Potholes are frequently encountered during mining operations in the Bushveld Igneous Complex in

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...

. Two orebodies, the Upper Group 2 (UG2) and the Merensky Reef, host about 70% of the world's platinum group metals (PGM), and pose major extraction problems for the mining industry in their faults, dykes, joints, domes, iron-rich

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...

pegmatoids, rolls and

dunite Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, with mi ...

pipes. The greatest mining problems, though, are presented by potholes. The massive intrusion of molten magma, predating the nearby Vredefort impact by at least 30 million years, led to partial to complete melting of the cumulus floor already in place. Flow and turbulence, high temperatures and chemical reactions, sculpted and potholed the surface of the floor in a process similar to the erosion caused by running water. With the end of the outpouring, when emplacement ceased, cooling of the magma started, leaving the potholes filled, and creating a fault surface at the interface of the pothole and the filling magma. The final phase was when crystallisation of

chromitite Chromitite is an igneous cumulate rock composed mostly of the mineral chromite. It is found in layered intrusions such as the Bushveld Igneous Complex in South Africa, the Stillwater igneous complex in Montana and the Ring of Fire discovery in O ...

was followed by that of

pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

and

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

, while the contents of the potholes, also consisting of UG2 or Merensky Reef, but having followed a different cooling profile, showed little or no crystallisation of chromitite. The problems posed by potholes to mining operations are rooted in the fracturing and fragmentation of the material in potholes and their surroundings. These lead to ground instabilities, especially in the hanging wall, and serious safety hazards. Mining is generally abandoned when more than about 40% of the raise line is potholed. A crew for developing new raise lines is usually on standby for such cases, incurring considerable development and labour costs. In horizontal section potholes are roughly circular to elliptical and vary in diameter from 20 m to more than 1 km. In vertical section their shape is generally dish-like and may be quite asymmetric.''The morphology of potholes in the UG2 chromitite layer and Merensky Reef (pothole reef facies) at Union Section, Rustenberg Platinum Mines'' - K. G. Lomberg, M. A. Patterson, J. E. Venter, and E. S. Martin
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References

*''Predictability of Pothole Characteristics and their Spatial Distribution at Rustenburg Platinum Mine'' - Chitiyo, Schweitzer, de Waal, Lambert and Ogilvie ''(Journal of the Southern African Institute of Mining and Metallurgy - December 2008)'' Mining terminology Structural geology Geology of South Africa {{mining-stub

See also

References