Virgin rock Temperature study of Venetia diamond mine

Abstract

Temperature surveys in 13 boreholes at Venetia diamond mine, Limpopo Province, South Africa, in 1994 (shortly after the mine opened) and in 2006 (during the feasibility phase of the Venetia underground project) have resulted in a comprehensive database of rock temperatures and geothermal gradients. The database permits clear identification of some factors that perturb rock temperatures, the most important being underground flow of water and recent climate change, and identification of depth intervals showing linear temperature trends that are useful for establishing geothermal gradients. The Venetia data demonstrates the value of such detailed virgin rock temperature studies for mine refrigeration purposes. The average ground surface temperature, estimated by upward extrapolation of the borehole data, is 25.9±0.5°C, and the geothermal gradient, estimated by averaging the gradients in linear sections of the boreholes, is 19.5±1.0 K/km. A plot of unperturbed borehole temperature versus depth below mine datum (696 m above mean sea level) yields a linear array with an intercept of 25.3°C and a slope of 20.1 K/km, which are compatible with the above averages. The average thermal conductivity of Limpopo Belt rocks of similar composition and metamorphic grade to the country rocks at Venetia is 3.01±0.55 W m-1 K-1. The geothermal heat flux calculated from this value and the measured geothermal gradients is 58±3 mW m-2.