Investment opportunities in solid minerals

By Idowu Olayinka

President Umaru Yar’Adua

A distinction can be made between mineral resources and mineral reserves. The term, resource, refers to hypothetical and speculative undiscovered, sub-economic mineral deposits or an undiscovered deposit of unknown economics (Peters, 1981).

On the other hand, reserves are concentrations of usable mineral or energy commodity, which can be economically and legally extracted at the time of evaluation. We are concerned primarily with both the mineral resources and reserves of South Western Nigeria. South Western Nigeria, in this context, covers Lagos, Ogun, Oyo, Osun, Ondo and Ekiti States.

Mineral resources are the major sources of raw materials in several manufacturing industries such as iron and steel, tin smelting, glass, chalk, fertilizer, cosmetics and construction.

About 75 per cent of South Western Nigeria is underlain directly by crystalline rocks of the basement complex, which lies to the east of the West African craton. The area has undergone late precambrian to early palaeozoic orogenesis. The remaining 25 per cent to the South, consists of mesozoic-recent sediments of the Dahomey and Niger Coastal Basins, which cover the basement complex. The occurrence and distribution of mineral resources in the area are intimately associated with the geology. The mineral resources can be grouped into three namely metalliferrous, industrial and energy minerals.

These mineral resources can be grouped into economic mineral deposits, which reserves are known or approximated and occurrences whose tonnage cannot be readily ascertained. The metallic mineral deposits have high unit values and are marketed internationally. Conversely, the industrial minerals have high place values and rarely enter international markets.

For most developing nations, small to medium-scale enterprises are the major springboards for massive industrialisation and development. The major operations that need to be undertaken include the following:

•Prospecting and exploration (field geological mapping, appraisal, geophysical and geochemical methods, quality, grades, reserves, drilling, surveying, sampling, analysis, testing and establishing feasibility parameters).

•Development and mining (appraisals, land acquisition, compensations, extraction by surface or underground methods, environmental management, stock piling, etc).

•Treatment and processing (crushing, classification, separation, beneficiation, blending, refinement, reduction, concentration, purification, industrial specification, etc).

•Application and utilisation (industrial raw material consumption for various functional uses and manufacture of products).

•Marketing and distribution (block raw materials, dressed items, by-products, manufactured and finished products for internal consumption and at times, for exportation).

The basement complex hosts important deposits of gold in the Ife-lIesha schist belt and tin-tantallum-niobium in the pegmatites of Ijero and Iregun. Gemstones such as aquamarine, zircon, rutile and emerald are mined by artisanal miners at Ofiki and Olode. Occurrences of nickel and chromite are known in the Ife-llesha area. Feldspars and quartz occur widely in Ogbomoso, Olode, Oshogbo, Iwo, Iregun and Ijero.

Extensive deposits of talc abound at Apomu and Ilesa. There is an important deposit of marble within the migmatite-gneiss complex of Igbeti. Construction aggregates are widely distributed. Similarly, there are occurrences of bauxite in Orin Ekiti and Oyan, sillimanite in Orin Ekiti, Oyan and Ibadan, as well as molybdenite in lkere-Ekiti.

In the sedimentary sector of South Western Nigeria, there are important deposits of limestone at Sagamu, Ewekoro and Ibese. There are silica sands at Aiyetoro, Ijero, Okitipupa, Lekki and Igbokoda. Extensive deposits of kaolin arc found at Ebe, lbese, and Imeko. Brick clay deposits are found in many locations. There are occurrences of phosphates, often associated with gypsum, at Oshoshun, Seriki-Oko, Idogo, Akinside and Fagbohun. Deposits of tar sand are found at Yemoji, Iwopin and Agbabu.

Some of the mineral deposits are small in size and may not be suitable for large-scale mining enterprises. With an increased interest in small-scale mining, and as the industrial infrastructure grows, small low-cost mining ventures can contribute to economic development in South Western Nigeria by providing raw materials that would otherwise have to be imported. In order for government or private entrepreneurs to reap the maximum benefits from their investment in mining ventures, the problem of artisanal mining, particularly of gold and gemstones will have to be addressed. The Federal Ministry of Mines and Steel has, in the past years, been very active in organising the artisanal miners into cooperatives

Geological distribution of mineral resources in South Westem Nigeria basement complex
The basement complex of South Western Nigeria consists of four broad groups of rocks:

•Gneiss-migmatite-guartzite complex

•Schists belts

•Pan-african granitcs (older granites) and associated granitic rocks

•Minor felsic and mafic untrusives

Gneiss-migmatites- quartzite complex
The gneiss-migmatites-quartzite complex constitutes more than 70 per cent of the basement complex. It is made up essentially of high grade metamorphic rocks, including schists, gneisses, marbles and quartzites of precambrian age (over two billion years). The gneisses and migmatites are so intimately associated that they are hardly separable on the field. They are ubiquitous and form the bulk of the rocks in this group. A characteristic feature of these rocks is a display of banding of varying width. Migmatite is a mixed rock composed of a gneissic host and intruded the granitic and pegmatitic rocks.

Both the quartzite and the marbles occur as relatively minor concordant layers within the gneiss-migmatite units. On account of their comprising a late crystal1ising mineral and consequently being resistant to weathering and erosion, the quartzites tend to stand out as prominent hills and ridges within the basement complex.

All the rock types in the gneiss-migmatite complex are sources of construction materials for roads, building, bridges, dams, airport, etc. The marbles can also provide valuable raw materials for the chemical industry.

Schist belts
The schist belts consist of approximately North to South trending narrow zones of low to medium grade metamorphic rocks of mainly sedimentary and minor igneous origin, which were deposited previously on the pre-existing gneiss-migmatite-quartzite basement. The rocks of the schist belts are also described as ‘Younger Metasediment’, composed of phyllite, schist, quartzite, amphibolite and flaggy gneiss of Palaeoproterozoic age (about two billion years). Associated with the schist belts in several localities is talc-bearing schist, which is considered to be remnant of mafic-ultra mafic complexes. The mineral resources associated with the rocks of the schist belts include gold in llesha area and talc in Apomu, Iregun and Iseyin districts.

Pan-African granites (older granites)
The Older granites occur as intrusive bodies of various dimensions in the pre-existing basement rocks, that is, the gneiss-migmatite-quartzite units and the schist belts. They are widely distributed in South Western Nigeria, being found in all the states except Lagos. They consist of a suite of porphyritic and non-porphyritic granite rocks of different textures. However, the medium to coarse-grained porphyritic variety is more common. One striking feature of the older granites is their occurrence as picturesque inselbergs, that is, prominent hills rising sharply above their surrounding plains. Such granite hills occur as Olumo rock in Abeokuta, ldanre hills in ldanre and Ikerre hills in lkere Ekiti.

In some places, charnockite, a bluish-green rock, and minor syenite are associated with the Pan African granites. The charnockite features prominently in the Akure and Ekiti districts while the syenites are found mainly in the area around Saki.

The older granite and associated rocks are sources of construction materials, particularly rock aggregates and powder, used for making roads, buildings, bridges and dams. Both the granites and the charnockites are used for making polished stones.

Minor felsic and mafic intrusive
The minor felsic and mafic intrusives consist of concordant and discordant dykes, veins and irregular bodies of pegmatite, aplite, quartz, dolerite, gabbro, pyroxenite and serpentinite. As the youngest members of the basement complex of South Western Nigeria, rocks of this unit intrude in places all the pre-existing rocks, namely the gneiss-migmatite quartzite complex, the schists belts and the Pan-African granites (older granites). The pegmatite hosts tourmaline and beryl gemstones, tantalite, columbite and cassiterite (tin ore). They also contain feldspar and micas in exploitable quantities

Sedimentary sector
The sedimentary rocks of South Western Nigeria extend from the Nigeria/Benin Republic boundary in the west to Makun-Omi and Irokun in the east. The Abeokuta group is the oldest stratigraphical unit in the Eastern Dahomey basin and consists of sands and intercalations of argillaceous sediments, which lie unconformably on the crystalline basement.

The Abeokuta group has been subdivided into three formations by Adegoke and Omatsola (1980), namely:

•Ise Formation, which overlies the basement complex and consists of pre-drift sediments of grits and siltstones and overlain by coarse to medium-grained, loose sands interbedded mostly with kaolinitic clays.

•Afowo Formation, which consists of transitional to marine sands and sandstone with variable but thick interbedded shales and siltstone. The shale to sand ratio increases upwards with the sediment becoming highly fossiliferrous. The entire sequence represents a paralic sedimentation.

•Araromi Formation, that is the youngest unit in the stratigraphic sequence and is composed of shales and siltstone with interbeds of limestone and sands. It is richly fossiliferrous.

The Ewekoro formation overlies the Abeokuta group and comprises grayish, white and greenish limestone and is sandy at the base. The sediments were deposited in a shallow marine environment. It is Palaeocene in age. The Akinbo Formation, which overlies the Ewekoro formation, comprises fissile and well-laminated shales.