Occurrence and genesis of hectorite and mixed-layer-illite/hectorite in argillaceous sediments interlayered with Neogene borate deposits, Emet, Türkiye

Abstract

The Li-rich claystone was formed within the Miocene volcano-sedimentary units in Emet borate deposits, in western Anatolia. This research identified hectorite and mixed-layer-illite/hectorite-abundant claystone that have technological and economic potential and have not been studied to date. The claystone was formed through the diagenetic alteration of rhyolitic and zeolitic tuffaceous units in a shallow lacustrine playa lake environment under arid climatic conditions. The parent rocks comprise altered and sericitized plagioclase and sanidine, opacited and chloritized biotite/muscovite, zeolite, quartz, and rock fragments cemented by argillizied, sericitized, and carbonatized matrix. Abundant hectoritic smectite and mixed-layer-illite/hectorite accompanied by accessory feldspar, quartz, locally calcite/dolomite, gypsum, and orpiment/realgar. The Li concentration reaches a max of 2200 ppm in the hectorite-abundant claystone and 218 ppm in the tuffaceous units. Smectite flakes are associated with illite fiber, feldspar, and amphibole. The degradation of sanidine, plagioclase, biotite/muscovite, and hornblende contributed to the positive correlation of SiO₂ vs. Al₂O₃, K₂O, TiO₂, and total rare-earth elements (ΣREE) vs. SiO₂, Al₂O₃, and K₂O; MgO vs. Li values, and enhancement of MgO + CaO, Li, Sr + Rb, and ΣREE values, and LREE/HREE ratio; negative Eu anomaly and high concentration of S and As. These physicochemical compositions favored the formation of hectorite and mixed-layer-illite/hectorite. The δD and δ¹⁸O values of hectorite and mixed-layer-illite/hectorite samples and the formation temperatures range from 98 to 119 °C reveal a burial diagenetic alteration process. The δ¹⁸O and δ¹³C values of the calcite samples suggest the mixing of thermal and fresh waters. The high ⁸⁷Sr/⁸⁶Sr ratios of calcite and gypsum reveal the consumption of Sr during the alteration processes. Additionally, the negative δ³⁴S values of realgar and orpiment suggest formation under acidic-neutral conditions via sulfate reduction or microbial cycling.