Birimian
Encyclopedia
The Birimian rocks are major sources of gold
and diamonds that extend through Ghana, the Ivory Coast, 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 metamorphosedvolcanic, 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 2.200Ga and 2.100Ga years ago.
, 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.
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.
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 cause environmental damage such as arsenic and mercury poisoning.
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
and diamonds that extend through Ghana, the Ivory Coast, Guinea
Guinea
Guinea , officially the Republic of Guinea , is a country in West Africa. Formerly known as French Guinea , it is today sometimes called Guinea-Conakry to distinguish it from its neighbour Guinea-Bissau. Guinea is divided into eight administrative regions and subdivided into thirty-three prefectures...
, Mali
Mali
Mali , officially the Republic of Mali , is a landlocked country in Western Africa. Mali borders Algeria on the north, Niger on the east, Burkina Faso and the Côte d'Ivoire on the south, Guinea on the south-west, and Senegal and Mauritania on the west. Its size is just over 1,240,000 km² with...
and Burkina Faso
Burkina Faso
Burkina Faso – also known by its short-form name Burkina – is a landlocked country in west Africa. It is surrounded by six countries: Mali to the north, Niger to the east, Benin to the southeast, Togo and Ghana to the south, and Côte d'Ivoire to the southwest.Its size is with an estimated...
. They are named after the Birim River
Birim River
The Birim River is one of the main tributaries of the Pra River in Ghana and the country's most important diamond-producing area, flowing through most of the width of the Eastern region. The river rises in the east of the Atewa Range, flows north through the gap between this range and the Kwahu...
, one of the main tributaries of the Pra River
Pra River (Ghana)
The Pra River is a river in Ghana, the easternmost and the largest of the three principal rivers that drain the area south of the Volta divide. Rising in the Kwahu Plateau near Mpraeso and flowing southward for 240 km through rich cocoa and farming areas and valuable forests in the Akan...
in Ghana
Ghana
Ghana , officially the Republic of Ghana, is a country located in West Africa. It is bordered by Côte d'Ivoire to the west, Burkina Faso to the north, Togo to the east, and the Gulf of Guinea to the south...
and the country's most important diamond-producing area. Ghana and Mali are the second and third largest producers of gold in Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
.
The Birimian terranes in the south part of the West African craton
West African craton
The West African craton is one of the five large masses, or cratons, of the Precambrian basement rock of Africa that make up the African Plate, the others being the Kalahari craton, Congo craton, Saharan Metacraton and Tanzania craton. These land masses came together in the late Precambrian and...
are a mix of metamorphosedvolcanic, 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 2.200Ga and 2.100Ga years ago.
Location
The Birimian rocks stretch across the countries to the north of the Gulf of GuineaGulf of Guinea
The Gulf of Guinea is the northeasternmost part of the tropical Atlantic Ocean between Cape Lopez in Gabon, north and west to Cape Palmas in Liberia. The intersection of the Equator and Prime Meridian is in the gulf....
, 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
Greenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
. 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
Tarkwa, Ghana
Tarkwa is a town in the southwest of Ghana, located about 120 miles west of Accra. As of 2005, it was estimated to have a population of 40,397. It is a noted centre of gold and manganese mining. Tarkwa Mine, which is a large open-caste gold mine, is situated to the northwest of the town, and...
, 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.
Origins
One view is that the Birimian rocks were formed by the collision of the ArcheanArchean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
Cupixi-Carajas craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
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
Man Shield
The Man Shield or Leo-Man shield or craton is a geological area in the southeast of the West African craton, in part overlaid by gold-bearing Birimian formations....
portion of the West African Craton, and was compressed to form the series of folds.
Mining Operations
Gold mining has been an important part of the economies of Ghana and Mali for centuries. In Ghana, over 90% of output originates from underground mines in the Ashanti regionAshanti Region
The Ashanti Region is the third largest of 10 administrative regions in Ghana, occupying a total land surface of 24389 square kilometers or 10.2 per cent of the total land area of Ghana. In terms of population, however, it is the most populated region with a population of 3,612,950 in 2000,...
of the country. The privatized Ashanti Goldfields Corporation
Ashanti Goldfields Corporation
This is the article on the mining company. For the football club of the same name see Ashanti Gold SC.The Ashanti Goldfields Corporation is a gold mining company based in Ghana that was founded by Edwin Cade. The Ashanti Mine, located at Obuasi, 56 km south of Kumasi, has been producing since 1897...
, formed in 1993 with
investment from Lonrho and now owned by AngloGold Ashanti
AngloGold Ashanti
AngloGold Ashanti Limited is a global gold mining company. It was formed in 2004 by the merger of AngloGold and the Ashanti Goldfields Corporation.AngloGold Ashanti Limited is now a global gold producer with 21 operations on four continents...
, controls most production and generally uses environmentally acceptable processes. However, there are many small-scale miners using techniques cause environmental damage such as arsenic and mercury poisoning.