CH 14 Mpodozis , Cornejo , 2012, Cenozoic Tectonics and Porphyry Copper Systemes of the Chilean Andes, SEGSP

© 2012 Society of Economic Geologists, Inc. Special Publication 16, pp. 329–360

Chapter 14 Cenozoic Tectonics and Porphyry Copper Systems of the Chilean Andes CONSTANTINO MPODOZIS† AND PAULA CORNEJO Antofagasta Minerals, Apoquindo 4001, Piso 18, Santiago, Chile

Abstract Subduction under South America has been active for the past 550 m.y. but large porphyry copper deposits were essentially emplaced during the Paleocene (60−50 Ma) in southern Peru, and mid-Eocene-early Oligocene (43−32 Ma) and late Miocene-Pliocene (10−6 Ma) in north and central Chile. Although the tectonic setting of the Paleocene porphyry deposits is still poorly understood, those of the northern Chile EoceneOligocene belt were emplaced along the margin-parallel Domeyko fault system, where active compressional and/ortranspressional deformation and block rotations took place during the formation of the Bolivian orocline. Eocene-early Oligocene oroclinal bending was a consequence of differential tectonic shortening focused along a mechanically weak zone of the Central Andean crust inherited from the Paleozoic. Deformation occurred during an episode of accelerated westward absolute motion of the South American plate, which coincided with very high rates of oceanic crust production in the eastern Pacific. The slow South American-Farallon convergence rates recorded for the Eocene-Oligocene suggest, however, that strong interplate coupling existed during that time. This permitted the transfer of horizontal stresses and large-scale deformation of the Andean margin, creating a favorable scenario for the generation and emplacement of porphyry copper magmas along the Domeyko fault system. The younger, Miocene-Pliocene porphyry copper deposits of central Chile-Argentina were emplaced in a different setting, after the initiation of compressional deformation within a volcano-tectonic depression (Abanico basin) that evolved during another, late Oligocene to early Miocene, period of increased East Pacific oceanic crust production. Nevertheless, in contrast to the Eocene-Oligocene situation in northern Chile, the relatively stationary position of the South American plate compared to the mantle reference frame and weak interplate coupling that permitted rapid subduction, increased volcanism, and overriding plate extension. Tectonic inversion of the basin and compressional deformation along with crustal thickening and mountain building began at around 20 m.y. ago as interplate coupling increased when the westward motion of South America accelerated and the Nazca-South America convergence velocity decreased in the mid-Miocene. Compression was accompanied, as during the Eocene-Oligocene in northern Chile, by slab shallowing and increased forearc subduction erosion. In both cases, the largely structurally controlled, syn- to post-tectonic porphyry copper deposits are associated with long-lived magmatic systems that were active for more than 10 m.y. In northern Chile, the deposits occur as parts of discrete intrusive clusters that comprise a suite of precursor plutons emplaced during multiple events since the Cretaceous. Porphyry copper mineralization is linked to multistage, amphibole-bearing intrusions of intermediate composition derived from hydrous, oxidized magmas with adakitic geochemical signatures. These intrusions appeared when crustal thickness increased to a critical threshold in the course of deformation. Production of magmas with high metal-carrying capacity was fostered as fluids were liberated when amphibole became unstable and was destroyed as the crust thickened. At the same time, source regions within the mantle were contaminated by hydrated fragments of fore-arc continental crust, as the result of enhanced subduction erosion during peaks of compressional deformation.

Introduction THE STUDY of the tectonic setting of porphyry copper deposits is fundamental to understanding their genesis (e.g., Sillitoe, 1998; Kay and Mpodozis, 2001; Cooke et al., 2005; Sillitoe and Perelló, 2005; Richards, 2009, 2011a; Tosdal et al., 2009). Some Cenozoic porphyry copper deposits are known to have formed during or shortly after continent-continent, continent-island arc, or island arc-island arc collisions in the Himalayas-Tibet, the Kerman arc in Iran, and Papua New Guinea (Solomon, 1990; Zenqiang et al., 2003; Shaifei et al., 2009). A Paleozoic example of this type of deposit may be Oyu Tolgoi in Mongolia (Perelló et al., 2001). In contrast, other large porphyry deposits such as Bingham Canyon in the western United States formed during the earliest stages of † Corresponding

author: e-mail, [email protected]

Basin and Range extension in the Eocene, far inland from the Pacific margin of North America (Kloppenburgh et al., 2010). Noncollisional porphyry copper deposit examples in subduction-related arc settings include those from the Chagai belt in Pakistan, the Laramide porphyry copper province of the western United States and northern Mexico (Lang and Titley, 1998; Valencia-Moreno et al., 2007; Perelló et al., 2008) and the Central Andes province, which host some of the largest known porphyry copper deposits in the world (Camus, 2003; Cooke et al., 2005; Sillitoe and Perelló, 2005). The Andes has long been considered as the type example of a noncollisional orogenic system (e.g., Jordan et al., 1983), where subduction of Pacific oceanic crust beneath South America has been active for the past 570 m.y. (Cawood, 2005). Nevertheless, the largest porphyry copper deposits are the result of anomalous magmatic systems that developed

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during short periods at specific locations within the Andean orogen. These include the Paleocene to early Eocene (66−52 Ma) and middle Eocene to early Oligocene (43−32 Ma) belts in southern Peru and northern Chile, and the late Miocene to early Pliocene (10−5 Ma) porphyry systems in central Chile and contiguous Argentina (Perelló et al., 2003a; Sillitoe and Perelló, 2005). In this contribution, with emphasis on the Chilean belts, we will try to demonstrate how major Cenozoic tectonic events along the central Andean convergent margin, prompted by large-scale reorganizations of the global tectonic system, were the main triggers for the formation of large porphyry copper deposits. Pre-Andean History: From Rodinia Dispersal to Pangea Breakup The western margin of South America underwent magmatic and tectonic activity at least since the late Neoproterozoic breakup of Rodinia (800−700 Ma), when the separation of Laurentia from Gondwana produced the opening of the proto-Pacific (Iapetus) ocean (Dalziel, 1997). East-directed subduction of newly formed ancestral Pacific crust below western Gondwana began at ~570 Ma and was fully active along the proto-Andean margin by 485 to 465 Ma (Pankhurst et al., 1998; Cawood, 2005; Chew et al., 2007). Plate convergence in the Central Andes region during the Ordovician to Devonian included the progressive collision and accretion of a group of tectonostratigraphic terranes of Laurentian and/or Gondwanan affinities (e.g., Ramos et al., 1986; Astini et al., 1995) against the western South American margin. Terrane amalgamation contributed to the formation of the accretionary Terra Australis orogen, which extended for more than 18,000 km along the Pacific margin of Gondwana from Australia to South America (Cawood, 2005). The accretionary stage was followed, in the Central Andes, by the buildup of a late Carboniferous to Early Permian (320? -280 Ma) suprasubduction magmatic arc on top of the newly accreted terranes, as well as the development of an outboard fore-arc subduction complex that extended for more than 1,000 km along the Chilean segment of the Gondwana margin south of 27° S (Mpodozis and Kay, 1992; Hervé, 1988; Willner et al., 2005; Chew et al., 2007). Magmatism continued from the Permian to the Middle Triassic (280−240 Ma), when great volumes of intrusive and mostly felsic volcanic rocks, including the Choiyoi large igneous province in Chile and Argentina and the Mitu Group in southern Peru (Kay et al., 1989, Sempere et al., 2002), were emplaced along the western South American margin. Although geochronologic and geochemical data are still incomplete, several competing hypotheses, such as normal or oblique subduction, postcollision extension-driven crustal melting, slab breakoff or slab shallowing, have been proposed to explain the prevailing tectonic regime along different segments of the Andean margin at that time (Mpodozis and Kay, 1992; Kleiman and Japas, 2009; Ramos and Folguera, 2009, and references therein). From the Middle Triassic to earliest Jurassic (240− 190 Ma), rifting associated with the incipient stages of Pangea dispersal (e.g., Veevers, 1989), accompanied by a decreasing volume of bimodal magmatism, seems to have occurred along the western margin of South America (e.g., Ramos and Kay, 1991; Franzese and Spalletti, 2001; Rosas et al., 2007). 0361-0128/98/000/000-00 $6.00

Diverse, yet basically subeconomic, porphyry copper deposits formed during these events in northern Chile and along the Frontal Cordillera in west-central Argentina (Sillitoe, 1977; Sillitoe and Perelló, 2005; Cornejo et al., 2006; Munizaga et al., 2008). Jurassic to Early Eocene Tectonics and Metallogeny of the Central Andes After the Triassic rifting event, subduction was reestablished in northern Chile and southern Peru during the Early Jurassic when a new magmatic arc developed west of the extinct late Paleozoic arc front. Since then, subduction has proceeded uninterrupted to date. Initial Jurassic to Early Cretaceous arc magmatism occurred under extensional conditions that permitted the formation of a series of interconnected back-arc basins to the east of the main arc, which were progressively filled with marine and continental sedimentary strata (Mpodozis and Ramos, 1989, 2008). Transpressional deformation along the arc axis created the intra-arc Atacama fault system in northern Chile (Scheuber and González, 1999) and was accompanied in the Early Cretaceous by the emplacement, in northern Chile, of some porphyry copper deposits at ~140 to 130 Ma (e.g., Antucoya-Buey Muerto, 141−139 Ma; Puntillas-Galenosa, 135−132 Ma; Perelló et al., 2003b; Maksaev et al., 2006, 2010). Fast convergence rates during the global mid Cretaceous superplume event (Larson, 1991) produced an upsurge in volcanism along the Andean margin, accompanied by intra-arc extension and transtension which fostered iron oxide-copper-gold (IOCG)−type mineralization between 120 and 100 Ma in northern Chile and southern Peru (Marschik and Fontboté, 2001; Sillitoe, 2003; Sillitoe and Perelló, 2005; Chen et al., 2010). Small, low-grade, gold-rich porphyry copper deposits such as Andacollo (104 Ma), Domeyko-Dos Amigos (108−104 Ma), and Pajonales (97 Ma) were emplaced under extensional conditions during the same general period in north-central Chile (Sillitoe and Perelló 2005; Maksaev et al., 2010). The extensional and transtensional conditions that dominated early Andean subduction ended in the early Late Cretaceous, when the back-arc basins were tectonically inverted (Mpodozis and Ramos, 1989; Tomlinson et al., 2001a). The shift to a more contractional, subduction-related regime occurred together with the accelerated westward drift of South America, in response to the final opening of the Atlantic (Russo and Silver, 1996; Somoza and Zaffarana, 2008). Subsequently, the coastal magmatic arc was abandoned (Fig. 1b) and the magmatic front jumped to the east in the Late Cretaceous, where it remained relatively stationary until the early Eocene. Abrupt shifts in the magmatic front such as this has been accompanied throughout the Andean history, by transient geochemical changes during and after arc migration (e.g., Cornejo and Matthews, 2001; Haschke et al., 2006: Fig. 1d). Stern (1991, 2011), Kay and Mpodozis (2002), and Kay et al. (2005) suggested that changes of this type reflect mantle contamination from fore-arc crust removed during enhanced subduction erosion processes associated with major contractional events along the Andean margin. In northern Chile, the Cretaceous-Tertiary boundary was marked by another short pulse of contractional deformation, the K-T event of Cornejo et al. (2003). Paleocene to early

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FIG. 1. (a). Main morphotectonic units of the central Andes, between 15° and 30° S. FA = modern fore-arc zone, including the Coastal Range and, farther to the east, the Precordillera (or Cordillera de Domeyko) shown in Figure 2; WC = Western Cordillera which, north of 27° S, is essentially formed by the active magmatic arc of the Central Andean volcanic zone (CVZ); EC = Eastern Cordillera; SP = Sierras Pampeanas; SA = sub-Andean fold-and-thrust belt. (b). Relationship between age and longitude (distance to the trench) for 1,000-km-long, 40- to 60-km-wide, orogen-parallel zone of deformation composed of a complex array of strikeslip, normal, and reverse faults, together with thin- and thickskinned folds and thrusts, which extends along the Cordillera de Domeyko (also known as Precordillera) in northern Chile between 20° and 27° S (e.g., Reutter et al., 1991, 1996; Cornejo et al., 1997). Some authors (e.g., Amilibia and Skarmeta, 2003; Amilibia et al., 2008) proposed that most of these faults and folds initiated during Late Cretaceous as a consequence of the inversion of normal faults inherited from the Mesozoic back-arc extension. However, others (e.g., Tomlinson et al., 2001a; Mpodozis et al., 2005) interpreted that the Andean back-arc basins were first inverted during the early Late Cretaceous to form a proto-Cordillera de Domeyko, while a second main tectonic pulse along the Domeyko fault system, coincident with the Incaic event, produced its final uplift (Reutter et al., 1991, 1996; Scheuber and Reutter 1992; Tomlinson et al., 1993; Maksaev and Zentilli, 1999). Parts of the Domeyko fault system were subsequently reactivated during the Oligocene and the Quaternary (Tomlinson and Blanco, 1997a, b; Audin et al., 2003; Soto et al., 2005). The kinematics of the middle Eocene to early Oligocene deformation along the Domeyko fault system is a matter of controversy; evidence for both left- and right-lateral displacements, including reversal in the sense of shear, has been reported along different parts of the faulted domain (Reutter et al., 1996; Dilles et al., 1997; Tomlinson and Blanco 1997a, b; Hoffman-Rothe et al., 2004; Niemeyer and Urrutia, 2009). Fission-track age data show that the Cordillera de Domeyko was exhumed between 40 and 30 m.y. ago (Maksaev and Zentilli, 1999; Nalpas et al., 2005) in association with surface tectonic uplift and profound erosion, the products of which accumulated in syntectonic basins east and west of the area of deformation (Mpodozis et al., 2005; Hong et al., 2007; Wotzlaw et al., 2011). Origin of the Domeyko fault system At first glance, the Domeyko fault system could be considered as a trench-linked fault system (Woodcock, 1986) that nucleated in the thermally weakened crust of the middle Eocene to early Oligocene magmatic arc of northern Chile during a period of suggested fast Eocene oblique convergence between the Farallon and South America plates (PardoCasas and Molnar, 1987; Somoza, 1998). However, when recent paleomagnetic and structural studies are taken into account it becomes apparent that tectonic activation of the Domeyko fault system during the Incaic episode is essentially 0361-0128/98/000/000-00 $6.00

a consequence of the formation of the sharp bend of the western South American margin, known as the Arica elbow or Bolivian orocline (Fig. 3). Paleomagnetic studies have been essential in obtaining a more constrained view of the deformational history of this segment of the Central Andes and support the tectonic model for orocline formation first proposed by Isacks (1988). Figure 3a is a simplified regional map showing the distribution of paleomagnetic (declination) vectors measured for the Central Andes. Importantly, independent of age, Mesozoic and Paleogene rocks have been rotated up to 50°. In contrast, rotations measured in Miocene and younger rocks (1,000-km strike of the fault system. Deformation in this area seems to have been dominated by tectonic escape linked to passive rotation and transport of brittle upper crustal blocks over hot and ductile lower crust in a way similar to the so-called orogenic float or clutch tectonics models discussed by Oldow et al. (1990), Lamb (1994), and Tikoff et al. (2002; see below). At this latitude, the Cordillera de Domeyko appears as a discontinuous mountain range formed by a group of discrete basement blocks bounded to the west by a 150-km-long shear lens (Escondida shear lens) developed between the regional Sierra de Varas and Escondida faults (Figs. 6a, 7). To the east, the Domeyko range abuts the Salar de Atacama depression, which is a deep subsiding basin filled by >9 km of Cretaceous to Tertiary continental sedimentary strata (Pananont et al., 2004; Mpodozis et al., 2005). The basin was built on top of a large positive gravimetric anomaly (Central Andean Gravity High; Götze and Krausse, 2002; Fig. 6b), which indicates the occurrence, at depth, of dense crustal rocks that may help to explain its long-lived subsidence basin history, recorded at least since the Cretaceous. The isolated basement blocks that form the core of the range are separated by small, triangular basins, with interior drainage (Fig. 6a). The southern rhomboid-shaped blocks (e.g., San Carlos and Imilac; Fig. 6a) are bounded along their northwestern margins by high-angle, SE-dipping reverse faults. 0361-0128/98/000/000-00 $6.00

The blocks at Quimal, Los Morros, and Mariposas are limited to the west and north by left-lateral strike-slip faults (Mpodozis et al., 1993a, b). Along the El Bordo Escarpment, the eastern margin of the Imilac and Mariposas blocks are thrust over the sedimentary fill of the Salar de Atacama basin (Fig. 6a), which includes, among other units, a 2,500-m-thick sequence of Eocene to early Oligocene continental conglomerates and poorly consolidated gravels. Internal progressive unconformities and Ar/Ar ages between 44 and 43 Ma from a tuffaceous horizon just above the base of this sequence (Loma Amarilla Formation) indicate that these strata-accumulated syntectonically during the regional Incaic deformation (Hammerschmidt et al., 1992; Mpodozis et al., 2005). The tectonics of this segment of the Cordillera de Domeyko (Fig. 6a) can be interpreted as a result of the displacement of a 250- × 50-km basement sliver that was transported northward during the Incaic deformation. According to Mpodozis et al. (1993a, b), the continuous northward shift of the displaced block was impeded by a buttress located to the north of the moving sliver as the displacement was transferred to the east by tectonic escape (cf. Mann, 1997) toward the deeply subsiding Salar de Atacama basin. In this model, the Salar de Punta Negra depression (Fig. 6c) would have formed as an extensional basin at the trailing edge of the displaced block. Displacement transfer seems to have occurred by clockwise rotation of small detached blocks, which in turn generated the local triangular-shaped extensional basins between the rotating blocks as well as contractional deformation in their northeastern corners where basement was thrust over the Salar de Atacama basin fill (Fig. 6c). Mpodozis et al. (1993a, b) located this buttress at Sierra de Limón Verde, which is a N-plunging basement half dome that attains one of the highest elevations (3,500 m.a.s.l.) in the Cordillera de Domeyko (Fig. 6a). However, if along-strike changes in local stresses resulting from the formation of the Bolivian orocline are considered, the buttressing effect may have been provided by the nonrotated paleomagnetic domain C, located north of Calama (Fig. 3), where initial Eocene deformation was taken up by pure east-west shortening (Tomlinson et al., 2001a; see Figs. 3, 4). Figure 7 is a more detailed map showing the geologic setting and distribution of the barren and mineralized intrusions that form part of the Escondida cluster (labeled “LE,” Fig. 2). The area encompasses the widest part of the regional Escondida shear lens, which is separated to the east from the Sierra Imilac and Sierra San Carlos basement blocks by the Escondida fault (the Panadero-Portezuelo fault is an alternative name used by Hervé et al., 2012). These two blocks were then separated by the intervening triangular Salar de Hamburgo depression (Fig. 7). Drilling shows that the Salar de Hamburgo fill includes >1,200 m of red beds, lahars, and pyroclastic rocks with U-Pb zircon ages of 38 Ma (San Carlos Strata, Fig. 7; Marinovic et al., 1995; Urzúa, 2009; Hervé et al., 2012); therefore, these units are equivalent to the upper portion of the syntectonic Loma Amarilla Formation in the Salar de Atacama. The Hamburgo fault is a NE-trending, high-angle reverse fault that places the late Paleozoic basement of the San Carlos block over the sedimentary sequences of the Salar de Hamburgo (Fig. 7).

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FIG. 6. (a). Main structural elements of the Cordillera de Domeyko (between Escondida and Sierra Limón Verde and Salar de Punta Negra (22° 30°−25° S; see location in Fig. 2). Note the large shear lens (Escondida shear lens) flanked by the Escondida and Sierra de Varas strike-slip faults along the western edge of the range and the discontinuous basement blocks (labeled with letters) forming the core of the range. (b). Tectonic sketch of the Cordillera de Domeyko between 21° and 25° S, indicating major Eocene-Oligocene Incaic structures (Tomlinson and Blanco, 1997a). Note contrast between clockwiserotated blocks in rotated domain D (Fig. 3) and deformation associated with reverse faults in nonrotated domain C. (c). Model of lateral transfer of displacement of a tectonic sliver bounded by a buttress and a free face moving northward along a left-lateral strike-slip fault system. Displacement is impeded, as shown, by a buttress at the leading edge of the block, and transferred toward the right by means of clockwise block rotations. Note extensional basins created between the rotating blocks. A-A' and r-r = position of points and lines before and after rotation (α = rotation angle). Adapted from Beck et al. (1993).

A protracted, >40-m.y. Cretaceous to Tertiary history of magmatism is recorded in the Escondida region. The oldest intrusive events produced Late Cretaceous (81−71 Ma; Fig. 7) tholeiitic to alkaline pyroxene gabbros and diorites as well as hornblende-pyroxene monzodiorites and diorites, in addition to early Paleocene (66−64 Ma) pyroxene diorites. These rocks intruded the sedimentary strata of the Mesozoic backarc basin in the Escondida shear lens to the south and west of Escondida (Fig. 7); Paleocene to early Eocene volcanic rocks (59−53 Ma) are also present in the area (Marinovic et al., 1995; Richards et al., 2001; Urzúa, 2009). All of this focused and recurrent magmatic activity took place east of the Andean arc front, which during the Late Cretaceous to early Paleocene (85−50 Ma) was located farther west, in the Central depression of the Antofagasta region (Boric et al., 1990). 0361-0128/98/000/000-00 $6.00

Incaic magmatism in the Escondida cluster began in the middle Eocene (~44 Ma), when left-lateral displacements along the Escondida fault and differential rotation between the San Carlos and Imilac blocks created the triangular Salar de Hamburgo depression (Fig. 7). The oldest intrusions are a group of 44 to 41 Ma pyroxene-biotite monzodiorites and pyroxene-hornblende granodiorites, emplaced within the Escondida shear lens to the north of Escondida (Marinovic et al., 1995; Richards et al., 2001; Urzúa, 2009). Together, they likely represent the roof of an underlying, partially eroded pluton nearly 20 km in diameter (Fig. 7). Most of these rocks, like the Los Picos complex and Pajonal diorite of the ChuquicamataEl Abra region, are barren, although geochronologic data and relationships between intrusive phases in the vicinity of the Chimborazo porphyry copper deposit indicate, according to

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Upper Eocene (39-38 Ma) hb dioritic porphyry intrusions Eocene (44-41 Ma) px-bt monzodiorites and px-hb granodiorites Upper Paleocene to Lower Eocene (59-53 Ma) volcanic sequences

Triassic (240-220 Ma) intrusive rocks Upper Paleozoic (300-270 Ma) basement

FIG. 7. Simplified geologic map of the area around Escondida, highlighting major regional faults and the different intrusive phases that form part of the Escondida intrusive cluster. Compiled and adapted from Marinovic et al. (1995), Richards et al. (2001), Urzúa (2009), Hervé et al. (2012), and field data from the authors (px = pyroxene, hb = hornblende, bt = biotite).

Hervé et al. (2012 ), that an early phase of copper mineralization probably occurred at ~41 Ma. The second episode of Eocene-Oligocene magmatism began with the emplacement of a closely spaced group of small intrusions distributed across the Escondida fault (Fig. 7). These more evolved, amphibole-bearing dioritic stocks, with U-Pb zircon ages of 39 to 38 Ma (Richards et al., 2001; Urzúa, 2009), intrude both the late Paleocene to early Oligocene volcanic rocks of the Escondida shear lens and the late Paleozoic basement units of the Imilac block (Fig. 7). Their distribution suggests that they could be apophyses of a larger pluton at depth that intruded along the Escondida fault. The slightly younger group of porphyry copper stocks include a series of multiphase, NE- to N-NE−trending, dikelike intrusions that were emplaced at or near the Escondida fault at 38 to 37 Ma; these include the deposits at Zaldívar, Escondida Norte, Escondida, and Pinta Verde, and, farther away, at Baker (Richards et al., 2001; Urzúa, 2009; Hervé et al., 2012; Fig. 7). 0361-0128/98/000/000-00 $6.00

The final event of Incaic magmatism in the Escondida cluster was related to the emplacement of the Escondida Este and Pampa Escondida deposits, immediately to the east of the Escondida fault (Fig. 7), between 36.0 and 34.5 Ma (Hervé et al., 2012). The mineralized porphyries of the Escondida cluster, with the exception, perhaps, of Chimborazo, postdate the earlier phase of sinistral faulting and block rotations along this segment of the Cordillera de Domeyko. Deformation seems to have begun at ~42 Ma (age of the base of the Loma Amarilla Formation; see above), although the lack of offset on any of the porphyry intrusions across the local fault strands (Panadero-Portezuelo fault) of the larger Escondida fault indicates that major along-strike fault activity had ceased by 38 Ma, as shown by the across-fault 38−37 Ma porphyry dikes (Hervé et al., 2012). Centinela Porphyry copper mineralization in the Centinela district (labeled “CE,” Fig. 2) occurs within a 25-km-wide, fault-bounded

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belt of Late Cretaceous to early Eocene volcanic rocks, located between the Paleozoic basement exposures of the Cordillera de Domeyko to the east and an early Cretaceous volcanic sequence in the Coastal Range to the west (Fig. 8). The Centinela district hosts one of the more recently discovered porphyry copper clusters in northern Chile. Although the occurrence of exotic copper mineralization at El Tesoro was known for a long time, the full potential of the district only began to be assessed in the mid-1990s (Perelló et al., 2010, and references therein). The district records again, a lengthy history (almost 80 m.y.) of magmatic activity, from the Early Cretaceous to the Eocene. The oldest plutonic rocks emplaced within the confines of the Centinela cluster comprise a group of Early Cretaceous olivine-pyroxene gabbros and hornblende-bearing quartz diorites, with U-Pb zircon and K-Ar ages between 124 and 100 Ma (Mpodozis et al., 1993b; Marinovic and García, 1999).

These rocks, emplaced within Jurassic marine limestones of the northern Chile back-arc basin (Fig. 8), are almost 100 km east of the Early Cretaceous magmatic front, which, as noted above, was situated at that time in the Coastal Range (Boric et al., 1990). Younger volcanic and intrusive events occurred in the Late Cretaceous, when a volcanic sequence (Quebrada Mala Formation) and a group of coeval 70 to 66 Ma pyroxene diorites to rhyolite porphyries and flow domes, dated (U-Pb zircon) between 70 and 66 Ma, were generated after the Andean arc front migrated eastward into the Centinela region (see Figs. 1b, 8). Volcanism continued after the CretaceousTertiary boundary deformation event, with eruptions from stratovolcanoes and small collapse calderas that were active between the early Paleocene (64 Ma) and the early Eocene (53 Ma). During this interval, a diverse group of epizonal intrusions composed mainly of pyroxene-biotite quartz diorite to monzodiorite (60 Ma) and hornblende-biotite granodiorite Sierra Limón Verde

69º00

Orión (44-41) as

Mirador (41-39) Llano (41) Esperanza (42-40) Llano fault

Telégrafo (42-40) Caracoles (42-41)

Esperanza fault

Centinela (45-44)

ra e er lc Si Du ua Ag

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Coronado fault

Penacho Blanco (42)

Los

Toro s

ult

fault

Las Lomas fault

0

Centinela

Sierr

a de

Sherezade (44-43)

l Buit

re fa

Las Lomas duplex

fault

Pilar (43) Polo Sur (42-41)

5

10 km

Upper Eocene (44-40 Ma) syntectonic, sedimentary and volcanic rocks

Undifferentiated Cretaceous granitoids Upper Cretaceous (78-66 Ma) sedimentary and volcanic sequences (Quebrada Mala fomation)

Eocene (44-40 Ma) px-hb monzodioritic to hb-bt granodioritic stocks and mineralized dacitic porphyry intrusions (Esperanza-Telégrafo and Centinela-Polo Sur) Paleocene (60-56 Ma) px-bt monzodiorites and rhyolitic porphyry intrusions Paleocene to Lower Eocene (64-53 Ma) volcanic rocks (Cinchado formation ) Lower Paleocene (65-64 Ma) diorites and dacitic porphyry intrusions

Lower Cretaceous (?) volcanic rocks Lower Cretaceous (124-100 Ma) ol-px gabbros to diorites and hb granodioritic porphyry intrusions Jurassic to Lower Cretaceous back-arc sedimentary and volcanic rocks Upper Triassic (210-200 Ma) volcanic and sedimentary rocks Upper Paleozoic (290-270 Ma) basement

Upper Cretaceous (78-68 Ma) px diorites and (minor) rhyolitic porphyry intrusions

FIG. 8. Regional geologic map of the Centinela cluster area. Note the 35-km-long NNE trend of 42−40 Ma porphyry copper deposits emplaced during earlier stages of the Incaic event. Multiple superimposed intrusive pulses and volcanic episodes between 120 and 40 Ma show a remarkable recurrence of magmatic events for >80 m.y. All ages are based on recently acquired U-Pb zircon data. (p = prospects, ol = olivine, bt = biotite, px = pyroxene). 0361-0128/98/000/000-00 $6.00

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(58−57 Ma) plus andesitic to dioritic porphyritic intrusions, were emplaced into the Mesozoic units and Paleogene volcanic edifices. Incaic magmatism and mineralization in the Centinela district occurred between 45 and 39 Ma (Mpodozis et al., 2009a; Perelló et al., 2010) and began, as revealed by numerous new U-Pb zircon ages in the intrusive rocks and Re-Os ages in molybdenite, ~12 to 10 m.y. after the termination of volcanism in the early Eocene. This event coincides with the mostly copper-barren early phase of Incaic intrusions at Chuquicamata-El Abra (45−42 Ma) and Escondida (44−41 Ma). The oldest Incaic intrusive rocks include a small group of 45 Ma pyroxene-biotite and quartz diorites yet, in contrast to Chuquicamata-El Abra and Escondida, at Centinela, numerous mineralized porphyry centers were emplaced between 44 and 39 Ma. They form, together with some barren stocks, a 40-km-long, N- to NEtrending belt, which includes at least 10 discrete intrusive complexes (Fig. 8). A syntectonic sequence of conglomerates and volcaniclastic sandstones, which accumulated at the same time as porphyry copper emplacement, comprises interbedded layers of dacitic block-and-ash deposits and tuffs with UPb zircon ages between 42 and 39 Ma. The oldest porphyry systems (45−43 Ma) occur along the southwest end of the belt, and the age decreases systematically to the northeast until reaching 39 Ma at the northeast edge of the porphyry trend (Fig. 8). The geometry of the porphyry complexes is controlled by their position relative to the main structural feature of the district, a 3- to 5-km-wide, N-S−trending fault zone that cuts obliquely across the porphyry belt. This zone of intense deformation constitute to the northern termination of the Sierra de Varas fault, which stretches for >250 km along the western border of the Cordillera de Domeyko (Mpodozis et al., 1993b; Soto et al., 2005; Figs. 2, 6a), and was active both during and after porphyry emplacement (Fig. 8). Porphyry deposits located west and east of this zone of concentrated deformation are largely undeformed. Copper mineralization in mineralized porphyry systems, located west of the fault zone, are related to subvertical, hornblende-biotite dacites dike swarms intruded into Paleocene volcanic/subvolcanic units (e.g., Centinela) or early Eocene rhyolitic dome complexes (Polo Sur, Perelló et al., 2010). The oldest deposits where emplaced at 45 to 44 Ma (Centinela) and 44 to 43 Ma (Shererezade) to be followed by the intrusion by several barren pyroxene-hornblende dioritic stocks and lacoliths dated at 43 Ma, although a porphyry copper system with the same age has been also recognized at Pilar. A new pulse of copper-bearing intrusions occurred, finally, between 42 to 41 Ma, at Polo Sur, while dacitic porphyries with a similar age (42 Ma) but apparently barren have been documented at Penacho Blanco (Fig. 8). Mirador, the youngest porphyry deposit recognized so far in the district (41−39 Ma; Mora et al., 2009) and located east of the fault zone (Fig. 8), is also structurally undisturbed. The copper mineralization, hosted within Jurassic marine limestones and evaporites (Mora et al., 2009) is associated with a group of multiphase, W- to NW-trending intrusions that, as in the older Centinela and Polo Sur deposits, appear to be subvertical. By contrast, deposits emplaced along the fault zone (Fig. 8) have intermediate ages of 42 to 40 Ma (Caracoles, 42−41 Ma; Telégrafo, 42−40 Ma; Esperanza, 42−40 Ma; Llano, 41 Ma; 0361-0128/98/000/000-00 $6.00

Perelló et al., 2004, 2010; Bisso et al., 2009; Münchmeyer and Valenzuela, 2009; Swaneck et al., 2009); they are all associated with tilted porphyry dike swarms. These deposits are emplaced into moderately to steeply dipping strata, which are disrupted by major postmineral faults that exhibit reverse, normal, and strike-slip displacement components (Figs. 8, 9). Although the widespread gravel cover makes it difficult to satisfactorily resolve the structural relationships within the whole district, the regional structure around the Esperanza and Telégrafo deposits (Fig. 8) comprises a long-wavelength, asymmetric, basement-cored anticline, bounded to the west by a moderately E-dipping yet unexposed thrust fault that was discovered during exploration drilling (Telégrafo fault; Perelló et al., 2004, 2010; Bisso et al., 2009; Münchmeyer and Valenzuela, 2009; Fig. 9). The hinge zone of the anticline is, in turn, sliced by two subvertical faults (Coronado and Llano faults; Figs. 8, 9) linked to the N-S−trending regional fault zone. Figure 9 includes a west-east structural section across the Esperanza deposit, where mineralization is associated with a group of easterly inclined porphyry dikes emplaced within the ~40° to 50° W-dipping Triassic to Upper Cretaceous strata that form the frontal limb of the anticline. This panel, containing in part mineralized and altered host rocks to the porphyry deposits, is upthrown to the west, along the Telégrafo fault, over barren, unaltered mid-Eocene (42−39 Ma) sedimentary and volcanic rocks that accumulated when porphyry intrusions were being emplaced at depth. The tilted, frontal-limb panel of the anticline is, in turn, bounded to the east by the subvertical Esperanza fault (Fig. 9) that places Jurassic limestones over Late Upper Cretaceous strata. The rectilinear fault trace and the mismatch of the lithology and age of the Late Cretaceous volcanic rocks across the Esperanza fault show it includes an important component of strike-slip movement, although the precise age of deformation and genetic links between both faults remain to be determined. The Llano and Coronado faults are, however, as shown in Figure 9, younger faults that are superimposed over the Telégrafo-Esperanza system, which exhibits both left-lateral and large, down-to-the-east components of displacement, part of which has a late Miocene or younger (56%) with abundant hydrous mineralogy, dominated by hornblende-bearing granodiorites and dacites; these intrusions show geochemical and isotopic signature (SiO2 >56%, Sr >400 ppm, high Sr/Y ratios, low HREE contents, high La /Yb ratios, 87Sr/86Sr 50 km (Ramos et al., 2004) and enhanced subduction erosion. Contamination of the asthenosphere through subduction of fore-arc crust created favorable conditions to produce water-rich mafic melts with high sulfur and metal contents; these melts had the capacity to ascend and evolve within an upper crustal magma chamber to generate large porphyry copper deposits. Concluding Remarks There are few studies that consider the relationships between the regional-scale tectonic evolution of the Andes and the formation of giant Cenozoic porphyry copper deposits. However, it is apparent that these deposits formed during critical moments in the tectonic evolution of the Andean margin. The emplacement of the large middle Eocene to early Oligocene porphyry copper intrusions in northern Chile seems to be associated with the formation of the Bolivian orocline during the Incaic event, which was the result of an unusual combination of factors. One critical factor was the acceleration of the absolute westward motion of South America concurrent with strong mechanical coupling between the South American and Farallon plates at a time when the rate of ocean-crust production in the eastern Pacific was very high. Bending of the Chilean margin during the Incaic event activated the Domeyko fault system in northern Chile and triggered the accompanying crustal thickening, slab shallowing, and increased subduction erosion. Volcanism virtually ceased and favorable tectonomagmatic conditions (i.e. enhanced, subduction erosion, crustal thickening, lower crust dehydration) permitted the formation of fertile hydrous magmas, while the transpressional and/or compressional upper plate tectonic regime contributed to the establishment of long-lived, upper-crustal magma chambers from which concentrating copper evolved, mostly below the Domeyko fault system. Younger, late Miocene to early Pliocene porphyry copper deposits of central Chile and contiguous Argentina were emplaced after inversion and collapse of the extensional, intra-arc Abanico basin. The basin evolved between the late Eocene and early Miocene when a relatively stable position of South America over the mantle, linked to weak interplate coupling, permitted fast subduction of the Nazca plate under the Andean margin. Acceleration of the westward motion of South America relative to the mantle reference frame at 20 Ma induced contractional deformation, accompanied by 0361-0128/98/000/000-00 $6.00

crustal thickening and eastward migration of the magmatic front. At the same time, the subduction angle shallowed, leading to the formation of a flat-slab region between 27° and 33° S as the Juan Fernández Ridge was being subducted beneath the western edge of South America. These changes again created favorable conditions for the formation of fertile hydrous magmas. The relationships described above demonstrate that the concentration of huge porphyry copper deposits in the Chilean Andes resulted directly from the tectonic evolution of the margin and indicate that a tectonic trigger is essential for the formation of giant porphyry coppers systems. Acknowledgments This contribution is the result of long years of work with many colleagues at the Chilean Geological Survey, Antofagasta Minerals, and various universities both in Chile and abroad. We are especially grateful to Sue Kay, Andy Tomlinson, Terry Jordan, Cesar Arriagada, Moyra Gardeweg, Rick Allmendinger, Victor Ramos, Pierrick Roperch, Francisco Camus, Stephen Matthews, Nicolás Blanco, Francisco Hervé, Reynaldo Charrier, Carlos Münchmeyer, Ricardo Muhr, José Cembrano, Carlos Arévalo, and many others who, for lack of memory, we are here unable to mention. We thank Jeff Hedenquist, Dick Sillitoe, José Perelló, and Francisco Camus for pushing us through this endeavor, and Antofagasta Minerals for providing time and support for the writing. Francisco Morales helped with the preparation of the figures. Victor Ramos, Sue Kay, José Perello, Jeff Hedenquist, and Dick Sillitoe carefully edited the manuscript and made numerous suggestions that helped to greatly improve earlier versions of the manuscript. REFERENCES Amilibia, A., and Skarmeta, J., 2003, La inversión tectónica en la Cordillera de Domeyko en el norte de Chile y su relación con la intrusión de sistemas porfíricos de Cu-Mo [ext.abs.]: X Congreso Geológico Chileno, Concepción, Extended Abstracts, v. 2, p. 1−7. Amilibia, A., Sabat, F., McClay, K.R., Muñoz, J.A., and Chong, G., 2008, The role of inherited tectono-sedimentary architecture in the development of the central Andean mountain belt: Insights from the Cordillera de Domeyko: Journal of Structural Geology, v. 30, p. 1520−1539. Anderson, M., Alvarado, P., Zandt, G., and Beck, S., 2007, Geometry and brittle deformation of the subducting Nazca Plate, Central Chile and Argentina: Geophysical Journal International, v. 171, p. 419–434. Arriagada, C., Roperch, P., Mpodozis, C., and Cobbold, P.R., 2008, Paleogene building of the Bolivian orocline: Tectonic restoration of the Central Andes in 2-D map view: Tectonics, v. 27, TC6014, 14 p., doi:10.1029/2008 TC002269. Arriagada, C., Mpodozis, C., Yañez, G., Charrier, R., Farías, M., and Roperch, P., 2009, Rotaciones tectónicas en Chile central: El oroclino de Vallenar y el “megakink” del Maipo [ext.abs.]: XII Congreso Geológico Chileno Santiago, Extended Abstracts (CD), 4 p. Astini, R.A., Benedetto, J.L., and Vaccari, N.E., 1995, The early Paleozoic evolution of the Argentine Precordillera as a Laurentian rifted, drifted, and collided terrane: A geodynamic model: Geological Society of America Bulletin, v. 107, p. 253–273. Audin, L., Hérail, G., Riquelme, R., Darrozes, J, Martinod, J., and Font, E., 2003, Geomorphological markers of faulting and neotectonic activity along the western Andean margin, northern Chile: Journal of Quaternary Science, v. 18, p. 681–694. Ballard, J., Palin, J.R., Palin, M., Williams, I., and Campell, I., 2001, Two ages of porphyry intrusion resolved for the super-giant Chuquicamata copper deposit of northern Chile by ELA-ICP-MS: Geology, v. 29, p. 383–386. Barra, F., 2011, Assessing the longevity of porphyry Cu-Mo deposits: Examples from the Chilean Andes: SGA Biennial Meeting, 11th, Antofagasta, Proceedings, p. 399−401.

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