A comparison of black claystones, lignites and dump materials from the Maritsa Iztok Coal Basin, Bulgaria, using organic geochemical proxies

Abstract

Geochemical analysis of dump materials from the opencast Maritsa Iztok mines, Bulgaria, was carried out based on biomarker assemblages of hydrocarbon fractions. Organic matter (OM) and secondary transformations in three representative samples (massive black claystones and materials from the Iztok and Staroselets dump sites) were studied using geochemical proxies.A number of differences were recognised in the respective OM compositions of the samples compared to both published data and between the individual dump samples themselves. The ОM of the studied samples was found to be polar, but also contains some apolar compounds. It consists mainly of resins and asphalthenes. Claystone OM is of the dispersed type, with intense oxidative-reductive interactions in a lacustrine environment resulting in its transformation into an inert material. Dump sample kerogen is of Type II and mixed Type II/III. In all samples, “odd” numbered n-alkanes are found in higher amounts. Diterpenoids (С19, С20) with pimarane, abietane and phyllocladane skeletons are preponderant. Tri- and tetracyclic terpenoids and steranes have been identified in the black claystones OM only. Hopanes are present in low amounts in extractable OM from all three samples. Biomarkers indicate that black claystone OM is formed from aqueous flora, with a minor supply of gymnosperms (mainly G. Sequoia). Iztok Dump OM is structured by higher plants with an aqueous vegetation input. The Staroselets Dump OM formation is assigned to an active microbial reworking of aqueous vegetation and bacteria with a minor coniferous supply. Different geochemical parameters admit anoxic stratified bottom waters for the black claystones with an addition of deep water stagnation for Staroselets sample in a Maritsa Iztok Basin (MIB) aqueous environment.An attempt was also made to track the effect of secondary processes (oxidation, destruction, dearomatisation), temperature, water drainage and wash-out on dump materials. Leaching and weak degradation processes in the MIB dump environment are likely for a time span of ca. 40–50 years, considering the low percentage of short-chain n-alkanes, long-chain prevalence and low Pr/nC17 and Ph/nC18 ratios, with the Iztok Dump sample experiencing more advanced transformations.