GEOLOGY, GEOCHEMISTRY, AND GEOCHRONOLOGY OF THE ÇöPLER PORPHYRY-EPITHERMAL GOLD DEPOSIT, CENTRAL EASTERN TURKEY

The Çöpler deposit is located about 120 km west of the city of Erzincan in central eastern Turkey, and it consists of a subeconomic porphyry-type Cu-Au mineralization overlain by paragenetically later low-sulfidation epithermal-style Au mineralization. The porphyry-epithermal mineralization is spatially related to middle Eocene intrusive rocks, which have been emplaced into a succession of Permian-Cretaceous metasedimentary and carbonate rocks within an ENE-WSW-trending structural window exposed in the northeastern part of the Tauride-Anatolide orogenic block.

The intrusive suite at Çöpler consists of different phases of diorite porphyry which contain phenocrysts of plagioclase and hornblende ± biotite, set in a groundmass of quartz and plagioclase. Major and trace element data suggest an I-type calc-alkaline magmatic affinity for the diorite porphyries with strong resemblance to magmatic rocks formed in continental arc settings.

⁴⁰Ar/³⁹Ar dating of fresh hornblende and biotite phenocrysts from diorite porphyries yielded ages between 43.75 ± 0.26 Ma and 44.13 ± 0.38 Ma. ⁴⁰Ar/³⁹Ar ages of hydrothermal biotite (43.84 ± 0.26 Ma) and sericite (44.44 ± 0.28 Ma), combined with Re-Os ages from two molybdenite samples (43.9 ± 0.2 Ma and 44.6 ± 0.2 Ma) reveal that the copper-gold mineralizing hydrothermal system was coeval with magmatism. The short life span (≤1 m.y.) of the magmatic-hydrothermal events at Çöpler is consistent with age relationships at similarly-sized porphyry-epithermal systems globally.

Although weakly developed, the porphyry-style mineralization at Çöpler displays characteristic alteration patterns of classical porphyry Cu systems, with a central zone of potassic alteration surrounded by successive zones of phyllic and propylitic alteration. Early potassic alteration is mainly developed in the intrusive rocks, and is characterized by the presence of secondary biotite, and lesser amounts of K-feldspar, and magnetite in close association with quartz ± magnetite ± chalcopyrite ± pyrite ± molybdenite veins, whereas phyllic alteration is characterized by an assemblage of sericite-quartz ± pyrite, and is commonly associated with quartz ± pyrite veinlets.

Superimposed epithermal-style mineralization, on the other hand, is mainly localized in zones of carbonate alteration, and occurs either as carbonate-sulfide veinlets or as manto-type replacement bodies of sulfides along the basal contact of the overlying Munzur limestone. In epithermal assemblages, gold appears to be mainly in solid solution within arsenical pyrite, which is accompanied by lesser arsenopyrite, chalcopyrite, tennantite/tetrahedrite, galena, sphalerite, realgar, and orpiment. Supergene oxidation of the sulfide mineralization following uplift and unroofing of the Çöpler window was critical for the formation of high grade and easily processed gold mineralization at Çöpler.

New geochemical and geochronological data presented in this study demonstrate that the intrusive rocks at Çöpler were formed through remelting of previously subduction-modified lower crustal sources, following Cretaceous arc magmatism and Paleocene continent-continent collision along the northern Neo-Tethys suture in Anatolia. Thus, the Çöpler deposit may belong to the recently recognized group of postsubduction porphyry and epithermal deposits, which tend to be relatively Au-rich compared to their arc-related counterparts.