Ore facies, mineral chemical and fluid inclusion characteristics of the Hossein-Abad and Western Haft-Savaran sediment-hosted Zn-Pb deposits, Arak Mining District, Iran

The Arak basin in the northern part of the Malayer-Esfahan metallogenic belt (MEMB) of Iran hosts 9 Zn-Pb sulfide deposits within a Jurassic sedimentary succession of black, fine-grained sandstone, siltstone and shale. This succession was deposited in an extensional back-arc environment between the Sanandaj- Sirjan Zone (SSZ) and the Central Iranian microcontinent. Two of the 9 deposits (Western Haft-Savaran and Hossein-Abad) are the focus of study. Here, three ore facies can be distinguished, based on the morphology of the orebodies and the sulfide mineralogy and textures: (a) stockwork, (b) massive, and (c) bedded mineralization. The stockwork and massive mineralization are interpreted to have formed in the subsurface, whereas bedded mineralization was sedimented on the seafloor. The Fe contents of sphalerites from massive and stockwork mineralization from the two deposits, together with the presence of pyrrhotite inclusions, indicate that sphalerite precipitated from relatively low fS(2) (< 10(-11)) fluid. Homogenization temperatures of fluid inclusions in sphalerite, quartz and siderite in massive mineralization range from 105 to 309 degrees C, and average 208 degrees C, with salinities between 4.03 and 11.1 wt% NaCl eq., and averaging 5.77 wt% NaCl eq. These widely ranging salinities and homogenization temperatures reflect mixing of shallow connate water with venting metalliferous fluid that is of deep basinal origin, and deposition of mineralization predominantly below the seafloor. Collectively, the host rock types, ore facies and textures, deposition in an reduced ambient environment, fluid inclusion temperatures and salinities, and mineralizing fluid behaviour at the seafloor are all compatible with a vent-proximal sedimentary exhalative (SEDEX) origin for the Western Haft-Savaran and Hossein-Abad deposits.