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Contrasting Tectonic Settings and Sulfur Contents of Magmas Associated with Cretaceous Porphyry Cu ± Mo ± Au and Intrusion-Related Iron Oxide Cu-Au Deposits
in Northern Chile*
Jeremy P. Richards,1,† Gloria P. López,1 Jing-Jing Zhu,1,2 Robert A. Creaser,1 Andrew J. Locock,1 and A. Hamid Mumin3
1 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada 2 State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, PR China
3 Department of Geology, Brandon University, 270-18th Street, Brandon, Manitoba R7A 6A9, Canada
AbstractPorphyry Cu ± Mo ± Au and iron oxide copper-gold (IOCG) deposits share many similarities (e.g., Fe, Cu, and Au contents), but also have important differences (e.g., the predominance of sulfide minerals in porphyry deposits and iron oxides in IOCG deposits). Genetic comparisons are complicated by the broad definition of IOCG deposits; here we restrict our study to IOCG deposits that are related to igneous intrusive systems. In the Mesozoic Coastal Cordillera of northern Chile, both porphyry and IOCG deposits occur in close spatial and temporal proximity, offering the chance to examine what controls their different modes of formation. From detailed examination of the timing, geochemistry, and tectonic setting of associated igneous rocks, based on new and published data, we find that rocks associated with mid-Cretaceous IOCG deposits (~125–110 Ma) are largely indistinguishable from those associated with slightly earlier (>125 Ma) and later (<110 Ma) porphyry Cu ± Mo ± Au deposits. Magmas related to IOCG deposits were formed during a brief period of back-arc transtension in the mid-Cretaceous and are, on average, somewhat more mafic (dioritic), locally alkaline, and isotopically primitive compared to granodioritic magmas associated with porphyry deposits formed during nor-mal contractional arc tectonics in the later Cretaceous. However, these compositional ranges overlap, and the differences are not clear enough to be diagnostic.
We measured the SO3 content of igneous apatite from selected samples of these rocks to test the hypothesis that the difference in sulfur content of the ore deposits was due to differences in sulfur content of the associated magmas. Early igneous apatite crystals occurring as inclusions in silicate phenocrysts from the Carmen de And-acollo porphyry Cu-Au deposit (Re-Os molybdenite ages of 103.9 ± 0.5 Ma, 103.6 ± 0.5 Ma) are significantly richer in S (0.25 ± 0.17 wt % SO3, n = 69) than similar apatite crystals from two IOCG deposits (Candelaria, Casualidad) and a sample of regional mid-Cretaceous igneous rock from near Productora (0.04 ± 0.02 wt % SO3, n = 76). Using published partition coefficients for S between apatite and oxidized silicate melt, we semi-quantitatively estimate corresponding magmatic sulfur contents of ~0.02 wt % S in the Carmen de Andacollo magmas versus ~0.001 to 0.005 wt % S in the IOCG-associated magmas. This is an order of magnitude dif-ference, and the opposite of what would be expected if the difference were due to bulk magma composition (sulfur solubility is generally higher in mafic magmas, whereas here the S content is higher in the more felsic porphyries). We conclude that the porphyry-forming magmas indeed had higher S contents than the IOCG-related magmas and suggest that these differences reflect different petrogenetic processes. During normal sub-duction, magmas derived from the metasomatized mantle wedge are hydrous, moderately oxidized, and S rich, and have the potential to generate S-rich porphyry-type deposits. In contrast, in back-arc extensional settings, upwelling asthenospheric melts carry a weaker subduction signature, including lower S contents. Interaction of these S-poor magmas with previously subduction modified upper plate lithosphere is more likely to give rise to S-poor IOCG deposits.
IntroductionSulfur-rich porphyry Cu ± Mo ± Au deposits are formed by the precipitation of sulfide minerals (pyrite, chalcopyrite, molybdenite) from hydrothermal fluids exsolved from shal-lowly emplaced calc-alkaline magmas in volcanic arcs, typi-cally generated in response to oceanic lithosphere subduction (Burnham, 1979; Richards, 2003; Cooke et al., 2005; Sil-litoe, 2010). In contrast, sulfur-poor iron oxide copper-gold (IOCG) deposits include a wide range of different deposit types, broadly linked by the prevalence of hydrothermal Fe
oxides (as opposed to Fe sulfides), with or without Cu and Au mineralization (Hitzman, 2000; Williams et al., 2005, 2010; Hunt et al., 2007; Groves et al., 2010). The broadness of this definition, as well as the capacity of oxidized saline fluids to transport Fe ± Cu ± Au in a variety of geologic settings, has led to controversy over the origin(s) of this group of deposits (e.g., Barton and Johnson, 1996, 2000; Pollard, 2000; Williams et al., 2005; Williams, 2010; Barton, 2014). However, within this group there is a subset of deposits that is more clearly associated with igneous rocks and fluids of possible magmatic-hydrothermal origin (Pollard, 2000; Sillitoe, 2003). Richards and Mumin (2013a, b) have referred to these as magmatic-hydrothermal IOCG deposits, but, for simplicity, we use the general term IOCG below. They share many features with
0361-0128/17/4470/295-24 295Submitted: October 2, 2015
Accepted: August 8, 2016
Economic Geology, v. 112, pp. 295–318
† Corresponding author: e-mail, [email protected]*A digital supplement to this paper is available at http://economicgeology.org/
and at http://econgeol.geoscienceworld.org/.
© 2017 Gold Open Access: this paper is published under the terms of the CC-BY license.
296 RICHARDS ET AL.
porphyry systems (e.g., Cu-Au-Mo-Fe metal association and broad tectonic setting and magmatic affinity) but also have important differences (such as more extensive development of high-temperature sodic, sodic-calcic, and potassic iron alteration envelopes, and more restricted development of lower-temperature phyllic and argillic alteration in IOCG systems compared to porphyry systems; Hitzman et al., 1992; Mumin et al., 2010).
Richards and Mumin (2013b) explained some of these dif-ferences, in particular the smaller acidic alteration zones in IOCG deposits, as reflecting lower abundances of S (SO2) in the ore-forming fluids compared to porphyry fluids. As S-rich porphyry fluids cool, the SO2 disproportionates to form H2S and H2SO4 (sulfuric acid), leading to widespread develop-ment of acidic alteration at shallow levels (Burnham, 1979; Candela, 1992; Field et al., 2005; Richards, 2011b, 2015); this happens to a lesser extent in S-poor IOCG fluids. Another important difference is the broader range of metals found in IOCG versus porphyry deposits, including, in some deposits, the presence of abundant U, rare earth elements (REEs), P, Co, Ni, and Bi. The increased variety of metals is attributed to the much greater influence of fluid reactions with crustal rocks and the resultant metal fluxing that occurs in giant IOCG hydrothermal systems (in addition to magmatic contri-butions), and the more common occurrence of mafic country rocks around many IOCGs (Somarin and Mumin, 2012; Rich-ards and Mumin, 2013b; Barton, 2014).
The Mesozoic Coastal Cordillera of northern Chile is uniquely suited to compare the characteristics and controls on ore formation in porphyry and IOCG deposits because both deposit types occur in a broadly coeval (Cretaceous), 50- to 80-km-wide belt that runs parallel to the coast for over
1,000 km (Fig. 1a; Sillitoe, 2003). The largest IOCG depos-its within this part of the belt are Candelaria (116–110 Ma; 501 Mt at 0.54% Cu, 0.13 g/t Au, and 2.06 g/t Ag) and Man-toverde (121–117 Ma; 440 Mt at 0.56 % Cu and 0.12 g/t Au). The largest porphyry Cu-Au deposit is Carmen de Andacollo (104 Ma; proven and probable reserves of 417 Mt at 0.34% Cu and 0.12 g/t Au; Table 1).
Small Early Cretaceous (>125 Ma; Berriasian-Barremian) porphyry Cu-Au deposits occur at 22°S (e.g., Antucoya, 142 Ma; Tovaku, 132 Ma; Maksaev et al., 2006) and at 33°S (e.g., Colliguay, ~129 Ma; Maksaev et al., 2010) and appear to have formed during a short period of synchronous transpres-sion at 22°S (Maksaev et al., 2006) or continental arc extension at 33°S (Creixell, 2007). In contrast, IOCG (and magnetite-apatite) deposits formed predominantly in the mid-Creta-ceous (~125–~110 Ma; Aptian-Albian; Gelcich et al., 2005; Arévalo et al., 2006; Rieger et al., 2010; Tornos et al., 2010) and are located in a tectonically distinct but spatially super-imposed belt from 25° to 34°S. These deposits have been described as spatially and temporally related to the culmina-tion of a period of back-arc extension that developed along the continental margin from the Late Jurassic to mid-Cretaceous (Oyarzun et al., 1999; Grocott and Taylor, 2002; Sillitoe, 2003) or, alternatively, as having formed in response to the initiation of basin inversion in the mid-Cretaceous (Chen et al., 2013).Porphyry Cu-Au-(Mo) deposits, including the large Carmen de Andacollo porphyry Cu-Au deposit (104 Ma), again began to form in the later Cretaceous (<110 Ma; Cenomanian-Turonian) between 25° and 32°S, following the resumption of arc magmatism during or after basin inversion (Maksaev et al., 2010). We group igneous rocks and associated ore deposits into three broad temporal groupings that relate to
Tropezon (110 Ma)Casualidad (100–94 Ma)
Santo Domingo (124 Ma)Todos los Santos (118 Ma)
Mantoverde (121–117 Ma)Cerro Negro Norte (116 Ma)
Candelaria (116–110 Ma)Punta del Cobre (116 Ma)
Productora (129 Ma)
Trapiche (122–120 Ma)
Panulcillo (115 Ma)
Espino (93–86 Ma)
Colliguay (129 Ma)
Llahuin (92 Ma)
Frontera (112 Ma)Andacollo (104 Ma)
Cachiyuyo (111 Ma)
Los Loros (91 Ma)La Verde (88 Ma)
Los Toros (98 Ma)
N
Porphyry Cu±Mo±Au (> or < 1 Mt Cu)
IOCG (> or < 1 Mt Cu)
Transitional
Extension
Contraction
Extension
Structural setting based onstructural evidence
Structural setting based ontectonostratigraphic evidence
Transtension
Contraction
Transpression
Age (Ma)
Inca de Oro(90–88 Ma)
Dos Amigos (108–104 Ma)Cortadera (87 Ma)
Pajonales (117 Ma)Totora (120 Ma)Las Campanas (90 Ma)
Chile
ArgentinaPaci�cOcean
(1) (i)
(ii)
(iv)
(vi)
(viii)
(vii)
(v)
(viii)
(iii)
(i)
(i)
(6)
(4)
(5)
(7)(7)
(7)
?
(9)
(11)
(8)
(10)
(3) (1)(1,2)
72°W34°S
32°S
30°S
28°S
26°S
24°S
70°W 68°W 145 135 125 115
Extension Sinistral transtension ContractionLegend
105 95 85
Fig. 1. Distribution, timing, and tectonic setting of Cretaceous porphyry Cu-Au and IOCG deposits in the Mesozoic Coastal Cordillera of northern Chile. (a) Geographic distribution of deposits and spatial relationship to the Atacama fault system; ages of deposits in Ma are shown in parentheses. (b) Temporal and tectonic separation of porphyry and IOCG deposits. Sources of data are provided in Table 1 and Appendix 1.
TECTONIC SETTINGS AND S CONTENTS OF MAGMAS ASSOCIATED WITH CRETACEOUS DEPOSITS, CHILE 297
tectonic setting as follows: early Cretaceous (extension), mid-Cretaceous (transtension), and late Cretaceous (contraction). Because the tectonic setting changed diachronously from north to south over intervals of 10 to 15 m.y., these terms do not correspond exactly to formal stratigraphic period subdivi-sions (epochs), and there is some temporal overlap between groups.
Evidence for a magmatic hydrothermal origin for several of these Chilean IOCG deposits has been reported, including at Candelaria and Mantoverde (Marschik and Kendrick, 2015) and Tropezon (Tornos et al., 2010, 2012), and several smaller IOCG deposits occur in close proximity to coeval intrusions with alteration patterns apparently centered on those plutons (e.g., El Trapiche veins, Creixell et al., 2009; El Espino, López et al., 2014). In contrast, Barton and Johnson (1996, 2000) argue that the ore-forming fluids were basinal brines, albeit with convection driven by the heat from coeval magmatism.
In this paper, we adopt the hypothesis that the Chilean IOCG and porphyry deposits are both of magmatic-hydrothermal
origin, and that their contrasting styles of ore formation relate to differences in the tectonic setting and chemistry of the associated magmas. We combine new and published geochro-nological and geochemical data for igneous rocks associated with Cretaceous porphyry and IOCG deposits in the Coastal Cordillera with published structural information to show that porphyry and IOCG deposits formed in distinct tectono-magmatic settings. The bulk compositions of the associated igneous rocks are almost indistinguishable, but we present data from analysis of igneous apatite to suggest that the por-phyry-forming magmas were S rich compared to their IOCG counterparts, consistent with the difference in S content of porphyry (S rich) and IOCG (S poor) ore deposits.
Mesozoic Tectonomagmatic Setting and Metallogeny of Northern Chile
Arc magmatism related to subduction has taken place along the Chilean segment of the Gondwana supercontinental margin since the late Paleozoic (Parada et al., 2007). Mid-Jurassic to
Table 1. Cretaceous Porphyry and IOCG Deposits of the Coastal Cordillera of Northern Chile
MetalDeposit/prospect association Resource Age (Ma) References
PorphyryInca de Oro Cu-Au-Ag 389 Mt at 0.39% Cu, 0.1 g/t Au 90–88 (U-Pb Zr) Maksaev et al. (2010)Los Toros Cu 98 Maksaev et al. (2007)Dos Amigos Cu-Au 36 Mt at 0.36% Cu 108–106 (U-Pb Zr) Almonacid (2007)Totora Cu-Au 120–121 (U-Pb Zr) Maksaev and Llaumet (2015)La Union (Frontera) Cu-Au 50.5 Mt at 0.4% Cu, 0.2 g/t Au 112 (U-Pb Zr) Creixell et al. (2015); Maksaev and Llaumet (2015)Pajonales Cu-Au 117 (U-Pb Zr) Morelli (2008)Punta Colorada Cu-Au 109 (U-Pb Zr) Creixell et al. (2015)Cachiyuyo Cu-Au 111 (U-Pb Zr) Creixell et al. (2015)La Verde Cu-Au 88 (U-Pb Zr) Creixell et al. (2015)Elisa Cu-Au 92 (U-Pb Zr) Creixell et al. (2015)Cortadera Cu-Au 87 (U-Pb Zr) Creixell et al. (2015)Las Campanas Cu-Au 90 (U-Pb Zr) Creixell et al. (2015)Los Loros Cu-Au 91 Maksaev et al. (2010)Carmen de Andacollo Cu-Au-Mo Proven and probable reserves 104 (U-Pb Zr; Re-Os Mo) Maksaev et al. (2010); Teck of 417 Mt at 0.34% Cu and Resources Limited, 2016; 0.12 g/t Au this studyLlahuin Cu-Au-Mo 145 Mt at 0.4% Cu equiv 92 (Ar-Ar Bt) Maksaev et al. (2010)Colliguay Cu-Au 129 (K-Ar WR) Maksaev et al. (2010)
IOCGTropezon Cu-(Mo-Au) ~1.5 Mt Cu 110 (U-Pb Zr) Tornos et al. (2010)Casualidad Cu-Au 400 Mt at 0.55% Cu 100 (U-Pb Zr), 100–94 (Ar-Ar Bt, Kovacic (2014) Kspar), 94–84 (Ar-Ar Act)Todos Los Santos Cu-Au 118 (Ar-Ar Act) Gelcich et al. (2005)Santo Domingo Cu-Fe-Au 417 Mt at 0.25% Cu, 27% Fe, 124 (U-Pb Zr, Tn) Daroch et al. (2015) 0.032 Au g/tMantoverde Cu-Au 440 Mt at 0.56% Cu, 0.12 g/t Au 121–117 (K-Ar Ser) Rieger et al. (2010)Cerro Negro Norte Fe-(Cu-Au) 100 Mt at 65% Fe 116 (U-Pb Tn) Raab (2002)Candelaria-Punta del Cobre Cu-Au-Ag 501 Mt at 0.54% Cu, 0.13 g/t Au 116–110 (Ar-Ar Bt; Re-Os Mo) Arévalo et al. (2006); Marschik and Fontboté (2001)Productora1 Cu-Au-(Mo) 214.3 Mt at 0.48% Cu, 0.1 g/t Au, 128.9 (Re-Os Mo), 130 (U-Pb Zr) Marquardt et al. (2015) 138 ppm MoTrapiche Cu-Au 122–120 (Ar-Ar Act) Creixell et al. (2015)Panulcillo Cu-Au 0.5 Mt at 2.75–3% Cu, 0.5–1 g/t Au 115 (K-Ar Phl) Diaz and Corvalán (2015)El Espino Cu-Au 145 Mt at 0.55% Cu, 0.22 g/t Au 93–89 (U-Pb Zr); 88–86 Del Real and Arriagada (2015); (Ar-Ar Act, Ser, Kspar) López et al. (2014)
Abbreviations: Act = actinolite, Bt = secondary biotite, Kspar = secondary potassium feldspar, Mo = molybdenite, Phl = phlogopite, Ser = sericite, Tn = titanite, WR = whole rock, Zr = zircon
1Deposit type is transitional between porphyry and IOCG
298 RICHARDS ET AL.
Cretaceous magmatism mostly developed ~100 km to the west of the Paleozoic arc (Parada et al., 2007). Extensional tecton-ics affected the arc from the mid-Jurassic to early Cretaceous, with intra-arc and back-arc basin formation occurring in the mid-Cretaceous, followed by tectonic inversion in the mid- to late Cretaceous (Charrier et al., 2007). These tectonic changes are interpreted to reflect alternating periods of subduction coupling and decoupling between the down-going and overrid-ing plates, with the generation of periods dominated by con-tractional and extensional tectonics, respectively, in the upper plate (Scheuber and Gonzalez, 1999). Of particular relevance to this study, early Cretaceous extension and mid-Cretaceous transtension occurred during a period of low convergence rate and weak plate coupling that may reflect slab rollback. This was followed by contraction in the mid- to late Cretaceous, caused by an increase in convergence rate in response to global-scale plate reorganization (Matthews et al., 2012).
Early Cretaceous magmatism in the Coastal Cordillera between 25° and 34°S was generated in a broadly extensional tectonic regime that caused rifting and crustal thinning (Parada et al., 2007). It was characterized by relatively primitive calc-alkaline to shoshonitic lavas that built thick (5–10 km) subaerial volcanic sequences (Vergara et al., 1995; Morata and Aguirre, 2003; Parada et al., 2005; Charrier et al., 2007; Girardi, 2014). By the mid-Cretaceous, the tectonic setting had become pre-dominantly transtensional (Brown et al., 1993; Arévalo et al., 2003) and was characterized by episodic volcanism and sedi-ment deposition in intra-arc and back-arc shallow marine and continental basins (Fig. 1b; Morata and Aguirre, 2003). The transition from extensional to transtensional tectonics was slightly diachronous from north to south, and represents the progression of continental arc rifting southward with time, beginning in the north in the Barremian (~130 Ma) and reach-ing its maximum in the Aptian-Albian (~120 Ma). Plutonic com-plexes were emplaced during both tectonic stages, controlled by crustal-scale fault zones that evolved into the Atacama fault sys-tem in the Valanginian-Barremian (144–126 Ma; Brown et al., 1993). The Atacama fault is an early dip-slip and later sinistral transtensional N-trending fault system that extends for more than ~1,000 km parallel to the continental margin (Fig. 1a) and is interpreted to have formed in response to oblique subduction (Scheuber and Andriessen, 1990; Brown et al., 1993; Palacios et al., 1993); it is the primary structural control on the location of IOCG deposits in Chile (Sillitoe, 2003; Creixell et al., 2009). Crustal extension ended in the Albian-Cenomanian (110–95 Ma) with a return to contractional tectonics that produced crustal shortening and thickening, basin closure, rapid uplift, and a marked eastward shift of magmatism (Parada et al., 2002, 2005; Arancibia, 2004; Maksaev et al., 2010).
The spatiotemporal distribution of Cretaceous IOCG and porphyry Cu-Au deposits in northern Chile between 25° and 34°S is shown in relation to these tectonomagmatic periods in Figure 1b. Here it can be seen that, although the deposits roughly overlap spatially, IOCG deposits are broadly sepa-rated in time from younger porphyry Cu-Au deposits at ~110 to 100 Ma, which also marks the time of the major tectonic change from transtension to contraction. During the initial stages of arc rifting in the Late Jurassic-early Cretaceous, only a few small porphyries were developed (e.g., Antu-coya, Colliguay). This was followed by the early formation
of magnetite-apatite deposits (Gelcich et al., 2005; Creix-ell et al., 2009) and then IOCG deposits during the main stage of rifting and back-arc basin development in the mid-Cretaceous (~125–~110 Ma, Aptian-Albian; Oyarzun et al., 1999). This tectonic relationship appears to be consistent with IOCG deposits globally, which are commonly found to be associated with extensional events, including postcollisional, intracontinental, back-arc, and intra-arc rift settings (Williams et al., 2005; Corriveau and Mumin, 2010; Skirrow, 2010). Porphyry Cu-Au-(Mo) deposit formation returned in the late Cretaceous with the resumption of subduction-related mag-matism during and after tectonic inversion. Apparent excep-tions to this three-part deposit distribution (e.g., the small, mid-Cretaceous Totora, Pajonales, and Cachiyuyo porphy-ries, and the late Cretaceous Casualidad and El Espino IOCG deposits; Fig. 1) may reflect uncertainties in age determina-tion, lithospheric heterogeneities, or delayed response to tec-tonic changes.
SamplingFifty-four samples of volcanic and intrusive igneous rocks associ-ated with the Carmen de Andacollo, Frontera, and Dos Amigos porphyry Cu ± Au ± Mo deposits, the transitional Productora deposit, and the Candelaria, Casualidad, Espino, and Man-toverde IOCG deposits from the Coastal Cordillera of north-ern Chile between 26° and 32°S (Fig. 1) were collected from drill core and outcrops in July 2015. Where possible, intrusive rocks thought to be directly related to mineralization (coeval, cospatial) were collected; other samples were from broadly coeval intrusions or volcanic rock outcrops in the vicinity of the deposits (based on regional geologic maps). Least-altered samples were targeted for collection, but most of the rocks have undergone either low-grade regional metamorphism (result-ing in minor chloritization of ferromagnesian silicate miner-als and partial saussuritization of plagioclase) or hydrothermal alteration due to proximity to the ore deposits. Two samples of quartz-molybdenite veins were collected from the Carmen de Andacollo porphyry Cu-Au deposit for Re-Os dating.
The rocks were studied petrographically to determine the degree of alteration and to seek unaltered accessory minerals such as zircon and apatite for electron microprobe analysis.
Analytical Methods
Whole-rock geochemistry
Thirty-nine least-altered samples from the collected suite were submitted to Activation Laboratories Ltd. (Ancaster, Ontario) for analysis using the 4E-Research analytical pack-age, which combines instrumental neutron activation analysis and lithium metaborate/tetraborate fusion inductively coupled plasma-mass spectrometry for determination of 62 elements. Analyses of standards and duplicates indicate that accuracy for major elements is typically within 5 relative %, and 10 rela-tive % for minor and trace elements. The ferrous iron (FeO) content of the rocks was also determined by titration.
Electron microprobe analyses
Polished thin sections of all samples were examined for the presence of igneous accessory minerals such as zircon and apatite. Because of the relatively mafic nature of many of
TECTONIC SETTINGS AND S CONTENTS OF MAGMAS ASSOCIATED WITH CRETACEOUS DEPOSITS, CHILE 299
the samples, few contained zircon in thin section, so zircon chemistry was not attempted. A larger number of samples contained apatite, although care was needed to distinguish between igneous and hydrothermal (or late-stage) apatite. The latter was common in the groundmass of altered samples, and clearly hydrothermal apatite (intergrown with hydrother-mal minerals such as quartz and sulfides) typically had thin, elongated prismatic habits (≤200-µm length; Fig. 2a). Apatite crystals with unequivocally igneous origin were distinguished by their inclusion within phenocrysts (mainly plagioclase and biotite). The igneous apatite crystals had stubby prismatic habits (20–50-µm length; Fig. 2b-d).
Compositional data were acquired with a Cameca SX100 electron microprobe using wavelength-dispersive spectros-copy and Probe for EPMA software (Donovan et al., 2015). For sessions in which seven elements (F, Na, Si, P, S, Cl, and Ca) were measured, the following conditions were used: 10-kV accelerating voltage, 20-nA beam current, and 5- to 10-µm beam diameter. Total count times of 30 s were used for both peaks and backgrounds. The X-ray lines, analyzing
crystals, and standards were as follows: F Kα, PC0, topaz; Na Kα, TAP, tugtupite; Si Kα, TAP, topaz; P Kα, PET, fluorapatite; S Kα, PET, anhydrite (intensity data aggregated from two spectrometers; Donovan et al., 2011); Cl Kα, PET, tugtupite; and Ca Kα, PET, fluorapatite. The calculated limits of detection (as element, rounded to the nearest 10 ppm) at 99% confidence were as follows: F, 570 ppm; Na, 130 ppm; Si, 120 ppm; P, 170 ppm; S, 80 ppm; Cl, 290 ppm; and Ca, 280 ppm. To check for interference from third-order P Kα on F Kα, the signal from synthetic GaP was examined with the PC0 crystal: no significant interference from P on F was detected under the conditions of analysis.
For sessions in which nine elements (F, Si, P, S, Cl, Ca, Mn, Fe, and As) were measured, the following conditions were used: 15-kV accelerating voltage, 20-nA beam cur-rent, and 5-µm beam diameter. The X-ray lines, analyzing crystals, standards, and count times (seconds) on both peaks and backgrounds were as follows: F Kα, PC0, topaz, 60 s; Si Kα, TAP, topaz, 40 s; P Kα, PET, fluorapatite, 30 s; S Kα, PET, anhydrite, 30 and 40 s (intensity data aggregated from
0.01%
0.02%
0.07%
0.06%
0.27%
0.15%
0.08%
0.45%
0.41%
0.01%
Amph
Biotite
Biotite
Amph
Ap
Ap
Ap
Ap
Ap
Plag
Chl
Chl
Plag
Plag
Plag
PlagQz
Qz
Qz
Qz
Qz
Qz
Qz
sulfide
sulfide
sulfide
(a) CAN-2 (b) CAS-2
(c) CDA-8 (d) CDA-2
Ap
Fig. 2. Photomicrographs of apatite crystals in samples from (a) Candelaria (CAN-2), (b) Casualidad (CAS-2), and (c, d) Carmen de Andacollo (CDA-8, CDA-2). Concentrations of SO3 in apatite crystals are shown in red (wt %); higher concentra-tions are observed in apatite from porphyry-related samples (Carmen de Andacollo) compared to the IOCG-related samples (Candelaria, Casualidad). Some apatite microphenocrysts from Carmen de Andacollo show zonation from SO3-rich cores to SO3-poorer rims (d). Abbreviations: Amph = amphibole, Ap = apatite, Chl = chlorite, Plag = plagioclase, Qz = quartz.
300 RICHARDS ET AL.
measurements on two spectrometers; Donovan et al., 2011); Cl Kα, PET, tugtupite, 40 s; Ca Kα, PET, fluorapatite, 30 s; Mn Kα, LIF, spessartine, 40 s; Fe Kα, LIF, spessartine, 40 s; and As Lα, TAP, synthetic GaAs, 40 s. The calculated limits of detection (as element, rounded to the nearest 10 ppm) at 99% confidence were as follows: F, 460 ppm; Si, 110 ppm; P, 260 ppm; S, 70 ppm; Cl, 150 ppm; Ca, 170 ppm; Mn, 170 ppm; Fe, 160 ppm; and As, 180 ppm.
In all sessions, time-dependent intensity corrections for F and Cl were carried out (peak count times divided into six intervals) with Probe for EPMA software (Donovan et al., 2015), following Nielsen and Sigurdsson (1981), Stormer et al. (1993), and Henderson (2011). Intensity data for all ele-ments were reduced following the methods of Armstrong (1995). Oxygen was calculated by stoichiometry and included in the data reduction, as was the correction for oxygen equiva-lence of the halogens (F and Cl).
Re-Os dating
A molybdenite mineral separate was made for each sample by metal-free crushing followed by gravity and magnetic con-centration methods described in detail by Selby and Creaser (2004). The 187Re and 187Os concentrations in molybdenite were determined by isotope dilution mass spectrometry using Carius tube, solvent extraction, anion chromatography, and negative thermal ionization mass spectrometry techniques. A mixed double spike containing known amounts of isotopi-cally enriched 185Re, 190Os, and 188Os was used (Markey et al., 2007). A ThermoScientific Triton mass spectrometer with a Faraday collector was used for isotopic analysis. Total blanks for Re and Os are less than 3 and 2 pg, respectively, which are insignificant for the Re and Os concentrations in molybde-nite. The molybdenite HLP-5 (Markey et al., 1998) was ana-lyzed as a standard, and over a period of two years an average Re-Os date of 221.56 ± 0.40 Ma (1 s.d. uncertainty, n = 10) was obtained. This value is within the uncertainty of the 221.0 ± 1.0 Ma age reported by Markey et al. (1998).
Re-Os Age of the Carmen de Andacollo Porphyry Cu-Au Deposit
The Carmen de Andacollo porphyry Cu-Au deposit has previ-ously been dated at 104 ± 3.3 Ma by U-Pb analysis of zircons in the ore-forming porphyry intrusions (Maksaev et al., 2010), and at 104 ± 3 and 98 ± 2 Ma by K-Ar analysis of phyllic- and potassic-altered rocks, respectively (Reyes, 1991). We have dated two samples of molybdenite from B-type quartz veins in the deposit, which yielded statistically indistinguishable ages of 103.9 ± 0.5 and 103.6 ± 0.5 Ma (2s errors; Table 2), in good
agreement with the U-Pb date for magmatism of Maksaev et al. (2010).
Cretaceous Igneous GeochemistryWhole-rock major and trace element geochemical data for 125 igneous rocks coeval with either IOCG or porphyry deposits from the Coastal Cordillera between 25° and 34°S were com-piled from the literature and combined with our 40 new anal-yses (new data are listed in Table 3 and Supplementary Table S1, and data from the literature are listed in Supplementary Table S2). These rocks are divided into three groups, based on the tectonomagmatic periods described above: early Cre-taceous, related to a few small porphyry Cu-Au deposits and early stages of arc rifting; mid-Cretaceous, related to IOCG deposits and back-arc transtension; and late Cretaceous, related to porphyry Cu-Au-(Mo) deposits and a return to con-tractional tectonics and arc magmatism. Note that, because of diachronous changes in tectonic style from north to south over periods of 10 to 15 m.y., the terms “early,” “mid-,” and “late Cretaceous” do not correspond exactly to formal strati-graphic divisions, but are approximately separated at ~125 and ~110 Ma, respectively.
Volcanic and plutonic rocks from the three tectonomag-matic groups are mostly metaluminous, calc-alkaline to high-K calc-alkaline, and range in composition from andesite (diorite) to dacite (granodiorite). Samples of igneous rocks directly associated with porphyry and IOCG deposits show a similar compositional range on a total alkali-silica diagram (Fig. 3), although it is evident that the porphyry-related rocks are mostly more felsic (diorite to granodiorite) compared to those associated with IOCG deposits (mostly gabbroic diorite to diorite). The apparently alkaline composition of several of these samples (especially from IOCG deposits) might be due to variable degrees of hydrothermal alteration (sodic-calcic or potassic), which was unavoidable in these suites of ore-asso-ciated rocks. However, on a plot of immobile element ratios (Zr/Ti versus Nb/Y; Fig. 4; Winchester and Floyd, 1977), a small number of mid-Cretaceous and IOCG-related samples do plot in more alkaline fields (alkali gabbro and syenite), sug-gesting that some of these samples are genuinely alkaline in composition. Nevertheless, the majority of the samples over-lap in the gabbro-diorite-granodiorite fields in Figure 4, with no clear distinction between age groups or association with porphyry and IOCG deposits.
On primitive mantle-normalized extended trace element (Fig. 5) and chondrite-normalized REE (Fig. 6) diagrams, the porphyry and IOCG suites display almost indistinguish-able patterns: enrichments in large-ion lithophile elements
Table 2. Re-Os Molybdenite Age Data from the Carmen de Andacollo Porphyry Cu-Au Deposit
Total Model Re 187Re 187Os common age ±2s withSample no. Location Description (ppm) ±2s (ppb) ±2s (ppb) ±2s Os (pg) (Ma)1 λ (Ma)2
CDA-4 DDH 11-41 Quartz-molybdenite vein 405.2 1.3 254,683 805 441.4 0.3 2.6 103.9 0.5 at 396.4 m in Porphyry DCDA-12 DDH 13-09 Quartz-molybdenite vein 556.9 1.8 350,059 1,106 604.8 0.4 1.0 103.6 0.5 at 383.2 m in andesitic country rock
1Model age calculated from the equation t = ln (187Os/187Re + 1)/λ, where t = model age and λ = 187Re decay constant, and assuming no initial radiogenic Os2λ = 187Re decay constant, 1.666 × 10–11 yr–1 (Smoliar et al., 2006)
TECTONIC SETTINGS AND S CONTENTS OF MAGMAS ASSOCIATED WITH CRETACEOUS DEPOSITS, CHILE 301Ta
ble
3. W
hole
-Roc
k an
d Tr
ace
Ele
men
t Geo
chem
ical
Dat
a fo
r Ig
neou
s R
ock
Sam
ples
from
the
Coa
stal
Cor
dille
ra o
f Nor
ther
n C
hile
Bet
wee
n 25
° an
d 34
°S
Sam
ple
no.
ESP
-1
ESP
-2
ESP
-3
CD
A-1
C
DA
-2
CD
A-3
C
DA
-7
CD
A-9
C
DA
-10
FR
-1
Fro
nter
a-M
ine
Loc
ality
E
spin
o E
spin
o E
spin
o A
ndac
ollo
A
ndac
ollo
A
ndac
ollo
A
ndac
ollo
A
ndac
ollo
A
ndac
ollo
L
a U
nion
Bio
tite,
Les
s al
tere
d, g
ypsu
m-
Qua
rtz-
C
hlor
ite,
diss
emin
ated
chal
copy
rite
- ch
alco
pyri
teA
ltera
tion/
ca
lcite
vei
ns
C
hlor
ite,
chal
copy
rite
- C
hlor
ite-
iron
oxi
de v
einl
ets,
ve
inle
ts, g
ypsu
m-
Epi
dote
, cla
ys,
min
eral
izat
ion
(ska
rn in
wal
l roc
k)
K
-fel
dspa
r py
rite
(s
eric
ite)
2° b
iotit
e ir
on o
xide
w
eak
pota
ssic
TAS
Gab
broi
c G
abbr
oic
Gab
broi
c
Bas
altic
trac
hyan
desi
tecl
assi
ficat
ion1
di
orite
di
orite
di
orite
M
onzo
nite
G
rano
dior
ite
Gra
nodi
orite
(m
ugea
rite
) G
rano
dior
ite
Mon
zoni
te
Gra
nodi
orite
SiO
2 (w
t %)
51.4
5 51
.85
49.7
8 57
.38
62.9
2 62
.79
49.9
2 62
.05
54.6
7 63
.54
Al 2O
3 (w
t %)
15.5
5 16
.81
16.7
8 16
.23
15.7
3 15
.85
18.1
9 15
.86
14.2
4 14
.63
Fe 2
O3 (
wt %
) 0.
98
0.69
1.
35
2.33
0.
97
0 5.
83
0.56
1.
07
3.79
FeO
(wt %
) 6.
3 6.
4 6.
8 2.
8 2.
7 4.
1 4
3.7
4.9
2.5
MnO
(wt %
) 0.
119
0.16
8 0.
167
0.13
0.
074
0.04
8 0.
152
0.09
5 0.
118
0.17
3M
gO (w
t %)
7.87
8
7.43
3.
22
1.06
2.
63
4.28
3.
39
6.77
1.
72C
aO (w
t %)
8.06
9.
43
7.15
5.
03
4.75
4.
49
3.78
5.
27
5.74
3.
77N
a 2O
(wt %
) 3.
44
3.11
2.
98
4.05
2.
68
3.46
4.
86
4.34
2.
83
4.11
K2O
(wt %
) 1.
36
0.35
1.
3 2.
64
2.89
2.
31
2.91
1.
48
2.9
2.36
TiO
2 (w
t %)
0.64
7 0.
693
0.71
1 0.
558
0.37
5 0.
489
0.92
9 0.
517
0.65
1 0.
368
P 2O
5 (w
t %)
0.12
0.
12
0.12
0.
2 0.
13
0.13
0.
25
0.2
0.33
0.
12L
OI
(wt %
) 2.
33
2.25
5.
59
5.37
5.
19
3.31
4.
54
2.76
4.
29
1.31
LO
I 2
(wt %
) 1.
63
1.53
4.
82
5.06
4.
89
2.85
4.
1 2.
35
3.74
1.
03To
tal (
wt %
) 98
.94
100.
6 10
0.9
100.
3 99
.78
100.
1 10
0.1
100.
6 99
.06
98.6
6To
tal 2
(wt %
) 98
.23
99.8
7 10
0.2
99.9
4 99
.48
99.6
2 99
.64
100.
2 98
.51
98.3
8F
e 2O
3(T
) (w
t %)
7.99
7.
8 8.
91
5.45
3.
97
4.56
10
.28
4.67
6.
52
6.57
Cs
(ppm
) 9.
4 1.
8 3.
5 4.
4 4
2.2
4.1
2 3.
8 0.
7T
l (pp
m)
0.27
<0
.05
<0.0
5 0.
23
0.24
0.
29
0.08
0.
2 0.
66
0.1
Rb
(ppm
) 71
9
35
68
78
63
68
43
105
37B
a (p
pm)
153
209
293
508
611
683
638
217
270
626
Th
(ppm
) 0.
52
0.49
0.
48
2.9
5.76
5.
21
2.44
6.
07
4.96
3.
13U
(ppm
) 0.
24
0.2
0.28
1.
1 1.
48
2.07
0.
71
1.75
1.
99
0.74
Nb
(ppm
) 1.
8 1.
7 1.
7 2.
7 2.
9 2.
6 2
2.8
3.3
2.5
Ta (p
pm)
0.14
0.
16
0.14
0.
22
0.33
0.
31
0.16
0.
28
0.25
0.
28L
a (p
pm)
7.46
6.
11
7.7
15.5
18
.8
16.3
16
15
.9
23
11.8
Ce
(ppm
) 15
.9
14.2
16
.9
32.7
36
31
.9
32.7
35
.3
49.6
23
.1Pb
(ppm
) <5
<5
<5
6
<5
<5
18
<5
<5
10Pr
(ppm
) 2.
26
2.02
2.
26
4.15
4.
26
3.86
4.
33
4.75
6.
3 2.
87Sr
(ppm
) 48
9 49
9 36
5 62
1 23
4 58
4 73
8 50
7 35
8 34
8N
d (p
pm)
9.61
8.
96
10.4
16
.8
16.1
15
.4
18
19.6
24
.9
11.4
Zr (p
pm)
58
60
60
92
91
86
66
113
115
69H
f (pp
m)
1.5
1.7
1.6
2.4
2.5
2.4
2 2.
9 3
1.9
Sm (p
pm)
2.74
2.
31
2.58
3.
55
3.16
3.
09
4.25
4.
06
5.26
2.
54E
u (p
pm)
0.86
3 0.
748
0.84
5 1.
17
0.84
1.
03
1.27
1.
19
1.42
0.
836
Sb (p
pm)
1.7
0.8
1.6
1.7
1 <0
.1
1.6
0.2
<0.1
2.
6G
d (p
pm)
2.58
2.
55
2.57
2.
9 2.
43
2.55
3.
42
3.28
4
2.38
Tb
(ppm
) 0.
46
0.44
0.
43
0.41
0.
35
0.38
0.
54
0.44
0.
56
0.39
Dy
(ppm
) 2.
91
2.71
2.
77
2.27
1.
91
2.06
3.
13
2.45
2.
9 2.
4Y
(ppm
) 15
15
14
13
12
13
15
14
15
15
Ho
(ppm
) 0.
58
0.56
0.
55
0.44
0.
36
0.39
0.
59
0.46
0.
54
0.5
Er
(ppm
) 1.
65
1.73
1.
67
1.26
1.
08
1.09
1.
73
1.29
1.
49
1.5
Tm
(ppm
) 0.
256
0.25
8 0.
237
0.19
0.
161
0.16
3 0.
262
0.20
1 0.
214
0.23
7Yb
(ppm
) 1.
74
1.68
1.
66
1.2
1.02
1.
07
1.74
1.
33
1.4
1.7
Lu
(ppm
) 0.
29
0.27
7 0.
263
0.18
3 0.
163
0.17
0.
285
0.21
0.
223
0.28
Cr
(ppm
) 36
9 38
9 37
5 26
.5
32.5
43
.1
3.4
102
372
8.4
Ni (
ppm
) 13
0 14
0 12
3 17
11
16
13
38
12
8 3
Sc (p
pm)
28.1
28
.6
28.7
13
.8
9.34
13
.9
22.3
13
.9
22.9
8.
22V
(ppm
) 19
7 20
8 20
7 14
5 10
0 14
1 33
5 13
3 18
1 83
Co
(ppm
) 31
.5
28.6
32
.1
15.4
8.
1 12
.3
26.3
13
23
.5
10.7
Cu
(ppm
) 84
25
0 62
1 5
813
1,14
0 12
37
6 1,
430
655
302 RICHARDS ET AL.Ta
ble
3. (
Con
t.)
Sam
ple
no.
PR-1
PR
-2
PR-3
PR
-4
PR-5
PR
-6
PR-7
PR
-8
CA
N-1
Pr
oduc
tora
-
Pr
oduc
tora
Pr
oduc
tora
Pr
oduc
tora
R
anch
o H
ill,
Prod
ucto
ra-
Prod
ucto
ra-
Pr
oduc
tora
(ir
on d
istr
ict
(iron
dis
tric
t (ir
on d
istr
ict
Loc
ality
C
achi
yuyi
to s
tock
A
lice
porp
hyry
A
lice
porp
hyry
Pr
oduc
tora
(R
uta
5 ba
thol
ith)
regi
onal
) re
gion
al)
regi
onal
) C
ande
lari
a
Wea
k di
ssem
inat
ed
py
rite
-cha
lcop
yrite
, So
dic
Alte
ratio
n/
Wea
k al
bite
- bu
t oth
erw
ise
(alb
ite-c
hlor
ite)
Mod
erat
ely
W
eak
albi
te-
min
eral
izat
ion
actin
olite
fa
irly
fres
h re
plac
ing
horn
blen
de
alte
red
ac
tinol
ite
Min
or e
pido
te
TAS
clas
sific
atio
n1
Gra
nodi
orite
G
rano
dior
ite
Gra
nodi
orite
B
asal
tic a
ndes
ite
Gra
nodi
orite
D
iori
te
Gra
nodi
orite
D
iori
te
Mon
zoni
te
SiO
2 (w
t %)
62.3
3 66
.72
62.2
9 51
.69
64.1
60
.49
65.2
5 57
.03
58.9
3A
l 2O3 (
wt %
) 14
.57
15.7
2 14
.26
14.9
3 16
.38
16.4
1 15
.17
17.2
3 17
.87
Fe 2
O3 (
wt %
) 2.
1 0.
43
6.87
2.
02
1.26
0.
69
2.19
0.
68
2.59
FeO
(wt %
) 2.
5 2
4.3
4.3
3.3
2.2
3 3
2.7
MnO
(wt %
) 0.
102
0.04
2 0.
105
0.13
1 0.
116
0.05
4 0.
09
0.15
3 0.
055
MgO
(wt %
) 2.
44
1.8
1.53
9.
41
2.02
3.
87
1.73
4.
31
1.89
CaO
(wt %
) 6.
17
5.77
3.
66
6.12
5.
64
8.66
4.
39
10.1
3.
81N
a 2O
(wt %
) 3.
58
4.02
3.
46
4.19
3.
69
4.26
3.
79
4.61
6.
08K
2O (w
t %)
2.97
0.
75
1.12
0.
5 1.
63
0.41
2.
29
0.58
2.
62Ti
O2 (
wt %
) 0.
953
0.50
1 0.
413
0.71
7 0.
42
1.08
3 0.
705
1.03
2 0.
437
P 2O
5 (w
t %)
0.19
0.
09
0.1
0.1
0.13
0.
22
0.14
0.
19
0.23
LO
I (w
t %)
0.89
1.
94
0.86
4.
41
1.14
1.
07
1.1
1.41
1.
15L
OI
2 (w
t %)
0.61
1.
72
0.38
3.
93
0.77
0.
82
0.76
1.
08
0.85
Tota
l (w
t %)
99.0
7 10
0 99
.44
99.0
1 10
0.2
99.6
7 10
0.2
100.
7 98
.67
Tota
l 2 (w
t %)
98.7
9 99
.8
98.9
6 98
.53
99.8
2 99
.42
99.8
3 10
0.3
98.3
7F
e 2O
3(T
) (w
t %)
4.88
2.
65
11.6
5 6.
8 4.
94
3.14
5.
53
4.02
5.
6
Cs
(ppm
) 0.
6 0.
8 1.
1 0.
4 1.
3 0.
2 0.
3 0.
5 0.
3T
l (pp
m)
<0.0
5 <0
.05
<0.0
5 <0
.05
0.09
<0
.05
<0.0
5 <0
.05
<0.0
5R
b (p
pm)
56
20
38
17
39
6 33
14
34
Ba
(ppm
) 64
3 11
8 13
0 41
43
2 12
8 59
0 15
0 84
3T
h (p
pm)
12
3.19
3.
94
1.63
3.
61
7.13
4.
44
2.8
3.33
U (p
pm)
1.44
0.
61
1.16
2.
4 1.
09
1.58
0.
53
0.73
1.
05N
b (p
pm)
6.8
2.3
5.5
1.2
3.8
4.4
4 3.
3 3.
1Ta
(ppm
) 0.
63
0.24
0.
3 0.
1 0.
45
0.41
0.
35
0.27
0.
24L
a (p
pm)
10.9
6.
9 12
.5
10.8
14
.6
12.2
11
.8
4.22
18
.3C
e (p
pm)
27.2
15
.7
25.5
21
.4
30.8
33
.4
29.8
12
.4
36.8
Pb (p
pm)
<5
<5
<5
<5
<5
<5
<5
<5
<5Pr
(ppm
) 3.
88
2.24
3.
14
2.37
3.
72
4.6
4.37
2.
06
4.59
Sr (p
pm)
285
313
184
144
384
364
228
330
426
Nd
(ppm
) 18
.2
9.13
12
.5
8.97
14
.3
19.5
20
.4
10.5
18
.9Zr
(ppm
) 18
9 89
95
61
11
3 17
1 18
1 13
3 12
7H
f (pp
m)
4.8
2.2
2.4
1.7
2.8
4.2
4.6
3.5
3Sm
(ppm
) 4.
96
2.06
2.
68
2.04
2.
99
4.83
5.
24
3.39
3.
7E
u (p
pm)
1.2
0.79
4 0.
896
0.62
8 0.
918
1.41
1.
24
1.12
1.
06Sb
(ppm
) <0
.1
0.3
0.5
0.5
0.5
0.5
<0.1
0.
2 0.
7G
d (p
pm)
5.29
2.
17
2.6
2.42
2.
58
4.64
5.
54
4.28
3.
28T
b (p
pm)
0.88
0.
38
0.43
0.
44
0.41
0.
76
0.97
0.
81
0.49
Dy
(ppm
) 5.
5 2.
37
2.67
2.
82
2.48
4.
67
6.09
5.
27
2.91
Y (p
pm)
29
18
12
17
16
28
36
31
19H
o (p
pm)
1.12
0.
49
0.56
0.
61
0.5
0.95
1.
26
1.08
0.
58E
r (p
pm)
3.2
1.51
1.
75
1.9
1.5
2.84
3.
62
3.16
1.
76T
m (p
pm)
0.47
3 0.
236
0.27
0.
302
0.24
2 0.
433
0.53
4 0.
46
0.27
9Yb
(ppm
) 3.
12
1.67
1.
86
2.19
1.
68
3.05
3.
59
3.11
1.
91L
u (p
pm)
0.50
6 0.
284
0.30
7 0.
371
0.28
5 0.
502
0.57
6 0.
502
0.31
6C
r (p
pm)
47.1
25
.7
230
434
34.8
86
.8
26.5
31
.3
9.1
Ni (
ppm
) 13
5
17
96
9 15
5
7 6
Sc (p
pm)
20
13.8
11
.7
34.5
9.
7 27
.5
19.9
31
.2
9.25
V (p
pm)
126
94
82
251
77
196
110
190
99C
o (p
pm)
11.4
6.
9 18
.4
8.9
8.2
11.3
12
.9
6.5
7.7
Cu
(ppm
) 25
19
4 1,
300
7 11
5
19
15
42
TECTONIC SETTINGS AND S CONTENTS OF MAGMAS ASSOCIATED WITH CRETACEOUS DEPOSITS, CHILE 303Ta
ble
3. (
Con
t.)
Sam
ple
no.
CA
N-2
C
AN
-3
CA
N-4
C
AS-
1 C
AS-
2 C
AS-
3 C
AS-
4 C
AS-
5 C
AS-
8 C
AS-
9
C
ande
lari
a-
Coq
uim
bana
Loc
ality
C
ande
lari
a C
ande
lari
a m
ine
Cas
ualid
ad
Cas
ualid
ad
Cas
ualid
ad
Cas
ualid
ad
Cas
ualid
ad
Cas
ualid
ad
Cas
ualid
ad
A
lbiti
c al
tera
tion
Pota
ssic
+
Prop
yliti
cA
ltera
tion/
+
hem
atite
-Cu
Wea
k W
eak
prop
yliti
c
chal
copy
rite
(c
hlor
ite-e
pido
te)
min
eral
izat
ion
vein
s ne
arby
prop
yliti
c (c
hlor
ite-e
pido
te)
ve
inle
ts
on
pota
ssic
TAS
G
abbr
oic
Gab
broi
c
F
oid
clas
sific
atio
n1
Mon
zoni
te
dior
ite
dior
ite
Mon
zodi
orite
D
iori
te
Mon
zodi
orite
G
rano
dior
ite
mon
zosy
enite
M
onzo
dior
ite
Dio
rite
SiO
2 (w
t %)
58.3
4 52
.88
52.8
52
.87
57.8
2 51
.26
65.0
3 51
.48
54.1
9 56
.66
Al 2O
3 (w
t %)
18.3
8 17
.53
16.5
8 15
.12
17.9
9 16
.96
16.7
15
.99
18.1
4 16
.93
Fe 2
O3 (
wt %
) 3.
01
5.86
3.
55
5.83
2.
03
6.5
0.19
4.
52
0.96
2.
62F
eO (w
t %)
3.4
3.9
6 5.
9 3.
3 4.
9 3.
5 7.
3 2.
1 3.
3M
nO (w
t %)
0.10
6 0.
051
0.10
1 0.
146
0.15
3 0.
063
0.04
4 0.
085
0.09
2 0.
126
MgO
(wt %
) 1.
68
4.13
4.
45
5.55
3.
16
5.83
1.
54
5.38
3.
5 4.
01C
aO (w
t %)
5.67
8.
04
8.46
4.
12
6 1.
84
4.77
1.
22
10.4
6 8.
02N
a 2O
(wt %
) 4.
27
4.05
3.
85
3.94
4.
26
6.13
4.
03
2.08
4.
25
4.12
K2O
(wt %
) 3.
66
1.1
1.17
2.
29
1.15
0.
71
1.25
8.
12
1.49
1.
19Ti
O2 (
wt %
) 0.
84
1.10
4 1.
123
0.82
2 0.
478
0.78
3 0.
351
0.76
8 0.
854
0.74
6P 2
O5 (
wt %
) 0.
45
0.45
0.
42
0.18
0.
12
0.15
0.
13
0.17
0.
32
0.14
LO
I (w
t %)
0.76
1.
07
0.62
2.
29
1.98
3.
35
1.2
1.5
2.43
2.
09L
OI
2 (w
t %)
0.38
0.
63
-0.0
6 1.
63
1.61
2.
8 0.
81
0.68
2.
19
1.72
Tota
l (w
t %)
100.
9 10
0.6
99.8
99
.72
98.8
99
.02
99.1
3 99
.42
99.0
3 10
0.3
Tota
l 2 (w
t %)
100.
6 10
0.2
99.1
2 99
.06
98.4
3 98
.47
98.7
4 98
.61
98.8
99
.95
Fe 2
O3(
T) (
wt %
) 6.
79
10.1
9 10
.22
12.3
9 5.
7 11
.95
4.08
12
.64
3.29
6.
29
Cs
(ppm
) 1.
7 0.
6 0.
7 1.
5 1.
4 0.
6 0.
7 2.
8 0.
3 0.
4T
l (pp
m)
0.05
<0
.05
<0.0
5 0.
08
<0.0
5 <0
.05
0.06
0.
33
<0.0
5 0.
05R
b (p
pm)
90
28
31
48
26
16
26
178
44
31B
a (p
pm)
776
263
346
269
352
67
341
3,13
7 13
4 14
7T
h (p
pm)
7.47
4.
95
4.76
4.
53
0.59
0.
83
2.31
2.
47
4.06
2.
39U
(ppm
) 2.
2 1.
41
1.65
1.
23
0.23
0.
28
0.36
1.
31
0.59
0.
78N
b (p
pm)
7.4
4 4.
1 3.
2 1.
7 1.
6 3.
1 2.
8 5.
5 2.
5Ta
(ppm
) 0.
57
0.29
0.
3 0.
27
0.16
0.
13
0.41
0.
2 0.
31
0.23
La
(ppm
) 33
25
.5
22.4
15
.5
7.71
7.
89
9.72
9.
88
6.37
12
.6C
e (p
pm)
72.6
57
.8
51
36.8
16
.9
18
19.2
23
.3
28
30.2
Pb (p
pm)
6 <5
<5
<5
<5
<5
<5
<5
<5
<5
Pr (p
pm)
9.47
7.
71
6.99
4.
94
2.17
2.
6 2.
16
3.15
5.
24
4.09
Sr (p
pm)
500
561
503
236
713
77
460
209
409
409
Nd
(ppm
) 39
.5
33
30.2
20
9.
29
12.3
7.
83
13.8
26
.9
16.6
Zr (p
pm)
100
124
130
135
75
66
77
125
96
116
Hf (
ppm
) 3
3.3
3.5
3.7
1.9
2 2.
2 3.
3 2.
8 3.
1Sm
(ppm
) 8.
81
7.4
7.06
4.
6 2.
27
3.17
1.
28
3.78
8.
07
3.92
Eu
(ppm
) 2.
05
1.69
1.
86
1.18
0.
852
0.92
3 0.
628
0.95
3 1.
66
1.16
Sb (p
pm)
0.6
0.7
0.8
0.7
0.6
0.5
<0.1
0.
2 0.
6 <0
.1G
d (p
pm)
6.78
6.
38
6.2
4.25
2.
37
3.06
1.
07
3.72
7.
93
3.39
Tb
(ppm
) 1.
03
0.98
0.
94
0.63
0.
37
0.48
0.
15
0.67
1.
28
0.51
Dy
(ppm
) 5.
63
5.52
5.
32
3.88
2.
33
2.98
0.
89
4.14
7.
79
3.07
Y (p
pm)
30
27
26
19
14
14
7 20
40
17
Ho
(ppm
) 1.
08
1.06
1.
06
0.79
0.
46
0.6
0.17
0.
84
1.46
0.
59E
r (p
pm)
3.06
3.
08
2.89
2.
24
1.38
1.
69
0.46
2.
39
4.14
1.
86T
m (p
pm)
0.44
1 0.
452
0.41
1 0.
326
0.22
4 0.
249
0.06
8 0.
352
0.64
5 0.
281
Yb (p
pm)
2.96
2.
91
2.77
2.
26
1.59
1.
67
0.45
2.
44
4.08
1.
92L
u (p
pm)
0.48
0.
449
0.46
7 0.
362
0.25
9 0.
262
0.07
3 0.
39
0.56
9 0.
308
Cr
(ppm
) 16
.1
32.7
26
18
3 26
.2
76.8
35
.7
57
26.6
39
.2N
i (pp
m)
6 14
12
42
14
32
5
33
12
15Sc
(ppm
) 15
.2
29.4
33
25
.4
13
29.1
6.
84
22.3
23
.7
24.4
V (p
pm)
127
316
315
201
117
233
59
83
171
213
Co
(ppm
) 14
.6
21.3
22
.5
28.2
15
.3
32.7
20
.2
37.9
8.
9 17
.8C
u (p
pm)
154
21
125
886
90
16
1,13
0 45
2 76
18
304 RICHARDS ET AL.Ta
ble
3. (
Con
t.)
Sam
ple
no.
CA
S-10
C
AS-
11
MV-
1 M
V-5
MV-
10
MV-
11
MV-
12
DA
-1
DA
-2
DA
-3
Man
tove
rde-
M
anto
verd
e-
Dos
Am
igos
- D
os A
mig
os-
Loc
ality
C
asua
lidad
C
asua
lidad
L
aura
L
aura
M
anto
verd
e M
anto
verd
e M
anto
verd
e D
os A
mig
os
Tric
olor
Tr
icol
or
2° K
-fel
dspa
r Po
tass
ic
Pota
ssic
(K-f
elds
par,
Pota
ssic
(som
e Po
tass
ic +
min
or
Pota
ssic
+
Alte
ratio
n/
Wea
k W
eak
prop
yliti
c w
ith c
hlor
ite
(K-f
elds
par,
biot
ite, c
hlor
ite) +
Pota
ssic
(som
e w
eath
erin
g to
ch
alco
pyri
te-
chal
copy
rite
-m
iner
aliz
atio
n ch
lori
te
(chl
orite
-epi
dote
) ov
erpr
int
biot
ite, e
pido
te)
min
or c
halc
opyr
ite
Pota
ssic
w
eath
erin
g)
clay
on
frac
ture
s)
pyri
te
pyri
teTA
S
Bas
altic
trac
hy-
Bas
altic
trac
hy-
clas
sific
atio
n1
Gra
nodi
orite
D
iori
te
Mon
zodi
orite
M
onzo
dior
ite
Syen
ite
ande
site
(sho
shon
ite)
ande
site
(sho
shon
ite)
Gra
nodi
orite
G
rano
dior
ite
Gra
nodi
orite
SiO
2 (w
t %)
61.6
4 59
.77
46.5
4 50
.79
57.3
9 51
.21
49.4
1 64
.7
65.9
2 66
.5A
l 2O3 (
wt %
) 14
.94
15.4
6 15
.14
16.8
16
.64
14.7
5 16
.08
13.9
1 14
.88
14.6
9F
e 2O
3 (w
t %)
2.22
2.
12
1.29
2.
92
2.37
8.
54
6.3
4.74
2.
18
2.15
FeO
(wt %
) 3.
7 4.
4 4.
7 5.
9 4.
1 2.
9 2.
9 4.
3 3.
2 4
MnO
(wt %
) 0.
096
0.14
6 0.
647
0.27
9 0.
061
0.16
4 0.
411
0.06
5 0.
109
0.12
4M
gO (w
t %)
2.44
2.
94
4.65
5.
25
3.06
3.
23
3.27
1.
76
1.55
1.
5C
aO (w
t %)
4.68
5.
54
9.42
6.
67
0.6
4.65
5.
2 1.
7 3.
02
3.04
Na 2
O (w
t %)
3.14
2.
96
3.2
3.74
0.
17
1.18
3.
7 3.
88
3.9
3.57
K2O
(wt %
) 2.
97
2.76
2.
17
1.85
9.
81
4.05
4.
13
1.83
1.
48
1.94
TiO
2 (w
t %)
0.81
9 0.
801
0.67
8 0.
787
1.19
6 1.
001
1.02
3 0.
401
0.36
8 0.
351
P 2O
5 (w
t %)
0.19
0.
19
0.1
0.13
0.
04
0.18
0.
24
0.09
0.
12
0.16
LO
I (w
t %)
1.42
1.
33
9.96
2.
99
2.91
6.
76
5.86
1.
19
1.41
1.
39L
OI
2 (w
t %)
1 0.
84
9.43
2.
33
2.45
6.
43
5.54
0.
71
1.05
0.
94To
tal (
wt %
) 98
.67
98.9
99
.03
98.7
6 98
.81
98.9
4 98
.86
99.0
5 98
.51
99.8
6To
tal 2
(wt %
) 98
.26
98.4
1 98
.51
98.1
98
.36
98.6
2 98
.53
98.5
7 98
.15
99.4
1F
e 2O
3(T
) (w
t %)
6.34
7.
01
6.52
9.
48
6.93
11
.77
9.52
9.
52
5.74
6.
6
Cs
(ppm
) 1.
8 1.
7 1.
8 0.
5 0.
7 1
1.2
2.1
0.6
0.5
Tl (
ppm
) 0.
11
0.2
<0.0
5 <0
.05
0.12
<0
.05
0.13
0.
33
0.1
0.09
Rb
(ppm
) 97
10
1 37
36
16
2 93
64
40
24
30
Ba
(ppm
) 42
7 36
8 2,
230
801
1,44
0 37
7 2,
753
303
654
615
Th
(ppm
) 13
.3
11.6
2.
33
3.25
3.
14
2.58
4.
16
2.66
3.
97
3.94
U (p
pm)
3.62
3.
38
0.65
0.
93
1.08
0.
92
1.22
0.
3 0.
98
0.85
Nb
(ppm
) 5.
9 5.
2 2
2.9
7.1
4 5.
5 3.
6 3.
3 3.
3Ta
(ppm
) 0.
56
0.44
0.
16
0.22
0.
54
0.3
0.38
0.
3 0.
36
0.35
La
(ppm
) 22
.1
20
6.32
12
.9
3.43
6.
87
14.7
5.
81
13.4
10
.8C
e (p
pm)
50.4
45
.4
16.1
26
.5
6.76
16
.4
34
12.3
26
.9
21.6
Pb (p
pm)
<5
8 <5
7
<5
<5
<5
7 <5
<5
Pr (p
pm)
6.53
5.
92
2.34
3.
57
0.81
2.
16
4.53
1.
68
3.13
2.
64Sr
(ppm
) 29
7 32
3 41
7 49
0 63
50
22
4 26
9 32
9 32
1N
d (p
pm)
26.9
24
.2
10.7
14
.5
3.55
9.
89
19.8
7.
18
12.7
10
.3Zr
(ppm
) 25
9 20
3 62
81
10
8 13
2 12
4 62
69
64
Hf (
ppm
) 6.
8 5.
6 1.
8 2.
2 3.
1 3.
4 3.
2 1.
8 2.
1 1.
9Sm
(ppm
) 6.
45
5.64
2.
69
3.44
0.
87
2.4
4.7
1.8
2.75
2.
31E
u (p
pm)
1.09
1.
16
0.88
9 1.
07
0.22
9 0.
636
1.46
0.
567
0.86
8 0.
753
Sb (p
pm)
1.4
1.8
5.3
1.3
0.8
1.9
1.3
0.5
0.7
1G
d (p
pm)
5.57
5.
26
2.57
3.
36
0.95
2.
82
4.51
1.
83
2.21
2.
03T
b (p
pm)
0.92
0.
87
0.43
0.
52
0.19
0.
51
0.71
0.
31
0.38
0.
34D
y (p
pm)
5.57
5.
2 2.
57
3.03
1.
25
3.52
4.
38
1.92
2.
46
2.14
Y (p
pm)
29
27
13
14
8 18
21
11
16
14
Ho
(ppm
) 1.
12
1.05
0.
5 0.
6 0.
27
0.8
0.86
0.
4 0.
52
0.45
Er
(ppm
) 3.
24
3.04
1.
34
1.68
0.
88
2.51
2.
43
1.2
1.61
1.
37T
m (p
pm)
0.47
7 0.
455
0.19
2 0.
247
0.14
4 0.
385
0.36
5 0.
192
0.26
1 0.
218
Yb (p
pm)
3.23
3
1.27
1.
64
1 2.
61
2.44
1.
35
1.85
1.
55L
u (p
pm)
0.51
6 0.
473
0.20
1 0.
256
0.16
8 0.
415
0.37
5 0.
229
0.31
0.
268
Cr
(ppm
) 32
.1
43.6
63
.3
54.3
36
.3
94.7
59
.7
34.4
13
.4
23N
i (pp
m)
8 11
22
26
24
36
15
9
3 3
Sc (p
pm)
20
23.4
27
.8
27.7
22
.7
24.6
27
.9
8.78
7.
32
7.01
V (p
pm)
147
169
198
228
143
209
261
92
74
68C
o (p
pm)
14.8
20
.5
25.7
28
.4
29
15.4
22
.3
5 7.
5 10
.3C
u (p
pm)
111
122
12
122
214
14
230
3,19
0 58
0 1,
580
LO
I =
loss
on
igni
tion
1 Cla
ssifi
catio
n sc
hem
es o
f Le
Mai
tre
et a
l. (2
002)
for
extr
usiv
e ro
cks
and
Mid
dlem
ost (
1994
) for
plu
toni
c ro
cks
TECTONIC SETTINGS AND S CONTENTS OF MAGMAS ASSOCIATED WITH CRETACEOUS DEPOSITS, CHILE 305
Fig. 3. Total alkali-silica diagram (Le Maitre et al., 2002) showing the compositions of Cretaceous igneous rocks associated with porphyry (blue symbols) and IOCG deposits (red symbols) in the Coastal Cordillera of northern Chile; Productora (purple symbol) shows characteristics of both porphyry and IOCG deposits. Data are from Table 3, excluding the following samples for reasons of extreme alteration or lack of correlation with the main suites: CAS-4 (clasts in igneous breccia); CAS-8, 9, 10, 11 (regional plutons, relationship to Casualidad deposit unclear); CDA-7 (Quebrada Marquesa Formation, andesitic country rock); MV-10 (strong potassic alteration); MV-11, MV-12 (altered volcanic rock outcrops, relationship to Mantoverde deposit unclear); PR-1 (Cachiyuyito stock, possibly unrelated). All of the remaining samples shown are nevertheless altered to varying degrees, and we attribute the apparently alkaline compositions of some rocks to this effect.
Rhyolite/Granite
Trachyandesite/Syenite
Rhyodacite, dacite/Granodiorite
Andesite/Diorite
Andesite, basalt/Diorite, gabbro
Sub-alkaline basalt/Sub-alkaline gabbro
Alkali basalt/Alkali gabbro
Trachyte
Zr/T
i
Nb/Y0.001
0.01
0.1
1
0.01 0.1 1 10
early Cretaceousmid-Cretaceouslate CretaceousIOCGPorphyry
Fig. 4. Zr/Ti vs. Nb/Y discrimination diagram (Winchester and Floyd, 1977) for Cretaceous igneous rocks from the Coastal Cordillera of Chile between 25° and 34°S. Regional igneous geochemical data from the literature are grouped according to the three temporal-tectonomagmatic groups, as defined in the text. Igneous rocks related to mid-Cretaceous IOCG and late Cretaceous porphyry deposits collected during this study are plotted for comparison. Regional Cretaceous data from Irwin et al. (1988), Vergara et al. (1995), Cisternas et al. (1999), Parada et al. (1999, 2002), Grocott and Taylor (2002), Morata and Aguirre (2003), Creixell (2007), Hasler (2007), and López et al. (2014).
306 RICHARDS ET AL.
(LILEs: Rb, Ba, Th, U, K) and light rare earth elements (LREEs); negative anomalies for Nb, Ta, and Ti; relative depletions in compatible elements and middle to heavy rare earth elements (MREEs, HREEs); and flat to listric-shaped patterns from MREEs to HREEs. Such patterns are typical
of subduction-related igneous rocks and reflect enrichments in fluid-mobile LILEs, retention of Nb, Ta, and Ti in insoluble Fe-Ti oxides, and fractionation of amphibole (which preferen-tially partitions MREEs; Gill, 1981; Green and Pearson, 1985; Klein et al., 1997). The only noticeable difference between
Fig. 5. Primitive mantle-normalized extended trace element diagrams (normalization values of Sun and McDonough, 1989) for selected igneous rocks associated with (a) porphyry and (b) IOCG deposits in the Coastal Cordillera of northern Chile.
TECTONIC SETTINGS AND S CONTENTS OF MAGMAS ASSOCIATED WITH CRETACEOUS DEPOSITS, CHILE 307
these suites is the slightly greater depletion of MREEs-HREEs in the porphyry-related suite, which can be attributed to the more felsic (fractionated) nature of these rocks. This characteristic is also observed in the ratios of Sr/Y and La/Yb, which are generally more elevated in the felsic porphyry suite compared to the more mafic IOCG suite (Fig. 7).
In an attempt to assess the relative oxidation states of the various suites of rocks, we have assessed whole-rock Fe2O3/FeO ratios where reported. While many of these samples are altered to varying degrees, which will clearly affect the Fe2O3/FeO ratio, most of the samples have values ranging from 0.18 to 5.10, corresponding to moderately or strongly oxidized
Fig. 6. C1 chondrite-normalized extended trace element diagrams (normalization values of Sun and McDonough, 1989) for selected igneous rocks associated with (a) porphyry and (b) IOCG deposits in the Coastal Cordillera of northern Chile.
308 RICHARDS ET AL.
rocks (using the criteria of Blevin, 2004). Importantly, there is no clear difference in oxidation state between the three temporal-tectonomagmatic groups and porphyry- and IOCG-related rocks, which all appear to be similarly oxidized.
Published Sr-Nd isotope compositions for 53 Cretaceous igneous rocks from the Coastal Cordillera between 25° and 34°S (Supplementary Table S3) are plotted in Figure 8. Early Cretaceous rocks have evolved isotopic compositions
Fig. 7. (a) Sr/Y versus Y and (b) La/Yb versus Yb plots for selected igneous rocks associated with porphyry and IOCG deposits from the Coastal Cordillera of northern Chile. The more “adakite like” compositions of many of the porphyry rocks compared to many of the IOCG-related rocks likely reflect the more felsic compositions and greater degrees of fractionation (especially of amphibole) of the former. Note that, with only one exception, all of these rocks have lower La/Yb ratios than adakites. Fields for adakite-like rocks from Richards and Kerrich (2007); regional Cretaceous data from Irwin et al. (1988), Vergara et al. (1995), Cisternas et al. (1999), Parada et al. (1999, 2002), Grocott and Taylor (2002), Morata and Aguirre (2003), Creixell (2007), Hasler (2007), and López et al. (2014).
TECTONIC SETTINGS AND S CONTENTS OF MAGMAS ASSOCIATED WITH CRETACEOUS DEPOSITS, CHILE 309
suggesting extensive crustal contamination. In contrast, the mid- and later Cretaceous rocks cluster at relatively primitive compositions, albeit still with some crustal contamination.
Apatite CompositionsElectron microprobe analyses of magmatic and hydrothermal apatite from three samples from the Carmen de Andacollo porphyry and seven samples from or near IOCG deposits (Productora, Candelaria, and Casualidad) are listed in Supple-mentary Table S4, and SO3 analyses of magmatic apatite are summarized in Table 4 (along with data from the literature). The results show a clear difference between igneous apatite from rocks associated with the Carmen de Andacollo porphyry deposit (0.25 ± 0.17 wt % SO3, n = 69) compared with those from IOCG deposits (0.04 ± 0.02 wt % SO3, n = 76). A simi-lar relationship is observed for apatite from porphyry deposits (0.12–0.60 wt % SO3) and IOCG deposits (0.07–0.13 wt % SO3) reported in the literature (Table 4). Hydrothermal or late-stage igneous apatite (e.g., edges of apatite micropheno-crysts; Fig. 2d) showed lower and more variable SO3 contents (Supplementary Table S4), consistent with observations else-where in the literature (Streck and Dilles, 1998; Van Hoose et al., 2013).
The SO3 content of apatite varies as a complex function of magmatic temperature, oxidation state, and sulfur fugac-ity (Peng et al., 1997; Parat and Holtz, 2005; Parat et al., 2011), and an accurate calculation of magmatic sulfur con-tent from apatite SO3 compositions is not currently possible.
However, the apatite-melt partition coefficient formula of Peng et al. (1997), which is derived for relatively oxidized arc magmas, can be used to obtain a semiquantitative esti-mate of magmatic S content. We have estimated the apatite saturation temperature of four samples from the Carmen de Andacollo porphyry, the Candelaria and Casualidad IOCG deposits, and a sample of regional mid-Cretaceous igneous rock near Productora (using the equation of Piccoli and Can-dela, 1994, 2002, which is derived from the data of Harrison and Watson, 1984) and used this temperature in the equa-tions of Peng et al. (1997) and Parat et al. (2011) to derive estimates of magmatic S content. The data reported in Table 5 suggest that the average S content of the magma associ-ated with the Carmen de Andacollo porphyry deposit was ~0.02 wt % S (up to 0.06 wt % S)—significantly higher than the average values for magma associated with IOCG depos-its in the region (0.001–0.005 wt % S; Table 5). An alterna-tive method for calculating magmatic sulfur content from apatite SO3 compositions is provided by Parat et al. (2011), and these values are also listed in Table 5. The results differ somewhat in absolute values compared to the results using the Peng et al. (1997) formula, but not in the relative enrich-ment in S of the porphyry-related magmas compared to the IOCG-related magmas.
In order to eliminate the possibility that contrasting mag-matic oxidation state was responsible for this difference in apatite SO3 content, we have estimated magmatic fO2 values for these samples from the average MnO contents of apatite
ε Nd
(87Sr/86Sr)i
Jurassic igneous rocks
Crustal contamination trend
Depleted MORB mantle
Late Paleozoicintrusive rocks
-5
0
5
10
0.702 0.703 0.704 0.705 0.706 0.707 0.708 0.709 0.710
early Cretaceous
mid-Cretaceous
late Cretaceous
Fig. 8. εNd vs. initial 87Sr/86Sr ratios for Cretaceous igneous rocks from the Coastal Cordillera of Chile between 25° and 34°S. Mid-Cretaceous rocks associated with IOCG deposits mostly fall at the primitive end of a range of data reflecting various degrees of crustal contamination of mantle-derived magmas, whereas early and late Cretaceous rocks associated with porphyry deposits range to significantly more isotopically evolved compositions. Data from Parada et al. (2005), Hasler (2007), Morata et al. (2008), and Girardi (2014). Depleted MORB mantle field at ~110 Ma from Pilet et al. (2011). Jurassic igneous rock field from Parada et al. (1999), Creixell (2007), and Girardi (2014). Paleozoic intrusive rock field from Parada et al. (1999).
310 RICHARDS ET AL.
using the equation of Miles et al. (2014, Table 5). These data confirm that all of the rocks are moderately oxidized (DFMQ ≈ 0.4–2.7), although we note that this fO2 calculation relies on electron microprobe analyses of Mn in apatite at concentra-tions close to the detection limit, and so cannot be considered highly accurate. There is also debate about the applicability of this method to rocks of different magmatic temperature and composition (Marks et al., 2016; Miles et al., 2016), although the samples compared here are of broadly similar composi-tion and likely temperature. Consequently, we consider that the formula of Peng et al. (1997), derived for oxidized arc rocks, should be approximately valid, and the calculated abun-dances of magmatic S should be relatively correct, even if the absolute values are questionable (cf. Streck and Dilles, 1998). These data support the hypothesis that the porphyry-forming magmas were richer in S than the IOCG-related magmas.
Discussion
Igneous geochemistry
Whole-rock geochemical compositions of Cretaceous igneous rocks from the Coastal Cordillera of Chile between 25° and 34°S show minimal differences beyond those expected from normal fractionation processes (Figs. 3–5). Rocks associated with early Cretaceous arc rifting (with minor porphyry Cu-Au deposits), mid-Cretaceous back-arc transtension (with IOCG deposits), and later Cretaceous contractional arc magmatism (with porphyry Cu-Au-(Mo) deposits) have compositions typi-cal of subduction-related magmatism. On average, however, later Cretaceous arc rocks are more felsic (Fig. 3), with pro-nounced listric-shaped REE patterns (Fig. 6) and higher Sr/Y and La/Yb ratios compared to mid-Cretaceous rocks associ-ated with IOCG deposits (Fig. 7). There is also a suggestion that some of the mid-Cretaceous magmas and those associ-ated with IOCG deposits may have had slightly more mafic, alkaline compositions (alkali gabbros to syenites).
We interpret these data to indicate that there was little fun-damental change in the source of magmatism throughout the Cretaceous, except for a hint of more mafic alkaline magma-tism during the mid-Cretaceous transtensional rifting phase. Published Sr and Nd isotope compositions of Cretaceous rocks from the Coastal Cordillera indicate mixing between depleted, mantle-derived magmas and isotopically evolved continen-tal crust, with early Cretaceous magmas showing the highest degrees of contamination, likely by late Paleozoic lower and mid-crustal rocks (Fig. 8, Damm et al., 1990; Lucassen et al., 1999, 2001, 2002). Although the isotopic ranges of mid- and late Cretaceous rocks overlap, there is a significant clustering of mid-Cretaceous data at the most primitive end of the range (εNd ≈ 6, 87Sr/86Sri ≈ 0.7033). We suggest that back-arc exten-sional tectonics in the mid-Cretaceous may have facilitated the greater involvement of primitive, mildly alkaline magmas derived from partial melting of upwelling mantle astheno-sphere during crustal extension and thinning (Fig. 9). Never-theless, a significant amount of crustal processing is evident for all of these Cretaceous magmas, as is commonly observed in continental arc rocks (Hildreth and Moorbath, 1988). The more felsic nature and higher Sr/Y and La/Yb ratios of the later Cretaceous porphyry-associated magmas (Fig. 7) may reflect longer residence times in deep crustal magma chambers under contractional tectonic conditions (Hildreth and Moorbath, 1988; Haschke et al., 2002; Richards and Kerrich, 2007), with more extensive fractionation of amphibole (Lang and Titley, 1998; Richards and Kerrich, 2007; Richards, 2011a).
We co nclude that it is not possible uniquely to distinguish between igneous rocks associated with porphyry and IOCG deposits in the Coastal Cordillera of Chile using their whole-rock geochemical or isotopic compositions alone, although the IOCG-associated rocks are, on average, more mafic (locally alkaline) and isotopically slightly more primitive than the por-phyry-related rocks, consistent with their back-arc as opposed to main arc tectonomagmatic setting.
Table 4. Summary of SO3 Compositions of Igneous Apatite from Chilean and Other Porphyry and IOCG Deposits
SO3 (wt %)
Locality Deposit type Minimum Maximum Average s.d. n Source
Carmen de Andacollo Porphyry Cu-Au 0.05 0.81 0.25 0.17 69 This studyProductora Transitional IOCG 0.02 0.07 0.04 0.02 14 This studyCandelaria IOCG 0.02 0.09 0.03 0.02 32 This studyCasualidad IOCG 0.02 0.08 0.03 0.02 30 This studyYerington Porphyry Cu 0.26 0.99 0.60 0.36 4 Streck and Dilles (1998)Santo Tomas II (Philex) Porphyry Cu n.a. 0.45 0.30 0.07 n.a. Imai (2002)Santo Tomas II (Philex) Porphyry Cu n.a. 0.62 0.25 0.07 n.a. Imai (2002)Clifton Porphyry Cu n.a. 0.76 0.29 0.14 n.a. Imai (2002)Clifton Porphyry Cu n.a. 0.48 0.27 0.08 n.a. Imai (2002)Bumolo (Waterhole) Porphyry Cu n.a. 0.59 0.21 0.15 n.a. Imai (2002)Camp 6 (Black Mountain) Porphyry Cu n.a. 0.48 0.12 0.09 n.a. Imai (2002)Dizon Porphyry Cu n.a. 0.57 0.17 0.08 n.a. Imai (2002)Taysan Porphyry Cu n.a. 0.55 0.20 0.11 n.a. Imai (2002)Duolong Porphyry Cu-Au 0.44 0.82 n.a. n.a. n.a. Li et al. (2012)Mt. Isa IOCG n.a. n.a. 0.13 0.09 n.a. Piccoli and Candela (2002)Mt. Isa IOCG n.a. n.a. 0.07 n.a. n.a. Belousova et al. (2002)Tjårrojåkka IOCG 0.02 0.13 0.08 0.03 n.a. Edfelt (2007)Se-Chahun IOCG (IOA) <0.03 0.11 n.a. n.a. n.a. Bonyadi et al. (2011)
n.a. = data not available, s.d. = standard deviation
TECTONIC SETTINGS AND S CONTENTS OF MAGMAS ASSOCIATED WITH CRETACEOUS DEPOSITS, CHILE 311
Tabl
e 5.
Est
imat
es o
f Mag
mat
ic T
empe
ratu
re a
nd O
xida
tion
Stat
e fr
om I
gneo
us A
patit
e an
d W
hole
-Roc
k C
ompo
sitio
ns
Apa
tite
SO3
Who
le-
Who
le-
A
patit
e
(w
t %) (
n)3
Ave
rage
A
vera
ge
Max
imum
M
axim
umD
epos
it Sa
mpl
e ro
ck S
iO2
rock
P2O
5
MnO
E
stim
ated
E
stim
ated
[m
axim
um,
mag
mat
ic S
m
agm
atic
S
mag
mat
ic S
m
agm
atic
S (t
ype)
no
. (w
t %)1
(w
t %)1
A
ST2
(wt %
) (n)
3 lo
g f O
24 D
FM
Q5
min
imum
] co
nten
t (w
t %)6
co
nten
t (w
t %)7
co
nten
t (w
t %)6
co
nten
t (w
t %)7
Car
men
de
C
DA
-2
66.7
4 0.
14
905°
C
n.a.
n.
a.
n.a.
0.
23 ±
0.1
8 (3
4)
0.01
60 ±
0.
0088
±
0.05
71
0.13
08A
ndac
ollo
[0.0
5, 0
.81]
0.
0120
0.
0230
(por
phyr
y)
CD
A-8
8 63
.67
0.21
91
7°C
0.
12 ±
0.0
4 (1
8)
–11.
8 ±
1.5
0.4
± 1.
7 0.
27 ±
0.1
6 (3
5)
0.02
30 ±
0.
0079
±
0.05
13
0.03
64
[0
.08,
0.6
1]
0.01
37
0.01
02
Prod
ucto
ra
PR-5
64
.95
0.13
87
8°C
n.
a.
n.a.
n.
a.
0.04
(1)
0.00
17
0.00
10
0.00
17
0.00
10(t
rans
ition
al
PR-8
57
.66
0.19
83
0°C
0.
08 ±
0.0
1 (8
) –1
1.1
± 0.
9 2.
7 ±
1.1
0.04
± 0
.02
(13)
0.
0009
±
0.00
10 ±
0.
0015
0.
0012
IOC
G)
[0
.02,
0.0
7]
0.00
03
0.00
01
Can
dela
ria
C
AN
-2
58.4
5 0.
45
951°
C
n.a.
n.
a.
n.a.
0.
04 ±
0.0
2 (3
) 0.
0051
±
0.00
10 ±
0.
0077
0.
0011
(IO
CG
)
[0.0
3, 0
.06]
0.
0022
0.
0001
C
AN
-3
53.3
6 0.
45
880°
C
0.12
± 0
.06
(6)
–11.
8 ±
1.9
1.1
± 2.
1 0.
03 ±
0.0
2 (2
9)
0.00
17 ±
0.
0010
±
0.00
43
0.00
13
[0
.02,
0.0
9]
0.00
07
0.00
01
Cas
ualid
ad
CA
S-4
66.6
7 0.
13
897°
C
n.a.
n.
a.
n.a.
0.
06 ±
0.0
3(3)
0.
0030
±
0.00
11 ±
0.
0050
0.
0013
(IO
CG
)
[0.0
2, 0
.08
] 0.
0015
0.
0002
C
AS-
10
63.6
5 0.
20
911°
C
n.a.
n.
a.
n.a.
0.
04 ±
0.0
2(4)
0.
0035
±
0.00
10 ±
0.
0046
0.
0011
[0.0
2, 0
.06]
0.
0020
0.
0001
C
AS-
11
61.5
6 0.
20
886°
C
0.08
± 0
.03
(10)
–1
1.0
± 1.
2 1.
7 ±
1.4
0.03
± 0
.01(
23)
0.00
16 ±
0.
0009
±
0.00
34
0.00
12
[0
.02,
0.0
7]
0.00
06
0.00
007
n.a.
= n
o da
ta a
vaila
ble
1 Nor
mal
ized
to 1
00 w
t % (S
uppl
emen
tary
Tab
le S
1)2 A
patit
e sa
tura
tion
tem
pera
ture
(AST
) cal
cula
ted
from
who
le-r
ock
SiO
2 and
P2O
5 con
cent
ratio
ns u
sing
the
equa
tion
of P
icco
li an
d C
ande
la (1
994)
, rec
ast f
rom
the
data
of H
arri
son
and
Wat
son
(198
4)3 A
vera
ge o
f all
igne
ous
apat
ite a
naly
ses
(Sup
plem
enta
ry T
able
S4)
4 Ave
rage
log
f O2 c
alcu
late
d fr
om M
nO c
onte
nts
of ig
neou
s ap
atite
cry
stal
s (S
uppl
emen
tary
Tab
le S
4) u
sing
the
equa
tion
of M
iles
et a
l. (2
014)
:
log
f O2 =
–0.
0022
(± 0
.000
3) M
n (p
pm) –
9.7
5 (±
0.4
6)5 A
vera
ge lo
g f O
2 val
ue r
elat
ive
to th
e fa
yalit
e-m
agne
tite-
quar
tz (Δ
FM
Q),
whe
re th
e va
lue
of F
MQ
at d
iffer
ent t
empe
ratu
res
(pro
vide
d by
the
AST
est
imat
e) is
cal
cula
ted
usin
g th
e eq
uatio
n of
Mye
rs
and
Eug
ster
(198
3): l
og f O
2 = –
24,4
41.9
/T (K
) + 8
.290
(± 0
.167
); th
e ΔF
MQ
err
or is
the
sum
of t
he ±
0.16
7 er
ror
on F
MQ
and
the
unce
rtai
nty
in th
e sa
mpl
e lo
g f O
2est
imat
e fr
om th
e pr
evio
us c
olum
n6 E
stim
ated
from
apa
tite
SO3 c
onte
nts
(Sup
plem
enta
ry T
able
S4)
usi
ng th
e te
mpe
ratu
re-d
epen
dent
apa
tite-
mel
t par
titio
n co
effic
ient
form
ula
of P
eng
et a
l. (1
997)
:
lnK
D =
21,
130/
T –
16.
2 (w
here
T is
in K
elvi
n)7 E
stim
ated
from
apa
tite
SO3 c
onte
nts
(Sup
plem
enta
ry T
able
S4)
usi
ng th
e ap
atite
-mel
t par
titio
n co
effic
ient
form
ula
of P
arat
et a
l. (2
011)
: SO
3 apa
tite
(wt %
) = 0
.157
× ln
[SO
3] (m
elt,
wt %
) + 0
.983
4 (r
2 =
0.62
)8 S
iO2 a
nd P
2O5 c
ompo
sitio
ns fo
r th
is s
ampl
e ta
ken
from
sam
ple
CD
A-9
(sam
e lit
holo
gy a
s C
DA
-8 b
ut le
ss a
ltere
d)
312 RICHARDS ET AL.
Magmatic sulfur content
Porphyry and IOCG deposits are most fundamentally dif-ferentiated by the much greater abundance of sulfur in the former (predominantly fixed as pyrite in phyllic alteration zones) compared to the latter (where Fe oxides, magnetite and hematite, predominate). We have hypothesized that this might reflect a difference in the chemistry of magmas asso-ciated with these contrasting deposit types, but the analysis presented above shows that no clear differences can be found in their major and trace element compositions, although the IOCG-related rocks are somewhat more mafic than the por-phyry intrusions.
Two other possible explanations are differences in the oxi-dation state or sulfur content of the original magma, but it is
difficult to constrain these parameters due to the widespread effects of oxidation during hydrothermal alteration in ore-associated rocks and the loss of S (as SO2) from igneous rocks as they crystallize. Nevertheless, Fe2O3/FeO ratios of igneous rocks from these different associations show no clear differ-ence, and all are moderately to strongly oxidized (acknowl-edging that many of these rocks are hydrothermally altered, which likely leads to some secondary oxidation). We would expect the oxidation state of the magma to make a difference to the behavior of dissolved sulfur only if the rocks were sig-nificantly reduced (resulting in S being dominantly present as sulfide) compared to oxidized (with S dissolved as sulfate; Carroll and Rutherford, 1985; Jugo et al., 2005). All of the rocks in these suites appear to be at least moderately oxidized, as expected in arc rocks, so S should have behaved in a similar way in the magma.
In order to test the hypothesis that the magmas that formed S-rich porphyry deposits were more S rich than those form-ing S-poor IOCG deposits, we analyzed the compositions of igneous apatite crystals trapped as inclusions in silicate phe-nocryst phases. Our results show that apatite from intrusive rocks associated with the Carmen de Andacollo porphyry Cu-Au deposit contains significantly more S (0.25 ± 0.17 wt % SO3; Table 4) than that from rocks associated with or near IOCG deposits (Productora, Candelaria, and Casualidad; 0.04 ± 0.02 wt % SO3; Table 4). No reliable formula currently exists to convert apatite SO3 compositions to magmatic S con-tents, but the partition coefficients of Peng et al. (1997) can be used to provide a semiquantitative estimate. The results of these calculations suggest that Carmen de Andacollo por-phyry magmas contained ~0.02 wt % S, compared to 0.001 to 0.005 wt % S in the IOCG-associated magmas—an order of magnitude difference (Table 5). This is the opposite to what would be expected if the difference were simply due to the felsic versus more mafic nature of the porphyry- versus IOCG-related rocks, because S solubility decreases with decreasing FeO content of magmas (Wallace and Carmichael, 1992). We conclude that the IOCG-related magmas were significantly S undersaturated compared to the porphyry-forming mag-mas and, consequently, could only generate S-poor magmatic hydrothermal fluids and ore deposits.
Metallogenic implications
The differences in tectonic setting and S content of magmas associated with porphyry and IOCG deposits in the Meso-zoic Coastal Cordillera of Chile suggest contrasting models for their formation (Fig. 9). Tectonomagmatic processes that give rise to magmas with the potential to form porphyry Cu ± Mo ± Au deposits are well established (Richards, 2003; Cooke et al., 2005) and involve the evolution of hydrous, oxidized, S-rich, calc-alkaline magmas derived from partial melting in the metasomatized suprasubduction zone asthenospheric mantle wedge (Fig. 9a, c). A continuous flux of oxidized sulfur and water (and other fluid-mobile components such as Cl and LILEs) from the subducting slab supplies the large amount of S that is typically present in Phanerozoic arc magmas and gives them the potential to form S-rich porphyry deposits. An input of oxidized sulfur is specifically required because reduced sulfur will separate early from the magma as sulfide liquid and will deplete the magma of chalcophile elements
(b) Mid-Cretaceous
Back-arcasthenospheric
upwelling
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lithosphere
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wedge
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(c) Late Cretaceousto Cenozoic
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wedge
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Upper crustalbatholith and IOCG
formation
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Partial-melting in lower crust, amphibolite (black), orhydrated lithospherePartial-melt flow lines
Fluid flow lines
0 km
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Fig. 9. Schematic model illustrating the evolution of the tectonomagmatic setting along the margin of northern Chile between 25° and 34°S during the Cretaceous, and its relationship to porphyry and IOCG deposit formation. SCLM = subcontinental lithospheric mantle.
TECTONIC SETTINGS AND S CONTENTS OF MAGMAS ASSOCIATED WITH CRETACEOUS DEPOSITS, CHILE 313
prior to their potential segregation into late-exsolving mag-matic hydrothermal fluids (Hamlyn et al., 1985; Spooner, 1993; Richards, 1995, 2003).
If this supply of oxidized sulfur ceases (e.g., subduction stops due to arc migration, reversal, or collision) or moves away from the region of magma production (e.g., subduction rollback), then derived magmas will be relatively S poor (Wal-lace and Edmonds, 2011). This may occur in back-arc settings, where slab rollback and back-arc extension result in the gen-eration of magmas from upwelling asthenosphere distal to the subduction zone. These magmas have compositions reflect-ing more primitive intra-plate characteristics, although they retain some arc geochemical features (Kimura and Yoshida, 2006; Caulfield et al., 2012; Timm et al., 2012).
We hypothesize that this may have been the case during the mid-Cretaceous back-arc extensional phase in northern Chile, and that magmas formed at this time had lower magmatic S contents than normal arc magmas due to their distance from the subduction zone. Interaction of these relatively primi-tive, S-undersaturated asthenospheric magmas with previ-ously subduction metasomatized upper plate lithosphere may have resulted in remobilization of metals and volatiles by dissolution of residual Cu-Au–rich sulfides and melting of hydrous silicates from lower crustal arc cumulates (Keays, 1987; Ackerman et al., 2009; Richards, 2009). Derivative mag-mas would be compositionally similar to normal arc magmas (being largely derived from subduction-modified lithosphere) but would only have the potential to form S-poor magmatic-hydrothermal ore deposits upon upper crustal emplacement (Fig. 9b; Core et al., 2006; Richards, 2009; Pettke et al., 2010; Richards and Mumin, 2013a, b). Deposits formed under such conditions would share many traits with normal arc porphy-ries but would be deficient in sulfur and sulfide minerals, and rich instead in Fe oxides. We refer to these as IOCG deposits and note that they share many similarities with magnetite-rich, alkalic-type porphyry Cu-Au deposits formed in back-arc settings (Harris et al., 2013), suggesting a continuum of broadly subduction related magmatic-hydrothermal deposit types, differentiated largely by their sulfur contents and the sulfur contents of the generative magmas (Richards and Mumin, 2013b).
ConclusionsBy studying an area where porphyry and IOCG deposits occur in relatively close spatial and temporal proximity, we have shown that small changes in tectonic conditions can gener-ate magmas with broadly similar bulk compositions but sub-stantially different sulfur contents, such that distinctive ore deposit types are formed. Specifically, porphyry Cu ± Mo ± Au deposits form from relatively S rich arc magmas under conditions of upper plate contraction or transpression (e.g., the late Cretaceous period in Chile) and suboptimally under extension (early Cretaceous; Tosdal and Richards, 2001). In contrast, magmas generated in back-arc (extensional) settings distal to the subduction zone are likely to be relatively S poor and somewhat more mafic (locally alkaline, and isotopically primitive) in composition. Where such magmas interact with previously subduction modified lithosphere during ascent, they may remobilize metals, leading to the potential forma-tion of S-poor ore deposits, such as IOCG and alkalic-type
porphyry Cu-Au deposits. Some overlap in space and time between these deposit styles may result from local mantle and crustal heterogeneities, as well as delayed magmatic responses to tectonic changes. We suggest that other deposit types in the wider IOCG family diverge from this point of intersection with porphyry systems by the greater involvement of external fluids and crustal metal fluxing.
AcknowledgmentsThis paper is based in part on the published findings of numerous researchers, whose work we hope we have appro-priately cited. This research was funded by a Discovery Grant (RGPIN203099) and an Engage Grant (EGP 485693-15) from the Natural Sciences and Engineering Research Council of Canada to JPR, and a postdoctoral fellowship from the University of Alberta to GPL. Claire Chamberlain and Andrew Davies from Teck Resources Ltd. are particu-larly thanked for their support of the Engage Grant and for access to and accommodation at the Carmen de Andacollo mine. We also thank all the geologists who helped us at the mine or project sites, particularly Andres Castillo from Teck Resources Ltd., Cristian Vasquez from Hot Chili Limited, Osvaldo Beltran from Pucobre, Cristian Leal from the Dos Amigos Mine, Bernardino Garay and Orlando Rivera from Codelco Chile, and Jose Armando Rodriguez from Anglo American. Special thanks go to Brian Townley for lending us regional maps. Finally, we are also grateful to managers Jorge Skarmeta (Codelco Chile), Vicente Irarrazabal (Anglo American), Rodrigo Arce (Anglo American), Wilfredo Tabilo and Marcelo Bruna (Pucobre), Rodrigo Diaz (Hot Chili), and Melanie Leighton (Hot Chili) for giving permission to access their sites and to sample drill core. Jared Geiger is thanked for his help with sample preparation at the University of Alberta. Garry Davidson is thanked for his careful review of an ear-lier version of the manuscript, and Murray Hitzman and John Dilles are thanked for their editorial contributions, which have helped to improve its quality.
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Tectonostratigraphic time boundaries in Figure 1b (gray lines) were defined based on correlation of similar formations and their interpreted tectonic settings, as follows:
i. Punta del Cobre, Arqueros, and Veta Negra formations (Charrier et al., 2007);
ii. Contact between Punta del Cobre and Bandurrias forma-tions (Marschik and Fontboté, 2001);
iii. Lower member of Cerrillos Formation (Maksaev et al., 2009);
iv. Ka1/Ka2 boundary, Arqueros Formation (Morata et al., 2008);
v. Quebrada Marquesa/Viñita formations (Emparan and Pineda, 2006);
vi. Arqueros Formation (Ferrando et al., 2014);vii. Quebrada Marquesa Formation (López, 2012);viii. Las Chilcas Formation (Wall et al., 1999).
Black lines represent the time boundaries of different tec-tonic settings based on a compilation of kinematics and tim-ing of movement on structures along the Coastal Cordillera, as follows:
1. Atacama fault system at 25.5° to 27°S (Grocott and Tay-lor, 2002);
2. Atacama fault system at 25° to 27°S (Brown et al., 1993; Taylor et al., 1998);
3. Atacama fault system at 26° to 27.5°S (Dallmeyer et al., 1996);
4. Atacama fault system at 29°S (Arévalo et al., 2003); 5. Dos Amigos fault system (Almonacid, 2007); 6. Bahia Agua Dulce shear zone at 32°S (Ring et al., 2012); 7. El Romeral fault at 30°S (Emparan and Pineda, 2006); 8. Silla del Gobernador fault at 32°S (Arancibia, 2004); 9. Magnetic fabric in Illapel plutonic complex (Ferrando et
al., 2014);10. Dike swarms at 33°S (Creixell et al., 2011);11. Rapid exhumation of intrusions at 33°S (Gana and
Zentilli, 2000).
APPENDIXSources of Data for Figure 1
