ABSTRACT
Ground magnetic and geoelectrical resistivity methods were used to prospect for gold mineralisation within Tsoho-Gurusu area of Minna, Niger state. The study area is bounded with latitudes 09o 37׳ 13״N to 09o37׳28.6״ N and longitudes 006o 36׳ 29.4״ E to 006o 36׳ 13.1״E, with a total area of 250,000 m2. Magnetic was collected using Geometric 857AX proton precession magnetometer. Prior to the geophysical survey, geological mapping was conducted to map out the surface structural features. Eleven traverses were occupied and aligned in North-South direction. Each traverse is 500 m long and uniformly spaced 50 m apart. The measurement stations were position East-West, across the traverses at 10 m apart in the mineral foliation direction. The magnetic data acquired were subjected to International Geomagnetic Referenced Field to obtained the residual field. The residual field was then processed and analysed using Euler and Analytical signal methods. The Electrical Resistivity Tomography (ERT) data was collected along two profile of anomalously high residual values (-99 to 82 nT) and low TMI value. The result from the analytical signal shows both high and low amplitude anomalies with the high amplitude anomalies dominates the entire study area. The result from Euler deconvolution shows a variation of depths between 8.0 to 25.0 m. The results from the electrical resistivity method shows a depth variation between 5 – 13.4 m. Low electrical resistivity values were found to associate with the high TMI values in an East-West pattern. This anomaly pattern agreed with the major East-West and Northeast-Southwest fracture pattern captured in the rose diagram and the lineament map. These indicates that fractures are potential mineralization zones for conductive minerals. Two possible mineralised zones were identified from the study area based on 2D electrical resistivity and magnetic data. The South Eastern part along the ERT profile shows eluvial and primary gold veins. The primary veins are trending in the NE-SW in consistent with the general trend of the geological formation likewise as revealed in the ERT profile in the North Eastern part of the study area. The surface structure pattern, magnetic anomaly pattern and electrical anomaly pattern are indicative of quartz vein and eluvial deposits. Both the quartz vein and the eluvial gold deposits could be found within 5 m and 13.4 m depths.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background to the study
The Basement Complex is a group of igneous and metamorphic rocks that range from Precambrian to Palaeozoic age in Nigeria. The major lithologic units within this complex are migmatite, gneiss, quartzite, schist, granite and acid and basic dykes (Obaje, 2009). The group of rocks are also known as massifs. They constitute the Northern Nigeria basement in North central Nigeria, the Western Nigeria basement in Western Nigeria, Adamawa Sarduana in North Eastern and Eastern Nigeria basement complex. It is these Basement complex that delimit the Nigeria sedimentary basins (Obaje, 2009).
Figure 1.1: Geological map of Nigeria showing Basement complex (Obaje, 2009). Figure 1.1 illustrates the spatial distribution of Basement rock of the Nigeria complexes. Various industrial and ore minerals are hosted within the Basement Complex rocks. One of such ore minerals is gold. Economic gold deposits are hosted in alluvial placers, eluvial bodies and primary veins within Nigeria schist belts. Reported locations of gold ore in Northern Nigeria in Schist belts include; Anka, Bin Yauri, Gurmana, Iperindo, Malele, Maru, Okolom-Dogondaji and Tsoho Birni Gwari-Kwaga. Primary gold mineralisation is explored for within the neighbourhood of igneous intrusions. It occurs in association with pyrrhotite, magnetite and ilmenite (Mbonimpa et al., 2007). These associated minerals possess high magnetic susceptibility. Consequently, the composite of these minerals and gold in rocks impart high magnetic susceptibility on the host rocks. It is this attribute of gold hosted rocks that makes the magnetic an effective tool for the prospecting for them (Fareed, 2016).
The use of magnetic methods is germane in metal ores which can be magnetic as well as electrically conductive with clear signature (Lyasky, 2010). Thus, the magnetic method is commonly employed to delineate areas with potential for gold mineralisation (Peter, 2019). High conductivity of gold makes electrical resistivity prospecting technique also effective in prospecting for them. The combinations of magnetic and electrical resistivity method do effectively delineate eluvial and primary gold ores. Co-ordinated development of delineated gold ores and associated minerals constitute a potent approach to the Nigeria mono-economy that has been hitched on petroleum exploitation for too long.
1.2 Statement of the Research Problem
For over a decade, artisanal miners have employed surface excavation and tunnelling to search for gold mineralisation in Tsoho Gurusu. This involves heavy manual labour which has been little rewarded because their effort has been based on intuition, and not on geoscience principles and geophysical information. The surface excavation and tunnelling have resulted in environmental degradation of the area (Plate I and II). The coincidence of high total magnetic field intensity anomaly with low electrical resistivity anomaly delineates areas for mining activates from the larger barren areas, thus minimising prospecting costs and avoiding widespread environmental degradation. Such requisite geophysical information for improving miner’s success and minimising environmental degradation in Tsohon Gurusu is yet to be systematically generated.
1.3 Aim and Objectives of the Study
The aim of this research is to delineate areas with potential for eluvial and primary gold ores from surface total magnetic field intensity and geoelectric resistivity tomographic data interpretation.
The objectives of the study are to:
i. Produce geological map of Tsohon-Gurusu on the scale of 1: 25,000
ii. Ascertain orientation pattern of the various fracture systems from surface expression and magnetic data
iii. Produce total magnetic field intensity map as well as its residual map
iv. Produce spatial depth variation maps for the magnetic source and 2D geoelectric section for the area.
1.4 Justification of the Study
The result of this work will delineate potential areas with eluvial and primary gold ore deposits. This will improve miners’ rewards by minimising prospecting costs. It will also minimise widespread environmental degradation as mining activities are concentrated on delineated areas rather being conducted on blind excavation and tunnelling. Successful application of combined techniques will stimulate similar application for gold prospecting within other parts of Northern Nigeria.
1.5 Limitation of the Study
The study will be limited to ground magnetic and geoelectrical investigation of 250,000 m2 areal extent of Tsohon-Gurusu.
1.6 Location of the Study Area
The study area lies within latitudes 09o 37׳ 13״N to 09o37׳28.6״ N and longitudes 006o 36׳ 29.4״ E to 006o 36׳ 13.1״E of North-Central Nigeria (Figure 1.2). It covers surface area of 250,000m2.
1.7 Geological Synopsis of Niger State
About one half of the land mass of Niger State is underlain by the basement complex rocks while the other half is occupied by the Cretaceous sedimentary rocks of the Bida Basin and part of the Sokoto Basin. Figure 1.3 is the geological map of Niger State showing the lithological units as adapted from Idris-Nda (2015). Oyawoye (1972), McCurry (1989) and Ajibade (1989) described basement rocks of Precambrian gneisses, migmatites and schists intruded by granitoids. The migmatite-gneiss complex includes migmatites, gneisses, mylonites and amphibolites. Truswell and Cope (1963) opined that the mylonites are major shear zones which mark the stratigraphic breaks between the gneissic basement complex and the cover rocks of the Birnin–Gwari Schist formation. metasedimentary
The schist belts area occurs as two elongated bodies separated by the older granite suite. The tips of the two formations are separated by a 40 km expanse of the Older Granite suite (Ajibade et al., 2008). However, this study indicates a much smaller separation of less than 10 km. The Birnin-Gwari formation lies to the west of the older granite (the Minna Batholith) while the Kushaka formation lies to the East.
1.8 Geology of Study Area
Tsohon Gurusu area belongs to the part of Nigeria Basement Complex called North Central Basement (Obaje, 2009) The lithological units in this area are Schist, Older granite suites and quartzites. It lies within the Kushaka Schist belt and its main rock type is semi-pelitic biotite- muscovite schist, which occasionally garnet and staurolite. Other rocks are phyllites, metasiltstones and graphitic schists. Several thick units of banded garnetgrunerite iron formation are interbedded with the schists.
The schist in the area is commonly intruded by quartzites which are mostly iron-stained on weathered surfaces and along joints. The quartzites are fine grained and represent original siltstone or sandstone often of high ferruginous origin.
In Nigeria, the primary veins and eluvial gold deposit appears to be Orogenic type which is controlled by deep seated curvilinear trans-crustal fracture system (Haruna, 2017). These deep-seated fracture systems are believed to serve as conduits for the subsidiary fractures which are linked to these major fracture and forms sites for gold depositions which is the main source of gold for humanity. During Orogenesis, basic and acid igneous rocks are formed and subsequently acted upon by tectonic activities that lead to deformation and metamorphism during this episode (Abubakar, 2012). Most ore deposits are hosted by metamorphic rocks and metamorphic fluids serve as basic source for various deposits (Bruce et al., 2013).
This material content is developed to serve as a GUIDE for students to conduct academic research
GROUND MAGNETIC AND GEOELECTRICAL PROSPECTING FOR GOLD MINERALISATION WITHIN TSOHO- GURUSU AREA OF MINNA, NIGER STATE, NIGERIA
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