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1. Age of the Cerro Quema dacite dome complex
There is ample evidence in the geologic literature of the region to suspect that the available Ar-based geochronological data should be treated cautiously (e.g., Lissinna, 2005; Wegner et al., 2011; Buchs et al, 2011a, b; Corral et al., 2016), a situation that finds further support in the analysis conducted by Corral (2020, Figure 3). Our selection of 71 Ma as the age of the Cerro Quema dacite dome was not arbitrary, but rather based on the recommendation by Wegner et al. (2011) for hornblende-bearing dacite rocks in the vicinity of the prospect area. This date has also been amply used in the literature to record one of the first vestiges of arc magmatism in the Central American Arc (e.g., Wörner et al., 2009; Wegner et al., 2011; Buchs et al., 2010, 2011a; Whattam et al., 2020 and references therein), including Corral et al. (2016).
Corral (2020) argues that this date is dubious and unreliable on analytical grounds, and that the age of the dome complex is younger, between ~65 and 68 Ma. However, his Figure 3 shows a wider range of acceptable ages, from 64 to ~70 Ma, and Corral et al. (2016) also accepted a ~71-66-Ma range for the same unit. According to Corral (2020), his new preferred range, and the U-Pb zircon ages of 68 to 66 Ma for the El Montuoso batholith (Montes et al., 2012; Ramírez et al., 2016), would make both the Cerro Quema domes and the El Montuoso batholith coeval with and part of the Azuero Arc volcano-plutonic complex. Although this interpretation may be correct in broad terms, regional geologic maps show the El Montuoso batholith to be transgressive to lower Azuero Arc stratigraphy, including the Río Quema Formation, and its structural grain (Buchs et al., 2011a; Corral et al., 2016), making such correlation unlikely. A simple alternative explanation for the similar 40Ar-39Ar and U-Pb dates referred to by Corral (2020) would be that all 40Ar-39Ar ages are compromised by the heating of the El Montuoso and the younger Valle Rico batholiths, as suggested in our paper and in consideration of the regional evidence, noting that single-mineral, Ar-based chronometers reflect the closure of the clock of the dated mineral at specific temperatures, rather than primary crystallization (e.g.,Stein, 2014 and references therein). In this respect, given the high sensitivity of the 40Ar-39Ar clock to thermal metamorphism, the younger 40Ar-39Ar ages can be better interpreted in the context of the thermal and uplift history of the region during emplacement of the El Montuoso batholith (e. g.,Lee, 2009). Clearly, precise U-Pb zircon dates for the Cerro Quema dacite domes are required, an aspect with which we fully agree with Corral (2020).
2. Age of the Cerro Quema mineralization
Corral (2020) states that our Re-Os molybdenite dates of 70.74 ± 0.29, 70.70 ± 0.29, and 70.66 ± 0.29 Ma for La Pava are older than his preferred age of 66.6 ± 1.8 Ma for the Cerro Quema dacite domes that host the mineralization, and claims that our molybdenite data can be compromised by: 1) thermal resetting; 2) alteration due to low-salinity and low-temperature hydrothermal fluids; and 3) the action of supergene fluids related to near-surface weathering processes.
Corral (2020) suggests that the three overlapping Re-Os molybdenite ages of 70.7 Ma are anomalously old as a result of weathering processes affecting the Re-Os system in molybdenite. Independently of the fact that our samples are entirely devoid of supergene sulfide and silicate mineralogy (Perelló et al., 2020; Figure 1), and that these younger events would have to remove an exact amount of Re from each sample to yield three identical inaccurate ages, this suggestion is counter to the current understanding of the Re-Os system in molybdenite, which is well summarized by Stein (2014). The salient points regarding development, testing and calibration of the Re-Os molybdenite geochonometer are that, prior to a series of major advances from ~1997 to 2007, Re-Os dating of molybdenite generally yielded erratic and imprecise ages resulting from low-precision isotopic analysis using ICP-MS and lack of knowledge regarding 'decoupling' of Re from radiogenic Os internally within molybdenite (e.g.,Stein, 2014). As an example, using the same analytical methods employed by Suzuki et al. (2000), as cited by Corral (2020) as support for anomalous Re-Os ages), Suzuki et al. (2001) concluded that inaccurate and imprecise Re-Os molybdenite ages from the Galway Granite in Ireland resulted from younger hydrothermal fluid interaction. However, Selby et al. (2004) showed that using different analytical protocols, reproducible and geologically plausible Re-Os molybdenite ages were produced from the same molybdenite samples. The citation for Re-loss in molybdenite by McCandless et al. (1993) similarly suffers from the issues noted above, as does the assumption that comparison ages, mostly K-Ar, are correct (e.g., Stein, 2014). Georgiev et al. (2012) studied Re and Os release from organic-rich shales, in which Re is hosted almost exclusively in organic matter, and is known to be readily leached during laboratory experiments (Selby and Creaser, 2003), but this topic is not relevant to Re-Os geochronology in molybdenite. Modern Re-Os molybdenite geo-chronology in porphyry systems typically yields accurate, precise ages which are well-correlated to the U-Pb ages of causative intrusions (Selby et al., 2007; Rosera et al., 2013; Li et al., 2017), and in many cases older than local and regional 40Ar-39Ar and K-Ar ages (e.g., Selby et al., 2002).
Figure 1 Textural aspects of the molybdenite-bearing mineralization from the La Pava area at Cerro Quema. a. Hand specimen showing intensely sericite-pyrophyllite altered Cerro Quema dacite with ill-defined veinlets of pyrite, enargite and molybdenite plus disseminations of molybdenite. b. Binocular view of an internal face of a veinlet with quartz, sericite, and pyrophyllite plus pyrite, molybdenite, and enargite. Note the complete absence of supergene silicate and sulfide mineralogy in these samples. Abbreviations: en: enargite; mo: molybdenite; py: pyrite.
Similarly, the problems associated with thermal resetting are nearly irrelevant to Re-Os molybdenite geochronology (Stein, 2014), while the complexities of the temperature-dependent 40Ar-39Ar method in the ore-forming environment were clearly evidenced by the work of Arribas et al. (2011) at Pueblo Viejo, Dominican Republic (see also Nelson et al., 2015 and Torró et al., 2017). Corral (2020) defends a new single 40Ar-39Ar alunite date of ~49 Ma obtained by him from a pyrite-alunite cemented breccia at Cerro Quema, and claims he did not detect any evidence of resetting in his samples. However, neither did Arribas et al. (2011), despite their ultra-detailed work on hypogene alunite at Pueblo Viejo and neighboring alteration zones. Furthermore, Arribas et al. (2011) concluded that the wide variability of the obtained ages, ranging from 70 to 40 Ma, was independent of the many textural and compositional features of the dated alunite, but it rather seemed to depend on "the actual 40Ar-39Ar geochemistry of the alunite, with individual alunite crystals or crystal groupings yielding different ages" (Arribas et al., 2011, p. 1965).
Interestingly, the wide variability of hypogene alunite ages determined at Pueblo Viejo and many other deposits and prospects (e. g.,Arribas et al., 2011 and references therein), can also be anticipated to constitute a problem at Cerro Quema, where three alunite separates from a high-sulfidation vein of coarse-grained alunite intimately intergrown with pyrite and enargite from La Pava yielded K-Ar ages of 62 ± 2, 58.0 ± 1.9, and 62 ± 3 Ma (J. Perelló, unpub. data). Irrespective of the Ar-based method, these and Corral's new data imply an age range from ~65 to 49 Ma for Cerro Quema hypogene alunite, and further confirm the complicated nature of the Ar systematics of alunite and support the interpretation of a complex thermal history for the region.
Rocks of the Azuero peninsula expose a complicated pattern of accretion, magmatism, and subduction during the development of the Central American margin (e. g.,Buchs, 2008), hence major regional thermal events that may have played a role in the formation and resetting of alunite at Cerro Quema are numerous. These include not only the thermal heating associated with the intrusion of the El Montuoso and Valle Rico batholiths mentioned above, but also additional post-mineralization arc magmatism, the ~2 km stratigraphic load of volcano-sedimentary and sedimentary sequences that once must have covered the prospect area and the region, regional events of tectonic accretion and associated metamorphism, tectonically induced uplift and erosion, and far-field strain accommodation associated with large-scale subcontinental collisions (Kolarsky et al., 1995; Wörner et al., 2009; Buchs et al., 2010, 2011a, b; Montes et al., 2012; Ramírez et al., 2016; Andjić et al., 2018; Perelló et al., 2020).
3. Conclusion
In conclusion, we interpret the ~71 Ma age of our 3 molybdenite samples as meaningful and that the Re-Os molybdenite method accurately dates the hypogene ore-stage mineralization at Cerro Quema. Both the Re-Os and Ar-based methods point to a more complex magmatic and mineralization history at Cerro Quema that is either currently understood, or as presented by Corral (2020). In this context, the ~71-Ma age of the hypogene mineralization at Cerro Quema effectively constrains the metallogenic birth of the Central American Arc.
