Arlit (Arlette), Akouta, Ariege, Taza, Tamou, Akola, Afasto, Azelik, Abokorum, Madaouela, Imouraren
Sandstone hosted uranium deposits are known and mined in two main districts in Niger at Arlit (Arlette), including the individual open pit deposits of Ariege, Taza, Takriza, Tamou, Artois and Tamgak and at Akouta, including the individual underground deposits of Akouta, Akola and Afasto. Other deposits include Azelik, Abokorum, Madaouela and Imouraren. These deposits are 150 to 200 km north to NNW of Agadez in north-western Niger.
In 2005, Niger produced 3093 t U. Total production to the end of 2002 was 87 859 t U (33 433 t from open pit mining; 48 641 t from underground mining; and 5785 t from heap leaching).
These deposits are developed over the Aïr Massif, the south-eastern segment of the Taureg Shield. The Taureg Shield is composed of three main physical massifs, the Aïr massif, the main Hoggar Mountains to the north and the Adrar des Iforas to the south-west.
The Taureg Shield, with the Benin-Nigeria Shield to the south are exposed parts of the Trans-Saharan (or Central Africa) Mobile Belt which was deformed during the Pan African (600 Ma) orogeny. This mobile belt is developed along the eastern margin of the West African Craton and is up to 1000 km wide and extends for 3000 km from the Mediterranean coast to the north to the southern West African coast in Nigeria to the south. The boundary between the West African Craton and the mobile belt is marked by a zone of gravity highs which has been interpreted to reflect an eastward dipping zone of un-rooted, dense mafic and ultramafic bodies, representing the Pan-African collision between the passive western margin of the West African Craton and the cordilleran-type margin that was the Trans-Saharan Mobile Belt of the Archaean to Paleoproterozoic eastern continent. The Taureg and Benin-Nigeria shields have subsequently been reworked during the Pan-African tectono-thermal events. Much of the mobile belt is concealed below a major, shallow, Mesozoic-Cenozoic downwarp, the Iullemmeden Basin which separates the The Taureg and Benin-Nigeria shields.
The Taureg Shield is divided into three main segments by a series of north-south oriented major, broad shear zones, the western Pharusian, the central Hoggar-Aïr and eastern Hoggar-Tenéré domains.
The 200 x 400 km crystalline Aïr Massif is the southern half of the Hoggar-Aïr domain and is composed of a core of Pan African granitoids intruding pre-Pan African crystalline rocks comprising probable Paleoproterozoic schists, gneiss and leptynites which were metamorphosed to amphibolites or upper greenschist facies. The crystalline core is separated from only mildly metamorphosed rocks of the flat lying, latest Neoproterozoic to Ordovician Proche Tenure molasse by overthrusts at Tafadek to the west and Aouzegeur to the east.
Caledonian uplift formed the Aïr Massif, a major horst block, leading to the erosion of the basal Palaeozoic sediments to expose the crystalline basement, where they are transgressively overlain by Lower Devonian sediments. The Aïr Massif is also an anticlinorium with kilometre width isoclinal folds, all dipping to the east. It is also characterised by north-south Pan-African thrusts which dip inwardly, related to the collision of the West African Craton and the Tran-Sahara Mobile Belt.
Over a dozen Palaeozoic - Ordovician to Devonian - (gabbro)-syenite-granite ring dyke complexes intrude the Paleoproterozoic basement, in central and southern Niger and northern Nigeria. This activity commenced with an acid volcanic phase, followed by gabbros, anorthosites, syenites and alkaline to peralkaline granites, all the result of the differentiation of basic magmas. It is suggested that these ring dyke complexes represent the eroded roots of volcanic edifices that were removed and deposited as part of the late Palaeozoic to Mesozoic basins that host the uranium mineralisation.
The Iullemmeden Basin is a large (0.36 million sq. km) structural depression covering much of western Niger, southern Algeria, Mali, Benin and Nigeria. It is filled by a poorly deformed, flat lying, 1500 to 2000 m thick, early Palaeozoic to Pleistocene succession of alternating marine and continental sediments. Tertiary volcanism is represented by more than 30 trachyte and phonolite plugs and associated basaltic lavas.
Along the western margin of the Aïr Massif, the Iullemmeden Basin commences with Devonian sediments which rest unconformably on crystalline basement and are overlain by Carboniferous, Permian and Mesozoic sequences to the late Cretaceous. The earliest Palaeozoic facies were epicontinental marine sediments which became lagoonal, deltaic and eventually continental in the lower Carboniferous. This change in sedimentation corresponded with major uplift along the Aïr axis.
Deeper in the basin, the Palaeozoic succession commenced with Cambro-Ordovician coarse siliciclastics, followed by Silurian graptolitic shales
Throughout the Permian and Mesozoic, the uplifted Aïr Massif was subjected to erosion by intermittent streams and rivers to form continental arkosic sandstones, siltstones, shales, and conglomerates known as the 'continental intercalaire' in the Iullemmeden Basin.
The Palaeozoic sediments in the Iullemmeden Basin, on the western margin of the Aïr Massif, are organised as a succession of wedges which each thin from north to south, controlled by north-south block faulting, possibly related to Hercynian tectonics. During the late Cretaceous, marine conditions were returned to the southern Sahara.
The Upper Cretaceous and Lower Tertiary sequences are marine, and comprise argillites, marls and fossiliferous limestones with lesser silty, sandy and gritty beds.
The sedimentary deposits adjacent to the western edge of the Aïr Massif, in the Tim Mersoi sub-basin (at Arlit) and the more southerly In Gall basin (at Agades) are the most important host to uranium mineralisation. Upper Carboniferous units, especially the sandstones, host uranium occurrences and also coal beds. Similarly, sandstone beds within the Jurassic and Cretaceous of the 'continental intercalaire' host uranium, copper and saline units which are mineable for salt. Much of the uranium in these units is hosted within sandstone and conglomerate units between layers of siltstone and shale which are rich in organic material and pyrite, lapping onto an erosional surface. The host units young to the south and west. The Arlit and Akouta deposits are hosted by Permo-Carboniferous units, while Abokorum and Azelik further to the west in Lower Cretaceous. Mineralisation occurs at depths of 10 to 75 m.
The uranium is mainly present as pitchblende and coffinite associated with reduced hosts.
The main deposits known in 2005 were within the Tim Mersoi sub-basin, the mainly continental part of the Iullemmeden Basin. These include the Ariege, Artois, Arlette, Taza, Takriza, Tamou and Akouta. The principal hosts are the Lower Carboniferous (Visean) Guezouman conglomerates and the late Lower Carboniferous (Namurian) Tarat Sandstone. Higher in the succession the oxide Imouraren deposit is hosted by the Jurassic Tchirezrine sandstone.
The source of the uranium appears uncertain. Some suggest that it is derived from Pan African granites of the Aïr Massif, while others attribute it to eroded volcanic piles that were deposited above the Palaeozoic ring dyke complexes mapped within the massif. Remnant beds of Palaeozoic ignimbrites on the massif associated with ring-dyke centres contain uranium concentrated in the matrix and on secondary iron-oxide coatings surrounding lithic and crystal fragments. Based on variable Th/U ratios and degree of oxidation, it has been concluded that the original ignimbrite field was enriched in uranium, but that a considerable proportion was leached during the weathering of the volcanic pile.
Grades of ore reserves in the underground mines average 5 kg/tonne U, while the open pit deposits average 5 kg/tonne U. The Imouraren oxide deposit averages 1.1 kg/tonne U
Total reserve and resource figures for the Niger deposits in 2002 include:
Reserve at <USD 40/kg - 89 800 t U
Reserve at <USD 80/kg - 102 227 t U
Resource at <USD 40/kg - 125 337 t U
Total production to 2002 was 87 859 t U
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.
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