Introduction Kaolin has been one of the important industrial minerals and is frequently utilized in many aspects of our lives (Bundy, 1993). Kaolin, which comprises predominantly kaolinite, has unique properties such as crystalline morphology, natural whiteness, fine particle size, nonabrasiveness, low surface area, cation exchange capacity, softness, and chemical stability (Kuşcu and Yıldız, 2016). Kaolin deposits have been extracted and mined historically. Kaolin is used as a raw material in the ceramic, paper, paint, rubber, abrasive, and pharmaceutical industries (Murray, 1991; Manju, 2002). Kaolin quality changes from one deposit to another. In this regard, utilization potentials of a kaolin deposit depend on factors such as chemical composition (Al, Si, and iron oxide concentrations) and the color index. Kaolin deposits are found in primary (residual) and secondary (sedimentary) types (Prasad et al., 1991; Murray and Keller, 1993). The kaolin resources of Iran have a wide distribution throughout the country. Hydrothermal kaolin deposits are found at many points, such as the northwest and southeast of the Iranian plateau, while sedimentary kaolin deposits are seen in central Iran and associated with huge sedimentary basins. Hydrothermal alterations caused by Tertiary volcanic activities are observable in many parts of NW Iran (Figure 1). For example, the Tarom-Hashtjin alteration zone is considered as one of the most significant mineralized zones in Iran. This altered zone, which has experienced Tertiary magmatism and volcanism, has a high distribution of metallic mineralizations such as Cu, Pb, Zn, Fe, Au, and Ag and nonmetallic deposits such as kaolin, bentonite, zeolite, and perlite. There are some kaolinized spots in the Tarom-Hashtjin altered zone, with the Kejal kaolin deposit being the most important one having medium-to high-quality kaolin Abstract: The Kejal kaolin deposit, situated in the northwest of Iran, is considered a small part of the Hashtjin hydrothermal zone. The kaolinization process in this deposit has occurred in ignimbritic and volcanic tuff parent rocks in different grades such that severely kaolinized samples can be found in the middle section of the kaolinization profile. Kaolinite is the most abundant clay mineral in the studied samples. Quartz and cristobalite are the main minerals while anatase is the minor mineral of the kaolin samples. Among the major oxides, SiO 2 , Al 2 O 3 , and TiO 2 show the highest concentrations in kaolin samples. The mass change calculations based on Ti show enrichment of Si, Al, Sr, and LREEs and depletion of alkali and alkali earth elements, HREEs, and HFSEs. The evaluation of REEs normalized to chondrite represents the higher concentration of LREEs comparing to HREEs. A remarkable positive Gd anomaly is seen in spider diagrams, which can be attributed to the primary CaO in the composition of ignimbritic precursor and also the Gd release due to the decomposition of Gd complexes and adsorption by clay minerals. The Eu negative anomaly is the other obvi...

This study evaluates the rare earth element (REE) geochemistry in altered trachyandesitic ignimbrites, tuff and lava flows in the Hashtjin area by assessing chondrite-normalized REE patterns and Y/Ho geochemical ratios. Modifications in the REE patterns took place along altered fault zones that were affected by hypogene and supergene alterations. The precursor volcanic and pyroclastic rocks contain phenocrysts of plagioclase accompanied by augite, zircon, apatite and pyrite. Based on X-ray diffraction analysis, the main mineral assemblages of the altered units consist of kaolinite as the main clay mineral, SiO 2 polymorphs (quartz and cristobalite) and anatase as a minor constituent. The chondrite-normalized REE patterns of argillic samples reveal fractionation of light REEs (LREEs) compared to heavy REEs (HREEs), together with a marked Eu anomaly and a weak W-type tetrad effect related to the weak non-charge radius control (CHARAC) behaviour of REEs and slightly higher Y/Ho and Zr/Hf ratios. The relationship between the Y/Ho and Zr/Hf ratios and recognizable T 3 and T 4 effects (tetrad effect) suggests that an increasing degree of water-rock interaction occurred during hypogene alteration processes by acidic hydrothermal fluids that were overprinted by supergene alteration. Water-rock interaction and adsorption by Mn-oxides and clay minerals are considered to have played important roles in determining the close to non-CHARAC behaviour of REEs during the argillic alteration of the pyroclastic rocks in the Hashtjin area.

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The Mount Sabalan district is regarded as the best place to investigate geothermal activities in northwest Iran. Since the last episode of volcanic activity in the Plio-Quaternary time, hot springs and surficial steams as conspicuous manifestation of geothermal activities have appeared around the slopes of Mount Sabalan. The hot fluids circulating in this geothermal field contains anions chiefly of HCO 3 and Cl -; however, SO 4 2content in some water samples is relatively high, imparting sulfate characteristics to such fluids. Geothermometric studies provided compelling evidence for estimation of the reservoir temperature (~150 °C) in the study areas. Thus, in this respect, the geothermal systems in the east of Mount Sabalan were categorized as high-temperature. The composition of stable isotopes of oxygen (δ 18 O) and hydrogen (δD) indicated that the waters involved in this geothermal field have mainly meteoric origin. On the basis of 3 H isotopes, only a few water samples exhibited a residence time of ~63 years, which can be grouped as old waters.

<p>The studied area is situated as a narrow strip NE Iran and belongs to Central Iran structural zone and also the Kavir-Sabzevar magmatic zone which is related to <em>continental arc subduction zone of NE Iran. The mineralization is hosted in Eocene andesite-basalt, trachyandesite, andesite, and monzodiorite with high potassic shoshonite affinities that have undergone propylitic alteration. The </em><em>disseminated copper oxide and sulfide mineralization occurs in the form of stratabound, vein-veinlet and open space filling</em><em>,</em><em> </em><em>resulted from replacement of volcanic units simultaneously with sericitization and silicification of the host rocks. Two stages of mineralization have been recognized, namely a hydrothermal stage and supergene enrichment stage. Chalcocite, covellite, chalcopyrite, bornite, digenite, and tetrahedrite are the main hypogene minerals, that were altered during the supergene enrichment into bornite, secondary chalcocite, and Cu-carbonates. The range of </em>δ<sup>34</sup>S values in primary chalcocites (-7.7<em>‰</em>-9.3<em>‰</em>) and the δ<sup>34</sup>S <sub>H2S </sub>(-3.5<em>‰-5.2‰</em>) indicate that the source of sulfur in ore forming fluids was most likely sedimentary. The hypogene ore minerals are enriched in Ag, As, Ba, Sr, Fe, Mn, Zn, and Ce compared to their supergene counterparts. The enrichment of Cu took place during the supergene stage. The study of hornblende pyroxene olivine andesite basaltic zircons, show the average age of 5.47 ± 0.22 my for this unit. Also, based on the study of zircons, the average age of the pyroxene hornblende monzodiorite is 2.44 ± 0.79 my.</p> <p><strong>Key words: </strong><em>Gol-Cheshmeh; </em><em>Manto-type; Cu mineralization; sulfur isotopes; Nd-Sr isotopes; NE Iran.</em></p> <p> </p>