عنوان مقاله [English]
Introduction The Tanbo Village is located in the Senderk region, south of Minab, Iran .Based on the Geological map of Taheruie (scale: 1/250,000), rocks of the study area are cropped out in the most western parts of Makran Ophiolite Zone where it crosses Zagros Fold Belt. Serpentinite, talc, harzborgite, conglomerate and calcareous Al-silicate rocks are the most Miocene exposed rocks occurring in Tanbo region. A harzborgite conglomeratic rock sequence exposed in light green color and includes long fibrous secondary minerals. The main scope and interest of this work is to characterize the fibrous minerals properties and their genetic relation. In the preliminary studies of this research, the fibrous minerals were assumed to be as “white asbestos group” belonging to the fibrous serpentinite (chrysotile). However, until now there is not any publication/information available about the lengthy fibrous minerals of this part of Iran. Based on the field observations, thin section studies, XRD and XRF analyses the fibrous silicate minerals are type of clay, namely “sepiolite”. Petrological and geochemical data indicate that the Tanbo sepiolite developed through hydrothermal fluids which strongly affected harzborgite. The dissolution of Fe2+, Ca2+ and Mg2+ cations at high fCO2 resulted in carbonation of chrysotile in the presence of fluids rich in CO2, H2O and SiO2. Carbonation of chrysotile at lower Al-activities and chemical reactions of the above mentioned fluids with high Fe-Mg rocks provided conditions to perform calcite, dolomite and Fe-sepiolite assemblage. The length of fibers in sepiolite sometimes reaches 60 cm and the ratio of the length to width is more than 1200. According to the XRF analyses on fibrous minerals of Tanbo Village and comparing data with published paper for sepiolite from NE Iran and international countries, the presence of sepiolite in Tanbo Village is confirmed. The formation of Tanbo sepiolite seems to be secondary because it has been developed during and after crystallization of carbonaceous minerals.Materials and MethodThis study is organized by three stages including literature review, field geology and experimental laboratory works. The literature studies indicate that there is not any published data regarding to mineralization and the presence of sepiolite in the region. After careful interpretation of field evidence, 10 microscopic thin sections from main rock assemblage and 10 thin sections from co-genetic minerals were prepared and studied by polarized microscope Leitz Laborlux in Shahid Rajaee Teacher Training University (SRTTU). The impurities were removed for all hand specimens, then samples were prepared for XRD and XRF analyses, undertaken by Geological Survey of Iran and Binalood Kansars Lab.Results and DiscussionMicroscopic studies reveal two groups of phyllosilicates including fine fibrous serpentinite, talc, calcite and dolomite. into the ultramafic rocks; and long fibers (up to 60 cm in length) of sepiolite into veins in contact with carbonates and harzborgitic rocks. The formation of sepiolite in veins hosted by carbonates (mostly calcite and dolomite). The selected objective is x100 and the light is XPL (and PPL ). The XRD patterns distinguished mineral peak intensities at 7-9º, 11-12º and 29-30º positions (º2 Theta) for sepiolite. For all analyzed samples, the patterns confirm distinct sepiolite mineral with chemical formula of Mg4Si6O15(OH)2.6H2O which is the major mineral component among hydrous fiber minerals. Minor mineral components recognized by XRF include montmorillonite, talc, calcite, aragonite, dolomite, antigorite, clinocrysotile and tremolite, all were identified by microscopic studies. According to XRF analyses the average chemical composition of selected hand-purified sepiolite includes SiO2 (52.6 wt%), MgO (19.3 wt%), LOI (18.2 wt%) and Fe2O3 (4.9 wt%), representing Fe-sepiolite which is a new composition compared with analogous sepiolite distinguished in Iran and global countries. The high content of LOI indicates a very hydrous structure as for the para genetic clay minerals.ConclusionFor the first time, this work identified a secondary mineral namely ‘Fe-sepiolite’ in Tanbo village according to systematic field and laboratory experiments. It is found in Eocene ultramafic rocks (rich in Fe, Mg and Ca elements) belonging to Makran ophiolite, mainly serpentinite and harzborgite. It occurs within conglomerate and carbonate rocks which have undergone intense hydrothermal alteration in the presence of liquids rich in Ca++, Mg++ and Fe++ solutions.