جستجوی مقالات مرتبط با کلیدواژه « سمنان » در نشریات گروه « زمین شناسی »

The Gano bauxite deposit is located 90 km northeast of Semnan city in the eastern Alborz Mountains, northern Iran. The bauxite ores occur as stratiform discrete lenses with a length of 6 km and thickness of 2–20 m along the contact between carbonates of the Elika Formation and shale, sandstone, siltstone, and coal of the Shemshak Formation. Mineralogical analyses revealed that the bauxite ores consist of diaspore, hematite, kaolinite, chlorite, anatase, illite, zunyite, goethite, quartz, and dolomite minerals. Fluctuations of the groundwater table level, acidic atmospheric waters, and an increase in pH of the weathering solutions close to carbonate bedrocks played an important role in the concentration of Fe-poor ores in the upper parts and Fe-rich ores in the lower parts of the studied profile. An increase in oxidation, the possible presentence of secondary phosphate minerals, fluctuations of the groundwater table level, and the role of carbonate bedrock as an active buffer played an important role in the extent of Ce anomaly in the ores (0.79–12.25). The pH variations of weathering solutions, fluctuations of the groundwater table level, the role of carbonate bedrock as a geochemical barrier, and simultaneous precipitation of Fe-bearing minerals and preferential scavenging of LREE(La–Eu) by hematite played an important role in the distribution and fractionation of rare earth elements in the bauxite ores. According to geochemical considerations (Eu/Eu* vs. TiO2/Al2O3 and Sm/Nd bivariate diagrams), the Gano bauxite deposit probably derived from the weathering of intermediate igneous rocks.

In order to study the environmental characteristics of Semnan spas, water, sediment, and slime samples were collected. The concentration of Ag, As, Cd, Pb and Zn (0.7, 33, 0.4, 128.5, and 297 mg/kg, respectively) in the sediment samples are much higher than their respective values in mean crust and mean sediment compositions. The sediments are not enriched in Mo, Ni, Th, Co, Cr, Mn, Fe and U, while moderately enriched in Sb and Cu, significantly enriched in Cd, As, Zn and Pb, and highly enriched in Ag. The Bioconcentration Factor (BCF) values of As, Mn, Cu, Ni, Pb and Zn in slime samples are very high (1286, 2863, 2340, 1053, 1236, and 976, respectively). The water samples are classified as neutral-high-metal, and the concentrations of B, Li, Ba, Zn, Pb, Se, Cu, As, Ni and Mn in all samples are higher than their mean concentrations in unpolluted natural waters. The values of Contamination Degree (Cd) and Heavy Metal Pollution Index (HPI) in all samples are higher than 3 and 75, respectively, confirming the high level of pollution in the samples. The Water Hazard Index (WHI) values of the studied samples are lower than 5, indicating the non-toxicity of the target elements. The high concentration of potentially toxic elements in water, sediments and slime samples of Semnan Spa should be taken into account by people who use the Semnan spas for therapeutic purposes.

The study rocks outcropped in the northeast Semnan area. These rocks, composed of monzogabbro-monzodiorite, was intruded in the Eocene sedimentary and volcanic rocks. Plagioclase and clinopyroxene are the main constituent minerals of these rocks. The study rocks represent granular and poiiklitic textures. Based on EMPA data, clinopyroxenes, and plagioclases are diopside and augite and andesine, labradorite and bitonite in compositions and formed in temperatures ranged from 1104 to 1168 and less than 700 °C respectively. On the basis of tectonic discrimination diagrams, the investigated samples fall into volcanic arc domain resulted in subduction of Neothetian oceanic lithosphere beneath Central Iran block.

The Vanakan (Sokan) barite-zinc-lead-copper deposit is located at 23 km northeast of Semnan, in the North Central Iran magmatic belt. It has occurred within the Eocene volcanic-sedimentary sequence. The host rocks of the ores mainly consist of tuff, shale and shaly tuff. Volcanic rocks in the district at the Ahovan region involve both mafic and felsic compositions including basalt, andesite, dacite, rhyolite and tuff. Many studies have been conducted on ore deposits in the Semnan region including Poshteh barite- base metals volcanogenic massive sulfide (VMS) deposit (Ghaffari, 2017), Hamyard (Haji-Bahrami, 2012) and northeast Semnan (e.g., Ghiasvand et al., 2009; Shahri, 2011) iron skarn deposits. Therefore, studying the barite-metal deposits in the Central Iran magmatic belt such as the Vanakan deposit, can provide exploratory keys to discover new reserves, which is one of the main goals of this research study. In this work, study on ore facies,mineralogy, alteration, geochemistry and genesis of the Vanakan barite-zinc-lead-copper deposit are considered.

First, regional and local geology, alteration, ore textures and structures and mineralogy of ore horizons in the Vanakan ore deposit were carefully checked out and studied during field studies. Then, the samples were systematically collected from trenches and open pit of the mine. Mineralogical studies were conducted on 24 thin sections and 8 polished samples in the microscopic laboratory at the Shahrood University of Technology. For geochemical studies, about 16 systematic samples from different ore facies and ore horizons were collected. Then, the samples were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method, and a few samples were studied by X-ray diffraction (XRD) method in the Aria Sharif Laboratories Company.

The host sequence in the Vanakan deposit involves three units, from bottom to top: Unit1: conglomerate, limestone, sandstone; Unit2: andesitic to dacitic lava-rich, and unit3: acidic tuff-rich. Mineralization as the Vanakan 1 and 2 deposits occurred at top of unit 2 and within unit 3. The entire Vanakan area involves a local syncline with northeast-southwest axial trend, in which the Vanakan 1 and Vanakan 1 deposits are located in the northern and southern limbs of the syncline, respectively. Based on structural,  textural and mineralogical studies, five different ore facies were distinguished  in Vanakan 1, from bottom to: 1) vein-veinlet and breccia: involving barite-pyrite-quartz vein-veinlets, 2) massive sulfide: composed of massive sphalerite, galena, barite, chalcopyrite and pyrite, 3) layered-banded sulfide ore: involving alternations of ore and sericite altered tuff-rich bands, 4) baritic ore: comprising of mainly barite and little sulfides, and 5) banded-exhalative cherty sediments. The ore facies in the Vanakan 2 from bottom to top are 1) barite -(galena)-rich vein-veinlets and 2) banded cherty iron oxide-hydroxides -rich red exhalative sediment. From a mineralogical point of view, the ores in the Vanakn 1 mainly consist of barite, sphalerite, galena, pyrite, chalcopyrite and marcasite accompanied with secondary minerals such as malachite, chrysocolla, smithsonite, cerussite, hematite, limonite, goethite.

Based on different characteristics of mineralization in the Vanakan district, such as geometry of ore bodies, textures and structures, ore facies, wall rock alterations, mineralogy, metal zonation and geochemical features, the Vanakan deposit can be classified as a bimodal- felsic or Kuroko-type volcanogenic massive sulfide (VMS) deposit, similar to those of the Mount Read volcanic deposits of Tasmanian Australia such as Rosebery (Large, 1992; Large et al., 2001) and Hokuroko basin in Japan (Huston et al., 2011; Ohmoto and Skinner, 1983).

The Nokeh intrusion exposed in the northern Semnan area. The intrusion is composed of monzonite-quartz monzonite and granite-granodiorite and was intruded in the Eocene carbonaceous tuffs, where the country rocks converted to magnetite-skarn. Plagioclase, orthoclase, quartz, biotite, amphibole and clinopyroxene are the constituent minerals of Nokeh intrusion. The study rocks represent granular, granophiric and mirmekitic textures. The Nokeh intrusion is metaluminous to peraluminous, calc-alkaline, I-type and belongs to subalkaline magmatic series. Based on EMPA data, clinopyroxenes, amphiboles, biotites and plagioclases are diopside, edenite, Mg-biotite and oligoclase to labradorite in compositions and formed in temperatures ranged from 1110 to 1160, 700, more than 800 and less than 700 °C respectively. Clinopyroxene, amphibole and biotite calc-alkaline affinity, low Ti and Ca-Si enrichment in the clinopyroxene composition and amphibole formation in a high-fugacity environment, confirm that Nokeh intrusion formed in a magmatic arc of active continental margin. On the basis of tectonic discrimination diagrams, the investigated samples fall into volcanic arc domain resulted in subduction of Neothetian oceanic lithosphere beneath Central Iran block.

Eocene volcanic and volcano- sedimentary rocks cover a large area in the Gardaneh Ahovan northeast of Semnan. These rocks comprise mainly of basalt, andesite, trachyandesite, rhyolite and dacite with hyalo porphyry, porphyry and glomeroporphyry textures. The basaltic rocks are composed of plagioclase and clinopyroxene phenocrysts. Sericite, kaolinite, calcite and chlorite also form the secondary minerals in these rocks. The size of the clinopyroxenes of these rocks range from fine to medium and their compositions include iron-magnesium-calcium type and plot in the diopside field. Medium-size clinopyroxenes show reverse zoning and compositional changes so the middle zone (btween core and rim) is depleted in Fe, Al and Ti whereas enriched in the Mg, Ca and Si. The determination diagrams of the magmatic series and tectonic environments, based on the chemical composition of clinopyroxenes, suggest that they are crystallized from alkaline to sub-alkaline magma, contianing 2-5% water and high oxygen fugacity. Thermo-barometery calculations also show that these clinopyroxenes were crystallized in pressures of ~1 kbar and temperatures of 1110-1230 °C.

Today, Soil erosion has become one of the biggest problems in the country, especially in arid and semi-arid including Semnan. Effective and long term water and soil conservation programs require the concentration of resources on limited areas. For that purpose, regional-scale assessments of erosion risk are required. There are various methods to studying, evaluating, calculate and prevention with soil erosion. In addition, a number of parameters such as lithology, slope, aspect, land cover, elevation, and distance to stream, drainage density, vegetable cover, land use, river banks, and human activities are recommended to analyze the mechanism of soil erosion. So a rapid and cost effective methodological erosion assessment for these regions is required to describe and monitor the processes that control erosion. This study uses one of the remote sensing analyses to describe the contribution of several factors that control erosion in Semnan drainage basin where erosion is the major environmental problem. Remote sensing monitoring has been carried out by using aero photos, or multispectral images, DTM (Digital Terrain Model) or ALS (Airborne LASER Scanning) data. Semnan basin, study area, is located in north of Kavir plain and south of Alborz mountain range.
This study was conducted to evaluate the potential of analyzing regional erosion risk Topography, land use; vegetation density, soil properties and climatic proxies are used to determine erosion risk and to provide basic maps of water and soil conservation practices. A hierarchical decision tree is used to sum and combine the weight of parameters controlling the erosion. The assigned weights of each spatial unit express the susceptibility to erosion. The most important attributes in the definition of erosion landforms like gullies were selected using decision tree induction algorithms, being these attributes spectral, altimetry and texture. Classifications hierarchical and by decision trees were carried out. Using decision tree the classification is performed only by a factor of scale, not allowing the identification of all the constituent features of the erosion landforms system. One of the advantages of this method is that it can be used if there are insufficient experimental data. The lack of experimental data can be compensated for through the use of expert evaluations.
Three different combinations of the three dominant controlling factors are yielded in this study. In order to optimize the qualitative erosion risk assessment, each combination is discussed and evaluated depending on the contribution of parameters involved in the erosion process. As different erosion landforms erosion is similar when presents the same evolution stage and soil type, it is not possible to select attributes to classify all erosion landform systems, being necessary to investigate attributes for each erosion landform erosion, based on available data and existing land use classes in the area. The erosion landforms are the biggest erosive processes and, consequently, responsible for ambient, social and financial damages. Corrective and preventive measures need mapping and monitoring, which can be made by local measurements or by remote sensing. In relation to the remote sensing, the erosion landform erosion presents spectral heterogeneity (soil, vegetation, shade and water mix), spatial heterogeneity (existence of features as head, canals and digits with irregular forms and variable dimensions) and altimetry variation (with high declivity on the edges). Due to spectral heterogeneity, it is not enough use only spectral data, being necessary auxiliary data, as altimetry and texture data. This clearly shows that the study area is generally exposed to a high hazard of soil erosion. Nevertheless, there is a probability that the rate of erosion will increase in the future, as hazard is the probability of occurrence of a potential damaging phenomenon, within a period of time and a given area. As known, there is always an interest to depend on latest developments when making subjective judgments. In spite of the results obtained in this study, the development of a susceptibility map is usually determined by the needs and available resources, and AHP method can be equally important for all sorts of susceptibility zoning practices.
The purpose of this study was to assess the soil erosion hazard in the Semnan province for planning appropriate conservation measures. The integrated GIS-AHP model was used to define spatial distribution of soil erosion hazard. In this area, erosion risk mainly was related with vegetation and also, it anticipated that the southern and south-eastern region due to the poverty of vegetation associated with increased levels of erosion. In each of the three mapped models, the area of the class with high erosion sensitivity was more than 75% and for observational data, the area in all three maps is above 71%. Also, the results of the assessment show that in all three maps there are over 99% correlation between the data obtained from the modeling and the test data. The erosion landforms present spectral and spatial heterogeneity and altimetry variation. This research demonstrates that the model developed was an effective tool for fast assessment of soil erosion hazard by the integration of remote sensed data, AHP, and GIS techniques. Nevertheless, the results obtained in this study are valid only for generalized planning and assessment purposes. They may be less useful at the site-specific scale, where local geological and geographic heterogeneities may prevail. Finally, any proposed decision-making tool in erosion control studies should also include local experimentation data to better simulate the erosion hazard, resulting thereby in the most appropriate and efficient choice of soil conservation works.

Hamyerd iron deposit is located about 74 km north east of Semnan, in the Jam area. This area is situated in the northern part of Central Iran structural zone. The outcrops are Middle-Eocene volcanic and pyroclastic rocks with andesite to andesite-basalt and tuff composition, which are intruded by some intrusive bodies of monzonite to monzodiorite composition in this area. Iron mineralization is occurred at the contact between intrusive bodies with volcanic rocks. Widespread presence of hematite with minor contents of magnetite, pyrite, barite, and calcite is the main characteristic of mineralization in Hamyerd deposit. Mineralization is occurred as veins and lenses of hematite with leser amounts of magnetite. According to geochemical properties, the sub-volcanic intrusive body of Hamyerd is calc-alkaline and high K calc-alkaline and meta-aluminous in nature, belonging to I-type volcanic arc granitoids. Enrichment of LIL elements compared with HFS and negative anomaly of Nb and Ti and the samples plot within the VAG field suggest that the sub-volcanic intrusive body of Hamyerd has formed in a volcanic arc environment, related to the subduction zone.

The Nukeh iron deposit is situated at the north of Semnan and at south of Central Alborz structural zone. Volcano-pyroclastic rocks with Eocene age are the host of this deposit. Iron mineralization occurs as massive, disseminated, vein and breccia types in the Nukeh deposit and magnetite, hematite, pyrite, chalcopyrite, garnet, epidote, quartz and calcite are the main minerals in this deposit. Fluid inclusions and stable isotopes (O, C, S) have been used to reveal the physico-chemical characteristics of hydrothermal fluids and genesis of the Nukeh Fe deposit. Seven types of fluid inclusions are identified in quartz, according to the phase numbers, which include, liquid inclusions (L), liquid-rich inclusions (L), vapor-rich inclusions (V), vapor inclusions (V), simple brine inclusions (L埤), halite-bearing liquid inclusions (L) and opaque-bearing liquid-rich inclusions (L埣). The ranges of homogenization temperature and salinity of liquid-rich fluid inclusions in quartz are 100-200˚C and 10-20 wt. % NaCl equivalent, respectively, whereas the ranges of homogenization temperature and salinity of vapor-rich fluid inclusions are 350-500˚C and 10-30 wt. % NaCl equivalent, respectively. Also homogenization temperature and salinities of liquid-rich fluid inclusions in calcite in garnet (type a) and magnetite (type b) zones is 75-125 ˚C but the salinity of fluid inclusions in calcite in garnet zone (15-25 wt. % NaCl) is more than salinity of these inclusions in magnetite zone (10-20 wt. % NaCl). δ13C and δ18O values of calcite (n=15) vary between -1.9 to .1 ‰ (VPDB) and -19.4 to -14.9‰ (SMOW), respectively. The average value of δ18OWater is of .85‰ (SMOW) in the Nukeh Fe deposit is different from the values for the primary magmatic fluid. Pyrite is the main sulfide mineral in the Nukeh Fe deposit and δ34S values of pyrite (n=9) is within the range of .9 to .4 ‰ CDT . The source of sulfur is considered to be magmatic on this basis. Fluid inclusions and stable isotopic (O, C, S) data suggest that the ore-forming fluids evolved by the various mixtures of magmatic brines and meteoric water and probably the genesis of the Nukeh Fe deposit is similar to skarn deposits.

The Garmab Cu mineralization, in the east of Semnan and southeast of Shahroud, is situated in the Eocene conglomerate association with weakly hydrothermal altered. The weak alteration, mineralization and geochemical haloes is locally controlled by faults and late fractures. At the surface, secondary copper minerals including malachite and azurite and in the depth, primary sulfides generally are pyrite, arsenopyrite and chalcopyrite. Gangue minerals of the veins are commonly quartz, chalcedony, opal and minor calcite. Other components contain goethite, limonite and hematite with manganese oxides and minor jarosite smithsonite and gypsum. The results of statistical geochemical analyses indicate a positive correlation between copper with Au, Ag and As. According to host rock, mineralization, spatial relationships and geochemical dispersion patterns, deep drilling is suggested in the depth in the east of mineralization. The study district is related to buried intrusive bodies and epithermal vein-type Cu deposits.

The Hamyerd iron deposit is located in the northeast of Semnan in the boundary of the southern Alborz and Central Iran structural zones. A sub-volcanic body of monzonite and monzodiorite composition intruded limestone and pyroclastic rocks (equivalent to the middle Eocene Karaj formation). The iron mineralization occurred at the contact between intrusive bodies and these sedimentary rocks. The extensive hematite content along with minor amounts of magnetite، goetite، limonite، pyrite، dolomite، barite and calcite are important characteristics of the Hamyerd ore deposit. Mineralization occurred as veins and also hematite lenses with minor magnetite content. Fluid inclusion and stable isotope (S، C and O) studies were integrated to explore the Hamyerd iron ore genesis. Petrographic studies display five types of fluid inclusions in quartz and 4 types in barite. Fluid inclusions in quartz include single-phase liquid، single-phase gas، two-phase liquid-rich، two-phase gas-rich، and three-phase (liquid-solid-gas) inclusions. Three-phase liquid-solid-gas inclusions were not detected in barite. Microtermometry studies in two-phase liquid-rich inclusions revealed homogenization temperatures of 200-250 ˚C and 100-200 ˚C، and salinities of 10-20 and 0. 5-5 wt% NaCl equivalent for quartz and barite fluid inclusions، respectively. Microthermometry of halite-bearing three-phase fluid inclusions showed homogenization temperature from 200 to 350 ˚C and salinity from 30 to 40 wt% NaCl equivalent. δ34SCDT values of pyrites at Hamyerd iron deposit are in the range of 2. 2 to 7. 4‰. The isotopic values of barites range from 13. 6‰ to 20. 2‰ for δ34SCDT and 10. 2‰ to 12. 1‰ for δ18OVSMOW، respectively. The carbon and oxygen isotopic values of calcite are in the range of -3. 4‰ to -4. 5‰ and 17. 7‰ to 19. 1‰، respectively. Microthermometry of fluid inclusions and stable isotopes (S، O، C) at Hamyerd iron deposit suggested mixing of magmatic and meteoric fluids as origin of hydrothermal solutions. Mineralization in the Hamyerd iron deposit is probably similar to Fe-skarn deposits.

The Kuh Zar Au- Cu mine is located south of Semnan province within volcano-plutonic belt of Troud-Chah-Shirin with upper Eocene to Mio-Pliocene age. The tourmaline of the study area occurred as concentration or massive forms within altered zone in contact with the granodioritic intrusion and related to the hydrothermal system. In the area, the tourmalinization process is pervasive and in association with mineral assemblage sericite+quartz+tourmaline+pyrite+feldspars as well as Cu ore deposit. The composition of tourmalines is Shorle–dravite series with more tendency towards dravite component. These phases belong to alkaline tourmaline and are associated with quartz- tourmaline, meta-pelites, Ca-depleted meta-psamiteand and meta-pelites associated with Al-phase. The studied tourmalines contain X-Ca=61.07-1.77, X-Mg=0.01-0.1 and Al>6a.p.f.u. Therefore, the replacements of AlO(Mg)(OH), CaMgNa-1Al-1 and Al(Na-Mg) is possible in their structure. High Mg content and chemical zoning of these minerals point to their hydrothermal origin. The FeO/MgO+FeO ratio <0.6 in the studied tourmalines confirms the role of hydrothermal fluids.

The iron deposits in north of Semnan are located in the south of Central Alborz structural zone. Stratigraphically, the area consists of Paleozoic to Quaternary rock series exposures. The area has been affected by Semnan, Darjazin, Attari and Diktash faults. An intermediate to acidic granitoid body of calc-alkaline and metaluminous composition, representing I-type granite characteristics, has intruded the Eocene volcanopyroclastic rocks in the north of Semnan. Skarn development and iron mineralization have occurred at the contact of the intrusive body and the volcanopyroclastic rocks. Mineral Paragenesis consists of magnetite accompanied by hematite, oligist, pyrite, chalcopyrite, garnet, pyroxene and epidote. Geometry of the ore bodies is massive, lenticular and vein type and their texture is disseminated, brecciated, vein-veinlet and massive. Dominant alterations in the area are propylitic, argillic, silicic, sericitic, chloritic and pyritic, respectively. The intrusive body has many similarities with intrusive bodies which form Fe-skarn deposits. Variations in the calculated parameters for REE indicate contribution of magmatic origin hydrothermal fluids to mineralization and that the intrusive body has had the dominant role as source of the skarn ore materials. Along with the intrusion, emplacement and crystallization of intrusive body, Fe-bearing fluids have intruded the volcanopyroclastic rocks, forming sodic metasomatism and deposited iron ores in the north of Semnan which have many similarities with calcic Fe-skarn deposits.

The Dalichai Formation at Balu section in Payambaran area, northern Semnan, was studied by using palynomorphs and organic matter contents in order to determine its Palaeonvironmental conditions. A total of 55 samples were collected and treated for palynological studies. The formation at this section consists mainly of shale, calcareous shale and limestone. Palynologically, productive samples contain spore, pollen grains, dinoflagellate cysts, foraminiferal linings, acritarchs, wood debris (plant tissue) and Amorphous Organic Matters (AOM). The high amount of the AOM indicates a shallow, low oxygen levels and relatively quiet environment. Based on palynofacies data, such as proportion of blade-shape to equal dimensional opaque palynomacerals, the percentage of AOM to marine palynomorphs was determined. This indicates deposition of Dalichai Formation in a shallow, low oxygenated condition with a slow rate of sedimentation. However, occurrence of marine elements testifies intermittent marine incursions. The sharp increases in abundance of chorate dinoflagellate cysts in two parts of the section indicate short-term marine incursions in these parts.

In this study, sediments of the Qom Formation were investigated from the two stratigraphical sections in the northeast (Attari section) and southeast (Garmab section) of Semnan. The Attari section that consists mainly of limestone, sandy limestone, sandstone, calcareous shale and marl has a thickness measured 280 meters. In this section, the Qom Formation lies nonconformably on basic subvolcanic rocks and is disconformably overlain by the Upper Red Formation. Garmab section, with 97.5 m thick is composed of limestone and lies disconformably between Tuff at the base and Upper Red Formations at the top. The study of 167 samples taken from the studied sections led to the identification of 31 genera and 36 species of benthonic foraminifera, 1 genus and species of planktonic foraminifera, 3 genera and species of red algae and 2 genera and species of bryozoa. Among them, benthonic foraminifera have more variety and abundance, so the benthonic foraminifera from the measured sections are used to determine the age of successions and justify their correlation. Based on the identified foraminifera, the Attari section is comparable to biozonation introduced by Adams & Bourgeois (1967). It is analogous to Borelis melo group-Meandropsina iranica Assemblage Zone (1). According to the distribution of the index foraminifera, the age of sediments belonging to studied formation in Attari section is Burdigalian. The age of sediments of the Qom Formation in Garmab section based on the occurrence of Triloculina tricarinata, Triloculina trigonula and Globigerinoides triloba is probably Aquitanian.

The sequence of sedimentary rocks of Aftar syncline (35 Km west of Semnan) be longs to Karadj, Semnan and Kond formations. Celestite occurs at the lower part of the Kond formation. The Kond formation itself is mainly made of gypsum, limestone and sandy limestone deposited in an evaporitic environment. Based on the type of fossils found in the limestones of the Kond formation, the age of this sequence is attribnted to upper Eocene. The formation was formed during a slow and fluctuating subsidence in an evaporitic shallow basin. The layered, rhythmic and elongated lenticular celestite crystals are positioned parallel to the general bedding of the sequence and formed at early principal diagenetic stage. Two forms of celestite crystals are distinguished: a) long automorphic, to xenomorphic, slabs, and b) elongated lenticular gypsum pseudomorphs. The act of silicification, at late diagenetic stages, affected the celestite bearing layers more than other layers. Based on the shape and arrangement of celestite in the evaporitic sequence the genesis of major portion is believed to be diagenetic; while a small portion has been probably formed synsedimentarily by saturation of strontium ions in solutions of evaporitic environment. It seems that strontium sulphate (celestite) was formed due to the abundant gypsum in shallow evaporitic environment. Furthermore; celestite could have been formed due to the migration of strontium ions from lower layers (Semnan and Karadj formations) or by transport of the ions by meteoric fluids. Relatively high strontium ion content of the Semnan and Karadj formations suggests a high probability of ion supply to form celestite from these formations.

The bentonites of Aftar region, with average of 15 meters thickness and about 10 km long, are volcanoclastic sequence of Semnan Formation that formed in a shallow sea water environment. Based on the field, XRD data, microscopic and scanning electron microscopic (SEM) studies, as well as chemistry, the bentonite beds contain zeolites (clinoptilolite and mordenite), gypsum, calcite, celestine, opal, quartz, crystobalite and aragonite associated with clay minerals. XRD data of air-dried, glycolated and heated of clay fractions show the majority of the clay minerals are of swelling and of dioctahedral, smectite types. FTIR spectra, in agreement with chemical analyses data, reveal that the smectites are of Wyoming type (SWy-2) montmorillonite. The structural formula unit of representative clay of Aftar region, based on 11 oxygens, is:(Ca0.057Na0.270K0.030)(Al1.515Mg0.313Fe0.109Ti0.010)(Si4.015O10)(OH)2.Based on the composition of tuff and bentonites, the bentonites appear to be derived from alteration of acidic (dacitic to rhyolitic) tuffs of Eocene age in a relatively basic environment. Relatively stable smectites and zeolites are formed by dissolutim of less stable glass of vitric ash that was present in tuff during variation in basic environmental conditions.