Mineralogy and geochemistry of Bobov Dol coals, Bulgaria

Abstract

The mineralogy, elemental composition, and modes of occurrence of 49 elements in nine composite samples of Bobov Dol high-ash coals were studied by optical microscopy, scanning and transmission electron microscopy, X-ray diffractometry and chemical analysis. The major minerals were quartz, kaolinite, illite, plagioclase and K-feldspar and the minor minerals and phases were pyrite, marcasite, siderite, calcite, dolomite, gypsum, muscovite, montmorillonite and volcanic glass. The accessory minerals include a wide variety of minerals, such as galena, pyrrhotite, magnetite, hematite, goethite, chromite, rutile, anatase, corundum, gibbsite, biotite, chlorite, zircon, enstatite, garnet, jarosite, alunite, barite, polyhalite, aragonite, ankerite, witherite, apatite, halite and sylvite. The modes of occurrence and some genetic peculiarities of the above-mentioned minerals are described. Thirty-three elements occurred in concentrations higher than the respective Clarke values; especially S, Rb, Nb, Hf, Zn, Cu, Pb, Mn, Ti and U. The concentration trends and modes of occurrence of the trace elements are also discussed. In decreasing order of significance, the trace elements probably occur as element-organic compounds, as impurities in the mineral constituents, as major components in the minerals, as major and impurity components in the inorganic amorphous constituents, and in the fluid constituents. The Bobov Dol coals have undergone complex syngenetic, diagenetic and epigenetic mineralization processes associated with hydrothermal and volcanic activities. These processes were also accompanied by intensive tectonic movements and, possibly, by a later change from continental to marine sedimentation in the area after burial of the coal.