Extended abstract

Introduction

The Majidabad area is located approximately 20 km southwest of Nir city, Ardabil Province, northwestern of Iran. Based on the division of sedimentary-structural zones of Iran (Nabavi, 1976), this area is considered a part of the Alborz-Azarbaidjan zone. In Azarbaidjan, deposits of lead, zinc, iron, copper, and molybdenum are known to be among the most important ore minerals. The metallogenic zones of Takab, Ahar-Arasbaran, and Tarom-Hashtjin can be identified in Azarbaidjan. The Majidabad area is located in eastern part of Azarbaidjan. Among the research works carried out in the Majidabad area, one can mention the exploration operations such as geological, geophysical and geochemical studies by the Industry, Mining, and Trade Organization of Ardabil Province (IMTOAP, 2012). Based on these studies, the highest and lowest amounts of iron oxide (magnetite) in this area were determined to be about 93.29 and 27.3%, respectively, with an approximate average of 54.47%. However, no detailed and scientific study has been conducted so far regarding the characteristics of alteration and mineralization zones and the characteristics of hydrothermal fluids in this area. In this study, an attempt is made to investigate the geological, alteration, mineralization, and fluid inclusions characteristics in the Majidabad area. This research also attempts to discuss the physiochemical properties and conditions governing the ore-forming fluid using the results of fluid inclusions studies. The results of this research can play an effective role in the exploration and identification of ore-bearing zones in other similar areas in the country.



Materials and methods

This research was conducted in two stages: field and laboratory studies. In the field stage, a field visit to the area was first conducted to identify rock units, alteration and mineralization zones, and the relationship of ore-bearing veins to host rocks. Then, 75 samples were taken from rock units and alteration and mineralization zones. In the laboratory stage, 10 thin-section samples and 20 thin-polished sections were prepared and studied. Microthermometry studies of fluid media were conducted at Tarbiat Modares University, Tehran, using a Linkam model THMSG600 device mounted on a ZEISS microscope. This device has the ability to change temperatures from +600 to -200°C. Calibration of the device was performed during the heating stage (with an accuracy of 0.6°C) using a cesium nitrate standard with a melting point of +414°C and during the cooling stage (with an accuracy of 0.2°C) using an n-hexane standard with a melting point of -94.3°C.



Discussion and results

Based on its geological location, the Majidabad area is located on the 1:100000 Sarab geological map (Behrouzi and Amini Azar, 1992). The oldest exposed unit in this area is related to Eocene volcanic and pyroclastic rocks, including tuff, volcanic breccias, and andesitic and rhyolitic tuffs along with basaltic lavas and basaltic andesite, which are covered by the Oligocene marl and conglomerate unit and then by Miocene andesitic eruptions (in the form of tuff and lava). All of these volcanic-pyroclastic complexes are cut by post-Miocene subvolcanic bodies, dykes, and diorite stocks. Based on field and microscopic studies conducted in the Majidabad area, types of silicic, sericitic, argillic, and propylitic alterations have developed in this area. These alterations have mostly affected Eocene rock units. According to field studies conducted in the Majidabad area, mineralization in this area has occurred mainly in the form of magnetite veins. Magnetite veins are often hosted by Eocene tuff units and their thickness usually varies from a few centimeters to several meters. These veins are mostly in the form of masses and mainly have a north-south trend. Based on field and microscopic studies conducted in the Majidabad area, mineralization in this area occurred in two stages: hypogene and supergene. During the hypogene stage, as a result of the activity of hydrothermal fluids, magnetite veins and veinlets were formed in the area, and various types of hydrothermal alterations (silicic, sericitic, argillic, and propylitic) developed around the ore-bearing veins. Ore minerals such as magnetite, pyrite, and chalcopyrite formed during this stage of mineralization within the ore-bearing veins. During the supergene stage, the reaction of descending surface fluids with primary minerals (magnetite, pyrite, and chalcopyrite) in the area has caused the formation of secondary minerals, including iron oxides and hydroxides (goethite, hematite, and jarosite) and secondary copper sulfides (bornite, chalcocite, covellite, and digenite). According to field and microscopic studies, the most important gangue mineral in this area is calcite. On the basis of content of the main phases and the classifications provided (Shepherd et al., 1985), types of mono-phase vapor (V), liquid-rich two-phase (LV) and vapor-rich two-phase (VL) fluid inclusions were identified in the investigated inclusions. The melting temperature of the first piece of ice in the fluid inclusions of the study area was obtained in the range of -26 to -28 °C. These melting temperatures of the first ice fragments indicate that the ore-forming fluid of the Majidabad area contained amounts of KCl and MgCl2 in addition to NaCl (Goldstein, 2003; Prokofiev et al., 2010). The melting temperature of the last ice pieces (Tmice) in the fluid inclusions was measured in the temperature range between -1.2 and -4.8 °C. According to the equation proposed by Bodnar (2003) and according to the melting temperature of the last ice pieces (Tmice), the salinity values of the fluid inclusions of the Majidabad area were determined to be in the range of 2.07 to 7.59 wt% NaCl eq; however, the highest frequency is related to the range of 2 to 3 wt% NaCl eq. The homogenization temperatures of fluid inclusions in the study area vary in the temperature range between 123 and 235 °C; however, the highest frequency belongs to the temperature range between 120 and 140 °C. The coexistence of mono-phase vapor, liquid-rich two-phase, and vapor-rich two-phase fluid inclusions in samples from the Majidabad area indicates the occurrence of ore-forming fluid boiling in this area (Albinson et al., 2001; Moncada et al., 2017; Simmons et al., 2005). In addition, the presence of plumose and comb textures in ore-bearing veins and veinlets may indicate the occurrence of boiling of the ore-forming fluid in the study area during mineralization (Hedenquist et al., 2000; Moncada et al., 2012). Therefore, considering the occurrence of ore-forming fluid boiling in the area, the homogenization temperatures of fluid inclusions do not require pressure correction (Simeone and Simmons, 1999); because in this case, the ore-forming fluid was most likely experiencing hydrostatic pressure at the time of trapping. Therefore, the homogenization temperature of fluid inclusions can be approximately equivalent to their entrapment temperature (Roedder and Bodnar, 1980; Simmons et al., 2005; Simeone and Simmons, 1999). Hydrostatic pressure values for the hydrothermal fluid of the Majidabad area were estimated to be less than 50 bar. These hydrostatic pressure values can be equivalent to depths of less than 500 meters below the water table. To determine the depth of mineralization in the study area, the homogenization temperature versus depth diagram (Haas, 1971) was also used. Considering the highest homogenization temperature (235 °C) and highest salinity (7.59 wt% NaCl eq) of the fluid inclusions, the approximate depth of mineralization in this area was determined to be about 300 m. To determine the evolutionary trends of the ore-forming fluid in the Majidabad area, the salinity versus homogenization diagram of fluid inclusions (Shepherd et al., 1985; Wilkinson, 2001) was used. According to this diagram, the fluid inclusions of the study area indicate two evolutionary processes of boiling and dilution by surface waters, as the most important mechanisms for the deposition of ore metals in the study area. The density of the fluid inclusions in the study area can be determined using the homogenization temperature versus salinity diagram (Wilkinson, 2001) without considering their trapping conditions. According to this diagram, the density of ore-forming fluids in the Majidabad area mainly varies between 0.88 and 0.97 g/cm3. The salinity versus homogenization temperature diagram (Pirajno, 2009) was used to determine the type of effective complexes in transporting ore metals in the study area. According to this diagram, sulfide complexes have played an important role in transporting ore metals in the Majidabad area.



Conclusion

The most important rock units in the Majidabad area include Eocene to Oligocene volcanic-pyroclastic rocks, which are cut by post-Miocene diorite bodies. The most important alterations in this area are silicic, sericitic, argillic, and propylitic alterations. Mineralization in this area has often occurred in the form of magnetite veins hosted by Eocene pyroclastic units. Mineralization in this area has occurred in two stages: hypogene and supergene. In the hypogene stage, the primary minerals magnetite, pyrite, and chalcopyrite are formed. Secondary minerals such as iron oxides and hydroxides (goethite, hematite, and jarosite) and secondary copper sulfides (bornite, chalcocite, covellite, and digenite) were formed during the supergene stage. Calcite is the most important gangue mineral in this area. The salinity values of fluid inclusions in the Majidabad area vary between 2.07 and 7.59 wt% NaCl eq. The homogenization temperatures of the fluid inclusions in the study area were measured in the temperature range between 123 and 235 °C. The approximate depth of mineralization in this area was determined to be about 300 m. Boiling and dilution processes by surface waters are the most important mechanisms for the deposition of ore metals in the study area. The density of ore-forming fluids in the Majidabad area mostly varies between 0.88 and 0.97 g/cm3. Sulfide complexes have played an effective role in transporting ore metals in the Majidabad area. Considering the geological, alteration, mineralization, mineralogy, and texture characteristics, as well as the characteristics of the ore-forming fluid in the Majidabad area, mineralization in this area can be considered hydrothermal (vein).