ogah, vincent e. pg/ph.d/07/42866 vincent e.pdf · · 2015-08-31metallic mineral exploration in...
TRANSCRIPT
1
Digitally Signed by: Content manager’s Name
DN : CN = Weabmaster’s name
O= University of Nigeria, Nsukka
OU = Innovation Centre
Nwamarah Uche
Faculty of PHYSICAL SCIENCES
Department of GEOLOGY
METALLIC MINERAL EXPLORATION IN
OTUKPO AREA OF BENUE STATE
OGAH, VINCENT E.
PG/Ph.D/07/42866
2
METALLIC MINERAL EXPLORATION IN
OTUKPO AREA OF BENUE STATE
BY
OGAH, VINCENT E.
PG/Ph.D/07/42866
A Ph.D THESIS SUBMITTED TO
THE DEPARTMENT OF GEOLOGY,
FACULTY OF PHYSICAL SCIENCES,
UNIVERSITY OF NIGERIA, NSUKKA.
APRIL, 2014.
TITLE PAGE
METALLIC MINERAL EXPLORATION IN OTUKPO AREA OF
BENUE STATE
3
BY
OGAH, VINCENT E.
PG/Ph. D/07/42866
BEING A THESIS SUBMITTED TO THE DEPARTMENT OF
GEOLOGY, IN PARTIAL FULFILMENT OF THE
REQUIREMENTS FOR THE AWARD OF THE DEGREE OF
DOCTOR OF PHILOSOPHY IN MINERAL EXPLORATION
AND ECONOMIC GEOLOGY
FACULTY OF PHYSICAL SCIENCES,
UNIVERSITY OF NIGERIA, NSUKKA.
4
DEDICATION
This work is dedicated to my children and my late parents Mr. and
Mrs. Emmanuel Ogah Edaba. whose nursing care and diligent guidance saw
me through early years of life.
5
ACKNOWLEDGEMENT
To God be the glory and greatest appreciation.
I am extremely thankful to my supervisors Professor M.C. Ezepue and
Dr. L. I. Mamah both of the Department of Geology, University of Nigeria,
Nsukka for many valuable discussions and useful suggestions during this
work.
My gratitude further goes to my Head of Department, Prof. Mrs. O.P.
Umeji for her guidance and moral support.
I appreciate with gratitude the contributions of Dr. B.S. Jatau; General
Manager, Nassarawa Minerals Development Company Ltd, Lafia.
I am also thankful to Geological Survey Agency of Nigeria (GSAN)
Abuja, for the airborne magnetic survey map. Thanks are also due to Mr. Oha
Ifyeanyi, Dr. O. Igwe, and other staff of the Department of Geology,
University of Nigeria, Nsukka who assisted me in so many ways.
I am also grateful to Dr. Kola Lawal and Mr. B. Nwosu both of the
Department of Physics, Ahmadu Bello University, Zaria.
The preparation of this thesis would have been impossible without the
generosity, cooperation and moral support of Mallam Labo of GSAN
Kaduna, Mr. Onche Okopi of GSAN, Makurdi. Miss Alache Idoko, members
of my family and others too numerous to mention.
This study was supported by Tertiary Education Trust Fund
(TETFUND) 2009 Training Programme Intervention.
6
CERTIFICATION
I Ogah, Vincent Edaba, a postgraduate student in the department of
Geology, University of Nigeria, Nsukka with registration number:
PG/Ph.D/07/42866 has satisfactorily completed the requirements for the
Degree of Doctor of Philosophy in Mineral Exploration and Economic
Geology.
It is hereby certified that the research work embodied in this
presentation is an original work conducted by me under the supervision of
Prof. M.C. Ezepue and Dr. L.I. Mamah and that it has never before been
submitted in part or in full by any person for any other degree or diploma of
University of Nigeria, Nsukka or any other higher institution.
_____________________ ____________________
Vincent E. Ogah Prof. M.C. Ezepue
Student Supervisor
_____________________
Dr. L.I. Mamah
Co-Supervisor
_____________________ ____________________
Prof. Mrs. O. P. Umeji Prof. E. Okwueze
Head of Department External Examiner
7
TABLE OF CONTENTS
Page
Title page i
Dedication
ii
Acknowledgement
iii
Certification
iv
Table of contents
v
List of figures
x
List of tables
xi
Abstract
xii
CHAPTER ONE - INTRODUCTION
1.0 General Introduction
1
1.2 Location, Accessibility, Climate
3
1.3. The aims and objectives of study
7
CHAPTER TWO - REVIEW OF RELEVANT LITERATURE
(LITERATURE REVIEW)
2.1 Review of previous work
9
8
2.2 Regional Geologic/Tectonic setting of Benue Trough
10
2.3 Mineralization in the Benue Trough
14
2.4 Regional stratigraphic setting of south Benue valley
15
2.5 The Geology of Otukpo
21
2.5.1 Asu River group
21
2.5.2 Eze-Aku Group
23
2.5.3 The Awgu Group
24
2.6 Hydrogeology
26
CHAPTER THREE - METHODOLOGY
3.1 Remote sensing
29
3.1.1 Remote sensing digital image processing
31
3.1.2 Resource Exploration
33
3.1.3 Dataset and method used
34
3.1.4 Trend lines
36
3.1.5 Rose plot
39
9
3.2 Aeromagnetic Studies
41
3.2.1 The Polynomial Fitting Method
41
3.2.2 The Least Square Method
42
3.3 Geochemical Analysis
44
3.3.1 Neutron Activation Analysis
44
3.3.2 Limitation of NAA
45
3.3.3 Principles of NAA Method
46
3.3.4 Equipment And Materials
47
3.3.5 Choosing the Appropriate Procedure
47
3.3.6 Procedure
48
3.3.7 Irradiation Facilities
49
3.3.8 Kinetics of Activation
50
3.3.9 Methods of Standardization
50
3.3.10 Classic Relative Method of Standardization
52
3.3.11 Measurement and Evaluation
52
10
3.3.12 Analysis of the Gamma Spectra
52
3.4 XRF Spectrometry, Applications and Analysis
53
3.4.1 Interaction of X-Rays with Matter
54
3.4.2 The Different XRF Spectrometers
55
3.4.3 XRF Analysis: Sample Preparations
57
3.4.4 Analysis Method
57
3.5 Volumetric Method
58
3.5.1 Procedure
59
CHAPTER FOUR - DATA ACQUISITION AND ANALYSIS
4.1 Aeromagnetic Data Acquisition
61
4.1.1 Aeromagnetic Data Analysis
61
4.1.2 The Regional – Residual Separation
64
4.1.3 Analytical Signal Method of Regional-Residual Separation
66
4.1.4 Methods of Aeromagnetic Survey Data Interpretation
68
4.1.5 Depth Estimation of Magnetic Sources by Means of
11
Spectral Analysis
69
4.1.6 Depth to the Magnetic Body
72
4.1.7 Upward and Downward Continuation of Magnetic
Observation
72
4.1.8 Regional Magnetic Anomaly
75
4.1.9 Development of Model
78
4.2 Ground Magnetic Survey Data Acquisition
79
4.2.1 The Instrument, Its Principle and Limitations
80
4.2.2 The Principle of Proton Magnetometer
80
4.2.3 Limitations of a Proton Magnetometer
81
4.2.4 Field Magnetic Survey Procedures and Data Reduction
82
4.2.5 Materials used during the field work
83
4.2.6 Survey Operation
84
4.2.7 Instrument Storage
87
4.2.8 Data Reduction
87
4.2.9 Time Variations
87
12
4.2.10 Correction for Time Variations
88
4.2.11 Other Corrections
89
4.2.12 Ground Magnetic Interpretation
89
4.2.13 Magnetic Effects of Geometric Models
91
4.2.14 Estimation of source parameters
92
4.2.15 Estimation of body dip (d) or direction of magnetization (Ij)
94
4.2.16 Estimation of Susceptibility
98
4.3 Ground Magnetic Results Interpretation
102
4.4 Exploration
113
4.4.1 Sampling and Estimation of Reserves
115
4.4.2 Pitting
115
4.4.3 Physico-Chemical Analysis
117
4.4.4 Moisture Content
118
CHAPTER FIVE - RESEARCH RESULTS AND DISCUSSION
5.1 Remote sensing results interpretation and analysis
127
13
5.1.1 Rock Classification Based on the Landsat Imagery
127
5.1.2 Geomorphic Configuration of the Study Area
127
5.1.3 The Drainage Networks
131
5.1.4 Lineaments
134
5.2 Interpretation of Aeromagnetic Profiles
137
5.2.1 Profile AB (Model AB)
137
5.2.2 Model CD (Profile CD)
140
5.2.3 Profile EF (Model EF)
143
5.2.4 Dykes
144
5.2.5 The Influence of Dykes
144
5.3.1 NAA, XRF and Volumetric Results Analysis
145
5.4 The Source of the Pyrite Mineral Investigated
145
5.4.1 Pyrite as Chemical Raw Material
147
5.4.2 Origin of Sedimentary Pyrite Deposit and the
Chemistry of the Transporting Solutions
149
5.5 Summary of Research Findings and Analysis
150
14
5.5.1 Discussion
150
5.5.2 Igneous bodies
153
5.5.3 Summary of Research Findings
153
CHAPTER SIX - CONCLUSION AND RECOMMENDATIONS
6.0 Conclusion and Recommendations
155
6.1 Conclusion
155
6.2 Recommendations
157
References
158
Appendix: Ground magnetic data
166
Pictorial Presentation of Exploration Works
180
15
LIST OF FIGURES
page
1. The location map of Otukpo Area
4
2. Topographical map of the study area
6
3. Regional geological map of the study area
17
4. Geological map of southern Benue Trough
18
5. Geological map of study area
22
6. Colour spectral contrast map
37
7. Map of trend lines
38
8. Rose plot of lineaments
40
9. Map showing profiles taking across prominent anomalies
within the area of study
43
10. Aeromagnetic map of Otukpo
62
11. Total magnetic field map of Otukpo
63
12. Residual field map for the area of study
65
13. Maps showing results of Analytical signal technique
67
14. Power spectrum graph for depth to magnetic body in the
area of study
70
15. Upward continued field 500m above flight height
73
16. Upward continued field 1km above flight height
74
17. Upward continued field 2km above flight height
77
18. Ground Magnetic survey traverse on residual map of the area
86
19. Straight and half slope tangent curves for depth estimation
93
20. Typical shapes of magnetic anomaly for various values of Ө
96
16
21. Shape of field profiles
97
22. Profile GH
104
23. Profile IJ
105
24. Profile KL
107
25. Profile MN
108
26. Profile OP
111
27. Sample points distribution
114
28. Sample location and points shown on the residual field map
of the area
116
29. XRF Analysis Result
126
30. Landsat composite map of the area
128
31. Enhancement filter band map
130
32. Digital elevation model of the study area
133
33. Lineaments draped on the drainage system of the area
136
34. Model of profile AB
139
35. Model of profile CD
141
36. Model of profile EF
142
17
LIST OF TABLES
page
1. Regional Stratigraphic sequence of South-West Benue Trough
20
2. Stratigraphic sequence of Otukpo area
25
3. Isotopic Neutron Sources
51
4. Comparison of EDXRF and WDXRF spectrometers
56
5. Magnetic susceptibility of rocks within the study area
99
6. Densities of variety of rocks found in the study area
101
7. Comparing results of Aeromagnetic and Ground magnetic studies
112
8. Ore block parameters
120
9. Summary of Analytical Results
121
10. XRF Sample results
122
11. X-Ray Fluorescene (XRF)
123
12. XRF Sample results
124
13. XRF Sample results
124
14. Geochemical, Pyrite analysis using XRF
125
18
15. Raw Material uses of Pyrite
148
19
ABSTRACT
Exploration was carried out in Otukpo Local Government Area of Benue State in North Central Nigeria. The area is underlain by Cretaceous
sediments of the lower Southern Benue Trough. The sediments and igneous rocks range in ages from Mid-Albian to Senonian times. They are made up of the Asu River, Eze-Aku and Awgu formations. Multispectral Satellite, aeromagnetic and field magnetic data were processed by the application of computer programs (ILWIS) 3.1, Arc view GIS Version 3.2, ‘SURFFER 8’, GM- SYS were used to map surface and subsurface geology of the area. With remote sensing studies, hydrological conditions (drainage patterns, variations in moisture content), linear trends and surface alteration zones in the study area were identified. The geomorphic features indicated areas of geological interest (suited for prospecting). The airborne and ground magnetic studies revealed major geological structures such as basement sill with variable lithology (intrabasement), magmatic intrusions, dykes, faults, folds, graben, faulted anticlines,
faulted-folded syncline traced to basement fault etc. These reservoir type structures provided evidence of the existence of a mineral ore body (pyrite) at Ogyoma Akpa. Calculated magnetic susceptibility contrast of the area under study is between 0.073-1.71 electromagnetic units. Depth to basement was estimated at 3-9km, while depth to magnetic sources within Cretaceous sediments is between 0.56-3.04km. The estimated direction of magnetization is 50-1850. Basic geotechnical analyses in terms of mineralogical characteristics, physical and chemical properties of the pyrite deposit were assessed to establish their exploitability, handling, storage and beneficiation for metallurgical and chemical industrial uses. The pyrite deposit is overlain by 2.50m thick overburden on the plain surface and 1.40m within stream channels. The ore is disseminated in shales. It occurs as irregular aggregates, dull and rusty, with uneven
fracture. The lump sizes are between 2-16 cm long, 3-10 cm wide and 4-9 cm thick. The pyrite mineral grains are interlocking, fine to medium size with dark grey (almost black) streak and irregular grain particles. The average moisture content of the pyrite ore is 1.5%, the water absorption is 2 g/l. The adsorption test conducted for one week gave 0.35 g/l. Exploration work for an area of 25 km2 gave a total reserve of 11,215 metric tons of pyrite. X-ray fluorescence (XRF) spectrometry, volumetric and neutron activation analysis (NAA) carried out to determine elemental concentrations of the pyrite mineral gave an average grade of 51.94% iron and 27.60% sulphur. The size, shape, grade, tonnage and depth of burial are favourable conditions for its extraction once practical constrains such as social, legal and political factors are taken care of.
20
CHAPTER ONE
INTRODUCTION
1.0 General Introduction
The area is occupied by Cretaceous sediments of the Benue Trough,
mainly comprising shales, sandstones and limestones considered to be non
magnetic. The Benue trough has great potential for resources of raw materials
of economic significance which include deposits of limestone, gypsum,
laterite, coal etc. while others are occurrences of base metal sulphides (lead,
zinc, with smaller amounts of copper), cadmium and silver and associated
minerals (Ford, 1981).
Exploration embraces a whole complex of investigations essential for
determining the industrial importance of a deposit (Mead and Alan 1981).
The prime objective is to determine the quantity and quality of the
mineral and to ascertain the natural and economic conditions in which it
occurs.
The quantity of the mineral is determined by the volume it occupies,
consequently, the aim of exploration in this respect is to ascertain the shape
and dimensions of the deposit.
Quality must be ascertained not only by determining the chemical and
mineralogical composition and natural types of the ores, but also by
establishing their physical properties and grades.
21
Apart from establishing the purely geological data about the
occurrence of a mineral body, the aim is also to discover any other mining
and economic information relevant to the conditions in which the deposit is
located.
Although exploration is concerned with answering a complex range of
geological, mining and economic problems, it is based chiefly on geology.
Every mineral body varies to a greater or less extent in its different
parameters. All modern exploration methods and techniques, however, have
been developed precisely in order to reckon with these variations such as
mode of occurrence of mineral bodies, host rocks, structures and other
properties.
Other factors important for evaluating a deposit in exploration are:
1. The depth and altitude of all parts of the deposit. These questions demand
accurate answers since they determine the choice of methods for opening
up and working the deposit.
2. The physical properties of the mineral and its host rocks. The chief
characteristics to be established being unit weight, strength, durability,
moisture content, lump size, factor of looseness, dustiness and gas
content.
3. The basic geotechnical and hydrological conditions – This information
may be used in assessing possible dilution of the ore, water inflow at the
deposit and the pumping capacity required for future exploitation, supply
of drinking water (Charles et al, 2006).
22
4. The transport facilities both for moving loads over the territory of the
future mine (roads, railway spurs, ropeways, etc.) and for communicating
with the outside world (railway trunk lines, water arteries, air lines, etc.).
5. Special features and economic life of the local population; related
industries, profitability and direction of local agriculture and the
possibility of supplying the future mine from local resources.
From these points it will be seen that the exploration geologists must
investigate most diverse problems, from purely geological ones to specialized
questions of mining, technology and economics.
1.2 Location, Accessibility, Climate.
Otukpo is one of the local government areas in Benue State in north
central Nigeria; Figure 1. The area is covered by sheet 270 of the 1:100,000
scale topographical map published by the Federal Survey of Nigeria (1970).
Otukpo is in Benue south senatorial district in the lower Benue trough.
The area is bounded by longitude 8o 00` E to 8
o 30`E and latitude 7
o
00` N to 7o 30`N respectively. The local government area has a total land area
of about 3,025Km2 and has 13 council areas.
23
NIGERIA
5 0 5 10 15 20 km
24
The distribution of villages and towns are as shown in Figure 2.
Population estimate for Otukpo area is about 430,000 (MacDonald et al,
1997).
There are networks of main pathways connecting minor roads that run
through out the local government areas. The minor roads join the main road
in Otukpo town leading to Makurdi, the state capital to the North and Enugu
to the South. The state is endowed with a tropical climate. The rainy season
starts from April and lasts till October while the dry season begins from
November and ends in March. The mean annual rainfall is between 1500mm
to 1800mm. Temperature fluctuates between 25oC to 33
oC most of the year
(Tahal Consultants, 1982). Geographically the area is generally a low-lying
plain with few hills that reach a height of 275m above sea level (asl).
The plains range from 58-153m asl and are gently undulating. Ferrallitic and
ferruginous soils supporting savannah woodland type vegetation cover much
of the area. The combination of climate, soil and geology produce distinctive
hydrology. During the wet season, short-term flash floods result in rapid
surface water run-off. Most of the rivers and streams are seasonal, drying up
soon after the rains stop. The largest and only perennial river is Okpokwu
river that flows north-south west across the local government area.
The main crops grown in this area are yams, cassava, rice, sorghum,
maize, groundnuts, oranges and plantain.
25
Figure 2: Topographical map of the study area
26
1.3 Aims and Objectives of Study
Aims:
To carry out remote sensing studies, acquire and interpret
aeromagnetic map of Otukpo to delineate structures associated with
mineralization in the area. Ground magnetic survey was carried out to
correlate the airborne magnetic survey. Hence to explore and evaluate
mineral resource(s) found.
The Objectives are:
i. Determination of depth to the basement and thickness of
sediments within the study area.
ii. Determine the configuration of rocks in the ground and susceptibility
of the magnetic bodies responsible for the anomalies.
iii. To determine local relief of the basement surface capable of producing
sub surface structural relief.
iv. Modeling of individual basement feature.
v. The location and delineation of massive magnetic ore body or
disseminated ore body in the area
vi. To identify probable faults/fractures and weak zones within the deposit
as may influence mining considerations.
27
vii. Geochemical sample collection and analysis to determine on semi
quantitative level mineralogical / elemental concentrations and other
parameters from which reserve establishment and result analysis are
carried out.
viii. A correct appraisal of the research results in order to determine
ultimately whether a discovered deposit merits further development
that will ensure industrial and commercial development of the local
government area.
28
CHAPTER TWO
LITERATURE REVIEW
2.1 Review of Previous Work
Shell Petroleum Development Company carried out extensive
geological mapping of Nigeria using aerial photographs and ground controls
to produce 250,000 geological maps of areas in Nigeria including the study
area in the Benue Trough. The British Geological Survey exploratory
drilling, as well as MacDonald (2001) revealed intrusive diorite rocks in the
area.
On the basis of the analysis of regional geophysical study (gravity
anomaly), the crustal thickness underneath the Benue Trough is estimated to
be in the range, 22-37km (Artsybashev and Kogbe 1974, Adighije, 1981).
Analysis of both ground and airborne magnetic data over the Benue Trough
carried out by Ajakaiye (1981), Ofoegbu (1984) have also shown extensive
block faulting in the Trough. Typical example of this basinal structure is the
Anambra Basin, which has a sediment thickness of 3-7km as described by
Ajakaiye (1981), and Ofoegbu (1984). Ofoegbu (1984) found the thickness of
sediments in the lower and middle Benue Trough, to vary between 0.5km and
7km.
29
2.2 Regional Geologic/Tectonic Setting of Benue Trough
The area under study is underlain by Cretaceous sediments of the
lower Benue Trough. The Benue Trough has often been described as an
intracontinental Cretaceous basin, occupied by up to 6,000m of marine and
fluviodeltaic sediments that have been compressionally folded in a non-
orogenic shield environment (Wright 1976). The Benue Trough origin in
terms of rift faulting and the folding of the Cretaceous associated with a
basement flexuring is seen as a direct consequences of the opening of the
south Atlantic ocean (Carter et al, 1963).
Nigeria consist of an uplifted continental land-mass made up of pre-
Cambrian basement rocks which were then unconformably overlain by lower
Cretaceous continental sediments (Kogbe, 1981). The earliest dated marine
transgression occurred during Albian times with the opening of the Gulf of
Guinea under the Niger Delta along lines of weakness at edges of the West
African and Congo cratons (Nwachukwu, 1972, Peters, 1978). Sinking along
this linear depression (which became the Benue Trough) began by mid-
Albian time and continued until late Senonian, interrupted by uplift and
folding during late Albian time (Agumanu and Enu, 1990).
Currently, the Benue Trough is bound by crystalline basement rocks,
the Jos Plateau granites to the north and the Cameroon Basement Massif to
the south. Cretaceous sediments and igneous rocks, ranging in age from
Albian to Maastrichtian, infill the trough to a depth of 3-6km (Cratchley and
Jones, 1965; Benkhelil, 1989).
30
The Trough is envisaged as being due to a combination of
downwarping and rift type faulting of an attenuated sialic crust with
subsidence enhanced as a result of isostatic loading by the sediments filling
the Trough, and overlapping the marginal faults (Artsybashev and Kogbe,
1974).
The lower Benue Trough has two main structural units, the N60oE
trending Abakaliki Anticlinorium flanked by the Anambra syncline trending
N30oE. Lower Cretaceous sediments are presumed to overlie unconformably
pre-Cambrain basement rocks along the Benue valley (Reyment, 1964).
The oldest sediments present belong to the Albian marine
transgression. Albian sediments constitute the Asu River group and its lateral
equivalents (Agumanu, 1989; Ojoh, 1990). The deposits consist of
alternating shales and siltstones with occurrence of sandstone. The Arufu
limestone and the Awe formation consisting mostly of fine sandstones with
carbonaceous shales have been dated as upper Albian on the basis of a
gastropod fauna (Offodile, 1976). The Nigeria Albian is rich in ammonites as
well as foraminifers, radiolarian and pollens.
In the lower Benue Trough, the Cenomanian regression was
characterized by extensive deltaic developments (base of the Eze-Aku
formation) with the continued deposition of the Bima sandstone in the lower
Gongola and upper Benue regions (Carter et al, 1963). Nwachukwu (1972)
suggested possible slight tectonic movement in the southern portion of the
31
Benue valley during the Cenomanian, hence the frequent occurrence of
deformed cross stratification in the Bima sandstone.
The Asu River group is locally tightly folded trending N 60o E with
local deflections due to the influence of transcurrent faulting. The
deformations from the South East basin edge towards the centre are a
diversity of structural styles which include; fracturing, slumping, folding,
tight folding with associated cleavages.
In the lower Benue Trough, the Turonian consists of fossiliferous
marine series outcropping throughout the Benue Trough. In the lower and
middle Benue, it is represented by the Eze-Aku formation, which consists of
hard grey and black calcareous shale, limestone and siltsone. The Eze-Aku
group is divided into two formations: the Eze-Aku shale formation where
mudstone dominates. Locally the shales grade into sandstone (Amaseri
sandstone) near Afikpo. The second is the Makurdi Sandstone formation
which comprise hard, well cemented fine to medium grained sandstones,
interbeded with varying thicknesses of soft shales and occasional limestone
(Nwajide, 1986). The Eze-Aku Formation varies in thickness up to 1200m
(Dessauvagie 1975).
The Eze-Aku Formation is overlein by 900m thick Agwu Shale
Formation. This group comprises bluish-grey, very soft, shallow marine
bedded carbonaceous mudstones with occasional muddy limestone and
siltstones as well as a narrow band of sandstone known as the Agbani
32
Sandstone Formation, which is generally fine to medium-grained and
moderately cemented (Agagu and Adighije, 1983; Peters 1978).
The generalized folding which affected the Cretaceous sediments in
the Benue Trough resulted from a compressional phase (Benkhelil, 1989).
The presence of marine Maastrichtian beds in the Benue Valley,
suggest that an arm of the Maastrichtian sea passed through the Benue
Valley and extended northwards into Damergou in the Niger Republic.
Tectonic activity remained localized along the major fault zones but
also resulting in a sub meridian mineralized fractures. Ofoegbu and Odigi
(1990) recognised that structural lineaments in the Benue Trough are
dominantly N-S, NE-SW and NW-SE, often crossing one another forming a
strong network of shearing fissures and fractures.
Fault bounded basins filled with alluvial to fan delta deposits are
common in the lower Benue Trough (Zaki-Biam, Katsina-Ala) but few data
are available on their tectono-sedimentary evolution (Maurin et al, 1986)
Magmatic activity was contemporaneous with the opening and infilling
of the Benue Trough. Various types of Volcanic occurrences especially
dolerite intrusions transect the area. These igneous intrusions are associated
with both pre and post-Turonian tectonic episodes (Nwachukwu, 1972).
Although few can be observed at outcrop in the study area their presence
throughout the central area can be inferred from aeromagnetic measurements.
The rhyolites intruding the Basement complex near Gboko and dated at
113my (Umeji and Caen Vachette, 1983) are the oldest known traces of a
33
magmatic activity in this part of the Benue Trough. Various types of volcanic
occurrences have been recorded around the Workum Hills in the lower Benue
Trough. These include intrusive bodies, dykes, lava flows mixed with
breccias and tuffs. They are mainly restricted to the Albian sediments
(Wright, 1976).
2.3 Mineralization in the Benue Trough
The potential of the Benue Trough for resources of raw materials of
economic significance include deposits of limestone, laterite, coal, etc.
Others are occurrences of base metal sulphides (lead and zinc, with smaller
amounts of copper), cadmium and silver, and the associated minerals.
Barytes and fluorspar are known to occur locally in spatial, and probably
genetic relation to salt water springs. Other evaporate deposits such as
gypsum and anhydrite may also occur (Olade, 1976; Kogbe, 1981).
The lead-zinc mineralization is attributed to the circulation of heated
brines, leaching base metals from sediments and underlying basement. There
is a perception associated with escape of magma and mineralization in Benue
Trough (Wright, 1976). Outliving this igneous activity, has been the rising of
(juvenile) mineralizing waters and volatile materials, leading to the
deposition of certain minerals, including sulphates, carbonates and fluorides
variously of iron, calcium, magnesium and barium, etc. The activity has been
strongest in the central
34
(axial) parts of the Trough where anticlinal structures, including the
Abakaliki Anticlinorium associated with a gravity “high” along the middle of
the Trough, may indicate upwelling of sub-crustal materials.
Olade (1976) appeals to a deeper and more widespread heat source
than localized magmatism to drive the hydrothermal solutions that leached
base metals from sediments and underlying basement. The hydrothermal
solutions were partly connate pore waters in the sediments. They may have
been mainly seawater percolating down onto the sediments and underlying
basement, to be warmed and supplied with dissolved metals, before migrating
upwards to precipitate the sulphides in suitable structural locations. Here
again, the emplacement of the ores by space –filling rather than by
replacement argues a tensional regime at the time of mineralization (Olade,
1976). Igneous bodies presumably acted both as subsidiary heat sources for
the circulating fluids and eventually as sites for emplacement of the mineral
veins. The numerous salt springs in the Benue Valley represent the dying
stages of the hydrothermal activity giving rise to the veins.
2.4 Regional Stratigraphic setting of South Benue Valley
In the lower or South Benue Valley (South and West of Makurdi) three major
depositional cycles are noted (Reyment, 1965).
Ammonite faunas are used to subdivide the stratigraphy of these Cretaceous
rocks.
35
The first major sedimentary cycle in the lower Cretaceous started
during the middle Albian. Middle Albian transgression caused the deposition
of very thick marine, dark, grey shales, siltstone and subordinate limestones
of the Asu River group. The Asu River group is known to be the oldest
formation within the study area and they uncomfomably overlie the
crystalline basement of pre-Cambrian age (Nwchukwu, 1972).
The second sedimentary cycle began in the upper Cretaceous with a
transgression at the end of the Cenomanian and ended with a regression in
the early or beginning of Turonian. This second cycle resulted in the
deposition of Eze-Aku formation. The Eze-Aku formation consists of thick
flaggy calcareous and non-calcareous shales, sandy or shaly limestones, and
calcareous sandstones. It overlies the Asu River group.
The third major sedimentary cycle in the South Benue Valley occurred
between late Turonian to lower Santonian. Up in the Senonian stage, the
Agwu formation was deposited.
Reyment (1965) described a large number of lithostratigraphic and
biostratigraphic divisions of these Cretaceous rocks. The lithologies and
Stratigraphical relationships of the sediments found in Southern Benue
Trough are as shown in a simplified geological map of the area shown in
Figure 3.
36
Figure 3: Regional Geological Map of the study area.
37
4:
38
These consist of marine fossiliferous grey-blue shales associated with
subordinate limestone and calcareous sandstones (Kogbe, 1981).
Apart from these major depositional cycles, Geological studies have
shown that there were other minor cycles during the Maastrichtian age (Table
1). Maastrichtian cycle caused the deposition of Enugu-Nkporo shales,
Mamu, Ajali and Nsukka formations. The Awgu formation is succeeded by
the Nkporo shales which are found in the South sedimentary basin to
constitute marine shales. Mamu formation (Lower coal measures) overlie
Nkporo shale and consist mainly of sandstones, carbonaceous shales, sandy
shales, and some coal seams. This mark a phase of deltaic-lagoonal
environment (Offodile, 1976). Ajali formation (False bedded sandstone)
overlies the Mamu formation. It consists of about 330m of coarse grained
current bedded sandstones (Cratchley and Jones, 1965). The last Cretaceous
sediment in the area is the Nsukka formation (Upper coal measures). This
formation is marked by the deposition of carbonaceous shales and sandstone
and some thin coal seams.
Figure 3: Regional geological map of the study area
39
Table 1. Regional stratigraphic sequence of the area. (South-West Benue
Trough, Otukpo-Makurdi).
Age Group Formations Description
Maastrichtian
Nsukka Upper coal measures. Carbonaceous shales,
sandstones and thin coal
seams
Ajali False bedded sandstones Coarse grained
current bedded sandstones
Mamu Lower coal measures Sandstones, carbonaceous
shales, sandy shales and coal
seams
Sen
on
ian
Campanian
Awgu
Group
Enugu-Nkporo Shales Marine shales
Santonian
NE-SW Trending elongate
folds and faulting; igneous
intrusions
Coniacian Awgu shale F M Shaley carbonaceous
mudstones with thin shaley
limestones and sandstones
Agbani Sandstone FM Fine to medium sandstones
with siltstones and mudstone
Upper Turonian
Eze-Aku
Group
Upper Eze-Aku FM Shaley mudstones and
siltstone with thin sandstones
and limestones
Makurdi/Otobi Fine to coarse sandstones
with siltstones and mudstone
Lower Turonian
Lower Eze-Aku FM Shaley mudstones and
siltstone with thin sandstones
and limestones
Cenomanian Makurdi Hiatus/unconformity
Upper Albian Asu River
Group
Asu River FM Carbonaceous shaley
mudstone, limestone,
sandstone and siltstone
Lower Albian Metamorphased Asu River
FM
Pyroclastics and intrusives
with contact metamorphosed
mudstone, shale and
sandstone
PreCambrian
Basement
Crystalline Basement N-S trending fault
Modified after MacDonald 2001
40
2.5 The Geology of Otukpo
The area is underlain by the Cretaceous sediments of the Benue
Trough; located in the Southern area of Benue valley. Lower Cretaceous
sediments are presumed to overlie Precambrian basement rocks
unconformably along the Benue valley (Reyment, 1964). The following
geological formations are outlined in the investigation carried out in Otukpo
area. They are namely: Asu River formation, Eze-Aku formation and the
Awgu formation. Their distribution is shown in Figure 5.
2.5.1 Asu River Group
The oldest sediments present, belong to the Asu River group which
crops out in the south east (Agumanu, 1989, Ojoh, 1990). There are two
distinct parts to the Asu River group:
i. The metamorphosed Asu River Group which comprises hard splintery,
slatey carbonaceous mudstones, subordinate calcareous meta-
sandstones, siltstone to very fine sandstones and limestones with
interbedded pyroclastic and intrusive igneous rocks.
The Asu River Group, composed of hard, deep marine shales, laminated
coarse siltstones, sandstones and limestones deposited in a tectonically active
environment.
41
42
These sediments show convoluted and folded bedding and have been lithified
by the effects of burial. Fractures within the Asu River group rocks are
common and generally remain open, regardless of their orientation. Much
horizontal fracturing is associated with base of the weathered zone
(MacDonald et al, 1997). Much secondary disseminated iron pyrite has been
deposited mainly within sandstone layers because of the metamorphism
(Hoque, 1984.
ii. The shales are mainly composed of kaolinite and illite clays (Murat,
1970). Rocks of this formation are exposed within stream beds and
gullies along flanks of hills where they crop out as hard dark grey
lithified mudstones with interbedded hard quartzitic sandstones and
thin limestones.
2.5.2 Eze-Aku Group
The Eze-Aku group overlies the Asu-River group to the north. This group is
composed of mudstones with occasional limestone, siltstone and sandstone.
The mudstone is generally lithified but becomes soft towards the north
(Peters, 1978; Nwajide, 1990).
The lower Eze-Aku shale formation, although softer contains open
fractures. These tend to be more widely spaced than in the Asu River group
and are associated with faults (MacDonald, 2001). The lower part of the
lower Eze-Aku sequence is composed mainly of shaly to silty mudstones
where as the upper part of the sequence contains more sand.
43
The upper Eze-Aku group is divided into two formations; the Eze-Aku
shale formation where sandstone dominates. The Makurdi sandstone
formation comprises hard, well cemented fine to medium-grained sandstones,
interbedded with varying thickness of soft shales and occasional limestones
(Nwajide, 1986). The Otobi sandstones mainly arenaceous sandstones also
occur.
2.5.3 The Awgu Group
The Awgu group, north of the area, are the youngest rocks present.
This group comprises very soft, shallow marine, carbonaceous mudstones
with occasional muddy limestones and siltstones as well as a narrow band of
sandstone known as the Agbani sandstone formation which is generally fine
to medium grained and moderately cemented (Agagu and Adighije, 1983).
The lithologies and stratigraphical relationships of the sediments found in the
area are outlined in Table 2.
Dolerite intrusions transect the area. These igneous intrusions are
associated with both pre and post Turonian tectonic episodes (Nwachukwu,
1972). There presence can be inferred from the regional aeromagnetic maps
that are ground truthed using a combination of magnetic and electromagnetic
surveys (MacDonald, 2001).
The area has mineral resources such as gas (inferred), Brick clay,
kaolinite clays, crushed and dimension stones, construction materials (laterite
and sandstones), pyrite, coal, limestone etc.
44
Table 2: Stratigraphic Sequence of Otukpo Area
Age Group Formations Description S
eno
nia
n
Santonian
Awgu
Group
NE-SW Trending elongate
folds and faulting; igneous
intrusions
Coniacian Awgu shale
Formation
Shaley limestone and
calcareous mudstone, thin
siltstone
Agbani
Sandstone FM
Felspathic fine to medium
clay rich sandstone with
slatey mudstone
Upper
Turonian
Eze-
Aku
Group
Upper Eze-Aku
FM
Carbonaceous mudstones,
arkosic sandstone, shaley
limestone with thin siltstone.
Lower
Turonian
Makurdi/Otobi
sandstone FM
Felspathic sandstone,
interbedded with
carbonaceous mudstone,
siltstone and thin muddy
limestone.
Lower Eze-Aku
FM
Laminated mudstone, with
interbeds of siltstone,
Sandstone, limestone and
clay
Cenomanian Makurdi Hiatus/unconformity
Upper Albian Asu
River
Group
Asu River FM Splintery mudstones,
laminated siltstone,
Sandstone, thin limestone
Lower Albian Metamorphosed
Asu River FM
Slatey carbonaceous
mudstones, calcareous meta-
sandstones, limestones,
pyroclastic and igneous rocks
PreCambrian
Basement
Crystalline
Basement
N-S trending fault
45
2.6 Hydrogeology
The thick tropical soil present throughout the area has an important
bearing on the hydrogeology. During the rainy season, water flows through
shallow nodular zone few metres deep to the river, causing extensive river
flows and flooding. Shallow traditional wells tap this water source and can
supply high yields when the shallow zone is saturated. However, as the rains
stop, the shallow tropical soil quickly dries out and water-levels in the wells
rapidly decline.
The clay zone beneath the shallow ferruginous soil impedes downward
movement of ground water. Survey has shown that clay lenses are
predominantly interbedded with mudstones/ siltstones and muddy limestones
(Daagu, 2001). Hence has poor groundwater bearing capacity. The clays can
be fractured and contain significant discontinuities which enhance recharge
in some locations.
Exploratory boreholes strike groundwater only from fractures and / or
weathered layer in the Asu River Formation at depths between 11m and 36m
(Alan, 2001). The mudstones have negligible intergranular permeability or
porosity, but with high degree of fracturing, they make good aquifers.
46
Groundwater occurs with fractures and fault zones in the Eze-Aku
shale. However, unlike the Asu River Formation, fracture zones are not
widespread. The sediments are too soft for small stress- release fractures
caused by weathering - denudation to remain open. Limited groundwater also
occurs within the shallow ferricrete.
Drilling at Adum East into Makurdi sandstone showed that the
sandstone is complex, highly variable and interlayered with thick mudstones
(Macdonald, 2001). The best targets for groundwater are the fractures at the
base of the weathered zone (8-15m deep) and fractured limestone layers
where present. The sandstone has moderate porosity. Core samples taken
from seven boreholes gave measurements of porosity varying from 9% to
34% (median value 16%) (Macdonald, 2001). Sandstone porosity is
enhanced within the weathered zone by dissolution of feldspar crystals
leading to the formation of intergranular voids. Unfortunately, these voids
may not be well interconnected so that although porosity is high, hydraulic
conductivity is often low.
Negligible portable groundwater exists within the Awgu shales. The
mudstones are too soft to contain open fractures. Sufficient groundwater for
rural water supply is only found within thin sandstone layers and dolerite
intrusions (Sonderegger, 1970).
47
CHAPTER THREE
METHOLODY
To achieve the objectives of this study, the combination of landsat,
aeromagnetic and field magnetic data were analyzed to determine structures
responsible for magnetic anomalies within the study area. These have great
potential for mineral exploration which is demonstrated in this work and in
several test cases.
The topographical map sheet 270 on a scale of 1:100,000 was digitized
from its hard copy. The drainage system of this area was reproduced from
this dataset.
The datasets used in this work were either in analog or digital format.
The satellite imagery and field data were in a digital format, the others were
in analog format. Those in the analog format were converted to their digital
equivalence and together with the satellite data were processed with a
computer programme (ILWIS 3.1) and interpreted to reveal their geological
importance. Several physico-chemical properties of the mineral (pyrite)
found in the area were investigated.
Densities of the prevailing rocks in the area were determined.
Neutron Activation and X-Ray Fluorescence mineralogical analyses were
carried out.
48
Finally, general considerations and calculations demonstrating, the
industrial potential of the pyrite minerals found in the area, its origin and
classification are major requirements fulfilled in this work. The data used are
based on exploration work.
3.1 Remote Sensing
Remote sensing is a regional reconnaissance study that indicates target
areas for follow-up surveys by more detailed and costly methods (Meer,
1997). In addition to location of specific mineral target areas or fracture
intersection or rock alteration, remote sensing provides data which are
fundamental exploration tools. Landsat is important in its own right as a
remote sensing system that has contributed greatly to earth resources studies.
Landsat imagery
The use of satellite imagery is now standard technique in mineral
exploration (Nash et al, 1980, Goez and Rowan, 1981; Peters, 1983). It has
also been used in structural investigations (Drury, 1986) and in hydrogeology
(Deutsch et al, 1981). The most common structures, lineaments of uncertain
nature, drainage patterns, deductions regarding stress in the area are made.
These studies are used to select exploration target zones on the basis of
favourable geology and structure. These structures yield clues to the location
of concealed mineral deposits (Abram and Hook , 2002). Within the
49
mineralized belts, potential mining sites are commonly localized by
intersecting fracture systems, which are mapped as lineaments.
In oil and mineral exploration the reconnaissance surveys are followed
by field mapping and by geophysical and geochemical surveys that
eventually define a prospect suitable for drilling.
Remote sensing digital image processing is greatly facilitated by
application of computer processing programs. The principal advantages of
digital processing methods are their versatility, repeatability and the
preservation of their original data precision. Digital image processing lends
the image analyst the ability to carry out the following functions.
i. Correct the data for geometric and radiometric imperfections.
ii. Improve the visual quality of the image data
iii. Carry out appropriate user custom manipulations to enhance or
suppress certain details vital for information extraction.
iv. Conduct computer assisted thematic mapping from digital
imageries.
These functions are conventionally referred to as image rectification,
enhancement, transformation and classification respectively.
There is a broad range of image enhancement methods but only three
are fundamental to geological applications. These methods include, contrast
50
stretching, spatial filtering and colour composite generation. Filters are
designed to enhance features, which are oriented in specific directions.
Hence, they are useful in the enhancement of linear geological structures.
Basic image transformation procedures useful for geological applications
include Principal Component Analysis (PCA) and Vegetation Index (VI)
(Meer, 1997).
3.1.1 Remote Sensing Digital Image Processing
Digital system for processing landsat and other images have been
developed by a number of Universities, government facilities, and
commercial organizations. One of these is the Video Image Communication
and Retrieval (VICAR) system. The Laboratory for Applications of Remote
Sensing (LARS) at Purdue University, USA has developed the LARS system
(LARSYS) for digitally processing a variety of multispectral data including
landsat (Sabins, 1997). VICAR, LARSYS and other image processing
software systems have been placed in the public domain.
Digitized image can be read into a computer for various processing
operations. Image processing methods may be grouped into the functional
categories of restoration, enhancement and information extraction.
1. Image restoration (cosmetic processes)
The objective is to make the image resemble the original scene. The
processes involved are (a) Sixth line dropout (b) Sixth offsets (c) Scan
corrections (f) Synthetic stereo images.
51
2. Image enhancement:
Image enhancement is the modification of an image to alter its impact
on the viewer. Image enhancement processes include (a) Contrast
enhancement (b) Density slicing (c) Edge enhancement (d) Spatial and
directional filtering (e) Simulated normal color images (f) Digital Mosaics
Image restoration and enhancement processes utilized computers to
provide correct and improved images for viewing by human interpreters
3. Information extraction:
Information extraction processes, however utilize the decision –
making capability of computers to identify and extract specific piece of
information.
The human operator must provide specific pieces of information. The
human operator must provide training data and instructions for the computer
and must evaluate the significance of the extracted information.
The evaluation involves (a) Band ratio images (b) Other ratio images
(c) Multispectral classification (d) Change detection images (Nash et al,
1980).
The computer implements the programs to process the image and
produce a tape of the new data which is then plotted as an image on film or
line printer display.
52
3.1.2 Resource Exploration
Landsat images have proven valuable for mineral exploration in three
ways:
i. Mapping of regional and local fracture systems that controlled ore
deposits: Landsat imagery is useful for mapping both regional lineaments
and local fractures (faults and zones of weakness). Prospectors have long
realized that mineralized belts or zones occur along linear trends and
many mines have been found by exploring along the projections of such
trends. Within the mineralize belts, potential mining sites are commonly
localized by intersecting fracture systems (Goetz et al, 1983).
ii. Detection of surface alteration effects associated with ore deposits:
Many ore bodies are deposited by hot watery fluid called hydrothermal
solutions that invade the host rock or country rock. During formation of ore
minerals these solutions also interact chemically with the country rock to
alter the mineral composition for considerable distances beyond the site of
ore deposition. This hydrothermal alteration is marked by distinctive
assemblages of secondary minerals that commonly are laterally and vertically
zoned with respect to the ore body. The zoning is due to changes in
temperature, pressure and chemistry of the hydrothermal solution at
progressively greater distances from the ore body.
53
At the time of ore deposition, alteration of the country rock may not
extend to the surface of the ground. Later uplift and erosion expose
successively deeper alteration zones and eventually the ore body itself.
Hydrothermal alteration zones are more areally extensive and generally less
conspicuous. However alteration zones are valuable indicators of possible
deposits (Charles et al, 2006).
iii. Providing basic data for geologic mapping:
One very important advantage of landsat images is that the low to
intermediate sun angle enhances many subtle geologic features. If digital
filtering, for example, reveals a previously unrecognized fracture system or
alteration zones that leads to the discovery of major ore deposits, the cost
benefits are obvious.
3.1.3 Data Set and Method Used
The multispectral satellite data of landsat Thematic Mapper (TM)
constitute by far the most important data used in the present study. The data
included black and white images, false colour composites at various scales
and digital data. The data set in the New National Landsat Archive
Production System (NLAPS) provided in the National Data Format (NDF)
was obtained from the National Center for Remote Sensing, Jos. The image
organization is in band sequential (BSQ) and the same data in Raster format
is presented in seven bands. Each scene was also radiometrically corrected.
54
The image scene used in this study belongs to path number 188 and row
number 56 obtained in September 2009.
This work involved digital image processing of landsat 5 TM data. The
computer which was used, is a HP pavilion dv 6500 Intel Centrino Duo
Laptop with 2GB of RAM and 160GB hard disk space. The programes run
on “WINDOW” vista. Digital image processing was carried out for feature
enhancement and extraction to facilitate its geological interpretations by
using one of the Integrated Land and Water Information Systems (ILWIS)
3.1 academic and Idrisi 32 Raster based software programmes. These
aggregate the imagery scene by their natural spectral values. Most vector
based analysis were carried out using Arc view GIS version 3.20. Various
enhancement techniques were employed, such as linear contrast stretching,
principal component analysis, colour compositing. Edge enhancement is
carried out by a process called spatial filtering, performed by pixel
transformation of an image. The enhanced digital images is next displayed on
a view device and converted into hard copies and visually interpreted.
Information extraction methods, however, simultaneously process
corresponding pixels from two or more bands. For instance band 5 responds
to variations in ferric iron (Fe03) content in rocks and soils, which show
higher reflectance as the iron content increases. Band 7 likewise reacts to
moisture contents and is especially suited to detecting hydrous minerals (such
as clays or certain alteration products) in geologic settings (Meer, 1997).
55
The processing helped to enhance the various landform features and
extract a stronger expression of lineaments and fault traces (Figure 6) than in
the original raw data. Therefore, lineaments found on the landsat imagery
were digitized to form a lineament map. This map was draped on the
drainage system of this area in a GIS environment for geological
interpretation.
And finally selected ground-truth checks were carried out to verify and
reassess the interpretation.
3.1.4 Trend Lines
Trend lines, which represent the observed magnetic lineaments, were
drawn along their strike directions to form a separate trend map, (Figure 7).
These trend lines were drawn from the visual inspection of the Digital
Elevation Model (DEM), with the aid of a computer. This trend map is
draped on the gradient filtered map. Drury (1986) discussed some of the
reasons trends may parallel a structure or geological boundary but may also
be offset from them.
56
Fig 6: Showing of colour spectral contrasts
57
Figure 7: Map of trend lines draped on topographical map of the area
58
3.1.5 Rose Plot
A rose diagram is prepared to depict the strike frequency distribution
(Figure 8).
The rose plot of these lineaments is essentially a radial histogram since
all the lineaments were given equal weights. The histogram of these data
show the number of lineaments frequency against a range of orientation
(azimuth). The northeast and northwest trends of the landsat lineaments are
parallel with magnetic positive trends. The rose plot shows three major
lineament trends: E-W, NE-SW and NW-SE (Figure 8). The
rose plot of these fractures show tri-modal distributions with NE-SW and
NW –SE for the major trends, and an E-W trend representing the minor one.
These confirm the established structural trends of this region
After landsat studies have defined areas of exploration interest. The
first geophysical work is typically an airborne magnetic survey. After that
magnetic surveys were made on the ground.
59
Number of data plotted =131
Sector Interval Angle = 100
Scale spacing = 2% (3 data)
Maximum = 13.0% (17 data)
Mean Resultant dir’n = 028
Circular mean Dev. = 560
Figure 8: Rose plot of the lineaments
60
3.2 AEROMAGNETIC STUDIES
Aeromagnetic Study
On a larger scale, aeromagnetic surveys are used for mapping
geological structures. In areas where sedimentary sequence is very thick, it is
sometimes possible to delineate major structural features because the
succession includes magnetic horizons which may be ferruginous sandstones
or shales, tuffs, or possibly lava flows. In many regions, however, the
igneous and metamorphic ‘basement’ which underlies the sedimentary
sequence is the predominant factor controlling the pattern of the anomaly
field, for it is usually far more magnetic than the sediments. Where the
basement rocks are brought nearer to the surface in structural highs, the
magnetic anomalies are large and characterized by strong relief (Griffiths and
King,1983).
3.2.1 The Polynomial Fitting Method
This is the most commonly used analytical method for determining regional
magnetic field (Johnson, 1969; Deton, 1976). The fitting is based on
statistical theory; since the observed data are computed by least square
method to obtain a surface that has the closest fit to the magnetic field. This
surface is considered to be the regional field while the residual is the
difference between the magnetic field value obtained in the field and the
regional field value computed.
61
3.2.2 The Least Square Method
The least square method was applied to the study because the area does
not have complex geology and has limited spatial extent.
What is needed in this method is to fit a straight line with equation Y=
a+bx to the data, whereby the sum of squares of the vertical distance from the
points to the line is a minimum. The observations are represented as point
Pi=(xi; yi) with ‘i’ ranging from 1 to N. If the vertical projection of Pi on the
line is indicated as Qi, then distance PiQi represents the residual or deviation
from the line. The advantage of this method includes:
1. Many points of the maps or profile are used to obtain the solution
2. Bodies of arbitrary shapes are considered
3. The solution may take known geological structures into consideration
4. Solutions can be made simultaneously for anomalies caused by more
than one body, hence taking interference into consideration (Johnson,
1969).
However, since the regional field is a first polynomial surface, all the
regional were therefore calculated as a two dimensional first degree
polynomial surface. A computer program was used to subtract values of the
regional field from the total magnetic field value at grid points.
Figure 9 shows the residual magnetic anomaly map of the study area, with
profiles taking across prominent anomalies.
62
Fig 9: Map showing profiles taking across prominent anomalies within the area of study.
8.0 8.1 8.1 8.2 8.2 8.3 8.3 8.3 8.4 8.4 8.5
longitude (degrees)
7.0
7.0
7.1
7.2
7.2
7.3
7.3
7.3
7.4
7.5
7.5
latitu
de
(degre
es)
B C
D E
F
8 8 30
7
7 30
A
Fig 17: Map showing profiles taking across prominent anomalies within the area of study.
Fig. 9
63
3.3 Geochemical Analysis
3.3.1 Neutron Activation Analysis
Neutron Activation Analysis (NAA) is a quantitative and qualitative
method of high efficiency for the precise determination of a number of main-
components and trace elements in different types of samples. NAA, based on
the nuclear reaction between neutrons and target nuclei, is a useful method
for the simultaneous determination of about 25-30 major, minor and trace
elements of geological, environmental, biological samples in ppb-ppm range
without or with chemical separation.
In NAA, samples are activated by neutrons. During irradiation the
naturally occurring stable isotopes of most elements that constitute the rock
or mineral samples, biological materials are transformed into radioactive
isotopes by neutron capture. Then the activated nucleus decays according to a
characteristic half-life; some nuclides emit particles only, but most nuclides
emit gamma-quanta, too, with specific energies. The quantity of radioactive
nuclides is determined by measuring the intensity of the characteristic
gamma-ray lines in the spectra. For these measurements a gamma-ray
detector and special electronic equipment are necessary. As the irradiated
samples contain radionuclides of different half-lives different isotopes can be
determined at various time intervals (Soete et al, 1972).
NAA is still competitive in many areas in analytical chemistry
compared to new methods of analytical techniques. The indisputable
64
advantage of the method is its sensitivity and accuracy especially in respect
of some trace elements. The method is of a multielement character, i.e. it
enables the simultaneous determination of many elements without chemical
separation. In the case of instrumental determination, the preparation of
samples involves only the preparation of representative samples i.e.
pulverization or homogenization in most cases, and this reduces the danger of
contamination to a minimum and accelerates the whole analytical process.
During NAA the neutrons get into interaction with the nucleus, therefore, the
chemical composition and crystal structure of the substance under analysis
will have an effect on the result only in exceptional cases.
The development of the method has contributed to the elaboration of
some very simple and accurate methods of standardization, which lead to a
surpassingly accurate analysis.
3.3.2 Limitation of NAA
The widespread application of NAA is hindered, however, by some
conditions. Among the different fields of application, the Instrumental
Neutron Activation Analysis (INAA) following a reactor irradiation is the
most competitive. In view of the increasing protest against nuclear energy, a
number of research reactors have been shut down; therefore, the possibilities
of irradiation are limited in many countries. The equipment needed for the
analysis is rather expensive and requires special laboratories and a highly
qualified staff.
65
3.3.3 Principles of NAA Method
In the process of NAA the neutrons interact with the stable isotopes of
the target element converting them to radioactive ones. The so-called
compound nucleus emits gamma rays promptly with extremely short lives
and these can be measured during irradiation through a technique called
Prompt Gamma Activation Analysis (PGAA). In most cases, the radioactive
isotopes decay and emit beta particles accompanied by gamma quanta of
characteristic energies, and the radiation can be used both to identify and
accurately quantify the elements of the sample (Vertes et al, 1998).
Subsequent to irradiation the samples can be measured instrumentally
by a high resolution semi conductor detector, or for better sensitivity,
chemical separations can also be applied to reduce interferences. The
qualitative characteristics are; the energy of the emitted gamma quanta (E)
and the half life of the nuclide (T1/2).
The quantitative characteristic is: the I intensity, which is the number
of gamma quanta of Energy E measured per unit time.
1. Step of the analysis: Sample preparation means in most cases only
pulverizing, homogenizing, mass determination, packing, as well as the
selection of the best analytical process and the preparation of the
standards, if any.
2. Step of the analysis: For irradiation one can choose from the various
types of neutron sources according to need and availability.
66
3. Step of the analysis: Measurement, evaluation and calculation involve
taking the gamma spectra and calculating trace element concentrations
of the sample. The most widely used gamma spectrometer consists of
germanium based semiconductor detectors connected to a computer
used as a multichannel analyzer for spectra evaluation and calculation.
3.3. 4 Equipment and Materials
- Sample for analysis (obtained from the study area-pyrite)
- Standard solutions (Ca, Mn, Na standard solutions)
- Analytical balance
- Micropipette
- Reactor for irradiation
- HPGe detector, spectrometer
3.3.5 Choosing the Appropriate Procedure
When solving an analytical problem by means of activation analysis,
or any other method, the analyst must select an appropriate procedure. In
attempting an optimization, one must consider a number of aspects. A set of
experimental parameters must be chosen for adjustment, the others being
fixed by practical considerations.
A number of elements have more than one isotope which can be
activated by neutrons. Each activation product has its own cross- section,
isotopic abundance and decay scheme. The first decision is to choose the
most selective nuclear reaction in order to optimize the procedure.
67
In nuclear reactors there are several irradiation channels with different
neutron energy spectra. The use of a thermal neutron filter is an important
option of selection. This type of analysis is called Epithermal Neutron
Activation Analysis (ENAA).
3.3.6 Procedure
1. Before starting the irradiation the following have to be ascertained
a. Determine Ca, Na, Cl in the samples in the presence of interfering
components (e.g, Mg, Al, Si).
b. Choose the proper
(i) nuclear reaction
(ii) analytical gamma line
(iii) irradiation, decay and measuring times
c. Calculate the quantity of the elements to be used for standardization
2. Sample preparation:
a. Weigh the samples into polyethylene bags using analytical balance
b. Prepare standards using micropipettes
3. Irradiation of the samples using pneumatic system of the reactor
4. Measure the gamma-spectra, evaluate the spectra (determine the peak
areas at the given gamma lines).
5. Identify the isotopes in the spectra using gamma library. Determine the
elemental concentrations and their uncertainties using standard method.
68
3.3.7 Irradiation Facilities
i) Neutron Sources: Applied here is isotopic neutron sources.
In the case of the most frequently used isotopic neutron sources an alpha
emitting radioactive material is mixed with beryllium and an (,n) reaction
generates the neutrons. Isotopic neutron sources include Actinium (Ac),
Radium (Ra), Plutonium (Pu), Polonium (Po).
ii) Neutron generators: These neutron sources are accelerators where a
convenient target material is bombarded by accelerated charged particles
and the neutrons are produced in a nuclear reaction. In the most
frequently used and commercially available neutron generators,
deuterons are accelerated and the target material is tritium. Due to the
emitted fast neutrons, in NAA the neutron generators are used for the
determination of elements of high cross section in this energy region.
Examples of elements of geochemical interest determined by the fast
neutrons of generators are Magnesium, Aluminium, Silicon, Titanium,
Iron, Zirconium, Nickel.
iii) Nuclear reactors
Owing to the high neutron flux, experimental nuclear reactors operating
in the maximum thermal power region of 100KW-10MW with a
maximum thermal neutron flux of 1012
- 1014
neutrons cm-2
S-1
are
the most efficient neutron sources for high sensitivity activation analysis
induced by epithermal and thermal neutrons. The reason for the high
69
sensitivity is that the cross section of neutron activation is high in the
thermal region for the majority of the elements (Faanhof et al, 1989).
3.3.8 Kinetics of Activation
In the case of nuclear reactions induced by neutrons the radioactivity
of the examined isotope depends on the flux of the neutrons and the cross
section of the given nuclear reaction. The cross section and the neutron flux
highly depend on the energy of neutrons.
3.3.9 Methods of Standardization
The analytical procedure can be made faster and more economical by
simplifying the standardization procedure. In this analysis, the “absolute”
standardization procedure was applied. The quantitative measurement can be
effected by determining the neutron flux and counting the absolute gamma
rays.
By optimizing the irradiation, decay and measuring times, a lot of
elements can be determined with higher sensitivity. A number of
interferences can be avoided in this way too. For elements with short half
lives the shortest irradiation and cooling times are determined by the
technical limitations.
70
Table: 3. Isotopic Neutron Sources
Emitter Half life Neutrons
S-1
ci-1 emitted
Average
Neutron energy (MeV)
227 Ac 22y 1.5x107 4
226 Ra 1620y 1.3x107 3.6
239 Pu 2.4x104y 1.4x107 4.5
210 Po 138d 2.5x106 4.3
Source: Soete et al, 1972
71
The radioactive isotopes of long half lives produced after irradiating
the elements for long time e.g. eight hours in a thermal channel of the reactor
are measured several times. The cooling times are one week and one month
or longer in special cases. By this way, usually 25-30 elements can be
determined in different types of samples.
3.3.10 Classic Relative Method of Standardization
The method is based on the simultaneous irradiation of the sample with
standards of known quantities of the elements in question in identical
positions, followed by measuring the induced intensities of both the standard
and the sample in a well known geometrical position.
3.3.11 Measurement and Evaluation
The modern gamma measuring systems consist of a gamma detector,
usually a HPGe type and sometimes NaI (TI) scintillation crystals. The
detectors are connected to a Multi Channel Analyzer (MCA) by an
appropriate electronic system (pre-amplifier, spectroscopy amplifier, etc.).
Nowadays, the MCAs are computer based systems with the ability of an
automatic spectrum evaluation.
3.3.12 Analysis of the Gamma Spectra
The usual objective of the measurements by gamma ray spectrometers
is the determination of the number and energy of the photons emitted by the
72
source. The peak location and the peak area in the spectra have to be
determined. The peak location is a measure of the gamma energy, while the
peak area is proportional to the photon emission rate. For the energy
measurement, the pulse height scale must be calibrated with standard sources
emitting photons of known energies. In order to calculate the activities, the
full-energy peak efficiencies of the source-detector system have to be
determined by using sources of known activities.
For the determination of the peak areas the background under the peak
interval has to be subtracted. The net count (Np) results from Np = Nint -
NB, (Nint integral under the peak and NB refers to the background).
The peak area can also be calculated by computer programs which fit
an analytical function to the peak. Thus all the peaks including also the
multiplets can be automatically analyzed.
3.4 XRF Spectrometry, Applications and Analysis
The theory of X-Ray Fluorescence (XRF) Spectrometry
XRF is an analytical method for determining the chemical composition
of all kinds of materials. The materials can be in solid, liquid, powder,
filtered or other form. XRF can also sometimes be used to determine the
thickness and composition of layers and coatings (Peter, 2006).
The method is fast, accurate and non-destructive, and usually requires
only a minimum of sample preparation. Applications are very broad and
include the metal, cement, oil, polymer, plastic and food industries, along
73
with mining, mineralogy and geology, and environmental analysis of water
and waste materials. XRF is also a very useful analysis technique for research
and pharmacy (Lachance and Claisse, 1995).
The precision and reproducibility of XRF analysis is very high. Very
accurate results are possible when good standard or no specific standard
specimens are available. The analysis time varies between seconds and 30
minutes.
In XRF, x-rays produced by a source irradiate the sample. In most
cases, the source is an x-ray tube but alternatively it could be a synchrotron
or a radioactive material. The elements present in the sample will emit
fluorescent x-ray radiation with discrete energies (equivalent to colours in
optical light) that are characteristic of these elements. A different energy is
equivalent to a different colour. By measuring the energies (determining the
colours) of the radiation emitted by the sample it is possible to determine
which elements are present. This step is called Qualitative Analysis. By
measuring the intensities of the emitted energies (colours) it is possible to
determine how much of each element is present in the sample. This step is
called Quantitative Analysis.
3.4.1 Interaction of X-Rays with Matter
X-Rays can be seen as Electro Magnetic (EM) waves with their
associated wavelengths, or as beams of photons with associated energies.
74
There are three main interactions when x-rays contact matter:
Fluorescence, compton scatter and Rayleigh scatter. If a beam of x-ray
photons is directed towards a slab of material, a fraction will be transmitted
through, a fraction is absorbed (producing fluorescent radiation) and a
fraction is scattered back. Scattering can occur with a loss of energy or
without a loss of energy. The first is known as Compton scatter and the
second Raleigh scatter. The fluorescence and the scatter depend on the
thickness (d), density (ρ), composition of the material, and on the energy of
the x-rays.
3.4.2 The Different XRF Spectrometers
The basic concept for all spectrometers is a source, a sample and a
detection system. The source irradiates a sample, and a detector measures the
radiation coming from the sample. In most cases the source is an x-ray tube.
Spectrometer systems are generally divided into two main groups: energy
dispersive systems (EDXRF) and wavelength dispersive systems (WDXRF).
The difference between the two systems is found in the detection systems.
The EDXRF and WDXRF spectrometers have their advantages and
disadvantages.
75
Table 4. Comparison of EDXRF and WDXRF spectrometers
EDXRF WDXRF
Elemental range Na — U (Sodium –
Uranium
Be — U (Beryllium-
Uranium
Detection limit Less optimal for light
elements. Good for
heavy elements
Good for Be and all
heavier elements
Sensitivity Less option for light
elements
Good for heavy elements
Reasonable for light
elements
Good for heavy
elements
Resolution Less optimal for light
elements
Good for heavy elements
Good for light elements
less optimal for heavy
elements
Cost Relatively Inexpensive Relatively expensive
Power consumption 5-1000W 200-4000W
Measurement Simultaneous Sequential/Simultaneous
Critical Moving
parts
No Crystal, Goniometer
After Peter Brouwer, 2006.
76
3.4.3 XRF Analysis
Sample Preparation
A good analysis starts with a well prepared sample and a good
measurement. Often, only a small sample of material is analyzed. The sample
must be representative of the entire material. Another basic requirement is
that a sample must be homogeneous.
Most spectrometers are designed to measure samples that are circular
disks with a radius between 5 and 50mm. The sample is placed in a cup, and
the cup is placed in the spectrometer. Powders can be placed on a supporting
film and measured directly. Another technique is to press them under very
high pressures (20,000kg) into a tablet. A binding material is sometimes
added to improve the quality of the tablet. The tablet is then measured and
analyzed. The analysis is done in two steps: Qualitative analysis followed by
the Quantitative analysis. Qualitative analysis determines which elements are
present and their net intensities from the measured spectra. The net intensities
are used in the Quantitative analysis to calculate the concentrations of the
elements present.
3.4.4 Analysis Method
The sample was powdered to pass through 60µm sieve. 10g of the
powdered sample was thoroughly mixed with 1g of stearic acid (binder) and
77
transferred into a circular disk 40mm in diameter and pressed into a pellet at
a pressure of 25tons using special hydraulic pressure to yield a specimen
pellet of the sample.
The pellet was measured for major and minor elements using Energy
Dispersive X-ray fluorescence Spectrometer (Mimi pal 4). The system
condition set for the analysis was, 14kv, Kapton Filter used, the measurement
was done in Air medium of a measurement time of 60 seconds. The system
consists of Rh X-ray tube; the detector type is
Silicon drift detector. The detector can measure photon energies from
1KeV (Naka) to 17.4 KeV (Moka) efficiently. The maximum count rate is
70,000 – 90,000 count per seconds (CPS). The Spectrometer (mimi pal 4) can
determine elements from Sodium to Uranium at various condition sets. The
results of these analyses are shown in chapter 4.
3.5 Volumetric Method
This method is used for the estimation of the total iron. Sulphide ores
containing iron are decomposed with fuming nitric acid. Treat the ore
(cautiously) with 10cm3 strong HCl in a porcelain evaporating dish.
Ferric chloride formed from the dissolution of the ore is converted to
ferrous condition by adding a solution of stannous chloride in the boiling hot
solution of the ore. The excess of stannous chloride is neutralized by adding a
solution of mercuric chloride to the cold ferrous solution. This is then titrated
78
with a standard potassium dichromate solution, using potassium ferricyanide
as an external indicator or diphenylamine as internal indicator.
3.5.1 Procedure
1. Weigh out in duplicate, 5g of the ore and grind to the finest powder in
an agate mortar.
2. Weigh accurately, 5g of the ore sample into a 250ml conical flask.
3. Add 1ml concentrated HCI and warm gently for 10-15 minutes. and
then gradually increase the heat to bring to a gentle boil.
4. Evaporate to dryness and again add about 10ml of concentrated HCl.
5. Heat the solution in (3) to boiling and add to it stannous chloride
solution drop by drop, all the time shaking the conical flask till the red
colour of ferric iron is completely discharged.
6. Add a few drops more of SnCl2 solution and cool quickly to room
temperature by putting the flask under running tap water.
i. The addition of SnCl2 should be made by a dropping bottle to avoid
any large excess in the solution, because its presence will consume
more K2Cr2O7 solution during titration and would thus register a
higher percentage of iron.
ii. Quick cooling of the solution after the addition of SnCl2 is
necessary to prevent re-oxidation of the reduced solution by
atmospheric oxygen.
79
7. Add about 10ml of mercuric chloride (HgCl2) solution to the cold
solution in (5) and dilute to about 2ml.
8. Add 2-3 drops of the indicator (diphenylamine) and 20ml solution of
H2SO4 and orthophospheric acid given.
9. Now run standard potassium dichromate solution in the cold reduced
solution through a burette, first liberally and then in small amounts,
stirring all the time with a glass rod until there is an intense violet-blue
colour. This shows that the oxidation of ferrous to ferric iron is
completed.
The important reactions involved are:
2FeCl2+SnCl2 2FeCl2 + SnCl2
SnCl2 + 2HgCl2 = SnCl4 + Hg2Cl2
6FeCl2 + H2Cr2O7 + 14HCl = 6FeCl3 + 2HCl + Cr2Cl6 + 7H2O.
NB. Since 1ml of 5/56 N H2Cr2O7 solution corresponds to 0.005g iron, then
number of ml of this solution used will directly give the percentage of iron on
a 5g iron, the number of ml of this solution used will directly give the
percentage of iron on a 5g sample of the ore. The result gave 55.1% iron.
This analysis was carried out in Mineralogy Laboratory of the Department of
Mineral Resources Engineering, Kaduna Polytechnic, Kaduna.
80
CHAPTER FOUR
DATA ACQUISITION AND ANALYSIS
4.1 Aeromagnetic Data Acquisition
The aeromagnetic map of the area, Otukpo sheet 270 was supplied by
the Geological Survey of Nigeria. The total intensity aeromagnetic map was
produced as part of nationwide airborne geophysical survey (1974 survey)
sponsored by the Government of Nigeria; Figure 10. The survey was
conducted along NW-SE direction with a nominal flight height of 500ft
(152.4m) and flight lines spacing of 2km apart.
The data are published in the form of aeromagnetic map on a scale of
1:100,000. The magnetic values were plotted at 5nT (gamma) interval.
4.1.1 Aeromagnetic Data Analysis
The aeromagnetic map was digitized at an equal spacing of 1km on a 52 by
52 grid lines. The data was fed into a computer file (MS DOS), which serves
as the input file for the computer program. This program picks all the data
intersection points row by row, calculate their longitudes and latitudes and
bring out result as column of X,Y, and Z representing longitude, latitude and
the magnetic value of the co-ordinates. X, Y, Z is then accepted by the
contouring package “SURFER”. This SURFER package produces a contour
map that is similar to the original aeromagnetic map. This contour map
produced is known as composite map (Figure11). This user friendly
81
Figure 10: Aeromagnetic Map of Otukpo Area
82
Fig1:Total magnetic field. 25,000 gamma is to be added to give the actual field values.
Contour interval is 5 nT.
8.0 8.1 8.1 8.2 8.2 8.3 8.3 8.3 8.4 8.4 8.5
longitude (degrees)
7.0
7.0
7.1
7.2
7.2
7.3
7.3
7.3
7.4
7.5
7.5la
titu
de
(de
gre
es
)
8 8 30
7
7 30
Figure 11: Total Magnetic Field 25,000 gamma is to be added to give the actual field values.
Contour interval is 5 gamma
83
computer program was also used to effect residual separation. The second
stage involves residual separation. This serves as a filter which emphasizes
the expressions of local features, and removes the effects of large anomalies
or regional influences.
4.1.2 The Regional – Residual Separation
The regional may be defined as the value of the field which would
exist if there were no local disturbance due to the source we are trying to
interpret.
A composite magnetic map is one that shows the superposition of
disturbance of noticeably different order of site and pattern. The larger
features, which are caused mainly by the deeper heterogeneity of the earth’s
crust, show up as trends over considerable distance. These trends are known
as regional and are frequently distorted by smaller local disturbance, which
are of primary interest in many magnetic interpretations. These smaller, local
disturbances are known as the residual anomalies and may provide evidence
of the existence of mineral ore bodies or reservoir type structures. For a
proper interpretation, the residual anomalies must be separated from the
regional background field.
84
Fig3 :Residual field for the area of study. Contour interval is 5 nT.
8.0 8.1 8.1 8.2 8.2 8.3 8.3 8.3 8.4 8.4 8.5
longitude (degrees)
7.0
7.0
7.1
7.2
7.2
7.3
7.3
7.3
7.4
7.5
7.5
lati
tud
e(d
eg
ree
s)
8 8 30
7
7 30
Figure 12: Residual field for the area of study. Contour interval is 5 nT
85
Regional residual separation is analogous to filtering in other
geophysical techniques like seismic, but the difference is that the noise that is
removed as a result of filtering is the object of importance in magnetic
studies. This noise or residue is of little or no interest in seismic data and
other geophysical techniques. There are several methods of removing the
unwanted regional, from the total field map. This includes graphical and
analytical.
4.1.3 Analytical Signal Method of Regional-Residual Separation
In this study the analytical method was applied. The analytical method
of determining the residual anomalies involves the use of numerical operation
on the observed data to isolate the residual anomalies without relying on the
visual method of smoothing in graphical method. The analytical methods
require the magnetic values to be spaced in a regular array or grid.
The analytical methods commonly used in regional-residual separation
include; fitting, the direct calculation of residual techniques such as, the
centre-point and ring method, the determination of second derivatives,
polynomial and downward continuation (Mohan et al, 1982).
86
Fig
13
: M
aps
sho
win
g th
e r
esu
lts o
f th
e A
na
ltytic
sig
na
l te
chn
iqu
e. T
he
re
sults
are
a c
om
bin
atio
n o
f th
e o
utp
ut
fro
m t
he
an
aly
tic s
ign
al t
ecn
ique
sup
eri
mp
osed
on
th
e z
ero
co
nto
urs
ob
tain
ed b
y a
plly
ing
a L
apl
acia
n o
pe
rato
r. (
a)
Th
e te
chn
iqu
e a
ppl
ied
at
flig
ht
he
igh
t.
(b)
Th
e t
echn
iqu
e a
pp
lied
for
th
e d
ata
co
ntin
ued
1 k
m a
bov
e th
e f
ligh
t h
eig
ht.
88
.05
8.1
8.1
58
.28
.25
8.3
8.3
58
.48
.45
7
7.0
5
7.1
7.1
5
7.2
7.2
5
7.3
7.3
5
7.4
7.4
5
(a)
88
30
7
7 3
0
88
.05
8.1
8.1
58
.28
.25
8.3
8.3
58
.48
.45
7
7.0
5
7.1
7.1
5
7.2
7.2
5
7.3
7.3
5
7.4
7.4
5
(b)
88 3
07
7 3
0
87
4.1.4 Methods of Aeromagnetic Survey Data Interpretation
The interpretation, explanation and guide presented here is directed
primarily towards a qualitative and quantitative interpretation for both
geological reasons as well as search applications, i.e., an understanding of
what causes the anomaly, its approximate depth, configuration, perhaps
magnetic content or mass, and other related factors.
Qualitative interpretation involves the description of the survey results and
the explanation of the major features revealed by a survey in terms of the
types of likely geological formations and structures which give rise to the
evident anomalies.
Quantitative interpretation involves making numerical estimates of the depth
and dimensions of the sources of anomalies and this often takes the form of
modeling of sources which could, in theory, replicate the anomalies recorded
in the survey. That is a model which is a suitable physical approximation to
the unknown geology.
The quantitative method is further divided into two groups (I) Graphical
method and (II) statistical/computer method. Here the second option is used.
It involves second order polynomial fitting which requires the computation of
power spectrum; this is usually done via fast Fourier transformation of the
data. The analysis of the transformed data yields depth to magnetic layer
anomalies (Bhattachryya, 1965).
88
Furthermore, computer program is used for the modeling of magnetic
anomaly. This is done along side statistical analysis, because it requires
statistical steps to transform a method into a computer program. This
computer method includes:
(i) Spectral Analysis method
(ii) Two-dimensional, (3) dimensional Hilbert Transformation
Another technique is upward/downward continuation of
magnetic observation
(iii) 2-D (and optionally 21/2 - D GM-Sys modeling.
4.1.5 Depth Estimation of Magnetic Sources by Means of Spectral
Analysis
Any process that quantifies the various amounts of light, sound, radio
waves etc. versus frequency can be called spectrum analysis. It can be done
on many short segments of time, or less often on longer segments or just once
for a deterministic function such as the Fourier transform.
The Fourier analysis of magnetic data, with the application of
computerized procedure is a standard technique for analyzing aeromagnetic
data. Spector and Grant (1970), Hahn et al, (1976); and many others have
analyzed one and two dimensional aeromagnetic data using this algorithm.
89
depth = 1.8 km
depth = 0.37 km
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Frequency (rad/km)
Lo
go
fP
ow
er(n
T2
)
Fig 7: Power spectrum graph for the area of study. Two layers can be observed with their respective depths as shown.
Fig 14: Power spectrum graph for the area of study. Two layers can be observed with their respective depths as shown
90
The Fourier transform of a function produces a spectrum from which
the original function can be reconstructed by an inverse transform, making it
reversible. In order to do that, it preserves not only the magnitude of each
frequency component, but also its phase. This information can be represented
as a 2-dimensional vector or a complex number, or as magnitude and phase
(polar coordinates). In graphical representations, often only the magnitude (or
squared magnitude) component is shown. This is also referred to as a power
spectrum.
The Fourier transform is a frequency domain representation. Linear
operations that could be performed in the time domain have counterparts that
can often be performed more easily in the frequency domain. It is also helpful
just for understanding and interpreting the effects of various time–domain
operations, both linear and non linear (Negi et al, 1983).
Fourier transform swaps the dimension of time with the dimension of
frequency. One can think of the Fourier transform as a combination of slow
and fast oscillations with different amplitude. The Fourier transform analysis
assumes the life of a signal from - ∝ to ∝. The power spectrum itself is the
Fourier transform of the auto-correlation function. Auto-correlation function
represents the relationship of long and short term correlation within the signal
itself.
91
4.1.6 Depth to the Magnetic Body
To estimate depth, the average amplitude together represent a spectrum
which when plotted in a semi-logarithmic co-ordinate system (log amplitude
versus frequency) often shows straight segments which decrease with
increasing frequency. By continuing the given field downwards these straight
segments become horizontal at a certain depth, the so-called “white depth”
(Hahn, 1965). This white depth may be used as a first estimate for the depth
to magnetic bodies of the anomalous field under consideration.
It is more often that estimated depths provide a good starting point for
a genuine structural interpretation (e.g. an interactive modeling of a
constrained inversion).
Basement depth (or equivalently, sedimentary thickness) is a primary
exploration risk parameter. Magnetic depth estimates provide insight into the
evolution of more recent sedimentary features.
4.1.7 Upward and Downward Continuation of Magnetic Observation
Peter’s (1949) devised two rule of thumb techniques for depth
estimation, the error curve method and the slope method.
The error curve method requires a contour map and program for
continuing the mapped data upward and downward. The depth to the top of
the body can be estimated from the result. Upward/downward continuation of
magnetic observation in aeromagnetic interpretation technique is sometimes
used in order to simplify the appearance of magnetic maps by suppressing
local features.
92
Fig 4: Upward continued field 500 m above flight hieht. Contour interval is 10 nT.
8.0 8.1 8.1 8.2 8.2 8.3 8.3 8.3 8.4 8.4 8.5
longitude (degrees)
7.0
7.0
7.1
7.2
7.2
7.3
7.3
7.3
7.4
7.5
7.5
latit
ud
e(d
eg
ree
s)
8 8 30
7
7 30
Fig 15: Upward continued field 500 m above flight height
93
Fig 5: Upward continued field 1 km above flight hieht. Contour interval is 10 nT.
8 8.05 8.1 8.15 8.2 8.25 8.3 8.35 8.4 8.45 8.5
longitude (degrees)
7
7.05
7.1
7.15
7.2
7.25
7.3
7.35
7.4
7.45
7.5la
titu
de(d
egr
ees
)
7
7 30
8 8 30
Fig 16: Upward continued field 1 km above flight height. Contour interval is 10 nT
94
The proliferation of local magnetic anomalies often obscures the
regional picture, but upward continuation is used to smooth out these
disturbances without repairing the main regional features.
Furthermore, downward continuation will smooth out the regional
features and gives a sharp picture of the shallow bodies or localized body.
This corresponds to the well-known fact that the field at lower depths, where
the short waves have relatively high amplitudes, shows more details than at
high levels where the long waves clearly dominate.
4.1.8 REGIONAL MAGNETIC ANOMALY
From the total aeromagnetic intensity anomaly map of Otukpo, it is
seen that the largest anomaly is 27.5km long and 19.3km wide at its
maximum. It has a North East – South West strike and a dip 55o North East.
This is named Benue trend and is almost always dominant in the entire
trough and the surrounding basement complex (Ojo and Ajakaiye 1976,
Nwachukwu, 1972). This trend is seen to have controlled the early structures,
the Cretaceous magmatism and the orientation of the fold belts in Benue
Trough (Ajakaiye, 1986).
95
The geomagnetic inclination of the earth field in this area is 10oN, and
the magnetic declination is 5oE. The residual magnetic map for the study area
resembles the total magnetic intensity map closely.
The upward continued field in Figure 17 clearly amplifies the major
anomaly which was filtered to reduce the other unwanted signals. The
regional anomalies show strong positive gradients which appears to be due to
a deeper regional features (structures deeper than the Cretaceous sediment i.e.
the basement). The broad magnetic closures seen on total magnetic intensity
anomaly map are often due to changes in the rock composition within the
basement (Grant et al, 1965).
The sources of the regional magnetic anomalies in this area are
interpreted as igneous activity. As the area is occupied by Cretaceous
sediments mainly comprising shales, sandstones and limestones considered to
be non magnetic.
The Cretaceous sediment depth has an average maximum of 1.8km
while the average minimum depth is 0.37km, it is clear from the figures that
the results obtained from 2-D harmonic transformation of the aeromagnetic
data conform well with other geophysical estimates.
Thus the mean depth to the top of the basement obtained by the
spectral method over and around Otukpo area may provide some useful
information with regard to first order geologic interpretation, and thickness
parameter useful for the indirect quantitative interpretation of magnetic data
in the region.
96
Fig 6: Upward continued field 2 km above flight hieht. Contour interval is 10 nT.
8.0 8.1 8.1 8.2 8.2 8.3 8.3 8.3 8.4 8.4 8.5
longitude (degrees)
7.0
7.0
7.1
7.2
7.2
7.3
7.3
7.3
7.4
7.5
7.5
lati
tud
e(d
eg
ree
s)
8 8 30
7
7 30
Fig 17: Upward continued field 2 km above flight height. Contour interval is 10 nT.
97
4.1.9 Development of Model
GM – SYS Modeling Method
There are several methods of modeling in aeromagnetic interpretation.
Choice of technique in any survey depends on the structures intended to
model and also the purpose of the survey.
The reason for this work is to first determine depth to basement and
then model the shape and depth of structures in the study area.
The development of the models involved the following steps
1. Filtering the bouguer anomaly to obtain the residual anomaly which
arises from sources shallower than 4km (Mareschal, 1985).
1. Inversion of the residual anomaly with a constant density contrast
to obtain a model of the basement.
2. Adjustment of the inversion parameters and of the basin sediment
density to obtain a model for basement depths which satisfies the
profiles taken across prominent anomalies within the study area.
Conditions for the modeling to succeed are as follows:
1. The basin (valley) stratigraphy can be represented by two layers,
Miocene and younger sediments and the Cretaceous basement
with the primary density contrast between them.
The sediments contain no large-scale magnetic susceptibility variations as
shown in table 3. This condition is satisfied as the basin lacks large thrust
sheets which complicates the expected stratigraphy.
98
In GM- SYS, the methods used to calculate magnetic model response
are based on the methods of Talwani and Heirtzler (1964), and makes use of
the algorithms described in Won and Bevis (1987). The results from GM-
SYS have been analyzed and found correct by several organizations who use
it for geophysical consulting work. This method uses a two dimensional flat
earth model for the gravity and magnetic calculations, here each structural
unit or block extends to plus and minus infinity in the direction perpendicular
to the profile. The model extends plus and minus 30,000km along the profile
to eliminate edge effect. This GM- SYS makes use of interactive graphics to
significantly speed up the interpretation processes.
From the residual total intensity aeromagnetic map of the study area
and the analytical signal map, three major anomaly profiles were modeled.
They are labeled AB, CD, EF for easy identification, profile AB runs across
anomaly A & B, profiles CD runs across anomaly C & D and profile EF runs
across anomaly E & F.
4.2 Ground Magnetic Data Acquisition
The magnetic data for the study were collected during field work
carried out in November, 2009 to complement existing airborne (magnetic
and landsat) data obtained from GSAN, Abuja and the National Centre for
Remote Sensing, Jos respectively.
99
4.2.1 The Instrument, Its Principle and Limitations
The Instrument
The model G-816 portable proton magnetometer was used for the field
survey. The G-816 is accurate and stable to within ±1 gamma over a range
from 20,000 to 90,000 gammas. The instrument measures total field intensity,
the accuracy of each measurement is independent of sensor leveling.
Furthermore, the measurement is based upon an atomic constant and is
independent of temperature, humidity and battery conditions. The inherent
simplicity of the G-816 proton magnetometer allows rapid, accurate, high
resolution measurements of the field to be obtained from a rugged, compact
field instrument.
Among very diverse applications of portable magnetometers,
especially the total proton (nuclear precession) magnetometers are their uses
in mineral and petroleum exploration, geological mapping, search for buried
or sunken objects, magnetic field mapping, geophysical research, magnetic
observatory use, measurement of magnetic properties of rocks or
ferromagnetic object, paleomagnetism, archaeological prospecting,
conductivity mapping, gradiometer surveying, and magnetic modeling etc.
4.2.2 The Principle of Proton Magnetometer
The proton precession magnetometer is so named because it utilizes
the precession of spinning protons or nuclei of the hydrogen atom in a sample
of hydrocarbon fluid to measure the total magnetic intensity. The spinning
100
protons in a sample of water, kerosene, alcohol etc, behave as small, spinning
magnetic dipoles. These magnets are temporarily aligned or polarized by
application of a uniform magnetic field generated by a current in a coil of
wire. When the current is removed, the spin of the protons causes them to
precess about the direction of the ambient or earth’s magnetic field. The
precessing protons then generate a small signal in the same coil used to
polarize them, a signal whose frequency is precisely proportional to the total
magnetic field intensity and independent of the orientation of the coil, i.e.
sensor of the magnetometer. The proportionality constant which relates
frequency to field intensity is a well known atomic constant. The
gyromagnetic ratio of the proton. The precession frequency, typically
2000Hz, is measured by modern digital counters as the absolute value of the
total magnetic field intensity with an accuracy of 1 gamma (Breiner, 1973).
4.2.3 Limitations of a Proton Magnetometer
Several operational restrictions exist, which may be of concern under
special field condition. First, the proton procession signal is sharply degraded
in the presence of a large magnetic field gradient greater than 200 gammas
per foot (approximately 600 gammas per meter). Also small signal can be
rendered immeasurable by the effects of nearby alternating current electrical
power sources. For this reason, it is important that the earth’s magnetic field
is not disturbed by allowing magnetic objects to come close to the sensor.
Such articles include rings, keys, watches, belt buckles, pocket knives, metal
101
pencils, zipper, some hats, steel chips, magnetic dirt etc. Do not make
readings in a highly magnetic environment e.g. inside buildings (Telford et
al, 1990).
4.2.4 Field Magnetic Survey Procedures and Data Reduction
Field Procedure
Prior to survey, the following steps should be performed to correctly
tune and turn on the magnetometer.
1. Attach signal cable to sensor. There are two cables provided. A long
coiled cable for staff use and a shorter cable for use with the
“backpack”.
2. Attach sensor to staff and assemble sections or place sensor in
“backpack” pouch attached to carrying harness.
3. Place G-816 console in harness, attach to shoulder harness, and adjust
for snug fit on operator’s person.
4. Connect sensor signal cable to console.
Caution: Be sure the following is performed in a clean magnetic
environment.
5. Adjust Tunning-Kilogammas switch to a position that produces
maximum signal. To save time, start with a setting near the known
field. The field value at any location can be estimated using the world
map provided in the operator’s manual. The average ambient magnetic
102
field and magnetic inclination used in this work is 33000nT and 10o
respectively.
6. Depress the READ button momentarily and release.
7. The battery indicator Lamp (BAT) will light immediately and blink
throughout the 3 second polarize interval.
8. Shortly after the polarize interval, the magnetometer will display the
total intensity of the earth’s magnetic field directly in gammas.
9. Simultaneously, the signal Lamp (SIG) will light and blink to give a
relative indication of signal strength.
10. Switch the tunning - kilogammas switch to an adjacent position in
either direction, repeat step 6, and count the number of SIG blinks. A
properly tuned signal should provide approximately 7 or more blinks
on the SIG indicator.
11. The instrument is now ready for field survey operation.
4.2.5 Materials used during the field work
Instruments: Magnetic compass, 12 channels Global Positioning System
(GPS), Proton Magnetometer, Sensor, Sensor staff, Micro Kappa meter, Time
piece.
Field Vehicles: Motor car, Motor cycle, bicycle.
Other materials: Geological hammer (1kg), Sample bags, Field note book,
shovels, Tape, Cutlasses, Pegs, Ranging poles, Pick axes, Bucket, Rubber
pail, Ruler, Masking tape, Marker, Weighing scale.
103
4.2.6 Survey Operation
To insure optimum results, the sensor is marked with an arrow and the
letter “N”. The arrow should be roughly pointed north or south. In the field, I
used the North direction for my sensor in all my traverses. This procedure
will allow the sensor axis to be placed perpendicular to the earth’s field and
produce optimum Signal.
During survey operation and after the instrument is tuned to the local
field intensity, the operator need only depress the READ button and note the
reading each time in a field notebook. If a reading is in question, ie. a sudden
shift of several hundred gammas, another reading should be taken. This
account for four readings in some locations. The one count repeatability and
sensitivity of the G-816 can always be verified by repeating a measurement
with the sensor in the exact same location.
Measurements are made at regular intervals along a grid or otherwise
selected path whose locations are noted for subsequent plotting. In this
survey, traverses were selected along pathways or other accessible routes. At
each station, the time, magnetic readings, altitude and coordinates of the
location is noted. See appendix.
The average of the readings per station would later be used to draw a
profile.
104
In this way, some of the surface noise is averaged out. A number of advanced
techniques for data enhancement or filtering as employed in airborne surveys
or ground surveys will not be discussed any longer as some of them e.g.
spectral analysis, analytical signal analysis, bandpass filtering, upward and
downward continuation etc. have been discussed earlier.
The pacing for Utonkon to Otukpo, a distance of 30km were at 1km
and 100m intervals separately and this gave same slope of profile in Figure
29 despite the different pacing for the same traverse and route. The traverse
from Upu/Otukpo to Okpamaju covering 15km, was at 50m pacing. Traverse
between Otukpo to Adoka and Otukpo to Aliade covering distances of 32km
and 51.5km respectively were both surveyed at 1km intervals. The profiles
are seen in figures 29 to 31. Traverse line Asa/Ogyoma close was at 50m
interval and covers a distance of 27km. while within the mineralized area
(sample location) in Ogyoma forest the pacing was at 20m interval (Figure
18).
105
8.0 8.1 8.1 8.2 8.2 8.3 8.3 8.3 8.4 8.4 8.5
longitude(degrees)
7.0
7.0
7.1
7.2
7.2
7.3
7.3
7.3
7.4
7.5
7.5
latitu
de
(de
gree
s)
Figure: GroundMagneticSurveyTraverses
Superimposedonresidualmapofthearea
Figure25: GroundMagneticSurveyTraverses
Superimposedonresidualmapofthearea
G
HI
J
L
K
M
N
18:
106
4.2.7 Instrument Storage
After use, all of the components were stored in the shipping container
to prevent damage, loss of components, or possible contact with magnetic
particles that could be imbedded in the sensor. The sensor signal must be
disconnected from the console to prevent constant battery drainage. If long
term storage is anticipated, the batteries should be removed from the console
to prevent any damage from electrolytic leakage or corrosion of contacts.
After long storage, always inspect the batteries.
4.2.8 Data Reduction
The origin of the Earth’s field magnetism is not well understood, but
thought to be due to currents in a fluid conductive core (Breiner, 1973).
There are spatial variations in the earth’s magnetic field, but the most
relevant deviation from a symmetric field is the anomalous set of features in
the earth’s crust caused by local variations in the magnetic minerals or other
features of interest which distort the local earth’s magnetic field.
4.2.9 Time Variations
Time variations with periods of seconds, minutes and hours are the
direct or indirect effect of the solar wind as it distorts the magnetosphere or
external magnetic field of the earth. Daily or diurnal variations are primarily
seen during the local daylight hours shown for typical days. The daily
107
variation is caused by electric currents due to tidal movements and ionization
in the ionosphere.
For very important field measurements, particularly for higher
resolution measurements, a recording base station or reference monitor is
often used which is examined at the start of each day for an indication of
magnetic storm activity and also for subsequent removal of the diurnal
variations from field data using time as a correlation. Magnetic storms are
caused by the circulation round the earth of charged particles from the sun in
a region beyond the atmosphere known as the magnetosphere. Large storms
produce changes of as much as several hundred gammas and the initial
disturbance takes several days to die away (Griffiths et al, 1983). The raw
data were corrected for diurnal variation of the earth’s magnetic field.
4.2.10 Correction for Time Variations
The simplest method for correcting for time variations involves
repeated readings in the same orientation at the same station at different times
during the survey. If a smooth curve is drawn through the readings plotted as
a function of time (every two hours), these values can be subtracted from all
other readings provided that each reading also includes the time at which it
was observed. In this case readings are repeated at base or reference stations
every two hours. It is also possible to ‘double-back’ to take a second or third
reading on each giving traverse to determine at least the time variations for
that traverse.
108
In this survey a local recording base station, i.e. diurnal station monitor
was used, is the most ideal method and certainly the most accurate for
removing time variations (Hood and Mcchure, 1965). The time variations are
removed from each reading on the traverse to within a minute or so of the
base station. The base station should not be further away than 161km
(100miles) from the area of the survey for agreement within a few gammas
and should be positioned more than 61m (200ft,) away from local traffic and
other disturbances.
4.2.11 Other Corrections
In proton magnetometer there are no calibration problems and
inherently it is drift free. Exact orientation is not necessary since the total
field is measured rather than any component.
Elevation and Terrain corrections are therefore insignificant. In local
surveys, corrections for changes in the main field with position are also often
too small to be of importance and even if appreciable may be extracted as a
‘regional’.
4.2.12 Ground Magnetic Interpretation
Minor variation in magnetic field from place to place caused by magnetic
inhomogeneities of the earth’s crust is the chief interest of magnetic
prospecting and we call it the anomaly part.
109
An anomaly represents local disturbance in the earth’s magnetic field
which arises from a local change in magnetization, or magnetization contrast.
The form of magnetic anomaly from a given body depends on the
following factors:-
1. The geometry of the structure
2. Magnetic inclination
3. Direction of the line of observation with respect to axis of the body
4. Depth and involvement of basement rocks and /or sedimentary
warping
The anomalies due to structure will always exhibit higher frequencies.
The steep and linear form of the gradient being in places strongly suggestive
of major faulting.
Area in which magnetic rocks are at or near the surface will be
characterized by marked and usually sharp anomalies. Those anomalies
which are smoother or of longer “wavelength” will reflect the presence of
non-magnetic rocks down to considerable depth (Grant and West, 1965).
Increase in density contrast sharpens the frequency of the anomalies.
The process does not imply that the distribution of magnetism in the earth is
necessarily related to the interpretation of magnetic field data by thinking of
magnetization in the same way as density. For instance basalt or mafic
intrusive would produce a significant positive gravity and magnetic anomaly.
110
Information of this kind is very valuable in prospecting for minerals to
locate areas favourable and to obtain some knowledge of their gross
structure.
4.2.13 Magnetic Effects of Geometric Models
Simple normalized characteristic curve for evaluating the general
nature of magnetic anomalies are in use (figure 27). These maps show total
magnetic intensity and its vertical derivative (or curvature) for different
rectangular bodies with unit depth to top and infinite depth to bottom, with
various orientations with respect to the earth’s field, with various ratios of
length to width and with magnetic inclinations of 0, 20, 30, 45, 60, 75 and 90.
Geological anomalies are interpreted in terms of much simplified
geological models which very much facilitate interpretation procedures. The
first simplification is the assumption that magnetization is uniform within
some elementary prismatic form. Typical of the kind of geologic sources that
are assumed to cause anomalies are those of dipping dyke, vertical dyke,
fault, intrusive, shallow wide dyke, anticline, graben (void), sphere etc.
Among many simple models available for magnetic interpretation, the thick
dyke and sill are the most widely used because of their suitability in many
geologic situations (Steenland et al, 1970). Interpretation of magnetic maps is
usually done to determine the depth, dip, size and susceptibility of causative
sources. The amplitude of the anomalies is directly related to the strength of
magnetization of the source.
111
For simplicity, we choose the width of the dyke to be equal to twice the
depth, which is generally true for many intrabasement anomalies (Steenland
et al, 1970).
It must be emphasized that not only simplification is required, but a
reasonable geologic frame work must be used as a guide when considering
the various possible sources.
i. The weight of the surrounding and overlying rocks.
ii. The internal stress of the included gases and steam.
iii. The stress due to the molecular movement of the constituents during
the process of cooling and
iv. Gravitational stresses, magma which cool slowly under pressure in
most cases assume a crystalline structure.
4.2.14 Estimation of Source Parameters
Estimation of Depth
A. The “Straight-Slope” has been widely used for determining the depth
to the top of the magnetic source.
The tangent is drawn to the steepest gradient of an individual magnetic
anomaly on a section of profile (Figure 19). The horizontal distance, Ss, over
which the tangent line is coincident with the anomaly profile is measured.
112
113
A depth estimate is then obtained by multiplying S by a factor which
usually falls in the range 1.2 to 1.6 for a vertical dyke-like body with various
values of width to depth of burial (α = w/h) the factor values are tabulated in
tables. For various effective magnetic inclinations. (The table repeats
symmetrically for effective magnetic inclinations from 450 to 0
0). For an
approximation which disregards the geometry of the source, it may be said
that:
h = 1.4SS ± 20%.
B. Peter’s “Half-slope” method: The same tangent is drawn as in the
straight-slope but ambiguity is reduced by drawing two more tangents
at half the slope of the first (figure 26b). Now the horizontal distance
between these two new points of tangency is measured, S1/2
. The depth
estimate is then h=0.63 S1/2
In the case where h=2W, note that S1/2
= 2.2Ss. These methods are useful
for aeromagnetic map of a new area, or with an anomaly on a field profile.
4.2.15 Estimation of body dip (d) or direction of magnetization (Ij)
Measurement of magnetization may also be useful in mapping certain
members of volcanic formation, particularly where magnetization reversals
are present. Measurements of the orientation of permanent magnetization of
rocks provide the basis for paleomagnetic measurements for study of the
changes and reversals of the earth’s magnetic field.
114
The dip of the dyke or the direction Ij of resultant magnetization may be
evaluated when Ө is known. Since Ө controls the shape and relative position
of the maximum and minimum (Figure 20), it can be evaluated from the
amplitudes and positions of the maximum and minimum curves. After
finding Ө, the next step is to find either ‘d’ or Ij. Where magnetization is due
solely to induction or when the remanent vector aligned along the Earth’s
present field, Ij=Ie and hence‘d’ may be evaluated using the following
relations.
D=2Ie- Ө-900
And Ie= Angle of inclination of the Earth’s magnetic field which varies from
– 90 to 900, positive in the Northern Hemisphere. Here it is 10
0
Ө= Angle between magnetic north and the positive X-axis of ones profile
Ij may be found using the following relation
Ij= Ө-Ie+d+900
115
20:
116
Figure 21: Shape of field profiles
117
4.2.16 Estimation of Susceptibility
Susceptibility (k) is a fundamental parameter in magnetic prospecting.
Magnetic susceptibility is a measure of how strongly magnetic a rock will
become in the inducing earth’s field. The susceptibility of rocks and mineral
is determined mainly by the amount of ferromagnetic materials present in
them. However the susceptibility of a single rock type can be very variable
and these ranges reflect the different amount of magnetic minerals present in
different samples of the same rock type.
Magnetic susceptibility of rock samples obtained from the study area
were determined by the use of Microkappa meter (a field instrument). The
average magnetic susceptibility of rocks in the area is 1.65emu, while, that of
the pyrite ore is 0.14emu.
The average susceptibility contrast of the basement and the sediments
is 0.71 versus 0.31 emu.
Table 5 lists susceptibility for various rocks and pyrite mineral, which
is basically non-magnetic.
Densities and magnetic susceptibility of rock units were ascertained
from hand dug pits and surface samples.
118
Table 5. Magnetic susceptibility of rocks within the study area
Rock Magnetic Susceptibility (emu)
Shale 0.07
Granite diorite 12.7
Slate/Limestone 0.24
Siltstone 0.01
Sandstone 0.13
Limonite/Garnet 0.64
Pyrite 0.07 – 0.20
emu = Electromagnetic units (Gaussian units)
119
The variations in the density of the formations and their relationships
to the diagenetic facies are expressed in table 6, because diagenesis proceeds
more rapidly with burial depth.
The average density of rocks found within the study area is 2.72g/cm3.
Density of the pyrite mineral found in the area is 6.3g/cm3.
Knowledge of the magnetic susceptibility is useful in ground follow up of
aeromagnetic survey to ascertain the source of observed anomalies,
determine possible magnetite-associated mineralization, and in mapping
several rock units as a function of their susceptibility.
The susceptibility contrast K of the survey area can be extracted from
the amplitude term ‘A’ using the following relation.
A= 2KTWPSind
= 3.33KTPsind
= ≥ 0.3 A/T
Where A = Total amplitude (i.e. peak to peak deflection)
S= 1/2P where P=25
T= Total intensity of the inducing field (33,000 gamma)
Being the average ambient magnetic field of the area
D= Body dip
W= body width
120
Table 6. Densities of variety of rocks found in the study area.
Rock Density (g/cm3)
Shale 2.07
Diorite 3.90
Slate/Limestone 2.66
Siltstone 2.20
Sandstone 2.38
Limonite/ Garnet 3.13
121
4.3 Ground Magnetic Results Interpretation
Comments on magnetic signal interpretation at low latitudes
Magnetic signal is dependent on its location on the globe. This is
because anomaly shape varies with field inclination and many other
parameters.
Interpretation of magnetic field data at low magnetic latitudes is
difficult because the vector nature of the magnetic field increases the
complexity of anomalies from magnetic rocks. The pole reduced anomaly
could be interpreted as a shallow North dipping body, which might explain
the strong low. Reduction to the pole attempts to simplify the magnetic field
by rotating the magnetizing vector to be vertical (Hansen et al, 1989).
The profiles obtained from plotting ground magnetic survey traverses namely
profile GH, IJ, KL, MN and OP is interpreted.
Profile GH
The anomaly produced an increased magnetic gradient towards ‘H’.
The signature portrayed a major positive anomaly toward negative X-axis. It
is dipping N3oW and striking in the direction NW-SE. the direction of
magnetization is 650. The depth to the magnetic source is between 1.12km to
1.26km, while the magnetic susceptibility is between 0.3-0.32 emu. The
increase in the density contrast of the layer which is faulted could represent a
carbonate or igneous formation. The lower density layers could represent a
clastic sequence.
122
Profile IJ
The magnetic signature shown above is an indication of a faulted-
folded syncline traced to basement faults. It is a folded sedimentary syncline
with a non uniform density with depth due to sedimentary warping. The
magnetic anomaly within the basin of the sedimentary section is shown as
magnetic. The profile shows a symmetric positive anomaly, dipping at 00 and
striking East-West. The direction of magnetization (Ij) is 950. The depth to
the source of anomaly at the lower frequency (basin) area is 3.04km while at
the faulted higher frequency area it is 0.28km. The magnetic susceptibility is
between 0.31-0.32 emu.
123
Latitude (degrees)
Latitude (degrees)
Fig. 22: Profile GH
Gam
ma
(nT
)
32500
33000
33500
34000
34500
35000
6.95 7 7.05 7.1 7.15 7.2 7.25
G
H
124
Longitude (degrees)
Fig. 23: Profile IJ
Gam
ma
(n
T)
33400
33600
33800
34000
34200
34400
34600
34800
35000
35200
8.15 8.2 8.25 8.3 8.35 8.4 8.45 8.5
I
J
Red
125
A syncline produce a minimum closure on most geophysical maps.
The amplitude and characteristics of the magnetic anomaly associated with a
syncline are dependent on (1) depth, (2) the type of sedimentary fill (3) the
amount of sedimentary warping, (4) the involvement of the basement rocks
(5) the magnetic inclination which is dependent on its location on the globe.
The areal or lateral extent of the basin graben is delineated by identification
of magnetic basin bounding fault signatures. The basin is expressed as a
minimum magnetic closure surrounded by higher frequency maximum
closures.
Profile KL
The profile shows a symmetric positive anomaly. A dipping bed
produces anomaly similar to a fault. The main difference is that as the bed
nears the surface, the gradient of the anomaly becomes steeper and the
magnitude of the anomaly tends to be greater. The high structural model
(amplitude) towards ‘L’ uses density contrast which could represent the
thrusting of older beds over younger beds, or a dyke intruding into a
sedimentary sequence. A basalt or mafic intrusive would produce a
significant positive anomaly as shown above. The magnetic susceptibility is
0.30 emu at the minimum frequency and 0.34 at the maximum frequency.
The body strikes in the East-West direction and dipping 00 with Ij at 95
0.
Depth to the magnetic body is 1.68km for the beds and 0.98km for the
intrusion.
126
Latitude (degrees)
Fig. 24: Profile KL
Gam
ma
(nT
)
32000
33000
34000
35000
36000
37000
38000
7.15 7.2 7.25 7.3 7.35 7.4 7.45 7.5
K
L
Light Blue
127
Latitude (degrees)
Fig. 25: Profile MN
Gam
ma
(nT
)
32500
33000
33500
34000
34500
35000
35500
36000
36500
37000
7.2 7.22 7.24 7.26 7.28 7.3 7.32
M
N
Green
128
Profile MN
The profile shows an antisymmetric anomaly with negative toward
negative X axis. It strikes in the North East-South West direction, dips at
N900W and has a magnetization direction of 5
0. Dept to magnetic source is
estimated at 0.56 to 0.7km for the high amplitude areas and 1.26km for the
low amplitude areas. Magnetic susceptibility (K) is 0.3 emu for the low
frequency and 0.33 emu for the higher frequency areas respectively. The
magnetic anomaly is for an anticline with a basement structure. Two faulted
anticlines as shown consist of a sedimentary sequence of density values that
increase with depth and a faulted basement uplift. This structure produces
narrow maximum anomalies indicating dyke intrusion (volcanic flow) and
the areal extent of the entire uplifted section. The magnetic susceptibility
assigned to the high-density layer suggest it to be magnetic volcanic rocks.
The minimum between the two high frequency closures is due to the wedge
of relatively lower density material between the fault and the rollover. This is
an anomaly characteristics generally observed on faulted anticlines. The
intra-sedimentary magnetic sill has almost equal amplitude anomaly implying
a uniform magnetization and density.
Sample Location: Profile OP
The magnetic signature here describes a faulted-folded antisymmetric
anticline with negative toward positive X-axis where the basement or a
129
magnetic sedimentary layer is faulted. The anomaly amplitude is dependent
on both dept and magnetic susceptibility. The gradient is so high due to the
increase in the density contrast of the layer which is faulted. The high density
layers could represent the thrusting of older beds over younger beds or a dyke
intruding into a sedimentary sequence. In this case the basement is involved
in the faulting. Here pyrite is found deposited within shale sediments at
Ogyoma in Akpa District of Otukpo Local Government Area of Benue State.
The source of the pyrite is hydrothermal and it is mineable quantity.
Higher frequency positive anomalies superimposed on the broad
anomaly indicate the areal extent of the entire uplifted section. The lower
density layers could represent clastic sequences. The minimum between the
high frequency closures is due to the wedge of relatively low density material
between the faults.
The magnitude of this minimum anomaly is governed by the density
contrast and the thickness of the wedge, which is an anomaly characteristic
generally when the basement is involved in the structure. The anomaly is
dipping N900 W and striking in the North West-South East direction. The
magnetization direction is estimated at 1850. The depth to the body causing
anomaly is 2.08 – 2.63km. Magnetic susceptibility for the pyrite area is
between 0.299 to 0.32emu.
130
Latitude (degrees)
Fig. 26: Profile OP
Gam
ma
(nT
)
33000
33200
33400
33600
33800
34000
34200
34400
34600
34800
35000
6.95 7 7.05 7.1 7.15 7.2 7.25
Deep Blue Spot
O
P
131
Table 7. Comparing results of the Aeromagnetic and Ground magnetic
Studies.
S/No Aeromagnetic Result Ground magnetic Result
1. Profiles exhibit lower frequency and less
anomalous features
Profile exhibit higher frequency
anomalies due to intrusions and
effect of basement uplifts.
2. Portray interpretation of magnetic
anomalies arising from regional crustal
structure
Emphasizes the expressions of
local features and removes the
effect of large anomalies or
regional influences
3. Permanent remanent magnetization for
basement
Magnetization is induced for
sediments
4. Magnetic susceptibility contrast is
between 0.073-1.71 emu
Magnetic susceptibility is
between 0.299-0.34 emu
5. Dip differences between 30-30
0 Dip is between -90
0 to 90
0
6. Geometry of the anomalous source-
Basement sill, basement uplift and
syncline
Geometry of the source body
- Dykes, faults, folds, sedimentary
warps, Graben (void) etc.
7. There are few basement structures hence
less variable suprabasement anomalies
(basement topography) and less
intrabasement (basement lithology).
Variable lithology due to high
intra-sedimentary structures and
non-uniform sub-basement
topography.
8. Depth to anomalous source is between
3-9.4km (depth to basement).
Depth to anomalous source is
between 0.56-3.04km within
Cretaceous sediments
9. The direction of magnetization is 650-
1250
The direction of magnetization is
between 50-185
0
10. The average sedimentary thickness by
Fourier analysis (power spectrum) of
aeromagnetic data are 0.37km and
1.8km for the first and second observed
layers respectively.
The average estimated depth to
magnetic bodies of the anomalous
field for two observed layers (first
and second) are 0.61 and 1.5km
respectively.
132
4.4 Exploration
A logically designed exploration program progresses through a number
of stages, from regional reconnaissance to semi-detailed follow up and thence
to detailed evaluation. The reconnaissance often is airborne and the final
evaluation perhaps involving down whole techniques.
Airborne anomaly indications have been so clear and definitive that
ground-follow-up (ground magnetics) was limited to defining the major sites,
which appeared geologically promising. The ground geophysical work
involved ranging (surveying), clearing and cutting and magnetic data
collection. Survey parameter such as line orientation took its guidance from
existing foot and road paths.
To aid evaluation, exploration sample pits were dug along
pegged lines and the locations are plotted at regular intervals by the use of a
planimeter. The purpose of sampling is purely qualitative, where it is
necessary to prove the presence or absence of certain minerals or chemical
elements and quantitative to determine the tonnage of the mineral present.
The survey used sample spacing of 50m and 1km intervals. However
borehole spacing for stratiform deposit is 100m (Grayson, 2001).
Samples were collected in black polyethylene bags, labeled and
numbered from locations shown in Figures 34 and 35 and then tucked into
bagco bags.
133
Longitude
Figure 27: Sample Points
134
4.4.1 Sampling and Estimation of Reserves
Ore valuation is a process whereby the economic value of a mineral
deposit is determined. Sample values provide geologist with quantitative data
for estimating ore reserve and on which he can take decisions.
The selection of exploration target zone is based on favourable geology
and structure. This work is based on sample results obtained by prospecting
on a sector of the major anomaly 25km2 where the tonnage of the ore block is
estimated. Sample is taken from materials excavated. The exploration
sample collection work provides a great opportunity to initiate geotechnical
investigations. The collection of geotechnical data will provide mining
engineers with essential information for design of the mining method.
4.4.2 Pitting
In the case of very shallow mineralization, the resource is proved by
digging pits. Pits are dug using local equipment, pick axes (diggers), shovels,
hoes, headpans, buckets, calabashes, etc. Seven labourers were used for this
work for 3months between February- April 2010. Sample pits of 1m2 up to
1.5m-2.5m depth were excavated.
Representative handfuls of the mined mineral are picked at some
convenient location and these form the samples.
135
Fig : Residual field for the area of study. Contour interval is 5 nT. Insert showing sample location and points
8.1 8.1 8.2 8.2 8.3 8.3 8.3 8.4 8.4 8.5
longitude (degrees)
7.0
7.0
7.1
7.2
7.3
7.3
7.3
7.4
7.5
7.5
lati
tud
e(d
eg
ree
s)
8o 8 30
7 30`o
8 30`0
7o
Figure 28: Residual field for the area of study. Contour interval is 5 nT. Insert showing sample
location and points
136
Samples are dried in the sun. Drying is generally followed by crushing,
grinding and sieving and the resultant finer material is split, or separated, into
descrete mass components for further reduction until the assay portion is
obtained for chemical analysis. The analysis is aimed at the determination of
the elemental concentrations in the sample and of trace metals. The choice of
analytical method will aim at optimizing contrast of the main target elements.
The Neutron Activation Analysis (NAA) was carried out at the Center for
Energy Research and Training (CERT), Ahmadu Bello University, Zaria. The
X-Ray fluorescence analysis was carried out at Geological Survey Agency of
Nigeria, Research Laboratory, Kaduna. The differences between the methods
shown are the detection limits of analysis, speed of analysis and the need to
take material into solution.
4.4.3 Physico-Chemical Analysis
The Nature and Morphology of the Ore body
The size, shape and nature of ore bodies affects the workable grade, hence
the following information are necessary in exploration.
The ore deposit is found overlain by 2.50m thick overburden and
1.40m within stream channels. The pyrite is grey to dark in colour with
yellowish specks, with uneven fracture. The dark colour is caused by
carbonaceous impurities. Rapid oxidation of the sulphide mineral as soon as
137
they are exposed to air as they absorb large quantities of oxygen is noticed.
The mineral surface is dull and rusty, with a specific gravity of 5. It displays
no cleavage but has various forms of complex interpenetrant crystals. Pyrite
commonly crystallizes in cubes and octahedral, but not infrequently occurs as
irregular aggregates.
The mineral in its mode of occurrence is found disseminated through
out the body of the host rock. It occurs in concordance with the lithological
banding (often bedding) in the enclosing rocks. The segregated, irregularly
shaped lumps occur in sizes between 2-6cm in length and 1-3cm in width.
The ore deposit is an epigenetic infilling of pore spaces.
It is a stratiform deposit (ore body show a considerable development
parallel to the bedding and a limited development perpendicular to it). The
mineral grains are interlocking and fine to medium size. The shapes of the
grain particles are irregular. The streak is dark grey (almost black).
4.4.4 Moisture Content
Moisture determinations are important for selecting the system of
transportation and storage for minerals.
Moisture content for ore reserve is given by the formula
138
Where W = Moisture content
P1 = weight of moist specimen
P2 = weight of dried specimen
The average moisture content of the mineral is 1.5%. The density is
6.3g/cm3, the water absorption is 2g/L. The adsorption test conducted for one
week gave 0.35g/L.
Ore reserve
Average depth (m) Σ Dept/No of pits =50.6/23=2.2m
Average value (kg/m3)= Σ (DepthXvalue)/ΣDepth = 1031.69kg/m
2/50.6m
= 20.39kg/m3
Area of block by planimeter = 250000m2
Volume of block = Area x Average depth = 250000m2 x 2.2m = 550000m
3
Ore reserve (Tonnage) = Average value x volume
20.39kg/m3 x 550000m
3
11215000kg
11215000x10-3
tons
11215 tons
139
Ore Reserve Calculation
Table 8: Showing Ore block parameters
No Line/Pit No. Depth (m) Value Kg/m3 Depth X Value
Kg/m2
1. Al-1 2.5 20.88 52.2
2. Al-2 2.2 33.14 72.91
3. Al-3 1.5 20.88 52.2
4. Al-4 2.0 24.97 49.94
5. Al-5 1.4 38.59 54.03
6. Al+6 1.7 21.79 37.04
7. Al+7 2.1 19.52 40.99
8. Al+8 1.9 34.96 66.42
9. Al+9 2.7 10.01 27.03
10. Al+10 1.6 30.87 49.39
11. Al+11 2.4 15.89 38.14
12. Bl-12 2.6 14.53 37.78
13. Bl-13 2.6 13.17 34.24
14. Bl-14 2.5 22.70 56.75
15. Bl-15 2.3 25.88 59.52
16. Bl-16 2.1 27.24 57.20
17. Bl+17 2.0 28.15 56.30
18. Bl+18 1.9 29.51 56.07
19. Bl+19 2.4 22.25 53.40
20. Bl+20 2.4 15.89 38.14
21. Bl+21 2.5 11.80 29.50
22. Bl+22 2.5 5.00 12.50
23. Bl+23 2.8 0 0
50.6 1031.69
140
Table 9 : Summary of Analytical Result
141
Table 10: XRF SAMPLE RESULT
142
Chemical Analysis Result of Pyrite Sample from Vincent Ogah
Method of Analysis X-Ray Fluorescene (XRF) technique was used for the
analysis.
Date: 28th AND 30th NOVEMBER, 2011
TABLE 11
ELEMENTAL COMPOSITION %
Fe 56.91%
S 28.70
AI 4.40
Ca 0.18
K 0.12
Ti 0.41
V 0.017
Mn 0.17
Cu 0.025
Eu 0.20