Occurrence and genesis of tonsteins in the Miocene lignite, Tunçbilek Basin, Kütahya, western Turkey

Abstract

Abundant beige and white kaolinite claystone layers, 2–50 cm thick from the Tunçbilek basin have the characteristics of tonsteins. The aim of this study was to determine the mode of formation of the tonsteins, together with their mineralogical and geochemical characteristics, in order to identify their environment of deposition. The Miocene lacustrine units in the Tunçbilek lignite deposit associated with tuff and tuffite are comprised of organic-rich shale, organic-rich marl, marl, claystone, silty claystone, siltstone, sandstone, coal seam, and conglomerate deposited in a fluvial and lacustrine environment. Sanidine, plagioclase, biotite and chlorite are partly to completely argillized and enclosed in an authigenic clay matrix in the rhyolitic to rhyodacitic tuff/tuffite, sandstone, organic-rich shale, and serpentinite host rocks of the clay deposit. Abundant kaolinite and quartz are associated with smectite, illite, feldspar, calcite, dolomite, siderite, pyrite, barite, and gypsum. Development of oriented 7 Å halloysite tubes on/in altered volcanogenic materials associated with kaolinite, and the occurrence of vermiform kaolinite edging disordered kaolinite, locally siderite and pyrite crystals may suggest formation of halloysite following kaolinite. The reaction pathway is: volcanic glass + feldspar + muscovite/biotite → 7 Å halloysite → kaolinite. The slight enrichment of light rare earth elements (LREE) compared to heavy rare earth elements (HREE) with positive Eu anomalies (based on Upper Continental Crust (UCC) normalized values) reflects fractional crystallization of feldspar and amphibole in the parent volcanic rocks and organic-rich shale. Alteration of feldspar, biotite and volcanic glass resulted in the concentration of Al, Si, Fe, Mg, and K and precipitation of kaolinite in an acidic open hydrological system, while in a stagnant and alkaline environment smectite and illite are the alteration products. The Al2O3/TiO2 and low Cr/Ni ratios suggest felsic parent rocks. The trace elements ratios such as Ni/Co, V/Cr, Cu/Zn and U/Th among the basement complex source rocks units also suggest oxic, suboxic, and anoxic conditions. The occurrence of Pinus, Alnus, Quercus, Alnus, Carya, Taxodiaceae and Myrica palymorphs reflect a subtropical to warm temperature, humid climate, low elevations and swamp vegetation condition during the Early–Middle Miocene period in the study area. Association of argillization and silicification with coal, pyrite, and gypsum indicates a swampy environment and hydrothermal processes. This interpretation is corroborated by the calculated formation temperatures from stable isotope analysis of 148.8–168 °C for kaolinite and 145–169.2 °C for smectite. The low δS isotope values of pyrite (−19 to 2.9‰) and high positive values of gypsum (11.5–21.7‰) indicate a mixed diagenetic and hydrothermal origin for the sulfide and sulfate mineral.