Geological setting of silica in Dehnow-Abid region (Eshghabad northeast) using fluid inclusions studies

Dehnow-Abid area is a part of the geological map of Eshghabad with scale 1:100000 (Aghanabati, 1994) that is located about 20 kilometers northeast of Eshghabad and in the coordinates of 57° 6´ 0" to 57° 10´ 0" eastern longitude and 34° 28´ 0" to 34 21´ 0" northern latitude. The Dehnow-Abid area is located in Tabas block and east of central Iran structural zone. The small continent east central Iran (Takin, 1972) includes blocks: Loot, Tabas and Yazd that constitute Iran's eastern part (Davoudzadeh and Schmidt, 1982). In geology, we can acquire more information about temperature forming minerals and rocks, pressure, density of the fluid and the chemical composition of the ore bearing fluids by fluid inclusions studies. Properties as well as their role in our understanding of the sources and evolution of ore bearing hydrothermal fluids and genesis of mineral deposits are very important (Rodder, 1979). In this study, we tried to use both field and laboratory studies, including petrography and thermometry studies of fluid inclusions, environment formation of quartz in the specified Dehno-Abid.
At first, in order to identify the area, the 1:100000 map of Eshghabad was used. Then, for a complete cognition of mentioned area, after a few field visits and sampling of outcrops of quartz, we prepared 16 double polishing sections from some crystalline and milky quartz. Then, 10 thin sections of sandstones of that area were prepared for identification the host rock. Microscopic examinations on fluid inclusions were done by a LEICA DMLSP polarizing light microscope. Fluid inclusion micro-thermometry studies were done by using a Linkam THM S600 heating and freezing stage and with a TMS94 controller. Also, a cooling LNP which is mounted on an Olympus BX-41 microscope in Laboratory Fluid inclusion of Earth Sciences, Damghan University was used.
Discussion and Lithology of the Dehnow-Abid area included dark shale, fine and coarse grains arkosic and lithic arkose sandstones (Tucker, 1994) that show low grade metamorphic texture which may be attributed to these sediments metamorphosed Jurassic age, but their equivalence is more with Shemshak forrnation (Aghanabati, 1994). Silica mineralization occurred as veins, veinlets and in some parts show silica mass of lense. In this area, there are two generations of faults with trends of northeast-southwest and north-south. Based on the geological section, the North – South faults is the second generation that cut system North-East to South-West faults (Kosari, 2004). In microscopic studies of fluid inclusions different characteristics such as their relationship with the host mineral, phase contents, size, shape, necking down and degree of filling were investigated. Microscopic investigates at room temperature, based on the criteria provided by Van den Kerkhof and Hein, 2001; Shepherd et al., 1985; Rodder, 1984 were performed. These studies indicated that as genetically point fluid inclusions in quartz area are able to divide into three groups (primary, secondary and pseudo secondary). The shapes of fluid inclusions are very different, but partly follow mineral crystallization system. Size of fluid inclusion varies between 5 to 120 microns, but most abundant fluid inclusions have size of 20 to 60 microns. According to a survey done on double polishing sections in laboratory conditions, the phase contents of fluid inclusions may be divided into six groups that include : monophase liquid (L), monophase vapor (V), two phases rich of liquid (L), two phases rich of vapor (V), three phases (L埤) and immiscible liquid (L1⽯). The presence of CO2 around gas bubbles can represent metamorphism environments (Yardley and Bodnar, 2014; Van den Kerkhof et al., 2014). Heating analysis was done on 113 samples of fluid inclusions studied in order to investigate the situation homogenization temperature, and cooling analysis was done on 99 selected samples. In addition, 38 samples were tested as heating in order to obtain a homogenization temperature of CO2 phase fluid inclusions (L1⽯). Micro-thermometric fluid inclusion investigates shows that the homogenization temperature of the CO2 is varied between 26.1 to 30.6. Fluid inclusion micro-thermometry studies on mineral quartz shows that the homogenization temperature is varied between 247 to 336 ° C, salinty is varied between 0.9 to 15.8 % NaCl eq and mineralizing fluid density range is between 0.7 to 0.9 gr/cm3. Based on evidences from field and laboratory studies, especially microthermometry studies and also the presence of carbonic phase in fluid inclusions, probably silica in the Dehnow-Abid region was formed under metamorphism conditions. On the basis of Wilkinson diagram (Wilkinson, 2001), regional data have been plotted in low temperature range and gold veins that shows metamorphic environments partly. Also plotting these data on a Kessler diagram (Kesler, 2005) suggests a metamorphic source for fluids which have made the veins, lenses and quartz mass of studied area.