2. OVERVIEW OF THE GEOLOGY AND THE MINERAL POTENTIAL
2.1 Geological Overview
The Precambrian history of this part of Africa is commonly described as a process of progressive cratonisation, or the gradual addition (also called accretion) of a series of successively younger mobile or orogenic zones or belts to the oldest crustal nuclei (older land) of early Archean age. Such zones are related to major orogenic events, like regional metamorphism (a process by which the original composition of rocks is being severely altered by pressure and heat), folding and the emplacement of plutonic or volcanic rocks. Occasionally, subsequent orogenic belts developed inside existing cratons (land, which has already stabilised), but more commonly they added to the size of older cratons by addition of new crustal material along their margins. Most of the cratons in the area under discussion stabilised already during the Archean after the accretion of only Archean mobile zones subsequent to orogenic events. The North Gabon Archean nucleus for example stabilised around 2.7 Ga. An exception is the West African craton, which stabilised much later at about 1.6 Ga after the accretion of vast areas of Lower Proterozoic (or Birimian) formations at the end of the Eburnean orogenic event.
Due to extensive cover by intracratonic basins and deep crustal reactivation during the Pan-African orogenic event, only segments of the original Archean-Lower Proterozoic cratons are recognisable today. The principal remaining segments of the West African craton are Reguibate in Mauritania, Kayes in Mali, Kedougou-Kenieba in Senegal and Mali and the vast Man-Leo terranes, which extend from Guinea in the west to Benin and Niger in the east. Parts of the Congo craton subcrop in the Congo Republic, Gabon, Equatorial Guinea, Cameroon and the Central African Republic. Segments of the ill-defined Nilotic craton are preserved in the northern part of the Central African Republic and in Chad. Remnants of Archean-Lower Proterozoic cratons are present in all countries under review of this study, with the exception of Gambia, Cape Verde Islands and possibly Guinea-Bissau.
The Archean-Lower Proterozoic cratons consist essentially of granitic-gneissic
terranes and of volcano-sedimentary and sedimentary greenstone belts (greenstone
is a field term, which is applied to altered basic igneous rocks which
owe their green colour to chlorite and other minerals). The greeenstone
belts, which are of either Archean or Lower Proterozoic age, are host to
significant precious metal, base metal and bulk mineral deposits world-wide.
Archean greenstone belts occur in most of the fragmented cratons of West-
and Central Africa, particularly in Sierra Leone, Guinea, Liberia, and
Ivory Coast. The second most significant gold resource of Africa (second
only to the Witwatersrand basin) is associated with greenstone belts
of the Lower Proterozoic Leo terrain of the West African craton. These
belts are also known as Birimian greenstone belts, named after the
Birimian River Valley in Ghana, where gold and diamonds occur. They encompass
a vast area of approximately 350 000 km² covering parts of Niger,
Burkina Faso, Benin, Togo, Ghana, Ivory Coast, Mali, Guinea, Liberia and
Intracratonic sedimentary basins, which are virtually uneffected by any orogenic event, cover extensive parts of the region. They range from Proterozoic to Quaternary age. Coastal basins of Cretaceous to Quaternary age occur along sections of the Atlantic coast.
West and Central Africa was part of Gondwanaland (the ancient continent
which combined most of today’s southern hemisphere), until the break-up
of the super-continent about 120 Ma ago. The basement geology of the mineral
rich Guayana and Brazilian massifs of South America is, therefore, comparable
to the West African and Congo massifs. This fact can be applied to mineral
The Precambrian greenstone belts of West- and Central Africa, being host to a number of producing mines already, have excellent potential for additional gold deposits. Their exploration potential is comparable to other greenstone belts in the world (Canada, South America, Australia).
The most promising of the greenstone belts are those of Birimian or Lower Proterozoic age. The Birimian belts also host the famous gold from the “Gold Coast” (now Ghana), first noted by Portuguese traders in 1471. Gold mineralization occurs in a large variety of local geological environments, such as:
There is also potential for gold associated with Archean greenstone belts, which occur in various parts of the region and also with other Lower Proterozoic greenstone belts, outside of the better-known Birimian. Such belts are known particularly from the Central African Republic, Congo, Gabon, Chad, Liberia, Sierra Leone and Mauritania. A number of primary and alluvial gold showings are known.
Gold potential also exists within the Pan-African, as the Akjouit  gold deposit in Mauritania demonstrates. Greenstone belts of Middle or Upper Proterozoic age and associated gold mineralization occur in the Pan-African as well. The Ilesha  greenstone belt in southwest Nigeria is an example. Outside of the region, similar Pan-African auriferous greenstone environments occur in Algeria, Sudan and Ethiopia. The known gold mineralization of Pan-African terranes has rarely resulted in economic gold deposits of any size. In comparison with the older Lower Proterozoic and Archean terranes the overall gold potential of the Pan-African is, therefore, rated lower.
The potential for economic silver concentrations is limited, except as a by-product of certain gold and base metal deposits. In Ghana the average Au/Ag ratio is approximately 20/1 (1997 Ag by-production of gold mining in Ghana is estimated at 30 000 oz).
184.108.40.206 Platinum Group Elements (PGE)
There is potential for economic PGE mineralization in West and Central Africa owing to the extensive subcrop of Archean-Lower Proterozoic mafic-ultramafic sequences and also to similar younger mafic Proterozoic hosts, including ophiolites. Exploration for PGE minerals is generally difficult due to the very low-level PGM contents, even in ore grade rock. Also, present-day PGM production is virtually restricted to a few deposits worldwide, most of them being associated with huge igneous complexes. The main producer is related to the15 km thick Bushveld Igneous Complex of South Africa, a laccolith of basic and acidic igneous rocks measuring 200 x 400 km on surface. Such deposits are difficult to duplicate. Nevertheless, it appears that exploration for primary PGM mineralization in the regions has a good chance and has hardly been attempted so far. Smaller alluvial PGM/ gold deposits can also be a target. PGM mineralization has been reported from the Archean Man terrane (Freetown area, Sierra Leone; Bopolu area in Lofa County, Liberia).
West- and Central Africa has a long history of diamond production, originating for the most part from fluviatile placers. Only a small fraction of total production derives from altered kimberlites. In the Kindia area in Guinea, some artisanal mining of relict heavy mineral concentrations is associated with older Paleozoic quartzites. The main producers have been Sierra Leone, Ghana, Guinea, Liberia, C.A.R. and Ivory Coast. Excellent geological potential for diamondiferous kimberlites and associated eluvial and smaller fluviatile placer deposits exists not only in the mentioned main producing countries, but also in many others of the regions, including Mauritania, Burkina Faso, Nigeria, Niger, Benin, Togo, Senegal, Cameroon, Congo, Gabon, and Chad. There is also a more speculative potential for marine on- and offshore diamonds, in particular in Sierra Leone, Liberia, Guinea, and perhaps also in Gabon and Congo.
The regions appear underexplored, in particular for diamondiferous kimberlites. Areas underlain by Archean terranes (>2.5 Ga) are particularly favourable, such as the western half of the Regibat in northern Mauritania as an example. As has been demonstrated in South Africa and Siberia, central areas of old Archean cratons, overlain by thick platform covers, have a high potential for economic kimberlites (“modified Clifford’s rule”). On the other hand, economic lamproites appear to occur along the periphery of Archean nuclei or in surrounding mobile belts of Proterozoic age (Argyle deposit, Australia and lamproites in Brazil). Potential zones in the regions include Lower Proterozoic, but possibly also Pan-African belts (Janse, A. J. A., Fifth International Kimberlite Conference, Brazil 1991).
The (plate)tectonic similarity of the Brazilian-Guayana massifs with West- and Central Africa, enhances the diamond potential for economic kimberlites and lamproites. Most of the numerous lamproites in Brazil occur in Proterozoic belts. They are frequently controlled by reactivated major continental structures, which also host a variety of alkali intrusives. A very similar environment exists in West- Central Africa. At Yengema  in Sierra Leone, one of the largest diamond centres in the region, diamondiferous primary orebodies are mainly controlled by extensional lower order structures, which form part of a regional shear array of prominent deep ancient reactivated structures. This favourable geologic environment, which exists over larger parts of the region, has been recognised for some time, but it has never been followed up by serious exploration - yet.
2.2.2 Non-Ferrous Metalliferous Minerals
220.127.116.11 Copper, Lead and Zinc
There is a good and virtually unrealised potential for volcanic associated
massif sulphide (VMS) copper, lead and zinc deposits, associated with
Archean and Lower Proterozoic greenstone belts of the West- and Central
African massifs. Similar massifs in other parts of the world, in particular
Canada host important VMS deposits, mostly of Archean age like Timmins
in Ontario, as an example. The only potential VMS deposit known presently
within the region is the Perkoa  zinc deposit in Burkina Faso, discovered
by UNDP in 1982. There are, however, a number of showings in several countries,
including Burkina Faso, Ivory Coast and Gabon.
Nickel mineralisation is primarily associated with Archean, but also with Lower and Upper Proterozoic meta-volcanosedimentary sequences. Nickel mineralisation is typically related to basic and ultrabasic rocks including amphibolites. Good and largely unrealised potential exists for low-grade, open-pittable lateritic ore particularly where cobalt credits add to the value. Falconbridge is developing a Ni/Co deposit in Ivory Coast. Numerous nickel occurrences are known within the Archean-Lower Proterozoic basement (Ivory Coast, Burkina Faso), but also within younger Pan-African belts (Mauritania, Senegal).
18.104.22.168 Tin and Tungsten
Tin mineralisation is related to various generations of granites, alkali-granites arranged as ring complexes and associated rhyolites and pegmatites. Tin occurrences are essentially associated with the Late Proterozoic Central Pan-African belt in Cameroon, Nigeria and Niger. Known deposits are of relatively moderate size. The well-known tin mining camp in Central Nigeria near Jos  has been an important producer for tin in the past. In consideration of present market conditions for tin and the comparatively small size of the deposits, the remaining reserves suggest mainly potential for smaller artisanal mining operations. Smaller and more isolated tin occurrences are also known from other parts of the regions, including Liberia, Ivory Coast, Senegal and Gabon. Tungsten mineralisation, although frequently associated with tin, rarely occurs in any concentration and is, therefore, of minor economic importance.
22.214.171.124 Other Metals
Indications of a variety of other metals are known. Significant columbite and tantalite mineralisation occurs in Central Nigeria near Jos  in a similar geological environment as tin mineralisation. Host rocks are granites, alkali granites and pegmatites of the the Central Pan-African belt. Chromite occurs generally in a similar environment as nickel. Known chromite showings are of small size and are considered to offer only limited economic potential at present (Mauritania, Sierra Leone, Liberia, Gabon). Cobalt may be closely associated with nickel deposits, as mentioned above (Ivory Coast). Vanadium is a byproduct of Tsumeb type stratabound copper, lead and zinc mineralisation in the M’passa area in the Congo Republic. Molybdenum is produced as a byproduct of uranium in Niger. Lithium (Burkina Faso) and rare earth mineralisation (Gabon) is known.
2.2.3 Bulk Minerals
The African bauxite province comprises the Guinea and Cameroon massifs. The Guinea massif area extends from Guinea-Bissau to Togo and north-northeasterly into Burkina Faso and Hoggar in the Central Sahara. The Cameroon area ranges from the west coast and across Central Africa to Mozambique. Bauxite formed from a variety of Precambrian, Paleozoic and Mesozoic rocks during a long period of Mesozoic and Cenozoic peneplanation and laterisation of the stable massif areas. The Guinea massif hosts one the world’s most important economic bauxite concentrations. The Guinea bauxite is not only important in terms of tonnage (2/3 of the world’s reserves), but also on account of its high grade alumina (Al2O3: 57 - 60%) and low grade silica (average SiO2: <1%). By contrast, the bauxite of Cameroon is of comparatively inferior quality (Al2O3: 43%, SiO2: 3.4%), which is one of the reasons why it has not been developed in the past. There is also potential for economic bauxite deposits outside the known Guinea and Cameroon bauxite provinces, mentioned above.
126.96.36.199 Iron Ore
Precambrian iron formations of the regions contain significant economic and sub-economic concentrations of iron ore of the Lake-Superior type, which are similar in size and grade as the world-class deposits of Brazil and Western Australia. Many deposits have been outlined, and further exploration would mainly be required to further define the quantity and quality of the ores. The most important deposits are associated with the Archean terranes of Man (Liberia, Guinea, Ivory Coast, Sierra Leone), and of the Congo (Gabon, Congo). Important iron-ore concentrations are related to Late Proterozoic terranes (Mauritania, Senegal, Niger) and other deposits with the Central Pan-African belt (Cameroon, Nigeria). The average grade of the commonly magnetite-rich ore is rather low (30 - 45 % iron), and requires upgrading to concentrates and pellets by enriching the iron-content and lowering the content of silica. Some of the deposits, however, have been upgraded by weathering to high-grade (60 - 70% iron) hematite ore. It was mostly high-grade ore, now largely depleted, which has been mined in the past. The lower grade-ores require considerable capital investment. This is one of the reasons, why most of the known deposits have not been developed.
The region has good potential for additional manganese deposits within
Proterozoic terranes. The unique manganese deposit of Moanda, Gabon 
is located within a sequence of 2 000 m of clastic sediments of the unmetamorphosed
Franceville basin of Middle Proterozoic age. The same basin is also
host to the uranium deposits of Gabon. Manganese bearing beds are 150-200
m thick, contain black shales, iron formation and sometimes dolomite.
The process of Mn accumulation is not well understood, but it is probably
associated with the weathering of older Mn-enriched basement and slow,
undisturbed deposition within a chemically suitable (black shales, dolomites)
environment. The presently mined deposits at Moanda constitute only a fraction
of the vast manganese-rich basin: It has been estimated that 20-27 t of
manganese has been deposited for every square metre over the entire 400-500
km² of the basin (Harben P. W. et al., 1997). The manganese mineralisation
at Nsuta, Ghana  is hosted by Proterozoic Upper Birimian carbonate-rich
sedimentary rocks, containing lower-grade primary or carbonate ore (30%
Mn). Higher grade (50% Mn) oxide ore represents residually concentrated
primary ore by laterisation of the peneplain during Tertiary time. A manganese
resource at Tambao, Burkina Faso  occurs in the upper zone of a kaolinised
granite. Manganese deposits at Ziémougoula (1.2 Mt at 47%
Mn)  and Korhogo (1.5 Mt at 20-35% Mn) , both in Ivory Coast remain
Several sedimentary phosphate deposits and significant phosphate mineralisation [mostly from the Tertiary (Eocene) age] are situated in marine coastal sedimentary basins (Senegal, Mauritania, Togo, Benin, Congo). Other phosphate deposits occur further inland, associated with Tertiary and older intracratonic marine sediments (Mali, Burkina Faso, Niger, Nigeria). Some of the deposits are of high grade (> 30% P2O5 in Togo), and there is excellent potential to increase the existing resource in a number of areas. Mining of phosphate is presently restricted to deposits, which are located in Togo, Senegal and Mali. A seven million tonne resource of offshore marine phosphorite in sea bed sediments with a high absorption rate of P2O5 in soil from raw material was identified by UNDP/ BRGM in the Congo . The Mabounié carbonatite  near Lambaréné (the place where Albert Schweitzer worked) in Gabon contains a significant resource of phosphate and columbium in its weathered residual soil.
2.2.4 Solid Fuel Minerals
188.8.131.52 Coal, Lignite and Peat
Although only a few economically expoitable coal or lignite deposits have been found in the region so far, there is certainly geological potential associated with the various intracratonic basins of Phanerozoic age. Exploration for coal has not been attempted on any systematic scale. Bituminous coal deposits are being exploited in Niger and Nigeria, several other occurrences and indications exist in Nigeria. Indications of bituminous coal and lignite were found in drill-cuttings for oil in some of the Cretaceous basins in Chad. Lignite occurs also in the Central African Republic. Due to very difficult infrastructure of many of the potential areas, the feasibility for large-scale export oriented production may perhaps not be feasible. The main potential, at least for the intermediate term, appears to lie with small-scale production intended for the local industry, domestic heating and fertilising. Geological potential for economic peat deposits is associated with bogs and marshlands, particularly along the coast (from Dakar to San Louis in Senegal, for example).
The sandstone-type uranium mineralisation of Niger and Gabon has been
of critical economic importance for both countries. Uranium is controlled
by specific lithological units within intracratonic basins of the Middle-Proterozoic
Franceville basin of Gabon and the basin of Agades of the Carboniferous-
Cretaceous age in Niger. In addition to the reserves being mined in Gabon
and Niger, supplementary reserves have been outlined, mainly in Niger.
Remaining economic reserves in Gabon are nearing depletion due to low grades.
There is an excellent exploration potential to increase the uranium resource
base of the Agades basin in Niger, but with prevailing market conditions
for uranium and sufficient available reserves, further exploration is probably
not recommendable. A few uranium-rich phosphate deposits have been located
in C.A.R. and particularly in Chad.
184.108.40.206 Beach Sand Placers
Marine placer potential for titanium (rutile, ilmenite) and zircon is
evident along major parts of the Atlantic coast, in particular along the
stretch from Liberia in the south to Mauritania in the north but also in
Ghana and Cameroon. Considering the vast areas of potential ground which
appear comparatively underexplored combined with potential export markets,
this target is recommended as a priority for assessment and possible future
Gypsum, salt and soda-ash are the only industrial minerals mined at present. There is excellent geological potential for increasing production of gypsum and salt in other areas of the region. Both salt and soda-ash have good export potential and should, therefore, be considered as priority targets.
220.127.116.11 Kaolin Clays
It is believed that there is good potential for economic kaolin deposits.
A number of resources have already been outlined, for example 1 Mt of good
quality kaolin near Adakpla in Benin  and 0.53 Mt near Mongo  in
Chad. Artisanal mining of kaolin for local construction and ceramic usage
is quite widespread. Kaolin is considered to have high export potential
and should, therefore, be considered a key target for exploration and possible
There is geological potential for a great variety of other industrial minerals. Their assessment has only been marginal in the past. Potential indications of industrial minerals in the region include graphite, kyenite, rare earth minerals, diatomite, silica sand, lithium, talc and barium. A few examples: A resource of 3 Mt good quality diatomite over 115 000 km² near Lake Chad  (in Chad), potential graphite schists near Goereda  (Chad) with grades of 13% to 18% C, and a resource of 10 000 t of silica sand (quartz) in Liberia .
2.2.6 Construction Minerals
Aggregate resources are available in all countries of the region. Potential
for ornamental marble, granite and other stones is widespread and particularly
known in Benin, Burkina Faso, Cameroon, Chad, Ivory Coast, Ghana, Mali,
Nigeria and Togo.