Geology, geochemistry, geochronology and genesis of the late Miocene porphyry Cu-Au-Mo mineralization at Afyon-Sandıklı (AS) prospect, western Anatolia, Turkey

Abstract

Afyon-Sandıklı ‘AS’ is an example of a porphyry Cu-Au-Mo systems hosted in post-collisional late Miocene magmatic rocks within the Afyon-Ören Zone of Turkey. The systems were generated by multi-phase intrusions emplaced into early stage volcanic and volcanoclastic rocks of the Afyon volcanics. The REE and radiogenic isotope data from these rocks suggest a magma source containing residual garnet with a low hydrous phase. These collectively, favor a metasomatized mantle source for magmas generated in either subduction or post-collisional environments. Four main alteration types have been identified in the AS porphyry Cu prospect; potassic, phyllic, epidote-chlorite and advanced argillic. The alteration zones are centered around monzonite porphyry intrusives. Chalcopyrite and bornite are the main copper minerals in the potassic alteration zone with subordinate molybdenite. All are replaced by pyrite. The U-Pb SHRIMP analyses on a post-mineral micromonzonite porphyry dike yielded an age of 10.97 ± 0.09 Ma, whereas the host monzonite porphyry yielded an age range between 10.5 and 12.5 Ma. Ar-Ar geochronology on alunite constrains the timing of the argillic alteration at 11.2 ± 0.5 Ma. Fluid inclusion studies were performed on primary inclusions in quartz veins from the potassic and phyllic alteration zones. The primary fluid inclusions in quartz veins in both zones were classified as Type 1, single-phase vapor (V); Type 2, two-phase liquid-vapor (L-V); and Type 3, three-phase liquid-vapor-solid (L-V-S). Type 3 inclusions in the potassic alteration zone with weak phyllic overprint have higher homogenization temperatures compared to those in pervasive phyllic alteration zones. The majority group of fluid inclusions in the AS prospect are Type 2 inclusions with variable vapor and liquid abundances. This suggests boiling (phase separation) of the hydrothermal fluids, and post boiling trapping of the vapor and liquid. The presence of abundant multiphase (Type 3) inclusions in the AS prospect marks the stage when phase separation took place. The phase separation appears to have resulted in intermediate to high-salinity multiphase brines and daughter crystals in the residual fluid. Additionally, we suggest that superposition of phyllic and potassic alteration zones with a variety of Type 2 and Type 3 inclusions in the AS prospect should mark the stage at which mixing by relatively cooler and dilute meteoric water coupled with phase separation occurred. The calculated ?18O(fluid) vs ?D(fluid) values for the alteration minerals display a wide range between the primary magmatic field to the meteoric water line. The ?18O(fluid) vs ?D(fluid) values for biotite suggest a strong magmatic water influence during the formation of potassic alteration. Likewise, a very narrow range of ?34S, close to the sulfur isotope values of magmatic sulfur indicate a magmatic source, and may suggest that potassic alteration was caused by a uniform magmatic fluid source. Additionally, the enrichment in the ?D(fluid) from biotite suggests that phase separation (boiling) was also active during potassic alteration. The overlapping pattern of the ?18O(fluid) vs ?D(fluid) compositions for biotite and sericite indicate an isotopic similarity, and may suggest that they have been formed from a similar, predominantly magmatic fluid mixed to some degree with the meteoric water. © 2020 Elsevier B.V.