MINERALOGY AND CHEMICAL EVALUATION OF GROUNDWATER POTENIAL OF GRANITOIDS IN MINNA AREA, NORTHWESTERN NIGERIA

Abstract

The chemical and mineralogical variations of granitoids are related to a large number of factors, such as the composition of magmatic source and process of formation or crystallization. Granitoids form a dominant component of the Precambrian basement complex rocks of Nigeria. Water is usually tapped from the weathered and fracture zones of these rocks, and the tendency of any basement rock to trap enough water depends on its depth of weathering and aquifer connectivity system. The study of granitoids around Minna, is with the aim of evaluating the mineralogy, chemical and its alteration index so as to understand its groundwater potential. The study, further elucidates the depth of weathering on granitoids in the study rea. Field study revealed that the granitoids consists of medium - coarsed grained biotite granite, biotite- muscovite granite and porphyirtic granite. These rocks occured as flat lane and as batholith and ridges. Structurally, most joints run parallel to one another and they are either healed or open joints. XRD study has revealed four granitic lithologic varieties namely ; syenite, granodiorite, alkali - feldspar and monzonite. X-RF analysis also revealed percentage major element ozides of SiO2 range from 62.54% - 74.7%, Al2O3, 6.22% -13.90%, Fe2O3 1.76% - 12.60%, K2O 1.97% - 9.12%, Na2O 0.31% - 4.21%, CaO 2.29% - 7.94%, while other ozides are in trace. Mineralogical, the rocks contain quartz, albite, muscovite, microline, sanidine and siderophyllite. Comparism of weathering index of granitic rocks n the study area show a relative weathering index range between 55% and 60% in granodiorite, and alkali - feldspar granite, 50% and 54% in syenite, granite and tonalite, and 46% in monzonite. Relative abundance of joints, small scale fractures and intense weathering of granodiorite and alkali - feldspar granite revealed from chemical index of Alteration (CIA) combined with field evidence show a reliable groundwater potential for these rocks.