The Birimian rocks are major sources of gold and diamonds that extend through Ghana, Côte d'Ivoire, Guinea, Mali and Burkina Faso. They are named after the Birim River, one of the main tributaries of the Pra River in Ghana and the country's most important diamond-producing area. Ghana and Mali are the second and third largest producers of gold in Africa.
The Birimian terranes in the south part of the West African craton are a mix of metamorphosed volcanic, sedimentary and plutonic rocks and low grade metavolcanics and metasediments. Almost half of the terranes consist of alkaline granites. The rocks formed over a period of about 50 million years between 2.200 Ga and 2.100 Ga years ago.
The Birimian rocks stretch across the countries to the north of the Gulf of Guinea, forming parallel belts that generally trend northeasterly and are 40 to 50 km wide and about 90 km apart. They consist of interlayered sedimentary and volcanic flow rocks metamorphosed to low greenschist facies. Most rivers draining the Birimian rocks hold alluvial gold deposits. They are overlaid in places by quartz-pebble conglomerates within the Tarkwaian System, name after Tarkwa, the second largest source of gold in Ghana. However, recent research indicates that the gold found in the Tarkwaian rocks is not derived from the Birimian terranes.
One view is that the Birimian rocks were formed by the collision of the Archean Cupixi-Carajas craton in the Southern Guiana shield and the Kenema-Man craton in the Western Africa shield. However, it is more generally accepted that the rocks 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 the series of folds.
Gold mining has been an important part of the economies of Ghana and Mali for centuries. In Ghana, over 90 percent of output originates from underground mines in the Ashanti region of the country. The privatized Ashanti Goldfields Corporation, formed in 1993 with investment from Lonrho and now owned by AngloGold Ashanti, controls most production and generally uses environmentally acceptable processes. However, there are many small-scale miners using techniques causing environmental damage such as arsenic and mercury poisoning.
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