Major Oxides Geochemistry of Tourmaline from Selected Gem-Mineral Deposits in Southwestern Nigeria

Abstract

Tourmaline occurs in associations with other gem minerals such as beryl, topaz, garnet and kunzite alongside rock-forming minerals like quartz, feldspar and muscovite in various gem mineral deposits across southwestern Nigeria. These occurrences of tourmaline in varieties of colors resulting from its range in chemical compositions remain puzzling, more so when there is a lack of sufficient information on the chemistry of the Nigerian tourmalines. This study is aimed at the major oxides investigation of tourmalines from selected gem-mineral deposits in southwestern Nigeria and how the chemistry can be used to infer their environment and processes of mineralization. A total of 22 differently colored tourmaline samples obtained from different locations in southwestern Nigeria were prepared and analyzed, using the inductively coupled plasma - mass spectrometry (ICP-MS) to determine the elemental constituents. Komu and Ijero-Ekiti, being the two areas with highest concentrations of the study samples were specially chosen and mapped, at least to reveal the geology of some gem mineral deposits in the area. Results of the geochemical analysis of the tourmaline samples showed enrichment in SiO2 and Al2O3, but depletion in the remaining major oxides. However, a few of the tourmaline samples showed unusual enrichment in CaO, revealing the fact that the samples have crystallized from highly fractionated melt. The major oxide compositions of the tourmalines exhibited wide and distinct variations, especially in SiO2, Al2O3 and CaO, indicating magmatic origin for the southwestern Nigerian tourmalines. Similarly, the strong negative correlations observed between CaO and the oxides Al2O3, Na2O, K2O and SiO2 in the tourmaline samples, coupled with the relatively high Fe/Fe+Mg ratio, averaging 0.89,clearly reinforced magmatic sourcing for the tourmalines. Almost all the southwestern tourmaline samples plot in the predefined field of Li-rich granitoidpegmatites and aplites in the Al-Fe-Mg and Ca-Fe-Mg plots. Southwestern Nigerian tourmalines are thus magmatic in origin with their chemistry reflecting control by host rocks and granitic-pegmatitic fluid. The study also revealed that the intensity of coloration in tourmalines can be linked to the presence or otherwise of iron concentration. The iron content varies directly and proportionately with the depth of color in the greens, and inversely in the pinks and reds.