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1.0    Introduction

1.1    General Statement

         Geological field mapping exercise is the preliminary study usually carried out to understand the geology of an area. A geological map identifies and shows the distribution of various rock types/lithology in an area. Geologic mapping is essential for both the economic, academic and financial growth of a nation.

Geological field mapping is a very important exercise in the field of geology and geosciences generally, geological field mapping is an exercise carried out to collect information about the different rock lithologies in an area, structures exhibited in them, type of rock, mode of occurrence, texture, color and mineralogy. Samples are also analyzed to determine the petrology which helps to confirm the rock name and origin as well as general age of emplacement and metamorphism in the case of a metamorphic rock.

The Research methodology used, materials employed, observations, results and conclusions are discussed in this report. This report also includes the list of different rocks found, their economic importance, mineralization and petrography.

The field mapping and petrographic study of the rock sample was done to delineate the different rock type present in the area and to integrate the previous works on the geology of the area in order to delineate the mineralization zones.

1.2    Aim and Objectives

The aim of this work is to thoroughly understand and establish the geologic history and events that led to the formation of the present geology of the area. The objectives of this study are:

        i.                        To produce an accurate geological map of the area.

      ii.                        To carry out petrographic analysis of the rocks encountered.

    iii.                        To identify the different rock lithologies present in the study area

    iv.                        To understand and establish the geologic history of the study area

      v.                        To analyze structural data acquired and their relevance to the current geology of the study area

    vi.                        To generally analyze/itemize the economic value of the rock lithologies found in the area.

1.3    Location of the Study Area

The study area covers Bolorunduro community and its environs, Plot20b of the master sheet. Ife north local government, Osun state, Nigeria. The area lies approximately between latitudes 7015’N and 70 12.5’N and longitudes 4037.5’E and 4040’E, covering an estimated area of 25km2. The study area is located in Atakumausa local government area of Osun state in southwestern Nigeria. The important settlements are Bolorunduro, Egbejoda, Baba Ibadan, Abusoro, Kajola, Onikanga. Fig 2 shows the study area.

1.4    Accessibility and Topography

Traversing the study area was fairly easy due to the presence minor roads and footpaths. Traversing the area was also made easy with the assistance of the local people in the area who helped to locate existing outcrops in initially inaccessible areas, stream channels were also used as traverses. Communities and hamlets are interconnected by footpaths. Accessibility by these footpaths is fair and further enhanced by the extensive cocoa plantations which have well worn footpaths.

The topography of the study area is uneven and characterized by ridges, hills and valleys. Fig 2 shows the topographic map of the study area. The uneven topography is caused by the crystalline nature of the rocks found in the area which make them resistant to weathering.

1.5    Climate and Vegetation

The study area falls within the tropical humid climate region where the wet and dry seasons are noticed prominently in the area. Just like any other parts of the south western region of Nigeria, The dry season is between November and February while the wet season is mostly between April and October. The mean minimum temperature observed ranges from 20°C in January to 23°C in February, whilst the mean maximum temperature observed is 36.20C Iloeje (1972). The mean annual rainfall is between 1100mm and 1500mm.

The vegetation of the area is similar to that of the tropical rainforest where there are high trees and shrubs. The vegetation here is characterized by the presence of thick tropical evergreen forest. Trees like iroko and obeche are common with undergrowth that includes ferns, epiphytes and grasses. The major grown crops in this area include cocoa and palm fruits. This explains the major occupation of the people living in the area which is farming, oil milling and lumbering.

Figure 1: Geological map of Nigeria: basement complex, younger granites and sedimentary basins. (adapted from Geology of Nigeria, Obaje et al.)

Figure 2: Topographic map of the study area.

Figure 3: Drainage map of the study area

1.6        Geology of the study area

The study area is located within the crystalline basement complex terrain of southwestern Nigeria. The area is generally underlain by basement rocks categorized by Rahaman (1976) as migmatite gneiss, quartzite, pelitic schist, biotite granite, charnockite, granite gneiss and porphyritic granites. The area is dominated by partly weathered crystalline rocks, which show evidence of several episodes of deformation and metamorphism and is also covered by reddished brown lateritised clay probably derived from weathering of the crystalline basement rocks.

1.7    Previous Work

Nigeria lies approximately between longitudes 40N and 150N and latitudes 30E and 140E, within the pan African mobile belt in between the West African and Congo cratons in the region of late Precambrian to early Paleozoic orogenesis. The basement complex is made up of Precambrian rocks and consists of schist belts folded in them.

Previous works have been carried out on a regional scale on the basement Complex of Nigeria and it has been shown that this is the most abundant lithology in Nigeria such work includes, Rahaman (1988), Odeyemi (1977) and Oyawoye (1964), Grant (1978), Anifowose (2006), Oyinloye(2011) among others. They gave an account of the geology of this area under a broader work “The geology of the basement complex of southwestern Nigeria”. Oyawoye (1964), Rahaman (1976), Odeyemi (1977) noted that the rocks in the study area show evidence of polyphase deformation with the plutonic episode of the Pan African event being the most pervasive. Rahaman (1988) noted that the south western basement complex of Nigeria lies within the rest of the Precambrian rocks in Nigeria, He grouped the rocks in this region as migmatite – gneiss – quartzite complex comprising largely of sedimentary series with associated minor igneous rock intrusions which have been altered by metamorphic, migmatitic and granitic processes. Odeyemi (1999) suggested that almost all the foliation exhibited by rocks of southwestern Nigeria excluding the intrusive are tectonic in origin, because pre-existing primary structures have been obliterated by subsequent deformation. Anifowose (2004) also noted that joints ranging from minor to major ones are found in all the rock types, some of which are filled with quartz, feldspars or a combination of both which lie generally in the NE-SW direction, while Boesse and Ocan (1992) reported that the south western basement complex of Nigeria has been affected by two phases of deformation namely D1, D2, the first phase (D1) produced tight to isoclinals folds while the second phase (D2) is characterized by more open folds of variable style and large vertical NNE-SSW trending fault. Oluyide (1988) gave evidence that within the basement complex, tectonic deformation has completely obliterated primary structures except in a few places where they survived deformation.

Figure 4: Geological map of previous work carried out in the study area. (adapted from Geology of Nigeria, Obaje et al.)

1.8    Regional Geology of Nigeria

         Nigeria is situated in the region east of the West African craton and the North West of the Congo Craton which has been affected by the Pan African Orogeny about 600ma (Ajibade and Woakes, 1976 and Rahaman 1976). The Nigeria basement lies to the south of the Taureg shield. Evidences from the eastern and northern margin of the west Africa craton suggest that the pan Africa belt was developed by plate tectonics which involved the collision of the passive continental margin of the west Africa craton and the Active continental margin. The collision of the plate margins was believed to have led to the reactivation of the internal region of the belt. The Nigerian basement complex lies in the reactivated part of the belt.

         The basement geology of the southwestern Nigeria has been divided into different major rock groups based on the result of detailed geological and petrological investigations by various authors. Rahaman (1988) divided the rock in this area into six major units as

1.         Migmatite ­- gneiss - quartzite complex.

2.         Slightly migmatized to non-migmatized meta-sedimentary and meta-igneous rocks, often referred to as newer metasediments (Oyawoye, 1964) or the schist belts (Ajibade, 1980).

3.         Charnockitic, gabbroic and dioritic rocks.

4.         Members of the Older Granite suite.

5.         Metamorphosed to non-metamorphosed calc-alkaline volcanic and hypabyssal rocks.

6.         non-metamorphosed dolerite dykes, basic dykes and syenite dykes etc.

1.8.1 The Migmatite - Gneiss - Quartzite Complex

         The Migmatite-Gneiss-Quartzite Complex is the most widespread making up about 60% of the total surface area of the Nigerian Basement (Rahaman and Ocan 1978). These rocks record three major geological events (Rahaman and Lancelot 1984): the earliest, at 2,500Ma, involved initiation of crust forming processes (e.g the banded Ibadan grey gneiss of mantle origin) and of crustal growth by sedimentation and orogeny; next came the Eburnean, 2,000±200Ma, marked by Ibadan type granite gneisses, this was followed by ages in the range from 900-450Ma which represent the imprint of the Pan-African event which not structurally overprinted and re-set many geochronological clocks in the older rocks but also gave rise to granite gneisses, migmatites and other similar lithological units.

1.8.2 Slightly Migmatised To Non-Migmatised Paraschist And Metaigneous Rocks

Rocks in this group constitute the schist belt of Nigeria (Ajibade, 1980). They show lithologic similar to schist belts from the other part of the world (Anhaussever et al) which are known to harbor important economic mineral deposit. The lithological variation of schist belts have been summarized by Tuner(1978) and they include coarse to fine grained politic schist, phyllite, banded iron formation, carbonate rocks, mafic metavocalnics, now (amphibolites). Grants (1979) and Rahaman (1976) for instance, suggested that there were several basins of deposition whereas; Oyawoye (1972) and M.c Curry (1976) considered the schist belts as relicts of a single supracrutal cover. Olade and Elueze (1979) considered the single belts as the fault controlled rift-like structures. The schist belts are best developed in the Western part of Nigeria, though smaller occurrences are found to the east but only sporadically.

1.8.3 Charnockitic Rocks

Charnockitic rocks can be identified on the basis of structure and petrographic characteristics viz: banded and/or gneissic charnockites; coarse often homogenous sometimes porphyritic, foliated charnockites and bauchites; fine-medium grained foliated basic charnockites; dioritic rocks of charnockitic affinity often associated with more acid coarser grained charnockites or granites or less frequently as bosses.

1.8.4 The Older Granites

The Older Granites are the most obvious manifestation of the Pan-African orogeny in this part of West Africa. The Older Granite suite represents a varied and long lasting (750-450 Ma) magmatic cycle associated with the Pan-African orogeny. The rocks of this suite range in composition from tonalites and diorites through granodiorites to true granites and syenites. They are generally high level intrusions and anatexis has played an important role (Rahaman 1981). In southwestern Nigeria, Older Granites make up about 20% of the basement outcrop, eastwards and northwards in the Akure-Okene –Abuja area and account for between 40-60% of the surface area.

Further eastwards in the 300km diameter area east of Kaduna rocks of the Older Granite suite accounts for over 70% of the basement outcrop. In Nigeria, they occur largely as distinct plutons often of batholitic dimensions and of limited composition and petrographic type whilst further west in Togo, the Pan African orogeny is manifested by widespread development of axial migmatite.

According to Olarewaju and Rahaman (1981), several types of Older granites have been recognized namely: migmatite or migmatitic granite; granite gneiss or anatectic migmatite or granite (Carter et al, 1964); early pegmatite dykes and fine-grained granites; homogenous medium-coarse grained granite; medium-coarse porphyritic biotite and biotite hornblende granite; slightly deformed pegmatite and aplite dykes and veins; undeformed pegmatite, mica granite and quartz veins. The Older Granite suite is notable for its general lack of associated mineralization although the thermal effects may play a role in the remobilization of mineralization fluids.

1.8.5 Volcanic And Hyperbasal Rocks

Volcanic and hyperbasal rocks of possibly late Pan-African age occur in several localities within a 200km by 30km linear zone associated with the Anka metasedimentary belt in northwestern Nigeria. The rocks are frequently associated with faults which controlled their emplacement.

According to Ajibade et al. (1982) Nigerian Basement Complex was re-activated, the supracrustal formations deformed and metamorphosed and involved by granitic rocks during the Pan-African thermo-tectonic event that resets the isotopic clock in the other rocks.

1.8.6 Non-metamorphosed Dolerite Dykes, Basic Dykes And Syenite Dykes

Dolerite dykes are widespread in the basement complex and have been described and occur as tabular, unmetamorphosed bodies across, cutting the foliation in the host rock and are regareded by most authors as the youngest member of the basement complex.

Dolerite dykes range in the thickness from a few millimeters to half a meter. In some cases, the dykes are stepped and there is an exact correspondence of opposite walls.

The general trend of all dykes observed is between NE-SW and ENE-WSW (jones and Hockey 1964, Freeth 1971, Rahaman 1973).

In most cases, closely spaced joints parallel to the trend of the dykes is observed in the host rocks in the immediate surroundings of the dykes, suggesting forceful intrusion of the dyke.

1.9        Economic Potentials of the Area

The major economic potential of the area of study is farming and the major crops produced are cassava, kolanut, fruit crops like oranges, mangoes and cash crops like cocoa are also grown. Most of these farming activities are carried out on the valleys which have very fertile soils. Sawmilling is very common in the study area due to the availability of timber in the area. There is widespread occurrence of palm trees in the study area from which palm oil and palm wine are produced. The rock types of economic importance in the area are the gneisses, quartzites and  pegmatites. Massive extrusive gneiss outcrops can be quarried for use as construction aggregates and as dimension stones. The pegmatites on the other hand have to be subjected to further detailed petrographic analysis to determine the presence of minerals of economic value that can be mined from them. Associated host minerals in the pegmatites are: muscovite, feldspars, tourmaline and quartz which can be mined for a number of industrial and ornamental purposes. Quartzite can be quarried for road dressing, stone aggregates, building and engineering purposes. Feldspars can also be gotten from the quartzofeldspartic intrusions and used in ceramic and glass industries.

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