Abstract

Three contiguous drainage basins in the Guinean diamond fields all contain alluvial diamond deposits. The basins are similar in size and underlying geology and all occur within the same morphogenetic and tectonic environment. However, despite these similarities their geomorphological histories are quite distinct and this, together with variations in known occurrences of kimberlitic diamond sources, explains the differences in the distribution and concentration of diamonds in the alluvial deposits. One of the basins has been captured in the last 1.5 m.y. and the consequent lowering of base level has resulted in the reworking of sediment, flushing of tributaries and slopes, and concentration of diamonds in the trunk channel. In contrast, the other two basins have been reduced in size by capture of portions of their headwaters. As a consequence the diamonds have been retained in tributaries close to their source and only low to moderate concentrations of diamonds are found in the trunk channels.

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