Back arc Setting for the Middle Eocene Çöpler porphyry epithermal gold deposit Turkey

The Çöpler epithermal Au deposit and related subeconomic porphyry Cu-Au deposit is located in central eastern Turkey, about 120 km west of the city of Erzincan. Porphyry-epithermal Au-Cu mineralization is hosted by the middle Eocene Çöpler-Kabataş magmatic complex which has been emplaced into a succession of Permian–Cretaceous metasedimentary and carbonate rocks within an ENE-trending structural window exposed in the northeastern margin of the Tauride-Anatolide Block. 40Ar/39Ar dating of three samples of igneous biotite and/or hornblende from least altered intrusive rocks from the deposit yielded plateau ages between 44.13 ± 0.38 and 43.75 ± 0.26 Ma. 40Ar/39Ar ages of hydrothermal biotite (43.84 ± 0.26 Ma) and sericite (44.44 ± 0.28 Ma), and Re-Os ages from two molybdenite samples (43.9 ± 0.2 Ma and 44.6 ± 0.2 Ma) broadly overlap with the 40Ar/39Ar ages of intrusive phases suggesting a life span of ≤1 m.y. for the magmatic-hydrothermal system at Çöpler. The Çöpler-Kabataş intrusive rocks range in composition from granodiorite to diorite, and are characterized by abundant phenocrystic plagioclase, and hornblende ± biotite, set in a fine-grained quartzofeldspathic groundmass. Whole-rock major and trace element compositions of these intrusive lithologies are characteristic of magmatic rocks that are formed in continental arcs in that they are medium- to high-K calc-alkaline with enrichments in large ion lithophile and light rare earth elements, and relative depletions in medium and heavy rare earth elements. Based on new geochemical and geochronological data, the middle Eocene calc-alkaline magmatism at the Çöpler-Kabataş magmatic complex is interpreted to have formed in a back arc setting with respect to the contemporaneous Helete volcanic arc in southeast Anatolia. The middle Eocene magmatism in the region was developed in response to the roll-back of the southern Neotethys slab in the Paleocene. This later facilitated extension in the upper plate and northward broadening of magmatism mainly along deep-seated, ENE-trending transcurrent fault systems. The rather unusual tectonic setting of the Çöpler Au deposit is analogous to some other Au-rich back-arc porphyry deposits such as Bajo de la Alumbrera (Argentina) and Bingham Canyon (USA)