The origin of the Bentonite deposits of Tashtab Mountains (Central Iran): Geological, Geochemical, and Stable Isotope evidences

Bentonite deposits of economic interest are widespread in Tashtab Mountains (Khur), east of Isfahan province, Iran. Several bentonite deposits have been developed in this area as a result of Eocene volcanic alteration. These deposits are classified as Khur bentonite horizon. XRD analyses reveal that alteration products consist of Na-montmorillonite, kaolinite, quartz, calcite, and crystobalite. The studied volcanic rocks predominantly consist of andesite and basalts, which have developed in back arc basin. Field studies demonstrate that there are many active faults that have had a significant role in transporting great volumes of silicic liquids to the surface and precipitated as geode, jasperoid and silicic veins. Alteration of volcanic parent rocks to bentonite is accompanied by gains and losses of some elements. The concentration of major and trace elements displays variable reduction in analogy to the parent rock. Most of the Large Ion Lithophile Elements (LILEs) are markedly depleted; Nb, U, Cr, Ni and V. The aforementioned elements have relatively decreased, whereas Al, Ti and Zr are rather immobile. Cesium is essentially derived from hydrothermal fluids and incorporated in bentonite structure. Both argillic and silicic zones are enriched in Cs. The chondrite –normalized Rare Earth Element (REE) patterns show similar trends in the andesite-basalt as well as bentonite and silicic zone. Slight depletion of REE’s, and in particular heavy REE’s, is observed in the bentonite samples in comparison with the unaltered rocks. Hydrogen and oxygen stable isotope results indicate a meteoric source of fluids responsible for transformation of volcanic rocks into bentonite at a temperature of about 83°C. The hydrothermal fluids have reached the surface by fault system activity, and silicic liquids have escaped frequently in a sedimentary basin, caused probably by Miocene compressive stresses.