Mineral chemistry and geothermometry of chlorite in Dalayon Area (East of Dorud- Sanandaj-Sirjan Zone)

The main aim of this paper is to identify the formation environment and the physicochemical conditions of chlorite minerals in Dalayon area by their mineralogical and geochemical features (Ciesielczuk, 2002). The geochemical characteristics, mineralogical and formation mechanism of granitic rocks can be investigated using the biotite hydrothermal alteration to chlorite (Morad et al., 2011). The temperature data obtained from chlorite geo-thermometry can be compared with the data obtained through fluid inclusion (Cathelineau and Nieva, 1985; Kordi and Shahrokhi, 2021; Shahrokhi, 2021).

Regional Geology:

Dalayoun area is located in the east of Drood city and is a small part of Sanandaj-Sirjan metamorphosed zone. Lithologically, the oldest units belonging to the Upper Triassic-Jurassic and are including a relatively uniform sequence of slate, schists with siliceous veins and veinlets, and cordierite and sillimanite mica schists along with black hornfelses and metamorphosed sandstones. (Shahrokhi, 2002).

In order to study the mineralogical and geochemical composition of chlorites in the area of study, 10 thin-polish sections were studied. In addition to, chemical analyses of 10 samples by XRD method and 20 points by EPMA of chlorite mineral as well as the study of the fluid inclusion on 3 quartz mineral samples were carried out. The structural formula was calculated based on 14 oxygens for chlorite.Petrography, Mineral chemistry, Whole rocks chemistryThe intrusive bodies in the studied area have infiltrated volcanic and metamorphic rocks (Shahrokhi, 2020). Based on field Geology and microscopic studies, the mineralogy of these rocks includes quartz, plagioclase, orthoclase, microcline and chlorite as the main minerals and biotite, muscovite, apatite, garnet, rutile, leucoxene, titanium minerals, sphene and zircon as the secondary minerals. Chlorite can be seen in the form of coarse crystals, needles and very narrow in the background of the rocks, and they have been replaced in the remains of crystalline molds of the primary biotite type. Based on this, it can be said that the transformation of biotite to chlorite is associated with the depletion of K2O and the reduction of SiO2 which gave rise to the formation of potassium feldspar (Czamanske et al., 1988). Completely rutile and sphene crystalline can be seen in the process of chlorite cleavage. The presence of a small amount of garnet in the form of crystalline and cubic indicates low pressure or poor magnesium in the primary rock (Yardley, 1989). Also, the study of polished sections shows the presence of minerals such as pyrite, hematite, corundum and Au. The amount of pyrite and gold increases when approaching granodiorites, which can indicate the effect of intrusive bodies and post-magmatic fluids in mineralization in these rocks. X-ray diffraction (XRD) mineralogical study shows that the most important variation in chlorite is in Dalayon area. The main minerals in the host rock include chlorite, quartz, muscovite, illite, microcline, and secondary minerals are rutile and biotite. It can be concluded that biotite has been transformed into chlorite in the presence of hot fluids containing Fe and Mg. With the help of Fe2+/ (Fe2++Mg2+) versus 2*Si diagram (Pflumio, 1991) and also the Si content of Dalayon chlorites is of ripidolite-pychnochlorite type and show the presence of iron-bearing chlorites. The amount of Si indicates the purity of chlorites, and the very small amounts of calcium and also the amount of Xc points to the absence of smectite and the high purity of chlorites (Lori et al., 1988). The very low Ti content of chlorites and the presence of rutile, sphene, and leucoxene indicate that the Ti content of primary biotite is formed secondarily in the form of Ti-bearing minerals and in the form of thin blades parallel to the chlorite cleavage and orthogonal to it (Czamanske, 1988; Parry and Downey, 1982). Considering that the sum of octahedral cations in the samples is very close to 6, which indicates that all octahedral sites are occupied by divalent cations and are of triple octahedral type (Xie et al., 1997). Although the existence of a vacancy cannot be proven with certainty (Jiang et al., 1994), but with the help of the structural formula of ideal chlorite, the vacancy is calculated as 0.26-0.66 apfu (Xie et al., 1997).

The formation conditions of chlorite reflect its structure and chemical composition and can be used as a geothermometer (Jiang et al., 1994). The crystallization temperature of chlorites with the help of T-Al (IV) diagram (Cathelineau, 1988) is in the range of 379⁰C -305⁰C, with an average of 353⁰C and does not show much change. As the available data, display it can be said that by moving away from the granitoid intrusive bodies, the formation temperature of chlorites decreases. Therefore, the points located in the far area and in the metamorphic rocks with ore have a lower formation temperature. The existence of an inverse correlation between the crystallization temperature of chlorites and their silica content can be due to the substitution of silica instead of aluminum. The fluid inclusion studies on the quartz mineral co-growing with the chlorites of the Dalayon area shows the thermal range of homogenization between 305 and 384 ⁰C with an average of 345.6⁰C, which is consistent with the temperature of formation obtained through thermometric chlorites and indicates the effectiveness of the thermometer by chlorites. The fluid inclusions studies and chlorite thermometry show the influence of mesothermal or orogenic hydrothermal fluids in the formation of chlorite and placement within the skarn range, which confirms each other and is consistent with mineralogical studies and field observations. Overall., the formation of chlorite under study at a temperature equivalent to the high temperature of the hydrothermal stage of granites demonstrates the influence of hot fluids derived from the granitic magma and the metamorphic function in the formation of chlorite.