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Earth Sciences Research Journal

Print version ISSN 1794-6190

Earth Sci. Res. J. vol.12 no.1 Bogotá Jan./June 2008

 

MINERALIZATION CONTROLS AND PETROGENESIS OF THE RARE METAL PEGMATITES OF NASARAWA AREA, CENTRAL NIGERIA

Akintola, O.F.1 and Adekeye, J.I.D.2
1 Raw Materials Research and Development Council, P.M.B. 232, Garki, Abuja, Nigeria. Fax: 234 9 4136034 E-mail: akintolaolatunde@yahoo.com
2 Geology and Mineral Sciences Department, University of Ilorin, P.M.B. 1515, Ilorin
E- mail: adekeye2001@yahoo.com

Manuscript Received May 11, 2008. Accepted for publication June 19, 2008.


Abstract

The pegmatites of Nasarawa area occur in the central part of Nigeria. They are mainly hosted by phyllonites in a NNE-SSW trending shear zone lying east of some foliated Pan-African and West of Jurassic Afu Complex Younger Granites. A geological mapping of the area was followed by petrographic and mineralogical studies of selected rock and mineral samples. A total of 72 samples consisting of 25 rocks, 22 feldspars and 25 white micas were analyzed for various elements.

The pegmatites are peraluminous and are genetically linked to the late Pan-African leucogranite with the shear zone. The Pan-African granites have very low REE abundances and non-chondritic ratios of Zr/Hf and Y/Ho and low Nb/Ta ratios indicating crystallization from a liquid-rich melt. Barren pegmatites are closely associated with the primitive hornblende biotite Pan-African synorogenic granites while Sn- Nb - Ta mineralized granites are correspondingly enriched in pegmatites spatially associated with Pan-African synorogenic granites with enhanced values of rare lithophile elements such as Rb, Cs, Mn, Sn and Nb-Ta. The primary control of rare metal mineralization in the pegmatites is the composition of the source rock since the Ta-Nb-Sn-Li-Be-W mineralized pegmatites crystallized from fluid (H2O-B-P-F) rich melts.

It is hereby proposed that the late Pan-African tectonic granite which is parental to the highly mineralized pegmatites in this area originated from anatexis of undepleted mica-rich metasediments at depth, followed by a magmatic fractionation of the fluid rich melt as it ascended through reactivated ancient fractures. The heat for the partial melting might have been supplied mainly by the reactivation of ancient fractures, which controlled the emplacement of the fertile granites and the related pegmatites.

Keywords: Pegmatites, Nasarawa, shear zones, mineralization, anatexis, magmatic fractionation, Nigeria.


Resumen

Las pegmatitas del área de Nasarawa se dan en la parte central de Nigeria. Ellas están principalmente emplazadas en filonitas de una zona de cizalla con una tendencia NNE, SSW reposando al E de algunos complejos graníticos como el Pan Africano joven y al W el Complejo Jurasico AFU. Un mapa geológico del área fue seguido mediante estudios petrográficos y mineralógicos de rocas seleccionadas y muestras minerales. Se realizó un análisis de varios elementos sobre un total de 72 muestras compuestas por 25 rocas, 22 feldespatos y 25 micas blancas.

Las pegmatitas son peraluminosas y están relacionadas genéticamente con el leucogranito Pan Africano tardío y con la zona de cizalla. Los granitos Pan Africanos tienen muy bajos contenidos REE y proporciones no condríticas de Zr/Hf y Y/Ho y las bajas relaciones de Nb/Ta indican cristalización a partir de un fundido rico en líquido. Las pegmatitas Barren están muy relacionadas con la biotitas y orblendas primitivas de los granitos sinorogénicos Pan Africanos, mientras que los granitos mineralizados con Sn-Nb-Ta son correspondientes con las pegmatitas enriquecidas espacialmente relacionadas con los granitos sinorogénicos Pan Africanos con valores altos de elementos litófilos raros tales como_: Rb, Cs, Mn, Sn y Nb-Ta. El control primario de la mineralización de metales raros en las pegmatitas es la composición de la roca fuente a partir de las pegmatitas mineralizadas en Ta,-Nb-Sn-Li-Be-W cristalizadas a partir de un fundido rico en fluidos (H2O-B-P-F).

Aquí se propone que el granito tectónico Pan Africano tardío, el cual es padre de las pegmatitas altamente mineralizadas en esta área se originó a partir de la anatexia de metasedimentos no empobrecidos en micas, seguido por un fraccionamiento magmático del fundido rico en fluidos que ascendió a través de fracturas antiguas reactivadas, las cuales controlaron el emplazamiento de granitos fértiles y las pegmatitas relacionadas.

Palabras clave: Pegmatitas, Nasarawa, zonas de cizalla, mineralización, anatexia, fraccionamiento magmático, Nigeria.


Introduction

The pegmatite field belongs to the pegmatites related to syn to late Pan-African tectonic granites occurring in the Pan-African Mobile Belt east of the West African Craton. The field occurs in an area bounded by 70351E - 70 051E, 80081N - 80301N covering an area of 531 km2 (Fig. 1). Nb-Ta-Sn-Be-Li- W primary mineralization is hosted in quartz-feldspar-muscovite pegmatites. Intrusion of the Older Granites into the reactivated Archean to Lower Proterozoic crust of central and southwestern Nigeria have been shown by Rb-Sr whole rock and U-Pb zircon age determinations) to have lasted at least 630 to 530Ma. Pegmatites in the same area have been dated 562-534 Ma (Matheis and Caen-Vachette, (1983) indicating that the pegmatite emplacement occurred at the end of Pan-African magmatic activity.

The Nasarawa pegmatite field is also in close spatial relationship with the granites of Afu Complex, which is the southernmost occurrence of the 1250 km-long belt of ring complexes extending across Niger and Nigeria. Rb/Sr age decreases from Ordovician in Northern Niger to Late Jurassic (141 Ma) of the Afu Complex in Nigeria. The Younger Granites as this later suite of rocks are called, are notably mineralized in Sn and Nb. The two geochemically distinct and economically important types of primary Sn-Nb-Ta mineralization were already recognized by Raeburn (1924).

In Wamba area, Kuster (1990) has shown that the emplacement of late Pan African granites with similar geochemical characteristics with the mineral-ized pegmatites was fracture-controlled and mylonitized along a conjugate set of NE-SW and NW-SE to NNW-SSE- striking faults. In more recent times, new rare-metal pegmatite fields have been discovered both within a NE-SW belt recognized by the earlier workers, Jacobson and Webb (1946) and Wright (1970) as well as other areas already known for gold mineralization northwest of the pegmatite province especially the Kushaka schist belt, the Magami and Maradun areas of northwestern Nigeria, Garba (2003).

All the rare metal and gold mineralizations are associated with prominent regional faults in the Basement Complex of Nigeria. This paper discusses the geology and geochemistry of the Nasarawa area in relation to the source and controls of mineralization of rare metal pegmatites in the Nasarawa area.

Regional Geology

Nigeria lies within the zone of Pan-African reactivation (ca.550 Ma) to the east of the West African Craton, which has been stable since approximately 1600Ma. This mobile belt extends from Algeria across the Southern Sahara into Nigeria, Benin and Cameroon. Rocks of the Nigerian Basement Complex which is part of the Pan African Mobile Belt are intruded by Mesozoic ring complexes of Jos area and overlain unconformably by Cretaceous to Quaternary sediments forming the sedimentary basins. Three broad lithological groups have been distinguished in the Nigeria Basement Complex: A polymetamorphic Migmatite-Gneiss Complex with ages ranging from Liberian (ca. 2800 Ma) to Pan-African (ca. 600Ma). Ages >3000Ma have lately been obtained from some of the rocks (Dada, 2006). Metamorphism is generally in the amphibolite to granulite facies grade. Younger members of this group are N-S to NNE-SSW trending belts of low grade (greenschist to amphibolite facies) metasedimentary and minor metavolcanic supracrustals of Late Proterozoic age. The schist belts which are concentrated in the western half of Nigeria are seldomly found east of 80 E longitude, (Ajibade and Wright, 1989). The schistbelt rocks host the gold and rare metal mineralized pegmatites and veins, which are associated with prominent regional fractures. The Older Granites, which are Pan-African orogeny-related, range from syn- through late to postorogenic granitoids of upper Proterozoic to Lower Paleozoic age (ca. 873-500 Ma). They intrude both the Schist Belts and Migmatite-Gneiss Complex rocks and comprise diorites, tonalites, granodiorites, granites, syenites, gabbros and charnockites.

The end of the Pan-African tectonic event is marked by a conjugate fracture system of strike-slip faults (Ball, 1980). Fault directions have a consistent trend and sense of displacement; i.e. a NNE-SSW trending system having a dextral sense of movement and a NW-SE trending system with a sinistral sense (McCurry, 1971; Ball, 1980). Both sets crosscut all the main Pan-African structures, including older N-S trending shear zones (mylonites) (Ball, 1980; Ajibade and Wright 1989, Kuster 1990, Garba 1996). Other parallel Pan-African fracture systems with structural trends (N300E and N600E) appear to have been precursors to the development of the Cretaceous Benue Trough and its associated volcanics. The pattern of these fracture systems was probably established during the Pan-African orogeny (McCurry, 1971), and the main transcurrent movement probably occurred then - but may well represent lineament of much greater age. Wright (1970) was of the opinion that the regional faults had some influence on the direction of migration of hot spots within the mantle that culminated in the formation of the Mesozoic ring complexes.

Late Pan-African granites parental to rare metal pegmatites and gold-bearing veins are closely associated with the fractures in the Pan African mobile belts (Kuster, 1990; Ekwueme and Matheis, 1995; Garba, 2002; Okunlola, 2005). The pegmatites, both rare metal mineralized and non-mineralized, are associated with the Older Granites. The pegmatites were initially thought to be concentrated in a NE-SW zone extending from Ago-Iwoye in the southwest through Wamba-Jema'a to Bauchi area in the north- east. However, other pegmatite fields have more recently been known around Zuru-Gusau in the northwest (Garba, 2002 and Okunlola, 2005), and Obudu area in the southeast of Nigeria (Ekwueme and Matheis, 1995).

“Younger Granites”- a 1250km-long belt of ring complexes extending across Niger and Nigeria, with the Rb/Sr age decreasing from Ordovician in northern Niger to Late Jurassic in Central Nigeria. These are high level anorogenic volcanic-plutonic ring complexes (Jacobson, Snelling et al, 1964) intruded into the older Precambrian to Paleozoic Basement Complex rocks. Granites overwhelmingly predominate in the province, but in some complexes their emplacement was preceded by basic and intermediate intrusions, ranging from olivine-gabbro to quartzmonzonite and syenite. The basic, intermediate and porphyritic members of the Younger Granites carry Pb-Zn-Cu-Fe sulphide mineralization. The granitic members are mainly peralkaline arfvedsonite granites and the metaluminous to peraluminous biotite granites: These are the commonest and carry most of the Sn-Nb mineralization.

Chemical analysis

Geological maping of the area was followed by petrographic and mineralogical studies of the rocks. Whole rocks chemical analysis of selected representative samples of the granites and the simple graphic quartz-feldspar pegmatites was done using Phillips 1404 automatic X-ray fluorescence (XRP) spectrometer on their powder pellets and glass discs in the Geochimistry Laboratories of the Technical University of Berlin. ICP-MS measurements of Ta, Nb and REEs were performed for 14 samples in Geoforschungs Zentrum Potsdam, using an ELAN 5000A quadrupole ICPmass spectrometer (Perkin- Elmer/SCIEZ), Canada. Details of laboratory procedures used in analyzing some of the samples by ICP-MS are as published by Dulski (2001) in Geostandars Newsletter. Few samples with peculiar assays were analyzed by the XRDmethod to determine their mineralogy. The framed powder samples using Phillips PW 1820 diffractometer in the Technical University of Berlin X-rays were generated at 50kv, 30mA. Analitycal condition for each sample were: 0.02° 2Ø/step, 2.5 seconds per step with analysis completed from 3-80°2 Ø.

Results

Geology and petrology of the area The Nasarawa area comprises metasedimentary rocks (mainly mica schists and sericitized/chloritized phyllonites) intruded by a Pan-African granodiorite/ granite batholith (ca. 600Ma), fracture controlled elongate Late Pan-African granite and pegmatites (Fig. 2). Abutting the mica schist southeast of the area is the Afu Complex of Late Jurassic (ca. 141Ma) composed mainly of biotite granites with minor quartz porphyry. There is a shear zone that trends north-north east in the area within which the rocks are mylonitized. Below is the detailed description of the geology of the area and Figure 2 is the geological map of the area. The schists occur as relics and xenoliths in the Older Granites and pegmatites with thick successions in a north-south trending low-lying area that lies between the Older Granite and granodiorite/tonalite gneisses rock suite in the west and the Jurassic Younger Granite (Afu) Complex in the east (see Figure 2). The schists generally have measurable north-south trending foliations that dip to the east at low angles (250-300). The north-south foliations are interpreted as Pan-African structures superimposed on earlier tectonite fabrics, which sometimes give contorted appearances to the schists.

Compositionally, the rocks range between metamorphosed pelitic to semi-pelitic and psammitic rocks with biotite, quartz and minor muscovite, as the major minerals. Compositional changes related to pegmatites' intrusion are noticeable at the contacts of the pegmatites and the schist. Towards the south of the area, the schist becomes gneissic with appearance of feldspars and pale amphiboles. Accessory minerals found in the schist include opaques (ilmenite and magnetite), sphene and garnet. Tourmalines and apatites are common accessory minerals in the schist at contact zones with pegmatites and in some cases may constitute more than 20% of the rock The tour-malines in the pegmatites' exocontact zones in the schist are usually zoned which shows that they crystallized from highly fluid-rich melts (London and Manning, 1995). Radiogenic haloes are formed around inclusions of radioactive minerals (monazite and zircon) in biotites. The schists as well as other rocks within the shear zone are mylonitized. The mylonitized mica schist-phyllonite is composed of porphyroclasts of biotite and chlorite in a matrix of fine-grained groundmass of muscovite and quartz.

Within the schist at the center of the area, and in proximity to the schist at the northern part of the area are tourmalinites, which are essentially composed of tourmalines and quartz with accessory to minor apatites. Within the schist are fragments of foliated amphibolites that are too small in dimensions to be represented as discrete bodies on the map. The mineral assemblages of biotite, muscovite, and garnet in the schists as well as amphiboles and feldspar porphyroblasts indicate that the rocks must have reached amphibolite grade of regional (Barroviantype) metamorphism, during the Pan African orogenic cycle. The greenschist facies minerals such as chlorite and green biotite, recrystallized fine-grained muscovite and quartz within the shear zone are products of retrograde metamorphism of the rocks by the post-tectonic processes of shearing/ mylonitization that probably accompanied the emplacement of the pegmatites. Coincidentally, the boundary of the shear zone marks the boundary of the zone of occurrences of the mineralized pegmatites within the schists.

The mylonitic micro-textures of these rocks within the shear zone provide evidence of fracturing/ shearing of the rocks. This phenomenon is observable in the leucogranitic samples and schistose samples in which porphyroclasts of biotite, chlorite and quartz are set in a groundmass of recrystallized fine grained muscovite and quartz. The shear zone is occupied by schistose metapelites/metapsammites in a north-northeast belt. Within this belt are also the leucocratic pegmatitic granite. Ocan and Okunlola (2001) have also observed zones of mylonitization in the rocks (granite and schist) associated with the mineralized pegmatites at Angwan Doka, north-east of this area. Similarly at Wamba, about 100kilometers northeast of this area (Kuster, 1990), there are elongated granitic plutons that are partly affected by deformation (mylonitization) along a conjugate set of strike-slip (transcurrent) faults. The emplacement of these granites appears to be fault-controlled and the directions of relative movements are dextral along NE-SW striking faults and sinistral along NW-SE to NNW- SSE striking- faults. It thus appears that there is a regional northeast trending shear/fracture-zone characterized by mylonitization of the rocks coinciding with the zone of mineralized pegmatites, and movement along the faults must have been active before and after the emplacement of the granites and the related pegmatites. Older Granites of batholitic dimensions intrude these schists, which range in composition from hornblende-biotite granodiorite/tonalite gneiss to biotite granites at higher elevations. This suite of rocks appears to represent the first major episode of granite plutonism in the area. While the granodiorite/tonalite gneiss occupies the western part of the area, the biotite granites, which outcrop as inselbergs, occur in the northwestern part of the area. The granodiorites are bly foliated with the quartzo-feldspathic phenocrysts developed into porphyroblasts or augen structures.

In thin sections, the granodiorite/tonalite gneiss consists of quartz, plagioclase, bluish and brownish amphiboles, biotites with accessory titanite (sphene) and apatite. Feldspar and quartz sometimes form wart-like intergrowths -myrmekites. In the biotite granite, quartz, biotite, microcline and plagioclase feldspar are the essential minerals. There may be minor or no hornblende. At the northernmost part of the area, the biotite granite is fluid-rich. Some plagioclase shows some sericitisation and pegmatites close to this granite have enhancement of the rare lithophile elements compared with pegmatites close to a less fluid-rich granite. Granites at the south-central part of the area are smaller bodies than the main phase granites and have some distinct characteristics in their mode of occurrence in the field. The pegmatitic granite in thin section consists of phenocrysts of quartz in a groundmass of felsic quartz, alkali feldspar and white mica, with very little biotite. The quartz phenocrysts are bly deformed showing wavy/undulose extinction in cross-polarized light; and in some cases, they are recrystallized due to shear movement. The rock is however not foliated and the elongated mode of emplacement is obviously controlled by a northeast-southwest trending fault. Some mineralized pegmatites are close to this pegmatitic granite.

Simple pegmatites with mineral assemblage of microcline-quartz and minor plagioclase (albiteoligoclase) with accessory garnet, tourmaline (schorl), biotite and magnetite intruded the biotite granites- granodiorite suites. Within the schists, the pegmatites become richer in muscovites and the rare metal minerals. There is a tendency towards the arrangement of the pegmatites in sub-parallel groups akin to an en-echelon emplacement, and in some cases there are two or more intersecting sets of dykes. A rose diagram plot of the pegmatites indicate two major directions, viz:east-west and north to north north-east. Many of the richly mineralized pegmatites occur as sill-like bodies. The swellings are generally loci of intense albitization and mineralization. While majority of the pegmatites in the study area strike north-east/south-west, some have north-westsouth- east and east-west strike directions. Strike and dip may change even in one dyke, following planes of weakness (joints, fractures and foliation planes) in the country rock. Majority of the pegmatites generally cut across the foliation of the host schists and gneisses. Many of the complex pegmatites display a textural and mineralogical zonation parallel with the walls of the intrusion. A zone of tourmalinization (black tourmalines) within the host rock at the contact with the pegmatites is followed by a prominent zone of quartz-mica margins of the dykes. In the complex pegmatites the marginal facies may be up to two feet or more in thickness and as observed in the Liberia pegmatite with a paragenesis of cloudy (and in some rare cases smoky) quartz, mica, microcline, albite and accessory large crystals of alkali enriched (blue-green) tourmalines and fluorapatite. The mica is coarse-grained and oriented at right angles to the contacts. Within the quartz-mica marginal zone is the quartz-microcline-albite-muscovite-beryl zone. This is followed by a quartz-muscovite-albite- tourmaline- amblygonite-montebrasite zone. At the inner zone, there is albite-fine grained muscovite-quartz (clear and colourless). In Liberia pegmatite, an albite- rich footwall zone with finely disseminated Nb-Ta mineralization was observed. From the outer to the inner zones there is enrichment in Ta, Li and Cs and their ores, and the toumalines become albite with increasing contents of Na and attractive colours. Most of these zones are observable in the complex pegmatites with some minor variations due to variations in their bulk chemistry; at Kilimanjaro, hydrated lithium-aluminosilicates (cookeite) were crystallized (no lithium aluminophosphates was sampled from this pegmatite) with albite, mica and quartz in the inner zone.

Pegmatite-country rock relationships (sharp contacts, unfractured wall rocks, variations in strike, dip and thickness of the dykes) suggest an emplacement level transitional between ductile to brittle host-rock behaviour (Kuster, 1990). Xenoliths of the foliated host rock, quartz-biotite schist, are present in some pegmatites, suggesting that the pegmatites are younger than the schists. The barren simple quartz-feldspar pegmatites found in proximity to the biotite Older Granites at the western part of the area are composed essentially of quartz, microcline-microperthite and minor plagioclase (albite-oligoclase). The minor plagioclase appears to be replacing the perthite with sericite by-product. Garnet, magnetite and tourmaline are accessory minerals observed in the simple pegmatites.

The more complex and mineralized pegmatite deposits occurring in the area show a more pervasive albitization. Some pegmatites show subparallel micro cracks with a large perthite crystal, which are filled with albite and sericite. Such cracks provide the channel ways by which the soda-rich late stage mineralizing fluids deposit the ores of Nb-Ta-Sn- Li-Be. In a favourable environment especially in the middle to inner zones (close to the quartz cores of the mineralized, complex and zoned pegmatites), replacement of microcline by albite is complete, giving rise to the formation of secondary feathery albites and fine-grained muscovite- “gilbertile”. East of the area is the western flank of the Afu Younger Granite Complex. The Complex is the southernmost occurrences of the Nigerian anorogenic ring complexes, which extend through Jos and northwards to the Arid region of Niger Republic. The Complex was dated 144+ 2 Ma (Bowden et al., 1976). It is elliptical in outline and about 50km in maximum diameter. It shows broad similarities (geochemical, mineralogical, etc.) to the other Younger Granite Complexes emplaced during the Early to Late Jurassic (Jacobson et al., 1958 and Macleod et al., 1971).

The Afu Complex is composed mainly of biotite granites with minor quartz porphyry (Imeokparia, 1982). The biotite granites show a somewhat fractionation trend with an enrichment of Nb, Li, F, Sn, in themore evolved albitized granites with low biotite contents. Mineralogically, the biotite granites are composed of quartz, K-feldspar, albite, and biotite, with fluorite, zircon, cryolite, magnetite, hematite and less commonly cassiterite, columbite, thorite, apatite and monazite as accessory minerals.

Geochemistry

In the Older Granites G8, G9 and G18b on the one hand have similar geochemical characteristics, which differentiate them from G25 and G27 (see Table 1). G8, G9 and G18b are calc-alkali granites with higher contents of Ca and Mg. Their K/Rb ratios range between 155 and 261. The lowest value 155 in the range is that of G18b. Field evidence shows that these three granites belong to the main-phase Pan African Older Granites. Sample G18b with the lowest K/Rb ratio as well as Ce among these three samples has relatively enhanced values of Rb-254ppm, W 241ppm, Ta 2ppm, Mn 700ppm, Sn 32ppm, Cs 11ppm, and Nb 36ppm. The spatially associated pegmatites 18P and 18aP have enhaced concentrations of Rb, 917ppm and 1718ppm; Cs, 62ppm and 779ppm, low K/Rb ratios 73 and 40 and correspondingly enriched in the ore elements Sn, 13ppm and 17ppm; Nb, 65ppm and 80ppm; and Ta, 15ppm and 21ppm, respectively. On the other hand, the pegmatite 10P which is spatially associated with the less geochemically evolved Older Granite in the area has low concentrations of the rare elements Rb, 450ppm; Cs, 2ppm; Sn, 9ppm ; and Ta 3ppm and a high K/Rb of 155.

Major elements composition of SiO2 73.16, 71.1; Fe2O3 1.54, 2.02; CaO 0.88, 1.75 and MgO 0.42, 0.5 respectively show that G25 and G27 are more leucocratic than the main phase Pan African Granites. However, trace elements' compositions of the two granites show a lot of differences and indicate different levels of fractionation and possibly origins for the two granites. G27 has very high Ba (1377ppm), Sr (677ppm), Ba/Rb (17.21), and very low K/Ba (22) and Rb/Sr (0.12), which may indicate a metamorphic origin of the rock. The G25 has enhanced Mn (600ppm), Rb (295ppm), Ta (13ppm), Nb (24ppm) low K/Rb, Al/Ga, Zr/Hf and Nb/Ta ratios of 134, 2486, 19.96 and 1.85, respectively. Such high values of lithophile rare elements and low K/Rb, Al/Ga, Zr/Hf and Nb/Ta ratios are characteristic of highly evolved granites parental to rare metal pegmatites (Raimbault et al., 1995). Thus the major and trace element distribution in the sampled Older Granites show that samples G25 and G18 are more highly evolved with LCT geochemical affinities (Cerny, 1991c) than G8, G9 and G27. Both granites also have negative Ce anomaly which may be an indication of oxidizing conditions during rare-metal mineralization (Piper, 1974).

The sampled Afu Complex Younger Granites G4 and G5 are depleted in Ca, Mg and Sr; have high Fe/Mg, and are enriched in Nb, Y, F and Zr, thereby showing the characteristics of the NYF suites (Cerny, 1991c) compared with the Older Granites. They have lower ASI (aluminum saturation indices) and more alkaline. In both the Older and the Younger Granites, Mg, Ti, Ba and Zr are depleted in the granites with enhanced values of Rb and therefore amply depict the degree of magmatic fractionation within the suites (Figures 3 and 4). The granites with enhanced values of Rb are also enriched in Cs and the ore elements of Sn-Nb-Ta. In the Younger Granites, G5 is more leucocratic and coarser grained with less biotite than G4 It also has more enhanced values of the lithophile rare elements like Ta, Nb, F, P, Rb, Mn, Y,U, Cs, Th, Mo, W, and low K/Rb (104) and Al/Ga (2280) ratios. It however has a higher Nb/Ta ratio (4.65) when compared with that of 1.85 of G25.

The Younger Granites have on the average, higher F content that the Older Granites while negative Ce anomaly (very low Ce content) is observed only in the mineralized Older Granites G25 and G18. Kinnaird (1984) and Barchelor (1987) have observed similarly distinct geochemical characteristics in the Older and Younger Granites of Nigeria. The REE concentration (Fig. 5) of pegmatites 10p and 11p show a decrease of an order of magnitude from those of the granite G25 while the REE in the pegmatitic white mica are the least (see Table 2). The REE generally have sub-horizontal to heavy rare earth element- depleted patterns; minerals with the lowest REE abundances have nearly horizontal patterns. REE contents of the granite, G25, which is highest in the samples analyzed, is very low (< 2x chondritic). Such low REE abundances (mostly between 20x and 1x chondritic) with sub-horizontal to heavy rare earth element-depleted patterns are typical of rare metal granites and associated pegmatites, (Cerny, 1991c; Raimbault et al, 1995; and Morteani et al, 1995; Preinfalk et al., 2000). Despite the low REE abundances in the rocks/minerals, evidence of magmatic fractionation is given by the negative Eu anomalies in the pegmatites and muscovites.

tabla 3

G25 shows a rather horizontal REE pattern and slight inflections with minima corresponding to Nd, Gd, and Ho, which indicate a fractionation reflecting the lanthanide tetrad effect (Bau, 1996). The tetrad effect is more noticeable in white micas and the pegmatitic samples with some showing the V-shaped pattern with b negative Eu anomaly. According to Zhao and Cooper (1993), V-shaped patterns indicate an extensive crystal fractionation involving feldspar, biotite and accessory REE minerals such as monazite and Zircon. The extremely negative EUanomalies also correlate positively with the rare-element accumulation (Matheis, 1991). The REE-depleted and Rb-enriched nature of the G25 is also characteristic of peraluminous LCT (enriched in Rb, Be, Ga, Sn, Mn, Li, Cs, Nb, and Ta) granite intrusions (Cerny, 1991c).

Petrogenesis of the Rare Metal Pegmatites

The peraluminous pegmatite granites parental to the rare-metal pegmatites were formed by partial melting of mica-rich metasediments along the regional fracture zones as enunciated by Wright (1970), Matheis (1991) and Garba (2002). Although, the anatexis of the metasediments occurred at deeper levels below the currently exposed surface, evidence of shearing of the rocks at the earth's surface is provided by mylonitization of the rocks within the fracture zone. High heat flow and shear movement along the regional fractures might have contributed significantly to the heat for the partial melting of the metasediments. An evolution of the magma as it ascends through the fractures would be toward an increase of the depolymerizing elements F, P and Al (Raimbault et al., 1995). These elements would depress the liquidus of the magma, thereby reducing the viscosity of the melt while aiding both its flow along the fractures and the extreme fractionation of the elements. Roofward enrichment of rare elements (such as B, Li, Rb, Cs, Ti, Be, Mn, Sc, Y, H, REEs, Sn, Th, Mo, Ta>Nb, and W) can be expected ( Cerny, 1991c) in the LCT granite-pegmatite suite. The magmatic evolution of the melts is towards an increase in the Al, Rb and Cs as documented in granites and pegmatites in the area while evolution from silicate- dominated melt to water-dominated B, F, Li and P-rich fluids is marked: petrologically by common accessory apatites and zoned tourmalines in the pegmatites' exocontact host schists, as well as deuteric alteration, sericitisation and albitization of feldspars in the granitic rocks parental to the mineralized pegmatites. The most complex of the pegmatites in this area belong to the amblygonite subtype of the classification of Cerny (1991b) enriched in P, F, Li, Rb, Cs, Be, Ta>Nb. Geochemically by low Mg, Ti, Ba, Zr and Ce, as well as low Ba/Rb, low K/Rb, Nb/Ta and K/Cs ratios in the pegmatites and the related granites as well as non-chondritic Y/Ho and Zr/Hf ratios, high negative Eu anomaly and lanthanide tetrad effect in the REE distribution patterns as documented in the pegmatitic granite G25.

As already noted by Bau (1996) the rare elements are transported as complexes in such fluid-rich melts. It is also clear from this area that there are two distinct rare metal generating events associated with the Pan African orogeny viz: Enrichment of the rare metals in the fluid-rich and deuterically altered main phase Pan African granite G18 and the spatially/genetically related pagmatites. This granite is the northernmost extension of Older Granites in this area and obviously represents marginal part of the batholith. It is characterized by a widespread b alteration of plagioclase (a replacement of the plagioclase by perthite, and finally albitisation/sericitization). This deuteric alteration resulted from the late metasomatic fluids that mobilized the ore elements, Sn and Nb, and concentrated them in the granite. Such deuteric alteration is marked in the border zones and in the uppermost parts of granitic bodies (Pedrosa and Siga, 1987). Enhancement of the rare metals in the geochemically distinct G25 is fracture controlled and postdates the emplacement of the main phase Pan African granites.

The fact that the pegmatitic granite G25 was emplaced into fractures and again mylonitized after emplacement (see also Kuster, 1990), shows that these fractures were active before and after the emplacement and were probably reactivated during the emplacement of the Younger Granites during the Jurassic. Matheis and Caen Vachettee (1983) have documented biotite ages of 185/183Ma from southwestern Nigeria. Basalt intrusions of 165Ma north of Zaria in northwestern Nigeria, which as inferred by Matheis and Caen Vachette (0p cit) indicate a regionally more extensive thermal event in association with the central Nigeria Younger Granites emplacement. The low P and F contents of the pegmatitic granite G25 (P2O5 0.068%, F35ppm Table 1) may be explained in that the elements which would have been concentrated at the roof of the granite, would at the current level of exposure of the granite have been lost to erosion. It appears, based on the low elevation of the G25 and the outcrop of NE-SW and NW-SE trending tourmalinites, which are most probably related to the pegmatitic granite intrusion in the area that some of the granites parental to the rare metal pegmatites are not yet exposed at the current erosional level (that is they are still lying buried). Similar views were very recently expressed by Garba (2002) who inferred from the studies of gold and rare-metal pegmatite occurrences in the Kushaka Schist belt of North-western Nigeria that the mineralizations are controlled by, postdates the Pan-African tectonism and related to NE-SW and NNE-SSW trending regional fractures. It is worth noting that Pan-African Sn-W bearing quartz-veins occur in the reactivated crust of the central Hoggar and are probably related to a 521Ma old peraluminous differentiated granitic plutons. Helba et al. (1996) also reported higher Ta/Nb ratios in the more differentiated albitized Eastern part of Nuweibi albite granite, in the Eastern Desert region of Egypt. 207Pb/206Pb ratios in zircon from the granite yielded 450 - 600Ma-a post-kinematic Pan-African age.

Discussion

Cerny (1991c) and Douce (1999) have observed that anatexis of mica-rich supracrustal sequences as well as ortho and para-lithologies of their basement in both the classical orogenic cycles and non-orogenic magmatic events give rise to peraluminous granites. These observations are corroborated by high intial 87Sr / 86Sr ratios (0.710 to 723) of the Pan African Older Granites as well as the post-kinematic intru- sives i.e. the Pan African Older Granites (Matheis and Caen-Vachette, 1983; Bertrand et al., 1987), which bly suggest crustal influence in the generation of the late Pan African peraluminous granites. Thus the Older Granites share some geochemical affinities with the LCT suites while the Younger Granites share geochemical affinities with the AYF suites as recognized by Cerny (1991c.)

Kuster (1990) observed that the late Pan African tectonic granites at Wamba (about 100km northeast of Nasarawa) are all subalkaline, peraluminous, and highly siliceous rocks with their peraluminosity more pronounced with increasing differentiation. The major elements Si, Al, K, and Na show only slight variations; only Na is enhanced toward the end of granite evolution. In the course of evolution from the biotite granites through biotite-muscovite granites, muscovite granites to the apogranites, there is a pronounced enrichment of Rb, Li, Cs, Sn, Nb, Mn, and P whereas B is only slightly enhanced. b depletion is evident for Ba, Sr, Zr, Y, La, and Ce together with Ti, Mg, Ca, and Fe. These results support the observation that the rare-metals are related to highly differentiated granitic magmas and represent bly fractionated residual melts rich in silica, alumina, alkali elements, water and other volatiles, lithophile elements, and rare metals, (Cerny, 1991b and London, 1990).

Conclusions

It is conclude that anatexis of mica-rich supracrustal sequences as well as ortho and paralithogies of their basement give rise to peraluminous granites. These observation are corroborated by high initial 83 Sr /86Sr, ratios (0.710 to 723) of the Pan African Older Granitesas well as the Younger Granites which bly suggest crustal influence in the generation of the late Pan African peraluminous granites. Thus the Older Granites share some geochemical affinities with the LCT suite while the Younger Granites share geochemical affinities with the AYF suites.

In the course of evolution fromthe biotite granites through biotite-muscovite granites, muscovite granites to the apogranites, there is pronounced enrichment of Rb, Li, Sc, San, Nb, Mn and P whereas B is only slightly enhanced. b depletion is evident for Ba, Sr, Zr, Y, La, and Ce together with Ti,Mg, Ca, and Fe. These results support the observation that the raremetals are related to highly differentiated graniticmagmas and represent bly fractionated residual melts rich in silica, alumina, alkali elements, water and other volatiles, lithophile elements, and rare metals.

The NE-SW and NNE-SSW regional fractures controlling themineralization are deep seated. Reactivation of the fractures in the Mesozoic probably influence the emplacement of the anorogenic Younger Granites and initiated the formation of Benue Trough in the Mesozoic period. The Benue Trough, which is parallel to the NE-SW trending belt of mineralized pegmatites hosts Pb-Zn-Cu-Fe sulphides, fluorites and barites.

The Afu Complex Younger Granites are more alkaline than the other granites as reflected in their lower A/N+K ratios; they have high Fe/Mg ratios and low TiO2 contents which tend to agree with Lameyre and Bowden's (1982) documentation of the Younger Granites of Nigeria as continental epeirogenic uplift granitoids (CEUG). Peraluminous granites are known, according to Cerny (1991c) to be parental to the granite- pegmatite suites. Trace element studies of the suites also show that extreme igneous fractionation aided by the fluids rich in B, P and F leads to the concentration of the rare metals in the residual melts that form the pegmatites.

Acknowledgements

Financial Assistance for the fieldwork by the Nigerian Government through the Raw Materials Research and Development Council, RMRDC IS GRATEFULLY ACKNOWLEDGED. The German Government through the German Academic Exchange Programme (DAAD) provided financial assistance for the rocks/minerals chemical analyses in the Technical University of Berlin (TUB). We are grateful toDr. G. Matheis of the Technical University of Berlin (TUB) for his assistance on the research and review of this paper. We are also grateful for the assistance of Professor Peter Moller and Dr. Peter Dulski of the Geoforshungs Zentrum (GFZ), Potsdam for their assistance in analyzing the Rare Earth and trace elements of some of the samples.

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