Geology and petrochemistry of Gwon-Gwon pegmatite field Wamba, Nasarawa state

Chapter One

Introduction

1.1 General Introduction

Pegmatites are coarse grained igneous and metamorphic rocks. They represent the end product of magmatic stage 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) and Matheis and Kuster,(1989), Matheis,(1989), Matheis and Cean- Vanchette,( 1983), 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; 1987] which trends from NW to the central North. The pegmatite “belt” in Nigeria is part of the Pan- African reactivation zone which extends in a SW-NE direction for about 400km and finally passes into the Jos ‘Tin fields’ associated with the Younger Granites (fig, 2), Matheis and Cean-Vanchette (1983).

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Fig 1. Distribution of Pegmatites in the Nigerian Tin Province

(Adopted from Matheis, 1987)

Some metallogeetic features of the Nigerian Basement

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Fig.2 Tin bearing Pegmatite zone and other regional features of Nigeria.

(Adopted from Matheis and Caen Vanchette; 1983).

This pegmatite belt in Nigeria (fig.1) is part of the Benin-Nigeria shield, which is the southern extension of the Pan-African reactivation zone (fig. 3a). The Pan-African mobile belt on which Nigeria is located lies between the West African Craton and the Congo craton and at the south of the Tuareg shield [(Fig. 3a), Black 1980]. The Tuareg shield is the southern prolongation of the Pan-African mobile belt (Trompette, 1980).

The Gwon-Gwon pegmatite at Wamba, is the third mineralized pegmatite field in Nigeria (others being at Egbe, Ijero) but the first mineralized in northern Nigeria. 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 4), therefore falls between longitude 8030I to 8o40I east and latitude 9o00I to 9o 071 North covering an area of about 45km2 within the Pan-African reactivation zone.

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

There was no serious exploration to ascertain the economic potential of the pegmatite of the study area (Gwon-Gwon).

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Fig.3(a) Regional geological settings of Nigeria. (Adopted from Black, 1980)

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Fig.3b Schematic Map of West Africa Coast showing Cratonic Areas and Mineralized

Fractures Zones (modified after Burkeetal, 1970; Black, 1980)

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Fig.4 Geological Map of the study area part of Sheet 189 Kurra.

Mining activity started as early as 1912 with the discovery of Tin-bearing pegmatite in the neighboring Jema’a area (approximately 40km northwest of Wamba}, and ceased in the Wamba area in 1960s. The Wamba area is also in close spatial relationship to the economically far more important Jurassic rare-metal mineralization (Sn, Nb) which is related to the anorogenic “Younger Granites” of Nigeria.

This two periods and styles of rare-metal mineralization in Nigeria have been recognized by Raeburn, (1924) and the pegmatites genetically linked to the “Older Granite” (Jacobson & Webb, 1946). This link was established by workers based on field relationships, isotope data studies and a derivation of the rare-metal pegmatites of Southern Nigeria from reactivation of tectonic lineaments combined with partial melting and external fluid supply.

This research work gives geological and geochemical data from the Gwon-Gwon pegmatite field of Wamba, central Nigeria and will also provide information on economic potential of the pegmatite, the petrography and petrology of the pegmatite and geochemistry of the pegmatite of the study area.

All this information will help us to identify other possible economic minerals associated with the pegmatite of the study area besides the cassiterite, columbite and Tantalum that was reported and partially worked on.

1.2 Statement of the Problem/Justification

Extensively the Nigerian Basement complex has been studied, but most of the research on pegmatite has been dealing with the economic parameters of their rare-metal contents (or rare-elements).

The classification of pegmatite in Nigeria into rare-metal bearing pegmatite and barren pegmatite within the Pan-African reactivation zone by Matheis and Cean Vanchette, (1983) and Matheis and Kuster ,(1989), was based on the above fact i.e. the economic parameters/benefits. The areas investigated by such workers (fig.1) were Iregun, Ijero, Egbe and a reconnaissance work at Wamba and Jema’a.

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”.

The least investigated is the Wamba rare-metal pegmatite both in terms of host lithology, petrology and Petro-chemistry. Also, little is known about the adjoining parent granite and its economic potential.

1.3 Objectives of the study

The objectives of the study include:

a. To characterize the Pegmatite based on their petrology, mineralogy
and petrochemical characteristics.
b. To develop the geological map of the pegmatite areas and the
possible mineralized areas.
c. Identify all areas of economic minerals in the study area
within the pegmatite field.
d. To propose the best method of mining such minerals.
e. To know the mining potential of the study area.

1.4 Location, extent and accessibility

The study area is located within Wamba Local Government Area of Nasarawa State and situated SE of Sheet 189 Kurra.It lies precisely within latitude 9o 001N and 90 07’N and longitude 8o 301E and 8o 401E covering a total area of about 145km2 on a map scale of 1:100,000 (fig.4). To the North of the study area lies River Arum and River Gudi all trending in the NE-SW direction draining from the surrounding rock units and cutting across some of the pegmatite in the area.

The area is accessible through the Jos-Fadan Karshi-Wamba high way which runs across the central pegmatite zone of the Basement sensu-strictor of Nigeria. Other minor roads, link Gwon-Gwon with Kuraga, Ungwan Rimi and Kejeri villages all within the study area. These minor roads (such as tracks and footpath) linking this different settlements make accessibility quite easy to the study area. Bush tracks were used to gain access to some of the outcrops.

1.5 Relief and Drainage

The relief of central Nigeria ranges between a mean elevations of 490-900 meters above sea level (Peneplain of Plateau of central Nigeria). (Falconer, 1911). The study area is within these ranges and is generally characterized by low-lying dome shaped hills which are very common. These hills are made up of granitic rocks that intruded the undifferentiated basement.

The pegmatite of the study area has not been exposed i.e. the pegmatite is buried. Study was made possible through the ancient abandoned pits that were blasted enabling in-situ small scale mining of the economic minerals of the area.

The drainage is generally dendritic and is controlled by the structural features as well as the topography. The minor tributaries flow in accordance to structural patterns trending NE-SW direction and all flow into River Gudi and River Arum.

1.6 Climate and vegetation

The study area lies within the Guinea Savannah of Nigeria characterized with tall grasses and tall/thick trees with some thorny shrubs, mostly along river channels.

It experiences rainfall for a period of 4 – 6 months in a year, from April to September. The mean annual rainfall is approximately 1,825mm with maxima in the months of July and August. Mean annual temperatures range between 18oC and 24oC, though there may be local variations (Kalumba, 1987). The Harmattan winds (North-Easterly dusty winds) are experienced from late November to February.

1.7 Extent and Scope of work

The area of study (Gwon-Gwon) is one of the areas in Nigeria of which little or no geological information is available. The work that has been done on the area is mainly on regional scale. Among the earliest geological works are those of the Geological Survey of Nigeria (GSN) (1954 and 1964) which discusses the basement complex on a scale of 1:200,000 and 1:100,000 respectively. Jacobson and Webb, (1946) wrote on the pegmatites of central Nigeria and more details by Wright, (1970).

Detailed work was done on the structural and metamorphic rocks (McCurry, 1971); Ajibade, (1980) and Okunola, (1989) carried out work on the geological and heavy minerals survey for Sn-Ta mineralization, and Kinnaird,(1984) also related the early mineralization of economically important primary Sn-Nb-Ta-Zn to the Paleozoic pegmatite occurring along the 400km NE trending belt characterized by extensive tourmalinisation. Hockey and Jones, (1964); Oyawoye, (1970); Rahaman and Malomo, (1983) established the subdivision of the basement complex rocks.

The delineation of the pegmatite zone was given by Rahaman et al, (1984) as being in the ENE-WSW direction. Woakes, (1981) worked on the metallogenetic features of the basement and Kuster, (1985) highlighted the economic viability of the pegmatites of central Nigeria (Wamba in particular) in his detailed geological work. Matheis and Cean- Vanchette, (1983) were able to distinguish between the barren and the mineralized pegmatites. Adediran et al, (1989) established the presence of emerald, aquamarine, topaz and tourmaline in the Wamba pegmatite of central Nigeria.

Others include the works of Van Breeman et al, (1977), Ogezi, (1977) on the pegmatite of central Nigeria. The pegmatite and the Aplites are usually mineralized with amounts of tin, tantalum and columbite which have been worked in the past and around Egbe, Jema’a and Abuja areas, and usually discordant and concordant varieties in bodies that range from sub-elliptical plutons less that 10km2 in area to batholiths with areas of up to 500km2 are known.

Minor components that exists include (but not necessarily) generally dark even-textured diorite, syenite and a variety of fayalite quartz Monzoite (“Bauchite”) [Oyayowe and Makanjuola, (1972); McCurry, (1976); Rahaman, (1976)].

Pegmatites of central Nigeria are basically peraluminous and usually foliated. Boundaries of these granites are often not well defined as they are generally surrounded by extensive aureoles of migmatisation. It is believed that they have been emplaced at relatively deep crustal levels during Pan-African orogeny, also age determination of Pan-African granites shows clearly that they are not the oldest granite rocks in Nigeria [Grant, (1970); Ogezi, (1977)] hence the suggestion that such rocks should be called Pan-African (Older) granites.

Geological, geochemical and isotopic evidence suggest that Pan-African granites were derived largely from pre-existing rocks (Ogezi, 1977). The Nigerian basement lies within the region which is supposed to have been subjected only to a thermo-tectonic event (Kroner, 1977; Grant, 1973).

1.8 Limitations

The study area (Gwon-Gwon), has no visible outcrops, sampling was carried out inside the abandoned mining pits. They used dynamites in blasting to get to the economic minerals they sought. Mining was done/carried out in-situ in the study area.

Few low-lying outcrops were found to the North, Northeast, Southeast and South of the study area (i.e. around Chessu, Chini and Ungwan Rimi villages/ settlements), and when found they were highly weathered in most cases.

Also past detailed work on the area of study (Gwon-Gwon) or literature was not available and the topographical map that covered the area, were long overdue/ most of the foot path and minor motorable roads have gone out of use and some new ones have emerged. All these pose difficulties during the field work.

Chapter two

Sub-Regional Geology

2.1 Introduction

The migmatite gneissic complex of Nigeria have quite a limited geological survey, and in view of the limited geologically and structurally controlled studies, geochronological data available up to 1982 has been summarized by Ajibade et al (in press).

Although, early rock and mineral age determinations suggested that rocks of the basement complex were formed during the Gwarian complex cycle (Truswell and cope, 1963). It was quickly shown that the basement complex is polycyclic and has been affected by up to about four thermotectonic events, some of which are erogenic in character (Grant, 1990, 1971, 1978; McCury, 1978; Oversby, 1976; Ogezi; 1975; 1977, in press). Together with these events at Liberian (2800 + 200 m.y) Eburnean (2000 + 200 m.y) Kibaran (1100 + 200 m.y) and Pan-African (600 + 150 m.y), a number of other minor non-orogenic events and sedimentations have affected the rocks in Nigeria

Table 1: GEOLOGIC SETTING IN NIGERIA(AFTER KOGBE).

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