Geology, petrography and petrogenesis of pegmatite from Gwon-Gwon, Nigeria

Table of Contents

1. Introduction

2. Metallogenetic Features Of The Study Area.

3. Geology
3.1 Hand Specimen Description of Rock Types
3.2 Field Relationship And Petrography
3.2.1 Migmatite (Host Rock)
3.2.2 Biotite-Granite
3.2.3 Biotite-Muscovite Granite
3.2.4 Pegmatite

4.0 Discussion
4.1 Interpretation of Photomicrograph Plates
4.2 Trace Element Geochemistry
4.3 Petrogenesis of the Rare Metal Pegmatite of Gwon-Gwon.

Acknowledgement

References

GEOLOGY, PETROGRAPHY AND PETROGENESIS OF THE RARE METAL PEGMATITE OF GWON-GWON, NASARAWA STATE-NORTH CENTRAL NIGERIA.

BY

Abdullahi, A.M 1, Haruna A.I 1 and Usman, A. M1

Department of Applied Geology, Abubakar Tafawa Balewa University Bauchi.

ABSTRACT: The pegmatite of the Gwon-Gwon area of Nasarawa state occurs in central Nigeria and is mainly hosted by the late Pan-African leucogranites and the migmatites. A geological mapping of the area was followed by petrographic and mineralogical studies of selected rock samples. Petrographic studies show that there is consistent enrichment in colour (within the rock mass) of muscovite from blue to greenish colour which is also persistent with increase in quantity of rare metals. This could be an indication of fractionation. The petrographic studies also show the presence of two mica (muscovite and biotite) in the granitoids which suggest that it is a two mica granitoid. Trace element geochemistry was carried out using the X-Ray Fluorescence (XRF) Spectrometry method. The samples were analyzed for their trace elements. The development of the rare-metal pegmatite of Gwon-Gwon is related to granite magmatism. The Pan-African granites have very low REE abundances and low Nb/Ta ratios indicating crystallization from a liquid rich melt. The Sn-Nb-Ta mineralized granites are correspondingly enriched in pegmatites genetically associated with Pan-African synorogenic granites with enhanced values of rare elements such as Rb, Cs,Li,Be,Sn and Nb-Ta.The primary control of rare metal mineralization in the pegmatite of Gwon-Gwon,is the composition of the source rocks, since the Ta-Nb-Sn-Li-Be-W mineralized pegmatites crystallized from fluid (H2O-B-P-F) rich melts.

Key words: petrographic, pegmatite, mineralogical, REE, Gwon-Gwon, Nigeria

1. Introduction

Pegmatites are coarse grained igneous rocks. They represent the end product of magmatic crystallization in the evolution of granitic melt. The rare-metals (rare-elements) that serve as petrogenetic indicators (geochemical indicators) and potential ore indicators are Rb, Cs, Li, Sn, Ta, Nb, Be and W, also volatiles, like B,F,H2O play a very important role in the whole process. Pegmatites research in Nigeria from 1946 to 1989 by Jacobson and Webb( 1946), Matheis and Cean- Vanchette(1983), Matheis(1987), and Matheis and Kuster(1989), use the rare-elements as tin indicators but petrology was not the main target. The Gwon-Gwon pegmatite at Wamba is the third mineralized pegmatite in Nigeria. It falls within the Nigeria’s 400km stretch of pegmatite belt [(fig.1), Matheis and Cean- Vanchette,1983] which trends from NW to the central North. The pegmatite “belt” in Nigeria is part of the Pan- African reactivation zone.

2. Metallogenetic Features Of The Study Area.

The Gwon-Gwon pegmatite at Wamba, is the third mineralized pegmatite field in Nigeria (others being at Egbe, Ijero). The second in the north being the Jema’a pegmatite (Matheis and Cean-Vanchette, 1983). The study area which is part of a pegmatite belt (fig 3), falls between longitudes 8030I to 8o40I east and latitudes 9o00I to 9o 071 North covering an area of about 45km2 within the Pan-African reactivation zone.

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Fig 1: Tin Bearing Pegmatite Zone and other Regional features of Nigeria (Adopted from Matheis and Caen Vanchette; 1983).

The Wamba pegmatites are complex and have been investigated for its Tin, Columbite and Tantalum content.

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Fig.3: Geological Map of the study area part of Sheet 189 Kurra.

3. Geology

Geological mapping of the study area reveals the presence of the Basement Complex rocks.The Basement Complex rocks identified are the Migmatite and members of the Older Granite series which include Biotite-Granite, Biotite-Muscovite Granite and the Pegmatites. Field description and petrographic study of these rocks are given below.

3.1 Hand Specimen Description of Rock Types

Two samples of the 2 mica granites shows that they are medium to fine grain. They are grayish white in colour. Biotite is more pronounced in hand specimen but the muscovite is only visible under the petrographic microscope. (See plate 1).

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Plate 1: Hand specimen of Sample F24 (mica granite)

The hand specimen samples of the migmatite shows that the rock is partly granitic and partly schistose implying that granitic magma intruded the schist to form the migmatite. (See plate 2).

The hand specimen samples of the pegmatite are divided into potassic pegmatites (F9, F21) and albitized pegmatities (F3, F4 and F7) as representatives of the two main groups. (See plate 3 and 4).

The potassic pegmatites have pegmatitic crystals of orthoclase and this is evident from the pinkish brown colour. Large blebs of muscovite and coarse grains of quartz are the associated minerals and are also visible in hand sample. The albitized pegmatite is whitish in colour with tints of pink colour. The quartz and muscovite seems assimilated and so are not too coarse. (See plate 4).

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Plate 2: Hand specimen of Sample F5 (migmatite)

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Plate 3: Hand Specimen of Sample F9 (Potassic pegmatite)

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Plate 4: Hand Specimen of Sample F7 (albitized pegmatite)

3.2 Field Relationship And Petrography.

3.2.1 Migmatite (Host Rock);

The Migmatite is the host rock for the granitic pegmatite in the research/study area. This Migmatite occupies about 55% of the area. In hand sample, there are traces of layering of the felsic and mafic minerals (Plate 2),but it is difficult to identify distinct minerals in hand specimen due to its closeness to the metasomatized areas. Plates 3 and 4 shows the two types of pegmatite samples (Potassic and Albitized pegmatites) in hand specimens within the study area. Under plain polarized light (PPL),biotite,hornblende and the multicoloured (purplish green) mica show high relief and were pleochroic. Biotite display dark brown colour while hornblende display medium brown colour. The multicoloured mica display purplish green colours. Quartz and feldspars were colourless. Under cross polarized light (CPL),(Plate 9 )biotite and multicoloured mica maintain their brown and purplish green colours respectively as the stage is rotated. Quartz display light grey interference colours while albite display dark grey colours, and in some parts, the albite display a clear albite twinning that goes into extinction at different angles due to orientation. Plate 11 shows another part of the migmatite displaying large blebs of multicoloured mica changing colour to greenish-purple (under cross polarized light).

3.2.2 Biotite-Granite;

The rock occupies part of the north-western study area around Amartita, Kontagora areas down towards river magama tributary (Figure 3).The rock is massive and medium grained in texture and outcrops as low-lying. Akintola et al.(2008) reported that this rock appears to represent the first major episode of granite plutonism in the area. The rock is composed of essentially quartz, biotite and plagioclase feldspar in hand specimen sample (Plate 1). Microscopically,quartz,biotite,microcline and plagioclase feldspar is observed. There may be minor or no hornblende in thin section study. Quartz occur as anhedral crystals and give grey interference colour with complete extinction. Biotite occurs as flaky brown crystals and could be up to 22% by composition in some samples (Plate 13), and are sericitized. They are brown and pleochroic from pale brown to dark brown. Some crystals show bent twin lamellae, which could indicate deformation. Microcline is much more than plagioclase ranging from 7% to 12% by composition. Microcline, which are fractured, occur as tabular crystals some of which contained myrmekitic structures. Microcline shows cross-hatched twinning. Some plagioclase shows some sericitization.

3.2.3 Biotite-Muscovite Granite;

The rock outcrops as low-lying around Malati at the northern extreme of the study area and around Ungwan-Rimi at the southern extension (Figure 3).The rock is massive and ranges from fine-medium grained and medium grained in texture, comprising of quartz, albite and biotite in hand specimen sample. Microscopically it consist of quartz,albite,orthoclase,plagioclase,biotite and muscovite. Iron-oxide constitute the accessory mineral of this rock. The quartz occur as anhedral crystals with grey interference colour with complete undulose extinction which may be as a result of deformation. Orthoclase exhibits carlsbad twinning and occurs in tabular form. Biotite occurs as flakes or in tabular form with perfect one directional cleavage. They are brown and pleochroic from pale brown to dark brown. Muscovite is present but much less in quantity compared to biotite. The muscovite displayed faint multicolours (blue-purplish) with a faint one directional cleavage under plain polarized light (PPL).Under cross polarized light (CPL),quartz displays second order grey and goes into extinction as dark grey. There is no cleavage. Biotite shows dark brown interference colour with a perfect one directional cleavage. The traces of the blue musvcovite turns to multicoloured (blue-purplish-green) lath as the stage is rotated. In some part of the slide, muscovite shows a clear navy blue colour embedded on albite matrix (Plate 17 and 18),while the albite matrix partly shows perfect lamella twinning in one direction and partly looking metasomatized.

3.2.4 Pegmatite;

The pegmatite occupies about one-third of the study area (Fig. 2).Hand specimen sample study indicate two types of pegmatites within the study area; the potassic and albitized pegmatites (Plate 3 and 4 respectively). Samples F9,F21 represents the potassic pegmatites, while samples F3,F4 and F7 are the representatives of the albitized pegmatite types. The potassic pegmatites have pegmatitic crystals of orthoclase and this is evident from the pinkish-brown colour. Hand specimen study also reveals large blebs of muscovite and coarse grains of quartz as associated minerals. The albitized pegmatite is whitish in colour with tints of pink colour. The quartz and muscovite are not too coarse (Plate 4).

Microscopic study shows quartz, orthoclase, plagiocase and minor biotite. Chlorite constitute the accessory mineral. Quartz occurs as anhedral crystals with grey interference colour and undulose extinction. Orthoclase displays carlsbad twinning under cross polarized light (Plate 17). It is colourless with a general low relief in most points of the slides in plane polarized light. Plate 14 is plagioclase lath (under cross polarized ligth) covering the whole slide and displaying perfect lamellae twinning. This suggest a primary albite that is quite distinct from the massive cloudy plagioclase that forms from albite metasomatism. Biotite is one of the minor minerals here and occurs as flaky/tabular brown crystal displaying a perfect one directional cleavage. They are brown and pleochroic from pale browm to dark brown. Chlorite possesses a thin nature and bluish colour which disappears upon rotation under cross polarized light.

4.0 Discussion

Granites and pegmatites basically contain quartz, feldspars and micas. Infiltration of trace elements converts them to many minerals ranging from uneconomic to economic. The colour and texture of the minerals and their changes are the factor that determines their economic viability. Selway et al, (2005) reported that K-feldspar is abundant in barren granite, fertile granite and pegmatite. K-feldspar is pink and medium grain in barren granite, whereas in fertile granite and pegmatite, they are white and blocky. K-feldspar in pegmatite can also be gray, peach, pink or green. The colour and texture of muscovite also change with fractionation. Muscovite in barren granites tends to be silver coloured and medium grain. In fertile granite and pegmatite they are green, brown, silver, rarely pink and coarse grain. The green muscovite usually contains lithium and it shows fractionation, Selway et al, (2005).

4.1 Interpretation of Photomicrograph Plates

Plates 13, 14,15,16,17 and 18 are photmicrograph of the granitoides within the sample area. Granitoid 13, 14 and 15 showed both the primary muscovite (in blue) and secondary muscovite (in green). Some granitoids (plates 13, 14 and 15) fractionated and this is due to increase in the transformation of muscovite in the granitoids.

Plates 5, 6, 7,8,9,10,11 and 12 shows the phtomicrograph of the pegmatite bodies in the research area. Pegmatite (plate 5, 6 and 8) look barren because there is no evidence of development of greenish secondary muscovite implying that there is no enrichment of rare elements, even though plate 5 and 9 displayed a very coarse primary muscovite. Pegmatite (plates 7 and 8) display random spindle of perthite which according to London,(1986), implies crystallization at elevated temperatures. Here also there are traces of blue primary muscovite but there is no development of green secondary muscovite and so there is no enrichment of rare elements. This pegmatite (plates 7 and 8) are also barren.

Pegmatite (plates 9 and 10) display enrichment zones where the blue primary muscovite is converting to the greenish secondary muscovite at their transitional boundary within an albite matrix. These pegmatites show enrichment of rare elements and could be fertile.

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Plate 5: Photomicrograph of Primary Muscovite on Albite Matrix: cross polarized light. X40

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Plate 6: Photomicrograph of fragments of Primary Muscovite on Albite matrix : cross polarized light. X40

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Plate 7: Photomicrograph of Sub-Parallel Spindles of Perthite trending in uniform direction: cross polarized light. X40

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Plate 8: Photomicrograph of Scattered Perthites on Albite Matrix: cross polarized light. X40

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Plate 9: Photomicrograph of Primary Muscovite changing into Secondary Muscovite: cross polarized light. X40

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Plate 10: Photomicrograph of partially developed Secondary Muscovite displaying unidirectional Cleavage: cross polarized light. X40

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Plate 11: Photomicrograph of well-developed Secondary Muscovite Flanking remnant Muscovite: cross polarized light. X40

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Plate 12: Photomicrograph of Muscovite in Albite Matrix; cross polarized. X40

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Plate 13: Photomicrograph of Mica Granite hosting Primary and Secondary Muscovite. cross polarized light; X40

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Plate 14: Photomicrograph of Large Bleb of Partially formed Secondary Muscovite in two Mica Granite: cross polarized light. X40

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Plate 15: Photomicrograph of Secondary Muscovite embedding Primary Muscovite with a transitional boundary: cross polarized light. X40

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Plate 16: Photomicrograph of Perthite and Albite coarse-grained Granite: cross polarized light. X40

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Plate 17: Photomicrograph of Granite showing distinct Orthoclase with carlsbad twinning; cross polarized light. X40

Plate 11 displayed very coarse well developed green secondary mucovite lath with purple rims. This is the most mineralized pegmatite of the muscovite and shows perfect one directional cleavage. The change in colour of muscovite from blue to green shows enrichment of rare elements according to Selway, (2005). Granitoid plate 13 is a two mica granite and it is one of the properties of fertile granite that is known to be parental to pegmatite. Another good property of the micas in fertile granite is that they should be coarse grain and not medium to fine grain according to Selway, (2005). Plate 13, 14 and 15 display coarse grain secondary micas especially plates 14 and 15. These shows that they could be parental to any mineralized pegmatite body in the research area and they are reasonably albitized.

Plates 16, 17 and 18 display granites with abundant K-feldspar due to the presence of distinct orthoclase and perthite. They contain traces of the bluish weathered primary muscovite but there is no evidence of the greenish secondary muscovite. This shows that there is no enrichment of rare elements in the granitoids. Plate 18 display additional exsolved micas with Albite and Quartz matrix. These granitoids (Plates 16, 17 and 18) could be the primitive granitoid from which the fertile pegmatite evolved in the area and this is the most productive and economical pegmatite body. The quantities of rare elements are the highest here. This area should be investigated for productive muscovites.

4.2 Trace Element Geochemistry

The samples F23(Granite),F2, F8 and F13 (Migmatite) have similar geochemical characteristics, which differentiate them from the others (granites and the migmatites).The granites are silicic (quartz-rich) with high contents of Ca and Mg. Samples F23 and F2, F8, F13 have low K/Rb ratios. They have enhanced values of Rb ranging between 20-30ppm and are well fractionated. They also have enhanced values of Sn and Cs (Table 1).

On the other hand, the pegmatite sample F20 which is spatially associated with the geochemically /genetically evolved granite (the older granite) in the area has low concentrations of the rare elements Rb-5.0ppm,Cs-4.8ppm, and Sn-47 ppm andK/Rb of 0.022ppm,K/Cs-0.023ppm.

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Plate 18: Photomicrograph of exsolved Mica embedded in Albite matrix within Granite; cross polarized light. X40

4.3 Petrogenesis of the Rare Metal Pegmatite of Gwon-Gwon.

The Parental granites to the rare metal pegmatites of Gwon-Gwon area, Wamba are peraluminous and were formed by partial melting of mica-rich metasediments along the fracture zones,(Wright,1970; Matheis, 1991; Garba, 2002; and Abdullahi, 2013); High heat flow and shearing along the regional fractures contributed significantly to the heat for the partial melting of the metasediments.

As magma ascends through the fractures, an increase in the elements F, P and Al will occur (Akintola et al. 2008). These elements would cause to reduce the viscosity of the melt ( Raimbault et al, 1995) thereby aiding not only the flow of this melt but also the extreme fractionation of the elements causing an enrichment of the rare elements such as Rb, Cs, Li, B, Ti, Be, Mn, Sc, Y, H, REEs, Sn, Mo, Ta, Nb and W (Cerny, 1991). These rare-elements are transported as complexes in fluid rich melts.

Table 1: Element Ratios and Trace Element Distribution in the Gown-Gown Pegmatite Field (ppm)

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Table 2: Main Element Distribution in the Gwon-Gwon Pegmatite Field (wt. %).

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The Gwon-Gwon area and its related rare-metal generating event is associated with the Pan-African Orogeny, which includes enrichment of rare-metals in the fluid-rich melts related to the granites of this area, and also their spatially /genetically related pegmatites (Abdullahi, 2013). The granites of this area is characterized by a strong alteration of plagioclase (a replacement of the plagioclase by perthite and finally albitization). This alteration possibly resulted from the late metasomatic fluids that mobilized the ore-elements (e.g Nb and Sn) and concentrated them in the granites of the area (Gwon-Gwon). The enhancement of the rare-metals in the geochemically distinct granites is fracture controlled and post-dates the emplacement of the main phase Pan-African granites.

The granites parental to the rare-metal pegmatite of this area are not exposed at the current erosional level (i.e they are still lying buried).This explains the low P contents of the pegmatitic granites. (See table 2). This element (P) would have been concentrated at the roof zones of the granites.

Raimbault et al., (1995) concluded on the evolution of the magma by stating that, as the magma ascends through the fractures it would be towards an increase of the depolymerizing elements F, P and Al.

Acknowledgement

We wish to acknowledge the efforts of the following in the preparation of this manuscript.

1. Dr. Ismanil Vella Haruna of Geology Department, Federal University of Technology, Yola, Adamawa State-Nigeria.
2. Dr. Jalo Muhammad El-Nafaty of the Depatment of Geology, University of Maiduguri, Borno State-Nigeria.

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