Consideration of mineralization and characterization of fluid inclusions in the Gharehkand sediment-hosted gold-bearing vein-veinlets, southeast of Maragheh, East Azarbaidjan

The Gharehkand area is located in the southeast of Maragheh city and in Takab metallogenic district. According to the structural divisions of Iran, this area is situated south of the volcanic Mount Sahand in the Central Iran zone (Stocklin, 1968). This paper aims to investigate the characteristics of gold-bearing vein-veinlets and physicochemical conditions of ore-forming fluids in the Gharehkand area.

About 40 rock samples were collected from mineralized zones and host rocks. The thin and thin-polished sample sections were studied in the geology department of the Mohaghegh Ardebili University, Ardebil. Microthermometric data were obtained from fluid inclusions using the Linkam THMS600 stage at the Payame Noor University of Tabriz.

1) The mineralization in the Gharehkand area took place principally as vein-veinlets within the host sedimentary rocks during two main stages. Mineralization occurred in quartz vein-veinlets during the first stage. The quartz crystals within the quartz vein-veinlets display brecciated, boxwork, drusy, and comb textures. During the second stage barite mineralization occurred as vein-veinlets. The mineralization of sulfides (galena, sphalerite, pyrite, and chalcopyrite) and gold occurred within the quartz vein-veinlets of the first stage.2) The values of homogenization temperature (Th) and salinity of fluid inclusions vary from 80°C to 220ºC and 6 to 13 wt.% NaCl eq., respectively.3) The study of fluid inclusions demonstrated that boiling and simple cooling were the most important mechanisms in the ore deposition at Gharehkand, and the sulfide complexing ligands played an effective role in transporting metals within the ore-forming hydrothermal fluids.4) The quartz crystals textures (brecciated, boxwork, and comb) in quartz vein-veinlets and microthermometric data (low salinity and Th) of the ore-forming fluids indicate that mineralization at Gharehkand is of epithermal type with low-sulfidation style.

The mineralization in the Gharehkand area took place principally as vein-veinlets within the host sedimentary rocks. Two main stages of mineralization were distinguished at the Gharehkand area. Stage-1 mineralization is represented by the quartz vein-veinlets. The quartz crystals within the quartz vein-veinlets display brecciated, boxwork, drusy, and comb textures. The formation of quartz crystals in quartz veinlets and micro-veinlets were temporally divided into three sub-stages: early, middle, and late. Stage-2 mineralization is represented by barite vein-veinlets. The hypogene alteration is mainly observed as the development of silicic halos around quartz vein-veinlets within the host sedimentary rocks. The mineralization of sulfides (galena, sphalerite, pyrite, and chalcopyrite) and gold have occurred within the late sub-stage quartz veinlets and micro-veinlets. Oxidized processes have caused formation of goethite, hematite, jarosite, malachite, and azurite. Copper sulfides (covellite, chalcocite, and digenite) are formed in the supergene zone. Based upon phase content, the studied fluid inclusions in late sub-stage quartz crystals may be classified into three types: liquid-rich two-phase (L+V), mono-phase vapor (V), and vapor-rich two-phase (V+L). The values of homogenization temperature (Th) and salinity of the analyzed liquid-rich two-phase fluid inclusions vary from 80°C to 220ºC and 6 to 13 wt.% NaCl eq., respectively (Fig. 11A, C). Given the Th and salinity values of the fluid inclusions, ore-forming hydrothermal fluids during development of the quartz vein-veinlets at Gharehkand experienced hydrostatic pressures within the range of 20-25 bars. This is almost equivalent to depths of 200-250 meters below the underground water table level. It is consistent with the estimated depths of most epithermal deposits (Bodnar et al., 2014). The Gharehkand microthermometric data points on the bivariate plot of Th versus salinity (Wilkinson, 2001) show that the densities of fluid inclusions vary from 0.9 to 1 g/cm3. The occurrence of boiling and simple cooling are the most effective mechanisms in deposition of ore and gangue minerals at Gharehkand.  The Gharehkand microthermometric data points on the bivariate plot of Th versus salinity (Pirajno, 2009) indicate that the bisulfide complexing ligand most likely played the important role in transporting ore metals (particularly gold).