Summary:
The ECC complex comprises the Dorstfontein, Forzando and Tumelo operations.
Dorstfontein geology.
Coal measures of the Highveld and Witbank coalfields are hosted in the Ecca Group, which includes the Vryheid and Volksrust coal-bearing formations. All coal seams in the ECC area are hosted in the Vryheid formation which ranges in thickness from 80m to 300m. There are five major coal seams present in the area, named from the base upwards as S1, S2, S3, S4 and S5.
A granitic basement high, which forms part of the Smithfield Ridge and is referred to below as the Central plateau, divides the DCM complex into two separate geological domains and the western and eastern limbs can be found on either side of this plateau. S1 is only developed in the central portions of the paleo-valleys, ranging in thickness from 0.1m to 2.5m. Underground extraction of this seam is considered uneconomic due to its highly variable thickness, inferior quality, isolated occurrence and coal devolatilisation. In the DCM complex, seam splitting is generally a provenance of detrital material resulting largely from the proximity of the seam to the Smithfield Ridge and, as a result, the S2 and S4 are further subdivided into S2L, S2U and S4L and S4U, respectively.
S2L is thinnest over palaeo-highs and thickest over troughs of palaeo-valleys. Two S2L thickness domains exist in the complex. In the west (DCMW), seam thickness ranges from 0.1m to 5m and, in the east (DCME) from 0.2m to 6.0m. In the north of DCMW, the S2L exists as a single coal horizon, generally devoid of stone partings. To the south of this block, an arenaceous parting splits the seam into an upper and lower subseam. The parting separating S2U and S2L in both the west and east attains a maximum thickness of 4.5m and, in the northern portion, a minimum of 0.2m with an average of 1.5m. S2U is considered uneconomic to mine separately by underground methods but is included for the opencastable blocks. Generally, S2L is the thicker of the two subseams and has better quality coal. As such, S2L is theoretically the mining target. However, practical mining of S2L is often problematic due to parting between two subseams S2L and S2U.
On the basis of seam thickness and coal quality, S4L is the main underground exploitation target in S4, which comprises coal and minor in-seam partings, and is characteristically banded with alternating dull and bright coal. The seam contains one in-seam parting of significant thickness and lateral extent with a thickness average of 0.2m but can reach 0.5m. Generally, the average thickness of S4L is 3.0m. S4U has an average thickness of 1.7m in the west and, in the east, the thickness is generally more than 1.0m, reaching 3.0m in the centre of the palaeo-valley. Seam S4U has a higher ash content and thus lower CV compared to S4L. Thickness of the S5 is generally over 1.5m. Due to a significant parting, the seam is considered uneconomic for underground extraction but can be considered for selective extraction in opencast.
Forzando geology.
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 lowerquality coal.
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.
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. 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.
Tumelo geology.
Tumelo is to the north of the Smithfield Ridge on the northeastern edge of the Springs-Witbank coalfield. The area is part of the Karoo basin with stratigraphy similar to that of other ECC operations, Dorstfontein and Forzando with subtle location-induced differences.
Six coal seams are developed in the area, named from the base up as S1, S2, S4L, S4U, S4A and S5. S1 is very thin, being only developed in the deepest part of the palaeo-valley. The thickest average seam is the S2 (0.5m to 5.3m), developed mainly in a palaeo-valley where it is either thin or absent over the adjacent palaeo-ridges. S2 has been exploited since Tumelo production started in 2009. S4L is the most continuous and secondthickest seam while S4U is thin and has mostly been eroded. S4A is thin and patchily developed and S5 is only present in a topographically high area in the south east. The only economically exploitable seam is S2.
Late Jurassic dolerite sills and dykes occur in the Tumelo area. The intrusions resulted in displacement of the various seams and devolatilisation of extensive areas of coal. The geological complexity and associated challenges are proactively managed by extensive infill drilling, downhole wireline logging for better contact definition and seam correlation as well as surface mapping, particularly of basement outcrops. This information is captured in a structural model, together with potential risks captured in a GIS-based risk and opportunity domain analysis (RODA), allowing for a more integrated approach to risk management.