| Summary: | The study forms part of the COALTECH 2020 research program, a collaborative study which aims
to ensure the continued viability of the South African Coal Mining Industry well beyond the year
2020. It participates in the Geology and Geophysics Technology Area of the COALTECH 2020
Technology Wheel. The mission statement of this Working Group is to facilitate applied research to
identify, quantify and qualify the remaining Coal Resources, starting with the Witbank-Highveld
Coalfield, to enable informed decisions when defining and extracting Coal Reserves. The structural
investigation of dolerites in the south-eastern part of the Witbank Coalfield contributes to Task
1.1.1; Sedimentological and Structural Model of the Witbank-Highveld Coalfield.
The Witbank Coalfield in the Mpumalanga Province of South Africa is situated on the northern
sector of the main Karoo basin. The main Karoo basin is described as an asymmetric depository
with a stable, passive cratonic platform (Kaapvaal Craton) in the northwest and a foredeep to the
south with the Cape Fold Belt on its southern margin.
The study area is situated south of the prominent ï± 15m thick Ogies Dyke, which strikes from Ogies
in the west to Optimum Colliery in the east. The east-west trending pre-Karoo Smithfield Ridge,
consisting of Rooiberg Felsites, bounds the study area to the south and also separates the Witbank
Coalfield from the adjacent Highveld Coalfield to the south. The study was conducted on four
collieries, namely Bank, Goedehoop, Koornfontein and Optimum Collieries in the south-eastern part
of the Witbank Coalfield.
The objective of the study is to investigate the intrusion mechanism of the dolerites and the
metamorphic effect the dolerite intrusions had on the coal in order to quantify the impact on mining
and coal utilisation in the south-eastern part of the Witbank Coalfield.
The most important effects of dolerites on mining with a decreasing order of importance are:
1. Decrease in the safety conditions and an increase in the risk of roof failures, pillar and floor
stability.
2. Increase in the overall production and mining costs with a decrease in the potential profit.
3. Decrease in saleable tonnages with a decrease in the profit margin.
4. Increase in waste product generation and an increase in the environmental risk.
From the objective two separate studies were identified: the first study (A) focuses on the
relationship between geological structures and dolerite intrusions and the second study (B)
determines the metamorphic effect the dolerite intrusions had on the coal. The structural
investigation of the relationships between geological structures and the dolerites is contained in this
document.
Regional scale information was acquired by using various remote-sensing techniques. The CSIR
Miningtek through the COALTECH 2020 Research Program provided this state of the art
information. In conclusion to the regional scale study probable relationships between certain
Karoo-age dyke, sill and lineament trends that are associated with the northern main Karoo basin
and surroundings could possibly provide insight into better understanding of the intrusion
mechanism of dolerites in the south-eastern Witbank Coalfield. It is therefore probable that some
of the Karoo-age intrusives in the south-eastern Witbank Coalfield followed older basement
structures inherited by the Karoo strata and/or syn-tectonic structures related to Gondwana
fragmentation which was synchronous with dolerite intrusion (Encarnación et al., 1996).
The EW striking Ogies Dyke, which is the main structure in the Witbank Coalfield, most probably
pre-dates its associated smaller scale dykes and sills. Conclusions for this relative age difference
are summarised as being the following:
Its association with EW basement Pre-Karoo diabase, which probably acted as a plane of
weakness and might have triggered its earlier intrusion.
Difference in geochemical and mineralogical characteristics.
Absence of sills immediately to its north.
Should the NS striking dykes north and south of the Ogies Dyke be favoured by cooling
joints which developed as a result of its earlier intrusion, the age difference is evident.
Comparing the physical appearance of the ± 20m sill (main sill) in the Witbank Coalfield with the B8
sill in the Secunda Coalfield, the two sills have a number of properties in common. However, these
physical property comparisons are not precise and it is therefore suggested a detailed geochemical
analysis be undertaken focussing on the mineralogy, major and trace elements.
The sedimentary sequence is reconstructed by removing the main sill from the stratigraphy. The
reconstruction is aimed at determining if a spatial relationship exists between the coal seams, the
intra-seam strata and the main sill prior to the intrusion event. Borehole information on the elevation of the pre-Karoo basement is sparse as borehole penetration was terminated at the
bottom of the coal seam of interest. The removal of the dolerite convincingly reveals the pre-Karoo
basement topography, palaeo-floor and -roof morphology, as well as the width distribution of the
sedimentary units.
The following reconstructed sedimentary units were examined individually:
No. 2 Coal Seam;
Facies between the No. 2 Coal Seam and No. 4L Coal Seam;
No. 4L Coal Seam;
Facies between No.4L and No. 5 Coal Seam;
No. 5 Coal Seam.
The examination process of the data of each unit starts with the statistical analyses thereof which
includes histogram and probability plots of the palaeo floor, width and palaeo roof.
The investigation resulted in nearly direct linear correlation curves which disclose the existing
relationships between the palaeo floor elevations of the No.2, No.4L and No.5 Coal Seams.
Considering the range of correlation coefficient values of 0.81 to 0.99 for the palaeo floor elevations
it convincingly reveals the co-existing relationship in the geometry of elevations throughout the
entire stratigraphy of the sedimentary sequences. Several sedimentological factors contributed to
the present day geometries and widths of coal and associated clastical sedimentary rocks of
sequence of succession. The evidence in the relationship of the geometries of the palaeo floor and
roof elevations concludes that irrespective of variable intra-seam strata and coal seam widths the
pre-Karoo topography is reflected throughout the entire stratrigraphic sequence.
Prior to sediment burial, plant growth took place most probably on similar structural relief of gentle
attitudes. To conclude, a four-stage model is proposing how burial could have influenced widths
and aerial distribution of peat and intra-seam clastic sedimentary rocks of sequence of succession.
At the time of peat formation, the unconsolidated sediments had not yet undergone a great deal of
lithification in that the floor structure of the peat might have been without undulations. In conclusion, a reasonable inverse relationship between the net width of the stratigraphic
sequence from the palaeo floor of the No. 2 Coal Seam to the palaeo roof of the No.5 Coal Seam
and the floor elevation of the main sill exists. A Quantile-Quantile-plot and the regression slope
analysis of the data sets convincingly conclude the inverse relationship that exists between the floor
elevation of the main sill and the net width of the almost entire sediment sequence. In this context
the reasonable negative correlation coefficient of -0.57 is good.
This negative correlation implies that where the main sill is present in the lower stratigraphic levels
it underlies thicker sedimentary sequences and conversely where the sill had stepped up to higher
stratigraphic levels it underlies the thinner sedimentary sequences. In conclusion the differential
compaction of the sedimentary strata was in the main controlled by the the Pre-Karoo basement
topography. This in turn resulted in the fracturing and jointing of the sedimentary rocks over the
flanks of the pre-Karoo basement topography which to a large extent controlled the propagation
path of the main sill.
Evidence established in this study suggests the effect of basin tectonics to be the overriding
controlling factor of the stratigraphic position of the main sill in the Vryheid Formation sedimentary
rocks of sequence of succession of the south-eastern Witbank Coalfield. Other factors i.e. the
influence of the Ogies Dyke and syn-tectonic related regional scale structures seem to have had
some control in the propagation paths of the associated offshoots of the main sill.
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