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

Examination of the diamondiferous sediments of the Birim River floodplain in Ghana using radiocarbon dating allows the recognition of three chronostratigraphic units: 13000–11000, 9000–7000, and 2100 years BP to the present, while older sediments rest on bedrock benches and form terraces. The late Pleistocene sediments infill deeper, scour channels, are generally both thicker and coarser, and return higher diamond grades. One thousand nine hundred 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 bedrock, valley morphology, and the location of tributary junctions and diamond sources. The influence of palaeoenvironmental conditions on fluvial deposition and placer formation in W Africa appears widespread and relates to the known late Quaternary climatic changes established from Lake Bosumtwi, Ghana, and from the Sahara.