Abstract

In the Qarachilar Cu-Mo-Au deposit of the Ahar–Arasbaran metallogenic zone (AAMZ), northwest Iran, mineralization occurs as three quartz-sulfide veins that cut granodiorite-quartz monzodiorite rocks of the Qaradagh batholith (QDB). Ore formation can be divided into three stages, with chalcopyrite, molybdenite, and goldbearing pyrite appearing mainly in thefirst two stages. The main wall-rock alteration is silicification, and intermediate argillic, carbonate, and propylitic alteration. Fluid inclusion microthermometry indicates trapping of medium- to high-salinity (9.2–55 wt% NaCl equiv.)fluids at Qarachilar. Fluid inclusion trapping conditions are estimated to be 190 °C–530 °C and 0.1–3 kbar. The variable phase ratios as well as spatial coexisting of liquidand vapor-rich two-phase and halite-bearing multiphasefluid inclusions homogenizing over the same temperatures are consistent withfluid boiling during ore formation. Obtainedδ 18 OH2Ovalues of quartz from orestage veins are +5.7‰to +9.7‰, signifying that the ore–fluid system was predominantly magmatic water. The average calculatedδ 34 SH2Svalues are 1 ± 1‰for pyrite, chalcopyrite and molybdenite, consistent with a magmatic source for sulfur. Combined, thefluid inclusion and stable isotope data indicate that the ore-forming fluids at Qarachilar were magmatic in origin and were subsequently cooled and diluted by meteoric water. Fluid boiling and mixing facilitated hydrothermal alteration and mineralization. Molybdenite Re–Os dating shows that mineralization occurred at 42.35 ± 0.16 Ma, coincident with formation of porphyry Cu-Mo mineralization at Agarak deposit, and Hanqasar, Aygedzor and Dastakert prospects in the Lesser Caucasus. However, Qarachilar is older than all porphyry Cu-Mo mineralization in the AAMZ and Urumieh-Dokhtar magmatic arc (UDMA), which suggests that collision between Arabia and Eurasia were oblique and thus diachronous. Our data suggest that mineralization at Qarachilar is related to collisional Eocene magmatic–hydrothermal activity related to NeoTethys subduction, and shares a number of similarities with the vein-type Cu-Mo-Au mineralization related to Cu-Mo porphyries.