Geology of the Caspiche Porphyry Gold-Copper Deposit, Maricunga Belt, Northern Chile*

RICHARD H. SILLITOE,1,† JUSTIN TOLMAN,2,** AND GLEN VAN KERKVOORT3,*** 1 27 West Hill Park, Highgate Village, London N6 6ND, England 2 Exeter Resource Corporation, Suite 1660 - 999 W. Hastings St., Vancouver, BC V6C 2W2, Canada 3 Exeter Resource Corporation, Suite 701, 121 Walker St., North Sydney, NSW 2060, Australia

Abstract The Caspiche porphyry gold-copper deposit, part of the Maricunga gold-silver-copper belt of northern Chile, was discovered in 2007 beneath postmineral cover by the third company to explore the property over a 21-year period. This company, Exeter Resource Corporation, has announced a proven and probable mineral reserve of 1,091 million tonnes (Mt) averaging 0.55 g/t Au, all but 124 Mt of which also contain 0.23% Cu, for a total of 19.3 Moz of contained gold and 2.1 Mt of copper. The deposit was formed in the latest Oligocene (~25 Ma) during the first of two volcanic and corresponding metallogenic epochs that define the Maricunga belt. The gold-copper mineralization is centered on a composite diorite to quartz diorite porphyry stock, within which five outward-younging phases are rou- tinely distinguished. The centrally located, early diorite porphyry (phase 1) hosts the highest-grade ore, averaging ~1 g/t Au and 0.4% Cu. The subsequent porphyry phases are quartz dioritic in composition and characterized by progressively lower gold and copper tenors. Stock emplacement was both pre- and postdated by the generation of large-volume, andesite-dominated breccias, with tuffaceous matrices, which are believed to be shallow portions of diatremes. The deposit is characterized by a central gold-copper zone and partially overlapped but noneconomic molybdenum halo. The gold-copper mineralization in the lower half of the deposit accompanies quartz ± magnetite-veined, potassic-altered rocks, whereas the shallower mineralization occurs within quartz-kaolinite–dominated, advanced argillic alteration. Upper parts of the advanced argillic zone are characterized by siliceous ledges, some auriferous, composed of vuggy residual quartz and/or silicified rock. The chalcopyrite-pyrite mineralization in the potassic zone was partially transformed to high sulfidation-state sulfides and sulfosalts during the advanced argillic overprint, although the underlying chalcopyrite-bornite assemblage was mainly too deep to be affected. The deposit terminates downward in a sulfide-deficient, potassic-calcic zone defined by K-feldspar, actinolite, and magnetite, which formed at the expense of biotite. A relatively minor, shallowly inclined zone of intermediate sulfidation epithermal gold-zinc mineralization, comprising narrow veinlets and disseminations, abuts the late-mineral diatreme contact. Supergene sulfide oxidation throughout the deposit is relatively shallow, and chalcocite enrichment extremely minor. The Caspiche deposit is thought to have been emplaced at relatively shallow paleodepths, within the southern, flat-bottomed part of the premineral diatreme vent. Earliest porphyry system development, probably north of the present deposit, appears to have been aborted by diatreme formation. Much of the gold and copper in the Caspiche deposit was introduced during the potassic alteration stage, with the highly telescoped, advanced argillic overprint being responsible for only minor redistribution of the two metals, and the addition of arsenic. The late-mineral diatreme was emplaced west of the Caspiche deposit, and caused destruction of only its uppermost peripheral parts. The late-mineral diatreme was both pre- and postdated by advanced argillic alteration. Finally, the intermediate sulfidation epithermal gold-zinc zone was localized by the enhanced perme- ability provided by intense fracturing along the underside of the upward-flared, late-mineral diatreme contact.

Introduction and Exploration History Oligocene to Miocene volcanic complexes and comagmatic THE CASPICHE porphyry gold-copper deposit is the latest dis- subvolcanic stocks associated with porphyry gold ± copper covery in the Maricunga metallogenic belt, located between and high sulfidation epithermal gold ± silver deposits and latitudes 26° and 28°S in the high volcanic Andes of the Ata- prospects (Vila and Sillitoe, 1991; Mpodozis et al., 1995; Fig. cama Region, northern Chile (Fig. 1). The belt, some 200 km 2). The alignment of mineralized centers continues to the long and 30 km wide, is an N-trending alignment of latest south, via the Caserones (formerly Regalito) porphyry copper deposit (Perelló et al., 2003) and nearby prospects, to the El Indio belt, where high-grade vein (e.g., El Indio) and bulk- † Corresponding author: e-mail, [email protected] tonnage (e.g., Pascua-Lama, Veladero), high sulfidation epi- *A digital supplement to this paper is available at http://economicgeology. thermal gold ± silver deposits predominate (e.g., Maksaev et org/and at http://econgeol.geoscienceworld.org/. al., 1984; Jannas et al., 1999; Chouinard et al., 2005; Fig. 1). **Present address: New Gold Inc., 12200 E Briarwood Ave., Suite 165, Miocene porphyry gold prospects also occur farther east, in Centennial, CO 80112. ***Present address: Rio Pison Gold Ltd., P.O. Box 758, Buderim, Queens- westernmost Argentina, where they were generated in the land 4556, Australia. backarc to the Maricunga belt.

Submitted: February 12, 2012 0361-0128/13/4108/585-20 585 Accepted: September 15, 2012 586 SILLITOE ET AL.

Bolivia Potrerillos -70°

-26°30’ ESPERANZA Argentina Chile Antofagasta

LA COIPA Fig. 2 PUREN

Copiapó CASPICHE Maricunga belt -27° Salar de CASERONES (REGALITO) Maricunga MARTE

PASCUA -LAMA VELADERO EL INDIO -30° -30° El Indio belt LOBO LA PEPA

VOLCAN

PANTANILLOS

-27°30’ Laguna del Negro Santiago REFUGIO Francisco 200 km -70°

FIG. 1. Location of the Caspiche porphyry gold-copper deposit in the CASPICHE Maricunga belt with respect to its southern continuation—the El Indio belt—and some key deposits. Area shown in Figure 2 is marked.

The Maricunga belt was defined during the mid- to late CERRO CASALE 1980s during a regional exploration program of areally extensive alteration zones for gold and silver conducted by Minera Anglo American Chile Ltda. and joint-venture partners. 10 km However, it was not until one of these zones, the host to the -69°30’ Marte deposit (Fig. 2), was systematically explored that the 16 to 11 Ma volcanic rocks presence of porphyry-type mineralization was first recognized (Vila and Sillitoe, 1991; Vila et al., 1991). The porphyry de- 26 to 17 Ma volcanic rocks posits, including those at Marte, Lobo, Refugio, La Pepa, and Mineral deposit Volcán (Fig. 2), typically contain gold as the only economi- Reverse fault cally extractable metal, whereas those at Caspiche and Cerro Other fault Casale (Fig. 2) contain gold plus by-product copper potential. Caspiche is located at 4,200 to 4,560 m above sea level, im- FIG. 2. Location of the Caspiche porphyry gold-copper deposit in the mediately east of the topographically and visually prominent Maricunga belt, showing the two ages of mineralized volcanic rocks and the Santa Cecilia alteration zone, from which it is separated by a main porphyry and epithermal deposits. The underlined deposits accompa- 250-m-wide valley (Fig. 3). Santa Cecilia was extensively nied the younger volcanism. Compiled from Kay et al. (1994).

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West Santa Cecilia

Ledge Ledge Surface projection of Caspiche Au-Cu deposit

FIG. 3. View of Caspiche looking west toward the highest point of the contiguous Santa Cecilia alteration zone (~4,600 m above sea level). The surface projection of the concealed porphyry gold-copper deposit, ~800 m north-south (red line), lies between two prominent silicified and vuggy residual quartz ledges (pale tones). The late-mineral diatreme lies beneath the valley (midview) separating Caspiche and Santa Cecilia.

investigated, but with only limited success, by Anglo Ameri- present at depth alongside the main siliceous ledge might can from 1985 to 1990 under an option agreement with a contain higher-grade, structurally controlled gold ore. During Chilean mining entrepreneur. At the same time, Anglo Amer- the 1996 to 1997 season, Newcrest conducted detailed geo- ican also claimed surrounding open ground, within which the logic mapping, rock and trench geochemical sampling, and an company recognized and then explored the Caspiche induced polarization-resistivity survey over the Caspiche and prospect, with a focus on the prominently exposed, gold- and newly pegged contiguous claims, preparatory to drilling 12 re- silver-bearing, siliceous ledges of high sulfidation epithermal verse-circulation holes to a nominal depth of 300 m at type (Fig. 3). Anglo American collected 22 rock-chip samples Caspiche. The results suggested the presence of bulk-ton- from the ledges during the 1985 to 1986 summer field season, nage, high sulfidation epithermal gold mineralization grading obtaining gold values ranging from 0.5 to 4 g/t. Grid talus- 0.3 to 0.6 g/t, but one of the holes, collared in an area of post- fines geochemistry and more detailed rock-chip sampling mineral cover between the siliceous ledges (Fig. 3), report- were followed by 12 50-m air-track holes during the 1986 to edly intersected potassic-altered “microdiorite porphyry” av- 1987 season. Based on the reasonably encouraging results eraging 0.73 g/t Au and 0.23% Cu from 232 to 326 m, with from two of the holes (including 48 m at 1.03 g/t Au), detailed values increasing near the bottom of the hole. Later in 1997, geologic mapping and six deeper (150–200 m) reverse-circu- a district-wide, helicopter-borne aeromagnetic survey was lation holes were completed in the 1989 to 1990 season, but flown, resulting in definition of a prominent magnetic high the best intercept was 148 m at 0.49 g/t Au. A preliminary re- centered immediately south of the previously drilled area. source of 6.3 million tonnes (Mt) averaging 0.45 g/t Au was Newcrest geologists appreciated that the 94-m gold inter- estimated, leading to all work at Caspiche being terminated. cept was of porphyry rather than epithermal style, and rec- The Caspiche prospect lay fallow for six years before Minera ommended follow-up core drilling to test the gold-copper Newcrest Chile Ltda. entered into a purchase option agree- mineralization beneath the covered area farther south and ment with Anglo American, with the aim of following up the west and at greater depth. Eventually, only two reverse-cir- low-grade, disseminated gold intercepts obtained previously culation holes, totaling 532 m, were drilled on the porphyry as well as testing the possibility that a “microdiorite” intrusion gold-copper target during the 1997 to 1998 season, both

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designed to test the magnetic high. The best results, 224 m Frontal farther east in Argentina (Mpodozis and Ramos, averaging 0.14 g/t Au, were discouraging and led to the option 1990). These rocks are capped by Triassic bimodal volcanic being terminated, but without it being appreciated that the and sedimentary rocks deposited in extensional rift basins and holes had penetrated a weakly mineralized, intermineral por- Jurassic to Early Cretaceous marine and continental sedi- phyry intrusion (see below). mentary rocks that accumulated in a backarc basin developed Following an additional eight years of inactivity, the prop- throughout northern Chile (Mercado, 1982; Mpodozis et al., erty was again optioned by Anglo American, this time to Ex- 1995; Cornejo et al., 1998; Iriarte et al., 1999; Arriagada et al., eter Resource Corporation, a Toronto-listed junior explorer. 2006). Late Cretaceous to Eocene arc to backarc volcanic During the 2005 to 2006 field season, Exeter conducted geo- rocks and continental redbeds formed after the tectonic in- logic mapping, rock-chip sampling, and a controlled source version of the basin and eastward migration of the Andean arc audio-frequency magnetotelluric (CSAMT) survey, followed (Cornejo et al., 1998; Iriarte et al., 1999; Mpodozis and in early 2007 by reverse-circulation drill testing of high sulfi- Clavero, 2002). In the southern Maricunga belt, between dation targets beyond the immediate Caspiche porphyry Caspiche and Cerro Casale (Fig. 2), as well as farther east and prospect. As documented more fully by Van Kerkvoort et al. south, the Paleocene volcanic units are covered by Oligocene (2009), only at the very end of the 2006 to 2007 season was it continental sedimentary sequences, composed of red sand- elected to drill the Caspiche prospect itself, with time remain- stone, siltstone, and conglomerate, and including evaporitic ing for just one hole, which returned 304 m averaging 0.90 g/t horizons (Mpodozis and Kay, 2003). The siliciclastic redbeds, Au and 0.26% Cu. In early 2008, this discovery hole was fol- which crop out west of Caspiche and are a major host rock to lowed up with a campaign of core drilling, with the third hole the porphyry deposit at depth (see below), are most likely reporting 793 m at 0.96 g/t Au and 0.40% Cu (Van Kerkvoort Oligocene in age because, farther south, they unconformably et al., 2009). By the beginning of 2012, ~63,000 m of core overlie Eocene redbeds and volcanic rocks (Mpodozis et al., drilling had been completed at Caspiche, the results of which, 1991; C. Mpodozis, written comm., 2011). coupled with engineering studies, enabled estimation of a The late Oligocene and Miocene Maricunga volcanic proven plus probable sulfide reserve of 967 Mt grading 0.57 rocks, predominantly medium- to high-K calc-alkaline an- g/t Au and 0.23% Cu, using an economic cutoff formula based desite to dacite in composition (Kay et al., 1994; McKee et on recovery, mining cost, and other parameters, but approxi- al., 1994; Mpodozis et al., 1995), are chiefly the products of mating 0.3 g/t Au equiv. To this should be added an overlying extinct stratovolcanoes and volcanic dome complexes, which proven plus probable oxide reserve of 124 Mt averaging 0.38 display varied degrees of erosional dissection (Vila and Silli- g/t Au, at a 0.18 g/t Au equiv cutoff. The reserves contain a toe, 1991; Kay et al., 1994; Mpodozis et al., 1995). The por- total of 19.3 million ounces of gold and 2.1 Mt of copper (Ex- phyry and high sulfidation epithermal deposits, closely eter Resource Corporation press release, 17 January 2012). linked in places (e.g., Caspiche, La Pepa; Fig. 2), have been This paper describes the geologic setting, alteration-miner- shown by K-Ar and 40Ar/39Ar dating (Sillitoe et al., 1991; alization characteristics, and evolutionary history of the com- Mpodozis et al., 1995) to have been generated beneath both pletely concealed Caspiche porphyry gold-copper deposit over these main volcanic landforms during two principal metallo- its known 1,400-m vertical extent, with particular emphasis on genic epochs: latest Oligocene to earliest Miocene (26–21 a spectrum of variably mineralized breccias. The geologic de- Ma) and mid-Miocene (14–10 Ma). The Caspiche deposit is scription and interpretation, based on logging of all drill core, assigned to the older of these two epochs on the basis of an reflects the latest version of the model that has guided the Re-Os age for molybdenite of 25.38 ± 0.09 Ma (H. Stein, exploration program; it complements the detailed discovery unpub. report for Gold Fields Chile S.A., 2009; see online case history presented elsewhere (Van Kerkvoort et al., 2009). Supplementary Data), which was determined using the methodology described by Zimmerman et al. (2008). Regional Setting Caspiche is penecontemporaneous with and probably geneti- The Maricunga belt, situated along the southwestern edge cally related to the contiguous Santa Cecilia alteration zone of the Altiplano-Puna plateau, spans the boundary between (Fig. 3), which is hosted by a stratified volcanic sequence, in- the currently active Central Volcanic Zone of the Andes and, cluding ignimbrite flows, and at least one dacite dome. Santa to the south, the nonvolcanic segment between 28° and Cecilia is known to host minor, albeit locally high grade, high 33°S—a change caused by the transition from moderate- sulfidation epithermal gold mineralization, which returned angle to flat-slab subduction (Thorpe et al., 1982; Cahill and essentially identical K-Ar ages of 24.3 ± 0.7 and 24.1 ± 0.8 Ma Isacks, 1992; Kay and Mpodozis, 2002). Arc magmatism in for hydrothermal sericite and alunite, respectively (Sillitoe et the Maricunga belt was initiated in the late Oligocene to early al., 1991). Miocene, at ~26 Ma, when reorganization of plate motions The latest Oligocene to earliest Miocene (26–21 Ma) vol- beneath the Pacific Ocean led to breakup of the Farallon canic activity and precious and base metal mineralization plate into the Nazca and Cocos plates (Lonsdale, 2005). likely took place in a neutral to weakly extensional tectonic These processes were accompanied by increased oceanic setting, which gave way to contraction consequent upon ini- crust generation at the East Pacific Rise (Conrad and Lith- tial slab flattening between 20 and 18 Ma (Mpodozis et al., gow-Bertelloni, 2007) and acceleration of plate convergence 1991, 1995; Kay et al., 1994, 2008; Kay and Mpodozis, 2001). at the Peru-Chile trench (Pardo-Casas and Molnar, 1987). The contraction resulted in diminished volcanism, compres- The basement to the Maricunga belt comprises metasedi- sive deformation, and crustal thickening (Kay et al., 1994; mentary and felsic volcanic rocks and granitoids of late Pale- Mpodozis et al., 1995). The gold-deficient Caserones por- ozoic age, like those that form large tracts of the Cordillera phyry copper-molybdenum deposit and nearby prospects,

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immediately south of the Maricunga belt (Fig. 1), were gen- rocks; volcanic breccia, which also predates all the potentially erated during this compressive event (Mpodozis and Kay, economic mineralization; five discrete porphyry intrusions, 2003; Perelló et al., 2003; Sillitoe and Perelló, 2005). Volcanic spanning the alteration-mineralization sequence; and a late- activity under more extensional conditions was again wide- mineral diatreme breccia (Figs. 4, 5). To these may be added spread and voluminous between 17 and 12 Ma, an interval relatively small volumes of hydrothermal breccia. that concluded with the second precious and base metal mineralization event. This was followed by renewed contractional Sedimentary rocks tectonism and marked geographic restriction of volcanism be- Sedimentary rocks surround the composite Caspiche por- tween 11 and 5 Ma, when all volcanic activity ceased in the phyry stock on all sides below elevations ranging from 3,870 Maricunga belt prior to reestablishment at the site of the pre- to 3,700 m above sea level, which is roughly 500 to 750 m sent-day magmatic arc, some 60 km farther east (Kay et al., below the surface (Figs. 4, 5). The rocks typically comprise a 1994, 2008; Mpodozis et al., 1995). Throughout the Mari- monotonous sequence of black, hornfelsed, and highly altered cunga belt, the volcanic centers and associated mineralization sandstone and siltstone, which may display little obvious tex- that formed during both of the noncompressive epochs dis- tural variation over tens of meters. In places, however, shal- play a clear control by steep, NW-, NE-, and E-striking faults, lowly dipping, relict bedding is observed, confirming their particularly the first of these (Mpodozis et al., 1995; Fig. 2); sedimentary origin (Fig. 6a). If, as seems probable, these sili- however, E-striking faults are most prominent in the ciclastic rocks are correlative with the nearby redbed se- Caspiche area (Mpodozis et al., 1991). Glaciation affected the quence of inferred Oligocene age (see above), the original higher parts of the Maricunga belt during the Plio-Pleis- diagenetic hematite was transformed to magnetite consequent tocene, giving rise to the final exhumation stages of the late upon the hornfelsing and pervasive potassic alteration, which Oligocene to mid-Miocene alteration zones and any con- also gave rise, in places, to a prominent, mottled texture pro- tained mineralization. duced by biotite clots. Notwithstanding the ubiquitous presence of magmatic-hydrothermal anhydrite throughout the Deposit Geology deeper parts of the Caspiche deposit (see below), concentra- The principal rock types defined at Caspiche may be as- tions of centimeter-sized anhydrite nodules in several thin signed to four broad units: premineral sedimentary host siltstone horizons may be evaporitic in origin, possibly formed

North A’

6,937,500 mE

6,937,000 mE

400 m 471,500 mE 471,000 mE A 470,500 mE EarlyEarly intermineralintermineral qquartzuartz ddioriteiorite porpporphyryhyry aandnd assassoci-oci- Late-mineralLate-mineral diatremediatreme brecciabreccia atedated brecciabreccia (phase(phase 2)2) Late-mineralLate-mineral quartzquartz dioritediorite porphyryporphyry (phase(phase 5)5) EarlyEarly dioritediorite porphyryporphyry (phase(phase 1)1)

HydrothermalHydrothermal brbrecciaeccia SandstoneSandstone andand ssiltstoneiltstone

Late-mineralLate-mineral quartzquartz dioritediorite porphyryporphyry (phase(phase 4)4) FaultFault LateLate iintermineralntermineral quarquartztz ddioriteiorite porpporphyryhyry andand DDrillrill holehole on planplan associatedassociated brecciabreccia (phase(phase 3)3)

FIG. 4. Geologic level plan at 3,700-m elevation of the Caspiche porphyry gold-copper deposit. Note that the porphyry phases tend to become progressively younger outward from the early phase 1 diorite porphyry, which constitutes the high- grade core to the system. Projections of drill holes also shown.

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SW NE

4,000 masl

3,500 masl

10,500 mE 9,500 mE 10,000 mE 9,000 mE 400 m

OverburdenOverburden VolcanicVolcanic brbrecciaeccia

Late-mineralLate-mineral diatremediatreme brecciabreccia SandstoneSandstone andand ssiltstoneiltstone

Late-mineralLate-mineral quartzquartz dioritediorite porphyryporphyry (phase(phase 5)5) SSedimentaryedimentary brbrecciaeccia

HHydrothermalydrothermal brbrecciaeccia AAndesiticndesitic vvolcanicolcanic rocksrocks LaLatete iintermineralntermineral quartzquartz dioritediorite porphyryporphyry andand assassociatedociated brecciabreccia (phase(phase 3)3) FauFaultlt EEarlyarly intermineralintermineral quartzquartz ddioriteiorite porpporphyryhyry aandnd assassoci-oci- DDrillrill hholeole on sectionsection atedated brecciabreccia (phase(phase 2)2) EarlyEarly dioritediorite porphyryporphyry (phase(phase 1)1)

FIG. 5. Representative geologic section through the Caspiche porphyry gold-copper deposit, showing all the main rock types recognized. Section line shown in Figure 4. during diagenesis (e.g., Dean et al., 1975) in a former playa- the late Paleozoic and Triassic basement, in which such felsic lake environment (cf. Mpodozis and Kay, 2003). volcanic rock types are widespread (e.g., Mpodozis and The deeper parts of the sedimentary sequence, beneath ap- Ramos, 1990; see above). The lower sedimentary breccia proximately 400 m of the sandstone and siltstone, include two horizon is directly underlain by andesitic volcanic rocks, sedimentary breccia horizons (Fig. 6b). The thicker (~100 m), chiefly flows (Fig. 5). upper horizon has a deposit-wide, ~2-m-thick bed of calcare- Minor bodies of probable andesite porphyry, characterized ous rock, transformed to garnet-diopside-epidote skarn, at its by centimeter-sized plagioclase phenocrysts in a black, fine- base. Correlation of these marker horizons over distances of grained, and highly altered groundmass, cut the siliciclastic several hundred meters confirms that the sedimentary se- sedimentary rocks locally, particularly near their upper con- quence is essentially flat lying (Fig. 5). The breccias, transi- tact. The bodies are clearly intrusive because of the presence tional to conglomerate in places, contain abundant clasts of of chilled margins, but it is still uncertain if they constitute distinctive rhyolite porphyry, almost certainly derived from sills, dikes, or a combination thereof.

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a b

1 cmcm 1 cmcm

c d

1 cmcm 1 cmcm

e Qz veinlet xenolith f

1 cmcm 1 cmcm

FIG. 6. Selected rock and alteration types from the Caspiche porphyry gold-copper deposit. a. Intensely biotitized, siliciclastic sedimentary wall rock from below 3,800-m elevation. Stratification is highlighted by more intense biotitization of siltstone than of sandstone beds. b. Sedimentary breccia, containing large rhyolite clast of probable late Paleozoic or Triassic age, from biotitized siliciclastic sedimentary wall-rock sequence at ~3,400-m elevation. Note that veinlets cut the breccia. c. Premineral volcanic breccia, inferred to be part of an early diatreme, displaying patchy texture and advanced argillic alteration. The patchy texture, including the amoeboid clast form, is inherited from the original fragmental rock texture. Clasts are altered to kaolinite and pyrite, whereas the matrix is pervasively silicified. d. Potassic-altered part of the same premineral volcanic breccia unit as depicted in c, containing flattened and aligned andesite porphyry clasts (fiamme) interpreted as former magma blobs or pumice. e. Potassic-altered, premineral volcanic breccia containing an A-type quartz (Qz) veinlet xenolith indicative of a preexisting porphyry system. Note the typical concentration of hydrothermal biotite (black) in the clasts and K-feldspar in the matrix. f. Early phase 1 diorite porphyry affected by intense K-feldspar-dominant potassic alteration and associated magnetite and A-type quartz veinlet stockwork. g. Potassic-altered, early intermineral phase 2 quartz diorite porphyry cut by weakly developed A-type quartz veinlet stockwork, and containing quartz (Qz) veinlet xenolith derived from nearby phase 1 diorite porphyry. Note prominent quartz (Qz) phenocryst. h. Potassic-altered, early intermineral phase 2 quartz diorite porphyry intrusion breccia developed near contact of the quartz diorite porphyry with sedimentary wall rocks. Sandstone and siltstone clasts are biotitized, whereas the quartz diorite porphyry cement is K-feldspar rich. Note D-type pyrite veinlet at top of image. i. Late-mineral, polymict hydrothermal breccia from peripheral chlorite-sericite alteration zone. The breccia contains abundant silicified, veinlet quartz (Qz), and quartz-veined clasts in a silicified rock-flour matrix. j. Late-mineral, polymict hydrothermal breccia displaying advanced argillic alteration, and cemented by alunite (al) and kaolinite (ka). Note quartz (Qz)-veined clast. k. Late-mineral diatreme breccia containing polymict clast population, including vuggy residual quartz (Qz), silicified, and pyrite (py)-veined clasts. Matrix is dominated by dacitic tuff. l. Phase 2 quartz diorite porphyry, from ~3,400-m elevation, affected by potassic-calcic alteration. The veinlet contains actinolite (Ac)-magnetite (Mg) and quartz (Qz)-K-feldspar (Kf)-rich segments, and has a K-feldspar alteration halo.

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g Qz veinlet xenolith Qz phenocryst h D veinlet

Siltstone clast

Sandstone clast

1 cmcm 1 cmcm

i Silicified pyritic clasts j Truncated qz veinlet

1 cmcm Al Al Ka

Qz veinlet clast Truncated qz veinlet 1 cmcm

k Vuggy qz clast l

Kf Qz Ac Mg Truncated py veinlet

Silicified clast

1 cmcm 1 cmcm

FIG. 6. (Cont.)

Volcanic breccia juvenile in origin. Intense alteration of the breccia precludes Throughout the prospect area, the siliciclastic sequence is certain identification of most component clast rock types, but overlain by a 500- to 750-m-thick volcanic breccia (Fig. 5), the remanent textures and characteristic absence of mag- which has an apparently restricted areal extent of approxi- matic quartz grains suggest that andesite/diorite predomi- mately 5 km2. nates. Clasts of hornfelsed siliciclastic sedimentary rock and, The breccia is polymict and mainly composed of rounded very locally, the intrusive andesite porphyry are particularly to subangular clasts surrounded by an obscure, highly altered, prominent in the lower parts of the breccia, within ~50 m of fine-grained, fragmental matrix, which is suspected to be contacts with these rock types, although suspected sedimen- dominated by a tuff component (Fig. 6c-e). The clasts are tary clasts also occur elsewhere in the breccia. The youngest typically 1 to 4 cm in size, with isolated examples attaining 10 zircon population in a representative breccia sample yielded cm, but with no hint of bedding or size sorting. In parts, some a U-Pb age of 24.7 ± 0.7 Ma (V. Valencia, unpub. report for of the clasts have an amoeboid shape defined by cuspate mar- Exeter Resource Corporation, 2010; see online Supplemen- gins (Fig. 6c), and, very locally, aligned, lenticular clasts (fi- tary Data), determined using the laser ablation-inductively amme in a descriptive sense; Bull and McPhie, 2007) are coupled plasma-mass spectrometry (LA-ICP-MS) method prominent, some up to 8 cm long (Fig. 6d). The fiamme are detailed by Chang et al. (2006). This age is essentially the either magma blobs or pieces of pumice flattened by com- same, within experimental error (2σ), as the Caspiche molyb- paction and, along with the cuspate clasts, are considered denite age (25.38 ± 0.09 Ma; see above).

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The breccia clasts typically display different alteration min- abundant phase 1 diorite porphyry or sedimentary wall-rock erals to the matrix (Fig. 6c-e; see below), thereby enhancing clasts and quartz veinlet fragments in a porphyry rock matrix appreciation of the fragmental texture. Although most clasts (Figs. 4, 5, 6h). In contrast, the phase 3 quartz diorite por- are internally homogeneous, recognition of veinlets confined phyry is only weakly veined, and displays low alteration inten- to a few of them is important because it confirms that some sity and partial preservation of magmatic biotite and mag- hydrothermal activity took place prior to breccia formation. netite. The late-mineral phase 4 and 5 quartz diorite The isolated, clast-confined veinlets comprise hydrothermal porphyries are texturally well preserved and nearly veinlet biotite, specularite, or granular A-type quartz (as defined by free. The earlier phase 4 displays extremely weak potassic al- Gustafson and Hunt, 1975). In addition, isolated clasts of ei- teration, whereas phase 5 is characterized by intense propy- ther A-type veinlet quartz (Fig. 6e) or silicified dioritic/an- litic alteration; both contain <0.02 g/t Au and <100 ppm Cu. desitic material are also widespread, particularly in the lower- The composite porphyry stock has a highly irregular form most 100 m or so of the breccia. in plan and is associated with a number of dikes, which to- The origin of this volcanic breccia remains to be fully re- gether suggest both northwest and northeast structural con- vealed, but it seems most likely to be part of a small, localized trols on its emplacement (Fig. 4). The phase 1 diorite porphyry, volcanic center that overlapped both spatially and temporally constituting the core of the stock, occurs as a ~300-m-long, with development of the Caspiche porphyry gold-copper de- sigmoidal body ranging from ~50 to 150 m wide (Fig. 4), but posit. In combination, the textural homogeneity, lack of bed- is blind and concealed beneath up to 150 m of premineral vol- ding and size sorting over several hundred vertical meters, canic breccia (Fig. 5). The phase 2 quartz diorite porphyry, and localized presence of mineralized material would appear comprising at least two discrete pulses, occurs as a discontin- to preclude accumulation of the breccia in stratovolcano or uous shell around phase 1; however, the largest volume of ash-flow caldera settings. A local origin is also supported by phase 2 occurs as an NE-trending, 400-m-long body along the the appearance of clasts of the underlying rock types on ap- southeastern side of phase 1 (Fig. 4). The internal position of proach to its basal contact. The breccia throughout much of the phase 1 porphyry relative to the enveloping phase 2 is un- the deposit area overlies a low-relief surface (Fig. 5), which usual and, in map pattern, might be taken to suggest that the gives way northward to a much steeper slope, as implied by former is younger than the latter. Nonetheless, the reverse is the presence of the breccia below 3,700-m elevation. This de- amply confirmed by the higher veinlet intensity and metal pressed area could be interpreted as one sector of an upward- content in phase 1 as well as the truncation of some A-type flared vent, which fed a restricted basin, although an incised quartz veinlets by, and presence of A-type quartz veinlet frag- paleosurface cannot yet be discounted. ments within, phase 2 (Fig. 6e). The form and diameter of phase 1 is roughly the same irrespective of whether it is en- Porphyry intrusions veloped by the sedimentary basement or phase 2 quartz dior- Two main porphyry intrusions, early and early intermineral ite porphyry (Fig. 5), implying that little or no phase 1 por- in timing (phases 1 and 2), constitute the well-mineralized phyry was assimilated or otherwise removed during phase 2 part of the composite Caspiche stock, which is abutted to emplacement. The phase 3 quartz diorite porphyry intrusion both east and west by a late intermineral phase 3 (Figs. 4, 5). measures at least 400 m in a northeast direction, and is nested In contrast, the two late-mineral phases 4 and 5 have so far into the western margin of phase 2 (Fig. 4). The phase 4 only been encountered farther west (Fig. 4). Together, the quartz diorite porphyry locally truncates the western side of well-mineralized phases 1 and 2 measure roughly 500 × 500 phase 2 and, in turn, is cut by phase 5 quartz diorite porphyry, m in plan view (Fig. 4), with little appreciable change in size which also occurs as large blocks in the late-mineral diatreme over their defined 1,400-m vertical extent (Fig. 5). (Fig. 5). The early phase 1 porphyry is dioritic in composition and contains sparse, 1-mm-sized quartz phenocrysts. Larger pla- Hydrothermal breccias gioclase and biotite phenocrysts are abundant and accompa- Several, volumetrically minor hydrothermal breccias are nied by subordinate hornblende. The original texture of the present at Caspiche, chiefly in the peripheral and shallow phase 1 porphyry is partly obliterated by intense alteration parts of the deposit (Figs. 4, 5). The best-defined breccias are and veining (Fig. 6f), the latter dominated by a multidirec- clearly dike like and <10 m wide. All the breccias are polymict tional, A-type quartz veinlet stockwork, which, near the apex and contain a variety of veined and mineralized clasts, which of the intrusion, constitutes >50% of rock volume. range from angular to subrounded in form. The other four intrusions, phases 2 to 5, are quartz diorite The peripheral breccias are mainly cemented by fine- porphyries. The intermineral phases 2 and 3 are texturally grained quartz (Fig. 6i) or chlorite, whereas the shallow ex- better preserved and coarser grained than the dioritic phase amples, within the advanced argillic zone (see below), are 1; they contain prominent quartz, besides plagioclase, biotite, characterized by alunite ± kaolinite cement accompanied lo- and hornblende phenocrysts (Fig. 6g). The phase 3 porphyry cally by minor pyrite and enargite (Fig. 6j). Most of the brec- is noticeably coarser grained and contains larger quartz phe- cias are largely barren, reflecting their late timing, although nocrysts than the better-mineralized phase 2. The latter is cut those that display the advanced argillic alteration can contain by abundant, relatively narrow (mostly <1 cm), A-type quartz minor copper and gold. veinlets (Fig. 6g), but truncates many of the A-type quartz veinlets in the phase 1 diorite porphyry, including all those Late-mineral diatreme with widths of 2 to 4 cm. In several places, the peripheral parts A large breccia body, which dips west at approximately 30° of the phase 2 porphyry body comprise intrusion breccia: to 40° and then steepens downward, truncates the western

0361-0128/98/000/000-00 $6.00 593 594 SILLITOE ET AL. side of the Caspiche porphyry deposit at shallow levels (Figs. pieces of vuggy residual quartz (Fig. 6k), showing that brec- 4, 5). The breccia extends for a distance of at least 1,100 m in ciation postdated development of some of the high sulfidation a north-south direction and has a maximum drilled thickness epithermal ledges (see below). Away from the contact zone, of 700 m. The east-west dimension remains undefined, al- tuffaceous material, apparently compositionally similar to the though the breccia is anticipated to underlie the full width of late-mineral phase 5 quartz diorite porphyry, is readily recog- the valley that separates Caspiche from the Santa Cecilia al- nizable as a matrix to texturally varied andesite and andesite teration zone (Fig. 3). porphyry clasts. The breccia contains blocks of phase 5 por- The breccia is highly polymict, matrix supported, and con- phyry, up to ~50 m in size, some of them displaying intense tains abundant mineralized clasts in proximity to its contact epidote-rich, propylitic alteration (Figs. 5, 7). Many of the with the Caspiche porphyry deposit. Besides quartz-veined blocks were also brecciated in situ, with the introduced ma- clasts and quartz-veinlet fragments, there are also prominent trix comprising fine-grained, hematite-impregnated quartz.

SW NENE

4,000 masl

3,500 masl

10,500 mE 9,500 mE 10,000 mE 9,000 mE 400 m

AdvancedAAddvanced aargillic:rgillic: ssilicification,ilicificaattion, vvuggyuggy quarquartztz ± alunitea lun ite BaseBase ooff lleachedeached cacappingpping

AAdvanceddvanced aargillic:rgillic: quartzquartz -kakaolinite olinite ± ppyrophyllite yrophyllite ± ddickiteickite SulfateSulfaatte ffrontront

CChloritehlorite --sericite sericite BorniteBornite ffrontront

PPotassic-calcic:otassic-calcic: acactinolitetinolite - KK-feldspar -feldspar OOverburdenverburden

PPropylitic:ropylitic: epidoteepidote --chloritec hlorite FauFaultlt

PPotassic:otassic: KK-feldspar-feldspar--albitea lbite-biotite-b iotite DDrillrill holehole on sectionsection

Potassic:Potassic: bbiotiteiotite

FIG. 7. Alteration section showing the spatial relationships between the principal alteration assemblages and the rock types depicted in Figure 5, Caspiche porphyry gold-copper deposit. The positions of the base of the leached capping (base of supergene sulfide oxidation), sulfate front (base of anhydrite and gypsum removal), and bornite front (top of megascopically visible chalcopyrite-bornite assemblage) are also shown. Note confinement of vuggy residual quartz ledges to shallow parts of the system.

0361-0128/98/000/000-00 $6.00 594 GEOLOGY OF THE CASPICHE PORPHYRY GOLD-COPPER DEPOSIT, MARICUNGA BELT, NORTHERN CHILE 595

Although much of the breccia is nonbedded and nonsorted, everywhere subordinate to chalcopyrite (chalcopyrite/bornite there are local intervals of normally graded, silt- to pebble- ~7). The bornite front is marked in Figure 7. The sulfide min- sized material, which are interpreted as surge deposits. One erals occur as disseminated grains in the quartz ± magnetite such interval has abrupt contacts against the nonbedded brec- veinlets as well as throughout the surrounding rock. A wide cia, suggesting an origin as a sunken block derived from sub- variety of pyrite-dominated veinlets, some having sericitic aerial tuff accumulations (cf. Sillitoe, 1985; Davies et al., halos (D-type; Gustafson and Hunt, 1975), cut the potassic al- 2008), whereas other bedded intervals may have formed in teration and quartz ± magnetite veinlet stockwork, as do late- place. stage anhydrite veinlets, some containing a few grains of On the basis of these characteristics and close similarities to pyrite, chalcopyrite, and/or molybdenite, but others com- late diatreme breccias in porphyry systems elsewhere (e.g., pletely barren. Sillitoe, 1985), it seems clear that the breccia fills a diatreme vent. To date, only the upward-flared, eastern contact has Propylitic been partly defined, with the bulk of the body, including the Propylitic alteration was encountered by drilling 600 m throat of the vent, lying farther west. The diatreme is a late, west of the mineralized Caspiche stock (Fig. 7), and is ex- but not the final manifestation of the Caspiche system be- pected to completely encircle it at depth, but also occurs as cause it not only contains clasts of ledge material but is also the earliest recognizable alteration type in the late-mineral, cut by a few residual vuggy quartz ledges, one at least 300 m phase 5 quartz diorite porphyry and parts of the late-mineral long. diatreme breccia (Fig. 7). The alteration is characterized by chlorite replacement of mafic minerals, epidote after plagio- Hypogene Alteration and Mineralization Features clase phenocrysts, and several percent of magnetite. The epi- Five alteration types are depicted in Figure 7: potassic, dote content is >5 vol %, with both veinlets and dispersed propylitic, potassic-calcic, chlorite-sericite, and advanced grains being commonplace. The pyrite content also exceeds 5 argillic, with the first and last of these being by far the most vol % in the propylitic halo, but attains only 1 to 2 vol % in the widely developed and economically important. Because the late-mineral porphyry and diatreme breccia. alteration zones overprint one another, commencing with the The peripheral propylitic alteration, most prominently the potassic and ending with the advanced argillic, combinations contained epidote veinlets, overprints the fringe of the potas- of alteration types are commonplace, although, for mapping sic zone, although the two alteration types are assumed to be purposes, the predominant assemblage everywhere takes broadly contemporaneous. A similar situation was reported precedence. from the Bajo de la Alumbrera porphyry copper-gold deposit, Argentina (Proffett, 2003). Potassic Potassic alteration becomes progressively more widespread Potassic-calcic downward in the Caspiche system, and is essentially the only Potassic-calcic alteration, defined by the presence of K- alteration type present between the ~3,600- and 3,400-m el- feldspar and actinolite and largely lacking biotite, is mainly evations (Fig. 7). The potassic zone comprises two distinct al- confined to the central parts of the early intermineral, phase teration assemblages, the earlier biotite dominated and the 2 quartz diorite porphyry and its host rocks below ~3,400-m younger containing K-feldspar, albite, and subordinate biotite elevation, although it is not widely developed in the part of (Fig. 7). Both contain >5 vol % of hydrothermal magnetite the deposit depicted in Figure 7. The downward change from where not appreciably overprinted by the later alteration potassic to potassic-calcic assemblages takes place relatively types. The biotite-rich type dominates the hornfelsed silici- abruptly, over just 10 to 20 m. The potassic-calcic zone is clastic sedimentary rocks (Fig. 6a) as well as affecting many characterized by actinolite intergrown with 10 vol % mag- clasts in the volcanic breccia (Fig. 6e). The K-feldspar-rich netite as veinlets, clots, and disseminations. The wider vein- potassic alteration is widely developed in the early and inter- lets, up to 2 cm across, tend to be segmented, with alternat- mineral porphyries as well as in the matrix to the volcanic ing actinolite-magnetite and granular quartz intervals (Fig. breccia (Fig. 6d-h). K-feldspar flooding is far more abundant 6l). The K-feldspar forms prominent veinlet halos, some of it than is its confinement to veinlet selvages. The potassic zone as centimeter-sized crystals, as well as occurring as a ground- is characterized by widespread quartz ± magnetite veinlets, mass alteration product. Diopside, albite, calcite, anhydrite, the quartz being granular and, hence, typical of A-type vein- chalcopyrite, and trace bornite are subsidiary veinlet compo- let generations (Gustafson and Hunt, 1975; Fig. 6f, g). Sul- nents. Some of the calcite fills numerous fractures in coarse- fide-free, magnetite-only veinlets predate most of the A-type grained diopside. veinlet generations. Banded quartz veinlets, typical of the In the transition between the potassic-calcic and overlying Maricunga porphyry gold deposits (Vila and Sillitoe, 1991; potassic zones, the actinolite and magnetite are observed to Muntean and Einaudi, 2000), are absent. overprint and destroy preexisting biotite, a process that also The potassic alteration contains extremely fine grained eliminates most of the contained chalcopyrite and bornite, al- chalcopyrite and subordinate pyrite in its shallower parts, though minor amounts of these sulfide minerals are retained with the pyrite/chalcopyrite ratio decreasing gradually down- in the wider actinolite-magnetite-quartz veinlets. Conse- ward and increasing outward. At a depth of about 700 m in quently, there is a marked drop in both total sulfide and cor- the phase 1 diorite porphyry and its immediate wall rock, vis- responding gold and copper contents on passing from the ible pyrite disappears entirely from the potassic alteration and potassic to potassic-calcic zones, with the latter averaging 0.13 bornite becomes progressively more abundant, although g/t Au and 0.06% Cu.

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Chlorite-sericite digenite, chalcocite-group minerals, and covellite (Fig. 8). The Chlorite-sericite alteration occurs marginal to the potassic chalcocite group and covellite are both hypogene and should core of the system, beyond the gold-copper orebody, but in- not be confused with the minor chalcocite resulting from su- ternal to the propylitic zone (Fig. 7). The late intermineral, pergene enrichment (see below). This dominantly dissemi- phase 3 quartz diorite porphyry is particularly susceptible to nated, high sulfidation sulfide assemblage is largely confined to this alteration type. Mafic minerals are transformed to chlo- the mineralized porphyry stock and its immediate wall rocks. rite and plagioclase to illite and/or sericite (fine-grained mus- Siliceous ledges occur within the quartz-kaolinite zone at covite). The dominance of illite over sericite was confirmed shallow depths, generally above 4,000-m elevation (Fig. 7), using a portable spectrometer. Pyrite was also introduced in where they represent the main upflow conduits within the ad- both veinlet and disseminated forms. vanced argillic lithocap. The drilled ledges, up to 35 m wide, The upper parts of the potassic zone in both the volcanic comprise silicification, vuggy residual quartz, and/or semi- breccia and porphyries have been partially overprinted by illite massive pyrite and marcasite where unoxidized. Enargite, ± smectite, the latter causing the sawn drill core to take on a orpiment, and native sulfur can occur as paragenetically late yellow stain after only a few months of storage (Fig. 6g). The additions in unoxidized parts of the ledges. Several ledges are main effects of this overprint are to convert plagioclase to illite auriferous whereas others are totally barren. Some of the and magnetite to hematite, the latter in the form of both mar- ledges have prominent, but typically narrow (<5 cm) alunite tite and accompanying specularite. Much of the biotite re- halos, although these can coalesce locally into broader quartz- mains stable, although it is locally retrograded to chlorite. Car- alunite zones where ledges are closely spaced along the bonate is also present in places. Only below the top of the zone southern margin of the system (see below). The alunite, like containing unaltered magnetite is the K-feldspar-rich potassic much of the kaolinite, replaces plagioclase phenocrysts. Dick- zone relatively free from the effects of the illite overprint. ite is most evident as a final infill of quartz-pyrite ± enargite Where the chlorite-sericite/illite alteration is strongest, it is veins interpreted as the roots of the siliceous ledges. mapped separately (Fig. 7), although it is commonly exten- The surface exposures (Fig. 3) and information from above sively overprinted by kaolinite and, hence, included in the ad- 4,200-m elevation suggest that the siliceous ledges are pref- vanced argillic zone. This progression is recorded by plagio- erentially developed around the periphery of the Caspiche clase phenocrysts that have illite cores and irregular kaolinite porphyry deposit, particularly to the south. However, it is sus- rims. pected that ledge development simply penetrated more deeply around the system margins, thereby leading to earlier Advanced argillic erosional removal of the ledges that once capped the central The shallower parts of the Caspiche system, to an average parts of the system. depth of 300 to 400 m, but a maximum depth of approximately Advanced argillic alteration, including siliceous ledges, is 1,000 m (3,400-m elevation) within steep fractures, are over- developed far beyond the Caspiche deposit, at least 3 km east printed by advanced argillic alteration dominated by a quartz- and 500 m north, and to depths of at least 400 m in places. kaolinite assemblage; however, it also contains widespread Some of the ledges are irregularly mineralized with enargite, albeit subordinate pyrophyllite and dickite as well as trace luzonite, and associated gold. Late-stage addition of orpiment diaspore (Fig. 7). Tiny (1 mm), disseminated rosettes of bluish- colored dumortierite occur as a widespread accessory mineral in the deeper parts of the advanced argillic zone, but give way 0.050.05 mmmm to black, centimeter-sized rosettes of black tourmaline (schorl) closely associated with kaolinite, sericite, chalcopyrite, and En pyrite at the base of the central portion of the advanced Cv argillic zone. The martite and specularite, products of the Py illite overprint, along with any remanent magnetite, are entirely converted to pyrite. Cv Where advanced argillic alteration affects the volcanic breccia, patchy texture is developed as a result of replace- Cp Py ment of the clasts by kaolinite and pyrite and the matrix by ex- Te tremely fine grained, gray-colored chalcedony (Fig. 6c; cf. Yanacocha; Gustafson et al., 2004). The patchy texture mim- ics the similar texture developed during the preceding potassic alteration, in which biotite-rich clasts are surrounded by K-feldspar-rich matrix (Fig. 6d, e; see above). Such inheri- tance of patchy-textured advanced argillic alteration from a Py preexisting alteration type affecting fragmental rock is also described from the Oyu Tolgoi porphyry copper-gold deposit, FIG. 8. Copper mineralization from the lower part of the advanced argillic Mongolia (Khashgerel et al., 2008). alteration zone, Caspiche deposit. Chalcopyrite (Cp) and associated pyrite The advanced argillic alteration contains a high sulfidation- (Py) grains, formed during potassic alteration, are rimmed by a high sulfidation-state sulfide assemblage, comprising covellite (Cv), enargite (En), and state sulfide assemblage in which chalcopyrite and pyrite are tennantite (Te). With a ×20 hand lens, the high sulfidation species appear as rimmed and, possibly, partially replaced by complex, micro- an indivisible black rim. Polished section under plane-polarized, reflected scopic intergrowths of enargite, luzonite, tennantite, bornite, light.

0361-0128/98/000/000-00 $6.00 596 GEOLOGY OF THE CASPICHE PORPHYRY GOLD-COPPER DEPOSIT, MARICUNGA BELT, NORTHERN CHILE 597 is also widespread in some of these peripheral ledges, ac- Hypogene Metal Distribution counting for arsenic contents of several percent in places. Porphyry deposit Late-stage gold-zinc mineralization The Caspiche deposit displays a classic hypogene metal- Scattered quartz ± calcite veinlets, containing straw-col- zoning pattern over its known 1,400-m vertical extent (Figs. 9, ored, low-iron sphalerite, galena, chalcopyrite, and pyrite, cut 10). Gold and copper contents correlate well, with the high- a spectrum of alteration types, ranging from pyrite-rich, est average tenors (~1 g/t Au, 0.4 % Cu) being largely con- quartz-kaolinite to pyrite-poor, illite-overprinted, potassic as- fined to the phase 1 diorite porphyry intrusion and its imme- semblages. The veinlets lack megascopically observable asso- diate wall rocks (Fig. 9a, b). The phase 2 and 3 quartz diorite ciated alteration. Along with accompanying sulfide dissemi- porphyries are characterized by roughly 50 and 20%, respec- nations, they are particularly concentrated and gold rich in a tively, of the phase 1 diorite porphyry grades. Notwithstand- shallowly dipping zone that abuts the eastern flared contact of ing the unusually high Au/Cu ratio at Caspiche, the close the late-mineral diatreme breccia. A few such veinlets also cut Au/Cu correlation is typical of that observed in most gold-rich overlying parts of the breccia, thereby confirming that the porphyry copper deposits (e.g., Sillitoe, 2000, 2010). At depth gold-zinc zone is the product of an end-stage hydrothermal in the phase 1 diorite porphyry intrusion, where bornite ac- event. companies chalcopyrite (Fig. 7), copper contents increase, as

a b

Au ppm Cu % 0.3 0.1 0.5 0.2 1.0 0.3 c d

As ppm Zn ppm 150 500 300 750 500 1000

FIG. 9. Element distribution contour plots for the same Caspiche section shown in Figures 5 and 7. a. Gold. b. Copper. c. Arsenic. d. Zinc. Note the central, broadly coincident positions of gold and copper and shallow concentration of arsenic associated with the advanced argillic alteration overprint. Appreciable zinc is confined to the gold-zinc zone immediately beneath the eastward-flared, late-mineral diatreme contact. Background geologic outline taken from Figure 5.

0361-0128/98/000/000-00 $6.00 597 598 SILLITOE ET AL.

>2,000 ppm (Fig. 9d); lead and copper can also attain several hundred ppm locally. This zone, lying immediately beneath and parallel to the W-dipping contact of the late-mineral diatreme breccia (Fig. 9a, d), coincides with the greatest concentration of the auriferous sphalerite-galena-chalcopyrite mineralization noted above.

Supergene Effects Sulfide oxidation The supergene profile at Caspiche is relatively thin (Fig. 7), in common with those developed over most deposits in the Maricunga belt (Vila and Sillitoe, 1991). The leached capping is typically thickest over the core of the system, where it is hosted by the uppermost parts of the advanced argillic zone, but tends to shallow progressively northward from 150 to 200 m in the southeast to 40 m in the northwest. All pyrite and Mo ppm other sulfide minerals, including any contained copper, have 50 Au 0.5 ppm 100 Cu 0.2 % been completely removed from the oxidized rock, whereas the gold remains largely in situ within derivative jarosite to consti- FIG. 10. Plan view of gold, copper, and molybdenum distributions at se- tute the oxide gold reserve cited above. The acidic solutions lected cutoff grades, 3,700-m elevation, Caspiche porphyry gold-copper generated by the pyrite oxidation produced an unquantified deposit. Note the peripheral location of molybdenum with respect to gold amount of supergene kaolinite, although most of this is impos- and copper. Background geologic outline and drill hole traces taken from Figure 4. sible to distinguish from the preexisting hypogene component. Most of the copper leached from the oxidized zone appears to have been lost to the paleoground-water system, but a thin expected, but gold grades tend to remain constant. Both gold (≤30 m) zone of incipiently developed chalcocite enrichment and copper grades decrease more or less progressively out- was intersected by a few, centrally located drill holes. ward to the inner edge of the propylitic halo, which contains only low-order metal values, whereas, downward, they dimin- Sulfate front ish abruptly as the potassic gives way to sulfide-poor, potassic- In common with most porphyry copper deposits, all the de- calcic alteration. fined alteration zones at Caspiche formerly contained a few Molybdenum at Caspiche is concentrated in a broad, annu- percent of hypogene anhydrite, much of it confined to the A- lar zone that surrounds and partially overlaps the gold-copper type quartz and late pyrite-bearing veinlets as well as occur- core to the deposit (Fig. 10). The molybdenite responsible for ring as the end-stage, anhydrite-only veinlets. The former the outer parts of this annulus occurs in a variety of forms, in- presence of this anhydrite is shown by characteristic open cluding quartz-molybdenite-pyrite and molybdenite-only vein- spaces present in many of the veinlets. lets and disseminated molybdenite flakes. The molybdenite is The anhydrite was removed by ingress of cool groundwater not present in B-type quartz veinlets, the commonest site of after erosion had exhumed the porphyry gold-copper miner- molybdenum concentration in gold-poor porphyry copper de- alization. Gypsum formed as an initial hydration product of posits (e.g., Gustafson and Hunt, 1975; Seedorff et al., 2005). the anhydrite, but was then completely dissolved. The resul- Arsenic distribution at Caspiche is closely related to the ad- tant sulfate front, present at depths of 600 to 900 m below the vanced argillic overprint because of its concentration in the surface within the deposit but locally much shallower on the associated high sulfidation-state sulfide assemblages, princi- margins (Fig. 7), is the level beneath which the rocks retain pally as enargite and lesser luzonite and tennantite. These sul- their calcium sulfate content. This is in the form of supergene fosalt minerals are commonest in the shallower parts of the gypsum and residual hypogene anhydrite in proximity to the advanced argillic zone, above 3,800-m elevation, where the front, but exclusively as anhydrite at depth. The sulfate front highest arsenic contents occur (Fig. 9c), as observed in sev- is typically sharp, but uncommon patches of sulfate-cemented eral other gold-rich porphyry copper deposits (e.g., Wafi- rock also remain stranded hundreds of meters above it. Golpu, Papua New Guinea; Sillitoe, 1999). Interestingly, the shallow parts of the phase 3 quartz diorite porphyry contain Genetic Considerations only low arsenic values (Figs. 5, 9c), suggesting that it was em- The Caspiche porphyry gold-copper deposit shares many placed later than most of the arsenic introduction. geologic features with gold-rich porphyry deposits elsewhere in the world (e.g., Sillitoe, 2000), with metal contents being Peripheral gold-zinc zone closely similar to those of the nearby Cerro Casale deposit, West of the main Caspiche deposit, the drilling defined an where an extensive advanced argillic lithocap is also partially ovoid, shallowly W dipping zone of elevated (locally >1 g/t) preserved (Vila and Sillitoe, 1991; Fig. 2). Interestingly, as gold values, which coincides with and is enveloped by a simi- noted above, Cerro Casale is part of the mid-Miocene metal- larly shaped, 500-m-long zone of high zinc values, typically in logenic epoch in the Maricunga belt and, therefore, some 12 the 500 to 1,000 ppm range, but with isolated values attaining m.y. younger than Caspiche.

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Caspiche is unique among well-described porphyry de- place in a relatively shallow setting, possibly beneath only a posits in that intrusion of the composite diorite to quartz dior- few hundred meters of extant volcanic cover (Fig. 12b-d). ite porphyry stock and gold-copper mineralization are brack- Perhaps the most enigmatic aspect of the volcanic breccia, eted by emplacement of large-volume vent breccias, in the context of a diatreme model, is its subhorizontal base interpreted here as diatremes (Fig. 11), the products of throughout much of the Caspiche deposit and its immediate phreatomagmatic (hydrovolcanic) eruptive activity caused by surroundings, which implies accumulation within a flat-bot- interaction between subsurface magma and external water tomed, basin-like depression rather than an upward-flared, (e.g., Sheridan and Wohletz, 1983; Lorenz, 1986). Although cone-shaped vent. However, evidence from several porphyry late-stage diatreme formation is a relatively widespread phe- copper-related diatremes elsewhere, including the late-mineral nomenon in gold-rich porphyry copper deposits (Sillitoe, examples at Lepanto-Far Southeast, Philippines (Garcia, 1991), 1985), premineral volcanic vents are uncommon, albeit well and Río Blanco-Los Bronces, Chile (J.C. Toro, pers. comm., documented (e.g., MacDonald and Arnold, 1994; Braxton et 2010; E. Wettke, pers. comm., 2011), as well as the prem- al., 2008). ineral example at Resolution, Arizona (G. Zulliger and R.H. Sillitoe, unpub. data, 2006; Hehnke et al., 2012), suggests that Premineral development thick, basin-like tuff and breccia accumulations above re- The up to 750-m-thick volcanic breccia that hosts the upper stricted and, in some cases, poorly documented vents may be half of the Caspiche porphyry stock and its associated gold- more common than previously thought. The shallow portions copper mineralization is best interpreted, for the reasons out- of diatreme vents tend to approximate broad, bowl-shaped lined above, as the southern manifestation of a diatreme vent, and flat-bottomed structures, rather than being steeply conical the center of which seems likely to be situated north of in form, where enclosing wall rocks are relatively incompe- Caspiche (Fig. 12a). The near synchronism of the U-Pb zir- tent (e.g., Auer et al., 2007), which may have been the case of con age of the volcanic breccia (24.7 ± 0.7 Ma) and Re-Os age the fine-grained sedimentary strata of Oligocene age at of Caspiche molybdenite (25.38 ± 0.09 Ma) adds further Caspiche. Furthermore, such thick but areally restricted tuff weight to a genetic connection between breccia accumulation and breccia accumulations could also represent the transitions and the porphyry system and, by the same token, argues between true diatremes and small calderas (Lipman, 1997). against an external source for this thick but apparently localized volcaniclastic accumulation. Porphyry gold-copper formation This genetic connection is further strengthened by the evi- The main stages of gold and copper introduction at Caspiche dence, in the form of veined and altered clasts (Fig. 6e; see accompanied the potassic alteration that spanned emplace- above), for at least minor porphyry copper formation before ment of phases 1 and 2 of the five readily recognizable por- the volcanic breccia was generated (Figs. 11, 12a). This evi- phyry intrusions (Figs. 11, 12b, c). With only few exceptions, dence may further imply that initial porphyry copper forma- the highest-grade ore occurs within the centrally positioned, tion was aborted by the catastrophic brecciation event, only to phase 1 diorite porphyry and its immediate wall rocks: the be resumed farther south at the site of the Caspiche deposit volcanic breccia and, at depth, siliciclastic sedimentary se- following completion of diatreme emplacement (Figs. 11, quence. Nonetheless, the encircling phase 2 quartz diorite 12b). This early porphyry copper system appears to have been porphyry, in particular its interior parts, also hosts a substan- aborted relatively early in its development, judging by the tial part of the orebody, albeit generally with lower average absence of any pyrite-bearing clast material in the volcanic grades. Minor metal introduction followed intrusion of the breccia. The close temporal association between the phase 3 quartz diorite porphyry, but had essentially ceased by Caspiche deposit and the volcanic breccia that hosts its upper the time the late-mineral, phase 4 and 5 quartz diorite por- half further suggests that the gold-copper mineralization took phyries were emplaced (Figs. 11, 12d, e).

Aborted system Au-Cu IS Au-Zn MINERALIZATION Cumulative 1 g/t Au 0.6 g/t Au 0.2 g/t Au ADVANCED ARGILLIC ALTERATION POTASSIC ALTERATION

PORPHYRIES Diorite 1 QDP 2 QDP 3 QDP 4 QDP 5 BRECCIAS Early diatreme Hydrothermal Late-mineral breccias diatreme

EARLY LATE

FIG. 11. Schematic time line diagram to show the relative age relationships between the main brecciation, porphyry intrusion, hydrothermal alteration, and metal introduction events at Caspiche. Note that the average gold grades in the two earliest porphyries (phases 1 and 2) are cumulative rather than being the products of only the host intrusions. Widths of bars approximate the importance of events. IS = intermediate sulfidation epithermal, QDP = quartz diorite porphyry.

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N S N S

a b

W E W E

c d

W E W E

e f

Late-stage gold-zinc zone

Late-mineral diatreme breccia Late quartz diorite porphyries (phases 3, 4, & 5) Intermineral quartz diorite porphyry (phase 2)

Early diorite porphyry (phase 1) Early diatreme breccia Present surface Aborted porphyry system Advanced argillic Siliciclastic sedimentary rocks Strong potassic

FIG. 12. Schematized evolution of the Caspiche porphyry gold-copper deposit. a. Formation of the premineral diatreme, showing the possible prediatreme position of weakly developed porphyry-type mineralization. b. Emplacement of the early phase 1 diorite porphyry, with associated potassic and advanced argillic alteration, into the southern part of the premineral diatreme breccia. c. Emplacement of the early intermineral phase 2 quartz diorite porphyry and continued potassic and advanced argillic alteration. d. Emplacement of the three later quartz diorite porphyries, phases 3, 4, and 5, following appreciable erosion and initiation of telescoping. e. Continued erosion and telescoping resulting in massive overprinting of the advanced argillic lithocap over the porphyry intrusions and potassic alteration, leading to high sulfidation-state gold-copper mineralization. f. Emplacement of the late-mineral diatreme, localized advanced argillic alteration within it, and intermediate sulfidation gold-zinc mineralization beneath the flared contact. The diatreme breccia covered and helped to preserve the deposit, with its lower contact inferred to have been subhorizontal. The blue line approximates the present erosion surface, at which the porphyry intrusions and potassic alteration are largely blind. Note that sections a and b are oriented north-south, with b offset to the south of a, whereas c-f are east-west. See text for further discussion.

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There is a crude outward decrease, most pronounced to the likely dictated by the subjacent magma column, possibly as a west, in the relative age of the Caspiche porphyry intrusions result of preferential copper sequestration by magmatic sul- from the phase 1 diorite to the propylitized, phase 5 quartz fides (Simon et al., 2010; Audétat and Simon, 2012). diorite, the latter emplaced after potassic alteration had completely ceased. This intrusion sequence is the opposite of that Late-stage evolution observed in many gold-rich porphyry copper deposits, in which Advanced argillic lithocap development in the shallower the porphyry phases tend to become progressively younger parts of the Caspiche system is assumed to have commenced inward (e.g., Panguna, Papua New Guinea, Batu Hijau, In- once potassic alteration was initiated at depth (Figs. 11, 12b, donesia, and Bajo de la Alumbrera, Argentina; Clark, 1990; c; Sillitoe, 2010). Nonetheless, much of the preserved ad- Clode et al., 1999; Proffett, 2003). vanced argillic zone, which overprints and obliterates pre- Notwithstanding the unusually high Au/Cu ratio at Caspiche existing potassic assemblages over the uppermost 300 to 400 compared to most gold-rich porphyry copper deposits, the vertical meters (Fig. 7), formed later in system development observed close correlation of the gold and copper contents during extreme telescoping. This is believed to have been a and the external but partly overlapped position of the main direct consequence of progressive but rapid erosional degra- molybdenum concentration are typical features of the deposit dation of the paleosurface (Figs. 11, 12d, e), leading to an class (Figs. 9a, b, 10). This lateral metal zoning is widely ob- equally rapid transition from ductile to brittle conditions served in gold-rich porphyry copper deposits, including Bajo within and around the porphyry stock. A similar situation ex- de la Alumbrera (Proffett, 2003), Esperanza, Chile (Perelló et ists at the Marte porphyry gold deposit farther north in the al., 2004), and elsewhere (Sillitoe, 2000, 2010), and suggests Maricunga belt (Sillitoe, 1994; Fig. 2). control by outward temperature decline and/or different This progressive overprinting of potassic by advanced argillic metal complexing of molybdenum compared to that of cop- alteration caused total to partial destruction of the intermedi- per and gold (e.g., Ulrich and Mavrogenes, 2008). ate sulfidation-state chalcopyrite ± pyrite assemblages in the The bottom of the Caspiche ore zone is defined by the upper parts of the potassic zone by high sulfidation-state cop- abrupt transition from potassic to potassic-calcic alteration, per ± iron sulfides and copper-arsenic sulfosalts (Fig. 8). This the latter being sulfide and gold deficient, albeit substantially reconstitution of the potassic zone and its contained mineral- richer in magnetite. The potassic-calcic alteration is tenta- ization was effective notwithstanding the relatively low tively considered to have followed emplacement of the phase temperature, probably not much greater than ~200°C 2 quartz diorite porphyry intrusion, and to have caused biotite (Hedenquist et al., 1996; Seedorff et al., 2005), implied by the and copper-bearing sulfide destruction. The downward ter- dominant quartz-kaolinite alteration assemblage. The arsenic, mination of the Caspiche ore zone as a result of increased absent from the pristine potassic zone, must have been intro- magnetite at the expense of sulfide contents differs markedly duced by the fluid responsible for the advanced argillic over- from that described from the Batu Hijau gold-rich porphyry print, whereas much of the gold and copper appear to have copper deposit, Indonesia, where downward grade decrease been recycled from the preexisting sulfide assemblages (cf. corresponds to increased pyrite/chalcopyrite ratios (Setyand- Brimhall, 1979). The latter conclusion is supported by the haka et al., 2008). In the context of a magmatic versus exter- general coincidence between the highest-grade gold and cop- nal basinal brine origin for actinolite-rich alteration assem- per mineralization in the advanced argillic zone and the phase blages in porphyry copper deposits (e.g., Seedorff et al., 2005; 1 diorite porphyry intrusion and its related high-intensity Sillitoe, 2010), the deep potassic-calcic assemblage at Caspiche quartz veinlet stockwork. Nonetheless, several prominent ex- might be assigned either to the former, in view of its confine- ceptions to this generalization appear to require at least local ment to the deep core of the system, or to the latter because redistribution of gold and copper. of the sedimentary host rocks, which potentially could have Hydrothermal breccias at Caspiche are typically small and acted as a connate brine source; however, in view of the ele- commonly of dike-like form, in common with those in many vated potassium content, in the form of K-feldspar, the first of other gold-rich porphyry copper deposits (Sillitoe, 2000). The these alternatives is preferred. brecciation, whether within (Figs. 5, 6j) or beneath (Fig. 5, 6i) The gold grade at Caspiche remains approximately constant the advanced argillic zone, took place relatively late in system over ~1,000 vertical meters, without any noticeable change evolution (Fig. 11), as evidenced by the abundance of miner- where copper contents increase on passing from the chalcopy- alized clasts and paucity of crosscutting veinlets (Fig. 6i, j). rite-only to deeper bornite-bearing zones (Figs. 7, 9a). This The breccias are consistently barren, and ascribed to phreatic situation is contrary to that predicted experimentally, whereby brecciation processes instigated by either late-mineral por- higher gold contents in the high-temperature precursor to phyry intrusion or, within the lithocap, by localized fluid over- bornite than in intermediate solid solution—the high-temper- pressuring (Sillitoe, 1985, 2010). ature chalcopyrite precursor—should be reflected by in- Development of the Caspiche system concluded with em- creased gold tenors with greater bornite abundance (Kesler et placement of the late-mineral diatreme along its western side. al., 2002). However, the vertical constancy of the gold grade Although the diatreme breccia contains abundant wall-rock combined with the clear gold-copper correlation may be clasts, including previously mineralized material, the breccia taken as further support for relatively shallow emplacement in the western, deeper known parts of the vent has a more ob- of the Caspiche deposit (Fig. 12), which would have favored vious juvenile tuffaceous matrix and, hence, is more clearly coprecipitation of gold and copper from a buoyant, sulfur-rich phreatomagmatic (hydrovolcanic) in origin. The magma cham- vapor plume as a result of depressurization (Murakami et al., ber from which the juvenile component was erupted is 2010). The uncommonly high Au/Cu ratio at Caspiche was inferred to be that from which the phase 5 quartz diorite

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porphyry was also derived, a proposal supported by the parts of the phase 2 quartz diorite porphyry (Fig. 6h), and pyrite-poor, propylitic alteration affecting both the breccia several texturally and mineralogically distinct, late-stage and phase 5 porphyry. Groundwater ingress to this late-min- hydrothermal breccias (Fig. 6i, j). Brecciation processes con- eral porphyry stock could have been the trigger for the cluded with emplacement of the late-stage diatreme breccia phreatomagmatic activity (e.g., Sheridan and Wohletz, 1983). along the western side of the deposit (Figs. 4, 5), within which The upward- and eastward-decreasing angle of the diatreme a variety of mineralized clasts are commonplace near its east- contact suggests that it may have approached horizontal over ern contact (Fig. 6k). To these may be added a number of the Caspiche deposit, in common with the situation described steep zones of tectonic brecciation caused by relatively minor above for the premineral diatreme (Fig. 12a, f). The subsided postmineral faulting (Figs. 4, 5). block of subaerial surge deposits recognized in the diatreme breccia implies that the vent may formerly have been sur- Acknowledgments mounted by a tuff ring or tuff cone (cf. Lorenz, 1986). The unstinting support of Exeter Resource’s senior man- Late-mineral diatreme formation overlapped in time with agement—the instigators of this paper—throughout the the waning stages of advanced argillic lithocap development Caspiche exploration program is greatly appreciated, as is the (Fig. 11), which resulted in fault-controlled, quartz-kaolinite geologic input provided at various times by Celena Ibáñez, alteration and localized siliceous ledge formation within the Leandro Sastre, Oscar Hernández, Marcia Reyes, Mauricio breccia. The final hypogene event at Caspiche was introduction Arce, Cristian Salgado, Felipe Jiménez, Matt Houston, and of the gold-zinc-(lead-copper) mineralization, the main con- the late Martín Rodríguez. Petrographic descriptions were centration of which was localized by the heavily fractured vol- provided by E. Tidy y Cía. Ltda. and Paul Ashley. Discussions canic breccia immediately beneath the eastward-flared, late- with Juan Carlos Toro and Enrique Viteri clarified historical mineral diatreme contact. The relatively impermeable nature details. Influential reviews were provided by Constantino of the diatreme breccia may have acted as an aquitard (cf. Mpodozis and Pepe Perelló and, on behalf of Economic Davies et al., 2008), thereby impeding upward fluid flow and Geology, Pete Hollings, Jeremy Richards, and Brian Townley. facilitating gold, zinc, and subordinate lead and copper precip- itation along the diatreme margin. 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