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Continental J. Environmental Sciences 4: 12- 19, 2010
©Wilolud Journals, 2010
ISSN: 2141 - 4084
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GEM MINERALIZATION IN THE PEGMATITES OF THE KEFFI AREA, NORTH CENTRAL NIGERIA
Piwuna, R. M., Aga, T and Chup, A. S.
Department of Geology and Mining, University of Jos, Nigeria
ABSTRACT
The mineralized pegmatites of south-east of Keffi town was mapped and the gem minerals
studied using Lapidary equipments such as dichroscope, refractometer, loupe, Chelsea
filter, veneer caliper, microscope and polariscope. These gems include blue, green and
pink beryls, tourmaline, topaz and amethyst. The striking characteristics of these gems and
their host rocks coupled with field observations clearly indicate that the pegmatites were
probably from a different magmatic source and are concentrated along young set of fault
structures.
KEYWORDS: gem, mineralization, pegmatites, lapidary equipment, Keffi
INTRODUCTION
The area of study covers 104km2 situated in Keffi area in Northcentral Nigeria. It lies between longitudes 8O
00’00” and 8o04’00”E and latitudes 8o41’12” and 8o48’30”N (Fig.1 ). Major settlements in the area include
Kokona, Ungwan Doka, Jamilu, Kofan Gwari and Bakin Ahini .This area under study falls within the late PanAfrican rare-metal pegmatite zone of central Nigeria characterized by tin, columbite and tantalum
mineralization (Jacobson and Webb 1946, Mathies 1987, Kuesta 1990). The basement rocks include gneisses,
migmatitic lenses and Older Granites which host numerous mineralized and barren pegmatites as well as quartz
veins. (Fig .1). Wright (1970) suggested that the pegmatites had no spatial relationship with the Older Granites
despite their proximity. The mineralized pegmatites in the area are concentrated in the central parts forming a
narrow zone running NW-SE. They are structurally and mineralogically zoned while the barren ones are simple
pegmatites not continous and trending E-W. The former appear younger because they cross-cut the latter.
Dolerites are younger than the pegmatites as indicated by similar field relationship at Takawa.
The study of gem minerals has been of increasing interest since after the second world war due to the role they
play in modern scientific development. For example, Available geological reports and visits to some mine sites
tend to suggest that Nigeria host the largest quantity of colourless topaz in the world (Aga, 2004). The reported
occurrence of these gem minerals in a NE-SW pegmatite zone in Central Nigeria prompted the interest and
selecting the area for further investigation.
MATERIALS AND METHODS
Field observations and geological mapping on the study area were carried out on the scale of 1:25,000.
Photographs were taken and sketches made for interpretation. Due to the reported occurrences of rare-metals
and gemstones in the region ( Jacobson and Webb 1946, Kinnaird 1984, Mathies 1987, Kuester 1990 and Abaa
1991) emphasis was placed on the pegmatite outcrops in order to bring out any structural patterns associated
with the mineralizations.
Traverses were taken mainly along road-cuts, gullies, river valleys, mining pits and randomly were outcrops
exist. Fresh samples of all rock types were collected, out of which sixteen (16) thin sections and six (6) polished
sections were prepared for microscopic studies to determine the various elements, structures and alterations.
Gem minerals were studied using lapidary equipments like dichroscope, loupe, refractometer, polariscope,
Chelsea filter, Veneer clipper and UV lamps.
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Piwuna, R. M.et al.,: Continental J. Environmental Sciences 4: 12- 19, 2010
Keffi
Fig .1. Simplified Geological Map of Nigeria showing the Study Area within Pegmatite Belt of Central
Nigeria.
RESULTS AND DISCUSSIONS
GEM MINERALS
Though gem minerals are known to occur in nearly all kind of rocks, most of them like some rare-elements, are
found in pegmatites. The pegmatites are known to contain the greatest variety of gem minerals other than
diamonds (Jahns, 1960). In the Keffi area, the gem minerals are found only in the complex pegmatites and the
miarolitic cavities of the biotite granites. Known occurrences in Nigeria have been reported to be small deposits
and very irregular in character (Adediran et al 1989). In central Nigeria especially in the study area, the crystals
of neat quality are concentrated in well-defined “pockets” which are ellipsoidal to highly irregular in form of
pegmatite pockets. The miarolitic cavities in the biotite-granites are similar to those reported by Abaa (1991)
and some well known world occurrences (Jahns, 1960). The mineralized pegmatites in this area lie between
Ungwan Doka in the west across to Takwa in the south east. They host most of the gem minerals. Some of the
pegmatite pockets are true cavities but others are partly or completely filled with aggregates of quartz, alkali
feldspars and some mica minerals and clay. Most of the well forms crystals of gem minerals are often found in
such aggregates though some of low qualities are occasionally found sporadically scattered throughout the solid
pegmatite. From field evidence, the gem minerals are released after the source rocks have undergone chemical
and mechanical weathering. They are then recovered by local mining from the alluvial surrounding the granitic
and pegmatite bodies. This is also a world-wide phenomenon
(Kuester, 1990). Based on Jahns (1960) classification (Table 1) only gem minerals of the silicate groups are
found in the area under study. In order of importance, they include beryls (emerald, aquamarine, and morganite),
tourmaline, topaz, amethyst and rock crystal.
Beryl (Be3Al2 (SiO3)6
They are found in both the pegmatites and the miarolitic cavities in the biotite granites, but the latter producing
the higher quality gems though smaller in size. The different varieties obtained include the green (emerald), pale
blue (aquamarine), pink (morganite) and the colourless (goshenite) (Plates 1 and 2). They range from a few
centimeters to over 12 cm in length (Plate 3).
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Table 1. Classification of Gemstones modified after Jahns (1960)
Native Elements
Diamond Gold
Silicates
Beryl, Garnets, Jadite, Nephrite, Olivine,
Opal, Quartz, Spodumene, Topaz, Zircon
Aluminosilicates
Feldspars, Lazurite, Jade-Jadeite, Kunzite,
Kyanite
Oxides
Corundum, Haematite, Spinel, Ruby,
Chromite
Aluminate
Chrysoberyl, Benitoite
Borosilicate
Tourmaline, Datolite, Danburite
Carbonate
Calcite, Azurite, Malachite
Phospahte
Turquoise, Apatite
Sulfate
Gypsum
Fluorite
Fluorite
Organic
Amber, Jet, CaCO3, in form of Coral and
Pearl
Emerald
Though few gems of good quality have been won, they are encounterd in the pegmatites. As the pockets are
approached, uncrystallized form of the green beryl is found. This always indicates the presence of emerald at
greater depths. The crystals are in the form of six-sided prisms terminated by flat or pyramidal faces. They are
grass-gresh or gresh in colour which is thought to be the presence of chromatic oxide and or vanadium
(Webster, 1962). The yield is small due to crude methods of mining, and even where they are found, they are
damaged or found covered with a tenacious coating of limonitic red iron oxides, especially in the alluvials.
Inclusions are really seen which suggest that further tests are necessary to distinguish them from green beryls
(Webster , 1962). They are associated with minerals such as quartz, schorl, topaz and feldspar.
Aquamarine
These beryls are the most abundant in the area (Plates 1 and 2). They are seen as pale-blue to sky blue or even
bluish green hexagonal crystals with prismatic faces striated parallel to the prism edge (Plate 1).
The colour is known to be due to the presence of iron. They range from very thin needle shaped crystals to large
crystals measuring up to 9cm in length. They have a weak basal cleavage and a tendency to brittleness. Many of
the gem quality varieties have been won in the area since 1968 (Kalumbu, 1987).
Plate 1: Pale-Blue Beryls (Aquamarine) from Ungwan Doka.
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Piwuna, R. M.et al.,: Continental J. Environmental Sciences 4: 12- 19, 2010
Plate 2: One of the beryls (aquamarine) about 9 cm in length (aaI) associated with quartz from Angwan Doka
Morganite
These are the pink beryls, the colour being ascribed to the presence of traces of lithium oxide, alkali metals,
caesium, and rubidium. They are hexagonal with prismatic faces found almost exclusively in the pegmatites in
association with green tourmaline and other beryls. Large crystals are often seen but not of gem quality due to
fracturing and weathering. Some crystals measure up to 8.5cm in length (Plate 3). Inclusions are seen in the
form of straight tubes in parallel arrangement and lying parallel to the prism faces.
Plate 3: Pink-Beryl (Morganite) marked XXI , in one of the Pegmatite also from Ungwan Doka.
Topaz ( Al2, F, OH)2 SiO4
They are seen as prismatic crystals with striated faces parallel to their length. The faces are terminated with
pyramidal and dome-shaped faces with two of the dome faces often so enlarged that they meet at the top of the
crystal forming a ridge (Plate 4). They are recovered from the pegmatites and cavities or ‘gem pockets’ in the
biotite granites. They are known to be associated with highly acid igneous rocks probably deposited by the
action of hot floriferous gases after solidification of the magma (Webster 1962, Abaa 1991). The gem varieties
are colourless, with hardness 8, exhibiting good cleavage. Transparent varieties are common though inclusions
could produce a cloudy effect in some specimen. One of the inclusions, which is typical of Nigerian varieties
(Webster 1962), is cubic shaped which was suggested to be fluorite crystals (Webster 1962). Topaz is found to
be associated with quartz and tourmaline. Similar varieties are known from the Younger Granites of the Jos
Plateau (Abaa, 1991)
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Plate 4: Topaz Crystals ( Centre of the Photograph) measuring 3cm in length from Bakin Ahini
Tourmaline (Al, Fe, Mn)6 (BO3)3 (Si6O18) (OH)4
These are the most widely recoverable gemstones in the area. Varieties, such as the pink (rubellite), green
(verdelite), colourless (achroite) and opaque (schorl) are often encountered (Plates 5 and 6). Some crystals are
seen as long prismatic crystals with vertical striations while others could be fibrous with fine fibre or carnals
running through the length of the crystals. The prisms in some crystals tend to be short thus giving them a
tabular habit. The characteristic rounded triangular section as viewed down the length is common to all well
formed crystals. The prisms are often capped by rhombohedral or scalenohedral faces which may differ in the
angle with the prism of the opposite ends of the crystal; thus one end appears flatter than the other. Cleavage is
poor but subconchoidal tracture is evident. Crystals vary from perfectly transparent varieties to translucent to
opaque as in schorl. In most cases, however, the clear varieties are marred by many cracks and flaws. Inclusions
are threadlike cavities filled with liquid and often contain bubbles or gas (two-phase inclusions) (Chup, 1993).
The tourmaline is associated with quartz, mica, topaz, beryl, cassiterite and feldspar.
Plate 5: Green and Black Tourmaline won from Ungwan Doka.
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Piwuna, R. M.et al.,: Continental J. Environmental Sciences 4: 12- 19, 2010
Plate 6: Black Tourmaline (Schorl) Crystals showing Characteristic Faces from Kokona.
Amethyst
Only few crystals were found in some of the vughs or hollow cavities in the biotite granites and some of the
surrounding quartz veins. They are found as drussy masses showing only the rhombohedral tops. In few of
them, only the tips of the crystals are deeply coloured, the remainder grading into milky quartz or rock crystal
(plate 7)
Inclusions seen are in the form of feathers giving a fibrous appearance especially at the base. They are
associated with topaz and beryls.
Plate 7: Amethyst Crystals from Kofan Gwari (The darker Crystals is Smoky Quartz). They range in size from
1.5 – 3cm
Rock-Crystal
This is seen as colourless-water-clean or transparent variety of quartz in the pegmatites and cavities. Crystals are
hexagonal terminated by pyramids at one end and characterized by horizontal striations on the prism faces. They
display the characteristic conchoidal fracture. There are two phase inclusions in the form of crystal shaped
cavities which could contain in a liquid or bubble of gas. Hairlike needles of rutile are seen in some of the
varieties.
PARAGENESIS AND EVOLUTION OF THE MINERALIZATION
The Ungwan Doka mineralized pegmatites, which are mineralized with tourmaline, topaz, tin and some clay
minerals suggest two types of mineralizations. The first being pneumatolytic and the second hydrothermal. One
of these processes may have lagged behind the other or both of them may have occurred simultaneously
resulting in the occurrence of a mixture of tourmaline, topaz, quartz and muscovite in such a coherent mixture
(Chup, 1993). Field observation shows the upper horizon contains extensive and enormous mica flakes and at
the lower horizon, there is a mixture of tourmaline, topaz and quartz. Kinnaird (1987) reported that
tourmalization was dependent on the metamorphic grade of the country rock.
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The above late stage igneous activities were rich in volatiles such as figurine, hydrofluoric acid and boron
fluorides (Kalumbu 1987). Their continuous action on the country rock in some localities gave rise to the
formation of tourmaline as well as topaz. The area has various types of tourmaline suggesting that due to the
wide range of cation in the structure as these boron-rich volatiles came in contact with pre-existing minerals of
the country rocks which are granitic, the above was formed. Similar reactions have been described by Abaa
(1985). The tourmaline is seen as radiating crystals in quartz and feldspar or greisens which are mica-rich
pneumatolysed rocks. The kaolin found in association is a product of hydrothermal processes. The fedlspars
were broken down by these superheated waters in the presence of active fluids. Therefore, kaolinization appears
the important hydrothermal process that led to the production of clay minerals from the feldspars.
Jacobson and Webb (1946), Mathies (1984) had also related the rare-metal mineralization in these pegmatites to
the degree of albitization especially in the cupola type massive pegmatites. Mathies (1984) observed that either
high K or Na concentration correlated with rare metal enrichment. However, the Ungwan Doka mineralized
pegmatites show little or no degree of albitization and from field occurrence and mineralogical similarities with
the host rocks. It is almost conclusive that the Pan African tectonic events introduced concentrations of ore
metals into the continental crust by accretion and reworking during ocean closure (Kinnaird 1984). Some of
these fluorine rich ones, later, crystallized and were emplaced as the pegmatites. Kinnaird (1984) pointed out
these these metals might have been subjected to further recycling during the subsequent ‘ring-complex’ alkaline
magmatism.
CONCLUSION
Precious and semi-precious gem mineralization occurs within the Keffi mineralized pegmatites. Studies on these
gems which include beryls, tourmaline, topaz and quartz show that they are of good quality. Also, the host
pegmatites formed from late stage magmatism.
ACKNOWLEGMENT
The authors wish to thank The School of Preliminary Studies for sponsoring this studies and Geotess Nigeria
Limited is acknowledged for the use of their Lapidary situated in Jos. The guidance provided by Prof. S.I. Abaa
of the Department of Geology at Ahmadu Bello University, Zaria in the course of this work is appreciated.
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Received for Publication: 02/04/2010
Accepted for Publication: 04/05/2010
Corresponding Author:
Aga, T
Department of Geology and Mining, University of Jos, Nigeria
Email: [email protected]
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