Geochemical Variations During Alteration of an Andesite-Basalt to Bentonite, In Khur, East of Isfahan, Iran

Abstract

Bentonite deposits of economic interest are widespread in Tashtab Mt (Khur), east of Isfahan, Iran. These deposits have developed at the expense of the Eocene andesite-basalt. The bentonite from the study area is essentially composed of montmorillonite. Other components are α-quartz, calcite, cristobalite, feldspar and minor amount of kaolinite. Mineralogically, bentonite varies from beidellite to Na-montmorillonite, implying the existence of a solid solution. Alteration of the parent rocks to bentonite is accompanied by gains and losses of some elements. Except for Cs, the concentration of major and trace elements displays variable reduction in analogy to the parent rock. LILEs are markedly depleted. Nb, U, Cr, Ni and V are relatively decreased, whereas, the amounts of Al, Ti, and Zr are to some extent immobile. The chondrite-normalized rare earth element (REE) pattern displays similar trends in the andesites-basalt as well as bentonite and siliceous zone. Slight depletion of REE in particular, HREE is observed in bentonite samples in comparison to the unaltered rocks. All REE particularly HREE are highly depleted in the silicic zone. HREE mobility is ascribed to the availability of SO ^-₄ ligands in the siliceous fluids. Fractionation of the REE in silicic zone is controlled by the existence of montmorillonite in the argillic zone which host for LREE. Cs is generally fixed during bentonite formation by structural incorporation in clays. Gains and losses of elements are attributed to alteration processes occurring under open-system conditions.