MINERALOGICAL, GEOCHEMICAL AND STABLE ISOTOPE STUDIES OF CLAY DEPOSITS IN THE LOWER BENUE TROUGH, NIGERIA

Abstract

The Lower Benue Trough (LBT) is a clay-shale rich basin with lots of industrial clay deposits with considerable economic value. Previous studies focused mainly on the geochemistry and mineralogy of the clay deposits for industrial applications, while detailed information on the geochemical characterisation, genetic delineation and tectonic setting of the clay deposits are generally lacking. Therefore, the aim of this study was to undertake detailed mineralogical, chemical and stable isotope composition of the clay deposits in part of the LBT. Clay samples were collected in the LBT purposively on the basis of the lithologic character at Aloji (10), Agbenema (5), Udane-Biomi (5), Ofejiji (10), Okpokwu (5), Otukpa (5) and Enugu (10) areas. Mineralogy and morphology of the samples were determined using X-ray diffractometry and scanning electron microscopy. Fourier Transform Infrared (FTIR), differential thermal and thermogravimetric analyses were used to determine the structure and the thermal conditions of the clays. Major elements composition was determined by inductively coupled plasma- atomic emission spectroscopy, while trace and rare-earth elements were by inductively coupled plasma-mass spectrometry. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW) of the clay samples were calculated. Hydrogen and oxygen isotopic compositions were measured by Delta Plus XP isotope ratio mass spectrometer (IRMS) and Thermo Scientific MAT 253 mass-spectrometer, respectively. Descriptive statistics was used to analyse the data obtained, while geochemical discrimination diagrams were used to interprete the geochemical data. Kaolinite (40-49%), quartz (37-55%), vermiculite (1-4%), muscovite (1-8%) and microcline (2-5%), were the minerals in the clay samples. The book-like morphology of the kaolinite and the characteristic FTIR bands at 3694.6-3660.1 cm-1 confirmed that it has not undergone diagenesis but rather suffered weight loss at temperature range of 499 to 504 °C. The SiO2 (66.8-90.3%), Al2O3 (6.7- 22.7%), Fe2O3 (0.3-3.9%) and LOI (3.3-11.1%) were characteristic of quartz-rich kaolinitic clay mineral assemblage. The concentrations (in ppm) of Co (0.6-7.6), Ni (0.1-1.1), and V (42.0-90.0) indicated a felsic source comparable to the upper continental crust, while the calculated CIA (83.1- 99.8), PIA (98.5-99.9) and CIW (99.2-99.9) values suggested derivation of the clay from intense chemical weathering of the source rocks. The La/Sc (3.7-9.3), Th/Sc (0.6-3.1), and Th/Co (2.0-21.0); the light rare-earth enrichment and depleted heavy rare-earth with negative Eu anomaly confirmed a felsic crustal origin. The La-Th-Sc discrimination diagram showed typical granitic gneiss source. The log of (K2O/Na2O) vs SiO2, La-Co-Zr/10 and Th-Sc-Zr/10 indicated a passive margin tectonic setting, while the Eu/Eu* vs (Gd/Yb)N diagram suggested a Proterozoic age for the granitic gneiss source. The oxygen and hydrogen isotope values of +15.4 to +21.2‰ and -66.4 to -50.8‰ were consistent with chemically weathered residual material deposited under an oxic environmental condition at high temperature of formation (54-91°C), which revealed formation of the kaolinitic clay under a hot and humid paleo-climatic condition. The quartz-rich kaolinitic clay deposits in the Lower Benue Trough were derived from the chemical weathering of Proterozoic granitic gneiss. The clay was deposited in a passive margin tectonic setting, under a tropical climatic condition.