ABSTRACT
Minerals serve a very important economic commodity to the growth and development of any nation, state or community. In an attempt to delineate potential zones for gold mineralisation, a geophysical investigation was carried out around Shakwatu area of Niger State, Nigeria. Mineral potential (especially gold) is evident by the extensive artisanal mining activities in the area. The Very Low Frequency Electromagnetic-VLF EM and electrical resistivity method was employed in the study. A total of six (6) profile lines (500 m in length), and 100 m inter-profile spacing were established on the study area at a sampling rate of 20 m each. The VLF-EM and the geoelectric data were qualitatively interpreted using the Karous Hjelt Fraser Filtering (KHFF) and RES2DINV Software respectively. VLF-EM data revealed a number of subsurface zones with high real component current density which define the potential subsurface structural features (fractures/ faults zones) with possible gold mineralisation. These zones were interpreted as the potential or inferred structurally controlled fracture zones with possible gold mineralisation, with profiles 1 – 6 showing significantly conductive mineralised zones at a depth of 20-80m. Results from 2D resistivity imaging show an elevated (high) resistivity zones and a variation in shape due to heterogeneous nature of mineralisation within the study area at different depths. The two results when compared show a degree of correlation. The high resistivity zones which could infer a quartz veining structure.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background to the Study
Mineral development is vital to any nation’s economic and technological advancement and therefore, its importance can’t be over emphasized because it is one in all the sources of commercial material provide. The Nigerian Extractive Industries and Transparency Initiative, NEITI reported that there are about forty (40) different kinds of solid minerals and precious metals buried in Nigerian soil that are yet to be exploited. The commercial value of Nigeria’s solid minerals has been estimated to run into hundreds of trillions of dollars, with 70 per cent of these buried in the bowels of Northern Nigeria (Vanguard Newspaper, 2018).
Mineral exploration is the initial stage of the mining cycle, the aim of which is to locate a new source of useful minerals. It is the search for mineral deposits, and it begins with identifying large areas that may show the occurrence of certain type of ore deposit that could be developed as a resource. Mineral exploration involves the process of finding commercially viable concentrations of minerals to mine. It is a much more intensive, organised and professional form of mineral prospecting. Mineral exploration which is the initial phase of the mining cycle is geared towards locating a new source of useful minerals (Idowu, 2013).
The exploitation of mineral resources has experienced prime importance in several developing countries which includes Nigeria. Mineral resources are an important channel of wealth generation for any nation but before they are exploited, they need to pass through the stages of exploration, mining and processing (Ajakaiye, 1985; Adekoya, 2003).
In recent times, mineral mining in Nigeria accounts for only 0.3% of its GDP, due to the nation’s huge interest in the oil sector, domestic mining industry is underdeveloped, leading to the nation’s having to import minerals that it ought to have been able to produce domestically (Idowu, 2013). Gold in Nigeria usually exists in alluvial and eluvia placers, and primary veins from several parts in the northwest and southwest of Nigeria. The most prominent of such are found in the Maru, Anka and Malele areas of Zamfara State; Tsohon Birnin Gwari and Kwaga area in Kaduna State; Gurmana area in Niger State; Bin Yauri in Kebbi State; Okolom-Dogondaji in Kogi State; and Iperindo area in Osun state. Other smaller occurrences of gold beyond these major areas include: Ajegunle-Awa area in Ogun State; Korobiri and Degeji area in Kwara State; Oban Massif area in Cross River State; and Ilasa area in Oyo State (Idowu, 2013).
In Niger State, the geological location endows it with mineral resources. The state boasts of commercial quantity of large mineral deposit such as gold, talc, kaolin, tantalite, granite, marble, copper and lead (NSEZP, 2017). According to officials of the ministry of Mining and Mineral Resources, there is no local government area in the state that does not have one or more deposits of solid minerals. The development of the solid mineral sector opens up the state to economic opportunities such as capturing the interest of foreign investors which thereafter result into job creation though the establishment of various industries and provision of social amenities, thereby improving the standard of living of her citizens. However, what is witnessed today is that most of the mineral development, especially the exploitation is done by informal and in most cases illegal miners using very crude techniques with no consideration for the environment or human health (Idowu, 2013).
The Nigerian Geophysical Survey Agency (NGSA) and private exploration/mining companies have continued to shed more light on the endowments and potentials of gold mineralisation in the country through its recent investigation.
The primary gold mineralisation is associated with veins, stringers, lenses, reefs and similar bodies of quartz, quartz feldspar and quartz-tourmaline rocks in both the supracrustal rocks and basement. The veins range in thickness from several centimeters to a few metres, often displaying lenticular or pinch and swell (boundinage) structure and invariably steeply inclined occurring as isolated body or as parallel or echelon veins system (Kankara and Darma, 2016).
Applied Geophysics tends to provide applicable investigative methods to exploring, as well as the development of the Earth’s confined resources through geophysical approaches in studying and enhancing the usage of such resources for human capital development. In mineral exploration, integrated geophysical methods can be used to investigate and explore ore minerals irrespective of mode of occurrence, either massive or disseminated.
According to Unuevho et al. (2016), an ore body is a rock from which one or more metals can be profitably extracted. Examples of such metals include gold, silver, platinum, copper, lead, zinc and tin. Exploration Geophysical method, aims at detecting or inferring the presence and position of ore minerals, hydrocarbons, geothermal reservoirs, ground water reservoirs and other geological structures using surface methods to measure the physical properties of the earth along with the anomalies in these properties (Alisa,1990).
In the exploration for subsurface resources the methods are capable of detecting and delineating local features of potential interest that could not be discovered by any realistic drilling programme. A wide range of geophysical surveying methods exists, for each of which there is an ‘operative’ physical property to which the method is sensitive. Integrated geophysical methods have proven to be very effective in the field of exploration geophysics in recent times. Hence, geophysical methods are often used in combination.
In the recent times, integrated geophysical investigations have been found to be useful and also experienced increased application in many geological studies ranging from shallow engineering studies, groundwater and mineral deposits explorations as well as in a variety of geo-environmental studies such as investigations of contaminated sites or waste disposal areas (Olorunfemi and Mesida, 1987; Sharma, 1997; Frohlich and Parke, 1989; Steeples, 2001). The ground electromagnetic (Very low frequency- VLF) and electrical resistivity methods are part of the primary geophysical tools for investigating the subsurface for ore bodies (Kearey et al., 2002).
1.1.1 VLF Electromagnetic Surveying
The Very Low Frequency (VLF) method is a passive method that utilises radiations from military communication transmitters within the frequency band (15-30 kHz). These transmitters generate plane electromagnetic waves that can induce secondary eddy currents particularly in electrically conductive elongated targets. Radio waves at VLF frequencies could be used to prospect for conductive mineral deposits. The VLF method is particularly useful for mapping steeply dipping structures such as faults, fractures and shallow areas of potential mineralisation.
The well-established very low frequency electromagnetic (VLF-EM) method is a rapid, wide coverage and cost effective technique for locating both hidden ores and the structures associated with the mineralisation, in use for over 30 years (McNeill et al., 1991; Ogilvy et al.,1991; Bayrak , 2002).
According to Paterson and Ronka (1971), the advantages of VLF-EM method include: lightweight and inexpensive equipment design, speed of field operation, ease in equipment handling, and low overall operation cost. VLF-EM method has proved to be an effective exploration tool for quick mapping of the resistivity, phase and other VLF- EM parameters (the real and imaginary components) of the vertical magnetic field which contain valuable diagnostic information and tilt angle of the near surface features using only 5 m of electric dipole (Bayrak, 2002) .
The VLF-EM method offers a relatively fast approach to delineate the fractures and this fractured zone has high conductivity which could be zones of mineralisation or water aquifer (Benson et al., 1997).
1.1.2 Electrical Resistivity Surveying
The electrical resistivity, which is a commonly used method, is based on the apparent resistivity measurements along the earth surface (Frohlich and Parke, 1989; Sporry, 2004).
Electrical survey is aimed at determining the subsurface resistivity distribution by taking measurements on the ground surface. The true resistivity of the subsurface can be estimated from these measurements. The ground resistivity has a relationship with various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity method has been employed for many decades in hydrogeological, mining and geotechnical investigations (Loke, 1999). More recently, it has been used for environmental surveys.
1.2 Study Area
In this section we shall be discussing the location, climate, vegetation, land use and land resources of the study area.
1.2.1 Location of the Study Area
The study area is located between Latitudes 9ᵒ 40՜ 10.1843՜՜ to 9ᵒ 40՜ 21.7524՜՜ and Longitudes 6ᵒ 42 ՜02.0897՜՜ to 6ᵒ 42՜13.3764՜՜ (Figure 1.1) and fall within the Kushaka schist belt. The survey site covers an expanse of 250,000 square-meters (m2). The Kushaka schist belt occupies a belt of about 50 km wide and stretching from the Minna area up to the TsohonBirninGwari area of northwestern Nigeria.
1.3 Statement of the Research Problem
The mineral sector over the years has been a potential wealth reserve. But what we experience so far is government’s diversion of attention into crude oil thereby causing the mineral sector to suffer set back. The oil market today is seen to have experience some level of fluctuation in terms of market price and value. Following the invention of solar driven cars, solar powering systems which has replaced our fuel generating sets and many other upcoming technological inventions. We can therefore conclude that the oil sector will one day become a thing of the past. If nothing is done to salvage the situation, then the nation’s economy will be left to suffer.
This therefore calls for government’s attention to diversifying her effort towards developing the mineral sector. The development of the mineral sector will enable the government meet up with some current challenges ranging from job creation, security challenges (because a very good number of our youths will be gainfully employed), infrastructural development, basic amenities, revenue generation and many more. Niger state is said to be endowed with so many minerals of high economic value which cuts across virtually all the local government areas in the state. These minerals include: gold, talc, kaolin, tantalite, granite, marble, copper and lead, iron ore (high quality), quartzite, granite, tourmaline, topaz, tantalite, hydro carbon (oil & gas) and many more. The revenue to be generated from some of these minerals is enough to sustain not just the state but even the nation at large.
One of the major challenges bordering our mineral sector is the incessant artisan (illegal/unskilled) mining activities that has so far damaged most of our farm lands and also exposed the environment to environmental hazards. In the study area for instance, there are several mine pits that were left uncovered over time which has resulted into erosion. Most of these minerals are structurally controlled, so as these artisans (unskilled) miners engage in their activities may end up damaging these geologic structures. Minerals such as gold for instance are found in veins and pegmatite cavities, which are no larger than a few meters in diameters and due to their small size, local miners most times, miss out these mineral hosts during their mining activities.
1.4 Justification for the Study
With the dwindling of the price of crude oil in recent times, and the abundance of economic solid minerals in Northern Nigeria, it is pertinent for Nigeria to exploit, explore and harness these vast minerals for economic and sustainable development. Exploiting and harnessing these rich solid mineral deposits would open the gate for establishing small scale industries which would go a long way in curbing the unemployment and insecurity menace in the country most especially in the North.
In Shakwatu, the potential for gold mineralisation is seen by numerous artisan workings in the area, several geologic exposures (outcrops) of different rock formations, rock intrusions and many more. The area is predominantly underlain by schists which is composed of quartzite, most of which are fractured. These findings prove that the area has potential for mineralisation necessitating more work to be done to further confirm these findings. This work will be important to better define the mineralisation zones and also aid any further drilling program in the area. These therefore call for the need to carry out an intensive geophysical survey over the area and use data processing techniques that enhance interpretation in order to identify suitable drilling sites.
The harnessing of imaging techniques in geophysics becomes significant because for instance many economically viable minerals such as those in veins or pegmatite cavities are small in size (measuring up to a few meters in diameter). Due to their small size,
these deposits are easily missed with conventional mining methods (such as trenching or tunneling), and most geophysical techniques which are one-dimensional methods could probe the subsurface in only one dimension, vertical or horizontal and cannot produce the resolution necessary to detect them. Such as Electrical resistivity to resolve geologic features that are as small as a fraction of a meter to a few meters in longest dimension and very shallow, within a few meters of the surface.
The very low frequency electromagnetic and electrical (2D) resistivity methods are some of the most effective geophysical methods to map this kind of mineralisation.
Both Very Low Frequency (VLF) and resistivity (2D) methods are well-known geophysical exploration techniques, due to their conceptual simplicity, low equipment cost and easy to use, the methods are routinely used in mineral exploration. They are environmentally friendly because the methods do not induce adverse effect during and after use. Results from both VLF and electrical method (2D resistivity) will be able to delineate geological structure like faults/fractures, which are possible host for mineral deposits.
1.5 Aim and Objectives of the Study
The aim of this research is to delineate potential mineralised zones in Shakwatu area of Niger State, Nigeria, using Very Low Frequency Electromagnetic (VLF-EM) and electrical method (2D resistivity). The objectives of the study are to:
(i) delineate the trend of the structures in the study area
(ii) identify and interpret anomalous (fractured) regions from the real and imaginary component of the VLF data and geoelectric pseudosection.
(iii) correlate the current density pseudosection from the VLF data and the geolectric pseudosection to delineate potential mineralisation zones.
1.6 Significance of the study
This research will serve an important endeavour in delineating potential mineral zones which will in turn help to define mineral resource in the study area. Having known the economic value of minerals, the study has the potential to attract positive economic effect to the local community.
This research will also save an investor from spending so much as it utilises effective geophysical processing techniques for better interpretation and producing reliable follow-up targets, by so doing reducing costs during the drilling phases.
Furthermore, results from this research will serve as a future reference for other researchers who may wish to embark on a similar research in the study area and also help enlighten mining investors on the mineral potential of the area.
1.7 Scope of Research
This research is constrained to survey grid set up, Electromagnetic (VLF) and electrical (2D) resistivity data acquisition, analysis and interpretation using KHFF (Karous Hjelt Fraser Filtering) software, Res2DINV software. This research seeks to delineate both geological, structural features associated with mineralisation in the area.
This material content is developed to serve as a GUIDE for students to conduct academic research
GEOPHYSICAL INVESTIGATION FOR POTENTIAL GOLD MINERALISATION IN SHAKWATU AREA OF NIGER STATE, NIGERIA USING VERY LOW FREQUENCY AND ELECTRICAL RESISTIVITY METHODS
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