Late Quaternary alluvial placer development in the humid tropics; the case of the Birim diamond placer, Ghana

Radiocarbon dating of the diamondiferous sediments of the Birim River floodplain indicate three chronostratigraphic units: 13 000- 11 000, 9000-7000 and 2100 yrs BP to the present. The late Pleistocene sediments infill deeper, scour channels, are generally both thicker and coarser, and return higher diamond grades. Some 1900 samples from 700 boreholes indicate that diamond grade is influenced by bedrock consistency, gravel thickness, gravel calibre and stratigraphic position, but other external factors include supply of coarse quartz from bed-rock valley morphology and the location of tributary junctions and diamond sources.