Geochemical Signatures and Mineralization Potentials …article.aascit.org/file/pdf/8990726.pdfGeochemical Signatures and Mineralization Potentials of Precambrian Pegmatites of Southern

International Journal of Geophysics and Geochemistry

2015; 2(3): 53-67 Published online May 20, 2015 (http://www.aascit.org/journal/ijgg)

Keywords Pegmatites, Petrography, Rare-Metals, Mineralization, Nigeria, Obudu Received: March 10, 2015 Revised: April 1, 2015 Accepted: April 2, 2015

Geochemical Signatures and Mineralization Potentials of Precambrian Pegmatites of Southern Obudu, Bamenda Massif, Southeastern Nigeria

Grace O. Edem, Barth N. Ekwueme, Bassey E. Ephraim

Department of Geology, University of Calabar, Calabar, Nigeria

Email address [email protected] (B. N. Ekwueme)

Citation Grace O. Edem, Barth N. Ekwueme, Bassey E. Ephraim. Geochemical Signatures and Mineralization Potentials of Precambrian Pegmatites of Southern Obudu, Bamenda Massif, Southeastern Nigeria. International Journal of Geophysics and Geochemistry. Vol. 2, No. 3, 2015, pp. 53-67.

Abstract Pegmatites constitute an important lithologic unit in the Precambrian Basement complex of Southern Obudu, Southeastern Nigeria. The primary aim of this study is to present field, petrographic and geochemical data with which to evaluate the rare-metal mineralization potential of the pegmatites. Petrographic investigations revealed the presence of microcline, quartz, muscovite, biotite and plagioclase as parts of the modal mineralogy of the pegmatites. Major element geochemistry show that SiO2, Al2O3 and K2O constitute the bulk of the rock composition, while trace element analysis show pronounced enrichment in Ba (ca.2727ppm) and W (ca.84ppm), together with low concentrations of Be, Ga, Nb, Sn, Rb, Ta and Cs among many other rare metals. The pegmatites document very low Na/K and Rb/Sr ratio values, as well as high K/Cs, K/Rb and Th/U ratios, which are in sharp contrast with the pattern recorded for rare metal mineralized pegmatites in the Basement Complex of Nigeria and elsewhere in the world. Variation plots of K/Rb versus Cs plots revealed that the pegmatite belongs to the muscovite class. Also, when both whole rock and muscovite extracts data are projected onto the Th/U versus K/Cs and Ta versus Cs diagram, it becomes obvious that the pegmatites of Southern Obudu area exhibits low–level rare metal mineralization potential.

1. Introduction

The Precambrian Basement Complex of Nigeria forms part of the late Achaean to early Proterozoic Pan-African mobile belt, which lies to the east of the West African Craton. The area South of Obudu constitutes parts of the Nigeria basement complex (Figure 1). One characteristic feature of the Precambrian Basement Complex of Nigeria is the occurrence of pegmatite bodies with rare metal mineralization potentials. Garba (2003) mapped mineralized pegmatites enriched in columbite-tantalite minerals from the Basement Complex of Northern Nigeria. Okunlola and Udoudo (2010) documents moderately high Ta, Nb, Sn, Rb, Li and Cs in Precambrian pegmatites occurring in Komu area of southwestern Nigeria. Okunlola and Akinola (2010) investigated the Precambrian pegmatite of Oke-Asa area of Ijero Ekiti and revealed that these pegmatites are variably enriched in rare metals, such as Ta, Nb, Li, Rb and Sr. Akintola et al. (2011) revealed Precambrian pegmatites with moderate to high Ta-Nb mineralization in the Awo Area of Southwestern Nigeria. Agunleti et al. (2014) worked on the Precambrian

54 Grace O. Edem et al.: Geochemical Signatures and Mineralization Potentials of Precambrian Pegmatites of Southern Obudu, Bamenda Massif, Southeastern Nigeria

pegmatites exposed at Agwan Rimi, in North Central Nigeria, and concluded that the deposit holds high potential for tantalum mineralization. Okunlola and Oyedokun (2009) observed that the pegmatites of Igbeti area in Southwestern Nigeria display Ta-Nb mineralization potential that is comparable to world class Tanco deposits of Canada and Noumas pegmatites of South Africa. Also, Ero and Ekwueme (2009) reported of mineralization of pegmatites in parts of

the Oban Massif of Southeastern Nigeria. Most of these pegmatites evolved during the time span of 600±150Ma (Matheis and Caen-Vachette, 1983; Okunlola and Udoudo, 2006, 2010), which indicates that the Nigerian pegmatites were formed during the Pan-African Orogeny. Rahaman (1988) and Elueze (2002) had earlier pointed out that the Nigerian pegmatites belong to the terminal stage of Pan-African magmatism.

Fig. 1. Geological map of Southern Obudu Plateau, Southeastern Nigeria (Modified after Ekwueme and Kroener, 1997).

In the past, the search for mineralized pegmatites in Nigeria was concentrated within the NE-SW Regions, within the so-called ‘Tin Province or Pegmatite Belt of Nigeria’ (Jacobson and Webb, 1946). However, recent studies by Ekwueme and Matheis (1995), Ero and Ekwueme (2009), Garba (2003), Okunlola (2004), Okunlola and King (2003), Okunlola and Ocan (2002), Okunlola and Ogendengbe (2003) and Okunlola and Somorin (2006) have demonstrated that the Precambrian pegmatites of Nigeria are not restricted to the so–called ‘Tin Province or Pegmatite Belt of Nigeria’, but occur also in southeastern Nigeria, extending into northeastern Brazil (Araújo et al., 2001; Da Silva et al., 1995). Ekwueme and Schlag (1989) and Ekwueme (1990) observed that the volume of pegmatites in the Obudu Plateau is greater than the volume of pegmatites in the Oban Massif, both in Southeastern Nigeria. They attributed this to the higher metamorphic grade in the Plateau since most rocks in the Plateau document upper amphibolite to granulite facies grade (Ephraim et al., 2006; Ukwang et al., 2003). Ekwueme (2005) observed that the muscovite-bearing types and the biotite-bearing type constitute the two dominating pegmatite types in the Obudu area. The third pegmatite type, referred to as the muscovite-biotite pegmatites is less common, according to Ekwueme (2005).

Research work on the mineralogy, classification geochemistry and economic aspects of pegmatites in

southwestern and northcentral regions have in the past received considerable attention (e.g., Adekeye and Adeoyin, 2007; Akintola and Adekeye, 2008; Okunlola and King, 2003; Okunlola, 2005), while those pegmatites in the southeastern region are almost non-existent. Only a few documented works exist on the pegmatites of southeastern Nigeria, e.g., Ekwueme and Matheis (1995) and Ekwueme and Schlag (1989), Ero and Ekwueme (2009), Igonor et al. (2012) and Oden et al. (2013). This paucity of documentary works on these pegmatites informed the decision to work on characterization and evaluation of mineralization potential of pegmatites occurring in Southern Obudu area of southeastern Nigeria.

2. Geological Background

Nigeria is underlain by Precambrian Basement Complex rocks, younger granites of Jurassic age and Cretaceous to Recent sediments. The Basement rocks, which occupy about half of the landmass of the country, forms part of the late Archaean to early Proterozoic Pan-African mobile belt, situated between the West African Craton and the Congo-Gabon Craton (Black1980). The Nigerian Basement has undergone polymetamorphism with upper amphibolite to slight granulite facies being the highest grade of metamorphism attained (Ephraim et al., 2006). The polycyclic evolutionary nature of the Nigerian Basement is

International Journal of Geophysics and Geochemistry 2015; 2(3): 53-67 55

supported by Ajibade (1988), Ekwueme and Caen–Vachette (1992) and Ekwueme (1994a; 1994b; 2003a, b and c).

The Obudu Plateau is a vast N-S trending high-grade metamorphic terrain that constitutes part of the Pan-African Trans-Saharan belt exposures in southeastern Nigeria. It is essentially characterised by the occurrence of regionally metamorphosed rock successions, pervasive migmatization, and granite plutonism (Ferre et al. 1996). The Plateau, which is very rugged, is often viewed as the terminal end of the western prolongation of the Precambrian Bamenda Massif which stretches from Cameroun Republic into Eastern Nigeria (Toteu et al, 2004; Ephraim, 2005; 2009a).

The study area in Southern Obudu is delimited by latitudes 6°30’N and 6°42’N and longitudes 9°08’E and 9°30’E, and enclosed within the Nigerian topographic sheet 291 (Obudu SE) (Fig.1). The area constitutes parts of the Precambrian Basement Complex of Nigeria, in present-day Cross River State. The distribution of major rock units occurring within the study area is shown in Fig.1. For brevity, Southern Obudu Plateau is composed of Precambrian rocks, notably, schists, gneisses, metaperidotite, granites, dolerites, charnockites, amphibolites, quartz veins, and pegmatites (Ekwueme, 1985).

The paragneisses and paraschistose rocks in the area constituted the host rocks for the granitic pegmatites in this study (Ephraim, 2005, 2009b). Both para schists and paragneisses are quartzofeldspathic in composition, medium to coarse-grained in texture, inter-banded on a megascopic scale with quartzites, and sheared with garnetiferous components (Ephraim, 2009a). The schistose rocks are of two types and both types exhibits widespread folding, while the gneisses frequently displays migmatitic characteristics,

intense folding and shearing. Ephraim et al. (2006) observed that migmatite schists and gneisses occurring in Obudu area constitute parts of an extensive migmatite gneiss complex in the region. The migmatitic character in these rocks reflects long, protracted and possibly poly cyclic evolutionary history. The schistose and gneissose rock units have attained at least the upper amphibolite facies metamorphism (Ephraim et.al. 2006; Ukwang et al., 2003). The dominant orientation of foliation in these basement rocks is the N–S to NE–SW trends, thereby pointing to the relevance of the Pan–African orogeny in the evolutionary history of the area.

The associated dolerites are neither metamorphosed nor deformed or altered, but constitute ubiquitous features in Southern Obudu area (Beka and Ukaegbu, 2006).

3. Materials and Methods

The method employed involved systematic geological mapping and sampling of the various pegmatite outcrops occurring within the study area, followed by petrographic and geochemical studies. More than 30 fresh and uncontaminated whole rock pegmatite samples were collected from pegmatites outcrops occurring within the study area. In the collection of the pegmatite samples, effort was made to sample mostly the central parts of the pegmatite bodies in order to collect fresh samples, and a total of seven samples, which were considered representative of the different major pegmatite veins in the study area were eventually adopted for whole rock geochemical analysis. The bulk samples were of the minus 25mm fraction, obtained by screening crushed pegmatites.

Fig. 2. Sample location map of Southern Obudu Plateau, South eastern Nigeria


It was also considered necessary to include feldspars and

Muscovites as sampling medium because Muscovites do not weather easily and indicator elements in both minerals have been utilised in exploration for mineralised pegmatite deposits (Gaupp et al., 1984; Moller and Morteani, 1987; Selway et al., 2005). In particular, Rb and Cs contents and K/Rb ratio in potassium feldspar can discriminate between various types of rare-element pegmatites and those that are barren of rare elements (Černý et al., 1981). Also, Ta, Nb and Cs contents of primary muscovite are useful in assessing the potential presence of tantalum-and niobium-bearing oxide minerals in pegmatites (Beus, 1966; Gordiyenko, 1971; Heinrich, 1962; Odikadze, 1958). The mineral samples were obtained from 2 major pegmatite veins that were considered best in terms of freshness of rocks outcrop. Care was taken to ensure that the mineral extracts were obtained from coarse grained pegmatites containing grains that are >2cm of the relevant mineral. A total of 4 mineral extract samples, comprising 2 muscovite extract samples and 2 feldspar extract samples were considered. Sample locations are given in Fig. 2, while further details on the location, treatments and other details of the representative samples are preserved in Edem (2014).

The representative samples, comprising whole rock pegmatite (WRP) and mineral extracts were processed into thin sections for petrographic examination at the Department of Geology, University of Calabar, Nigeria, while analysis for

major, trace and rare earth elements were carried out using ICP-AES analytical methods at the Acme Analytical Laboratories, Vancouver, Canada.

4. Results and Discussions

4.1. Field Geology and Petrography

The pegmatite in the study area, believed to be late intrusive members of the Pan African Older Granite suite (Ephraim, 2012), frequently occur in close spatial association with criss-crossing quartz veins and parent granitic intrusions, which are of monzogranitic composition (Ephraim, 2012; Ukaegbu, 2003).The pegmatites are widely distributed as boulders (Fig.3a), veins (Fig.3b) and shallow dipping dykes in mostly the schistose and gneissose rocks of the area (Fig.1). The traceable pegmatite vein length ranges from about 6 to 20m, though the most frequently occurring vein length is between 7m–8m. Similarly, the width of the pegmatite veins range from about 0.1to1.2m, with 1.0–1.2m being the most consistent vein width. Some of the veins have been folded and deformed together with their host rocks (Fig.3c), thereby revealing that the pegmatites were formed before the last phase of deformation of the country rocks .A similar observation was made in earlier geologic investigations of pegmatite veins in parts of the Obudu Basement complex (Igonor et al., 2012).

Fig. 3. Outcrop characteristics of pegmatite exposures in southern Obudu area. (A)Boulder of pegmatites in Shikpeshi Mountain. (B) Pegmatite vein in Kundeve town. (C) Deformed and folded pegmatite vein in a highly weathered outcrop of banded biotite gneiss at Shikpeshi Mountain. (D) Highly weathered gneiss outcrop containing multiple pegmatite veins at Shikpeshi Mountain.


Towards the Cattle Ranch area (Fig. 2), the pegmatite

bodies form knobs (Fig.3d) in otherwise low-lying areas. Most of these knobs are weathered and intensely lateritized pegmatitic bodies, and are rampant in the areas of Amunga, Beggiaba, and Ogevaav villages on Shikpeshi Mountain. Despite the widespread nature of the pegmatite boulders, they were not considered in the present study since it could not be ascertained how far these materials have been transported.

Megascopically, the pegmatites are composed of mostly coarse grains of approx. 20mm – 0.5m average diameter, which grades into patches of aplite. No pegmatite zones were observed in any of the pegmatites structure studied. Pegmatite structures in the area are commonly lensoidal veins which are composed predominantly of large crystal of microcline and subordinate quartz, plagioclase and muscovite. Most of the pegmatites in southern Obudu are pink in colour, possibly due to the presence of orthoclase feldspar.

The modal compositions of the whole rock pegmatite

samples and feldspar extracts are presented in Table 1, and the photomicrographs are shown in Figs. 4, 5, 6. As seen in these figures (Figs.4, 5, 6), the major mineralogy of the Obudu pegmatites (in order of decreasing abundance) are: microcline, quartz, muscovite, biotite and plagioclase with subordinate zircon and opaque oxide. The microcline typically occurs as very coarse crystal having characteristic strong cross hatched twinning and variable micro-perthitic intergrowth (Fig.6) and is colourless to pale green in thin section. Biotite is pleochroic and ranges in colour from yellowish brown to reddish brown, under plane polarised light (PPL). Plagioclase is colourless with poly synthetic twinning. Zircon occurs mostly as inclusions in biotite where it forms pleochroic halos (Fig. 5). Apart from biotite, zircon, and opaque oxides which are mostly magnetite and subordinate ilmenite, the present study did not reveal much in terms of accessory minerals present. However, Igonor et al. (2012) documents the occurrence of accessory minerals, such as beryl (var. Aquamarine), tourmaline, garnet, and amethystine pegmatites of Southern Obudu area.

(a)

(b)

Fig. 4. Photomicrograph of pegmatite sample (P1) from pegmatites of Southern Obudu area (X100), under (a) crossed polarized light t(CPL), plane polarized light (PPL), with (c) illustration of modal compositions (Q=Quartz; MU=Muscovite; B=Biotite)


Fig. 5. Photo micrograph of pegmatite sample (P2) from pegmatites of Southern Obudu area (X100), under plane polarized light (PPL), with accompanying illustration of modal compositions (Q=Quartz; MU=Muscovite; B=Biotite)

Fig. 6. Photomicrograph of pegmatite sample (P3) from pegmatites of Southern Obudu area (X100), under plane polarized light (PPL), with accompanying illustration of modal compositions. (Q= Quartz; MU= Muscovite; B=Biotite)

Table 1. Modal composition (vol. %) of pegmatites from southern Obudu area, southeastern Nigeria

Minerals PMT-1(%) PMT-2(%) PMT-3(%)

Muscovite(Mu) 25 15 5 Quartz(Q) 15 25 10 Plagioclase(Pl) 5 10 5 Microcline(M) 25 40 77 Biotite(B) 25 5 - Zircon(Z) 2 - - Opaque(O) 3 5 3 Total 100 100 100

4.2. Geochemistry

4.2.1. Major Elements Composition and

Variation

The concentrations of the major element oxides of whole rock and mineral extracts (muscovite and feldspar) of pegmatites from Southern Obudu Plateau are presented in Table 2. A comparison of major elements composition of the whole rock pegmatite samples from Southern Obudu with similar rock from other locations in Nigeria are given in

Mu

sco

vite

(M

u)

Qu

art

z (

Q)

Pla

gio

cla

se

(P

l)

Mic

roclin

e (

M)

Bio

tite

(B

)

Zirco

n (

Z)

Op

aq

ue

(O

)

%M

od

al c

om

po

sit

ion


Table 3. The pegmatitic rocks compared with the Southern Obudu pegmatites were chosen based on geographical similarity and the fact that these rocks were also produced during the Pan-African orogeny.

With respect to SiO2, the 71.90 wt.% value recorded for the Obudu pegmatites (Table 2) compare favourably with the SiO2 values recorded for pegmatites from Oke-Asa (Okunlola

and Akintola, 2010), Oban massif (Ero and Ekwueme, 2009), Itakpe (Okunlola and Somorin, 2006) and Komu areas (Okunlola and Udoudo, 2006) (Table 3). However, when compared with pegmatites from Awo (Okunlola and Akintola, 2010), Sepeteri (Okunlola and Akintola, 2007) and Igbeti areas (Okunlola and Oyedokun, 2009), the investigated pegmatites appear to be richer in SiO2 composition.

Table 2. Major elements and CIPW norm composition of whole rock pegmatites and mineral extracts (muscovite and feldspar) of pegmatites from Southern Obudu Plateau, south eastern Nigeria

Elements Whole Rock Pegmatite Muscovite Feldspar

SOP-1 SOP-2 SOP-3 SOP-4 SOP-5 SOP-6 SOP-7 AVG SOP-8 SOP-9 AVG SOP-11 SOP-12 AVG

SiO2 71.01 66.84 71.84 72.37 74.11 76.57 70.57 71.90 59.41 54.44 56.93 65.1 64.36 64.73 Al2O3 15.77 17.99 15.03 16.07 14.21 13.14 15.89 15.44 23.24 28.99 26.12 19.05 18.99 19.02 Fe2O3 0.13 0.55 0.36 0.59 0.53 1.04 0.14 0.48 1.62 1.32 1.47 0.07 0.09 0.08 MgO 0.03 0.12 0.1 0.16 0.2 0.2 0.03 0.12 0.6 0.63 0.62 <0.01 <0.01 <0.01 CaO 0.11 0.33 0.41 2.26 1.55 1.35 0.14 0.88 0.81 0.26 0.54 0.11 0.1 0.11 Na2O 1.76 1.88 1.49 4.47 2.94 2.54 1.95 2.43 0.5 0.62 0.56 3.32 2.54 2.93 K2O 10.04 10.84 9.25 2.95 5.21 3.6 10.05 7.42 8.71 9.06 8.89 11.67 13.02 12.35 TiO2 0.01 0.05 0.04 0.07 0.06 0.11 0.01 0.05 0.37 0.39 0.38 <0.01 <0.01 <0.01 P2O5 0.05 0.09 0.02 0.03 0.03 0.09 0.05 0.05 0.57 0.18 0.38 0.03 0.03 0.03 MnO <0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.02 0.02 0.02 <0.01 <0.01 <0.01 Cr2O3 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.0 0.004 0.002 0.003 <0.002 <0.002 <0.002 LOI 1 1.1 1 0.9 0.7 1.2 0.7 0.94 4 4 4 0.6 0.5 0.55 Total 99.5 99.8 99.5 99.83 99.52 99.85 99.52 - 99.86 99.88 - 99.98 99.64 - Analytical Procedures

ICP-AES

ICP-AES

ICP-AES

ICP-AES

ICP-AES

ICP-AES

ICP-AES

- ICP-AES

ICP-AES

- ICP-AES ICP-AES

-

Table 3. Comparisons of Southern Obudu pegmatites with similar rock from other locations in Nigeria.

Oxides Present Study A* B** C*** D**** E***** F****** G*******

SiO2 71.90 71.46 71.02 70.17 68.26 66.17 63.02 54.95 Al2O3 15.44 13.90 15.99 15.68 14.02 25.51 15.03 25.77 Fe2O3 0.48 1.49 1.5 1.15 1.95 3.91 5.58 2.44 MnO <0.01 0.12 0.056 - 0.16 0.19 0.18 1.61 MgO 0.12 0.03 0.063 0.13 1.49 0.37 2.75 0.06 CaO 0.88 1.11 0.38 1.19 1.38 0.35 4.25 0.30 Na2O 2.43 2.08 4.43 3.26 3.42 1.18 2.93 1.28 K2O 7.42 8.77 4.83 6.05 4.64 1.16 4.10 3.74 TiO2 0.05 0.01 0.01 0.04 0.20 0.73 0.75 0.06 P2O5 0.05 0.26 0.51 0.02 0.03 0.20 0.30 0.14

*A= Oke – Asa pegmatites (Okunlola and Akintola, 2010), **B= Oban pegmatites, (Ero and Ekwueme 2009), ***C= Itakpe pegmatites (Okunlola and Somorin, 2006), ****D= Komu pegmatites (Okunlola and Udoudo, 2006), *****E= Sepeteri pegmatites (Okunlola and Akintola, 2007), ******F= Igbeti pegmatites (Okunlola and Oyedokun, 2009), ******G= Awo pegmatites (Akintola et al, 2011).

Like the SiO2 contents, the Al2O3 contents of Obudu pegmatites (av.15.44wt. %; Table 2) are comparable to Al2O3

contents of rocks from Oban massif (Ero and Ekwueme, 2009), Itakpe (Okunlola and Somorin, 2006), Komu (Okunlola and Udoudo, 2006) and Igbeti areas (Okunlola and Oyedokun, 2009), but lower than those of Awo (Akintola et al, 2011) and Sepeteri (Okunlola and Akintola, 2007) areas.

The Obudu pegmatites have a K2O content that is much higher than the K2O values recorded for Awo (Akintola et al, 2011), Itakpe (Okunlola and Somorin, 2006), Komu (Okunlola and Udoudo, 2006), Sepeteri (Okunlola and Akintola, 2007) and Igbeti (Okunlola and Oyedokun, 2009) areas (Table 3). It is also shown on Table 3 that Oke–Asa pegmatites have the highest relative concentration of K2O (Okunlola and Akintola, 2010).

The Fe2O3 contents of Southern Obudu pegmatites (av. 0.48wt. %; Table 2) is very low when compared to the average recorded for pegmatites from Oban massif (Ero and

Ekwueme, 2009), Itakpe (Okunlola and Somorin, 2006), Awo (Akintola et al, 2011), Komu (Okunlola and Udoudo, 2006), Igbeti (Okunlola and Oyedokun, 2009) and Sepeteri (Okunlola and Akintola, 2007) areas, respectively. The pegmatites from Igbeti area of Okunlola and Oyedokun (2009) documents the highest Fe2O3 content of all the compared pegmatites.

When the average MgO concentrations of Southern Obudu pegmatites (Table 2) is compared with values recorded for MgO for pegmatites from Oke–Asa (Okunlola and Akintola, 2010), Oban massif (Ero and Ekwueme, 2009) and Awo (Akintola et al, 2011) areas (Table 3), it becomes obvious that Southern Obudu pegmatites are richer in terms of MgO. However, these pegmatites are considerably lower in MgO concentrations, with regards to the MgO composition of pegmatites from Komu (Okunlola and Udoudo, 2006) and Igbeti (Okunlola and Oyedokun, 2009) areas.

As also shown in Table 3, Southern Obudu pegmatite have


higher CaO (0.88wt. %av.) and Na2O (2.43wt. %av.) contents (Table 2) than pegmatites from Awo (Akintola et al, 2011) and Sepeteri (Okunlola and Akintola, 2007) areas. These CaO and Na2O contents of Southern Obudu pegmatites are low, when compared with pegmatites from Itakpe (Okunlola and Somorin, 2006), Komu (Okunlola and Udoudo, 2006) and Igbeti (Okunlola and Oyedokun, 2009) areas (Table3).

In terms of the mineral extracts, muscovite samples from Southern Obudu pegmatites display SiO2 contents (56.93wt. %)(Table2) that is higher than the SiO2 values recorded for muscovite extracts (ME) from Ago-Iwoye (Akintola et al., 2012) and Awo (Akintola et al, 2011) pegmatites. Conversely, the Al2O3 contents (26.12wt.%) of ME from Southern Obudu pegmatites (Table2) is lower with regards to the value documented for ME of pegmatites from Ago-Iwoye (Akintola et al, 2012), and Awo (Akintola et al, 2011) areas. In addition, ME from Southern Obudu pegmatites have the lowest K2O and Fe2O3 contents (K2O = 8.89wt.% and Fe2O3 = 1.47wt.%) with regards to ME of pegmatites of Ago-Iwoye (Akintola et al., 2012) and Awo (Akintola et al, 2011) areas. The MgO, CaO MnO and Na2O contents in the muscovite extracts from pegmatites of the three areas under comparison (Obudu, Ago Iwoye and Awo areas) are in low concentration.

Feldspar extracts (FE) from Southern Obudu pegmatites have the highest Al2O3 and Na2O contents (Al2O3=19.02wt. % and Na2O=2.93wt.%; Table 3) with regards to K-feldspars from other pegmatite deposits, notably, the Kaatiala pegmatites of Western Finland (Nieminen, 1978), pegmatites of Thomaston Barnesville District of Georgia (Cocker, 1992) and granitic pegmatites from the Evje–Iveland pegmatites field of southern Norway (Larsen, 2002) (Table 3). In terms of SiO2

contents, the 64.73wt.% value recorded for FE from Southern Obudu pegmatites compares favourably with SiO2 contents of FE from Kaatiala pegmatite of Western Finland (Nieminen, 1978), pegmatites of Thomaston- Barnesville District of Georgia and granitic pegmatites from the Evje–Iveland pegmatite field of southern Norway (Table 3). Similarly, the12.35wt.% K2O contents of FE from Southern Obudu pegmatites compares favourably with K2O contents of FE from Kaatiala pegmatite of Western Finland (K2O = 12.99 wt.%) (Nieminen, 1978), pegmatites of Thomaston- Barnesville District of Georgia (K2O = 12.30 wt. %) (Cocker, 1992) and granitic pegmatites from the Evje–Iveland pegmatite field of southern Norway (K2O=12.67wt. %) (Larsen, 2002). Other components, notably, TiO2, Fe2O3, MnO, MgO and CaO contents in the feldspars from the four pegmatite groups considered (Southern Obudu pegmatites, Kaatiala pegmatites of Western Finland, pegmatites of Thomaston-Barnesville District of Georgia, and granitic pegmatites from the Evje–Iveland pegmatite field of southern Norway) are insubordinate concentration of less than 1 wt. % each.

4.2.2. Trace Element Composition and

Variation

Concentrations of the trace element composition of whole rock pegmatites and mineral extracts (muscovite and feldspar) of pegmatites from Southern Obudu Plateau are shown in Table 4, together with the concentration range for trace elements in bulk rock analysis of fertile granites after Černý (1989). Ratios of selected trace elements have been computed and are shown, with the acceptable range for bulk-rock values of fertile granites (Černý1989) in Table 5.

Table 4. Trace element composition (ppm) of pegmatites and associated minerals extracts from Southern Obudu area

Elements Whole Rock Pegmatite [ppm] Muscovite[ppm] Feldspar[ppm] FERTILE

GRANITE SOP-1 SOP-2 SOP-3 SOP-4 SOP-5 SOP-6 SOP-7 AVG SOP-8 SOP-9 AVG SOP-11 SOP-12 AVG

Ba 3392 1572 4208 1236 4038 892 3753 2727 656 661 659 211 3050 1631 6–900 Be <1 <1 <1 2 <1 1 <1 1 4 1 2.5 <1 <1 <1 1–604 Co 11 9 8 5 12 14 13 10.29 17 13 15 7 8 8 Cs 0.5 0.9 1.9 0.9 0.3 0.6 0.4 0.79 4.2 4.4 4.3 1.8 8.1 5 3–51 Cu 0.4 1.8 3 1.4 3.1 11.8 0.4 3.13 7.5 2.6 5.1 0.2 0.2 0.2 – Ga 12 16 12 17 13 14 11 13.57 36 52 44 26 22 24 19–90 Hf 0.1 0.2 1.8 1.5 0.2 4.5 0.3 1.23 4 0.4 2.2 <0.1 <0.1 <0.1 K 83342 89983 76784 24488 43248 29884 83425 61593 72302 75207 73755 96873 108079 102476 Na 13057 13948 11054 33163 21812 18844 14467 18049 3710 4600 4155 24631 18844 21738 Nb 0.3 1.4 1 1.4 0.7 2 0.7 1.07 19.6 27.6 23.6 <0.1 <0.1 <0.1 – Pb 2.2 4.8 8.5 1.1 2.2 6.9 4.9 4.37 2.6 1 1.8 0.4 1.3 0.9 – Rb 150 187 151 63 82 70 190 128 276 274 275 666 614 640 32–5775 Sc <1 1 <1 <1 <1 2 <1 1 18 25 22 <1 <1 <1 Sn <1 <1 <1 <1 <1 <1 <1 <1 19 36 28 <1 <1 <1 <1–112 Sr 369 253 316 295 418 208 431 327 46 34 40 44 244 144 <1–445 Ta 0.2 0.2 0.2 0.1 0.3 0.5 0.3 0.26 3.5 4.2 3.9 0.2 0.2 0.2 – Th <0.2 2.1 0.7 1.9 0.5 3 0.4 1.43 6.2 1.5 3.85 <0.2 <0.2 <0.2 Ti 60 300 240 420 360 659 60 300 2218 2338 2278 <59.94 <59.94 <59.94 <100–4300 U <0.1 0.9 0.9 0.2 <0.1 1.2 0.2 0.68 3.7 0.9 2.3 <0.1 <0.1 <0.1 V <8 <8 <8 <8 13 <8 <8 <8 86 113 100 <8 <8 <8 – W 103 62 68 34 102 107 115 84 107 111 109 72 70 71 – Y 0.3 14.2 1.8 1.8 0.7 4.9 1 3.5 34 14.2 24.1 <0.1 3.7 1.9 3–102 Zn <1 6 4 11 9 14 <1 8.8 15 6 10.6 <1 <1 <1 – Zr 3 7 54 49 4 170 11 43 110 13 62 0 1 1 <1–77

*FERTILEGRANITES=Range of trace elements ratios in bulk whole rock analysis of fertile granites (Černý1989).


Table 5. Ratios of selected trace elements in whole rock pegmatites and mineral extracts (muscovite and feldspar) of pegmatites from Southern Obudu Plateau,

Whole Rock Pegmatite Muscovites Feldspars

*Fertile Granites SOP-1 SOP-2 SOP-3 SOP-4 SOP-5 SOP-6 SOP-7 SOP-8 SOP-9 SOP-11 SOP-12

Al/Ga 6956 5842 6521 5033 5655 4968 7716 3389 2957 3878 4632 1180–3100 K/Ba 25 57 18 20 11 34 22 110 114 459 35 48–18200 K/Cs 166684 99981 40413 27209 144161 49806 208563 17215 17093 53818 13343 1600–15400 K/Rb 557 480 509 391 527 429 438 262 275 146 176 42–270 Rb/Sr 0.40 0.74 0.48 0.21 0.20 0.33 0.44 6.05 8.12 15.27 2.52 1.6–185 Zr/Hf 28 34 30 32 19 38 35 28 33 4 5 14–64 Na/k 0.16 0.16 0.14 1.35 0.50 0.63 0.17 0.05 0.06 0.25 0.17 – Ba/Rb 23 8 28 20 49 13 20 2 2 0 5 – Nb/Ta 1.50 7.00 5.00 14.00 2.30 4.00 2.30 5.60 6.60 0.50 0.50 – Rb/Ba 0.04 0.12 0.04 0.05 0.02 0.08 0.05 0.42 0.41 3.15 0.20 – Rb/Cs 299 208 79 70 274 116 476 66 62 370 76 – Sr/Rb 2.47 1.35 2.09 4.71 5.09 2.99 2.27 0.17 0.12 0.07 0.40 – Ta/W 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.04 0.00 0.00 – Th/U 2 2.33 0.78 9.5 5 2.5 2 1.68 1.67 2 2 –

*FERTILEGRANITES=Range of trace elements ratios in bulk whole rock analysis of fertile granites (Černý1989).

The trace element data presented in Table 4 shows pronounced enrichment in trace elements like Ba, and W. On the other hand, fairly low values are recorded for Rb, Ta, Cs, Sn, and Nb (Table 4). However, despite relatively low values recorded for whole rocks and feldspar extracts, the concentration of elements such as Be, Cu, Ga, Nb, Sn, Ta, Ti, V, W, Y, Zn and Zr are relatively higher in the muscovite extracts (Table 4). The preferential enrichment of these elements in the muscovite can probably be explained by the ability of muscovite to accommodate a wide range of substitutions at various sites in its crystal structure (Belyankina and Petrov, 1983; Bailey, 1984).

When the whole rock data of the investigated pegmatites are compared with trace elements concentration values quoted for fertile granites (Černý, 1989) in Table 4, it becomes obvious that Southern Obudu pegmatites have low concentration of many important trace elements. For instance, Be contents of Southern Obudu pegmatites is generally less than 1ppm (excluding samples SOP-6 and SOP-4 which respectively display 1 and 2 ppm), which is low, compared to the 1–604 ppm Be quoted for fertile granite. Similarly Cs, Ga and Y documents lower values, in comparison to the values quoted as the range of concentrations of each of these elements in fertile granites (Table 4).

It can therefore be deduced that pegmatites from Southern Obudu have relatively low concentration of important trace elements/rare metals, notably: Nb, Rb, Ta, Cs and Ga. Incidentally, these elements are the major mineralisation indices for pegmatites bodies in the Nigerian pegmatites (Akintola et al., 2011).

A cursory appraisal of the trace elements ratio values presented in Table 5 show that it is only the Zr/Hf ratio of bulk rock samples of the Southern Obudu pegmatites that fall within the range acceptable for fertile granites after Černý (1989) (Table 5). All other ratio values of Southern Obudu pegmatites are either higher or lower than what obtains for fertile granites. For instance, K/Ba and Rb/Sr values of bulk rocks of Southern Obudu pegmatites are quite low when compared to the values quoted for fertile granites Černý

(1989) (Table 5). In the same light, Al/Ga, K/Cs and K/Rb values are very much higher than relevant values listed for fertile granites in Table 5. It is also shown in Table 5 that Na/K display very low ratio values for the whole-rock pegmatites, muscovite extracts, and feldspar extracts, while the recorded values for Th/U fall and Nb/Ta are not only high but vary over a wide range for mostly the whole rock samples (Table 5). These observed ratio values for Southern Obudu pegmatites are in sharp contrast with the pattern recorded for rare metal mineralized pegmatites in the Basement Complex of Nigeria (Agunleti et al., 2014; Akintola et al., 2011,2012; Okunlola and Akinola, 2010; Okunlola and King, 2003; Okunlola and Ocan, 2002; Okunlola and Oyedokun, 2009; Okunlola and Udoudo, 2006, 2010; Okunlola, 2004), and elsewhere (Breaks et al., 2003; Černý, 1989; DeKun, 1965). Černý (1982) observed that the ratios, K/Rb, K/Ti, Ba/Rb, Rb/Sr and Na/Ta all tend to decrease to extremely low values with increasing pegmatite fractionation. Accordingly, the values recorded for the trace element ratios, together with the earlier observed relative low concentration of rare metals, notably, Ga, Nb, Ta, Rb and Zn supports a low degree of fractionation of the pegmatites. Furthermore, the very low Cs values recorded for Southern Obudu pegmatites imply paucity of alkali metal fractionation (Černý, 1982, 1989, and 1991).

4.2.3. Rare - Earth Elements (REE)

The rare-earth elements abundance of bulk rock and mineral extracts (muscovite and feldspar) of pegmatites from Southern Obudu Plateau are presented in Table 6. These REE abundances were normalized to chondrite (Haskin et al., 1968; Nakamura, 1974), and the relevant ratios computed from the chondrite-normalized values are also presented in Table 6). The chondrite normalised plots are shown as Figs 7–10 respectively for all sample types, whole rock samples alone feldspar extracts and muscovite extracts samples.

Generally, the REE patterns of the bulk rock pegmatite, presented as Fig 8 show tetrad effect and possess low total REE abundances (∑REE 5.89–44.28 ppm), strong positive Eu anomalies (Eu/Eu*: 2.21–40.96), weak negative Ce


anomalies and relative enrichment of heavy REE (LaN/YbN = 5.33–92). The M-type REE tetrad effect is vaguely discernible in the bulk rock pegmatite. Similar features have been observed on the REE patterns of the feldspar extracts (Fig. 9); the mineral extract also show vague M-type tetrad

effects, and possess low total REE abundances (∑REE 2.15–9.09 ppm), strong positive Eu anomalies (Eu/Eu*: 4.28–6.76), weak negative Ce anomalies and relative enrichment of heavy REE (LaN/YbN= 3.15–10.67).

Table 6. Rare earth element composition and relevant chondrite normalised ratios for pegmatites and associated minerals extracts from Southern Obudu area (ppm)

Elements Whole Rock Pegmatite Muscovite Feldspar

SOP-1 SOP-2 SOP-3 SOP-4 SOP-5 SOP-6 SOP-7 SOP-8 SOP-9 SOP-11 SOP-12

La 1.8 5.6 3.2 13.8 6.9 8.1 3.5 27.1 7.2 0.8 1.7 Ce 1.6 9.5 4.1 16.1 11.9 14.3 4.3 43.8 12.1 0.6 1.7 Pr 0.16 1.15 0.57 2.39 1.2 1.79 0.3 6.52 1.76 0.06 0.4 Nd 0.5 3.7 2.3 8.7 3.6 6.9 0.6 25.9 7.1 <0.3 1.7 Sm 0.14 1.17 0.41 1.1 0.54 1.43 0.14 5.66 1.74 <0.05 0.56 Eu 1.41 1.07 1.26 0.65 0.47 1.04 1.63 0.91 0.4 0.11 0.78 Gd 0.08 1.75 0.35 0.74 0.41 1.15 0.2 5.41 1.98 <0.05 0.56 Tb <0.01 0.33 0.04 0.07 0.02 0.16 <0.01 0.93 0.33 <0.01 0.11 Dy <0.05 2.12 0.39 0.27 0.12 1.05 0.18 5.75 2.33 <0.05 0.66 Ho <0.02 0.44 0.04 0.06 <0.02 0.18 <0.02 1.11 0.45 <0.02 0.11 Er 0.05 1.34 0.19 0.15 0.05 0.65 0.03 3.68 1.65 <0.03 0.39 Tm <0.01 0.13 0.02 0.02 <0.01 0.07 <0.01 0.58 0.24 <0.01 0.03 Yb <0.05 0.84 0.4 0.21 <0.05 0.62 0.06 4.7 1.79 <0.05 0.36 Lu <0.01 0.09 0.03 0.02 0.01 0.08 <0.01 0.59 0.25 <0.01 0.03 ∑REE 5.89 29.23 13.30 44.28 25.30 37.52 10.99 132.64 39.32 2.15 9.09 Eu/Eu* 40.96 2.30 10.22 2.21 3.07 2.49 29.94 0.51 0.66 6.76 4.28 LaN/LuN 18.79 6.40 11.02 70.88 72.10 10.45 36.59 4.73 2.97 8.34 5.85 LaN/YbN 24.00 4.44 5.33 43.81 92.00 8.71 38.89 3.84 2.68 10.67 3.15 LaN/SmN 7.91 2.94 4.80 7.72 7.86 3.48 15.38 2.95 2.55 9.84 1.87 EuN/YbN 8.14 2.88 2.61 19.50 60.53 5.87 18.23 2.37 1.72 3.05 1.20 CeN/YbN 2.68 1.91 2.35 3.43 5.17 2.35 7.21 1.82 1.63 2.82 0.71 CeN/SmN 80.57 3.64 9.00 8.84 26.86 4.79 77.62 0.55 0.64 6.29 6.19

Fig. 7. Chondrite normalize REE pattern for average values of bulk-rock pegmatite, muscovite and feldspar samples for Southern Obudu pegmatite

Fig. 8. Chondrite normalize REE pattern for bulk-rock pegmatites of Southern Obudu

Fig. 9. Chondrite normalize REE pattern of feldspar extract from Southern Obudu pegmatites

The REE patterns of the muscovite extracts (Fig. 10), on the other hand, possess moderate to high total REE abundances (∑REE 39.32–132.64 ppm), negative Eu anomalies (Eu/Eu*: 0.51–0.66), prominent negative Ce anomalies and relative enrichment of heavy REE (LaN/YbN = 2.68–3.84). The muscovite extracts also display vague W-type tetrad effects (Fig. 10). Taylor et al. (1986) advanced the views that weak negative Ce signature and strong negative Eu signature denotes considerable fractionation and metasomatism. However, this is not the case with the pegmatites of Southern Obudu which displays weak negative Ce signature and strong positive Eu anomaly on the REE pattern diagram for bulk sample (Fig.8). Indication therefore is that the Southern Obudu pegmatites experienced low


degree of fractionation and metasomatism.

Fig. 10. Chondrite normalize REE pattern of muscovite extract from Southern Obudu pegmatites

4.2.4. Mineralization Potentials

The metallogenic potential for rare metals such as Rb, Cs, Be, Y, REE, Zr, Hf, Nb, Ta (Smirnov et al., 1986) in the Southern Obudu pegmatite can be assessed using geochemical criteria and representative diagrams for the whole rock pegmatites and mineral extracts (Androne, 2005; Černý and Ercit, 2005; Černý, 1992; Murariu et al., 2008; Murariu and Răileanu, 2006; Murariu et al., 1999, 2007; Shmakin, 1973, 1979). Accordingly, the southern Obudu pegmatites have been appraised using various geochemical criteria and representative diagrams. When the chemical data of the investigated pegmatites are projected on the K/Rb versus Rb diagram (Fig.11), the pegmatites plots across both the mineralized and barren field. A clearer picture is obtained on the Th/U versus K/Cs diagram (Fig.12), where samples of the whole rock pegmatites and mineral extracts of muscovite and feldspars of the investigated pegmatites plotted below the Ta-Nb mineralization line of Beus (1966) and Gordiyenko (1971) (Fig.12). Indication is that Southern Obudu pegmatites are not Ta–Nb mineralized as many Precambrian pegmatites in the Basement Complex of Nigeria (see for instance, Agunleti et al., 2014; Akintola et al., 2011, 2012; Okunlola and Akinola, 2010; Okunlola and King, 2003; Okunlola and Ocan, 2002; Okunlola and Oyedokun, 2009; Okunlola and Udoudo, 2006, 2010; Okunlola, 2004).

Several field and petrographic data exist to support this position. In particular the close spatial association of the pegmatite bodies with parent granitic rocks, and the complete absence of zonation structures in Southern Obudu pegmatites is a confirmation of the barren nature of the pegmatites. 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 (Trueman and Černý, 1982). Also, classical pegmatite zonation is typical of highly fractionated and mineralized pegmatites (Sweetapple, 2000).

The composition of samples of Southern Obudu pegmatites on the variation plots of Ta versus Cs (Fig.13) is in agreement with the low–level rare metal mineralization potential of the Southern Obudu pegmatites.

Fig. 11. K/Rb versus Rb distribution pattern for Southern Obudu Pegmatite. The Arrow indicate normal differentiation trend, after Staurossssv et al.,

(1969)

Fig. 12. Plot of Th/U versus K/Cs ratio for the Southern Obudu Pegmatites (Gordiyenko, 1971; Beus, 1966);

Furthermore, the paucity or relative low–level rare metal mineralization potential of the investigated pegmatites can equally be seen in the very low values recorded as Rb/Ba ratio for potassium minerals (muscovite and feldspar extracts) in the rock. Following the criteria outlined in Murariu et al., (2008), low Rb/Ba ratio values such as the ones recorded for the investigated pegmatites rules out the consideration of the pegmatites as a rare-metal pegmatites or as a rare-metal muscovite pegmatites.

Fig. 13. Plot of Ta versus Cs for the Southern Obudu Pegmatites


Finally, on the K/Rb versus Cs variation diagram, the studied pegmatites plot in the field of muscovite class (Fig.14). This classification into the muscovite class is consistent with the occurrence of rare accessory minerals like garnet, biotite, tourmaline, and probably beryl, together with the association with upper amphibolites facies grade country rocks (Černý, 1982, 1991; Ephraim et al., 2006).

Fig. 14. Plot of K/Rb versus Cs for Southern Obudu Pegmatite (Černý, 1982)

Additionally, the classification corroborates an earlier sorting of the pegmatites of Obudu into muscovite-bearing types, biotite-bearing types and subordinate muscovite-biotite pegmatites (Ekwueme, 2005).

5. Conclusions

Field investigation revealed that the pegmatites of Southern Obudu area frequently occur in close spatial association with criss-crossing quartz veins and granitic intrusions. They are widely distributed as boulders and veins in mostly the schistose and gneissose basement rocks that have attained at least the Upper Amphibolites facies metamorphism. Some of the veins have been deformed alongside the country rocks, indicating that the pegmatites were formed before the last phase of deformation of the country rocks (Igonor et al., 2012). The major mineral composition (quartz-feldspar-mica) of the pegmatites are mostly not different from those of rare-metal bearing pegmatites of Nigeria. Other traits, shared with rare-metal bearing pegmatites include very low mean values of MnO, MgO, CaO, Na2O, TiO2, and P2O5.The trace elements analysis shows that the pegmatites documents relatively low contents of trace elements and rare metals. Notably, the pegmatites exhibit low contents of Nb, Rb, Ta, Cs and Ga, which are the major mineralisation indices for pegmatites bodies in the Nigerian Basements. The trace elements ratio supports a low degree of fractionation and metasomatism in the evolutionary history of the pegmatites, and the very low Cs values recorded for the pegmatites indicate paucity of alkali metal fractionation. These features agrees with the weak negative Ce signature and strong positive Eu anomaly REE pattern exhibited by mostly the bulk rock pegmatites.

The overall geochemical signatures of pegmatites of Southern Obudu generally indicate low fractionation, and

low level of rare metal mineralization potential, unlike the rare metal bearing pegmatites in the Basement Complex of Nigeria (Agunleti, et al., 2014; Akintola et al, 2011, 2012; Okunlola and Akinola, 2010; Okunlola and King, 2003; Okunlola and Ocan, 2002; Okunlola and Oyedokun, 2009; Okunlola and Udoudo, 2006, 2010; Okunlola, 2004). Furthermore, the classification of the pegmatites into the muscovite class is in order since abyssal and muscovite classes have no significant tantalum mineralization (Ginsburg et al., 1979; Ginsburg, 1984; London, 1986).

Acknowledgement

This work has benefitted immensely from the support and encouragement of Prof. E. O. Esu, Dr. T. N. Nganje, Dr. N. U. Essien, and many other staff of Geology Department, University of Calabar, Nigeria. A staff of NUT Calabar Branch, Mr. Orlando E. Edem is also acknowledged for his role towards the smooth and successful completion of the work. Others, including Dr. Effanga, Emmanuel Igonor and I. Umoren were exceptionally supportive. Finally, we thank the anonymous reviewers for a thorough review of the manuscript. Their views, comments and suggestions have substantially improved the draft of this paper.

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