DEVELOPMENT OF A CONCEPTUAL MODEL FOR MINERALISATION POTENTIAL IN PART OF ILESA SCHIST BELT, SOUTHWESTERN NIGERIA

Abstract

Multi Criteria Decision Analysis (MCDA) in the context of Analytical Hierarchy Process (AHP) was adopted as the data mining technique to develop a conceptual model for mineralisation potential in part of Ilesa Schist Belt southwestern Nigeria. The research integrated geophysical mapping involving aeromagnetic and electrical resistivity methods. This study involved the qualitative and quantitative analysis of aeromagnetic and pseudo-gravity data using Oasis MontajTM software. The application of data enhancement techniques such as total horizontal derivative, tilt derivative, source edge detection and 3-D Euler deconvolution assisted in the production of lineament map of the study area. Depth to magnetic sources were obtained from the spectral frequency analysis performed on the aeromagnetic data. The electrical resistivity method involved the application of Vertical Electrical Sounding (VES) technique. Schlumberger configuration was adopted for the study. Two hundred and sixty five (265) VES points were occupied across the study area. Geo-electric parameters were obtained and coefficients of anisotropy were obtained from the VES data. Drainage and lithologic information were obtained respectively from the existing topographic and geological maps of the area. MCDA in the context of AHP assisted in the assignment of weight to each of the mineralogical indices. Four parameters were identified to favour mineralisation in the study area. The assigned weight was normalized and its consistency was established. Validation of the model was done using the existing mining pits and geochemical data available. The consistency ratio of the model was 2.18% which is lower than the recommended limit of 10%. The orientations of lineaments extracted suggest predominantly ENE-WSW trending structures, characteristic of primary geologic events in the area. The few NE-SW trending lineaments are imprints of secondary structural alteration in magnetic fabrics in this direction. Furthermore, the Euler deconvolution solutions estimated depth to the magnetic sources ranging from 94 – 600 m for the shallow and intermediate sources. Spectral frequency analysis categorised the area into shallow, intermediate and deep magnetic sources. The coefficients of anisotropy obtained from the geo-electric parameters vary from 1.00 to 2.87. Lineament density (Ld), lithology (Lt), coefficient of anisotropy (An) and drainage density (Dd) are the parameters selected to favour mineralisation in the study area. Results obtained from the application of AHP assigned weight of 0.5194, 0.2009, 0.2009 and 0.0789 respectively for lineament density, lithology, anisotropy and drainage density. Finally, the MCDA was used to develop the mineralisation potential map for the area. The area was classified into very low, low, moderate, high and very high mineralisation potential zones. Preliminary validation was done using the existing mining pits in the area and it showed about 86 % accuracy. Validation of the model using the existing geochemical information from previous studies suggests about 78% agreement with the conceptual model thus confirming the reliability of the model. The developed model will serve as a guide to future investments in the Ilesa Schist Belt and areas of similar geologic setting in Nigeria and around the globe.