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