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0361-0128/10/3868/155-31 155
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 1Z4
3 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, Perú
5 Rio Tinto Mining and Exploration, Manco Capac 551, Miraflores, Lima 18, Perú
Abstract
The 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%) at
Marcona was controlled by northeast-striking faults transecting an active andesitic-dacitic, shallow-marine
Middle 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 the
earliest, 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 (171
Ma) generated, respectively, cummingtonite and phlogopite-magnetite assemblages through high-temperature
Mg-Fe metasomatism of previously metamorphosed Lower Paleozoic Marcona Formation siliciclastic rocks
and 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 an
en 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 of
andesitic eruption and dacitic intrusion which terminated the growth of the arc, but was hosted largely by
quartz-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) at
ca. 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 pyrite
at 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 of
breccia 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 ± 0.25
Ma 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, 2009
Accepted: 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 Olympic
Dam Cu-U-Au (-REE) deposit in 1975. Although most early
identified IOCG systems, e.g., those of the Gawler craton of
South Australia, the eastern Mount Isa inlier of Queensland,
and the northern Fennoscandian Shield, are of Proterozoic
age, the central Andean orogen, and especially the vol-
canoplutonic arcs of Jurassic and Cretaceous age exposed in
the Cordillera de la Costa of northern Chile and central and
southern Peru, are now recognized as hosting major IOCG
mineralization (Fig. 1). The well-defined tectonomagmatic 
environment in this region provides an ideal context for clar-
ification of the genesis and metallogenic relationships of this
problematic class of mineralization (Sillitoe, 2003). Marschik
and Fontboté (2001), de Haller et al. (2006), and Sillitoe
(2003) interpreted central Andean IOCG deposits on the
basis of magmatic-hydrothermal models, although Sillitoe
emphasized their distinction from the magnetite-rich por-
phyry Cu-Au group. In contrast, the incursion of “exotic,” in
part evaporite-sourced, brines has been argued to be essential
to economic Cu (-Au) mineralization, and the involvement of
such 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
156 CHEN ET AL.
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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 the
inversion of back-arc volcanosedimentary basins. The latter environment recurred episodically in the Mesozoic
Andes, as in comparable orogenic settings elsewhere, and extended histories of hydrothermal alteration and
mineralization, incorporating numerous barren events, may therefore represent a salient feature of the IOCG
deposit clan.
12 o
14 o
77 o 75 o
14 o
12 o
Marcona
Pampa de
Pongo
Lima
Monterrosas
Eliana
Raul-Condestable
Neogene to Quaternary
Sediments
Cenozoic
Subaerial Volcanics
MesozoicCoastal Batholith
Mina JustaMesozoic Basinal Volcanics
and Sediments
Paleozoic
San Nicolas Batholith
Precambrian (includes some Paleozoic)
Metamorphic Basement
Bolivia
Argentina
N
Eliana
Monterrosas
Marcona
Pampa de Pongo
Mantos Blancos
Candelaria-Punta del Cobre
El Espino
Mina Justa
Pacific
Ocean
Chile
Peru
20
S
o
70 Wo
A B
B
Pacific
Ocean
Fig 2
El Laco
Raul-
Condestable
Licona
SantiagoValpara so
Cerro Pelado
El Algarrobo
30
S
o
Cristales
El Romeral
Los lorados
Boqueron Chanar
s
ntoverde
Cerro Negro
Chilean iron belt
Tocopilla
Guanillos
Gatico
Maguayan
Montecristo-Julia
Carrizalillo de Las Bombas
Teresa de Colmo
El Salado
Las Animas
Rosa Maria
Galleguillos Dulcinea
Ojancos Nuevo
FarolaCarrizal Alto
Quebradita
La Higuera
Brillador San Antonio
Tamaya Panulcillo
Los 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
C
anete
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 Benavides
et 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 IOCG
mineralization is provided by the proposal that the majority of
magnetite-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 arguments
through documentation of the Marcona district of littoral
south-central Peru, which juxtaposes major IOCG-style ore
deposits with widely variable proportions of iron oxides and
copper sulfides. Marcona itself, representing much the
largest-known concentration of high-grade magnetite ore in
the central Andes, is centered in Nazca Province, Ica Depart-
ment, at latitude 15°12' S, longitude 75°7' W (Figs. 1, 2), 10
to 15 km from the Pacific coast and below 800 m a.s.l. Hosted
by 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 Hierro
Perú 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) at
latitude 15°10' S, longitude 75°5" W and an altitude of 785 to
810 m a.s.l., has an indicated open pit resource of 346.6 Mt at
an average grade of 0.71 percent Cu, 3.8 g/t Ag and ~ 0.03 g/t
Au 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 giant
magnetite deposit, Pampa de Pongo, located 30 km southeast
of Marcona-Mina Justa (Fig. 2; Hawkes et al., 2002) and with
an 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 Acarí
Hierro 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 in
the Acarí-Cobrepampa district (Fig. 2; Caldas, 1978; Injoque,
1985). This paper documents the geology and evolution of the
Marcona 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 Marcona
magnetite orebodies and apply light stable isotope geochem-
istry and fluid inclusion microthermometry and chemistry to
the identification of fluid sources.
Regional and District Geological Setting
The subdued coastal cordillera of south-central Perú (Fig.
1B) exposes remnants of a succession of volcanoplutonic arcs
which 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 the
South American plate. However, the Andean magmatic
record in the immediate Marcona area (Fig. 2) is dominated
EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 157
0361-0128/98/000/000-00 $6.00 157
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 de
Pongo
Pampa de
Pongo
Mina JustaMina Justa
Pacific
Ocean
Faults
San JuanSan Juan
San NicolasSan Nicolas
Towns
Active mines
inactive mines
(including prospects)
A Stratigraphic columnsin figure 4
B
C
Argent inaArgent ina
Treinta Lib
ra
s Fa
ult
Lechuza Fault
Tung
a
 Fa
ult
Pampa Las Galgas
Pampa de Poroma
Pampa El Choclon
Pampa de PongoPampa Colorado
Pampa Pajayuna
Pampa Lagunal
Pampa Lagunal
Grande
Rio Grande CanyonRio Grande CanyonA
5k
m
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 units
and by mid-Cretaceous granitoid plutons. Stratigraphic rela-
tionships in the wider Marcona area, incorporating data from
Caldas (1978), Vidal et al. (1990), Hawkes et al. (2002) and
this study, are summarized in Figure 3. 
The discontinuous Peruvian IOCG belt (Fig. 1) is under-
lain by high-grade metamorphic rocks of the allochthonous
Paleoproterozoic-to-Mesoproterozoic Arequipa Massif (Waste -
neys et al., 1995; Loewy et al., 2004), comprising schists,
gneisses, granites, and migmatites cut by basic and pegmatitic
158 CHEN ET AL.
0361-0128/98/000/000-00 $6.00 158
C
e
n
o
zo
ic
M
e
so
zo
ic
P
re
-M
e
so
zo
ic
Era Series Formation Li thology Intrusive
Quaternary1,2
Sencca
Formation2Miocene-
Pliocene Millo
Formation2
Miocene
Pisco
Formation2
1 2- including Miocene and Pliocene sediments; - the estimated thicknesses for the Quaternary, Sencca, Millo, Pisco and Chiquerio
formations are: 350 m, 50m, 3m, 500 m, and 100-800 m, respectively (not to scale in this column).
Aptian to
lower Albian
(110-125 Ma)
(Fossi l age range)
Copara
Formation
(1000 m)
Neocomian
(125-146 Ma)
(Fossi l age range)
Yauca
Formation
(1500 m)
Kimmeridgian
to Tithonian
(146 -155 Ma)
(Fossi l age range)
Jahuay
Formation
(1000 m)
Callovian
to Oxfordian
(155-164 Ma)
(Fossi l, K-Ar ages)
R
io
 G
ra
n
d
e
 Fo
rm
a
ti o
n
(4000
m
)
Lower
Paleozoic
( > 425 Ma)
Marcona
Formation
(1500 m)
Neoproterozoic
San JuanFormation
(3000 m)
Chiquerio
Formation 2
Paleo-to-
Mesoproterozoic
940, 1200 and
1820 Ma
(metamorphism,
U-Pb age)
Arequipa
Massif
Mina Justa
Cu orebodies
Marcona
Fe orebodies
Widespread marine terraces, aeolian sands; alluvium
White to rose colored tuffs of dacitic to rhyolitic
composition
Loosely consolidated marine sandstones
and conglomerates
Thick conglomerates, yellow and reddish sandstones,
shales, bentonite beds, fine-grained volcaniclastics
Conglomerates, with mainly volcanic fragments, feldspathic
sandstones, violet graywackes, red shales, minor tuffs, lava
flows and limestones with chert nodules
Shales, mudstones and sandstones
Agglomerates, brecciated lava flows,
conglomerates and sandstones, quartzites, shales and
limestones. Sills with compositions similar to the
lava flows
Conglomerates, dolomitic marbles, siltstones, sandstones,
silicified limestones with chert laminations and quartz layers
Hornblende and pyroxene metamorphism
Dolomitic marbles and chloritic schists
Base: calcareous schists, dolomitic marbles, calcareous
marls and turbidites
Top: dolomitic marbles and chloritic schists
Central: Pelitic rocks
Tillites with dolomites near top
Gneisses, granites, migmatites and schists cut by multiple-
stage basic and pegmatitic dikes
?
Dacite
San Nicolas
Batholith
(425 4Ma)
Aalenian to
Bajocian
(166-179 Ma)
(Fossi l age range)
Upper
Rio Grande
Formation
Lower
Rio Grande
Formation
(Cerritos
Formation
at Marcona)
Porphyritic, partly pillowed, K-rich, calc-alkaline andesites. Minor
intercalations 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)
Marcona
Fe orebodies
unconformity
unconformity
unconformity
unconformity
unconformity
unconformity
unconformity
?
Coastal
Batholith
(80- < 109 Ma) Tunga
Andesite 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 by
Neoproterozoic and Paleozoic sedimentary strata and, more
extensively, 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 the
majority of the economic magnetite orebodies at Marcona, is
dominated by quartz-rich siltstones and sandstones, interca-
lated with minor quartz arenites and impure limestones and
dolostones (Atchley, 1956; Injoque, 1985). It is intruded and
metamorphosed by the post-kinematic, 425 ± 4 Ma (Mukasa
and Henry, 1990; Vidal et al., 1990), San Nicolás granitoid
batholith (Fig. 2), and is therefore at least Early Silurian in
age. 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 diopside
and forsterite porphyroblasts record the local attainment of
the pyroxene hornfels metamorphic facies.
The Jurassic and Cretaceous strata of the wider Marcona
area 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 Formation
and the dominantly hypabyssal andesitic-dacitic Bella Unión
complex 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 a
polymictic 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 folded
red sandstones, shales, limestones, and brecciated andesitic
flows with high K calc-alkaline-to-shoshonitic compositions
EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 159
0361-0128/98/000/000-00 $6.00 159
? ?
Canon Rio Grande
(Aguirre, 1988; Ruegg, 1956)
Marcona area
(modified from Injoque, 1985,
and Atchley, 1956)
Pampa de Pongo
(Caldas, 1978)
200
m
300
m
N
o
tto
s
c
a
le
in
th
is c
o
lu
m
n
Arequipa Massif Marcona Formation Sandstone Conglomerate
Limestone Tuff Andesite
Volcanic
Breccias
Magnetite
orebodies
Inferred
boundary
Inferred
unconformity
Lo
w
er R
i o
 G
ra
n
d
e
 Fo
rm
a
t io
n
U
p
p
e
r R
io
 G
r a
n
d
e
 Fo
rm
a
tio
n
Lo
w
er R
io
 G
ra
n
d
e
 Fo
rm
a
t io
n
M
a
rc
o
n
a
 Fo
rm
a
tio
n
Upper
Rio Grande
Formation
U
p
p
e
r R
io
 G
r a
n
d
e
 Fo
rm
a
tio
n
Callovian-Oxfordian
151- 161 Ma
(Not to scale)
?
Mina Justa
Cu-orebodies
Aalenian-Bajocian
166-178 Ma
Lower
Rio Grande
Formation
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 Pampa
de 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 the
formation is established by Aalenian fauna (W.J. Arkell, in
Rüegg, 1956; Roperch and Carlier, 1992), indicating that
shallow-marine sedimentation was underway by 174.0+1.0–7.9 Ma
and 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 of
177.1 ± 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 to
154.7+3.8–3.3 Ma, but was interrupted between ca. 166 and 164
Ma, which is recorded by the unconformity between lower
and upper Río Grande Formation. All of the formation
records nondeformational, very low grade, zeolite or prehnite-
pumpellyite facies metamorphism (Aguirre and Offler, 1985;
Aguirre, 1988). However, the accurate deposition ages for the
host rocks of the Marcona magnetite deposit (i.e., Marcona For-
mation and lower Río Grande Formation) and Mina Justa Cu
deposit (i.e., upper Río Grande Formation) are still unknown.
Dike swarms, sills, and small plugs assigned to the Tunga
Andesite intrude the Yauca Formation and older units (Cal-
das, 1978; Fig. 3). The most characteristic lithology is a
coarsely porphyritic rock with large (≤1.5 cm) glomerocrysts
of labradorite and sparse augite phenocrysts, informally
termed “ocöite” (Hawkes et al., 2002) by analogy with the
broadly contemporaneous, strikingly porphyritic andesites of
the Ocoa Formation in the Copiapó area of northern Chile
(Thomas, 1958). Essentially identical textures are, however,
shown by several Río Grande Formation andesitic flows in
the Mina Justa area, a potential source of stratigraphic confu-
sion. Ages for bothTunga andesite and upper Río Grande
Formation andesite are not well defined. 
Granitoid plutons of the Cretaceous Coastal batholith
(Pitcher and Cobbing, 1985) intrude Neocomian and older
strata in the Acarí-Cobrepampa area (Fig. 2; Dunin-
Borkowski, 1970; Caldas, 1978). U-Pb zircon age data are
lacking for this part of the Arequipa segment of the batholith,
but K-Ar (Cobbing, 1998) and Rb-Sr (Sánchez, 1982) dates
for, respectively, the Acarí diorite and Cobrepampa mon-
zonite-monzogranite suggest that granitoid intrusion locally
began at ca. 109 ± 4 Ma, shortly after emplacement of the
Bella Unión complex. Small, undated, dioritic stocks, 7 to 8
km east-southeast and southeast of the Mina Justa prospect
(Caldas, 1978), may be correlative with the larger intrusions
to the east.
The Marcona Magnetite Deposit
The 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 magnetite
orebodies (“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, and
the 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 and
siltstone with minor limestone. The hypogene grades (Table
1) of the larger orebodies hosted by the Paleozoic metasedi-
ments average 57 to 58 percent Fe, significantly exceeding
those of 41 to 48 percent for the orebodies in Jurassic strata.
Whereas the total sulfur content of the orebodies is consistent
at ~3 wt percent, the copper content is more variable, aver-
aging 0.06 to 0.18 percent, but attaining 0.4 wt percent in
Mina 1 and 0.9 percent in the upper part of the easternmost,
Mina 11, orebody (Fig. 5). Pyrrhotite occurs mainly in the
lower, and chalcopyrite in the upper levels of the orebodies.
As exemplified by the schematic cross section of the Mina 4
orebody (Fig. 7A), most orebodies at Marcona yield higher
Cu grades as well as elevated total sulfide contents in their
upper parts, although sulfides are locally enriched in the
lower parts of some orebodies. Sphalerite and galena, nor-
mally subordinate to chalcopyrite, are abundant in the Mina
14 orebody. The clearly epigenetic orebodies are dominated
by essentially massive magnetite, and most original contacts
with both Paleozoic and Jurassic host rocks are abrupt, only
locally complicated by disseminated mineralization, stock-
work veining, or hydrothermal breccias. 
The mineralized area is intruded by a swarm of hypabyssal
bodies (Fig. 7). These range from apparently syn- to clearly
postmineralization 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, in
part “ocöitic,” andesite dikes are largely postmineralization
(Fig. 7A), dacitic porphyry bodies have complex, amoeboid
relationships with massive magnetite orebodies, possibly ev-
idence for the comingling of silicate and oxide melts (Chen,
2008). 
160 CHEN ET AL.
0361-0128/98/000/000-00 $6.00 160
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 35 224
Fe grade (%)3 58.5 60.2 57.3 58.1 57.0 54.4 41.9
Cu grade (%)3 0.17 0.06 0.06 0.11 0.08 0.45 0.04
S content (%)3 3.55 2.57 3.10 2.51 2.97 3.51 2.86
Zn 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), but
new observations show that at least four are represented. The
oldest, Pista normal faults, strike 295° and dip 60° to the north.
Together with the coeval or younger Repetición faults, they
are inferred to record east-southeast-west-northwest contrac-
tion during the Jurassic, perhaps linked to sinistral shear along
the regionally important, northwest-trending, Treinta Libras
fault zone northeast of Marcona (Figs. 2, 5). Emplacement of
the 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 Repeticíon
fault system may include a series of faults that formed before
mineralization 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 the
Repetición faults at Marcona, but they dip shallowly southeast
rather than northwest, and show normal displacement. Recog-
nized herein in the Marcona mine, where they segment the
orebodies (Fig. 7), these faults may record a change to dextral
transtension on the Treinta Libras fault. The youngest, Huaca,
normal faults strike 335° and dip 60° to the east. They are
postmineralization at both Marcona and Mina Justa, but are
commonly followed by porphyritic andesitic (ocöite) dikes. 
Paragenetic relationships
Numerous stages of hydrothermal alteration and hypogene
mineralization, M-I through M-VII, are recognized, largely
on the basis of megascopic and microscopic textural relation-
ships and mineral assemblages (Fig. 8). Representative elec-
tron microprobe analyses of alteration minerals are recorded
in Table 2, complementing the data of Injoque (1985). 
EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 161
0361-0128/98/000/000-00 $6.00 161
15 10 So,
75 05 Wo,
Marcona
Mina Justa
1 Km
N
M
in
a 
5
M
in
a 
7
Mina 9-10
Min
a 1
4
Mina 11
(M
ina
2 -
3-4
)
Mina 1
Min
a 8
Mina 20
Figure 10
A
AFigure 6
Figure 7B
Figure 7A
Treinta Libras Fault
Neogene to Quarternary sediments
Pisco 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
TuffRi
o
 G
ra
n
d
e
 F
m
Lower
Paleozoic
Cenozoic
Mesozoic
(J-K)
Precambrian
Marcona Fm
(metasediments)
Arequipa Massif
(Gneisses, K-rich Granite, migmatites)
Dacite
Tunga Andesites
San Nicolas granitoids
Porphyritic andesite
dikes
Coastal Batholith
Tung
a
 Fa
ult
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). Insert
shows 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-g
rid)
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
650
350
400
450
500
550
600
300
DDM4-7
Cu(%)
in the hypogene ore
Ele
v a
tio
n
(m
)
Legend
Marcona Formationmetasediments
porphyritic andesite
dykes
Diabase dikes
cutting orebodies
Hypogene
magnetite orebody
Transitional ore
Leached ore
Mina Justa
Fault
Pit outline (2004 )
DDM4-6
DDM4-6
Sampled drillcores
NWSE
250
Neogene Sediments
A
Legend
Marcona Formation
metasediments
Dacite
magnetite
orebody
Repeticion Fault
(late)
Pit outline (2004) Mina Justa Fault
B
porphyritic andesite
dykes
NWSE
?
650
700
750
600
Mina 1
Mina 4
Huaca
Fault
Ele
va
tio
n
(m
)
Repeticion
Faults
Repeticion Fault
(early)
FIG. 7. (A) Cross section of the Mina 4 orebody, Marcona. Copper grade distribution on the right is for >50 percent Fe
orebody. 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) Cross
section of the Mina 1 orebody, Marcona. Two sets of Repetición faults are recognized: postmineralization and displacing the
orebody, 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 aggregates
of fine-grained cummingtonite (Table 2; Fig. 9A, B) occur in
feldspathic 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 (Table
2), 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 apparently
restricted to the upper Marcona Formation. Coarse-grained
phlogopite, 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 of
Mina 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-Cl
metasomatism widely generated albite and subordinate Na-
rich scapolite, particularly in Marcona Formation siliciclastic
rocks and lower Río Grande Formation sedimentary units
and andesites. Patches of coarse, white albite with clusters of
bladed white scapolite are widely developed along the folia-
tion of metaclastic host rocks, in places adjacent to bodies of
massive 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 this
stage 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 the
dacite porphyries which are interpreted as contemporaneous
with stage M-III magnetite mineralization (Fig. 8). Rock
staining 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 by
actinolite-sulfide alteration (Fig. 9E). Na-rich scapolite lo-
cally replaced original feldspars in andesite in contact with
the orebodies and, with a composition of meionite29–38 and 2.8
to 3.3 wt percent Cl, has been identified in the lower Río
Grande Formation north of the Marcona mine (Injoque,
1985), where it was subsequently replaced by amphibole and
magnetite.
Stages M-III and M-IV—Main magnetite and magnetite-
sulfide mineralization: Magnetite in the massive orebodies
and local stockwork breccia mineralization is associated with
varying proportions of calcic amphibole, phlogopite, biotite,
K-feldspar, apatite, calcite, diopside, and sulfides. The major
mineral associations in the main magnetite orebodies are
magnetite-actinolite (or tremolite) ± phlogopite and mag-
netite-biotite (± actinolite), both assigned to a sulfide-free
stage 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
0361-0128/98/000/000-00 $6.00 163
Minerals
Albite-scapolite
alteration
Quartz
Pyrite
Pyrrhotite
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-serpentine
alteration
Diopside*
Rhodochrosite
Phlogopite
Actinolite
Tremolite
* only in host rocks
Sericite
Greenalite
Early Mg-silicate alteration
Stage M-I-A Stage M-I-B
Magnetite-sulfide stage
Stage M-IV
? ?
Polymetallic sulfide
mineralization
FIG. 8. Alteration and mineralization paragenesis of the Marcona magnetite deposit.
164 CHEN ET AL.
0361-0128/98/000/000-00 $6.00 164
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Ti
0.
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00
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01
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26
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83
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99
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95
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97
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0.
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1.
81
1.
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79
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77
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01
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l
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0.
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N
ot
es
: *
 =
 to
ta
l i
ro
n;
 n
um
be
r 
of
 io
ns
 c
al
cu
la
te
d 
on
 th
e 
ba
si
s 
of
 F
, C
l, 
an
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23
 O
 fo
r 
cu
m
m
in
gt
on
ite
 (c
um
), 
tr
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ite
 (t
re
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) a
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 a
ct
in
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ite
 (a
ct
); 
22
 O
 fo
r 
bi
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ite
 (b
t)
 a
nd
 p
hl
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op
ite
 (p
hl
); 
28
 O
 fo
r 
ch
lo
-
ri
te
 (
ch
l),
 2
2 
O
 fo
r 
ta
lc
 (
tlc
), 
an
d 
14
 O
 fo
r 
gr
ee
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lit
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 R
**
: M
g/
M
g 
+ 
F
e 
ra
tio
 fo
r 
am
ph
ib
ol
es
; F
e/
F
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+ 
M
g 
fo
r 
m
ic
as
 a
nd
 c
hl
or
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s
1 
A
l–
te
tr
ah
ed
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l 
2 
A
l–
oc
ta
he
dr
al
FIG. 9. Marcona alteration and mineral-
ization stages. (A) Cummingtonite (stage
M-IA), partially altered to greenalite, occurs
interstitially to coarse-grained biotite and
magnetite (stage M-III) (#MA5-9, Mina 5
open pit, 670 m, main orebody; plane-polar-
ized transmitted light). (B) Electron back -
scatter image illustrating the replacement of
cummingtonite (darker) by fine-grained stage
M-III biotite. (C) Stage M-IB phlogopite
and magnetite (Mt-1) replaced by pyrite and
magnetite (Mt-2). Pyrite was emplaced along
the cleavage of early phlogopite (#MA5-2,
Mina 5 open pit, 670 m, main orebody; plane-
polarized reflected light). (D) Albitization of
Marcona Formation metasediments. Coarse
white albite, locally with pockets of bladed
scapolite, 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 stage
M-V actinolite (+ sulfide, dark-green). The
major sulfide is pyrite (#MA3-24, Mina 3
open 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 transmitted
light). (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 phlogopite
with interstitial magnetite and apatite. Chlo-
rite and minor talc replace phlogopite along
cleavages (#MA3-18, Mina3 open pit, 600
m, orebody, transmitted light, crossed nicols).
(I) Electron backscattered image showing
the local replacement of stage M-III biotite
(paler, massive aggregates and veins) by stage
M-IV phlogopite (#MA3-19, Mina 3 open
pit, 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 or
without 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 and
pyrite. Stage M-V actinolite-tremolite is com-
monly coarse-grained. Magnetite occurs as
traces in stage M-V veins (#DDM5-4-2, drill
core 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, adjacent
to a magnetite orebody; transmitted light,
crossed nicols). (N) Late magnetite veins
(Mt-2) cut massive magnetite (Mt-I) and late
tremolite. Magnetite in veins is commonly
fine grained and locally associated with cal-
cite (#DDM3-3-3, drill core DDM3-3, 343
m, Mina 3 orebody). (O) Late quartz vein
(with erratic calcite and Mn oxides) cuts
massive 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
Bt
Mt
Cum
Bt
A
Bt Bt
Cum
Cum
Gre
Mt
B
B
Phl
Mt-1
PyPhl
Py
Mt-2
Cum
Cum
C
Ab (Stage M-II)
Act-Sulfide veins (Stage M-V)
Kfs (Stage M-III)
Marcona
Formation
Magnetite
orebody
Albite
Albite
D E
Mt
Mt Phl
Chl
Chl Ap
Mt
Amph
Py
Py
Phl
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
Bt
Mt
Bt
Kfs+ Ab alteration
J
Massive Mt-Amph
Cp-Py-Amph veins
Cp
PyCal
K
1 cm
Cal
Mt
Mt
L
Tlc
Tlc
Tlc
Cp
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 stage
M-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-green
tremolite or dark-green actinolite (Table 2; the classification
of 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, occurring
largely as subhedral to anhedral crystals interstitial to mag-
netite, calcic amphibole, and phlogopite (Fig. 9G), but locally
showing microscopic replacement textures. Although no un-
ambiguous replacement of pyrrhotite by pyrite and chalcopy-
rite was observed, the common association of pyrrhotite and
magnetite without other sulfides, especially in the lower
parts of the orebodies, and the absence of pyrrhotite in late
sulfide veins suggest that it largely formed prior to pyrite and
chalcopyrite. 
Red-brown stage M-III biotite (Ann36–38) commonly occurs
as 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 of
both 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 magnetite
and/or sulfides. Stage M-III K-feldspar alteration is dominant
in dacite and fine-grained andesite, generally has the ortho-
clase structure, and commonly occurs within an outer zone to
biotitization (Fig. 9J). Metasomatic magnetite, associated with
secondary K-feldspar, is only locally observed in host rocks.
Although biotite, phlogopite and amphibole are also common
skarn-type alteration minerals, the immediate metasiltstone
host rock for major magnetite orebodies and the absence of
extensive hydrothermal magnetite in the alteration envelope
indicate a carbonate-replacement skarnization (Injoque,
1985) is unlikely for the Marcona main magnetite formation. 
Stage M-V—Polymetallic sulfide mineralization: The major
sulfides in stage M-V are again pyrite, chalcopyrite, and
pyrrhotite. 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 of
stages M-IV and M-V are rarely clear. Stage V sulfides and co-
existing minerals widely occur as aggregates replacing stage
M-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-calcite
assemblage also occurs locally. Stage M-V sulfides are gener-
ally euhedral to subhedral and coarse grained, and commonly
have planar contacts with amphibole and calcite, which may
indicate 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 in
sulfide 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), both
coexisting with sulfides. Tremolite, without associated metal-
lic minerals, also developed late in stage M-V, forming veins
cutting massive magnetite orebodies. Hydrothermal breccias,
in which coarse-grained, late-stage M-V tremolite cements
magnetite-sulfide clasts, are widespread in the Cu-poor Mina
5 and Mina 7 orebodies, but are only locally developed else-
where. Tremolite which formed late in stage M-V has a lower
iron 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 cuts
calcite, locally also replacing stage M-IV phlogopite (Fig.
9M). Lizardite and, locally, chrysotile also replace stage M-V
actinolite and tremolite, and talc and serpentine replace
coarse-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 records
the retrograde alteration of phlogopite, actinolite and tremo-
lite, the replacement of calcite and calcic amphiboles by talc
and serpentine is evidence for Mg metasomatism following
the main stage M-V sulfide precipitation.
Stage M-VII—Late veins: Late-stage hydrothermal veins
are abundant at Marcona, but their mutual age relationships
are ambiguous. Fine-grained subhedral magnetite and sul-
fides form narrow veins cutting both late-stage M-V tremolite
and stage M-III magnetite (Fig. 9N). Rare chalcopyrite veins
lacking gangue minerals cut late magnetite veins. Late mag-
netite is widely weathered to powdery hematite, but some
hematite (± gypsum) veins which cut the main magnetite ore-
bodies and late quartz veins (Fig. 9O) are interpreted as hy-
pogene. The majorsulfides in hematite veins are pyrite and
chalcopyrite. Although the main mineralization stages at Mar-
cona are almost free of quartz, barren quartz ± calcite veins
cut magnetite orebodies and host-rock alteration zones (Fig.
9O). Rhodochrosite locally occurs in these veins. Calcite veins
up to 5 cm thick cut the magnetite orebodies and reopen ser-
pentine veins. Tourmaline-quartz-pyrite veins locally cut
Marcona Formation metasediments. In the Mina 11 orebody,
anhydrite veins, commonly replaced by gypsum and bassan-
ite, cut all previous stages. Locally, anhydrite with abundant
pyrite 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 oxidation
profiles, comprising lower, 4 to 6 m horizons of sulfate-rich
“transitional” ore (Fig. 7A), in which martitized magnetite is
intergrown with jarosite, botyrogen, amarantite and parabut-
lerite, and surficial leached, martite-dominated zones. The
supergene profiles are eroded by a regionally extensive pedi-
ment overlain by a 9.13 ± 0.25 (2σ) Ma rhyodacitic ash-flow
tuff (Quang et al., 2001).
The Mina Justa Cu (-Ag) Deposit
The 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 is
dominated by porphyritic andesite flows and medium to fine-
grained andesitic volcaniclastic rocks with minor horizons of
sandstone, siltstone and limestone. Callovian to Oxfordian
166 CHEN ET AL.
0361-0128/98/000/000-00 $6.00 166
fauna have been identified in inferred equivalent strata of the
upper 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-grained
matrix. 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 southeast
part of the area, but host no economic mineralization. The
abundant secondary hydrothermal biotite and sericite inhibit
definition of magmatic chemistry (Hawkes et al., 2002), but
the correlative andesites at the base of the upper section of
the Río Grande Formation in Cañón Río Grande (40 km
northwest 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, constituting
up to 35 percent of the rock volume in the main mineraliza-
tion center, was emplaced following mineralization. These
plagioclase-phyric, “ocöitic” rocks are texturally and miner-
alogically similar to the Río Grande Formation flows, but
record only weak K feldspathization and sericitization.
The Mina Justa deposit incorporates two principal orebod-
ies, the Main and Upper (Figs. 10 and 11A). The mineralized
lensoidal bodies characteristically comprise a massive mag-
netite-sulfide core enclosed by hydrothermal breccias with
strongly altered host rock clasts in a magnetite+sulfide matrix,
in turn surrounded by extensive stockwork (Fig. 12). They are
controlled by subparallel, northeast-trending and shallowly
southeast dipping faults and range from 10 m to 200 m in ver-
tical extent (Baxter et al., 2005). The Main mineralized body
EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 167
0361-0128/98/000/000-00 $6.00 167
B
C
B
,
C
,
Upper
orebody
Main
orebody
Neogene to Quaternary
Post-mineralization
ocoite
Amygdaloidal Andesite
Tuff
Sandy volcaniclastics
(sandstone/siltstones)
Fine-grained andestie
Massive Magnetite bodies
with minor Cu-oxides
Alteration
(Ab+ Kfs+ Act)
with Cu-oxide
Faults
Cross Sections
50 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.
168 CHEN ET AL.
0361-0128/98/000/000-00 $6.00 168
350m
550m
C
B
C100m
BB MA 27 MA 35 MA 17 MA 64
Upper orebodyMain orebody
100m
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 body
with minor Cu-oxides
Main orebody
750m
FIG. 11. Cross sections through the Mina Justa orebodies. (A) Northwest-southeast section through the Main and Upper
orebodies (from Baxter et al., 2005). (B) Southwest-northeast section through the Main orebody (sulfide zones modified after
Moody et al., 2003).
MA-64
Graphic
logging
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
Hydrothermal
Breccias
Veins+ massive
Magnetite+ sulfide bodies
Hydrothermal Breccias + massive
Magnetite+ 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
Graphic
logging
Mineral-
ization
Graphic
logging
Mineral-
ization
Graphic
logging
Mineral-
ization
Graphic
logging
Mine Graphic
logging
Mineral-
ization
Graphic
logging
Mineral-
ization
M-C
M-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. The
locations 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 800
m southeast of the upper zone and out of the map area in Figure 10. *Magnetite either occurs erratically as haloes around
coarse-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 and
albite-K-feldspar-actinolite alteration (Fig. 10), which dips
10° to 30° to the southeast, i.e., at a high angle to the bedding
of the host andesites. It has been intersected to a depth of 500
m, where it remains open (Fig. 11A). The Upper mineralized
body, cropping out subparallel to and approximately 400 m
southeast of the Main zone (Fig. 10), has a similar concave-
upward, “spoon-shaped” form in section, and a similar dip of
10° to 30° to the southeast. On surface, this zone has been
identified 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 are
also exposed on surface (Fig. 10). They commonly contain
minor Cu oxides and are locally cut by the southeast-dipping
Mina Justa normal faults (Fig. 11A). 
Copper oxide minerals, predominantly chrysocolla with
lesser atacamite, dominate the upper 200 m of the deposit,
giving way gradually to sulfides with depth (Fig. 11A, B). The
oxide zone, with an average grade of 0.54 percent Cu, hosts
approximately 40 to 50 percent of the recoverable Cu in the
measured-plus-indicated reserves. In individual orebodies,
the major sulfides are zoned upward, and locally laterally, but
not strictly concentrically (cf. Moody et al., 2003), from
pyrite-chalcopyrite to bornite-chalcocite (± digenite), with a
concomitant increase in Cu grade(Figs. 11, 12). Around the
magnetite-sulfide orebodies, the alteration is zoned outward
from 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 the
orebodies.
Paragenetic relationships
Seven stages of hypogene alteration-mineralization, J-I
through 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 matrix
of andesites (Fig. 14A), recording Na metasomatism. 
Stage J-II—K-feldspar–magnetite alteration: Rocks affected
by this event generally appear massive in hand specimen, and
range from pink to black. K-feldspar commonly occurs as ex-
tremely small grains (<0.05 mm) replacing both fresh and
previously albitized plagioclase (Fig. 14A), and the associated
magnetite 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-Fe
metasomatism, was probably contemporaneous with the de-
velopment of lenses of sulfide-free magnetite which strike
northeast and dip northwest, subparallel to stratigraphy, and
are 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 is
common (Fig. 14C).
Stage J-III—actinolite (± magnetite ± diopside) alteration:
Green actinolite (Table 3), associated with minor magnetite,
EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 169
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Minerals
Albite-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
Late
hematite
Abundant Local Trace
K-feldspar-magnetite
alteration
Stage 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
Early
hematite
Stage J-IV
?
FIG. 13. Alteration and mineralization paragenesis of the Mina Justa Cu (-Ag) deposit.
170 CHEN ET AL.
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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 MA64
no. –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 0.10
TiO2 0.20 0.02 0.02 S 20.99 21.38 25.18 20.13
Al2O3 3.25 0.95 14.41 Fe 0.14 0.08 11.17 0.04
FeO* 11.76 10.73 20.41 Ni 0.00 0.00 0.00 0.00
MnO 0.10 0.10 1.06 Zn 0.00 0.00 0.00 0.00
MgO 15.71 16.88 17.71 Ag 0.24 0.26 0.12 0.14
CaO 12.72 13.16 1.56 Cu 78.40 78.25 63.41 80.46
Na2O 0.44 0.15 0.12 Co 0.00 0.00 0.00 0.00
K2O 0.23 0.06 0.03
Cl 0.10 0.04 0.04
F 0.33 0.17 0.32
Total 97.70 98.41 88.31 Total 99.773 100.088 99.880 100.865
Si 7.63 7.95 6.67
Al 1 0.37 0.05 1.33
Al 2 0.18 0.10 2.14
Ti 0.02 0.00 0.00
Fe 1.42 1.27 3.49
Mn 0.01 0.01 0.18
Mg 3.38 3.56 5.40
Ca 1.97 2.00 0.34
Na 0.12 0.04 0.05
K 0.04 0.01 0.01
Cl 0.02 0.01 0.01
F 0.15 0.08 0.21
R** 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–tetrahedral
2 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 (stained
yellow) 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-polarized
transmitted light). (C) Magnetite-sulfide-calcite veins with K-feldspar haloes (red) cut stage J-III actinolite and stage J-II
fine-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 and
locally as veins (# MA64-3, drill core MA64, 220.1 m). (E) Stage J-V magnetite (Mt-2)-bornite-chalcocite assemblage occurs
as a matrix to pinkish-red stage J-II K-feldspar-magnetite (Mt-1) altered clasts cut by stage J-III actinolite (green) veins. The
magnetite-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 extensively
replaces pyrite and locally occurs along fractures in magnetite. Chloritized stage J-III actinolite relics occur between the
magnetite crystals. Stage J-IV calcite has planar contacts with platy magnetite, but is locally replaced by stage J-V granular
magnetite 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 magnetite
and 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 reflected
and 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 calcite
veins 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 host
rocks (#MA64-4, drill coreMA64, 248.3 m, plane-polarized reflected light).
EVOLUTION OF THE GIANT MARCONA-MINA JUSTA IOCG DISTRICT, PERU 171
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Mt
Kfs
Kfs
Act
Act
Mt - Cal-Sulfid e vein
Kfs halo
Act -(Kfs- Mt)
altered host rock
Ab
Pl
Ab
Kfs
Pl
Act
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 fide
Kfs -
Mt-1
G
Mt
Act
Mt - Py
Chl
0.5 cm
H
150 m
Py Mt
Qtz
Act
150 m
Mt (II) - Sulfid e- Qtz
spotty mineralizat ion
Act - (Kfs-Mt-I) altered
host rock
J
0.5 cm
Mt
PyAct
Chl
Qtz
K
150 m
I
Act - (Kfs + Mt) altered
host rock
Cp
Cal vein with
Kfs halos
L
1 cm
Py
Cp
Mt
M
150 m
Cp
Cv
Qtz
150 m
Bn
Hm
Mt
Cc
Dg
N O
150 m
occurs throughout the deposit, commonly as massive aggre-
gates along the contacts of stage J-V magnetite bodies or as
coarse, 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 of
K-feldspar-magnetite–altered host rocks. Actinolite inter-
growths occur as irregular clasts in a magnetite-sulfide matrix
along the contacts of the stage J-V magnetite bodies with their
actinolitic alteration haloes. A temporal evolution is evident
from K-feldspar–magnetite, through actinolite, to magnetite-
pyrite alteration (Fig. 14E). Actinolite is strongly chloritized
and carbonatized, and locally replaced by quartz. It replaced
both albite and K-feldspar, evidence for Ca metasomatism.
Diopside is spatially associated with and locally replaced by
actinolite in the albitized and K-Fe–metasomatised host
rocks. 
Stage J-IV—early hematite-calcite alteration: “Mushke-
tovite”, i.e., magnetite unambiguously pseudomorphous after
specular hematite, occurs commonly in the main magnetite
bodies, evidence for a now obliterated hematite-dominant
stage which temporally separated the actinolite alteration and
the 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 intergrown
with the pseudomorphs, and is locally replaced by quartz and
magnetite. Coarse-grained, subhedral-to-euhedral allanite
(stage J-V) occurs rarely as inclusions in stage J-IV calcite in
contact with stage J-V magnetite and pyrite.
Stage J-V—magnetite-pyrite-K-feldspar alteration: The
massive, lensoid, and brecciated magnetite-pyrite bodies
which host the highest grade copper sulfide mineralization at
Mina Justa were controlled by the northeast-striking, south-
east-dipping, Mina Justa system faults, but are dislocated by
the northwest-striking, northeast-dipping Huaca faults and
associated ocöite dikes. Magnetite-pyrite veins, varying from
0.1 to 5 cm in width, cut alteration assemblages of stages J-II
and J-III adjacent to the massive magnetite bodies. Hy-
drothermal breccias commonly occur at the margins of the
magnetite bodies, and comprise a magnetite-pyrite–dominant
matrix 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 occurs
as 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) and
subhedral 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 and
pyrite 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 or
red K-feldspar, predominantly microcline, is a common alter-
ation mineral in rocks associated with stage J-V magnetite-
pyrite mineralization, forming haloes to magnetite-pyrite
veins 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, and
quartz. Titanite commonly forms medium-grained subhedral
crystals or aggregates enclosed by chlorite, as well as euhedral
grains in magnetite-pyrite-quartz-chlorite veins. Fluorapatite
locally occurs in stage J-V veins, but more commonly forms
coarse-grained, subhedral to euhedral grains in stage J-III
actinolite 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 inherent
Cu 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 associated
assemblages more commonly occur in massive magnetite-
pyrite bodies or veins with which they exhibit unambiguous
microscopic replacement textures (Fig. 14M). Locally, stage
J-V magnetite-pyrite aggregates in stage J-III actinolite veins
have 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 by
chalcopyrite 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 above
or in the upper parts of the main magnetite bodies, commonly
forming veins that cut the host rocks and earlier alteration as-
semblages. Covellite, entirely supergene, occurs mainly in the
lower part of the oxide zone, replacing bornite and chalcopy-
rite (Fig. 14N). Chalcocite, digenite, and bornite typically
form large patches with complex vermicular intergrowths
(Fig. 14O), such as are inferred to form through noncoherent
exsolution at low temperature (<250°C) and under protracted
cooling (Brett, 1964). Similar hypogene relationships were
documented at the Olympic Dam deposit by Roberts et al.
(1983). Copper sulfides exhibiting vermicular textures are all
rich in silver (Table 3), and represent the major Ag host in the
ores. 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 which
generally 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 chalcopyrite
along the boundaries of earlier magnetite grains or stage J-IV
coarse-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 in
chalcopyrite-calcite veins cutting altered andesite host rock.
Sparse epidote veins with chalcopyrite