Sillitoe, 2003. Iron Oxide-copper-gold Deposits an Andean View

Mineralium Deposita (2003) 38: 787–812 DOI 10.1007/s00126-003-0 10.1007/s00126-003-0379-7 379-7

ARTICLE

Richard H. Sillitoe

Iron oxide-copper-gold deposits: an Andean view

Received Received:: 3 March March 2003/ Accepted Accepted:: 22 July 2003 / Publishe Published d online: online: 17 Septembe Septemberr 2003 Ó Springer-Verlag 2003

oxide-copper pper-gold -gold (IOCG) (IOCG) deposits, deposits, deAbstract Iron oxide-co fined primarily by their elevated magnetite and/or hematite tite conten contents, ts, consti constitu tute te a broad broad,, ill-de ill-define fined d clan clan relat related ed to a vari variety ety of tectono-m tectono-magma agmatic tic settings. settings. The youngest youngest and, therefore, most readily understandable IOCG belt is located in the Coastal Cordillera of northern Chile and southern Peru, where it is part of a volcano-plutonic arc of Jurass Jurassic ic throug through h Early Early Cretac Cretaceou eouss age. age. The arc is characte characterised rised by voluminou voluminouss tholeiit tholeiitic ic to calc-alka calc-alkaline line pluton plutonic ic comple complexes xes of gabbro gabbro throug through h gra granod nodior iorite ite compositi composition on and primitiv primitive, e, mantle-d mantle-deriv erived ed parentag parentage. e. Major arc-parallel fault systems developed in response to extension and transtension induced by subduction rollback at the retreating convergent margin. The arc crust was attenuated and subjected to high heat flow. IOCG deposits deposits share share the arc with massive massive magnetite magnetite deposits, deposits,the the copper-deficient end-members of the IOCG clan, as well as with manto-type copper and small porphyry copper deposits to create a distinctive metallogenic signature. The IOCG IOCG deposi deposits ts displa display y close close relati relations ons to the plutonic plutonic complexe complexess and broadly broadly coeval coeval fault fault systems. systems. Based on deposit morphology and dictated in part by lithological and structural parameters, they can be separated into several styles: veins, hydrothermal breccias, repl replac acem emen entt mant mantos os,, calc calcic ic skar skarns ns and and comp compos osit itee deposi deposits ts that that combin combinee all or many many of the preced preceding ing types. The vein deposits tend to be hosted by intrusive rocks, especially equigranular gabbrodiorite and diorite, whereas the larger, composite deposits (e.g. CandelariaPunta del Cobre) occur within volcano-sedimentary sequences up to 2 km from pluton contacts and in intimate mate asso associ ciat atio ion n with with majo majorr orog orogen en-p -par aral alle lell faul faultt systems. Structurally localised IOCG deposits normally share faults and fractures with pre-mineral mafic dykes, many of dioritic composition, thereby further emphas-

ising the close connection with mafic magmatism. The depos deposits its formed formed in ass associ ociati ation on with with sodic, sodic, calcic calcic and potassic alteration, either alone or in some combination, reveal reveal eviden evidence ce of an upward upward and outwar outward d zonati zonation on from magnetite-actinolite-apatite magnetite-actinolite-apatite to specular hematitechlorite-sericite and possess a Cu-Au-Co-Ni-As-Mo-U(LREE) (light rare earth element) signature reminiscent of some some calcic calcic iron iron skarns skarns around around diorit dioritee intrus intrusion ions. s. Scant Scant observati observations ons suggest suggest that massive massive calcite calcite veins veins and, at shallower palaeodepths, extensive zones of barren pyritic pyritic feldsparfeldspar-dest destruct ructive ive alterati alteration on may be indiindicators of concealed IOCG deposits. The balance of evidence strongly supports a genetic connection of the central Andean IOCG deposits with gabbr gabbrodi odiori orite te to diorit dioritee magma magmass from from which which the ore fluid fluid may have been been channe channelle lled d by major major ductil ductilee to brittle fault systems for several kilometres vertically or perha perhaps ps even even latera laterally lly.. The large, large, compo composit sitee IOCG IOCG deposits originated by ingress of the ore fluid to relatively tively permeabl permeablee volcano-s volcano-sedim edimentar entary y sequences sequences.. The mafic magma may form entire plutons or, alternatively, may underplate more felsic intrusions, as witnessed by the ore-related diorite dykes, but in either case the origin of the ore fluid at greater, unobserved depths may be inferr inferred. ed. It is conclu concluded ded that that extern external al ‘basin ‘basinal’ al’ fluids fluids were were not not a requ requir irem emen entt for for IOCG IOCG form format atio ion n in the the central Andes, although metamorphic, seawater, evaporitic or meteoric fluids may have fortuitously contaminated inated the magma magmatic tic ore ore fluid fluid locall locally. y. The propo proposed sed linkag linkagee of centra centrall Andean Andean and probab probably ly some some other other IOCG IOCG deposi deposits ts to oxidis oxidised ed diorit dioritic ic magma magmass may be compa compared red with with the well-d well-docu ocumen mented ted depend dependenc ency y of  several other magmatic-hydrothermal deposit types on igneous petrochemistry. The affiliation of a spectrum of  base-metal poor gold-(Bi-W-Mo) deposit styles to relatively tively reduced reduced monzogra monzogranitenite-grano granodiori diorite te intrusion intrusionss may be considered as a closely analogous example.

Editorial handling: B. Lehmann R. H. Sillitoe 27 West Hill Park, Highgate Village, London N6 6ND, UK E-mail: [email protected] [email protected]

Keywords Iron oxide-copper-gold deposits Æ Metallogeny Æ Central Andes Æ Diorite Æ Extensional tectonics Æ Volcano-plutonic arcs

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Introduction

Iron Iron oxideoxide-cop copper per-go -gold ld (IOCG) (IOCG) deposi deposits ts compri comprise se a broad and ill-defined clan of mineralization styles which, as the the name name impl implie ies, s, are are grou groupe ped d toge togeth ther er chie chiefly fly because because they contain contain hydrothe hydrothermal rmal magnetite magnetite and/or and/or specular hematite as major accompaniments to chalcopyrite±bo pyrite±bornite rnite (e.g. Ray and Lefebure Lefebure 2000). 2000). Besides Besides the copper and by-product gold, the deposits may also contain appreciable amounts of Co, U, REE, Mo, Zn, Ag and and othe otherr elem elemen ents ts.. IOCG IOCG depo deposi sits ts curr curren entl tly y account for <5 and <1%, respectively, of the world’s annually mined copper and gold production, much of it deri derive ved d from from Olym Olympi picc Dam Dam and and Erne Ernest st Henr Henry y in Austra Australia lia and Candel Candelari aria a and and Manto Mantover verde de in Chile. Chile. Notwithstanding their modest economic contributions, IOCG deposits deposits have become become fashiona fashionable ble explorati exploration on and research objectives over the past few years. Fig. 1 Position of the central Andean IOCG belt of northern Chile–southern Peru with respect to the Jurassic–Early Cretaceous magmatic arc and a series of interconnected backarc basins along its eastern side. Approximate locations of arc segments and intra- and backarc basins mentioned in the text are shown. Also marked are: two post-Early Cretaceous IOCG deposits located east of  the main IOCG belt; axes of the two main belts of ’Kiruna-type’ massive magnetite deposits; the two main concentrations of  manto-type copper-(silver) copper-(silver) deposits; the area occupied by VHMS deposits; and selected Jurassic and Early Cretaceous porphyry copper-(gold)  ´ a Marı ´a;  ´ a; 2 deposits (1 Tı ´a Galenosa-Puntillas; 3 Antucoya-Buey Antucoya-Buey Muerto; 4 Mercedita; 5 Andacollo)

One of the best developed, but perhaps rather poorly appre apprecia ciated ted,, IOCG IOCG provin provinces ces is locate located d in the South South American Coastal Cordillera and immediately adjoining areas of northern Chile and southern Peru (latitudes 13–  33°30¢S; Fig. Fig. 1), 1), wher wheree it is clos closel ely y asso associ ciat ated ed with with Mesozoic Mesozoic batholith batholithss and major major arc-para arc-parallel llel fault systems. The origin of IOCG deposits has recently become the the subjec subjectt of consid considera erable ble debate debate,, with with both both metal metal-bear bearin ing g magm magmat atic ic brin brinee (e.g (e.g.. Hitz Hitzma man n et al. al. 1992 1992;; Polla Pollard rd 2000) 2000) and extern external al ‘basin ‘basinal’ al’ brine brine heated heated by intrus intrusion ionss (e.g. (e.g. Barton Barton and Johnso Johnson n 199 1996; 6; Hitzma Hitzman n 2000) 2000) being being proposed proposed as viable viable ore-form ore-forming ing fluids. fluids. In view of the fact that the central Andean IOCG province is the world’ world’ss younge youngest, st, is largel largely y unaffec unaffected ted by the complicating effects of later metamorphism and deformation and is relatively well documented geologically, it prov provid ides es an idea ideall exam exampl plee with with whic which h to asse assess ss the the competing genetic models. Understanding the origin of 

789

IOCG deposits is, of course, also fundamental to their effective exploration. This article reviews the geological and metallogenic settings of the IOCG province in the Coastal Cordillera of Chile and Peru and then the styles and salient features of the the IOCG IOCG depo deposi sits ts them themse selv lves es,, with with part partic icul ular ar emphasis on smaller, higher-grade deposits as well as the large, better-documented examples like Candelaria and Mantoverde (Table 1). On balance, the evidence favours a magmaticmagmatic-hydr hydrothe othermal rmal origin origin for central central Andean Andean IOCG IOCG depo deposi sits ts,, besi beside dess reve reveal alin ing g seve severa rall feat featur ures es and relationship relationshipss of potentia potentiall use during during depositdeposit- and district-scale exploration.

and and Aguirr Aguirree 199 1992), 2), were were active active during during the Mesoz Mesozoic oic volc volcan anis ism m and and plut pluton onis ism. m. Wide Widesp spre read ad exte extens nsio ion n induced induced tilting tilting of the volcano-s volcano-sedime edimentar ntary y sequences sequences.. Immed Immediat iately ely east east of the Mesozo Mesozoic ic arc terran terranee of the Coastal Cordillera in northern Chile and southern Peru, sedimentary sequences accumulated in a series of interconne connecte cted, d, predom predomina inantl ntly y marine marine backback-arc arc basins basins (Mpodozis and Ramos 1990). Early to mid-Juras mid-Jurassic sic through through mid-Creta mid-Cretaceou ceouss volcanism canism and plutonism plutonism througho throughout ut the Coastal Coastal CordilCordillera and immediate immediately ly adjoinin adjoining g regions regions are generally generally considere considered d to have taken place place under variably variably extenextensional sional conditio conditions ns in response response to retreatin retreating g subducti subduction on boundarie boundariess (slab (slab roll-back roll-back)) and steep, steep, MarianaMariana-type type subductio subduction n (Mpodozis (Mpodozis and Ramos Ramos 1990; 1990; Grocott Grocott and Taylor 2002). Nevertheless, Atherton and Aguirre (1992) questioned the existence of subduction during the Early Cretaceous in southern Peru and favoured extension at a passive passive continen continental tal margin. margin. Throughou Throughoutt much of the Coastal Cordillera of northern Chile and southern Peru, western portions of the Mesozoic arc terrane (and the corr corres espo pond ndin ing g fore fore-a -arc rc)) seem seem like likely ly to have have been been removed removed by subductio subduction n erosion erosion or lateral lateral translati translation on (Rutla (Rutland nd 197 1971; 1; Dalzi Dalziel el 198 1986; 6; Mpodoz Mpodozis is and Ramos Ramos 1990) or, at the very least, lie below sea level.

Geological setting 

General features In the Coasta Coastall Cordil Cordiller lera a and immedi immediate ately ly adjoin adjoining ing physiogra physiographic phic regions of northern northern Chile and southern southern Peru, major Mesozoic plutonic complexes are emplaced into broadly contemporaneous arc and intra-arc volcanic produc products ts and under underlyi lying ng penetr penetrati ativel vely y deform deformed ed metasedimentary units of Palaeozoic age. Early Proterozoic rozoic craton cratonic ic basem basemen entt of the Arequi Arequipapa-Ant Antofa ofalla lla massi massiff underp underpins ins the centra centrall segmen segmentt of the Coasta Coastall Cordiller Cordillera a (Shacklet (Shackleton on et al. 1979) 1979) and the adjoining adjoining Andean Cordillera, between about latitudes 14 and 26 °S (Ramo (Ramoss and Aleman Aleman 200 2000). 0). Extens Extensive ive longit longitudi udinal nal brittle fault systems and/or ductile shear zones, including the Atacama Fault System in northern Chile (e.g. Scheuber and Andriessen 1990) and deeply penetrating faults that localised the Can ˜ ete basin in Peru (Atherton

Volcano-sedimentary rocks The Middle to Late Jurassic La Negra Formation, up to 5,000–10,000 m of subaerial to locally shallow-submarine basalt, basaltic andesite and andesite lavas, tuffs and minor intercalated sedimentary rocks, and correlative tive form format atio ions ns comp compri rise se the the arc arc and and intr intraa-ar arcc

Table 1 Tonnage and grade of selected IOCG deposits, central Andes

Deposit (Fig. 4)

Tonnage a Cu(%) (million tonnes)

Au(g/t)

Ag(g/t) Data source

Rau´ ll-Condestable, Peru Eliana, Peru Monterrosas, Peru Mina Justa, Peru

>25 0.5 1.9 209

0.9

6

Cobrepampa, Peru Tocopilla, Chile Montecristo, Chile Cerro Negro, Chile Teresa de Colmo, Chile Mantoverde, Chile

3–5 2.4 (0.31) 15 249 (49) 70 230 oxide, >400 sulphide Candelaria, Chile 470 Punta del Cobre, Chile 120 Carrizal Alto, Chile 3 Panulcillo, Chile 3 (10.4) Tamaya, Chile >2 (0.9) Los Mantos de Punitaqui, Chile 2 (gold zone only) El Espino, Chile 30 La Africana, Chile 3.3 $ $

$

$

$

1.7 2.7 1.0–1.2 0.86 2–5 3.1 (16) 1.6 0.4 (0.71) 0.8 0.55 oxide, 0.52 sulphide 0.95 1.5 5 2.7–3.5 (1.45) 12 (20)

a

Present Present 0.6 0.15 Trace 0.11 $

0.22 0.2–0.6 Up to 0.1 4

$

$

6 Minor

1.2 2.5

de Haller et al. (2002) Injoque (2002) 20 Injoque (2002) Present Rio Tinto Mining and Exploration Ltd. (unpublished data, 2003) 15 Injoque (2002) locally Ruiz and Peebles (1988) J. Esquivel (personal communication, 2003) Atna Resources (press release, 2002) Hopper and Correa (2000) Zamora and Castillo (2001)

0.15

3.1 2–8

Marschik et al. (2000) Marschik and Fontbote ´ (2001b) Ruiz et al. (1965) Hopper and Correa (2000) Ruiz and Peebles (1988) R. Muhr (personal communication, 1998) Correa (2003) N. Saric (personal communication, 2003)

Cumulative production and/or reserves, only approximate for mines active before the 20th century Alternative tonnage and corresponding Cu grade

790

succes successio sions ns in north northern ern Chile Chile (Boric (Boric et al. 199 1990; 0; PicPichowiak 1994; Figs. 1 and 2). La Negra lavas overlap the tholeiiti tholeiiticc and calc-alkal calc-alkaline ine compositi compositional onal fields (Pichowiak et al. 1990). The volcanic arc appears to have been topographica topographically lly subdued subdued and to have developed developed close to sea level (Fig. 2). Late Jurassic to Early Cretaceous arc volcanism occurred along the eastern side of  the Coastal Cordillera, at least from latitudes 26–29 °S, wher wheree it is repr repres esen ente ted d by up to 3,00 3,000 0 m of basa basalt ltic ic andesi andesite, te, andesi andesite te and dacite dacite volca volcanic nic rocks rocks now now assigned to the Punta del Cobre Group (Lara and Godoy 1998), 1998), host to the Candelaria-P Candelaria-Punta unta del Cobre Cobre IOCG district (e.g. Marschik and Fontbote ´ 2001a). The Jurassic and Early Cretaceous back-arc domain betwee between n latitu latitudes des 21 and 27 °S in north northern ern Chile, Chile, the Tarapaca ´ ´ basin basin (Fig. (Fig. 1), 1), is domina dominated ted by marine marine carcarbonate and continental terrigenous sequences, although interbedded andesitic volcanic rocks also occur locally (Mun ˜ oz et al. 1988; Mpodozis and Ramos 1990; Ardill et al. 1998). Evaporite horizons appear locally, especially in the Late Jurassic (Fig. 2; Boric et al. 1990; Ardill et al. 1998). In the back-arc basin of central Chile (Aconcagua Platfo Platform rm;; Fig. Fig. 1) 1),, south south of about about latitu latitude de 31 °30¢S, a Jurassic Jurassic marine marine carbonate carbonate sequence, sequence, including including a thick gypsum horizon, is overlain by Late Jurassic continental red beds and Early Cretaceous marine carbonates, while

Fig. 2 Schematic tectonic sections of the central Andean margin at A latitudes 21–26 °S in the Late Jurassic–Ea Jurassic–Early rly Cretaceo Cretaceous us and B latitudes latitudes 12–14 12–14°S in the the Earl Early y Cret Cretac aceo eous us,, show showin ing g stee steep p subduction at a retreating convergent boundary. Note in A that IOCG deposits occur in a subaerial arc paralleled eastwards by a sediment-dominated back-arc basin, whereas in B IOCG deposits occur occur in a subaqu subaqueou eouss intraintra-arc arc basin. basin. Approx Approxim imate ate ages ages of  evaporites and IOCG deposits (see text) are also shown. Sections adapted from Ramos and Aleman (2000)

farther west up to 5,000 m of Early Cretaceous volcanic and volcaniclastic sedimentary rocks accumulated in an intra-arc basin formed in response to vigorous extension (Aberg et al. 1984; Mpodozis and Ramos 1990; Ramos 2000). Most of the volcanic rocks range in composition from basalt to andesite and are high-K calc-alkaline to shoshoni shoshonitic tic in composit composition; ion; parts of the sequence sequence display compositional bimodality (Levi et al. 1988). Mesozoic arc rocks in southern Peru include the Rı ´ Rı ´o o Grande and Chala Formations of mid-Jurassic age, both of which comprise basaltic andesite of medium- to highK calc-alkaline affinity (Romeuf et al. 1995). The backarc arc doma domain in incl includ udes es the the Areq Arequi uipa pa basi basin n (Fig (Fig.. 1) 1) in which up to 1,500 m of Early Jurassic basaltic volcanic rocks belonging to the Chocolate Formation are overlain by several thousand metres of mainly terrigenous, Middl Middlee to Late Late Jurass Jurassic ic sedim sediment entary ary rocks rocks (Vicen (Vicente te 1990; Sempere et al. 2002a, 2002b). The Can ˜ ete basin (Fig. (Fig. 1), the southe southern rn portio portion n of the the West West Peruv Peruvian ian trough (Wilson 1963), is dominantly Early Cretaceous in age (Cobbing 1978) and probably best interpreted as a product product of advanced advanced intra-ar intra-arcc extension extension (Ramos and Aleman 2000). The Copara´ and Quilmana´ Formations in the Can ˜ ete ete bas basin are are dom dominat inateed by highigh-K K calc-alkaline to shoshonitic basalt and basaltic andesite, although although subordinate subordinate dacite and rhyolite rhyolite impart a bimodal signature to the latter formation (Atherton and Aguirre 1992). These volcanic formations are underlain by a clastic-carbonate succession containing very minor amounts of evaporite minerals (Palacios et al. 1992). The Jurassic and Early Cretaceous arc and intra-arc sequences throughout the Coastal Cordillera are dominated nated by basalt basaltic ic andesi andesite, te, appear appear to posses possesss greate greaterr amounts amounts of lava than other other volcanic volcanic or volcanicla volcaniclastic stic products products and lack volumetr volumetricall ically y importan importantt felsic felsic volcanic canic rocks. rocks. Furthe Furthermo rmore, re, there there is little little eviden evidence ce of  major major volcanic volcanic edifices edifices typical typical of most subducti subduction-r on-relate elated d arc arc terran terranes, es, and the volcan volcanic ic enviro environm nment ent may well well have been more akin to flood basalt provinces. Low-grade, non-deformative, diastathermal (burial) metamorp metamorphism hism induced induced by elevated elevated geotherm geothermal al gradigradients consequent upon crustal thinning was active during accum accumula ulatio tion n of all the Mesozo Mesozoic ic arc and intraintra-arc arc volca volcanic nic sequen sequences ces,, with with the result resulting ing metam metamorp orphic hic grade grade common commonly ly attain attaining ing the prehni prehnitete-pum pumpel pellyi lyite te facies and, at depth, greenschist facies (Levi et al. 1989; Atherton Atherton and Aguirre Aguirre 1992). 1992). This low-grade low-grade regional regional meta metamo morp rphi hism sm is not not dire direct ctly ly rela relate ted d to plut pluton on emplaceme emplacement, nt, which gave rise to fairly fairly restricte restricted d and easily easily distingu distinguishab ishable le contact contact aureoles aureoles similar similar to that rela relate ted d to the the Tier Tierra ra Amar Amaril illa la bath bathol olit ith h near near the the Candelaria IOCG deposit (Tilling 1976; Marschik and Fontbote´ Fontbote´ 1996, 2001b).

Plutonic rocks The plutonic plutonic complexes, complexes, ranging ranging in composit composition ion from primitive early gabbro and diorite through quartz diorite

791

and quartz quartz monzodio monzodiorite rite to tonalite tonalite and granodior granodiorite ite and, and, unco uncomm mmon only ly,, monz monzog ogra rani nite te were were empl emplac aced ed throughout the Jurassic and Early Cretaceous as a series of relatively short pulses, each estimated to last roughly 3 to14 M.Y. where extensively dated between latitudes 25°30¢ and and 27°30¢S (Dallm (Dallmeye eyerr et al. 1996; 1996; Lara Lara and Godoy 1998; Grocott and Taylor 2002). Hence, unsurprisingly, multiple ages of any particular intrusive rock type type occu occurr thro throug ugho hout ut the the Coas Coasta tall Cord Cordil ille lera ra;; for for example, example, the early early gabbros gabbros from latitudes latitudes 23–24 23–24 °S in northern Chile are dated at 196–185 Ma (Early Jurassic) (Pichowiak et al. 1990), whereas those farther north in the Can ˜ ete basin of Peru are clearly assignable to the mid-Cretaceous (Regan 1985). Plutons are irregular in outline outline but markedly markedly elongate elongate parallel parallel to the orogen, nort northe herl rly y in nort northe hern rn Chil Chilee and and nort northw hwes este terl rly y in southern Peru. Typical plutons exceed 50 km in longitudinal dimensions. During the Mesozoic, the locus of  plutonis plutonism m in northern northern Chile migrated 50 km or so eastwa eastwards rds to reach reach the easter eastern n border border of the Coasta Coastall Cordillera by the Early Cretaceous (Farrar et al. 1970; Berg and Baumann 1985; Parada 1990; Dallmeyer et al. 1996; Lara and Godoy 1998; Fig. 3), and an apparently simila similarr but still still poorly poorly defined defined progre progressi ssion on also also took took place in southern Peru (Clark et al. 1990). Abunda Abundant nt andes andesite ite,, basalt basaltic ic andesi andesite te and basalt basalt dykes cut many of the plutons and their host rocks (e.g. Pichowiak and Breitkreuz 1984; Regan 1985; Scheuber and Gonzalez 1999; Taylor and Randall 2000; Sempere et al. 2002b). 2002b). Both synpluto synplutonic nic emplacem emplacement ent features features (Moore (Moore and Agar 1985; 1985; Regan Regan 1985) 1985) and radiometric radiometric dating (Dallmeyer et al. 1996) show that the dykes are broadl broadly y synchr synchron onous ous with with host host or nearby nearby pluton plutons. s. Furt Furthe herm rmor ore, e, indi indivi vidu dual al dyke dyke swar swarms ms tend tend to be centre centred d on single single pluton plutonic ic comple complexes xes,, beyond beyond which which they they cannot cannot be traced traced very very far (Taylo (Taylorr and and Randal Randalll 2000). $

In northern Chile and southern Peru, the Jurassic and Early Early Cretaceou Cretaceouss intrusive intrusive rocks, most of them hornblende blende-be -beari aring, ng, are largel largely y metal metalum umino inous us and calccalcalkaline (Parada 1990; Pichowiak 1994), although early gabb gabbro ross are are thol tholei eiit itic ic in char charac acte terr (Reg (Regan an 1985 1985;; Pichowiak et al. 1990). All the intrusive rocks are oxidised and belong to the magnetite-series (Ishihara and Ulriksen 1980). Initial strontium isotope ratios for plutonic tonic rocks rocks decrea decrease se marke markedly dly eastw eastward ardss across across the Coastal Coastal Cordillera Cordillera of northern northern Chile, in general general from 0.704–0.705 for the Middle–Late Jurassic to 0.703–0.704 for the Early Cretaceous rocks (McNutt et al. 1975; Berg and and Baum Bauman ann n 1985 1985;; Pich Pichow owia iak k 1994 1994;; Para Parada da et al. al. 1999), a pattern that may be interpreted to imply maximal imal extens extension ion and and crusta crustall thinni thinning ng and, and, as a conseconsequence, minimal crustal contamination during the Early Cretaceous period. Nevertheless, the mantle wedge remained the main site of magma generation throughout the the Jurass Jurassic– ic–Ear Early ly Cretac Cretaceou eouss interv interval al (Roger (Rogerss and Hawkesworth 1989). The pluton plutonic ic comple complexes xes of the Coasta Coastall Cordil Cordiller lera a between about latitudes 26 and 27 °30¢S were emplaced syntecton syntectonical ically ly during during the Early to Middle Middle Jurassic Jurassic as gently inclined, sheet-like bodies up to several kilometres thick, thick, controlle controlled d by east-dip east-dipping ping extension extensional al fault fault systems (Grocott et al. 1994; Grocott and Taylor 2002), but thereafter probably as steep, slab-like bodies localised by ductile ductile shear zones zones (Grocott (Grocott and Wilson Wilson 1997). 1997). Roof  lift liftin ing g and and floor floor depr depres essi sion on both both enab enable led d plut pluton on emplacement (Grocott and Taylor 2002). In accord with this intrusion mechanism, the outcropping plutons were empla emplaced ced at relati relativel vely y high high crusta crustall levels levels,, < 10 km between about latitudes 22 and 28 °S (Dallmeyer et al. 1996; Scheuber and Gonzalez 1999), and cooled rapidly as shown by concordance between U-Pb zircon ages and 40 Ar/39Ar isotope-correlation ages (Berg and Baumann 1985; 1985; Dallmeye Dallmeyerr et al. 1996), 1996), as well well as by relati relativel vely y restricted (<4 km) contact-metamorphic aureole development. The Coasta Coastall Cordil Cordiller lera a became became amagm amagmati aticc after after 90 Ma, and most of the Late Cretaceous and younger plutonism, including emplacement of the main Coastal Batholith of southern Peru, was restricted to belts farther east (Cobbing 1985; Taylor et al. 1998; Grocott and Taylor 2002). $

$

Structural elements

Fig. 3 Generalised spatial and temporal distributions of magmatic arc rocks (from Hammerschmidt et al. 1992) and IOCG deposits (this (this study) study) in northe northern rn Chile. Chile. Note Note the system systemati aticc eastwa eastward rd migration of both the arc and contained mineralization, and the marked decline of IOCG mineralization from the Late Cretaceous onwards

The The Atac Atacam ama a Faul Faultt Syst System em foll follow owss the the axis axis of the the Coastal Cordillera for >1,000 km between about latitudes 20 and 30 °S where it is made up of a series of  conca concaveve-wes westt segmen segments ts compri comprisin sing g NNW-, NNW-, N- and NNE-striking ductile and brittle faults which underwent varia variable ble motion motion,, includ including ing sinist sinistral ral strike strike-sl -slip ip (e.g. (e.g. Herve´ 1987; Scheuber and Andriessen 1990; Brown et al. 1993 1993). ). Tran Transi sien entt duct ductil ilee defo deform rmat atio ion, n, char charte ted d by greenschist and amphibolite facies mylonites (Scheuber and Andriessen 1990; Scheuber et al. 1995), occurred at

792

shallow crustal levels (<10 km) in close association with Mesozoic pluton emplacement, but gave way to brittle behaviour during arc cooling (Brown et al. 1993). The brittle faults tend to be localised by pre-existing mylonite zones, zones, commonly commonly along pluton margins (Brown et al. 1993). Fault displacement on the Salado segment of the Atacama Fault System, between latitudes 25 and 27 °S, changed from normal slip to left-lateral transtension at 132 132 Ma (Gro (Groco cott tt and and Wils Wilson on 1997 1997;; Groc Grocot ottt and and Taylor 2002), as apparently it also did as far north as latitude 22 °S (Scheuber and Gonzalez 1999). Nevertheless, less, sinistra sinistrall motion motion of Jurassic Jurassic (pre-155 (pre-155 Ma) Ma) age has been been inte interp rpre rete ted d betw betwee een n lati latitu tude dess 22 and and 26 °S (Scheuber et al. 1995; Scheuber and Gonzalez 1999). The Atacama Fault System is the best documented of three principal principal orogen-paral orogen-parallel lel fault systems in the Coastal Coastal Cordillera between latitudes 25 °30¢ and 27°S, where it is paralleled westwards and eastwards, respectively, by the ductile to brittle Tigrillo and Chivato systems (Grocott and Taylor Taylor 200 2002; 2; Fig. Fig. 5). 5). The three three fault fault sys system tems, s, in concer concertt with with the pluton plutonism ism,, young young eastwa eastwards rds from from Jurassic–Early Cretaceous in the case of the normal-slip Tigrillo system to Early Cretaceous for the left-oblique extensio extensional nal Chivato Chivato system. system. East-sideEast-side-down down displace displace-ment on the fairly shallowly inclined Tigrillo fault exceeds ceeds 1 km, km, and and only only a few kilome kilometre tress of strike strike-sl -slip ip offse offsett are are dedu deduce ced d for for the the Atac Atacam ama a Faul Faultt Syst System em (Grocott and Taylor 2002). In southern Peru, a series of poorly known, orogenparall parallel el faults faults exist exist in the arc terran terrane, e, includ including ing the Can ˜ ˜ ete intra-arc basin, as well as localising the Arequipa back-arc basin. The prominent Treinta Libras fault zone along along the easter eastern n margin margin of the Coasta Coastall Cordil Cordiller lera a underwent underwent dextral dextral strike-sli strike-slip p motion motion in the Jurassic–  Jurassic–  Early Cretaceous, and is marked by a broad dyke swarm (Caldas 1978; Injoque et al. 1988). During During the the early early Late Late Cretac Cretaceou eous, s, transp transpres ressio sion n triggered by final opening of the Atlantic Ocean basin caused caused tectonic tectonic inversio inversion n of the formerly formerly extension extensional al back-arc back-arc basins (Mpodozis (Mpodozis and Ramos 1990; Ladino Ladino et al. 1997). At the same time, the Chivato fault system, a set of northwest-striking transverse faults throughout the Coasta Coastall Cordil Cordiller lera a and other other struct structura urall elemen elements ts between at least latitudes 18 and 30 °S, underwent reactivation in the transpressive regime (Taylor et al. 1998; Grocott Grocott and Taylor Taylor 2002). 2002). Positive Positive tectonic tectonic inversio inversion n also affected southern Peru in the early Late Cretaceous (Benavides-Ca ´ ´ cere ceress 1999 1999)) and and caus caused ed demi demise se of the the Can ˜ ˜ ete basin basin (Cobbi (Cobbing ng 198 1985). 5). The far more more subdue subdued d deformation in the Coastal Cordillera of northern Chile and southern Peru since the mid-Cretaceous took place in a fore-arc setting. $

Metallogenic setting 

The Coastal Cordillera of northern Chile and southern Peru is endowed with iron, copper and subordinate gold, silver and zinc resources, all of mainly Early Jurassic to

mid-Cretaceous age. In addition to the IOCG deposits highligh highlighted ted in this article, article, ‘Kiruna-t ‘Kiruna-type’ ype’ massive massive magnetite-(a netite-(apati patite), te), porphyry porphyry copper-( copper-(gold) gold),, manto-ty manto-type pe copper-(s copper-(silve ilver) r) and volcanic volcanic hosted hosted massive massive sulphide sulphide (VHMS) (VHMS) zinc-cop zinc-copper-b per-barite arite deposits deposits are the principal principal ore types.

Magnetite deposits The massive magnetite deposits occupy the same belt as many of the IOCG deposits over a longitudinal distance of near nearly ly 700 700 km betw betwee een n lati latitu tude dess 25 and and 31 °S in northern Chile and a similar distance in southern Peru, although large deposits there are far more restricted in latitudinal extent (Fig. 1). Many investigators subscribe to a hydrothermal-replacement origin for the magnetite and minor associat associated ed actinolit actinolitee and apatite (e.g. Ruiz et al. 1965, 1968) and locally developed clinopyroxene (e.g (e.g.. Fier Fierro ro Acar Acarıı´, sout southe hern rn Peru Peru;; Injo Injoqu quee 2001 2001), ), although some advocate emplacement mainly as intrusion sionss and and mino minorr extr extrus usio ions ns of iron iron oxid oxidee melt melt (e.g (e.g.. Espinoza 1990; Nystro ¨ m and Henrı´quez 1994; Naslund et al. 2002). A number of small magnetite deposits occur as veins in diorite intrusions, which both Me ´ nard (1995) and Injoque (2001) favour as the magmatic-fluid source for for the magne magnetit titee deposi deposits ts in genera general. l. Severa Severall large large magn magnet etit itee depo deposi sits ts,, incl includ udin ing g El Rome Romera rall and and El Algarrobo in northern Chile, are steep, lens-like bodies within within intrusionintrusion-boun bounded ded screens screens of Early Cretaceous Cretaceous andesitic volcanic rocks along strands of the Atacama Faul Faultt Syst System em (Rui (Ruizz et al. al. 1968 1968;; Book Bookst stro rom m 1977 1977). ). However, the major Marcona deposit in Peru is different, ent, being being a series series of strata strata-bo -bound und bodies bodies (manto (mantos) s) replacing principally early Palaeozoic but also Jurassic (Rı´o (Rı´o Grande Grande Formation Formation)) carbonat carbonatee horizons horizons west of, rather than within, the major Treinta Libras fault zone (Injoque et al. 1988; Injoque 2002). Inclusion of the magnetite deposits as end-members of the IOCG clan (Hitzman et al. 1992) is supported by the abundance of early-stage magnetite in many IOCG deposits, the occurrence of late-stage pyrite, chalcopyrite and gold in and near some massive magnetite deposits (e.g (e.g.. Marc Marcon ona, a, El Rome Romera ral, l, Cerr Cerro o Negr Negro o Nort Norte; e; Bookstrom 1977; Injoque et al. 1988; Vivallo et al. 1995) and the commonality of certain alteration and gangue minerals, especially actinolite and apatite, although nowhere are the two deposit types observably transitional. Nevertheless, magnetite veins and lens-like bodies occur widely in both Jurassic and Early Cretaceous IOCG vein districts, including some of the more important ones like Los Pozos Pozos (Manto (Mantover verde; de; Vila Vila et al. 199 1996), 6), Nagua Naguaya ya ´ n and and Montec Montecris risto to (Boric (Boric et al. 199 1990). 0). There There are also also several several examples examples of IOCG deposits deposits located located alongsid alongsidee major major concentra concentration tionss of massive massive magnetite magnetite (e.g. Mina Justa in the Marcona district; Moody et al. 2003). Thus, the genetic model for the massive magnetite bodies in the Coastal Cordillera is likely to possess major components in common with that preferred for the IOCG deposits.

793

Porphyry copper deposits

Tertiary porphyry copper belts farther east (Fig. 4), and hypogene grades are relatively low (up to 0.4% Cu). Porphyry Porphyry copper copper deposits, deposits, some relative relatively ly enriched enriched in The deposits are related to small stocks of quartz diorite gold, are distributed throughout the Coastal Cordillera to granodiorite porphyry emplaced into arc plutonic or of northern Chile (Fig. 1), where most of them appear to volcanic rocks, and tend to be dominated by potassic be of Early Cretaceous age ( 135–100 Ma) (Munizaga alteration. et al. 1985; 1985; Boric et al. 1990; 1990; Perello Perello´ et al. 2003). 2003). The The The porp porphy hyry ry copp copper er and and IOCG IOCG depo deposi sits ts in the the best-known deposit, and the only producer, is Andacollo Coastal Coastal Cordiller Cordillera a are readily readily distingui distinguishab shable le because because where where a zone zone of superg supergene ene chalco chalcocit citee enrich enrichme ment nt is the potassic alteration and copper-(gold) mineralization exploited; however, several others have been extensively in the former are centred on, and largely confined to, drill drill tested tested (e.g. (e.g. Galenosa Galenosa-Pun -Puntilla tillas, s, AntucoyaAntucoya-Buey Buey porphyry stocks, which are absent from the latter. FurMuerto, Mercedita; Fig. 1). Several prospects also exist thermore, the characteristic quartz veinlets containing all in the Coasta Coastall Cordil Cordiller lera a of southe southern rn Peru Peru (Fig. (Fig. 1), or part part of the the chal chalco copy pyri rite te in the the porp porphy hyry ry copp copper er  ´ a Marı  ´ a is like where where Tı ´a Marı ´a likely ly to be Jura Jurass ssic ic in view view of  deposits deposits as well as the pyrite-d pyrite-domina ominated, ted, sericitesericite-borboravaila ava ilable ble radiom radiometr etric ic ages ages for nearby nearby pluto plutonic nic rocks rocks dered D-type veinlets are also absent from the IOCG (Clark (Clark et al. 199 1990). 0). The depos deposits its are typica typically lly much much deposits, and the iron oxides that define the IOCG class smal smalle lerr (< 300 300 mill millio ion n tonn tonnes es)) than than thos thosee in the the are sparsely represented in the porphyry copper deposits. $

$

$

Fig. 4 Subdivision of the central Andean IOCG province into western Middle–Late Jurassic and eastern Early Cretaceous belts, showing distribution of different deposit styles discussed in the text. Also marked are axes of Palaeocene–  Early Eocene, Late Eocene–  Early Oligocene and Late Miocene–Pliocene Miocene–Pliocene porphyry copper belts, including locations of principal deposits

794

Manto-type copper deposits Manto Manto-ty -type pe copper copper deposi deposits ts occur occur as strata strata-bo -bound und dissem dissemina inated ted bodies bodies,, as steep steep hydrot hydrother hermal mal brecci breccias as around barren, finger-like gabbro to diorite plugs and as related veins, mostly within basaltic to andesitic arc volc volcan anic ic sequ sequen ence cess of the the La Negr Negra a Form Format atio ion n betwee between n latitu latitudes des 22 and 25 °S (Fig. (Fig. 1). 1). Howev However, er, the larges largestt deposi deposit, t, Manto Mantoss Blanco Blancos, s, is unusua unusuall in being being part partly ly host hosted ed by fels felsic ic volc volcan anic ic rock rockss and and plug plugss  ´ rez 1996). Broadly similar copper-silver deposits, (Ramı ´rez incl includ udin ing g El Sold Soldad ado, o, are are wide widesp spre read ad in the the Earl Early y Cret Cretac aceo eous us volc volcan anic ic and and sedi sedime ment ntar ary y rock rockss of the the Central Chile intra-arc basin (e.g. Fig. 1; Maksaev and Zentilli 2002). The highes highest-g t-grad radee parts parts of mantomanto-typ typee deposi deposits, ts, typica typically lly contro controlle lled d by the permea permeabil bility ity provid provided ed by faults, faults, hydrothe hydrothermal rmal breccias breccias,, dyke contacts, contacts, vesicular vesicular flow tops and flow breccias, are characterised by hypogene chalcocite and bornite, which grade outwards and downwards downwards through chalcopyrite chalcopyrite to minor minor distal distal concentratio centrations ns of pyrite. pyrite. The chalcocit chalcocite-bo e-bornite rnite cores of  large deposits commonly abut original redox boundaries in the host stratigraphic packages and are overlain or flanked by sulphide-deficient zones containing hypogene hematite (Sillitoe 1992; Kirkham 1996). Albite, quartz and chlorite are the main alteration minerals in the cores of the deposits. Opinion is divided between magmatichydrother hydrothermal mal (e.g. Holmgren Holmgren 1987; Wolf et al. 1990) and metamo metamorph rphoge ogenic nic (e.g. (e.g. Sato Sato 198 1984; 4; Sillit Sillitoe oe 199 1990, 0, 1992) fluid origins for the manto-type deposits, although the latter alternative alternative is favoured favoured by the obvious similarities lari ties to stratifor stratiform, m, sedimentsediment-host hosted ed copper copper deposits deposits (Kirkham 1996). Nevertheless, emplacement of plutonic complexes complexes may have been instrumen instrumental tal in causing causing the fluid fluid circul circulati ation on that that result resulted ed in mantomanto-typ typee copper copper formation (Maksaev and Zentilli 2002). The manto-type deposits comprise a distinctive class of copper mineralization uncommon outside the Coastal Cordillera of northern and central Chile (Sillitoe 1992; Kirkha Kirkham m 199 1996). 6). Althou Although gh many many manto manto-ty -type pe copper copper deposi deposits ts contai contain n albite albite altera alteratio tion, n, calci calcite te and minor minor hematite, and some are spatially related to gabbro and diorite diorite bodies—fe bodies—featur atures es shared shared with some central Andean IOCG deposits (see below)—the manto-type style appear appearss to be distin distingu guish ished ed by its asy asymme mmetri trical cal sulsulphide-ox phide-oxide ide zonation zonation and marked marked deficiency deficiency in gold. gold. Caution is necessary, however, because it will be recalled that the breccia-hosted IOCG deposit at Olympic Dam in Sout South h Aust Austra rali lia a is also also char charac acte teri rise sed d by simi simila larr asymmetrical sulphide-oxide zonation, with distal pyrite giving way through chalcopyrite and bornite-chalcocite to overlying hematite (Reeve et al. 1990). Notwithsta Notwithstandin nding g these these apparent apparent difference differences, s, some invest investiga igator torss treat treat at least least select selected ed large large mantomanto-typ typee deposits (e.g. Mantos Blancos) as members of the IOCG class (Williams 1999; Pollard 2000), include the two deposit types in a broader manto-type category (Injoque 2000) or propose that manto-type deposits are shallow

manifestations of the IOCG type (Vivallo and Henrı´quez 1998; Orrego et al. 2000). However, manto-type deposits are apparently nowhere observed to be directly related or transitional to IOCG deposits; while an intimate genetic connection cannot be precluded at present, substantive geological support is a clear necessity.

VHMS deposits Several Several VHMS deposits deposits of Kuroko Kuroko type were formed formed in centra centrall and northern northern Peru Peru during during the Early Early CretaCretaceou ceous. s. The The VHMS VHMS belt belt over overla laps ps with with the the nort northe hern rn recognised limit of the IOCG belt in the Can ˜ ete intraarc basin (Fig. 1; Injoque Injoque 2000). 2000). The deposits deposits display classic massive and stringer types of mineralization and are particularly noted for their zinc and barite contents (Vidal 1987), although copper besides zinc is important at Cerro Lindo, the most southerly deposit (Ly 2000). Thes Thesee ore ore text textur ures es and and meta metall cont conten ents ts,, besi beside dess the the defici deficienc ency y of magnet magnetite ite and hemati hematite, te, clearl clearly y distin distin-guish the Peruvian VHMS from central Andean IOCG deposits.

IOCG deposits

Sites of mineralization In northern Chile, mainly between latitudes 22 and 31 °S, most of the IOCG deposits are hosted by the La Negra Formation arc volcanics and their stratigraphic equivalents farther south as well as by the Late Jurassic and Early Early Cretaceous Cretaceous plutons that intrude intrude them (Table 2). Candelari Candelaria-Pu a-Punta nta del Cobre Cobre and some smaller IOCG depos deposits its,, howeve however, r, were were genera generated ted near near Early Early CretaCretaceous plutons emplaced near the contact between Late Juras Jurassic sic–Ea –Early rly Cretac Cretaceou eouss volcan volcanoge ogenic nic sequen sequences ces (Punta del Cobre Group) and Neocomian marine carbonat bonatee sequen sequences ces.. Most Most IOCG IOCG deposi deposits ts docume documente nted d from southern Peru, between latitudes 12 °30¢ and 14°S, are confined to the Can ˜ ete intra-arc basin (Fig. 1). The copper-be copper-bearin aring g Marcona Marcona magnetite magnetite district, district, including including the Mina Justa IOCG deposit, and several minor magnetite netite and IOCG deposits deposits farther south pre-date pre-date formation of the Can ˜ ete basin and occur within the Jurassic arc terrane (Fig. 4). The latitudinal extent of Mesozoic IOCG deposits in the the centra centrall Andes Andes is closel closely y compar comparabl ablee with with that that of  Tertiary Tertiary porphyry porphyry copper copper deposits deposits (Fig. 4), although although known IOCG deposits are apparently few and relatively minor minor betwee between n latitu latitudes des 16 and and 22 °S where where the westwesternmost part of the IOCG belt may now lie beneath sea level. The Coastal Cordillera IOCG province spans three structurally, stratigraphically stratigraphically and metallogenically distinct tectonic segments of long standing, and is delimited by fund fundam amen enta tall tran transv sver erse se segm segmen entt boun bounda dari ries es at roughly latitudes 13 ° (the Pisco-Abancay deflection) and 33°30¢S.

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Fig. 5 Schemati Schematicc east–wes east–westt sections sections of the Middle–L Middle–Late ate Jurassic Jurassic and Early Early Cretac Cretaceo eous us IOCG IOCG belts belts in northe northern rn Chile, Chile, showin showing g distributions of plutonic, volcanic and sedimentary rocks and main fault fault systems. systems. Selected Selected IOCG and massive massive magnetit magnetitee deposits, deposits, coded on the basis of deposit style, are projected onto the sections. Note the eastward migration of plutonic and volcanic rocks and their contained mineralization with time, the structural localisation of some of the deposits and the close spatial association between IOCG and magnetite deposits. Taken with slight modification from Espinoza et al. (1999) and Gelcich et al. (1998), with fault additions schematised from Grocott and Taylor (2002)

Metallogenic epochs The relatively restricted radiometric age data, provided for gangue gangue (actinoli (actinolite) te) or alteratio alteration n (biotite, (biotite, sericite) sericite) minerals by the K-Ar method unless stated otherwise, suggest suggest that the principa principall IOCG deposits in northern northern Chil Chilee and and sout southe hern rn Peru Peru were were main mainly ly gene genera rate ted d in Middle–La Middle–Late te Jurassic Jurassic (170–150 (170–150 Ma) and Early CretaCretaceous ceous (130–1 (130–110 10 Ma) Ma) epochs epochs (Fig. (Fig. 4), althou although gh a few Late Late Cretac Cretaceou eouss and Palaeo Palaeocen cenee exampl examples es are also also know known n (Fig (Figs. s. 1 and and 4). 4). The The meta metall llog ogen enic ic epoc epochs hs migra migrated ted eastwa eastwards rds in concer concertt with with spatia spatially lly relate related d plutonic belts (Figs. 3, 4 and 5). The Middle–Late Jurassic deposits are located near the the Paci Pacific fic coas coast. t. In nort northe hern rn Chil Chile, e, they they incl includ udee Toco Tocopi pill lla a (165 (165±3 ±3 Ma), Ma), Guan Guanil illo loss (167 (167±7 ±7 Ma), Ma), Naguaya ´ n (153±5 Ma), Montecristo and Julia (164± 11 Ma; Ma; Bori Boricc et al. al. 1990 1990), ), Las Las Anim Animas as (162 (162±4 ±4 Ma; Ma; Gelcich et al. 1998) and, based on ages of  150 Ma for the host diorite pluton (Moscoso et al. 1982), probably Carrizal Carrizal Alto. In southern southern Peru, the large Mina Justa deposit and other copper mineralization in the Marcona dist distri rict ct (154 (154±4 ±4.0 .0 and and 160± 160± 4.0 4.0 Ma; Ma; Injo Injoqu quee et al. al.  ´ a (ca. 160 or 145 Ma; Clark et al. 1988) and Rosa Marı ´a 1990) 1990) are ass assign igned ed to the same same overal overalll metall metalloge ogenic nic epoch. $

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Most Most of the major major IOCG IOCG deposi deposits ts and numerous numerous smaller examples are located farther east in the Coastal Cordillera and are Early Cretaceous in age (Fig. 4). This epoch includes: Candelaria (116–114 Ma, 40Ar/39Ar and Re-Os; Re-Os; Marschik Marschik and Fontbote Fontbote´ 2001b; 2001b; Mathur Mathur et al. 40 39 2002; 200 2; or 112 112–11 –110 0 M Ma, a, Ar/ Ar; Ar; Ullr Ullric ich h and and Clar Clark k 1999; 199 9; Are´ val valo o et al. 200 2000; 0; Ullric Ullrich h et al. 2001) 2001),, MantMantover overde de (123 (123±3 ±3,, 121± 121±3 3 and and 117± 117±3 3 Ma; Ma; Vila Vila et al. al. 1996; 199 6; Orr Orrego ego et al. 2000) 2000),, Galle Gallegui guillo lloss (121±4 (121±4 Ma; Ma; R.H. Sillitoe and M. Orrego, unpublished data, 1999), Bril Brilla lado dorr (con (conti tigu guou ouss plut pluton on date dated d at 108. 108.5 5 Ma; Ma; Moscoso et al. 1982), Panulcillo (115±3 Ma; R. Ardila in Sugak Sugakii et al. 2000) 2000) and El Espino Espino (nearby (nearby pluton pluton dated dated at 108 108±3 ±3 Ma; Ma; Riv Rivano ano and Sepu Sepu´ lveda lveda 199 1991) 1) in northern Chile; and Rau´ l-Condestable (116.5–113 Ma, U-Pb on sphene; de Haller et al. 2002; A. de Haller, personal personal communic communicatio ation, n, 2003) 2003) and Eliana Eliana (115±5.0 (115±5.0 and 113±3.0 Ma; Vidal et al. 1990) in southern Peru. Additiona Additionally, lly, district-w district-wide ide hydrother hydrothermal mal alteratio alteration n at the Productora IOCG occurrence, near latitude 29 °S, is centred on an albitised diorite intrusion (Ray and Dick 2002) dated at 129.8±0.1 Ma (U-Pb, zircon; Fox 2000), although K-feldspar associated with the IOCG mineraliz lizatio ation n retu return rned ed an avera erage age of  91 Ma 40 39 ( Ar/ Ar; Fox 2000; Fox and Hitzman 2001), probably bly due due to re-s re-set etti ting ng duri during ng subs subseq eque uent nt bath bathol olit ith h empla emplacem cement ent (G.E. (G.E. Ray Ray,, person personal al commun communica icatio tion, n, 2003). The largest of the younger IOCG deposits is Dulcinea, situated about 12 km east of the eastern border of  the Coastal Cordillera (Figs. 1 and 4), which is hosted by a dior diorit itee-mo monz nzod odio iori rite te intr intrus usio ion n and and ande andesi siti ticc metavolcanic rocks assigned ages of 65–60 Ma (Iriarte et al. 1996). The La Africana deposit, at the southern extremity of the IOCG belt and also previously mined formally, cuts a diorite of presumed Late Cretaceous age (Saric 1978). The El Espino deposit may also be Late Cretaceous or Palaeocene rather than Early Cretaceous, as stated above, if the Late Cretaceous–Palaeocene age favoured for the nearby diorite intrusion by Rivano and Sepu´ lveda (1991) is substantiated. A few small and lessimpo import rtan antt IOCG IOCG depo deposi sits ts,, as well well as seve severa rall smal smalll massive magnetite deposits, of Late Cretaceous or Palaeocen aeocenee age age are also also presen presentt immedi immediate ately ly east east of the Coastal Cordillera in northern Chile. Minor IOCG vein deposi deposits ts also also occur occur in the coasta coastall bathol batholith ith of Peru, Peru, where they are likely to be of Late Cretaceous age (Vidal 1985). The age distri distribut bution ion of IOCG IOCG deposi deposits ts and occuroccurrences in the Coastal Cordillera is certainly more complex than the two broad epochs defined above would sugg sugges est, t, as witn witnes esse sed d by K-Ar K-Ar ages ages obta obtain ined ed for for numerous numerous mineral mineral occurrenc occurrences, es, many containin containing g iron oxides, copper and gold, in the Coastal Cordillera between latitudes 26 and 28 °S (Dı´az ´az and Vivallo 2001). 2001). Base Based d on the the ages ages,, thes thesee work worker erss prop propos osed ed four four metalloge metallogenic nic epochs: epochs: 188–172, 188–172, 167–153, 167–153, 141–132 141–132 and 130–98 130–98 Ma, Ma, which coincide coincide with four eastwardeastward-youn youn-ging plutonic belts of broadly the same ages. $

Tectonic controls Only Only limite limited d inform informati ation on is ava availa ilable ble on the detail detailed ed tect tecton onic ic envi enviro ronm nmen ents ts of IOCG IOCG form format atio ion n in the the Coastal Coastal Cordillera Cordillera,, although although all major major deposits deposits were generated during regional extension or transtension, and locali localised sed by ductil ductilee to brittl brittlee faults faults and fractu fractures res of  varied strike (Table 2). At least between latitudes 22 and 27°30¢S, the Late Late Jurass Jurassic ic deposi deposits ts were were genera generated ted in associati association on with normal normal fault fault systems systems displayin displaying g easteastside-down displacements, whereas those dated as later than 132 132 Ma, Ma, essent essential ially ly all the Early Early Cretac Cretaceou eouss depos deposits its,, were were locali localised sed by sinist sinistra rall transt transten ensio sional nal structures within or related to the Atacama and Chivato fault systems (Grocott and Wilson 1997; Scheuber and Gonzalez 1999; Grocott and Taylor 2002), but typically beyond the main north–south splays (Fig. 5). The control of the Mantoverde deposit, for example, has been interpre interpreted ted as a strike-sli strike-slip p duplex duplex or side-wall side-wall ripout (Brown et al. 1993; Taylor et al. 1998) or, alternatively, as a strike-slip relay ramp breached by the ore-controlling Mantoverde fault (C. Bonson in Grocott and Taylor 2002). In contra contrast st to the steep attitude attitudess of most most IOCGIOCGcontr controll olling ing faults faults,, a low-an low-angle gle listri listricc normal normal fault, fault, >1 km in downdown-di dip p exte extent nt but but givi giving ng rise rise to litt little le apparent apparent offset, in combinat combination ion with a series series of steep hangi hangingng-wal walll splay splayss locali localised sed the Mina Mina Justa Justa IOCG IOCG deposit in the Marcona district of southern Peru (Moody et al. 2003). 2003). The fault may merge merge eastwa eastwards rds with the major Treinta Libras strike-slip fault zone (see above). Most, but perhaps not all, of the ductile deformation in indivi individua duall fault fault sys system temss pre-da pre-dated ted relate related d IOCG IOCG deposit deposit formation formation,, as clearly clearly observed observed at Mantover Mantoverde de and elsewhere. Syn-mineralization ductile shearing was, however, proposed at the Panulcillo deposit by Hopper and Correa (2000), and is observed to have been active during during early early magnetite magnetite introductio introduction n at several several of the IOCG deposits. Opinion concerning the tectonic setting of the the major major Candel Candelari aria-P a-Punt unta a del Cobre Cobre depos deposit it is divided: Martin et al. (1997) and Are ´ valo et al. (2000) believed believed that minerali mineralizatio zation n took place place during during transtranstension while low-angle ductile shearing was still active because because of thermal thermal mediation mediation by the nearby nearby plutonic plutonic complex (cf. Dallmeyer et al. 1996; Grocott and Taylor 2002). Biotite schist is believed to have formed concurrently with the early stages of magnetite and chalcopyrite precipitation (Are ´ valo et al. 2000). In stark contrast, Marschik and Fontbote ´ (2001b) and Ullrich et al. (2001) proposed proposed a less-like less-likely ly interpret interpretatio ation n that copper-go copper-gold ld mineraliz mineralizatio ation n post-dat post-dated ed schist schist formation formation and coincoincided cided with with initia initiall backback-arc arc basin basin invers inversion ion and conconcomitant comitant uplift. A similar similar notion, notion, gabbro gabbro and diorite empla emplacem cement ent and, and, by assoc associat iation ion,, IOCG IOCG genera generatio tion n during initial tectonic inversion, was favoured by Regan (1985) and Injoque (2001) for the Can ˜ ete basin, although an extensional setting for the mafic magmatism would seem to be more reasonable. The relatively minor Late Cretaceous and Palaeocene IOCG deposits were formed $

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after the early Late Cretaceous tectonic inversion event, during during subseque subsequent nt extensio extensional nal episodes episodes (e.g. (e.g. Cornejo Cornejo and Matthews 2000).

Deposit styles The IOCG deposits of northern Chile and southern Peru include representatives of most common mineralization styles, either alone or in varied combinations (Table 2, Fig. 4). Vein deposits are by far the most abundant, with many many hund hundre reds ds of them them occu occurr rrin ing g thro throug ugho hout ut the the Coasta Coastall Cordil Cordiller lera a belt, belt, especi especiall ally y in north northern ern Chile. Chile. There, the IOCG veins accounted for Chile’s position as the world’s leading copper producer in the 1860–1870s, alth althou ough gh most most of them them have have not not been been the the focu focuss of  attention over the last 40 years or so because of their relatively small size (Table 1) and the fact that many of  the mines are severely depleted. The veins, products of  both both replac replaceme ement nt and ass associ ociate ated d openopen-spa space ce filling filling,, typically occur as swarms of up to 40 occupying areas up to several tens of square kilometres (Fig. 6). The principal veins are 1–5 km long and 2–30 m wide, with ore shoots worked for at least 500 m down the dip of the veins, and attaining 700 m at Tocopilla and 1,200 m at Dulcinea. In addition to the veins, isolated breccia pipes (Carrizali rizalillo llo de las Bombas Bombas,, Teresa Teresa de Colmo Colmo)) and calcic calcic skarns [San Antonio, Panulcillo, Farola (Las Pintadas)] also occur locally (Fig. 4). 4). The major IOCG deposits, deposits, however, are typically composite in style and comprise varied combinations of breccias, stockworked zones and

Pluton-hosted sted IOCG veins veins and controll controlling ing faults in the Fig. 6 Pluton-ho Tocopilla Tocopilla district district,, northern northern Chile. Chile. Principa Principall mined mined deposits deposits are named. Taken from Boric et al. (1990)

replacement mantos besides veins, as observed at Candelaria-Punta del Cobre (Martin et al. 1997; Marschik and and Fontbo Fontbote te´ 200 2001b) 1b),, Mantov Mantoverd erdee (Vila (Vila et al. 199 1996; 6; Zamora Zamora and Castil Castillo lo 200 2001), 1), Cerro Cerro Negro Negro and Rau´ lCondestable (Vidal et al. 1990; de Haller et al. 2002). Breccias, both hydrothermal and tectonic in origin, are comm common on comp compon onen ents ts of the the comp compos osit itee depo deposi sits ts (Tab (Table le 2), 2), in part partic icul ular ar at Mant Mantov over erde de wher wheree they they compr comprise ise the the shallo shallower wer,, curren currently tly mined mined parts parts of the main fault-contro fault-controlled lled vein structur structuree (Vila et al. 1996) 1996) and the associate associated d Manto Manto Ruso strata-bo strata-bound und deposit deposit (Fig. (Fig. 7; Orrego Orrego and Zamora Zamora 1991). 1991). The predomina predominant nt brecc breccias ias at Cerro Cerro Negro Negro also also compri comprise se strata strata-bo -bound und manto mantos. s. Compos Composite ite deposi deposits, ts, includ including ing Candel Candelari ariaaPunta Punta del Cobre Cobre (Marschi (Marschik k and Fontbote ´ ´ 2001b) 2001b) and Rau´ l-Condestable (Vidal et al. 1990), contain bodies of  dispersed dispersed mineralizati mineralization on controlle controlled d in part by stratal stratal

Fig. 7 Geological sketch of the Los Pozos (Mantoverde) district, showing showing its confinemen confinementt to a fault-bo fault-bounde unded d screen screen of Jurassic Jurassic volcanic volcanic rocks (La Negra Formation) Formation) and tight tight control control by the transten transtensiona sionall Atacama Atacama Fault System. Also shown shown are the two contiguous plutonic complexes and the different styles and relative sizes of IOCG and massive magnetite mineralization that comprise the district, along with their respective radiometric ages. Note the temporal temporal and probable probable genetic genetic relation relationship ship between between the Sierra Sierra Diecioc Dieciocho ho diorite-m diorite-monzo onzodiori diorite te complex complex and the Mantover Mantoverde de IOCG deposit based on their age similarities. Map slightly modified after Espinoza et al. (1999) and radiometric ages summarised from sources cited in the text

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permeability provided by fragmental volcanic or volcaniclastic horizons. The large Mina Justa deposit in the Marcona Marcona district district consists consists of irregular irregular patches, veinlets and breccia fillings of well-zoned sulphide mineralization within a low-angle fault zone that transgresses the host stratigraphy (Moody et al. 2003). Hornfelsi Hornfelsing ng of volcanovolcano-sedim sedimenta entary ry host rocks rocks to IOCG deposits is ubiquitous and may have predisposed them to widesprea widespread d brittle brittle fracturin fracturing g and conseque consequent nt permea permeabil bility ity enhanc enhanceme ement. nt. Typica Typically lly,, howeve however, r, the thermal effects are difficult to discriminate from metasomatic products, including widespread and pervasively developed biotite, actinolite, epidote, albite and related minerals. Permeability barriers, especially marbleised or even little-altered carbonate sequences, may have played an important role in the confinement and ponding of  hydrothermal fluid in some deposits, such as CandelariaPunta del Cobre and El Espino (Correa 2003). Nevertheless, if fluid penetration is more effective, carbonate rocks may be transformed to skarn and constitute integral gral part partss of some some comp compos osit itee depo deposi sits ts (e.g (e.g.. Ra Rau u´ lCondestable; Vidal et al. 1990).

Intrusion relations In common with many IOCG deposits worldwide (e.g. Ray Ra y and and Webs Webste terr 2000 2000), ), a numb number er of the the Ande Andean an examples examples lack clear clear genetic genetic relations relations to specific specific intruintrusions despite being located in close proximity (<2 km) to outcro outcroppi pping ng pluton plutonic ic comple complexes xes,, includ including ing early early diorit dioritic ic phases phases (e.g. (e.g. Sierra Sierra Diecio Dieciocho cho pluton pluton east east of  Manto Mantover verde; de; Fig. Fig. 7; 7; Zamora Zamora and Castil Castillo lo 2001; 2001; and Ojancos Ojancos plutonic plutonic complex complex west of Candelaria Candelaria;; Fig. 8; Marschik and Fontbote ´ 2001b). Mantoverde and Candelaria-Punta del Cobre are typical examples of deposits where where the IOCG IOCG minera mineraliz lizati ation on and nearby nearby pluton plutonic ic complexes are not observed to be in contact, although radiometric dating has shown that the intrusive activity and alteratio alteration-mi n-mineral neralizat ization ion episode episode overlap overlap tempotemporally (Fig. 9). For example, at Mantoverde, K-Ar ages of 123± 123±3, 3, 121± 121±3 3 and and 117± 117±3 3 Ma Ma (Vil (Vila a et al. al. 1996 1996;; Orre Orrego go et al. al. 2000 2000)) for for hydr hydrot othe herm rmal al seri serici cite te are are encompas encompassed sed by U-Pb zircon, whole-rock whole-rock Rb-Sr isochron, 40Ar/39Ar hornblende and K-Ar ages of  127–  120 Ma for the contiguo contiguous us Sierra Sierra Diecioch Dieciocho o plutonic plutonic comp comple lex x (Fig (Figs. s. 7 and and 9; Berg Berg and and Baum Bauman ann n 1985 1985;; Dallmeyer et al. 1996; Espinoza et al. 1999). Similarly Similarly,, preferred preferred ages of 116–114 116–114 Ma (Marschik (Marschik and Fontbote 2001b; Mathur et al. 2002) or 112–110 Ma (Ullr (Ullrich ich and Clark Clark 1999; 1999; Are ´ ´ val valo o et al. 2000; Ullrich Ullrich et al. 2001) for copper mineralization at Candelaria fall within within the 117. 117.2±1. 2±1.00- to 110. 110.5±1. 5±1.7-Ma 7-Ma emplaceme emplacement nt span span for the contig contiguou uouss Oja Ojanco ncoss pluton plutonic ic comple complex x 40 39 ( Ar/ Ar; Ullrich et al. 2001). In clear clear contra contrast, st, howeve however, r, most most of the princi principa pall IOCG IOCG vein vein deposi deposits ts in north northern ern Chile, Chile, such such as TocoTocopilla, Gatico, Montecristo, Julia, Las Animas, Ojancos Nuevo Nuevo,, Carriz Carrizal al Alto, Alto, Quebra Quebradit dita a and La Africa Africana na $

Fig. 8 Spatial Spatial relation relationss of IOCG and carbonate carbonate-host -hosted ed massive massive magneti magnetite te deposits, deposits, including including the major major Candelari Candelaria a deposit deposit and related related deposits deposits in the Punta Punta del Cobre district, district, to the Ojancos Ojancos plutonic complex, in particular the diorite phase. The rest of the complex comprises monzodiorite, tonalite and monzogranite. The Farola deposit is a garnet-rich skarn (Ruiz et al. 1965). Although a case may be made for a genetic relationship between the diorite and deposits deposits hosted by both volcanic volcanic (Punta (Punta del Cobre Group) Group) and sedimentary (Chan ˜ arcillo Group) rocks, it should be noted that the pluton pluton-ho -hoste sted d IOCG IOCG veins veins share share faults faults with with pre-or pre-oree ‘diaba ‘diabase’ se’ dykes, dykes, suggestin suggesting g that the unobserve unobserved d magmatic magmatic source of the dykes may be more closely related genetically to the mineralisation  ´ az et al. than the outcroppi outcropping ng diorite diorite itself. itself. Compiled Compiled from Dı ´az al. (1998) and Marschik and Fontbote ´ (2001b)

(Fig. 4), are hosted by plutons, most of them dioritic in compo composit sition ion.. Severa Severall of the IOCG IOCG vein vein deposi deposits ts and their host intrusions have been shown to possess similar ages, ages, a relati relations onship hip that that is partic particula ularly rly clear clear at Las Animas where alteration biotite is dated at 162±4 Ma (K-Ar) by Gelcich et al. (1998) and the nearby diorite at 161±4 161±4 (K-Ar, (K-Ar, biotite), biotite), 159.7±1.6 159.7±1.6 (U-Pb, (U-Pb, zircon) zircon) and 157.6±2.6 157. 6±2.6 Ma (Rb-Sr, (Rb-Sr, whole rock; rock; Fig. 9; 9; Dallmeyer Dallmeyer et al. al. 1996 1996). ). Furt Furthe herm rmor ore, e, in sout southe hern rn Peru Peru,, the the Monte Monterro rrosas sas and Eliana Eliana veins veins are hosted hosted mainl mainly y by gabbrodiorite (Atkin et al. 1985; Vidal et al. 1990). Irresp Irrespect ective ive of whethe whetherr host host rocks rocks are diorit dioritic ic or more felsic plutons (e.g. Julia) or their nearby wall rocks (e.g. Brillador, Tamaya; Table 2), the IOCG veins normally share localising faults with mafic to intermediate dykes. They are variously described as andesite, basalt, doler dolerite ite,, diabas diabase, e, diorit diorite, e, gabbro gabbro or simply simply mafic mafic in composition, and are typically of pre- or syn-ore timing (Table 2; Ruiz et al. 1965; Boric et al. 1990; Espinoza et al. 1996), 1996), but locally locally mappe mapped d as post-o post-ore re (e.g. (e.g. La Africana; Saric 1978). Additionally, syn- to late-mineralizat alization ion diorit dioritee dykes dykes occur occur alongs alongside ide the volcan volcanicichoste hosted d Manto Mantover verde de vein-b vein-brec reccia cia deposi depositt (Vila (Vila et al.

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Fig. 9 Comparisons of radiometric ages for hydrothermal minerals from from IOCG IOCG deposi deposits ts and magm magmati aticc minera minerals ls from from host host (Las (Las Animas) Animas) or contiguo contiguous us (Mantove (Mantoverde, rde, Candelari Candelaria) a) plutonic plutonic complexes plexes determin determined ed using a vari variety ety of methods. methods. Note the overall temporal coincidence of hydrothermal and intrusive events within the error limits limits of the metho methods ds employ employed. ed. See text text for furthe furtherr details. Data compiled from Gelcich et al. (1998), Espinoza et al. (1999), Ullrich and Clark (1999), Orrego et al. (2000), Marschik and Fontbote ´ ´ (2001b), Ullrich et al. (2001) and Mathur et al. (2002)

1996; M. Orrego, personal communication, 2002), preand post-breccia post-breccia diorite dykes are associated associated with the Teresa Teresa de Colmo Colmo brecci breccia a (Corre (Correa a 200 2000; 0; Hoppe Hopperr and Correa Correa 2000) 2000) and diorite diorite dykes are prese present nt in the El Salado vein district (Browne et al. 2000) and at Punta del Cobre. Typically, the dykes observed by the writer are best best desc descri ribe bed d in hand hand samp sample le as finefine- to medi medium um-grained diorite porphyries. It is partic particula ularly rly instru instructi ctive ve to point point out out that that the Ojancos Nuevo veins lie within, and the Farola copper skarn skarn abuts, abuts, an areall areally y extens extensive ive diorit dioritee phase phase of the Ojanco Oja ncoss pluton plutonic ic comple complex, x, which, which, as noted noted above, above, is <2 km from the Candelari Candelaria-Pu a-Punta nta del Cobre Cobre deposit deposit and and of broa broadl dly y the the same same age age (Fig (Figs. s. 8 and and 9). 9). The The Panulcillo copper-gold skarn deposit, the southernmost in an 80-km long, north-trending belt of small copper skarns skarns extend extending ing as far as San San Anton Antonio io (Fig. (Fig. 4 4), ), lies lies adjacent to an albitised diorite intrusion (Sugaki et al. 2000), 200 0), as do the breccia breccia mantos mantos at the Cerro Negro Negro deposi deposit. t. Drilli Drilling ng also also inters intersect ected ed an albiti albitised sed diorit dioritee intrusion containing low-grade chalcopyrite mineralization about 500 m beneath the Teresa de Colmo chalcopyrite-bearing breccia (Correa 2000; Hopper and Correa 2000). Notwithstanding the apparently widespread association between IOCG deposits and broadly dioritic plutons tons and and mino minorr intr intrus usio ions ns,, some some of them them inte intens nsel ely y albiti albitised sed,, in the Coasta Coastall Cordil Cordiller lera, a, it should should also also be mentioned mentioned that volumetr volumetricall ically y minor minor dacite dacite porphyry porphyry dykes, dykes, docume documente nted d as either either synsyn- or interinter-min minera erall in timing, occur within the Punta del Cobre (R.H. Sillitoe, unpublished data, 1992; Marschik and Fontbote ´ Fontbote ´ 1996; Pop et al. 200 2000), 0), Rau´ l-Cond l-Condest establ ablee (de Halle Hallerr et al. 2002) and Mina Justa (Moody et al. 2003) deposits. At Rau´ l-Cond l-Condest establ able, e, zircon zircon from from the dacite dacite porph porphyry yry yields U-Pb ages of  115 Ma, closely similar to that for $

hydrothermal sphene associated with the IOCG mineralization (de Haller et al 2002; A. de Haller, personal communication, 2003).

Geochemistry and mineralogy Long before the iron-oxide-bearing copper deposits of  northern Chile were assigned to the IOCG clan, Ruiz and Ericksen (1962) and Ruiz et al. (1965) (see also Ruiz and and Peebl Peebles es 198 1988) 8) subdiv subdivide ided d them them into into magne magnetit titeedominated and (specular) hematite-dominated subtypes. Most Most member memberss of their their two subtyp subtypes es are chalco chalcopypyrite±born rite±borniteite-beari bearing ng veins, veins, but the hematitehematite-rich rich subtype also includes the vein breccia at Mantoverde and the veins, breccias and mantos at Punta del Cobre. No doub doubtt Cand Candel elar aria ia woul would d have have been been assi assign gned ed to the the magnetite-rich category had it been known at that time! Subsequent work has shown that at least some of the hematitehematite-rich rich veins veins are transitio transitional nal downwards downwards to the magne magnetit tite-r e-rich ich var variet iety y (Fig. (Fig. 1 10), 0), as observ observed ed at Julia Julia (Espinoza et al. 1996), Las Animas (Gelcich et al. 1998) and, as a result of recent deep drilling, at both Mantover overde de (Zam (Zamor ora a and and Cast Castil illo lo 2001 2001)) and and El Sala Salado do (Brown (Brownee et al. 200 2000) 0),, in keepin keeping g with with the genera generalis lised ed vert vertic ical al zona zonati tion on of IOCG IOCG depo deposi sits ts prop propos osed ed by Hitzman et al. (1992). A similar upward and outward change from magnetite to hematite is also documented at the distri district ct scale scale at Candel Candelari aria-P a-Punt unta a del Cobre Cobre (Marschi (Marschik k and Fontbote Fontbote ´ ´ 2001b). 2001b). An appreciab appreciable le proportion of the magnetite in the hematite-rich veins is the mushketo mushketovite vite vari variety: ety: pseudomo pseudomorpho rphous us after after specular specular hematite (Ruiz et al. 1965). Late-stage hematite also cuts and replaces some of the magnetite. Widespread developmen opmentt of magnet magnetite ite after after hemati hematite te was recent recently ly reemphasise emphasised d at Candelari Candelaria-Pu a-Punta nta del Cobre Cobre (Marschik (Marschik and Fontbote ´ ´ 2001b), Rau´ l-Condestable (de Haller et al. 2002) 2002) and Mina Mina Justa Justa (Moody (Moody et al. 2003). 2003). The iron oxides oxides are typically typically post-dated post-dated by pyrite pyrite and copperbearing sulphides (e.g. Ruiz et al. 1965), although temporal overlap is observed locally. The magnetite-rich veins contain appreciable actinolite, biotite and quartz, as well as local apatite, clinopyroxene, garnet, hematite and K-feldspar, and possess narro narrow w altera alteratio tion n haloe haloess contai containin ning g one or more more of 

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Fig. 10 Idealised section of an IOCG vein in the Coastal Cordillera showing upward zonation from magnetite to hematite domination, and the possibility of coarse calcite (± silver mineralisation) in its top parts and copper-poor massive magnetite at depth. Much of the magnetite is the mushketovite variety. Hematite zone may display hydrother hydrothermal/ mal/tect tectonic onic brecciat brecciation. ion. Note shared shared fault/fra fault/fractur cturee control with pre-vein mafic dyke. Expanded from Espinoza et al. (1996)

actinolit actinolite, e, biotite, biotite, albite, albite, K-feldsp K-feldspar, ar, epidote, epidote, quartz, quartz, chlorite, sericite and scapolite (Table 2; Ruiz et al. 1965; Boric et al. 1990; Espinoza et al. 1996; Injoque 2001, 2002). In contrast, the hematite-rich veins tend to contain sericite and/or chlorite, with or without K-feldspar or albite, and to possess alteration haloes characterised by these same minerals (Table 2). Tourmaline may be a constituent of either subtype, but is perhaps most common where hematite is more abundant than magnetite. Both IOCG subtypes tend to be relatively poor, but by no means deficient, in quartz, while especially the specular hematite-rich variety is commonly associated with coarse-grained calcite and ankerite, either as early or late additions or as a distal equivalent (Fig. 10; Ruiz et al. 1965). Monomineralic chalcopyrite may be intergrown with these carbonate minerals. Both Both the the magn magnet etit itee- and and spec specul ular ar hema hemati tite te-r -ric ich h IOCG veins contain chalcopyrite and generally subordinate pyrite, but in a few cases bornite accompanies the chal chalco copy pyri rite te (Tab (Table le 2). 2). The The main main Tama Tamaya ya vein vein was was dominated by bornite to a depth of 400 m (Ruiz et al. 1965). 1965). The irregular irregular but broadly broadly vein-like vein-like Mina Justa Justa deposit deposit contains contains concentri concentrically cally zoned zoned sulphide sulphide assemassemblages, with a bornite-chalcocite core grading outwards through bornite-chalcopyrite and chalcopyrite-pyrite to a broad pyrite halo (Moody et al. 2003). Similar zoning,

albeit without chalcocite in the central zone, is also described from Panulcillo (Hopper and Correa 2000). As in many many vein vein deposi deposits, ts, the coppe copperr is concen concentra trated ted in well-defined ore shoots separated by barren or low-grade vein segments. Copper contents, without any influence by supergene processes, tend to diminish in some vein systems at depths of several hundred metres, in response to increasing pyrite (La Africana) or pyrrhotite (Carrizal Alto) contents. Gold contents are higher, but typically undetermined, in the hematite-rich than in the magnetite tite-r -ric ich h depo deposi sits ts (Rui (Ruizz et al. al. 1965 1965). ). A few few of the the hematitehematite-rich rich veins veins were worked worked as small, small, stand-alo stand-alone ne gold gold depo deposi sits ts,, incl includ udin ing g Los Los Mant Mantos os de Puni Punita taqu quii (Table 2) where the economically dominant metals are uniquely uniquely zoned from copper through through gold to mercury mercury over a distance of 4.5 km (McAllister et al. 1950; Ruiz et al. 1965). Both IOCG vein subtypes are characterised by highly anomalous amounts of Co, Ni, As, Mo and U (Table 2), as show shown n by the the wide widesp spre read ad occu occurr rren ence ce of mino minorr amounts of cobaltite, safflorite, danaite (all with Co and As), As), nicc niccol olit ite, e, chlo chloan anth thit itee (bot (both h with with Ni and and As), As), molybdenite and uraninite (Ruiz et al. 1965). The Carrizal Alto veins contain as much as 0.5% Co in places (Ruiz (Ruiz et al. 1965). Arsenic, Arsenic, as arsenopyrite arsenopyrite,, may also occur occur common commonly, ly, especi especiall ally y at Tocopi Tocopilla lla,, and is also also reported reported at Candelari Candelaria-Pu a-Punta nta del Cobre Cobre (Hopf (Hopf 1990). 1990). Cobalt and Mo contents are also anomalously high at Rau Ra u´ l-Co l-Cond ndes esta tabl blee (Atk (Atkin in et al. al. 1985 1985), ), Cand Candel elar aria ia (Marschik and Fontbote ´ 2001b) and El Espino (Correa 2003). Ilmenite is recorded as an ancillary hydrothermal mineral in several deposits, especially in southern Peru (Injoq (Injoque ue 200 2002), 2), althou although gh the magne magnetit titee from from IOCG IOCG deposits is typically low in titanium (Hitzman et al. 1992; G.E. Ray, personal communication, 2003). Minor, typical ically ly late late-s -sta tage ge Zn and, and, in some some exam exampl ples es,, Pb are are present in several of the vein deposits (e.g. Espinoza et al. 1996) 1996) as well as at Rau´ l-Condesta l-Condestable ble (Vidal et al. 1990; de Haller et al. 2002), whereas the anomalously high zinc conte contents nts in parts parts of the Candel Candelari aria-P a-Punt unta a del Cobre Cobre deposit appear to accompany the final stage of copper introduc introduction tion (N. Pop, personal personal communic communication ation,, 1999; 1999; Marschik and Fontbote ´ 2001b). Several hundred parts per million of LREE are reported in parts of the Candelaria-Punta del Cobre deposit and Productora prospect, pect, at least least partly partly in allani allanite te (Marsc (Marschik hik et al. 200 2000; 0; C. Osterman Osterman in Ray and Dick 2002), as well as at Rau´ lCondesta Condestable ble (A. de Haller, Haller, personal personal communic communication ation,, 2003). Alteration related to the large composite deposits is typi typica call lly y comp comple lex x and and rath rather er vari varied ed in char charac acte terr (Table 2). Widespread, early sodic or sodic-calcic alteration characterised by albite with or without actinolite occurs in some of the IOCG districts (e.g. CandelariaPunta del Cobre; Marschik and Fontbote´ 1996, 2001b), but is apparentl apparently y absent absent elsewhere elsewhere (e.g. (e.g. Mantover Mantoverde; de; Vila et al. 1996; Cornejo et al. 2000). Pervasive biotitequartz-ma quartz-magnet gnetite±K ite±K-feld -feldspar spar alteratio alteration n immediate immediately ly preceded preceded copper copper introduc introduction tion at Candelari Candelaria-Pu a-Punta nta del

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Cobre, an event associated even more closely with formation mation of actinoli actinolite te (Ullrich (Ullrich and Clark Clark 1999; 1999; Are´ valo et al. 200 2000; 0; Marsc Marschik hik and Fontb Fontbote ote 2001b 2001b). ). Signifi Signifi-cantly, cantly, the same minerals minerals also comprise comprise narrow alteration haloes to the IOCG veins within the contiguous Ojanco Oja ncoss pluton plutonic ic comple complex x (Dı (Dı´az et al. 199 1998). 8). Albite Albite,, chlori chlorite te and calci calcite te becom becomee predom predomina inant nt in the shalshallowest parts of the Punta del Cobre deposit (Marschik and Fontbote 2001b) as they are in the Teresa de Colmo breccia breccia pipe (Correa (Correa 2000; 2000; Hopper Hopper and Correa 2000). 2000). High-grade mineralization at Mina Justa is intergrown with actinolite, clinopyroxene and apatite, and is closely associated with K-feldspar-chlorite-actinolite alteration (Moody (Moody et al. 2003). 2003). In the Mantover Mantoverde de vein-brec vein-breccia, cia, however, sericite besides K-feldspar and chlorite is closely associated with copper mineralization, and biotite is scarce (Vila et al. 1996; Cornejo et al. 2000). In contrast, at Rau´ l-Condesta l-Condestable, ble, potassic potassic alteratio alteration n is not evident evident and early albite, scapolite and a variety of calcic amphiboles are followed by iron oxides, chlorite and sericite (Vidal et al. 1990; de Haller et al. 2002). Potassic alteration is also unreported at El Espino where early albite albite is overpr overprint inted ed by epidot epidote, e, chlori chlorite te and lesser lesser amounts amounts of actinolit actinolitee and sericite (Correa 2003). Prograde garnet dominates the skarn-type IOCG deposits (Fre ´ ´ raut and Cuadra 1994) and, at Panulcillo (Table 2), is observed to be paragenetically equivalent to K-feldspar-albite-quartz and biotite-magnetite assemblages in contiguous andesitic volcanic rocks (Hopper and Correa 2000).

Metallogenic model

basaltic to andesitic sequences were tilted above zones of  exten extensio sional nal detach detachmen ment, t, and subjec subjected ted to prehni prehnitetepumpellyite and greenschist facies diastathermal (burial) metamorphism in response to elevated geothermal gradients prior to and possibly also during IOCG ore formati mation on.. The The IOCG IOCG depo deposi sits ts,, alon along g with with mass massiv ivee magnetite, manto-type copper and small porphyry copper deposits, provide a distinctive metallogenic signature to the Jurassic and Early Cretaceous Coastal Cordillera (cf. Oyarzu´ n 1988; 1988; Maksaev Maksaev and Zentilli Zentilli 2002). 2002). Once compr compress ession ion,, crusta crustall thicke thickenin ning g and more more evolve evolved d magma magmass became became widesp widesprea read d in respon response se to the early early Late Cretaceous Cretaceous tectonic inversion, inversion, IOCG (as well as massive magnetite and manto-type copper) deposit formation diminished dramatically in the Late Cretaceous and only very locally persisted into the Palaeocene. In contra contrast st to many many IOCG IOCG provin provinces ces worldw worldwide ide,, especially those of Precambrian age, the relationship of  the the Andean Andean IOCG deposi deposits ts to intrus intrusive ive rocks rocks is subsubstantially clearer. In particular, a number of the deposits are are hosted hosted by or occur occur near near gabbr gabbrodi odiori orite te or diorit dioritee intrusio intrusions. ns. Even where somewhat somewhat more felsic felsic plutonic plutonic phase phasess act as host host rocks, rocks, broadl broadly y contem contempor porane aneous ous diorite dykes commonly share controlling faults with the IOCG veins, veins, implying implying that relatively relatively primitiv primitivee magma magma sourc sources es existe existed d at depth depth just just before before and potent potential ially ly during copper mineralization. Therefore there is strong suggestion of an intimate connection between relatively primitiv primitive, e, poorly poorly fractiona fractionated ted and little-co little-contam ntaminate inated d gabbrodiorite to diorite magmas and the IOCG deposits (Table 2; Fig. 11). In this regard, it should be remarked that that Marsc Marschik hik and Fontbo Fontbote te ´ ´ (1996; (1996; but not 2001b 2001b)) linked the Candelaria-Punta del Cobre IOCG deposit to nearby diorite of the Ojancos plutonic complex, which,

Geologic Geological al synthesis synthesis of the Coastal Coastal Cordiller Cordillera a IOCG provin province ce in northe northern rn Chile Chile and southe southern rn Peru Peru at regional, district and deposit scales enables construction of  a preliminary metallogenic model.

Regional- and district-scale aspects Most of the IOCG deposits were generated during the early development of the ensialic Andean orogen, when the crust crust was var variab iably ly extend extended ed and and attenu attenuate ated d and unusually hot, and magmatism was relatively primitive. IOCG formation took place during both extensional and transtensional tectonic regimes. The greatest number of  IOCG IOCG deposi deposits, ts, includ including ing some some of the larges largest, t, were were genera generated ted durin during g the Early Early Cretac Cretaceou eouss when when crusta crustall attenuation attained a maximum. The deposi deposits ts are contro controlle lled d princi principal pally ly by brittl brittlee faults, although ductile deformation locally overlapped with the early stages of mineralization. The voluminous tholeiitic to calc-alkaline intrusions that either host or occu occurr in prox proxim imit ity y to the the IOCG IOCG depo deposi sits ts poss posses esss a domina dominantl ntly y mantle mantle source source,, lack lack apprec appreciab iable le crusta crustall contamination and are oxidised, in common with their thick thick volcanic volcanic-dom -dominat inated ed host-roc host-rock k sequences sequences.. These

Fig. 11 Cartoon of Jurassic IOCG vein in the La Negra arc of  northern northern Chile to show possible possible sources sources of ore fluid. Vertical Vertically ly ascendant ascendant magmatic magmatic fluid supplied supplied from an unobserve unobserved d diorite diorite magma source at depth is preferred. See text for further discussion. Note Triassic Triassic evaporite evaporitess are very restricte restricted d in both volume and extent, extent, whereas those in both the overlyin overlying g intra-ar intra-arcc basin and back-arc basin are too young to have contributed to the ore fluid, being deposited after IOCG formation

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as noted noted above, above, itself itself hosts hosts IOCG IOCG veins veins and skarns. skarns. Similarly, the IOCG deposits in the Can ˜ ete basin have been been relate related d geneti genetical cally ly to gabbro gabbrodio diorit ritee intrus intrusion ionss (Vidal et al. 1990; Injoque 2001, 2002). Furthermore, it should should also be recalled recalled that Me´ nard (1995) conclude concluded d that the massive magnetite deposits within the Atacama Fault Fault System System are also also geneti genetical cally ly relate related d to diorit dioritee intrusions. Notwithstanding the compelling evidence in favour of a gabbrodiorite to diorite intrusive source for the IOCG ore fluids, fluids, it should should be re-emp re-emphas hasise ised d that that very minor volumes of dacitic magma, in addition to the more more mafic mafic melt, melt, were were also also clearl clearly y ava availa ilable ble during during copper mineralization in the case of at least three of the deposits (Candelaria-Punta del Cobre, Rau´ l-Condestable, Mina Justa), although not necessarily sourced from the same part of the parental magma chamber as the metalliferous fluid. This inferred genetic association with relatively mafic plutonis plutonism m would nicely nicely explain explain the Cu-Au-Co-N Cu-Au-Co-Ni-Asi-AsMo-U signature, bearing in mind that a similar metal suite, suite, albeit albeit subecono subeconomic mic with respect to copper, copper, characterises some calcic iron skarns associated with dioritic intrusions (Einaudi et al. 1981; Meinert 1992; Ray and Lefebure 2000). The Larap magnetite deposit in southeastern Luzon island, the Philippines, part of a Neogene island isla nd arc, provides provides an instructi instructive ve example. example. The magnetite skarn, developed from both carbonate and noncarbonat carbonatee lithologie lithologies, s, is part of a low-grade low-grade porphyry porphyry copper copper system system related related to diorite diorite porphyry porphyry intrusion intrusions, s, and is enriched in Co, Ni and U, besides Cu, Mo and Au (Sillitoe (Sillitoe and Gappe Gappe 1984). 1984). Wang and Williams Williams (2001) reported reported a similar similar Cu-Au-NiCu-Au-Ni-Co-Te Co-Te-Se -Se suite suite from the Mount Mount Elliot Elliottt skarn skarn deposi depositt in the Cloncu Cloncurry rry IOCG IOCG district of Queensland, Australia. Magmatic-hydrothermal provision of copper and, to a lesser degree, gold in the Coastal Cordillera province is unequivocally confirmed locally by the existence of the Mesozoic Mesozoic porphyry porphyry copper-( copper-(gold) gold) deposits deposits in associaassociation with volumetrically restricted albeit somewhat more felsic felsic porphyry porphyry stocks. Indeed, the extensio extensional, nal, as opposed posed to compress compressive, ive, stress stress regime regime prevalen prevalentt during during porphyry copper formation in the Coastal Cordillera is believed to be a major factor responsible for the small sizes sizes and low hypog hypogene ene gra grades des of these these deposi deposits ts (cf. (cf. Sillitoe 1998; Tosdal and Richards 2001). Therefore, the widely widely exposed, exposed, deeper plutonic plutonic complexes, complexes, from the tops of which porphyry copper stocks may have already been eroded, may also reasonably be expected to have had the capacity to generate broadly similar magmatichydrot hydrother herma mall fluids fluids for IOCG IOCG genesi genesiss (cf. (cf. Oya Oyarzu rzu´ n 1988). Available sulphur isotopic results for several of  the IOCG deposits fall in a fairly narrow range centred around 0 per mil (Fig. 12; Fox 2000; I. Ledlie in Hopper and Correa Correa 2000; 2000; Marschik Marschik and Fontbote Fontbote´ 2001b), 2001b), entirely consistent with a largely magmatic source for the sulphide sulphur; however, an origin by leaching from Mesozoic Mesozoic igneous rocks cannot cannot be entirely entirely ruled ruled out. Sulphu Sulphurr isotop isotopic ic val values ues consis consisten tentt with with a magma magmatic tic origin (Fig. 12) even characterise the Teresa de Colmo

Fig. 12 Sulphu Sulphurr isotop isotopee values values for sulphi sulphide de minera minerals, ls, mainl mainly y chal chalco copy pyri rite te and and pyri pyrite te,, from from sele select cted ed IOCG IOCG depo deposi sits ts and and prospects in northern Chile and southern Peru. Data taken from  ´ quez (1998), Fox (2000), I. Ledlie in Hopper and Vivallo and Henrı ´quez Correa (2000), Injoque (2001), Marschik and Fontbote ´ (2001b) and de Haller et al. (2002). The markedly high and low values at Rau´ lCondestable are attributed to reduction of evaporitic or seawater sulpha sulphate te (Riple (Ripley y and Ohmoto Ohmoto 197 1977; 7; de Haller Haller et al. 2002) 2002) and biogenic biogenic sulphur (de Haller Haller et al. 2002), 2002), respectiv respectively, ely, whereas values values for the other other deposits deposits suggest a dominanc dominancee of magmati magmaticc sulphur

deposit, which cuts an evaporite sequence (Correa 2000; Hopper Hopper and Correa Correa 2000). 2000). Rau´ l-Condesta l-Condestable ble presents presents an apparent exception, however, with both highly positive and highly negative d34S values being interpreted in terms of the involvement of seawater or evaporitic sulphur and biogenic sulphur, respectively (Fig. 12; Ripley and Ohmoto 1977; de Haller et al. 2002). In porphyry porphyry copper copper deposits, deposits, metal-bea metal-bearing ring,, magmatic-hyd matic-hydroth rotherma ermall fluid is channelle channelled d upwards upwards from parent magma chambers via steep, typically cylindrical porphyry stocks, within and around which much of the coppe copperr and gold gold are eventu eventuall ally y concen concentra trated ted in response sponse to declining declining fluid temperat temperature. ure. Alteratio Alteration n and mineraliz mineralizatio ation n are, therefor therefore, e, relativel relatively y confined, confined, al2 though zones of >5 km may be affected by potassic altera alteratio tion n in gia giant nt sys system temss (e.g. (e.g. El Tenien Teniente, te, Chile; Chile; Skewe Skewess et al. al. 2002 2002). ). In the the case case of larg largee comp compos osit itee IOCG IOCG deposi deposits, ts, such such confin confineme ement nt of altera alteratio tion n and mineralization is not so apparent, especially where sodic-calcic alteration either presages or accompanies the copper copper mineraliz mineralizatio ation. n. Alteration Alteration in the Candelari CandelariaaPunt Punta a del del Cobr Cobree dist distri rict ct,, for for exam exampl ple, e, occu occupi pies es >30 km2 (Marschi (Marschik k and Fontbote Fontbote 2001b). 2001b). Neverthe Neverthe-less, the alteration associated with simple IOCG veins, breccia breccia pipes and skarns is generally generally just as volumetvolumetricall rically y restri restricte cted d as that that with with non-IO non-IOCG CG depos deposits its of  these types. The tendency for alteration and mineralization to be unusually widespread in many IOCG districts, especially in association with large composite IOCG deposits, may be ascribed to the existence of magmatic-hydrothermal fluid sources at considerable depth within either the host or contiguous plutonic complexes. Fluid ascent on approach to ore-forming levels appears to be guided by second- and lower-order splays of the major localising

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fault fault zones, zones, intrus intrusive ive contac contacts ts and permea permeable ble strati strati-graphic graphic horizons horizons,, and therefore therefore may not be as tightly tightly focused as in most porphyry copper deposits. The precise cise locati locations ons of the source source intrus intrusion ionss remain remain to be clarified, although these could potentially be at considerable palaeodepths, perhaps as great as 10 km, given inferred depths of pluton emplacement and the association of the IOCG deposits with crustal-scale ductile to brittle fault zones (e.g. Grocott and Wilson 1997). In the case of the vertically extensive veins and other deposit styles hosted by gabbrodiorite and diorite, it may reasonably be presumed that the mineralizing fluids were exsolved exsolved during final consolida consolidation tion of the deep, compositiona positionally lly similar similar portions portions of the plutons plutons (Fig. (Fig. 11 11). ). However, where diorite dykes and the IOCG veins share controlling faults cutting more felsic plutons, derivation of both the dyke magma and metalliferous fluid from deeper, deeper, more mafic and less-frac less-fractiona tionated ted parts of the plutonic complexes may be inferred. Replenishment of  magma chambers by more primitive mantle melts could result result in underplat underplating ing of plutonic plutonic complexes complexes by more mafic material as well as acting as a potential trigger for liberatio liberation n of sulphursulphur- and metal-cha metal-charged rged fluid, in the manner proposed recently by Hattori and Keith (2001). Hypothetically, IOCG ore fluids might be supplied by mafic intrusive phases anywhere within host or nearby plutonic complexes, which range from several to perhaps 10 km in vertical extent (e.g. Grocott and Taylor 2002), assumi ass uming ng that that they they were were efficie efficientl ntly y tapped tapped by steep, steep, through-going faults. Such a deep origin for IOCG fluids in the central Andes accords with a postulated deeper source of magmatic fluids in IOCG than in porphyry copper copper deposi deposits, ts, inferr inferred ed from from higher higher CO 2 contents (Polla (Pollard rd 2001) 2001),, in keepin keeping g with with those those docume documente nted d in fluid fluid inclus inclusion ionss from from Candel Candelari aria a (Ullri (Ullrich ch and Clark Clark 1999). The elevated geothermal gradients that existed in the extens extension ional al Mesozo Mesozoic ic arc terran terranes es of the Coasta Coastall Cordiller Cordillera a would would have favoured favoured prolonged prolonged ascent and even lateral flow of the deeply derived magmatic fluid before before cooling cooling was sufficient to cause cause wholesale wholesale metal precipitation.

Consideration of alternative fluid sources A non-magmatic origin for IOCG ore fluids and their contai contained ned metals metals in the Andean Andean provin province ce gains gains little little support from the overall geological settings of many of  the deposits. deposits. Hitzman Hitzman (2000) and Kirkham Kirkham (2001) (2001) favoured voured chloridechloride-rich rich basinal basinal brine brine produced produced by evapevaporite orite disso dissolut lution ion as perha perhaps ps the most most likely likely fluid fluid for copper copper and gold transpor transportt and IOCG deposit deposit formation tion in the the Coas Coasta tall Cord Cordil ille lera ra of nort northe hern rn Chil Chile. e. Evaporites, albeit predominantly sulphates rather than halite, halite, are preserved preserved in the Tarapaca Tarapaca´ and Aconcagua Aconcagua back-arc basins, as noted above (Fig. 11; Mun ˜ oz et al. 1988; 198 8; Mpodo Mpodozis zis and Ramos Ramos 199 1990; 0; Ardill Ardill et al. 1998) and and occu occurr in mino minorr amou amount ntss at dept depth h in the the Can Can ˜ ete intr intraa-ar arcc basi basin n (Pal (Palac acio ioss et al. al. 1992 1992). ). Alth Althou ough gh a

genetic model involving such external fluids is perhaps not unreasonable for IOCG deposits within the Can ˜ ete basin basin (e.g. (e.g. Rau´ l-Cond l-Condest establ able) e) and along along the easter eastern n edge of the Coastal Cordillera (e.g. Teresa de Colmo) and, hence, close to the back-arc environment, it seems a far far less less like likely ly poss possib ibil ilit ity y for for the the majo majori rity ty of the the depos deposits its occurr occurring ing within within the volcan volcanic ic arc itself itself.. It would seem to be especially convoluted to invoke basina sinall brin brinee acce access ss as a mean meanss of form formin ing g the the IOCG IOCG veins veins,, many many of which which are sealed sealed within within sizeab sizeable le pluplutonic tonic complexes complexes and possess possess original depth extents of  >1 km (Fig (Fig.. 11) 11).. Most Most of thes thesee vein veinss were were form formed ed immediat immediately ely following following emplaceme emplacement nt of their their host plutons tons (Fig (Fig.. 9), 9), clea clearl rly y whil whilee they they were were stil stilll hot hot and and ther theref efor oree even even less less like likely ly to perm permit it the the ingr ingres esss of  external brine. Provision of brine from a back-arc sedimentary basin by means of gravity-induced flow is precluded by palaeo-topog aeo-topograph raphic ic considera consideration tions, s, given given that an at least least partially subaerial arc must be higher in elevation than a marin marinee back-a back-arc rc basin. basin. The extens extension ional al settin setting g also also preclude precludess tectonic tectonically ally induced induced brine brine expulsion expulsion at the times when most of the IOCG deposits were generated. The only only other other altern alternati ative, ve, crusta crustal-s l-scal calee convec convectio tion n (Fig. 11; Barton and Johnson 1996, 2000), also seems implausible, especially in the case of the Middle to Late Jurassic La Negra arc, because fluid circulation across an 50-km width of a pluton-dominated arc would need to be invoked. Furthermore, some of the oldest (latest Middl Middlee Jurass Jurassic) ic) IOCG IOCG veins veins in northe northern rn Chile Chile were were generated at least 7 M.Y. before evaporite formation at < 155 Ma in the adjoin adjoining ing back-a back-arc rc basin basin (Fig. (Fig. 2; Ardill et al. 1998). Finally, it is important to note that the Mesozoic IOCG belts of the central Andes span 19 ° of latitu latitude, de, within within and alongs alongside ide only only parts parts of which which evaporites are documented. Even less likely is an evaporite or formational brine source source within within or beneath beneath the Mesozoic Mesozoic plutonic plutonic complexes of the IOCG-bearing La Negra arc in northern Chile. Chile. Triassic Triassic sediment sedimentary ary sequence sequencess locally locally beneath beneath the arc and the thin Jurassic sedimentary intercalations within within it are preser preserved ved only only discon discontin tinuou uously sly and are volum volumetr etrica ically lly minor minor (Fig. (Fig. 11). 11). Moreo Moreover ver,, the only only known potential brine sources would appear to be the extremely limited sabkha facies described locally as part of Triassic rift sequences, but such material would be restricted to the region between approximately latitudes 24 and 27 °S (Sua ´ ´ rez and Bell 1992, 1994). 1994). Palaeozoi Palaeozoicc meta-sedimentary sequences and older crystalline basement ment are the most most common common substrat substratee to the pluton plutonic ic compl complexe exess and clear clearly ly could could not have have acted acted as brine brine sources during the Mesozoic (Fig. 11). Indeed, the host pluton plutonss for the Las Animas Animas,, Carriz Carrizal al Alto Alto and Quebradita vein deposits directly intrude the metasedimentary rocks (Ruiz et al. 1965). Descent of brine from overlying sources, proposed in some shield areas (Gleeson et al. 2000) and elsewhere (Haynes 2000), might be invoked as a means of generating the IOCG deposits within the arc, but widespread $

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descent of fluid for at least 1 km down pluton-hosted faults to generate veins at palaeodepths as great as 5 km or more seems highly improbable (Fig. 11). Indeed, the widespread widespread pseudomo pseudomorphin rphing g of specular specular hematite hematite by magnetite magnetite,, suggestiv suggestivee of thermally thermally progradin prograding g hydrohydrothermal systems, not to mention formational temperatures of >500 °C for the magnetite (e.g. Marschik and Fontbote ´ ´ 2001b), would seem to be more easily explicable in terms of ascent rather than descent of the ore fluid, fluid, in keepin keeping g with with more more conven conventio tional nal concep concepts ts of  vein vein form format atio ion. n. Furt Furthe herm rmor ore, e, ther theree are are no know known n Mesoz Mesozoic oic sedime sedimenta ntary ry accumu accumulat lation ionss that that are either either capa capabl blee of copi copiou ouss brin brinee gene genera rati tion on or suffic sufficie ient ntly ly wide widesp spre read ad to have have over overla lain in the the nume numero rous us IOCG IOCG deposits in the Mesozoic arc of northern Chile, bearing in mind that significant copper mineralization was active during both Middle–Late Jurassic and Early Cretaceous epochs. Moreover, the well-known evaporite occurrence in the restri restricte cted d Coloso Coloso basin, basin, at latitu latitude de 23 °50¢S, is appreciably younger than nearby IOCG deposits formed in the Middle Middle–La –Late te Jurass Jurassic ic epoch epoch (Flint (Flint and Turne Turnerr 1988). A perhaps more reasonable non-magmatic ore fluid would would be metamo metamorph rphic ic brine brine of the type type believ believed ed by some some invest investiga igator torss to have have been been respon responsib sible le for the manto-type copper deposits (see above). Generation of  metamorphic fluid accompanied subsidence of the intraarc basins basins transg transgres ressed sed by the centra centrall Andean Andean IOCG IOCG belts (Aguirre et al. 1999), and such a fluid has been considered as a possible, even the sole, contributor to the IOCG IOCG deposi deposits ts in the Can ˜ ete basin basin of southe southern rn Peru Peru (Vidal et al. 1990; Injoque 2000), and perhaps also to thos thosee in the the Coas Coasta tall Cord Cordil ille lera ra of nort northe hern rn Chil Chilee (Hitzman 2000). However, ingress of metamorphic fluid to large large coolin cooling g pluton plutonss to genera generate te the IOCG IOCG veins veins confronts some of the same difficulties as those considered above for other externally derived brines. Heated seawater is another fluid that may have been available avai lable during pluton pluton emplaceme emplacement nt and IOCG formation in the intra-arc basin environment, and has been proposed proposed at Rau´ l-Condesta l-Condestable ble in the northern northern part of  the Can ˜ ete basin on the basis of the sulphur isotopic values (Ripley and Ohmoto 1977; de Haller et al. 2002). Certainly, seawater may be inferred to have played a key role in VHMS formation only slightly later in the same part of the Can ˜ ete basin (Vidal 1987). On the basis of this discussion of possible external fluid fluid source sources, s, it is conclu concluded ded that that evapo evaporit ritic, ic, metametamorphogen morphogenic ic and seawater brines all seem unlikely unlikely to have have been been sole solely ly resp respon onsi sibl blee for for the the gene genesi siss of the the IOCG IOCG deposi deposits ts in the Coasta Coastall Cordil Cordiller lera, a, althou although gh their their local local involveme involvement, nt, along with that of locally locally derived rived meteor meteoric ic water, water, remain remainss feasib feasible. le. Given Given that that a single fluid type rather than different or blended fluids woul would d seem seem to be requ requir ired ed to expl explai ain n the the comm common on charac character terist istics ics of the IOCG IOCG deposi deposits ts throu througho ghout ut the 1,700-km 1,70 0-km-lon -long g Coastal Coastal Cordillera Cordillera belt, belt, the magmaticmagmatichydrothermal model discussed above is believed to gain further support.

Deposit-scale aspects Pluton-hosted IOCG deposits in the Coastal Cordillera, chiefly veins, tend to be localised by minor faults and fractu fractures res and and to be relati relativel vely y small small in size, size, albeit albeit of  appreciab appreciable le horizont horizontal al and vertical vertical extents extents (Fig. 13). Nevertheless, the large vein districts, like Tocopilla, may be areally extensive (Fig. 6). In contrast, IOCG deposits in volcanic and sedimentary host rocks to plutons are

Fig. 13 Schem Schemati atised sed styles styles of IOCG IOCG deposi deposits ts in the Coasta Coastall Cordille Cordillera ra of the central central Andes. Andes. Note the fundamen fundamental tal control imposed by faults, commonly shared with pre-ore mafic (basaltic andesite/diorite) dykes. Large deposits are composite, in the sense of comprisin comprising g several several closely closely spaced spaced minerali mineralizati zation on styles, styles, and localised by zones of high structural and lithological permeability, possib possibly ly confine confined d benea beneath th carbo carbonat natee or other other litho litholog logica ically lly determin determined ed aquitards aquitards.. Vein breccias breccias (and breccia breccia mantos mantos and pipes) tend to occur at relatively shallow palaeodepths and, hence, are typica typically lly confine confined d to volca volcanog nogeni enicc wallro wallrocks cks.. There There is an upward change in the predominant hydrothermal iron oxide from magn magnet etit itee to spec specul ular ar hema hemati tite te.. The The IOCG IOCG syst system em may may be concealed beneath an extensive zone of barren feldspar-destructive alterati alteration on containi containing ng pyrite. pyrite. A deeply deeply derived derived magmati magmaticc fluid guided upwards along the dyke-filled faults, and possibly sourced from from the same magma magmatic tic reservoi reservoirr as the dyke rock rock itself itself,, is hypoth hypothesi esised sed.. Note Note that that telesc telescopi oping ng of alter alterati ation on types types (e.g. (e.g. potassic potassic over sodic-cal sodic-calcic) cic) and minerali mineralisati sation on styles, styles, especial especially ly within within composite composite deposits, deposits, is a possibili possibility ty locally; locally; however, however, the phenomenon is considered to be far less widely developed in the low-relief extensional arcs of the Coastal Cordillera than it is in the highly highly uplifted uplifted compressi compressive ve arcs that host the Tertiary Tertiary porphyry copper deposits farther east

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variable in size, but include all the largest deposits. The larges largestt hosthost-roc rock k deposi deposits ts appear appear to be those those where where fault-gu fault-guided ided magmatic magmatic-hyd -hydrothe rothermal rmal fluid permeates permeates one or more more porou porouss strati stratigra graphi phicc horizo horizons ns (Fig. (Fig. 13), 13), which in the case of Candelaria span a 350-m-thick rock packag packagee (Ryan (Ryan et al. 199 1995; 5; Marsc Marschik hik and and Fontbo Fontbote te´ 2001 2001b) b).. LowLow-an angl glee faul faultt or shea shearr zone zoness may may also also enhance syn-mineralization permeability. Fluid ponding bene beneat ath h aqui aquita tard rds, s, such such as marb marble leis ised ed carb carbon onat atee sequ sequen ence ces, s, may may also also favo favour ur the the form format atio ion n of larg largee composite deposits (Fig. 13). Known copper-gold skarn deposits in the Coastal Cordillera are small, but clearly an integral component of the IOCG spectrum, thereby rendering redundant any discussion of the generic difference between these skarns and other IOCG deposits in the belt. This assertion is amply supported by observations at Panulcillo, where Hopper and Correa (2000) charte charted d the equiva equivalen lence ce of gar garnet net skarn skarn and potas potassic sic assemblages developed in contiguous andesitic volcanic rocks. Marschik and Fontbote´ Fontbote´ (1996) considered the Punta del Cobre IOCG deposit to be intermediate in overall style style between between massive massive magnetite magnetite and porphyry porphyry copper depo deposi sits ts,, both both of whic which h occu occurr fair fairly ly clos closee by in the the Coastal Coastal Cordiller Cordillera a (Fig. 1). However, However, IOCG and porphyry phyry copper copper deposits, deposits, as discussed discussed above, are clearly clearly distinct and apparently not directly related; nevertheless, they may display certain features in common, including occurrence of hydrothermal magnetite and/or hematite and potassic, potassic, potassicpotassic-calci calcicc and/or and/or sodic-ca sodic-calcic lcic alteralteration (cf. Pollard 2000; Lang and Thompson 2001). Many gold-rich porphyry copper deposits worldwide contain abundant hydrothermal magnetite±hematite as a component of both early, barren sodic-calcic and later, ore-related potassic-(calcic) alteration assemblages (e.g. Sillit Sillitoe oe 200 2000). 0). Magne Magnetit titee conten contents ts at Grasbe Grasberg, rg, for exam exampl ple, e, atta attain in 15 vol% vol% in part partss of the the pota potass ssic ic altera alteratio tion n zone zone (MacD (MacDona onald ld and Arnold Arnold 1994) 1994),, not volu volume metr tric ical ally ly diss dissim imila ilarr to some some IOCG IOCG depo deposi sits ts.. Furthermore, in a few porphyry copper deposits, sodiccalcic alteration, defined by sodic plagioclase, clinopyroxene, amphibole and magnetite, rather than the more normal potassic assemblages directly hosts all or part of  the copper-gold mineralization (Sillitoe 2000), especially in the case of deposits in the Intermontane belt of British Columbia, Canada (Lang et al. 1995). Moreover, two of  the deposits in the Intermontane belt (Afton and Ajax) constitute late stages of the Iron Mask batholith, which happens to contain magnetite-apatite veins like those in the Coasta Coastall Cordil Cordiller lera a belt belt (Cann (Cann and Godwi Godwin n 198 1983; 3; Snyder and Russell 1995). Interestingly, copper mineralization in several central Andean Andean IOCG IOCG deposi deposits ts (e.g. (e.g. Rau´ l-Cond l-Condest establ able, e, El Espino) Espino) is exclusive exclusively ly present present in zones zones of sodic-ca sodic-calcic lcic alteration, although potassic alteration or combinations of this this with with sodicsodic-cal calcic cic altera alteratio tion n phases phases are more more typical hosts. This situation may be a product of vertical alte altera rati tion on zoni zoning ng,, not not just just radi radica call lly y diffe differe rent nt fluid fluid chemistries, as it is in the case of at least some porphyry

copper-(gold) deposits (e.g. Sillitoe 2000). Moreover, if  this comparison comparison between between the upward upward transitio transition n from sodic-ca sodic-calcic lcic to potassic potassic alteration alteration in some IOCG and porph porphyry yry copper copper deposi deposits ts is val valid, id, then then simila similarr fluid fluid evolutions, perhaps controlled by declining temperature, migh mightt be invo invoke ked d in both both case cases. s. Zoni Zoning ng in IOCG IOCG deposits of the Coastal Cordillera is still poorly documented, mented, although although observat observations ions from several several vein distric tricts ts and and Cand Candel elar aria ia-P -Pun unta ta del del Cobr Cobree show show that that magnetit magnetite-ac e-actinol tinoliteite-apat apatite ite is transitio transitional nal upwards upwards to hematite-chlorite-sericite at both the individual vein and district scales (Figs. 10 and 13). Sizeable hydrothermal brecc breccia ia veins, veins, pipes pipes and manto mantoss appear appear to be largel largely y restricted to this shallower, hematite-dominated IOCG zone (Fig. 13), where fluid overpressures overpressures may develop develop more readily. The close association of magnetite-dominated IOCG and massive magnetite-(apatite) veins containing minor copper in several districts, especially but not limited to those of Middle–Upper Jurassic age, may be taken to suggest that the two deposit types are transitional and, furthermore, that copper contents of IOCG veins may decrease decrease downwards downwards,, giving giving rise to massive massive magnetite magnetite veins (Fig. 10; cf. Espinoza et al. 1999; Naslund et al. 2002; Ray and Dick 2002). The same relationship is also favou favoured red by the tenden tendency cy of IOCG IOCG minera mineraliz lizati ation on to occur alongside some massive magnetite deposits, perhaps suggestive of a crude zonal relationship (e.g. Mina Justa Justa,, Manto Mantover verde) de).. The deeper deeper massiv massivee magnet magnetite ite bodies, with or without copper, lack hydrothermal biotite and K-feldspar and are accompanied by sodic-calcic alteratio alteration, n, in keeping keeping with the conclusio conclusions ns of several several previous previous workers (Hitzman (Hitzman et al. 1992; 1992; Pollard Pollard 2000; 2000; Ray and Dick Dick 2002) 2002).. The distri districtct- and and deposi deposit-s t-sca cale le geologic geological al evidence, evidence, especiall especially y the intimate intimate associat association ion between IOCG and massive magnetite deposits in parts of the Coasta Coastall Cordil Cordiller lera, a, does does not suppor supportt radica radically lly different fluid sources for the two deposit types, as recent cently ly prop propos osed ed on the the basi basiss of diffe differe renc nces es in thei theirr 187 Os/188Os ratios (Mathur et al. 2002). Hydrothermal magnetite in porphyry copper deposits is normally normally considered considered to result result from precipitatio precipitation n of  iron partitioned partitioned directly from the source magma magma into magmatic-hydrothermal brine (e.g. Arancibia and Clark 1996). However, at least part of the iron present as iron oxide oxidess in some some of the massive massive magne magnetit titee (Ruiz (Ruiz et al. 1968; 1968; Me ´ ´ nard nard 199 1995) 5) and IOCG (Corne (Cornejo jo et al. 2000) depos deposits its of the Coasta Coastall Cordil Cordiller lera a may have have result resulted ed from from leachi leaching ng by hot hot hypers hypersali aline ne magmat magmatic ic fluid fluid of  ferromagnesian minerals in igneous rocks adjoining the sites sites of minera mineraliz lizati ation. on. Zones Zones of maficmafic-poo poor, r, albite albite-K-feldspa K-feldspar-al r-altere tered d rocks rocks developed developed in the vicinities vicinities of  many large massive magnetite and some IOCG deposits, includin including g Candelaria Candelaria (Marschi (Marschik k and Fontbote Fontbote ´ 2001b) and and Manto Mantover verde de (Corne (Cornejo jo et al. 200 2000), 0), provid providee the supporting evidence. The upward extensions of IOCG deposits are even less well known than their roots, although there is limited observational evidence for occurrence of coarse-grained

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calcit calcitee veins veins (Fig. (Fig. 1 10), 0), even even immedi immediate ately ly above above large large composite composite deposits deposits like Candelari Candelaria-Pu a-Punta nta del Cobre. Cobre. Small copper-gold skarns located above the Candelaria deposit (Ryan et al. 1995) are probably more a reflection of the carbonat carbonatee protolith protolith than uppermos uppermostt manifest manifestaations of the entire Candelaria-Punta del Cobre district. Ray and Dick Dick (2002 (2002)) conclu concluded ded that that a 1.5 1.5-km -km-wi -wide, de, down-faulted block of massive silicified tuff containing pyrite, pyrite, sericite sericite and minor dumortierite dumortierite represent representss the shallo shallowes westt altera alteratio tion n facies facies at the Produc Productor tora a IOCG IOCG prospect. This proposal would be in keeping with the widesp widesprea read d occurr occurrenc encee of extens extensive ive zones zones of pyriti pyriticc feldsp feldsparar-des destru tructi ctive ve altera alteratio tion n affecti affecting ng volcan volcanic ic sequences locally throughout the Coastal Cordillera, some of them in proximit proximity y to IOCG districts. districts. Silicifica Silicification tion accompanied by sericitic and/or advanced argillic alteration is commonly recorded.

Exploration consequences

The precedin preceding g discussion discussion highlight highlightss several several geological geological featur features es and relati relations onship hipss of possib possible le use in IOCG IOCG explor explorati ation on in the Coasta Coastall Cordil Cordiller lera a of the centra centrall Andes and, potentially, in similar extensional environments elsewhere: 1. Middle–La Middle–Late te Jurassic Jurassic and Early Cretaceous Cretaceous plutonic plutonic belts in the Coastal Cordillera are more prospective for IOCG deposits than the younger magmatic arcs farthe fartherr east. east. The latter latter coinci coincide de with with the princi principa pall porphyry copper belts of the central Andes (Fig. 4), thereby thereby underlin underlining ing an inverse inverse correlati correlation on between between major IOCG and porphyry copper deposits. 2. Larg Largee IOCG IOCG depo deposi sits ts seem seem more more like likely ly to form form within major orogen-parallel, ductile to brittle fault system sys temss that that underw underwent ent extens extension ion or transt transtens ension ion than than in ass associ ociati ation on with with either either minor minor or compre compresssional fault structures. 3. Receptive rock packages packages cut by gabbrodiorite, gabbrodiorite, diorite or more more felsic felsic pluton plutonss contai containin ning g IOCG IOCG veins veins or bordered by skarns may be especially prospective for large composite IOCG deposits. The intrusive rocks are likely to display at least localised zones of weakly developed potassic-(calcic) and/or sodic-calcic alteration. 4. Fragm Fragment ental al volcan volcanic ic or volcan volcanicl iclast astic ic host host rocks rocks character characterised ised by high intrinsic intrinsic and/or and/or structur structurally ally imposed permeability favour the formation of large compos composite ite IOCG IOCG deposi deposits ts if suitab suitable le progen progenito itorr intrusions intrusions and deeply deeply penetrat penetrating ing feeder feeder faults faults are presen present. t. HighHigh- or low-an low-angle gle faults faults or shears shears may create the structural permeability. 5. Relatively Relatively impermeabl impermeablee rocks, rocks, such as massive marbleised bleised carbonate carbonate units, units, may be conducive conducive to fluid ponding and the consequent development of immediatel diately y subjac subjacent ent IOCG IOCG deposi deposits ts (Fig. (Fig. 13). 13). Such Such impe imperm rmea eabl blee unit unitss may may even even stil stilll conc concea eall IOCG IOCG deposi deposits ts and, and, as at Candel Candelari aria a (Ryan (Ryan et al. 199 1995), 5),

mino minorr copp copper er skar skarn n occu occurr rren ence cess may may repr repres esen entt hanginghanging-wall wall leakage leakage anomalies anomalies (Fig. 13). The possible relationship of calcic skarns to IOCG deposits should not be overlooked. 6. Broad Broad,, stron strongly gly develo developed ped contac contact-m t-meta etamo morph rphic ic (hornf (hornfels els)) and metaso metasomat matic ic (sodic (sodic-ca -calci lcicc and/or and/or potass potassic ic altera alteratio tion) n) aureol aureoles es to gabbro gabbrodio diorit ritee or diorite intrusions are a favourable indicator for large composite IOCG deposits. 7. Intense Intense and pervasive pervasive hydrothe hydrothermal rmal alteration alteration is a prerequisite for large, composite IOCG deposits, althou though gh the the copp copper er-g -gol old d mine minera rali liza zati tion on may may be accompani accompanied ed by potassic, potassic, potassicpotassic-calc calcic ic or sodicsodiccalcic assemblages. 8. Mineraliz Mineralized ed hydrothe hydrothermal rmal breccia and the predomipredominance of specular hematite over magnetite both suggest gest relati relativel vely y shallo shallow w palaeo palaeodep depths ths and, and, hence, hence, persistence of IOCG potential at depth (Fig. 13). By the same token, widespread development of magnetite and actinolite indicate fairly deep levels in IOCG system sys tems, s, with with less less likeli likelihoo hood d of encou encounte nterin ring g ecoeconomic copper-gold contents at appreciable depth. 9. Some, but by no means means all, composite IOCG IOCG deposits have irregularly and asymmetrically developed pyrite haloes that may provide useful vectors to ore. 10. Coarsely Coarsely crystalline crystalline calcite calcite or ankerite veins may be either either the tops tops or distal distal manif manifest estati ations ons of IOCG IOCG deposits. 11. Speculatively Speculatively,, extensive extensive zones zones of barren barren feldspar feldspar-destructi destructive ve alterati alteration, on, including including silicifica silicification, tion, serisericite, pyrite and even advanced argillic assemblages, within within volcanovolcano-sedim sedimenta entary ry sequences sequences may either either conceal underlying IOCG deposits or intimate their presence presence nearby. nearby. In essence, essence, such zones zones are lithocaps, caps, comparab comparable le to those those well documented documented from the porphy porphyry ry copper copper enviro environm nment ent (e.g. (e.g. Sillit Sillitoe oe 2000). 12. The distal fringes and immediate immediate surrounding surroundingss of  massive magnetite deposits may be prospective for IOCG deposits if suitable structural preparation and volcano-sedimentary host rocks are present. 13. Notwiths Notwithstandi tanding ng point point 12, districts districts dominate dominated d by massive magnetite bodies or veins may imply relatively tively deep erosion erosion levels levels unfavoura unfavourable ble for major major IOCG deposit preservation. Concluding remarks

This This revi review ew of the the cent centra rall Ande Andean an IOCG IOCG prov provin ince ce concludes that the most likely ore fluid is of magmatic parentag parentage, e, although although inadverte inadvertent nt participa participation tion of nonmagmatic fluids, of the types generated during low-grade diastathermal (burial) metamorphism, seawater circulation or evaporite dissolution, cannot be ruled out locally and, indeed, have been proposed proposed at Candelari Candelaria-Pun a-Punta ta del Cobre Cobre (Ullrich (Ullrich et al. 2001) 2001) and Rau´ l-Condesta l-Condestable ble (Ripley and Ohmoto 1977; Vidal et al. 1990; de Haller et al. 2002). Metals, with the possible exception of some

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of the iron, are also thought most likely to have been provid provided ed direct directly ly by the same same magma magmatic tic source source,, for which a primitive gabbrodiorite to diorite composition at appreciable depths beneath the deposit sites is preferred. ferred. It is salutary to recall recall that Buddington Buddington (1933) proposed this same genetic relationship between diorite intrusions and veins rich in magnetite, Cu, Au, Co and Ni. No evidence for involvement of alkaline magmas, as implie implied d for IOCG IOCG deposi deposits ts in genera generall by Groves Groves and Vielre Vielreich icher er (2001) (2001),, is presen presentt in the centra centrall Andean Andean province. Notwithsta Notwithstandin nding g this fundament fundamental al genetic genetic concluconclusion, sion, the dispar disparate ate and and poorly poorly define defined d nature nature of the IOCG deposit clan does not necessarily imply that all other other iron oxide-ric oxide-rich h copper-g copper-gold old deposits deposits worldwide worldwide are generat generated ed in the same or even even a simila similarr manne manner. r. Indeed, Indeed, a non-magm non-magmatic atic brine origin origin for some IOCG provinces, as advocated by Barton and Johnson (1996, 2000), Haynes (2000) and others, may remain a possibility. Nevertheless, although the IOCG deposit class is too all-encompassing as presently defined, the obvious simi simila lari riti ties es betw betwee een n seve severa rall larg largee IOCG IOCG depo deposi sits ts,, including Candelaria-Punta del Cobre in Chile, Sossego in the Caraja Caraja´ s district, district, Brazil and Ernest Ernest Henry Henry in the Cloncurry district of Queensland, Australia (e.g. Mark et al. 2000; Leveille and Marschik 2001), may be taken to suggest that broadly similar, probably pluton-related hydrothermal systems were operative periodically from the Archaean to the Mesozoic. Mesozoic. Indeed, Indeed, diorite diorite and/or and/or gabbro also abut both the Sossego and Ernest Henry orebodies, although current wisdom would consider it to possess no direct genetic connection with, and to predate, the copper-gold introduction. In all three cases, the plutonism and deeply penetrating fault zones proximal to ore ore are are prod produc ucts ts of regi region onal al exte extens nsio ion, n, in eith either er subductio subduction-re n-relate lated d arc or intracon intracontinen tinental tal rift settings settings (cf. Hitzman 2000). If the geneti geneticc relati relations onship hip to oxidis oxidised, ed, primit primitive ive,, diorit dioritic ic magma magmatis tism m propos proposed ed herein herein for for the Coasta Coastall Cordiller Cordillera a province province and, possibly, possibly, some deposits deposits elseelsewhere where is correc correct, t, then then at least least a select selection ion of deposi deposits ts assigned to the IOCG class would seem to constitute an exte extend nded ed clan clan in the the same same mann manner er as prop propos osed ed by Thompson et al. (1999) and Lang et al. (2000) for base metal metal-po -poor, or, litho lithophi phile le elemen elementt (Bi-W(Bi-W-Mo Mo)-e )-enri nriche ched d gold gold depos deposits its in ass associ ociati ation on with with highly highly fracti fractiona onated ted and relatively reduced felsic intrusions. In both cases, a broad broad range range of deposit deposit styles, styles, including including pluton-hosted pluton-hosted veins, veins, skarns skarns,, brecci breccias as and replac replaceme ement nt manto mantos, s, is evident. The even broader spectrum of copper and gold deposi deposits ts linked linked to alkali alkaline ne magma magmatis tism m (Jense (Jensen n and Barton 2000; Sillitoe 2002) may be cited as yet another example example of the same fundamental fundamental metallogenic metallogenic influence exerted by magma type, as detailed by Blevin and Chappell (1992) and others for common mineralization types types like porphyry porphyry copper and tin-tung tin-tungsten sten deposits. deposits. Accept Acceptanc ancee of this this propos proposal al might might provid providee an explaexplanation for the occurrence of IOCG deposits in association ation with petrochem petrochemicall ically y similar similar intrusion intrusionss in both

subductio subduction-re n-relate lated d arcs and intraplat intraplatee settings settings,, in an analogous analogous manner manner to formation formation of lithophile lithophile-elem -element ent enriched gold deposits both along the landward sides of  Cordilleran arcs and in collisional settings (Thompson et al. 1999). Acknowledgements This article is an expanded version of a keynote address address presented presented at the 11th Quadrennial Quadrennial IAGOD IAGOD Symposiu Symposium m and Geocongre Geocongress ss 200 2002 2 in Windhoek Windhoek,, Namibia. Namibia. The Organising Organising Committee, and especially its chairman, Roy Miller, are thanked for the invitati invitation on to attend attend and, indire indirectl ctly, y, for the impetus impetus to prepare this review. Thanks are due to the many companies and geologists with whom I have had the pleasure of working on IOCG deposits and prospects in Chile, Peru and elsewhere over the last three three decades. decades. Special acknowledg acknowledgemen ementt is also due to the late Carlos Ruiz Fuller and his colleagues, and their several generations of successors, at the Chilean Geological Survey (Servicio Nacional  ´ a y Mine  ´ a, formerly de Geologı Geologı ´a Minerı rı ´a, formerly Instituto Instituto de Investiga Investigacion ciones es Geolo ´ gicas) for pioneering studies of regional geology and IOCG deposits in the Coastal Cordillera. The manuscript was improved as a result result of reviews reviews by Constanti Constantino no Mpodozis Mpodozis,, Pepe Perello Perello ´ , Gerry Ray, John Thompson and, on behalf of  Mineralium Deposita , Lluı´ Lluı´s Fontbote´ and Peter Pollard.

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