ali lotfi bakhsh

Zeolites, which form the largest group of silicate minerals, belong to the tectosilicate family, in which the [SiO4]4- and [AlO4]5- tetrahedrals are connected to each other in the form of a three-dimensional network so that an empty space is created between them. Channels are created in the structure of zeolites by placing the empty spaces one after the other. Zeolites have unique physical and chemical properties that have led to their widespread use in various fields. Most zeolites are secondary minerals formed in a water-rich environment in the temperature range of 40 to 250 °C. Zeolites can form during the reaction of aqueous fluids with rocks in various geological environments. They are formed during diagenetic processes in sedimentary rocks (including volcanic deposits) that can be grouped into several geological environments or hydrological systems, such as open hydrological systems, closed hydrological systems, soil and surface sediments, and deep marine sediments. Zeolites in volcanic lava cavities are formed during burial metamorphism of lava masses, hydrothermal alteration of continental basalts, or diagenesis in areas with high heat flow caused by active geothermal systems. The purpose of this research is to determine the type of zeolite minerals formed in the volcanic host rock located in the magmatic belt of Alborz-Azerbaijan in the northwest of Iran and their formation based on the data obtained from stable isotopes of oxygen and deuterium.

The studied area is located 25 km southeast of Ardabil and east of Hir, which according to the map of the main tectonic subdivisions of Iran is located in the Tertiary-Quaternary volcanic zone on the western Alborz-Azerbaijan. In this research, X-ray fluorescence analysis (XRF) has been used to determine the type of host rocks. Petrographic studies were done using microscopic thin sections by polarizing microscope. X-ray diffraction (XRD) and electron microprobe (EMP) analysis have also been used to study the mineralogy of the samples and their chemical composition. The analysis of stable isotopes of oxygen and hydrogen has been performed to determine the isotopic composition of zeolite minerals by mass spectrometer (MS) method.

Lithology in the studied area do not have high diversity and young volcanic rocks cover almost the entire area. The analysis of samples taken from the host rock showed that zeolites were formed in the Eocene andesitic volcanic unit with porphyry to megaporphyry texture. Plagioclases make up the most important phenocrysts of the host rock. Based on the results of XRD analysis and the study of thin sections, the zeolites of Hir region are composed of stilbite, barrerite, stellerite, chabazite, scolecite and mesolite minerals. The zeolite crystals are milky to pale yellow in color and often with radial, sugar cube, fascicled, intergrown blades and needles in the form of veins, filling the pore space and druses are scattered inside the cavities and fractures of the volcanic host rock. Zeolite mineralization in thin sections occurs as massive, open- space filling and inclusion. The massive types of stilbite often have a mosaic and intergrown texture, and its space filling types often have a radial and fan-like texture. Needle crystals of mesolite are formed in limited form as inclusions inside some stilbite crystals. The results of the microprobe analysis on the Ca+Mg–Na–K+Ba+Sr diagram showed that the studied zeolites including stilbite, scolecite and chabazite have a calcic nature, and stilbite and chabazite contain some amounts of sodium in addition to calcium as the main component. Also, the results of the analysis of oxygen and hydrogen isotopes of three selected zeolites showed that their isotopic values are close to each other and they are located in the δ18O-δD diagram in the field of meteoric hydrothermal waters and close to the kaolinite line. Being close the kaolinite line indicates their formation under surface conditions. Stilbite, scolecite and chabazite are formed at temperatures below 100 degrees Celsius in the order of stilbite → scolecite → chabazite.

The presence of evidences such as the formation of large and euhedral zeolite crystals, their limitation to the cracks and fractures of young volcanic rocks, and the absence of metamorphic zeolite facies minerals (such as perhenite) show the hydrothermal origin of zeolites in the Hir area. According to the appearance of zeolite minerals in the Hir area, the presence of suitable host rock and the results obtained from stable isotope studies, it is thought that the meteoric fluids penetrated into the volcanic units and were gradually heated. Then, by decomposition the glass matrix of the rocks and minerals prone to alteration such as plagioclase, they have provided the necessary materials (CaO, Al2O3, SiO4, Na2O) for the formation of zeolites. Zeolite minerals have formed by circulation inside the cavities, fractures and open spaces in the volcanic units in areas near the surface that have lower temperature.

The ever-increasing global demand for energy is associated with the more consumption of fossil fuels, which has caused the widespread release of CO2 gas into the earth's atmosphere. The process of carbonation in geological formations is one of the safest and most promising approaches for CO2 storage. Majdar area is located in the northwest of Iran on the Alborz-Azerbaijan magmatic belt. Eocene volcanic units consisting of andesite, basaltic andesite, basalt, agglomerate and tuff form the thickest and widest rock units in the study area. Basalt and basaltic andesite rocks have been studied in this area. Petrographic studies showed that plagioclase, pyroxene and some olivine minerals are the main phenocrysts of host rock placed in a glassy to microcrystalline matrix. The XRF results showed that Majder basalt samples contain adequate amounts of Ca oxides (10.29–12.27 wt%), Fe (9.83–12.66 wt%) and Mg (5.75–7.45 wt% ) that can react with fluids containing CO2 to form stable carbonate minerals. According to the XRD results, three carbonate minerals including calcite, manganocalcite and ankrite are formed along with secondary alteration minerals clinochlore, clinoptilolite, illite, montmorillonite, glauconite and quartz. The study of microscopic sections showed that the alteration of calcium and magnesium minerals such as augite and anorthite by CO2-rich fluids has caused the release of cations needed for the formation of carbonate minerals. The value of δ18O isotopes in the studied samples ranges from -10.14 to -12.54. The calculations based on the results of δ18Ocalcite show the temperature range of 71.35–88.35 °C for the formation of calcite, which corresponds to a depth of 2.4-3 km. According to the measured average porosity of rocks, the possibility of storing 154,000 tons of CO2 in a block measuring 1000 meters long, 1000 meters wide and 70 meters high in the studied area can be estimated.

Travertine refers to all non-marine carbonate sediments consisting of calcite/aragonite formed by calcium and CO2-rich fluids under surface conditions and low pressure. Travertine deposition takes place as a result of the decomposition of calcium bicarbonate and the release of CO2 gas. Based on elemental geochemistry, origin of carbon dioxide gas and composition of stable isotopes, travertines are divided into two groups: thermogenic and meteogenic. Studies of stable isotopes of carbon and oxygen play a fundamental role in determining the origin and type of travertine. Thermogenic travertines, which are often associated with young volcanic areas, were deposited from medium to high temperature fluids and their carbon isotope composition is heavy. While meteogenic travertines are formed from relatively low temperature fluids and their carbon isotope composition is light.

In this study, the mineralogical, geochemical and isotopic characteristics of Borjlu travertine located in the Alborz-Azerbaijan magmatic belt in the northwest of Iran have been investigated based on field and laboratory studies. In this research, petrographic studies were done using microscopic thin sections by polarizing microscope. The samples were analyzed by inductively coupled plasma mass spectroscopy (ICP-MS) to determine Ba and Sr contents. X-ray diffraction (XRD) and scanning electron microscope (SEM) imaging have also been used to study the mineralogy of the samples. The analysis of stable isotopes of carbon and oxygen has been done to determine the isotopic composition of carbonate minerals by mass spectrometer (MS) method.

Three facies including crystalline crust in the form of alternating light and dark layers, raft in the form of discontinuous layers, and shrub consisting of branching structures with rounded and buttons ends can be seen in Barjelo travertine. The result of XRD analysis from the crystalline crust shows that they are formed from pure calcite. SEM image shows the limited presence of microbial accumulations on the surface of calcite crystals. Petrographic studies of the crystalline crust part of travertine show that calcite crystals have grown in three forms of lamellar prisms, fibrous crystals and micrite grains. Some sections also indicate the alternating growth of crystalline crust and shrub facies. The average values of Ba and Sr in the travertine samples are 40.22 and 563.89 ppm, respectively. The values of δ13C(VPDB) and δ18O(VPDB) isotopes of the samples range from +1.44‰ to +2.19‰ and -14.39 to -16.51‰, respectively. The calculated values of δ18O(SMOW) and δ13C(CO2) for the samples show changes from +13.84‰ to +16.02‰ and -7.87 to -8.77‰ respectively.

The comparison of Ba and Sr contents indicates the thermogenic origin of Borjlu travertine. Although the δ13C(VPDB) positive values in Borjlu travertine indicate its thermogenic origin, its values are lower than typical thermogene travertines. Comparison of δ13C(VPDB) and δ18O(SMOW) values showed that Borjlu travertine samples located in the field of thermogene and meteogene travertines, which indicates the mixing of two fluids with heavy and light isotopic composition. The calculated values of δ13C(CO2) indicate the inorganic and internal origin of CO2 in the travertine-forming fluid. Also, in the isotope composition diagram of δ13C(VPDB) versus δ18O(VPDB), carbonate rocks are shown as the source of CO2 gas for the formation of Borjlu travertine. Some field evidences, such as the formation of crystalline crust and shrub facies, the presence of a hot spring with a relatively high temperature in the studied area, and the presence of a geothermal reservoir in the depth are signs of active hydrothermal phenomena in the area. It is thought that the presence of limestone units in the region and their contact with the fault system has provided the possibility of infiltration and circulation of hydrothermal fluids containing CO2 inside them. In this way, the bicarbonate ions necessary for the formation of travertine has been provided by decarbonation and dissolution of these carbonates. It seems that the fault system acted as a conduit for the migration and ascent of fluids containing calcium bicarbonate towards the surface. The mixing of this fluid with meteoric waters near the surface caused lighter isotopic composition of the ascending fluid. Based on isotope composition of travertine samples, the temperature of outflowing fluid is estimated about 70 °C.

Geochemical halos are regions around mineral deposits where the concentration of elements decreases to a constant level called the background. In the meantime, in order to find out the unusual concentration of elements (anomaly), the upper limit of the background limit (threshold value), must be determined. To achieve this goal, it will be effective and efficient to use new exploration methods. In this article, to draw the potential areas of stream sediments, modeling the data of element enrichment index with the U-spatial statistic method has been used. According to the morphological characteristics of the stream sediments environment, the origin of the samples taken from these sediments goes back to the upstream of the watersheds, so the analysis of the geochemical data of these samples is different from the rock environments. A preliminary modeling should be done on the data to consider the geological features upstream of the samples. The isotropic nature of the window in which the weighted averaging is performed in the algorithm of the U-spatial statistics method is such that all the upstream and downstream samples participate in this averaging and this case can be the point considered a significant weakness for this method. To reduce this weakness and to improve the method, first by considering the upstream rock units, the element enrichment index was obtained on the raw data, and then the U statistics was modeled on these indices. With this method, while the indices of the area were more accurately identified, the false potential areas obtained from the U-spatial statistic method were also removed and the results were acceptable and close to the field realities for determining mineralization potential areas.

In the north of Ardabil (from Namin to Lahroud) there are widespread sequences of Eocene and Quaternary mafic to intermediate and felsic magmatic activities with different compositions. The composition of these rocks varies from basaltic lavas as well as dacitic and rhyolitic domes in Namin to basalt and basaltic andesite in Lahroud area. The chemical composition of olivine from olivine basaltic lavas indicates a forsterite composition changing from 67.8 to 92.7. Clinopyroxenes show diopside composition whereas plagioclase has labradorite to bytownite composition. Garnet xenocrysts in the rhyolitic domes have an almandine composition. These rocks are characterized by the enrichment in LREEs compared to the HREEs. Mafic-intermediate rocks show shoshonitic to high-K calc-alkaline composition whereas dacitic and rhyolitic domes show adakitic signature. Geochemical and isotopic characteristics of basaltic-andesitic rocks indicate their genesis are related to the partial melting of a metasomatized mantle wedge, re-fertilized by sediments and fluids from the subducting slab in the Eocene subduction zone of Iran. The geochemical and isotopic signatures of dacitic-rhyolitic domes indicate their origin from partial melting of the lower parts of the thickened continental crust of Iran.