Summary:
Highveld coalfield hosts up to five coal seams in the middle Ecca Group sediments of the Karoo Supergroup. The stratigraphic sequence in the mine area includes five coal seams that can be correlated with seams found in the Witbank coalfield, named from the base upwards as seam 1 (S1), seam 2 (S2), seam 3 (S3), seam 4 (S4) and S5. Seam splitting is common feature in the area. This is fundamentally attributed to the proximity to the Smithfield Ridge and thus the provenance of detrital material. S2 may be split into seam 2 upper (S2U) and seam 2 lower (S2L) while S4 is split into three subseams, seam 4 lower (S4L), seam 4 upper (S4U) and S4A. Furthermore, S4A may be split into S4A1 and S4A2. S5 is generally split into the S5 and seam 5 lower (S5L).
The thickest and most ubiquitous being the S2, S4 and S5 seams. S1 is restricted to palaeo-lows while S3 only occurs on the western side of the complex. The principal economic seams currently exploited are S2L and S4L, with the remaining seams being either too thin, laterally discontinuous, poor quality or impractical/uneconomical to mine.
FZO complex is characterised by two adjoining palaeo-valleys, one in FZON and the other in FZOS. The valley in FZON has higher-quality coal while FZOS is characterised by lower-quality coal. The different coal qualities are indicative of different depositional environments and thus the different geological domains. Remnants of the Smithfield Ridge exist in the north-western edge of FZON and northern extremity of Schurvekop.
S2 occurs over most of the Forzando area, except for areas of extreme palaeo-highs along the Smithfield Ridge, along the central portion and in an isolated palaeo-hill in the north-east of the complex. The seam varies in thickness from under 1.0m to over 6.0m, with siltstone and sandstone parting typically found in areas where the seam thickness exceeds 3.5m.
The seam has been extensively devolatilised, leaving disconnected pockets of unaffected coal. Within these pockets, coal quality is extremely variable with high ash content resulting from the prevalence of thin shaly partings. The lack of continuity of unburnt coal renders it of no economic value.
Throughout the area the S4 consists, from top down, of S4A (a gritstone parting), S4U (siltstone and coarse-grained sandstone parting) and the S4L. In the east, the siltstone and coarse-grained sandstone below have been eroded, leaving only the overlying grit. Both S4A and S4U are rarely thick enough and too poor in quality to be considered economic. S4L is the only seam that is thick enough, of sufficient quality and developed throughout the area to warrant economic extraction in prevailing economics. In the FZO complex area, the thickness of the seam ranges from 0.5m to 4.0m with an average of 2.1m.
The total FZO area has been intruded by Karoo dolerites even though the impact on S4 is comparatively small and confined to those areas where they come close to or intersect the seams. Transgressive sills with a thickness of 5m to 30m have been noted, resulting in extensive burning of coal in areas, particularly where the sill either closely underlies or overlies the seam. Seams are also burnt and devolatilised near the sill transgression zones with burnt zones ranging from a few metres to 20m.
Dolerite dykes and stringers with a thickness ranging from 0.5m to 5.0m are commonly encountered in the FZO complex. Dolerite structures have a tendency to pinch and swell, even over very short distances. Dolerite stringers are thought to be offshoots from the dykes which in turn feed from the dolerite sills below. Dykes in the area are noted as having a general preferred orientation of north-west to south-east and north-east to south-west. Burnt coal zones associated with dykes/stringers vary considerably from zero to ±10m with the width of the burnt zone not necessarily relating to the thickness of the dyke/stringer. These geological risks are well managed through extensive surface vertical and underground horizontal drilling in areas of concern, use of downhole wireline logging for better correlation, underground channel sampling incorporated in the short-term planning model, underground face mapping to track both vertical and lateral variations in the lithology, which assists with reconciliations and structural interpretation, and surface mapping, particularly of basement outcrops.