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Evolution of the Giant Marcona-Mina Justa Iron Oxide-Copper-Gold District, South-Central Peru

HUAYONG CHEN,1,†,* ALAN H. CLARK,1 T. KURTIS KYSER,1 THOMAS D. ULLRICH,2 ROBERT BAXTER,3,** YUMING CHEN 4,‡ AND TIMOTHY C. MOODY 5,‡‡

1 Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6 2 Pacific Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences,

University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z43 Chariot Resources Limited, Avenida Benavides N° 1180, Miraflores, Lima 18, Perú

4 Shougang Hierro Perú S.A.A, Avenida República de Chile 262, Jesús Maria, Lima 11, Perú5 Rio Tinto Mining and Exploration, Manco Capac 551, Miraflores, Lima 18, Perú

AbstractThe Mesozoic iron oxide-copper-gold (IOCG) subprovince of littoral south-central Perú, centered at latitude

15°11' S, longitude 75°6' W, incorporates Marcona, the preeminent central Andean iron oxide deposit (1.9 Gt@ 55.4% Fe), and Mina Justa, one of the few major Andean IOCG deposits with economic copper grades(346.6 Mt @ 0.71% Cu). The emplacement of magnetite orebodies with uneconomic Cu grades (avg 0.12%) atMarcona was controlled by northeast-striking faults transecting an active andesitic-dacitic, shallow-marineMiddle Jurassic (Aalenian to Oxfordian) arc. In contrast, hypogene Cu sulfide (~15 g/t Ag, 0.12 g/t Au) miner-alization at Mina Justa was emplaced along reactivated listric-normal detachment faults during the mid-Creta-ceous inversion of the contiguous, plate boundary-parallel, Aptian to Albian Cañete basin, accompanied by theearliest, largely granodioritic-dioritic, stocks of the Coastal batholith. Alteration and mineralization assem-blages, supported by 40Ar/39Ar geochronology of biotite, phlogopite, actinolite, cummingtonite, and K-feldspars,reveal a history of magmatic and hydrothermal processes extending episodically for at least 80 m.y., from ca.177 to 95 Ma, wherein metal-rich mineralization events were preceded and separated by episodes of barren alteration.

At Marcona, precursor, subocean-floor hydrothermal activity in the Aalenian (177 Ma) and Bajocian (171Ma) generated, respectively, cummingtonite and phlogopite-magnetite assemblages through high-temperatureMg-Fe metasomatism of previously metamorphosed Lower Paleozoic Marcona Formation siliciclastic rocksand minor carbonate units underlying the nascent Río Grande Formation arc. Subsequent areally widespread,albite-marialite alteration (Na-Cl metasomatism) largely predated but overlapped with the emplacement of anen echelon swarm of massive magnetite orebodies, in turn overprinted by subordinate magnetite-sulfide assemblages. Magnetite and weak Cu and Zn sulfide mineralization coincided with a 156 to 162 Ma episode ofandesitic eruption and dacitic intrusion which terminated the growth of the arc, but was hosted largely byquartz-rich metaclastic rocks. From 162 to 159 Ma, iron oxide mineralization evolved from magnetite-biotite-calcic amphibole ± phlogopite ± fluorapatite to magnetite-phlogopite-calcic amphibole-pyrrhotite-pyrite assemblages. These were overprinted at 156 to 159 Ma by chalcopyrite-pyrite-calcite ± pyrrhotite ± sphalerite± galena assemblages, locally resulting in grades of 0.45 percent Cu and 0.5 percent Zn.

Hydrothermal activity was thereafter focused in the Mina Justa area, 3 to 4 km to the northeast of Marcona,where Middle Jurassic andesites experienced intense albite-actinolite alteration at ca. 157 Ma, i.e., contempo-raneous with sulfide mineralization at Marcona, and magnetite-microcline alteration (K-Fe metasomatism) atca. 142 Ma. Development of the Mina Justa Cu (-Ag) deposit proper, however, began much later, with, suc-cessively, actinolitization at ca. 109 Ma, the deposition of calcite and specular hematite, now entirely pseudo-morphed by magnetite, and the metasomatic emplacement of bodies of barren, massive magnetite and pyriteat 101 to 104 Ma. Finally, at 95 to 99 Ma, chalcopyrite-bornite-digenite-chalcocite mineralization, with abun-dant calcite and hematite, was emplaced as two ~400-m-long, ~200-m-wide, gently dipping, tabular arrays ofbreccia and stockwork, cored by preexisting magnetite-pyrite lenses. Supergene oxidation generated a chryso-colla-atacamite-covellite blanket, hosting ~40 percent of the Cu reserve, prior to the eruption of a 9.13 ± 0.25Ma rhyodacitic ignimbrite flow.

Although areally contiguous, the major magnetite and copper-rich centers of the Marcona district record independent metallogenic episodes widely separated in age. Further, whereas the Cu-poor magnetite miner-alization at Marcona was integral to the terminal eruptions of the Middle Jurassic arc, representing a shallow-marine analog of the Pliocene El Laco magnetite deposits of northern Chile, the Mina Justa Cu sulfide

† Corresponding author: e-mail, huayong.chen@utas.edu.au*Present address: CODES, University of Tasmania, Private Bag 126, Hobart, TAS, 7001, Australia.**Present address: Norsemont Mining, 507-700 West Pender St., Vancouver, British Columbia, Canada V6C 1G8.‡Present address: Development & Research Center, China Geological Survey, 45 Fuwai Street, Xicheng District, Beijing, P.R. China, 100037. ‡‡Present address: Rio Tinto plc, 6 St. James’s Square, London, United Kingdom SW1Y 4LD.

©2010 Society of Economic Geologists, Inc.Economic Geology, v. 105, pp. 155–185

Submitted: May 1, 2009Accepted: December 23, 2009

Introduction

IRON OXIDE-COPPER-GOLD (IOCG) mineralization, first for-mally defined by Hitzman et al. (1992), has been a major ex-ploration target since the discovery of the enormous OlympicDam Cu-U-Au (-REE) deposit in 1975. Although most earlyidentified IOCG systems, e.g., those of the Gawler craton ofSouth Australia, the eastern Mount Isa inlier of Queensland,and the northern Fennoscandian Shield, are of Proterozoicage, the central Andean orogen, and especially the vol-canoplutonic arcs of Jurassic and Cretaceous age exposed inthe Cordillera de la Costa of northern Chile and central andsouthern Peru, are now recognized as hosting major IOCGmineralization (Fig. 1). The well-defined tectonomagmatic

environment in this region provides an ideal context for clar-ification of the genesis and metallogenic relationships of thisproblematic class of mineralization (Sillitoe, 2003). Marschikand Fontboté (2001), de Haller et al. (2006), and Sillitoe(2003) interpreted central Andean IOCG deposits on thebasis of magmatic-hydrothermal models, although Sillitoeemphasized their distinction from the magnetite-rich por-phyry Cu-Au group. In contrast, the incursion of “exotic,” inpart evaporite-sourced, brines has been argued to be essentialto economic Cu (-Au) mineralization, and the involvement ofsuch nonmagmatic fluids has been confirmed in Raúl-Con-destable (Ripley and Ohmoto, 1977; de Haller and Fontboté,2009), La Candelaria (Ullrich and Clark, 1999; Ullrich et al.,2001) and Mantoverde (Benavides et al., 2007). A radically

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orebodies—like the other economic, mid-Cretaceous, Cu-rich IOCG deposits of the central Andes, e.g., Can-delaria-Punta del Cobre, Mantoverde, and Rául-Condestable—was the product of brines released during theinversion of back-arc volcanosedimentary basins. The latter environment recurred episodically in the MesozoicAndes, as in comparable orogenic settings elsewhere, and extended histories of hydrothermal alteration andmineralization, incorporating numerous barren events, may therefore represent a salient feature of the IOCGdeposit clan.

12 o

14 o

77 o 75 o

14 o

12 o

Marcona

Pampa dePongo

Lima

Monterrosas

Eliana

Raul-Condestable

Neogene to QuaternarySediments

CenozoicSubaerial Volcanics

MesozoicCoastal Batholith

Mina JustaMesozoic Basinal Volcanicsand Sediments

PaleozoicSan Nicolas Batholith

Precambrian (includes some Paleozoic)Metamorphic Basement

Bolivia

Argentina

N

ElianaMonterrosas

MarconaPampa de Pongo

Mantos Blancos

Candelaria-Punta del Cobre

El Espino

Mina Justa

Pacific

Ocean

Chile

Peru

20S

o

70 Wo

A BB

Pacific

Ocean

Fig 2

El Laco

Raul-Condestable

LiconaSantiagoValpara so

Cerro Pelado

El Algarrobo

30S

o

Cristales

El Romeral

Los loradosBoqueron Chanar

s

ntoverde

Cerro Negro

Chilean iron belt

TocopillaGuanillosGatico

Maguayan

Montecristo-Julia

Carrizalillo de Las BombasTeresa de Colmo

El SaladoLas Animas

Rosa Maria

Galleguillos DulcineaOjancos Nuevo

FarolaCarrizal AltoQuebradita

La HigueraBrillador San Antonio

Tamaya PanulcilloLos Mantos de Punitaqui

Small IOCG Deposits

Large, Cu-rich IOCG Deposits(> 30 Mt; Cu grade> 0.5% )Iron Oxide Deposits

200 km

s

50 km

N

IOCG Deposits

Acari

Cobrepampa

Argentina

Canete

basin

San Juan

Manto-type Deposits

El Soldado

Talcuna

Santo Domingo

San Domingo Sur

Cobrepampa

Amolanas

Productora

Acari

Morritos

Co

Ma

FIG. 1. (A) Locations of Cu-rich IOCG deposits, principal iron deposits, and manto-type Cu-Ag deposits in Peru and Chile(from Clark et al., 1990; Hawkes et al., 2002; Maksaev and Zentilli, 2002; Oyarzún et al., 2003; Sillitoe, 2003; and Benavideset al., 2007). (B) Simplified geologic map of the IOCG mineralization belt of south-central Peru (modified from Vidal et al.,1990), illustrating the extent of the mid-Cretaceous Cañete intra-arc extensional basin (Atherton and Aguirre, 1992).

different perspective on the genesis of cental Andean IOCGmineralization is provided by the proposal that the majority ofmagnetite-dominated, so-called “Kiruna-type” (Geijer, 1931)deposits are the product of silica-poor, iron oxide-rich melts(e.g., Nyström and Henríquez, 1994; Naslund et al., 2002;Henríquez et al., 2003), although such deposits have been re-cently divorced from Cu-rich IOCG systems (Williams et al.,2005).

Our purpose herein is to contribute to these argumentsthrough documentation of the Marcona district of littoralsouth-central Peru, which juxtaposes major IOCG-style oredeposits with widely variable proportions of iron oxides andcopper sulfides. Marcona itself, representing much thelargest-known concentration of high-grade magnetite ore inthe central Andes, is centered in Nazca Province, Ica Depart-ment, at latitude 15°12' S, longitude 75°7' W (Figs. 1, 2), 10to 15 km from the Pacific coast and below 800 m a.s.l. Hostedby Paleozoic metasedimentary and Jurassic andesitic and sed-imentary strata, and with present reserves of 1,551 Mt grad-ing 55.4 percent Fe and 0.12 percent Cu (Shougang HierroPerú SA., Resource Estimate of the Marcona iron mine,unpub. report, 2003, in Chinese). The Mina Justa Cu-(Ag)prospect, 3 to 4 km northeast of the Marcona mine (Fig. 2) atlatitude 15°10' S, longitude 75°5" W and an altitude of 785 to810 m a.s.l., has an indicated open pit resource of 346.6 Mt atan average grade of 0.71 percent Cu, 3.8 g/t Ag and ~ 0.03 g/tAu at a cutoff grade of 0.3 percent Cu, and an inferred re-source of 127.9 Mt at 0.6 percent Cu (Mining Journal, Nov.

24, 2006, p. 8). The district includes (Figs. 1, 2) several ap-parently less important Cu prospects as well as a second giantmagnetite deposit, Pampa de Pongo, located 30 km southeastof Marcona-Mina Justa (Fig. 2; Hawkes et al., 2002) and withan inferred resource of 953 Mt grading 44 percent Fe(Cardero Resource Corp., news release, September 6, 2005).In addition, numerous magnetite and/or hematite-rich de-posits, some rich in Cu and Au and including the small AcaríHierro magnetite vein and the formerly productive La Ar-gentina Cu vein swarm, are hosted by dioritic-to-monzo-granitic plutons of the mid-Cretaceous Coastal batholith inthe Acarí-Cobrepampa district (Fig. 2; Caldas, 1978; Injoque,1985). This paper documents the geology and evolution of theMarcona magnetite and Mina Justa Cu (-Ag, Au) deposits.Complementary studies (Chen, 2008), to be reported else-where, assess the evidence for a melt origin for the Marconamagnetite orebodies and apply light stable isotope geochem-istry and fluid inclusion microthermometry and chemistry tothe identification of fluid sources.

Regional and District Geological SettingThe subdued coastal cordillera of south-central Perú (Fig.

1B) exposes remnants of a succession of volcanoplutonic arcswhich regionally range in age from latest Triassic to Holocene,evidence for a protracted but episodic history of suprasub-duction zone magmatism along the convergent margin of theSouth American plate. However, the Andean magmaticrecord in the immediate Marcona area (Fig. 2) is dominated

EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 157

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15 00 So,

15 30 So,

15 15 So,

75 30 Wo, 74 30o,75 00o,

5 km

N

Neogene to Quaternary gravels

Coastal Batholith(80- < 109 Ma)

Tunga Andesite/Bella Union Volcanics (Albian to Upper Cretaceous)Mesozoic Formations(Rio Grande; Jahuay; Yauca; Copara)

San Nicolas Batholith(425 4Ma)

Marcona Formation

Cenozoic Formations(Pisco, Millo and Sencca)

Precambrian units(San Juan, Chiquerio Fm.and Arequipa Massif)Main Marcona Orebodies

MarconaMarcona

Hierro AcariHierro Acari

CobrepampaCobrepampa

Pampa dePongoPampa dePongo

Mina JustaMina Justa

Pacific

Ocean

Faults

San JuanSan Juan

San NicolasSan Nicolas

Towns

Active minesinactive mines(including prospects)

A Stratigraphic columnsin figure 4

B

C

Argent inaArgent ina

Treinta Libra

s Fault

Lechuza Fault

Tunga

Fault

Pampa Las Galgas

Pampa de Poroma

Pampa El Choclon

Pampa de PongoPampa Colorado

Pampa Pajayuna

Pampa Lagunal

Pampa LagunalGrande

Rio Grande CanyonRio Grande CanyonA

5km

FIG. 2. Geology of the Marcona-Mina Justa district (modified from Caldas, 1978; Hawkes et al., 2002; and Chew et al.,2007).

by Middle Jurassic volcanosedimentary and hypabyssal unitsand by mid-Cretaceous granitoid plutons. Stratigraphic rela-tionships in the wider Marcona area, incorporating data fromCaldas (1978), Vidal et al. (1990), Hawkes et al. (2002) andthis study, are summarized in Figure 3.

The discontinuous Peruvian IOCG belt (Fig. 1) is under-lain by high-grade metamorphic rocks of the allochthonousPaleoproterozoic-to-Mesoproterozoic Arequipa Massif (Waste -neys et al., 1995; Loewy et al., 2004), comprising schists,gneisses, granites, and migmatites cut by basic and pegmatitic

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Ce

no

zoic

Me

sozo

icP

re-M

eso

zoic

Era Series Formation Li thology IntrusiveQuaternary1,2

SenccaFormation2

Miocene-Pliocene Millo

Formation2

MiocenePisco

Formation2

1 2- including Miocene and Pliocene sediments; - the estimated thicknesses for the Quaternary, Sencca, Millo, Pisco and Chiquerioformations are: 350 m, 50m, 3m, 500 m, and 100-800 m, respectively (not to scale in this column).

Aptian tolower Albian(110-125 Ma)(Fossi l age range)

CoparaFormation(1000 m)

Neocomian(125-146 Ma)(Fossi l age range)

YaucaFormation(1500 m)

Kimmeridgianto Tithonian(146 -155 Ma)(Fossi l age range)

JahuayFormation(1000 m)

Callovianto Oxfordian

(155-164 Ma)(Fossi l, K-Ar ages)

Rio

Gra

nd

e Fo

rma

ti on

(4000m

)

LowerPaleozoic

( > 425 Ma)

MarconaFormation(1500 m)

NeoproterozoicSan Juan

Formation(3000 m)

ChiquerioFormation 2

Paleo-to-Mesoproterozoic

940, 1200 and1820 Ma

(metamorphism,U-Pb age)

ArequipaMassif

Mina JustaCu orebodies

MarconaFe orebodies

Widespread marine terraces, aeolian sands; alluvium

White to rose colored tuffs of dacitic to rhyoliticcomposition

Loosely consolidated marine sandstonesand conglomerates

Thick conglomerates, yellow and reddish sandstones,shales, bentonite beds, fine-grained volcaniclastics

Conglomerates, with mainly volcanic fragments, feldspathicsandstones, violet graywackes, red shales, minor tuffs, lavaflows and limestones with chert nodules

Shales, mudstones and sandstones

Agglomerates, brecciated lava flows,conglomerates and sandstones, quartzites, shales andlimestones. Sills with compositions similar to thelava flows

Conglomerates, dolomitic marbles, siltstones, sandstones,silicified limestones with chert laminations and quartz layersHornblende and pyroxene metamorphism

Dolomitic marbles and chloritic schists

Base: calcareous schists, dolomitic marbles, calcareousmarls and turbidites

Top: dolomitic marbles and chloritic schistsCentral: Pelitic rocks

Tillites with dolomites near top

Gneisses, granites, migmatites and schists cut by multiple-stage basic and pegmatitic dikes

?

Dacite

San NicolasBatholith

(425 4Ma)

Aalenian toBajocian

(166-179 Ma)(Fossi l age range)

UpperRio GrandeFormation

LowerRio GrandeFormation

(CerritosFormationat Marcona)

Porphyritic, partly pillowed, K-rich, calc-alkaline andesites. Minorintercalations of reddish conglomerates, and brick-red, cross-laminated, volcanogenic sandstones

Red conglomerates, conglomeratic sandstones, and fine- to medium-grained, red volcanogenic sandstones intercalatic with ignimbrites,foss iliferous limestones, calcareous sandstones, and greenish tuffs(A meta-volcanic breccia base is present in the Marcona area)

MarconaFe orebodies

unconformity

unconformity

unconformity

unconformity

unconformity

unconformity

unconformity

?

CoastalBatholith

(80- < 109 Ma) TungaAndesite and

Bella Union complex(Albian to Upper Cretaceous)

Neoproterozoic

FIG. 3. Summarized stratigraphic column for the Marcona-Mina Justa district (modified after Caldas, 1978; Injoque,1985; Hawkes et al., 2002, and Loewy et al., 2004).

dikes. This basement complex is unconformably overlain byNeoproterozoic and Paleozoic sedimentary strata and, moreextensively, volcanic and sedimentary rocks of Mesozoic age(Fig. 2; Caldas, 1978; Hawkes et al., 2002). The ~1,500-m-thick metasedimentary Marcona Formation, which hosts themajority of the economic magnetite orebodies at Marcona, isdominated by quartz-rich siltstones and sandstones, interca-lated with minor quartz arenites and impure limestones anddolostones (Atchley, 1956; Injoque, 1985). It is intruded andmetamorphosed by the post-kinematic, 425 ± 4 Ma (Mukasaand Henry, 1990; Vidal et al., 1990), San Nicolás granitoidbatholith (Fig. 2), and is therefore at least Early Silurian inage. Where unaffected by hydrothermal alteration, metaclas-tic and metacarbonate members in the mine area widely ex-hibit, respectively, hornblende hornfels cordierite + biotite ±muscovite and tremolite ± quartz assemblages, but diopsideand forsterite porphyroblasts record the local attainment ofthe pyroxene hornfels metamorphic facies.

The Jurassic and Cretaceous strata of the wider Marconaarea are subdivided (Figs. 2, 3), in decreasing age, into the

Río Grande, Jahuay, Yauca, and Copara formations (Caldas,1978). The ages of the three older formations are well estab-lished on faunal grounds, but those of the Copara Formationand the dominantly hypabyssal andesitic-dacitic Bella Unióncomplex which intrudes it, as well as the post-Yauca Forma-tion hypabyssal Tunga Andesite, are poorly defined (Caldas,1978). The Río Grande Formation hosts the Mina Justa de-posit and several orebodies of the Marcona mine (Injoque,1985; Hawkes et al., 2002; Moody et al., 2003). The type sec-tion of this ~ 3,000- to 4,000-m-thick, generally northeast-striking and northwest-dipping (45°–60°) succession is ex-posed in the Monte Grande area in the Cañón Río Grande,northwest of Marcona (Fig. 2; Rüegg, 1956, 1961). It incor-porates (Fig. 4) a 500-m lower member made up of apolymictic basal conglomerate overlain successively by mud-stones, sandstones, limestones, rhyolitic to andesitic breccias,and rhyolitic to andesitic flows (Romeuf et al., 1993). This as-sociation is itself overlain by at least 2,000 m of gently foldedred sandstones, shales, limestones, and brecciated andesiticflows with high K calc-alkaline-to-shoshonitic compositions

EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 159

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? ?

Canon Rio Grande(Aguirre, 1988; Ruegg, 1956)

Marcona area(modified from Injoque, 1985,and Atchley, 1956)

Pampa de Pongo(Caldas, 1978)

200m

300m

No

ttos

ca

lein

this c

olu

mn

Arequipa Massif Marcona Formation Sandstone Conglomerate

Limestone Tuff Andesite VolcanicBreccias

Magnetiteorebodies

Inferredboundary

Inferredunconformity

Low

er Ri o

Gra

nd

e Fo

rma

t ion

Up

pe

r Rio

Gr a

nd

e Fo

rma

tion

Low

er Rio

Gra

nd

e Fo

rma

tion

Ma

rco

na

Form

atio

nUpper

Rio GrandeFormation

Up

pe

r Rio

Gr a

nd

e Fo

rma

tion

Callovian-Oxfordian151- 161 Ma

(Not to scale)

?

Mina JustaCu-orebodies

Aalenian-Bajocian166-178 Ma

LowerRio GrandeFormation

K-Ar age

164 Ma

A B C

s

FIG. 4. Schematic stratigraphic columns of the Río Grande Formation in the Cañón Río Grande, Marcona, and Pampade Pongo areas (Atchley, 1956; Rüegg, 1956; Caldas, 1978; Injoque, 1985; Aguirre, 1988).

(upper Río Grande Formation in Fig. 4; Aguirre, 1988;Romeuf et al., 1993, 1995). The age of the basal units of theformation is established by Aalenian fauna (W.J. Arkell, inRüegg, 1956; Roperch and Carlier, 1992), indicating thatshallow-marine sedimentation was underway by 174.0+1.0

–7.9 Maand after 178.0+1.0

–1.5 Ma (Pálfy et al., 2000). Roperch and Car-lier (1992) report a quasi-plateau 40Ar/39Ar whole-rock age of177.1 ± 2.2 Ma for a basal basalt of the correlative Chala For-mation 120 km to the southeast. Río Grande Formation vol-canism persisted into the Oxfordian, i.e., ca. 156.5+3.1

–5.1 to154.7+3.8

–3.3 Ma, but was interrupted between ca. 166 and 164Ma, which is recorded by the unconformity between lowerand upper Río Grande Formation. All of the formationrecords nondeformational, very low grade, zeolite or prehnite-pumpellyite facies metamorphism (Aguirre and Offler, 1985;Aguirre, 1988). However, the accurate deposition ages for thehost rocks of the Marcona magnetite deposit (i.e., Marcona For-mation and lower Río Grande Formation) and Mina Justa Cudeposit (i.e., upper Río Grande Formation) are still unknown.

Dike swarms, sills, and small plugs assigned to the TungaAndesite intrude the Yauca Formation and older units (Cal-das, 1978; Fig. 3). The most characteristic lithology is acoarsely porphyritic rock with large (≤1.5 cm) glomerocrystsof labradorite and sparse augite phenocrysts, informallytermed “ocöite” (Hawkes et al., 2002) by analogy with thebroadly contemporaneous, strikingly porphyritic andesites ofthe Ocoa Formation in the Copiapó area of northern Chile(Thomas, 1958). Essentially identical textures are, however,shown by several Río Grande Formation andesitic flows inthe Mina Justa area, a potential source of stratigraphic confu-sion. Ages for both Tunga andesite and upper Río GrandeFormation andesite are not well defined.

Granitoid plutons of the Cretaceous Coastal batholith(Pitcher and Cobbing, 1985) intrude Neocomian and olderstrata in the Acarí-Cobrepampa area (Fig. 2; Dunin-Borkowski, 1970; Caldas, 1978). U-Pb zircon age data arelacking for this part of the Arequipa segment of the batholith,but K-Ar (Cobbing, 1998) and Rb-Sr (Sánchez, 1982) datesfor, respectively, the Acarí diorite and Cobrepampa mon-zonite-monzogranite suggest that granitoid intrusion locallybegan at ca. 109 ± 4 Ma, shortly after emplacement of theBella Unión complex. Small, undated, dioritic stocks, 7 to 8km east-southeast and southeast of the Mina Justa prospect(Caldas, 1978), may be correlative with the larger intrusionsto the east.

The Marcona Magnetite DepositThe Marcona mine now exploits eight open pits in a ~25

km2 area elongated from west-northwest to east-southeast(Fig. 2). A crudely en echelon array of 12 major magnetiteorebodies (“minas”) and 55 smaller “cuerpos” is recognized(Fig. 5). However, the three zones exploited by the largest, 3-km-long pit, i.e., Mina 2, Mina 3, and Mina 4, represent in-terconnected segments of a single orebody (Table 1). Approx-imately 60 percent of the reserve, making up the so-called“E-grid” orebodies, is hosted by the Marcona Formation, andthe remainder, the N-13 type orebodies, by the lower mem-bers of the Río Grande Formation (Figs. 5, 6). The immedi-ate host rock for “E-grid” is dominantly metasandstone andsiltstone with minor limestone. The hypogene grades (Table1) of the larger orebodies hosted by the Paleozoic metasedi-ments average 57 to 58 percent Fe, significantly exceedingthose of 41 to 48 percent for the orebodies in Jurassic strata.Whereas the total sulfur content of the orebodies is consistentat ~3 wt percent, the copper content is more variable, aver-aging 0.06 to 0.18 percent, but attaining 0.4 wt percent inMina 1 and 0.9 percent in the upper part of the easternmost,Mina 11, orebody (Fig. 5). Pyrrhotite occurs mainly in thelower, and chalcopyrite in the upper levels of the orebodies.As exemplified by the schematic cross section of the Mina 4orebody (Fig. 7A), most orebodies at Marcona yield higherCu grades as well as elevated total sulfide contents in theirupper parts, although sulfides are locally enriched in thelower parts of some orebodies. Sphalerite and galena, nor-mally subordinate to chalcopyrite, are abundant in the Mina14 orebody. The clearly epigenetic orebodies are dominatedby essentially massive magnetite, and most original contactswith both Paleozoic and Jurassic host rocks are abrupt, onlylocally complicated by disseminated mineralization, stock-work veining, or hydrothermal breccias.

The mineralized area is intruded by a swarm of hypabyssalbodies (Fig. 7). These range from apparently syn- to clearlypostmineralization and, in composition, from silicic to,rarely, ultramafic (hornblende pyroxenite: Atchley, 1956),but magmatic chemistry and mineralogy are almost every-where disguised by alteration. Whereas andesine-phyric, inpart “ocöitic,” andesite dikes are largely postmineralization(Fig. 7A), dacitic porphyry bodies have complex, amoeboidrelationships with massive magnetite orebodies, possibly ev-idence for the comingling of silicate and oxide melts (Chen,2008).

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TABLE 1. Selected Tonnage-Grade Data for Marcona Orebodies 1

Orebody Minas 2-3-4 Mina 5 Mina 7 Minas 9-10 Mina 14 Mina 11 Mancha N-13

Host rock Marcona Fm. Marcona Fm. Marcona Fm. Marcona Fm. Marcona Fm. Marcona Fm. Río Grande Fm.

Reserve (Mt)2 399 190 18 110 110 35 224Fe grade (%)3 58.5 60.2 57.3 58.1 57.0 54.4 41.9Cu grade (%)3 0.17 0.06 0.06 0.11 0.08 0.45 0.04S content (%)3 3.55 2.57 3.10 2.51 2.97 3.51 2.86Zn grade (%)3 – – – – 0.5 – ——-

1 From “The Resource Estimate of the Marcona Iron Mine,” Shougang Hierro Perú, unpub. report, 2003 (in Chinese)2 2003 3Fe, Cu, Zn, and S grades of hypogene ore; Pb grade is not available

Three principal fault systems were documented in the Mar-cona mine by Atchley (1956) and Hawkes et al. (2002), butnew observations show that at least four are represented. Theoldest, Pista normal faults, strike 295° and dip 60° to the north.Together with the coeval or younger Repetición faults, theyare inferred to record east-southeast-west-northwest contrac-tion during the Jurassic, perhaps linked to sinistral shear alongthe regionally important, northwest-trending, Treinta Librasfault zone northeast of Marcona (Figs. 2, 5). Emplacement ofthe majority of the Marcona magnetite orebodies was con-trolled by the multiple-stage Repeticíon fault system (Fig. 7),striking N 45° E and dipping 30° to 60° NW. The Repeticíonfault system may include a series of faults that formed beforemineralization and persisted after magnetite emplacement,and varied from reverse movement in the early stages to nor-mal movement in later stages. The younger faults which con-trolled the Cu mineralization at Mina Justa, herein termed

Mina Justa faults, have strike directions similar to those of theRepetición faults at Marcona, but they dip shallowly southeastrather than northwest, and show normal displacement. Recog-nized herein in the Marcona mine, where they segment theorebodies (Fig. 7), these faults may record a change to dextraltranstension on the Treinta Libras fault. The youngest, Huaca,normal faults strike 335° and dip 60° to the east. They arepostmineralization at both Marcona and Mina Justa, but arecommonly followed by porphyritic andesitic (ocöite) dikes.

Paragenetic relationships

Numerous stages of hydrothermal alteration and hypogenemineralization, M-I through M-VII, are recognized, largelyon the basis of megascopic and microscopic textural relation-ships and mineral assemblages (Fig. 8). Representative elec-tron microprobe analyses of alteration minerals are recordedin Table 2, complementing the data of Injoque (1985).

EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 161

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15 10 So,

75 05 Wo,

Marcona

Mina Justa

1 KmN

Min

a 5

Min

a 7

Mina 9-10Mina 14

Mina 11(M

ina2 -3-4)

Mina 1

Mina 8

Mina 20

Figure 10

A

AFigure 6

Figure 7B

Figure 7A

Treinta Libras Fault

Neogene to Quarternary sedimentsPisco Fm (sandstones, shales, bentonite beds)and Sencca Fm (rhyolitic to dacitic tuffs)

Copara Fm(conglomerates, sandstones, some tuffs)

Yauca Fm(Shales, mudstones, sandstones)Jahuay Fm(mixed calcareous sediments and volcanics)

Amygdaloidal Andesite

Fine and Hornblende Andesite

Sandstones/sillstones

TuffRio

Gra

nd

e F

m

LowerPaleozoic

Cenozoic

Mesozoic(J-K)

Precambrian

Marcona Fm(metasediments)Arequipa Massif(Gneisses, K-rich Granite, migmatites)

Dacite

Tunga Andesites

San Nicolas granitoids

Porphyritic andesitedikes

Coastal Batholith

Tunga

Fault

Ocoite

FIG. 5. Geology of the area surrounding the Marcona deposit and Mina Justa prospect. Line A-A' illustrates the cross sec-tion (see Fig. 6) through the Marcona mine (modified from Rio Tinto, Marcona JV exploration report, June 2003). Insertshows area of Figure 10.

162 CHEN ET AL.

0361-0128/98/000/000-00 $6.00 162

A A

1000 m

Mina 20

(N-13 type)Mina 5Mina 3 (E-grid)

FIG. 6. Schematic cross section of Marcona mine area (A-A' in Fig. 5). Ornaments as in Figure 5. The magnetite ore-bodies are extensively dislocated by faults (modified from Hawkes et al., 2002).

0.10

0. 00

0.20

0.30

650350

400450

500550

600300

DDM4-7Cu(%)in the hypogene ore

Eleva

tion

(m)

LegendMarcona Formationmetasediments

porphyritic andesitedykes

Diabase dikescutting orebodies

Hypogenemagnetite orebody

Transitional ore

Leached ore

Mina JustaFault

Pit outline (2004 )

DDM4-6

DDM4-6

Sampled drillcores

NWSE

250

Neogene Sediments

A

LegendMarcona Formationmetasediments

Dacite

magnetiteorebody

Repeticion Fault(late)

Pit outline (2004) Mina Justa Fault

B

porphyritic andesitedykes

NWSE

?

650700

750600

Mina 1

Mina 4

HuacaFault

Eleva

tion

(m)

RepeticionFaults

Repeticion Fault(early)

FIG. 7. (A) Cross section of the Mina 4 orebody, Marcona. Copper grade distribution on the right is for >50 percent Feorebody. Porphyritic andesite and basaltic dikes are common. The main and subsidiary orebodies are controlled by north-east-striking and northwest-dipping Repetición faults, and displaced by later Mina Justa and Huaca system faults. (B) Crosssection of the Mina 1 orebody, Marcona. Two sets of Repetición faults are recognized: postmineralization and displacing theorebody, in turn cut by Mina Justa system faults; and controlling the emplacement of the orebody and dacite porphyry in-trusions. Porphyritic andesite dikes are displaced by late Repetición and Huaca faults. Locations of sections are shown in Fig-ure 5 (modified after Shougang Hierro Perú cross sections of Mina 4 and Mina 1, 2004).

Stage M-I—Early Mg-silicate alteration: Felted aggregatesof fine-grained cummingtonite (Table 2; Fig. 9A, B) occur infeldspathic metasiltstones of the Marcona Formation, origi-nally ~300 m vertically below the base of the Río Grande For-mation, and are assigned to paragenetic stage M-IA (Fig. 8).The cummingtonite is locally replaced by biotite and mag-netite (Fig. 9B), and has an Mg/Mg + Fe ratio of 0.74 (Table2), exceeding those of most metamorphic and all igneous ex-amples (Deer et al., 1997). Cummingtonite alteration, mega -scopically indistinguishable from the more widespread acti-nolitic facies and not previously recorded, is apparentlyrestricted to the upper Marcona Formation. Coarse-grainedphlogopite, in part intergrown with magnetite but also re-placed by magnetite and pyrite (Fig. 9C), talc and chlorite,also developed at an early stage in the alteration envelopes ofMina 5 and other orebodies hosted by the Marcona Forma-tion. This magnesian mica alteration is assigned to stage M-IB(Fig. 8).

Stage M-II—albite-scapolite alteration: At Marcona, Na-Clmetasomatism widely generated albite and subordinate Na-rich scapolite, particularly in Marcona Formation siliciclasticrocks and lower Río Grande Formation sedimentary unitsand andesites. Patches of coarse, white albite with clusters ofbladed white scapolite are widely developed along the folia-tion of metaclastic host rocks, in places adjacent to bodies ofmassive magnetite (Fig. 9D). In such zones, scapolite is re-stricted to within 1 to 1.5 m of the magnetite bodies. Albite

and scapolite do not occur within the latter, however, and thisstage M-II alteration is inferred to have largely predated mag-netite mineralization. Nonetheless, replacive pink albite man-tles plagioclase phenocrysts both in andesites and in thedacite porphyries which are interpreted as contemporaneouswith stage M-III magnetite mineralization (Fig. 8). Rockstaining and X-ray study are commonly required to distin-guish this pink albite alteration from the widespread K-feldspathization. Albitization everywhere predated K-feldspar development which was, in turn, overprinted byactinolite-sulfide alteration (Fig. 9E). Na-rich scapolite lo-cally replaced original feldspars in andesite in contact withthe orebodies and, with a composition of meionite29–38 and 2.8to 3.3 wt percent Cl, has been identified in the lower RíoGrande Formation north of the Marcona mine (Injoque,1985), where it was subsequently replaced by amphibole andmagnetite.

Stages M-III and M-IV—Main magnetite and magnetite-sulfide mineralization: Magnetite in the massive orebodiesand local stockwork breccia mineralization is associated withvarying proportions of calcic amphibole, phlogopite, biotite,K-feldspar, apatite, calcite, diopside, and sulfides. The majormineral associations in the main magnetite orebodies aremagnetite-actinolite (or tremolite) ± phlogopite and mag-netite-biotite (± actinolite), both assigned to a sulfide-freestage M-III, and magnetite-actinolite (or tremolite)-sulfides(± apatite ± calcite), and magnetite-phlogopite-sulfides (±

EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 163

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MineralsAlbite-scapolite

alteration

Quartz

PyritePyrrhotite

Albite*

Biotite

Sphalerite

Chalcopyrite

Apatite

Scapolite*

Prehnite

Magnetite

Calcite

Cummingtonite

Tourmaline

Serpentine

Talc

Hematite

Anhydrite

K-feldspar *

Stage M-II

Magnetite stage

Stage M-III Stage M-V Stage M-VI I

Late veins

Abundant Local Trace

Chlorite

Stage M-VI

Chlorite-talc-serpentinealteration

Diopside*

Rhodochrosite

Phlogopite

Actinolite

Tremolite

* only in host rocks

SericiteGreenalite

Early Mg-silicate alteration

Stage M-I-A Stage M-I-B

Magnetite-sulfide stage

Stage M-IV

? ?

Polymetallic sulfidemineralization

FIG. 8. Alteration and mineralization paragenesis of the Marcona magnetite deposit.

164 CHEN ET AL.

0361-0128/98/000/000-00 $6.00 164

TAB

LE

2. R

epre

sent

ativ

e E

lect

ron

Mic

ropr

obe

Dat

a fo

r A

ltera

tion

Min

eral

s, M

arco

na I

ron

Dep

osit

Min

eral

cum

cum

gre

btbt

trm

trem

act

phl

phl

phl

phl

phl

phl

phl

act

trem

trem

trem

trem

trem

chl

chl

tlc

Stag

eM

-IM

-I?

M-I

IIM

-III

M-I

IIM

-III

M-I

IIM

-III

M-I

IIM

-IV

M-I

VM

-IV

M-I

VM

-IV

M-V

M-V

M-V

M-V

M-V

M-V

M-V

IM

-VI

M-V

I

Sam

ple

MA5

-9M

A5-9

MA5

-9M

A5-9

MA5

-9M

A3-1

8M

A7-2

3D

DM

3-M

A3-1

8M

A3-1

8M

A3-1

9M

A3-1

9M

A3-1

9M

A3-1

9M

A3-1

9D

DM

5D

DM

5D

DM

3D

DM

3D

DM

5M

A91

MA3

-19

MA3

-19

MA3

-18

no.

II-2

I-1

II-3

II-1

I-2

II-2

II-2

3-1-

IIII

-1I-

1I-

1I-

2II

-2II

-3II

-1-4

-2-2

-4-2

-1-3

-8-3

-1-I

-4-3

-1I-

4I-

4I-

2

SiO

257

.68

57.6

435

.72

39.3

40.1

958

.54

58.7

55.3

944

.73

43.5

145

.41

44.8

944

.51

43.3

541

.91

55.6

158

.63

57.8

59.7

758

.19

57.9

836

.74

37.5

962

.11

TiO

20.

010

0.01

1.23

1.44

00

0.06

0.03

0.04

0.07

0.05

0.32

0.69

1.62

0.02

0.01

0.02

0.05

0.05

00

0.03

0A

l 2O3

0.83

0.8

0.09

12.4

612

.32

0.56

0.48

2.43

10.9

912

.62

11.2

10.8

811

.43

12.7

313

.09

0.22

0.33

1.17

0.29

0.73

0.37

10.2

38.

830.

37F

eO*

13.0

414

.01

49.9

817

.03

15.7

11.

442.

257.

262.

082.

242.

451.

912.

452.

296.

5814

.69

3.83

3.6

1.8

2.83

2.55

5.17

5.72

1.11

MnO

00.

050.

560.

020.

060.

010.

080.

040

00

0.04

0.01

0.02

0.07

0.11

0.06

0.08

0.08

0.05

0.05

0.08

0.06

0.02

MgO

21.1

220

.91

2.84

15.1

116

.07

23.5

123

.01

19.0

827

.25

26.4

227

.17

27.4

426

.81

26.1

522

.03

14.4

222

.15

22.3

323

.49

22.9

221

.59

33.6

333

.52

29.6

8C

aO0.

690.

130.

030

013

.66

13.3

413

.44

0.01

00

00

00

12.8

113

.27

13.9

13.7

312

.68

13.4

50.

110.

10.

04N

a 2O

0.13

0.13

0.01

0.07

0.04

0.2

0.12

0.32

0.1

0.18

0.11

0.1

0.05

0.06

0.08

0.1

0.12

0.25

0.09

0.22

0.15

0.05

0.13

0.13

K2O

0.09

0.09

0.01

9.45

9.43

0.12

0.13

0.16

9.65

10.2

49.

799.

7910

.11

9.88

9.74

0.03

0.09

0.17

0.06

0.1

0.11

0.14

0.05

0.1

Cl

0.03

0.05

0.92

1.06

0.98

00.

020.

050.

140.

180.

140.

160.

120.

120.

30.

010.

030.

040.

010.

020.

020.

050.

030.

01F

0.35

0.28

0.01

0.53

0.73

0.47

0.4

0.37

1.97

1.55

1.77

1.8

1.64

1.59

1.14

0.31

0.35

0.54

0.53

0.58

0.58

0.66

0.78

0.71

Tota

l93

.95

94.0

990

.19

96.2

796

.97

98.5

98.5

298

.696

.94

96.9

798

.197

.05

97.4

496

.87

95.6

798

.32

98.8

699

.89

99.8

998

.38

96.8

586

.85

86.8

294

.28

Si8.

288.

294.

205.

915.

967.

967.

997.

746.

246.

086.

256.

256.

196.

055.

978.

038.

007.

858.

017.

968.

057.

037.

218.

02A

l 10.

000.

002.

092.

040.

040.

010.

261.

761.

921.

751.

751.

811.

952.

030.

000.

000.

150.

000.

040.

000.

970.

790.

00A

l 20.

140.

140.

010.

120.

110.

050.

070.

140.

050.

160.

070.

030.

060.

150.

170.

090.

060.

040.

070.

080.

161.

331.

210.

10Ti

0.00

0.00

0.00

0.14

0.16

0.00

0.00

0.01

0.00

0.00

0.01

0.01

0.03

0.07

0.17

0.00

0.00

0.00

0.01

0.01

0.00

0.00

0.00

0.00

Fe

1.57

1.68

4.92

2.14

1.95

0.16

0.26

0.85

0.24

0.26

0.28

0.22

0.29

0.27

0.78

1.77

0.44

0.41

0.20

0.32

0.30

0.83

0.92

0.12

Mn

0.00

0.01

0.06

0.00

0.01

0.00

0.01

0.01

0.00

0.00

0.00

0.01

0.00

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.00

Mg

4.52

4.48

0.50

3.39

3.55

4.76

4.67

3.98

5.67

5.50

5.58

5.69

5.55

5.44

4.68

3.10

4.51

4.52

4.69

4.67

4.47

9.59

9.59

5.72

Ca

0.11

0.02

0.00

0.00

0.00

1.99

1.95

2.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

1.98

1.94

2.02

1.97

1.86

2.00

0.02

0.02

0.01

Na

0.04

0.04

0.00

0.02

0.01

0.05

0.03

0.09

0.03

0.05

0.03

0.03

0.01

0.02

0.02

0.03

0.03

0.07

0.02

0.06

0.04

0.02

0.05

0.03

K0.

020.

020.

001.

811.

780.

020.

020.

031.

721.

821.

721.

741.

791.

761.

770.

010.

020.

030.

010.

020.

020.

030.

010.

02C

l0.

010.

010.

180.

270.

250.

000.

000.

010.

030.

040.

030.

040.

030.

030.

070.

000.

010.

010.

000.

000.

000.

020.

010.

00F

0.16

0.13

0.00

0.25

0.34

0.20

0.17

0.16

0.87

0.68

0.77

0.79

0.72

0.70

0.51

0.14

0.15

0.23

0.22

0.25

0.25

0.40

0.47

0.29

R**

0.74

0.73

0.36

0.38

0.97

0.95

0.82

0.04

0.05

0.05

0.04

0.05

0.05

14.3

0.64

0.91

0.92

0.96

0.94

0.94

0.1

0.1

Not

es: *

= to

tal i

ron;

num

ber

of io

ns c

alcu

late

d on

the

basi

s of

F, C

l, an

d 23

O fo

r cu

mm

ingt

onite

(cum

), tr

emol

ite (t

rem

) and

act

inol

ite (a

ct);

22 O

for

biot

ite (b

t) a

nd p

hlog

opite

(phl

); 28

O fo

r ch

lo-

rite

(ch

l), 2

2 O

for

talc

(tlc

), an

d 14

O fo

r gr

eena

lite;

R**

: Mg/

Mg

+ F

e ra

tio fo

r am

phib

oles

; Fe/

Fe

+ M

g fo

r m

icas

and

chl

orite

s1

Al–

tetr

ahed

ral

2 A

l–oc

tahe

dral

FIG. 9. Marcona alteration and mineral-ization stages. (A) Cummingtonite (stageM-IA), partially altered to greenalite, occursinterstitially to coarse-grained biotite andmagnetite (stage M-III) (#MA5-9, Mina 5open pit, 670 m, main orebody; plane-polar-ized transmitted light). (B) Electron back -scatter image illustrating the replacement ofcummingtonite (darker) by fine-grained stageM-III biotite. (C) Stage M-IB phlogopiteand magnetite (Mt-1) replaced by pyrite andmagnetite (Mt-2). Pyrite was emplaced alongthe cleavage of early phlogopite (#MA5-2,Mina 5 open pit, 670 m, main orebody; plane-polarized reflected light). (D) Albitization ofMarcona Formation metasediments. Coarsewhite albite, locally with pockets of bladedscapolite, is concentrated along the foliation(Mina 2 open pit, 700 m, south wall). (E)Stage M-II albitized dacite porphyry (white)cut by stage M-III K-feldspar (microcline;pink-red) veins, in turn reopened by stageM-V actinolite (+ sulfide, dark-green). Themajor sulfide is pyrite (#MA3-24, Mina 3open pit, 600 m, ~ 30 m from the main mag-netite orebody). (F) Stage M-III tremolite(with actinolite) occurs interstitially to sub-hedral magnetite. Chloritization of amphi-bole is common. (#MA3-22, Mina 3 open pit,600 m, combined reflected and transmittedlight). (G) Stage M-IV magnetite, pyrite,tremolite and phlogopite. The smooth con-tacts suggest contemporaneous formation(#MA2-9, Mina 2 open pit, 600 m, orebody,combined reflected and transmitted light).(H) Coarse-grained stage M-III phlogopitewith interstitial magnetite and apatite. Chlo-rite and minor talc replace phlogopite alongcleavages (#MA3-18, Mina 3 open pit, 600m, orebody, transmitted light, crossed nicols).(I) Electron backscattered image showingthe local replacement of stage M-III biotite(paler, massive aggregates and veins) by stageM-IV phlogopite (#MA3-19, Mina 3 openpit, 600 m). (J) Alteration at the contact be-tween magnetite (Mt) orebody and dacite,zoned outward from biotitization to K-feldspathization and albitization (all with orwithout minor magnetite) (Mina 3 open pit,east end of south wall, 620 m). (K) Actinolite-tremolite-sulfide veins cut stage M-III mas-sive magnetite-calcic amphibole aggregate.The major sulfides are chalcopyrite andpyrite. Stage M-V actinolite-tremolite is com-monly coarse-grained. Magnetite occurs astraces in stage M-V veins (#DDM5-4-2, drillcore DDM5-4, 210 m, main Mina 5 ore-body). (L) Stage M-V pyrite, chalcopyrite,and calcite occur as aggregates superim-posed on stage M-III magnetite (#MA5-3,Mina 5 open pit, 670 m, orebody). (M) Fine-grained talc replaces stage M-IV phlogopite(#MA3-11, Mina 3 open pit, 580 m, adjacentto a magnetite orebody; transmitted light,crossed nicols). (N) Late magnetite veins(Mt-2) cut massive magnetite (Mt-I) and latetremolite. Magnetite in veins is commonlyfine grained and locally associated with cal-cite (#DDM3-3-3, drill core DDM3-3, 343m, Mina 3 orebody). (O) Late quartz vein(with erratic calcite and Mn oxides) cutsmassive magnetite-amphibole-sulfide assem-blage. A hematite vein cuts both (#MA3-35,Mina 3 open pit, 580 m, south wall).

EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 165

0361-0128/98/000/000-00 $6.00 165

Mt

Mt

BtMt

Cum

Bt

A

Bt Bt

CumCum

Gre

Mt

BB

Phl

Mt-1

PyPhl

Py

Mt-2

Cum

Cum

C

Ab (Stage M-II)

Act-Sulfide veins (Stage M-V)

Kfs (Stage M-III)

MarconaFormation

Magnetiteorebody

Albite

Albite

D E

MtMt Phl

Chl

Chl Ap

Mt

Amph

Py

PyPhl

Mt

150 m

Mt

Mt Amph

150 m

Amph

Chl

Bt

Phl

Phl

Phl

F

1 m

150 m150 m

G

150 m

H I

Mt

BtMt

Bt

Kfs+ Ab alteration

J

Massive Mt-Amph

Cp-Py-Amph veins

Cp

PyCal

K

1 cm

CalMt

Mt

L

Tlc

Tlc

TlcCp

Mt

CalPhl

Phl TlcPhl

M

Mt(2) veins

Mt(1)

Late tremolit e

Late tremolite

Mt - Amph -Sulfide - Cal

Qtz vein

Hm vein

150 m

N

0.5 cm

0.5 cm

O

actinolite or tremolite ± apatite ± calcite), assigned to stageM-IV. In both stages, magnetite forms euhedral to subhedral,0.3 to 5 mm grains and massive aggregates, commonly inter-grown (Fig. 9F) with fine-grained (0.1–0.5 mm), light-greentremolite or dark-green actinolite (Table 2; the classificationof Leake et al., 1997). However, these amphiboles rarely co-exist. The major stage M-IV sulfides are pyrite, chalcopyrite,and both hexagonal and monoclinic pyrrhotite, occurringlargely as subhedral to anhedral crystals interstitial to mag-netite, calcic amphibole, and phlogopite (Fig. 9G), but locallyshowing microscopic replacement textures. Although no un-ambiguous replacement of pyrrhotite by pyrite and chalcopy-rite was observed, the common association of pyrrhotite andmagnetite without other sulfides, especially in the lowerparts of the orebodies, and the absence of pyrrhotite in latesulfide veins suggest that it largely formed prior to pyrite andchalcopyrite.

Red-brown stage M-III biotite (Ann36–38) commonly occursas coarse flakes in the main orebodies and their envelopes.Phlogopite, locally occurring in stage M-III but more abun-dant in stage M-IV assemblages, has a composition of Ann4–5

(Table 2) and is widely replaced by chlorite and talc (Fig. 9H).The locally developed stage M-IV ferroan phlogopite (Ann14)may record the alteration of stage-III biotite. Replacement ofboth massive and vein biotite by phlogopite in Marcona For-mation metasediments is also observed in electron backscat-ter images (Fig. 9I). Accessory minerals in the magnetite ore-bodies include fine-grained stage M-III fluorapatite (Fig. 9H)and stage M-IV calcite, both widely coexisting with magnetiteand/or sulfides. Stage M-III K-feldspar alteration is dominantin dacite and fine-grained andesite, generally has the ortho-clase structure, and commonly occurs within an outer zone tobiotitization (Fig. 9J). Metasomatic magnetite, associated withsecondary K-feldspar, is only locally observed in host rocks.Although biotite, phlogopite and amphibole are also commonskarn-type alteration minerals, the immediate metasiltstonehost rock for major magnetite orebodies and the absence ofextensive hydrothermal magnetite in the alteration envelopeindicate a carbonate-replacement skarnization (Injoque,1985) is unlikely for the Marcona main magnetite formation.

Stage M-V—Polymetallic sulfide mineralization: The majorsulfides in stage M-V are again pyrite, chalcopyrite, andpyrrhotite. Sulfide-rich veins, commonly with calcic amphi-boles, occur in the upper parts of the orebodies and cut mas-sive stage M-III and M-IV magnetite-amphibole associations(Fig. 9K). However, the relationships between the sulfides ofstages M-IV and M-V are rarely clear. Stage V sulfides and co-existing minerals widely occur as aggregates replacing stageM-III magnetite and amphibole. The characteristic assem-blages include chalcopyrite-pyrite-calcic amphibole (±pyrrhotite) and less abundant, chalcopyrite-pyrite-calcite(Fig. 9L). The chalcopyrite-pyrite-calcic amphibole-calciteassemblage also occurs locally. Stage M-V sulfides are gener-ally euhedral to subhedral and coarse grained, and commonlyhave planar contacts with amphibole and calcite, which mayindicate broadly coeval precipitation. Pyrrhotite mainly oc-curs as aggregates replacing stage M-III or M-IV magnetite-amphibole and is subordinate to chalcopyrite and pyrite insulfide veins. Accessory stage M-V sulfides include sphalerite,abundant in the Mina 14 orebody and commonly associated

with pyrite and chalcopyrite. Calcic amphibole formed exten-sively in stage M-V as tremolite and actinolite (Table 2), bothcoexisting with sulfides. Tremolite, without associated metal-lic minerals, also developed late in stage M-V, forming veinscutting massive magnetite orebodies. Hydrothermal breccias,in which coarse-grained, late-stage M-V tremolite cementsmagnetite-sulfide clasts, are widespread in the Cu-poor Mina5 and Mina 7 orebodies, but are only locally developed else-where. Tremolite which formed late in stage M-V has a loweriron content than that associated with sulfides (Table 2).

Stage M-VI—Chlorite-talc-serpentine alteration: Talc com-monly occurs as fine-grained aggregates and replaces or cutscalcite, locally also replacing stage M-IV phlogopite (Fig.9M). Lizardite and, locally, chrysotile also replace stage M-Vactinolite and tremolite, and talc and serpentine replacecoarse-grained stage M-V apatite. Serpentine veins com-monly cut magnetite and sulfides in the cores of the orebod-ies. Whereas the chlorite-talc -serpentine assemblage recordsthe retrograde alteration of phlogopite, actinolite and tremo-lite, the replacement of calcite and calcic amphiboles by talcand serpentine is evidence for Mg metasomatism followingthe main stage M-V sulfide precipitation.

Stage M-VII—Late veins: Late-stage hydrothermal veinsare abundant at Marcona, but their mutual age relationshipsare ambiguous. Fine-grained subhedral magnetite and sul-fides form narrow veins cutting both late-stage M-V tremoliteand stage M-III magnetite (Fig. 9N). Rare chalcopyrite veinslacking gangue minerals cut late magnetite veins. Late mag-netite is widely weathered to powdery hematite, but somehematite (± gypsum) veins which cut the main magnetite ore-bodies and late quartz veins (Fig. 9O) are interpreted as hy-pogene. The major sulfides in hematite veins are pyrite andchalcopyrite. Although the main mineralization stages at Mar-cona are almost free of quartz, barren quartz ± calcite veinscut magnetite orebodies and host-rock alteration zones (Fig.9O). Rhodochrosite locally occurs in these veins. Calcite veinsup to 5 cm thick cut the magnetite orebodies and reopen ser-pentine veins. Tourmaline-quartz-pyrite veins locally cutMarcona Formation metasediments. In the Mina 11 orebody,anhydrite veins, commonly replaced by gypsum and bassan-ite, cut all previous stages. Locally, anhydrite with abundantpyrite and minor chalcopyrite forms the matrix of hydrother-mal breccias.

Supergene alteration: Most magnetite orebodies at Mar-cona were mantled by 10- to 40-m-thick supergene oxidationprofiles, comprising lower, 4 to 6 m horizons of sulfate-rich“transitional” ore (Fig. 7A), in which martitized magnetite isintergrown with jarosite, botyrogen, amarantite and parabut-lerite, and surficial leached, martite-dominated zones. Thesupergene profiles are eroded by a regionally extensive pedi-ment overlain by a 9.13 ± 0.25 (2σ) Ma rhyodacitic ash-flowtuff (Quang et al., 2001).

The Mina Justa Cu (-Ag) DepositThe Mina Justa Cu oxide and sulfide orebodies are hosted

entirely by the mid-late Jurassic upper Río Grande Formation(Fig. 10). This unit dips at 40° to 60° to the northwest and isdominated by porphyritic andesite flows and medium to fine-grained andesitic volcaniclastic rocks with minor horizons ofsandstone, siltstone and limestone. Callovian to Oxfordian

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fauna have been identified in inferred equivalent strata of theupper Río Grande Formation in the Pampa de Pongo district,30 km south of Mina Justa (fig. 4, Caldas, 1978; Hawkes et al.,2002; Baxter et al., 2005). The volcaniclastic rocks locally in-corporate rounded plagioclase phenoclasts in a fine-grainedmatrix. Subordinate host rocks include plagioclase- and horn-blende-phyric andesite with vesicles filled by chlorite and car-bonates. Lensoid marble bodies occur mainly in the southeastpart of the area, but host no economic mineralization. Theabundant secondary hydrothermal biotite and sericite inhibitdefinition of magmatic chemistry (Hawkes et al., 2002), butthe correlative andesites at the base of the upper section ofthe Río Grande Formation in Cañón Río Grande (40 kmnorthwest of Mina Justa) are K rich and have high Cu con-tents (avg 400 ppm: Aguirre, 1988). A swarm of northwest- to

north-striking, 20- to 50-m-wide andesitic dikes, constitutingup to 35 percent of the rock volume in the main mineraliza-tion center, was emplaced following mineralization. Theseplagioclase-phyric, “ocöitic” rocks are texturally and miner-alogically similar to the Río Grande Formation flows, butrecord only weak K feldspathization and sericitization.

The Mina Justa deposit incorporates two principal orebod-ies, the Main and Upper (Figs. 10 and 11A). The mineralizedlensoidal bodies characteristically comprise a massive mag-netite-sulfide core enclosed by hydrothermal breccias withstrongly altered host rock clasts in a magnetite+sulfide matrix,in turn surrounded by extensive stockwork (Fig. 12). They arecontrolled by subparallel, northeast-trending and shallowlysoutheast dipping faults and range from 10 m to 200 m in ver-tical extent (Baxter et al., 2005). The Main mineralized body

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BC

B

,

C,

Upperorebody

Mainorebody

Neogene to Quaternary

Post-mineralizationocoite

Amygdaloidal Andesite

Tuff

Sandy volcaniclastics(sandstone/siltstones)

Fine-grained andestie

Massive Magnetite bodieswith minor Cu-oxides

Alteration(Ab+ Kfs+ Act)with Cu-oxide

Faults

Cross Sections50 m

N

B B

FIG. 10. Geologic map of Mina Justa Cu deposit, hosted by the upper Río Grande Formation. B-B' and C-C' show loca-tions of the Figure 11 cross sections (modified from Rio Tinto 1: 10,000 mapping of Mina Justa prospect, February 2003,unpub. report). Ab = albite, Act = actinolite, Kfs = K-feldspar.

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350m

550m

C

B

C100m

BB MA 27 MA 35 MA 17 MA 64

Upper orebodyMain orebody100m

600m

A

600m

?

800m

200m

400m

600m

Upper orebody

Main orebody

Upperorebody

Main orebody

Cu-oxide zone

Bornite+ Chalcocite(+ Magnetite) zone

Bornite+ Chalcopyrite(+ Magnetite) zone

Chalcopyrite+Pyrite(+ Magnetite) zone

Late ocoite

Altered volcaniclastics(Rio Grande Formation)

Massive Magnetite bodywith minor Cu-oxides

Main orebody

750m

FIG. 11. Cross sections through the Mina Justa orebodies. (A) Northwest-southeast section through the Main and Upperorebodies (from Baxter et al., 2005). (B) Southwest-northeast section through the Main orebody (sulfide zones modified afterMoody et al., 2003).

MA-64Graphiclogging

Mineral-ization640m

480m

560m

Mt+Hm+Bn + Cc

Mt+Bn

MA-17

495m

395m

445m

Mt+Bn

Mt+Cp+Py

MA-35

320m

260m

290m

Mt+Cp+Py

MA-27

320m

260m

290m

Mt+Cp+Py

MA-45

550m

380m

467m

Mt+Cp+Py

MA-89

490m

430m

460m

Veins

HydrothermalBreccias

Veins+ massiveMagnetite+ sulfide bodies

Hydrothermal Breccias + massiveMagnetite+ sulfide bodies

Massive (Mt+ Bn)

Massive (Mt+ Cp)

Pyrite-rich zone*

Pyrite-rich zone*+ hydrothermal Breccias

Altered sandy volcanoclastics

Late ocoite

M-B

M-C

M-B

Graphiclogging

Mineral-ization

Graphiclogging

Mineral-ization

Graphiclogging

Mineral-ization

Graphiclogging

Mine Graphiclogging

Mineral-ization

Graphiclogging

Mineral-ization

M-CM-C

Mt+Cp+Py

Mt+Cp+Py

NW SE

FIG. 12. Mineralogical and structural zonation of the Mina Justa orebodies, based on logging of selected drill cores. Thelocations of holes MA-64, MA-17, MA-35, and MA-27 are shown in Figure 11A. MA-45 and MA-89 are collared 600 to 800m southeast of the upper zone and out of the map area in Figure 10. *Magnetite either occurs erratically as haloes aroundcoarse-grained pyrite or is absent in this zone. Bn = bornite, Cc = chalcocite, Cp = chalcopyrite, Mt = magnetite, Py = pyrite.

crops out as a 400 m long, discontinuous belt of Cu oxides andalbite-K-feldspar-actinolite alteration (Fig. 10), which dips10° to 30° to the southeast, i.e., at a high angle to the beddingof the host andesites. It has been intersected to a depth of 500m, where it remains open (Fig. 11A). The Upper mineralizedbody, cropping out subparallel to and approximately 400 msoutheast of the Main zone (Fig. 10), has a similar concave-upward, “spoon-shaped” form in section, and a similar dip of10° to 30° to the southeast. On surface, this zone has beenidentified over a distance of at least 400 m and it has been in-tersected to a depth of 300 m (Fig. 11A). The similarly north-east-trending, but northwest-dipping magnetite lenses arealso exposed on surface (Fig. 10). They commonly containminor Cu oxides and are locally cut by the southeast-dippingMina Justa normal faults (Fig. 11A).

Copper oxide minerals, predominantly chrysocolla withlesser atacamite, dominate the upper 200 m of the deposit,giving way gradually to sulfides with depth (Fig. 11A, B). Theoxide zone, with an average grade of 0.54 percent Cu, hostsapproximately 40 to 50 percent of the recoverable Cu in themeasured-plus-indicated reserves. In individual orebodies,the major sulfides are zoned upward, and locally laterally, butnot strictly concentrically (cf. Moody et al., 2003), frompyrite-chalcopyrite to bornite-chalcocite (± digenite), with aconcomitant increase in Cu grade (Figs. 11, 12). Around themagnetite-sulfide orebodies, the alteration is zoned outwardfrom potassic (K-feldspar dominant), through calcic (actino-lite) to sodic (albite). Hypogene hematite, in part as specular-ite, commonly occurs in the upper parts of the zones of Cu

mineralization, particularly in the northeast quadrant of theorebodies.

Paragenetic relationships

Seven stages of hypogene alteration-mineralization, J-Ithrough J-VII, are herein recognized at Mina Justa (Fig. 13).

Stage J-I—albite-actinolite alteration: The earliest hy-drothermal event at Mina Justa generated widespread albite-actinolite alteration in andesitic lavas and volcaniclastic in-terbeds. Light pink albite and green, fine-grained actinolite(Table 3) replace both plagioclase phenocrysts and the matrixof andesites (Fig. 14A), recording Na metasomatism.

Stage J-II—K-feldspar–magnetite alteration: Rocks affectedby this event generally appear massive in hand specimen, andrange from pink to black. K-feldspar commonly occurs as ex-tremely small grains (<0.05 mm) replacing both fresh andpreviously albitized plagioclase (Fig. 14A), and the associatedmagnetite is mainly fine to medium grained (0.05–0.1 mm),locally forming aggregates interstitial to the feldspar (Fig.14B). Stage J-II alteration, unambiguously the result of K-Femetasomatism, was probably contemporaneous with the de-velopment of lenses of sulfide-free magnetite which strikenortheast and dip northwest, subparallel to stratigraphy, andare locally crosscut by massive magnetite-pyrite bodies (Fig.11A). Overprinting of stage J-II alteration by stage J-III acti-nolite and stage J-V coarse-grained K-feldspar ± magnetite iscommon (Fig. 14C).

Stage J-III—actinolite (± magnetite ± diopside) alteration:Green actinolite (Table 3), associated with minor magnetite,

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MineralsAlbite-actinolite

alteration

Quartz

Pyrite

Albite

Chlorite

Sphalerite

Chalcopyrite

Apatite

Magnetite

Calcite

Actinolite

Microcline

Diopside

Stage J-I

Magnetite-pyrite-K-feldspar alteration

Stage J-V

Cu mineralization

Stage J-VI Stage J-VII

Latehematite

Abundant Local Trace

K-feldspar-magnetitealterationStage J-II

Actinolite-diopside-magnetite alteration

Stage J-III

Epidote

Bornite

Chalcocite

Carrollite

Ti tanite

Note: supergene minerals are omitted

Allanite

Hematite

Prehnite

Galena

Barite

Molybdenite

Clinozoisite

Earlyhematite

Stage J-IV

?

FIG. 13. Alteration and mineralization paragenesis of the Mina Justa Cu (-Ag) deposit.

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TABLE 3. Representative Electron Microprobe Data for Hydrothermal Silicates and Sulfides from Mina Justa

Mineral act act chl dg dg bn cc

Stage J-I J-III J-V J-VI J-VI J-VI J-VI

Sample MA64 MA89 MA89 MA64 MA64-4 MA64 MA64no. –3 –4-1 –4-2 –4-II-1 –II-1-A –4-II-2 –4-I-1

SiO2 52.88 56.14 32.62 As 0.00 0.12 0.000 0.10TiO2 0.20 0.02 0.02 S 20.99 21.38 25.18 20.13Al2O3 3.25 0.95 14.41 Fe 0.14 0.08 11.17 0.04FeO* 11.76 10.73 20.41 Ni 0.00 0.00 0.00 0.00MnO 0.10 0.10 1.06 Zn 0.00 0.00 0.00 0.00MgO 15.71 16.88 17.71 Ag 0.24 0.26 0.12 0.14CaO 12.72 13.16 1.56 Cu 78.40 78.25 63.41 80.46Na2O 0.44 0.15 0.12 Co 0.00 0.00 0.00 0.00K2O 0.23 0.06 0.03Cl 0.10 0.04 0.04F 0.33 0.17 0.32Total 97.70 98.41 88.31 Total 99.773 100.088 99.880 100.865

Si 7.63 7.95 6.67Al 1 0.37 0.05 1.33Al 2 0.18 0.10 2.14Ti 0.02 0.00 0.00Fe 1.42 1.27 3.49Mn 0.01 0.01 0.18Mg 3.38 3.56 5.40Ca 1.97 2.00 0.34Na 0.12 0.04 0.05K 0.04 0.01 0.01Cl 0.02 0.01 0.01F 0.15 0.08 0.21R** 0.70 0.74 0.39

Notes: * = Total iron; number of ions calculated on the basis of F, Cl, and 23 O for actinolite (act); 28 O for chlorite (chl), bn = bornite, cc = chalcocite,dg = digenite; R**: Mg/Mg + Fe ratio for amphiboles; Fe/Fe+Mg for chlorite; detection limits for sulfides (in wt percent): Fe = 0.02; Ag = 0.04; As = 0.07;Cu = 0.02; S = 0.02; Ni, Co, and Zn = 0.06

1 Al–tetrahedral2 Al–octahedral

FIG. 14. Mina Justa alteration and mineralization stages. (A) Light-pink albite (not stained) and fine-grained actinolite ex-tensively replace original phenocrystic and groundmass plagioclase (stained pink to red). Stage J-II red microcline (stainedyellow) replaces albite. Stage J-III actinolite is superimposed on albite and microcline (#MA64-7, drill core MA64, 394.4 m,80 m from main orebody). (B) Fine-grained microcline coexists with magnetite in a clast cemented by stage J-III actinolite.Subhedral to euhedral actinolite crystals locally replace microcline (#MA64-3, drill core MA64, 220.1 m, plane-polarizedtransmitted light). (C) Magnetite-sulfide-calcite veins with K-feldspar haloes (red) cut stage J-III actinolite and stage J-IIfine-grained K-feldspar-magnetite (gray to pink) alteration. Actinolite is extensively chloritized. (#MA17-7, drill core MA17,364 m). (D) Red-green breccia in which stage J-III actinolite (green) matrix cements clasts of stage J-II fine-grained K-feldspar - magnetite (Mt-1) (pinkish red to dark gray). Coarse-grained stage J-V magnetite (Mt-2) occurs with actinolite andlocally as veins (# MA64-3, drill core MA64, 220.1 m). (E) Stage J-V magnetite (Mt-2)-bornite-chalcocite assemblage occursas a matrix to pinkish-red stage J-II K-feldspar-magnetite (Mt-1) altered clasts cut by stage J-III actinolite (green) veins. Themagnetite-sulfide matrix was reopened and partially replaced by late specularite (#MA64-6, drill core MA64, 276 m). (F)Platy stage J-V magnetite (after stage J-IV hematite) occurs with calcite, quartz and chalcopyrite. Chalcopyrite extensivelyreplaces pyrite and locally occurs along fractures in magnetite. Chloritized stage J-III actinolite relics occur between themagnetite crystals. Stage J-IV calcite has planar contacts with platy magnetite, but is locally replaced by stage J-V granularmagnetite and quartz veins. (#MA17-6 from drill core MA17, 355.1 m, combined reflected and transmitted light). (G) Hy-drothermal breccia at the margin of the Main orebody. Magnetite (Mt-2)-sulfide occurs as a matrix around angular stage J-II microcline-magnetite (Mt-1) clasts. Actinolite relics occur in matrix (#MA35-0, drill core MA35, 484.3 m). (H) Replace-ment of stage J-III actinolite by stage J-V magnetite-pyrite. Strong chloritization of actinolite is locally evident (lower-right)(#MA27-2, drill core MA27, 366.9 m; transmitted light, crossed nicols). (I) Spotty magnetite-chalcopyrite-quartz mineral-ization in earlier actinolite and microcline-magnetite (gray to pink) - altered host rocks. Chalcopyrite coexists with magnetiteand quartz (#MJ-38, drill core MA54, 341.8 m). (J) Magnetite-pyrite-quartz alteration. Quartz is coarse grained and euhe-dral. Stage J-III actinolite crystals occur as relics in quartz grain (#MA17-6, drill core MA17, 355.1 m, combined reflectedand transmitted light). (K) Magnetite alteration and related chloritization. Magnetite coexists with pyrite, quartz, and chlo-rite. Chlorite (locally with quartz) extensively replaces stage J-III actinolite (#MA89-4, drill core MA89, 360.2 m, plane-po-larized transmitted light). (L) Chalcopyrite-calcite veins cut altered host rocks. Microcline occurs as haloes around calciteveins and locally cuts calcite (#MA45-6, drill core MA45, 404.2 m). (M) Chalcopyrite replaces stage J-V pyrite and magnetite(#MA17-9, drill core MA35, 507.9 m, plane-polarized reflected light). (N) Supergene covellite replaces chalcopyrite (#MA14-3, drillcore MA14, 394.7 m, plane-polarized reflected light). (O) Fine-grained bladed hematite coexists with bornite,digenite and chalcocite with vermicular and eutectic-like textures, occurring as patches in a magnetite vein which cuts hostrocks (#MA64-4, drill core MA64, 248.3 m, plane-polarized reflected light).

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Mt

Kfs

Kfs

Act

ActMt - Cal-Sulfid e vein

Kfs halo

Act -(Kfs- Mt)altered host rock

Ab

Pl

Ab

Kfs

PlAct

Act

Kfs

A

0.5 cm

B

150 m

C

Kfs - Mt-1Act

Mt-2

Kfs - Mt-1

Act

Mt-2-Sul fide

D

0.5 cm

E

0.5 cm

Mt

Cal

Cp

Qtz

F

150 m

Kfs - Mt-1

Mt-2-Sul fideKfs -Mt-1

G

Mt

ActMt - Py

Chl

0.5 cm

H

150 m

Py Mt

Qtz

Act

150 m

Mt (II) - Sulfid e- Qtzspotty mineralizat ion

Act - (Kfs-Mt-I) alteredhost rock

J

0.5 cm

Mt

PyAct

Chl

QtzK

150 m

I

Act - (Kfs + Mt) alteredhost rock

Cp

Cal vein withKfs halos

L

1 cm

PyCp

Mt

M

150 m

CpCv

Qtz

150 m

Bn

Hm

MtCc

Dg

N O

150 m

occurs throughout the deposit, commonly as massive aggre-gates along the contacts of stage J-V magnetite bodies or ascoarse, acicular crystals in veins cutting stage J-II K-feldspar-magnetite alteration. More locally, it forms the matrix of hydrothermal breccias (Fig. 14D) which incorporate clasts ofK-feldspar-magnetite–altered host rocks. Actinolite inter-growths occur as irregular clasts in a magnetite-sulfide matrixalong the contacts of the stage J-V magnetite bodies with theiractinolitic alteration haloes. A temporal evolution is evidentfrom K-feldspar–magnetite, through actinolite, to magnetite-pyrite alteration (Fig. 14E). Actinolite is strongly chloritizedand carbonatized, and locally replaced by quartz. It replacedboth albite and K-feldspar, evidence for Ca metasomatism.Diopside is spatially associated with and locally replaced byactinolite in the albitized and K-Fe–metasomatised hostrocks.

Stage J-IV—early hematite-calcite alteration: “Mushke-tovite”, i.e., magnetite unambiguously pseudomorphous afterspecular hematite, occurs commonly in the main magnetitebodies, evidence for a now obliterated hematite-dominantstage which temporally separated the actinolite alteration andthe main magnetite alteration in andesite. The hematite orig-inally formed fractured plates (Fig. 14F). Anhedral-to-subhe-dral, and medium- to coarse-grained calcite is intergrownwith the pseudomorphs, and is locally replaced by quartz andmagnetite. Coarse-grained, subhedral-to-euhedral allanite(stage J-V) occurs rarely as inclusions in stage J-IV calcite incontact with stage J-V magnetite and pyrite.

Stage J-V—magnetite-pyrite-K-feldspar alteration: Themassive, lensoid, and brecciated magnetite-pyrite bodieswhich host the highest grade copper sulfide mineralization atMina Justa were controlled by the northeast-striking, south-east-dipping, Mina Justa system faults, but are dislocated bythe northwest-striking, northeast-dipping Huaca faults andassociated ocöite dikes. Magnetite-pyrite veins, varying from0.1 to 5 cm in width, cut alteration assemblages of stages J-IIand J-III adjacent to the massive magnetite bodies. Hy-drothermal breccias commonly occur at the margins of themagnetite bodies, and comprise a magnetite-pyrite–dominantmatrix and angular clasts of andesite altered to microcline(stage J-II) or actinolite (stage J-III) (Fig. 14G), and mag-netite-pyrite intergrowths locally replace stage J-III actinolite(Fig. 14H). Rarely, stage J-V magnetite-rich alteration occursas spots in altered host rocks peripheral to the main mag-netite bodies (Fig. 14I).

Magnetite and pyrite of stage J-V are medium- to coarse-grained (0.5–10 mm, with some pyrite exceeding 1 cm) andsubhedral to euhedral. Magnetite commonly occurs intersti-tially to pyrite and has planar grain boundaries (Fig. 14J).Abundant quartz is associated with magnetite-pyrite alter-ation in the main magnetite bodies, occurring as 0.1 to 1 mm,subhedral to euhedral, crystals interstitial to magnetite andpyrite and commonly with actinolite inclusions (Fig. 14J). Ac-cessory calcite is generally anhedral to subhedral and medium-grained, coexisting with magnetite, pyrite, and quartz. Pink orred K-feldspar, predominantly microcline, is a common alter-ation mineral in rocks associated with stage J-V magnetite-pyrite mineralization, forming haloes to magnetite-pyriteveins or patches incorporating medium- to fine-grained mag-netite crystals and superimposed on early alteration (Fig.

14C). Chlorite, largely diabantite (classification of Hey, 1954;Table 3), extensively replaces actinolite or diopside (Fig.14K), and locally occurs in veins with magnetite, pyrite, andquartz. Titanite commonly forms medium-grained subhedralcrystals or aggregates enclosed by chlorite, as well as euhedralgrains in magnetite-pyrite-quartz-chlorite veins. Fluorapatitelocally occurs in stage J-V veins, but more commonly formscoarse-grained, subhedral to euhedral grains in stage J-IIIactinolite in contact with magnetite bodies. The mutual rela-tionships of apatite and actinolite are ambiguous.

Stage J-VI—copper sulfide mineralization: Stage J-V mag-netite alteration zones, although rich in pyrite, lack inherentCu sulfides. Copper sulfide-bearing veins, assigned to stage J-VI, locally cut altered host rocks and stage J-V magnetite-pyrite-quartz (Fig. 14L), but the Cu sulfides and associatedassemblages more commonly occur in massive magnetite-pyrite bodies or veins with which they exhibit unambiguousmicroscopic replacement textures (Fig. 14M). Locally, stageJ-V magnetite-pyrite aggregates in stage J-III actinolite veinshave been almost completely replaced by chalcopyrite or bornite, giving rise to the common actinolite-Cu sulfide asso-ciation, or in some cases, pyrite was intensively replaced bychalcopyrite and generated the magnetite-chalcopyrite asso-ciation in hand specimens (e.g., Fig. 14G, I).

The main hypogene Cu sulfides at Mina Justa are, in de-creasing abundance, chalcopyrite, bornite, chalcocite, and di-genite. Except for chalcopyrite, these are concentrated aboveor in the upper parts of the main magnetite bodies, commonlyforming veins that cut the host rocks and earlier alteration as-semblages. Covellite, entirely supergene, occurs mainly in thelower part of the oxide zone, replacing bornite and chalcopy-rite (Fig. 14N). Chalcocite, digenite, and bornite typicallyform large patches with complex vermicular intergrowths(Fig. 14O), such as are inferred to form through noncoherentexsolution at low temperature (<250°C) and under protractedcooling (Brett, 1964). Similar hypogene relationships weredocumented at the Olympic Dam deposit by Roberts et al.(1983). Copper sulfides exhibiting vermicular textures are allrich in silver (Table 3), and represent the major Ag host in theores. Chalcocite, bornite, and chalcopyrite locally occur to-gether, with no unambiguous mutual replacement relation-ships. Accessory stage J-VI sulfides include sphalerite, galena,molybdenite, and rare fine-grained (<25 µm), carrollite whichgenerally coexist with chalcopyrite and locally replace pyrite.

The iron oxide associated with both chalcopyrite and bor-nite-chalcocite mineralization is fine-grained platy hematite,commonly occurring as aggregates around Cu sulfides (Fig.14O). Locally, stage J-VI hematite formed with chalcopyritealong the boundaries of earlier magnetite grains or stage J-IVcoarse-grained hematite (“mushketovite”). Calcite is the dom-inant nonmetallic mineral associated with Cu mineralization,generally occurring in veins which cut the host rocks and mag-netite mineralization (Fig. 14L). Calcite-Cu sulfide assem-blages dominate these veins but give way upward to hematite-bearing assemblages. Albite (± microcline) locally occurs inchalcopyrite-calcite veins cutting altered andesite host rock.Sparse epidote veins with chalcopyrite or bornite cut stageJ-V magnetite-pyrite mineralization and associated chloriticalteration zones. Red microcline and subordinate light-pinkalbite occur as narrow haloes around calcite-chalcopyrite

172 CHEN ET AL.

0361-0128/98/000/000-00 $6.00 172

veins, and locally cut calcite, evidence that they partially post-dated Cu mineralization. Epidote and clinozoisite commonlyoccur in calcite in contact with K-feldspar. Barite locally oc-curs in these late K-feldspar veins, but the temporal relation-ships between K-feldspar veins and specular hematite (stageJ-VII) are ambiguous.

Stage J-VII—specular hematite: In the upper parts of theorebodies, a Cu sulfide-barren hematite stage locally devel-oped. Medium- to coarse-grained (0.1–0.5 mm), specularhematite forms veins cutting stage J-III actinolite alterationzones and stage J-V magnetite mineralization. Locally, stageJ-VII hematite replaces stage J-V (magnetite) and stage J-VI (Cu) mineralization in hydrothermal breccias. The re-placement and crosscutting relationships between stage J-VII hematite and Cu sulfides confirm the distinction ofstage J-VII.

40Ar/39Ar GeochronologyConventional K-Ar ages of 154 ± 4 (2 ) Ma and 160 ± 4 Ma

for, respectively, sericite and phlogopite from Marcona were re-ported by Injoque (1985). Although supporting a mid-Jurassic

age for the main period of magnetite mineralization, thesedata provide no evidence of the duration of hydrothermal ac-tivity in this multistage center. Moreover, the age relation-ships of the Marcona and Mina Justa deposits have remaineduncertain: only problematic K-Ar whole-rock dates rangingfrom 104 ± 3 to 132 ± 5 Ma have been reported for the latter(N. Hawkes, 2003, Marcona district geochronology and newage date results: unpub. memo, Rio Tinto Mining and Explo-ration Ltd., 2 p.). Laser-induced incremental-heating 40Ar/39Ar techniques were herein applied to high-quality mineralseparates from samples well documented in terms of field re-lationships and mineralogy. The dated materials include bi-otite, phlogopite, cummingtonite, tremolite, actinolite andmicrocline from Marcona, and actinolite and microcline fromMina Justa. The age spectra and corresponding Ca/K andCl/K ratios and inverse-isochron plots for Marcona and MinaJusta are illustrated in Figures 15 and 16, respectively; ana-lytical details are provided in the Appendix. The locations anddescriptions of the dated Marcona and Mina Justa samplesare documented in Table 4. All dates are quoted with an uncertainty of ± 2σ (95 percent confidence level). An age

EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 173

0361-0128/98/000/000-00 $6.00 173

TABLE 4. Summary of 40Ar/39Ar Ages from Marcona and Mina Justa

Sample no. Location Sample description Mineral dated Stage Plateau age ± 2 (Ma)

Marcona

MA5-9A* Marcona, Mina 5 open pit Fine-grained mt occurs interstitially cum M-I-A 177.0 ± 1.5(670 m a.s.l., NE corner) with cum and bt 175.2 ± 2.3

MA5-2 Marcona, Mina 5 open pit Fine-grained mt occurs with sulfide, (620 m a.s.l., center) cum and Phl phl M-I-B 171.5 ± 1.11

MA5-9B Marcona, Mina 5 open pit Fine-grained mt occurs interstitially bt M-III 161.4 ± 0.9(670 m a.s.l., NE corner) with cum and bt

MA3-30 Marcona, Mina 3 open pit Fine-grained mc alteration in dacite mc M-III 109.2 ± 0.6(580 m a.s.l., south wall)

MA3-24 Marcona, Mina 3 open pit Mc veins cut albitized dacite and are mc M-III 101.0 ± 0.6(580 m a.s.l., SW corner) refilled by act-sulfide veins

MA3-19 Marcona, Mina 3 open pit Coarse-grained mt-sulfides-phl phl M-IV 159.7 ± 0.8(580 m a.s.l., SW corner)

DDM3-3-8 Marcona, Mina 3 Trm-sulfide aggregates replacing trm M-V 158.5 ± 1.9drill core DDM3-3; 418 m massive mt

MA91-2* Marcona, Mina 11, Fine-grained mt occurs interstitially with trm M-V 157.3 ± 3.2drill core MA91, 92m trm and sulfides 156.8 ± 2.9

DDM5-4-2 Marcona, Mina 5 Act-sulfide veins cutting mt act M-V 156.6 ± 4.2drill core DDM5-4; 210 m

DDM3-3-1 Marcona, Mina 3 Trm cementing mt-sulfide-act clasts trm M-V 156.2 ± 2.4drill core DDM3-3; 340 m

Mina Justa

MA45-2* Mina Justa, Act alteration in andesite act J-I 157.3 ± 3.5drill core MA45, 262m 154.3 ± 5.5

MJ-6 Mina Justa, Hem-cp-ep-ab veins cut stage II mc (+ ab) J-II 142.4 ± 6.7drill core MA64 mc-altered andesite

MA64-3* Mina Justa, Coarse-grained act (± mt) brecciated act J-III 110.9 ± 0.7drill core MA64, 220.1m K-Fe metasomatised andesite 109.9 ± 1.0 1)

MA17-7 Mina Justa, Py-mt-mc veins mc J-V 103.7 ± 0.6 1)

drill core MA17; 364 mMA14-3 Mina Justa, Mt-py-mc aggregates in act-altered mc J-V 101.5 ± 0.7

drill core MA14; 394.7 m andesiteMA45-6 Mina Justa, Cal-cp veins (with ab-mc envelope) mc (+ ab) J-VI 99.1 ± 0.9

drill core MA45; 404.2 mMA17-9 Mina Justa, Cal-cp veins and late-filling mc veins mc Late-stage J-VI 95.0 ± 0.6

drill core MA17; 408.7 m cut act veins

* Samples with double runsMineral abbreviations: ab = albite, act = actinolite, bt = biotite, cal = calcite, cp = chalcopyrite, cum = cummingtonite, ep = epidote, hem = hematite,

mc = microcline, mt = magnetite, phl = phlogopite, py = pyrite, trm = tremolite 1 Plateau represents less than 50% of 39Ar released

174 CHEN ET AL.

0361-0128/98/000/000-00 $6.00 174

MA

5-9A

. Cu

mm

ing

ton

ite +

Gre

en

alit

e

177.

01.

5

MSW

D=

0.4

961

%A

r re

lea

sed

39

Inte

gra

ted

Ag

e: 1

61.8

11.

29

1C

a /

K

A24

0

200

160

120

80 40 00

4020

8060

100

0.00

15

0.00

13

0.00

11

0.00

09

0.00

07

0.00

05

3640Ar/ ArAge(Ma)

Cu

mu

lativ

eA

r pe

rce

nt

39

Inve

rse

iso

ch

ron

Ag

e:

177.

13.

7In

itia

l A

r/A

r=29

2

15

4036

MSW

D=

0.5

7

Mg

/Mg

+Fe

= 0

.74

3940

Ar/

Ar

Cl /

K

B17

5.2

2.3

MSW

D=

1.0

673

.8%

Ar

rele

ase

d39

Inte

gra

ted

Ag

e: 1

62.0

52.

89

Ca

/ K

1

Inve

rse

iso

ch

ron

Ag

e:

175.

33.

9

Initi

al

Ar/

Ar=

294

1

040

36

MSW

D=

1.4

Cl /

K

Cu

mu

lativ

eA

r p

erc

en

t39

MA

5-9A

. Cu

mm

ing

ton

ite +

Gre

en

alit

e

3940

Ar/

Ar

0.05

20.

060

0.06

40.

068

0.07

60.

084

0.00

22

0.00

18

0.00

14

0.00

10

0.00

06

240

200

160

120

80 40 00

4020

8060

100

MA

5-2.

Phl

og

op

ite

+C

hlo

rite

-Ta

lc

171.

51.

1

Inte

gra

ted

Ag

e: 1

55.2

40.

53

MSW

D=

1.6

35.5

%A

r re

lea

sed

39

0.1

Ca

/ K

C

0.09

30.

094

0.09

50.

096

0.09

70.

098

Inve

rse

iso

ch

ron

Ag

e:

175.

32.

3

Initi

al

Ar/

Ar=

123

8

940

36

MSW

D=

0.2

1

Cl /

K

Cu

mu

lativ

eA

r p

erc

en

t39

3940

Ar/

Ar

0.00

045

0.00

035

0.00

025

0.00

015

0.00

0050

4020

8060

100

0.09

20.

094

0.09

60.

098

0.10

00.

102

MA

5-2.

Phl

og

op

ite+

Ch

lorit

e-T

alc

No

pla

tea

u a

ge

Inve

rse

iso

ch

ron

Ag

e:

171.

01.

9

Initi

al

Ar/

Ar=

734

1

30

4036

MSW

D=

0.2

5

DR

eru

n

Ca

/ K

Cl /

K

Cu

mu

lativ

eA

r p

erc

en

t39

3940

Ar/

Ar

0.00

012

0.00

010

0.00

008

0.00

006

0.00

004

0.00

002

0.00

000

040

2080

6010

0

MA

5-9B

. Bio

tite

161.

4

MSW

D=

0.7

180

%A

r re

lea

sed

39

Inte

gra

ted

Age

: 160

.21

0.47

0.9

Ca

/ K

EM

g/F

e =

1.6

Ann

57

Cl /

KC

um

ula

tive

Ar

pe

rce

nt

39

0.09

90.

101

0.10

30.

105

0.10

7

Inve

rse

iso

ch

ron

Age

:16

0.6

1.2

Initi

al

Ar/

Ar=

328

4

940

36

MSW

D=

0.4

9

3940

Ar/

Ar

0.00

024

0.00

020

0.00

016

0.00

012

0.00

008

0.00

004

0.00

000

0.11

0.11

MA

3-30

. M

icro

clin

e

109.

2

MSW

D=1

.676

%A

r re

lea

sed

39

Inte

gra

ted

Age

:107

.83

0.47

0.6

Cu

mu

lativ

eA

r p

erc

en

t39

0.1

0.01

0.14

60.

126

0.13

00.

134

0.13

80.

142

Inve

rse

iso

ch

ron

Age

:11

0.2

3.2

Initi

al

Ar/

Ar=

279

6

240

36

MSW

D=

1.7

3940

Ar/

Ar

0.00

055

0.00

045

0.00

035

0.00

025

0.00

015

101.

0

MSW

D=1

.061

.2%

Ar r

ele

ase

d39

Inte

gra

ted

Age

:

0.6

Cu

mu

lativ

eA

r p

erc

en

t39

100.

610.

34

MA

3-24

. M

icro

clin

e

040

2080

6010

00

4020

8060

100

040

2080

6010

0

0.01

Ca

/ K C

l / K

Cl /

KC

a /

K

0.15

10.

141

0.14

30.

145

0.14

70.

149

0.00

028

Inve

rse

iso

ch

ron

Age

:10

2.8

1.9

Initi

al

Ar/

Ar=

255

4

040

36

MSW

D=0

.37

3940

Ar/

Ar

0.00

048

0.00

044

0.0

004

0

0.00

036

0.00

032

0.05

20.

060

0.06

40.

068

0.07

60.

084

159.

70.

8

Inte

gra

ted

Age

: 158

.82

0.30

MA

3-19

. Ph

log

op

ite

MSW

D=

1.0

797

.8%

Ar

rele

ase

d39

Ca

/ K

0.01 0.

1068

0.10

720.

1096

Inve

rse

iso

ch

ron

Ag

e:

159.

50.

9

Initi

al

Ar/

Ar=

452

3

30

4036

MSW

D=

1.1

9

Cu

mu

lativ

eA

r p

erc

en

t39

Mg

/Fe

= 2

1.5

Ann

4.5

3940

Ar/

Ar

0.1

Cl /

K

3640Ar/ ArAge(Ma)

240

200

160

120

80 40 00

4020

8060

100

H

0.00

003

0.00

002

0.00

001

0.00

000

DD

M3-

3-8.

Tre

mo

lite

158.

51.

9

MSW

D=

0.1

898

.7%

Ar

rele

ase

d39

100

Inte

gra

ted

Ag

e: 1

59.6

92.

31

0.00

4

0.00

3

0.00

2

0.00

1

0.00

0

Inve

rse

iso

ch

ron

Age

:15

8.0

4.7

Initi

al

Ar/

Ar=

295

2

340

36

MSW

D=

0.0

37

Mg

/Fe

+M

g =

0.9

2

3940

Ar/

Ar

1

Cu

mu

lativ

eA

r p

erc

en

t39

040

2080

6010

0

Ca

/ K

Cl /

K

0.00

0.02

0.04

0.06

0.08

0.10

0.12

DD

M5-

4-2

. Act

ino

lite

156.

64.

2

MSW

D=

0.3

680

.5%

Ar

rele

ase

d39

Ca

/ K

ratio

is o

ff s

ca

le

Inte

gra

ted

Ag

e: 1

48.7

54.

94

0.00

18

0.00

14

0.00

10

0.00

06

0.00

020.

064

0.06

80.

072

0.07

60.

080

0.08

4

Inve

rse

iso

ch

ron

Ag

e:

162

69

Initi

al

Ar/

Ar=

272

2

80

4036

MSW

D=

0.0

41

Mg

/Mg

+Fe

= 0

.64

3940

Ar/

Ar

1

10

Cl /

KCu

mu

lativ

eA

r p

erc

en

t39

040

2080

6010

0

JD

DM

3-3-

1. T

rem

olit

e

156.

22.

4

MSW

D=

1.0

100%

Ar

rele

ase

d39

Inte

gra

ted

Ag

e: 1

55.2

63.

52

Ca

/ K

rati

o is

off

sc

ale

0.00

4

0.00

3

0.00

2

0.00

1

0.00

00.

000.

020.

040.

060.

080.

100.

1239

40A

r/ A

r

Inve

rse

iso

ch

ron

Ag

e:

156.

17.

6In

itia

l A

r/A

r=29

3

13

4036

MSW

D=

0.1

03

Mg

/Mg

+Fe

= 0

.96

1C

l / K

Cu

mu

lativ

eA

r p

erc

en

t39

040

2080

6010

0

157.

3

MSW

D=

0.9

199

.9%

Ar

rele

ase

d39

Inte

gra

ted

Ag

e:

3.2

157.

923.

71

MA

91-2

Act

ino

lite

0.00

45

0.00

35

0.00

25

0.00

15

0.00

0.02

0.04

0.06

0.00

05

Inve

rse

iso

ch

ron

Ag

e:

155

22

Initi

al

Ar/

Ar=

300

4

040

36

MSW

D=

0.1

3

3940

Ar/

Ar

1

1010

0

L

Cu

mu

lativ

eA

r p

erc

en

t39

040

2080

6010

0

Ca

/ K

Cl /

K

Re

run

156.

8

MSW

D=

0.4

99.9

%A

rre

lea

sed

39

Inte

gra

ted

Age

:

2.9

155.

613.

64

MA

91-2

Act

ino

lite

0.00

5

0.00

4

0.00

3

0.00

2

0.00

0.02

0.04

0.06

0.00

1

Inve

rse

iso

ch

ron

Ag

e:

158

27

Initi

al

Ar/

Ar=

293

3

240

36

MSW

D=

0.0

42

3940

Ar/

Ar

1

10

100

Ca

/ K

Cl /

K

040

2080

6010

0

Cu

mu

lativ

eA

r p

erc

en

t39

240

200

160

120

80 40 0

240

200

160

120

80 40 0

240

200

160

120

80 40 0

240

200

160

120

80 40

0

F24

0

200

160

120

80 40

0

G

240

200

160

120

80 40

240

200

160

120

80 40

I24

0

200

160

120

80 40

240

200

160

120

80 40

000

0

K24

0

200

160

120

80 40 0

MR

eru

n

Sta

ge

M-I

-ASt

ag

e M

-I-A

Sta

ge

M-I

-BSt

ag

e M

-I-B

Sta

ge

M-I

IISt

ag

e M

-III

Sta

ge

M-I

II

Sta

ge

M-I

VSt

ag

e M

-VSt

ag

e M

-VSt

ag

e M

-VSt

ag

e M

-VSt

ag

e M

-V

FIG

.15.

Las

er-in

duce

d 40

Ar/

39A

r ag

e sp

ectr

a, w

ith C

a/K

and

Cl/K

rat

ios

for

each

hea

ting

step

, and

inve

rse

isoc

hron

plo

ts, f

or s

ampl

es fr

om M

arco

na a

ltera

tion

and

min

eral

izat

ion

stag

es.

EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 175

0361-0128/98/000/000-00 $6.00 175

MA

17-7

. M

icro

clin

e

103.

70.

6

MSW

D=

0.4

333

.9%

Ar

rele

ase

d39

Inte

gra

ted

Ag

e: 1

07.6

0.3

0.01

Ca

/ K

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0

100 0

50

0.00

074

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072

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068

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060

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30.

134

0.13

50.

136

0.13

70.

138

0.13

9

0.00

070

0.00

062

MA

14-3

. M

icro

clin

e

101.

50.

7

MSW

D=

0.4

383

.3%

Ar r

ele

ase

d39

Inte

gra

ted

Ag

e: 1

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Ca

/ K

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11

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100.

110.

120.

130.

14

MA

45-6

. M

icro

clin

e +

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ite

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MSW

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384

.9%

Ar r

ele

ase

d39

Inte

gra

ted

Ag

e: 9

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Ca

/ K

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0.00

20

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0.00

120.

000.

020.

040.

060.

080.

100.

12

MA

17-9

. M

icro

clin

e

95.0

0.6

MSW

D=

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72.6

%A

r re

lea

sed

39

0.1

Ca

/ K

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gra

ted

Ag

e: 9

3.8

0.4

J15

0

100 50

0.00

115

0.0

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0.0

009

5

0.00

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0.00

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0.00

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122

0.12

60.

130

0.13

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138

0.14

20.

146

0.15

0

MJ-

6. M

icro

clin

e +

Alb

ite

142.

46.

7

MSW

D=

0.3

489

.4%

Ar

rele

ase

d39

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gra

ted

Ag

e: 1

35.0

512

.35

1C

a /

K

0.00

32

0.00

28

0.00

24

0.00

20

0.00

160.

000.

010.

020.

030.

040.

05

Inve

rse

iso

ch

ron

Age

:10

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8.7

Initi

al

Ar/

Ar=

267

1

00

4036

MSW

D=

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0

Inve

rse

iso

ch

ron

Ag

e:

100.

92.

0In

itia

l A

r/A

r=30

0

13

4036

MSW

D=

1.0

4

Inve

rse

iso

ch

ron

Ag

e:

98.6

1.7

Initi

al

Ar/

Ar=

296.

4

2.9

4036

MSW

D=

1.2

Inve

rse

iso

ch

ron

Ag

e:

92.7

2.2

Initi

al

Ar/

Ar=

317

19

4036

MSW

D=

1.0

Inve

rse

iso

ch

ron

Ag

e:

140

11

Initi

al

Ar/

Ar=

297.

4 6

.340

36

MSW

D=

0.1

7

3940

Ar/

Ar

3940

Ar/

Ar

3940

Ar/

Ar

3940

Ar/

Ar

00

(060)

(204

)41

.550

.551

.0

42.0

mic

roclin

e

Orthocla

seSa

nidine

C

I G H

J

A15

7.3

MSW

D=

1.0

795

.2%

Ar

rele

ase

d39

Inte

gra

ted

Age

:

Age(Ma)

Cu

mu

lativ

eA

r p

erc

en

t39

156

.05.

7

3.5

MA

45-2

Act

ino

lite

0.00

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16

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50.

035

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08

Inve

rse

iso

ch

ron

Age

:16

322

Initi

al

Ar/

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284

4

040

36

MSW

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0.1

5

0.00

12

0.06

5

3640Ar/ Ar

1

10

100

Ca

/ K

Cl /

K

154.

3

MSW

D=

0.7

599

.8%

Ar

rele

ase

d39

Inte

gra

ted

Age

:

5.5

154.

068.

51

B

0.00

5

0.00

4

0.00

3

0.00

2 0.00

0.01

0.04

0.05

0.00

1

Inve

rse

iso

ch

ron

Ag

e:

161

35

Initi

al

Ar/

Ar=

288

3140

36

MSW

D=

0.0

56

0.02

0.03

0.06

1

10

C

Cu

mu

lativ

eA

r p

erc

en

t39

Ca

/ K

rati

o is

off

sc

ale

Cl /

K

MA

45-2

Act

ino

lite

Re

run

D

040

2080

6010

0

240

200

160

120

80 40 0

240

200

160

120

80 40 0

Sta

ge

J-I

I24

0

200

160

120

80 40 00

4020

8060

100

Cu

mu

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eA

r p

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en

t39

040

2080

6010

0

Sta

ge

J-I

Sta

ge

J-I

No

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tea

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ge

0.00

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0.00

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0.00

15

0.00

0.04

0.08

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05

Inve

rse

iso

ch

ron

Age

:99

.42.

7

Initi

al

Ar/

Ar=

302

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7.5

4036

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D=0

.21

The

last

ste

p1

10.9

0

.7

(Lo

w-s

tep

s 1-

4)T

1

10

3940

Ar/

Ar

3940

Ar/

Ar

Cl /

K

Ca

/ K

240

200

120

80 40 0

160

040

2080

6010

0

MA

64-3

. A

cti

no

lite

Sta

ge

J-I

II

3940

Ar/

Ar

E

109.

9M

SWD

=0

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28.2

%A

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sed

39

Inte

gra

ted

Ag

e:

1.0

121.

20.

6

0.00

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0.00

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0.00

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0.00

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104

0.10

80.

112

0.11

60.

0006

5

Inve

rse

iso

ch

ron

Age

:10

7.2

9.1

Initi

al

Ar/

Ar=

317

7

340

36

MSW

D=

1.0

9

1

10

Age(Ma) 3640Ar/Ar

040

2080

6010

0C

um

ula

tive

Ar

pe

rce

nt

39

MA

64-3

. A

cti

no

lite

Sta

ge

J-I

II

Re

run

Sta

ge

J-V

150

100 0

50

150

100 0

50

040

2080

6010

0C

um

ula

tive

Ar

pe

rce

nt

39

3940

Ar/

Ar

3940

Ar/

Ar

040

2080

6010

0C

um

ula

tive

Ar

pe

rce

nt

39

Sta

ge

J-V

G

Ca

/ K

Cl /

K

040

2080

6010

0C

um

ula

tive

Ar

pe

rce

nt

39

Sta

ge

J-V

ISt

ag

e J

-VI

040

2080

6010

0C

um

ula

tive

Ar

pe

rce

nt

39

FIG

.16.

(A

) X

-ray

pow

der

diff

ract

ion

data

for

hydr

othe

rmal

K-f

elds

pars

from

Min

a Ju

sta

(dat

a fa

ll in

the

mic

rocl

ine

field

s in

the

Al/S

i ord

erin

g di

agra

m o

f Wri

ght,

1968

). (B

-J)

Las

er-in

duce

d 40

Ar/

39A

r ag

e sp

ectr

a, w

ith C

a/K

and

/or

Cl/K

rat

ios

for

each

hea

ting

step

, and

inve

rse

isoc

hron

plo

ts fo

r ea

ch s

ampl

e.

plateau is defined as at least three contiguous outgassing stepswith ages that are concordant at 2σ errors, that account for atleast 50 percent of the 39Ar released, and show no monotonicincrease or decrease with temperature (Dalrymple and Lan-phere, 1974).

40Ar/39Ar spectra and age relationships

Marcona: Two acceptable and concordant plateau ages of177.0 ± 1.5 Ma and 175.2 ± 2.3 Ma are given by stage M-IAcummingtonite. The cummingtonite is locally replaced bystage M-III biotite (Fig. 9B), which gave a significantlyyounger plateau age of 161.4 ± 0.9 Ma (Fig. 15E; Table 4).The 175 to 177 Ma age for the cummingtonite hosted by theupper Marcona Formation demonstrates that it represents aprecursor alteration event during the initial deposition of theoverlying Río Grande Formation in the Aalenian, i.e., prior to174.0+1.0

–7.9 Ma and after 178.0+1.0–1.5 Ma (Pálfy et al., 2000). Stage

M-IB phlogopite gives a quasiplateau age of 171.5 ± 1.1 Ma(Fig. 15C), but the inverse-isochron age (171.0 ± 1.9 Ma) ob-tained for a duplicate sample (Fig. 15D) indicates that the“plateau age” is probably acceptable. In the latter sample, theage spectrum exhibits a configuration suggestive of reactor-induced 39Ar recoil, and therefore the apparent age of thehighest-temperature step, i.e., 172.6 ± 1.8 Ma, may representa maximum for the age of crystallization.

Coarse-grained stage M-III biotite gives a good plateau ageof 161.4 ± 0.9 Ma (Fig. 15E). Two stage M-III K-feldspars(maximum microcline) from potassic alteration zones indacite porphyry give acceptable plateau ages of 109.2 ± 0.6Ma and 101.0 ± 0.6 Ma (Figs. 15F and G), which aremarkedly younger than those for M-III biotite. Microclinehas a low closure temperature of 130° to 160°C (Harrison andMcDougall, 1982), and the two age plateaus are therefore in-terpreted as recording resetting by late thermal events. TheMarcona deposit area was therefore probably heated, albeitnon-pervasively, to over ~130°C in the mid-Cretaceous. Thelowest-temperature step in the spectrum for MA3-30 mayrecord a second, ca. 90 Ma resetting (Fig. 15F). A stage M-IVphlogopite gave a good plateau age of 159.7 ± 0.8 Ma (Fig.15H). This age, slightly younger than that for stage M-III bi-otite, is in conformity with the textural evidence for replace-ment of biotite by phlogopite (Fig. 9I).

Stage M-V tremolite and actinolite, both associated withchalcopyrite and pyrite, gave acceptable and similar plateauages of 158.5 ± 1.9 and 156.6 ± 4.2 Ma (Figs. 15I and J). Asecond stage M-V tremolite, unassociated with sulfides, givesa plateau age of 156.2 ± 2.4 Ma (Fig. 15K). An actinolite fromthe Mina 11 orebody, probably corresponding to either stageM-V or stage M-IV in the more westerly Marcona orebodies,gave a good plateau age of 157.3 ± 3.2 Ma (Fig. 15L). Anidentical plateau age of 156.8 ± 2.9 Ma (Fig. 15M) was ob-tained for a duplicate sample. These actinolite ages are con-cordant with those for stages M-IV and M-V actinolite fromMinas 3 and 5, demonstrating that, although mineralogicallydistinctive, the Cu-rich Mina 11 orebody represents an inte-gral part of the Marcona deposit.

Despite the possibility of resetting of biotite due to its rel-atively low closure temperature (ca. 310°C: Harrison et al.,1985), the plateau ages for the alteration minerals of stagesM-III, M-IV, and M-V at Marcona are in conformity with

paragenetic relationships (Fig. 8; Table 4), and imply that themain episode of base-metal mineralization shortly followedthe emplacement of the economic magnetite bodies.

Mina Justa: Two actinolite and five alkali feldspar separateswere dated from Mina Justa (Table 4). X-ray powder diffrac-tion study shows that all of the feldspars are strongly ordered,with the microcline structure (Fig. 16A).

Two heating runs for a fine-grained, stage J-I actinolite(MA45-2) gave good plateau ages of 157.3 ± 3.5 and 154.3 ±5.5 Ma (Figs. 16B and C). The stage J-I actinolite ages are,within error, concordant with those for the magnetite andpolymetallic sulfide mineralization at Marcona (Table 4).

An alkali feldspar separate (Fig. 16D) from the envelope ofa stage J-VI hematite-chalcopyrite (-epidote) vein-cutting al-tered Río Grande Formation andesite yields a significantlyyounger plateau age of 142.4 ± 6.7 Ma (Table 4). The low Kcontent is responsible for the large error (Fig. 16D). Feldsparstaining of this sample reveals intergrown albite, and the rel-atively high Ca/K ratio may record the presence of albitic pla-gioclase or minor epidote in the separates. The Berriasian ageis therefore interpreted as that of stage J-II (Fig. 13), imply-ing that the hematite, chalcopyrite, and epidote in the samplewere emplaced during stage J-VI through reopening of a frac-ture with previously feldspathized margins.

A stage J-III coarse-grained actinolite gives a disturbed ap-parent age spectrum (Fig. 16E), with a configuration sugges-tive of Cl-derived excess argon (McDougall and Harrison,1999) in the low-temperature steps. The age of the highest-temperature step, 110.9 ± 0.7 Ma, may therefore approachthe true age. The disturbed spectrum of a duplicate sampleincorporates a trough at 109.9 ± 1.0 Ma (Fig. 16F), which isconcordant within 2σ error with the inverse-isochron age(107.2 ± 9.1 Ma) given by the three steps constituting a “miniplateau.”

Microcline associated with the main Mina Justa magnetitestage (Table 4) has a quasi-plateau age of 103.7 ± 0.6 Ma (Fig.16G). In contrast, a second stage J-V microcline (MA14-3)gave a good plateau age of 101.5 ± 0.7 Ma (Fig. 16H). Micro-cline (+ albite) from the stage J-VI Cu mineralization (MA45-6) associated with calcite and chalcopyrite, but not with mag-netite, similarly yields an acceptable plateau age of 99.1 ± 0.9Ma (Fig. 16I), which is slightly younger than those of the twodated stage J-V microclines. A microcline vein (MA17-9)which reopened a calcite-chalcopyrite vein gives an age of95.0 ± 0.6 (Fig 16J).

Microcline has a low argon closure temperature and thefive apparent age plateaus determined for this mineral atMina Justa could therefore be interpreted as recording reset-ting by postmineralization thermal events, ascribable to thenumerous postmineralization ocöitic andesite dikes assignedto the Upper Cretaceous Tunga andesite (Caldas, 1978).However, the considerably older plateau age (142.4 ± 6.7 Ma)yielded by the stage J-II microcline indicates that Ar resettingof triclinic feldspar was not pervasive within the deposit,which was possibly due to fast cooling of andesitic dikes. Con-sidering the errors, two stage J-V microclines give an agerange of 100.8 to 104.3 Ma, at least 0.8 m.y. older than thoseof two stage J-VI microclines (94.4–100.0 Ma), which per-fectly conforms to the defined paragenesis (Table 4). Partiallyor complete resetting of K-feldspar Ar-Ar ages by Tunga

176 CHEN ET AL.

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andesite in the same district can not reasonably explain theseinternally consistent results. In addition, regarding the highclosure temperature of amphibole (~550°C; McDougall andHarrison, 1999), the age data for stage J-III actinolite mayimply a maximum age of ca. 110 Ma for stages J-V and VI al-teration and mineralization and the younger feldspar datesare therefore interpreted as representing crystallization ages.We conclude that the main hydrothermal events (stages J-IV–J-VI) at Mina Justa, i.e., those responsible for the forma-tion of hematite and magnetite as well as the Cu mineraliza-tion, occurred at ca. 100 Ma, in the Albian, which is generallyconcordant with the major IOCG epoch in the central Andes(Fig. 17). Stages J-I (albitization) and J-II (K-feldspathiza-tion), however, were much older and were not directly relatedto the development of the orebodies, but probably were tem-porally associated with the Marcona mineralization systemand subsequent hydrothermal events. Two young ages (109.2± 0.6 Ma and 101.0 ± 0.6 Ma) from stage M-III K-feldspar atMarcona may record the resetting by later Mina Justa hy-drothermal events or Tunga andesite. Nonetheless, the possi-bility of resetting of Mina Justa K-feldspar and actinolite byTunga andesite can not be entirely excluded and the MinaJusta mineralization and alteration system may have devel-oped prior to the Albian, or coincided with the Upper Creta-ceous Tunga andesite event.

Evolution of the Marcona-Mina Justa DistrictThe Mesozoic magnetite-rich mineralization of the Mar-

cona-Mina Justa district, extending over an area exceeding 75km2 and encompassing numerous dispersed orebodies, oc-curred episodically over a period of ca. 80 m.y., extendingfrom 176 Ma to, at least, 95 Ma, with the uncertainities ofpossible Ar-Ar resetting. Moreover, Cu mineralization atMina Justa is shown to have been emplaced at least 60 m.y.after that in the Marcona deposit and therefore cannot be ge-netically related to it. Figures 18 and 19 illustrate the majorevents in the district recorded by the newly defined parage-netic and age data.

(1) Precursor alteration, ca. 176 Ma (Fig. 18A)

Cummingtonite-dominated alteration (stage M-IA, Fig. 8)in metasedimentary units of the Marcona Formation ~300m below the unconformity with the Río Grande Formation,was apparently unassociated with either magnetite or Cusulfide mineralization. Development of an alkali-poor Mg-Fe amphibole implies either that the metaclastic, in part cal-careous, host rocks had earlier suffered feldspar destructionwithout the formation of muscovite, possibly as a result ofchloritization, or, more probably, were directly affected byMg-Fe metasomatism and concomitant Si, Ca, and alkali de-pletion. Similar early development of Mg-Fe clinoamphi-bole has been documented from several other IOCG de-posits, including Vähäjoki, Finland (Liipo and Laajoki,1991); Osborne, Cloncurry (Adshead, 1995); La Candelaria,Chile (Ullrich and Clark, 1997, 1999); Salobo, Carajás (Re-quia and Fontboté, 1999; Requia et al., 2003); and GuelbMoghrein, Mauritania (Kolb et al., 2006). At Osborne, am-phibole alteration, in part magnesian, may have occurred ca.55 m.y. prior to the main Cu mineralization event (Gauthieret al., 2001).

The two Ar/Ar age spectra for cummingtonite confirm thatthis precursor alteration took place in the Aalenian, contem-poraneously with the initial sediment-dominated, shallow-marine, accumulation of the overlying Río Grande Forma-tion. Hydrothermal activity is therefore inferred to haveoccurred at very low pressure and at temperatures consider-ably below 760°C (Evans and Ghiorso, 1995). A plausible en-vironment would be the southwest margin of an arc-parallelrift delimited by segments of the northeast-dipping Pista faultsystem (Fig. 18A), recording northeast-southwest extensionwithin the West Peruvian trough at the outset of the MiddleJurassic (Benavides-Cáceres, 1999).

(2) Precursor phlogopite-magnetite mineralization, 171 Ma (Fig. 18B)

40Ar/39Ar age spectra for hydrothermal phlogopite coexist-ing with magnetite in the upper Marcona Formation are in-terpreted as evidence either for a discrete alteration-mineral-ization event (M-IB) in the Bajocian or, less probably, for acontinuation of the hydrothermal activity recorded by the 177Ma cummingtonite. The mineral assemblage records intenseK-Fe metasomatism. At this stage, intermediate to silicic py-roclastic activity was well underway and a significant thick-ness of the lower Río Grande Formation had locally accumu-lated, in permissive agreement with a magmatic contributionto this apparently minor magnetite mineralization, the first ofat least four such episodes in the district. Localization by con-tinued displacement on the Pista fault system is favored (Fig.18B).

(3) Albite-scapolite alteration

No age data are available for the Na-Cl metasomatism re-sponsible for the widespread stage M-II albite ± marialiticscapolite alteration predating stage M-III: this can only bebracketed between ca. 171 and 162 Ma. Albitization at Mar-cona is markedly less intense than in most other IOCG cen-ters, e.g., the La Candelaria deposit (Ullrich and Clark, 1999),but is similarly unassociated with either magnetite or sulfidedeposition. Albite (-scapolite) alteration is also developed inboth dacite porphyry and Marcona Formation rocks in im-mediate contact with the Minas 2–4 magnetite orebody, andtherefore persisted into stage M-III.

(4) Main magnetite and magnetite-sulfide mineralization,159–162 Ma (Figs. 18C-D)

The excellent 40Ar/39Ar age plateaus determined for stageM-III biotite (161.4 Ma) and stage M-IV phlogopite (159.7Ma) directly associated with, respectively, the major Mina 5and Mina 3 magnetite orebodies, indicate that the most im-portant magnetite mineralization at Marcona took place closeto the Bathonian-Callovian boundary (Pálfy et al., 2000).They are also in agreement with the development of biotitelargely prior to the phlogopite. The initial deposition of chal-copyrite occurred at ca. 160 Ma. Iron oxide and sulfide min-eralization at Marcona therefore shortly followed the ca. 164to 166 m.y. hiatus in andesitic volcanism documented in theupper Río Grande Formation (Fig. 18C). This erosional eventrecords a regional, terminal-Bajocian (ca. 166 Ma), upliftevent which interrupted the protracted subsidence of theWestern Peruvian trough (Hosmer, 1959). In the Marcona

EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 177

0361-0128/98/000/000-00 $6.00 177

178 CHEN ET AL.

0361-0128/98/000/000-00 $6.00 178

Aa

len

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JurassicCretaceous

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Stra

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Min

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EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 179

0361-0128/98/000/000-00 $6.00 179

Ve

rtic

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FIG

.18.

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the

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osit.

180 CHEN ET AL.

0361-0128/98/000/000-00 $6.00 180

Ve

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FIG

.19.

Gra

phic

dep

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the

evol

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Min

a Ju

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Mar

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area, uplift was accommodated by the predominantly reverse,east-northeast –trending Repetición faults, which developedalong the main linear array of the earlier Pista faults as a re-sult of east-southeast–west-northwest contraction, probablyas a response to major sinistral transcurrent displacement onthe Treinta Libras fault system to the northeast (Figs. 2, 18C).Following the uplift and erosion, the late Bathonian-Oxfor-dian (ca. 155–164 Ma) submarine upper Río Grande Forma-tion volcanic and volcanosedimentary units accumulatedwithin a newly developed rift contiguous with the continentalmargin. Magnetite and the subsequent sulfide mineralizationat Marcona were emplaced along the Repetición faults duringaccumulation of the upper Río Grande Formation from ca.162 Ma (Fig. 18D).

(5) Polymetallic sulfide mineralization at Marcona and early albite-actinolite alteration at Mina Justa, 154–159 Ma (Fig. 18E)

During the main base-metal sulfide stage in the Marconadeposit there was a decreased deposition of pyrrhotite and amarked increase in pyrite and chalcopyrite, particularly in theMina 11 orebody. Sphalerite is abundant in the Mina 14 ore-body, where it is associated with minor galena. Polymetallicsulfide mineralization accompanied continued crystallizationof calcic amphiboles, both as pervasive alteration zones and asveins, but there was also a major increase in calcite deposi-tion, commonly in cavities in vuggy magnetite. The mag-netite-free and sulfide-dominated stage M-V assemblagesformed in a relatively reduced (pyrite-pyrrhotite fO2 buffer)and possibly cooler environment.

Stage M-V calcic amphiboles from the Mina 3 and Mina 5orebodies yield acceptable Ar/Ar age plateaus in the range of156.2 to 158.5 Ma, which implies that intense hydrothermalactivity at Marcona, initiated following the ca. 164 to 166 Mauplift event at the Bajocian-Bathonian boundary, persisted forca. 5 m.y., until the termination of andesitic volcanism in theextensional marginal rift (Fig. 18E). The ages of 154.3 and157.3 Ma for two stage J-I actinolites from Mina Justa indi-cate that stage J-I hydrothermal alteration of andesitic hostrocks, the albite-actinolite (Na-Ca) assemblage recording Nametasomatism, was here initiated in the upper Río GrandeFormation simultaneously with the polymetallic sulfide and/or magnetite mineralization in the lower Río Grande Forma-tion and the Marcona Formation at Marcona (Fig. 18E).

(6) Potassic metasomatism in the Mina Justa area, 142 Ma (Fig. 19A)

This probably weak Tithonian-Berriasian alteration event isrecorded by a microcline from an andesite of the upper RíoGrande Formation. The structural context of the potassic al-teration is uncertain, but it is likely that it was focused by fault-ing related to renewed movement on the Treinta Libras faultsystem to the northeast (Figs. 2, 19A). By ca. 142 Ma, JahuayFormation volcanic activity had apparently terminated (Cal-das, 1978) and no potentially parental intrusive bodies havebeen recognized in the immediate area. Injoque (1985), how-ever, reports whole-rock K-Ar dates of 137.4 ± 3 and 136.4 ±3 Ma for two “basic dykes” in the Marcona mine area, evi-dence for an otherwise undocumented Neocomian magmaticevent. The barren K metasomatism may have been a response

either to fluid expulsion resulting from continued or reneweddetachment faulting along the southwest margin of the evolv-ing Jahuay basin, comparable to that documented by Chapinand Lindley (1986) and Roddy et al. (1988), or to latest-Tithonian inversion of the basin.

(7) Actinolite-diopside-magnetite alteration at Mina Justa,ca. 110 Ma (Fig. 19B)

This early Albian Ca-metasomatism with minor magnetitedevelopment is recorded by stage J-III coarse-grained actino-lite which cements clasts of the 142 Ma K-feldspar-magnetitealteration assemblage. This age coincides with the early stagesof emplacement of the Coastal batholith in the Acarí area tothe east (Fig. 2), i.e., ≤109 ± 4 Ma, and is contemporaneouswith biotitic alteration at the Jahuay Formation-hostedPampa del Pongo magnetite prospect (Hawkes et al., 2002; G.Lester, pers. commun., 2007). By the late Albian, accumula-tion of the probably Aptian to early Albian, dominantly clasticCopara Formation had probably ceased in the Cañete basin(Caldas, 1978); the closest outcrop of this unit is ~12 to 15 kmeast of Mina Justa (Fig. 19B). Further, the hypabyssal intru-sion of the andesitic Bella Unión Complex (Caldas, 1978), ex-tensively exposed north and east of Mina Justa, probably sig-nificantly antedated mineralization.

(8) Hematite-calcite mineralization at Mina Justa

This now covert, undated, stage, not illustrated in Figure19, records the incursion of oxidized, CO2-rich fluids separat-ing the relatively reduced, magnetite-rich stages J-III (110Ma) and J-V (≤104 Ma). Comparable oscillations in fO2 arecharacteristic of the IOCG environment (e.g., Pop et al.,2000), evidence for the involvement of multiple hydrother-mal fluids. This represents the initial deposition of calcite atMina Justa. No sulfides were precipitated in stage IV, al-though the hematite-calcite assemblage is identical to that ofthe ore-forming stage J-VI. Because depositional conditionsin these stages were probably similar, we infer that reducedsulfur was not present in significant abundance at stage J-IV.

(9) Magnetite and Cu sulfide mineralization at Mina Justa, 95–104 Ma (Fig. 19C, D)

Four Ar/Ar dates for microcline record the major eventswhich generated the Mina Justa orebodies, defining a pro-tracted Albian-to-Cenomanian history of hydrothermal activ-ity. Thus, magnetite-pyrite-quartz–dominated assemblagesassigned to stage J-V give ages of 103.7 and 101.5 Ma, signif-icantly postdating the ca. 110 Ma stage J-III actinolitic alter-ation. These thermal events apparently also affected the Mar-cona deposit, in which stage M-III microclines were locallyreset at 101–109 Ma. Feldspathic alteration envelopes tochalcopyrite-calcite veins representing the main, stage J-VICu mineralization formed at 99.1 Ma, and terminal, late-stageJ-VI microcline veins associated with minor chalcopyrite-cal-cite veins record the apparent termination of sulfide deposi-tion at ca. 95 Ma (Table 4).

Numerous authors (e.g., Jaillard et al., 2000; Oyarzun etal., 2003) have emphasized the geodynamic impact of the re-organization of plate interactions in late-Neocomian time,probably triggered by development of a mid-Pacific super-plume (Larson, 1991). Coupling of the converging plates,

EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 181

0361-0128/98/000/000-00 $6.00 181

with a northeast vector, ended a long period of orthogonal ex-tension and sinistral transtension along the central SouthAmerican littoral, and gave rise to dextral transtension andthe formation of mid-Cretaceous basins (Polliand et al.,2005). The subsequent increasing plate coupling resulted inuplift and basin inversion, accompanied by emplacement ofthe earliest, ca. 109 Ma, plutons of the Arequipa segment ofthe Coastal batholith (Vidal et al., 1990; Atherton, 1990) andthe associated magnetite-actinolite (-biotite) alteration atPampa de Pongo and Mina Justa. In the Marcona area, dex-tral transtension on the Treinta Libras fault system (Pope,2003) generated the east-northeast–striking Mina Justa nor-mal faults which controlled the emplacement of both mag-netite lenses and Cu orebodies (Fig. 19C, D). However, Cumineralization at Mina Justa significantly postdated mag-netite-pyrite alteration. The Upper Cretaceous Tunga an-desite developed during or after Mina Justa mineralizationand was emplaced mainly along the younger Huaca faults.

It is unlikely that exotic, basin-derived fluids contributed tothe early high-temperature (~600°C: Chen, 2008) magnetite-pyrite alteration stage at Mina Justa, despite the proximity ofthe Cañete basin to a well-developed fault system cutting theRío Grande Formation (Fig. 19C) which could have served asa pathway for fluid circulation. The high-T, Cu-barren, mag-netite-pyrite assemblage may be evidence for either a Cu-poor magmatic fluid or, less probably, the suppression of Cusulfide deposition by high temperatures (Hezarkhani et al.,1999). During further compression and basin inversion, ex-otic, possibly basinal fluids (Chen, 2008), are inferred to haveinvaded the Jurassic andesite succession along the deep de-tachment fault depicted in Figure 19C and D. However,these exotic fluids were plausibly driven by heat from deep-seated intrusions, represented by the dioritic stocks in theMina Justa area, and the possibility of mixing with magmaticfluids cannot be excluded. Ore-forming fluids rose along theMina Justa fault system and replaced the stage J-V magnetite-pyrite bodies (Fig. 19D). The coexistence of hematite withbornite-chalcocite-chalcopyrite assemblages indicates a rela-tively low T and P (i.e., <500°C and <1 kbar: Hemley et al.,1992) and a medium to high pH (calcite stable) environmentfor Cu mineralization. This mineralization event is un-recorded in the Marcona deposit, and the stage J-VI hy-drothermal system was therefore restricted areally to themargin of the Cañete basin.

ConclusionsThe Middle Jurassic Marcona Fe and mid-Cretaceous

Mina Justa Cu deposits, although both controlled by majorfault systems transecting the Mesozoic, dominantly andesiticarcs of the Cordillera de la Costa, developed in contrastedtectonomagmatic environments. Thus, the swarm of majormagnetite orebodies at Marcona formed within an exten-sional rift during the eruption of the upper-Río Grande For-mation arc, following a brief episode of uplift and erosion,whereas the weak superimposed Cu-Zn (Pb) sulfide assem-blages were emplaced during the terminal stage of arc devel-opment. Mineralization was focused both within the shallow-marine volcanic pile and in the underlying Paleozoicmetaclastic basement, and was associated directly with smallhypabyssal dacite porphyry plugs rather than with granitoid

plutons. Early hydrothermal alteration (stage M-IA: Mg-Femetasomatism) and weak magnetite mineralization (stage M-IB) developed up to 300 m beneath the ocean floor, 9 to 15m.y. prior to the main magnetite emplacement, precisely con-temporaneous with the intial development of the arc. Noother central Andean IOCG deposit displays a comparably in-timate relationship with shallow marine volcanism, althoughthe large El Laco cluster of magnetite orebodies (Park, 1961;Naslund et al., 2002) may be regarded as a Pliocene analog,similarly associated with andesitic volcanism and shallowdacitic intrusion, albeit in an entirely subaerial setting.

The Mina Justa Cu (-Ag, Au) prospect is, however, repre-sentative of the small number of large Cu-rich IOCG centersin the central Andes. Salient features include the following:emplacement in significantly older, largely volcanic stratarather than in broadly synmineralization granitoids; the local-ization of sulfide mineralization, dominated by hydrothermalbreccia and stockwork, by older, sulfide-deficient and tenu-ously related, magnetite bodies; and the channeling of brinesby regionally extensive fault systems accommodating the con-tractional inversion of arc-parallel volcano-sedimentary basins.

The pre-cordilleran Late Triassic through mid-Cretaceouscentral Andean orogen repeatedly generated tectonomag-matic and shallow marine stratigraphic environments, foster-ing both magnetite-dominant, “Kiruna-type,” and Cu-richIOCG mineralization. Hydrothermal activity in the Marcona-Mina Justa district occurred episodically over a period of ca.80 m.y. and Cu mineralization at Mina Justa is shown to havebeen emplaced at least 60 m.y. after magnetite deposition atMarcona. Moreover, at Mina Justa, the Cu mineralization oc-curred betweem 5 and 10 m.y. after magnetite-pyrite forma-tion. The protracted history of hydrothermal activity in theMarcona district is matched by those of other major IOCGcenters in the Andes. In the La Candelaria-Punta del Cobredistrict, a U-Pb zircon age for intramineralization dacite indi-cates that Cu (-Au) mineralization had commenced by 125Ma (Pop et al., 2000), whereas the major Cu-Au orebodieswere emplaced between ca. 115 (Ullrich and Clark, 1999;Mathur et al., 2002) and 112 Ma (Ullrich and Clark, 1999;Marschik and Fontboté, 2001). In the wider Mantoverde dis-trict, mineralization was initiated at 131 to 126 Ma, definedby a U-Pb isochron age for apatite-magnetite from the Car-men iron deposit and a U-Pb age for titanite from Man-toverde (Gelcich et al., 2002, 2005), but K-Ar ages of sericitefrom Mantoverde (Vila et al., 1996) and 40Ar/39Ar plateau agesof actinolite from the Todos Los Santos Cu-Au veins and theJerusalem magnetite-apatite deposit may indicate that IOCGactivity persisted at least to 117 Ma.

AcknowledgmentsField and laboratory research was funded by Natural Sci-

ences and Engineering Research Council of Canada grants toA.H.C and T.K.K. Chariot Resources, Shougang Hierro PerúSA, and Rio Tinto Mining and Exploration Ltd. generouslyprovided unstinting logistical support for the field work.Kerry Klassen was particularly helpful in the laboratories ofthe Queen’s University Facility for Isotope Research (QFIR).Peter Jones, Carleton University, is thanked for the electronmicroprobe analyses. Joan Charbonneau uncomplaininglyprepared numerous versions of the manuscript. The authors

182 CHEN ET AL.

0361-0128/98/000/000-00 $6.00 182

greatly appreciate the comments from two Economic Geologyreviewers, Antoine de Haller and Roger Skirrow, which con-siderably improved the quality of this paper.

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40Ar/ 39Ar techniques

Mineral separates were hand picked, washed in acetone,dried, wrapped in aluminum foil and stacked in an irradiationcapsule with similar-aged samples and neutron flux monitors(Fish Canyon Tuff sanidine, 28.02 Ma (Renne et al., 1998)).The samples were irradiated on July 4 through July 6, 2006,at the McMaster Nuclear Reactor in Hamilton, Ontario, for90 MWH, with a neutron flux of approx. 3 × 1016 neu-trons/cm2. Analyses (n = 57) of 19 neutron flux monitor posi-tions produced errors of <0.5 percent in the J value. The sam-ples were analyzed on July 25–August 1 and August 13–14,2006, at the Noble Gas Laboratory, Pacific Centre for Iso-topic and Geochemical Research, University of British Co-lumbia, Vancouver, BC, Canada. The mineral separates werestep-heated at incrementally higher powers in the defocusedbeam of a 10W CO2 laser (New Wave Research MIR10) untilfused. The gas evolved from each step was analyzed by aVG5400 mass spectrometer equipped with an ion-countingelectron multiplier. All measurements were corrected for

total system blank, mass spectrometer sensitivity, mass dis-crimination, radioactive decay during and subsequent to irra-diation, as well as interfering Ar from atmospheric contami-nation and the irradiation of Ca, Cl, and K.

The plateau and correlation ages were calculated using Iso-plot ver. 3.09 (Ludwig, 2003). Errors are quoted at the 2-sigma (95% confidence) level and are propagated from allsources except mass spectrometer sensitivity and age of theflux monitor. The best statistically justified plateau andplateau age were picked based on the following criteria: (1)three or more contiguous steps making up more than 50 per-cent of the 39Ar; (2) probability of fit of the weighted meanage greater than 5 percent; (3) slope of the error-weightedline through the plateau ages equals zero at 5 percent confi-dence; (4) ages of the two outermost steps on a plateau arenot significantly different from the weighted-mean plateauage (at 1.8σ six or more steps only); (5) Outermost two stepson either side of a plateau must not have nonzero slopes withthe same sign (at 1.8σ nine or more steps only).

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APPENDIX

Analytical Techniques