Fluid inclusion study of the ore-quartz veins at Haftcheshmeh porphyry copper (Mo) deposit, Ahar–Arasbaran Magmatic Belt, NW Iran

Abstract

The Haftcheshmeh porphyry copper deposit in Alborz–Arasbaran Magmatic Belt (AAMB) of northwestern Iran is hosted by I-type granitoids of calcalkaline series of Eocene to Miocene age. The mineralization consists of chalcopyrite, molybdenite, rare amounts of covellite, pyrite, and magnetite that are commonly found in silicified veins of the potassic–sericitic alteration zones in the host quartzdiorite, quartzmonzonite, and granodiorite porphyries. The obtained fluid inclusion data (TmCO2 −57.5 to −60.5°C) in ore-quartz veins shows enrichment in CO2. The coexistence of Cu-sulfide daughter crystals together with halite crystals and the CO2-bearing fluid inclusions suggest the presence of two immiscible fluids: a high salinity, CO2-rich magmatic fluid with high metal contents (31.1 and 48.4wt.% NaCl equivalent, (Th) 169 to 500°C), and a low salinity fluid (0.88 and 11.1wt.% NaCl equivalent, (Th) 242 to 418°C), whereas the presence of liquid and gaseous CO2 implies fluid immiscibility. Measured first melting temperatures of −21 to −40°C in fluid inclusions of the ore-quartz veins at Haftcheshmeh deposit are below the eutectic point of NaCl–H2O indicating the presence of bivalent cations including the copper salt CuCl2.4H2O in the ore solution. The salinity of the magmatic fluids in the A-type quartz veins (31.8 to 43.1wt.% NaCl equivalent) is close to the salinities in the D-type quartz veins (31.1 to 48.4wt.% NaCl equivalent), and the homogenization temperatures of (LV) inclusions in the A-type quartz veins (242 to 327°C) are only slightly lower than in the D-type quartz veins (261 to 387°C). These data together with the very close homogenization temperatures of (VL) inclusions in the A-type (302 to 418°C) and the D-type quartz veins (300 to 395°C) suggest similar ore fluid sources for both quartz vein types. The δ18O values of the ore-quartz veins showing a typical magmatic fluid and suggests that the early fluid inclusions were of magmatic origin. It is envisaged that CO2 degassing in the primary magma at Haftcheshmeh could have led to separation of the vapor phase and destabilization of the metal transporting complexes and the ore formation.