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The diamond deposits of the Mandala basin, SE Guinea, West Africa

Abstract The Mandala drainage basin lies in the central part of the West African craton immediately to the south of the regional watershed. There has been frequent kimberlite dyke intrusion in the basin. The kimberlites range in grade from apparently barren to having a high diamond content. The presence of other undiscovered kimberlites can be inferred from the distribution, chemistry and abundance of kimberlite satellite minerals and variations in diamond size and character. Secondary diamond deposits are widespread with the main concentrations in the present and former Mandala valley bottoms. Tributaries have lower diamond contents. There are systematic variations in the alluvial diamond characteristics which can be explained in terms of diamond sources as well as transport of the diamonds away from those sources. The occurrence of the secondary diamond deposits is explained by the form and distribution of the primary sources, the nature of the drainage network and the long-term evolution of the drainage basin. Since the Pliocene or Early Pleistocene the Mandala has been rejuvenated with incision of the main channel by up to 26 m. This incision has led to flushing of the tributaries and storage of sediment, including diamonds, in the principal channel.