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Late Eocene biostratigraphic age for andesitic volcanic host rocks formed in an island environment at the Cobre Panama porphyry Cu-Mo-Au-Ag deposit, Panama

Redwood, Stewart D., Bryan, Jonathan R., Buchs, David M. ORCID: and Burge, Colin 2021. Late Eocene biostratigraphic age for andesitic volcanic host rocks formed in an island environment at the Cobre Panama porphyry Cu-Mo-Au-Ag deposit, Panama. Economic Geology and the Bulletin of the Society of Economic Geologists 116 (1) , 199–209. 10.5382/econgeo.4782
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The giant Cobre Panama porphyry Cu-Mo-Au-Ag deposit in western Panama is hosted by an undated andesitic volcanic sequence, the Petaquilla batholith (32.20 ± 0.76–28.28 ± 0.61 Ma), and porphyry stocks (28.96 ± 0.62–27.48 ± 0.68 Ma). Here we present a biostratigraphic age for the volcanic sequence based on stratigraphically diagnostic large foraminifera from thin limestone beds within kilometer-thick andesitic rocks. These yield a late middle to late Eocene biostratigraphic age (41.2–33.9 Ma), with a probable late Eocene age (Priabonian stage, 37.8–33.9 Ma), which is slightly older than the age of the batholith and porphyry intrusions. The volcanic sequence is dominated by fine-grained, massive basalt to andesite lavas with subordinate volcaniclastic deposits. A preliminary description of volcanic textures based on macroscopic observation of drill core and quarry/road exposures supports the occurrence of lavas, fallout tuffs, volcanic breccias, and possible pyroclastic density current deposits. Rare polymictic conglomerates with well-rounded clasts of igneous rocks attest to minor sedimentary reworking from a nearby subaerial volcanic environment. The dated limestone that is interbedded with the submarine volcanic sequence was deposited in an estimated water depth of 50 to 80 m, probably in a middle- to outer-shelf large foraminiferal shoal. These results support deposition on the flank of an active volcanic island during early shallowing of the Isthmus of Panama. The Cobre Panama volcanic center is interpreted to have formed in the final stages of the latest Cretaceous-Eocene volcanic arc before, or possibly during, the 175-km sinistral offset of the Panama volcanic front in the late Eocene-Oligocene. However, it remains unclear whether the volcanic center formed on the western continuation of the San Blas-Chagres arc segment or the eastern termination of the Azuero-Soná arc segment and whether it was emplaced during broadening of the pre-Oligocene volcanic front or in a back-arc setting.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Publisher: Society of Economic Geologists
ISSN: 1554-0774
Date of First Compliant Deposit: 17 November 2021
Date of Acceptance: 22 April 2020
Last Modified: 18 Nov 2023 17:44

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