ch 14 mpodozis , cornejo , 2012, cenozoic tectonics and porphyry copper systemes of the chilean...

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IntroductionTHE STUDY of the tectonic setting of porphyry copper depositsis fundamental to understanding their genesis (e.g., Sillitoe,1998; Kay and Mpodozis, 2001; Cooke et al., 2005; Sillitoeand Perelló, 2005; Richards, 2009, 2011a; Tosdal et al., 2009).Some Cenozoic porphyry copper deposits are known to haveformed during or shortly after continent-continent, conti-nent-island arc, or island arc-island arc collisions in the Hi-malayas-Tibet, the Kerman arc in Iran, and Papua NewGuinea (Solomon, 1990; Zenqiang et al., 2003; Shaifei et al.,2009). A Paleozoic example of this type of deposit may beOyu Tolgoi in Mongolia (Perelló et al., 2001). In contrast,other large porphyry deposits such as Bingham Canyon in thewestern United States formed during the earliest stages of

Basin and Range extension in the Eocene, far inland from thePacific margin of North America (Kloppenburgh et al., 2010).

Noncollisional porphyry copper deposit examples in sub-duction-related arc settings include those from the Chagaibelt in Pakistan, the Laramide porphyry copper province ofthe western United States and northern Mexico (Lang and Ti-tley, 1998; Valencia-Moreno et al., 2007; Perelló et al., 2008)and the Central Andes province, which host some of thelargest 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 ofa noncollisional orogenic system (e.g., Jordan et al., 1983),where subduction of Pacific oceanic crust beneath SouthAmerica has been active for the past 570 m.y. (Cawood,2005). Nevertheless, the largest porphyry copper deposits arethe result of anomalous magmatic systems that developed

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

AbstractSubduction 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-earlyOligocene (43−32 Ma) and late Miocene-Pliocene (10−6 Ma) in north and central Chile. Although the tectonicsetting of the Paleocene porphyry deposits is still poorly understood, those of the northern Chile Eocene-Oligocene belt were emplaced along the margin-parallel Domeyko fault system, where active compressionaland/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 alonga mechanically weak zone of the Central Andean crust inherited from the Paleozoic. Deformation occurredduring an episode of accelerated westward absolute motion of the South American plate, which coincided withvery high rates of oceanic crust production in the eastern Pacific. The slow South American-Farallon conver-gence rates recorded for the Eocene-Oligocene suggest, however, that strong interplate coupling existed dur-ing that time. This permitted the transfer of horizontal stresses and large-scale deformation of the Andean mar-gin, creating a favorable scenario for the generation and emplacement of porphyry copper magmas along theDomeyko fault system.

The younger, Miocene-Pliocene porphyry copper deposits of central Chile-Argentina were emplaced in adifferent setting, after the initiation of compressional deformation within a volcano-tectonic depression (Aban-ico basin) that evolved during another, late Oligocene to early Miocene, period of increased East Pacificoceanic crust production. Nevertheless, in contrast to the Eocene-Oligocene situation in northern Chile, therelatively stationary position of the South American plate compared to the mantle reference frame and weakinterplate coupling that permitted rapid subduction, increased volcanism, and overriding plate extension. Tec-tonic inversion of the basin and compressional deformation along with crustal thickening and mountain build-ing began at around 20 m.y. ago as interplate coupling increased when the westward motion of South Americaaccelerated and the Nazca-South America convergence velocity decreased in the mid-Miocene. Compressionwas accompanied, as during the Eocene-Oligocene in northern Chile, by slab shallowing and increased fore-arc subduction erosion.

In both cases, the largely structurally controlled, syn- to post-tectonic porphyry copper deposits are associ-ated with long-lived magmatic systems that were active for more than 10 m.y. In northern Chile, the depositsoccur as parts of discrete intrusive clusters that comprise a suite of precursor plutons emplaced during multi-ple events since the Cretaceous. Porphyry copper mineralization is linked to multistage, amphibole-bearing in-trusions of intermediate composition derived from hydrous, oxidized magmas with adakitic geochemical sig-natures. These intrusions appeared when crustal thickness increased to a critical threshold in the course ofdeformation. Production of magmas with high metal-carrying capacity was fostered as fluids were liberatedwhen amphibole became unstable and was destroyed as the crust thickened. At the same time, source regionswithin the mantle were contaminated by hydrated fragments of fore-arc continental crust, as the result of en-hanced subduction erosion during peaks of compressional deformation.

† Corresponding author: e-mail, [email protected]

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

during short periods at specific locations within the Andeanorogen. These include the Paleocene to early Eocene (66−52Ma) and middle Eocene to early Oligocene (43−32 Ma) beltsin southern Peru and northern Chile, and the late Miocene toearly Pliocene (10−5 Ma) porphyry systems in central Chileand contiguous Argentina (Perelló et al., 2003a; Sillitoe andPerelló, 2005). In this contribution, with emphasis on theChilean belts, we will try to demonstrate how major Cenozoictectonic events along the central Andean convergent margin,prompted by large-scale reorganizations of the global tectonicsystem, were the main triggers for the formation of large por-phyry copper deposits.

Pre-Andean History: From Rodinia Dispersal to Pangea Breakup

The western margin of South America underwent mag-matic and tectonic activity at least since the late Neoprotero-zoic breakup of Rodinia (800−700 Ma), when the separationof Laurentia from Gondwana produced the opening of theproto-Pacific (Iapetus) ocean (Dalziel, 1997). East-directedsubduction of newly formed ancestral Pacific crust belowwestern Gondwana began at ~570 Ma and was fully activealong the proto-Andean margin by 485 to 465 Ma (Pankhurstet al., 1998; Cawood, 2005; Chew et al., 2007). Plate conver-gence in the Central Andes region during the Ordovician toDevonian included the progressive collision and accretion ofa group of tectonostratigraphic terranes of Laurentian and/orGondwanan affinities (e.g., Ramos et al., 1986; Astini et al.,1995) against the western South American margin. Terraneamalgamation contributed to the formation of the accre-tionary Terra Australis orogen, which extended for more than18,000 km along the Pacific margin of Gondwana from Aus-tralia to South America (Cawood, 2005). The accretionarystage was followed, in the Central Andes, by the buildup of alate Carboniferous to Early Permian (320? -280 Ma) supra-subduction magmatic arc on top of the newly accreted terranes,as well as the development of an outboard fore-arc subduc-tion complex that extended for more than 1,000 km along theChilean 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 andmostly felsic volcanic rocks, including the Choiyoi large ig-neous province in Chile and Argentina and the Mitu Group insouthern Peru (Kay et al., 1989, Sempere et al., 2002), wereemplaced along the western South American margin. Althoughgeochronologic and geochemical data are still incomplete,several competing hypotheses, such as normal or oblique sub-duction, postcollision extension-driven crustal melting, slabbreakoff or slab shallowing, have been proposed to explainthe prevailing tectonic regime along different segments of theAndean margin at that time (Mpodozis and Kay, 1992; Kleimanand Japas, 2009; Ramos and Folguera, 2009, and referencestherein). From the Middle Triassic to earliest Jurassic (240−190 Ma), rifting associated with the incipient stages of Pangeadispersal (e.g., Veevers, 1989), accompanied by a decreasingvolume of bimodal magmatism, seems to have occurredalong the western margin of South America (e.g., Ramos andKay, 1991; Franzese and Spalletti, 2001; Rosas et al., 2007).

Diverse, yet basically subeconomic, porphyry copper depositsformed during these events in northern Chile and along theFrontal Cordillera in west-central Argentina (Sillitoe, 1977;Sillitoe and Perelló, 2005; Cornejo et al., 2006; Munizaga etal., 2008).

Jurassic to Early Eocene Tectonics and Metallogeny of the Central Andes

After the Triassic rifting event, subduction was reestab-lished in northern Chile and southern Peru during the EarlyJurassic when a new magmatic arc developed west of the extinct late Paleozoic arc front. Since then, subduction hasproceeded uninterrupted to date. Initial Jurassic to EarlyCretaceous arc magmatism occurred under extensional con-ditions that permitted the formation of a series of intercon-nected back-arc basins to the east of the main arc, which wereprogressively filled with marine and continental sedimentarystrata (Mpodozis and Ramos, 1989, 2008). Transpressionaldeformation along the arc axis created the intra-arc Atacamafault system in northern Chile (Scheuber and González,1999) and was accompanied in the Early Cretaceous by theemplacement, in northern Chile, of some porphyry copperdeposits 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 ratesduring the global mid Cretaceous superplume event (Larson,1991) produced an upsurge in volcanism along the Andeanmargin, accompanied by intra-arc extension and transtensionwhich fostered iron oxide-copper-gold (IOCG)−type mineral-ization between 120 and 100 Ma in northern Chile and south-ern Peru (Marschik and Fontboté, 2001; Sillitoe, 2003; Silli-toe and Perelló, 2005; Chen et al., 2010). Small, low-grade,gold-rich porphyry copper deposits such as Andacollo (104Ma), Domeyko-Dos Amigos (108−104 Ma), and Pajonales (97Ma) were emplaced under extensional conditions during thesame general period in north-central Chile (Sillitoe andPerelló 2005; Maksaev et al., 2010).

The extensional and transtensional conditions that domi-nated early Andean subduction ended in the early Late Cre-taceous, when the back-arc basins were tectonically inverted(Mpodozis and Ramos, 1989; Tomlinson et al., 2001a). Theshift to a more contractional, subduction-related regime oc-curred together with the accelerated westward drift of SouthAmerica, in response to the final opening of the Atlantic(Russo and Silver, 1996; Somoza and Zaffarana, 2008). Sub-sequently, the coastal magmatic arc was abandoned (Fig. 1b)and the magmatic front jumped to the east in the Late Creta-ceous, where it remained relatively stationary until the earlyEocene. Abrupt shifts in the magmatic front such as this hasbeen accompanied throughout the Andean history, by tran-sient 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 etal. (2005) suggested that changes of this type reflect mantlecontamination from fore-arc crust removed during enhancedsubduction erosion processes associated with major contrac-tional events along the Andean margin.

In northern Chile, the Cretaceous-Tertiary boundary wasmarked by another short pulse of contractional deformation,the K-T event of Cornejo et al. (2003). Paleocene to early

330 MPODOZIS AND CORNEJO

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Eocene magmatism included the emplacement of a porphyrycopper belt between southern Peru (Cerro Verde, 61 Ma;Quellaveco, 54 Ma; Cuajone, 52 Ma) and Cerro Colorado (52Ma), Spence (57 Ma), and Relincho (61 Ma) in northern Chile(Sillitoe and Perelló, 2005). Structural and geochemical datagathered by the authors suggest that these deposits in north-ern Chile formed in a neutral stress to mildly extensional arc

built on a relatively thin crust over a steep subduction zone.This is consistent with the very low rates of and oblique con-vergence between the South American and Farallon plates(Pardo-Casas and Molnar, 1987), and also with within-plategeochemical signatures of Paleocene to early Eocene rocks inthe Inca de Oro-El Salvador region (Cornejo and Matthews,2001).

CENOZOIC TECTONICS AND PORPHYRY COPPER SYSTEMS OF THE CHILEAN ANDES 331

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Late CretaceousPeruvian Event KT

Event

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Eoc-E Olig.Incaic intrusions

Post Incaic intrusionsIncaic Event

90 80 70 60 50 40 30 20 10 0AGE (Ma)

26 Ma

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TRANSEC T(20 - 23ºS)

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(c) (d)

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FIG. 1. (a). Main morphotectonic units of the central Andes, between 15° and 30° S. FA = modern fore-arc zone, in-cluding 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 volcaniczone (CVZ); EC = Eastern Cordillera; SP = Sierras Pampeanas; SA = sub-Andean fold-and-thrust belt. (b). Relationship be-tween age and longitude (distance to the trench) for <200 Ma volcanic and intrusive rocks of the Central Andes (19°−28° S;Haschke et al., 2002, CVZ = modern Central volcanic zone of the Andes). (c). Longitude vs. age for Cenozoic intrusive andvolcanic rocks at 20° to 23° S (Iquique to Antofagasta transect; Trumbull et al., 2006). The ~32 to 26 Ma gap may be relatedto a transient episode of flat subduction produced as a consequence of the Incaic tectonic episode. The east to west migra-tion of the magmatic front during the Miocene is considered to record late-stage slab steepening (Kay et al., 1999; Kay andCoira, 2009). (d). Geochemical changes since the Late Cretaceous near 26° S (Copiapó-El Salvador region; data fromCornejo and Mathews, 2001). Note the short-lived peaks in the Sm/Yb ratio during major tectonic events, superimposed overa more subdued long-term trend to increased values. Peak values may reflect contamination of the mantle magma source re-gions as a result of massive removal of fore-arc continental crust during periods of enhanced subduction erosion (Kay andMpodozis, 2002; Kay et al., 2011).

Middle Eocene to Early Oligocene Tectonics of the Central Andes

The Domeyko fault system

One of the most relevant tectonic and metallogenicepisodes in the Central Andes correlates with the middleEocene to early Oligocene (45−33 Ma) Incaic tectonic event(Noble et al., 1979; Maksaev and Zentilli, 1988; Mpodozis andPerelló, 2003; Sillitoe and Perelló, 2005); during this period,bending of the continental margin generated the Bolivianorocline and the Domeyko fault system along the Pre-cordillera of northern Chile. The Domeyko fault system (Fig.2) is a >1,000-km-long, 40- to 60-km-wide, orogen-parallelzone of deformation composed of a complex array of strike-slip, normal, and reverse faults, together with thin- and thick-skinned folds and thrusts, which extends along the Cordillerade Domeyko (also known as Precordillera) in northern Chilebetween 20° and 27° S (e.g., Reutter et al., 1991, 1996;Cornejo et al., 1997). Some authors (e.g., Amilibia andSkarmeta, 2003; Amilibia et al., 2008) proposed that most ofthese faults and folds initiated during Late Cretaceous as aconsequence of the inversion of normal faults inherited fromthe Mesozoic back-arc extension. However, others (e.g., Tom-linson et al., 2001a; Mpodozis et al., 2005) interpreted thatthe Andean back-arc basins were first inverted during theearly Late Cretaceous to form a proto-Cordillera deDomeyko, while a second main tectonic pulse along theDomeyko fault system, coincident with the Incaic event, pro-duced its final uplift (Reutter et al., 1991, 1996; Scheuber andReutter 1992; Tomlinson et al., 1993; Maksaev and Zentilli,1999). Parts of the Domeyko fault system were subsequentlyreactivated during the Oligocene and the Quaternary (Tom-linson and Blanco, 1997a, b; Audin et al., 2003; Soto et al.,2005).

The kinematics of the middle Eocene to early Oligocenedeformation along the Domeyko fault system is a matter ofcontroversy; evidence for both left- and right-lateral displace-ments, including reversal in the sense of shear, has been re-ported along different parts of the faulted domain (Reutter etal., 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 Domeykowas exhumed between 40 and 30 m.y. ago (Maksaev and Zentilli, 1999; Nalpas et al., 2005) in association with surfacetectonic uplift and profound erosion, the products of whichaccumulated in syntectonic basins east and west of the area ofdeformation (Mpodozis et al., 2005; Hong et al., 2007; Wot-zlaw et al., 2011).

Origin of the Domeyko fault system

At first glance, the Domeyko fault system could be consid-ered as a trench-linked fault system (Woodcock, 1986) thatnucleated in the thermally weakened crust of the middleEocene to early Oligocene magmatic arc of northern Chileduring a period of suggested fast Eocene oblique conver-gence between the Farallon and South America plates (Pardo-Casas and Molnar, 1987; Somoza, 1998). However, when re-cent paleomagnetic and structural studies are taken intoaccount it becomes apparent that tectonic activation of theDomeyko fault system during the Incaic episode is essentially

a consequence of the formation of the sharp bend of the west-ern South American margin, known as the Arica elbow or Bo-livian orocline (Fig. 3). Paleomagnetic studies have been es-sential in obtaining a more constrained view of thedeformational history of this segment of the Central Andesand support the tectonic model for orocline formation firstproposed by Isacks (1988).

Figure 3a is a simplified regional map showing the distrib-ution of paleomagnetic (declination) vectors measured for theCentral Andes. Importantly, independent of age, Mesozoicand Paleogene rocks have been rotated up to 50°. In contrast,rotations measured in Miocene and younger rocks (<18−11Ma in northern Chile; <20 Ma in southern Peru) are negligi-ble, suggesting that most of the rotations were acquired dur-ing a single Paleogene episode of deformation (Roperch et al.,2006, 2011; Arriagada et al., 2008, and references therein).Rotations are counterclockwise in southern Peru (Domain B;Fig. 3a), clockwise in northern Chile south of Antofagasta(domain D), and almost nonexistent in the intermediate re-gion (domain C) between Antofagasta and Arica (Taylor et al.,2005; Arriagada et al., 2008). The magnitude of the counter-clockwise rotations decreases significantly at the Abancay De-flection in central Peru (domain A; Fig. 3a), the latter breakbeing interpreted as a zone of intense Eocene-early Oligoceneleft-lateral shear along the boundary between the Arequipaand Paracas basement terranes (Ramos, 2009; Roperch et al.,2011; Fig. 4b). In northern Chile, clockwise rotations de-crease progressively south of Antofagasta and essentially dis-appear near Vallenar (28°30' S) upon entering the essentiallynonrotated domain E (Fig. 3a), which extends southward tothe latitude of Santiago (33° S).

In his landmark paper, Isacks (1988) proposed that the ob-served paleomagnetic rotations and the formation of the sea-ward concave Bolivian orocline are related to along-strikevariations in the amount of late Cenozoic shortening pro-duced during contractional deformation focused along a mechanically and thermally weakened zone located in theoverriding South American plate. Recent structural studiesindicate, however, that the bulk of the shortening (~60%), atand near the Arica bend (13°−19° S; Fig. 3), is pre-Neogenein age and occurred between 40 and 20 Ma (Lamb, 2001;Müller et al., 2002; Kley et al., 2005; McQuarrie, 2006). In-caic shortening concentrated within the Eastern Cordillera ofBolivia where >12 km of terrigenous sedimentary strata accu-mulated in a Paleozoic marine basin above highly attenuatedcontinental crust (Figs. 1, 3). The amount of horizontal short-ening reaches a maximum near the axis of the orocline, wherethe Paleozoic sedimentary sequence is thickest, and decreasessymmetrically along-strike (Oncken et al., 2006; Gotberg etal., 2010, and references therein, Fig. 4a) as the Paleozoicsedimentary rocks of the Eastern Cordillera become thinnerand give way to metamorphic and crystalline rocks in bothPeru (Cordillera de Marañón) and northwest Argentina (Sier-ras Pampeanas, see Fig. 3).

Arriagada et al. (2008) attempted to remove the combinedeffects of accumulated horizontal shortening and block rota-tions. Figure 3b shows their preferred solution for the restoredshape of the continental margin (Peru-Chile trench) at 45Ma, before the formation of the Bolivian orocline. As shownin Figure 4b, the ca. 30° E azimuth of the Farallon-South


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FIG. 2. Sketch map of the Cordillera de Domeyko (or Precordillera) and the Domeyko fault system, showing main faultstraces, exposures of Paleozoic basement, clusters of Eocene plutonic rocks (YP = Yabricoya-La Planada intrusive cluster;QBC = Quebrada Blanca-Collahuasi; CA = Chuquicamata-El Abra; CE = Centinela, LE = La Escondida; SE = Sierra Ex-ploradora-Juncal; PS = Potrerillos- El Salvador; RF = Río Figueroa) and the locations and ages (in parentheses, Ma) ofEocene-early Oligocene porphyry copper deposits. More detailed maps of CA, LE, and CE clusters are shown in Figures 5,7, and 8. Based on the 1:1,000,000 geologic map of Chile (SERNAGEOMIN, 2002).


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(200

8).

America Eocene to Oligocene convergence vector calculatedby Somoza (1998, Fig. 4b) was nearly orthogonal to the (re-stored) NW-SE−trending section of the margin along thecenter and northern limb of the Bolivian orocline. Neverthe-less plate convergence was, at the same time, highly obliquealong the southern limb of the orocline fostering margin-parallel shear, and (theoretically dextral) strike-slip faulting innorthern Chile where the Domeyko fault system was formed(Fig. 4b). Bending of the margin seems to have been accom-panied at the southern limb of the Bolivian orocline by whole-sale NE-directed crustal flow (Fig. 3b), which is also requiredto explain the excess orogenic volume and crustal thicknessbelow the Altiplano-Puna reported by Kley and Monaldi(1998) and Hindle et al. (2005).

Mass transfer, likely associated with lower crustal flow to-ward the center of the orocline (Hindle et al., 2005), increasesthe possibility of strike-slip faulting along the Domeyko faultsystem. Continued displacement was likely initially blockednear the orocline axis where deformation was dominated bymargin-normal contraction (Figs. 3b, 4b). As suggested byMcQuarrie (2002) and Boutelier and Oncken (2010), masstransfer toward the core of the orocline implies crustal thin-ning and stretching in central Chile, which may have inhib-ited mountain building south of 28° S. Crustal thinning and amore extensional tectonic regime prevailed in that region(south of 28° S) during the Eocene to early Miocene (e.g.,Jordan et al., 2001; Charrier et al., 2002). These contrastshelp to explain the termination of the Incaic porphyry copper

belt at approximately 30° S and the possibly segmented na-ture of the belt along its strike length between southern Peruand central Chile (Mpodozis and Perelló, 2003; see below).

Middle Eocene to Early Oligocene Porphyry CopperProvince of Northern Chile

Overview

Middle Eocene to early Oligocene porphyry copper de-posits of the Central Andes were emplaced contemporane-ously with the Incaic tectonic event, when the entire Andeanmargin was being reshaped during the formation of the Aricabend. Mineralized centers occur in Peru near the eastern endof the Abancay Deflection (Andahuaylas-Yauri cluster; Perellóet al., 2003a; Fig. 3a), although the vast majority are locatedin Chile along the Domeyko fault system (Sillitoe and Perelló,2005; Figs. 2, 3a). El Morro, the southernmost porphyry copper-gold deposit of economic importance (Perelló et al.,1996), is located where the rotated and thickened southernlimb of the Bolivian orocline (domain D; Fig. 3a) terminatesand merges with the nonrotated central Chile domain E (seebelow). Most porphyry copper deposits were emplaced alongthe Domeyko fault system at long-lived zones of focused mag-matism, which in some cases were active well before theEocene. They occur as parts of discrete intrusive clusters separated by large barren areas where only the Paleozoicbasement and back-arc basin sedimentary cover is exposed(Fig. 2).


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Initial (non rotated)Domeyko fault

system

45 Ma Trench

PARACAS

TER

RAN

E

AREQUIPA

TERRANE

0 Ma Trench

BrazilianShield

Shear zone along

terrrane boundary

Mechanicallyweak crust(Paleozoicsedimentaryrocks

Fold andThrust

Belt

Domeykofault system

Orthogonal

convergence

Orthogonal

convergence

Oblique

convergence

(b)

?

21ºS

(a)

N S

Distance from orocline axis

1500 1000 500 0 500 1000 1500 km

Tecto

nic

sh

ort

en

ing

(km

)

Latitude (ºS)

0º 10º 15º 20º 25º 30º 35º

400

300

200

100

1

2

4

3b

3a

FIG. 4. (a). Different horizontal tectonic shortening estimates for the Central Andes, showing how values decrease sym-metrically north and south of the orocline axis. 1 = Neogene deformation in the Subandean belt (Oncken et al., 2006); 2 =total shortening (Peloegene + Neogene, Oncken et al., 2006); 3a = shortening needed to accommodate crustal area assum-ing initial crustal thickness of 40 km; 3b = same, but considering 35 km as initial thickness (Gotberg et al., 2010); 4 = short-ening needed to balance paleomagnetically determined rotations (Arriagada et al., 2008). (b). Tectonic sketch showing howoblique convergence along the southern limb of the orocline may drive strike-slip displacements along the Domeyko faultsystem at the beginning of the Incaic event. The general scheme comes from figures taken from Isacks (1988) and Lamb(2001). Reverse faults prevailed north of 21° S as a consequence of the original N-NW trend of the continental margin. Noteleft-lateral shear along the Abancay Deflection along the boundary between the Arequipa and Paracas terranes.

The structural control of the porphyry systems is strikinglydiverse (cf. Sillitoe and Perelló, 2005). At Chuquicamata, por-phyry copper-related intrusions were syntectonically emplacedalong an extensional jog in an active dextral strike-slip faultsystem (Lindsay et al., 1995); at Potrerillos, intrusions relatedto porphyry copper deposits ascended along subvertical, left-lateral strike-slip faults and migrated laterally near the surfacealong an active low-angle thrust (Tomlinson et al., 1993;Niemeyer and Munizaga, 2008). In contrast, porphyry stocksrelated to mineralization at Escondida or El Abra-Fortunawere emplaced along premineralization reverse and strike-slipfaults formed during earlier stages of the Incaic event (Dilleset al., 2011; Hervé et al., 2012). Other mineralized intrusionswere cut and displaced by late-stage faults (Esperanza-Telé-grafo, Chuquicamata; Dilles et al., 1997; Tomlinson et al.,1997a, b).

Not all porphyry copper deposits are, however, obviouslyrelated to major Domeyko fault splays (Sillitoe and Perelló,2005), although the deposits of the Collahuasi-QuebradaBlanca cluster, which crop out to the east of the main faultsystem (Fig. 2), seem to have been emplaced over a basementdiscontinuity marked by regional isotopic changes in Neo-gene magmatic rocks (Mamani et al., 2010). At El Salvadorand Polo Sur, 42 to 41 Ma porphyry copper-related intrusionsare encapsulated in preexisting, up to 10-m.y. older, caldera-related rhyolite domes, indicating that the middle Eocene toearly Oligocene magmas effectively reused the same conduits(Cornejo et al., 1997).

These differences in the structural controls of porphyrycopper deposits can be attributed to the changing tectonicconditions along different segments of the Domeyko faultsystem during the >10-m.y. Incaic event. Camus (2003) rec-ognized three temporal groupings of mineralized intrusionsalong the Domeyko fault system at 43 to 42, 39 to 37, and 36to 33 Ma.

In the Chuquicamata, El Abra, and Quebrada Blanca-Col-lahuasi districts, where the three generations of intrusionsoccur, only the two youngest were fertile in copper (Campbellet al., 2006; Maksaev et al., 2009; Dilles et al., 2011), whereasin the Escondida district all three events are related to mineralization (Hervé et al., 2012). In contrast, most of the copper-bearing intrusions recognized in the Centinela districtseem to have been emplaced during the early event. In orderto illustrate the changing nature of the diverse porphyry clus-ters and explore the links between mineralized centers andthe evolution of the Domeyko fault system, the salient geo-logic features of three porphyry copper districts, Chuquica-mata-El Abra, Escondida, and Centinela (Figs. 2, 5), are ex-amined below.

Chuquicamata-El Abra

The Chuquicamata-El Abra intrusive cluster (Fig. 5a) is sit-uated along the northern Domeyko fault system, near theboundary between paleomagnetic domains C and D (Fig. 3a).The geology of the region records superimposed tectonic andmagmatic events beginning in the Mesozoic. In this region,the Precordillera (Cordillera de Domeyko) is formed by twofault-bounded, N-S−trending basement ranges. The westernrange, Sierra de Moreno, is a thick-skinned block uplifted inthe early Late Cretaceous (~85−84 Ma), during the inversion

of the Mesozoic back-arc basin (Ladino et al., 1997; Tomlin-son et al., 2001a). It is bounded to the west by high-angle,west-vergent reverse faults that place Neoproterozoic to Pa-leozoic basement units on top of Jurassic sedimentary strata(Fig. 5a). The east-vergent Arca reverse fault that bounds theeastern side of the Sierra de Moreno (Fig. 5a) has been in-terpreted as a reactivated normal fault, which formed nearthe eastern edge of the Mesozoic back-arc basin (Tomlinsonet al., 2001a; Fig. 5a). Syn- to post-tectonic red beds (Tolarand Tambillos Formations; Fig. 5a) were shed to the east andwest of the uplifting block during the Late Cretaceous. Afterdeformation, volcanic rocks, which unconformably cover thebasement units and Mesozoic sedimentary strata at Sierra deMoreno, developed in two separate episodes during the latestCretaceous (Cerro Empexa and Quebrada Mala Formations)and early to middle Eocene (Icanche Formation; Fig. 5a;Tomlinson et al., 2001a, 2010).

The eastern, Sierra del Medio basement block (Fig. 5a) wasuplifted between 43 and 38 Ma during the Incaic event (Tom-linson et al., 1997a) along a new set of west- and east-vergent,high-angle reverse faults. As in other regions of northernChile, volcanism in the Chuquicamata-El Abra region sharplydiminished at this time; however, a syntectonic sedimentarysequence (Sichal Formation; Fig. 5a) that accumulated in anarrow basin between Sierra de Moreno and Sierra delMedio contains a few intercalations of tuffs and volcanic brec-cias that yield K-Ar and Ar/Ar ages between 43 and 36 Ma(Tomlinson et al., 2001a). The Incaic deformation also includesa strike-slip component that produced segmented N-NE− toNE-trending, dextral strike-slip faults (e.g., the Mesabi faultin Fig. 5a; Tomlinson et al., 1997a). These faults becamemore important in the southern part of the area nearChuquicamata (Fig. 5a), in accordance with increasing plateconvergence obliquity to the south along the Andean marginduring the Eocene (Fig. 4b).

As shown in Figure 5, the region between Chuquicamataand El Abra is one of the anomalous zones along the Cor -dillera de Domeyko (Fig. 2), where magmatism was recurrentsince the Late Cretaceous. Despite lacking evidence for largevolumes of middle Eocene to early Oligocene volcanic prod-ucts, intrusive magmatism of this age is well recorded. A large(>500 km2), composite intrusive complex (Fortuna-El Abra)was emplaced syntectonically between 45 and 38 Ma near thesouthern termination of the Sierra de Moreno (Tomlinson etal., 2001a, 2010; Dillles et al., 2011). Figure 5b shows the re-stored shape of the Fortuna-El Abra batholith before beingseverely dismembered by left-lateral displacements along theWest fault (Tomlinson et al., 2007a; Dilles et al., 2011). Fieldrelationships indicate that the batholith was intruded alongthe trace of the Quetena reverse fault, an inverted normalfault inherited from Mesozoic back-arc extension. A flatteningfoliation in metaclastic rocks in the contact aureole is consis-tent with emplacement during regional Incaic E-W shorten-ing (Tomlinson and Blanco, 1997a).

The Fortuna-El Abra batholith, described by Dilles et al.(1997) as a porphyry copper batholith, is a long-lived, com-posite magmatic system that contains intrusive phases em-placed during different stages of the Incaic event; it is similarto the Andahuaylas-Yauri batholith of southern Peru, de-scribed by Perelló et al. (2003a). The batholith comprises an


0361-0128/98/000/000-00 $6.00 336

older group of intrusions, namely the Los Picos complex andPajonal diorite, emplaced between ~45 and 42 Ma (Tomlin-son et al., 2001a; Campbell et al., 2006), which include pre-dominantly mafic pyroxene- and biotite-bearing quartz mon-zodiorites, monzodiorites, and quartz monzonites that arecopper barren (Fig. 5b). The second intrusive group includesthe Fortuna-El Abra granodiorite complex and is largely com-posed of two hornblende-bearing granodioritic phases (An-tena, 39.5−39.0 Ma and Fiesta, 38−37.5 Ma) as well as por-phyritic components (San Lorenzo porphyries; Fig. 5b). TheFortuna-El Abra Complex intrusions, derived from hydrous,oxidized, and sulfur-rich magmas (Dilles et al., 2011), are as-sociated with large porphyry copper deposits with U-Pb agesbetween 39 and 36 Ma (El Abra, 38−37 Ma; the Toki clusterincluding Toki, 39 Ma and Opache, 38−37 Ma; AlejandroHales, 39−36 Ma; Conchi, 36 Ma; Perelló, 2003; Campbell et

al., 2006; Boric et al., 2009; Marquardt et al., 2009; Barra,2011; Fig. 5b).

The youngest intrusive event in the Chuquicamata clusterwas related to the emplacement, beyond the southern limits ofthe Fortuna-El Abra batholith, of the Chuquicamata (36−32Ma) and Radomiro Tomic (36−34 Ma) porphyry copper de-posits (Lindsay et al., 1995; Reutter et al., 1996; Ballard et al.,2001; Ossandón et al., 2001; Campbell et al., 2006; Barra, 2011;Fig. 5). Sibson (1987), who relied on maps of Perry (1952), in-terpreted the Chuquicamata porphyry copper deposit to havebeen emplaced syntectonically, following an extensionalstepover linking two separate but overlapping, parallel, dextralstrike-slip faults. Alternatively, Lindsay et al. (1995) favored amodel in which the required space for magma emplacementcoincided with a releasing bend (cf. Cunningham and Mann,2007) along a single, continuously linked dextral fault.


0361-0128/98/000/000-00 $6.00 337

WEST FAULT

21º30'

22º

22º30'

CALAMA

69º

El Abra

Conchi Viejo

Clara Granodiorite

Llareta Granodiorite

FutureWest Fault

Porphyry Cu depositsunder post mineral cover

porphyrydikes

Small copperprospects

Opache

69º

22º00'

(Toki Cluster)

Fiesta Granodiorite(38-37.5 Ma)

Los PicosComplex

(45-42 Ma)

Upper Cretaceous volcanic rocks(Quebrada Mala fm.)

Upper Cretaceoussedimentary rocks

(Tolar fm)

Lower to MiddleEocene volcanic

rocks(Icanche fm)

Arca fault

Co.

Jas

pe

R. Tomic

Alejandro Hales

Chuquicamata

Mesabi Fault

Toki

El Abra

SIE

RR

A D

EL

ME

DIO

Cretaceous reverse fault

SIE

RR

A D

E M

OR

ENO

Upper Cretaceous volcanosedimentary sequences (Cerro Empexaand Quebrada Mala formation)

Lower to Middle Eocene volcanic rocks (Icanche formation)

Upper Eocene-Oligocene, syntectonic, sedimentary rocks (Sichaland Calama formations)

Eocene (45-37 Ma) monzodiorites to granodiorites (includingFortuna El Abra batholith)

Chuquicamata Porphyry Complex (36-33 Ma)

Cretaceous intrusive rocks

Cretaceous reverse faultwith Eocene reactivation

Eocene reverse faultrelated to EW shortening

(a) (b)

San Lorenzo

Early Abra-AntenaGanodiorite

Alejandro Hales

GenovevaTokiQuetena

Miranda

Quetenafault

AntenaGranodiorite

(39.5-39 Ma)

Pajonal Diorite

+++

++++

+

++

++

++++ +++

+

++

+

++ +

+

+ +++

+ + ++ + +

+ +

++ +

+ +

++

+

++ +++ + ++

+ +++ ++

++ ++ ++

+

+ ++++ +++

++ ++

+ ++ ++

+++

+

+

++++++

++

+

++

+

+ +

+ +

++

+

+++

++ + ++

+

+++

+

++

+

+

Quetena Fault

+

++

Paleocene intrusive rocks

Jurassic to Lower Cretaceous, back-arc, sedimentary sequences

0 10 20km

0 5 10km

+ +++

++

+

+

++ +

+

++

+ ++ +

+

+

+

+

+

++

++

Triassic intrusive rocks (ca. 230 Ma; Elena granodiorite)

Sierra del Medio basement block (Upper Paleozoic)

Triassic volcanic and sedimentary rocks

Sierra de Moreno basement block (Neoproterozoic to Paleozoic)

Upper Cretaceous, syntectonic, red beds (Tambillos and Tolarformations)

FIG. 5. (a). Simplified geologic map of the El Abra-Chuquicamata region, indicating age of faults (based on Tomlinson etal., 2001). (b). Restored map of the El Abra-Fortuna batholith after removal of 35 km of Oligocene-early Miocene left-lat-eral motion on the West fault, showing main intrusive phases and mineralized centers (Dilles et al., 2011). Names of intru-sive units east of the future trace of the West fault follow the nomenclature that has been employed for intrusive phases nearEl Abra.

After porphyry emplacement at the Chuquicamata andRadomiro Tomic, to the south of their present location, thedeposits were displaced as a consequence of late Oligocene toearly Miocene (31.0−16.3 Ma) left-lateral movements alongthe throughgoing regional West fault or West Fissure. Dis-placement along the West fault, which includes, at Chuquica-mata, according to McInnes et al. (1999), a 600 ± 100 m westsideup vertical displacement component (contested by Tom-linson et al., 2001b), was able to offset the Fortuna-El Abrabatholith 35 to 37 km in a left-lateral sense (Dilles et al., 1997;Tomlinson and Blanco, 1997b; Tomlinson at al., 2001a; Fig.5). South of Chuquicamata, at least part of the strike-slip dis-placement component was transferred to a set of normalfaults that bound the Cenozoic Calama extensional and/ortranstensional basin (Blanco, 2008; Blanco and Tomlinson,2009). Seismic data reveal that a buried normal fault with 1-to 1.5-km down-to-the-east displacement limits the north-western margin of the basin, where up to 2,500 m of silici-clastic continental strata accumulated between the Oligoceneand Miocene (Jordan et al., 2004; Blanco, 2008).

The West fault forms, at present, the sharp western limit ofthe Chuquicamata orebody. Nevertheless, structural inter-pretations by Sibson (1987) and Lindsay et al. (1995) consid-ered that the internal architecture of second-order faults andveins indicates that the deposit is essentially intact. Accordingto Lindsay et al. (1995), the Oligocene to early Miocene Westfault propagated northward, along the western edge of theporphyry complex (previously emplaced at 36−33 Ma), with-out displacing the mineralized intrusive units within the for-mer dextral releasing bend and/or stepover.

Escondida

Despite also being along the Domeyko fault system, thetectonic history of the Escondida region (Fig. 2) is markedlydifferent, highlighting the significant changes in tectonicstyles along the >1,000-km strike of the fault system. Defor-mation in this area seems to have been dominated by tectonicescape linked to passive rotation and transport of brittleupper crustal blocks over hot and ductile lower crust in a waysimilar to the so-called orogenic float or clutch tectonics mod-els discussed by Oldow et al. (1990), Lamb (1994), and Tikoffet al. (2002; see below). At this latitude, the Cordillera deDomeyko appears as a discontinuous mountain range formedby a group of discrete basement blocks bounded to the westby a 150-km-long shear lens (Escondida shear lens) devel-oped between the regional Sierra de Varas and Escondidafaults (Figs. 6a, 7). To the east, the Domeyko range abuts theSalar de Atacama depression, which is a deep subsiding basinfilled by >9 km of Cretaceous to Tertiary continental sedi-mentary strata (Pananont et al., 2004; Mpodozis et al., 2005).The basin was built on top of a large positive gravimetricanomaly (Central Andean Gravity High; Götze and Krausse,2002; Fig. 6b), which indicates the occurrence, at depth, ofdense crustal rocks that may help to explain its long-lived sub-sidence basin history, recorded at least since the Cretaceous.

The isolated basement blocks that form the core of therange are separated by small, triangular basins, with interiordrainage (Fig. 6a). The southern rhomboid-shaped blocks(e.g., San Carlos and Imilac; Fig. 6a) are bounded along theirnorthwestern margins by high-angle, SE-dipping reverse faults.

The blocks at Quimal, Los Morros, and Mariposas are lim-ited 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 arethrust over the sedimentary fill of the Salar de Atacama basin(Fig. 6a), which includes, among other units, a 2,500-m-thicksequence of Eocene to early Oligocene continental con-glomerates and poorly consolidated gravels. Internal pro-gressive unconformities and Ar/Ar ages between 44 and 43Ma from a tuffaceous horizon just above the base of this se-quence (Loma Amarilla Formation) indicate that thesestrata-accumulated syntectonically during the regional Incaicdeformation (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 ofa 250- × 50-km basement sliver that was transported north-ward during the Incaic deformation. According to Mpodoziset al. (1993a, b), the continuous northward shift of the dis-placed block was impeded by a buttress located to the northof the moving sliver as the displacement was transferred tothe east by tectonic escape (cf. Mann, 1997) toward thedeeply subsiding Salar de Atacama basin. In this model, theSalar de Punta Negra depression (Fig. 6c) would have formedas an extensional basin at the trailing edge of the displacedblock. Displacement transfer seems to have occurred byclockwise rotation of small detached blocks, which in turngenerated the local triangular-shaped extensional basins be-tween the rotating blocks as well as contractional deformationin their northeastern corners where basement was thrust overthe 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 ofthe highest elevations (3,500 m.a.s.l.) in the Cordillera deDomeyko (Fig. 6a). However, if along-strike changes in localstresses resulting from the formation of the Bolivian oroclineare considered, the buttressing effect may have been pro-vided by the nonrotated paleomagnetic domain C, locatednorth of Calama (Fig. 3), where initial Eocene deformationwas 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 set-ting and distribution of the barren and mineralized intrusionsthat form part of the Escondida cluster (labeled “LE,” Fig. 2).The area encompasses the widest part of the regional Escon-dida shear lens, which is separated to the east from the SierraImilac and Sierra San Carlos basement blocks by the Escon-dida fault (the Panadero-Portezuelo fault is an alternativename used by Hervé et al., 2012). These two blocks were thenseparated by the intervening triangular Salar de Hamburgodepression (Fig. 7). Drilling shows that the Salar de Ham-burgo fill includes >1,200 m of red beds, lahars, and pyro-clastic rocks with U-Pb zircon ages of 38 Ma (San CarlosStrata, Fig. 7; Marinovic et al., 1995; Urzúa, 2009; Hervé etal., 2012); therefore, these units are equivalent to the upperportion of the syntectonic Loma Amarilla Formation in theSalar de Atacama. The Hamburgo fault is a NE-trending,high-angle reverse fault that places the late Paleozoic base-ment of the San Carlos block over the sedimentary sequencesof the Salar de Hamburgo (Fig. 7).


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A protracted, >40-m.y. Cretaceous to Tertiary history ofmagmatism is recorded in the Escondida region. The oldestintrusive events produced Late Cretaceous (81−71 Ma; Fig.7) tholeiitic to alkaline pyroxene gabbros and diorites as wellas hornblende-pyroxene monzodiorites and diorites, in addi-tion to early Paleocene (66−64 Ma) pyroxene diorites. Theserocks intruded the sedimentary strata of the Mesozoic back-arc basin in the Escondida shear lens to the south and west ofEscondida (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 focusedand recurrent magmatic activity took place east of the Andeanarc front, which during the Late Cretaceous to early Pale-ocene (85−50 Ma) was located farther west, in the Centraldepression of the Antofagasta region (Boric et al., 1990).

Incaic magmatism in the Escondida cluster began in themiddle Eocene (~44 Ma), when left-lateral displacementsalong the Escondida fault and differential rotation betweenthe San Carlos and Imilac blocks created the triangular Salarde Hamburgo depression (Fig. 7). The oldest intrusions are agroup of 44 to 41 Ma pyroxene-biotite monzodiorites and pyroxene-hornblende granodiorites, emplaced within the Es-condida shear lens to the north of Escondida (Marinovic et al.,1995; Richards et al., 2001; Urzúa, 2009). Together, they likelyrepresent the roof of an underlying, partially eroded plutonnearly 20 km in diameter (Fig. 7). Most of these rocks, like theLos Picos complex and Pajonal diorite of the Chuquicamata-El Abra region, are barren, although geochronologic data andrelationships between intrusive phases in the vicinity of theChimborazo porphyry copper deposit indicate, according to


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Salar de AtacamaEl

Bo

rdo

Esca

rpam

en

Bla

ncas

faul

Salar de los Morros

Salar Verónica

Barr

anca

s

Salar dePunta Negra

N

Cen

tinel

a F

ault

Cor

don

de L

ila

Sier

raLim

ón Verde

Salar deHamburgo

Sie

rra

de

Va

ras

faul

Esc

ond

ida

faul

t

Sier

ra d

e A

lmei

da

Salarde Elvira

(a)

I

M

LM

Q

SC

ESCONDIDA

GABY

ESPERANZA-TELEGRAFO

Los

Toro

s fa

ult

Cen

tinel

a fa

ult

23º

24º

69º

ESL

Fortuna-El Abrabatholith

CALAMA

Sierra Limón Verde

Paleozoicbasement

blocks

ANTOFAGASTA

Sie

rra

de

Var

asf a

ult

Sie

rra

de

lMed

io ?

?

N

24º

70º 68º

Non-rotatedDomain C

Extensional(rotational)

basins

BUTTRESS ZONE

Free face

GravityHigh

RotatedDomain D

Bolivia

Arg

entin

a

Boundaryfault

Salarde

Atacama

Salarde

Punta Negra

(b) (c)

Dee

p B

asin

ThrustsA'

La Escondida shear lens (ESL )

Cordillera de Domeyko clockwise rotatedbasement blocks (Q: Quimal, LM: Los Morros,M: Mariposas, I: Imilac, SC: San Carlos)

Mesozoic to Paleocene strata ofthe Centinela District

Outcrops of Lower Cretaceousto Upper Oligocene continentalclastic strata of the Salar deAtacama basin

Cordón de Lila-Sierra deAlmeida stable domain

Strike-Slip faults Reverse faults Normal faults0 20 40 km

25º

A

rr'

α

0 50 km

Transition zone betwenC and D domains

N

FIG. 6. (a). Main structural elements of the Cordillera de Domeyko (between Escondida and Sierra Limón Verde andSalar de Punta Negra (22° 30°−25° S; see location in Fig. 2). Note the large shear lens (Escondida shear lens) flanked by theEscondida 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 clockwise-rotated 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 alonga left-lateral strike-slip fault system. Displacement is impeded, as shown, by a buttress at the leading edge of the block, andtransferred toward the right by means of clockwise block rotations. Note extensional basins created between the rotatingblocks. A-A' and r-r = position of points and lines before and after rotation (α = rotation angle). Adapted from Beck et al.(1993).

Hervé et al. (2012 ), that an early phase of copper mineral-ization probably occurred at ~41 Ma.

The second episode of Eocene-Oligocene magmatismbegan with the emplacement of a closely spaced group ofsmall 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 earlyOligocene volcanic rocks of the Escondida shear lens and thelate Paleozoic basement units of the Imilac block (Fig. 7).Their distribution suggests that they could be apophyses of alarger pluton at depth that intruded along the Escondidafault. The slightly younger group of porphyry copper stocksinclude a series of multiphase, NE- to N-NE−trending, dike-like intrusions that were emplaced at or near the Escondidafault at 38 to 37 Ma; these include the deposits at Zaldívar,Escondida Norte, Escondida, and Pinta Verde, and, fartheraway, at Baker (Richards et al., 2001; Urzúa, 2009; Hervé etal., 2012; Fig. 7).

The final event of Incaic magmatism in the Escondida clus-ter was related to the emplacement of the Escondida Esteand Pampa Escondida deposits, immediately to the east ofthe Escondida fault (Fig. 7), between 36.0 and 34.5 Ma(Hervé et al., 2012). The mineralized porphyries of the Es-condida cluster, with the exception, perhaps, of Chimborazo,postdate the earlier phase of sinistral faulting and block rota-tions along this segment of the Cordillera de Domeyko. De-formation seems to have begun at ~42 Ma (age of the base ofthe Loma Amarilla Formation; see above), although the lackof offset on any of the porphyry intrusions across the localfault strands (Panadero-Portezuelo fault) of the larger Escon-dida fault indicates that major along-strike fault activity hadceased by 38 Ma, as shown by the across-fault 38−37 Ma por-phyry dikes (Hervé et al., 2012).

Centinela

Porphyry copper mineralization in the Centinela district (la-beled “CE,” Fig. 2) occurs within a 25-km-wide, fault-bounded


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a fa

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ault

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c fa

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Pinta Verde

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rra

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ilac

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Chimborazo (41)

Zaldivar (38-37) Baker (38-37)

Escondida(38-37)

Escondida Norte (38-37)

Salar deHamburgo

Salar dePunta Negra

Pampa Escondida (36-34.5)

0 5 10 km

Upper Paleocene to Lower Eocene (59-53 Ma) volcanicsequences

Eocene (44-41 Ma) px-bt monzodiorites and px-hbgranodiorites

Upper Eocene (42?-36 Ma) sedimentary-volcanicsequence (San Carlos strata)

Upper Eocene (39-38 Ma) hb dioritic porphyry intrusions

Upper Eocene (38-35 Ma) dacitic to granodioriticmineralized porphyry intrusions (Escondida cluster)

Upper Triassic-Lower Cretaceous sedimentary sequences

Upper Paleozoic (300-270 Ma) basement

Upper Cretaceous (81-72 Ma) gabbro-diorites and hb-pxdiorites

Upper Cretaceous (74-70 Ma) rhyolitic ignimbrites

Lower Paleocene (66-64 Ma) px diorites

Triassic (240-220 Ma) intrusive rocks

Escondida Este(36-34.5)

FIG. 7. Simplified geologic map of the area around Escondida, highlighting major regional faults and the different in-trusive 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).

belt of Late Cretaceous to early Eocene volcanic rocks, lo-cated between the Paleozoic basement exposures of theCordillera de Domeyko to the east and an early Cretaceousvolcanic sequence in the Coastal Range to the west (Fig. 8).The Centinela district hosts one of the more recently discov-ered porphyry copper clusters in northern Chile. Althoughthe occurrence of exotic copper mineralization at El Tesorowas known for a long time, the full potential of the districtonly 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 theCentinela cluster comprise a group of Early Cretaceousolivine-pyroxene gabbros and hornblende-bearing quartz dior-ites, with U-Pb zircon and K-Ar ages between 124 and 100Ma (Mpodozis et al., 1993b; Marinovic and García, 1999).

These rocks, emplaced within Jurassic marine limestones ofthe northern Chile back-arc basin (Fig. 8), are almost 100 kmeast of the Early Cretaceous magmatic front, which, as notedabove, was situated at that time in the Coastal Range (Boricet al., 1990). Younger volcanic and intrusive events occurredin the Late Cretaceous, when a volcanic sequence (QuebradaMala Formation) and a group of coeval 70 to 66 Ma pyroxenediorites to rhyolite porphyries and flow domes, dated (U-Pbzircon) between 70 and 66 Ma, were generated after the An-dean arc front migrated eastward into the Centinela region(see Figs. 1b, 8). Volcanism continued after the Cretaceous-Tertiary boundary deformation event, with eruptions fromstratovolcanoes and small collapse calderas that were activebetween the early Paleocene (64 Ma) and the early Eocene(53 Ma). During this interval, a diverse group of epizonal in-trusions composed mainly of pyroxene-biotite quartz dioriteto monzodiorite (60 Ma) and hornblende- biotite granodiorite


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Upper Paleozoic (290-270 Ma) basement

Upper Triassic (210-200 Ma) volcanic and sedimentaryrocks

Jurassic to Lower Cretaceous back-arc sedimentaryand volcanic rocks

Lower Cretaceous (124-100 Ma) ol-px gabbros todiorites and hb granodioritic porphyry intrusions

Lower Cretaceous (?) volcanic rocks

Upper Cretaceous (78-66 Ma) sedimentary andvolcanic sequences (Quebrada Mala fomation)

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

Paleocene to Lower Eocene (64-53 Ma) volcanic rocks(Cinchado formation )

Paleocene (60-56 Ma) px-bt monzodiorites andrhyolitic porphyry intrusions

Eocene (44-40 Ma) px-hb monzodioritic to hb-btgranodioritic stocks and mineralized dacitic porphyryintrusions (Esperanza-Telégrafo and Centinela-Polo Sur)

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

Lower Paleocene (65-64 Ma) diorites and dacitic porphyryintrusions

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Esperanza (42-40)

Sierra Limón Verde

Las Lomasduplex

Los

Toro

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Cen

tinel

a fa

ult

Llano (41)Mirador (41-39)

Orión (44-41)

Telégrafo (42-40)

Caracoles (42-41)

Centinela (45-44)

Penacho Blanco (42)

Pilar (43)

Polo Sur (42-41)

Sierra

Agua Dulce

Undifferentiated Cretaceous granitoids

0 5 10 km

as

Las Lomas fault

Esperanza fault

Coronado fault

Llano fault

Sherezade (44-43)

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

(58−57 Ma) plus andesitic to dioritic porphyritic intrusions,were emplaced into the Mesozoic units and Paleogene vol-canic edifices.

Incaic magmatism and mineralization in the Centinela dis-trict occurred between 45 and 39 Ma (Mpodozis et al., 2009a;Perelló et al., 2010) and began, as revealed by numerous newU-Pb zircon ages in the intrusive rocks and Re-Os ages inmolybdenite, ~12 to 10 m.y. after the termination of volcan-ism in the early Eocene.

This event coincides with the mostly copper-barren earlyphase of Incaic intrusions at Chuquicamata-El Abra (45−42Ma) and Escondida (44−41 Ma). The oldest Incaic intrusiverocks include a small group of 45 Ma pyroxene-biotite andquartz diorites yet, in contrast to Chuquicamata-El Abra andEscondida, at Centinela, numerous mineralized porphyrycenters were emplaced between 44 and 39 Ma. They form,together with some barren stocks, a 40-km-long, N- to NE-trending belt, which includes at least 10 discrete intrusivecomplexes (Fig. 8). A syntectonic sequence of conglomeratesand volcaniclastic sandstones, which accumulated at the sametime as porphyry copper emplacement, comprises interbed-ded layers of dacitic block-and-ash deposits and tuffs with U-Pb zircon ages between 42 and 39 Ma.

The oldest porphyry systems (45−43 Ma) occur along thesouthwest end of the belt, and the age decreases systematicallyto the northeast until reaching 39 Ma at the northeast edge ofthe porphyry trend (Fig. 8). The geometry of the porphyrycomplexes is controlled by their position relative to the mainstructural feature of the district, a 3- to 5-km-wide, N-S−trend-ing fault zone that cuts obliquely across the porphyry belt. Thiszone of intense deformation constitute to the northern termi-nation of the Sierra de Varas fault, which stretches for >250km along the western border of the Cordillera de Domeyko(Mpodozis et al., 1993b; Soto et al., 2005; Figs. 2, 6a), and wasactive both during and after porphyry emplacement (Fig. 8).

Porphyry deposits located west and east of this zone of con-centrated deformation are largely undeformed. Copper miner-alization in mineralized porphyry systems, located west of thefault zone, are related to subvertical, hornblende-biotite dacitesdike 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 em-placed at 45 to 44 Ma (Centinela) and 44 to 43 Ma (Sher-erezade) to be followed by the intrusion by several barren py-roxene-hornblende dioritic stocks and lacoliths dated at 43 Ma,although a porphyry copper system with the same age has beenalso recognized at Pilar. A new pulse of copper-bearing intru-sions occurred, finally, between 42 to 41 Ma, at Polo Sur, whiledacitic porphyries with a similar age (42 Ma) but apparentlybarren have been documented at Penacho Blanco (Fig. 8).Mirador, the youngest porphyry deposit recognized so far in thedistrict (41−39 Ma; Mora et al., 2009) and located east of thefault zone (Fig. 8), is also structurally undisturbed. The coppermineralization, hosted within Jurassic marine limestones andevaporites (Mora et al., 2009) is associated with a group ofmultiphase, W- to NW-trending intrusions that, as in the olderCentinela 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;

Perelló et al., 2004, 2010; Bisso et al., 2009; Münchmeyer andValenzuela, 2009; Swaneck et al., 2009); they are all associ-ated with tilted porphyry dike swarms. These deposits areemplaced into moderately to steeply dipping strata, which aredisrupted by major postmineral faults that exhibit reverse,normal, and strike-slip displacement components (Figs. 8, 9).

Although the widespread gravel cover makes it difficult tosatisfactorily resolve the structural relationships within thewhole district, the regional structure around the Esperanzaand Telégrafo deposits (Fig. 8) comprises a long-wavelength,asymmetric, basement-cored anticline, bounded to the westby a moderately E-dipping yet unexposed thrust fault thatwas discovered during exploration drilling (Telégrafo fault;Perelló et al., 2004, 2010; Bisso et al., 2009; Münchmeyerand Valenzuela, 2009; Fig. 9). The hinge zone of the anticlineis, in turn, sliced by two subvertical faults (Coronado andLlano faults; Figs. 8, 9) linked to the N-S−trending regionalfault zone. Figure 9 includes a west-east structural sectionacross the Esperanza deposit, where mineralization is associ-ated with a group of easterly inclined porphyry dikes em-placed within the ~40° to 50° W-dipping Triassic to UpperCretaceous strata that form the frontal limb of the anticline.This panel, containing in part mineralized and altered hostrocks 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 accumu-lated 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 Creta-ceous strata. The rectilinear fault trace and the mismatch ofthe lithology and age of the Late Cretaceous volcanic rocksacross the Esperanza fault show it includes an importantcomponent of strike-slip movement, although the preciseage of deformation and genetic links between both faults re-main to be determined. The Llano and Coronado faults are,however, as shown in Figure 9, younger faults that are su-perimposed over the Telégrafo-Esperanza system, which ex-hibits both left-lateral and large, down-to-the-east compo-nents of displacement, part of which has a late Miocene oryounger (<10 Ma) age.

Structural relationships at Caracoles, in spite of being only10 km to the south, are entirely different and difficult tomatch with those observed at Esperanza-Telégrafo. In thesouthern part of the district (Fig. 8), the fault zone is dis-placed to the west, out of strike with the fault zone to thenorth, and includes a strike-slip duplex (Las Lomas duplex)bounded by two N-S−trending subvertical faults (Las Lomasand Centinela faults; Fig. 8). Kinematic indicators show evi-dence for an episode of left-lateral displacement, even if theinternal geometry of the duplex is compatible with an earlier(Late Cretaceous?) event of dextral shear (Marinovic andGarcía, 1999; Mpodozis et al., 2009a). The Caracoles deposit(42−40 Ma, Swaneck et al., 2009) is hosted in early Paleocene(64−60 Ma) volcaniclastic rocks beneath the gravel cover.Copper mineralization is linked to a steeply SE-dippingswarm of thin dacite porphyry dikes that follow the internalstructural trends of the Las Lomas duplex; this is consistentwith emplacement of the porphyry along a zone of (earlier orsynmineral?) strike-slip deformation.


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Long-lived intrusive clusters and porphyry deposits: What is the relationship?

As discussed above, Eocene to Oligocene porphyry copperdeposits in northern Chile are located in districts of focusedand long-lived magmatic activity distributed along the Cor -dillera de Domeyko (Fig. 2). In some districts, magmatismwas active for 50 m.y. (Chuquicamata-El Abra, Escondida) oreven 80 m.y. (Centinela). Magmatism began in a back-arc set-ting while the arc front was located much farther west in theCoastal Range or Central depression (Boric et al., 1990); todate, the cause of these zones of long-term magmatism is un-certain. Yañez and Maksaev (1994) suggested that porphyryspacing may be related to Rayleigh-Taylor diapirism along themid Eocene-early Oligocene arc. Behn et al. (2001) notedthat a correlation exists between the location of porphyry cop-per deposits and regional E-W−trending negative magneticanomalies that extend from the coast to the modern Andeanarc, and which may represent crustal structures favorable formagma ascent as the arc migrated eastward since the Jurassic.Richards (2003) speculated that porphyry copper depositswere emplaced at the intersections of the Domeyko fault sys-tem with NW-trending transcordilleran crustal-scale struc-tures, yet with the exception of Potrerillos (Fig. 2), none ofthese has been documented on the western (Chilean) side ofthe Andes. To address these observations, Tomlinson andCornejo (2012) proposed a hybrid model to explain porphyryspacing, which combines Rayleigh-Taylor diapirism withcrustal-scale structural control.

Even if these models explain porphyry spacing, they do notelucidate why clustered magmatism began long before theEocene. As more data are gathered, an even more strikingspatial relationship is emerging in several districts (Escon-dida, Centinela, Chuquicamata, Quebrada Blanca-Collahuasi,Sierra Exploradora; Figs. 5, 7, 8), between extended, Creta-ceous and younger magmatic activity, porphyry copper de-posits, and Triassic intrusions which, near Escondida and Col-lahuasi, show evidence of weakly developed porphyry-stylecopper mineralization (Cornejo et al., 2006; Munizaga et al.,2008). In addition, indistinguishable geochemical signaturesreported in a pilot study by Wilson et al. (2011), which com-pared Triassic granodiorites to mineralized Eocene porphyrydeposits at Alejandro Hales (Fig. 5), may suggest tapping of acommon source region at depth. The problem remains open.

Neogene Tectonic Province of Central Chile and Argentina

Late Eocene (?) to early Miocene extension and the Abanico intra-arc basin

The tectonic evolution of the Andean orogen in centralChile (31°−34° S) and contiguous Argentina differs from thatof northern Chile in that the main deformation events areyounger, the tectonic style is different, and the principal ageof porphyry copper mineralization is much younger (lateMiocene to early Pliocene). At present, a zone of flat subduc-tion (Central Chile or Pampean flat-slab region; Cahill andIsacks, 1992; Kay and Mpodozis, 2002; Ramos et al., 2002;


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Ks

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2400

2000

1600

1200

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Ol Ep

0 0.5 1 km

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Lower Paleocene dacitic domes (64-63 Ma)

49000

Miocene to Pliocene gravelsMi-Pl

Eocene sedimentary and volcanic strataEo

UTM (E )m.a.s.l.

Ep Esperanza copper porphyry intrusions (41-40 Ma)

Pal

Upper Cretaceous volcanic rocks (Quebrada Malaformation, 70-66 Ma)Ks

Trv

Jurassic marine sedimentary sequences

Upper Triassic volcanic rocks (210-200 Ma)

Jur

Left-lateral displacement on faults 0.5% CuT

Oligocene sedimentary rocks

Ol

Ol

FIG. 9. Schematic structural section across the Esperanza porphyry Cu deposit, Centinela district. The Esperanza ore-body is part of a W-dipping sliver of Jurassic and Late Cretaceous strata intruded by Eocene porphyry dikes (41−40 Ma),thrust to the west (Telégrafo fault) on top of Eocene (42−37 Ma) sedimentary and pyroclastic sequences. The Esperanza andCoronado faults have a complex and younger displacement history, including strike-slip components. The down-to-the-eastdisplacement shown along the Coronado and Llano faults corresponds only to the youngest (late Miocene-Pliocene?) episodeof deformation. Location of section shown in Figure 8.

Fig. 10) that was formed by progressive slab shallowing, begin-ning in the early Miocene (Kay et al., 1987), extends from 27°to 33° S. As in northern Chile, the Coastal Range is formed bythe volcanic and intrusive remnants of a Jurassic to Cretaceousarc system. From 32° S southward, a thick sequence of coevalmarine and terrestrial sedimentary rocks exposed along theeastern slope of the Cordillera Principal corresponds to sedi-ments that accumulated within the Mesozoic Neuquén back-arc basin (Mpodozis and Ramos, 2008; Figs. 10−12).

The most notable geologic feature, however, is a severalkilometer-thick volcano-sedimentary sequence that formsmost of the western part of the Cordillera Principal between32° and 37° S (Figs. 10b, 11, 12), which has been traditionally

assigned to the Abanico, Coya-Machalí, and Cura-Mallín For-mations (e.g., Charrier et al., 1996, 2002; Jordan et al., 2001;Kay et al., 2005; Farías et al., 2008). These sequences accu-mulated in extensional volcano-tectonic depressions or intra-arc basins that are referred to as the Abanico basin; Ar/Arages at the latitude of Santiago (33° S) range from latestEocene to early Miocene (35−21 Ma; Muñoz et al., 2006).Volcanic rocks are calc-alkaline to tholeiitic in composition,and the overall geochemical signature suggests that volcanicactivity occurred over a relatively thin crust (<35 km; Kay etal., 2005; Muñoz et al., 2006).

Equivalent units extend for more than 1,500 km to thesouth along the crest of the Andean range into the northern


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Figure 11

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Oligocene to middle Miocene volcanicsequences and interbeddedsedimentary strata

Late Oligocene to middle Miocenecontinental strata

Late Oligocene to middle MioceneSomuncurá plateau volcanic rocks

Eocene volcanic and intrusive rocks

Oligocene to Miocene intrusive rocks

IslaMagdalena

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Bariloche

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Per

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40º

(a)

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CVZ

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tre

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Flat-slabR

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Antofagasta

Santiago

Concepción

FIG. 10. (a). Map showing position of the Chilean-Pampean flat-slab region and the distribution of Quaternary volcanoes(CVZ = Central Andean volcanic zone, SVZ = Southern Andes volcanic zone). Light-colored area shows region with eleva-tions >3 km. (b). Simplified geologic map showing the distribution of Oligocene-Miocene volcanic and sedimentary strata insouth-central Argentina and Chile from 32° to 45° S. Based on the compilation by Jordan et al. (2001) and the 1:1,000,000geologic map of Chile (SERNAGEOMIN, 2002). Arrows north and south of the Maipo orocline (or Maipo mega kink; Arriagada, et al., 2009) indicate the average values of paleomagnetically determined block rotations (Arriagada et al., 2009).

Patagonian Andes (Fig. 10b). Even though the erosion levelincreases and the volume of preserved volcanic products decreases southward, these sequences define a progressivelysouthward-widening belt that extends from the CoastalRange to the eastern slope of the Andes at the latitude ofPuerto Montt (41° S, Fig. 10b). The nature of interbeddedsedimentary rocks changes along strike, from continental tolacustrine facies in the northern part of the belt to marine facies toward the south; geochemical signatures also displayevidence for crustal thinning and extension increasing to thesouth. Muñoz et al. (2000) indicated that 37 to 20 Ma maficlavas exposed at the latitude of Puerto Montt (41° S; Fig. 10b)and erupted in an extensional setting possess island-arc geochemical affinities and were likely produced by deep

asthenospheric upwelling during a transient Oligocene toearly Miocene event of very rapid subduction. Deep mantleupwelling has also been suggested by Kay et al. (2007a) to ex-plain the origin of the 33 to 17 Ma basaltic flows of the largeback-arc Somuncurá volcanic plateau in Patagonia (Fig. 10b).Oligocene to Miocene submarine basaltic pillow lavas withMORB geochemical features in the Aysén region indicate ex-treme crustal thinning along the arc farther south, between43° and 46° S (Silva et al., 2003).

Miocene compressional failure of the Abanico basin

The late Eocene to early Miocene extensional period ter-minated at ~20 to 18 Ma, followed by compressional defor-mation leading to the emergence of the modern Andes in


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Rancagua

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Tupungato

San José

Maipo MAIPO OROCLINESYMMETRY AXIS

CHILE ARGENTINA71º 69º

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Cretaceous intrusive rocksLate Paleozoic to Jurassic batholiths(Coastal Cordillera) Eocene? - Early Miocene volcano-

sedimentary sequences (Coya - Machalí/Abanico formations)

Miocene volcanic rocks (Farellonesformation)

CO

AS

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A N

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M

Early Cretaceous sedimentary strata(Neuquén basin)Triassic to Jurassic sedimentary strata(Neuquén basin)

Quaternary stratovolcanoes

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Fore arc Miocene sedimentary strata

Miocene intrusive rocks (PrincipalCordillera)

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0 25 50 km

Los Andes

EL TENIENTE

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Jurassic to Cretaceous volcano-sedimentarysequences (Coastal Codillera)

Santiago

RIO BLANCO -LOS BRONCES

A

FIG. 11. Geologic map of the area around the Maipo orocline from 32°30' to 35° 30' S (location in Fig. 10). Note changein the structural trend of the Coastal Range and Principal Cordillera across the Maipo orocline (Farías et al., 2008; Arriagadaet al., 2009), from N-S, to the north, to N-NE, to the south. A = Aconcagua fold-and-thrust belt, M = Malargüe fold-and-thrust belt (adapted from Farías et al., 2008). See text for more details.


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32º

33º

34º

Piuquenes (11)

Los Azules (10-8)

Rincones de Araya (9)

El Altar (12-10)

Los Pelambres (14-10)

Cerro Bayo del Cobre(12-10)

El Yunque (15)

Aconcagua

Amos- Andrés (9-8)

Vizcachitas (11-10)Morro Colorado Pimentón (11-9)

San Felipe

West Wall (11-9)

Novicio (15-13)

Rio Blanco-Los Bronces(7.7-4.7)

Los Sulfatos (7-6)

Tupungato

San José

Maipo

Santiago

El Teniente(6.5-4.6)

Rancagua

Valparaiso

0 50 km

El Pachón (9-8)

Los Machos (15-14)Los Piches (14-12.5)

Cerro Mercedario

Mercedario ( 13)

Quaternary stratovolcanoesQuaternary volcanic rocksUpper Cenozoic foreland sedimentarysequencesMiocene intrusive rocksVolcanic rocks of the Abanico basin (Eocene? to Miocene) include the Abanico, Coya-Machali and Farellones formationsJurassic to Cretaceous sedimentarysequences of the Neuquén basinCoastal Range block (Paleozoic to earlyCretaceous)Frontal Cordillera Paleozoic basement

71º 70º

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FIG. 12. Tectonic sketch of the northern end of the Abanico intra-arc basin (31°−34° S), showing the location and age(Ma, in parentheses) of Miocene to early Pliocene porphyry copper deposits of central Chile and contiguous Argentina andthe composite fold-and-thrust belt developed along the eastern margin of the basin (LR = La Ramada, A = Aconcagua, M =Malargüe fold-and-thrust belts).

central and southern Chile and contiguous Argentina (e.g.,Giambiagi et al., 2003). Between 31° and 34° S, much of thisdeformation was focused along the eastern border of theAbanico basin, which finally collapsed and was inverted by com-pressional deformation in response to the collision between

the colder and rigid western Coastal Range and easternFrontal Cordillera blocks (Fig. 13). Under these conditions,the late Eocene to early Miocene volcanic sequences weretectonically transported to the east and juxtaposed over thesedimentary sequences of the Neuquén back-arc basin to


0361-0128/98/000/000-00 $6.00 347

Rapid ConvergenceWeak intraplate

coupling

Intra-arcAbanico basin Mesozoic

sedimentary wedge

Lithosphere

Asthenosphere

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Steep slab35-21 Ma

Slow ConvergenceStrong intraplate

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Abanico basin

Zone of partial melting

12-5 Ma

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Strong intraplatecoupling

Syntectonic intrusions

El TenienteLos Bronces Los Pelambres

Lower crust melting andmixing with mantle-derived magmas

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10-5 Ma

Moho

W E

Slow Convergence

AcceleratingSAM plate

Aconcagua-La Ramada

FTB

Aconcagua-La Ramada

FTB

Frontal Cordillera block

Frontal Cordillera

FIG. 13. Schematic diagram near 33° to 34° S, showing the evolution of the Abanico intra-arc basin during the Oligoceneand Miocene (SAM = South America, FTB = fold and thrust belt). Major porphyry copper deposits began to be formed at10 Ma when the deformation front migrated to the east and the Frontal Cordillera was uplifted as a consequence of shal-lowing of the subducted Nazca plate. Even though the diagrams combine geologic relationships observed at different lati-tudes it shows the tectonic position of the Los Pelambres intrusions along the boundary thrusts of the deformed Abanicobasin and the Río Blanco-Los Bronces and El Teniente intrusions in the less deformed rocks near the center of the formerbasin farther to the west. Red arrows below the Abanico basin show hypothetical magma paths. (SAM = South Americanplate, FTB= fold and thrust belt)

form the La Ramada, Aconcagua, and Malargüe fold-and-thrust belts (Ramos et al., 1996; Figs. 11−13). In central Chile,a sharp decrease in the volume of volcanism ensued, the arcfront migrated eastward, and the geochemical and isotopicsignatures of younger, middle to late Miocene volcanic sequences (e.g., Farellones Formation; Figs. 10−11) indicateprogressive crustal thickening as a consequence of increasedhorizontal shortening (Ramos et al., 1996; Kay and Mpodozis,2002; Stern et al., 2010).

Between 33° and 35° S, the amount of Miocene andyounger back-arc shortening decreased along strike fromnorth to south, as the tectonic style in the deformed back-arcsequences changed from the narrow, thin-skinned Aconcaguafold-and-thrust belt to the wider, mixed thin- and thick-skinned style of the Malargüe fold-and-thrust belt to thesouth (Ramos et al., 1996, 2004; Giambiagi et al., 2011; Figs.11−12). The transition zone, at 34° S, coincides with the sym-metric axis of the W-NW−trending Maipo orocline (Farías etal., 2008; Arriagada et al., 2009), and is revealed by thechange in orientation of both the Chile trench and the mainstructural trends of the Principal Cordillera, from N-S to N-NE (Fig. 11). The Maipo orocline, a more subtle feature thanthe Bolivian orocline (see above), is also shown in paleomag-netic studies, as the magnitude of paleomagnetic block rota-tions determined in all rocks older than 10 Ma changes from4°clockwise north of the orocline to 32° clockwise to thesouth (Arriagada et al., 2009; see Fig. 10b). These changes co-incide with a rapid fall of the absolute elevation of the Andeanrange, as well as an overall decrease of crustal thickness, from50 km at 32° S to <40 km at 36° S (Introcaso et al., 1992;Gilbert et al., 2006; Anderson et al., 2007).

These along-strike differences seem to reflect the effect of themore contractional conditions prevailing during the Neogenenorth of latitude 33° S, as shallowing of the subducting Nazcaslab progressed. This, in turn, lead to the establishment of themodern Chilean or Pampean flat-slab region between 28°and 33° S (e.g., Kay and Mpodozis, 2002). Slab shallowing isgenerally attributed to the buoyancy effect introduced by thesubduction of the E-NE−trending Juan Fernández Ridgeduring the late Miocene (Yañez et al., 2001; Ramos et al.,2002, and references therein), although recent analogue andnumerical experiments (e.g., Martinod et al., 2005) show thatmoderate-sized, buoyant ridges that impinge on a trench arenot able to alone induce formation of flat-slab segments of thedimensions observed in central Chile and contiguous Ar-gentina. Other authors (Manea et al., 2012) suggest that acombination of trenchward motion of thick cratonic lithos-phere accompanied by trench retreat may better explain theformation of the Chilean-Pampean flat slab during theMiocene.

Late Miocene to Early Pliocene Porphyry Copper Deposits of Central Chile and Contiguous Argentina

Overview

The major porphyry copper deposits in central Chile andcontiguous Argentina are preferentially located at or near thetransition zone between the Chilean (or Pampean) flat-slabzone and the steeper subduction zone beneath southernChile. This region contains three of the world’s largest copper

deposits, at Los Pelambres, Río Blanco-Los Bronces, and ElTeniente (Camus, 2003; Cooke et al., 2005; Sillitoe and Perelló,2005), as well as numerous smaller deposits and prospectsthat straddle the Chile-Argentina border region from 31° to35° S (Fig. 12). Although the first evidence of mineralizationthat postdates the beginning of Miocene deformation is as oldas 15 to 13 Ma (Novicio, 15–13 Ma; Los Machos, 14−12.5 Ma;Los Piches, 14−12.5 Ma; Maksaev et al., 2009; Toro et al.,2009; Fig. 12), most of the deposits north of 33° S (LosAzules, Rincones de Araya, El Altar, Los Pelambres, ElPachón, Vizcachitas, Amos-Andrés, Pimentón, Novicio, plusWest Wall in the San Felipe cluster) were emplaced between12 and 8 Ma (Sillitoe and Perelló, 2005; Maksaev et al., 2009;Toro et al., 2009; Maygadán et al., 2011; Perelló et al., 2012).

At this time, the E-NE−trending fragment of the Juan Fer-nández Ridge began to subduct below the Los Pelambres re-gion (Yañez et al., 2001; Kay and Mpodozis, 2002), causingthe Andean deformation front to move eastward; uplift of theFrontal Cordillera (Fig. 11) commenced and was accommo-dated by regional thick-skinned faults (Ramos et al., 1996,2004; Pérez, 2001; Giambiagi et al., 2003; Fig. 12). By con-trast, south of 33° S, the youngest and largest deposits, in-cluding Río Blanco-Los Bronces and El Teniente (Maksaev etal., 2004; Deckart et al., 2005; Maksaev et al., 2009; Fig. 12),were emplaced from 7 to 4 Ma during a period (7−5 Ma) oftransient shallowing of the subducting slab below theNeuquén basin, when volcanism with arc-like geochemicalsignatures expanded up to 500 km east of the present dayPerú-Chile trench (Ramos and Folguera, 2005; Kay et al.,2006).

From a structural point of view, porphyry copper depositsof central Chile and contiguous Argentina include a group ofstocks that postkinematically intruded regional thrusts alongthe boundary between the former Abanico basin and the thin-skinned La Ramada fold-and-thrust belt (Los Pelambres,Amos-Andrés, Pimentón; Figs. 12, 14a), along the northernpart of the belt. Another, southern group (e.g., Vizcachitas,West Wall, Río Blanco-Los Bronces, and El Teniente; Figs.11−12) comprises intrusive complexes emplaced farther westof the thrust belt in gently folded sequences of the Abanicobasin, with no obvious relationship to regionally significanttectonic structures. A distinctive feature of the porphyry sys-tems at Los Pelambres, Río Blanco-Los Bronces, and El Te-niente is the occurrence of barren and/or mineralized mag-matic hydrothermal breccia complexes (Skewes and Stern,1994).

Los Pelambres

The late Miocene Los Pelambres porphyry copper-molyb-denum deposit and its smaller gold-bearing satellite, theFrontera deposit (Fig. 12), are located in a narrow belt of in-tense deformation that involves Oligocene to early Miocene(33−18 Ma) volcanic rocks of the Abanico basin; these vol-canic rocks, previously described as the Los Pelambres For-mation, form part of the northern termination of the La Ra-mada fold-and-thrust belt at 31°42' S (Mpodozis et al., 2009b;Perelló et al., 2012 ; Fig. 14a). The deposit is formed by mul-tiple magmatic-hydrothermal centers with ages of 12.3 to 10.8Ma, hosted by a precursor quartz diorite pluton (Los Pelam-bres stock, 13.6−13.0 Ma) and adjacent andesitic (Abanico


0361-0128/98/000/000-00 $6.00 348


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Formation) country rocks. The precursor stock and its con-tained porphyry copper mineralization were postkinematicallyemplaced along the high-angle, Los Pelambres reverse fault,which constitutes the eastern limit of the above-mentionedzone of concentrated deformation (Mpodozis et al., 2009b;Perelló et al., 2009, 2012). At a more regional scale, however,the Los Pelambres stock appears to be a satellite intrusivebody of a much larger (>250 km2) and long-lived, multistagepluton located a short distance to the west (Chalinga intrusivecomplex; Fig. 14), which was active for at least 8 m.y. between23 and 15 Ma. The complex includes a pretectonic (with ref-erence to regional deformation) suite of gabbros, pyroxenediorites, and granodiorites, with U-Pb zircon ages between 23and 21 Ma; a group of 18 Ma syntectonic olivine gabbro-dior-ites and granodiorites; and a younger and larger group ofpost-tectonic, 16 to 15 Ma hornblende-bearing granodiorites.Rocks of this latter group form the eastern margin of theChalinga intrusive complex, where they cut the traces of re-gional thrust faults (Fig. 14a).

Several small granodiorite to quartz diorite stocks andhornblende-bearing dacite porphyry intrusions, dated at 15 to13 Ma (Fig. 14a), form a NW-SE−trending string that extendsfrom the eastern Chalinga intrusive complex across the inter-national frontier to Cerro Mercedario, ~70 km to the south-east in Argentina showing a southeastward propagation of intrusive magmatism from the Chalinga complex during themiddle to late Miocene (Figs. 12, 14a). Some of these intru-sions are associated with large porphyry-related hydrothermalalteration zones, such as El Yunque (Fig. 14a). Porphyry mag-matism and copper mineralization at Los Pelambres evolvedalong this trend between ~14 and 10 Ma, whereas much morelimited data suggest that El Pachón was active between 9.2and 8.4 Ma, and the Cerro Mercedario porphyry copper de-posit at ~13 Ma (Sillitoe, 1977; Bertens et al., 2006; Perelló etal., 2012).

Río Blanco-Los Bronces

The world’s largest copper district at Río Blanco-Los Bronces(Serrano et al., 1996; Skewes et al., 2003; Frikken et al., 2005;Irarrázaval et al., 2010; Toro et al., 2012), is located, farthersouth, near the center of the former Abanico basin (Figs.11−12). Although younger than Los Pelambres, it also formedduring the final stages of evolution of a long-lived, >10-m.y,magmatic system, which includes a large premineral intrusivecomplex (San Francisco batholith; Fig, 14b); this is remarkablysimilar to the Chalinga intrusive complex of the Los Pelambresarea and was emplaced within gently folded, early Miocene(18−15 Ma) volcanic rocks (Farellones Formation; Fig. 14b).

Deckart et al. (2005, 2012) recognized that the San Fran-cisco batholith includes three main intrusive phases, emplacedbetween12 and 8 Ma, although an older U-Pb zircon age (14.7Ma) for pyroxene monzodiorites (Jerez, 2007; Deckart 2012)indicates that magma emplacement began during the middleMiocene. The main mineralization at Río Blanco-Los Broncesoccurred along an 8-km-long, N-NW−trending corridor,which commences in the San Francisco batholith and extendsfrom Río Blanco in the northwest to Los Sulfatos in the south-east (Fig. 14b). Multiple magmatic-hydrothermal brecciacomplexes associated with porphyry copper-bearing, amphi-bole-rich quartz diorite porphyry intrusions were emplaced

along this trend, beginning at 7.7 Ma, during the waningstages of the San Francisco batholith. Intrusive activity termi-nated with the emplacement of late-mineral dacitic porphyryintrusions at ~5 Ma, and the postmineral La Copa rhyolitebreccia complex at 4.7 Ma (Skewes et al., 2003; Deckart et al.,2005, 2012; Irarrázaval et al., 2010; Toro et al., 2012; Fig.14b).

El Teniente

El Teniente, 100 km south of Río Blanco-Los Bronces, islocated near the axis of the Maipo orocline, at the center ofthe deformed Coya-Machalí (Abanico) basin and 30 km westof the most internal thrust sheets of the Aconcagua fold-and-thrust belt (Fig. 15). The deposit is hosted by gentlyfolded volcanic rocks of the El Teniente volcanic complex(informal unit equivalent to the Farellones Formation),which unconformably overlies Oligocene to early Miocenevolcanic rocks of the Coya-Machalí Formation (Kay et al.,2005; Stern et al., 2010). Mineralization at El Teniente is as-sociated with a magmatic-hydrothermal center that recordsat least 6 m.y. of continuous activity between ~9 and 3 Ma.As at Los Pelambres and Río Blanco-Los Bronces, magma-tism at El Teniente includes a large premineral intrusivecomplex containing older mafic facies (andesite intrusivesills and olivine-pyroxene gabbros), described as the Te-niente Mafic Complex, and a younger core of hydrous, horn-blende-bearing intrusions (Sewell Tonalite; Cannell et al.,2005; Stern et al., 2010; Vry et al., 2010; Fig. 15). Ages forthis group of intrusions are still not well constrained (Te-niente Mafic Complex: 8.4 Ma K-Ar whole-rock; SewellTonalite: 7.05 Ma total gas, Ar/Ar; Maksaev et al., 2004,Stern et al., 2010, and references therein).

The copper mineralization at El Teniente is associated witha >2-km-long, N-NW−trending body of dacite porphyry anda series of small magmatic-hydrothermal breccias (Vry et al.,2010 ), which in contrast to Los Pelambres or Río Blanco-LosBronces, were emplaced within the earlier intrusions (Te-niente Mafic Complex and Sewell Tonalite; Fig. 15). DetailedU-Pb zircon, Ar/Ar, and Re-Os geochronology (Maksaev etal., 2004) has allowed recognition of five pulses of felsic in-trusions linked to mineralization emplaced between 6.5 and4.6 Ma. Subsequently, the late-mineral 4.81 Ma Braden brec-cia pipe and a family of narrow, E-NE−oriented olivine-horn-blende lamprophyre dikes; ages from 3.9 to 2.9 Ma (Stern etal., 2011) mark the last magmatic pulses in the district beforethe magmatic front migrated ~50 km eastward to the Chile-Argentine frontier to form the northernmost active volcanoesof the modern Andean Southern volcanic zone (Fig. 12).

Origin of Miocene-Pliocene porphyry copper alignments

One of the most striking features of porphyry copper de-posits in central Chile is their relationship to N-NW−trendingalignments of multiple magmatic-hydrothermal centers, em-placed during the final stages of long-lived magmatic systemsthat include breccia complexes whose formation has beenconsidered to be triggered by decompression during rapid ex-humation and tectonic uplift (Warnaars et al., 1985; Skewesand Stern, 1994).

When the overall geometry of the porphyry alignments ofthe three main centers described in this paper are considered,


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their orientation is similar and almost perpendicular to the di-rection of the Miocene and Pliocene plate convergence (So-moza and Ghidella, 2005). In contrast, the late lamprophyredikes at El Teniente (Fig. 15) are nearly parallel to theMiocene plate convergence vector (see Fig. 16). Recent stud-ies on the relationships between volcanism and tectonics inthe modern Southern volcanic zone of the Andes (Sepúlvedaet al., 2005; Cembrano and Lara, 2009) have shown thatprimitive mantle-derived basalts ascend along NE-trendingfractures and faults, parallel to σHmax, which is regionallyclose to the orientation of plate convergence. Nevertheless,during the recent eruptions of southern Andes volcanoes likePuyehue-Cordón Caulle (40°30' lat S, 1960, 2011) evolvedrhyolites erupted along NW-trending basement faults thatare, in theory, severely disoriented in relationship to re-gional stresses to allow magma ascent. To overcome thisparadox, Sepúlveda et al. (2005) and Lara et al. (2006) havesuggested that coseismic or postseismic stress relaxationduring large subduction-zone earthquakes can producetransient episodes of extension that allow ascent of evolvedmagmas that otherwise would remain trapped in crustalreservoirs. Similarly, decompression during seismic events(see Sibson, 1987, 1994) appears to be a plausible mecha-nism to explain repeated cycles of breccia formation andporphyry intrusion along now-concealed N-NW− to NE-ori-ented faults that may have tapped the roofs of overpres-sured, deeper seated magma chambers during the evolutionof the central Chile porphyry coppers systems.

Discussion

Linking geochemistry and tectonics: the suggested adakite connection

Both the middle Eocene to early Oligocene and late Mioceneto early Pliocene porphyry copper-bearing intrusions includeintermediate rocks (SiO2 >56%) with abundant hydrous min-eralogy, dominated by hornblende-bearing granodiorites anddacites; these intrusions show geochemical and isotopic sig-nature (SiO2 >56%, Sr >400 ppm, high Sr/Y ratios, lowHREE contents, high La /Yb ratios, 87Sr/86Sr <0.704) similarto those described for typical adakitic rocks (cf. Defant andDrumond, 1990; Castillo, 2012). Concave middle REE pat-terns in Chilean porphyry coppers indicate hornblende frac-tionation, whereas the lack of negative europium anomaliesdenotes a high oxidation state of the magmas (see Kay et al.,2005).

A relationship between adakites and mineralized porphyrysystems was proposed by Thiéblemont et al. (1997) and Kayand Mpodozis (2001), while some authors (e.g., Sun et al.,2011) have suggested that the adakitic signatures of copper-rich magmas are indicative of direct melting of subductedoceanic crust. In accord with these views Oyarzún et al.(2001) proposed that the middle Eocene to early Oligoceneporphyry copper belt of northern Chile may have beenformed when fast, oblique convergence led to flat subductionand direct melting of the downgoing plate, whereas Reich etal. (2003) proposed that the porphyry copper intrusions at


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Teniente Volcanic Complex

Braden Pipe (4.81±0.10 Ma)

Hydrothermal brecciasMafic Complex (8.9±14.4 Ma)

Sewell Tonalite (7.05 0.14 Ma)±

Laguna La Negra

Laguna La Huifa

BradenPipe

6232

6228

6224

N Mine

1600

800

00

800

MarginalBreccia

1200

Level Teniente 5(2.284m)

Teniente Dacite Porphyry5.28±0.10 Ma

Latite Dike4.82±0.09 Ma

Porphyry A5.67±0.19 Ma

Daciticporphyries

6.09±0.18 Ma

Braden Pipe4.81±0.10 Ma

Sewell Tonalite7.05±0.14 Ma

Mafic Complex8.9±1.4 Ma?

400

Biotite breccia Igneous breccia Anhydrite breccia Tourmaline breccia

Mafic Complex Sewell Tonalite Porphyry "A" Dacite Porphyry

Braden Pipe

Supergene zone

(a)(b)

Lamprophyre dykes

0 2 km

0 400 km

Unconsolidated deposits

FIG. 15. Intrusive complexes in the area of El Teniente porphyry copper deposit (Stern et al., 2010). Note the N-NWtrend of mineralized porphyry intrusions and magmatic-hydrothermal breccias, hosted within the precursor Teniente maficcomplex and Sewell Tonalite. Late-stage lamprophyre dikes are perpendicular to the trend of the copper-bearing porphyrydeposits and breccias.

Los Pelambres formed by melting of the subducting east-northeast arm of the Juan Fernández Ridge under flat-slabconditions. However, as noted by Kay and Kay (2002) andCastillo (2012), thermal models for subducting plates showthat sufficiently high pressure-temperature conditions forslab fusion can be reached only in exceptional circumstances,including subduction of very young and hot oceanic crust, asprevailed during emplacement of the 12 Ma Cerro Pampaadakites near the Chile Triple Junction in Patagonia (Kay andKay, 2002).

Richards and Kerrich (2007) and Richards (2011b) stronglyargued against slab melts being a necessary ingredient in porphyry copper-gold mineralization, pointing out that thecritical factors for adakite genesis include elevated water andsulfur contents as well as high oxidation state of the magmas,which together result in hornblende fractionation and sup-pression of plagioclase crystallization. Richards (2011b) con-sidered that such hydrous, oxidized conditions are typical innormal arc settings. Nevertheless, this view is inconsistentwith the fact that giant porphyry copper deposits are notwidespread throughout the Andean history; as we have shown

they formed during and/or after the most important tectonicepisodes that reshaped the whole Andean margin leading tomountain building and crustal thickening.

Kay et al. (1999) and Kay and Mpodozis (2001) argued thatpart of the water content of mineralizing Andean magmas canbe derived from the exsolution of fluids during the transfor-mation of hydrous lower crustal amphibolite to dry garnet-bearing eclogite during crustal thickening. When the crust ofthe arc thickens to a critical value of ~45 km, amphibole andplagioclase break down, water is liberated, and eclogite forms.These relationships explain why the largest porphyry copperdeposits preferentially form during contractional events, suchas the Incaic episode of southern Peru and northern Chile,and the late Miocene to early Pliocene event in central Chileand contiguous Argentina.

Another process that can also contribute to generation ofwater-rich adakitic magmas during periods of deformation issubduction erosion (von Heune and Scholl, 1991; Stern, 1991,2011; Kay et al., 2005). Subduction of oceanic crust, pelagicand terrigenous sediments, and crust tectonically erodedfrom the edge of the continent into the mantle-source region


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140 130 120 110 100 80 70 60 50 40 30 20 1090

East PacificWest PacificSouth Pacific

10

7

6

5

4

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half-spreadingrate

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and present-daySouth American

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FIG. 16. Temporal changes of critical plate parameters that can be linked to adjustments in the tectonic regime along theAndean margin. (a). Ocean crust production (average half-spreading rates since 140 Ma) for different regions of the Pacificbasin (Conrad and Lithgow-Bertelloni, 2007). (b). Absolute velocity of the South American plate since 80 Ma, treated as anangular velocity vector decomposed into its ~ E to W and S to N components (Silver et al., 1998). (c). Cenozoic convergencerates between the Farallon-Nazca and South American plates. Data from: 1 = Sdrolias and Muller (2006), 2 = Pardo-Casasand Molnar (1987), 3 = Somoza (2008), 4 = Somoza and Ghidella (2005). (d). Direction of the Farallon-Nazca plate motion,shown as the deviation angle from the E-W path (0°, north = positive, south = negative). Adapted from Somoza and Ghidella(2005). Shaded areas in all graphics indicate the time of porphyry copper emplacement during the Eocene to Oligocene andMiocene to Pliocene.

of Andean magmas may provide large amounts of water (e.g.,Stern et al., 2010). Transient geochemical changes, such asthose depicted in Figure 1d, showing adakitic signatures(Haschke et al., 2002; Kay et al., 2005) are consistent with theloss of slivers of fore-arc crust by subduction erosion (Fig. 13)during, or immediately following, major Mesozoic to Ceno-zoic deformation events. Intermittent and massive loss offore-arc crust is a possibility that has been recognized in re-cent numerical models of subduction erosion reported byKeppie et al. (2009) and may explain the abrupt eastwardshifts of the magmatic front that punctuated the tectonic his-tory of the Central Andes. Without ignoring other models foradakite formation (e.g., Castillo, 2012), high Sr/Y hydrousmagmas linked to the middle Eocene to early Oligocene andlate Miocene to early Pliocene porphyry copper deposits ofnorthern and central Chile, respectively, are most likely at-tributed to a combination of melting of mantle derived mag-mas, including asthenosphere contaminated with pieces offore-arc crust that entered the mantle through subductionerosion, that mixed with fluids derived from dehydration ofthe base of thickened amphibole-rich lower crust duringthese two main periods of Andean deformation (e.g., Kay etal., 2007b).

Plate tectonics and Andean tectonic regimes

As discussed above, giant Andean porphyry copper de-posits were emplaced during regional deformation eventsthat reshaped the entire Andean margin. Regional tectonicand tectonomagmatic events are, ultimately, the result ofchanges in plate interaction parameters that could influencethe state of stress in the overriding plate. Among thesechanges on sea-floor spreading and plate convergence rates,hot-spot activity, absolute plate velocity, and shifts in the po-sition of the plate contact (trench roll-back), together withvariations in slab age, width, and/or dip has been consideredto strongly influence the dynamics of convergent marginsworldwide (e.g., Uyeda and Kanamori, 1979; Heuret andLallemand, 2005; Schellart and Rawlinson, 2010, and refer-ences therein). In recent years, variations in absolute platevelocities (Russo and Silver, 1996; Silver et al., 1998) as wellas variations on the degree of mechanical interplate cou-pling (Yañez and Cembrano, 2004; Luo and Liu 2009a, b;Iaffaldano et al., 2012) have been proposed as some of thefundamental controls on Andean orogeny. Lamb and Davis(2003) even suggested that differences in plate coupling arepossibly linked to along-strike differences in climate thatmodulated the supply of sediments to the trench along theAndean margin which, upon subduction, lubricated theplate interface and, as a result, determined the degree ofmechanical coupling.

Tectonic regime: Northern Chile

Figure 16 shows time-dependent variations for some platetectonic parameters that can be compared with the geologicrecord of the Andean margin. A correlation can be made be-tween the Incaic compressional event and an episode of veryrapid oceanic crust production in the eastern Pacific, whichpeaked in the middle Eocene at ~40 Ma (Fig. 16a; Conradand Lithgow-Bertelloni, 2007). At the same time the east-west component of the absolute velocity vector of the South

American plate (Silver et al., 1998) sharply increased (Fig.4b). However, as shown in Figure 4c these effects were notbalanced, as should have occurred, by an associated increasein the Farallon-South America convergence rate. Accordingto Somoza (1998) and Somoza and Ghidella (2005), the Far-allon-South America convergence velocity was rather low(6−7 cm/yr) or, as alternatively suggested by Pardo-Casas andMolnar (1987) and Sdrolias and Muller (2006), was rapidlydecreasing (Fig. 16c).

This apparent paradox can be resolved if a high degree ofcoupling existed at the plate interface at that time. If this wasthe case for northern Chile and southern Peru during theEocene, the mechanically weak margin of the Central Andes,pushed from the east and west, may have started to bend andcontract to form the Bolivian orocline and the Domeyko faultsystem. High interplate coupling may also explain the forma-tion of a flat-slab zone and the subsequent Oligocene mag-matic lull recorded in southern Peru and northern Chile (e.g.,Sandeman et al., 1995; James and Sacks, 1999; Kay et al., 1999;Perelló et al., 2003a; Hasckhe et al., 2006; Kay and Coira,2009). Slab flattening intensifies interplate frictional forces,which increases the possibility of subduction erosion; this, inturn, would promote eastward migration of the shortening andmagmatic fronts toward the Andean foreland (e.g., Espurt etal., 2008; Keppie et al., 2009; Martinod et al., 2010). Conver-gence rates increased during the Oligocene and Miocene,leading to steepening of the Incaic flat slab; accumulation ofrelated mafic magmas below the already thickened crust ledto the production of crustal melts and the eruption of largevolumes of ignimbrites during the inception of volcanismalong the modern Central Andean volcanic zone during theMiocene (Kay and Coira, 2009).

Tectonic regime: Central Chile

Although the tectonic evolution of central and south-cen-tral Chile appears to be different from that of northern Chile,it is in fact complementary and preconditioned by the forma-tion of the Bolivian orocline. As noted above, crustal flux to-ward the north along the southern limb of the orocline duringthe middle Eocene to early Oligocene Incaic contraction mayhave caused stretching and thinning of the upper crust in cen-tral Chile, thereby facilitating the opening of the Abanicobasin (McQuarrie, 2002; Arriagada et al., 2008; Boutelier andOncken, 2010). The along-strike differences in tectonic stylealso agree with the numerical simulations by Schellart (2008),which show that higher compression occurs near the center ofwide subduction zones where the trench remains stationaryor advances toward the continent. Rapid trench retreat (roll-back) along the lateral slab edges may explain extension in theoverriding plate and be compatible with the opening of theAbanico basin in central Chile.

The late Eocene to Oligocene opening of the Abanico basinoccurred during a period of steady westward displacement ofthe South America plate (Silver et al., 1998), a period that alsocoincided with another transient episode of very rapid gener-ation of oceanic crust in the eastern Pacific, which com-menced immediately after the formation of the Nazca plate(Conrad and Lithgow-Bertelloni, 2007; Fig. 16a). In contrastto the situation during the Incaic event, the convergence ratebetween the Nazca and South American plates reached, at


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this time, a record high (15 cm/yr; Somoza, 1998; Sdrolias andMuller, 2006; Fig. 16b). Such a disparity may be explained ifweak plate coupling permitted rapid subduction and, as aconsequence, the generation of large volumes of magma andextension in the overriding plate during the Oligocene toearly Miocene in central Chile and contiguous Argentina.

The beginning of contraction in central Chile and contigu-ous Argentina at ~20 Ma coincides (as shown in Fig. 16d)with an acceleration of the absolute motion of South America(see discussion in Kay and Copeland, 2006), drastic decreasein oceanic crust production in the eastern Pacific, and a dropin plate convergence rates between the Nazca and SouthAmerican plates (Somoza, 1998; Conrad and Lithgow-Bertel-loni, 2007; Fig. 16). Inversion of the Abanico basin resulted,north of 35° S in continued deformation during the Miocene,causing an increase in crustal thickness to >50 km (Ramos etal., 2004) and enhanced subduction erosion. Contaminationof the asthenosphere through subduction of fore-arc crustcreated favorable conditions to produce water-rich maficmelts with high sulfur and metal contents; these melts hadthe capacity to ascend and evolve within an upper crustalmagma chamber to generate large porphyry copper deposits.

Concluding RemarksThere are few studies that consider the relationships be-

tween the regional-scale tectonic evolution of the Andes andthe formation of giant Cenozoic porphyry copper deposits.However, it is apparent that these deposits formed duringcritical moments in the tectonic evolution of the Andean mar-gin. The emplacement of the large middle Eocene to earlyOligocene porphyry copper intrusions in northern Chileseems to be associated with the formation of the Bolivian orocline during the Incaic event, which was the result of anunusual combination of factors. One critical factor was the ac-celeration of the absolute westward motion of South Americaconcurrent with strong mechanical coupling between theSouth American and Farallon plates at a time when the rateof ocean-crust production in the eastern Pacific was veryhigh. Bending of the Chilean margin during the Incaic eventactivated the Domeyko fault system in northern Chile andtriggered the accompanying crustal thickening, slab shallow-ing, and increased subduction erosion. Volcanism virtuallyceased and favorable tectonomagmatic conditions (i.e. en-hanced, subduction erosion, crustal thickening, lower crustdehydration) permitted the formation of fertile hydrous mag-mas, while the transpressional and/or compressional upperplate tectonic regime contributed to the establishment oflong-lived, upper-crustal magma chambers from which con-centrating copper evolved, mostly below the Domeyko faultsystem.

Younger, late Miocene to early Pliocene porphyry copperdeposits of central Chile and contiguous Argentina were emplaced after inversion and collapse of the extensional,intra-arc Abanico basin. The basin evolved between the lateEocene and early Miocene when a relatively stable position ofSouth America over the mantle, linked to weak interplatecoupling, permitted fast subduction of the Nazca plate underthe Andean margin. Acceleration of the westward motion ofSouth America relative to the mantle reference frame at 20Ma induced contractional deformation, accompanied by

crustal thickening and eastward migration of the magmaticfront. At the same time, the subduction angle shallowed,leading to the formation of a flat-slab region between 27° and33° S as the Juan Fernández Ridge was being subducted be-neath the western edge of South America. These changesagain created favorable conditions for the formation of fertilehydrous magmas.

The relationships described above demonstrate that theconcentration of huge porphyry copper deposits in theChilean Andes resulted directly from the tectonic evolutionof the margin and indicate that a tectonic trigger is essentialfor the formation of giant porphyry coppers systems.

AcknowledgmentsThis contribution is the result of long years of work with

many colleagues at the Chilean Geological Survey, AntofagastaMinerals, and various universities both in Chile and abroad.We are especially grateful to Sue Kay, Andy Tomlinson, TerryJordan, Cesar Arriagada, Moyra Gardeweg, Rick Allmendinger,Victor Ramos, Pierrick Roperch, Francisco Camus, StephenMatthews, Nicolás Blanco, Francisco Hervé, Reynaldo Char-rier, Carlos Münchmeyer, Ricardo Muhr, José Cembrano,Carlos Arévalo, and many others who, for lack of memory, weare here unable to mention. We thank Jeff Hedenquist, DickSillitoe, José Perelló, and Francisco Camus for pushing usthrough this endeavor, and Antofagasta Minerals for provid-ing time and support for the writing. Francisco Moraleshelped with the preparation of the figures. Victor Ramos, SueKay, José Perello, Jeff Hedenquist, and Dick Sillitoe carefullyedited the manuscript and made numerous suggestions thathelped to greatly improve earlier versions of the manuscript.

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