermediate sulfidation, massive sulfide veins and phreatic breccia dikes. Since 41 Ma, the faults in the district have undergone no appreciable displacement because none of the porphyry copper deposits shows significant lateral or vertical offset. Renewed uplift and denudation characterized the late Oligocene to early Miocene, during which the extensive former lithocap was largely stripped and incorporated as detritus in a thick piedmont gravel sequence. Development of hematitic leached capping and attendant chalcocite enrichment zones, along with subsidiary oxide copper ore, was active beneath the topographic prominences at Escondida, Escondida Norte-Zaldívar, and, to a lesser degree, Chimborazo from ∼ 18 to 14 Ma, but supergene activity was much less important at the topographically lower, gravel-covered Pampa Escondida deposit. After ∼ 14 Ma, supergene processes were soon curtailed by the onset of hyperaridity throughout much of northern Chile.

Abstract A review of the record of copper discovery in the Andes over the past four decades reveals that discoveries peaked in the 1990s when 14 discoveries were made. The 1980s and 1990s were the most important decades in terms of amount of copper discovered, accounting for 115 million metric tons (Mt) of copper. During the most recent decade, discoveries have dropped by 57 percent due to exploration maturity in well-exposed terrain, lack of effective exploration methods in concealed terrain, and a greater focus of exploration expenditures on known resources rather than new discovery. The first concealed deposit was recognized in 1986 at Candelaria, and since then nine additional concealed deposits have been discovered, accounting for 47.8 Mt of copper. Geochemistry played an important role in several of these copper discoveries, but unfortunately the published record on these case histories is sparse. In the late 1960s, recently developed geochemical exploration methods using field-based analytical techniques were applied for the first time in South America. Although crude, these early applications were effective in aiding discoveries at Santa Clara, Argentina, and Los Pelambres, Chile. At Los Pelambres, the rugged topography and extensive talus deposits led to testing the use of finegrain talus as a sample medium. A subsequent talus survey effectively outlined the extent of mineralization. Reconnaissance stream-sediment geochemistry and leached cap geochemistry were instrumental in the discovery of Escondida and later Zaldivar-Escondida Norte. Although the discovery of the completely gravelcovered Spence deposit is mainly attributed to systematic grid-based drilling of vast covered regions, base-ofgravel copper geochemistry did provide a vector to nearby mineralization. The use of panned-concentrate geochemistry in southern Ecuador, initially designed for gold exploration, highlighted several areas of base metal mineralization, leading to the discovery of the San Carlos deposit. The discovery of the Haquira oxide mineralization in southern Peru, and subsequent discovery of primary mineralization, resulted from follow-up of anomalous molybdenum and copper in stream sediments. The Haquira deposit is hosted in nonreactive siliciclastic rocks and does not show visual signs of alteration, despite the fact that the deposit subcrops. The use of unconventional geochemical methods, such as partial extraction geochemistry and ground-water geochemistry, has not yet resulted in a copper discovery in the Andes. Future discoveries in the Andes are likely to be in covered regions, deeper settings in outcropping areas where subtle signs of mineralization and alteration are present, and in poorly explored regions due to remoteness or political, social, or security concerns. An improved understanding of supergene enrichment processes in northern Chile, involving leaching and enrichment under semiarid conditions, and subsequent saline metasomatism under hyperarid conditions, help clarify secondary geochemical dispersion processes. Geochemical methods optimized to detect this dispersion should lead to greater success in exploring covered areas in this environment. The discovery of deep, high-grade hypogene mineralization at Los Sulfatos demonstrates the importance of this target type. Improved three-dimensional vectoring methods and zoning models are needed to aid in defining these targets. Other methods, such as ground-water geochemistry and the use of porphyry copper indicator minerals, are exciting developments that should contribute to future copper discoveries in the Andes.

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.