1. 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.

2. 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, [21] [22] [23] [24] [25] ).

The migmatite gneiss complex consists mainly of migmatites and gneisses of different origin as well as meta-igneous rocks of basic to ultrabasic composition; the schist belts consists of fine grained clastics, pelitic schists, phyllites, banded iron formations, marble and amphibolites with imprints of the Kibaran (ca. 1200 Ma) and Pan African tectonic events [26] , while the older granites consist of

orogenic (Pan-African) igneous rocks emplaced in the migmatite gneiss complex and the schists belts with varied lithology of granites, granodiorites, diorites, tonalities, syenites and pegmatites.

3. 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 granitic, the southeastern flank and south central flank is dominantly by granite gneisses and migmatites with the central flank of the study area being dominantly schistose in nature with intercalations of quartzite and dolerite dykes occurring in the northeastern flank of the study area as intrusions associated with quartzites while the charnokites and amphibolites occurs as pods mostly within the schists (Figure 3).

Pegmatites occur as intrusions of various dimensions, varying length and thicknesses some of which crosscut the host rocks, it has interlocking grains of quartz, feldspars and in some cases minor traces of biotite, muscovite, garnet, tourmaline and agate; it follows a NW/SE trend with massive deposits occurring in the central portion of the study area as intrusions within the mica schist (Figure 4).

Pegmatites are typically coarse grained but they show great variation in grain size, the contact between pegmatite and host rock are sharp in the granites, migmatites and granite gneisses while the contact in the schists was rarely observed due to the friable nature of the schists. Pegmatites associated with the porphyritic granites, migmatites and granite Gneisses are simple pegmatites dykes with central quartz bounded by feldspar, they are not extensive, they have similar mineralogy of quartz, microcline and occurrence of schorl in some areas while the pegmatites associated with the schist are mostly extensive and possess varying mineralogy with feldspars varying from orthoclase to albite, tourmaline, garnet, fluorite and tourmaline amongst others.

The polycyclic nature of the Nigerian basement complex is evident in the area with structural features such as faults and folds observed mainly in the migmatite gneiss and the NW/SE trend of the pegmatite as well as the NW/SE and NE/SW orientation of the joints with the NW/SE direction the main orientation of joints in the area. Some of the pegmatite bodies predate some of the structural features, this evident by structural deformation which manifest by brittle/ductile fault offsets by the structural episode.

They lack any form of foliation and they show no form of regional zoning, this could be due to the lower volatile contents of parent melts and the emplacement of their parental granitic melts at relatively shallow depth. Although variations in mineraology and field features in the pegmatites are obvious, it was not possible to investigate the changes in the bulk chemistry of the host rocks related to pegmatite formation due to the lack of clear cut contact zones between the pegmatites and the schists in some areas and the level of weathering of the metasediments. Presently, no geochronological data is available for rocks in the study area but studies are ongoing to acquire absolute age for the granites and pegmatites.

4. 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' - 8˚00' with an area extent of about 829 km². It involved the direct observation of the rocks in the study area, their structures features as well as their basic mineralogy.

During the study, lithological relationship between the pegmatites and host rocks was established; pegmatites samples were collected, thin sections as well as polished sections of the pegmatites were carried out at the department of geology, Rhodes University, Republic of South Africa and geochemical analysis was carried at Bureau Veritas Mineral Commodities, Canada.

Samples for polished sections were cut into ~2 cm diameter x ~1 cm rock fragments and mounted in epoxy. They were then polished with 3 µm and then 1 µm diamond paste, carbon coated with a Q150T Turbo-Pumped Sputter Coater/ Carbon and analysed with a Jeol JSM6610 SEM equipped with a Thermo Fisher Ultradry EDS detector. Polished sections were examined using back-scattered electron imagery in an attempt to detect phases with high back-scattered electron intensity. These were subsequently analysed by means of energy dispersive X-ray spectrometry in order to determine the chemical composition of the phases. Analysis was carried out at the University of Free State, South Africa.

5. Results and Discussion

5.1. Mineralogy and Petrography

Hand samples of pegmatites vary from samples with a graphic texture composed of orthoclase and quartz with low percentage of mica, to pegmatites with albite/othoclase bond to a central quartz and muscovite with rare biotite; the presence of muscovite over biotite in the pegmatites is an indication of the general peraluminous nature of the pegmatite.

Petrographic studies revealed a mineral assemblage of quartz, microcline and tourmaline with accessory minerals that are opaque. Quartz display a low positive relief under crossed nicols while microcline displayed cross hatched twinning; albite displayed a polysynthethic albite twinning with perthitic habit indicating the intergrowth of two feldspars.

Potassium feldspars consist mostly of orthoclase which shows varying degrees of perthitic intergrowth, sodium felspars which are mainly albite; feldspars probed by SEM are mainly albites with very few of the plagioclase probed having compositions tending towards oligoclase.

Quartz occurs in all the pegmatites as an intergrowth with feldspars in some outcrops and as well developed euhedral crystals in some locations; it is mainly colorless in the pegmatites and composed almost entirely of SiO₂ based on SEM analysis. Micas occurs as muscovites and rarely as biotite with the presence of muscovite over biotite in the pegmatites an indication of the general peraluminous nature of the pegmatites.

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; ${\text{Mn}}_3^{2+}{\text{Al}}_2{\left({\text{SiO}}_4\right)}_3$ ; garnet chemistry reveals the chemical evolution of the phase from which the garnets crystallized and the presence of a garnet end member

can be used to draw inference on the possible petrogenesis of granitic magma and the approximate depth of formation and garnets from the study plots in the field of igneous (Figure 7).

SEM analysis revealed garnets have high concentration of SiO₂, MnO, Fe₂O₃ and Al₂O₃, and low concentration of Na₂O and CaO while P₂O₅, K₂O, Ta₂O₅, TiO₂ CaO concentrations were not obtained from the analysis (Table 1) while inclusions of apatite contains high concentration of P₂O₅ (39.9%), CaO (50.8%), MnO (4.48%) and F (3.45%); a composition indicative of flouroapatite.

5.2. Tourmaline

Analysis of tourmaline did not reveal the crystal structures of the tourmaline grains (Figure 8); tourmalines have high concentration of Al₂O₃, Fe₂O₃ and SiO₂ with average concentration of 36.2%, 14.9% and 40.6% respectively; low concentration of Na₂O and MgO with average concentration of 2.7% and 4.5% respectively while concentration of CaO, TiO₂, MnO and BaO are below 0.6%.

The tourmaline falls into the class of Fe rich quartz tourmaline rocks (hydrothermally altered granites), an indicating of boron metasomatism during the late stage crystallization of the pegmatites ( [30] , Table 2 and Figure 9).

SEM analysis of feldspars reveal average concentration of 9.99%, 20.50%, 64.50%, 0.03%, 0.08%, 0.26% and 0.41% for Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, Fe₂O₃ and ZnO respectively indicative of albite as the dominant plagioclase in the pegmatites (Table 3).

5.3. Geochemistry of Pegmatites

In terms of major elements the whole rock pegmatite samples have the highest concentration of silica and CaO with average of 73.91% and 0.57% respectively, the feldspar extracts have the highest concentration of Na₂O and P₂O₅ with a

verage concentration of 5.77% and 0.14% while the mica extracts have the highest concentration of Al₂O₃ (16.42%), Fe₂O₃ (2.11%), MnO (0.08%), MgO (0.42%) and K₂O (5.66%).

In terms of trace elements, for Nb, Sn and W average concentration in whole rocks exceeds that of the other extracts (whole rock > mica > feldspar > tourmaline). For Ta, elemental abundance is highest in micas and least in tourmaline (mica > feldspar > whole rock > tourmaline), Cs abundance is highest in the

feldspars and least in tourmaline (feldspars > whole rock > mica > tourmaline) and Li abundance is highest in micas and lowest in the whole rock samples (mica > tourmaline > feldspar > whole rock).

Generally the average REE content of the pegmatites is low; ∑REE ranges from 3 - 203 ppm, 2 - 26 ppm, 4 - 18 ppm and 25 - 33 ppm in whole rocks, mica, feldspar and tourmaline extracts respectively with an average of 31 ppm, 13 ppm, 7 ppm and 29 ppm in whole rocks, mica, feldspar and tourmaline extracts. Analytical results of the major oxide, trace and rare elemental composition of pegmatites and extracts is presented in Tables 4-6.

5.4. Major Element Variation

CaO-Na₂O-K₂O diagram reveals a wide distribution of samples along the Na₂O-K₂O 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 Na₂O and CaO composition due to the difference in feldspar type and composition (Figure 10).

5.5. 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] .

Al₂O₃/Na₂O + K₂O vs. Al₂O₃/CaO + Na₂O + K₂O (A/NK vs. A/CNK) shows pegmatites are strongly peraluminous (Figure 10(e)) and K/Rb values range from 24 - 575 with an average of 177 ppm (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

zonation is typical of highly fractionated and mineralized pegmatites. The low level of rare earth mineralisation in this area is supported by lack of definate zonation patterns in the pegmatites and proximity to parent granitic bodies.

Bdl-below detection limit. -No data

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.

6. Conclusion

The study area which is majorly underlain by schist and potassic porphyritic

granites has been intruded by low lying NW-SE trending pegmatites. Results of geochemical analysis as well as trace elemental plots revealed low to medium enrichment in rare metals and REEs though there is potential for gem winning based on previous winnings of gems from the study area, unfortunately, there is no clear cut geochemical method to ascertain the presence or absence of gemstones in pegmatites from surface samples.

Further Studies

Understanding the genesis of pegmatites still remains a controversial topic. Further studies can be carried out using oxygen and hydrogen isotopes in conjunction with fluid inclusion studies (in progress) to obtain more information on the petrogenesis of these pegmatites and the mineralogy can further be studied to understand further the mineral species and delineate minerals which might be used to understand the genesis of the pegmatite and delineate the gem bearing zones.

Acknowledgements

This study is part of the PhD thesis of OOG. OOG acknowledges the research grant and PhD scholarship received from the African University Commission to pursue his studies. Olajide-Kayode, J. O., Popoola, O. and Sobhy, M. and others who assisted during the field work are appreciated. Two anonymous reviewers whose comments improved this manuscript are appreciated.