Richard H. Sillitoe,1 Fionnuala A.M. Devine,2 Martin I.
Sanguinetti,3 and Richard M. Friedman4
1 27 West Hill Park, Highgate Village, London N6 6ND, England 2
178-6th Street, Atlin, British Columbia, Canada V0W 1A0
3 NGEx Resources Inc., Gorriti 4855, CABA, Buenos Aires, Argentina
4 Pacific Centre for Isotopic and Geochemical Research, University
of British Columbia, 6339 Stores Road,
Vancouver, British Columbia, Canada V6T 1Z4
Abstract The Josemaría porphyry copper-gold deposit is located in
the Frontal Cordillera of San Juan Province, Argen- tina, near the
present-day northern limit of the Chilean-Pampean flat-slab segment
of the central Andes, and midway between the Maricunga and El Indio
metallogenic belts. The deposit is centered on small, multiphase
dacite porphyry intrusions that were emplaced at the contact
between rhyolitic volcanic and tonalitic plu- tonic rocks of Late
Permian to Triassic age. The earlier, more intensely quartz ±
magnetite-veined porphyry phases and contiguous wall rocks display
a telescoped sequence of alteration-mineralization zones, from
shallow advanced argillic (mainly quartz-pyrophyllite) and
underlying sericitic to deeper chlorite-sericite and minor remnant
potassic. All the alteration types are mineralized, but the highest
copper and gold grades are present as a low-arsenic,
high-sulfidation assemblage in the quartz-pyrophyllite and
sericitic zones. The outermost parts of the copper-gold zone are
overlapped by a pronounced molybdenum-bearing annulus.
New U-Pb zircon ages show that the deposit was formed at ~25 to
24.5 Ma, partially unroofed during contin- ued NNE-striking,
high-angle reverse faulting, and then unconformably overlain by
red-bed conglomerate and sandstone capped by andesitic and dacitic
tuff and lava. The andesite reported an age of ~22.35 Ma. A second,
discrete pulse of currently undated, advanced argillic alteration,
accompanied by minor high-sulfidation enar- gite mineralization,
locally affected the southern periphery of the deposit, including
its postmineral cover. Fol- lowing erosional removal of the
volcano-sedimentary strata from the northern and central parts of
the deposit, the NNE-trending fault zone underwent minor normal
displacement and localized economically significant supergene
chalcocite enrichment. However, probably because of the rapidity of
deposit unroofing, supergene processes were barely able to keep
pace with erosion, resulting in a thin supergene profile over much
of the exposed deposit. The southern part of the deposit remains
beneath the postmineral cover and, hence, escaped the
enrichment.
Josemaría is unusual among the many central Andean porphyry copper
deposits formed during rapid uplift because it preserves evidence
for not only alteration-mineralization telescoping but also
exceptionally rapid postmineral exhumation and subsequent burial
beneath thick volcano-sedimentary cover. Unroofing of por- phyry
copper deposits in 1 to 2 m.y. is more typical of the high erosion
rates that characterize pluvial tropical climates than the semiarid
conditions that prevailed during and since the formation of
Josemaría.
Introduction Josemaría is a porphyry copper-gold deposit located
4,400 to 4,800 m above sea level in the Frontal Cordillera of San
Juan Province, Argentina, ~8 km east of the border with Chile (lat
28°26'S, long 69°32'W). The Frontal Cordillera—the main Andean
mountain range at this latitude—is a fault-bounded massif
consisting of late Paleozoic to Triassic basement and
partially preserved cover sequences (Ramos, 1999). Josemaría lies
roughly midway between the porphyry and high-sulfidation epithermal
deposits of the Maricunga and El Indio metallo- genic belts and,
along with nearby porphyry copper deposits and several early-stage
prospects, effectively links the two (Pan- teleyev and Cravero,
2001; Mpodozis and Kay, 2003; Jones and Martínez, 2007; Rode and
Carrizo, 2007; Fig. 1a).
The Josemaría deposit was discovered in 2004 by Desarrollo de
Prospectos Mineros S.A. (Deprominsa), the Argentinian
Submitted: December 27, 2018 / Accepted: March 17, 2019 ©2019
Society of Economic Geologists; Economic Geology, v. 114, no. 3,
pp. 407–425 ISSN 0361-0128; doi:10.5382/econgeo.4645; 19 p. Digital
appendices are available in the online Supplements section.
407
† Corresponding author: e-mail,
[email protected]
Vol. 114 May No. 3
© 2019 Gold Open Access: this paper is published under the terms of
the CC-BY license.
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°S 31
MINERAL DEPOSITS
Porphyry Au
18 - 5 Ma
25 - 20 Ma
35° 65°70° 65°70°75° W
S P e r u - C h i l e T r e n c h
A R
G E
N T
I N
A
P E R U B O L I V I A
C H
I L
E C
H I
L E
A R
G E
N T
I N
A
P E R U B O L I V I A
South America
Los Azules
200 km
C H
Mid-late Miocene volcanic and sedimentary rocks Late Oligocene -
early Miocene volcanic and sedimentary rocks Late Eocene -
Oligocene intrusive and volcanic rocks Late Paleozoic - Triassic
basement and Triassic-Cretaceous sedimentary rocks
Pliocene - Holocene sediments
(
(
a b
Fig. 1. a. Location of the Josemaría porphyry copper-gold deposit
in the late Oligocene-Miocene volcanic belt of northern Chile and
contiguous Argentina, showing the age assignment of the main
porphyry and epithermal deposits. Insets show the location of the
flat-slab portion of the subducted Nazca plate (contours from
Cahill and Isacks, 1992) and position of Figure 1a in South
America. b. More detailed geology of the Josemaría area and
vicinity. Volcanic rocks compiled mainly from Maksaev et al.
(1984), Mpodozis et al. (1995), Fauqué (2001), Panteleyev and
Cravero (2001), Zappettini et al. (2001, 2008), SERNAGEOMIN (2003),
Sanguinetti (2006), Martínez et al. (2015a), Mpodozis et al.
(2018), and C. Mpodozis (writ. commun., 2018). The age of older
volcanic rocks west of the Los Helados fault is uncertain, but
between Paleocene and Oli- gocene. Deposit age assignments based on
Sillitoe et al. (1991, 2013, 2016), Kay et al. (1994), Mpodozis et
al. (1995), Bissig et al. (2001), Mpodozis and Kay (2003), Perelló
et al. (2012), Maydagán et al. (2014), Cáceres (2015), Y. Kapusta
in Rode et al. (2015), Yoshie et al. (2015), Holley et al. (2016),
Astorga et al. (2017), and NGEx Resources Inc. (unpub. data).
Background shaded-relief image from Esri (2014 version).
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JOSEMARIA PORPHYRY Cu-Au DEPOSIT, ARGENTINA 409
subsidiary of Tenke Mining Corporation—a Lundin Group company,
during a regional program to explore the apparently poorly
mineralized gap between the Maricunga and El Indio belts (Jones,
2007). Follow-up of a LANDSAT TM false-color anomaly, first defined
a decade earlier by Norwest Mine Ser- vices, Inc. on behalf of the
Secretaría de Minería de la Nación of Argentina (Decker, 1994), led
to definition of a promising 400- × 400-m, talus-fines geochemical
copper-gold-molybde- num anomaly and eventual drill testing (Rode
and Carrizo, 2007). Since then, Josemaría has been the subject of a
series of drilling campaigns, for a total of 61,100 m in 142 holes,
culminating in the 2016 announcement by NGEx Resources Inc.—a
successor company to Tenke Mining Corporation—of an indicated
sulfide mineral resource of 835 Mt at 0.35% Cu and 0.25 g/t Au, at
a 0.3% Cu equiv cutoff, for 6,500 Mlb of copper and 6.6 Moz of gold
(Ovalle et al., 2016).
In the context of its geographic location, the Josemaría deposit
displays several interesting geologic features, includ- ing its
age, localized supergene enrichment, and limited time spanning
deposit formation, exhumation, and subse- quent burial beneath
postmineral volcano-sedimentary rocks. These topics are the main
focus of this paper, which is based on detailed field mapping,
drill core logging, and support- ing petrography and U-Pb zircon
geochronology. The paper builds on an extended abstract presented
previously (Ortiz et al., 2015).
Regional Geologic Setting The Josemaría deposit is situated near
the northern limit of the present-day amagmatic Chilean-Pampean
flat-slab seg- ment of the Andean Cordillera (~28°–33°S), which is
char- acterized by low-angle subduction of the Nazca plate beneath
South America (Jordan et al., 1983; Cahill and Isacks, 1992; Kay
and Mpodozis, 2002; Ramos et al., 2002; Fig. 1a, inset). Josemaría
is an integral part of a N-trending, late Oligocene to Miocene
magmatic arc, containing numerous porphyry and epithermal deposits
and prospects, which spans the northern transition zone between the
Chilean-Pampean flat-slab seg- ment and the central Andean steep
slab, including the Mari- cunga belt to the north (Fig. 1a). In the
Maricunga and El Indio belts, volcanic rocks are widespread whereas
between them, where Josemaría is situated, the volcanic pile is
rather less extensive, possibly because of erosional removal during
kilometer-scale Miocene uplift to expose the late Paleozoic to
Triassic basement (Ribba et al., 1988; Mpodozis and Kay, 1992,
2003; Fig. 1a).
The late Oligocene to early Miocene (~26–20 Ma) part of the
volcanic arc was constructed during relatively steep sub- duction,
which since ~18 Ma became progressively shallower and, south of
latitude 28°S, evolved to the current flat-slab geometry (Kay and
Mpodozis, 2002; Kay et al., 2014). The slab shallowing is generally
attributed to subduction of the Juan Fernández aseismic ridge, a
bathymetric high, result- ing in increased interplate mechanical
coupling and conse- quent contraction, thick-skinned reverse
faulting, and crustal shortening and thickening (Jordan et al.,
1983; Gutscher et al., 2000; Horton, 2018). Nonetheless, one of the
anomalous geologic features of the transition zone between the
flat-slab segment and steeper slab to the north, at least along the
east- ern side of the Maricunga belt around latitude 27°–28°S, is
an
important earlier pulse of compressive deformation at ~26 to 25 Ma
(Mpodozis and Clavero, 2002; Mpodozis et al., 2018) that is thought
likely to have extended at least as far south as Josemaría.
Most porphyry and epithermal deposits in the flat-slab segment were
formed during the process of flattening, from ~18 to 5 Ma (Bissig
et al., 2001; Mpodozis and Kay, 2003; Y. Kapusta in Rode et al.,
2015; Yoshie et al., 2015; Holley et al., 2016; Sillitoe et al.,
2016; Astorga et al., 2017; Fig. 1a). The only known exceptions,
emplaced prior to slab flatten- ing, are Josemaría and several
prospects near the Caserones porphyry copper deposit (Perelló et
al., 2003a; Yoshie et al., 2015; Fig. 1a). However, north of the
flat-slab segment, in the main part of the Maricunga belt, several
deposits and prospects formed during the 26 to 20 Ma interval, most
notably the Caspiche porphyry gold-copper, Refugio (Mari- cunga)
porphyry gold, and La Coipa, Esperanza (Nueva Esperanza), and La
Pepa high-sulfidation epithermal gold- silver deposits (Sillitoe et
al., 1991, 2013; Kay et al., 1994; Mpodozis et al., 1995; Fig.
1a).
Deposit Geology
Premineral lithologies
The Josemaría deposit is located within a 25-km-wide, fault-
bounded block of volcanic and plutonic basement rocks (Fig. 1b),
the former considered Late Permian in age by Marcos et al. (1971).
These volcanic rocks are now assigned to the Guanaco Sonso
Formation, of Late Permian-Early Triassic age (Martin et al., 1999;
Coloma et al., 2017), which is wide- spread along the Chilean side
of the international border, west of Josemaría (Martínez et al.,
2015a). In the western parts of the Josemaría area, the Guanaco
Sonso Formation comprises rhyolitic volcaniclastic rocks, including
welded ignimbrite flows (Figs. 2, 3), although systematic
subdivision is difficult because of the masking effects of
hydrothermal alteration.
East of and locally intruded beneath the rhyolitic sequence is an
equigranular hornblende-biotite tonalite pluton (Figs. 2, 3), which
consists of several phases distinguishable on the basis of grain
size and total quartz content. A granodioritic phase is also
present north of the deposit (Fig. 3) and melano- cratic diorite
was encountered by drilling to the southeast. A U-Pb zircon age of
259.11 ± 0.21 Ma was obtained for a sam- ple of the tonalite from
the southern part of the deposit, using the chemical abrasion,
isotope dilution, thermal ionization mass spectrometry (CA-TIMS)
technique on single grains, as detailed in Scoates and Friedman
(2008) and the Electronic Appendix. This result makes it part of
the regionally exten- sive Montosa-El Potro Plutonic Complex of
Late Permian to Early Triassic age (265–245 Ma; Martínez et al.,
2015a). The tonalite is cut by N- to NE-trending, andesitic to
basaltic dikes (Fig. 3), most too narrow to be shown in the
figures. Such dikes are a typical feature of many Permo-Triassic
plutons in the Frontal Cordillera (e.g., Martin et al.,
1999).
These Late Permian to Triassic volcanic and plutonic rocks are
currently considered to represent the shallow and deep parts,
respectively, of igneous complexes emplaced during crustal
extension induced by either a hiatus in subduction (Mpodozis and
Kay, 1992) or slow subduction with slab roll- back (del Rey et al.,
2016).
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410 SILLITOE ET AL.
Pre- and synmineral structure
The main premineral feature at Josemaría is the contact between the
tonalite pluton and rhyolitic volcanic rocks. Although this contact
is inferred to be generally subhorizon- tal, there is a suggestion
from three-dimensional modeling of the immediate deposit area that
it may be marked by a steep, N-striking, premineral fault that was
plugged by por- phyry intrusions, although the evidence is largely
obscured by postmineral faulting (Fig. 3). Judging by the northerly
elon- gation of the porphyry intrusions, the suspected fault may
have influenced their emplacement (Fig. 3). Furthermore, the
Josemaría deposit lies at the southern end of a >16-km- long,
N-trending alteration corridor, within which there are at least two
additional porphyry copper-gold prospects (Fig. 1b), including
N-trending porphyry dikes, suggesting that the putative structure
could be regionally significant.
At the time of and immediately following deposit forma- tion, and
broadly contemporaneous with the 26 to 25 Ma compressive tectonism
(Mpodozis et al., 2018), a slightly oblique, NNE-trending fault
zone transected the Josemaría area and acted as an east-over-west
reverse structure, thereby accounting for preservation of the
rhyolitic volcanic rocks and shallow-level alteration features (see
below) on its west- ern footwall side (Fig. 3). Similarly oriented
faults, showing most recent W-vergent reverse displacement, are
mapped in the region; some of them had early normal motion dur- ing
late Eocene extension when a major parallel structure, the Mogotes
fault, ~5 km east of Josemaría, was active along the western margin
of the Macho Muerto basin (C. Mpodozis, writ. commun., 2018; Fig.
1b).
Porphyry intrusions
The Josemaría porphyry copper-gold deposit is centered on several
small dacite porphyry intrusions, which are subdivided
into early, inter-, and late-mineral phases. All the phases are
characterized by 60 to 80 vol % of well-formed plagioclase,
hornblende, and biotite phenocrysts, the last as well-formed
“books,” along with scattered quartz phenocrysts in a fine- grained
groundmass.
The early porphyry phases are intensely quartz veined and well
mineralized, whereas the intermineral phases are notice- ably less
strongly veined; both phases were originally potassic altered. In
contrast, the late-mineral phases display propylitic alteration,
have few quartz veinlets, and are essentially bar- ren. The
porphyry phases were distinguished using standard geologic criteria
(Sillitoe, 2000, 2010; Proffett, 2003), partic- ularly the presence
near intrusive contacts of quartz veinlet xenoliths (Fig. 4) in
combination with quartz veinlet intensi- ties and copper and gold
contents.
The greatest volume of intrusive porphyry occupies the cen- tral
parts of the deposit, where the largest body of the early phase is
flanked to the northeast and southwest by the largest late and
intermineral bodies, respectively (Figs. 2, 3). Farther north
still, the porphyries are confined to steep dikes, with the early
and intermineral phases only a few meters wide.
The early and intermineral porphyries (Fig. 3) returned U-Pb zircon
ages of 24.98 ± 0.04 Ma (youngest grain) and 24.66 ± 0.04 Ma (MSWD
= 1.12), respectively, employing the same CA-TIMS method;
analytical details are given in the Electronic Appendix.
Hypogene Alteration and Mineralization Five alteration
types—potassic, chlorite-sericite, sericitic, advanced argillic,
and late propylitic—are systematically rec- ognized within the
Josemaría deposit, each with its own dis- tinctive sulfide
assemblage where not subjected to supergene oxidation. Together,
these define an alteration footprint mea- suring 4 km north-south
and 2 km east-west (Fig. 5).
Tonalite Rhyolitic volcanic rocks
Postmineral volcanic and sedimentary rocks
Approx. limit of >0.3% Cu equiv. resource projected to surface
(not shown under postmineral cover)
WNWESE
Granodiorite
Fig. 2. View, looking south, of the Josemaría alteration zone,
showing exposed pre- and synmineral rock types, surface projec-
tion of the >0.3% Cu equiv zone, unconformably overlying
postmineral volcanic rocks, and NNE-striking, postmineral fault
zone that controls supergene chalcocite enrichment.
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JOSEMARIA PORPHYRY Cu-Au DEPOSIT, ARGENTINA 411
MIOCENE (post - 22 Ma) Josemaría dacite porphyry intrusions LATE
OLIGOCENE (~25 Ma)
PERMIAN - TRIASSIC
Late mineral Intermineral Early mineral
Tonalite / granodiorite Rhyolitic volcanic rocks
Andesitic dike (Ad)
Early reverse: certain, inferred Late normal: certain,
inferred
Fault
Limit of >0.3% Cu equiv. projected to surface, dashed where
under postmineral cover
Basaltic plug Hydrothermal breccia Rhyolite dike
445000 445500 446000 446500 447000
68 53 00 0
68 53 50 0
68 53 50 0
68 54 00 0
68 54 00 0
68 54 50 0
68 54 50 0
68 55 00 0
68 55 00 0
68 55 50 0
68 55 50 0
56005600
50005000
Ad
Ad
Ad
Ad
Ad
Ad
Fig. 3. Geology of the Josemaría porphyry copper-gold deposit and
environs, simplified from 1:5,000-scale mapping by the second
author (FAMD). Also shown is the surface projection of the deposit
at a >0.3% Cu equiv cutoff (after Ovalle et al., 2016).
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412 SILLITOE ET AL.
The Josemaría deposit is broadly centered on an exposed
chlorite-sericite alteration zone, approximately 800 m across,
which gives way westward and, below postmineral cover, southward to
sericitic and advanced argillic alteration (Figs. 5, 6b, 7b). The
chlorite-sericite assemblage formed at the expense of preexisting
potassic alteration, which is preserved at depths of anywhere
between 400 and 600 m below the present surface and as a steeply
inclined, roughly cylindrical body, up to 200 m across, which
extends to the surface alongside the dikes in the central-northern
part of the deposit (Figs. 5, 6b). Approach to the potassic zone is
heralded by weakening of the chlorite-sericite overprint and
presence of remnant biotite.
Results of the surface mapping show that the alteration zone is
much larger than the deposit itself, with advanced argillic and
sericitic alteration extending 700 m west and >1,500 m north and
south; to the east, this distal alteration is concealed beneath
postmineral cover (Fig. 5). The advanced argillic alteration
occupies the highest ground, dominates the rhyo- litic volcanic
rocks (see below), and transitions downward to and overprints the
sericitic zone or, where the latter is absent, the
chlorite-sericite or potassic zones. The sericitic zone is
encircled by a weakly developed propylitic halo (Fig. 5).
Potassic alteration
The potassic alteration is preserved at depth in the tonal- ite as
well as in the preporphyry andesitic to basaltic dikes and early
and intermineral dacite porphyry intrusions that cut it (Fig. 6b).
The preporphyry dikes can display potassic alteration even where
enclosing tonalite was overprinted by chlorite-sericite alteration,
reflecting their greater contents of hydrothermal biotite and
fine-grained, relatively imperme- able nature.
Hydrothermal biotite defines the potassic zone (Fig. 8a), although
apparently unaltered magmatic biotite is also pres- ent.
Plagioclase is bleached white in parts of the potassic zone as a
result of replacement by albite associated in part with K-feldspar,
an assemblage denominated sodic-potassic by Lang et al. (2013).
Hydrothermal magnetite constitutes up to 5 vol % or more of the
potassic-altered tonalite and early and intermineral porphyries, in
veinlets with or without quartz as well as in disseminated
form.
Multidirectional quartz veinlets were introduced during the
potassic event and become less abundant as the porphyries
become younger and, in their host rocks, with distance from the
core of the deposit; they attain ~15 vol % of the early porphyry
and were everywhere preserved during the super- position of the
chlorite-sericite, sericitic, and advanced argillic alteration
(Fig. 8b, d). The more prominent exposed quartz veinlets are mapped
in Figure 5. The outer limit of appre- ciable quartz veining acts
as a useful proxy for the external limit of copper-gold
mineralization (Fig. 6b). Following the classification of Gustafson
and Hunt (1975), the quartz veinlets in the potassic zone are
mainly A-type (Figs. 4, 8a), containing varied amounts of
chalcopyrite ± magnetite, but molybdenite-bearing B-type quartz and
D-type quartz-pyrite veinlets (Fig. 8c) are also present. Where not
dissolved by cool descendant groundwater under supergene conditions
(see below), anhydrite is present in the potassic zone, most of it
in monomineralic veinlets that were introduced late during deposit
formation.
The potassic alteration is characterized by veinlet and dis-
seminated chalcopyrite, with little or no accompanying pyrite. The
relatively low-sulfidation state of the sulfide zone is fur- ther
emphasized by the local presence, mainly in the A-type quartz
veinlets, of minor bornite. Chalcopyrite/bornite ratios are
estimated to be >30:1.
Chlorite-sericite alteration
The chlorite-sericite alteration is dominant throughout the
tonalite and early and intermineral porphyry intrusions in the
northern and central parts of Josemaría where it is transitional to
and overprints the potassic alteration; however, farther south in
the deposit it is present mainly at depth. Plagioclase is wholly or
partially replaced by sericite, and biotite and horn- blende by
chlorite (Fig. 8b). On the margins of the chlorite- sericite zone,
the sericite gives way in places to pale-green illite. Magnetite is
largely transformed to hematite, either as pseudomorphic martite or
specularite. Anhydrite veining is similar to that in the potassic
alteration. The low-sulfidation chalcopyrite ± bornite assemblage
in the potassic alteration is variably reconstituted to
pyrite-chalcopyrite in the chlorite- sericite zone, typically
giving rise to pyrite/chalcopyrite ratios of ~3 to 10:1.
Sericitic alteration
The sericitic alteration is present in the shallower levels of the
deposit and commonly underlies the advanced argillic altera- tion,
a relationship best seen in the southern parts. However, as
illustrated by Figure 6b, it can be volumetrically subordi- nate to
advanced argillic alteration in the rhyolitic volcanic rocks.
Sericite-bordered, D-type veinlets are a characteristic feature of
the sericitic zone (Fig. 8c).
The sericitic alteration within the copper-gold deposit is
characterized by a prominent high-sulfidation sulfide assem- blage
in which disseminated grains of pyrite are surrounded by black
sulfidic rims composed of intergrown hypo- gene chalcocite,
bornite, and/or covellite along with trace amounts of tennantite
and enargite. All preexisting magne- tite and hematite were
pyritized and pyrite/copper sulfide ratios are roughly 10:1.
Downward, the sericitic alteration and high-sulfidation
mineralization are observed in places to grade into the pyrite- and
chalcopyrite-bearing, chlorite- sericite zone.
veinlet xenolith
A-type quartz veinlets
Fig. 4. Intermineral dacite porphyry containing a xenolith of
chalcopyrite- bearing A-type quartz veinlet derived from the early
porphyry. The rock was subsequently cut by narrow A-type quartz
veinlets and then overprinted by sericitic alteration. Veinlet
nomenclature follows Gustafson and Hunt (1975). Scribe tip for
scale.
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Josemaría porphyry system alteration LATE OLIGOCENE (~25 Ma)
Basaltic plug (unaltered)
Postporphyry cover rocks Volcanic rocks Advanced argillic
Sericitic (strong/moderate)
68 53 00 0
68 53 50 0
68 53 50 0
68 54 00 0
68 54 00 0
68 54 50 0
68 54 50 0
68 55 00 0
68 55 00 0
68 55 50 0
68 55 50 0
48 00
47 50
47 00
4250
Drill hole Limit of >0.3% Cu equiv. projected to surface, dashed
where under postmineral cover
Early reverse: certain, inferred Late normal: certain,
inferred
Fault
56005600
50005000
Fig. 5. Hydrothermal alteration of the Josemaría porphyry
copper-gold deposit and environs. Also shown is the surface pro-
jection of the deposit at a >0.3% Cu equiv cutoff (after Ovalle
et al., 2016).
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Sulfate front
Drill hole, with Cu (red) and Au (yellow) grades
Late-mineral porphyry
Quaternary colluvium
Fig. 6. Representative sections at 5,600N, approximately the middle
of the Josemaría copper-gold deposit. a. Lithologies. b.
Hydrothermal alteration. c. Supergene profile. Section line shown
in Figures 3 and 5.
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D H 51
Fault (NW): late normal motion
Drill hole, with Cu (red, right) and Au (green, left) grades
LITHOLOGY
Intermineral porphyry
Early porphyry
a E5000NW
259.11 ± 0.21 Ma
Fig. 7. Representative sections at 5,000N, in the southern part of
the Josemaría copper-gold deposit where it is concealed beneath
postmineral rocks. a. Lithologies. b. Hydrothermal alteration. c.
Supergene profile. Note the position of the dated tonalite sample
in (a). Section line shown in Figures 3 and 5.
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Advanced argillic alteration
The advanced argillic alteration is largely but not exclusively
restricted to the rhyolitic volcanic unit (Fig. 6b) although,
locally, narrow, structurally controlled zones penetrate the
underlying tonalite to depths of up to 350 m. The advanced argillic
alteration preferentially affected the rhyolite because of its
silicic nature and consequent low acid-buffering capac- ity
relative to the other lithologies present. The zone is domi- nated
by quartz and pyrophyllite (Fig. 8d) although in places, at shallow
depths, alunite rather than pyrophyllite replaces plagioclase
phenocrysts. Other minerals typical of the advanced argillic
alteration include dickite, observed together with pyrite filling a
few veinlets, dumortierite, in the form of tiny pale-blue rosettes,
and diaspore, identified only in thin sections.
The advanced argillic alteration contains the same high-
sulfidation sulfide assemblage as the sericitic zone and is simi-
larly pyrite rich and devoid of magnetite and hematite. Native
sulfur coats fractures in places, accompanied locally by crys-
talline, hypogene covellite.
Propylitic alteration
In addition to the weakly developed propylitic halo to the
Josemaría deposit (Fig. 5), the late-mineral dacite porphyry
intrusions are also pervasively albeit weakly propylitized (Figs.
5, 6b), with all mafic minerals altered to chlorite and plagioclase
containing replacive patches of epidote. The late propylitization,
in common with that in late-mineral por- phyry copper intrusions in
general (e.g., Sillitoe, 2000), barely affects the enclosing rocks.
Pyrite contents of both the early halo and late-mineral porphyry
propylitization are low, typi- cally <1 vol %.
The propylitic halo, thought to have formed contemporane- ously
with the potassic zone (e.g., Proffett, 2003), is not distin-
guished from this late-stage propylitization of the late-mineral
porphyry in Figures 5 and 6b because of their mineralogic
similarity.
Grade distribution
Using a >0.3% Cu equiv cutoff projected to surface, the cop-
per-gold zone at Josemaría measures a maximum of ~1,500 m
north-south and ~1,000 m east-west (Figs. 3, 5, 9). Within this
zone, the highest hypogene copper and gold values lie in its
southern part, eccentrically with respect to the deposit out- line
(Fig. 9). The external parts of the copper-gold deposit are
overlapped by a molybdenum-rich annulus, with >50 ppm Mo, which
is higher in grade around the northern and east- ern sides (Fig.
9). Although the molybdenite in the annulus formed during potassic
alteration, as shown by much of it occurring in B-type quartz
veinlets, it was retained during the subsequent alteration
overprints.
The hypogene copper and gold values generally correlate reasonably
well in all the alteration-mineralization types, sug- gesting joint
introduction and precipitation of the two met- als during the early
potassic event and their continued close association during the
alteration-mineralization overprinting. However, the Cu-Au
correlation coefficient for the entire deposit is only 0.57, at
least in part due to mobility of copper in the central and northern
parts of the deposit during supergene
b
a
c
d
Aq
Aq
Aq
Aq
py
ser
ser
Aq
Aq
Aq
Aq
Aq
py
ser
ser
Aq
Fig. 8. Alteration and veinlet types in tonalite, except for (d) in
early por- phyry, Josemaría porphyry copper-gold deposit. a.
Potassic alteration with A-type quartz (Aq) veinlets. b.
Chlorite-sericite alteration with inherited A-type quartz (Aq)
veinlet. c. Moderate-intensity sericitic alteration with D-type
veinlets, in which pyrite (py) center lines have
texture-destructive ser- icitic (ser) halos. d. Advanced argillic
alteration with inherited A-type quartz (Aq) veinlets. Veinlet
nomenclature follows Gustafson and Hunt (1975). Scribe tip for
scale.
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S urfac e p
Cu
Au
Fault: early reverse, late normal displacement
Main supergene Cu enrichment
Maximum limit of the 0.3% Cu equiv. resource projected to
surface
Hypogene Hypogene + supergene
Supergene Cu domain
N
Fig. 9. Horizontal slices through the Josemaría porphyry
copper-gold deposit at 4,500-, 4,300-, and 4,100-m elevations,
show- ing distributions of hypogene gold (>0.3 g/t) and
molybdenum (>50 ppm) and hypogene plus supergene copper
(>0.2%). Note the well-formed molybdenum annulus. The
progressive northward deepening of the highest copper values
reflects the fault-localized supergene chalcocite enrichment. Grade
boundaries are interpolated from downhole assay values.
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418 SILLITOE ET AL.
processes. The overall similarity of copper (~0.35%) and gold (~0.2
g/t) contents in the potassic and chlorite-sericite zones suggests
that there was little metal addition or removal during the
reconstitution of the chalcopyrite ± bornite assemblage to
pyrite-chalcopyrite.
At the deposit scale, the highest hypogene grades, particu- larly
of copper but, in the south, also of gold, tend to occur in the
advanced argillic and sericitic zones in which they are present in
the high-sulfidation sulfide assemblage; this clearly gave rise to
hypogene enrichment, defined as a grade increase relative to the
preexisting potassic and chlorite-sericite altera- tion (Fig. 8a,
b). In the south, average copper and gold val- ues of ~0.6% and
~0.7 g/t are attained locally, accounting for the highest grade
part of the deposit. The minor tennantite and enargite result in
arsenic values typically between 10 and 100 ppm. Distinction
of the hypogene enrichment from that of supergene origin at
Josemaría is not straightforward. None- theless, hypogene-enriched
zones are commonly cut by pris- tine pyrite veinlets lacking
copper-bearing sulfide minerals, whereas such veinlets are the
preferred sites for supergene chalcocite enrichment (see
below).
Postmineral Volcano-Sedimentary Cover The southern part of the
Josemaría deposit is unconformably overlain by a
volcano-sedimentary sequence, much of which is completely unaltered
and devoid of sulfide minerals (Figs. 5, 7b). Contacts between the
porphyry copper alteration and mineralization and postmineral
sequence are abrupt and readily mappable at surface and in drill
core (Figs. 2, 3, 5). The sequence, which comprises lower
sedimentary and upper volcanic components (Fig. 10a), thickens
dramatically southeastward to >500 m, much of it attributable to
the sedi- mentary unit (Fig. 7a). The paleotopography upon which
the sediments were deposited dips southeast at ~30°, steepen- ing
farther southeastward (Fig. 7a). The fact that the por- phyry
intrusions are subvertical strongly suggests that this is an
original slope angle rather than a product of postmineral
tilting.
The lower part of the postmineral sequence is entirely composed of
stratified siliciclastic rocks, both polymict, clast-supported,
cobble-pebble conglomerate and imma- ture, poorly bedded,
quartz-rich sandstone, the latter becoming more abundant eastward
(Figs. 3, 7a). Clasts vary from well rounded to subangular, with
the term brec- cia perhaps more appropriate where the latter
predominate. These lithofacies suggest deposition in a proximal
alluvial fan environment (e.g., Blair and McPherson, 1994). Above
the southern part of the deposit, the conglomerate horizon
Bd
Be
Ba
Bb
Bc
hem
jar
Fig. 10. Postmineral features near the Josemaría porphyry
copper-gold deposit. a. Hematitic basal conglomerate overlain by
felsic ignimbrite. Note the thin jarosite layer along the contact.
b. Hematitic polymict conglomer- ate, with the largest clast being
rhyolitic volcanic rock containing chalcopyrite (cpy). c. Hematitic
(hem) conglomerate cut by pyrite veinlets with kaolinized halos
(kao) containing disseminated pyrite (py). Note that pyrite
overprints and destroys the hematite. d. Advanced argillic-altered
conglomerate impreg- nated with quartz (q), pyrite (py), and
enargite (en); kaolinite (kao) fills cavi- ties. e. Hematitic (hem)
conglomerate overprinted by supergene jarosite (jar) after former
disseminated pyrite. Note the A-type quartz (Aq) veinlet in the
centrally located clast. Scribe tip for scale in (b), (c), (d), and
(e).
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JOSEMARIA PORPHYRY Cu-Au DEPOSIT, ARGENTINA 419
and a thinner conglomerate bed intercalated in the overlying
volcanic unit have a hematite-impregnated matrix and are considered
as red-bed sediments (Figs. 7a, 10b). At surface, the main
hematitic horizon can be traced continuously for 2,300 m along the
base of the postmineral sequence (Fig. 3). The conglomerate clasts
consist mainly of rhyolitic vol- canic rocks, some displaying
advanced argillic alteration, but clasts containing A-type quartz
veinlets (Fig. 10e) or specular hematite are also present. Pyrite,
chalcopyrite, and high-sulfidation sulfide mineralization occur in
many incom- pletely oxidized or unoxidized clasts (Fig. 10b).
Indeed, the unaltered conglomerate contains elevated copper and
gold values, typically ~50 to 500 ppm and 0.1 to 0.2 g/t,
respec- tively, which are assumed to be contributed by the mineral-
ized clasts. Local concentrations of detrital magnetite grains also
occur in places. These lithologic and geochemical fea- tures
support a local source contribution from the eroding Josemaría
deposit.
The upper, volcanic part of the postmineral sequence com- prises
andesitic and dacitic lavas underlain by poorly welded, lithic-rich
and -poor ignimbrites (Figs. 7a, 10a). An andesite sample,
collected at surface (Fig. 3), returned a U-Pb zircon age of 22.35
± 0.03 Ma (MSWD = 1.3; Electronic Appen- dix), in keeping with the
postmineral timing of the volcano- sedimentary sequence. Based on
this age, the postmineral volcano-sedimentary rocks may be
correlated with the Río La Gallina-Refugio sequence along the
eastern side of the south- ern Maricunga belt, between latitude
27°30' and 28°S (26– 21 Ma; Mpodozis et al., 2018) and the
Tilito Formation of the Doña Ana Group in the El Indio belt
(24.1–21 Ma; Maksaev et al., 1984; Kay et al., 1988; Bissig et al.,
2001; C. Mpodozis, pers. commun., 2019).
Postmineral Normal Faults A series of NW-striking, high-angle
faults transect the eastern parts of the deposit, stepping the
postmineral volcano-sedi- mentary rocks down to the northeast but
also accommodat- ing some sinistral motion (Fig. 3). The NW faults
appear to terminate at the NNE-striking reverse fault zone within
the deposit, which was reactivated at this time as a series of
normal faults. The reactivated faults are marked by 5 to 10 cm of
gouge surrounded by damage zones a few meters wide. East-side-down
displacement on the easternmost of the three main reactivated
faults is on the order of 120 m (Fig. 6a).
The age of this post ~22 Ma normal faulting is uncon- strained,
although it could have been broadly synchronous with the
postmineral red-bed sedimentation and volcanism and, therefore,
correlative with the regional extension that accompanied deposition
of the Doña Ana Group in the El Indio belt (24.1–17.5 Ma; Bissig et
al., 2001; C. Mpodozis, writ. commun., 2019). Alternatively, it
could have been in the mid-Miocene when extension affected the
eastern side of the southern Maricunga belt around latitude
27°–28°S (Mpodozis et al., 2018).
Younger Hypogene Alteration and Mineralization The upper, volcanic
part of the postmineral cover sequence is everywhere little altered
except for weak propylitization of the andesitic units; however,
part of the siliciclastic red-bed
sequence unconformably overlying the Josemaría deposit,
particularly the hematitic basal conglomerate, is pervasively
kaolinized and pyritized (Fig. 7b). Although most of this later
alteration and mineralization lies beyond the Josemaría deposit, it
did affect it locally along faults but with minimal addition to
grade.
Hydrothermal breccia dikes, 1 to 50 m wide and cemented by
chalcedony, alunite, kaolinite, pyrite, and enargite, are prominent
just south of the Josemaría deposit. The main dike at surface
strikes west to northwest and is ~2 km long (Figs. 3, 5). Locally,
the advanced argillic alteration and mineralization associated with
this breccia dike invade fault zones, with a particular affinity
for the NW faults, as well as the hematitic conglomerate at the
base of the postmin- eral sequence (Fig. 5), causing the latter to
undergo bleach- ing and hematite destruction. In drill core, pyrite
veinlets cutting the conglomerate have kaolinized halos containing
disseminated pyrite, which clearly overprints and sulfidizes the
hematitic matrix (Fig. 10c). The conglomerate in several places,
including that at the base of one of the thickest silici- clastic
intervals, is cut by a stockwork of quartz, pyrite, and enargite
associated with cavity-filling kaolinite and lesser alunite (Figs.
7b, 10d), with all former hematite entirely destroyed. Importantly,
this arsenic-rich copper mineraliza- tion lacks accompanying gold
(<40 ppb), thereby making it not only temporally but also
geochemically distinct from the high-sulfidation mineralization in
the upper parts of the Josemaría deposit.
Supergene Oxidation and Enrichment The supergene profile at
Josemaría is generally thin, but in the central and northern parts
of the deposit can attain a thickness of 400 m over widths of
<200 m because of downward exten- sion along the three main
NNE-striking, steeply E-dipping, now normal postmineral faults
(Figs. 2, 7c, 9). The supergene profile consists of a jarosite- and
goethite-dominated leached capping—within which 43 Mt average 0.32
g/t Au (at a 0.2 g/t Au cutoff; Ovalle et al., 2016)—underlain
by a northward- deepening, chalcocite enrichment blanket, ranging
from 100 to 200 m thick (Fig. 6c). In places, however, where the
oxida- tion affected pyrite-poor potassic alteration, there are
zones containing oxide copper minerals (Fig. 6c), mainly fracture-
coating malachite and neotocite. Nonetheless, the leached capping
is generally depleted in copper, typically reporting <0.1%, in
marked contrast to the underlying enrichment with typical values of
0.8 to 1.5% Cu.
In the concealed southern parts of the deposit, the postmin- eral
volcano-sedimentary rocks overlie a 30- to 140-m-thick, E-thinning
supergene profile, comprising mainly leached and partially leached
zones; underlying chalcocite enrichment is minimal (Fig. 7c). The
leached and partially leached zones contain more supergene hematite
than the leached capping farther north, the reason for which is
uncertain. The southern part of the supergene profile and its host
rocks may be dis- rupted by the NW-striking normal faults, which
may also have facilitated groundwater circulation (Figs. 3,
7c).
In places, where the postmineral hematitic conglomerate was
affected by the later pyritization (Fig. 10c), supergene oxidation
gave rise to formation of jarosite. Since hematite is stable in the
weathering environment, the oxidation of
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partially pyritized zones resulted in jarosite veining of the
hematite-cemented conglomerate (Fig. 10e). The unaltered and
pyrite-free conglomerate and overlying volcanic rocks were
incapable of developing leached capping, thereby giv- ing the
erroneous impression that they postdate all leached- capping
development (Fig. 7c).
The Josemaría deposit is characterized in places by an abrupt
sulfate front: the boundary between gypsum ± anhy- drite-cemented
rock below and sulfate-free rock above (Sil- litoe, 2005). The
anhydrite is an abundant hypogene mineral, probably formerly
present in all the alteration types described above, with the
possible exception of the propylitic halo. The gypsum is its
supergene hydration product, which persists until it is eventually
dissolved by continued descent of cool groundwater, leaving open
cavities. Although the anhydrite front is typically deeper than 600
m, the lower limit of drilling, it occurs locally at relatively
shallow depths in the northern and central parts of the deposit.
There, pillars of imperme- able, sulfate-cemented rock exist as
shallowly as the base of the leached capping, a situation that
effectively prevents the downward passage of supergene solutions
and development of chalcocite enrichment (Fig. 6c).
Discussion
Evolution of Josemaría hypogene mineralization
Josemaría displays the main attributes of porphyry copper- gold
deposits worldwide, including elevated hydrothermal magnetite
content, reasonably good copper-gold correlation, and a
molybdenum-rich halo to the central copper-gold zone (Sillitoe,
1979, 2000). Furthermore, notwithstanding their very different host
rocks, these features, along with the tele- scoped nature of the
alteration and mineralization zoning, are shared with the
contemporaneous Caspiche porphyry gold- copper deposit at the
southern end of the Maricunga belt (Sil- litoe et al., 2013; Fig.
1a).
In combination, various lines of evidence, including the trend of
elongate porphyry bodies and dikes (Fig. 3), suggest that the
Josemaría porphyry copper-gold deposit, along with the two
prospects farther north (Fig. 1b), was localized by a N-striking
zone of structural weakness in the late Paleozoic to Triassic
basement (Fig. 11a). This basement feature was transected by a
slightly oblique, NNE-trending fault zone, which underwent reverse
motion following and probably also during deposit formation at ~25
to 24.5 Ma (Fig. 11b), in gen- eral accord with the timing of
compressive tectonism along
~25 Ma
~23 Ma
~22 Ma
Post-22 Ma
Late Oligocene sedimentary rocks
El Potro fault
Fig. 11. Evolutionary schema for the Josemaría porphyry copper-gold
deposit. a. Premineral development of N-striking zone of basement
weakness, possi- bly followed by normal faulting. The Mogotes
normal fault was active at this time along the western margin of
the Macho Muerto basin (C. Mpodozis, writ. commun., 2018; Fig. 1b).
b. Initiation of compression, NNE-striking reverse faulting, and
emplacement of dacite porphyry intrusions and asso- ciated porphyry
copper system. c. Continued compression, telescoping of alteration
and copper-gold mineralization, unroofing, and deposition of red-
bed siliciclastic sequence. d. Accumulation of dacitic and
andesitic volcanic rocks. e. Localized post-Josemaría hydrothermal
brecciation, advanced argil- lic alteration, and high-sulfidation
copper mineralization. f. Reexhumation of the deposit and supergene
copper enrichment, probably later than the nor- mal motion on the
NNE-striking fault zone. g. Further erosion, particularly during
Pleistocene glaciation.
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JOSEMARIA PORPHYRY Cu-Au DEPOSIT, ARGENTINA 421
the eastern side of the southern Maricunga belt (26–25 Ma; Mpodozis
et al., 2018) and elsewhere in the Andean backarc at latitude
~27°–28°S (late Oligocene onward; Kraemer et al., 1999; Carrapa et
al., 2005; Zhou et al., 2017).
As a direct result of the late Oligocene-early Miocene compressive
deformation and consequent uplift, Josemaría underwent synmineral
erosion, resulting in progressive paleo- surface lowering and
alteration-mineralization telescoping (cf. Sillitoe, 1994; Fig.
11c). This caused extensive reconstitu- tion of early potassic by
lower temperature chlorite-sericite alteration, which, in turn, was
widely and deeply overprinted by sericitic and, at shallower
levels, advanced argillic altera- tion. Although there was little
obvious grade change dur- ing the conversion of potassic to
chlorite-sericite alteration, the superposition of the
high-sulfidation sulfide assemblage during the sericitic and
advanced argillic events gave rise to hypogene enrichment of copper
and, less extensively, also gold. A significant proportion of the
copper and gold in the high-sulfidation assemblage appears to have
been inherited from the preexisting chlorite-sericite zone: a
conclusion sup- ported by the consistently higher grade of the
early compared to intermineral porphyries where both host
high-sulfidation mineralization.
Once hydrothermal activity had ceased, the Josemaría deposit
continued to be eroded until concealment beneath the synorogenic
red-bed sediments that constitute the lower unit of the postmineral
cover sequence (Fig. 11c). The rhyo- litic volcanic rocks appear to
have contributed much of the detritus. The steep slopes existing at
the time (Fig. 6) would have favored mass wasting and accumulation
of the coarse conglomerate and breccia. The local presence at
surface of potassic alteration and A-type quartz veinlets along
with the quartz-pyrophyllite domination of the advanced argillic
alter- ation suggests that relatively deep parts of a formerly more
areally extensive lithocap are currently exposed; this likely
implies erosional removal of at least 1,000 m of shallower,
probably more silicic and alunitic advanced argillic altera- tion
that may have been hosted by coeval volcanic rocks and contained
higher grade, high-sulfidation epithermal gold min- eralization
(cf. Sillitoe, 2010; Hedenquist and Taran, 2013). However, such
material is not abundant in the observed hematitic conglomerate,
suggesting that, if ever present, it was not transported in a
southeasterly direction or deposited nearby Josemaría.
The duration of deposit exhumation cannot be precisely determined
on the basis of current data. Nonetheless, the fact that the early
and intermineral porphyries, separated by only ~0.3 m.y., are
followed first by postmineral red-bed sediments (Fig. 11c) and
then, <2.4 m.y. later, by the volcanic rocks (Fig. 11d) shows
that it was a short-lived event that is unlikely to have exceeded
~2 m.y.
Post-Josemaría copper mineralization
After concealment of the Josemaría deposit beneath the postmineral
volcano-sedimentary cover, advanced argillic alteration and minor
high-sulfidation copper mineralization affected mainly the area
south and southeast of the deposit (Figs. 5, 7b, 10d, 11e). The
event has not yet been radiometri- cally dated but must be >~2.4
m.y. later than formation of the Josemaría deposit. Some of the
NW-striking faults cutting the
postmineral sequence guided ascent of the high-sulfidation fluid,
with the siliciclastic sediments, probably poorly consoli- dated at
the time, providing the permeability for more wide- spread fluid
ingress (Figs. 7b, 11e).
Although at least 2 m.y. of erosion, sedimentation, and vol- canism
intervened between the main porphyry copper-gold and late
high-sulfidation epithermal events, their proximity suggests that
the latter is the product of delayed resurgence of the Josemaría
system, presumably linked to a currently undocumented
intrusion.
Supergene development at Josemaría
The supergene chalcocite enrichment at Josemaría was pref-
erentially developed in the central and northern parts of the
deposit and most deeply within the reactivated, NNE- striking,
postmineral normal fault zone (Figs. 6c, 9), which appears to have
acted as a channelway for focused descent of copper-charged
solutions. In contrast, the rest of the deposit, especially its
southern part, has much thinner leached and partially leached zones
beneath which there is negligible chal- cocite enrichment despite
the presence of apparently suitable hypogene pyrite and copper
contents (Fig. 7c). Furthermore, in places, the oxidation also
affected pyritized parts of the postmineral siliciclastic
sedimentary rocks (Figs. 7c, 10e).
In combination, these features strongly suggest that the supergene
profile developed since the postmineral cover sequence was
deposited, but after it was eroded from the central and northern
parts of the deposit (Fig. 11f). However, precisely when the cover
sequence was removed is unknown although it could potentially have
been relatively recently, depending upon its original thickness and
the erosion rate. Percolation of water along the top and base of
the hematitic conglomerate horizon and down the normal faults (Fig.
7c) could have caused the weak sulfide oxidation that affected the
southern part of the deposit, a possibility supported by the
jarosite layer along the top contact (Fig. 10a). The steepness of
the topography that existed during unroofing of Josemaría (Fig. 7)
and the fact that many mineralized clasts in the con- glomerate are
sulfidic suggest that little sulfide oxidation took place during
initial deposit exhumation, prior to concealment beneath the
postmineral cover, probably because of the rapid- ity of the
unroofing process.
Erosional stripping of the postmineral volcano-sedimentary cover
from the northern and central parts of the deposit and initiation
of the oxidation and enrichment must have taken place during the
continued Miocene uplift of the 25-km-wide block of late Paleozoic
to Triassic basement in which Josemaría is located, facilitated by
major displacements on the El Potro and subsidiary Los Helados
faults (Fig. 1b). Nonetheless, as noted above, an extensional
episode must have intervened either before or during the active
enrichment. The erosional dissection would have progressively
lowered paleoground- water tables, resulting in exposure of the
Josemaría sulfide mineralization to the effects of oxidation and
the generation of subjacent chalcocite enrichment. Nonetheless, the
overall thinness and immaturity of the supergene profile over much
of the Josemaría deposit suggests that supergene processes may have
barely kept pace with the rapid erosion, resulting in confinement
of appreciable chalcocite enrichment to the per- meable
NNE-striking fault zone. Preservation of the shallow,
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anhydrite-cemented rock volumes (Fig. 6c) further supports the
notion of supergene immaturity.
An alternative possibility is that much of the supergene profile,
except for the more deeply developed parts in the NNE-striking
fault zone, was removed during the Pleistocene when the high
altitude (>4,350 m) parts of the region under- went intense
glacial and periglacial dissection (Ammann et al., 2001; Perucca
and Angillieri, 2008). Since then, and notwith- standing ongoing
active sulfide oxidation and enrichment in this high elevation
Andean region (Sillitoe, 2005), insufficient time has elapsed to
redevelop a deep supergene profile if, indeed, one ever
existed.
Rapidity of porphyry copper exhumation
Advanced argillic lithocaps are widely distributed in the Mari-
cunga and El Indio belts and the transition zone between them. Some
contain high-sulfidation epithermal gold ± silver deposits (Fig.
1a), whereas many are largely barren (Bissig et al., 2001) even
where they flank or overlie known por- phyry gold and gold-copper
deposits (e.g., Cerro Casale and Marte; Vila and Sillitoe, 1991;
Vila et al., 1991; Fig. 1a). The abundance of lithocaps provides
firm evidence that erosion levels are generally relatively shallow
(say, <0.5–1 km; Silli- toe 2010; Bissig et al., 2015)
throughout large parts of these belts, notwithstanding the Miocene
(post-18 Ma) compres- sive tectonism and concomitant rock uplift
and denudation south of approximately latitude 28°S (e.g., Martínez
et al., 2015a, b; Lossada et al., 2017; Mpodozis et al., 2018;
Rossel et al., 2018). In the absence of dated postmineral cover—and
approximations based on either radiometric age of supergene
minerals (Sillitoe, 2005) or thermochronometry (McInnes et al.,
2005)—the only available constraint on the timing of erosional
unroofing is provided by the formational ages of the deposits
themselves, one of the youngest being El Indio (~8–5 Ma; Bissig et
al., 2001, 2015; Fig. 1a). Therefore, Jose- maría is the standout
exception because the partly preserved postmineral cover can be
used to show that initial exposure and subsequent burial were
accomplished within ~2 m.y. of deposit formation. Accepting the
removal of at least 1,000 m of overlying lithocap prior to deposit
burial, the erosion rate could at times have exceeded 0.5
km/m.y.
Rapidity of exhumation is assured where exposed porphyry copper
deposits are extremely young (<3 Ma), with Ok Tedi in the Papua
New Guinea Highlands being arguably the best example because it was
formed at 1.1 to 1.2 Ma (K-Ar; Page and McDougall, 1972), an age
confirmed by U-Pb zircon dat- ing (1.187 ± 0.022 Ma; Large et al.,
2018). There is no evi- dence that Ok Tedi was ever concealed
beneath more recent cover rocks and, given its prominent
topographic position atop Mount Fubilan and the absence of any
nearby active volcanic center, it seems unlikely to be in the
foreseeable future. At the late Pliocene Boyongan and Bayugo
porphyry copper-gold deposits in Mindanao, southern Philippines,
however, intru- sion, mineralization, exhumation, and concealment
beneath postmineral debris flows (containing mineralized clasts),
vol- canic material, and fluviolacustrine sediments took place in
<2.3 m.y. (Braxton et al., 2012), a similarly brief interval to
that documented at Josemaría.
In the Plio-Pleistocene Tombulilato porphyry copper- gold district
of North Sulawesi, Indonesia, high-sulfidation
epithermal copper-gold mineralization at Motomboto was at least
intermittently active from 1.45 to 0.93 Ma, when unroofing was
responsible for synmineral incorporation of enargite-bearing clasts
in boulder- and cobble-bearing col- luvial deposits interbedded
with tuffs (Perelló, 1994). This situation is reminiscent of that
at Josemaría where high-sul- fidation mineralization both pre- and
postdated the unroofing and associated burial event. Such
repetitive advanced argillic alteration and any associated
high-sulfidation mineralization may well be relatively commonplace
in porphyry copper dis- tricts (e.g., Hervé et al., 2012).
Once porphyry deposits are concealed beneath postmin- eral cover,
their preservation potential is inevitably enhanced. Indeed, in the
case of Josemaría, located within a fault- bounded, basement block
that continued to undergo pro- nounced uplift and erosion
throughout much of the Miocene (Martínez et al., 2015a, b; Lossada
et al., 2017; Mpodozis et al., 2018; Rossel et al., 2018), the
deposit could well have been completely removed in 2 to 3 m.y. had
it not been for burial beneath the postmineral cover.
In order to preserve Mesozoic, Paleozoic, and older por- phyry
copper systems, burial beneath postmineral cover is even more
likely to be necessary than in the case of younger deposits,
although precise measurement of the time required to unroof them
tends to be more difficult. This is exempli- fied by the Cretaceous
deposits at Pebble, Alaska, and Tie- gelongnan, Tibet, which were
exhumed sometime during the ~25- and 8-m.y. intervals,
respectively, that preceded their concealment beneath postmineral
cover (Lang et al., 2013; Song et al., 2018). Nonetheless,
extremely rapid unroofing was documented at the Late Devonian Hugo
Dummett porphyry copper-gold deposit in the Oyu Tolgoi district,
Mongolia, where as little as 1 m.y. could have separated deposit
forma- tion and an unconformably overlying volcano-sedimentary
sequence within which polymictic conglomerate and brec- cia contain
mineralized clasts (Wainwright et al., 2017). Fur- thermore, the
postmineral sequence, including the advanced argillic alteration
that affected it, is cut by weakly altered and mineralized, late
mineral porphyry intrusions, showing con- clusively that exhumation
took place during the waning stages of the magmatic-hydrothermal
system (Wainwright et al., 2017).
Paleoclimatic context
Yanites and Kesler (2015) showed that the world’s youngest exposed
porphyry copper deposits occur in tropical regions where rainfall
is higher and erosion rates generally faster than in other climatic
regimes. Nonetheless, exhumation of the Josemaría deposit was just
as fast as that of porphyry depos- its in tropical regions, such as
the southwestern Pacific island arcs, notwithstanding the semiarid
conditions that prevailed in this region of the Andes during the
late Oligocene-early Miocene, as documented by widespread
vegetation-free alluvial fan, evaporitic playa, and eolian deposits
(Mpodozis and Clavero, 2002; Voss, 2002; Carrapa et al., 2005;
Nalpas et al., 2008; Mpodozis et al., 2018; this study). This
apparent anomaly may reflect the fact that relief and, hence,
erosion rate are maximized immediately following pulses of Andean
uplift (Carretier et al., 2015), in accord with the limited time
that separated the compressive deformation (~26–25 Ma)
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JOSEMARIA PORPHYRY Cu-Au DEPOSIT, ARGENTINA 423
from the rapid erosion responsible for deposit telescoping
(~25–24.5 Ma) and subsequent unroofing (24.5–22.3 Ma) at
Josemaría.
Therefore, although few porphyry copper deposits world- wide
possess (or at least preserve) direct geologic evidence bearing on
their exhumation histories, it is clear that deposits in magmatic
arc terranes subject to compressive tectonism and rapid rock uplift
can be unroofed in 1 to 2 m.y. Although this is most common under
the tropical climatic conditions that prevailed during the
unroofing at Ok Tedi, Boyongan- Bayugo, Tombulilato, and probably
also Oyu Tolgoi, given its low-latitudinal Devonian position
(Copper and Scotese, 2003), the Josemaría deposit demonstrates that
it is also pos- sible in semiarid, mountainous terrain.
Conclusions The Josemaría porphyry copper-gold deposit is located
in a reverse fault-bounded block of Permian to Triassic igneous
basement that forms part of the Frontal Cordillera of west- ernmost
Argentina. The deposit is genetically related to small, multiphase
dacite porphyry intrusions that were emplaced in a reverse fault
zone concurrently with compression and con- comitant uplift in the
late Oligocene (~25–24.7 Ma). A tele- scoped sequence of alteration
zones, from early potassic to a late advanced argillic lithocap, is
broadly centered on the por- phyry intrusions and, together, host
the potentially economic copper-gold mineralization.
During continued compressive tectonism and uplift, the deposit was
unroofed, with some of the resultant detritus contributing to a
nearby red-bed conglomerate, breccia, and sandstone sequence. These
strata and overlying volcanic rocks, dated at ~22.3 Ma, eventually
buried the Josemaría deposit. The postmineral volcano-sedimentary
package was then locally subjected to weakly developed,
high-sulfidation epithermal copper mineralization, which, in marked
con- trast to the high-sulfidation mineralization in the lithocap,
is gold deficient. The deposit was subsequently reexhumed from
beneath its volcano-sedimentary cover and, during or after minor
extensional reactivation of the reverse fault zone, underwent
supergene sulfide oxidation and enrichment, with the highest grades
confined to the proximity of the fault.
Many porphyry copper deposits in the central Andes were formed
during uplift resulting from the crustal shortening and thickening
that accompanied compressive tectonism (e.g., Maksaev and Zentilli,
1999; Perelló et al., 2003b; Sillitoe and Perelló, 2005; Maksaev et
al., 2009) and, as a consequence, are likely to have been rapidly
exhumed; however, Josemaría is the only known example in which the
presence of post- mineral cover allows estimation of the actual
time required, ~2 m.y., for the unroofing to take place.
Josemaría joins sev- eral other porphyry copper-gold deposits,
predominantly in the western Pacific region, that were exhumed
extremely rap- idly, in 1 to 2 m.y., and then protected from
further erosion by concealment beneath postmineral cover. However,
Josemaría appears currently to be unique in that the erosion took
place under semiarid rather than high-rainfall, tropical
conditions.
Acknowledgments The senior management of NGEx Resources Inc.,
Wojtek Wodzicki, Bob Carmichael, and Alfredo Vitaller, are
thanked
for their support and instigation of this paper; Patricio Jones,
Ricardo Martínez, Max La Motte, and Martín Rode for the dis- covery
and early-stage exploration of Josemaría; Juan Arrieta and Leo
Ortiz for conducting the subsequent exploration, which resulted in
definition of the supergene enrichment; Japan Oil, Gas and Metals
National Corporation (JOGMEC) for financial and technical
contributions during nine years (2009–2017) as joint-venture
partner; Constantino Mpodozis for transformative comments on an
early manuscript draft; and Constantino Mpodozis, Pepe Perelló,
and, on behalf of Economic Geology, Thomas Bissig, Laura Maydagán,
and Jer- emy Richards for constructive reviews.
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Richard Sillitoe gained B.Sc. and Ph.D. degrees from the University
of London, England. After working for the Geological Survey of
Chile and then returning to the University of London as a Shell
postdoctoral research fellow, he has operated for over 45 years as
an independent consultant to more than 300 mining companies,
international agencies, and foreign governments. He has worked on a
wide variety of pre- cious-, base-, and lithophile-metal deposits
and prospects in 100 countries worldwide, but focuses primarily on
the epithermal gold and porphyry copper environments.
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