45

Models and Exploration

Metallogenic aspects of giant porphyry systems of theAndes

J. Perell1 and R.H. Sillitoe2

1Antofagasta Minerals S.A., Ahumada 11, Oficina 602, Santiago, Chile227 West Hill Park, Highgate Village, London, N6 6ND, England

The Andes hosts the worlds premier porphyry copper province, ~6000-km long and containing a variety ofdeposits distributed in a number of metallogenic belts. Current aggregate output amounts to approximately30 percent of world mined copper metal. Metallogenic belts range in age from Permian to Pliocene, butmajor 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 includethe worlds largest supergene chalcocite blankets at Escondida and Chuquicamata in northern Chile, butalso the high-grade hypogene deposits at Ro Blanco-Los Bronces and El Teniente in central Chilethelatter arguably the worlds largest single copper depositand Toquepala and Cuajone in Peru, as well ascopper-gold mineralisation from Bajo de la Alumbrera, Argentina, and copper-zinc ore from the worldslargest skarn deposit at Antamina, Peru. All producing deposits are of conventional porphyry copper typewith additional, important amounts of ore hosted by porphyry-related hydrothermal breccia complexes andskarn at Ro Blanco-Los Bronces and Antamina, respectively. This article reviews some of the salientmetallogenic features of the three premier Andean porphyry belts by focusing on selected tectonomagmaticand alterationmineralisation aspects that are considered critical for, but not exclusive of, the formation ofgiant and even larger (cf. Clark 1993) porphyry deposits.

Tectonomagmatic SettingGiant, and larger, Andean porphyry deposits are characteristically associated with multiphase porphyriticstocksup to five at Escondidaof quartz monzonitic to dioritic composition that intrude a variety ofMesozoicCenozoic plutonic, volcanic and sedimentary rocks. Locally, the volcanic sequences were intrudedby medium- to large-sized batholiths immediately prior to porphyry stock emplacement, as in segments ofthe Palaeocene to early Eocene and middle Eocene to early Oligocene belts of southern Peru, and in theMiocene to early Pliocene belt of central Chile. Gold-rich examples exhibit a marked tendency to be associatedwith more mafic intrusions, dominantly quartz diorite to diorite, although the only producing porphyrycopper-gold deposit in the Andes, Bajo de la Alumbrera, Argentina (Fig. 1c), is centered on a dacitic torhyodacitic stock. Evolution from intermediate-composition intrusions pre-mineralisation to more felsicphases, including dacite to rhyodacite domes, in the intermediate and late stages of mineralisation is typicalof 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 Andesare all I-type, magnetite-series, moderate- to high-K calc-alkaline in compositionincluding shoshoniticvarieties in the Faralln Negro district, Argentina. The stocks are typically characterised by high Fe2O3/FeOratios that imply high oxidation states and possess, together with associated ore-bearing hydrothermal breccias,restricted Sri (0.70410.7046) and

iNd (0 to +4) values. The relatively high La/Yb data for deposits of the

southern Peru Palaeocene to early Eocene (2022), northern Chile middle Eocene to early Oligocene (1535), and central Chile Miocene to early Pliocene (2060 for El Teniente) belts suggest that the magmas thatfed 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 (610) areconsistent with regional geologic evidence, including caldera settings, that suggest evolution under extensionalconditions over a relatively thin crust. Tectonic relaxation between contractional episodes, albeit over athickened crust (La/Yb: 1522), accompanied porphyry copper-gold mineralisation at Cerro Casale in thesouthern part of the Maricunga region of northern Chile and at Bajo de la Alumbrera in the Faralln Negrodistrict, Argentina, but synmineralisation regional uplift and concomitant deformation were responsible forthe more complex internal geometry of the telescoped porphyry Cu-Mo-Au mineralisation at the nearbyAgua Rica deposit (Fig. 1c).

Regional fault systems are important in parts of some belts. For example, the majority of the Palaeocene toearly Eocene deposits of southern Peru lie along the Incapuquio fault system and many of the largest depositsin the middle Eocene to early Oligocene belt of northern Chile are located on or near main faults of theDomeyko regional fault system or at intersections with transverse lineaments. However, irrespective of the

46

SEG 2004 Predictive Mineral Discovery Under Cover

metallogenic belt, many other deposits possess no apparent association with regional faults, although foldthrust belts occur in the Miocene to early Pliocene belt, as in northern Peru and central Chile-northwesternArgentina (e.g. Aconcagua foldthrust belt). Where present, regional fault systems were demonstrably activeduring emplacement and mineralisation of porphyry copper stocks and, through a combination of high- andlow-angle reverse faulting, accommodated contraction and regional uplift (e.g. Skarmeta et al. 2003), withor 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 southernPeru and northern Chile, the major deposit-localising faults, including transverse lineaments, aremanifestations of rejuvenated crustal discontinuities of Mesozoic, Palaeozoic, or even older ancestry. In allcases, erosion consequent upon the regional surface uplift gave rise to accumulation of several kilometers ofterrestrial sediments in peripheral, structurally controlled synorogenic basins (e.g. Maksaev & Zentilli 1999,Perell et al. 2003).

Figure 1. Selected deposits of the main Andean metallogenic copper belts (ages from Sillitoe 1988, 1992, Noble &McKee 1999, Camus 2003, Perell et al. 2003). a) Palaeocene to early Eocene. b) Middle Eocene toearly Oligocene. c) Miocene to early Pliocene. d) Total copper contents and production from the maincopper belts of the Andes compared to the aggregate copper resources of other Andean belts (afterCamus 2003 and other unpubl. sources).

47

Models and Exploration

Independent of the metallogenic belt and the presence or absence of regional fault systems, structural controlis appreciable in most major porphyry deposits of the Andes, with both intrusion and alterationmineralisationgeometries accommodating themselves to pre-existing faults during porphyry development (e.g. Lindsay etal. 1995). Clusters or alignments of three or more deposits are common in certain belts, as in the middleEocene to early Oligocene belt of southern Peru and northern Chile, with the greatest number of largediscrete porphyry copper centerssevenbeing 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 tourmalinebreccia pipes being notably older and falling in the 66 to 62 Ma interval (Sillitoe 1988). Major deposits ofthe middle Eocene to early Oligocene belt formed between ~44 and 30 Ma (Maksaev & Zentilli 1999,Perell et al. 2003), whereas those from the Miocene to early Pliocene belt of central Chile and contiguousArgentina evolved between ~11 and 4 Ma (Camus 2003 and references therein). In northern and centralPeru, hydrothermal alteration and mineralisation appear to have taken place simultaneously at many differenttimes and places during the Miocene, although pulses at 2018, 1513 and 107 Ma are suspected to havebeen the most fecund (Noble & McKee 1999). The metallogenic epochs along certain belt segments, as inthe Palaeocene to early Eocene of southern Peru, the middle Eocene to early Oligocene of southern Peru andnorthern Chile, and the Miocene to early Pliocene of central Chile and contiguous Argentina, werecharacterised by periods of overall volcanic quiescence accompanied by eastward translation of the magmaticfronts in northern and central Chile, with porphyry copper mineralisation effectively concluding arcdevelopment (e.g. Sandeman et al. 1995, Maksaev & Zentilli 1999, Kay et al. 1999).

Tectonic implicationsContractional deformation involving crustal shortening and thickening caused most of the regional uplift inthe 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 lengthsof the copper belts described above are inferred to have been responses to flattening of subducted slabs (e.g.Mpodozis & Perell 2003) and consequent arc migrations, possibly combined with subduction erosion ofthe forearc. Slab flattening may be linked to accelerated convergence rates between plates or, as in northernPeru and central Chile, to subduction of buoyant oceanic features (Pilger 1981, Gutscher et al. 1999). Severecontractional conditions are further inferred to impede rapid magma ascent and venting, therefore favouringefficient magma storage in large, confined, shallow-level chambers (Sillitoe 1998, Perell et al. 2003).

AlterationMineralisationPorphyry copper mineralisation in the Andes is associated with one or more potassic, calc-silicate, intermediateargillic, sericitic and advanced argillic alteration assemblages. Potassic assemblages containing albite oractinolite constitute either separate hybrid potassic-sodic and potassic-calcic zones in some deposits, as atToquepala, Cerro Colorado and Cotabambas (e.g. Bouzari & Clark 2002, Perell et al. 2003; Fig. 1), or acombination of both, as in the deep parts of El Salvador, Chile (Gustafson & Quiroga 1995). At Ro Blanco-Los Bronces and El Teniente, however, copper-poor and sulphur-poor assemblages characterised by calcicactinolite 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, wasintroduced in most deposits during early stage potassic alteration, in intimate association with multiphasequartz stockworks and fine-grained disseminations. For example, nearly 50 percent of the mineralisation atRo Blanco-Los Bronces and >80 percent at El Teniente (Camus 2003 and references therein) are containedin 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, butonly 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 alterationwas responsible for reconstituting the original copper mineralogy with the consequent reduction of theoverall copper content (e.g. Perell et al. 2003).

Sericitic alteration occupies appreciable rock volumes in the shallower parts of many less deeply erodedmajor deposits in the three premier Andean copper belts. In the southern Peru part of the Palaeocene to earlyEocene 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 northernChile middle Eocene to early Oligocene belt possess major sericitic zones that overprint the central parts ofpre-existing copper-bearing potassic assemblages, most notably at Chuquicamata and Escondida (Sillitoe

48

SEG 2004 Predictive Mineral Discovery Under Cover

1992). In a number of deposits, including Rosario, Chuquicamata, Chimborazo, Escondida Norte-Zaldvar,Escondida, El Salvador and La Fortuna, there is a tendency for sericitic alteration to grade upward to advancedargillic lithocaps, most of which possess fault-controlled copper-rich massive sulphide veins and minorhydrothermal breccias (Sillitoe 1992 and references therein) including, at La Fortuna, Au mineralisation ofhigh-sulphidation epithermal type in vuggy quartz ledges (Perell et al. 1996). Some of the highest hypogenecopper contents are invariably contained in high-sulphidation overprint assemblages contained in massivesulphide veins and the intervening host rocks. Grades at Chuquicamata and Escondida were enhanced severaltimes through episodic overprinting (e.g. Padilla et al. 2001). The massive sulphide veins are dominated byenargite, bornite and pyrite, although chalcocite, covellite, digenite and tennantite are also commonconstituents. Sericitic alteration is present to varied degrees in many Miocene to early Pliocene deposits inthe Andes, including copper-barren annuli around potassic centers, as at Bajo de la Alumbrera and LosPelambres, or confined to certain hydrothermal breccia bodies as at Agua Rica, Ro Blanco-Los Broncesand El Teniente.

Advanced argillic lithocaps have been eroded from many porphyry copper deposits in the Andes owing tothe high rates of erosion consequent upon intense tectonic uplift, climatic conditions, or a combination ofboth. However, localised pyrophyllite-bearing assemblages in many deposits, irrespective of metallogenicepoch (e.g. Minas Conga, Toquepala, Cerro Colorado, Polo Sur, Chimborazo, Escondida Norte-Zaldvar,Escondida, El Salvador, Cerro Casale, Agua Rica, La Fortuna and Famatina; Fig. 1), are interpreted as thedeep roots of lithocaps and document their former existence in the upper parts of porphyry copper systemsthroughout the Andes. With the exception of the middle Eocene to early Oligocene porphyry copper depositsof northern Chile and the late Miocene deposit at Agua Rica, Argentina, porphyry-related lithocaps aregenerally poorly mineralised in the Andes, although that above porphyry copper-gold centres at Yanacochahosts one of the worlds largest high-sulphidation gold deposits.

An outstanding feature of some, but by no means all, Miocene to early Pliocene porphyry copper deposits incentral Chile and contiguous Argentina porphyry copper deposits is the presence of voluminous hydrothermalbreccias (e.g. Skewes & Stern 1994), which include both ore-bearing phases of magmatichydrothermalorigin and post-ore events associated with phreatomagmatic processes. The breccias formed throughout theevolution of the porphyry systems, as evidenced by the presence of actinolite, biotite, chlorite, anhydrite, ortourmaline 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 asmuch as 50 percent of the contained copper, at Ro Blanco-Los Bronces (Camus 2003).

Gold-rich porphyry copper deposits of the region share all the geologic features of such copper-gold systemselsewhere, including dominance of magnetite-rich potassic alteration variably overprinted by intermediateargillic assemblages, and an overall sympathetic relationship between copper and gold grades. Gold-richporphyry copper and porphyry gold (

49

Models and Exploration

supergene solutions during active enrichment generated exotic copper deposits where suitably receptivepiedmont gravel sequences were available (Fig. 1c). In contrast, where pyrite-bearing, feldspar-destructivealteration was only poorly developed, in situ oxidation in reactive rocks was the main supergene process.Maintenance of a balance between denudation rates and supergene processes under appropriate semi-aridclimatic and surface uplift conditions results in optimal supergene profile development, as in much of themiddle Eocene to early Oligocene belt of northern Chile (Maksaev & Zentilli 1999; Fig. 2).

Figure 2. Section showing present-day elevation of tops of supergene sulphide zones at selected deposits in themiddle Eocene to early Oligocene belt of northern Chile. Note main breaks at 22 and 27 S and thepreferred elevation ranges in which supergene chalcocite blankets are present and preserved (Perell,unpubl. data).

AcknowledgementsOur thanks to Hctor Poblete for his drafting work.

ReferencesAlpers C.N. & Brimhall G.H., 1988, Middle Miocene climatic change in the Atacama Desert, northern Chile: Evidence from supergene

mineralization at La Escondida, Geol. Soc. Amer. Bull., 100, 1640-1656.Bouzari F. & Clark A.H., 2002, Anatomy, evolution, and metallogenic significance of the supergene orebody of the Cerro Colorado

porphyry copper deposit, I Regin, northern Chile, Econ. Geol., 97, 1701-1740.Camus F., 2003, Geologa de los Sistemas Porfricos en los Andes de Chile, Santiago, Chile, Servicio Nacional de Geologa y

Minera, 267.Clark A.H., 1993, Are outsized porphyry copper deposits either anatomically or environmentally distinctive? SEG Spec. Publ. 2,

213-283.Gustafson L.B. & Quiroga J., 1995, Patterns of mineralization and alteration below the porphyry copper orebody at El Salvador,

Chile, Econ. Geol., 90, 2-16.Gutscher M.-A., Olivet J.-L., Aslanian D., Eissen J.-P. & Maury R., 1999, The lost Inca Plateau: cause of flat subduction beneath

Peru? Earth Planet. Sci. Lett., 171, 335-341.Kay S.M., Mpodozis C. & Coira B., 1999, Neogene magmatism, tectonics, and mineral deposits of the central Andes (22 to 33 S

Latitude), SEG Spec. Publ. 7, 27-59.Kurtz A., Kay S.M., Charrier R. & Farrar E., 1997, Geochronology of Miocene plutons and Andean uplift history in the El Teniente

region, central Chile (34-35S), Revista Geolgica de Chile, 24, 75-90.Lindsay D.D., Zentilli M. & Rojas de la Rivera J., 1995, Evolution of an active ductile to brittle shear system controlling the

mineralization at the Chuquicamata porphyry copper deposit, northern Chile, Inter. Geol. Rev., 37, 945-958.Love D., Clark A.H. & Lipten E., 2003, Genesis of the Antamina Cu-Zn skarn deposit, Ancash, Peru, Congreso Geolgico Chileno,

10th, Concepcin, 2003, CD-ROM, 1.Maksaev V. & Zentilli M., 1999, Fission track thermochronology of the Domeyko Cordillera, northern Chile: Implications for

Andean tectonics and porphyry copper metallogenesis, Exploration and Mining Geology, 8, 65-89.Mpodozis C. & Perell J., 2003, Porphyry copper metallogeny of the middle Eocene-early Oligocene arc of western South America:

Relationships with volcanism and arc segmentation, Congreso Geolgico Chileno, 10th, Concepcin, 2003, CD-ROM, 1.Noble D.C. & McKee E.H., 1999, The Miocene metallogenic belt of central and northern Peru, SEG Spec. Publ. 7, 155-193.Padilla R., Titley S.R. & Pimentel F., 2001, Geology of the Escondida porphyry copper deposit, Antofagasta Region, Chile, Econ.

Geol., 96, 307-324.Perell J., Urza F., Cabello J. & Ortiz F., 1996, Clustered, gold-bearing Oligocene porphyry copper and associated epithermal

mineralization at La Fortuna, Vallenar region, northern Chile, SEG Spec. Publ. 5, 81-90.

50

SEG 2004 Predictive Mineral Discovery Under Cover

Perell J., Rojas N., Devaux C., Fava L., Etchart E. & Harman P., 1998, Discovery of the Agua Rica porphyry Cu-Mo-Au deposit,Catamarca Province, northwestern Argentina. Part I: Geology, in Porphyry and Hydrothermal Copper and Gold Deposits: AGlobal Perspective, Porter T.M., ed., Australian Mineral Foundation, 117-132.

Perell J., Garca A., Ramos P., Glover K., Neyra C., Muhr R., Fuster N. & Caballero A., 2001, The Magistral porphyry-skarn Cu-Mo deposit, Ancash, Peru, ProExplo 2001, Congreso Internacional de Prospctores y Exploradores, 2nd, Lima, 2001, CD-ROM, 3.

Perell J., Carlotto V., Zrate A., Ramos P., Posso H., Neyra C., Caballero A., Fuster N. & Muhr R., 2003, Porphyry-style alterationand mineralization of the middle Eocene to early Oligocene Andahuaylas-Yauri belt, Cuzco region, Peru, Econ. Geol., 98, 1575-1605.

Pilger R.H., 1981, Plate reconstructions, aseismic ridges, and low angle subduction beneath the Andes, Geol. Soc. Amer. Bull., 92,448-456.

Sandeman H.A., Clark A.H. & Farrar E., 1995, An integrated tectono-magmatic model for the evolution of the southern PeruvianAndes (13-20S) since 55 Ma, Inter. Geol. Rev., 60, 153-174.

Sillitoe R.H., 1988, Epochs of intrusion-related copper mineralization in the Andes, J. South Amer. Earth Sci., 1, 89-108.Sillitoe R.H., 1992, Gold and copper metallogeny of the central Andes past, present, and future exploration objectives, Econ.

Geol., 87, 2205-2216.Sillitoe R.H., 1998, Major regional factors favouring large size, high hypogene grade, elevated gold content and supergene oxidation

and enrichment of porphyry copper deposits, in Porphyry and Hydrothermal Copper and Gold Deposits: A Global Perspective,Porter T.M., ed., Australian Mineral Foundation, 21-34.

Sillitoe R.H. & McKee E.H., 1996, Age of supergene oxidation and enrichment in the Chilean porphyry copper province, Econ.Geol., 91, 164-179.

Skarmeta J., McClay K. & Bertens A., 2003, Structural controls on porphyry copper deposits in northern Chile: New models andimplications for Cu-Mo mineralization in subduction orogens, Congreso Geolgico Chileno, 10th, Concepcin, 2003, CD-ROM, 1.

Skewes M.A. & Stern C.R., 1994, Tectonic trigger for the formation of Late Miocene Cu-rich megabreccias in the Andes of centralChile, Geology, 22, 551-554.

Skewes M.A., Arvalo A.G., Floody R., Ziga P. & Stern C.R., 2002, The giant El Teniente breccia deposit: Hypogene copperdistribution and emplacement, SEG Spec. Publ. 9, 299-332.

Zweng P.L. & Clark A.H., 1995, Hypogene evolution of the Toquepala porphyry copper-molybdenum deposit, Moquegua, southeasternPeru, in Porphyry Copper Deposits of the American Cordillera, Pierce F.W. & Bolm J.G., eds, Arizona Geol. Soc. Digest, 20,566-612.