Palaeogeomorphology and the occurrence of diamondiferous placer deposits in Koidu, Sierra Leone

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Abstract
The geomorphology and diamond distribution within the Koidu alluvial diamond field in Sierra Leone are examined to provide a methodology for palaeoplacer appraisal within a humid tropical environment of diverse relief and headwater drainage containing local diamond sources. A morphogenetic terrain model is proposed for use in devising prospecting programmes, based on geomorphological mapping and sediment analysis of terrain units mapped at 1:1250. Valley floors including headwater swamps and stream floodplains provide a radiocarbon-dated late Quaternary stratigraphic framework for deductions concerning interfluvial domains containing remnant river terraces, and colluvial stoneline deposits interspersed with non-alluvial erosional slopes. Planar interfluves as well as piedmont ‘glacis’ contain alluvial indicators within gravels composed of bedrock quartz and iron concretions derived from former landsurfaces. A process history for the interfluvial domain indicates downwasting and diagenesis of ancient degraded alluvials, lateral shifts of drainage lines and continued supply of diamonds from erosion of local sources, the ‘stonelines’ functioning as feeders of coarse clasts from the interfluves towards the valley floors. The use of geomorphology to provide descriptive and genetic terrain models to guide prospecting for alluvial placers is advocated as a standard procedure for use by mining geologists.