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TheWest African Craton (WAC) is one of the fivecratons of thePrecambrianbasement rock ofAfrica that make up theAfrican Plate, the others being theKalahari craton,Congo craton,Saharan Metacraton andTanzania Craton.[1] Cratons themselves are tectonically inactive, but can occur near active margins,[2] with the WAC extending across 14 countries in Western Africa, coming together in the late Precambrian and earlyPalaeozoic eras to form the African continent. It consists of twoArchean centers juxtaposed against multiplePaleoproterozoic domains made ofgreenstone belts,sedimentary basins, regional granitoid-tonalite-trondhjemite-granodiorite (TTG) plutons, and large shear zones. The craton is overlain byNeoproterozoic and younger sedimentary basins. The boundaries of the WAC are predominantly defined by a combination ofgeophysics and surface geology, with additional constraints by thegeochemistry of the region.[1] At one time, volcanic action around the rim of the craton may have contributed to a major global warming event.[3]
The craton appears to have formed when threeArchean cratons fused: Leo-Man-Ghana, Taoudeni and Reguibat.
The first two docked around 2.1 Ga (billion years ago), and the Reguibat Craton docked with the craton around 2 Ga. The roots of the combined craton extend to a depth of over 300 km (190 mi) in the sub-continental lithospheric mantle.[4] The WAC stretches from theLittle Atlas mountains ofMorocco to theGulf of Guinea, and is bounded by mobile belts of much younger rocks to the north, east and west. The oldest rocks weremetamorphosed 2.9 to 2.5 billion years ago. In theSahara it is mostly covered by more recent sediments from thePhanerozoic Eon. Further south, youngervolcanic andsedimentary rocks outcrop inGhana,Ivory Coast, andSierra Leone, surrounded by even younger sediments laid down in thePrecambrian.[5]
The WAC is made of two distinct regions north and south of each other; the Reguibat shield and theMan shield respectively. Both of these regions are mainly made of rocks that are eitherArchean orPaleoproterozoic in age,[1] with westernArchean nuclei,[2] and rock types are separated by major shear zones.[6] The fold belts surrounding the WAC were folded and metamorphosed during thePan-African and/or theVariscan orogenies.[7]
The WAC underlies the modern countries ofMorocco,Algeria,Mauritania,Senegal,The Gambia,Guinea Bissau,Guinea,Mali,Burkina Faso,Sierra Leone,Liberia,Ivory Coast,Ghana,Togo andBenin.
The metamorphic record of the craton is characteristic ofPaleoproterozoic plate tectonics. A definitive evolution of the area has not been determined as a result of conflicting interpretation of the relationship between low-gradegreenstone belts and high-gradegneissic terranes dominated byTTG suites.[8] The three major, widely accepted tectonothermal events for the WAC; the 3.5 to 2.9 Ga Pre-Leonean and Leonean Orogeny, the 2.9 to 2.8 Ga Liberian Orogeny, and the 2.15-1.8 GaEburnean Orogeny.[6] A definitive answer has strong implications on thegeodynamic processes controlling the craton stabilization and maturation after theArchean-Proterozoic transition. Limited geochronological data indicate a prolonged period of metamorphic overprint, lasting approximately 70-million-years, with the support ofSm-Nd garnet-whole-rock isochron age data andU-Pb andPb-Pb crystallization ages of zircon, monazite, and titanite.[8]
Overprinting relationships in the area indicate coppermineralization is associated with the first deformation event in the WAC, with gold mineralization occurring during subsequentdeformation events via reactivation ofmagmatic andhydrothermal fluids.[1]
Themetamorphic rock of the WAC include, but are not limited to, high-gradeamphibolite faciesamphibolite,gneiss, paragneissschist,calc-silicate rock, andmigmatites. Portions of the region have also been metamorphosed to thegreenschist facies,[2] generally termed agreenstone belt in anArchean terrane.[9] The timing of the facies is constrained by the in-situU-Pb dating, with garnet composition providing constraints forprograde evolution at theblueschist-amphibolite facies transition.[8] The tectonic environment is constrained by a combination ofgeophysics, surface geology,geochemistry, and metallogenesis.[10]
The Earth formed about 4.54 billion years ago.[11] As it cooled, thelithosphere, consisting of thecrust and the rigid uppermost part of themantle, solidified. The lithosphere rides on theasthenosphere, which is also solid but can flow like a liquid on geological time scales. The lithosphere is broken up intotectonic plates, which slowly move in relation to one another at speeds of 50–100 mm annually, colliding, combining intocontinents, splitting and drifting apart to form new continental configurations.[12]
It is difficult to reconstruct the early wanderings of the West African Craton, but around 1.13–1.071 billion years ago it seems to have been one of the cratons that came together to formRodinia, asupercontinent. At that time, the Congo Craton lay to the west of theAmazonian Craton, and the West African Craton lay to the south where both were rotated about 180° and retain this relative configuration.[13]
Around 750 million years ago Rodiniarifted apart into three continents:Proto-Laurasia, theCongo craton andProto-Gondwana.[14] The West African Craton may then have combined with other cratons to formPannotia, a hypotheticalsupercontinent that existed from thePan-African orogeny about 600 million years ago to the end of thePrecambrian about 539 million years ago.[15] Later it became part ofGondwana,[16] and later still part ofPangaea, thesupercontinent that existed during thePaleozoic andMesozoic eras about between 335 and 175 million years ago, before North and South America separated from Eurasia and Africa and the continents started to drift towards current configurations.[17]
Proponents of the Snowball Earth theory claim that sometime before 650 million years ago the Earth was going through an extremely cold period. The oceans were frozen to great depths, and snow covering reflected the heat from the sun through cloudless skies. Only simple forms of life could survive in locations such as deep oceanichydrothermal vents.[18] At the end of this period the edges of the West African craton became highly active, throwing up a ring ofvolcanoes. The thermal activity was caused by excessivemantle heat that had built up below the craton, insulated by thelithosphere. The volcanic eruptions created agreenhouse effect on massive scale, melting the ice and releasing CO2 into the atmosphere. The climate quickly switched to one much warmer than today, resulting in theCambrian explosion of life forms.[3]
During its wanderings, at different times covered by ice sheets, forests, marshes or arid desert, the surface of the West African Craton has been heavily eroded by ice, water and wind. In most places the original rocks are buried far below more recent volcanic and sedimentary deposits. The visible features are usually of comparatively recent origin.
TheLittle Atlas range formed about 300 million years ago whenEuramerica andGondwana ground against one another during theAlleghenian orogeny, a process that also formed theAppalachians in present-dayNorth America. More recently, in theCenozoic Era from 66 million to about 1.8 million years ago, the mountain chains that today compose theAtlas Mountains were lifted up as the European and African plates collided at the southern end of theIberian Peninsula. Erosion has reduced theLittle Atlas range so that it is today lower than theHigh Atlas range to the north.[19][20]
South of the mountains, the West African Craton is relatively flat, mostly desert or drysavanna apart from the areas near the Atlantic or Gulf of Guinea. However, below the surface there are ancient sedimentary basins such as theTaoudeni basin that may contain large reserves of oil and gas.[21]
The northern Reguibat shield covers parts ofAlgeria,Western Sahara,[22][23]Morocco andMauritania.[1] It contains exposedPaleoproterozoic units to the east, andArchean to the west, includingkimberlites[2] making it a primarydiamond source withgold andcopper deposits occurring in a sub-domain of the shield (Paleoproterozoic Yetti).[6] It experienced at least one major cooling event during theMesozoic and three minor cooling events from thelate Cretaceous to present day. Low temperaturethermochronology data indicates that the western side of the shield has anigneous differentiation post-Triassic thermal history, largely controlled by vertical movements of the crust via burial and exhumation processes.[7]
The southernMan shield covers the countriesIvory Coast,Mali,Burkina Faso,Ghana,Sierra Leone,Liberia, andGuinea.[1] In this shield, a large portion of the WAC consists ofPaleoproterozoicBirimian continent.[2] TheBirimian units were initially produced in an immaturevolcanic arc setting, which was later metamorphosed during theEburnean orogeny. The poor exposure of thegreenstone belt in the area limits interpretations of a broad structural context. The limitedgreenstone belt exposure indicates a majordeformation event associated withgranitoidintrusions.[9]
The shield hosts world-class gold deposits, importantiron ore concentrations, and themineralization of aluminum ore, lead-zinc,manganese,phosphate, anduranium. A majority of the gold deposit formations occurred during theEburnean orogeny, but a number of the remaining gold deposits formed before this orogeny during a period of oceanic arc-back-arc basin formation anderosion during theNeoproterozoic andCretaceous.[6]
Within the West African Craton, there is a large amount of mining activity covering resources such as gold, copper, cobalt, silver, tin, and zinc.[24]
Artisanal mining activity in the craton dates back to the early 1960s which used quartz vein debris as a gold indicator.[25] Placer gold is primarily exploited by the artisanal miners, with other deposit types being mined byopen-pit or by excavation mining (surface mining orsub-surface mining).[6] In 1985 the State ofBurkina Faso created an official structure, known as Le Projet Orpaillage, for the management of gold mining and buying in the region.Metallurgical studies on thegold panning rejects were funded by theUnited Nations Development Programme (UNDP) and theBureau de Recherches Géologiques et Minières (BUMIGEB). The treatment of mine tailings collected from artisanal working gold was the principle operation conducted by the Compagnie d'Exploitation des Mines d'Or du Burkina (CEMOB), operating aheap leach plant which processed approximately 500 tonnes of gold panning rejects per day.[25]
Following the Mineral Act, organized mining began in 1927 with significant mineral discovery in the 1930s and 40s resulting in medium to moderately sized mines; iron ore, two extensive alluvial gold field deposits, placer gold, alluvial diamonds, and chromite deposits. Quarrying and mining provides a livelihood for greater than 250 thousand people inSierra Leone alone, employing approximately 15% of its population, producing a significant amount of material to qualify as a resource-rich country. The mining of the WAC can establish a strong economy and government development support, helping the alleviation of poverty.[26]
{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)The Reguibat Shield is distributed between Mauritania , Western Sahara and Algeria
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