REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN NIGERIA)

7/28/2019 REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN

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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.


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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


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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)


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(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.


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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.


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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


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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.


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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)


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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).


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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


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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.


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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)


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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.


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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.


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Fig.2 Flowchart for Gypsum Sample Pelletization

Gypsum samples from field

Cleaning, crushing/rolling

Grinding

Sieving to


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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


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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


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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:


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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


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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


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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


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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


Mining site 8 Garin Ali Zangaya

Thin Mudstone

Grey shales containing Selenite gypsum



Mining site 9 Fika

Thin Mudstone

Grey shales containing Selenite gypsum


Blue-Black shales containing massive Alabaster

gypsum

Fig. 4 contd


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Fig. 4 Sections of Fika Formation Hosting Gypsum Forms from all the

Mining sites.


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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).


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Plate. 1 Detriatal Gypsun Form

Fig. 5 Detrital Gypsun with Pyramid Structure


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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.


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Plate. 2 Balatino Laminated Gypsum Form

Fig. 6 Baltino or Laminated Gypsum


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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.


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Fig. 7 Sketched Photomicrograph of Balatino or Laminated Gypsum

Fig. 8 Sketched Photomicrograph of Satinspar of Fibrous Gypsum


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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).


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Plate. 3 Satinspar Gypsum Form

Fig. 9(a) Satinspar Gypsum Form

Fig. 9(b) Enterolithic Satinspar


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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.


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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).


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Plate. 4 Selenite & Alabaster Gypsum Form

Fig. 10 Selenite Gypsum Form

Fig. 11 Alabaster Gypsum Form


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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).


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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


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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)


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Fig. 12 Sketched Photomicrograph of Selenite Gypsum

Fig.13 Sketched Photomicrograph of Alabaster Gypsum


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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:


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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


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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


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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.


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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


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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).


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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


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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.


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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;


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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


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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.


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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 %)


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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


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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)


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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.


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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)


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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


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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)


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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.


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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).


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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).


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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


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restricted entrance (Zambuk Ridge) during Upper Turonian-Maastrichtian

marine transgression. Fig.18 illustrates this relationship.


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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)


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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.


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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.


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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.


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Fig. 19 Model for the mechanism of anhydritization of primary gypsum

during early diagenesis.

(Adopted and modified after, Sonnenfeld, 1991).


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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.


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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


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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.


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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).


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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


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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.


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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


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(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


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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 (


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(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


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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.


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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.

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Documents

REVIEW OF GEOLOGY, GEOCHEMISTRY AND ORIGIN OF GYPSUM MINERALIZATION IN CHAD BASIN (NORTH EASTERN NIGERIA)