doi: 10.1111/j.1751-3928.2010.00150.x Resource Geology Vol. 61, No.

1: 91–100

Original Article rge_150 91..100

Molybdenite Mineralization and Re-Os Geochronology of

the Escondida and Escondida Norte Porphyry Deposits,
Northern Chile

Bárbara Romero,1,2 Shoji Kojima,1 Chilong Wong,2 Fernando Barra,3* Walter Véliz2 and
Joaquin Ruiz3
1
Department of Geological Sciences, University of Católica del Norte, 2Minera Escondida Ltd, Antofagasta, Chile and
3
Department of Geosciences, University of Arizona, Tucson, Arizona, USA

Abstract
Molybdenum is an economically important subproduct of North Chilean porphyry-type deposits, and thus
spatial and temporal distribution of molybdenite as the primary Mo-bearing mineral in the Escondida and
Escondida Norte deposits were characterized using several mineralogical and chemical techniques and the
Re-Os dating method. Molybdenum is distributed extensively in the two deposits, and high molybdenum
concentrations (>500 ppm) are recognized particularly in the chlorite-sericite transitional zone between the
potassic and sericitic zones. Two modes of occurrence of molybdenite are observed in the Escondida deposit:
aggregates with Cu-Fe-sulfide minerals in fine veinlets (sulfide-veinlet type), and monomineralic microvein-
lets associated with NE-trending faults. The former and the latter yielded ages of 36.1 ⫾ 0.2 Ma and 35.2 ⫾
0.2 Ma, respectively. Re-Os dating of Escondida Norte molybdenites also show two distinct episodes, at 37.7 ⫾
0.3 Ma and a younger episode at 36.6 ⫾ 0.2 Ma. These data indicate that the Escondida Norte is older than the
main Escondida deposit. The Re-Os age data combined with those of the porphyry emplacement suggest that
the molybdenite mineralization in the Escondida district occurred as several short episodic pulses during the
late-magmatic to hydrothermal transition, and that the Cu-Mo deposits were formed in a variable overall
period spanning 1 to 5 m.y.
Keywords: Chile, Escondida district, molybdenite mineralization, porphyry-type deposits, Re-Os
geochronology.

1. Introduction index.asp). This large Cu deposit has been the subject

of several geological and geochemical studies since its
The Escondida porphyry Cu-Mo deposit is located in discovery in 1981 (e.g. Ojeda, 1986, 1990; Alpers &
the Eocene-Early Oligocene porphyry copper belt Brimhall, 1988; Ortíz, 1995; Richards et al., 1999, 2001;
along the Domeyko fault system in northern Chile Padilla-Garza et al., 2001, 2004; Vergara, 2002; Quiroz,
(Fig. 1), and is the largest copper-producing mine in 2003; Véliz, 2004; Contreras, 2006; Tascón, 2007). In
the world with more than 1.2 Mt of fine copper pro- September, 2003, the Escondida Norte deposit, which
duced in 2008 (http://www.escondida.cl/mel/en/ is regarded as the eastern extension of the Zaldívar

Received 10 March 2010. Accepted for publication 18 August 2010.

Corresponding author: S. KOJIMA, Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida. Angamos 0610,
Casilla 1280, Antofagasta, Chile. Email: skojima@ucn.cl
*Present address: Departamento de Geología, Universidad de Chile, Plaza Ercilla 893, Casilla 13518 Correo 21, Santiago, Chile.

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Resource Geology © 2010 The Society of Resource Geology 91
B. Romero et al.

Fig. 1 Location map of the Escon-

dida district and the North
Chilean Eocene-Early Oligocene
porphyry copper belt associated
with the Domeyko fault system.

porphyry deposit, was discovered 5 km north of the approach the aforementioned characteristics of molyb-
Escondida pit as the result of an extensive exploration denite mineralization in the Escondida and Escondida
campaign (Williams, 2003). The overall Cu resources Norte deposits. The results obtained are discussed in
contained in this porphyry copper cluster, including the light of the geologic context so as to provide con-
the Escondida, Escondida Norte and Zaldívar deposits straints on the timing of formation of this porphyry
(Fig. 1), is then estimated to exceed 3000 Mt. Molyb- copper cluster.
denite is widely distributed as an economically impor-
tant subproduct in these deposits, but its temporal- 2. Geologic framework
spatial characteristics have not yet been investigated.
This study provides new data on mode of occurrence The geology of the Escondida district has been
and Re-Os ages of molybdenite with the objective to described in detail by several authors (e.g. Richards

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92 Resource Geology © 2010 The Society of Resource Geology
 Molybdenite of the Escondida deposits Chile

Fig. 2 Geologic maps for the Escondida (below) and Escondida Norte (above) deposits, (a) Geologic units, (b) Hydrothermal
alteration, (c) Distribution of molybdenum (ppm Mo).

et al., 1999, 2001; Padilla-Garza et al., 2001, 2004; granodiorite to tonalite. The earlier phase of the stock,
Campos et al., 2009). The geology of the Escondida dis- termed Colorado Grande porphyry and Escondida
trict consists predominantly of Paleozoic quartz por- porphyry, occur in the Escondida deposit. Several U-Pb
phyry (Antigua porphyry), the Paleozoic La Tabla zircon ages that range from 37.9 ⫾ 1.1 to 37.2 ⫾ 0.8 Ma
Formation, the Paleogene Augusta Victoria Formation, have been obtained for the granodiorite unit (Richards
feldspathic porphyry stocks (Escondida and Escondida et al., 1999; Padilla-Garza et al., 2004). In the Escondida
Norte porphyries), the Colorado Chico rhyolite dome, Norte deposit a similar complex porphyritic stock
and gravels (Fig. 2a). The Paleozoic quartz porphyry is intrudes the Paleozoic quartz porphyry. This unit,
distributed at the west of the Escondida Norte area, named Escondida Norte porphyry, is granodioritic to
extending to the adjacent Zaldívar mine (Richards dacitic in composition, and is considered equivalent to
et al., 1999). A U-Pb zircon age of 294.2 ⫾ 2.2 Ma was the Zaldívar granodiorite or Llamo porphyry (38.7 ⫾
obtained for the Escondida Norte quartz porphyry 1.3 Ma; Richards et al., 1999) in the adjacent Zaldívar
(Escondida Internal Report, 2002). A similar rock unit, mine. The Colorado Chico rhyolite occurs in the north-
the Cerro Sureste rhyolite (Padilla-Garza et al., 2004), is ern sector of the principal Escondida pit, and has a
present in the eastern part of the Escondida pit younger U-Pb zircon age of 34.7 ⫾ 1.7 Ma (Richards
(Fig. 2a). A small coarse porphyritic unit (not shown in et al., 1999). Magmatic-hydrothermal breccia zones
Fig. 2a) located deep within the central area of the occur sporadically in the two pits, and form ~5% of the
Escondida Norte pit has a U-Pb zircon age of 298.9 ⫾ total of rocks (Fig. 2a).
2.6 Ma (Escondida Internal Report, 2002). The La Tabla The typical types of hydrothermal alteration
Formation comprises mainly biotite-hornblende- observed in porphyry Cu-Mo deposits are present at
bearing andesitic to rhyolitic lavas and tuffs (García, the Escondida deposit (Fig. 2b), i.e. potassic
1967). A 40Ar/39Ar age of 267.6 ⫾ 4.3 Ma was obtained (K-feldspar and biotite), phyllic (chlorite-sericite,
from hornblende in a dacite porphyry unit outside the quartz-sericite), propylitic, argillic and advanced argil-
Escondida district (Richards et al., 2001). The Augusta lic. The potassic alteration zone is represented by fine
Victoria Formation is composed of andesitic lavas (55.0 veinlets of pinkish K-feldspar with biotite, sericite,
⫾ 1.4 Ma; Marinovic et al., 1992) with subordinate vol- quartz and anhydrite, locally with small amounts of
canoclastic sandstone, rhyolitic tuff and dacitic breccia, albite and apatite in the Escondida deposit, and
and is distributed extensively in the Escondida district K-feldspar, quartz and sericite in the Escondida Norte
(Fig. 2a). deposit. A biotite subzone present in the Escondida
The Escondida feldspathic porphyry is a complex deposit is characterized by aggregates of fine second-
porphyritic intrusion that varies in composition from ary biotite with minor chlorite and sericite. The

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Resource Geology © 2010 The Society of Resource Geology 93
B. Romero et al.

chlorite-sericite alteration corresponds to a transitional oxide zone is relatively rich in the Escondida Norte
zone from the biotite to quartz-sericite alteration zones, deposit. The secondary sulfide subzone consists mainly
and chlorite and sericite occur as partial selective of secondary chalcocite-group minerals and covellite.
replacement of biotite and plagioclase, respectively. At the Escondida Norte deposit, the secondary sulfide
The quartz-sericite alteration zone has fine quartz vein- subzone is more irregular than that of the Escondida
lets and stockworks with sericitic halos, and is distrib- deposit which attains a maximum thickness of ~600 m
uted extensively as the principal area in the two pits (Ortíz, 1995).
(Fig. 2b). The propylitic alteration, which is character- The Escondida deposit is characterized by three
ized by a mineral association of epidote, calcite and main fault sets: NS, NW and NE (Padilla-Garza et al.,
chlorite is observed in the distal zone outside the pit 2001; Vergara, 2002). The NS faults are regarded as the
area (Padilla-Garza et al., 2004). The supergene argillic oldest and are possibly related to the regional
alteration zone (not shown in Fig. 2b) is present at the Domeyko fault system (Padilla-Garza et al., 2001). The
higher levels of the Escondida pit and overprints the NW fault system controls the distribution of the early
quartz-sericite zone. This zone is characterized by quartz-sericite to advanced argillic alteration zones,
kaolinite and illite with lesser amounts of sericite and and the polymetallic veins (Padilla-Garza et al., 2001,
quartz. The hypogene advanced argillic alteration is 2004). The NE faults are concentrated in the eastern part
represented by kaolinite-dickite, pyrophyllite, alunite, of the pit, and displace the NW-trending faults with a
and quartz (vuggy silica) with diaspore, andalusite and dextral sense of movement.
minor corundum (Véliz, 2004; Romero, 2008). This late
hydrothermal alteration event is distributed in the 3. Analytical methods
upper marginal areas of both pit areas (Fig. 2b).
The hydrothermal alteration pattern associated with Samples were first studied under the microscope, and
the Escondida Norte deposit is similar to that observed a total of 56 polished sections from both deposits were
in the Escondida deposit, and consists of a deep-core selected for mineralogical and chemical analyses. Crys-
potassic (K-feldspar, biotite), transitional chlorite- tallographic and chemical characterization of molyb-
sericite and extensive quartz-sericite alteration, with denite from the deposits was carried out using a
local intermediate argillic (sericite-chlorite-clay) and Siemens D5000 X-ray powder diffractometer (XRD;
advanced argillic (illite-prophyllite-alunite) alterations Siemens, Karlsruhe, Germany) and JEOL JSM6360LV-
(Williams, 2003; Romero, 2008). The hypogene type scanning electron microscope (SEM; JEOL,
advanced argillic zone is superimposed on all earlier Akishima, Japan) equipped with an Oxford Inca
alteration zones. Energy EDS system (Oxford, Concord, USA) at the
In the potassic to quartz-sericite zones, hypogene ore Universidad Católica del Norte, Chile. The XRD analy-
minerals normally occur as fine sulfide stockwork sis and IR spectroscopy were applied to identification
veinlets or disseminations of chalcopyrite, pyrite, of alteration minerals. Mo abundances of whole rock
bornite, molybdenite and magnetite. A polymetallic samples were determined using an inductively
mineralization with a high-sulfidation mineral associa- coupled plasma-mass spectrometer (ICP-MS) at ALS-
tion of enargite, tennantite, chalcocite-digenite, covel- Chemex commercial Laboratory (Vancouver, Canada).
lite, pyrite, hematite, chalcopyrite, bornite, sphalerite Five molybdenite separates from the Escondida and
and trace amounts of electrum, occurs in the advanced Escondida Norte pits (Fig. 3) were selected for Re-Os
argillic alteration zone as late-hydrothermal veins dating. All samples were digested using the Carius
(Quiroz, 2003; Williams, 2003; Véliz, 2004). tube method (Shirey & Walker, 1995), and were ana-
A supergene copper zone is widely developed in the lyzed following the procedure described by Barra et al.
upper levels of both deposits, and comprises an oxide, (2003, 2005). About 50 to 70 mg of pure molybdenite
mixed oxide-sulfide and sulfide subzones in descend- was loaded in the Carius tube with Re and Os spikes,
ing order of elevation. The oxide zone occurs mainly in and then dissolved in inverse aqua regia by heating in
the southwestern sector of the Escondida deposit, and an oven at 220°C for ~12 h. After homogenization of the
contains brochantite and antlerite with lesser amounts solution, Re and Os were separated using a distillation
of atacamite, chrysocolla, malachite, azurite, technique (Nägler & Frei, 1997), in which Os was col-
pseudomalachite, libethenite, turquoise, tenorite, lected into cold HBr. Later, the dried Os was purified
cuprite, native copper, black copper (copper wad) and using the microdistillation technique of Birck et al.
“almagrado” (Cu-Fe sulfohydroxide). This type of (1997), while Re was purified using AG1-X8 anion

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94 Resource Geology © 2010 The Society of Resource Geology
 Molybdenite of the Escondida deposits Chile

Fig. 3 Map of the principal Escondida (below) and

Escondida Norte (above) deposits showing the sample
locations of molybdenite used for dating analysis.
Open squares indicate sampling points.

exchange resin. Along with Ba salts to enhance ioniza-

tion, Re and Os were loaded on Ni and Pt filaments,
respectively. Measurements were carried out by nega- Fig. 4 Photomicrographs of the two types of molybden-
ite occurrence in the Escondida deposit, (a) aggregates
tive thermal ion mass spectrometer (NTIMS) at the with Cu-Fe-sulfide minerals in veinlets (cp: chalcopy-
University of Arizona, USA. rite, mlb: molybdenite) and (b) fine monomineralic
veinlets associated with N-E-trending faults.
4. Results
4.1 Mode of occurrence, mineral chemistry and later than the sulfide veinlet-type, and has not yet been
distribution of molybdenite found in the Escondida Norte deposit. Molybdenite
Molybdenite occurs as subhedral to anhedral grains remains as an unaltered product in the secondary
with variable size of 10 mm to 2.5 mm. General obser- enrichment zone of the two deposits, and is preserved
vations and ore microscopy studies reveal that molyb- more abundantly in the Escondida Norte deposit.
denite occurs as aggregates with other sulfides (pyrite, The XRD analysis indicates that all molybdenite
chalcopyrite, bornite) in veinlets (sulfide veinlet-type) samples dated in this study are of the 2H polytype,
with no discrete alteration halo in the chlorite-sericite which is commonly present in porphyry copper depos-
and superimposed quartz-sericite zones, and as fine its (Newberry, 1979a, b). EDS point analysis showed
monomineralic veinlets in the host rocks (Fig. 4). The that molybdenite grains are composed mainly of Mo, S,
latter occur as smear-like specks related to the and Re, and that the highest Re value was ~0.33 wt%.
NE-trending fractures in the quartz-sericite zone asso- Mo is distributed extensively in the two pits, and the
ciated with the advanced argillic alteration, and have elemental concentrations of Mo range mostly between
not been observed in the chlorite-sericite zone. Thus, 20 and 600 ppm (Fig. 2c). The drill-core samples with
the fine veinlet molybdenite appears to have formed the highest Mo concentration occur particularly in the

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Resource Geology © 2010 The Society of Resource Geology 95
B. Romero et al.

transitional chlorite-sericite zone. High Mo concentra- 5. Discussion

tions are also detected in the leached and oxide zones,
where no molybdenite is observed. The XRD and Age data for the Escondida district are summarized in
IR spectroscopy analyses indicate that secondary Table 2 and Figure 5. The published zircon U-Pb ages
molybdate minerals including lindgrennite for the Escondida deposit show that both phases of the
(Cu3(MoO4)2(OH)2) and ilsemanite (Mo3O8 · nH2O) are Escondida granodiorite stock (Colorado Grande and
present in the zones. Escondida porphyries) were emplaced between 39.0
and 36.4 Ma (Richards et al., 1999; Padilla-Garza et al.,
2004). On the other hand, 40Ar/39Ar ages for igneous
4.2 Re-Os geochronology biotite from both phases are identical within error, and
As mentioned earlier, two types of molybdenite occur- are at least a few hundred of thousands years younger
rence are recognized in the Escondida deposit: molyb- than the U-Pb zircon ages. 40Ar/39Ar data for secondary
denite associated with Cu-Fe-sulfides in veins (Sample biotite (Padilla-Garza et al., 2004) indicate that the
E-1 associated with the chlorite-sericite alteration zone) biotitic alteration occurred over an extensive period of
and monomineralic molybdenite veinlets (Sample E-2 c. 4 m.y., partially overlapping with the U-Pb and 40Ar/
39
from the quartz-sericite zone). Three samples (Samples Ar ages of the Escondida stock. This means that the
EN-1 to EN-3) were selected from different sectors of biotitic alteration occurred during the later stage of
the quartz-sericite zone in the Escondida Norte pit. The the Escondida stock emplacement, and also that the
molybdenite Re-Os ages were calculated using a 187Re magmatic-hydrothermal events of the porphyry con-
decay constant of 1.666 ¥ 10-11yer-1 (Smoliar et al., tinued over a long duration. Due to no valid plateau or
1996). Errors in the age determination are at ~0.5%, and inverse isochron, poorly constrained ages of 40.7 to
include uncertainties in spike calibrations, uncertainty 35.7 Ma were obtained for the sericitic alteration
in the rhenium decay constant (0.31%), weighing and (Padilla-Garza et al., 2004). The youngest age deter-
measurement errors. mined does not overlap with the U-Pb zircon ages,
As listed in Table 1, the analyzed Escondida samples implying that some sericitic alteration was caused as a
have total Re and Os concentrations ranging between post-emplacement event. However, it is highly likely
1450 and 331 ppm, and 546 and 122 ppb, respectively, that 40Ar/39Ar ages have been variably reset by later
whereas the Escondida Norte samples show Re and Os hydrothermal or intrusive activity. Thus, the sericitic
concentrations varying from 1804 to 433 ppm and 689 alteration ages are not shown in Figure 5, and an
to 171 ppb, respectively. These results lead to the radio- attempt to further discuss the ages is not made here.
metric ages of 36.1 ⫾ 0.2 Ma (Sample E-1) and 35.2 ⫾ The 40Ar/39Ar alunite ages show that the advanced
0.2 Ma (Sample E-2) for the Escondida deposit, and argillic alteration occurred during the waning stages of
37.8 ⫾ 0.2 Ma (Sample EN-1) to 36.6 ⫾ 0.2 Ma (Sample the biotitic and sericitic alterations (Fig. 5).
EN-3) for the Escondida Norte deposit, respectively. Two stages of Mo mineralization are recognized in
The data show that there is no correlation between the Escondida deposit: aggregates with Cu-Fe-sulfide
molybdenite age and Re concentration (Table 1), and minerals in fine veinlets (sulfide veinlet-type) and sub-
clearly indicate that the Escondida Norte molybdenite ordinate fine monomineralic veinlets associated with
mineralization is slightly older than that in the Escon- the NE-trending fault fractures. The first stage of major
dida deposit. Mo-mineralization occurred at 36.1 ⫾ 0.2 Ma, whereas

Table 1 Re-Os data for molybdenite samples from the Escondida and Escondida Norte deposits
187 187
Deposit Sample Weight (mg) Total Re (ppm) Re (ppm) Os (ppb) Age (Ma)
Escondida
E-1 65 1449.5 907.4 546.4 36.1 ⫾ 0.2
E-2 51 331.0 207.2 121.5 35.2 ⫾ 0.2
Escondida Norte
EN-1 56 432.5 270.8 170.7 37.8 ⫾ 0.2
EN-2 72 595.2 372.6 233.6 37.6 ⫾ 0.2
EN-3 54 1804.4 1129.5 689.0 36.6 ⫾ 0.2

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 Molybdenite of the Escondida deposits Chile

Table 2 Summary of geochronological data for the Escondida district

Deposit/sample Method, material Age (Ma) Reference
Escondida
Colorado grande porphyry U-Pb, zircon 37.9 ⫾ 1.1 Richards et al. (1999)
U-Pb, zircon 37.2 ⫾ 0.8 Padilla-Garza et al. (2004)
Ar-Ar, igneous biotite 35.8 ⫾ 0.2 Padilla-Garza et al. (2004)
Escondida porphyry U-Pb, zircon 37.7 ⫾ 0.8 Padilla-Garza et al. (2004)
Ar-Ar, igneous biotite 35.9 ⫾ 0.3 Padilla-Garza et al. (2004)
Biotite alteration (andesite) Ar-Ar, biotite 37.5 ⫾ 0.6 Padilla-Garza et al. (2004)
Biotite alteration (andesite) Ar-Ar, biotite 34.8 ⫾ 0.4 Padilla-Garza et al. (2004)
Biotite alteration (vein) Ar-Ar, biotite 36.0 ⫾ 0.4 Padilla-Garza et al. (2004)
Quartz-sericite alteration† Ar-Ar, sericite 35.7 - 40.7 Padilla-Garza et al. (2004)
Advanced argillic zone Ar-Ar, alunite 35.7 ⫾ 0.3 Padilla-Garza et al. (2004)
Advanced argillic zone Ar-Ar, alunite 35.4 ⫾ 0.2 Padilla-Garza et al. (2004)
Advanced argillic zone K-Ar, alunite 35.7 ⫾ 1.4 Alpers and Brimhall (1988)
Sulfide veinlet-type Re-Os, molybdenite 36.1 ⫾ 0.2 This study
Monomineral-type Re-Os, molybdenite 35.2 ⫾ 0.2 This study
Polymetallic vein Ar-Ar, alunite 35.2 ⫾ 0.2 Véliz (2004)
Late Polymetallic vein Re-Os, molybdenite 33.7 ⫾ 0.3 Padilla-Garza et al. (2004)
Colorado Chico rhyolite U-Pb, zircon 39.0 ⫾ 1.5 Padilla-Garza et al. (2004)
U-Pb, zircon 34.7 ⫾ 1.7 Richards et al. (1999)
K-Ar, sericite 31.6 ⫾ 1.6 Alpers and Brimhall (1988)
Supergene alteration K-Ar, alunite 14.7–18.0 Alpers and Brimhall (1988)
Escondida Norte
Escondida Norte porphyry
(granodioritic phase) U-Pb, zircon 37.5 ⫾ 0.5 Escondida Internal Report (2002)
(dacitic phase) U-Pb, zircon 38.7 ⫾ 0.4 Escondida Internal Report (2002)
Biotite alteration (granodiorite) Ar-Ar, biotite 37.3 ⫾ 0.1 Escondida Internal Report (2002)
Quartz-sericite zone Ar-Ar, sericite 36.8 ⫾ 0.1 Escondida Internal Report (2002)
Mo-mineralization Re-Os, molybdenite 37.8 ⫾ 0.2 This study
Re-Os, molybdenite 37.6 ⫾ 0.2 This study
Re-Os, molybdenite 36.6 ⫾ 0.2 This study
Zaldívar
Llamo porphyry U-Pb, zircon 38.7 ⫾ 1.3 Richards et al. (1999)
Llamo porphyry (early) U-Pb, zircon 38.0 ⫾ 0.5 Jara et al. (2009)
Llamo porphyry (intermédiate) U-Pb, zircon 35.5 ⫾ 0.8 Jara et al. (2009)
Llamo porphyry (late) U-Pb, zircon 36.0 ⫾ 0.8 Jara et al. (2009)
Llamo porphyry Ar-Ar, igneous biotite 37.4 ⫾ 0.2 Richards et al. (1999)
Ar-Ar, igneous biotite 37.7 ⫾ 0.4 Campos et al. (2009)
Ar-Ar, igneous biotite 37.1 ⫾ 0.5 Campos et al. (2009)
Ar-Ar, igneous biotite 36.6 ⫾ 0.9 Campos et al. (2009)
Ar-Ar, igneous biotite 36.5 ⫾ 0.5 Campos et al. (2009)
Ar-Ar, igneous biotite 36.0 ⫾ 0.3 Campos et al. (2009)
Biotite alteration Ar-Ar, biotite 35.6 ⫾ 0.7 Campos et al. (2009)

†No valid plateau or inverse isochron obtained.

the second episode occurred about 1 m.y. later at 35.2 ⫾ lic vein mineralization in the Cerro Sureste rhyolite, for
0.2 Ma. This is consistent with the observation that which a Re-Os age of 33.7 ⫾ 0.3 Ma is given (Padilla-
molybdenite is abundant in the transitional chlorite- Garza et al., 2004). The age is clearly younger than those
sericite zone. The second event (monomineralic obtained in this study, implying several pulses of Mo
molybdenite veinlets) is closely related to, and contem- mineralization in the Escondida area. Taken together,
poraneous with the advanced argillic alteration. This the age data indicate that there were at least three epi-
observation raises the possibility that the monominer- sodes of Mo mineralization, and that this type of min-
alic molybdenite mineralization was triggered by the eralization occurred within a ~ 3 m.y. interval. Similar
advanced argillic alteration. It is also known that episodic Mo mineralization events have been reported
molybdenite is locally associated with a late polymetal- in several younger giant porphyry Cu-Mo deposits,

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B. Romero et al.

Fig. 5 Summary of geochronologi-

cal information for the Escondida
district. Data from Table 2.

such as El Teniente (Maksaev et al., 2004) and Los (2009) imply that mineralization at the Zaldívar area
Pelambres (Bertens et al., 2003; Hannah et al., 2007). could have occurred over a much longer period.
On the other hand, the molybdenite dating from The age data in Table 2 suggest that the stocks at the
Escondida Norte yielded older ages of 37.8 ⫾ 0.2 Ma, three deposits comprise two or more phases that were
37.6 ⫾ 0.2 Ma and 36.6 ⫾ 0.2 Ma. The first two ages are emplaced in some cases within <1 m.y. (Colorado
the same within error, and are consistent with a U-Pb Grande and Escondida porphyries at Escondida, later
zircon age of 37.5 ⫾ 0.5 Ma for the Escondida Norte phases at Zaldívar), whereas other intrusions are over a
granodiorite porphyry phase (Fig. 5) and slightly older few million years (granodioritic and dacitic phases at
than a 40Ar/39Ar hydrothermal biotite age of 37.3 ⫾ Escondida Norte, earlier phases at Zaldívar). Cooling
0.1 Ma (Table 2). The Re-Os molybdenite age of 36.6 ⫾ from ~800°C (closure temperature of U-Pb in zircons)
0.2 Ma overlaps within error with a 40Ar/39Ar age of to below 300 ⫾ 50°C (40Ar/39Ar biotite closure tempera-
36.8 ⫾ 0.l Ma for the sericitic alteration associated with ture) appears to have occurred rapidly at the Zaldívar
the copper mineralization at the Escondida Norte deposit (Campos et al., 2009), and this is emphasized
deposit (Escondida Internal Report, 2002). As sug- more clearly by the overlap of 40Ar/39Ar igneous biotite
gested earlier, the Escondida Norte porphyry is essen- ages with the U-Pb zircon ages. The limited range of
tially coeval with the Llamo porphyry at the Zaldívar the geochronologic data and the overlap of Re-Os
deposit. Recent U-Pb zircon dating of the dacitic phases molybdenite ages with the 40Ar/39Ar and U-Pb ages
of the Llamo porphyry (Jara et al., 2009) show that the (Fig. 5) also suggest a fast cooling of the Escondida
Llamo porphyry is a multiphase stock with an early Norte porphyry system. By contrast, the Escondida
phase emplaced at 38.0 ⫾ 0.5 Ma, and an intermediate deposit appears to have cooled more slowly or was
and late phase emplaced at least 0.7 m.y. later (Table 2). disturbed by a later intrusive event, such as the Colo-
Campos et al. (2009) have provided a series of 40Ar/39Ar rado Chico rhyolite, judging from the slightly younger
40
igneous biotite ages for the Llamo porphyry that range Ar/39Ar igneous biotite ages and the wide range
from 37.7 ⫾ 0.4 to 36.0 ⫾ 0.3 Ma. They concluded, of ages for the biotitic and sericitic alteration. Thus,
based on these data and the less precise U-Pb zircon the age values presented here combined with the
age of 38.7 ⫾ 1.3 Ma (Richards et al., 1999), that previously-published data suggest that most porphyry
the Llamo porphyry cooled approximately within Cu-Mo deposits were formed by several short-period
~1.5 m.y. However, the new U-Pb ages of Jara et al. episodic pulses of mineralizing activity within an

© 2010 The Authors

98 Resource Geology © 2010 The Society of Resource Geology
 Molybdenite of the Escondida deposits Chile

overall period of igneous and hydrothermal activity Barra, F., Ruiz, J., Mathur, R. and Titley, S. R. (2003) A Re-Os study
spanning a few (1–2 m.y.) to several million years on sulphide minerals from the Bagdad porphyry Cu-Mo
deposit, northern Arizona, USA. Mineral. Deposita, 38, 585–
(4–5 m.y.).
596.
Barra, F., Ruiz, J., Valencia, V. A., Ochoa-Landin, L., Chesley, J. T.
6. Conclusions and Zurcher, L. (2005) Laramide porphyry Cu-Mo mineral-
ization in northern Mexico: age constraints from Re-Os geo-
Molybdenite occurs extensively as the primary chronology in molybdenite. Econ. Geol., 100, 1605–1616.
Mo-bearing mineral in both Escondida and Escondida Bertens, A., Deckart, K. and González, A. (2003) Geocronología
Norte deposits, and is spatially enriched in the transi- U-Pb, Re-Os y 40Ar-39Ar del pórfido de Cu-Mo Los Pelambres,
Chile Central. Acta X Chile. Geol. Congr., Concepción (abst.).
tional chlorite-sericite zone. Due to supergene oxida- Birck, J. L., Roy Barman, M. and Capmas, F. (1997) Re-Os mea-
tion reactions, molybdenite was altered into lindgrenite surements at the femtomole level in natural samples. Geo-
and ilsemanite in the leached/oxide zones. stand. Newslett., 20, 19–27.
The age data presented suggest that the principal Campos, E., Wijbrans, J. and Andriessen, P. A. M. (2009) New
sulfide veinlet-type molybdenite mineralization at the thermochronologic constraints on the evolution of the Zaldí-
var porphyry copper deposit, Northern Chile. Mineral.
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late-magmatic to hydrothermal transition events. At gena del yacimiento Escondida. Unpubl. Graduate thesis,
least three episodes of Mo mineralization are observed Universidad Católica del Norte, Antofagasta, 147p.
in the Escondida deposit, whereas only two have been Escondida Internal Report (2002) SHRIMP U/Pb and 40Ar/39Ar
geochronology of intrusive rocks and mineralization at the
recognized so far in the Escondida Norte deposit. These Escondida Norte Cu deposit, northern Chile. Internal Rept
available data indicate that the Escondida porphyry Escondida Ltd., 16p.
system may have cooled at a slower rate, because the García, F. (1967) Geología del Norte Grande de Chile. In Symp.
lifespan of the larger Escondida deposit is apparently sobre el Geosinclinal Andino, Soc. Geol. Chile, Santiago, No 3,
long in comparison with the Zaldívar and Escondida 138p.
Hannah, J. L., Stein, H. J., Wieser, M. E., de Laeter, J. R. and
Norte deposits. The magmatic-hydrothermal lifespan Varner, M. D. (2007) Molybdenum isotope variations in
of the Zaldívar-Escondida Norte area is regarded to be molybdenite: vapor transport and Rayleigh fractionation of
approximately 1–2 m.y., as indicated for several major Mo. Geology, 35, 703–706.
porphyry deposits (Maksaev et al., 2004; Sillitoe & Jara, C., Rabbia, O. and Valencia, V. (2009) Petrología y dataciones
Perelló, 2005; Campos et al., 2009). In contrast to these U-Pb del pórfido cuprífero Zaldivar, II Región de
Antofagasta, Chile. Acta XII Chile. Geol. Congr., Santiago
deposits, the larger Escondida porphyry system has a (abst.).
characteristically protracted history with at least three Maksaev, V., Munizaga, F., McWilliams, M., Fanning, M., Mathur,
pulses of primary mineralization spanning 4–5 m.y. R., Ruiz, J. and Zentilli, M. (2004) New chronology for El
Teniente, Chilean Andes, from U-Pb, 40Ar/39Ar, Re-Os, and
Acknowledgments fission-track dating: implications for the evolution of a super-
giant porphyry Cu-Mo deposit. In Sillitoe, R. H., Perelló, J.
We thank Omar Cortés and Guillermo Vergara of and Vidal, C. E. (eds.) Andean metallogeny: new discoveries,
concepts, and updates, Vol. 11. Soc. Econ. Geol. Spec. Publ.,
Minera Escondida for their helpful support. Thanks are White Plains, NY, 15–54.
due to Nelson Guerra and Eduardo Medina for their Marinovic, N., Smoje, I., Maksaev, V., Hervé, M. and Mpodozis,
kind assistance in X-ray powder diffraction and EDS C. (1992) Hoja de Aguas Blancas, Región de Antofagasta. Serv
analyses. Also we would like to thank Leonel Jofre who Nac Geol Minería, Carta Geol. Chile, 70p.
completed figures of this article. The earlier manu- Nägler, T. F. and Frei, R. (1997) Plug in plug osmium distillation.
Schweiz. Mineral. Petrogr. Mitt., 77, 123–127.
script was greatly improved through critical reading by Newberry, J. (1979a) Polytypism in molybdenite (I): a non-
Jeremy Richards and Holly Stein, to whom we express equilibrium impurity-induced phenomenon. Am. Mineral.,
our sincere thanks. This study was carried out as a part 64, 758–767.
of the graduate thesis of the first author (BR). Newberry, J. (1979b) Polytypism in molybdenite (II): relationship
between polytypism, ore deposition alteration stages and
rhenium contents. Am. Mineral., 64, 768–775.
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