Trace elements geochemistry of the Dobrudza coal basin, Bulgaria

The paper discusses the abundance, mode of occurrence and origin of trace elements in the Dobrudza coal deposit. The deposit is the deepest in Bulgaria - it is overburden by Paleozoic up to Cenozoic sediments of 1300 m to 1600 m thickness. The coal rank reaches the stage of high volatile A bituminous coal. The contents of Ba, Cl, Br, Mn, Ge, Pb, W, Mo, As, Zr, Li, Cs, Cu, Zn, Sb, Ti, V, Th. U, Ag. Be and Y are enriched from 1.5 to 9.2 times as compared with the Clarkes for bituminous coals. The content of the other elements are slightly higher/lower than the respective world-wide means. Chondrite-normalized patterns of the REE reveal a strong fractionation among the HREE and LREE, most pronounced for the coals than for the coaly shales and partings. A specific feature is the small Eu anomaly in the shales and the tonsteins underlying the coal bed. Most elements are of mixed, organic and inorganic association. Ge, Cl, Br, Mo, Be and Au are predominantly bound to the organic matter. Arsenic correlates strongly with Fe suggesting presence in pyrite. Ba occurs mainly as barite which is most probably of epigenetic origin. Sr and Mn are presented mainly as isomorphous admixtures in barite and calcite. The abundance of the lithophile elements (excluding Ge, Mo, Ba, Cl, Br) is controlled largely by the detrital and clay mineral content of the coals Many elements of different groups correlate positively each other. Calculation of partial correlation coefficients excluding ash shows that many of these correlations are spurious, due to the positive correlation of every single element with ash. For this reason, the correlation coefficients must be critically evaluated when deducing the mode of occurrence of elements. The accumulation of the trace elements in the Dobrudza coals was influenced by multi-component geological factors: 1. As a result of Upper Pennsylvanian volcanism, there are widespread tonsteins, underlying the coal layers and as thin layers in the coal. Most likely, many elements were leached from the tonsteins and trapped by the coal. 2. Olivinobasalt and andesito-basalt dikes intruding the coal-bearing sediments may cause enrichment or depletion of many elements. 3. Highly mineralized underground waters are the source of Cl, Br, B, Sr and probably some other elements. 4. The numerous regional faults favor the penetration of solutions of different origin which were carriers of trace elements. 5. Postgenetic erosive processes may cause leaching of trace elements from the rocks which are consequently fixed by the coal. (C) 2009 Elsevier B.V. All rights reserved.