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Econ. Environ. Geol. 2023; 56(6): 799-816

Published online December 29, 2023

https://doi.org/10.9719/EEG.2023.56.6.799

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Mineralogy and Geochemistry of Shale Deposits in the Lower Anambra Basin, Nigeria: Implication for Provenance, Tectonic Setting and Depositional Environment

Olugbenga Okunlola1, Agonsi Udodirim Lydia1, Aliyu Ohiani Umaru2,*, Raymond Webrah Kazapoe3, Olusegun G. Olisa4

1Department of Geology, University of Ibadan, Ibadan, Nigeria
2Department of Geology, University of Maiduguri, Maiduguri, Borno State
3Department of Geological Engineering, University for Development Studies, Nyankpala, Ghana
4Department of Earth Sciences, Olabisi Onabanjo University, Ago Iwoye, Ogun state, Nigeria

Correspondence to : *ualeey@gmail.com

Received: September 10, 2023; Revised: November 24, 2023; Accepted: November 27, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Mineralogical and geochemical studies of shales within the Lower Anambra Basin was conducted to unravel the depositional environment, provenance, maturity, paleo-weathering conditions, and tectonic settings. Mineralogical studies conducted using X-ray diffraction analysis revealed that the samples were composed of kaolinite, montmorillonite, chlorite, and illite. KaolinIite is the dominant mineral, constituting approximately 41.5% of the bulk composition, whereas the non-clay minerals are quartz, ilmenite, and sillimanite. Geochemical analysis showed a predominance of SiO2, Al2O3, and Fe2O3 contents of the shale samples with mean values of 52.29%, 14.09%, and 6.15% for Imo Shale (IS); 52.31%, 16.70%, and 7.39% for Mamu Shale (MS); 43.21%, 21.33%, and 10.36% for Enugu Shale (ES); 53.35%, 15.64%, and 7.17% for Nkporo Shale (NS); and 51.24%, 17.25%, and 7.78% for Agwu Shale (AS). However, the shales were depleted in Na2O, MgO, K2O, MnO, TiO2, CaO, and P2O5. The trace element ratios of Ni/Co and Cu/Zn of the shale suggest an oxic depositional environment. The average SiO2 vs. Al2O3 ratio of the shales indicated textural maturity. Compared to the PAAS standard, the shales plot below the PAAS value of 0.85, suggesting a high degree of maturity and intensive chemical weathering, further confirmed on a CIA vs. PIA plot. On log (K2O/Na2O) against SiO2 and tectonic setting discriminant function diagrams, the shales plot mostly in the field of passive continental margin tectonic setting. The discriminant function diagrams as well as Al2O3/TiO2 ratio of the shales showed that they were derived from a mixed source (mafic and intermediate igneous rocks).

Keywords Anambra Basin, provenance, tectonic setting, mineralogical, chemical, shale

Article

Research Paper

Econ. Environ. Geol. 2023; 56(6): 799-816

Published online December 29, 2023 https://doi.org/10.9719/EEG.2023.56.6.799

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Mineralogy and Geochemistry of Shale Deposits in the Lower Anambra Basin, Nigeria: Implication for Provenance, Tectonic Setting and Depositional Environment

Olugbenga Okunlola1, Agonsi Udodirim Lydia1, Aliyu Ohiani Umaru2,*, Raymond Webrah Kazapoe3, Olusegun G. Olisa4

1Department of Geology, University of Ibadan, Ibadan, Nigeria
2Department of Geology, University of Maiduguri, Maiduguri, Borno State
3Department of Geological Engineering, University for Development Studies, Nyankpala, Ghana
4Department of Earth Sciences, Olabisi Onabanjo University, Ago Iwoye, Ogun state, Nigeria

Correspondence to:*ualeey@gmail.com

Received: September 10, 2023; Revised: November 24, 2023; Accepted: November 27, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Mineralogical and geochemical studies of shales within the Lower Anambra Basin was conducted to unravel the depositional environment, provenance, maturity, paleo-weathering conditions, and tectonic settings. Mineralogical studies conducted using X-ray diffraction analysis revealed that the samples were composed of kaolinite, montmorillonite, chlorite, and illite. KaolinIite is the dominant mineral, constituting approximately 41.5% of the bulk composition, whereas the non-clay minerals are quartz, ilmenite, and sillimanite. Geochemical analysis showed a predominance of SiO2, Al2O3, and Fe2O3 contents of the shale samples with mean values of 52.29%, 14.09%, and 6.15% for Imo Shale (IS); 52.31%, 16.70%, and 7.39% for Mamu Shale (MS); 43.21%, 21.33%, and 10.36% for Enugu Shale (ES); 53.35%, 15.64%, and 7.17% for Nkporo Shale (NS); and 51.24%, 17.25%, and 7.78% for Agwu Shale (AS). However, the shales were depleted in Na2O, MgO, K2O, MnO, TiO2, CaO, and P2O5. The trace element ratios of Ni/Co and Cu/Zn of the shale suggest an oxic depositional environment. The average SiO2 vs. Al2O3 ratio of the shales indicated textural maturity. Compared to the PAAS standard, the shales plot below the PAAS value of 0.85, suggesting a high degree of maturity and intensive chemical weathering, further confirmed on a CIA vs. PIA plot. On log (K2O/Na2O) against SiO2 and tectonic setting discriminant function diagrams, the shales plot mostly in the field of passive continental margin tectonic setting. The discriminant function diagrams as well as Al2O3/TiO2 ratio of the shales showed that they were derived from a mixed source (mafic and intermediate igneous rocks).

Keywords Anambra Basin, provenance, tectonic setting, mineralogical, chemical, shale

    Fig 1.

    Figure 1.Location of the Anambra Basin within the Lower Benue Trough (Modified after Murat, 1972).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 2.

    Figure 2.Geological map of the Lower Anambra Basin and adjacent areas showing sample locations (Modified after Ojo et al., 2009).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 3.

    Figure 3.X-Ray diffractogram for Agwu shale.
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 4.

    Figure 4.X-Ray diffractogram for Enugu shale.
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 5.

    Figure 5.X-Ray diffractogram for Nkporo shale.
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 6.

    Figure 6.X-Ray diffractogram for shale at Mamu Formation.
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 7.

    Figure 7.X-Ray diffractogram for Imo shale.
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 8.

    Figure 8.Geochemical classification of the shales using log (SiO2/Al2O3) vs log (Fe2O3/K2O) diagram (after Herron, 1988).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 9.

    Figure 9.Major element discriminant function diagram for Anambra shales (Roser and Korsch, 1988).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 10.

    Figure 10.Discrimination plot for provenance Al2O3 (wt%) vs TiO2 (wt%) plot (after Krzeszowska, 2019).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 11.

    Figure 11.CIA versus ICV plot showing weathering intensity and maturity of Anambra shales (after Long et al.,2012).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 12.

    Figure 12.CIA vs PIA plot showing Palaeo-weathering condition (after Suttner and Dutta 1986).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 13.

    Figure 13.Plot of SiO2 versus Al2O3 + K2O + Na2O discrimination diagram for paleoclimate condition of sedimentation (after Suttner and Dutta, 1986).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 14.

    Figure 14.Bivariate diagrams of Na, Ca, and K against PIA showing the mobility of feldspars due to weathering in the lithological components of shales.
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 15.

    Figure 15.Discriminant function diagrams based on high –silica for shales of the Lower Anambra Basin (after Verma and Armstrong-Altrin 2013).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 16.

    Figure 16.Discriminant function multidimensional diagram of low-silica for shales of the Lower Anambra Basin (after Verma and Armstrong-Altrin 2013).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Fig 17.

    Figure 17.Bivariate plot of SiO2 versus K2O/Na2O indicating tectonic setting discrimination diagram (After Roser & Korsch, 1986).
    Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

    Stratigraphic Setting of the Anambra Basin (Modified after Nwajide and Reijers,1996).


    AgeSouthern Benue/Anambra BasinCycles of sedimentation
    TertiaryEoceneAmeki/Nanka formationThird (3rd) cycle of sedimentation
    PaleoceneImo shale
    Upper CretaceousMaastrichtianNsukka formation
    Ajali formation
    Mamu Shale
    CampanianEnugu/Nkporo formation
    Santonian-ConiacianAgwu formationSecond (2nd) cycle of sedimentation
    TuronianEze-Aku Group, (Keana, Markudi, Agala and Amaseri formations)
    CenomanianOdukpani formation
    Lower CretaceousAlbianAsu River GroupFirst (1st) cycle of sedimentation
    Aptian
    PrecambrianBasement complex


    Average of mineralogical composition of the shale samples.


    MINERALAMOUNT (%)
    Kaolinite group41.5
    Chlorite17.8
    Montmorillonite15.8
    Sillimanite10.2
    Illite6.8
    Ilmenite4.0
    Quartz3.7


    Major elements distribution of shale samples from Anambra Basin.


    Parameters (% Oxide)Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo ShaleMean value
    L4S1L4S2L1S1L1S4L5S1L5S4L3S1L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17
    SiO251.4151.0654.7851.9241.944.5152.3175.8750.1945.760.2653.1347.9122.8564.750.0751.15
    Al2O316.8717.6314.8216.4621.4121.2416.79.4817.1117.8414.8417.1718.874.649.5417.315.75
    Fe2O37.647.927.077.2611.489.237.393.927.488.337.474.625.51.669.277.116.83
    MgO0.570.590.330.530.510.470.560.482.623.081.132.893.251.5922.471.44
    CaO0.260.230.070.060.390.440.060.040.10.410.170.130.3234.50.190.232.35
    Na2O0.280.270.370.360.120.140.360.040.020.030.010.030.040.010.060.030.14
    K2O1.321.31.331.320.790.851.320.440.440.760.390.840.810.160.90.780.86
    TiO21.431.451.441.141.651.721.161.241.051.171.211.091.260.770.731.11.23
    P2O50.070.080.080.090.170.180.110.050.120.070.080.060.070.280.190.110.11
    MnO0.060.060.030.030.030.030.030.020.020.210.020.030.030.220.060.120.06
    Fe2O3/K205.786.095.315.514.5310.855.598.91710.9619.155.56.7910.3710.39.11-
    SiO2/Al2O34.052.893.693.152.952.093.1386.937.564.065.092.534.926.782.894.35
    K2O/Na2O4.714.813.693.676.586.073.67112225.33392820.25161526-
    Al2O3/TiO211.7912.1610.2914.4412.9712.3514.397.6516.2915.2412.2615.7514.976.0313.0615.72-
    CIA90.0690.7389.3390.4394.2793.6990.5694.896.8393.6996.394.4994.1681.889.2494.3292.16
    ICV0.60.5880.620.580.6220.5250.5820.5210.6240.7180.6190.4970.5278.2191.3080.62-
    PIA96.6497.0296.8497.397.5897.2397.3499.1299.2897.4898.7699.0298.0481.4997.1898.4596.79
    CIW96.8997.2497.1197.5197.6797.3497.5499.1699.397.5998.899.0798.1291.8597.4498.5197.57
    LOI19.919.219.420.621.32119.88.220.622.214.219.821.73312.120.4-

    CIA: Chemical Index of Alteration = Al2O3Al2O3+CaO*+Na2O*100 after (Nesbitt and Young, 1982), CaO*=CaO103*P2O5 after (McLennan et al., 1993), PIA: Plagioclase index of alteration = 100*Al2O3K2OAl2O3+CaO*+Na2OK2O (after Fedo et al., 1995), CIW: Chemical index of weathering = Al2O3Al2O3+CaO*+Na2O*100 (After Harnois, 1988), ICV: Index of Chemical Variability = Fe2O3+K2O+Na2O+CaO+MgO+MnOAl2O3.



    Trace elements (ppm) distribution of shale samples from Anambra Basin.


    AgwuShaleNkporoShaleEnuguShaleMamu ShaleImo Shale
    Trace (ppm)L4S1L4S2L1S1L1S4L5S1L5S4L3S1L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17
    Ni23.127.424.32822.224.730.14.123.423.67.723.713.2245.827.5
    Ba334298376353211254351204268325119199120541317329
    Co22.227.435.626.423.12431.35.421.252.710.126.616.632632.5
    Cu27.534.177.458.750.429.842.11.84.27.75.28.45.613.83.812.5
    Pb19.116.620.819.819.71820.97.68.811.710.95.57.74.91212.1
    Zn8510355661529984191011164112170409876
    Ni/Co1.0410.681.060.961.020.960.761.10.450.760.890.790.671.760.85
    Cu/Zn0.320.331.410.890.330.30.50.090.040.060.130.070.080.350.040.16


    Rare earth element (ppm) distribution of shales from Anambra Basin.


    ElementsNkporoShaleImo ShaleMamu ShaleAgwuShaleEnuguShale
    L1S1L1S4L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17L3S1L4S1L4S2L5S1L5S4
    La19.822.410.216.118.115.022.818.815.417.822.422.218.018.520.621.6
    Ce116.1129.155.791.8107.387.4159.4109.644.1109.5129.0128.0102.3104.8117.0120.6
    Nd34.136.415.826.032.320.658.932.528.139.940.437.430.030.135.335.9
    Sm1.962.00.861.511.901.113.302.01.562.422.452.11.661.792.122.14
    Eu0.160.150.060.120.150.080.240.150.110.190.210.160.130.150.170.17
    Tb0.070.050.030.050.060.040.090.060.050.090.100.060.060.060.070.08
    Yb0.860.730.430.510.550.500.710.50.260.721.00.760.740.780.810.82
    Lu0.020.020.010.010.010.010.020.010.0060.020.020.020.020.020.020.02
    Total173.1190.983.1136.1160.4124.7245.5163.689.6170.6195.6190.7152.9156.2176.1181.3


    Average chemical composition of Anambra shales compared to shale from other sedimentary basins in Nigeria.


    OxidePresent studyBida Shale (Okunlola & Idowu,2012)Asu River Group (Amajor, 1987)Ezeaku Shale (Amajor, 1987)Auchi Shale (Fagbamigbe, 2013)Ifon Shale (Ajayi et al.,1989)
    Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
    SiO251.2453.3543.2152.3152.2961.2669.9444.9151.6863.3
    Al2O317.2515.6421.3316.7014.0816.881015.7118.7618.47
    Fe2O37.787.1710.367.396.153.754.046.244.671.26
    MgO0.580.430.490.562.170.160.872.584.390.82
    CaO0.250.070.420.064.010.053.3815.421.90.09
    Na2O0.280.370.130.360.030.060.40.420.930.42
    K2O1.311.330.821.320.611.391.152.361.162.36
    TiO21.441.291.691.161.071.740.520.651.951.02
    P2O50.080.090.180.110.110.080.170.460.250.46
    MnO0.060.030.030.030.080.020.040.060.060.01
    LOI19.552021.1519.819.114.29.2111.114.0511.6
    Total99.8299.7799.8199.899.799.5999.6999.9199.8799.81


    Comparing average chemical composition of Anambra shale to published average shales.


    OxidePresent studyAverage Bida shale (Okunola &Idowu, 2012)Average shale (Pettijohn, 1957)Turekan & Wedephol (1961)PAAS (Taylor and McLennan, 1985)NASC (Gromet et al., 1984)
    Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
    SiO251.2453.3543.2152.3152.2961.2658.158.562.4064.82
    Al2O317.2515.6421.3316.7014.0816.8815.41518.7817.05
    Fe2O37.787.1710.367.396.153.756.94.727.185.7
    MgO0.580.430.490.562.170.162.42.52.192.83
    CaO0.250.070.420.064.010.053.13.11.293.51
    Na2O0.280.370.130.360.030.061.31.31.191.13
    K2O1.311.330.821.320.611.393.23.13.683.97
    TiO21.441.291.691.161.071.740.60.770.990.8
    P2O50.080.090.180.110.110.080.20.160.160.15
    MnO0.060.030.030.030.080.02Trace---
    K2O/Na2O4.673.596.303.6720.3323.16
    K2O/Al2O30.080.090.040.080.040.08
    Al2O3/TiO211.9812.1212.6214.3913.159.70
    Cu/Zn0.331.040.320.500.140.12
    Ni/CO2.040.840.990.960.890.58

    *PAAS= Post Archean Australian shales *NASC= North American shale composite.



    Average trace element chemical composition of Anambra shale compared to shale from other sedimentary basins.


    Present studyBida Shale (Okunlola & Idowu,2012)Levinson (1974)Vine & Tourtelot (1970)Turekan & Wedephol (1961)PAAS(Taylor and McLennan, 1985)NASC (Gromet et al., 1984)
    Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
    Ni25.2526.1523.4530.11919.375070685558
    Ba316364.5232.5351269.1394.23300700580650636
    Co24.83123.5531.321.533.421020_23n.a
    Cu30.868.0540.142.1714.4570504550n.a
    Pb17.8520.318.8520.99.0222.2820n.an.a20n.a
    Zn9460.5125.58475.77116.393001009585n.a
    Srn.an.an.an.an.a59.39200300300200142
    Vn.an.an.an.an.a108.77150130130150130
    Yn.an.an.an.an.a70.693025--n.a
    Zrn.an.an.an.an.a1156.5470160160210200
    Mon.an.an.an.an.a0.72103--n.a
    Nbn.an.an.an.an.a52.462020n.a1.90n.a
    Rbn.an.an.an.an.a46.19140n.an.a160n.a
    Thn.an.an.an.an.a29.2212n.an.a14.60n.a
    Un.an.an.an.an.a13.074n.an.a3.10n.a
    Cu/Zn0.331.040.320.500.140.12
    (Cu+Mo)/Zn-----0.13
    Ni/Co2.040.840.990.960.890.58
    Rb/K2O-----33.23
    U/Th-----0.45

    *PAAS= Post Archean Australian shales *NASC= North American shale composite *n.a= not analyzed.



    Average rare earth elements of Anambra shale compared to world averages.


    Present studyBida Shale (Okunlola & Idowu,2012)PAAS (Taylor and McLennan, 1985)Codo Shale (McLennan, et al., 1990)Average shale (Levinson,1974)
    Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
    La18.2521.121.122.217.477.4038.229.7121
    Ce103.55122.6118.8128.099.3170.4279.663.450
    Nd30.0535.2535.637.432.767.6633.927.924
    Sm1.7251.982.132.11.9012.465.55--
    Eu0.140.1550.170.160.142.251.08--
    Tb0.060.060.0750.060.061.980.744--
    Yb0.760.7950.8150.760.57----
    Lu0.020.020.020.020.011.180.433--
    Dy-----11.794.68--
    Pr-----18.718.83--
    Gd-----11.014.66--
    Ho-----2.480.991--
    Er-----7.692.85--
    Tm-----1.140.405--


    Two-tailed Pearson correlation matrix.


    SiO2Al2O3Fe2O3MgOCaONa2OK2OTiO2P2O5MnONiBaCoCuPbZn
    SiO21
    Al2O3-.0611
    Fe2O3.122.6571
    MgO-.154.026-.2211
    CaO-.683-.664-.621.0391
    Na2O.057.225.255-.677-.2421
    K2O.162.429.412-.393-.495.8801
    TiO2.002.720.499-.509-.443.381.3791
    P2O5-.616-.393-.030-.072.721-.211-.380-.2831
    MnO-.589-.381-.301.359.655-.314-.318-.412.4081
    Ni.185.316.641.028-.491.376.634-.045.003-.1681
    Ba-.503-.486-.257-.187.642.330.138-.362.531.629.1311
    Co-.019.501.512.208-.463.314.555.155-.302.206.655.1481
    Cu-.155.284.330-.666-.119.859.691.495-.011-.245.258.325.3071
    Pb.027.474.666-.699-.400.853.790.597-.088-.314.486.172.432.8361
    Zn-.258.643.661.220-.327-.002.264.286.070-.024.576-.143.550.105.2531

    KSEEG
    Jun 30, 2024 Vol.57 No.3, pp. 281~352

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