Influence of an igneous intrusion on the surrounding coal body

Abstract

On the north-western edge of the Witbank Coalfield, near the town of Delmas (Mpumalanga Province) a prominent and undulating sill is believed to affect quality of the coal in the seams mined in the area. The study area was separated into two subsections, Study Area A and Study Area B. In Study Area A the sill overlays the coal seams. The top of coal and the sill are separated by ~11 m thick sandstone with intermittent bands of shale, siltstone and carbonaceous shale. Study Area B, on the other hand, is located where the same sill underlies the coal seams. In this area, Dwyka tillites form the parting between the sill and the bottom coal seam. 51 composited samples were received from Study Area A and 26 uncomposited samples were received from Study Area B. In both areas, Seam 2 (bottom coal) was sampled due to its consistency and economic importance, as other seams were irregular over the complete area. Three parameters were assessed and compared to the thickness of the sill and the distance between the sill and the bottom coal; i.e. volatile matter, calorific value and ash. The volatile matter behaviour was further investigated using an empirical equation postulated by an industry specialist to determine whether the samples had been devolatilised by an external mechanism, i.e. contact metamorphism. The experimental calculation, based on a regression slope in relation to the South African baseline, indicated that two very different events had occurred on the coal seam. In Study Area A, the calculation indicated that the coal in the area was not devolatilised, thus implying that the sill had no influence on the coal. Conversely the results of the experimental equation in Study Area B indicated that the volatile matter of the lower coal seam had been greatly reduced by an external factor and the coal was devolatilised. Global studies of similar conditions point out that the content of volatile matter in the coal is the most affected variable during the intrusion of an igneous body into coal. This variability is governed by proximity to the sill, the shape of the intrusion, the thickness of the intrusion, orientation, location of the sill or lastly by the thermal conductivity of the country rock. In Study Area A the bottom coal seam was insulated from the heat conducted away from the sill by the presence of strata of different conductivities separating the sill intrusion from the bottom coal. On the other hand, in Study Area B heat was successfully transmitted to the bottom coal by highly conductive strata which separated the sill from the bottom coal. Therefore the study concluded that the sill had a significant influence on the bottom coal when it was underlying the coal (Study Area B) and a conductive layer separated the sill from the coal. In addition, the results indicate that neither the distance from the sill nor the thickness of the sill influence the effect which the sill had with the bottom coal seam in Study Area A.