Petrographic and geochemical characterization of part of a permian coal deposit of the South-Eastern Kalahari sub-basin, Moiyabana area, eastern Botswana

Abstract

The Moiyabana Coalfield, located ~80 km west of Mahalapye village in the South Eastern Kalahari sub-basin, is one of the potential areas for coal mining in Botswana. This project discusses the petrographic and geochemical results of a detailed investigation carried out on a borehole core of the Moiyabana area, eastern Botswana. Geologically, the Moiyabana coal seams occur in sedimentary rocks of Permian Age and are hosted in the Karoo Supergroup sequence. All coal seams are found within the Ecca Group, which comprises of fluvio-deltaic mudstones and siltstones. A total of 8 coal samples from one borehole core were collected from the western part of Moiyabana Coalfield. The maceral analysis reveals a predominance of inertinite group macerals, with minor vitrinite and liptinite group macerals, suggesting possible oxidizing conditions for a prolonged period of time when accumulated vegetal matter was exposed to the surface during peatification. Mineralogical analysis shows that these coals are characterized by high mineral matter dominated by silicate minerals. In terms of coal rank, using vitrinite reflectance in the range of 0.84-1.04 (mean RoVmr% values), placing the Moiyabana coal samples in the medium rank C bituminous coal, borderline B, using ECE-UN rank classification. Proximate analysis data indicate low contents of moisture and volatile matter, moderate fixed carbon and high ash content. The low to medium sulphur content suggests a possible fresh water environment after the deposition of precursor peat. Major oxides of Si, Ti and Al positively correlate with ash content, indicating inorganic affinity, while other oxides have negative correlation with ash content revealing organic modes of occurrence. Elements As, Al, B, Ba Cd, Cu, Cr, F, Ge, Hg, Li, Mo, Ni, Pb, Sb, Si, Ti, U and V exhibit positive correlations with ash yield, suggesting inorganic affinity, whereas elements Ca, Cl, K, Mg, Mn, Na, P, S, Se, Sr, Th and Zn negatively correlate with ash yield, signifying a potential organic mode of occurrence. When compared to the concentration ranges (Coal Clarkes) in coals worldwide, the generally low concentration of trace elements in the Moiyabana coals, particularly those considered to be hazardous air pollutants, gives an advantage for this coal should it be used for power generation.