A geologicalpothole is a natural depression in the rock filled with cooled magma, creating a fault. 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.[1]
Potholes are frequently encountered during mining operations in theBushveld Igneous Complex inSouth Africa. The massive intrusion of molten magma, predating the nearbyVredefort impact by at least 30 million years,[2] led to partial to complete melting of thecumulus 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 that results in ariverbed pothole. 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 ofchromitite was followed by that ofpyroxene andplagioclase, 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.
In theBushveld Igneous Complex, two orebodies, the Upper Group 2 (UG2) and theMerensky Reef, host about 70% of the world'splatinum group metals (PGM), and pose major extraction problems for the mining industry in their faults, dykes, joints, domes, iron-richultramafic pegmatoids, rolls anddunite pipes. The greatest mining problems, though, are presented by potholes.
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.
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