Comment to "Porphyry-related high-sulfidation mineralization early in Central American Arc Development: Cerro Quema deposit, Azuero Peninsula, Panama" by Perelló et al., (2020)

The Cerro Quema Au-Cu deposit is hosted by a dacite dome complex of the Río Quema Formation, a Late Campanian-Maastrichtian volcano-sedimentary sequence of the Panamanian magmatic arc. Its formational age is constrained at ~49 Ma by field evidences, crosscutting relationships and 40Ar/39Ar geochronology (Corral et al., 2016, Corral, 2021). The recent molybdenite Re-Os dates by Perelló et al. (2020) claim that ore is spatially and temporally related to the host volcanic domes at ~71 Ma. After a thorough review of the geologic, geochemical and geochronological data from the Cerro Quema area, it is concluded that the Re-Os dates of Perelló et al. (2020) are not representative of the Cerro Quema formational age. Their proposed formational age at ~71 Ma is significantly older than the age of the host rock (~67 Ma). Furthermore, they invoke a previously unrecognized regional-scale magmatic event solely based on their molybdenite Re-Os dates. Instead, the Cerro Quema genetic 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.

Reply to “Porphyry-related high-sulfidation mineralization early in Central American Arc development: Cerro Quema deposit, Azuero Peninsula, Panama” by Corral (2021)

Dear Editor, we thank Corral (2020) for his anticipated interest in our paper on the timing of the porphyry-related high-sulfidation epithermal mineralization at Cerro Quema in the Azuero peninsula of southwestern Panama. Our study, based on three Re-Os ages for molybdenite intimately associated with Cu-bearing sulfide minerals from the hypogene roots of the La Pava center (Figure 1), shows that the main event of high-sulfidation Cu mineralization took place during the earliest Maastrichtian at ~71 Ma. The reported ages, together with the geologic relationships described in our paper (Perelló et al., 2020), plus a series of regional geologic, structural, petrochemical, and geotectonic considerations, not only precisely date the porphyry-related nature of the Cerro Quema high-sulfidation mineralization, but are also significant in that they confirm the rapid evolution of the earliest stages of the Central American Arc – from subduction initiation at 75-73 Ma to arc stability and maturation at 71 Ma (e.g., Buchs et al., 2011a and references therein) – and place the mineralization in a regional geodynamic setting. Irrespective of the regional geologic arguments reiterated by Corral (2020) in support of his previous genetic interpretation (e.g., Corral et al., 2016) and to invalidate our conclusions, Corral´s real concern is the reliability of our molybdenite ages, which are much older than his preferred age of mineralization for Cerro Quema. We believe that many of the points raised by Corral (2020), including the regional and local geologic backgrounds of the deposit and the dated samples, were properly addressed in Perelló et al. (2020), and that it would be redundant to repeat them here. Additional petrochemical evidence in support can be found in Whattam and Stern (2015, 2020) and Whatam (2018).

Porphyry-related high-sulfidation mineralization early in Central American Arc development: Cerro Quema deposit, Azuero Peninsula, Panama

The 70.74 to 70.66 Ma age range for three molybdenite samples accompanying pyrite- and enargite-bearing assemblages effectively constrains an earliest Maastrichtian age for the high-sulfidation Au-Cu mineralization at Cerro Quema, Panama. The epithermal system was contemporaneous with emplacement of a composite dacite dome complex in a geotectonic setting transitional from mafic, primitive intraoceanic (Azuero Protoarc) to more evolved island arc magmatism (Azuero Arc), during initial construction of the Central American land bridge at the trailing edge of the Caribbean Large Igneous Province (CLIP). The molybdenite ages confirm the rapid evolution of the earliest stages of the Central American Arc, from subduction initiation at 75–73 Ma to arc maturation at 71 Ma. A porphyry connection is apparent at Cerro Quema and characterized by highly contorted, banded, and planar quartz-veinlet stockworks and sheeted zones in pyrophyllite- and sericite-bearing patchy-textured rock. These are cut by ledges of quartz, alunite, and dickite, which implies overprinting of the advanced argillic lithocap onto the underlying porphyry environment. Hydrothermal telescoping resulted from synmineralization uplift congruent with an actively emerging volcanic arc, which the Re-Os molybdenite dates accurately constrain at 71 Ma, presumably as a far-field effect of collision between the leading edge of the CLIP with parts of North and South America.

Rhyolite ignimbrite boulders and cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—age, composition, transport, and stratigraphic setting

A stratigraphic layer containing rhyolite cobbles and boulders in the Middle Jurassic Carmel Formation of southern Utah represents a singular, unusual event in the otherwise low-energy sedimentation of this formation. A laser-fusion, single-crystal 40Ar/39Ar age of 171.73 ± 0.19 Ma obtained from sanidine in one of the clasts is about 8 m.y. older than a zircon U-Pb age obtained on a fallout tuff from the sediments surrounding the clasts (163.9 ± ~3.3 Ma). The volcanic clasts are poorly-welded rhyolite ignimbrites that may have been deposited as much as 200 km from the eruptive center, perhaps along pre-existing valleys. The tuff deposits then remained in place for several million years during which time they were subjected to weathering, alteration, and perhaps topographic inversion, creating mesas capped with tuff underlain by soft Middle Jurassic silt and mud. Triggered by unusual rainfall or earthquakes, debris flows carried the clasts a few 10s of kilometers from their outcrops to the depositional site. Earlier work proposed that the Middle Jurassic arc was a low-standing, arc-graben. If this was the case, then the tectonic setting was likely similar to the modern Central American arc in the vicinity of Nicaragua where tuffs erupted from a low-standing arc deposited onto an adjacent highland and were then eroded by streams flowing to the east onto a fluvial plain that is near the sea.

A Calcium-in-Olivine Geohygrometer and its Application to Subduction Zone Magmatism

High-precision electron microprobe analyses were obtained on olivine grains from Klyuchevskoy, Shiveluch and Gorely volcanoes in the Kamchatka Arc; Irazú, Platanar and Barva volcanoes of the Central American Arc; and mid-ocean ridge basalt (MORB) from the Siqueiros Transform. Calcium contents of these subduction zone olivines are lower than those for olivines from modern MORB, Archean komatiite and Hawaii. A role for magmatic H2O is likely for subduction zone olivines, and we have explored the suggestion of earlier workers that it has affected the partitioning of CaO between olivine and silicate melt. We provide a provisional calibration of DCaOOl/L as a function of magmatic MgO and H2O, based on nominally anhydrous experiments and minimally degassed H2O contents of olivine-hosted melt inclusions. Application of our geohygrometer typically yields 3–4 wt % magmatic H2O at the Kamchatka and Central American arcs for olivines having ∼1000 ppm Ca, which agrees with H2O maxima from melt inclusion studies; Cerro Negro and Shiveluch volcanoes are exceptions, with about 6% H2O. High-precision electron microprobe analyses with 10–20 μm spatial resolution on some olivine grains from Klyuchevskoy and Shiveluch show a decrease in Ca content from the core centers to the rim contacts, and a sharp increase in Ca in olivine rims. We suggest that the zoning of Ca in olivine from subduction zone lavas may provide the first petrological record of temporal changes that occur during hydration of the mantle wedge and dehydration during ascent, and we predict olivine H2O contents that can be tested by secondary ionization mass spectrometry analysis.