The influence of depositional environment and other dolerite intrusions on the quality of coal : a study of the Twistdraai mine area near Secunda, Southeastern Transvaal

Abstract

The objective of the present study is to establish the influence of syngenetic (sedimentary environment) and of epigenetic (dolerite intrusions) factors on the quality of coal. The effect of dolerite intrusions is investigated in order to develop a prediction method for the location of such dykes and sills within the seam. Both factors are dealt with separately, but their respective impacts on the exploration and mining phases of a coalfield are compared. The sedimentary environment was established by identifying and describing eight vertically successive lithofacies associations. They are each laterally continuous and represent a certain depositional phase throughout the study area. These are related to sedimentary processes, from which a three-dimensional paleoenvironmental model is derived. Subsequently coal distribution and quality parameters are linked to the depositional features of the model. The identified depositional phases commenced with subglacial, glaciofluvial and glaciolacustrine settings, with associated Gilberttype deltas. These are followed by meandering and minor braided fluvial settings, characterized by laterally and vertically highly variable lithofacies, in which the main coal-bearing strata were formed. Coal distribution and quality parameters (ash, volatile matter and calorific value) are closely related to paleoenvironmental characteristics. Major changes of coal distribution and quality parameters are mainly associated with active and abandoned channels and these properties are less variable in floodplain settings. The effect of dolerite intrusions on the quality of coal is investigated by analysing a series of coal samples, which were taken at certain distance intervals from a dyke/coal contact. The analytical procedures include maceral analysis, vitrinite reflectivity measurements, proximate analysis, ultimate analysis and inorganic geochemistry of the coals and dolerites by means of XRF spectrometry. The lateral extent of metamorphic effects of the dolerite intrusions on the parameters of the coal are largely restricted to the width of the visually distinguishable contact aureole. Petrographic composition mainly changes due to the formation of natural coke; ultimate and proximate analysis results are primarily dependent on the petrographic composition of the unaltered coals. With increasing metamorphism they change in accordance with the rank increase and corresponding loss of volatile matter in the coals. Major and trace element concentrations become more variable with decreasing distance from an intrusion; however, little significant depletion or enrichment tendencies are noticeable. Metasomatic processes are most probably limited to an increased formation of carbonates in the cindered coals along the dyke. The vitrinite reflectivity proves to be the most sensitive parameter in the search for an anlytical tool for the prediction of intrusions. In apparently unaffected coals a distinct reflectivity gradient is discernible from bottom to top of the seam. This gradient becomes enhanced and may even be partially inverted in proximity to an intrusion. The sedimentary processes are found to be of more regional importance in respect to their impact on the type and grade of a coal. Intrusions only have very localized effects and determine small-scale variations in the rank of the surrounding coals. They necessitate, however, more attention in view of the short and medium-term mine planning.