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Ouelle...UNIVERSITY OF CAMBRIDGE Rocks for Crops - 157
Guinea-Bissau
Total population (July 2000 estimate): 1,286,000
Area: 36,120 km2
Annual population growth rate (2000): 2.4%
Life expectancy at birth (1998): 44.9 years
People not expected to survive to age 40 (1998): 40.6% of total population
GDP per capita (1998): US $616
158 - Guinea-Bissau
Large parts of Guinea-Bissau consist of low-lying areas with numerous islands, rivers and mangrove
swamps. Further inland are savanna and small mountainous areas along the border with Guinea.
Agriculture dominates the economy of Guinea-Bissau. In 1999, the agricultural sector contributed 62% of
the GDP and provided a livelihood to more than 80% of the population. Rice is the staple food of the
people of Guinea-Bissau, followed by cassava, beans, sweet potatoes, yams and fruits. Cashew nuts, oil
palm kernels and groundnuts are the main export crops.
To date, the mining industry of Guinea-Bissau is insignificant, with only some small-scale mining for sand
and gravel meeting domestic construction needs. Low-grade bauxite deposits exist in the country but have
not been developed.
A major boost to the mining industry came when Champion Mineral Resources Inc., Canada, announced
the signing of a memorandum of understanding to develop the extensive phosphate resources of the Farim
deposit (Industrial Minerals 2000).
Geological outline
Geologically, Guinea-Bissau can be divided into an eastern zone with predominantly Paleozoic rocks and
a western zone with mainly upper Mesozoic to Cenozoic sediments. The sedimentary facies of the
Paleozoic are mainly clastic, with carbonates occurring only in the Silurian. The Cretaceous to Cenozoic
sediments are mainly of marine origin.
AGROMINERALS
Phosphates
Phosphates in the Farim area (Farim-Saliquinhe) of northern Guinea-Bissau were discovered during oil
drilling in the early 1950s. The Directorate of Geology and Mines drilled seven boreholes in the same area
in the late 1970s, one of which intersected a 4.90 m thick sandy phosphate layer. Subsequent drilling (in
excess of 100 boreholes) outlined a phosphate deposit with reserves of 112 million tonnes at 29.85% P2O5,
7.8% SiO2, and 7.1% Fe2O3 + Al2O3 (Sustrac 1986; Prian 1989). The CaO/P2O5 ratio of the phosphate ore
is 1.41. The age of the deposit is Eocene (Prian 1989).
The phosphate deposit of Guinea-Bissau represents the southern equivalent of phosphate deposits in
Senegal and Mauritania and was deposited at the flanks of the Mauritania-Senegal-Guinea basin (Figure
2.7).
The Saliquinhe phosphate deposit (also known as Farim deposit due to its location close to the town of
Farim) formed during middle Eocene in a shallow sheltered bay. The 1-6 m thick phosphate bed (mean
thickness 3.2 m) is made up of greyish-whitish unconsolidated phosphates with no calcareous cement.
Some limited aluminum phosphates have formed at the top of the deposit due to weathering. The Fe
content of the ore is high due to the presence of 1-8% pyrite. Uranium concentrations range from
80-160 mg kg-1 and cadmium values of 80 mg kg-1 were found in a composite drill hole sample (Prian
1989). Beneficiation tests in France produced a concentrate assaying 37.8% P2O5, 1.02% Fe, and 0.1% S.
A major problem for the extraction of the ore is the overburden, which ranges from 26 m near Farim to 50
m in the northern part of the deposit (Prian 1989). The stripping ratio of over 6:1 is high.
Rocks for Crops - 159
A re-evaluation of the resources amounted to 105 million tonnes of phosphorites grading 29.8% P2O5
(Industrial Mineral 1997). Three possible ways to develop the deposit have been discussed:
• extract the unprocessed phosphate ore at US $39 per tonne and ship it for processing,
• concentrate the phosphates on-site to at least 32% P2O5 and ship it for further processing,
• fully process the phosphates on-site, at US $208 per tonne fertilizer product.
The company decided to develop 32% P2O5 concentrates at 750,000 tonnes per year for the first 5 years
(Industrial Minerals 2000).
Figure 2.7: Geological setting of the Farim-Saliquinhe phosphate deposit.
Agromineral potential
The Farim deposit is a large sedimentary phosphate deposit with considerable overburden. As such, it does
not lend itself to small-scale agromineral development. Agricultural direct application tests with ground
Farim phosphates have been conducted in Guinea Bissau on cereal and groundnut crops (Prian 1989), but
no results have been reported. The potential for the development of other agromineral resources is unclear
due to the paucity of detailed geological and soils information.
160 - Guinea-Bissau
References:
Industrial Minerals 1997. New phosphate deposit under evaluation. Oct. 1997, p.19.
Industrial Minerals 2000. Champion to boost phosphate capacity. Jan. 2000, p.10.
Prian JP 1989. The Farim-Saliquinhe Eocene phosphate deposit Guinea Bissau, West Africa. In: Notholt AJG,
Sheldon RP and DF Davidson (eds.) Phosphate deposits of the world. Vol. 2. Phosphate rock resources.
Cambridge University Press, Cambridge, UK: 277-283.
Sustrac G 1986. BRGM phosphate prospecting methods and results in West Africa. Trans. Instn. Min. Metall. (Sect.
A: Min. Industry) 95:A134-A143.
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