GEOCHEMISTRY OF ARSENIC AND ANTIMONY IN BULGARIAN COALS

Arsenic and Sb were quantified by instrumental neutron activation analysis in similar to 400 individual full channel samples comprising coals, coaly shales and partings from 14 Bulgarian coal deposits. The mean values for As in the coal deposits range from 2 to 58 ppm, while mean values for Sb range from 0.2 to 16 ppm. The As means by coal rank are: lignites, 11 ppm; subbituminous, 19 ppm; bituminous, 37 ppm. The values for Sb are: lignites, 0.5 ppm; subbituminous, 4.2 ppm; bituminous, 1.1 ppm. Regional geological factors have more important influence on As and Sb content than does the rank of the coal. The highest As and Sb contents were found in single vitrain lenses hosted in sediments: 4011 ppm As and 137 ppm Sb. Arsenic is present mainly in pyrite, which was demonstrated by analysis of pyrite hand-picked from coals and microprobe analysis of pyrite grains in coal samples. Presence of organically associated As is supposed as well. Indirect evidence shows that Sb in Bulgarian coals has a dominant organic affinity. There is no correlation between As and Sb. The enhanced Sb and As content of the Vulce pole, Pcelarovo and Meden buk deposits is related to hydrothermal and volcanic activity in the region. The deposits are located in a Paleogene mollasse formation with Sb mineralization (stibnite containing > 1000 ppm As). One factor controlling enrichment of coals in As and Sb could be paleothermal solutions penetrating coal beds. The source for background levels of As and Sb in coals could be detrital material containing Fe-oxides which are hosts of both elements. In a peat-forming environment (low pH, reducing conditions, abundant H2S) some As and Sb bound to Fe-oxides could be redistributed, partly coprecipitated with pyrite and partly accumulated by the organic matter.