In central Benin, fluoride concentrations in groundwater generally exceed the national and WHO guideline values (1.5 mg/L) and are locally above 7 mg/L (max. 7.19 mg/L). In this area, aquifers are found in a Precambrian bedrock made of migmatitic gneiss and granites. Recent hydrogeochemical studies have shown that the occurrence of fluoride in groundwater in this area is of geogenic origin. The aims of this investigation are to determine the sources of fluorine in the bedrock and to assess the role of geology on the mineralization and high fluoride concentrations in groundwater.

Thirty-five rock samples were collected in different areas that display contrasting concentrations of fluoride in groundwater. We carried out analyses of petrology, mineralogy and geochemistry on the samples. The results show that fluorine concentrations vary between 100 and 2900 ppm. Several fluorine-bearing minerals such as (biotite, muscovite, amphibole), titanite, fluorite, fluor-apatite, fluor-allanite, epidote and chlorite were identified. Groundwaters with elevated concentrations of fluoride are found in geological aquifers units and watershed rocks that contain fluorine bearing minerals. Biotite occurring in the granitic formations has the highest fluorine concentrations and appears as the most important contributor to the total fluorine from the bedrock to the groundwater.

在贝宁中部，地下水中的氟化物浓度通常超过国家和世卫组织的指导值（1.5 毫克/升），当地高于 7 毫克/升（最高 7.19 毫克/升）。在该地区，含水层位于由混合片麻岩和花岗岩构成的前寒武纪基岩中。最近的水文地球化学研究表明，该地区地下水中氟化物的存在是地质成因的。本次调查的目的是确定基岩中氟的来源，并评估地质对矿化和地下水中高氟浓度的作用。

在不同地区收集了 35 个岩石样品，这些样品显示了地下水中氟化物的不同浓度。我们对样品进行了岩石学、矿物学和地球化学分析。结果表明，氟浓度在 100 到 2900 ppm 之间变化。几种含氟矿物，如（黑云母、白云母、角闪石）、钛铁矿、萤石, 氟磷灰石, 氟铝榴石, 绿帘石和绿泥石被确定。在含有含氟矿物的地质含水层单元和流域岩石中发现氟化物浓度升高的地下水。出现在花岗岩地层中的黑云母具有最高的氟浓度，并且似乎是从基岩到地下水的总氟的最重要贡献者。