Perelló , Sillitoe, ...., Metallogenic Aspects of Giant Porphyry Systems of the Andes

Models and Exploration

Metallogenic aspects of giant porphyry systems of the Andes J. Perelló1 and R.H. Sillitoe2 1

Antofagasta Minerals S.A., Ahumada 11, Oficina 602, Santiago, Chile 2 27 West Hill Park, Highgate Village, London, N6 6ND, England

The Andes hosts the world’s premier porphyry copper province, ~6000-km long and containing a variety of deposits distributed in a number of metallogenic belts. Current aggregate output amounts to approximately 30 percent of world mined copper metal. Metallogenic belts range in age from Permian to Pliocene, but major producing deposits are of Cenozoic age and define spatially restricted Palaeocene to early Eocene, middle Eocene to early Oligocene, and Miocene to early Pliocene belts (Fig. 1). These belts not only include the world’s largest supergene chalcocite blankets at Escondida and Chuquicamata in northern Chile, but also the high-grade hypogene deposits at Río Blanco-Los Bronces and El Teniente in central Chile—the latter arguably the world’s largest single copper deposit—and Toquepala and Cuajone in Peru, as well as copper-gold mineralisation from Bajo de la Alumbrera, Argentina, and copper-zinc ore from the world’s largest skarn deposit at Antamina, Peru. All producing deposits are of conventional porphyry copper type with additional, important amounts of ore hosted by porphyry-related hydrothermal breccia complexes and skarn at Río Blanco-Los Bronces and Antamina, respectively. This article reviews some of the salient metallogenic features of the three premier Andean porphyry belts by focusing on selected tectonomagmatic and alteration–mineralisation aspects that are considered critical for, but not exclusive of, the formation of giant and even larger (cf. Clark 1993) porphyry deposits. Tectonomagmatic Setting

Giant, and larger, Andean porphyry deposits are characteristically associated with multiphase porphyritic stocks—up to five at Escondida—of quartz monzonitic to dioritic composition that intrude a variety of Mesozoic–Cenozoic plutonic, volcanic and sedimentary rocks. Locally, the volcanic sequences were intruded by medium- to large-sized batholiths immediately prior to porphyry stock emplacement, as in segments of the Palaeocene to early Eocene and middle Eocene to early Oligocene belts of southern Peru, and in the Miocene to early Pliocene belt of central Chile. Gold-rich examples exhibit a marked tendency to be associated with more mafic intrusions, dominantly quartz diorite to diorite, although the only producing porphyry copper-gold deposit in the Andes, Bajo de la Alumbrera, Argentina (Fig. 1c), is centered on a dacitic to rhyodacitic stock. Evolution from intermediate-composition intrusions pre-mineralisation to more felsic phases, including dacite to rhyodacite domes, in the intermediate and late stages of mineralisation is typical of many systems, but reversals to more mafic magmatism are also apparent in many others. Independent of the metallogenic belt, copper-bearing porphyry stocks and comagmatic rocks of the Andes are all I-type, magnetite-series, moderate- to high-K calc-alkaline in composition—including shoshonitic varieties in the Farallón Negro district, Argentina. The stocks are typically characterised by high Fe2O3/FeO ratios that imply high oxidation states and possess, together with associated ore-bearing hydrothermal breccias, restricted Sri (0.7041–0.7046) and εiNd (0 to +4) values. The relatively high La/Yb data for deposits of the southern Peru Palaeocene to early Eocene (20–22), northern Chile middle Eocene to early Oligocene (15– 35), and central Chile Miocene to early Pliocene (20–60 for El Teniente) belts suggest that the magmas that fed the porphyry copper stocks evolved during periods of crustal thickening (e.g. Kay et al. 1999). In contrast, lower La/Yb ratios of Palaeocene to early Eocene deposits and related rocks of northern Chile (6–10) are consistent with regional geologic evidence, including caldera settings, that suggest evolution under extensional conditions over a relatively thin crust. Tectonic relaxation between contractional episodes, albeit over a thickened crust (La/Yb: 15–22), accompanied porphyry copper-gold mineralisation at Cerro Casale in the southern part of the Maricunga region of northern Chile and at Bajo de la Alumbrera in the Farallón Negro district, Argentina, but synmineralisation regional uplift and concomitant deformation were responsible for the more complex internal geometry of the telescoped porphyry Cu-Mo-Au mineralisation at the nearby Agua Rica deposit (Fig. 1c). Regional fault systems are important in parts of some belts. For example, the majority of the Palaeocene to early Eocene deposits of southern Peru lie along the Incapuquio fault system and many of the largest deposits in the middle Eocene to early Oligocene belt of northern Chile are located on or near main faults of the Domeyko regional fault system or at intersections with transverse lineaments. However, irrespective of the 45

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Figure 1.

Selected deposits of the main Andean metallogenic copper belts (ages from Sillitoe 1988, 1992, Noble & McKee 1999, Camus 2003, Perelló et al. 2003). a) Palaeocene to early Eocene. b) Middle Eocene to early Oligocene. c) Miocene to early Pliocene. d) Total copper contents and production from the main copper belts of the Andes compared to the aggregate copper resources of other Andean belts (after Camus 2003 and other unpubl. sources).

metallogenic belt, many other deposits possess no apparent association with regional faults, although fold– thrust belts occur in the Miocene to early Pliocene belt, as in northern Peru and central Chile-northwestern Argentina (e.g. Aconcagua fold–thrust belt). Where present, regional fault systems were demonstrably active during emplacement and mineralisation of porphyry copper stocks and, through a combination of high- and low-angle reverse faulting, accommodated contraction and regional uplift (e.g. Skarmeta et al. 2003), with or without participation of Palaeozoic or older basement units as elsewhere along the entire Andean orogen. In most belts, but particularly in certain segments of the middle Eocene to early Oligocene belt of southern Peru and northern Chile, the major deposit-localising faults, including transverse lineaments, are manifestations of rejuvenated crustal discontinuities of Mesozoic, Palaeozoic, or even older ancestry. In all cases, erosion consequent upon the regional surface uplift gave rise to accumulation of several kilometers of terrestrial sediments in peripheral, structurally controlled synorogenic basins (e.g. Maksaev & Zentilli 1999, Perelló et al. 2003). 46

Models and Exploration

Independent of the metallogenic belt and the presence or absence of regional fault systems, structural control is appreciable in most major porphyry deposits of the Andes, with both intrusion and alteration–mineralisation geometries accommodating themselves to pre-existing faults during porphyry development (e.g. Lindsay et al. 1995). Clusters or alignments of three or more deposits are common in certain belts, as in the middle Eocene to early Oligocene belt of southern Peru and northern Chile, with the greatest number of large discrete porphyry copper centers—seven—being present at Radomiro Tomic, Chuquicamata, MM, Quetena, Toki, Genoveva and Opache in the Chuquicamata district. All major productive porphyry belts of the Andes were generated during restricted intervals of 7 to 13 m.y. The porphyry copper deposits of the Palaeocene to early Eocene belt formed from 62 to 52 Ma, with tourmaline breccia pipes being notably older and falling in the 66 to 62 Ma interval (Sillitoe 1988). Major deposits of the middle Eocene to early Oligocene belt formed between ~44 and 30 Ma (Maksaev & Zentilli 1999, Perelló et al. 2003), whereas those from the Miocene to early Pliocene belt of central Chile and contiguous Argentina evolved between ~11 and 4 Ma (Camus 2003 and references therein). In northern and central Peru, hydrothermal alteration and mineralisation appear to have taken place simultaneously at many different times and places during the Miocene, although pulses at 20–18, 15–13 and 10–7 Ma are suspected to have been the most fecund (Noble & McKee 1999). The metallogenic epochs along certain belt segments, as in the Palaeocene to early Eocene of southern Peru, the middle Eocene to early Oligocene of southern Peru and northern Chile, and the Miocene to early Pliocene of central Chile and contiguous Argentina, were characterised by periods of overall volcanic quiescence accompanied by eastward translation of the magmatic fronts in northern and central Chile, with porphyry copper mineralisation effectively concluding arc development (e.g. Sandeman et al. 1995, Maksaev & Zentilli 1999, Kay et al. 1999). Tectonic implications

Contractional deformation involving crustal shortening and thickening caused most of the regional uplift in the Andes, including rates of as much as 3 km/m.y. at the latitude of El Teniente, central Chile (Kurtz et al. 1997). Contractional events coincident with giant porphyry copper formation along parts or entire lengths of the copper belts described above are inferred to have been responses to flattening of subducted slabs (e.g. Mpodozis & Perelló 2003) and consequent arc migrations, possibly combined with subduction erosion of the forearc. Slab flattening may be linked to accelerated convergence rates between plates or, as in northern Peru and central Chile, to subduction of buoyant oceanic features (Pilger 1981, Gutscher et al. 1999). Severe contractional conditions are further inferred to impede rapid magma ascent and venting, therefore favouring efficient magma storage in large, confined, shallow-level chambers (Sillitoe 1998, Perelló et al. 2003). Alteration–Mineralisation

Porphyry copper mineralisation in the Andes is associated with one or more potassic, calc-silicate, intermediate argillic, sericitic and advanced argillic alteration assemblages. Potassic assemblages containing albite or actinolite constitute either separate hybrid potassic-sodic and potassic-calcic zones in some deposits, as at Toquepala, Cerro Colorado and Cotabambas (e.g. Bouzari & Clark 2002, Perelló et al. 2003; Fig. 1), or a combination of both, as in the deep parts of El Salvador, Chile (Gustafson & Quiroga 1995). At Río BlancoLos Bronces and El Teniente, however, copper-poor and sulphur-poor assemblages characterised by calcic actinolite and magnetite formed during a distinct event that predated potassic alteration (Skewes et al. 2002). Copper mineralisation, chiefly in the form of chalcopyrite and subordinate bornite and digenite, was introduced in most deposits during early stage potassic alteration, in intimate association with multiphase quartz stockworks and fine-grained disseminations. For example, nearly 50 percent of the mineralisation at Río Blanco-Los Bronces and >80 percent at El Teniente (Camus 2003 and references therein) are contained in biotite-dominated potassic assemblages. Abundant hydrothermal magnetite occurs in Au-rich deposits, irrespective of the metallogenic epoch. Intermediate argillic alteration is a component of ore zones in several deposits throughout the Andes, but only at Antamina, Cerro Colorado and Escondida do such assemblages constitute the main copper event (Padilla et al. 2001, Bouzari & Clark 2002, Love et al. 2003). In all other cases, intermediate argillic alteration was responsible for reconstituting the original copper mineralogy with the consequent reduction of the overall copper content (e.g. Perelló et al. 2003). Sericitic alteration occupies appreciable rock volumes in the shallower parts of many less deeply eroded major deposits in the three premier Andean copper belts. In the southern Peru part of the Palaeocene to early Eocene belt, sericitic assemblages contributed between 60 and 90 percent of the copper grade at Cuajone, Quellaveco and Toquepala (e.g. Zweng & Clark 1995), and many giant and larger deposits of the northern Chile middle Eocene to early Oligocene belt possess major sericitic zones that overprint the central parts of pre-existing copper-bearing potassic assemblages, most notably at Chuquicamata and Escondida (Sillitoe 47

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1992). In a number of deposits, including Rosario, Chuquicamata, Chimborazo, Escondida Norte-Zaldívar, Escondida, El Salvador and La Fortuna, there is a tendency for sericitic alteration to grade upward to advanced argillic lithocaps, most of which possess fault-controlled copper-rich massive sulphide veins and minor hydrothermal breccias (Sillitoe 1992 and references therein) including, at La Fortuna, Au mineralisation of high-sulphidation epithermal type in vuggy quartz ledges (Perelló et al. 1996). Some of the highest hypogene copper contents are invariably contained in high-sulphidation overprint assemblages contained in massive sulphide veins and the intervening host rocks. Grades at Chuquicamata and Escondida were enhanced several times through episodic overprinting (e.g. Padilla et al. 2001). The massive sulphide veins are dominated by enargite, bornite and pyrite, although chalcocite, covellite, digenite and tennantite are also common constituents. Sericitic alteration is present to varied degrees in many Miocene to early Pliocene deposits in the Andes, including copper-barren annuli around potassic centers, as at Bajo de la Alumbrera and Los Pelambres, or confined to certain hydrothermal breccia bodies as at Agua Rica, Río Blanco-Los Bronces and El Teniente. Advanced argillic lithocaps have been eroded from many porphyry copper deposits in the Andes owing to the high rates of erosion consequent upon intense tectonic uplift, climatic conditions, or a combination of both. However, localised pyrophyllite-bearing assemblages in many deposits, irrespective of metallogenic epoch (e.g. Minas Conga, Toquepala, Cerro Colorado, Polo Sur, Chimborazo, Escondida Norte-Zaldívar, Escondida, El Salvador, Cerro Casale, Agua Rica, La Fortuna and Famatina; Fig. 1), are interpreted as the deep roots of lithocaps and document their former existence in the upper parts of porphyry copper systems throughout the Andes. With the exception of the middle Eocene to early Oligocene porphyry copper deposits of northern Chile and the late Miocene deposit at Agua Rica, Argentina, porphyry-related lithocaps are generally poorly mineralised in the Andes, although that above porphyry copper-gold centres at Yanacocha hosts one of the world’s largest high-sulphidation gold deposits. An outstanding feature of some, but by no means all, Miocene to early Pliocene porphyry copper deposits in central Chile and contiguous Argentina porphyry copper deposits is the presence of voluminous hydrothermal breccias (e.g. Skewes & Stern 1994), which include both ore-bearing phases of magmatic–hydrothermal origin and post-ore events associated with phreatomagmatic processes. The breccias formed throughout the evolution of the porphyry systems, as evidenced by the presence of actinolite, biotite, chlorite, anhydrite, or tourmaline as principal cementing minerals. High-grade copper ore in breccias is present in nearly all deposits, and ranges from metre-scale examples at Los Pelambres to large, kilometre-scale complexes, hosting as much as 50 percent of the contained copper, at Río Blanco-Los Bronces (Camus 2003). Gold-rich porphyry copper deposits of the region share all the geologic features of such copper-gold systems elsewhere, including dominance of magnetite-rich potassic alteration variably overprinted by intermediate argillic assemblages, and an overall sympathetic relationship between copper and gold grades. Gold-rich porphyry copper and porphyry gold (