1 International Research Journal of Geology and Mining (IRJGM) (2276-6618) Vol. 6(1) pp. 009 - 0027, May, 2016 DOI: http://dx.doi.org/10.14303/irjgm.2016.107 Available online http://www.interesjournals.org/irjgm Copyright©2016 International Research Journals Full Length Research Paper Geochemical Study of Tourmalines from Some Parts of Southwestern Nigeria Olatunji A.S. 1 and Jimoh R. O. 2 * 1 Department of Geology, University of Ibadan, Oyo State, Nigeria 2 Department of Chemical and Geological Sciences, Al-Hikmah University, Ilorin, Kwara State, Nigeria *Corresponding author email address: rojgems@yahoo.com 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, Al 2O3, 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. Keywords: Southwestern Nigeria, Tourmaline, Pegmatite, host rock, magmatic deposits INTRODUCTION Tourmaline, a group of boron aluminum cyclosilicate with a typical complex chemical formula (Na,Ca)(Mg,Li,Fe)3Al6(BO3)3(Si6O18)(OH)4 is a very common accessory mineral occurring in some mineralized pegmatites across southwestern Nigeria. It is usually associated with other gem minerals such as beryl, kunzite, garnet and topaz, alongside rock-forming minerals like quartz, feldspar and mica. Prominent locations with significant occurrences of tourmaline in southwestern Nigeria include Komu and Ofiki in Oyo state, Ijero in Ekiti state and Oro in Kwara state, where characteristic occurrences of euhedral gem tourmaline crystals are contained in granitoids intrusive rocks and their associated pegmatites and aplites. Sometimes tourmaline may be found as detrital grains in alluvial deposits where they have been deposited after being released and transported from their parent pegmatites. Mining of tourmaline and other gem minerals in pegmatites from these locations and some other areas in southwestern Nigeria has started many years ago and it is still continuing till date. Tourmaline varies greatly in composition and consequently the colour varies with the chemical composition of the crystals. It has a wide range in chemical composition with major and trace elements substituting for one another in the crystal structure. This ability of tourmaline to accommodate major and trace elements of widely varying ionic charge and radius in its