review of geology, geochemistry and origin of gypsum mineralization in chad basin (north eastern...
TRANSCRIPT
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
1/81
ABSTRACT
A field trip was conducted in order to evaluate the geology, geochemistry and
origin of gypsum mineralization in the Nigerian sector of Chad Basin, part of
which is the research area. The gypsum mineralization underlain the whole area
of about 2247.75km2
within the Chad Basin and comprises of five different
forms; Detrital, Balatino Laminated, Selenite, SatinSpar, and Alabaster. The
thickness of the gypsum forms increases with depth, and they are emplaced at
various depths. All the gypsum forms retained traces of former lamination
except detrital gypsum therefore lamination is a primary structure.The chemical
composition of the gypsum forms showed that they are all high grade (over 70%
CaSO4 . 2H2O). The moisture content could be used to show fractured areas
where circulation of water is easier. Alabaster gypsum is the heaviest as suchmore suitable for cement works. The chemical composition of all the gypsum
forms revealed high trace elements content which could be the product of the
brine mixing phenomenon. The petrographic studies of the Balatino, Selenite,
SatinSpar and Alabaster gypsum forms revealed that they all recrystallized from
the primary laminations during the early diagenesis. The recrystallization
appeared to be perpendicular to the primary laminations. There are
dissemination of anhydrite in the matrix of the gypsum forms which shows the
effect of incomplete rehydration after upliftment. The origin of gypsummineralization in Chad Basin is both sedimentary and diagenetic, so based on
high trace element content and primary lamination, a brine mixing hypothesis
was proposed for the origin of the gypsum mineralization in the Chad Basin.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
2/81
TABLE OF CONTENTS
Page
Chapter 1
Introduction 1
Chapter 2
Literature Review 4
Chapter 3
Materials and Methods 8
Chapter 4
Results 16
Chapter 5
Discussion of Results 56
Chapter 6
Conclusion and Recommendation 69
References 76
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
3/81
1
CHAPTER ONE: INTRODUCTION
1.1 Necessity of GypsumGypsum mineralization is one of the pivot for Nigerias industrial
revolution for a self reliant and durable economy especially in the building
and agricultural and construction industries.
The cement industries, as well as chemical, ceramic, pharmaceutical,
paints and may other industries in Nigeria need gypsum as one of the most
important raw material for their productions. However, prior to the 1990s,
gypsum mineral has been imported from Spain and Morocco. Nigeria spent
about N900 million annually on the importation of gypsum for her cement
Anonymous (1996).
The earliest work on evaporates in the North Eastern region was done
by Vischer (1910) in a geographical account of an early expedition into the
areas of the Chad Basin and immediate environment. The author described
two classes of evaporate mineral deposits. Namely;
(a) The magma salt (an admixture of sodium carbonate and bicarbonate,sodium sulphate and sodium chloride in concentrations of
approximately equal magnitude)
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
4/81
2
(b) Magma natron (mainly sodium carbonate with subordinate amount ofsodium sulphate and sodium chloride).
The earliest reconnaissance traverse of the area was made by Falconer
(1911) during the Mineral survey of northern Nigeria in the first decade of the
century. Gypsum occurrence in North Eastern Nigeria was first reported by
Carter et al., (1963) as occurring within a sequence of blue black shales,
containing few, thin, impersistent limestone beds and occasional interbedding
with thin siltstone beds and lava flows. Reyment (1965) confirmed this by
reporting the Fika Formation as consisting of blue-black shales, occasionally
gypsiferous with a thickness exceeding 430 meters. Maglione (1981) also
confirmed the presence of gypsum mineralization in well drained, well aerated
environments within the Nigerian sector of Chad Basin (part of which is the
research area). Gypsum occurrence at Nafada Bajoga areas was reported by
Orazulike (1988).
All these workers confirmed that the Fika shales are gypsiferous. Since
then, not much work has been done in this area to determine the economic
viability of the gypsum mineralization at various depths and in various places
within the Chad Basin. Only the illegal miners patronized the gysiferous areas.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
5/81
3
1.2 ObjectivesThis research work is in line with the renewed interest in the search for
gypsum in various parts of the country in order to feed the Nigerias industrial
sector like, cement, chemical and ceramics industries. This would help to
attain maximum utilization of gypsum resources by the said industries in order
to hasten development in Nigeria.
Another objective of the study was to conduct a detailed investigation
on the gypsum mineralization in the research area (Fig.1). the detailed
research entails studying the geology, geochemistry, origin, as well as the
mineralogical and textural evolution of the gypsum prospects in order to
assess its economic significance.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
6/81
4
CHAPTER TWO: LITERATURE REVIEW
2.1 Fika Shales
The name Fika Shales was assigned by Raeburn and Brynmor (1934) to
the Limestone Shale group which also includes the Gongila and Pindiga
Formations of the present work. The Fika Shles is a sequence of blue black
shales occasionally guypsiferous and containing one or two thin impersistent
limestone beds (Carter et al., 1963). The Formation underlies a broad belt of
country in the north-western part of the Mutwe plain extending westwards to
Fika and south-westwards to form the narrow outerop which strikes
southwards from Nafada. Although the beds are poorly exposed, sections are
known from wells, boreboles and stream channels (Carter et al., 1963)
The shales contain abundant fish fossils and also crocodile remains and
Chelonian fragments. The blue-black nature of shale may be indicative of
attendant reducing conditions at the time of deposition of the unit. The blue-
blackk shales were deposited during the middle Cretaceous world-wide marine
transgression in both the Benue Trough and Chad Basin (Petters, 1978). These
sediments feature sparse population of benthic foraminifera assemblages as
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
7/81
5
well as high organic matter content suggesting deposition under anoxic
conditions (Petters and Ekweozpr, 1982b).
However, blackness of sediment can also be due primarily to high
abundance of pyrite and may not necessarily signal high organic content.
Ekweozor et al., (1989) analyzed many shale cuttings from the Fika Shales
and reported that they contain fluffy, biodegraded humic matter (non-
fluorescent amorphous organic matter, humosapropelinite showing
intergrowth of micrinite and framboidal purite in some places. This organic
matter is inferred to have been derived mostly from oxic paralic swamp or
lacustrine depositional environment.
The relative abundance of arenaceous benthic foraminifera within Fika
Shales point to the prevalence of near shore environment. Petters (1983)
dated Fika Formation as Coniacian to early Maastrichtian. The thickness of
Fika Shales overlies the Gongila Formation and underlies the Gombe
Sandstone in the Chad Basin, Table 1.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
8/81
6
Table. 1 Stratigraphic succession of Chad Basin, Zambuk Ridge and
Upper Benue Basin
Upper Benue Basin Zambuk Ridge Chad Basin
Pleistocene Gulani AreaKerri-KerriFormation
Gombe Area
Upper
Cretaceous
Maestric htian
Senonian
Turonian
cenomanian
Companian
Santonian
Coniacian
Upper
Lower
LamjaSanstone
NumanhaShale
Sekule Form
Jessu Form
Dukul Form
Upper MiddleLower
Pindiga
Formation
Yolde
Formation
Bima
Sandstone
GombeSandstone
Fika Shales
Gongila
Formation
Lower Palaeozoic To Pre-cambrian Crystalline Baseme`nt
(After Carter et al., 1963)
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
9/81
7
2.2 Chad Basin
The Chad Basin which is the largest intracratonic area of inland
drainage basin in Africa (Raeburn and Brynmor, 1934) and (Barber, 1965)
occupies an area of about 230, 000km2
in the central Sahara and southern
Sudan. The western limit is the water divide which divides the Niger and the
Chad drainage systems and the southern limit is the watershed between the
Chad and Benue systems. About one tenth of the basin is situated in the
northern part of Nigeria which lies between latitude 100N-14
0N and longitude
100E13
0E.
The Chad Basin is endoreic i.e. it does not drain to the outside. It is
separated from Upper Benue by a basement dome (Zambuk Ridge) and it also
contains Albian-recent sedimentary rocks among which are the Fika Shales
that host gypsum mineralization. Some of the sedimentary rocks; Bima
Sandsone, Gongila Formation and Fika Shales have been folded and uplifted
during the Maastrichtian orogenic event which trends NW-SE at right angle
to Santonian orogenic event which trends NESW (Benkhelil, 1982).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
10/81
8
CHAPTER THREE: MATERIALS AND METHOD
3.1 Field Work
The field work was conducted in the first half of the month of June,
1997. Seven mining sites were visited. The mode of mining in all the mining
sites are pitting,( ranging from 0.2 to 13m depth (Table 2, Fig.1) and
trenching. In most of the mining sites, the carrierbeds (Fika Shales and
Mudstone) area shallow, so the pits are not very deep. The mineralization is
continuous with minor discontinuities as such it is intercepted by the different
mining pits in different mining sites. The sections are shown in Fig.2. The
continuity suggests a uniform depositional environment over a wide region.
While on the field, observations were made on the gypsum samples
along the following lines:
(a)different gypsum forms and their various carriers beds(b)structural and textural relationship between the different gypsum forms.
The mining sites are confined to stream slopes and areas liable to flood near
stream channels between the villages. The streams are part of the tributaries of
the River Gongola which drain 90% of the water in the region, Fig.1. Table 2
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
11/81
9
summarizes the measurement in different mining sites and Fig.2. gives the
measured section in the different mining sites.
The following are the findings based on the field work:
(i) Five different gypsum forms are recognised: Detrital, Balatino,Selenite, SatinSpar and Alabaster.
(ii) The changes in gypsum forms is vertical with depth and nothorizontal with distance
(iii) The thickness and deformation of the gypsum forms increases withdepth
(iv) All the different gypsum forms at any depth have peculiar carrierbeds.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
12/81
10
Fig. 1 Map of the Research Areas Showing Mudflat areas (Shoreline
Environment) hosting the Gypsum Mineralization within the exposed
Fika Shales. (modified after Carter et al., 1963)
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
13/81
11
3.2 Geochemical Analysis
Ten samples were analyzed using X-Ray Fluorescence analytical
technique. This was done to determine the chemistry and hence assess the
quality of the gypsum.
3.2.1 Sample Preparation
Gypsum samples were cleaned and ground into powder using agate
mortar and piston. Agate mortar was used to prevent silica contamination. For
every sample ground, the agate mortar was washed and dried before grinding
another sample. This was also done to prevent contamination of samples.
When dealing with samples containing heavy elements in the light (low
density) matrix, which is often the case in gypsum, the grain size effect can be
an additional source of error in XRF analysis. This was overcome by grinding
to very fine particles.
The60 mesh sieves was used to ensure that the powdered particles are
in size. The sieve was made of nylon as iron and steel sieves can introduce Zn,
Pb, Ag, Cu or Co contamination into the sample during sieving. 2g of the
sample was weighed using a sensitive digital scale.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
14/81
12
The 2g of the sample was placed into a pelletizer device for pelletization. No
binder was added to the ground gypsum powder because the water of
crystallization act as a binder. The pelletizing device compacted the 2g of the
powdered sample into pellets and the pellets were used directly for the XRF
analysis. The analysis was conducted at the laboratory of the Centre for
Energy Research and Training (CERT), ABU Zaria.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
15/81
13
Fig.2 Flowchart for Gypsum Sample Pelletization
Gypsum samples from field
Cleaning, crushing/rolling
Grinding
Sieving to
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
16/81
14
3.3 Petrographic Studies
Four different gypsum forms were studied in hand and thin section to
assess their textures and textual evolution.
3.3.1 Sample preparation
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
17/81
15
Gypsum samples from field
Samples; washed, dried and clean
Samples polished using polishing machine
Samples gumed on a polished glass surface
using Canada Balsam or Araldite
Samples polished to 3mm
Samples grind with Carborundum of0.6mm size
Samples grind with powdered caborundum
Samples grind with powdered caborundum
Polished surface washed, and dried
Polished gypsum surface covered with gum
(Canada balsam or Araldite) and thin glass
The slide is then placed on heater to expel
air bubbles and dried up
Gypsum slide is then allowed to cool (thin
section)
Petrographic studies
Fig.3 Flowchart for sample preparation
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
18/81
16
CHAPTER FOUR: RESULTS
4.1 Field Studies
The results of the field studies are summarized in Table 2 and all the
information and findings in Table 2 are plotted in Fig. 2:
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
19/81
17
Table 2: Summary of the Field Studies.
Names of
Villages
Site
No.
Gypsum
Forms
Depth of
Occurrence
Thickness of
Gypsum
Lithology of Carrier Beds
Daura 1 Detrital 0.23m 0.5 -1cm Mudstone and Grey shales.Satinspar 56m 67cm Blue-Black shales
Kwayaye
Manawaji
2 Detrital 13m 0.5-0.6cm Mudstone
Balatino-
Laminated
24m 23cm Mudstone and Grey shales.
Selenite 45m 22.5cm Mudstone and Grey Shales.
Satinspar 7m 45cm Blue-Black Shales.
Kanwaram 3 Detrital
Balatino
Laminated
0.2 -2M
24M
0.1-0.4cm
13cm
Mudstone
Mudstone and Gyrey Shale.
Satinspar 57m 23cm Blue-Black Shales.
Alangafe 4 Selenite
SatinSpar
Alabaster
45m
5m
8m
23cm
56cm
5 8cm
Grey Shales
Grey Shales
Blue-Black Shale
Bulgaje 5 Detrital
Balatino
Laminated
0.22m
14m
0.31.5cm
13cm
Mudstone
Mudstone
Selenite
satinSpar
alabaster
45m
67m
1213m
23cm
2.53cm
810cm
Grey Shales
Blue-Black Shales
Blue-Black Shales
Nyakire 6 Detritali
Balatino
Laminated
24m
23 m
0.22cm
12cm
Mudstone
Mudstone
satinSpar
alabaster
68m
911m
45cm
78cm
Blue-Black Shales
Blue-Black Shales
TurmiMalori 7 Detrital
Balatino
Alabaster
0.24m
1011m
0.51cm
57cm
Mudstone
Blue-black Shales
Zangaya Garin
Ari
8 SatinSpar
Selenite
Alabaster
56m
45m
78m
1.53cm
12cm
45cm
Blue-Black Shales
Grey Shales
Bluee-Black Shales
Fika 9 Selenite
satinSpar
Alabaster
4m
56m
67m
12cm
34cm
45cm
Grey Shales
Blue-Black Shales
Blue-Black Shales
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
20/81
18
Mining Site 1 Daura
Thin mudstone containing disseminated detrital
gypsum crystals
Grey shales and mudstone containing detritalgypsum crystals
Blueblack shale containing satinspar gypsum
Mining site 2 Kwayaya Mainamaji
Mudstone containing laminated and detrital gypsum
Grey shales containing selenite gypsum
BlueBlack shale containing Satinspar
Mining site 3 Kanwaram
Mudstone containing disseminated detrital gypsum
Mudstone & Grey shales containing laminatedgypsum
Blue-Black shales containing SatinSpar gypsum
Fig. 4 contd
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
21/81
19
Mining Site 4 Alangafe
Thin layer and mudstone grey shale containing
selenite
Grey shales containing Satinspar
Blue-Black shales containing massive Alabastergypsum
Mining site 5 Balagaye
Laminated gypsum and disseminated detritalgypsum in Mudstone
Selenite gypsum in Mudstone and grey shales
Satinspar in BlueBlack shales
Enterolithic satinspar in BlueBlack shales
Marine Alabaster gypsum in Blue-Black shales
Mining site 6 Nyakire
Mudstone containing laminated gypsum anddisseminated detrital gypsum crystals
Blue-black shales containing Satinspar gypsum
Blue black alabaster gypsum
Fig. 4 contd
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
22/81
20
Mining site 7 Turmi Malori
Mudstone containing laminated gypsum and
disseminated detrital gypsum
Mudstone, grey shale containing Selenite gypsum
Blue-Black shales containing Satinspar gypsum
Blue-Black shales containing massive Alabastergypsum
Mining site 8 Garin Ali Zangaya
Thin Mudstone
Grey shales containing Selenite gypsum
Blue-Black shales containing Satinspar gypsum
Blue-Black shales containing massive Alabastergypsum
Mining site 9 Fika
Thin Mudstone
Grey shales containing Selenite gypsum
Blue-Black shales containing Satinspar gypsum
Blue-Black shales containing massive Alabaster
gypsum
Fig. 4 contd
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
23/81
21
Fig. 4 Sections of Fika Formation Hosting Gypsum Forms from all the
Mining sites.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
24/81
22
4.2 Petrography and Geochemistry of Gypsum Forms
Hand specimen description was merged with the petrographic studies in
order to determine the textural similarities and differences. The chemical
composition of the gypsum forms also showed their level of similarities.
Five different gypsum forms are recognized on the basis of difference in
depth, carrier beds and structures.
They are as follows;
4.3 Petrography
4.3.1 Detrital gypsum form: This gypsum form has hemipyramidal to
pyramidal crystal structures. The length range from 0.5 1cm and they occur
disseminated in mudstone and shallow shales of the Fika Formation.
Monocrystalline gypsum grains are far more abundant than twined crystals.
The apices and edges of the pyramidal crystals are commonly abraded and/or
broken (plate 1).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
25/81
23
Plate. 1 Detriatal Gypsun Form
Fig. 5 Detrital Gypsun with Pyramid Structure
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
26/81
24
The long axes of the detrital pyramidal crystal always lie parallel to the shale
bedding. The shapes of the crystals range from auhedral to anhedral. The
crystal edges and apices seemed to have suffered partial dissolution by ground
water. Petrographic studies were impossible because the crystal were too small
for thin section preparation.
4.3.2 Balatino Lamnated Gypsum forms: they are finely bedded and the
fine beds range in thickness from 1-3mm. They occur within mudstone and the
bedding plane between mudstone and shale. Sometimes particles of blue-black
shale were seen in between the thin gypsum beds. The surface of the Balatino
laminated gypsum is transparent, smooth and tabular (Fig. 6; Plate 2). Some
samples of this gypsum form showed evidence of dissolution from the edges
of the lamination hence such areas have slightly rough texture. They are found
mainly in mudstone at the depth of 2 4m with much pyramidal detrital
gypsum around them within some of the pits. The individual laminae look
equal in size (mainly, 2mm) and extend laterally conforming with the fine
bedding of the shale and pseudo-bedding of the mudstone. Petrographically
the Balatino laminated gypsum is colourless under plane polarized light but
the cleavages are very perfect and distinct. The direction of the cleavage
follows the direction of thin lamination.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
27/81
25
Plate. 2 Balatino Laminated Gypsum Form
Fig. 6 Baltino or Laminated Gypsum
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
28/81
26
Under cross polarized light, the individual laminae showed an offlaping
relationship with each other. The limit of each laminae showed a kind of
mamillary structure. The perfect cleavages are parallel to each other, and
conformed to the primary laminations. Each laminae showed white colour
with low relief under cross polarized light. The matrix of gypsum laminae
showed inclined extinction as the stage is rotated Fig. 7.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
29/81
27
Fig. 7 Sketched Photomicrograph of Balatino or Laminated Gypsum
Fig. 8 Sketched Photomicrograph of Satinspar of Fibrous Gypsum
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
30/81
28
There wer few anhydtrite crystals disseminated within the lamination. These
crystals are distinct when the stage is rotated because they have high relief
with a strong birefringence. These are an evidence of dissolution from the
sides of the laminations. This is marked by tiny fiber-like structures of gypsum
then there are small gypsum particles beside those structures which could be
detrital crystals of gypsum that recrystallized after dissolution.
4.3.3 SatinSpar Gypsum Form: These are tabular beds of fibrous gypsum
commonly parallel to subparallel to the beddings of the blue-black shales. The
thickness range from 2cm-6cm and consist of vertically arranged fibers or
acicular crystals which are perpendicular to the bedding of the shale (Plate 3;
Fig.8, Fig.9a & 9b).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
31/81
29
Plate. 3 Satinspar Gypsum Form
Fig. 9(a) Satinspar Gypsum Form
Fig. 9(b) Enterolithic Satinspar
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
32/81
30
This gypsum form still retained the traces of former laminations which appear
as primary structures. The gypsum tabular thick bed alternative with black
shale beds from depths of 48m in the deeper parts of shales. The deeper the
shale bed, the thicker the SatinSpar. This shows an obvious probable
nucleation of fibrous crystals with depth due to overburden pressure.
At greater depth of 9 11m, the thick beds of SatinSpar are folded,
which appeared to be due to nucleation of the fibrous crystals. Tucker
(1981)and Leeder (1982) called it enterolithic folding which could be due to
contortion of fibrous gypsum beds under high confining pressure at depth due
to lack of space. This phenomenon causes the deformation of blue-black
shales as there are many fragments of blue black shale within the folded
SatinSpar. This might have happened during the early diagenesis. The
enterolithic SatinSpar was found to alternate with deformed blue-Black shales
which increase in thickness with depth, it was also obvious that the
deformation of shales increase with depth. This may be due to increase in
confining pressure coupled with the folding phenomenon. The former
lamination was partially preserved. The orientation of the fibrous crystals
along the folded side is; inclined-horizontal-inclined. The carrier beds are
exclusively blue-black shales.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
33/81
31
Petrographic studies of the SatinSpar gypsum showed that, under
plane polarize light, the gypsum is colourless. Under cross polars the
cleavages are perfect, distinct and seemed to define the direction of lamination
on the matrix. These is growth of parallel fibers of gypsum from anterior and
posterior sides of the slide towards the centre. These fibers are growing
inclined to the laminae and the cleavage. This structure is called gneissic
structure by Shearman et.al, (1972). The cleavages seemed to persist from the
original matrix into the recrystallized fibers. Most of the fibers show low
relief, low birefringence when the stage is rotated under cross polars. They
showed inclined extinction. This indicates that they are gypsum fibers. A few
fibers showed strong birefringence with high relief, so they are remnant
anhydrite crystals.
4.3.4 Selenite Gypsum Form: This consist of laterally continues beds of
prismatic gypsum crystals that are arranged vertically or obliquely to the
bedding planes of mudstones. The thickness of the selenite beds depends on
the length of the vertically arranged crystals (Fig.10: plate 4a).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
34/81
32
Plate. 4 Selenite & Alabaster Gypsum Form
Fig. 10 Selenite Gypsum Form
Fig. 11 Alabaster Gypsum Form
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
35/81
33
The length of the crystals range from 2-3cm. the selenite crystals are
associated with minor fragments of pyramidal detrital gypsum and fragments
of mudstone between the prismatic crystals. The bed of the selenite crystals
show traces of former lamination. Sometimes, the prismatic crystals are well
developed to the extent that they persist through the former laminations
without breaking (Fig.10; Plate 4a). The surface of the selenite bed is irregular
and rough and this could be due to leaching by ground water. It is found at the
depth of 46m within mudstone and grey shales in the mining pits.
Petrogrphic studies of selenite indicated that under plane polarized light
the gypsum is colourless with traces of cleavage. Under cross polars the
matrix contain well developed, distinct, perfect cleavage which are oriented
in one direction showing the direction of lamination as well as the bedding.
The prismatic selenite crystals grow perpendicular to the cleavage and the
lamination. The gypsum crystal (prismatic) goes into incline extinction at
different times when the stage is rotated indicating probable recrystallization
at different times. They all show weak birefringence, low relief, and weak
interference colours of white and dark under cross polars. Some of the gypsum
crystals go into inclined extinction at 450
while others go into extinction at
400(Fig. 12).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
36/81
34
There were very few anhydrite relics in the slide. Some parts of the matrix is
recrystallized into megacrysts of gypsum which is further fragmented into
selenite and arranged perpendicular to the cleavage and lamination. Most of
these megacrysts of gypsum goes into extinction at the same time when the
stage is rotated. At the edge of the laminations, there is evidence of dissolution
marked by tiny fibers of gypsum. Besides the tiny fibers, there are small
angular crystals which could have developed during recrystallization.
The petrographic characteristic of the detrital gypsum in the slide
(birefringence) is similar to that of the selenite prismatic crystals but there is
no cleavage on the tiny angular crystals which recrystallized after dissolution.
Similar gypsum crystals going into extinction at different times on the same
slide have been described as porphyroblast by Holliday (1973).
4.3.5 Alabaster gypsum Form: This is exclusively massive gypsum. It is
internally laminated but the laminations are thicker than that of the Balatino
laminated gypsum. It was found at greater depth of 1213m within the highly
deformed blue-black shales. It is very heavy and has irregular, rough texture
from outside surface. It is internally laminated and the laminations have
thickness of 3-4mm. There are lots of blue-black shale fragments trapped
between most of the internal gypsum laminations. The internal gypsum
laminations appear undisturbed. It looks similar to Selenite only that here
there is no evidence of recrystalization in hand specimen. The internal
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
37/81
35
laminations are thick, well developed, well preserved, and transparent (Fig.11;
Plate 4b). The inner laminations are characteristic of meganodule of gypsum
at great depth Holliday (1970).
The petrographic studies of the Alabaster gypsum showed a marked
similarity of that of the Balatino laminated gypsum. Under plane polarize
light, the gypsum was colourless and the traces of cleavages were seen. Under
cross polarize light; the gypsum appeared to be laminated with the limit of
each laminae forming a mammilary structure. The mammillary structures
displayed an offlap relationship. All the laminations have distinct, well
developed, perfect cleavages which defined the direction of lamination.
The recrystallization is marked by the growth of small interlocking crystals
which are arranged perpendicular to the laminations as well as the cleavage.
Some of the crystals showed low relief, weak birefringence of white and dark
colours. They showed inclined extinction when the stage is rotated. Some of
the few crystals showed a high relief, strong birefringence of white and dark
colours. They go into extinction at 720
as the stage is rotated; this implies that
they are remnant anhydrite crystals that were not hydrated, (Fig. 13)
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
38/81
36
Fig. 12 Sketched Photomicrograph of Selenite Gypsum
Fig.13 Sketched Photomicrograph of Alabaster Gypsum
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
39/81
37
The cleavages persist from the matrix of gypsum into the recrystallized
interlocking crystals. The recrystallize crystals therefore consist of small to
large often poorly defined interlocking crystals, many with irregular
extinction. This is typically restricted to Alabaster gypsum and is sometimes
called Alabastrine texture. Holliday, (1973). There is no evidence of
dissolution in the slide; this could be due to lack of enough circulation of
water at depth. This is evident from the presence of the remnant anhydrite
crystals that were not hydrated and hardness of the gypsum.
4.4 Geochemistry of gypsum Forms
The result of the chemical analysis of the different gypsum forms are given in
Table 3:
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
40/81
38
Table 3: Chemical Composition of Gypsum Forms
SAMPLE IN WEIGHT% PPM
chemical
compound wt (0/0)
Sample
Sample Sample Sample Sample Sample
SO3 41.4 37.8 37.8 34.2 35.1 34.4
CaO 28.4 24.4 26.6 23.4 24.7 23.7
K2O 0.53 0.33 0.26 0.36 0.32 0.41
TiO2 0.14 0.15 0.15 0.06 0.18 0.07
MnO2 0.03 0.04 0.05 0.13 0.03 0.02
Fe2O3 0.19 0.12 0.04 0.04 0.10 0.02
SrO 0.008 0.007 0.008 0.01 0.007 0.02
Combined water 15.12 14.80 14.31 19.84 14.54 15.03
Trace elements in
(ppm)
CO 61.9 69.6 70.5 68.1 107 458
CU 31.6 33.2 44.2 33.7 42.9 23.4
Zn 28.2 21.3 22.9 25.1 22.4 26.9
Rb 4.68 3.82 6.84 3.96 4.56 2.31
Y 3.25 2.83 3.14 2.27 2.64 1.33
Zr 7.79 5.23 7.47 3.97 2.89 5.39
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
41/81
39
Table 3: Contd
SAMPLE IN WEIGHT% PPM
Chemical
compound wt (0/0)
Sample
Sample Sample Sample
SO3 35.3 46.3 48.3 46.7
CaO 23.5 30.9 31.7 31.7
K2O 0.47 0.5 0.5 0.72
TiO2 0.17 0.18 0.2 0.43
MnO2 0.04 0.05 0.04 0.1
Fe2O3 0.03 0.05 0.02 0.07
SrO 0.02 0.02 0.04 0.02
Combined water 14.56 14.50 14.07 15.0
Trace elements in (ppm)
CO 88.5 75.4 112 165
CU 87.5 37.8 123 85.5
Zn 29.7 38.7 38.1 70.3
Rb 5.47 4.76 5.03 14.6
Y 3.69 4.28 3.38 6.85
Zr 6.67 7.46 8.73 13.4
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
42/81
40
Sample identification by numbers, name and locality
# 1 Balatino Laminated Kanwaram
# 2 Balatino Laminated TurmiMalori
# 3 Balatino Laminated Kwayaya
# 4 SatinSpar Garin Ari
# 5 SatinSpar Bulagaje
# 6 Selenite TurmiMalori
# 7 Selenite Zangaya
# 8 Alabaster Bulagaje
# 9 Alabaster TurmiMalori
# 10 Alabaster Fika
The total CaSO4 . 2H2O content in each gypsum form determine the grade, so
it is obtain in Table 4.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
43/81
41
Table 4: Average weight of the gypsum forms
Gypsum Forms CaSO4 . 2H2O content in wt.
(%)
Average weight in wt. (%)
A 85.3
77.0
78.7
80.3
B 77.4
74.3 75.9
C 73.1
73.4 73.3
D 91.7
94.1
93.4
93.1
A = Balatino Laminated Gypsum Form
B = SatinSpar Gypsum Form
C = Selenite Gypsum Form
D = Alabaster Gypsum Form
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
44/81
42
The results of this computation are plotted in Fig. 14. The results showed that
the Alabaster gypsum form is the heaviest as such very suitable for the cement
works. All the gypsum forms are high grade because the CaSO4. 2H2O content
in each gypsum form exceeds 70% (Table 4; Fig.14).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
45/81
43
Alabaster > Laminated > Satinspar > Selenite
Fig. 14 A Bar chart of average weight % against Gypsum forms
L
AMINATEDGYPSUM
S
ELENITEGYPSUM
SATINSPARGYPSUM
ALABASTERGYPSUM
20
40
60
80
100
Y
AVERAGE
wt%
(CaSO
4.
2H
2O)
GYPSUM FORMS
X
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
46/81
44
Water determines the stability field of gypsum/anhydrite. Loss of water by
gypsum convert it to anhydrite which lower the grade of gypsum unless if it is
rehydrated to gypsum. The assessment of water content indicates which
gypsum form is most stable (i.e with high water content). This estimation is
illustrated in Table 5.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
47/81
45
Table 5: Average moisture content of the gypsum forms
Gypsum Forms Moisture content in wt. (%) Average Moisture content in
wt. (%)A 15.2
14.8
14.3
14.8
B 19.8
14.5 17.2
C 15.0
14.6
14.8
D 14.5
14.1
15.0
14.5
A = Balatino Laminated Gypsum
B = SatinSpar Gypsum
C = Selenite Gypsum
D = Alabaster Gypsum
The result of these computations is also plotted in Fig. 15;
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
48/81
46
Fig.15 A Bar chart average moisture content against Gypsum forms
LAMINATEDGY
PSUM
SELENITEGYPSUM
SATINSPAR
GYPSUM
ALABASTERGY
PSUM
4
8
12
16
20
Y
AVERAGEMOISTURECONTNET(wt%)
GYPSUM FORMS
X
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
49/81
47
Fig.15 showed that the SatinSpar has the highest water content. This can be
interpreted as being the most stable gypsum form in terms of water content.
The rest of the gypsum forms also contain appreciable quantities of water and
are also stable. The quantity of water can be used to show the uplifted areas or
fractured zones which are areas of circulation of ground water.
The combined results of the average weight (%) and average moisture content
in weight (%) is plotted in Fig. 16.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
50/81
48
SSSatinspar Gypsum
SESelenite Gypsum
BLBalatino Laminated
ALAlabastar Gypsum
Fig.16 Graph of average moisture content against average weight of Gypsum
4
8
12
1
20
Y
AVERAGEMOIST
URECONTENT(inwt%)
70
X
80 90 100
ALBL
SS
SE
AVERAGE TOTAL WEIGHT OF GYPSUM (in wt %)
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
51/81
49
It clearly indicated that: Satinspar, Selenite and Balatino laminated gypsum
are located in areas of circulation of water. This may explain why they are
light weighted than Alabaster. It is possible that they are lighter than Alabaster
due to dissolution by ground water as seen in the thin section (Fig.7 & Fig.12).
All the gypsum the gypsum forms shows appreciable amount of minor and
trace elements in their composition. This is common in gypsum that primarily
precipitated as laminations Sonnenfield (1991)
4.5 Economic Significance of the Gypsum Forms
The Balatino Laminated Gypsum, SatinSpar gypsum, Selenite gypsum and
Alabaster gypsum are all economically significant because they can be used as
raw materials in many industries; cement chemical, pharmaceutical, ceremics,
chalk, paper, and etc.
The cement companies prefer the heaviest gypsum form to enable them
improve the quality of their products. In the present work, the alabaster
gypsum form is very suitable for the cement works because it is comparable in
composition and weight to the imported gypsum from Spain and Morocco.
Table 6 give the relationship
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
52/81
50
Table 6: Comparison of chemical composition of gypsum from Nigeria
(Research Area) Morocco and Spain
A B C
Nigeria wt. (%) Damagun-
Fika Gypsum
Morocco wt. (%)
Moroccan Gypsum
Spain wt. (%) Spanish
Gypsum
SO3 46.7 47.32 44.85
CaO 31.7 33.89 31.44
K2O 0.72 - -
TiO2 0.43 - -
MnO2 0.1 0.07 0.2
Fe2O3 0.07 0.05 0.16
SrO 0.02 - -
Al2O2 - 0.12 0.53
M2O - 1.05 0.50
SiO2 - 0.60 1.50
Combined H20 15.0 15.67 18.38
CaSO4 . 2H2O 93.4 95.9 94.7
A = Data from present work
B = Data from Sillo and Okunsenogu (1994)
C = Data from Orazulike (1988)
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
53/81
51
The three gypsum forms have very identical weight (above 90%) as such they
are all very suitable for cement works prescribed by the Nigeria Industrial
Standards (N15.11/1974).
All the gypsum forms are suitable for the chemical industry because the
average CaSO4 . 2H2O content is over 70%. The Pharmaceutical companies
need very clean, undeformed gypsum to produce high quality Plaster of Paris.
The Balatino laminated gypsum is suitable because it contain appreciable
quantity of CaSO4 . 2H2O (Table 4.). It is clean and transparent as such it
would be very good for making Plaster of Paris (P.O.P) for medicine as well
as ceramics, paint, and paper industries.
A comparison of chemical composition of Alabaster gypsum from Chad Basin
(research area), Adoka gypsum (Mid-Benue Trough) and Sokoto gypsum
(Sokoto Basin) revealed that the gypsum in Chad Basin is the best in quality
for cement works because it is the heaviest (93.4wt.%), followed by Adoka
gypsum (89.3wt.%) and the lightest is the Sokoto gypsum (42.4wt.%). Table 7
illustrates this relationship.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
54/81
52
Table 7: Comparison of chemical composition of gypsum in Nigeria
(Chad Basin, Mid-Benue and Sokoto Basin)
A B C
Yobe State
(Chad Basin)
Alabaster gypsum
Benue State
(Mid Benue Basin)
Adoka gypsum
Sokoto State
(Sokoto Basin)
Sokoto gypsum
SO3 46.7 41.6 17.4
CaO 31.7 29.2 15.21
K2O 0.72 - -
Fe2O3 0.43 0.09 0.63
SiO2 - 5.50 26.55
Combined H20 15.0 18.52 9.78
CaSO4 . 2H2O 93.4wt.% 89.3wt.% 42.4wt.%
AData from the present work (Chad Basin)
BData from Sillo and Okunsenogu (1994)
CData from Sillo and Okunsenogu (1994)
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
55/81
53
All the gypsum samples are suitable for raw materials in chemical industries.
Also a comparison of chemical composition of SatinSpar gypsum (Garin Ari)
and that of Pindiga gypsum shows that they are very similar. They also have
similar carrier beds (Shales), Table 8 illustrate that relationship.
s
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
56/81
54
Table 8: Comparison of chemical composition of gypsum in Chad
basin and Upper Benue Trough.
A B
Chemical Compound wt (%) Garin Ari Gypsum (Chad Basin) Pindiga gypsum; (Upper Benue)
SO3 34.65 33.88
CaO 24.05 24.87
K2O 0.34 0.22
TiO2 0.12 0.08
MnO2 0.08 -
Fe2O3 0.07 1.16
Combined water 17.19 17.25
CaSO4 . 2H2O 76.5wt.% 76.0wt.%
Trace elements in (ppm)
Mn 542.0 16.0
Zn 23.8 16.0
Rb 4.3 21.0
Y 2.5 11.0
Zr 3.4 10.0
Sr 146.0 207.0
AData from present work (Chad Basin)
BData from Orazulike (1988)
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
57/81
55
The carrier beds (Fika Shales and Upper Pindiga Shales) are time equivalent
of each other, therefore the gypsum could be time equivalent of each other.
The evaluation of the tonnage of the gypsum mineralization in the research
area was not possible because of shortage of fund but the geographical spread
of the mineralization in Chad Basin suggest that it is in economic quantity.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
58/81
56
CHAPTER FIVE: DISCUSSION
5.1 Genetic Model
It has long been recognized that simple evaporative concentration of sea
water is insufficient to produce the great thickness of evaporate salts observed
in the geologic record Borchert and Muir (1964). Fig.17 showed that the ideal
sequence of salts precipitation from seawater differs somewhat from the actual
sequence recorded in evaporates deposit in the geological record. It shows
increase in proportion of NaMg sulphates.
Raup (1970) proposed a brine mixing hypothesis for the salt in the Paradox
basin of Colorado. The model is similar to that proposed by Adams (1944) and
King (1947) for the Delaware basin; by Dellwig (1955), Briggs (1957, 1958)
and Dellwig and Evans (1969) for the Michigan basin; Kuhn (1955) and
Richter Bernburg (1955) for the Werra and Lower Saxony basins of the
German Zechstein; and by Hite (1968, 1970). Peterson and Hite (1969).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
59/81
57
Fig. 17 Comparative precipitation profiles from
(a) Experimental evaporation of sea water
(b) From a Zechstein evaporate sequence in Germany and
(c) From the average of numerous other marine evaporate
sequences around the world(After Borchert and Muri, 1964).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
60/81
58
Raup (1982) conducted an experiment by mixing two brines (sea waters) of
different evaporative history with different composition and specific gravity.
He concluded that,
(1)Precipitation of gypsum can occur in a marine evaporate basin bymixing brines of different composition and specific gravity
(2)Precipitation occurs without further water loss by evaporation(3)Precipitation can occur from a brine that was under-saturated before
mixing
(4)The only form of calcium sulfate to precipitate in this case is gypsum.5.2 Geologic Model
The presence of gypsum mineralization in Chad Basin and Upper Benue
separated by the Zambuk Ridge suggest a brine mixing hypothesis for the
origin of the gypsum mineralization in north-eastern Nigeria Fig.18. This is
based on the works of Raup (1970; 1982).
The simplest model for sub-aqueous gypsum precipitation is the
Shallow Water Barred Basin where evaporation proceeds in semi -isolated
Chad Basin with replenishment of sea water from Upper Benue Trough over a
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
61/81
59
restricted entrance (Zambuk Ridge) during Upper Turonian-Maastrichtian
marine transgression. Fig.18 illustrates this relationship.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
62/81
60
Fig. 18 Diagrammatic cross section of Chad Basin, Zambuk Ridge and
Upper Benue Trough showing effects of sea level on influxing
currents and formation of stratified brine layers which depositedGypsum. Movement of current shown by dashed arrows.
(Adopted and modified from Raup, 1970 & 1982)
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
63/81
61
The timing of the gypsum mineralization is consistent with the time of
the deposition of the marine Fika Shales in the Chad Basin. Carter et al.
(1963), Petters (1978) reported independently that Black Shales were
deposited during Mid-Cretaceous marine transgression in both upper Benue
and Chad Basin. During the precipitation of gypsum, the epicontinental
shallow areas are characterized by the oxidizing conditions due to periodic
transgression. It is obvious that there were interruptions during shales
deposition. These are marked by gypsum interbedding with shales. The shales
are reported to have been deposited under oxidizing condition (because of its
composition). These create a conducive and continuous ideal environment
for gypsum deposition under oxidizing condition.
The inflow of water (transgression) from Upper Benue into the Chad
Basin led to salinity changes due to mixing of sea water brines of different
stages of evaporation (i.e. sea water brines from Upper Benue and Chad
Basin), Fig.18. This led to salinity and density stratification due to the brine
mixing phenomenon. The total salinity of the brine in Chad Basin and reflux
of dense high salinity brine into the Upper Benue. This marked the period of
low sedimentation rate of shales possibly due to change in Ph/Eh as well as
salinity conditions.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
64/81
62
As the surface layers of water becomes more and more dense, part of it will
sink until it reach water of the same specific gravity. This process formed a
series of stratified brines, the densest at the bottom and the least dense at the
top. Precipitation of gypsum would occur where brines mixed at the interface
between dense basal layers and the less dense overlying layers. This is one
laudable mechanism for gypsum precipitation as pure salt laminations. This
model is similar to that of Adams (1944); and Raup (1970; 1982).
During the period of high sea level (Fig.18 Y); the depth of water in
Chad Basin and over the barrier (Zambuk Ridge) is greatest. Water flowing
into the Chad Basin (transgression) is little restricted, and because of the high
water level, space is sufficient for dense high salinity brines to flow out as
an under-current (reflux). If the sea level continues to lower, eventually the
refluxing brines cannot overcome the friction of the influxing surface currents
and reflux stops (Fig. 18z). This produces an increase in both salinity and rate
of gypsum precipitation in the Chad Basin and the parts of Upper Benue
adjacent to the Zambuk Ridge.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
65/81
63
5.3 Mechanism for the evolution of the Gypsum Forms
The main process of formation of the present gypsum forms is
anhydritization during early diagenesis after the precipitation of gypsum. All
the gypsum forms (except detrital forms) have traces of lamination both in
hand specimen and in thin section so they all evolved from a primary
laminated gypsum. The present structures of the gypsum forms, mammillary
structures, prismatic structures, fibrous structures and enterolithic structures
are all secondary structures imparted on the primary laminated gypsum during
anhydritization (diagenesis). This is shown in Fig.19.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
66/81
64
Fig. 19 Model for the mechanism of anhydritization of primary gypsum
during early diagenesis.
(Adopted and modified after, Sonnenfeld, 1991).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
67/81
65
Diagenesis of gypsum occurred simultaneously with the diagenesis of
the mudstone and shale (the host rock). During the early diagenesis, the main
physical post-depositional process affecting the shale and gypsum as a whole
is compaction due to the overburden pressure. Compaction is the main process
that converts sediment into sedimentary rock and the salt into a rock salt. The
compaction led to the loss of water by both shale and the gypsum. The loss of
water in the gypsum led to the increase in strength, structural changes
resulting from chemical changes and yields to deformation by reaction hence
the different structures at various depths. This is the main process of
anhydritization as well as the mechanism of formation of the present gypsum
forms. At greater depth under higher overburden pressure, the deformation is
higher than at shallow depth, and after the anhydritization all the primary
gypsum must have been converted to anhydrite. At shallow depth laminated
gypsum was converted to laminated anhydride with mammillary structure. At
the upper parts of intermediate depth, the laminated gypsum was converted to
prismatic anhydrite. At the lower parts of the intermediate depth, the
laminated gypsum is converted to fibrous enterolithic anhydrite under medium
overburden pressure. At greater depth, the thick laminated gypsum was
converted to massive anhydrite.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
68/81
66
During the late diagenesis, there was upliftment of the whole area
followed by erosion; as a result the different anhydrite forms were converted
back to the present gypsum forms by rehydration. The level of rehydration
depends on the level of upliftment.
Dehydration
CaSo4 . 2H2O CaSO4 + 2H2O
Rehydration
Gypsum Anhydrite
The process of rehydration is accompanied by dissolution of anhydrite and
gypsum as all of them are slightly soluble in water. The dissolved gypsum
recrystallized again within the mudstone and shale to form detrital pyramidal
crystals of gypsum in disseminated form. So at the end we have; Balatino
laminated gypsum form laminated anhydrite; Selenite gypsum from prismatici
anhydrite; SatinSpar gypsum from fibrous anhydrite; Alabaster gypsum from
thick massive anhydrite and finally the detrital pyramidal crystal of gypsum
precipitating from solution within mudstones, bedding planes, fissile
structures and cracks. The summary of the whole process is:
Dehydration RehydrationGypsum Anhydrite Gypsum
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
69/81
67
5.4 Paleaoenvironment of Gypsum Deposition
The suitable environment of gypsum precipitation is a shallow marine,
epicontinental, near-shore sabkha environment. This environment is
characterized by fluctuation of reducing and oxidizing condition (but mostly
oxidizing condition), mostly due to periodic incursion of water from the
adjacent sea. This could be the condition under which the gypsum carrier beds
(Fika Shales) were deposited.
Ekweozor et al,. (1989) reported that the organic matter in Fika shales
were derived mostly from oxic paralic swamps. This confirm that there was
persistent oxidizing condition during the deposition of shallow parts of Fika
Shales and this is evident by the alternate bedding of the shales with gypsum.
Ekweozor et al., (1989) reported that the organic matter in Fika shales were
derived mostly from oxic paralic swamps. This confirm that there was
periodic oxidizing condition during the deposition of Fika Shales and this is
evident by the interbedding of the shales with gypsum.
Reyment (1965) have suggested deposition of Fika Shales under a
shallow, epicontinental sea due to the low diversity of benthic foraninifera
assemblages and the scarcity of planktonic species.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
70/81
68
Barber (1965) reported that only 10% of the whole Chad Basin lies in
Nigeria. This area or most parts of it could be the epicontinental shallow areas
sloping from the Zambuk Ridge into the Chad Basin. This could be the reason
why only the Nigerian sector of Chad Basin contains economic gypsum
deposit.
The Zambuk Ridge is basement dome that separated the Chad Basin
from Upper Benue. This done seem to be the structural barrier that created a
conducive, shallow, epicontinental environment on its flanks which led to the
gypsum precipitation in both the Chad Basin and Upper Benue, Fig.18. All
this events most have happened before any diagenesis and/or orogeny took
place. Sennenfield (1991) reported that gypsum precipitation occur in the
shallow areas within the photic zone (about 300-400m depth), below which
the gypsum scavenging continues by the anaerobic bacteria (Disulfovibrio).
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
71/81
69
CHAPTER SIX: SUMMARY AND CONCLUSION
6.1 Summary
The primary gypsum that was deposited in the Chad Basin during the
brine mixing phenomenon was laminated gypsum. It was deposited alternately
with shale or as shale interbeddings. Sonnenfeld (1991) reported that the
position or precipitation was always accompanied by subsidence that is why
gypsum alternates with shale at various depths.
During the early diagenesis, the shale and laminated gypsum were
affected concurrently by compaction. The laminated primary gypsum at
various depths reacted differently depending on the pressure regime. This
process imparted different structures on the laminated gypsum depending on
the level of emplacement. So the structures are; laminated, prismatic, fibrous
and enterolithic, and massive. All these are secondary structures. They are
mainly anhydrite structures. During the late diagenesis and probably during
the Maastrichtian orogeny which trends NW SE, the marine sequence in the
Chad Basin were folded and upliftment (Benkhelil, 1982). This suggest why
the different anhydrite forms were uplifted within the Fika Shales. This
upliftment led to the rehydration of the anhydrite at various depths by ground
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
72/81
70
water circulation and meteoric water which led to the formation of the present
gypsum forms (Balatino Lamined, Selenite, SatinSpar and Alabaster). The
rehydration is always accompanied by slight dissolution and so the dissolved
gypsum later recrystallized in mudstone and shale to form the detrital
pyramidal crystals of gypsum in disseminated form.
Many studies of gypsum-anhydrite relationship have shown that the
stable phase is determined by the activity of water and temperature (Hardie,
1967). Recrystallization of equant and lath anhydrite produces coarse granular
mosaics (pile of bricks texture), large fibrous crystals and fibro-radiating
aggregates (Holliday, 1973). Alabaster gypsum consists of small to large,
often poorly-defined interlocking crystals, many with irregular extinction
(Holliday, 1970). Veins of SatinSpar (fibrous) gypsum grew under pressure in
water-filled veins induced by hydraulic fractures (Shearman et al., 1972). The
detrital pyramidal gypsum results from crystals growth in more permeable
zones under phreatic condition (Arakel, 1980). So the transformation is;
gypsum anhydrite gypsum by
compaction dehydration rehydration.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
73/81
71
The petrographic studies of all the gypsum forms (except detrital form)
indicated traces of lamination which are defined by perfect cleavage. All the
recrystallized crystals occur inclined-perpendicular to the lamination which
confirmed that they are secondary and the lamination is a primary structure.
There are very few disseminated anhydrite crystals within the gypsum crystals
and this showed the remnant anhydrites that were not completely hydrated.
The chemical composition of the gypsum forms showed that all the
gypsum forms Balatino laminated, Selenite, SatinSpar and Alabaster are high
grade gypsum because the contained over 70% CaSO4 . 2H2O. Economically
they are good gypsum raw materials but the Alabaster being the heaviest, is
the most suitable for cement works. Balatino, Selenite, SatinSpar and
Alabaster could serve as good raw materials for the chemical industries
especially in manufacture of fertilizer. The pure Balatino laminated gypsum is
the best for the pharmaceutical, ceramic, paint and paper industries because it
makes a very good, Plaster of Paris (Calcined gypsum) because of its pure
quality.
A brine mixing hypothesis is proposed to account for the origin of
gypsum mineralization in northeastern Nigeria (part of which is the research
area (Chad Basin)). This is based on evidence from recent works of Raup
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
74/81
72
(1970; 1982), the presence of gypsum mineralization on both sides of the
Zambuk Ridge (Chad Basin and Upper Benue), the similarity of carrier beds
of gypsum (mudstones and shales) from Chad Basin into the Upper Benue,
and similarity in structure and composition of the gypsum in Chad Basin and
Upper Benue. Sonnenfeld (1991) reported that most primary laminated
gypsum have very high concentration of minor and trace elements like Sr, Zr,
Y, Rb, Zn, Cu, Fe, Co etc. All the gypsum forms have high concentration of
these elements and it could have been concentrated by more than one brine.
This adds more evidence to the brine mixing hypothesis. A model for the
evolution of the present gypsum forms is proposed based on Sonnenfeld
(1991).
Ekweozor et al. (1989) reported that the organic matter in Fika Shales
have been derived most probably from on oxic parallic swamp which is
poor in oil generation. This condition seem favourable for the gypsum
mineralization as low organic content have been shown by (Sonneenfeld,
1991) to influence what gypsum form will be present depending on
prevailing Ph/Eh conditions. Sonnenfeld (1991) reported that proteins,
naphthenic acids, amino acids, resins and sugars delay gypsum precipitation or
even prevent it. The higher the Ph (>7) during gypsum precipitation, the
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
75/81
73
stubbier are the crystals. Alkanes, phenols, and fatty acids foster the formation
of more tabular, equidimensional, or discoidal lenticular crystals of gypsum.
Elongate, prismatic gypsum crystals grow in low Ph (
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
76/81
74
(c) The gypsum mineralization in Chad Basin have shale and mudstone as
carrier beds and the one in Upper Benue also have shales and mudstone
as carrier beds. The carrier beds are horizontal and lateral equivalent of
each other so the gypsum mineralization in both basins is time
equivalent of each other.
(d) The abnormally high concentration of minor and trace elements in all
the gypsum forms originate from more than one source because
evaporation alone cannot concentrate that much.
(2) Five different gypsum forms are recognized; Balatino Laminated
Gypsum; Selenite gypsum; SatinSpar gypsum, Alabaster gypsum; and
Detrital gypsum. With the exception of detrital gypsum, all the past
high grade because the contained over 70% CaSO4 . 2H2O. This implies
that they are economically significant.
(3) With the exception of the detrital gypsum, all the other forms showed
traces of lamination both in thin section and hand specimen. This
suggest that lamination is a primary structures.
(4) Balatino Laminated, Selenite, SatinSpar and Alabaster gypsum forms
all have secondary structures (like; mammillary, prismatic, fibrous and
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
77/81
75
granular) which are imparted on the primary gypsum during early
diagenesis.
(5) The main agents of evolution of present gypsum forms are; chemical
changes, loss of water due to compaction which increases with depth.
(6) Dissolved organic matter is an important steering mechanism of
evaporative precipitation; it influences gypsum precipitation and
determines the type of gypsum form and thickness.
(7) The rehydration and transformation of anhydrite to the present gypsum
forms could not have involved complete dissolution stage but there
could be a gelatinous stage. This is because most of the gypsum forms
preserved the primary laminations which would have been obliterated if
there is a complete dissolution stage.
(8) The changes in the gypsum forms is not horizontal with distance but
vertically with depth
(9) So it is justifiable that the importation of gypsum into Nigeria should
be banned.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
78/81
76
REFERENCES
Adams, J.E. 1944. Upper Permian Ochoa Series of Delware basin, West
Texas, and Southeastern New Mexio; American Association ofPetroleum Geologist Bulletin,28: 15961625.
Anonymous, 1996. Minister for Solid Minerals. The Guardian, October 26th
,1996.
Arakel, A.V. 1980, Genesis and diagenesis of Holocene evaporitic sediments
in Hutt and Leeman Lagoons, Western Australia; Journal ofSedimentary Petrology, 50: 13051326.
Barber, W. 1965. Pressure water in the Chad Formation of Bornu and Dikwa
Emirate, northeastern Nigeria, Geological Survey of Nigeria, Bulletin35.
Benkhelil, M.J. 1982. Structural Map of the Upper Benue valley. Journal ofMining and Geology, 18: 140151.
Briggs, L.I.Jr. 1957. Quantitative aspects of evaporates deposition;MichiganAcademic Science Papers, 1956, 42: 4656.
Briggs, L.I. 1958 Evaporite facies.Journal of Sedimentary Petrology, 28: 46
56.Borchert, H. and Muir, R.O. 1964. Salt deposits; The Origin, Metamorphsm
and Deformation of Evaporites, Van Nostrand, London, 338 pp.
Carter, J.D., Barber, W.M. and Tait, E.A. 1963. The geology of Parts of
Adamawa, Bauchi and Borno provinces in North Eastern Nigeria.Bulletin of Geological survey of Nigeria 30: 1134.
Dellwig, L.F. 1955, Origin of the Salina salt of Michigan, Journal of
sedimentary petrology, 25: 83110.
Dellwig, L.F., and Evans, R. 1969. Depositional Processes in Salina Salt of
Michigan, Ohio, and New York: American Association of PetroleumGeologist Bulletin 53: (4), 949956.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
79/81
77
Ekweozor, C.M. and Mukhopadhyay, P.K. 1989. Initial Assessment of the
Palaeoenvironment and Petroleum Potential of Cretaceous Black Shales
in the Lower Benue Trough and Soughwestern Chad Basin. NigerianAssociation of Petroleum Explorationist Bulletin 4: (1) 5680.
Falconer, J.D. 1911. The Geology and Geography of Northern Nigeria. MacMillan Publishers, London. Pp. 3336.
Hardie, L.A. 1967. The GypsumAnhydride equilibrium at one atmosphericpressure.American Mineralogy. 52: 171200.
Hite, R.J. 1968, Salt deposits of the Paradox basin, Southeast Utah and
Southwest Colorado in R.B. Mattox, ed., Saline deposits, a symposium
based on paper from the international conference of Saline deposits:
Geological Society of American Spectacular paper. 88: 319330.Hite, R.J. 1970. Shelf carbonate sedimentation controlled by salinity in the
Paradox basin, Southeast Utah, in Third symposium on salt: NorthernOhio Geological Society,1: 4866.
Holliday, D.W. 1970. The petrology of secondary gypsum rock; a review.
Journal of sedimentary Petrology 40: 734744.
Holliday, D.W. 1973. Early diagenisis in nodular anhydride rocks. Trans.Institute of Mining and Metallurgy. 82: 8184.
Analytical Technique for Geological material, in Bulletin of InternationalAtomic Energy Agency Autria. June 1997.
King, R.H. sedimentation in Permian Castile Sea (U.S.); AmericanAssociation of Petroleum Geologist Bulletin, 31: 470477.
Kuhn, R. 1955. Tiefenberechnung des Zechsteinmecres nach dem Bromgehalt
der Salze:Zeitschr. Deutschen Geologischen Gessellschaft, 105: 646663.
Leeder, M.R. 1982. Sedimentology Process and Product. pp. 30 34.George Allen and Unwin publishers Ltd. London.
Maglione, g. 1981. An Example of Recent Continental Evaporitic
Sedimentation; The Chadian Basin (Africa), in Evaporite Deposits, Gulf Publishing Company, Houston, Texas, 59.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
80/81
78
Orazulike, D.M. 1988. The Prospect of Gypsum as an Ore in Pindiga,
Bauchi State. Unpublished interim Technological report and a proposal
for detailed prospecting of the commodity submittd to the AshakaCement Co. Bauchi State. pp. 13.
Petters, S.W. 1978. Stratigraphic Evolution of the Benue Trough and its
implication for the Upper Cretaceous Paleogeography of West Africa.
American Association of Petroleum Geologist Bulletin. 66: 1441 1449.
Peterson, J.A., and Hite, R.J. 1969. Pennsylvanian evaporate carbonate
cycles and their relationship to petroleum occurrence, southern Rocky
Mountains:American Association of Petroleum Geologist Bulletin. 53:884908.
Petters, S.W. and Ekweozor, C.M. 1982. Origin of Mid-cretaceous Black
shales in the Benue Trough, Nigeria.Nigerian Association of PetroleumExplorationist Bulletin 4: 4680.
Racburn, C. and Brynmor, J. 1934. The Chad Basin; Geology and Watersupply. Bulletin of geological Survey of Nigeria. 15.
Raup, O. B. 1982. Gypsum Precipitation by Mixing Seawater Brines.
American Association of Petroleum Geologist Bulletin. 66: (3) 363 367.
Reyment, R.A. 1965. Aspects of the Geology of Nigeria Ibadan UniversityPress. 133.pp
Richtar Bernburg, G. 1955. Uber Salinare Sedimentation; ZeitschriftDuetschen Geologischen Gessellschaft, 105: 593645.
Shearman, D.J., Mossop, G., Dunsmore, H. and Martin, M. 1972. Origin of
Gypsum veins by hydraulic fracture. Trans. Institute of Mining andMetallurgy. 81: 149155.
Sillo, C.E. and Okunsenogu, C.A. 1994; Adoka gypsum and its suitability forCement Production.Journal of Mining and Geology. 30: (2) 169174
Sonnenfeld, P. 1991. Evaporite basin analysis.A review in Economic Geology,5: 159169.
-
7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN
81/81
Tucker, M.E. 1981. Sedimentary petrology, an Introduction. BlackwellScientific publication Australia 33: 158173.
Vischer, H. 1910. Across the Sahara from Tripoli to Borno, Edward
Arnold, London. 808 pp.