Geology and Mineral Resources of Nigeria

In 1914 the Protectorates of Southern and Northern Nigeria were amalgamated with the Colony (Lagos) by Lord Lugard to form what is now known as Nigeria. There are thirty six (36) federating states alongside the Federal Capital Territory (FCT) that make up the Federal Republic of Nigeria (Fig. 1). A total of 774 local government councils are recognized in the Constitution of the Federal Republic of Nigeria. Since 1991, the Nigeran capital has been the centrally-located city of Abuja; previously, the Nigerian government was headquartered in Lagos. Nigeria is famous for her huge population of about 140 million people – the largest national population on the African continent.

The basement complex is one of the three major litho-petrological components that make up the geology of Nigeria (Fig. 1.1). The Nigerian basement complex forms a part of the Pan-African mobile belt and lies between the West African and Congo Cratons (Fig. 1.2) and south of the Tuareg Shield (Black, 1980). It is intruded by the Mesozoic calc-alkaline ring complexes (Younger Granites) of the Jos Plateau and is unconformably overlain by Cretaceous and younger sediments. The Nigerian basement (Fig.1.3) was affected by the 600 Ma Pan-African orogeny and it occupies the reactivated region which resulted from plate collision between the passive continental margin of the West African craton and the active Pharusian continental margin (Burke and Dewey, 1972; Dada, 2006). The basement rocks are believed to be the results of at least four major orogenic cycles of deformation, metamorphism and remobilization corresponding to the Liberian (2,700 Ma), the Eburnean (2,000 Ma), the Kibaran (1,100 Ma), and the Pan-African cycles (600 Ma).

The Mesozoic Younger Granite ring complexes of Nigeria (Fig. 2.1) form part of a wider province of alkaline anorogenic magmatism. They occur in a zone 200 km wide and 1,600 km long extending from northern Niger to south central Nigeria. Rb/Sr whole rock dating indicates that the oldest complex of Adrar Bous in the north of Niger is Ordovician in age, with progressively younger ages southwards. The most southerly ring complex of Afu is Late Jurassic in age (Bowden et al., 1976). Aeromagnetic anomalies suggest that a series of buried NE–SW lineaments of incipient rifts controlled the disposition of the individual complexes (Ajakaiye, 1983).

Basaltic lava plateaus, trachyte plugs and domes, large central volcanoes and small basalt cinder cones with thin flows are all found among the more southerly manifestations of Cenozoic volcanism in West Africa (Wright, 1985). This province also includes the remarkable offshore continuation of the Cameroon volcanic line, the four islands situated in the Gulf of Guinea itself. Areas of basement doming include the Jos Plateau, southeast of the Benue Trough, with probably still greater uplifts; and the Adamawa Highlands further east, where lavas of the Ngaoundere Plateau overlie the Ngaoundere fault zone, which was reactivated in the Cretaceous. The flood basalts of the Biu Plateau are situated on the Zambuk Ridge of the Upper Benue Trough, and the smaller Longuda Plateau lies near the bifurcation of the Upper Benue Trough, which has been identified as a possible secondary triple junction (Fig. 3.1).

The Benue Trough of Nigeria is a rift basin in central West Africa that extends NNE–SSW for about 800 km in length and 150 km in width. The southern limit is the northern boundary of the Niger Delta, while the northern limit is the southern boundary of the Chad Basin (Fig. 4.1). The trough contains up to 6,000 m of Cretaceous – Tertiary sediments of which those predating the mid-Santonian have been compressionally folded, faulted, and uplifted in several places. Compressional folding during the mid-Santonian tectonic episode affected the whole of the Benue Trough and was quite intense, producing over 100 anticlines and synclines (Benkhelil, 1989). Major such deformational structures include the Abakaliki anticlinorium and the Afikpo syncline in the Lower Benue, the Giza anticline and the Obi syncline in the Middle Benue, and the Lamurde anticline and the Dadiya syncline in the Upper Benue Trough.

The Nigerian sector of the Chad Basin, known locally as the Bornu Basin, is one of Nigeria’s inland basins occupying the northeastern part of the country. It represents about one-tenth of the total area extent of the Chad Basin, which is a regional large structural depression common to five countries, namely, Cameroon, Central African Republic, Niger, Chad, and Nigeria. The Bornu Basin falls between latitudes 11°N and 14°N and longitudes 9°E and 14°E, covering Borno State and parts of Yobe and Jigawa States of Nigeria.

The Iullemmeden Basin in north-western Nigeria is known locally as the “Sokoto Basin”. It consists predominantly of a gently undulating plain with an average elevation varying from 250 to 400 m above sea-level. This plain is occasionally interrupted by low mesas. A low escarpment, known as the “Dange Scarp” is the most prominent feature in the basin and it is closely related to the geology.

The Mid-Niger Basin otherwise known as the Bida Basin or the Nupe Basin is a NW–SE trending intracratonic sedimentary basin extending from Kontagora in Niger State of Nigeria to areas slightly beyond Lokoja in the south. It is delimited in the northeast and southwest by the basement complex while it merges with Anambra and Sokoto basins in sedimentary fill comprising post orogenic molasse facies and a few thin unfolded marine sediments (Adeleye, 1974). The basin is a gently downwarped trough whose genesis may be closely connected with the Santonian orogenic movements of southeastern Nigeria and the Benue valley, nearby. The basin is a NW–SE trending embayment, perpendicular to the main axis of the Benue Trough and the Niger Delta Basin (Fig. 4.1). It is frequently regarded as the northwestern extension of the Anambra Basin, both of which were major depocentres during the third major transgressive cycle of southern Nigeria in Late Cretaceous times. Interpretations of Landsat images, borehole logs, as well as geophysical data across the entire Mid-Niger Basin suggest that the basin is bounded by a system of linear faults trending NW–SE (Kogbe et al., 1983). Gravity studies also confirm central positive anomalies flanked by negative anomalies as shown for the adjacent Benue Trough and typical of rift structures (Ojo, 1984; Ojo and Ajakaiye, 1989).

The Dahomey Basin is a combination of inland / coastal / offshore basin that stretches from southeastern Ghana through Togo and the Republic of Benin to southwestern Nigeria. It is separated from the Niger Delta by a subsurface basement high referred to as the Okitipupa Ridge. Its offshore extent is poorly defined. Sediment deposition follows an east-west trend. In the Republic of Benin, the geology is fairly well known (Billman, 1976; De Klasz, 1977). In the onshore, Cretaceous strata are about 200 m thick (Okosun, 1990). A non-fossiliferous basal sequence rests on the Precambrian basement. This is succeeded by coal cycles, clays and marls which contain fossiliferous horizons. Offshore, a 1,000 m thick sequence consisting of sandstones followed by black fossiliferous shales towards the top has been reported. This was dated by Billman (1976) as being pre-Albian to Maastrichtian. The Cretaceous is divisible into two geographic zones, north and south. The sequence in the northern zone consists of a basal sand that progressively grades into clay beds with intercalations of lignite and shales. The uppermost beds of the Maastrichtian are almost entirely argillaceous. The southern zone has a more complicated stratigraphy with limestone and marl beds constituting the major facies.

The Cenozoic Niger Delta is situated at the intersection of the Benue Trough and the South Atlantic Ocean where a triple junction developed during the separation of the continents of South America and Africa in the late Jurassic (Whiteman, 1982). Subsidence of the African continental margin and cooling of the newly created oceanic lithosphere followed this separation in early Cretaceous times. Marine sedimentation took place in the Benue Trough and the Anambra Basin from mid-Cretaceous onwards. The Niger Delta started to evolve in early Tertiary times when clastic river input increased (Doust and Omatsola, 1989). Generally the delta prograded over the subsidizing continental-oceanic lithospheric transition zone, and during the Oligocene spread onto oceanic crust of the Gulf of Guinea (Adesida et al., 1997). The weathering flanks of out-cropping continental basement sourced the sediments through the Benue-Niger drainage basin. The delta has since Paleocene times prograded a distance of more than 250 km from the Benin and Calabar flanks to the present delta front (Evamy et al., 1978). Thickness of sediments in the Niger Delta averages 12 km covering a total area of about 140,000 km².

Nigeria as a nation is blessed with abundant solid mineral resources distributed fairly in all the states of the federation (Fig. 10.1; Table 10.1). According to reports by the Geological Survey of Nigeria Agency, Nigeria has some 34 known major mineral deposits distributed in locations across the country and offers considerable attraction for investors. Exploration in Nigeria for several solid minerals, e.g. tin, niobium, lead, zinc and gold, goes back for more than 90 years but only tin and niobium production have ranked on a world-wide scale. While the major international exploration groups have seldom paid more than a passing interest, there has been general exploration carried out by the tin mining groups and since the mid 1970s by several parastatal organizations and in particular the Nigerian Mining Corporation. Throughout its long history the Geological Survey of Nigeria Agency has played an active role in the exploration for mineral deposits many of which have been first reported by its officers.

The attached explanatory notes show that Nigeria is the world’s 8th largest producer and 6th largest exporter of petroleum. It is the largest producer and exporter on the African continent and is currently closely followed in production level by Angola, Libya and Gabon. Current reserves (as at 2008) are put at 35 billion barrels for oil and 180 trillion standard cubit feet for gas. At the moment production is entirely from fields in the Niger Delta region while exploration campaigns have been carried and/or ongoing in different segments of Nigeria’s sedimentary basins.

Prior to the emergence of petroleum in the mid nineteen seventies as a major foreign exchange earner, the solid minerals subsector ranked second only to the agricultural sector as a source of export earnings. The subsector also contributed substantially to national output, accounting for about 10% of the GDP in 1970 (Kogbe, 1976). The annual average output in the subsector was put at some 130.8 thousand metric tonnes over the years 1970–1973. It employed, on the average, about 49 thousand workers per annum over the period 1958–1970. However, with the exit of foreign multinational mining companies and their expatriate professionals in the wake of the 1972 Indigenisation Decree, the performance of the subsector began to dwindle. Annual production declined considerably, particularly in metallic minerals. The tempo of mining activities shifted to industrial non-metallic minerals needed for construction, building and industrial applications in domestic industries.