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Review

Physiographic and tectonic settings of high-suldation epithermalgold–silver deposits of the Andes and their controls onmineralizing processes

Thomas Bissig a,⁎, Alan H. Clark b, Amelia Rainbow b, Allan Montgomery c

a Mineral Deposit Research Unit, University of British Columbia, 2020-2207 Main Mall, Vancouver, BC V6T 1Z4, Canadab Queen's University, Bruce Wing/Miller Hall, Kingston, ON K7L 3N6, Canadac Riverside Resources Suite 1110, 1111 West Georgia Street, Vancouver, BC V6E 4M3, Canada

a b s t r a c ta r t i c l e i n f o

Article history:

Received 6 July 2014Received in revised form 10 September 2014Accepted 17 September 2014Available online 28 September 2014

Keywords:

High-suldationEpithermalAndesLandscape evolutionErosionUpliftFlat subductionNeogene

Gold and silver ores in the vast majority of Andean high-suldation epithermal Au–Ag deposits occur at highpresent day elevations and typically 200–500 m below low relief landforms situated at 3500 to 5200 m a.s.l.Most deposits are middle Miocene and younger and include, El Indio, Tambo, Pascua–Lama, Veladero (El Indiobelt, Chile/Argentina), Cerro de Pasco (Central Peru), Pierina, Lagunas Norte, Yanacocha (northern Peru),Quimsacocha (Ecuador), and the California–Vetas mining district (Santander, Colombia), jointly accounting forN130 MozAu resources. Slightlyolderexamples are only preserved in the AtacamaDesertand include themiddleEocene El Guanaco and El Hueso and the late Oligocene/early Miocene La Coipa deposits. The absence of Paleo-cene and older high-suldation epithermal deposits can be explained by limited preservation potential imposedby transpressional tectonics withinoverall contractileepisodes andsurface uplift.These conditions prevailed pre-dominantly in segments of shallow-angle subduction of the Nazca or Caribbean plate below the South Americancontinent, a tectonic settingalso common for porphyry-style Cu (–Au, Mo) deposits. Stratovolcanoes are uncom-mon ore hosts and volcanic rocks coincident with mineralization are in most cases volumetrically restricted orabsent, recording the terminal stages of local arc magmatism. However, dacitic domes are important at,e.g., Yanacocha and LaCoipa. At Lagunas Norte, a small stratovolcano largely pre-dating but temporally overlap-ping with mineralization occurs immediately east of the deposit and volcanic sector collapse may have occurredduring hydrothermal activity.Mineralization is typically located near the backscarp of pediments or the heads of valleys incising now high-elevation, low-relief surfaces. In the California–Vetas Mining District and El Indio belt, hydrothermal aluniteages become generally younger upstream along the incising valleys, indicating that hydrothermal activity and,by inference, ore deposition were facilitated by erosion. The lowering of the water table and reduction of hydro-static and lithostatic pressure at these sites of high local relief are believed to have enhanced both boiling andmixing of magmatic with meteoric uids, ultimately enhancing ore deposition.The host rock composition, permeability and location of the water table control the distribution of alterationzones and ore. Intermediate volcanic rocks are the most common ore-hosts but they typically pre-date mineral-ization by several Ma. However, high-suldation epithermal mineralization can be hosted in any conceivablerocktype including highgrade metamorphic rocks (California–Vetas mining district), signicantly older plutonicrocks (Pascua–Lama) or quartzites (Lagunas Norte). Large vuggyquartzalteration zones and commonly oxidizedlow-grade large-tonnage mineralizationare bestdeveloped in relatively permeable volcaniclastic rocks or hydro-thermal breccia bodies, whereas coherent volcanic, plutonic, or metamorphic rocks may host fault- and breccia-

controlled ores. The near-surface steam-heated zone can attain a thickness of several hundred meters in dryclimates (e.g. Veladero, Pascua–Lama, Tambo) but is typically poorly developed and less than 20 m thick inhumid climatic zones.The physiographic and tectonic settings of high-suldation epithermal deposits are distinct from low-suldationepithermal districts such as those of Patagonia, El Peñón (Chile) or Fruta del Norte (Ecuador). The latter range tosignicantly older ages (Jurassic to early Eocene) occur at mainly lower elevations and were emplaced in exten-sionalsettings. A temporal coincidence between uplift,erosion and mineralizing processes as well as a spatialandtemporal association with porphyry style mineralization is not evident for these low-suldation districts.

© 2014 Elsevier B.V. All rights reserved.

Ore Geology Reviews 65 (2015) 327–364

⁎ Corresponding author.E-mail address: [email protected] (T. Bissig).

http://dx.doi.org/10.1016/j.oregeorev.2014.09.027

0169-1368/© 2014 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

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http://dx.doi.org/10.1016/j.oregeorev.2014.09.027http://dx.doi.org/10.1016/j.oregeorev.2014.09.027http://dx.doi.org/10.1016/j.oregeorev.2014.09.027mailto:[email protected]://dx.doi.org/10.1016/j.oregeorev.2014.09.027http://www.sciencedirect.com/science/journal/01691368http://www.sciencedirect.com/science/journal/01691368http://dx.doi.org/10.1016/j.oregeorev.2014.09.027mailto:[email protected]://dx.doi.org/10.1016/j.oregeorev.2014.09.027http://crossmark.crossref.org/dialog/?doi=10.1016/j.oregeorev.2014.09.027&domain=pdf

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3282. Epithermal deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3283. Distribution of high-suldation deposits in the Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3294. El Indio belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

4.1. The Tambo deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3314.2. El Indio deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3314.3. Pascua–Lama . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

4.4. Veladero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3375. Maricunga belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338

5.1. La Coipa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3405.2. La Pepa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343

6. High-suldation epithermal deposits of the Domeyko fault system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3436.1. El Hueso . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3436.2. El Guanaco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344

7. Late Miocene high-suldation epithermal Au deposits of the western Cordillera of northern Chile and southern Peru . . . . . . . . . . . . . . . 3448. Central to northern Peruvianat slab segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3459. High-suldation epithermal deposits of the central Peruvian polymetallic belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345

9.1. Julcani . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3459.2. The Marcapunta–Colquijirca district . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3459.3. Cerro de Pasco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3459.4. Quicay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346

10. The high-suldation epithermal Au (–Cu, Ag) deposits of northwestern Peru . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34610.1. Pierina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34610.2. Lagunas Norte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34610.3. Yanacocha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34810.4. Tantahuatay, Sipan and La Zanja . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351

11. The northern Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35111.1. Quimsacocha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35311.2. California Vetas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353

12. Summary and comparison to low-suldation deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35513. Controls of geomorphic processes and climate on mineralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35714. Igneous rocks, volcanology and magmatic uids related to high-suldation epithermal deposits . . . . . . . . . . . . . . . . . . . . . . . . 36015. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361

1. Introduction

The Andes are the world's most endowed region with respect togiant magmatic-hydrothermal ore deposits (Cooke et al., 2005). Theyhost the largest-known porphyry copper deposits (e.g., Rio Blanco–LosBronces–Los Sulfatos, El Teniente, Chuquicamata) as well as many of theworld's largest epithermal Au–Ag deposits (e.g.,Yanacocha,LagunasNorte, Pascua–Lama, Veladero: Sillitoe, 2008). The vast majority of Andean epithermal deposits containing N10 Moz Au are of high-suldation type. These deposits have a close link to a magmatic sourcefor uids, volatiles and metals (e.g., Deyell et al., 2004; Rye, 1993) butform at depths of typically less than 1 km (e.g.Sillitoe, 2010) and conse-

quently mineralizing processesare inuenced by the near-surface phys-icochemical environment. The main focus of this review is on depositsand districts where the bulk of the precious metal is contained inthe epithermal environment, i.e., the shallow part of magmatic-hydrothermal systems, and concentrates on the physiographic environ-ment of epithermal mineralization. This paper does not discuss majorporphyry Cu deposits in detail, although the shallow portions of manyof these have been overprinted by epithermal mineralization or alter-ation (e.g., Masterman et al., 2004; Ossandón et al., 2001). Similarly,the deposits hosting Sn, W, Ag and Au ores in the eastern Cordillera of Bolivia and Peru are not discussed. Following a general summary of epithermal deposit types and their terminology, this article presents acomprehensive overview of the major high-suldation epithermal dis-tricts and mineral belts of the Andes. It focuses on the links betweenlandscape evolution, climatic setting, volcanology and tectonics, and

discusses the inuence these factors can have on both mineralizingprocesses and the preservation of the deposits.

2. Epithermal deposits

Epithermaldeposits are usually classied into sub-typesbasedon ei-ther ore sulde assemblage or characteristic associated alteration; bothschemes have inherent limitations. Some of the most widely referencedreview papers on the topic (Hedenquist et al., 2000; Sillitoe andHedenquist, 2003; Simmons et al., 2005) prefer a classication intohigh, low and intermediate-suldation types. This classicationscheme

can, however, be problematic, because sulde assemblages may be dif-cult to classify in a eld exploration setting, particularly if the deposithas been oxidized. Moreover, sulde assemblages within a single de-posit may represent precipitation over the entire breadth of suldationstate, from high to intermediate and low-suldation, depending onuid–wall rock interaction (e.g., El Indio: Heather et al., 2003a, 2003b;Cerro de Pasco: Baumgartner et al., 2008; Lagunas Norte: Cerpa et al.,2013). Alternative classication is based on dominant alterationand gangue assemblages and includes quartz–adularia–sericite andquartz–alunite or acid sulfate type epithermal deposits (Heald et al.1987, Tosdal et al., 2009), the former typically including low tointermediate-suldation sulde assemblages and the latter associatedwith high-suldation deposits. The limitation of this classicationscheme is that, particularly in high-suldation deposits, alteration maypre-date and may not be directly related to mineralization (see below).

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For this paper we use the widely applied classication scheme of high-suldation (normally associated with acidic quartz–alunitealteration) and low-suldation type (associated with near neutralquartz–adularia gangue and illite–adularia alteration) deposits. Theterm intermediate suldation is used to describe suldation state butis not used in the sense of a distinct subtype of epithermal deposit dueto the inherent variation of sulde assemblages within a single depositor vein.

In most high-sul

dation epithermal deposits, two hydrothermalstages can be distinguished (e.g., Simmons et al., 2005). An early stageof intense acid leaching of the wall-rock by a magmatic vapor that con-densed into groundwater, cooled to temperatures below 300 °C andwas acidied by dissociation of acids such as HCl, or H2SO4(Hedenquist and Taran, 2013); and, a subsequent stage of weakly-acidic uid incursion, from which the bulk of the suldes and preciousmetals, together with euhedral quartz, precipitated (e.g., Heinrichet al., 2004, 2007; Holley, 2012; Stoffregen, 1987). The intense acidleaching generates the zoning from vuggy quartz to quartz–aluniteand more distal kaolinitic alteration. This assemblage is commonly re-ferred to as a lithocap because it “caps” potential porphyry Cu mineral-ization (Sillitoe, 1995). Although mineralization is widely related to theinltration of the less acidic uids into the precursor lithocap(e.g., Holley, 2012; Stoffregen, 1987), it has been shown that the acidicmagmatic vapors can also transport gold into the shallow epithermalenvironment (Scher et al., 2013; Taran et al., 2000). At Pascua–Lamasuch uids were responsible forat least some of the gold mineralization(Chouinard et al., 2005). Conversely, there are examples of high-suldation epithermal deposits where vuggy quartz alteration is largelyabsent (e.g., La Bodega, Colombia: Rodriguez, 2014).

The precipitation mechanisms for precious metals in high-suldation epithermal deposits are not as well understood as for low-suldation systems. Forthe latter, ample textural anduidinclusionev-idence leaves little doubt that uid boiling leads to precipitation of metals due to the loss of the dominant ligands (H2S) to the gas phase(Moncada et al., 2012; Simmons et al., 2005). In contrast,uid inclusionstudies of high-suldation deposits are dif cult, as the appropriatetransparent minerals are rare. Several possible precipitation mecha-

nisms such as boiling, and uid mixing of a magma derived uid withoxidized groundwater have been proposed (e.g., Rye et al., 1992;

Jannas et al., 1999). Recent work on the La Bodega deposit inColombia, where the high-suldation sulde assemblage is hosted inveins with similar textures as described for low-suldation deposits(see below), supports that boiling is a viable mechanism to precipitatesuldes (Rodriguez, 2014). Moreover, steam-heated alteration, presentabove many Andean high-suldation systems, forms by H2S-rich steamthat condenses and oxidizes to form sulfuric acid in the vadose zone,and is considered evidence for uid boiling at depth (Simmons et al.,2005).

A close association of high-suldation epithermal deposits with aparent intrusion and porphyry-style mineralization has been proposedby Hedenquist and Loewenstern (1994) and documented in detail

for the Lepanto–Far Southeast porphyry system in the Philippines(Hedenquist et al., 1998). In earlier literature, stratovolcanoes, calderasand extrusive or subvolcanic dacitic to andesitic domes have beenconsidered the dominant hosts, and large volcanic centers have beenpresumed to be temporally and genetically associated with high-suldation epithermal deposits (e.g. Hayba et al. 1985, Arribas, 1995).Indeed, stable S, H and O isotopic evidence (e.g., Deyell et al., 2005a,2005b; Rye, 1993, 2005) as well as thermodynamic considerations(Heinrich, 2005, 2007) clearly indicate that magma-derived uids orcondensed magmatic vapors constitute the dominant component of the mineralizinguids. However, as will be discussed below,bettergeo-logical understanding and new geochronological information, togetherwith exploration discoveries made since 1995, suggest that stratovol-canoes and calderas are uncommon hosts to epithermal ore in the

Andes.

3. Distribution of high-suldation deposits in the Andes

High-suldation epithermal deposits occur mostly in the CentralAndes (~34° to 3° Lat. S) with few examples in the Northern Andes(~3° Lat. S to 11.3° Lat. N) and no known example in the southern vol-canic zone of the Central Andes (south of 34° Lat. S) and the southern,a.k.a. Patagonian Andes, south of 46° Lat. S (Fig. 1). The overwhelmingmajority of high-suldation epithermal deposits are Miocene or youn-

ger, and are located in segments of the Andean arc where magmatismis now absent or subdued (e.g., Bissig et al., 2008; Kay and Mpodozis,2001; Rosenbaum et al., 2005). The lack of magmatism is attributed totwo zones of shallow angle subduction of the Nazca plate below theSouth American continent in the Central Andes (the Pampean at slabin northern Chile and the Peruvian at slab in central and northernPeru: Ramos, 2009) as well as theBucaramangaat slabwhereCaribbe-an plate is subducting below northern Colombia (Vargas and Mann,2013). Flat subduction coincideswithshortening and crustalthickeningof the overriding continental plate (Martinod et al., 2010) and has beenspatially linked to the subduction of aseismic ridges or oceanic plateauson the oceanic plate (e.g. Gutscher et al., 1999b; Yañez et al., 2001).However the causative relationship between ridge impingement andonset of at subduction remains a matter of debate (Skinner andClayton, 2013). Thus, crustal shortening, uplift and porphyry Cumineralization pre-dates the arrival of the subducting aseismic JuanFernández ridge in Central Chile (Deckart et al., 2013), a situationcomparable to northern Peru where high-suldation epithermal goldmineralization at Lagunas Norte occurred at 17 Ma, coincident with up-lift, but several Ma prior to the inferred onset of Nazca ridge subduction(Montgomery, 2012, see below).

TheNorthern Andes are characterized by a seriesof oceanic terranesaccreted to the South American continent dominantly between the Me-sozoic and Miocene (Cediel et al., 2003). The Central Andes, in contrast,consist of a collage of terranes accreted during the Proterozoic-to-Paleozoic (Ramos, 2008). Here, Andean typesubduction has dominatedthe active continental margin since Permo-Triassic times (Ramos,2009), and in both the Northern Andes south of ~5.5° Lat. N and theCentral Andes, the Nazca plate is currently subducting eastward

beneath the South American continent.High-suldation epithermal deposits occur in the same segments of

the Andean arc as porphyry-style deposits (e.g. Sillitoe, 2008, 2010).However, a directtemporal, spatialand genetic relationshipto porphyrydeposits containingeconomic Cu mineralization,as demonstratedat FarSoutheast and Lepanto (Hedenquist et al., 1998), has not been docu-mented for any Andean high-suldation epithermal deposits discussedherein, although the two mineralization styles may overlap eitherspatially or temporally (e.g. Yanacocha district: Longo et al., 2010, LaPepa, Maricunga belt: Muntean and Einaudi, 2001).

All high-suldation epithermal deposits of the Andes nowcrop out at high elevation, and with few exceptions, above~3500 m a.s.l. Most are also located near the water-shed along thecrest of the orogen, or what appears to have been the water-shed

at the timeof hydrothermal activity, in areas dominated by relativelyat landforms which are interpreted as uplifted sub-planarpaleosurfaces. In the semi-arid and arid portions of the Andes,these paleosurfaces probably represent pediplains (regionally inter-connected pediment surfaces) formed under semi-arid conditionsand several pulses of pediment erosion separated by uplift pulsescan commonly be distinguished (Bissig and Riquelme, 2009; Bissiget al., 2002a; Riquelme et al., 2007; Rodriguez et al., 2014; Sillitoeet al., 1968), resulting in a series of pediment surfaces, vertically sep-arated up to several 100 m by steeper back-scarps. Similar relation-ships between high-level sub-planar landforms incised by broad,at bottomed valleys are present in southern Peru (Quang et al.,2005; Tosdal et al., 1984) and northern Peru (Montgomery, 2012).

Themajorhigh-suldation deposits anddistrictsare describedwith-

in their geomorphologic and volcanologic context from south to north

329T. Bissig et al. / Ore Geology Reviews 65 (2015) 327 – 364

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in the following section. Key characteristics, locations, ages and approx-imate contained resources are summarized in Table 1.

4. El Indio belt

The El Indio belt straddles the Chile–Argentina Border between 29°and 30° Lat. S. which corresponds to the middle of the Pampean atslab segment. The belt is named after the El Indio Au (–Ag, Cu) depositwhich was mined between 1978 and 2001. The belt also includes thesmall Tambo and the giant Veladero and Pascua–Lama deposits aswell as other prospects (Fig. 2), jointly accounting for ~ 40 Moz of Auresources. Additionally, the belt contains numerous alteration zonesranging in age from Eocene to late Miocene but, with the exception of

Veladero (12.7–

10.3 Ma; Holley, 2012), economic mineralization is

only known to have occurred between 9.5 and 5 Ma (Bissig et al.,2001; A.H. Clark,unpubl. data).Alteration andmineralizationages quot-ed herein for the El Indio belt are all 40Ar/39Ar plateau ages on alunite orsericite mostlywith analytical errorsof b1to5%onindividualdates.Itisworth noting that the small epithermal Cu–Au district at Famantina(Table 1), is located in the same Andean segment, albeit some 220 kmE of Veladero in Argentina, and is at 5 to 3.8 Ma just slightly youngerthan El Indio (Lozada-Calderón et al., 1994; Pudack et al., 2009).

Volcanism in the Oligocene and early Miocene was voluminous inthe El Indio belt (Bissig et al., 2001; Martin et al., 1995; Winocur et al.,2014) initially generating the 27–23 Ma Tilito Formation dacitic-to-andesitic pyroclastic and volcaniclastic rocks and small volumes of basalts in an extensional setting (Winocur et al., 2014). The Tilito

Formation is unconformably overlain by the dominantly-andesitic

12

3456

7

8

9

1011

121314

1516

18

1921

17

22

20

1: Tambo

2: El Indio

3: Pascua-Lama

4: Veladero

5: Famatina

6: La Coipa

7: El Hueso8: El Guanaco

9: Choquelimpie

10: Santa Rosa

11: Tucari

12: Julcani

13: Marcapunta/Colquijirca

14: Cerro de Pasco

15: Quicay

16: Pierina

17: Lagunas Norte

18: Yanacocha

19: Tantahuatay

20: Sipán

21: La Zanja

22: Quimsacocha

23: La Bodega

24: Angostura

NR

JFR

IR

CaR

CoR

IP

N o r t h e

r n A n d e

s

C e n t r a l

A n d e s

epithermal deposits

Active volcano

Flat slab segments

Aseismic ridges

Subducted aseismic

ridges/plateaus

90° W 80° W 70° W 60° W 50° W

20° N

10° N

0°

10° S

10° S

23/24

30° S

Fig. 1. Mapof South America, showing locations of major high-suldation epithermal depositsdiscussed in this paper. Alsoindicatedare segments of at subduction (dashed black lines),volcanoes, aseismic ridges and inferred subducted portions of aseismic ridges and oceanic plateaus. The boundary between the northern and central Andes is indicated. Abbreviations:CoR: Cocos Ridge; CaR: Carnegie Ridge; IP: Inca Plateau; NR: Nazca Ridge; IR: Iquique Ridge; JFR: Juan Fernandez Ridge.


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Escabroso (21–17 Ma) and Cerro de las Tórtolas (16.6–14 Ma) Forma-tions (Fig. 2), both associated with large volcanic edices andsubvolcanic intrusions (Bissig et al., 2001; Martin et al., 1995). Eruptedmagma volumes decreased markedly after ca. 14 Ma and volcanismwas conned to isolated eruptive centers at ~12.7–11 Ma (VacasHeladas formation andesite to dacite) and the rhyodacitic Pascua(8–7.6 Ma) and Vallecito (6–5.5 Ma) Formations (Bissig et al., 2001).An upper Pliocene rhyolite dome (Cerrode Vidrio) has been document-

ed ~7 km E of the Veladero deposit (Bissig et al., 2001, 2002a, 2002b).Thevoluminous upper Oligoceneto middleMiocene andesitesshowan increasing modal abundance of phenocrystic hornblende, relative toclinopyroxene, with time (Bissig et al., 2003). Further, igneous rocksyounger than ca. 14 Ma are characterized by elevated Sr/Y (N40) anddepleted HREE contents, indicative of garnet fractionation at the baseof the crust and elevated pressure equivalent to N45 km crustal thick-ness (Bissig et al., 2003). This change in both the erupted volumes andchemistry of the igneous rocks is attributed to the onset of at subduc-tion and associated increased coupling between the Nazca slab andoverlying continental plate, which led to contractile deformation andcrustal thickening(Bissig et al., 2003; Kay et al., 1999). Epithermal min-eralization is generally hosted in Cerro de las Tórtolas Formation andolder rocks but occurred several Ma after the emplacement of the hostrocks. Mineralization of the Filo Federico ore zone of the Veladerodeposit is partly hosted in volcaniclastic deposits of the Vacas HeladasFormation (Holley, 2012).

The geomorphology in the El Indio belt preserves evidence of a his-tory of episodic uplift and pediplain erosion throughout the middle tolate Miocene (Bissig et al., 2002a; Figs. 3, 4, 5, 6). Relics of three low-relief erosional surfaces, vertically separated from each other by200–400 m, have been documented. These are the 17–15 MaFrontera–Deidad surface, the 14–12.5 Ma Azufreras–Torta surface andthe 10–6 MaLos Ríossurface (Bissig et al., 2002a). The Frontera–Deidadsurface dominating the higher interuves is a relic of a regionally exten-sive pediplainon thewestern Andeanslopeat thelatitudeof theEl Indiobelt (Aguilar et al., 2011; Rodriguez et al., 2014), subsequently incisedby the Azufreras–Torta and Los Ríos pediment surfaces. These nestedpaleosurfaces in the main Cordillera are now separated from the land-

forms of the Coastal Cordillera by the north striking, west verging Vicu-ña–San Félix fault system (Aguilar et al., 2011, 2013). Verticaldisplacement along this fault system starting in the early–middle Mio-cene is thought to be responsible forthethree distinct paleosurfaces ob-served in the El Indio belt (Aguilar et al., 2011). Steam-heated alterationassociated with epithermal mineralization in all deposits of the El Indiobelt is almost exclusively exposed on the Azufreras–Torta surface(Figs. 3,4,5, 6), whereas the top of the mineralized zonetypically occurs~200 m below this surface (Bissig et al., 2002a).

4.1. The Tambo deposit

At Tambo, mineralization is hosted mainly in three diatreme

breccia bodies (Wendy, Kimberly and Canto Sur; Figs. 3, 4), as wellas a number of satellite breccia bodies and replacement veins, all of which were emplaced into dacitic pyroclastic and volcanoclasticrocks assigned to the upper Oligocene Tilito Formation. The orezone has an overall vertical extent of about 300 m (Deyell et al.,2005a; Jannas et al., 1999). The paleosurface level is located at4500 m a.s.l. for Kimberly and Wendy and has been downfaulted~100 m relative to Canto Sur, located at 4600 m a.s.l. elevation(Deyell et al., 2005a). Alunite, some intergrown with native Au, hasbeen dated at 8.8 to 8 Ma although at Canto Sur it yields ages asyoung as 7.1 Ma (Bissig et al., 2001; Deyell et al., 2005a). Aluniteages coincide with the inferred age of the Los Ríos surface incision(10–6 Ma) and the Tambo area is situated near the back scarp of aLos Ríos valley where it incises the older Azufreras–Torta surface

(Figs. 3, 4).

4.2. El Indio deposit

Mineralization at El Indio, in contrast to Tambo, is hosted in veins( Jannaset al., 1999) and includes sulde-rich massive enargite ± pyriteveins as well as quartz-rich veins with tetrahedrite/tennantite andchalcopyrite. For some veins, an along-strike zonation from high- tointermediate-suldation sulde assemblages, and from pyrophyllite–sericite–diaspore ± alunite to illite-dominated alteration has been doc-

umented (Heather et al., 2003a; K.B. Heather, pers. commun. 2012). ElIndio is world-renowned due to exceptional bonanza grades of up toN1% Au in the quartz-rich, intermediate-suldation El Indio Sur 3500vein which permitted shipping of the ore directly to the smelter (directshipping ore) in the early days of exploitation. Gold occurred mainly asnative Au, auricupride and as tellurides (Heather et al., 2003a; Jannaset al., 1990, 1999). As is the case for Tambo, the top of the mineralizedzone at El Indio is located about 200 m below the Azufreras–Torta Sur-face, here located at ~4400 m a.s.l. Steam-heated alteration is exposedlocally on Cerro Campana above the western part of the deposit(Fig. 3). The mineralized zone has a vertical extent of 500 m extendingdown to 3700 m a.s.l, below which the gold grade decreases markedly(Bissig et al., 2002a; Jannas et al., 1999). The veins are best developedin dacitic pyroclastic rocks of the Tilito Formation, which in the area of the deposit are uncomformably overlain by andesitic lavas andvolcaniclastic rocks of the lower Miocene Escabroso Group. Most veinsalso cross-cut the Escabroso Group but typically horsetail out abovethe contact (Bissig et al., 2002a; Heather et al., 2003a). Low volumesof ca. 8 Ma dacitic hypabyssal intrusions spatially associated with min-eralization have also been reported (Heather et al., 2003b). The miner-alization ages range from 8 to 5 Ma and are inferred from sericiteimmediately adjacent to and alunite from within veins (Bissig et al.,2001; A.H. Clark and K.B. Heather, unpubl. data) and a late stage 3.5–2.5 Ma barren hydrothermal overprint has also been identied (A.H.Clark and K. B. Heather, unpubl. sericite 40Ar/39Ar data). The age rangeindicates that multiple hydrothermal pulses related to successive pres-sure release, likely facilitated by erosion, from a sizeable batholith scalemagma system were responsible for mineralization. Contrary to theconclusion of Jannas et al. (1999) enargite-rich veins are not systemat-

ically older than gold-rich veins, since the youngest data comefrom 6.2 Ma alunite in the enargite-rich Campana B vein (Bissig et al.,2001) and sericite from the 5 Ma Viento Oeste/Cuarzo Uno high tointermediate-suldation veins whereas the El Indio Sur 3500 veinyielded the oldest sericite ages of ca. 8 Ma (A.H. Clark and K.B. Heatherunpubl. data). The geochronological data demonstrate a signicantage gap of at least 7 Ma between volcanic rock deposition (~26–15 Ma) and mineralization (8–5 Ma). Only small amounts of igneousrocks contemporaneous with mineralization are known and these in-clude ca. 8 Ma dacitic plugs and 5.5 Ma Vallecito Formation rhyolitetuffs in the vicinity of El Indio (Bissig et al., 2001; Heather et al.,2003a; A.H. Clark and K.B. Heather unpubl. data). Like Tambo, it isplausible that El Indio formed near the back-scarp of the incising RíoMalo valley (part of the Los Ríos surface) and is hosted below the

Azufreras–Torta surface (Bissig et al., 2002a; Fig. 3).

4.3. Pascua–Lama

Pascua–Lama is the largest deposit in the El Indio belt and is locatedsome 50 km N of El Indio on the Chile–Argentina border. It is hostedwithin an uplifted Paleozoic basement block (Figs. 5, 6), with minerali-zationcentered on a series of diatreme breccia complexes. Themost im-portant of these is Brecha Central, which cuts Paleozoic and Jurassicgranitoids(Chouinardet al., 2005). Hydrothermal activity is constrainedto 9.1–8 Ma based on 40Ar/39Ar dating of alunite and jarosite; alunitefrom pyrite–enargite veins constraining the gold mineralization to8.8 ± 0.6 Ma (Bissig et al., 2001; Deyell et al., 2005b). Slightly older al-unite of 9.5 Ma is documented from the Penelope ore zone some 2 km

SE of Brecha Central (Bissig et al., 2001; Deyell et al., 2005b). No igneous


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Table 1 (continued)

Deposit/district

Lat. Long. Elevation of mineralization(m.a.s.l.)

Elevation of paleosurface(if known)

Mineraliza-tionage (method)

Totalresources(approx.)

Mineralization Host rocks Volcanolo

Cerro dePasco

−10.68 −76.27 4350–3900 n/a 15.4–14.4(40Ar/39Ar,alunite)

12,250 ktZn, 3500kt Pb,

15,750 tAg, 1000kt Cu

Early pyrrhotite-pyrite–galena–Ferich sphalerite (stage I) and laterCu–Ag (Au–Pb–Zn) enargite–pyrite

veins as well as carbonatereplacement Pb–Zn ore (stage II).

Late Triassic to early JurassicPucara group limestone andmid Miocene diatreme

breccias.

Mineralizborder ofdiatreme

stage II mbut with urelationshNo volcandome com

Pierina −9.46 −77.59 n/a 14.2–14.7(40Ar/39Ar,alunite)

9 Moz Au Mineralization hosted in permeablepumiceous rock units and associatedwith intense acid sulfate alteration,including vuggy quartz and quartzalunite. Suldes include enargite,pyrite as well as galena, sphaleriteand stibnite, and lesserparagenetically late digenite andcovellite.

Oligocene to Middle Miocene(29.3–14.8 Ma) andesitic todacitic tuffs, lavas andsubvolcanic domes of theHuaraz group

Late dacitimmediatmineralizvolcanic r

LagunasNorte

−7.95 78.25 n/a 17.4–16.5 Ma(40Ar/39Ar,alunite)

13.1 MozAu

Disseminated and fracture hostedmineralization, centered on diatremebreccias. Mineralization in volcanic

rocks hosted in vuggy quartz andquartz–alunite zones. Dominantsulifdes include enargite, pyrite anddigenite.

Lower Cretaceous ChimúFormation quartzites (twothirds) and early Miocene

dacite tuffs and breccias (onethird).

Volcanic rmineralizbetween

mineralizwere likebreccias wmineralizdome adjsame age

Yanacocha −7 −78.58 4100–3500 ~4200 11–8.2 Ma(40Ar/39ArAlunite)

N70 MozAu

Massive and vuggy quartz zoneshosting pyrite ± enargite–tennantite–covellite

Calipuy group (19.5–15.1 Ma)andesitic to dacitic lavas andpyroclastic deposits and14.5–8.4 Ma Yanacocha unitdacite lavas and pyroclasticrocks

Mineralizvoluminovolcanic r(N47 Mozwaning strepresentdacite do

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Sipan −6.9 −78.79 ~3500 n/a n/a n/a n/a n/a n/aLa Zanja −6.83 −78.89 3700–3400 n/a n/a 0.73 Moz

Au, 6Moz Ag

n/a n/a n/a

Tantahuatay −6.72 −78.67 4050–3800 n/a 12.4 Ma(K/Ar alunite)

5.1 MozAu; 108.3Moz Ag

Gold is mostly associated to pyrite–enargite hosted in intensely quartz–pyrophyllite–alunite and vuggy tomassive quartz alteration. Much of the ore is oxidized

Miocene andesite domes andto a lesser degree underlyingpyroclastic rocks

Volcanic mineralizconstrainrestricted8.6 Ma (K

Quimsacocha −3.04 −79.22 3700–3450 3900 7.5–7.3 Ma(40Ar/39ArAlunite)

3.6 MozAu, 27Moz Ag

Stratabound Au-Ag mineralizationassociated with pyrite and enargite,hosted in vuggy quartz zones

Mineralization located inpyroclastic units between lesspermeable coherent andesitelava ows of the QuimsacochaFormation

Andesite pyroclasistratovolcis ~ 9 Maby 1.5 Madacite do6.7 Ma, 0

La Bodega/LaMascota

7.38 −72.9 2900–2300 ~3600 3.5–1.7 Ma(40Ar/39Ar,alunite, sericite)

3.5 MozAu

Dominantly structurally controlledbreccias cemented by quartz, aluniteand high-suldation suldeassemblage, overprinting earlierquartz–pyrite ± chalcopyrite veinsin phyllic alteration. Most Au–Ag inlate stages. Banded, colloform andlattice textures normally typical forlow to intermediate suldationdeposits are common.

Proterozoic gneisses and lateTriassic leucogranite.

No igneoumineraliz

juvenile c

Angostura 7 .39 −72.88 3500–2600 ~3600 4–1.9 Ma(40Ar/39Ar,

alunite, sericite)

2.7 MozAu

Widespread sheeted quartz–pyrite ± chalcopyrite veins

associated with pervasive quartz–sericite alteration. Locally cut byenargite–Cu sulde bearing brecciasand veins associated with quartzalunite alteration. The latter hostshighest grade Au zones.

Proterozoic gneisses and lateTriassic leucogranite.

No igneoumineraliz

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rocks contemporaneous with mineralization have been conrmed butlow volumes of juvenile igneous material probably played a role duringemplacement of the diatreme complexes and rare, locally weaklyargillically altered, 7.8 Ma rhyodacitic dykes post-dating mineralizationhave been recognized at Pascua (Bissig et al., 2001).

The alteration and sulde paragenesis reveals that unusually acidicuids were responsible for alteration and some of the mineralization.Gold mineralization is associated with pyrite and enargite, as well asalunite, jarosite and szomolnokite (Chouinard et al., 2005). Jarosite

is locally overprinted by hypogene alunite–

enargite bearing veins

(Chouinard et al., 2005). Based on light stable isotope evidence, jarositeprecipitated from mixed meteoric and magmaticuids whereas alunitehas a magmaticuid signature (Deyell et al., 2005b). This indicates thatuidsources variedthroughtime,plausiblyas a function of a uctuatingbut overall decreasing water table and short lived magma-deriveduidpulses, as also documented for El Indio (see above).

Near Pascua–Lama, the Frontera–Deidad surface reaches elevationsof 5200–5300 m a.s.l. (Fig. 5) The Azufreras–Torta surface is incisedinto this high-elevation pediplain at 4950–5100 m and extensive

steam-heated alteration and clastic deposits, both probably related to

6 , 8

0 0 , 0

0 0

6 , 7

8 0 , 0 0 0

6 , 7

6 0 , 0

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6 , 7

4 0 , 0

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0 0

6 , 7

0 0 , 0

0 0

6 , 6

8 0 , 0

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380,000 400,000 420,000 440,000 460,000

Paleozoic and Mesozoic basement

27-23 Ma Tilito Fm. and equivalentsdacite, andesite and minor basalt

21-14 Ma Escabroso and Cerrode las Tórtolas Fm. andesiteand subvolcanic diorite

12.7-11 Ma Vacas Heladas Fm. dacite

6-5.5 Vallecito Fm. rhyolite

8-7.6 Ma Pascua Fm. dacite

2 Ma Cerro de Vidrio rhyolite dome

Pascua LamaPascua-Lama

VeladeroVeladero

SancarrónSancarrón

El IndioEl Indio

TamboTambo

Cerro Doña Ana

5625 m)

Cerro Doña Ana(5625 m)

Cerro de las Tórtolas 6160 m)Cerro de las Tórtolas (6160 m)

Cerro del Toro 6168 m)Cerro del Toro (6168 m)

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Thrust/reverse fault (symbol on hanging wall side)

Fault (unspecified)

Lineament from satellite image

El Indio belt geology

High-sulfidation epithermal deposit

Major Mountains

20 km

N

A u m i n

e r a l i z a t i o n

Fig. 2. Geological mapof theEl Indio beltandlocation of principal epithermal deposits. Compiled from Martinet al.(1995), Bissig etal. (2001), Zappettini (2008),and Winocuret al.(2014).Coordinates in UTM Zone 19S, Provisional South American Datum 56.


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phreatomagmatic eruptions derived from the mineralized breccia pipes,areexposed on this landform (Bissig et al., 2002a). Mineralization occursbelowthe steam-heated zone at 4850 to 4600 m (Chouinard et al.,2005).As at El Indio and Tambo, a valley (Río Turbio valley: Fig. 5) incised theAzufreras–Torta surface from the east. Ferricrete-cemented conglomer-ates at thevalley-bottom display glacial striations, showing that this val-ley pre-dated Plio-Pleistocene glacial erosion. Further, steam-heated

alteration also overprints the eastern side of the deposit to lower

elevations (to 4800 m) than to the west (to 5000 m: Chouinard et al.,2005) indicating that incision of the Río Turbio valley from the eastmay have occurred during hydrothermal activity.

4.4. Veladero

Veladero is situated about 8 km to theSE of Pascua in a NNE striking

graben (Charchaié et al., 2007). Gold and silver mineralization is

Tambo

400000 410000 420000

6 7 0 0 0 0 0

6 7 1 0 0 0 0

69°50'0"W69°52'0"W69°54'0"W69°56'0"W69°58'0"W70°0'0"W70°2'0"W

2 9 ° 4 0 ' 0 " S

2 9 ° 4 2 ' 0 "

S

2 9 ° 4 4 ' 0 " S

2 9 ° 4 6 ' 0 " S

2 9 ° 4 8 ' 0 " S

2 9 ° 5 0 ' 0 " S

2 9 ° 5 2 ' 0 " S

ArgentinaChile

0 1 2 3 4 5 km

Elevation (m)

High : 6107

Low : 1775

Slope (Degrees)

0 - 10

AT

AT

AT

AT

FD

LR

LR

LR

El Indio/Tambo Area, Chile & Argentina

El Indio

Canto Sur

Wendy

Kimberly Veta Veronica

Paleosurfaces

Los Ríos (LR); 10-6 Ma

Azufreras-Torta (AT); 14-12.5 Ma

Frontera-Deidad (FD); 17-15 Ma

Río del Medio

Cerro Campana

Fig. 3. El Indio Tambo area topography and landscape elements. Paleosurfaces after Bissig et al. (2002a). UTM Zone 19S, WGS84.


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hosted in two principal breccia bodies: Amable and Filo Federico(Fig. 6). Ore mineralogy comprises electrum and silver-bearing jarositewhich were deposited together with euhedral quartz, and aresuperimposed on earlier acidic residual quartz alteration (Holley,2012). Jarosite is abundant, especially at Amable, whereas suldes areexceedingly rare. At Amable, jarosite forms euhedral crystals andgrows in open spaces and veins, and is locally overgrown by barite.Stable-isotope evidence indicates that most jarosite precipitated frommeteoric uids above the water table, with sulfur derived from precur-sor suldes. However, one location in the Fabiana zone to the east of Veladero (Figs. 5, 6) shows unambiguous hypogene jarosite, replacing

plagioclase and biotite phenocrysts in the host rock and displayingstable isotope signatures of magmatic uids (Holley, 2012).

Mineralization at Amable is constrained between a 12.7 Ma alunite40Ar/39Ar age and a 12.14 Ma U/Pb zircon date of a narrow andesitedyke that cross-cuts mineralization (Holley, 2012). Mineralization atFilo Federico is at 11.1 to 10.3 Ma (40Ar/39Ar, alunite, n = 4) somewhatyounger than at Amable (Bissig et al., 2001; Holley, 2012). Jarosite agesat Amable range from 11.8 to 8.6 Ma and overlap with hypogene aluniteand gold deposition at Filo Federico (Holley, 2012). These data areinterpreted as evidence for an overall lowering of the water table withtime, episodically interrupted by pulses of magma derived uids.

Jarosite at Veladero almost entirely pre-dated hypogene alunite alter-ation and sulde mineralization at nearby Pascua−Lama.

The host rocks of the Amable ore zone consist of crudely stratied

volcaniclastic breccias assigned to the Cerro de las Tórtolas formation,unconformably overlying Tilito Formation andesites and dacites aswell as Permian felsic tuffs (Charchaié et al., 2007; Holley, 2012). TheFilo Federico orezone is partlyhostedin volcaniclastic deposits assignedto the Vacas HeladasFormation (Holley, 2012). Mineralization is closelyassociated with hydrothermal breccias intruding the volcaniclastic de-posits and occurs between 4400 and 3900 m a.s.l. in a subhorizontal,N−S elongated ore body, extending extends to lower elevations atFilo Federico than at Amable. A steam-heated alteration zone of up to200 m thickness overlies the orebodies and denes the paleosurface at~4400–4600 m a.s.l. At Filo Federico, steam-heated alteration is local-ized along fracture zones, and overprints the mineralized zone downto more than 500 m below the inferred paleosurface (Holley, 2012).Here, the Frontera–Deidad surface is marked by the lower contact of

the Cerro de las Tórtolas formation which here has been downfaulted

from 5300 m to about 4100 m in the N–S oriented Río de las Taguasgraben (Charchaié et al., 2007, Fig. 6), whereas the Azufreras–Tortasurface has been cut into Cerro de las Tórtolas volcaniclastic depositsat an elevation of ~4500 m (Charchaié et al., 2007, Fig. 6), i.e. at thetime of, or immediately prior to, hydrothermal activity in that area.Conversely, the Filo Federico mineralization probably formed at a timewhen incision of the Los Ríos surface was well advanced, as evidencedby the steam-heated overprint of the ore zone and the mineralizationextending to lower elevations than at Amable. The lowering of thewater table in the Veladero area during Los Ríos surface incision is alsoreected in the formation of jarosite in the vadose zone at Amable

between 11.8 and 8.5 Ma.

5. Maricunga belt

The Maricunga belt is dened as the region of the Andean Cordillerabetween 26° and 28° Lat S (Vila andSillitoe, 1991), located at the north-ern margin of the Pampean at slab segment. Gold-rich porphyry typedeposits (e.g., Cerro Casale (a.k.a. Adelbarán), Caspiche, Lobo, Marte)are the dening mineralization style and the gold-rich nature of porphyry-style mineralization is thought to be related to the shallowemplacement of the porphyry intrusions (Muntean and Einaudi,2001). Although barren quartz–alunite alteration is commonly foundin the shallow portions of these porphyry systems, La Pepa is the onlyporphyry deposit where signicant gold mineralization hosted in

quartz–alunite veins that overprint porphyry-style mineralization hasbeen documented (Muntean and Einaudi, 2001). In contrast, La Coipa,located at the northern end of the Maricunga belt, hosts Ag–Au miner-alization exclusively as epithermal mineralization of largely high-suldation type (Cecioni and Dick, 1992; Oviedo et al., 1991).

Porphyry and epithermal deposits of the Maricunga belt occur intwo partly overlapping parallel N–S striking belts, with mineralizationtraditionally considered to have been emplaced during two distinct ep-isodes at 25–20Maand15–13Ma(Sillitoe et al., 1991). However, morerecently published, but still limited geochronology requires modica-tion of this model. The Caspiche porphyry Au deposit was dated at~25.4± 0.01 Maby Re/Oson molybdenite (Sillitoe et al., 2013) whereasthe Caserones porphyry Cu deposit in the southern Maricunga beltformed at 20–18 Ma (Mpodozis and Kay, 2003). Further, alunite K/Ar

ages from the La Coipa district range from ~20 to ~17 Ma (Oviedo

Canto Sur pitFrontera Deidad surface (17-15 Ma)

Azufreras-Torta surface (14-12.5 Ma)Los Ríos surface (10-6 Ma)

Steam-heated alteration

Veta Verónica o C

E l e

f a n t e

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C d e

l a s

T ó r t o l a

s

Tambo(Kimberly pit)

AzufrerasDeidad

oC Doña Ana

Looking S from Canto Sur, Tambo

A

B

Fig. 4. Landscape in the Tambo area. A) Panoramic view to the south, B) line drawing of the photograph in A) showing the principal landscape elements, locations of steam-heatedalteration zones as well as the Kimberly and Canto Sur orebodies. See Fig. 3 for locations.


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et al., 1991) which disagree with the range between 25 and 20 Maestablished on the basis of alunite K/Ar ages (Sillitoe et al., 1991) andbiotite and hornblende K/Ar ages for dacite domes that were thoughtto be closely related to mineralization (Oviedo et al., 1991; Sillitoeet al., 1991).

Thelarge-scale morphotectonic features of theAndesaredifferent atthe latitude of the Maricunga belt to those of the El Indio belt, probably

inuenced by the differing Paleozoic and older basement architecture

(Ramos, 2008). Thus, the Precordillera (a.k.a. Cordillera Domeyko),resulting from the late Eocene Incaic deformation is present north of 29° Lat. S but absent south of it (Rodriguez et al., 2014), a locationwhich roughly coincides with the inferred southern limit of theAntofalla massif and the northern limit of the Chilenia terrane (Ramos,2008). Oligocene extensional tectonics are documented from south of the Vallenar Orocline (Abanico basin, Tilito Fromation: Winocur et al.,

2014; Charrier et al., 2002) whereas a contractile tectonic regime at

Cerro de Vidrio

69°55'0"W70°0'0"W

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400000 410000

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R í o T u r b i o

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Veladero

Pascua-Lama

Los Ríos (LR); 10-6 Ma

Azufreras-Torta (AT); 14-12.5 Ma

Frontera-Deidad (FD); 17-15 Ma

ArgentinaChile

Elevation (m)

High : 5678

Low : 2877

Slope (Degrees)

0 - 13

0 1.5 3 6 km

Pascua-Lama/Veladero Area, Chile & Argentina

Fabiana

PenelopeLama Central

Fig. 5. Veladero and Pascua area topographic map and landscape elements. Paleosurfaces after Bissig et al. (2002a) and Charchaié et al. (2007). UTM Zone 19S, WGS84.


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roughlythe same time is documented from theCordillera Claudio Gayatthe latitude of the Maricunga belt during the late Oligocene (Mpodozisand Clavero, 2002). The dominant landscape elements at the latitude of thenorthern Maricungabelt include theprobably Oligocene relictSierraChecos de Cobre surface which is incised by the upper Oligoceneto ~ 18 Ma Asientos pediplain in the eastern Precordillera (Bissig andRiquelme,2009;Mortimer,1973). TheOligoceneto early Miocene land-scape was incised by the middle- to early-upper Miocene Atacamapediplain and late Miocene canyons (Bissig and Riquelme, 2009)which record a progressive tilting of the western Andean slope and up-lift of the Andean Cordillera since the early Miocene (Riquelme et al.,

2007).

5.1. La Coipa

TheLa Coipa district contains multiple epithermal deposits of mostlyhigh-suldation type including Ladera Farellón, Can Can, Coipa Norte,Purén, Purén Sur and Pompeya (Fig. 7). With the exception of Purénand Pompeya, mineralization is hosted in the Triassic La Ternera forma-tion, consisting of black shales and sandstones, as well as in overlyingupper Oligocene (24–22 Ma) dacitic tuffs, volcaniclastic breccias anddomes (Cecioni and Dick, 1992; Oviedo et al., 1991). Hydrothermalbreccias hosting ore are probably penecontemporaneous with mineral-ization. AluniteK/Aragesrange between ca. 20 and 17 Ma(Oviedoet al.,

1991). A relict, large, upper Oligocene volcanic center, Cerros Bravos

Amable Filo FedericoCerro Pelado

Cerro NevadoRío TurbioPenelope

Lama Central

Pascua/Lama

g r a b e n b o u

n d i n g f a u l t s

Fabiana

Frontera Deidad surface (17-15 Ma)

Azufreras-Torta surface (14-12.5 Ma)Los Ríos surface (10-6 Ma)

Steam-heated alteration

Looking W from Fabiana

A

B

Fig. 6. Landscape in theVeladero andPascua–Lama area. A) panoramic viewfromthe Fabianaprospecttoward thewest.B) Linedrawing of thelandscapeshown in A. Landscape elements

are shown as well as steam-heated alteration zones. See also Fig. 5 for locations.


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(Cornejo et al., 1993), is located 16 km N of Ladera Farellón. Unaltered,16–14.7 Ma dacitic domes and lavas post-date mineralization (Oviedoet al., 1991) and locally overlie the mineralized zones at Pompeya(Figs. 7, 8). The Pompeya and the Purén ore bodies were discoveredafter 2010, and only Purén has been mined. Pompeya is located ~3 kmNE of Ladera Farellón and is of high-suldation type. Purén, 8 kmNE of Ladera Farellón features high grade Pb–Zn mineralization

representing an intermediate suldationsulde assemblage, an absence

of advanced argillic alteration and the occurrence of various carbonatesincluding rhodochrosite as alteration minerals (S. Gamonal, pers.commun. 2014). At Purén Sur, some 2.5 km south of Purén, nativesulfur associated with steam-heated alteration was historically mined.Steam-heated alteration is also present at Pompeya, Purén and CoipaNorte and is generally preserved at 4250 to 4400 m a.s.l. ( Figs. 7, 8).

Mineralization at Ladera Farellón, Can Can and Coipa Norte is largely

controlled by subvertical fractures when hosted in Triassic shales, but

Ladera Farellón

Can Can

Coipa Norte

Purén

Purén Sur

Pompeya

470000 480000

7 0 3 0 0 0

0

7 0 4 0 0 0 0

69°12'0"W69°14'0"W69°16'0"W69°18'0"W

2 6 ° 4 6

' 0 " S

2 6 ° 4 8 ' 0 " S

2 6 ° 5 0 ' 0 " S

2 6 ° 5 2 ' 0 " S

Outline of post-mineral

dacite domes and lavas

0 1 2 3 4 50.5 km

La Coipa Area, Chile

Elevation (m)

High : 4907

Low : 3013

Slope (Degrees)

0 - 10

L. Oligocene/E. Miocene paleosurface

Fig. 7. Topographic map and landscape elements of the La Coipa district. Some geological information from Cornejo et al. (1998). UTM Zone 19S, WGS84.


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expands into mushroom-shaped zones in the overlying volcanic rocks.The latter hosts higher Ag grades, whereas the former is relativelyAu-rich (Oviedo et al., 1991). The top of the exposed orebodies lies

around 4300 m a.s.l., with sulde oxidation extending downwards

to ~ 3800 m a.s.l. (Cecioni and Dick, 1992). The dominant alterationis vuggy–quartz and quartz–alunite in the volcanic rocks and quartz–alunite ± dickite and pyrophyllite in the shale and sandstones.

There is a close association of alunite vein stockworks with gold

Pompeya16 Ma dacite lava

16 Ma dacite dome

Ladera FarellónCan Can Coipa Norte 16 Ma

dacite dome

A

B

C

D

Steam-heated alteration zone

Late Oligocene/early Miocene erosion surface

Fig. 8. Landscape and physical setting of mineralization in the La Coipa district. A) Panoramic view to the southwest from Pompeya. Dark unaltered rocks at left margin of photographbelong to a post mineral dacite dome. B) Line drawing of photograph shown in A) highlighting the relationship of landscape and mineralization. Steam-heated alteration is exposed ona late Oligocene to early Miocene erosion surface (light blue). C) Panoramic view looking northwest at the Ladera Farellón, Can Can and Coipa Norte ore bodies. D) Line drawing of thephotograph shown in C). Light blue indicates the late Oligocene to early Miocene paleosurface.


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mineralization (Oviedo et al., 1991). The suldes observed at depth in-clude pyrite, chalcocite, covellite, bornite, chalcopyrite, tennantite,tetrahedrite, galena and sphalerite (Oviedo et al., 1991), representativeof precipitation at intermediate to high-suldation states. No porphyry-style mineralization related to epithermal mineralization is known at LaCoipa.

Thegeomorphology of theLa Coipa district is dominatedby a planar,slightly west-inclined landscape constructed on the upper Oligocene

pyroclastic rocks and domes, but covered by the post mineral16–14.7 Ma dacite lavas and extrusive domes which form the topo-graphic highs in the district (Cornejo et al., 1998; Figs. 7, 8). This planarsurface is located at present day elevations of 4400–4200 m a.s.l., andappears to have been the dominant paleosurface at the time of epithermalmineralization as steam-heated mineralization is commonlyexposed on it (Fig. 8). The paleosurface that plausibly controlledepithermal activity can be correlated with the upper Oligoceneto ≥ 18 Ma Asientos pediplain mapped on the northern side of theCerros Bravos volcanic complex (Bissig and Riquelme, 2009). The upliftand subsequent erosion of the Asientos pediplain is probably related tothe late Oligocene contractile tectonic phase described from the Cordil-lera Claudio Gay some 50 km E of La Coipa ( Mpodozis and Clavero,2002). Gold and silver mineralization at Ladera Farellón and Pompeyais located near the upper reaches of a NE oriented valley, incised intothe probable upper Oligocene to lower Miocene paleosurface, but thetiming of valley erosion is currently unknown.

5.2. La Pepa

Within the Maricunga belt, besides la Coipa, only La Pepa containssignicant high-suldation epithermal mineralization. Here it is hostedin NNW-trending quartz alunite veins that cross-cut porphyry-Au stylemineralization centered on the Cavancha porphyry (Muntean andEinaudi, 2001). Similar quartz–alunite veins are widespread in otherporphyry Au deposits of the Maricunga belt, but only those at La Pepacontain gold grades high enough to sustain small-scale historic produc-tion (Muntean and Einaudi, 2001). La Pepa is also the only one of thesedeposits where alunite precipitation (between 23.5 and 23.25 Ma,

based on 40Ar/39Ar plateau ages), did not overlap temporally with thepotassic alteration, here dated at 23.81 ± 0.08 Ma (hydrothermal biotite40Ar/39Ar plateau age,Cavancha porphyry; Muntean and Einaudi, 2001).The late Oligocene landscape at La Pepa is obscured by extensive upperMiocene ignimbrite deposits through which La Pepa is exposed in anerosional window (Muntean and Einaudi, 2001).

6. High-suldation epithermaldeposits of the Domeyko fault system

Two noteworthy middle to late Eocene high-suldation epithermaldeposits, El Hueso and El Guanaco, are located in the southern AtacamaDesert. These deposits occur near the Domeyko fault system of theChilean Precordillera, which also contains the behemothian Escondida,Chuquicamata, and Collahuasi porphyry Cu districts. All of these

deposits were emplaced during, or immediately after the late EoceneIncaic orogenic event which has been ascribed to an inferred episodeof slab attening, crustal thickening, volcanic lull and subsequent east-ward shift of the volcanic arc (Mpodozis and Cornejo, 2012; O'Driscollet al., 2012).

Regional uplift and formation of large scale erosional surfaces of Incaic age are implied from constraints on supergene activity andlarge scale morphotectonic features such as the Precordillera whichformed during the Incaic orogeny in the southern Atacama Desert of northern Chile. Uplift in the Eocene to elevations similar to the presentday is also evidenced from stable-isotope paleoaltimetry for the Punaplateau (Canavan et al., 2014) as well as for the pre-Cordillera at ~ 26°Lat. S (Bissig and Riquelme, 2010). Supergene oxidation and Cu enrich-ment in Paleocene to middle Eocene porphyry deposits in response to

uplift and erosion, initiated as early as 45 to 36 Ma at Cerro Verde

(Peru), Cerro Colorado, Spence and El Salvador (Bissig and Riquelme,2010; Bouzari and Clark, 2002; Gustafson and Hunt, 1975; Quanget al., 2003, 2005). The late Eocene climate was probably semi-aridand considerably wetter than the present day since suf cient water isnecessary for economically signicant supergene enrichment anderosion.

6.1. El Hueso

Arguably, nowhere in the Andes is the interrelationship betweenmajor orogenic episodes and porphyry and epithermal mineralizationbetter demonstrated than in the Potrerillos–El Hueso district (Cornejoet al., 1993; Marsh et al., 1997; Niemeyer and Munizaga, 2008;Thompson et al., 2004). At El Hueso, epithermal mineralization is locat-ed about 3 km E of the Potrerillos porphyry Cu deposit, in thehangingwall of the SE-verging Potrerillos Mine thrust fault which wasactive during and immediately after porphyry and epithermal mineral-ization (Marsh et al., 1997; Niemeyer and Munizaga, 2008). The districtalso hosts the Jerónimo carbonate-hosted epithermal gold deposit(Thompson et al., 2004), located some 1.5 km to the east of El Hueso,in the footwall of the Potrerillos Mine Thrust fault.

Epithermal alteration and mineralization at El Hueso are hosted incalcareoussilt and sandstones of the Upper Jurassic Asientos formation,as well as the uncomformably-overlying andesitic-to-rhyoliticlavas andvolcaniclastic rocks of thePaleoceneHornitos Formation(Marsh, 1997).Hydothermal activity occurred in two stages. An early mineralizationstage spatially associated with a 40.8 Ma porphyry stock was dated atca. 40.2 Ma by hydrothermal sericite and alteration at this time wasdominated by sericite and illite, but incorporated only minor advancedargillic alteration. A second stage of barren quartz–alunite alterationand silicication occurred at ca. 36.2 Ma (King, 1992; Marsh et al.,1997; Olson, 1984). The early stage deposited gold, accompanied by1–2% pyrite, stibnite, chalcopyrite, galena and arsenopyrite, all hostedin quartz veinlets (Marsh et al., 1997). The principal porphyry Cu–Aumineralization centered on the Cobre porphyry was emplaced about3 kmwest of El Hueso andimmediately after theepithermal mineraliza-tion and alteration at 35.5–35.9Ma(Marsh et al., 1997). Structural anal-

ysis of the district suggests that the emplacement depth of porphyryCu–Au mineralization at the Cobre porphyry was shallow, as little as1 km below thepaleosurface(Niemeyer and Munizaga,2008). Althougha number of Eocene porphyry stocks intrude the Jurassic and Paleocenerocks in the district, no age-equivalent volcanic rocks are recognized(Marsh et al., 1997). In the El Salvador porphyry Cu district, locatedsome 20 km NW of Potrerillos, hypogene mineralization occurred at43–41 Ma (Gustafson et al., 2001), but the deposit was exposed to oxi-dation and supergene modication at ca. 36 Ma (Bissig and Riquelme,2010; GustafsonandHunt,1975; Mote et al., 2001). Thus, upliftandero-sion exhumed porphyry style mineraliztion at El Salvador about 6 Maafter hypogene mineralization, i.e. at the time of hydrothermal activityat el Hueso and Potrerillos.The Precordillera (a.k.a. CordilleraDomeyko)at the latitude of the present day southern Atacama Desert is located at

the southewestern boundary of the Puna Plateau and probably formedpart of theAndean crest from late Eoceneuntilthe late Oligocene, beforelate Oligocene thrusting in the Cordillera Claudio Gay, some 70 km eastof thePrecordillera, established the main Andean range as known today(Bissig and Riquelme, 2010).

The geomorphology of the area around El Hueso is dominated by asubplanar erosion surface, now situated at 3900–4000 m elevationwhich has been cut into the Paleocene Hornitos formation (Fig. 9).This erosion surface is interpreted as a pediplainand its age is uncertainbut probably represents a somewhat erosion-degraded relic of apediplain that formed in response to uplift related to the late Eoceneto early Oligocene Incaic orogeny (Fig. 9). In a broad context, El Huesois located below a topographic high, near a scarp that separates itfrom topographically lower areas to the west and may have been

emplaced in a physiographic setting comparable, albeit less well


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preserved, to e.g., Pascua–Lama or El Indio. Given the widespread pres-ervation of late Eocene to early Oligocene epithermal alteration andmineralization styles at most about 200–300 m of erosion occurredsince hydrothermal activity in the El Hueso area. Erosion modicationof the original Incaic erosion surface probably occurred after 33 Ma asit affects the granodiorite stock of Cerro El Hueso dated at ca. 33 Ma(Cornejo et al., 1993), but prior to 18 Ma, which is the age of the~100–200 m lower Asientos pediplain cutting into the high level postIncaic surface at El Hueso (Bissig and Riquelme, 2009). The base levelof the Asientos pediplain is the Salar de Pedernales which wasestablished in the late Oligocene (Bissig and Riquelme, 2009). TheOligoceneto early Miocene landscape elements and underlyingshallowlevel epithermal mineralization in the El Hueso area are preserved dueto the arid climate. Erosion since the early Miocene was mostly concen-trated in canyons and average denudation rates were low (~14 m/Ma:Riquelme et al., 2008).

6.2. El Guanaco

El Guanaco is a second example of a high-suldation epithermal de-posit spatially associated with the Domeyko fault system. It is located30 km W of the crest of the Precordillera at 25.1° Lat. S. Mineralizationis hosted in roughly E to ENE-trending quartz replacement veins witha narrow advanced argillic and argillic alteration halo. Suldes includeenargite, pyrite and subordinate chalcopyrite. Veins were emplacedin andesite lavas and overlying dacite tuffs of Paleocene age (ca.54–60 Ma; Puig et al., 1988). The veins are well-dened, subvertical,tabular structures when hosted in the andesite, but assumean irregularbehavior in the overlying, more permeable felsic pyroclastic rocks.Supergene oxidation affected the deposit up to 300 m depth and gaverise to a large variety of arsenate minerals, El Guanaco being the type-

locality for some of these (Witzke et al., 2006).At El Guanaco, hydrothermal alunite yields K/Ar ages between ~48

and 42 Ma (Puig et al., 1988). No age-equivalent intrusive or volcanicrock or porphyry-style mineralization has been documented in the dis-trict. El Guanaco is the oldest-known high-suldation epithermal de-posit of the Andes, but overlaps in age with the El Salvador andEsperanza porphyry Cu (–Au) districts (Gustafson et al., 2001; Perellóet al., 2004), similarly located west of the main Domeyko fault system.

Mineralization at El Guanaco is located below Cerro Estrella, a localtopographic high at ~2900 m a.s.l., and extends to depths of at least2500 m a.s.l. Cerro Estrella rises about 300 m above surrounding relictpediment surfaces of unknown age. However, El Guanaco's physio-graphic setting is consistent with a position below a high-level Incaicor pre-Incaic erosion surface being incised by pediments either during

or after hydrothermal activity.

7. Late Miocene high-suldation epithermal Au deposits of the

western Cordillera of northern Chile and southern Peru

High-suldation epithermal deposits arerare in theCentral Volcaniczone, between ~25° Lat S in northern Chile and 15° Lat S in Central Peru,an area that coincides with the major Chile–Peru orogenic bend. Here,thesubduction angle is at ~ 30° steeper than in thepresentat-slab seg-ments of northern Chile and central and northern Peru (Cahill andIsacks, 1992). The current slab geometry resulted from steepening of an Eocene shallow angle slab (Sandeman et al., 1995). The Andes atthe Chile–Peru oroclinal bend are metallogenetically diverse and in-clude W,Sn, Ag, Pb, Zn and U deposits inthe Inner Arc (partly coincidingwith theCordillera Oriental)rangingfrom Jurassic to Pleistocene age, aswell as porphyry and skarn type mineralization of Paleocene andEocene ages (Clark et al., 1990; Perelló et al., 2003). However, a fewMiocene epithermal gold deposits occur in the Cordillera Occidental of northern Chile and southern Peru, all of which are located at local topo-graphic highs with mineralization exposed at elevations of 4800 to

5200 m a.s.l. Signicant deposits include the 6.6 Ma Choquelimpiehigh-suldation deposit in Chile (Groepper et al., 1991) and the SantaRosa and Tucarí high-suldation deposits of the Aruntani district insouthern Peru (Barreda et al., 2004; Morche et al., 2008). Hypogenealunites from Santa Rosa and Tucari have been dated at 6.4 and4.6 Ma, respectively (Morche et al., 2008).

Mineralization at Choquelimpieis hostedin thecore of a stratovolca-no interpreted as broadly cogenetic with epithermal mineralization(Groepper et al., 1991). At Aruntani, multiple episodes of volcanism,represented by dacitic plugs and subvolcanic intrusions as well as an-desitic lavas both pre-dated and overlapped with epithermal minerali-zation (Barreda et al., 2004). Thus, the volcanologic setting of thesedeposits is distinct from the deposits farther south in that mineraliza-tion is temporally more closely associated with voluminous volcanism

and does not post-date the age of their host rocks by 5 or more Ma.The geodynamic setting of late Miocene high-suldation deposits of the Cordillera Occidental alsodiffers from the current at slab segmentsin that magmatism is associated with contraction of the arc due toslab steepening rather than arc-broadening and cessation related toshallowing subuduction angles as observed in the Pampean andPeruvian at slab segments (cf. Bissig et al., 2003, 2008; Kay andMpodozis, 2001; Sandeman et al., 1995).

The landscape evolution documented on the western Andean slopeindicates that multiple stages of pediment bevelling due to uplift of the Incaic paleosurface occurred between ca. 24 Ma and 8 Ma (Quanget al., 2005; Tosdal et al., 1984) followed by a major pulse of Altiplanouplift and incision of deep Canyons (Thouret et al., 2007). The majorityof workers agree that most of the Altiplano uplift to average elevations

of 3800 m, occurred during the Miocene. Evidence from both

El Hueso epi-thermal Au Deposit

Cerro El Hueso

>18 Ma Asientos pediplain with gravel cover

Late Eocene to Oligocene paleosurface

Fig. 9. Panoramic view of the physiographic setting of the El Hueso epithermal Au deposit. Photograph taken from the East looking West. The Potrerillos porphyry Cu (–Au) deposit islocated behind the Horizon West of El Hueso and is not visible in this view. See Bissig and Riquelme (2009).


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geomorphology (Hoke et al., 2007; Jordan et al., 2010; Schildgen et al.,2007) and paleoaltimetry (based on stable isotope and paleobotany;Garzione et al., 2008) suggest that at least 1000 m or more of this eleva-tion gain occurred after 10 Ma,butprior to about 5 Ma.Thus,epithermalgold deposits of the Central Volcanic zone formed during a major upliftpulse.

8. Central to northern Peruvian at slab segment

The Andean segment extending from~ 15° Lat S in Peru, to 2° Lat S insouthern Ecuador, lacks recent arc volcanism, which, as in Chile, hasbeenattributedtoat-slab subductionspatially coincidingwithsubduc-tion of aseismic ridges and oceanic plateaus (e.g., Gutscher et al., 1999a;Martinod et al., 2010; Skinner and Clayton, 2013). The high-suldationepithermal deposits of this region occur in two distinct belts (Nobleand McKee, 1999). The southeastern belt, located east of the CordilleraOccidental, which forms the Continental divide, and extends from

Julcani (~13° Lat S) to the latitude of Antamina (~9.5° Lat S). It coincideswith the central Peruvian polymetallic province and contains a varietyof porphyry-related and carbonate hosted deposits emplaced at differ-ent paleodepths (Bissig and Tosdal, 2009; Escalante, 2008; Escalanteet al., 2010; Love et al., 2004). This segment includes the cordilleranbase metal lode deposits (i.e. sulde rich polymetallic deposits:Bendezú et al., 2003) of Marcapunta–Colquijirca and Cerro de Pascowhich are centered on high-suldation epithermal deposits emplacedin reactive host rocks (Bendezú and Fontboté, 2009; Bendezú et al.,2008). The northwestern metallogenetic belt has its southern terminusat Pierina at 9.5° Lat S, west of the Cordillera Blanca and is dominated byhigh-suldation epithermal and porphyry Cu–Au deposits but alsoincludes a number of smaller vein hosted Ag–Au deposits such asQuiruvilca (Gustafson et al., 2004; Noble and McKee, 1999). It includes,from south to north, Pierina, Lagunas Norte and Yanacocha, the latter of which contains upwards of 70 Moz contained Au (Longo et al., 2010;Teal and Benavides, 2010) and which is by far the largest epithermaldeposit cluster of the Andes.

Subduction of the Nazca aseismic ridge in Peru commenced in themiddle Miocene at 14–10 Ma (Hampel, 2002) and is the inferred

cause of at subduction (e.g.,Martinod et al., 2010). However, estimateson the timing of crustal thickening and uplift of the Cordillera Occiden-tal in central and northern Peru vary from the early to the middle Mio-cene and do not coincide everywhere with the onset of aseismic ridgesubduction and at subduction. Thus, Noble et al. (1990) on the basisof the age of tuff deposits in deeply incised paleovalleys in northernPeru, suggested that uplift and erosion occurred in the early–lateMiocene. Crustal thickening inferred from whole rock geochemicaldata of volcanic rocks in the central Peruvian polymetallic province isinferred to have occurred around 12–15 Ma (Bissig and Tosdal, 2009).In contrast, Montgomery (2012)proposed that a major episodeof upliftcommenced at about 17 Ma, i.e., before the inferred onset of ridge sub-duction, but coincidentwith high-suldationepithermal mineralizationat Lagunas Norte (Cerpa et al., 2013; Montgomery, 2012).

9. High-suldation epithermal deposits of the central Peruvian

polymetallic belt

The central Peruvian polymetallic belt contains a number of lateMiocene silver and base metal-rich vein deposits that were emplacedin the epithermal environment. These include: San Cristóbal (Beuchatet al., 2004), Morococha (Catchpole et al., 2011) and Uchucchacua(Bussell et al., 1990; Escalante, 2008). At Morococha an evolution fromthe porphyry to the epithermal environment occurred concurrentlywith erosion, and late Miocene epithermal Ag mineralization probablyoccurred only a few 100 m below the paleosurface (Catchpole et al.,2011). All of these epithermal deposits have sulde assemblages of in-termediate suldation state and veins are largely hosted in Mesozoic

carbonaceous rocks as well as Triassic volcanosedimentary rocks.

However, the polymetallic belt also contains a number of epithermaldeposits with important high-suldation characteristics; these aredescribed in the following.

9.1. Julcani

The Julcani district, mined since colonial times, consists of several-vein hosteddeposits from which Ag andbase metals, as well as subordi-

nate Au,have been produced.Themineralization is hosted in coalescingdomes of andesitic-to-dacitic composition and postdates a large zone of vuggy quartzand quartzalunite alterationin thecenterof thedistrict, aswell as quartz–tourmaline–pyrite breccia bodies (Deen et al., 1994;Petersen et al., 1977). Veins are zoned along strike from highersuldation stateenargite–tennantite bearing assemblages near the cen-ter of the district to lower-suldation assemblages containing galenaand tennantite–tetrahedrite more distally (Petersen et al., 1977).Magmatism largely predated the epithermal mineralization but a9.7 Ma syn-mineral dyke and small, 7 Ma, post-mineral rhyolitedomes have been recognized (Deen et al., 1994; Petersen et al., 1977).

Julcani is hosted below a sub-planar, slightly east-inclined, land surfacesituated at elevations of 4000–4500 m a.s.l.

9.2. The Marcapunta–Colquijirca district

Marcapunta is a high-suldation epithermal Au (–Ag, Cu) deposit.The district also contains cordilleran base metal mineralization at theSmelter deposit, northward contiguousto Marcapunta,and moredistal-ly the Colquijirca Ag–Pb–Zn deposit, some 5 km north of Marcapunta.The San Gregorio deposit is another cordilleran base metal depositsoutherly adjacent to Marcapunta (Bendezú et al., 2008). Vuggy quartzalteration, phreatomagmatic breccias and disseminated gold minerali-zation occur in the central Marcapunta dome, whereas strataboundbase metal and silver rich mineralization at Smelter and Colquijirca,extending as far as 5 km north from Marcapunta, are hosted in Eocenelimestones and marls (Bendezú et al., 2008). There is a district-scalelateral zoning fromproximal high-suldationmineralization containingenargite and pyrite to intermediate pyrite, chalcopyrite and tennantite-

bearing ore, to a distal pyrite, sphalerite and galena dominated suldeassemblage, the later reecting an intermediate suldation state(Bendezú et al., 2008; Vidal and Ligarda, 2004). Alunite associatedwith the central high-suldation gold mineralization yielded ages of 11.9 to 11.1 Ma, whereas that associated with the base metal minerali-zation yielded slightly younger ages of 10.8 to 10.5 Ma ( Bendezú et al.,2008). The dacitic dome complex hosting some of the epithermalmineralization was emplaced at 12.9 to 12.1 Ma (40Ar/39Ar biotiteages). No syn- or post mineral volcanic or intrusive rocks are exposedin the district.

Colquijirca is located near the eastern margin of a regionally exten-sive low-relief plain situated at an elevation of 4200 to 4300 m a.s.l.whereas the Cerro Marcapunta summit at 4450 m a.s.l. constitutes thehighest feature in the deposit area. Vuggy quartz hosted gold minerali-

zation extends from 4450 m to about 4000 m a.s.l. or 450 m below sur-face, whereas distalbase metal mineralization occursat depths less than~400 m below surface. Proximal enargite-rich mineralization adjacentto the dome complex, however, has been drilled to a depth of N600 mbelow surface and attains thickness of up to 100 m (Bendezú et al.,2008; Vidal and Ligarda, 2004).

9.3. Cerro de Pasco

Cerro de Pasco is located about 11 km N of Marcapunta and sharesmany similarities to the geologic setting and mineralization style of Colquijirca. The deposit is spatially related to a 15.4 to 15.2 Ma daciticdiatreme dome complex which was emplaced at theboundary of Devo-nian phyllites to the west and Triassic-to-Jurassic limestones to the east

(Baumgartner et al., 2008, 2009). Mineralization occurred in two stages


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(Baumgartner et al., 2008; Einaudi, 1977). Stage 1 was focused at theeastern margin of the diatreme dome complex and is characteriz

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