Geology and Petrochemistry of Granitic Pegmatite in Jema’a Pegmatite Field, Kaduna State

CHAPTER ONE

1.0 INTRODUCTION

1.1 General Statement

The topic of the research is: the Geology and the petrochemistry of granitic pegmatites in Jema’a pegmatite field, Kaduna State. The area is located within longitude 8o 11’ to 8o 23’ and latitude 9o 18’ to 9o 30’ covering an area of about 117km2 (Fig. 1) within the Nigeria’s pegmatite belt. The Nigerian rare elements pegmatite belt contains many pegmatite fields concentrating within the broad belt that stretches for over 400km of a distance from Jema’a through Wamba (in cenral northern basement complex) to Ago-Iwoye area through Iregun- Ijero- Igbe and Aromoko (in southwestern Nigeria) (Jacobson and Webb, (1946); Wright, (1970); Kinnaird, (1984); Matheis, (1979; 1981); Kuster, (1987); Matheis and Caen Vanchette, (1983); Kuster, (1990); Garba, (2002; 2003); Okunlola, (2005); Adekeye and Akintola, (2007); and Akintola and Adekeye, (2008) (Figs. 2 ). Ekwueme and Matheis, (1995) have also reported mineralized pegmatites in the Precambrian basement of southeastern Nigeria. Many new rare elements pegmatite fields have been discovered within the known belt and other areas especially in Kushaka schist belt and Magami and Maradun areas of the southwest Nigeria (Garba, 2003) (Fig.2). The Nigeria’s pegmatite fields were named Older Tin Fields to differentiate them from Newer Tin Fields of the Jos, Plateau by Wright, (1970) (Fig. 3). The pegmatite belt in Nigeria is known for Sn-Ta-Nb mineralization besides tourmaline and other gems stones. The pegmatite belt crosscutt the Nigeria’s schist belt that host the gold mineralization and quartz gold veins, hence the rare elements pegmatites are often associated with gold mineralization in schist belt. The Jema’a pegmatite field is no exception to the occurrence of rare elements pegmatites and the associated mineralizations even though it’s not in record.

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Fig. 1: Location map showing sampling points in the research area

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Fig. 2: Regional Geology of the Nigerian Pan-African Basement, showing locations of areas of rare-metal and barren pegmatites (modified from Geological Survey of Nigeria map, 1994), NS, Nassarawa; KS, Kushaka;M, Magami; MD,Maradun; RC, Richifa; BD, Badafi; KM, Kafin Maiyaki (adopted from Ibrahim Garba, 2003)

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Older Pegmatite Tin fields within the belt Newer Jos Tin Fields

Fig. 3: Nigeria’s Pegmatite belt containing the six Pegmatite fields (Modified after Matheis and Caen Vanchette, 1983)

1.2 Statement of the Problem

The Nigerian Basement complex has been extensively studied and research on granitic pegmatite is equally vast, both on pegmatites lying within pegmatite belt (Fig. 2) and those outside the pegmatite belt (in both Pan African granitoids and Jurassic Younger Granite) by many workers (Jacobson and Webb, (1946); Wright, (1970); Kinnaird, (1984); Matheis, (1979; 1981); Kuster, (1987}; Matheis and Caen Vanchette, (1983); Kuster, (1990); Ekwueme and Matheis, (1995) ;Garba, (2002; 2003); Okunlola, (2005); Adekeye and Akintola, (2007); and Akintola and Adekeye, (2008)). Most of the studies of pegmatites from 1960s to 1990s were in relation to Sn – Ta – Nb exploration and exploitation. From 1990s to present times, there are a lot of research on the geochemical characteristic of rare element pegmatite by Kuster, (1990); Garba, (2003); Okunlola, (2005); Adekeye and Akintola, (2007); and Akintola and Adekeye, (2008) and others.

For all that was done, the Jema’a pegmatite field even though was exploited for Sn – Ta – Nb in the 1970s (Kuster, 1987), has not received any attention, so information on the granitoids and pegmatites and their geology, geochemistry and economic significance in scanty or not available. Matheis and Kuster, (1989) later concluded that “Detailed Geochemistry on heavy mineral concentrates; whole rock samples and rock-forming minerals will determine the final economic potential of the areas of the Wamba and Jema’a pegmatite fields within the Nigeria’s pegmatite belt”. Jema’a pegmatite field is a northwestern terminal point of the 400km Nigeria’s pegmatite belt that stretches from southwest-northern Basement complex. This research will attempt to evaluate the geology, petrochemistry and economic significance of pegmatites in the Jema’a pegmatite field (Fig. 4).

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Fig. 4: Location of Jema’a and Wamba Pegmatite Fields. (modified after Kuster, 1990)

1.3 Aims and Objectives of the Present Work

The aims and objectives of this research are to conduct systematic geologic study of the pegmatites in relation to the adjoining granitoids with a view to;

a. Understanding the Geology and geodynamic evolution of the pegmatites
b. Determine their mineralogical, geochemical characteristics and petrogenetic affinity.
c. Evaluate their economic mineral potential with emphasis on rare elements mineralization.
d. Relate pegmatites to tectonic regimes and the related magma generating processes

1.4 Scope of the Present Work

In order to achieve the above objectives, the work entails systematic mapping on a scale of 1:50.000 (because of the size of the area) to delineate the different rocks (granitoids and metamorphic rocks) and pegmatites within the research area. The whole samples (rocks and mineral separates from pegmatites) were divided into two groups; one group for thin section/petrographic study while the other group was used for geochemical analysis/studies. Three types of analytical instruments were used to generate the geochemical data (Atomic Absorption Spectrometry, X-Ray Fluorescence and Insrumental Neutron Activation Analysis). The chemical data was used for petrogenesis which involves petrochemistry and trace element geochemistry, to establish the relationship between the rocks (granitoids and metamorphic hosts) and the pegmatites. The chemical data was also used to determine Aluminum Saturation Index (ASI) and Differenttion Index (DI) for both the granite rocks and pegmatites.

Combined petrographic and overall geochemical data interpretations were used to determine the regional zoning of the fertile granite and pegmatites using fractionation, economic potential of the pegmatite bodies using rare elements enrichments, establish the geochemical families of the pegmatite groups and the model for origin of the pegmatites.

CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 Geologic Setting

The Nigerian Basement complex is the southern extension of the Pan-African orogenic belt (Figs. 5&6). The Trans-Saharan Pan-African orogen is characterized by early thrust-nappe development, high grade metamorphism, voluminous granite plutonism and late orogen-parallel tectonics (Boullier et al., (1986); Caby, (1989); Black and Liegeois, (1993)). This resulted from the oblique convergence between the West African Craton and a domain to the East of the Sahara that represents Neoproterozoic juvenile crust possibly connected with the Arabian-Nubian Shield (Ferre et al., (1998)). The Air-Hoggar segment of this belt (Fig. 5) was formed, mostly between 730 and 580 Ma, by amalgamation of contrasted north-south elongated terranes (Black et al., (1994); Liegeois et al., (1994)). The Nigerian segment, further south (Fig. 6), also consist of accreted terranes (Ajibade and Wright, (1989); Ferre et al., (1996)) and was connected to the Air-Hoggar, Cameroon and Borborema Pan-African provinces.

A set of major N-S lineaments, identified on Landsat images (Ananaba and Ajakaiye, (1987)), separates Nigeria Basement in two terranes thoroughly affected by Pan-African tectono-metamorphic events (Ferre et al., (1996); Fig.5):

a. Eastern Nigeria, a tin metallogenic province (Woakes et al., (1987)), dominated by Eburnean protoliths (Dickin et al., (1991); Ferre et al., (1996)); and
b. Western Nigeria, a gold bearing province, characterized by numerous Archaean features such as tonalites-trondjhemites-granodiorites, greenstone belts and Nd model ages older than 2.7 Ga (Klemm et al., (1984); Bruguier et al., (1994)). The Archaean Basement of Western Nigeria is overlain by a Proterozoic metasedimentary cover with Pan-African metamorphism ranging from upper green-schist to lower amphibolites facies (Grant, (1978); Turner, (1983); Caby, (1989)).

The Togo-Benin-Nigeria shield (Fig. 6) (Trompette, (1980); Wright, (1985)) is the southern prolongation of the Pan-African mobile belt. To the west, it is thrust onto the West African craton or as in most cases, on the Volta basin. To the south, Benin-Nigerian shield is directly thrust on to the passive margin of the craton. Here, the oceanic opening was either very small or the oceanic crust and active margin have been entirely subducted (Black, (1984); Wright et al., (1985)).The Basement Complex is dominated by mainly amphibolite-grade quartz-feldspar biotite (± hornblende) gneisses and migmatites. They range in composition between granite and diorite, but probably average out as granodiorite. Layers and lenses of other rocks types such as quartzite, marbles and amphibolite also occur within the Basement but, although they may be widespread, they are quantitatively subordinate. The complex structures and geochronological relationships among the Basement rocks provide much of the evidence for crustal reactivation. In other words, the Basement is made up of rocks that have experienced the effect of more than one thermotectonic event (Wright et al., (1985)).

Elongate supracrustal belts consist of mainly greenschist to amphibolite facies, phyllite, greesnstones and schists. These rocks occupy synclinorial structures within the basement. They have a varied lithology and mineralogy and represent sediments and volcanics of many different types. They are called supracrustals because they look as though they were originally deposited upon the Basement and are therefore younger.

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Fig. 5: Subdivision of Basement Complex (after Ferre’ et al., 1996)

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Fig. 6: Generalized map to show the extent of correlation between

Precambrian Tuareg Shield, of Hoggar and the eastern Pan African domain in the southern part of West Africa. (After Wright et al., 1985)

Their contact relationships with basement rocks are (where exposed) commonly gradational or sheared due to faulting or thrusting and direct evidence of superposition is rare. Though tightly folded and commonly faulted, they generally have less complex structural and geochronological relationships than surrounding Basement rocks and are generally believed to have been deformed and metamorphosed during the last of the thermotectonic events which reactivated that Basement (Wright et al., (1985)). Such supracrustal belts are widely developed in Western Nigeria, but there is still considerable argument over their true age. Several authors regard them as middle Proterozoic and on the basis of Rb-Sr dating of metasediments (which has given ages of 1100 – 1200 Ma) believe in the existence of a “Kibaran” events. Others class these belts as Upper Proterozoic, the metamorphism and deformation being entirely Pan – African. It should be noted however that, all Kibaran ages (and this also applies to the Tuareg shield) have been obtained by Rb-Sr and K-Ar methods on metasediments, so far no granites have yielded this age (Black, (1984)).

Syntectonic to late tectonic plutonic intrusions are mainly of granite to granodiorite in composition, but include smaller masses of diorite, gabbro, syenite and related rocks. They intruded both Basement and supracrustals. Contacts with basements rocks vary from concordant and gradational to sharp and cross cutting. Contact with supracrustals is normally sharp and cross-cutting. Their field and geochronological relationship indicate that emplacement occurred during the last of the reactivation event that affect the basement i.e. during deformation and metamorphism of the supracrustals (Wright et al., (1985)).

Metamorphic grades are generally similar throughout the Precambrian, with amphibolite to granulite facies rocks characterising the Basement, greenschist to amphibolite facies more typical of the supracrustals. There are often steep metamorphic gradients between Basement gneisses and migmatites and supracrustals, both of schist and phyllite (Wright et al., (1985)). The schist and phyllite, which now make up the supracrustal belts represent an assortment of sedimentary and mainly volcanic igneous lithologies, the proportion of which differ considerably from belt to belt and in particular from region to region. Among the metasediments, mica-schist, and phyllite generally predominate. These are rocks of pelitic composition, signifying their originally argillaceous nature as muds, silts and shales. Also common are psammitic composition, quartzite and quartzo – feldspathic rocks, originally quartz – sandstone, arkoses and greywackes. These are arenaceous rocks also sometimes called arenites. Conglomeritic facies may be locally abundant. Among the metavolcanic greenstones, amphibolites and chlorite-rich schists are predominant rock types, representing basaltic lavas, ashes and pyroclastic and minor intrusive, and are often called metabasic rocks or simply metabasites. Amphibolites of andesitic composition rather than basaltic composition are plentiful in some belts (Wright et al., (1985)).

CHAPTER THREE

3.0 MATERIALS AND METHODS

3.1 Materials Required

Topographic map, Aeromagnetic map, Satellite image of the area, ILWIS software, Sigma plot software, Surfer 8 software, Microsoft Excel programme, geologic hammer, masking tape, marker, measuring tape, compass-clinometers, GPS, sample bag, field notebook.

3.2 Exploration Method

Pegmatite exploration techniques can be divided into two types: grassroots and advanced exploration (Selway et al, (2005)). Grassroot exploration is chosen because it entails mapping a region looking for pegmatite dikes and to evaluate their mineralization potential. Grassroots exploration includes examining the regional zoning of fertile granites and pegmatite dikes, and using bulk whole-rock composition and bulk K-feldspar and Muscovite composition to determine the degree of fractionation of the granite and pegmatite, and identifying the presence of Ta minerals. Grassroots exploration also includes bulk sampling of metasomalized host rocks, involving chip samples, channel samples and drill core samples.

3.2.1 Field Methods

The field work and mapping was conducted on a scale of 1:50.000 (because of the size of the area) to delineate the different rocks (granitoids and metamorphic rocks) and granitic pegmatites within the research area. Accessibility was rather difficult because the area is rocky and has very thick vegetation cover but most of the exposed pegmatite bodies mapped were located on elevated areas (Fig. 1). Only areas of granitic intrusions that were located on topographical map, aeromagnetic map and satellite image were earmarked for the present work.

3.2.1.1 Sampling

The field work was conducted for two months; two weeks in November 2005, two weeks in February 2006 and then subsequent sporadic visits in April and June, 2007. 64 samples of rocks (Granitoids, migmatites and amphibolite) and mineral separates (feldspars and muscovite) from pegmatite dykes are collected from different parts of the research area (Fig. 1 and plate 1) based on textures, colours, structures and mineralogy and placed in sample bags and taken to the laboratory. The GPS readings of sampling points were taken and compared with the similar readings on the topographical map of the area.

3.2.2 Laboratory Methods

3.2.2.1 Field Analysis

The field analysis entails developing the area in both two and three dimensions using the longitude, latitude and elevation readings taken by GPS in the field and the contour values on the topographical map in order to identify prospective areas and also establish the field relationships of the rocks. The longitude and latitude readings were tabulated and plotted using Surfer 8 software to produce the area in two dimensions while longitude, latitude and elevation readings were tabulated and plotted into the Sigma-plot software to produce the area in three dimensions. The satellite image of the area was used to locate the outcrops, study remote areas and develop the structural trends using ILWIS software which was plotted on the rosette diagram (Fig. 41). The aeromagnetic map of the area (Fig. 40) was used as layers on the satellite image (Plate 1) on the same scale to identify the low magnetic anomalous areas which are perceived to host the fertile granites and the pegmatites within amphibolites and migmatites. The field analysis was used to develop the lithological and field relationships.

3.2.2.2 Sample Preparation

The 64 samples were regrouped into 33 and were further apportion into two parts; first part was taken to laboratory for thin section and the second part was ground and sieved using -60mesh and placed in small sample bags and sent to laboratory for geochemical analysis at Technical University, Berlin.

3.2.2.3 Sample Analysis

33 samples of representative rocks and mineral groups (12 rocks, 10 feldspars, 9 micas and 2 tourmalines (groups)) were analysed using Atomic Absorption Spectrometry (National Metallurgical Center, Jos) and X-Ray Flourescence (Technical University Laboratory, Berlin, Germany) for major and minor elements and Instrumental Neutron Activation Analysis (National Center for Energy Research and Training, ABU Zaria) for trace elements. The total number of Granites used were eight. The feldspars and micas were extracted from pegmatite dykes.

3.3 Conceptual Model Theories

The model for the origin of pegmatite has always been very controversial but some geologists and pegmatologists have come up with suggestive models for the evolution of pegmatite in all their environments of occurrence. In any case, model should incorporate the granitoids and their adjoining pegmatites in order to buttress on their geochemical, mineralogical and economical relationship and peculiarities and whether or not they were related to any orogeny.

There is a general consensus that granitic pegmatites originate from residual fluid that emanates after the crystallization of granitoids but the controversy lies in establishing the origin of the granitic melt, composition of the granitic melt, the time it takes as melt; whether or not there was equilibration after partial melting and what happen to it prior to crystallization. All this information collectively explains the geochemistry, mineralogy, structures and the cogenetic relationships. There are basically two positions for the origin of pegmatite;

3.3.1 Past:

The first attempt to propose an origin for pegmatite was made by Brogger, (1890), suggesting that “Distinctive feature of pegmatite arises from the interplays of existing silicates melt and water vapour”. This view was held up to the end of 19th century. This model was letter ascribed to Jahns and Burnham, (1963). According to this model “Segregation of major alkalis can occur in significant degree when pegmatite magma becomes saturated with volatile constituents i.e. if both silicates melt and vapour is present in the system” (Jahns and Burnham, (1963)). In the 1970s and 80s, the most widely acceptable model of pegmatite genesis was that proposed by Jahns and Burnham, (1969) who proposed that “pegmatite formed by equilibrium crystallization of coexisting granitic melt and hydrous fluid at or slightly below the hydrous granite liquidus” (in Simmons, (2007)). This group held the view that pegmatite because of their pegmatitic texture crystallized slowly like normal granite.

3.3.2 The Present

More recently, the detailed mineralogical and geochemical investigations of Petr Cerny and the experimental studies of David London have led to a much improved understanding of the details of pegmatite genesis (Simmons, (2007)). Cerny’s (1991a) pegmatite classification scheme (Tables 1& 2), which is a combination of emplacement depth, metamorphic grade and minor element content, is now widely used (Simmons, (2007)). Almost all recent pegmatite descriptions classify pegmatites according to its Lithium-Cesium-Tantalum (LCT)– families and Niobium-Yttrium-Fluorine (NYF) - families, types and subtypes. The classification provides insight into the origin of the melts and relative degrees of fractionation (Tables 1 and 2).

The role of fluxes in the crystallization of pegmatites has been demonstrated by London’s experimental work which has shown that water saturation is neither necessary nor likely in the early crystallization of pegmatites (London, (2005)). Other fluxes such as B, F, P and Li in addition to H2O play a critical role in the formation of rare element pegmatites by lowering the crystallization temperature, decreasing nucleation rates, decreasing melt polymerization, decreasing viscosity, increasing diffusion rates, and increasing solubility (Simmons el at. (2003); London., (2005)). The fluxes act as network modifiers that prevent or hinder the formation of nuclei and increase the diffusion rates of ions to the few nuclei that do survive and begin to grow (Simmons, (2007)). These two effects combine to facilitate ion migration over greater distances and promote the growth of the few nuclei that do manage to form, resulting in fewer, much larger crystals (Simmons, (2007)). Until recently, pegmatites were widely believed to be products of extremely slow cooling. Studies of the Harding pegmatite, New Mexico, by (Chakoumakos and Lumpkin, (1990), the Himalaya—San Diego pegmatite dike system, Mesa Grande, CA, by Webber et al., (1997; l999) and the Little Three dike, Ramona, CA, by Morgan and London, (1999) all show that these pegmatite bodies cooled to their solidus in days to months. Cerny, (2005) has recently suggested that the very large Tanco pegmatite also solidified quite rapidly, in decades to a few hundred years (Simmons, (2007)).

Table 1: Classification of Pegmatites of the Rare-Elements Class (Cerny, 1991a)

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Table 2: The Three Petrogenetic Families of Rare-Element Pegmatites (Cerny, 1991a)

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These proposed cooling rates of rare element pegmatites are radically more rapid than previously believed and represent an extraordinary contradiction of the paradigm that large crystal size requires long cooling times. London’s constitutional zone refining model of pegmatite evolution involves disequilibrium crystallization from a flux—bearing granitic melt that is undercooled by about l00—300oC (Simmons, (2007)). The melt is not necessarily flux rich and the presence or absence of an aqueous vapour phase is not required (London, (2005)). A lag time between cooling and the initiation of crystallization produces a supersaturated melt. When nucleation and crystallization commence, excluded fluxes accumulate in a boundary layer ahead of the crystallization front (London, (2005)). The solidus of’ the boundary layer is lowered by the fluxes and, as crystallization continues, this boundary layer liquid becomes progressively enriched in fluxes, water and other incompatible elements relative to the bulk melt composition. Boundary layers advancing from the wall zones inward may merge especially in thin dikes. In the final stages of crystallization an aqueous vapour phase may evolve, giving rise to the formation of miarolitic cavities and evolved suites of pegmatitic minerals (London, (2005)).

These recalculated cooling rates are nothing short of revolutionary because most textbooks and dictionaries define pegmatite as the products of very slow cooling (London, (2005)). Geologist have presumed that a positive correlation exists between the size of a crystal and the time frame in which it grows; therefore in comparison with the much finer grain size of slowly cooled granite plutons, crystal of pegmatitic dimension should require geologic eons to form(London,(2005)).

CHAPTER FOUR

4.0 RESULTS

4.1 Petrographic Studies

Three main granitoids were identified representing three batholiths (biotite-muscovite granite (BMG), albite granite (AG), and granite aplite (GA)). The granite aplite is smaller and most of the pegmatites are emplaced around it. The batholiths are collectively called granitoid because they contain rare elements higher than the normal granite that is barren. The granitoids are homogenous in composition so samples were taken from fresh unweathered portions. When sampling the pegmatite dikes for the thin section, the monomineralic areas were avoided because one crystal could cover the whole slide. So areas that have relative combination of mineral growth were targeted within the dyke. The petrographic studies were used to establish the field relationships, degree of undercooling of the liquidus through changes in orientation and colour of rock-forming minerals and also the degree of the development of the pegmatitic textures. The enrichment of rare elements in both feldspars and micas give them additional coloration (Selway et al., (2005)). Also the petrographic study of host rocks (in this research amphibolites and migmatites; Fig. 52) of pegmatites is also very important, because they contribute a lot in the mineralogy and chemistry of pegmatites, especially at their margin and also host metasomatized zones. For this purpose, samples of the hosts of pegmatites from various locations were collected to see the possible mineralogical changes in thin sections.

4.1.1 Macro and Microscopic Studies

The hand specimen description and petrographic studies were merged together for each representative sample for easy comparison and interpretation.

4.1.1.1 Granitoids

Biotite-Muscovite Granite (BMG)

Samples of granitoid (biotite-muscovite granite (BMG)) were taken at and around Kogun Dutse (Fig. 1; appendix1, Plate II). It is medium to coarse grained. It appeared to be two mica granitoid; quartz, albite, muscovite and biotite are seen in hand specimen using handlens.

Under plane polarized light (PPL), only quartz is colourless but muscovite, biotite and opaque minerals appeared to take up colours. Biotite shows brown colour with perfect one directional cleavage and also hosting pleochroic halo of a fragment of quartz. Muscovite displayed faint multicolours (blue - purplish) with a faint one directional cleavage. The opaque minerals appeared dark (appendix 1. plate II.Ib). Under cross polarized light (CPL), quartz display 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 Muscovite turn to multicoloured (blue-purplish-green) lath as the stage is notated. The opaque minerals remain dark (appendix 1, Plate II.Ia). In some parts of the slide, muscovite shows a clear navy blue colour embedded on albite matrix while the albite matrix partly shows perfect lamella twining in one direction and partly looking metasomalized. Fragment of prismatic crystals of tourmaline are also visible which all pointing towards the NE direction from SW direction (appendix 1, Plate II.III)

Albite Granite (AG)

Samples of granitoid (albite granite (R2 group)) displaying the coarse grained to pegmatitic texture. Only quartz, albite and little muscovite are seen even with handlens (appendix 1, Plate III).

Under plane polarized light, Plate III.Ib shows only two minerals (Muscovite and Albite) in the view because the rock is pegmatitic in texture. The thick lath of mica show blue muscovite containing multicolour mica, and also development of purplish green colours, and the plagioclase display traces of one directional lamellae twining on its white colour. Under cross polarization (CPL) (Plate III.Ia), the blue muscovite, and purplish and green colours persisted on the mica lath, and only exchange position as the stage is rotated, but plagioclase shows dark grey extinction while quartz shows medium grey extinction within the albite matrix.

Granite Aplite (GA)

Samples of granitoid (granite aplite (GA)) displaying a fine grained texture. It looks aplitic and is poorly exposed and partially weathered. Quartz and feldspars are fine (appendix 1, Plate IV).

Under plane polarization (PPL), Plate IV.Ib displaying blue muscovite on one side and the well developed purplish green mica on the other side. The multicoloured mica displayed a faint one directional cleavage (at the center of Plate IV.Ib). Other minerals were colourless. Under cross polarization (CPL)(appendix 1, Plate IV.Ia), all the primary minerals display their interference colours; blue muscovite persisted as blue with rims of purplish brown colours while the multicoloured mica maintain the purplish green interference colours which exchange position as the stage is rotated. Quartz display white to medium grey interference colours within albite matrix while the albite displays white to dark position of extinction as the stage is rotated.

4.1.1.2 Host Rocks

There are two host rocks (amphibolites and migmatite) for the granitic oegmatites in the research area. The occupy more than 70% of the area;

Amphibolite (AMP)

Hand specimen of the metasomatized portion of the host rock (amphibolite) displaying patches of felsic pegmatitic minerals as white spots along the weak foliation planes of amphibolites (appendix 1, Plate V).

Under plane polarized light (ppl), on plate V.Ib, the minerals on the whole slide looks cloudy and white except the stunted brown biotite bleb and the elongated brown hornblende. The cleavages are not clear so their structures and colours were the only yardstick for identification. Hornblende shows elongated structure with dark brown colour while biotite shows light brown colour with a stunted and thick bleb. The whole groundmass looked bluish white which could be assimilation of muscovite and albite (Plate V.Ib), Under cross polarized light (CPL) biotite and hornblende maintain their dark brown colour while in the groundmass, portion of plagioclase appeared distinct from portion of blue muscovite (Plate V.Ia).

Plate (V.IIb) is another similar slide, under plane polarized light (PPL), faint multicoloured mica, biotite and hornblende were visible within the whitish cloudy matrix because they show different reliefs. Multicoloured mica display low relief with faint purplish green, while biotite and hornblende shows a good relief. They are all pleochroic. Under cross polarized light (CPL), (Plate V.IIb) fragments or remnants of blue muscovite appear and the other multicoloured mica only shows increase in intensity of purplish green colours. Biotite maintain its dark brown colour and goes into extinction of very dark brown colour as the stage is rotated while hornblende also showed medium brown colour. The albitized portion also appeared cloudy white with tints of colours of other minerals.

Migmatite (MG)

Hand sample of migmatites (appendix 1, plate. 6) coexisting with the amphibolite (host) in the field. There are traces of layering of the felsic and mafic minerals but it is difficult to identify distinct minerals in hand specimen. These are close to the metasomatized areas.

Under plane polarized light (PPL), (appendix 1, Plate VI.Ib) 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. Other minerals (quartz and feldspars) were colourless (appendix 1, Plate VI.Ib). Under cross polarized light (cpl), (Plate VI.Ib) biotite and the multicoloured mica maintain their purplish green and brown 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 twining that goes into extinction at different angles due to orientation (Plate VI.Ib). Plate VI.II shows plagioclase with clear albite twining been affected partially by metasomatism.

Plates VI.IIIa and VI.IIIb shows another part of migmatite displaying large bleb of the multicoloured mica changing colour from purplish green (under plane polarized light) to greenish purple (under Cross Polarized Light). The plagioclase shows the development of open, straight and parallel one directional crack through a thin film of the multicoloured mica.

4.1.1.3 Feldspars

There are three representative feldspar groups. Hand samples correspond to the thin sections; Microcline perthite, albite antiperthite, albite;

Microcline Perthite (group 1)

Hand sample of feldspar (appendix 1, Plate VII). It is fine grained to glassy. It is reddish and is apparently fresh and unweathered. The reddish mineral disseminations could be K-feldspars.

Under plane polarized light (PPL), the minerals display no colouration and so the slide contain only anisotropic minerals. Under cross polarized light (CPL), the sample shows prismatic crystals, most of which display black-white band of colours. This is a perthitic texture. The dark thick bands of the prismatic crystal are the K-feldspars, while the thin white bands are the position of the exsolved plagioclase (Plates VII.I and VII.II), the bigger bands of K-feldspar shows interference colours of dark yellow and black. The blue to pink disseminated fragments are muscovite fragments. The muscovite shows inner navy blue colours and purplish rims (Plate VII.I and VII.II). This is microcline perthite displaying random orientation of crystals.

Albite Antipethite (group 2)

Hand sample of feldspar (Plate VIII) displaying multi colors (red, blue and yellow) of disseminated minerals. The sample is medium to fine grained with a combination of many coloured minerals.

Under plane polarized light, the minerals appear colourless with only traces of the perthitic texture. Under cross polarized light (CPL) (Plates VIIIa and VIII b), the sample display parallel thick light bands and dark thin bands. The thick bands represent plagioclase while the thin bands represent the K-feldspar. The plagioclase bands show blue, brown, yellow and white interference colours on the different bands due to assimilation of blue muscovite. The bands exchange the colours as the stage is rotated.

Plates VIII.II and VIII.III shows perpendicular arrangements perthitic texture which shows relative organization compared to the microcline perthite. This is albite antiperthite with evidence of metasomalism at the edges (Northern and eastern edges) under cross polarized light (CPL).

Plates VIII.IV and VIII.V shows the texture of albite antiperthite bands that are partially metasomalized. The metasomatism appeared to remove or annulled the perthitic textures of the feldspars. The bluish colouration appeared to be imposed by reaction of muscovite and the invading metasomatic fluid.

Albite (group 3)

Plate IX is a plagioclase lath (under cross polarized light) covering the whole slide and displaying perfect lamellae twinning. This suggests a primary albite that is quite distinct from the massive cloudy plagioclase that forms from albite metasomatism.

4.1.1.4 Mica

The only common mica is muscovite and were grouped into three categories based on their shapes;

Coarse Muscovite (group 1)

The coarse muscovite samples (appendix 1, Plate X) were taken from within the pegmatite dykes associated mostly with plagioclase and quartz in the field. The hand sample display plagioclase-muscovite association within the pegmatite dike. In some parts they look mixed up while in other parts they are discrete.

Plate X.Ia display the muscovite-plagioclase association under cross polarized light, muscovite display blue interference colours with brown edges while plagioclase display a faint characteristic lamella twining. Plates X.Ia and X.Ib display plagioclase groundmass hosting muscovite and purplish green mica under plane and cross polars. The plagioclase display white to grey colours with a faint lamella twining as the stage is rotated. Muscovite was colourless under plane polarized light, and it display blue colouration under cross polarized light. The coloured mica (purplish green) persisted under plane and cross polarized light with high relief.

Plates X.IIa and X.IIb is another part of the same coarse muscovite displaying multicoloured mica and biotite on quartz and plagioclase matrix under cross and plane polarized lights. The multicoloured (purplish green) mica and the brown biotite were pleochroic and show high relief under plane polarized light while plagioclase and quartz were colourless. Biotite hosts a big pleochroic halo of quartz. The purplish green mica and brown biotite persisted in their colours (purplish green and brown) under cross polarized light but with higher intensity,

Plates X.IIIa and X.IIIb show the same relationship at close range displaying a thick bleb of coloured mica (purplish green) with little brown biotite under cross and plane polarized light. The persistence of the colouration is considerable. This conforms to the coarse texture of the muscovite in the pegmatite sample.

Muscovite (group 2)

Hand sample of well developed muscovite book (appendix 1, Plate XI) exposed at Farin Hawa (road cut) (Fig. 1). It looks mixed up a little with quartz and plagioclase because it is not loose. Plates XI.Ia, XI.Ib, XI.IIa, XI.IIb and XI.III all display a clear relationship between blue muscovite, muticoloured mica and the purplish green mica. The contact between them is transitional especially on plates XI.Ia, XI.Ib, XI.IIa, and XI.IIb which suggest transformation probably through enrichment of rare elements.

Recrystallized Medium Muscovite (group 3)

The sample of recrystallized medium bleb muscovite (appendix 1, Plate XII) was taken from distal pegmatite dykes. They contain more quartz, little plagioclase and the muscovite blebs are medium size. The large pegmatite dyke hosting this micaIt was found cross cutting a migmatite ridge. It was exclusively found at the areas of intersection or multiple intrusions. They appeared recrystallized.

Plates XII.Ia and XII.Ib display quartz and plagioclase groundmass coated with thin film of blue muscovite within some part of the sample under cross polarized light. Purplish green mica is well developed and it appeared to be forming directly from biotite which intersects it at right angle.

Plates XII.Ia, XII.Ib, XII.IIa, XII.IIb, XII.IIIa and XII.IIIb all display similar transitional relationships between blue muscovite and purplish to green mica. The transformation from blue muscovite through multicoloured to purplish green mica is clearer in plates XII.IIa, XII.IIb, XII.IIIa and XII.IIIb because of the vivid transitional boundary between them on one side and the presence of the purplish green mica within the blue muscovite on the other side especially in plates XII.IIIa and XII.IIIb.

4.1.1.5 Tourmaline

There are two groups of tourmaline which are very fine and poorly developed as such studying them under the microscope was better. The first group (T1) was found at contact of the intruded pegmatite and the host (migmatite), while the second group (T2) was found in the metasomatized zone within the other host (amphibolite);

Tourmaline (group T1)

Plate XIII shows the hand sample of aggregate of small prismatic crystals of black to purple tourmalines cemented by quartz and plagioclase. The tourmaline crystals are randomly oriented and poorly developed. It occurs at the contact of an intruding pegmatite dyke and the migmatite. It is called contact tourmaline after its origin.

Plate XIII.Ia display the contact tourmalines under cross polarized light (CPL), The tourmalines here are poorly developed and mostly stick to themselves as aggregate of tiny prismatic crystals. They change in colour from blue to brown. Under plane polarized light, (appendix 1, Plate XIII.Ib) the tourmalines show low relief as such their outline could be clearly seen. They seem to have a relationship with biotite.

Plate XIII.Ic shows some of the tourmaline crystals are well developed and they displayed colour zonation (green colour at the core, purple to brown color at the rim). Some of the tourmaline crystals are destroyed and transformed to biotite.

Tourmaline (group T2)

Plate XIII.II is slide of sample from the tourmalinized portion of the metasomatized zone within amphibolites .In hand sample; they are very small prismatic crystals with black to purplish colours.

Under Cross Polarized Light (PPL), (appendix 1, Plate XIII.II) displays metasomatized portion of the host rock (amphibolites) hosting these prismatic tourmaline crystals. They were poorly developed. Biotite is also associated with them in this metasomalized zone. They show light brown coloration with very good relief. This is a product of metasomatism. Plate (XIII.II) also shows that most of the holmequisite crystals are brown to black in colour while the underdeveloped tourmaline crystals display light blue colours. Biotite maintains its brown colouration.

4.2 Geochemistry

The overall geochemical results of total of 33 samples representating the rocks (biotite-muscovite granite (BMG), albite granite (AG), granite aplite (GA), amphibolites (AMP), migmatite (Mg))) and pegmatites mineral groups

(feldspars, micas and tourmalines) are presented in the following tables;

Table 3: Major and Minor Elements in Granitoids

Abbildung in dieser Leseprobe nicht enthalten

(biotite muscovite granite, albite granite and granite aplite)

CIPW-NORM

Abbildung in dieser Leseprobe nicht enthalten

CIPW Normative: q = quart; c = corundum; or = orthoclase; ab = albite; ap = apatite.

derived from the data of Granitoids

DI = Differentiation Index calculated from normative minerals

LOI = Lost on Ignition recorded from the laboratory

A/CNK = Aluminum Saturation Index calculated from chemical data

BMG = Biotite-Muscovite Granite; AG= Albite Granite; GA= Granite Aplite

Table 4: Major and Minor Elements in Host Rocks

(Amphibolite and Migmatites)

Abbildung in dieser Leseprobe nicht enthalten

AMP4 = Amphibolite host from the metasomatized zone Mg5-8 = Migmatite Host

Table 5: Trace Elements in Granitoids and Amphibolite Host

(biotite muscovite granite, albite granite and granite aplite)

Abbildung in dieser Leseprobe nicht enthalten

BMG1 – 3 = Biotite-Muscovite Granite Groups

AG1 – 3 = Albite Granite Group

GA = Granite Aplite

AMP4 = Amphibolite host (metasomatized zone)

Table 6: Major and Minor Elements in Feldspars

Abbildung in dieser Leseprobe nicht enthalten

F5, F1, IF3 = Microcline perthites

F10, F7 = Albite antiperthite

F3, F4, F6, F2 = Plagioclase from albitized portions

IF1 = Mainly quartz

LOI = Lost on Ignition recorded from the laboratory

A/CNK = Aluminum Saturation Index calculated from chemical data

Table 7: Trace Elements in Feldspars

Abbildung in dieser Leseprobe nicht enthalten

Table 8: Major and Minor Elements in Micas (muscovites and biotites)

Abbildung in dieser Leseprobe nicht enthalten

LOI = Lost on ignition recorded from the laboratory

Table 9: Trace Elements in Micas (muscovites and biotites)

Abbildung in dieser Leseprobe nicht enthalten

Table 10: Major and Minor Elements in Tourmalines

Abbildung in dieser Leseprobe nicht enthalten

Table 11: Trace Elements in Tourmaline Groups

Abbildung in dieser Leseprobe nicht enthalten

4.2.1 Major and Minor Elements Geochemistry

The presence of common rock-forming minerals (quartz, feldspars, mica) with elevated contents of rare elements in fertile granites is often the first clue in exploring for blind and buried pegmatite deposits (Selway et al., (2005)). Fractionation of a granitic melts is an important process for concentrating incompatible elements (Selway et al., (2005)). Bulk whole rock analysis is an excellent method to distinguish between barren and fertile granite and to evaluate the degree of fractionation of the rare-element pegmatites (Selway et al., (2005)). Regional bulk sampling and analysis of granites may provide a vector towards mineralized pegmatite dykes, because fertile granites become more enriched in rare elements closer to the pegmatite dyke (Selway et al., (2005)). Bulk whole-rock analysis can also be used to identify to the presence of Tantalum mineralization in a rock (especially aplites), because Ta-oxide minerals tend to be very small and difficult to identify with the naked eye (Selway et al., (2005)). .Most of the major and minor compatible elements per take in the normal fractionation process that produce the residual fluids that form the pegmatites. The seven selected granitic samples (BMG1, BMG2, BMG3c, AG1, AG2, AG3 & GA) were group into three granitoids (biotite-muscovite granite, albite granite and granite aplite) were considered for their vivid contrasting shapes, mineralogy and texture which may correspond to different stages of a dynamic evolution considering the fact that Ferre et al., (1998) reported that “elongate plutons are often syntectonic and rounded ones generally post-tectonic”;

4.2.1.1 Interpretations of CIPW norm

The chemical data was used to calculate the CIPW norms for all the granitoids using XM norm software. The interpretations of the result are as follows;

Biotite-Muscovite Granite (BMG)

This group contain a reasonable quantity of normative quartz (BMG1 = 25.81% CIPW norm, BMG2 = 28.75% CIPW norm & BMG3 = 25.76% CIPW norm) and so the average normative quartz for biotite-muscovite granite is 26.77%CIPW norm. Normative orthoclase (BMG1 = 31.91% CIPW norm, BMG2 = 32.26% CIPW norm & BMG3 = 31.50% CIPW norm) with an average of 31.89% CIPW norm which predominates over normative albite (BMG1 = 29.46% CIPW norm, BMG2 = 29.68% CIPW norm, BMG3 = 33.54 % CIPW norm) with an average of 30.89% CIPW norm (Table 3). This suggests that the area is mostly granite and some parts have slightly started transforming to granodiorite. Biotite is present but its composition add up to hypersthene and that led to the proliferation of many minor minerals like apatite, ilmenite, wollastonite, enstatite and ferrosilite according to CIPW norm procedures. The normative corundum in the granitoids (BMG1 = 4.00% CIPW norm, BMG2 = 2.15% CIPW norm and BMG3 = 1.68% CIPW norm) with an average of BMG = 2.62% CIPW norm are high enough to produce a rich melt. Biotite-Muscovite Granite therefore could be the most primitive and partially differentiated. Apatite (BMG1= 0.77% CIPW norm, BMG2= 0.41% CIPW norm, BMG3= 0.24% CIPW norm) with an average of 2.98% CIPW norm is adequate for the evolving melt.

Albite Granite (AG)

In this group, the quantity of normative quartz (AG1 = 33.00 % CIPW norm, AG2 = 30.76 % CIPW norm, & AG3 = 26.75 % CIPW norm) with an average of AG = 30.17 % CIPW norm has increase critically and that could be due to crystallization of orthoclase in biotite-muscovite granite. Normative orthoclase is as low as AG1 = 6.56 % CIPW norm, AG2 = 19.56 % CIPW norm & AG3 = 21.33 % CIPW norm with an average of AG = 16.15 % CIPW norm which is lower than normative albite (AG1 = 41.72 % CIPW norm, AG2 = 41.72 % CIPW norm & AG3 = 42.82 % CIPW norm) with an average of AG = 42.09 % CIPW norm, may be due to sudden saturation brought by removal or crystallization of orthoclase in biotite-muscovite granite. . The normative corundum also increase (AG1 = 9.50 % CIPW norm, AG2 = 3.20 % CIPW norm, & AG3 = 3.19 % CIPW norm) with an average of AG = 5.3 % CIPW norm compared to that of biotite-muscovite granite. Apatite (AG1 = 2.12 % CIPW norm, AG2 = 1.32 % CIPW norm, & AG3 = 0.94 % CIPW norm) with an average of AG = 3.39 % CIPW norm act as a fluxing agent besides other rare elements and volatiles. This suggests magmatic albitization than a post magmatic alteration process.

Granite Aplite (GA)

In this group, there is relative decrease in normative quartz (GA = 18.68% CIPW norm) compared to albite granite but the quantit y is high enough to consider the rock granitic. The quantity of normative orthoclase (GA =16.15% CIPW norm) has decreased and that suggest development of more muscovite prior to extraction of residual fluid. Normative albite (GA = 53.82 % CIPW norm) is highest here probably because the system is gradually losing temperature and albite is the stable species at lower temperature. This also suggests magmatic albitization rather than post magmatic albitization alteration process. The quantity of normative corundum (GA = 5.50 % CIPW norm) is also high and reasonable for an involving fractionating melt. Apatite (GA = 1.73 % CIPW norm) is also high and the quantity is reasonable as a fluxing agent besides other rare elements and volatiles.

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