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

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Chapter 4 Geologic Setting and Evolution of the Porphyry Copper-Molybdenum and Copper-Gold Deposits at Los Pelambres, Central Chile © 2012 Society of Economic Geologists, Inc. Special Publication 16, pp. 79–104 Chapter 4 Geologic Setting and Evolution of the Porphyry Copper-Molybdenum and Copper-Gold Deposits at Los Pelambres, Central Chile JOSÉ PERELLÓ,1,† RICHARD H. SILLITOE,2 CONSTANTINO MPODOZIS,1 HUMBERTO BROCKWAY,1 AND HÉCTOR POSSO3 1 Antofagasta Minerals S.A., Apoquindo 4001, piso 18, Las Condes, Santiago, 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 potas- sic center grades laterally to an annular sericitic zone surrounded by a propylitic halo. The bulk of the hypo- gene metal resource is hosted by multiple veinlet generations within potassic alteration, of which type 4 (quartz ± K-feldspar ± biotite ± sericite ± phengite ± andalusite ± corundum), A, and B types are volumetrically 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 intermin- eral 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 ore- body. This coalescence also led to deposit-scale sulfide zoning, from internal chalcopyrite-bornite through chal- copyrite-pyrite to external pyrite. Abundant hydrothermal magnetite accompanies the gold-bearing 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 con- strained by Re-Os molybdenite geochronology. The entire system cooled to nearly ambient temperatures by ~8 Ma, as supported by temporally overlapping K-Ar, Ar/Ar, and (U-Th)/He ages, and was exposed to the effects of supergene oxidation and immature enrichment by ~5 Ma. Plio-Pleistocene glaciation partially 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 cen- ters and northeast structural corridor defined by sericitic and advanced argillic alteration, are ascribed to syn- mineral tilting. The tilting accompanied regional tectonic uplift during crustal shortening and thickening, which were controlled by thick-skinned reverse faults active ~60 km farther east in Argentina. 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). 79 Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/3812113/9781629490410_ch04.pdf by guest on 05 August 2020 80 PERELLÓ ET AL. h 80 100 175 200 74º c La Serena 69º 64º n e 120 r 30º t Wadati-Benioff u zone contours r 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 cop- per 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 con- tours 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. 0361-0128/98/000/000-00 $6.00 80 Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/3812113/9781629490410_ch04.pdf by guest on 05 August 2020 PORPHYRY Cu-Mo & Cu-Au DEPOSITS, LOS PELAMBRES, CENTRAL CHILE 81 (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).
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