Geology of the Josemaría Porphyry Copper-Gold Deposit, Argentina: Formation, Exhumation, and Burial in Two Million Years

Home , Jorquera (caldera), Jotabeche, Zona Sur

BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

Vol. 114 May No. 3

Richard H. Sillitoe,1 Fionnuala A.M. Devine,2 Martin I. Sanguinetti,3 and Richard M. Friedman4 1 27 West Hill Park, Highgate Village, London N6 6ND, England 2 178-6th Street, Atlin, British Columbia, Canada V0W 1A0 3 NGEx Resources Inc., Gorriti 4855, CABA, Buenos Aires, Argentina 4 Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

Abstract The Josemaría porphyry copper-gold deposit is located in the Frontal Cordillera of San Juan Province, Argen- tina, near the present-day northern limit of the Chilean-Pampean flat-slab segment of the central Andes, and midway between the Maricunga and El Indio metallogenic belts. The deposit is centered on small, multiphase dacite porphyry intrusions that were emplaced at the contact between rhyolitic volcanic and tonalitic plutonic rocks of Late Permian to Triassic age. The earlier, more intensely quartz ± magnetite-veined porphyry phases and contiguous wall rocks display a telescoped sequence of alteration-mineralization zones, from shallow advanced argillic (mainly quartz-pyrophyllite) and underlying sericitic to deeper chlorite-sericite and minor remnant potassic. All the alteration types are mineralized, but the highest copper and gold grades are present as a low-arsenic, high-sulfidation assemblage in the quartz-pyrophyllite and sericitic zones. The outermost parts of the copper-gold zone are overlapped by a pronounced molybdenum-bearing annulus. New U-Pb zircon ages show that the deposit was formed at ~25 to 24.5 Ma, partially unroofed during continued NNE-striking, high-angle reverse faulting, and then unconformably overlain by red-bed conglomerate and sandstone capped by andesitic and dacitic tuff and lava. The andesite reported an age of ~22.35 Ma. A second, discrete pulse of currently undated, advanced argillic alteration, accompanied by minor high-sulfidation enargite mineralization, locally affected the southern periphery of the deposit, including its postmineral cover. Fol- lowing erosional removal of the volcano-sedimentary strata from the northern and central parts of the deposit, the NNE-trending fault zone underwent minor normal displacement and localized economically significant supergene chalcocite enrichment. However, probably because of the rapidity of deposit unroofing, supergene processes were barely able to keep pace with erosion, resulting in a thin supergene profile over much of the exposed deposit. The southern part of the deposit remains beneath the postmineral cover and, hence, escaped the enrichment. Josemaría is unusual among the many central Andean porphyry copper deposits formed during rapid uplift because it preserves evidence for not only alteration-mineralization telescoping but also exceptionally rapid postmineral exhumation and subsequent burial beneath thick volcano-sedimentary cover. Unroofing of porphyry copper deposits in 1 to 2 m.y. is more typical of the high erosion rates that characterize pluvial tropical climates than the semiarid conditions that prevailed during and since the formation of Josemaría.

Introduction partially preserved cover sequences (Ramos, 1999). Josemaría Josemaría is a porphyry copper-gold deposit located 4,400 to lies roughly midway between the porphyry and high-sulfidation 4,800 m above sea level in the Frontal Cordillera of San Juan epithermal deposits of the Maricunga and El Indio metallo- Province, Argentina, ~8 km east of the border with Chile (lat genic belts and, along with nearby porphyry copper deposits 28°26'S, long 69°32'W). The Frontal Cordillera—the main and several early-stage prospects, effectively links the two (Pan- Andean mountain range at this latitude—is a fault-bounded teleyev and Cravero, 2001; Mpodozis and Kay, 2003; Jones and massif consisting of late Paleozoic to Triassic basement and Martínez, 2007; Rode and Carrizo, 2007; Fig. 1a). The Josemaría deposit was discovered in 2004 by Desarrollo † Corresponding author: e-mail, [email protected] de Prospectos Mineros S.A. (Deprominsa), the Argentinian

© 2019 Gold Open Access: this paper is published under the terms of the CC-BY license. ©2019 Society of Economic Geologists; Economic Geology, v. 114, no. 3, pp. 407–425 ISSN 0361-0128; doi:10.5382/econgeo.4645; 19 p. Submitted: December 27, 2018 / Accepted: March 17, 2019 Digital appendices are available in the online Supplements section. 407

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408 SILLITOE ET AL. 70° 69° 68°W 69°40’ ( 69°30’ 69°20’ a b lt au f ro ot P Salares Norte El Porphyry 26° S Cu-Au Los Helados prospects 28°20’

Nueva Esperanza n

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s o o Refugio L M Maricunga belt Caspiche T I N Cerro Casale 28°

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Los Helados 28°40’ Filo del Sol JosemariJosemaría 20 km GEOLOGY A R G E N Pliocene - Holocene sediments

29° Valeriano P E R U Mid-late Miocene belt Taguas B O L I V I A 15° volcanic and sedimentary rocks Pascua-Lama Late Oligocene - early Miocene

Veladero S P e volcanic and sedimentary rocks r u

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A MINERAL DEPOSITS 35° 75° 70° W 65° 31 °3 Los Azules 18 - 5 Ma HS epithermal Au ± Ag Porphyry Au El Altar - Piuquenes South Porphyry Cu-Au America Porphyry Cu-Mo Los Pelambres - El Pachón 25 - 20 Ma HS epithermal Au ± Ag

32°S Porphyry Au, Au-Cu Porphyry Cu-Au 200 km

Fig. 1. a. Location of the Josemaría porphyry copper-gold deposit in the late Oligocene-Miocene volcanic belt of northern Chile and contiguous Argentina, showing the age assignment of the main porphyry and epithermal deposits. Insets show the location of the flat-slab portion of the subducted Nazca plate (contours from Cahill and Isacks, 1992) and position of Figure 1a in South America. b. More detailed geology of the Josemaría area and vicinity. Volcanic rocks compiled mainly from Maksaev et al. (1984), Mpodozis et al. (1995), Fauqué (2001), Panteleyev and Cravero (2001), Zappettini et al. (2001, 2008), SERNAGEOMIN (2003), Sanguinetti (2006), Martínez et al. (2015a), Mpodozis et al. (2018), and C. Mpodozis (writ. commun., 2018). The age of older volcanic rocks west of the Los Helados fault is uncertain, but between Paleocene and Oli- gocene. Deposit age assignments based on Sillitoe et al. (1991, 2013, 2016), Kay et al. (1994), Mpodozis et al. (1995), Bissig et al. (2001), Mpodozis and Kay (2003), Perelló et al. (2012), Maydagán et al. (2014), Cáceres (2015), Y. Kapusta in Rode et al. (2015), Yoshie et al. (2015), Holley et al. (2016), Astorga et al. (2017), and NGEx Resources Inc. (unpub. data). Background shaded-relief image from Esri (2014 version).

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subsidiary of Tenke Mining Corporation—a Lundin Group important earlier pulse of compressive deformation at ~26 to company, during a regional program to explore the apparently 25 Ma (Mpodozis and Clavero, 2002; Mpodozis et al., 2018) poorly mineralized gap between the Maricunga and El Indio that is thought likely to have extended at least as far south as belts (Jones, 2007). Follow-up of a LANDSAT TM false-color Josemaría. anomaly, first defined a decade earlier by Norwest Mine Ser- Most porphyry and epithermal deposits in the flat-slab vices, Inc. on behalf of the Secretaría de Minería de la Nación segment were formed during the process of flattening, from of Argentina (Decker, 1994), led to definition of a promising ~18 to 5 Ma (Bissig et al., 2001; Mpodozis and Kay, 2003; Y. 400- × 400-m, talus-fines geochemical copper-gold-molybde- Kapusta in Rode et al., 2015; Yoshie et al., 2015; Holley et num anomaly and eventual drill testing (Rode and Carrizo, al., 2016; Sillitoe et al., 2016; Astorga et al., 2017; Fig. 1a). 2007). Since then, Josemaría has been the subject of a series The only known exceptions, emplaced prior to slab flatten- of drilling campaigns, for a total of 61,100 m in 142 holes, ing, are Josemaría and several prospects near the Caserones culminating in the 2016 announcement by NGEx Resources porphyry copper deposit (Perelló et al., 2003a; Yoshie et al., Inc.—a successor company to Tenke Mining Corporation—of 2015; Fig. 1a). However, north of the flat-slab segment, in an indicated sulfide mineral resource of 835 Mt at 0.35% Cu the main part of the Maricunga belt, several deposits and and 0.25 g/t Au, at a 0.3% Cu equiv cutoff, for 6,500 Mlb of prospects formed during the 26 to 20 Ma interval, most copper and 6.6 Moz of gold (Ovalle et al., 2016). notably the Caspiche porphyry gold-copper, Refugio (Mari- In the context of its geographic location, the Josemaría cunga) porphyry gold, and La Coipa, Esperanza (Nueva deposit displays several interesting geologic features, includ- Esperanza), and La Pepa high-sulfidation epithermal gold- ing its age, localized supergene enrichment, and limited silver deposits (Sillitoe et al., 1991, 2013; Kay et al., 1994; time spanning deposit formation, exhumation, and subse- Mpodozis et al., 1995; Fig. 1a). quent burial beneath postmineral volcano-sedimentary rocks. These topics are the main focus of this paper, which is based Deposit Geology on detailed field mapping, drill core logging, and support- ing petrography and U-Pb zircon geochronology. The paper Premineral lithologies builds on an extended abstract presented previously (Ortiz et The Josemaría deposit is located within a 25-km-wide, fault- al., 2015). bounded block of volcanic and plutonic basement rocks (Fig. 1b), the former considered Late Permian in age by Marcos Regional Geologic Setting et al. (1971). These volcanic rocks are now assigned to the The Josemaría deposit is situated near the northern limit of Guanaco Sonso Formation, of Late Permian-Early Triassic the present-day amagmatic Chilean-Pampean flat-slab seg- age (Martin et al., 1999; Coloma et al., 2017), which is wide- ment of the Andean Cordillera (~28°–33°S), which is char- spread along the Chilean side of the international border, west acterized by low-angle subduction of the Nazca plate beneath of Josemaría (Martínez et al., 2015a). In the western parts of South America (Jordan et al., 1983; Cahill and Isacks, 1992; the Josemaría area, the Guanaco Sonso Formation comprises Kay and Mpodozis, 2002; Ramos et al., 2002; Fig. 1a, inset). rhyolitic volcaniclastic rocks, including welded ignimbrite Josemaría is an integral part of a N-trending, late Oligocene flows (Figs. 2, 3), although systematic subdivision is difficult to Miocene magmatic arc, containing numerous porphyry and because of the masking effects of hydrothermal alteration. epithermal deposits and prospects, which spans the northern East of and locally intruded beneath the rhyolitic sequence transition zone between the Chilean-Pampean flat-slab seg- is an equigranular hornblende-biotite tonalite pluton (Figs. ment and the central Andean steep slab, including the Mari- 2, 3), which consists of several phases distinguishable on the cunga belt to the north (Fig. 1a). In the Maricunga and El basis of grain size and total quartz content. A granodioritic Indio belts, volcanic rocks are widespread whereas between phase is also present north of the deposit (Fig. 3) and melano- them, where Josemaría is situated, the volcanic pile is rather cratic diorite was encountered by drilling to the southeast. A less extensive, possibly because of erosional removal during U-Pb zircon age of 259.11 ± 0.21 Ma was obtained for a sam- kilometer-scale Miocene uplift to expose the late Paleozoic ple of the tonalite from the southern part of the deposit, using to Triassic basement (Ribba et al., 1988; Mpodozis and Kay, the chemical abrasion, isotope dilution, thermal ionization 1992, 2003; Fig. 1a). mass spectrometry (CA-TIMS) technique on single grains, as The late Oligocene to early Miocene (~26–20 Ma) part of detailed in Scoates and Friedman (2008) and the Electronic the volcanic arc was constructed during relatively steep sub- Appendix. This result makes it part of the regionally exten- duction, which since ~18 Ma became progressively shallower sive Montosa-El Potro Plutonic Complex of Late Permian to and, south of latitude 28°S, evolved to the current flat-slab Early Triassic age (265–245 Ma; Martínez et al., 2015a). The geometry (Kay and Mpodozis, 2002; Kay et al., 2014). The tonalite is cut by N- to NE-trending, andesitic to basaltic dikes slab shallowing is generally attributed to subduction of the (Fig. 3), most too narrow to be shown in the figures. Such Juan Fernández aseismic ridge, a bathymetric high, result- dikes are a typical feature of many Permo-Triassic plutons in ing in increased interplate mechanical coupling and conse- the Frontal Cordillera (e.g., Martin et al., 1999). quent contraction, thick-skinned reverse faulting, and crustal These Late Permian to Triassic volcanic and plutonic rocks shortening and thickening (Jordan et al., 1983; Gutscher et are currently considered to represent the shallow and deep al., 2000; Horton, 2018). Nonetheless, one of the anomalous parts, respectively, of igneous complexes emplaced during geologic features of the transition zone between the flat-slab crustal extension induced by either a hiatus in subduction segment and steeper slab to the north, at least along the east- (Mpodozis and Kay, 1992) or slow subduction with slab roll- ern side of the Maricunga belt around latitude 27°–28°S, is an back (del Rey et al., 2016).

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ESE WNW

Approx. limit of >0.3% Cu equiv. resource Intermineral projected to surface porphyry intrusion (not shown under postmineral cover)

Postmineral S r N r r volcanic and sedimentary rocks r r r r y r mit Rhyolitic r nfor unco gocene Tonalite volcanic rocks te Oli La r r Postmineral Tonalite normal fault zone r that localized supergene Cu enrichment Granodiorite

Fig. 2. View, looking south, of the Josemaría alteration zone, showing exposed pre- and synmineral rock types, surface projection of the >0.3% Cu equiv zone, unconformably overlying postmineral volcanic rocks, and NNE-striking, postmineral fault zone that controls supergene chalcocite enrichment.

Pre- and synmineral structure into early, inter-, and late-mineral phases. All the phases are The main premineral feature at Josemaría is the contact characterized by 60 to 80 vol % of well-formed plagioclase, between the tonalite pluton and rhyolitic volcanic rocks. hornblende, and biotite phenocrysts, the last as well-formed Although this contact is inferred to be generally subhorizon- “books,” along with scattered quartz phenocrysts in a fine- tal, there is a suggestion from three-dimensional modeling grained groundmass. of the immediate deposit area that it may be marked by a The early porphyry phases are intensely quartz veined and steep, N-striking, premineral fault that was plugged by por- well mineralized, whereas the intermineral phases are notice- phyry intrusions, although the evidence is largely obscured by ably less strongly veined; both phases were originally potassic postmineral faulting (Fig. 3). Judging by the northerly elon- altered. In contrast, the late-mineral phases display propylitic gation of the porphyry intrusions, the suspected fault may alteration, have few quartz veinlets, and are essentially bar- have influenced their emplacement (Fig. 3). Furthermore, ren. The porphyry phases were distinguished using standard the Josemaría deposit lies at the southern end of a >16-km- geologic criteria (Sillitoe, 2000, 2010; Proffett, 2003), partic- long, N-trending alteration corridor, within which there are ularly the presence near intrusive contacts of quartz veinlet at least two additional porphyry copper-gold prospects (Fig. xenoliths (Fig. 4) in combination with quartz veinlet intensi- 1b), including N-trending porphyry dikes, suggesting that the ties and copper and gold contents. putative structure could be regionally significant. The greatest volume of intrusive porphyry occupies the cen- At the time of and immediately following deposit forma- tral parts of the deposit, where the largest body of the early tion, and broadly contemporaneous with the 26 to 25 Ma phase is flanked to the northeast and southwest by the largest compressive tectonism (Mpodozis et al., 2018), a slightly late and intermineral bodies, respectively (Figs. 2, 3). Farther oblique, NNE-trending fault zone transected the Josemaría north still, the porphyries are confined to steep dikes, with the area and acted as an east-over-west reverse structure, thereby early and intermineral phases only a few meters wide. accounting for preservation of the rhyolitic volcanic rocks The early and intermineral porphyries (Fig. 3) returned and shallow-level alteration features (see below) on its west- U-Pb zircon ages of 24.98 ± 0.04 Ma (youngest grain) and ern footwall side (Fig. 3). Similarly oriented faults, showing 24.66 ± 0.04 Ma (MSWD = 1.12), respectively, employing the most recent W-vergent reverse displacement, are mapped same CA-TIMS method; analytical details are given in the in the region; some of them had early normal motion dur- Electronic Appendix. ing late Eocene extension when a major parallel structure, the Mogotes fault, ~5 km east of Josemaría, was active along the Hypogene Alteration and Mineralization western margin of the Macho Muerto basin (C. Mpodozis, Five alteration types—potassic, chlorite-sericite, sericitic, writ. commun., 2018; Fig. 1b). advanced argillic, and late propylitic—are systematically rec- ognized within the Josemaría deposit, each with its own dis- Porphyry intrusions tinctive sulfide assemblage where not subjected to supergene The Josemaría porphyry copper-gold deposit is centered on oxidation. Together, these define an alteration footprint mea- several small dacite porphyry intrusions, which are subdivided suring 4 km north-south and 2 km east-west (Fig. 5).

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0 470 1 km 0 4550 UTM 19S WGS84 685300 MIOCENE (post - 22 Ma) EARLY MIOCENE (~22 Ma) LATE OLIGOCENE (~25 Ma) Basaltic plug Basaltic-andesitic flows Josemaría dacite porphyry intrusions Hydrothermal breccia Andesitic-dacitic flows Late mineral Rhyolite dike Andesitic-dacitic tuff Intermineral Early mineral Fault Dacitic tuff Early reverse: certain, inferred Redbed sandstone/conglomerate PERMIAN - TRIASSIC Late normal: certain, inferred Andesitic dike (Ad) U-Pb zircon sample Limit of >0.3% Cu equiv. projected to surface, Tonalite / granodiorite dashed where under postmineral cover Drill hole Rhyolitic volcanic rocks

Fig. 3. Geology of the Josemaría porphyry copper-gold deposit and environs, simplified from 1:5,000-scale mapping by the second author (FAMD). Also shown is the surface projection of the deposit at a >0.3% Cu equiv cutoff (after Ovalle et al., 2016).

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become younger and, in their host rocks, with distance from veinlet xenolith the core of the deposit; they attain ~15 vol % of the early porphyry and were everywhere preserved during the superposition of the chlorite-sericite, sericitic, and advanced argillic alteration (Fig. 8b, d). The more prominent exposed quartz veinlets are mapped in Figure 5. The outer limit of appreciable quartz veining acts as a useful proxy for the external limit of copper-gold mineralization (Fig. 6b). Following A-type quartz veinlets the classification of Gustafson and Hunt (1975), the quartz veinlets in the potassic zone are mainly A-type (Figs. 4, 8a), Fig. 4. Intermineral dacite porphyry containing a xenolith of chalcopyrite- containing varied amounts of chalcopyrite ± magnetite, but bearing A-type quartz veinlet derived from the early porphyry. The rock was molybdenite-bearing B-type quartz and D-type quartz-pyrite subsequently cut by narrow A-type quartz veinlets and then overprinted by sericitic alteration. Veinlet nomenclature follows Gustafson and Hunt (1975). veinlets (Fig. 8c) are also present. Where not dissolved by Scribe tip for scale. cool descendant groundwater under supergene conditions (see below), anhydrite is present in the potassic zone, most of it in monomineralic veinlets that were introduced late during The Josemaría deposit is broadly centered on an exposed deposit formation. chlorite-sericite alteration zone, approximately 800 m The potassic alteration is characterized by veinlet and dis- across, which gives way westward and, below postmineral seminated chalcopyrite, with little or no accompanying pyrite. cover, southward to sericitic and advanced argillic alteration The relatively low-sulfidation state of the sulfide zone is fur- (Figs. 5, 6b, 7b). The chlorite-sericite assemblage formed ther emphasized by the local presence, mainly in the A-type at the expense of preexisting potassic alteration, which is quartz veinlets, of minor bornite. Chalcopyrite/bornite ratios preserved at depths of anywhere between 400 and 600 m are estimated to be >30:1. below the present surface and as a steeply inclined, roughly cylindrical body, up to 200 m across, which extends to the Chlorite-sericite alteration surface alongside the dikes in the central-northern part of The chlorite-sericite alteration is dominant throughout the the deposit (Figs. 5, 6b). Approach to the potassic zone is tonalite and early and intermineral porphyry intrusions in the heralded by weakening of the chlorite-sericite overprint and northern and central parts of Josemaría where it is transitional presence of remnant biotite. to and overprints the potassic alteration; however, farther Results of the surface mapping show that the alteration zone south in the deposit it is present mainly at depth. Plagioclase is much larger than the deposit itself, with advanced argillic is wholly or partially replaced by sericite, and biotite and horn- and sericitic alteration extending 700 m west and >1,500 m blende by chlorite (Fig. 8b). On the margins of the chlorite- north and south; to the east, this distal alteration is concealed sericite zone, the sericite gives way in places to pale-green beneath postmineral cover (Fig. 5). The advanced argillic illite. Magnetite is largely transformed to hematite, either as alteration occupies the highest ground, dominates the rhyo- pseudomorphic martite or specularite. Anhydrite veining is litic volcanic rocks (see below), and transitions downward to similar to that in the potassic alteration. The low-sulfidation and overprints the sericitic zone or, where the latter is absent, chalcopyrite ± bornite assemblage in the potassic alteration the chlorite-sericite or potassic zones. The sericitic zone is is variably reconstituted to pyrite-chalcopyrite in the chlorite- encircled by a weakly developed propylitic halo (Fig. 5). sericite zone, typically giving rise to pyrite/chalcopyrite ratios of ~3 to 10:1. Potassic alteration The potassic alteration is preserved at depth in the tonal- Sericitic alteration ite as well as in the preporphyry andesitic to basaltic dikes The sericitic alteration is present in the shallower levels of the and early and intermineral dacite porphyry intrusions that deposit and commonly underlies the advanced argillic altera- cut it (Fig. 6b). The preporphyry dikes can display potassic tion, a relationship best seen in the southern parts. However, alteration even where enclosing tonalite was overprinted by as illustrated by Figure 6b, it can be volumetrically subordi- chlorite-sericite alteration, reflecting their greater contents nate to advanced argillic alteration in the rhyolitic volcanic of hydrothermal biotite and fine-grained, relatively imperme- rocks. Sericite-bordered, D-type veinlets are a characteristic able nature. feature of the sericitic zone (Fig. 8c). Hydrothermal biotite defines the potassic zone (Fig. 8a), The sericitic alteration within the copper-gold deposit is although apparently unaltered magmatic biotite is also pres- characterized by a prominent high-sulfidation sulfide assem- ent. Plagioclase is bleached white in parts of the potassic zone blage in which disseminated grains of pyrite are surrounded as a result of replacement by albite associated in part with by black sulfidic rims composed of intergrown hypo- K-feldspar, an assemblage denominated sodic-potassic by gene chalcocite, bornite, and/or covellite along with trace Lang et al. (2013). Hydrothermal magnetite constitutes up to amounts of tennantite and enargite. All preexisting magne- 5 vol % or more of the potassic-altered tonalite and early and tite and hematite were pyritized and pyrite/copper sulfide intermineral porphyries, in veinlets with or without quartz as ratios are roughly 10:1. Downward, the sericitic alteration well as in disseminated form. and high-sulfidation mineralization are observed in places Multidirectional quartz veinlets were introduced during the to grade into the pyrite- and chalcopyrite-bearing, chlorite- potassic event and become less abundant as the porphyries sericite zone.

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MIOCENE (post - 22 Ma) EARLY MIOCENE (~22 Ma) LATE OLIGOCENE (~25 Ma) Josemaría porphyry system alteration Basaltic plug (unaltered) Postporphyry cover rocks Advanced argillic Hydrothermal breccia Volcanic rocks (alunite & kaolinite cement) Redbed Sericitic (strong/moderate) sandstone/conglomerate Fault Propylitic Early reverse: Drill hole certain, inferred Limit of >0.3% Cu equiv. Chlorite-sericite Late normal: Quartz veinlets projected to surface, dashed certain, inferred where under postmineral cover Potassic

Fig. 5. Hydrothermal alteration of the Josemaría porphyry copper-gold deposit and environs. Also shown is the surface projection of the deposit at a >0.3% Cu equiv cutoff (after Ovalle et al., 2016).

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W 5600N E a LITHOLOGY Quaternary colluvium

4500m Late-mineral porphyry Intermineral porphyry Early porphyry Tonalite Rhyolitic volcanic rocks

Fault (NNE): early reverse, 4100m late normal movement

b ALTERATION Advanced argillic

4500m Sericitic Chlorite-sericite

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Drill hole, with Cu (red) and 4100m Au (yellow) grades

c SUPERGENE Leached capping 4500m Oxide Cu enrichment (>1% Cu equiv.)

Sulfate front

GRADE: Cu equiv., % > 0.5 > 0.4 4100m > 0.3

500 m Fig. 6. Representative sections at 5,600N, approximately the middle of the Josemaría copper-gold deposit. a. Lithologies. b. Hydrothermal alteration. c. Supergene profile. Section line shown in Figures 3 and 5.

W 5000N E a LITHOLOGY D H 51 259.11 ± 0.21 Ma Andesitic-dacitic flows Andesitic-dacitic tuff Redbed sandstone Redbed conglomerate Intermineral porphyry 4500m Early porphyry Andesitic dike Tonalite Rhyolitic volcanic rocks

4200m Fault (NNE): Fault (NW): early reverse, late normal late normal motion b motion

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4200m Drill hole, with Cu (red, right) and Au (green, left) grades c SUPERGENE Leached capping Partly leached

GRADE: Cu equiv., % > 0.5 4500 > 0.4 > 0.3

4200m

500 m Fig. 7. Representative sections at 5,000N, in the southern part of the Josemaría copper-gold deposit where it is concealed beneath postmineral rocks. a. Lithologies. b. Hydrothermal alteration. c. Supergene profile. Note the position of the dated tonalite sample in (a). Section line shown in Figures 3 and 5.

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a Advanced argillic alteration The advanced argillic alteration is largely but not exclusively restricted to the rhyolitic volcanic unit (Fig. 6b) although, locally, narrow, structurally controlled zones penetrate the underlying tonalite to depths of up to 350 m. The advanced argillic alteration preferentially affected the rhyolite because of its silicic nature and consequent low acid-buffering capac- ity relative to the other lithologies present. The zone is domi- Aq nated by quartz and pyrophyllite (Fig. 8d) although in places, at shallow depths, alunite rather than pyrophyllite replaces plagioclase phenocrysts. Other minerals typical of the advanced argillic alteration include dickite, observed together with pyrite filling a few veinlets, dumortierite, in the form of b tiny pale-blue rosettes, and diaspore, identified only in thin sections. The advanced argillic alteration contains the same high- sulfidation sulfide assemblage as the sericitic zone and is similarly pyrite rich and devoid of magnetite and hematite. Native Aq sulfur coats fractures in places, accompanied locally by crys- talline, hypogene covellite. Propylitic alteration In addition to the weakly developed propylitic halo to the Aq Josemaría deposit (Fig. 5), the late-mineral dacite porphyry intrusions are also pervasively albeit weakly propylitized (Figs. 5, 6b), with all mafic minerals altered to chlorite and c plagioclase containing replacive patches of epidote. The late py propylitization, in common with that in late-mineral porphyry copper intrusions in general (e.g., Sillitoe, 2000), barely affects the enclosing rocks. Pyrite contents of both the early ser halo and late-mineral porphyry propylitization are low, typically <1 vol %. The propylitic halo, thought to have formed contemporane- ser ously with the potassic zone (e.g., Proffett, 2003), is not distinguished from this late-stage propylitization of the late-mineral porphyry in Figures 5 and 6b because of their mineralogic similarity. Grade distribution d Using a >0.3% Cu equiv cutoff projected to surface, the copper-gold zone at Josemaría measures a maximum of ~1,500 m north-south and ~1,000 m east-west (Figs. 3, 5, 9). Within this Aq zone, the highest hypogene copper and gold values lie in its southern part, eccentrically with respect to the deposit out- line (Fig. 9). The external parts of the copper-gold deposit Aq are overlapped by a molybdenum-rich annulus, with >50 ppm Mo, which is higher in grade around the northern and eastern sides (Fig. 9). Although the molybdenite in the annulus formed during potassic alteration, as shown by much of it occurring in B-type quartz veinlets, it was retained during the subsequent alteration overprints. The hypogene copper and gold values generally correlate Fig. 8. Alteration and veinlet types in tonalite, except for (d) in early por- reasonably well in all the alteration-mineralization types, sug- phyry, Josemaría porphyry copper-gold deposit. a. Potassic alteration with gesting joint introduction and precipitation of the two met- A-type quartz (Aq) veinlets. b. Chlorite-sericite alteration with inherited als during the early potassic event and their continued close A-type quartz (Aq) veinlet. c. Moderate-intensity sericitic alteration with association during the alteration-mineralization overprinting. D-type veinlets, in which pyrite (py) center lines have texture-destructive sericitic (ser) halos. d. Advanced argillic alteration with inherited A-type quartz However, the Cu-Au correlation coefficient for the entire (Aq) veinlets. Veinlet nomenclature follows Gustafson and Hunt (1975). deposit is only 0.57, at least in part due to mobility of copper in Scribe tip for scale. the central and northern parts of the deposit during supergene

4500 m 4300 m 4100 m N 4 50 0

m s u r face c on to ur

Supergene Cu domain

Cu Postmineral volcanic rocks 6855000

e c r u h o .) s v e i r u f q o e n u o i C t Limited drilling to c % e 6854000 j this dept 3 Postmineral o . r 0 Au p (

volcanic rocksc e e rfa in Su utl o 445700 446700

1 km

Drill hole pierce point on Hypogene Hypogene + supergene slicing plane Au, g/tMo, ppm Cu, % Fault: early reverse, > 0.5 > 150 > 0.5 late normal displacement

> 0.4 > 90 > 0.4 Main supergene Cu enrichment > 0.3 > 50 > 0.3 Maximum limit of the > 0.2 > 0.2 0.3% Cu equiv. resource projected to surface Fig. 9. Horizontal slices through the Josemaría porphyry copper-gold deposit at 4,500-, 4,300-, and 4,100-m elevations, showing distributions of hypogene gold (>0.3 g/t) and molybdenum (>50 ppm) and hypogene plus supergene copper (>0.2%). Note the well-formed molybdenum annulus. The progressive northward deepening of the highest copper values reflects the fault-localized supergene chalcocite enrichment. Grade boundaries are interpolated from downhole assay values.

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processes. The overall similarity of copper (~0.35%) and gold (~0.2 g/t) contents in the potassic and chlorite-sericite zones Ba suggests that there was little metal addition or removal during Postmineral volcanic rocks the reconstitution of the chalcopyrite ± bornite assemblage to pyrite-chalcopyrite. At the deposit scale, the highest hypogene grades, particularly of copper but, in the south, also of gold, tend to occur in the advanced argillic and sericitic zones in which they are present in the high-sulfidation sulfide assemblage; this clearly Conglomerate gave rise to hypogene enrichment, defined as a grade increase relative to the preexisting potassic and chlorite-sericite alteration (Fig. 8a, b). In the south, average copper and gold values of ~0.6% and ~0.7 g/t are attained locally, accounting for the highest grade part of the deposit. The minor tennantite Bb and enargite result in arsenic values typically between 10 and 100 ppm. Distinction of the hypogene enrichment from that of supergene origin at Josemaría is not straightforward. None- theless, hypogene-enriched zones are commonly cut by pris- tine pyrite veinlets lacking copper-bearing sulfide minerals, whereas such veinlets are the preferred sites for supergene chalcocite enrichment (see below). cpy Rhyolite Postmineral Volcano-Sedimentary Cover The southern part of the Josemaría deposit is unconformably overlain by a volcano-sedimentary sequence, much of which is completely unaltered and devoid of sulfide minerals (Figs. 5, 7b). Contacts between the porphyry copper alteration Bc and mineralization and postmineral sequence are abrupt and readily mappable at surface and in drill core (Figs. 2, 3, 5). The sequence, which comprises lower sedimentary and py upper volcanic components (Fig. 10a), thickens dramatically southeastward to >500 m, much of it attributable to the sedimentary unit (Fig. 7a). The paleotopography upon which the kao + sediments were deposited dips southeast at ~30°, steepen- py ing farther southeastward (Fig. 7a). The fact that the porphyry intrusions are subvertical strongly suggests that this is an original slope angle rather than a product of postmineral tilting. Bd The lower part of the postmineral sequence is entirely py composed of stratified siliciclastic rocks, both polymict, clast-supported, cobble-pebble conglomerate and imma- en ture, poorly bedded, quartz-rich sandstone, the latter becoming more abundant eastward (Figs. 3, 7a). Clasts vary from well rounded to subangular, with the term breccia perhaps more appropriate where the latter predominate. These lithofacies suggest deposition in a proximal alluvial q fan environment (e.g., Blair and McPherson, 1994). Above kao the southern part of the deposit, the conglomerate horizon Be

Fig. 10. Postmineral features near the Josemaría porphyry copper-gold hem deposit. a. Hematitic basal conglomerate overlain by felsic ignimbrite. Note the thin jarosite layer along the contact. b. Hematitic polymict conglomer- Aq ate, with the largest clast being rhyolitic volcanic rock containing chalcopyrite jar (cpy). c. Hematitic (hem) conglomerate cut by pyrite veinlets with kaolinized halos (kao) containing disseminated pyrite (py). Note that pyrite overprints and destroys the hematite. d. Advanced argillic-altered conglomerate impregnated with quartz (q), pyrite (py), and enargite (en); kaolinite (kao) fills cavities. e. Hematitic (hem) conglomerate overprinted by supergene jarosite (jar) after former disseminated pyrite. Note the A-type quartz (Aq) veinlet in the centrally located clast. Scribe tip for scale in (b), (c), (d), and (e).

and a thinner conglomerate bed intercalated in the overlying sequence unconformably overlying the Josemaría deposit, volcanic unit have a hematite-impregnated matrix and are particularly the hematitic basal conglomerate, is pervasively considered as red-bed sediments (Figs. 7a, 10b). At surface, kaolinized and pyritized (Fig. 7b). Although most of this the main hematitic horizon can be traced continuously for later alteration and mineralization lies beyond the Josemaría 2,300 m along the base of the postmineral sequence (Fig. deposit, it did affect it locally along faults but with minimal 3). The conglomerate clasts consist mainly of rhyolitic vol- addition to grade. canic rocks, some displaying advanced argillic alteration, Hydrothermal breccia dikes, 1 to 50 m wide and cemented but clasts containing A-type quartz veinlets (Fig. 10e) or by chalcedony, alunite, kaolinite, pyrite, and enargite, are specular hematite are also present. Pyrite, chalcopyrite, and prominent just south of the Josemaría deposit. The main high-sulfidation sulfide mineralization occur in many incom- dike at surface strikes west to northwest and is ~2 km long pletely oxidized or unoxidized clasts (Fig. 10b). Indeed, the (Figs. 3, 5). Locally, the advanced argillic alteration and unaltered conglomerate contains elevated copper and gold mineralization associated with this breccia dike invade fault values, typically ~50 to 500 ppm and 0.1 to 0.2 g/t, respec- zones, with a particular affinity for the NW faults, as well tively, which are assumed to be contributed by the mineral- as the hematitic conglomerate at the base of the postmin- ized clasts. Local concentrations of detrital magnetite grains eral sequence (Fig. 5), causing the latter to undergo bleach- also occur in places. These lithologic and geochemical fea- ing and hematite destruction. In drill core, pyrite veinlets tures support a local source contribution from the eroding cutting the conglomerate have kaolinized halos containing Josemaría deposit. disseminated pyrite, which clearly overprints and sulfidizes The upper, volcanic part of the postmineral sequence com- the hematitic matrix (Fig. 10c). The conglomerate in several prises andesitic and dacitic lavas underlain by poorly welded, places, including that at the base of one of the thickest silici- lithic-rich and -poor ignimbrites (Figs. 7a, 10a). An andesite clastic intervals, is cut by a stockwork of quartz, pyrite, and sample, collected at surface (Fig. 3), returned a U-Pb zircon enargite associated with cavity-filling kaolinite and lesser age of 22.35 ± 0.03 Ma (MSWD = 1.3; Electronic Appen- alunite (Figs. 7b, 10d), with all former hematite entirely dix), in keeping with the postmineral timing of the volcano- destroyed. Importantly, this arsenic-rich copper mineraliza- sedimentary sequence. Based on this age, the postmineral tion lacks accompanying gold (<40 ppb), thereby making volcano-sedimentary rocks may be correlated with the Río La it not only temporally but also geochemically distinct from Gallina-Refugio sequence along the eastern side of the south- the high-sulfidation mineralization in the upper parts of the ern Maricunga belt, between latitude 27°30' and 28°S (26– Josemaría deposit. 21 Ma; Mpodozis et al., 2018) and the Tilito Formation of the Doña Ana Group in the El Indio belt (24.1–21 Ma; Maksaev Supergene Oxidation and Enrichment et al., 1984; Kay et al., 1988; Bissig et al., 2001; C. Mpodozis, The supergene profile at Josemaría is generally thin, but in the pers. commun., 2019). central and northern parts of the deposit can attain a thickness of 400 m over widths of <200 m because of downward exten- Postmineral Normal Faults sion along the three main NNE-striking, steeply E-dipping, A series of NW-striking, high-angle faults transect the eastern now normal postmineral faults (Figs. 2, 7c, 9). The supergene parts of the deposit, stepping the postmineral volcano-sedi- profile consists of a jarosite- and goethite-dominated leached mentary rocks down to the northeast but also accommodat- capping—within which 43 Mt average 0.32 g/t Au (at a 0.2 g/t ing some sinistral motion (Fig. 3). The NW faults appear Au cutoff; Ovalle et al., 2016)—underlain by a northward- to terminate at the NNE-striking reverse fault zone within deepening, chalcocite enrichment blanket, ranging from 100 the deposit, which was reactivated at this time as a series to 200 m thick (Fig. 6c). In places, however, where the oxida- of normal faults. The reactivated faults are marked by 5 to tion affected pyrite-poor potassic alteration, there are zones 10 cm of gouge surrounded by damage zones a few meters containing oxide copper minerals (Fig. 6c), mainly fracture- wide. East-side-down displacement on the easternmost of coating malachite and neotocite. Nonetheless, the leached the three main reactivated faults is on the order of 120 m capping is generally depleted in copper, typically reporting (Fig. 6a). <0.1%, in marked contrast to the underlying enrichment with The age of this post ~22 Ma normal faulting is uncon- typical values of 0.8 to 1.5% Cu. strained, although it could have been broadly synchronous In the concealed southern parts of the deposit, the postmin- with the postmineral red-bed sedimentation and volcanism eral volcano-sedimentary rocks overlie a 30- to 140-m-thick, and, therefore, correlative with the regional extension that E-thinning supergene profile, comprising mainly leached and accompanied deposition of the Doña Ana Group in the El partially leached zones; underlying chalcocite enrichment is Indio belt (24.1–17.5 Ma; Bissig et al., 2001; C. Mpodozis, minimal (Fig. 7c). The leached and partially leached zones writ. commun., 2019). Alternatively, it could have been in the contain more supergene hematite than the leached capping mid-Miocene when extension affected the eastern side of the farther north, the reason for which is uncertain. The southern southern Maricunga belt around latitude 27°–28°S (Mpodozis part of the supergene profile and its host rocks may be dis- et al., 2018). rupted by the NW-striking normal faults, which may also have facilitated groundwater circulation (Figs. 3, 7c). Younger Hypogene Alteration and Mineralization In places, where the postmineral hematitic conglomerate The upper, volcanic part of the postmineral cover sequence was affected by the later pyritization (Fig. 10c), supergene is everywhere little altered except for weak propylitization of oxidation gave rise to formation of jarosite. Since hematite the andesitic units; however, part of the siliciclastic red-bed is stable in the weathering environment, the oxidation of

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partially pyritized zones resulted in jarosite veining of the a Macho Muerto basin hematite-cemented conglomerate (Fig. 10e). The unaltered

and pyrite-free conglomerate and overlying volcanic rocks o

g Late Eocene - ? o t were incapable of developing leached capping, thereby giv- e s

early Oligocene f a

ing the erroneous impression that they postdate all leached- u

l capping development (Fig. 7c). t The Josemaría deposit is characterized in places by an abrupt sulfate front: the boundary between gypsum ± anhy- b drite-cemented rock below and sulfate-free rock above (Sil- litoe, 2005). The anhydrite is an abundant hypogene mineral, ~25 Ma probably formerly present in all the alteration types described above, with the possible exception of the propylitic halo. The gypsum is its supergene hydration product, which persists until it is eventually dissolved by continued descent of cool groundwater, leaving open cavities. Although the anhydrite c Macho Muerto front is typically deeper than 600 m, the lower limit of drilling, basin ~23 Ma it occurs locally at relatively shallow depths in the northern and central parts of the deposit. There, pillars of imperme- able, sulfate-cemented rock exist as shallowly as the base of the leached capping, a situation that effectively prevents the downward passage of supergene solutions and development of chalcocite enrichment (Fig. 6c). d ~22 Ma Discussion

Evolution of Josemaría hypogene mineralization ?? Josemaría displays the main attributes of porphyry copper- gold deposits worldwide, including elevated hydrothermal e magnetite content, reasonably good copper-gold correlation, and a molybdenum-rich halo to the central copper-gold zone Post-22 Ma (Sillitoe, 1979, 2000). Furthermore, notwithstanding their very different host rocks, these features, along with the telescoped nature of the alteration and mineralization zoning, are ?? shared with the contemporaneous Caspiche porphyry gold- copper deposit at the southern end of the Maricunga belt (Sil- f litoe et al., 2013; Fig. 1a). In combination, various lines of evidence, including the Mid-Miocene trend of elongate porphyry bodies and dikes (Fig. 3), suggest E l that the Josemaría porphyry copper-gold deposit, along with P otr the two prospects farther north (Fig. 1b), was localized by a o fault N-striking zone of structural weakness in the late Paleozoic to Triassic basement (Fig. 11a). This basement feature was g transected by a slightly oblique, NNE-trending fault zone, which underwent reverse motion following and probably also Present during deposit formation at ~25 to 24.5 Ma (Fig. 11b), in general accord with the timing of compressive tectonism along

Fig. 11. Evolutionary schema for the Josemaría porphyry copper-gold deposit. Supergene Cu enrichment a. Premineral development of N-striking zone of basement weakness, possi- Hydrothermal breccia and bly followed by normal faulting. The Mogotes normal fault was active at this high-sulfidation mineralization time along the western margin of the Macho Muerto basin (C. Mpodozis, writ. commun., 2018; Fig. 1b). b. Initiation of compression, NNE-striking Late Oligocene - Miocene volcanic rocks reverse faulting, and emplacement of dacite porphyry intrusions and associated porphyry copper system. c. Continued compression, telescoping of Late Oligocene sedimentary rocks alteration and copper-gold mineralization, unroofing, and deposition of red- bed siliciclastic sequence. d. Accumulation of dacitic and andesitic volcanic Lithocap rocks. e. Localized post-Josemaría hydrothermal brecciation, advanced argillic alteration, and high-sulfidation copper mineralization. f. Reexhumation of Porphyry Cu-Au deposit the deposit and supergene copper enrichment, probably later than the normal motion on the NNE-striking fault zone. g. Further erosion, particularly during Pleistocene glaciation. Eocene - Oligocene volcanic rocks

the eastern side of the southern Maricunga belt (26–25 Ma; postmineral sequence guided ascent of the high-sulfidation Mpodozis et al., 2018) and elsewhere in the Andean backarc fluid, with the siliciclastic sediments, probably poorly consoli- at latitude ~27°–28°S (late Oligocene onward; Kraemer et al., dated at the time, providing the permeability for more wide- 1999; Carrapa et al., 2005; Zhou et al., 2017). spread fluid ingress (Figs. 7b, 11e). As a direct result of the late Oligocene-early Miocene Although at least 2 m.y. of erosion, sedimentation, and vol- compressive deformation and consequent uplift, Josemaría canism intervened between the main porphyry copper-gold underwent synmineral erosion, resulting in progressive paleo- and late high-sulfidation epithermal events, their proximity surface lowering and alteration-mineralization telescoping suggests that the latter is the product of delayed resurgence (cf. Sillitoe, 1994; Fig. 11c). This caused extensive reconstitu- of the Josemaría system, presumably linked to a currently tion of early potassic by lower temperature chlorite-sericite undocumented intrusion. alteration, which, in turn, was widely and deeply overprinted by sericitic and, at shallower levels, advanced argillic altera- Supergene development at Josemaría tion. Although there was little obvious grade change dur- The supergene chalcocite enrichment at Josemaría was pref- ing the conversion of potassic to chlorite-sericite alteration, erentially developed in the central and northern parts of the superposition of the high-sulfidation sulfide assemblage the deposit and most deeply within the reactivated, NNE- during the sericitic and advanced argillic events gave rise to striking, postmineral normal fault zone (Figs. 6c, 9), which hypogene enrichment of copper and, less extensively, also appears to have acted as a channelway for focused descent of gold. A significant proportion of the copper and gold in the copper-charged solutions. In contrast, the rest of the deposit, high-sulfidation assemblage appears to have been inherited especially its southern part, has much thinner leached and from the preexisting chlorite-sericite zone: a conclusion sup- partially leached zones beneath which there is negligible chal- ported by the consistently higher grade of the early compared cocite enrichment despite the presence of apparently suitable to intermineral porphyries where both host high-sulfidation hypogene pyrite and copper contents (Fig. 7c). Furthermore, mineralization. in places, the oxidation also affected pyritized parts of the Once hydrothermal activity had ceased, the Josemaría postmineral siliciclastic sedimentary rocks (Figs. 7c, 10e). deposit continued to be eroded until concealment beneath In combination, these features strongly suggest that the the synorogenic red-bed sediments that constitute the lower supergene profile developed since the postmineral cover unit of the postmineral cover sequence (Fig. 11c). The rhyo- sequence was deposited, but after it was eroded from the litic volcanic rocks appear to have contributed much of the central and northern parts of the deposit (Fig. 11f). However, detritus. The steep slopes existing at the time (Fig. 6) would precisely when the cover sequence was removed is unknown have favored mass wasting and accumulation of the coarse although it could potentially have been relatively recently, conglomerate and breccia. The local presence at surface of depending upon its original thickness and the erosion rate. potassic alteration and A-type quartz veinlets along with the Percolation of water along the top and base of the hematitic quartz-pyrophyllite domination of the advanced argillic alter- conglomerate horizon and down the normal faults (Fig. 7c) ation suggests that relatively deep parts of a formerly more could have caused the weak sulfide oxidation that affected the areally extensive lithocap are currently exposed; this likely southern part of the deposit, a possibility supported by the implies erosional removal of at least 1,000 m of shallower, jarosite layer along the top contact (Fig. 10a). The steepness probably more silicic and alunitic advanced argillic altera- of the topography that existed during unroofing of Josemaría tion that may have been hosted by coeval volcanic rocks and (Fig. 7) and the fact that many mineralized clasts in the con- contained higher grade, high-sulfidation epithermal gold min- glomerate are sulfidic suggest that little sulfide oxidation took eralization (cf. Sillitoe, 2010; Hedenquist and Taran, 2013). place during initial deposit exhumation, prior to concealment However, such material is not abundant in the observed beneath the postmineral cover, probably because of the rapid- hematitic conglomerate, suggesting that, if ever present, it ity of the unroofing process. was not transported in a southeasterly direction or deposited Erosional stripping of the postmineral volcano-sedimentary nearby Josemaría. cover from the northern and central parts of the deposit and The duration of deposit exhumation cannot be precisely initiation of the oxidation and enrichment must have taken determined on the basis of current data. Nonetheless, the fact place during the continued Miocene uplift of the 25-km-wide that the early and intermineral porphyries, separated by only block of late Paleozoic to Triassic basement in which Josemaría ~0.3 m.y., are followed first by postmineral red-bed sediments is located, facilitated by major displacements on the El Potro (Fig. 11c) and then, <2.4 m.y. later, by the volcanic rocks (Fig. and subsidiary Los Helados faults (Fig. 1b). Nonetheless, as 11d) shows that it was a short-lived event that is unlikely to noted above, an extensional episode must have intervened have exceeded ~2 m.y. either before or during the active enrichment. The erosional dissection would have progressively lowered paleoground- Post-Josemaría copper mineralization water tables, resulting in exposure of the Josemaría sulfide After concealment of the Josemaría deposit beneath the mineralization to the effects of oxidation and the generation postmineral volcano-sedimentary cover, advanced argillic of subjacent chalcocite enrichment. Nonetheless, the overall alteration and minor high-sulfidation copper mineralization thinness and immaturity of the supergene profile over much affected mainly the area south and southeast of the deposit of the Josemaría deposit suggests that supergene processes (Figs. 5, 7b, 10d, 11e). The event has not yet been radiometri- may have barely kept pace with the rapid erosion, resulting in cally dated but must be >~2.4 m.y. later than formation of the confinement of appreciable chalcocite enrichment to the per- Josemaría deposit. Some of the NW-striking faults cutting the meable NNE-striking fault zone. Preservation of the shallow,

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anhydrite-cemented rock volumes (Fig. 6c) further supports epithermal copper-gold mineralization at Motomboto was the notion of supergene immaturity. at least intermittently active from 1.45 to 0.93 Ma, when An alternative possibility is that much of the supergene unroofing was responsible for synmineral incorporation of profile, except for the more deeply developed parts in the enargite-bearing clasts in boulder- and cobble-bearing col- NNE-striking fault zone, was removed during the Pleistocene luvial deposits interbedded with tuffs (Perelló, 1994). This when the high altitude (>4,350 m) parts of the region under- situation is reminiscent of that at Josemaría where high-sul- went intense glacial and periglacial dissection (Ammann et al., fidation mineralization both pre- and postdated the unroofing 2001; Perucca and Angillieri, 2008). Since then, and notwith- and associated burial event. Such repetitive advanced argillic standing ongoing active sulfide oxidation and enrichment in alteration and any associated high-sulfidation mineralization this high elevation Andean region (Sillitoe, 2005), insufficient may well be relatively commonplace in porphyry copper dis- time has elapsed to redevelop a deep supergene profile if, tricts (e.g., Hervé et al., 2012). indeed, one ever existed. Once porphyry deposits are concealed beneath postmineral cover, their preservation potential is inevitably enhanced. Rapidity of porphyry copper exhumation Indeed, in the case of Josemaría, located within a fault- Advanced argillic lithocaps are widely distributed in the Mari- bounded, basement block that continued to undergo pro- cunga and El Indio belts and the transition zone between nounced uplift and erosion throughout much of the Miocene them. Some contain high-sulfidation epithermal gold ± silver (Martínez et al., 2015a, b; Lossada et al., 2017; Mpodozis et deposits (Fig. 1a), whereas many are largely barren (Bissig al., 2018; Rossel et al., 2018), the deposit could well have been et al., 2001) even where they flank or overlie known por- completely removed in 2 to 3 m.y. had it not been for burial phyry gold and gold-copper deposits (e.g., Cerro Casale and beneath the postmineral cover. Marte; Vila and Sillitoe, 1991; Vila et al., 1991; Fig. 1a). The In order to preserve Mesozoic, Paleozoic, and older por- abundance of lithocaps provides firm evidence that erosion phyry copper systems, burial beneath postmineral cover is levels are generally relatively shallow (say, <0.5–1 km; Silli- even more likely to be necessary than in the case of younger toe 2010; Bissig et al., 2015) throughout large parts of these deposits, although precise measurement of the time required belts, notwithstanding the Miocene (post-18 Ma) compres- to unroof them tends to be more difficult. This is exempli- sive tectonism and concomitant rock uplift and denudation fied by the Cretaceous deposits at Pebble, Alaska, and Tie- south of approximately latitude 28°S (e.g., Martínez et al., gelongnan, Tibet, which were exhumed sometime during the 2015a, b; Lossada et al., 2017; Mpodozis et al., 2018; Rossel ~25- and 8-m.y. intervals, respectively, that preceded their et al., 2018). In the absence of dated postmineral cover—and concealment beneath postmineral cover (Lang et al., 2013; approximations based on either radiometric age of supergene Song et al., 2018). Nonetheless, extremely rapid unroofing was minerals (Sillitoe, 2005) or thermochronometry (McInnes documented at the Late Devonian Hugo Dummett porphyry et al., 2005)—the only available constraint on the timing of copper-gold deposit in the Oyu Tolgoi district, Mongolia, erosional unroofing is provided by the formational ages of where as little as 1 m.y. could have separated deposit forma- the deposits themselves, one of the youngest being El Indio tion and an unconformably overlying volcano-sedimentary (~8–5 Ma; Bissig et al., 2001, 2015; Fig. 1a). Therefore, Jose- sequence within which polymictic conglomerate and brec- maría is the standout exception because the partly preserved cia contain mineralized clasts (Wainwright et al., 2017). Fur- postmineral cover can be used to show that initial exposure thermore, the postmineral sequence, including the advanced and subsequent burial were accomplished within ~2 m.y. of argillic alteration that affected it, is cut by weakly altered and deposit formation. Accepting the removal of at least 1,000 m mineralized, late mineral porphyry intrusions, showing con- of overlying lithocap prior to deposit burial, the erosion rate clusively that exhumation took place during the waning stages could at times have exceeded 0.5 km/m.y. of the magmatic-hydrothermal system (Wainwright et al., Rapidity of exhumation is assured where exposed porphyry 2017). copper deposits are extremely young (<3 Ma), with Ok Tedi in the Papua New Guinea Highlands being arguably the best Paleoclimatic context example because it was formed at 1.1 to 1.2 Ma (K-Ar; Page Yanites and Kesler (2015) showed that the world’s youngest and McDougall, 1972), an age confirmed by U-Pb zircon dat- exposed porphyry copper deposits occur in tropical regions ing (1.187 ± 0.022 Ma; Large et al., 2018). There is no evi- where rainfall is higher and erosion rates generally faster than dence that Ok Tedi was ever concealed beneath more recent in other climatic regimes. Nonetheless, exhumation of the cover rocks and, given its prominent topographic position atop Josemaría deposit was just as fast as that of porphyry depos- Mount Fubilan and the absence of any nearby active volcanic its in tropical regions, such as the southwestern Pacific island center, it seems unlikely to be in the foreseeable future. At arcs, notwithstanding the semiarid conditions that prevailed the late Pliocene Boyongan and Bayugo porphyry copper-gold in this region of the Andes during the late Oligocene-early deposits in Mindanao, southern Philippines, however, intru- Miocene, as documented by widespread vegetation-free sion, mineralization, exhumation, and concealment beneath alluvial fan, evaporitic playa, and eolian deposits (Mpodozis postmineral debris flows (containing mineralized clasts), vol- and Clavero, 2002; Voss, 2002; Carrapa et al., 2005; Nalpas canic material, and fluviolacustrine sediments took place in et al., 2008; Mpodozis et al., 2018; this study). This apparent <2.3 m.y. (Braxton et al., 2012), a similarly brief interval to anomaly may reflect the fact that relief and, hence, erosion that documented at Josemaría. rate are maximized immediately following pulses of Andean In the Plio-Pleistocene Tombulilato porphyry copper- uplift (Carretier et al., 2015), in accord with the limited time gold district of North Sulawesi, Indonesia, high-sulfidation that separated the compressive deformation (~26–25 Ma)

from the rapid erosion responsible for deposit telescoping for their support and instigation of this paper; Patricio Jones, (~25–24.5 Ma) and subsequent unroofing (24.5–22.3 Ma) at Ricardo Martínez, Max La Motte, and Martín Rode for the dis- Josemaría. covery and early-stage exploration of Josemaría; Juan Arrieta Therefore, although few porphyry copper deposits world- and Leo Ortiz for conducting the subsequent exploration, wide possess (or at least preserve) direct geologic evidence which resulted in definition of the supergene enrichment; bearing on their exhumation histories, it is clear that deposits Japan Oil, Gas and Metals National Corporation (JOGMEC) in magmatic arc terranes subject to compressive tectonism for financial and technical contributions during nine years and rapid rock uplift can be unroofed in 1 to 2 m.y. Although (2009–2017) as joint-venture partner; Constantino Mpodozis this is most common under the tropical climatic conditions for transformative comments on an early manuscript draft; that prevailed during the unroofing at Ok Tedi, Boyongan- and Constantino Mpodozis, Pepe Perelló, and, on behalf of Bayugo, Tombulilato, and probably also Oyu Tolgoi, given Economic Geology, Thomas Bissig, Laura Maydagán, and Jer- its low-latitudinal Devonian position (Copper and Scotese, emy Richards for constructive reviews. 2003), the Josemaría deposit demonstrates that it is also possible in semiarid, mountainous terrain. REFERENCES Ammann, C., Jenny, B., Kammer, K, and Messerli, B., 2001, Late Quaternary Conclusions glacial response to humidity changes in the arid Andes of Chile (18–29°S): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 172, p. 313–326. The Josemaría porphyry copper-gold deposit is located in a Astorga, D., Crosato, S., Gallardo, M., Griffiths, S., Guerra, R., Jorquera, C., reverse fault-bounded block of Permian to Triassic igneous and Plasencia, C., 2017, Alturas: A unique discovery within a mature district basement that forms part of the Frontal Cordillera of west- through integrating sound geological practices, multidisciplinary expertise ernmost Argentina. The deposit is genetically related to small, and leading technology, in NewGenGold 2017, Case histories of discovery: West Perth, Paydirt Media Pty Ltd., p. 219–235. multiphase dacite porphyry intrusions that were emplaced in Bissig, T., Lee, J.K.W., Clark, A.H., and Heather, K.B., 2001, The Cenozoic a reverse fault zone concurrently with compression and con- history of volcanism and hydrothermal alteration in the Central Andean comitant uplift in the late Oligocene (~25–24.7 Ma). A tele- flat-slab region: New40 Ar-39Ar constraints from the El Indio-Pascua Au (-Ag, scoped sequence of alteration zones, from early potassic to a Cu) belt, 29°20'–30°30' S: International Geology Review, v. 43, p. 312–340. Bissig, T., Clark, A.H., Rainbow, A., and Montgomery, A., 2015, Physio- late advanced argillic lithocap, is broadly centered on the por- graphic and tectonic settings of high-sulfidation epithermal gold-silver phyry intrusions and, together, host the potentially economic deposits of the Andes and their controls on mineralizing processes: Ore copper-gold mineralization. Geology Reviews, v. 65, p. 327–364. During continued compressive tectonism and uplift, the Blair, T.C., and McPherson, J.G., 1994, Alluvial fans and their natural dis- deposit was unroofed, with some of the resultant detritus tinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages: Journal of Sedimentary Research, v. 64A, contributing to a nearby red-bed conglomerate, breccia, p. 450–489. and sandstone sequence. These strata and overlying volcanic Braxton, D.P., Cooke, D.R., Dunlap, J., Norman, M., Reiners, P., Stein, H., rocks, dated at ~22.3 Ma, eventually buried the Josemaría and Waters, P., 2012, From crucible to graben in 2.3 Ma: A high-resolution deposit. The postmineral volcano-sedimentary package was geochronological study of porphyry life cycles, Boyongan-Bayugo copper- then locally subjected to weakly developed, high-sulfidation gold deposits, Philippines: Geology, v. 40, p. 471–474. Cáceres, M., 2015, Nuevos antecedentes geocronológicos del Distrito Casale, epithermal copper mineralization, which, in marked con- sur de la Franja Maricunga, Región de Atacama, Chile [ext. abs.]: Con- trast to the high-sulfidation mineralization in the lithocap, greso Geológico Chileno, 14th, La Serena, 2015, Extended Abstracts, v. 2, is gold deficient. The deposit was subsequently reexhumed p. 44–47. from beneath its volcano-sedimentary cover and, during or Cahill, T., and Isacks, B.L., 1992, Seismicity and shape of the subducted Nazca Plate: Journal of Geophysical Research: Solid Earth, v. 97, p. 17,503–17,529. after minor extensional reactivation of the reverse fault zone, Carrapa, B., Adelmann, D., Hilley, G.E., Mortimer, E., Sobel, E.R., and underwent supergene sulfide oxidation and enrichment, with Strecker, M.R., 2005, Oligocene range uplift and development of plateau the highest grades confined to the proximity of the fault. morphology in the southern central Andes: Tectonics, v. 24, TC4011. Many porphyry copper deposits in the central Andes were Carretier, S., Tolorza, V., Rodríguez, M.P., Pepin, E., Aguilar, G., Regard, formed during uplift resulting from the crustal shortening and V., Martinod, J., Riquelme, R., Bonnet, S., Brichau, S., Hérail, G., Pinto, L., Farías, M., Charrier, R., and Guyot, J.L., 2015, Erosion in the Chilean thickening that accompanied compressive tectonism (e.g., Andes between 27°S and 39°S: tectonic, climatic and geomorphic control: Maksaev and Zentilli, 1999; Perelló et al., 2003b; Sillitoe and Geological Society, London, Special Publication 399, p. 401–418. Perelló, 2005; Maksaev et al., 2009) and, as a consequence, Coloma, F., Valin, X, Oliveros, V., Vásquez, P., Creixell, C., Salazar, E., and are likely to have been rapidly exhumed; however, Josemaría Ducea, M.N., 2017, Geochemistry of Permian to Triassic igneous rocks is the only known example in which the presence of post- from northern Chile (28º-30º15’S): Implications on the dynamics of the proto-Andean margin: Andean Geology, v. 44, p. 147–178. mineral cover allows estimation of the actual time required, Copper, P., and Scotese, C.R., 2003, Megareefs in Middle Devonian super- ~2 m.y., for the unroofing to take place. Josemaría joins sev- greenhouse climates: Geological Society of America Special Paper 370, eral other porphyry copper-gold deposits, predominantly in p. 209–230. the western Pacific region, that were exhumed extremely rap- Decker. J.H., 1994, Mining investment in Argentina, in Mining Latin Amer- ica: London, Institution of Mining and Metallurgy, p. 3–16. idly, in 1 to 2 m.y., and then protected from further erosion by del Rey, A., Deckart, K., Arriagada, C., and Martínez, F., 2016, Resolving concealment beneath postmineral cover. However, Josemaría the paradigm of the late Paleozoic-Triassic Chilean magmatism: Isotopic appears currently to be unique in that the erosion took place approach: Gondwana Research, v. 37, p. 172–181. under semiarid rather than high-rainfall, tropical conditions. Esri, 2009, World shaded relief [basemap]: Redlands, California, Environ- mental Systems Research Institute, https://www.arcgis.com/home/item. Acknowledgments html?id=9c5370d0b54f4de1b48a3792d7377ff2 (accessed April 7, 2019). Fauqué, L., 2001, Hoja geológica 2969-I Pastillos, La Rioja y San Juan. The senior management of NGEx Resources Inc., Wojtek 1:250,000: Servicio Geológico Minero Argentino [SEGEMAR] Boletín 316 Wodzicki, Bob Carmichael, and Alfredo Vitaller, are thanked (preliminar).

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Gustafson, L.B., and Hunt, J.P., 1975, The porphyry copper deposit at El provincia de La Rioja: Simposio Nacional de Geología Económica, 1st, San Salvador, Chile: Economic Geology, v. 70, p. 857–912. Juan, Argentina, 1971, v. 2, p. 305–318. Gutscher, M.-A., Spakman, W., Bijwaard, H., and Engdahl, E.R., 2000, Geo- Martin, M.W., Clavero, J., and Mpodozis, C., 1999, Late Paleozoic to Early dynamics of flat subduction: Seismicity and tomographic constraints from Jurassic tectonic development of the high Andean Principal Cordillera, El the Andean margin: Tectonics, v. 19, p. 814–833. Indio region, Chile (29-30°S): Journal of South American Earth Sciences, Hedenquist, J.W., and Taran, Y.A., 2013, Modeling the formation of advanced v. 12, p. 33–49. argillic lithocaps: Volcanic vapor condensation above porphyry intrusions: Martínez, F., Peña, M., and Arriagada, C., 2015a, Geología de las áreas Iglesia Economic Geology, v. 108, p. 1523–1540. Colorada-Cerro del Potro y Cerro Mondaquita, Región de Atacama. Escala Hervé, M., Sillitoe, R.H., Wong, C., Fernández, P., Crignola, F., Ipinza, M., 1:100,000: Servicio Nacional de Geología y Minería, Carta Geológica de and Urzúa, F., 2012, Geologic overview of the Escondida porphyry copper Chile, Serie Geología Básica 179–180, 67 p. district, northern Chile: Society of Economic Geologists Special Publica- Martínez, F., Arriagada, C., Valdivia, R., Deckart, K., and Peña, M., 2015b, tion 16, p. 55–78. Geometry and kinematics of the Andean thick-skinned thrust systems: Holley, E.A., Bissig, T., and Monecke, T., 2016, The Veladero high-sulfidation Insights from the Chilean Frontal Cordillera (28°–28.5°S), Central Andes: epithermal gold deposit, El Indio-Pascua belt, Argentina: Geochronology Journal of South American Earth Sciences, v. 64, p. 307–324. of alunite and jarosite: Economic Geology, v. 111, p. 311–330. Maydagán, L., Franchini, M., Chiaradia, M., Dilles, J., and Rey, R., 2014, Horton, B.K., 2018, Tectonic regimes of the central and southern Andes: The Altar porphyry Cu-(Au-Mo) deposit (Argentina): A complex magmatic- Responses to variations in plate coupling during subduction: Tectonics, hydrothermal system with evidence of recharge processes: Economic Geol- v. 37, p. 402–429. ogy, v. 109, p. 621–641. Jones, J.P., 2007, Exploración minera en la República Argentina, áreas exami- McInnes, B.I.A., Evans, N.J., Fu, F.Q., and Garwin, S., 2005, Application nadas por Deprominsa: Asociación Argentina de Geólogos Economistas, of thermochronology to hydrothermal ore deposits: Reviews in Mineralogy Publicación Especial 1, p. 10–20. and Geochemistry, v. 58, p. 467–498. Jones, J.P., and Martinez, R.D., 2007, Aspectos regionales de nuevos sistemas Mpodozis, C., and Clavero, J., 2002, Tertiary tectonic evolution of the south- porfíricos en Argentina, desde Salta hasta Mendoza: Asociación Argentina western edge of the Puna Plateau: Cordillera Claudio Gay (26°-27° S), de Geólogos Economistas, Publicación Especial 1, p. 21–30. northern Chile [ext. abs.]: International Symposium on Andean Geodynam- Jordan, T.E., Isacks, B.L., Allmendinger, R.W., Brewer, J.A., Ramos, V.A., and ics, 5th, Toulouse, 2002, Extended Abstracts, p. 445–448. Ando, C.J., 1983, Andean tectonics related to geometry of subducted Nazca Mpodozis, C., and Kay, S.M., 1992, Late Paleozoic to Triassic evolution of the plate: Geological Society of America Bulletin, v. 94, p. 341–361. Gondwana margin: Evidence from Chilean Frontal Cordilleran batholiths Kay, S.M., and Mpodozis, C., 2002, Magmatism as a probe to the Neogene (28°S to 31°S): Geological Society of America Bulletin, v. 104, p. 999–1014. shallowing of the Nazca plate beneath the modern Chilean flat-slab: Journal ——2003, Neogene tectonics, ages and mineralization along the transition of South American Earth Sciences, v. 15, p. 39–57. zone between the El Indio and Maricunga mineral belts (Argentina and Kay, S.M., Maksaev, V., Moscoso, C., Mpodozis, C., Nasi, C., and Gordillo, Chile 28°-29°S) [abs.]: Congreso Geológico Chileno, 10th, Concepción, E., 1988, Tertiary Andean magmatism in Chile and Argentina between 28°S 2003, CD-ROM, 1 p. and 33°S: Correlation of magmatic chemistry with a changing Benioff zone: Mpodozis, C., Cornejo, P., Kay, S.M., and Tittler, A., 1995, La Franja de Mari- Journal of South American Earth Sciences, v. 1, p. 21–38. cunga: síntesis de la evolución del Frente Volcánico Oligoceno-Mioceno Kay, S.M., Mpodozis, C., Tittler, A., and Cornejo, P., 1994, Tertiary magmatic de la zona sur de los Andes Centrales: Revista Geológica de Chile, v. 21, evolution of the Maricunga mineral belt in Chile: International Geology p. 273–313. Review, v. 36, p. 1079–1112. Mpodozis, C., Clavero, J, Quiroga, R., Droguett, B., and Arcos, R., 2018, Kay, S.M., Mpodozis, C., and Gardeweg, M., 2014, Magma sources and tec- Geología del área Cerro Cadillal-Cerro Jotabeche, región de Atacama. tonic setting of Central Andean andesites (25.5-28°S) related to crustal Escala 1:100,000: Servicio Nacional de Geología y Minería, Carta Geológica thickening, forearc subduction erosion and delamination: Geological Soci- de Chile, Serie Geología Básica 200, 94 p. ety, London, Special Publication 385, p. 303–334. Kraemer, B., Adelmann, D., Altena, M., Schnurr, W., Erpenstein, K., Kiefer, Nalpas, T., Dabard, M.P., Ruffet, G., Vernon, A., Mpodozis, C., Loi, A., and E., van den Bogaard, P., and Görler, K., 1999, Incorporation of the Pale- Hérail, G., 2008, Sedimentation and preservation of the Miocene Atacama ogene foreland into the Neogene Puna plateau: The Salar de Antofalla Gravels in the Pedernales-Chañaral area, northern Chile: Climatic or tec- area, NW Argentina: Journal of South American Earth Sciences, v. 12, tonic control?: Tectonophysics, v. 459, p. 161–173. p. 157–182. Ortiz, L., Arrieta, J., and Rode, M., 2015, El pórfido de Cu-Au de Josemaría, Lang, J.R., Gregory, M.J., Rebagliati, C.M., Payne, J.G., Oliver, J.I., and Rob- San Juan, Argentina [ext. abs.]: Congreso Geológico Chileno, 14th, La Ser- erts, K., 2013, Geology and magmatic-hydrothermal evolution of the giant ena, 2015, Extended Abstracts, v. 2, p. 377–380. Pebble porphyry copper-gold-molybdenum deposit, southwest Alaska: Ovalle, A., Quiñones, C., Quezada, C., Frost, D., Khera, V., and Zandonai, G., Economic Geology, v. 108, p. 437–462. 2016, Constellation project incorporating the Los Helados deposit, Chile Large, S.J.E., von Quadt, A., Wotzlaw, J.-F., Guillong, M., and Heinrich, and the Josemaría deposit, Argentina: NI 43-101 technical report on pre- C.A., 2018, Magma evolution leading to porphyry Au-Cu mineralization at liminary economic assessment: http://www.ngexresources.com/site/assets/ the Ok Tedi deposit, Papua New Guinea: Trace element geochemistry and files/3467/Josemaríaresourceestimate.pdf, 315 p. Accessed 8 March 2018. high-precision geochronology of igneous zircon: Economic Geology, v. 113, Page, R.W., and McDougall, I., 1972, Ages of mineralization of gold and por- p. 39–61. phyry copper deposits in the New Guinea Highlands: Economic Geology, Lossada, A.C., Giambiagi, L., Hoke, G.D., Fitzgerald, P.G., Creixell, C., v. 67, p. 1034–1048. Murillo, I., Mardonez, D., Velásquez, R., and Suriano, J., 2017, Thermo- Panteleyev, A., and Cravero, O., 2001, Faja del Potro and Cordón de la Brea chronologic evidence for late Eocene Andean mountain building at 30°S: ore deposits, La Rioja and San Juan. Radiometric dating, analytical results Tectonics, v. 36, p. 2693–2713. and sample documentation: Servicio Geológico Minero Argentino (SEGE- Maksaev, V., and Zentilli, M., 1999, Fission track thermochronology of the MAR), Serie Contribuciones Técnicas, Recursos Minerales 11, 56 p. Domeyko Cordillera, northern Chile: Implications for Andean tectonics Perelló, J.A., 1994, Geology, porphyry Cu-Au, and epithermal Cu-Au-Ag and porphyry copper metallogenesis: Exploration and Mining Geology, v. 8, mineralization of the Tombulilato district, North Sulawesi, Indonesia: Jour- p. 65–89. nal of Geochemical Exploration, v. 50, p. 221–256. Maksaev, V., Moscoso, R., Mpodozis, C., and Nasi, C., 1984, Las unidades Perelló, J., Mpodozis, C., and Urzúa, F., 2003a, Late Oligocene-early Mio- volcánicas y plutónicas del Cenozoico superior en la Alta Cordillera del cene porphyry Cu-Mo and Cu-Au mineralization, Pulido cluster, Atacama Norte Chico (29°-31°S): Geología, alteración hidrotermal y mineralización: Region, northern Chile [abs.]: Congreso Geológico Chileno, 10th, Concep- Revista Geológica de Chile, no. 21, p. 11–51. ción, 2003, CD-ROM, 1 p. Maksaev, V., Munizaga, F., Zentilli, M., and Charrier, R., 2009, Fission track Perelló, J., Carlotto, V., Zárate, A., Ramos, P., Posso, H., Neyra, C., Caballero, thermochronology of Neogene plutons in the Principal Andean Cordillera A., Fuster, N., and Muhr, R., 2003b, Porphyry-style alteration and miner- of central Chile (33-35°S): Implications for tectonic evolution and porphyry alization of the middle Eocene to early Oligocene Andahuaylas-Yauri belt, Cu-Mo mineralization: Andean Geology, v. 36, 153–171. Cuzco region, Peru: Economic Geology, v. 98, p. 1575–1605. Marcos, O.R., Faroux, R., Alderete, M., Guerrero, M.A., and Zolezzi, R., Perelló, J., Sillitoe, R.H., Mpodozis, C., Brockway, H., and Posso, H., 2012, 1971, Geología y prospección geoquímica de la Cordillera Frontal en la Geologic setting and evolution of the porphyry copper-molybdenum and

copper-gold deposits at Los Pelambres, central Chile: Society of Economic Song, Y., Yang, C., Wei, S., Yang, H., Fang, X., and Lu, H., 2018, Tectonic Geologists Special Publication 16, p. 79–104. control, reconstruction and preservation of the Tiegelongnan porphyry and Perucca, L., and Angillieri, Y.E., 2008, A preliminary inventory of periglacial epithermal overprinting Cu (Au) deposit, central Tibet, China: Minerals, v. landforms in the Andes of La Rioja and San Juan, Argentina, at about 28°S: 8 (9), article 398. Quaternary International, v. 190, p. 171–179. Vila, T., and Sillitoe, R.H., 1991, Gold-rich porphyry systems in the Mari- Proffett, J.M., 2003, Geology of the Bajo de la Alumbrera porphyry copper- cunga belt, northern Chile: Economic Geology, v. 86, p. 1238–1260. gold deposit, Argentina: Economic Geology, v. 98, p. 1535–1574. Vila, T., Sillitoe, R.H., Betzhold, J., and Viteri, E., 1991, The porphyry gold Ramos, V.A., 1999, Las provincias geológicas del territorio argentino, in deposit at Marte, northern Chile: Economic Geology, v. 86, p. 1271–1286. Haller, M., ed., Geología argentina: Instituto de Geología y Recursos Mine- Voss, R., 2002, Cenozoic stratigraphy of the southern Salar de Antofalla region, rales [Buenos Aires], Anales 29 (3), p. 41–96. northwestern Argentina: Revista Geológica de Chile, v. 29, p. 151–165. Ramos, V.A., Cristallini, E.O., and Pérez, D.J., 2002, The Pampean flat-slab Wainwright, A.J., Tosdal, R.M., Lewis, P.D., and Friedman, R.M., 2017, of the Central Andes: Journal of South American Earth Sciences, v. 15, Exhumation and preservation of porphyry Cu-Au deposits at Oyu Tolgoi, p. 59–78. South Gobi region, Mongolia: Economic Geology, v. 112, p. 591–601. Ribba, L., Mpodozis, C., Hervé, F., Nasi, C., and Moscoso, R., 1988, El basa- Yanites, B.J., and Kesler, S.E., 2015, A climate signal in exhumation patterns mento del Valle del Tránsito, cordillera de Vallenar: eventos magmáticos y revealed by porphyry copper deposits: Nature Geoscience, v. 8, p. 462–465. metamórficos y su relación con la evolución de los Andes chileno-argenti- Yoshie, T. Otsubo, T., Oku, N., and Ueda, Y., 2015, Exploration and resource nos: Revista Geológica de Chile, v. 15, p. 129–149. estimation of the Caserones porphyry Cu-Mo deposit in III Region, Chile: Rode, M., and Carrizo, M., 2007, La faja de pórfidos del Salado, entre Valle Shigen-Chishitsu, v. 65, p. 53–79 (in Japanese with English abs.). Ancho y Vicuña, provincias de Catamarca, La Rioja y San Juan: Asociación Zappettini, E., Miranda Angeles, V., Rodrigues, C., Palacios, O., Cocking, R., Argentina de Geólogos Economistas, Publicación Especial 1, p. 31–50. Godeas, M., Uribe-Zeballos, H., Vivallo, W., Paz, M.M., Seggiaro, R., Heu- Rode, M., Guitart, A., Sanguinetti, M., and Richard, F., 2015, El depósito schmidt, B., Gardeweg, M., Boulangger, E., Koreniewski, L., Mpodozis, C., tipo pórfido Cu-Au de Los Helados, III Región, Atacama, Chile [ext. abs.]: Carpio, M., and Rubiolo, D., 2001, Mapa metalogénico de la región fronter- Congreso Geológico Chileno, 14th, La Serena, 2015, Extended Abstracts, iza entre Argentina, Bolivia, Chile y Perú (14°S y 28°S), escala 1:1,000.000: v. 2, p. 373–376. Buenos Aires, Servicio Geológico Minero Argentino [SEGEMAR], Pub- Rossel, K., Aguilar, G., Salazar, E., Martinod, J., Carretier, S., Pinto, L., and licación Geológica Multinacional 2, 222 p., https://sigam.segemar.gov.ar/ Cabré, A., 2018, Chronology of Chilean Frontal Cordillera building from visor. geochronological, stratigraphic and geomorphological data insights from Zappettini, E., Ferpozzi, F., and Olmos, M., 2008, Geología de frontera Miocene intramontane‐basin deposits: Basin Research, v. 30, p. 289–310. Argentina-Chile, escala 1:500,000: Buenos Aires, Servicio Geológico Sanguinetti, M.I., 2006, Geología del sector El Potro y Los Helados, Cordil- Minero Argentino [SEGEMAR], Web Map Service, http://sig.segemar.gov. lera Frontal (28°20’, 69°30’), Provincias de La Rioja y Copiapó, Argentina y ar/wms. Chile: Buenos Aires, Universidad de Buenos Aires, unpub. Tesis de Licen- Zhou, R., Schoenbohm, L.M., Sobel, E.R., Davis, D.W., and Glodny, J., 2017, ciatura, 138 p. New constraints on orogenic models of the southern Central Andean Pla- Scoates, J.S., and Friedman, R.M., 2008, Precise age of the platiniferous teau: Cenozoic basin evolution and bedrock exhumation: Geological Soci- Merensky Reef, Bushveld Complex, South Africa, by the U-Pb zircon chem- ety of America Bulletin, v. 129, p. 152–170. ical abrasión ID-TIMS technique: Economic Geology, v. 103, p. 465–471. SERNAGEOMIN, 2003, Mapa geológico de Chile, versión digital: Servicio Nacional de Geología y Minería, Publicación Digital 4 (CD-ROM, versión 1.0, 2003). Sillitoe, R.H., 1979, Some thoughts on gold-rich porphyry copper deposits: Mineralium Deposita, v. 14, p. 161–174. ——1994, Erosion and collapse of volcanoes: Causes of telescoping in intrusion-centered ore deposits: Geology, v. 22, p. 945–948. ——2000, Gold-rich porphyry deposits: Descriptive and genetic models and their role in exploration and discovery: Reviews in Economic Geology, v. 13, p. 315–345. ——2005, Supergene oxidized and enriched porphyry copper and related deposits: Economic Geology 100th Anniversary Volume, p. 723–768. ——2010, Porphyry copper systems: Economic Geology, v. 105, p. 3–41. Sillitoe, R.H., and Perelló, J., 2005, Andean copper province: Tectonomag- Richard Sillitoe gained B.Sc. and Ph.D. degrees matic settings, deposit types, metallogeny, exploration, and discovery: Eco- from the University of London, England. After nomic Geology 100th Anniversary Volume, p. 845–890. working for the Geological Survey of Chile and Sillitoe, R.H., McKee, E.H., and Vila, T., 1991, Reconnaissance K-Ar geo- then returning to the University of London as a chronology of the Maricunga gold-silver belt, northern Chile: Economic Shell postdoctoral research fellow, he has operated Geology, v. 86, p. 1261–1270. for over 45 years as an independent consultant to Sillitoe, R.H., Tolman, J., and Van Kerkvoort, G., 2013, Geology of the Caspi- more than 300 mining companies, international che porphyry gold-copper deposit, Maricunga belt, northern Chile: Eco- nomic Geology, v. 108, p. 585–604. agencies, and foreign governments. He has worked on a wide variety of pre- Sillitoe, R.H., Burgoa, C., and Hopper, D.R., 2016, Porphyry copper discov- cious-, base-, and lithophile-metal deposits and prospects in 100 countries ery beneath the Valeriano lithocap, Chile: Society of Economic Geologists worldwide, but focuses primarily on the epithermal gold and porphyry copper Newsletter 106, p. 1, 15–20. environments.

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