1. Introduction

The recent molybdenite Re-Os dating study by ^{Perelló et al., (2020)} of the Cerro Quema deposit (Azuero Peninsula, Panama) claims that ore is spatially and temporally related to the volcanic domes of the Río Quema Formation at ~71 Ma. This interpretation contrasts with early observations indicating that mineralization is the product of a hydrothermal system (e.g., porphyry system) driven by concealed intrusions associated with the Valle Rico batholith at 55 - 49 Ma (^{Corral et al., 2016}) as well as with the recently obtained alunite ⁴⁰Ar/³⁹Ar date of ~49 Ma (^{Corral, 2021}). Although the ~71 Ma age would indicate that Cerro Quema is similar to other high-sulfidation epithermal deposits in which mineralization is coeval with volcanic dome emplacement (^{Arribas, 1995}; ^{Bissig et al., 2001}; ^{Nelson et al., 2015}; ^{Holley et al., 2016}; ^{Torró et al., 2017}; ^{Sahlström et al., 2018}), it conflicts with geologic evidences as well as with previous geochronologic studies of the Azuero Peninsula (^{Lissinna, 2005}; ^{Buchs et al., 2010}, ²⁰¹¹; ^{Wegner et al., 2011}; ^{Montes et al., 2012}; ^{Corral et al., 2013}, ²⁰¹⁶). Since Cerro Quema stands as the first discovery of a high-sulfidation epithermal Au-Cu deposit in Panama, its correct geologic contextualization, including the formational age and the genetic model, will aid exploration for similar deposits in Panama.

My concern with the manuscript presented by ^{Perelló et al., (2020)} centers on their interpretation of the previous field-based, geochemical and geochronological studies of the Azuero Peninsula, the significance and reliability of the new Re-Os molybdenite ages, and the role of the dacite domes in the gold and copper mineralization. Here, an alternative interpretation to explain the formation of Cerro Quema, consistent with key field evidences and geochronological studies of the Azuero Peninsula, is presented.

2. Geology and geochemistry of the Cerro Quema area

^{Perelló et al., (2020)} present a complete and updated literature review of the geological and geochronological studies carried out in the Azuero Peninsula. However, less attention has been paid to the geology of the Cerro Quema area in particular. Contrary to the geologic relationships reported by ^{Perelló et al., (2020)}, the dacite dome complex is interfingered with volcanic, volcaniclastic rocks and pelagic to hemipelagic limestones (Río Quema Formation upper unit) that sits atop of a ~100 m thick hemipelagic limestone (Río Quema Formation Limestone unit) that in turn sits atop of a ~300 m thick pile of turbidites, siltstones and mudstones (Figure 1; ^{Corral et al., 2011}, ²⁰¹³, ²⁰¹⁶). The Río Quema Formation discordantly overlaps the Azuero Protoarc. Therefore, there is no contact and close relationship between the dacite dome complex and the Azuero Protoarc as they have >500 m of igneous and sedimentary rocks in between.

Figure 1 Stratigraphic section of the Río Quema Formation showing the emplacement of the Cerro Quema Au-Cu deposit (modified from ^{Corral et al., 2016}). 

The Río Quema Formation volcanic and volcaniclastic rocks have been geochemically characterized by ^{Corral et al., (2010}, ²⁰¹¹) ^{and Corral (2013)}. Their results indicate that rocks of the Río Quema Formation have calc-alkaline signatures consistent with their emplacement in an intra-oceanic magmatic arc (^{Lissinna, 2005}; ^{Wörner et al., 2009}; ^{Buchs et al., 2010}, ²⁰¹¹; ^{Wegner et al., 2011}). This contrasts with the interpretation of ^{Perelló et al., (2020)} that the dacite dome complex of the Río Quema Formation was emplaced in a geotectonic setting transitional from mafic, primitive intra-oceanic to more evolved arc magmatism. By the time the dacite dome complex emplaced, the intra-oceanic magmatic arc was well developed and mature enough to be geochemically distinguishable from the Azuero Protoarc and the Azuero Igneous basement.

3. Age of the Cerro Quema host rock

Several hornblende ⁴⁰Ar/³⁹Ar spectra should be considered to cogently constrain the age of the Cerro Quema host rock (Figure 2). The dacite dome complex of the Río Quema Formation is the main mineralization host rock (^{Leach, 1992}; ^{Horlacher and Lehmann, 1993}; ^{Torrey and Keenan, 1994}; ^{Nelson, 1995}; ^{Corral et al., 2011}, ²⁰¹⁶; ^{Perelló et al., 2020}). The volcanic domes have been dated at Cerro Quema by ^{Corral et al.,(2016}; hornblende ⁴⁰Ar/³⁹Ar) obtaining a weighted average date of 66.5 ± 1.6 Ma (Table 1, Figure 2). The ^{Corral et al., (2016)} hornblende ⁴⁰Ar/³⁹Ar total gas age was not included in the calculation as it was considered less reliable than plateau ages. Two amphibole-bearing dacitic rocks located 10 and 5 km from Cerro Quema were dated by ^{Wegner et al., (2011}; hornblende ⁴⁰Ar/³⁹Ar) yielding a 71.0 ± 2.5 Ma weighted average date and a 67.5 ± 1.9 Ma plateau date, respectively (Table 1). Although the sample PAN-05-017A studied by ^{Wegner et al., (2011)} shows a bumpy spectrum between 20 - 40% of cumulative ³⁹Ar fraction that compromises the age, a plateau date of 72.3 ± 2.5 Ma was calculated (Figure 2A). A duplicate sample (PAN-05-017B; Figure 2B) has a disturbed spectrum between 10 - 30% of cumulative ³⁹Ar fraction with no apparent plateau that compromises the reported plateau date of 68.9 ± 3.3 Ma.

Figure 2 Summary of the hornblende ⁴⁰Ar/³⁹Ar dating of the Río Quema Formation dacite domes and equivalent rocks from the Azuero Peninsula (^{Corral, 2021}). A - C: Samples form ^{Wegner et al. (2011)} collected at 10 and 5 km away from Cerro Quema, respectively. D - F: Samples from ^{Corral et al. (2016)} collected at La Pava orebody. Plateau ages of A and B have been discarded, and C to F are the representative ages of the Río Quema Formation (as discussed in the text). 

The final reported age of this sample is an average of the two plateau dates, which is questionable given the nature of their spectra. Therefore, the dacite sample reported by ^{Wegner et al., (2011}; PAN-05-008) and the samples of ^{Corral et al., (2016}; Figure 2C to 2F) dated by ⁴⁰Ar/³⁹Ar on hornblende showing well-defined plateaus with no disturbed spectra, are considered here to be more reliable ages of the Cerro Quema host rocks, constraining their age to 66.6 ± 1.8 Ma (weighted average of plateaus).

These data and geochronological studies of the El Montuoso batholith (e.g., U-Pb in zircon, ⁴⁰Ar/³⁹Ar in hornblende and plagioclase; Table 1; Figure 3), indicate that they were coeval and represent the Late Cretaceous magmatic arc.

Figure 3 Comparative diagram showing the geochronological data of the El Montuoso batholith, the dacites of the Río Quema Formation and the Re-Os molybdenite ages (^{Corral, 2021}). Avg. = average, CQ = Cerro Quema, hbl = hornblende, mo = molybdenite, RQF = Río Quema Formation. 

On the contrary, ^{Perelló et al., (2020)} assumed the 71.0 ± 2.5 Ma date of ^{Wegner et al., (2011)} as the age of the Cerro Quema host rock. They discarded the younger dacite dome complex ages (Figure 2C to 2F; ^{Wegner et al., 2011}; ^{Corral et al., 2016}) interpreting them to be likely compromised by disturbed isotopic systems, including the presence of excess argon and partial to complete resetting and neocrystallization during younger heating events. However, samples from Figure 2C to 2F used to calculate the age of the dacite dome complex of the Río Quema Formation do not reflect any of the abovementioned processes.

4. Age of the Cerro Quema deposit

^{Perelló et al., (2020)} performed Re-Os dating on molybdenite samples from La Pava orebody. Four samples were analyzed yielding dates of 70.74 ± 0.29, 70.70 ± 0.29, 70.66 ± 0.29 and 68.72 ± 0.29 Ma being the youngest date discarded by the presence of significant common Os.

They presented a good and consistent data establishing ~71 Ma as the age of the Cerro Quema mineralization. However, their results conflict with the age range of 55 - 49 Ma proposed in ^{Corral et al., (2016)}, and the ~49 Ma alunite ⁴⁰Ar/³⁹Ar date of ^{Corral (2021)}. Considering the 66.6 ± 1.8 Ma age of the Cerro Quema host rock, the molybdenite Re-Os age of ~71 Ma is considerable older than the host rocks (Figure 3).

If as mentioned in ^{Perelló et al., (2020)}, the younger hornblende ⁴⁰Ar/³⁹Ar dates of ^{Wegner et al., (2011)} and ^{Corral et al.,(2016)} are compromised by resetting, their molybdenite Re-Os ages would also be compromised by the same process as the closure temperature of hornblende ⁴⁰Ar/³⁹Ar and molybdenite Re-Os isotopic systems is the same, ~500 °C (^{Harrison, 1982}; ^{Suzuki et al., 1996}). Other processes that may compromise the Re-Os systematics are surface weathering (e.g., oxidation) and alteration by low salinity (< 1 %NaCl eq.) and low temperature (~180°C) hydrothermal fluids containing NaCl and/or CO2 (McCandless et al.,1992; ^{Suzuki et al., 2000}; ^{Georgiev et al., 2012}; ^{Stein and Hannah, 2014}). According to the location of molybdenite samples studied by ^{Perelló et al., (2020)}, some of them are likely to be affected by weathering processes as they are in the range of the weathering profile that has affected Cerro Quema to depths of 150 m (^{Corral et al., 2011}, ²⁰¹⁶, ²⁰¹⁸).

However, no further speculation can be done to explain the anomalous old molybdenite Re-Os ages as molybdenite occurrence at Cerro Quema is only documented by two pictures of the same hand sample, with no supporting optical microscope or SEM-BSE images by ^{Perelló et al., (2020)}.

Although most of the alunite ⁴⁰Ar/³⁹Ar dating studies considered the obtained age as representative of the age of hypogene mineral precipitation (^{Rye et al., 1992}; ^{Itaya et al., 1996}; ^{Bissig et al., 2001}; ^{Holley et al., 2016}; ^{Sahlström et al., 2018}), alunite dating problems may arise due to thermal resetting during deformation and/or magmatism (e.g., Pueblo Viejo, ^{Arribas et al., 2011}; Mt. Carlton, ^{Sahlström et al., 2018}). However, ^{Corral (2021)} could not prove the hypothesis of resetting. If resetting could be proven, the obtained alunite ⁴⁰Ar/³⁹Ar age would be interpreted as a minimum age of Cerro Quema.

In this case, the maximum age of Cerro Quema is that of the host rocks: 66.6 ± 1.8 Ma (^{Wegner et al., 2011}; ^{Corral et al., 2016}). Therefore, the ~49 Ma alunite ⁴⁰Ar/³⁹Ar date of ^{Corral (2021)} is interpreted as representative of the age of hypogene mineral precipitation (at ~240 °C; ^{Corral et al., 2017}).

5. Alternative genetic model for Cerro Quema

Subduction initiation of the Farallon plate beneath the Caribbean plate and ensuing protoarc volcanism are dated at ~75 - 73 Ma (i.e., late Campanian; ^{Buchs et al., 2010}). From Late Cretaceous to middle Eocene (68 - 40 Ma), the first stage of magmatism occurred within the Caribbean plate.

This stage is characterized by the development of the Soná-Azuero (Western Panama) and the Chagres-Bayano (Eastern Panama) arcs, which record abundant tholeiitic to calc-alkaline arc magmatism and emplaced on an oceanic plateau crust (Lissina, 2005; ^{Buchs et al., 2010}; ^{Wörner et al., 2009}; ^{Wegner et al., 2011}; ^{Corral et al., 2011}; ^{Montes et al., 2012}). El Montuoso batholith constitutes the main arc-related plutonic rocks of the Soná-Azuero arc, whereas the dacite domes of the Río Quema Formation emplaced in its forearc basin at ~67 Ma (^{Del Giudice and Recchi, 1969}; ^{Kesler et al., 1977}; ^{Wegner et al., 2011}; ^{Montes et al., 2012}; ^{Corral et al., 2013}, ²⁰¹⁶; ^{Ramírez et al, 2016}). During the lower Eocene (~55 - 49 Ma), diorites and quartz-diorites associated with the Valle Rico batholith intruded the Soná-Azuero arc along E-trending regional faults (^{Del Giudice and Recchi, 1969}; ^{Lissinna, 2005}; ^{Montes et al., 2012}; ^{Corral et al., 2016}). These E-trending structures and contacts around the dacite dome complex probably served as conduits for magmatic-hydrothermal fluids derived from porphyry copper-like intrusions associated with the Valle Rico batholith to form the Cerro Quema Au-Cu deposit (^{Corral et al., 2011}, ²⁰¹⁶; ^{Corral, 2021}).

The dacite domes played a key role on the Au-Cu mineralization as they are the most permeable and porous rocks cropping out in the area, facilitating the mineralizing fluid flow preferentially through this lithology (permeability: 2.0 10- 11 kg/s*mPa; porosity: 10.8%; ^{Corral et al., 2017}). After this tectono-magmatic event, the magmatic arc migrated ~50 km to the north, where it is represented by the Parita batholith (48 - 36 Ma; ^{Lissinna, 2005}; ^{Montes et al., 2012}; ^{Corral et al., 2016}; ^{Ramirez et al., 2016}). Additionally, this genetic model mirrors the metallogenic evolution of the Chagres-Bayano arc (Eastern Panama) where diorites and quartz-diorites intruded Cretaceous volcanosedimentary sequences at ~49 Ma forming the Río Pito porphyry copper deposit (^{Kesler et al., 1977}; ^Nelson,1995).

This alternative model is consistent with the Azuero Peninsula geology and geochemistry, as well as with previous geochronological studies (U-Pb zircon, ^{Montes et al., 2012}; ^{Ramirez et al., 2016}; ⁴⁰Ar/³⁹Ar hornblende and plagioclase, ^{Lissinna, 2005}; ^{Corral et al., 2016}; K-Ar hornblende and plagioclase, ^{Del Giudice and Recchi, 1969}; ^{Kesler et al., 1977}). It differs from the genetic model proposed by ^{Perelló et al., (2020)} that invokes a previously unrecognized regional-scale magmatic event to explain the mineralization at Cerro Quema based on their molybdenite Re-Os ages.

6. Reinforcement of the Cerro Quema genetic model

Further geochronological work would help to improve the Cerro Quema genetic model. U-Pb dating of zircons from the Cerro Quema host rock (e.g., LP-204; ^{Corral et al., 2016}) will help to better constrain its age and therefore, provide insightful information to validate the proposed genetic models. This is an opportunity for a collaborative work between the two groups that worked on the Cerro Quema deposit to propose a more robust genetic model based on further geochronological data.

7. Conlcusions

After a thorough review of the geologic, geo-chemical and geochronological data form the Cerro Quema area, it is concluded that the Re-Os molybdenite dates reported by ^{Perelló et al., (2020)} are not representative of the formational age of Cerro Quema. The geology interpreted by ^{Perelló et al., (2020)} differs from the geology reported by field-based studies, especially when describing the stratigraphic relationship between the Azuero Protoarc unit and the dacite dome complex of the Río Quema Formation. They also invoke a previously unrecognized regional-scale magmatic event based on their molybdenite Re-Os ages. Furthermore, their proposed formational age at ~71 Ma is significantly older than the age of the hosting dacite dome complex (~67 Ma). Instead, the Cerro Quema formational model discussed here, in which magmatic-hydrothermal fluids derived from porphyry copper-like intrusions associated with the Valle Rico batholith produced the Au-Cu mineralization at ~49 Ma, is consistent with the geology, geochemistry and geochronology of the Azuero Peninsula.