Journal of South American Earth Sciences 126 (2023) 104289

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Journal of South American Earth Sciences

journal homepage: www.elsevier.com/locate/jsames

Evidence for transpression during formation of the Candelaria Punta del

Cobre IOCG -district and regional implications
Irene del Real a, *, Richard W. Allmendinger b, John F.H. Thompson c, Christian Creixell d
a
Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Avenida Eduardo Morales Miranda, Edificio Emilio Pugín, Valdivia, Chile
b
Department of Earth and Atmospheric Sciences, Cornell University, Snee Hall, Ithaca, NY, 14853, USA
c
PetraScience Consultants, Vancouver, BC, Canada
d
Servicio Nacional de Geología y Minería, Av. Santa María 0104, Providencia, Santiago, Chile

A R T I C L E I N F O A B S T R A C T

Keywords: The youngest and best exposed Iron Oxide Cu–Au (IOCG) deposits currently recognized are found in the coastal
IOCG belt of the Andes. Their formation has been attributed to back-arc extension or transtension associated with the
Structural controls mineralization convergent Andean margin. Here we document transpressional deformation synchronous with Cu mineralization
Candelaria
in the Candelaria-Punta del Cobre district, the largest IOCG district of the Andean belt. A northwest-southeast
shortening direction is recorded by north-northwest sinistral strike-slip faults that host mineralization, north­
west dikes, and north-northeast compressive structures. Batholith emplacement synchronous with mineralization
formed a north-northeast oriented foliation zone parallel to the intrusive contact and associated folds in the host
rock sequences that face inwards towards the intrusive contact. Age constraints indicate that transpressional
deformation in the Cretaceous arc, at least locally, begun earlier than previously documented, and IOCG
mineralization may have spanned the transition from extension to the initial phase of compression.

1. Introduction IOCG deposits in the Andean belt.

Important Andean IOCG deposits include Candelaria and Man­
The distribution of mineral deposits through time and space is inti­ toverde in Chile, and Raúl Condestable and Mina Justa in Peru. The
mately related to geodynamic settings. For example, porphyry deposits interpreted age of these and other smaller deposits in the Andes range
are associated with convergent margins (Cooke et al., 2005; Richards, from Late Jurassic to Early Cretaceous, a period characterized by two
2003) and volcanogenic massive sulfides deposits are associated with cycles of volcanic arc magmatism, back-arc extension and associated
mid-ocean spreading centers, intra-continental and arc rifts (Huston back-arc basins (Mpodozis and Allmendinger, 1993; Mpodozis and
et al., 2010; Ohmoto, 1996). Iron Oxide-Copper–Gold (IOCG) deposits, Ramos, 1989), and discrete events of Late Jurassic intra-arc trans­
characterized by abundant iron oxides (>10%, magnetite and/or he­ pression (Creixell et al., 2011; Ring et al., 2012; Scheuber et al., 1994).
matite), and economically important concentrations of Cu and Au Many of the IOCG deposits in Northern Chile are spatially associated
formed from the Late Archean to the Mesozoic. In contrast with other with faults within the Atacama Fault System (Fig. 1) (Arabasz, 1971;
major types of mineral deposits, the tectonic setting of IOCG deposits is Espinoza et al., 1996; Grocott et al., 1994; Wilson and Grocott, 1999;
poorly constrained. Previous researchers have proposed an orogenic, Grocott and Taylor, 2002), a continental, trench parallel series of
post-orogenic and continental margin arc settings for the formation of interconnected, dominantly north-northwest and west-northwest strik­
this deposit type (Groves et al., 2010; Skirrow, 2022). The Andean IOCG ing sinistral strike-slip faults. Faults associated with several IOCG de­
belt, the youngest IOCG belt discovered up to date, is well preserved and posits in the Andes have been documented to be part of extensional to
therefore ideally suited for potentially constraining the tectonic settings transtensional periods (Arévalo et al., 2006; Cembrano et al., 2009;
related to the formation of these deposits. Both arc-related extension Groves et al., 2010; Lopez et al., 2014; Richards et al., 2017; Sillitoe,
(Arévalo et al., 2006; Richards et al., 2017; Skirrow, 2022; Veloso et al., 2003; Skirrow, 2022). From the mid-Cretaceous onward, the Andean arc
2017) and arc-related compression (Chen et al., 2010, 2013) have been has been dominated by compression (Mpodozis and Ramos, 1989) and
proposed previously as the tectonic setting at the time of formation of multiple arcs and associated major porphyry copper deposits that young

* Corresponding author.
E-mail address: irene.delreal@uach.cl (I. del Real).

https://doi.org/10.1016/j.jsames.2023.104289
Received 30 October 2022; Accepted 2 March 2023
Available online 7 March 2023
0895-9811/© 2023 Published by Elsevier Ltd.
I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

to the east. The age of onset of compression has been interpreted by most hydrothermal system (del Real et al., 2018) (Fig. 1). Mineralization is
workers to be 107-105 Ma (Arancibia, 2004; Arévalo et al., 2006; predominantly hosted in the Lower Cretaceous (~135–132 Ma)
Arriagada et al., 2006; Bascuñán et al., 2016; Grocott et al., 2009; volcanic-sedimentary Punta del Cobre Formation and the basal parts of
Grocott and Taylor, 2002; Skirrow, 2022). the overlying sedimentary marine sequences belonging to the Lower
Mineralization in the Candelaria-Punta del Cobre district has previ­ Cretaceous (132–130 Ma) Chañarcillo Group (del Real et al., 2018;
ously been associated with a transtensional structural setting (Arévalo Marschik et al., 1997). The Punta del Cobre Formation can be divided
et al., 2006), which occurred between 115 and 110 Ma. Here, we show into four volcanic and volcaniclastic members, from older to younger:
that the deposits of the Candelaria-Punta del Cobre district formed in a (1) Lower Andesite; (2) Dacite; (3) Volcanic-sedimentary unit, and (4)
transpressional structural setting, contemporaneous with batholith Upper Andesite (Marschik and Fontboté, 2001). The Chañarcillo Group
emplacement, and the onset of basin inversion and folding. This is includes a mixed package of mostly marine sedimentary rocks (Seger­
consistent with emplacement of IOCG deposits in a similar setting strom and Parker, 1959; Segerstrom and Ruiz, 1962) that are divided
further south (29◦ –30◦ S) (Creixell et al., 2012, 2020) during a regional into four formations (from bottom to top): Abundancia, Nantoco,
episode of left-lateral displacements along segments of the Atacama Totoralillo and Pabellón (Fig. 2). The Chañarcillo Group is interpreted to
Fault System that started at 127-115 Ma after a previous period of be a discrete marine basin that formed during back-arc extension
extension-transtension (Scheuber and Gonzalez, 1999; Seymour et al., (Arévalo, 1999; Mourgues, 2004) with a north-northeast depocenter and
2020; Veloso et al., 2017). Recognition of the transpressional setting interfingering relationship with volcanic units towards the northwest
suggests an alternative interpretation of the tectonic setting for Andean that are assigned to the Bandurrias Group (Fig. 1).
IOCG deposits, at least in this area, and also provides a potential new The most important intrusions in the district belong to the Copiapó
constraint for the onset of compression in the western Andean margin. composite batholith, located on the western side of the district. The
main phases of the Copiapó batholith are defined as La Brea diorite
2. Geology and mineralization in the Candelaria-Punta del (~118 Ma), San Gregorio monzodiorite (~115 Ma), and Los Lirios
Cobre district granodiorite to tonalite (~110 Ma) (Fig. 1) (Arévalo, 1999; Marschik
and Söllner, 2006). La Brea intrusion, the most extensive phase in the
The Candelaria-Punta del Cobre district is located south of the city of batholith, has an overall north-northeast orientation (Arévalo, 1999),
Copiapó in northern Chile and comprises more than nine active mines and is in contact with rocks from the Punta del Cobre formation and the
exploiting IOCG mineralization that is interpreted to be part of a large Chañarcillo Group towards the east. The San Gregorio phase, just

Fig. 1. Location of IOCG, Iron-Apatite and porphyry deposits formed during the Upper Jurassic–Lower Cretaceous in the Atacama region; Simplified geological map
of the Candelaria-Punta del Cobre district with the main IOCG deposits (modified from Arévalo, 1999). UTM coordinates are in datum PSAD56. Stars: 1-Mantos de
Cobre, 2-Alcaparrosa, 3-Santos, 4-Granate, 5-Punta del Cobre, 6-Carola, 7-Candelaria, 8-Atacama Kozan.

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I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

Fig. 2. Schematic cross-sections of the main IOCG deposits in the district. Ore bodies towards the western side of the Copiapó valley are more stratigraphically
controlled whereas ore bodies towards the eastern side of the valley are structurally controlled with a strong north-northwest orientation.

northwest of the Candelaria deposit, forms a north-northwest orientated district (both at depth and laterally) occurs as disseminated and perva­
body with a well-developed north-northeast syn-emplacement fabric sive alteration that can completely replace the volcanic host rocks and is
(Arévalo et al., 2006). The San Gregorio phase is in fault contact with La interpreted to represent a distinct higher temperature hydrothermal
Brea intrusion and forms a high strain zone when in contact with the pulse (del Real et al., 2021). Early magnetite-actinolite and calcic-sodic
rocks of the Punta del Cobre Formation and Chañarcillo Group towards alteration stages are overprinted by a biotite–K-feldspar–chalcopyrite ±
the south-southeast (Arévalo et al., 2006). Los Lirios comprises a magnetite–actinolite alteration that represents the main Cu minerali­
discrete intrusive body in the southern part of the district and is in zation phase in the area (del Real et al., 2018; Marschik and Fontboté,
contact with the Chañarcillo Group towards the east. 2001). Most economic Cu mineralization in the Candelaria-Punta del
Smaller intrusive bodies, dikes and sills are also present in the area Cobre district is focused around the contact between the Lower Andesite
with varying temporal relations to the Copiapó batholith, deformation and the Volcanic-sedimentary/Dacite units, extending up to ~100 m
and mineralization. Dikes were emplaced pre-, syn- and post- above and more than 300 m below the contact. Stratigraphically
mineralization, providing key information on the timing of Cu- controlled mineralization (“manto” style) is observed in the
mineralization. The main phase of Cu mineralization is interpreted Volcanic-sedimentary unit, or in basal breccias of the Dacite and Lower
have formed at ~115 Ma (Re–Os on molybdenite; Mathur et al., 2003) Andesite units. Smaller manto ore bodies are present locally in the Upper
and hydrothermal alteration continued to at least until 112–111 Ma, Andesite and in the lower part of the Chañarcillo Group, particularly in
constrained by a late-mineralization dike dated by U–Pb zircon at 112.8 the southern part of the district (del Real et al., 2018). Mineralization in
± 1.3 Ma (strike = 320◦ ) (del Real et al., 2018), and Ar–Ar dates of the district also occurs in narrow structures (veins) and in broad struc­
syn-alteration biotite (110.0 ± 1.4 and 110.7 ± 1.6 Ma) (Arévalo et al., turally controlled bodies (e.g., deeper parts of Candelaria) (del Real
2006). Pre-main phase and late-mineralization dikes have et al., 2018). Narrow mineralized structural zones typically have a
north-northwest orientations. The onset of early pre-mineralization north-northwest orientation and can extend for up to 2 km (e.g. Carola,
alteration is currently not well defined, but is has been proposed to be Punta del Cobre, Santos, Mantos de Cobre; Fig. 2) (del Real et al., 2018).
as early as 120 Ma (del Real et al., 2018). North-northwest structures associated with mineralization show
Regional early calcic-sodic alteration is observed in the northwestern left-lateral strike-slip movement (Arévalo et al., 2006). Manto-style
part of the district followed by a widespread magnetite-actinolite mineralization is prevalent in the western side of the Copiapó valley
alteration formed between ~120 and 116 Ma (del Real et al., 2018). (e.g. Candelaria, Granate and Alcaparrosa mines; Fig. 2) whereas
Early magnetite-rich alteration extends beyond all deposits in the structurally controlled ore bodies are prevalent on the eastern side

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I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

(Punta del Cobre, Santos and Carola mines; Fig. 2) (del Real et al., 2018). intrusive phases; Fig. 1). The Lar fault displays sub horizontal slicken­
Differences in the styles of mineralization are directly related to the sides (del Real et al., 2018). Both the Rocio and Lar faults also display
presence of the Volcanic sedimentary unit in the west, which is a normal vertical fault movement with some degree of inverse reac­
particularly favorable host for manto-style mineralization, whereas vein tivation (del Real et al., 2018). Most structurally controlled ore bodies
and breccia mineralization are better developed in the Lower Andesite on the eastern side of the Copiapó Valley strike north-south to
and Dacite that are prevalent in the eastern part of the district (del Real north-northwest and are hosted in faults that predominantly record
et al., 2018). sinistral strike-slip kinematics.
The Tierra Amarilla anticlinorium (Fig. 1) (Arévalo, 1999) is a
3. Deformation styles map-scale structure with a fold axis that trends north-northeast, roughly
parallel to the margin of the Copiapó batholith. Smaller north-northeast
Major structures exposed in the Candelaria-Punta del Cobre district oriented folds occur east of the Tierra Amarilla anticlinorium (Fig. 1).
(Fig. 1) include early north-northeast striking normal faults, north- Where in contact with the San Gregorio intrusive phase and segments of
northwest-striking, high-angle, sinistral faults, and a major northeast- La Brea, the Tierra Amarilla anticlinorium has a northwestern limb that
trending anticline with a highly attenuated northwest limb. Normal faces inwards toward the pluton and steepens forming an overturned
faults interpreted to be related to basin formation are locally visible and fold south of the Candelaria deposit (Fig. 1) (Arévalo, 1999). At the
widely interpreted within the Punta del Cobre Formation. Where map­ contact with the San Gregorio phase, Arévalo (1999) reports limb dips as
ped, these faults have a predominant north-northeast strike, but are also steep as 74◦ NW (Fig. 3C). The most prominent thrust fault previously
observed having east-west, east-northeast and west-northwest strikes. interpreted in the district, the Paipote fault (Arévalo et al., 2006), has a
These faults locally display horizontal to sub-vertical slickensides north-northeast strike and top to the southeast sense of shear, and is
(Fig. 3A and B), suggesting a strike-slip component of movement at some mapped between the San Gregorio intrusive phase and the Chañarcillo
point during their history (del Real et al., 2018). Scarce outcrop and Group strata (Fig. 1). Other thrust faults in the area also have a pre­
complex volcanic stratigraphy, facies changes, and significant variations dominant north-northeast strike, but have restricted extents and minor
in unit thickness limit the interpretation of syn-volcanic and displacements.
syn-sedimentary faults. Two high strain foliation zones are recognized in the area. A low
Prominent sub-vertical faults in the district display a left-lateral angle syn-mineralization foliated zone is developed locally at the Lower
sense of motion (Arévalo et al., 2006). The most important Andesite – Volcanic-sedimentary unit contact with reported asymmetric
north-northwest strike-slip faults include the Lar fault (which cuts the strain shadows showing a top-to-the-southeast sense of shear (Cande­
Candelaria deposit), Rocío fault (which cuts the Alcaparrosa deposit), laria shear zone; Arévalo et al., 2006). The second, termed the Ojancos
and the San Gregorio fault (which juxtaposes La Brea and San Gregorio shear zone (Fig. 1) (Arévalo et al., 2006) strikes north-northeast, and

Fig. 3. (A) Sub vertical slickensides in north-northwest oriented fault above the Mantos de Cobre deposit; (B) Non-vertical to sub-horizontal slickensides in north-
northwest oriented fault above the Mantos de Cobre deposit, same fault measured in (A); (C) Strata of the Chañarcillo Group steeply dipping just beside the contact
with the San Gregorio pluton UTM E370364 N6955621 WGS84; (D) Outcrop of the Ojancos “shear” zone.

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I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

extends along the contact between the San Gregorio intrusive phase with Table 1
the Punta del Cobre Formation and Chañarcillo Group for at least 5 km UTM coordinates (WGS 84).
(Fig. 3D). The Ojancos shear zone is largely coincident with the steep Structure type strike dip East North
northwest limb of the Tierra Amarilla anticlinorium. Shear indicators
Fault 132 82 373366 6963833
reported by Arévalo et al. (2006) are consistent with expected layer Fault 162 90 372970 6964219
parallel shear during folding. Fault 220 69 373464 6963748
Fault 170 84 373565 6963801
4. Methods Fault 220 50 375436 6964581
Fault 325 88 376147 6962003
Fault 327 89 373897 6962104
4.1. Field methods and sampling Fault 25 90 373920 6962019
Fault 325 90 373427 6958913
For this study, 78 measurements of brittle fault planes, slickensides Fault 163 70 377037 6959614
Fault 340 90 377141 6959527
and vein orientation were obtained from surface. Measurements are
Fault 170 60 377122 6959720
distributed throughout the district. Structural data were plotted and Fault 200 70 376927 6960121
analyzed with Stereonet (Cardozo and Allmendinger, 2013). Fault 165 30 377108 6959710
Sampling focused on the Ojancos and Candelaria shear zone. Two Fault 340 90 376248 6954373
oriented samples were taken from surface from the Ojancos shear zone Fault 270 65 370190 6948339
Fault 335 45 370196 6948333
in order to evaluate potential strike slip deformation within the high Fault 340 90 370146 6948360
strain zone (UTM 370532E, 6955475N; Fig. 3D). Thin sections Fault 43 82 376497 6957410
perpendicular to the dip and to the foliation direction were made from Fault 30 80 376437 6957378
these samples in order to identify simple shear or slip movement. One Fault 85 51 376416 6957366
Fault 120 68 375870 6959709
drill core (drill hole LS1556) which crosses the main Cu mineralization
Fault 125 60 375892 6959685
in Candelaria and the Ojancos shear zone was examined and docu­ Fault 315 55 375892 6959685
mented in order to identify timing relationships between mineralization Fault 280 80 375934 6959673
and the shear zone. The Candelaria shear zone was intercepted and Fault 330 65 370151 6948359
documented in an exploration drill core (drill hole ES064) south from Fault 330 60 370135 6948369
Fault 350 80 370134 6948370
the Candelaria deposit. Fault 110 35 376354 6962106
Fault 310 90 376598 6957391
4.2. U–Pb geochronology Fault 80 60 376409 376409
Fault 330 58 6957360 6957360
Fault 60 85 374077 6963794
A mineralized dioritic dike from the Candelaria Norte mine was
Fault 222 83 373472 6963783
sampled for better constraining the age of the main Cu event. Since this Fault 130 50 373415 6964394
dike was emplaced prior to or during mineralization, the age of the dike Fault 275 85 376278 6962017
provides a constraint on the timing of the main-stage Cu mineralizing Fault 155 60 376387 6961057
event which can be integrated with geochronological data from the Fault 90 82 377860 6960176
Fault 320 35 377968 6959936
district. The dike was intercepted in six different drill holes over in­ Fault 90 80 378040 6959879
tervals ranging from 2 to 3 m. The mineralized dike has a north- Fault 141 84 370601 6955588
northwest strike, similar to unmineralized dacitic dikes that cut the Fault 165 81 370583 6955613
main-stage Cu mineralization and have been dated previously (del Real Fault 330 69 370808 6955603
Fault 274 71 370643 6955615
et al., 2018). The dioritic dike was dated at the Pacific Centre for Isotopic
Fault 230 71 374077 6963795
and Geochemical Research (PCIGR), at the University of British Fault 222 83 373466 6963782
Columbia. Zircons were analyzed using laser ablation (LA) ICP-MS, Fault 189 81 373441 6963775
employing methods as described by Tafti et al. (2009). Fault 149 83 376116 6962009
Reference materials were analyzed throughout the sequence to allow Fault 155 79 376110 6962011
Fault 181 71 376128 6962010
for drift correction and to characterize downhole fractionation for U–Pb Fault 157 85 376147 6962004
isotopic ratios. For U–Pb analyses, natural zircon reference materials Fault 134 77 376141 6962002
were used, including Plešovice (Sláma et al., 2008) or 91500 (Wie­ Fault 183 76 377913 6960149
denbeck et al., 1995, 2004) as the internal reference material and both Fault 91 83 378044 6959878
Fault 295 46 370597 6955533
Temora2 and/or 91500 as monitoring reference materials; the zircon
Fault 294 22 370604 6955538
reference materials were placed between the unknowns. Raw data was Fault 329 42 370616 6955601
reduced using the Iolite 3.4 extension (Paton et al., 2011) for Igor Pro™ Fault 330 69 370733 6955632
yielding U/Pb ages, and their respective uncertainties. Final interpre­ Fault 23 45 370644 6955623
tation and plotting of the analytical results employed the ISOPLOT Vein 10 90 371393 6962965
Vein 5 90 372730 6963677
software of (Ludwig, 2001). Vein 160 90 376057 6965095
Vein 315 44 377211 6962921
5. Results Vein 314 71 375978 6961888
Vein 325 90 376083 6961956
Vein 324 90 373303 6958998
We document faults and veins in the district with distinct orienta­
Vein 334 90 375966 6959423
tions. Systematic measurement of faults, including small meter scale Vein 324 90 375985 6959397
structures, indicate a dominant north-northwest strike (Fig. 5A; Vein 90 71 368757 6954260
Table 1). North-northwest faults measured and identified throughout Vein 90 75 368867 6954184
the district can contain sub-vertical and horizontal slickensides sug­ Vein 340 90 374108 6955663
Vein 325 90 376467 6959354
gesting both vertical and horizontal components of deformation. These Vein 325 90 370686 6955261
faults can show evidence of normal but also reverse sense of displace­ Vein 345 90 369200 6952234
ment, suggesting some degree of reactivation in the normal faults. Sys­ (continued on next page)
tematic measurement of mineralized veins containing iron oxides

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I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

Table 1 (continued ) anticlinorium facing inward toward the San Gregorio pluton. As
Structure type strike dip East North reviewed, below, this inward facing geometry, combined with the minor
structures observed in drill core, suggest that the emplacement of the
Vein 343 90 369310 6952453
Vein 345 90 369673 6952491
pluton and formation of the anticlinorium, were broadly coeval. Drill­
Vein 355 90 370023 6952495 hole data collected since the publication of Arévalo et al. (2006),
Vein 305 85 376279 6957314 together with surface strike and slip data and unconstrained inversion
Vein 350 55 370073 6948305 models from aeromagnetic surveys (Daniel Brake, pers. Comm., 2018),
were used to construct an interpretive cross-section perpendicular to the
Ojancos shear zone (Fig. 9). The Ojancos shear zone is sub-vertical to
(magnetite and/or hematite) ± copper sulfides in the district, both
steeply northwest-dipping and is surrounded by a broad zone of strong
within mineralized and non-mineralized centers also display similar
foliation that forms the inward-facing fold that is interpreted to have
north-northwest strikes which differ from the average strike of faults by
formed during the emplacement of the San Gregorio intrusion (Figs. 9
10–15◦ (Fig. 5B).
and 10) and associated top-to-the south-southeast thrusts faults with the
The Ojancos shear zone, where intersected in drill hole LS1556,
same shear sense.
displays a steep shear fabric parallel to the steeply dipping strata. In the
The Punta del Cobre Formation and Chañarcillo Group were depos­
main Cu mineralization zone, veins related to early alteration are
ited in an extensional to transtensional environment during the Early
sheared and folded parallel within the fabric, while late veins containing
Cretaceous (~135–130 Ma) (Arévalo, 1999; del Real et al., 2018)
specularite and sulfides cut the foliation (Fig. 4A, B and C). Oriented
(Fig. 11). The structural regime in the area shifted to a transpressive
samples from the Ojancos shear zone show high strain in the foliation
environment between 118 and 115 Ma, as evidenced by the
zone but no significant simple shear or slip movement when samples
north-northeast strike of thrust faults, the trend of the major fold axis (e.
were examined petrographically. These samples show a consistent foli­
g., Fig. 10A and B and Fig. 11), the north-northwest orientation of
ation parallel to the dip of the host rock and rare shear features such as
sinistral strike-slip faults, and the northwest dikes (Fig. 10A and B). A
fish tails in some of the amphibole or biotite crystals, but with no
principal shortening axis with a northwest-southwest orientation would
dominant sense of slip (Fig. 6A and B). Similarly, we found no
produce strain partitioning with left-lateral north-northwest strike slip
compelling evidence for a distinct sense of shear in drill core samples
faulting, northeast shortening and northwest extension consistent with
from the Candelaria Shear zone. Mineralization and biotite alteration
features observed in the district. The north-northwest sinistral strike slip
are deformed, but more competent parts of the rock (such as volcanic
faults host mineralization (Fig. 10a) in the district and mineralization in
clasts in a breccia) are largely undeformed (Fig. 7). The pre-
the Ojancos shear zone shows textural evidence for formation during the
mineralization dioritic dike yielded a U–Pb zircon age of 114.9 ± 1.6
development of foliation (Fig. 4B). The pre-to syn-mineralization
Ma. Results and the concordia plot of the geochronological data are
northwest diorite dike dated in this study at ~115 Ma therefore dem­
displayed in Table 2 and Fig. 8 respectively.
onstrates that compressive or transpressive deformation also occurred
by ~115 Ma. The Candelaria shear zone formed dominantly within the
6. Discussion
Volcanic Sedimentary unit above the contact with the Lower Andesite.
Deformation, intense biotite alteration and mineralization in the shear
Based on structural and lithological observations, combined with
zone appear to be focused in a zone of significant rheological contrast.
geochronological data and the distinct styles of mineralization, we
The geometry of an inward-facing fold related to emplacement of an
propose that IOCG mineralization in the Candelaria-Punta del Cobre
intrusion has been documented previously for the Eureka Valley-Joshua
formed in a transpressional structural regime. Core from drill holes
Flat-Beer Creek pluton in eastern California (Morgan et al., 2013), and
intersecting the Ojancos shear zone show intense axial planar foliation
the Cordilleran batholiths on the western margin of North America
subparallel to the dipping strata, which is interpreted to be related to the
(Paterson and Farris, 2006). In these models, pluton emplacement
formation of the steep northwest limb of the Tierra Amarilla

Fig. 4. Photos of mineralization pre, syn and post-deformation in the Ojancos shear zone: (A) Early alteration pre-mineralization wavy quartz vein stretched in the
direction of foliation. (B) Deformed chalcopyrite–pyrite-actinolite-magnetite vein parallel to foliation in magnetite-biotite altered Volcanic-sedimentary member in
the Ojancos shear zone photo from the Candelaria Norte mine; (C) Undeformed specularite–pyrite–chalcopyrite vein cutting the foliated San Gregorio intrusive phase
in the Ojancos shear zone.

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I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

Fig. 5. Rose diagrams of (a) faults and (b) veins orientations measured in the district. Both sets display a north-northwest orientation and a subsidiary east-west
orientation. Rose diagrams from Stereonet 10 software (Cardozo and Allmendinger, 2013).

Fig. 6. (A) Oriented sample SZ-1, the sample has a strike of 26◦ E and the dip of foliation of 70◦ NE, it’s location (in UTM, WGS84) is 370532 E, 6955475 N. Thin
section is perpendicular to foliation following its strike, the sample shows no clear cinematic indicators; (B) Oriented sample SZ-2, the sample has a strike of 34◦ E and
the dip of foliation is 54◦ NE, it’s location (in UTM, WGS84) is 370516 E, 6955453 N. The thin section is perpendicular to foliation following its strike, sample shows
porphyroclasts with conflicting “fish tails”, indicating no consistent shear orientation.

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I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

main phase mineralization in the dioritic dike dated by U–Pb zircon at

114.9 ± 1.6 Ma and also by a late-mineralization dacite dike with the
same orientation previously dated by U–Pb zircon at 115.2 ± 1.8 Ma
(strike = 315◦ ) (del Real et al., 2018). These ages overlap within error.
The main Cu mineralization event was therefore coeval with the
emplacement of the San Gregorio pluton and with foliation developed in
the Ojancos shear zone as suggested by pre-, potentially syn-, and
post-foliation mineralized veins (Fig. 4).
The temporal relationship of mineralization in the Candelaria-Punta
del Cobre district and the emplacement of most of the Copiapó batholith
(~118-110 Ma) has been proposed previously (Marschik and Fontbote,
2001; Mathur et al., 2003). There is, however, no evidence that any of
the phases of the Copiapó batholith were the major source of hydro­
thermal fluids, at least at current levels of exposure, with the batholith
only being a host for minor fault-controlled zones of mineralization (del
Real et al., 2018). Previous Os and Cl isotopic data (Chiaradia et al.,
2006) and high contents of Ni and Co in pyrite from the district (del Real
et al., 2020) indicate that hydrothermal fluids may have a mantle
signature, possibly reflecting more primitive mafic intrusions at depth as
Fig. 7. Candelaria shear zone sample from drill hole ES064, south from the
the source for fluids and metals. Further research would be needed in
Candelaria deposit. The sample displays deformed biotite and mineralization,
and undeformed volcanic clasts.
order to characterized how the transpressional structural setting influ­
enced the emplacement of the Copiapó batholith and the fluid flow from
deep sources that resulted in IOCG mineralization in structures and
during compression forms monoclinal to anticlinal folds that face the
permeable horizons.
pluton margin, and ductile strain also occurs in the adjacent contact
Following IOCG mineralization, minor porphyry-style mineraliza­
aureole, similar to geometry and fabrics observed in the Ojancos shear
tion formed in the Los Lirios intrusion, the latest phase of the Copiapó
zone. An inward facing to overturned fold against La Brea intrusive
batholith (Kreiner and Barton, 2009). Porphyry deposits also occur in
phase is also locally observed in the Candelaria district (Arévalo, 1999),
several areas south of Candelaria-Punta del Cobre, with the earliest
showing a similar structural configuration to the San Gregorio intrusive
examples at ca. 116 Ma followed by a younger group at 108-104 Ma
phase, but without the development of a distinct foliated high strain
(Creixell et al., 2020; Maksaev et al., 2010). Good examples of these
zone, suggesting that La Brea may also have been emplaced in a trans­
deposits are Tricolor-Dos Amigos (Maksaev et al., 2010); Andacollo
pressive regime at ~118 Ma.
(Richards et al., 2017), Punta Colorada, Cachiyuyo and La Unión
The main Cu mineralization event is further constrained by the pre-

Table 2
Zircon U–Pb laser ablation ICP-MS analytical data New.
Sample no. Isotopic Ratios Isotopic Ages
Analysis ID 207
Pb/235U 2σ 206
Pb/238U 2σ (abs) 207
Pb/206Pb 2σ 207
Pb/235U 2σ 206
Pb/238U 2σ 207
Pb/206Pb 2σ
(abs) (abs) (Ma) (Ma) (Ma)

Diorite 0.107 0.025 0.0182 0.0012 0.012645 0.042 0.01 100 24 116 7.4 − 130 430
Dike_1
Diorite 0.108 0.034 0.0196 0.0013 0.19541 0.037 0.011 100 31 125.2 8.5 − 460 460
Dike_2
Diorite 0.092 0.028 0.0181 0.0012 0.083737 0.033 0.01 86 27 115.9 7.6 − 500 440
Dike_3
Diorite 0.151 0.044 0.0201 0.0016 0.18292 0.056 0.017 137 40 128.4 10 190 550
Dike_4
Diorite 0.107 0.026 0.0177 0.0012 0.062073 0.043 0.011 101 23 112.8 7.7 − 200 420
Dike_5
Diorite 0.12 0.016 0.01804 0.00061 0.35334 0.0489 0.0053 117 14 115.3 3.9 140 230
Dike_6
Diorite 0.142 0.045 0.0207 0.0014 0.13131 0.055 0.017 128 40 132.1 9.1 − 20 570
Dike_7
Diorite 0.134 0.098 0.02 0.0011 0.27035 0.05 0.026 127 69 127.7 6.7 150 470
Dike_8
Diorite 0.127 0.023 0.0179 0.00062 0.33199 0.0523 0.008 123 20 114.4 3.9 220 320
Dike_9
Diorite 0.121 0.014 0.01749 0.00055 0.12979 0.0494 0.0051 115.6 13 111.7 3.5 160 200
Dike_10
Diorite 0.095 0.033 0.0183 0.0012 0.23558 0.044 0.014 94 31 116.9 7.8 − 410 540
Dike_11
Diorite 0.124 0.027 0.0188 0.0011 0.014554 0.048 0.0088 117 24 120.1 6.7 − 20 380
Dike_12
Diorite 0.115 0.028 0.01814 0.00082 0.1076 0.045 0.011 108 25 115.9 5.2 − 10 450
Dike_13
Diorite 0.103 0.042 0.0185 0.0013 0.11383 0.043 0.017 94 39 117.8 7.9 − 340 570
Dike_14
Diorite 0.164 0.063 0.0187 0.0012 0.25548 0.065 0.019 150 49 119.1 7.7 510 420
Dike_15
Diorite 0.124 0.071 0.01788 0.00092 0.31989 0.047 0.018 118 52 114.3 5.8 20 390
Dike_16

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I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

6.1. Regional tectonic implications

The onset of the inversion in the Chañarcillo basin was previously

proposed to be ~110 Ma (Maksaev et al., 2009) based on the age of the
continental Cerrillos formation which uncomformably overlies the
Charñacillo Group and contains clasts from lower units (Amilibia, 2009;
Maksaev et al., 2009). Our results suggest that uplift and initial erosion
could have predated this time (Fig. 11). Consistent with this hypothesis,
a series of slumps and an increase in volcanoclastic deposits in the lower
part of the Pabellón Formation has been suggested as evidence of
incipient tilting in the Chañarcillo Basin during the Aptian (Mourgues
et al., 2009). Basin instability continued until the Albian with olitos­
tromes, deposition of fan-deltas, and volcanic deposits below the
erosional contact with the Cerrillos Formation, indicating the emersion
of the marine basin and tilting of the platform (Mourgues et al., 2009).
Recent research has identified angular and erosional uncomformities
between the Pabellón Formation of the Chañarcillo Group and the
Cerrillos Formation between 28◦ 30′ and 29◦ 30’ S, which are interpreted
according to stratigraphic and geochronological data, to represent the
tilting, erosion and probably uplift of the Chañarcillo beds before 110
Ma (Creixell et al., 2013, 2020). Moreover, an increase in volcanic
Fig. 8. Concordia diagram for pre-mineralization dioritic dike dated for this detritus towards the upper section of the Chañarcillo Group (Pabellón
study. Black ellipses correspond to zircon measurements used for calculating Formation) and U–Pb ages from detrital zircons in these rocks, suggest
the samples age. Red ellipses in the diagram correspond to zircon measure­ that denudation and exhumation of the Early Cretaceous magmatic arc
ments that were discarded for calculating the age of the unit due to potential Pb took place during the Albian ca. between 120 and 110 Ma (Creixell et al.,
loss or mixing causing discordance in the system. 2020) during sinistral transpression (Arevalo and Creixell, 2009;
Creixell et al., 2012). The evidence from these areas supports a
(Creixell et al., 2020). Most Andean Cenozoic porphyry deposits are spatial-temporal link between Lower Cretaceous sinistral transpression
interpreted to have formed in compressional or transpressional arc set­ and a major paleogeographic change in the arc-backarc system, where
tings (Richards, 2003; Tosdal et al., 2009), similar to the setting pro­ the marine basin environment during the Early Cretaceous changed to
posed for the Candelaria-Punta del Cobre district. Based on this study, subaerial depositional conditions that existed from the Albian to present
therefore, IOCG and porphyry copper systems do not necessarily form in times. The main inversion event of the Chañarcillo basin has been pro­
distinctly different structural settings. Potentially, the differences be­ posed to have occurred between 80 and 67 Ma (Martínez et al., 2021),
tween these deposit types reflects differences in crustal architecture, significantly later that the initial transpression event proposed in this
such as changes in crustal thickness or composition. The Andean IOCG work.
deposits are associated with a juvenile Jurassic-Early Cretaceous South of the Candelaria-Punta del Cobre district, El Tofo Fault Sys­
magmatic arc emplaced on a thin crust composed of a Paleozoic accre­ tem documents sinistral transpression along reverse and strike-slip
tionary prism (Creixell et al., 2020; Jara et al., 2021; Lucassen et al., brittle faults that overprint previous extensional structures (Creixell
2002) whereas the younger Cenozoic porphyry deposits formed on a et al., 2012). These displacements are consistent with left-lateral and
more-evolved crust already thickened in previous Paleozoic and Meso­ compressive S–C–C’ kinematic indicators along syn-plutonic shear zones
zoic orogenies (Amilibia et al., 2008; Haschke et al., 2002; Lee and Tang, located between 28◦ and 30◦ S (Algarrobo and La Higuera shear zones)
2020). (Creixell et al., 2012). Sinistral transpression along the El Tofo Fault
The structural framework observed in the Candelaria-Punta del System was active starting at ca. 121 Ma and accommodated the
Cobre district have been described in other IOCG districts in north- emplacement of composite plutons and numerous IOCG veins around
central Chile. For example, at La Higuera IOCG district, left-lateral 29◦ S (Creixell et al., 2012, 2020). As mentioned above, it is possible to
transpression is interpreted to be coeval with mineralization (Creixell establish a temporal and spatial link between the deformation that
et al., 2020) and at El Espino strike-slip conjugate faults and an inward occurred along the arc and depositional and paleogeographic changes
facing fold against an intrusion appear to be coeval with mineralization along the back arc basin (Chañarcillo Group). The possible exhumation
(del Real and Arriagada, 2015). and denudation of the magmatic arc affected depositional dynamics in

Fig. 9. A-A′ cross-section from Fig. 1. Strike and dip measurements obtained in the field combined with those published by Arévalo (1999), in combination with drill
core data from Lundin Mining, were used for the construction of the section. Horizontal: vertical = 1:1. * projected 1.2 km NNE, **projected 200 m N.

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I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

Fig. 10. (A) Current distribution of lithologies, structures and mineralization in the Candelaria-Punta del Cobre district; (B) on plan map and schematic cross section
showing preferred northwest σ1 deformation vector; this preferred direction of compression would generate northeast compressional structures (e.g. Tierra Amarilla
anticlinorium, Paipote thrust faults), northwest extension (e.g. northwest dikes) and north-northwest sinistral strike-slip deformation (e.g. Lar, San Gregorio, Rocio
faults). The strain ellipse is schematic.

the back-arc with the increase of terrigenous material supply into the the fault system (Ruthven et al., 2020). An overall
basin and deformation and erosion taking place during the Albian sinistral-transtensional regime for the Atacama Fault System until the
(Creixell et al., 2013, 2020). Lower Cretaceous has been proposed by many workers (Cembrano et al.,
A similar change in tectonic and paleogeographic setting is recog­ 2005; Grocott and Taylor, 2002; Jensen et al., 2011; Scheuber and
nized to the north and south. Based on analysis of sedimentary com­ Gonzalez, 1999; Veloso et al., 2015), therefore, it is also possible that
ponents, sequence stratigraphy and U/Pb detrital zircons ages in the local sinistral transpressional events may have formed by stepovers
Tonel Formation, it has been suggested that the first pulse of Cretaceous along the strike of the main Atacama Fault System (González et al.,
compressive deformation and subsequent erosion occurred along a 2012; McClay and Bonora, 2001). Further research would need to be
foreland basin located in the western margin of the Salar de Atacama done in order to establish if transpression in the Lower to
(Bascuñán et al., 2016). This episode was followed by a second pulse of mid-Cretaceous is a local or more extended structural event.
deformation estimated at 79 Ma. Arancibia (2004) preliminarily dated a
regional scale pulse of compressive deformation at 109 ± 11 Ma evi­ 7. Conclusions
denced along the in the Silla del Gobernador Shear Zone, in the Coastal
Cordillera at 32◦ S. Further south at 33◦ S, the Las Chilcas Formation Mineralization in the Candelaria-Punta del Cobre IOCG district in
shows a stratigraphic record of terrigenous strata of Early Cretaceous northern Chile occurred during transpression with shortening in a
age that suggest sedimentary filling of a proximal foreland basin asso­ northwest-southeast direction associated with north-northwest striking
ciated with several compressive pulses that took place between 105 and sinistral strike-slip faulting. Mineralization was coeval with emplace­
83 Ma (Boyce et al., 2020). ment of the Copiapó batholith demonstrated by syn-emplacement
Early Cretaceous transpressional deformation has also been recently mineralization in the Ojancos shear zone.
recorded associated with the Atacama Fault System north of the Punta IOCG deposits in the Andean belt have been interpreted to be asso­
del Cobre district (Mavor et al., 2020; Ruthven et al., 2020). Northwest ciated with extensional-transtensional structural settings, but results
transpression is recorded in the Taltal segment of the Atacama Fault presented here indicate that the important Candelaria-Punta del Cobre
system at ~119 Ma, which then transitioned to east-west shortening at district formed under transpressional conditions. A compressive envi­
~114 Ma (Mavor et al., 2020). This is consistent with suggested lateral ronment was also previously proposed for the Marcona-Mina Justa IOCG
slip along this segment of the Atacama Fault System ending around district in southern Perú based on geochronology linked to hydrother­
114-105 Ma (Seymour et al., 2021). In the Paposo southern segment of mal alteration (Chen et al., 2010). These examples suggest that trans­
the Atacama Fault System an east-southeast-dipping faulting has been pressive kinematics may be more important, at least locally, for IOCG
documented to have accommodated sinistral transpression from ~139 mineralization than previously recognized. We hypothesize that the
Ma to at least ~117 Ma, although the Paposo segment shows coeval pronounced shift from extensional-transtensional to transpressional
transtension in its northern segment consistent with arcuate geometry of sinistral deformation on high-angle faults promoted more adequate

10
I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

Fig. 11. Candelaria-Punta del Cobre district Tectono-stratigraphic chart. The main IOCG mineralization event as well as geodynamic context and structural styles are
represented.

stress conditions in the crust for the emplacement of large volume of review & editing. John F.H. Thompson: Investigation, Writing – review
magmas and hydrothermal IOCG deposits during the early-mid & editing. Christian Creixell: Investigation.
Cretaceous.
Declaration of competing interest
CRediT authorship contribution statement
The authors declare the following financial interests/personal re­
Irene del Real: Conceptualization, Data curation, Investigation,
lationships which may be considered as potential competing interests:
Writing – original draft, Writing – review & editing. Richard W. All­
Irene del Real reports financial support was provided by National
mendinger: Investigation, Conceptualization, Data curation, Writing –
Agency for Research and Innovation. Irene del Real reports financial

11
I. del Real et al. Journal of South American Earth Sciences 126 (2023) 104289

support was provided by Lundin Mining Corporation. Creixell, C., Ortiz, M., Arevalo, C., 2012. Geología del área Carrizalillo-El Tofo. Servicio
Nacional de Geología y Minería, Carta Geológica de Chile, Serie Geología Básica,
Santiago.
Data availability Creixell, C., Parada, M.Á., Morata, D., Vásquez, P., de Arce, C.P., Arriagada, C., 2011.
Middle-Late Jurassic to early Cretaceous transtension and transpression during arc
Al the data used is public in the manuscript building in central Chile: evidence from mafic dike swarms. Andean Geol. 38, 37–63.
del Real, I., Arriagada, C., 2015. Inversión tectónica positiva en el distrito El Espino:
Relaciones entre deformación, magmatismo y mineralizacion IOCG, Provincia de
Acknowledgments Choapa. XIV Congr. Geol. Chileno, La Serena.
del Real, I., Reich, M., Simon, A.C., Deditius, A., Barra, F., Rodríguez-Mustafa, M.A.,
Thompson, J.F.H., Roberts, M.P., 2021. Formation of giant iron oxide-copper-gold
Lundin Mining and Fondo Nacional de Desarrollo Científico y Tec­ deposits by superimposed, episodic hydrothermal pulses. Commun. Earth Environ.
nológico postdoctoral grant #3200532 is acknowledged and thanked for 21 (2), 1–9. https://doi.org/10.1038/s43247-021-00265-w, 2021.
funding and field support for the project. The staff of the Candelaria del Real, I., Thompson, J.F.H., Carriedo, J., 2018. Lithological and structural controls on
the genesis of the Candelaria-Punta del Cobre Iron Oxide Copper Gold district,
exploration and Lundin exploration are thanked for their support. Northern Chile. Ore Geol. Rev. 102, 106–153. https://doi.org/10.1016/j.
Thanks to Richard Friedman of the Pacific Centre of Isotopic and oregeorev.2018.08.034.
Geochemical Research of the University of British Columbia for U–Pb del Real, I., Thompson, J.F.H.F.H., Simon, A.C., Reich, M., 2020. Geochemical and
Isotopic Signature of Pyrite as a Proxy for Fluid Source and Evolution in the
analysis. Two anonymous reviewers are acknowledged for their com­ Candelaria-Punta del Cobre Iron Oxide Copper-Gold District, Chile. Econ. Geol. 115,
ments on the manuscript. 1493–1518. https://doi.org/10.5382/econgeo.4765.
Espinoza, S., Véliz, H., Esquivel, J., Arias, J., Moraga, A., 1996. The cupriferous province
of the coastal range, northern Chile. Andean Copp, 5. SEG Special Publication,
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