Fluid inclusion and stable isotope studies of the Kighal porphyry Cu–Mo prospect, East-Azarbaidjan, NW Iran

Abstract

The Kighal porphyry Cu–Mo prospect is located to the north of Varzeghan, NW Iran. It is hosted by a quartz monzonite porphyry stock that intruded Eocene volcanic rocks. Hydrothermal activity and associated liquid–vapor immiscibility caused hydro-fracturing and generation of various kinds of quartz–sulfide veinlets. Almost all quartz crystals within these veinlets are fluid inclusion-rich. The entrapped fluids are of four main compositional types: mono-phase vapor-rich, two-phase liquid-rich, two-phase vapor-rich, and multiphase (L + V + S). Based on microthermometric studies of fluid inclusions, the salinity ranges from 1.4 to 70 wt% NaCl equivalent, while most of the values cluster between 35 and 70 wt% NaCl equiv. The T H values of two-phase inclusions homogenizing into liquid and vapor states range between 180–440 and 360–540 °C, respectively. Most of the multiphase inclusions homogenize by halite dissolution with T m(NaCl) varying from 360 to 600 °C, while some of them homogenize by simultaneous disappearance of halite and vapor which show a wider range of T H (300–600 °C). Inclusions with T m(NaCl) ≫ T H(L–V) and higher salinity values may have suffered necking down and/or liquid loss or trapped solid halite crystals. The approximate depth of mineralization and entrapment of the studied inclusions is estimated about >1.0 to 2 km. The principal process responsible for mineralization was liquid–vapor immiscibility and consequent supersaturation of the hydrothermal fluids. Based on stable isotope analyses carried out on quartz–sulfide veinlets, δ18O (5.4–5.8 ‰) and δD (−77 to −61 ‰) values of the ore-bearing fluids, as well as the δ34S value of a representative pyrite sample (1.3 ‰) are consistent with a predominantly magmatic source.