Chapter 3 Geologic Overview of the Escondida Porphyry Copper District, Northern Chile

Home , Andean Geology, Chuquicamata, Domeyko Fault, Escondida, Minera Escondida

MIGUEL HERVÉ1,* RICHARD H. SILLITOE,2 CHILONG WONG,1 PATRICIO FERNÁNDEZ,1,** FRANCISCO CRIGNOLA,1,*** MARCO IPINZA,1 AND FELIPE URZÚA3 1 Minera Escondida Limitada, Avenida de la Minería 501, Antofagasta, Chile 2 27 West Hill Park, Highgate Village, London N6 6ND, England 3 BHP Billiton, 10 Marina Boulevard 50-01, Marina Bay Financial Centre Tower 2, Singapore 018983

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 km2 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. De- velopment 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.

† Corresponding author: e-mail, [email protected] Present addresses: *Antofagasta Minerals S.A., Avenida Apoquindo 4001, piso 18, Las Condes, Santiago, Chile. **BHP Billiton, Avenida Américo Vespucio 100, piso 8, Las Condes, Santiago, Chile. ***BHP Billiton, 10 Marina Boulevard 50-01, Marina Bay Financial Centre Tower 2, Singapore 018983.

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Introduction enriched sulfide leach operation (Table 1) is owned and oper- THE SUPERGIANT Escondida porphyry copper district is lo- ated by Barrick Gold Corporation. The Escondida-Escondida cated ~170 km southeast of the port city of Antofagasta, the Norte operation is the world’s largest copper producer, and capital of Antofagasta Region, northern Chile (Fig. 1). The since 2004 has shipped >1 Mt of copper per year for a 20-year district comprises the Escondida cluster and, 15 km north- total of 17.7 Mt (Comisión Chilena del Cobre, 2011). The west, the Chimborazo deposit. The Escondida cluster is made Pampa Escondida, Chimborazo, Baker, and Pinta Verde de- up of the Escondida (including Escondida Este), Escondida posits, also owned by Mineral Escondida Limitada, remain Norte-Zaldívar, Pampa Escondida, and small Baker and Pinta under study. Verde deposits. Escondida Norte and Zaldívar are the eastern This overview of the Escondida district provides geologic and western parts of a single deposit, separated by a property and alteration-mineralization descriptions of the Chimbo- boundary. The district is located in the Atacama Desert, razo, Escondida (including Escondida Este), Escondida where it lies beneath rounded, talus-mantled hills and broad, Norte-Zaldívar, and Pampa Escondida deposits, the last three alluviated plains of the Domeyko Cordillera (Fig. 2), part of over 1,500 to 2,000 vertical meters. The deposit descriptions the Precordillera morphostructural province, at elevations of are prefaced by summaries of the exploration history and ge- 2,600 to 3,500 m above sea level. ologic setting and followed by a synthesis of the district The giant status of the Escondida district is underscored by chronology and general discussion. The present work is based its current resources plus past production of 144 million met- on district-scale geologic remapping at 1:25,000 scale and log- ric tons (Mt) of Cu, as detailed in Table 1. The resources also ging and interpretation of ~1,000,000 m of diamond drill contain locally elevated molybdenum and gold contents. The core, over half from 430 holes drilled to depths of >1,000 m, Escondida and Escondida Norte-Zaldívar deposits are mined during a six-year brownfields exploration program led by the from two open pits. Escondida and Escondida Norte, owned first author; however, the results reported here must still be and operated by Minera Escondida Limitada (BHP Billiton considered as work in progress. 57.5%, Rio Tinto 30%, JECO Corp. 10%, and JECO 2 Ltd. Exploration History 2.5%), produce enriched sulfide ore, treated by conventional flotation, and oxide and low-grade enriched sulfide ores, sub- The discovery history of the Escondida deposit by a joint jected to heap leaching. The much smaller Zaldívar oxide and venture between Utah International and Getty Minerals was exhaustively described by Ortíz et al. (1985, 1986), Lowell (1990, 1991), and Ortíz (1995), and briefly summarized by Sillitoe (1995). In 1979, the joint venturers began a search for chalcocite enrichment zones concealed beneath postmineral cover in northern Chile. Initial fieldwork included a regional drainage geochemical survey, which defined a copper-molybdenum anomaly centered on a well-known alteration zone at Cerro Colorado, now the Escondida deposit. Before the drainage geochemical results became available, the joint-venture’s landman drew attention to the prominent color anomaly at Cerro Colorado (Fig. 2a), and two claims were filed immediately. A field inspection confirmed porphyry copper potential, leading to a campaign of grid rock-chip geochemical sampling. Zoned alteration and geochemical patterns, presence of peripheral veins, high molybdenum geochem- istry, and partially hematitic leached capping led in early 1981 to the decision to drill five rotary holes at 1-km intervals. The holes were collared in the gravel-filled depression between what are now the Escondida and Escondida Norte-Zaldívar deposits, but copper values encountered were <0.25%. Four additional holes were then drilled to test the sulfide zone beneath Cerro Colorado, the first of which cut 52 m at 1.51% Cu beneath 241 m of leached capping to discover the Escon- dida deposit. In 1984, Utah International was purchased by BHP Minerals and Getty Oil by Texaco, which led a year later to reassignment of Texaco’s share and the formation of Min- era Escondida Limitada. By end 1984, an initial feasibility study was completed, and Escondida entered production at the end of 1990. The Utah International-Getty Minerals joint venture optioned the claims covering the prominently altered Cerro Zaldívar (Fig. 2b) from a local company in early 1981 and FIG. 1. Location of the Escondida district with respect to the Domeyko fault system and other major middle Eocene to early Oligocene porphyry conducted rock-chip geochemical sampling, which led to de- copper districts in northern Chile. Faults after Cornejo and Mpodozis (1996). finition of an extensive molybdenum anomaly (Ortíz et al.,

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FIG. 2. Geologic images of the Escondida district. a. Cerro Colorado Grande (lithocap remnant) and Cerro Colorado Chico (lithocap remnant developed within and around the rhyolite dome): the surface expressions of the Escondida deposit, looking northwest (cf. Brimhall et al., 1985). Photograph taken in 1984. b. Escondida Norte-Zaldívar deposit, looking south- west. The Escondida deposit is marked by the waste dumps in the upper left corner of the image. Photograph taken in 1998. c. Cerro Chimborazo advanced argillic lithocap, looking north. The Chimborazo deposit lies farther north, concealed behind the lithocap. Photograph taken in 2009. d. Surface projection of the gravel-covered Pampa Escondida deposit, looking north, with Escondida Norte waste dumps behind and Escondida Norte-Zaldívar deposit on the skyline. Photograph taken in 2009.

TABLE 1. Copper Resources and Past Production, Escondida District

Deposit Ore type Total resource (Mt) Grade (% Cu) Contained Cu (Mt) Past Cu production1 (Mt)

Escondida, Escondida Este, Oxide 213 0.63 1.34 and Escondida Norte2 Mixed 183 0.81 1.48 Sulfide + sulfide leach 13,540 0.57 77.12 17.703 Pampa Escondida4 Sulfide 7,378 0.47 34.68 Pinta Verde4 Oxide 103 0.63 0.65 Sulfide 72 0.46 0.33 Zaldívar5 Supergene oxide + sulfide 829 0.53 4.39 2.04 Hypogene6 ~1,000 ~0.27 2.70 Chimborazo2 Supergene sulfide 223 0.54 1.20 Escondida district total Supergene + hypogene 123.89 19.74

1 Comisíon Chilena del Cobre (2011) 2 BHP Billiton news release, 14 February 2012 3 Includes minor oxide Cu 4 BHP Billiton 2011 Annual Report 5 Barrick Gold Corporation 2010 Annual Report 6 Mineral inventory

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1985, 1986). Following additional work, drilling was conducted the Escondida district) and coeval granitoid plutons that form from 1981 to 1984, resulting in discovery and partial delin- part of a magmatic belt built along the Gondwana margin eation of the Escondida Norte-Zaldívar enrichment zone; how- (e.g., Bahlburg and Hervé, 1997). After cessation of magma- ever, the deposit was accorded a lower priority than Escon- tism in the Triassic, accompanied by regional extension and dida. In early 1989, Sociedad Minera La Cascada, a Chilean rifting, a new magmatic arc was built along the present company, purchased outright the Zaldívar claims, leaving the Coastal Cordillera of northern Chile (Fig. 1). The magmatic Utah-Getty joint venture with the eastern (Escondida Norte) front remained there through the Early Cretaceous, and dur- part of the deposit. Percussion drilling by Sociedad Minera ing this period prevailing extensional/transtensional tectonic La Cascada indicated the existence of a viable orebody, but conditions led to formation of a large backarc sedimentary the company became insolvent and, in late 1989, creditor basin (Mpodozis and Ramos, 1990; Ardill et al., 1998). In the banks sold the property at public auction to Outokumpu Re- Escondida district and beyond, a thick Jurassic to Early Cre- sources (Services) Limited (Sillitoe, 1995). Outokumpu sold a taceous marine carbonate sequence and continental siliciclas- 50% interest in the deposit to Placer Dome Inc. in late 1992, tic strata (El Profeta and Santa Ana Formations; Maksaev et and the resulting joint company brought the mine into pro- al., 1991) accumulated in the backarc basin over the Paleozoic duction in 1995. Outokumpu’s share was bought in 1999 by to Triassic basement. At the beginning of the Late Creta- Placer Dome, which was acquired in 2006 by Barrick Gold ceous, a major episode of compressive deformation produced Corporation, the current operator. Minera Escondida Limi- inversion of the backarc basin and formation of a proto- tada resumed exploration at Escondida Norte in 1995 and Domeyko Cordillera. At the same time, sediments derived brought a large open-pit mine on stream 10 years later. from erosion of the uplifted range accumulated in the eastern The prominent alteration zone capping Cerro Chimborazo basins (Mpodozis and Perelló, 2003; Mpodozis et al., 2005; (Fig. 2c) was mined historically for both copper and gold but Fig. 1). This deformation episode, triggered by initial opening was first formally explored in 1968 by ITT Corporation (Pe- of the South Atlantic Ocean and consequent westward ad- tersen et al., 1996). During the 1980s, the property was op- vance of the South American plate (Samoza and Zaffarana, tioned from the local owner by Chevron and then Freeport, 2008), terminated the arc magmatism in the Coastal the latter company intersecting chalcocite enrichment grad- Cordillera and changed the mainly extensional tectonic con- ing 2.2% Cu over 22 m during its gold exploration program ditions (Mariana-type subduction) to a more compressive (Petersen et al., 1996). In 1988, Minera Orion Chile, a joint regime (Chilean-type subduction) throughout most of the venture between Echo Bay Mines and Coeur d’Alene Mines, Chilean Andes (Uyeda and Kanamori, 1979). optioned the property and confirmed the existence of the en- After this wholesale change in tectonic regime, the mag- richment zone discovered by Freeport. Minera Orion Chile, matic front migrated eastward and extensive volcanic se- purchased by Cyprus Minerals Company in late 1989, went quences, including ignimbrite flows erupted from caldera on to define the chalcocite enriched deposit (Petersen et al., complexes, accumulated during the Late Cretaceous and Pa- 1996). Minera Escondida Limitada purchased the Chimbo- leocene (Augusta Victoria Formation in the Escondida dis- razo property from Cyprus in 2000. trict; Maksaev et al., 1991). The magmatic front again shifted The district-wide brownfields exploration conducted by eastward in the Eocene, after the orogen-wide Incaic tectonic Minera Escondida Limitada led to discovery of the concealed event, which caused bending of the Andean orogen and for- Pampa Escondida deposit in late 2006 (Sillitoe, 2010; Fig. 2d) mation of the Bolivian orocline (Arriagada et al., 2008). This and the deep, hypogene Escondida Este deposit beneath the episode was accompanied by renewed uplift and erosion and eastern edge of the Escondida open pit in early 2009. Both the development of the porphyry copper belt (Maksaev and discoveries resulted from relogging of core from previously Zentilli, 1999; Nalpas et al., 2005). Porphyry copper forma- drilled holes that had intersected anomalously high copper tion coincided with a decrease in volcanism and the reactiva- values over their final intervals, leading to geologic reap- tion of the Domeyko fault system, which includes reverse, praisals and the decision to drill deeper. strike-slip, and normal fault components along with associated folds (Chong, 1977; Maksaev and Zentilli, 1988; Reutter Geologic Setting et al., 1991; Mpodozis et al., 1993a). After a magmatic lull during the Oligocene, when extensional tectonic conditions Regional context seem to have prevailed (Pananont et al., 2004), the magmatic The Escondida district lies within the narrow middle front shifted eastward yet again, this time to the present Cen- Eocene to early Oligocene (44–33 Ma) porphyry copper belt tral Volcanic Zone along the Chile-Argentina frontier (Fig. 1). of northern Chile, which is >1,000 km long and coincides Finally, compression and tectonic uplift resumed in the late with the Domeyko Cordillera and its northern and southern Oligocene and Miocene, giving rise to accumulation of thick extensions between latitudes 21° and 31° S (Sillitoe, 1988; sequences of piedmont gravel (Pampa de Mulas Formation at Camus, 2003; Sillitoe and Perelló, 2005; Fig. 1). At the lati- the latitude of Escondida; Chong, 1977; Marinovic et al., tude of Escondida, the Domeyko Cordillera is a 35-km-wide, 1995). up to 4,500-m-high, internally broken, N-trending range that The epizonal intrusions associated with some of the middle separates the Central Depression, to the west, from large en- Eocene to early Oligocene porphyry copper deposits, includ- dorheic sedimentary basins to the east (Atacama and Punta ing those in the Escondida cluster, are spatially related to Negra salars; Fig. 1). main fault traces of the Domeyko fault system (see below), Most of the Domeyko Cordillera is formed by Late Car- which originated as normal faults along the eastern side of the boniferous to Triassic volcanic rocks (La Tabla Formation in Mesozoic backarc basin prior to undergoing inversion during

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the Late Cretaceous and Incaic events (Amilibia and Escondida Este, Escondida Norte-Zaldívar, and Pampa Es- Skarmeta, 2003; Amilibia et al., 2008). Richards et al. (2001) condida deposits is well established by 20 U-Pb zircon ages proposed that the main porphyry copper clusters in the determined for both the andesitic and rhyolitic volcanic rocks Domeyko Cordillera were located at intersections between and coeval monzogranite, monzogranite porphyry, rhyolite the fault system and NW-trending, trans-Andean lineaments porphyry, tonalite, quartz monzodiorite porphyry, and quartz (Salfity, 1985)—possible translithospheric fault zones—that diorite porphyry intrusions, which range in age from ~300 to are well marked farther east in Argentina as linear Miocene to 282 Ma (Richards et al., 1999; Jara et al., 2009; Urzúa, 2009; Recent volcanic complexes. Nonetheless, no field evidence this study). The youngest Early Permian rhyolitic and an- for major NW-trending faults has been recognized to date in desitic strata may postdate the intrusive suite (Table 2). Com- the Domeyko Cordillera (Maksaev et al., 1991; Mpodozis et parable latest Carboniferous to Early Permian ages, for both al., 1993b; Gardeweg et al., 1994; Urzúa, 2009). igneous rocks and associated alteration, were obtained immediately north of the Escondida Norte-Zaldívar deposit Escondida district geology (Cornejo et al., 2006). Figure 3 is a geologic map of the Escondida district grossly The Augusta Victoria Formation in the vicinity of the Es- simplified from Urzúa (2009). The district is dominated by condida cluster is dominated by calc-alkaline andesitic flows, two units: La Tabla Formation basement rocks in the east and also shallowly dipping and calc-alkaline in composition, al- the Mesozoic backarc sedimentary succession (El Profeta and though more felsic units occur farther west. The volcanic Santa Ana Formations) and unconformably overlying Augusta rocks were dated by the U-Pb zircon method at ~58 to 53 Ma Victoria Formation in the west. The Escondida cluster spans (Urzúa, 2009; Table 2), confirming a Paleocene age, in accord the faulted contact between these two environments, whereas with previous, more regional work (Marinovic et al., 1995). Chimborazo is hosted exclusively by the Augusta Victoria Hand-sample distinction between hydrothermally altered La strata. Tabla and Augusta Victoria andesitic volcanic rocks in and La Tabla Formation comprises generally shallowly dipping, around the deposits is impossible (cf. Jara et al., 2009); how- andesitic and rhyolitic volcanic rocks, the former principally ever, rare earth element patterns for the two units are dis- flows and the latter dominated by welded ignimbrite, as pro- tinctive (Richards et al., 2001) and are used routinely to sep- posed by Richards et al. (2001). Both the volcanic and coeval arate them for mapping purposes. intrusive rocks are calc-alkaline in composition (Richards et The oldest post-Paleozoic intrusive rocks in the Escondida al., 2001; Urzúa, 2009). The latest Carboniferous to Early district are alkaline gabbro and associated diorite, monzodi- Permian age of La Tabla Formation host rocks within the orite, monzonite, and granite of Late Cretaceous age (~77–72

TABLE 2. U-Pb Zircon Ages for Late Paleozoic Intrusive Rocks and La Tabla and Augusta Victoria Formations, Escondida District

Sample Location Age (Ma) Reference

La Tabla Formation volcanic rocks Andesite Escondida Este 282.0 ± 2.0 Urzúa (2009) Andesitic tuff Baker-Escondida Norte 284.0 ± 4.0 Urzúa (2009) Andesite Escondida 286.0 ± 2.0 Urzúa (2009) Rhyolitic tuff Escondida Este 287.0 ± 3.0 Urzúa (2009) Andesite Pampa Escondida 288.0 ± 2.4 This study Biotite granodiorite porphyry Zaldívar 287.1 ± 4.4 Jara et al. (2009) Andesite Pampa Escondida 288.8 ± 2.4 This study Andesite Zaldívar 294.4 ± 4.6 Jara et al. (2009) Rhyolite Zaldívar 298.2 +5.5/-4.9 Jara et al. (2009)

Late Paleozoic intrusive rocks Quartz monzodiorite porphyry Pampa Escondida 287.6 ± 3.3 This study Monzogranite Zaldívar 289.9 ± 3.5 Morales (2009) Rhyolite porphyry Zaldívar 290.0 ± 4.0 Richards et al. (1999) Monzogranite Zaldívar 291.1 ± 2.3 Morales (2009) Monzogranite porphyry Escondida Norte 293.0 ± 6.0 This study Quartz diorite porphyry Pampa Escondida 293.0 ± 4.3 This study Rhyolite porphyry Escondida Norte 294.2 ± 2.4 P.J. Pollard and R.G. Taylor, unpub. rept., 2002 Monzodiorite porphyry Escondida Este 296.8 ± 3.2 This study Monzogranite porphyry Escondida Este 296.6 ±4.4/-3.8 This study Monzogranite Escondida Norte 298.8 ± 2.6 P.J. Pollard and R.G. Taylor, unpub. rept., 2002 Tonalite Escondida Este 300.1 ± 3.5 This study

Augusta Victoria Formation volcanic rocks Andesitic flow Escondida cluster 56.8 ± 0.5 Urzúa (2009) Andesitic flow Escondida cluster 55.8 ± 0.8 Urzúa (2009) Andesitic flow Escondida cluster 54.9 ± 0.8 Urzúa (2009) Andesitic flow Escondida cluster 53.9 ± 0.8 Urzúa (2009) Andesitic flow Escondida cluster 57.0 ± 0.6 Urzúa (2009)

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Recent alluvium (Holocene) Salar (Miocene-Holocene) Pampa de Mulas Formation: Piedmont gravel (late Oligocene-middle Miocene) Granodiorite, dacite, andesite, + rhyolite porphyries (middle-late Eocene, ~38-34 Ma) San Carlos strata: Siliciclastic + volcaniclastic rocks Eocene) Diorite, quartz diorite, manzodiorite, + quartz monzonite intrusions (middle Eocene, ~43-41 Ma) Augusta Victoria Formation: Andesitic + dacite volcanic rocks (late Paleocene-early Oligocene) Gabbro, diorite, + monzodiorite intrusions (Late Cretaceous, ~77-72 Ma) Santa Ana Formation: Siliciclastic rocks (Late Jurassic-Neocomian) El Profeta Formation: Limestone + gypsum (Late Triassic-Kimmeridgian) Diorite to granite intrusions (late Paleozoic-Late Triassic) La Tabla Formation: Rhyolitic + andesitic volcanic rocks (Late Carboniferous-Early Permian) Subsurface limit of San Carlos strata Porphyry copper deposit (>0.3 % Cu) Observed fault Inferred fault High-angle reverse fault Syncline

FIG. 3. Geology of the Escondida district, showing the locations of the Chimborazo, Escondida-Escondida Este, Escon- dida Norte-Zaldívar, Pampa Escondida, Baker, and Pinta Verde porphyry copper deposits. Note the subsurface extent of the San Carlos strata immediately east of Escondida Este and Escondida Norte and the NE-striking reverse fault that bounds them to the southeast. Grossly simplified and slightly modified from Urzúa (2009).

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Ma; U-Pb zircon) in the southwestern quadrant of Figure 2, The formation is assigned to the Oligocene to mid-Miocene which are part of a N-trending, 15-km2 pluton (Urzúa, 2009). interval by Marinovic et al. (1995) and Urzúa (2009), which Their geotectonic significance remains uncertain, although a accords well with ages of 8.7 ± 0.4 to 4.2 ± 0.2 Ma for overly- backarc setting is suspected (Richards et al., 2001; Urzúa, ing felsic air-fall tuff horizons at Escondida and Zaldívar 2009) by comparison with alkaline gabbro intrusions of simi- (Alpers and Brimhall, 1988; Morales, 2009). lar age farther north in the Domeyko Cordillera (Mpodozis et al., 2005). Two additional gabbro-to-granite complexes of Escondida district structure Late Cretaceous to early Paleocene age and backarc affilia- The principal faults and associated fold axes in the Escon- tion are present along the western side of the Escondida dis- dida district are orogen parallel and strike N to NNE trict (Urzúa, 2009). (Mpodozis et al., 1993b; Marinovic et al., 1995; Richards et The next intrusive activity in the district gave rise to the al., 2001; Urzúa, 2009; Fig. 3). The faults constitute the east- epizonal complexes associated with the porphyry copper de- ern side of a giant cymoid loop, ~180 km long and up to 20 posits. These commence with fine-grained hornblende dior- km wide (Mpodozis et al., 1993a, b; Fig. 1). In the Escondida ite and monzodiorite, which constitute a series of stocks, oc- district, the pre-eminent Portezuelo-Panadero fault is a 65° cupying an area of 45 km2, in the northwestern part of the E-dipping, reverse structure which places La Tabla over Au- district (Fig. 3) as well as occurring as small bodies within the gusta Victoria units (Navarro et al., 2009; Urzúa, 2009; Fig. 3). Chimborazo and Pampa Escondida deposits and near Escon- Its total vertical and transcurrent displacement remains un- dida Este and Baker. Based on U-Pb zircon ages, most of certain, but the movement history is complex, as discussed these rocks were emplaced from ~43 to 41 Ma (Urzúa, 2009; below. cf. ~38–36 Ma Ar/Ar ages of Richards et al., 2001), although The NE-striking, 70° SE-dipping Hamburgo reverse fault those at Baker and Pampa Escondida returned ages of 38.2 ± (Fig. 3) is the southernmost of several such structures along 0.5 and 37.6 ± 0.5 Ma, respectively (Table 3; see below). the eastern side of the giant shear lens (Fig. 1) that were pro- These diorites and monzodiorites (including those at Baker duced by clockwise rigid-block rotations about vertical axes and Pampa Escondida) are clearly precursors to the ore-re- coeval with a component of sinistral motion on the N-striking lated intrusions (Richards et al., 2001). The ore-related intru- faults (Mpodozis et al., 1993a, b; Arriagada et al., 2000). The sions in the Escondida cluster are multiphase biotite granodi- accommodation space generated incrementally during this orite porphyries, which report U-Pb zircon ages between ~38 rotation was filled by the San Carlos strata (see above). and 34.5 Ma; however, the undated porphyry at Chimborazo is older, probably ~41 Ma (Table 3; see below). The last siz- Chimborazo Porphyry Copper Deposit able intrusion in the district is the rhyolite porphyry at Es- condida Este, dated at ~34 Ma (Table 3), which is composi- Deposit summary tionally the most evolved and plots nearer the quartz apex in Chimborazo is a large porphyry copper system, within a Streckeisen diagram. which the only resource defined to date is a supergene en- Recent drilling has defined a remarkably thick sequence of richment zone (Table 1). The enrichment is situated in the siliciclastic sedimentary and andesitic volcanic rocks, immedi- northern part of the system, north of Cerro Chimborazo, ately east of Escondida Este and Escondida Norte, which ac- which is formed by an advanced argillic lithocap (Figs. 2c, 4). cumulated both before and during porphyry copper deposit The entire Chimborazo system to a depth of ~1,000 m is formation (Fig. 3). These rocks crop out in the immediate mainly hosted by dacitic tuffs and lesser andesitic breccias footwall of the northwest-vergent Hamburgo reverse fault and flows assigned to the Augusta Victoria Formation (Pe- (Fig. 3), where they were designated as San Carlos strata by tersen et al., 1996). North of the lithocap, the Chimborazo Urzúa (2009). The strata attain maximum thicknesses of system is concealed beneath an extensive, northward-thick- >1,200 m and comprise gray-green and red sandstone and ening wedge of Pampa de Mulas gravels, which are omitted conglomerate, which in their upper parts are interbedded from Figure 4. with a cumulative thickness of up to 500 m of andesitic laharic Precursor, phaneritic hornblende diorite and monzodiorite breccia, ignimbrite, and subsidiary flows, which reported two intrusions, dated at ~42 Ma (Table 3), cut the volcanic suc- U-Pb zircon ages of 38.0 ± 2.1 and 37.7 ± 0.6 Ma (Urzúa, cession, both south (Fig. 4) and north of the alteration zone 2009). Urzúa (2009) compared the San Carlos strata with sim- as well as at depth (Fig. 5a). At ~1,000 m below the surface, ilar siliciclastic units farther north, which include the upper drill holes have intersected the uppermost parts of an early bi- Calama (Blanco et al., 2003; May et al., 2005) and Loma otite granodiorite porphyry intrusion, the isotopic age of Amarilla (Mpodozis et al., 2005) Formations in the Calama which is pending. Minor bodies and dikes of late mineral and Salar de Atacama basins, respectively (Fig. 1). These sed- hornblende diorite, dated at 38.1 ± 0.3 Ma by the Ar/Ar imentary units are the erosional products resulting from method (Richards et al., 1999), cut the lithocap immediately Domeyko Cordillera uplift (Jordan et al., 2007; Wotzlaw et south of the supergene copper deposit and appear to be al., 2011). closely related to the inter- to late mineral Chimborazo intru- The final stratigraphic unit in the district is the Pampa de sive complex that flanks the deposit to the northwest (Figs. 4, Mulas Formation, a widespread, flat-lying, crudely stratified, 5a). This intrusive complex, dated at ~39 to 37 Ma (Table 3), poorly consolidated, piedmont gravel sequence of mass-flow comprises fine-grained, inequigranular diorite to granodiorite origin, which is up to 240 m thick. In proximity to the por- phases, which are controlled and cut by NE-striking faults, phyry copper deposits, the sequence contains abundant some displaying ductile synintrusion deformation textures at clasts of altered rocks, especially advanced argillic lithocap. depth.

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TABLE 3. U-Pb, Re-Os, Ar/Ar, and K-Ar Ages for Porphyry Copper Deposits in the Escondida District

Sample Location Method, Material Age (Ma) Reference

Precursor diorite Pampa Escondida U-Pb, zircon 37.6 ± 0.5 This study Precursor monzodiorite Chimborazo U-Pb, zircon 42.0 ± 0.9 Urzua (2009) Precursor monzodiorite Baker U-Pb, zircon 38.2 ± 0.5 This study Early granodiorite porphyry Escondida U-Pb, zircon 37.9 ± 1.1 Richards et al. (2009) Early granodiorite porphyry Escondida U-Pb, zircon 37.7 ± 0.8 Padilla-Garza et al. (2004) Early granodiorite porphyry Escondida U-Pb, zircon 37.2 ± 0.8 Padilla-Garza et al. (2004) Rhyolite dome Escondida U-Pb, zircon 37.5 ± 0.6 This study Early granodiorite porphyry Zaldívar U-Pb, zircon 38.0 ± 0.5 Jara et al. (2009) Early granodiorite porphyry Escondida Norte U-Pb, zircon 37.5 ± 0.5 P.J. Pollard and R.G. Taylor, unpub. rept., 2002 Early granodiorite porphyry Escondida Este U-Pb, zircon 34.5 ± 0.6 This study Early granodiorite porphyry Escondida Este U-Pb, zircon 34.5 ± 0.5 This study Early granodiorite porphyry Pampa Escondida U-Pb, zircon 35.0 ± 0.6 This study Early granodiorite porphyry Pampa Escondida U-Pb, zircon 36.1 ± 0.7 This study Early granodiorite porphyry Pampa Escondida U-Pb, zircon 36.0 ± 1.2 This study Early granodiorite porphyry Baker U-Pb, zircon 37.1 ± 0.7 This study Intermineral granodiorite porphyry Escondida U-Pb, zircon 35.4 ± 0.7 This study Late-mineral rhyolite porphyry Escondida Este U-Pb, zircon 34.2 ± 0.7 This study Late-mineral dacite porphyry Escondida Norte U-Pb, zircon 35.7 ± 0.7 This study Late-mineral dacite porphyry Zaldívar U-Pb, zircon 36.0 ± 0.8 Jara et al. (2009) Late-mineral dacite porphyry Zaldívar U-Pb, zircon 35.5 ± 0.8 Jara et al. (2009) Intermineral granodioritie porphyry Pampa Escondida U-Pb, zircon 35.0 ± 0.8 This study Intermineral granodioritie porphyry Pampa Escondida U-Pb, zircon 34.5 ± 0.4 This study Hornblende dacite late mineral porphyry Pampa Escondida U-Pb, zircon 35.2 ± 0.8 This study Inter- to late mineral granodiorite porphyry Chimborazo U-Pb, zircon 38.5 ± 0.5 This study Inter- to late mineral monzodiorite Chimborazo U-Pb, zircon 37.5 ± 0.5 This study Inter- to late mineral monzodiorite Chimborazo U-Pb, zircon 37.4 ± 0.5 This study Inter- to late mineral quartz diorite Chimborazo U-Pb, zircon 37.9 ± 0.7 This study Inter- to late mineral quartz monzodiorite Chimborazo U-Pb, zircon 38.9 ± 0.8 This study Molybdenite mineralization Escondida Re-Os, molybdenite 36.1 ± 0.2 Romero et al. (2010) Molybdenite mineralization Escondida Re-Os, molybdenite 35.2 ± 0.2 Romero et al. (2010) Molybdenite mineralization Escondida Este Re-Os, molybdenite 33.7 ± 0.3 Padilla-Garza et al. (2004) Molybdenite mineralization Escondida Norte Re-Os, molybdenite 37.8 ± 0.2 Romero et al. (2010) Molybdenite mineralization Escondida Norte Re-Os, molybdenite 37.6 ± 0.2 Romero et al. (2010) Molybdenite mineralization Escondida Norte Re-Os, molybdenite 36.6 ± 0.2 Romero et al. (2010) Molybdenite mineralization Chimborazo Re-Os, molybdenite 41.9 ± 0.4 This study Molybdenite mineralization Chimborazo Re-Os, molybdenite 36.6 ± 0.4 This study Early granodiorite porphyry Escondida Ar/Ar, igneous biotite 35.8 ± 0.2 Padilla-Garza et al. (2004) Early granodiorite porphyry Escondida Ar/Ar, igneous biotite 35.9 ± 0.3 Padilla-Garza et al. (2004) Biotitic alteration Escondida Ar/Ar, hydrothermal biotite 37.5 ± 0.6 Padilla-Garza et al. (2004) Biotitic alteration Escondida Ar/Ar, hydrothermal biotite 36.0 ± 0.4 Padilla-Garza et al. (2004) Biotitic alteration Escondida Ar/Ar, hydrothermal biotite 37.5 ± 0.6 Padilla-Garza et al. (2004) Biotite from granodiorite porphyry Zaldívar Ar/Ar, igneous biotite 37.4 ± 0.2 Campos et al. (2009) Biotite from granodiorite porphyry Zaldívar Ar/Ar, igneous biotite 37.7 ± 0.4 Campos et al. (2009) Biotite from granodiorite porphyry Zaldívar Ar/Ar, igneous biotite 37.1 ± 0.5 Campos et al. (2009) Biotite from granodiorite porphyry Zaldívar Ar/Ar, igneous biotite 36.6 ± 0.9 Campos et al. (2009) Biotite from granodiorite porphyry Zaldívar Ar/Ar, igneous biotite 36.5 ± 0.5 Campos et al. (2009) Biotite from granodiorite porphyry Zaldívar Ar/Ar, igneous biotite 36.0 ± 0.3 Campos et al. (2009) Biotite from granodiorite porphyry Zaldívar Ar/Ar, biotite 35.6 ± 0.7 Campos et al. (2009) Advanced argillic alteration Escondida Ar/Ar, alunite 35.7 ± 0.3 Padilla-Garza et al. (2004) Advanced argillic alteration Escondida Ar/Ar, alunite 35.9 ± 0.3 Padilla-Garza et al. (2004) Advanced argillic alteration Escondida Ar/Ar, alunite 35.2 ± 0.2 Véliz (2004) Supergene alunite Zaldívar K-Ar, alunite 14.7 ± 0.7 Morales (2009) Supergene alunite Zaldívar K-Ar, alunite 16.8 ± 0.6 Morales (2009) Supergene alunite Escondida K-Ar, alunite 17.7 ± 0.7 Alpers and Brimhall (1988) Supergene alunite Escondida K-Ar, alunite 14.7 ± 0.6 Alpers and Brimhall (1988) Supergene alunite Escondida K-Ar, alunite 16.4 ± 0.7 Alpers and Brimhall (1988) Supergene alunite Escondida K-Ar, alunite 18.0 ± 0.7 Alpers and Brimhall (1988) Supergene alunite Chimborazo K-Ar, alunite 17.6 ± 1.1 This study Supergene alunite Chimborazo K-Ar, alunite 15.9 ± 1.0 This study

The volcanic rocks and monzodiorite are cut by a series depth, an assemblage that grades upward through quartz, of pipe-like, magmatic-hydrothermal breccias (Petersen et schorlitic tourmaline, sericite, pyrite, and gypsum (after an- al., 1996), which are now known to be developed over a hydrite) to quartz, alunite, pyrite, enargite, native sulfur, 1,000-m vertical interval (Fig. 5a). The breccias are ce- and abundant open space (produced by gypsum dissolu- mented by anhydrite with chalcopyrite and molybdenite at tion) in the advanced argillic lithocap, in conformity with

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FIG. 4. Geology of the Chimborazo porphyry copper deposit, at sub-Pampa de Mulas Formation bedrock surface. Outer limit of advanced argillic lithocap is also shown.

the system-scale alteration-mineralization zoning pattern Farther from the center of the deposit, illite dominates over (Fig. 5b, c). sericite (fine-grained muscovite). The chlorite-sericite alteration is gradational downward to and overprints a deep potas- Hypogene alteration and mineralization sic zone, comprising pervasive biotitization in the volcanic The topographically higher parts of the lithocap, beneath rocks and orthoclase, minor quartz, and varied amounts of an- the upper slopes of Cerro Chimborazo (Figs. 2c, 4, 5b), are hydrite in the granodiorite porphyry intrusion (Fig. 5b). A lo- dominated by fine-grained quartz-alunite, which grades calized zone of gray sericite development, associated with downward nearer its exposed base to quartz-alunite-kaolinite quartz and andalusite, overprints the biotitized zone (Fig. plus minor dickite. Crystalline hypogene alunite from an out- 5b). lying lithocap exposure, 6 km east, was dated at 42.4 ± 2.0 Ma The potassic-altered rocks contain minor amounts of chal- (Table 3). copyrite, pyrite, and subordinate magnetite, whereas the The lithocap includes a series of downward-penetrating overprinted gray sericite zone is characterized by chalcopyrite prongs, some attaining a depth of ~1,000 m below the surface and bornite (Fig. 5c, d) plus elevated gold values. The chlo- (Fig. 5b). These prongs grade downward from quartz-alunite- rite-sericite zone is dominantly pyritic (Fig. 5c), shows clear dickite to quartz-pyrophyllite-dickite, the latter assemblage evidence for hypogene leaching of preexisting chalcopyrite, containing minor diaspore, topaz, and andalusite. Green tour- and in its external parts contains a well-defined zinc halo. The maline coexisting with lesser dumortierite is present through- advanced argillic alteration is associated with early pyrite and out the advanced argillic prongs, the upper parts of which also enargite ± tennantite as disseminations, veinlets, and veins, contain native sulfur. and late chalcopyrite-sphalerite ± pyrite ± galena veins, which The advanced argillic alteration passes downward and out- tend to occur peripherally. The veins consistently strike NE ward through sericitic alteration to an earlier chlorite-sericite and follow minor faults and fractures (Petersen et al., 1996; assemblage, containing minor schorlitic tourmaline (Fig. 5b). Fig. 4).

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FIG. 5. Chimborazo sections. a. Geology. b. Alteration. c. Mineralization zones. d. Copper distribution. Section line shown in Figure 4. In this and all following sections, only selected drill holes are shown to avoid obscuring geologic detail.

Supergene mineralization Este center is developed within andesitic volcanic rocks of La The Chimborazo supergene enrichment zone is low grade Tabla Formation and coeval intrusions. The latter comprise a (0.4–0.7% Cu), laterally continuous, ~2 × 1 km in areal ex- large stock of quartz monzodiorite porphyry, dated at 296.8 ± tent, and averages ~100 m thick (Figs. 4, 5c, d); it is chalcocite 3.2 Ma, monzogranite porphyry dikes, dated at 296.6 + 4.4/- dominated at shallow levels but contains more covellite at 3.8 Ma, and, at a depth of 2,000 m, a foliated phaneritic depth. The zone is strongly controlled by the NE-striking tonalite, dated at 300.1 ± 3.5 Ma (Fig. 7a; Table 2). faults, containing the high-sulfidation pyrite-enargite veins, The Augusta Victoria volcanic sequence at Escondida is cut along which the enrichment extends considerably deeper by a composite biotite granodiorite porphyry stock, within (Petersen et al., 1996; Fig. 5c). The enrichment zone is over- which the NE-trending, early phases, traditionally referred to lain by hematitic leached capping, up to 150 m thick, but only as Colorado Grande and Escondida porphyries (Ojeda, 1986, minor oxide copper (brochantite-dominated) mineralization 1990; Padilla-Garza et al., 2001, 2004; Quiroz, 2003a), are (Fig. 5c). Supergene alunite veins at and near Chimborazo dated at 37.9 ± 1.1, 37.7 ± 0.8, and 37.2 ± 0.8 Ma (Richards yielded ages of ~18 to 16 Ma (Table 3). et al., 1999; Padilla-Garza et al., 2004; Table 3). At depths of >500 m and westward, the early granodiorite porphyries are Escondida Porphyry Copper Deposit cut by a large stock of late intermineral biotite granodiorite porphyry (Figs. 6, 7a), which explains the low-grade of the hy- Geologic summary pogene mineralization reported beneath the high-grade su- Recent exploration at Escondida shows that it comprises two pergene enrichment zone by previous workers (e.g., Ojeda, discrete porphyry copper centers. The western center devel- 1986; Fig. 7d). This intrusion was dated at 35.4 ± 0.7 Ma oped the major chalcocite enrichment blanket exploited to date, (Table 3). The granodiorite porphyry stock and copper min- whereas the eastern center, informally named Escondida Este, eralization are cut northward by a biotite rhyolite dome (Figs. lies deeper and east of the Panadero fault (Figs. 6, 7a). The 2a, 6), which is notably richer (>10 vol %) in quartz phe- two centers formed at different times, as documented below. nocrysts (Ojeda, 1986, 1990; Padilla-Garza et al., 2001); it is The Escondida center is hosted, at least at shallow levels, dated at 37.5 ± 0.6 Ma (Table 3). by andesitic flows and subordinate breccias of the Augusta Numerous small magmatic-hydrothermal breccia bodies Victoria Formation (Ojeda, 1986), whereas the Escondida constitute roughly 5% of the Escondida deposit and invariably

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FIG. 6. Geology of the Escondida-Escondida Este porphyry copper deposit, compiled from various sources: bench mapping supplied by the mine geologists and relogging of core from exploration drill holes within pit limits; top of bedrock geology from historic drill holes and surface exposures beyond pit limits.

host the highest grade hypogene and supergene copper min- existence of this deep Escondida Este porphyry copper cen- eralization (Ojeda, 1986, 1990; Véliz, 2004). The breccia ter was presciently predicted by Padilla-Garza et al. (2004) on clasts, commonly polymict in nature, are surrounded by var- the basis of shallow high-sulfidation mineralization affecting ied proportions of quartz-sulfide cement and rock-flour ma- the rhyolite porphyry. The Escondida Este center, comprising trix (Ojeda, 1986, 1990; Véliz, 2004). There are also late, early porphyry, dated at 34.5 ± 0.6 and 34.5 ± 0.5 Ma (Table poorly mineralized pebble dikes of phreatic origin (cf. Silli- 3), and at least two intermineral phases (Fig. 7a), is therefore toe, 1985), most of them striking NW (Ojeda, 1986, 1990). ~2 m.y. younger than the Escondida center. The Escondida deposit is bounded eastward by a late min- The upward-flared rhyolite porphyry (Ojeda, 1986, 1990; eral biotite rhyolite porphyry, measuring 3 × 1.5 km at sur- Padilla-Garza et al., 2001), displaying local flow foliation and face, which follows the trace of the N-striking Portezuelo- partly broken quartz phenocrysts, was emplaced at 34.2 ± 0.7 Panadero fault (Figs. 6, 7a). Recent deep drilling east of the Ma (Table 3), an age preferred to that (34.7 ± 1.7 Ma) re- fault encountered a new biotite granodiorite porphyry center ported by Richards et al. (1999) because of the smaller error. at a depth of ~800 m beneath the rhyolite porphyry. The The N-trending, dike-like form of the rhyolite porphyry

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FIG. 7. Escondida-Escondida Este sections. a. Geology. b. Alteration. c. Mineralization zones. d. Copper distribution. Sec- tion line shown in Figure 6.

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suggests control by the Portezuelo-Panadero fault, which bor- chlorite-sericite and sericitic zones (Fig. 7c). Molybdenum ders it westward (Figs. 6, 7a); however, postmineral fault dis- values average ~70 ppm (Romero, 2008). Padilla-Garza et al. placement is relatively minor because rhyolite porphyry dike (2001) recognized that copper, as chalcopyrite, was added at offshoots occur both within and immediately west of the the chlorite-sericite stage. High sulfidation-state assemblages broad zone of fractured rock that defines the fault zone. A se- occur in the advanced argillic zone. In the underlying late in- ries of NW-striking rhyodacite porphyry dikes cut all the late termineral porphyry intrusion, pyrite dominates over chal- Eocene porphyry phases at Escondida and are especially copyrite and copper grades are 0.05 to 0.25%, decreasing abundant in the rhyolite porphyry (Ojeda, 1986, 1990; downward (Fig. 7c, d). Padilla-Garza et al., 2001, 2004; Quiroz, 2003a; Véliz and Ca- The deep, potassic-altered rocks in the Escondida Este macho, 2003; Véliz, 2004). center contain chalcopyrite-pyrite mineralization, whereas Small magmatic-hydrothermal breccias are widespread at the gray sericite zone is bornite rich at depth, but contains Escondida Este, with most of them being clast supported, more chalcopyrite in its upper parts (Fig. 7c); both sulfide monomict, and cemented by high-sulfidation sulfide assem- minerals occur mainly as disseminated grains in the veinlet blages and anhydrite; hence, they are late-stage additions to halos. The gray sericite zone gives rise to the highest copper the system. Even later, barren, NW-striking pebble dikes are and molybdenum grades (Fig. 7d), the latter in preexisting B- also present. type quartz veinlets and later pyrite veinlets. The overlying advanced argillic zone at Escondida Este contains dissemi- Hypogene alteration and mineralization nated, high-sulfidation-state assemblages (Fig. 7c), which are Much of the early granodiorite porphyry at Escondida dis- higher grade than the surrounding mineralization (hypogene plays sericitic alteration, although an advanced argillic zone is enrichment). A series of NW-trending, banded, massive sul- also widely developed at the shallowest levels (Cerro Col- fide veins, up to 3 m wide, cut the quartz porphyry as well as orado Grande and Chico; Fig. 2a) and at much greater depths occurring in faults and fractures farther east and west. The along fault zones (Fig. 7b). Quartz, pyrophyllite, and subordi- veins contain a variety of high-sulfidation assemblages, which nate alunite, diaspore, and svanbergite are reported (Brimhall include abundant pyrite accompanied by one or more of et al., 1985; Alpers and Brimhall, 1988), with some of the enargite, tennantite, chalcopyrite, bornite, covellite, and chal- quartz-pyrophyllite displaying a distinctive patchy texture cocite; they also include molybdenite and minor amounts of (Ojeda, 1986; J. Perelló, pers. commun., 2009), comparable gold. Sphalerite is especially abundant in the youngest vein to that reported from Yanacocha, Peru (Gustafson et al., fillings (Padilla-Garza et al., 2001, 2004; Quiroz, 2003a, b). 2004). At depth and as remnants in the sericitic zone, there Vein halos, up to 1 m wide, include pyrophyllite, dickite, are patches of chlorite-sericite alteration, which give way kaolinite, and alunite, with subordinate diaspore, andalusite, downward to biotite in andesitic volcanic rocks and K- and corundum (Padilla-Garza et al., 2001; Quiroz, 2003a, b; feldspar>biotite in the early porphyries (cf. Padilla-Garza et Véliz, 2004), which are transitional to sericitic alteration at al., 2001; Fig. 7b). The potassic and overprinted sericitic al- depth. teration contain abundant A- and B-type quartz veinlets (cf. Gustafson and Hunt, 1975). The late intermineral porphyry Supergene mineralization underwent weak potassic alteration and veining, but typically The Escondida deposit is dominated by a mature, kaolinite- displays a pervasive chloritic overprint within which rema- rich supergene profile, whereas Escondida Este displays very nent hydrothermal K-feldspar is prominent (Fig. 7b). limited supergene effects (Fig. 7c). The Escondida profile The Escondida Este center is characterized at depths of comprises a hematitic leached capping, averaging ~200 m >~1,800 m by K-feldspar and biotite alteration, which is cut thick but double this figure in places, underlain by a NW- by a variety of veinlet types, including early biotite and sparse trending enrichment zone, with an areal extent of 4.5 × 1 km early dark micaceous (EDM) veinlets (cf. Meyer, 1965). The and maximum thickness of ~400 m (Ojeda, 1986, 1990; Fig. potassic zone is overprinted at shallower levels (~1,000–1,800 7c). The NW-striking faults, fractures, and veins combined m below surface) by gray sericite (±andalusite), which is de- with the highest hypogene copper contents appear to have veloped as centimetric halos to discontinuous quartz-anhy- been the main controls on both the form and depth of the en- drite veinlets that partly coalesce resulting in pervasive seric- richment zone (Ojeda, 1986, 1990; Padilla-Garza et al., 2001; itization (Fig. 7b). Similar alteration is reported at Fig. 6). The zone is dominated by chalcocite-group minerals Chuquicamata, Chile (Ossandón et al., 2001). Outward, these in its upper, highest grade parts, with lower grade covellite zones are transitional to chlorite-sericite alteration. The rhyo- and remanent hypogene sulfides becoming dominant down- lite porphyry underwent white sericitic and advanced argillic ward (Alpers and Brimhall, 1989). The initial mineable re- alteration, which completely dominate the shallow parts of serve comprised 662 Mt at 2.12% Cu, parts of which averaged the Escondida Este center (Fig. 7b). The early gray sericite is >3% (Ojeda, 1990). Supergene alunite from the leached cap- richer in magnesium and iron (phengite) than the shallower, ping and underlying enrichment zone ranges in age from ~18 lower temperature white variety (muscovite) based on sys- to 14 Ma (Alpers and Brimhall, 1988; Table 3). tematic portable SWIR spectrometer readings (cf. Pontual et The relatively small volumes of oxide copper mineralization al., 1997) and confirmed by QEMSCAN analysis. at Escondida (Table 1) tend to be concentrated in biotite- and The hypogene sulfide assemblage at Escondida is largely chlorite-sericite-altered andesitic volcanic rocks, in which obliterated by the effects of the supergene enrichment. None- brochantite and antlerite are the principal minerals, along theless, chalcopyrite and bornite are reported from potassic with minor amounts of chrysocolla, atacamite, several copper remnants, and chalcopyrite and pyrite from the overprinted phosphate minerals, cuprite, and native copper, the last two

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concentrated at the top of the enrichment zone (Ojeda, 1986; small Pinta Verde satellite deposit, at the southwestern ex- Véliz and Camacho, 2003). tremity of Escondida Norte-Zaldívar, is associated with a markedly different, porphyritic biotite tonalite intrusion (Fig. Escondida Norte-Zaldívar Porphyry Copper Deposit 8), which is yet to be dated. Small bodies of polymict magmatic-hydrothermal breccia Geologic summary are associated with the early and intermineral porphyries Escondida Norte and the eastern part of the contiguous (Figs. 8, 9a). These breccias display sericitic or chlorite- Zaldívar property are hosted by volcanic rocks of La Tabla sericite alteration and are cemented by quartz, pyrite, and Formation and coeval intrusive phases. In the east and at varied amounts of chalcopyrite at shallow depths but by peg- depth, La Tabla Formation comprises andesitic rocks, dated matoidal quartz-biotite-anhydrite ± K-feldspar ± magnetite at 294.4 ± 4.6 Ma (Jara et al., 2009; Table 2), which are over- along with chalcopyrite and bornite at depth (e.g., Monroy, lain westward by a rhyolitic sequence, principally welded ign- 2000; Navarro et al., 2009). Hydrothermal breccia and por- imbrite, dated at 290.0 ± 4.0, 294.2 ± 2.4, and 298.2 + phyry bodies appear to be controlled by the Portezuelo- 5.5/−4.9 Ma (Richards et al., 1999; Jara et al., 2009; Table 2). Panadero fault (Navarro et al., 2009). The breccias are typi- The intrusions comprise coarse-grained monzogranite (298.8 cally higher grade than surrounding rocks. ± 2.6, 293.0 ± 6.0, 291.1 ± 2.3, 289.9 ± 3.5 Ma; Morales, 2009; Table 2), granodiorite porphyry (287.1 ± 4.4 Ma; Jara et al.; Hypogene alteration and mineralization 2009; Table 2), and diorite (Figs. 8, 9a). Additionally, in the Potassic alteration is developed at depth throughout the southeastern part of the deposit, the Early Permian quartz deposit (Fig. 9b). A biotite-K-feldspar assemblage is best diorite and quartz monzodiorite porphyry defined and dated developed in the felsic rocks, whereas biotite and minor mag- at Pampa Escondida are also present (see below). The west- netite predominate in the andesitic volcanic rocks and diorite. ern part of the Zaldívar property, west of the demonstrably The potassic alteration contains early biotite and magnetite west-vergent Portezuelo-Panadero reverse fault, is underlain veinlets and abundant K-feldspar and quartz-K-feldspar vein- by andesitic volcanic rocks, which are assigned to La Tabla lets, the latter of A type. Gray sericite veinlets overprint the Formation at depth and the Augusta Victoria Formation at potassic zone but do not constitute coherent, mappable zones shallow levels (Navarro et al., 2009; Fig. 8). like that at Escondida Este (see above). The above rocks are intruded by a series of NE-striking At shallower levels, widespread chlorite-sericite alteration, dikes and larger bodies of biotite granodiorite porphyry, characterized by chlorite-sulfide veinlets, overprints and which include early, inter-, and late mineral phases (Figs. 8, largely destroys the potassic assemblage (Fig. 9b). This, in 9a). The early and intermineral phases, not differentiated in turn, is overlain by a sericitic zone, which is overprinted lo- Figures 8 and 9a, yielded ages of 38.0 ± 0.5 and 37.5 ± 0.5 cally by quartz-pyrophyllite ± alunite alteration, closely asso- Ma, whereas the late mineral phase gave ages of 36.0 ± 0.8, ciated with NW-striking, high-sulfidation veinlet zones, some 35.7 ± 0.7, and 35.5 ± 0.8 Ma (Jara et al., 2009; Table 3). The of them sheeted.

FIG. 8. Geology of the Escondida Norte-Zaldívar porphyry copper deposit, 2,750-m elevation. Escondida Norte geology from logging of exploration drill holes, Zaldívar geology from Monroy (2009).

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FIG. 9. Escondida Norte sections. a. Geology. b. Alteration. c. Mineralization zones. d. Copper distribution. Section line shown in Figure 8.

Most of the hypogene sulfide mineralization at Escon- the potassic zone (Fig. 9c, d), implying that appreciable dida Norte-Zaldívar consists of chalcopyrite and pyrite (Fig. copper was introduced at the chlorite-sericite stage (cf. 9c), with development of only localized centers of chal- Escondida, see above). Molybdenum values average ~100 copyrite-bornite ± chalcocite mineralization in the potassic ppm (Romero, 2008). Pyrite, enargite, tennantite, and zone, mainly recognized in the Zaldívar part of the deposit. sphalerite are observed in the high-sulfidation veinlet zones There is a tendency for copper tenors to drop at depth in (Williams, 2003).

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Supergene mineralization A well-developed supergene profile is present at Escon- dida Norte-Zaldívar (Fig. 9c), comprising a hematitic leached capping, averaging 100 to 200 m (up to 350 m) thick, underlain by an enrichment zone from 20 to 250 m thick, which constitutes the main orebody (Fig. 9c, d). The enrichment zone is 2 × 1.5 km in areal extent, trends northeast, and roughly parallels the present topography (Monroy, 2000; Williams 2003; Fig. 2b); it is divided into a high-grade, chalcocite-dominated, upper zone and a lower grade, covellite- bearing, basal part in which pyrite is little replaced. Super- gene kaolinite is ubiquitous, and associated supergene alunite returned ages of ~17 to 14 Ma (Morales, 2009; Table 3). South of Escondida Norte, a 5-km-long paleochannel contains low-grade, exotic copper mineralization, hosted by both the basal Pampa de Mulas gravels and immediately sub- jacent bedrock. Oxide copper mineralization is irregularly developed above the enrichment zone, principally antlerite and brochantite in the central, highest grade parts (Maturana and Saric, 1991; Monroy, 2000; Williams, 2003), but chrysocolla and atacamite peripherally (Fig. 9c). In contrast, the oxide copper mineralization developed at the expense of low-grade chalcopyrite mineralization in andesitic volcanic rocks at Pinta Verde is dominated by chrysocolla, atacamite, black oxide phases, and FIG. 10. Geology of the Pampa Escondida porphyry copper deposit, copper-bearing clay (cf. Monroy, 2000). 2,000-m elevation. Pampa Escondida Porphyry Copper Deposit Geologic summary Hypogene alteration and mineralization The large Pampa Escondida deposit (Table 1), completely The alteration and mineralization at Pampa Escondida have concealed beneath 10 to 130 m of Pampa de Mulas piedmont a classic dome-shaped form centered on the early granodiorite gravel (Fig. 2d), is hosted by La Tabla andesitic tuffs and porphyry stock (Fig. 11b). The core of the deposit displays flows, dated at 288.0 ± 2.4 Ma, which are intruded at depth potassic alteration, composed of K-feldspar > biotite in the by dark-colored quartz diorite and quartz monzodiorite por- early porphyry and, locally, also in the intermineral phase. Out- phyries, dated at 293.0 ± 4.3 and 287.6 ± 3.3 Ma, respectively ward, biotite > K-feldspar alteration affects the Early Permian (Figs. 10, 11a; Table 2). Shallowly dipping lenses of calcare- porphyries, whereas intense biotitization characterizes the ous sedimentary rock, dacitic tuff, and andesitic lava are in- Early Permian andesitic volcanic rocks. Local garnet-diopside terbedded with the upper parts of La Tabla succession (Fig. skarn is developed in the calcareous lenses within the upper 11a). part of the andesitic sequence. Beyond the potassic core zone, A multiphase porphyry stock is present at Pampa Escon- chlorite-sericite alteration is widely developed in the andesitic dida, most of it at depths of >600 to 700 m below the surface sequence as well as affecting the late mineral dacite porphyry (Fig. 11a). The first phase, the precursor hornblende diorite dike (Fig. 11b). Sericitic alteration is not abundant but is dated at 37.6 ± 0.5 Ma (Table 3), occurs as dikes cutting La mapped at the shallowest levels of the deposit (Fig. 11b). Tabla Formation. The copper mineralization spans several The inner potassic zone with K-feldspar > biotite contains phases of biotite granodiorite porphyry, which intrude all the the highest grade mineralization, ranging between 0.5 and Early Permian units. The earliest porphyry, dated at 36.1 ± 1.0% Cu, as chalcopyrite and bornite (2/1) along with lesser 0.7, 36.0 ± 1.2, and 35.0 ± 0.6 Ma, constitutes a NE-oriented, amounts of hypogene digenite, chalcocite, covellite, and 2- × 0.5-km stock, which is cut centrally, at depths of >1,000 molybdenite (Fig. 11c, d); however, maximum molybdenite m, by an intermineral porphyry, dated at 35.0 ± 0.8 and 34.5 concentrations constitute an annular zone, partly overlapping ± 0.4 Ma (Figs. 10, 11a; Table 3). These, in turn, are cut in the and external to the copper-rich core, in common with many northeast of the deposit by a late intermineral hornblende gold-rich porphyry copper deposits (e.g., Sillitoe, 1979; Perelló dacite porphyry dike, 20 to 30 m wide, dated at 35.2 ± 0.8 Ma et al., 2004). The sulfides occur in early K-feldspar and later (Figs. 10, 11a; Table 3). In the northeast of the deposit there A- and B-type quartz veinlets as well as in disseminated form. are also several minor monomict magmatic-hydrothermal Minor gray sericite veinlets are also present. The chalcopy- breccias as well as NW-striking dikes of barren, polymict rite-bornite mineralization grades outward to chalcopyrite- breccia cutting the granodiorite porphyry stock (Figs. 10, pyrite, averaging 0.2 to 0.5% Cu, which in turn is transitional 11a). The latter breccias have rock-flour cement, postdate all to a pyrite halo, containing 2 to 3 vol % pyrite and <0.1% Cu alteration and mineralization, and are considered phreatic in (Fig. 11c, d). The late mineral dacite porphyry dike is also origin (cf. Sillitoe, 1985). pyritic (Fig. 11c). The Pampa Escondida deposit contains

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FIG. 11. Pampa Escondida sections. a. Geology. b. Alteration. c. Mineralization zones. d. Copper distribution. Section line shown in Figure 10.

more gold than the others in the district, with the highest which oxide copper mineralization is present in places, and grades accompanying the central, bornite-bearing zone. The an underlying enrichment zone averaging ~15 m thick (Fig. copper-bearing part of the system barely attains the bedrock 11c). The oxide copper minerals, chiefly chrysocolla, mala- surface (Fig. 11c). chite, and brochantite, but also including dioptase, atacamite, and cuprite, are largely confined to the calcareous Supergene mineralization horizons. Supergene montmorillonite, nontronite, and ver- The supergene profile at Pampa Escondida is thinly devel- miculite, partly copper bearing, accompany the oxide copper oped, comprising leached capping averaging ~40 m thick, in mineralization.

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Chronology of Intrusion and Mineralization comprising early, inter-, and late mineral phases, all of similar granodioritic composition. The early porphyries tend to be District chronology volumetrically small, particularly at Escondida and Escondida Based on the U-Pb zircon and Re-Os molybdenite ages Este, and the later intrusions substantially larger, particularly (Table 3), porphyry copper emplacement in the Escondida at Escondida and Chimborazo (Figs. 4, 5a, 6, 7a). The late district spanned a maximum interval of 9.4 m.y., whereas the mineral rhyolite porphyry at Escondida Este, however, is Escondida cluster was generated in a maximum of 5.1 m.y., compositionally distinct. Copper grades are consistently high- which is increased to 5.3 m.y. if the dated monzodiorite pre- est in the early porphyries and decrease progressively through cursor at Baker is included (Fig. 12a). The modern K-Ar and the inter- to late mineral phases. Ar/Ar ages, not plotted in Figure 12a, fall well within these in- Magmatic-hydrothermal breccias of intermineral timing tervals (Padilla-Garza et al., 2004; Véliz, 2004; Campos et al., typify all deposits of the Escondida district, including Chimb- 2009; Table 3). orazo, but tend to occur as small bodies (Figs. 5a, 7a, 9a, 11a), As shown in Figure 12a, the Chimborazo deposit formed albeit commonly with higher hypogene and, where applica- first, at ~41 Ma, followed some 2 to 3 m.y. later by the initia- ble, supergene copper grades than those in surrounding tion of the Escondida cluster. The cluster appears to have rocks. The alteration in the breccias broadly reflects that in formed in two separable, but partly overlapping stages: the the rocks within which they are developed, from shallow ad- early Escondida, Escondida Norte-Zaldívar, and Baker por- vanced argillic (at Chimborazo and Escondida Este) through phyries at ~38 to 37 Ma, followed by the early Pampa Escon- sericitic and chlorite-sericite to deep potassic. dida and Escondida Este porphyries at ~36 to 34.5 Ma (Fig. Remanent advanced argillic zones, partly overprinted on 12a). No further magmatism is recorded in the district after the underlying sericitic, chlorite-sericite, and, locally, potassic ~34 Ma. alteration types, occur at the shallowest levels, except at The lifespans of the individual porphyry copper deposits in Pampa Escondida where it is assumed to have been lost to the district cannot be precisely determined because of the erosion (Figs. 5b, 7b, 9b, 11b). Highest hypogene copper val- errors inherent in the U-Pb zircon ages and their consequent ues occur as chalcopyrite and bornite in either potassic alter- overlap (Fig. 12a). Nonetheless, lifespan maxima of ~4 m.y ation or overprinted, pervasive to veinlet-controlled, gray and minima of ~1 m.y. are apparent, although Escondida Este sericite alteration; however, the latter alteration type consti- formed in a much shorter time (~0.2−0.4 m.y.). These lifes- tutes an appreciable rock volume only at Escondida Este (Fig. pans fall within the range for other central Andean porphyry 7b). Elevated copper tenors are also suspected in the high- copper deposits (Sillitoe and Perelló, 2005, and references sulfidation assemblages associated with advanced argillic alter- therein). ation at Escondida, although supergene enrichment has obliterated much of the evidence (Alpers and Brimhall, 1989). Escondida chronology High- and subordinate intermediate-sulfidation veins com- Available U-Pb zircon ages emphasize that two discrete posed of pyrite-dominated, massive sulfides are widely devel- porphyry copper centers are present at Escondida, with initi- oped at Chimborazo, Escondida-Escondida Este (Fig. 7c), ation of the Escondida Este deposit being ~2 m.y. younger and, to a lesser extent, Escondida Norte. The veins are com- than initial porphyry emplacement at the Escondida deposit monplace as overprints to the deposits themselves, but also (Fig. 12b). The Re-Os and Ar/Ar ages fit reasonably well with occur peripherally, particularly southeast of Escondida Este this temporal scheme (Fig. 12b). where they extend into the synorogenic San Carlos strata. It is particularly noteworthy that the advanced argillic alter- Both vein types can be auriferous. The Chimborazo veins ation, recorded by the hypogene alunite ages, formed in a strike NE, whereas those throughout the Escondida cluster minimum of two stages, as concluded previously by Quiroz are predominantly northwest. (2003a, b) and Padilla-Garza et al. (2004). Alunite from the Notwithstanding these geologic similarities, the deposits youngest advanced argillic event, which affects the quartz por- also have some notable hypogene and supergene differences. phyry, has not been dated, although molybdenite from a high- At the early Chimborazo deposit and late-stage Escondida sulfidation vein in this zone yielded the youngest age obtained Este and Pampa Escondida deposits (Fig. 12a), the early gra- to date (33.7 ± 0.3 Ma; Padilla-Garza et al., 2004; Fig. 12b). nodiorite porphyry intrusions lie 600 to 1,000 m below the present surface, whereas at Escondida, Escondida Norte- Discussion Zaldívar, and Baker they lie close to or at the surface (Figs. 5a, 7a, 9a, 11a). The deep position of the Chimborazo porphyry Porphyry copper deposit evolution is further emphasized by the 600-m thickness of chlorite- The Escondida-Escondida Este, Escondida Norte-Zaldívar, sericite alteration that separates the base of the main lithocap and Pampa Escondida deposits display a number of common from the underlying potassic zone (Fig. 5b). This marked dif- characteristics. All were emplaced into latest Carboniferous ference in the relative elevations of intrusions is attributed to to Early Permian volcanic and intrusive rocks, which within different degrees of telescoping, which, in turn, result mainly the confines of the Escondida and Zaldívar deposits are jux- from different degrees of synmineral uplift and erosion (Silli- taposed with, as well as probably overlain by, late Paleocene toe, 1994). Furthermore, deep, mature supergene profiles are andesitic volcanic rocks assignable to the Augusta Victoria developed at Escondida and Escondida Norte-Zaldívar and a Formation (cf. Navarro et al., 2009; Figs. 6, 8). thinner profile characterizes Chimborazo, whereas only minor The mineralized porphyry stocks, spatially related to pre- supergene mineralization overlies Escondida Este and Pampa cursor diorite to monzodiorite intrusions, are multiphase, Escondida (Figs. 5c, 7c, 9c, 11c; see below).

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FIG. 12. Summary geochronology of intrusion, alteration, and mineralization in the Escondida district. a. U-Pb zircon ages for precursor, early, inter-, and late mineral intrusions at Chimborazo, Escondida Norte-Zaldívar, Escondida, Baker, Pampa Escondida, and Escondida Este. Re-Os molybdenite ages for Chimborazo, Escondida Norte, Escondida, and Escondida Este are also plotted. Note the age difference between Chimborazo and the Escondida cluster. U-Pb ages from Richards et al. (1999), P.J. Pollard and R.G. Taylor (unpub. rept. for Minera Escondida Limitada, 2002), Padilla-Garza et al. (2004), Jara et al. (2009), and this study (Table 3); Re-Os ages from Padilla-Garza et al. (2004), Romero et al. (2010), and this study (Table 3). Box heights indicate 2σ errors of all dated samples. U-Pb ages with errors of >1.2 m.y. are excluded from consideration. b. U-Pb zircon, Re-Os molybdenite, and Ar/Ar biotite ages for intrusion, alteration, and mineralization events in the Escon- dida and Escondida Este deposits. Note the ~2-m.y. interval between early porphyry emplacement at Escondida and Es- condida Este. Data sources detailed in Table 3.

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Tectonic setting Most early interpretations of the tectonic setting of middle Eocene to early Oligocene porphyry copper emplacement in northern Chile assumed that the N-striking, controlling faults of the Domeyko system underwent transcurrent motion (e.g., Reutter et al., 1991, 1996; Richards et al., 2001). It is now ap- preciated, however, that the faults are transpressional structures that underwent kilometer-scale reverse displacement as well as varied degrees of transcurrent motion (McInnes et al., 1999; Amilibia and Skarmeta, 2003; Amilibia et al., 2008; Niemeyer and Urrutia, 2009). Indeed, fission-track ther- mochronologic results suggest that 4 to 5 km of uplift and consequent unroofing took place in the Domeyko Cordillera during the Incaic orogeny (Maksaev and Zentilli, 1999). During the Incaic orogeny in the Escondida district, sinistral transpression—to generate the clockwise block rotation implicated in the initial opening and synorogenic filling of the San Carlos depocenter (Mpodozis et al., 1993a, b; Arriagada et al., 2000; see above)—prevailed for an indeterminate time prior to formation of the Chimborazo deposit at ~41 Ma (Fig. 13a). Comparisons with the Salar de Atacama basin suggest that the initial synorogenic sedimentation could have com- menced at ~45 Ma or even earlier (Hammerschmidt et al., 1992; Mpodozis et al., 2005). Most of the San Carlos strata were clearly in place at the time of porphyry copper formation because their upper, partly volcanic parts are dated at ~38 Ma (Urzúa, 2009) and cut by polymetallic veins peripheral to the porphyry copper deposits (see above). A dextral transcurrent stress regime then seems to have been imposed during porphyry copper formation, between ~41 and 34.5 Ma, in order to explain the systematic NNE to

NE orientation of the early, inter-, and late mineral porphyry FIG. 13. Schematic structural evolution of the Escondida district. a. Pre- intrusions at Chimborazo, Escondida, Escondida Norte- 41 Ma: Large-scale sinistral transpression, giving rise to clockwise rigid-block Zaldívar, Baker, and Pampa Escondida as well as the late- rotations and opening of accommodation space progressively filled by the stage Chimborazo veins (Fig. 13b). However, this dextral San Carlos strata. b. ~41–34.5 Ma: Cessation of sinistral transpression, imposition of a transient dextral transpressive regime, localized andesitic volcan- transpression was only incipiently developed in the Escon- ism, and porphyry copper emplacement. c. ~34 Ma: Switchover to a transient dida district because there was no appreciable offset of ~38 to sinistral transpressive regime and formation of end-stage high-sulfidation 34.5 Ma porphyry copper elements across the Portezuelo- massive sulfide veins and phreatic breccia dikes. d. Late Oligocene-Miocene: Panadero and subsidiary N-striking faults (Figs. 3, 6, 8). Reimposition of transient sinistral transpression, continued uplift and unroofing of porphyry copper deposits, accumulation of Pampa de Mulas pied- The Escondida Este rhyolite porphyry was intruded as a N- mont gravels, and supergene oxidation and enrichment (until ~14 Ma). See trending body, apparently controlled by the Portezuelo- text for further details. Inspired by Mpodozis et al. (1993a, b). Panadero fault (Figs. 6, 7a), at ~34 Ma, implying that by then the dextral stress regime no longer prevailed. Immediately after ~34 Ma, when the NW-striking, high-sulfidation veins progressive bending of the central Andean orogen to produce and phreatic breccia dikes at Escondida-Escondida Este and the Bolivian orocline (Arriagada et al., 2008), rather than to Pampa Escondida (Fig. 10), were formed, the stress regime changes in plate vectors at the Andean margin (e.g., Reutter had once again become sinistral transpressive, although again et al., 1991; Richards et al., 2001). the amount of fault displacement within the district was min- This tectonic evolution implies that the Escondida por- imal (Fig. 13c). phyry copper cluster was not emplaced during either sinistral Therefore, the porphyry copper deposits of the Escondida transcurrent faulting (Padilla-Garza et al., 2001) or change district developed immediately after the main phase of Incaic from regional-scale dextral to sinistral transcurrent motion sinistral transpression, in a transient dextral transpressive (Richards et al., 2001). Nor were the deposits sinistrally offset regime that was transitional to transient sinistral transpression by postmineral transcurrent faulting as appears to have oc- before the end-stage, high-sulfidation vein emplacement and curred farther north in the Chuquicamata porphyry copper phreatic brecciation. These systematic district-scale and time- district (e.g., Lindsay et al., 1995; Tomlinson and Blanco, progressive changes in the orientation of porphyry copper el- 1997; Astudillo et al., 2008; Fig. 1), although appreciable ements, from NE through N to NW in perhaps as little as 0.5 Oligo-Miocene offset may have taken place on parallel struc- m.y., preclude deposit formation in a static stress regime. tures well beyond the immediate Escondida district (e.g., Such rapid tectonic switchovers are best explained by the Niemeyer and Urrutia, 2009).

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Alteration-mineralization telescoping The profoundly telescoped Escondida, Escondida Norte- Zaldívar, and Baker deposits, in which the advanced argillic alteration and attendant high-sulfidation copper mineralization affected quartz-veinlet stockworked early porphyry intrusions (Fig. 7b), formed from ~38 to 36 Ma (Fig. 12a, b); a situation suggesting that deposit generation coincided tempo- rally with the maximal rates of Incaic uplift and unroofing. The younger Escondida Este and Pampa Escondida deposits underwent little telescoping, perhaps implying that exhumation rates were in decline by the time they formed at ~36 to 34 Ma (Fig. 12a, b). Furthermore, by analogy, the little-telescoped Chimborazo deposit, emplaced at ~41 Ma (Fig. 12a), may have immediately preceded onset of the main phase of tectonic uplift and related denudation. These data suggest that maximal exhumation in the Escondida district com- menced sometime between ~41 and 38 Ma and was already waning by ~36 Ma.

Synmineral volcanism The laharic breccias, ignimbrite flows, and subsidiary lava flows, all andesitic in composition, that are a major component of the upper parts of the San Carlos strata within the Es- condida cluster seem likely to be distal, ring plain products of a stratovolcano or large dome complex (cf. Fisher and Schmincke, 1994). The most likely former position for the source volcano is within the confines of the Escondida cluster, the only known nearby magmatic center of appropriate FIG. 14. Inferred former extents of lithocaps in the Escondida district age. Certainly, there is no evidence for an eruptive source based on exposed advanced argillic alteration. Isotopic ages of hypogene alu- within the San Carlos depocenter itself. However, no in situ nite (Table 3) and locations of porphyry copper deposits are also shown. volcanic remnants have yet been recognized within the cluster, although it is possible that they could have been confused with andesitic rocks of the late Paleocene to early Eocene Au- component at Escondida Este where the late mineral quartz gusta Victoria Formation. Furthermore, if the volcano overlay porphyry displays extensive but currently undated advanced all or part of the Escondida cluster, the latter must have re- argillic alteration. Additional isotopic dating may well docu- mained concealed because of the notable absence of altered ment even more complex temporal histories. and mineralized clasts in the San Carlos strata. The contemporaneity of the subaerial volcanism and initial Supergene history formation of the Escondida cluster at ~38 Ma opens the pos- Supergene enrichment is spectacularly developed at Escon- sibility of more widespread volcanic activity during the Incaic dida and Escondida Norte-Zaldívar, although oxide copper ore orogeny and associated porphyry copper emplacement than is also present, particularly peripherally in potassic- altered has been documented to date in the Domeyko Cordillera (e.g., andesitic host rocks characterized by lower neutralization po- Mpodozis and Ramos, 1990; Mpodozis and Perelló, 2003). tentials (Figs. 7c, 9c). Enrichment is also more modestly and thinly developed at Chimborazo (Fig. 5c). In contrast, Escon- Advanced argillic lithocaps dida Este and Pampa Escondida possess little enrichment An appreciable volume of advanced argillic lithocap is pre- (Figs. 7c, 11c). served at Chimborazo (Fig. 2c) and more restricted remnants There are three main reasons for the high-grade enrich- occur at Escondida-Escondida Este (Fig. 2a), Escondida ment zones at Escondida and Escondida Norte-Zaldívar: The Norte (Fig. 2b), and Baker, as described above. If these par- zones are hosted by advanced argillic and sericitic alteration tially mineralized remnants are combined with other, appar- with low neutralization potentials, high pyrite contents, and ently barren lithocap exposures, two large lithocaps could have copper-bearing, high sulfidation-state sulfide assemblages, existed at ~34 Ma, when porphyry copper formation ended partly occurring in steep veins (cf. Véliz and Camacho, 2003; (Fig. 14). In fact, the two lithocaps could easily have coalesced Padilla-Garza et al., 2004). The zones underlie topographic to form a single advanced argillic zone, covering ~200 km2. highs (Fig. 2a, b) that were undergoing erosion and paleo - On the basis of the available isotopic age data (Fig. 14; ground-water table depression as a result of the renewed tec- Table 3), the northern lithocap may have formed at ~42 Ma, tonic uplift that gave rise to accumulation in flanking depres- in association with the Chimborazo deposit, whereas the sions of the Pampa de Mulas piedmont gravels (Fig. 13d), southern one is composite and includes 37 to 36 Ma parts at which contain abundant limonitic clasts of advanced argillic Escondida, Escondida Norte, and Baker and a post-34 Ma lithocap. Lateral transport of copper to form exotic copper

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deposits, a common process in northern Chile (Münchmeyer, peak of the Incaic uplift underwent extreme telescoping, with 1996), was apparently only incipiently developed in the dis- advanced argillic alteration overprinting originally deeply em- trict. Enrichment was inhibited at Pampa Escondida because placed, early porphyry intrusions, potassic alteration, and the rocks subjected to oxidation were pyrite poor and in- chalcopyrite-bornite mineralization. The earlier Chimborazo cluded calcareous horizons with a high acid-neutralizing ca- and later Escondida Este and Pampa Escondida deposits dis- pacity; and the deposit lies beneath a gravel-filled, topo- play much lesser degrees of telescoping. Synchronously, the graphic low (Figs. 2d, 3) and, judging by the limited oxidation uppermost synorogenic San Carlos strata were accumulating, depth (Fig. 11c), had a relatively near-surface paleoground- including andesitic volcanic rocks dated at ~38 Ma and there- water table. Enrichment was poorly developed at Escondida fore coeval with early porphyry intrusion at Escondida and Este because of a deficiency of near-surface copper and is rel- Escondida Norte-Zaldívar; this situation implies that volcan- atively low grade and thin at Chimborazo for the same reason, ism, probably associated with construction of a stratovolcano except in the pyrite-enargite veins, and because most of it un- or major dome complex, was active nearby in the magmatic derlies low, gravel-covered terrain. arc, possibly above the Escondida cluster. Oxidation and enrichment were active in the Escondida The porphyry copper deposits were likely still overlain at district between ~18 and 14 Ma based on ages of supergene ~34 Ma by extensive, composite advanced argillic lithocaps, alunite, a proxy for pyrite oxidation (Alpers and Brimhall, which, judging by the localized high-sulfidation epithermal 1988; Morales, 2009; this study; Table 3), in keeping with the gold mineralization preserved in the remnants at Escondida supergene chronology throughout the middle Eocene to early and Chimborazo may have been auriferous. The main strip- Oligocene porphyry copper belt of northern Chile (Sillitoe ping of the lithocaps took place during renewed uplift and and McKee, 1996; Arancibia et al., 2006). Contemporane- erosion in the late Oligocene to middle Miocene, as charted ously, the Pampa de Mulas Formation was accumulating in by the abundance of advanced argillic-altered clasts in the nearby depressions (Marinovic et al., 1995). Since about 14 piedmont gravels of the coeval Pampa de Mulas Formation. Ma, aridity intensified, erosion rates decreased, and ground- Once high-sulfidation copper mineralization in the basal parts water recharge and supergene processes ceased, as reflected of the lithocaps became exposed to weathering, the thick by the absence of younger supergene alunite development leached cappings and their underlying chalcocite enrichment and near-surface preservation of the tuff horizons as old as 8.7 blankets began to form. Their recorded age spans the ~18 to Ma (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996; 14 Ma interval, contemporaneous with gravel accumulation. Arancibia et al., 2006). The topographically prominent Escondida and Escondida Norte-Zaldívar deposits developed high-grade enrichment Conclusions zones, whereas the topographically recessive, gravel-covered The Escondida district is the largest of several giant por- Pampa Escondida deposit underwent only shallow oxidation phyry copper clusters that characterize the middle Eocene to and incipient enrichment. early Oligocene magmatic arc of northern Chile (Fig. 1). De- posits of the Escondida cluster are assumed to have formed Acknowledgments above a single parental magma chamber, from which ascent The writers wish to acknowledge the contributions to geo- of the first magma batches and related fluid involved in de- logic understanding of the Escondida district by many past and posit generation took place in two discrete pulses separated present members of the Escondida brownfields team. Javier by ~1 to 2 m.y. (Fig. 12a). These initial magma batches, rep- Urrutia of Minera Escondida Limitada, Ricardo Muhr of resented by the early porphyries, were followed by additional Antofagasta Minerals, and Geoff McKinley, Geoff Woad, and discrete aliquots of magma and fluid, giving rise to the inter- Angus Campbell of BHP Billiton MinEx facilitated the collec- and late mineral porphyry sequences and their associated, but tive preparation of the first draft of the manuscript in Santiago, progressively declining intensities of alteration and mineral- Chile. Constantino Mpodozis provided valuable instruction on ization. Nonetheless, there was temporal overlap between the the regional geologic setting of the Escondida district. Re- two porphyry copper events. The deeply concealed Chimbo- views of the first manuscript draft were provided by Leyla razo porphyry initiated intrusion and copper introduction at Vaccia and Walter Véliz of Minera Escondida Limitada and the district scale, but it remains uncertain if the magma and Jean des Rivières and Rick Preece of BHP Billiton Base Met- fluid were supplied by the same parental chamber as that re- als. Geoff Ballantyne and Rubén Padilla carried out the formal sponsible for the Escondida cluster. manuscript reviews. Edgar Basto, President of Minera Es- Chimborazo was formed prior to the main phase of Incaic condida Limitada, and BHP Billiton authorized publication. uplift and exhumation, possibly toward the end of an episode of sinistral transpressional faulting, related clockwise rigid- REFERENCES block rotation, and the consequent opening and initial filling Alpers, C.N., and Brimhall, GH., 1988, Middle Miocene climatic change in of the San Carlos sedimentary depocenter. 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