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Airborne geophysical data show major future energy in the Waterberg coalfield
WATERBERG COALFIELD lineament interpretation of phasemag data. A surface geology interpretation was done by using a ternary radiometric image. It was calculated by using potassium count in the 'red channel', total count in the 'green channel' and uranium count in the 'blue channel'.
Posted in our July 2010 issue
South Africa is highly dependent on coal to generate 75% of its electricity. The majority of this coal comes from the Witbank Coalfield, but this source will be exhausted in the next century (Prevost 2007).
The Waterberg Coalfield in Limpopo province contains vast resources of coal and is the next area that will supply South Africa with energy well into the future.
The coalfield is in the Karooage Ellisras Basin. Coal was discovered in the Basin in 1920, but little exploration was conducted since.
Coaltech Research Association commissioned an airborne geophysical survey of the area to enhance the structural understanding of the basin. The magnetic and radiometric methods were applied.
The datasets collected were magnetics, radiometric counts uranium, thorium, potassium, and total, as well as digital elevation model.
Magnetic susceptibility in the Ellisras Basin is low; however the application of a phase operator on this data reveals a large amount of weakly magnetised anomalies that could be due to pre-Karoo features in the basin floor.
Radiometric data complemented the magnetic data by delineating the basin boundaries and indicated large block faulting.
IMPORTANCE OF THE STUDY
Eskom currently uses 110 million metric tonnes (Mt) of coal in its power stations per year (2007). Export coal is currently 70Mt and 50Mt is used for synthetic fuel (Prevost 2007). There is currently only one mine in the Waterberg Coalfield (Grootegeluk), which is the sole supplier to the Matimba Power Station.
Medupi Power Station of similar size is currently under construction in the same area and will also source its coal from Grootegeluk, which could cause delivery capacity problems.
Waterberg Coalfield infrastructure raises various concerns and the precise location of the faults and structures will greatly impact on the estimation of both the shallow and deep coal resources. This has not been investigated to date. Information regarding the basement underneath the Waterberg will also help understanding the genesis of the basin.
Geological setting
The coal-bearing rocks belong to the Karoo Supergroup and were deposited between 260 and 190Ma ago. They formed as a large graben structure bounded by these basin edge faults:
o In the north on older basin rocks (the Melinda Fault zone) that belong to the Limpopo mobile belt
o In the south with the Waterberg Group (Eenzaamheid and Ellisras Faults)
o Post Karoo faults (Daarby Fault) that disrupt coal seams.
The formation of the basin was controlled by structures that were formed and reactivated over time (Daarby Fault) and is the basis for the block faulting that occurs through the basin. To avoid confusion, the term 'Ellisras Basin' will be used for coal-bearing rocks (Johnson et al, 2006a).
Stratigraphy
The area covered by the geophysical investigation constitutes mainly the following geological terrains:
o Limpopo Mobile Belt - highly metamorphosed gneiss at 2700Ma (Kramer et al, 2006)
o Ellisras Basin, consisting of Waterberg Group (Barker et al, 2006) and Karoo Supergroup (Johnson et al, 2006b), containing coal, mostly in the Grootegeluk Formation, 110m thick in the south.
o Recent cover is from the weathering of gneiss of the Limpopo Mobile Belt and the Karoo rock in the north, and from Waterberg sandstones in the south
o Intrusive rocks cut through coal-bearing rocks and disrupt the seams. They occur less frequently in the Ellisras Basin.
Geophysical survey
This survey was conducted in 2007 and covered eight 1:50 000 sheets. The survey was flown in a north-southerly direction at a 200m line spacing. The flying height was 80m at a speed of 230km/h. Sampling frequency was 10Hz, implicating a measurement every 6.5m.
The survey was flown in blocks of 5km by 5km with a tie-line every 1km in an east-westerly direction. The purpose of the tie lines is to facilitate the levelling of the data. The magnetic data was collected with a caesium vapour magnetometer (resolution 100pT). The radiometric data was collected with an 80 litre NaI crystal and the elevation was measured using a laser altimeter.
Contact between the Limpopo Mobile belt and the Ellisras Basin can easily be seen on the magnetic data. It also shows the absence of strongly magnetised structures in the basin.
Total count radiometric data shows the northern contact of the Ellisras Basin clearly. It also shows the large block faulting and radioactive material eroding from the source (Waterberg Sandstones) in the south into the sediment load of the north-flowing Mokolo River.
These sandstones are mainly the feldspar-enriched kranskop sandstones, which indicate that the original source was granitic in composition. The source in the north is the gneiss of the Limpopo Mobile Belt.
However, the concentrations are low. The higher thorium count suggests that the source of the radioactivity is much older; i.e. the current source is a second generation source. Potassium count is the highest and maps the distribution of the radioactive isotope of Feldspar. High concentrations are due to the sandstones, gneiss and granites in the area.
Geophysical interpretation
A phase operator was calculated on the magnetic data. The enhanced data show all the smaller anomalies in the data. This confirms that the structure in the Ellisras Basin is either weakly magnetic or all structures are below the Karoo cover in the basement material (Waterberg).
Fieldwork indicated that dykes are scarce and highly weathered. The complete dataset was used to conduct a lineament interpretation. Interpreted dykes and faults were indicated.
Physical properties
To conduct a credible geophysical
interpretation and modelling, it is always necessary to sample the geology and measure the physical properties of the different lithologies. Samples were taken of the Karoo lavas (Letaba Basalt), Karoo sandstones (Molteno Formation) and Waterberg sandstones (Kranskop, Holkrans and Mogolakwena formations).
A specimen was taken from what we first believed was a Hydrothermal or Felsitic Dyke. Microscope studies showed that it is an iron rich pisolite with growth rings around a quartz nucleus. This means that the origin was not hydrothermal, but hygroscopic.
Physical properties show that the basalts and shale are magnetic and conductive. The sandstones are mostly non-magnetic. Densities of the shale and sandstones vary.
Conclusions
Airborne geophysical survey was a major contribution towards gaining knowledge about the Ellisras Basin. Data and first interpretation have shown that the basin has undergone much more structural disturbance than what was previously suspected. The survey also revealed a large amount of previously unknown structure in the basin.
This poses the question of how many of these structures remain undetected because they are non-magnetic. The question can most likely be answered by airborne electromagnetic survey. This would produce much more additional data and greatly enhance the structural understanding of the Ellisras Basin. This will also ensure better coal delineation and ultimately better mining practices with a better coal recovery.