Benavides Et Al-2007

©2007 Society of Economic Geologists, Inc. Economic Geology, v. 102, pp. 415–440

The Mantoverde Iron Oxide-Copper-Gold District, III Región, Chile: The Role of Regionally Derived, Nonmagmatic Fluids in Chalcopyrite Mineralization JORGE BENAVIDES,†,* T. K. KYSER, ALAN H. CLARK, Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6

CHRISTOPHER J. OATES, Geochemistry Division, Anglo American plc, 20 Carlton House Terrace, London, United Kingdom SW1Y 5AN

RICHARD ZAMORA, RAÚL TARNOVSCHI, AND BORIS CASTILLO** Anglo American Chile Ltda, Avenida Pedro de Valdivia 291, Santiago, Chile

Abstract Located in the Cordillera de la Costa of northern Chile, the mines of the Mantoverde district exploit supergene oxide ore developed over several Lower Cretaceous, hematite-rich, iron oxide-Cu-Au (IOCG) deposits with an average protore grade of 0.52 percent Cu and 0.11 g/t Au (e.g., Mantoverde proper, Manto Ruso). The geologic setting and genesis of this productive IOCG district are clarified herein through regional petrologic and lithogeochemical study and light stable isotope analysis of paragenetically constrained samples from Mantoverde and its satellite deposits. Together with chalcopyrite-bearing, but subeconomic, bodies of metasomatic magnetite (e.g., Montecristo and Franco) and Cu-barren magnetite-fluorapatite-pyrite bodies (e.g., Ferrífera), the deposits of the Mantoverde district were emplaced along the main and, more commonly, subsidiary segments of the plate boundary-parallel Atacama fault system. They are hosted by Middle to Upper Jurassic andesites of the La Negra Formation and diorites and monzodiorites assigned to the Lower Cretaceous Sierra Dieciocho plutonic complex. Prior to mineralization, the Jurassic and Neocomian igneous rocks of this Andean transect were subjected to moderate albitization (spilitization) and hydrolytic alteration and, subsequently, to regional, nondeformational metamorphism, which locally attained the lower greenschist facies. Both processes, however, were focused along the western margin of a Neocomian marginal basin, 25 to 30 km east of the Atacam fault system, and there is no evidence of widespread albitization in the vicinity of the major IOCG centers. An extensively revised paragenetic model for Mantoverde and its satellite deposits incorporates four stages. Stage I was dominated by widespread potassium and iron metasomatism which converted granitoid and volcanic rocks to orthoclase and magnetite, respectively. Stage II comprises chloritic and sericitic alteration and veining. The deposition, early in stage II, of marialitic scapolite, subsequently largely replaced by chlorite, was probably contemporaneous with regional scapolitization in the area between the Atacama fault system and the marginal basin. Chalcopyrite deposition was restricted to the ensuing stage III, hosted by calcite veins and, particularly, specular hematite-dominated hydrothermal breccias and stockworks. Stage IV barren calcite-quartz vein swarms record the terminal hydrothermal activity. Stable isotope fractionation relationships and published fluid inclusion microthermometry define a retrograde thermal evolution, from above ~460°C in stage I, through ~350°C in stage II, to ~210° to 280°C in ore stage III, and ~110° to 240°C in stage IV. The δ34S values of chalcopyrite and pyrite from Mantoverde and its associated orebodies and prospects range overall from –6.8 to +11.2 per mil, overlapping extensively. However, the narrow range, –0.6 to +2 per mil, of δ34S values of pyrite associated with stage I magnetite contrasts with the much wider range, –1.2 to +9.1 per mil, of that deposited in stage II. The compositional variability increases from +1.4 to –11.2 per mil in the mineralized assemblages of stage III, chalcopyrite generally having higher values than pyrite. The iron oxides in the district have δ18O values that vary overall from –1.9 to +4.1 per mil, the highest values, +1.4 to +4.1 per mil, occurring in stage I metasomatic magnetite, whereas stage III hematite has lower values of –2.0 to +1.7 per mil. Estimated equilibrium δ34Sfluid values increased dramatically with time, from +0.4 to +4 per mil in stage I, through +9.1 to +14.9 per mil during stage II, to +26.4 to +36.2 per mil for the most richly mineralized hematitic breccias. Stage III hematite equilibrated with a fluid with δ18O values of +3.0 to +8.0 per mil, significantly lower than those of fluids from which stage I magnetite crystallized (i.e., +7.3 to +9.9‰). The fluids responsible for barren stage I magnetite-pyrite assemblages, with δ34S and δ18O values close to 0 and +8 per mil, respectively, may have been products of the second boiling of granitoid magmas, possibly of the Sierra Dieciocho complex. Markedly higher δ34S and lower δ18O values in stages II and, particularly, stage III, in which all significant chalcopyrite and gold were deposited, are interpreted as evidence for the incursion of modified seawater, possibly via evaporitic sediments. Such externally derived fluids, probably mobilized by marginal basin inversion and recorded by the district-wide scapolitization (Na-Cl metasomatism), may have been a prerequisite for hypogene Cu(-Au) mineralization in the Mantoverde district.

† Corresponding author: e-mail, [email protected] *Present address: Cambria Geosciences Inc., 303–5455 West Boulevard, Vancouver, Canada V6M 3W5. **Present address: Rómulo J. Peña no. 170, Departamento 21-B, Condominio Las Palmas, Copiapó, Chile.

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IQUIQUE

O

72

70

O

O

68

SYMBOLS Fe oxide Cu-Au deposits Volcanic-hosted Cu-(Ag) dep.

INA ENT ARG CHILE

AFS

Magnetite-apatite deposits Fault Town/City O

22 S

TOCOPILLA Buena Esperanza (Cu-Ag) Mantos de Luna Cu-Ag Michilla District Cu-(Ag) Naguayan Cu-(Au) Mantos Blancos (Cu-Ag) ANTOFAGASTA 0

O

OCEAN

100 km SCALE

AFS

24 S

Julia (Cu)

AFS

Santo Domingo Cu-(Ag) TALTAL

O

26 S CHANARAL

Cerro Negro Cu-(Au)

O

28 S

A

Punta del Cobre (Cu-Au) Candelaria (Cu-Au)

ILE AR GE NT IN

COPIAPO

Fig. 2

CH

Mantoverde Cu-(Au)

AFS

Boqueron Chanar (Fe)

Los Colorados (Fe) VALLENAR Algarrobo (Fe) SOUTH AMERICA

N EA

TIC

CHILE

AN

O

30 S

Romeral (Fe)

OC

Santiago

LA SERENA

FIG. 1 ARGENTINA

AT L

Los Cristales (Fe)

PACIFIC OCEAN

Introduction THE CENTRAL Andean orogen provides a unique context for the clarification of the genetic factors responsible for the economic concentration of copper sulfides and gold in deposits of the iron oxide-copper-gold (IOCG) clan. The majority of such systems in Chile and Perú are of Mesozoic age, and the geologic record provides better constraints on their geodynamic setting and on the petrogenesis of the spatially and temporally associated granitoid rocks than are afforded by most Precambrian IOCG provinces. Moreover, the deposits embrace the entire spectrum of IOCG composition, from centers with negligible Cu and Au (e.g., the magnetite deposits of the Chilean iron belt, such as El Romeral: Bookstrom, 1977), through large magnetite deposits with proportionately minor associated Cu-Au mineralization (e.g., Carmen, Chile: Espinoza, 1990; Marcona and Pampa de Pongo, Perú: Hawkes et al., 2002), to major copper sulfide deposits in which the associated Fe oxide mineralization is uneconomic (e.g., La Candelaria: Ryan et al., 1995; Mantoverde: Vila et al., 1996). A fundamental uncertainty in the evolving genetic model for IOCG mineralization is whether it is generated by metalbearing brines exsolved from crystallizing granitoid magmas, and therefore controlled primarily by melt-aqueous fluid equilibria, or alternatively, whether the intervention of nonmagmatic waters is a prerequisite for chalcopyrite and gold enrichment (Williams et al., 2005, and references therein). Numerous authors (e.g., Sillitoe, 2003; Sillitoe and Perelló, 2005) interpreted the geological and geochemical relationships of the IOCG deposits of northern Chile in entirely magmatic-hydrothermal terms and argued that their characteristic metal association (i.e., Cu, Au, Co, Ni, As, Mo, and U) reflects the basic, dioritic to gabbroic, nature of the inferred parental magmas. Pollard (2006) proposed that IOCG systems in the Andes and elsewhere differ from porphyry copper deposits in that vapor saturation in parental magmas occurred at higher pressures, owing to the abundance of CO2, and that the evolution of the hydrothermal fluids was controlled by unmixing of the carbonic phase. In contrast, Ullrich and Clark (1999) and Ullrich et al. (2001) concluded that temporal changes in the sulfur and oxygen isotope compositions of the hydrothermal fluids at La Candelaria resulted from incursion of water from contiguous evaporitic strata of the Chañarcillo Group during emplacement of the chalcopyrite-gold ore. Their findings were consistent with the conclusion of Barton and Johnson (1996, 2000) that many salient features of IOCG deposits are difficult to reconcile with straightforward magmatic-hydrothermal models. Fluid mixing has been advocated by Haynes et al. (1995) and Johnson and McCulloch (1995) for Olympic Dam, the most Cu- and Au-rich large IOCG deposit, whereas Menuge et al. (2002) recorded late-stage, saline, oxidized fluids of possible evaporite origin in the Pea Ridge magnetite-hematite deposit in Missouri. However, Marschik and Fontboté (2001a) discounted the stable isotope data presented by Ullrich and Clark (1999), a decision later supported by Pollard (2006). In this study, we document sulfur and oxygen isotope data for representative, paragenetically constrained, samples from the Mantoverde mining district, located in the Coastal Cordillera, III Región, northern Chile (26°30'40"–26°36'03" S, 70°17'39"–70°20'05" W; Figs. 1, 2). The Coastal Cordillera

PACIFI C

416

FIG. 1. Location map of the Mantoverde district. Numerous Fe oxide (i.e., magnetite-apatite), Fe oxide-copper-gold, and volcanic-hosted, strata-bound Cu (Ag) deposits, located between latitudes 22° and 27° S, are controlled by the main or subsidiary structures in the Atacama fault system. Area of Figure 2 is also shown. After Sillitoe and Perelló (2005).

416

o

26 45’ S 7.040

o

7.050

7.060

7.070

26 25’ S

Jigf

Q.

Jgla

o

tern bran

ch

Jkgm

Kglt

Jln

Kgm

Jln

Jln

Jln

Kgm

Kgm

Mantoverde

Kglt

Manto Ruso

Kglt

Kgsd

Fig. 3

Kgsd

Q. d e

Jln

380

Kgr

Pirula

Kgr

390

Kpc

Chivato

400 o

Kpc

10 km

Kgch

Berta

Jln

Kch

Sierra Santo Domingo district

Rodados Negros

Kpc

Palmira

Kgsm

UTM Coordinates (x 1.000) UTM 19S PSAD 56

nga

Jln

Santa Rosa

Gua ma

as

im

An

Kgsm

Kgsm

Q. La s

Ferrifera

Kgsd

Jln

Q. del Salado

Jln

70 00’ W

26 45’ S 7.040

o

N

Magnetic Declination

7.050

7.060

7.070

o

26 25’ S

7.080

LEGEND

IOCG Mine/Prospect

NW-SE lineament Contact Mafic dike

Reverse fault

Fault, observed; covered

SYMBOLS

Metasedimentary rocks (Devonian-Carboniferous, DCce)

Metamorphic Rocks

Lower Jurassic Jigf (Flamenco, ca. 190-200 Ma)

Middle-Upper Jurassic Jgla (Las Animas, ca. 150-160 Ma)

Jurassic-Cretaceous JKgm (Cerro Moradito, ca. 140-145 Ma)

Lower Cretaceous Kgsd (Sierra Dieciocho, 120-126 Ma) Kglt (Las Tazas, 125-130 Ma) Kgm (Cerro Morado, 130-135 Ma)

mid-Cretaceous Kgsm (Sierra Merceditas, ca. 90-110 Ma) Kgr (Remolino, ca. 90-110 Ma) Kgch (Chivato, ca. 111-114 Ma)

Plutonic Complexes

La Negra Fm., Jln (Middle-Upper Jurassic)

Punta del Cobre Fm., Kpc (L. Cretaceous)

Chanarcillo Group, Kch (L. Cretaceous)

Alluvial deposits (Neogene-Quaternary)

FIG. 2. Regional geologic setting and location of the Mantoverde district, modified from Lara and Godoy (1998) and Godoy and Lara (1998). Black stars indicate the main IOCG deposits and prospects in the area. AFS, MVF, and ChF refer to the Atacama fault system, Mantoverde fault, and Chivato fault, respectively. For sources of geochronologic data for plutonic complexes, see Lara and Godoy (1998) and Godoy and Lara (1998). The age of the Punta del Cobre Formation is based on Pop et al. (2000) and Marschik and Fontboté (2001b). Area of Figure 3 is also shown.

Jgla

a

Jgla

Jgla

Jgla

os

litr

Sa

DCce

DCce

DCce

entral

branch AFS, c

AFS, wes

Salado district

AFS, eastern branch

7.080

F

70 25’ W 360

360

Ch

370 370

417 380

MVF 390

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between latitudes 22° and 27° S hosts numerous Fe oxide (e.g., magnetite-apatite), Fe oxide-Cu-(Au), and volcanichosted strata-bound Cu-(Ag) deposits and constitutes a distinctive metallogenic subprovince of the Central Andes (Ruiz et al., 1965; Boric et al., 1990; Sillitoe, 2003, and references therein; Fig. 1). The Mantoverde district is one of only three Andean IOCG camps (including the La Candelaria-Punta del Cobre district in Chile and Raúl-Condestable in Perú), which have supported significant Cu production in recent years. This research is a part of a regional study of a 3,500-km2 area surrounding Mantoverde (Benavides, 2006), with the objective of establishing lithogeochemical vectors to, specifically, Cu-rich IOCG centers. Mining in the district is now focused in three open pits, Manto Ruso, Mantoverde, and Mantoverde Sur (Fig. 3), operated by Anglo American plc. The district has measured reserves of 140 million metric tons (Mt) at 0.63 percent of Cu, at a cutoff grade of 0.38 percent and an annual production of ~60,000 t of Cu, entirely from supergene oxide ore. Gold, with an average grade of 0.4 ppm, is not recovered (C. Astudillo, pers. commun., 2005). In the Mantoverde deposit itself the geologic resource of hypogene protore is 400 Mt at 0.52 percent Cu (at a cutoff grade of 0.2%) and 0.11 ppm Au (C. Astudillo, pers. commun., 2005). Regional Geology The Mantoverde district is located in an ensialic calc-alkaline volcanoplutonic arc terrane of Mesozoic age (Dallmeyer et al., 1996; Grocott and Taylor, 2002), hosted by Devonian to Carboniferous metasedimentary strata (Lara and Godoy, 1998; Fig. 2) and Permo-Triassic plutonic and volcaniclastic rocks (Brown, 1991; Lara and Godoy, 1998). Both the arc and basement are transected by the regionally extensive Atacama fault system and widely covered by Neogene to Quaternary alluvial and coluvial deposits (Fig. 2). The most voluminous volcanism in the wider Mantoverde area has been assigned to the La Negra and Punta del Cobre Formations (Fig. 2). The former is a Middle to Upper Jurassic succession (García, 1967) of basaltic andesitic to andesitic lava flows with subordinate volcaniclastic and marine sedimentary units (Lara and Godoy, 1998; Vivallo and Henríquez, 1998). This formation constitutes either fault-bounded blocks separating the central and eastern branches of the Atacama fault system or roof pendants in Neocomian plutons (Fig. 2). The younger Punta del Cobre Formation comprises a thick package of andesitic flows with intercalations of tuffs, tuffaceous sandstones, and welded tuffs, and thin beds of lithic arenites and limestones (Lara and Godoy, 1998). A Neocomian U-Pb zircon date of 131.3 ± 1.4 Ma (i.e., Valanginian/Hauterivian boundary age) has been reported by Pop et al. (2000) for a near-basal andesitic member of the type section in the La Candelaria-Punta del Cobre mining district, ~120 km south of Mantoverde. In the study area, the Punta del Cobre Formation is restricted to the eastern part of the district (Fig. 2), where it concordantly overlies the La Negra Formation and exhibits gradational contacts with the sedimentary Chañarcillo Group (Segerstrom and Parker, 1959; Lara and Godoy, 1998). Exposures of the latter, preserved east of the Mantoverde mining district proper (Fig. 2), comprise mudstones, calcareous sandstones and siltstones, chert 0361-0128/98/000/000-00 $6.00

and fossiliferous limestones with intercalations of tuffs and conglomerates (Lara and Godoy, 1998), recording marine sedimentation in a marginal back-arc basin. Naranjo (1978) reported a Valanginian age (i.e., 132–137 Ma) for limestones in the Sierra Santo Domingo district area, 30 km north-northeast of the Mantoverde mining district (Fig. 2). Faunal assemblages documented by Moraga (1977) indicate a range of ages from Berriasian to Barremian (i.e., ca. 121–144 Ma) for Chañarcillo Group strata cropping out 60 km southeast of the Mantoverde district (location not shown in Fig. 2). In the wider Mantoverde district (Fig. 2), the intrusive rocks range in age from Late Triassic to mid-Cretaceous (Dallmeyer et al., 1996; Lara and Godoy, 1998; Gelcich et al., 2002). On the basis of Rb-Sr whole-rock isochron and U-Pb zircon dates (Berg and Breitkreuz, 1983) and, more extensively, 40Ar/39Ar hornblende age spectra (Dallmeyer et al., 1996), Lara and Godoy (1998) delimit the following pre-Albian intrusive complexes (Fig. 2): Flamenco monzogranites, granodiorites, and tonalites at 190 to 200 Ma; Las Animas pyroxene quartz diorites at 150 to 160 Ma; Cerro Moradito hornblende quartz diorites and hornblende granodiorites at 140 to 145 Ma; Cerro Morado quartz monzodiorites and granodiorites with hornblende, biotite, and pyroxene at 130 to 135 Ma; Las Tazas biotite-hornblende granodiorites at 125 to 130 Ma; Sierra Dieciocho hornblende-biotite quartz diorites at 120 to 126 Ma. Although the main intrusive focus was displaced eastward from the Early Jurassic to the mid-Cretaceous, there was considerable areal overlap throughout the Cretaceous (Fig. 2). The granitoid rocks are I-type (Chappell and White, 1974), members of the magnetite series (Ishihara, 1977) and characteristic of volcanic arcs (Pearce et al., 1984). In the western parts of the area, the La Negra Formation, the plutonic bodies, and basement units are cut by swarms of dioritic and/or andesitic dikes with north-south, northwest, and northeast strikes (Fig. 2; Lara and Godoy, 1998). Tectonic relationships The major tectonic features in the area are assigned to the north-south Atacama fault system (Fig. 2), which extends for more than 1,000 km from Iquique to La Serena (Naranjo, 1987; Thiele and Pincheira, 1987; Brown et al., 1993, and references therein). Initiated in the Early Jurassic, this arc-parallel structure (Scheuber and Andriessen, 1990; Brown et al., 1993) records a complex kinematic evolution, but dip-slip and left-lateral strike-slip displacements predominated during the Early Cretaceous (Brown et al., 1993, and references therein; Dallmeyer et al., 1996). The Atacama fault system controlled both the emplacement of the Upper Jurassic and Lower Cretaceous plutons (Grocott et al., 1994; Wilson and Grocott, 2001; Grocott and Taylor, 2002) and the development of iron oxide-copper-gold deposits, including Mantoverde (Fig. 2) and the sulfide-poor magnetite deposits of the Chilean iron belt (Espinoza, 1990; Grocott and Taylor, 2002). The Chivato fault in the southeast part of the area (Fig. 2) exhibits a reverse displacement, with a dominant northeast to northnortheast strike and northwest-directed tectonic transport that translated the La Negra Formation over the Punta del Cobre Formation (Fig. 2; Lara and Godoy, 1998). During the late Neocomian, this structure behaved as a ductile shear zone, with both dip- and strike-slip displacements, that

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70 17’39”W

AFS, central branch

VOLCANIC/SEDIMENTARY ROCKS

A’

01GT11

Neogene/Quaternary alluvial deposits

01DS07

A

RCH04MR66

99CN12

Manto Ruso

La Negra Formation, andesites (Middle-Upper Jurassic)

RCH04MR68

PLUTONIC ROCKS

MVF B’ 01CN03

Mafic dikes Sierra Dieciocho complex (ca. 120-126 Ma)

B N 7.064.000

LEGEND LITHOLOGIC UNITS

99MS01

N 7.065.000

N 7.066.000

E 371.000 26 30’40”S

E 370.000

E 369.000

E 368.000

E 367.000

Laura

Las Tazas complex (ca. 125-130 Ma)

01DS22

Ferrifera

00CN02 01CN04

Cerro Morado complex (ca. 130-135 Ma)

N 7.063.000

ALTERATION UNITS Tectonic Breccia (Mantoverde body, Vila et al., 1996; mineralized cataclastic rock along the Mantoverde Fault)

99MV06 99MS03

Green Breccia (Chlorite/quartz- bearing hydrothermal breccia)

01DS13

00DS05

N 7.062.000

C’

“Felsic Body” (Fine- grained aggregate of potassium feldspar and quartz)

Mantoverde main open pit C

MINERALIZATION UNITS

N 7.061.000

Sulfide-bearing stockwork of specular hematite veins (Transition Zone, Vila et al., 1996) Sulfide-bearing specular hematite-cemented hydrothermal breccias (Manto Atacama, Vila et al., 1996)

01GT01 00DS02

Bodies of magnetite-pyrite (Stage I, this study) Mantoverde Sur N 7.060.000

01GT02

Calcite vein system

N 7.059.000 N 7.058.000

Franco

SYMBOLS Fault: observed; covered

AFS, easter

D’

D

n branch

Montecristo

96GM21

NW - SE lineaments Drill-hole collar

01DS18

A

A’

Geological cross section (Fig. 8)

MVF Trillizos

70 20’05”W

E 367.000

Bodies of massive magnetite-apatite-pyrite

01DS18

01DS16

E 368.000

E 369.000

E 370.000

N

MAGNETIC DECLINATION 4 59’

26 36’03”S E 371.000

0

SCALE

1,000 m

Datum: PSAD56 Projection: UTM 19S

FIG. 3. Geology of the Mantoverde mining district. Black stars indicate the locations of the collars of drill holes selected for sulfide and iron oxide sampling. Modified from Zamora and Castillo (2001). See text for details. Note: Reverse circulation drill holes 04DT01, 04DT04, and 04DT02, not shown in the map, are located, respectively, 1.5, 2.2, and 2.3 km south of Trillizos. AFS and MVF refer to Atacama fault system and Mantoverde fault, respectively. 0361-0128/98/000/000-00 $6.00

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controlled the emplacement of the Remolino pluton (Grocott and Taylor, 2002: Fig. 2) as well as the small Berta and Chivato Cu-Au deposits (Fig. 2). In addition to the above structures, southwest-northeast– to north-south–striking, eastverging, reverse faults to the east of the main Mantoverde district exerted a primary control on the location of Palmira and several other IOCG centers (Fig. 2). Finally, a series of northwest-southeast lineaments may be shallow expressions of episodically reactivated older structures rooted in the basement (Bonson, 1998). In the Andes of northern Chile between 22° to 27° S, extensional tectonism during the Jurassic led to the formation of the Tarapacá basin in the foreland of the magmatic arc (Mpodozis and Ramos, 1990). In the Early Cretaceous, however, changes in both the rate and obliquity of convergence at the plate boundary and steepening of the subducting oceanic slab generated a transtensional tectonic regime that resulted in the formation of the Atacama fault system and an aborted marginal back-arc basin (Åberg et al., 1984; Aguirre et al., 1989; Mpodozis and Ramos, 1990; Grocott and Taylor, 2002). In this setting, arc magmatic activity and sedimentation were broadly contemporaneous (Grocott and Taylor, 2002). The former is represented by both the Las Tazas and Sierra Dieciocho plutonic complexes and the Punta del Cobre Formation, and the sedimentary filling by the Chañarcillo Group. The present areal separation (Fig. 2) of Neocomian granitoid rocks to the west and volcanic strata to the east may be an effect of erosion but probably reflects an inherent separation of large-scale intrusive and eruptive activity. The tectonic inversion of the Neocomian basin took place in the mid-Cretaceous (Mpodozis and Ramos, 1990). Regional alteration and metamorphism Igneous rock compositions and alteration features have been documented by Benavides (2006) for 460 outcrop samples from an area extending 32 km east, 50 km north, and 33 km south of the Mantoverde mine. The major alteration and metamorphic domains are shown in Figure 4, subdivided into early albitization and argillic alteration and later lowgrade metamorphism. Neither is associated with penetrative deformation. Albitization: Weak to moderate replacement of magmatic plagioclase by albite is widespread in the andesites of the Punta del Cobre Formation, in the eastern part of the wider Mantoverde area (Fig. 4), whereas the La Negra Formation volcanic rocks only locally exhibit such effects. Secondary albite initially developed as patches in plagioclase phenocrysts (Fig. 5A), with concomitant destruction of albite twin lamellae, whereas more intense albitization resulted in complete pseudomorphism of plagioclase crystals (Fig. 5B). However, most associated augite phenocrysts are unaltered, with only local marginal replacement by probably magmatic magnesiohornblende. Such selective alteration is interpreted as subocean floor spilitization. Complete replacement of the rocks by albite, such as occurred in the Upper Andesite at La Candelaria (Ullrich and Clark, 1999), is not observed. Weak to moderate albitization is focused in a 10- to 15-kmwide, north-south zone extending for 55 km from the Rodados Negros area to the latitude of the Cerro Negro deposit (Figs. 2, 4), where it affects units of the La Negra Formation 0361-0128/98/000/000-00 $6.00

as well as the overlying Punta del Cobre Formation (Fig. 4). The zone of Na metasomatism therefore rims the Neocomian basin and lies immediately west of the outliers of Chañarcillo Group sedimentary rocks. There is no evidence of extensive secondary albite development in either volcanic or plutonic rocks in the vicinity of the Atacama fault system and the major IOCG centers along that structure (see below). Hydrolytic alteration: The regionally developed albitic alteration in the north-south domain between Rodados Negros and Sierra Aspera (Fig. 4) is extensively overprinted by both intermediate argillic assemblages (illite, chlorite, and smectite: Fig. 5C) and, particularly in the Mina Berta and Diego del Almagro areas (Fig. 4), by muscovite and quartz (Fig. 5D). Low-grade metamorphism: As documented by Palacios (1977), the Jurassic volcanic strata of the region are widely affected by low-grade metamorphism, comparable in all salient aspects to that described for central Chile by Aguirre et al. (1989). In this study, similar effects have been confirmed in the Neocomian Punta del Cobre Formation. The most widespread metamorphic assemblages may be assigned to the prehnite-pumpellyite facies (Alt, 1999) but locally both prehnite-actinolite and pumpellyite-actinolite facies are represented. Characteristic mineralogical relationships in andesites of the La Negra and Punta del Cobre Formations are shown in Figure 6. As emphasized by Frey et al. (1991) and Alt (1999), this range of assemblages, although extending from the sub- to the lower greenschist facies, may not record a significant temperature range. The metamorphic assemblages clearly developed after the formation of secondary albite and even the superimposed hydrolytic alteration (Fig. 6). As with those processes, however, metamorphism was most intense in the eastern part of the study area along the western margin of the marginal basin (Fig. 4) and only weakly affected areas adjacent to the Atacama fault system. The replacement of metamorphic actinolite by magnetite in the Santa Rosa Cu-Au prospect (Figs. 4, 7) shows that the regional metamorphism predated the initial stages of hydrothermal alteration, at least in the minor IOCG centers located east of the main Mantoverde district. Geology of the Mantoverde Mining District Host-rock units The least altered andesites of the La Negra Formation in the Mantoverde district proper contain plagioclase (An60–70) phenocrysts in a microcrystalline to aphanitic groundmass. The main mafic mineral is augite, and titaniferous magnetite is finely disseminated in the groundmass. In the northern part of the district, amygdules are filled with calcite, chlorite, and chalcedony (López, 2002). Numerous beds of fine-grained lithic and crystal tuffs are intercalated with the andesites. The major plutonic rocks of the immediate district are equigranular, medium-grained, hornblende and biotite-bearing diorites and monzodiorites assigned to the Hauterivian-Barremian (i.e., ca. 121–132 Ma) Sierra Dieciocho complex (Lara and Godoy, 1998). Structural relationships Mineralization in the Mantoverde district developed within an intensely fractured structural block delimited by the

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26 06’ 20”S

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LEGEND

26 06’ 20”

7.110

7.110

Trgc Jln

Alluvial deposits (Neogene-Quaternary)

?

Jln

Cerro Negro

Kpc

Chanarcillo Group, Kch (L. Cretaceous) Punta del Cobre Fm., Jln (L. Cretaceous)

Jln

7.100

Jln

7.090

Jln

mid-Cretaceous complexes Kgsm (Sierra Merceditas, ca. 90-110 Ma) Kgr (Remolino, ca. 90-110 Ma) Kgch (Chivato, ca. 111-114 Ma) Kglb (La Borracha, ca. 105-110 Ma)

Jln

7.090

Sierra Aspera district

Kgsa

Carmen Kgsa

Jln

Jgla

Plutonic Complexes

Jln

AFS, centra

Kglt

Jgla

La Negra Fm., Kpc (Middle-Upper Jurassic) Kgsa

l branch

Jgla

DCce

AFS, weste rn bra

nch

7.100

Kpc

Ester

7.080

Diego de 7.080 Almagro

Jln

Salado district

Sierra Santo Domingo district

Santa Rosa

Q. del Salado

Jgla

Lower Cretaceous complexes Kgsd (Sierra Dieciocho, ca.120-126 Ma) Kgsa (Sierra Aspera, ca. 125-130 Ma) Kglt (Las Tazas, ca. 125-130 Ma) Kgm (Cerro Morado, ca.130-135 Ma)

Jln

Jln

Kglt

Jurassic-Cretaceous complexes JKgm (Cerro Moradito, ca. 140-145 Ma)

Kpc Jln

Kglt

Kpc

Kch

7.070

7.070

Middle-Upper Jurassic complexes Jgas (Agua de Sol, ca. 150 Ma) Jgla (Las Animas, ca. 150-160 Ma)

Kgsm Jln

Manto Ruso

Kgsm

Kglt

Ferrifera Mantoverde 7.060

La sA nim

Kgm

7.050

AFS, eastern branch

Kgsd

Kglt

Triassic plutons, Trgc (Capitana, ca. 215 Ma)

Pirula

Q.

Kpc

7.060

Metamorphic Rocks

Palmira

as

Metasediments, DCce (DevonianCarboniferous)

Kgsm Jln

Kpc

Ch F

Jgla

Rodados Negros

Q. d e

SYMBOLS

Kgr

7.050

Gua ma

Western limit of moderate albitization

nga

Remolino Kgsd

Jln

Kgr

Western limit of pervasive low-grade metamorphism

Chivato

Kgr Kgm

Berta

Zones of scapolitization: intense; moderate

Kgch

7.040

Q. S

Jkgm

7.040

Fault, observed; covered

ali tr

Kgr

os

Kgm

a

Jln

Reverse fault; NW-SE lineament

Kgr

Sierra Desiertito

390

380

370

360

70 25’W

Jgas

10 km

Magnetic Declinatio n

N

IOCG Mine/Prospect

7.030 400

Kglb

7.030

Contact 70 00’W

Ch FS

? 26 50’ 35”S

Sedimentary/Volcanic Rocks

26 50’ 35”S

UTM Coordinates (x 1.000) UTM 19S PSAD 56

FIG. 4. Geologic map showing the western boundaries of regional albitization (thick dashed line) and low-grade metamorphism (thick dashed-dotted line) in the wider Mantoverde district. Based on regional petrographic studies by Benavides (2006). These earlier alteration events affected predominantly Neocomian and Jurassic volcanic rocks in the vicinity of the western margin of the back-arc basin. Areas of volcanic rocks with moderate and strong development of marialitic scapolite are also shown. Symbols for lithologic units as in Figure 2, with the addition of the following plutonic complexes: La Capitana (Trgc, Triassic), Agua de Sol (Jgas, Jurassic), Sierra Aspera and La Borracha (Kgsa and Kglb, respectively, Lower Cretaceous). Base map modified from Lara and Godoy (1998) and Godoy and Lara (1998). The area of the Mantoverde mining district is also shown. 0361-0128/98/000/000-00 $6.00

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421

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BENAVIDES ET AL.

FIG. 5. Regional albitization. (A). Patchy albitization of plagioclase phenocryst in Punta del Cobre Formation andesite. Sample 124633, Diego de Almagro sector (coordinates: x = 398,684, y = 7,083,747; transmitted light, crossed nicols). (B). More intense albitization, accompanied by the destruction of twinning. Sample 9795, Rodados Negros sector (coordinates: x = 391,529, y = 7,051,142; transmitted light, crossed nicols). (C). Regional hydrolysis. Na-metasomatized plagioclase phenocrysts are overprinted by assemblages dominated by clays and illite. Sample 124682, Santa Rosa mine area (coordinates: x = 386,991, y = 7,075,031; plane-polarized transmitted light). (D). Hydrolytic assemblage dominated by muscovite and quartz (Ms-Qtz), overprinting a moderately albitized plagioclase phenocryst. Sample 124633, Diego de Almagro sector (coordinates: x = 398,684, y = 7,083,747; transmitted light, crossed nicols).

subvertical central and eastern branches of the Atacama fault system (Figs. 2, 3). These are connected by the Mantoverde fault (MVF, Fig. 3), which strikes N15° to 20° W and dips 40° to 50° E (Figs. 3, 8, 9). With their subsidiary structures, the Mantoverde fault and the eastern branch of the Atacama fault system exerted a strong control on the location and morphology of both barren magnetite-apatite-pyrite bodies and sulfide-bearing specular hematite-cemented breccias (Fig. 8). The Mantoverde deposit itself has been interpreted as being hosted by a releasing strike-slip duplex located in a transfer zone between the central and eastern branches of the Atacama fault system (A. Sanhueza and W. Robles, 1999, Estudio Estructural del Distrito Mantoverde, unpublished report for Anglo American Chile, Santiago, 1999, 25 p.). Distribution of mineralization The Mantoverde mining district extends for 10 km in a north-south direction and comprises several mineralized 0361-0128/98/000/000-00 $6.00

centers (Figs. 3, 8), which range from chalcopyrite- and specular hematite-rich breccias and stockworks to massive magnetite-pyrite and magnetite-apatite ± pyrite bodies. The mineralized breccias and mineralogically identical stockworks are commonly located in the hanging wall of the Mantoverde fault and in the northern half of the district, whereas crudely tabular, massive bodies of magnetite-pyrite occur predominantly in the footwall of the Mantoverde fault and in the southern half of the district (Fig. 3). Massive and irregular bodies of magnetite-apatite ± pyrite are developed along the eastern branch of the Atacama fault system (Figs. 3, 8). From north to south, the most important deposits and prospects are Manto Ruso, a sulfide-bearing hematite-cemented breccia pipe and associated stockwork (Fig. 8A); Laura, a sulfide-bearing specularite-cemented breccia (Fig. 8B); Ferrífera, a magnetite-apatite ± pyrite body (Fig. 3); Mantoverde proper, the largest known deposit in the district, comprising mineralized tectonic breccias, tabular chalcopyrite-rich,

422

FLUIDS IN THE MANTOVERDE IOCG DISTRICT, III REGION, CHILE

423

FIG. 6. Mineral associations and textural relationships of regional, nondeformational, low-grade metamorphism. (A). Vesicle filled with prehnite (Prh) and calcite (Cal) in a fine-grained andesitic volcanic rock of the La Negra Formation, east of Cerro Negro. Sample 124579 (coordinates: x = 382,696, y = 7,104,867; transmitted light, crossed nicols). (B). Actinolite disseminated in the matrix of an andesite (upper-right corner) and as thin veinlets cutting moderately albitized plagioclase phenocrysts. Sample 124670 (coordinates: x = 381,720, y = 7,075,744; transmitted light, crossed nicols). (C). Irregular patches of metamorphic chlorite (Chl) in the matrix of a fine-grained andesite of the Punta del Cobre Formation. Note the moderately albitized plagioclase phenocryst altered to clays (right side), prior to the development of chlorite veinlets. Sample 124699, Sierra Santo Domingo district (coordinates: x = 394,593, y = 7,074,909; plane-polarized transmitted light). (D). Aggregate of epidote (Ep), chlorite (Chl), and actinolite (Act) in fine-grained andesite of the Punta del Cobre Formation. Sample 9769, Palmira mine area (coordinates: x = 388,499, y = 7,060,987; transmitted light, crossed nicols).

hematite-cemented, hydrothermal breccias, and stockworks (Fig. 8C); and Montecristo and Franco-Trillizos, chalcopyritebearing subvertical veins and lenses of magnetite (Fig. 8D). Chalcopyrite and pyrite, the dominant hypogene sulfide minerals, are associated with minor bornite and pyrrhotite. Hematite, chlorite, apatite, calcite, quartz, potassium feldspar, sericite, and tourmaline are the major associated minerals. Vila et al. (1996) defined three main alteration-mineralization units in the Mantoverde deposit itself (Fig. 8C).

FIG. 7. Metasomatic magnetite (Mag) of hydrothermal stage I forms irregular patches overprinting metamorphic actinolite (Act) and epidote (Ep) in an andesite of the Punta del Cobre Formation. Sample 9815, west of the Sierra Santo Domingo district (coordinates: x = 392,394, y = 7,067,006; plane-polarized transmitted light). 0361-0128/98/000/000-00 $6.00

423

0361-0128/98/000/000-00 $6.00

1368

A

1378

2469

1373

0

424 100

C

200 m

SCALE H:V 0

2447

DDH00DS05

2357 2358 2359

2470

2465

DDH01DS13

200 m

0

50

150

250

350

450

550

650

750

850

950

1050

m.a.s.l. 1150

50

150

250

350

1361 1362B 1363

1358

B

SCALE H:V

BLSP

D

100

2384

0

200 m

100 m

SCALE H:V

DDH96GM21

Mantoverde Fault

SECTION D-D’ (Montecristo)

0

700

750

800

850

900

950

m.a.s.l. 1000

150

250

350

450

550

1356

750

850

950

450

DDH01CN03

1050

650

BLSP

Mantoverde Fault

m.a.s.l. 1150

550

650

750

850

m.a.s.l. 950

SECTION B-B’ (Laura)

FIG. 8. Representative east-west geologic profiles in the Mantoverde district. Selected drill holes have been projected to show the locations of some analyzed samples. The bodies of metasomatic magnetite deepen in a northward direction, and the Montecristo sector (Section D-D’) is inferred to represent the deepest parts of the hydrothermal system. Most specular hematite-cemented breccias are located in the hanging wall of the Mantoverde fault. Massive bodies of magnetite-apatite ± pyrite along the eastern branch of the Atacama fault system are not shown. See text for details. BLSP records the base of the supergene profile. Legend as in Figure 3. Redrawn with permission from sections prepared by the Mantoverde Geology Division, Anglo American-Chile.

BLSP

Mantoverde Fault

100

SCALE H:V

DDH01GT11

SECTION C-C’, (Mantoverde deposit)

BLSP

DDH01DS07

Mantoverde Fault

SECTION A-A’ (West Manto Ruso)

424 BENAVIDES ET AL.

425

FLUIDS IN THE MANTOVERDE IOCG DISTRICT, III REGION, CHILE

FIG. 9. Photograph of supergene copper oxide zone, showing the structural relationships of the Mantoverde and Manto Atacama breccias with respect to the Mantoverde fault, which dips moderately to the east. View to north, Mantoverde main open pit, 960 m bench (for location see Fig. 3).

From west to east, these are (1) Mantoverde, a mineralized tectonic breccia in the footwall of the Mantoverde fault (Fig. 9); (2) Manto Atacama, a specularite-cemented, sulfide-bearing hydrothermal breccia in the hanging wall of, and elongated parallel to, the Mantoverde fault and grading eastward into a sulfide-bearing specular hematite vein system cutting moderately altered andesites and diorites (the Transition zone of Vila et al., 1996; Figs. 8C, 9); and (3) Brecha Verde, a chlorite-quartz-sericite–cemented hydrothermal breccia body developed on both sides of the Mantoverde fault (Fig. 8C).

STAGE I K and Fe Metasomatism

Paragenetic Relationships The paragenetic relationships of hydrothermal alteration and hypogene mineralization in the district are herein modified from the observations of Vila et al. (1996), Cornejo at al. (2000), and López (2002). The sequence of events is based primarily on drill core samples and exposures in the Mantoverde mine (Figs. 3, 8, 9), but very similar associations and crosscutting relationships are observed at Manto Ruso, Laura, Montecristo, and Franco (Fig. 3). Four stages are distinguished (Fig. 10). Of these, stage I, dominated by widespread potassium and iron metasomatism of both plutonic and volcanic rocks, and stage II, chlorite-rich hydrolytic alteration and veining (Fig. 11A-B), preceded the emplacement of stage III chalcopyrite-bearing, specular hematite-cemented hydrothermal breccias and stockworks (Fig. 11C-D). Barren calcite-quartz veining represents the latest paragenetic stage at Mantoverde (stage IV; Figs. 10, 11D). The magnetite-apatite ± pyrite bodies cropping out along the eastern branch of the Atacama fault system (Fig. 3) have somewhat different mineral compositions and textural relationships and are considered separately. Albitization: There is no evidence that intense Na metasomatism occurred along the Atacama fault system, and in this respect Mantoverde differs markedly from other major central Andean IOCG districts (e.g., Ullrich and Clark, 1999; Hawkes et al., 2002). Secondary albite has, however, been observed in a single clast in a chloritic hydrothermal breccia from the Franco area (Figs. 3, 12), where it is extensively replaced by orthoclase, presumably representing stage I of the hydrothermal paragenesis (Fig. 10). Stage I potassium and iron metasomatism: Intense potassium metasomatism is recorded by the assemblage K-feldspar

STAGE II Chlorite-Sericite Quartz

Magnetite Orthoclase Biotite Tourmaline Titanite Quartz Muscovite Chlorite Anhydrite Scapolite Rutile Epidote Hematite Pyrite Chalcopyrite Gold Siderite Calcite

ORE STAGE III Specular HematiteChalcopyrite

STAGE IV Late calcite veins

(v)

FIG. 10. Paragenetic sequence of hypogene mineralization and alteration in the Mantoverde deposit (modified after Vila et al., 1996, Cornejo et al., 2000, and López, 2002). See text for details (thick line = major mineral, thin line = minor mineral). (v) = veins. 0361-0128/98/000/000-00 $6.00

425

426

BENAVIDES ET AL.

FIG. 11. Samples showing the crosscutting relationships of alteration and mineralization units in the Mantoverde district. (A). Stage II, chlorite-bearing hydrothermal breccia (i.e., Brecha Verde: Vila et al., 1996) containing fragments of Kfeldspathized host rock. Sample 2397, drill hole 00DS18, 190 m. (B). Subangular fragments of stage I metasomatic magnetite are enclosed in a pyrite-bearing, chlorite-dominated stage II assemblage, which in turn is cut by a stage III hematite vein (left side). Sample 2357, drill hole 01DS13, 967.7 m. (C). Hand sample showing moderately K-feldspathized and chloritized granitoid rock cut by stage III hematite veins and assigned to the Transition zone of Vila et al. (1996). Note the aggregates of fine-grained chrysocolla (Ccl) in the hematitic veins. Mantoverde main open pit. (D). Stage III chalcopyrite intergrown with hematite fills fractures in host rock altered to K-feldspar and chlorite of stages I and II, respectively. Note the thin, irregular, stage IV calcite veins. Sample 1400, drill hole 99CN12, 388 m.

+ quartz, with subordinate magnetite, titanite, and tourmaline. Although López (2002) reported the presence of microcline, X-ray powder diffraction analyses indicate that orthoclase is the dominant secondary feldspar. K-feldspar selectively replaced plagioclase, with preservation of the original textures of volcanic and plutonic host rocks. More intensely altered rocks were transformed into fine-grained,

FIG. 12. Fragment of an altered host rock included in a stage II hydrothermal breccia with chlorite-rich matrix. In the fragment, secondary albite (Ab), possibly associated with early regional albitization, is partially replaced by orthoclase (Kfs) of metasomatic stage I. A planar stage IV quartz vein is visible in the upper right corner. Sample 124720, northwest of the Franco sector (Fig. 3; coordinates: x = 369,480, y = 7,059,145; plane-polarized transmitted light). 0361-0128/98/000/000-00 $6.00

426

FLUIDS IN THE MANTOVERDE IOCG DISTRICT, III REGION, CHILE

pink, granular aggregates (i.e., the “felsic bodies” of Vila et al., 1996, and Zamora and Castillo, 2001; Figs. 3, 8, 13). Hydrothermal biotite developed more erratically in the groundmass of the andesites. Although there is little textural evidence that magnetite and K-feldspar formed contemporaneously, oxygen isotope data, discussed below, and crosscutting relationships indicate that, as proposed by López (2002), both are the products of high-temperature metasomatism by magmatic fluids. Fe metasomatism is manifested as the progressive replacement of the groundmass of andesites and tuffs by magnetite (Fig. 14A-C). Plagioclase phenocrysts, quartz, and lithic fragments are generally less altered (Fig. 14A-B, D). In the Mantoverde mine, fine-grained massive bodies of magnetite and pyrite occur mainly in the deepest parts of the deposit, in the footwall of the Mantoverde fault (Fig. 8), although in the Franco and Montecristo sectors, tabular and subvertical bodies of

FIG. 13. Stage I, texturally destructive, K-feldspathization in the Mantoverde district. (A). Andesitic host rock southeast of Manto Ruso replaced by fine-grained orthoclase (Kfs). A stage II chlorite vein and patches (Chl) and stage IV calcite (Cal) developed along the chlorite vein. Sample 1399, drill hole 99CN12, 340 m. (B). Intense K-feldspathization of a volcanic host rock located east of the Mantoverde main open pit (see Fig. 3). Orthoclase (Kfs) is cut by stage II muscovite veins (Ms). Sample 2464, drill hole 01DS13, 317 m. Both in plane-polarized transmitted light. 0361-0128/98/000/000-00 $6.00

427

magnetite occur in both the footwall and hanging wall of the Mantoverde fault (Figs. 3, 8D). Small irregular cavities in magnetite are widely filled with chlorite, calcite, and quartz, and magnetite bodies are cut by thin veins of chlorite (Fig. 14D), quartz, orthoclase, and calcite. Magnetite is locally replaced by hematite (Fig. 14C). The major magnetite bodies lie progressively deeper from south to north, suggesting that the southern part of the district has been uplifted relative to the northern part since ore formation through the northward tilting of the hanging-wall block of the Mantoverde fault (Zamora and Castillo, 2001; López, 2002; Figs. 3, 8). The pyrite associated with stage I magnetite (Fig. 10) forms Py Cpy Cpy>>Py Cpy Cpy Cpy>>Py Py Py>>Cpy Py Py>>Cpy Py>>Cpy

Specular hematite Specular hematite Quartz Specular hematite Quartz Specular hematite Specular hematite Specular hematite Quartz Specular hematite Potassic alteration

Irregular aggregate Irregular aggregate Irregular and fine aggregates Fine aggregate Irregular and fine aggregates Irregular aggregate Rounded crystals Subhedral grains Irregular and fine aggregates Irregular aggregates Vein

8.9 10.0 –6.6 7.2 –5.1 9.3 1.9 4.6 –6.8 5.5 6.8

294

368,560

7,065,570

III

Cpy

Calcite vein

Fine/rounded aggregates

–4.7

386 255 340 340 388 386 386 386

370,065 370,065 370,065 370,065 370,065 370,065 370,065 370,065

7,065,490 7,065,490 7,065,490 7,065,490 7,065,490 7,065,490 7,065,490 7,065,490

III III III III III III III III

Cpy Cpy>>Py Cpy Cpy Cpy Cpy Cpy Py

Calcite vein Specular hematite Specular hematite Specular hematite Specular hematite Calcite vein Calcite vein Calcite vein

Irregular aggregate Irregular filling Irregular filling Irregular filling Open space fillings Aggregate Irregular and massive aggregates Irregular aggregates

2.2 2.5 6.1 5.0 3.7 –0.9 –0.4 2.8

758 732 757 798 798 752 448 591

369,300 369,300 369,300 369,300 369,300 369,300 369,300 369,300

7,064,660 7,064,660 7,064,660 7,064,660 7,064,660 7,064,660 7,064,660 7,064,660

III III III III III III II III

Cpy Cpy Cpy>>Py Cpy Cpy Py>>Cpy Py>>Cpy Py>>Cpy

Calcite vein Calcite vein Calcite vein Calcite/Fe oxide Calcite/Fe oxide Calcite vein Chlorite Specular hematite

Irregular aggregate Irregular aggregate Aggregate Irregular aggregate Irregular vein Irregular aggregate Irregular fillings Irregular aggregates

0.4 0.1 –0.1 –2.5 0.9 5.1 4.9 5.1

549 167 167 404 404 404 404 311 404

369,620 369,620 369,620 369,360 369,360 369,360 369,360 369,360 369,360

7,062,950 7,062,950 7,062,950 7,063,785 7,063,785 7,063,785 7,063,785 7,063,785 7,063,785

III III III III III III III III III

Cpy>>Py Cpy Py>>Cpy Cpy>>Py Cpy>>Py Cpy>>Py Cpy>>Py Cpy>>Py Cpy>>Py

Calcite vein Specular hematite Specular hematite Specular hematite Specular hematite Specular hematite Specular hematite Specular hematite Specular hematite

Fine aggregates Open space fillings Irregular filling Irregular aggregate Irregular aggregate Irregular fracture fillings Irregular fracture fillings Irregular fracture fillings Irregular aggregate

–5.4 4.0 11.2 8.7 5.3 3.8 3.9 1.4 4.5

302 448 448 301 236

370,090 370,090 370,090 370,090 370,180

7,063,930 7,063,930 7,063,930 7,063,930 7,063,950

III III III III III

Cpy>>Py Py Py Py Py

Specular hematite Specular hematite Specular hematite Specular hematite Quartz vein

Irregular aggregate Irregular aggregate Irregular aggregate Subhedral cumulates Subhedral cumulates

3.9 3.6 3.1 5.0 8.0

324 453 453 453 453

370,525 370,525 370,525 370,525 370,525

7,062,700 7,062,700 7,062,700 7,062,700 7,062,700

II II II II II

Py Py>>Cpy Py Py>>Cpy Py>>Cpy

Chl/sericite Chl/sericite Chl/sericite Chlorite Chlorite

Disseminations Subhedral cumulates Subhedral cumulates Subrounded grains Subrounded grains

1.5 1.7 –0.9 0.7 1.2

830 830 830 970 427 954 968 290

369,930 369,930 369,930 369,930 369,930 369,930 369,930 369,535

7,062,510 7,062,510 7,062,510 7,062,510 7,062,510 7,062,510 7,062,510 7,062,490

III III III I II II II III

Cpy>>Py Cpy>>Py Cpy>>Py Py Py Py>>Cpy Py Cpy

Calcite vein Calcite vein Calcite vein Magnetite Sericite Chlorite Chl/sericite Specular hematite

Irregular aggregate Irregular aggregate Irregular aggregate Subhedral cumulates Fracture filling Irregular aggregates Rounded disseminations Irregular fillings/veins

–2 –1.8 –1.4 –0.5 9.1 1.0 –0.9 4.5

290

369,610

7,061,130

II

Py

Altered host rock

Matrix of breccia

–1.2

216 357

369,880 369,880

7,061,050 7,061,050

I II

Cpy Cpy

Magnetite Altered host rock

Irregular aggregate Discontinous veinlet

2.0 –0.9

433

434

BENAVIDES ET AL. TABLE 2. (Cont.)

Sample no. Drill hole

Depth (m)

UTM_E

UTM_N

Stage

Sulfide

Host mineral/rock

Habit/texture

δ34S

216 302

369,880 369,880

7,061,050 7,061,050

I II

Cpy Py>>Cpy

Magnetite Chlorite

Irregular fillings/veins Subhedral grains

0.6 –0.5

169 220

370,445 370,445

7,060,290 7,060,290

II II

Py>>Cpy Py

Chlorite Chlorite

Subhedral cumulates Subhedral grains

–0.8 0.0

194 296 296

370,860 370,860 370,860

7,057,050 7,057,050 7,057,050

III II II

Cpy Py Py

Calcite vein Sericite Sericite

Irregular and fine aggregates Subrounded grains Subrounded grains

242 210 238 232 110

371,040 371,040 371,040 371,040 371,230

7,056,550 7,056,550 7,056,550 7,056,550 7,055,650

II II II II II

Py>>Cpy Py Py Py Py

Chl-qtz/magnetite Chl-qtz/magnetite Chl-qtz/magnetite Chl-qtz/magnetite Chl-qtz/magnetite

Irregular aggregate Irregular aggregate Dissemination/aggregates Fine disseminations Disseminations

370,180

7,063,950

Py

Magnetite/apatite

Fine disseminations

1.7

370,525 370,525

7,062,700 7,062,700

Py Py

Magnetite/apatite Magnetite/apatite

Irregular aggregates Irregular aggregates

0.6 0.9

2425A DDH00DS02 2426 DDH00DS02 SE Mantoverde pit south 2395 DDH01DS18 2398 DDH01DS18 Trillizos 2386 DDH01DS16 2394 DDH01DS16 2394-1 DDH01DS16 S-SSW of Franco 155687 RCH04DT01 155671 RCH04DT01 155685 RCH04DT01 155682 RCH04DT01 155762 RCH04DT02

Magnetite-apatite-pyrite bodies NW Ferrifera 2483A DDH01DS22 236 Ferrifera 2389 DDH99MS03 264 2389-1 DDH99MS03 264

4.1 1.1 0.6 0.5 –0.4 1.4 0.9 0.6

Samples are tabulated from north to south; for locations of the drill hole collars, see Figure 2

Extraction of oxygen from purified separates was carried out at 600°C, using the BrF5 technique of Clayton and Mayeda (1963). The oxygen isotope compositions, measured with a Finnigan MAT 252 mass spectrometer, are reported using the δ notation in units of per mil relative to VSMOW and have a precision of ±0.2 per mil. Using these techniques, analyses of NIST-28 gave a value of 9.6 per mil. Sulfur isotope compositions The δ34S values of sulfides in the Mantoverde district range from –6.6 to +11.2 per mil overall (Fig. 19). Values of pyrite range from –6.8 to +11.2 per mil and those for chalcopyrite from –6.6 to +10.0 per mil. Despite the overlap, the δ34S values vary systematically with the paragenetic stage, with markedly wider variations characterizing the younger hydrothermal activity (Figs. 10, 19). Pyrite associated with the magnetite-apatite bodies along the eastern branch of the Atacama fault system (Fig. 3) has δ34S values of +0.6 to +1.7 per mil (Fig. 15, Table 2). A similar narrow range and relatively low values from –0.6 to +2 per mil are exhibited by sulfides associated with the stage I magnetite bodies located in the footwall of the Mantoverde fault (Figs. 3, 8, 14, 19). In contrast, pyrite and chalcopyrite spatially associated with stage II sericitic and chloritic alteration have δ34S values ranging from –0.9 to +4.9 per mil (Figs. 11B, 19; Table 2). Similarly, the values for stage II sulfides filling fractures in altered host rocks range from –1.2 to as high as +9.1 per mil (Fig. 19, Table 2). The greatest isotopic variability is shown by chalcopyrite and pyrite directly associated with stage III specular hematite-cemented hydrothermal breccias, with δ34S values ranging from +1.4 to +11.2 per mil, chalcopyrite generally having the higher values (Fig. 19, Table 2). In contrast, sul0361-0128/98/000/000-00 $6.00

fides associated with stage III calcite veins tend to have lower values, –5.5 to +5.1 per mil, with chalcopyrite having lower values than pyrite (Fig. 19, Table 2). Sulfides from the central and northern parts of the district, (i.e., north of the Mantoverde main open pit; Fig. 3), have the widest range of δ34S values. In this sector, the lowest δ34S value (i.e., –6.8‰ in sample 1378: Table 2) was found in pyrite from a deep stage III quartz-calcite vein at Manto Ruso, whereas the highest values (i.e., +7.2–+10‰: Table 2) occur in sulfides in shallow specular hematite-cemented breccias and veins in the vicinity of Manto Ruso (Fig. 3). Pyrite associated with intensely chloritized volcanic host rocks and massive bodies of magnetite in the southern part of the district (i.e., Montecristo, Franco, and Trillizos sectors: Fig. 3) has δ34S values close to 0 per mil (Table 2). Oxygen isotope compositions of iron oxides A histogram of the δ18O values of iron oxide phases is shown in Figure 20. The magnetites include samples from both the magnetite-apatite-pyrite bodies at Ferrífera and the stage I magnetite bodies associated with the Cu-mineralized orebodies contiguous with the Mantoverde fault. Hematite samples are from chalcopyrite-rich hydrothermal breccias assigned to paragenetic stage III (Fig. 10, Table 3). The iron oxides have δ18O values which vary overall from –1.9 to +4.1 per mil (Fig. 20, Table 3). The highest values are shown by magnetite and range from +1.4 to +4.1 per mil. Magnetite in the sulfide-bearing orebodies has slightly lower values, from +1.4 to +3.1 per mil, than those in the magnetite-apatite-pyrite bodies, which vary from +2.2 to +4.1 per mil. Stage III hematite has lower isotopic values of –2.0 to +1.7 per mil (Fig. 20).

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435 164 296 296 296 164 128 218 236 264 180 252 180 264 236

01DS16 01DS16 01DS17 01DS18 01DS16 04DT041 04DT041 01DS22 99MS03 99MS03 99MS03 99MS03 99MS04 01DS22

180

99MS03

158

968 968

01DS13 01DS13

96GM21

404 500 167 223 223

01CN04 01CN04 99MV06 01DS05 01DS05

172 190

732 798 798 301

01CN03 01CN03 01CN03 00CN02

01DS18 01DS18

294

01DS07

216 302

387 254

99CN12 99CN12

00DS02 00DS02

351

Depth (m)

01GT11

Drill hole

370,180 370,525 370,525 370,525 370,525 370,525 370,180

371,100 371,100

370,860 370,860 370,860 370,860 370,860

370,110

370,445 370,445

369,880 369,880

370,525

369,930 369,930

369,360 369,360 369,620 369,540 369,540

369,300 369,300 369,300 370,090

368,570

370,065 370,065

369,200

UTM_E

7,063,950 7,062,700 7,062,700 7,062,700 7,062,700 7,062,700 7,063,950

7,055,800 7,055,800

7,057,050 7,057,050 7,057,050 7,057,050 7,057,050

7,059,370

7,60,290 7,60,290

7,061,050 7,061,050

7,062,700

7,062,510 7,062,510

7,063,785 7,063,785 7,062,950 7,062,500 7,062,500

7,064,660 7,064,660 7,064,660 7,063,930

7,065,570

7,065,490 7,065,490

7,065,700

UTM_N

II II

I II II II II

I

II II

I I

II

I III

III III III II II

III III III III

II

III III

III

Stage

Notes: for locations of the drill hole collars, see Figure 3; δD and δ13C values from Benavides (2006) 1 Reverse circulation drill holes

Manto Ruso 1375 SE Manto Ruso 1400 1398A SW Manto Ruso 1368 NE-E Laura sector 1361 1363 1363 2496 NNE-Mantoverde main pit 2475 2477 2488 2446A 2446B E-Mantoverde main pit 2358 2358A South Ferrífera-E Mantoverde main pit 2454 NW-Mantoverde pit south 2425 2426 SE-Mantoverde pit south 2396 2397 Montecristo 2384 Trillizos 2387 2394 2394 2394 2387 South Trillizos 156613 156680 Eastern branch, AFS (Ferrífera) 2483 2389 2454 2456 2454A 2390 2483

Sample/location

Magnetite Magnetite Magnetite Magnetite Magnetite Apatite Apatite

Chlorite Chlorite

Magnetite Hematite Muscovite Calcite Chlorite

Magnetite

Chlorite Chlorite

Magnetite Magnetite

Chlorite

Magnetite Hematite

Hematite Hematite Hematite Chlorite Chlorite

Hematite Hematite Calcite Hematite

Chlorite

Hematite Hematite

Hematite

Mineral

Magnetite-apatite-pyrite bodies Magnetite-apatite-pyrite bodies Magnetite-apatite-pyrite bodies Magnetite-apatite-pyrite bodies Magnetite-apatite-pyrite bodies Magnetite-apatite-pyrite bodies Magnetite-apatite-pyrite bodies

Chloritized volcanic rock Chloritized volcanic rock

Massive bodies Breccia Intergrown Intergrown Chloritized volcanic rock

Massive bodies

Chlorite-bearing breccia Chlorite-bearing breccia

Massive bodies Massive bodies

Fine grained

Fractured bodies Vein

Breccia Breccia Breccia Chloritized granitoid Chloritized granitoid

Vein Breccia Breccia Breccia

Chloritized volcanic rock

Breccia Breccia

Breccia

Texture/host

TABLE 3. Oxygen, Hydrogen, and Carbon Isotope Compositions of Selected Minerals, Mantoverde District

4.1 2.2 3.5 2.5 3.1 7.2 8.2

8.4 7.8

2.5 –1.5 9.3 12.5 9.5

3.1

11.2 9.0

1.4 3.4

5.6

1.4 –1.9

–1 0.5 0.0 9.7 8.7

–1.7 –1.9 11.3 –1

7.6

–1.7 1.0

1.7

δ18O

–54 –46

–71

–54

–62 –62

–55

–70 –60

–66

δD

–5.3

–5

δ13C

FLUIDS IN THE MANTOVERDE IOCG DISTRICT, III REGION, CHILE

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12

FREQUENCY

10

8

6

4

2

0 -7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

δ S (‰, CDT) 34

LEGEND

Py with ironstones at Ferrifera Cpy "associated" with Stage I-Mt Cpy with Stage II-Chl/Ser Py with Stage III-Cal Cpy with Stage III-Cal/Hem Cpy with Stage III-Qtz Cpy with Stage III-Breccia

Py with Stage I-Mt Py with Stage II-Chl/Ser Sulfide veins/fillings-Stage II Cpy with Stage III-Cal Py with Stage III-Qtz Py with Stage III-Breccia

FIG.19. Sulfur isotope composition of pyrite and chalcopyrite in the Mantoverde district, including sulfides associated with the magnetite-apatite-pyrite bodies at Ferrífera. The δ34S values range widely from –6.8 to +11.2 per mil (n = 73) and, despite the extensive overlap, δ34S values vary systematically with paragenetic stage. Sulfides associated with stage III hydrothermal breccias exhibit a wider range (+1.4 to +11.2‰, n = 25) and higher δ34S values than those of stage I sulfides (–0.6 to +2‰, n = 3). The isotopic composition of sulfides associated with both chlorite and sericite of stage II ranges from –0.9 to +4.9 per mil (n = 18), and that for stage III calcite veins is –5.0 to +5.1 per mil (n = 14). The δ34S values of sulfides associated with bodies of magnetite-apatite at Ferrífera (n = 3) and stage I metasomatic magnetite in the Mantoverde district (n = 3) are similar and close to 0 per mil. See text for details. Abbreviations: Ap = apatite, Cal = calcite, Chl = chlorite, Cpy = chalcopyrite, Mag = magnetite, Py = pyrite, Qtz = quartz, Ser = sericite.

Isotopic characterization of the hydrothermal fluids Estimation of the stable isotopic chemistry of the fluid requires knowledge of mineral and/or water isotope fractionation factors, temperature, and, for sulfur, oxidation state (Ohmoto and Rye, 1979; Ohmoto and Goldhaber, 1997). The parameters and information relevant to isotopic composition and paragenetic stage are recorded in Table 4. Stage I: The massive bodies of magnetite in the Mantoverde district are the product of high-temperature Fe metasomatism (e.g., 460°–550°C, see Table 1) and the coexistence of magnetite and pyrite suggests that oxygen fugacity was close to 10–18 bars (see fig. 10.7, Ohmoto and Rye, 1979). These conditions imply that fluids with δ34S values of +0.4 to +4.0 per mil were responsible for preore magnetite deposition, a range consistent with a magmatic origin (Ohmoto and Goldhaber, 1997; Table 4). For the magnetiteapatite-pyrite bodies along the eastern branch of the Atacama fault system (Fig. 3), calculations based on the oxygen isotope geothermometer for the pair apatite-magnetite (Table 3) suggest a formation temperature of ~650°C. The 0361-0128/98/000/000-00 $6.00

sulfur isotope composition of pyrite from Ferrífera, +0.6 to +1.7 per mil, similarly is consistent with the involvement of a magmatic fluid. Crosscutting relationships indicate that magnetite and hypogene hematite formed almost entirely at different paragenetic stages in the Mantoverde district (Fig. 10). Using oxygen isotope fractionation factors for magnetite-water (Bottinga and Javoy, 1973) and a temperature range of 460° to 550°C (Table 1), stage I magnetite is inferred to have equilibrated with a fluid with an oxygen isotope composition of +7.3 to +10 per mil. The whole-rock δ18O value of +12 per mil for a K-feldspathized host rock (i.e., sample 2464, drill hole 01DS13, 371 m: Figs. 3, 13B) and the oxygen isotope fractionation for the pair K-feldspar-water between 460° and 550°C (O’Neil and Taylor, 1967) indicate a fluid with δ18O value of about +10 per mil, indistinguishable from that in equilibrium with magnetite. Similarly, the massive bodies of magnetite-apatite-pyrite along the eastern branch of the Atacama fault system (e.g., Ferrífera: Fig. 3) are inferred to have equilibrated with a fluid with δ18O values of +8 to +9 per mil.

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5

FREQUENCY

4

3

2

1

0 -3

-2

-1

0

1

2

3

4

5

delta 18O

δ O (‰, SMOW) 18

LEGEND Stage III-Hematite (Mantoverde) Stage I- Magnetite (Mantoverde) Magnetite-apatite-pyrite (Ferrifera) FIG. 20. Frequency histogram of δ18O values of magnetite and hematite in the Mantoverde district (n = 20). The average value for magnetite is +3.5 per mil (n = 10), whereas hematite tends to have lower values, with an average of –0.6 per mil (n = 10). The isotopic composition of metasomatic magnetite (stage I, Figs. 11, 14) at Mantoverde is indistinguishable from that of massive bodies of magnetite-apatite ± pyrite located along the eastern branch of the Atacama fault system (i.e., Ferrífera, see Figs. 3, 15).

Stage II: The associations of muscovite-calcite ± hematite ± pyrite and chlorite-quartz-pyrite ± rutile ± hematite indicate that the hydrothermal fluids became more acidic in stage II, following cooling to ~350°C (see Table 1). The dominance of pyrite and the incipient to moderate development of hematite suggest that conditions fell between the hematite-magnetitepyrite and hematite-pyrite-FeSO4 oxygen buffers, with a ƒO2 of ~10–22.5 bars (Shi, 1992). The stage II fluids would therefore have had δ34S values of +9.1 to +14.9 per mil (Table 4; i.e., significantly heavier than those of fluids with a magmatic derivation: Ohmoto and Goldhaber, 1997). Additionally, the δ18O and δD values of stage II chlorite range widely from

+5.6 to +11.2 and –71 to –46 per mil, respectively (Table 3). The oxygen and hydrogen isotope fractionation factors defined for the pair chlorite-water (Graham et al., 1987; Cole and Ripley, 1998) indicate that at 350°C the fluid in equilibrium with stage II chlorite would have had δ18O and δD values of +5.7 to +11.2 and –38 to –13 per mil, respectively (Benavides, 2006). Such compositions indicate that the hydrothermal system experienced a significant incursion of nonmagmatic fluids during stage II (see Taylor, 1997). Stage III: Megascopic and, particularly, microscopic crosscutting relationships indicate that stage III pyrite and chalcopyrite were not coprecipitated, precluding temperature estimation on the basis of sulfur isotope fractionation (Ohmoto and Rye, 1979). However, the δ18O values of coexisting calcite and hematite (Table 1) define temperatures of ~230° to 250°C. The coexistence of hematite and pyrite indicates that at those temperatures, the oxygen fugacity would be between the hematite-magnetite-pyrite and hematite-pyrite-FeSO4 buffers, with calculated fO2 values of ~10–33 bars (Shi, 1992). On this basis and Ohmoto and Rye (1979), Cu-rich stage III assemblages with δ34S values of +1.4 to +11.2 per mil (Tables 2, 4) are inferred to record deposition of fluids with δ34S values ranging from +26.4 to as high as +36.2 per mil (Table 4). Chalcopyrite and pyrite in the matrix of stage III hematite-cemented breccias equilibrated with fluids having δ34S values of +26.4 to +35 and of +26.9 to +36.2 per mil, respectively (Table 4). The oxygen isotope fractionation for the pair hematite-water (Yapp, 1990) at 200° to 250°C indicates that stage III hematite equilibrated with a fluid with δ18O values in the range +3.0 to +8.0 per mil, overlapping with, but mainly lighter than, that of the fluid in equilibrium with stage I magnetite. Discussion and Conclusions Role of “exotic”sulfur The new light stable isotope data for the Mantoverde deposit and its satellites are directly germane to the ongoing debate concerning the genesis of central Andean Mesozoic iron oxide-copper-gold mineralization and, more generally, to the understanding of the IOCG clan globally. Despite the inherent uncertainties in both isotope fractionation factors and the temperature and oxygen fugacity ranges for the various paragenetic stages, the data are sufficiently robust to conclude

TABLE 4. δ34S Values of Sulfides from Paragenetic Stages I to III and Inferred Sulfur Isotopic Compositions of Ore Fluid Paragenetic stage association δ34Ssulfides Estimated depositional temperature1 fO2 indicator(s) fO2 in bars 2 ∆= δ34Ssulfide - δ34Sfluid δ34Sfluid

I (Mag-Py)

II (Chl-Ms-Py ± Hem)

III (Hem-Cpy-Py)

–0.6 to +2.0‰

–0.9 to +4.9‰

+ 1.4 to +11.2‰

460º to 550ºC Mag-Py 10–18 –1‰ to –2‰ + 0.4 to + 4‰

~350ºC Py-(Hem) 10–22.5 –10‰ + 9.1 to +14.9‰

~230º–250ºC Hem-Py 10–33 – 25‰ + 26.4 to +36.2‰

Notes: The isotopic compositions of the fluid are calculated according to the procedures of Ohmoto and Rye (1979) and Ohmoto and Goldhaber (1997); abbreviations: Py = Pyrite, Cpy = Chalcopyrite, Mag = Magnetite, Ms = Muscovite, Chl = Chlorite, Hem = Hematite, Cal = Calcite 1 For depositional temperature see Table 1 2 For stage I, based on Ohmoto and Rye (1979). For stages II and III, according to phase diagrams of Shi (1992) 0361-0128/98/000/000-00 $6.00

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that a major change in the isotopic chemistry, and hence predominant reservoir, occurred between stages I and II. Taken in conjunction with the observations of Ullrich and Clark (1999) and Ullrich et al. (2001) on the larger and higher grade La Candelaria deposit, and those of Ripley and Ohmoto (1977) on the small, high-grade, Raúl-Condestable deposit, the sulfur and oxygen isotope compositions for the different paragenetic stages in the Mantoverde district strongly imply that, at the least, exotic sulfur input was a prerequisite for economic, or even subeconomic, copper mineralization in the Andean IOCG centers. Moreover, the direct correlation between Cu and Au in these deposits implies that gold enrichment may also have been influenced by the incursion of such nonmagmatic fluids. The requirement for the introduction of sulfur from the wider exocontact environment is indeed a satisfactory explanation for the “fortuitous” involvement of externally derived brines in Cu-rich IOCG mineralization, in the central Andes and elsewhere (Sillitoe, 2003). The geochemical relationships documented in this study are consistent with the concepts of Barton and Johnson (1996, 2000), in that seawater, plausibly mediated through evaporitic processes, is the most likely reservoir for the high δ34S and low δ18O fluids implicated in the later stages of the Mantoverde and La Candelaria (Ullrich et al., 2001) hydrothermal systems. It should be emphasized that Cornejo et al. (2000) earlier proposed a fluid mixing model for the Mantoverde deposit but envisaged meteoric water rather than evaporite-sourced brines as the nonmagmatic component. However, the isotopic relationships preclude major meteoric water contributions, even in the terminal stages of hydrothermal activity. Whereas the involvement of sulfur-bearing fluids with high δ34S values (≤36.2‰ at Mantoverde; ≤20.2‰ at La Candelaria) is unambiguous in these two largest documented Andean IOCG deposits, there is no evidence that Cu and associated minor ore metals were similarly derived from either the postulated evaporitic reservoir or from andesitic host rocks. In this regard, the temporal relationships between initial chalcopyrite deposition and the introduction of “exotic” sulfur are critical. At Mantoverde, marked changes in δ34S fluid occurred during the chlorite-dominated stage II (Fig. 10, Table 4), and therefore all significant Cu mineralization took place from fluids with high δ34S values. In contrast, Ullrich and Clark (1999) showed that chalcopyrite deposition at La Candelaria was initiated by fluids with magmatic δ34S values of –0.4 to +5.7 per mil, preceding marked increases in δ34Sfluid values. Moreover, this “polymetallic” substage also involved the deposition of minor sphalerite and molybdenite. These relationships suggest that at Mantoverde the dominant ore metal reservoir may similarly have been magmatic. Regional setting of mineralization Sillitoe (2003) has emphasized that evaporitic successions are either absent or minimally exposed in the vicinity of most central Andean IOCG centers. This would constitute as a cogent objection to the involvement of evaporite-derived brines in the genesis of these deposits. However, the Chañarcillo Group is probably vestigially preserved in northern Chile as a result of erosion during inversion of the back-arc basins, and any evaporites also would have been eroded. In the case of 0361-0128/98/000/000-00 $6.00

Mantoverde, the widespread circulation of brines in the area now separating the remnants of the Neocomian basin and the Atacama fault system may be recorded by the numerous areas of strong scapolitization (Fig. 4) and, as argued previously, the marialitic scapolite deposited early in stage II in the Mantoverde deposit and its satellites may represent this event. Such district-scale or even regional Na-Cl metasomatism is characteristic of several Precambrian IOCG provinces (Frietsch et al., 1997) and has been ascribed to the large-scale dissolution of evaporites. Despite the relative youth of the Andean environment, erosion has certainly disguised the original distribution of metasomatic scapolite within and behind the Neocomian arc, and there is no clear evidence for the source and flow paths of the fluids. On the assumption, however, that the back-arc basin, when developing, would have extended farther west than the existing outcrops of the Punta del Cobre Formation and Chañarcillo Group, a fluid source along the basin margin is reasonable, possibly with subsidiary flow within the Atacama fault system. Because scapolite deposition occurred at the beginning of stage II, and therefore coincided with the initial input of 34S-enriched fluids, it is plausible that the relict distribution of scapolite in the wider district partially records the migration of the brines responsible for chalcopyrite deposition at Mantoverde and elsewhere. The age of Cu mineralization (stage III) along the Atacama fault system and in the vicinity of the Neocomian basin is poorly constrained. However, as documented herein, the ≤131.3 ± 1.4 Ma volcanic strata of the back-arc basin successively experienced spilitization and hydrolytic alteration and low-grade metamorphism (Fig. 4) prior to the formation of magnetite in the IOCG prospects of the Sierra Santo Domingo district (Fig. 7). If we assume that all IOCG centers in the district developed broadly simultaneously, the U-Pb date (126.4 ± 0.3 Ma) determined for stage I titanite from Mantoverde by Gelcich et al. (2002) implies that the subocean-floor alteration and ensuing metamorphism were accomplished in no more than ca. 7 m.y. and, perhaps, in considerably less time. The metamorphism, therefore, was probably diastathermal (i.e., extensional, see Robinson, 1987; Alt, 1999), rather than due to burial. This would require extreme heat flow during the extensional opening of the backarc basin, and residual heat expelled subsequently during initial basin closure could have promoted destruction of evaporitic strata and driven the scapolitizing brines westward. Despite the absence of evaporites in the remnant outliers of the Chañarcillo Group at this latitude, tentative correlations between events in the back-arc basin and hydrothermal activity along the Atacama fault system may be proposed. Concluding statement The first comprehensive documentation of the stable isotope chemistry of hydrothermal minerals from the IOCG deposits and prospects of the Mantoverde district, in the framework of a refined paragenetic model, provides unambiguous evidence that nonmagmatic sulfur played a critical role in the formation of the chalcopyrite protore. We propose that the incursion of brines with a major component of seawater, possibly mediated by evaporites, occurred after high-temperature Fe and K metasomatism by magmatic-hydrothermal

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fluids and at the outset of a period of intense hydrolytic alteration. Rather than representing merely a retrograde acidification of hydrothermal fluids, the chlorite-dominated stage II assemblages are therefore envisaged as a key link between the barren magnetite-rich mineralization and the stage III hematitic breccias and stockworks, which host all significant chalcopyrite. Mantoverde therefore adheres in fundamental respects to the genetic model advocated by Barton and Johnson (1996, 2000), as do La Candelaria (Ullrich and Clark, 1999; Ullrich et al., 2001) and Raúl-Condestable (Ripley and Ohmoto, 1977; de Haller et al., 2002, 2006). Whereas the ore metals in central Andean iron oxide-copper-gold deposits are probably magma derived (e.g., Ullrich and Clark, 1999; Sillitoe, 2003), an influx of sulfur from a nonmagmatic source was apparently essential for even subeconomic copper concentration in these hydrothermal centers. The paths followed by such regionally derived brines may be partially revealed by the district-scale distribution of scapolitic alteration. Acknowledgments This study is a component of the senior author’s Ph.D. research and constitutes a contribution to the Queen’s University Central Andean Metallogenic Project (Q CAMP). Field studies were generously facilitated by the geology staff of the Mantoverde division of Anglo American-Chile, which has authorized publication of the paper. Kerry Klassen, Queen’s Facility for Isotopic Research, is greatly thanked for her valuable support during isotopic analysis. Joan Charbonneau assisted with the preparation of the revised manuscript. Critical and constructive reviews by R.H. Sillitoe, J. Perelló, S.J. Matthews, and P.J. Pollard have considerably improved the manuscript, as did Mark Hannington, editor. This project has received generous financial and logistic support from AngloAmerican plc and was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), and the Ontario Innovation Trust (OIT). Anglo American has also contributed to the costs of color printing. December 2, 2005; April 9, 2007 REFERENCES Åberg, G., Aguirre, L., Levi, B., and Nyström, J.O., 1984, Spreading-subsidence and generation of ensialic marginal basins: An example from the Early Cretaceous of central Chile: Geological Society of London Special Publication 16, p. 185–193. Aguirre, L., Levi, B., and Nyström, J.O., 1989, The link between metamorphism, volcanism and geotectonic setting during the evolution of the Andes: Geological Society of London Special Publication 43, p. 223–232. Alt, J.C., 1999, Very low-grade hydrothermal metamorphism of basic igneous rocks, in Frey, M., and Robinson, D., eds., Low-grade metamorphism: Oxford, U.K., Blackwell Science, p. 169–201. Árkai, P., 1991, Chlorite crystallinity: An empirical approach and correlation with illite crystallinity, coal rank and mineral facies as exemplified by Palaeozoic and Mesozoic rocks of northeast Hungary: Journal of Metamorphic Geology, v. 9, p. 723–734. Astudillo, C., 2001, Distribución y Caracterización de Carbonatos en Mantoverde, Provincia de Chañaral, Tercera Región, Chile: Unpublished B.Sc. thesis, Antofagasta, Chile, Universidad Católica del Norte, 103 p. Barton, M.D., and Johnson, D.A., 1996, Evaporitic source model for igneous-related Fe oxide-(REE-Cu-Au-U) mineralization: Geology, v. 24, p. 259–262. ——2000, Alternative brine sources for Fe-oxide (-Cu-Au) systems: Implications for hydrothermal alteration and metals, in Porter, T.M., ed., 0361-0128/98/000/000-00 $6.00

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