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Establishing A Solid Mineral Database for A Part of Southwestern Nigeria

A.Y.B. ANIFOWOSE

Federal University of Technology,
Nigeria
Email: yanifowose@yahoo.com

ABSTRACT
Nigeria is endowed with varieties of solid minerals in addition to the abundant oil and gas reserves that have been driving the country’s economy since the mid-70s. About 35 minerals of proven reserves abound in the country. Due to the vastness of these resources, there is the need to establish a database that would enhance and optimize their exploration and exploitation with the aim of deriving maximum benefits from such venture.

The study area, Igarra, is located in Edo State, southwestern Nigeria. While the northern part is underlain by crystalline rocks of Precambrian age, the southern part is covered by Cretaceous to Tertiary sedimentary rocks. Landsat-7 ETM imagery was utilized in ILWIS® environment to delineate lithological boundaries in the study area. Field observations were carried out at 37 localities in order to evaluate the rock/mineral type and mining activity at each locality.

Finally, a Geographic Information System database of the mineral resources of the study area was established, and utilized to carry out a digital update of the existing geological map. This is expected to contribute accurate and up-to-date metadata to Nigeria’s National Geospatial Data Infrastructure (NGDI) which is at its formative stage.

Introduction
For many centuries, maps were prepared in analogue form which were very difficult to store and preserve. This is because such papers continued to deteriorate with age, thus defacing the content. In the past few years, experience has shown that maps which were hitherto available on request from custodian agencies were either eaten by rodents or termites, outdated or out of stock. This has continuously made desk studies for many aspects of investigation difficult if not impossible, because existing information on the study area may be dispersed, poorly catalogued or no longer in existence. However, with the invention of the computer and its rapid transformation, many aspects of the geoscience profession are being speedily computerized.

Geological surveys are established basically to collect information on the mineral resources of countries in which they are situated. The Geological Survey of Nigeria Agency, in pursuit of its mandate, has strived to carry out geological mapping of most parts of the country since the early 1960s. It has, over the years, made major contributions to the development of the solid minerals sector, and has been a source of information through its publications. For example, such published geological maps and bulletins form an indispensable guide to geologists and mineral prospectors in Nigeria. Until recently, these maps and reports were traditionally kept in various file cabinets, shelves, drawers, loose-leaf binders and folders. This method of data management is fraught with a lot of drawbacks, some of which are:

• Loss to animal and insect attacks over time;
• Availability of data to only a limited number of users;
• Data storage in incompatible formats and scales, thereby creating difficulty in transfer and integration of data from various sources;
• Duplication of efforts in data collection. Even when there are no duplications, data may be so scattered that access becomes difficult, if not impossible.

With the widening of scientific horizon consequent on new technological developments, the need to provide information in digital format has risen tremendously. According to Culshaw et al. (2006), the key strategies that geological surveys should adopt for their information resources include, among others,

• Managing data responsibly as a long-term asset;
• Converting as much analogue data as possible to digital format; and
• Working routinely in three-dimensional space rather than in two.

In order for any geological survey to be relevant in procurement and sharing of data, it must improve the interoperability of data so as to facilitate integration with data from other sources. Adegoke and Ajayi (2003) emphasized the need for the integration of GPS and GIS into geo-exploration efforts in order to considerably reduce the time required to acquire and interpret geological and mineral exploration data. Since these are rather new innovations, it is incumbent on the geology profession in Nigeria as in other parts of the world to establish a platform for the entrenchment of the GIS concept and successfully combine such with the experience of the older generation. This is one of the basic requirements for establishing a geospatial data infrastructure.

The Study Area
The Igarra area lies within Latitudes 7o00’N-7030’N and Longitudes 6o00’E-6o30’E at the northern fringe of Edo State, Nigeria (Figure 1). It is underlain in the north by Precambrian Basement Complex, and in the south by Cretaceous and Tertiary sediments. The northern part is rich in industrial and metallic minerals which are currently at various stages of exploitation (Table 1). The area has been sufficiently one of studied by Odeyemi, 1977, 1988, 1990; Rahaman, 1976, 1988) due to the relatively unweathered and well-exposed outcrops (Figure 2).

Figure 1: Location map of the study area.


Figure 2: The geological map of the study area (modified after Odeyemi, 1977)

Table 1: Some of the mineral deposits in the study area


Rahaman (1988) grouped the rocks of the basement complex into six:

• Migmatite-gneiss-quartzite complex;
• Younger metasediments, commonly referred to as the schist belts;
• Charnockitic, gabbroic and dioritic rocks;
• Older Granites;
• Volcanics and hypabyssal rocks; and
• Unmetamorphosed dolerite dykes, basic dykes and syenite dykes.

The Precambrian Basement Complex in Igarra area is made up of a metasupracrustal suite comprising of quartzites, quartz-schists, metaconglomerates, marble and calc-silicate rocks. Within this area is the Igarra schist belt A detailed study (Odeyemi, 1988) shows that the said schist belt can be classified into four groups:

• Quartz-biotite schist with intercalated quartz-pebble conglomerate;
• Calc-silicate gneiss and marble;
• Polymict metaconglomerate; and
• Phyllites and muscovite schists.

All the above groups of rocks have undergone various degrees of deformation with the adjacent migmatite-gneiss-quartzite complex. There have also been a lot of migmatization and granitization in some places as a result of emplacement of Pan African granite plutons which marks the last of the Precambrian activities to affect the Igarra area (Odeyemi et al., 1999). Anifowose (2004) was of the opinion that the granitic emplacement was probably controlled by fractures within the basement, and also showed outcrop pattern indicating that the Older Granites cut across all other structures with sharp and chilled contacts. The dominant structural trend in the study area is approximately N-S, and this is defined by mineralogy, lithology, axial planes and cleavages.

The geological complexity of the study area is therefore not in doubt. This probably has been responsible for the existence of diverse rock and mineral types, particularly industrial minerals that are being exploited by several industries providing employment for hundreds of people. For example, the area has some of the largest deposits of marble in Nigeria.

Materials and Methods
A Landsat-TM imagery over the study area was acquired courtesy of the Regional Centre for Training in Aerospace Survey (RECTAS), Obafemi Awolowo University Campus, Ile-Ife, Nigeria. The imagery was georeferenced and processed by combining the spectral values of Bands 4, 5 and 6 in order to enhance the structural and lithological features. Information retrieval was based on tone, colour, texture, structure, rock associations and spectral characteristics exhibited by different rock types. ILWIS 3.3 Academic® was employed in digitizing different features from the image. Existing geological and topographical maps were used for checks to ensure proper tying of points since the geology of the area is sufficiently known through fieldwork to permit a sound interpretation and correlation with the Landsat image. All the detected geological features in addition to drainage and cultural features were subsequently digitised as individual layers.

The Digital Elevation Model (DEM) of the study area was produced by creating a sub-map of the digitised topographic map (Figure 3). According to Banerjee and Mitra (2004), the draping of satellite imagery over DEM enables the identification and subsequent delineation of geological features. In the mapping of lithological boundaries using remote sensing technique, it has been observed that there is only a little difference between the technique and field mapping (Hennier and Spang, 1983; Banerjee and Mitra, 2004) but complementing with DEM has improved the map information quality particularly in the study area with rugged terrains with ridges and valleys. Therefore such DEM data will form part of the required metadata in NGDI for mineral exploration purposes. Also, this method can be conveniently extended to areas where there are no geological maps. This technique was adopted in order to generate a surface for draping and visualization of the terrain it represents.

The field survey was based on 1:50,000 geological map of the project area produced by Odeyemi (1977), the 1:100,000 topographical map (Auchi Sheet 266), and the 1:250,000 geological map of Lokoja Sheet No. 62. During the fieldwork, a total of 37 localities were visited to investigate the economic rocks and minerals within the study area in terms of lithological/mineralogical characteristics, the state of exploration, lateral extent of each occurrence, type of mining activity and the company involved. These attributes were created and linked to the digital mineral map (Figure 4). The two datasets that were generated (Landsat image interpretation and fieldwork data) were input into the GIS to generate thematic maps based on each mineral occurrence. GPS enables accurate reading of geographical coordinates of mineral occurrences, which can then be combined with other datasets mentioned above to form a Geographical Information System (GIS) database.

Figure 4: Mineral map draped over the geological map of study area

Discussion
The use of GIS database in mineral resources data management has been found to be an improvement over the conventional resource management technique. Thematic maps that were generated indicate the results of the field survey, remote sensing image analysis and interpretations as well as the result of some data analysis that were carried out. The thematic maps generated to show the mineral occurrences reveal the accurate location of each of the rock/mineral outcrops. The result of the study shows that marble, calc-gneiss, granites, vein quartz and kaolin occur in large quantities at different localities within the study area.

There is no gainsaying the fact that geospatial information technology has contributed a lot to sustainable development in developed countries. According to Boroffice and Akinyede (2005), over 80% of environmental management decisions in developed countries are based on quality and accurate geospatial information collected from different facets of human activities on the Earth. On the other hand, Africa has been lagging behind in the collection and archiving of reliable spatial data and this has significantly resulted in the failure of development projects on the continent.

With the launching of NigeriaSat-1 Earth observation satellite in September 2003, the Nigerian scientific community finds it less difficult having access to satellite imageries. Also, the periodical liberalization of satellite data access for scientific purposes has made them available to African scientists who are using the opportunity to develop some strategies in tackling the problem of sustainability in response to the New Partnership for Africa’s Development (NEPAD) initiative. In this regard, a series of pilot projects are being implemented with focus on boosting food production, mineral and water resources management, deforestation, desertification and land use/cover studies.

In view of the fact that all sustainable development issues require geospatial data utilization and management, it became imperative for countries to establish national geospatial data infrastructure (NGDI). The United Nations Economic Commission for Africa (UNECA) considered that the development of NGDI be mandatory for all African countries. This geared the necessary government agencies towards the establishment of better data collection and archiving techniques in preparation for a geospatial data infrastructure policy. Sequel to this, a meeting was convened by the National Space Research and Development Agency (NASRDA) in February 2003 with the objective of charting a road map for the implementation of an NGDI for Nigeria. Nigeria’s Earth Observation Satellite, NigeriaSat-1, is in a good stead to provide the stream of image data for the NGDI. The details of the project, which is being anchored by NASRDA, have been well documented in terms of implementation and update. The draft policy Nigeria’s NGDI was said to be at the final stage of presentation to the National Assembly that is expected to pass a bill on it (Agbaje, 2006). This would consolidate the availability and standardization of data for sustainable national development.

Conclusions
This study has revealed that there is improvement in mineral resources data management in the study area compared with the conventional resource management technique. With the creation of a GIS database of the mineral resources in the study area, accurate representation of locations of mineral deposits and their characteristics can be provided and can be easily updated over time. This will put an end to the problem of inadequate mineral resources database that has discouraged local and foreign private investment in the solid minerals sector of Nigeria’s economy. Also, with the eventual legislation of Nigeria’s Geospatial Data Infrastructure policy, the country will be in a good stead to tackle the problems generally associated with data collection and access for planning purposes and decision making. 

Acknowledgements
The first author is grateful to the Federal University of Technology, Akure for offering teaching assistantship during this study.

References

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