Abstract

The Marsfontein M-1 kimberlite pipe in the Mokopane (Potgietersrus) area is the highest grade kimberlite ever mined in southern Africa, and forms part of an extensive east-northeast striking pipe and fissure swarm. The kimberlites are of late Jurassic age and were emplaced into basement and formerly overlying Karoo Supergroup rocks. The area hosting the kimberlites was subject to erosion, which has removed the upper portions of the kimberlites and their Karoo host rocks. This phase of erosion commenced in the Pliocene as a result of the onset of subsidence of the Bushveld Basin. Subsidence was accommodated in part by the Zebediela fault, which passes immediately south of the kimberlite swarm. Material eroded from the vicinity of the swarm was deposited south of the fault, burying down-faulted Karoo Supergroup rocks and forming an extensive bahada, which includes up to 30 m of gravel in proximity to the fault. Alluvial deposits consisting of sand and gravel extend as much as 20 km south of the fault. It is estimated that some 9 billion tonnes of gravel occur in the section of the deposit immediately south of the kimberlite swarm with an average grade possibly exceeding 0.2 c/100 tonnes, and it is possible that economically viable diamond accumulations are locally developed within this gravel sheet.

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