Abstract

JONES, M.Q.W.. Virgin rock temperatures and geothermal gradients in the Bushveld Complex. J. S. Afr. Inst. Min. Metall. [online]. 2018, vol.118, n.7, pp.671-680. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/2018/v118n7a1.

Knowledge of virgin rock temperature (VRT) is essential for planning refrigeration and ventilation requirements of deep mines. Geothermal studies in the goldfields of the Witwatersrand have been in progress for approximately 80 years, with the result that a very extensive rock temperature database is available. However, it is only in the last three decades that deep-level platinum mining in the Bushveld Complex, north of the Witwatersrand Basin, has made mine refrigeration an important issue. This paper presents temperature profiles in 31 boreholes in the Bushveld Complex that were surveyed as part of a project, conducted between 1985 and 2005, aimed at establishing the geothermal characteristics of the Complex. Most of the boreholes are located in platinum mining areas or prospects situated around the perimeter of the Complex. The geothermal gradient in rocks of the Main Zone of the Complex, which immediately overlie the platinum reefs, is remarkably constant at 20.7±1.3 K/km. Data from the Upper Zone and the Bushveld granites yield lower gradients, 16.5±1.1 K/km and 16.4±2.4 K/km respectively. Geothermal gradients in the Bushveld Complex are approximately double the thermal gradients in the Witwatersrand. The geothermal heat flux in the platinum mining areas, calculated from the thermal gradients and thermal conductivity data measured on samples of borehole core, is also remarkably uniform at 45±4 mW/m²; this is somewhat less than the average for Witwatersrand gold mining areas, 51±6 mW/m². The main reason for the higher gradients in the Bushveld Complex is the low thermal conductivity of the rocks. Bottom-hole temperature measurements in boreholes in the Northam mining area of the Complex yield the highest virgin rock temperatures (up to 70°C at 2.2 km depth). This is 30°C hotter than the temperature at the same depth in mines in the Carletonville area of the Witwatersrand Basin. The surface temperature data and heat flow data, together with an extensive thermal conductivity database, make it possible to predict VRTs in new platinum mining areas as well as chromium and vanadium mines if such mining proceeds to substantial depths.

Keywords : Bushveld Complex; platinum mines; virgin rock temperature; geothermal gradient; heat flux.

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