Bissig 2015 Au Ag

Ore Geology Reviews 65 (2015) 327–364

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Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeorev

Review

Physiographic and tectonic settings of high-sulfidation epithermal gold–silver deposits of the Andes and their controls on mineralizing processes Thomas Bissig a,⁎, Alan H. Clark b, Amelia Rainbow b, Allan Montgomery c a b c

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

a r t i c l e

i n f o

Article history: Received 6 July 2014 Received in revised form 10 September 2014 Accepted 17 September 2014 Available online 28 September 2014 Keywords: High-sulfidation Epithermal Andes Landscape evolution Erosion Uplift Flat subduction Neogene

⁎ 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.

a b s t r a c t Gold and silver ores in the vast majority of Andean high-sulfidation epithermal Au–Ag deposits occur at high present 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 Indio belt, 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 for N 130 Moz Au resources. Slightly older examples are only preserved in the Atacama Desert and include the middle Eocene El Guanaco and El Hueso and the late Oligocene/early Miocene La Coipa deposits. The absence of Paleocene and older high-sulfidation epithermal deposits can be explained by limited preservation potential imposed by transpressional tectonics within overall contractile episodes and surface uplift. These conditions prevailed predominantly in segments of shallow-angle subduction of the Nazca or Caribbean plate below the South American continent, a tectonic setting also common for porphyry-style Cu (–Au, Mo) deposits. Stratovolcanoes are uncommon ore hosts and volcanic rocks coincident with mineralization are in most cases volumetrically restricted or absent, recording the terminal stages of local arc magmatism. However, dacitic domes are important at, e.g., Yanacocha and La Coipa. At Lagunas Norte, a small stratovolcano largely pre-dating but temporally overlapping with mineralization occurs immediately east of the deposit and volcanic sector collapse may have occurred during hydrothermal activity. Mineralization is typically located near the backscarp of pediments or the heads of valleys incising now highelevation, low-relief surfaces. In the California–Vetas Mining District and El Indio belt, hydrothermal alunite ages 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 hydrostatic and lithostatic pressure at these sites of high local relief are believed to have enhanced both boiling and mixing of magmatic with meteoric fluids, ultimately enhancing ore deposition. The host rock composition, permeability and location of the water table control the distribution of alteration zones and ore. Intermediate volcanic rocks are the most common ore-hosts but they typically pre-date mineralization by several Ma. However, high-sulfidation epithermal mineralization can be hosted in any conceivable rock type including high grade metamorphic rocks (California–Vetas mining district), significantly older plutonic rocks (Pascua–Lama) or quartzites (Lagunas Norte). Large vuggy quartz alteration zones and commonly oxidized low-grade large-tonnage mineralization are best developed in relatively permeable volcaniclastic rocks or hydrothermal breccia bodies, whereas coherent volcanic, plutonic, or metamorphic rocks may host fault- and brecciacontrolled ores. The near-surface steam-heated zone can attain a thickness of several hundred meters in dry climates (e.g. Veladero, Pascua–Lama, Tambo) but is typically poorly developed and less than 20 m thick in humid climatic zones. The physiographic and tectonic settings of high-sulfidation epithermal deposits are distinct from low-sulfidation epithermal districts such as those of Patagonia, El Peñón (Chile) or Fruta del Norte (Ecuador). The latter range to significantly older ages (Jurassic to early Eocene) occur at mainly lower elevations and were emplaced in extensional settings. A temporal coincidence between uplift, erosion and mineralizing processes as well as a spatial and temporal association with porphyry style mineralization is not evident for these low-sulfidation districts. © 2014 Elsevier B.V. All rights reserved.

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Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Epithermal deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distribution of high-sulfidation deposits in the Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . El Indio belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. The Tambo deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. El Indio deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Pascua–Lama . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Veladero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Maricunga belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. La Coipa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. La Pepa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. High-sulfidation epithermal deposits of the Domeyko fault system . . . . . . . . . . . . . . . . . . . . . . 6.1. El Hueso . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. El Guanaco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Late Miocene high-sulfidation epithermal Au deposits of the western Cordillera of northern Chile and southern Peru 8. Central to northern Peruvian flat slab segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. High-sulfidation epithermal deposits of the central Peruvian polymetallic belt . . . . . . . . . . . . . . . . . 9.1. Julcani . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. The Marcapunta–Colquijirca district . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3. Cerro de Pasco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4. Quicay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. The high-sulfidation epithermal Au (–Cu, Ag) deposits of northwestern Peru . . . . . . . . . . . . . . . . . . 10.1. Pierina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2. Lagunas Norte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3. Yanacocha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4. Tantahuatay, Sipan and La Zanja . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. The northern Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1. Quimsacocha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2. California Vetas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Summary and comparison to low-sulfidation deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13. Controls of geomorphic processes and climate on mineralization . . . . . . . . . . . . . . . . . . . . . . . 14. Igneous rocks, volcanology and magmatic fluids related to high-sulfidation epithermal deposits . . . . . . . . . 15. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The Andes are the world's most endowed region with respect to giant magmatic-hydrothermal ore deposits (Cooke et al., 2005). They host the largest-known porphyry copper deposits (e.g., Rio Blanco–Los Bronces–Los Sulfatos, El Teniente, Chuquicamata) as well as many of the world's largest epithermal Au–Ag deposits (e.g., Yanacocha, Lagunas Norte, Pascua–Lama, Veladero: Sillitoe, 2008). The vast majority of Andean epithermal deposits containing N 10 Moz Au are of highsulfidation type. These deposits have a close link to a magmatic source for fluids, volatiles and metals (e.g., Deyell et al., 2004; Rye, 1993) but form at depths of typically less than 1 km (e.g. Sillitoe, 2010) and consequently mineralizing processes are influenced by the near-surface physicochemical environment. The main focus of this review is on deposits and districts where the bulk of the precious metal is contained in the epithermal environment, i.e., the shallow part of magmatichydrothermal systems, and concentrates on the physiographic environment of epithermal mineralization. This paper does not discuss major porphyry Cu deposits in detail, although the shallow portions of many of these have been overprinted by epithermal mineralization or alteration (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 a comprehensive overview of the major high-sulfidation epithermal districts and mineral belts of the Andes. It focuses on the links between landscape evolution, climatic setting, volcanology and tectonics, and

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discusses the influence these factors can have on both mineralizing processes and the preservation of the deposits.

2. Epithermal deposits Epithermal deposits are usually classified into sub-types based on either ore sulfide assemblage or characteristic associated alteration; both schemes have inherent limitations. Some of the most widely referenced review papers on the topic (Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003; Simmons et al., 2005) prefer a classification into high, low and intermediate-sulfidation types. This classification scheme can, however, be problematic, because sulfide assemblages may be difficult to classify in a field exploration setting, particularly if the deposit has been oxidized. Moreover, sulfide assemblages within a single deposit may represent precipitation over the entire breadth of sulfidation state, from high to intermediate and low-sulfidation, depending on fluid–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 classification is based on dominant alteration and gangue assemblages and includes quartz–adularia–sericite and quartz–alunite or acid sulfate type epithermal deposits (Heald et al. 1987, Tosdal et al., 2009), the former typically including low to intermediate-sulfidation sulfide assemblages and the latter associated with high-sulfidation deposits. The limitation of this classification scheme is that, particularly in high-sulfidation deposits, alteration may pre-date and may not be directly related to mineralization (see below).

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

For this paper we use the widely applied classification scheme of high-sulfidation (normally associated with acidic quartz–alunite alteration) and low-sulfidation type (associated with near neutral quartz–adularia gangue and illite–adularia alteration) deposits. The term intermediate sulfidation is used to describe sulfidation state but is not used in the sense of a distinct subtype of epithermal deposit due to the inherent variation of sulfide assemblages within a single deposit or vein. In most high-sulfidation epithermal deposits, two hydrothermal stages can be distinguished (e.g., Simmons et al., 2005). An early stage of intense acid leaching of the wall-rock by a magmatic vapor that condensed into groundwater, cooled to temperatures below 300 °C and was acidified by dissociation of acids such as HCl, or H2SO4 (Hedenquist and Taran, 2013); and, a subsequent stage of weaklyacidic fluid incursion, from which the bulk of the sulfides and precious metals, together with euhedral quartz, precipitated (e.g., Heinrich et al., 2004, 2007; Holley, 2012; Stoffregen, 1987). The intense acid leaching generates the zoning from vuggy quartz to quartz–alunite and more distal kaolinitic alteration. This assemblage is commonly referred to as a lithocap because it “caps” potential porphyry Cu mineralization (Sillitoe, 1995). Although mineralization is widely related to the infiltration of the less acidic fluids into the precursor lithocap (e.g., Holley, 2012; Stoffregen, 1987), it has been shown that the acidic magmatic vapors can also transport gold into the shallow epithermal environment (Scher et al., 2013; Taran et al., 2000). At Pascua–Lama such fluids were responsible for at least some of the gold mineralization (Chouinard et al., 2005). Conversely, there are examples of highsulfidation epithermal deposits where vuggy quartz alteration is largely absent (e.g., La Bodega, Colombia: Rodriguez, 2014). The precipitation mechanisms for precious metals in highsulfidation epithermal deposits are not as well understood as for lowsulfidation systems. For the latter, ample textural and fluid inclusion evidence leaves little doubt that fluid 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, fluid inclusion studies of high-sulfidation deposits are difficult, as the appropriate transparent minerals are rare. Several possible precipitation mechanisms such as boiling, and fluid mixing of a magma derived fluid with oxidized groundwater have been proposed (e.g., Rye et al., 1992; Jannas et al., 1999). Recent work on the La Bodega deposit in Colombia, where the high-sulfidation sulfide assemblage is hosted in veins with similar textures as described for low-sulfidation deposits (see below), supports that boiling is a viable mechanism to precipitate sulfides (Rodriguez, 2014). Moreover, steam-heated alteration, present above many Andean high-sulfidation systems, forms by H2S-rich steam that condenses and oxidizes to form sulfuric acid in the vadose zone, and is considered evidence for fluid boiling at depth (Simmons et al., 2005). A close association of high-sulfidation epithermal deposits with a parent intrusion and porphyry-style mineralization has been proposed by 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, calderas and extrusive or subvolcanic dacitic to andesitic domes have been considered the dominant hosts, and large volcanic centers have been presumed to be temporally and genetically associated with highsulfidation 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 fluids or condensed magmatic vapors constitute the dominant component of the mineralizing fluids. However, as will be discussed below, better geological understanding and new geochronological information, together with exploration discoveries made since 1995, suggest that stratovolcanoes and calderas are uncommon hosts to epithermal ore in the Andes.

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3. Distribution of high-sulfidation deposits in the Andes High-sulfidation epithermal deposits occur mostly in the Central Andes (~ 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 volcanic 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 overwhelming majority of high-sulfidation epithermal deposits are Miocene or younger, and are located in segments of the Andean arc where magmatism is now absent or subdued (e.g., Bissig et al., 2008; Kay and Mpodozis, 2001; Rosenbaum et al., 2005). The lack of magmatism is attributed to two zones of shallow angle subduction of the Nazca plate below the South American continent in the Central Andes (the Pampean flat slab in northern Chile and the Peruvian flat slab in central and northern Peru: Ramos, 2009) as well as the Bucaramanga flat slab where Caribbean plate is subducting below northern Colombia (Vargas and Mann, 2013). Flat subduction coincides with shortening and crustal thickening of the overriding continental plate (Martinod et al., 2010) and has been spatially linked to the subduction of aseismic ridges or oceanic plateaus on the oceanic plate (e.g. Gutscher et al., 1999b; Yañez et al., 2001). However the causative relationship between ridge impingement and onset of flat subduction remains a matter of debate (Skinner and Clayton, 2013). Thus, crustal shortening, uplift and porphyry Cu mineralization pre-dates the arrival of the subducting aseismic Juan Fernández ridge in Central Chile (Deckart et al., 2013), a situation comparable to northern Peru where high-sulfidation epithermal gold mineralization at Lagunas Norte occurred at 17 Ma, coincident with uplift, but several Ma prior to the inferred onset of Nazca ridge subduction (Montgomery, 2012, see below). The Northern Andes are characterized by a series of oceanic terranes accreted to the South American continent dominantly between the Mesozoic and Miocene (Cediel et al., 2003). The Central Andes, in contrast, consist of a collage of terranes accreted during the Proterozoic-toPaleozoic (Ramos, 2008). Here, Andean type subduction has dominated the active continental margin since Permo-Triassic times (Ramos, 2009), and in both the Northern Andes south of ~ 5.5° Lat. N and the Central Andes, the Nazca plate is currently subducting eastward beneath the South American continent. High-sulfidation epithermal deposits occur in the same segments of the Andean arc as porphyry-style deposits (e.g. Sillitoe, 2008, 2010). However, a direct temporal, spatial and genetic relationship to porphyry deposits containing economic Cu mineralization, as demonstrated at Far Southeast and Lepanto (Hedenquist et al., 1998), has not been documented for any Andean high-sulfidation epithermal deposits discussed herein, although the two mineralization styles may overlap either spatially or temporally (e.g. Yanacocha district: Longo et al., 2010, La Pepa, Maricunga belt: Muntean and Einaudi, 2001). All high-sulfidation epithermal deposits of the Andes now crop out at high elevation, and with few exceptions, above ~ 3500 m a.s.l. Most are also located near the water-shed along the crest of the orogen, or what appears to have been the water-shed at the time of hydrothermal activity, in areas dominated by relatively flat landforms which are interpreted as uplifted sub-planar paleosurfaces. In the semi-arid and arid portions of the Andes, these paleosurfaces probably represent pediplains (regionally interconnected pediment surfaces) formed under semi-arid conditions and several pulses of pediment erosion separated by uplift pulses can commonly be distinguished (Bissig and Riquelme, 2009; Bissig et al., 2002a; Riquelme et al., 2007; Rodriguez et al., 2014; Sillitoe et al., 1968), resulting in a series of pediment surfaces, vertically separated up to several 100 m by steeper back-scarps. Similar relationships between high-level sub-planar landforms incised by broad, flat bottomed valleys are present in southern Peru (Quang et al., 2005; Tosdal et al., 1984) and northern Peru (Montgomery, 2012). The major high-sulfidation deposits and districts are described within their geomorphologic and volcanologic context from south to north

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70° W

80° W

90° W

60° W

50° W

epithermal deposits

20° N

10° N

23/24 CoR

0°

des

n An

CaR

22 Norther al Andes ntr Ce

IP 21 20 19 18 17 16 15 14 13 12

10° S

1: Tambo 2: El Indio 3: Pascua-Lama 4: Veladero 5: Famatina 6: La Coipa 7: El Hueso 8: 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 Active volcano Flat slab segments Aseismic ridges Subducted aseismic ridges/plateaus

11 10 9 10° S

8

NR

IR 30° S

7 6 4 35 2 1

JFR

Fig. 1. Map of South America, showing locations of major high-sulfidation epithermal deposits discussed in this paper. Also indicated are segments of flat 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.

in the following section. Key characteristics, locations, ages and approximate 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 flat slab segment. The belt is named after the El Indio Au (–Ag, Cu) deposit which was mined between 1978 and 2001. The belt also includes the small Tambo and the giant Veladero and Pascua–Lama deposits as well as other prospects (Fig. 2), jointly accounting for ~ 40 Moz of Au resources. Additionally, the belt contains numerous alteration zones ranging 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 and mineralization ages quoted herein for the El Indio belt are all 40Ar/39Ar plateau ages on alunite or sericite mostly with analytical errors of b1 to 5% on individual dates. It is worth noting that the small epithermal Cu–Au district at Famantina (Table 1), is located in the same Andean segment, albeit some 220 km E of Veladero in Argentina, and is at 5 to 3.8 Ma just slightly younger than El Indio (Lozada-Calderón et al., 1994; Pudack et al., 2009). Volcanism in the Oligocene and early Miocene was voluminous in the El Indio belt (Bissig et al., 2001; Martin et al., 1995; Winocur et al., 2014) initially generating the 27–23 Ma Tilito Formation dacitic-toandesitic 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

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

Escabroso (21–17 Ma) and Cerro de las Tórtolas (16.6–14 Ma) Formations (Fig. 2), both associated with large volcanic edifices and subvolcanic intrusions (Bissig et al., 2001; Martin et al., 1995). Erupted magma volumes decreased markedly after ca. 14 Ma and volcanism was confined to isolated eruptive centers at ~ 12.7–11 Ma (Vacas Heladas 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 (Cerro de Vidrio) has been documented ~7 km E of the Veladero deposit (Bissig et al., 2001, 2002a, 2002b). The voluminous upper Oligocene to middle Miocene andesites show an increasing modal abundance of phenocrystic hornblende, relative to clinopyroxene, with time (Bissig et al., 2003). Further, igneous rocks younger than ca. 14 Ma are characterized by elevated Sr/Y (N40) and depleted HREE contents, indicative of garnet fractionation at the base of the crust and elevated pressure equivalent to N 45 km crustal thickness (Bissig et al., 2003). This change in both the erupted volumes and chemistry of the igneous rocks is attributed to the onset of flat subduction and associated increased coupling between the Nazca slab and overlying continental plate, which led to contractile deformation and crustal thickening (Bissig et al., 2003; Kay et al., 1999). Epithermal mineralization is generally hosted in Cerro de las Tórtolas Formation and older rocks but occurred several Ma after the emplacement of the host rocks. Mineralization of the Filo Federico ore zone of the Veladero deposit is partly hosted in volcaniclastic deposits of the Vacas Heladas Formation (Holley, 2012). The geomorphology in the El Indio belt preserves evidence of a history of episodic uplift and pediplain erosion throughout the middle to late Miocene (Bissig et al., 2002a; Figs. 3, 4, 5, 6). Relics of three lowrelief erosional surfaces, vertically separated from each other by 200–400 m, have been documented. These are the 17–15 Ma Frontera–Deidad surface, the 14–12.5 Ma Azufreras–Torta surface and the 10–6 Ma Los Ríos surface (Bissig et al., 2002a). The Frontera–Deidad surface dominating the higher interfluves is a relic of a regionally extensive pediplain on the western Andean slope at the latitude of the El Indio belt (Aguilar et al., 2011; Rodriguez et al., 2014), subsequently incised by the Azufreras–Torta and Los Ríos pediment surfaces. These nested paleosurfaces in the main Cordillera are now separated from the landforms of the Coastal Cordillera by the north striking, west verging Vicuña–San Félix fault system (Aguilar et al., 2011, 2013). Vertical displacement along this fault system starting in the early–middle Miocene is thought to be responsible for the three distinct paleosurfaces observed in the El Indio belt (Aguilar et al., 2011). Steam-heated alteration associated with epithermal mineralization in all deposits of the El Indio belt is almost exclusively exposed on the Azufreras–Torta surface (Figs. 3,4,5, 6), whereas the top of the mineralized zone typically 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 well as a number of satellite breccia bodies and replacement veins, all of which were emplaced into dacitic pyroclastic and volcanoclastic rocks assigned to the upper Oligocene Tilito Formation. The ore zone has an overall vertical extent of about 300 m (Deyell et al., 2005a; Jannas et al., 1999). The paleosurface level is located at 4500 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, has been dated at 8.8 to 8 Ma although at Canto Sur it yields ages as young as 7.1 Ma (Bissig et al., 2001; Deyell et al., 2005a). Alunite ages 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 a Los Ríos valley where it incises the older Azufreras–Torta surface (Figs. 3, 4).

331

4.2. El Indio deposit Mineralization at El Indio, in contrast to Tambo, is hosted in veins (Jannas et al., 1999) and includes sulfide-rich massive enargite ± pyrite veins as well as quartz-rich veins with tetrahedrite/tennantite and chalcopyrite. For some veins, an along-strike zonation from high- to intermediate-sulfidation sulfide assemblages, and from pyrophyllite– sericite–diaspore ± alunite to illite-dominated alteration has been documented (Heather et al., 2003a; K.B. Heather, pers. commun. 2012). El Indio is world-renowned due to exceptional bonanza grades of up to N1% Au in the quartz-rich, intermediate-sulfidation El Indio Sur 3500 vein which permitted shipping of the ore directly to the smelter (direct shipping ore) in the early days of exploitation. Gold occurred mainly as native Au, auricupride and as tellurides (Heather et al., 2003a; Jannas et al., 1990, 1999). As is the case for Tambo, the top of the mineralized zone at El Indio is located about 200 m below the Azufreras–Torta Surface, here located at ~4400 m a.s.l. Steam-heated alteration is exposed locally on Cerro Campana above the western part of the deposit (Fig. 3). The mineralized zone has a vertical extent of 500 m extending down 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 developed in dacitic pyroclastic rocks of the Tilito Formation, which in the area of the deposit are uncomformably overlain by andesitic lavas and volcaniclastic rocks of the lower Miocene Escabroso Group. Most veins also cross-cut the Escabroso Group but typically horsetail out above the contact (Bissig et al., 2002a; Heather et al., 2003a). Low volumes of ca. 8 Ma dacitic hypabyssal intrusions spatially associated with mineralization have also been reported (Heather et al., 2003b). The mineralization ages range from 8 to 5 Ma and are inferred from sericite immediately 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 identified (A.H. Clark and K. B. Heather, unpubl. sericite 40Ar/39Ar data). The age range indicates that multiple hydrothermal pulses related to successive pressure release, likely facilitated by erosion, from a sizeable batholith scale magma system were responsible for mineralization. Contrary to the conclusion of Jannas et al. (1999) enargite-rich veins are not systematically older than gold-rich veins, since the youngest data come from 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 to intermediate-sulfidation veins whereas the El Indio Sur 3500 vein yielded the oldest sericite ages of ca. 8 Ma (A.H. Clark and K.B. Heather unpubl. data). The geochronological data demonstrate a significant age gap of at least 7 Ma between volcanic rock deposition (~ 26– 15 Ma) and mineralization (8–5 Ma). Only small amounts of igneous rocks contemporaneous with mineralization are known and these include ca. 8 Ma dacitic plugs and 5.5 Ma Vallecito Formation rhyolite tuffs 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 is plausible that El Indio formed near the back-scarp of the incising Río Malo 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 located some 50 km N of El Indio on the Chile–Argentina border. It is hosted within an uplifted Paleozoic basement block (Figs. 5, 6), with mineralization centered on a series of diatreme breccia complexes. The most important of these is Brecha Central, which cuts Paleozoic and Jurassic granitoids (Chouinard et al., 2005). Hydrothermal activity is constrained to 9.1–8 Ma based on 40Ar/39Ar dating of alunite and jarosite; alunite from pyrite–enargite veins constraining the gold mineralization to 8.8 ± 0.6 Ma (Bissig et al., 2001; Deyell et al., 2005b). Slightly older alunite 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

332

Table 1 Summary of main HS deposits. Mineralization Total resources (approx.)

Host rocks

Volcanology

Selected references

8.5 to 8 Ma

0.8 Moz Au

Hydrothermal breccia hosted Au–Ag mineralization associated with intense acid sulfate alteration. Subordinate replacement veins.

Late Oligocene Tilito Formation dacite tuffs

Bissig et al. (2002a, 2002b); Deyell et al. (2005a); Jannas et al. (1999)

40

8 to 5 Ma Ar/39Ar, alunite, sericite

4.2 Moz Au

Largely vein hosted Au (–Cu, Ag) mineralization. Most gold produced from quartz rich veins with sulfide assemblage of intermediate sulfidation state, most Cu produced from massive enargite veins. Dominant alteration is advanced argillic to phyllic (sericite, diaspore, locally pyrophyllite, alunite). Mineralization in two principal ore zones and associated with intense acid sulfate alteration and silicification: Amable (12.1 to 12.7 Ma) and Filo Federico (10.7– 11 Ma). Apart from minor argentite no primary sulfides recognized. Most Ag in jarosite at Amable. Breccia and stockwork vein-hosted mineralization associated with high sulfidation state sulfide assemblage and intense acid sulfate alteration Vein hosted mineralization above porphyry center. Ore mineralogy indicates high-sulfidation state and includes famatinite, enargite, pyrite and lesser, tennantite, tetrahedrite, sphalerite, gold, tellurides,covellite, and chalcopyrite. Primary sulfides include pyrite– enargite, at depth also bornite, covellite, fahlore, galena and spalerite. Much of the ore is oxidized

Late Oligocene Tilito Formation dacite tuffs and overlying early Miocene Escabroso Group andesite lavas and breccias

No igneous rocks age equivalent to mineralization recognized, dacite tuffs of the Tilito Formation pre-date mineralization by N 14 m.y. Minor, undated pre-mineral diorite porphyry has locally been intersected below mineralization. Phreatomagmatic activity pre to syn mineralization evidenced by breccia bodies. No igneous rocks age equivalent to mineralization recognized, except for possible 8 Ma dacite dyke. Host rocks pre-date mineralization by ~7 to 17 m.y.

Elevation of Mineraliza-tion Elevation of mineralization paleosurface age (method) (if known) (m.a.s.l.)

Deposit/ district

Lat.

Long.

Tambo

−29.8

−69.95 4300–4100

4500

El Indio

−29.74 −69.97 4200–3700

4400

−29.36 −69.95 4300–3900

4600

12.7–10.7 Ma (40Ar/39Ar, alunite)

12.2 Moz Au; 180 Moz Ag

Pascua-Lama

−29.32 −70.01 4850–4550

5000–5150

9.1–8 Ma (40Ar/39Ar, alunite)

17.6 Moz Au; 585 Moz Ag

Famatina

−29.00 −67.78 4750–4350

3.8 Ma (40Ar/39Ar, sericite)

0.4 Moz Au; 3 Moz Ag

La Coipa

−26.81 −69.27 4150–3800

4350

20–17 Ma K/Ar, alunite

3.5 Moz Au; 230 Moz Ag

El Hueso

−26.49 −69.53 3950–3700

n/a

40–36.2 Ma (40Ar/39Ar, alunite)

0.84 Moz Au; 3.9 Moz Ag

Mineralization is associated with illite–sericite and quartz alteration. Sulfide minerals include pyrite, chalcopyrite, galena, native bismuth and Bi–Pb sulfide minerals. This assemblage is overprinted by advanced argillic alteration (alunite, pyrophyllite) and pyrite. Most gold was introduced during the first stage

Cerro de las Tórtolas Formation (16–14 Ma) volcano sedimentary breccias, subvolcanic domes

Permo-Triassic to Jurassic granitic rocks

Volcano sedimentary host rocks pre-date hydrothermal breccias and mineralization by ~ 2–4 Ma. Locally 12.1 Ma andesite dykes cutting mineralization at Amable zone

Phreatomagmatic breccias, pre and syn mineralization. Post mineral dacitic dyke (7.8 Ma) cutting mineralization. Dacitic porphyries dated at 5 Ma are Cambrian weakly metamorphosed marine shale associated with porphyry mineralization but no extrusive and siltstones of the Negro volcanic rocks known from the Peinado formation district but ash-flow tuffs from the eastern flank of the Famatina range may be age equivalent to porphyries. Black shales and sandstones of Domes and lava flows of the Triassic Ternera formation intermediate composition were dated at 24–22 as well as 16–14.7 Ma and overlying Oligocene dacitic domes and pyroclastic and thus both pre- and post date rocks. Diatreme breccias are a hydrothermal alunite ages. No common ore host throughout volcanic rocks coincident with mineralization known the district as well Calcareous silt and Host rocks pre-date mineralization sandstones, limestones of the by ~15 m.y. No age equivalent Asientos Formation (u. volcanic rocks known but Jurassic) and overlying subvolcanic porphyries are Paleocene andesite lavas and widespread and associated with volcaniclastic rocks porphyry Cu mineralization at Potrerillos

Charchaflie et al. (2007); Holley (2012)

Bissig et al. (2002a, 2002b); Chouinard et al. (2005); Deyell et al. (2005b) Lozada Calderón et al. (1994); Pudack et al. (2009).

Cecioni and Dick (1992); Oviedo et al. (1991)

Marsh et al. (1997)

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

Veladero

Bissig et al. (2002a, 2002b); Heather et al. (2003a); Jannas et al. (1999)

−25.1

n/a

48–42 Ma (K/Ar, alunite)

1.6 Moz Au

Choquelimpie −18.49 −69.39 4860–4700

n/a

6.6 Ma?? (K/Ar, host rock)

0.5 Moz Au

Santa Rosa (Aruntani)

−16.63 −70.02 4950–4750

n/a

7.2 Ma (40Ar/39Ar, alunite)

0.5 Moz Au

Tucari (Aruntani)

−16.57 −70.19 5200–4900

n/a

4.6 Ma (40Ar/39Ar, alunite)

2.1 Moz Au

Julcani

−12.95 −74.95 4500–4000

~9.7 Ma (K/Ar on biotite)

8 Moz Ag

Marcapunta/ Colquijirca

−10.78 −76.27 4450–4850

n/a

11.9–10.6 Ma (40Ar/39Ar, alunite)

Quicay

−10.7

n/a

37.5 Ma (K/Ar) alunite (?)

2345 kt Zn, 910 kt Pb, 1510 kt Cu, 6620 t Ag, 64 t Au n/a

−76.26 4350–?

Vein hosted mineralization of enargite, pyrite and minor chalcopyrite with quartz gangue in andesite, smaller irregular veins in overlying pyroclastic rocks. Oxidaton and supergene enrichment affected the ore to ~ 300 m depth and is characterized by the presence of a variety of rare arsenate minerals. Mineralization associated with hydrothermal breccia bodies in the core of a late Miocene stratovolcano Mineralization is hosted in vuggy quartz zones centered on magmatic hydrothermal breccia bodies and is surrounded by quartz–alunite alteration zones. Primary sulfides include pyrite and enargite but the deposit has been oxidized to a depth of 300–400 m below surface. Highgrade mineralization is controlled by NNE oriented fractures. Mineralization is hosted in vuggy quartz zones centered on hydrothermal breccias and is surrounded by quartz–alunite alteration zones. Primary sulfides include pyrite and enargite but the deposit has been oxidized to a depth of 300–400 m below surface. Pre-ore acid sulfate alteration with vuggy quartz core and surrounding quartz–alunite zone in the center of the district, cut by discontinuous quartz–tourmaline–pyrite breccias. Mineralization is hosted by laterally zoned veins with quartz–pyrite– wolframite–enargite–tetrahedrite– galena and barite rich zones. Zoning is from high sulfidaton near the core to distal intermediate sulfidation assemblages). Zoned polymetallic district with a high-sulfidation Au–Ag prospect and distal carbonate replacement Pb–Zn–Ag mineralization.

Mineralization is hosted in an oxidized diatreme–dome complex. Highest grades up to 3 g/t are hosted in a central vuggy quartz zone.

Andesite lava flows and overlying dacitic pyroclastic rocks. Both units were dated by K–Ar at ~60.2–60.7 Ma

Volcanic host rocks pre-date mineralization by 12 or more m.y. No volcanic rocks age equivalent to mineralization known.

Puig et al. (1988); http:// www.australgold.com.au accessed Nov 2013.

Late Miocene andesite lavas and dacite subvolcanic dome

Andesitic stratovolcano. Mineralization largely hosted in late dacitic subvolcanic dome Mineralization occurs adjacent to rhyolitic flow/dome complex and is closely associated with magmatichydrothermal breccias.

Groepper et al. (1991)

Miocene (9–7 Ma) felsic pyroclastic deposits are dominant ore host.

Barreda et al. (2004); Morche et al. (2008)

Intrusions and domes of dacitic composition.

Mineralization is spatially associated with magmatic-hydrothermal breccias emplaced at the margin of dacitic domes. The latter were emplaced in the core of an andesitic stratovolcano.

Barreda et al. (2004); Morche et al. (2008)

10.1 Ma andesitic to dacitic domes overlying paleozoic phyllites of the Excelsior group

Voluminous extrusive domes of andesitic to dacitic composition, covering 16 km2. Syn mineralization 9.7 Ma dykes and post mineralization 7 Ma rhyolite dome are also documented.

Deen et al. (1994); Noble and Silberman, 1984

Eocene limestone and marls and 12.9–12.4 Ma diatreme dome complex.

Mineralization is spatially associated with a 12.9–12.4 Ma diatreme dome complex (Marcapunta) pre-dating alunite by N0.8 m.y. A dacite dyke dated at 14.1 Ma has been documented W of Marcapunta. No other igneous rocks recognized.

Bendezu et al. (2008); Vidal and Ligarda (2004);

Intermediate volcanic rock and hydrothermal breccias

Subvolcanic to extrusive dome complex of uncertain age hosting mineralization. Apparently no postmineral volcanic rocks.

Rossell et al. (2006); Ingemmet.gob.pe, accessed Jan 2014.

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

−69.53 2400–2900

El Guanaco

(continued on next page)

333

334

Long.

Elevation of Mineraliza-tion Elevation of mineralization paleosurface age (method) (if known) (m.a.s.l.)

Mineralization Total resources (approx.)

Host rocks

Volcanology

Selected references

Late Triassic to early Jurassic Pucara group limestone and mid Miocene diatreme breccias.

Mineralization occurs at eastern border of a large 15.4 to 15.15 Ma diatreme–dome complex pre-dating stage II mineralization by N 0.8 Ma but with unclear temporal relationship to stage I mineralization. No volcanic rocks pre-dating the dome complex known. Late dacite flow dome complexes immediately pre-date mineralization. No post mineral volcanic rocks reported.

Baumgartner et al. (2008, 2009)

Deposit/ district

Lat.

Cerro de Pasco

−10.68 −76.27 4350–3900

n/a

15.4–14.4 (40Ar/39Ar, alunite)

12,250 kt Zn, 3500 kt Pb, 15,750 t Ag, 1000 kt Cu

Pierina

−9.46

−77.59

n/a

14.2–14.7 (40Ar/39Ar, alunite)

Oligocene to Middle Miocene (29.3–14.8 Ma) andesitic to dacitic tuffs, lavas and subvolcanic domes of the Huaraz group

Lagunas Norte

−7.95

78.25

n/a

17.4–16.5 Ma (40Ar/39Ar, alunite)

9 Moz Au Mineralization hosted in permeable pumiceous rock units and associated with intense acid sulfate alteration, including vuggy quartz and quartz alunite. Sulfides include enargite, pyrite as well as galena, sphalerite and stibnite, and lesser paragenetically late digenite and covellite. 13.1 Moz Disseminated and fracture hosted Au mineralization, centered on diatreme breccias. Mineralization in volcanic rocks hosted in vuggy quartz and quartz–alunite zones. Dominant sulifdes include enargite, pyrite and digenite.

Yanacocha

−7

−78.58 4100–3500

~4200

11–8.2 Ma (40Ar/39Ar Alunite)

N70 Moz Au

Calipuy group (19.5–15.1 Ma) andesitic to dacitic lavas and pyroclastic deposits and 14.5–8.4 Ma Yanacocha unit dacite lavas and pyroclastic rocks

Early pyrrhotite-pyrite–galena–Fe rich sphalerite (stage I) and later Cu–Ag (Au–Pb–Zn) enargite–pyrite veins as well as carbonate replacement Pb–Zn ore (stage II).

Massive and vuggy quartz zones hosting pyrite ± enargite– tennantite–covellite

Lower Cretaceous Chimú Formation quartzites (two thirds) and early Miocene dacite tuffs and breccias (one third).

Volcanic rocks hosting mineralization were emplaced between early and main mineralization stage. Volcanic rocks were likely derived from diatreme breccias which also host mineralization. Andesite lava and dome adjacent to main diatreme has same age as mineralization. Mineralization overlaps in age with voluminous andesitic to dacitic volcanic rocks. Most mineralization (N47 Moz Au) emplaced in the waning stages of volcanism represented by the felsic Coriwachay dacite domes (10.9–8.4 Ma).

Fifarek and Rye (2005); Rainbow et al. (2005, 2006)

Cerpa et al. (2013); Montgomery (2012).

Longo et al. (2010)

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Table 1 (continued)

−6.9 −6.83

−78.79 ~3500 −78.89 3700–3400

n/a n/a

n/a n/a

n/a 0.73 Moz Au, 6 Moz Ag

Tantahuatay

−6.72

−78.67 4050–3800

n/a

12.4 Ma (K/Ar alunite)

Quimsacocha

−3.04

−79.22 3700–3450

3900

7.5–7.3 Ma (40Ar/39Ar Alunite)

Gold is mostly associated to pyrite– 5.1 Moz Au; 108.3 enargite hosted in intensely quartz– pyrophyllite–alunite and vuggy to Moz Ag massive quartz alteration. Much of the ore is oxidized Stratabound Au-Ag mineralization 3.6 Moz associated with pyrite and enargite, Au, 27 hosted in vuggy quartz zones Moz Ag

La Bodega/La Mascota

7.38

−72.9

2900–2300

~3600

3.5–1.7 Ma (40Ar/39Ar, alunite, sericite)

3.5 Moz Au

Angostura

7.39

−72.88 3500–2600

~3600

4–1.9 Ma (40Ar/39Ar, alunite, sericite)

2.7 Moz Au

n/a n/a

Dominantly structurally controlled breccias cemented by quartz, alunite and high-sulfidation sulfide assemblage, overprinting earlier quartz–pyrite ± chalcopyrite veins in phyllic alteration. Most Au–Ag in late stages. Banded, colloform and lattice textures normally typical for low to intermediate sulfidation deposits are common. Widespread sheeted quartz– pyrite ± chalcopyrite veins associated with pervasive quartz– sericite alteration. Locally cut by enargite–Cu sulfide bearing breccias and veins associated with quartz alunite alteration. The latter hosts highest grade Au zones.

n/a n/a

Miocene andesite domes and to a lesser degree underlying pyroclastic rocks

Mineralization located in pyroclastic units between less permeable coherent andesite lava flows of the Quimsacocha Formation

Proterozoic gneisses and late Triassic leucogranite.

Proterozoic gneisses and late Triassic leucogranite.

n/a n/a

Gustafson et al. (2004) Compañía minera Buenaventura 2012 Annual report, Gustafson et al. (2004) Gustafson et al. (2004); Volcanic domes hosting the Noble and Mckee mineralization are not age (1999); Compañía constrained. Post mineral dyke of minera Buenaventura restricted volume was dated at 2012 Annual report 8.6 Ma (K/Ar, biotite). MacDonald et al. (2011, Andesite lavas and intercalated pyroclasitc units associated to a large 2012) stratovolcano complex. Andesite is ~ 9 Ma, pre-dating mineralization by 1.5 Ma. A central caldera and dacite domes were emplaced at 6.7 Ma, 0.5 Ma after mineralization Rodriguez (2014) No igneous rocks age equivalent to mineralization recognized. No juvenile clasts in breccias observed.

No igneous rocks age equivalent to mineralization recognized.

Felder et al. 2005, Rodriguez (2014).

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

Sipan La Zanja

335

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

6,800,000

336

El Indio belt geology

Cerro del Toro (6168 m)

6-5.5 Vallecito Fm. rhyolite

Au mineralization

6,780,000

2 Ma Cerro de Vidrio rhyolite dome

8-7.6 Ma Pascua Fm. dacite 12.7-11 Ma Vacas Heladas Fm. dacite 21-14 Ma Escabroso and Cerro de las Tórtolas Fm. andesite and subvolcanic diorite

Cordille ra Ortíg a

6,760,000

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

Pascua-Lama

Paleozoic and Mesozoic basement Thrust/reverse fault (symbol on hanging wall side) Fault (unspecified) Lineament from satellite image High-sulfidation epithermal deposit Major Mountains

6,720,000

Cordille ra Zanc

Sancarrón

Cordille ra de la

arrón

Brea

6,740,000

Veladero

Tambo

6,700,000

Cerro Doña Ana (5625 m)

Cordille ra Cola ngüil

El Indio

Cerro de las Tórtolas (6160 m)

6,680,000

N 20 km

380,000

400,000

420,000

440,000

460,000

Fig. 2. Geological map of the El Indio belt and location of principal epithermal deposits. Compiled from Martin et al. (1995), Bissig et al. (2001), Zappettini (2008), and Winocur et al. (2014). Coordinates in UTM Zone 19S, Provisional South American Datum 56.

rocks contemporaneous with mineralization have been confirmed but low volumes of juvenile igneous material probably played a role during emplacement of the diatreme complexes and rare, locally weakly argillically altered, 7.8 Ma rhyodacitic dykes post-dating mineralization have been recognized at Pascua (Bissig et al., 2001). The alteration and sulfide paragenesis reveals that unusually acidic fluids were responsible for alteration and some of the mineralization. Gold mineralization is associated with pyrite and enargite, as well as alunite, 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, jarosite precipitated from mixed meteoric and magmatic fluids whereas alunite has a magmatic fluid signature (Deyell et al., 2005b). This indicates that fluid sources varied through time, plausibly as a function of a fluctuating but overall decreasing water table and short lived magma-derived fluid pulses, as also documented for El Indio (see above). Near Pascua–Lama, the Frontera–Deidad surface reaches elevations of 5200–5300 m a.s.l. (Fig. 5) The Azufreras–Torta surface is incised into this high-elevation pediplain at 4950–5100 m and extensive steam-heated alteration and clastic deposits, both probably related to

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

337

El Indio/Tambo Area, Chile & Argentina 69°58'0"W

69°56'0"W

29°42'0"S 29°44'0"S

Cerro Campana

69°52'0"W

69°50'0"W

Argentina

Chile

Río del Medio

69°54'0"W

LR AT

6710000

70°0'0"W

29°40'0"S

70°2'0"W

El Indio

LR

29°46'0"S

AT

AT Canto Sur

29°48'0"S

Kimberly

FD Veta Veronica

Tambo

Wendy 6700000

29°50'0"S

AT

29°52'0"S

LR

0

1

2

3

4

5 km

400000

410000

Elevation (m)

Slope (Degrees)

420000

Paleosurfaces

High : 6107

Los Ríos (LR); 10-6 Ma 0 - 10

Azufreras-Torta (AT); 14-12.5 Ma

Low : 1775

Frontera-Deidad (FD); 17-15 Ma

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

phreatomagmatic eruptions derived from the mineralized breccia pipes, are exposed on this landform (Bissig et al., 2002a). Mineralization occurs below the 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 the Azufreras–Torta surface from the east. Ferricrete-cemented conglomerates at the valley-bottom display glacial striations, showing that this valley 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 east may have occurred during hydrothermal activity. 4.4. Veladero Veladero is situated about 8 km to the SE of Pascua in a NNE striking graben (Charchaflié et al., 2007). Gold and silver mineralization is

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B

Veta Verónica

Co Ele fan te Co Tó de la rto las s

A

Deidad

Steam-heated alteration Frontera Deidad surface (17-15 Ma) Azufreras-Torta surface (14-12.5 Ma) Los Ríos surface (10-6 Ma)

Tambo (Kimberly pit)

o

Azufreras

C Doña Ana

Canto Sur pit

Looking S from Canto Sur, Tambo

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-heated alteration zones as well as the Kimberly and Canto Sur orebodies. See Fig. 3 for locations.

hosted in two principal breccia bodies: Amable and Filo Federico (Fig. 6). Ore mineralogy comprises electrum and silver-bearing jarosite which were deposited together with euhedral quartz, and are superimposed on earlier acidic residual quartz alteration (Holley, 2012). Jarosite is abundant, especially at Amable, whereas sulfides are exceedingly rare. At Amable, jarosite forms euhedral crystals and grows in open spaces and veins, and is locally overgrown by barite. Stable-isotope evidence indicates that most jarosite precipitated from meteoric fluids above the water table, with sulfur derived from precursor sulfides. 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 displaying stable isotope signatures of magmatic fluids (Holley, 2012). Mineralization at Amable is constrained between a 12.7 Ma alunite 40 Ar/39Ar age and a 12.14 Ma U/Pb zircon date of a narrow andesite dyke that cross-cuts mineralization (Holley, 2012). Mineralization at Filo Federico is at 11.1 to 10.3 Ma (40Ar/39Ar, alunite, n = 4) somewhat younger than at Amable (Bissig et al., 2001; Holley, 2012). Jarosite ages at Amable range from 11.8 to 8.6 Ma and overlap with hypogene alunite and gold deposition at Filo Federico (Holley, 2012). These data are interpreted as evidence for an overall lowering of the water table with time, episodically interrupted by pulses of magma derived fluids. Jarosite at Veladero almost entirely pre-dated hypogene alunite alteration and sulfide mineralization at nearby Pascua−Lama. The host rocks of the Amable ore zone consist of crudely stratified volcaniclastic breccias assigned to the Cerro de las Tórtolas formation, unconformably overlying Tilito Formation andesites and dacites as well as Permian felsic tuffs (Charchaflié et al., 2007; Holley, 2012). The Filo Federico ore zone is partly hosted in volcaniclastic deposits assigned to the Vacas Heladas Formation (Holley, 2012). Mineralization is closely associated with hydrothermal breccias intruding the volcaniclastic deposits and occurs between 4400 and 3900 m a.s.l. in a subhorizontal, N − S elongated ore body, extending extends to lower elevations at Filo Federico than at Amable. A steam-heated alteration zone of up to 200 m thickness overlies the orebodies and defines the paleosurface at ~4400–4600 m a.s.l. At Filo Federico, steam-heated alteration is localized along fracture zones, and overprints the mineralized zone down to 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 Taguas graben (Charchaflié et al., 2007, Fig. 6), whereas the Azufreras–Torta surface has been cut into Cerro de las Tórtolas volcaniclastic deposits at an elevation of ~ 4500 m (Charchaflié et al., 2007, Fig. 6), i.e. at the time of, or immediately prior to, hydrothermal activity in that area. Conversely, the Filo Federico mineralization probably formed at a time when incision of the Los Ríos surface was well advanced, as evidenced by the steam-heated overprint of the ore zone and the mineralization extending to lower elevations than at Amable. The lowering of the water table in the Veladero area during Los Ríos surface incision is also reflected 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 defined as the region of the Andean Cordillera between 26° and 28° Lat S (Vila and Sillitoe, 1991), located at the northern margin of the Pampean flat slab segment. Gold-rich porphyry type deposits (e.g., Cerro Casale (a.k.a. Adelbarán), Caspiche, Lobo, Marte) are the defining mineralization style and the gold-rich nature of porphyry-style mineralization is thought to be related to the shallow emplacement of the porphyry intrusions (Muntean and Einaudi, 2001). Although barren quartz–alunite alteration is commonly found in the shallow portions of these porphyry systems, La Pepa is the only porphyry deposit where significant gold mineralization hosted in quartz–alunite veins that overprint porphyry-style mineralization has been documented (Muntean and Einaudi, 2001). In contrast, La Coipa, located at the northern end of the Maricunga belt, hosts Ag–Au mineralization exclusively as epithermal mineralization of largely highsulfidation type (Cecioni and Dick, 1992; Oviedo et al., 1991). Porphyry and epithermal deposits of the Maricunga belt occur in two partly overlapping parallel N–S striking belts, with mineralization traditionally considered to have been emplaced during two distinct episodes at 25–20 Ma and 15–13 Ma (Sillitoe et al., 1991). However, more recently published, but still limited geochronology requires modification of this model. The Caspiche porphyry Au deposit was dated at ~25.4 ± 0.01 Ma by Re/Os on molybdenite (Sillitoe et al., 2013) whereas the Caserones porphyry Cu deposit in the southern Maricunga belt formed 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

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Pascua-Lama/Veladero Area, Chile & Argentina 70°0'0"W

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0 - 13

Azufreras-Torta (AT); 14-12.5 Ma Frontera-Deidad (FD); 17-15 Ma

Low : 2877 Fig. 5. Veladero and Pascua area topographic map and landscape elements. Paleosurfaces after Bissig et al. (2002a) and Charchaflié et al. (2007). UTM Zone 19S, WGS84.

et al., 1991) which disagree with the range between 25 and 20 Ma established on the basis of alunite K/Ar ages (Sillitoe et al., 1991) and biotite and hornblende K/Ar ages for dacite domes that were thought to be closely related to mineralization (Oviedo et al., 1991; Sillitoe et al., 1991). The large-scale morphotectonic features of the Andes are different at the latitude of the Maricunga belt to those of the El Indio belt, probably influenced 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 location which roughly coincides with the inferred southern limit of the Antofalla 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

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A

Cerro Pelado Amable

Filo Federico

Penelope Lama Central

Río Turbio

Cerro Nevado

B Pascua/Lama

nb grabe

oundin

g fault

s

Fabiana Steam-heated alteration Frontera Deidad surface (17-15 Ma) Azufreras-Torta surface (14-12.5 Ma) Los Ríos surface (10-6 Ma)

Looking W from Fabiana

Fig. 6. Landscape in the Veladero and Pascua–Lama area. A) panoramic view from the Fabiana prospect toward the west. B) Line drawing of the landscape shown in A. Landscape elements are shown as well as steam-heated alteration zones. See also Fig. 5 for locations.

roughly the same time is documented from the Cordillera Claudio Gay at the latitude of the Maricunga belt during the late Oligocene (Mpodozis and Clavero, 2002). The dominant landscape elements at the latitude of the northern Maricunga belt include the probably Oligocene relict Sierra Checos de Cobre surface which is incised by the upper Oligocene to ~ 18 Ma Asientos pediplain in the eastern Precordillera (Bissig and Riquelme, 2009; Mortimer, 1973). The Oligocene to early Miocene landscape was incised by the middle- to early-upper Miocene Atacama pediplain and late Miocene canyons (Bissig and Riquelme, 2009) which record a progressive tilting of the western Andean slope and uplift of the Andean Cordillera since the early Miocene (Riquelme et al., 2007).

5.1. La Coipa The La Coipa district contains multiple epithermal deposits of mostly high-sulfidation type including Ladera Farellón, Can Can, Coipa Norte, Purén, Purén Sur and Pompeya (Fig. 7). With the exception of Purén and Pompeya, mineralization is hosted in the Triassic La Ternera formation, consisting of black shales and sandstones, as well as in overlying upper Oligocene (24–22 Ma) dacitic tuffs, volcaniclastic breccias and domes (Cecioni and Dick, 1992; Oviedo et al., 1991). Hydrothermal breccias hosting ore are probably penecontemporaneous with mineralization. Alunite K/Ar ages range between ca. 20 and 17 Ma (Oviedo et al., 1991). A relict, large, upper Oligocene volcanic center, Cerros Bravos

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La Coipa Area, Chile 69°16'0"W

69°14'0"W

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Purén L. Oligocene/E. Miocene paleosurface Purén Sur Coipa Norte

26°48'0"S

Pompeya Can Can

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0

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480000

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High : 4907 0 - 10

Outline of post-mineral dacite domes and lavas

Low : 3013 Fig. 7. Topographic map and landscape elements of the La Coipa district. Some geological information from Cornejo et al. (1998). UTM Zone 19S, WGS84.

(Cornejo et al., 1993), is located 16 km N of Ladera Farellón. Unaltered, 16–14.7 Ma dacitic domes and lavas post-date mineralization (Oviedo et al., 1991) and locally overlie the mineralized zones at Pompeya (Figs. 7, 8). The Pompeya and the Purén ore bodies were discovered after 2010, and only Purén has been mined. Pompeya is located ~3 km NE of Ladera Farellón and is of high-sulfidation type. Purén, 8 km NE of Ladera Farellón features high grade Pb–Zn mineralization representing an intermediate sulfidation sulfide assemblage, an absence

of advanced argillic alteration and the occurrence of various carbonates including rhodochrosite as alteration minerals (S. Gamonal, pers. commun. 2014). At Purén Sur, some 2.5 km south of Purén, native sulfur associated with steam-heated alteration was historically mined. Steam-heated alteration is also present at Pompeya, Purén and Coipa Norte 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

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A

16 Ma dacite lava

16 Ma dacite dome

Pompeya

B Steam-heated alteration zone

Late Oligocene/early Miocene erosion surface

C

Ladera Farellón

Can Can

Coipa Norte

16 Ma dacite dome

D

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 photograph belong 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 on a 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 the photograph shown in C). Light blue indicates the late Oligocene to early Miocene paleosurface.

expands into mushroom-shaped zones in the overlying volcanic rocks. The latter hosts higher Ag grades, whereas the former is relatively Au-rich (Oviedo et al., 1991). The top of the exposed orebodies lies around 4300 m a.s.l., with sulfide oxidation extending downwards

to ~ 3800 m a.s.l. (Cecioni and Dick, 1992). The dominant alteration is 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

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mineralization (Oviedo et al., 1991). The sulfides observed at depth include pyrite, chalcocite, covellite, bornite, chalcopyrite, tennantite, tetrahedrite, galena and sphalerite (Oviedo et al., 1991), representative of precipitation at intermediate to high-sulfidation states. No porphyrystyle mineralization related to epithermal mineralization is known at La Coipa. The geomorphology of the La Coipa district is dominated by a planar, slightly west-inclined landscape constructed on the upper Oligocene pyroclastic rocks and domes, but covered by the post mineral 16–14.7 Ma dacite lavas and extrusive domes which form the topographic highs in the district (Cornejo et al., 1998; Figs. 7, 8). This planar surface is located at present day elevations of 4400–4200 m a.s.l., and appears to have been the dominant paleosurface at the time of epithermal mineralization as steam-heated mineralization is commonly exposed on it (Fig. 8). The paleosurface that plausibly controlled epithermal activity can be correlated with the upper Oligocene to ≥ 18 Ma Asientos pediplain mapped on the northern side of the Cerros Bravos volcanic complex (Bissig and Riquelme, 2009). The uplift and subsequent erosion of the Asientos pediplain is probably related to the late Oligocene contractile tectonic phase described from the Cordillera Claudio Gay some 50 km E of La Coipa (Mpodozis and Clavero, 2002). Gold and silver mineralization at Ladera Farellón and Pompeya is located near the upper reaches of a NE oriented valley, incised into the probable upper Oligocene to lower Miocene paleosurface, but the timing of valley erosion is currently unknown. 5.2. La Pepa Within the Maricunga belt, besides la Coipa, only La Pepa contains significant high-sulfidation epithermal mineralization. Here it is hosted in NNW-trending quartz alunite veins that cross-cut porphyry-Au style mineralization centered on the Cavancha porphyry (Muntean and Einaudi, 2001). Similar quartz–alunite veins are widespread in other porphyry Au deposits of the Maricunga belt, but only those at La Pepa contain gold grades high enough to sustain small-scale historic production (Muntean and Einaudi, 2001). La Pepa is also the only one of these deposits where alunite precipitation (between 23.5 and 23.25 Ma, based on 40Ar/39Ar plateau ages), did not overlap temporally with the potassic alteration, here dated at 23.81 ± 0.08 Ma (hydrothermal biotite 40 Ar/39Ar plateau age, Cavancha porphyry; Muntean and Einaudi, 2001). The late Oligocene landscape at La Pepa is obscured by extensive upper Miocene ignimbrite deposits through which La Pepa is exposed in an erosional window (Muntean and Einaudi, 2001). 6. High-sulfidation epithermal deposits of the Domeyko fault system Two noteworthy middle to late Eocene high-sulfidation epithermal deposits, El Hueso and El Guanaco, are located in the southern Atacama Desert. These deposits occur near the Domeyko fault system of the Chilean 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 Eocene Incaic orogenic event which has been ascribed to an inferred episode of slab flattening, crustal thickening, volcanic lull and subsequent eastward shift of the volcanic arc (Mpodozis and Cornejo, 2012; O'Driscoll et al., 2012). Regional uplift and formation of large scale erosional surfaces of Incaic age are implied from constraints on supergene activity and large scale morphotectonic features such as the Precordillera which formed during the Incaic orogeny in the southern Atacama Desert of northern Chile. Uplift in the Eocene to elevations similar to the present day is also evidenced from stable-isotope paleoaltimetry for the Puna plateau (Canavan et al., 2014) as well as for the pre-Cordillera at ~26° Lat. S (Bissig and Riquelme, 2010). Supergene oxidation and Cu enrichment in Paleocene to middle Eocene porphyry deposits in response to uplift and erosion, initiated as early as 45 to 36 Ma at Cerro Verde

343

(Peru), Cerro Colorado, Spence and El Salvador (Bissig and Riquelme, 2010; Bouzari and Clark, 2002; Gustafson and Hunt, 1975; Quang et al., 2003, 2005). The late Eocene climate was probably semi-arid and considerably wetter than the present day since sufficient water is necessary for economically significant supergene enrichment and erosion. 6.1. El Hueso Arguably, nowhere in the Andes is the interrelationship between major orogenic episodes and porphyry and epithermal mineralization better demonstrated than in the Potrerillos–El Hueso district (Cornejo et al., 1993; Marsh et al., 1997; Niemeyer and Munizaga, 2008; Thompson et al., 2004). At El Hueso, epithermal mineralization is located about 3 km E of the Potrerillos porphyry Cu deposit, in the hangingwall of the SE-verging Potrerillos Mine thrust fault which was active during and immediately after porphyry and epithermal mineralization (Marsh et al., 1997; Niemeyer and Munizaga, 2008). The district also 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 in calcareous silt and sandstones of the Upper Jurassic Asientos formation, as well as the uncomformably-overlying andesitic-to-rhyolitic lavas and volcaniclastic rocks of the Paleocene Hornitos Formation (Marsh, 1997). Hydothermal activity occurred in two stages. An early mineralization stage spatially associated with a 40.8 Ma porphyry stock was dated at ca. 40.2 Ma by hydrothermal sericite and alteration at this time was dominated by sericite and illite, but incorporated only minor advanced argillic alteration. A second stage of barren quartz–alunite alteration and silicification occurred at ca. 36.2 Ma (King, 1992; Marsh et al., 1997; Olson, 1984). The early stage deposited gold, accompanied by 1–2% pyrite, stibnite, chalcopyrite, galena and arsenopyrite, all hosted in quartz veinlets (Marsh et al., 1997). The principal porphyry Cu–Au mineralization centered on the Cobre porphyry was emplaced about 3 km west of El Hueso and immediately after the epithermal mineralization and alteration at 35.5–35.9 Ma (Marsh et al., 1997). Structural analysis of the district suggests that the emplacement depth of porphyry Cu–Au mineralization at the Cobre porphyry was shallow, as little as 1 km below the paleosurface (Niemeyer and Munizaga, 2008). Although a number of Eocene porphyry stocks intrude the Jurassic and Paleocene rocks in the district, no age-equivalent volcanic rocks are recognized (Marsh et al., 1997). In the El Salvador porphyry Cu district, located some 20 km NW of Potrerillos, hypogene mineralization occurred at 43–41 Ma (Gustafson et al., 2001), but the deposit was exposed to oxidation and supergene modification at ca. 36 Ma (Bissig and Riquelme, 2010; Gustafson and Hunt, 1975; Mote et al., 2001). Thus, uplift and erosion exhumed porphyry style mineraliztion at El Salvador about 6 Ma after hypogene mineralization, i.e. at the time of hydrothermal activity at el Hueso and Potrerillos. The Precordillera (a.k.a. Cordillera Domeyko) at the latitude of the present day southern Atacama Desert is located at the southewestern boundary of the Puna Plateau and probably formed part of the Andean crest from late Eocene until the late Oligocene, before late Oligocene thrusting in the Cordillera Claudio Gay, some 70 km east of the Precordillera, established the main Andean range as known today (Bissig and Riquelme, 2010). The geomorphology of the area around El Hueso is dominated by a subplanar erosion surface, now situated at 3900–4000 m elevation which has been cut into the Paleocene Hornitos formation (Fig. 9). This erosion surface is interpreted as a pediplain and its age is uncertain but probably represents a somewhat erosion-degraded relic of a pediplain that formed in response to uplift related to the late Eocene to early Oligocene Incaic orogeny (Fig. 9). In a broad context, El Hueso is located below a topographic high, near a scarp that separates it from topographically lower areas to the west and may have been emplaced in a physiographic setting comparable, albeit less well

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Cerro El Hueso

El Hueso epithermal Au Deposit

Late Eocene to Oligocene paleosurface

>18 Ma Asientos pediplain with gravel cover

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 is located behind the Horizon West of El Hueso and is not visible in this view. See Bissig and Riquelme (2009).

preserved, to e.g., Pascua–Lama or El Indio. Given the widespread preservation of late Eocene to early Oligocene epithermal alteration and mineralization styles at most about 200–300 m of erosion occurred since hydrothermal activity in the El Hueso area. Erosion modification of the original Incaic erosion surface probably occurred after 33 Ma as it 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 post Incaic surface at El Hueso (Bissig and Riquelme, 2009). The base level of the Asientos pediplain is the Salar de Pedernales which was established in the late Oligocene (Bissig and Riquelme, 2009). The Oligocene to early Miocene landscape elements and underlying shallow level epithermal mineralization in the El Hueso area are preserved due to the arid climate. Erosion since the early Miocene was mostly concentrated 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-sulfidation epithermal deposit spatially associated with the Domeyko fault system. It is located 30 km W of the crest of the Precordillera at 25.1° Lat. S. Mineralization is hosted in roughly E to ENE-trending quartz replacement veins with a narrow advanced argillic and argillic alteration halo. Sulfides include enargite, pyrite and subordinate chalcopyrite. Veins were emplaced in andesite lavas and overlying dacite tuffs of Paleocene age (ca. 54–60 Ma; Puig et al., 1988). The veins are well-defined, subvertical, tabular structures when hosted in the andesite, but assume an irregular behavior in the overlying, more permeable felsic pyroclastic rocks. Supergene oxidation affected the deposit up to 300 m depth and gave rise to a large variety of arsenate minerals, El Guanaco being the typelocality 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 volcanic rock or porphyry-style mineralization has been documented in the district. El Guanaco is the oldest-known high-sulfidation epithermal deposit of the Andes, but overlaps in age with the El Salvador and Esperanza porphyry Cu (–Au) districts (Gustafson et al., 2001; Perelló et al., 2004), similarly located west of the main Domeyko fault system. Mineralization at El Guanaco is located below Cerro Estrella, a local topographic high at ~ 2900 m a.s.l., and extends to depths of at least 2500 m a.s.l. Cerro Estrella rises about 300 m above surrounding relict pediment surfaces of unknown age. However, El Guanaco's physiographic setting is consistent with a position below a high-level Incaic or pre-Incaic erosion surface being incised by pediments either during or after hydrothermal activity.

7. Late Miocene high-sulfidation epithermal Au deposits of the western Cordillera of northern Chile and southern Peru High-sulfidation epithermal deposits are rare in the Central Volcanic zone, 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, the subduction angle is at ~ 30° steeper than in the present flat-slab segments of northern Chile and central and northern Peru (Cahill and Isacks, 1992). The current slab geometry resulted from steepening of an Eocene shallow angle slab (Sandeman et al., 1995). The Andes at the Chile–Peru oroclinal bend are metallogenetically diverse and include W, Sn, Ag, Pb, Zn and U deposits in the Inner Arc (partly coinciding with the Cordillera Oriental) ranging from Jurassic to Pleistocene age, as well as porphyry and skarn type mineralization of Paleocene and Eocene ages (Clark et al., 1990; Perelló et al., 2003). However, a few Miocene epithermal gold deposits occur in the Cordillera Occidental of northern Chile and southern Peru, all of which are located at local topographic highs with mineralization exposed at elevations of 4800 to 5200 m a.s.l. Significant deposits include the 6.6 Ma Choquelimpie high-sulfidation deposit in Chile (Groepper et al., 1991) and the Santa Rosa and Tucarí high-sulfidation deposits of the Aruntani district in southern Peru (Barreda et al., 2004; Morche et al., 2008). Hypogene alunites from Santa Rosa and Tucari have been dated at 6.4 and 4.6 Ma, respectively (Morche et al., 2008). Mineralization at Choquelimpie is hosted in the core of a stratovolcano 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 andesitic lavas both pre-dated and overlapped with epithermal mineralization (Barreda et al., 2004). Thus, the volcanologic setting of these deposits is distinct from the deposits farther south in that mineralization 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-sulfidation deposits of the Cordillera Occidental also differs from the current flat slab segments in that magmatism is associated with contraction of the arc due to slab steepening rather than arc-broadening and cessation related to shallowing subuduction angles as observed in the Pampean and Peruvian flat slab segments (cf. Bissig et al., 2003, 2008; Kay and Mpodozis, 2001; Sandeman et al., 1995). The landscape evolution documented on the western Andean slope indicates that multiple stages of pediment bevelling due to uplift of the Incaic paleosurface occurred between ca. 24 Ma and 8 Ma (Quang et al., 2005; Tosdal et al., 1984) followed by a major pulse of Altiplano uplift and incision of deep Canyons (Thouret et al., 2007). The majority of workers agree that most of the Altiplano uplift to average elevations of 3800 m, occurred during the Miocene. Evidence from both

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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 elevation gain occurred after 10 Ma, but prior to about 5 Ma. Thus, epithermal gold deposits of the Central Volcanic zone formed during a major uplift pulse. 8. Central to northern Peruvian flat slab segment The Andean segment extending from ~15° Lat S in Peru, to 2° Lat S in southern Ecuador, lacks recent arc volcanism, which, as in Chile, has been attributed to flat-slab subduction spatially coinciding with subduction of aseismic ridges and oceanic plateaus (e.g., Gutscher et al., 1999a; Martinod et al., 2010; Skinner and Clayton, 2013). The high-sulfidation epithermal deposits of this region occur in two distinct belts (Noble and McKee, 1999). The southeastern belt, located east of the Cordillera Occidental, which forms the Continental divide, and extends from Julcani (~13° Lat S) to the latitude of Antamina (~9.5° Lat S). It coincides with the central Peruvian polymetallic province and contains a variety of porphyry-related and carbonate hosted deposits emplaced at different paleodepths (Bissig and Tosdal, 2009; Escalante, 2008; Escalante et al., 2010; Love et al., 2004). This segment includes the cordilleran base metal lode deposits (i.e. sulfide rich polymetallic deposits: Bendezú et al., 2003) of Marcapunta–Colquijirca and Cerro de Pasco which are centered on high-sulfidation epithermal deposits emplaced in reactive host rocks (Bendezú and Fontboté, 2009; Bendezú et al., 2008). The northwestern metallogenetic belt has its southern terminus at Pierina at 9.5° Lat S, west of the Cordillera Blanca and is dominated by high-sulfidation epithermal and porphyry Cu–Au deposits but also includes a number of smaller vein hosted Ag–Au deposits such as Quiruvilca (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 epithermal deposit cluster of the Andes. Subduction of the Nazca aseismic ridge in Peru commenced in the middle Miocene at 14–10 Ma (Hampel, 2002) and is the inferred cause of flat subduction (e.g., Martinod et al., 2010). However, estimates on the timing of crustal thickening and uplift of the Cordillera Occidental in central and northern Peru vary from the early to the middle Miocene and do not coincide everywhere with the onset of aseismic ridge subduction and flat subduction. Thus, Noble et al. (1990) on the basis of the age of tuff deposits in deeply incised paleovalleys in northern Peru, suggested that uplift and erosion occurred in the early–late Miocene. Crustal thickening inferred from whole rock geochemical data of volcanic rocks in the central Peruvian polymetallic province is inferred to have occurred around 12–15 Ma (Bissig and Tosdal, 2009). In contrast, Montgomery (2012) proposed that a major episode of uplift commenced at about 17 Ma, i.e., before the inferred onset of ridge subduction, but coincident with high-sulfidation epithermal mineralization at Lagunas Norte (Cerpa et al., 2013; Montgomery, 2012). 9. High-sulfidation epithermal deposits of the central Peruvian polymetallic belt The central Peruvian polymetallic belt contains a number of late Miocene silver and base metal-rich vein deposits that were emplaced in the epithermal environment. These include: San Cristóbal (Beuchat et al., 2004), Morococha (Catchpole et al., 2011) and Uchucchacua (Bussell et al., 1990; Escalante, 2008). At Morococha an evolution from the porphyry to the epithermal environment occurred concurrently with erosion, and late Miocene epithermal Ag mineralization probably occurred only a few 100 m below the paleosurface (Catchpole et al., 2011). All of these epithermal deposits have sulfide assemblages of intermediate sulfidation state and veins are largely hosted in Mesozoic carbonaceous rocks as well as Triassic volcanosedimentary rocks.

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However, the polymetallic belt also contains a number of epithermal deposits with important high-sulfidation characteristics; these are described in the following. 9.1. Julcani The Julcani district, mined since colonial times, consists of severalvein hosted deposits from which Ag and base metals, as well as subordinate Au, have been produced. The mineralization is hosted in coalescing domes of andesitic-to-dacitic composition and postdates a large zone of vuggy quartz and quartz alunite alteration in the center of the district, as well as quartz–tourmaline–pyrite breccia bodies (Deen et al., 1994; Petersen et al., 1977). Veins are zoned along strike from higher sulfidation state enargite–tennantite bearing assemblages near the center of the district to lower-sulfidation assemblages containing galena and tennantite–tetrahedrite more distally (Petersen et al., 1977). Magmatism largely predated the epithermal mineralization but a 9.7 Ma syn-mineral dyke and small, 7 Ma, post-mineral rhyolite domes have been recognized (Deen et al., 1994; Petersen et al., 1977). Julcani is hosted below a sub-planar, slightly east-inclined, land surface situated at elevations of 4000–4500 m a.s.l. 9.2. The Marcapunta–Colquijirca district Marcapunta is a high-sulfidation epithermal Au (–Ag, Cu) deposit. The district also contains cordilleran base metal mineralization at the Smelter deposit, northward contiguous to Marcapunta, and more distally the Colquijirca Ag–Pb–Zn deposit, some 5 km north of Marcapunta. The San Gregorio deposit is another cordilleran base metal deposit southerly adjacent to Marcapunta (Bendezú et al., 2008). Vuggy quartz alteration, phreatomagmatic breccias and disseminated gold mineralization occur in the central Marcapunta dome, whereas stratabound base metal and silver rich mineralization at Smelter and Colquijirca, extending as far as 5 km north from Marcapunta, are hosted in Eocene limestones and marls (Bendezú et al., 2008). There is a district-scale lateral zoning from proximal high-sulfidation mineralization containing enargite and pyrite to intermediate pyrite, chalcopyrite and tennantitebearing ore, to a distal pyrite, sphalerite and galena dominated sulfide assemblage, the later reflecting an intermediate sulfidation state (Bendezú et al., 2008; Vidal and Ligarda, 2004). Alunite associated with the central high-sulfidation gold mineralization yielded ages of 11.9 to 11.1 Ma, whereas that associated with the base metal mineralization yielded slightly younger ages of 10.8 to 10.5 Ma (Bendezú et al., 2008). The dacitic dome complex hosting some of the epithermal mineralization was emplaced at 12.9 to 12.1 Ma (40Ar/39Ar biotite ages). No syn- or post mineral volcanic or intrusive rocks are exposed in the district. Colquijirca is located near the eastern margin of a regionally extensive 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 the highest feature in the deposit area. Vuggy quartz hosted gold mineralization extends from 4450 m to about 4000 m a.s.l. or 450 m below surface, whereas distal base metal mineralization occurs at depths less than ~400 m below surface. Proximal enargite-rich mineralization adjacent to the dome complex, however, has been drilled to a depth of N600 m below 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 shares many similarities to the geologic setting and mineralization style of Colquijirca. The deposit is spatially related to a 15.4 to 15.2 Ma dacitic diatreme dome complex which was emplaced at the boundary of Devonian 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 the eastern margin of the diatreme dome complex and is characterized by a low-sulfidation sulfide assemblage consisting of a massive quartz–pyrite ± pyrrhotite body with associated distal carbonate replacement ore containing galena and Fe-rich sphalerite. Alteration related to stage 1 is quartz–sericite. The second mineralizing stage reflects high-sulfidation states and has proximal E–W trending enargite–pyrite veins, hosted in part by the diatreme breccia body, and distal carbonatehosted Pb–Zn–Ag veins and replacement bodies containing Fe-poor sphalerite (Baumgartner et al., 2008). Alunite is part of the alteration assemblage of stage 2 and gives 40Ar/39Ar ages of 14.5 to 14.2 Ma, whereas stage 1 is bracketed between 15.4 and 14.5 Ma (Baumgartner et al., 2009). Like the nearby Marcapunta and Colquijirca deposits, Cerro de Pasco is associated with a dome complex which forms a local topographic high above the regionally-extensive, low-relief surface to the west of it. The pre-mining land surface was situated at about 4350 m a.s.l. and mineralization extends about 450 m below this original land surface (Baumgartner et al., 2008). 9.4. Quicay Quicay is a high-sulfidation epithermal center located 14 km W of Cerro de Pasco. It is being mined by the Peruvian private company Chancadora Centauro SAC and little information on its geology and resources is publically available. According to information obtained through the INGEMMET website Peru (Rossell et al., 2006) gold mineralization is hosted in a diatreme–dome complex and the highest gold grades are found in the central vuggy quartz zones (average 3 ppm). The ore is oxidized. A mineralization age of 37.5 Ma based on a K/Ar alunite age is suggested by Noble and McKee, (1999) which is significantly older than Cerro de Pasco, despite the similar inferred shallow level of formation. Quicay mineralization is centered on a small hill that contains outcrops of vuggy residual quartz. It has a summit elevation of 4350 m, which is roughly 100 m above the same low-relief land surface described for Cerro de Pasco. If the age constraint is reliable, it can be inferred that no significant erosion affected the area since the Eocene. 10. The high-sulfidation epithermal Au (–Cu, Ag) deposits of northwestern Peru 10.1. Pierina Pierina is located in the Cordillera Negra, Ancash, west of the Cordillera Blanca. The coeval, dominantly intermediate sulfidation, Santo Toribio Ag-base metal vein systems occur ~ 5 km south of Pierina (Rainbow, 2009), and both deposits are located beneath the shoulder of an erosional surface overlooking the Callejón de Huaylas valley (Figs. 10, 11). At Pierina, disseminated mineralization forms a subhorizontal body, and is largely hosted in a lithologically-controlled vuggy quartz alteration zone, focused in a ca. 16.9 ± 0.6 Ma (40Ar/39Ar, weakly chlorite-altered biotite total gas age) pumice tuff and an underlying dacitic flow dome complex (Rainbow, 2009); both members of the Oligocene to mid-Miocene Huaraz Group (Rainbow et al., 2005; Strusievicz et al., 2000), the upper succession of the Calipuy Supergroup subaerial volcanic arc. Hydrothermal breccias and small dacitic domes cut these rocks but appear to pre-date mineralization (Rainbow et al., 2005). Gold–silver mineralization was introduced after initial acidic alteration, and is associated with enargite, pyrite, bismuthinite–stibnite, galena and low-Fe sphalerite (Rainbow et al., 2005). However, sulfides have largely been oxidized. Sub-microscopic Au and Ag are now hosted in a goethite–hematite dominated oxide assemblage (Rainbow et al., 2005), the formation of which was facilitated by microbial activity during supergene oxidation. During this process, the local reduction of supergene fluids led to the formation of Aubearing acanthite (Rainbow et al., 2006). Supergene minerals at Pierina do not include jarosite.

Hydrothermal alunite 40Ar/39Ar ages range from 15.08 ± 0.09 to 13.89 ± 0.13 Ma (n = 19), clustering in two pulses around 15 Ma and 14.4 Ma (Rainbow, 2009), whereas an age of rare, vug filling porcellaneous alunite from the oxide zone, yielded a large-error 14.12 ± 1.59 Ma plateau age and may be of supergene origin (Rainbow, 2009). This suggests that oxidation closely followed hypogene mineralization. The host rocks of the Huaraz group range in age from 29.3 to 14.8 Ma (Rainbow et al., 2005; Strusievicz et al., 2000). The mineralized part of Pierina is located between about 4000 and 3800 m a.s.l., starting about 100 below the nearest topographic highs. A horizontally extensive steam-heated alteration blanket has not been documented at Pierina, and the presence of any steam-heated alunite remains controversial (Fifarek and Rye, 2005; Rainbow et al., 2005, 2006). However, paragenetic relationships and stable-isotope geochemistry demonstrate that meteoric waters played an increasingly important role in deposit formation, from early alteration to subsequent oxidation, and that these fluids became progressively less isotopically exchanged over time (Rainbow et al., 2006). This suggests that the water table was progressively lowered during the lifetime of the hydrothermal system (Fifarek and Rye, 2005; Rainbow et al., 2005, 2006). In the Pierina–Santo Toribio area, components of middle Miocene planar erosional landforms both preceding and broadly contemporaneous with mineralization at Pierina can be recognized (Fig. 11). The deposit formed at the crest of the Cordillera Occidental and Calipuy arc facing the Amazonian low-lands to the east, the latter not yet separated hydrographically from the Cordillera Negra as the intervening Cordillera Blanca was only uplifted in the late Miocene (González and Pfiffner, 2012; Petford and Atherton, 1992). An erosion surface, marked by an angular unconformity, now tilted and dipping at ~ 20° to the ENE, underlies the andesites below the Pierina deposit. The andesites have been dated at ca. 21 Ma, suggesting that this angular unconformity records uplift and erosion in the late Oligocene to early Miocene, probably representing the Aymará orogenic event (Sébrier et al., 1988). The upper slope of the Cordillera Negra, immediately south of the Santo Toribio deposit is faceted by four erosional surfaces, one of which is constrained by the overlying 14.10 ± 1.33–14.99 ± 0.50 Ma (40Ar/39Ar hornblende plateau ages) lava flows of the unaltered Santo Toribio Formation andesite package. This constrained surface also intersects the extensive area of phyllic alteration surrounding the Santo Toribio vein system. This shows that hydrothermal activity (at both Santo Toribio and Pierina, dated at ca. 15 to 14.4 Ma) was penecontemporaneous with both andesite eruption and the incision of the pediment. Volcanism terminated after 14 Ma in the area. The mineralization at Pierina (and Lagunas Norte—see below), the cessation of volcanism and onset of uplift around 14 Ma pre-dated the arrival of the Nazca ridge at the subduction zone by at least 2 Ma (Hampel, 2002). Uplift in response to crustal thickening in central and northern Peru has, instead, been linked to increased mid Miocene plate convergence and may not be directly related to initiation of flat subuduction (Montgomery, 2012; Pardo‐Casas and Molnar, 1987). 10.2. Lagunas Norte Lagunas Norte, La Libertad, is located about 200 km NNW of Pierina and 100 km SSE of Yanacocha in the northeastern mineral belt of northern Peru. Mineralization is largely hosted in quartzites with scarce interspersed coal beds of the Lower Cretaceous Chimú Formation and, to a lesser degree, in overlying dacitic pyroclastic and volcaniclastic rocks of the Lagunas Norte Formation (Fig. 12) Montgomery, 2012; Cerpa et al., 2013). Lagunas Norte Formation volcanic units are volumetrically minor and are restricted in distribution to the immediate Lagunas Norte deposit area (Fig. 12) Mineralization is centered on at least 2 diatremes which are also considered the source of the pyroclasitc rocks overlying the quartzites (Cerpa et al., 2013; Montgomery, 2012). Hydrothermal alunite 40Ar/39Ar data constrain the age of mineralization at Lagunas Norte to between 17.4 and 16.5 Ma (Cerpa et al., 2013; Montgomery,

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Pierina Area, Peru 77°40'0"W

77°35'0"W

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Pierina

Co

rd

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ra

Bl

8940000

ille

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an

ca

no

rm

al

fa

ul

t

Huaraz

8930000 8920000

9°40'0"S

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5

7.5

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-450000

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-440000

-430000

-420000

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High : 6280 0 - 10

Low : 1565 Fig. 10. Topographic map flat landscape elements and main physiographic features of the Pierina area. The approximate trace of the Cordillera Blanca normal fault is shown. Note that this fault was not active prior to the late Miocene and areas to the east of it have been uplifting relative to the Cordillera Negra since the Late Miocene to the present (González and Pfiffner, 2012; Petford and Atherton, 1992). UTM Zone 18S, WGS84.

2012), whereas a minimum age of 16.5 Ma and maximum age of 17.2 Ma for volcanic units of the Lagunas Norte Formation have been determined (Montgomery, 2012). Extensive lower Miocene (21.1 Ma to 16.4 Ma; Montgomery, 2012) andesitic-to-dacitic volcanic rocks of the Sauco Volcanic Complex (age- and compositional-equivalent to Huaraz Group strata of the Pierina district; Strusievicz et al., 2000; Montgomery,

2012) were deposited east of Lagunas Norte. These rocks largely predate hydrothermal activity at Lagunas Norte (Fig. 12) but mapping together with geomorphology and geochronology suggests that the Sauco volcanic complex is the result of sector collapse contemporaneous with mineralization at Lagunas Norte. At Lagunas Norte a reduction of erupted magma volumes and increased SiO2 content over time is

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Quebrada Pacchac Quebrada Huellap (with heap leach facility) Santo Toribio Callejón de Huaylas 15.59 ± 0.23

14.10 ± 1.33

15.21 ± 0.23 14.81 ± 1.27

14.60 ± 0.08

Looking S from Pierina Surface IV (~3970 m) Surface III (~4040 m) Surface II (~4260 m) Surface I (~4500 m)

andesite hydrothermal alteration

hornblende Ar-Ar plateau age sericite Ar-Ar plateau age sericite Ar-Ar total fusion age alunite Ar-Ar plateau age

Fig. 11. Field photograph of the landscape and physical setting of the Pierina area. A) Panorama photographs taken at Pierina looking south across Santo Toribio. B) Line drawing of principal landscape elements and locations. See text for further detail.

documented leading up to the main period of high-sulfidation epithermal activity, with restricted, dacitic eruptions overlapping temporally and spatially with mineralization (Montgomery, 2012). The volcanic rocks at Lagunas Norte exhibit alteration zonation typical of highsulfidation epithermal deposits with a vuggy quartz core surrounded by quartz–alunite and kaolinite–dickite alteration zones. The quartzites, in contrast, are non-reactive, and display only subtle alteration that includes kaolinite, minor pyrophyllite and traces of alunite (Cerpa et al., 2013; Montgomery, 2012). Mineralization in the quartzites is fracturecontrolled, and hypogene ore minerals include digenite, enargite and pyrite, the latter hosting some of the gold. Locally, within coal beds, low-sulfidation sulfide assemblages are present (Cerpa et al., 2013). The ore is oxidized to a depth of over 80 m. The orebody forms a tabular body between ~4250 to 4000 m a.s.l., although sulfide mineralization extends to greater depths. It occurs below relics of the extensive ca. 26–25 Ma subplanar erosional Pampa La Julia surface (Montgomery, 2012) into which the ca. 18–16 Ma Río Chicama surface, consisting of steep-walled, but flat-bottomed valleypediment was cut. The Pampa La Julia surface may be correlative with the upper Oligocene to lower Miocene angular unconformity recognized 200 km to the SSE at Pierina in the Huaraz District (see above), attributed to the regional Aymará orogenic event (Sébrier et al., 1988). The eastern Sauco Volcanic Complex in the Lagunas Norte district erupted onto the older Pampa La Julia surface, whereas the Lagunas Norte deposit is laterally contiguous with pronounced scarp between this older surface and the younger Rio Chicama valley pediment (Figs. 13, 14). Active erosion and scarp retreat during hydrothermal activity are constrained in age by truncated a 17.05 Ma alunite veins of at the western margin of the Lagunas Norte deposit and 16.75 Ma volcanic rocks emplaced on the Rio Chicama surface immediately west of the deposit (Figs. 13, 14; Montgomery, 2012). Incision of the Rio Chicama

surface at Lagunas Norte occurred during, and was likely a response to, the initial stages of an episode of major, mid- to late Miocene (ca. 17–5 Ma) regional uplift (e.g., Montario et al., 2005a, 2005b) and crustal thickening, during which, as much as 2–3 km of surface uplift may have occurred (Montgomery, 2012). Hydrothermal activity and ore deposition at Lagunas Norte occurred during the latter stages of development of the Sauco Volcanic Complex, centered 4 km east of the deposit, during waning volcanism, and immediately post-dated collapse of the volcanic edifice. Regional contractile tectonism and uplift may have ultimately triggered the collapse of this volcanic edifice, and may also have led to marked decrease in magmatic activity which set the stage for development of the Lagunas Norte ore body (Montgomery, 2012). Inferred emplacement of a shallow intrusion subjacent to Lagunas Norte within this evolving tectonomagmatic setting, and its subsequent cooling, downward contraction and associated fluid release, is envisaged to have resulted in early acid leaching and subsequent ore deposition at Lagunas Norte. Incision of the Rio Chicama pediment at the margin of the Lagunas Norte deposit at this time, would have favored decompression and fluid release, and subsequent fluid mixing and/or boiling at the site of ore deposition. 10.3. Yanacocha Gold mineralization occurs in about 10 individual centers within broad NE-trending zones of residual quartz, massive quartz, quartz– alunite–pyrophyllite and distal kaolinite alteration (Longo et al., 2010; Teal and Benavides, 2010). Early pervasive alteration also includes quartz–pyrophyllite featuring the wormy or gusano texture characteristic of the lower portion of the lithocap environment (Sillitoe, 2010; Teal and Benavides, 2010). Gold was introduced in several pulses and is largely associated with permeable pyroclastic rocks, subvertical

5 km

810000 E

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805000 E

795000 E

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9130000 N

N

Unmapped

Lagunas Norte

CSVC

9120000 N Shulcahuanga dome

9115000 N

Quiruvilca mine

8° S

Quaternary Undifferenated glacial deposits Middle Miocene Las Princesas volcanic domes. 16.3-15.2 Ma

Quiruvilca-Tres Cruces Volcanic Complex (> 25 -18.4 Ma) Andesite domes and lavas Cretaceous Undifferenated sedimentary rocks

Lagunas Norte Formaon, dacite to andesite, domes lavas and tuffs. 17.3-16.4 Ma Sauco Volcanic Complex (21.1-17.1 Ma) Dacite to andesite lavas and domes (CSVC - Central Sauco Vent complex) Andesite volcaniclasc rocks

Structures Reverse fault (observed/inferred) Normal fault (observed/inferred) Fault (observed/inferred) Syncline Ancline

Early Andesite Dome/Lava Fig. 12. Simplified geological map of the Lagunas Norte area. Geology and age constraints from Montgomery (2012). Coordinates in UTM Zone 18S, Provisional South American Datum 56.

fractures or breccia zones as well as margins of domes (Teal and Benavides, 2010). The highest gold grades are associated with replacive cream-colored chalcedony veins containing barite and fine grained rutile and Fe oxides. These are interpreted as representing precipitation under intermediate-sulfidation conditions (Teal and Benavides, 2010). Much of the ore mined to date came from the oxidized portion of the deposit where gold is associated with supergene Fe oxides. Sulfide assemblages associated with precursor hypogene epithermal mineralization include enargite and covellite and, at depth, chalcopyrite (Teal and Benavides, 2010). Late-stage veins of base metal sulfides associated with rhodochrosite have locally been reported from the Cerro Yanacocha deposit and reflect higher pH and intermediate-sulfidation state fluids (Teal and Benavides, 2010). Besides high-sulfidation and intermediate-sulfidation epithermal mineralization, porphyry style Cu–Au ore has been described from Kupfertal, an area exposed at ~3800 m a.s.l., 300 m below the summit of Cerro Yanacocha (Gustafson et al., 2004; Teal and Benavides, 2010).

The rocks hosting the epithermal mineralization largely belong to the 14.5–11.2 Yanacocha Volcanics (Longo et al., 2010) which consist of andesitic-to-dacitic lavas and pyoclastics and associated subvolcanic rocks. As at Lagunas Norte, the volcanic pile records a reduction of erupted magma volumes and increased SiO2 content over time, with over 90% of the volcanic rocks erupted prior to 11 Ma (Longo et al., 2010). Hydrothermal alunite ages (13.6 to 8.2 Ma) temporally overlap with the Yanacocha Volcanics, but less than 20% of the gold is associated with hydrothermal activity older than 11 Ma. The bulk of the gold was introduced after 10.9 Ma and temporally overlaps with the volumetrically-minor Coriwachay dacite domes dated at 10.9 to 8.4 Ma (Longo et al., 2010). Thus, high-sulfidation epithermal gold mineralization clearly post-dated much of the volcanism and occurred in multiple pulses over a period of 2–3 Ma. Additionally, hydrothermal biotite associated with porphyry Cu–Au mineralization at Kupfertal yielded an 40Ar–39Ar age of 10.7 Ma. Although both porphyry and high-sulifdation epithermal style mineralization are present and

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Lagunas Norte Area, Peru 78°15’0”W

7°55’0”S

RCh surface 18-16 Ma

Sauco volcanic complex 9120000

PLJ surface 26-25 Ma

Lagunas Norte

Q Constructional surface

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PLJ surface 26-25 Ma

0

1

2

3

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5 km 800000

Elevation (m)

810000

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High : 4254 0-8

Low : 2275 Fig. 13. Topographic map, flat landscape elements and main physiographic features of the Lagunas Norte area. PLJ: Pampa La Julia 26–25 Ma paleosurface, RCh: Río Chicama 18–16 Ma paleosurface, Q: Quesquenda constructional surface composed of lower Miocene volcanic rocks, including those of the Sauco volcanic complex which is indicated. Surfaces and location of Sauco Volcanic complex from Montgomery (2012). UTM Zone 18S, WGS84.

spatially overlapping, they are probably formed in distinct magmatichydrothermal settings (cf. La Pepa, Muntean and Einaudi, 2001). The Yanacocha deposits underlie a 3900 to 4000 m a.s.l. subplanar land surface (Fig. 15) evident in digital elevation models, with local topographic highs, such as Cerro Yanacocha, reaching 4150 m. The high planation surface has a similar elevation to the Pampa la Julia surface

in the Lagunas Norte district (Montgomery, 2012) and could conceivably be age-equivalent. The area hosting mineralization is incised by flat bottomed valleys of ~ 3500 m elevation to the west and south which themselves have been incised up to 250 m by deep canyons. No detailed documentation and direct age constraints on erosional surfaces are available. However, steam-heated alteration is locally preserved

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A

351

view looking southwest

Lower Miocene Sauco volcanic complex

Lagunas Norte (early stages of open pit)

~ 16-18 Ma Río Chicama valley-pediment network ~ 25-26 Ma Pampa La Julia pediplain

B

Alexa Zone 17.05 Ma alunite Shulcahuanga dome 16.95 Ma Lagunas Norte orebody

Lagunas Norte Fm.

Looking south

Tres Amigos dome 16.75 Ma

4200 m a.s.l.

Pampa La Julia surface Chimú Fm. 4000 m a.s.l.

Tres Amigos lava flows

Chimú Fm.

Río Chicama valley-pediment

Fig. 14. Field photographs of the landscape and physical setting of the Lagunas Norte area.

above mineralization (Teal and Benavides, 2010), suggesting that only limited erosion has occurred since the late Miocene. Although glaciation affected parts of the district above 3900 m a.s.l., glacial erosion was only locally significant and led to the formation of the detrital La Quinua gold deposit hosted in glacial till (Mallette et al., 2004). 10.4. Tantahuatay, Sipan and La Zanja Besides Yanacocha, northern Peru hosts a number of porphyry Cu–Au and high-sulfidation epithermal deposits of late–middle to late Miocene age (Gustafson et al., 2004; Noble and McKee, 1999; Noble et al., 2004). Most of these occur within 50 km of Yanacocha (Fig. 15). The high-sulfidation epithermal deposits include Tantahuatay, Sipan and La Zanja, whereas Cu ± Au ± Mo porphyry style mineralization includes El Galeno and Cerro Corona (Gustafson et al., 2004). At Tantahuatay, mineralization is hosted in an intensely vuggy to, locally, massive quartz and quartz–pyrophyllite–alunite alteration zone, hosted in andesitic domes and underlying pyroclastic rocks (Gustafson et al., 2004). The andesites constitute the local topographic highs at 4050 m a.s.l. Gold is associated with pyrite and enargite and is concentrated in areas of secondary permeability (vuggy quartz or fracture

zones). The deposit has been affected to a more significant degree by glaciation than Yanacocha, and it is thought that a significant part of the mineralization has been eroded (Gustafson et al., 2004). Little detailed geological information has been published for La Zanja, operated by Compañia Minera Buenaventura and for Sipán. However, Tantahuatay as well as Sipán and La Zanja lie beneath a high-elevation planar landform similar to- and apparently contiguous with the one at Yanacocha (Fig. 15). This surface is slightly inclined to the west and Sipan and La Zanja are exposed at about 400 m lower elevation than Tantahuatay and Yanacocha, the latter being located about 30–40 km to the E of Sipan and La Zanja.

11. The northern Andes A major break in the basement architecture in southern Ecuador separates the Northern Andes (3.5° Lat S. to 11° Lat N.) from the Central Andes (Cediel et al., 2003; Gansser, 1973) and is reflected in the differing metallogeny of the two regions. The western Cordillera of the northern Andes of Ecuador and Colombia is comprised of terranes accreted during the Cretaceous and Miocene, whereas the basement of the

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Yanacocha/Sipan/Tantahuatay/La Zanja Area, Peru 78°50'0"W

78°45'0"W

78°40'0"W

78°35'0"W

9260000

78°55'0"W

6°50'0"S

Cerro Corona P. Cu-Au

9250000

6°45'0"S

Tantahuatay

9240000

La Zanja

9230000

6°55'0"S

Sipan

0 1 2

4

6

730000

8

9220000

7°0'0"S

Yanacocha District

10 km

740000

Elevation (m)

750000

760000

770000

Slope (Degrees)

High : 4203 0-8

Porphyry Cu-Au deposit

Low : 1041 Fig. 15. Topographic map, flat landscape elements and geomorphologic setting of Yanacocha and other epithermal deposits of northern Peru. UTM Zone 18S, WGS84.

Central Andes was assembled during the Paleozoic and earlier (Cediel et al., 2003; Ramos, 2009). In the northern Andes, in contrast to the Central Andes, high-sulfidation epithermal deposits are scarce, although numerous other gold-rich deposits are known. The latter occur along the eastern margin of the western Cordillera in northern Ecuador and particularly throughout Colombia (Leal Mejía, 2011; Leal Mejía et al., 2011; Schuette, 2010) and include the late Miocene gold-only La Colosa porphyry deposit (Lodder et al., 2010) as well as the Marmato (Tassinari

et al., 2008) and Buriticá (Lesage, 2011) low-sulfidation epithermal deposits. Large high-sulfidation epithermal deposits include Quimsacocha in Ecuador and the California–Vetas district in the eastern Cordillera of Colombia. The northern Andes are now dominated by humid tropical climate and thus are wetter than the western cordillera of the Central Andes. Areas at high elevations record 1–2 m annual rainfall, but some of the adjacent low-lying areas record even higher precipitation rates of up to 10 m/yr (Alvarez Villa et al., 2011).

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The uplift history and landscape evolution of the Northern Andes is less well established than in the Central Andes and the geomorphology is influenced by the terrane architecture. However, extensive planation surfaces have been documented in the Ecuadorian Andes (Coltorti and Ollier, 2000) and for the central Cordillera in Colombia (RestrepoMoreno et al., 2009). In Ecuador, relict planation surfaces are commonly exposed at 3500–3800 m a.s.l. The nature of these and the timing of uplift are controversial. Coltorti and Ollier (2000) interpret them have formed as planation surfaces near sea-level as recently as early Pliocene and having been uplifted rapidly since the Pliocene. Conversely, Steinmann et al. (1999) documented the onset of major uplift in the early–late Miocene on the basis of stratigraphic relationships in intramontane basins. A major tectonic change and increased uplift and contractile deformation in the Andes has been associated with the arrival of the Carnegie Ridge at the subduction trench but age estimates for this event vary widely between 15 Ma (Spikings et al., 2001) and 2 Ma (e.g., Gutscher et al., 1999b). The Colombian Andes, comprise three separate ranges, the Cordilleras Occidental, Central and Oriental, each of which experienced differing geomorphologic histories. Planar surfaces have been described from the Central Cordillera and the Eastern Cordillera. A high plateau, the Antioqueño Plateau, is located at the northern limit of the central Cordillera (6° ± 1° Lat. N). It consists of an extensive upper Oligocene to lower Miocene planar erosion surface which was cut from the NE into an older, uplifted early Eocene landscape (Restrepo-Moreno et al., 2009; Villagomez and Spikings, 2013). The Antioqueño plateau is separated from higher landscape elements by a relict back-scarp and may have formed originally as a pediplain. A renewed late Miocene to early Pliocene uplift pulse and associated incision of deep canyons is identified on the basis of apatite U–Th/He data (Villagómez and Spikings, 2013). The magnitude of late Miocene uplift and exhumation increases south of the Antioqueño Plateau and contiguous central Cordillera (Villagómez and Spikings, 2013) but there the planar nature of the uplifted surface is obscured by recent voluminous volcanic deposits and presumably by more intense landscape dissection in response to uplift. In contrast to the central Cordillera, the Eastern Cordillera was established in the late Oligocene when the tectonic regime changed from extensional to contractile and the frontal thrust separating it from the Llanos Foreland was established (Horton et al., 2010). However, based on paleobotanical evidence, much of the uplift to the present day elevations of around 2600–3600 m a.s.l. probably took place in the late Miocene-toPliocene (Gregory-Wodzicky, 2000). The northern extension of the eastern Cordillera which hosts the California Vetas Mining District, may have experienced uplift pulses in the Paleocene and early Miocene but, most importantly, in the late Miocene-to-Pleistocene (Villagómez et al., 2011). 11.1. Quimsacocha Quimsacocha is located in the northern Andean block (sensu Gansser, 1973), about 20 km NW of the NE-trending Girón fault which constitutes an important terrane boundary separating the Upper Cretaceous Piñón terrane to the northwest, from the Lower Creteaceous Anaime terrane to the southeast (Ramos, 2009; Schuette et al., 2012). The Quimsacocha deposit adheres closely to the model for high-sulfidation epithermal deposits (MacDonald et al., 2011). Mineralization is hosted within a large vuggy quartz, massive quartz and quartz–alunite ± pyrophyllite alteration zone which is centered on the N-striking Río Falso fault zone. The ore is confined in pyroclastic strata between subhorizontal andesite lava flows of the 9 Ma Quimsacocha Formation (all age dates are 40 Ar/39Ar plateau ages: MacDonald et al., 2011, 2012). The host rocks underwent intense acid leaching resulting in vuggy quartz alteration, prior to the deposition of pyrite, enargite and associated gold and minor quartz (MacDonald et al., 2011). Alunite associated with the vuggy quartz and advanced argillic alteration is dated at 7.6 to 7.3 Ma and biotite from a post-mineral dacite dome yielded an age of 6.7 Ma

353

(MacDonald et al., 2012). Steam-heated alteration zones of limited thickness have locally been preserved in the highest parts of the deposit. The ore-controlling Rio Falso fault is located along the eastern margin of a caldera in which the post mineral dacitic domes lie but is a districtscale feature and does not constitute the caldera margin (Fig. 16). Mineralization at Quimsacocha is located between 3500 and 3700 m a.s.l. The paleosurface delimited by the steam-heated alteration zone is located at 3800–3900 m a.s.l. The deposit is located near the eastern margin of a large plateau at 3800 ± 100 m elevation with steep margins relative to the adjacent valleys (Fig. 16; Coltorti and Ollier, 2000). Based on sedimentological evidence and zircon fissiontrack age data for tuff layers (Hungerbuehler et al., 2002; Steinmann et al., 1999), the area was at sea-level as recently as 15–11 Ma, with uplift to almost 4000 m a.s.l. occurring in the Late Miocene, broadly coeval with mineralization. Quimsacocha also overlies the subducting Carnegie ridge and no recent volcanism has been documented from the area (Chiaradia et al., 2004; Schuette et al., 2010). 11.2. California Vetas The California Vetas Mining District is located in the northeastern Cordillera of Colombia, some 30 km N of the city of Bucaramanga. The modern climate in the area is tropical and has two pronounced rainy seasons per year, with annual rainfall of around 2000 mm. The location corresponds to the southern tip of the triangular Maracaibo block which is bounded by the NNW-striking sinistral Santa Marta–Bucaramanga fault and the NE-striking dextral Boconó fault. Mineralization is hosted by Grenvillia-aged Bucaramanga gneisses as well as upper Triassic to lower Jurassic peraluminous granites (Mantilla Figueroa et al., 2013). Locally, at the El Cuatro prospect (Fig. 17), small volumes of metaluminous, coarsely-porphyritic granodiorite dykes of late Miocene age (10.9–8.4 Ma: Mantilla F. et al., 2009, 2011; Mantilla Figueroa et al., 2013; Bissig et al., 2014) are associated with porphyry Mo mineralization (Bissig et al., 2012). Gold mineralization is mainly vein-hosted and is largely of highsulfidation epithermal type, overprinting earlier porphyry-style Mo mineralization (Rodriguez, 2014). Numerous artisanal mines have been operating since colonial times, but most current resources are contained in the La Bodega/La Mascota and contiguous Angostura deposits. Mineralization was largely controlled by the NE-trending La Baja fault trend, Angostura focused in a dextral strike-slip sigmoidal loop (Fig. 17; Rodriguez, 2014). Several hydrothermal stages can be distinguished. Early quartz–pyrite ± chalcopyrite veins associated with pervasive sericitic alteration represent precipitation in the porphyry environment and are associated with low-grade Au mineralization (b1 g/t). These are overprinted by several stages of veins and fault-controlled breccias which introduced the bulk of the gold mineralization (Rodriguez, 2014). Ore minerals of the epithermal stages include covellite, bornite, chalcopyrite, hubnerite, enargite and pyrite, all of which are associated with native gold and gold–silver tellurides, as well as late Fe-poor sphalerite. Epithermal veins and breccias have quartz and alunite gangue that form banded, colloform and cockade textures (Rodriguez, 2014), such as are more typically associated with low-sulfidation type deposits (Simmons et al., 2005). Conversely, vuggy residual quartz and aeriallyextensive, pervasive, quartz–alunite–kaolinite alteration typical of high-sulfidation deposits is largely absent in the California–Vetas district. Both fluid inclusion and textural evidence indicates that boiling was an important ore depositional mechanism, occurring at 200–250° C (Rodriguez, 2014). Early muscovite alteration associated with quartz– pyrite veins at Angostura and La Bodega gives 40Ar/39Ar ages of ca. 4 to 3.5 Ma and one alunite sample from Los Laches, at ~ 3500 m a.s.l., some 800 m above the muscovite sample locations, was dated at 4.02 ± 0.06 Ma and likely was deposited close to the paleosurface (Fig. 17, Rodriguez, 2014). The age of mineralization inferred from alunite in mineralized veins and breccias ranges from 2.6 to 2 Ma at La Bodega and Angostura, but alunite the La Plata, San Celestino and El

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Quimsacocha Area, Ecuador 79°10’0”W

Quimsacocha caldera

Río Falso fault zone

3°0’0”S

9670000

79°15’0”W

3°5’0”S

9660000

Quimsacocha Deposit

0 690000

Elevation (m)

1

2

3

4

5 km

700000

Slope (Degrees)

High : 4079 0-8

Low : 2925 Fig. 16. Topographic map of the Quimsaochca area. The approximate trace and orientation of the Río Falso fault is indicated.

Cuatro prospects downstream from La Bodega (Fig. 17) yielded ages of 3.5 to 3.2 Ma. Post-mineralization alunite associated with sphalerite yielded ages from 1.9 to 1.6 with Ma (Rodriguez, 2014). Paragenetically late U mineralization has been reported from San Celestino (Polania, 1980) 2 km SW of La Bodega, along the same mineralized trend. No igneous rocks similar in age to the epithermal mineralization have been documented, but stable-isotope data indicate a magmatic source for the mineralizing fluid (Rodriguez, 2014).

The geomorphology in the La Bodega and Angostura area is characterized by a deeply-incised, steep-walled valley paralleling the NEtrending dextral La Baja fault zone (Figs. 18, 19). Angostura is located at the upper termination of this valley, La Bodega/La Mascota at 2800 to 2350 m, and Angostura between 3500 and 2700 m a.s.l. More subdued topography characterizes the topographically-high areas from ~ 3400 to 4000 m a.s.l. (Figs. 18, 19) representing the highest parts of the northeastern cordillera of Colombia. This suggests that,

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

7°22'0"N

m

4.02 +/- 0.06 Ma 2.77 +/- 0.15 Ma

Fault Rio Cucutil la

fau ill a

Veta de Barro ura ost Los Laches Ang La Picota o

3.91 +/- 0.15 Ma

Perezosa PaLa ez f ault

La Mascota

2.1- 1.8 Ma (n=2)

2.5 - 1.6 Ma (n=9)

El Cuatro

Pie de Gallo

ult fa

R

La Bodega

San Celestino

ra tu

e ralCu cu t

An go s

La

3.4 +/- 0.06 Ma

72°52'0"W lt

72°54'0"W

Ba ja f aul t

72°56'0"W

355

3.26+/- 0.30 Ma 3.23+/- 0.06 Ma

1305000

La Plata 3.43 +/- 0.07 Ma

Violetal

Geologic Units Pliocene

California

Hydrothermal breccia/veins

Late Miocene Hydrothermal breccia

Faults & Contacts Normal fault Normal fault, inferred Thrust fault

7°20'0"N

Strike-slip, dextral Porphyry

Non conformity

Late Cretaceous

Santander

Tambor Formation

Late Triassic to Early Jurassic

Bucaramanga Antioquia

Rosablanca Formation

California Vetas Mining District

Bolivar

Arauca

Town Prospect/Artisanal Mine 40Ar/39Ar ages

Diorite to Granodiorite

Alunite

Leucogranite

Sericite

Boyaca

Casanare

1125000

1300000

Proterozoic Bucaramanga Gneiss

1130000

1135000

Fig. 17. Geological map of the La Baja Trend, California Vetas Mining District, Santander Colombia. Prospect locations as well as alunite and sericite ages are shown. Modified from Rodriguez (2014). Coordinates given are geographic and Colombian Gauss with Bogota Observatory datum.

despite the wet climate, the topographically high areas are not in erosional equilibrium with the present day base-level, as in northern Chile and Peru. This high elevation surface is interpreted as a relict lower to middle Miocene paleosurface. Based on apatite fission track data, uplift and valley incision commenced around 17 Ma (Van Der Lelij, 2013) but the most important uplift pulse probably occurred in the Pliocene (Gregory-Wodzicky, 2000; Shagam et al., 1984; Villagomez et al., 2011). Thus, uplift, erosion and epithermal mineralization overlapping in age and the fact that the alunite ages along the La Baja trend become younger upstream strongly suggests that erosion is directly stimulating epithermal mineralization. 12. Summary and comparison to low-sulfidation deposits High-sulfidation epithermal deposits of the Andes occur over a geographically wide area, yet were emplaced in remarkably predictable geologic and geomorphologic settings. Virtually all high-sulfidation epithermal deposits formed during episodes of major uplift in the respective Andean segment in which they are hosted. The uplifted landforms hosting epithermal mineralization are mostly extensive low

relief surfaces, in many cases demonstrably pediplains, located at the crest of the Andes at the time of mineralization. The low-relief surfaces were commonly subject to erosion and valley pediments or valleys were being incised during mineralization. A direct temporal and spatial relationship of erosion and mineralization has been documented for a number of epithermal districts including the El Indio belt deposits, Pierina, Lagunas Norte and California–Vetas. Most high-sulfidation epithermal deposits were emplaced above segments of flat subduction where no volcanism is presently observed. The few exceptions to the latter occur in the western Cordillera near the Andean orocline in southern Peru and northern Chile. The vast majority of deposits are 17 Ma or younger, but exceptions include the Eocene El Hueso, and Guanaco deposits in the southern Atacama Desert near the Domeyko fault system, and possibly the poorly documented Quicay deposit as well as the late Oligocene to early Miocene La Coipa deposit for which age constraints are ambiguous. Large volcanic edifices are uncommon hosts to ore. Host rocks to high-sulfidation systems commonly pre-date mineralization by several Ma to as much as 1 Ga, as at La Bodega and Angostura in Colombia. However, examples exist where the ages of ore-hosting volcanic rocks overlap with those of

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California Vetas Mining District, Colombia 72°50’0”W

7°25’0”N

72°55’0”W

Angostura

La Bodega/La Mascota El Cuatro San Celestino

La Plata

7°20’0”N

California

Vetas

0

1

2

3

Elevation (m)

4

5 km

Slope (Degrees)

High : 4272 0 - 16

Low : 1358

High elevation low relief paleosurface

Town Prospect

Fig. 18. Topographic map of the California–Vetas Mining District, showing steep valleys incised into a high-elevation relatively low relief landscape. WGS84.

alteration and mineralization (e.g., Yanacocha, Lagunas Norte, Aruntani) but where this temporal overlap exists, the volcanic rocks contemporaneous with mineralization are, with exception of Aruntani, of low volume and volcanism ceased shortly after mineralization. Volcanic rocks of significant volume covering and/or post-dating mineralization are reported from La Pepa and La Coipa.

Andean low-sulfidation epithermal deposits are commonly smaller and many occur in geologic settings distinct from those of their highsulfidation counterparts. Some of the larger examples include the Jurassic Fruta del Norte deposit in Ecuador (Henderson, 2009), the Paleocene El Peñón deposit of northern Chile (Warren et al., 2004, 2007). The Late Jurassic to Early Cretaceous deposits of the Deseado and Patagonian

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

California San Celestino El Cuatro

La Mascota La Bodega

Fig. 19. Photograph looking SW from Angostura, showing the approximate locations of prospects. Note the steeply incised La Baja valley and the less rugged higher elevation terrain.

Massifs (Dietrich et al., 2011; Echavarria et al., 2005; Fernández et al., 2008) are, strictly speaking, located outside the Andes but the Cerro Bayo low-sulfidation deposit considered the westernmost deposit of this low sulfidation epithermal province is located in the Chilean Patagonian Andes (Poblete et al., 2014). Mineralization here took place in several pulses between 144 and 113 Ma and partly overlaps in age with the slightly older low-sulfidation epithermal deposits of the Deseado and north Patagonian massifs (Poblete et al., 2014). The low-sulfidation deposits mentioned above occur at elevations below 2400 m a.s.l., are hosted by dominantly rhyolitic volcanic and volcaniclastic rocks, lack a spatial or temporal association with porphyry systems (as defined by Sillitoe, 2010) and occur in extensional tectonic settings without documented contractile deformation and surface uplift during mineralization. However, there are also examples of epithermal deposits containing low-sulfidation mineralization that occur within larger magmatic-hydrothermal systems in arc segments also containing porphyry style or high-sulfidation epithermal mineralization. Examples of these include Cerro de Pasco (Peru: Baumgartner et al., 2008), Marmato and Buriticá (Colombia: Tassinari et al., 2008; Lesage, 2011) which are all mid to late Miocene in age. The low-sulfidation nature of epithermal deposits of some contractile arc settings may be attributed to more reactive, locally-reducing host or basement rock characteristics (e.g., Marmato: Tassinari et al., 2008). Thus, as proposed earlier (Sillitoe and Hedenquist, 2003) low-sulfidation deposits may be associated with both, overall contractile arc as well as rift settings. Those associated with rift settings and rhyolitic magmatism evidently are more likely to be preserved over longer time intervals than epithermal deposits in contractile settings. 13. Controls of geomorphic processes and climate on mineralization Epithermal deposits are emplaced at shallow crustal levels, in the case of high-sulfidation epithermal deposits at depths commonly less than 500 m (see above). Such shallow hydrothermal environments are directly influenced by surface processes and climatic conditions. Precious metals in the epithermal environment are typically precipitated by processes of boiling or fluid mixing (Simmons et al., 2005). The depth of boiling in the epithermal environment is controlled by the hydrostatic pressure which depends on the elevation of the water table. A lowering of the water table during hydrothermal activity such as would occur during catastrophic erosion or volcanic sector collapse would enhance boiling and can be important for the efficiency of precious metal deposition (Bissig et al., 2002a; Simmons, 1991). This can

357

lead to the superposition of epithermal mineralization on the porphyry environment during the life-span of a single magmatic-hydrothermal system, a process commonly referred to as “telescoping” (Sillitoe, 1994). At the giant Ladolam deposit in Papua New Guinea, volcanic sector collapse eliminated more than 500 m of rock cover very rapidly, and is thought to be integral to epithermal ore formation (Blackwell et al., 2014; Carman, 2003; Sillitoe, 1994). While Ladolam has a lowsulfidation sulfide assemblage and is associated with alkalic magmatism and has, thus, fluid chemistry differing from that of Andean highsulfidation epithermal systems, the gold transport and precipitation processes are still comparable, Au bisulfide complexes being probably important in both (e.g., Heinrich et al., 2004). Sector collapse and “telescoping” have also been proposed for Marte and other porphyry Au deposits in the Maricunga belt (Muntean and Einaudi, 2001; Sillitoe, 1994), indicating that catastrophic erosion during hydrothermal activity is not unique to Ladolam. However, in the absence of large volcanic landforms which are prone to rapid degradation, erosion in response to uplift events can enhance mineralizing processes (Bissig et al., 2002a) and may exert a first-order control on exsolution of fluids from magmas. This hypothesis is supported by the fact that Eocene and Miocene porphyry and high-sulfidation epithermal deposits in the Central Andes were emplaced during contractile deformation, uplift and largescale erosion on the western Andean slope, which occurred during the Eocene Incaic, late Oligocene–early Miocene Aymará and Miocene Quechua orogenic phases. On a more local scale, valley or pediment incision during hydrothermal activity can lead to lowering of the water-table near the head of the incising valleys and, by generating local topography, may lead to increased lateral groundwater flow and enhanced mixing of magmatic with meteoric fluids. This process can enhance boiling and fluid mixing especially near the shoulders or back-scarps of incising pediments (e.g. El Indio belt: Bissig et al., 2002a, Lagunas Norte: Montgomery, 2012) or fault induced steep topographic gradients (potentially the case in the Famatina district: Losada-Calderón et al., 1994; Pudack et al., 2009). The time scale of valley incision is well within the proposed duration of hydrothermal activity of individual hydrothermal systems, which is in the order of 10 to 100 k.y. (e.g., Simmons, 2002; Simmons and Brown, 2006). Erosion can in part influence the locus of mineralization over time. The example of Veladero and Pascua as well as the deposits along the La Baja trend in the California Vetas district illustrate this relationship (Figs. 20, 21). In both cases, older mineralization occurs downstream from younger mineralization. Perhaps the best temporal control is available for the La Baja trend where at 3.5 Ma, alunite and associated Au mineralization formed to the west and downstream of contemporaneous higher temperature phyllic alteration, which formed under 500 m or more of cover rock (Rodriguez, 2014; Fig. 20). At a later stage, the La Baja valley incised northeastward and subsequent epithermal hydrothermal alteration and mineralization overprinted the phyllic alteration at La Bodega and Angostura (Fig. 20). In the case of Veladero and Pascua, no constraints for phyllic alteration overprinted by high-sulfidation epithermal mineralization are available. However, mineralization at the Filo Federico zone at Veladero is at 11–10.3 Ma distinctly older than 9.5 Ma mineralization at the Penelope ore zone and 9.1–8.1 Ma mineralization at Pascua which occur upstream from Filo Federico (Fig. 21). At the time of mineralization at Pascua, Veladero was already subject to oxidation, as suggested by the jarosite ages of Veladero, roughly contemporaneous with hypogene alunite formation at Pascua (Deyell et al., 2005b; Holley, 2012). Andean high-sulfidation epithermal deposits occur over a wide range of climatic zones from humid tropical to hyper-arid climate. The Miocene climate of the Central Andes was probably slightly more humid than at present but probably not fundamentally different as South America was located at similar latitudes as at present (Clarke, 2006). Thus, the availability of meteoric fluids during hydrothermal activity does not seem to be the principal controlling factor for mineralization. This is in agreement with the growing number of stable-isotope studies, all of which propose a dominantly magmatic source for the

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La Bodega

10 Ma

NE Los Laches

A

Angostura

Paleosurface

es en td ay su rfa ce

Perezosa

La Bodega

La Mascota

El Cuatro

San Celestino

3,000

Pr

Elevation (m)

3,500

La Plata

California

SW

2,500

Cu?

2,000 0

B

Elevation (m)

3,500

Mo Mo Cu?

500

1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000 7,500

Distance (m)

4 - 3.25 Ma

4 Ma

3,000

~

3.

5

M

a

p

e al

os

ur

f

e ac

~ 2,500

2 3.

5

M

a

Au Ag

2,000 0

500

rfa

ce

Au Au

3.25 Ma

3.9 Ma Cu

3.4 Ma

Au Au Ag Ag

Au Cu

?

1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000 7,500

C

Elevation (m)

le

3.25 Ma 3.5 Ma

3,500

pa

u os

Distance (m)

~ 2.5 - 2 Ma

2.7 Ma

Advanced argillic Sericite/illite quartz Potassic, K-feldspar, biotite Porphyries Ar/Ar on alunite age Alunite < 2 Ma Ar/Ar on sericite Re/Os molybdenite U/Pb zircon

3,000

2,500

2.1 Ma 2.2 Ma 2.6-2.3 Ma

U 2,000 0

500

Au Ag 1.9 Ma Zn

1.6 Ma

Au Ag (Cu)

Au

1.8 Ma Ag

(Cu)

1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000 7,500

Distance (m) Fig. 20. Schematic relationships between erosion, tectonics and mineralization along the La Baja trend, California–Vetas Mining District, Colombia. Based on data from Rodriguez (2014); Refer to Figs. 17 and 18 for context. A) snap-shot of the landscape configuration in relation to hydrothermal systems at ~10 Ma, at the time of porphyry intrusion and Mo (–Cu) mineralization. B) Distribution of hydrothermal alteration in relation to the eroding La Baja valley at ~3.5 to 3.25 Ma. Note that at La Bodega and Angostura sericite forms at a depth of 700–1000 m below surface, whereas further downstream at alunite, spatially associated with Au mineralization forms at only a few 100 m depth at the same time. Alunite also forms near the paleosurface at Los Laches which is the topographically highest prospect of the area. C) At 2.5–2 Ma, during the main stage of epithermal Au (–Ag, Cu) mineralization at La Bodega, La Mascota and Angostura, erosion is further advanced and overprints 3.5–3.9 Ma sericite. Note, Los Laches has not been affected by significant erosion since the early Pliocene and 4 Ma and 2.7 Ma alunite formed at the same place.

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A

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NW Pascua 9.1-8 Ma

SE Penelope 9.5 Ma

Río de las Taguas N-S valley

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Lama Central barren alteration 13.3 Ma

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4000 Frontera-Deidad surface Azufreras-Torta surface Los Ríos surface

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B 5000 4750 4500 4250 4000

Valley profile at 9.5 Ma Present day Río Turbio profile

C 5000 4750 4500 4250 4000

Valley profile at 10.5 Ma Present day Río Turbio profile

Fig. 21. Schematic relationship between erosion, tectonics and mineralization at Veladero and Pascua–Lama, Argentina, Chile. The cartoon is based on data from Bissig et al. (2002a), Charchaflié et al. (2007) and Holley (2012). A) Present-day configuration of landscape elements, faults, mineralized zones. B) Snap-shot of the landscape configuration at 9.5 Ma, i.e., at the initiation of hydrothermal activity at Penelope. At that time, Veladero was already subject to oxidation and jarosite formation. C) Snap-shot of the landscape configuration at 10.5 Ma, during hypogene mineralization at Filo Federico.

precious metal-bearing fluids (e.g., Cerpa et al., 2013; Deyell et al., 2004; Rainbow et al., 2005; Rye, 2005). However, the hydrothermal systems overall are influenced by the climate. Thus, the position of the water table is influenced by the climate and the extent of near surface steam-heated alteration formed in the vadose zone varies. Up to several 100 m thick steam-heated blankets overlying and overprinting mineralization are observed in the El Indio belt and La Coipa. These are interpreted as evidence for dry climate at the time of mineralization and general water table lowering during hydrothermal activity (e.g., Bissig et al., 2002a; Holley, 2012), but short term water table fluctuations due to episodic incursion of magmatic fluid or periods of more humid climate has been inferred from hypogene alunite or barite overprinting jarosite (Chouinard et al., 2005; Deyell et al., 2005b; Holley, 2012). In contrast to deposits of northern Chile, in the more humid Miocene climates of Peru, Ecuador or Colombia where the water-table was probably much closer to the land-surface, steamheated alteration blankets tend to be less well developed or even lacking (McDonald et al., 2011; Rainbow et al., 2005; Rodriguez, 2014; Teal and Benavides, 2010). Although this difference may be due to better preservation under dry climatic conditions, the persistence of high-elevation paleosurfaces in northern Peru and Ecuador that predate hydrothermal activity suggests that steam-heated blankets should have been forming but were probably never as extensive in those areas as in dryer climatic zones, and could therefore have been removed by modest erosion.

Most high-sulfidation epithermal deposits were emplaced between 17 and 6 Ma, irrespective of the climatic zone in which they are located. There is no clearcut overall correlation of younger age with better preservation within this age range. The location and age of currently exposed Miocene high-sulfidation epithermal deposits is, thus, not only a function of the preservation potential but also due to tectonomagmatic factors. However, the Eocene deposits of El Guanaco and El Hueso are located in one of the most arid climatic zone of the planet, whereas the youngest deposits La Bodega and Angostura in Colombia are located in the wettest climatic zone in which high-sulfidation epithermal deposits are known in the Andes. This indicates that a longer-term control on preservation potential does exist. The fact that no high-sulfidation epithermal deposits older than Eocene are known, together with the fact that they generally were emplaced near surface during uplift indicates that the preservation of high-sulfidation epithermal deposits older than the Eocene is unlikely except in the driest climatic zones. Oxidation of primary sulfide assemblages is an important factor for the economic viability of high-sulfidation epithermal deposits, as it may liberate encapsulated gold. Oxidation occurs where sulfides are exposed to atmospheric oxygen and water and is favored by decreasing water table as well as sufficient rock permeability. Some examples of high-sulfidation epithermal deposits of Northern Peru as well as the Pampean flat slab are oxidized to considerable depth (e.g., Yanacocha, Lagunas Norte, Pierina, Veladero), whereas others (e.g., Pascua–Lama, Cerro de Pasco) have undergone considerably less

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Although low-density magmatic vapor may be responsible for early vuggy quartz alteration, later high-sulfidation epithermal mineralization hosted in vuggy quartz likely precipitated from contracted magmatic vapor exsolved from a deeper porphyry intrusion (Heinrich et al., 2004; Pudack et al., 2009). This evolution from shallow toward deeper locus of fluid exsolution is consistent with waning magmatism and increasing SiO2 content of magmas; as is evident for the majority of Andean high-sulfidation epithermal districts. Large-scale tectonic changes such as the onset of flat subduction, increased plate coupling, contractile deformation and uplift result in decreasing volumes of eruptive magmatism and increasing depth of porphyry emplacement and may favor high-sulfidation epithermal Au and associated deeper porphyry Cu over porphyry Cu–Au mineralization.

extensive supergene oxidation. In the case of Pascua–Lama and Veladero as well as the La Coipa district, oxidized and non-oxidized deposits occur within b 15 km from each other, which indicates that the degree of oxidation cannot simply ascribed to climatic factors. Rock-permeability or physiographic setting as well as the posthydrothermal history influence the oxidation and hence economic viability of epithermal deposits as well. 14. Igneous rocks, volcanology and magmatic fluids related to high-sulfidation epithermal deposits Although magmas are the primary contributors of metals and volatiles in porphyry systems (e.g., Audétat and Simon, 2012), extrusive magmatism is commonly waning, of low volume or absent during the formation of high-sulfidation epithermal Au–Ag deposits in the Andes. Conversely, Au-rich porphyry deposits in the Maricunga belt and in northern Peru are typically emplaced in volcanic centers or associated with shallowly emplaced magmatic bodies (e.g. Cerro Casale: Muntean and Einaudi, 2001). Murakami et al. (2010) showed that Cu/Au ratios increase with greater depth of emplacement of porphyry systems and suggested that high-sulfidation epithermal Au mineralization occurs where significant amounts of Au has been physically separated from Cu, a process which is apparently favored where the depth of intrusion of the magma providing the magmatic volatiles is N 3 km (Murakami et al., 2010). A compelling explanation for this is that precious metals are better transported in high-density magmatic vapors released from magmas emplaced at more than 3–4 km depth than in low-density vapors exsolved at depths of less than 2 km (Heinrich, 2005, 2007; Fig. 22). Fluid evolution at depth can lead to porphyry Cu mineralization but allows for efficient Au transport in contracted magmatic vapor. If condensed magmatic vapors are efficiently transported and focused, a high-sulfidation epithermal Au deposit may form several km above.

A

High-sulfidation epithermal deposits of the Andes formed in predictable geological and geomorphological settings. They are located at high elevation and largely near the crest of the Andean cordillera in segments where volcanism is currently absent or subdued and where subduction angles are shallow. All were emplaced during major periods of contractile deformation and uplift and most coincided with the terminal stages of local arc magmatism. The vast majority of deposits are between ca. 17 and 5 Ma old. Although the tectonic setting in which these deposits form makes them prone to erosion, their restricted age range is not only a function of preservation potential but also attributed to favorable tectonomagmatic settings at the time of hydrothermal activity. Eocene high-sulfidation epithermal deposits are only known from the driest regions of the Atacama Desert but they plausibly formed under similar tectonomagmatic conditions as the Miocene ones. Conversely, the youngest deposits, La Bodega and Angostura, Colombia,

B v

1 km

15. Conclusions

vvvv vvvv

Au

v

v v

v Au

C

Cu

v

Au

Au v

v

vvvv

v

v

vvvv v

2 km

3 km Cu Mo

4 km

Cu Mo

Cu Mo

Steam-heated alteration

Diatreme breccia

High-sulfidation epithermal Au

Water table

Porphyry Cu-Au

vvvv

Volcanic rock

Porphyry Cu-Mo Fig. 22. Schematic relationships between depth of intrusion volcanic setting and mineralizatiion style, inspired by Murakami et al. (2010) and relationships observed in the Andes. Three scenarios are shown. A) Porphyry Au–Cu hosted in a volcanic edifice such as observed in the Maricunga belt (e.g. Cerro Casale). Depth of intrusion and fluid exsolution is b2 km, magmatic vapor cannot dissolve significant Au and temperature gradient to surrounding rocks is steep, leading to bulk co-precipitation of Cu and Au. B) High-sulfidation epithermal deposit forms N3–4 km above porphyry intrusion. At that depth, magmatic vapor has a higher density and is, after contraction, capable of transporting significant Au as bisulfide complexes at low fluid temperatures (Heinrich et al., 2004), whereas Cu and Mo precipitate in the higher-T porphyry environment. Erosion at surface stimulates precipitation of precious metals but through reduction of lithostatic load may enhance fluid release from the magma and the structural pathways permitting efficient separation of magmatic vapor derived fluids from the porphyry Cu environment (this scenario reflects the situation at La Bodega and Angostura, Colombia as well as El Indio and Pascua–Lama). C) A scenario where multiple pulses of both, porphyry Cu–Au and high-sulfidation epithermal mineralization occurred. This scenario reflects the situation at Yanacocha. Volcanism in the form of flow-domes and pyroclastic flow deposits occurred intermittedly as well.

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were emplaced in the late Pliocene in an area with pluvial tropical climate with low preservation potential. High-sulfidation epithermal Au–Ag deposits can be emplaced in a range of host rocks, and volcanic rocks both pre- or syn-hydrothermal activities, are not a pre-requisite for epithermal mineralization. However, a magma from which the mineralizing magmatic fluids are derived must have present, albeit fluid exsolution typically occurred N3–4 km below the surface at the time of hydrothermal activity. Significant erosion during hydrothermal activity is documented for a number of districts (e.g., Lagunas Norte, Pierina, El Indio belt, California Vetas Mining District) and mineralization ages generally become younger upstream in both, the El Indio belt and the California Vetas Mining District. Fluid boiling and mixing with meteoric water are enhanced near topographic breaks such as the heads of incising valleys or backscarps of pediments, ultimately stimulating mineralization in those locations.

Acknowledgments This paper reviews and summarizes many years of work on Andean epithermal systems, much of it carried out at Queen's University. The authors would like to express their special thanks to Barrick Gold Corp. and former chief geologist Jay Hodgson for the continued support and funding of TB's, AM's and AR's PhD theses at Queen's University. Barrick also funded projects at MDRU and Universidad Católica del Norte, Chile in which the senior author continues to be involved. Without Barrick's support, our understanding of Andean high-sulfidation epithermal deposits would be far less advanced. We also acknowledge the contributions and student support from NSERC in the form of grants to AHC and to Kurt Kyser, as well as from many other companies who supported research at Queen's University and MDRU, including (but not limited to) IamGold, Ventana Gold, Eco Oro minerals and Kinross. Sara Jenkins is acknowledged for help with the figures and we thank the OGR editors Franco Piranjo and Tim Horscroft for the invitation of this review article. Comments provided by reviewers Stuart Simmons and Dick Tosdal helped improve clarity of the paper.

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