Identification of major sources controlling groundwater geochemistry in Mount Makabaï in the Far-North of Cameroon (the northernmost part of the Pan-African Belt)

The present work focuses on the volcanic basement rock geochemistry of Mount Makabaï (Maroua-Cameroon city) in relation to groundwater geochemistry. Investigations were centered on an approach involving an association of several methods such as geological cartography, hydrogeology, and geochemistry. Three rock samples, four disturbed soil samples, three undisturbed soil samples, and 20 samples of groundwater in 10 wells were collected during fieldwork and prepared for laboratory analyses. Bedrocks exhibit porphyritic, aphyric, and sub-aphiric textures. The phenocryst phases are mainly olivine, clinopyroxene, and plagioclase. Based on whole rock major element chemical compositions, the Makabaï samples, plotted on Na 2 O + K 2 O versus SiO 2 diagram, are mugearite, hawaiite, and basalt. The ratio of (MgO + CaO)/(Na 2 O + K 2 O) exceeds unity and shows dominance of alkaline Earth metals (MgO: 2.99–4.48 %; CaO: 6.19–9.80 %) relatively to alkali metals (Na 2 O: 2.27–4.46 %; K 2 O: 2.09–2.51 %). Exchangeable base contents are high, mostly represented by Ca and Mg, whose contents vary from 14.6 to 35.28 cmol(+) kg −1 and from 3.28 to 9.48 cmol(+) kg −1 respectively. The CEC clay is likewise high, fluctuating between 116.80 and 181.38 cmol(+) kg −1 , values in line with the presence of 2:1 clay minerals represented in the study area by montmorillonite. In the water samples, the Ca 2+ and Na + contents are generally higher than K + and Mg 2+ , reflecting the relative abundance of Na 2 O and CaO oxides to K 2 O and MgO oxides in rocks where different waters were sampled. The waters of Makabaï are acidic to neutral (6.3 ≤ pH ≤ 7.4) and weakly to highly mineralized (110.0 ≤ EC ≤ 1190.0 µS/cm). Hydro-geochemical classification by using the Piper diagram revealed two water types: (1) CaMg–HCO 3 (40%) and (2) CaMg–Cl or CaMg–SO 4 (60%). The average content of ions in all the analyzed samples was HCO 3 − ≫ NO 3 − > SO 4 2− > Cl − for anions and Ca 2+ ≫ Mg 2+ > Na + > K + for cations. Major mechanisms governing ionic constituents of groundwater in the study area are water–rock interactions, silicate weathering, and ion exchange.