Geochemical Study of Tourmalines from Some Parts of Southwestern Nigeria

Abstract

Tourmaline is a common gem mineral found in association with beryl, topaz, garnet and kunzite alongside other rock-forming minerals like quartz, feldspar and muscovite in pegmatite across southwestern Nigeria. This study undertook a geochemical study of tourmalines from southwestern Nigeria with a view of ascertaining their origin and petrologic significance. Twenty-two (22) differently coloured tourmaline samples from different locations in southwestern Nigeria were prepared and analyzed using inductively coupled plasma - mass spectrometry (ICP-MS) to determine elemental constituents. The tourmalines are enriched in SiO2, with wide compositional variability in CaO, Al2O3, and FeO, with low contents of P2O5, TiO2, MgO and the alkalis. This wide variation in chemical compositions is believed to be a reflection of the systematic changes associated with the heterogeneity in compositions of their host rocks and mineralizing fluids brought about by the diversity in their geologic settings. The strong negative correlations observed between CaO and the oxides Al2O3, Na2O, K2O and SiO2 in the mineral samples clearly revealed features of magmatic deposits for the tourmalines, with their chemistry reflecting control by host mafic rocks and/or granitic-pegmatitic fluid. The tourmaline samples are extremely depleted in Heavy Rare Earth Elements (HREE) relative to Light Rare Earth Elements (LREE) suggesting that the crystallizing fluids are sourced from the upper continental crust rather than the mantle. The intensity of colouration in tourmaline is linked to the presence or otherwise of iron concentration.