Abstract

Qarachilar is a small (0.2 Mt) vein-type Cu‒Mo‒Au deposit related to Cu‒Mo porphyries in the Ahar–Arasbaran metallogenic zone (AAMZ), north-western Iran. It is occurs as three NW-trending quartz-sulfide veins hosted by granodiorite-quartz monzodiorite rocks of the Qaradagh batholith (QDB). Three stages of ore-formation could be identified at Qarachilar: (I) chalcopyrite-molybdenite-pyrite-quartz veins; (II) Au-bearing pyrite-quartz veins and breccias; and (III) barren post-ore carbonate veinlets. Microthermometric measurement on quartz-hosted fluid inclusions indicate that the mineralization may have taken place between 190 to 530 °C, from a medium- to high-salinity (9.2 to 55 wt.% NaCl equiv.) fluids. Oxygen isotopic data (+5.7‰ to +9.7‰) signifying that the fluid may have had a magmatic source, but was ¹⁸O-depleted by mixing with meteoric water. The average calculated δ³⁴S_H2S values are 1 ± 1‰ for pyrite, chalcopyrite and molybdenite, consistent with a magmatic source for sulfur. Combined, the fluid inclusion and stable isotope data indicate that fluid boiling and mixing facilitated hydrothermal alteration and mineralization at Qarachilar. Molybdenite Re–Os dating shows that mineralization occurred at 42.35 ± 0.16 Ma. This middle Eocene age is much older than porphyry Cu‒Mo mineralization in the AAMZ and Urumieh-Dokhtar magmatic arc (UDMA), but overlap the age ranges of porphyry Cu‒Mo mineralization at Agarak, Hanqasar, Aygedzor and Dastakert deposits in the Lesser Caucasus, which suggests that collision between Arabia and Eurasia were oblique and thus diachronous.