Petrogenetic characterization of pegmatites and their host rocks in southern Akwanga, North-Central Basement Complex, Nigeria

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Petrogenetic characterization of pegmatites and their host rocks in southern Akwanga, North-Central

Basement Complex, Nigeria

ANTHONY CHUKWU^1,* and SMART C OBIORA²

1Department of Geology, Ebonyi State University, Abakaliki, Nigeria.

2Department of Geology, University of Nigeria, Nsukka, Enugu State, Nigeria.

*Corresponding author. e-mail: achukwu1@gmail.com

MS received 25 February 2020; revised 20 July 2020; accepted 11 September 2020

The pegmatites in southern Akwanga occur within the reactivated belt of the basement complex of Nigeria. The pegmatites consist of dominantly albite–muscovite pegmatites (rare-metal), southern parts of the map areas and biotite-microcline pegmatites (barren) central parts of the map. The pegmatites intruded gneiss-migmatitic complex consist of metasedimentry rocks; granitic gneisses and biotite gneisses and rarely meta-igneous, amphibolites. The rare metal pegmatites are composed of quartz, albite and muscovites and tourmaline. Garnets, ilmenites and minor tin–columbite–tantalite mineralization constitute accessory minerals in contrast to the biotite-microcline pegmatites. The host rocks are composed of quartz, plagioclase (An5–21; albites–oligoclase), microcline and muscovite. Minor constituents include biotites, cordierites and hornblendes. Ilmenite occurs as opaques. The pegmatites and their host rocks are corundum and hypersthene normative, highly peraluminous, exhibiting similar geochemical signatures; however, the rare metal pegmatites are more fractionated than the host rocks and the biotite- microcline pegmatites. The rare metal pegmatites are relatively enriched in Rb, Li, Cs, B, Be, Nb and Ta, low in K/Rb and Al/Ga ratios than the biotite–microcline–pegmatites and their host rocks. The pegmatites are products of crustal anatexis of sedimentary origin. This indicates that the rare metal pegmatites are source rock controlled (product of post-collision activities) rather than fractional crystallization.

Keywords. Nigerian basement complex; southern Akwanga rare metal pegmatites; LCT and peraluminous; crystal fractionation; anataxis.

1. Introduction

There is increasing eAort for the exploration of granitic pegmatites around the world because of their significant host for rare metals such as Sn, Li, Nb, Ta and Cs. These metals are currently in high demand because of their applications in new tech- nologies globally. However, the major issues sur- rounding pegmatite studies for decades among geologists remain their petrogenetic classiBcations.

Earlier works on Nigerian pegmatites predominantly believed that they are genetically related to the Proterozoic to Paleozoic Pan-African granites of Nigeria (Jacobson and Webb 1946; Matheis 1987; Kuster 1990; Akintola and Adekeye 2008;

Akoh et al. 2015). However, recent Bndings cast some doubts on this genesis. The pegmatitic Beld in the southern Akwanga, northcentral basement complex constitutes part of the unmetamorphosed intrusions of acid and basic dykes of late to

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post-tectonic Pan-African rocks (600 ± 150 Ma;

Grant 1970; Matheis and Caen-Vachette 1983;

Obiora and Ukaegbu 2008). The pegmatites are reported in broad belt extending from Ago–Iwoye area in the southwest to Jos, northcentral basement Nigeria (Bgure 1; Jacobson and Webb 1946;

Wright 1970; Matheis 1987; Kuster 1990). Other areas are around Zuru-Gusau, northwest (Garba 2003; Okunlola and King 2003) and Obudu area, southeastern Nigeria (Ekwueme and Matheis1995;

Ibe and Obiora2019). Presently, rare metal mining of these pegmetite Belds around southern Akwanga are not known; however, there are evidences of alluvial panning of rare metals down streams by locals in the area. Furthermore, Cassiterite and surbordinate columbite-tantalite artisanal mining have been ongoing for over 10 decades around parts of Wamba (NE Akwanga) pegmatites and eluvial placer (Kuster 1990), which is about 30 km northeast of southern Akwanga. The pegmatities of the southwest and part of Wamba are dated to range from 580 to 530 Ma by Rb/Sr method (Matheis 1987).

There are notable works in Nasarawa northcentral basements, but most were concentrated in northwest part of the Nasarawa, mostly around

KeD and partly of Wamba area. Onyeagocha (1984) and Obiora and Ukaegbu (2008) described the basement rocks around the boundary between the basement and sediments as migmatitic banded gneisses and granitic rocks of calc-alkaline and peraluminous, but no account on pegmatites mineralization potentials. Other workers on pegmatites mostly around KeD area, NW of Akwanga include Adekeye and Akintola (2007, 2008); Akintola and Adekeye (2008) who sug- gested that the pegmatites were structurally controlled with high potential for rare metals.

Okunlola and King (2003); Okunlola and Ocan (2009) revealed the occurrence of Ta–Sn–Li–Be mineralization in vertical and lateral pegmatites around KeD area. Jacobson and Webb (1946), Kuster (1990) and Akintola and Adekeye (2008) opined that the rare metal pegmatites in part of Wamba, Nigeria are genetically related to the Pan-African granitoids based on Beld observation and available data; however, earlier age differences from isotopic data from Matheis and Caen- Vachette (1983) proved otherwise, which led Matheis (1987) to suggest that the pegmatites in southwest Nigeria resulted from reactivation of tectonic trends in addition to partial melting and external volatiles. Most of these workers on the pegmatites in Nigeria attributed its formation to the model of extended fractional crystallization of fertile parent granites of Stilling et al. (2006) even when such granite bodies are enigmatic, but recent work of Goodenough et al. (2014) considering the age differences (more than 100 Ma differences) between the pegmatites in Sarkin Pawa, northcentral Nigeria and Pan-African granitiods conclude that they are not genetically related, and rather suggest a pegmatite conundrum. Therefore, there are still existing controversies regarding the origin of the granitic pegmatites in Nigeria. Fur- thermore, apart from the generalized classiBcation of the pegmatites in northcentral basement as Wamba–Nasarawa Beld of the ‘Older Tin-Beld’ by Jacobson and Webb (1946), there is no known literature account of the pegmatite rocks, southern Akwanga. Finally, comprehensive data on whole-rock geochemistry of the pegmatities in Nigeria in relation to their host rock petrogenesis is very rare. This work used detailed Beld studies, mineralogical and bulk rock chemistry to characterize the pegmatites, petrogenesis, mineralization potential and their relationship with the host rocks in the southern part of Akwanga in order to infer their sources.

Figure 1. Position of the Precambrian basement complex of Nigeria in relation to the West African, Congo Cratons and southern Tuareg Shield. (A) Represents the western Tuareg shield (Pharusian belt); (B) central Tuareg Sheild (Hoggar- Air segment); (C) eastern Tuareg shield (East-Saharan Craton, modiBed after Obiora and Ukaegbu2008).

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2. General geology

The Nigeria basement complex is part of the Upper Proterozoic Pan-African mobile belt, located between the West African and Congo Cratons and south of the Tuareg Shield (Bgure 1). The Pan- African reactivated mobile belt extends from Algiers across southern Sahara into Nigeria, Benin and Cameroon to northeastern Brazil, where comparable rare-metal mineralized pegmatites occurred (Casting et al. 1994; Garba 2002; Jacobs and Thomas 2004; Arthaud et al.2008). The Pan- African trans-Saharan mobile belt evolved by collision of the active margin of the Pharusian belt (Taureg shield) and the passive plate of the conti- nental margin of the West African craton approx- imately 600 Ma (Burke and Dewey 1972; Black et al.1979; Caby et al.1981). McCurry and Wright (1977) noted that the subduction led to ocean closure and extensive melting of the older rocks which consequently emplaced the predominantly calc-alkline granitoids and basaltic intrusions, while Black and Liegeois (1993); K €uster and Harms (1998) characterize the belts to mainly post-colli- sional granitoids. The plutons are potassic in nature and parental magmas probably derived from mixed mantle and crustal sources (Black and Liegeois 1993; K €uster and Harms1998).

Rocks of the Nigerian Basement Complex are believed to have undergone four major orogenic cycles characterized by intense deformation, metamorphism, reactivation and remobilization (resulting to extensive migmatization) corresponding to the Liberian (2650 ± 150 Ma), the Eburnean (2000 ± 50 Ma), the Kibaran (1100 ± 200 Ma) and the Pan-African cycle (600 ± 150 Ma). Generally, three major lithologic units are mapped in the Nigerian Basement Complex (Bgure 1), they are migmatite-gneiss complex (biotite and biotite hornblende gneisses, quartzites and quartz schist and small lenses of calc-silicate rocks), slightly migmatized to unmigmatized paraschists and metaigneous rocks (pelitic schists, quartzites, amphibolites, talclose rocks metaconglomerates, marbles, banded iron-formations and calc-silicate rocks) and older granites (granodiorites, granites and potassic syenites (Akoh et al. 2015; Ibe and Obiora2019).

Migmatite-gneiss complex represents the basement in the real sense of it. It is believed to be the oldest rocks of the Nigerian Basement Complex and is dominant in all parts of the Nigerian Base- ment Complex. They exhibit great variation in

composition which is as a result of the different protolith and pressure–temperature conditions which they were formed (Obiora2005) and also due to the intense Pan-African deformation and remobilization which the basement had undergone. The components of the migmatite-gneiss include: the leucosome, mesosome and paleosome. The schist belt represents the younger metasediments with occasional metaigneous units. The schist trends mostly N–S and appeared to have been restricted to western half of Nigerian Basement although;

metasediments have been mapped around the Nigerian Oban massif (southeastern basement complex). The Pan-African (Older) granite suites intruded the migmatite-gneiss and the schist complex. Other rocks of the Pan-African granites include: porphyritic/porphyroblastic muscovites- granites, aplites, tonalities, diorites, syenites and chanockites (Obiora2005). They are often slightly foliated, gradational contacts which suggests non- magmatic. They are also calc-alkaline and peraluminous and metaluminous in composition. Jones and Hockey (1964) identiBed three phases of the Pan-African granite suites based on Beld relationship, mineralogy and textures: (1) Early phase comprising gabbroic rock, dolerites, granodiorites and quartz diorites. (2) Main phase made up of coarse porphyritic hornblende granite, syenites and coarse porphyritic biotites granite. (3) Late phase made up of homogenous granite, pegmatite and aplite dykes. However, Dada and Respaut (1989) using Beld and geochemical evidences opined that the dolerites and pegmatites, classiBed as unmetamorphosed rocks of the basement complex since they cross-cut the Pan-African rocks and represent the youngest rocks of the basement complex. The Jurassic Younger granites of Jos area intruded the basement complex. In contrast to the older granites, it has sharp contact with the basement rocks which shows that it is magmatic, occurring as ring dykes and alkaline in nature. It serves as the major source of the tin and other rare metal mineralization around the basement especially around Jos Plateau, Nigeria.

3. Geology and petrography

The basement rocks of the southern Akwanga area consist of predominantly migmatitic gneisses, granitic gneisses (most host of the pegmtaite bodies) and sparse amphibolites (Bgure 2). The grade of metamorphism ranges from greenschist to

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amphibolite facies. They are intruded by unmetamorphosed syn- to post-collision rocks such as biotite granites, dolerites and the pegmatites swarm. The Mada hills located at the southern parts of the map, Wogan community trends E–W direction and cover an area of more than 50,000 km²long beyond the mapped area. The migmatitic gneiss generally shows coarse-grained, mesocratic and gneissose foliations in NNE–SSW directions (Bgure3a). The foliation planes dip ranges from about 18° to 55° mostly SE direction except otherwise folded. There are obvious alignments of biotites and conspicuous alternation of leucocratic and melanocratic components resulting to banding of the gneisses. The leucosome is composed mainly of quartz, microcline and plagioclase, while the melanosome composed of mainly biotites and minor hornblendes. There is also evidence of undifferentiated older metasediment (paleosome) in the migmatitic gneisses. The migmatitic gneisses also possess quartzo-feldspathic veins ranging from 0.8 to 5 cm, mostly concordant with few cross- cutting the foliation trends. Ptygmatic folds of the quartzo-feldspathic veins and pinch and swell structures with lit per lit ejections are common especially around the Wowyen hill (Bgure3a).

There are also strike-slip faults, dextral and sinistral dislocating veins and veinlets. The

migmatite-gneisses are cross-cut by basic dykes, basaltic, rhyolitic porphyries and thin pegmatite dykes, although some occur as sills (Bgure 2). The variable dip directions of the foliations also show tectonic remobilization of the area. Some of the biotite gneisses are porphyroblastic in composition (Bgure3b) with phenocrysts of albites forming augen structures. The plagioclase crystals occur as albite in the leucosome and andesine in the melanocratic components and are characterized mostly by albite polysynthetic twinning with few showing Carlsbad-albite combine twinning.

Microcline is colourless, subhedral to euhedral with distinct cross-hatch twinning.

3.1 Granitic gneisses

The granitic gneisses are leucocratic to mesocratic, medium to coarse-grained and possess gneissose foliation in mostly NNE–SSW directions. There are also augen structures (Bgure 3b) of cleavendite and rarely orthoclase mineral. The granitic gneisses contain major mineral constituents as quartz, plagioclase (An_5–12; albites), microcline and muscovite. Minor constituents include biotites, cordierites and hornblendes. Ilmenite occurs as opaques. Biotites and hornblende are the common maBc minerals in the rocks. Biotites and

Figure 2. Geological map of the study area showing location of the pegmatites and the host rocks.

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muscovites are mostly aligned to the foliation trends in the granitic gneisses and biotite gneisses within the study area. Quartz is milky coloured, low relief and undulatory extinction showing deformation similar to the migmatites. Microcline is anhedral with cross-hatch twinning. Muscovite and biotites are colourless and brownish pleo- chrioc, respectively; they are elongate with thin cleavages in one direction, subhedral to euhedral

and show parallel extinction to the cleavages. The mineralogical compositions and presence of relicts of metasediment show that the protolith is most probably pelitic.

3.2 Amphibolite

The amphibolites are less abundant in the study area and occurred mostly at the southern parts of a

c

d e

b

Figure 3. Field photographs of the granitic pegmatites and the host rocks in south of Akwanga. (a) Migmatitic banded gneisses at the base of Wowyen hill showing the leucosome, paleosomes and quartzo-feldspatic Cow veins, (b) porphyroblastic banded gneisses outcrop at Hawonkibo hill Wogan (road section along Akwanga–LaBa road), showing augen structures composed of cleavendites, (c) wall zone showing contact between the granitic pegmatites and granitic gneisses occurring as roof pendant, (d) mineralized portion of the pegmatite showing sporadic mineralization (black tourmaline, tin, Nb–Ta) in red colour, and (e) portion of the pegmatites showing graphic texture of quartz and albite with patches of garnets (dark brown).

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the map at the base of the migmatitic gneisses hills.

They occurred in two forms: the coarse grained type and the pegmatitic type that is extremely coarse grained. The coarse-grained amphibolites are localized within the migmatitic gneisses ter- rains in the Wowyen quarry; it is dark greenish and holocrystalline and consists of grains of hornblendes with lustrous surfaces up to 6 mm and the cleavages can be observed even megascopically using hand lens in network of albites and quartz in almost equigranular relationship. There is also minor calcite, which may result as a breakdown of pyroxenes. There are also cross-cutting quartzo- feldspathic veins. This amphibolite most possibly has a protolith of metabasites thereby making the migmatitic-gneiss terrain, complex and heteroge- neous protoliths but predominantly pelites. The pegmatitic-amphibolites somewhat occur sparsely as weathered boulders forming swarm dykes radi- ating down the base of the Mada hills following the trend of the rare metal pegmatites (NW–SE) but are rare compared to the granitic pegmatites. The grain sizes of the hornblendes in these pegmatitic- amphibolites are up to 1.4 cm. The dominant minerals in the amphibolites are hornblende, quartz, albite and biotite. Calcites occur as secondary from breakdown of clino-pyroxene and calcic-plagioclase. Magnetites occur as opaque. The hornblende is brown to pale green and pleochroic.

It is subhedral to euhedral, high relief and shows cleavages in two directions. Longitudinal sections possess cleavages in one direction with maximum extinction of about 12°.

3.3 Granitic pegmatites

Granitic pegmatities are widely distributed within the granitic gneisses and migmatitic gneisses in the study area and less common in other rock types in the area except few pegmatitic veins and dykes of about 10–30 cm width. Prominent bodies of the pegmatites are predominantly conBned to the Wogan–Wowyen axis of the study area (southern parts of the map area; Bgure 2). The pegmatites occur as discrete swarm dykes radiat- ing down the base of the Mada hill while some occur as lenses on the surface. Pegmatites generally occur as tabular bodies, numbering more than 25 in the area (table 1). The major trend of the pegmatites is NNW–SSE, although E–W and rarely NE–SW directions also occur. The strike and dip of the pegmatites are highly variable due

to joints and fractures. The thickness of the Cat lying dykes mostly in migmatitic gneisses ranges from about 30 to 60 cm, while the tabular and massive dykes range from 4 to 30 m; the length of the dykes ranges from 20 to more than 300 m and some dykes continue along the trend beyond 800 m with interruptions between the dykes (table1).

The pegmatites generally show sharp contacts with the host rocks and roof pendants of the granitic gneiss are observed close to the periph- eral of the pegmatites (Bgure 3c). The granitic pegmatites in the southern Akwanga are grouped into two, based on their Beld and mineralogy studies; the biotite-microcline pegmatites, which represent simple pegmatites and barren and the albite–muscovite pegmatites, which represent the complex pegmatites and rare metal pegmatites (table1).

The boitites-microcline-quartz pegmatites occurred mostly towards the central parts of Akwanga and towards Andaha along new Jos road. Apart from the extremely coarse and euhedral grains of quartz, biotite, and microcline, other mineralogical compositions are albites, muscovites, sparse garnets and magnetites. The quartz is colourless to smoky, while the feldspars are pinkish, blocky with two perpendicular cleavages given ways for parting. The biotites are mostly primary and some replaced by secondary muscovites. Biotites are brownish and pleochroic from pale brown to deep brown and exhibits parallel extinction. Albite is colourless with thin albite polysynthetic twinning. Microcline is colourless with its unique cross-hatch twinning.

Small graphic intergrowths of myrmekite are also observed in thin section.

The albite-muscovite pegmatites are prominent towards the southern parts of the mapped area (Bgure2) around Wogan–Wowyen and Sabon Gida areas of Akwanga areas (the Wogan–Wowyen pegmatites Beld). There is no clear zoning in these pegmatites, but careful observations can trace large crystals up to 15 cm of quartz, muscovite-cleavan- dite and occasional black tourmalinized irregular core zones while coarse to aplitic quartz-muscovite and minor biotites wall zones can be observed.

These pegmatites have potential for Sn, Nb and rare Ta mineralization in contrast to the assertion of Matheis (1987) who disputed the potential of columbite around the Nasarawa pegmatites. The pegmatites are characterized by colourless to smoky quartz, light-pinkish albite and minor microcline, and large books of colourless – brownish

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muscovites (more than 10 cm). Black tourmaline patches (up to 16 cm in diameter) with striations also occur as accessory (Bgure3d) and garnets are sporadically distributed in the pegmatites. Miaro- litic textures were observed in few locations of the pegmatites and graphic textures of quartz and albites are common than the simple pegmatites (Bgure3e). From the mineralogical studies, extremely coarse grained and euhedral quartz, albite and muscovites constitute the major mineral constituents of the pegmatites. Biotites also occurred in

\4% of the mode. Quartz is characterized by colourless and undulating extinction in some of the samples. Albite shows albite twinning with anorthite composition of An_7–13. Graphic-myrmekitic texture (intergrowth of albite and quartz) is also common and albitisation is the product of metasomatic replacement in the pegmatites, where some of the slides in modal analysis recorded almost 65–80%

albite (cleavendites) in place of microcline and other K-feldspars which occur as patchy in most of the slides. Other accessory minerals are fractured almandine-garnet, black tourmalines (schorl) and tiny dark patchy minerals possibly cassiterite–columbite–tantalite minerals which are mostly associated with the albitisation. The muscovites are primary with no obvious deformations showing that the pegmatites are post-tectonic activ- ity in the region. SpeciBc mineral species were not determined since there are no mineral chemistry data.

4. Sampling and analytical methods

Eighteen representative samples of large granitic pegmatites (*4 kg to optimize the very coarse grains) and the host rocks were collected. The

samples were collected according to their abundances in the Beld; they consist of nine granitic pegmatites and nine host rocks comprising of seven granitic gneisses, biotite gneiss and amphibolite.

Although the pegmatites are massive and poorly zoned, the surface wall zones were systematically identiBed and sampled because of their average representative of the mineral assemblages of the entire body and to maintain analytical consistency.

The sampling was conducted along traverse across the ridges of the pegmatites according to Beld grouping in locations. Each sample is made up of rock fragments obtained using sledge hammer and chisel up to 4 kg. The samples were grinded using jaw crushed to minimal grain size of \5 mm to achieve homogeneity, the crushed sample materials were thoroughly mixed with ceramic ball mill for more than 15 min and rifCe splinter used to split 100 g sub-samples each, which was thereafter pulverized down to 200 mesh using tungsten mill at the Inorganic Geochemistry Laboratory of the Department of Geology, University of Nigeria, Nsukka. The samples were shipped to Genalysis (Intertek) Laboratory Services Pty Ltd, Madding- ton, Australia for analysis. The major oxides were determined using X-Ray Fluorescence (XRF) with analytical precision from duplicate samples better than 1%. The trace elements and rare earth elements were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Boron and lithium were determined using sodium peroxide fusion ICP-MS instead of lithium metaborate/

tetraborate fusion ICP-MS. Loss-on-ignition (LOI) was determined by robotic thermogravi- metric analyzer (TGA). Detection limits for the major oxides is 0.01 wt.%, 20 ppm for B, 5 ppm for Li, 2 ppm for Sn, 0.5 ppm for Rb, 0.1 ppm for Cs, 10 ppm and 0.1 ppm for Nb and Ta, respectively.

Table 1. Pegmatites locations and dimensions as occurred in the study area.

Pegmatite group

No. of pegmatite

bodies Location name Trends

Dimension in length

(m) Rock name

1 6 Wogan I NNW–SSE 10–200 Albite–muscovite pegmatites

2 3 Alhala Utsu stream

(Wogan)

NW–SE 5–30 m Albite–muscovite pegmatites

3 6 Wogan II NNW–SSE 20–850 m Albite–muscovite pegmatites

4 5 Wowyen river NW–SE 4–120 m Albite–muscovite pegmatites

5 2 Wowyen E–W 2–8 m Albite–muscovite pegmatites

6 4 Agwadi river NW–SE 2–10 m Biotite–microcline pegmatites

7 2 Andaha road NE–SW 2–4 m Biotite–microcline pegmatites

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5. Results

5.1 Bulk-rock geochemistry 5.1.1 Host rocks

The various host rocks of the granitic pegmatites include: metasedimentary rocks (granitic gneisses and biotite gneisses) and amphibolite. The major element oxides of the rocks from the southern Akwanga with CIPW norms are presented in table2. Aluminium Saturation index (A/CNK) and modiBed Alkali–Lime index (MALI/N+K–Ca) and other important parameters were calculated and included in the table. Considering the CIPW norms (table 2), the percentages of quartz, orthoclase and albite are [90% in granitic gneisses conBrming the high leucocratic components of the rocks. The host rocks are hypersthene and corundum-normative except the biotite gneisses and the amphibolite which only showed hypersthene – normative. The alumina saturation index (ASI;

A/CNK) of the host rocks is strongly peraluminous (ASI; 1.32 in biotite gneiss to 1.60 in granitic gneisses) except the amphibolites (ASI; 0.68) which is metaluminous. On the R1–R2 variation diagram of De la Roche et al. (1980) is based on the cation proportion (Bgure 4). The granitic gneisses plot is predominantly in the granite Beld, while one sample of the granitic gneiss plot is in granodiorite Beld. The biotite gneiss plots are in the Beld of monzodiorite.

The R₁–R2 variation diagram is more reliable over other discrimination diagrams for rock clas- siBcations due to the use of cation proportions of the rocks instead of the oxides and the involvement of entire major element chemistry for the classiB- cation [R₁ = 4Si 11(Na+K) 2(Fe+Ti), R₂ = 6Ca+Mg+Al]. In Harker diagrams (Bgure 5), the values of Al₂O₃, P₂O₅, TiO₂, MnO, Fe₂O₃(t) and CaO show inverse relationship with SiO2 in the host rocks except the amphibolite, which behaves differently from the metasediments like in Na2O and MgO having mild inverse relation with SiO₂, while K₂O exhibits positive correlation with SiO₂. These characteristics indicate that the host rocks have similar origin probably from partial melting and crystallization from crustal materials while the amphibolite could originate from metaigneous rocks.

Trace element concentration (table3) varies across the host rocks. The values of Rb, Ba and Sr across the host rocks are relatively higher than

average crustal abundances of Wedepohl (1995) except in the amphibolites which have low values.

The values of Nb and Ta which range from 6 to 19 and 0.2–1.8 ppm, respectively, in the metasediments are also relatively low compared to Wede- pohl (1995) of 19 and 1.1 ppm, respectively. The host rocks are generally depleted in transitional metals and base metals such as Ag, Cr, Co, Ni, Cu, Mo, Pb and Zn with exceptions in the amphibolites, which are relatively enriched in Cr (465 ppm), Co (54.2 ppm), Cu (96.4 ppm), Ni (74 ppm).

The average values of Cs and Ga in the host rocks (Cs: 9.25 ppm, Ga: 27.0 ppm in granitic gneisses;

Cs: 6.0 ppm, Ga: 19.0 ppm in biotite gneisses; Cs:

12.6 ppm, Ga: 16 ppm in amphibolites) are relatively high compared to the average crustal abundances of Wedepohl (1995). Li concentrations in the metasediments (granitic gneisses and biotite gneiss) range from 19 to 90 ppm, while it is 80 ppm in amphibolites. Some of the trace elements and the REEs are normalized to primitive mantle and CI chondrite, respectively, after McDonough and Sun (1995) and presented in Bgure6. The patterns show that the host rocks (granitic gneisses, biotite gneiss and amphibolite) are characterized by positive anomalies for Rb, Th, K, Cs, P and minor positive Ba, La and Zr and depleted Nb, Ta, Hf and Ti, Sn, Sr and Pb (Bgure 6a). This geochemical characteristic is similar to rocks from other Pan- African orogenic belts (Goodenough et al. 2014;

K €uster and Harms 1998). The study rocks are enriched in the LREE relative to HREE [(La/Yb)n

= 3.27–54.91; Bgure 7a) with the granitic gneisses showing the highest value, while the amphibolite shows the lowest value. The host rocks also show mild negative Eu anomaly [(Eu/Eu*=0.21–1.21;

0.78 average)]. CA38 sample of the granitic gneisses has the lowest Eu anomaly with Eu/Eu* value of 0.21.

5.1.2 Granitic pegmatites

The granitic pegmatites (albite–muscovite–pegmatites and biotite-microcline-pegmatites) have higher SiO2contents (63.88–78.81 wt.%; average of 72.3 wt.%) compared to the host rocks. The values of TiO2, MgO and CaO\ 1 wt.%, and Na₂O concentrations dominate K₂O in contrast to the host rocks conBrming albitization in the pegmatites, also observed in the modal composition where albite dominate k-feldspars. Similarly to the host rocks, the light coloured minerals (quartz, albite

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Table 2. Major element oxides (wt.%) composition of the pegmatites and host rocks from southern Akwanga areas.

Albite–muscovite–pegmatite Biotite–microcline–pegmatite

CA01 CA02 CA03 CA04 CA05 CA06 CA07 CA18 CA19

SiO₂ 74.75 74.43 74.83 74.71 63.88 68.74 78.81 75.19 66.06

TiO₂ 0.03 0.03 0.01 0.02 0.02 0.05 0.03 0.04 0.04

Al₂O₃ 14.69 13.79 14.5 14.75 21.62 17.49 11.84 14.21 19.92

Fe₂O₃^T 1.07 2.18 1.01 1.13 0.94 3.56 1.15 0.69 0.87

MnO 0.02 0.03 0.05 0.02 0.04 0.12 0.02 0.02 0.04

MgO 0.1 0.23 0.08 0.09 0.12 0.41 0.02 0.06 0.17

CaO 0.31 0.11 0.17 0.35 0.53 0.12 0.87 0.64 1.73

Na2O 4.87 2.64 4.75 5.18 9.51 3.6 3.37 5.08 7.12

K2O 3.59 5.7 4.22 3.15 2.04 4.47 3.8 3.89 2.66

P2O5 0.14 0.189 0.156 0.147 0.416 0.154 0.022 0.062 0.593

LOI 0.47 0.38 0.47 0.53 0.8 0.76 0.23 0.2 0.65

Total 100.1 99.79 100.31 100.13 99.97 99.57 100.23 100.12 100.02

Na2O+K2O 8.46 8.34 8.97 8.33 11.55 8.07 7.17 8.97 9.78

A/NK 1.74 1.65 1.62 1.77 1.87 2.17 1.65 1.58 2.04

A/CNK 1.68 1.63 1.59 1.70 1.79 2.14 1.47 1.48 1.73

K₂O+NaO–CaO 8.15 8.23 8.8 7.98 11.02 7.95 6.3 8.33 8.05

CIPW norm

Q 31.67 43.60 34.90 31.44 7.35 42.51 42.19 29.09 11.68

C 2.56 3.53 2.18 2.53 3.80 6.88 0.65 0.62 3.59

Or 21.22 33.68 24.94 18.62 12.06 26.42 22.46 22.99 15.72

Ab 41.21 21.04 39.86 43.83 80.30 29.69 28.52 42.99 60.25

An 0.62 0.78 4.17 2.78 4.73

Di

Hy 1.54 0.82 0.26 1.58 0.33 1.17 1.46 0.96 1.50

Ol 0.40 0.14 0.21 0.72

mt 0.36 5.46 2.87 0.43 2.39 9.14 0.38 0.23 0.29

il 0.06 0.06 0.02 0.04 0.04 0.09 0.06 0.08 0.08

ap 0.33 0.45 0.37 0.35 0.99 0.36 0.05 0.14 1.40

Granitic-gneiss Biotite-gneiss

Amphibolite

SiO2 76.12 71.1 70.56 69.27 73.65 70.68 60 72.36 50.63

TiO2 0.2 0.26 0.21 0.45 0.06 0.45 0.83 0.24 1.09

Al₂O₃ 12.47 15.5 15.61 15.58 14.66 15.17 16.71 15.05 9.89

Fe₂O₃^T 1.35 1.45 1.45 3.11 0.51 2.08 6.07 1.71 10.86

MnO 0.05 0.04 0.03 0.06 0.01 0.04 0.14 0.04 0.2

MgO 0.16 0.25 0.54 0.98 0.11 0.39 2.36 0.29 11.47

CaO 0.61 1.22 1.35 2.94 0.83 1.3 4.95 1.17 11.61

Na2O 2.67 4.03 4.04 3.77 3.62 3.87 4.17 3.86 1.9

K2O 6.08 5.6 5.34 3.03 6 5.32 3.51 4.41 1.12

P2O5 0.03 0.1 0.115 0.176 0.046 0.14 0.313 0.234 0.19

LOI 0.24 0.35 0.36 0.59 0.24 0.53 0.49 0.54 0.97

Total 100.16 100.12 99.9 100.2 99.91 100.25 99.94 100.04 100.22

Na2O+K2O 8.75 9.63 9.38 6.8 9.62 9.19 7.68 8.27 3.02

A/NK 1.43 1.61 1.66 2.29 1.52 1.65 2.18 1.82 3.27

A/CNK 1.33 1.43 1.45 1.60 1.40 1.45 1.32 1.59 0.68

K2O+Na2O–CaO 8.14 8.41 8.03 3.86 8.79 7.89 2.73 7.1 8.59

CIPW norm

Q 35.21 22.87 22.57 27.02 27.57 23.97 7.44 29.93 0.00

C 0.46 0.83 1.00 1.17 0.81 1.02 0.00 2.35 0.00

Or 35.93 33.09 31.56 17.91 35.46 31.44 20.74 26.06 6.62

Ab 22.59 34.10 34.19 31.90 30.63 32.75 35.29 32.66 16.08

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and orthoclase) constitute more than 92% of the norm minerals in the CIPW norms which reCect leucocratic nature of the rocks (table 2). The average normative anorthite composition in the pegmatites is \3% which is consistent with high percentage of albites observed in the modal analysis. The pegmatites are hypersthene and corundum-normative. The corundum normative is as high as 6% in individual samples of the albite–muscovite–pegmatites. The A/CNK of the pegmatites in southern Akwanga is strongly peraluminous (A/CNK = 1.45–2.14) with the albite–muscovite–pegmatites having the highest value. In the R₁–R₂ diagram of De la Roche et al.

(1980; Bgure 4), the pegmatites plot mostly in the granite and alkali granite Belds except one sample (CA19) of the biotite-microcline–pegmatite, which plots in quartz syenite Beld. Similar to the host rocks, Al₂O₃, CaO, P₂O₅, Fe₂O₃(t), Na₂O and MnO show inverse relationship with SiO₂ in the pegmatites and overlapping with the trends of the host rocks especially with the metasediments, MgO show mild inverse with SiO2, while K2O shows positive correlation with SiO₂. TiO₂ and MgO show low concentrations in relation to the SiO2

compared to the host rocks. The similarities in characteristics between the pegmatites and the host rocks in Harker diagrams (Bgure 5) show that they are probably related from their sources except the amphibolites.

Rb contents in the albite–muscovite–pegmatites (277–533.8 ppm) and biotite–microcline-pegmatites (126.5–216.1 ppm) are higher compared to the host rocks (table 3). In contrast, the values of Ba and Sr in the pegmatites are very low compared to the host rocks and below average crustal abundances of Wedepohl (1995). The values of Sr in the pegmatites are mostly below detection limit of 10 ppm. Sn, Nb and Ta values in the pegmatites are higher in the albite–muscovite–pegmatites with values up to 170 ppm Sn, 59 ppm Nb and 24.2 ppm Ta compared to average fertile pegmatitic leucogranite of Superior Province (Cerny and Meintzer1988; Selway et al.2005). Sn, Nb and Ta values in biotite-microcline pegmatite range from 4 to 35, 7–15 and 0.8–10.7 ppm, respectively. The pegmatites are also depleted in REEs and base metals Ag, Cr, Co, Ni, Cu, Mo, Pb and Zn. LREEs are relatively higher in the host rocks compared to the pegmatites, while HREEs are generally depleted across the rocks. Cs, Ga and B values are enriched in the albite-muscovite-pegmatites compared to the host rocks. The highest values of Cs, Ga and B are up to 56.5, 59 and 4205 ppm,

Table 2. (Continued.)

Amphibolite

An 2.83 5.40 5.95 13.44 3.82 5.53 16.51 4.30 15.15

Di 5.00 33.21

Hy 1.81 2.05 2.84 5.66 0.81 2.87 9.81 2.50 14.77

Ol 7.10

Mt 0.45 0.48 0.48 1.01 0.17 0.68 1.97 0.57 3.52

Il 0.38 0.49 0.40 0.85 0.11 0.85 1.58 0.46 2.07

Ap 0.07 0.24 0.27 0.42 0.11 0.33 0.73 0.54 0.45

1000 1500 2000 2500 3000 3500

R1 = 4Si-11(Na+K)-2(Fe+Ti) 0

1000 2000

R2 = 6Ca+Mg+Al Gabbro-diorite Diorite

Tonalite

Granodiorite

Granite Quartz

monzonite

Quartz syenite

Alkali granite Monzonite

Monzo diorite

Figure 4. Nomenclature of the host rocks and the granitic pegmatites using the R1–R2discrimination diagram of De la Roche et al. (1980). Shaded triangle = granitic gneisses, shaded square = biotite gneiss, shaded rhombus = amphibolite, shaded circle = complex pegmatites, unshaded circle = simple pegmatites.

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respectively. Li concentrations range from 5.00 ppm in the biotite–microcline–pegmatites to 101 ppm in the albite–muscovite–pegmatites. These variations in elemental concentrations show that degrees of fractionation increases with increase in rare element (Li, Be, B, F, P, Ga, Rb, Cs, Y, Nb, Sn and Ta) content in fertile granites ( Cerny and Meintzer 1988), while Ti, Sr, Ba and Zr decreases with fractionation.

The strong peraluminous nature of the pegmatites and their host rocks from southern

Akwanga is clearly observed on the Shand index diagram (Bgure8), Al2O3/(Na2O+K2O) vs. Al2O3/ (CaO+Na₂O+K₂O) of Maniar and Piccoli (1989). The albite–muscovite–pegmatites are most evolved and show extremely peraluminous (A/

CNK=1.5–2.1) than the biotite–microcline–pegmatites and the host rocks which overlap within the peraluminous Beld. The pegmatites and the host rocks are associated with S-type granites except the amphibolits which is metaluminous and associated with I-type granites (Bgure8). Samples

a b c

d e f

g h i

Figure 5. Harker diagrams of major oxides for the host rocks and the granitic pegmatites. Symbols are as in Bgure4.

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of the pegmatites plot mostly in the high-K calc- alkaline Beld of Peccerillo and Taylor (1976), while the host rocks fall mostly within the shoshonite

Beld and partly high-K calc-alkaline Beld (Bgure9a). The amphibolite plots in the medium- K calc-alkaline Belds. On the SiO2vs. Fe* of Frost

Table 3. Trace and rare-earth elements composition (ppm) of the pegmatites and host rocks from southern Akwanga areas.

Albite–muscovite–pegmatite Biotite–microcline–pegmatite

Li 101 27 30 42 29 66 5 31 40

B 31 2191 73 21 35 4205 106 33 420

Ba 14 19 31 28 21 11 112 10 571

Be 4 20 4 5 6 198 5 4 85

Bi 0.34 0.12 0.09 0.08 0.26 0.48 0.05 0.16 0.39

Co 7.8 8.6 8.7 7.1 7.9 9.9 19.2 11.3 4.8

Cr 2 2 bd 1 bd 1 1 1 1

Cs 8.4 26.5 5.1 6.7 56.5 45 4.3 7.1 32.1

Cu 2.5 6.5 3.8 1.7 3.2 2.1 2 1.4 1.7

Ga 36 33 37 37 59 58 17 20 23

Hf 0.5 1.7 0.5 0.1 0.5 1.8 0.9 2.1 3.1

Mo 1.9 0.3 0.2 0.2 bd 0.4 0.4 0.2 0.1

Nb 27 33 43 30 41 59 8 7 15

Ni 7.8 bd 3.6 bd 3.3 1.7 2.2 2 1.7

Pb 21 12.7 14.6 13.5 6.6 9.2 32.6 27.4 33.5

Rb 277 499 438 249 296 534 118 216 127

Sn 44 18 170 36 141 120 4 3 35

Sr bd bd bd bd 35 bd 79 bd 234

Ta 1.8 24.2 2.8 2.4 7.4 20.9 1.1 0.8 10.7

Th 2.4 1.6 2.2 1.6 1.2 1.6 63.3 3.5 1.2

Tl 1.48 3.02 2.1 1.23 1.27 2.79 0.63 1.24 0.71

U 3.1 1.2 10.6 6.2 1.2 1.2 12 5 2.1

W 94 118 93 91 69 95 438 142 61

Y 8 2.5 2.7 3.3 1.7 2.9 2.5 9.4 6.5

Zn 53 125 34 40 29 240 23 25 48

Zr 15 39 11 0 8 21 26 49 77

La 1.9 1.3 1 1.2 0.7 2 0.8 3.1 4.3

Ce 4.6 2.3 2.4 3.4 1.5 3.5 1.5 6.6 8.9

Pr 0.6 0.3 0.3 0.4 0.2 0.5 0.2 0.8 1

Nd 2.6 1.3 1.1 1.3 0.6 2 0.9 2.6 4

Sm 1.1 0.6 0.6 0.6 0.4 0.8 0.4 0.5 1.3

Eu bd bd bd bd bd bd 0.2 bd 0.5

Gd 1.3 0.4 0.5 0.5 0.4 0.6 0.5 0.7 1.5

Tb 0.3 0.1 0.1 0.2 0.1 0.1 0.1 0.2 0.3

Dy 1.8 0.4 0.6 0.6 0.5 0.7 0.5 1.3 1.4

Ho 0.3 bd bd 0.1 bd 0.1 0.1 0.2 0.2

Er 0.7 0.3 0.2 0.3 0.1 0.2 0.3 1 0.5

Tm 0.1 bd bd bd bd bd bd 0.1 bd

Yb 0.8 0.4 0.2 0.3 0.2 0.3 0.3 0.9 0.5

Lu 0.1 bd bd bd bd bd bd 0.2 bd

PREE 16.29 7.72 7.31 9.11 4.95 10.98 5.93 18.24 24.55

PLREE 10.8 5.8 5.4 6.9 3.4 8.8 3.8 13.6 19.5

PHREE 5.4 1.84 1.83 2.12 1.5 2.12 1.93 4.6 4.55

PLR/HR 2.00 3.15 2.95 3.25 2.27 4.15 1.97 2.96 4.29

La/Ybn 1.61 2.21 3.40 2.72 2.38 4.53 1.81 2.34 5.84

Eu/Eu* bd bd bd bd bd bd 1.36 bd 1.09

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et al. (2001), the pegmatites and the host rocks (granitic gneisses, biotite gneiss and biotite granite) fall predominantly in the ferroan Beld except

three samples of the granitic gneisses and the amphibolites, which fall in the magnesian Beld (Bgure9b). The ferroan aDnity of the rocks is

Table 3. (Continued.)

Amphibolite

Li 30 71 70 89 27 19 90 36 80

B 20 20 23 28 28 40 47 30 42

Ba 577 928 1623 891 631 1303 1570 307 151

Be 3 8 8 10 4 4 4 3 3

Bi 0.07 0.04 0.03 0.12 0.07 0.04 0.12 0.06 0.44

Co 7.9 10.6 8.9 15.4 6.1 10.7 19.2 8.1 54.2

Cr 1 2 6 16 2 3 20 3 465

Cs 13 8.8 7.5 10.1 16.1 2.5 6 3.3 12.6

Cu 21.5 5.8 1.2 3.6 1.4 1.8 22.2 0.8 96.4

Ga 13 26 23 23 24 22 19 27 16

Hf 4.2 13 4.3 4.5 1.8 6.3 4.6 4.3 2.4

Mo 0.5 0.1 0.1 0.3 0.1 0.2 0.9 0.4 0.4

Nb 8 9 6 6 7 19 12 14 bd

Ni 1.8 2.5 3.9 7.7 1 2.7 14.7 1.3 74.9

Pb 28.2 52.5 54.3 35.2 56.2 32.9 16.6 29 13.6

Rb 156 154 244 158 173 172 80 274 47

Sn 8 8 15 8 4 3 5 6 5

Sr 142 355 354 409 265 320 588 75 220

Ta 0.4 1.1 0.2 1.3 0.6 1.8 0.9 1 0.6

Th 10.2 18.4 20.9 11.6 5.5 19.8 8.6 33.4 2.8

Tl 0.77 0.78 1.41 1.02 0.99 0.84 0.41 1.4 0.28

U 1.3 3.4 3.4 4.1 1.7 1.5 2.2 3.7 1.7

W 93 89 36 173 69 87 124 55 33

Y 19.9 10.2 7.7 13.1 2.6 30 33.9 15.4 28.5

Zn 119 44 62 76 10 43 92 75 106

Zr 143 970 145 160 46 250 174 131 72

La 24.7 41.6 48.5 35.9 8.5 58.8 31.2 43.7 14

Ce 56.8 80.5 87.7 67.3 17 111.9 67.6 100.4 36.6

Pr 5.5 8.8 9.1 7.4 1.9 11.8 8.3 11.6 5.2

Nd 19.8 33.6 31.4 30.4 7.2 41.2 35.7 46.3 23.1

Sm 4.2 5.8 4.7 4.9 1.7 6.4 7.4 7.6 5.7

Eu 0.6 1 1 1.3 0.6 1.3 2.2 0.4 1.6

Gd 3.2 3.1 2.4 3.7 1.3 4.5 6.5 4.4 5.7

Tb 0.5 0.4 0.3 0.5 0.1 0.8 1 0.6 0.9

Dy 3.6 2 1.6 2.6 0.6 5.9 6.4 3.3 5.9

Ho 0.8 0.3 0.3 0.4 0.1 1.2 1.2 0.5 1.1

Er 2.4 0.8 0.8 1.2 0.2 3 3.7 1.2 3.2

Tm 0.4 0.1 bd 0.2 bd 0.4 0.5 0.2 0.5

Yb 2.9 0.7 0.6 1 0.2 2 3.6 1 2.9

Lu 0.4 0.1 bd 0.1 bd 0.2 0.5 0.1 0.4

PREE 125.8 178.8 188.54 156.9 39.45 249.4 175.8 221.3 106.8

PLREE 111 170.3 181.4 145.9 36.3 230.1 150.2 209.6 84.6

PHREE 14.2 7.5 6.14 9.7 2.55 18 23.4 11.3 20.6

PLR/HR 7.82 22.71 29.54 15.04 14.24 12.78 6.42 18.55 4.11

La/Ybn 5.79 40.37 54.91 24.39 28.87 19.97 5.89 29.69 3.28

Eu/Eu* 0.50 0.72 0.91 0.93 1.23 0.74 0.97 0.21 0.86

*bd stands for below detection.

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consistent with the presence of magnetites both in the mode and norm of the rocks. In MALI diagram of Frost et al. (2001), the pegmatites and the host rocks plot dominantly in alkalic-calcic Beld except the amphibolite, which plots in calcic Beld (Bgure9c). These geochemical properties are similar to the Bndings of Goodenough et al. (2014) in pegmatites around the northcentral basement, Nigeria. The peraluminous characteristic of the pegmatites in this study area are consistent with the observations on other rare metal and barren pegmatites in other parts of Nigeria such as southwestern Nigeria, Oke-Asa and Igbeti pegmatites (Okunlola and Oyedokun 2009); northcentral Nigeria, Sarkin Pawa–Minna pegmatites (Goodenough et al. 2014) and Angwan Doka

pegmatites (Akoh et al. 2015); southeastern Nige- ria, Oban and Bamenda massif pegmatites (Ibe and Obiora 2019). The peraluminous and ferroan characters of the host rocks in the study area are also similar to the Bndings of Rahaman et al. (1988) and Ibe and Obiora (2019) on granitic rocks around Oban massif, southeastern Nigeria, and Okonkwo and Folorunso (2013) on granitic gneisses around Aderan area, southwestern Nigeria. However, metaluminous and magnesian characteristics are also reported in granitic basement rocks around Igbeti, southwestern Nigeria (Rahaman et al.1988;

Olarewaju 1987), Hyowanye and Katsina Ala, southeastern Nigeria (Obiora2012) and Eggon and Abuja, northcentral Nigeria (Obiora and Ukaegbu 2008; Goodenough et al.2014).

a b

Figure 6. Primitive mantle-normalized multi-element diagrams for the (a) host rocks and (b) granitic pegmatites. Normalizing factors from McDonough and Sun (1995). Symbols are as in Bgure4.

a b

Figure 7. Chondrite-normalized rare earth element diagrams for the (a) host rocks and (b) granitic pegmatites. Normalizing factors from McDonough and Sun (1995). Symbols are as in Bgure4.

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The pegmatites in the study area show more variable trace element and REE patterns and are strongly fractionated especially the albite–muscovite–pegmatites compared to the host rocks (Bgure7b). They are enriched in Cs, Rb, B, Th, K, Nb, Ta and Li relative to the host rocks, but show typical depletions in Ba, Sn, Pb, and Ti and REEs similar to the host rocks. The major difference between the pegmatites and host rock diagrams are the enrichment in B, Nb and Li in the pegmatites which can be attributed to the fractionation of pegmatites, while the enrichment in P in the host rocks is aAected by high grade of metamorphism, since P tend to increase with intensity of metamorphism (Wedepohl 1995). The pegmatites generally show lower concentrations of REE compared to the host rocks (Bgure 8) with some of the values below detection limit. Though the pegmatites have minor higher LREE relative to the HREE [(La/

Yb)_n = 1.62–5.84], the LREE and HREE concentrations are very low compared to the host rocks (Bgure7a and b) because the REEs in pegmatites are partitioned into the accessory phases (Chris- tiansen et al. 1993). Furthermore, the Eu concentrations in the pegmatites are very low compared to the host rocks. The Eu in most pegmatites are below detection limit (\0.1 ppm) in contrast to that of the host rocks (Eu = 0.6–2.2 ppm) except two samples of simple pegmatites (CA07 and

Figure 8. Shand index plot for the host rocks and pegmatites in southern Akwanga. A/NK = Al₂O₃/(Na₂O+K₂O) and A/CNK = Al2O3/(CaO+Na2O+K2O) after Maniar and Piccoli (1989).

c a

b

Figure 9. Binary plots for the granitic pegmatites and their host rocks in southern Akwanga. (a) K2O vs. SiO2 of Peccerillo and Taylor (1976); (b) FeO_t/(FeO_t+MgO) vs.

SiO2after Frost et al. (2001); (c) modiBed alkali-lime index (MALI) vs. SiO₂of Frost et al. (2001).