Episodic porphyry Cu (-Mo-Au) formation and associated magmatic evolution in Turkish Tethyan collage

Abstract

Continental collision between Eurasia and Afro-Arabia in Turkey along the Pontide subduction zone, Izmir-Erzincan-Ankara suture zone, Bitlis-Zagros suture zone, and Aegean subduction zone generated a fertile metallogenic environment with calc-alkalic and alkalic porphyry Cu (-Mo-Au) and magmatic hydrothermal systems emplaced in narrow arc segments over limited time durations within major magmatic events in the late Cretaceous to late Miocene. The major magmatic events represent multiple subduction zones that gradually stepped from north to south as small oceans closed during collisional events and the trench retreated or shifted southward as a result. Based on geological, new geochemical, and Ar-40/Ar-39, U-Pb LA-ICP-MS geochronological data presented herein along with published data for igneous and hydrothermally altered rocks, four metallogenic episodes are defined; late Cretacecous, early-late Eocene, late Oligocene- early Miocene, and middle-late Miocene, in four geographically distinct regions, the Pontides in the north, the western Anatolian province in the west, central Anatolian Crystalline Complex, and the Southeastern Anatolian Orogenic Belt in the east central part of Turkey. Within those regions, porphyry Cu and intrusion-related hydrothermal systems begin in the late Cretaceous to the north, and generally young southward. They begin as porphyry Cu-Mo type, and tend to be more Au-rich from late Cretaceous to late Miocene. Published and new ages indicate early-late Cretaceous magmatic rocks in the Pontides were emplaced between ca. 131 and 65 Ma, peaking between ca. 88-76 Ma. Apart from the Re-Os molybdenite ages, published and new ages for porphyry Cu-related rocks and hydrothermal minerals from the Ayder, Konak, Esendal, Borekli, and Derekoy prospects indicate porphyry Cu formation near the end of that magmatic event (ca. 83-76 Ma). Published data indicate a somewhat shorter late Cretaceous magmatic event (86.5 to 70.3 Ma) peaking at ca. 86 to 80 Ma at the southeastern Anatolian orogenic belt with porphyry Cu systems reported to have formed contemporaneously. The latest Paleocene to early-late Eocene magmatism and porphyry-style hydrothermal systems are common across Turkey, with porphyry Cu systems formed in the eastern Pontides (58-37.9 Ma) based on the ages of their host rocks and syn-mineral stocks; in the western Anatolian province in the Tav artli porphyry belt (ca. 49.1-46.7 Ma) and the Biga porphyry belt (40.1-39.5 Ma), and in the Southeastern Anatolian orogenic belt (ca. 47 to 43 Ma). Oligocene-Miocene magmatism (30-18 Ma) and associated hydrothermal systems dominate during ca. 27.4 to 24.8 Ma the western Anatolian province in the Biga porphyry belt, but appear less common in other parts of Turkey except for the Cevizlidere porphyry deposit in the Southeastern Anatolian orogenic belt. Middle-late Miocene magmatism (ca. 16-8 Ma) and associated porphyry Cu systems predominate within the Afyon-Konya porphyry belt in western Anatolia, and contains the youngest porphyry related magmatic and hydrothermal systems (14.4-8.9 Ma). Calc-alkalic porphyry Cu +/- Mo-Au systems dominate all belts except the Afyon-Konya porphyry belt where alkalic porphyry Au-Cu systems are present. The porphyry systems are dominantly metaluminous and medium to high-K rocks with a range in composition from diorite to granite, with monzonite and syenite associated with the alkalic porphyry systems. Late Cretaceous rocks have typical arc magma compositions except the Central Anatolian Crystalline complex, whereas Eocene to Miocene collisional to post-collisional rocks have compositional characteristic of arc magmas that over time interacted and assimilated by ancient crust and evolved toward slightly higher Sr/Y and La/Yb trace element characteristics. The trace element compositions along with rare earth element patterns share characterristics, typical of rocks associated with porphyry Cu-related magmas reflecting the hydrous and oxidized nature of the magmas. The late Cretaceous porphyry systems common in the Pontides form a portion of a belt of porphyry Cu deposits that included world-class deposits in Serbia and Bulgaria, and extend eastward into the Caucasus of Georgia, Armenia and Iran. Early-late Eocene porphyry systems in Pontides, western Anatolian province and southeastern Anatolian orogenic belt form clusters not extending the in the Balkans. Oligocene and Miocene porphyry systems are common in the adjoining countries in the Balkans and Iran, but are only known in the western Anatolia region of Turkey. Overall, the distribution of porphyry Cu systems reflects complex plate interactions involving subduction, collision of continental blocks, closure of small oceanic basins, and rapidly shifting axes of magmatic activity.