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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
Correspondence to : *ualeey@gmail.com
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
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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
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
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Stratigraphic Setting of the Anambra Basin (Modified after Nwajide and Reijers,1996).
Age Southern Benue/Anambra Basin Cycles of sedimentation Tertiary Eocene Ameki/Nanka formation Third (3rd) cycle of sedimentation Paleocene Imo shale Upper Cretaceous Maastrichtian Nsukka formation Ajali formation Mamu Shale Campanian Enugu/Nkporo formation Santonian-Coniacian Agwu formation Second (2nd) cycle of sedimentation Turonian Eze-Aku Group, (Keana, Markudi, Agala and Amaseri formations) Cenomanian Odukpani formation Lower Cretaceous Albian Asu River Group First (1st) cycle of sedimentation Aptian Precambrian Basement complex
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Average of mineralogical composition of the shale samples.
MINERAL AMOUNT (%) Kaolinite group 41.5 Chlorite 17.8 Montmorillonite 15.8 Sillimanite 10.2 Illite 6.8 Ilmenite 4.0 Quartz 3.7
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Major elements distribution of shale samples from Anambra Basin.
Parameters (% Oxide) Agwu Shale Nkporo Shale Enugu Shale Mamu Shale Imo Shale Mean value L4S1 L4S2 L1S1 L1S4 L5S1 L5S4 L3S1 L2S1 L2S2 L2S3 L2S6 L2S7 L2S11 L2S14 L2S16 L2S17 SiO2 51.41 51.06 54.78 51.92 41.9 44.51 52.31 75.87 50.19 45.7 60.26 53.13 47.91 22.85 64.7 50.07 51.15 Al2O3 16.87 17.63 14.82 16.46 21.41 21.24 16.7 9.48 17.11 17.84 14.84 17.17 18.87 4.64 9.54 17.3 15.75 Fe2O3 7.64 7.92 7.07 7.26 11.48 9.23 7.39 3.92 7.48 8.33 7.47 4.62 5.5 1.66 9.27 7.11 6.83 MgO 0.57 0.59 0.33 0.53 0.51 0.47 0.56 0.48 2.62 3.08 1.13 2.89 3.25 1.59 2 2.47 1.44 CaO 0.26 0.23 0.07 0.06 0.39 0.44 0.06 0.04 0.1 0.41 0.17 0.13 0.32 34.5 0.19 0.23 2.35 Na2O 0.28 0.27 0.37 0.36 0.12 0.14 0.36 0.04 0.02 0.03 0.01 0.03 0.04 0.01 0.06 0.03 0.14 K2O 1.32 1.3 1.33 1.32 0.79 0.85 1.32 0.44 0.44 0.76 0.39 0.84 0.81 0.16 0.9 0.78 0.86 TiO2 1.43 1.45 1.44 1.14 1.65 1.72 1.16 1.24 1.05 1.17 1.21 1.09 1.26 0.77 0.73 1.1 1.23 P2O5 0.07 0.08 0.08 0.09 0.17 0.18 0.11 0.05 0.12 0.07 0.08 0.06 0.07 0.28 0.19 0.11 0.11 MnO 0.06 0.06 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.21 0.02 0.03 0.03 0.22 0.06 0.12 0.06 Fe2O3/K20 5.78 6.09 5.31 5.5 14.53 10.85 5.59 8.9 17 10.96 19.15 5.5 6.79 10.37 10.3 9.11 - SiO2/Al2O3 4.05 2.89 3.69 3.15 2.95 2.09 3.13 8 6.93 7.56 4.06 5.09 2.53 4.92 6.78 2.89 4.35 K2O/Na2O 4.71 4.81 3.69 3.67 6.58 6.07 3.67 11 22 25.33 39 28 20.25 16 15 26 - Al2O3/TiO2 11.79 12.16 10.29 14.44 12.97 12.35 14.39 7.65 16.29 15.24 12.26 15.75 14.97 6.03 13.06 15.72 - CIA 90.06 90.73 89.33 90.43 94.27 93.69 90.56 94.8 96.83 93.69 96.3 94.49 94.16 81.8 89.24 94.32 92.16 ICV 0.6 0.588 0.62 0.58 0.622 0.525 0.582 0.521 0.624 0.718 0.619 0.497 0.527 8.219 1.308 0.62 - PIA 96.64 97.02 96.84 97.3 97.58 97.23 97.34 99.12 99.28 97.48 98.76 99.02 98.04 81.49 97.18 98.45 96.79 CIW 96.89 97.24 97.11 97.51 97.67 97.34 97.54 99.16 99.3 97.59 98.8 99.07 98.12 91.85 97.44 98.51 97.57 LOI 19.9 19.2 19.4 20.6 21.3 21 19.8 8.2 20.6 22.2 14.2 19.8 21.7 33 12.1 20.4 - CIA: Chemical Index of Alteration =
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Trace elements (ppm) distribution of shale samples from Anambra Basin.
Agwu Shale Nkporo Shale Enugu Shale Mamu Shale Imo Shale Trace (ppm) L4S1 L4S2 L1S1 L1S4 L5S1 L5S4 L3S1 L2S1 L2S2 L2S3 L2S6 L2S7 L2S11 L2S14 L2S16 L2S17 Ni 23.1 27.4 24.3 28 22.2 24.7 30.1 4.1 23.4 23.6 7.7 23.7 13.2 2 45.8 27.5 Ba 334 298 376 353 211 254 351 204 268 325 119 199 120 541 317 329 Co 22.2 27.4 35.6 26.4 23.1 24 31.3 5.4 21.2 52.7 10.1 26.6 16.6 3 26 32.5 Cu 27.5 34.1 77.4 58.7 50.4 29.8 42.1 1.8 4.2 7.7 5.2 8.4 5.6 13.8 3.8 12.5 Pb 19.1 16.6 20.8 19.8 19.7 18 20.9 7.6 8.8 11.7 10.9 5.5 7.7 4.9 12 12.1 Zn 85 103 55 66 152 99 84 19 101 116 41 121 70 40 98 76 Ni/Co 1.04 1 0.68 1.06 0.96 1.02 0.96 0.76 1.1 0.45 0.76 0.89 0.79 0.67 1.76 0.85 Cu/Zn 0.32 0.33 1.41 0.89 0.33 0.3 0.5 0.09 0.04 0.06 0.13 0.07 0.08 0.35 0.04 0.16
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Rare earth element (ppm) distribution of shales from Anambra Basin.
Elements Nkporo Shale Imo Shale Mamu Shale Agwu Shale Enugu Shale L1S1 L1S4 L2S1 L2S2 L2S3 L2S6 L2S7 L2S11 L2S14 L2S16 L2S17 L3S1 L4S1 L4S2 L5S1 L5S4 La 19.8 22.4 10.2 16.1 18.1 15.0 22.8 18.8 15.4 17.8 22.4 22.2 18.0 18.5 20.6 21.6 Ce 116.1 129.1 55.7 91.8 107.3 87.4 159.4 109.6 44.1 109.5 129.0 128.0 102.3 104.8 117.0 120.6 Nd 34.1 36.4 15.8 26.0 32.3 20.6 58.9 32.5 28.1 39.9 40.4 37.4 30.0 30.1 35.3 35.9 Sm 1.96 2.0 0.86 1.51 1.90 1.11 3.30 2.0 1.56 2.42 2.45 2.1 1.66 1.79 2.12 2.14 Eu 0.16 0.15 0.06 0.12 0.15 0.08 0.24 0.15 0.11 0.19 0.21 0.16 0.13 0.15 0.17 0.17 Tb 0.07 0.05 0.03 0.05 0.06 0.04 0.09 0.06 0.05 0.09 0.10 0.06 0.06 0.06 0.07 0.08 Yb 0.86 0.73 0.43 0.51 0.55 0.50 0.71 0.5 0.26 0.72 1.0 0.76 0.74 0.78 0.81 0.82 Lu 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.006 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Total 173.1 190.9 83.1 136.1 160.4 124.7 245.5 163.6 89.6 170.6 195.6 190.7 152.9 156.2 176.1 181.3
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Average chemical composition of Anambra shales compared to shale from other sedimentary basins in Nigeria.
Oxide Present study Bida Shale (Okunlola & Idowu,2012) Asu River Group (Amajor, 1987) Ezeaku Shale (Amajor, 1987) Auchi Shale (Fagbamigbe, 2013) Ifon Shale (Ajayi et al.,1989) Agwu Shale Nkporo Shale Enugu Shale Mamu Shale Imo Shale SiO2 51.24 53.35 43.21 52.31 52.29 61.26 69.94 44.91 51.68 63.3 Al2O3 17.25 15.64 21.33 16.70 14.08 16.88 10 15.71 18.76 18.47 Fe2O3 7.78 7.17 10.36 7.39 6.15 3.75 4.04 6.24 4.67 1.26 MgO 0.58 0.43 0.49 0.56 2.17 0.16 0.87 2.58 4.39 0.82 CaO 0.25 0.07 0.42 0.06 4.01 0.05 3.38 15.42 1.9 0.09 Na2O 0.28 0.37 0.13 0.36 0.03 0.06 0.4 0.42 0.93 0.42 K2O 1.31 1.33 0.82 1.32 0.61 1.39 1.15 2.36 1.16 2.36 TiO2 1.44 1.29 1.69 1.16 1.07 1.74 0.52 0.65 1.95 1.02 P2O5 0.08 0.09 0.18 0.11 0.11 0.08 0.17 0.46 0.25 0.46 MnO 0.06 0.03 0.03 0.03 0.08 0.02 0.04 0.06 0.06 0.01 LOI 19.55 20 21.15 19.8 19.1 14.2 9.21 11.1 14.05 11.6 Total 99.82 99.77 99.81 99.8 99.7 99.59 99.69 99.91 99.87 99.81
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Comparing average chemical composition of Anambra shale to published average shales.
Oxide Present study Average Bida shale (Okunola &Idowu, 2012) Average shale (Pettijohn, 1957) Turekan & Wedephol (1961) PAAS (Taylor and McLennan, 1985) NASC (Gromet et al., 1984) Agwu Shale Nkporo Shale Enugu Shale Mamu Shale Imo Shale SiO2 51.24 53.35 43.21 52.31 52.29 61.26 58.1 58.5 62.40 64.82 Al2O3 17.25 15.64 21.33 16.70 14.08 16.88 15.4 15 18.78 17.05 Fe2O3 7.78 7.17 10.36 7.39 6.15 3.75 6.9 4.72 7.18 5.7 MgO 0.58 0.43 0.49 0.56 2.17 0.16 2.4 2.5 2.19 2.83 CaO 0.25 0.07 0.42 0.06 4.01 0.05 3.1 3.1 1.29 3.51 Na2O 0.28 0.37 0.13 0.36 0.03 0.06 1.3 1.3 1.19 1.13 K2O 1.31 1.33 0.82 1.32 0.61 1.39 3.2 3.1 3.68 3.97 TiO2 1.44 1.29 1.69 1.16 1.07 1.74 0.6 0.77 0.99 0.8 P2O5 0.08 0.09 0.18 0.11 0.11 0.08 0.2 0.16 0.16 0.15 MnO 0.06 0.03 0.03 0.03 0.08 0.02 Trace - - - K2O/Na2O 4.67 3.59 6.30 3.67 20.33 23.16 K2O/Al2O3 0.08 0.09 0.04 0.08 0.04 0.08 Al2O3/TiO2 11.98 12.12 12.62 14.39 13.15 9.70 Cu/Zn 0.33 1.04 0.32 0.50 0.14 0.12 Ni/CO 2.04 0.84 0.99 0.96 0.89 0.58 *PAAS= Post Archean Australian shales *NASC= North American shale composite.
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Average trace element chemical composition of Anambra shale compared to shale from other sedimentary basins.
Present study Bida Shale (Okunlola & Idowu,2012) Levinson (1974) Vine & Tourtelot (1970) Turekan & Wedephol (1961) PAAS(Taylor and McLennan, 1985) NASC (Gromet et al., 1984) Agwu Shale Nkporo Shale Enugu Shale Mamu Shale Imo Shale Ni 25.25 26.15 23.45 30.1 19 19.37 50 70 68 55 58 Ba 316 364.5 232.5 351 269.1 394.23 300 700 580 650 636 Co 24.8 31 23.55 31.3 21.5 33.42 10 20 _ 23 n.a Cu 30.8 68.05 40.1 42.1 7 14.45 70 50 45 50 n.a Pb 17.85 20.3 18.85 20.9 9.02 22.28 20 n.a n.a 20 n.a Zn 94 60.5 125.5 84 75.77 116.39 300 100 95 85 n.a Sr n.a n.a n.a n.a n.a 59.39 200 300 300 200 142 V n.a n.a n.a n.a n.a 108.77 150 130 130 150 130 Y n.a n.a n.a n.a n.a 70.69 30 25 - - n.a Zr n.a n.a n.a n.a n.a 1156.54 70 160 160 210 200 Mo n.a n.a n.a n.a n.a 0.72 10 3 - - n.a Nb n.a n.a n.a n.a n.a 52.46 20 20 n.a 1.90 n.a Rb n.a n.a n.a n.a n.a 46.19 140 n.a n.a 160 n.a Th n.a n.a n.a n.a n.a 29.22 12 n.a n.a 14.60 n.a U n.a n.a n.a n.a n.a 13.07 4 n.a n.a 3.10 n.a Cu/Zn 0.33 1.04 0.32 0.50 0.14 0.12 (Cu+Mo)/Zn - - - - - 0.13 Ni/Co 2.04 0.84 0.99 0.96 0.89 0.58 Rb/K2O - - - - - 33.23 U/Th - - - - - 0.45 *PAAS= Post Archean Australian shales *NASC= North American shale composite *n.a= not analyzed.
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Average rare earth elements of Anambra shale compared to world averages.
Present study Bida Shale (Okunlola & Idowu,2012) PAAS (Taylor and McLennan, 1985) Codo Shale (McLennan, et al., 1990) Average shale (Levinson,1974) Agwu Shale Nkporo Shale Enugu Shale Mamu Shale Imo Shale La 18.25 21.1 21.1 22.2 17.4 77.40 38.2 29.7 121 Ce 103.55 122.6 118.8 128.0 99.3 170.42 79.6 63.4 50 Nd 30.05 35.25 35.6 37.4 32.7 67.66 33.9 27.9 24 Sm 1.725 1.98 2.13 2.1 1.90 12.46 5.55 - - Eu 0.14 0.155 0.17 0.16 0.14 2.25 1.08 - - Tb 0.06 0.06 0.075 0.06 0.06 1.98 0.744 - - Yb 0.76 0.795 0.815 0.76 0.57 - - - - Lu 0.02 0.02 0.02 0.02 0.01 1.18 0.433 - - Dy - - - - - 11.79 4.68 - - Pr - - - - - 18.71 8.83 - - Gd - - - - - 11.01 4.66 - - Ho - - - - - 2.48 0.991 - - Er - - - - - 7.69 2.85 - - Tm - - - - - 1.14 0.405 - -
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Two-tailed Pearson correlation matrix.
SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO Ni Ba Co Cu Pb Zn SiO2 1 Al2O3 -.061 1 Fe2O3 .122 .657 1 MgO -.154 .026 -.221 1 CaO -.683 -.664 -.621 .039 1 Na2O .057 .225 .255 -.677 -.242 1 K2O .162 .429 .412 -.393 -.495 .880 1 TiO2 .002 .720 .499 -.509 -.443 .381 .379 1 P2O5 -.616 -.393 -.030 -.072 .721 -.211 -.380 -.283 1 MnO -.589 -.381 -.301 .359 .655 -.314 -.318 -.412 .408 1 Ni .185 .316 .641 .028 -.491 .376 .634 -.045 .003 -.168 1 Ba -.503 -.486 -.257 -.187 .642 .330 .138 -.362 .531 .629 .131 1 Co -.019 .501 .512 .208 -.463 .314 .555 .155 -.302 .206 .655 .148 1 Cu -.155 .284 .330 -.666 -.119 .859 .691 .495 -.011 -.245 .258 .325 .307 1 Pb .027 .474 .666 -.699 -.400 .853 .790 .597 -.088 -.314 .486 .172 .432 .836 1 Zn -.258 .643 .661 .220 -.327 -.002 .264 .286 .070 -.024 .576 -.143 .550 .105 .253 1
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