02005 Society ol FA:ononUe CeoIogim, loc. EmnomK: GeOlogy lOOt" Anniwnary Voillme pp . ........

Andean Copper Province: Tectonomagmatic Settings, Deposit Types, Metallogeny, Exploration, and Discovery

RICHARD H. SILLITOE'

27 West Hill Park. Highgate WInge. Umdon N66ND. Engkmd

AND JOSE PERELLO

Antofagasta Minerals S.A. . Ahunuufn 11. oficina 602. Santiago. Chile

Abstract

The Andes, in particular their central parts, have been known as a preeminent Cu province for more than 100 years and have been the source of many innovative metallogenic concepts and models directly applicable to Cu deposits worldwide. The central Andes currently produce 44 percent of world-mined Cu.

The -6.000-km-Iong Andean Cu province oomprises severnllong and markedly Unear. orogen-parallel metallo­genic belts. each developed during a restricted metallogenie epoch. Belts in the northern Andes are still poorly ex­plored. those in the central Andes are the focus of current Cu exploration and mining. and the southern Andes have little Cu potential. In the central Andes, from southern Peru to central Chile and contiguous Argentina. an incipi­ently developed belt oflate Paleozoic to early Mesowic porph)'l)' Cu mineralization is partly overlapped by foureas!­ward-younging Cu belts, middle to late Mesowic on the Paci£c coast. Paleocene to early Eocene. middle Eocene to early Oligocene. and, along the eastern border of the orogen. Miocene to earIy Pliocene. all but the first dominated by porph)'l)' Cu mineralization. Porph)'l)' Cu deposits in the northern part of the Paleocene to early Eocene bel~ in southern Peru. and the southern part of the premier middle Eocene to early Oligocene bel~ in northern Chile. 00-

incide with major orogen-parallel fault systems thal underwent synmineralization reverse displacement. The middle to late Mesowic belt also oontains major orogen-parallel faults but with nonnal and nonnal-oblique motions syn­chronous with Cu mineralization of Fe oxide-Cu-Au. manto-type Cu. and subordinate porph)'l)' Cu types. In oon­trast. remaining portions of the Tertiary Cu belts. along with the central Chile segment of the Miocene to early Pliocene belt. lack evidence for such clearcut structurnl control on deposit location. The spatial distribution of Cu belts farther north is different. with only the Miocene to early Pliocene belt recognized in the central Andes of north­ern Peru and at least three belts developed semioontinuously in the northern Andes of Ecuador and Colombia.

Compositions of host porph)'l)' stocks and alteration-mineralization types and geometries in porph)'l)' Cu­Mo and Cu-Au deposits throughout the Andes are grossly similar to those encountered elsewhere and do not appear to control either deposit size or hypogene ore grade. Nevertheless, deposits in the middle Eocene to early Oligocene belt of northern Chile. in particular. are characterized by telescoping of structurally localized high-sulfidation mineral assemblages over earlier and deeper alteration types. Hydrothennal breccias occur in many porph)'l)' eu centers, but ore-bearing varieties are volumetrically important in only three widely scat­tered deposits of different ages. Porphyry eu-Au depoSits and prospects, although concentrated in several dis­crete sub-belts and districts, also occur randomly throughout most of the belts. Geochronologic studies of sev­eral major depoSits suggest that magmatic-hydrothermallifespans commonly approximate 1 to 2 m.y.

The three most productive porph)'l)' Cu belts developed syntectonically during oontractional events and crustaI thickening. possibly linked to shallow subduction. forearc subduction erosion. and consequent arc migration. Sup­pression of volcanism during compression, high surface uplift rates, and rapid exhumation optimized the conditions for aocumulation of fluid-rich magma in large. shallow-level chambers propitious for giant porph)'l)' Cu develop­ment. The uplift was also ultimately responsible for the supergene upgrading of many Cu deposits. particularly in northern Chile. The concept of giant porph)'l)' Cu deposit fonnation by superposition of two temporally discrete magmatic-hydrothennal systems lacks geologie support. Crustal oomposition appears to have exerted little influ­ence on porphYl)' Cu genesis. In marked contrast to these contrnctional settings, extensional arcs in the Meso­Genomic Andes gave rise to smaller. lower grade porph)'l)' Cu deposits. The attenuated crust. high heat-flow regime. and abundance of basaltic to intenned.iate-composition magmatism, characteristic of the middle to late Mesozoic belt in coastal southern Peru and Chile. provided optimal oonditions for Fe oxide-Cu-Au and manto-type Cu forniation. although the role of magmatic versus basinal brines in deposit genesis remains unresolved.

A variety of geologie. geoche.nical. and geophysical techniques have been employed in Andean Cu explo­ration, but it is the combined routine geologic-geochemical approach that bas resuJted in most discoveries. in­cluding those during the past few years. Continued reliance on these tried-and-tested techniques, combined with timely drilling, is likely to be the best means of ensuring future exploration su(,'(''ess. During the last 13 years, more than half of discoveries in the central Andes have been made beneath pre- or postmineral cover, a trend that is thought likely to continue. Nevertheless, undiscovered, at least partially exposed mineralization is also considered to exist, even in the premier middle Eocene to early Oligocene belt, which has accounted. for apprOximately 65 percent of all Andean discoveries over the last three decades. Con(''eptual geology, capable of predicting deposit locations. has played a very subordinate role in Cu discovery to date but is believed to be perhaps the single most underappreciated parameter for increasing the future discovery rate.

I Corresponding author: e-mail.aucu@compuserve.com

845

846 SILLlTOE AND PERELL6

Introduction

THE SOUTH AMERICAN ANDES (Fig. I), in particular the northern Chile-southern Peru part (latitudes 13"-33" S), has the world's largest eu endowment, a situation that was pre­saged by the results of early investigations by Domeyko (1876), Miller and Singewald (1919), and Little (1926). Northern Chile, southern Peru, and northwestern Argentina possess >40 percent of the world's Cu resources and prOvide 44 percent of mined production of Cu metal. Chile, the lead­ing producer with 36 percent of market share, first dominated the world Cu scene during the 1850s to 1880s when a series of high-grade vein deposits were worked. Chile regained its premier status in 1980, this time based on exploitation of an ever-increasing number of major porphyry Cu deposits.

Pioneering metallogenie stuelies of the Chilean Andes were presented by Domeyko (1876) and Little (1926), both of whom elivided northern parts of the country into longituelinal metallogenic belts. Little (1926) further classified the de­posits on the basis of genetic type and age of formation, al­though the latter parameter has been largely superseded.

PRINCIPIU.

~ Coastal Ccx'dillera and Plains

_ Western Cordillera

IT::::!] Central Cordillera

R1 lnterandean Basins

_ Eastem Cordillera

~ Frontal Cordillera

!:55l P<eeo«lille<a

~ Subandean Ranges

~ Sierras Pampeanas

CENTRAL ANDES

SOUTHERN ANDES

o 1000 km , ,

FIG. 1. The South American Andes, showing main physiographic provinces. Simplified from Corvahin (1990), with additions.

Flores (1942) erected a practical mineralogic classification of Chilean are depoSits, which was subsequently expanded and incorporated into a more elaborate metallogenie scheme by Ruiz and Ericksen (1962) and, with modifications, Ruiz et al. (1965). They recognized five main types and several subtypes of Cu depoSits in northern Chile, as well as a variety of others containing Fe, Au, Pb-Zn, Ag, or Mn. The dominance of the country's metallogeny by Cu is inherent in use of the terms "Chilean copper province" (Turneaure, 1955) and "Chilean copper belt" (Stoll, 1964, 1965). The northward extension into southern Peru was emphasized by denomination of the Pacific Cu subprovince by Bellido and de Montreuil (1972). In a prescient metallogenic analysiS, using the criteria of Rad­kevich (1961), Stoll (1965) considered the Andes as a sialic province characterized by polycyclic emplacement of grani­toid rocks and a scarcity of mafic and ultramafic intrusions over much of their length. He further suggested that tempo­rally and compositionaliy elistinct igneous suites, each giving rise to elistinctive mineralization, could explain the linear elis­tribution of the various deposit types. Stoll (1965) also pointed out the gabbrophile (Cu, Fe, Mn), as opposed to granitophile, character of the major metals, which was taken to inelicate their ultimate source in the mantle.

FollOwing the formulation of the plate tectonic hypotheSiS in the late 1960s, Sillitoe (1972a) attempted to link the longi­tuelinal metallogenic belts of magmatic affiliation in the cen­tral Andes to the eastward subduction of oceanic lithosphere and, in the specific case of the region's porphyry Cu depoSits, he proposed that the uppermost parts of the downgoing slab contributed metals, S, and CI for eventual are formation in the upper crust (Sillitoe, 1970, 1972b). In an overall plate tec­tonic context, Farrar et al. (1970), Clark and Zentilli (1972), and Clark et al. (1976) used K-Ar ages to confirm and better define the post-Paleowic eastward migration of longituelinal plutOnic and metallogeniC belts proposed by Ruiz et al . (1965) in northern Chile, soon after this method was first applied to elirect dating of Andean Cu deposits (Laughlin et al., 1968). Sillitoe (1981, 1988) used raeliometric ages determined by Quirt et al. (1971) and others to define and delineate Early Cretaceous, Paleocene to early Eocene, middle Eocene to early Oligocene, and Miocene to early Pliocene porphyry Cu belts in northern Chile, southern Peru, and northwestern Ar­gentina, an eastward-younging array that effectively accom­modates the ages of all subsequently eliscovered deposits (see below). It is now widely accepted that Cu deposits through­out the Andes formed during relatively restricted metallo­genic epochs, each coincieling with a longituelinal metallo­genic belt (Sillitoe et al., 1982; Beckinsale et al. , 1965; Clark et al. , 1990; Noble and McKee, 1999; Gendall et al., 2000; Pere1l6 et al ., 2oo3a). Petersen (1970, 1972, 1979) and Sillitoe (1976, 1990) emphasized the broad spectrum of Andean Cu deposit types, including those clearly or poSSibly unrelated di­rectly to intrusive activity.

Stoll's (1965) notion of a deep source for Andean magmas and metals gained geochemical support from the first com­prehensive Sr and Pb isotope stuelies of igneous rocks and ores in northern Chile (McNutt et al. , 1975; Tilton et al. , 1981). During the subsequent 25 years, copious petrochemi­cal and isotopic research further documented the close asso­ciation of central Andean Cu deposits with mantle-derived

•

ANDEAN Cu PROVINCE. 847

magmas, with contributions from the subducted slab (e.g., Maksaev and Zentilli, 1988; Puig, 1988; Macfarlane et aI., 1990; Skewes and Stem, 1995; Kay et aI" 1999; Mathur et aI" 2000). The link between middle Eocene to early Oligocene porphyry Cu-bearing intrusions in northern Chile and coeval crustal thickening, uplift, and exhumation processes (Mak­saev, 1990) was subsequently extended to those of late Miocene to early Pliocene age in central Chile (Skewes and Holmgren, 1993; Skewes and Stem, 1994). This concept was expanded to explain the generation of exceptionally large, high-grade porphyry Cu deposits throughout the central Andes and elsewhere during contractional tectonjc events (Sillitoe, 1998). Kay et al. (1999) and Kay and Mpodozis (2001, 2002) related trace element geochemistry to degree of crustal thickening in the central Andes and proposed that ore­forming fluids were a direct product of the consequent tran­sition from amphibole to gamet stability in the lower crust.

Numerous studies of individual Andean Cu deposits have been conducted during tlle last century, with those hy Lind­gren and Bastin (1922) and Gustafson and Hunt (1975) at the EI Teniente and EI Salvador porphyry Cu depoSits, respec­tively, being arguably the truly ground-breaking studies. Ver­tical wning of central Andean porphyry Cu systems, from vol­canic tops to plutOniC roots , was proposed by Sillitoe (1973a), although contractional deformation coincident with emplace­ment of many giant deposits appears to have inhibited coeval volcanism (Mpodozis and Ramos, 1990). Development of ad­vanced argillic lithocaps as shallow manifestations of porphyry Cu formation (e.g., Sillitoe, 1992, 1995a) is now widely ac­cepted throughout the Andes. Several attempts have been made to define the magmatic-hydrothermallifespans of cen­tral Andean porphyry Cu deposits, culminating in the com­bined use of40ArP"Ar, U-Pb, and Re-Os methods to show that the giant EI Teniente deposit formed over a -2-m.y. interval (Maksaev et al., 2004). Recently, integrated models have been presented for porphyry Cu (Camus, 2003), manto-type Cu (Maksaev and Zentilli, 2002), and Fe oxide-Cu-Au (Sillitoe, 2003) deposits in the central Andes.

Aim

The aim of this article is to provide an overview of the Andean Cu provinoe, from Colombia in the north to Argentina and Chile in the south. An initial section briefly introduoes all hypo­gene Cu deposit types and belts throughout the Andean orogen, preparatory to more detailed oonsiderations of the econOmically most Significant Cu belts in the oentral Andes of northern and oentral Chile and contiguous parts of Argentina and Peru. The tectonomagmatic settings and salient mineralization features of these premier belts are summarized, leading to an evaluation of the geolOgiC oontrols of Cu deposit formation and an attempt to explain why the oentral Andes are so highly endowed with this metal. The article ooncludes with a prOCis of Cu exploration in the Andes and assesses the methods and strategies that have led to success and are thought likely to oontinue to do so. Porphyry Cu deposits, including both Mo- and Au-rich varieties, are far and away the principal mineralization type and dominate the discussion. Supergene oxidation and enrichment wnes and prooesses, critical to economic viability of most Cu deposits in the central Andes, are mentioned only briefly because they are dealt with elsewhere in this volume (Sillitoe, 2005).

Andean Cu Distribution in Space and Time

11ltrrxllictory staterrwnt

The Andes have long been subdivided into northern, cen­tral , and southern parts by transverse tectonomagmatic dis­continuities (e.g., Gansser, 1973; Sillitoe, 1974). The most important discontinuities from a metallogenic standpoint are taken here to be those at approximately lat 5° S (between the northern and central Andes) and 36° S (between the central and southern Andes; Figs. 1, 2). The central Andes are the most highly mineralized of the three segments and contain all economically important Cu depOSits and numerous prospects, whereas Cu occurrences are limited in number in the northern Andes and even scarcer in the southern Andes (Fig. 1).

Five metallogenic epochs and their corresponding Cu belts are introduced in the remainder of this section. The epochs span the late Paleowic, Mesowic, and Cenowic, and the belts show progreSSively greater metal endowment with de­creasing age (Sillitoe, 1992; Fig. 3). Outcropping pre-Permian rocks within the Andean orogen lack important Cu prospects, with the exception of the volcanogeniC massive sulfide (VMS) occurrence at Bailadores in the Merida Andes (Fig. 1) of Venezuela (Carlson, 1977).

Late Paleozoic to early Mesozoic

At least 13 porphyry Cu-Mo systems in the central and southern Andes have been dated as Early Permian to Early Jurassic (286-198 Ma; Sillitoe, 1977; Marquardt et al. , 1997; Williams et aI., 1999; Camus, 2003; Figs. 2, 4a). The systems define a discontinuous, composite helt in which potassic al­teration wnes (e.g., La Voluntad, Cerro Samenta) and exten- _ sive advanced argillic lithocaps (e.g., Lila) are prominent. Documented Cu contents are generally <0.2 percent, al­though the San Jorge prospect, containing both oxidized and enriched mineralization, is an exception (Williams et al., 1999). The Cobriza Cu deposit in central Peru (Fig. 4a), a re­duced calcic skarn containing pyrrhotite and arsenopyrite (Petersen, 1965), has a reported K-Ar age of 263 ± 8 Ma for hydrothermal amphibole (Noble et al. , 1995). This determi­nation supports assignment of Cobriza, the only presently ex­plOited Cu deposit of pre-Jurassic age in the Andes, to a Late Permian to Triassic rifting event characterized by bimodal magmatism (Kontak et al. , 1985; Sempere et al., 2002).

The six Early Permian (286-272 Ma) porphyry Cu <x:cur­rences in Chile and western Argentina (Fig. 4a) define a tem­porally restricted metallogenic epoch and clearly all ()(.'Cur in the less eroded parts of a volcanoplutonic arc of this age that developed along the Pacific margin of Gondwana (M podozis and Ramos, 1990). At least part of the arc was G"Onstructed on relatively thin continental crust (Mpodozis and Kay, 1992), al ­though the region studied by them lacks known porphyry­type manifestations. In contmst. those occurrences in Chile and western Argentina that yielded Late Permian to Early JurasSiC ages, if correct, are an integral part of the granite­and rhyolite-dominated Choiyoi large igneous province (Kay et al. , 1989), a mnsequence of lower-crustal anatexis and re­gional extension that intervened between the Carboniferous to Early Permian and Mesozoic subduction regimes (Kay et al ., 1989; Mpodozis and Kay, 1992). Such nonsubduction

848

80' la'

PANTANOS-PEGAOORCI TO ( '-____ ,-'IIt.

la'

ESCONDIDA

30'

SILL/TOE AND PERELL6

BRAZIL

60'

MAIN Cu BELTS

c:::J Miocene-early Pliocene

o Middle Eocene-early Oligocene

Iiiiii:I Paleocene-early Eocene

Late Cretaceous

G:J Early Cretaceous

II!fJI Jurassic

Fe oxide-Cu-Au

~ Early Cretaceous o Middle-LateJurassic

6:iJ l ate Paleozoic-early Mesozoic

DEPOSIT TYPES

• Porphyry Cu-{Mo, Au)

• Porphyry-related skarn Cu-(Mo, Au, Zn)

t::.. Skarn

... Tourmaline breccia

Enargite vein

~ Enargite-bearing replacement * Fe oxide-Cu-Au o Volcanogenic massive sulfide

CJ Red bedeu

Qlj Group of manto-typeCu deposits

• Group of red bed Cu deposits

Major tectoni::: discontinuity

BOLIVIA

ARGENTINA

r --j-SAN JORGE (263-257)

~j."j~"'OC'ZA·'f'" am VERDE (1().9)

SOOkm

50' LA VOLUNT AD (287)

FIG. 2. Coppe r be lts of the Andes, showing selected deposits and prospects and their genetic type (modified from 5illi­toe, 1988, 1992). Obvious longitudinal gaps (e .g., in Colombia and Ecuador) reflect paucity of data. Also shown nre the three main tran sverse discontinuities in the Andes. Numbers in parentheses after deposit names are isotopic ages (approximated ), taken from compilations by Sillitoe (1 988, 2003), Vila and Sillitoe (1991), Noble and Mc Kee (1999), Cendall e t al. (2000), and Perell6 et al. (2003a), with additions from Losada-Calderon e t ai, (1994), Noble et a!. (1995, 2004), Urbina et al. (1997), R. H. Sillitoe (unpub. data, 1997), Perel16 et al. (1998, 2001 , 2003b), Sasso and Clark (1998), J. D. Lowell (unpub. rept. , 1999), Bissig et al. (2001 ), Howland and Clark (2001), Douzari and Clark (2002), Camus (2003), Cotton (2003), Quang et al. (2003), Bendezu et al. (2004), Gustafson et at. (2004), and Rivera and Pardo (2004).

ANDEAN Cu PROVINCE

Fine Cu (million metric tons)

300 250 200 150 100

Time (Ma) ~ Mlocene-earty Pliocene bett

Middle Eocene-earty Oligocene belt

~ Paleocene-earty Eocene be~

~ Middle-late Mesozoic belts

~ Late Paleozoic-early Mesozoic bett

50 o

FIC. 3. Total Cn contents of the late Paleozoic to early Mesozoic, middle to late Mesozoic, Paleocene to early Eocene, middle Eocene to early Oligocene, and Miocene to early Pliocene Cn belts of the Andes. Only geo­logic resources and past production are included. Copper present almost ex­clusively in porphyry Cn deposits except for the middle to late Mesozoic belts, in which Fe oxide-Co-Au, manto-type. and VMS deposits are also im­portant. Note that part of the Cll contained in the middle Eocene to early Oligocene deposits underwent superge ne concentration and redistribution during the Miocene (see text for discussion). Compiled from Camus (2003) and sources listed in Tables 1 through 4.

settings are not normally considered to be conducive to por­phyry Cu generation (Sillitoe, 1999).

Middle to late Mesozoic

Two principal middle to late Mesozoic Cu belts are defined in the Andes (Figs. 2, 4). The best documented and most im­portant economically occurs in the Coastal Cordillera of the central Andes from southern Peru through northern to cen­tral Chile (lat 13°-34° S). The other, without Cu production to date, occupies an inland position within the northern Andes, where it follows the Eastern Cordillera of Colombia and the Subandean belt of southern Ecuador (lat 7" N~ S); it has not been recognized farther south than the border with Peru. The Middle Jurassic and Early Cretaceous parts of the coastal southern Peru to central Chile belt, described in greater de­tail in the follOwing section, are characterized by varied Cu metallogeny but relatively unimportant porphyry Cu mineral­ization. In contrast, the inland Colombia-Ecuador belt hosts exclusively porphyry Cu and small skarn prospects (Sillitoe et al., 1982; Gendall et al. , 2000; FIg. 4a).

The intra- or backarc Lancones basin of Early Cretaceous age in northernmost Peru is host to a cluster of Cu-rich VMS prospects in the Tambogrande district (Fig. 4a), which could be considered as a northern extension of the coastal middle to

late Mesozoic Cu belt in southern Peru. Unlike VMS d poIll at the northern extremity of this latte r belt (e.g., Cerro Undo; Fig. 4a), which are of Kuroko type (Vidal, 1987; G.ri~py ond Hinostroza, 2(03), those at Tambogrande are assigned 10 Ih bimodal mafic category (Winter et al., 2004).

In contrast to the western Middle to Late JurasSiC and Early Cretaceous parts of the coastal southern Peru to central Chile belt, its eastern fringe, of Late Cretaceous age, is rather poorly endowed with Cu. Nevertheless, at least six porphyry Cu oc­currences in northern Chile, as well as three more in the southern Andes of Chile and adjOining Argentina (lat 28"--39" S), have Late Cretaceous ages (98-73 Ma; Fig. 4a). The only Cu production is from a small supergene enrichment horizon in the Domeyko district, although the enriched Zafranal prospect in southern Peru may also be of this age (Fig. 40).

In the Mesozoic Cu belt of Colombia and Ecuador, the prinCipal porphyry Cu prospects are Mocoa in southern Colombia (Sillitoe et al ., 1984) and Panantza, San Carlos, and Mirador in the Pangui trend of southern Ecuador (Gendall et al., 2000), all of Middle to Late JurasSiC age (Fig. 4a). The mineralized centers are associated with small porphyry stocks that intrude mainly I-type Jurassic batholiths. The prospects in Ecuador contain relatively high hypogene Cu grades (-0.7% Cu) and variable but modest Mo and Au contents (up to 0.2 glt). Supergene oxidation and enrichment are imma­turely aeveloped because of high erosion rates. The inland position of the Mesozoic belt in Colombia and Ecuador is a result of latest JurasSiC and younger accretion of oceanic ter­ranes to the western margin of the northern Andes (Aleman and Ramos, 2000 and references therein).

Small sediment-hosted Cu deposits occur in two Late Cre­taceous backarc settings in Argentina-the Salta basin in the central Andes and the Neuqu~n basin of the southern Andes (Fig. 2). The main Cu-(Ag) prospects of the Salta basin are of Kupferschiefer type and occur in shallow-marine or lacus­trine limestone and calcareous siltstone above a rift-fi.lling, subaerial red-bed sequence (Peral and Wormald, 1999). In contrast, the red-bed Cu-(U -V) mineralization in the Neuquen basin is hosted by bleached zones in a red subaerial sandstone-siltstone package (Lyons, 1999). The disseminated mineralization, rich in hypogene chalcocite, was preCipitated from basinal brines by mainly diagenetic pyrite in the Salta basin and plant material in the Neuquen basin (Peral and Wormald, 1999; Lyons, 1999).

Cenozoic

The Cenozoic Cu metallogeny of the Andes is characterized by three principal epochs giving rise to spatially restricted, east­ward-younging belts (Sillitoe, 1988; Fig. 2): (1) a Paleocene to early Eocene belt from southern Peru to central Chile, (2) a middle Eocene to early Oligocene bell over broadly the same latitudinal extent of southern Peru and c-entnu Chile, and (3) a Miocene to early Pliocene belt extending from northern Colombia to central Chile and adjOining west-central Ar­gentina. These three e<;onomically preeminent belts are de­scribed in greater detail in succeeding sections of this paper.

An additional, but poorly defined belt of Paleocene to mid­dle Eocene deposits in the northern Andes of northwestern Colombia (Fig. 2) overlaps in age the two oldest Cenozoic belts defined in the central Andes. The Paleocene to middle

850

c.mog;. Ridge

(102-1001

a

o I

80·

500km I

SILL/TOE AND PEREU6

b BRAZIL

BOUVIA

Fe oxide-Cu-Au belts _ Early Cretaceous

_ Middle-Late

Jurassic

Fe oxide-Cu-Austyles • Vein .. S!<am 6, Breccia • Composite

_ Manto-typeCu

.:.:-:. =g~~=:s """'" (123-1'1)

ARGENTJNA

DEPOSIT TYPES

• Porphyry Cu-M::l • Porphyry Cu-Au ... Breccia pipe

'" Skarn • Volcanogenic

massive sulfide

Cu BELTS .. Late Cretaceous

~ Early Cretaceous

Middle-lateJurassic

1+ .. .. +1 P8f1l1ian-Earty Jurassic

_ Major tectonic discontinuity

Flc.4. Late Paleozoic to early Mesozoic and middle to late Mesozoic Cll belts of the Andes. a. Porphyry Cu. breccia pipe, skarn, and VMS deposits and prospects. h. Iron oxide-Cu-Au and manto-type Cll deposits and prospects. Also shown are the three main transverse discontinuities in the Andes. Numbers in parenthesis after deposit names are isotopic ages (approxi­mated). taken from compilations by Sillitoe (1988, 20(3) and Camus (2003), with additions from Noble et al. (1995), R. H. SiUitoe (unpub. data, 1997), Gendall et al . (2000), Pere1l6 et al. (2003b), J. Pere1l6 (unpub. data, 20(4), Winter et aI. (2004),

and Quang et 01. (2005).

ANDEAN C .. PROVINCE 851

Eocene belt occurs in an allochthonous island-arc terrane ac­creted to the northern Andean margin (Sillitoe et al., 1982; Aspden et al., 1987). If formed prior to accretion, as seems likely (Cediel et al., 2(03), the belt would be the only oceanic island-arc porphyry Cu environment in the Andes. The four principal known prospects that define the belt, including Rio Pito in contiguous southern Panama, are spatially related to precursor batholiths and major orogen-parallel faults with ev­idence for ductile as well as brittle motion. The prospects are geologically varied, with M urind6 being the most noteworthy because of the Au- and magnetite-rich potassic alteration wne (Sillitoe et al., 1982).

Small red-bed Cu deposits and prospects occur in the cen­tral Andes, particularly at Corocoro and Chacarilla in north­western Bolivia (Entwistle and Gouin, 1955; Ljunggren and Meyer, 1964), San Bartolo in neighboring Chile (Flint, 1986), and in southern Peru (Fig. 2); none of them is currently in production. The mineralization is hosted by subaerial sand­stone and siltstone sequences that accumulated in Oligocene to Miocene molasse basins and is closely associated with evaporitic diapirism (Evernden et al., 1977; Cox et al. , 1992). The chalcocite and native Cu mineralization, in part associ­ated with elevated Ag contents, is present chiefly in dissemi­nated form and was probably introduced by evaporitic brines (Sillitoe, 1990; Cox et al. , 1992).

Middle Jurassic to Early Cretaceous Belt of the Central Andes

Mineralization types and economic significance The Middle jurassic to Early Cretaceous Cu belt of the cen­

tral Andes is semicontinuous for a distance of 2,300 lan, from southern Peru to central Chile, where it follows the Coastal Cordillera along the Pacific littoral (Fig. 1) throughout, and av­erages -30 Ian in width (Fig. 4). As noted above, the belt may be continued northward to include the Tarnbogrande VMS de­posits (Fig. 4a). The belt contains a variety of Cu deposits , in­cluding porphyry Cu, Fe oxide-Cu-Au, and manto Cu types and, north of latitude 13" S, Cu-bearing VMS deposits, the largest in southern Peru being at Cerro Lindo (Gariepy and Hi­nostroza, 2003; Fig. 4a; Table 1). The largest historic and cur­rent Cu producers in the belt (Table 1) are porphyry Cu (An­dacollo), Fe oxide-Cu-Au (RaUl-Condestable, Mantoverde, Candelaria-Punta del Cobre), and manto-type Cu (Michilla, Mantos Blancos, El Soldado, Lo Aguirre) deposits. The por­phyry Cu prospects contain Mo but are relatively poor in Au, with the notable exception of Andacollo (Uaumett et al. , 1975).

The main Middle Jurassic to Early Cretaceous deposits had a total combined production in 2003 of -600,000 metric tons (t) of fme Cu (Table I ), which represented 10 percent of total Andean production. In addition, local companies and arti­sanal miners exploit widespread small deposits in the north­ern Chile part of the belt.

Tectonomagmatic setting

The Middle Jurassic to Early Cretaceous Cu belt of the central Andes is hosted by volcanoplutonic arcs constructed on Precambrian metamorphic rocks in southernmost Peru and on a late Paleowic subduction complex, Permian grani­toids, and Triassic volcanosedimentary sequences south of

latitude 26° S in northern Chile (Ramos, 2000, and references therein). The arcs and intra-arc basins comprise several thou­sand meters of subaerial to shallow submarine volcanic rocks dominated by lavas of basaltic to undesitic composition (e.g., Boric et al. , 1990; Mpodozis and Ramos. 1990, Pichowiak, 1994). The volcanic sequences underwent prehnite-pumpel­lyite to greenschist facies (burial) metamorphism during arc construction (Levi et al., 1989; Atherton and Aguirre, 1992). Several interconnected marine-sedimentary hm.:karc basins bound the arc terrane to the east (M podozis und RUlllos, 1990).

A series of orogen-parallel major fault systems cut the Cu­bearing volcanoplutonic arcs in southenl Pent and northern Chile. The best known of these is the Atacama fault system that extends for >1,000 Ian between lat 20" and 30" S (Fig. 5), where it consists of concave-west, ductile and brittle fault seg­ments that underwent variable motion (Herve, 1987; Scheuber and Andriessen, 1990; Brown et al., 1993). Tran­sient ductile deformation, charted by mylonite formation (e.g., Scheuber and Andriessen, 1990; Scheuber et al., 1995), occurred at shallow to intermediate crustal levels (dO Ian) in close association with syntectonic pluton emplacement but gave way to brittle behavior during arc cooling (Brown et al ., 1993). Between lat 22° and 27" S, motion on the Atacama fault system switched from normal slip to sinistral transten­sion at -132 Ma (Scheuber and Gonzalez, 1999; Grocott and Taylor, 2(02), a change that apparently instigated an impor­tant epoch of Cu mineralization (see below). Normal faults along the entire eastern margin of the Coastal Cordillera and farther east were inverted during early Late Cretaceous trans­pression (Mpodozis and Ramos, 1990; Lara and Godoy, 1998; Benavides-Caceres, 1999).

The plutOniC complexes and extensional and transtensional faulting in the northern Chile part of the belt young system­atically eastward from Early JurassiC through Early Creta­ceous (Farrar et al., 1970; Berg and Baumann, 1985; Dallmeyer et al., 1996), although the pattern is still poorly re­solved in southern Peru (Clark et al ., 1990). Copper deposits have the same overall ages as their host or nearby plutons and, hence, also young systematically eastward. On the basis of available geochronologic data, Sillitoe (2003) defined Mid­dle to Late Jurassic (170-150 Ma) and Early Cretaceous (130-110 Ma) sub-belts of Fe oxide-Cu-Au deposits (Fig. 4b; Table 1). Dated porphyry Cu deposits also fall broadly into these same two time intervals (Sillitoe, 2003; Fig. 4a). Two main groups of manto-type Cu deposits occur, one in the JurasSiC arc of northern Chile at apprOximately 150 to 140 Ma (Michilla, Mantos Blancos) and the other in the Early Creta­ceous intra-arc basin of central Chile at - 110 to 100 Ma (EI Soldado, Lo Aguirre; Figs. 2, 4b; Table I ), although exact ages are somewhat equivocal (Maksaev and Zentilli, 2(02). The Cu-rich VMS deposits in central and northern Peru formed at approximately 100 Ma (Tegart et aI., 2000; Gariepy and Hi­nostroza, 2(03).

The plutons in the Middle Jurassic to Early Cretaceous Cu belt range in composition mainly from gabbro and diorite, through quartz diorite and tonalite, to granodiOrite, locally with phases as felsic as monwgranite; they constitute the coastal batholith . Individual plutonic complexes were em­placed over intervals of 3 to 14 m.y. (Dallmeyer et al., 1996).

TABLE 1. Selected Geologic Characteristics of POrphyry Cu, Fe oxide-Cu-Au, Manto-type Cu, and VMS Deposits and Principal Prospects, Middle Jurassic to Early Cretaceous Belt, Central Andes ~ '" Status and Production +

2003 reserves (million Ore-related Deposit! production' metric tonnes) hypogene prospect (metric tons and grade (%) Intrusion alteration Mineralization Structural (type) Cu x 1,000) (cutoff, % Cu) composition Host rocks (J!lineralization) Age (Ma) style control Key references

Quartz diorite Cretaceous RaUl- porphyry body, marine volcano- Veins, mantos, Vidal el aI. (1990), Condestable Mine Sulf: >25@ 1.7 Cu, dacite porphyry sedimentary Zircon: and disseminated Im~rtant : NW de Haller et al. (Fe oxide-Cu-Au) 11.2 0.9 gil Au, 6 gil Ag dikes rocks Calcic (ep) 116.5-113 bodies an NE faults (2002)

Massive sulfide Present: Cerro Lindo Sulf, 41.6@0.8Cu, Andesite Cretaceous Late Early lenses and intersecting NW Cari~py and (VMS) Prospect 5.1 Zn, 35 glt Ag porphyry dikes volcanic rocks Sericitic (cp) Cretaceous stockworks and NE faults Hinostroza (2003)

Cretaceous Irre~lar Andesite and volcaniclastic Calcic + vein ike Injoque et at.

Mina Justa Ox + sulf: 209 @ dadle porphyry rocks and K-feldspar Biotite: 160 ± 4; replacement Important: NE (1988), Moody el (Fe oxide-Cu-Au) Prospect 0.86 Cu, 0.1 gil Au dikes andesitic sill (ep, bn, ee) Sericite: 154 ± 4 body listric fault aI. (2003)

Potassic, inter-Composite Jurassic volcanic mediate argillic, Important:

~ diorite to rocks and sericitlc, and intersecting Buey Muerto 0" 219@ granodiOrite monzodiorite advanced Biotite: 137 ± 4; Porphyry Cu NNE and ~

(porphyry Cui Prospect 0.36 Cu (0.2) porphyry slock stock argiIlie (ep) Sericite: 132 ± 4 stockwork NW faults Perell6 et aI. (2003b) Cl '" Microdiorite ~ intrusions: Important:

Michilla Mine Ox + sulf: 65 @ Microdionte and JurasSiC Sodie (ee-dg, 159.9.0.7- Steep breccias ENE-trending Wolf el aI. (1990), ."

(manto-type Cu) 52.7 1.0 Cu andesite dikes volcanic rocks bn, cp) 137.4. l.l and mantas dikes TIistd el aI. (2003) ~ t-

Tassinari et al. t-o. Ox: 154 @ 1.04 Cu. JurasSiC volcanic Sericite: ISO ± 4; Important: (1993), Ramirez

Mantos Blancos Mine Sulf, 115.9 @ 1.20 and sub- Sodie (bn, Rb-Sr error- Mantas, lenses, NW,NS, (1996), O' rego el (manto-type Cu) 147.1 Cu, -14 gil Ag Andesite dikes volcanic rocks ee,cp) chron: 158 ± 6 and veins and N E faults al. (2003)

V;la el aI . (1996), 0" 212 @ 0.63 Cu. Potassic, Vein breccia, Orrego et al.

Mantoverde Mine Sulf: >400 @ 0.52 JurasSiC chloritic, and SeriCite: 123 ± 3, stockworks, and Important: (2000), Zamora and (Fe oxide-Cu-Au) 60.2 Cu, 0.11 gil Au Diorite dikes volcanic rocks sericitic (cp) 121±3,1l7±3 breccia manto NNE fault Castillo (2001)

Biotite: 116.51 ± 0.26,115.14. 0.18, 114.9 • 1.0, 114.6. 1.6,

Candelaria sulf: 114.1 ± 0.7; 470 @ 0.95 Cu, Amphibole: 116.6 0.22 gil Au; Cretaceous ± 1.2, 111.7 ± 0.8; lly.n el aI. (1995),

Candelaria-Punta Punta del Cobre volcanic and Molybdenite: Mantos, breccias, Important: Marschik and del Cobre (Fe Mines sulf: - 120 @ 1.5 Diorite and volcaniclastic Potassic and 115.2.0.6, veins, and NWand FonlboM (2001), onde-Cu-Au) 257.0 Cu, 0.2-<J.6 gil Au dacite dikes rocks sadie-calde (ep) 114.2.0.6 stockwarks NNW faults Mathur et aI. (2002)

SulE 300 @ 0.70 Uaumett et al. AndacoUo Mine Cu, 0.015 Mo, Tonalite Cretaceous Potassic and Sericite: Po~yryCu Present: NW (1975), Munizaga (porphyry Cui 20.0 0.23 gil Au porphyry volcanit rocks sericitic (cp) 112 ± 10 sloc ork and NNW faults el aI. (1985)

TABLE 1. (Cont.)

Status and Production + 2003 reserves (million Ore-related

Depositl production! metric tonnes) ~ene prospect (metric tons and grnde (%) Intrusion teration Mineralization Structural (type) Cu x 1,000) (cutoff. % Cu) composition Host rocks (mineralization) Age (Ma) style control Key references

Holmgren (1987), Cretaceous Steep. irregular Important: Wilson and Zentilli volcanic and K-feldspar: structurally con- intersecting NS. (1999), Boric e' aI.

Mine volcaniclastic Sooic + chlorite 106.0. l.l- IOO.5 trolled bodies EW, and (2000), Wilson et EI Soldado 70.5 Sul[ 476@ 1.11 Cu Andesite dikes rocks (bn, ce, cp) ;t 1.5 and veins NW faults 01. (2003)

Albite: 102 :% 5 Whole rock: Important:

Sodic + 110 * 4; NW-EW, NS- Munizaga et al. Abendoned Sulf. 19@ Diorite body and Cretaceous chlorite-sericite Rb-Sr isochron: Mantos and NNE, and (1988). Sanc et aI.

1..0 Aguirre mine 1.66 Cu (0.4) andesite dikes volcanic rocks (bn, ce, cp) 113.3 breccias NE faults (2003)

Abbreviations: bn '" bornite . cc E chalcocite, cp '" chalcopyrite. dg IE digenite. Ox ". oxide. Sulf :: sulfide ! Figures from Comisi6n Chilena del Cobre and Direccl6n General de Mine ria (Peru)

or. Z g' gs,;;;-"Il S'~ S' fl So-rg~ ~;'~CI) "" til III ..... S:-cq3 2 ~~n2' ;- -.< g;: eL 3 Q!= ~ · ~ '" ..... ~ cr ::r~ =. · ~ , " '" ;;;- ::5' 0 '" e (11:1 -.

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854 SILLITOE AND PERELL6

Abundant basaltic to andesitic dikes cut many of the plutons and their country rocks. Iron oxide-Cu-Au deposits, particu­larly those of vein type, share faults with diorite porphyry dikes, whereas the porphyry Cu deposits are centered on small stocks of diorite to granodiOrite porphyry (Sillitoe, 2003). These minor intrusions, in common with the Jurassic and Early Cretaceous plutOniC rocks, are hornblende bearing, metaluminous, and calc-alkaline, although the gabbros are tholeiitic in character (e.g., Pichowiak et aI., 1990; Pichowiak, 1994). All the intrusions are I type (Ishihara and Ulriksen, 1980), with Sri of 0.704 to 0.705 in the Middle to Late Juras­sic sub-belt and 0.703 to 0.704 in the Early Cretaceous sub­belt, the more primitive Early Cretaceous ratios taken to in­dicate maximal crustal thinning (McNutt et aI. , 1975; Berg and Baumann, 1985; Pichowiak, 1994).

Principal mineralization features

The Fe oxide-Cu-Au mineralization is characterized by a variety of styles, including veins through breccia pipes, calcic skarns, and composite deposits comprising varied combina­tions of hydrothermal and tectonic breccias, stockwork wnes, and replacement horiwns besides veins (Sillitoe, 2003; Table 1). The largest depoSits are composite, as exemplified by Mantoverde (Vila et aI., 1996) and Candelaria-Punta del Cobre (Marschik and Fontbote, 2001). Laterally and verti­cally extensive Fe oxide-Cu-Au veins commonly occur as swarms cutting diorite or, locally, more felsic plutons, with diorite porpbyry dikes along the same controlling structures being immediately pre- to synmineralization in timing. In contrast, the composite deposits are hosted by volcanosedi­mentary sequences in proximity to plutons, with some exam­ples concealed beneath largely barren carbonate units (Ryan et aI., 1995; Sillitoe, 2003).

The Fe oxide-Cu-Au deposits are defined by their abun­dance of magnetite and/or specular hematite, which are ac­companied by mainly chalcopyrite, pyrite, pyrrhotite, and ar­senopyrite, although bornite is recorded locally. A marked feature of several depoSits (e.g., Candelaria-Punta del Cobre, Mantoverde) is an upward and/or outward change from mag­netite to hematite. A distinctive Fe-Cu-Au-Co-Ni-As-Mo-U geochemical signature characterizes the depoSits. Alteration in the composite deposits is complex and comprises varied combinations of potassic, calcic, and sodic assemblages (Table 1). Magnetite-dominated deposits tend to have a gangue composed of actinolite and apatite ± clinopyroxene, whereas those with hematite are typified by chlorite and sericite (SilIi­toe, 2003, and references therein).

The manto-type Cu deposits occur as strata-bound dissem­inated bodies, steep hydrothermal breccias surrounding bar­ren, fingerlike, diorite plugs, and related veins, mostly within the basaltic to andesitic volcanic sequences (Table 1). How­ever, the largest deposit, Mantos B1ancos, is unique in being partly hosted by felsic volcanic rocks and subvolcanic intru­sions (Ramirez, 1996; Orrego et aI., 2003). The highest grade parts of manto-type deposits, typically controlled by highly permeable faults, hydrothermal breccias, dike contacts, vesic­ular flow tops, and flow breccias, are characterized by hypo­gene chalcocite and bornite, which grade outward and down­ward through chalcopyrite plus pyrite to minor distal concentrations of pyrite, reflecting depletion of Cu in the

mineralizing fluids (Sillitoe, 1990; Maksaev and Zentilli, 2002). The mineralization typically contains byproduct Ag but no appreciable Au. The chalcocite-bornite cores of the larger deposits are commonly localized at original redox boundaries in the host stratigraphic packages and are overlain or flanked by sulfide-deficient wnes containing hypogene hematite (SiI­Iitoe, 1992; Kirkham, 1996). Albite, chlorite, and quartz are the main gangue minerals in the cores of deposits (Table 1).

The Middle Jurassic to Early Cretaceous porphyry Cu de­posits tend to be dominated by potassic (biotite, K-feldspar) and overprinted intermediate argillic (chlorite, greenish sericite, illite, and/or smectite) assemblages, although sericitic (quartz-sericite [fine-grained muscovite]) alteration occurs at Andacollo (Uaumett et aI., 1975) an1 Buey Muerto, where traces of pyrophyllite and dickite are also present (Pere1l6 et aI., 2003b; Table 1).

Paleocene to Early Eocene Belt of the Central Andes

Mineralization types and economic Significance

The Paleocene to early Eocene belt, the third largest Cu concentration in the central Andes, extends for -1,900 km from southern Peru to central Chile (Iat 17"-32° S) and aver­ages 30 to 50 km in width (Fig. 6). The belt is defined princi­pally by porphyry-type depoSits but also includes several groups of generally small, tourmaline-bearing breccia pipes, especially south of latitude 26° S (e.g., Ruiz et aI. , 1965; SilIi­toe and Sawkins, 1971), an enargite-bearing vein deposit at EI Guanaco and an Fe oxide-Cu-Au vein at Dulcinea. The largest breccia deposits are Cerro Negro, near Cerro Verde­Santa Rosa, southern Peru, and Santa Catalina in the Sierra Gorda district of northern Chile, both located in porphyry Cu districts. The largest porphyry Cu deposits are all Mo rich and occur near the northern termination of the belt, at Cerro Verde-Santa Rosa, Cuajone, Quellaveco, and Toquepala (Fig. 6). The most important porphyry Cu-Mo deposits farther south are at Cerro Colorado and Spence. Porphyry Cu-Au prospects are small, such as Carmen (Rivera and Peri, 1991; Camus, 2003) and others plotted in Figure 6.

The Paleocene to early Eocene deposits have accounted for much of Peru's Cu production since 1960, when Toquepala came on stream, and still contribute roughly 55 percent of the nation's output. In contrast, the Cerro Colorado and Lomas Bayas depOSits, the main producers in the northern Chile part of the belt, had startups in 1994 and 1998, respectively. In 2003, the porphyry Cu deposits of the belt were responSible for production of 619,000 t of fine Cu (Table 2).

Tectonomagmatic setting

The Paleocene to early Eocene belt in southern Peru is sit­uated along the oceanward slopes of the Western Cordillera at elevations of 3,000 to 3,700 m, whereas in Chile it lies im­mediately east of the Longitudinal Valley at lower elevations (Fig. 1). All the main porphyry Cu-Mo deposits in the belt are hosted by Late Cretaceous to Paleocene, calc-alkaline, an­desitic to rhyolitic volcanic rocks, which are intruded in places by batholiths emplaced immediately prior to the porphyry stocks (e.g., Beckinsale et aI., 1985; Clark et aI., 1990; Marinovic and Garda, 1999). The silicic volcanic rocks in the northern Chile part of the belt define a series of large ash-flow calderas

ANDEAN Cu PROVINCE 855

10'

0'

10'

o I

80"

NOPlTO~ L DEPOSIT TYPES

• Porphyry Cu-Mo

10'

• Porphyry CrAu

t:. Tourmaline brecca

= Enargite vein

- K>CG

_ Major tectoni:: dSCOfltiflUity

COLOMBIA

BRAZIL

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SANTA AOSA.IG)

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"""", .. +-""1::" """"''''''''''''''' .... 20'

ARGENnNA

Flc . 6. PaJeocene to early Eocene Cu belts of the Andes, showing main deposits and prospects and their genetic type. Also shown are the three main transverse discontinuities in the Andes. Numbers in parenthesis after deposit names are isotopic ages (approximated), taken from compilation by Sillitoe (1988). with .dwtions from Sillitoe et aI. (1982). Boric et aI. (1990). J. D . Lowell (unpub. rept.. 1999). Rowland and Clark (2001). J. Perell6 (unpub. data, 2002, 2004). Bouzari and Clark (2002), Camus (2003), P. Fem4ndez (pe ..... commun .• 20(3). and Quang et aI. (2003).

(e.g., Arevalo e l al.. 1994 ; Hivem and Mpodozis. 1994; Cornejo et al .. 1997). Precambrian basement crops out only in the northernmost part of the belt but is assumed to extend to at least latitude 26" S (Hamos. 2000, anu references therein). Substantial parts of the belt nurth of llItitude 21 ° S are concealed beneath thick sequences of lute Oligocene and younger ignimbrite sheets (e.g., Tosdal et III.. 1984).

The southern Peru part of the belt . except fo r the Cerro Verde-Santa Rosa deposit, lies alongside the steeply dipping, orogen-parallel Incapuquio fault system. which Is Imceable for 140 Ian to the vicinity of the border with Chile. The poorly understood fault system was clearly active during p0'l,hyry Cu emplacement (Zweng aod Clark, 1995; Concha and Valle. 1999). The Incapuquio fault itself shows evidence for normal. sinistral transcurrent, and reverse motion (Wilson and Garda, 1962; Jacay et al. , 2(02). It is assumed that the Incapuquio fault system facilitated at least part of the contraction and up­lift that accompanied porphyry Cu emplacement (Noble et al. , 1985; Sandeman et al , 1995; Benavides-C~ceres, 1999). In northern Chile, however, ma/'or orogen-parallel faults have not been recognized in the be t, which was underlain by rela­tively thin crust under a moderately extensional stress regime and characterized by discontinuous normal faulting (Arevalo et al ., 1994; Cornejo et al., 1994; Cornejo and Mpodozis, 1997).

The dated hydrothermal breccia pipes in the northern Chile part of the belt range in age from 66 to 60 Ma (Sillitoe, 1988; Fig. 6), whereas most of the large porphyry Cu-Mo de­posits are appreciably younger (62--51 Ma; Boric et aI. , 1990; Rowland and Clark, 2001; Bouzari and Clark, 2002; Camus, 2003; Quang e t aI. , 2003; Fig. 6, Table 2). The porphyry Cu­Au prospect at Carmen yielded a 66 Ma age (Camus, 2(03), similar to that of nearby tourmaline breccia pipe clusters (Fig. 6). The largest porphyry Cu deposits in the belt concluded arc development (e.g., Saodemao et al ., 1995).

The porphyry Cu stocks are multiphase intrusions with variable compositions ranging from tonalite to quartz mon­zonite (Table 2). These stocks in the southern Peru part of the belt reported Sr; between 0.7044 and 0.7054 (Beckinsale et al., 1985). The granodiOrite porphyry at the Cerro Verde­Santa Rosa deposit has a LaIYb ratio of 20 to 22 (Le Bel. 1985), indicating thickened crust, and substantially higher than those (&--10; Williams, 1992) for porphyry Cu and brec­cia pipe districts underlain by attenuated crust in the north­ern Chile part of the belt.

Principal mineralization features

The Paleocene to early Eocene porphyry Cu depoSits are characterized by potassic, sericitic, intermediate argi llic, and advanced argillic (quartz, alunite, andlor pyrophyllite, dickite) alteration (Table 2). Potassic assemblages are ubiquitous and typically formed Simultaneously \vith stockwork chalcopyrite and minor bornite mineralization , except at Cerro Colorado where early biotite-albite alteration is deficient in Cu and in­termediate argillic assemblages constitute the main Cu-bear­ing stage (Bouzari and Clark. 2(02). Sericitic alteration over­printing the potassic assemblages is also commonly Cu bearing and was responsible for approximately 60 to >90 per­cent of hypogene Cu tenor at Cuajone, Quellaveco, and Toquepala (Zweng and Clark, 1995; Concba and Valle, 1999).

TABLE 2. Selected Geologic Characteristics of POrphyry Cu Deposits and Principal Prospects, Paleocene to Early Eocene Belt, Central Andes ~ Ol

Status and Production + 2003 reseJ'Ves (million Ore~related production l metric tonnes) hypogene

Deposit! (metric tons and grade ('h) Po<phyry Host alteration Hydrothennal Massive Structural prospect C u x 1,(00) (cutoff, % Cu) composition rocks (mineralization) Age (Ma) breccias su16de veins control Key references

Cretaceous~ Sericite Cerro Pa1eogene Verde: 61 .8 :t

Suli, 1,470 @ volcanic and Potassic 0.7,62.0:t 1.1; 0.55 Cu, 0.015 Mo 1\vo phases: intrusive rocks, overprinted Sericite Santa Severn! Kihien (1975), (including super~ quam. Precambrian and surrounded Rosa: 62.2 :t 2.9; phases with Absent. but Important: Mathur et aI.

Cerro Verde· Mine gene, 270 @0.8) monzonite metamorphic by sericitic Molybdenite: tounnaline and tetrahedrite NW regional (2001), Quang et Santa Rosa 86.4 Ox: 85@ 1.1 Cu rocks (cp, bn') 58.6-58.9 • 0.3 dumortierite. present fa ults aI. (2003)

Manrique and Suli, 1,400 @ Plazolles (1974), 0.64 Cu, 0.033 Sericite: Satchwell (1983), Mo (0.4) (in- Cretaceous~ Potassic 52.3. 1.6; SeveraJ phases: Absent. but Important: Concha and Valle

Mine c1uding super· Three phases: Paleogene overprinted by Molybdenite: igneous enargite majorNW (1999). Mathur et Cuajone 168.0 gene, 75 @ 1.5 Cui quartz latite volcanic rocks sericitic (cp. bn) 53.4 :t 0.3 breccias present regional fau lts aI. (2001)

Poorly defined Severa] phases: concentric pattern '" monzonite. Cretaceous~ with potassiC NW.trending Estrada (1975), ~ quartz mon~ Paleogene surrounded Minor igneous intrusions and Clark et al. § Suli, 1.670 @0.56 zonite. and volcanic and and overprinted breccias and alteration (1990), Kihie n

'" Quellaveco Prospect Cu, 0.02 Mo (0.3) quartz diorite intrusive rocks by sericitic (cp) Biotite (?), 54.5 pebble dikes Absent geometry (1995)

~ Potassic and Ma~or complex Cretaceous· potassic·sodic wit five phases; Zweng and Clark tll Paleogene overprinted by Sericite(?): tounnaline brec~ Absent, but Important: (1995), Mattos '" '" volcanic and sericitic and 56 % 1; cias important. tetrahedrite along major and Valle (1999), ...

Mine SulE 778 @ 0.96 Three phases: Paleocene minor advanced Molybde nite: Barren late~min· and enargite NW regional Mathur et aI. ... Q.

Toquepala 176.3 Cu, 0.047 Mo dacite intrusive rocks argillic (cp) 57.1 .0.3 eral diatreme present faults (2001)

Intennediate Late argillic overprinted Biotite: Important: Important:

Ox (including Cretaceous- by sericitic and 51.8.0.5; several phases; intersecting Bouzari and Cerro Mine enrichment): SeveraJ phases: Paleocene minor advanced Molybdenite: tourmaline NE and Clark (2002), Colorado 128.3 228@ 1.0 Cu dacite volcanic rocks argillic (cp) 55:t 0.3 breccias Absent NW faults Cotton (2003)

Sulf (hypogene), Late Creta~ 186 @ 0.54 Cu. Three phases: ceous volcanic Important: Important: Sulf (supergene): quartz mon~ and volcano~ Potassic over~ Biotite: 57.00 several NE alignment Rowland and

Mine 230 @ 1.14 Cu. zonite to sedimentary printed by • 0.6!l-56.6 1 tounnaline of hydrothe rmal Clark (2001), Spence development Ox: 79@ 1.18 Cu granodiOrite rocks sericitic (cp, brl) :t 0.63 breccias Absent centers Tapia (2003a. b)

Important: sericite. chlorite. Important :

Ox: 363 @ 0 .34 Cu Paleocene Potassic with biotite. and NW, NE,and Boric et aI . Mine + 252 @ 0.27 Cu One main granodiOrite. minor sericitic Biotite (?): tounna]ine EW intersecting (1990). Camus

Lomas BardS 60.2 (ROW) (0.08) phase: dacite tonalite pluton overprint (cp) 57.9-57.1 • 1.8 breccias Absent faults (2003)

Abbreviations: bn :: bornite. cp = chalcopyrite. Ox :: oxide, Sulf = sulfide I Figures from Comisi6n Chilena de l Cobre and Direcci6n General de Minerfa (Peru) ! M illor su16de minerals italicized 3 ROM ", run of mine orc

ANDEAN Cu PROVINCE 857

Tourmaline-rich sericitic alteration characterizes a l-km' hy­drothermal breceia body at Toquepala (Zweng and Clark, 1995), whereas smaller breccias at Cerro Verde-Santa Rosa contain tourmaline plus dumortierite (Kihien, 1975; Table 2). At Lomas Bayas and Re~ncho, however, sericitic alteration is subordinate and Cu is chiefly a component of potasSiC zones.

Advanced argil~c-altered ~thocaps have been eroded from above the main porphyry Cu depoSits in the Paleocene to early Eocene belt, although they remain widespread in the northern Chile part of the belt, as at Anillo and EI Guanaco (Marquardt et al. , 1994; Fig. 6), the latter ~thocap cut by the major enargite-bearing veins. Pyrophyllite-bearmg assem­blages at Toquepala (Zweng and Clark, 1995) and Cerro Col­orado (Bouzari and Clark, 2002) are interpreted as the deep roots of ~thocaps.

Supergene chalcocite enrichment was a requirement for ore generation at Cerro Colorado and Spence and deep oxi­dation played the same role at Lomas Bayas. In southern Peru, chalcocite enrichment produced the higher grade ores mined initially at Cuajone and Toquepala, whereas oxide ore has been more important to date at Cerro Verde-Santa Rosa. Enrichment took place where pyrite-rich sericitic alteration is well developed, whereas the oxide Cu mineralization is con­fined to potassic zones containing only minor pyrite. The su­pergene profiles developed since -42 Ma (Bouzari and Clark, 2002; Quang et al ., 2003, 2005) and, at least at Cerro Verde­Santa Rosa, Cerro Colorado, and Spence, were active before the hypogene porphyry Cu deposits in the middle Eocene to early O~gocene belt were formed (S@toe and McKee, 1996). Fossilization of the currently observed supergene profIles in the northern Chile deposits commenced at -14 Ma, but those in southern Peru probably remain active (S@toe, 2005).

Middle Eocene to Early Oligocene Belt of the Central Andes

Mineralization types and economic significance

The middle Eocene to early Oligocene belt of the central Andes contains the largest concentration of Cu resources in the world. The belt is defined by a ~near, orogen-parallel array of Cu deposits and prospects extending for -2,500 km between approximately lat 13°30'S in southern Peru and 31°S in nortllern Chile (Fig. 7). Its width varies between 130 km in southern Peru and 30 to 50 km in northern Chile, although the late . O~gocene age of the Taca Taca Baja prospect in northwestern Argentina (Fig. 7) suggests an even locally greater width. The majority of the deposits and prospects in the belt are of porphyry type (Fig. 7, Table 3), ranging from Au-rich, Mo-poor examples (Cotabambas, Esperanza) through deposits carrying both Au and Mo (Los Chancas, Antapaceay, Tintaya, Escondida) to relatively Mo rich, Au poor end mem­bers (Rosario, Chuquicamata).

The southern Peru part, histOrically known as the An­dabuaylas-Yauri belt (Santa Cruz et al ., 1979), also includes Significant mineralization of skarn type, typically around low-grade porphyry Cu stocks. The TIntaya skarn is the only deposit being mined in this part of the belt, although recent porphyry and skarn Cu discoveries at Antapaccay, Corocco­huayco, Los Chancas, and Cotabarnbas also contain important resources (Perell6 et al ., 2oo3a; Fig. 7, Table 3). In contrast,

80"

'0'

20'

DEPOsrr lYPES

• Porphyry Cu-tle • Porphyry Co-Au

... Porphyry-related

Cu ""'"'

PERU

""" .... "" Q1IM8ORo\ZO ~I """"""",,,,,,,,,. UUlNNl Pl-37l ,""""""...., ~.IAAIlIH (cz-3lt

""""""-" on _ Major tectoni::: dsoontinuity

30"

o , SOOkm ,

' 0"

VENEZUELA

0"

'0"

BOUVIA

3!J

FIG. 7. MiddJe Eocene to early Oligocene Cu belt of the Andes, showing main deposits and prospects and their genetiC type. Also shown are the three main transverse discontinuities in the Andes. Numbers in parenthesis after deposit names are isotopic ages (approximated), taken from compilations by Sillitoe (1988) and Perell6 et al. (2OO3a). with additions from Carow (2003), Masterman et aI. (2004), J. Pere1l6 (unpub. data, 2004. 2005). Rivera and Paroo (2004), and mvcOl et aI. (2004).

TABLE 3. Selected Geologic Characteristics of POrphyry and POrphyry~related Skarn Cu Deposits and Principal Prospects, Midd1e Eocene to Early Oligocene Belt, Central Andes 81 '" Status and Production oj.

2003 reselVes (million Ore-related production I metric tonnes) hypogene

Deposit! (metric tons and grade (%) PorphY')' Host alteration Hydrothermal Massive Structural prospect Cu x 1,(00) (cutoff, % Cu) composition Rocks (mineralization) Age (Ma) breccias sulfide veins control Key references

'IWo phases: Concentric Sulf + 0" 200 @ granodiOrite Cretaceous pattern:J:tassic Corrales (2001), 1.0 Cu, 0.08 Mo, to quartz marine clastic surraun ed by Biotite: Important: Pere1l6 et al.

Los Chancas Prospect 0.12 Wi Au monzonite rocks sericitic (cp, hnt.) 32.0.0.8 n." Absent NS faults (2OO3a)

Potassic and 1Wo phases: Middle Eocene potassic~cic granodiorite granodiorite overprinted by Minor igneous Important:

Sulf: 112 @0.62 to quartz and diorite intermediate Biotite: breccias and NE and Pere1l6 et al. Cotabambas Prospect Cu, 0.36 glt Au (0.2) monzodiorite plutons argillic (cp, bn) 35.7:t: 0.9 pebble dikes Absent NW faults (2OO3a, 2OO4b)

Six phases: Potassic over- Jones el ,). (2000), monzonite Cretaceous printed by minor La.ge, Important: Fierro et al.

SulE 3B3 @ 0.89 to quartz marine, cal- intennediate ~stmineral NWand (2002), Perell6 el Antapaccay Prospect Cu, 0.16 Wi Au monzonite careous rocks w-g;llic (cp, bnl' Midd1e Eocene latreme Absent NE faults ,). (2003a)

Potassic over~ Zwenf et al. Cretaceous printed by minor (1997 . Fierro et '" Mine SulE 139 @ 1.23 Two phases: marine. cal- intennediate Molybdenite: ,). (1997), PereU6 t=:

t-TIntaya 17.2 Cu, 0.23 Wi Au monzonite careous rocks argil~c (cp, bnl' 41.9.0.2 Pebble dikes Absent Important cl ,). (2003a) ~

6 Biotite: '" 34.4 • 0.3;

~ Permo--Triassic Potassic over- Illite: 34.5 :t: 0.5; volcanic and prinled by Alunite: Important: Important: Dick el ,). (1994), ."

Sulf 1,094@ sedimentary sericitic and 32.6.0.3; py. enarg, NE and NW Bisso el ,). (1998), '" f;l 1.03 Cu, Several phases: rocks. Paleocene advanced Molybdenite: Minor igneous tenn. bn. intersecting Masterman et al. t-Rosario Mine 0.025 Mo (0.45) granodiOrite intrusive rocks argilliC (cp, bnl' 33.3.0.2 breccia dg, ce. c:v faults. THled (2004) t-o.

Sui£, 741 @ 0.81 Cu Permo. Triassic Concentric Biotite: 35.2 :t: Important: Dick el ,). (1993), (including super- volcanic and pattem:J:lassiC 0.3,34.7.0.3; NWandNE Bisso el aI. (1998),

Min~ gene: ISO @ Several phases: sedimentary surroun ed by Late intrusion: Minor igneous Minor: enarg, intersecting Masterman et aI. Ujina 433.5 I. 71 CuI (0.45) granodiOrite rocks sericitie (cp. bn) 34.23.0.13 breccia tenn. cp faults (2004)

Potassic over- Important: Sulf (h)1X>gene); prinled by mod- Important ENE-trending -500 @ 0.5-0.6 Cu. Two main erate sericitic and igneous and breccias and

Quebnoda Mine Sulf (supe'Sene); phases: quartz Paleozoic minor advanced Biotite: 35.0 :t: tounnaline dikes. NW and Hunl el ,). (1983), Blanca 73.8 166@ 1.36 Cu monzonite intrusive rocks a<giIIic (cp, bn) 0.2,34.9 • 0.4 breccias Absent NNE faults Maksaev (1990)

Penno-Triassic volcanic and Potassic with Ambrus (1977), volcanosew- moderate Important: Maksaev (1990).

Sulf 650 @ 0.53 Four phases: mentary rocks. structurally Minor. igneous Present WNW and DiUes et aI. Mine Cu; 0" B36@ quartz diorite JurassiC sedi· controlled sericitic Biotite: and biotite-rich lal.,,)ly; py, NW faults and (1997), Gerwe el

El Abra 225.2 0.50 C" (0.26) to granodiOrite mentary rocks (cp, bn, ee) 39.1.1.2 breccias enarg. tenn late veins aI. (2003)

Potassic over~ printed by sericitic. inter- Biotite:

Thr~hases: Penno-TriassiC mediate argilliC, 36.2:t: 1.8; Important: ~ 'orite, volcanic and and minor ad· Molybdenite: Minor Igneous WNW-trending Pe,eU6 (2003),

Sulf; 648 @ acite. and volcanosedi- vanced w-g;llic 35.1.0.12, breccias and intrusions and J. Perell6, unpub. Conchi Prospect 0.58 Cu (0.3) rhyodacite mentary rocks (cp, bn , ee) 35.34 • 0.13 pebble dikes Minor vein sets dala, 2004

J

TABLE 3. (Cem' .)

Status and Production + 2003 rese rves (million Ore-related production I metric tonnes) hypogene

DCJ.X>Sitl (metric tons and grade (%) Porphyry Host alteration Hydrothermal Massive Stmctural prospect Cu x 1.(00) (cutoff, % Cu) composition Rocks (m ineralization) Age (Ma) breccias sulfide veins control Key references

Cuadra et a!. Sulf (hypogene), (1997 •. b). Cm,drn 1.300 @ 0.53 Cu. One main Triassic Potassic with Biotite: and Camus (1998), Sulf (supergene); phase: volcanic rocks, minor structurally 32.7. 0.3; Important: Cuadra and Rojas

Radomiro Mine 180 @ 0.93 Cu. granodiorite to Cretaceous controlled Sericite: NE and (2001), Ossand6n Tamie 297.1 0 " 850 @0.62 Cu monzogranite granodiOrite sericitic (cp. bn) 31.8:i: 0.3 Absent Absent NS faults et.1. (2001)

Biotite: Sulf (hypogene), Paleozoic Potassic 33.4 :t 0.3; 5.400@ 0.48 Cu. amphibolite overprinted Sericite: Ambrus (1979). 0.024 M o. and granitoids. by intense 31.1 :i: 0.2; Mathur et al. Sulf (sup"gene), Triassic structurally Molybdenite: Important: (2000). Bali.,d et 2.229 @ 1.41 Cu. Three phases: granodiorite controlled 34.8:i: 0.2; Important: NNE st ruc- al. (2001).

Mine Ox:506@ granodiorite to and volcanic sericitlc (cp. Zircon: 34.6 :i: py, enarg, tures, vein sets, Ossand6n et al . Chuquicamata 596.8 1.56 Cu (0 .2) monzogranite rocks bn. dg. cv. ee) 0.2-33.3 • 0.3 Absent ee.cv and intrusions (2001)

Potassic over-

~ printed by Important in Sillitoe et al. Two phases: Triassic sericitic and Biotite: 32.5; upper part: (1996). Millie, tl

Mine Sulf, 882 @ granodiorite intrusive and advanced Alunite: Important in py, e narg, Important: and QUiroga ~ MM development 1.02 Cu and dacite volcanic rocks acgilUc (ep. bn) 31.11-,'11.4 shallow parts bu, cc, cv NS rault pane ls (2003) <:

Q Potassic with ~

Several phases: minor structurally Important: '" 0 Sulr + ox: 856 @ granodiOrite to Paleogene controlled N E-trending Rivera e t al. :s

Quetena Prospect 0.42 Cu monzogranite granodiorite sericitic (cp, bn) Late Eocene Absent Absent intrusions (2003b) <:

" Sulf (hypogene), Important: '" 1.830 @ 0.5 Cu Triassic volcanic Potassic with Biotite: NE-trending (0.3). Sulf (supe'- Several phases: rocks and structurally con- 37.3 . 1.3; Minor intrusions and Rivera et al. gene) + ox: 720 granodiOrite to Paleoge ne trolled serititic Sericite: igneous NW-trending (2003a), Rive ra

Toki Prospect @ 0.4 Cu (0.2) monzogranite granodiorite (ep. bn. (I;) 34.52.0.20 breccia Absent vein sets and Pardo (2004 )

Late Cretaceous Concentric pat-volcanic and tern : potassic

Sulf, 443 @ 0.63 volcanosedi- surrounded by Biotite: Important: Cu. 0 .26 glt Au. Two phases: mentary rocks. intenned.iate 41.3.0.3; Important NE-trending 0 " 71 @0.42 granodiorite Middle Eocene argillic and Molybde nite: igneous intrusions and Perell6 e t al.

Esperanza Prospect Cu (0.3) and dacite dacite domes sericitic (cp. bn}l -4l.BO:i: 0.13 breccias Absent raults. Tilted (2004.)

Permian intru-sive rocks, Important:

Two phases: Carbonirerous Potassic, very NE-trending 0" 890@ 0.4 granodiOrite volcanosedi- minor sericitic intrusions and Camus (200 1,

G.by Prospect C" (0 .2) and dacite mentary rocks (ep. bn) Biotite: 43-40 Absent Absent minor raul ts 2003)

Potassic over-printed by Late-mineral sericitic and in trusion, hom- Important :

One main widespread ad- blende: 38.8 :i: Abundant and Important : NE-trending Pete rsen et al. phase: diorite- Paleocene vanced argillic 3.4; Zircon: important py, enarg. breccias and (1996), Richards

~ Chimborazo Prospect Sulf 180 @ 0.8 Cu tonalite volcanic rocks (cp, cc, enarg, bn ) 38.09 .0.30 bast to ore ce, cv, bn dikes et "I. (1999) CO

TABLE 3. (Cont.) 00

~ Status and Production + 2003 reserves (million Ore-related production! metric tonnes) hypogene

Deposit! (metric tons and grade (%) Pmphyry Host aJteration Hydrothermal Massive Structural prospect Cu X 1,000) (cutoff, % Cu) composition Rocks (mineraJization) Age (Ma) breccias sulfide veins control Key references

Maturana and Potassic widely Monomict Sarlc (1991), overprinted by Biotite: breccias, pebble Important: Richards et al.

Swf (supergene): Three phases: Penno-Triassic sericitic, chloritic, 37.40 • 0.18; dikes, local Important: NE-trending (1999), Monroy Escondida Mine 1,280 @ 1.24 Cu. granodiorite to and Paleocene and advanced Zircon: barren tour- py, enarg, intrusions and (2000), Williams Norte-Zaldivar 147.8 0" 330 @ 0.77 Cu monzodiorite volcanic rocks argillic (cp, bn, cv) 38.7.0.3 maJine breccias tenn breccias (2003)

Biotite: 34.8 1; 0.4,38.0 • 0.4, 37.5.0.6; Alunite: 35.7 1; 0.3,35.4 • 0.2; Molybdenite: 33.7.0.3; Zircon: 37.2 1; Ojeda (1986),

Potassic over- 0.8,37.9 • 1.3, Padilla et al. Five phases: printed by 37.7.0.8, Important: (2001), Richards '" Sulf (supergene): quartz mon- intenneruate 39.0 • 1.5; Late· Important: NW-trending et aI. (1999, 2001). t::

" 1,670 @ 1.59 Cu, zonite to argillic, sericitic, mineral dike, py, enarg, intrusions, Quiroz (2003), ~

Mine 0.021 Mo, granodiorite Paleocene and advanced zircon: 35.6 Abundant tenn. ce. pebble dikes, Padilla-Garza g Escondida 758.0 0.24 glt Au (0.7) and rhyolite volcanic rocks argillic (cp, bn) 1; 1.2 pebble dikes cv, bn and veins et aI. (2004)

~ Biotite: 41.6 :t 0.6, 41.2 • 0.6; Gustafson and ."

Sericite: 41.9 1; Hunt (1975), "' f:; 0.5, 40.84 • 0.46, Gustafson and " 41.40 • 0.17; QUiroga (1995), S. Sulf. 974 @ 0.63 Potassic over- Zircon: 41.8 1; Cornejo et al.

Cu, 0.022 Mo, printed by 2.3,41.2; Minor, but Important: (1997). Watanabe 0.10 Wt Au (in- Paleocene and sericitic and Molybdenite: cy. enarg. intersecting et aI. (1999),

Mine eluding supergene: Four phases: early Eocene advanced 42.21;0.2. Abundant n,dg NEand NW Gustafson et al. EI SaJvador 72.8 340@ 1.5 Cu) granodiOrite volcanic rocks a<gillic (cp, bn) 42.0 :t 0.2 pebble dikes present structures (200 1)

Potassic over- Biotite: Important: Ma<ch (1935), Sulf (supergene) Several phases: Middle jurassic printed by 36.65 :t 0.03; Absent, NW fawts Marsh et al.

Abandoned + ox: 31O@ monzodiorite marine cal- sericitic Sericite: but enarg and NNE (1997). Camus Potrerillos mine 0.95 Cu and dacite careous rocks (cp, bn, en"'1ll' 35.64 • 0.03 n.a. present thrusts (2003)

Potassic over-Sulf (hypogene). printed by 362 @ 0.58 Cu, intenneruate 0.54 glt Au. Several phases: jurassic argillic, sericitic, Sericite: Important: Perell6 et al. Sulf (supergene): quartz mon- volcano- and advanced 35.0.0.6; NWand NNE (1996), Palec-lek 103 @0.71 Cu, zonite to diorite sedimentary argillic (~. Alunite: Abundant Important: structures and and Caceres

La Fortuna Prospect 0.35 glt Au (0.4) and dacite rocks enarg, bn 32.8.0.3 pebble dikes py. enarg, ce intrusions (2003)

Abbreviations: bn '" bornite. cc '" chalcocite, cp '" chalcopyrite, cv '" covelJite, df '" digenite, enarg :II enargite, Ox ::0: oxide. Sulf '" sulfide, tenn = tennantite I Figures from Comisi6n Chilena del Cobre and Direcci6n General de Minerfa Peru) 2 Minor sulfide minerals italicized 3 Minor skarn-type mineralization present • Skarn-~ mineralization dominant .5 Enriche ore mined out during 2004

ANDEAN Cu PROVINCE 1

the belt in northern Chile is made up largely of porphyry Cu deposits that contain major supergene oxidized and enriched zones (Fig. 7, Table 3), as well as several exotic Cu deposits formed by lateral drainage of supergene Cu-bearing solutions from actively forming enriched rones (e.g., Miinchmeyer, 1996; Sillitoe, 2005).

The principal deposits of the northern Chile part of the belt are at least an order of magnitude larger than most others in the central Andes. This region dominated Chilean Cu pro­duction for most of the last century as a result of bulk mining of Chuquicamata (since 1915), Potrerillos (1927), and EI Sal­vador (1959). Output experienced a dramatic rise in the 1990s when Escondida (1990), Quebrada Blanca (1994), Zaldivar (1995), Radomiro Tomic (1996), EI Abra (1996), and Ujina (1998) entered production. Current output comes essentially from the Chilean part of the belt and Tintaya in southern Peru (Table 3). Production from the Chilean mines in 2003 totaled 2.6 Mt of fine Cu or approximately 57 percent of Chile's total (Fig. 7). The largest deposits are Chuquicamata and Escondida, which together produced 1.4 Mt of fine Cu during 2003, about 12 percent of world-mined Cu produc­tion.

Tectonomagmatic setting

Northern Chile: In northern Chile, between lat 20' and 26' S, the middle Eocene to early Ougocene belt coincides with an elevated (3,500-5,ooo m) longitudinal range known as the Cordillera de Domeyko or Precordillera (Fig. 1). The range is cored by late Paleoroic plutonic and volcanic rocks and flanked by intensely folded Late Triassic through Early Cre­taceous volcanic and marine sedimentary sequences, which, in tum, are overlain by Late Cretaceous continental sedi­mentary and Paleogene volcanic units (Boric et al., 1990; Maksaev, 1990; Mpodozis et al., 1993a).

The porphyry Cu centers are genetically related to small epizonal stocks, many of which are emplaced in the > 1,000-km-Iong, orogen-parallel Domeyko fault system (Maksaev, 1990), which also acted as a control on the internal arrange­ment of basement blocks in the Cordillera de Domeyko and its northern and southern extensions (Fig. 8). The fault sys­tem was llkely active during and subsequent to porphyry Cu stock emplacement (Maksaev and Zentilli, 1988; Reutter et al., 1991, 1996; Lindsay et al., 1995). Some of the porphyry Cu centers are localized on or near the main faults of the sys­tem (e.g., Radomiro Tomie, Chuquieamata, MM, Escondida, Exploradora), whereas others are farther away-9 km at EI Salvador and 13 to 17 km at Rosario and Ujina (Fig. 8).

The Domeyko fault system is segmented in nature, with in­dividual strands shOwing complex and unique histories. Fault displacements and senses of motion are still relatively poorly constrained, with dominant dextral (Maksaev, 1990; Reutter et al., 1991; Lindsay et al., 1995) or sinistral (Mpodozis et al., 1993a, b; Tomunson et al., 1994; Tomlinson and Blanco, 1997a; Dilles et al., 1997) strike-slip displacement, as well as reverse motion (Mpodozis and Ramos, 1990; Skarmeta et al., 2003a, b; McClay, 2004), with lateral translation of basement blocks by tectonic rafting and block rotation (Yanez et al., 1994; Arriagada et al., 2000, 2003), being proposed. Notwith­standing these discrepancies, it is generally accepted that three main segments of the fault system exist (Fig. 8): (I)

T 9

DEPOSIT TYPES • PorphyIy Cu-t.tJ • Porphyry Cu·Au

-"'"'" '" ._;-!: IneaIe synorogenk: ,:,"­IL-l"'. FWJlt ;r ............ ,t FoIO-and-ttwustbeit

VQou"""""

IZJ TtrtiWy IJ)"'IOfOgeOIc depoIita _ Porphyry otlPP'f lJ10ck

E::;:] Paleogenli YOIcank: rocks 0 Early T.,-tlMy ~ rockI;

Maozoic~nxks

Paleozoic basement

Flc. 8. Structural setting of the middle Eocene to early Oligocene Cu belt in northern Chile, shO\Ving deposit types . Domeyko fault system, and syn­orogenic clastic sedimentary depoSits within and near the arc. The exotic oxide Cu deposits, sourced by porphyry ell deposits in this belt during early to middle Miocene supergene activity. are shown for reference only. Note that hypogene Cu deposit fonnaHon is syntectonic. Schematic geologie sec­tion (along line A-B in map) emphasizes importance of reverse faulting. Structura1 segments I. 2. and 3 are discussed in text. Fault systems from SERNAGEOM1N (2002), synorogenic deposits from Pere1l6 et al. (2003a), and section from Mpodozis and Ramos (1990) and Skarmeta et at. (2OO3a).

862 SILLITOE AND PERELL6

from Chuquicamata northward, a group of north-trending faults with Miocene reverse and sinistral motion is superim­posed on older structures possessing evidence for Late Cre­taceous, Paleocene. and Eocene reverse, Eocene sinistral, and Oligocene dextral motion (Lindsay et al., 1995; Reutter et al., 1996; Dilles et al. , 1997; GUnther et al. , 1997; Ladino et al., 1997; Tomlinson and Blanco, 1997b; McInnes et al., 1999); (2) between Calama and just north of Exploradora, the predominant structural fabric appears to result from major clockwise rotation of basement blocks during the Eocene (Mpodozis et al., 1993a, b; Aniagada et al., 2000, 2(03); and (3) south of Exploradora, in the EI Salvador-Potrerillos re­gion, there is evidence for sinistral transpressive deformation during the Eocene (Tomlinson et al., 1994), superimposed on Late Cretaceous and Paleocene contractional faults, with strain partitioned between strike-slip and thrust structures in a fold-and-thrust belt cutting Mesozoic marine sedimentary rocks (Cornejo and Mpodozis, 1996). South ofPotrerillos, be­yond these three formal segments, deformation was accom­modated by a series of Eocene through early Miocene high­angle reverse faults (Martin et al., 1995, 1997), which involved large crystalline basement blocks and gave rise to an overall thick-skinned structural style (Moscoso and Mpodozis, 1988; Nasi et al., 1990).

There is general agreement that the Domeyko fault system was intimately associated with the Incaic contractional orogeny, active from the middle Eocene to early Oligocene, and responsible for much of the uplift of the Cordillera de Domeyko and its southern extensions (Maksaev and Zentilli, 1988, 1999; Pere1l6 et al., 1996). At least some of the strands of the Domeyko fault system, as well as several oblique, northwest-trending lineaments (e.g., Salfity, 1985), are reacti­vated Mesozoic, Paleozoic, or older faults, including Jurassic backarc basin-bounding normal faults that were inverted as high-angle reverse structures (Sillitoe, 1981; Mpodozis and Ramos, 1990; Cornejo and Mpodozis, 1996; Cornejo et al., 1997; GUnther et al., 1997; Skarmeta et al., 2003a, b; McClay, 2004). The erosion products generated by middle Eocene through middle Miocene uplift of the Cordillera de Domeyko led to deposition of several kilometers of synorogenic terres­trial sediments in adjacent basins (Maksaev and Zentilli, 1988, 1999; Maksaev, 1990; Tomlinson et aI. , 1999; Mpodozis et al., 2000; M podozis and Pere1l6, 2003; Fig. 8).

The middle Eocene to early Oligocene porphyry Cu de­posits and prospects of northern Chile characteristically occur in clusters or alignments of three or more, some apparently located at the intersections between faults of the Domeyko system and broadly northwest-trending lineaments (e.g., Richards et al., 2(01). The greatest number of discrete por­phyry centers is obseIVed in the Chuquicamata cluster, com­prising Radomiro Tomic, Chuquicamata, MM, and the re­cently discovered Quetena, Toki, Genoveva, and Opache deposits (Camus, 2003; Rivera et al. , 2003a, b). Structural control is important in all major deposits of the belt (Table 3) but is perhaps most clearly developed in the 13-km-Iong, north-northeast- trending Chuquicamata-Radomiro Tomic system, where intrusion and alteration-mineralization geome­tries are both strongly influenced by fault structures (Lindsay et al., 1995; Ossand6n et al. , 2001; Fig. 9a). Other notable examples include the northwest-trending main porphyry

intrusion at Escondida (Ojeda, 1986; Padilla et al ., 2001 ), the dike array at Esperanza (Pere1l6 et al., 2004a), and the thrust­controlled geometry of the Potrerillos and Esperanza stocks and their associated alteration-mineralization (Tomlinson, 1994; Pere1l6 et al., 2004a; Fig. 9b).

Independent of size, all porphyry Cu deposits and prospects in the belt formed in a brief time interval of -13 m.y., between apprOldmately 44 and 31 Ma (Fig. 7, Table 3). This remarkably brief metallogenic epoch, characterized by overall volcanic quiescence, followed eastward translation of the magmatic front from the Paleocene to early Eocene belt to tl,e Cordillera de Domeyko and was abruptly terminated by post-31-Ma migration of the arc to the east (Maksaev and Zentilli, 1988, 1999; Maksaev et al ., 1988; Maksaev, 1990; Mpodozis and Ramos, 1990). Coeval volcanism along the Cordillera de Domeyko was restricted to a few dome centers and related pyroclastic and lava flows (Mpodozis and Pere1l6, 2(03), whereas the more widespread intrusive activity seems to have been confmed to clustered or aligned centers sepa­rated by relatively large amagmatic gaps. The porphyry Cu districts are commonly spatially associated with much older or precursor magmatic centers, such as Late Cretaceous alkalic gabbro at Escondida (Marinovic et al., 1995; Richards et al., 2(01), Late Cretaceous granite in the Collahuasi district (Masterman et al. , 2(04), Paleocene felsic caldera at EI Sal­vador (Cornejo et al. , 1997), and middle to late Eocene gran­odiorite and diorite at many localities (Mpodozis et al., 1993a; Cornejo and Mpodozis, 1996; Dilles et al., 1997; Richards et al., 2001).

Porphyry Cu-bearing stocks in the belt are multi phase, with as many as five porphyritic intrusions recorded at Escondida. The intrusions range in composition from biotite ± horn­blende-bearing quartz diorite to quartz monzonite and mon­zogranite, although the majority includes one or more phases of granodioritic composition (Table 3). Intermineralization dacitic intrusions are present in a number of deposits, and rhyodacitic dikes and domelike intrusions are locally impor­tant as late-mineralization phases. Although the porphyry Cu­bearing intrusive complexes generally show an evolution from preore intermediate composition to more felsic inter- and late-mineralization phases richer in SiO, and K,O (Camus and Dilles, 2(01), a reversal of the trend to more mafic mag­matism is also locally present, as evidenced by the evolution from granitic through granodioritic to quartz dioritic intru­sions in the EI Salvador district (Cornejo et al., 1997; Gustafson et al., 2(01). The porphyry Cu-bearing stocks are I-type and belong to the magnetite series of Ishihara (1981). They are high to moderate K calc-alkaline in composition, with high Fe,O,fFeO ratios indicative of oxidized melts (Ishi­hara et al., 1984). They are also characterized by remarkably restricted variations of Sr and Nd isotope ratios, with low Sri of 0.7042 to 0.7045 and positive t~d ratios of 1 to 4 (Rogers, 1985; Zentilli et al., 1988, 1995; Maksaev, 1990). Avatlable LalYb data for ore-bearing stocks at EI Salvador (20-25; Cornejo et al., 1997) and other northern Chile porphyry Cu­Mo deposits (15-35; Mpodozis et al., 1995; Haschke et al., 2(02) suggest that magmas evolved during a period of crustal thickening when amphibole-bearing lower crust was being transformed into anhydrous gamet-bearing eclogite (e.g., Kay et al., 1999; Cornejo and Matthews, 2000).

ANDEAN eu PROVINCE

Flc. 9. Features of porphyry ell deposits in the middle Eocene to early Oligocene belt of northem Chile. ll. The Chuquicamata porphyry Cu-Mo deposit, looking south across the 4-km-long open pit, showing the postmineralil' .. ation \Alest fault that delimits Cu-Mo mineralization to th e west (indicated by arrows), a broad zone of pervasive sericitic alteration (white) related to a swarm of fault-controlled D veins extending eastward from the fault, and potassic alteration on the benches farther east. 1994 photograph. b. Drill core sample from the thnlst-controlled, massive crystalline anhydrite body that transecl'i the deep (500-600 Ill ) parts of the Esperanza porphyry Cu-Au deposi t, with 45°-dipping tectonic fabric de­fi ned by gamet-sulflde schlieren. Sample 20 cm long. c. Drill core sample showing the three main alteration types at the Es­perarw..a porphyry Cu-Au deposit. Early potassic (biotite-K-feldspar-magnetite-chalcopyrite-bomite) alteration (K) is cut by an intermediate argillic (sericite-chlorite-pyrite) assemblage (IA) zoned around a chlorite-pyrite-chalcopyrite vein!et, which , in tUI11 , is cut by a D-t)1>e pyrite-quartz veinlet enveloped by a sericitic (quartz-sericite-pyrite) assemblage (S). Sample 10 cm long. d. Northwest-striking swarm of sheeted 0 veins (between and parallel to arrows and marked by jarositic limonite) overprinting oxidc Cu-bearing potassic altemtion at the El Abra porphyry Cu-Mo deposit. e. The advanced argillic Iithocap above the Escondida porphyry Cu-Mo depOSit, looking northwest. Exploration shaft is located in area of maximum distu r­bance in the foreground. at foot of Cerro Colomdo Chico. 1984 photograph.

863

Southern Peru: The middle Eocene to early Oligocene belt of southern Peru (Fig. 7) encompasses parts of lhe in­termontane depressions between the Eastern and \"'estern Cordille ra and the northernmost Altiplano at elevations be­tween 3,400 and 4,700 m (Fig. 1). Copper mineralization is spatially and temporally associated with the middle Eocene to early Oligocene (-48--32 Ma), calc-alkaline Andahuaylas­Yauri batholith, a composite body emplaced into clastic and carbonate strata of Jurassic to Cretaceous age (Pere1l6 et a!. , 2003a). Batholith phases include cumulate gabbro and diorite, emplaced between 48 and 43 Ma, and granodiorite and quartz monzodiorite, between 40 and 32 Ma. Coeval Eocene to early Oligocene volcanic and sedimentary rocks occur in th e region and are interpreted to have accu mulated

mainly in transtensional and contractional synorogenic basins (e.g., Pere1l6 et aI. , 2003a). Major faul t systems, some >300 km long, occur in the region and most display evi­dence for both high-angle reverse and strike-slip motion . The southern Peru part of the belt diffe rs in two main ways from its counterpart in northern Chile: lack of intimate as­sociation between regional faults and porphyry Cu-bearing stocks, and domination by thin-skinned, fold-and-thrust belt style structures, without involvement of crystalline base­ment, although uplifted Paleozoic plutoniC blocks are com­monplace farthe r north and northeast (Pere1l6 et a!. , 2003a). In com mon with the northern Chile part of the belt, how­ever, many of the regional faults , in particular those that de­fin e the limit between the Western Cordillera and Altiplano,

864 SILLITOE AND PERELL6

are inferred to possess Mesozoic and/or Paleozoic ancestry (Pere1l6 et al., 2003a).

A wealth of K-Ar data, coupled with Re-Os and apatite fis­sion-track ages, demonstrate that porphyry Cu alteration and mineralization in the southern Peru part of the belt took place during the middle Eocene to early Oligocene (-4WO Ma; Pere1l6 et al., 2003a, 2004b; Table 3). Therefore, ore forma­tion accompanied batholith emplacement, volcanism, and sedimentation during a period of intense deformation, crustal shortening, and regional uplift defining the Incaic orogeny (Perell6 et al., 2003a).

The porphyry Cu stocks, comprising cylindrical, dikelike, and stratigraphically controlled bodies of sill-like form, are bi­otite- and/or hornblende-bearing intrusions of mainly gran­odioritic composition, although monzogranite, quartz mon­zonite, monzonite, and monzodiorite occur locally (Table 3). Limited geochemical data show that the stocks are moderate K calc-alkaline in composition and moderately to highly en­riched in light (L)REE, with La/Yb ratios ranging from 15 to 20 but with local higher values (20-45) in some late-mineral­ization intrusions at Tmtaya (Carlier et al ., 1989), which else­where in the Andes are taken as indicative of thickening crust (e.g., Kay et al., 1999).

Principal mineralization features

Porphyry Cu mineralization in the belt is associated with potassic, intermediate argillic, sericitic, and advanced argilliC alteration assemblages (Table 3). In addition, a deep-level al­bite-actinolite-magnetite assemblage is reported beneath Cu ore at EI Salvador (Gustafson and Quiroga, 1995), and a hy­brid calcic-potassic assemblage, defined by actinolite, horn­blende, clinopyroxene, biotite, and K-feldspar, is prominent at Cotabambas (Pere1l6 et al., 2004b).

All deposits in the belt contain potassic alteration, in which biotite and K-feldspar are the dominant alteration minerals (Fig. 9c), and uni- or multidirectional quartz veinlets. Chal­copyrite, with or without bornite and digenite, is dispersed throughout the quartz veinlets and is present, to lesser de­grees, as disseminated grains in the enclosing rocks (Table 3). The bornite and digenite are concentrated in the deeper, cen­tral parts of deposits, where Cu tenors are typically higher. The potassic alteration accompanied the prinCipal Cu intro­duction event in most of the deposits. In several major de­posits of the Chilean segment, including Rosario (Bisso et al. , 1998), El Abra (Ambrus, 1977), Radomiro Tomic (Cuadra and Camus, 1998), Quetena (Rivera et al. , 2003b), and Gaby (Camus, 2(01), the potassic alteration predominates over all other alteration types (Table 3). Hydrothermal magnetite is an additional common component of Au-rich potassic assem­blages, as at Cotabambas and Esperanza (Perell6 et al. , 2004a, b; Fig. 9c). In the many porphyry Cu centers hosted by carbonate rocks in southern Peru, calc-silicate alteration, dominated by gamet, diopside, and actinolite, developed syn­chronously with the potassic alteration, and associated skam­type Cu-Mo and/or Au mineralization is commonplace (e.g., Las Bambas, Katanga, Tmtaya; Perell6 et al., 2003a; Fig. 7; Table 3).

Intermediate argillic alteration is a component of ore zones in several depoSits and prospects throughout the belt, includ­ing Cotabambas and Tintaya in southern Peru (Pere1l6 et al.,

2003a) and Conchi (Pere1l6, 2(03), Esperanza (Pere1l6 et al. , 2004a; Fig. 9c), Escondida Norte (Williams, 2003), Escon­dida (Padilla et al., 2(01), and La Fortuna (Pere1l6 et al ., 1996) in northern Chile. Chalcopyrite, generally accompa­nied by pyrite, occurs in some intermediate argillic vein let as­semblages but locally constitutes monomineralic veinlets. Copper contents are generally lower than those of the earlier potassic alteration zones, with a Significant remobilization of chalcopyrite and bornite documented at Cotabambas and Es­peranza (Pere1l6 et al., 2004a, b) and suspected elsewhere (cf. Sillitoe, 2000a). At Escondida and Ujina, however, intermedi­ate argillic alteration accompanied renewed input of both Cu and Mo (Padilla et al., 2001; Quiroz, 2003; Masterman et al. , 2004).

Sericitic alteration occupies appreciable rock volumes in the shallower parts of several less deeply eroded major de­posits in the Chilean segment (Table 3). Broad sericitic halos to potassic cores (e.g., Lowell and Guilbert, 1970) are not typ­ical but are developed at Esperanza and Ujina (Bisso et al ., 1998; Pere1l6 et al., 2004a). In marked contrast, the majority of the porphyry Cu deposits, particularly those defined as giant and behemothian by Clark (1993), possess major sericitic alteration zones that overprint the central parts of Cu-bearing potassic assemblages (Sillitoe, 1992), most no­tably at Chuquicamata (Ossand6n et al., 2(01), MM (Sillitoe et al., 1996), Escondida Norte-Zaldivar (Maturana and Saric, 1991; Williams, 1993; Monroy, 2000), Escondida (Ojeda, 1986), and El Salvador (Gustafson and Hunt, 1975). The ver­tical extent of the structurally controlled sericitic zone at Chuquicamata is > 1,000 m (Fig. 9a). Swarms of sericite-bor­dered, D-type veins (Gustafson and Hunt, 1975) that over­print potassic alteration are interpreted as the structurally lo­calized roots of formerly more extensive sericitic alteration zones that were lost to erosion (Sillitoe, 1992). Those at El Abra (Fig. 9d), Radomiro Tomic, and Quetena are barren, al­though elsewhere they may contain appreciable hypogene Cu (see below).

Sericitic alteration zones are typically the roots of advanced argillic-altered lithocaps (e.g., Sillitoe, 1995a, 2000a), the transition being characterized by pyrophyllite, dickite, anellor alunite, as well as generally subordinate andalusite and dias­pore. Such transitional alteration assemblages are docu­mented at Chuquicamata (Ossand6n et al. , 2(01), MM (Silli­toe et al., 1996), Chimborazo (Petersen et al. , 1996), Escondida Norte (Williams, 2(03), Escondida (Padilla et al. , 2001; Fig. ge), El Salvador (Gustafson and Hunt, 1975; Watanabe and Hedenquist, 2(01), and La Fortuna (Perell6 et al., 1996). Similar advanced argillic assemblages are also typ­ically integral parts of fault-controlled massive sulfide veins and hydrothermal breccias, including pebble dikes, as at Que­brada Blanca, Rosario, MM, Escondida Norte-Zaldivar, Es­condida, and La Fortuna (Ojeda, 1986; Sillitoe, 1992; Dick et al., 1993; Perell6 et al., 1996; Sillitoe et al., 1996; Padilla et al., 2001; Milller and Quiroga, 2003; Williams, 2003).

Some of the highest hypogene Cu tenors are inVariably in the form ofhigh-sulfidation overprint assemblages contained in the late-stage massive sulfide veins and their adjacent host rocks. Where multiple overprinting took place, as at Es­condida and Chuquicamata, Cu grades may be enhanced sev­eral times (e.g., Padilla et al., 2(01). Such veins are typically

ANDEAN Gil PROVINCE 865

dominated by pyrite, enargite, and bornite, but chalcocite, covellite, digenite, and tennantite are also common con­stituents at Rosario, EI Abra (Maria vein), Chuquicamata, MM, Chimborazo, Escondida, Escondida Norte-Zaldivar, and La Fortuna (Table 3).

The middle Eocene to early Oligocene belt in northern Chile undelWent leaching, oxidation, and cumulative super­gene enrichment, mainly during the early to middle Miocene under arid to semiarid climatic conditions (Alpers and Brimhall, 1988; Maksaev, 1990; Sillitoe, 1990, 1992; Sillitoe and McKee, 1996). The supergene profiles developed during a period of steady and moderate surface uplift and exhuma­tion (Maksaev, 1990; Sillitoe and McKee, 1996; Maksaev and Zentilli, 1999; Sillitoe, 2005). Fossilization and preservation of the resulting oxidized zones and any underlying chalcocite enrichment were caused by onset of hyperaridity in the late Miocene (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996), with climatic desiccation being attributed to the cou­pling of coastal upwelling of cold Antarctic water delivered by a north-flawing ancestral Humboldt Current and tectonic up­lift of the central Andes (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996).

Key factors influencing effective leaching and Cu sulfide enrichment, as exemplified by the Chuquicamata and Escon­dida depoSits, include: (1) permeability enhancement due to structural preparation by major fault zones andlor fault inter­sections; (2) low neutralization potentials induced by sericitic andlor advanced argillic alteration assemblages; and (3) acidic conditions generated by oxidation of pyrite-rich zones, partic­ularly where high hypogene Cu contents exist in massive sul­fide veins. In contrast, where these pyritic, feldspar-destruc­tive alteration assemblages are poorly preserved, as in much of southern Peru and at EI Abra, Radomiro Tomic, Quetena, Esperanza, and Gaby in northern Chile, supergene chalcocite enrichment was minimal and in situ oxidation in reactive host rocks was the main supergene process. Lateral flow of surfi­cial water, controlled by local hydraulic gradients, caused ap­preCiable transport of dissolved Cu beyond some actively forming enrichment blankets to generate major exotic oxide Cu deposits, as at Sagasca in the Paleocene to early Eocene belt, and Mina Sur, EI Tesoro, and Damiana in the middle Eocene to early Oligocene belt (Sillitoe, 1992, 2005; Miinch­meyer, 1996; Sillitoe and McKee, 1996; Fig. 10).

Flc. 10. Miocene to early Pliocene Cu belt of the Andes. showing main deposits and prospects and their genetic type. Also shown are the northern to central Peru segment, the Maricunga-EI Indio and central Chile sub-belts, and the Fara1l6n Negro district emphasized in the text (gray shading) and the three main transverse discontinuities in the Andes. Note that the exotic Cu deposits were fonned during supergene oxidation and enrichment of por­phyry Cu deposits in the Paleocene to early Eocene and middle Eocene to early Oligocene belts. Numbers in parentheses after deposit names are iso­topic ages (approximated), take n from compilations by Sillitoe (1988) and Noble and McKee (1999), with additions from Vila and SilJitoe (1991 ), Losada-Calderon et aI. (1994 ), R.H . Sillitoe (unpub. data, 1997, 2002), U,bina et aI. (1997), Perell6 et aI. (1998), S.,.", and CIMk (1998), Cendall et aI. (2000), Muntean and Einaudi (2001), Perell6 et aI. (2001, 2003c, d), J. Pere1l6 (unpub. data, 2002), Spencer et al. (2002), Camus (2003), C. Feebrey (pers. commun., 20(3) , F. Malbnin (pers. commun., 2(03), Mpodozis and Kay (2003), Rasmussen et aI. (2003), Bendezu et al. (20(4), Gustafson et a1. (2004), and Noble et aI. (2004).

1\1 iocene to Early Pliocene Belt of the Central Andes

MineralizatiOll types and economic significance

The Miocene to early Pliocene belt of the northern and central Andes extends semicontinuously for -6,000 km, be­tween southwestern Colombia and central Chile-west-central

o !

80"

500km !

""

DEPOSIT TYPES

• Porphyry Cu-Mo

• Porphyry Cu-Au

.. Porphyry-f8lated skarn

WI Enargite-bearing replacement

= Enargite vein l1 Tourmaline breccia _ Exotic D.J

p Red bedCu

_ Major tectonic discontinuity

BRAZJL

BOUVIA

866 SILLITOE AND PERELL6

Argentina (Fig. 10; Sillitoe, 1988, 1990). The economically most important and best defined segments of the belt are pre­sent in northern and central Peru and northern and central Chile and contiguous northwestern and west·central Ar­gentina, where the Maricunga-EI Indio and central Chile (Los Pelambres-EI Teniente) sub-belts and the Farall6n Negro district are preeminent (Fig. 10). Only these most im­portant parts of the belt are described in this section.

The northern Peru to central Chile part of the belt contains the most varied eu metallogeny of the entire Andes, with por­phyry, breccia, skarn, enargite-bearing carbonate replace­ment, high-sulfidation epithermal enargite vein, red-bed, and exotic oxide Cu deposits al l present. Porphyry Cu-Au deposits are common and include Cerro Corona, Minas Conga, and several others in the Cajamarca sub-belt, Cerro Casale in the Maricunga-EI Indio sub-belt, Bajo de]a Alumbrera and Agua Rica in the Fara1l6n Negro district, and Piuquenes and other deposits in west-central Argentina (Fig. 10, Table 4). Copper skarns, particularly the giant Antamina Cu-Zn-Mo-Ag deposit (Fig. lla)-the world's largest Cu skarn- and much smaller

Magistral Cu-Mo prospect, as well as complex, polymetallic, enargite-bearing veins and replacements in carbonate and as­sociated rocks are important metallogenic constituents of the northern and central Peru segment, as at Cerro de Pasco, Marcapunta (Colquijirca), Yauricocha, and Morococha (Fig. 10). These deposit types are rare elsewhere in the Miocene to early Pliocene belt (Petersen, 1970), although volcanic rock­hosted, enargite-bearing, high-sulfidation epithermal veins are present at Laurani, Famatina (La Mejicana), and EI Indio (Fig 10).

The Cu resource of the belt is mainly contained in the giant porphyry Cu-Mo deposits of central Chile, Los Pelambres, Rio Blanco-Los Bronces (Fig. lIb, c), and EI Teniente; the latter the world's largest. These deposits produced 1.14 Mt of fine Cu in 2003, a quarter of Chile's total (Table 4). Mining at EI Teniente began in 1906 (Camus, 2003), whereas small­scale production commenced at Los Bronces in 1916. Copper output from the central Chile sub-belt increased dramatically in the second half of the last century, with major expansions at EI Teniente and Los Bronces and startup of large-scale

Flc. 11. Features of Miocene to early Pliocene porphyry Cll deposits in the central Andes. a. Antamina skarn Cu-Zn-Mo­Ag deposit, showing the west-dipping contact (indicated by arrows) between gossan replacing garnet skarn (lower valley side) and overlying marbleized limestone. 1997 photograph. b. Donoso orthomagmatic breccia, Rio Blanco-Los Bronces porphyry Cu-Mo deposit, showing angular clasts of sericitized quartz mon'l.Onite cemented by tounnaline (black), quartz (white ), and chalcopyrite (bronze) . Sample 30 cm long. c. Steep contact of the postmineraliz.'l.tion diatreme (indicated by arrow) cutting altered and mineralized porphyry and volcanic rocks, Rio Blanco-Los Bronces porphyry Cu-Mo deposit. 1970 photograph. d. Bajo de la Alumbrera porphyry Cu-Au deposit, shOWing potassic core (brown ), sericitic annulus (white), and propylitic pe­riphely (dark). The prominent low, dark hill within the potassic zone (indicated by arrow) comprises an early porphyry phase hosting an intensely developed stockwork of quartz-magnetite vein le ts rich in Cu and Au. 1970 photograph.

ANDEAN Cu PROVINCE 867

production at Rio Blanco (1969) and Los Pelambres (1999). Several Miocene porphyry CuoMo prospects in northern and central Peru (Rio Blanco, Canariaco, La Granja; Fig. 10) are also large but nonetheless remain unexploited. The Bajo de la Alumbrera porphyry Cu-Au deposit (Fig. 10d) entered pro­duction in 1997, but Cu-Au prospects elsewhere in the belt, including those in northern Peru, the Maricunga-El Indio sub-belt, and the Farall6n Negro district, have yet to demon­strate their economic viability (Fig. 10; Table 4).

Tecton01lUlgl1wtiC setting

Central Chile sub-belt: The late Miocene to early Pliocene sub-belt of central Chile extends for approximately 400 km along the Principal Cordillera, between lat 32° and 35°S (Fig. 1). It is principally defined by a narrow, north-trending array of major deposits that formed between approximately 12 and 4 Ma, although numerous porphyry Cu prospects of the same general age occur beyond the sub-belt, especially immedi­ately east of the border with Argentina (Fig. 10).

Volcanic and plutOniC rocks within the sub-belt are asSigned to three main stages (Kurtz et al. , 1997; Kay et al. , 1999; Kay and Mpodozis, 2001; Maksaev et al., 2004): (1) mafic to silicic flows and volcaniclastic strata that accumulated in a large, fault-controlled extensional basin during the late Eocene to early Miocene (37-21 Ma; Charrier et al., 1996; Godoy et al., 1999; Kay et al. , 1999), which are cut by approximately 20 to 16 Ma plutons at Rio Blanco-Los Bronces and in the El Te­niente area; (2) andesitic to dacitic flows and pyroclastic units of middle to late Miocene age (-16-7 Ma), intruded by co­magmatic granodiOrite plutons and porphyry Cu-bearing stocks between -12 and 8 Ma; and (3) Rio Blanco-Los Bronces and El Teniente porphyry complexes oflate Miocene to early Pliocene age (-7-4 Ma), followed, at EI Teniente, by postmineralization hornblende-bearing dikes between 4 and 3 Ma. Contractional deformation events, involving crustal shortening, thickening, and regional uplift, took place at 19 to 16 and 8 to -5 Ma (Kurtz et al. , 1997). The latter event caused most of the uplift and exhumation in the sub-belt, with surface uplift rates of 3 krnlm.y. claimed at the latitude of EI Teniente (Kurtz et al., 1997), and concomitant eastward mi­gration of the arc front (Kay et al. , 1999). The erosional prod­ucts generated during the regional uplift were deposited in synorogenic basins along both sides of the orogen, but only the foreland sites are preserved, east of the continental divide in Argentina (Perez and Ramos, 1996; Giambiagi et al., 2001; Perez, 2001; Fig. 12), where they underwent hybrid, thin­and thick-skinned deformation (Giambiagi and Ramos, 2002).

In marked contrast to the late Eocene to early Oligocene belt of deposits in northern Chile, these younger, giant Cu de­posits are not observed to lie on major regional faults, al­though the Aconeagua fold-and-thrust belt is located imme­diately to the east (Cegarra and Ramos, 1996; Fig. 12). Along the western side of the sub-belt, the regional Pocuro fault may have controlled Miocene uplift and inversion of Meso­wic and Cenowic basins (Godoy et al. , 1999; Camus, 2003; Fig. 12). Nevertheless, smaller scale structures, structural corridors, and intersecting faults are important in all depOSits, as exemplified by the -14-km-long, northeast-trending EI Te­niente fracture zone (Garrido et al. , 1994; Skewes et al. , 2002) and the -12-km-Iong, north-northwest alignment of the

entire Rio B1anc-o-Los Bronc""s system (Serrano et al., 1996; Table 4).

The porphyry Cu deposits lind prospects in the sub-belt are associated with multi phase porphyry !ntn.slons comprising quartz monwnite, quartz monzodiorite. quartz diorite, dior­ite, and/or dacite that intrude Tertiary vole"n!c and plutonic rocks of the different magmatic stages described above (Table 4). Cretaceous volcanic rocks are also present at Los Pelam­bres (Sillitoe, 1973b) and marine sedimentary rocks of Juras­sic age occur at El Pach6n (Pach6n S.A. Minera, 1999; Tuble 4). The porphyry Cu-bearing stocks are I-type and be long to the magnetite series of Ishihara (1981). They lire typical medium to high K calc-alkaline in composition and possess chemical affmities typical of central Andean Tertiary igneous rocks (Stem and Skewes, 1995). Their high Fe,O,tFeO ratios (1-3; Garrido et al. , 2002) imply a high oxidation state. Asso­ciated intermineralization intrusions and hydrothermal brec­cia complexes are characterized by restricted ranges of Sr. (0.7041-0.7046) and Ei;J (0-4; Skewes and Stem, 1994, 1995) and in the case of El Teniente, high LaIYb ratios (2()...Q()), in­terpreted to reflect thickened crust (Camus, 2003). Intermin­eralization dacite intrusions are common in all deposits and late-mineralization latite dikes are recognized at Rio Blanco­Los Bronces and EI Teniente, with the former deposit also having rhyodacite porphyry and related diatreme breccia (Fig. 11e; Table 4). There is a general trend for younger in­trusions to display more evolved compositions, with higher SiO. and K,O contents, as is observed at Los Pelambres (Atkinson et al., 1996) and Rio Blanco-Los Bronces (Stem and Skewes, 1995; Serrano et aI., 1996). However, the reverse is apparent at EI Teniente, which contains postminerali7.ation, hornblende-bearing andesite dikes (Skewes et aI. , 2002), in­cluding possible Jamprophyric varieties (Cuadra, 1986; Skewes et al., 2002), suggestive of the existence at depth of more mafic magma.

Maricunga-El Indio sub-belt: The northern continuation of the central Chile sub-belt follows the Argentina-Chile fron­tier between lat 2SO and 31° S, where it constitutes the Mari­cunga-El Indio sub-belt (Vila and Sillitoe, 1991; Fig. 10). Calc-alkaline magmatism was active at broadly the same times as in central Chile, although shallower erosion levels preserve widespread stratovolcanoes, dome complexes, and shallowly emplaced stocks. Several discrete pulses of volcan­ism, each followed by magmatic lulls coincident with contrac­tional deformation and crustal thickening events, took place between 26 and 7 Ma (Kay et aI., 1994, 1999; Martin et al., 1995, 1997; Mpodozis et aI., 1995; Clavero et aI., 1997; Kay and Mpodozis, 2001). The late Oligocene to early Miocene rocks possess low to moderate LaIYb ratios (7-21 ), whereas younger rocks display higher ratios, commonly >20 until 16 Ma, indicative of progressively thickening crust. Cessation of contractional events in the Maricunga area at -12 Ma is sug­gested by normal faulting and LalYb ratios of 15 to 22 (Kay et aI., 1994; Mpodozis et al ., 1995). The second and prinCipal contractional event is recorded by progressively increasing LaIYb ratios, which reach maxima of 55 to 75 in trivial vol­umes of rhyolite and basaltic andesite erupted at -6 Ma (Kay et al. , 1994; Mpodozis et al. , 1995).

The porphyry Cu-Au and CuoMo prospects (e.g., Cerro Casale, Regalito) in the northern part of the Maricunga-El

TABLE 4. Selected Geologic Characteristics of POIphyry and Porphyry-related Skarn Cu Deposits and Principal Prospects, Miocene to Early Pliocene Belt, Central Andes '" &l Status and Production + ZOO3 reserves (million Ore-related production l metric tonnes) hypogene

Deposit! (metric tons and gmde (%) Porphyry Host alteration Hydrothennal Massive Structural prospect Cu x 1,(00) (cutoff, % Cu) composition Rocks (mineralization) Age (Ma) breccias sulfide veins control Key references

Potassic over-Cretaceous printed by inter-marine ca]- mediate argillic "",eous and sericitic. Uosa et aI. (1999), rocks and Periphera] L10sa and Veliz

Perol SulE 429 @ 0.31 Thr~hases: Eocene advanced Orthoclase: Minor igneous Important: (2000), Gustafson (Minas Conga) Prospect Cu, 0.78 r!/I Au gran 'orite intrusions argillic (cp, bnl' 15.80.0.09 breccias Absent NW faults el aI. (2004)

Potassic over-Cretaceous printed by inter- Uo,a el aI. (1999), marine cal- mediate argillic Important: Uosa and Veliz

Chailhuag6n SulE 190 @ 0.28 One phase: careous and sericitic Biotite: Minor igneous NNE-trending 2000), Gustafson (M inas Conga) Prospect Cu, 0.77 r!/I Au granodiOrite rocks (cp, bn) 15.58.0.1Z breccias Absent intrusions el aI. (ZOO4)

Biotite: 10.15 %

0.04; lllitized Important: SulE 559 @ 1.24 Cretaceous Potassic over- &Iafoclase; severa] phases, O'Connor (1999), Cu, 1.03 Zn, At least five marine caI- printed by inter- .9 .0.06; i~eous and Important: Love et aI. (2003, '" Mine 0.OZ9 Mo, 13.7 phases: careous mediate argillic Sericite(?): c. rea.tic NW thrusts 20(4), Redwood " Antamina 341.4 r!/I Ag (0.7) monzogranite rocks (cp, bnl' 9.8.0.Q7 Absent and fold axes (ZOO4) " recctas ~

Potassic over- Present: py, Important: Cl '" printed by inter- Biotite: tet, stibi- domlnantlr NS ~ Cretaceous mediate argilliC, 15.3.0.7; Important: oluzonite, thrusts and

marine caI- sericitic, and Mo~bdenite: igneous brec- luzonite, ENE-trending Perell6 et al. " Sul[; lOS@0.74 Two phases: =eou, advanced 14 . 3:l 0.02, cias and few realgar, intrusion (ZOOl), J. Perell6, <:l Magistral Prospect Cu, 0.05Z Mo (0.5) granodiOrite rocks argillic (cp) 14.63.0.02 pebble wke, orpiment corridor unpub. data, 2001 f;l

Permian Poorly deAned [:: volcanic concentric pat- Apparently

Q.

Four phases: and Triassic- tern: potassic absent, but diorite, grano- JurasSiC surrounded and py,ena~ Alvarez (1999),

Sul[; 618 @0.71 diorite. quartz calcareous overprinted by Biotite: tenn, an Important: Noble and Toromocho Prospect Cu (0.5) monzonite rocks sericitic (cp) 7.4 % 0.3 Important cv present NW faults Mckee (1999)

Potassic over- Vila and Sillitoe printed by Biotite: (1991), Mpedoz;,

SuiI', 1,285 @0.35 Miocene sericitic and ad- 13.89 • 0.04; Minor: igneous el aI. (1995), Cu. 0.02 Mo. severa]lh~es: volcanic vanced argillic in Alunite: and tourmaline Present: Present: NNE Muntean and

Cerro Casale Prospect 0.7 r!/I Au quartz onte rocks upper parts (cp) 13.91 .0.04 breccias tet-tenn. cp and NW faults Einauw (2001)

Biotite: 7.1O:l 0.13,6.98 .0.08, Sasso and Clark 6.83 .0.07; (1998), Ulrich

Concentric pat- Sericite: 6.75 % and Heinrich Seven phases: Miocene tern: potasSiC 0.09; Zircon, 7.10 (ZOOl ), ProlTelt

Bajo de la Mine Sulf, 805 @ 0.54 dacite to volcanic surrounded % 0.07, 7.98 % Minor: igneous Present: (ZOO3), Harris et Alumbrera 203.7 Cu, 0.64 Wt Au rhyolite rocks by sericitic (cp) 0.14 , 8.0Z • 0.14 breccias Absent NNW faults aI. (2004)

Precambrian- Important: early Potassic over- Biotite: phreatiC and Pere1l6 et aI . Paleozoic printed by in- 5.10.0.OS; phreato- Important: (1998), Rojas et

Sul[; 750 @0.66 Severa] phases: metasedi- tense sericitic Alunite: magmatic, Important: intersecting aI. (1998), Cu, 0.037 Mo, monzonite mentary and advanced 4.96 % 0.08, barren late- Cy, cv, enarg. NE and Landtwing et aI.

Agua Rica Prospect 0.23 r!/I Au (0.4) to dacite rocks argillic (cp) 4.88.0.08 mineral diatreme n, cc NW faults (ZOOZ)

-------;;

T ABLE 4 . (Cont .)

Status and Production + 2003 reserves (million Ore-related production I metric tonnes) hypogene

Deposit! (metric tons and grade (%) Porphyry Host altemtion Hydrothe rmal Massive Stlllctural prospect ell x 1,000) (cu toff, % Cu) composition Rocks (m ineraliz.:ltion) Age (Ma) breccias su lfide veins control Key rere rences

Biotite: 10.2", 0.2, 9.9 :I: 0.2; Biotite and sericite: J 2.39 :i:

0.10- 10.19.0.09, Molybdenite:

Concentric IU8.0.04. Sule, 3.300 @ 0.63 Cretaceous pattem: 11.08 :t 0.04; Absent, Si llitoe (1973, Cu, 0.016 Mo (in- Four phases: volcanic potassic Late-minernl, Present: but minor Present : 1988), Atkinson eluding supergene: quartz diorite rocks and surrounded and zircon : pe3mlltoidal As-rich intersecting el al. (1900).

Mine 560 @ 0.93 Cu) to quartz tonalite overprinted by 11 .56 :t: 0.15, an tounnaline stnlctures NNE and Be rtens et al. Los Pelmnbres 335.5 (0.4) monzonite pluton sericitic (cp, bn) 11 .24 .0.12 breccias recorded NNW faults (2003)

Ju rassic Poorly defined marine ca1. concentric pat· Absent. but Important:

SulE 2.106 @ Two phases: careous and tem: potassic Importnnt: py. e narg, NW-trending 0.55 C u, diorite·tonalite. Cretaceous surrounded by tOllnnaline tenn , and tet intrusion Pach6n M illera ).

EI Pach6n Prospect 0.013 Mo (0.45) dacite volcanic rocks sericitic (cp, bn") Latc Miocene breccias in breccias corridor S.A. (1999) <: 0

Bioti te: 4.59-~ <:

5. 12; Sericite: Important: " " 4.40 ± 0.15. major con:r.lex :i 4.37 .0.06, with abun ant Absent, but Important : Wamaars et al. 0 Several phases: Zircon: 6.16- tourmaline py. luzon ile. NNW-trending (1985). Se rnmo el <: quartz mon- Miocene 5.08; Late- bret:cills. enarg. and mine ralize d al. (1996). Vargas :;: zonile. quartz volcanic and Potassic mineral dia· barren late- tenn in corridor and el aI. (1999). " '" IHo Blanco- Mines SulE 6.991 @ 0.75 monzod iorite, intnlsive overprinted by treme. zircon: mineml minor late NE faults and Deckart et al.

Los Bronces 468.5 Cu. 0.02 Mo (0.3) dacite, latite rocks sericitic (cp, bll) 4.92 ± 0.09 diatreme veinlets fractures (2005)

Biotite and sericite: 5.06 ±

0.12-4.37.0.10, Molybdenite: 6.30 • 0.03. 5.60 .0.02.5.01-4.96. 4.89 • 0.01>-4.78 ± 0.03, 4.42 ± Important: 0.02; Zirt"On: 6.46 igneous, biotite,

Miocene ± 0.11-5.28 ± an hydrit e, and Camus (19'7S. Sul f: 12,480 @ volcanic 0.10; Late· tourmaline 2003). Cuadr. 0.63 Cu, 0.02 Mo Five phases: rocks and mineral in tru- breccias, low- Absent, but Present (1_1_ .. (including supe r- quart'1: diorite- mafic Potassic locally sions, zircon: gmde late- py. tet-tenn, intersecting ~S. ... (2OOl).

Mine gene: 956@ tonalite and laccoliths overprinted by 4.82 .0.09. mineml stibnile, and NE. and M.-.· ..... EI Te niente 334.3 1.68Cn) dacite and sills sericitic (cp. bn) 4.58. 0.10 diatreme bn in vein lets NW r.aults (iOOf)

Abbreviations: btl '" bornite, cc '" chalcocite. cp '" chalcopyrite. cv '" t.'ovellite. enarg '" enargite, Sulf ", sulfide, tenn '" tennantite, tet • tetrahedrite I Figures from Comisi6n Chilena de l Cobre and Dirccci6n General de Mineria (Peru) 2: Minor skarn-type mineralization present 3 Skarn-type minemlizlltion dominant Co ~ Minor sulfide minernls italicized 0>

<D

870 SILL/TOE AND PERELL6

LOS PEL""~'S

CERRO BAY" De. C9~~s

A

G Neogene synorogenic deposits

CORDON DE LA RAMADA

A

[=:J Other geologic units

~ Neogene volcanic arc

1:::::::::1 Neogene synorogenic deposits

[±] Permo·Triassic basement

? Reverse fault

? sNormaJ fault

/' Cross section

• Porphyry Cu-M>

• Porphyry Cu-Au

SOkm

B

+

_ Porphyry copper stock (projected)

~ Dominantly late Mesozoic and Tertiary volcanic rocks

~ Mesozoic volcanic and sedimentary rocks ./ Reverse fault

[::±J Permo-Triassic basement

Flc. 12. Structural setting of the Miocene to early Pliocene Cu belt in northern Chile, showing deposit types, main faults of the Aconcagua fold and thrust belt, and synorogenic clastic sedimentary deposits within and alongside the arc. Note that eu deposit formation is syntectonic. Schematic geologiC section (along line A-B in map) emphasizes importance of reverse faulting. Fold and thrust belt and synorogenic deposits rrom Ramos et al. (1996), raults in Chile rrom SERNACEOMIN (2002), and section Simplified from Cristallini (1996).

Indio sub-belt were generated during the modest crustal thickening event between 18 and 14 Ma (Fig. 10), whereas mineralization coincident \vith the extension-related volcan­ism between 23 and 21 Ma (Sillitoe et aI. , 1991) and during tectonic relaxation at 14 to 12 Ma (Sillitoe et aI., 1991; Mpodozis et aI., 1995) gave rise to Au- and Ag-rich, but Cu­poor porphyty and high-sulfidation epithermal deposits (Vila and Sillitoe, 1991). Farther south, at EI Indio, the enargite­bearing veins formed during the contractional event in the late Miocene (9--6 Ma; Bissig et aI., 2001).

The Cu-Au-bearing pOlphyry stocks are multiphase, horn­blende ± biotite-bearing, fine - to medium-grained quartz diorite porphyries intruded at subvolcanic levels (Vila and Sillitoe, 1991; Mpodozis et aI. , 1995; Muntean and Einaudi , 2001). In contrast, the porphyry Cu-Mo prospects are re­lated to stocks of dacitic composition (e.g., Pe re1l6 et aI ., 2003c). The porphyty depOSits and prospects tend to occur as clusters or alignme nts localized at or near key structural sites, especially regional faults cutting basement rocks and fault intersections.

ANDEAN Gil PROVINCE 871

FarallOn Negro district: The FaralJ6n Negro district lies in the Andean foreland, -500 km east of the Chile-Peru trench, at the transition between two Neogene physiographic provinces, the Puna and Sierras Pampeanas (Figs. 1, 10). This transition w ne coincides with a discontinuity in Mesozoic and Cenozoic geology, geomorphology, and metallogeny (Sillitoe, 1974; de Urreiztieta et aI., 1996; Sasso and Clark, 1998). The district broadly overlaps the Farall6n Negro volcanic complex, a large, multicentered edifice of shoshonitic to high K calc-al­kaline composition, made up of several basaltic to silicic vol­canic units with ages mainly between 8.5 and 6.8 Ma (Sasso and Clark, 1998; Proffett, 2003; Harris et aI., 2004). These rocks unconformably overlie Miocene continental sequences and Paleozoic crystalline basement (Caelles et aI., 1971; McBride et aI., 1976), the latter exposed in the contiguous Sierra de Aconquija block, host to the Agua Rica porphyry Cu­Au-Mo deposit. Stocks and dikes of dacitic to rhyolitic compo­sition were emplaced between -8 and 5 Ma in both the Faral-16n Negro volcanic complex and contiguous areas (Perell6 et aI., 1998; Sasso and Clark, 1998; Harris et aI., 2004).

Regional uplift and contractional defonnation were active since 10 Ma (Jordan et aI. , 1983; Allmendinger, 1986; Cough­lin et aI. , 1998; Perell6 et aI., 1998; Sasso and Clark, 1998; Ramos et aI., 2002), resulting in juxtaposition of the Sierra de Aconquija basement block and FaralI6n Negro volcanic com­plex on regional reverse faults between - 10 and 5 Ma and since 3 Ma (Coughlin et aI., 1998; Ramos et aI. , 2002).

Northe", and central Peru segment: The segment extends for - 1,000 km along the Western Cordillera between lat 6' and 13' S (Figs. 1, 10), where it is centered east of the Meso­zoic and early Paleogene coastal batholith and formed in stratigraphically and structurally complex Paleozoic to Ceno­zoic rocks dominated by marine-carbonate sequences. M ulti­pIe episodes of contractional tectonism affected these rocks, with the most important and Widespread events being in the Late Cretaceolls (Peruvian), middle Eocene (Incaic), early Miocene (Quechua I), middle to late Miocene (Quechua II and III), and latest Pliocene to early Pleistocene (Megard, 1984; Sebrier et aI., 1988; Sebrier and Soler, 1991; Benavides­Caceres, 1999; Noble and McKee, 1999).

Magmatic rocks along the belt are of typical calc-alkaline composition. Several pulses of volcanism and intrusive activ­ity, broadly contemporaneous with mineralization, took place during the Miocene (-24-4 Ma), since when magmatism was inactive. Intermediate-composition lavas and pyroclastic rocks are dominant, \vith deposition of local silicic ash-fl ow tuff accompanying emplacement of late phases of the Cordillera Blanca batholith in north-central Peru and farther north. Minor, but conspicuous amounts of basalt in the volcanic se­quences (Noble and McKee, 1999) show that mafic magma existed at depth. Limited chemical and Sr; and £i;J isotope data for the igneous rocks from the belt are consistent with magma derivation from relatively discrete sources in the lithospheriC mantle or underplated mafic crust (Noble and McKee, 1999).

Porphyry Cu prospects and related ClI-bearing skarn de­posits are associated with calc-alkaline, granodiOrite to mon­zogranite, Single to multiphase porphyry stocks emplaced into intensely folded and tllrusted, mainly Cretaceous carbonate and clastic strata (Table 4). Coarse-grained dacite or quartz

latite porphyry, typically occurring IlS dome c'Omplexes, is prominent in many shallowly eroded deposits (e.g., Cerro de Pasco, Marcapunta, Julcnni; Fig, [0), Regional thrusting and reverse faulting during the Miocene Quechuu cv nts reacti­vated older structures of the Eoc'Cne Incule fold-nnd-thrust belt defined by Megard (1984), althollgh distinction between these two deformation ages is not eusy (e,g" SUllins, 1977). Evidence for syntectonic emplac'C ment of' porphyry Cu stocks and dikes during Quechuan phases comes rrom Anhornin" (Love et aI., 2003), EI Galeno (C6rdova and Hoyos, 2000), Magistral (Pere1l6 et aI. , 2001), and Pachag6n (Per 116 et nl. , 2oo3d), and is suspected elsewhere.

All ClI deposits in ti,e northern and central Peru segm ' nl formed between -20 and 6 Ma (Fig. 10). Mineralization ap­pears to have taken place Simultaneously throughout the seg­ment at many different times and places during the Miocene, altilOugh the 20 to 18, 15 to 13, and 10 to 7 Ma intervals are suspected to have been the most fertile (Noble and McKee, 1999).

Principal mineralization features

The major porphyry Cu-Mo deposits of central Chile are characterized by large areal extents (3-6 km'), multistage hy­drothermal breccias, and very low Au tenors. Early alteration is principally potassic and associated with Cu mineralization, although Cu- and S-poor assemblages characterized by calcic actinolite, oligoclase-albite, and magnetite define an even earlier event at Rio Blanco-Los Bronces (Skewes et aI., 1994; Serrano et aI., 1996) and EI Teniente (Skewes et aI., 2002), Biotitization. particularly intense in andesitic and more mafic host rocks, is associated with the first major stage of chal­copyrite and bornite introduction. At Los Pelambres, potassic alteration also contains andalusite and corundum (Skewes and Atkinson, 1985; Atkinson et aI. , 1996), whereas at Rio Blanco-Los Bronces magnetite, specular hematite, and tour­maline are accessory alteration minerals (Serrano et aI., 1996). Approximately 50 percent of the hypogene mineraliza­tion at Rio Blanco-Los Bronces (Serrano et aI., 1996) and >80 percent at EI Teniente (Camus, 1975, 2003) are contained in biotite-dominated potassic alteration, which also hosts a major part of ti,e hypogene ore at Los Pelarnbres (Atkinson et aI., 1996) and the contiguous EI Pach6n deposit (PacMn S.A. Minera, 1999).

Sericitic alteration is present in all the central Chile por­phyry Cu deposits. At Los Pelambres and EI PacMn, it occurs mainly as barren, pyritic halos surrounding the potassic cen­ters (Sillitoe, 1973b; PacMn S.A. Minera, 1999; Table 4), whereas at Rio Blanco-Los Bronces and EI Teniente it is mainly confined to certain hydrothennal breccia bodies. Ad­vanced argillic alteration is scarce in the major central Chile porphyry Cu depOSits, probably because of relatively deep erosion levels, although the basal parts of lithocaps are pre­served in several prospects within and east of the sub-belt (e.g., Piment6n, EI Altar, and Los Bagres Sur; Fig. 10). High­sulfidation sulfide assemblages are widespread at EI Altar and Los Bagres Sur but in ti,e major deposits are restricted to rel­atively minor occurrences of e nargite, iuzonite, tetrahedrite, and tennantite in sericitic breccias and veinlets (Table 4).

An outstanding featu re of some, but not all , central Chile porphyry Cu deposits and prospects is the presence of

872 SILL/ TOE AND PERELL6

voluminous hydrothennal breccias (Howell and Molloy, 1960; Wamaars et al. , 1985; Skewes and Stern, 1994, 1995; Serrano et al., 1996; Vargas et al., 1999; Skewes et al., 2(02). The ore­bearing breccias are considered of orthomagmatic origin, whereas later barren breccias are products of phreatomag­matic processes (Sillitoe, 1985; Fig. llb, c). The breccias vary in fonn from dikelike bodies to well-defined funnel-shaped pipes, with diameters ranging from tens of meters at Los Pelambres (Sillitoe, 1973b) to >1,200 m in the case of the Braden pipe at El Teniente, which has a known vertical extent of 1,800 m (Camus, 1975, 2003; Cuadra, 1986). At RIO Blanco-Los Bronces, multiple centers of texturally diverse breccias, occupying a total volume of -3 km3, define a -12-km-long and >1-km-wide, north-northwest-striking corridor (Warnaars et al., 1985; Serrano et al., 1996; Vargas et al., 1999; Fig. llb, c). The breccia complexes formed throughout the evolution of the porphyry Cu systems, as suggested by the presence of actinolite, biotite, chlorite, anhydri te, or tourma­line as principal cementing minerals (Warnaars et al., 1985; Serrano et al., 1996; Vargas et al. , 1999; Skewes et al., 2(02). Tourmaline-cemented, sericitic-altered breccia is transitional downward to potassic-altered breccia (e.g., Vargas et al. , 1999). High-grade (> 1 % Cu), breccia-hosted ore is present in all deposits of the sub-belt but is most Significant at Rio Blanco-Los Bronces where an estimated 50 percent of the Cu is contained in breccia (Serrano et al ., 1996). The proportion of the Cu resource hosted by breccia decreases appreciably to 10 to 15 percent at El Teniente (Camus, 2(03) and to only 2 to 3 percent at Los Pelambres (A. Gonzalez, pers. commun., 2(03), although larger percentages have been proposed (Skewes and Stern, 1994, 1995; Skewes et al ., 2(02). More­over, the volumetrically dominant hreccia at El Teniente is the predominantly subore-grade Braden pipe.

Porphyry e u-Au deposits and prospects in the Miocene to early Pliocene belt (Fig. 10) share all the geolOgiC features of such systems elsewhere. These include dominance of mag­netite-rich potassic alteration variably overprinted by inter­mediate argillic assemblages and an overall sympathetic relationship between Cu and Au grades. The alteration­mineralization zoning at Bajo de la Alumbrera and other por­phyry eu prospects in the Farall6n Negro district (Fig. lld), in common with the Au-poor Los Pelambres deposit (see above), confonns closely to the classic geometry defined by Lowell and Guilbert (1970), with potassic cores surrounded by annular sericitic zones (Sillitoe, 1973c; Proffett, 2003). Hy­pogene Cu-Au mineralization at the Bajo de la Alumbrera de­posit (Sillitoe, 1979; Ulrich and Heinrich, 2001; Proffett, 2003), and most other prospects in the belt (James and Thompson, 1997; Llosa et al. , 1999; Muntean and Einaudi, 2(01), occurs as quartz-magnetite-chalcopyrite stockworks contained within the potassic zones (Fig. lld). In contrast, the Agua Rica porphyry Cu-Au-Mo deposit in the Farall6n Negro district displays more complex alteration-mineraliza­tion relationships, with early potassic alteration intensely overprinted by the main Cu-bearing sericitic and advanced argillic assemblages and numerous hydrothermal breccias, in­cluding a diatreme complex (Koukharsky and Mirre, 1976; Pere1l6 et al. , 1998; Landtwing et al., 2002; Table 4). Else­where in the belt, advanced argillic lithocap remnants, such as those at Minas Conga (Llosa et al. , 1999) and Cerro Casale

(Vila and Sillitoe, 1991), are poorly mineralized, although that above porphyry eu-Au centers at Yanacocha hosts the world's largest high sulfidation Au deposit (Gustafson et al. , 2(04).

The Antamina eu-Zn-Mo-Ag and other much smaller skarn deposits in the northern and central Peru segment abut composite porphyry stocks displaying potassic alteration and low-grade porphyry Cu-Mo mineralization (e.g., Redwood, 2(04). Garnet-rich exoskarn hosts much of the ore (Fig. 11a). In other similar systems, such as Magistral, La Granja, and Pashpap (Fig. 10), the porphyry-type mineralization is higher grade but still subeconomic (e.g., Schwartz, 1982; Torres and Enriquez, 1997; Pere1l6 et aI. , 2001; Table 4).

Elsewhere in the northern and central Peru sub-belt, enar­gite-bearing, carbonate-replacement bodies occur in the cen­tral parts of complexly zoned polymetallic districts, as at Cerro de Pasco, Marcapunta, Yauricocha, and Morococha (Petersen, 1965, 1970; Sillitoe, 1990; Noble and McKee, 1999; Bendezll et al. , 2004; Vidal and Ligarda, 2(04). Some of these bodies are located alongSide stocks containing porphyry eu alteration and mineralization, as observed at Morococha­Toromocho (Alvarez, 1999). Petersen (1970) included such deposits in his zoned Cu-Zn-Pb-Ag deposit category, and Noble and McKee (1999) considered them as a hallmark of the metallogeny of northern and central Peru. The deposits are extremely varied in form and include veins, breccia pipes, mantos, and irregular bodies, the larger examples displaying conversion of limestone to quartz and pyrite. This replace­ment assemblage, which may be considered as a low-pH eqUivalent of skarn (Einaudi, (982), is accompanied by sericitic and/or advanced argillic alteration in adjacent oon­carbonate litholOgies (Einaudi, 1977; Sillitoe, 1990; Vidal and Ligarda, 2004).

Mature, multicyclic enrichment blankets are absent at the eu deposits in the Miocene to early Pliocene belt, primarily due to their youthfulness and the commonly unsuitable late Cenozoic and presently prevailing climate, particularly in northern and central Peru and central Chile, where steep, deeply incised topography and Quaternary glaCial erosion are additional inhibiting factors (e.g. , Redwood, 2004; Sillitoe, 2(05). Reactive host rocks and low pyrite contents also mili­tate against enrichment in some of the deposits (e.g. , Baja de la Alumbrera). Nevertheless, there are exceptions, such as the supergene chalcocite additions to shallow hypogene zones at several major depOSits and important prospects (e.g., RIO Blanco, La Granja, Toromocho, Agua Rica, Los Pelambres, Rio Blanco-Los Bronces, El Teniente; Braun et aI. , 1999; Schwartz, 1982; Alvarez, 1999; Pere1l6 et al., 1998; Atkinson et al ., 1996; Warnaars et al., 1985; Cuadra, 1986, respec­tively). The enrichment is believed to have taken place during the last 3 m.y. and to be still active (Sillitoe, 2005).

Metallogenic Discussion

Porphyry Cu deposits

Allert/tion-minerali;;ation ;;aning: Andean porphyry Cu ±

Mo ± Au depOSits, in common with those elsewhere, display a gross vertical alteration-mineralization zoning from deep potassic through sericitic zones of varied geometry to over­lying advanced argillic lithocaps (e.g., Sillitoe, 1995a, 2oooa; Fig. 13). Pyrophyllite is particularly characteristic of the

ANDEAN CII PRO VINCE 873

: . : . .' )---:c-• ....:. x , '

x

x , '.'

.- :: , , , • t • , , +0+ • , , , + +, ' + ' , , • 0+" ,

, + ,

x x • .+~ " x x + , ,

• "+ 0

, x x x ' + x . + ' + ' , , ' + ' +

+ <- x x ,+ ' x : + + + ' , X '+. . t . , '1\ x x ° +0+

X 00 + 0" x , , H '~ x x 0 ,

~ Coeval volcanic rocks

~ Multip/1asestock

I".' .' :1 Advanced argillic tithocap

C8 Sericitic zone / Massive sulfide vein

¢ Reverse fault ~ Undifferentiated Mesozoic rocks

~ Paleozoic basement

I', ': : 1 Potassic zone

FIG. 13. Cartoon showing typical alteration zoning and degrees o f te lescoping in Andean porphyry Cu deposits. Note the telescoped deposits in the uplifted fault block, contrasting with the lack of telescoping in the deposit beyond it, where coeva1 volcanism is well developed. The telescoping in the uplifted block varies from strongly to nonfault controlled. Pro~mity of deposits is not necessarily implied. See text for further discussion .

sericitic to advanced argillic transition. Early. Cu-deficient ca1cic-sadic assemblages may be preselVed as remnants, es­pecially at deep levels, within or beneath potassic zones. Fur­thermore. chlOrite-bearing intermediate argillic alteration acts as a transition between the potassic and sericitic zones in some systems, including many of those rich in Au. Where car­bonate host rocks predominate, calcic skarn abuts the por­phyry stocks and quartz-pyrite alteration characterizes the ad­vanced argillic environment. Therefore. the fundamental controls on the dominant alteration type obselVed are erosion level and host-rock composition. Neveltheless, shallowlyex­posed systems, with preserved lithocap remnants. are known from all the Andean metallogenic epochs: Triassic (Lila). Early Cretaceous (Buey Muerto), Paleocene to early Eocene (Sierra Gorda), middle Eocene to early Oligocene (Escon­dida), and Miocene to early Pliocene (Agoa Rica).

The overall alteration-mineralization architecture is heavily dependent on the structural setting and degree of telescoping of Andean porphyry Cu systems (Fig. 13). These factors vary between structural blocks, as a result of stress partitioning, and do not appear to be constant throughout entire segments of a metallogeniC belt. in structural blocks characterized by high rates of synmineralization tectonic uplift, at least 100 to 200 mlm.y. (Maksaev and Zentilli. 1999), the vertical alter­ation-mineralization sequence tends to be highly telescoped, with appreciable overprinting and mineralOgic reconsti tution of earlier and deeper zones during younger and shallower hy­drothennal events. The overprint geometry. espeCially the form and position of the sericitic and deep parts of advanced argillic wnes, is highly influenced by faults active during de­posit genesis (Fig. 13).

The mineralogic reconstitution of earlier alteration-miner­alization zones can either add or remove Cu and Au . Super­position of intermediate argillic or sericitic on potassic alte r­ation in porphyry Cu-Au deposits commonly causes partial leaching of Au, although exceptions exist (e.g., Perol at Minas Conga and Agoa Rica; Fig. 10, Table 4). Sericitic alteration overprinting potassic zones, common in porphyry Cu-Mo de­posits, may be pyritic and essentially barren or, alternatively, contain Cu-bearing assemblages dictated by a range of sulfi­dation states. These latter assemblages, also pyrite rich. may be dominated by chalcopyrite (e.g., Paleocene-early Eocene belt of southern Peru) or bornite. chalcocite, and/or covelUte, with or without major amounts of enargite (e.g., middle Eocene-early Oligocene belt of northern Chile). Where structural control is particularly severe, as in the largest deposits of the middle Eocene to early Oligocene belt of northern Chile (Rosario, Chuquicamata, MM . Escondida Norte-Zaldivar, Escondida), these high-sulfidation sulfide as­semblages constitute massive sulfide veins (Fig. 13). com­monly rich in enargite. but elsewhe re are of mainly dissemi­nated habit (e.g., Miocene-early Pliocene belt: Canariaco. El Altar. and Los Bagres Sur; Fig. 10). Enargite also dominates the massive sulfide bodies that replace carbonate rocks in the lithocap environment. In other deposits where structural con­trol is less evident, chalmpyrite-bearing sericitic alteration may be confined to hydrothermal breccias (e.g .. Rfo Blanco­Los Bronces). In structural domains characterized by high synmineralizatioll uplift rates. perhaps attaining 3 kmlm.y. at El Teniente (Kurtz et al. , 1997), lithocaps and subjacent sericitic zones could be lost to e rosion in as little as 1 m.y. to reveal the deeper potassic cores. In contrast, porphyry Cu

874 SILL/TOE AND PERELL6

deposits in structural sites that apparently underwent slower exhumation during mineralization tend to be less telescoped and to display classic alteration symmetry, with potassic cores surrounded by sericitic halos in their upper parts (Fig. 13). In such deposits, ti,e sericitic alteration is typically Cu deficient (e.g., Esperanza, Bajo de la Alumbrera).

Hydrothennal breccia.s: Many porphyry Cu deposits in the Andes, as elsewhere, contain relatively small volumes of hy­drothermal breccia as parts of their Cu ore w nes or as post­mineralization diluents, although some exceptionally large deposits lack breccia (e.g., Chuquicamata; Ossand6n et a1. , 2001; Tables 2-4). Three depoSits, Toquepala, Agua Rica, and Rio Blanco-Los Bronces, are notably rich in Cu-bearing brec­cias, with or without tourmaline, which constitute upwardly flared bodies that exceed 1 krn3. The largest breccias, 1 to 2 Ian across at surface . are the phreatomagmatic diatre mes in the Antapaccay, Toquepala, Exploradora, Agua Rica, RIo Hurtado, Rio Blanco-Los Bronces, and EI Teniente deposits, all notably late- to postmineralization in timing (Tables 2-4). Notwithstanding their large sizes, all these breccias are ac­companiments to normal porphyry Cu stockwork-style min­eralization. Assignment of such breccia-rich porphyry Cu de­posits to a discrete megabreccia deposit type (Skewes and Stem, 1994, 1995; Skewes et al., 2(02) lacks convincing geo­logic support.

The porphyry Cu depoSits exceptionally rich in Cu-bearing andlor postore breccias, as well as hydrothermal breccias in general, are widely scattered throughout the central Andean Cu belts (e.g., Toquepala, Antapaceay, Exploradora, and Agua Rica; Figs. 6, 7, 10), and are not an exclusive feature of the Miocene to early Pliocene belt in central Chile as implied by Skewes and Stem (1994, 1995). This fact casts serious doubt on the concept of rapid exhumation-induced "tectonic trig­gering" as a unifying explanation for hydrothermal breccia formation in the central Chile sub-belt (Skewes and Stern, 1994, 1995).

Lifespans and deposit size: A comprehensive isotopic study of the giant EI Teniente porphyry Cu deposit showed that multiphase intrusion, pre- and postore brecciation, and four distinct alteration-mineraliL1tion events spanned approxi­mately 2 m.y. (Maksaev et al ., 2(04). Compilation of other re­cent estimates of the lifespans of central Andean Cu deposits suggests that most of them were active for 1.2 to 2. 1 m.y., al­though estimates range from as little a. 0.27 m.y. at Potreril­los to - 3.6 m.y. at Escondida (Fig. 14).

Apparent time intervals of - 2 m.y. between ore-bearing potassic alteration and overprin ted high-sul fidation Cu stages at the Rosario, Chuquicamata, and Escondida deposits (e.g., Ballard et aI. , 2001; Masterman et aI. , 2004; Padilla-GaI-,w et al ., 2004) have been interpreted by some investigators to imply that tI,ese giant deposits resulted from the superposi­tion of two temporally discrete porphyry Cu systems (Zentill i et al., 1995; Reynolds et al., 1998; Ballard et aI. , 2001; Os­sand6n et al ., 2001 ; Padilla et al ., 2001; Padilla-Garza et aI. , 2(04). Harris et al . (2004) used the same concept to explain the I-m.y. separation between the two main Cu-mineralized porphyry stages that constitute the Bajo de la Alumbrera deposit. This two-stage concept seems an unlikely explanation for either the longevity or large size of Andean porphyry Cu depOSits because of the improbability of two discrete porphyry

ESCONDIDA

BA.JO DE ~AlUM8REAA

RiO BLANCO.;.LOS BAONCES

o 2 3 4 5 6

Flc. 14 . Compilation of the approximate lifespans of selected porphyry CU systems in the central Andes. Durations (dots) and errors (bars) taken from Marsh et a1. (1997). Ballard et aJ. (2001), Custafson et aJ. (2001), Os­sand6n et aJ. (ZOO1), Love et aJ. (2003), Harris et aJ. (2004 ), Maksaev et aJ. (2004), Masterman et aJ. (2004), Padilla-Carza et al. (2004), and Deckart et aJ. (2005). Note the preponderance o f depOSits with calculated li fespans of 1 to 2 m.y.

Cu deposits commonly being superimposed one on the other; and the failure to encounter at any of these supposed two­stage depOSits any evidence to support two potassic through advanced argillic alteration sequences and the corresponding reversals of veinlet generations (e.g., A follOwing 0 type). Rather, these 1- to 2-m.y. lifespans are believed to reneet the protracted , pulsed histories of intrusive and hydrothermal ac­tivity at most of the dated Andean porphyry Cu centers, irre­spective of their size and Cu content (cf. Maksaev et aI. , 2004; Masterman et al ., 2004).

Fe oxide-Cu-Au and "11m to- type Cu depOSits

In contrast to the porphyry Cu deposits, the main issues surrounding Andean Fe oxide-Cu-Au and manto-type Cu de­posits are genetic in nature, in large measure because of the ir commonly uncertain relationships to intrusive rocks. Iron oxide-Cu-Au deposits of the middle to late Mesowic belt are currently considered to be the products of either magmatic nuids (Ruiz et al ., 1965; Sillitoe, 2(03) or basinal brines heated by intrusions (Haynes, 2000; Hitzman, 2000). The spatial and temporal relationships of many Fe oxide-Cu-Au depOSits, especially those of vein type, to diorite plutons sug­gest a connection to mafic magmatism. The intimate relation­ships ,vith the slightly older or synmineralization diorite dikes underscores this connection still further (Sillitoe, 2003; Table 1). A deep source for the magmatic fluids seems likely, with delive,y to the mineralization sites utilizing the closely related major brittle-ductile fault systems (Fig. 5).

NOhvithstanding the distinctive geolOgiC featu res of the manto-type Cu deposits, some investigators consider them as shal low variants of Fe oxide-Cu-Au depoSits (e.g., Vivallo and Henriquez, 1998; Haynes, 2000; Orrego et al ., 2000). This in­terpretation is supported by the presence of widespread al­bite alte ration, calcite, and minor hematite, as well as the spa­tial relationship of a number of deposits to diorite bodies, features shared with some Fe oxide-Cu-Au deposits. How­ever, the asymmetric sulfide-oxide zoning and lack of Au, Co,

ANDEAN Gil I'ROVINCF. 875

Mo, Ni, and U in the manto-type Cu deposits contrasts with the Fe oxide-Cu-Au systems.

The origin of the manto-type Cu depOSits may be linked di­rectly to magmatic fluid exsolution (e.g. , Holmgren, 1987; Wolf et aI. , 1990) or to fluids generated during regional low­grade burial metamorphism of the volcano-sedimentary piles (Sato, 1984; Sillitoe, 1990, 1992). Maksaev and Zentilli (2002) suggested that Cu-bearing basinal fluids were mobilized dur­ing pluton emplacement, which is broadly coincident with the JurasSiC and Early Cretaceous manto deposit events. Manto­type Cu mineralization is localized where S- and Fe-poor flu­ids, of whatever origin, cross redox fronts and encounter or­ganic matter or diagenetic pyrite or undergo cooling andlor mixing with meteoric water (Sillitoe, 1992; Maksaev and Zen­tilli , 2002; Wilson and Zentilli, 1999). Genetic debate is likely to continue until any linkage of Fe oxide-Cu-Au and manto­type Cu deposits to stocks or plutons, as well as to one an­other, is satisfactorily resolved.

Tectonomagmatic Processes and Andean Cu Metallogeny

H'Ipogene deposits

Regional-, district-, and depoSit-scale geolOgiC relationships and isotopic ages for Cenomic Cu deposits of the central Andes suggest that the three economically preeminent por­phyry Cu belt segments-the middle Eocene to early Oligocene of northern Chile, Miocene to early Pliocene of central Chile, and Paleocene to middle Eocene of southern Peru-were generated within intervals of -7 to 13 m.y., dur­ing which contractional tectonism was active. The contraction is manifested by high- and low-angle reverse faults, which had only modest transcurrent components of motion (Camus, 2003; Skarmeta, 2oo3a, b; McClay, 2004). Although the faults were crucial to the location and geometry of some deposits (e.g., Chuquicamata), they are not considered to have been a basic prerequisite for porphyry Cu formation. Rather, ex­treme upper crustal shortening and thickening accommo­dated by these regional fault systems, and the consequent rapid surface uplift and exhumation (e.g., Maksaev and Zen­tilli, 1988, 1999; Maksaev, 1990, Skewes and Holmgren, 1993; Skewes and Stern, 1994; Perell6 et aI., 1996; Kurtz et aI. , 1997; Kay and Mpodozis, 2001), are considered as the funda­mental controls on the genesis of giant porphyry Cu deposits (cf., Sillitoe, 1998). Nevertheless, contractional events result­ing in lesser amounts of crustal thickening (say, to 40 km), such as that which produced the early Late Cretaceous inver­sion of the Mesozoic backarc basins (Mpodozis and Ramos, 1990; see above), appear to be metallogenically rather infer­tile. Severe contractional conditions can impede magma vent­ing and thus favor efficient magma storage in large, confined, shallow-level chambers, from which unusually voluminous amounts of magmatic fluid are eventually released for por­phyry Cu formation (Sillitoe, 1998; Perell6 et aI ., 2oo3a; Stem and Skewes, 2003; Fig. 15). Decompression, linked to rapid exhumation, may contribute to the fluid release (Sillitoe, 1998). The giant depoSits appear to be confined to magmatic fronts rather than to backarc regions, because that is where both Andean magmatism and deformation are focused.

IsotopiC data, summarized by Maksaev (1990), Sillitoe (1990), and Camus (2003), show that the ore-related magmas

a

b

c

Active vOlcanlam .........

Manto-type Cu depoaI1 Smo'_Co_

in volcanic sequence

Small porphyry Cu deposit

?

l.a<ge """""'" Co_ atop large parental dlamber

ubducted crustal sliver

Flc. 15. Cartoon tectonomagmatic sections of the centraJ Andean margin of northern and central Chile, shOwing selected features relevant to Cu belt fonnation . a. Exte nsionaJ to transtensionaJ arc (e.g., Middle JurasSic-Early Cretaceous belt), characterized by abundant andesitic to basaltic volcanism and intra- and backarc marine basins. developed over thin crust during steep subduction. Iron oxide-Cu-Au and manto-type Cu deposits predominate over the small porphyry Cu deposits. b. Neutral to mildlyertensionaJ mag­matic arc (e.g., PaJeocene--early Eocene belt), characterized by abundant in­te nnediate-composition volcanism, developed over fairly thin crust during relatively steep subduction. Porphyry Cu deposits are relatively small and low grade. c. ContractionaJ magmatic arc (e.g., middle Eocene-early Oligocene and Miocene-early Pliocene belts) without Significant volcanism developed over a thickening crust during low-angle subduction. Porphyry C li depoSits include giant. high-grade examples. Note active subduction erosiOIl . Inspired by Skewes and Stem (1994 ), Sillitoe (1998), and Ramos (2000).

are of unexceptional oxidized, calc-alkaline composition and dominantly mantle parentage (see above), albeit with volu­metrically restricted contributions from the lower crust and subducted slab. The elevated LalYb ratios (>20; see above) of ore-related magmatic rocks in the (.'Ontractional arc segments may be products of basalt-induced partial melting at the base of thickened lower crust, or alternatively, of partial melting of the mantle wedge contaminated by crustal material tectoni­cally eroded from beneath the forearc (e.g., Cornejo et aI., 1997; Kay et al. . 1999; Haschke et al. , 2002; Stem and Skewes, 2003; Kayet aI. , 2005; Fig. 150). The adakite-Iike sig­nature (steep REE patterns, heavy REE depletion, and high

876 SILL/TOE AND PERELLO

Sr and Na contents) emphasized recently for intrusions host­ing giant porphyry Cu deposits in nortllern and central ChUe (e.g., Oyarzun et al., 2001; Haschke et al., 2002; Kay, 2003), may be an additional indicator of magma genesis during crustal tluckening and/or subduction erosion (Kay, 2003; Kay et al., 2005), although Stem and Skewes (2003) attributed it to crystal-liquid fractionation and lIuid transfer processes in upper-crustal magma chambers. Volumetrically minor basaltic lava broadly synchronous with Cu mineralization is recog­nized in the contractional arcs (Mpodozis et al., 1993a; Noble and McKee, 1999; Skewes et al., 2002), suggesting that rela­tively primitive mafic magma was able to penetrate the thick­ened crust locally.

The contractional events coincident with Cenozoic por­phyry Cu formation, in southern Peru and nortllern and cen­tral Chile, are inferred to be responses to lIattening of sub­ducted slabs, leading eventually to cessation of magmatism and eastward arc migrations (Skewes and Stem, 1994; Sande­man et al., 1995; Kay et al., 1999; Haschke et al. , 2002; Mpodozis and Pere1l6, 2003; Pere1l6 et al., 2003a), in combi­nation with subduction erosion of the forearc (Stem, 1991; Kay et al., 2005; Fig. 15c). In the case of the middle Eocene to early Oligocene belt, the slab lIattening may be linked to accelerated convergence rates between the Farallon and South America plates (Pardo Casas and Molnar, 1987). In central Chile, in contrast, slab lIattening has been related di­rectly to progressive oblique subduction of the Juan Fernan­dez aseismic ridge (e.g., PUger, 1981; Yanez et al., 2001), in part based on an inferred southward-younging of the Cu de­posits (Skewes and Stem, 1994, 1995; Kay et al., 1999; Kay and Mpodozis, 2001). We note, however, that there is a lack of systematic longitudinal changes in age of the Cu deposits within each of the three Cenozoic belts (Figs. 6, 7, 10), sug­gesting that the slab lIattening and contraction were essen­tially synchronous along their fulllengtllS during metallogenie epochs. Therefore, although ridge subduction may have played a role in slab lIattening, it would appear to have no di­rect relationship with Cu deposit generation in central Chile and contiguous Argentina.

The Quechua II and III contractional phases of northern and central Peru may have been a consequence of slab lIat­tening caused by collision of buoyant oceanic features , in­cluding the Inca Plateau at -12 Ma (Gutscher et al. , 1999) and the aseismic Nazca Ridge at -7 to 5 Ma (Machare and Ortlieb, 1992; JaUlard et al. , 2000). Notwithstanding forma­tion during the Quechuan orogeny, the Miocene to early Pliocene Cu belt in northern and central Peru contains por­phyry Cu depOSits of only moderate size and grade, although the Antamina porphyry-related skarn, formed during uplift (Love et al., 2003), does attain giant status. The intermediate­size, but relatively high grade, hypogene porphyry CuoMo de­posits in the inland Middle to Late Jurassic belt of southern Colombia and southern Ecuador form ed in an uncertain tec­tonic setting, which may have either just preceded or over­lapped the regional contraction induced by latest jurassic col­lision of an exotic terrane at the continental margin (Aspden and Litherland, 1992). The telescoping of the broadly con­temporaneous Zamora batholith and porphyry Cu stocks in the Pangui trend of southern Ecuador may be taken to favor the latter possibility.

Although most major Cu depoSits in the Andes formed dur­ing contractional tectonic regimes, those of middle to late Mesozoic age in southern Peru and northern Chile and Pale­ocene to early Eocene age in northern Chile were developed during periods of regional extension (Fig. 15a, b). The por­phyry Cu deposits in these belts are characterized by low hy­pogene Cu contents « 0.4%) and typically small size «250 Mt), although Spence is exceptionally large. Furthermore, there is a strong suggestion tllat ti,e porphyry Cu-Au deposits and prospects were generated where coeval volcanism was ac­tive, implying that tectonic conditions were either extensional or, at most, moderately contractional (e.g., Maricunga-El Indio sub-belt and Farall6n Negro district; Fig. 10; see above), Crustal thinning during extension favored extensive volcanism and prevented formation of the large, shallow magma chambers and voluminous derivative lIuids thought to be the fundamental requirement for the genesis of giant por­phyry CuoMo deposits (Fig. 15).

The middle to late Mesowic extensional belt in southern Peru and northern Chile is also the site of Fe oxide-Cu-Au and manto-type Cu deposits (Fig. 15a), both of which are conspicuously absent in much of the Andes, particularly in the Cenowic contractional metallogenie belts (Sillitoe, 2003). It would appear tllat the attenuated crust and consequent high heat-llow regime in the Middle to Late JurasSiC and Early Cretaceous arcs were especially propitious for Fe oxide­Cu-Au and manto-type Cu formation, with at least the former deposit type being closely linked to the mafic plutonism that was Widespread under these tectonic conditions. However, a fuller understanding of why extension favors the formation of Fe oxide-Cu-Au and manto-type Cu deposits awaits resolu­tion of the genetic debate. The Mesozoic metallogeny of northern and central Peru is also typified by VMS deposits, which worldwide are favored by submarine extensional set­tings (e.g., Lentz, 1998; Sillitoe, 1999).

The Cenozoic evolution of the southern Andes, south of ap­prOximately latitude 36" S, is also mainly extensional in char­acter (Ramos, 2000). Hence, as with the middle to late Meso­zoic coastal belt in the central Andes, the crust remained thin and volcanism was Widespread, pOlticularly from the Eocene through Neogene. These characteristics, combined with ex­ceSSively deep erosion levels SOUtll of latitude 43" S, lead to only vestigial development of porphyry Cu deposits (Mpodozis and Perell6, 2003).

Supergene modification

The overriding controls on the fonnation of supergene pro­files in Andean Cu deposits are uplift and erosion rates, and hence climate, since at least 42 Ma (e.g., Alpers and Brimhall, 1988; Sillitoe, 2005). Cenozoic uplift may be a control of cen­tral Andean paleoclimate (see above) but the opposite may also be true, with aridity actually being instrumental in the rise of the central Andes. Lamb and Davis (2003) suggested that climate-controlled sediment starvation of the Peru-ChUe trench deprived the subduction interface of its lubrication and, in conjunction with tectonic erosion of the forearc, in­creased shear stresses to the levels needed to raise and sup­port the high Andes.

Supergene profiles in the more plUvial nortllern and south­ern Andes, where erosion rates are high, are typically stunted

ANDEAN ell PROVINCE i!77

compared to those in the arid central Andes. In the central Andes, however, the difTerential Cenozoic uplift of tectonic blocks described above also exerts a powerful influence on su­pergene oxidation and enrichment. Where tectonic uplift rates are excessive, erosion tends to outpace supergene processes (e.g., Famatina in the Miocene-early Pliocene belt of northwestern Argentina; Fig. 10), whereas relatively low uplift rates result in restricted supergene development (e.g., Bajo de la Alumbrera in the same belt). Maintenance of a bal­ance between rates of denudation and supergene processes under appropriate uplift mnditions results in optimal super­gene profile development, as in much of the middle Eocene to early Oligocene belt of northern Chile, where exhumation rates during early Miocene supergene activity approximated 50 m/m.y. (Maksaev and Zentilli, 1999). Development of the immature PHo-Pleistocene enrichment in the central Chile sub-belt took place during less violent uplift and exhumation than accompanied deposit emplacement.

Exploration and Discovery

Exploration targets and methodologies

The first wave of Cu exploration in the central Andes, dur­ing the first half of the twentieth century, chiefly involved ap­praisal of known Cu occurrences, many of them sites of small­scale mining of high-grade, oxidized Cu are. Cuajone, Toquepala, Chuquicamata, Potrerillos, Rio Blanco-Los Bronces, and EI Teniente are major porphyry CuoMo de­posits that were first recognized in this way (e.g., Richard and Courtright, 1958; Lacy, 1991; Camus, 2003). Other known Cu prospects that were preliminarily explored at the time but did not attain deposit status until the 1960s or 1970s, include the Antamina and Tintaya Cu skarns; Cerro Verde-Santa Rosa, EI Abra, Andacollo, and Los Pelambres porphyry Cu deposits; Mantos Blancos manto-type Cu deposit; and Mantoverde Fe oxide Cu-Au deposit (Concha and Valle, 1999; O'Connor, 1999; Ambrus, 1977; Llaumett et aI., 1975; Sillitoe, 1973b; Ramirez, 1996; Vila et al., 1996). Copper exploration in the northern and southern Andes before 1950 was minimal.

From the 1950s onward, numerous efforts were made in arid parts of the central Andes to identify previously unrecog­nized c'Olor anomalies, which might represent leached cap­pings, using aerial photography, fIXed-wing overflights and, since the early 1980s, false-color satellite imagery. The leached capping at Quebrada Blanca was inspected in 1957 during ground follow-up of one of the first color aerial photo­graphic surveys conducted in the world for the purpose of porphyry Cu exploration (Hunt et al., 1983). The leached capping above the Escondida porphyry CuoMo deposit was claimed in the early 1960s, after identification from the air, but was not seriously tested for nearly two decades (Sillitoe, 1995b). Overflights conducted in 1980 and 1981 to target al­teration wnes of epithermal affiliation in the high Andes of northern Chile and northwestern Argentina paved the way for discovery of the Cerro Casale porphyry Cu-Au deposit (Vila and Sillitoe, 1991).

Detailed geologic mapping was the main tool used to ex­plore a number of well-exposed leached cappings in the 1950s and 1960s, with discovery of the El Salvador and Cerro Colorado porphyry CuoMo deposits and Michiquillay and

Quellaveco prospects (e.g., Swayne and Trask, I \)f,(); Ilolllsll'[ and Sirvas, 1974; Hart, 1991). Some of the earliesl illlluc...,d. polarization (IP) geophysical surveys were also applied 10 por. phyry Cu exploration during this period, and successfully otll · lined the Cerro Verde-Santa Rosa, Cuajone, and Quclluvec<I porphyry CUoMo depoSits (Lacy, 1991; Arc", J999). In II." late 1960s and early 1970s, additional porphyry deposits. in particular EI Abra, Bajo de la Alumbrera, and Los Pelamhr"s. were discovered by drilling geolOgically and geochemically defined targets within previously known altered and leached zones. Prospect-specific ex-ploration, reliant upon compre­hensive geolOgiC and geochemical studies, was recommenced in northern Chile during the late 1970s. The leached capping identified at Quebrada Blanca 20 years earlier, using aerial photography, was subjected to detailed geolOgiC, alteration , leached-capping, and geochemical appraisal and drill tested in 1976 to 1977 to discover a supergene-enriched porphyry CuoMo deposit (Hunt et al. , 1983). The nearby Collahuasi prospect, the site of Chile's largest vein-type Cu mining oper­ation at the beginning of the twentieth century and recog­nized for its porphyry potential since the 1950s, was subjected to similar geolOgiC study and drill tested in 1978 and 1979 to discover the Rosario porphyry CuoMo deposit (Hunt, 1985).

During the 1970s, in the pluvial, jungle-covered northern Andes, reconnaissance drainage geochemistry was lvidely conducted and proved a very successful tool for porphyry Cu recognition. Regional surveys detected the Pantanos­Pegadorcito and Mocoa prospects in Colombia (Ramfrez et al., 1979; Sillitoe et al. , 1984), the Chaucha prospect in south­ern Ecuador (Fozzard, 1991), and the La Granja, Caiiariac'O, and other prospects in northern Peru, in addition to a Pb·Zn­Ag anomaly that was shown subsequently to be derived from ti,e Yanacoeha high-sulfidation Au deposit (Baldock, 1977; Cobbing et al., 1981). More recent, but similar, drainage geo­chemical surveys during ti,e 1990s led to discovery of the Pangui porphyry Cu trend of southern Ecuador (Gendall et aI. , 2000) and the Rio Blanm and Minas Conga prospects in contiguous Peru (Braun et aI., 1999; Llosa et al., 1999).

Systematic regional exploration programs in the Chilean porphyry Cu belt, as broadly defined at that time (Ruiz et ,J., 1965), were initiated at the end of the 1970s but did not be­come commonplace in southern Peru until the 1990s because of serious SOCiopolitical impediments during the preceding two decades. The first carefully conceived regiomJ e,,,lo­ration program for supergene-enriched porphyry Cu deposits beneath gravel and/or volcanic cover was initiated in )979 and resulted in discovery of the (partly outcropping) Esmndida and Escondida Norte-Zaldivar porphyry Cu·Mo deposits two years later (Lowell , 1991; Ortiz, 1995).

Porphyry Cu exploration, espeCially in nurthern Chile, be­came highly c'Ompetitive during the J980s and 1990s, and re­mains so today, with oxidized and/or enriched zones con­cealed beneath gravel or volcanic cover being the prime objective. Exploration e/1tlrts are usually tightly focused on the Paleocene to early Eoce ne and/or middle Eocene to early Oligocene porphyry ClI belts defined by Sillitoe (1981, 1988). Three direct exploration approaches, used separately or in various combinations, have been adopted: (1) careful geolOgiC and geochemical apprai"J of exposed bedrock peripheral to covered areas in search of distal porphyry Cu features (e.g.,

878 SILL/TOE AND PERELL6

alteration along faults, propybtic alteration halos, peripheral veins, or geochemical anomalies), which is essentially the basic premise of the program that resulted in the Escondida discovery (Lowell , 1991); (2) geochemical surveys, using par­tial-extraction analytical techniques, conducted over covered areas with the aim of detecting subtle, low-order, multiele­ment anomalies derived from buried porphyry Cu centers (e.g., Cameron et al. , 2002; Kelley et aI. , 2003); and (3) ground or airborne geophysical surveys designed to target sul­fide concentrations beneath covered areas. The geophysical methods tl13t have been tried include ground magnetics, IP, electromagnetics, controlled-source audio magnetotellurics, and audio magnetotellurics; and airborne magnetics, electro­magnetics, and gravity gradiometry. Some explorers have used faults, transverse lineaments, and/or structural intersec­tions interpreted from geolOgiC maps and satellite imagery as targeting guides. Nevertheless, all modem explorationists used satellite imagery as a field aid to location and geolOgiC interpretation. A radically different exploration approach, adopted by a single company in 1993, involved pattern drilbng of available covered areas, hundreds of square kilo­meters in size, in the Paleocene to early Eocene belt of north­ern Chile. The program, now discontinued, resulted in dis­covery of the concealed Spence deposit (Sillitoe, 2ooob). The only other belt that has been seriously investigated for its por­phyry Cu potential over the last two decades is the Miocene to early Pbocene belt in central Chile and adjoining Ar­gentina, where at least partly outcropping alteration zones and hypogene mineralization continue to be the main focus. Several already-known porphyry Cu prospects in the north­ern and central Peru part of this belt underwent renewed ex­ploration, including drilling, but without demonstration of economic viability.

Iron oxide-eu-Au mineralization is the only other deposit type that has seen intense regional exploration effort in the Andes, \vith the first programs being organized shortly after discovery of the Candelaria deposit in 1986. The programs, which focused on the Early Cretaceous part of the coastal belt in northern Chile and, subsequently, southern Peru, em­ployed electrical geophysical metl,ods andlor magnetometry, because of the intense IP and magnetic responses associated witl, the concealed Candelaria deposit and its immediate en­virons (Ryan et al., 1995). More recently, airborne gravity gra­diometry has been tested. Integrated approaches have been less commonly adopted, although discovery of the only signi f­icant Fe oxide-Cu-Au deposit in the central Andes since Can­delaria, at Mina Justa (Marcona district) in southern Peru (Fig. 4b, Table 1), resulted from drill testing geochemical and IP chargeability anomalies in the vicinity of small , abandoned oxide Cu workings (Moody et aI. , 2003). Formalized search for manto-type and VMS Cu minerilization in the middle to late Mesozoic belt of Chile and Peru, respectively, has been mainly confined to the vicinities of known districts.

Recent discovert} case histories-(I sfJnthesis

Case histories of probably the 23 most important Cu dis­coveries in the Andes during the last 30 years (Fig. 16a), since formal exploration programs were widely implemented, re­veal a clear pattern and emphasize the superiority of certain exploration methods over others, as compiled and analyzed by

Sillitoe (1995b, 2ooob). More than 80 percent of the discover­ies are porphyry Cu-Mo and Cu-Au deposits, with the re­mainder being of exotic oxide Cu or Fe oxide-Cu-Au types (Fig. 16b). Fifteen, more than two-thirds, are located in the middle Eocene to early Oligocene belt , four in the Miocene to early Pliocene belt, three in the Mesozoic belts, and one in the Paleocene to early Eocene belt (Fig. 16c). All but San Carlos, Mirador, and other deposits in the Pangui trend, southern Ecuador and Rio Blanco in contiguous Peru (Figs. 2, 4, 10), are in the arid, bttle vegetated parts of the central Andes.

GeolOgiC work, mainly involving standard recording and in­terpretation of geologic, alteration, and leached-capping fea­tures in outcrop, was directly responsible for siting discovery drill holes at 19 of the 23 deposits (Fig. 16d). In the case of Damiana. however, the geolOgiC work was conceptual in na­ture and led to prediction of the existence of the exotic oxide Cu mineralization using a genetically based deposit model (Sillitoe, 1995b). In 11 of these 23 discoveties, conventional rock-chip, soil, andlor talus geochemistry, with analysiS for Cu, Mo ± Au, was closely integrated wi th the geolOgiC work and a crucial component of the case histOlY (Fig. l6d). More­over, at San Carlos , Mirador, and other prospects in the Pan­gui trend, and at Rio Blanco and Minas Conga, follow-up of c1earcut drainage Cu geochemical anomilies (5->10 times re­gional background) resulted in initial target identification (Braun et al. , 1999; Llosa et al., 1999; Gendall et aI. , 2000). Three of the discovery sites were first inspected because of vi­sually distinctive color anomalies caused by leached cappings, two of them identified from the air and one using specially flown color aerial photography (see above). Vse of satellite imagery and airborne scanners have not led directly to dis­covery in the Andes, perhaps surprisingly in view of their ap­parent efficacy, although Dick et aI. (1993) did emphasize the role of the fonner in delimitation of the mineralized systems in the ColiallUasi district. Two of the discoveries, the Cande­laria Fe oxide-Cu-Au and Antapaccay porphyry Cu-Au de­posits, were made serendipitously during exploration for ex­tensions to small skarn Cu prospects (Fig. 16d), whereas initial evidence for Cu mineralization at MM came from a condemnation drill hole. Spence, the Outcome of a regional program to test covered areas (see above), and Quetena, dis­covered during step-out drilling near the already known Toki and Genoveva deposits (Rivera et aI., 2oo3b), are two exam­ples of exploration success resulting from pattern drilling.

Ten of the discoveries are entirely concealed below un min­eraJizecl cover rocks, which are here taken to exclude leached capping, and eight of the 14 discoveries made during the last 13 years are totally blind (Fig. 16e). The unmineralized cover rocks are mainly gravel sequences, but pastore ignimbrite predominates over gravel at Vjina (Dick et al. , 1993) and a preore carbonate sequence caps Candelaria (Ryan et al., 1995). Categorization of the discoveries, in a similar manner to that proposed by Miller (1976; cf. Sillitoe, 1995b), reveals that seven of the 13 outcropping deposits are located in old mining districts or near known e u occurrences , six are in vir­gin areas distant from known mineralization, and none is close to an operating mine (Fig. 16e). In the case of the con­cealed deposits, five were found near operating mines, four in old mining districts or near known Cu mineralization, and only one in a virgin area (Fig. 16e).

MIDEAN Cli PROVINCE 879

e

d

1975

c

1975

b

~ ~ r.oJ U

• D ..

OUTCROPPING DEPOSIT

Near operating mine

In old mining districV near knoVv11 mineralization In virgin area

1980 1985

Geologic work Geologic work and geochemistry

1985

~ Paleocene-early Eocene belt

~ Mesozoic belt

1990

Near operating mine

In old mining districV near known mineralization In virgin area

2000

Geologic work and geophysics Drilling

Serendipity

1990 1995 2000

~ Miocene-early Pliocene belt

II Middle Eocene-early Oligocene belt

II Porphyry Cu-Mo ~ Porphyry Cu-Au D ExoticCu ~ Fe oxide-Cu-Au

16

1975 1980 1985 1990 1995 2000

DISCOVERY YEAR 1975-2004

1 Quebrada Blanca 2 Rosario 3 Escondida 4 Escondida Norte-Zaldivar 5 Candelaria 6 Cerro Casale

7MM 8 EITesoro 9 Ujina

10Damlana 11 La Fortuna 12 Agua Rica

13 Minas Conga 14 Spence 15 Gaby 16 Rro Blanco 17 San Carlos 18 Antapaccay

19 Los Chancas 20 Esperanza 21 Tokl 22 Mina Justa 23 Quetena

Flc. 16. Histograms summarizing type, location , and discovery methods of Andean eu deposits during the last 28 years. a. Deposit names and discovery year. b. Deposit types. c. Host metallogenic belts. d. Principal discovery methods. c. Loca­tions of outcropping versus concealed deposit discoveries. Taken in part from Sillitoe (l995b, 2000b).

Although expenditure on partial-extraction geochemical techniques and ground and airborne geophYSics has been ap­preciable over the last ten years or so, particularly in the search for blind deposits, these methods made no contribu­tion to the decisions to drill; however, at both Mina Justa and Esperanza, ground geophysics influenced the siting of the discovery holes (Moody et al. , 2003; Pere1l6 et al ., 2004a).

Nevertheless, an airborne magnetic low, judged to be similar to those that encompass several major porphyry Cu districts in northern Chile (Behn et ai. , 2001), contributed to selection of the Gaby area for detailed geologie work, albeit as only a third-priori ty target (Siliitoe, 2ooob; Camus, 2003). The hy­perarid climate and deep saline ground-water conditions over much of northern Chile and southernmost Peru Oat 18"-26°

880 SILL/TOE AND PERELL6

S), at least below about 3,500 m in elevation, militate against the use of electrical geophysical methods, which historically have had a poor record throughout this region. Geophysics proved more valuable in the postdiscovery delineation of de­posits: airborne electromagnetic conductivi ty and magnetics at San Carlos and nearby prospects (Gendall et al ., 2000), IP chargeability and magnetics at Minas Conga (U osa et al. , 1999), electromagnetic conductivity at Mina Justa (Moody et al., 2003), IP resistivity and chargeability at Vjina (Dick et al ., 1993), and magnetics at Esperanza (Pere1l6 et al ., 2oo4a).

Fu ture exploration. strategies

Bearing in mind Andean Cu metallogeny, the variety of ex­ploration approaches adopted previously, and the synthesis of discove ry case histories presented above, an overall strategy for Cu exploration becomes readily apparent. Clearly, por­phyry Cu deposits should constitute the principal exploration objective, \vith highest priority being asSigned to oxidized and enriched zones in the premier middle Eocene to early Oligocene belt of northern Chile and southern Peru. High­grade hypogene deposits, perhaps upgraded by immature en­richment, in the Miocene to early Pliocene belt, especially in central Chile and contiguous Argentina, constitute a second­priority target. On the basis of the recent history of Andean discovery, the Paleocene to early Eocene belt is assigned lower priority (Fig. 16c), and the late Paleozoic to early Meso­zoic be lt is discounted. Once the sociopolitical situation im­proves in Colombia, the Paleocene to middle Eocene belt of the Western Cordille ra could also provide new discoveries, as indeed could some othe r parts of the belts discussed above. Porphyry Cu-Au deposits are not considered particularly high-priority targets in the Andes because the known exam­ples have eithe r relatively low hypogene Cu tenors andior small sizes, as reflected by the fact that five of the six deposits of this type included in Figure 16b are yet to be developed. A case might also be made for undertaking Fe oxide-Cu-Au ex­ploration in the middle to late Mesozoic belt of northern Chile and southern Peru, although the apparently small to moderate size of most deposits of this type known to date , in common \vith Andean manto-type Cu and VMS depoSits, makes them a somewhat unattractive target for major compa­nies. In stark contrast to the northern and central parts, the southern Andes (Fig. 1) are not recommended for porphyry Cu exploration, although there may be potential for Fe oxide­Cu-Au and manto-type Cu deposits in the southern continu­ation of the middle to late Mesozoic belt as far south as lati­tude 39" S.

The projected trend is to seareh for concealed porphyry Cu deposits in northern Chile and southe rn Peru. The best means of achieving success is considered to be by extrapola­tion of exposed, but typically subtle, geolOgiC and geochemi­cal features beneath cover. However, direct geolOgiC dril ling of the covered areas can also bring success, particularly when conducted in the geolOgiC context of an already-known por­phyry Cu cluster (e .g., Toki cluste r; Rivera et aI. , 2oo3b). Lib­eral use of cheap drilling technology as a means of better understanding geolOgiC relationships has the added advan­tage that it may also invite serendipity! It is particularly evi­dent that exploration focused in the vicinities of known de­pOSits or prospects, wi th the concept of a porphyry Cu (or

VMS) cluste r or alignment in mind, is a particularly effective strategy. It is evident that search for concealed Cu deposits beyond known districts or occurrences is a far more di fficult undertaking (Fig. 16e), probably less likely to be rewarded with success.

Notwithstanding the necessity of explOring beneath cover, continued search for partially exposed porphyry Cu systems should also be part of any comprehenSive exploration pro­gram. Our recent experience in the Paleocene to early Eocene and middle Eocene to early Oligocene belts of north­e rn Chile and, in particular. southern Peru strongly suggests that outcropping porphyry CUoMo and Cu-Au centers remain to be found there. These are likely to be prospects dominated by pyrite-poor potassic alteration. which. upon exposure to supe rgene weathering, give rise to much less prominent sur­face color anomalies or, in some cases , none at all The Miocene to early Plioc'Cne be lt may be conside red even more likely to possess currently unrecognized porphyry Cu centers partially exposed at surface. as may the jungle-covered belts in Colombia, Ecuador, and northern Peru.

Careful geolOgiC and conventional geochemical work. in­cluding leached-capping inte rpretation (e.g .. Sillitoe. 2005) and alteration mapping. is judged as ti,e best means of ap­praising such outcropping prospects. Previous work by com­petitor companies in no way downgrades the potential of prospects; indeed the opposite may well be the case (Sillitoe. 1995b. 2oo0b). bearing in mind that 10 of ti,e 23 deposits an­alyzed here were the subject of previous drilling campaigns unrelated to the ones responSible for actual discovery. In view of the prolife ration of largely untested Cu shomngs through­out much of the middle to late Mesozoic coastal belt. a geo­lOgically based rather than geophYSical approach to Fe oxide­Cu-Au explo ration needs further e mphasiS, with special attention being paid to modeling alteration systems and their outcropping Cll mineralization, in a fashion similar to that ap­plied to porphyry Cu prospects.

A different approach to Cu exploration in northern Peru, l'cuador. and Colombia is dictated by the wetter climate and consequent demonstrated e fficacy of conventional drainage geochemistry. It might be remarked that drainage sampling is a task for experienced geolOgists because of the high p roba­bility that any mineralization exposed in a particular drainage catchment will be directly identi fied. and even followed to source. by means of float mapping before the stream-sedi­ment analytical results become avai lable. Nevertheless. as in the drier parts of the Andes. attention to old districts and prospects and known Cu occurrences. as well as careful prospect-scale geologie work. are like ly to be important for success.

We argue that ground geophysics is normally best reserved for speci fic prospect- or district-scale investigations that are undertaken once first-pass drilling in formation is available as a control on interpretation and anomaly definition, irrespec­tive of whether the area is exposed or concealed beneath cover. Partial-extraction geoche mical techniques e mployed in the central Andes have not lived up to initial claims by their proponents and are not recommended as a routine compo­nent of exploration in the region. Furthermore, given their dismal track record to date and current budgetary rest rictions in most exploration and mining companies. airborne geophysiCS

ANDEAN ell PROVINCE BBI

would appear difficult to justiry as a routine Cu exploration technique, although airborne magnetics clearly assists re­gional geologic mapping. The obvious exception to this state­ment, however, would be exploration for VMS deposits in the late Mesozoic belt of c-entral and northern Peru. Electromag­netic andlor gravity techniques have demonstrated utility in discovery of massive sulfide bodies, not only in the Tambo­grande district (Tegart et al. , 2000) but worldwide.

The great challenge in the Andes, as elsewhere, is to obtain an acceptable return on exploration investment, especially if programs are focused on the most mahIre parts of the region, in particular the Cu belts of northern Chile. This reality be­hooves Cu explorationists to optimize search strategies and approaches and to ensure they are underpinned by state-of­the-art geolOgiC concepts.

Conclusions

More than 95 percent of Andean Cu resources are present as porphyry Cu and, generally minor, related skarn deposits. The only other economically Significant Cu deposits are of Fe oxide­Cu-Au and manto Cu types. VMS and red-bed Cu mineraliza­tion is relatively minor and currently unworked. The largest porphyry Cu deposits define three linear, age-restricted metal­logeniC belts: the preeminent middle Eocene to early Oligocene belt in southern Peru and, particularly, northern Chile; ti,e Miocene to early Pliocene belt, particularly in cen­tral Chile; and the Paleocene to early Eocene belt, especially in soutllern Peru but also extending into northern Chile. Meso­zoic and older porphyry Cu belts are of reduced importance. Although all ti,e major Andean porphyry Cu deposits are Ter­tiary, there is no clear spatial or temporal trend of increasing size and grade of individual depoSits. Nevertheless, ti,e Cu belts and their host magmatic arcs in ti,e central Andes, from southern Peru to central Chile, young progressively eastward, potentially in response to a longstanding series of subduction erosion events at ti,e continental margin (Rutland, 1971; Mpodozis and Ramos, 1990; Stem, 1991; Haschke et al. , 2002). In the northern Andes, such systematic migration of ti,e Cu belts is absent because of Mesozoic and Tertiary accretion of oceanic terranes (Sillltoe et al. , 19B2). Witllin the individual Cu belts, however, tI,ere is no evidence for any temporal migration of deposits, \vitll entire belts apparently being Simultaneously active throughout tI,eir 1,600- to 6,000-km lengths.

The three premier porphyry Cu belts in the Andes are be­lieved to have formed during contractional tectonic events of regional extent characterized by high- and low-angle reverse faulting of both tI,ick- and thin-skinned types. The contrac­tional pulses may be related to flattening of underthrust slabs, at least in some cases involving the subduction of buoyant oceanic features. The contraction induced crustal tI,ickening, suppression of volcanism, surface uplift, and consequent ex­humation. Magma trapped in large, shallow chambers, which were favored by these tectonic conditions, is thought to have supplied the voluminous fluids necessary for generation of the giant porphyry Cu deposits. However, there is no con­vincing linkage between subduction of oceanic features, such as aseismic ridges, and speCific sites of giant Cu deposit for­mation. It is concluded that the exceptional Cu endowment of the central Andes is a direct result of the occurrence of tI,ese three contractional magmatic arc segments.

There is no euncretc evidence that the e"ireme e u ton­nages in giant hypogene porphyry depoSits are related to the details of tl,eir internal anatomies (Clark. 1993). Neverthe­less, it is recognized that telcsm ping induced hy rapid synmineralization exhumation is commonly an il11/xH1ant con­tributor to grade developme nt as are certain wa I-rock ("'OITI­

positions (e .g., massive limestone at Antapaccu), and mafic in­trusions at EI Teniente; Sillitoe, 2(04 ). Exceptional si'l.£ cannot be conSistently related to unusual geologie features (e.g., Ossand6n et al. , 2001) or to superposition of discrete magmatic-hydrotllermal systems (e.g., Reynolds e t aI. , 1998; Ballard et al. , 2001; Harris et aI., 2004). Copper grade and mineability in nortllern Chile, however, have been pro!ounuly influenced by supergene enhancement consequent upon tee· tonic uplift . NotWithstanding the overriding control by con­tractional tectonism, there is no obviously unique structural niche(s) that favor porphyry Cu localization. Nor is tI,e re a consistent control apparent by transverse lineament intersec· tions \vitll ti,e contractional arcs, such as those proposed by Richards et al. (2001 ).

Given that the three premier Cu belts span fundamentally different geolOgiC terranes (e.g., Ramos, 2000) and are both underlain and hosted by a variety of rock units, crustal influ­ences on the provision of Cll are not evident. Similarly, in view of the fact that Au-rich and -poor porphyry Cu deposits are scattered throughout the Tertiary belts and locally occur togetller \vithin individual deposit clusters, crustal influences on Au concentration (e.g. , Camus, 2003; Leveille and Williams, 2003) are also discounted. Instead, Cu, Mo, and Au have been episodically introduced into ti,e upper crust for al­most 300 m.y. from an ultimate source in eitller tlle mantle wedge andlor subducted material (e.g., Sillltoe, 1972b; Hedenquist and Richards, 199B), although it is accepted that this process has no practical bearing on eitl,er the shaUow for­mation or search for giant Cu deposits.

Appraisal of exploration strategies that have proved suc­cessful in the Andes during the last 25 years or so predicts that a geolOgically based approach, supplemented by stan­dard geochemical methods and complemented by timely drill testing, is likely to be the most cost-effective means of Cu de­posit discovery, irrespective of whether the target is in out­crop or under cover. Nevertheless, we feel that future explo­ration endeavors would benefit from a greater input of conceptual geology by leading to a more predictive approach to ti,e search procedure. The empirical geologie facts and four-dimensional relationships at regional, district, and de­posit scales, as emphasized in this paper, are thought likely to be the most useful exploration tools for future dismveries.

Acknowledgnlents

This article relies on our collective expe rience in Cli explo­ration throughout the Andes during which we have had the pleasure of working with numerous individuals from many companies. They are all thanked anonymously for unselfishly sharing information over the years, although none is held re­sponsible for the views expressed. In particular, however, we owe a particular debt of gratitude to Constantino Mpodozis for improving our understanding of regional geolOgiC rela­tionships. JI' also acknowledges Antofagasta Minerals for gen­erous provision of some of the time required for preparation

882 SILL/TOE AND PERELL6

of this article. Hector Poblete and Claudio Montecinos are thanked for preparation of the figures. Constantino Mpodozis, Juan Carlos Taro, referees Francisco Camus and Marcos Zentilli, and editors Rich Goldfarb and Jeff Heden­quist reviewed the manuscript and made numerous valuable suggestions for its improvement.

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