Phosphates in the Sokoto Basin, Nigeria
There is substantial literature on the phosphogenesis and phosphate resources in a number of countries in West Africa (Gabon, Togo, Senegal, Morocco and Tunisia Slansky 1985, among others.
The generally accepted genetic theories are upwelling in arid regions and upwelling in a north-south sea-way in the west coast of continents. The genesis of phosphates in the Sokoto Basin is inexplicable on the basis of the said theories. This paper notes the genetic model earlier proposed by the author at the IGCP 156 Phosphorites Field Workshop and Symposium; Oxford 1988.
The genetic model was used by the Geological Survey of Nigeria to establish, exploration model for phosphates in the basin. The results indicate the potential of substantial phosphate resources.
The Geological setting of the Sokoto
The Sokoto Basin is Nigeria sector of extensive lullemeden Basin. It predominantly consists of a gently undulating plain with elevation varying from 250 to 400 meters above sea level. The area is marked by high drainage density stream dissection resulting in deeply incised ravines.
The major stratigraphic subdivisions are recognized:
3. Continental deposits of Upper Eocene Miocene age [= Continental Terminal]- Gwandu Formation
2. Intermediate marine and brackish water deposits.
a) Sokoto Group [Palaeocene]
ii. Kalambina Formation
i. Dange Formation
b) Rima Group [Maestrichitian]
iii. Wurno Formation
ii. Dukamaje Formation
i. Taloka Formation
1. The Lower Continental Beds [ Continental Intercalarie] of Late Jurassic to Early Cretaceous age]
- Gundumi Group
Gundumi Formation and Illo Formation
The phosphates show features of an epicontinental setting. A unique set of conditions favoured the accumulation of phosphates availability of phosphorus in excess of normal in the shallow marine waters, tectonic setting, structural setting and post-depositional history. Deposition of phosphates was in the periods of changing tectonism in the Palaeocene which modified the regional chemistry of bottom waters. The regional tectonic framework determined the basin sediment environment, depositional processes and the resulting geometries. The intra-continental basin bordered by gravity faults provided the essential shallow water system. The structural controls were a number of highs, and subsiding areas related to shoaling sites of enhanced phosphate accumulation correspond to three structural highs and saddles connecting them. Six sub-basins related to shoaling respect the economic prospects. The subsequent phospogenic history was marked by weathering, reworking and winnowing [see also Nwabufo-Ene 1990 and 2004, Haynes and Nwabufo-Ene 1998].
Other genetic features
a) The key host lithologies for the phosphate nodules and pellets are grey shales and siltstones. Carbonate beds provide markers for mineralized horizons.
unconformities played some part in the genesis of some phosphates. For example the unconformable horizon between the Dange Formation and the Wurno Formation is characterized by a number of phosphate mineral assemblages; the key ones being secondary iron phosphates and aluminum phosphates. The pathway for the formation of the minerals may be as follows;
i) Periodical concentration of iron either on land or in shallow waters.
ii) Subsidence caused by the differential movement.
iii) Formation of H2S by bacterial reduction of sulphates in organic rich sediments and the presence of iron oxides and hydroxides.
iv) Subsequent oxidation of pyrite producing free iron and sulphuric acid.
b) Clay minerals
clay minerals are useful indicators of phosphate genesis in the epicontinental setting of the Sokoto Basin. Clay minerals complexes comprising mostly of kaolinite with subordinate hydrargillite are good indicators of phosphate bearing sediments. Conversely, complexes dominated by montomorillmite indicate non-phosphate sediments. The clay mineral distribution suggest that the phosphates bearing sequences were deposited during periods of intense chemical weathering of the continental landmass, in warm, humid climatic conditions.
Grade of phosphates
Two types of phosphates of economic value are identified as follows:
a) Nodules and pellets
- Up to 36.6% P2O5
- Commercial indices are excellent
- Potentially toxic metals [Cd, Cr, U, Se, Hg, etc] are absent or in very low concentrations.
b) Primary phosphatic rocks shales, siltstones etc
- P2O5 - 5% -30%
Economics of the phosphates
The high grade nodules and pellets are suitable for the production of chemical (commercial) fertilizers e.g. Superphosphate. The reserves appear substantial.
The lower grade phosphatic rocks [10% - 30% P2O5] could be applied directly to soils for P' minerals PR or after partial acidulation PAPR.
The several option should be very attractive to investors considering the low capital investment and potential high profits.
Artisanal mining is feasible for the deposits in the shallow horizons. The demand for P' nutrients is high in Sokoto State and the adjoining states of Kebbi and Zamfara. The markets in these locations will have minimal effects of the adverse economic effect of long-distance movement of products.
The exploration model should be based on the genetic model as proposed. The model should emphasize the paleogeographical disposition of the area during the Palaeocene, especially the middle Palaeocene. Economic phosphate accumulation requires an effective structural or stratigraphic trap which would limit the influx of diluting materials.
Geological mapping on large scales, supported by the use of appropriate geophysical techniques (to identify subsurface structures) is recommended. Drilling, pitting or trenching are sine qua non to obtaining any useful results. Chemical and mineralogical changes must constitute a major part of the programme.
Haynes, J.R and Nwabufo-Ene K.E 1998 Foraminifera from the Palaeocene phosphatic beds, Sokoto Basin, Nigeria- Revista Espanol Micropaleonotologie Vol. 2 No 30 pp56 -78
Nwabufo-Ene, K.E 1988 Genesis of the phosphates in the Sokoto Basin, Nigeria West Africa Proc. IGCP 156 Phosporites Field Workshop and Symposium Oxford, England
Nwabufo-Ene K.E.2004 Minerals and Industrial Development in Sokoto State, Nigeria- Special Paper for the Investment Retreat, Sokoto State Nigeria Sokoto 18p