GEOCHEMISTRY AND SEDIMENTOLOGY OF COAL SEAMS FROM THE PERMIAN WITBANK COALFIELD, SOUTH-AFRICA - A MEANS OF IDENTIFICATION

Sedimentological investigations of the coal-bearing Vryheid Formation (Karoo Sequence) in the Witbank Coalfield have revealed that coal-peat deposition was associated with both marine and non-marine palaeodepositional events. At the terminal stage of peat accumulation, swamps were inundated by marine transgressions which deposited mud and silt above the peat (coal) and these over-lying sediments contain typical marine features: glauconite associated with marine ichnofossil assemblages. In similar stratigraphic positions elsewhere in the basin, the peat swamps were invaded by basinward-prograding non-marine fluvial systems which deposited coarse sand and gravel over the peat. These overlying sediments are coarse-grained arkosic strata. Geochemical analysis of the No. 2, No. 4 and No. 5 coal seams in the region were undertaken using instrumental neutron activation analysis (INAA) and X-ray fluorescence spectrometry (XRFS). These analyses show that the geochemistry of the coal compliments the sedimentological interpretations. Elements which tend to be more concentrated in marine-derived sediments, e.g. boron, chlorine, lithium and bromine, occur in greater concentrations in the top of coals overlain by marine strata than in coals overlain by non-marine fluvial sequences. The latter do not show enrichments of these elements. Trace-element composition of the coal seams was further utilized as a means of identifying the seams using multivariant discriminant analysis. Each coal seam plots in a specific field thereby permitting its identification based on the trace-element content. If unknown areas of the coalfield are being explored, a knowledge of the palaeodepositional stratigraphic sequences together with quantitative geochemical analyses could permit rapid identification of the coals concerned.