Crushed rock, gravel, sand and clay are ubiquitous building materials widely used for rural and urban dwellings, and in the construction industry for civil engineering works such as roads, buildings, bridges and dams. The Basement Complex furnishes a rich source of crushed rock which is augmented with basalt, dolerite and limestone in some places. Laterite is a common building and road-surfacing material. River gravel of various sizes are widely available as aggregates and building material, especially in Akwa lbom and Cross River states.
Sand of glass-making quality has been found and assays in many parts of the country (Odukwe, 1981). In Port Harcourt, the Trans-Amadi sands contain over 99 per cent Silicon oxide (S,0g) and at Ughelli, another clean coastal plain, sand deposit assays 97 per cent Silicon oxide (S,0j. Both sands are used for the manufacture of bottles and window glass. The West African Glass Company at Aba utilises a local sand deposit. Many other glass sand deposits are known especially along the coast. Reserves are inexhaustible since full industrial use has hardly begun.
Clays of various kinds and grades abound throughout Nigeria's sedimentary basins and on the basement. Nigeria has a rich traditional pottery industry that dates from the Stone Age. In all parts of the country, native pottery is a vibrant traditional art form and an established cottage industry for earthenwares. Modern industrial uses of clay for ceramics and bricks obtain throughout the country. Ceramic works at Abeokuta (Ogun State), Ikorodu (Lagos State), Okigwe (Imo State), Umuahia (Abia State) and Suleja in Niger State produce glazed wares from local kaolin. Refractory clays for refractory bricks have been proven at Onibode near Abeokuta where the refractoriness is very high at about 1,750°C.
In Enugu, beneath the coal seams, are vast deposits of other fire clays, with reserves of about 130 million tonnes. Other major clay deposits occur in Akwa lbom (Etinan, ltu, Onna, Uyo); Anambra State at Ozubulu (3.5 million tonnes) and Ukpor (4.2 million tonnes); Edo/Delta States at lgoriakhi (over 14.0 million tonnes); Imo/Abia States (over 2.0 million tonnes) at lgbere and Ogbuebule; Kaduna State (over 5.3 million tonnes) at Miraba- Rido and Kaohia; Katsina State (3.4 million tonnes) at Kankara; Ogun State (over 2.0 million tonnes) at Oke-Eri and Illefun Hill; large deposits in Ondo State; over 18 million tonnes in Plateau State at Nafuta, Worrom, Major Porter and Sabon Gida; and, in Sokoto State 2.0 million tonnes of pisolitic clays occur at Girl, and clay bricks are made at Wurno (RMRDC, 1989). Bentonitic clays, which are used for drilling mud, were first produced in Bende in Abia State in 1961-62.
Recent investigations by the Nigerian Mining Corporation, aimed at locally sourcing the enormous amount of bentonite imported into the country by the petroleum industry, established the existence of bentonitic clay reserves of over 700 million tonnes in the country, with the largest single deposit atAfuze in Edo State holding 70-80 million tonnes. Limestone and gypsum, the principal raw materials for cement manufacture, are very essential for a rapidly growing nation like Nigeria.
And, indeed, cement is in very high demand in the country. While limestones abound in Nigeria more than in any other West African country, gypsum and other salts have not been found in commercial quantity. The nearest potential source of gypsum for Nigeria could be Mali Republic, where millions of tonnes of mineable salts occur at Agorgott. As for clinker (processed limestone for cement), Nigeria could export this material to Cameroun, which at present obtains its clinker from Europe.
Seven out of the eight cement factories in Nigeria are sited close to limestone deposits, except the plant at Okpilla in Edo State, which uses marble. The cement factory at D'Onigholo in Benin Republic is jointly owned by Benin (52 per cent) and Nigeria (46 per cent); It is located on the Ewekoro limestone deposit which extends from Ogun State in Nigeria to as far as Ghana. Other cement factories in Nigeria are located atAshaka, Yandev, Nkalagu, Shagamu, Okpilla, Ewekoro, Sokoto and Calabar.
The limestone at Mfamosing, near Calabar, is the largest and the purest deposit in Nigeria. It is about 50m thick, at the quarry site. West of Calabar, another carbonate body occurs in the subsurface that is 450m thick. The Calabar flank is the main carbonate province in Nigeria, with well-developed tropical karsts and caves (Reiljers and Potters, 1987). The Mfamosing limestone has over 97 per cent limestone (CaC03).
It is used by the Delta Steel plant at Aladja as a flux- ing agent and for making hydrated lime. Marble is found in the Basement Complex and is quarried at Jankura near Lokoja, at Ubo near Okene, and at Okpilla. The Jankura marble is utilised as decorative marble by the Nigerian Marble Industries. Dolomitic marble deposits are exploited at lgbeti in Oyo State and at Kwakuti in i Niger State. Other deposits are at Elehu in Kwara State, Kobe in Benue State, Muro Hill in Plateau State, and at Burum and Taka Lafia in the Federal Capital Territory.
Feldspar, a mineral that is used locally in the ceramic and glass industries, occurs abundantly in the Basement Complex. Economic deposits are found in the granitic intrusives in the basement and especially in pegmatites where potash feldspar is the commercial type. Important sources of feldspar rich pegmatites occur in Kwara and Kogi States and also extensively around Osogbo in Osun State.Near Oshogbo, potash feldspars attain lengths of 0.9m, but around Ijero in Ekiti state, gigantic crystals are up to 1.2m long.
Feldspar-rich pegmatities are common in the Qwozan area of Borno State and in parts of Adamawa/Taraba states. Although rock salt has not been found in Nigeria, highly saline brines in the Benue Trough provide potential local sources of salt except, of course, the open ocean and there are as yet no salinas on the Nigerian coast.But along the Benue Trough brines, saltsprings, saltponds and shallow subsurface brine pools are commonly associated with lead-zinc deposits.
Salt concentrations are up to 1.5 per cent in the Ikom area and 8.6 per cent at Okpoma in Cross River State, where the richest occur. At Uburu-Okposi in Abia State, the salt con- centration is up to 5 per cent and 5 - 6 per cent at Ameri.nearAbakaliki. Further north, in Nassarawa State (Keana, Awe) and in Taraba and Adamawa States, there are other brine fields. Geophysical investigations by the Geological Survey of Nigeria and other organisations aimed at locating salt domes in the Benue Trough, have so far not found any.
Therefore, the establishment of local, modern and large-scale salt industry in Nigeria is still a dream not realised. However, at Okpoma, Ikom, Uburu, Okposi, Abakaliki and in the Keana/Awe areas, the local people have established a long tradition of winning salt from their brines.
Among the metallurgical and refractory minerals needed for the reduction of ores from their metallic state and for making resistant materials for high temperature metallurgical furnace, which will be required by the local metallurgical industries (e.g. iron steel plant), only flourspar and graphite seem to have commercial prospects in Nigeria.
The lead-zinc lodes in the Benue Trough carry small amounts of flourspar which also occur in tin veins in the younger granites. Impure graphite is known in the younger metasediments near Birnin Gwari in Kaduna State, Taraba and Adamawa States, at Hayna (Ningi) in Bauchi State and in the Obudu area of Cross River State.
Other refractory minerals, such as sillimanite and kyanite, occur in the Birnin Qwari schists, while sillimanite is found in the vicinity of lbadan. Other industrial and manufacturing materials such as mica, talc, asbestos and barytes also occur in the country. There is a mica deposit around Egbe in Kwara and Kogi States which was mined in the past.
The crystal size of Nigerian mica is often not large enough to be economic. Low grade talc occurs in schists near lbadan (Oyo State), llesha (Osun State), Abuchi in Niger State, Zonkwa in Kaduna State and at Isanlu in Kwara State. Asbestos exists at Shemi in Kaduna State, and a large deposit occurs at Chafe in Sokoto State.
Barytes, a major weight filler for drilling mud, occurs in veins, up to 1.8m thick in association with lead- zinc deposits in Cross River, Benue, Plateau, Taraba and Adamawa States. Major baryte occurrences in Nigeria are at Lefin in Cross River State, at Aloshi, Akuri, Wuse, Azara and Gbane in Plateau State. Keana in Nassarawa State and at Ibi and Durngel in Taraba and Adamawa States.
At Azara, good quality baryte reserves of about 70,000 tonnes have been proven by the Nigerian Mining Corporation. Finally, the Nigerian basement also holds some prospects for gemstones. In Nassarawa, Plateau and Kaduna States, aquamarine, emerald, sap- phire, ruby, topaz, tourmaline, zircon and almandine have been won from pegmatites.
The solid mineral sector offers viable orosoects for mining, mineral processing and the manufacture of a host of intermediate raw materials for local industries as well as for foreign exchange earning. It has created the opportunity for diversifying Nigeria's hitherto petroleum - dominated economy. Intensive mineral exploration and appraisals are needed to attract foreign investors into the mining of gold, tin, columbite, lead and zinc. Moreover, coal, bitumen, barytes, feldspars and a host of other industrial minerals are awaiting exploitation by potential investors.