The drastic global shifts from the production of technologies with huge carbon emissions to eco-friendly and green technologies have become inevitable because of the numerous adverse effects of carbon emissions on the environment. Notably among the raw materials used in green technologies are the Rare Earth Elements (REEs) including Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb), Lutetium (Lu), Scandium (Sc) and Yttrium (Y). These elements are widely used currently as metal alloys in the production of rechargeable batteries, cell phones, catalysts, ceramics, glass polishing, metallurgy, magnets, and fluorescent lighting among several others
Geologically, the REEs occur in Carbonatites, Alkaline Igneous rocks, ion absorption clay deposits, monazite-xenotime-phosphate deposits, Flouroapatites, some pegmatites, tailings of red muds produced during bauxite mining, and as placer deposits in some laterites due to weathering of REEs rich rocks. All these sources are being explored in the production of REEs globally, with China being the world's largest producer accounting for about 62%, followed by the United States (12.2%), Myanmar (10.3%), Australia (9.9%), India (1.4%) and other countries (4.2%),. The increasing demand for REEs has led researchers into more exploration works on alternative sources to Carbonatites, with phosphates being one of such
These Sokoto phosphates have been studied for their agricultural usage, but no information about the potential for REEs mineralizations has ever been mentioned
The sample locations lie within the Sokoto Basin and are defined by Latitudes 13034'00'' – 13036'00''N and Longitudes 5034'00'' – 5037'00''E. Identified villages within the area include Chimmola, Miyalyako, Gidan Fako, Gaigawo, Dillingo and Bang'nawge (
Ground trotting of the areas was done to physically identify notable places with phosphate occurrences and the locations' geographic coordinates were determined using the Global Position System (G.P.S). A detailed description of each location was done by recording in-situ field characters such as depth of road cut exposures and pits, observable change in color, and lithology down the pits excavated to phosphatic layers. Representative phosphate samples were separated and handpicked from the road cuts/excavated pits at depths ranging from surface level to about 7m and bagged in well-labeled sample bags before being transported to the Laboratory (
Separate sets of pulverized portions of the phosphate samples were also analyzed for total elements contents using Inductively Coupled Plasma Mass Spectrometry (ICPMS) at Activation Laboratory Limited, Canada. Pulverized samples were mixed with excess lithium borate and heated until the mixture formed a homogenous mass then dissolved in 5% HNO3 before analysis. Blank fusion solutions were provided so that calibration standards and blanks could be matric matched while quality assurance and quality control procedures were carried out using standard reference materials: OREAS101b, NCSDC86318, BCR-2, USZ42-2006, REE-1, and W-2b. Detection limits for MnO and TiO2 were 0.001%; SiO2, Al2O3, Fe2O3(T), MgO, CaO, Na2O, K2O, and P2O5 were 0.01%; Bi 0.4 µg/g; Ag, Sb, and Cs was 0.5 µg/g; Sc, Be, Y, Co, Ga, Ge, Nb, Sn, and W was 1 µg/g; Ba, Sr, Zr, Rb, and Mo was 2 µg/g; V, As and Pb was 5 µg/g; Cu 10 µg/g: Cr and Ni were 20 µg/g while Zn was 30 µg/g. Duplicate samples and blank samples were also performed throughout the experiment.
Phosphates occur in all sample locations as nodules in shale with or without gypsum that can be easily handpicked occurring either as disseminated nodules at the surface most likely due to reworking of the environments or at some depth in the excavated pits. Phosphate occurrence at Chimmola village was at a roadcut session along the major road with a profile of about 1m overburden of reddish-brown ferroginized Ironstone overlying about 4m of light-brown Shale intercalated with phosphate nodules which are easily handpicked with a base of about 2m of dark-brown Shale which was almost turning to Marl due to erosion.
Phosphates at Miyal'yako village occur as surface exposure of about 500m by 300m together with Gypsum indicating that the area has been reworked. The area also had a significant amount of mud cracks showing a high level of evaporation while some of the phosphates had been calcified. The logged pit showed about 1.4m reddish-brown Ironstone overburden and about 7m Shale intercalated with clay and Gypsum. Two pits were logged at Gidan Fako with reddish-brown Ironstone overburden of depth ranging between 0.5-0.8m and Shale having intercalation of minute phosphates with a depth of 2-4m while the pit at Gaigawo village was characterized by 0.2m overburden of reddish-brown Ironstone and about 1m of light-brown Shale occurring together with minute phosphate and Gypsum.
Pit logged at Dillingo village had 0.25m overburden of reddish-brown Ironstone and about 2m of shale intercalated with phosphate and Gypsum was absent while Bang'nawge pit had Reddish-brown Ironstone overburden as low as 0.5m followed by Shale intercalated with phosphate. The phosphates are very close to the surface and artisanal mining and sale of phosphate are ongoing in this location (
The mineralogical composition (
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|
|
|
|
|
|
|
|
Chimmola |
P1 |
- |
- |
0.67 |
96.95 |
2.39 |
- |
- |
Miyal’yako |
P2 |
13.13 |
- |
5 |
- |
- |
79.28 |
2.59 |
Gidan Fako |
P3 |
- |
- |
0.44 |
97.89 |
- |
1.67 |
- |
Gaigawo |
P4 |
1.78 |
- |
0.47 |
97.74 |
- |
- |
- |
Gaigawo |
P5 |
7.89 |
2.3 |
8.68 |
79.8 |
- |
1.33 |
- |
Dillingo |
P6 |
9.03 |
- |
10.46 |
42.22 |
1.7 |
36.6 |
- |
Bang’nawge |
P7 |
2.39 |
- |
4.7 |
83.59 |
- |
8.41 |
0.92 |
Min |
1.78 |
2.3 |
0.44 |
42.22 |
1.7 |
1.33 |
0.92 |
|
Max |
13.13 |
2.3 |
10.46 |
97.89 |
2.39 |
79.28 |
2.59 |
|
Mean |
6.84 |
2.3 |
4.35 |
83.03 |
2.05 |
25.46 |
1.76 |
|
SD |
4.26 |
0 |
3.79 |
19.61 |
0.35 |
29.87 |
0.84 |
The results for the major, trace, and rare earth elements analysis are presented in Table 2. The oxides had average concentrations in the descending order of CaO (46.73wt%), P2O5 (11.17wt%), SiO2 (8wt%), Al2O3 (2.23wt%), Fe2O3T(1.84wt%), MgO (0.57wt%), MnO (0.21wt%), TiO2 (0.17wt%), Na2O (0.15wt%) and K2O (0.12wt%). Trace elements had the highest average concentrations of V (38 µg/g) followed by Zn (30 µg/g), U (27.52 µg/g), Cr and Ni (24 µg/g), Cu (12 µg/g), Pb (8.2 µg/g), and Co (4.7 µg/g).
Concentrations of P2O5 were correlated with values of other oxides and trace elements to determine their relationship (
A very high positive correlation between P2O5 and U may be due to the co-precipitation of phosphorus and Uranium during phosphatization and also further confirm the oxidizing state of the environment
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|
SiO2 |
11.95 |
3.35 |
15.71 |
4.25 |
4.56 |
Al2O3 |
2.51 |
1.26 |
4.36 |
1.11 |
1.91 |
Fe2O3T |
1.4 |
1.67 |
1.63 |
1.01 |
3.5 |
MnO |
0.22 |
0.27 |
0.04 |
0.15 |
0.37 |
MgO |
0.42 |
0.33 |
1.54 |
0.3 |
0.24 |
CaO |
44.84 |
50.89 |
39.03 |
50.98 |
47.9 |
Na2O |
0.19 |
0.15 |
0.04 |
0.11 |
0.24 |
K2O |
0.17 |
0.08 |
0.18 |
0.06 |
0.09 |
TiO2 |
0.3 |
0.08 |
0.23 |
0.1 |
0.12 |
P2O5 |
14.42 |
11.04 |
0.29 |
7.8 |
22.29 |
Ni |
20 |
30 |
30 |
10 |
30 |
V |
35 |
29 |
54 |
27 |
45 |
U |
24.6 |
35.8 |
2 |
21.2 |
54 |
Cr |
20 |
10 |
70 |
10 |
10 |
Cu |
10 |
10 |
10 |
10 |
20 |
Pb |
10 |
3 |
5 |
6 |
14 |
Co |
1 |
3 |
2 |
0.5 |
17 |
Zn |
15 |
30 |
50 |
15 |
40 |
The phosphate deposits in the study area occur within the Sokoto Basin, Northwestern Nigeria as nodular intercalations with Shale in the presence/absence of Gypsum at a depth between 0.25 – 7m, are mostly close to the surface and can be easily handpicked and separated from the Shale. Mineralogical and geochemical data were used to better understand the constituents of these samples and ultimately their REEs mineralization potentials.
Their mineralogical composition includes Fluoroapatite, Calcite, Smectite, Quartz, Kaolinite, Goethite, and Palygoskite with the highest concentration of Fluorapatite at Miyal'yako (79.28%) followed by Dillingo (36.60%) and the lowest at Gaigawo (1.33%). Interpretation of the relationship between P2O5 with other major oxides and trace elements revealed that phosphates are the primary type and are typically formed in an oxidizing environment with fluctuating pH in the presence of calcite or gypsum. Low Uranium contents and other similarities with reports from the Ogun phosphates may point to the fact that both deposits were formed in similar depositional.
The high concentrations of fluoroapatites in these two locations out of the seven locations studied are significant as fluoroapatites are known to be rich in REEs and thus a pointer to the possibility of these locations being enriched in REEs. Further studies should be done in these two locations particularly mineral chemistry to ascertain the exact concentrations of their REEs to provide more information for REE exploitation in Nigeria.
The authors acknowledge the funding gotten from the African Union Commission through Pan African University Institute for Life and Earth Sciences (including Health and Agriculture) (PAULESI), University of Ibadan, Nigeria for the purpose of this research. The writing skills acquired during the Early Career Researchers' development training and mentoring workshops (WW21100104) sponsored by the British Academy was highly helpful in the drafting of this manuscript. The authors also appreciate the help of Mr Mark and Mr Gada during the fieldwork of this research and critical contributions by the reviewers towards improving the quality of this article is greatly appreciated.