Cerro Quema (Azuero Peninsula, Panama): Geology, Alteration, Mineralization, and Geochronology of a Volcanic Dome-Hosted High Sulfidation Au-Cu Deposit

Cerro Quema (Azuero Peninsula, southwest Panama) is a high sulfidation epithermal Au-Cu deposit hosted by a dacite dome complex of the Río Quema Formation (late Campanian to Maastrichtian), a fore-arc basin sequence. Mineral resource estimates (indicated + inferred) are 30.86 Mt at 0.73 g/t Au, containing 728,000 oz Au (including 76.900 oz Au equiv of Cu ore). Hydrothermal alteration and mineralization are controlled by an E-trending regional fault system. Hydrothermal alteration consists of an inner zone of vuggy quartz with locally developed advanced argillic alteration, enclosed by a well-developed zone of argillic alteration, grading to an external halo of propylitic alteration. Mineralization produced disseminations and microveinlets of pyrite and minor chalcopyrite, enargite, and tennantite, with traces of sphalerite, crosscut by late-stage base metal veins. New 40Ar/39Ar data of igneous rocks combined with biostratigraphic ages of the volcanic sequence indicate a maximum age of lower Eocene (~55–49 Ma) for the Cerro Quema deposit. It was probably triggered by the emplacement of an underlying porphyry-like intrusion associated with the Valle Rico batholith. The geologic model suggests that in the Azuero Peninsula high sulfidation epithermal mineralization occurs in the Cretaceous-Paleogene fore arc. This consideration should be taken into account when exploring for this deposit type in similar geologic terranes. Introduction South Central America is a region characterized by a longlived intraoceanic subduction zone with a volcanic arc active since the Late Cretaceous (e.g., Lissinna, 2005; Buchs et al., 2010, 2011a). It displays many characteristics of zones where epithermal, porphyry copper, and volcanogenic massive sulfide (VMS) deposits are found around convergent plate boundaries (e.g., Roberts and Irving, 1957; Levy, 1970; Ferencic, 1971; Weyl, 1980; Nelson, 2007). High sulfidation epithermal deposits (Hedenquist et al., 2000) are commonly hosted by subaerial, calc-alkaline volcanic rocks that formed at convergent margins, generally within island or continental arcs as a direct result of plate subduction (Sillitoe, 1993, 2010; Arribas and Tosdal, 1994; Cooke and Simmons, 2000). Mineralization styles related to high sulfidation deposits display a wide variety, including veins, hydrothermal breccia bodies, stockworks, and dissemination or replacements (Arribas, 1995). A distinguishing feature of this deposit type is the presence of alteration halos (grading from the fluid conduit outward) characterized by quartz ± alunite ± pyrophyllite ± dickite ± kaolinite ± illite, and montmorillonite ± chlorite (Steven and Ratté, 1960; Stoffregen, 1987; Arribas, 1995; Hedenquist et al., 2000). The most common geologic setting documented for this deposit type is a volcanic dome complex; however, they may also occur in a central-vent volcano setting and in a spatial association with maar-diatremes or calderas (Sillitoe et al., 1984; Arribas, 1995; Sillitoe, 1999; Hedenquist et al., 2000). Furthermore, submarine high sufidation epithermal Au-Cu deposits have been reported in the Izu-Bonin-Mariana arc, the Tonga-Kermadec arc, and the Bismark archipelago (e.g., Binns and Scott, 1993; Hannington and Herzig, 1993; de Ronde et al., 2003; Embley et al., 2004). In Panama, gold and copper are the most economically important metals, and they are mainly hosted by epithermal (e.g., Cana, Santa Rosa, and Cerro Quema deposits; Woakes, 1923; Wleklinski, 1969; White, 1993; Nelson, 1995, 2007; Corral et al., 2011a) and porphyry copper deposits (e.g., Petaquilla and Cerro Colorado, Kesler et al., 1977; Kesler, 1978; Nelson, 1995; Speidel, 2001). The present study focuses on the Cerro Quema deposit, located in the Azuero Peninsula, 0361-0128/16/4367/1-24 1 Submitted: June 5, 2014 Accepted: September 18, 2015 †Corresponding author: e-mail, i.corral.geo@gmail.com ©2016 by Economic Geology, Vol. 111, pp. 1–000 BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS Vol. 111 March–April No. 2

We document the geologic setting, mineralogy, geochemistry, and 40 Ar/ 39 Ar geochronology of Cerro Quema. A geologic model is developed from these data that contributes to the understanding and exploration of high sulfidation Au-Cu deposits in ancient and modern terranes, with similar geologic features.

Regional geology
Panama, located in south Central America, is the youngest segment of the land bridge between the North and South American plates. It is a tectonic block that lies at the junction of the Caribbean, South American, Cocos, and Nazca plates (e.g., Duque-Caro, 1990;Kellogg et al., 1995;Harmon, 2005). A volcanic arc developed during the Late Cretaceous as a result of the subduction of the ancient Farallon plate (nowadays Cocos and Nazca plates) beneath the Caribbean plate. Volcanic arc magmatism continued until the Miocene (~23 Ma; Barckhausen et al., 2001;Werner et al., 2003;Lonsdale, 2005;Buchs et al., 2009Buchs et al., , 2010Pindell and Kennan, 2009;Wörner et al., 2009). The accretion and obduction of seamounts and oceanic plateaus (middle Eocene; Buchs et al., 2010) and the collision of the Panamanian volcanic arc with Colombia during the middle to late Miocene (e.g., Keigwin, 1978;Trenkamp et al., 2002;Coates et al., 2004;Barat et al., 2012Barat et al., , 2014 produced a change in the subduction direction and the migration of the volcanic arc toward the north Lissinna, 2005). The Cordillera Central in north Panama is the present-day expression of the active Panamanian volcanic arc.
The Azuero Peninsula is transected by several regionalscale subvertical faults (Fig. 1B). These include the NWtrending Soná-Azuero fault zone, the E-trending Ocú-Parita fault, and the Río Joaquín fault zone (Kolarsky et al., 1995;Buchs, 2008;Corral et al., 2011aCorral et al., , 2013. The Río Joaquín fault zone is 30 km in length, shows evidence for reverse dip-slip motion, and juxtaposed the Azuero igneous basement against the Azuero Arc Group (e.g., Río Quema Formation). Secondary NW-trending regional structures such as the Pedasí fault zone and the Punta Mala fault, both with a sinistral strike-slip motion, have disrupted the eastern Azuero Peninsula (Fig.  2). In the central Azuero Peninsula mesoscale open folds with SW-plunging fold axes and moderate limb dips indicate dextral transpression with dominant reverse dip-slip motion (Corral et al., 2011a. The local stratigraphy was initially defined by two units (C.F. Horlacher, pers. comm., 1993): (1) the Ocú Formation, composed of limestones and volcanosedimentary rocks, and (2) the Quema Formation, composed of dacites and massive andesites. Corral et al. (2011aCorral et al. ( , 2013 used new field, geochemical, and biostratigraphic data to define a new lithostratigraphic unit, the Río Quema Formation (Fig. 3). This newly defined unit, which hosts the Cerro Quema deposit, is a volcanosedimentary sequence enclosed within the Azuero Arc Group. It is interpreted as the volcaniclastic apron of the Panamanian Cretaceous volcanic arc. The volcanic sequence is exposed from the central to southeastern Azuero Peninsula and represents the fore-arc basin, the region between the subduction trench and the magmatic arc (e.g., Stern et al., 2012). On the basis of biostratigraphic data, the volcanosedimentary sequence is late Campanian to Maastrichtian in age . The Río Quema Formation is subdivided into three units (Fig. 3). The lower unit contains andesitic lava flows and well-bedded crystal-rich sandstone to siltstone turbidites, interbedded with hemipelagic thin limestone beds. The limestone unit is a thick, light gray biomicritic hemipelagic limestone, interlayered with well-bedded cherts, thin-bedded turbidites, and fine ash layers. The upper unit consists of volcaniclastic sediments interlayered with massive to laminar andesitic lava flows, dacite domes, dacite hyaloclastites (Fig. 3B), and polymictic conglomerates. Dacites are characterized by quartz and hornblende phenocrysts (up to 5 cm in hornblende) and smaller plagioclase crystals in a microcrystalline quartz-feldspar groundmass. The total thickness of the Río Quema Formation is approximately 1,700 m. It overlies both the Azuero igneous basement (Fig. 3A) and the Azuero primitive volcanic arc, and is discordantly overlain by the Tonosí Formation (Fig. 3C).
The Cerro Quema deposit is located in the center of the Azuero Peninsula. It covers an area of ~20 km 2 (Figs. 1B, 2) and is associated with an E-trending regional fault system, parallel to the Río Joaquín fault zone (Corral et al., 2011a). The deposit is hosted by the dacite dome complex of the Río Quema Formation and contains several orebodies. From east to west, these are Cerro Quema, Cerro Quemita, and La Pava (Fig. 4). Although mineralization and hydrothermal alteration persist to the east (e.g., Cerro Idaida, Pelona, and Peloncita), the economic potential of this zone is poorly known. Data from Cerro Idaida are presented below in order to complement the geologic characterization of Cerro Quema.

Hydrothermal Alteration
Wall-rock alteration at Cerro Quema was initially described by T. M. Leach (pers. comm., 1992) and, subsequently, by Corral et al. (2011a). We provide new data on hydrothermal alteration mineralogy and zoning based on field mapping and core    logging, and on analysis of surface and drill core samples by petrographic microscope, X-ray diffraction (XRD), scanning electron microscope-energy dispersive X-ray (SEM-EDX), and electron microprobe analysis (EMPA). Hydrothermal alteration at Cerro Quema appears mainly restricted to the dacite domes of the Río Quema Formation (Fig. 5) due to the difference in permeability and porosity with respect to other rock types of the volcanosedimentary sequence (Corral, 2013). Hydrothermal alteration follows an easterly trend that is parallel to secondary faults of the Río Joaquín fault zone.    Volcaniclastic sedimentary rocks and andesite lava flows affected by the E-trending faults to the east and west of Cerro Quema have also been weakly affected by hydrothermal alteration. Dacites are easily distinguished, due to their characteristic porphyritic texture, even when hydrothermally altered (Figs. 5, 6A, 7A). Although hydrothermal alteration had a strong structural control, a lithological control is also evident in the mushroom-shaped alteration domains at shallow levels (e.g., La Pava). The Cerro Quema alteration pattern consists of an inner zone of vuggy quartz (30-230 m wide), with local quartz-alunite and pyrophyllite (advanced argillic alteration, 30-200 m wide), enclosed by a kaolinite, illite, and illite/smectite-bearing widespread alteration zone (argillic alteration, 100-400 m wide; Fig. 5). Propylitic alteration has only been observed in some drill core samples, and forms a halo surrounding the argillic alteration zone.

Vuggy quartz
This innermost alteration zone ( Fig. 5) occurs as irregular, generally vertical, funnel-and tabular-shaped bodies, and is commonly found on top of mineralized zones. Patches of massive quartz and silicified breccias are also present in this zone.
Vuggy quartz is made up of a groundmass of microcrystalline anhedral quartz grains, disseminated pyrite, barite, and minor rutile, with traces of sphalerite. At depth, vuggy quartz contains disseminated pyrite, chalcopyrite, enargite, and tennantite. Vuggy quartz texture is characterized by voids preserving the crystal morphology of hornblende and plagioclase (Figs. 6B, 7B). Drusy quartz, pyrite, and rutile have partially filled some void spaces. Quartz phenocrysts preserved within dacite contain secondary two-phase (liquid rich) fluid inclusions, possibly recording the fluids responsible for hydrothermal alteration and mineralization.

Advanced argillic
The advanced argillic alteration zone is an irregular halo developed around the vuggy quartz alteration zone (Fig. 5). The advanced argillic alteration zone has different mineralogical expressions depending on its occurrence (surface/ subsurface).
Quartz-alunite alteration associated with a massive quartzcemented breccia zone is exposed at surface at La Pava (Fig.  6C). Alunite is a very fine grained minor component that is only identifiable by XRD and is associated with the breccia cement. A more representative association of the advanced argillic alteration at surface is characterized by quartz, dickite, pyrophyllite, barite, illite, and minor diaspore (at La Pava, Chontal Edge, and Cerro Quema). These minerals altered the massive and brecciated dacites (Fig. 6D) to quartz. Clay minerals (dickite, pyrophyllite, and illite) replaced hornblende and plagioclase, and also occur in the breccia as cement (Fig. 7C). Barite occurs along fractures and as part of breccia cement. Disseminated pyrite is characteristic of the advanced argillic alteration zone. At depth, the advanced argillic alteration assemblage consists of quartz, alunite-natroalunite, aluminum phosphate-sulfate (APS) minerals, dickite, pyrophyllite, barite, and rutile. This assemblage has only been observed in drill core samples, associated with hydraulic breccias (Fig. 7D).

Argillic
The argillic alteration zone defines a halo surrounding the vuggy quartz and advanced argillic alteration zones (Fig. 5). The argillic envelope generally bounds the vuggy quartz zone with a sharp contact, whereas the contact with the advanced argillic zone is gradational. The whitish-gray, hydrothermally altered rock typically preserves the original volcanic textures (Fig. 6E). Argillic alteration produced quartz, kaolinite, illite, and illite-smectite with minor chlorite, which replaced hornblende and plagioclase crystals (Fig. 7E). Disseminated pyrite is found locally.
Minerals within the argillic alteration zone are zoned outward from the mineralized centers. Kaolinite is dominant proximal to ore, and the assemblage grades to kaolinite ± illite, and then to ± illite-smectite. Kaolinite ± smectite ± chloritesmectite and chlorite have been recognized in distal locations. At La Pava, there are subvertical pipe-like structures where dacites with hornblende and plagioclase phenocrysts have been replaced by quartz, dickite, barite, and pyrite alteration (advanced argillic alteration; Fig. 6F). These pipes have crosscut the argillic altered rocks (Fig. 6F).

Propylitic
A propylitic assemblage constitutes the most distal alteration halo, affecting dacites, andesites, and volcaniclastic sedimentary rocks (e.g., turbidites and debris flows; Fig. 6G). It is characterized by chlorite, epidote, carbonate, rutile, pyrite, and chalcopyrite, with minor hematite and magnetite. Hornblende has been partially to completely replaced by chlorite and epidote, and plagioclase by carbonate (Fig. 7F). Carbonates also occur as patches and veinlets. Minor amounts of pyrite, chalcopyrite, rutile, magnetite, and hematite have replaced hornblende, and also occur as disseminated grains. The propylitic zone has a transitional contact with the argillic alteration zone, where clay minerals have partially overprinted propylitic alteration minerals.

Mineralization
Gold occurs as disseminated submicroscopic grains and as invisible gold within pyrite (Corral et al., 2011a). Copper is associated with hypogene chalcopyrite, enargite, bornite and tennantite, and supergene covellite and chalcocite. Mineralization (gold and copper) is mainly associated with the vuggy quartz and advanced argillic alteration zones. However, minor gold and copper occurrences have been found in the argillic and propylitic alteration zones.

Hypogene mineralization
Hypogene mineralization is generally developed below the oxidized zone, even though small (meter scale) outcrops are found at surface. Pyrite is the most abundant sulfide at the Cerro Quema deposit; however, there is a group of accompanying sulfides also associated with the Au-Cu mineralization.
Hypogene mineralization is divided into five stages (Fig. 8), where stages 3 and 4 are the main ore-forming stages. Stage 1 consists of disseminated, fine-grained, idiomorphic and subidiomorphic pyrite, accompanied by rutile and barite in voids and groundmass (Fig. 9A), with minor enargite, tennantite, and chalcopyrite at depth. Sphalerite is a trace mineral that   9D).
Weathering of the high sulfidation ores has produced a thick quartz-and iron oxide-rich zone that overprinted the primary sulfide-bearing zone. This zone developed in the upper part of mineral bodies, and is characterized by vuggy quartz containing abundant hematite and goethite within the groundmass, replacing the cement of hydrothermal breccias, and filling voids in the vuggy quartz zone (Fig. 9E). Supergene jarosite, kaolinite, halloysite, and gypsum are also found in fractures, vugs, and breccia matrix. Hypogene pyrite, barite, and rutile remain as trace minerals in the oxidation zone.
Below the oxidation zone, supergene enrichment has caused deposition of secondary Cu-bearing minerals such as chalcocite and minor covellite. The secondary Cu sulfides are found replacing chalcopyrite, tennantite, and enargite as well as filling small fractures (Fig. 9F).
The enrichment factor of the oxide zone with respect to the sulfide zone in terms of gold and copper is 2.41 and 0.61, respectively (Corral, 2013). At Cerro Quema, the oxidation zone has higher gold grades (up to 2,400 ppb Au), and the enrichment zone has higher copper grades (up to 1% Cu).

Trace Element Geochemistry
Trace element data from high sulfidation epithermal deposits are not abundant (e.g., Nansatsu, Japan- Rodalquilar, Spain-Hernandez et al., 1989;Pueblo Viejo, Dominican Republic-Kesler et al., 2003). Co/Ni and S/Se ratios in pyrite have been used as empirical indicators of the depositional environment (e.g., Edwards and Carlos, 1954;Goldschmidt, 1954;Loftus-Hills and Solomon, 1967;Bralia et al., 1979). Pyrite compositions combined with major and trace element contents of alunite and APS group minerals may provide significant information for understanding their origin. Chemical composition of enargite, alunite, and APS minerals can be used as an ore guide in mineral exploration as they can be related in time and space to epithermal and porphyry mineralization (e.g., Bove, 1990;Dill, 2003;Chang et al., 2009Chang et al., , 2011Deyell and Hedenquist, 2011).
Analyses of S, Fe, Co, Ni, Cu, As, Se, Ag, Cd, Sb, Au, and Hg have been performed by EMPA for 55 pyrites from six drill hole samples of the vuggy quartz and advanced argillic alteration. The contents of Al, Fe, Ca, Na, K, P, F, S, Cu, As, Sr, Ba, Ce, and Pb of 20 alunites and 21 APS minerals were analyzed by EMPA from two drill core samples of the advanced argillic alteration. All the analyses were performed at the Serveis Científics i Tecnològics of the University of Barcelona.

Pyrite
Several pyrite types have been analyzed (e.g., idiomorphic, subidiomorphic, zoned, massive, framboidal, and brecciated; Fig. 10A, B). The aim was to determine the chemical composition of the different pyrite types. However, they have similar Ag, Cd, Sb, and Se concentrations, but some differences exist in Co and Ni concentrations (Table 1; Appendix 1). Co/ Ni ratios (N = 11) range from 0.58 to 5.50 (Fig. 11A), and S/ Se ratios (N = 21) are between 1,050 and 2,694. Pyrites are generally Cu rich, varying from 0.03 to 3.67 wt % Cu. The Au, Hg, and As concentrations of pyrite are below detection limits.

Alunite and APS minerals
At Cerro Quema, alunite and APS minerals occur as cement in the hydraulic breccias associated with pyrite and dickite, filling voids in the vuggy quartz zone, and replacing plagioclase crystals (Fig. 10C, D). In general, alunite is zoned (~1-7 µm wide), which is mainly due to the variation in Na, K, and Ca contents. Alunites have typical flaky shapes, indicating a hypogene origin (e.g., Arribas et al., 1995a;Itaya et al., 1996), and commonly have a core of APS minerals (e.g., svanbergite; Fig.  10C). Representative chemical data for alunite and APS minerals from Cerro Quema are shown in Table 2 and Appendix 2. Alunite is Na rich, exhibiting a compositional range within the alunite-natroalunite solid solution (Fig. 11B). P is generally present as a trace, excepting few alunite crystals that show P enrichment, which is also correlated with an enrichment in Sr and Ba. In contrast, APS minerals show irregular element content (e.g., Na, Ca, Sr, Ba, and Fe), typically with enrichment in Sr, and locally in Ca and Ba (Fig. 11C), which is characteristic of the svanbergite-woodhouseite solid solution.

Ar/ 39 Ar Geochronology
The first geochronological studies of arc rocks in the Azuero Peninsula were conducted by Del Giudice and Recchi (1969) and Kesler et al. (1977). Recent studies have focused on dating igneous rocks such as those at El Montuoso, Valle Rico, and Parita batholiths, as well as quartz-diorites from the Punta Mala area, northeast Azuero basalts, and Central Azuero arc rocks ( Fig. 1; Lissinna, 2005;Wegner et al., 2011;Montes et al., 2012). Results of the previous geochronological studies are summarized in Table 3. Ar/Ar step-heating dating has been conducted in this study in order to complete the existing radiometric and biostratigraphic ages of the volcanic, volcaniclastic, sedimentary, and plutonic rocks of the Azuero Peninsula, and to constrain the age of the Cerro Quema deposit. Mineral separates of eight hornblende phenocrysts were prepared by crushing 1 kg of rock, sieving, washing, and handpicking to obtain 100 mg of optically pure mineral. The 40 Ar/ 39 Ar step-heating analyses were performed at the U.S. Geological Survey on samples irradiated at the U.S. Geological Survey TRIGA reactor in Denver, Colorado (Dalrymple et al., 1981). Dated samples are from the El Montuoso, Valle Rico, and Parita batholiths, the Cerro Quema host rock (dacite dome complex), and volcaniclastic andesite (Fig. 1). Results and sample locations are shown in Table 4 and in Figure 12.
Valle Rico 40 Ar/ 39 Ar dating of a sample of the Valle Rico quartz-diorite ( Fig. 12; Table 4) provided an integrated age of 54.8 ± 1.2 Ma, which is consistent with the hornblende K/Ar age of 53 ± 3 Ma (Del Giudice and Recchi, 1969). However, our age is considerably older than plagioclase 40 Ar/ 39 Ar ages of 49.5 ± 0.2 and 50.6 ± 0.3 Ma for the same batholith reported by Lissinna (2005). A recent zircon U/Pb age of 49.2 ± 0.9 Ma (Montes et al., 2012) suggests that this is the true age of this quartz diorite. The Valle Rico hornblende 40 Ar/ 39 Ar date is therefore interpreted to have been compromised by the presence of excess argon.

Río Quema Formation
Four hornblendes separated from the dacite dome complex of the Río Quema Formation yielded 40 Ar/ 39 Ar plateau ages of 67.9 ± 1.1, 66.0 ± 1.0, and 65.6 ± 1.3 Ma and an integrated age of 69.7 ± 1.2 Ma ( Fig. 12; Table 4). Wegner et al. (2011) reported hornblende 40 Ar/ 39 Ar ages of 71.0 ± 2.0 and 67.5 ± 1.9 Ma for two dacite samples found in the Tonosí River (central Azuero Peninsula), probably corresponding to boulders coming from the erosion of the dacite dome complex of the Río Quema Formation.
An attempt to date the volcaniclastic rocks of the Río Quema Formation was made. Unfortunately, the integrated age of 143 ± 11 Ma and the plateau age of 105 ± 3 Ma ( Fig.  12; Table 4) have no geologic sense within the geologic framework of the Azuero Peninsula (the 143 ± 11 Ma age indicates that the rock is older than the Azuero igneous basement).

Hydrothermal alteration and mineralization
Geochronologic dating of the Cerro Quema hydrothermal alteration-mineralization was attempted by the performance of 40 Ar/ 39 Ar step-heating analysis on alunite (advanced argillic alteration). Unfortunately, we had no success due to the finegrained size of the alunite crystals and their intergrowths with kaolinite, which prevented the preparation of a pure alunite sample.

Deposit type
Classification of Cerro Quema has been a matter of debate since the first studies were carried out in the area. T. M.
Leach (pers. comm., 1992) and Nelson (1995) considered the deposit to be a high sulfidation epithermal deposit potentially related to an underlying porphyry-style intrusion. In contrast, Nelson and Nietzen (2000) and Nelson (2007) proposed that Cerro Quema could be an oxidized Au-Cu deposit transitional between epithermal deposits and volcanogenic massive sulfide deposits, similar to the Pueblo Viejo deposit, Dominican Republic.

Alunite and APS origin
Analyzed alunite crystals have a flaky shape and are Na rich, covering a wide range of the alunite-natroalunite solid     Table 2; Appendix 2). APS minerals in the core of alunite and occurring as single crystals are mainly Sr rich, locally showing Ca enrichment ( Fig. 11C; Table 2; Appendix 2), characteristic of the svanbergite-woodhouseite solid solution. Studies focused on the alunite geochemistry (e.g., Stoffregen and Alpers, 1987;Arribas et al., 1995b;Deyell et al., 2005a, b;Chang et al., 2011) showed that supergene alunite and low-temperature alunite are generally K rich in comparison with higher-temperature occurrences, which are Na rich. Aoki et al. (1993) suggested that the core of hypogene alunite is commonly enriched in PO4 and multivalent cations such as Ca (crandalite, woodhouseite), Sr (svanbergite), and Ba (groceixite). These inclusions are typically rimmed by minamiite and rhythmic bands of alunite and natroalunite (Stoffregen and Alpers, 1987;Aoki et al., 1993). According to the aforementioned studies, texture and chemical composition of alunite (Na-rich, flaky shapes and with inner core of APS) and svanbergite-woodhouseite (Sr-and Ca-rich APS mineral, occurring as an alunite core as well as single crystals) from Cerro Quema (see Figs. 10, 11; Table 2) present all of the characteristics indicating a magmatic-hydrothermal origin related to an intrusion-driven hydrothermal system, such as a porphyry copper intrusion.

Geologic Evolution and Epithermal Mineralization
This section summarizes the events from Late Cretaceous to present times that constrained the geologic evolution of the Azuero Peninsula and the formation of Cerro Quema (Fig.  13).

Arc development
The late Campanian (~75-73 Ma) marked the initiation of Farallon plate subduction beneath the Caribbean plate (Buchs et al., 2010). The initial stages of an intraoceanic subduction are characterized by extension of the overriding plate (Stern and Bloomer, 1992;Stern, 2010). In the Azuero Peninsula, this extension controlled the morphology and evolution of the volcanic arc and fore arc. From late Campanian to Maastrichtian (~71-66 Ma), the first stage of magmatism occurred within the Caribbean plate. This stage is characterized by the intrusion of El Montuoso batholith and the development of the arc and fore-arc basin. The Río Quema Formation, of late Campanian to Maastrichtian age, represents the fore-arc basin. Contemporaneous intrusions of dacite domes (~71-66 Ma) into the Río Quema Formation resulted in the interstratified volcanic and sedimentary sequences of the fore-arc basin (Fig. 13A).

Arc maturation and emplacement of the Cerro Quema deposit
During the lower Eocene (~55-49 Ma), a second stage of magmatism occurred (Fig. 13B), where the Paleogene volcanic arc developed on top of the Cretaceous volcanic arc. Valle Rico-like batholiths intruded along E-trending regional faults to the north of the Cretaceous fore-arc basin. However, some Valle Rico-like intrusions (quartz-diorites, diorites, and trachyandesites) also occurred in the central and southern limit of the fore-arc basin. Emplacement of Valle Rico intrusions in the fore-arc led to the formation of Cerro Quema.

Age of the Cerro Quema deposit
The age of Cerro Quema has been constrained from field evidence coupled with biostratigraphic data of sedimentary rocks of the Río Quema Formation and geochronological data of the igneous rocks of the Azuero Peninsula. The age of the deposit is estimated to be ~55 to 49 Ma (lower Eocene), based on the following observations: 1. Crystal-rich sandstones and turbidites of the Río Quema Formation, a volcanosedimentary sequence of Campanian-Maastrichtian age, do not contain altered clasts derived from hydrothermally altered rocks. Dacite clasts in conglomerates derived from the erosion of the dacite dome complex that hosts Cerro Quema (~71-66 Ma; Wegner et al., 2011) show no signs of hydrothermal alteration. Therefore, hydrothermal alteration and mineralization should be younger than the age of the dacite dome complex (~71-66 Ma). 2. As a high sulfidation deposit, Cerro Quema will have been related to a magmatic event. In the Azuero Peninsula, the first recorded post-Cretaceous magmatic event occurred during the lower Eocene (~55-49 Ma; Del Giudice and Recchi, 1969;Kesler et al., 1977;Lissinna, 2005;Montes et al., 2012), corresponding to Valle Rico-like batholith intrusions. Based on correlations with this second magmatic event, the maximum age of Cerro Quema is lower Eocene (55-49 Ma).

Arc migration
During the middle Eocene (~45 Ma), the Azuero Peninsula was an area of accreted intraoceanic island arcs, such as la Hoya and Punta Blanca islands (Buchs et al., 2011b). Subduction erosion and possible slab flattening induced the migration of the arc front toward the Caribbean. The emplacement of the Parita batholith (~48-41 Ma) to the north of the Ocú-Parita fault (Fig. 1) supports arc migration toward the north during middle Eocene times. This migration is in agreement with geodynamic reconstructions of Buchs et al. (2010) and geochronological data of Lissinna et al. (2002).

Erosion and supergene enrichment
Some time before the emplacement of the Cerro Quema deposit (~55-49 Ma) and present day, erosion and supergene enrichment affected the Cerro Quema deposit (Fig. 13C). Consequently, oxidation and intense weathering generated a thick Au-bearing, silica-and iron-rich zone of up to 150-m depth, below which a Cu-rich zone was developed.

Conclusions
Cerro Quema is a high sulfidation epithermal deposit hosted by the dacite dome complex of the Río Quema Formation, and was emplaced into a Cretaceous fore-arc sequence.
Weathering and supergene oxidation processes at Cerro Quema produced two mineralized zones, an upper quartz and iron oxide zone enriched in Au and a lower supergene enrichment zone where Cu is concentrated.
Field observations as well as geochronologic and biostratigraphic data support a maximum lower Eocene (~55-49 Ma) age for Cerro Quema. The deposit is interpreted to be related to the emplacement of a porphyry-like intrusion associated with the Valle Rico batholith.
The formation of Cerro Quema in the lower Eocene fore arc has important implications for exploration in the Panamanian volcanic arc and elsewhere. Our results suggest that high sulfidation deposits can form in a fore-arc environment if acidic intrusions are emplaced between the volcanic arc front and the trench. Exploration for high sulfidation epithermal deposits in the Azuero Peninsula should therefore be focused in the Cretaceous fore-arc sequence, especially in the Río Quema Formation dacite domes, targeting E-trending regional faults (parallel to the Río Joaquín fault zone) and lower Eocene acidic intrusions (Valle Rico-like intrusions). This implies a potential zone about ~70 × 10 km for hosting porphyryrelated high sulfidation epithermal deposits (Fig. 14). These findings should also be considered when exploring for high sulfidation deposits in geologically similar terranes globally.