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0361-0128/07/3662/415-26 415
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 super-gene 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). Thegeologic setting and genesis of this productive IOCG district are clarified herein through regional petrologic andlithogeochemical study and light stable isotope analysis of paragenetically constrained samples from Mantoverdeand its satellite deposits. Together with chalcopyrite-bearing, but subeconomic, bodies of metasomatic mag-
netite (e.g., Montecristo and Franco) and Cu-barren magnetite-fluorapatite-pyrite bodies (e.g., Ferrífera), thedeposits 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 theLa Negra Formation and diorites and monzodiorites assigned to the Lower Cretaceous Sierra Dieciocho plu-tonic complex. Prior to mineralization, the Jurassic and Neocomian igneous rocks of this Andean transect weresubjected to moderate albitization (spilitization) and hydrolytic alteration and, subsequently, to regional, nonde-formational metamorphism, which locally attained the lower greenschist facies. Both processes, however, werefocused 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 vol-canic 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, wasprobably contemporaneous with regional scapolitization in the area between the Atacama fault system and themarginal basin. Chalcopyrite deposition was restricted to the ensuing stage III, hosted by calcite veins and, par-
ticularly, specular hematite-dominated hydrothermal breccias and stockworks. Stage IV barren calcite-quartz vein swarms record the terminal hydrothermal activity. Stable isotope fractionation relationships and publishedfluid 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 rangeoverall 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 permil, of that deposited in stage II. The compositional variability increases from +1.4 to –11.2 per mil in the min-eralized assemblages of stage III, chalcopyrite generally having higher values than pyrite. The iron oxides inthe district have δ18O values that vary overall from –1.9 to +4.1 per mil, the highest values, +1.4 to +4.1 permil, occurring in stage I metasomatic magnetite, whereas stage III hematite has lower values of –2.0 to +1.7per mil. Estimated equilibrium δ34Sfluid values increased dramatically with time, from +0.4 to +4 per mil instage I, through +9.1 to +14.9 per mil during stage II, to +26.4 to +36.2 per mil for the most richly mineral-ized 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 0and +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, stageIII, in which all significant chalcopyrite and gold were deposited, are interpreted as evidence for the incursionof 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 havebeen 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.
©2007 Society of Economic Geologists, Inc.Economic Geology, v. 102, pp. 415–440
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Introduction
THE CENTRAL Andean orogen provides a unique context forthe clarification of the genetic factors responsible for the eco-nomic concentration of copper sulfides and gold in deposits of the iron oxide-copper-gold (IOCG) clan. The majority of suchsystems in Chile and Perú are of Mesozoic age, and the geo-logic 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 Precam-brian IOCG provinces. Moreover, the deposits embrace theentire spectrum of IOCG composition, from centers with neg-ligible Cu and Au (e.g., the magnetite deposits of the Chileaniron belt, such as El Romeral: Bookstrom, 1977), throughlarge magnetite deposits with proportionately minor associ-ated 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 associatedFe oxide mineralization is uneconomic (e.g., La Candelaria:Ryan et al., 1995; Mantoverde: Vila et al., 1996).
A fundamental uncertainty in the evolving genetic modelfor IOCG mineralization is whether it is generated by metal-
bearing brines exsolved from crystallizing granitoid magmas,and therefore controlled primarily by melt-aqueous fluidequilibria, or alternatively, whether the intervention of non-magmatic waters is a prerequisite for chalcopyrite and goldenrichment (Williams et al., 2005, and references therein).Numerous authors (e.g., Sillitoe, 2003; Sillitoe and Perelló,2005) interpreted the geological and geochemical relation-ships of the IOCG deposits of northern Chile in entirely mag-matic-hydrothermal terms and argued that their characteris-tic metal association (i.e., Cu, Au, Co, Ni, As, Mo, and U)reflects the basic, dioritic to gabbroic, nature of the inferredparental magmas. Pollard (2006) proposed that IOCG sys-tems in the Andes and elsewhere differ from porphyry copperdeposits in that vapor saturation in parental magmas occurred
at higher pressures, owing to the abundance of CO2, and thatthe evolution of the hydrothermal fluids was controlled by un-mixing of the carbonic phase.
In contrast, Ullrich and Clark (1999) and Ullrich et al.(2001) concluded that temporal changes in the sulfur and oxy-gen isotope compositions of the hydrothermal fluids at LaCandelaria resulted from incursion of water from contiguousevaporitic strata of the Chañarcillo Group during emplace-ment of the chalcopyrite-gold ore. Their findings were consis-tent with the conclusion of Barton and Johnson (1996, 2000)that many salient features of IOCG deposits are difficult toreconcile with straightforward magmatic-hydrothermal mod-els. 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 Menugeet al. (2002) recorded late-stage, saline, oxidized fluids of pos-sible evaporite origin in the Pea Ridge magnetite-hematite de-posit in Missouri. However, Marschik and Fontboté (2001a)discounted the stable isotope data presented by Ullrich andClark (1999), a decision later supported by Pollard (2006).
In this study, we document sulfur and oxygen isotope datafor representative, paragenetically constrained, samples fromthe Mantoverde mining district, located in the CoastalCordillera, III Región, northern Chile (26°30'40"–26°36'03"S, 70°17'39"–70°20'05" W; Figs. 1, 2). The Coastal Cordillera
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TOCOPILLA
SYMBOLS
IQUIQUE
ANTOFAGASTA
TALTAL
C H I L E
C H
I L E
A R G E N T I N A
A R G E
N T I N A
VALLENAR
LA SERENA
Julia (Cu)
Santo Domingo Cu-(Ag)
Mantoverde Cu-(Au)
Naguayan Cu-(Au)
Cerro Negro Cu-(Au)
Punta del Cobre (Cu-Au) Candelaria (Cu-Au)
Los Colorados (Fe)
Algarrobo (Fe)
Los Cristales (Fe)
Romeral (Fe)
Buena Esperanza (Cu-Ag)
Fe oxide Cu-Au deposits
Volcanic-hosted Cu-(Ag) dep.
Magnetite-apatite deposits
Fault
Town/City
Cu-Ag Mantos de Luna
Mantos Blancos (Cu-Ag)
Michilla District Cu-(Ag)
P A C I F I C
O C E A N
A F S
A F S
A F S
A
F S
CHANARAL
COPIAPO
Fig. 2
Boqueron Chanar (Fe)
72
22 S
24 S
26 SO
O
O
O O O
O
O
28 S
30 S
70 68
Santiago
FIG. 1
CHILE
SOUTH AMERICA
ARGENTINA
P A C I F I C
O C E
A N
A T L A N T I C
O C E A N
0 100 km
SCALE
FIG. 1. Location map of the Mantoverde district. Numerous Fe oxide (i.e.,magnetite-apatite), Fe oxide-copper-gold, and volcanic-hosted, strata-boundCu (Ag) deposits, located between latitudes 22° and 27° S, are controlled by the main or subsidiary structures in the Atacama fault system. Area of Figure2 is also shown. After Sillitoe and Perelló (2005).
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between latitudes 22° and 27° S hosts numerous Fe oxide(e.g., magnetite-apatite), Fe oxide-Cu-(Au), and volcanic-hosted strata-bound Cu-(Ag) deposits and constitutes a dis-tinctive metallogenic subprovince of the Central Andes (Ruizet al., 1965; Boric et al., 1990; Sillitoe, 2003, and referencestherein; Fig. 1). The Mantoverde district is one of only threeAndean IOCG camps (including the La Candelaria-Punta del
Cobre district in Chile and Raúl-Condestable in Perú), whichhave supported significant Cu production in recent years.This research is a part of a regional study of a 3,500-km2 areasurrounding Mantoverde (Benavides, 2006), with the objec-tive 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), op-erated by Anglo American plc. The district has measured re-serves 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. As-tudillo, pers. commun., 2005). In the Mantoverde deposit it-
self the geologic resource of hypogene protore is 400 Mt at0.52 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-alka-line volcanoplutonic arc terrane of Mesozoic age (Dallmeyeret al., 1996; Grocott and Taylor, 2002), hosted by Devonian toCarboniferous metasedimentary strata (Lara and Godoy,1998; Fig. 2) and Permo-Triassic plutonic and volcaniclasticrocks (Brown, 1991; Lara and Godoy, 1998). Both the arc andbasement are transected by the regionally extensive Atacamafault system and widely covered by Neogene to Quaternary alluvial and coluvial deposits (Fig. 2).
The most voluminous volcanism in the wider Mantoverdearea has been assigned to the La Negra and Punta del CobreFormations (Fig. 2). The former is a Middle to Upper Juras-sic succession (García, 1967) of basaltic andesitic to andesiticlava flows with subordinate volcaniclastic and marine sedi-mentary units (Lara and Godoy, 1998; Vivallo and Henríquez,1998). This formation constitutes either fault-bounded blocksseparating the central and eastern branches of the Atacamafault system or roof pendants in Neocomian plutons (Fig. 2).The younger Punta del Cobre Formation comprises a thickpackage of andesitic flows with intercalations of tuffs, tuffa-ceous sandstones, and welded tuffs, and thin beds of lithicarenites and limestones (Lara and Godoy, 1998). A Neoco-mian U-Pb zircon date of 131.3 ± 1.4 Ma (i.e., Valangin-
ian/Hauterivian boundary age) has been reported by Pop etal. (2000) for a near-basal andesitic member of the type sec-tion in the La Candelaria-Punta del Cobre mining district,~120 km south of Mantoverde. In the study area, the Puntadel Cobre Formation is restricted to the eastern part of thedistrict (Fig. 2), where it concordantly overlies the La NegraFormation and exhibits gradational contacts with the sedi-mentary Chañarcillo Group (Segerstrom and Parker, 1959;Lara and Godoy, 1998). Exposures of the latter, preservedeast of the Mantoverde mining district proper (Fig. 2), com-prise mudstones, calcareous sandstones and siltstones, chert
and fossiliferous limestones with intercalations of tuffs andconglomerates (Lara and Godoy, 1998), recording marinesedimentation in a marginal back-arc basin. Naranjo (1978)reported a Valanginian age (i.e., 132–137 Ma) for limestonesin the Sierra Santo Domingo district area, 30 km north-north-east of the Mantoverde mining district (Fig. 2). Faunal as-semblages documented by Moraga (1977) indicate a range of
ages from Berriasian to Barremian (i.e., ca. 121–144 Ma) forChañarcillo Group strata cropping out 60 km southeast of theMantoverde district (location not shown in Fig. 2).
In the wider Mantoverde district (Fig. 2), the intrusiverocks 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-Pbzircon dates (Berg and Breitkreuz, 1983) and, more exten-sively, 40Ar/ 39Ar hornblende age spectra (Dallmeyer et al.,1996), Lara and Godoy (1998) delimit the following pre-Al-bian intrusive complexes (Fig. 2): Flamenco monzogranites,granodiorites, and tonalites at 190 to 200 Ma; Las Animas py-roxene quartz diorites at 150 to 160 Ma; Cerro Moraditohornblende quartz diorites and hornblende granodiorites at
140 to 145 Ma; Cerro Morado quartz monzodiorites and gra-nodiorites with hornblende, biotite, and pyroxene at 130 to135 Ma; Las Tazas biotite-hornblende granodiorites at 125 to130 Ma; Sierra Dieciocho hornblende-biotite quartz dioritesat 120 to 126 Ma. Although the main intrusive focus was dis-placed eastward from the Early Jurassic to the mid-Creta-ceous, there was considerable areal overlap throughout theCretaceous (Fig. 2). The granitoid rocks are I-type (Chappelland 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, theplutonic 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 thenorth-south Atacama fault system (Fig. 2), which extends formore than 1,000 km from Iquique to La Serena (Naranjo,1987; Thiele and Pincheira, 1987; Brown et al., 1993, and ref-erences therein). Initiated in the Early Jurassic, this arc-par-allel structure (Scheuber and Andriessen, 1990; Brown et al.,1993) records a complex kinematic evolution, but dip-slip andleft-lateral strike-slip displacements predominated during theEarly Cretaceous (Brown et al., 1993, and references therein;Dallmeyer et al., 1996). The Atacama fault system controlledboth the emplacement of the Upper Jurassic and Lower Cre-taceous plutons (Grocott et al., 1994; Wilson and Grocott,
2001; Grocott and Taylor, 2002) and the development of ironoxide-copper-gold deposits, including Mantoverde (Fig. 2)and the sulfide-poor magnetite deposits of the Chilean ironbelt (Espinoza, 1990; Grocott and Taylor, 2002). The Chivatofault in the southeast part of the area (Fig. 2) exhibits a re- verse displacement, with a dominant northeast to north-northeast strike and northwest-directed tectonic transportthat translated the La Negra Formation over the Punta delCobre Formation (Fig. 2; Lara and Godoy, 1998). During thelate Neocomian, this structure behaved as a ductile shearzone, with both dip- and strike-slip displacements, that
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FLUIDS IN THE MANTOVERDE IOCG DISTRICT, III REGION, CHILE 419
0361-0128/98/000/000-00 $6.00 419
26 3 6’03”S 7020 ’
05”W
7017’39”W
26 30’40”S
N
7 . 0 6 5 . 0 0 0
N
7 . 0 6 4 . 0 0 0
N
7 . 0 6 3 . 0 0 0
N
7 . 0 6 �
