GEOCHEMICAL AND FLUID INCLUSION STUDIES OF PEGMATITES FROM PARTS OF SOUTHWESTERN NIGERIA

Abstract

Geochemical assessment of fluid inclusions in minerals has been successfully used in the description of genesis and characteristics of different ore-forming fluids. However, there are few reported applications of fluid inclusions study in the delineation and characterisation of ore-forming fluids in pegmatites from southwestern Nigeria. This study was designed to determine the type, origin and characteristics of fluids involved in the genesis of pegmatites from parts of southwestern Nigeria.

Systematic geological mapping was carried out in the Olode, Komu and Idiyan areas. Forty-eight samples of pegmatite were subjected to petrographic studies. In addition, 16 whole rock samples of pegmatite and 32 mineral extracts (11 feldspar, 13 muscovite, 8 biotite) were analysed for major, trace and rare-earth elements using inductively coupled plasma optical emission spectrometry technique. Fluid inclusion study was carried out on pegmatitic quartz using microthermometry. Descriptive statistics and geochemical variation plots were used for data evaluation.

Identified lithological units were quartz muscovite schist, sillimanite quartzite, granite gneiss and amphibolite. The pegmatites contained mainly albite, microcline, quartz, biotite and muscovite. Major oxides concentration (%) for pegmatite and mineral extracts were: SiO2 (35.22 – 82.17%), Al2O3 (0.73 – 34.81%), Fe2O3 (0.1 – 19.79%), MnO (0.004 – 1.693), MgO (0.01 – 8.55%), CaO (0.01 – 2.61), Na2O (0.09 – 10.16%) and K2O (0.08 – 12.66%), respectively. The concentration (ppm) for Be, Zn, Nb, Sn, Cs, W and Ta ranged as 1.00 – 384.00, 30.00 – 3800.00, 0.60 – 330.00, 1.00 – 319.00, 0.70 – 475.00, 17.20 – 933.00 and 0.32 – 107.00, respectively. Elevated concentrations of Sn were observed in muscovite extracts while elevated concentrations of W were observed in whole pegmatite. Plots of Ta vs K/Cs, K/Rb vs Rb, K/Rb vs Ba, Ta vs K/Cs, Rb vs Ba and Ta vs Cs + Rb showed that the pegmatites were unmineralised to mineralised muscovite class pegmatites. Three types (I, II and III) of aqueous primary to pseudo-secondary inclusions were observed. Type I were two-phase liquid (L)-vapor (V); (L+V; L>V); ~2-100 μm occurring as isolated inclusions, clusters and trails. Type II inclusion was three-phase: Liquid+ Vapor + Solid (L+V+S), ~2 – 15 μm and they occurred as isolated individuals and in trails. Type III were monophase (liquid) at room temperature and occurred as isolated inclusions, clusters and in trails that also contained types I and II. Type I inclusion has salinity of 0.7-21.9wt% NaCleq. All inclusions homogenised into the liquid phase at temperature of total homogenisation (Th) of 80.1-335.1°C. Temperature-pressure modelling revealed type II inclusion with Th of 350°C as the earliest fluid trapped and were associated with late magmatic-hydrothermal fluids and type II inclusion were trapped at 250°C. Type I inclusion was trapped at 160-250°C, indicating dilution and interaction of ore-forming fluid with meteoric water while Type III inclusion was trapped at a much lower temperature of <50 °C based on Th values.

The pegmatites of Southwestern Nigeria are characterised by aqueous fluid inclusions of primary and pseudo-secondary types and they are of magmatic to meteoric origins.