Geochemical characteristics of the Jos-Plateau Basalts, North-Central Nigeria

The Jos Plateau basalts are subdivided into two subtypes based on their textures and mode of occurrence viz, Newer and Older Basalts. The Newer Basalts occur as cones and lava flows and are mainly built of basaltic scoria and pyroclastics. The Older Basalts are characterized by small eroded, partly decomposed remnants, and with a number of plugs or dome-like outcrops. These two sub-types, however, present the same mineralogical composition (phenocrysts of both olivine, plagioclase (bytwonite-labradorite) and rarely pyroxene (diopside-augite) set in a groundmass of labradorite laths, magnetite, ilmenite, biotite, minor k-feldpars, nepheline and volcanic glass.Geochemical data show that the basalts are nepheline normative and have relatively low SiO2 contents (39.8-46.49 wt%). These compositional characteristics are typical of alkaline olivine basalts and are consistent with their mineralogy (biotite, k-feldspar and nepheline), their deficiency in silica (understaturated) and their enrichment in incompatible elements (V, Y, Ti, Zr, Sr, Nb, to Th). This alkaline nature is further attested to by the positive Nb anomalies exhibited by these basalts. The narrow difference in the ratios AlSub>2O2/TiO2 (4.44-6.13); CaO/TiO2 (3.04-4.30); Al2O3/CaO (1.07-1.61) and the very similar chondrite normalized distribution exhibited by these basalts are in support of their formation by partial melting process. The Jos Plateau basalts, present Zr/Nb ratios (2.4-3.0) comparable to those of the alkali basalts of the lower Benue valley, and of the Cameroon volcanic line, suggesting that they were possibly derived from the same mantle source.Keywords: Jos Plateau, alkali basalt, mantle, partial melting, incompatible elements. Global Journal of Geological Sciences Vol. 3(2) 2005: 187-19

Hydrogeochemical appraisal of fluoride in groundwater of Langtang area, Plateau State, Nigeria

Consumption of high fluoride in groundwater of Langtang area, manifest in the inhabitants of the area in form of dental fluorosis and skeletal fluorosis in older group. The aim of this study was to appraise the hydrogeochemistry of fluoride in the groundwaters of Langtang area. Thirty seven surface and groundwater samples and nine rock samples were collected in Langtang area for geochemical analysis. The Inductively Coupled Plasma Emission Spectrometry (ICPOES) was used to detect cations. The anions (Cl-, SO4 = and Br-) were determine by Ion Chromatography method. Fluoride was determined by the Specific Ion Electrode and bicarbonate was determined by titration. Major oxides, trace elements and rare earth elements for the rock samples were determined by the XRF method and fluorine by the Fusion method. Polished thin sections for rocks were prepared and studied. Geochemical results from analysis of the samples (rock and water) show that four major rock units make up the geology of the area; coarse porphyritic biotite granite, migmatite, rhyolite and riebeckite granites, the minor ones are pegmatite, trachyte, aplite and fine to medium grained biotite granites. The rhyolite, the riebeckite granites and trachyte have the highest level of fluorine content in the area (1,470, 1000, 900 and 800 ppm) respectively. The fluorine mineral (Fluorite) crystallized in the late stage of the magma as replacement of Fe/Mg mineral probably hornblende or biotite. Fluorine is leached into the groundwater from the rhyolite under the slightly alkaline (Ca-Mg-HCO3 evolving Na-HCO3) water in the area. The two major groundwater types (Ca-Mg-HCO3 and Na-HCO3) in the area have good relationship with content of fluoride in water. Although, the riebeckite granites have high fluorine content, contribution of fluoride from them is towards the southern portion of the map, owing to the groundwater flow direction. The different water sources in the area do not show variation in content of fluoride in water. However, groundwater barriers (dykes) may be responsible for some area having low fluoride (<1.5 mg/l) content. The consumption of high content of fluoride in the area has resulted in severe dental fluorosis in both children and youths and bowing of legs (Genu Valgum) in children with no discrimination between the sexes.Keywords: Fluoride, Fluorite, Fluorine, Fluorosis, Riebeckite Granite, Groundwate