Occurrence and genesis of zeolites located in Hir in Ardabil province based on the stable isotopes findings

Introduction
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.
 
Materials and Methods
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.
 
Results and Discussion
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.
 
Conclusion
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.