Sillitoe[05 AndeanCu EG100thAV

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. 00incide 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: [email protected]

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.

~ Coastal Ccx'dillera

_

and Plains Western Cordillera

IT::::!]

Central Cordillera

R1

lnterandean Basins

_

Eastem Cordillera

~ Frontal Cordillera

!:55l

P90 percent of hypogene Cu tenor at Cuajone, Quellaveco, and Toquepala (Zweng and Clark, 1995; Concba and Valle, 1999).

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

~

Ol

Deposit! prospect

Status and 2003 production l (metric tons C u x 1,(00)

Production + reseJ'Ves (million metric tonnes) and grade ('h) (cutoff, % Cu)

O re~related

Po 1,000km-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)

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 syste m, and synorogenic clastic sedime ntary 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 e t at. (2OO3a).

862

SILLITOE AND PERELL6

from Chuquicamata northward, a group of north-trending faults with Miocene reverse and sinistral motion is superim-

posed 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; Mpodozis 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 geome-

tries 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

±

horn-

blende-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 PROVI NCE

863

Flc. 9. Features of porphyry ell deposits in the middle Eocene to early Oligocene belt of northem Chile. ll. The Chuquicamata porphyry Cu-M o 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 alte ration 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 fabri c defi ned by gamet-sulflde schlieren. Sample 20 cm long. c. Drill core sample showing the three main alte ration types at the Espe rarw..a porphyry Cu-Au deposit. Early potassi c (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 e nveloped 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.

Southern Peru: The middle Eocene to early Oligocene belt of southern Peru (Fig. 7) encompasses parts of lhe inte rmontane 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 Oligoce ne (-48--32 Ma), calc-alkaline AndahuaylasYau ri batholith, a composite body emplaced into clastic and carbonate strata of Jurassic to Cretaceous age (Pe re1l6 et a!., 2003a). Batholith phases include cumulate gabbro and diorite, e mplaced 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 syno rogenic basins (e.g., Pere1l6 et aI. , 2003a). Major faul t syste ms, some >300 km long, occur in the region and most display evidence for both high-angle reve rse and strike-slip motion . The southern Peru part of the belt diffe rs in two main ways from its counterpart in north ern Chile: lack of intimate association between regional fa ults and porphyry C u-bearing stocks, and domination by thin-ski nned, fold-and-thrust belt style structures, without involve ment of crystalline baseme nt, 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, howeve r, 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), E l Abra (Ambrus, 1977), Radomiro Tomic (Cuadra and Camus, 1998), Quetena (Rivera et al. , 2003b), and Gaby (Camus, 2(01), the potassic alte ration 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

865

ANDEAN Gil PROVINCE

dominated by pyrite, enargite, and bornite, but chalcocite, covellite, digenite, and tennantite are also common con-

stituents 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

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

80"

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

enrichment was minimal and in situ oxidation in reactive host BOUVIA

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).

o !

500km !

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 e ntire Andes, with porphyry, breccia, skarn, enargite-bearing carbonate replacement, high-sulfidation epithermal enargite vein, red-bed, and exotic oxide Cu deposits all 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 Piuque nes 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, e nargite-bearing veins and replaceme nts in carbonate and as sociated 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

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

867

entire Rio B1anc-o-Los Bronc""s system (Se rrano 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

Deposit! prospect

Perol (Minas Conga)

Chailhuag6n (M inas Conga)

Antamina

Status and ZOO3 production l (metric tons Cu x 1,(00)

Prospect

Prospect

Mine 341.4

Production + reserves (million metric tonnes)

and gmde (%)

Porphyry

Host

(cutoff, % Cu)

composition

Rocks

SulE 429 @ 0.31 Cu, 0.78 r!/I Au

SulE 190 @ 0.28 Cu, 0.77 r!/I Au

SulE 559 @ 1.24 Cu, 1.03 Zn, 0.OZ9 Mo, 13.7 r!/I Ag (0.7)

Thr~hases:

gran

'orite

One phase: granodiOrite

At least five phases: monzogranite

Cretaceous marine ca]"",eous rocks and Eocene intrusions

Magistral

Toromocho

Cerro Casale

Bajo de la Alumbrera

Agua Rica

Prospect

Prospect

Prospect

Mine 203.7

Prospect

Sul[; 618 @0.71 Cu (0.5)

SuiI', 1,285 @0.35 Cu. 0.02 Mo. 0.7 r!/I Au

Sulf, 805 @ 0.54 Cu, 0.64 Wt Au

Sul[; 750 @0.66 Cu, 0.037 Mo, 0.23 r!/I Au (0.4)

Potassic overprinted by intermediate argillic and sericitic. Periphera] advanced

argillic (cp, bnl'

Cretaceous marine calcareous

Potassic overprinted by intermediate argillic and sericitic

rocks

(cp, bn)

Cretaceous marine caIcareous

Potassic overprinted by intermediate argillic

rocks

(cp, bnl'

Cretaceous marine caI-

Sul[; [email protected] Two phases: Cu, 0.05Z Mo (0.5) granodiOrite

Ore-related hypogene alteration (mineralization)

=eou, rocks

argillic (cp)

breccias

Orthoclase: 15.80.0.09

Minor igneous breccias

Biotite: 15.58.0.1Z Biotite: 10.15 0.04; lllitized &Iafoclase; .9 .0.06; Sericite(?): 9.8.0.Q7 Biotite: 15.3.0.7; Mo~bdenite:

14 . 3:l 0.02, 14.63.0.02

Minor igneous breccias Important: severa] phases, i~eous and rea.tic recctas

c.

Important: igneous breccias and few

pebble wke,

rocks

severa]lh~es:

Miocene volcanic

Potassic overprinted by sericitic and advanced argillic in

quartz

rocks

upper parts (cp)

Biotite: 13.89 • 0.04; Alunite: 13.91 .0.04

rocks

Concentric pattern: potasSiC surrounded by sericitic (cp)

Biotite: 7.1O:l 0.13,6.98 .0.08, 6.83 .0.07; Sericite: 6.75 % 0.09; Zircon, 7.10 % 0.07, 7.98 % Minor: igneous 0.14 , 8.0Z • 0.14 breccias

Precambrianearly Paleozoic metasedimentary

Potassic overprinted by intense sericitic and advanced

rocks

argillic (cp)

onte

Seven phases: dacite to rhyolite

Severa] phases: monzonite to dacite

Miocene volcanic

Massive sulfide veins

Structural control

Key references

Uosa et aI. (1999), L10sa and Veliz (2000), Gustafson

Absent

Important: NW faults

el aI. (2004)

Absent

Important: NNE-trending intrusions

Uo,a el aI. (1999), Uosa and Veliz 2000), Gustafson el aI. (ZOO4)

Absent

Important: NW thrusts and fold axes

O'Connor (1999), Love et aI. (2003, 20(4), Redwood (ZOO4)

Present: py, tet, stibioluzonite, luzonite, realgar, orpiment

Important: domlnantlr NS thrusts and ENE-trending intrusion corridor

%

Poorly deAned concentric pattern: potassic surrounded and overprinted by sericitic (cp)

Four phases: diorite, granodiorite. quartz monzonite

Permian volcanic and TriassicJurasSiC calcareous

Potassic overprinted by intermediate argilliC, sericitic, and advanced

Hydrothennal Age (Ma)

Biotite: 5.10.0.OS; Alunite: 4.96 % 0.08, 4.88.0.08

Important

Minor: igneous and tourmaline breccias

Present: tet-tenn. cp

Perell6 et al. (ZOOl), J. Perell6, unpub. data, 2001

Important: NW faults

Alvarez (1999), Noble and Mckee (1999)

Present: NNE and NW faults

Vila and Sillitoe (1991), Mpedoz;, el aI. (1995), Muntean and Einauw (2001) Sasso and Clark

Absent

Important: phreatiCand phreatomagmatic, Important: barren lateCy, cv, enarg. mineral diatreme n, cc

~

"

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 Te niente 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 tennan tite 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 -12km-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 extre me ly 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 oon-

carbonate 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 Pelam bres, 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 over-

lying advanced argillic lithocaps (e.g., Sillitoe, 1995a, 2oooa; Fig. 13). Pyrophyllite is particularly characteristic of the

873

ANDEAN CII PRO VI NCE

: . : . .' )---:c• ....:. x , '

x

,

, , , ,

, ,

•t • +0+ , , +

, x

x

x

,

'.'

.- ::

•

+, '+ '

• 0+"

, +, • .+~ " + , , • "+

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