Geological field mapping on a scale of 1:12,500 on Wase Sheet 191 NW, was undertaken to determine the rock types and groundwater quality assessment within Langtang area, Nigeria. Rock samples were collected at various points and the coordinates were taken using the Global Positioning System (GPS) device. The samples were physically observed in the field and afterwards transported to the lab for thin section production and petrographic studies. Thin sections were observed under a petrological microscope in cross and polarized light and the minerals were described as shown in plates. The geological field mapping revealed that the area is made up of four (4) major rock types; Pan African older Granites, Migmatite-gneiss, Alluvium, and Agwu Formation. Analysis of water samples from the study area shows that major cations; Ca²⁺ had a value range of 1.053 to 60.346 mg/L, Mg²⁺ with values of 1.424 to 60.346 mg/L, Na⁺ had values of 0.212 to 2.66, and K⁺ with a value range of 2.13 to 27.22 mg/L. While the Anions; NO₃^- ranged from 0.56 to 87.52 mg/L, SO4- with values of 0.00 to 3.2 mg/L, HCO3-with values of 0.00 to 4.7 mg/L, and F-ranged from 0.35 to 3.47 mg/L. The pH value averaged at 6.4, Temperature 26.5° C, and EC 681.96 µS/cm. The Piper Trilinear Plot, Stiff and Durov plots classified the water of the study area into three major types: Ca-Mg-HCO₃, Na-HCO₃ and Ca-Mg-SO₄. The water quality and Geochemistry of the rocks of the study area show that fluoride is leached from biotite granite into the ground water (through weathering processes), thereby raising the levels above acceptable limits.