Chapter 4 Geologic Setting and Evolution of the Porphyry Copper-Molybdenum and Copper-Gold Deposits at Los Pelambres, Central Chile

Home , Chilenia, Choiyoi Group, Porphyry (geology), Porphyry copper deposit

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, Chile 2 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

Abstract The 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 (~36 million metric tons (Mt) Cu) in the Miocene to early Pliocene belt of central Chile. Los Pelambres is centered on 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 Cretaceous volcanic 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 mineralization at 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 potassic center grades laterally to an annular sericitic zone surrounded by a propylitic halo. The bulk of the hypogene 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 and economically the most important. The type 4 veinlets are regularly distributed throughout Los Pelambres and Frontera, 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 intermineral porphyry intrusions of dacitic (porphyry B) and andesitic (porphyry A) compositions along with igneous and hydrothermal breccias, the apical parts of which contain aplite and pegmatite pods. These centers acted as a series of miniature porphyry copper deposits whose coalescence generated the Los Pelambres-Frontera orebody. This coalescence also led to deposit-scale sulfide zoning, from internal chalcopyrite-bornite through chalcopyrite-pyrite to external pyrite. Abundant hydrothermal magnetite accompanies the gold-bearing copper mineralization 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 of Los Pelambres contains copper sulfosalts. The internal portions of this corridor are characterized by advanced argillic 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 ~3.8 m.y., from emplacement of the precursor Los Pelambres stock between ~14 and 12.5 Ma, through generation of numerous porphyry B and A phases and associated magmatic-hydrothermal centers between ~12.3 and 10.5 Ma, to intrusion of late mineral porphyry at Frontera at ~10.2 Ma. Similarly, the copper, molybdenum, and gold mineralization was introduced during a protracted interval of ~1.7 m.y., between 11.8 and 10.1 Ma, as constrained 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 eroded a former, more widespread supergene chalcocite blanket, the remnants of which accounted for the bulk of the ore 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 centers and northeast structural corridor defined by sericitic and advanced argillic alteration, are ascribed to synmineral 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.

Introduction at elevations between 3,200 and 3,600 m above sea level LOS PELAMBRES, the northernmost and third largest copper (Fig. 2a). Los Pelambres and Frontera along with the cop- concentration in the Miocene to early Pliocene belt of cen- per-molybdenum deposit at El Pachón, 5 km southeast tral Chile (Fig. 1), comprises two contiguous deposits, Los across the international frontier in Argentina, constitute the Pelambres copper-molybdenum and Frontera copper-gold. Los Pelambres-El Pachón porphyry copper cluster (Fig. 1). The deposits underlie the vegetation-free talus slopes of a U- The supergiant status of the Los Pelambres deposits is de- shaped glacial valley in the Principal Cordillera of the Andes, 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% † Corresponding author: e-mail, [email protected] Cu (Perelló et al., 2011).

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h 80 100 175 200 74º c La Serena 69º 64º n

e 120 r 30º t Wadati-Benioff

u zone contours

e (km)

P -

e LOS PELAMBRES-EL PACHON

l i

h Flat-slab

C segment Juan Fernández Mendoza 140

Ridge Santiago RIO BLANCO-LOS BRONCES Southern 100 km Principal deposits EL TENIENTE Other deposits and Volcanic prospects Zone

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

The deposits at Los Pelambres are owned by Minera Los setting and the geology and alteration-mineralization features Pelambres (Antofagasta Minerals S.A. 60%, Japanese consor- of Los Pelambres and Frontera, documents the lifespan of tium 40%). The copper-molybdenum deposit, mined in an the hydrothermal system, and discusses the geologic evolu- open pit (Fig. 2b) at a current ore throughput of 176,000 t/d tion of the porphyry mineralization within a regional tectono- averaging 0.74% Cu and 0.019% Mo, produced 411,800 t of magmatic framework. The paper is based on more than three copper, 9,900 t of molybdenum, and 39,800 oz of gold, and years of fieldwork by the authors during a brownfields explo- 1,774,300 oz of silver in 2011. The ore is processed by con- ration program that included 1:50,000-scale regional, 1:10,000- ventional flotation and the resulting copper concentrate is scale district, and 1:2,000-scale pit mapping as well as 1:100- transported ~120 km by slurry pipeline to the company’s port scale logging of 160,000 m of preexisting and newly obtained for shipment to overseas smelters. Los Pelambres and Fron- drill core. Previous published studies by Sillitoe (1973), tera are currently the subject of another major infill drilling Skewes (1985), Skewes and Atkinson (1985), Atkinson et al. campaign, which is likely to further increase resources for a (1996), Bertens et al. (2003, 2006), Perelló et al. (2007, 2009, planned future mine expansion. 2011), and Mpodozis et al. (2009) as well as extensive unpub- This contribution summarizes the historic and recent explo- lished in-house data provide the basis for this synthesis. The ration history of the district, describes the regional geologic porphyry and veinlet nomenclature of Skewes and Atkinson

a b

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

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(1985) and Atkinson et al. (1996) is followed throughout, but marine sedimentation in a backarc setting during the Jurassic- the relative timing of porphyry phases, the economic relevance Early Cretaceous, and subaerial, subduction-related, calc-al- of certain veinlet generations, and the volumetric importance kaline volcanism and associated plutonism during the Creta- of hydrothermal breccias are considered to be different. ceous through Cenozoic (Mpodozis and Ramos, 1990). Central Chile and contiguous parts of Argentina, including Exploration History the porphyry copper belt, underwent contractional tectonism In 1967, the Instituto de Investigaciones Geológicas pre- from the early Miocene through early Pliocene in response to pared the first geologic report for Los Pelambres, which for- subduction zone shallowing (Jordan et al., 1983). This trig- mally identified the porphyry copper affiliation of the gered crustal shortening and thickening through hybrid thin- prospect (Thomas, 1967). Exploration was resumed in 1969 and thick-skinned thrusting to generate the Aconcagua fold- under a joint program conducted by the United Nations and thrust belt (Ramos et al., 1996; see below). The slab shallow- Empresa Nacional de Minería (ENAMI), the state mining ing is generally ascribed to the diachronous oblique subduc- agency. Drilling defined a mineral inventory of 430 Mt at tion of the buoyant Juan Fernández ridge on the Nazca plate 0.80% Cu and 0.035% Mo (Sillitoe, 1995). No further explo- (Fig. 1; Pilger, 1981; Yáñez et al., 2001). The copper mineral- ration took place until 1979 when Anaconda South America ization in the belt took place between 12 and 4 Ma and ac- purchased the property from the local owners and undertook companied multikilometer, regional-scale uplift and concomi- detailed exploration, culminating in 1983 with completion of tant exhumation (Skewes and Holmgren, 1993; Kurtz et al., a feasibility study for a 60,000 t/d operation based on a re- 1997). The porphyry copper stocks have rare earth element source of 3,300 Mt at 0.63% Cu and 0.016% Mo (Sillitoe, signatures interpreted to reflect the thickening of the crust 1995; Atkinson et al., 1996). At the copper prices prevailing at (Kay et al., 1999; Kay and Mpodozis, 2002). the time, such a large-scale project was considered uneco- nomic and all work was discontinued. Tectonics and stratigraphy of the greater In 1985, Antofagasta Holdings acquired Anaconda’s inter- Los Pelambres region ests in Chile and a wholly owned subsidiary, Compañía Min- The greater Los Pelambres region, spanning the Chile-Ar- era Los Pelambres, developed a 5,000-t/d sublevel caving and gentina frontier between latitudes 31°35' and 32°03'S, com- flotation operation based on high-grade, breccia-hosted ore prises three main tectonic domains bounded by high-angle, (see below). Through 1999, Compañía Minera Los Pelambres E- or W-vergent, N- to NNW-striking, reverse faults, herein mined approximately 12 Mt averaging 1.5% Cu (Perelló et al., named the Los Pelambres, Totoral, and González faults (Fig. 2011). Simultaneously, a feasibility study for an 80,000-t/d 3). These structural elements form the northern termination plant was carried out, with completion in 1996 and, the fol- of the larger Aconcagua and smaller (e.g., La Ramada) fold- lowing year, a Japanese consortium acquired a 40% interest in thrust belts (Cegarra and Ramos, 1996; Cristallini and Ramos, Compañía Minera Los Pelambres and committed the same 2000). proportion of development funding. Project construction The eastern domain, east of the E-vergent Los Pelambres began in early 1998 and in January 2000 the plant attained its reverse fault, contains the large basement block of the rated capacity. Successive expansions through 2004 increased Cordillera de Santa Cruz (Fig. 3), composed of late Paleozoic ore throughput to 130,000 t/d, with the most recent expansion rhyolite and felsic tuff along with comagmatic granitoids, completed in 2010 increasing capacity to ~175,000 t/d. collectively assigned to the Choiyoi Group (Alvarez, 1996; Since the completion of Anaconda’s feasibility study in Cristallini and Ramos, 2000). Beyond the limits of Figure 3, 1983, no further exploration was carried out at Los Pelambres this basement block is thrust eastward over synorogenic, con- until late 2005 when the brownfields program was initiated by tinental, siliciclastic deposits of Miocene age (Jordan et al., Antofagasta Minerals S.A. on behalf of the operating com- 1996; Pérez, 2001). To the west of the Cordillera de Santa pany. The program resulted in discovery of the Frontera cop- Cruz block, the Paleozoic basement is overlain by Triassic per-gold deposit (700 Mt at 0.52% Cu and 0.1 g/t Au, using a continental volcanic and sedimentary strata and Jurassic to 0.4% Cu cutoff) and definition of additional resources at Los Early Cretaceous marine and continental sedimentary rocks Pelambres, which together constitute the current global re- (Alvarez, 1996), which together represent the northernmost source cited above (Perelló et al., 2011). exposures of the sedimentary fill to the Neuquén backarc basin, amply developed farther south (Mpodozis and Ramos, Tectonomagmatic Setting 1990; Cristallini and Ramos, 2000). These bedded units are unconformably overlain by continental volcaniclastic conglom- Central Chile porphyry copper belt erate and breccia, rhyolitic tuff, and pyroxene- and hornblende- The Miocene to early Pliocene porphyry copper belt of bearing andesite and dacite, herein informally grouped as the central Chile and contiguous Argentina extends for ~400 km Mondaca Strata (Fig. 3), which yield U-Pb zircon ages of 22.1 between latitudes 31° and 35° S and contains an exceptional ± 0.4 and 21.6 ± 0.4 Ma (Table 1). On the Chilean side of the copper endowment (~360 Mt) contained in a series of super- frontier, at Laguna del Pelado (Fig. 3), these strata are un- giant and smaller sized deposits (Fig. 1). The belt was con- conformably overlain by >400 m of subhorizontal, hornblende- structed within the Chilenia terrane, a microcontinental block bearing andesitic lava flows, which provide U-Pb zircon ages accreted to the Gondwana margin in the Devonian and un- between 21.3 +0.4/-0.3 and 18.3 ± 0.4 Ma (Table 1). Imme- derpinned by basement rocks of Proterozoic age (Ramos, 2009). diately to the north, these rocks are tectonically overlain by Following terrane accretion, the belt was the site of exten- Paleozoic basement and Mesozoic strata along the E-vergent sion-related, bimodal magmatism during the Permo-Triassic, Mondaca reverse fault (Fig. 3).

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345,000 370,000

C O R D

I L L E

6,500,000 A

Río Chalinga

R ío C arn SA icer ía N Los T A Pelambres

Frontera R

El Pachón Z

Totoral fault

Quebrada Piuquenes

Pachón Río Cross section

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Chile Argentina Meters above Río Santa Cruz Quebrada Piuquenes Los Pelambres El Pachón sea level Río Pachón 4,000

2,000 Mondaca Chalinga intrusive complex Totoral fault fault 0 STRATIFIED ROCKS INTRUSIVE ROCKS Cretaceous volcanic rocks Laguna del Pelado volcanic rocks (20-18 Ma) (Salamanca Formation) Middle-late Miocene intrusive rocks (14-10 Ma) Mondaca Strata (22 Ma) Early Cretaceous redbeds Totoral pluton (18 Ma) a a: Basal conglomerate Chalinga intrusive complex Pachón and Abanico Formations (28-21 Ma) Jurassic continental and a b c: Phase 3 (16-15 Ma) marine sedimentary sequences b: Phase 2 (18 Ma) Los Pelambres Formation (33-18 Ma) c Triassic volcanosedimentary rocks a: Phase 1 (23-22 Ma) Paleocene volcanic rocks Late Cretaceous-Eocene intrusive rocks Late Paleozoic-Triassic Reverse fault (teeth on upper plate) Late Cretaceous (75-70 Ma) basement (Choiyoi Group) volcanic and sedimentary rocks Normal fault a a: Intensely deformed sequences of the central domain Undifferentiated fault FIG. 3. Regional geologic map of the greater Los Pelambres region, based on Alvarez (1996), Mpodozis et al. (2009), and more recent mapping by the authors. UTM datum: Prov. S. Am 56, Zone 19 South.

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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 tuff PEL 681 21.6 ± 0.4 378,011 6,480,767 North of El Yunque Mondaca Strata Welded rhyolitic tuff prospect PEL 211 21.3 (+0.4/-0.3) 378,020 6,458,681 Laguna del Pelado Laguna del Pelado Daciandesitic, pumice-rich, Sequence lithic tuff PEL 161 18. 3 ± 0.4 379,343 6,461,318 Laguna del Pelado Laguna del Pelado Hornblende-bearing porphyritic Sequence andesite FRONT 42 22.7 ± 0.2 360,775 6,487,750 Los Pelambres Pachón Formation Fine-grained porphyritic andesite PELLAG 012 21.69 ± 0.26 358,375 6,494,200 Río Carnicería Pachón Formation Aphanitic andesitic lava DDH 71962 21.36 ± 0.80 350,000 6,491,440 Los Pelambres Pachón Formation Quartz-eye dacitic sill PEL 11 33.4 ± 0.5 357,720 6,485,634 Río Pelambres Los Pelambres Formation Fine-grained andesitic volcanic breccia PEL 411 18.5 ± 0.4 364,693 6,464,923 Río Totoral Los Pelambres Formation Recrystallized daciandesite PEL 471 24.9 ± 0.5 367,718 6,447,831 Río Chicharra Abanico Formation Andesitic lithic tuff PEL 511 22.2 ± 0.4 364,854 6,447,799 Río Chicharra Abanico Formation Fine-grained porphyritic andesite PEL 461 18.5 ± 0.4 359,196 6,460,543 Río Totoral Totoral pluton Biotite (pyroxene) monzogranite Sec1-12 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) monzodiorite PEL 1031 15.1 (+0.6/-0.6) 350,890 6,504,830 Los Helados, Chalinga intrusive complex Pyroxene-biotite quartz Río Chalinga (Phase 3) monzodiorite PEL 651 15.4 ± 0.4 373,153 6,483,804 Río Pachón Northwest-trending Hornblende-biotite dacitic intrusive belt porphyry PEL 661 15.0 ± 0.3 378,397 6,475,054 El Yunque prospect Northwest-trending Hornblende-biotite dacitic intrusive belt porphyry PEL 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 South 1 Dated at the University of Arizona, Tucson, Arizona 2 Dated at Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, Australia 3 Dated at University of Tasmania, Tasmania, Australia 4 Dated at Washington State University, Washington

A well-defined belt of pyroxene ± olivine-bearing basaltic González faults is intensely deformed, as indicated by vertical to andesitic lava flows and minor felsic tuff interbeds extends and overturned strata, anastomosing, thrust-bounded tec- continuously throughout the greater Los Pelambres region tonic lenses, and widespread mesoscale, subisoclinal folds. for ~60 km and constitutes the Pachón Formation (Fernán- Newly obtained U-Pb zircon ages for the Los Pelambres For- dez et al., 1974; Lencinas and Tonel, 1993). Near Los Pelam- mation, ranging from 33.4 ± 0.5 to 18.0 ± 0.4 Ma (Table 1), bres, as well as east of the international frontier, this unit confirm its early Oligocene to early Miocene age (Mpodozis yields U-Pb zircon ages between 21.69 ± 0.26 and 22.7 ± 0.2 et al., 2009; Perelló et al., 2009), contrary to previous Early Ma and, in the Los Pelambres open pit, it is intruded by a Cretaceous age assignments (Rivano and Sepúlveda, 1991; quartz-eye dacite sill with a U-Pb zircon age of 21.36 ± 0.80 Bertens et al., 2006). Ma (Perelló et al., 2009; Table 1). The Pachón Formation is West of the Totoral and González faults, the western domain the westernmost unit of the eastern domain, and throughout comprises a >2-km-thick, gently E-dipping sequence of conti- the area is delimited westward by the Los Pelambres fault nental volcanic, volcanosedimentary, and sedimentary rocks of (Fig. 3). Cretaceous age, including the Salamanca Formation (Rivano The central domain straddles the international frontier and and Sepúlveda, 1991) and other undifferentiated units (Fig. corresponds to a N- to NW-striking, ~5-km-wide, fault- 3). The steeply dipping Pocuro fault, first defined farther bounded zone of strongly deformed andesitic to basaltic lava south in central Chile by Carter and Aguirre (1965), consti- flows and tuffs, fluviatile epiclastic strata, and local lacustrine tutes the tectonic contact between the Salamanca Formation limestone of the Los Pelambres Formation (Rivano and to the west and the other volcanic units farther east, which Sepúlveda, 1991) as well as tectonic slivers of Cretaceous vol- become more intensely deformed on approach to the central canic rocks (Fig. 3). The domain, defined and bounded by the domain. The tectonic wedge of Cretaceous volcanic rocks E-vergent Los Pelambres and Totoral and W-vergent between the Totoral and González faults is an example of

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particularly intense deformation (Fig. 3). The volcanic rocks basaltic andesite, and basalt, the great majority with SiO2 con- exposed east of the Pocuro fault yield U-Pb zircon ages be- tents between 61 and 53%. They display FeO/MgO versus tween 75 and 70 Ma, similar to the 70.10 ± 1.50 Ma age SiO2 ratios transitional between the tholeiitic and calc-alka- (Table 1) obtained from the rhyolitic tuff at Los Pelambres line fields and possess flat rare earth element patterns, with (see below). The Cretaceous volcanic rocks of the western do- La/Yb between 5 and 19 and La/Sm between 3 and 6 (Fig. 4). main are unconformably overlain northwest of Los Pelambres Intrusive and volcanic rocks with ages between 18 and 15 Ma by andesitic volcanic sequences of Paleocene age and, at Río display more felsic compositions (67–62% SiO2), stronger arc- Totoral, by similar rocks of latest Oligocene to early Miocene like signatures, and higher La/Yb (6–34) but similar La/Sm age (Fig. 3). Immediately south of the area covered by Figure (3–7) ratios. 3, the latter rocks provided U-Pb zircon ages of 24.9 ± 0.5 and The Los Pelambres stock and related porphyry copper cen- 22.2 ± 0.4 Ma (Table 1) and are correlated with the north- ters (see below) have low Nb/Ta ratios (<11) and steep rare ernmost expressions of the Abanico Formation, widely dis- earth element patterns (La/Yb: 26–72; Fig. 4). The Sm/Yb ra- tributed farther south (Charrier et al., 2002). tios of 4 to 9 for these intrusive rocks indicate that amphibole and garnet were stable as residual or crystallizing phases in a Intrusive rocks of the greater Los Pelambres region deeper source region, in contrast to the Los Pelambres and Meso-Cenozoic intrusive magmatism is abundantly devel- Pachón Formations, which are characterized by Sm/Yb ratios oped in the western domain where Late Cretaceous, Pale- between 2 and 3. These petrochemical features, together ocene, Eocene, and Oligocene plutons and stocks of generally with very high Sr/Y ratios (61–92), place the porphyry copper- intermediate composition intrude the volcanic and vol- related Los Pelambres intrusive rocks in the adakite field canosedimentary sequences. However, intrusive activity is (Fig. 4; cf. Reich et al., 2003), whereas the absence of Eu markedly less abundant farther east. South of Los Pelambres, anomalies is compatible with the highly oxidized state of the at Río Totoral, the large, isolated, pyroxene-bearing, Totoral magmas. The petrochemical evolution of the Los Pelambres monzogranite pluton, with a single U-Pb zircon age of 18.5 ± magmas is similar to that documented for other porphyry 0.4 Ma (Fig. 3; Table 1), intrudes and effectively constrains copper deposits in central Chile (Kay and Mpodozis, 2002) the age of the Abanico Formation volcanism. Similarly, im- and supports emplacement during crustal thickening conse- mediately west of Los Pelambres, the large 25- × 15-km, quent upon the tectonic contraction described above (Kay multiphase, partly nested Chalinga intrusive complex (Fig. 3) and Mpodozis, 2002; Hollings et al., 2005). was initiated with pyroxene diorite and granodiorite phases, followed by olivine gabbro and diorite. The early intrusions Geology of Los Pelambres-Frontera Area have 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 and Country rocks 18.1 ± 0.5 Ma (Table 1). A third, quartz monzodiorite phase Los Pelambres and Frontera are spatially and genetically re- forms the northern part of the Chalinga intrusive complex, a lated to multiphase porphyry bodies that are located within and southeastern lobe of which cuts the Totoral fault at the west- immediately southeast of the Los Pelambres stock (Figs. 5, 6). ern edge of the central domain. This third phase yields U-Pb The stock is emplaced into the Late Cretaceous rhyolitic tuff zircon ages of 16.5 +0.3/−0.2 and 15.1 ± 0.6 Ma (Table 1), and late Oligocene to early Miocene Los Pelambres and thereby providing a minimum age for the Totoral reverse Pachón Formations (see above). The two formations are sepa- fault at this latitude. rated by the N-striking Los Pelambres reverse fault (Figs. 3, 5). A string of equigranular to porphyritic, hornblende-bearing East of the fault, the Pachón Formation is composed of quartz diorite and dacite stocks extends southeastward for gently to steeply dipping (20°−70°W), massive andesitic ~70 km from the youngest phase of the Chalinga intrusive flows and flow breccias, whereas moderately to steeply dip- complex to beyond the international frontier (Fig. 3) and in- ping (50°−80°W) epiclastic rocks and andesite flows consti- cludes, among others, the porphyry copper centers at Los tute the Los Pelambres Formation to the west (Figs. 3, 5). Pelambres, El Pachón, Yunque, and beyond the greater Los Several roof pendants and large blocks of massive andesite, Pelambres region, Cerro Mercedario (Sillitoe, 1977; Bertens some up to 500 m in vertical extent, are present in the Los et al., 2006; Mpodozis et al., 2009; Perelló et al., 2009). Dated Pelambres stock, defining a NW-trending corridor through intrusions of this transverse intrusive trend yield new U-Pb the north-central parts of the open pit (Fig. 5). Within the pit zircon ages of 15.4 ± 0.4 and 15.0 ± 0.3 Ma (Table 1). The and its immediate environs, both formations are also in- northwesternmost intrusion in this trend is the Los Pelam- truded by numerous, coarse-grained, porphyritic andesite bres stock and satellite bodies, for which Bertens et al. (2003, dikes and sills of premineral age and, more locally, as in the 2006) and A. Bertens (writ. commun., 2007) reported U-Pb northern high wall of the pit, by quartz-eye-bearing sills of zircon ages of 13.92 ± 0.15 and 12.51 ± 0.17 Ma. These in- dacitic composition (cf. quartz-feldspar porphyry of Atkinson trusions were controlled by and sealed the Los Pelambres re- et al., 1996). West of the Los Pelambres fault, the brownish- verse fault during the waning stages of Chalinga intrusive colored epiclastic sandstone and shale of the Los Pelambres complex emplacement, in accord with the timing of the To- Formation unconformably overlie the Late Cretaceous rhy- toral fault reported above. olitic tuff (Fig. 5). Igneous petrochemistry of the greater Los Pelambres region Los Pelambres stock The Oligocene to Miocene volcanic sequences of the greater The late Miocene Los Pelambres stock is a N-trending, 4.5- Los Pelambres region are predominantly composed of andesite, × 2.5-km intrusion that has been intersected by drilling to a

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10 200200

Adakites 8 160160

6 120120 Los Pelambres Tholeiite

Sr/Y 8080 FeO*/MgO 4

Normal arc 2 Los Pelambres 4040

Calc-alkaline 0 00 50 60 70 80 0 10 20 30 40 a SiO2 b Y Oligocene-early Miocene 1000 Oligocene-early Miocene volcanic rocks volcanic rocks Early Miocene Chalinga intrusive Early Miocene volcanic rocks complex (23-22 Ma) 100 Early Miocene Totoral pluton (18 Ma) Early Miocene Laguna del Pelado 10 volcanic rocks (21-18 Ma) Middle Miocene intrusive rocks (16-15 Ma)

1 Nonmineralized Late Miocene intrusive rocks (12-10 Ma) Los Pelambres intrusive rocks

0.1 PYX = Pyroxene HBL = Hornblende c La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu GAR = Garnet 10 Los Pelambres 80 Adakites 8

60 Los Pelambres 6

La/Yb La/Sm 4

20 Normal arc 2

00PYX HBL GAR 0013243648510 d Ybe Sm/Yb

FIG. 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 arc fields from Defant and Drummond (1990). c. Chondrite-normalized rare earth element patterns (Boynton, 1984). Note the steep configuration and concave shape of the Los Pelambres intrusive rock patterns, indicating residual or crystallizing amphibole/garnet in the magma source region. d. La/Yb vs. Yb diagram highlighting the adakitic nature of the Los Pelambres intrusive rocks. Fields are from Castillo (2012). e. La/Sm vs. Sm/Yb diagram displaying the strong petrochemical differences between Oligocene to early Miocene volcanic and >15 Ma intrusive rocks compared to <15 Ma intrusions. The magmas of the first group evolved at low pressures in the presence of pyroxene, whereas the younger group evolved at higher pressures across the transition zone from amphibole to garnet stability, as indicated by higher Sm/Yb ratios. Ranges of Sm/Yb ratios are from Kay et al. (1999).

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358,000 359,000 360,000 361,000 Late mineral

T Low-grade L

U stockpile and Diorite and dacite porphyry

F moraine deposits

S Intermineral E RAM-05

6,492,000 N Magmatic-hydrothermal centers

M Magmatic-hydrothermal breccia LA

E Porphyry A family

50° S Porphyry B family O L Premineral 3,500 a a : Los Pelambres quartz diorite stock MINA PORTEZUELO b (porphyritic within dotted line) b : Satellite phases 3,300 PORTEZUELO ESTE Country rocks 6,491,000 Pachón Formation 3,250 a a : Dominantly andesite 80° b : Dacite sill 70° b

2 A DAM-01 Los Pelambres Formation E A´

N I (Figs.6a, 9b)

L Cretaceous rhyolitic tuff

U 70°

O 3,200

T I Fault: mapped, inferred P DAM-18 Reverse fault (teeth on 6,490,000 CLUSTER 70° upthrown block) OESTE 500 m

MINA RGENTINA

VICTORIA MAM-03 CHILE

70° 70°

6,489,000 DAM-10

PEGMATITA SUR

FRONTERA 75° Low-grade (Figs.6b, 9c) stockpile 20°

B´

6,488,000 DAM-06

80° 70°

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

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Meters above sea level 3,800 A A´ Original topography 2008 pit

3,400

3,000

30 year pit c 1 cm 2,600 a 250 m

Meters above sea level B 3,800 Original topography B´

3,400 2008 pit 30 year pit d 1 cm 3,000

2,600

b 400 m a Late porphyry a: Porphyry A family b: Porphyry B family b Magmatic-hydrothermal centers Los Pelambres stock 1 cm UST e Pachón Formation Breccias Fault a a: Hydrothermal cement Los Pelambres Formation b: Igneous matrix b FIG. 6. Representative geologic sections and principal intrusive phases of Los Pelambres porphyry copper-molybdenum deposit. a. East-west transverse section. b. Northwest-southeast longitudinal section. Note the southeast plunge of the porphyry centers and associated features. c. Quartz diorite of Los Pelambres stock. d. Porphyry B, Los Pelambres. e. Porphyry A, Los Pelambres.

depth of 1.5 km (Figs. 5, 6). The stock is composed of sev- Intermineral porphyry phases eral discrete phases, including medium- and fine-grained, Numerous intermineral porphyry phases are present at Los equigranular as well as porphyritic varieties. The bulk of the Pelambres and Frontera, which, for practical reasons, are stock is made up of light- to medium-gray, subequigranular herein grouped into two main families: porphyry A and por- to hypidiomorphic equigranular quartz diorite (tonalite of phyry B (cf. Atkinson et al., 1996). The porphyry phases typi- Sillitoe, 1973, following IUGS classification [Streckeisen, cally comprise dikes and small bodies of irregular geometry, 1973]), with a dominance of plagioclase of andesine compo- the majority of which are clustered in the central parts of the sition over biotitized hornblende and biotite (Atkinson et al., Los Pelambres stock (Figs. 5, 6). A poorly defined alignment 1996; Table 2; Fig. 6c). Minor quartz and K-feldspar occur of minor intrusions cutting andesitic rocks of the Pachón For- interstitially in the equigranular phases, and aplitic ground- mation extends for ~2 km southeastward into the Frontera mass is common in the porphyritic phases. Porphyritic deposit and beyond (Fig. 5). At least 20 discrete intrusive cen- quartz diorite occupies an important volume in the east-centers form the central cluster of porphyry A and porphyry B, tral part of the Los Pelambres stock (Fig. 5), but its contact the largest ones attaining 250 m in diameter where subcircu- relationship with the rest of this intrusion is poorly defined. lar and 500 m in length where dike-like in form (Fig. 5). Both However, local observations suggest that the porphyritic the porphyry A and porphyry B families consist of multiple phases postdate emplacement of the equigranular quartz phases, many of which could not be unambiguously corre- diorite. lated on a hole-to-hole basis.

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Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/3812113/9781629490410_ch04.pdf by guest on 05 August 2020 88 PERELLÓ ET AL. hypidiomorphic; porphyritic (porphyritic quartz diorite) and fine-grained equigranular (microdiorite) phases present apparent on the basis of crosscutting relationships is light gray and medium-grained (1-4 mm) porphyritic, with dacitic composition amphibole (1-3 mm), subequigranular to equigranular, plagioclase, K-feldspar, quartz, biotite, grained (2-4 mm) crowded porphyritic amphibole multiple phases apparent on the basis of texture; miarolitic cavities locally present; crosscutting relationships plagioclase, rare interstitial quartz cavities locally present; biotite is dominant (1-3 mm) porphyritic texture; miarolitic component of groundmass; multiple phases amphibole distribution; the terminal phase at Frontera K-feldspar), medium-grained light to medium gray, Groundmass Vol %Vol Composition features Textural 2 40 40 -An -An 30 30 An An 2–6 mm (to 1 cm) 1–5 mm 1–5 mm Dominant phenocryst phase 2. Principal Characteristics of the Los Pelambres-Frontera Porphyry-Related Intrusive Units ABLE T 0.5–1 mm 1–2 mm 1–2 mm 1–2 mm plagioclase, dikes between 20 and 30 m wide of limited 0.5–2 mm Vol % Vol 1 Porphyritic phase (Fig. 5) All amphibole, dominantly hornblende, is moderately to completely altered biotite Notes: All phases contain variable amounts of apatite, magnetite, rutile, and zircon as accessories; hematite occurs as inclusions in rutile porphyry A and B phases Notes: All phases contain variable amounts of apatite, magnetite, rutile, and zircon as accessories; hematite occurs inclusions 1 2 Los Pelambres stock 70 15–25% 5–10% 60–65% 3–10% 3–10% 30 Aplitic (quartz- Quartz diorite comprises ~90% of the stock; Unit (rock type) phenocrysts Quartz K-feldsparPorphyry B Plagioclasefamily (PB) Biotite 50-65 Amphibole Porphyry A 1–5%family (PA) Minor to absent 1–2.5 mm 35 40–75%Late Porphyry (L) <2% 5–20% 70 <1 mm Minor to absent 5–20% 2–8 mm 20–40% 35–40 <1% Felsitic, 2–4 mm 10–20 % 2–4 mm Absent 10–20% Dominantly of dacitic composition; light to 1–6.5 mm 65 40–60% 1–6 mm microaplitic; Trachytic; 1–6 mm 5–10% medium gray or brownish, with medium- Dominantly of andesitic composition; dark 5–10% biotite, 30 Biotite, gray to brown, medium fine-grained Dominantly of microdioritic composition; as

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Porphyry B dikes and coalescent bodies are characterized chalcopyrite, are present in both of the porphyry families, im- by crowded, medium-grained, porphyritic textures, with an plying fluid segregation and entrapment during porphyry overall dacitic composition (Table 2; Fig. 6d). In contrast, consolidation (cf. Candela, 1997). porphyry A intrusions are finer grained, darker brown in color, and dominantly andesitic in composition (Table 2; Fig. Related magmatic-hydrothermal features 6e). Oriented plagioclase and biotite phenocrysts impart a At least 12 of the porphyry centers within the open pit at characteristic and readily identifiable, banded texture to Los Pelambres and beyond are characterized by one or more many of the porphyry A phases. Although porphyry B is locally closely related features indicative of the magmatic to hydro- observed to cut porphyry A, it is normally the case in por- thermal transition. These features, elaborated in Table 3, phyry centers containing both families that porphyry A post- comprise aplite, pegmatite, unidirectional solidification tex- dates porphyry B and truncates some of its contained veinlets. tures (cf. Shannon et al., 1982), and magmatic-hydrothermal This observation contradicts the relative age relationship pro- and igneous breccias (cf. Sillitoe, 1985) as well as the copper, posed by Atkinson et al. (1996, fig. 14), who preferred an molybdenum, and gold mineralization (see below). An ideal- early, premineral timing for the porphyry A family. These ap- ized reconstruction of the features with respect to a porphyry parently mutually crosscutting relationships suggest the pos- center is given in Figure 7a. The developmental sequence of sibility that, in fact, there may be more than one generation these features in each center is complex, and not necessarily of both porphyry A and porphyry B (see below). Localized the same in all centers, with exposure level clearly being an miarolitic cavities, containing biotite, chlorite, anhydrite, and important determinant of what is observed. As summarized in

Aplite-pegmatite Zone of pervasive pods and dikes sericite-chlorite alteration

Sheeted quartz veinlets and local Magmatic-hydrothermal UST development breccia with hydrothermal cement > igneous matrix

1 cm d Magmatic-hydrothermal breccia with dominantly igneous matrix bn bi Intermineral porphyry phase with irregular bn miarolitic cavities qz

a 100 m cp

1 cm e

b 3 cm

3 cm c 1 cm f

FIG. 7. Magmatic-hydrothermal features of Los Pelambres porphyry copper-molybdenum and Frontera copper-gold deposits. a. Idealized reconstruction of magmatic-hydrothermal and igneous breccias, aplite-pegmatite bodies, unidirectional solidification texture (UST), and sheeted quartz veinlets with respect to an underlying porphyry A or B center. b. Aplite with pegmatitic clots of brown and green biotite and K-feldspar. c. Sheeted quartz veinlets, associated with bornite and chalcopyrite (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 B matrix (dark, indicated by scratcher).

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Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/3812113/9781629490410_ch04.pdf by guest on 05 August 2020 90 PERELLÓ ET AL. 0.3 <0.002 0.1 0.6 0.02 <0.01 0.6 ~0.03–0.05 <0.01 >0.6 <0.01 <0.01 >1.0 ~0.05–0.08 0.05 0.4–0.5 <0.005 0.1 0.2–0.3 <0.005 <0.01 0.4–0.5 <0.002 0.1–0.2 0.4–0.6 <0.01 <0.01 0.4–0.6 0.02 <0.01 → → → → → → → Sulfide assemblage Cp-Bo ) Cp-Py 2 ) Py-Cp 0.2–0.3 <0.01 n.a. )Py ) 2 2 2 ) Bo-Cp ) 2 2 ) )) Cp-Bo ) ) ) Bo-Cp 2 2 2 2 2 2 high Bo-Cp high (10–>30/m moderate (3–10/m Cp-Bo very andesite stock to high andesite (3–10/m PA Pachón Low to PACp-Bo Pachón Low PB Los Pelambres Low to Cp-Bo → PB Los Pelambres Low to Cp-Py → Two families Two Los Pelambres Low (<3/m PB PB Los Pelambres Low (<3/m PA PB (10–30/m → → → → → → → PBPA Formation Formation (10–20/m PB, intruded Formation moderate Cp-Bo Formation PB, intruded PB igneous stockPy-Cp PB moderate (3–10/m PB (3–10/m PB, by PA PB, by igneous intruded stock igneous stock PA of PA stock PA Formation moderate Bo-Cp anhydrite PB Los Pelambres Moderate to Cp-Bo → → → → → → → bottom porphyry phase Host rocks (A and B type) zonation Cu (%) Mo (%) Au (ppm) → breccia pegmatite breccia pegmatite tourmaline Pachón (3–10/m tourmaline Pachón breccia 3. Main Features of Selected Magmatic-Hydrothermal Centers at Los Pelambres-Frontera cylinder-like breccia cylinder-like 10°/80° S UST → to tabular breccia → defined probable PA defined probable 0°/55° SE 0°/80° SE0°/65° E breccia → andesite 0°/ 80° SE UST → 30°/55° SE breccia → 10°/55° SE breccia → to tabular breccia → 0°/85° E probable PA by PA andesite (3–10/m limits poorly breccia → cylinder-like breccia cylinder-like irregular; breccia ABLE T exposure Shape of facies Causative intensity Veinlet plunge of highest zonation Vertical and down- grade Average 450 3,450 Subcircular; Anhydrite PA Pachón Moderate Cp-Bo 250 3,600 Subcircular Pegmatite 300 3,300 Cylinder-like Pegmatite 210 3,500 Irregular Pegmatite 300 3,400 Subcircular Tourmaline- PA Los Pelambres Moderate to Py-Cp → 250 2,900 Subcircular Anhydrite PB Los Pelambres Moderate Cp-Bo250Moderate 0.6–1.0 0.01–0.02 2,900 <0.01 SubcircularPelambres Anhydrite PB Los >600 3,800 Subcircular Quartz-anhydrite PB to Pachón Low Cp-Py >600 3,100 Highly Anhydrite PB Los Pelambres Low to Cp → >700 3,150 Subcircular; UST >300 3,150 Subcircular Pegmatite >400 3,300 Tabular Pegmatite >700 3,200 Subcircular Tourmaline PB Los Pelambres Low to Py → × × × × × × × × × × × × × × 70 80 140 120 100 130 90 70 90 80 100 240 vertical (m asl) strike/plunge top → × × width × × × × length × × × × × × Dimensions (m) Elevation Abbreviations: PA = Porphyry A family, PB = Porphyry B family, UST= Unidirectional solidification texture, n.a. = not available PB = Porphyry B family, = Porphyry A family, Abbreviations: PA Sulfides: Bo = bornite, Cp chalcopyrite, Py pyrite MAM-03 150 DAM-18 400 Mina 170 Portezuelo 0°/55° SE anhydrite stockSurDAM-06 high (10–20/m 120 0°/75° E stock Cp-Py Center RAM-05 160 DAM-01 180 Cluster 600 Mina 160 DAM-10 150 Pegmatita 470 DDH-1061 120 Victoria 0°/55° SE anhydrite-biotite- stock and moderate Bo-Cp Portezuelo 170 Este ~0°/ 55° E igneous stock moderate Cp Oeste

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Table 3, the features are preserved at various elevations be- display ramp structures and tight local folding about N-strik- tween 3,600 and 2,900 m above sea level and, in combination, ing axes. display irregular forms, particularly where clustered (e.g., A series of NE-striking, 55° to 70° SE-dipping faults, de- Cluster Oeste; Fig. 5). This clustering led previously to inter- fined by late-stage swarms of veins and veinlets and associated pretation of a single, large breccia body (the Central breccia; alteration and mineralization (see below), cut the west-central Atkinson et al., 1996). The breccias and related features are parts of the Los Pelambres stock (Fig. 5). A principal compo- generally <200 m in diameter, although larger (up to 600 m) nent of this fault set appears to displace the Los Pelambres bodies occur where clustered or elongated (e.g., Pegmatita stock contact and Los Pelambres fault (Fig. 5). Nonetheless, Sur; Fig. 5). Vertical extents, constrained by drilling, range there is no obvious structural control of the porphyry centers from 210 to 700 m; typically, they display a SE plunge of 55° and associated breccias and other features, which appear to on average, although steeper attitudes are also observed have an entirely random distribution; however, the three (Table 3). The magmatic-hydrothermal brecciation and asso- Frontera centers do define a southeast alignment. ciated features are better developed on the southeastern, hanging wall rather than northwestern, footwall sides of the Hydrothermal Alteration and Mineralization centers (Fig. 6b). In the idealized section (Fig. 7a), aplite and pegmatite con- Alteration-mineralization assemblages stitute composite bodies in which both products are inti- Los Pelambres and Frontera partly conform to the classic mately and irregularly mixed (Fig. 7b). These bodies occur to- Lowell and Guilbert (1970) hydrothermal alteration model, in gether with sheeted quartz veinlets (Fig. 7c) and which a potassic center grades laterally to an annular sericitic unidirectional solidification textures (Fig. 7d) in the roof zone surrounded by a propylitic halo (Sillitoe, 1973; Fig. 8a). zones of the breccia bodies and/or the associated causative in- The bulk of the hypogene metal resource is hosted by the trusions. The pegmatite portions comprise several of quartz, large potassic zone, shown by drilling to extend to a depth of K-feldspar, biotite, chlorite, tourmaline, sericite, anhydrite, at least 1.2 km, whereas the sericitic halo is largely pyritic. and copper-iron sulfides, an association similar to that in the The potassic alteration is defined by the presence of hydro- miarolitic cavities in the subjacent porphyry bodies (see thermal biotite and K-feldspar together with anhydrite, chal- above). Magmatic-hydrothermal breccia is normally polymict copyrite ± bornite ± trace digenite, and molybdenite (Sillitoe, and composed of clasts of several intrusive phases, including 1973). Hornblende in the igneous rocks at Los Pelambres is the Los Pelambres quartz diorite, as well as andesite in the almost totally converted to fine-grained, shreddy, brown bi- vicinity of volcanic country rock (e.g., Frontera deposit). The otite, and plagioclase is partially replaced by K-feldspar (Silli- hydrothermal cement is made up of the same minerals as toe, 1973; Skewes, 1985; Skewes and Atkinson, 1985; Atkin- those in the aplite-pegmatite bodies (Fig. 7e). Downward, the son et al., 1996). In contrast to Los Pelambres, the potassic hydrothermal cement becomes progressively less at the ex- alteration at Frontera is developed mainly in andesitic rocks pense of an igneous matrix of porphyry A or B composition of the Pachón Formation, which are transformed to gra- (Fig. 7f). noblastic aggregates of fine-grained, brown biotite accompanied by subordinate chlorite and apatite. Minor epiclastic Late mineral porphyry phases horizons in the Pachón Formation are replaced by alternating Late mineral diorite porphyry and microdiorite dikes are bands of garnet, hedenbergite, actinolite, and chlorite. exposed at surface and have been encountered by drilling, Hydrothermal magnetite is present locally at Los Pelambres predominantly in the zone between Los Pelambres and Fron- but is abundant (>5 vol %) in the biotitized rocks at Frontera tera (Figs. 5, 6b). The typically N- to NE-striking, steeply dip- (Sillitoe, 1973; Perelló et al., 2007, 2009, 2011). The mag- ping dikes along the southeastern margin of the Los Pelam- matic-hydrothermal and igneous breccias, described above, bres stock are 20 to 30 m wide and several hundred meters are integral parts of the potassic zone at both Los Pelambres long, and display equigranular, microphaneritic textures. The and Frontera. The breccias, unlike the pervasive potassic late mineral body at Frontera strikes northwest and has a zone, contain brown and green biotite as well as being char- dacitic composition and coarse-grained, porphyritic texture acterized by the development of sericite ± chlorite in their (Table 2). apical parts (Fig. 7a). The annular sericitic zone is best developed in the north- Structure western quadrant (Fig, 8a), where it coincides with the NE- The tectonic development of the Los Pelambres-Frontera striking structural zone, elements of which affect not only the area is dominated by the N-striking, high-angle Los Pelam- Los Pelambres stock but also extend for up to 1 km into the bres reverse fault, a brittle structure that places the Late Cre- propylitic andesitic host rocks. The sericitic alteration clearly taceous rhyolitic tuff and late Oligocene to early Miocene Los overprints and destroys the earlier formed potassic assem- Pelambres Formation over the early Miocene Pachón For- blage and comprises quartz, sericite, pyrite, and subsidiary mation (Figs. 3, 5). The western lobe of the Los Pelambres schorlitic tourmaline, the latter present as patches, veinlets, stock cuts the fault, thereby implying that final displacement and rosettes (Sillitoe, 1973). Tennantite, chalcopyrite, galena, took place prior to mid-Miocene intrusion (see above). The sphalerite, molybdenite, and trace digenite are minor con- westerly monoclinal dips of both the Los Pelambres and stituents. Locally, internal parts of the northeast structural Pachón Formations steepen markedly on approach to the corridor contain andalusite and kaolinite, minerals denoting fault. Immediately north and south of the Los Pelambres the existence of advanced argillic alteration. Silicified an- stock, the epiclastic units of the Los Pelambres Formation desitic rocks mapped previously in the northeastern quadrant

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359,000 361,000 359,000 361,000

3,500 N 3,500

3,300 3,300 8 8 0 0 0 0 2 3,250 2 3,250 E E N N I N I L L 3,200 T T

U U

O O

T T

I I

3,200 P P

6,490,000 LOS PELAMBRES LOS PELAMBRES

FRONTERA 1 km FRONTERA 1 km Veinlet intensity 2 6,488,000 Sericitic Low (up to 3 veinlets/m ) 2 Potassic Moderate (up to 10 veinlets/m ) 2 Propylitic High (up to 20 veinlets/m ) a b 359,000 361,000

6,492,000 N

3,500

3,300 08 0 2 3,250 E

N I 3,200

T I LOS PELAMBRES P 6,490,000

d 2 cm

1 km FRONTERA Veinlet intensity Low (up to 3 veinlets/m2 ) 6,488,000 Moderate (up to 10 veinlets/m2 ) High (up to 20 veinlets/m2 ) 2 Very high (up to 40 veinlets/m ) 2 cm c e

f 2 cm g 2 cm

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 quartz veinlet (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 containing pyrite and tennantite (dark), porphyry B, northeast structural zone, Los Pelambres. a, b, and c based on a combination of core logging and pit and surface mapping. UTM datum: Prov. S. Am 56, Zone 19 South.

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(Sillitoe, 1973) also contain andalusite and, in the topograph- contain appreciable amounts of magnetite and subsidiary ically higher (~3,700 m) parts of the northeast corridor, veins actinolite (Fig. 8e). The Frontera deposit also contains early, containing andalusite, sericite, barite, gypsum (after anhy- sulfide-poor magnetite-actinolite veinlets with prominent al- drite), and native sulfur cut the propylitic halo (cf. Sillitoe, bite halos. 1973). Sulfide and metal zoning Veinlet types and distribution The sulfide minerals at Los Pelambres and Frontera are Six main veinlet types are widely recognized throughout zoned within and around the individual porphyry centers the potassic zone at Los Pelambres and Frontera, which, from (Table 3), from the scale of the A and B veinlets to whole-rock oldest to youngest based on crosscutting relationships, com- volumes, and independently of the host rock (Atkinson et al., prise early biotite, green mica, type 4, A, B, and D (Skewes 1996). In contrast, the type 4 veinlets are not zoned around and Atkinson, 1985; Atkinson et al., 1996; Table 4; Fig. 8d-g); the individual porphyry centers but nonetheless do contribute only the last four types were systematically counted at the de- to the deposit-scale sulfide zoning pattern (Fig. 8b). The cores posit scale and incorporated in the resource model (Fig. 8b, of the porphyry centers, where A- and B-type veinlet intensi- c). The entire veinlet sequence is characterized by the pres- ties are highest but not necessarily everywhere the same, are ence of varied amounts of anhydrite and tourmaline (Table 4). characterized by chalcopyrite/bornite ratios of 2 to 3 at Los These six veinlet types, described in Table 4, are widely rec- Pelambres but as low as unity at Frontera where total sulfide ognized in porphyry copper deposits worldwide (e.g., See- contents are much less (<1 vol %). Bornite contents are high- dorff et al., 2005; Sillitoe, 2010). At Los Pelambres-Frontera, est in the cores of the porphyry centers and decrease both the most widespread veinlet types are type 4, A, and B, which outward and upward to give rise to chalcopyrite, chalcopyrite have a combined footprint of approximately 4 × 2 km (Fig. plus pyrite, and, eventually, pyrite alone. Nonetheless, total 8b, c). The distribution of these three veinlet types defines sulfide contents remain approximately the same (~2 vol %) NE and SE trends, which mirror the structural and geologic through to the pyrite halo, except within the overprinted elements described above (Fig. 8b, c). Within these trends, sericitic alteration along the NE-striking structural zone (Fig. there is a concentric distribution of veinlet intensities, culmi- 8a), where pyrite contents locally attain 5 vol %. The por- nating in discrete centers of greatest veinlet development phyry A centers are everywhere lower grade than porphyry B (Fig. 8b, c). The early biotite and green mica veinlets are also centers. Sulfides in the apical portions of the magmatic- widely, but irregularly, distributed and, hence, not readily hydrothermal breccias in both porphyry A and B centers tend countable. In contrast, the D veinlets are concentrated in the to be both higher grade and coarser grained than those in the northeast structural corridor in the northwestern quadrant, associated porphyry intrusion and immediate wall rock (Fig. where their latest representatives contain the base metal sul- 7e). A few porphyry centers situated near the margins of the fides listed above besides abundant pyrite (Table 4; Fig. 8g). Los Pelambres stock are less strongly veined and more The early biotite and green mica veinlets contain only triv- pyritic, in keeping with their position in the deposit-scale zon- ial amounts of copper-bearing sulfide minerals. The earliest ing pattern (e.g., RAM-05 and Pegmatita Sur; Fig. 5; Table 3). significant introduction of copper and molybdenum at Los The most remarkable aspect of metal zoning at Los Pelam- Pelambres and copper and gold at Frontera accompanied bres-Frontera is the distribution of molybdenum and gold, generation of the type 4 veinlets (Fig. 8d, e). The type 4 vein- with the former concentrated at Los Pelambres and the latter lets make a major contribution to the copper inventory at Los almost exclusively confined to Frontera (Fig. 9a). Copper, in Pelambres, probably accounting for approximately 0.4% of contrast, spans both deposits. At Los Pelambres, molybde- the deposit average grade of 0.56% Cu; this is somewhat less num tenors of >50 ppm define a N-trending zone, some 4 km than the figure estimated by Atkinson et al. (1996) for shallow long and up to 1 km wide (Fig. 9a). However, the highest levels of the orebody where chalcocite enrichment was ubiq- molybdenum values are confined to the north-central part of uitous (see below). In common with similar (early dark mica- this zone, which is characterized by a steep, SE-plunging ceous; Meyer, 1965) veinlets elsewhere, most of the con- cylindrical body containing >200 ppm Mo (Fig. 9a, c). The tained chalcopyrite and bornite are disseminated in the halos, high-grade, hypogene copper mineralization (>0.6%) also which are composed of admixed quartz, K-feldspar, biotite, plunges southeastward at a similar angle, in conformity with sericite, phengitic sericite, andalusite, anhydrite, corundum, the partly coalesced porphyry centers (Fig. 9b). There is a rutile, and plagioclase (Skewes and Atkinson, 1985; Atkinson clearcut correlation between gold and hydrothermal mag- et al., 1996; Proffett, 2009; Table 4). The A- and B-type vein- netite, which is most marked at Frontera but remains valid let generations, some with weakly developed K-feldspar even for the minor gold occurrences within the Los Pelam- halos (Table 4; Fig. 8d, f), contain the rest of the copper in bres deposit (Fig. 9a). Nonetheless, a single, small bornite- both deposits, and, in conjunction with the magmatic-hydro- rich zone within the apical part of the DAM-01 magmatic- thermal breccias, clearly give rise to the centers of highest hydrothermal breccia (Fig. 5) contains up to 0.1 g/t Au even grade. The type 4 veinlets contain relatively minor, but though it lacks magnetite. widely distributed, molybdenite at Los Pelambres, but the B Most of Los Pelambres and all Frontera are devoid of ar- veinlets account for at least 60% of the total; however, B senic, which is associated almost exclusively with the over- veinlets are more sparsely developed at Frontera, which, as a printed sericitic alteration and tennantite-bearing D vein- result, contains less molybdenite (50–60 ppm Mo; Perelló et lets that define the northeast structural corridor in the al., 2011). The gold at Frontera is hosted by the type 4 and A northwestern quadrant of Los Pelambres (Fig. 9d). Nonethe- veinlets, which, unlike their counterparts at Los Pelambres, less, values are relatively low (<550 ppm As).

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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, plagioclase, 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 laterally are continuous >5-10 cm sugary in texture; later veinlets of pure quartz representatives typically common planar, 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 Pelambres hydrothermal breccia; coarse-grained sulfide fill

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

Supergene Alteration and Mineralization jarosite rich peripherally as pyrite contents increase (Fig. 2a). In the goethitic capping, oxide copper minerals are relatively Supergene profile uncommon and consist mainly of malachite, although azurite, The supergene profile at Los Pelambres is thinnest beneath chrysocolla, chalcanthite, brochantite, and pseudomalachite the floor of the U-shaped glacial valley (Fig. 2a), where sul- are reported from the eastern parts of the potassic zone (Sil- fide oxidation is locally as little as 6 m and chalcocite enrich- litoe, 1973). Nevertheless, the close coincidence between the ment is vestigial (Sillitoe, 1973; Fig. 9b). However, the profile >300-ppm Cu isopleth in talus fines and the mapped potassic thickens as the pyrite halo is approached beneath the valley zone at surface (Maranzana, 1972) emphasizes that copper sides, where sulfide oxidation penetrates in places to depths removal was incomplete. of 200 m and the immaturely developed enrichment zone av- In contrast, at Frontera, copper leaching is minimal and an erages ~80 m thick, but locally attains a maximum of ~350 m oxidized zone, containing malachite and pitch limonite (Atkinson et al., 1996; Fig. 9b). (cupriferous goethite) and averaging 140 m thick, is underlain The leached capping is predominantly goethitic within the by immature supergene enrichment, which is best developed confines of the potassic zone but becomes increasingly within the uppermost 50 to 120 m of the sulfide zone, where

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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 Frontera B 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 crosscutting 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 Sph and Cp

the chalcocite occurs as thin replacement coatings on chal- Geochronology of Intrusive Rocks and copyrite, bornite, and minor pyrite grains (Perelló et al., Mineralization at Los Pelambres-Frontera 2011). Minor cuprite and native copper occur in the vicinity of the oxide-sulfide interface. U-Pb zircon ages of intrusive rocks This study is underpinned by 18 new LA-ICP-MS U-Pb zir- Anhydrite front con ages on intrusive rocks (Table 5). Two of these ages show Los Pelambres and Frontera are both characterized by an that the Los Pelambres quartz diorite stock was emplaced be- abrupt anhydrite front, above which nearly all the anhydrite tween 13.60 ± 0.30 and 13.00 ± 0.70 Ma, which are broadly in both the veinlets and host rock has been removed by the compatible with previously reported U-Pb zircon ages rang- passage of cool ground water to leave characteristic open ing from 13.92 ± 0.15 to 12.51 ± 0.17 Ma (Bertens et al., pores and cavities (Sillitoe, 1973). The anhydrite front com- 2003, 2006; A. Bertens, writ. commun., 2007; see above). monly coincides closely with the base of chalcocite enrich- Eight ages for six different porphyry B centers at Los ment (Fig. 9b), although in places it is developed up to 50 m Pelambres and Frontera range from 12.30 ± 0.30 to 10.80 ± deeper (Atkinson et al., 1996). 0.23 Ma, defining an interval of ~1.5 m.y. Seven ages for

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N N N 6,492,000

3,500 3,500 3,500

2008 2008 3,300 0 2008 3,300 3,30 E E E N N N I I 3,250 I

L L L

T T T 3,2 3 U U U ,25

O 5 O O 0 0

IT IT IT 6,490,000 P P P

Cu % Mo ppm Au ppm >1.0 > 200 > 0.2 0.6 - 1.0 100 - 200 0.1 - 0.2 6,488,000 0.4 - 0.6 50 - 100 0.08 - 0.1 0.1 - 0.4 1km 1km 1km a 358,500360,500 358,500 360,500 358,500 360,500

B B´

Meters above

sea level l 359,000359,000

u Original topography a

3,800 3,800 6,492,0006,492,000 f

s e

2008 pit r

m N a

3,400 3,400 l e

TS P 3,500

BE o L 3,000 3,000 3,300 Cu % 30 year pit 8 0 > 0.8 0 3,250 2 2,600 > 0.6 2,600 E N I

> 0.35 L

500 m U

b O

T I TS P B Top of sulfides B´ 3,200 BE Meters above Bottomof supergene enrichment sea level 6,490,0006,490,000 Original topography 3,800 3,800 2008 pit 3,400 3,400 As ppm TS 131-550 BE 51-130 3,000 Mo ppm 3,000 > 400 30 year pit Reverse fault > 300 Fault: mapped, inferred 2,600 > 200 2,600 1km > 100 d c 500 m

FIG. 9. Metal zoning at Los Pelambres-Frontera. a. Plan views of copper, molybdenum, and gold distribution at 3,000 m above sea level, simplified from the block model (all available data, including blast holes). b. NW-SE longitudinal section (same as 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.

seven different porphyry A centers at Los Pelambres and emplacement and initiation of porphyry center development. Frontera fall between 11.50 ± 0.30 and 10.53 ± 0.14 Ma, a The partial temporal overlap between the age ranges for the range of ~1.0 m.y. Two U-Pb zircon ages from surface expo- porphyry A and B centers is notable, bearing in mind that sures of porphyries considered to be porphyry B yielded ages where crosscutting relationships are observed porphyry A is that fall in the range defined above (Bertens et al., 2003, generally younger than porphyry B. Nevertheless, it is clear 2006). The single age for the late mineral dacite porphyry that the youngest porphyry B and porphyry A ages are for the dike at Frontera is 10.24 ± 0.55 Ma. Frontera samples (Table 5). The oldest age for a porphyry center at Los Pelambres (Pegmatita Sur; Fig. 5; Tables 3, 5) is, within error limits, es- Re-Os ages of molybdenite sentially the same as the two youngest ages obtained for the During this study, 12 Re-Os ages were determined for dif- Los Pelambres quartz diorite by Bertens et al. (2003, 2006) ferent molybdenite samples from Los Pelambres and Frontera and this study, implying a close relationship between stock (Table 6). The ages lie between 11.81 ± 0.06 and 10.14 ± 0.04

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

UTM Magmatic- 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 ± 0.30 FRONT - 22 (DDH817:122) 6,488,686 360,446 PB MAM-03 Frontera 11.29 ± 0.37 FRONT - 32 (DDH817:300) 6,488,619 360,495 PB MAM-03 Frontera 10.83 ± 0.23 FRONT - 12 (DDH817:30) 6,488,720 360,421 PB MAM-03 Frontera 10.80 ± 0.23 MAM - 3 (330)1 6,489,332 360,065 PA MAM-03 Frontera 10.70 ± 0.20 RAM - 051 6,491,900 359,972 PA RAM-05 Los Pelambres 11.50 ± 0.30 PELSE - 022 6,487,780 360,720 PL DAM-06 Frontera 10.24 ± 0.55 FRONT - 82 (DAM - 10:570) 6,488,833 360,511 PB DAM-10 Frontera 10.87 ± 0.07 PELSE - 012 6,488,630 360,500 PA DAM-10 Frontera 10.53 ± 0.14 FRONT - 102 (MAM - 6:432) 6,488,806 360,598 PA DAM-10 Frontera 11.13 ± 0.12 DDH 1030 (494)1 6,490,354 358,450 PA DAM-18 Los Pelambres 11.50 ± 0.30 DAM - 181 (DDH1030:247) 6,490,191 358,608 Los Pelambres stock DAM-18 Los Pelambres 13.00 ± 0.70 DDH 1052 (160)1 6,488,581 359,472 PB Pegmatita Sur Los Pelambres 12.30 ± 0.30 DDH 2424 (107)1 6,490,946 359,678 PB Portezuelo Este Los Pelambres 11.90 ±0.30 DDH 2268 (140)1 6,490,152 358,944 PB Cluster Oeste Los Pelambres 11.80 ± 0.30 DDH 2439 (80)1 6,489,949 358,969 PA Cluster Oeste Los Pelambres 11.50 ± 0.30 DDH 1012 (195)1 6,491,003 359,334 PA Mina Portezuelo Los Pelambres 10.80 ± 0.20 DDH 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 South 1 Dated at University of Arizona, Tucson, Arizona 2 Dated at Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, Australia

Ma. To these may be added 13 additional molybdenite ages the latest Miocene (Bertens et al., 2006). However, most of reported by Bertens et al. (2006), three by Mathur et al. the jarosite ages fall in the 3.06 to 0.93 Ma range, when su- (2001), two by Stein et al. (2002), and two by Hannah et al. pergene oxidation was clearly active (Bertens et al., 2006). (2007), all from Los Pelambres and all falling in the age range defined by this study. This database clearly shows that the (U-Th)/He zircon and apatite ages molybdenite mineralization at Los Pelambres-Frontera was Bertens et al. (2006) presented six (U-Th)/He zircon and introduced during a protracted interval of ~1.7 m.y., which apatite ages from Los Pelambres. The ages of 10.37 to 8.15 accords well with the ~1.4-m.y. interval defined by additional Ma (no errors reported) are not much younger than the K-Ar molybdenite ages from Los Pelambres reported by Stein et al. and Ar/Ar determinations, implying rapid cooling of the Los (2004) but not considered herein. The samples dated during Pelambres stock and its associated mineralization. this study show that the youngest ages are for Frontera molybdenites (Table 6), in good agreement with the U-Pb Discussion data (see above). District-scale controls K-Ar and Ar/Ar ages for silicate minerals The Los Pelambres and Frontera deposits are located along Six K-Ar ages reported by Atkinson et al. (1996) and two by the faulted eastern boundary of a narrow, N-NW-striking belt Quirt et al. (1971), all for biotite from the Los Pelambres of intensely deformed rocks, which are the product of tec- stock and porphyries, range from 10.7 to 8.9 Ma. Ten samples tonic inversion of the extensional Abanico intra-arc basin (Jor- of igneous and hydrothermal muscovite, biotite, and horn- dan et al., 2001; Charrier et al., 2002; Fig. 3). The compres- blende from within and immediately surrounding Los Pelam- sive collapse of the Abanico basin at ~20 Ma correlates with a bres were subjected to Ar/Ar age determination by Bertens et drastic decrease in crust generation in the eastern Pacific al. (2006) and yielded ages of 10.58 to 9.52 Ma (no errors re- Ocean and a drop in Nazca-South America plate convergence ported). These K-Ar and Ar/Ar ages overlap with the velocity (Pardo-Casas and Molnar, 1987; Somoza, 1998; Con- youngest Re-Os molybdenite ages and would appear to re- rad and Lithgow-Bertelloni, 2007). Increased mechanical flect initial cooling of the system below the argon blocking coupling along the plate interface (e.g., Yáñez and Cembrano, temperature. 2004) may have produced the change from extension to contraction that led to initiation of the Aconcagua and La Ra- Ar/Ar ages for supergene jarosite group minerals mada fold-thrust belts in the greater Los Pelambres region The earliest jarosite age determined at Los Pelambres is (Giambiagi and Ramos, 2002; Ramos et al., 2002). At Los 5.34 Ma, implying that the deposit was already unroofed by Pelambres, this event is bracketed by the Chalinga intrusive

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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 magnetite, with disseminated chalcopyrite; both veinlets cut magnetite-rich biotitized 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 chalcopyrite 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 disseminated 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 South 1 Dated at Department of Geosciences, University of Arizona, Tucson, Arizona 2 Dated at University of Alberta, Alberta, Canada 3 Dated at AIRIE - Colorado State University, Colorado

complex, which constrains the timing of the reverse motion with subduction of the Juan Fernández Ridge beneath the on the Los Pelambres and Totoral faults to pre-16.5 Ma (Fig. leading edge of the South America plate at this latitude 3; see above). (Yáñez et al., 2001; Kay and Mpodozis, 2002; see above). During the mid-Miocene (~14–13 Ma), emplacement of As shown above, the porphyry copper-related intrusions at the precursor Los Pelambres stock took place at the intersec- Los Pelambres display adakitic signatures (high Sr/Y and tion of the NW-striking belt of mid-Miocene intrusions and La/Yb), concave heavy rare earth element patterns, and lack the Los Pelambres fault. This NW-striking intrusive align- of europium anomalies indicative of hornblende fractionation ment may have been localized by a transverse, deeply pene- in hydrous, oxidized magmas. Slab flattening and continued trating basement feature, not currently evident at the surface deformation increased crustal thickness (Kay and Mpodozis, and comparable to those mapped but poorly understood far- 2002) under Los Pelambres and may have induced partial fu- ther north in the Andes (Salfity, 1985). Therefore, porphyry sion of the mantle lithosphere; a situation that seems to have copper formation at Los Pelambres took place ~10 m.y. after played a fundamental role in the generation of the giant Los the inversion of the Abanico basin when the deformation Pelambres porphyry copper system. front shifted eastward, giving rise to the Frontal Cordillera (e.g., Cordillera de Santa Cruz; Fig. 3) by thick-skinned re- Deposit-scale temporal development verse faulting (Jordan et al., 1996; Pérez, 2001; Giambiagi and Magmatism at Los Pelambres commenced with emplace- Ramos, 2002). The tectonic migration and emplacement of ment of the multiphase Los Pelambres quartz diorite stock the Los Pelambres porphyry copper deposits also coincide between ~14 and 12.5 Ma (Fig. 10a). Intrusion of the numerous

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PLEISTOCENE

PLIOCENE

5 Los Pelambres-Frontera activity supergene Los Pelambres-Frontera cooling hydrothermal El Pachón mineralization Los Pelambres-Frontera mineralization 10 Los Pelambres-Frontera centers magmatic-hydrothermal Precursor plutonism Precursor stock) (Los Pelambres

Premineral plutonism Premineral (Chalinga intrusive complex) RAM-05 MIDDLE15 LATE (11.5) N MIOCENE Regional volcanism Formations) (Los Pelambres-Pachón-Abanico MINA PORTEZUELO (10.8) 20 PORTEZUELO ESTE (11.9) [11.81-11.27]

EARLY DAM-1 (11.7)

25 DAM-18 (11.5) MAM-03 (11.3-10.7) CLUSTER [10.14] OESTE (11.8-11.5)

30 DAM-10 PEGMATITA (11.1-10.5) SUR (12.3) OLIGOCENE

EARLY LATE DAM-06 [10.77-10.51] DAM-06 (10.2)

(U-Pb) a b [Re-Os]

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 Los Pelambres-Frontera. The age of mineralization at the El Pachón deposit is also shown. Note the almost transitional or overlapping 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.

B and A porphyry centers commenced almost immediately deposit (9.16–8.43 Ma; Bertens et al., 2006), 5 km farther thereafter and lasted for ~1.5 m.y., terminating with the southeast (Figs. 3, 10a). Frontera late mineral porphyry at 10.24 ± 0.55 Ma (Fig. 10a). Based on the temporally overlapping arrays of U-Pb zir- Therefore, the Los Pelambres stock is clearly a precursor in- con and Re-Os molybdenite ages, the longevity of intrusion, trusion genetically related to porphyry copper formation. The magmatic-hydrothermal brecciation, potassic alteration, individual porphyry and associated magmatic-hydrothermal veining, and associated copper-molybdenum and copper- centers within the Los Pelambres stock and southeastward to gold mineralization at Los Pelambres and Frontera was ~1.7 Frontera appear to have been emplaced over intervals of 0.3 to 1.5 m.y., an interval which is expanded to ~3.8 m.y. if the to 0.6 m.y., but without any clearly discernable space-time precursor Los Pelambres stock is also included (Perelló et arrangement (Fig. 10b). However, the Frontera porphyry al., 2009). The continuity of molybdenum introduction for centers do tend to be younger than those within the Los ~1.7 m.y. argues against the existence of discrete metallifer- Pelambres stock (Fig. 10b), in accord with the southeastward ous pulses separated by quiescent intervals. The K-Ar, younging of the Re-Os molybdenite ages. This southeastward Ar/Ar, and (U-Th)/He ages document cooling of the Los migration appears to have been maintained at the district Pelambres-Frontera system to near ambient temperatures scale for a further ~2 m.y. to account for the El Pachón by ~8 Ma (Bertens et al., 2006; Fig. 10a) as a result of rapid

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exhumation during the tectonically induced uplift. Indeed, Deposit architecture and evolution the Los Pelambres deposit was exposed to the effects of su- A schematic model for the genesis of Los Pelambres-Fron- pergene oxidation and enrichment by ~5 Ma (Bertens et al., tera is presented as Figure 11. The Los Pelambres quartz 2006; Fig. 10a). The supergene profile was eroded from the diorite stock is hypothesized to be a cupola above a deeper, upstream Los Pelambres valley floor during the Plio-Pleis- parental intrusive complex (Fig. 11a). The widespread distri- tocene glaciation but preserved downstream and on the val- bution of the type 4 veinlets and the fact that early examples ley sides (Fig. 9b, c). are truncated by the intermineral B and A porphyries suggest

15 Ma 14-12.5 Ma Paleosurface ? Totoral fault

Los Pelambres fault Chalinga Type 4 veinlets intrusive

complex ?

Regional uplift Cupola 2 km 2 km

2 km Postulated intrusive 2 km complex High-temperature Regional single-phase uplift a b magmatic liquid

12.3-10.3 Ma ? 10 Ma? ? Advanced argillic ? lithocap Late-mineral dike

Meteoric Frontera water Low-temperature single-phase liquid Los Pelambres Porphyry B and Sericitic Porphyry A magmatic- alteration hydrothermal centers

2 km Two-phase 2 km brine/vapor input 2 km Regional 2 km Regional uplift uplift 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 inferred to 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 in the 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 active Los Pelambres fault (Fig. 3) and becomes tilted as uplift progresses throughout the region. c. The Los Pelambres stock magmas continue their ascent, with fresh magma being emplaced as multiphase porphyry B and porphyry A centers, each with its own magmatic-hydrothermal focus. Each center creates its own miniature porphyry system, including several generations of A- and B-type quartz veinlets. Coalescence of the centers is favored by physical proximity as well as by new generations of type 4 veinlets, which continue to form, albeit in lesser numbers. Most of the porphyry centers are confined within the Los Pelambres stock although some, like those at Frontera, are emplaced in the immediate andesitic wall rocks. Renewed magma input forms the late mineral dikes in the roof of the tilted system, particularly at Frontera, simultaneous with paleosurface 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 Pelambres stock 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).

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that the bulk of the type 4 veining is either linked to a con- Comparison with other central Chile porphyry cealed early porphyry intrusion or took place prior to any por- copper deposits phyry emplacement at Los Pelambres-Frontera (Fig. 11b). If The Los Pelambres-Frontera system shares a number of the latter alternative were the case, then the dilute, single- obvious intrusive and hydrothermal features with the other phase fluid responsible for the type 4 veining (Rusk et al., deposits in the central Chile porphyry copper belt, particu- 2008; Proffett, 2009) must have ascended alone from the sub- larly the giant Río Blanco-Los Bronces and El Teniente sys- jacent parental pluton. tems (Fig. 1). The individual multiphase porphyry centers appear to have The Los Pelambres and Río Blanco-Los Bronces deposits acted like a series of miniature porphyry copper deposits, were both generated near batholithic intrusions, the Chalinga which coalesced to form the Los Pelambres-Frontera ore- intrusive complex and San Francisco batholith, respectively, body (Fig. 11c). Each center gave rise to brecciation and as- the emplacement of both of which began several million years sociated magmatic-hydrothermal features (see above) as well before the onset of porphyry copper mineralization (Deckart as generating its own sulfide zoning pattern. These discrete et al., 2005; this study). Furthermore, the porphyry copper foci bring to mind the concept of emanative centers, first de- alignments in both the Los Pelambres-El Pachón and Río fined by Dines (1934, 1956), within the granitic plutons of the Blanco-Los Bronces districts project southeastward from the Cornubian tin-tungsten-copper province of southwestern England. The coalescence of the zoning patterns linked to in- respective batholithic intrusions (Warnaars et al., 1985; Ser- dividual porphyry centers led during potassic alteration to the rano et al., 1996; Fig. 3), presumably implying similar base- deposit-scale sulfide zoning from internal chalcopyrite-bor- ment tectonic controls. nite through chalcopyrite-pyrite to peripheral pyrite. Fur- The individual porphyry centers documented at Los thermore, A- and B-type veinlet intensities also attain their Pelambres-Frontera also bear close similarities with the maxima in some of the porphyry centers, in contrast to the aligned series of small, but partly coalesced magmatic-hydro- type 4 veinlets (Fig. 8b, c). thermal centers that combine to form the El Teniente por- There is a gross coincidence between copper and molybde- phyry copper-molybdenum deposit (Vry et al., 2010). As at num mineralization at the scale of the Los Pelambres-Fron- Los Pelambres, each of the El Teniente centers is character- tera system, although at the cutoff grades employed in Figure ized by magmatic-hydrothermal breccias and veinlet zoning. 9a, the correlation is far from perfect. In marked contrast, ap- All three of the main central Chile deposits contain well- preciable gold is confined to Frontera, where it correlates mineralized, magmatic-hydrothermal breccias, although well with copper. The gold-bearing bornite and chalcopyrite these are volumetrically and economically far more important at Frontera are associated with appreciable hydrothermal at Río Blanco-Los Bronces than at the other two deposits. In- magnetite, a mineral largely absent from the gold-poor Los deed, the magmatic-hydrothermal breccias at Los Pelambres- Pelambres deposit, thereby displaying the classic gold-rich Frontera constitute no more than 5% of the orebody (Fig. 5), porphyry copper deposit signature (Sillitoe, 1979; Perelló et thereby emphasizing that it cannot be considered as a al., 2011). The gold-bornite correlation proposed by Kesler et megabreccia deposit (Skewes and Stern, 1994, 1995). The al. (2002) is only evident in the apical part of a single porphyry abundance of magmatic-hydrothermal breccias at Río center (DAM-01; see above) and does not apply at the deposit Blanco-Los Bronces could be interpreted to imply a shal- scale. lower erosion level than at Los Pelambres, especially since The sericitic alteration and contained pyrite, best devel- much of the porphyry copper mineralization there is con- oped in the northwestern quadrant of the Los Pelambres de- cealed beneath pyritic rocks (R. H. Sillitoe, pers. observa- posit, is partly controlled by the contact of the Los Pelambres tions) and several of the major breccias display sericitic alter- stock and the NE-striking structural zone (Fig. 8a). The seric- ation and are pyritic and copper poor in their near-surface itization overprinted the potassic zone and partially depleted parts (Vargas et al., 1999; Irarrazaval et al., 2010). A similar its copper and molybdenum contents, although minor quan- situation is observed at the small Copper Creek porphyry tities of copper-bearing sulfosalts were precipitated. The copper deposit in Arizona, where numerous magmatic-hydro- presence of localized advanced argillic alteration in the north- thermal breccia pipes, displaying sericitic alteration at shallow east structural corridor suggests that the roots of a previously levels, overlie largely concealed porphyry copper mineraliza- far more extensive lithocap are preserved there (Fig. 11d). tion (Guthrie, 1994; Anderson et al., 2009). The SE-inclined geometry of both the porphyry centers and northeast structural corridor are explained by north- Conclusions westward tilting of the entire Los Pelambres-Frontera sys- The Los Pelambres porphyry copper-molybdenum and con- tem by as much as 35° (Fig. 11b-d). The preferential preser- tiguous Frontera porphyry copper-gold deposits were gener- vation of the sericitic alteration in the northwestern quadrant ated in a period of ~1.5 to 1.7 m.y., immediately following of the Los Pelambres deposit further supports this interpre- emplacement of a host precursor quartz diorite pluton. The tation. The fact that the brecciation and associated mag- latter, the Los Pelambres stock, was localized by the preexist- matic-hydrothermal features are best developed on the ing, N-NW–striking Los Pelambres reverse fault, which southeastern, hanging-wall sides of the porphyry centers bounds the eastern side of a narrow, intensely deformed struc- (Fig. 6b) implies that the tilting was synmineral in timing and tural corridor. The Los Pelambres fault and other subparallel presumably linked to the tectonic uplift of the greater Los reverse faults in the greater Los Pelambres region were active Pelambres region between 12 and 7 Ma (Giambiagi and 2 to 3 m.y. before Los Pelambres stock emplacement as a re- Ramos, 2002). sponse to orogen-wide, contractional deformation and crustal

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thickening. The Los Pelambres stock is the northwesternmost Héctor Poblete drafted the figures. Thorough manuscript re- of a series of intrusive centers that define a marked align- views by William Atkinson and John Proffett are much ap- ment, trending for 70 km southeast from the Chalinga intru- preciated. sive complex, a composite mafic to intermediate composition batholith assembled from 23 to 15 Ma. Basaltic to andesitic REFERENCES volcanism at and near Los Pelambres ceased ~4 m.y. prior to Alvarez, P., 1996, Los depósitos triásicos y jurásicos de la Alta Cordillera de emplacement of the Los Pelambres stock. 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