1.1 GENERAL INTRODUCTION
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 in area. Geological 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, its 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, colour 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 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;
1. To produce an accurate geological map of the area.
2. To carry out petrographic analysis of the rocks encountered.
3. To identify the different rock lithologies present in the study area.
4. To understand and establish the geologic history of the study area
5. To analyze structural data acquired and their relevance to the current geology of the study area.
6. To generally analyze 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. Plot 20B of the master sheet. Ife north local government Osun state , Nigeria. The area lies approximately between latitude 7015’n and 7012.5’N and longitude 4037.5’E and 40 40’E, covering an estimated area of 25km2. The study area is located in Atakumasa local government area of osun state n southwestern Nigeria. The important settlements are Bolorunduro, Egbejoda, baba Ibadan, Abusoro, kajola, Onikanga.
1.4 ACCESSIBILITY AND TOPOGRAPHY
Traversing the study area was fairly easy due to the presence of minor roads and footpaths and with the assistance of the villagers, who help to locate the existing outcrops in an initially inaccessible area, stream channels were also used as traverse, 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. 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 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.
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, politic schist, biotite granite, charnockite, granite, gneiss and porphyritic granite. The area is dominated by partly weathered crystalline rocks which show evidence of several episodes of deformation and metamorphism and is also covered by reddish brown laterite probably derived from weathering of the crystalline basement rocks.
1.7 PREVIOUS WORK
Nigeria lies approximately between longitude 40N and 150N and latitude 30E and 140E . Within the pan African mobile belt in between the west African craton 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 (19176), Odeyemi (1977) noted that the rocks in the study area show eveidence of polyphase deformation with the plutonic episode of the pan African event being the most pervasive
1.8 REGIONAL GEOLGY OF NIGERIA
The Nigerian Basement Complex is one of the three major lithological components that make up the geology of Nigeria. It forms the southern part of the Trans Saharan mobile belt east of the West African craton and northwest of the Congo craton (Caby, 1989; Ferre et al., 2002; Caby, 2003) and has also explained to be south of the Tuareg shield in Black (1980). This basement complex comprises Archean and Proterozoic rocks which is believed to be the results of three major orogenic cycles of deformation, metamorphism and remobilization, and basement reactivation corresponding to the radiometric ages indicated by Liberian (2700 ± 200 Ma), Eburnean (ca 2000 Ma), Pan-African (ca 600 Ma) which resulted from plate collision between the passive continental margin of the West African craton and the active Pharusian continental margin (Grant, 1970; Oversby, 1975; van Breemen et al., 1977; Fitches et al., 1985; Rahaman, 1988; Dada et al., 1994). Three broad lithological groups are usually distinguished within the Nigerian basement complex (Ajibade et al, 1987). These divisions are
(i.) polymetamorphic migmatite-gneiss complex,
(ii.) low grade sediment dominated schists and
(iii.) syntectonic to late tectonic granitic rocks which cut both the migmatite-gneiss complex and the schist belts.
The polymetamorphic migmatite-gneiss complex is composed largely of migmatites and gneisses of various compositions and amphibolites. Relict of meta-sedimentary rocks represented by medium to-high grade calcareous, pelitic and quartzitic rocks occur within the migmatites and gneisses, and they have been described as “Ancient Metasediments” by Oyawoye, (1972). The migmatite-gneiss complex is considered to be the basement sensu stricto, and isotopic ages varying from Liberian to Pan-African have been obtained from the rocks. The Pan-African ages have been interpreted as due to isotopic re-homogenization in preexisting rocks during the Pan-African orogeny. Low-grade sediment dominated schists form narrow belts in the eastern half of the country have been described as “Newer Metasediments” (Oyawoye, 1972), “Younger Metasediments” and “Unmigmatised to Slightly Migmatised Schists” (Rahaman, 1976). These schist belts are believed to be relicts of a supracrustal cover which was infolded into the migmatite gneiss complex. The schist belts are intruded by Pan-African granitoids. Further attempts on the classification of the Nigerian Basement Complex (e.g. Rahaman, 1988) distinguished four major petro-lithological units, namely:
(i.) The Migmatite – Gneiss- Quartzite Complex
(ii.) The Schist Belt (Metasedimentary and Metavolcanic rocks)
(iii.) Older Granites and associated granitic rocks(The Pan-African Granitoids
(iv.) Undeformed Acid and Basic Dykes or The minor felsic and mafic intrusives.
Ajibade et al (1982); Proterozoic crustal development in the Pan-African Regime of Nigeria. ILP working Group 3 Mid-term Report Dada , S.S (2006): Proterozoic evolution of Nigeria In ; Oshin. The Basement complex of Nigeria and its mineral resources ( A tribute to prof M.A. O Rahaman). Akin Jinad and Co. Ibadan , PP 29-44
Elueze, A.A. (1992); Rift system for proterozoic schist belt in Nigeria. Tectonophysics, 209, 12-14
Olarewaju, V.O., (1981) Geochemistry of the charnockitic and granitic rocks of the basement complex around Ado-Ekiti, Southwestern Nigeria
Oyawoye, M.O., (1964): The geology of the Nigerian Basement Complex. Journal. Nigeria min Geol. And metal. Soc. V.I, pp87-482
Rahaman, M.A., (1988): Recent advances in the study of the basement complex of Nigeria. In Precambrian Geology of Nigeria, pp 11-43