Petrological and geochemical composition of lignite from the D field, Kolubara basin (Serbia)

Abstract The Upper Miocene lignite from the Main coal seam in the D field, Kolubara basin, is a typical humic coal with huminite, liptinite and inertinite concentrations of up to 83.7 vol.%, 17.2 vol.% and 15.5 vol.%, respectively. In the huminite group, textinite and ulminite are the most abundant macerals with variable amounts of densinite and attrinite. Liptodetrinite and sporinite are the most common macerals of the liptinite group, while inertodetrinite is the most abundant maceral of the inertinite group. The mineral matter consists mostly of clay minerals. The main sources of organic matter were gymnosperms (conifers) and microbial biomass, followed by angiosperms. Based on composition of saturated and aromatic diterpenoids it has been established that coal forming plants belonged to the gymnosperm families Taxodiaceae, Podocarpaceae, Cupressaceae, Araucariaceae, Phyllocladaceae and Pinaceae. Peatification occurred in neutral to slightly acidic, fresh water environment. Composition and distribution of biomarkers show that diagenetic changes of the organic matter were mainly governed by bacterial activity in a suboxic to oxic environment. Based on distribution of aromatic diterpenoids a novel diagenetic pathway for transformation of abietane-type precursors under suboxic to oxic conditions is proposed. Variations in compositions of macerals and biomarkers are in concordance with pronounced seasonality during Pontian, which caused changes in the water level, redox conditions during peatification, and to some extent vegetation differences in the paleo-plant communities.