Articulo 8C.indd EARTH SCIENCES RESEARCH JOURNAL GEOCHEmISTRy Earth Sci. Res. SJ. Vol. 16, No. 2 (December, 2012): 151 - 164 The geochemistry of lignite from the Neogene Ogwashi-Asaba Formation, Niger Delta Basin, southern Nigeria Jude E. Ogala Geology Department, Delta State University, P.m.B.1, Abraka, Nigeria. E-mail: etunimogala@yahoo.com ABSTRACT major and trace element compositions of lignite from the Tertiary Ogwashi-Asaba formation, Southern Nigeria, have been investigated to determine the prevailing environmental conditions which controlled their formation. Seven samples were obtained from outcrops along river valleys, streams and springs in seven localities; these samples were subsequently analysed using fusion combined with inductively-coupled plasma (FUS-ICP) and total digestion with inductively-coupled plasma (TD-ICP). Geochemical analysis revealed the following concentration range in weight percent (wt. %) for major oxides: SiO 2 (0.04-9.78); Al2O3 (0.56-6.40), Fe2O3 (0.05-1.22), mnO (0.001-0.004), mgO (0.02-0.11), CaO (0.05-0.15), K2O (0.01 - 0.04), TiO2 (0.016-0.299) and S (0.08-0.39). Trace elements indicated the following concentration range in parts per million (ppm): Be (2-5), Zr (4-63), Sr (5-22), y (13-68), Ba (9-89), V (4-216), Zn (17- 176), Ni (8-28), Co (5-13), Cr (2-31), Cu (1-22) and Ga (1-14). The low silica (<10 wt. %) and alumina contents (<7 wt. %) were explained by very limited detrital input during coal formation. magnesium oxide and CaO content were relatively low thereby confirming the continental nature of the peat-forming envi- ronment. Redox-sensitive trace element ratios (Ni/Co, V/Cr and V/V+Ni) indicated predominantly oxic environments for coal deposition. The low trace element concentrations determined in the lignite samples did not point to any severe environmental impact from coal use. RESUmEN Las composiciones de elementos mayores y trazas de lignita de la formación terciaria Ogwashi-Asaba, en el sur de Nigeria, han sido investigadas para determinar las condiciones ambientales prevalecientes que con- trolaron su formación. Siete muestras fueron tomadas de afloramientos a lo largo de valles de ríos, arroyos y manantiales en siete localidades; estas muestras se analizaron posteriormente mediante fusión combinada (FUS-ICP) y la digestión total con plasma de acoplamiento inductivo (ICP-TD). El análisis geoquímico reveló el siguiente intervalo de concentración en porcentaje en peso (% en peso) para los óxidos mayores: SiO 2 (0,04 a 9,78); Al2O3 (0,56-6,40), Fe2O3 (0,05-1,22), mnO (0.001-0.004), mgO (0,02 -0,11), CaO (0.05-0.15), K2O (0,01 - 0,04), TiO2 (0.016-0,299) y S (0,08 a 0,39). Los elementos traza indican el siguiente rango de concentración en partes por millón (ppm): Be (2-5), Zr (4-63), Sr (5-22), y (13-68), Ba (9-89), V (4-216), Zn (17-176), Ni (8-28), Co (5-13), Cr (2-31), Cu (1-22) y Ga (1-14). El bajo con- tenido de sílice (<10 en peso.%) y el contenido de aluminio (<7 wt.%) fueron explicados por la entrada muy limitado de detrítos durante la formación de carbón. El óxido de magnesio y el contenido de CaO son relativamente bajos, lo que confirma el carácter continental del entorno de formación de turba. La relación de elementos traza Redox-sensibles (Ni/Co, V/ Cr y V/V + Ni) indicó ambientes predominantemente óxi- cos para la deposición de carbón. Las bajas concentraciones de trazas de elementos hallados en las muestras de lignito no indican un impacto ambiental severo de la utilización del carbón. Palabras claves: Geoquímica, Lignito, Formación Ogwashi- Asaba, ambientte continental, Nigeria Keywords: Geochemistry, lignite, Ogwashi-Asaba formation, continental environment, Nigeria. Record manuscript received: 12/01/2012 Accepted for publications: 06/05/2012 Introduction Coal accumulation in Nigeria took place mainly during upper Cre- taceous and Tertiary times, resulting in the formation of extensive lignite, sub-bituminous and bituminous coal deposits in the Benue trough and Anambra basin (Simpson, 1954; Reyment, 1965; Akande et al., 1992; Obaje and Ligouis, 1996; Obaje and Hamza, 2000; Obaje, 2009). The Ogwashi-Asaba formation occurs extensively within the Niger Delta basin Jude E. Ogala152 in southern Nigeria (Figure 1); it was first described by Wilson (1925) and named the “lignite group.” Lithologically, it consists of a sequence of coarse-grained sandstones, light-coloured clays and carbonaceous shales, within which are continental lignite seam intercalations (Reyment, 1965; Kogbe, 1976; Jan du Chene et al., 1978). Nigeria has the largest known lignite deposits in Africa, proven reserves exceeding 300 million tons (Orajaka et al., 1990; mOmSD, 2007). Lignite is mined in open pits in Nigeria and is used mainly as a substitute for wood as fuel in domestic cooking. The pertinent literature on Nigerian lignite deposits’ major and trace element composition is scarce (Adedosu et al., 2007; Olobaniyi and Ogala, 2011). Coal is heterogeneous sediment having both organic and inorganic fractions; it contains variable amounts of almost all the elements included in the periodic table (Christanis et al., 1998; Orem and Finkelman, 2003). most elements contained in coal usually show a close association with or- ganic matter or with the coal’s inorganic fraction. Changes in the elements’ affinity with either the organic or inorganic fraction may also occur during coalification (Christanis et al., 1998). Several researchers have attempted to study the geochemical features of trace elements contained in coal to understand and evaluate trace elements’ mode of occurrence as well as their behaviour during combustion (Gluskoter et al., 1977; Swaine, 1990; Finkelman, 1994; meij, 1995; Spears and Zheng, 1999; Davidson, 2000; Vassilev et al., 2001). Previous studies on trace elements in Nigerian coal have revealed their distribution (Olajire et al., 2007; Ogala et al., 2010a), composition (Ndiokwere et al., 1983; Olabanji, 1991; Ewa et al., 1996; Adedosu et al., 2007; Ogala et al., 2010b), characteristics (Nwadinigwe, 1992; Sonibare et al., 2005) and association ( Ewa, 2004; Ogala et al., 2009). Several researchers have studied trace element content in lignite and their behaviour during combustion (Foscolos et al., 1989; Goodarzi et al., Figure 1. map of Nigeria showing the location of Ogwashi-Asaba Formation and adjacent units. The geochemistry of lignite from the Neogene Ogwashi-Asaba Formation, Niger Delta Basin, southern Nigeria 153 1990; Georgakopoulos et al., 1994; Filippidis et al., 1996; Gentzis et al., 1990; Gerouki et al., 1996; Sakorafa et al., 1996; Christanis et al., 1998; Chatziapostolou et al., 2006; Adamidou et al., 2007). Trace elements are of great importance in ascertaining the depositional environment and matu- rity of geological samples within a particular basin (Adedosu et al., 2007). The present study was aimed at investigating the chemical compo- sition (major and trace elements) of lignite samples from the Ogwashi- Asaba formation, southern Nigeria, to predict the prevailing geochemi- cal environmental conditions as well as the processes responsible for their distribution. The study area’s geology and stratigraphy The Ogwashi-Asaba formation (Reyment, 1965) occurs extensively within the Niger Delta basin in southern Nigeria, covering a 4,900 km2 area (Fig. 1). The formation was originally referred to as a “lignite group” (Wilson, 1925; Wilson and Bain, 1928), “lignite series” (Simpson, 1949; 1954), and “lignite formation” (De Swardt and Casey, 1963). Lignite lay- ers have also been encountered in the lowermost strata of the Ameki group (Fig. 1) and the uppermost strata of the Benin formation in drill holes, streams/river banks and road-cut outcrops (Okezie and Onuogu, 1985). The southern Nigerian sedimentary basin’s formation began during the early Cretaceous period (Albian) following basement subsidence along the Benue and Niger troughs (Nwachukwu, 1972; Olade, 1975). Folding and uplift occurred during the Santonian along a northeast-southwest axis in the Abakaliki-Benue area. The Anambra platform, lying to the west and southwest of the Abakaliki folded belts, subsided to form the Anambra basin (Reyment, 1965; Short and Stauble, 1967; murat, 1972; Benkhelil, 1989). The upper Cretaceous stratigraphic succession in the Anambra basin began with the deposition of sediment from the marine Campanian-maas- trichtian Enugu/Nkporo shales and their lateral equivalent - deltaic Ow- elli sandstone (Fig. 2). These base units were successively overlain by the lower-middle maastrichtian deltaic coal-bearing mamu formation (lower coal measures) and middle maastrichtian tidal Ajali sandstone (false-bed- ded sandstones) and overlain by the fluvial-deltaic Nsukka formation (up- per coal measures) . The Imo and Nsukka formations marine shales were deposited during the Paleocene era and overlain by the regressive Ameki formation (Eocene); the paralic Ogwashi-Asaba formation (Oligocene- miocene) was capped by the continental Benin formation (miocene-re- cent) constituting the tertiary succession (Figures 1 and 2). The Ogwashi- Asaba formation consists of a succession of coarse-grained sandstone, clay and carbonaceous shale, containing continental lignite seam intercalations (Kogbe, 1976; Jan du Chene et al., 1978). Reyment (1965) suggested an Oligocene-miocene age for the Ogwashi-Asaba formation, but palynolog- ical study by Jan du Chene et al. (1978) proposed a late Eocene age for the base part. The lignite seams found within the Ogwashi-Asaba formation are usually brownish to black, varying in thickness from a few millimetres to a maximum of 6 meters. They are thinly laminated and fissile, hav- ing leaf and woody fragments on fresh cleats. The average overburden to Figure 2. Shows the stratigraphy of three basins in Southern Nigeria- Benue Trough, Anambra Basin and Niger Delta Basin (modified from Reyment, 1965). AGE FORMATION LITHOLOGY DEPOSITIONAL ENVIRONMENT BASIN Quaternary Benin Formation Ogwashi Asaba Formation Ameki F ormation/Nanka Sand Imo Formation Nsukka Formation Sandstones, Clay, Shales Clay, Shales, Sandstones, Lignite Sandstones, Cl ay, Shales, Limestone Clay, Shales, Limestone, Sandstone, Marl Sandstones, Clays, Shales, Coals, Limestone Continental Continental Estuarine, Shallow marine Shallow Marine, deltaic Fluvio - deltaic N ig er D el ta B as in T er ti ar y Pliocene Miocene Oligocene Eocene Paleocene U p p er C re ta ce o u s Maastrichtian Ajali Formation Sandstones, Claystones Fluvio – deltaic Mamu Formation Enugu/Nkporo/Owelli Formation Sandstones, Clays, Coals Shales, Sandstones, Clay, ironstones, Siltstones. Shallow Marine, deltaic Shallow Marine, deltaic Campanian MAJOR UNCONFORMITY Santonian Awgu Formation Sandstones, Limestones, Clays, Coals, Siltstones Shallow Marine, deltaic Conician M id d le C re t. Turonian Eze -Aku Formation Shales, Limestones, Sandstones Shallow Marine Cenomanian Odukpani Formation Sandstones, Limestones, Shales Shallow Marine Lo w er C re t. Albian Asu River Group Shales, Limestones, Sandstones Shal low Marine Aptian MAJOR UNCONFORMITY Basement complex Granites, Gneisses, Schists, Migmatites Igneous, Metamorphic A n am b ra B as in B en u e T ro u gh Jude E. Ogala154 and Okpuzu Rivers in Ibusa and at the mgbala and Ekete Springs in Okpa- nam and Nnewi, respectively. Lignite seams are also exposed along river val- leys at the Nkpunkpa and Iyiodo Springs as well as Nnem-Agadi River, all occurring in Obomkpa. The lignite beds are brownish to black and vary in thickness from a few millimetres to a maximum of 2 meters. They are thinly laminated and fissile with leaf and woody fragments on fresh cleats. The lith- ological sequence of lignite seam outcropping along the Nnem-Agadi River was logged (Figure 3); seven lignite samples were collected: three samples from Obomkpa, two from Ibusa and one each from Okpanam and Nnewi. Analytical procedure Samples (0.25 g) were powdered in an agate mortar and determined by a combination of methods to investigate lignite chemical composition (major and trace elements), i.e. fusion with inductively-coupled plasma (FUS-ICP) and total digestion with inductively-coupled plasma (TD- ICP). Loss on ignition (LOI) caused by escaping volatiles was measured after heating to about 1,050oC. Limits of detectable measurement (LDm) for major elements were 0.01 for SiO2, Al2O3, Fe2O3(T), mgO, CaO, Na2O, K2O and P2O5, 0.001 for mnO and TiO2 and 0.01% for S. LDm for trace elements were 2 ppm for Ba, Sr, Zr, Bi, Te, 1 ppm for y, Sc, Be, Co, Cr, Cu, Ga, Hg, Ni and Zn, 5 ppm for V, Sb, Tl and W, 0.3 ppm for Ag and Cd, 10 ppm for U, 3 ppm for As and Pb and 4 ppm for Sc Coal data was calculated on an ash basis (value in ash = element value in coal × 100/ash yield, where ash yield = 100 – LOI). major oxides and selected trace elements were analysed by the fusion technique (Table 1). Samples were mixed with a flux of lithium metaborate and lithium tetraborate and fused in an induction furnace. The molten melt was immediately poured into a 5% nitric acid solution containing an internal standard and mixed continuously until becoming completely dis- solved (c. 30 minutes). The samples were run for major oxides and selected trace elements using a combination fusion technique (FUS-ICP) followed by inductively-coupled plasma-optical emission spectrometry (ICP-OES) analysis using simultaneous/sequential Thermo Jarrell-Ash Enviro II ICP. A 0.25 g sample was digested with four acids for trace element analy- sis (Table 2), beginning with hydrofluoric acid (HF), followed by a mix- ture of nitric (HNO 3) and perchloric acid (HClO4), heated using precise Figure 3. Lithologic column of outcrop along Nnem-Agadi River at Obomkpa. lignite ratio is 6:1, thereby appearing to rule out open-cast mining (Da Swardt and Piper, 1957). Ogwashi-Asaba lignite outcrops along river valleys and streams/ springs in Ibusa (Okpunzu and mgbiligba River), Okpanam (mgbala Spring), Obomkpa (Nnem-Agadi River, Iyiodo and Nkpunkpa Springs) and Nnewi (Ekete Spring) (Fig. 1). Analysis methods Field work and sampling Field work for the present study took place in outcrops along river val- leys, streams and springs in seven localities (Fig. 1). Lignite seams do not easily outcrop in the field but good exposure was found along the mgbiligba Element Detection limit Analysis method Location 1 Location 2 Location 3 Location 4 Location 5 Location 6 Location 7 SiO 2 0.01 FUS-ICP 0.39 0.04 3.94 1.04 9.38 9.78 2.99 Al 2 O 3 0.01 FUS-ICP 0.56 0.65 2.47 1.31 6.4 2.13 2.7 Fe 2 O 3 T 0.01 FUS-ICP 0.05 1.22 0.13 0.21 0.42 0.38 0.17 MnO 0.001 FUS-ICP 0.001 0.004 0.002 0.004 0.002 0.004 0.001 MgO 0.01 FUS-ICP 0.02 0.02 0.04 0.04 0.04 0.11 0.02 CaO 0.01 FUS-ICP 0.08 0.08 0.09 0.15 0.05 0.43 0.05 Na 2 O 0.01 FUS-ICP < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 K 2 O 0.01 FUS-ICP < 0.01 < 0.01 0.02 0.01 0.04 0.03 0.02 TiO 2 0.001 FUS-ICP 0.016 0.017 0.095 0.025 0.299 0.07 0.095 P 2 O5 0.01 FUS-ICP < 0.01 < 0.01 < 0.01 < 0.01 0.02 < 0.01 < 0.01 LOI FUS-ICP 97.45 96.84 91.02 95.92 82.8 87 92.8 Total 0.01 FUS-ICP 98.6 98.88 97.82 98.71 99.45 99.94 98.86 Ash Yield FUS-ICP 2.55 3.16 8.98 4.08 17.2 13 7.2 Table 1. major element concentration in weight percent (wt. %) for lignites of the Ogwashi-Asaba Formation. The geochemistry of lignite from the Neogene Ogwashi-Asaba Formation, Niger Delta Basin, southern Nigeria 155 programmer-controlled heating in several ramping and holding cycles thus drying the samples. After dryness was attained, samples were brought back into solution using hydrochloric acid. The sample solution was then anal- ysed for element concentration using Varian Vista Pro inductively-coupled plasma-optical emission spectrometry (ICP-OES). USGS and CANmET certified reference materials were used for calibration. The Activation Lab- oratories in Ancaster, Canada, performed the chemical analysis. Statistical analysis The elements were subjected to univariate (minimum, maximum, mean and standard deviation, correlation coefficient) and multivariate sta- tistical analysis (factor and principal component) using statistical package for social sciences (SPSS) software (version 16.0). Since most parameters measured here were not normally distributed, Spearman’s rank correlation was used to examine the correlation between elements (the correlation coefficient matrix measures how well each constituent’s variance can be explained by its relationship to the others). Factor analysis (FA)/princi- pal component analysis (PCA) was applied to the experimental data stan- dardised through z-scale transformation to avoid misclassification due to differences in the units of measurement. Standardisation tends to increase the influence of variables whose variance is small and reduce the influence of variables whose variance is large (Liu et al., 2003). Factor analysis was applied to the data set to study the association of the trace elements and Table 2. Trace elements and sulphur concentration for lignites of the Ogwashi-Asaba Formation (Values are in parts per million except for S which is in weight percent); 1: Low-rank coal world average after Seredin and Finkelman (2008). Elements Detection Limit Analysis method Location 1 Location 2 Location 3 Location 4 Location 5 Location 6 Location 7 LRCWA1 Ba 2 FUS-ICP 17 44 54 27 60 93 23 150.0 Sr 2 FUS-ICP 5 6 12 7 22 20 10 110.0 Y 1 FUS-ICP 34 68 57 42 33 18 13 8.6 Sc 1 FUS-ICP < 1 < 1 4 2 7 6 4 4.1 Zr 2 FUS-ICP 4 6 31 10 63 32 24 36.0 Be 1 FUS-ICP 2 4 3 4 5 3 3 1.6 V 5 FUS-ICP < 5 < 5 47 23 197 45 33 25.0 Ag 0.3 TD-ICP < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 0.1 As 3 TD-ICP < 3 < 3 < 3 < 3 < 3 < 3 < 3 8.3 Bi 2 TD-ICP < 2 < 2 < 2 < 2 < 2 < 2 < 2 Cd 0.3 TD-ICP 0.4 < 0.3 < 0.3 0.3 < 0.3 < 0.3 < 0.3 0.2 Co 1 TD-ICP 7 5 13 8 9 6 6 5.1 Cr 1 TD-ICP 3 2 12 6 31 16 15 16.0 Cu 1 TD-ICP 1 1 21 3 22 12 5 16.0 Ga 1 TD-ICP 2 1 5 2 14 2 5 5.8 Hg 1 TD-ICP < 1 < 1 < 1 < 1 < 1 < 1 < 1 0.10 Mo 1 TD-ICP < 1 < 1 1 < 1 2 < 1 < 1 2.2 Ni 1 TD-ICP 17 8 28 22 21 17 10 13.0 Pb 3 TD-ICP < 3 < 3 5 < 3 8 3 < 3 7.8 Sb 5 TD-ICP < 5 < 5 < 5 < 5 < 5 < 5 < 5 0.84 S 0.01 TD-ICP 0.12 0.19 0.14 0.16 0.08 0.34 0.11 Te 2 TD-ICP < 2 < 2 < 2 < 2 < 2 < 2 < 2 Tl 5 TD-ICP < 5 < 5 < 5 < 5 < 5 < 5 < 5 0.68 U 10 TD-ICP < 10 < 10 < 10 < 10 < 10 < 10 < 10 2.4 W 5 TD-ICP < 5 < 5 < 5 < 5 < 5 < 5 < 5 1.1 Zn 1 TD-ICP 176 52 105 54 29 36 17 23.0 V/(V+Ni) 0.23 0.38 0.63 0.51 0.9 0.73 0.77 Ni/Co 2.43 1.6 2.15 2.75 2.33 2.83 1.67 V/Cr 1.66 2.5 3.92 3.83 6.36 2.81 2.2 Jude E. Ogala156 extract the principal factors governing trace element distribution (Lu et al., 1995). Components having > 1 eigenvalue were selected to explain association amongst the measured variables. Cluster analysis (CA) established site segregation and desegregation; such analysis is an unsupervised pattern recognition technique which un- covers a data set’s intrinsic structure or underlying pattern without pre- vious knowledge concerning the data being available. This is to enable classification being made based on measured objects’ nearness or similar- ity, thereby helping to establish relationships among sites in the form of a dendrogram. Hierarchical agglomerative CA involved normalising the data set by means of Ward’s method (i.e. using Euclidean distances as a measurement of similarity). CA was applied to the data set to group simi- lar sampling sites (spatial variation) spread over the region. Results and discussion The chemical composition of lignite The chemical composition of the lignite samples is presented in Tables 1 and 2. Thirty-seven parameters were measured, consisting of ten major elements, loss on ignition (LOI) and 26 trace elements. SiO 2 (0.04-9.78%), Al2O3 (0.56-6.40%) and Fe2O3 (0.05-1.22%) were the dominant oxides in most lignite samples; mgo, mnO, CaO, TiO2, Na2O, K2O and P2O5 all occurred in traces. LOI had high values (82.80- 97.45%). The low to moderate SiO2, Al2O3 and Fe2O3 concentration was due to quartz, feldspar and pyrite constituting the lignite samples’ main mineral phase (Adamidou et al., 2007; Kalaitzidis et al., 2010). The low SiO 2 (< 10%) and Al2O3 content (< 7%) suggested very limited detrital input during coal formation. The relatively low CaO (0.05-0.43%) and mgO content (0.02-0.11%) implied a continental origin for the lig- nite (Olobaniyi and Ogala, 2011); this agreed with the Ogwashi-Asaba formation’s continental setting (Reyment, 1965; Kogbe, 1976; Jan du Chene et al., 1978). The most abundant trace element was Zn (average 67 ppm), followed by V, Ba, y and Zr (50.71 ppm, 45.43 ppm, 37.86 ppm and 24.29 ppm average values, respectively) (Tables 2 and 6). The average mean values for Sr, Sc, Be, Cd, Co, Cr, Cu, Ga, mo, Ni and Pb ranged from 0.2 ppm to 18 ppm, Ag, As, Bi, Hg, Sb, Te, Tl, U and W concentration being below the detection limit in all samples. Compared to average worldwide coal concentration (Seredin and Finkelman, 2008), most trace element content was lower than world average values for low-ranking coal due to low ash yield (Table 2). y, Be, V, Co and Zn were the only trace elements having relatively higher concentrations in the coal samples used here than the world coal average. Redox-sensitive trace element ratios (Ni/Co, V/Cr and V/V+Ni) are usually considered powerful geochemical indicators for environmental dis- V /C r 5 8 6 4 2 0 0 5 10 15 20 Ni/Co V /( V + N i) 1 0,9 0,8 0,7 0,6 0,5 0 5 10 15 20 Ni/Co Oxic OxicDysoxic Dysoxic Su bo xi c/ A no xi c Suboxic/Anoxic Suboxic/Anoxic D ys ox ic O xi c Eu xi ni c A no xi c D ys ox ic (a) (b) Figure 4. Cross plots of redox-sensitive trace metal ratios (a) V/Cr vs. Ni/Co; (b) V/(V+Ni) vs. Ni/Co. Ranges for V/Cr and Ni/Co are from Jones and manning (1994); ranges for V/(V+Ni) are from Hatch and Leventhal (1992). Element N=7 Range Minimum Maximum Mean Standard Deviation SiO 2 9.74 .04 9.78 3.937 4.099 Al 2 O 3 5.84 .56 6.40 2.317 1.990 Fe 2 O 3 T 1.17 .05 1.22 .369 .398 MnO .003 .001 .004 .003 .001 MgO .09 .02 .11 .041 .032 CaO .38 .05 .43 .133 .135 Na 2 O .000 < 0.01 < 0.01 < 0.01 .000 K 2 O .031 < 0.01 .040 .020 .012 LOI 14.65 82.80 97.45 91.976 5.473 Table 3. Compositional average and standard deviation of major oxides for lignites of the Ogwashi-Asaba Formation. The geochemistry of lignite from the Neogene Ogwashi-Asaba Formation, Niger Delta Basin, southern Nigeria 157 crimination (Lewan, 1984; Hatch and Leventhal, 1992; Jones and man- ning, 1994; Hoffman et al., 1998; Rimmer, 2004; Rimmer et al., 2004; Algeo and maynard, 2004; Johnson et al., 2010; Saez et al., 2011). Jones and manning (1994) suggested that < 5 Ni/Co ratios inferred oxic condi- tions, 5-7 dysoxic conditions and > 7 suboxic to anoxic conditions. They also used < 2 V/Cr ratios to infer oxic conditions, 2-4.25 for dysoxic con- ditions and > 4.25 for suboxic to anoxic conditions. Lewan (1984) showed that V/V+Ni should be greater than 0.5 for organic matter accumulated in euxinic conditions. Hatch and Leventhal (1992) compared V/V+Ni ratios to other geo- chemical redox indicators, including degree of pyritisation, and suggested that ratios greater than 0.84 showed euxinic conditions, 0.54-0.82 an- oxic water and 0.46-0.60 for dysoxic conditions. According to Calvert and Pedersen (1993), Jones and manning (1994), Hoffman et al (1998) and Algeo and maynard (2004) anoxic environments are characterised by high V/Cr ratios and V/V+Ni values lying between 0.5 and 0.9 because of the disparate behaviour of V, Ni and Cr during redox processes in marine environments characterised by fine-grained detritic sedimentation. All the samples analysed and shown on the V/Cr cf Ni/Co and V/ V+Ni cf Ni/Co diagram (Figure 4) plotted within the oxic ranges for the four redox-sensitive trace element ratios proposed by Jones and manning (1994). Figure 5. Box plots showing the distributions of major elements. C on ce n tr at io n ( % ) 0,020 0,015 0,010 0,005 0 mnO P2O5 ly io do _S pr in g C on ce n tr at io n ( % ) Oxides 10 8 6 4 2 0 SiO2 Al2O3 Fe2O3T R iv er _m gb ili gb a ly io do _S pr in g C on ce n tr at io n ( % ) Oxides 0,5 0,4 0,3 0,2 0,1 0 mgO CaO K2O TiO2 R iv er _m ne m _A ga di R iv er _m ne m _A ga di ly io do _S pr in g Table 4. Correlation coefficients between the major elements in lignite samples from Ogwashi-Asaba Formation SiO 2 Al 2 O 3 Fe 2 O 3 T MnO MgO CaO K2O TiO 2 P 2 O 5 Ash_Yield SiO 2 1.000 Al 2 O 3 .744 1.000 Fe 2 O 3 (T) -.122 -.127 1.000 MnO .075 -.261 .577 1.000 MgO .736 .126 -.077 .503 1.000 CaO .513 -.166 -.016 .581 .951 1.000 K 2 O .955 .902 -.111 -.099 .505 .240 1.000 TiO 2 .717 .993 -.084 -.288 .077 -.214 .885 1.000 P 2 O 5 .585 .905 .057 -.180 -.020 -.270 .755 .937 1.000 Ash_Yield .963 .889 -.100 -.030 .547 .276 .994 .870 .739 1.000 Jude E. Ogala158 Table 5. Unrotated component matrix from loadings of 9 major elements and LOI on three significant factors. Elements Factor 1 Factor 2 Factor 3 SiO 2 0.929 0.341 -0.105 Al 2 O 3 0.935 -0.316 0.080 Fe2O3T -0.127 0.193 0.932 MnO -0.103 0.764 0.544 MgO 0.451 0.868 -0.203 CaO 0.175 0.956 -0.204 K 2 O 0.991 0.063 -0.028 TiO 2 0.923 -0.361 0.123 P 2 O 5 0.823 -0.386 0.301 LOI -0.988 -0.114 0.011 Eigen values 5.485 2.801 1.372 % of Variance 54.847 28.008 13.716 Cumulative % 54.847 82.855 96.571 Major element geochemistry-correlation matrix Table 3 shows the composition average and standard deviation for the lignite samples from the Ogwashi-Asaba formation analysed here. The data set was screened to identify outliers (Figure 5). The other elements were not normally distributed, except for SiO 2, K2O and mnO; a non-parametric Spearman test was thus used which applies variables’ rank- ing order in determining the correlation coefficient. Table 4 shows major elements’ correlation coefficients. There was a strong positive relationship between mgO, K 2O, TiO2 and P2O5 with SiO2, Al2O3 and ash yield, sug- gesting a common detrital source for these elements; the strong positive statistical correlation between Al 2O3 and TiO2 (r= 0.993) was typical of elements which probably came from clastic rocks. Factor analysis of the major elements studied here gained statistical information on the grouping and hence the processes responsible for their formation. The variance/covariance and factor loadings of the variables having eigenvalues were computed; factor analysis results are summa- rized in a factor matrix (Table 5). A 3-factor model was adopted, covering 96.6% of total variance. The first factor expresses 54.9% of the total data variance and showed high positive loading for elements SiO 2, Al2O3, K2O, TiO2, and P2O5 and negative loading for LOI (Table 6). The strong positive correlations be- tween SiO 2, Al2O3, and TiO2 were typical of clay minerals. The loadings in factor 1 reflected sediments believed to have their source probably in the mafic basement complex (Oban massif ) on the north-eastern flank of the Niger Delta basin (Edet et al., 2003; Elueze et al., 2009). The presence of TiO 2 and P2O5 was indicative of basaltic rocks of oceanic environment (Ogala et al., 2009); such interpretation is supported by the strong posi- tive correlation between TiO 2 and P2O5 (r= 0.937). The second factor ex- pressed 28% of the total variance and showed high positive loadings with mnO, mgO and CaO. The grouping in factor 2 suggests a lithogeochemi- cal input representative of weathering and composition of carbonate sedi- ments. The close proximity of the limestone bands/nodules in the Ameki Formation within the Niger Delta basin in close proximity may be the possible source. The strong positive correlation between CaO and mgO C om p on en t 2 Component 1 Component 3 1,0 0,5 0,0 -0,5 -1,0 mnO CaO P2O5 SiO2 K2O mgO LOI II I III Fe 2O3T Al2O3 -1,0 -1,0-0,5 -0,50,0 0,00,5 0,51,0 1,0 Figure 6. Principal component analysis for the first 3 components for oxides. Figure 7. The results of cluster analyses on the major elements contained in the lignite samples from Ogwashi-Asaba Formation. The geochemistry of lignite from the Neogene Ogwashi-Asaba Formation, Niger Delta Basin, southern Nigeria 159 Trace elements Trace elements Ba BeZr Pby CdCr SSr GaCu CoSc ScNi Zn River_Okpuzu Lyiodo_Spring Lyiodo_Spring Lyiodo_Spring mgbala_Spring River_Nnem_AgadiRiver_Okpuzu Figure 8. Box plots showing the distribution of trace elements. 200 150 100 50 0 12.5 10.0 7.5 5.0 2.5 0.0 C on ce n tr at io n ( p p m ) C on ce n tr at io n ( p p m ) Element N=7 Range Minimum Maximum Mean S.D. Ba 76.00 17.00 93.00 45.43 26.46 Sr 17.00 5.00 22.00 11.71 6.80 Y 55.00 13.00 68.00 37.86 19.76 Sc 6.01 .99 7.00 3.57 2.37 Zr 59.00 4.00 63.00 24.29 20.63 Be 3.00 2.00 5.00 3.43 .98 V 192.01 < 5 197.00 50.71 66.72 Ag .00 < 0.3 < 0.3 < 0.3 .00 As .00 < 3 < 3 < 3 .00 Bi .00 < 2 < 2 < 2 .00 Cd .11 .29 .40 .31 .04 Co 8.00 5.00 13.00 7.71 2.69 Cr 29.00 2.00 31.00 12.14 10.02 Cu 21.00 1.00 22.00 9.29 9.14 Ga 13.00 1.00 14.00 4.43 4.50 Hg .00 < 1 < 1 < 1 .00 Mo 1.01 < 1 2.00 1.14 .38 Ni 20.00 8.00 28.00 17.57 6.95 Pb 5.01 < 3 8.00 3.99 1.92 Sb .00 < 5 < 5 < 5 .00 S .26 .08 .34 .16 .09 Te .00 < 2 < 2 < 2 .00 Tl .00 < 5 < 5 < 5 .00 U .00 < 10 < 10 < 10 .00 W .00 < 5 < 5 < 5 .00 Zn 159.00 17.00 176.00 67.00 55.71 Table 6. Compositional average and standard deviation of trace elements for lignites of the Ogwashi-Asaba Formation. Jude E. Ogala160 Table 7. Correlation coefficients between trace elements and ash yield in lignite samples from Ogwashi-Asaba Formation. Table 8. Unrotated component matrix from loadings of 17 trace elements on four significant factors. Table 9. Varimax rotated component loadings of 17 trace elements on four significant components explaining 93.80% of the total variance. Ba Sr Y Sc Zr Be V Cd Co Cr Cu Ga Mo Ni Pb S Zn Ash_ Yield Ba 1.000 Sr .807 1.000 Y -.097 -.412 1.000 Sc .705 -973 -.503 1.000 Zr .568 .918 -.302 .940 1.000 Be .243 .423 .246 .381 .539 1.000 V .387 .803 -.195 .798 .940 .678 1.000 Cd -.507 -.468 -.079 -.511 -.467 -.629 -.322 1.000 Co .093 .204 .294 .266 .398 .054 .332 -.114 1.000 Cr .490 .903 -.453 .943 .981 .521 .932 -.432 .274 1.000 Cu .602 .795 -.026 .814 .886 .358 .767 -.432 .729 .791 1.000 Ga .188 .679 -.198 .722 .901 .596 .963 -.262 .397 .903 .741 1.000 Mo .245 .668 -.104 .639 .830 .709 .968 -.186 .220 .831 .620 .939 1.000 Ni .209 .297 .139 .311 .377 .056 .338 -.011 .902 .274 .663 .316 .224 1.000 Pb .298 .667 .061 .662 .874 .624 .945 .255 .551 .817 .829 .947 .925 .477 1.000 S .293 .242 -.125 .130 -.147 -.196 -.330 -.224 -.350 -.173 -.099 -.539 -.428 -.132 -.465 1.000 Zn -.386 -.500 .279 -.546 -.463 -.653 -.384 .863 .278 -.522 -.198 -.317 -.298 .295 -.178 -.196 1.000 Ash_ Yield .728 .987 -.380 .981 .969 .469 .870 -.475 .291 .952 .851 .780 .741 .338 .761 .091 -.491 1.000 Elements Factor 1 Factor 2 Factor 3 Factor 4 Ba .557 -.536 .536 .114 Sr .897 -.323 .218 -.172 Y -.216 .395 .060 .817 Sc .909 -.288 .182 -.219 Zr .990 -.026 .028 -.105 Be .636 -.089 -.453 .548 V .957 .130 -.197 -.075 Cd -.508 .498 .007 -.596 Co .421 .660 .511 .217 Cr .963 -.091 -.087 -.224 Cu .882 .185 .390 .073 Ga .907 .277 -.286 -.123 Mo .858 .209 -.363 -.068 Ni .412 .547 .618 .094 Pb .900 .384 -.106 .063 S -.165 -.763 .557 .057 Zn -.487 .686 .342 -.276 Eigen values 9.230 3.002 2.071 1.644 % of Variance 54.295 17.660 12.183 9.669 Cumulative % 54.295 71.954 84.138 93.806 Elements Component 1 Component 2 Component 3 Component 4 Ba 0.276 0.872 0.164 0.185 Sr 0.779 0.606 0.084 0.070 Y -0.437 -0.316 0.500 0.577 Sc 0.818 0.560 0.072 0.035 Zr 0.920 0.292 0.195 0.152 Be 0.507 -0.070 -0.040 0.809 V 0.954 0.025 0.151 0.209 Cd -0.220 -0.478 0.028 -0.764 Co 0.242 -0.048 0.928 0.008 Cr 0.958 0.260 0.034 0.088 Cu 0.691 0.340 0.600 0.129 Ga 0.961 -0.151 0.162 0.152 Mo 0.913 -0.164 0.074 0.226 Ni 0.235 0.102 0.883 -0.114 Pb 0.859 -0.117 0.406 0.239 S -0.386 0.866 -0.152 -0.009 Zn -0.374 -0.399 0.474 -0.614 Eigen values 7.898 3.028 2.798 2.223 % of Variance 46.46 17.81 16.45 13.07 Cumulative % 46.46 64.27 80.72 93.80 The geochemistry of lignite from the Neogene Ogwashi-Asaba Formation, Niger Delta Basin, southern Nigeria 161 (r= 0.951) indicates the presence of dolomite in the lignite samples (yazdi and Shiravani, 2004). Finally, the third factor expresses 13.7% of the total variance and shows high positive loadings with Fe 2O3. The high positive loading of Fe2O3 in factor 3 confirms the presence of iron bearing miner- als such as pyrite. Also the moderate positive correlation between Fe2O3 and mnO (r= 0.577) indicates that the element is present in carbonates, probably siderite. Principal component analysis extraction method was further ap- plied to characterize the geochemistry of major elements. The elements were plotted in rotated space yielding three components (Fig. 6) which explained 97% of the total data variance. The chemical compositions were categorized to three groups (Fig. 7) by cluster analysis. According to Horner and Krissek (1992), cluster analysis is a powerful tool that helps in the identification of groups with similar samples, while principal com- ponent analysis aids in the identification of elements (variables) that are responsible for the similarities or differences between groups of samples. The three components derived from factor analysis (Fig. 6) correspond to the three cluster groups of major elements (Fig. 7). Cluster I comprises of Al 2O3, TiO2, P2O5, SiO2, and K2O, indicating that the majority of these elements were mainly of terrigenous origin. Cluster II comprises of mgO and CaO, indicating sediments of carbonate origin, while cluster III com- prises of Fe 2O3, mnO and LOI indicating affiliation to epigenetic minerals such as pyrite and siderite. Trace element geochemistry-correlation matrix Table 6 shows the average composition and standard deviation of the trace elements in the lignite sample. The data set was screened to identify outliers (Figure 8). Among the trace elements analysed, Zn, Cd, Ga, mo, Pb, S and Co were not normally distributed. Spearman’s rank correlation determined the correlation coefficient between trace elements and ash yield (Table 8). most trace elements had strong positive correlation coef- ficients with ash yield and with each other (Table 7); this indicated a com- mon detrital source for most of these elements. y, Be, Cd, Co, Ni, S and Zn poorly correlated with ash yield (- 0.380 < r < 0.469) which could be explained by organic affinity and these elements’ association with sulphide Figure 9. Principal component analysis for the first 3 components for trace elements. Figure 10. The results of cluster analyses on the trace elements contained in the lignite samples from Ogwashi-Asaba Formation. C om p on en t 2 Component 1 Component 3 1,0 0,5 0,0 -0,5 -1,0 Ni Cd Co y Zn II I III -1,0 -1,0-0,5 -0,50,0 0,00,5 0,51,0 1,0 S Ba Sr Cr Sc Cu Zr Be mo Pb Ga phases. The poor correlation of S with Cu (r = - 0.099), Zn (r = - 0.196), Cd (r = - 0.224), Ga (r = - 0.539), Pb (r = - 0.465), Sr (r = 0.242), Sc (r = 0.130), y (r = -0.125), Zr (r = - 0.147), Be (r = - 0.196), V (r = - 0.330), Co (r = - 0.350), Cr (r = - 0.173), Ni (r = - 0.132) and mo (r = - 0.428) indicated that sulphur was not only present in its sulphide form but also as an organic form (Finkelman, 1995). There were highly significant correlations (r ≥ 0.90) between the ele- ments in the following couples: Sr and Sc, Sr and Zr, Sr and Cr, Sc and Zr, Sc and Cr, Zr and V, Zr and Cr, Zr and Ga, V and Cr, V and Ga, V and mo, V and Pb, Co and Ni, Cr and Ga, Ga and mo, Ga and Pb and mo and Pb (Table 7). This result was consistent with that recorded by Spears and Zheng (1999) and was interpreted as being essentially associated with detrital minerals and clay minerals. FA applied to the correlation matrix initially produced overlaps in Ba, Be, Co, Ni and Cd (Table 8), thereby necessitating further analysis. Jude E. Ogala162 This was achieved by rotating the axis, thereby producing a new set of fac- tors; this was attained after 11 iterations (Table 9). Each factor involved a sub-set of the original variables with as much minimal overlap as possible (Table 9). Since the differences between the unrotated and rotated compo- nents were significantly different, the rotated option was used for further analysis. A 4-factor model having 93.8% total cumulative variance was ob- tained (Table 9). Principal component 1 (PC1) expressed 46.5% of total data variance and showed high positive loadings with Sr, Sc, Zr, V, Cr, Cu, Ga, mo and Pb (Table 9). The inferred host phases for PC1 were a mixture of lithophile (Sr, Sc, Zr, V and Cr; r = 0.803-0.973) and chalcophile (Cu, Ga, mo and Pb; r = 0.620-0.829). The high positive loading of Pb in PC1 may have resulted from processes which led to the emplacement of Pb-Zn mineralisation in the Lower Benue trough (Akande and Erdtmann, 1998; Ogala et al., 2010b). The positive loading of Cu in both PC1 and PC3 indicated dual sources contributing to the presence of Cu. Further analysis using CA (Figure 10) confirmed that Cu belonged to PC1. PC1 had a high positive loading with Zr (Table 9), indicating a concentration of resistant heavy minerals, such as zircon. Principal component 2 (PC2) expressed 17.8% of total data variance and had the highest loadings with Ba, Sr and S. According to Finkelman (1995) and Christanis et al., (1998), Ba and S have a strong organic affili- ation, while Sr indicates a carbonate origin. Principal component 3 (PC3) expressed 16.5% of total data variance and showed positive loading with Co, Cu and Ni. It could be inferred from the data obtained that this group of elements was associated with Fe-sulphide (pyrite). The reported occurrence of chalcopyrite (CuFeS 2) in the Abakaliki area of the Lower Benue trough cannot be ruled out due to the presence of Cu (Elueze et al., 2009). Principal component 4 (PC4) expressed 13% of total variance and showed positive loading with y and Be and negative loading for Cd and Zn. PC4 was made up of a mixture of lithophile (Be and y) and chal- cophile (Cd and Zn), suggesting that the elements were associated with silicates and sulphides, respectively. The trace elements were plotted in rotated space yielding three com- ponents (Fig. 9). CA established trace element association and origin (Fig. 10); this clustering (Fig. 10) coincided with the results obtained from studying the correlation coefficient matrix (Table 7) and PCA (Fig. 9). Cluster 1 consisted of elements associated with detrital input (i.e. Zr, Cr, Sr, Sc, Cu, V, mo, Ga, Pb and Be) including quartz, clay and acces- sory minerals such as zircon and rutile commonly representing inorganic constituents (Finkelman, 1995). These elements’ strong positive correla- tion with each other (Table 7) and with ash yield indicated a common terrigenous origin. Cluster 2 was made up of Ba and S, suggesting a strong organic as- sociation. The positive statistical correlation between Ba and S (r = 0.693) also supported an organic association for these elements. Cluster 3 consist- ing of Cd, Zn, Co, Ni and y, indicated a strong association with sulphides such as pyrite (FeS 2), sphalerite (ZnS), chalcopyrite (CuFeS2) and galena (PbS). The occurrence of polymetallic sulphide lodes of PbS, ZnS and CuFeS2 within the Lower Benue trough cannot be ruled out for these ele- ments (Elueze et al., 2009). Conclusion The geochemical characteristics of the ignite seams within the Og- washi-Asaba formation were investigated; the main mineral phases con- tained in the lignite were quartz, feldspar and pyrite. The relatively low silica (SiO 2) and alumina (Al2O3) content suggested very limited detrital input during coal formation. The elements’ geochemical association was examined through sta- tistical correlation and principal component analysis (PCA) and cluster analysis (CA) of major and trace elements. The three components de- rived from FA corresponded to the three major elements’ cluster groups. The first group consisted of clastic rock-derived elements, whilst the sec- ond and third groups consisted of elements having carbonate and sul- phide affinity, respectively. The concentrations for most trace elements were lower than world average values for low-rank coals. 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