Mineralogy, Geochemistry, and Genesis of Bentonites in Miocene Volcanic-Sedimentary Units of the Ankara-Çankiri Basin, Central Anatolia, Turkey

Abstract

AbstractWidespread alteration in the Miocene lacustrine volcanic/sedimentary rocks of the Ankara-Çankiri basin of central Anatolia has resulted in the formation of sizeable (economic) quantities of bentonite deposits. No detailed characterization of the geological, mineralogical, and geochemical properties or the depositional environments of these primary and secondary bentonite deposits has been carried out to date. The present study was undertaken to close this knowledge gap. Two possible hypothetical processes were examined to explain the genesis of the bentonites: 1) The bentonites were formed by the devitrification of volcanic glass in a lacustrine setting; and 2) The bentonites were formed by the chemical weathering of previously deposited volcaniclastic sediments and ophiolitic materials. The characteristics of the bentonites were examined using X-ray diffractometry, scanning and transmission electron microscopy, energy dispersive spectroscopy, and chemical analyses of major and trace elements. The Ankara-Çankırı bentonites are found gradationally interbedded with parent Miocene volcanic and volcaniclastic rocks. These bentonites were deposited in a shallow lacustrine setting based on observed desiccation cracks, locally enclosed coal seams, plant rootlets, gypsum lenses, yellow sulfate-like fracture infillings, and ferric iron oxide stains. Smectite resulted from the chemical weathering of feldspar and possibly also the weathering of biotite and hornblende. This smectite was precipitated in situ on partially dissolved glass and feldspar. The average major-element composition of the smectite-rich clay fractions yielded the following montmorillonitic smectite structural formula: (Na0.33Ca0.31K0.18) (Al2.35Fe0.80Mg0.78)(Si7.79Al0.21)O20(OH)4.The interlayer cation occupancy in the smectite-rich clay fractions was based on the use of Na+/(Na++Ca2+) ratios and showed a composition between a Ca-smectite and a Na-smectite. The relative increases in several groups of elements according to the LREE/(MREE+HREE) ratio, Al2O3, the sum of Ni+Co+Cr, the sum of Fe2O3+MgO+TiO2, the positive correlation between Rb+Ba and Na2O+K2O, Sr and Rb, Rb/Sr and Zr, Zr/Sm and SiO2, the negative Eu anomaly, and the field and petrographic observations further showed that the Si, Al, Fe, and Mg required to form smectite were mainly supplied from the decomposition of feldspars, amphiboles, and volcanic glass from volcanic materials and were partially supplied from the chemical weathering of ophiolitic basement units. The bentonite deposits examined in this study are mainly primary bentonites derived from volcanics and local secondary bentonites from previously deposited volcaniclastic sediments and ophiolitic materials.