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Volta Basin - Kodjari, Arli, Tapoa, Mekrou, Aloub Djouana

Burkina Faso

Main commodities: P
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The northeastern NeoproterozoicVolta Basin in Burkina Faso, Benin and Niger, contains the largest of a series of deposits and sedimentary accumulations that occur in three Neoproterozoic phosphogenic basins/regions distributed along the margins of the West African craton. The other two are the Taoudeni Basin and the Anti-Atlas region.

Five main phosphatic areas include Kodjari and Arli in flat-lying sections of the basin, Tapoa and Mekrou (the most important) in the folded portion of the basin, and Aloub Djouana in a structurally complex folded and metamorphosed unit.

Kodjari and Arli are 60 km apart, both on the NE-SW trending northwestern margin of the basin, while Aloub Djouana, which is 30 km SE of Kodjari, is on the eastern side of the basin. All three are in far eastern Burkina Farso, around 485 km by road to the east of the capital Ouagadougou. Tapoa is also on the northwestern margin of the basin, 70 km NE of Kodjari, but in neighbouring Niger. The Mekrou series of deposits are generally 20 to 30 km south to SE of Tapoa in Benin and Niger.

The Volta basin is best developed in Ghana, narrowing to a fracture controlled depository that is only around 50 km wide to the north where it contains the deposits detailed above. The host sequence dips gently to the east where they overlie pre-1.7 Ga crystalline Birrimian basement to the west, and become more deformed to the SE, towards the NNE-SSW-trending Dahomeyide structural belt which is overthrust from the east onto the Volta basin sediments at ~615 Ma. The Dahomeyides represent a portion of the Pan-African (Cadomian) chain that extends northward into the Hoggar chain and are interpreted to comprise metamorphosed basement.

The stratigraphic sequence is divided into three units: (i) a ~1000 to 650 Ma Lower Voltaian Series of sandstones and conglomerates (possibly of glacial origin), which include hematitic claystone and conglomerate in Niger and Benin - overall regarded to be of shallow continental origin; (ii) a middle series of mainly claystones of the 675 to 615 Ma Oti Group, and equivalent shales, silty shales, sandstones and greywackes of the Pendjari Group in the Tapoa-Mekrou area in Benin and Niger; and (iii) the Upper Voltaian Series of predominantly arenaceous sediments of the Obosum Group which is Late Neoproterozoic the Cambrian in age.

The bulk of the mineralisation, particularly in the shallower dipping sections of the basin, are contained within the ~100 m thick Kodjari Formation in the lower half of the Oti Group. The type section at Kodjari commences with breccia lenses conaining fragments of older quartzite from the underlying Lower Voltaian deposited on a striated pavement. These are overlain by a green tillite containing polished and striated boulders of granite, rhyolite, quartz, sandstone, amphibolite, schist and gneiss in a matrix of clay-sand-carbonate. The tillites are overlain in turn by 0.5 to 3 m of limestone which is dolomitic (and baritic), often brecciated or slumped, followed by 25 to 30 m of green-orange chert with intercalations of silty-shale and tuffaceous beds/silica-rich volcanic ash. Overlying these are 0 to 30 m of argillaceous to micaceous sitstone that may contain some phosphate, passing upwards and laterally into the 0 to 15 m (generally >10 m) thick phosphate bed (20 to 30% P2O5) that comprises a fine to very fine-grained argillaceous siltstone, that rests directly into the bedded chert. These are all overlain by 1500 to 2000 m of green and black shale, pyritic mudstone, clayey-siltstone, argillaceous and feldspathic sandstone with thin intercalations of greywacke and limestone that comprises the Pendjari Group. The succeeding Upper Voltaian Obosum Group is absent in the Kodjari district.

The phosphorites at Kodjari are well bedded and essentially composed of structureless grains of brown, cryptocrystalline, isotropic apatite that range from 35 to 40, up to 200 µm, with a mean diameter of 80 µm. They are characterised by their fine grain size and good sorting, and are commonly rich in argillaceous and ferruginous impurities. Intraclasts and pseudo-oolites are present but rare, but where present are composed of nucleii of brown, cryptocrystalline isotropic apatite, encapsulated by a rim of colourless, fibrous, crystalline apatite or chalcedony. The detrital grains are essentially angular silt-sized quartz grains which make up ~1% of the rock. The cement, which represents 20 to 25% of the rock, is composed of microcrystalline quartz, crystalline and colourless apatite, small prisms of apatite, and some clay with associated iron oxides and hydroxides.

According to Trompette et al., 1980, these deposits contain in excess of 100 Mt of ore, including:
      Kodjari - 60 Mt @ 27.5% P
2O5,
      Arly - 2.8 Mt @ 29% P
2O5, both at a maximum 20 m overburden.

The reserves of the deposits, cited by McClellan and Notholt (1986), are:
      Aloub Djouana - 224 Mt 15% P
2O5,
      Kodjari - 80 Mt @ >18% P
2O5,
      Arly - 4 Mt @ similar grades.

The most recent source geological information used to prepare this summary was dated: 2005.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Trompette R, Affaton P, Joulia F, Marchand J  1980 - Stratigraphic and structural controls of Late Precambrian phosphate deposits of the northern Volta Basin in Upper Volta, Niger and Benin, West Africa: in    Econ. Geol.   v75 pp 62-70


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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