Rare Metals (Ta-Nb-Sn) Mineralization Potential of Pegmatites of Igangan Area, Southwestern Nigeria

The study was carried out to determine the rare metal mineralization potential of some pegmatites associated with metasediments in the Igangan 240 NW sheet. Geological mapping on a scale of 1:50,000 revealed the pegmatites intrude metasediments and geochemical analysis for major, trace and rare earth elements were carried out using ICP MS/AES. Petrographic studies reveal a mineral assemblage of quartz, microcline and tourmaline; SEM studies revealed garnet and tourmaline to be close to the spessartine end-member and schorl respectively with albite occurring as the dominant plagioclase feldspar in the pegmatites. Result of geochemical analysis revealed SiO2 with an average of 73.91% in the whole rock pegmatite Al2O3 with an average of 13.93%, and average concentration of 0.57%, 4.3% and 4.77% for CaO, Na2O and K2O respectively. It also revealed average concentration of 29 ppm, 153 ppm, 30 ppm, 118 ppm and 129 ppm for W, Li, Ta, Nb and Sn in the mica respectively which is above the average values in the whole rock, felspars and tourmaline extracts. REE abundance in the whole rock pegmatites is low to moderate with ∑REE varying between 8 220 ppm, 2 23 ppm in feldspars and 3 32 ppm in mica signifying no form of REE enrichment. Geochemical analysis results and trace elemental plots such as K/Rb vs. Rb, Ta vs. Ga, Ta vs. Cs were used to assess rare metal mineralization and it revealed the pegmatites have low level of rare metal and rare earth element mineralization with average k/Rb values of 177 indicative of low fractionation levels in the pegmatites.


Introduction
Pegmatites are economically important sources of ceramic materials and high technology metals such as Rb, Cs, Be and the high field strength elements [1]. Its genesis is a controversial topic based on the different ideas and schools of thought on its genesis. The entirety of pegmatite genesis is a continuum of increasing fractionation and increasing spatial order via segregation of internal mineral assemblages [2].
Granitic pegmatites are an important part of granitic intrusions in orogenic belts and are characterized by strong enrichment in incompatible elements such as Rb, Cs, Li, Be and Sn. They are also associated with Nb-Ta-Sn-W mineralisation and the petrogenesis of pegmatites can be studied from the feldspar, mica and tourmaline which are sensitive indicators of magmatic and post magmatic events responsible for pegmatite evolution [3].
Various studies have been carried out on pegmatites in Nigeria by [4] [5] [6]. In the southwestern part of Nigeria, pegmatites are hosted within varieties of rocks of metamorphic and igneous origin where they intrude the older lithologies discordantly [7] [8] [9] [10] but there is paucity of information on pegmatites in Igangan sheet 240 NW.
Recent studies in Nigeria by [11] and [12] revealed that Nigeria has a pegmatite belt which is beyond the confines of the 400 km NE-SW trending belt from Abeokuta, southwestern Nigeria to North Central Nigeria which previous authors thought they extend [13] [14]. [15] delineated the occurrence of new rare element pegmatites in Nigeria thereby revealing the possibility of more pegmatite bodies than previously thought. Geochronological studies have been carried out on Nigerian pegmatites with recent studies by [16] suggesting an age of 709 Ma for emplacement of pegmatites in Ede, southwestern Nigeria based on U-Pb zircon geochronology.

Regional Geological Setting
The study area falls within the basement complex of southwestern Nigeria, it lies within the Pan-African belt which resulted from the collision of the passive continental margin of the West-African Craton and the active margin of the Tuareg shield during the Pan African tectonic event (Figure 1, [17] [18] [19] [20]).
The Nigerian Precambrian basement complex bears the imprint of the Liberian (ca 2500 Ma), Eburnean (ca 2000 Ma) and Pan-African (ca 600 Ma) tectonic events; it is polycyclic and can be broadly grouped into the migmatite gneiss complex, the schists belts and the older granite suite (Figure 2

Local Geological Setting
The lithologies in the study areas include porphyritic granites, schists, biotite granites, granite gneisses as well as charnockites and amphibolites. These rocks are intruded by dolerite, granitic dykes, pegmatites quartz veins and aplites of varying length and thickness.
In the study area, the northwestern and northeastern flank is predominantly

Method of Study
Field work involved systematic geological mapping on the scale of 1:50,000 to delineate the geological units in an area extending from 3˚00' -3˚15' and 7˚45' -

Mineralogy and Petrography
Hand The predominant minor minerals are garnet and tourmaline. Garnet is very rare in the pegmatites and was only observed at the only pegmatite/schist contact while tourmaline is also rare but more common than garnet, it is black in color and presumed to be schorl.
Garnet is euhedral with inclusions of apatite ( Figure 5, Figure 6, Table 1) and composition close to the spessartine end-member with a formula; ( )

Tourmaline
Analysis of tourmaline did not reveal the crystal structures of the tourmaline grains ( Figure 8)

Geochemistry of Pegmatites
In

Major Element Variation
CaO-Na 2 O-K 2 O diagram reveals a wide distribution of samples along the Na 2 O-K 2 O sideline, coupled with the very low Ca contents of the pegmatites.
The samples show a significant departure from the Calc-alkaline trend of [31] as well as the Trondhjemitic trend of [32]. The plot shows that the samples are mainly potassium rich with a few exceptions showing high Na 2 O and CaO composition due to the difference in feldspar type and composition ( Figure 10).

Trace Elements and Mineralization Potential
Trace element data shows an enrichment of W, Li, Ta, Nb and Sn in the mica with an average of 29 ppm, 153 ppm, 30 ppm, 118 ppm and 128 ppm respectively which is above the average values in the whole rock, felspars and tourmaline extracts (Table 5). This prefential enrichment of some elements in the muscovite extracts can be explained by the ability of the muscovite to accommodate wide range of substitutions at various sites in its crystal structure [33] and [34]. K/Rb vs. Cs, Ta vs. Cs, Ta vs. Ga, Ta vs. Cs, Ta vs. K/Cs and Ta vs. Cs + Rb plots also reveals a low level of rare earth element mineralization ( Figures  11(a)-(d)), based on the line of mineralization proposed by [35] and [36].  (Table 7) with few samples having K/Rb ratios less than 100 ppm which is generally accepted as indicative of mineralization [37]. Plots of Cs vs. K/Rb and Ba vs. K/Rb show the pegmatites are non mineralized and belong to the muscovite class ( Figure 11(f), Figure 11(g)) and they have Nb/Ta ratios of 1 -19 (Table 7). [38] inferred that rare metal pegmatites are typically the most distant pegmatites from their parent granites while [39] reported that classical pegmatite O. G. Olisa et al.   The rare-earth elements abundance of bulk rock and mineral extracts (muscovite and feldspar) of pegmatites from the study area are presented in Table 5.
Generally, the REE pattern of the bulk pegmatite samples and the mineral extracts display different REE patterns (Figure 12), REE abundance in whole rock pegmatites is low to moderately high with ∑REE varying between 7.67 -220.37 ppm, a weak negative Eu anomy (Eu/Eu* = 0.15 -1.53), a slightly discernable negative Ce anomaly and relative enrichment of HREE (La N /Yb N = 0.96 -25.82).
Based on the normalization plots; the pegmatites did not undergo considerable fractionation and metasomatism as well as the study of [42] that advanced the views that weak negative Ce signature and a strong negative Eu signature denotes considerable fractionation and metasomatism.

Conclusion
The study area which is majorly underlain by schist and potassic porphyritic  Figure 11. (a)-(g). (a) Ta vs Ga plot for pegmatitess and extracts from the study area; (b) Ta vs Cs plot; (c) Ta vs Cs + Rb; (d) Ta vs K/Cs; (e) molar A/NK vs. A/CNK (after [40]); (f) and (g) mineralization potentials and characterization of the pegmatites (discrimination lines after [41]).