Abstract:
Shale is an important geological material useful in the production of ceramics, building materials and other chemical products. The Mamu and Nkporo shales have been studied mainly for their stratigraphic and sedimentary significance without much attention to their compositional, mineralogical and physical characteristics necessary for determining their economic importance. Therefore, this study was designed to investigate the mineralogical, geochemical and physical compositions of the shales, in the Afikpo Area, Southeastern Nigeria.
The Afikpo area was mapped on a scale of 1:100,000. Fifty-two shale samples were purposively collected, air-dried and pulverised. Thirty representative samples were selected and analysed for major, trace and rare earth elements using inductively coupled plasma mass spectrometry. Chemical compositions of the shale samples were compared with their Standard Units. Ten samples were selected for mineralogical analysis using X-ray diffractometer. Ten samples were also analysed using Total Organic Carbon TOC-LECO C-230 Carbon Analyser to quantify the organic content of source rocks. Geotechnical tests of fourteen selected samples were performed to determine the Atterberg limits and firing tests.
The Nkporo shale was observed to be fossiliferous and highly fissile with limestone nodules and crystals of pyrite, while Mamu shale was dark-grey with some lignite bands, cross-beddings, clay and intercalated oolitic ironstones. The percentage concentrations of SiO2, Al2O3, Fe2O3, K2O, and TiO2 for Nkporo shale were 42.6 – 64.4; 12.6 – 25.3; 4.1 – 18.0; 0.6 – 3.1 and 0.8 – 1.4, respectively, while Mamu shale were 45.1 - 73.1; 13.5 – 24.9; 1.4 – 6 -7; 0.18 – 1.8 and 0.9 – 1.5, respectively. The chemical compositions of the shale samples met the required industrial specifications for bricks, ceramics and refractory products. The plot of TiO2/Al2O3 showed that both shale sequences were derived from granitic source. The rare earth element pattern of the shale indicated a source similar to average upper continental crust. Kaolinite was the major clay mineral, while quartz was the major non-clay mineral, with minor amounts of hematite, limonite, jarosite, goethite and smectite. The average TOC, oil generation potential and thermal maturity level of the shale samples for Mamu and Nkporo were 2.68 wt% and 1.89 wt%; 0.88 mg/g and 1.81 mg/g; 428oC and 427oC, respectively, indicating good organic matter concentrations, poor source rock potential and immature organic sediment. The plot of plasticity indices against Liquid Limits (26.87% and 23.86%); (62.87% and 51.66%) for Nkporo and Mamu shales, respectively indicates that both shales are mostly inorganic clay and silt, of medium to high plasticity and compressibility. The linear shrinkage and the water absorption capacity of the shale samples were moderate, as they ranged from 2.0% - 9.0% and 2.0% - 6.0%; 3.65% - 9.82% and 5.90% -10.08% for Nkporo and Mamu shales, respectively.
The shale deposits in the Afikpo area were dominated by kaolinite and met the specifications for bricks, ceramics and refractory products
