Mineralogical and elemental composition of the Middle Miocene coal seams from the Alpu coalfield (Eskişehir, Central Türkiye): Insights from syngenetic zeolite formation

This study focuses on determining mineralogical and elemental compositions of coal seams (to the upwards, D, C, B, A, and S0) within seven coal exploration wells from the Alpu coalfield (Eskişehir, Central Türkiye). Furthermore, the special goal of the study is a comparative analysis of the relations between the elements by using agglomerative hierarchical clustering algorithm with different linkage methods as well as different similarity measures. Clay minerals and quartz are commonly detected as abundant to dominant phases, while natural zeolite formations were detected in the studied seams C, B, A, and S0. The SEM-EDX data shows that clinoptilolites in zeolite minerals were observed within the organic matter, while crystalline and non-crystalline analcime minerals along with syngenetic authigenic rhomboid K-feldspars were only detected in the seam A from one studied well. The existence of some micron-sized minerals such as apatite, monazite, and Ti-oxides within the smectite matrix and the measurable amount of Ti in smectite imply that alteration of epiclastic and contemporaneous volcanic inputs was developed under weak acidic to neutral conditions during peat accumulation. The lack of natural zeolite and carbonate minerals in the seam D could be an indicator of weak acidic to neutral conditions and semi-closed hydrogeological conditions. Nevertheless, the alkalinity of mire water, water table, and hydrogeological regime seem to be variable during the accumulation of precursor peats of seams C, B, A, and S0. In turn, alteration of volcanic inputs was observed under neutral to weak alkaline conditions and semi-closed to closed hydrogeological regime. Hence, syngenetic authigenic, micron-sized clinoptilolites were formed. Moreover, the existence of authigenic rhomboid K-feldspars and syngenetic authigenic analcimes in certain exploration well could suggest local increases on dissolved Na⁺ concentrations, alkalinity, and water table. Except for volcanogenic origin for minerals, accessory micron-sized minerals, like chromite, pentlandite, and allanite grains presumably originated from clastic influxes of ophiolitic rocks in the basement into palaeomires. The variations in mire water chemistry and clastic-influx source area could also control the elemental enrichments in the studied seams. Epiclastic and contemporaneous volcanic inputs into palaeomires seem to control enrichments of Li, B, Sc, and Ti in coal samples, while clastic influx from ophiolitic rocks into palaeomires caused to enrichments of Cr, V and Ni. Furthermore, the liberated Ba, Sr, and As ions from the alteration of epiclastic and contemporaneous volcanic inputs are absorbed by syngenetic zeolite minerals, while anoxic conditions in the palaeomires resulted in precipitation of Sr-barite and As-bearing pyrite grains during peat accumulation and/or early diagenetic stages. Overall, the differences in water chemistry of mire water, epiclastic and contemporaneous volcanic inputs, and clastic influx from the adjacent areas also caused several elemental enrichments and variations in mineralogical compositions of the Middle Miocene coal seams in the Alpu coalfield.