Ore facies, mineralogy, alteration, geochemistry and genesis of the Vanakan (Sokan) barite-zinc-lead-copper deposit, north east of Semnan

The Vanakan (Sokan) barite-zinc-lead-copper deposit is located at 23 km northeast of Semnan, in the North Central Iran magmatic belt. It has occurred within the Eocene volcanic-sedimentary sequence. The host rocks of the ores mainly consist of tuff, shale and shaly tuff. Volcanic rocks in the district at the Ahovan region involve both mafic and felsic compositions including basalt, andesite, dacite, rhyolite and tuff. Many studies have been conducted on ore deposits in the Semnan region including Poshteh barite- base metals volcanogenic massive sulfide (VMS) deposit (Ghaffari, 2017), Hamyard (Haji-Bahrami, 2012) and northeast Semnan (e.g., Ghiasvand et al., 2009; Shahri, 2011) iron skarn deposits. Therefore, studying the barite-metal deposits in the Central Iran magmatic belt such as the Vanakan deposit, can provide exploratory keys to discover new reserves, which is one of the main goals of this research study. In this work, study on ore facies,mineralogy, alteration, geochemistry and genesis of the Vanakan barite-zinc-lead-copper deposit are considered.

First, regional and local geology, alteration, ore textures and structures and mineralogy of ore horizons in the Vanakan ore deposit were carefully checked out and studied during field studies. Then, the samples were systematically collected from trenches and open pit of the mine. Mineralogical studies were conducted on 24 thin sections and 8 polished samples in the microscopic laboratory at the Shahrood University of Technology. For geochemical studies, about 16 systematic samples from different ore facies and ore horizons were collected. Then, the samples were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method, and a few samples were studied by X-ray diffraction (XRD) method in the Aria Sharif Laboratories Company.

The host sequence in the Vanakan deposit involves three units, from bottom to top: Unit1: conglomerate, limestone, sandstone; Unit2: andesitic to dacitic lava-rich, and unit3: acidic tuff-rich. Mineralization as the Vanakan 1 and 2 deposits occurred at top of unit 2 and within unit 3. The entire Vanakan area involves a local syncline with northeast-southwest axial trend, in which the Vanakan 1 and Vanakan 1 deposits are located in the northern and southern limbs of the syncline, respectively. Based on structural,  textural and mineralogical studies, five different ore facies were distinguished  in Vanakan 1, from bottom to: 1) vein-veinlet and breccia: involving barite-pyrite-quartz vein-veinlets, 2) massive sulfide: composed of massive sphalerite, galena, barite, chalcopyrite and pyrite, 3) layered-banded sulfide ore: involving alternations of ore and sericite altered tuff-rich bands, 4) baritic ore: comprising of mainly barite and little sulfides, and 5) banded-exhalative cherty sediments. The ore facies in the Vanakan 2 from bottom to top are 1) barite -(galena)-rich vein-veinlets and 2) banded cherty iron oxide-hydroxides -rich red exhalative sediment. From a mineralogical point of view, the ores in the Vanakn 1 mainly consist of barite, sphalerite, galena, pyrite, chalcopyrite and marcasite accompanied with secondary minerals such as malachite, chrysocolla, smithsonite, cerussite, hematite, limonite, goethite.

Based on different characteristics of mineralization in the Vanakan district, such as geometry of ore bodies, textures and structures, ore facies, wall rock alterations, mineralogy, metal zonation and geochemical features, the Vanakan deposit can be classified as a bimodal- felsic or Kuroko-type volcanogenic massive sulfide (VMS) deposit, similar to those of the Mount Read volcanic deposits of Tasmanian Australia such as Rosebery (Large, 1992; Large et al., 2001) and Hokuroko basin in Japan (Huston et al., 2011; Ohmoto and Skinner, 1983).