Investigations of sulfide-gold mineralization and fluid inclusions in the Nabijan area, southwest of Kaleibar, East-Azarbaidjan province

Introduction
The Nabijan region is located 20 km southwest of Kalybar in East Azerbaijan Province and in northwest Iran. According to the divisions of Iranian structural zones (Aghanabati, 2004), this region is considered a part of the western part of the Alborz-Azerbaijan magmatic belt and is metallogenically located in the Ahar-Arasbaran metallogenic zone (Castro et al, 2013). The Ahar-Arasbaran metallogenic zone is one of the most important and richest metallogenic zones, especially for gold, copper and molybdenum, in the northwest and Iran (Jamali et al, 2012). So far, numerous geological studies have been carried out by various researchers on geochemistry, mineralogy and alteration in the Arasbaran zone and around the Nabijan region. Based on exploration studies conducted by Shekouei (2003), a promising area for gold and copper elements has been introduced in Nabi Jan. Based on the aforementioned studies (trenching and boring) by the Geological and Mineral Exploration Organization of the country, about 300 thousand tons of gold ore have been estimated in silica zones and veins with an average grade of 1.37 grams per ton. Previous researchers have conducted studies on geology, geochemistry, mineralization and alteration in the Nabi Jan region. In this paper, an attempt has been made to describe the geological, mineralization and alteration characteristics of the Nabi Jan region, and to conduct new studies for the first time based on fluid intermediates in quartz-sulfide veins - veinlets in order to determine the physicochemical conditions of mineralizing fluids and the genesis of the deposit.
 
Materials and Methods
This research consists of two parts: field and laboratory investigations. In field studies, mineralized veins were identified and their relationship with host rocks and alteration zones was investigated, and samples were taken for laboratory studies. In this regard, 60 samples were collected from host rock units and mineralizing outcrops. During laboratory studies, 20 thin sections and 5 polished sections were prepared and subjected to petrographic and mineralographic studies at the University of Tabriz. In order to understand the physicochemical nature of the mineralizing fluid and to investigate the process of chemical and temperature changes of mineralizing fluids during ore deposition, petrographic and thermometric studies of fluid intercalations were carried out on 5 samples containing quartz crystals cognate with sulfide mineralization and gold (taken from quartz veins). Thermometric measurements were performed using a Linkam THMSG600 fluid interface device connected to an OLYMPUS BX-51 microscope with LD-LensX40 and equipped with a TMS94 thermal controller and LNP cooler at Payam Noor University of Tabriz.
The temperature range of the device is -190 (by liquid nitrogen) to +600 (by electrical energy) °C. Calibration of the device during the heating stage was performed with an accuracy of ±0.6 °C at +414 °C (melting temperature of cesium nitrate) and ±0.2 °C at -94.3 °C (melting temperature of n-hexane). The salinity of the fluid interfaces was calculated in terms of weight percent equivalent to common salt (wt% NaCl eq.) using the melting temperature of the last ice piece (Tmice) and using the equation (Hall et al, 1988).
Geology and Mineralization
The study area is part of the Lesser Caucasus-Arasbaran metallogenic zone. Magmatism in this metallogenic zone began during the Late Cretaceous and continued into the Cenozoic and Quaternary. Mineralization in this zone is mostly related to Cenozoic magmatic rocks. Cenozoic magmatic activities in the Arasbaran zone have led to the formation of alkaline to calc-alkaline plutons with porphyry, skarn, and epithermal mineral systems. The exposed rocks in the Nabi Jan area mostly consist of Cretaceous volcanic and sedimentary units that have been intruded by Oligocene intrusive masses with quartz-monzodiorite composition. Intrusive masses with dioritic to monzodiorite composition are the main factor of epithermal gold-silver mineralization in the Nabi Jan area and its surroundings from around the villages of Paigham and Alawiq to Jundshafq and Marzrud. Alteration zones around gold veins and zones in the Nabijan region include silicic, phyllic, and propolite types that extend to dimensions of 1 to 20 meters. Mineralization in the studied area has occurred in the form of stockwork and quartz veinlets-veins within the quartz monzodiorite host rock, which include sulfide minerals (pyrite, chalcopyrite, galena, and sphalerite) and native gold. Pyrite and chalcopyrite have been transformed into iron hydroxides (goethite and limonite) as a result of supergene processes. Quartz crystals within the quartz veinlets-veins exhibit comb and void-filling textures. Also, bipyramidal shape is common in crystalline quartz in this region. Based on the studies of the fluids involved, the homogenization temperature and salinity values of the fluid interlayers vary between 170 and 282 °C and 3.27 to 8.51% by weight of table salt, respectively. Based on the findings of the fluid interlayers, the boiling process is the most important process in the deposition of sulfide minerals and gold, and sulfide complexes have played a major role in the transport of ore elements. Based on the geological characteristics, mineralogical, structure, texture, and homogenization temperature and salinity values of the fluid interlayers, the Nabijan sulfide-gold mineralization can be classified as a low-sulfidation epithermal deposit.
 
Results and Discussion
Based on the studies conducted, the rock units exposed in the Nabijan region include Cretaceous volcanic and sedimentary rocks and Oligocene intrusive rocks. The Cretaceous volcanic and sedimentary rocks are cut by the Oligocene quartz-monzodiorite intrusion. The main minerals of this rock unit are plagioclase, potassium feldspar, and quartz, and its accessory minerals are biotite, amphibole, dark minerals, and rarely clinopyroxene. In the altered areas, most hornblens have been transformed into biotite and biotites into chlorite. The activity of hydrothermal fluids resulting from the intrusion has caused the formation of various siliceous, phyllic, and propolite alterations in the host rock. As a result of the passage of hydrothermal fluids along the fractured and faulted zones, while altering the quartz-monzodiorite host rock, it has caused the formation of stony zones and silica veins-veins. Gold-bearing silica veins and veins contain pyrite and small amounts of chalcopyrite, sphalerite and galena. Natural gold of 2 to 5 microns is observed in the altered parts of pyrite to iron hydroxide (goethite) in polished sections. Petrographic and thermometric studies of fluid intercalations were carried out inside coarse quartz crystals. Quartz crystals in these vein-veins show co-growth and twinning with sulfide minerals and gold. Fluid intercalations have polyhedral, elongated, acicular, spherical and negative crystal shapes and are observed as primary, secondary and pseudo-secondary intercalations from a paragenetic point of view. The size of the fluid interlayers varies from 10 to 24 microns. The studied fluid interlayers can be divided into 4 types: (1) two-phase liquid-vapor (L+V), (2) two-phase vapor-liquid (V+L), (3) single-phase vapor (V), and (4) single-phase liquid (L). The frequency of two-phase liquid-rich interlayers is higher than the other interlayers. Thermometric studies were carried out on two-phase liquid-rich interlayers during two cooling and heating stages. The range of homogenization temperatures for two-phase liquid-rich interlayers was between 170 and 282 °C. Based on the values of these temperatures, the salinity of the two-phase liquid-rich interlayers ranges from 3.27 to 8.51 with an average of 75.5% by weight equivalent to table salt. Based on the bivariate plot of salinity versus homogenization temperature, the points corresponding to the thermometry findings of the fluid intercalations at Nabijan show a trend of approximately threefold increase in salinity (from 3.2% to 8.5%) accompanied by a significant decrease in temperature (from 282°C to 170°C), which is somewhat similar to the boiling trend. Also, the presence of single-phase liquid intercalations could indicate that the activity of hydrothermal fluids continued down to temperatures below 70°C. The ice melting temperatures range from -2 to -5.5°C, which 
correspond to salinities between 3.27 and 8.51% by weight of table salt. The homogenization temperature-salinity trend is consistent with the boiling of ore-forming fluids. It seems that the deposition of sulfides and gold occurred during the same boiling process. Considering the average homogenization temperatures and salinities of the fluid interlayers and using the diagram of fluid interlayer pressure (during homogenization) versus homogenization temperature, the minimum fluid pressure during the deposition of waste and ore minerals was about 25 bar. Considering the occurrence of fluid boiling, this pressure should be considered hydrostatic, which is equivalent to a depth of about 250 meters. This depth can be considered as the lowest depth of sulfide and gold mineralization in the Nabijan area. According to the temperature-salinity diagram (Wilkinson, 2001), sulfide-gold mineralization in the Nabijan area is in the epithermal range.
 
Conclusion
Mineralization in the Nabijan area occurred in the form of stockwork and quartz veinlets within the quartz monzodiorite host rock. The epithermal gold mineralization agent is most likely a buried intrusive mass from which only the silica veins and veinlets originating from it were able to cut the quartz monzodiorite mass. Silicic, phyllic and propolite alterations have developed around the quartz veinlets. The quartz veinlets contain sulfide mineralization (pyrite, chalcopyrite, galena and sphalerite) and native gold, and the quartz crystals within these veinlets show comb and void-filling textures. The homogenization temperature of the two-phase fluid interlayers present in the mineralized quartz is in the range of 170 to 282 with the highest frequency between 170 and 210 °C. The geological characteristics, structure, and texture of mineralization and alteration zones, along with microthermometric findings in the Nabi Jan area, are consistent with low-temperature epithermal gold mineralization.