The Birimian rocks are major sources of

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

and

diamonds Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of electricity, and insol ...

that extend through

Ghana Ghana, officially the Republic of Ghana, is a country in West Africa. It is situated along the Gulf of Guinea and the Atlantic Ocean to the south, and shares borders with Côte d’Ivoire to the west, Burkina Faso to the north, and Togo to t ...

Côte d'Ivoire Ivory Coast, also known as Côte d'Ivoire and officially the Republic of Côte d'Ivoire, is a country on the southern coast of West Africa. Its capital city of Yamoussoukro is located in the centre of the country, while its largest city and ...

Guinea Guinea, officially the Republic of Guinea, is a coastal country in West Africa. It borders the Atlantic Ocean to the west, Guinea-Bissau to the northwest, Senegal to the north, Mali to the northeast, Côte d'Ivoire to the southeast, and Sier ...

Mali Mali, officially the Republic of Mali, is a landlocked country in West Africa. It is the List of African countries by area, eighth-largest country in Africa, with an area of over . The country is bordered to the north by Algeria, to the east b ...

and

Burkina Faso Burkina Faso is a landlocked country in West Africa, bordered by Mali to the northwest, Niger to the northeast, Benin to the southeast, Togo and Ghana to the south, and Ivory Coast to the southwest. It covers an area of 274,223 km2 (105,87 ...

. 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, respectively. The Birimian

terranes In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its d ...

in the southern part of the

West African craton The West African Craton (WAC) is one of the five 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.Jessell M.W., Li ...

are a mix of

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

, and

plutonic Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: Marc ...

rocks and low-grade metavolcanics and metasediments. Almost half of the terranes consist of alkaline

granites Granite ( ) is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

. The rocks formed over a period of about 50 million years between 2.200 Ga and 2.100 Ga years ago.

Location

The Birimian rocks stretch across the countries to the north of the

Gulf of Guinea The Gulf of Guinea (French language, French: ''Golfe de Guinée''; Spanish language, Spanish: ''Golfo de Guinea''; Portuguese language, Portuguese: ''Golfo da Guiné'') is the northeasternmost part of the tropical Atlantic Ocean from Cape Lopez i ...

, forming parallel belts that generally trend northeasterly and are wide and about apart. They consist of interlayered sedimentary and volcanic flow rocks metamorphosed to low

greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as Chlorite ...

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

. Most rivers draining the Birimian rocks hold

alluvial Alluvium (, ) is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is ...

gold deposits. They are overlaid in places by quartz-pebble conglomerates within the Tarkwaian System, name after

Tarkwa Tarkwa is a town and is the capital of Tarkwa-Nsuaem Municipal District, Tarkwa-Nsuaem Municipal district, a district in the Western Region, Ghana, Western Region southwest of South Ghana. Frequently dubbed as the "Golden City" by its indigenous ...

, 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. The Birimian forms two distinct and parallel

greenstone belt Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. The name comes from the green h ...

s in southwest Burkina Faso, the Hounde and the Boromo. The Hounde Greenstone Belt extends from the Ivory Coast border to the north of

Houndé Houndé is a city located in the Provinces of Burkina Faso, province of Tuy Province, Tuy (of which it is the capital) in the Hauts-Bassins Region of Burkina Faso. It is situated about east by northeast of Bobo-Dioulasso along the trunk road Ro ...

. The Boromo Greenstone Belt to the east of the Hounde Greenstone Belt, also extends from the Ivory Coast border through the Poura mine, Boromo and north to

Ouahigouya Ouahigouya () is a city in northern Burkina Faso, situated 182 kilometres northwest of Ouagadougou. It is the capital of the Yatenga Province and one of its subdivisions the Ouahigouya Department. It is also the biggest town in the Nord Region (B ...

and

Kaya Kaya may refer to: People *Kaya (given name) * Kaya (surname) Places *Kaya, Burkina Faso, a town in Burkina Faso, capital of the department * Kaya Airport, serving the town * Kaya Department, a department or commune of Sanmatenga Province in cen ...

Origins

One view is that the Birimian rocks were formed by the collision of the

Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

Cupixi-Carajas

craton A craton ( , , or ; from "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of contine ...

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.

Mining operations

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 The Ashanti Region is located in the southern part of Ghana and is the third largest of Regions of Ghana, 16 administrative regions, occupying a total land surface of and making up 10.2 percent of the total land area of Ghana. It is the List of ...

of the country. The privatized

Ashanti Goldfields Corporation The Ashanti Goldfields Corporation is a gold mining company based in Ghana that was founded by Joseph Ellis and Joseph Biney both from Cape Coast. The Ashanti Mine, located at Obuasi, 56 km south of Kumasi, has been producing since 1897. Du ...

, formed in 1993 with investment from

Lonrho Lonrho is a London-based conglomerate that was established in 1998 as Lonrho Africa plc. It is engaged in multiple business sectors in Africa, mainly agribusiness, infrastructure, transport, hospitality and support services. History Lonrho ...

and now owned by

AngloGold Ashanti AngloGold Ashanti Limited is a South African gold mining company, with global operations. With a diverse portfolio of cooperation, projects, and exploration activities, AngloGold Ashanti was formed in 2004 by the merger of AngloGold and the ...

, 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 Mercury poisoning is a type of metal poisoning due to exposure to mercury. Symptoms depend upon the type, dose, method, and duration of exposure. They may include muscle weakness, poor coordination, numbness in the hands and feet, skin rashe ...

. Gold

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

veins and lenticular reefs occur in the

shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear ...

between the Upper Birimian

phyllite Phyllite ( ) is a type of foliation (geology), foliated metamorphic rock formed from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation.Stephen Marshak ''Essentials of Geology'', 3rd ed. I ...

s and the Lower Birimian greenstones. Gold mines include the Obuasi Gold Mine, and those located at Prestea, Konongo, Hiré, Poura, Kalana, and Sabodala.

Placer gold Placer mining () is the mining of stream bed deposits for minerals. This may be done by open-pit mining or by various surface excavating equipment or tunneling equipment. Placer mining is frequently used for precious metal deposits (particularly ...

deposits are mined at

Siguiri Siguiri (N'Ko script, N’ko: ߛߌ߯ߙߌ߲߫; Arabic: سِجِرِ ِ) is a city in northeastern Guinea on the River Niger. It is a Sub-prefectures of Guinea, sub-prefecture and capital of Siguiri Prefecture in the Kankan Region. It is known for ...

Ity The pyramid of Ity was probably the tomb of Pharaoh who reigned during the 8th dynasty. It has never been discovered and is known only from a cliff-face inscription at Wadi Hammamat in the Eastern Desert, where there were several quarries in P ...

, and on the Ofin River.

References

{{Reflist Geology of Burkina Faso Geology of Ghana Mining in Ghana Geology of Guinea Geology of Ivory Coast Geology of Mali Mining in Mali

Location

Origins

Mining operations

See also

References