Abstract
Residual clays and laterite of economic values often occur within weathering profiles above basement rocks in tropical regions due to supergene enrichment and leaching of liable components. Previous studies in Ore area mainly on geochemistry of the basement rocks with scanty information on the weathered profiles. This study was carried out to determine the compositional characteristics of the basement rocks, the geochemical trends within the profiles above the parent rocks and the evaluation of their economic potentials.Petrographic study was carried out on thin sections of the rock samples. Elemental compositions of the rocks, clay, laterite, and top-soil were determined using Inductively Coupled Plasma-Mass Spectrometer (ICP-MS). Clay mineralogy was determined using X-ray Diffraction (XRD) analysis. Chemical index of Alterations (CIA) was calculated from geochemical data.Weathering of granite and banded gneiss in Ore resulted in the formation of soil layer, which ranged 0-0.5m, laterite 1.2-3m and clayey zone 2.9-3.0m. Quartz, plagioclase feldspars, microcline, biotite and hornblende were the essential minerals in the parent rocks. Granite and banded gneiss is high SiO2 (>65%) but low in MgO (<2.0%) and CaO (<4.0%). Kaolinite (60-80%), goethite (3-12%) and microcline (4-10%) were the dominant minerals in the XRD of the weathering profiles. Traces of illite were present only in granite. The CIA was generally > 85 indicating advanced state of weathering producing lateritic soil. The lateritic profiles over granite and banded gneiss of Ore area varied with the composition of the parent rocks. The clay layer has economic potential for ceramics, fertilizer and structural wares.
Highlights
Lateritic soil is a residual soil derived essentially from chemical weathering of igneous and metamorphic rocks under tropical climate
Ca and Mg are dominant in mafic and ultramafic rocks, whereas K and Na are released in greater proportions by felsic rocks and argillaceous sediments
The felsic minerals consist of feldspar and quartz, whereas the mafic mineral consists of biotite and hornblende and other accessory minerals. (Fig.4)
Summary
Lateritic soil is a residual soil derived essentially from chemical weathering of igneous and metamorphic rocks under tropical climate. The chemical compositions of the laterite within a soil profile vary according to the nature of the rock. Chromium is fairly stable during chemical weathering and is retained in chrome-chlorites and chrome-smectites. Some of the basement rocks have been greatly weathered to form soil as extensive occurrences of such residual bodies have been reported by various workers (Bolarinwa 2006, Elueze and Bolarinwa 1994, Emofurieta et al 1994, Adeola and Dada 2017). The average thickness of weathering profiles over the basement rocks in this area ranges between 6 to 8 meters. The variations in colour, texture and composition of the various zones within the profiles depend on the prevailing chemical environmental conditions, weather it is tending towards laterization or bauxitization. The main focus of this project is to investigate and determine the mineralogical and geochemical trend in the residual soil and evaluate the economic potentials of the weathered products
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