Mineralogy, geochemistry, and genesis of lithium-bearing argillaceous sediments associated with the Neogene Bigadiç borate deposits, Balıkesir, western Anatolia, Türkiye

The Li-bearing claystone and carbonate are sedimentary rocks deposited in the lacustrine environment hosting the Bigadiç borate deposits in western Anatolia. The purpose of this paper is to explain the mineralogical, geochemical, stable isotope characterizations and formation of Li-rich claystone (hectorite, saponite), and have strategic, technological, and economic importance for the country's economy, which have not been sufficiently studied previously. In the rhyolitic and dacitic tuffs, sanidine and plagioclase crystals were altered, biotite and hornblende, Fe-oxidized, locally opaque, and chloritized in a sericitized, Fe-oxidized, argillized, calcified, and zeolitized glassy matrix. The claystone consists mainly of smectite and minor illite, volcanogenic quartz, feldspar, mica, hornblende, and occasionally calcite, aragonite, dolomite, and gypsum/anhydrite. Hectorite and saponite were determined based on the expansion of their basal peaks following heating at 500 °C and solvation with ethylen-glycol; additionally, hectorite expanded, and saponite was not affected after glycerine-saturated Cs-smectite and heating at 100 °C for 20 h. Smectite shows webby to crenulated forms, and coexist with feldspar, volcanic glass, rhomboidal calcite, blocky gypsum/anhydrite, and Fe-oxide phases. The Li values increase up to 2650 ppm in the smectite-rich claystone and marl, and up to 449 ppm in the volcanic rocks. The positive correlation of REE with each of SiO2, Al2O3, and K2O; positive correlation of MgO vs. Li, and increase of MgO + CaO, Sr, Li, LREE relative to MREE and HREE; negative Eu anomaly and high values of As and S suggest that the feldspar, mica, and hornblende alteration originating from the Miocene volcanic and pyroclastic materials and presence of gypsum/anhydrite during the hydrothermal alteration activities were the sources for the smectite formation. The high negative δ18O values of calcite, the δD and δ18O values of smectite and δ34S‰ and δ18O‰ values of gypsum, and the 87Sr/86Sr isotope ratios range of calcite and gypsum suggest the contribution of mixing meteoric and hydrothermal environmental conditions during the depositional and diagenetic process(es) in the playa lake environment.