INTEGRATED GEOPHYSICAL AND FUZZY INFERENCE SYSTEM FOR GOLD MINERALISATION POTENTIAL MAPPING OF GURARA AREA, NORTHCENTRAL NIGERIA

Abstract

This work applied logistically-based fuzzy logic inference system to critical factors that could control orogenic gold mineralisation in Gurara area, Northcentral Nigeria, in order to develop a process-based mineralisation potential map (MPM) of the area. These factors, obtainable from geophysical and geological dataset, were weighted using logistic function. The research integrated geological and geophysical mapping with a view to assessing the sub-surface structural framework, hydrothermal alteration haloes and favourable lithologies that are related to orogenic gold mineralisation in the study area. The geophysical data were subjected to Reduction-to-the Equator (RTE), Upward Continuation (UC), Total Horizontal Derivative (THD), Analytical Signal (AS), Source Edge Detection (SED), and Tilt-angle Derivative of Total Horizontal Derivative (THD_TDR). Radioelements ratio and ternary maps were obtained from the airborne gamma ray spectrometric data to map hydrothermal alteration pathways and modify the existing geology in the area. The fuzzy logic inference system adopted in this work provides the capability to handle complex geological processes that culminated to orogenic gold mineralisation as well as minimizing systemic uncertainties/fuzziness that often affect MPM. The results revealed that, granitic intrusion with fuzzy score range of 0.67 – 0.90, played a major role in auriferous fluid enrichment in the area. Seventy percent of the existing gold mine sites in the area spatially coincide with metasedimentary rocks, having fuzzy score range of 0.7 – 0.9, thus suggesting metasedimentary rocks as most probably responsible for the entrapment of gold-rich fluid in the area. The evidence of hydrothermal activities with fuzzy score range of 0.53 and 0.91 corroborate the occurrence of mineralisation associated with geologic structures and granite rocks. Lithological contacts and faults, having fuzzy score range of 0.60 – 0.80 presumably contribute to the localization of orogenic gold mineralisation in the area. Deep-seated, brittle-ductile shear/fracture zones with characteristic depth varying from 427.96 – 607.61 m and lateral extent of 15.5 km that could serve as target zones of gold deposits in the area were delineated. The magnetic-derived structural framework in the area reveals E-W, NE-SW and NW-SE with NE-SW as the most dominant, which is characteristic of primary geologic events in the area. The few E-W and NW-SE trending lineaments are imprints of secondary structural deformation events. The spectral analysis on the magnetic data assisted in estimating depths to shallow, intermediate and deep magnetic sources. Emerging from the results, favourable zones for primary orogenic gold mineralisation in the area occurred predominantly on granite gneiss and quartz vein. The MPM was observed to be consistent with the local geology, structural styles and hydrothermal alteration signatures in the area. Its validation using existing locations of mine/mining pits and geochemical anomalies in the study area showed agreement of 89% and 75% respectively, thus confirming the reliability of the developed MPM for resource management.