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Econ. Environ. Geol. 2024; 57(6): 735-768

Published online December 31, 2024

https://doi.org/10.9719/EEG.2024.57.6.735

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Petrogenesis and Geochemical Evolution of Rocks and Pegmatites in Kwarra Area, Northcentral Nigeria: Implications for Rare Metal Mineralization

Adamu, Lukman Musa2,*, Sunday, Adedeji Ebenezer1, Ohiemi, Adukwu Fabian3, Ayuba, Rufai2, Ugbena, Kelvins Godfrey2, Baba, Yahaya2, Abraham, Templeman4, Ogunkolu, Bolade Ayodeji5, Ebeh, Austine2

1Department of Geology, Ahmadu Bello University, Zaria, Kaduna State, Nigeria
2Department of Earth Sciences, Kogi State University, Anyigba, Kogi State, Nigeria
3Department of Geosciences, Confluence University of Science and Technology Osara, Kogi State, Nigeria
4Department of History and International Relation Studies, Kogi State University Anyigba, Kogi State, Nigeria
5Department of Geography, Kogi State University, Anyigba, Kogi State, Nigeria

Correspondence to : *lukman10adamu@gmail.com, lukman10musa@yahoo.com, adamulm@ksu.edu.ng

Received: September 20, 2024; Revised: December 4, 2024; Accepted: December 17, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Although pegmatite rare metal deposits are significant sources of rare metals, their metallogenic mechanisms remain a topic of debate. Nigeria's Basement Complex and Younger Granite provinces have identified pegmatites, which comprise a variety of rock types. We classify pegmatites into two categories: quartz-muscovite pegmatite and quartz-feldspar pegmatite. The study area has three primary stress orientations: N-S, NE-SW, and NNE-SSW, with secondary trends of ENE-WSW and E-W. There are more high-ferromagnetic elements (HFS) in quartz-muscovite pegmatites than there are in the crust as a whole. These elements include Zr, Ga, Nb, and Ta. They have a high concentration of the lithophile element Rb but are deficient in Sr and Ba. Quartz-feldspar pegmatite doesn't have many high-field-strength (HFS) elements, but it has a lot of rubidium (Rb), though not as much as quartz-muscovite pegmatites. The pegmatites and albitized granite were subjected to sodic metasomatism, which changed the feldspars from K to Na and made them contain more uranium. The albitized rock types contain low total rare earth elements (REE), strontium (Sr), and barium (Ba), while the amount of rubidium (Rb) decreases with increasing albitization. It's not clear how the columbite-tantalite series or the Ta and Cs minerals formed, but the high Sn concentration in pegmatites is linked to the formation of cassiterite minerals. Rare metal pegmatite is formed when deeply buried S-type fertile granite is partially melted. On the other hand, barren quartz-feldspar pegmatite is formed from late-stage residual melts from a less evolved granitic parent.

Keywords pegmatite, hydrothermal, metasomatism, petrogenesis, Kwarra, Nigeria

  • Two types of pegmatites were mapped: pegmatites that are rich in rare metals and muscovite (quartz-muscovite) and pegmatites that are barren and low in muscovite (quartz-feldspar-pegmatite).

  • The quartz-feldspar-pegmatite represent late-stage residual melts of a less evolved granitic parent that have been derived by fractionation.

  • The quartz-muscovite-pegmatite are products of highly fractionated late-stage magma derived from fertile granite at depth and modified by interaction with a coexisting hydrothermal phase during crystallization beneath the amphibolite belt.

The Kwarra Area is located in the northeast central margin of the Basement Complex of Nigeria, underlain by Basement Complex rocks and parts of the Younger Granites (Fig. 1). The area is home to numerous pegmatites. Some of these pegmatites are rich in rare elements, such as Li, Be, Nb, Ta, and Sn, and are Li-, Cs-, and Ta-rich (LCT) pegmatites (Kuster 1990; Okunlola, 2005; Jatau et al., 2012).

Fig. 1. Topographical map of the study area parts of Kurra sheet 189SW modified after Fed. Surveys, Nigeria 1967.

Rare metals, including Li, Be, Nb, Ta, and Sn, are crucial for the advancement of several economically significant sectors, such as national economic development, defenserelated science and technology, and innovative scientific research. Numerous governments acknowledge them as strategic resources (Linnen et al., 2012; Chakhmouradian et al., 2015; Zhai et al., 2019; Hou et al., 2020). The increasing demand for rare metal resources has stimulated mineral exploration and investigation into the origins of rare metal deposits, elevating them to prominence in scientific study (Damdinova et al., 2018; Wang, 2019; Lyalina et al., 2019; Li et al., 2021, 2022; Ayuso and Foley, 2023; Xuanchi et al., 2024).

These metals are derived from diverse geological sources, each offering unique mineralogical and geochemical characteristics. Among these, pegmatites serve as one of the most significant and economically viable sources of rare metals. Pegmatites are intrusive igneous rocks characterised by their coarse-grained texture and the presence of rare minerals that host rare metals. They are particularly renowned for yielding lithium, tantalum, niobium, and beryllium, along with significant concentrations of rare earth elements (REEs). Lithium-bearing minerals such as spodumene and petalite are commonly associated with lithium-rich pegmatites, whereas columbite-tantalite minerals serve as primary carriers of tantalum and niobium (Černý & Ercit, 2005). Additionally, beryl, a beryllium-bearing mineral, is another hallmark of some pegmatite deposits. Beyond pegmatites, other sources of rare metals include carbonatites, which are a primary source of niobium and REEs, and lateritic deposits, which often host scandium and nickel-cobalt. Sedimentary deposits, such as placer deposits, also play a crucial role, particularly in concentrating REEs and heavy minerals like zircon and monazite.

Pegmatites, known for their exceptional mineralogical diversity and enrichment in rare metals, are of immense importance in the exploration and exploitation of critical resources. These igneous rocks, characterised by their large crystal sizes and coarse-grained textures, often serve as repositories of economically significant rare metals, including lithium, tantalum, niobium, and beryllium. Their unique geochemical and structural attributes establish a crucial link between pegmatites and rare metal sources, underscoring their relevance to resource development. One of the primary reasons pegmatites are vital is their ability to concentrate rare metals during the late stages of magma crystallisation. This occurs because incompatible elements, which do not fit into the crystal lattices of common rockforming minerals, are concentrated in the residual melts that form pegmatites (Černý & Ercit, 2005). For instance, lithium-bearing minerals such as spodumene and petalite are primarily found in lithium-enriched pegmatites. Similarly, tantalum and niobium, crucial for electronics and aerospace applications, occur in columbite-tantalite minerals hosted in specialised pegmatitic deposits. Furthermore, pegmatites often host rare earth elements (REEs) in smaller quantities, complementing other major REE sources like carbonatites. These deposits are strategically significant, given the rising demand for critical metals in renewable energy technologies, electric vehicles, and defence systems (Kesler, 2007). Their accessibility and typically high-grade ores make pegmatites economically viable. For example, regions such as Greenbushes in Australia and Bikita in Zimbabwe are globally recognised as prolific sources of rare metals, directly linked to pegmatite mineralisation.

Researchers are very interested in where rare metalbearing pegmatite deposits come from because they are major sources of these metals (Barros and Menuge, 2016; London, 2018; Müller et al., 2018; Bekele and Sen, 2020; Kaeter et al., 2021; Galliski et al., 2021; Zhang et al., 2021; Morozova et al., 2022; Li et al., 2023; Hong et al., 2022). The principal determinants governing the mechanisms of rare metal enrichment in pegmatites are of significant interest (Auley and Bradley, 2014; Melcher et al., 2017; Knoll et al., 2018; Keyser et al., 2023). Three ideas have been put forward by researchers to explain why pegmatites have a lot of rare metals: liquid immiscibility, fractional crystallization differentiation, and hydrothermal metasomatism (Thomas and Davidson, 2016; Ran and Li, 2021). Rare metals are typically converted into minerals through magmatic, transitional magmatic-hydrothermal, and hydrothermal processes. The parent magmas, crystallization fractionation, and the subsequent hydrothermal fluids influence these processes (Kuster 1990; Okunlola, 2005; Jatau et al., 2012). Consequently, the principal variables governing the enrichment of rare metal elements in pegmatites remain ambiguous. Recently, Okunlola (2005) did research on the metallogeny of Nigeria's rare metal (Ta-Nb) pegmatites. He identified seven main fields: Kabba-Isanlu, Ijero-Aramoko, Keffi-Nasarawa, Lema-Ndeji, Oke Ogun, Ibadan-Oshogbo, and Kushaka-Birnin Gwari. The Keffi-Nasarawa field, which includes the pegmatites around the Kwarra area, is currently exploiting other members, namely Wamba and Keffi pegmatites, for both metallic and gem minerals like cassiterite, columbite, tantalite, tourmaline, and beryl. The available data on the rocks suggests a genetic relationship with the well-studied Wamba pegmatites (Jatau et al., 2012). Previous research has mostly focused on regionally cataloging the pegmatites around the Kwarra area (Kuster, 1990; Matheis and Caen-Vachette, 1983; Matheis, 1987). However, we still don't fully understand how to classify pegmatites, their geochemical makeup, how old they are, or where the rare metals that are found in them come from. In this paper, we present new data on the mineralogy and geochemistry of muscovite, amphibolite, migmatite gneiss, banded gneiss, granite gneiss, alkali granite, and albitized granite in the quartz-muscovite-pegmatites and quartz-feldspar-pegmatites. We also discuss their petrogenesis and structural features, their impact on rare metal mineralization, the concentration of major and trace elements, and our petrogenetic evaluation using trace elements Rb, Ba, Sr, Zr, Y, and the rare earth elements. Finally, we classify the pegmatites based on their relationship, and conduct a structural analysis of the pegmatites and host rocks in the study area to determine how their trends relate to the geochemistry.

1.1. Regional Geology

The Nigerian Pan-African basement is part of an Upper Proterozoic-Lower Phanerozoic mobile belt situated between the West African and Congo cratons (Garba 2003). This mobile belt extends from Algeria across the Southern Sahara into Nigeria, Benin, and Cameroon. Mesozoic ring complexes of the Jos area intrude into the rocks of the Nigerian Basement Complex, part of the Pan African Mobile Belt, while Cretaceous to Quaternary sediments unconformably overlie them, forming the sedimentary basins (Akintola and Adekeye, 2008). The geology of the study area falls within the framework of the North-Central Basement Complex of Nigeria, which is underlain by Basement Complex rocks and parts of the Younger Granites (Fig. 2).

Fig. 2. Simplified map of the geology of Nigeria after Okunlola, (2005).

1.2. Nigerian Pegmatites

According to Garba (2003) and Rahaman et al. (1988), the Nigerian pegmatites date from the end of the Pan- African magmatic activity and belong to the terminal stage of Pan-African magmatism, which occurred between 562 and 534 Ma. Pegmatization was heavily influenced by progressive metamorphism and partial melting of country rocks at the end of Pan-African plutonism. (Adetunji et al.,. 2016). These pegmatite units' ages, mineralogy, and composition appear to be analogous to those of the pegmatite environment in Brazil, Canada, and Australia (Akintola and Adekeye, 2008). Both barren and mineralized pegmatites exist in the Nigerian basement (Fig. 3). The study area's pegmatites are located in Nigeria's North Central pegmatite province, which stretches from Abuja to Jos Plateau (Fig. 3) and primarily settle in the gneissic basement. Albitization is a dominant feature of the area's rare metal pegmatites, which are particularly rich in tin (Matheis, 1987). The pegmatites in the study area have a close, but sparse, relationship with the Pankshin anorogenic ring complex. In the study area, Rb/Sr dating for the Gwon-Gwon pegmatite gave an age of 555 +/-5 Ma, and three muscovite model ages range from 537–522 Ma (Matheis, 1987).

Fig. 3. Geological Sketch map of central and south-west Nigeria showing the location of the Wamba pegmatite field (study area) and the distribution of Pan-African Older Granites and pegmatites (underlined). Geochronogical data sources are van Breemen et. al. (1977), Rahaman et. al. (1983), Matheis and Caen-Vachette (1983), Tubosun et. al. (1984).

1.2.1. Barren and Rare Metal Pegmatites

Moller and Morteani (1987), Černy (1989), Kuster (1990), and Garba (2003) have helped us learn more about the pegmatite bodies in the southwestern and northern parts of Nigeria. They have helped us tell the difference between barren and rare metal-bearing pegmatites and shown that the pegmatites are not just found in the 400 km long NESW trending belt that goes from the Wamba area (near the Jos plateau) to the Ilesha area. Granitic pegmatites that are rich in rare metals form when large reservoirs of granitic magma separate very quickly. In these types of pegmatites, the amount of rare metals shows how much fractionation happened in the last stages of granitic differentiation (Černy, 1991). All the major lithologies of the basement, such as gneiss, migmatites, schists, and granitoids, are associated with barren pegmatites. The morphology and major mineral composition (quartz-feldspar-mica) are mostly the same as those of rare metal types (Garba, 2003). The chemistry of the pnuematolytic and/or hydrothermal fluids that are added will play a big role in telling the difference between the mineralized and bare pegmatites (Adedoyin et al., 2006). Researchers have observed that the degree of albitization and fractionation in pegmatites significantly influences the differentiation between barren and rare metal pegmatite (Oyebamiji, 2014; Matheis, 1987; Akintola et al., 2012 (a); Kuster, 1990; Oyebamiji et al., 2018, Jacobson and Webb, 1946). Garba (2003) observed the following common characteristics of rare metal pegmatites: sharp contacts with their host rocks, wall rock alteration (mostly tourmalinization), and close proximity to major and subsidiary fault structures. Given that rare metal pegmatites are associated with the major fault lineament systems in Nigeria, the albitization and rare-metal mineralization may have resulted from late-stage fluids present at the end of the Pan-African metamorphic cycle (Ekwueme and Matheis, 1995) or from a sodium-rich hydrothermal solution from the mantle along the ancient lineament (Wright, 1970). A study by Okunlola (2005) explained the metallogeny of Nigeria's rare metal pegmatites and named 7 main fields: the Kabba-Isanlu, Ijero-Aramoko, Keffi-Nasarawa, Lema-Share, Oke-Ogun, Ibadan-Oshogbo, and Kushaka-Birnin Gwarri. A broad zone extending NE from the Ago-Iwoye area towards the Younger Granite province (Akoh and Ogunleye, 2014) shows a marked concentration of these pegmatites, exhibiting an appreciable degree of mineralization. Rare metal pegmatites commonly exhibit late-stage albite and sericite (Garba, 2003).

1.2.2. Structural and Geochemical Characterization of Nigerian Pegmatites

The rare metal pegmatites are found in a clear belt that goes from Ife to Jos and seems to cross the line between the eastern and western Nigerian terranes, even though the pegmatite intrusions are oriented north to south (Kinnaird, 1984; Matheis and Caen-Vachette, 1983; Woakes et al., 1987). Individual pegmatites vary in length from 10 m to over 2 km and can be up to 200 m wide (Adetunji and Ocan, 2010). We have never observed a direct genetic link between the rare metal hearing pegmatites and proximal granite occurrences. The tecno-chemical characteristics of the host rock, not specific lithologies, determine mineralization (Adedoyin et al., 2006). Rotational stresses created by the Benue Trough appear to relate to the pegmatitic belt and the orientation of its units. From a more global perspective, this trend is probably the northern extension of the Brazilian pegmatite belt, which runs from Rio Grande del Sul to Rio Grande del Norte (Akintola and Adekeye, 2008). Researchers suggest that the Pan-African orogeny's reactivation of old tectonic lineaments provided excess heat and fluid, which concentrated rare-metal pegmatites through partial melting and selective leaching from the country rocks, or their lithological framework (Garba, 2003). Erny et al. (2012) support this suggestion. It is thought that the conjugate faults and shear belts that Garba (2003) suggested play a big part in how the magmatic fluid flows and how the pegmatites are arranged in a straight line or slightly offline. Rare metal pegmatites sometimes have big pinch and swell patterns that are linked to semi-ductile deformation, and minerals are well developed in the swells (Adedoyin et al., 2006). Different geochemical signatures, related to their spatial geological framework, distinguish the different pegmatite fields in Nigeria (Matheis, 1987; Kuster, 1990). Nigerian pegmatites show a marked difference in trace element and rare-earth element concentration and fractionation patterns between the rare-metal and barren pegmatites. It's very important to note that the rare-metal pegmatites have higher amounts of Rb, Cs, Ga, Nb, Ta, Sn, Li, and Be than the barren types and Pan-African granitoids (Garba, 2003). Some pegmatites in the amphibolite complex, like the Egbe pegmatites, have unusually high amounts of pathfinder elements like Li, Rb, F, and K/Rb, which are strongly linked to the rock's tin content (Mathei, 1987; Kuster, 1990). The pegmatites' origin also has a significant influence on the pegmatités geochemistry. The higher K/Ba ratios of the granitic pegmatites distinguish them from the metamorphic ones. The high K/Ba and Rb/Sr but low K/Rb ratios of the rare-metal pegmatites attest to their granitic origin (Garba, 2003).

1.2.3. Evolution of pegmatites from a granitic melt

The most accepted model for pegmatite formation is its derivation from granitic magmas via igneous differentiation processes. Granitic magmas refine their compositions through crystal fractionation and the separation of residual liquids from their crystalline products (Cerný et al., 2012). In granitic pegmatites, the chemical and textural segregation that makes them unique is caused by the uneven distribution of alkalis between the melt and an upwardly buoyant vapor phase (London, 2005). The pegmatite zonés slow rate of cooling will encourage the growth of large crystals, yet the slow rate of cooling cannot fully explain the large crystals characteristic of pegmatites. Undoubtedly, the central regions of the parent rock experienced a significantly slower rate of cooling during its formation, yet they did not yield extremely large crystals (Cern et al., 1985). Evidence from mineral compositions and thermal models indicates that crystallization within pegmatites commences at ~450 °C, which is ~200 – 250 °C below the liquidus temperature at which crystallisation should commence (London and Kontak, 2012). The rising melts containing H2O reach H2O saturation (possibly with some wall-rock dehydration as well), expel a fluid phase as they rise, and continue to do so as they crystallize. An increase in H2O saturation can dramatically reduce the melting point of silicate systems at elevated pressures. As crystal growth begins, the growing crystal interface of quartz and feldspar rejects incompatible components such as H2O, OH-, CO2, HCO-3, CO32-, SO42-, PO43-, H3BO3, F, and Cl, along with elements Li, Na, K, Rb, Cs, and Be. These components then concentrate along the margins of the growing crystal front, acting as fluxes (Cern et al., 2012). Because of its increasingly flux-rich composition, the boundary layer liquid will have a low solidus temperature and enhanced silicate-H2O miscibility, reducing the system's viscosity (London, 2005). Volatile species, such as B, F, and P, can individually and collectively lower the granite solidus temperature to below 500 °C and increase the range of temperatures over which magmatic crystallization occurs (London, 1996). Pegmatites have big grains because they don't have good nucleation and have very high diffusivity in the H2O-rich phase. This lets chemical species move around easily and join minerals that are growing quickly (Winter, 2014). The trace elements that are important for petrology can be found in granite-pegmatite systems. The levels to which they don't work with each other depend on the pressure, temperature, and mineral phases in the system. The process of fractional crystallization or the assimilation of material from external reservoirs can modify the relative and absolute abundances of the initial suite of trace elements, depending on their compatibility in the rock-forming minerals of granites (Cerne et al., 2012).

2.1. Field Method

We conducted field work in two phases. Firstly, we conducted a reconnaissance survey of the area to evaluate its accessibility and plan the logistics for the field mapping. The second phase of the field work involved geological field mapping on a scale of 1:50,000 as the base map. Geological features (textural and structural characteristics) were identified, as were field measurements (attitudes; strikes and dips), as well as their mode of occurrence. Textural characteristics of the rocks were elucidated using a magnifying hand lens. Photographs of the outcrops were also taken using a digital camera, and representative fresh samples were collected for laboratory analyses.

2.2. Sample Preparation for Geochemical Analysis

The fifteen (15) samples chosen for geochemical analysis were broken down into smaller pieces (11 whole rocks and 4 muscovite) at the activation laboratory in Ontario, Canada. The pieces were broken down to a size of less than 200 mesh. The pulverization of the samples was undertaken with the aid of Retsch Planetary Ball Mill 400. Rock chips were loaded into the planetary ball mill, and a time interval of 15 minutes was allowed for complete pulverization to <200 mesh size. After pulverization of each sample, the equipment was cleaned using acetone. Pulverization was later followed by weighing 15 grams of each sample using an electronic weighing balance. Samples were then sealed and labeled. We used the 4 lithos package inductively-coupled plasma mass spectrometry (ICP-MS) to look for major, trace, and rare earth elements in the samples.

2.3. Analytical Technique

For the main elements, inductively coupled plasma atomic emission spectrometry was used after lithium borate fusion and dilute acid digestion. For the trace and REE elements, inductively coupled plasma emission spectrometry (ICPES) was used after a multi-acid digestion method. Details of the analytical procedures adopted have been discussed in the work of Maja et al. (2011). As part of the analysis process, 5 ml of perchloric acid (HClO4), 15ml of hydrofluoric acid (HF), and trioxonitrate (V) HNO3 were added to 0.5 g of the sample to dissolve the minerals. The solution was stirred properly and allowed to evaporate to dryness after it was warmed at a low temperature for some hours. Four (4) ml of hydrochloric acid (HCl) was added to the cooled solution and warmed to dissolve the salts. The solution was cooled and then diluted to 50 mL with distilled water. The solution was then introduced into the ICP torch as an aqueous aerosol. The ions in the ICP emitted light, which was converted to an electrical signal by a photomultiplier in the spectrometer. The electrical signal produced by the emitted light from the ions was compared to a standard (a previously measured intensity of a known concentration of the elements), and the concentrations were then computed. Analytical precisions vary from 0.1% to 0.04% for major elements. Data obtained was processed using the following software: Microsoft Excel, Geochemical Data Toolkit (GCD Kit 3.0), and Surfer 12.

2.4. Lineament Analysis

O'Leary et al. (1976) defined lineaments as mappable linear features that differ distinctly from the pattern of adjacent features and presumably reflect subsurface phenomena. Lineament usually follows regional geology (e.g., intrusive bodies or large faults' strikes) and is thus useful in mapping structural trends. As a result, the structural lines in the area were primarily faults and fractures. The structural lineaments were extracted from the study area's landsat ETM+ image. The Global Land Cover Facility website (glcf.umd.edu/data/landsat/) was used to download Landsat band7. It was then enhanced with ENVI v4.5 software to make the lineaments more visible. Individual lineaments were traced out using Global Mapper v15 software, then exported to ArcGIS and Rockware software for statistical and orientation analysis. A total of 31 lineaments were extracted from the study area. The minimum length derived from these lines was 822.50 m; the maximum length was 6275.15 m. The non-geological lineaments such as paths, roads, power cables, and field boundaries in the study area were eliminated using the topographical map (Yassaghi, 2006). The mapped structural lineaments were analyzed using lineament density (LD) and lineament frequency (LF) parameters (Solomon and Ghebreab, 2006).

3.1. Geology and Petrology

Precambrian Basement Complex rocks and Younger Granites underlie the study area, according to systematic geological field mapping at a scale of 1:50,000. The Basement Complex rocks include amphibolite, migmatite gneiss, banded gneiss, granite gneiss, and albitized granite, while the Younger Granite rock is alkali granite (Figs. 4, 5, and 6). Relics of tectonic activity are visible in the area's structures. The structural features observed in the study area include foliation, fractures, veins, lineation, joint folds, and minor faults (Figs. 7 and 8). The predominant structures trend from N-S to NE-SW (Fig. 8), and this conforms to the Pan-African structural pattern. Late pegmatitic veins either fill the shear fractures observed on the granites or show evidence of free aperture.

Fig. 4. (a) Hand specimen of the alkali granite in the study area (Latitude 9°06'05"N and Longitude 8°44'30"E), (b) Field photograph of albitised granite (Latitude 9°04'10"N and Longitude 8°47'43"E), (c) Field photograph of a granite gneiss outcrop (Latitude 9°03'23"N and Longitude 8°33'43"E), (d) Field photograph of foliation structure in the banded gneiss with quartz and quartzo-feldspathic veins (Latitude 9°05'05"N and Longitude 8°31'36"E), (e) Field Photograph migmatite gneiss with ptygmatitic folding, upper arrow pointing at melanosome and lower arrow showing leucosome (Latitude 9°00'28"N and Longitude 8°42'30"E), (f) Hand Specimen of amphibolite rock in the study area (Latitude 9°07'17"N and Longitude 8°41'50"E).
Fig. 5. Photomicrograph of the studied rocks under Plane Polarized Light (PPL) (a) alkali granite, (b) albitised granite, (c) granite gneiss, (d) banded gneiss, (e) migmatite gneiss, (f) sheared amphibolite; Biotite (Bt), Perthite (Prth), Orthoclase (Or), Plagioclase (Pl), Quartz (Qtz), Augite (Aug), hornblende (Hbl), garnet (Grt), Sphene (Shen), and muscovite (Ms). Mag.0.25mmX10.
Fig. 6. Field Photograph of (a) quartz-feldspar-pegmatite intrusion into migmatite gneiss (Latitude 9°00'20"N and Longitude 8°42'16"E), (b) a narrow zone of tourmalinisation around the contact between quartz-feldspar-pegmatite and the host rock. (Latitude 9°00'30"N and Longitude 8°44'27"E), (c) Quartzmuscovite- pegmatites boulders (d) Whitish quartz-muscovitepegmatites intrusion in granite gneiss (Latitude 9°02'45"N and Longitude 8°34'38"E), (e) Highly deformed quartz-muscovitepegmatites trending NE (9°02'16"N and Longitude 8°35'36"E). (f) Deformed nature of quartz-muscovite-pegmatites in uplifted region (9°02'48"N and Longitude 8°31'10"E).
Fig. 7. Field Photograph of (a) Field photograph of joints in the granite gneiss of the study area (Latitude 9°02'09"N and Longitude 8°38'34"E), (b) Field photograph of dextral fault along a quartz veins in the banded gneiss (Latitude 9°05'35"N and Longitude 8°30'06"E).
Fig. 8. (c) Field photograph of pegmatite veins that intruded the migmatite gneiss of the study area (Latitude 9°01'15"N and Longitude 8°41'30"E). (d) Field photograph of quartzo-feldspathic veins in the banded gneiss within the study area (Latitude 9°05'05"N and Longitude 8°31'12"E), (c) Rose diagram of pegmatite veins in the study area showing the NE-SW dominant trend, (d) quartzo-feldspathic veins showing the NE-SW dominant trend in the banded gneiss of the study area.

3.2. Lineament Analysis

The results of the analysis are presented as structural lineament map (Fig. 9a) lineament density map (Fig. 9b) and rose diagram (Fig. 9). The qualitative study of the geological map, structural lineament map and Lineament density map of the study area showed that structural lineaments were more concentrated in the migmatites, banded gneisses and amphibolites (Pre-Cambrian basement rocks) than on the Jurassic Younger Granites. This was due to the effect of several tectonic deformations that accompanied the different orogenic events that have occurred throughout the Nigerian polycyclic basements. The orientation of structural lineaments on rocks can be correlated to past tectonic events that have affected them. The orientation of the structural lineaments on the structural lineament map was measured and plotted on a rose diagram using Georose software. The rose diagram of the lineament trends in the study area (Fig. 9c) revealed that well developed principal structural trends are N-S, NE-SW, and NNE-SSW with minor ENE-WSW ad NNW-SSE trends. This finding is in agreement with previous works suggesting that Nigeria has a complex network of fractures and lineaments with dominant trend directions of NE-SW, NW-SE, and N-S directions. (Ananaba and Ajakaye, 1987; Chukwu-Ike., 1977, Obiora 2009). These trends also agree with more recent works carried out in parts of the basement complex adjoining the Benue Trough by Olasehinde et al., (1990) and Anudu et al., (2012). Many of the lineaments coincided with drainage lines indicating that drainage in the study area may be structurally controlled (Samaila et al., 2011). According to Obiora (2009), NNE-SSW and N-S trends are Pan-African, while E-W and NNW-SSE trends are pre-Pan-African. Hence the NE-SW, N-S and NNE-SSW trends in the study area may probably have been Pan-African, whereas the NNW-SSE and E-W trends may have been relicts of pre- Pan- African events and hence less pronounced.

Fig. 9. (a) Structural lineament map of the study area, (b) Rose diagram of structural lineament trends of the study area.

3.3. Geochemistry

3.3.1. Major Element Geochemistry

1) The Major Oxides Composition of the Rocks in the Study Area

The major oxides composition of the rocks in the study area is presented in Table 1 while Table 2 shows the CIWP Norm values for the granitic rocks of the study area. The Harker’s plot for albitized granite, pegmatites and alkali granites are shown in Fig. 10.

Table 1 Major oxide compositions of the rocks in the study area


Table 2 CIWP Norm for the granitic rocks of the study area


Fig. 10. Harker plot of Al2O2, CaO, K2O and Na2O against silica (SiO2) for the granite suites in the study area. S2 and S6 (Alkali Granite), S4 and S12 (Albitized Granite), S2 and S6 (Alkali Granite), S4 and S12 (Albitized Granite).

The SiO2 concentration in the host rocks ranges from 50.43 to 74.48 wt%. The alkali granites exhibit the highest SiO2 concentration, averaging 74.18 wt%, while the albitized granite follows with a SiO2 concentration of 73.89 wt%. The high SiO2 content in granites is an indication of their acidic nature and enrichment in common rock-forming minerals such as quartz and feldspar. The migmatite gneiss has the highest SiO2 concentration among the metamorphic rocks in the study area, with a value of 71.97. Granite gneiss, banded gneiss, and amphibolite are the other metamorphic rocks with SiO2 concentrations of 69.76 wt%, 67.79 wt%, and 50.43 wt%, respectively. The Al2O3 concentration in the host rocks ranges from 11.06 wt% to 15.35 wt%, with albitized granites having the highest average concentration of 15.29 and the alkali granite with the lowest average concentration of 11.56. In this area, the metamorphic rocks like migmatite gneiss, banded gneiss, granite gneiss, and amphibolite all have about the same amount of Al2O3: 14.89, 14.96, 15.3, and 13.69. On the Harker’s diagram, the rocks form a somewhat vertical linear trend, with the younger granites plotting on the lower end of the trend. The FeO3 concentration in the host rocks ranges from 0.76 to 10.25 wt%. The albitized granites have the lowest Fe2O3 concentration, which is indicative of their acidic and felsic nature. The amphibolite has the highest Fe2O3 content of 10.25 wt%, which reflects its mafic and ferromagnesian-rich composition. The banded gneiss has a relatively high Fe2O3 composition of 5.2 Wt%, which may be due to its biotite-rich nature. The alkali granite has an average Fe2O3 concentration of 2.72, while the migmatite gneiss and granite gneiss have a concentration of 1.52 and 3.41, respectively. On the Harker's diagram, the rocks form a negative correlation with SiO2, and the Fe2O3 concentration drops steadily as SiO2 content increases from amphibolite to albitized granite. The alkali granite plot is slightly off the trend. The rocks rich in felsic phases, such as alkali granite and albitized granite, have the lowest MgO concentrations of 0.07 wt% and 0.02 wt%, respectively. The higher MgO concentration of the amphibolite (7.57 wt%) reflects the modal mineralogy of the rocks, which contains a higher proportion of mafic minerals. The migmatite gneiss, banded gneiss, and granite gneiss have MgO concentrations of 0.7 Wt%, 1.14 Wt%, and 0.71 Wt%, respectively. The rocks have a somewhat negative relationship with SiO2. The metamorphic rocks have a weak relationship, and the felsic-rich rocks (like habitized granite and alkali granite) are grouped together at the bottom of the trend. The rocks generally have a low MnO concentration with a range of 0.02–0.18 wt%. The migmatite gneiss has the lowest value, while the amphibolite has the highest concentration at 0.18 wt%. The CaO content in the rocks ranges from 0.11 to 12.35 wt%. The albitized granite has the lowest average CaO content of 0.12 wt%, while the amphibolite has the highest of 12.35 wt%. The banded gneiss has a relatively high concentration of 3.24 wt%, while the migmatite and the granite gneiss have concentrations of 2.96 wt% and 1.93 wt%, respectively. The SiO2 concentration appears to have little influence on the CaO concentration.

2) Major Oxide Geochemistry in the Pegmatites

The Muscovite extract samples have an average SiO2 concentration of 61.12 wt%, ranging from 54.35 wt% to 67.77 wt%. The whole rock pegmatites have an average SiO2 concentration of 73.9 wt%, ranging from 73.56 wt% to 74.29 wt%. The muscovite samples have the highest values of Al2O3, MgO, K2O, and MnO, with mean concentrations of 23.80 wt%, 0.23 wt%, 6.89 wt%, and 0.07 wt%. The muscovite extract also has a high value of Fe2O3 and P2O5, with mean values of 2.05 wt% and 0.21 wt%, respectively. However, the SiO2 and CaO values are relatively low, with mean values of 61.12 wt% and 0.16 wt%, respectively. Muscovite samples' high Al2O3 values attest to their peraluminous nature, which is consistent with rare metal pegmatites. The rocks show a negative correlation with Al2O3 on the Harker’s diagram (Fig. 10). The quartz-feldspar-pegmatite is SiO2 rich, with an average value of 73.92 wt%. It has the highest P2O5, with a mean value of 0.42 wt%. It also has relatively high Al2O3, CaO, and N2O values, with mean values of 14.13 wt%, 0.51 wt%, and 3.55 wt%, respectively. On the plot of K2O vs. SiO2, we observe a negative correlation for the quartzmuscovite- pegmatites, and a positive correlation for Na2O vs. SiO2 (Fig. 10). The pegmatites show a negative correlation with Fe2O3 on the Harker’s diagram. They also show a positive correlation with P2O5, indicating that P2O5 increases with SiO2 during magmatic differentiation. Phosphorous is an incompatible element that concentrates in the residual melt, where it acts as fluxes and enhances the growth of large crystals, as seen in pegmatites (London, 2008).

The Shand index plot (Fig. 11a) shows that the albitized granite and pegmatites are in the peraluminous (ASI > 1.0) field, with an aluminum saturated index (ASI) ranging from 0.99 wt% to 2.64 wt% and a modified alkali-lime index ranging from 3.07 wt% to 9.22 wt%. The albitized granites and muscovites had the strongest peraluminous values (ASI 1.26 and 2.40, respectively). The alkali granite has a mean ASI value of 0.98 and plots in a metaluminous field close to the border of the peralkaline field. A plot of Fe (total)/(Fe (total)+MgO) versus SiO2 (Fig. 11b) and Na2O+K2O-CaO versus SiO2 (Fig. 12a), following Frost et al. (2001), indicates a strong iron enrichment in pegmatites, alkali granites, and albitized granite. We interpret ferroan (Fe-enriched) as closely associated with conditions of limited H2O availability and low oxygen fugacity during partial melting of their source rocks (Frost et al., 2001). According to Frost et al. (2001) (Fig. 11b), the granitic rocks can be put into four groups: the alkali, the alkali-calcic, the calcic-alkali, and the calcic series. All the granites fall in the alkali-calcic field, as do the pegmatites, with the exception of one quartz-feldspar-pegmatite sample (S8) that falls in the calcic-alkali field. On the SiO2 vs. Na2O+K2O diagram of Middlemost (1985) (Fig. 12b), the quartz-feldspar-pegmatite, alkali granites, and albitized granite samples are also plotted in the granite field, indicating the bulk granite geochemistry of these rocks. The normative Ab-Or-An diagram illustrates the major element changes in the Ternary plot after O'Connor (1965) (Fig. 13a). The variable contents of albite, compared to the other feldspar components, delineate the rocks. The pegmatites, albitized granite, and alkali granite all plot within the granite field, with the albitized granite plotting much closer to the Ab corner due to its albite-rich nature. We plotted the granites on the A/CNK versus SiO2 discrimination diagram (after Chappel and White, 1974) (Fig. 13b). The albitized granite, quartz-feldspar pegmatite, and quartz-muscovite pegmatite all plotted within the Stype granite, while the alkali granite plotted within the Itype field. The partial melting of already peraluminous sedimentary source rocks imprinted by weathering at the Earth's surface produces S-type granitoids (Winter, 2014), while Collins et al., 1982, and White, 2005, and Whalen et al., 1987, interpret the I-type granitoids as those derived from recycled, dehydrated continental crust and those directly derived from melting of subducted oceanic crust or the overlying mantle.

Fig. 11. (a) A/CNK – A/NK plot for the granitoids in the study area after Shand (1943), (b) Fe (total)/(Fe (total)+MgO) versus SiO2 after Frost et al 2001.
Fig. 12. (a) Na2O + K2O-CaO versus SiO2 after Frost et al 2001, (b) Na2O+K2O vs. SiO2 for the granitoids diagram of Middlemost, (1985). S2 and S6 (Alkali Granite), S4 and S12 (Albitized Granite), S1, S5 and S8 (Quartz-Feldspar-Pegmatite).
Fig. 13. (a) Ternary normative Ab-Or-An diagram after O’Connor, 1965, (b) Fig. 16: A/CNK vs SiO2 plot of rocks from the study area after (Chappel and White, 1974).

3.3.2. Trace Element Geochemistry

Trace element levels in granitic melts change a lot as crystallization happens. These levels can show how the magma splits and how the chemicals in pegmatites change over time (London, 2008). The trace element composition and some important ratios are presented in Table 3, while Table 4 shows the concentration of rare earth elements in the samples. Table 5 displays the average abundances and ranges of some trace elements, rare earth elements, and selected ratios from Taylor and McLennan's (1985) upper continental crust values.

Table 3 Concentration of some selected trace elements and some important elemental ratios in the rocks of the study area

PetrologyAmphiboliteMigmatiteBanded GneissGranite GneissAlbatized GraniteAlkali GraniteQuartz-Feldspar-PegmatiteQuartz-Muscovite-Pegmatite
Elements (ppm)S10S3S9S7S4S12S2S6S1S5S8S13S14S15S16
Be123635131581354221435519
Ba4583684474173351379724892419735814
Sr1454774513231144450654831760405512
Y211710143294992122231
Zr65185250134292228029235231171221184
Co70595658695647536656296011911466
Zn100106110801631408091< 30< 30< 30120130100240
Ga141723264736252930221612797100179
Rb135485152465822214231542511833213294634823957
Nb48121430222834753187116164194
Tl0.20.20.51.14.21.0120.92.50.78.45.611.76.6
Sn1137816671125203328189585798
Cs0.71.63.215.15749.97.612.410.814.91.367.954.713362
Hf1.61.45.83.21.972.17.66.54.11.83.71.21.52.10.3
Ta0.51.11.41.75.34.24.26.37.5621.711.370.565.591.177.2
W142132298345412378298310285352187405745802429
Pb1018153218102835582752201278
Bi0.4< 0.4< 0.41.12.31.8< 0.4< 0.4< 0.4< 0.4< 0.42.33.1< 0.4< 0.4
Th1.21.312.47.70.80.426.4310.60.30.40.710.11
U0.30.41.62.77.46.26.47.61823.41.915.282.60.4
K/Rb280.97405.85213.89219.5567.4836.76197.84182.5688.8093.25463.0818.0615.6116.0217.43
Nb/Ta8.007.278.578.247.925.246.675.4012.502.9410.009.5910.093.9922.38
Rb/Sr0.090.110.190.4742.27205.504.864.628.3410.650.2653.5573.6563.31329.75
Na/K1.891.601.680.851.201.240.600.590.570.570.640.240.270.160.11
K/Ba81.1726.2221.5445.044482.8210072120.62111.27668.43992.7241.603054.08630.01961.834927.54

Table 4 Concentration of rare earth elements in the rocks of the study area


Table 5 Average abundances and ranges of some trace elements, rare earth elements and selected ratios from upper continental crust values from Taylor and McLennan (1985)



Fig. 14 shows spider diagrams for all the rock samples normalized to average crust after McDonough and Sun (1995). The normalized abundance patterns permit characterization of the rocks. Similar patterns are found in migmatite gneiss, banded gneiss, amphibolite, and granite. They are all characterized by negative anomalies of some incompatible elements, namely, Nb, Ce, Zr, and Ti, as well as positive anomalies for Pd, Ta, Sr, and Dy. The banded gneiss has a slight positive anomaly for Th, while the other metamorphic rocks all show a negative anomaly for Th. The amphibolite exhibits a slight negative Zr anomaly. The albitized granite exhibits a prominent negative anomaly for Ba, Ce, Sr, Nd, and Ti, as well as a marked positive anomaly for U, Ta, Pb, and Zr. The alkali granite is distinguished by negative anomalies for Sr, Nb, Ce, Zr, Ti, and Ba, as well as positive anomalies for Pb and the radioactive elements Th and U. All pegmatites samples have trace element patterns that are different from the norm. These patterns show that Rb, Ta, Pb, Zr, and U are more abundant than usual, while Ba, Nb, Ce, and Ti are less common. The quartz-feldspar-pegmatite samples are generally more HREE-enriched than the mica samples. The muscovite sample has the lowest K/Rb ratio (mean value of 16.78) as well as Ce (mean 1 ppm) among the other samples. It also has notably low levels of Ba (mean 41 ppm) and Sr (42 ppm) below the crustal abundances of 425 and 375, respectively (Taylor, 1964). The clear loss of Ba and Sr is likely caused by K and Na feldspar or plagioclase fractionation, respectively (Akoh et al., 2015). The quartz-muscovite-pegmatites are relatively enriched in Ga (mean 126 ppm), Rb (mean 3399 ppm), W (mean 595 ppm), Ta (mean 22 ppm), Sn (mean 475 ppm), Cs (mean 79.4 ppm), and Nb (mean 165 ppm), well above the crustal abundances of Ga 15 ppm, Rb 90 ppm, W 1.5 ppm, Ta 2 ppm, Sn 2 ppm, Cs 3 ppm, and Nb 20 ppm (Taylor, 1964). The relative concentrations of Nb and Ta are influenced by volatiles associated with the late-stage magma, so the quartz-muscovite-pegmatite represent the more evolved facies (Akoh et al., 2014). The quartzfeldspar- pegmatites that are close together have higher amounts of Ba (348 ppm), Rb (378 ppm), and Zr (58.33 ppm), and their Nb/Ta ratios are low (8.48).

Fig. 14. Spider diagrams for all the rocks samples normalized to average crust after McDonough and Sun (1995).

3.3.3. Rare Earth Element (REE) Geochemistry

We normalized all the REE values to those provided by Nakamura (1974) and then plotted them on a spidergraph (Figs. 15a, 15b, and 15c). The alkali granites have the highest sum of REE (ΣREE), with an average of 419.96 (Fig. 15b). The albitized granites have the lowest sum of REEs with an average value of 5.17. The low value of the sum of REEs for the albitized granite may be due to the effect of element mobility during hydrothermal alteration. The sum REEs for the other rock types are 65.07, 48.26, 97.78, and 187.03 for migmatite gneiss, amphibolite, granite gneiss, and banded gneiss, respectively (Table 6). The degree of negative Eu anomaly in the rocks varies widely (0.25–10.45). One way to find the Eu anomaly is to divide the chondrités normalized value of Eu concentration by half the sum of the normalized concentrations of Sm and Gd (Terekhov and Shcherbakova, 2006). This gives you Eu/Eu*. The alkali granités REE pattern is subparallel, LREE-enriched (LaN/YbN value of 6.38), and characterized by a nearly flat HREE profile. The alkali granite exhibits a weak negative Europium anomaly (Eu/Eu* = 0.4), suggesting that it originated from melts in equilibrium with a plagioclaserich phase during partial melting or fractional crystallization. This process either retained the plagioclase in the residual solid or removed it as phenocrysts during the earlier stages of crystallization (Winter, 2014). The banded gneiss and the granite gneiss show similar REE patterns (Fig. 15a), with ΣREE values of 187.03 and 97.78, respectively. These rocks have weak Eu anomalies (Eu/Eu* 0.87 and 0.94, respectively). Hugh (1993) attributes the weak negative Eu anomaly to the rock's formation from a protolith deficient in plagioclase. Both the migmatite gneiss and the amphibolite have a subparallel REE pattern with LREE enrichment. This may mean that there aren't any accessory phases like garnet and zircon to hold the HREEs. In the migmatite, the LREEs are more evenly distributed compared to the HREEs than in the amphibolite (LaN/YbN = 38.76 and 12.30, respectively). The notable downward trending slope from La to Lu on the REE spidergraph graphically indicates this. The albitized granite has nearly similar REE patterns as quarts-feldspar-pegmatite, but with lower total ΣREE (5.17 and 15.82, respectively). The REE diagram for albitized granite (Fig. 15c) and pegmatites (Fig. 15b) reveals the segmentation of neighboring elements into successive troughs and crests, creating a jagged-edge pattern known as the REE tetrad effect. The REE tetrad effect observed is the W-type, which begins with a downward convex from La, alternating progressively with increasing atomic number to Lu (Bea, 2015). The observed trend may be the result of hydrothermal fluids leaching the more mobile REEs relative to the less mobile ones. The albititzed granite and quartz-feldspar-pegmatite both have well-pronounced positive Eu anomalies (mean Eu/Eu* = 7.28 and 9.73, respectively). The abundance of early fractionated plagioclase feldspars in the rock, formed in a region of low oxygen fugacity, may explain this (Milord et al., 2000; Winter 2014). Albitized granite and quartzfeldspar pegmatite have average LaN/YbN values of 1.5 and 1.7, respectively, indicating a moderate fractionation of the LREEs relative to the HREEs. The quartz-muscovitepegmatite has a low mean ΣREE (5.07) and a REE tetrad effect pattern that is similar to that seen in albitized granite and quartz-feldspar-pegmatite. However, there is a clear negative Eu anomaly (Eu/Eu* 0.44). The LREEs in the rocks have a slight to moderate fractionation relative to the HREEs, with a LaN/YbN value ranging from 1.00 to 1.8. In the ternary Rb-Ba-Sr plot of El Bouseily and El Sokkary (1975) (Fig. 15d), the alkali granite, albitized granites, and pegmatites all fall into the field of strongly differentiated granites. The modified triangular discriminating plot of Ti- Sn-(Nb+Ta) after Kuster (1990) shows that the albitized granite, quartz-feldspar-pegmatites, and quartz-muscovitepegmatites all lie in the zone of albitization (Fig. 15e). The quartz-muscovite-pegmatite and albatized granite show how highly evolved they are by plotting close to the peak in the field of strongly differentiated granites. The alkali granite, on the other hand, plots close to the normal granite boundary, much closer to the Ba corner. After Kaur et al. (2012), the plot of Rb vs. Na/K (Fig. 15f) shows the progressive depletion of Rb with advancing albitization. We can see from the plot that the quartz-feldspar-pegmatite and the albitized granite have been albitized the most and have the least amount of Rb.

Fig. 15. Chondrite normalized plot (After Nakamura, 1974) rare elements (REE) pattern for the (a) metamorphic rocks, (b) Pegmatites, (c) granites from the study area, (d) Plot of Rb-Ba-Sr after El Bouseily and El Sokkary (1975) for granitic rocks, (e) Modified Triangular Ti-Sn-(Nb+Ta) Plot for albitized granite and pegmatites in the study area (after Kuster, 1990), (f) Plot of Rb vs Na/K after Kaur et al., (2012) showing the progressive depletion of Rb with the advancing albitisation of K-feldspar.

3.4. Mineral Potential Diagrams

We used plots of Ta versus Cs (Fig. 16a), Ta versus K/ Cs (Fig. 16b), and Ta versus Ga (Fig. 16c) to distinguish between mineralized and non-mineralized pegmatites, taking into account the Beus (1966) and Gordiyenko (1971) lines of mineralization. Both the Beus (1966) and Gordiyenko (1971) lines of mineralization are below the quartz-feldsparpegmatite plot. The quartz-muscovite-pegmatite plot, on the other hand, is mostly above the Beus (1966) line of mineralization but below the Gordiyenko (1971) lines of mineralization. We also used plots of K/Rb versus Cs (after Erny and Burt, 1984) (Fig. 16d) and K/Rb versus Cs (after Trueman and Erny, 1982) (Fig. 16e) to determine the mineralization potential in pegmatites. The plots revealed the presence of quartz-feldspar-pegmatite in the barren field, and the presence of muscovite extracts in the rare metal pegmatites, similar to the enrichment of Be and Li. Ballouard et al. (2016) evaluated the mineralization potential of the granites in the study area using a plot of Nb/Ta versus Zr/Hf, as shown in Fig. 16f. The albitized granite is located in the field of rare-metal granites and Sn-W-U-related granites, while the alkali granite, with Nb/ Ta values ranging between 5 and 16, is specifically located in the barren granite field. Therefore, we cannot classify any of the granites in the area as fertile granites capable of giving birth to rare metal pegmatites.

Fig. 16. (a) Plot of Ta Versus Cs tor The Muscovites of the pegmatites in the Kwarra area, (b) Plot of Ta Versus K/Cs For The Muscovites of Pegmatites in the study area. (After Beus 1968), Gordiyenko (1971), (c) Plot of Ta versus Ga for the pegmatites in the study area (After Černy and Burt, 1984), (d) Plot of K/Rb versus Cs for the pegmatites (after Černy and Burt, 1984), (e) Classification of the pegmatites using the plots of K/Rb versus Cs (after Trueman and Černy 1982), (f) Nb/Ta versus Zr/Hf diagram differentiating the barren granites and granites hosting ore deposits (after Ballouard et al., 2016).

3.5. Tectonic Discrimination Diagram

In the Rb vs. Y+Nb tectonic discrimination diagram, Pearce et al. (1984) (Fig. 17a) plot the pegmatites and albitized granite sample in the syn collisional granite field associated with orogenic events. The alkali granite plot within the within-plate field is consistent with A-type granites. Figure 17b shows the plot of FeOt/MgO vs. Zr+Nb+Ce+Y, which Whalen et al. (1987) used to separate A-type granites from other granitoids (M-, I-, and S-type). The alkali granite falls into the A-type granite field. The alkali granites in the study area are part of both the withinplate granite field (Fig. 17a) from Pearce et al. (1984) and the A-type granite field of the Ga/Al vs. Zn plots from Whalen et al. (1987). They also belong to a plot of Nb- Y-Ce (Fig. 17c) for A1, A2 granite discrimination after Eby (1992). The discrimination plot reveals the granités A2 nature, which is considered a subtype of the I-type and indicates a crustal source that is not metasedimentary. These magmas belong to the A2 group. They are made up of continental crust or underplated crust that has gone through a collisional or island arc magmatism cycle (Eby, 1992).

Fig. 17. (a) Rb vs. Y+Nb tectonic discrimination diagram after Pearce et al. (1984), (b) Plot of FeOt / Mgo vs. Zr+Nb+Ce+Y for discriminating A-type granites after Whalen et al., (1987), (c) Triangular plot of Nb-Y-Ce for distinguishing the alkali granites into A1 and A2 granite (After Eby, 1992).

4.1. Geochemistry and Structural Geology of the Polycyclic Basement Rocks

The pegmatites are displaced by the polycyclic migmatite gneiss complex, which includes migmatites, banded gneisses, granite gneiss, and amphibolite. As Obaje (2009) says, petrographic evidence shows that the Pan-African reworking partially melted and re-crystallized many of the minerals that make up the Migmatite-Gneiss Complex. Most of the rock types showed medium to upper amphibolite facies metamorphism. Faulting and fracturing marked the end of the orogeny, leading to regional shearing and tectonometamorphic evolution of the terrain (Gandu et al., 1986; Olayinka, 1992). The type of deformation varies greatly between rock types, as well as pressure, temperature, and strain rate. The variation between rock types is also due to the fact that the constituent minerals of each rock type have different mechanical properties. When the pressuretemperature (P-T) is low and the crustal level is high, rocks tend to be brittle and break apart at all scales (cataclasis), as seen in amphibolites. In the upper amphibolite facies and higher grades of metamorphism, on the other hand, grains can move around by diffusing atoms (diffusional creep). This can be a big process that helps with ductile deformation (Andy, 1998), as shown by the ptygmtitic folding (Plate 19b) in the migmatite. A "brittle-ductile" transition happens when rocks show signs of both brittle and ductile or semi-brittle conditions. This happens somewhere between the two end-member scenarios of brittle and ductile deformation (Murrell, 1990). The pinch and swell structures devolved in the banded gneiss clearly display this type of deformation. At higher temperatures, both quartz and feldspar experience ductile deformation. Other minerals, such as hornblende, also experience brittle deformation at low metamorphic grades but behave ductilely at high metamorphic grades. This gives rise to strain partitioning, with some areas experiencing only low strain while others become highly strained. This partitioning occurs from the macroscale right down to the microscale, with significant strain variations forming pinch and swell structures (Andy, 1998).

The amphibolite has the lowest SiO2 content among the rocks in the study area. It also has the lowest K2O and Na2O values of 0.93 and 0.44 wt%, respectively. The high Fe2O3 (10.65 wt%), MgO (7.57 wt%), and CaO (12.35 wt%) values of the amphibolite reveal its ferromagnesian nature. The rock also has the highest value of TiO2 at 0.93 wt%, indicating the presence of mafic-rich phases. The HSFE-depleted rock has low values for Zr, Y, Nb, Hf, and Ta, indicating that it was formed from protoliths deficient in garnet and other accessory phases such as zircon, rutile, and titanite. It also shows low values for the LILE, such as Rb, Ba, and Sr with values of 13, 145, and 45 ppm, respectively. The rock is generally REE depleted (ΣREE = 48.26). The rock exhibits a subparallel REE pattern, indicating a slight enrichment of the LREEs over the HREEs (LaN/YbN = 1.72). There is a slight positive Eu and Tb anomaly. Petrography reveals that the amphibolite consists of hornblende, augite, orthoclase, plagioclase, and quartz, with the majority of the quartz being highly fractured. Hornblende and augite crystals are primarily subhedral poikiloblastic crystals with quartz inclusion and two sets of cleavage. The outcrop's brecciated and scattered nature clearly demonstrates the rock's deformed nature, which is characteristic of rocks formed during faulting or other crustal deformation processes. The amphibolites coincide with major linear elements in the study area, attesting to their highly deformed nature.

The granite gneisses are the most common rock type in the mapped area. Its SiO2 value is 67.79 wt%, with relatively high Al2O3, Fe2O3, K2O, and TiO2 concentrations of 15.3, 3.41, 4.02, and 0.517 wt%, respectively. Ga levels are higher in granite gneiss (26 ppm) but lower in Nb, Zr, and Y (14, 134, and 14 ppm, respectively). This is different from the crustal abundance of Ga, Nb, Zr, and Y (15, 20, 165, and 33 ppm) (Taylor, 1964). The amount of Sr (323 ppm) in the rock is lower than the crustal abundance of 375 ppm. The amounts of the other lithophile elements, Ba (425 ppm) and Rb (90 ppm), are slightly higher than the crustal abundance (Taylor, 1964). The rock's REE pattern shows the fractionation of LREEs relative to HREEs (LaN/YbN = 12.3). It was noted by Hugh in 1993 that the minerals olivine, orthopyroxene, and clinopyroxene may separate the light REEs from the heavy REEs, with a partition coefficient that ranges from La to La. The banded gneisses have a medium-sized grain and show notable pinch and swell structures along with strongly lined and weakly to highly foliated gneissic structures. The banded gneiss mostly occurs as low laying and pavement outcrops and is characterized by well-developed mafic and felsic bands. It has been seen that diffusive mass transfer and in situ partial melting make any primary compositional banding stronger and create more separation of felsic and mafic minerals (Andy, 1998), which can be seen in the banded gneiss. Quartz and feldspars make up the felsic bands, while biotite makes up the dark bands. The prominent pinch and swell structures observed in the banded gneisses are indicative of semi-brittle deformation (Garner et al., 2015). The banded gneiss and migmatite gneiss both have a similar geochemistry. They are fairly rich in SiO2, with values of 67.79 and 70.25 wt%, respectively. Both have relatively high Al2O3 concentrations of 14.96 and 14.23 wt%, respectively. The banded gneiss is relatively rich in CaO (3.24 wt%) and Na2O (4.12 wt%) but poor in K2O (2.19 wt%).

4.2. Geochemistry and Structural Geology of the Granites

4.2.1. Alkali Granites

The alkali granite in the study area is part of the wellstudied Nigerian Younger Granites. It is an extension of the Pankshin Younger Granite Ring Complex, which extends from the Pankshin area on the Jos Plateau to parts of Wamba, Nasarawa State, northcentral Nigeria. The alkali granites represent the youngest lithologic unit in the study area and intrude the migmatite gneiss in the northeastern portion of the study area. The slightly lower AI for alkali granite indicates that it is less evolved than the pegmatites and albitized granite (Abdelfadil et al., 2016). The alkali granite consistently plots in the A-type granite field on the Ga/Al vs. Zn diagram of Whalen et al. (Fig. 17b). The alkali granites align with the I-type granite field on the A/CNK versus SiO2 (Fig. 13b) protolith discrimination diagram, following the work of Chappel and White in 1974. Whalen et al. (1987) noted that highly fractionated felsic I-type granites can have Ga/Al ratios, as well as some major and trace element values that overlap those of typical A-type granites. Therefore, extreme fractionation from calc-alkaline I-type magmas could potentially explain the A-type characteristics of the studied rocks. We interpret it to represent rocks that crystallized from magmas derived from continental crust or underplated crust that underwent a cycle of continent-continent collusion or island arc magmatism (Eby, 1992). It plots in the A2 subgroup (Fig. 17b) for A-type granite. It also plots the within-plate granite field of the Rb vs. Y+Nb tectonic discrimination diagram of Pearce et al. (1984) (Fig. 17a), which is characteristic of A-type granites (Winter, 2014). The alkali granite is rich in SiO2 and has the highest mean SiO2 value (74.18 wt%) amongst the rocks in the study area. It also has the highest mean values for Fe2O3 (2.72 wt%) and CaO (0.64 wt%). The HFSE elements Ga, Zr, Hf, Nb, and Y, with mean values of 27 ppm, 286 ppm, 7.05 ppm, 31 ppm, and 96.5 ppm, respectively, enrich it, indicating its production from melts rich in accessory phases like zircon, garnet, and sphene (Hanson, 1978; Hanson, 1980). The alkali granites have the highest mean Ba and Ce values, at 365 ppm and 164.5 ppm, respectively. In both megascopic and microscopic observations, the alkali granites show no signs of significant deformation. It occurs as high-rising plutons in a ridge-like manner, extending much further beyond the study area from the north-eastern portion. In El-Bouseily and El-Sokkary's (1975) Rb-Sr-Ba ternary diagram (Fig. 15d), the alkali granites are located in a strongly differentiated granite field, near the normal granite boundary and significantly closer to the Ba corner of the plot. The alkali granite REE pattern is subparallel, LREE enriched (LaN/ YbN value of 6.38), prominent negative Eu anomaly (mean Eu/Eu* = 0.27), and characterized by a nearly flat HREE profile consistent with REE patterns for A-type granite (Eby, 2011; El Hadek et al., 2016).

4.2.2. Albitised Granite

Ba, Pb, and Sr severely deplete the albitized granite. Hofman (1972) noted that the transformation of K-feldspar and calcic plagioclase to albite causes severe depletion in Rb, Ba, Sr, and Pb. The metasomatic process may have also led to the severe leaching of the REEs (ΣREE = 5.17) as observed in these rocks. The transformation of accessory phases, such as monazite to apatite and thorite in albitized granitoid, may contribute to the REE depletion (Boulvais et al., 2007). The albitized granite has a low K/ Rb (36.7–67.4) and a low Nb/Ta ratio (5.2–7.9), reflecting their highly evolved nature. The albitized granite is SiO2- rich, with a mean value of 73.89 wt%. It has the highest Na2O value of 5.06 wt%, reflective of its albite-rich nature. The rock contains a relatively low K2O value (3.71 wt%) but a high Na2O concentration, indicating the extent of K replacement by Na (Kaur et al., 2012). It also has a high P2O5 value of 035–038 wt%. The alkali granites exhibit extremely low CaO (0.11–0.14 wt%), TiO2 (0.002–0.004 wt%), and Fe2O3 (0.76–0.89 wt%) values, a result of leaching during hydrothermal alteration (Denies and Mark, 2000). The most noticeable thing about albitized granite is the presence of broad albite lamellas. These are thought to have formed when albite completely replaced Kfeldspar (Moody et al., 1985). According to Milord et al. (2000), the albitized granite exhibits a prominent positive Eu anomaly (Fig. 15c) and a low Rb/Sr ratio, indicating the accumulation of early-formed feldspar crystals rich in Eu. The REE plot for albitized granite shows that neighboring elements from Eu-Lu are getting more and less, making a pattern called a "REE tetrad." We interpret the W-type REE tetrad pattern as an open system fluidmelt reaction in a magmatic-hydrothermal system during the final stages of crystallization (Irber W., 1999; Zhao et al., 2002). Because the rock lacks biotite and is rich in feldspar, the positive Eu anomaly may indicate that it originated from melts depleted in other REEs. This is likely due to the accessory phases containing these REEs remaining armored in the biotite, preventing the melt from accessing them (Milord et al., 2000). The albitized granite occurs in an uplifted region as highly deformed and scattered outcrops. It contains several joints prominently running along the NE-SW direction. Along the joints, there is a thin zone of fine-grained greenish, possibly chlorite, resulting from hydrothermal alteration. The analysis of thin sections shows sericitization along the edges of the muscovite and feldspar grains, which is a sign of hydrothermal change. In the hand specimen, the rock is whitish and fine-grained. In the plot of Nb/Ta versus Zr/Hf (Fig. 16f), following Ballouard et al. (2016), the albitized granite separates from the field of rare-metal granites and Sn-WU related granites. Therefore, it appears highly improbable that it can serve as a fertile granite for rare metal pegmatites.

4.2.3. Geochemistry and Mineral Potential of the Pegmatites

It has been said that the trace element makeup of muscovite mica is another good way to tell how much fractionation there is in rare-metal pegmatites (Tischendorf et al., 2001; London, 2008). In muscovite mica, the amounts of Rb, Cs, Mn, Ga, Tl, Sn, and Ta go up, but the ratios of K/Rb, Ba/ Rb, Rb/Sr, Na/Ta, and Fe/Mn go down. This is typical of pegmatites that are highly fractionated (Černy et al., 1985; Shaw et al., 2016). The study area's muscovite samples exhibit enrichment in various trace elements, such as Rb (up to 3957 ppm), Be (up to 55 ppm), Sn (up to 798 ppm), Ga (up to 179 ppm), W (up to 802 ppm), and Nb (up to 194 ppm). The muscovite exhibits concentrations of other trace elements such as Cs (up to 133 ppm), Ta (up to 91.1 ppm), and Tl (up to 11.7 ppm), while it displays a relative depletion in Ba, Sr, REE, Th, and U. These patterns of enrichment and depletion could indicate the presence of micro-inclusions within the muscovite crystals (Shaw et al., 2016). On the other hand, the quartz-feldspar-pegmatite sample is observed to be enriched in Ba, Th, and Pb but depleted in Be, Nb, Ce, and Ti. The quartz-muscovite pegmatite has a higher content of Be, Mn, Nb, Ta, and W than the quartz-feldspar pegmatite. The presence of accessory minerals like beryl and columbite in the former is responsible for these differences. The muscovite samples are rich in Al, K, Sn, and Rb but low in Ca, Ba, and Sr. Rubidium contents in the muscovite samples range between 2946 and 3957 ppm. However, quartz-feldspar-pegmatites samples exhibit far lower concentrations, ranging between 83 and 542 ppm. In muscovite samples, K/Rb ratios are generally lower, ranging between 15.61 and 18.06, compared to quartz-feldspar-pegmatite samples with K/Rb ratios between 88.79 and 463.08 (Table 3). The low K/Rb ratio of the muscovite samples expresses their more evolved nature (Černy, 1982). El-Bouseily and El-Sokkary (1975) (Fig. 15d) cluster the quartz-muscovite-pegmatite and albitized granites near the peak of the strongly differentiated granite field. The alkali granites plot in the field of strongly differentiated granite close to the normal granite boundary and much closer to the Ba corner of the plot. Černy (1982) observed that the ratios K/Rb, Ba/Rb, Rb/ Sr, and Na/Ta all tend to decrease to extremely low values with increasing pegmatite fractionation. So, the K/Rb, Ba/ Rb, Rb/Sr, and Na/Ta values for the quartz-muscovitepegmatite trace element ratios are low, which means that the pegmatites are highly fragmented. We observe a negative correlation between K2O and SiO2 in the quartzmuscovite- pegmatites, and a positive correlation between Na2O and SiO2 (Fig. 10), which could potentially indicate Na- K exchange due to the influence of coexisting metasomatic fluid during the magma's evolution.

In particular, REE patterns for rare-metal pegmatites show strong negative Eu anomalies and more MREE and HREE than LREE. These strong negative Eu anomalies could mean that the plagioclase is being broken up, that a source material low in plagioclase is melting, or that a source material high in plagioclase is melting but the plagioclase is not melting (Zhao et al., 2002). On the other hand, the quartz-feldspar-pegmatite is marked by a prominent positive Eu anomaly (Fig. 15b), which is indicative of Eu fractionation into feldspars during the early stages of fractional crystallization, possibly in the lower crust (Shaw et al., 2016). Rudnick and Gao (2003) noted that the positive Eu anomaly is characteristic of rocks sourced from the lower crust. The occurrence of a weak negative Ce anomaly in the quartz-muscovite-pegmatite, accompanied by a significant negative Eu anomaly, indicates the role of late-stage or metasomatic fluids in the genesis of these rocks (Akintola et al., 2012 b; Taylor et al., 1986). The quartz feldspar pegmatites, on the other hand, have a weakly negative Ce signature and a strong positive Eu anomaly on the REE trend (Fig. 15b). The pegmatites and albitized granite display a noticeable W-type REE tetrad effect pattern. The W-type tetrad effect is interpreted to indicate open system conditions during granite crystallization (Irber W., 1999). Zhao et al. (2002) found that pegmatites mostly formed through fluid/melt processes and often have a strong Eu depletion and REE tetrad effect in the melt as well as in the rock-forming and accessory minerals that crystallized from this melt. This was seen in both types of the Kwarra pegmatite. A number of highly evolved granitic rocks, such as leucogranites and pegmatite, have been shown to have similar REE tetrad effects (Walker et al., 1986; Zhao, 1988; Jolliff et al., 1989; Yurimoto et al., 1990; McLennan, 1994; Irber, 1999). The main chemical differences between quartz-muscovite pegmatites and quartzfeldspar pegmatites are due to different compositions of the magmas that formed them. These differences may have been caused by the magmas mixing with rocks from the crust as they rose, or they could have come from different sources (London 2008, Černy 1991).

The mineral potential for rare metals such as Rb, Cs, Be, Y, REE, Zr, Hf, Nb, and Ta (Smirnov et al., 1986) in the area's pegmatite was assessed using geochemical criteria and representative diagrams for the whole rock pegmatites and mineral extracts. The amount of Ta and Cs in muscovite mica has also been used to find places where Ta-Nb minerals might be present in pegmatites (Selway et al., 2005). According to Selway et al. (2005), pegmatites containing muscovite with a Ta concentration greater than 65 ppm and a Cs concentration above 500 ppm are highly likely to contain Ta-Nb mineralization. The muscovite samples, on the other hand, have Ta levels between 65.5 and 91.1 ppm and Cs levels between 37.4 and 133 ppm, which are both much lower than 500 ppm. This means that they shouldn't be thought of as a major source of Nb-Ta mineralization. On the other hand, the quartz-feldsparpegmatite has a Ta concentration ranging from 11.3 to 21.7 ppm and a Cs concentration ranging from 1.3 to 14.9 ppm. When we look at the Ta-Nb mineralization potential trend of Ta versus Cs (Fig. 16a), Ta versus K/Cs (Fig. 16b), and Ta versus Ga (Fig. 16c), we can see that the quartzfeldspar- pegmatite plot always lies below the mineralization lines found by Beus (1966) and Gordiyenko (1971), while the quartz-muscovite-pegmatite plot usually lies above the Beus (1966) line of mineralization but below the Gordiyenko (1971) lines of mineralization. According to Ta-Nb mineralization potential plots for the pegmatites, the quartz-feldspar-pegmatite is interpreted as barren, while the quartz-muscovite-pegmatite can be described as not well developed.

We adapted a K/Rb versus Cs discrimination plot (Fig. 16d) from Erny and Burt (1984) to determine the mineralization potential of the pegmatites. We used a discrimination line to distinguish between the rare-metal class and the barren class. This plot showed that the quartz-quartz-muscovite-pegmatite belonged to the field of rare metal pegmatites, while the quartz-feldspar pegmatites belonged to the field of empty pegmatites. A second plot used to further separate the pegmatites into Be-class and Be-Li class pegmatites after Trueman and Černy (1982) shows that the quartz-muscovite-pegmatite are of the Be- Li class (Fig. 16e). The Kwarra pegmatite has a relatively high Sn concentration (up to 798 ppm) (Table 3), which is high enough to be associated with cassiterite mineralization. The method used to measure the amounts of different trace elements doesn't record the concentration of lithium. However, samples of quartz-muscovite-pegmatite show higher levels of lithium and beryllium in the plot of K/ Rb versus Cs (Fig. 16e) of their muscovite after Trueman and Černy (1982), so they can be thought of as higher in lithium. The fact that the quartz-muscovite-pegmatite has a mean concentration of 63.9 ppm of Cs and 22.3 ppm of Ta shows that the Cs-Ta mineralogy is not well developed in the rock. However, their concentrations significantly exceed the crustal abundance of 2 and 3 ppm, respectively (Taylor, 1964). Based on their chemistry and mineralogy, quartz-muscovite-pegmatite (rare metal) is most similar to LCT-type (Lithium, Cerium, and Tantalum) pegmatites and is therefore likely to have an ultimate source in sedimentary rocks (erny, 1991; Martin and De Vito, 2005; London, 2008; erny et al., 2012). The graph of A/CNK vs. SiO2 from Chappel and White (1974) (Fig. 12b) shows that the pegmatites came from an S-type source. According to Oyebamiji et al. (2018), LCT pegmatites have higher amounts of Be, B, F, P, Mn, Ga, Rb, Nb, Sn, and Hf. This is in line with the fact that the rock is high in Sn, Ga, Rb, Nb, and W.

4.3. The Role of Hydrothermal Alteration on the Geochemistry and Mineralogy of the Granitoids

Kuster (1990) modified the triangular Ti-Sn-(Nb+Ta) discriminant plot, showing the Pegmatites in the Kwarra area and the albitized granite plots in the albitization zone (Fig. 15c). The norm calculations (Table 2) show that the Ab value rises in the albitized granite, the quartz-feldsparpegmatite, and the quartz-muscovite-pegmatite, with mean Ab values of 12.1, 30, and 42.8, respectively. This increase in normative Ab is indicative of albitization (Kinnard et al. 1985). Albitization occurs during sodic metasomatism and is characterized by the exchange of Na for Ca or K and, to a lesser extent, Ca for Fe and Mg. Plagioclase and/ or K-feldspar are changed into almost pure albite by hydrothermal fluids during the albitization process (Kaur et al., 2012). Local albitization and local greisenization in the north-western portion of the studied area represent the effect of the sodic metasomatic stage. One of the notable megascopic indicators for strong and pervasive albitization is the whitened feldspars on the affected granitoid outcrops, a phenomenon commonly observed in albitized granite, quartz-feldspar-pegmatite, and muscovite granite (Baker, 1985; Charoy and Pollard, 1989; Petersson and Eliasson, 1997). The mineral groups that form during sodic metasomatism depend on how strongly the rock and fluid interact. Granites that are highly peralkali show the most impact (Bowden and Kinnaird, 1984). Kinnard et al. (1985) characterized soda metasomatism by increasing Ab, as observed in albitized granite, bulk granite, and muscovite granite, all of which exhibit strong albitized signatures with mean normative Ab values of 12.1, 30, and 42.8, respectively. Kuster (1990) conducted subsequent work on a regional scale covering the study area, clearly identifying these albites as products of Na-metasomatism. The amount of Na2O in the albitized granitoids goes from 3.55 wt% in the quartz-feldspar-pegmatite to 7.0 wt% in the albitized granite, while the amount of K2O goes down from 5.38 wt% to 3.71 wt%, which means that K and Na are exchanging places. As seen in the albitized granite and pegmatites, the metasomatic change of K-feldspar and calcic plagioclase to albite leads to a significant loss of Rb, Ba, Pb, and Sr (Kaur et al., 2012). The amount of Rb in the quartzmuscovite- pegmatite decreased from 3399 ppm to 41.5 ppm in the albitized granite and 21.6 ppm in the quartz-feldsparpegmatite. This is consistent with the progressive albitization of K-feldspar (Fig. 15d) and the loss of mafic phases, since Rb is more compatible with K-feldspar, biotite, and potassic hastingsite than it is with albite (Kaur et al., 2012). As Na/K levels rise, the graph of Rb vs. Na/K (Fig. 15d) shows that Rb levels decrease over time. This is called progressive albitization of K-feldspar. However, it is important to suggest that the initial Rb concentration in the rocks was significantly influenced by fractional crystallization, with albitization only slightly altering the Rb concentration. The metasomatic process may have also caused the REEs to be heavily leached (mean ΣREE = 5.17 for albitized granite, 15.82 for quartz-feldspar-pegmatite, and 5.07 for muscovite granite), as well as the W-type REE tetrad effect that can be seen in these rocks. The sodic metasomatic stage is characterized by the coexistence of crystals and supercritical fluid (El Hadek et al., 2016). The existence of a W-type REE tetrad effect supports this, indicating an open system, late-stage magmatic-hydrothermal interaction during the formation of the albitized rocks (Irber, 1999; Zhao et al., 2002). The loss of REE may be connected to the change of other minerals, like monazite to apatite and thorite in perauminous albitized granitoid (Boulvais et al., 2007). Along with the albitization process, the original Ti-Fe oxides are broken down, uranium levels rise, and columbite with small amounts of cassiterite, thorite, and xenotime are added (Obaje, 2009). Very little Fe and Ti are found in albitized granite, quartz-feldsparpegmatite, and quartz-muscovite-pegmatite (Fe2O3 = 0.82%, 0.73%, and 2.05% of the total weight, respectively). The concentrations of TiO2 are 0.003 wt%, 0.008 wt%, and 0.002 wt%, respectively. Albitization also concentrates HFSE, especially Nb and Ta, as observed in the albitized granite and pegmatites. While this may not be well pronounced in the albitized granitoids, it is however important to note that the albitized rocks have relatively notably elevated Nb and Ta concentrations compared to the other rocks in the study area (Table 3).

Sodic change is tightly controlled by fractures (Battles, 1994); becciation is well developed in places where albitization is strong, especially in the albitized granite and quartz-muscovite-pegmatite. Hydrothermal fluids migrate to areas of low pressure and in regimes of extensional and strike-slip tectonics. It is not surprising that fluids concentrate into dilatant fault zones. Apart from brecciation and cataclasis, we commonly observe intense silicification, sericitization, chloritization, or other chemical or mineralogical changes indicative of high fluid flux in these fault zones (Andy, 1998). In the Arum area, the albitized granite is well deformed and brecciated. The thin-section study reveals that the original K-feldspar has been albitized, and sericites are developing along the mica grain boundaries. We observe the most pronounced albitization along the NE-SW linearments in the northwestern flank of the study area, which are characterized by high lineament densities. Dillies and Einaudi (1992) suggest a depth of 1–4 km occurring at a low temperature ranging between 200 and 400°C. The fluid inclusions study conducted by El Hadek (2016) reveals that albitization occurred at high temperatures (350°C–410°C) in vapor-rich aqueous fluid. Sodic metasomatism's shallow depth and low temperature characteristic can explain the observed brittle deformation in the albitized rocks. Sodic metasomatism is important for the economy because it brings in ore minerals that contain nickel. These minerals show up as columbite in peraluminous biotite granites, pyrochlore in peralkali granites, and fergusonite in metaluminous hornblende biotite granites, but they are not as important (Obaje, 2009). The Nigerian provincés albitized granites have the highest uranium enrichment (Bowden et al., 1981); this is also evident on a smaller scale in the study area, where the albitized rock samples exhibit elevated uranium concentrations compared to the others. It ranges from 1.9 to 23.4 ppm in the quartzfeldspar pegmatite, from 0.4 to 15 ppm in the quartzmuscovite pegmatite, and from 6.8 ppm on average in the albitized granite. All the albitized rocks have a mean uranium concentration greater than the crustal abundance of 2.7 ppm (Taylor, 1964).

4.4. Petrogenesis

The Pan-African rare-metal pegmatites in central Nigeria formed because of post-kinematic, late-tectonic granite magmatism (Kuster, 1990). Multiple intrusive activities, being positioned in a way controlled by structure, and geochemical specialization patterns are some of the things that make this plutonism unique. It happens after a big orogenic event. Most geologists think that the pegmatites in the Kwarra area came from intermediate to felsic crustal sources because they are peraluminous (Miller and Mittlefehldt, 1985; Chappel and White, 2001). A lot of experiments have shown that biotite and muscovitecontaining metapelites melt to form peraluminous granites (Gardien et al., 1995; Patino Douce and Harris, 1998). As Chappell and White (1974) suggested, the Kwarra pegmatite is a type of granitic rock that comes from sedimentary protoliths (S-type). These were formed by the melting of sedimentary and/or metamorphosed sedimentary or supracrustal rocks, such as metapelites, according to Chappell and White (2001). In 1990, Kuster wrote that these kinds of metasedimentary rocks can be found in large amounts in the high-grade basements of the Wamba area and nearby areas. These rocks are thought to be a combination of semipelitic sediments from a metavolcano. This metapelitic rock provides a source of appropriate composition for the LCT rare-metal pegmatite end-members (Černy, 1991). Most LCT-type pegmatite fields are thought to have formed when melts from sedimentary rocks were split into very small granitic pieces (Erny, 1991; Martin and De Vito, 2005; London, 2008; Erny et al., 2012). Fractional crystallization is a feature of this type of magmatic process. As the leftover melt moves farther away from the solidifying parent granitic source, it picks up more rare elements and fluxes (London, 2008; Trueman and Černú, 1982). It is common for mica-rich melts to not separate completely in collisional zones, which creates LCT granitic pegmatites with a lot of trace elements (Černú et al., 2012). Different A/CNK values, trace element patterns, and mineralogy were found in the (rare-metal) quartzmuscovite pegmatites and the barren quartz-feldspar pegmatites. This suggests that the melting process used different materials. It is thought that the collision of the Pan-African continents and the thickening of the crust caused some of the older crust to melt. This caused several batches of magma to form and move in with slightly different chemical makeups, now making up major plutons (Kuster, 1990). The pegmatites' syncollisional setting supports their formation, which is linked to the process of crustal thickening (Pearce, 1996). Partial melting of compositionally distinct protoliths can also produce a wide compositional spectrum of granite magmas with the same degree of partial melting. Mineralogy, trace element chemistry, the stability field of sheet silicates or accessory minerals and their amount in the mineral/residuum phase could all change. This could also happen if the source metasedimentary lithology changed in a way that affected the somatics (Shearer, C.K. et al., 1992). Late Pan-African magmatic activity was multiphase, and successive plutons were able to release leftover pegmatitic melt that probably changed in different ways. This is how barren and mineralized pegmatites have been formed from different parent granites (Kuster, 1990).

The REE pattern of quartz-muscovite-pegmatites shows the impact of late-stage metasomatic fluid in an open system of magmatic and hydrothermal interaction while rocks were being formed. The quartz-feldspar-pegmatite REE trend also indicates a similar melt-fluid interaction. On average, the quartz-muscovite-pegmatite appears to be geochemically more evolved than the quartz-feldsparpegmatite. It shows higher contents of Rb, Cs, Mn, Ga, Tl, Sn, and Ta, while K/Rb, Fe/Mn, Ca, Ba, Sr, and Zr show a concomitant decrease. This trend is observed in the quartz-muscovite-pegmatites, attesting to their highly fractionated nature (Kuster, 1990; Shaw et al., 2016). Quartz-feldspar-pegmatite is very divided, which may have caused incompatible elements to accumulate in leftover melts. This could have led to a magmatic-hydrothermal system that was at the same time active. So, it appears that the metasomatic replacement and mineralization processes originated from the fluids that were left over in the pegmatite melts rather than fluids that were injected from outside (Oyebamiji et al., 2018; Kuster, 1990). However, the noticeable loss of REE, Ba, and Sr as the Na/K ratio rises may also be due to metasomatic recrystallization occurring during albitization and fluid interaction (Kaur et al., 2012). Trace elements that don't change easily during postmagmatic alteration (Ti, Nb, Ta, and Cs) are very low in the quartz-feldspar pegmatite but not as low in the quartz-muscovite pegmatite. This may be a better reflection of the magmatic compositions at the time the rocks were formed. The quartz-feldspar pegmatite has a positive Eu anomaly, while the quartz-muscovite pegmatite has a negative Eu anomaly. This strongly suggests that they came from different places, most likely the lower crust and the upper crust, respectively (Rudnick and Gao 2003). In terms of structure, the pegmatites are found in an area with a lot of stress. This is where the country rocks are severely deformed, foliated, and affected by brittle-ductile shear zones. Lineament analysis and structural trends observed in joints, faults, veins, and foliation show that NE-SW, NS, and NNE-SSW are the principal stress directions. The host rocks can be seen to have NE-SW (dextral) and NWSE (sinistral) conjugate systems. These are the results of late Pan-African brittle deformation (Ball, 1980). The migmatite gneisses are mostly made up of quartz-feldspar pegmatite that moves in NW-SE and E-W directions. The banded gneisses and granite gneisses are mostly made up of quartz-muscovite pegmatite that moves NE-SW. The albitization is observed to be most pronounced along the NE-SW lineaments in the northwestern flank of the study area, characterized by high lineament densities. Brecciation is well developed in the areas where the albitization is well pronounced, especially in the albitized granite and quartzmuscovite- pegmatite. Sodic metasomatism happens at low temperatures and shallow depths, which may explain why the rocks that have been altered by albitization are breaking apart easily. A depth of 1–4 km occurring at a low temperature ranging between 200 and 400°C has been suggested for such a sodic alteration (Dillies and Einaudi, 1992). Based on the geochemistry, geological structures, and mineralogy of the pegmatites, it is therefore suggested that the quartz-feldspar-pegmatite represent late-stage residual melts derived from less evolved granitic parent in the lower crust, which have been derived by fractionation, while the quartz-muscovite-pegmatite are products of highly fractionated late-stage magma derived from fertile granite at depth and modified by interaction with a coexisting hydrothermal phase during crystallization beneath the amphibolite belt. Reactivated ancient fractures, predominantly NE-SW, NNE-SSW, and N-S trending, as well as minor NNW-SSE and E-W, controlled the emplacement of these rocks. No fertile granite was mapped in the study area, suggesting that it is either buried deep within the subsurface or has a rather distal relationship with the rare metal pegmatite. Trueman and Černy (1982) noted that rare metal pegmatites are typically the most distant from their parent granites, having undergone increasing fractionation and concentration of rare elements and volatiles with increasing distance.

4.5. Relationship between Pegmatites and Rare Metal Mineralization in the Study Area

The relationship between pegmatites and rare metal mineralisation is intricately linked to their genesis, geochemistry, and associated host rocks. Pegmatites, particularly those of the Lithium-Cesium-Tantalum (LCT) type, are widely recognised as fertile sources of rare metals, including Rb, Cs, Ta, and Nb. In the studied region, the pegmatites are derived from highly fractionated melts, representing the final stages of granitic crystallisation. These magmas are enriched in incompatible elements, which concentrate in residual melts and subsequently crystallise to form mineralised pegmatites. The geological setting is defined by the polycyclic migmatite-gneiss complex, which has undergone extensive tectonometamorphic evolution, resulting in varying degrees of deformation and metamorphism. Pegmatites in the area often intrude migmatites, banded gneisses, and granite gneisses. The geochemical signature of pegmatites, particularly quartz-muscovite-pegmatites, reveals higher concentrations of rare metals compared to quartz-feldspar-pegmatites. This is attributed to the enrichment of elements like Rb, Cs, Sn, and Ta in late-stage magmatic fluids, coupled with fractional crystallisation and subsequent hydrothermal alteration. Structural controls, such as brittle-ductile shear zones and lineaments trending NE-SW and N-S, have facilitated the emplacement of pegmatites. Additionally, metasomatic processes, including albitisation, enhance the mobilisation and concentration of high-field-strength elements (HFSE), further contributing to mineralisation. These processes underscore the complex interplay between tectonics, petrogenesis, and hydrothermal fluid activity in determining the mineralisation potential of pegmatites (Beus, 1966; Gordiyenko, 1971; McDonough & Sun, 1995).

4.6. Conclusions

We detailed the mapping of the study area and documented the lithological units. Locally, the main lithological units in the study area are the migmatite-gneiss complex, Pan-African granitoids, and the Younger Granites. The rock types in the area are amphibolite, migmatite gneiss, banded gneiss, granitic gneiss, albitized granite, and alkali granite. Pegmatites and dolerites represent the minor rock types in the study area. The quartz-feldspar-pegmatite types are mostly found in migmatite gneiss, while the quartzmuscovite types are mostly found in banded gneiss and granite gneiss. Because they align with major study area faults, they are deformed. It was discovered that there were more structural lines in the pre-Cambrian basement rocks, such as migmatites, banded gneisses, and amphibolites, than in the Jurassic Younger Granites. The principal lineament trends are N-S, NE-SW, and NNE-SSW, with minor ENEWSW and E-W trends. The alkali granite is metaluminous, whereas the albitized granite and pegmatites are peraluminous. When the temperature and pressure are just right, the hornblende-plagioclase mineral forms in polycyclic basement rocks. This type of metamorphism is called amphibolite facies grade. There are two types of pegmatites that can be seen in the study area: pegmatites that are rich in rare metals and muscovite (quartz-muscovite) and pegmatites that are barren and low in muscovite (quartzfeldspar- pegmatite). The barren pegmatites tend to trend NE-SW, NNE-SSW, and N-S, while smaller amounts trend NNW-SSE and E-W. The quartz-muscovite-pegmatites are observed to be trending mainly along the NE-SW trend, while the quartz-feldspar-pegmatite predominantly along NW-SE and minor E-W trends. The pegmatites were formed during the Pan-African orogeny, which is characterized by crust thickening (Pearce, 1996) and syn-collisional setting (Kuster, 1990). The pegmatites are both peraluminous in nature (ASI > 1.0), with the quartz-muscovite-pegmatite showing the strongest peraluminous signature (ASI = 3.01). On average, the quartz-muscovite-pegmatites appear to be more evolved geochemically than the quartz-feldsparpegmatite. They show higher contents of Rb, Cs, Mn, Ga, Tl, W, Sn, and Ta, while K/Rb, Fe/Mn, Ca, Ba, Sr, and Zr show a concomitant decrease. The quartz-muscovitepegmatites exhibit this trend, indicating their highly fractionated nature (Kuster, 1990; Shaw et al., 2016). Because quartz-feldspar-pegmatite is very fragmented, it's possible that incompatible elements built up in the leftover melts. The W-type REE tetrad effect, present in quartz, muscovite, and pegmatites on the REE trend, suggests the simultaneous formation of a magmatic and hydrothermal system. After Kuster (1990), the pegmatites plot in the field of albitized pegmatites in the plot of Ti-Sn-(Nb+Ta) changed. This shows that hydrothermal fluids were involved in the rock formation. The amount of Rb decreases over time as albitization proceeds. This suggests that the process of albitization may have changed the Rb content of the pegmatites and albatized granite in some way. The albitified rocks indicate that the sodic metasomatism may have increased the abundance of W, Na, Al, U, and Sn while decreasing the concentration of Fe, Ti, and REE. In this study, the quartz-muscovite-pegmatites contain more of the rare elements Sn, W, Rb, and Be than REE, Sr, and Ba. They also have fewer of the rare elements Ta and Nb. The mineral potential plots indicate that the rare metal pegmatites are Li-enriched and LCT-type. The quartz-muscovitepegmatite rock group's LCT and peraluminous properties suggest that the rocks were formed by the melting of sedimentary rocks, most likely metapellites. Based on the geochemistry and mineralogy of the pegmatites, it is therefore suggested that the quartz-feldspar-pegmatite represent latestage residual melts of a less evolved granitic parent that have been derived by fractionation, while the quartzmuscovite- pegmatite are products of highly fractionated late-stage magma derived from fertile granite at depth and modified by interaction with a coexisting hydrothermal phase during crystallization beneath the amphibolite belt.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The first author is grateful to Dr. A.A Ibrahim, Dr. B.A. Jolly, Prof. Saidu Baba, Prof. Ogunleye Paul and Dr. Magaji for improving on the work.

During the preparation of this work the author(s) used Qullbot.com/ Grammer Checker in order to improve the grammer for better flow. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

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Article

Research Paper

Econ. Environ. Geol. 2024; 57(6): 735-768

Published online December 31, 2024 https://doi.org/10.9719/EEG.2024.57.6.735

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Petrogenesis and Geochemical Evolution of Rocks and Pegmatites in Kwarra Area, Northcentral Nigeria: Implications for Rare Metal Mineralization

Adamu, Lukman Musa2,*, Sunday, Adedeji Ebenezer1, Ohiemi, Adukwu Fabian3, Ayuba, Rufai2, Ugbena, Kelvins Godfrey2, Baba, Yahaya2, Abraham, Templeman4, Ogunkolu, Bolade Ayodeji5, Ebeh, Austine2

1Department of Geology, Ahmadu Bello University, Zaria, Kaduna State, Nigeria
2Department of Earth Sciences, Kogi State University, Anyigba, Kogi State, Nigeria
3Department of Geosciences, Confluence University of Science and Technology Osara, Kogi State, Nigeria
4Department of History and International Relation Studies, Kogi State University Anyigba, Kogi State, Nigeria
5Department of Geography, Kogi State University, Anyigba, Kogi State, Nigeria

Correspondence to:*lukman10adamu@gmail.com, lukman10musa@yahoo.com, adamulm@ksu.edu.ng

Received: September 20, 2024; Revised: December 4, 2024; Accepted: December 17, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Although pegmatite rare metal deposits are significant sources of rare metals, their metallogenic mechanisms remain a topic of debate. Nigeria's Basement Complex and Younger Granite provinces have identified pegmatites, which comprise a variety of rock types. We classify pegmatites into two categories: quartz-muscovite pegmatite and quartz-feldspar pegmatite. The study area has three primary stress orientations: N-S, NE-SW, and NNE-SSW, with secondary trends of ENE-WSW and E-W. There are more high-ferromagnetic elements (HFS) in quartz-muscovite pegmatites than there are in the crust as a whole. These elements include Zr, Ga, Nb, and Ta. They have a high concentration of the lithophile element Rb but are deficient in Sr and Ba. Quartz-feldspar pegmatite doesn't have many high-field-strength (HFS) elements, but it has a lot of rubidium (Rb), though not as much as quartz-muscovite pegmatites. The pegmatites and albitized granite were subjected to sodic metasomatism, which changed the feldspars from K to Na and made them contain more uranium. The albitized rock types contain low total rare earth elements (REE), strontium (Sr), and barium (Ba), while the amount of rubidium (Rb) decreases with increasing albitization. It's not clear how the columbite-tantalite series or the Ta and Cs minerals formed, but the high Sn concentration in pegmatites is linked to the formation of cassiterite minerals. Rare metal pegmatite is formed when deeply buried S-type fertile granite is partially melted. On the other hand, barren quartz-feldspar pegmatite is formed from late-stage residual melts from a less evolved granitic parent.

Keywords pegmatite, hydrothermal, metasomatism, petrogenesis, Kwarra, Nigeria

Research Highlights

  • Two types of pegmatites were mapped: pegmatites that are rich in rare metals and muscovite (quartz-muscovite) and pegmatites that are barren and low in muscovite (quartz-feldspar-pegmatite).

  • The quartz-feldspar-pegmatite represent late-stage residual melts of a less evolved granitic parent that have been derived by fractionation.

  • The quartz-muscovite-pegmatite are products of highly fractionated late-stage magma derived from fertile granite at depth and modified by interaction with a coexisting hydrothermal phase during crystallization beneath the amphibolite belt.

1. Introduction

The Kwarra Area is located in the northeast central margin of the Basement Complex of Nigeria, underlain by Basement Complex rocks and parts of the Younger Granites (Fig. 1). The area is home to numerous pegmatites. Some of these pegmatites are rich in rare elements, such as Li, Be, Nb, Ta, and Sn, and are Li-, Cs-, and Ta-rich (LCT) pegmatites (Kuster 1990; Okunlola, 2005; Jatau et al., 2012).

Figure 1. Topographical map of the study area parts of Kurra sheet 189SW modified after Fed. Surveys, Nigeria 1967.

Rare metals, including Li, Be, Nb, Ta, and Sn, are crucial for the advancement of several economically significant sectors, such as national economic development, defenserelated science and technology, and innovative scientific research. Numerous governments acknowledge them as strategic resources (Linnen et al., 2012; Chakhmouradian et al., 2015; Zhai et al., 2019; Hou et al., 2020). The increasing demand for rare metal resources has stimulated mineral exploration and investigation into the origins of rare metal deposits, elevating them to prominence in scientific study (Damdinova et al., 2018; Wang, 2019; Lyalina et al., 2019; Li et al., 2021, 2022; Ayuso and Foley, 2023; Xuanchi et al., 2024).

These metals are derived from diverse geological sources, each offering unique mineralogical and geochemical characteristics. Among these, pegmatites serve as one of the most significant and economically viable sources of rare metals. Pegmatites are intrusive igneous rocks characterised by their coarse-grained texture and the presence of rare minerals that host rare metals. They are particularly renowned for yielding lithium, tantalum, niobium, and beryllium, along with significant concentrations of rare earth elements (REEs). Lithium-bearing minerals such as spodumene and petalite are commonly associated with lithium-rich pegmatites, whereas columbite-tantalite minerals serve as primary carriers of tantalum and niobium (Černý & Ercit, 2005). Additionally, beryl, a beryllium-bearing mineral, is another hallmark of some pegmatite deposits. Beyond pegmatites, other sources of rare metals include carbonatites, which are a primary source of niobium and REEs, and lateritic deposits, which often host scandium and nickel-cobalt. Sedimentary deposits, such as placer deposits, also play a crucial role, particularly in concentrating REEs and heavy minerals like zircon and monazite.

Pegmatites, known for their exceptional mineralogical diversity and enrichment in rare metals, are of immense importance in the exploration and exploitation of critical resources. These igneous rocks, characterised by their large crystal sizes and coarse-grained textures, often serve as repositories of economically significant rare metals, including lithium, tantalum, niobium, and beryllium. Their unique geochemical and structural attributes establish a crucial link between pegmatites and rare metal sources, underscoring their relevance to resource development. One of the primary reasons pegmatites are vital is their ability to concentrate rare metals during the late stages of magma crystallisation. This occurs because incompatible elements, which do not fit into the crystal lattices of common rockforming minerals, are concentrated in the residual melts that form pegmatites (Černý & Ercit, 2005). For instance, lithium-bearing minerals such as spodumene and petalite are primarily found in lithium-enriched pegmatites. Similarly, tantalum and niobium, crucial for electronics and aerospace applications, occur in columbite-tantalite minerals hosted in specialised pegmatitic deposits. Furthermore, pegmatites often host rare earth elements (REEs) in smaller quantities, complementing other major REE sources like carbonatites. These deposits are strategically significant, given the rising demand for critical metals in renewable energy technologies, electric vehicles, and defence systems (Kesler, 2007). Their accessibility and typically high-grade ores make pegmatites economically viable. For example, regions such as Greenbushes in Australia and Bikita in Zimbabwe are globally recognised as prolific sources of rare metals, directly linked to pegmatite mineralisation.

Researchers are very interested in where rare metalbearing pegmatite deposits come from because they are major sources of these metals (Barros and Menuge, 2016; London, 2018; Müller et al., 2018; Bekele and Sen, 2020; Kaeter et al., 2021; Galliski et al., 2021; Zhang et al., 2021; Morozova et al., 2022; Li et al., 2023; Hong et al., 2022). The principal determinants governing the mechanisms of rare metal enrichment in pegmatites are of significant interest (Auley and Bradley, 2014; Melcher et al., 2017; Knoll et al., 2018; Keyser et al., 2023). Three ideas have been put forward by researchers to explain why pegmatites have a lot of rare metals: liquid immiscibility, fractional crystallization differentiation, and hydrothermal metasomatism (Thomas and Davidson, 2016; Ran and Li, 2021). Rare metals are typically converted into minerals through magmatic, transitional magmatic-hydrothermal, and hydrothermal processes. The parent magmas, crystallization fractionation, and the subsequent hydrothermal fluids influence these processes (Kuster 1990; Okunlola, 2005; Jatau et al., 2012). Consequently, the principal variables governing the enrichment of rare metal elements in pegmatites remain ambiguous. Recently, Okunlola (2005) did research on the metallogeny of Nigeria's rare metal (Ta-Nb) pegmatites. He identified seven main fields: Kabba-Isanlu, Ijero-Aramoko, Keffi-Nasarawa, Lema-Ndeji, Oke Ogun, Ibadan-Oshogbo, and Kushaka-Birnin Gwari. The Keffi-Nasarawa field, which includes the pegmatites around the Kwarra area, is currently exploiting other members, namely Wamba and Keffi pegmatites, for both metallic and gem minerals like cassiterite, columbite, tantalite, tourmaline, and beryl. The available data on the rocks suggests a genetic relationship with the well-studied Wamba pegmatites (Jatau et al., 2012). Previous research has mostly focused on regionally cataloging the pegmatites around the Kwarra area (Kuster, 1990; Matheis and Caen-Vachette, 1983; Matheis, 1987). However, we still don't fully understand how to classify pegmatites, their geochemical makeup, how old they are, or where the rare metals that are found in them come from. In this paper, we present new data on the mineralogy and geochemistry of muscovite, amphibolite, migmatite gneiss, banded gneiss, granite gneiss, alkali granite, and albitized granite in the quartz-muscovite-pegmatites and quartz-feldspar-pegmatites. We also discuss their petrogenesis and structural features, their impact on rare metal mineralization, the concentration of major and trace elements, and our petrogenetic evaluation using trace elements Rb, Ba, Sr, Zr, Y, and the rare earth elements. Finally, we classify the pegmatites based on their relationship, and conduct a structural analysis of the pegmatites and host rocks in the study area to determine how their trends relate to the geochemistry.

1.1. Regional Geology

The Nigerian Pan-African basement is part of an Upper Proterozoic-Lower Phanerozoic mobile belt situated between the West African and Congo cratons (Garba 2003). This mobile belt extends from Algeria across the Southern Sahara into Nigeria, Benin, and Cameroon. Mesozoic ring complexes of the Jos area intrude into the rocks of the Nigerian Basement Complex, part of the Pan African Mobile Belt, while Cretaceous to Quaternary sediments unconformably overlie them, forming the sedimentary basins (Akintola and Adekeye, 2008). The geology of the study area falls within the framework of the North-Central Basement Complex of Nigeria, which is underlain by Basement Complex rocks and parts of the Younger Granites (Fig. 2).

Figure 2. Simplified map of the geology of Nigeria after Okunlola, (2005).

1.2. Nigerian Pegmatites

According to Garba (2003) and Rahaman et al. (1988), the Nigerian pegmatites date from the end of the Pan- African magmatic activity and belong to the terminal stage of Pan-African magmatism, which occurred between 562 and 534 Ma. Pegmatization was heavily influenced by progressive metamorphism and partial melting of country rocks at the end of Pan-African plutonism. (Adetunji et al.,. 2016). These pegmatite units' ages, mineralogy, and composition appear to be analogous to those of the pegmatite environment in Brazil, Canada, and Australia (Akintola and Adekeye, 2008). Both barren and mineralized pegmatites exist in the Nigerian basement (Fig. 3). The study area's pegmatites are located in Nigeria's North Central pegmatite province, which stretches from Abuja to Jos Plateau (Fig. 3) and primarily settle in the gneissic basement. Albitization is a dominant feature of the area's rare metal pegmatites, which are particularly rich in tin (Matheis, 1987). The pegmatites in the study area have a close, but sparse, relationship with the Pankshin anorogenic ring complex. In the study area, Rb/Sr dating for the Gwon-Gwon pegmatite gave an age of 555 +/-5 Ma, and three muscovite model ages range from 537–522 Ma (Matheis, 1987).

Figure 3. Geological Sketch map of central and south-west Nigeria showing the location of the Wamba pegmatite field (study area) and the distribution of Pan-African Older Granites and pegmatites (underlined). Geochronogical data sources are van Breemen et. al. (1977), Rahaman et. al. (1983), Matheis and Caen-Vachette (1983), Tubosun et. al. (1984).

1.2.1. Barren and Rare Metal Pegmatites

Moller and Morteani (1987), Černy (1989), Kuster (1990), and Garba (2003) have helped us learn more about the pegmatite bodies in the southwestern and northern parts of Nigeria. They have helped us tell the difference between barren and rare metal-bearing pegmatites and shown that the pegmatites are not just found in the 400 km long NESW trending belt that goes from the Wamba area (near the Jos plateau) to the Ilesha area. Granitic pegmatites that are rich in rare metals form when large reservoirs of granitic magma separate very quickly. In these types of pegmatites, the amount of rare metals shows how much fractionation happened in the last stages of granitic differentiation (Černy, 1991). All the major lithologies of the basement, such as gneiss, migmatites, schists, and granitoids, are associated with barren pegmatites. The morphology and major mineral composition (quartz-feldspar-mica) are mostly the same as those of rare metal types (Garba, 2003). The chemistry of the pnuematolytic and/or hydrothermal fluids that are added will play a big role in telling the difference between the mineralized and bare pegmatites (Adedoyin et al., 2006). Researchers have observed that the degree of albitization and fractionation in pegmatites significantly influences the differentiation between barren and rare metal pegmatite (Oyebamiji, 2014; Matheis, 1987; Akintola et al., 2012 (a); Kuster, 1990; Oyebamiji et al., 2018, Jacobson and Webb, 1946). Garba (2003) observed the following common characteristics of rare metal pegmatites: sharp contacts with their host rocks, wall rock alteration (mostly tourmalinization), and close proximity to major and subsidiary fault structures. Given that rare metal pegmatites are associated with the major fault lineament systems in Nigeria, the albitization and rare-metal mineralization may have resulted from late-stage fluids present at the end of the Pan-African metamorphic cycle (Ekwueme and Matheis, 1995) or from a sodium-rich hydrothermal solution from the mantle along the ancient lineament (Wright, 1970). A study by Okunlola (2005) explained the metallogeny of Nigeria's rare metal pegmatites and named 7 main fields: the Kabba-Isanlu, Ijero-Aramoko, Keffi-Nasarawa, Lema-Share, Oke-Ogun, Ibadan-Oshogbo, and Kushaka-Birnin Gwarri. A broad zone extending NE from the Ago-Iwoye area towards the Younger Granite province (Akoh and Ogunleye, 2014) shows a marked concentration of these pegmatites, exhibiting an appreciable degree of mineralization. Rare metal pegmatites commonly exhibit late-stage albite and sericite (Garba, 2003).

1.2.2. Structural and Geochemical Characterization of Nigerian Pegmatites

The rare metal pegmatites are found in a clear belt that goes from Ife to Jos and seems to cross the line between the eastern and western Nigerian terranes, even though the pegmatite intrusions are oriented north to south (Kinnaird, 1984; Matheis and Caen-Vachette, 1983; Woakes et al., 1987). Individual pegmatites vary in length from 10 m to over 2 km and can be up to 200 m wide (Adetunji and Ocan, 2010). We have never observed a direct genetic link between the rare metal hearing pegmatites and proximal granite occurrences. The tecno-chemical characteristics of the host rock, not specific lithologies, determine mineralization (Adedoyin et al., 2006). Rotational stresses created by the Benue Trough appear to relate to the pegmatitic belt and the orientation of its units. From a more global perspective, this trend is probably the northern extension of the Brazilian pegmatite belt, which runs from Rio Grande del Sul to Rio Grande del Norte (Akintola and Adekeye, 2008). Researchers suggest that the Pan-African orogeny's reactivation of old tectonic lineaments provided excess heat and fluid, which concentrated rare-metal pegmatites through partial melting and selective leaching from the country rocks, or their lithological framework (Garba, 2003). Erny et al. (2012) support this suggestion. It is thought that the conjugate faults and shear belts that Garba (2003) suggested play a big part in how the magmatic fluid flows and how the pegmatites are arranged in a straight line or slightly offline. Rare metal pegmatites sometimes have big pinch and swell patterns that are linked to semi-ductile deformation, and minerals are well developed in the swells (Adedoyin et al., 2006). Different geochemical signatures, related to their spatial geological framework, distinguish the different pegmatite fields in Nigeria (Matheis, 1987; Kuster, 1990). Nigerian pegmatites show a marked difference in trace element and rare-earth element concentration and fractionation patterns between the rare-metal and barren pegmatites. It's very important to note that the rare-metal pegmatites have higher amounts of Rb, Cs, Ga, Nb, Ta, Sn, Li, and Be than the barren types and Pan-African granitoids (Garba, 2003). Some pegmatites in the amphibolite complex, like the Egbe pegmatites, have unusually high amounts of pathfinder elements like Li, Rb, F, and K/Rb, which are strongly linked to the rock's tin content (Mathei, 1987; Kuster, 1990). The pegmatites' origin also has a significant influence on the pegmatités geochemistry. The higher K/Ba ratios of the granitic pegmatites distinguish them from the metamorphic ones. The high K/Ba and Rb/Sr but low K/Rb ratios of the rare-metal pegmatites attest to their granitic origin (Garba, 2003).

1.2.3. Evolution of pegmatites from a granitic melt

The most accepted model for pegmatite formation is its derivation from granitic magmas via igneous differentiation processes. Granitic magmas refine their compositions through crystal fractionation and the separation of residual liquids from their crystalline products (Cerný et al., 2012). In granitic pegmatites, the chemical and textural segregation that makes them unique is caused by the uneven distribution of alkalis between the melt and an upwardly buoyant vapor phase (London, 2005). The pegmatite zonés slow rate of cooling will encourage the growth of large crystals, yet the slow rate of cooling cannot fully explain the large crystals characteristic of pegmatites. Undoubtedly, the central regions of the parent rock experienced a significantly slower rate of cooling during its formation, yet they did not yield extremely large crystals (Cern et al., 1985). Evidence from mineral compositions and thermal models indicates that crystallization within pegmatites commences at ~450 °C, which is ~200 – 250 °C below the liquidus temperature at which crystallisation should commence (London and Kontak, 2012). The rising melts containing H2O reach H2O saturation (possibly with some wall-rock dehydration as well), expel a fluid phase as they rise, and continue to do so as they crystallize. An increase in H2O saturation can dramatically reduce the melting point of silicate systems at elevated pressures. As crystal growth begins, the growing crystal interface of quartz and feldspar rejects incompatible components such as H2O, OH-, CO2, HCO-3, CO32-, SO42-, PO43-, H3BO3, F, and Cl, along with elements Li, Na, K, Rb, Cs, and Be. These components then concentrate along the margins of the growing crystal front, acting as fluxes (Cern et al., 2012). Because of its increasingly flux-rich composition, the boundary layer liquid will have a low solidus temperature and enhanced silicate-H2O miscibility, reducing the system's viscosity (London, 2005). Volatile species, such as B, F, and P, can individually and collectively lower the granite solidus temperature to below 500 °C and increase the range of temperatures over which magmatic crystallization occurs (London, 1996). Pegmatites have big grains because they don't have good nucleation and have very high diffusivity in the H2O-rich phase. This lets chemical species move around easily and join minerals that are growing quickly (Winter, 2014). The trace elements that are important for petrology can be found in granite-pegmatite systems. The levels to which they don't work with each other depend on the pressure, temperature, and mineral phases in the system. The process of fractional crystallization or the assimilation of material from external reservoirs can modify the relative and absolute abundances of the initial suite of trace elements, depending on their compatibility in the rock-forming minerals of granites (Cerne et al., 2012).

2. Material and Methods

2.1. Field Method

We conducted field work in two phases. Firstly, we conducted a reconnaissance survey of the area to evaluate its accessibility and plan the logistics for the field mapping. The second phase of the field work involved geological field mapping on a scale of 1:50,000 as the base map. Geological features (textural and structural characteristics) were identified, as were field measurements (attitudes; strikes and dips), as well as their mode of occurrence. Textural characteristics of the rocks were elucidated using a magnifying hand lens. Photographs of the outcrops were also taken using a digital camera, and representative fresh samples were collected for laboratory analyses.

2.2. Sample Preparation for Geochemical Analysis

The fifteen (15) samples chosen for geochemical analysis were broken down into smaller pieces (11 whole rocks and 4 muscovite) at the activation laboratory in Ontario, Canada. The pieces were broken down to a size of less than 200 mesh. The pulverization of the samples was undertaken with the aid of Retsch Planetary Ball Mill 400. Rock chips were loaded into the planetary ball mill, and a time interval of 15 minutes was allowed for complete pulverization to <200 mesh size. After pulverization of each sample, the equipment was cleaned using acetone. Pulverization was later followed by weighing 15 grams of each sample using an electronic weighing balance. Samples were then sealed and labeled. We used the 4 lithos package inductively-coupled plasma mass spectrometry (ICP-MS) to look for major, trace, and rare earth elements in the samples.

2.3. Analytical Technique

For the main elements, inductively coupled plasma atomic emission spectrometry was used after lithium borate fusion and dilute acid digestion. For the trace and REE elements, inductively coupled plasma emission spectrometry (ICPES) was used after a multi-acid digestion method. Details of the analytical procedures adopted have been discussed in the work of Maja et al. (2011). As part of the analysis process, 5 ml of perchloric acid (HClO4), 15ml of hydrofluoric acid (HF), and trioxonitrate (V) HNO3 were added to 0.5 g of the sample to dissolve the minerals. The solution was stirred properly and allowed to evaporate to dryness after it was warmed at a low temperature for some hours. Four (4) ml of hydrochloric acid (HCl) was added to the cooled solution and warmed to dissolve the salts. The solution was cooled and then diluted to 50 mL with distilled water. The solution was then introduced into the ICP torch as an aqueous aerosol. The ions in the ICP emitted light, which was converted to an electrical signal by a photomultiplier in the spectrometer. The electrical signal produced by the emitted light from the ions was compared to a standard (a previously measured intensity of a known concentration of the elements), and the concentrations were then computed. Analytical precisions vary from 0.1% to 0.04% for major elements. Data obtained was processed using the following software: Microsoft Excel, Geochemical Data Toolkit (GCD Kit 3.0), and Surfer 12.

2.4. Lineament Analysis

O'Leary et al. (1976) defined lineaments as mappable linear features that differ distinctly from the pattern of adjacent features and presumably reflect subsurface phenomena. Lineament usually follows regional geology (e.g., intrusive bodies or large faults' strikes) and is thus useful in mapping structural trends. As a result, the structural lines in the area were primarily faults and fractures. The structural lineaments were extracted from the study area's landsat ETM+ image. The Global Land Cover Facility website (glcf.umd.edu/data/landsat/) was used to download Landsat band7. It was then enhanced with ENVI v4.5 software to make the lineaments more visible. Individual lineaments were traced out using Global Mapper v15 software, then exported to ArcGIS and Rockware software for statistical and orientation analysis. A total of 31 lineaments were extracted from the study area. The minimum length derived from these lines was 822.50 m; the maximum length was 6275.15 m. The non-geological lineaments such as paths, roads, power cables, and field boundaries in the study area were eliminated using the topographical map (Yassaghi, 2006). The mapped structural lineaments were analyzed using lineament density (LD) and lineament frequency (LF) parameters (Solomon and Ghebreab, 2006).

3. Result

3.1. Geology and Petrology

Precambrian Basement Complex rocks and Younger Granites underlie the study area, according to systematic geological field mapping at a scale of 1:50,000. The Basement Complex rocks include amphibolite, migmatite gneiss, banded gneiss, granite gneiss, and albitized granite, while the Younger Granite rock is alkali granite (Figs. 4, 5, and 6). Relics of tectonic activity are visible in the area's structures. The structural features observed in the study area include foliation, fractures, veins, lineation, joint folds, and minor faults (Figs. 7 and 8). The predominant structures trend from N-S to NE-SW (Fig. 8), and this conforms to the Pan-African structural pattern. Late pegmatitic veins either fill the shear fractures observed on the granites or show evidence of free aperture.

Figure 4. (a) Hand specimen of the alkali granite in the study area (Latitude 9°06'05"N and Longitude 8°44'30"E), (b) Field photograph of albitised granite (Latitude 9°04'10"N and Longitude 8°47'43"E), (c) Field photograph of a granite gneiss outcrop (Latitude 9°03'23"N and Longitude 8°33'43"E), (d) Field photograph of foliation structure in the banded gneiss with quartz and quartzo-feldspathic veins (Latitude 9°05'05"N and Longitude 8°31'36"E), (e) Field Photograph migmatite gneiss with ptygmatitic folding, upper arrow pointing at melanosome and lower arrow showing leucosome (Latitude 9°00'28"N and Longitude 8°42'30"E), (f) Hand Specimen of amphibolite rock in the study area (Latitude 9°07'17"N and Longitude 8°41'50"E).
Figure 5. Photomicrograph of the studied rocks under Plane Polarized Light (PPL) (a) alkali granite, (b) albitised granite, (c) granite gneiss, (d) banded gneiss, (e) migmatite gneiss, (f) sheared amphibolite; Biotite (Bt), Perthite (Prth), Orthoclase (Or), Plagioclase (Pl), Quartz (Qtz), Augite (Aug), hornblende (Hbl), garnet (Grt), Sphene (Shen), and muscovite (Ms). Mag.0.25mmX10.
Figure 6. Field Photograph of (a) quartz-feldspar-pegmatite intrusion into migmatite gneiss (Latitude 9°00'20"N and Longitude 8°42'16"E), (b) a narrow zone of tourmalinisation around the contact between quartz-feldspar-pegmatite and the host rock. (Latitude 9°00'30"N and Longitude 8°44'27"E), (c) Quartzmuscovite- pegmatites boulders (d) Whitish quartz-muscovitepegmatites intrusion in granite gneiss (Latitude 9°02'45"N and Longitude 8°34'38"E), (e) Highly deformed quartz-muscovitepegmatites trending NE (9°02'16"N and Longitude 8°35'36"E). (f) Deformed nature of quartz-muscovite-pegmatites in uplifted region (9°02'48"N and Longitude 8°31'10"E).
Figure 7. Field Photograph of (a) Field photograph of joints in the granite gneiss of the study area (Latitude 9°02'09"N and Longitude 8°38'34"E), (b) Field photograph of dextral fault along a quartz veins in the banded gneiss (Latitude 9°05'35"N and Longitude 8°30'06"E).
Figure 8. (c) Field photograph of pegmatite veins that intruded the migmatite gneiss of the study area (Latitude 9°01'15"N and Longitude 8°41'30"E). (d) Field photograph of quartzo-feldspathic veins in the banded gneiss within the study area (Latitude 9°05'05"N and Longitude 8°31'12"E), (c) Rose diagram of pegmatite veins in the study area showing the NE-SW dominant trend, (d) quartzo-feldspathic veins showing the NE-SW dominant trend in the banded gneiss of the study area.

3.2. Lineament Analysis

The results of the analysis are presented as structural lineament map (Fig. 9a) lineament density map (Fig. 9b) and rose diagram (Fig. 9). The qualitative study of the geological map, structural lineament map and Lineament density map of the study area showed that structural lineaments were more concentrated in the migmatites, banded gneisses and amphibolites (Pre-Cambrian basement rocks) than on the Jurassic Younger Granites. This was due to the effect of several tectonic deformations that accompanied the different orogenic events that have occurred throughout the Nigerian polycyclic basements. The orientation of structural lineaments on rocks can be correlated to past tectonic events that have affected them. The orientation of the structural lineaments on the structural lineament map was measured and plotted on a rose diagram using Georose software. The rose diagram of the lineament trends in the study area (Fig. 9c) revealed that well developed principal structural trends are N-S, NE-SW, and NNE-SSW with minor ENE-WSW ad NNW-SSE trends. This finding is in agreement with previous works suggesting that Nigeria has a complex network of fractures and lineaments with dominant trend directions of NE-SW, NW-SE, and N-S directions. (Ananaba and Ajakaye, 1987; Chukwu-Ike., 1977, Obiora 2009). These trends also agree with more recent works carried out in parts of the basement complex adjoining the Benue Trough by Olasehinde et al., (1990) and Anudu et al., (2012). Many of the lineaments coincided with drainage lines indicating that drainage in the study area may be structurally controlled (Samaila et al., 2011). According to Obiora (2009), NNE-SSW and N-S trends are Pan-African, while E-W and NNW-SSE trends are pre-Pan-African. Hence the NE-SW, N-S and NNE-SSW trends in the study area may probably have been Pan-African, whereas the NNW-SSE and E-W trends may have been relicts of pre- Pan- African events and hence less pronounced.

Figure 9. (a) Structural lineament map of the study area, (b) Rose diagram of structural lineament trends of the study area.

3.3. Geochemistry

3.3.1. Major Element Geochemistry

1) The Major Oxides Composition of the Rocks in the Study Area

The major oxides composition of the rocks in the study area is presented in Table 1 while Table 2 shows the CIWP Norm values for the granitic rocks of the study area. The Harker’s plot for albitized granite, pegmatites and alkali granites are shown in Fig. 10.

Table 1 . Major oxide compositions of the rocks in the study area.


Table 2 . CIWP Norm for the granitic rocks of the study area.


Figure 10. Harker plot of Al2O2, CaO, K2O and Na2O against silica (SiO2) for the granite suites in the study area. S2 and S6 (Alkali Granite), S4 and S12 (Albitized Granite), S2 and S6 (Alkali Granite), S4 and S12 (Albitized Granite).

The SiO2 concentration in the host rocks ranges from 50.43 to 74.48 wt%. The alkali granites exhibit the highest SiO2 concentration, averaging 74.18 wt%, while the albitized granite follows with a SiO2 concentration of 73.89 wt%. The high SiO2 content in granites is an indication of their acidic nature and enrichment in common rock-forming minerals such as quartz and feldspar. The migmatite gneiss has the highest SiO2 concentration among the metamorphic rocks in the study area, with a value of 71.97. Granite gneiss, banded gneiss, and amphibolite are the other metamorphic rocks with SiO2 concentrations of 69.76 wt%, 67.79 wt%, and 50.43 wt%, respectively. The Al2O3 concentration in the host rocks ranges from 11.06 wt% to 15.35 wt%, with albitized granites having the highest average concentration of 15.29 and the alkali granite with the lowest average concentration of 11.56. In this area, the metamorphic rocks like migmatite gneiss, banded gneiss, granite gneiss, and amphibolite all have about the same amount of Al2O3: 14.89, 14.96, 15.3, and 13.69. On the Harker’s diagram, the rocks form a somewhat vertical linear trend, with the younger granites plotting on the lower end of the trend. The FeO3 concentration in the host rocks ranges from 0.76 to 10.25 wt%. The albitized granites have the lowest Fe2O3 concentration, which is indicative of their acidic and felsic nature. The amphibolite has the highest Fe2O3 content of 10.25 wt%, which reflects its mafic and ferromagnesian-rich composition. The banded gneiss has a relatively high Fe2O3 composition of 5.2 Wt%, which may be due to its biotite-rich nature. The alkali granite has an average Fe2O3 concentration of 2.72, while the migmatite gneiss and granite gneiss have a concentration of 1.52 and 3.41, respectively. On the Harker's diagram, the rocks form a negative correlation with SiO2, and the Fe2O3 concentration drops steadily as SiO2 content increases from amphibolite to albitized granite. The alkali granite plot is slightly off the trend. The rocks rich in felsic phases, such as alkali granite and albitized granite, have the lowest MgO concentrations of 0.07 wt% and 0.02 wt%, respectively. The higher MgO concentration of the amphibolite (7.57 wt%) reflects the modal mineralogy of the rocks, which contains a higher proportion of mafic minerals. The migmatite gneiss, banded gneiss, and granite gneiss have MgO concentrations of 0.7 Wt%, 1.14 Wt%, and 0.71 Wt%, respectively. The rocks have a somewhat negative relationship with SiO2. The metamorphic rocks have a weak relationship, and the felsic-rich rocks (like habitized granite and alkali granite) are grouped together at the bottom of the trend. The rocks generally have a low MnO concentration with a range of 0.02–0.18 wt%. The migmatite gneiss has the lowest value, while the amphibolite has the highest concentration at 0.18 wt%. The CaO content in the rocks ranges from 0.11 to 12.35 wt%. The albitized granite has the lowest average CaO content of 0.12 wt%, while the amphibolite has the highest of 12.35 wt%. The banded gneiss has a relatively high concentration of 3.24 wt%, while the migmatite and the granite gneiss have concentrations of 2.96 wt% and 1.93 wt%, respectively. The SiO2 concentration appears to have little influence on the CaO concentration.

2) Major Oxide Geochemistry in the Pegmatites

The Muscovite extract samples have an average SiO2 concentration of 61.12 wt%, ranging from 54.35 wt% to 67.77 wt%. The whole rock pegmatites have an average SiO2 concentration of 73.9 wt%, ranging from 73.56 wt% to 74.29 wt%. The muscovite samples have the highest values of Al2O3, MgO, K2O, and MnO, with mean concentrations of 23.80 wt%, 0.23 wt%, 6.89 wt%, and 0.07 wt%. The muscovite extract also has a high value of Fe2O3 and P2O5, with mean values of 2.05 wt% and 0.21 wt%, respectively. However, the SiO2 and CaO values are relatively low, with mean values of 61.12 wt% and 0.16 wt%, respectively. Muscovite samples' high Al2O3 values attest to their peraluminous nature, which is consistent with rare metal pegmatites. The rocks show a negative correlation with Al2O3 on the Harker’s diagram (Fig. 10). The quartz-feldspar-pegmatite is SiO2 rich, with an average value of 73.92 wt%. It has the highest P2O5, with a mean value of 0.42 wt%. It also has relatively high Al2O3, CaO, and N2O values, with mean values of 14.13 wt%, 0.51 wt%, and 3.55 wt%, respectively. On the plot of K2O vs. SiO2, we observe a negative correlation for the quartzmuscovite- pegmatites, and a positive correlation for Na2O vs. SiO2 (Fig. 10). The pegmatites show a negative correlation with Fe2O3 on the Harker’s diagram. They also show a positive correlation with P2O5, indicating that P2O5 increases with SiO2 during magmatic differentiation. Phosphorous is an incompatible element that concentrates in the residual melt, where it acts as fluxes and enhances the growth of large crystals, as seen in pegmatites (London, 2008).

The Shand index plot (Fig. 11a) shows that the albitized granite and pegmatites are in the peraluminous (ASI > 1.0) field, with an aluminum saturated index (ASI) ranging from 0.99 wt% to 2.64 wt% and a modified alkali-lime index ranging from 3.07 wt% to 9.22 wt%. The albitized granites and muscovites had the strongest peraluminous values (ASI 1.26 and 2.40, respectively). The alkali granite has a mean ASI value of 0.98 and plots in a metaluminous field close to the border of the peralkaline field. A plot of Fe (total)/(Fe (total)+MgO) versus SiO2 (Fig. 11b) and Na2O+K2O-CaO versus SiO2 (Fig. 12a), following Frost et al. (2001), indicates a strong iron enrichment in pegmatites, alkali granites, and albitized granite. We interpret ferroan (Fe-enriched) as closely associated with conditions of limited H2O availability and low oxygen fugacity during partial melting of their source rocks (Frost et al., 2001). According to Frost et al. (2001) (Fig. 11b), the granitic rocks can be put into four groups: the alkali, the alkali-calcic, the calcic-alkali, and the calcic series. All the granites fall in the alkali-calcic field, as do the pegmatites, with the exception of one quartz-feldspar-pegmatite sample (S8) that falls in the calcic-alkali field. On the SiO2 vs. Na2O+K2O diagram of Middlemost (1985) (Fig. 12b), the quartz-feldspar-pegmatite, alkali granites, and albitized granite samples are also plotted in the granite field, indicating the bulk granite geochemistry of these rocks. The normative Ab-Or-An diagram illustrates the major element changes in the Ternary plot after O'Connor (1965) (Fig. 13a). The variable contents of albite, compared to the other feldspar components, delineate the rocks. The pegmatites, albitized granite, and alkali granite all plot within the granite field, with the albitized granite plotting much closer to the Ab corner due to its albite-rich nature. We plotted the granites on the A/CNK versus SiO2 discrimination diagram (after Chappel and White, 1974) (Fig. 13b). The albitized granite, quartz-feldspar pegmatite, and quartz-muscovite pegmatite all plotted within the Stype granite, while the alkali granite plotted within the Itype field. The partial melting of already peraluminous sedimentary source rocks imprinted by weathering at the Earth's surface produces S-type granitoids (Winter, 2014), while Collins et al., 1982, and White, 2005, and Whalen et al., 1987, interpret the I-type granitoids as those derived from recycled, dehydrated continental crust and those directly derived from melting of subducted oceanic crust or the overlying mantle.

Figure 11. (a) A/CNK – A/NK plot for the granitoids in the study area after Shand (1943), (b) Fe (total)/(Fe (total)+MgO) versus SiO2 after Frost et al 2001.
Figure 12. (a) Na2O + K2O-CaO versus SiO2 after Frost et al 2001, (b) Na2O+K2O vs. SiO2 for the granitoids diagram of Middlemost, (1985). S2 and S6 (Alkali Granite), S4 and S12 (Albitized Granite), S1, S5 and S8 (Quartz-Feldspar-Pegmatite).
Figure 13. (a) Ternary normative Ab-Or-An diagram after O’Connor, 1965, (b) Fig. 16: A/CNK vs SiO2 plot of rocks from the study area after (Chappel and White, 1974).

3.3.2. Trace Element Geochemistry

Trace element levels in granitic melts change a lot as crystallization happens. These levels can show how the magma splits and how the chemicals in pegmatites change over time (London, 2008). The trace element composition and some important ratios are presented in Table 3, while Table 4 shows the concentration of rare earth elements in the samples. Table 5 displays the average abundances and ranges of some trace elements, rare earth elements, and selected ratios from Taylor and McLennan's (1985) upper continental crust values.

Table 3 . Concentration of some selected trace elements and some important elemental ratios in the rocks of the study area.

PetrologyAmphiboliteMigmatiteBanded GneissGranite GneissAlbatized GraniteAlkali GraniteQuartz-Feldspar-PegmatiteQuartz-Muscovite-Pegmatite
Elements (ppm)S10S3S9S7S4S12S2S6S1S5S8S13S14S15S16
Be123635131581354221435519
Ba4583684474173351379724892419735814
Sr1454774513231144450654831760405512
Y211710143294992122231
Zr65185250134292228029235231171221184
Co70595658695647536656296011911466
Zn100106110801631408091< 30< 30< 30120130100240
Ga141723264736252930221612797100179
Rb135485152465822214231542511833213294634823957
Nb48121430222834753187116164194
Tl0.20.20.51.14.21.0120.92.50.78.45.611.76.6
Sn1137816671125203328189585798
Cs0.71.63.215.15749.97.612.410.814.91.367.954.713362
Hf1.61.45.83.21.972.17.66.54.11.83.71.21.52.10.3
Ta0.51.11.41.75.34.24.26.37.5621.711.370.565.591.177.2
W142132298345412378298310285352187405745802429
Pb1018153218102835582752201278
Bi0.4< 0.4< 0.41.12.31.8< 0.4< 0.4< 0.4< 0.4< 0.42.33.1< 0.4< 0.4
Th1.21.312.47.70.80.426.4310.60.30.40.710.11
U0.30.41.62.77.46.26.47.61823.41.915.282.60.4
K/Rb280.97405.85213.89219.5567.4836.76197.84182.5688.8093.25463.0818.0615.6116.0217.43
Nb/Ta8.007.278.578.247.925.246.675.4012.502.9410.009.5910.093.9922.38
Rb/Sr0.090.110.190.4742.27205.504.864.628.3410.650.2653.5573.6563.31329.75
Na/K1.891.601.680.851.201.240.600.590.570.570.640.240.270.160.11
K/Ba81.1726.2221.5445.044482.8210072120.62111.27668.43992.7241.603054.08630.01961.834927.54

Table 4 . Concentration of rare earth elements in the rocks of the study area.


Table 5 . Average abundances and ranges of some trace elements, rare earth elements and selected ratios from upper continental crust values from Taylor and McLennan (1985).



Fig. 14 shows spider diagrams for all the rock samples normalized to average crust after McDonough and Sun (1995). The normalized abundance patterns permit characterization of the rocks. Similar patterns are found in migmatite gneiss, banded gneiss, amphibolite, and granite. They are all characterized by negative anomalies of some incompatible elements, namely, Nb, Ce, Zr, and Ti, as well as positive anomalies for Pd, Ta, Sr, and Dy. The banded gneiss has a slight positive anomaly for Th, while the other metamorphic rocks all show a negative anomaly for Th. The amphibolite exhibits a slight negative Zr anomaly. The albitized granite exhibits a prominent negative anomaly for Ba, Ce, Sr, Nd, and Ti, as well as a marked positive anomaly for U, Ta, Pb, and Zr. The alkali granite is distinguished by negative anomalies for Sr, Nb, Ce, Zr, Ti, and Ba, as well as positive anomalies for Pb and the radioactive elements Th and U. All pegmatites samples have trace element patterns that are different from the norm. These patterns show that Rb, Ta, Pb, Zr, and U are more abundant than usual, while Ba, Nb, Ce, and Ti are less common. The quartz-feldspar-pegmatite samples are generally more HREE-enriched than the mica samples. The muscovite sample has the lowest K/Rb ratio (mean value of 16.78) as well as Ce (mean 1 ppm) among the other samples. It also has notably low levels of Ba (mean 41 ppm) and Sr (42 ppm) below the crustal abundances of 425 and 375, respectively (Taylor, 1964). The clear loss of Ba and Sr is likely caused by K and Na feldspar or plagioclase fractionation, respectively (Akoh et al., 2015). The quartz-muscovite-pegmatites are relatively enriched in Ga (mean 126 ppm), Rb (mean 3399 ppm), W (mean 595 ppm), Ta (mean 22 ppm), Sn (mean 475 ppm), Cs (mean 79.4 ppm), and Nb (mean 165 ppm), well above the crustal abundances of Ga 15 ppm, Rb 90 ppm, W 1.5 ppm, Ta 2 ppm, Sn 2 ppm, Cs 3 ppm, and Nb 20 ppm (Taylor, 1964). The relative concentrations of Nb and Ta are influenced by volatiles associated with the late-stage magma, so the quartz-muscovite-pegmatite represent the more evolved facies (Akoh et al., 2014). The quartzfeldspar- pegmatites that are close together have higher amounts of Ba (348 ppm), Rb (378 ppm), and Zr (58.33 ppm), and their Nb/Ta ratios are low (8.48).

Figure 14. Spider diagrams for all the rocks samples normalized to average crust after McDonough and Sun (1995).

3.3.3. Rare Earth Element (REE) Geochemistry

We normalized all the REE values to those provided by Nakamura (1974) and then plotted them on a spidergraph (Figs. 15a, 15b, and 15c). The alkali granites have the highest sum of REE (ΣREE), with an average of 419.96 (Fig. 15b). The albitized granites have the lowest sum of REEs with an average value of 5.17. The low value of the sum of REEs for the albitized granite may be due to the effect of element mobility during hydrothermal alteration. The sum REEs for the other rock types are 65.07, 48.26, 97.78, and 187.03 for migmatite gneiss, amphibolite, granite gneiss, and banded gneiss, respectively (Table 6). The degree of negative Eu anomaly in the rocks varies widely (0.25–10.45). One way to find the Eu anomaly is to divide the chondrités normalized value of Eu concentration by half the sum of the normalized concentrations of Sm and Gd (Terekhov and Shcherbakova, 2006). This gives you Eu/Eu*. The alkali granités REE pattern is subparallel, LREE-enriched (LaN/YbN value of 6.38), and characterized by a nearly flat HREE profile. The alkali granite exhibits a weak negative Europium anomaly (Eu/Eu* = 0.4), suggesting that it originated from melts in equilibrium with a plagioclaserich phase during partial melting or fractional crystallization. This process either retained the plagioclase in the residual solid or removed it as phenocrysts during the earlier stages of crystallization (Winter, 2014). The banded gneiss and the granite gneiss show similar REE patterns (Fig. 15a), with ΣREE values of 187.03 and 97.78, respectively. These rocks have weak Eu anomalies (Eu/Eu* 0.87 and 0.94, respectively). Hugh (1993) attributes the weak negative Eu anomaly to the rock's formation from a protolith deficient in plagioclase. Both the migmatite gneiss and the amphibolite have a subparallel REE pattern with LREE enrichment. This may mean that there aren't any accessory phases like garnet and zircon to hold the HREEs. In the migmatite, the LREEs are more evenly distributed compared to the HREEs than in the amphibolite (LaN/YbN = 38.76 and 12.30, respectively). The notable downward trending slope from La to Lu on the REE spidergraph graphically indicates this. The albitized granite has nearly similar REE patterns as quarts-feldspar-pegmatite, but with lower total ΣREE (5.17 and 15.82, respectively). The REE diagram for albitized granite (Fig. 15c) and pegmatites (Fig. 15b) reveals the segmentation of neighboring elements into successive troughs and crests, creating a jagged-edge pattern known as the REE tetrad effect. The REE tetrad effect observed is the W-type, which begins with a downward convex from La, alternating progressively with increasing atomic number to Lu (Bea, 2015). The observed trend may be the result of hydrothermal fluids leaching the more mobile REEs relative to the less mobile ones. The albititzed granite and quartz-feldspar-pegmatite both have well-pronounced positive Eu anomalies (mean Eu/Eu* = 7.28 and 9.73, respectively). The abundance of early fractionated plagioclase feldspars in the rock, formed in a region of low oxygen fugacity, may explain this (Milord et al., 2000; Winter 2014). Albitized granite and quartzfeldspar pegmatite have average LaN/YbN values of 1.5 and 1.7, respectively, indicating a moderate fractionation of the LREEs relative to the HREEs. The quartz-muscovitepegmatite has a low mean ΣREE (5.07) and a REE tetrad effect pattern that is similar to that seen in albitized granite and quartz-feldspar-pegmatite. However, there is a clear negative Eu anomaly (Eu/Eu* 0.44). The LREEs in the rocks have a slight to moderate fractionation relative to the HREEs, with a LaN/YbN value ranging from 1.00 to 1.8. In the ternary Rb-Ba-Sr plot of El Bouseily and El Sokkary (1975) (Fig. 15d), the alkali granite, albitized granites, and pegmatites all fall into the field of strongly differentiated granites. The modified triangular discriminating plot of Ti- Sn-(Nb+Ta) after Kuster (1990) shows that the albitized granite, quartz-feldspar-pegmatites, and quartz-muscovitepegmatites all lie in the zone of albitization (Fig. 15e). The quartz-muscovite-pegmatite and albatized granite show how highly evolved they are by plotting close to the peak in the field of strongly differentiated granites. The alkali granite, on the other hand, plots close to the normal granite boundary, much closer to the Ba corner. After Kaur et al. (2012), the plot of Rb vs. Na/K (Fig. 15f) shows the progressive depletion of Rb with advancing albitization. We can see from the plot that the quartz-feldspar-pegmatite and the albitized granite have been albitized the most and have the least amount of Rb.

Figure 15. Chondrite normalized plot (After Nakamura, 1974) rare elements (REE) pattern for the (a) metamorphic rocks, (b) Pegmatites, (c) granites from the study area, (d) Plot of Rb-Ba-Sr after El Bouseily and El Sokkary (1975) for granitic rocks, (e) Modified Triangular Ti-Sn-(Nb+Ta) Plot for albitized granite and pegmatites in the study area (after Kuster, 1990), (f) Plot of Rb vs Na/K after Kaur et al., (2012) showing the progressive depletion of Rb with the advancing albitisation of K-feldspar.

3.4. Mineral Potential Diagrams

We used plots of Ta versus Cs (Fig. 16a), Ta versus K/ Cs (Fig. 16b), and Ta versus Ga (Fig. 16c) to distinguish between mineralized and non-mineralized pegmatites, taking into account the Beus (1966) and Gordiyenko (1971) lines of mineralization. Both the Beus (1966) and Gordiyenko (1971) lines of mineralization are below the quartz-feldsparpegmatite plot. The quartz-muscovite-pegmatite plot, on the other hand, is mostly above the Beus (1966) line of mineralization but below the Gordiyenko (1971) lines of mineralization. We also used plots of K/Rb versus Cs (after Erny and Burt, 1984) (Fig. 16d) and K/Rb versus Cs (after Trueman and Erny, 1982) (Fig. 16e) to determine the mineralization potential in pegmatites. The plots revealed the presence of quartz-feldspar-pegmatite in the barren field, and the presence of muscovite extracts in the rare metal pegmatites, similar to the enrichment of Be and Li. Ballouard et al. (2016) evaluated the mineralization potential of the granites in the study area using a plot of Nb/Ta versus Zr/Hf, as shown in Fig. 16f. The albitized granite is located in the field of rare-metal granites and Sn-W-U-related granites, while the alkali granite, with Nb/ Ta values ranging between 5 and 16, is specifically located in the barren granite field. Therefore, we cannot classify any of the granites in the area as fertile granites capable of giving birth to rare metal pegmatites.

Figure 16. (a) Plot of Ta Versus Cs tor The Muscovites of the pegmatites in the Kwarra area, (b) Plot of Ta Versus K/Cs For The Muscovites of Pegmatites in the study area. (After Beus 1968), Gordiyenko (1971), (c) Plot of Ta versus Ga for the pegmatites in the study area (After Černy and Burt, 1984), (d) Plot of K/Rb versus Cs for the pegmatites (after Černy and Burt, 1984), (e) Classification of the pegmatites using the plots of K/Rb versus Cs (after Trueman and Černy 1982), (f) Nb/Ta versus Zr/Hf diagram differentiating the barren granites and granites hosting ore deposits (after Ballouard et al., 2016).

3.5. Tectonic Discrimination Diagram

In the Rb vs. Y+Nb tectonic discrimination diagram, Pearce et al. (1984) (Fig. 17a) plot the pegmatites and albitized granite sample in the syn collisional granite field associated with orogenic events. The alkali granite plot within the within-plate field is consistent with A-type granites. Figure 17b shows the plot of FeOt/MgO vs. Zr+Nb+Ce+Y, which Whalen et al. (1987) used to separate A-type granites from other granitoids (M-, I-, and S-type). The alkali granite falls into the A-type granite field. The alkali granites in the study area are part of both the withinplate granite field (Fig. 17a) from Pearce et al. (1984) and the A-type granite field of the Ga/Al vs. Zn plots from Whalen et al. (1987). They also belong to a plot of Nb- Y-Ce (Fig. 17c) for A1, A2 granite discrimination after Eby (1992). The discrimination plot reveals the granités A2 nature, which is considered a subtype of the I-type and indicates a crustal source that is not metasedimentary. These magmas belong to the A2 group. They are made up of continental crust or underplated crust that has gone through a collisional or island arc magmatism cycle (Eby, 1992).

Figure 17. (a) Rb vs. Y+Nb tectonic discrimination diagram after Pearce et al. (1984), (b) Plot of FeOt / Mgo vs. Zr+Nb+Ce+Y for discriminating A-type granites after Whalen et al., (1987), (c) Triangular plot of Nb-Y-Ce for distinguishing the alkali granites into A1 and A2 granite (After Eby, 1992).

4. Discussion

4.1. Geochemistry and Structural Geology of the Polycyclic Basement Rocks

The pegmatites are displaced by the polycyclic migmatite gneiss complex, which includes migmatites, banded gneisses, granite gneiss, and amphibolite. As Obaje (2009) says, petrographic evidence shows that the Pan-African reworking partially melted and re-crystallized many of the minerals that make up the Migmatite-Gneiss Complex. Most of the rock types showed medium to upper amphibolite facies metamorphism. Faulting and fracturing marked the end of the orogeny, leading to regional shearing and tectonometamorphic evolution of the terrain (Gandu et al., 1986; Olayinka, 1992). The type of deformation varies greatly between rock types, as well as pressure, temperature, and strain rate. The variation between rock types is also due to the fact that the constituent minerals of each rock type have different mechanical properties. When the pressuretemperature (P-T) is low and the crustal level is high, rocks tend to be brittle and break apart at all scales (cataclasis), as seen in amphibolites. In the upper amphibolite facies and higher grades of metamorphism, on the other hand, grains can move around by diffusing atoms (diffusional creep). This can be a big process that helps with ductile deformation (Andy, 1998), as shown by the ptygmtitic folding (Plate 19b) in the migmatite. A "brittle-ductile" transition happens when rocks show signs of both brittle and ductile or semi-brittle conditions. This happens somewhere between the two end-member scenarios of brittle and ductile deformation (Murrell, 1990). The pinch and swell structures devolved in the banded gneiss clearly display this type of deformation. At higher temperatures, both quartz and feldspar experience ductile deformation. Other minerals, such as hornblende, also experience brittle deformation at low metamorphic grades but behave ductilely at high metamorphic grades. This gives rise to strain partitioning, with some areas experiencing only low strain while others become highly strained. This partitioning occurs from the macroscale right down to the microscale, with significant strain variations forming pinch and swell structures (Andy, 1998).

The amphibolite has the lowest SiO2 content among the rocks in the study area. It also has the lowest K2O and Na2O values of 0.93 and 0.44 wt%, respectively. The high Fe2O3 (10.65 wt%), MgO (7.57 wt%), and CaO (12.35 wt%) values of the amphibolite reveal its ferromagnesian nature. The rock also has the highest value of TiO2 at 0.93 wt%, indicating the presence of mafic-rich phases. The HSFE-depleted rock has low values for Zr, Y, Nb, Hf, and Ta, indicating that it was formed from protoliths deficient in garnet and other accessory phases such as zircon, rutile, and titanite. It also shows low values for the LILE, such as Rb, Ba, and Sr with values of 13, 145, and 45 ppm, respectively. The rock is generally REE depleted (ΣREE = 48.26). The rock exhibits a subparallel REE pattern, indicating a slight enrichment of the LREEs over the HREEs (LaN/YbN = 1.72). There is a slight positive Eu and Tb anomaly. Petrography reveals that the amphibolite consists of hornblende, augite, orthoclase, plagioclase, and quartz, with the majority of the quartz being highly fractured. Hornblende and augite crystals are primarily subhedral poikiloblastic crystals with quartz inclusion and two sets of cleavage. The outcrop's brecciated and scattered nature clearly demonstrates the rock's deformed nature, which is characteristic of rocks formed during faulting or other crustal deformation processes. The amphibolites coincide with major linear elements in the study area, attesting to their highly deformed nature.

The granite gneisses are the most common rock type in the mapped area. Its SiO2 value is 67.79 wt%, with relatively high Al2O3, Fe2O3, K2O, and TiO2 concentrations of 15.3, 3.41, 4.02, and 0.517 wt%, respectively. Ga levels are higher in granite gneiss (26 ppm) but lower in Nb, Zr, and Y (14, 134, and 14 ppm, respectively). This is different from the crustal abundance of Ga, Nb, Zr, and Y (15, 20, 165, and 33 ppm) (Taylor, 1964). The amount of Sr (323 ppm) in the rock is lower than the crustal abundance of 375 ppm. The amounts of the other lithophile elements, Ba (425 ppm) and Rb (90 ppm), are slightly higher than the crustal abundance (Taylor, 1964). The rock's REE pattern shows the fractionation of LREEs relative to HREEs (LaN/YbN = 12.3). It was noted by Hugh in 1993 that the minerals olivine, orthopyroxene, and clinopyroxene may separate the light REEs from the heavy REEs, with a partition coefficient that ranges from La to La. The banded gneisses have a medium-sized grain and show notable pinch and swell structures along with strongly lined and weakly to highly foliated gneissic structures. The banded gneiss mostly occurs as low laying and pavement outcrops and is characterized by well-developed mafic and felsic bands. It has been seen that diffusive mass transfer and in situ partial melting make any primary compositional banding stronger and create more separation of felsic and mafic minerals (Andy, 1998), which can be seen in the banded gneiss. Quartz and feldspars make up the felsic bands, while biotite makes up the dark bands. The prominent pinch and swell structures observed in the banded gneisses are indicative of semi-brittle deformation (Garner et al., 2015). The banded gneiss and migmatite gneiss both have a similar geochemistry. They are fairly rich in SiO2, with values of 67.79 and 70.25 wt%, respectively. Both have relatively high Al2O3 concentrations of 14.96 and 14.23 wt%, respectively. The banded gneiss is relatively rich in CaO (3.24 wt%) and Na2O (4.12 wt%) but poor in K2O (2.19 wt%).

4.2. Geochemistry and Structural Geology of the Granites

4.2.1. Alkali Granites

The alkali granite in the study area is part of the wellstudied Nigerian Younger Granites. It is an extension of the Pankshin Younger Granite Ring Complex, which extends from the Pankshin area on the Jos Plateau to parts of Wamba, Nasarawa State, northcentral Nigeria. The alkali granites represent the youngest lithologic unit in the study area and intrude the migmatite gneiss in the northeastern portion of the study area. The slightly lower AI for alkali granite indicates that it is less evolved than the pegmatites and albitized granite (Abdelfadil et al., 2016). The alkali granite consistently plots in the A-type granite field on the Ga/Al vs. Zn diagram of Whalen et al. (Fig. 17b). The alkali granites align with the I-type granite field on the A/CNK versus SiO2 (Fig. 13b) protolith discrimination diagram, following the work of Chappel and White in 1974. Whalen et al. (1987) noted that highly fractionated felsic I-type granites can have Ga/Al ratios, as well as some major and trace element values that overlap those of typical A-type granites. Therefore, extreme fractionation from calc-alkaline I-type magmas could potentially explain the A-type characteristics of the studied rocks. We interpret it to represent rocks that crystallized from magmas derived from continental crust or underplated crust that underwent a cycle of continent-continent collusion or island arc magmatism (Eby, 1992). It plots in the A2 subgroup (Fig. 17b) for A-type granite. It also plots the within-plate granite field of the Rb vs. Y+Nb tectonic discrimination diagram of Pearce et al. (1984) (Fig. 17a), which is characteristic of A-type granites (Winter, 2014). The alkali granite is rich in SiO2 and has the highest mean SiO2 value (74.18 wt%) amongst the rocks in the study area. It also has the highest mean values for Fe2O3 (2.72 wt%) and CaO (0.64 wt%). The HFSE elements Ga, Zr, Hf, Nb, and Y, with mean values of 27 ppm, 286 ppm, 7.05 ppm, 31 ppm, and 96.5 ppm, respectively, enrich it, indicating its production from melts rich in accessory phases like zircon, garnet, and sphene (Hanson, 1978; Hanson, 1980). The alkali granites have the highest mean Ba and Ce values, at 365 ppm and 164.5 ppm, respectively. In both megascopic and microscopic observations, the alkali granites show no signs of significant deformation. It occurs as high-rising plutons in a ridge-like manner, extending much further beyond the study area from the north-eastern portion. In El-Bouseily and El-Sokkary's (1975) Rb-Sr-Ba ternary diagram (Fig. 15d), the alkali granites are located in a strongly differentiated granite field, near the normal granite boundary and significantly closer to the Ba corner of the plot. The alkali granite REE pattern is subparallel, LREE enriched (LaN/ YbN value of 6.38), prominent negative Eu anomaly (mean Eu/Eu* = 0.27), and characterized by a nearly flat HREE profile consistent with REE patterns for A-type granite (Eby, 2011; El Hadek et al., 2016).

4.2.2. Albitised Granite

Ba, Pb, and Sr severely deplete the albitized granite. Hofman (1972) noted that the transformation of K-feldspar and calcic plagioclase to albite causes severe depletion in Rb, Ba, Sr, and Pb. The metasomatic process may have also led to the severe leaching of the REEs (ΣREE = 5.17) as observed in these rocks. The transformation of accessory phases, such as monazite to apatite and thorite in albitized granitoid, may contribute to the REE depletion (Boulvais et al., 2007). The albitized granite has a low K/ Rb (36.7–67.4) and a low Nb/Ta ratio (5.2–7.9), reflecting their highly evolved nature. The albitized granite is SiO2- rich, with a mean value of 73.89 wt%. It has the highest Na2O value of 5.06 wt%, reflective of its albite-rich nature. The rock contains a relatively low K2O value (3.71 wt%) but a high Na2O concentration, indicating the extent of K replacement by Na (Kaur et al., 2012). It also has a high P2O5 value of 035–038 wt%. The alkali granites exhibit extremely low CaO (0.11–0.14 wt%), TiO2 (0.002–0.004 wt%), and Fe2O3 (0.76–0.89 wt%) values, a result of leaching during hydrothermal alteration (Denies and Mark, 2000). The most noticeable thing about albitized granite is the presence of broad albite lamellas. These are thought to have formed when albite completely replaced Kfeldspar (Moody et al., 1985). According to Milord et al. (2000), the albitized granite exhibits a prominent positive Eu anomaly (Fig. 15c) and a low Rb/Sr ratio, indicating the accumulation of early-formed feldspar crystals rich in Eu. The REE plot for albitized granite shows that neighboring elements from Eu-Lu are getting more and less, making a pattern called a "REE tetrad." We interpret the W-type REE tetrad pattern as an open system fluidmelt reaction in a magmatic-hydrothermal system during the final stages of crystallization (Irber W., 1999; Zhao et al., 2002). Because the rock lacks biotite and is rich in feldspar, the positive Eu anomaly may indicate that it originated from melts depleted in other REEs. This is likely due to the accessory phases containing these REEs remaining armored in the biotite, preventing the melt from accessing them (Milord et al., 2000). The albitized granite occurs in an uplifted region as highly deformed and scattered outcrops. It contains several joints prominently running along the NE-SW direction. Along the joints, there is a thin zone of fine-grained greenish, possibly chlorite, resulting from hydrothermal alteration. The analysis of thin sections shows sericitization along the edges of the muscovite and feldspar grains, which is a sign of hydrothermal change. In the hand specimen, the rock is whitish and fine-grained. In the plot of Nb/Ta versus Zr/Hf (Fig. 16f), following Ballouard et al. (2016), the albitized granite separates from the field of rare-metal granites and Sn-WU related granites. Therefore, it appears highly improbable that it can serve as a fertile granite for rare metal pegmatites.

4.2.3. Geochemistry and Mineral Potential of the Pegmatites

It has been said that the trace element makeup of muscovite mica is another good way to tell how much fractionation there is in rare-metal pegmatites (Tischendorf et al., 2001; London, 2008). In muscovite mica, the amounts of Rb, Cs, Mn, Ga, Tl, Sn, and Ta go up, but the ratios of K/Rb, Ba/ Rb, Rb/Sr, Na/Ta, and Fe/Mn go down. This is typical of pegmatites that are highly fractionated (Černy et al., 1985; Shaw et al., 2016). The study area's muscovite samples exhibit enrichment in various trace elements, such as Rb (up to 3957 ppm), Be (up to 55 ppm), Sn (up to 798 ppm), Ga (up to 179 ppm), W (up to 802 ppm), and Nb (up to 194 ppm). The muscovite exhibits concentrations of other trace elements such as Cs (up to 133 ppm), Ta (up to 91.1 ppm), and Tl (up to 11.7 ppm), while it displays a relative depletion in Ba, Sr, REE, Th, and U. These patterns of enrichment and depletion could indicate the presence of micro-inclusions within the muscovite crystals (Shaw et al., 2016). On the other hand, the quartz-feldspar-pegmatite sample is observed to be enriched in Ba, Th, and Pb but depleted in Be, Nb, Ce, and Ti. The quartz-muscovite pegmatite has a higher content of Be, Mn, Nb, Ta, and W than the quartz-feldspar pegmatite. The presence of accessory minerals like beryl and columbite in the former is responsible for these differences. The muscovite samples are rich in Al, K, Sn, and Rb but low in Ca, Ba, and Sr. Rubidium contents in the muscovite samples range between 2946 and 3957 ppm. However, quartz-feldspar-pegmatites samples exhibit far lower concentrations, ranging between 83 and 542 ppm. In muscovite samples, K/Rb ratios are generally lower, ranging between 15.61 and 18.06, compared to quartz-feldspar-pegmatite samples with K/Rb ratios between 88.79 and 463.08 (Table 3). The low K/Rb ratio of the muscovite samples expresses their more evolved nature (Černy, 1982). El-Bouseily and El-Sokkary (1975) (Fig. 15d) cluster the quartz-muscovite-pegmatite and albitized granites near the peak of the strongly differentiated granite field. The alkali granites plot in the field of strongly differentiated granite close to the normal granite boundary and much closer to the Ba corner of the plot. Černy (1982) observed that the ratios K/Rb, Ba/Rb, Rb/ Sr, and Na/Ta all tend to decrease to extremely low values with increasing pegmatite fractionation. So, the K/Rb, Ba/ Rb, Rb/Sr, and Na/Ta values for the quartz-muscovitepegmatite trace element ratios are low, which means that the pegmatites are highly fragmented. We observe a negative correlation between K2O and SiO2 in the quartzmuscovite- pegmatites, and a positive correlation between Na2O and SiO2 (Fig. 10), which could potentially indicate Na- K exchange due to the influence of coexisting metasomatic fluid during the magma's evolution.

In particular, REE patterns for rare-metal pegmatites show strong negative Eu anomalies and more MREE and HREE than LREE. These strong negative Eu anomalies could mean that the plagioclase is being broken up, that a source material low in plagioclase is melting, or that a source material high in plagioclase is melting but the plagioclase is not melting (Zhao et al., 2002). On the other hand, the quartz-feldspar-pegmatite is marked by a prominent positive Eu anomaly (Fig. 15b), which is indicative of Eu fractionation into feldspars during the early stages of fractional crystallization, possibly in the lower crust (Shaw et al., 2016). Rudnick and Gao (2003) noted that the positive Eu anomaly is characteristic of rocks sourced from the lower crust. The occurrence of a weak negative Ce anomaly in the quartz-muscovite-pegmatite, accompanied by a significant negative Eu anomaly, indicates the role of late-stage or metasomatic fluids in the genesis of these rocks (Akintola et al., 2012 b; Taylor et al., 1986). The quartz feldspar pegmatites, on the other hand, have a weakly negative Ce signature and a strong positive Eu anomaly on the REE trend (Fig. 15b). The pegmatites and albitized granite display a noticeable W-type REE tetrad effect pattern. The W-type tetrad effect is interpreted to indicate open system conditions during granite crystallization (Irber W., 1999). Zhao et al. (2002) found that pegmatites mostly formed through fluid/melt processes and often have a strong Eu depletion and REE tetrad effect in the melt as well as in the rock-forming and accessory minerals that crystallized from this melt. This was seen in both types of the Kwarra pegmatite. A number of highly evolved granitic rocks, such as leucogranites and pegmatite, have been shown to have similar REE tetrad effects (Walker et al., 1986; Zhao, 1988; Jolliff et al., 1989; Yurimoto et al., 1990; McLennan, 1994; Irber, 1999). The main chemical differences between quartz-muscovite pegmatites and quartzfeldspar pegmatites are due to different compositions of the magmas that formed them. These differences may have been caused by the magmas mixing with rocks from the crust as they rose, or they could have come from different sources (London 2008, Černy 1991).

The mineral potential for rare metals such as Rb, Cs, Be, Y, REE, Zr, Hf, Nb, and Ta (Smirnov et al., 1986) in the area's pegmatite was assessed using geochemical criteria and representative diagrams for the whole rock pegmatites and mineral extracts. The amount of Ta and Cs in muscovite mica has also been used to find places where Ta-Nb minerals might be present in pegmatites (Selway et al., 2005). According to Selway et al. (2005), pegmatites containing muscovite with a Ta concentration greater than 65 ppm and a Cs concentration above 500 ppm are highly likely to contain Ta-Nb mineralization. The muscovite samples, on the other hand, have Ta levels between 65.5 and 91.1 ppm and Cs levels between 37.4 and 133 ppm, which are both much lower than 500 ppm. This means that they shouldn't be thought of as a major source of Nb-Ta mineralization. On the other hand, the quartz-feldsparpegmatite has a Ta concentration ranging from 11.3 to 21.7 ppm and a Cs concentration ranging from 1.3 to 14.9 ppm. When we look at the Ta-Nb mineralization potential trend of Ta versus Cs (Fig. 16a), Ta versus K/Cs (Fig. 16b), and Ta versus Ga (Fig. 16c), we can see that the quartzfeldspar- pegmatite plot always lies below the mineralization lines found by Beus (1966) and Gordiyenko (1971), while the quartz-muscovite-pegmatite plot usually lies above the Beus (1966) line of mineralization but below the Gordiyenko (1971) lines of mineralization. According to Ta-Nb mineralization potential plots for the pegmatites, the quartz-feldspar-pegmatite is interpreted as barren, while the quartz-muscovite-pegmatite can be described as not well developed.

We adapted a K/Rb versus Cs discrimination plot (Fig. 16d) from Erny and Burt (1984) to determine the mineralization potential of the pegmatites. We used a discrimination line to distinguish between the rare-metal class and the barren class. This plot showed that the quartz-quartz-muscovite-pegmatite belonged to the field of rare metal pegmatites, while the quartz-feldspar pegmatites belonged to the field of empty pegmatites. A second plot used to further separate the pegmatites into Be-class and Be-Li class pegmatites after Trueman and Černy (1982) shows that the quartz-muscovite-pegmatite are of the Be- Li class (Fig. 16e). The Kwarra pegmatite has a relatively high Sn concentration (up to 798 ppm) (Table 3), which is high enough to be associated with cassiterite mineralization. The method used to measure the amounts of different trace elements doesn't record the concentration of lithium. However, samples of quartz-muscovite-pegmatite show higher levels of lithium and beryllium in the plot of K/ Rb versus Cs (Fig. 16e) of their muscovite after Trueman and Černy (1982), so they can be thought of as higher in lithium. The fact that the quartz-muscovite-pegmatite has a mean concentration of 63.9 ppm of Cs and 22.3 ppm of Ta shows that the Cs-Ta mineralogy is not well developed in the rock. However, their concentrations significantly exceed the crustal abundance of 2 and 3 ppm, respectively (Taylor, 1964). Based on their chemistry and mineralogy, quartz-muscovite-pegmatite (rare metal) is most similar to LCT-type (Lithium, Cerium, and Tantalum) pegmatites and is therefore likely to have an ultimate source in sedimentary rocks (erny, 1991; Martin and De Vito, 2005; London, 2008; erny et al., 2012). The graph of A/CNK vs. SiO2 from Chappel and White (1974) (Fig. 12b) shows that the pegmatites came from an S-type source. According to Oyebamiji et al. (2018), LCT pegmatites have higher amounts of Be, B, F, P, Mn, Ga, Rb, Nb, Sn, and Hf. This is in line with the fact that the rock is high in Sn, Ga, Rb, Nb, and W.

4.3. The Role of Hydrothermal Alteration on the Geochemistry and Mineralogy of the Granitoids

Kuster (1990) modified the triangular Ti-Sn-(Nb+Ta) discriminant plot, showing the Pegmatites in the Kwarra area and the albitized granite plots in the albitization zone (Fig. 15c). The norm calculations (Table 2) show that the Ab value rises in the albitized granite, the quartz-feldsparpegmatite, and the quartz-muscovite-pegmatite, with mean Ab values of 12.1, 30, and 42.8, respectively. This increase in normative Ab is indicative of albitization (Kinnard et al. 1985). Albitization occurs during sodic metasomatism and is characterized by the exchange of Na for Ca or K and, to a lesser extent, Ca for Fe and Mg. Plagioclase and/ or K-feldspar are changed into almost pure albite by hydrothermal fluids during the albitization process (Kaur et al., 2012). Local albitization and local greisenization in the north-western portion of the studied area represent the effect of the sodic metasomatic stage. One of the notable megascopic indicators for strong and pervasive albitization is the whitened feldspars on the affected granitoid outcrops, a phenomenon commonly observed in albitized granite, quartz-feldspar-pegmatite, and muscovite granite (Baker, 1985; Charoy and Pollard, 1989; Petersson and Eliasson, 1997). The mineral groups that form during sodic metasomatism depend on how strongly the rock and fluid interact. Granites that are highly peralkali show the most impact (Bowden and Kinnaird, 1984). Kinnard et al. (1985) characterized soda metasomatism by increasing Ab, as observed in albitized granite, bulk granite, and muscovite granite, all of which exhibit strong albitized signatures with mean normative Ab values of 12.1, 30, and 42.8, respectively. Kuster (1990) conducted subsequent work on a regional scale covering the study area, clearly identifying these albites as products of Na-metasomatism. The amount of Na2O in the albitized granitoids goes from 3.55 wt% in the quartz-feldspar-pegmatite to 7.0 wt% in the albitized granite, while the amount of K2O goes down from 5.38 wt% to 3.71 wt%, which means that K and Na are exchanging places. As seen in the albitized granite and pegmatites, the metasomatic change of K-feldspar and calcic plagioclase to albite leads to a significant loss of Rb, Ba, Pb, and Sr (Kaur et al., 2012). The amount of Rb in the quartzmuscovite- pegmatite decreased from 3399 ppm to 41.5 ppm in the albitized granite and 21.6 ppm in the quartz-feldsparpegmatite. This is consistent with the progressive albitization of K-feldspar (Fig. 15d) and the loss of mafic phases, since Rb is more compatible with K-feldspar, biotite, and potassic hastingsite than it is with albite (Kaur et al., 2012). As Na/K levels rise, the graph of Rb vs. Na/K (Fig. 15d) shows that Rb levels decrease over time. This is called progressive albitization of K-feldspar. However, it is important to suggest that the initial Rb concentration in the rocks was significantly influenced by fractional crystallization, with albitization only slightly altering the Rb concentration. The metasomatic process may have also caused the REEs to be heavily leached (mean ΣREE = 5.17 for albitized granite, 15.82 for quartz-feldspar-pegmatite, and 5.07 for muscovite granite), as well as the W-type REE tetrad effect that can be seen in these rocks. The sodic metasomatic stage is characterized by the coexistence of crystals and supercritical fluid (El Hadek et al., 2016). The existence of a W-type REE tetrad effect supports this, indicating an open system, late-stage magmatic-hydrothermal interaction during the formation of the albitized rocks (Irber, 1999; Zhao et al., 2002). The loss of REE may be connected to the change of other minerals, like monazite to apatite and thorite in perauminous albitized granitoid (Boulvais et al., 2007). Along with the albitization process, the original Ti-Fe oxides are broken down, uranium levels rise, and columbite with small amounts of cassiterite, thorite, and xenotime are added (Obaje, 2009). Very little Fe and Ti are found in albitized granite, quartz-feldsparpegmatite, and quartz-muscovite-pegmatite (Fe2O3 = 0.82%, 0.73%, and 2.05% of the total weight, respectively). The concentrations of TiO2 are 0.003 wt%, 0.008 wt%, and 0.002 wt%, respectively. Albitization also concentrates HFSE, especially Nb and Ta, as observed in the albitized granite and pegmatites. While this may not be well pronounced in the albitized granitoids, it is however important to note that the albitized rocks have relatively notably elevated Nb and Ta concentrations compared to the other rocks in the study area (Table 3).

Sodic change is tightly controlled by fractures (Battles, 1994); becciation is well developed in places where albitization is strong, especially in the albitized granite and quartz-muscovite-pegmatite. Hydrothermal fluids migrate to areas of low pressure and in regimes of extensional and strike-slip tectonics. It is not surprising that fluids concentrate into dilatant fault zones. Apart from brecciation and cataclasis, we commonly observe intense silicification, sericitization, chloritization, or other chemical or mineralogical changes indicative of high fluid flux in these fault zones (Andy, 1998). In the Arum area, the albitized granite is well deformed and brecciated. The thin-section study reveals that the original K-feldspar has been albitized, and sericites are developing along the mica grain boundaries. We observe the most pronounced albitization along the NE-SW linearments in the northwestern flank of the study area, which are characterized by high lineament densities. Dillies and Einaudi (1992) suggest a depth of 1–4 km occurring at a low temperature ranging between 200 and 400°C. The fluid inclusions study conducted by El Hadek (2016) reveals that albitization occurred at high temperatures (350°C–410°C) in vapor-rich aqueous fluid. Sodic metasomatism's shallow depth and low temperature characteristic can explain the observed brittle deformation in the albitized rocks. Sodic metasomatism is important for the economy because it brings in ore minerals that contain nickel. These minerals show up as columbite in peraluminous biotite granites, pyrochlore in peralkali granites, and fergusonite in metaluminous hornblende biotite granites, but they are not as important (Obaje, 2009). The Nigerian provincés albitized granites have the highest uranium enrichment (Bowden et al., 1981); this is also evident on a smaller scale in the study area, where the albitized rock samples exhibit elevated uranium concentrations compared to the others. It ranges from 1.9 to 23.4 ppm in the quartzfeldspar pegmatite, from 0.4 to 15 ppm in the quartzmuscovite pegmatite, and from 6.8 ppm on average in the albitized granite. All the albitized rocks have a mean uranium concentration greater than the crustal abundance of 2.7 ppm (Taylor, 1964).

4.4. Petrogenesis

The Pan-African rare-metal pegmatites in central Nigeria formed because of post-kinematic, late-tectonic granite magmatism (Kuster, 1990). Multiple intrusive activities, being positioned in a way controlled by structure, and geochemical specialization patterns are some of the things that make this plutonism unique. It happens after a big orogenic event. Most geologists think that the pegmatites in the Kwarra area came from intermediate to felsic crustal sources because they are peraluminous (Miller and Mittlefehldt, 1985; Chappel and White, 2001). A lot of experiments have shown that biotite and muscovitecontaining metapelites melt to form peraluminous granites (Gardien et al., 1995; Patino Douce and Harris, 1998). As Chappell and White (1974) suggested, the Kwarra pegmatite is a type of granitic rock that comes from sedimentary protoliths (S-type). These were formed by the melting of sedimentary and/or metamorphosed sedimentary or supracrustal rocks, such as metapelites, according to Chappell and White (2001). In 1990, Kuster wrote that these kinds of metasedimentary rocks can be found in large amounts in the high-grade basements of the Wamba area and nearby areas. These rocks are thought to be a combination of semipelitic sediments from a metavolcano. This metapelitic rock provides a source of appropriate composition for the LCT rare-metal pegmatite end-members (Černy, 1991). Most LCT-type pegmatite fields are thought to have formed when melts from sedimentary rocks were split into very small granitic pieces (Erny, 1991; Martin and De Vito, 2005; London, 2008; Erny et al., 2012). Fractional crystallization is a feature of this type of magmatic process. As the leftover melt moves farther away from the solidifying parent granitic source, it picks up more rare elements and fluxes (London, 2008; Trueman and Černú, 1982). It is common for mica-rich melts to not separate completely in collisional zones, which creates LCT granitic pegmatites with a lot of trace elements (Černú et al., 2012). Different A/CNK values, trace element patterns, and mineralogy were found in the (rare-metal) quartzmuscovite pegmatites and the barren quartz-feldspar pegmatites. This suggests that the melting process used different materials. It is thought that the collision of the Pan-African continents and the thickening of the crust caused some of the older crust to melt. This caused several batches of magma to form and move in with slightly different chemical makeups, now making up major plutons (Kuster, 1990). The pegmatites' syncollisional setting supports their formation, which is linked to the process of crustal thickening (Pearce, 1996). Partial melting of compositionally distinct protoliths can also produce a wide compositional spectrum of granite magmas with the same degree of partial melting. Mineralogy, trace element chemistry, the stability field of sheet silicates or accessory minerals and their amount in the mineral/residuum phase could all change. This could also happen if the source metasedimentary lithology changed in a way that affected the somatics (Shearer, C.K. et al., 1992). Late Pan-African magmatic activity was multiphase, and successive plutons were able to release leftover pegmatitic melt that probably changed in different ways. This is how barren and mineralized pegmatites have been formed from different parent granites (Kuster, 1990).

The REE pattern of quartz-muscovite-pegmatites shows the impact of late-stage metasomatic fluid in an open system of magmatic and hydrothermal interaction while rocks were being formed. The quartz-feldspar-pegmatite REE trend also indicates a similar melt-fluid interaction. On average, the quartz-muscovite-pegmatite appears to be geochemically more evolved than the quartz-feldsparpegmatite. It shows higher contents of Rb, Cs, Mn, Ga, Tl, Sn, and Ta, while K/Rb, Fe/Mn, Ca, Ba, Sr, and Zr show a concomitant decrease. This trend is observed in the quartz-muscovite-pegmatites, attesting to their highly fractionated nature (Kuster, 1990; Shaw et al., 2016). Quartz-feldspar-pegmatite is very divided, which may have caused incompatible elements to accumulate in leftover melts. This could have led to a magmatic-hydrothermal system that was at the same time active. So, it appears that the metasomatic replacement and mineralization processes originated from the fluids that were left over in the pegmatite melts rather than fluids that were injected from outside (Oyebamiji et al., 2018; Kuster, 1990). However, the noticeable loss of REE, Ba, and Sr as the Na/K ratio rises may also be due to metasomatic recrystallization occurring during albitization and fluid interaction (Kaur et al., 2012). Trace elements that don't change easily during postmagmatic alteration (Ti, Nb, Ta, and Cs) are very low in the quartz-feldspar pegmatite but not as low in the quartz-muscovite pegmatite. This may be a better reflection of the magmatic compositions at the time the rocks were formed. The quartz-feldspar pegmatite has a positive Eu anomaly, while the quartz-muscovite pegmatite has a negative Eu anomaly. This strongly suggests that they came from different places, most likely the lower crust and the upper crust, respectively (Rudnick and Gao 2003). In terms of structure, the pegmatites are found in an area with a lot of stress. This is where the country rocks are severely deformed, foliated, and affected by brittle-ductile shear zones. Lineament analysis and structural trends observed in joints, faults, veins, and foliation show that NE-SW, NS, and NNE-SSW are the principal stress directions. The host rocks can be seen to have NE-SW (dextral) and NWSE (sinistral) conjugate systems. These are the results of late Pan-African brittle deformation (Ball, 1980). The migmatite gneisses are mostly made up of quartz-feldspar pegmatite that moves in NW-SE and E-W directions. The banded gneisses and granite gneisses are mostly made up of quartz-muscovite pegmatite that moves NE-SW. The albitization is observed to be most pronounced along the NE-SW lineaments in the northwestern flank of the study area, characterized by high lineament densities. Brecciation is well developed in the areas where the albitization is well pronounced, especially in the albitized granite and quartzmuscovite- pegmatite. Sodic metasomatism happens at low temperatures and shallow depths, which may explain why the rocks that have been altered by albitization are breaking apart easily. A depth of 1–4 km occurring at a low temperature ranging between 200 and 400°C has been suggested for such a sodic alteration (Dillies and Einaudi, 1992). Based on the geochemistry, geological structures, and mineralogy of the pegmatites, it is therefore suggested that the quartz-feldspar-pegmatite represent late-stage residual melts derived from less evolved granitic parent in the lower crust, which have been derived by fractionation, while the quartz-muscovite-pegmatite are products of highly fractionated late-stage magma derived from fertile granite at depth and modified by interaction with a coexisting hydrothermal phase during crystallization beneath the amphibolite belt. Reactivated ancient fractures, predominantly NE-SW, NNE-SSW, and N-S trending, as well as minor NNW-SSE and E-W, controlled the emplacement of these rocks. No fertile granite was mapped in the study area, suggesting that it is either buried deep within the subsurface or has a rather distal relationship with the rare metal pegmatite. Trueman and Černy (1982) noted that rare metal pegmatites are typically the most distant from their parent granites, having undergone increasing fractionation and concentration of rare elements and volatiles with increasing distance.

4.5. Relationship between Pegmatites and Rare Metal Mineralization in the Study Area

The relationship between pegmatites and rare metal mineralisation is intricately linked to their genesis, geochemistry, and associated host rocks. Pegmatites, particularly those of the Lithium-Cesium-Tantalum (LCT) type, are widely recognised as fertile sources of rare metals, including Rb, Cs, Ta, and Nb. In the studied region, the pegmatites are derived from highly fractionated melts, representing the final stages of granitic crystallisation. These magmas are enriched in incompatible elements, which concentrate in residual melts and subsequently crystallise to form mineralised pegmatites. The geological setting is defined by the polycyclic migmatite-gneiss complex, which has undergone extensive tectonometamorphic evolution, resulting in varying degrees of deformation and metamorphism. Pegmatites in the area often intrude migmatites, banded gneisses, and granite gneisses. The geochemical signature of pegmatites, particularly quartz-muscovite-pegmatites, reveals higher concentrations of rare metals compared to quartz-feldspar-pegmatites. This is attributed to the enrichment of elements like Rb, Cs, Sn, and Ta in late-stage magmatic fluids, coupled with fractional crystallisation and subsequent hydrothermal alteration. Structural controls, such as brittle-ductile shear zones and lineaments trending NE-SW and N-S, have facilitated the emplacement of pegmatites. Additionally, metasomatic processes, including albitisation, enhance the mobilisation and concentration of high-field-strength elements (HFSE), further contributing to mineralisation. These processes underscore the complex interplay between tectonics, petrogenesis, and hydrothermal fluid activity in determining the mineralisation potential of pegmatites (Beus, 1966; Gordiyenko, 1971; McDonough & Sun, 1995).

4.6. Conclusions

We detailed the mapping of the study area and documented the lithological units. Locally, the main lithological units in the study area are the migmatite-gneiss complex, Pan-African granitoids, and the Younger Granites. The rock types in the area are amphibolite, migmatite gneiss, banded gneiss, granitic gneiss, albitized granite, and alkali granite. Pegmatites and dolerites represent the minor rock types in the study area. The quartz-feldspar-pegmatite types are mostly found in migmatite gneiss, while the quartzmuscovite types are mostly found in banded gneiss and granite gneiss. Because they align with major study area faults, they are deformed. It was discovered that there were more structural lines in the pre-Cambrian basement rocks, such as migmatites, banded gneisses, and amphibolites, than in the Jurassic Younger Granites. The principal lineament trends are N-S, NE-SW, and NNE-SSW, with minor ENEWSW and E-W trends. The alkali granite is metaluminous, whereas the albitized granite and pegmatites are peraluminous. When the temperature and pressure are just right, the hornblende-plagioclase mineral forms in polycyclic basement rocks. This type of metamorphism is called amphibolite facies grade. There are two types of pegmatites that can be seen in the study area: pegmatites that are rich in rare metals and muscovite (quartz-muscovite) and pegmatites that are barren and low in muscovite (quartzfeldspar- pegmatite). The barren pegmatites tend to trend NE-SW, NNE-SSW, and N-S, while smaller amounts trend NNW-SSE and E-W. The quartz-muscovite-pegmatites are observed to be trending mainly along the NE-SW trend, while the quartz-feldspar-pegmatite predominantly along NW-SE and minor E-W trends. The pegmatites were formed during the Pan-African orogeny, which is characterized by crust thickening (Pearce, 1996) and syn-collisional setting (Kuster, 1990). The pegmatites are both peraluminous in nature (ASI > 1.0), with the quartz-muscovite-pegmatite showing the strongest peraluminous signature (ASI = 3.01). On average, the quartz-muscovite-pegmatites appear to be more evolved geochemically than the quartz-feldsparpegmatite. They show higher contents of Rb, Cs, Mn, Ga, Tl, W, Sn, and Ta, while K/Rb, Fe/Mn, Ca, Ba, Sr, and Zr show a concomitant decrease. The quartz-muscovitepegmatites exhibit this trend, indicating their highly fractionated nature (Kuster, 1990; Shaw et al., 2016). Because quartz-feldspar-pegmatite is very fragmented, it's possible that incompatible elements built up in the leftover melts. The W-type REE tetrad effect, present in quartz, muscovite, and pegmatites on the REE trend, suggests the simultaneous formation of a magmatic and hydrothermal system. After Kuster (1990), the pegmatites plot in the field of albitized pegmatites in the plot of Ti-Sn-(Nb+Ta) changed. This shows that hydrothermal fluids were involved in the rock formation. The amount of Rb decreases over time as albitization proceeds. This suggests that the process of albitization may have changed the Rb content of the pegmatites and albatized granite in some way. The albitified rocks indicate that the sodic metasomatism may have increased the abundance of W, Na, Al, U, and Sn while decreasing the concentration of Fe, Ti, and REE. In this study, the quartz-muscovite-pegmatites contain more of the rare elements Sn, W, Rb, and Be than REE, Sr, and Ba. They also have fewer of the rare elements Ta and Nb. The mineral potential plots indicate that the rare metal pegmatites are Li-enriched and LCT-type. The quartz-muscovitepegmatite rock group's LCT and peraluminous properties suggest that the rocks were formed by the melting of sedimentary rocks, most likely metapellites. Based on the geochemistry and mineralogy of the pegmatites, it is therefore suggested that the quartz-feldspar-pegmatite represent latestage residual melts of a less evolved granitic parent that have been derived by fractionation, while the quartzmuscovite- pegmatite are products of highly fractionated late-stage magma derived from fertile granite at depth and modified by interaction with a coexisting hydrothermal phase during crystallization beneath the amphibolite belt.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data Availability

Data will be made available on request.

Acknowledgements

The first author is grateful to Dr. A.A Ibrahim, Dr. B.A. Jolly, Prof. Saidu Baba, Prof. Ogunleye Paul and Dr. Magaji for improving on the work.

Declaration of Generative AI and AI-assisted Technologies in the Writing Process

During the preparation of this work the author(s) used Qullbot.com/ Grammer Checker in order to improve the grammer for better flow. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

Fig 1.

Figure 1.Topographical map of the study area parts of Kurra sheet 189SW modified after Fed. Surveys, Nigeria 1967.
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 2.

Figure 2.Simplified map of the geology of Nigeria after Okunlola, (2005).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 3.

Figure 3.Geological Sketch map of central and south-west Nigeria showing the location of the Wamba pegmatite field (study area) and the distribution of Pan-African Older Granites and pegmatites (underlined). Geochronogical data sources are van Breemen et. al. (1977), Rahaman et. al. (1983), Matheis and Caen-Vachette (1983), Tubosun et. al. (1984).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 4.

Figure 4.(a) Hand specimen of the alkali granite in the study area (Latitude 9°06'05"N and Longitude 8°44'30"E), (b) Field photograph of albitised granite (Latitude 9°04'10"N and Longitude 8°47'43"E), (c) Field photograph of a granite gneiss outcrop (Latitude 9°03'23"N and Longitude 8°33'43"E), (d) Field photograph of foliation structure in the banded gneiss with quartz and quartzo-feldspathic veins (Latitude 9°05'05"N and Longitude 8°31'36"E), (e) Field Photograph migmatite gneiss with ptygmatitic folding, upper arrow pointing at melanosome and lower arrow showing leucosome (Latitude 9°00'28"N and Longitude 8°42'30"E), (f) Hand Specimen of amphibolite rock in the study area (Latitude 9°07'17"N and Longitude 8°41'50"E).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 5.

Figure 5.Photomicrograph of the studied rocks under Plane Polarized Light (PPL) (a) alkali granite, (b) albitised granite, (c) granite gneiss, (d) banded gneiss, (e) migmatite gneiss, (f) sheared amphibolite; Biotite (Bt), Perthite (Prth), Orthoclase (Or), Plagioclase (Pl), Quartz (Qtz), Augite (Aug), hornblende (Hbl), garnet (Grt), Sphene (Shen), and muscovite (Ms). Mag.0.25mmX10.
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 6.

Figure 6.Field Photograph of (a) quartz-feldspar-pegmatite intrusion into migmatite gneiss (Latitude 9°00'20"N and Longitude 8°42'16"E), (b) a narrow zone of tourmalinisation around the contact between quartz-feldspar-pegmatite and the host rock. (Latitude 9°00'30"N and Longitude 8°44'27"E), (c) Quartzmuscovite- pegmatites boulders (d) Whitish quartz-muscovitepegmatites intrusion in granite gneiss (Latitude 9°02'45"N and Longitude 8°34'38"E), (e) Highly deformed quartz-muscovitepegmatites trending NE (9°02'16"N and Longitude 8°35'36"E). (f) Deformed nature of quartz-muscovite-pegmatites in uplifted region (9°02'48"N and Longitude 8°31'10"E).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 7.

Figure 7.Field Photograph of (a) Field photograph of joints in the granite gneiss of the study area (Latitude 9°02'09"N and Longitude 8°38'34"E), (b) Field photograph of dextral fault along a quartz veins in the banded gneiss (Latitude 9°05'35"N and Longitude 8°30'06"E).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 8.

Figure 8.(c) Field photograph of pegmatite veins that intruded the migmatite gneiss of the study area (Latitude 9°01'15"N and Longitude 8°41'30"E). (d) Field photograph of quartzo-feldspathic veins in the banded gneiss within the study area (Latitude 9°05'05"N and Longitude 8°31'12"E), (c) Rose diagram of pegmatite veins in the study area showing the NE-SW dominant trend, (d) quartzo-feldspathic veins showing the NE-SW dominant trend in the banded gneiss of the study area.
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 9.

Figure 9.(a) Structural lineament map of the study area, (b) Rose diagram of structural lineament trends of the study area.
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 10.

Figure 10.Harker plot of Al2O2, CaO, K2O and Na2O against silica (SiO2) for the granite suites in the study area. S2 and S6 (Alkali Granite), S4 and S12 (Albitized Granite), S2 and S6 (Alkali Granite), S4 and S12 (Albitized Granite).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 11.

Figure 11.(a) A/CNK – A/NK plot for the granitoids in the study area after Shand (1943), (b) Fe (total)/(Fe (total)+MgO) versus SiO2 after Frost et al 2001.
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 12.

Figure 12.(a) Na2O + K2O-CaO versus SiO2 after Frost et al 2001, (b) Na2O+K2O vs. SiO2 for the granitoids diagram of Middlemost, (1985). S2 and S6 (Alkali Granite), S4 and S12 (Albitized Granite), S1, S5 and S8 (Quartz-Feldspar-Pegmatite).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 13.

Figure 13.(a) Ternary normative Ab-Or-An diagram after O’Connor, 1965, (b) Fig. 16: A/CNK vs SiO2 plot of rocks from the study area after (Chappel and White, 1974).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 14.

Figure 14.Spider diagrams for all the rocks samples normalized to average crust after McDonough and Sun (1995).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 15.

Figure 15.Chondrite normalized plot (After Nakamura, 1974) rare elements (REE) pattern for the (a) metamorphic rocks, (b) Pegmatites, (c) granites from the study area, (d) Plot of Rb-Ba-Sr after El Bouseily and El Sokkary (1975) for granitic rocks, (e) Modified Triangular Ti-Sn-(Nb+Ta) Plot for albitized granite and pegmatites in the study area (after Kuster, 1990), (f) Plot of Rb vs Na/K after Kaur et al., (2012) showing the progressive depletion of Rb with the advancing albitisation of K-feldspar.
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 16.

Figure 16.(a) Plot of Ta Versus Cs tor The Muscovites of the pegmatites in the Kwarra area, (b) Plot of Ta Versus K/Cs For The Muscovites of Pegmatites in the study area. (After Beus 1968), Gordiyenko (1971), (c) Plot of Ta versus Ga for the pegmatites in the study area (After Černy and Burt, 1984), (d) Plot of K/Rb versus Cs for the pegmatites (after Černy and Burt, 1984), (e) Classification of the pegmatites using the plots of K/Rb versus Cs (after Trueman and Černy 1982), (f) Nb/Ta versus Zr/Hf diagram differentiating the barren granites and granites hosting ore deposits (after Ballouard et al., 2016).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Fig 17.

Figure 17.(a) Rb vs. Y+Nb tectonic discrimination diagram after Pearce et al. (1984), (b) Plot of FeOt / Mgo vs. Zr+Nb+Ce+Y for discriminating A-type granites after Whalen et al., (1987), (c) Triangular plot of Nb-Y-Ce for distinguishing the alkali granites into A1 and A2 granite (After Eby, 1992).
Economic and Environmental Geology 2024; 57: 735-768https://doi.org/10.9719/EEG.2024.57.6.735

Table 1 . Major oxide compositions of the rocks in the study area.


Table 2 . CIWP Norm for the granitic rocks of the study area.


Table 3 . Concentration of some selected trace elements and some important elemental ratios in the rocks of the study area.

PetrologyAmphiboliteMigmatiteBanded GneissGranite GneissAlbatized GraniteAlkali GraniteQuartz-Feldspar-PegmatiteQuartz-Muscovite-Pegmatite
Elements (ppm)S10S3S9S7S4S12S2S6S1S5S8S13S14S15S16
Be123635131581354221435519
Ba4583684474173351379724892419735814
Sr1454774513231144450654831760405512
Y211710143294992122231
Zr65185250134292228029235231171221184
Co70595658695647536656296011911466
Zn100106110801631408091< 30< 30< 30120130100240
Ga141723264736252930221612797100179
Rb135485152465822214231542511833213294634823957
Nb48121430222834753187116164194
Tl0.20.20.51.14.21.0120.92.50.78.45.611.76.6
Sn1137816671125203328189585798
Cs0.71.63.215.15749.97.612.410.814.91.367.954.713362
Hf1.61.45.83.21.972.17.66.54.11.83.71.21.52.10.3
Ta0.51.11.41.75.34.24.26.37.5621.711.370.565.591.177.2
W142132298345412378298310285352187405745802429
Pb1018153218102835582752201278
Bi0.4< 0.4< 0.41.12.31.8< 0.4< 0.4< 0.4< 0.4< 0.42.33.1< 0.4< 0.4
Th1.21.312.47.70.80.426.4310.60.30.40.710.11
U0.30.41.62.77.46.26.47.61823.41.915.282.60.4
K/Rb280.97405.85213.89219.5567.4836.76197.84182.5688.8093.25463.0818.0615.6116.0217.43
Nb/Ta8.007.278.578.247.925.246.675.4012.502.9410.009.5910.093.9922.38
Rb/Sr0.090.110.190.4742.27205.504.864.628.3410.650.2653.5573.6563.31329.75
Na/K1.891.601.680.851.201.240.600.590.570.570.640.240.270.160.11
K/Ba81.1726.2221.5445.044482.8210072120.62111.27668.43992.7241.603054.08630.01961.834927.54

Table 4 . Concentration of rare earth elements in the rocks of the study area.


Table 5 . Average abundances and ranges of some trace elements, rare earth elements and selected ratios from upper continental crust values from Taylor and McLennan (1985).


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KSEEG
Dec 31, 2024 Vol.57 No.6, pp. 665~835

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