The distribution and quantities of radioactive trace elements, in particular uranium and thorium, within the Waterberg Coalfield
Date
2018
Authors
Murdoch, Shaun
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Abstract
In an uncertain future with regards to coal utilisation due to environmental regulations, coal mining, and particularly the future of coal mining located in the Waterberg Coalfield of the Ellisras Basin in South Africa, needs to be aware of all the risks and possible pitfalls that could hamper its further development.
The radioactive elements, U and Th, which are typically associated with sedimentary environments where coal is formed, if found in elevated quantities, would be one of the risks that needs to be quantified. Based on the impact elevated U levels has had on the development of the Springbok Flats Basin, this study was instigated to evaluate if there was a similar risk to the coal resources and reserves in the Waterberg Coalfield, which lies 150 km to the northwest of the Springbok Flats Basin.
Due to limitations related to access to samples it was not possible to sample the entire Karoo Supergroup sequence within the Waterberg Coalfield. However, a database of downhole wireline geophysical logs was compiled and used as a first order study to identify the most suitable horizons for sampling for geochemical analysis. A total of 34 exploration holes and 7,451 blast holes were used to compile the geophysical database.
Trends related to gamma anomalies were determined where gamma values, here measured in American Petroleum Institute (API) values, are greater than 300. These were associated with the Eendragtpan (Beaufort) and Swartrant (Vryheid) Formations. The 54 anomalies (where API values greater than 300) within the Eendragtpan (Beaufort) Formation did not appear to follow any apparent trend. The 118 anomalies within the Swartrant (Vryheid) Formation conformed to two trends – the first was associated with a gradual increase in gamma values towards the base of the interburden between the coal zones 3 and 4A, Grootegeluk Bench 8. The second trend identified was that the anomalies were not associated with the coal seams, but instead with the interburden. The largest anomalies occurred in the interburden between the coal zones 2 and 3, at Grootegeluk Bench 10.
Geochemical analysis of the Bench 10 interburden yielded only two anomalous U and Th values (compared to U and Th values reported by Swaine (1994)), with the mean values for U and Th being 2.89 ppm and 2.47 ppm respectively. Various classifications and analysis were applied to the geochemical results to help ascertain if there was any correlations or trends within the results; 1) Th/U ratios indicate that Bench 10 was deposited under reducing conditions; and 2) the samples with anomalous U and Th values also report elevated values for Y, Zr, Nb and Ag, and are depleted in Na and Al.
The overall conclusion of the study was that while there was some U and Th, as would be expected in such an environment, there was not significant quantities found for exploitation purposes and it is unlikely that the radioactive elements will pose a health risk within the region. As such there is little cause for concern regarding the mining of coal within the Waterberg Coalfield, in particular pertaining to U or Th.
Description
A research report submitted in partial fulfilment to the degree of Master of Science to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa, 2018
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Citation
Murdoch, Shaun (2018) The distribution and quantities of radioactive trace elements, in particular uranium and thorium, within the Waterberg Coalfield, University of the Witwatersrand, Johannesburg, https://hdl.handle.net/10539/26732