Abstract
�,8:2;<; 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
Highlights
8:2;
Investigation of mine refrigeration and ventilation requirements depends on a knowledge of virgin rock temperature (VRT) and thermal properties of rocks involved in excavations
The primary purpose of this paper is to describe and discuss the borehole temperature database
Summary
Venetia mine is situated in the Limpopo Belt (Figure 1) which is situated between the Zimbabwe Craton to the north and the Kaapvaal Craton in the south(Kramers et al, 2007). The Limpopo Belt is a broad zone of highly deformed and high-grade metamorphic rocks It is separated from the cratons by prominent shear zones (Figure 1). Rocks through which the Venetia kimberlite cluster was extruded are predominantly biotite gneiss, biotite schist, amphibolite, some quartzofeldspathic gneiss, and a younger Proterozoic diabase sill (Barton et al, 2003; Kurszluakis and Barnett, 2003, a detailed geological log of one of the boreholes investigated here, DDH132). These rocks are enveloped by medium- to high-grade metasedimentary rocks. Four of the holes are collared within the kimberlites and the rest in country rock surrounding the kimberlites
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