The sedimentology and geochemistry (major and trace element compositions) of lignite and argillite (carbonaceous shale and claystone) sequences in a Basin in Bali Nyonga, west of the Bamenda Mountain have been investigated to determine their sequences and the prevailing environmental conditions which control their formation. Ten representative samples were obtained fromtrenches, pits, and river and stream valleys in the study area. These samples and their ashes were subsequently examined using X-ray fluorescence spectrometry (XRF), inductively coupled plasma spectrometry (ICP), and X-ray diffraction analysis (XRD). The geochemical results revealed that thelithophile, chalcophile and siderophile elements were dominantly epigenetic in origin, mainly from detrital sources supported by high silica and alumina concentrations in all the samples. The mineral phases identified were quartz, kaolinite, illite, pyrite, hematite, and minor phases of feldspars,pseudorutile. The relatively high silica (54.10 wt%) and alumina contents (27.77 wt%) in these samples can be explained by high detrital input during peat formation. The low contents of MgO and CaO in the analysed samples agree very much with the continental setting of the peat formating basin. A clayey microband derived from alkaline volcanic ashes was identified in the lignite and the dominant composition of these clayey microbands was mixed-layer clay minerals of illite and kaolinte, which were interlayered with organic bands. The modes of occurrence of ash bands indicated that the volcanic activities were characterized by multiple eruptions, short time interval and small scale for each eruption during peat accumulation. The ratios of redox-sensitive traceelement (V/Cr versus Ni/Co and V/V + Ni versus Ni/Co), Sr/Ba, and major oxides ratio (CaO + MgO + Fe 2 O 3 )/(SiO 2 + Al 2 O 3 ) from the analysed samples from Bali Nyonga indicate a terrestrial, reductive (oxic), littoral to brackish water environmental conditions which are characteristics of paludal-lacustrine basin that is filled by Tertiary volcanic materials.
Lithologically, this basin consists of a sequence of conglomerate, fine to meduim-grained sandstone, argillite (claystone and carbonaceous shale), beds within which are continental lignite seam intercalations. Coal is a highly heterogeneous organoclastic sedimentary rock consisting of a variety of humified plant-derived organic matter or debris which is deposited in layers, undergoes coalification and may have vertical and lateral facies changes [
The sedimentological and geochemical characteristics of lignite-argillite sequences in the study area are still unclear. It is at this background that this research work is to carry out the sedimentology and geochemistry (chemical composition of trace and major elements) of lignite and argillite samples from a basin in Bali-Nyonga. In this way, the prevailing geochemical environmental conditions, depositional and a stratigraphical model for the rocks will be designed and this will also aid in understanding their relationship with the volcanic rocks in this basin.
The study area lies between longitude 9˚59'9"E to 10˚0'512"E and latitude 5˚52'865"N to 5˚53'882"N and covers a surface area of about 4.151 square kilometers (
very important part of the continental sector of the CVL and lies mid-way between Bambouto in thesouth east and the Oku massif in the northwest
The Bamenda Mountain has an altitude of 2621 m and is characterized by two elliptical calderas: Santa-Mbu caldera (6 km by 4 km) and Lefo caldera (4 km by 3 km) wide [
Some reconnaissance studies were carried out between the periods 1924 to 1936 on lignite during the construction of the Dschang-Bana road [
Two main types of sampling were done: sub-surface rock sampling in pits and trenches and surface rock sampling on exposure and outcrops along rivers and stream valleys. A bed by bed sampling of the section in pits at Boh Muyangka (BM), Boh Muyangka-Banda (BMB) and Boh Etoma II (BEII) was carried out to collect fresh representative bulk samples from the top, middle and bottom parts of the lignite seams and argillites (carbonaceous shale, claystone) beds in these localities (
Ten air-dried representative samples of lignite asnd argillite were carefully selected, packaged, labelled; and taken to the Mission de Promotion de Materiaux Locaux (MIPROMALO) laboratory in Yaounde for crushing using a jaw crusher. X-ray fluorescence (XRF), ICP and XRD analytical techniques were used to analyse for major, trace elements and mineral phases respectively. To achieve the X-ray fluorescence (XRF) analysis, the crushed samples were again powdered in an agate mortar and 0.25 g and taken to the Scientific Services (CC) Consulting Analytical Laboratory in South Africa. Fused glass beads made from the powdered samples were compacted within lithium metaborate or tetraborate for major elements analysis. The specimens were bombarded with high-energy X-rays which emits secondary radiation, characteristic of the elements present. A suite of major elements SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, MnO, TiO2 and P2O5 (
Selected trace elements were analysed by fusion with inductively-coupled plasma (ICP) spectrometry technique in pulse counting mode (three points per peak) in the Consulting Analytical Laboratory in South Africa. Samples were mixed with a flux of lithium metaborate and lithium tetraborate and fused in an induction furnace.
Sample | Fe2O3 % | MnO % | Cr2O3 % | V2O5% | TiO2 % | CaO % | K2O % | P2O5 % | SiO2 % | Al2O3 % | MgO % | Na2O % | LOI % | Total |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
BMBS-2 | 9.99 | 0.19 | 0.01 | 0.01 | 0.78 | 0.41 | 2.00 | 0.25 | 45.36 | 22.84 | 0.00 | 0.47 | 16.78 | 99.07 |
BE11T-2 | 1.45 | 0.01 | 0.01 | 0.01 | 0.90 | 0.16 | 1.49 | 0.13 | 48.64 | 20.81 | 0.00 | 0.25 | 25.08 | 98.93 |
BE11T-6 | 1.13 | 0.01 | 0.01 | 0.00 | 0.87 | 0.18 | 2.03 | 0.11 | 50.44 | 18.83 | 0.00 | 0.85 | 24.66 | 99.12 |
BMS-2 | 5.28 | 0.06 | 0.00 | 0.01 | 1.38 | 0.44 | 1.77 | 0.27 | 54.10 | 21.94 | 0.07 | 0.22 | 13.52 | 99.05 |
BN-3 | 10.54 | 0.26 | 0.00 | 0.00 | 1.58 | 1.94 | 2.02 | 1.40 | 47.56 | 21.55 | 0.19 | 0.76 | 12.01 | 99.80 |
BMBPSb-1 | 3.24 | 0.03 | 0.01 | 0.02 | 0.96 | 0.18 | 1.26 | 0.27 | 44.17 | 27.77 | 0.00 | 0.00 | 20.51 | 98.41 |
BE11-3 | 3.61 | 0.03 | 0.02 | 0.04 | 3.00 | 0.12 | 0.55 | 0.29 | 37.09 | 26.81 | 0.01 | 0.00 | 27.84 | 99.42 |
BE11S-8 | 11.80 | 0.13 | 0.00 | 0.00 | 0.62 | 0.54 | 1.50 | 0.21 | 44.02 | 18.84 | 0.19 | 0.49 | 20.78 | 99.12 |
BMPSb-6 | 10.84 | 0.14 | 0.00 | 0.01 | 0.85 | 0.60 | 1.58 | 0.34 | 46.04 | 21.14 | 0.14 | 0.41 | 16.14 | 98.25 |
BMPSb-10 | 12.81 | 0.14 | 0.01 | 0.00 | 0.65 | 0.55 | 1.39 | 0.23 | 43.10 | 18.97 | 0.20 | 0.39 | 21.05 | 99.49 |
The molten melt was immediately poured into a 5% nitric acid solution containing an internal standard andmixed continuously until becoming completely dissolved for about 30 minutes. 0.25 g sample was digested with acids for trace element analysis, beginning with hydrofluoric acid (HF), followed by a mixture of nitric (HNO3) and perchloric acid (HClO4), heated using precise programmer-controlled heating at a temperature range of 6000˚C - 8000˚C in several ramping and holding cycles thus drying the samples. After dryness was attained, samples were brought back into solution using hydrochloric acid. The samples were dissociates in an argon plasma and a large number of atomic and ionic spectral lines are excited. The spectral lines are detected by a range of photomultipliers, which were compared with calibration lines, and their intensities converted into concentrations. A suite of trace elements (As, Ba, Be, Cd, Co, Cr, Cu, La, Mo, S, Ce, Pb, Sr, Y, V, Zn, Ni, Th, W and Tl (
The powder samples for X-ray diffraction (XRD) were analysis were in Tshwane University of Technology Pretoria South Africa. 0.5 g of powder was mixed with approximately 0.3 cm3 distilled water, pipetted onto a glass slide and allowed to dry. Analysis of specimens took place on a Siemens D5000 diffractometer using a Cu anode X-ray tube at 40 kV, 30 mA and a diffracted-beam graphite monochromator. A step size of 0.04˚ and a count time of 4 secs step gave optimum results in terms of time per scan and peak resolution. Samples lignite and argillites were scanned from 2˚ 2θ to allow analysis of any mixed layer clays. All diffractograms were scaled to give 750 counts for a full-scale peak, using Siemens “Diffrac-At” software. The mineral phases were identified by peak matching using Siemens search/match software. The presence of quartz, pyrite, hematite, kaolinite, illite, feldspar etc. was confirmed through peak breakdown after heating samples for 450˚C and 550˚C respectively.
The chemical compositions of lignite and argillite samples are presented in (
Sample # | As (ppm) | Ba (ppm) | Be (ppm) | Cd (ppm) | Ce (ppm) | Co (ppm) | Cr (ppm) | Cu (ppm) | La (ppm) | Mo (ppm) | Ni (ppm) |
---|---|---|---|---|---|---|---|---|---|---|---|
BEIIT-2 | 4.1 | 221 | 13.5 | 2.8 | 529 | 10 | 7 | 15 | 406 | 2 | 7 |
BMPSb-10 | 4.0 | 209 | 2.0 | 4.2 | 253 | 5 | 131 | 22 | 85 | 12 | 25 |
BEIII-3 | 3.8 | 240 | 9.5 | 2.2 | 655 | 6 | 13 | 20 | 400 | 2 | 9 |
BEIIT-6 | 4.8 | 218 | 5.5 | 5.4 | 446 | 6 | 1 | 1 | 248 | 3 | 5 |
BEIIS-8 | 3.8 | 238 | 7.8 | 6.2 | 426 | 6 | 1 | 0 | 287 | 3 | 4 |
BN-3 | 3.8 | 73 | 2.6 | 2.8 | 381 | 6 | 15 | 12 | 311 | 3 | 7 |
BMBS-2 | 8.6 | 87 | 2.8 | 4.1 | 667 | 4 | 2 | 4 | 398 | 15 | 5 |
BMS-2 | 6.2 | 84 | 2.8 | 3.3 | 454 | 9 | 10 | 10 | 352 | 4 | 10 |
BMBPSb-1 | 5.8 | 67 | 3.0 | 2.2 | 525 | 0 | 3 | 2 | 355 | 3 | 2 |
BMPSb-6 | 6.9 | 107 | 6.2 | 2.5 | 537 | 4 | 11 | 8 | 592 | 3 | 7 |
Sample # | Th (ppm) | Tl (ppm) | V (ppm) | W (ppm) | S (ppm) | Se (ppm) | Sr (ppm) | Pb (ppm) | Y (ppm) | P (ppm) | Zn (ppm) |
BEIIT-2 | 19 | 17 | 41 | 9 | 2340 | 2 | 58 | 20 | 268 | 240 | 651 |
BMPSb-10 | 26 | 9 | 226 | 1 | 1182 | 36 | 60 | 24 | 42 | 480 | 75 |
BEIII-3 | 53 | 21 | 47 | 4 | 2279 | 3 | 48 | 29 | 265 | 409 | 316 |
BEIIT-6 | 25 | 17 | 22 | 2 | 1582 | 8 | 57 | 19 | 165 | 894 | 238 |
BEIIS-8 | 5 | 16 | 29 | 2 | 1216 | 8 | 55 | 20 | 168 | 699 | 235 |
BN-3 | 15 | 13 | 27 | 5 | 1234 | 4 | 42 | 32 | 217 | 201 | 397 |
BMBS-2 | 43 | 24 | 25 | 18 | 7184 | 5 | 76 | 28 | 198 | 216 | 1354 |
BMS-2 | 25 | 16 | 21 | 4 | 2445 | 3 | 55 | 31 | 219 | 251 | 191 |
BMBPSb-1 | 47 | 19 | 4 | 4 | 136 | 5 | 70 | 19 | 164 | 217 | 333 |
BMPSb-6 | 14 | 17 | 25 | 5 | 935 | 3 | 70 | 26 | 393 | 243 | 292 |
variables whose variance is large using [
Elements | Factor 1 | Factor 2 | Factor 3 |
---|---|---|---|
Al2O3 | 0.111 | −0.780 | −0.452 |
CaO | 0.899 | 0.133 | 0.352 |
Fe2O3 | 0.311 | 0.094 | 0.904 |
K2O | 0.413 | 0.881 | −0.055 |
LOI | −0.762 | −0.351 | −0.169 |
MgO | 0.346 | 0.141 | 0.842 |
MnO | 0.708 | 0.137 | 0.620 |
Na2O | 0.338 | 0.719 | 0.220 |
P2O5 | 0.948 | −0.084 | 0.142 |
SiO2 | 0.215 | 0.807 | −0.415 |
TiO2 | 0.241 | −0.776 | −0.301 |
Elements | Component 1 | Component 2 | Component 3 | Component4 | Component 5 | Component 6 |
---|---|---|---|---|---|---|
As | −0.177 | 0.660 | 0.254 | −0.481 | −0.407 | 0.000 |
Ba | 0.212 | −0.159 | −0.273 | 0.890 | 0.059 | −0.030 |
Be | −0.218 | −0.026 | 0.336 | 0.902 | 0.082 | −0.046 |
Cd | −0.052 | 0.107 | −0.768 | 0.212 | −0.151 | −0.494 |
Ce | −0.513 | 0.531 | 0.411 | 0.190 | −0.082 | 0.468 |
Co | 0.044 | 0.105 | 0.077 | 0.462 | 0.664 | −0.449 |
Cr | 0.969 | −0.144 | −0.183 | −0.063 | −0.024 | −0.007 |
Cu | 0.743 | −0.074 | 0.316 | 0.206 | 0.488 | 0.243 |
La | −0.496 | 0.210 | 0.822 | −0.003 | −0.124 | −0.012 |
Mo | 0.531 | 0.704 | −0.306 | −0.290 | −0.188 | 0.038 |
Ni | 0.966 | −0.084 | −0.082 | −0.011 | 0.171 | −0.047 |
Pb | 0.121 | 0.250 | 0.311 | −0.506 | 0.700 | 0.069 |
S | −0.056 | 0.980 | −0.080 | −0.024 | 0.128 | 0.083 |
Se | 0.873 | −0.132 | −0.425 | −0.031 | −0.178 | −0.053 |
Sr | 0.033 | 0.485 | 0.158 | −0.179 | −0.827 | 0.040 |
Th | 0.017 | 0.260 | 0.014 | −0.071 | −0.066 | 0.945 |
Tl | −0.600 | 0.579 | 0.180 | 0.118 | −0.168 | 0.452 |
V | 0.966 | −0.077 | −0.219 | 0.091 | −0.010 | −0.017 |
W | −0.176 | 0.924 | 0.180 | −0.075 | −0.035 | 0.082 |
Y | −0.426 | 0.074 | 0.859 | 0.098 | 0.061 | −0.131 |
Zn | −0.223 | 0.912 | 0.056 | −0.025 | −0.064 | 0.114 |
distribution based on [
Lignite in most locations in the study area occurred in thin to thickly parallel seams alternating with dirt partings of either fine to medium-grained sandstone or clay, ranging in thickness from 0.9 cm to as thick as 25 cm (
Fe2O3 | MnO | TiO2 | CaO | K2O | P2O5 | SiO2 | Al2O3 | MgO | Na2O | LOI % | ||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Fe2O3 % | 1.000 | |||||||||||
MnO % | 0.845 | 1.000 | ||||||||||
TiO2 % | −0.312 | −0.162 | 1.000 | |||||||||
CaO % | 0.552 | 0.833 | 0.026 | 1.000 | ||||||||
K2O % | 0.191 | 0.437 | −0.601 | 0.437 | 1.000 | |||||||
P2O5 % | 0.345 | 0.714 | 0.258 | 0.951 | 0.286 | 1.000 | ||||||
SiO2 % | −0.213 | −0.053 | −0.434 | 0.146 | 0.766 | 0.046 | 1.000 | |||||
Al2O3 % | −0.372 | −0.235 | 0.593 | −0.220 | −0.566 | 0.048 | −0.441 | 1.000 | ||||
MgO % | 0.833 | 0.672 | −0.216 | 0.676 | 0.130 | 0.480 | −0.067 | −0.513 | 1.000 | |||
Na2O % | 0.278 | 0.461 | −0.394 | 0.523 | 0.787 | 0.371 | 0.412 | −0.734 | 0.352 | 1.000 | ||
LOI % | −0.544 | −0.726 | 0.252 | −0.702 | −0.618 | −0.600 | −0.486 | 0.128 | −0.463 | −0.293 | 1.000 | |
As | Ba | Be | Cd | Ce | Co | Cr | Cu | La | Mo | Ni | Pb | S | Se | Sr | Th | Tl | V | W | Y | Zn | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
As | 1.000 | ||||||||||||||||||||
Ba | −0.639 | 1.000 | |||||||||||||||||||
Be | −0.386 | 0.651 | 1.000 | ||||||||||||||||||
Cd | −0.119 | 0.445 | −0.077 | 1.000 | |||||||||||||||||
Ce | 0.508 | −0.120 | 0.366 | −0.367 | 1.000 | ||||||||||||||||
Co | −0.344 | 0.401 | 0.500 | 0.140 | −0.125 | 1.000 | |||||||||||||||
Cr | −0.281 | 0.224 | −0.327 | 0.071 | −0.661 | −0.040 | 1.000 | ||||||||||||||
Cu | −0.410 | 0.275 | 0.167 | −0.456 | −0.186 | 0.357 | 0.647 | 1.000 | |||||||||||||
La | 0.491 | −0.342 | 0.357 | −0.522 | 0.728 | −0.044 | −0.656 | −0.194 | 1.000 | ||||||||||||
Mo | 0.500 | −0.173 | −0.517 | 0.245 | −0.038 | −0.229 | 0.495 | 0.102 | −0.333 | 1.000 | |||||||||||
Ni | −0.274 | 0.261 | −0.245 | 0.026 | −0.597 | 0.186 | 0.958 | 0.760 | −0.578 | 0.445 | 1.000 | ||||||||||
Pb | 0.198 | −0.449 | −0.352 | −0.390 | 0.118 | 0.207 | 0.043 | 0.411 | 0.200 | 0.181 | 0.209 | 1.000 | |||||||||
S | 0.615 | −0.147 | −0.064 | 0.121 | 0.550 | 0.139 | −0.183 | −0.066 | 0.158 | 0.672 | −0.101 | 0.334 | 1.000 | ||||||||
Se | −0.265 | 0.300 | −0.374 | 0.329 | −0.701 | −0.136 | 0.952 | 0.416 | −0.778 | 0.546 | 0.862 | −0.156 | −0.169 | 1.000 | |||||||
Sr | 0.805 | −0.334 | −0.199 | −0.011 | 0.357 | −0.536 | −0.041 | −0.394 | 0.310 | 0.511 | −0.150 | −0.324 | 0.367 | 0.040 | 1.000 | ||||||
Th | 0.277 | −0.133 | −0.131 | −0.448 | 0.573 | −0.418 | −0.029 | 0.175 | 0.050 | 0.247 | −0.040 | 0.120 | 0.334 | −0.066 | 0.255 | 1.000 | |||||
Tl | 0.575 | −0.153 | 0.232 | −0.174 | 0.961 | −0.211 | −0.705 | −0.389 | 0.600 | 0.053 | −0.675 | 0.020 | 0.611 | −0.669 | 0.430 | 0.583 | 1.000 | ||||
V | −0.325 | 0.366 | −0.201 | 0.148 | −0.611 | 0.040 | 0.985 | 0.663 | −0.667 | 0.506 | 0.954 | −0.012 | −0.117 | 0.950 | −0.056 | −0.035 | −0.656 | 1.000 | |||
W | 0.679 | −0.354 | 0.029 | −0.148 | 0.646 | −0.011 | −0.329 | −0.139 | 0.410 | 0.531 | −0.307 | 0.233 | 0.886 | −0.356 | 0.505 | 0.297 | 0.666 | −0.290 | 1.000 | ||
Y | 0.281 | −0.194 | 0.457 | −0.502 | 0.587 | 0.160 | −0.585 | −0.041 | 0.950 | −0.470 | −0.463 | 0.265 | 0.032 | −0.740 | 0.067 | −0.113 | 0.416 | −0.586 | 0.245 | 1.000 | |
Zn | 0.617 | −0.284 | 0.042 | −0.053 | 0.638 | −0.061 | −0.350 | −0.206 | 0.334 | 0.532 | −0.357 | 0.145 | 0.881 | −0.337 | 0.482 | 0.309 | 0.684 | −0.299 | 0.985 | 0.163 | 1.000 |
accumulation. Dark gray to black argillite (carbonaceous shale) with significant amount of vegetal materials in the form of disseminated particles or flakes occurred as laminae and dirt partings in most of the lignite seams in this area; and in some locations, they were found embedded as fragmental clasts within fine to medium-grained volcanic ash.
A common feature observed on the field is the splitting of lignite seams through rhythmic deposition in sequence, especially conspicuous in samples (
seen interrupted by dirt partings in this basin, in locations such as BEIIT, BMP, and BMBP. A seam when trace vertically along the trench or pit, splits in to several thinner seams and lenses separated by sediment partings such as fine to medium-grained sandstone and more common are claystone from altered volcanic ash/tuffs. This splitting reflects the instability of the basin of deposition, with the deposition of volcanic air-fall deposits (ash/ tuffs) over part of the basin and the cessation of plant growth during its presence. It may also suggest a temporal change in the physical conditions, perhaps a temporal flooding with sediment-laden water in a shallow quiet water environment protected from intense wave agitation and abundant volcanic ash fall deposits [
The common interlayering of carbonaceous shale facies of BMP with sandstone, and volcanogenic sediments and complete churning of the upper portion of the interface of the sediments by burrowing organisms (
A sequence of lignite seams and argillite beds in BEIIT had polygonal desiccation cracks of varying dimen- sions, averagely 64 cm in depth. These cracks are filled with the overlying greyish brown fine-grained pelitic sandstone, forming a neptunean dykes. This regular pattern of the desiccation cracks (
The volcanic ash fall that occurred as dirt parting in the lignite seams, were intensively altered to claystone; were from pyroclastic current that emanate from the surrounding calderas such as the Santa-Mbu caldera, (6km by 4 km), the Lefo caldera (4 km by 3 km), and the Bamboutos caldera [
The deeper water interpretation for the facies unit in BMP is further supported by the absence of plant roots, desiccation features that are common in the lignite beds of the Boh Etoma II (BEII) facies and the upper portion of Boh Muyangka (BM) facies unit. The fine-grained size of rocks in BMP and BMBP lithologic units indicates deep water environments [
Clayey microbands derived from alkaline volcanic ashes were identified in the lignite. The dominant compositions of these clayey microbands were mixed-layer clay minerals of illite and kaolinte, which were interlayered with organic bands.Lignite seams alternating with altered fine to medium-grained volcanic ash/claystone laminae (
X-ray diffractograms from lignite and argillite samples from various localities are shown in (
margin. Some quartz might have been introduced to the peat by mixing of the silica-rich water with the volcanic sediments already existing on the swamp floor in the basin. The mixing of peat and swamp floor sediment may have arisen somehow from a combination of bioturbation by organisms and contemporaneous clastic deposition early in the history of peat accumulation. Some detrital particles may also have been blown by winds into the swamp, including air-borne material of pyroclastic origin which may penetrate more extensively into the peat-forming environment. Such materials may include wide spread deposits of altered volcanic tuffs/ash such as koalinite, illite, quartz and feldspars. Others minerals in the lignite and argillite samples may occur as biogenic mineral particles such as amorphous silica that are relatively soluble in water, that may result directly from biological activity in the peat swamp (including skeletal fragments from diatoms, molluscs and other organisms). Some from minerals formed within living plant tissues, and others possibly deposited in the peat swamp as faecal pellets.
A total of 10 samples were analysed for a whole suite of elements, in which ten (10) major elements and twenty one (21) trace elements were selected and are reported here and a complete geochemical data for the suite of trace elements can be obtained from the author. The geochemical data are summarised in (
The moderately high concentration of SiO2 (37.09 - 54.10 wt%), Al2O3 (18.83 - 27.77 wt%), and Fe2O3 (1.13 - 12.81 wt%), is due to quartz, feldspar, clay minerals, and pyrite that constitutes the main mineral phase in the lignite and argillite samples. The high concentrations of SiO2 (˃51 wt%), and Al2O3 content (˃25 wt%) compared with the concentrations of other major elements in the analysed samples, suggest a high detrital input from the surrounding of the basin during peat formation. This quartz-rich lignite of Bali-Nyonga occurs beneath intra-seam ash and claystone bands apparently of pyroclastic origin, with the possibility that the quartz may represent silica released by the complete alteration of volcanic glass, feldspars and other minerals in the pyroclastic sediments, due to interaction of the tuff/ ash with the water of the peat swamp environment [
Studies of major elements through factor analysis provided vital statistical information on the grouping of the elements and the processes responsible for their formation. The first factor shows a high positive loading for CaO, MnO, P2O5, suggesting input from carbonate-phosphate rich sources. Factor two showed a high positive loading for K2O, Na2O, SiO2 and a high negative loading for Al2O3 and TiO2, and the oxides of Si, Al, K, Na and Ti represent minerals phases such as quartz, feldspar, clay, rutile which are common detrital minerals in the lignite [
Cluster I, (
The poor correlation of sulphur (S) with Cu (r = 0.158), Cd (r = 0.121), Pb (r = 0.334), Sr (r = 0.04), Y (r = 0.032), Be (r = −0.064), V (r = −0.117), Co (r = 0.139), Cr (r = −0.183), Ni (r = −0.101) ; indicates that the sulphur was not only present in its sulphide form but also in organic form [
and Cu have a sulphide affinity and are organically associated because they may be extracted from the soil by photosynthesising plant for their biological needs and they are easily mobilised in the soils and aqueous environment. Figures 9(e)-(g) illustrate positive trends and therefore indicate that these elements are from the same source and have a sulphide affinity.
The principal component 1 (PC1) showed a positive loading for Cr, Cu, Ni, Se, V, Mo (
Cluster 1 (
Redox-sensitive trace element ratio (Ni/Co, V/Cr and V/(V + Ni) presented in the cross-box (
The sedimentology and geochemical characteristics of the lignite and argillite sequences within a Basin in Bali Nyonga, West of the Bamenda Mountain were investigated, and their lithological sequences together with the prevailing environmental conditions which controlled their formation were determined. Clayey microbands derived from alkaline volcanic ashes were identified in lignite and were examined to be mixed-layered clay minerals interbedded with organic bands, and their modes of occurrence indicated that the volcanic activities that deposited them were characterized by multiple eruptions, short time interval and small scale for each eruption during peat accumulation. The organic matter for the formation of the lignite deposits in Bali Nyonga, accumulated in place and this therefore indicates an autochthonous (growth in place) lignite deposit.
The weathering of alkaline volcanic ash/tuffs, mafic and felsic igneous rocks was considered to be the dominant factor for the enrichment of detrital inorganic input fraction, and this reflects the geology of the surrounding rocks to a high degree. The relatively high silica (54.10 wt%) and alumina contents (27.77 wt%) in these samples indicate a high detrital input during peat formation. The low contents of MgO and CaO on the other hand agree very much with the continental setting of the peat forming basin. The main mineral phases contained in the lignite and argillites were quartz, pyrite, hematite, aluminosilicate or clay minerals (kaolinite and illite), feldspar, and minor phase of pseudorutile. The study also reveals that trace elements such as Co, Pb, As, Se, Ni, Cu, and Zn have sulfide affinity and that the aluminosilicate and sulphide minerals might have been responsible for the occurrence and distribution of most trace elements in the lignite and argillites of Bali Nyonga.
Elements’ geochemical associations were examined through principal component analysis (PCA), factor analysis (FA), cluster (CA) and statistical correlation of major and trace elements. Factors 1 and 3 for major oxides are bound to the organic rich facies and factor 2 consists of clastic rock-derived elements which are bound to clay facies. The PCA for trace elements corresponds to the cluster groups with the first and third groups consisting of elements having a high sulfide affinity, and second group consisting mainly of clastic rock-derived elements with some of the elements having a high affinity for sulfides, and elements of these groups are either epigenetic or syngenetic in origin.
The ratios of environmental redox-sensitive elements (trace elements, Sr/Ba, and (CaO + MgO + Fe2O3)/ (SiO2 + Al2O3)) analysed from the Bali Nyonga lignites and argillites samples indicate a terrestrial and littoral- brackish water reducing environmental conditions which are characteristics of paludal-lacustrine basin that is filled by Tertiary volcanic materials.
This article is part of a M.Sc. thesis on sedimentology and geochemistry of lignite-argillite by the author. We are grateful for the support and contributions from the Department of Geology and Chemistry in the University of Buea. I also appreciate Dr. Mbom Divine and Dr. Che Vivian for their assistance in this research. The editorial comments of the editor are highly appreciated.
Roger NgongNgia,Christopher M.Agyingi,JosephaFoba-Tado,Germain M. M.Mboudou,AnitaNshukwi,Victorine N.Beckley, (2015) Sedimentology and Geochemical Evaluation of Lignite-Argillite Sequences in a Named Basin in Bali Nyonga, Northwest, Cameroon. International Journal of Geosciences,06,917-937. doi: 10.4236/ijg.2015.68074