chapter 4 geologic setting and evolution of the porphyry

79

IntroductionLOS PELAMBRES, the northernmost and third largest copperconcentration in the Miocene to early Pliocene belt of cen-tral Chile (Fig. 1), comprises two contiguous deposits, LosPelambres copper-molybdenum and Frontera copper-gold.The deposits underlie the vegetation-free talus slopes of a U-shaped glacial valley in the Principal Cordillera of the Andes,

at elevations between 3,200 and 3,600 m above sea level(Fig. 2a). Los Pelambres and Frontera along with the cop-per-molybdenum deposit at El Pachón, 5 km southeastacross the international frontier in Argentina, constitute theLos Pelambres-El Pachón porphyry copper cluster (Fig. 1).The supergiant status of the Los Pelambres deposits is de-fined by their current resource of 6,165 million metric tons(Mt) at 0.56% Cu and 0.011% Mo, using a cutoff of 0.35%Cu (Perelló et al., 2011).

Chapter 4

Geologic Setting and Evolution of the Porphyry Copper-Molybdenum and Copper-Gold Deposits at Los Pelambres, Central Chile

JOSÉ PERELLÓ,1,† RICHARD H. SILLITOE,2 CONSTANTINO MPODOZIS,1 HUMBERTO BROCKWAY,1 AND HÉCTOR POSSO3

1 Antofagasta Minerals S.A., Apoquindo 4001, piso 18, Las Condes, Santiago, Chile2 27 West Hill Park, Highgate Village, London N6 6ND, England

3 Anaconda Perú, Avenida Paseo de la República 3245, piso 3, San Isidro, Lima, Peru

AbstractThe porphyry copper mineralization at Los Pelambres is contained in two contiguous deposits, Los Pelam-

bres (Cu-Mo) and Frontera (Cu-Au), which together constitute the third largest copper concentration (~36million metric tons (Mt) Cu) in the Miocene to early Pliocene belt of central Chile. Los Pelambres is centeredon a composite, N-oriented, ~4.5- × 2.5-km precursor quartz diorite stock emplaced within the regional,NNW-striking, E-vergent Los Pelambres reverse fault. The fault places intensely deformed Late Cretaceousvolcanic and late Oligocene to early Miocene volcanic and volcanosedimentary rocks of the Los Pelambres For-mation over gently folded early Miocene volcanic rocks of the Pachón Formation. Copper-gold mineralizationat Frontera is hosted mainly by andesite of the Pachón Formation.

Hydrothermal alteration at Los Pelambres-Frontera conforms to the classic zonal pattern in which a potas-sic center grades laterally to an annular sericitic zone surrounded by a propylitic halo. The bulk of the hypo-gene metal resource is hosted by multiple veinlet generations within potassic alteration, of which type 4 (quartz± K-feldspar ± biotite ± sericite ± phengite ± andalusite ± corundum), A, and B types are volumetrically andeconomically the most important. The type 4 veinlets are regularly distributed throughout Los Pelambres andFrontera, whereas highest intensities of A and B veinlets display a spatial correlation with at least 20 small(~200-m diam), SE-plunging magmatic-hydrothermal centers. These centers comprise one or more intermin-eral porphyry intrusions of dacitic (porphyry B) and andesitic (porphyry A) compositions along with igneousand hydrothermal breccias, the apical parts of which contain aplite and pegmatite pods. These centers acted asa series of miniature porphyry copper deposits whose coalescence generated the Los Pelambres-Frontera ore-body. This coalescence also led to deposit-scale sulfide zoning, from internal chalcopyrite-bornite through chal-copyrite-pyrite to external pyrite. Abundant hydrothermal magnetite accompanies the gold-bearing coppermineralization in biotitized andesite at Frontera. The sericitic alteration is largely pyritic, but a NE-striking,SE-dipping corridor of D-type veinlets that overprints the potassic alteration in the northwestern quadrant ofLos Pelambres contains copper sulfosalts. The internal portions of this corridor are characterized by advancedargillic assemblages, defining the roots of a once more extensive lithocap.

On the basis of detailed U-Pb zircon dating, the intrusive magmatism at Los Pelambres-Frontera lasted ~3.8m.y., from emplacement of the precursor Los Pelambres stock between ~14 and 12.5 Ma, through generationof numerous porphyry B and A phases and associated magmatic-hydrothermal centers between ~12.3 and 10.5Ma, to intrusion of late mineral porphyry at Frontera at ~10.2 Ma. Similarly, the copper, molybdenum, and goldmineralization was introduced during a protracted interval of ~1.7 m.y., between 11.8 and 10.1 Ma, as con-strained by Re-Os molybdenite geochronology. The entire system cooled to nearly ambient temperatures by~8 Ma, as supported by temporally overlapping K-Ar, Ar/Ar, and (U-Th)/He ages, and was exposed to the effects of supergene oxidation and immature enrichment by ~5 Ma. Plio-Pleistocene glaciation partially erodeda former, more widespread supergene chalcocite blanket, the remnants of which accounted for the bulk of theore mined during the first 10 years of the Los Pelambres open-pit operation.

The southeast-inclined geometry of the entire Los Pelambres-Frontera system, including the porphyry cen-ters and northeast structural corridor defined by sericitic and advanced argillic alteration, are ascribed to syn-mineral tilting. The tilting accompanied regional tectonic uplift during crustal shortening and thickening,which were controlled by thick-skinned reverse faults active ~60 km farther east in Argentina.

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

© 2012 Society of Economic Geologists, Inc.Special Publication 16, pp. 79–104

Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/3812113/9781629490410_ch04.pdfby gueston 05 August 2020

The deposits at Los Pelambres are owned by Minera LosPelambres (Antofagasta Minerals S.A. 60%, Japanese consor-tium 40%). The copper-molybdenum deposit, mined in anopen pit (Fig. 2b) at a current ore throughput of 176,000 t/daveraging 0.74% Cu and 0.019% Mo, produced 411,800 t ofcopper, 9,900 t of molybdenum, and 39,800 oz of gold, and1,774,300 oz of silver in 2011. The ore is processed by con-ventional flotation and the resulting copper concentrate istransported ~120 km by slurry pipeline to the company’s portfor shipment to overseas smelters. Los Pelambres and Fron-tera are currently the subject of another major infill drillingcampaign, which is likely to further increase resources for aplanned future mine expansion.

This contribution summarizes the historic and recent explo-ration history of the district, describes the regional geologic

setting and the geology and alteration-mineralization featuresof Los Pelambres and Frontera, documents the lifespan ofthe hydrothermal system, and discusses the geologic evolu-tion of the porphyry mineralization within a regional tectono-magmatic framework. The paper is based on more than threeyears of fieldwork by the authors during a brownfields explo-ration program that included 1:50,000-scale regional, 1:10,000-scale district, and 1:2,000-scale pit mapping as well as 1:100-scale logging of 160,000 m of preexisting and newly obtaineddrill core. Previous published studies by Sillitoe (1973),Skewes (1985), Skewes and Atkinson (1985), Atkinson et al.(1996), Bertens et al. (2003, 2006), Perelló et al. (2007, 2009,2011), and Mpodozis et al. (2009) as well as extensive unpub-lished in-house data provide the basis for this synthesis. Theporphyry and veinlet nomenclature of Skewes and Atkinson

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FIG. 1. Location of the Los Pelambres-El Pachón porphyry copper cluster in the Miocene to early Pliocene porphyry cop-per belt of central Chile (diagonal shading). The principal deposits are named. The position of the belt with respect to thetransition between amagmatic flat-slab subduction and the Southern Volcanic Zone of the Andes is defined by depth con-tours on the present-day Wadati-Benioff zone (after Cahill and Isacks, 1992; Anderson et al., 2007).

a b

FIG. 2. Views of Los Pelambres porphyry copper deposit, looking north. a. In 1970 prior to mining. Note the jarositicleached capping (yellowish-brown) developed over the pyrite-rich sericitic halo. The ore-bearing potassic zone underlies theU-shaped glacial valley. b. The open pit in 2007 after seven years of large-scale mining.


(1985) and Atkinson et al. (1996) is followed throughout, butthe relative timing of porphyry phases, the economic relevanceof certain veinlet generations, and the volumetric importanceof hydrothermal breccias are considered to be different.

Exploration HistoryIn 1967, the Instituto de Investigaciones Geológicas pre-

pared the first geologic report for Los Pelambres, which for-mally identified the porphyry copper affiliation of theprospect (Thomas, 1967). Exploration was resumed in 1969under a joint program conducted by the United Nations andEmpresa Nacional de Minería (ENAMI), the state miningagency. Drilling defined a mineral inventory of 430 Mt at0.80% Cu and 0.035% Mo (Sillitoe, 1995). No further explo-ration took place until 1979 when Anaconda South Americapurchased the property from the local owners and undertookdetailed exploration, culminating in 1983 with completion ofa feasibility study for a 60,000 t/d operation based on a re-source of 3,300 Mt at 0.63% Cu and 0.016% Mo (Sillitoe,1995; Atkinson et al., 1996). At the copper prices prevailing atthe time, such a large-scale project was considered uneco-nomic and all work was discontinued.

In 1985, Antofagasta Holdings acquired Anaconda’s inter-ests in Chile and a wholly owned subsidiary, Compañía Min-era Los Pelambres, developed a 5,000-t/d sublevel caving andflotation operation based on high-grade, breccia-hosted ore(see below). Through 1999, Compañía Minera Los Pelambresmined approximately 12 Mt averaging 1.5% Cu (Perelló et al.,2011). Simultaneously, a feasibility study for an 80,000-t/dplant was carried out, with completion in 1996 and, the fol-lowing year, a Japanese consortium acquired a 40% interest inCompañía Minera Los Pelambres and committed the sameproportion of development funding. Project constructionbegan in early 1998 and in January 2000 the plant attained itsrated capacity. Successive expansions through 2004 increasedore throughput to 130,000 t/d, with the most recent expansioncompleted in 2010 increasing capacity to ~175,000 t/d.

Since the completion of Anaconda’s feasibility study in1983, no further exploration was carried out at Los Pelambresuntil late 2005 when the brownfields program was initiated byAntofagasta Minerals S.A. on behalf of the operating com-pany. The program resulted in discovery of the Frontera cop-per-gold deposit (700 Mt at 0.52% Cu and 0.1 g/t Au, using a0.4% Cu cutoff) and definition of additional resources at LosPelambres, which together constitute the current global re-source cited above (Perelló et al., 2011).

Tectonomagmatic Setting

Central Chile porphyry copper belt

The Miocene to early Pliocene porphyry copper belt ofcentral Chile and contiguous Argentina extends for ~400 kmbetween latitudes 31° and 35° S and contains an exceptionalcopper endowment (~360 Mt) contained in a series of super-giant and smaller sized deposits (Fig. 1). The belt was con-structed within the Chilenia terrane, a microcontinental blockaccreted to the Gondwana margin in the Devonian and un-derpinned by basement rocks of Proterozoic age (Ramos, 2009).Following terrane accretion, the belt was the site of exten-sion-related, bimodal magmatism during the Permo-Triassic,

marine sedimentation in a backarc setting during the Jurassic-Early Cretaceous, and subaerial, subduction-related, calc-al-kaline volcanism and associated plutonism during the Creta-ceous through Cenozoic (Mpodozis and Ramos, 1990).

Central Chile and contiguous parts of Argentina, includingthe porphyry copper belt, underwent contractional tectonismfrom the early Miocene through early Pliocene in response tosubduction zone shallowing (Jordan et al., 1983). This trig-gered crustal shortening and thickening through hybrid thin-and thick-skinned thrusting to generate the Aconcagua fold-thrust belt (Ramos et al., 1996; see below). The slab shallow-ing is generally ascribed to the diachronous oblique subduc-tion of the buoyant Juan Fernández ridge on the Nazca plate(Fig. 1; Pilger, 1981; Yáñez et al., 2001). The copper mineral-ization in the belt took place between 12 and 4 Ma and ac-companied multikilometer, regional-scale uplift and concomi-tant exhumation (Skewes and Holmgren, 1993; Kurtz et al.,1997). The porphyry copper stocks have rare earth elementsignatures interpreted to reflect the thickening of the crust(Kay et al., 1999; Kay and Mpodozis, 2002).

Tectonics and stratigraphy of the greater Los Pelambres region

The greater Los Pelambres region, spanning the Chile-Ar-gentina frontier between latitudes 31°35' and 32°03'S, com-prises three main tectonic domains bounded by high-angle,E- or W-vergent, N- to NNW-striking, reverse faults, hereinnamed the Los Pelambres, Totoral, and González faults (Fig.3). These structural elements form the northern terminationof the larger Aconcagua and smaller (e.g., La Ramada) fold-thrust belts (Cegarra and Ramos, 1996; Cristallini and Ramos,2000).

The eastern domain, east of the E-vergent Los Pelambresreverse fault, contains the large basement block of theCordillera de Santa Cruz (Fig. 3), composed of late Paleozoicrhyolite and felsic tuff along with comagmatic granitoids, collectively assigned to the Choiyoi Group (Alvarez, 1996;Cristallini and Ramos, 2000). Beyond the limits of Figure 3,this basement block is thrust eastward over synorogenic, con-tinental, siliciclastic deposits of Miocene age (Jordan et al.,1996; Pérez, 2001). To the west of the Cordillera de SantaCruz block, the Paleozoic basement is overlain by Triassiccontinental volcanic and sedimentary strata and Jurassic toEarly Cretaceous marine and continental sedimentary rocks(Alvarez, 1996), which together represent the northernmostexposures of the sedimentary fill to the Neuquén backarcbasin, amply developed farther south (Mpodozis and Ramos,1990; Cristallini and Ramos, 2000). These bedded units areunconformably overlain by continental volcaniclastic conglom-erate and breccia, rhyolitic tuff, and pyroxene- and hornblende-bearing andesite and dacite, herein informally grouped as theMondaca Strata (Fig. 3), which yield U-Pb zircon ages of 22.1± 0.4 and 21.6 ± 0.4 Ma (Table 1). On the Chilean side of thefrontier, at Laguna del Pelado (Fig. 3), these strata are un-conformably overlain by >400 m of subhorizontal, hornblende-bearing andesitic lava flows, which provide U-Pb zircon agesbetween 21.3 +0.4/-0.3 and 18.3 ± 0.4 Ma (Table 1). Imme-diately to the north, these rocks are tectonically overlain byPaleozoic basement and Mesozoic strata along the E-vergentMondaca reverse fault (Fig. 3).

PORPHYRY Cu-Mo & Cu-Au DEPOSITS, LOS PELAMBRES, CENTRAL CHILE 81

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Jurassic continental and marine sedimentary sequences

Early Cretaceous redbeds

Cretaceous volcanic rocks (Salamanca Formation)

Late Cretaceous (75-70 Ma) volcanic and sedimentary rocks a: Intensely deformed sequences of the central domain

Paleocene volcanic rocks Late Cretaceous-Eocene intrusive rocksReverse fault (teeth on upper plate) Normal faultUndifferentiated fault

Mondaca Strata (22 Ma) a: Basal conglomerate

Laguna del Pelado volcanic rocks (20-18 Ma)

Pachón and Abanico Formations (28-21 Ma)

STRATIFIED ROCKS INTRUSIVE ROCKS

Los Pelambres Formation (33-18 Ma)

Totoral pluton (18 Ma)Chalinga intrusive complex c: b: Phase 2 (18 Ma)a:

Phase 3 (16-15 Ma)

Phase 1 (23-22 Ma)

a

a

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Middle-late Miocene intrusive rocks (14-10 Ma)

FIG. 3. Regional geologic map of the greater Los Pelambres region, based on Alvarez (1996), Mpodozis et al. (2009), andmore recent mapping by the authors. UTM datum: Prov. S. Am 56, Zone 19 South.


A well-defined belt of pyroxene ± olivine-bearing basalticto andesitic lava flows and minor felsic tuff interbeds extendscontinuously throughout the greater Los Pelambres regionfor ~60 km and constitutes the Pachón Formation (Fernán-dez et al., 1974; Lencinas and Tonel, 1993). Near Los Pelam-bres, as well as east of the international frontier, this unityields U-Pb zircon ages between 21.69 ± 0.26 and 22.7 ± 0.2Ma and, in the Los Pelambres open pit, it is intruded by aquartz-eye dacite sill with a U-Pb zircon age of 21.36 ± 0.80Ma (Perelló et al., 2009; Table 1). The Pachón Formation isthe westernmost unit of the eastern domain, and throughoutthe area is delimited westward by the Los Pelambres fault(Fig. 3).

The central domain straddles the international frontier andcorresponds to a N- to NW-striking, ~5-km-wide, fault-bounded zone of strongly deformed andesitic to basaltic lavaflows and tuffs, fluviatile epiclastic strata, and local lacustrinelimestone of the Los Pelambres Formation (Rivano andSepúlveda, 1991) as well as tectonic slivers of Cretaceous vol-canic rocks (Fig. 3). The domain, defined and bounded by theE-vergent Los Pelambres and Totoral and W-vergent

González faults is intensely deformed, as indicated by verticaland overturned strata, anastomosing, thrust-bounded tec-tonic lenses, and widespread mesoscale, subisoclinal folds.Newly obtained U-Pb zircon ages for the Los Pelambres For-mation, ranging from 33.4 ± 0.5 to 18.0 ± 0.4 Ma (Table 1),confirm its early Oligocene to early Miocene age (Mpodoziset al., 2009; Perelló et al., 2009), contrary to previous EarlyCretaceous age assignments (Rivano and Sepúlveda, 1991;Bertens et al., 2006).

West of the Totoral and González faults, the western domaincomprises a >2-km-thick, gently E-dipping sequence of conti-nental volcanic, volcanosedimentary, and sedimentary rocks ofCretaceous age, including the Salamanca Formation (Rivanoand Sepúlveda, 1991) and other undifferentiated units (Fig.3). The steeply dipping Pocuro fault, first defined farthersouth in central Chile by Carter and Aguirre (1965), consti-tutes the tectonic contact between the Salamanca Formationto the west and the other volcanic units farther east, whichbecome more intensely deformed on approach to the centraldomain. The tectonic wedge of Cretaceous volcanic rocks between the Totoral and González faults is an example of


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TABLE 1. U-Pb Zircon Ages of Selected Geologic Units from Los Pelambres Area and Greater Los Pelambres Region

Sample no. Age (Ma ± 2σ) UTM E UTM N General location Geologic unit Comments

PEL 761 22.1 ± 0.4 376,152 6,490,687 Lower Río Carnicería Mondaca Strata Quartz-sanidine rhyolitic tuffPEL 681 21.6 ± 0.4 378,011 6,480,767 North of El Yunque Mondaca Strata Welded rhyolitic tuff

prospectPEL 211 21.3 (+0.4/-0.3) 378,020 6,458,681 Laguna del Pelado Laguna del Pelado Daciandesitic, pumice-rich,

Sequence lithic tuffPEL 161 18. 3 ± 0.4 379,343 6,461,318 Laguna del Pelado Laguna del Pelado Hornblende-bearing porphyritic

Sequence andesiteFRONT 42 22.7 ± 0.2 360,775 6,487,750 Los Pelambres Pachón Formation Fine-grained porphyritic

andesitePELLAG 012 21.69 ± 0.26 358,375 6,494,200 Río Carnicería Pachón Formation Aphanitic andesitic lavaDDH 71962 21.36 ± 0.80 350,000 6,491,440 Los Pelambres Pachón Formation Quartz-eye dacitic sillPEL 11 33.4 ± 0.5 357,720 6,485,634 Río Pelambres Los Pelambres Formation Fine-grained andesitic volcanic

brecciaPEL 411 18.5 ± 0.4 364,693 6,464,923 Río Totoral Los Pelambres Formation Recrystallized daciandesitePEL 471 24.9 ± 0.5 367,718 6,447,831 Río Chicharra Abanico Formation Andesitic lithic tuffPEL 511 22.2 ± 0.4 364,854 6,447,799 Río Chicharra Abanico Formation Fine-grained porphyritic

andesitePEL 461 18.5 ± 0.4 359,196 6,460,543 Río Totoral Totoral pluton Biotite (pyroxene) monzograniteSec1-12 23.32 ± 0.23 355,053 6,486,484 Río Pelambres Chalinga intrusive complex Pyroxene granodiorite

(Phase 1)PEL2603 21.62 ± 0.67 352,151 6,485,829 Quebrada del Perro Chalinga intrusive complex Pyroxene-hornblende-biotite

(Phase1) granodiorite PEL2473 18.59 ± 0.43 351,281 6,494,431 Upper Quebrada Chalinga intrusive complex Pyroxene-biotite diorite

Piuquenes (Phase 2)PEL2463 18.11 ± 0.52 353,490 6,494,503 Upper Quebrada Chalinga intrusive complex Hornblende-biotite granodiorite

Piuquenes (Phase 2)PEL 2014 16.5 (+0.3/-0.2) 351,270 6,509,363 Río Tres Quebradas Chalinga intrusive complex Pyroxene-biotite quartz

(Phase 3) monzodioritePEL 1031 15.1 (+0.6/-0.6) 350,890 6,504,830 Los Helados, Chalinga intrusive complex Pyroxene-biotite quartz

Río Chalinga (Phase 3) monzodioritePEL 651 15.4 ± 0.4 373,153 6,483,804 Río Pachón Northwest-trending Hornblende-biotite dacitic

intrusive belt porphyryPEL 661 15.0 ± 0.3 378,397 6,475,054 El Yunque prospect Northwest-trending Hornblende-biotite dacitic

intrusive belt porphyryPEL 1021 70.1 ± 1.5 358,755 6,488,350 Los Pelambres Country rock at Quartz-eye rhyolitic tuff

Los Pelambres

Notes: UTM datum for all samples is Prov. S. Am 56, Zone 19 South1 Dated at the University of Arizona, Tucson, Arizona2 Dated at Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, Australia3 Dated at University of Tasmania, Tasmania, Australia4 Dated at Washington State University, Washington


particularly intense deformation (Fig. 3). The volcanic rocksexposed east of the Pocuro fault yield U-Pb zircon ages be-tween 75 and 70 Ma, similar to the 70.10 ± 1.50 Ma age(Table 1) obtained from the rhyolitic tuff at Los Pelambres(see below). The Cretaceous volcanic rocks of the western do-main are unconformably overlain northwest of Los Pelambresby andesitic volcanic sequences of Paleocene age and, at RíoTotoral, by similar rocks of latest Oligocene to early Mioceneage (Fig. 3). Immediately south of the area covered by Figure3, the latter rocks provided U-Pb zircon ages of 24.9 ± 0.5 and22.2 ± 0.4 Ma (Table 1) and are correlated with the north-ernmost expressions of the Abanico Formation, widely dis-tributed farther south (Charrier et al., 2002).

Intrusive rocks of the greater Los Pelambres region

Meso-Cenozoic intrusive magmatism is abundantly devel-oped in the western domain where Late Cretaceous, Pale-ocene, Eocene, and Oligocene plutons and stocks of generallyintermediate composition intrude the volcanic and vol-canosedimentary sequences. However, intrusive activity ismarkedly less abundant farther east. South of Los Pelambres,at Río Totoral, the large, isolated, pyroxene-bearing, Totoralmonzogranite pluton, with a single U-Pb zircon age of 18.5 ±0.4 Ma (Fig. 3; Table 1), intrudes and effectively constrainsthe age of the Abanico Formation volcanism. Similarly, im-mediately west of Los Pelambres, the large 25- × 15-km,multiphase, partly nested Chalinga intrusive complex (Fig. 3)was initiated with pyroxene diorite and granodiorite phases,followed by olivine gabbro and diorite. The early intrusionshave U-Pb zircon ages between 23.3 ± 0.2 and 21.6 ± 0.6 Ma,whereas the later ones give U-Pb zircon ages of 18.6 ± 0.4 and18.1 ± 0.5 Ma (Table 1). A third, quartz monzodiorite phaseforms the northern part of the Chalinga intrusive complex, asoutheastern lobe of which cuts the Totoral fault at the west-ern edge of the central domain. This third phase yields U-Pbzircon ages of 16.5 +0.3/−0.2 and 15.1 ± 0.6 Ma (Table 1),thereby providing a minimum age for the Totoral reversefault at this latitude.

A string of equigranular to porphyritic, hornblende-bearingquartz diorite and dacite stocks extends southeastward for~70 km from the youngest phase of the Chalinga intrusivecomplex to beyond the international frontier (Fig. 3) and in-cludes, among others, the porphyry copper centers at LosPelambres, El Pachón, Yunque, and beyond the greater LosPelambres region, Cerro Mercedario (Sillitoe, 1977; Bertenset al., 2006; Mpodozis et al., 2009; Perelló et al., 2009). Datedintrusions of this transverse intrusive trend yield new U-Pbzircon ages of 15.4 ± 0.4 and 15.0 ± 0.3 Ma (Table 1). Thenorthwesternmost intrusion in this trend is the Los Pelam-bres stock and satellite bodies, for which Bertens et al. (2003,2006) and A. Bertens (writ. commun., 2007) reported U-Pbzircon ages of 13.92 ± 0.15 and 12.51 ± 0.17 Ma. These in-trusions were controlled by and sealed the Los Pelambres re-verse fault during the waning stages of Chalinga intrusivecomplex emplacement, in accord with the timing of the To-toral fault reported above.

Igneous petrochemistry of the greater Los Pelambres region

The Oligocene to Miocene volcanic sequences of the greaterLos Pelambres region are predominantly composed of andesite,

basaltic andesite, and basalt, the great majority with SiO2 con-tents between 61 and 53%. They display FeO/MgO versusSiO2 ratios transitional between the tholeiitic and calc-alka-line fields and possess flat rare earth element patterns, withLa/Yb between 5 and 19 and La/Sm between 3 and 6 (Fig. 4).Intrusive and volcanic rocks with ages between 18 and 15 Madisplay more felsic compositions (67–62% SiO2), stronger arc-like signatures, and higher La/Yb (6–34) but similar La/Sm(3–7) ratios.

The Los Pelambres stock and related porphyry copper cen-ters (see below) have low Nb/Ta ratios (<11) and steep rareearth element patterns (La/Yb: 26–72; Fig. 4). The Sm/Yb ra-tios of 4 to 9 for these intrusive rocks indicate that amphiboleand garnet were stable as residual or crystallizing phases in adeeper source region, in contrast to the Los Pelambres andPachón Formations, which are characterized by Sm/Yb ratiosbetween 2 and 3. These petrochemical features, togetherwith very high Sr/Y ratios (61–92), place the porphyry copper-related Los Pelambres intrusive rocks in the adakite field(Fig. 4; cf. Reich et al., 2003), whereas the absence of Euanomalies is compatible with the highly oxidized state of themagmas. The petrochemical evolution of the Los Pelambresmagmas is similar to that documented for other porphyrycopper deposits in central Chile (Kay and Mpodozis, 2002)and supports emplacement during crustal thickening conse-quent upon the tectonic contraction described above (Kayand Mpodozis, 2002; Hollings et al., 2005).

Geology of Los Pelambres-Frontera Area

Country rocks

Los Pelambres and Frontera are spatially and genetically re-lated to multiphase porphyry bodies that are located within andimmediately southeast of the Los Pelambres stock (Figs. 5, 6).The stock is emplaced into the Late Cretaceous rhyolitic tuffand late Oligocene to early Miocene Los Pelambres andPachón Formations (see above). The two formations are sepa-rated by the N-striking Los Pelambres reverse fault (Figs. 3, 5).

East of the fault, the Pachón Formation is composed ofgently to steeply dipping (20°−70°W), massive andesiticflows and flow breccias, whereas moderately to steeply dip-ping (50°−80°W) epiclastic rocks and andesite flows consti-tute the Los Pelambres Formation to the west (Figs. 3, 5).Several roof pendants and large blocks of massive andesite,some up to 500 m in vertical extent, are present in the LosPelambres stock, defining a NW-trending corridor throughthe north-central parts of the open pit (Fig. 5). Within the pitand its immediate environs, both formations are also in-truded by numerous, coarse-grained, porphyritic andesitedikes and sills of premineral age and, more locally, as in thenorthern high wall of the pit, by quartz-eye-bearing sills ofdacitic composition (cf. quartz-feldspar porphyry of Atkinsonet al., 1996). West of the Los Pelambres fault, the brownish-colored epiclastic sandstone and shale of the Los PelambresFormation unconformably overlie the Late Cretaceous rhy-olitic tuff (Fig. 5).

Los Pelambres stock

The late Miocene Los Pelambres stock is a N-trending, 4.5-× 2.5-km intrusion that has been intersected by drilling to a

84 PERELLÓ ET AL.

0361-0128/98/000/000-00 $6.00 84



0361-0128/98/000/000-00 $6.00 85

0.1

La

80 40

4 85 10

70 30

3 6

60 20

2 4

SiO2 Y

Yb Sm/Yb

Tholeiite

Oligocene-early Miocene volcanic rocks

Early Miocene volcanic rocks

Los Pelambres

Calc-alkaline

Adakites

Adakites

Los Pelambres

Los Pelambres

Los Pelambres

PYX HBL GAR

Normal arc

Normal arc

FeO

*/M

gO

Sr/

Y

La/Y

b

La/S

m

50 10

1 3

0

0 0

Ce

Pr

Nd

Sm

Eu

Gd

Tb Dy

Ho

Er

Tm Yb

Lu

1

10

100

10 200200

8 160160

6 120120

606

80

8

10

4 8080

404

2 4040

20 2

0 00

0 0

1000Oligocene-early Miocene volcanic rocks

Early Miocene Chalinga intrusive complex (23-22 Ma)

Early Miocene Totoral pluton (18 Ma)

Early Miocene Laguna del Pelado volcanic rocks (21-18 Ma)

Middle Miocene intrusive rocks (16-15 Ma)

Nonmineralized Late Miocene intrusive rocks (12-10 Ma)

Los Pelambres intrusive rocks

PYX = PyroxeneHBL = HornblendeGAR = Garnet

b

e

a

c

dFIG. 4. Petrochemical features of the Oligocene and Miocene igneous rocks of the greater Los Pelambres region. a.

FeO*/MgO vs. SiO2 plot. Calc-alkaline and tholeiitic fields after Miyashiro (1974). b. Sr/Y vs Y plot. Adakite and normal arcfields from Defant and Drummond (1990). c. Chondrite-normalized rare earth element patterns (Boynton, 1984). Note thesteep configuration and concave shape of the Los Pelambres intrusive rock patterns, indicating residual or crystallizing am-phibole/garnet in the magma source region. d. La/Yb vs. Yb diagram highlighting the adakitic nature of the Los Pelambresintrusive rocks. Fields are from Castillo (2012). e. La/Sm vs. Sm/Yb diagram displaying the strong petrochemical differencesbetween Oligocene to early Miocene volcanic and >15 Ma intrusive rocks compared to <15 Ma intrusions. The magmas ofthe first group evolved at low pressures in the presence of pyroxene, whereas the younger group evolved at higher pressuresacross the transition zone from amphibole to garnet stability, as indicated by higher Sm/Yb ratios. Ranges of Sm/Yb ratios arefrom Kay et al. (1999).


86 PERELLÓ ET AL.

0361-0128/98/000/000-00 $6.00 86

70° 70°

70°80°

70°

75°

80°

20°

50°

6,488,000

6,489,000

6,490,000

PIT

OU

T2

LIN

E

008

6,492,000

358,000 359,000 360,000 361,000

N

AR

GE

NT

INA

CH

ILE

3,500

3,300

3,250

3,20

0

6,491,000

70°

70°

Low-gradestockpile

FRONTERA

PORTEZUELOESTE

MINAVICTORIA

CLUSTEROESTE

MINA PORTEZUELO

LOS

L

PE

LAM

UB

RE

AS

FT Low-grade

stockpile andmoraine deposits

Diorite and dacite porphyry

Pachón Formation

a : Dominantly andesiteb : Dacite sill

a : Los Pelambres q stock (porphyritic within dotted line)b : Satellite phases

uartz diorite

Los Pelambres Formation

Cretaceous rhyolitic tuff

Magmatic-hydrothermal breccia

Porphyry A family

Porphyry B family

Intermineral Magmatic-hydrothermal centers

Late mineral

Premineral

Country rocks

a

a

b

b

Fault: mapped, inferred

Reverse fault (teeth onupthrown block)

500 m

A

B

A´(Figs.6b, 9c)

B´

PEGMATITASUR

DAM-10

DAM-06

MAM-03

DAM-18

DAM-01

RAM-05

(Figs.6a, 9b)

FIG. 5. Simplified geologic map of Los Pelambres-Frontera area, based on surface and pit mapping and core logging bythe authors. Basic geologic elements modified from Thomas (1967), Sillitoe (1973), and Atkinson et al. (1996). Positions ofsections presented in Figures 6 and 9 are also shown. UTM datum: Prov. S. Am 56, Zone 19 South.


depth of 1.5 km (Figs. 5, 6). The stock is composed of sev-eral discrete phases, including medium- and fine-grained,equigranular as well as porphyritic varieties. The bulk of thestock is made up of light- to medium-gray, subequigranularto hypidiomorphic equigranular quartz diorite (tonalite ofSillitoe, 1973, following IUGS classification [Streckeisen,1973]), with a dominance of plagioclase of andesine compo-sition over biotitized hornblende and biotite (Atkinson et al.,1996; Table 2; Fig. 6c). Minor quartz and K-feldspar occurinterstitially in the equigranular phases, and aplitic ground-mass is common in the porphyritic phases. Porphyriticquartz diorite occupies an important volume in the east-cen-tral part of the Los Pelambres stock (Fig. 5), but its contactrelationship with the rest of this intrusion is poorly defined.However, local observations suggest that the porphyriticphases postdate emplacement of the equigranular quartzdiorite.

Intermineral porphyry phasesNumerous intermineral porphyry phases are present at Los

Pelambres and Frontera, which, for practical reasons, areherein grouped into two main families: porphyry A and por-phyry B (cf. Atkinson et al., 1996). The porphyry phases typi-cally comprise dikes and small bodies of irregular geometry,the majority of which are clustered in the central parts of theLos Pelambres stock (Figs. 5, 6). A poorly defined alignmentof minor intrusions cutting andesitic rocks of the Pachón For-mation extends for ~2 km southeastward into the Fronteradeposit and beyond (Fig. 5). At least 20 discrete intrusive cen-ters form the central cluster of porphyry A and porphyry B,the largest ones attaining 250 m in diameter where subcircu-lar and 500 m in length where dike-like in form (Fig. 5). Boththe porphyry A and porphyry B families consist of multiplephases, many of which could not be unambiguously corre-lated on a hole-to-hole basis.


0361-0128/98/000/000-00 $6.00 87

a

3,800

Meters abovesea level

2008 pit

Original topography

30 year pit

3,400

3,000

2,600

B

a

b

c

d

e

A

B´

A´

2008 pit

Original topography

30 year pit

3,800

3,400

3,000

2,600


1 cm

250 m

400 m

1 cm

1 cm

a: Porphyry A familyb: Porphyry B family

bLate porphyry

UST

Breccias

Los Pelambres stock

Pachón FormationFault

a: Hydrothermal cementb: Igneous matrix

a

b

Magmatic-hydrothermal centers

Los Pelambres Formation

FIG. 6. Representative geologic sections and principal intrusive phases of Los Pelambres porphyry copper-molybdenumdeposit. a. East-west transverse section. b. Northwest-southeast longitudinal section. Note the southeast plunge of the por-phyry centers and associated features. c. Quartz diorite of Los Pelambres stock. d. Porphyry B, Los Pelambres. e. PorphyryA, Los Pelambres.


88 PERELLÓ ET AL.

0361-0128/98/000/000-00 $6.00 88

TAB

LE

2. P

rinc

ipal

Cha

ract

eris

tics

of th

e L

os P

elam

bres

-Fro

nter

a Po

rphy

ry-R

elat

ed I

ntru

sive

Uni

ts

Dom

inan

t phe

nocr

yst p

hase

Gro

undm

ass

Vol %

U

nit (

rock

type

)ph

enoc

ryst

sQ

uart

zK

-fel

dspa

rPl

agio

clas

eB

iotit

eA

mph

ibol

e2Vo

l %C

ompo

sitio

nTe

xtur

al fe

atur

es

Los

Pel

ambr

es

7015

–25%

5–10

%60

–65%

3–10

%3–

10%

30A

pliti

c (q

uart

z-Q

uart

z di

orite

com

pris

es ~

90%

of t

he s

tock

; st

ock1

0.5–

2 m

m2–

6 m

m (

to 1

cm

)1–

5 m

m1–

5 m

mK

-fel

dspa

r),

light

to m

ediu

m g

ray,

med

ium

-gra

ined

A

n 30-

An 4

0am

phib

ole

(1-3

mm

), su

bequ

igra

nula

r to

equ

igra

nula

r, hy

pidi

omor

phic

; por

phyr

itic

(por

phyr

itic

quar

tz d

iori

te)

and

fine-

grai

ned

equi

gran

ular

(m

icro

dior

ite)

phas

es p

rese

nt

Porp

hyry

B

50-6

51–

5%M

inor

to a

bsen

t40

–75%

5–20

%5–

20%

35–4

0F

elsi

tic,

Dom

inan

tly o

f dac

itic

com

posi

tion;

ligh

t to

fam

ily (

PB)

1–2.

5 m

m2–

8 m

m2–

4 m

m2–

4 m

mm

icro

aplit

ic;

med

ium

gra

y or

bro

wni

sh, w

ith m

ediu

m-

plag

iocl

ase,

gr

aine

d (2

-4 m

m)

crow

ded

porp

hyri

tic

K-f

elds

par,

text

ure;

mia

rolit

ic c

aviti

es lo

cally

pre

sent

; qu

artz

, bio

tite,

m

ultip

le p

hase

s ap

pare

nt o

n th

e ba

sis

of

amph

ibol

ecr

ossc

uttin

g re

latio

nshi

ps

Porp

hyry

A

35<2

%M

inor

to a

bsen

t20

–40%

10–2

0 %

10–2

0%65

Trac

hytic

; D

omin

antly

of a

ndes

itic

com

posi

tion;

dar

k fa

mily

(PA

)<1

mm

1–6.

5 m

m1–

6 m

m1–

6 m

mbi

otite

, gr

ay to

bro

wn,

med

ium

to fi

ne-g

rain

ed

An 3

0-A

n 40

plag

iocl

ase,

(1

-3 m

m)

porp

hyri

tic te

xtur

e; m

iaro

litic

ra

re in

ters

titia

l ca

vitie

s lo

cally

pre

sent

; bio

tite

is d

omin

ant

quar

tzco

mpo

nent

of g

roun

dmas

s; m

ultip

le p

hase

s ap

pare

nt o

n th

e ba

sis

of c

ross

cutt

ing

rela

tions

hips

Lat

e Po

rphy

ry (

L)

70<1

%A

bsen

t40

–60%

5–10

%5–

10%

30B

iotit

e,

Dom

inan

tly o

f mic

rodi

oriti

c co

mpo

sitio

n; a

s 0.

5–1

mm

1–2

mm

1–2

mm

1–2

mm

plag

iocl

ase,

di

kes

betw

een

20 a

nd 3

0 m

wid

e of

lim

ited

amph

ibol

edi

stri

butio

n; th

e te

rmin

al p

hase

at F

ront

era

is li

ght g

ray

and

med

ium

-gra

ined

(1-

4 m

m)

porp

hyri

tic, w

ith d

aciti

c co

mpo

sitio

n

Not

es: A

ll ph

ases

con

tain

var

iabl

e am

ount

s of

apa

tite,

mag

netit

e, r

utile

, and

zir

con

as a

cces

sori

es; h

emat

ite o

ccur

s as

incl

usio

ns in

rut

ile in

por

phyr

y A

and

B p

hase

s1

Porp

hyri

tic p

hase

(F

ig. 5

)2

All

amph

ibol

e, d

omin

antly

hor

nble

nde,

is m

oder

atel

y to

com

plet

ely

alte

red

to b

iotit

e


Porphyry B dikes and coalescent bodies are characterizedby crowded, medium-grained, porphyritic textures, with anoverall dacitic composition (Table 2; Fig. 6d). In contrast,porphyry A intrusions are finer grained, darker brown incolor, and dominantly andesitic in composition (Table 2; Fig.6e). Oriented plagioclase and biotite phenocrysts impart acharacteristic and readily identifiable, banded texture tomany of the porphyry A phases. Although porphyry B is locallyobserved to cut porphyry A, it is normally the case in por-phyry centers containing both families that porphyry A post-dates porphyry B and truncates some of its contained veinlets.This observation contradicts the relative age relationship pro-posed by Atkinson et al. (1996, fig. 14), who preferred anearly, premineral timing for the porphyry A family. These ap-parently mutually crosscutting relationships suggest the pos-sibility that, in fact, there may be more than one generationof both porphyry A and porphyry B (see below). Localizedmiarolitic cavities, containing biotite, chlorite, anhydrite, and

chalcopyrite, are present in both of the porphyry families, im-plying fluid segregation and entrapment during porphyryconsolidation (cf. Candela, 1997).

Related magmatic-hydrothermal features

At least 12 of the porphyry centers within the open pit atLos Pelambres and beyond are characterized by one or moreclosely related features indicative of the magmatic to hydro-thermal transition. These features, elaborated in Table 3,comprise aplite, pegmatite, unidirectional solidification tex-tures (cf. Shannon et al., 1982), and magmatic-hydrothermaland igneous breccias (cf. Sillitoe, 1985) as well as the copper,molybdenum, and gold mineralization (see below). An ideal-ized reconstruction of the features with respect to a porphyrycenter is given in Figure 7a. The developmental sequence ofthese features in each center is complex, and not necessarilythe same in all centers, with exposure level clearly being animportant determinant of what is observed. As summarized in


0361-0128/98/000/000-00 $6.00 89

100 m

Zone of pervasivesericite-chlorite alteration

Magmatic-hydrothermalbreccia with hydrothermalcement > igneous matrix

Magmatic-hydrothermalbreccia with dominantlyigneous matrix

Intermineral porphyry phase with irregularmiarolitic cavities

Sheeted quartzveinlets and localUST development

Aplite-pegmatitepods and dikes

b

a

c

d

e

bn

bn

bi

cp

qz

an

f

3 cm

1 cm

1 cm

3 cm

1 cm

FIG. 7. Magmatic-hydrothermal features of Los Pelambres porphyry copper-molybdenum and Frontera copper-gold de-posits. a. Idealized reconstruction of magmatic-hydrothermal and igneous breccias, aplite-pegmatite bodies, unidirectionalsolidification texture (UST), and sheeted quartz veinlets with respect to an underlying porphyry A or B center. b. Aplite withpegmatitic clots of brown and green biotite and K-feldspar. c. Sheeted quartz veinlets, associated with bornite and chal-copyrite (dark spots). d. Unidirectional solidification texture defined by quartz layers. e. Magmatic-hydrothermal breccia, cemented by quartz (qz), biotite (bi), bornite (bn), chalcopyrite (cp), and anhydrite (an). f. Igneous breccia, with porphyry Bmatrix (dark, indicated by scratcher).


90 PERELLÓ ET AL.

0361-0128/98/000/000-00 $6.00 90

TAB

LE

3. M

ain

Fea

ture

s of

Sel

ecte

d M

agm

atic

-Hyd

roth

erm

al C

ente

rs a

t Los

Pel

ambr

es-F

ront

era

Sulfi

de

Dim

ensi

ons

(m)

Ele

vatio

n as

sem

blag

e le

ngth

×of

hig

hest

Ve

rtic

al z

onat

ion

and

dow

n-A

vera

ge g

rade

wid

th ×

expo

sure

Sh

ape

of fa

cies

C

ausa

tive

Vein

let i

nten

sity

pl

unge

C

ente

rve

rtic

al(m

asl

)st

rike

/plu

nge

top

→bo

ttom

porp

hyry

pha

seH

ost r

ocks

(A a

nd B

type

)zo

natio

nC

u (%

)M

o (%

)A

u (p

pm)

RA

M-0

516

0 ×

120

×25

03,

600

Subc

ircu

lar

Pegm

atite

→Tw

o fa

mili

es

Los

Pel

ambr

es

Low

(<3

/m2 )

Py-C

p0.

2–0.

3<0

.01

n.a.

10°/

80°

SU

ST →

PAof

PA

stoc

k

Min

a 17

0 ×

70 ×

300

3,40

0Su

bcir

cula

r To

urm

alin

e-PA

Los

Pel

ambr

es

Mod

erat

e to

Py

-Cp

→>0

.6<0

.01

<0.0

1Po

rtez

uelo

0°/5

5° S

Ean

hydr

ite

stoc

khi

gh (

10–2

0/m

2 )C

p-Py

→br

ecci

a →

Cp-

Bo

pegm

atite

→PA

Port

ezue

lo

170

×13

0 ×

210

3,50

0Ir

regu

lar

Pegm

atite

→PB

Los

Pel

ambr

es

Low

to

Cp-

Py →

0.4–

0.6

<0.0

1<0

.01

Est

e~0

°/ 5

5° E

igne

ous

stoc

km

oder

ate

Cp

brec

cia

→PB

(3–1

0/m

2 )

DA

M-0

118

0 ×

90 ×

>700

3,15

0Su

bcir

cula

r;

UST

→an

hydr

ite

PBL

os P

elam

bres

M

oder

ate

to

Cp-

Bo

→>1

.0~0

.05–

0.08

0.05

cylin

der-

like

brec

cia

→ig

neou

s st

ock

very

hig

h B

o-C

p30

°/55

° SE

brec

cia

→PB

(10–

>30/

m2 )

DA

M-1

840

0 ×

80 ×

>700

3,

200

Subc

ircu

lar

Tour

mal

ine

PBL

os P

elam

bres

L

ow to

Py

→0.

6~0

.03–

0.05

<0.0

1to

tabu

lar

brec

cia

→ig

neou

s st

ock

mod

erat

e Py

-Cp

→10

°/55

° SE

brec

cia

→PB

(3–1

0/m

2 )C

p-B

o

Clu

ster

60

0 ×

240

×>6

003,

100

Hig

hly

Anh

ydri

te

PB

Los

Pel

ambr

es

Low

to

Cp

→0.

4–0.

60.

02<0

.01

Oes

teir

regu

lar;

br

ecci

a →

igne

ous

intr

uded

st

ock

mod

erat

e C

p-B

olim

its p

oorl

y br

ecci

a →

PB,

by P

A(3

–10/

m2 )

defin

ed

prob

able

PA

0°/5

5° S

E

Min

a 16

0 ×

70 ×

>300

3,15

0Su

bcir

cula

rPe

gmat

ite →

PBL

os P

elam

bres

L

ow to

C

p-B

o →

0.6

0.02

<0.0

1V

icto

ria

0°/5

5° S

Ean

hydr

ite-b

iotit

e-

stoc

k an

d m

oder

ate

Bo-

Cp

tour

mal

ine

Pach

ón

(3–1

0/m

2 )br

ecci

a →

PBF

orm

atio

n an

desi

te

MA

M-0

315

0 ×

140

×45

03,

450

Subc

ircu

lar;

A

nhyd

rite

PA

Pach

ón

Mod

erat

eC

p-B

o →

0.4–

0.5

<0.0

050.

1cy

linde

r-lik

e br

ecci

a →

PAF

orm

atio

n (1

0–20

/m2 )

Bo-

Cp

0°/8

0° S

Ean

desi

te

DA

M-1

015

0 ×

100

×30

03,

300

Cyl

inde

r-lik

ePe

gmat

ite →

PAPa

chón

L

ow to

C

p-B

o →

0.4–

0.5

<0.0

020.

1–0.

20°

/ 80°

SE

UST

→PA

For

mat

ion

mod

erat

e B

o-C

pan

desi

te(3

–10/

m2 )

Pegm

atita

47

0 ×

90 ×

>400

3,30

0Ta

bula

rPe

gmat

ite →

PBPB

Los

Pel

ambr

es

Low

(<3

/m2 )

Py →

0.2–

0.3

<0.0

05<0

.01

Sur

0°/7

5° E

stoc

kC

p-Py

DA

M-0

612

0 ×

100

×>6

003,

800

Subc

ircu

lar

Qua

rtz-

anhy

drite

PB

Pa

chón

L

ow to

C

p-Py

→0.

3<0

.002

0.1

to ta

bula

rbr

ecci

a →

PB,

intr

uded

F

orm

atio

n m

oder

ate

Cp-

Bo

→0°

/85°

Epr

obab

le P

Aby

PA

ande

site

(3–1

0/m

2 )B

o-C

p

DD

H-1

061

120

×80

×25

02,

900

Subc

ircu

lar

Anh

ydri

te

PBL

os P

elam

bres

M

oder

ate

Cp-

Bo

0.6–

1.0

0.01

–0.0

2<0

.01

0°/6

5° E

brec

cia

→st

ock

to h

igh

pegm

atite

→PB

(10–

30/m

2 )

Abb

revi

atio

ns: P

A =

Por

phyr

y A

fam

ily, P

B =

Por

phyr

y B

fam

ily, U

ST=

Uni

dire

ctio

nal s

olid

ifica

tion

text

ure,

n.a

. = n

ot a

vaila

ble

Sulfi

des:

Bo

= bo

rnite

, Cp

= ch

alco

pyri

te, P

y =

pyri

te


Table 3, the features are preserved at various elevations be-tween 3,600 and 2,900 m above sea level and, in combination,display irregular forms, particularly where clustered (e.g.,Cluster Oeste; Fig. 5). This clustering led previously to inter-pretation of a single, large breccia body (the Central breccia;Atkinson et al., 1996). The breccias and related features aregenerally <200 m in diameter, although larger (up to 600 m)bodies occur where clustered or elongated (e.g., PegmatitaSur; Fig. 5). Vertical extents, constrained by drilling, rangefrom 210 to 700 m; typically, they display a SE plunge of 55°on average, although steeper attitudes are also observed(Table 3). The magmatic-hydrothermal brecciation and asso-ciated features are better developed on the southeastern,hanging wall rather than northwestern, footwall sides of thecenters (Fig. 6b).

In the idealized section (Fig. 7a), aplite and pegmatite con-stitute composite bodies in which both products are inti-mately and irregularly mixed (Fig. 7b). These bodies occur to-gether with sheeted quartz veinlets (Fig. 7c) andunidirectional solidification textures (Fig. 7d) in the roofzones of the breccia bodies and/or the associated causative in-trusions. The pegmatite portions comprise several of quartz,K-feldspar, biotite, chlorite, tourmaline, sericite, anhydrite,and copper-iron sulfides, an association similar to that in themiarolitic cavities in the subjacent porphyry bodies (seeabove). Magmatic-hydrothermal breccia is normally polymictand composed of clasts of several intrusive phases, includingthe Los Pelambres quartz diorite, as well as andesite in thevicinity of volcanic country rock (e.g., Frontera deposit). Thehydrothermal cement is made up of the same minerals asthose in the aplite-pegmatite bodies (Fig. 7e). Downward, thehydrothermal cement becomes progressively less at the ex-pense of an igneous matrix of porphyry A or B composition(Fig. 7f).

Late mineral porphyry phases

Late mineral diorite porphyry and microdiorite dikes areexposed at surface and have been encountered by drilling,predominantly in the zone between Los Pelambres and Fron-tera (Figs. 5, 6b). The typically N- to NE-striking, steeply dip-ping dikes along the southeastern margin of the Los Pelam-bres stock are 20 to 30 m wide and several hundred meterslong, and display equigranular, microphaneritic textures. Thelate mineral body at Frontera strikes northwest and has adacitic composition and coarse-grained, porphyritic texture(Table 2).

Structure

The tectonic development of the Los Pelambres-Fronteraarea is dominated by the N-striking, high-angle Los Pelam-bres reverse fault, a brittle structure that places the Late Cre-taceous rhyolitic tuff and late Oligocene to early Miocene LosPelambres Formation over the early Miocene Pachón For-mation (Figs. 3, 5). The western lobe of the Los Pelambresstock cuts the fault, thereby implying that final displacementtook place prior to mid-Miocene intrusion (see above). Thewesterly monoclinal dips of both the Los Pelambres andPachón Formations steepen markedly on approach to thefault. Immediately north and south of the Los Pelambresstock, the epiclastic units of the Los Pelambres Formation

display ramp structures and tight local folding about N-strik-ing axes.

A series of NE-striking, 55° to 70° SE-dipping faults, de-fined by late-stage swarms of veins and veinlets and associatedalteration and mineralization (see below), cut the west-centralparts of the Los Pelambres stock (Fig. 5). A principal compo-nent of this fault set appears to displace the Los Pelambresstock contact and Los Pelambres fault (Fig. 5). Nonetheless,there is no obvious structural control of the porphyry centersand associated breccias and other features, which appear tohave an entirely random distribution; however, the threeFrontera centers do define a southeast alignment.

Hydrothermal Alteration and Mineralization

Alteration-mineralization assemblages

Los Pelambres and Frontera partly conform to the classicLowell and Guilbert (1970) hydrothermal alteration model, inwhich a potassic center grades laterally to an annular sericiticzone surrounded by a propylitic halo (Sillitoe, 1973; Fig. 8a).The bulk of the hypogene metal resource is hosted by thelarge potassic zone, shown by drilling to extend to a depth ofat least 1.2 km, whereas the sericitic halo is largely pyritic.

The potassic alteration is defined by the presence of hydro-thermal biotite and K-feldspar together with anhydrite, chal-copyrite ± bornite ± trace digenite, and molybdenite (Sillitoe,1973). Hornblende in the igneous rocks at Los Pelambres isalmost totally converted to fine-grained, shreddy, brown bi-otite, and plagioclase is partially replaced by K-feldspar (Silli-toe, 1973; Skewes, 1985; Skewes and Atkinson, 1985; Atkin-son et al., 1996). In contrast to Los Pelambres, the potassicalteration at Frontera is developed mainly in andesitic rocksof the Pachón Formation, which are transformed to gra-noblastic aggregates of fine-grained, brown biotite accompa-nied by subordinate chlorite and apatite. Minor epiclastichorizons in the Pachón Formation are replaced by alternatingbands of garnet, hedenbergite, actinolite, and chlorite.Hydrothermal magnetite is present locally at Los Pelambresbut is abundant (>5 vol %) in the biotitized rocks at Frontera(Sillitoe, 1973; Perelló et al., 2007, 2009, 2011). The mag-matic-hydrothermal and igneous breccias, described above,are integral parts of the potassic zone at both Los Pelambresand Frontera. The breccias, unlike the pervasive potassiczone, contain brown and green biotite as well as being char-acterized by the development of sericite ± chlorite in theirapical parts (Fig. 7a).

The annular sericitic zone is best developed in the north-western quadrant (Fig, 8a), where it coincides with the NE-striking structural zone, elements of which affect not only theLos Pelambres stock but also extend for up to 1 km into thepropylitic andesitic host rocks. The sericitic alteration clearlyoverprints and destroys the earlier formed potassic assem-blage and comprises quartz, sericite, pyrite, and subsidiaryschorlitic tourmaline, the latter present as patches, veinlets,and rosettes (Sillitoe, 1973). Tennantite, chalcopyrite, galena,sphalerite, molybdenite, and trace digenite are minor con-stituents. Locally, internal parts of the northeast structuralcorridor contain andalusite and kaolinite, minerals denotingthe existence of advanced argillic alteration. Silicified an-desitic rocks mapped previously in the northeastern quadrant


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92 PERELLÓ ET AL.

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3,500

3,300

3,250

3,200

3,500

P

ITI

OU

TL

NE

20

083,300

3,250

3,200

3,500

3,300

3,250

3,20

0

PU

IO

T

TLI

NE

2008

PU

IO

T

TLI

NE

2008

d

a b

c e

f g

359,000 361,000

359,000 361,000

6,488,000

6,490,000

359,000 361,000

6,488,000

6,490,000

6,492,000

1 km

1 km

1 km

2 cm

2 cm

2 cm 2 cm

Very high (up to 40 veinlets/m )2High (up to 20 veinlets/m )2Moderate (up to 10 veinlets/m )2

Low (up to 3 veinlets/m )2Veinlet intensity

High (up to 20 veinlets/m )2Moderate (up to 10 veinlets/m )2

Low (up to 3 veinlets/m )2

Veinlet intensity

PropyliticPotassicSericitic

N

N

N

LOS PELAMBRESLOS PELAMBRES

LOS PELAMBRES

FRONTERA

FRONTERA

FRONTERA

FIG. 8. Hydrothermal alteration and veinlet types at Los Pelambres and Frontera. a. Map of alteration zoning. b. Distri-bution of type 4 veinlets. c. Distribution of A- and B-type veinlets. d. Biotite-rich type 4 veinlet (dark) cut by an A-type quartzveinlet (pale), porphyry B, Los Pelambres. e. Magnetite-rich type 4 veinlets cut by magnetite-bearing A veinlet, porphyry B,Frontera. f. B veinlet with molybdenite along its margins (shown by scratcher tip), Los Pelambres stock. g. D veinlets con-taining pyrite and tennantite (dark), porphyry B, northeast structural zone, Los Pelambres. a, b, and c based on a combina-tion of core logging and pit and surface mapping. UTM datum: Prov. S. Am 56, Zone 19 South.


(Sillitoe, 1973) also contain andalusite and, in the topograph-ically higher (~3,700 m) parts of the northeast corridor, veinscontaining andalusite, sericite, barite, gypsum (after anhy-drite), and native sulfur cut the propylitic halo (cf. Sillitoe,1973).

Veinlet types and distribution

Six main veinlet types are widely recognized throughoutthe potassic zone at Los Pelambres and Frontera, which, fromoldest to youngest based on crosscutting relationships, com-prise early biotite, green mica, type 4, A, B, and D (Skewesand Atkinson, 1985; Atkinson et al., 1996; Table 4; Fig. 8d-g);only the last four types were systematically counted at the de-posit scale and incorporated in the resource model (Fig. 8b,c). The entire veinlet sequence is characterized by the pres-ence of varied amounts of anhydrite and tourmaline (Table 4).

These six veinlet types, described in Table 4, are widely rec-ognized in porphyry copper deposits worldwide (e.g., See-dorff et al., 2005; Sillitoe, 2010). At Los Pelambres-Frontera,the most widespread veinlet types are type 4, A, and B, whichhave a combined footprint of approximately 4 × 2 km (Fig.8b, c). The distribution of these three veinlet types definesNE and SE trends, which mirror the structural and geologicelements described above (Fig. 8b, c). Within these trends,there is a concentric distribution of veinlet intensities, culmi-nating in discrete centers of greatest veinlet development(Fig. 8b, c). The early biotite and green mica veinlets are alsowidely, but irregularly, distributed and, hence, not readilycountable. In contrast, the D veinlets are concentrated in thenortheast structural corridor in the northwestern quadrant,where their latest representatives contain the base metal sul-fides listed above besides abundant pyrite (Table 4; Fig. 8g).

The early biotite and green mica veinlets contain only triv-ial amounts of copper-bearing sulfide minerals. The earliestsignificant introduction of copper and molybdenum at LosPelambres and copper and gold at Frontera accompaniedgeneration of the type 4 veinlets (Fig. 8d, e). The type 4 vein-lets make a major contribution to the copper inventory at LosPelambres, probably accounting for approximately 0.4% ofthe deposit average grade of 0.56% Cu; this is somewhat lessthan the figure estimated by Atkinson et al. (1996) for shallowlevels of the orebody where chalcocite enrichment was ubiq-uitous (see below). In common with similar (early dark mica-ceous; Meyer, 1965) veinlets elsewhere, most of the con-tained chalcopyrite and bornite are disseminated in the halos,which are composed of admixed quartz, K-feldspar, biotite,sericite, phengitic sericite, andalusite, anhydrite, corundum,rutile, and plagioclase (Skewes and Atkinson, 1985; Atkinsonet al., 1996; Proffett, 2009; Table 4). The A- and B-type vein-let generations, some with weakly developed K-feldsparhalos (Table 4; Fig. 8d, f), contain the rest of the copper inboth deposits, and, in conjunction with the magmatic-hydro-thermal breccias, clearly give rise to the centers of highestgrade. The type 4 veinlets contain relatively minor, butwidely distributed, molybdenite at Los Pelambres, but the Bveinlets account for at least 60% of the total; however, Bveinlets are more sparsely developed at Frontera, which, as aresult, contains less molybdenite (50–60 ppm Mo; Perelló etal., 2011). The gold at Frontera is hosted by the type 4 and Aveinlets, which, unlike their counterparts at Los Pelambres,

contain appreciable amounts of magnetite and subsidiaryactinolite (Fig. 8e). The Frontera deposit also contains early,sulfide-poor magnetite-actinolite veinlets with prominent al-bite halos.

Sulfide and metal zoning

The sulfide minerals at Los Pelambres and Frontera arezoned within and around the individual porphyry centers(Table 3), from the scale of the A and B veinlets to whole-rockvolumes, and independently of the host rock (Atkinson et al.,1996). In contrast, the type 4 veinlets are not zoned aroundthe individual porphyry centers but nonetheless do contributeto the deposit-scale sulfide zoning pattern (Fig. 8b). The coresof the porphyry centers, where A- and B-type veinlet intensi-ties are highest but not necessarily everywhere the same, arecharacterized by chalcopyrite/bornite ratios of 2 to 3 at LosPelambres but as low as unity at Frontera where total sulfidecontents are much less (<1 vol %). Bornite contents are high-est in the cores of the porphyry centers and decrease bothoutward and upward to give rise to chalcopyrite, chalcopyriteplus pyrite, and, eventually, pyrite alone. Nonetheless, totalsulfide contents remain approximately the same (~2 vol %)through to the pyrite halo, except within the overprintedsericitic alteration along the NE-striking structural zone (Fig.8a), where pyrite contents locally attain 5 vol %. The por-phyry A centers are everywhere lower grade than porphyry Bcenters. Sulfides in the apical portions of the magmatic-hydrothermal breccias in both porphyry A and B centers tendto be both higher grade and coarser grained than those in theassociated porphyry intrusion and immediate wall rock (Fig.7e). A few porphyry centers situated near the margins of theLos Pelambres stock are less strongly veined and morepyritic, in keeping with their position in the deposit-scale zon-ing pattern (e.g., RAM-05 and Pegmatita Sur; Fig. 5; Table 3).

The most remarkable aspect of metal zoning at Los Pelam-bres-Frontera is the distribution of molybdenum and gold,with the former concentrated at Los Pelambres and the latteralmost exclusively confined to Frontera (Fig. 9a). Copper, incontrast, spans both deposits. At Los Pelambres, molybde-num tenors of >50 ppm define a N-trending zone, some 4 kmlong and up to 1 km wide (Fig. 9a). However, the highestmolybdenum values are confined to the north-central part ofthis zone, which is characterized by a steep, SE-plungingcylindrical body containing >200 ppm Mo (Fig. 9a, c). Thehigh-grade, hypogene copper mineralization (>0.6%) alsoplunges southeastward at a similar angle, in conformity withthe partly coalesced porphyry centers (Fig. 9b). There is aclearcut correlation between gold and hydrothermal mag-netite, which is most marked at Frontera but remains valideven for the minor gold occurrences within the Los Pelam-bres deposit (Fig. 9a). Nonetheless, a single, small bornite-rich zone within the apical part of the DAM-01 magmatic-hydrothermal breccia (Fig. 5) contains up to 0.1 g/t Au eventhough it lacks magnetite.

Most of Los Pelambres and all Frontera are devoid of ar-senic, which is associated almost exclusively with the over-printed sericitic alteration and tennantite-bearing D vein-lets that define the northeast structural corridor in thenorthwestern quadrant of Los Pelambres (Fig. 9d). Nonethe-less, values are relatively low (<550 ppm As).


0361-0128/98/000/000-00 $6.00 93


Supergene Alteration and Mineralization

Supergene profile

The supergene profile at Los Pelambres is thinnest beneaththe floor of the U-shaped glacial valley (Fig. 2a), where sul-fide oxidation is locally as little as 6 m and chalcocite enrich-ment is vestigial (Sillitoe, 1973; Fig. 9b). However, the profilethickens as the pyrite halo is approached beneath the valleysides, where sulfide oxidation penetrates in places to depthsof 200 m and the immaturely developed enrichment zone av-erages ~80 m thick, but locally attains a maximum of ~350 m(Atkinson et al., 1996; Fig. 9b).

The leached capping is predominantly goethitic within theconfines of the potassic zone but becomes increasingly

jarosite rich peripherally as pyrite contents increase (Fig. 2a).In the goethitic capping, oxide copper minerals are relativelyuncommon and consist mainly of malachite, although azurite,chrysocolla, chalcanthite, brochantite, and pseudomalachiteare reported from the eastern parts of the potassic zone (Sil-litoe, 1973). Nevertheless, the close coincidence between the>300-ppm Cu isopleth in talus fines and the mapped potassiczone at surface (Maranzana, 1972) emphasizes that copperremoval was incomplete.

In contrast, at Frontera, copper leaching is minimal and anoxidized zone, containing malachite and pitch limonite(cupriferous goethite) and averaging 140 m thick, is underlainby immature supergene enrichment, which is best developedwithin the uppermost 50 to 120 m of the sulfide zone, where

94 PERELLÓ ET AL.

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TABLE 4. Principal Veinlet Types at Los Pelambres-Frontera:

Veinlet fill

Veinlet type Dimensions Habit Gangue Sulfides

Early Biotite (EB) Few cm long and Planar to sub-planar seams Biotite, anhydrite, quartz; Minor; mainly Cp and Bo; <1 cm wide with diffuse margins local tourmaline and chlorite trace Po and Py in places

Green Mica (GM) Few cm long and Highly variable, from planar Typically white and/or green Minor; mainly Cp and Bo<0.5-1 cm wide and continuous central fill sericite (phengite), biotite,

to poorly defined alignments and chlorite; variable K-of intermittent clots feldspar, anhydrite, plagio-

clase, andalusite, corundum, and tourmaline

Type 4 (T4) Several tens of cm long; Central fill variable in Quartz, green phengitic Present; Cp, Bo, and Mol; halos up to 1 m wide outline; halos typically 5-10 sericite, K-feldspar, tourmaline, Py increases laterally

times wider than central and anhydrite; variable veinlet fill andalusite and corundum

A-type (A) Early veinlets are short Early representatives are Granular quartz, K-feldspar, Important; mainly Cp, Bo, and discontinuous, a few wavy with fuzzy edges; and anhydrite with variable Mol; Di locally present; cm long; later veinlets central fill is granular and biotite and trace tourmaline; Py increases laterallyare continuous >5-10 cm sugary in texture; later veinlets of pure quartz

representatives typically commonplanar, with sharp edges

B-type (B) Several cm to tens of cm Planar, with sharp edges; Quartz, K-feldspar, and Important; Cp, Bo, Mol, Py. long and 1-2 cm wide centerline well developed anhydrite with variable Py increases laterally; Mol-only

and continuous; comb amounts of tourmaline veinlets locally dominant textures common

D-type (D) Several tens of cm to Planar and continuous, with Typically quartz only, but Dominantly Py, but Cp and meters long and a few sharp edges and sericitic anhydrite and tourmaline Mol in places; Ten, Ga, Di, cm to tens of cm wide halos; in structurally con- locally present and Sph present in large

trolled zones at Los transgressive corridors at Pelambres, with minor Los Pelambreshydrothermal breccia; coarse-grained sulfide fill

Abbreviations: Bo = bornite, Cp = chalcopyrite, Di = digenite, Ga = galena, Mol = molybdenite, Po = pyrrhotite, Sph = sphalerite, Ten = tennantite


the chalcocite occurs as thin replacement coatings on chal-copyrite, bornite, and minor pyrite grains (Perelló et al.,2011). Minor cuprite and native copper occur in the vicinityof the oxide-sulfide interface.

Anhydrite front

Los Pelambres and Frontera are both characterized by anabrupt anhydrite front, above which nearly all the anhydritein both the veinlets and host rock has been removed by thepassage of cool ground water to leave characteristic openpores and cavities (Sillitoe, 1973). The anhydrite front com-monly coincides closely with the base of chalcocite enrich-ment (Fig. 9b), although in places it is developed up to 50 mdeeper (Atkinson et al., 1996).

Geochronology of Intrusive Rocks and Mineralization at Los Pelambres-Frontera

U-Pb zircon ages of intrusive rocks

This study is underpinned by 18 new LA-ICP-MS U-Pb zir-con ages on intrusive rocks (Table 5). Two of these ages showthat the Los Pelambres quartz diorite stock was emplaced be-tween 13.60 ± 0.30 and 13.00 ± 0.70 Ma, which are broadlycompatible with previously reported U-Pb zircon ages rang-ing from 13.92 ± 0.15 to 12.51 ± 0.17 Ma (Bertens et al.,2003, 2006; A. Bertens, writ. commun., 2007; see above).

Eight ages for six different porphyry B centers at LosPelambres and Frontera range from 12.30 ± 0.30 to 10.80 ±0.23 Ma, defining an interval of ~1.5 m.y. Seven ages for


0361-0128/98/000/000-00 $6.00 95

Summary of Alteration-Mineralization Assemblages

Veinlet halo

Gangue Sulfides Distribution Comments

Normally without halos, but None to minor Irregularly distributed throughout the Typically the earliest veinlet event; probably locally with incipient albite Los Pelambres stock, but never in accompanied by widespread biotitization of mag-and/or chlorite countable amounts; rare in Porphyry B matic ferromagnesian componets in all intrusive

and A phases phases, but dominantly in the Los Pelambres stock; some generations possibly transitional to T4 and GM veinlets; magnetite is an important veinlet constituent at Frontera

Quartz, K-feldspar plus trace Minor; Irregularly distributed within the Transitional between EB and T4 veinlets, and biotite and andalusite Cp and Bo Los Pelambres stock, but never in locally a variety of T4; magnetite is an important

countable amounts; rare in Porphyry veinlet constituent at FronteraB and A phases

Variable mixtures of quartz, Important; Cp, Widely distributed throughout Los Earliest significant contributor of Cu (Mo) at Los K-feldspar, biotite, sericite, Bo, Mol; minor Pelambres and Frontera, although most Pelambres and Cu-Au at Frontera; deposit-scale phengitic sericite, andalusite, Di in places; Py common within Los Pelambres stock and zonation includes central parts of Los Pelambres anhydrite, corundum, rutile, increases laterally Porphyry B phases; decreasing intensity with Cp-Bo T4 veinlets and peripheral parts and plagioclase in Porphyry A phases dominated by Py-bearing veinlets; everywhere T4

veinlets are crosscut by A-, B-, and D-type veinlets, although multiple generations and mutually cross-cutting relationships with A and B veinlets locally present

Uncommon in early wavy None to minor; Widely distributed throughout Los Significant contributors of Cu (Mo) at Los veinlets; later veinlets are Cp, Bo Pelambres and Frontera, but more Pelambres and Cu-Au at Frontera; laterally more planar with K-feldspar intense around emanative centers zoned, with Cp-Bo zones in central parts and

associated with porphyry B phases and Py-dominated veinlets in peripheral parts of Los UST zones; porphyry A phases contain Pelambres; later veinlet events transitional to B, proportionally lesser amounts although multiple generations present

Uncommon; locally K-feldspar Minor; Widely distributed throughout Los Principal contributor of Mo at Los Pleambres; halos are incipiently developed; Cp, Bo, Mol Pelambres where they occur together several generations present with mutual cross-weak sericitic halos present in with A veinlets, mainly in Porphyry B cutting relationships with A veinlets; almost outer parts of Los Pelambres phases in proximity to emanative centers; absent at Frontera, where they contribute to

a central zone at Los Pelambres contains background Mo contents (~50 ppm)high-grade Mo in emanative centers DAM-1 and Cluster Oeste

Important; sericite, muscovitic Important; dom- A several hundred-meter wide swarm cuts Although dominantly Py-rich, a terminal event of sericite, and montmorillonite, inantly Py with through the central and northwestern Cu introduction, together with As, along trans-plus local trace albite variable amounts quadrant of Los Pelambres with a gressive veinlet and vein swarms with Ten plus Ga

of Ten, Ga, Sph, dominatly 35°-40° strike, 50°-60°SE dip and Sphand Cp


seven different porphyry A centers at Los Pelambres andFrontera fall between 11.50 ± 0.30 and 10.53 ± 0.14 Ma, arange of ~1.0 m.y. Two U-Pb zircon ages from surface expo-sures of porphyries considered to be porphyry B yielded agesthat fall in the range defined above (Bertens et al., 2003,2006). The single age for the late mineral dacite porphyrydike at Frontera is 10.24 ± 0.55 Ma.

The oldest age for a porphyry center at Los Pelambres(Pegmatita Sur; Fig. 5; Tables 3, 5) is, within error limits, es-sentially the same as the two youngest ages obtained for theLos Pelambres quartz diorite by Bertens et al. (2003, 2006)and this study, implying a close relationship between stock

emplacement and initiation of porphyry center development.The partial temporal overlap between the age ranges for theporphyry A and B centers is notable, bearing in mind thatwhere crosscutting relationships are observed porphyry A isgenerally younger than porphyry B. Nevertheless, it is clearthat the youngest porphyry B and porphyry A ages are for theFrontera samples (Table 5).

Re-Os ages of molybdenite

During this study, 12 Re-Os ages were determined for dif-ferent molybdenite samples from Los Pelambres and Frontera(Table 6). The ages lie between 11.81 ± 0.06 and 10.14 ± 0.04

96 PERELLÓ ET AL.

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B

B

B´

B´

3,8003,800

3,4003,400TS

TS

BE

BE

3,0003,000

2,6002,600

3,8003,800

3,4003,400

3,0003,000

2,6002,600

> 0.8> 0.6> 0.35

Top of sulfidesTS

BEBottomof supergene enrichment

> 400> 300> 200> 100



358,500 360,500

6,492,000

6,490,000

6,488,000

358,500 358,500360,500 360,500

2008 pit

2008 pit

Original topography

Original topography

30 year pit

30 year pit

>1.0 > 200 > 0.2Cu %

Cu %

Mo ppm

Mo ppm Au ppm

0.6 - 1.0 100 - 200 0.1 - 0.20.4 - 0.6 50 - 100 0.08 - 0.10.1 - 0.4

N

N N N

51-130

Reverse faultFault: mapped, inferred

131-550As ppm

a

b

c d

3,500

PIT

IO

UT

LN

E20

08

2008

2008

2008

PITO

UT

LIN

E

PITO

UT

LIN

E

PITO

UT

LIN

E

6,490,0006,490,000

359,000359,0006,492,0006,492,000

1 km

500 m

500 m

1 km 1 km 1 km

Los

Pel

ambr

esfa

ult

3,300

3,250

3,20

0

3,500

3,3003,250

3,500

3,300

3,250

3,500

3,300

3,250

FIG. 9. Metal zoning at Los Pelambres-Frontera. a. Plan views of copper, molybdenum, and gold distribution at 3,000 mabove sea level, simplified from the block model (all available data, including blast holes). b. NW-SE longitudinal section (sameas Fig. 6b) at Los Pelambres, showing copper distribution. c. NW-SE longitudinal section (same as Fig. 6b) at Los Pelam-bres, showing molybdenum distribution. d. Plan view of arsenic distribution. UTM datum: Prov. S. Am 56, Zone 19 South.


Ma. To these may be added 13 additional molybdenite agesreported by Bertens et al. (2006), three by Mathur et al.(2001), two by Stein et al. (2002), and two by Hannah et al.(2007), all from Los Pelambres and all falling in the age rangedefined by this study. This database clearly shows that themolybdenite mineralization at Los Pelambres-Frontera wasintroduced during a protracted interval of ~1.7 m.y., whichaccords well with the ~1.4-m.y. interval defined by additionalmolybdenite ages from Los Pelambres reported by Stein et al.(2004) but not considered herein. The samples dated duringthis study show that the youngest ages are for Fronteramolybdenites (Table 6), in good agreement with the U-Pbdata (see above).

K-Ar and Ar/Ar ages for silicate minerals

Six K-Ar ages reported by Atkinson et al. (1996) and two byQuirt et al. (1971), all for biotite from the Los Pelambresstock and porphyries, range from 10.7 to 8.9 Ma. Ten samplesof igneous and hydrothermal muscovite, biotite, and horn-blende from within and immediately surrounding Los Pelam-bres were subjected to Ar/Ar age determination by Bertens etal. (2006) and yielded ages of 10.58 to 9.52 Ma (no errors re-ported). These K-Ar and Ar/Ar ages overlap with theyoungest Re-Os molybdenite ages and would appear to re-flect initial cooling of the system below the argon blockingtemperature.

Ar/Ar ages for supergene jarosite group minerals

The earliest jarosite age determined at Los Pelambres is5.34 Ma, implying that the deposit was already unroofed by

the latest Miocene (Bertens et al., 2006). However, most ofthe jarosite ages fall in the 3.06 to 0.93 Ma range, when su-pergene oxidation was clearly active (Bertens et al., 2006).

(U-Th)/He zircon and apatite ages

Bertens et al. (2006) presented six (U-Th)/He zircon andapatite ages from Los Pelambres. The ages of 10.37 to 8.15Ma (no errors reported) are not much younger than the K-Arand Ar/Ar determinations, implying rapid cooling of the LosPelambres stock and its associated mineralization.

Discussion

District-scale controls

The Los Pelambres and Frontera deposits are located alongthe faulted eastern boundary of a narrow, N-NW-striking beltof intensely deformed rocks, which are the product of tec-tonic inversion of the extensional Abanico intra-arc basin (Jor-dan et al., 2001; Charrier et al., 2002; Fig. 3). The compres-sive collapse of the Abanico basin at ~20 Ma correlates with adrastic decrease in crust generation in the eastern PacificOcean and a drop in Nazca-South America plate convergencevelocity (Pardo-Casas and Molnar, 1987; Somoza, 1998; Con-rad and Lithgow-Bertelloni, 2007). Increased mechanicalcoupling along the plate interface (e.g., Yáñez and Cembrano,2004) may have produced the change from extension to con-traction that led to initiation of the Aconcagua and La Ra-mada fold-thrust belts in the greater Los Pelambres region(Giambiagi and Ramos, 2002; Ramos et al., 2002). At LosPelambres, this event is bracketed by the Chalinga intrusive


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TABLE 5. U-Pb Zircon Ages of Some Intrusive Phases at Los Pelambres-Frontera

UTMMagmatic-

Sample no. N E Intrusive suite hydrothermal center Location Age (Ma ±2σ)

DAM - 11 6,490,501 358,864 PB DAM-01 Los Pelambres 11.70 ± 0.30FRONT - 22

(DDH817:122) 6,488,686 360,446 PB MAM-03 Frontera 11.29 ± 0.37FRONT - 32

(DDH817:300) 6,488,619 360,495 PB MAM-03 Frontera 10.83 ± 0.23FRONT - 12

(DDH817:30) 6,488,720 360,421 PB MAM-03 Frontera 10.80 ± 0.23MAM - 3 (330)1 6,489,332 360,065 PA MAM-03 Frontera 10.70 ± 0.20RAM - 051 6,491,900 359,972 PA RAM-05 Los Pelambres 11.50 ± 0.30PELSE - 022 6,487,780 360,720 PL DAM-06 Frontera 10.24 ± 0.55FRONT - 82

(DAM - 10:570) 6,488,833 360,511 PB DAM-10 Frontera 10.87 ± 0.07PELSE - 012 6,488,630 360,500 PA DAM-10 Frontera 10.53 ± 0.14FRONT - 102

(MAM - 6:432) 6,488,806 360,598 PA DAM-10 Frontera 11.13 ± 0.12DDH 1030 (494)1 6,490,354 358,450 PA DAM-18 Los Pelambres 11.50 ± 0.30DAM - 181

(DDH1030:247) 6,490,191 358,608 Los Pelambres stock DAM-18 Los Pelambres 13.00 ± 0.70DDH 1052 (160)1 6,488,581 359,472 PB Pegmatita Sur Los Pelambres 12.30 ± 0.30DDH 2424 (107)1 6,490,946 359,678 PB Portezuelo Este Los Pelambres 11.90 ±0.30DDH 2268 (140)1 6,490,152 358,944 PB Cluster Oeste Los Pelambres 11.80 ± 0.30DDH 2439 (80)1 6,489,949 358,969 PA Cluster Oeste Los Pelambres 11.50 ± 0.30DDH 1012 (195)1 6,491,003 359,334 PA Mina Portezuelo Los Pelambres 10.80 ± 0.20DDH 303 (121)1 6,489,898 359,479 Los Pelambres stock Mina Victoria Los Pelambres 13.60 ± 0.30

Notes: UTM datum: Prov. S. Am 56, Zone 19 South1 Dated at University of Arizona, Tucson, Arizona2 Dated at Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, Australia


complex, which constrains the timing of the reverse motionon the Los Pelambres and Totoral faults to pre-16.5 Ma (Fig.3; see above).

During the mid-Miocene (~14–13 Ma), emplacement ofthe precursor Los Pelambres stock took place at the intersec-tion of the NW-striking belt of mid-Miocene intrusions andthe Los Pelambres fault. This NW-striking intrusive align-ment may have been localized by a transverse, deeply pene-trating basement feature, not currently evident at the surfaceand comparable to those mapped but poorly understood far-ther north in the Andes (Salfity, 1985). Therefore, porphyrycopper formation at Los Pelambres took place ~10 m.y. afterthe inversion of the Abanico basin when the deformationfront shifted eastward, giving rise to the Frontal Cordillera(e.g., Cordillera de Santa Cruz; Fig. 3) by thick-skinned re-verse faulting (Jordan et al., 1996; Pérez, 2001; Giambiagi andRamos, 2002). The tectonic migration and emplacement ofthe Los Pelambres porphyry copper deposits also coincide

with subduction of the Juan Fernández Ridge beneath theleading edge of the South America plate at this latitude(Yáñez et al., 2001; Kay and Mpodozis, 2002; see above).

As shown above, the porphyry copper-related intrusions atLos Pelambres display adakitic signatures (high Sr/Y andLa/Yb), concave heavy rare earth element patterns, and lackof europium anomalies indicative of hornblende fractionationin hydrous, oxidized magmas. Slab flattening and continueddeformation increased crustal thickness (Kay and Mpodozis,2002) under Los Pelambres and may have induced partial fu-sion of the mantle lithosphere; a situation that seems to haveplayed a fundamental role in the generation of the giant LosPelambres porphyry copper system.

Deposit-scale temporal development

Magmatism at Los Pelambres commenced with emplace-ment of the multiphase Los Pelambres quartz diorite stockbetween ~14 and 12.5 Ma (Fig. 10a). Intrusion of the numerous

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TABLE 6. Re-Os (molybdenite) Ages from Los Pelambres-Frontera

Sample no.North/East Location Re (ppm) 187Re (ppm) 187Os (ppb) Age (Ma ±2σ) Comments

MAM-17 (699)1 Portezuelo Este, 309.65 193.84 38.32 11.81 ± 0.06 T4 quartz-molybdenite veinlet with (6,491,350/359,528) Los Pelambres grayish halos overprinting biotitized

Los Pelambres stock; cut by A-type quartz-sulfide veinlet without alteration halos

MAM-17 (714)1 Portezuelo Este, 163.01 102.04 19.25 11.27 ± 0.09 T4 halo-rich veinlet with centerline fill (6,491,350/359,528) Los Pelambres of chalcopyritye-molybdenite; cuts

biotitized Los Pelambres stock

MAM-17 (714)2 Portezuelo Este, 238.6 ± 0.6 150.0 ± 0.4 28.69 ± 0.02 11.48 ±0.05 T4 veinlet with grayish-greenish halo (6,491,350/359,528) Los Pelambres cutting biotitized Los Pelambres stock;

chalcopyrite, bornite, and molybdenite along center line, with quartz

DAM-06 (402.3)2 DAM-06 area, 535.4 ± 1.4 336.5 ± 0.9 60.10 ± 0.04 10.72 ± 0.04 B-type quartz-molybdenite veinlet cut-(6,487,750/360,599) Frontera ting through a poorly defined T4 seam

of K-feldspar, biotite, chlorite, and mag-netite, with disseminated chalcopyrite; both veinlets cut magnetite-rich bioti-tized andesite of the Pachón Formation

DAM-06 (420)2 DAM-06 area, 154.0 ± 0.4 96.77 ± 0.25 16.94 ± 0.02 10.51 ± 0.04 Classic halo-dominated T4 veinlet with (6,487,750/360,599) Frontera irrregular disseminations of chalcopy-

rite and molybdenite; veinlet cuts Porphyry B and is cut by thin A-type veinlets

DAM-06 (376.2)2 DAM-06 area, 475.6 ± 1.3 298.9 ± 0.8 53.65 ± 0.04 10.77 ± 0.04 A-type quartz veinlet with irregular (6,487,750/360,599) Frontera arrays of molybdenite and finely dis-

seminated chalcopyrite; veinlet cuts Porphyry B and earlier EB-type seams

DAM-09 (458.3)2 Northern part of 221.7 ± 0.6 139.3 ± 0.4 23.55 ± 0.02 10.14 ± 0.04 B-type quartz-molybdenite veinlet, with (6,489,175/360,401) Frontera, transitional molybdenite along a well-defined

to Los Pelambres centerline; veinlet cuts igneous breccia with biotitized andesite fragments and Porphyry B matrix

DDH-71 (106)3 Los Pelambres pit, 16.79 10.51 2 11.66 ± 0.04 Swarm of T4 veinlets, with grayish halos (6,491,330/359,052) north wall overprinting biotitized Los Pelambres

stock; cut by B- and D-type veinlets

Notes: UTM datum: Prov. S. Am 56, Zone 19 South1 Dated at Department of Geosciences, University of Arizona, Tucson, Arizona2 Dated at University of Alberta, Alberta, Canada3 Dated at AIRIE - Colorado State University, Colorado


B and A porphyry centers commenced almost immediatelythereafter and lasted for ~1.5 m.y., terminating with theFrontera late mineral porphyry at 10.24 ± 0.55 Ma (Fig. 10a).Therefore, the Los Pelambres stock is clearly a precursor in-trusion genetically related to porphyry copper formation. Theindividual porphyry and associated magmatic-hydrothermalcenters within the Los Pelambres stock and southeastward toFrontera appear to have been emplaced over intervals of 0.3to 0.6 m.y., but without any clearly discernable space-timearrangement (Fig. 10b). However, the Frontera porphyrycenters do tend to be younger than those within the LosPelambres stock (Fig. 10b), in accord with the southeastwardyounging of the Re-Os molybdenite ages. This southeastwardmigration appears to have been maintained at the districtscale for a further ~2 m.y. to account for the El Pachón

deposit (9.16–8.43 Ma; Bertens et al., 2006), 5 km farthersoutheast (Figs. 3, 10a).

Based on the temporally overlapping arrays of U-Pb zir-con and Re-Os molybdenite ages, the longevity of intrusion,magmatic-hydrothermal brecciation, potassic alteration,veining, and associated copper-molybdenum and copper-gold mineralization at Los Pelambres and Frontera was ~1.7to 1.5 m.y., an interval which is expanded to ~3.8 m.y. if theprecursor Los Pelambres stock is also included (Perelló etal., 2009). The continuity of molybdenum introduction for~1.7 m.y. argues against the existence of discrete metallifer-ous pulses separated by quiescent intervals. The K-Ar,Ar/Ar, and (U-Th)/He ages document cooling of the LosPelambres-Frontera system to near ambient temperaturesby ~8 Ma (Bertens et al., 2006; Fig. 10a) as a result of rapid


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RAM-05(11.5)

MINA PORTEZUELO(10.8)

PORTEZUELO ESTE(11.9)

[11.81-11.27]

DAM-1(11.7)

DAM-18(11.5)

MAM-03(11.3-10.7)

[10.14]CLUSTER OESTE

(11.8-11.5)

DAM-10(11.1-10.5)PEGMATITA

SUR (12.3)

DAM-06(10.2)

(U-Pb)[Re-Os]

DAM-06[10.77-10.51]

N

PLEISTOCENE

PLIOCENE

Ma

5

10

15

20

25

30

EA

RLY

LATE

EA

RLY

MID

DLE

LATE

OLI

GO

CE

NE

MIO

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NE

Reg

iona

l vol

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sm(L

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ico

Form

atio

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Los

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sup

erge

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ity

El P

achó

n m

iner

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atio

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Los

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res-

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hyd

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Los

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mag

mat

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Los

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amb

res-

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min

eral

izat

ion

Pre

curs

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stoc

k)

Pre

min

eral

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sm(C

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ive

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ple

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a b

FIG. 10. Summary chronology of magmatic, hydrothermal, and supergene events in the Los Pelambres district. a. Re-gional volcanism, premineral plutonism, porphyry intrusion and cooling, and hypogene and supergene mineralization at LosPelambres-Frontera. The age of mineralization at the El Pachón deposit is also shown. Note the almost transitional or over-lapping nature of the magmatic, hydrothermal, and supergene events. Data from Bertens et al. (2003, 2006) and Tables 1, 5,and 6. b. U-Pb zircon and Re-Os molybdenite ages or age ranges for ten of the porphyry centers at Los Pelambres-Frontera.Geologic elements taken from Figure 5 and data from Tables 5 and 6.


exhumation during the tectonically induced uplift. Indeed,the Los Pelambres deposit was exposed to the effects of su-pergene oxidation and enrichment by ~5 Ma (Bertens et al.,2006; Fig. 10a). The supergene profile was eroded from theupstream Los Pelambres valley floor during the Plio-Pleis-tocene glaciation but preserved downstream and on the val-ley sides (Fig. 9b, c).

Deposit architecture and evolution

A schematic model for the genesis of Los Pelambres-Fron-tera is presented as Figure 11. The Los Pelambres quartzdiorite stock is hypothesized to be a cupola above a deeper,parental intrusive complex (Fig. 11a). The widespread distri-bution of the type 4 veinlets and the fact that early examplesare truncated by the intermineral B and A porphyries suggest

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Cupola

?

Chalinga

intrusive

complex

2 km

15 Ma

12.3-10.3 Ma

Los Pelambres

Frontera

14-12.5 Ma

10 Ma?

Meteoric water

Regionaluplift

Regionaluplift

Regionaluplift

Regionaluplift

Totoral fault

Los Pelambres fault

High-temperaturesingle-phase

magmatic liquid

Type 4 veinlets

Late-mineral dike

Porphyry B andPorphyry A magmatic-hydrothermal centers

Advanced argilliclithocap

Sericiticalteration

Low-temperaturesingle-phase liquid

Two-phase brine/vapor input

2 km

2 km

2 km

Paleosurface

?

??

2 km

2 km

2 km

2 km

Postulated intrusive complex

a b

c d

?

FIG. 11. Schematic evolution of Los Pelambres-Frontera system over its ~3.8-m.y. lifespan. a. The precursor Los Pelam-bres quartz diorite stock commences differentiation as the cupola of a postulated parental plutonic complex; this is inferredto have taken place following final emplacement of the Chalinga intrusive complex, which was rapidly uplifted along the re-gionally extensive Totoral fault system (Fig. 3). b. The Los Pelambres stock magmas separate, ascend, and focus flow of high-temperature, single-phase liquid which, upon cooling, reacts with the quartz diorite and develops early potassic alteration inthe form of widespread biotitization of magmatic hornblende along with formation of early biotite, green mica, and, most im-portantly, type 4 veinlets. The Los Pelambres stock is emplaced along the structural break provided by the formerly activeLos Pelambres fault (Fig. 3) and becomes tilted as uplift progresses throughout the region. c. The Los Pelambres stock mag-mas continue their ascent, with fresh magma being emplaced as multiphase porphyry B and porphyry A centers, each withits own magmatic-hydrothermal focus. Each center creates its own miniature porphyry system, including several generationsof A- and B-type quartz veinlets. Coalescence of the centers is favored by physical proximity as well as by new generationsof type 4 veinlets, which continue to form, albeit in lesser numbers. Most of the porphyry centers are confined within theLos Pelambres stock although some, like those at Frontera, are emplaced in the immediate andesitic wall rocks. Renewedmagma input forms the late mineral dikes in the roof of the tilted system, particularly at Frontera, simultaneous with paleo-surface degradation in response to continued regional surface uplift. d. The system cools and renewed aqueous liquid as-cends, preferentially along fracture swarms, and continues upward into the lithocap. The upper parts of the Los Pelambresstock and its contained magmatic-hydrothermal centers are then overprinted by the roots of the lithocap, as synhydrother-mal erosion and surface degradation progresses. Based on theoretical modeling by Rusk et al. (2008), Redmond and Einaudi(2010), and Sillitoe (2010).


that the bulk of the type 4 veining is either linked to a con-cealed early porphyry intrusion or took place prior to any por-phyry emplacement at Los Pelambres-Frontera (Fig. 11b). Ifthe latter alternative were the case, then the dilute, single-phase fluid responsible for the type 4 veining (Rusk et al.,2008; Proffett, 2009) must have ascended alone from the sub-jacent parental pluton.

The individual multiphase porphyry centers appear to haveacted like a series of miniature porphyry copper deposits,which coalesced to form the Los Pelambres-Frontera ore-body (Fig. 11c). Each center gave rise to brecciation and as-sociated magmatic-hydrothermal features (see above) as wellas generating its own sulfide zoning pattern. These discretefoci bring to mind the concept of emanative centers, first de-fined by Dines (1934, 1956), within the granitic plutons of theCornubian tin-tungsten-copper province of southwesternEngland. The coalescence of the zoning patterns linked to in-dividual porphyry centers led during potassic alteration to thedeposit-scale sulfide zoning from internal chalcopyrite-bor-nite through chalcopyrite-pyrite to peripheral pyrite. Fur-thermore, A- and B-type veinlet intensities also attain theirmaxima in some of the porphyry centers, in contrast to thetype 4 veinlets (Fig. 8b, c).

There is a gross coincidence between copper and molybde-num mineralization at the scale of the Los Pelambres-Fron-tera system, although at the cutoff grades employed in Figure9a, the correlation is far from perfect. In marked contrast, ap-preciable gold is confined to Frontera, where it correlateswell with copper. The gold-bearing bornite and chalcopyriteat Frontera are associated with appreciable hydrothermalmagnetite, a mineral largely absent from the gold-poor LosPelambres deposit, thereby displaying the classic gold-richporphyry copper deposit signature (Sillitoe, 1979; Perelló etal., 2011). The gold-bornite correlation proposed by Kesler etal. (2002) is only evident in the apical part of a single porphyrycenter (DAM-01; see above) and does not apply at the depositscale.

The sericitic alteration and contained pyrite, best devel-oped in the northwestern quadrant of the Los Pelambres de-posit, is partly controlled by the contact of the Los Pelambresstock and the NE-striking structural zone (Fig. 8a). The seric-itization overprinted the potassic zone and partially depletedits copper and molybdenum contents, although minor quan-tities of copper-bearing sulfosalts were precipitated. Thepresence of localized advanced argillic alteration in the north-east structural corridor suggests that the roots of a previouslyfar more extensive lithocap are preserved there (Fig. 11d).

The SE-inclined geometry of both the porphyry centersand northeast structural corridor are explained by north-westward tilting of the entire Los Pelambres-Frontera sys-tem by as much as 35° (Fig. 11b -d). The preferential preser-vation of the sericitic alteration in the northwestern quadrantof the Los Pelambres deposit further supports this interpre-tation. The fact that the brecciation and associated mag-matic-hydrothermal features are best developed on thesoutheastern, hanging-wall sides of the porphyry centers(Fig. 6b) implies that the tilting was synmineral in timing andpresumably linked to the tectonic uplift of the greater LosPelambres region between 12 and 7 Ma (Giambiagi andRamos, 2002).

Comparison with other central Chile porphyry copper deposits

The Los Pelambres-Frontera system shares a number ofobvious intrusive and hydrothermal features with the otherdeposits in the central Chile porphyry copper belt, particu-larly the giant Río Blanco-Los Bronces and El Teniente sys-tems (Fig. 1).

The Los Pelambres and Río Blanco-Los Bronces depositswere both generated near batholithic intrusions, the Chalingaintrusive complex and San Francisco batholith, respectively,the emplacement of both of which began several million yearsbefore the onset of porphyry copper mineralization (Deckartet al., 2005; this study). Furthermore, the porphyry copperalignments in both the Los Pelambres-El Pachón and RíoBlanco-Los Bronces districts project southeastward from therespective batholithic intrusions (Warnaars et al., 1985; Ser-rano et al., 1996; Fig. 3), presumably implying similar base-ment tectonic controls.

The individual porphyry centers documented at LosPelambres-Frontera also bear close similarities with thealigned series of small, but partly coalesced magmatic-hydro-thermal centers that combine to form the El Teniente por-phyry copper-molybdenum deposit (Vry et al., 2010). As atLos Pelambres, each of the El Teniente centers is character-ized by magmatic-hydrothermal breccias and veinlet zoning.

All three of the main central Chile deposits contain well-mineralized, magmatic-hydrothermal breccias, althoughthese are volumetrically and economically far more importantat Río Blanco-Los Bronces than at the other two deposits. In-deed, the magmatic-hydrothermal breccias at Los Pelambres-Frontera constitute no more than 5% of the orebody (Fig. 5),thereby emphasizing that it cannot be considered as amegabreccia deposit (Skewes and Stern, 1994, 1995). Theabundance of magmatic-hydrothermal breccias at RíoBlanco-Los Bronces could be interpreted to imply a shal-lower erosion level than at Los Pelambres, especially sincemuch of the porphyry copper mineralization there is con-cealed beneath pyritic rocks (R. H. Sillitoe, pers. observa-tions) and several of the major breccias display sericitic alter-ation and are pyritic and copper poor in their near-surfaceparts (Vargas et al., 1999; Irarrazaval et al., 2010). A similarsituation is observed at the small Copper Creek porphyrycopper deposit in Arizona, where numerous magmatic-hydro-thermal breccia pipes, displaying sericitic alteration at shallowlevels, overlie largely concealed porphyry copper mineraliza-tion (Guthrie, 1994; Anderson et al., 2009).

ConclusionsThe Los Pelambres porphyry copper-molybdenum and con-

tiguous Frontera porphyry copper-gold deposits were gener-ated in a period of ~1.5 to 1.7 m.y., immediately followingemplacement of a host precursor quartz diorite pluton. Thelatter, the Los Pelambres stock, was localized by the preexist-ing, N-NW–striking Los Pelambres reverse fault, whichbounds the eastern side of a narrow, intensely deformed struc-tural corridor. The Los Pelambres fault and other subparallelreverse faults in the greater Los Pelambres region were active2 to 3 m.y. before Los Pelambres stock emplacement as a re-sponse to orogen-wide, contractional deformation and crustal


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thickening. The Los Pelambres stock is the northwesternmostof a series of intrusive centers that define a marked align-ment, trending for 70 km southeast from the Chalinga intru-sive complex, a composite mafic to intermediate compositionbatholith assembled from 23 to 15 Ma. Basaltic to andesiticvolcanism at and near Los Pelambres ceased ~4 m.y. prior toemplacement of the Los Pelambres stock.

The Los Pelambres and Frontera deposits comprise at least20 discrete, multiphase, intermineral porphyry centers in-truded within and just beyond the Los Pelambres stock. Par-tial coalescence of these centers, many of them blind, accountsfor the giant status of the deposit (36 Mt Cu). Although thereis no clearcut spatial or temporal arrangement of the individ-ual porphyry centers, there does appear to be a southeastwardyounging from Los Pelambres to Frontera, which continuesin the same direction for 5 km to the El Pachón porphyry cop-per-molybdenum deposit. Each center is associated withmagmatic-hydrothermal breccia and associated features in-dicative of the magmatic-to-hydrothermal transition as well asbeing the focus of quartz veining and sulfide zoning. How-ever, early generations of type 4 (early dark micaceous) vein-lets are present throughout the Los Pelambres stock and donot appear to be directly related to the known porphyry cen-ters. Pervasive potassic alteration affected the entire LosPelambres stock but is overprinted by sericitic alteration inthe northwestern quadrant of the Los Pelambres depositwhere the roots of an advanced argillic lithocap are preserved.The gold-rich character of the Frontera deposit correlateswell with its abundant hydrothermal magnetite content. Theporphyry centers and structurally controlled parts of thesericitic zone were generated during northwest tilting of up to35°, linked to the regional contractional tectonism.

Continued contractional tectonism and consequent upliftcooled the Los Pelambres deposit to ambient temperatureswithin ~2 m.y. of its final emplacement and exposed it to su-pergene processes within a further ~3 m.y. The leached cap-ping and underlying chalcocite enrichment zone were par-tially eroded during U-shaped glacial valley development inthe Plio-Pleistocene. Supergene processes at Los Pelambreswere penecontemporaneous with giant porphyry copper for-mation farther south in the central Chile Miocene to earlyPliocene belt, at Río Blanco-Los Bronces and El Teniente(e.g., Howell and Molloy, 1960; Warnaars et al., 1985).

AcknowledgmentsPatricio East and Felipe Matthews are thanked for their

contributions to the brownfields program that resulted in thediscovery of Frontera, an effort that received unstinting sup-port from Ricardo Muhr. Edmundo Martínez, HéctorPoblete, Esteban Acuña, Jorge Artal, and Tammy Verdugo as-sisted with various technical aspects of the program duringwhich the staff of Compañía Minera Los Pelambres, particu-larly Francisco Carvajal, Alvio Zuccone, and FernandoGonzález provided invaluable logistic assistance. AlfredoBertens shared his ideas and U-Pb data for Los Pelambres,and Paula Cornejo and Enrique Tidy provided petrographicdescriptions of selected samples from the area. Much of theU-Pb geochronology reported here was conducted by VictorValencia, whereas Robert Creaser, Fernando Barra, and HollyStein separately undertook the Re-Os molybdenite dating.

Héctor Poblete drafted the figures. Thorough manuscript re-views by William Atkinson and John Proffett are much ap-preciated.

REFERENCESAlvarez, P., 1996, Los depósitos triásicos y jurásicos de la Alta Cordillera de

San Juan, in Geología de la Región del Aconcagua, Provincias de San Juany Mendoza: Dirección Nacional del Servicio Geológico [Argentina], Anales,v. 24, no. 14, p. 59–137.

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chapter 4 geologic setting and evolution of the porphyry

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