Coal characteristics, elemental composition and modes of occurrence of some elements in the İsaalan coal (Balıkesir, NW Turkey)

Abstract

This study uses coal petrography, micro-FTIR, mineralogical, and geochemical analyses to determine the coal characteristics and controlling factors of elemental enrichments in the İsaalan coal (NW Turkey). The obtained data show that this subbituminous coal has a relatively low ash yield (avg. 19%), high hydrogen content (avg. 5.4%) and total S content (avg. 2.4%), on an air-dry basis. The maceral composition, along with micro-FTIR data on ulminite, suggests that lignin- and resin-rich vegetation was predominant. The coal facies data imply the precursor peat was accumulating under, mesotrophic anoxic conditions where water level was high and fluctuating. The mineralogical composition of the bulk coal samples is composed mainly of quartz, pyrite, and clay minerals and lesser amounts of feldspars and carbonates.The statistical analysis of the bulk compositional data shows the major elements, including Al, Fe, K, Na, Mg, P, and the vast majority of trace elements, have inorganic affinity, whereas B presumably has organic affinity. The trace element concentrations are variable, and especially As and elements including B, Ba, Cr, Cs, Ni, Sb, and V are enriched compared to most world coals. The enrichments and distributions of Cr, Cs, Ni, and V were controlled by clastic inputs into a palaeomire, whereas As enrichments were controlled mainly by epigenetic and, to a lesser extent, syngenetic factors. Introduced SO42− and As-rich surface- and groundwater from the basin margin area, along with anoxic conditions in the palaeomire, resulted in syngenetic formation of As-bearing pyrite. The circulating intra-seam solutions later liberated As from syngenetically formed pyrite, which resulted in As- and Ni-rich overgrowths around framboidal and massive pyrite crystals. These solutions could also easily mobilize B that was absorbed by organic matter. Therefore, B enrichment appears to be related to epigenetic factors. Tectonic activities during coalification allowed hydrothermal solutions from neighbouring epithermal mineralisation to penetrate and circulate along with underground waters that leached overlying volcanoclastic-volcanic strata and supplied a number of elements to the studied coal. Thus, infilling As-bearing pyrite and silica were formed mainly during coalification. All these results suggest that As-enrichment in the study area is more likely a combination of epigenetic factors and syngenetic-detrital rather than volcanogenic contributions, as suggested earlier for some other Turkish coal deposits.