Summary:
The Elandspruit, Khanyisa, Vanggatfontein and Moabsvelden Collieries are located within the Witbank Coalfield. The northern extent of the Coalfield is defined by the limit of coal-bearing sediments of the Vryheid Formation which abut against pre-Karoo basement rocks of the Transvaal Sequence, the Waterberg Group and Bushveld Complex. The southern limit of the central sector of the Coalfield is marked by a series of Bushveld Complex inliers known as the Smithfield Ridge while in the east and west sectors, the boundary between the Witbank Coalfield and the Highveld Coalfield to the south is somewhat arbitrary.
The five major coal seams of the coalfield are contained within a stratigraphic interval averaging approximately 70m in thickness. The seams are numbered, from the base upwards and include the No. 1 Coal Seam, the No. 2 Coal Seam (split into the No. 2 Lower Coal Seam and No. 2 Upper Coal Seam), the No. 3 Coal Seam, the No. 4 Coal Seam (split into the No. 4 Lower Coal Seam and No. 4 Upper Coal Seam) and the uppermost No. 5 Coal Seam.
Locally, three coal seams are developed across the licence area – namely the No. 5 Seam, No. 4 Seam, and the No. 2 Seam. Within the opencast resource areas, the No. 2 and No. 4 Seams are the primary economic targets, with the addition of the No. 5 Seam where present.
No. 2 Seam
The No. 2 Seam is the bottom most economic coal seam. The No. 2 Seam sequence includes the succession from the top of the Transvaal Supergroup basement to the top of the No. 2 coal seam. Rocks of the Dwyka group and overlying No. 1 and No. 2 Coal seams are included as well.
The No. 2 Seam is well developed over the majority of the licence area, until it subcrops towards the east. The seam comprises zones of alternating bright and dull coal, with occasional shaley coal bands in places. On average the seam is 6.3m thick. The floor of the seam is variable, ranging from a mudstone, to a highly silty sandstone, and in places to a medium grained sandstone. The roof of the seam is also variable, generally comprising a coarsegrained sandstone to granulestone, however, in some areas it comprises a carbonaceous mudstone. The average depth to the No. 2 Seam is 46m across the eastern resource block (“ERB”).
The parting between the No.2 and No. 4 Seam above varies greatly in thickness between VG4 in the north and VG5 in the south. In VG4, the parting ranges between 4 – 8 m, whereas in VG5 it ranges between 14 – 20 m. The parting material comprises a basal, highly carbonaceous siltstone, interbedded siltstone, and fine-grained sandstone, which is frequently bioturbated that grades upwards into a medium to coarse-grained arkosic sandstone. In many places this succession is truncated by an erosive based channel sandstone, which forms a fining-upwards sequence to the base of the No.4 Seam. The No 2 Seam is separated from the No 4 Seam above by a parting of approximately 11 m in thickness. The No 2 Seam is only considered economic in the ERB, as it has been heat affected in the western resource block (“WRB”).
No. 4 Seam
The No. 4 Seam sequence comprises the predominantly sandstone succession from the immediate roof of the No. 2 Seam, to the top of the No. 4 Coal Seam. The seam comprises zones of dull coal with bands of shaley coal increasing towards the top (washing the coal ensures the removal of the shale partings). The seam averages 6.8 m in thickness across the ERB area. It is, however, thickest (8.4 m) in VG4 and thinnest in VG5 (5.9 m). Within the WRB resource area, the seam averages 6.2 m thick, however for resource estimation purposes, it is limited to 4.8 m.
The average depth to the No. 4 Seam roof is 28 m within the ERB, and 44m within VG6. The No 4 Seam depths range from less than 20 m to over 100 m in the WRB. The floor of the seam may be formed by a carbonaceous siltstone, a bioturbated fine to medium grained sandstone, or a well-developed medium to coarse grained and gritty sandstone.
The sequence between the No.4 Seam and the base of the No.5 Seam is formed by a thick succession of interbedded sandstones and carbonaceous siltstones, culminating in the rocks that form the immediate floor to the No.5 Seam. This parting is consistently between 16 – 20 m thick.
No. 5 Seam
The No. 5 Seam is only present across VG4 and a small portion of VG3. It has been removed by erosion in VG5. The seam averages 1.3 m in thickness, and comprises bright coal, interbedded with thin stringers of carbonaceous siltstone.
The immediate roof of the No.5 seam is formed by a succession of interbedded siltstones and sandstone or coarsegrained glauconitic sandstone, here informally referred to as the “Glauconite Marker”. This marker horizon is important in opencast mining operations as it forms an easily recognizable unit immediately above the No.5 seam coal. The No. 5 Seam is predominantly mined out within the ERB, with only the small portion in VG4 remaining.
Within the ERB there have been a number of boreholes which have intersected dolerites, many having a notable effect on the coal qualities. Within VG3 and VG4 there is a transgressive sill which has affected the coal seams. The affected areas are reasonably well delineated and are for the most part, removed from the coal resources. Within VG4, both dolerite and faulting have been encountered. Within VG5, there is a large transgressive sill which has had an effect on the No. 4 Seam, and to a less extent, the No. 2 Seam.