Age and geochemistry of host rocks of the Cobre Panama porphyry Cu–Au deposit, central Panama: Implications for the Paleogene evolution of the Panamanian magmatic arc

Abstract

The Cobre Panama porphyry Cu–Au deposit, located in the Petaquilla district of central Panama, is hosted by a sequence of medium- to high-K calc-alkaline volcanic and sub-volcanic rocks. New crystallisation ages obtained from a granodiorite Petaquilla batholith and associated mineralised diorite to granodiorite porphyry stocks and dikes at Cobre Panama indicate that the batholith was emplaced as a multi-phase intrusion, over a period of ~4million years from 32.20±0.76Ma to 28.26±0.61Ma, while the porphyritic rocks were emplaced over a ~2 million year period from 28.96±0.62Ma to 27.48±0.68Ma.Both the volcanic to sub-volcanic host rocks and intrusive rocks of the Cobre Panama deposit evolved via fractional crystallisation processes, as demonstrated by the major elements (e.g. Al2O3, Fe2O3, TiO2 and MgO) displaying negative trends with increasing SiO2. The Petaquilla intrusive rocks, including the diorite–granodiorite porphyries and granodiorite batholith, are geochemically evolved and appear to have formed from more hydrous magmas than the preceding host volcanic rocks, as evidenced by the presence of hornblende phenocrysts, higher degrees of large-ion lithophile element (LILE) and light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion, and higher Sr/Y and La/Yb values. However, the degree of LREE enrichment, HREE depletion and La/Yb values are insufficient for the intrusive rocks to be considered as adakites. Collectively, the volcanic and intrusive rocks have LILE, REE and mobile trace element concentrations similar to enriched Miocene-age Cordilleran arc magmatism found throughout central and western Panama. Both the Petaquilla and Cordilleran arc magmatic suites are geochemically more evolved than the late Cretaceous to Eocene Chagres–Bayano arc magmas from northeastern Panama, as they display higher degrees of LILE and LREE enrichment. The geochemical similarities between the Petaquilla and Cordilleran arc magmas suggest that evolved calc-alkaline arc magmatism may extend to the late Eocene, at least ~10million years earlier than previously estimated.The crystallisation ages for intrusive rocks associated with mineralisation at Cobre Panama imply that the deposit formed in the early Oligocene, between a period of late Cretaceous to Eocene magmatism (ca. 66–42Ma; Chagres–Bayano arc) and Cordilleran arc magmatism (22–7Ma). Similarities in the timing of intrusive suite emplacement and the fingerprinting of magmatic fractionation processes between the Cobre Panama porphyry deposit and the Cerro Colorado porphyry deposit in western Panama (ca. 5.3Ma) suggest that these features provide favourable geodynamic and geochemical prerequisites for the formation of porphyry deposits along the Panamanian magmatic arc during the Cenozoic.