Assessment of the mineralogy of granitoids and associated granitic gneisses responsible for groundwater fluoride mobilization in the Vea catchment, Upper East Region, Ghana

The minerals responsible for the high fluoride content in groundwaters in the Vea catchment of the broader Bongo–Sumbrungu areas of the Upper East Region of Ghana were evaluated through petrographic studies using point counting and SEM–EDX analysis of the granitic rocks within the study area. The F-bearing minerals observed in the samples were in the order of microcline > plagioclase > muscovite > biotite > hornblende > apatite > pyroxene and titanite from the petrography. Among the F-bearing minerals, biotite, hornblende, apatite and titanite contain fluoride as observed in the SEM–EDX patterns and hence are the minerals responsible for the fluoridation of groundwater within the Vea catchment through water–rock interaction within the aquifer. The order in which fluoride is been released into the aquifers is apatite > biotite > hornblende > titanite. Ion exchange reaction, wherein the process of smectization converts biotite, hornblende and other mafic minerals into clay, is the other geological process having the potency of contributing to the overall groundwater fluoride content in the area. Biotite and hornblende are minerals commonly distributed within the Vea catchment and form major components of basin-type granitoids, granitic gneisses and granites in the area making these two minerals the main culprits for the fluoridation in the broader Bongo areas through water–rock interaction as well as ion exchange reaction processes. The study agrees with earlier workers on lithology but disagrees with mineralogy as fluoride sources.