Geology
The Birim River basin lies in the Man Shield area of the West African craton, which has been overlaid by Early Proterozoic metasedimentary Birimian rocks. These rocks appear to have originated in mid-oceanic arcs of volcanoes, which formed a crust that collided with and rode over the Man shield portion of the West African Craton and was compressed to form a series of folds generally trending northeasterly. The Birimian rocks include Akwatian formations, named after the town of Akwatia in the Birim valley, which have yielded more than 100,000,000 carats (20,000 kg) of diamonds. The great majority of the diamonds are found in Harzburgitic rocks and seem to have crystallized at unusually high temperatures and pressures deep in the lithosphere.
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