EXPLORATION FOR IRON ORE IN THE GUNUNG JERAI AREA BY GEOPHYSICAL METHODS

EXPLORATION FOR IRON ORE IN THE GUNUNG JERAI AREA BY GEOPHYSICAL METHODS

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ABDOUL-FATAH IBRAHIM HASSAN

Thesis submitted in fulfillment of the requirements for .- the degree of Master of Science

January, 2003

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ACKi'iOWLEDGEMENTS

All pmise and thanks to ivlah who created me and made it possible for me to do this research. J would like to express my sincere gratitude to my pl1nciple supervisor Prof.

Lee Chong Yan for providing me an opportunitj \:0 conduct this re~earch with regular contributions and valuable COIILments throughout my entire studies. I am very much grateful to Mr Yaacob, Mr Bala, rvlr Maydin, l'v'~r Low Weng Leng who helped me during my data collection.

:liP.

I am also indebted to my colleagues - lVIr Faysal, Jv!uhmmnad Syukri and ,\IIr Abdi Nouh at the school for their unlimited support to my research especially at the time of discussions. I extend my love and regards to my intimate ii'iends -- Mr Ahmed O. Awil, Mr Ornar Dhollawaa, Mr Aluned Hussein (Hurre), Mr Papa Secka, Mr Omar Gassama,

Mr Abdollie Barrow, Mr Mustappa Dibba, Dr Usman Karofi, Dr Mohamed Aminu, Dr Ibrahim Danjaji, Dr Bila, Mr Alphanso, Dr Gabrial Ako, Dr Abshir, Dr Ali Abdallah, all of them gave me a very useful an.notation about the campus and their sense of humor created picture in my mmd.

I would like to thank ivir Kaysar Abdillahi Mohained, who gave me a prompt support during my studies at Universiti Sains Malaysia. I acknowledge the assistant from Lee jau Shya during preparation of my thesis

My heartfelt acknowledgment goes to my beloved mother Sam'a Mohamed and Dad Ibrahim Shiekh, and all my siblings for their moral suppmt to my life and educations.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS LIST OF FIGURES

LIST OF TABLES LIST OF PLATES ABSTRAK ABSTRACT CHAPTER 1

1.1 Location 1.2 Climate 1.3 Vegetation 1.4 Drainage 1.5 Topography 1.6 Previous work

1.7 Objective of the survey CHAPTER 2

2 GEOLOGY OF THE GUNlJNG JERAI AREA

2.1 General geology of the area 2.2 Jerai Fonnation

2.3 Mahang Fonnation 2.3.1 The argillaceous facies 2.3.2 The arenaceous facies 2.3.3 The minor siliceous facies 2.4 Semanggul Fonnation 2.5 Granite and allied intmsives 2.6 Superficial deposits

2.7 Mineral resources in the Gunung Jerai area 2.7.1 Tin

2.7.2 Columbite-tantalite 2.7.3 Iron ore

2.7.3.1 History of prospecting for iron ore 2.7.3.2 Occurrence of iron OIe

2.7.3.3 Chemical composition cfiron ore 2.7.3.4 Iron ore genesis

2.7.3.5 Production of iron ore CHAPTER 3

3 GEOPHYSICAL METHODS OF INVESTIGATION

3.1 Introduction

3.2 The magnetic method 3.2.1 Introduction

3.2.2 Basic concept ofthe magnetic method 3.2.3 Magnetism of the Earth

3.2.4 Magnetic susceptibility and remanence 3.2.4.1 Magnetic susceptibility

3.2.4.2 Remanence

3.3 Reduction of magnetic data 3.3.1 Diurnal correction

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1 1 1 4 4 4 5 7 8 8 8 8 10 10 11 13 13 15 16 17 18 18 19 19 21 23 24 25

26

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27 27 29 29 33 34 35 36

3.3.2 Elevation and latitude corrections 3.3.3 Terrain correction

3.3.4 Temperature correction

3.4 Application of the magnetic method to iron ore exploration 3.5 The gravity method

3.5.1 Introduction

3.5.2 Basic theory of the gravity method 3.6 Reduction of gravity data

3.6.1 Drift and tidal correction 3.6.2 Latitude correction 3.6.3 Elevation correction 3.6.3.1 Free air correction 3.6.3.2 Bouguer correction 3.6.4 Terrain correction 3.6.5 Bouguer anomaly

3.7 Application of the gravity method to iron ore exploration 3.8 Electrical resistivity method

3.8.1 Introduction

3.8.2 Basic electrical theory 3.8.3 Electrical imaging survey

3.9 Application ofthe resistivity method to iron ore exploration CHAPTER 4

4 FIELD DATA ACQUISITION AND ANALYSIS 4.1 Introduction

4.2 Magnetic survey 4.2.1 Instrumentation 4.2.2 Field procedure 4.3 Gravity survey 4.3.1 Instrumentation 4.3.2 Field procedure

4.4 Resistivity imaging survey 4.4.1 Instrumentation

4.4.2 Field procedure 4.5 Laboratory analysis 4.6 Data analysis

4.6.1 Magnetic data processing 4.6.2 Gravity data processing 4.7 Potential field data modelling

4.8 Resistivity imaging data processing and modelling CHAPTER 5

5 DAT A MODELLING AND INTERPRET A TION 5.1 Area A

5.2 Area B 5.3 AreaC 5.4 Area D 5.5 Modelling

5.5.1 Magnetic data modelling and interpretation '.5.1.1 Profile PBEL 3

5.5.1.2 Profile PBEL 4

37 37 37 38 38 38 39 40 40 40 41 41 41 42 42 43 45 45 45 49 52 54 54 54 54 54 57 59 59 59 61 62 62 64 66 66 68 69 73 75 75 75 77 79 79 84 86 87 88

5.5.1.4 Remarks on the magnetic method 94

5.5.2 Gravity data modelling and interpretation 94

5.5.2.1 Profile PBEU 95

5.5.2.2 Profile PBEL4 97

5.5.2.3 Profile PBEL5 97

5.5.2.4 Remarks on the gravity method 99

5.5.3 Combined magnetic a.nd gravity modelling 101

5.5.3.l Profile PBEU 103

5.5.3.2 Profile PBEL 4 106

5.5.3.3 Profile PBEL5 107

5.5.3.4 Remarks on the combined magnetic and gravity modelling 109

5.6 Electrical imaging results of Area D 111

5.6.1 Remarks on the electrical imaging method 121

CHAPTER 6 122

6 SUMMARY AND CONCLUSION 122

6.1 Summary 122

6.2 Conclusions 124

REFERENCES 126

Appendix A Total Magnetic Field & Gravity Profiles 134

Appendix B Magnetic Data Of Surveyed Area 144

APPENDIX C P ARAl'v1ETER DESCFJPTIONS OF MODELLED PROFILES 156 Appendix D Observed And Calculated Magnetic And Gravity Data Of Modelled

Profiles 183

LIST OF FIGURES

Figure 1 ~ 1 Location of the Gunung Jerai area 2

Figure 1-2 Location of the area surveyed 3

Figure 1-3 Topography of Gtmung Jerai area and location of the study area in Bedong 6 Figure 2-1 Geological map of the Gunung Jerai area

Figure 2-2 Iron ore occurrences in the Gutllmg lerai area.

Figure 3-1 Components of the total magnetic field vector

Figure 3-2 Variation of geomagnetic and geographic components Figure 3-3 Ternary system composition diagram

Figure 3-4 Diurnal variations in the total magnetic field intensity recorded at the base 9 22 31

32 32

station in Site B 36

Figure 3-5 Terrain correction

Figure 3-6 Summary of the corrections for gravity meter readings Figure 3-7 Rock resistivities

Figure 3-8 Current flows through the earth resistor.

Figure 3-9 Current flows through a cylinder and its effect.

Figure 3-10 The three different models used in the interpretation of resistivity measurements

44 44 46 46

47

49 Figure 3-11 Common electrode arrays (a) V!emler, (b) Schlumberger, (c) dipole-dipole.

51 Figure 4-1 Map of the surveyed area

Figure 4-2 The resistivity survey lines

Figure 4-3 Sequence of measurements us~d to build up a pseudosectioll.

55

60 62

Figure 4-4 Four-cable layout for resistivity imaging survey 63 Figure 4-5 Contour map of magnetic field at Site D, showing IDeation of survey lines 67

Figure 4-6 Flow-chart for the modelling 72

Figure 4-7 Field data set of Profile PBEL4 widl a few bad data points Figure 5-1 Examples of boreholes drilled in the Area A

74 76 Figure 5-2 Map showing the locations of the magnetic survey lines in Areas A, B &

c.

78 Figure 5-3 Map showing the location of the survey lines in the Area D. 80 Figure 5-4 Total magnetic intensity r~su1ts of ground magnetic survey over Area D. 82

Figure 5-6 A 3-dimensional perspective of anomalies in the Earth's total magnetic field

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Figure 5-7 Magnetic model of Prot lIe PBEL3 89

Figure 5-8 Magnetic model of Profile PBEL4 91

Figure 5-9 Magnetic model of Profile PBEL5 93

Figure 5-10 Gravity model of Profile PBEL3 Figure 5-11 Gravity model of Profile PBEL4 Figure 5-12 Gravity model of Profile PBEL5

Figure 5-13 3-dimensional bodies for combined Profiles ciPBELJ Figure 5-14 3-dimensional bodies for combined Profiles ofPBEL4 Figure 5-15 3-dimensional bodies for combined Profiles ofPBELS Figure 5-16 Combined model of Profile PBEU

Figure 5-17 Combined model of Profile PBEL4 Figure 5-18 Combined model of Profile PBEL5

Figure 5-19 Inverse model resistivity section of Profile PBEL2 Figure 5-20 Inverse model resistivity section of Profile PBEL3 Figure 5-21 Inverse model resistivity section of Profile PBEL4 Figure 5-22 Inverse model resistivity section of Profile PBEL5 Figure 5-23 Inverse model resistivity section of Profile PBEL8 Figure 5-24 Inverse model resistivity section of Profile PBEL9 Figure 5-25 Inverse model resistivity section of Profile CBELl Figure 5-26 Inverse model resistivity section of Profile CBEL2 Figure 5-27 Inverse model resistivity section of Profile CBEL3 Figure 5-28 Inverse model resistivity section of Profile CBEL4

96 98 100 104 104 104 105 108 110 113 113 115 115 117 117 119 119 120 120

LIST OF TABLES

Table 2-1 Analysis of black indurated mudstone from Mahang Fonnation 12 Table 2-2 Analysis of rock from the siliceous facies ofthe Mahang Fonnation. 14 Table 2-3 Description of 15 alluvial mines which were been operated during the period

1946-1963 20

Table 2-4 Amounts of impurities (%) 23

Table 2-5 Composition of iron ore from selected areas of Gllllllg Jerai. 24 Table 2-6 Annual production iron ore jn the Gunung Jerai area 25

Table 3-1 The range of iron ore resistivities 53

Table 4-1 The susceptibilities and. densities of the examined iron ore samples 65 Table 4-2 Observed and calculated combined modelling data of Profile PBEL3 71 Table 5-1 The chemical analysis of typical iron ore collected in Area A 75 Table 5-2 Calculated magnetic model parameters of Profile PBELJ 88 Table 5-3 Calculated magnetic model parameters of Profile PBEL4 90 Table 5-4 Calculated magnetic model parameters of Profile PBEL5 92 Table 5-5 Density and volume of model bodies of Profile PBEL3 95 Table 5-6 Density and volume of model bodies of Profile PBEL4 97 Table 5-7 Density and volume of model bodies of Profile PBELS 99 Table 5-8 Summary of combined model parameters of Profile PBEL3 106 Table 5-9 Summary of combined model parameters of Profile PBEL4 106 Table 5-10 Summary of combined model parameters of Profile PBEL5 109

LIST OF PLATES Plate 4.1 Theodolite and measuring tape

Plate 4.2 Bison Model 3101 Susceptibility Meter Plate 4.3 Scintrex CG2 Worden gravity meter Plate 4.4 Resistivity measuring equipment

56 56 60 63

PENCARIGALIAN BIJIH DES I DI KA WASAN GUNUNG JERAI DENGAN KAEDAH-KEADAH GEOFIZIK

ABSTRAK

Pencarigalian mineral pesat berkembang dalam empat dekad kebelakangan ini dengan penggablmgan peralatan canggih ke dalam sistem pencarigalian praktik. Kajian geofizik ini dijalankan di tapak Sungai Tok Pm,vang, lebih kurang 8 km di tenggara Gunung Jerai, dekat bandar Bedong, Kedah. Kawasan ini terletak di tanah perlombongan South Island Mining Company (SIMCO) serta kawasan sekeliling.

Kajian ini melibatkan sambutan terperinci geofizik oleh longgokan bijih besi (di empat tapak) kepada tiga teknik geofizik. Tujuannya adalah unhLk mengesan sisa atau sambllngan, jib ada, longgokan bijill yang telah di!ombong atau kewujudan bijih primer yang lebih dalam. Pada mulanya kajian ini melibatkan tiJUauan awal magnet darat di tapak bekas lombong dan kawasan berhampiran yakni Tapak A, B, C dan D.

Tinjauan tersebut tidak rnengesan sebarang anomali bererti di A, B dan C. Akan tetapi, Tapak D yang tidak pemah dilombong, berhampiran dengan beberapa kolam perlombongan lama, dijelajahi clengan suatu rangkaian rentasan tinjauan magnet, graviti dan pengimejan kerintcmgan. Di sini snatu longgokan bijih besi mllllgkin, yang tidak diketalmi setakat ini, telail dikeSfu'1. Pemode1an matematik bersepadu (dengan ModelVision Pro) atas data magnet dan graviti menunjukkan suam jasad bijih yang agak besar dengan ciri-ciri fizikal serupa dengan longgokan yang pemall dilombong di kawasan ini. Tinjauan magnet menunjukkan. bahawa anomali-anomali utama magnet menand&\:an keluasan jasad-jasad itu, manakala data graviti memberi maklumat kuantitatif seperti ketumpatan magnetit, hematit dan/atau limonit (3.5 g cm-³ hingga

Juga, anomali-anomali magnet dan gravi~i memperlihatkan korelasi baik antaranya.

Keputusan tinjauan pen!,Timejan elektrik tidal( banyak menyumbang kepada penentuan bentuk dan kedalaman Ionggokan bijih besi ini kerana ia dipengaruhi oleh faktor-faktor lain seperti kewujudan air tanah.

ABSTRACT

Mineral exploration has advanced significantly over the last four decades with the incorporation of sophisticated instrument developments into practical exploration systems. This geophysical study was conducted in the Sungai Tok Pawang locality, approximately 8 Ian southeast of Gunung Jerai, near the town of Bedong, Kedah. This area is located on land held under mining lease by the South Island Mining Company (SIMCO) and the adjacent areas.

The study discusses the detailed geophysical responses of iron are deposits (in four sites) to three geophysical techniques. The aim was to detect the remnants or extensions, if any, of known ore deposits which have been mined or the possible presence of primary are material at depth. The study initially involved reconnaissance ground magnetic surveys over old mining sites and their adjacent areas namely Sites A, B, C and D. The ground magnetic survey did not locate any significant anomaly over A, B, and C. However, an lmmined area, adjacent to some old mining pools, namely site D, was prospected "vith a network of grmmd magnetic traverses c.S well as gfavity and resistivity imaging surveys. On this site a previously unlmown probable iron ore deposit was detected. Furthennore, integrated mathematical modelling (with ModelVison Pro) of the magnetic and gravity data shows a sizeable ore body with physical characteristics similar to previollsly mined deposits ill the area. The magnetic survey in this study shows that the prominent magnetic anomalies outline the extent of the bodies, while gravity data provided quantitative information such as the densities of magnetite, hematite and/or limonite (3.5 g cm-³ to 3.62 g cm-³) and that they differ significantly from the density of the s1m-ounding rocks. Besides, the magnetic and gravity anomalies show strong correlations with one another. The results of the elecnical imaging survey

could not contribute much to the detennination of the shape and depth of the iron are deposit since they were affected by several other factors like the presence of groundwater.

1.1 Location

CHAPTER 1 INTRODUCTION

Glmung Jerai or Kedah Peak lies on the west coast of Kedah in northwest Peninsular Malaysia. It is an isolated mountain mass, which rises abruptly to an elevation of 1217m (3992 ft) from the low coastal alluvial plains of south Kedah. Figure 1-1 shows the location of the Gtmung IeTai area. The co-ordinates of the summit are SO 47'N and 100'26' E. The study area in Betiong is located on the southeastern flanks of the Gunung Jerai massif It lies within the area bounded by latitudes 4°4S'21"N and S046'26"N, and longitudes 100°29'31 "E and lOO^o30'17"E (Figure 1.2). This is within the area held under mining lease by the South Island Mining Company (SIMCO).

1.2 Climate

The Gummg lerai area enjoys an equatorial rain forest climate characterized by constant high temperature and high rainfall rather well distributed throughout the year.

Temperatures show more diurnal than seasonal range; while the average daily temperature is always between 80°F and 83°F, the daily range is of the order of 70°F to 90°F. The mean annual rainfall is 245.4 em. Although more than 7.5 em of rain nonnally fall each month, there is an obvious seasonality. The wettest period is from September to November when, on average, 38% of the fu'illUal rain is received.

Following this is a relatively dry season from December to March, with only about 21 % of the yearly rainfall. The dry season is followed by a subsidiary rainy spell in April and May, succeeded by an intermediate period from June to August. The relative lUIDlidity averages about 84%. Monsoonal \vinds blow from the northeast during October to February and from the southwest during May to September (Bradford, 1972).

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1.3 Vegetation

The natural vegetation of the Gunung J erai area consists primarily of tropical evergreen rain forest with a great variety of plants. In both lowlands and hills up to 914m elevation, the dominant tree family is that of the Dipterocarpaceae, which includes a variety of valuable timber trees such as Shorea spp and Dipterocarpus spp. At around 914m the forest thins out, and along with this change much Aga/his Alba and various members of the genus Quercus put in an appearance (Bradford, 1972). The Bedong area has proved very suitable for agriculture, particularly for plantation rubber. Extensive deforestation has therefore taken place (Burton, 1988).

1.4 Drainage

The radial drainage pattern of the Kedah Peak massif is typical of an area which has a dome-shaped structure. Superimposed on this radial pattern are sets of dendritic drainage patterns. Drainage to the north of Gtillung Jerai is mainly through the Sungai Gurun, which rises on the northeastern slope of the massif. To the south of Kedah Peak the two main rivers are the Slmgai Merbok and Sungai Muda. Many of the streams in the area exhibit linearity in their flow pattern, at ler..st along parts of their courses, suggesting structural control. This structural f\)ntrol could possibly be due to joints or fracture zones (Bradford, 1972). The area around Bedong is drained by the Sungai Bangkok and its tributaries Sungai Ladang and S11llgai Getak.

1.5 Topography

The Gunung Jerai massif is circular in plan and has the appearance of huge dome. It is . extremely ntgged and rises abntptly from sea level to an elevation of 1217m. There are some extraordinarily steep-sided valleys and it exhibits numerous scarp faces, cliffs and

waterfalls. Elsewhere, near the coast, the land consists of a flat alluvial plain below the 15m contour level, while farther hiland it forms loy ^v' lmdulating hills rarely exceeding 75m elevation, but with occasional summits of up to 230 m. nle topography oftlIe area is governed largely by the geology with steepest cliffs being fanned of a hard, metamorphosed, well-jointed quartzite. The somewhat gentler southern slopes of the peak are underlain by granite or schist and gently rolling hills at the foot of the peak including the Bedong are underlain by shale with occasional arenaceous bands (Bradford, 1972). Figure 1-3 shows the topography of the Gunung Jerai area.

1.6 Previous work

The general structure of Gunung Jerai was briefly mentioned by Scrivenor (1919) and brief accounts of the geology of the area, especially in relation to mining, have been made by a number of authors. The earliest of these accounts was by WiUbourn (1926).

Courtier (1962) and Burton (1967) mapped the area. The first detailed study of the Glmung lerai area was carried out from 1953 to 1956 by Bradford (1972). He described the general geology and the mineral re30urces ofthe area.

Other contributions to the understanding of the stratigraphy and metamorphic geology of the area were made by Alexander (1962), Gobbett and Hutcbison (1973) a.'1d Burton (1988), while Bean (1969) conducted some research on the iron-ore de;Josits. In addition to the above contributors, there are some unpublished academic exercises in the adjacent areas by Parmnanathan (1964) who wrote on geology of the Gunung Jerai massif, Rao (1972) who wrote on the geology and geochemistry of the southern part of Keda.h Peak, Almashoor (1974) who studied the geoJ.ogy of the

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Gummg Jerai area and Awalludin (1981) who mapped the eastern part of the Gunung Jerai area providing invaluable information to the present investigation.

Agocs and Paton (1958) described the airbome magnetometer and scintillation counter survey over parts of Perlis and Kedah, which L.l1cluded the Bedong area, to investigate the possibilities of opening new areas for mining. Lim (1980-a) conducted hlTOtmd magnetic surveys on Kedah Peak to study and detennine the cause of the magnetic anomaly over the area as described by Agocs and Paton. Jamalludin (1983) and Ong (1983) conducted geophysical SlU'Veys over parts of the South Island Mine in Bedong.

Rajan (1998) was involved in an integrated grOtmd geophysical study to interpret the granite structure in the subsurface around the Gunung Jerai massif

1.7 Objective of the survey

The main objective oftrus survey is to investigate the iron ore deposits in the vicinity of Bedong. For this purpose various geophysical methods are used to locate and delineate the ore bodies in the areas under mining lease of the South Island Mining Company.

From the interpretation of data acquired it is hoped that a better understanding of the geology of the ore deposits can be obtained.

A related objective is to see if an appropriate combination of geophysical methods can serve as a cost-effective tool to prospect fer yet unknmvn iron ore deposits in the area.

CHAPTER 2

GEOLOGY OF THE GlJl'IlJNG JERAl AREA

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2.1 General geology of the area

The Gunung Jerai area consists of a series of sediments which have been domed and metamorphosed by an intrusion of granite (Bea.'1, 1969). It can be described presently as a hill of quartzite and schist (Jerai Formation) with a core of granite (Figure 2-1).

Based on Bradford (1972) and Burton (1988) the main geological units of the Gummg Jerai area are:

Jerai Formation.

Mahang Fonnation.

Semanggul Formation.

Granite and Allied Intrusives Superficial Deposits.

2.2 Jerai Formation

Tbis formation occupies a well-defined area of about 80 kIn^2, and makes up the largest part of the Gunung Jerai complex. By correlation with the Machinchang Formation of Langkawi Island the age of the formation was estimated as Cambrian (the oldest known sedimentary deposits in the Malay Peninsula) (Gobbett and Hutchison, 1973; Tan and Khoo, 1978). Bradford (1972) distinguished two different facies in the Jerai Fonnation, that is, an argillaceous facies (metamorphosed facies consisting of schist and semi- schist) and an arenaceous facies (metamorphosed facies consisting of qurutzite, granulite and grit).

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Bradford (1972) distinguished two different facies in the Jerai F0n11ation, that is, an argillaceous facies (metamorphosed facies consisting of schist and semi-schist) and an arenaceolls facies (metamorphosed facies consisting of quartzite, granulite and grit).

Furthermore, he mentioned that the distribution of the two facies could not be sharply delineated, due to complex interbedding and the gradational character of contacts between them throughout the fonnation.

2.3 Mahang Formation

The term Mahang Formation was initially used by Courtier (1974) to specify a sequence of shale, mudstone, "flags" and chert with minor sandstone occurring around the town of Mahang·. This fonnation covers more then 1000km² of central and south Kedah, and is also by far the most extensive rock in the Bedong area. 'DIe tme age of the Mahang Fonnation is Lower Silurian (Burton, 1967).

The main lithology of the Mahang Formation can be divided into three members as follows:

2.3.1 The argillaceolls facies

The argillaceous facies fonn the predominant rocks in the Mahang Fonnation. The Mahang Formation adjoins rocks mapped as "Slmgai Patani Fonnation" by Braford (1972) and Courtier (1974). These investigations and fossil discoveries, however, have sho\'.'I1 that the "Sungai Patani Fonnation" is homologous to the Mahang Fonnation. It comprises 64% shales, 25% mudstone, 3% siltstone and 8% meta-argillites. Its

components are generally carbonaceolls, more or less siliceous and moderately metamorphosed. \Vhile the argillaceous rocks rich in carbon and silica give the fonnation a rather resistant nature, this argillaceous facies exhibits a wide range in fissility. The Mahang argillites are highly fissile or thinly bedded rocks often with obvious high carbon content whilst those rich in silica are rather slabby or massive (Burton, 1967 and 1988). Burton (1967) stated that the rocks rich in carbon often carry considerable pyrite, while Bradford (1972) stated that the degree of concentration of iron oxides in the shale or in material derived from the shale, is very variable and in some cases almost reaches economic proportion, the equivalent of a content of about 5 per cent.

Table 2-1 is shown the analysis of black indurated mudstone. The table shows 0.68% of iron oxides according to Burton, while Bradford stated that the red shale of the Sungai Patani Fonnation contains about 5% Fe.

2.3.2 The arenaceOllS facies

The arenaceous rocks in the Gummg J erai area are strongly recrystallized, well jointed and rather fine grained metaquartzite. Burton (1988) stated that the arenaceous facies of the 1v1ahang Fonnation covers snme 31 krn² of the cour.try, fonning 8% of the Malllli1g Fornlation, and 4.1 % of the 13edong area. It compris~s :3 number of lenses or multiple- lenses from a few hundred meters tQ 10 km L'1 length, scattered through the eastern half of the argillaceous facies outcrops. On the evidence of the outcrops examined by Burton (1988) in Bedong the arenaceous facies appears to cotJprise proto quartzite, subgreywacke, greywacke and orthoquartzite. The typical arenaceous expression is a.

Table 2-1 Analysis of black indurated mudstone from Mahang Fonnation

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(Riverside Estate, Bedong, Kedah)

white, pink or pale grey medium protoquartzite in which rather coarse cross-bedding is sometimes seen. When fresh the arenites are rather hard and fonn hills, ridges, scarps and other positive topographic features. On weathering, in common with other West Malaysian quartzite, they become incoherent and are broken into friable sandy rock.

Because of their maturity the Mahang arenites are a little more resistant than those of the Triassic Semanggul Formation ofthis area.

2.3.3 The minor siliceous facies

The siliceous facies of the Mahang Formation fonns 0.08% of the Mahang outcrops. It comprises a number of small scattered lenticular bodies of chert, radiolarite porcellanite etc; ranging up to 1.6 km in length and 30 to 60 meter in thickness and usually intimately associated with larger but similarly lenticular development of arenaceous facies. Rocks of the siliceous facies typically contain appreciable sericite and clay minerals and are often more or less carbonaceous (Burton, 1967). Table 2-2 shows the analysis of the siliceous facies.

2.4 Semanggul Formation

The name Semanggul Fonnation has been used to designate the sediments exposed in the Semanggul range some l3lan north west of Taiping. The Semanggul Formation is predominantly arenaceous, consisting of sandstone interbedded with shale, and containing minor intercalations of siltstone and chert. Within the Bedong area, the Semanggul Fonnation is a rather rapidly altemating sequence of arenaceous and argillaceous rocks with one major and several minor intercalations of chert. Rudaceous and carbonate rocks are unkno'WTI. Outcrops of arenaceous rocks are three times as abundant as those of argillaceous rocks;.this is in palt a consequence of the selective

Table 2-2 Analysis of rock from the siliceous facies of the Mahang Fonnation.

Constituent Percentage

I

^A

*

^B*

I

^C*

lD* .'

IF¥ .••.. '.

I

I :~~ '--"j ~_25_7~_1 ~6_~:_O ----I :~O __ ~, ~~66_·30

^__

I~~:O ________ J

I-IF_e_²0_³ ----'"_-+. _1_.9_2 _____

i

^{0.87 : 1.49 fo.31}

I

^{0.59 . ___}

-=

FeO 0.01

I

0.01

i

0.01

I

tr 10.05 ^Ii

I-M-gO-~··. ~-+-0.17

^{I 0.05 i tr}

i

0.06

I

O. f4 i

1-1 ----+---fr--

^!

---+-- I ~

I

^{CaO 0.18}

I

^{0_ .08 i 0.22}

---L

^{0.07 i 0.35 i}

1-, N-al-O---+-- ; _ I - I

^{0.10 ---j}

~--~-~---+I

_{KiO - -}

i - - I - I

^{j 0.03}

~

^I

HW+ . 0.13 0.15 ^I, 0.15 10.24 -to'16---i

I I I

f-,----'--:----t----~~---_+__~--_+___:__:_::---I---j

lH 2

^{0- .... 0.63 i 0.27 i 0.13}

I

^0.08

J

^1.06

I

If-I T_i_O_2,...,..· ---'-,-.,---,-f-:O-:.O:-:::9 _ _ _+__0 .-:-09=--_ _ _ --+I_o-:.0_5 _ _ ^+-1 O __

~. °

¹

1

^0.13

~

I

P205 0.05

I

0.02

!

0.05 0.01

I

^0.01

I

I-_Mn-. -O--'-'-''---'-'---tr: tr

i

tr --- tr

-jNU--- ---~

I-~-O-. 2_·~·.~

^{.. .C-}..•..•• ...,.. ..

-+--~---t-~---

^{_ _ _ +i}

-~ ---+1-~---~~:1

~~~····~····~~---I---~---+I---tl~~----

Cu '. .•... - -

I - -

^I' 0.004

...

^{. ... .}

·"VZ():iii - - - - I

^0.04

U ...•..

>.. ....

_---t---_ --.---~. - I N o t detected - -

~

..

~"~"".~~~~~---I-~~---+--=-~---t~~--~

L-T_o_ta~,l,-,-." .~.

,,-,-,-,-",--"-9-,--8_.3,---8

_---'-~_9

^{.. _92} _____ L.9_8_.7 __ 0 _ _ .L1_9_8._73 __

J 10~---'

After Burton (1967)

A* = Dark chert with Foraminifera. Location: 5° 20' OS" N; 100° 42' 06"E;

B* = Black chert. Location: 5° 18' 07" N; 100° 43' 33"E. Tonghurst Estate, Kedah.

C* == Dark chert. Location: 5° 23' 00" N; 100° 42' 27"E. Dublin Estate Ridge, Kedah.

D* =Black chert. Location: 5° 18' 30" N; 100° 46' 05"E.

F* = Dark grey (carbonaceou::.) argillaceous cbert. Location 5° 22' 09" N; 100⁰43 '04" E.

Dublin Estate, Kcdah (Mahang Formation type location)

action of intense tropical weathering; Compared with those of arenite, outcrops oflutite are preferentially disintegrated, eroded and concealed. Apart from several chert bands which fonn ridges rising up to 132m in height, the Semanggul Fonnation constitutes low-lying cOlmtry wherein exposures are generally poor. Fossil evidence from the Bedong area suggests that the Semanggul Fonnationis Ladinian or Carnian in age (Courtier, 1974; Burton, 1988).

According to Burton (1988) the argillaceous facies, arenaceous facies and mmor siliceous facies are recognized in the Bedong area. The argillaceous facies represents 8% of the area consisting mostly of protoquartzite with some subgreywackes, closely associated with outcrops of the siliceous facies. The siliceous facies (0.8%) comprises porcellanite, argillaceous chert, radiolarite and chert, and the arenaceous facies (91%) consist of black shales and mudstone with minor siltstone.

2.5 Granite and allied intrusives

Granite is exposed over about 32km², mostly on the southern and western slopes of the Kedah Peak complex, where the sedimentary cover has been removed by erosion. Most of the rock is medium-grained biotite granite, while some containing tounnaline and garnet have been found to be of common local occurrence. Pegmatite and quartz veins are abundant both in the granite and in the sediments of the J erai Formation (Bradford, 1972).

In the Bedong area granite outcrops cover over 9.1km2 and at greater or lesser depths it probably underlies the whole area. Airborne magnetometer surveys extending into the Bedong area suggested a 'basement' rising up to 210m below ground level in the

northwest. However, two different types of granite have been recognized in the Bedong area, namely:

The Inas Granite: It is named for the mountai.'l of Gunung Inas, whose summit is situated some 9.5km southeast of the Bedong area 'Dle lnas Granite is by far the dominant type and covers an area of 8.3km2

out of the 9.1km2 of the total granite outcrops. The Inas Granite is a coarsely porphyritic, pale grey rock consisting of a matrix of white feldspar, translucent quartz and black biotite set with coarse prismatic phenocrysts of white feldspar. The lnas Granite is bounded by L1e Mahang Formation (Silurian) sediments to the southwest and by the Damar Granite to the east.

The Damar Granite: The name Damar Granite is derived from the mountain referred to as Damar II some 22.5km east of the Bedong area. The Damar Granite is restricted to very small outcrops, about O.8km^L in area The Damar Granite is a fine" to medium- grained equigranular alkali granite evidently associated with abillldant microcline and usually with both muscovite and biotite. Similar granite also occurs in the GUJlung Jerai mountain.

The only other rock of any importance under this heading is quatiz. Dykes and veins are very limited. Apart from tlle ubiquitous quartz veins, nmnerous larger bodies of quartz occur in outcrops in the area, generally eloagate in plan, infilling linear fissures (Burton, 1988).

2.6 Superficial deposits

The superficial deposits of the Gunung Jerai area are mainly alluvium that covers some 600km². Also there is outwash detritus that is confined to an area about 6.5km² .

immediately to the east of Yan. This detritus is a poorly sorted deposit of huge boulders, derived partly from rocks of the Jerai Fonnation and partly from granite and pegmatite of the Kedah Peak complex.

Burton (1988) stated that the superficial deposit of the survey area (Bedong) is alluviwn, which covers 92km² of the area. Three facies of alluviwn can be recognized in the area as follows:

Pre-Recent or Older Alluvium: Semi-consolidated, oxidized, iron-stained, usually fonned of pale-coloured sand, in part with a certain amount of clay often rich in organic material.

Sub-Recent Alluvium: Slightly consolidated, somewhat iron-stained deposit, comprising sand and clayey sand. It is intermediate in character between Pre-Recent Alluvium and Recent Alluvium.

Recent Alluvium: A loose, unconsolidated deposit, without iron stains, usually formed of pale-coloured sand, in part with a certain amount of clay often rich in orga.Tlic material.

2.7 Mineral resources in the Gunung Jerai area

Cassiterite, colwnbite-tantalite and iron are appear to be the only economically impol1ant deposits in the area. There is also available a large supply of good quality road metal, building stone and ornamental-stone including granite, limestone and quartzite.

2.7.1 Tin

Tin was undoubtedly one of the first metals to be used by man, although it was not until the nineteenth century that tin was employed in industry on a large scale. The metal continues to be used in the manufacture of various bronzes containing up to 25 per cent tin and of other non-ferrous alloys, but the largest uses are now in the tin-plate industry and production of soft solders. The only important ore mineral of tin is cassiterite (Sn02), which when pure contains 78.6 % Sn (Arther, 1978).

The only tin mineral identified in the area is cassiterite (obtained mainly from alluvial placer deposits) and it is of widespread occurrence in the Gunung Jerai area. The tin mineralization is found within or in the vicinity of the Gummg Jerai granite. The tin (Bradford, 1972 a.l1d Awalludin, 1981) of the Gunung Jerai area occurs in the weathered pegmatite and as alluvial deposits. For tIlis reason it is believed that the pegmatite is the source of the tin. It is believed to be genetically related to the pegmatite, which nonnally carries coarse muscovite, tounnaline and garnet, together with the ore minerals. The partial analyses of pegmatite from Tanjong Jaga and Bukit Karong show that tin contents of the order of 0.02 to 0.06% are common in the local granite, pegmatite and celtain metamo~Vilosed cr metasomatized sediments.

2.7.2 Columbite-tantalite

Columbite-tantalite has been known to be present as impurities in tin concentrates from the Senliling area since before the 1939-45 war. Because of their content of these minerals such concentrates used to be paid for at a reduced rate by tin smelters.

The only known deposits of columbite of economic interest in West Malaysia are in th~

minerals of niobium and tantalum found in the Kedah Peak area are believed to be genetically related to the pegmatite dykes. Unfortunately the advanced stage of weathering of most of the pegmatite exposures in the area has precluded the collection of in situ crystals of the niobium-tantalum minerals, all the known specimens of these minerals having been found in alluvial, or in a few cases eluvial, detritus. Table 2-3 gives the description of 15 alluvial mines.

2.7.3 Iron ore

2.7.3.1 History of prospecting for iron ore

Next to aluminium, iron is the most abundant metal in the earth's crust. It is also the most indispensable of metals. Because of its many useful properties, including hardness, strength and durability, it is rightly regarded as the backbone of modem industry. Iron ore is consumed in very large quantities in the manufacture of iron and steel, the uses of which are too numerous to mention. Unlike aluminium, however, which has become economically important only since the beginning of the present century, man has used iron fer at least 3000 years. The initial development of iron mining in Malaysia, and the whole production up to the end of World War II, was in the hands of Japanese-owned companies. At the end of World War I the Japanese began an intensive search for iron-ore deposits in Southeast Asia to supplement their own home production.

Iron are, most of it high grade, was mined in Malaya by the Japanese for about 20 years before 1941 with gradually increasing outputs, and during their last six years of operation they were exporting to Japan at the rate of more than 1 Yz million tons a year (Willboum, 1946). Furthermore the presence of iron deposits in the Gunung Jerai area

Table 2-3 Description of 15 alluvial mines which were been operated during the period 1946-1963

! F . .. I .

I I

I

I

I l

\1 ; Tungku Daud Mine

12

ⁱ Loke Hee Tin Mines

1 Limited

3 Tong Loong Tin Mine Company

52 1949- 63 1952/56

9,820.23 1,44l.33

4 ! Choong Chum Fah 1954- 9,554.43

! Mines Limited 63

1

5 i Poh Sung Tin Mine 1955-

i 60

1,083.08

278.20 103.21

808.21 77.89

35:1 97

!

3

I I

14:1 93

i

7

I

I I !

12:1 92 14:1 93 7 12:1 92 24:1 96 16

I

Siau Hin Tin Mining \1949- 6~624.72 ^540.85

I ; Company 5 6 . - : - ' : - - - - j - - - + - - - - t - - c - - , - - - - j - - - {

; Ship In Mining ):6950- 3,368.12 1139.20

Com any Limited .

I

⁸ i Yik Nyen Tin Mines No production pre-1964

~:

_I ^{, !\bdullah Ghaffaf 11951163}

I

^{9600 ,}

-

^{. 01}

-

¹⁵⁷⁸⁸⁵

.

: Mining ComRany

I

¹

10 ' Thai Nyen Tin Mines 1951/63 132,448.77 ' .., ^{i j , j} ~ It.! l' ^{c. '+}

i

^10:1

; Limited ^I

!w8.72 I III

' Hon K wok Tin Mine 1956/57 462.48 14:1

1

!

12 . Semiling Tin Limited 1946/63 28,704.22 2,242.10 ₁13 :1

I

; Seng Chow Tin Mine 11955/63 ' 9,153.02

I

113

425.29 122:1

I . Com.Qany Limited

I

I 14 _{: Bedong Tin} Mine 1951/59 3,534.71 702.63 5:1 Limited

15 N.S.At Tin Mine 1955/63 5,369.46 884.62

1

6:1

Limited

I

I

Total

I

^121,285.07

1

10,203.64

!

i2:1

I

^I

i

After Bradford (1972) Cass

=

Cassiterite Colum

=

Columbite

I

91 9

'So-tw-

93 7

I

96 4 83 17 86 14

~

I

92 8

has been known for a long time where veins of magnetite in this area were mentioned by Scrivenor (1919), and Willboum (1926) described three occurrences of iron deposits, two to the west (Bukit Ahan and Kampong Merbok area) of Gunung Jerai and one to the east (the Sungei Tok Pawang estate) of the sth'l1mit.

2.7.3.2 Occurrence of iron ore

A large number of iron-bearing minerals are known, but only four are important commercial sources of the metal. These are magnetite (Fe304), hematite (FC203), goeth:\te (Fe203.H20), and to a lesser extent, siderite (FeC03) sometimes known as spathic iron ore (Arther, 1978).

Gunung J erai iron mineralisation is abundant, but the chief minerals are magnetite, hematite and goethite. Most of the iron deposits lie close to or at the contact with intrusive igneous rocks in Peninsular Malaysia with which they are considered to be genetically related (Geological Survey of Malaysia Atmual Report 1991). The greatest concentration of iron ore is in the Gunung Jerai area, found in the schist and metaquartzite which surround the granite.

Bradford (1972) stated that thcr,:~ are sporadic disseminations along the bedding planes of the Jerai Formation quartzite. It is less frequently found in the Jerai Formation schist, and has not been recorded in the Sungai Patani Formation shale. In a number of localities the magnetite is associated with pegmatite intrusives, and in many places it is accompanied by vein quartz.

1---- ---·_·1

^{ICV~ 30'}

s·

45'

...---,-

?:7 .

P. Bu r^tl:1)!

STRAITS OF A·IALACCA

Tg. ;aga

Scale 1 Inch to -4 Miles

Miles 5 4 3. 2 I 0 5 Miles

E~~~~~~~_~~=-~±Ec=±=s=~~-= .. -~~ ----.-_===J~~

LEGEND

• •

. :.

•

..

•

~ ••

.

<>

Magnditc

G

Mining Area

0

Figure 2-2 Iron ore occurrences in the Gunung Jerai area.

After Bean (1969)

,

H

"\5' • ^,So

I

I

I

I I

_I

Hematite commonly occurs with the magnetite.

In

the Jerai Fonnation

it

appears to

be

more common in the schist tilan

in

the quartzite. \Vhere fOlmd together with magnetite it usually is a secondary alteration product of the mineral. Hematite also occurs without magnetite in what are believed to be primary bodies. Figure

2·2

shows all the recorded occurrences of magnetite and hematite (reduced from a map prepared by Bradford).

Most of these may not have economic value at the time.

2.7.3.3 Chemical composition of iron ore

Although the mineralogy of the Gummg Jerai iron ores has not been investigated in detail, a number of chemical analyses are available showing the grade of ore and the amount of impurities present. The maximum recorded Fe content of bulk ore is about

69%

with most of it varying from about

60 %

down to the minimum saleable

Fe

content of

58 %.

The average Fe content of the iron-stained soil (the estimation was before it was mined out) containing detrital hematite ore

at

Bukit Merah, Harvard Estate, has been estimated at about

15%.

The impurities vary from place to place, but the following data in Table

2·4

can be regarded as more or

less

average percentage figures (Bradford,

1972)

Table

2·L~Amounts

of impurities

(%)

Al

203 4.0

S

^{0.02 I}

--

I

eu

0.004 Si02 6.0

Mn I 1.0

Sn

0.02 ^I _I

I

i

As

₁0.04

TiO

0.1

I

^{p 1005 10.05}

j

I I

Bradford

(1972) J

2.7.3.4 Iron ore genesis

Though Bradford (1965) believed that the ore is all of magmatic origin genetically related to the local intrusions of pegmatite and vein quara, from a study by Bean (1962) of

some

other West Malaysian iron deposits, it

was

suggested that the magnetite possibly formed slightly earlier and at a higher temperature than the hematite, but this has not been proved for the Gtilltmg J erai area. More recently, it is believed that the iron deposits in Gunung Jerai are epigenetic in origin, having been fanned either as a result of pyrometasomatic or hydrothermal activities (Geological Survey of Malaysia Annual Report 1991). Table 2-5 shows the chemical composition of iron ore in the Gunung J erai area.

Table 2-5 Composition of iron are from selected areas of Gunung Jerai.

Sample location

I

Constituent Percentages

I Fe

ⁱ Mn

--"Si0

2 __

po

- - I - - - l -

I

^{67.8 i Nil} 2.61 0.02 0.02

~

^{t "}

.~.---.-.h--_-+---1r---+----l---1

68.2

i

O.16;~.20 0.00) 0.027 Bukit Ahan

Is

I

4 I

f - - - - ' - - - .. I - - - I - ---+-c----I---+----+----I

59.1

!

0.15 12.52 0.038 0.044 5.99

1

i

^L9

i O~07

0017-rm11114

~H=a-~-ar~d--E-s-ta-te~lr6~3-.6~i---+-l--.3-0--~IIIO-.-63--11.O.Ol--4-_----4-·-~_·--~1

B~~~

I

Sungei Tal< ^,1 62.7 ^{< -} 3.02

I

tr

~l-

^{-3.83 I'} O.24-'-Nil ^III

Pawang Estate

I

^Ii'll.

~---.----~I--~----+I---_+--_+---~---.L--4_-~

Kampong ,67.7, - l.63 0.01 0.01 2.10 ¹0.29 Nil

II

Merbok 5

(Average of 5

I

a5says)

~--~~~

Bean (1969)

__ Lil----~-- __ L---L---L---L---LII--~--~

0.14 -

10.04 -

2.7.3.5 Production of iron ore

Post-war production commenced during late 1955 and reached a peak of just over 12 million tons per an..'1um in 1961, falling to Y4 million tons by 1963.

TIle greater

pa.li: of the total production has been derived from the Sungei Tok Pawang area. Table 2-6 shows the annual production of iron are in the GUflung Jerai area between 1955 and 1964, according to Bean (1969) and Bradford (1972).

Table 2-6 Annual production iron ore in the GlL'lUng Jerai area

Year Production (Tons)

=]

1955 5,290

-

----_.

1956 33,901

---J

1957 168,367

1958

-1

61 ,060

I

1959 ₁168,080

I

----l

1960 225,893

I

1961 563,725

- -

1962 416,643

1963 ' 242,752

!

^I

I F4

^Total

-J

-J

^I _i

- ___ ----1

Bean (1969), Bradford (1972).

CHAPTER 3

GEOPHYSICAL METHODS OF Il"~VESTIGATION

3.1 lntroduction

Geophysical surveying is basically an a.."sortrnent of methods utilizing physic,>j principles to enable the prospector/surveyor to sec underground and is based on the fact that soils, rocks and minerals of different types may display large differences in physical properties.

Geophysical methods C(lJl be either active (e.g. seismic profiling) or passive (e.g, gravity survey). The passive methods utilize the natural gravitational, magnetic, electrical and electromagnetic fields of the Earth, searching for perturbations in these fields caused by geological anomalies. The active met:.hods involve the introduction of energy into the Earth and may be used analogously to natural fields, for example the generation of seismic waves whose propagation velocities and transmission paths through the subsurface are mapped to provide infopllation on the distribution of geological boundaries at depth. However, such Ifl0thods on their own can lead to erroneous conclusions. Therefore, one needs direct observations to constrain geophysical models, i.e. geological knowledge from rock outcrops, boreholes andlor trial pits.

However, no single geophysical method can be used by itself for the exploration of iron ore. But a combination of various methods can be successfi111y applied to provide information on the occurrence of ore. 11ms, the ll1aill geophysical techniques which were utilized in this projeti: are ground magnetics, gravitj and 2-dimensional resistivity·

L,l1aging.

3.2 The magnetic method 3.2.1 Introduction

The study of the Earth's magnetism is the oldest branch of geophysics. Sir William Gilbert in 1600 analysed the Earth's magnetic field and found it roughly equivalent to that a of pennanent magnet lying in a general north-south direction near the Earth's rotational axis. Basically magnetic methods are based on the measurcm~nts of small variations in the magnetic field due to the effect of variations in the distribution of magnetic minerals in rocks. Magnetism of rocks and natural minerals (Carmichael, 1989) involves the study of (1) geomagnetism - the Earth's magnetic field: origin and source, character and configuration, and change with time; (2) magnetism - the magnetic properties and behaviour of magnetic minerals and rocks; and (3) paleomagnetism - the study of the remanent magnetization retained in rocks as a means of deducing the history and nature of the Earth's field through geological time. Of interest are the direction, intensity, polarity and configuration of the geomagnetic field.

These variations 1I1 the magnetic field can be measured by magnetometers. Tllis instrument, required for magnetic surveys, is simple yet sophlsticated. It usually detects the vertical component (Z), the horizontal component (H), or the total intensity (F) of the Earth's field. The most commonly used magnetometers in exploration surveys are:

• Fluxgatc magnetometer: The fhL'{gate magnetometer is capable of measuring the strength of any component of the Earth's magnetic field by simply reorienting the instrument in the required direction. Fluxgate magnetometers are capable of measuring the strength of the magnetic field to about 0.5 to 1.0 nT. They show no appreciable instrument drift with time.

• Proton magnetometer: The proton precession magnetometer measures the total strength of the Earth's magnetic field. The sensor component of the proton precession magnetometer is a cylindrical container filled with a liquid rich in protons (nuclei of hydrogen atoms) surrounded by a coil. Commonly used liquids include water, kerosene and alcohol. The sensor is connected by a cable to a small unit in which is housed a power supply, an electronic switch, an amplifier and a frequency counter. The strength of the total field can be measured down to about 0.1 nT. Proton precession magnetometers show little appreciable instrument drift with time. One of the important advantages of the proton precession magnetometer is its ease of use and its reliability. Sensor orientation need only be set to a high angle with respect to the Earth's magnetic field. No precise levelling or orientation is needed. If, however, the magnetic field changes rapidly from place to place (larger than about 600 nT/m), different portions of the cylindrical sensor will be influenced by magnetic fields of various magnitudes, and readings will be seriously degraded.

Finally, because the signal generated by precession is small, this instrument cannot be used near AC power sources.

Some geophysical methods are more suitable for regional-scale targets; while some others can be successfully applied at the mine scale. Hence, magnetic survey may be undertaken from the air (aeromagnetic survey), which is fast and is used for regional or large-scale survey to cover a wide area. They may be undertaken on the ground (ground survey); this is used for small-scale studies and gives more detail.

3.2.2 Basic concept of the magnetic method

Modern and classical magnetic theories differ in basic concept. Classical magnetic theOlY is similar to electrical and gravity theory; its basic concept is that point magnetic poles are analogous to point electrical charges and point masses, \J;1th a similar inverse·

square law for the force between the poies, charges, or masses. Magnetic units in the centimetre-gra.m--second and electromagnetic units (cgs·emu) system are based on this concept. Systeme International (SI) units are based on the fact that a magnetic field is electrical in origin. Its basic unit is the dipole, which is created by a circular electrical current, rather than the fictitious isolated monopole of the cgs-emu system. Both emu and SI units are in current use. The egs-emu system begins with the concept that the force F acting on two poles having values

of

pole strengdl PI and P2, and separated by

a

distance r, is expressed by Coulomb's law:

F=(PIP2/M?) r

F is the force on P_2, in dynes, the poles of strength PI and P2 are r em apart, M is the magnetic penneability, and r, is a tmit vector dinil~ted from P^j toward P2. As in the electrical case (but unJi..~e the gravity case, il.l_ which the force is always attractive), lhe magnetostatie force is attractive for poles of opposite sign and repulsive for poles of like sign (Telford et ai, 1990).

3.2.3 Magnetism of the Earth

The ~rth acts as a giant magnet, generating a field about itself that influences or captures other objects that are either magnetic or

may

be ill:lgnctized. As the magnetic method is concerned with detecting anomalies in the Earth's field, it is necessary to have an idea of the earth's field. Telford et al (1990) s~parated the Earth's field into three parts as follows:

(a) TIle main field, which varies relatively slO\v1y und is of inten1al origin;

(b) A small field (compared to the main field), which varies rather rapidly and originates outside the Earth. Like solar diurnal variations and sunspot activities these variations do not affect the Earu'l's main field significantly except for occasional magnetic storms. Diurnal variations can be cOITected for by the use of a base-station magnetometer;

(c) Spatial variations of the main field, which are usually smaller than the main field, are nearly constant in time and place, and are caused by local magnetic anomalies in the near-surface crust of the Earth. These are the targets in magnetic prospecting.

In addition, there are many geological factors that influence magnetic properties, directly or indirectiy, including lithology, depositional environment, tectonic setting, geochemical affinities, hydrothermal alteration, metamorphic grade, structures and rock age (Clark and et aI, 1992).

As the magnetic method of prospecting depends on detecting the deviations (anomalies) in the Earth's field, it is best to refer to it with seven components. The seven field relations are shown in Figure 3-1. Magnetic north is not equal to geographic north as shown in Figure 3-2. The difference betvi'een the two, in the horizontal direction (Figure 3-1) is referred to as the dec1inatior!. Inc1il1ation is the a.Tlgle the Eart..~'S magnetic field makes with the horizontll. TIle vector of the Rqrth's field is completely specified at any point by its" elements":

F

=

total magnetic field, FH

=

horizontnl component Df total fieid vector,

FN = north component of horizontal vector, FE = east component of horizontal vector,

. . . , • ~ , • • • . i·4'·I~~·~~,~,,~;~~·

~ , .. ^~

..

^{~ , ,}

. . F

I • I ~ • • ~ , • I I ~ • • • ' • ~ ~ , • ~ , • • • • • • '" .. ' J

: : : : :

~

: : : :

^~

: : : : : : :

. ~

.

^,

. : : : : : : : : : : : :

~ .:

: :

~ :.:

:

^:.

: :

.

• ~ • ~ • , • • . . . , . , . ~ • • • • ~ r • , I • . f I ~ ~ , t ~ ~ 1

. :::::::: :::::.,:::: ::;: ~ :~:;:~: ~ :~::;} ~: ~ :~:;t :.;.'~~ .. ~~.;~:>'

, • • • • • • • • • ~ , • • • • • 4

Figure 3-1 Components of the total magnetic field vector After Lillie (1999)

www.earthsci

N Geomogneti c

north pole (Ge09raphiea\ pole)

... _--t--_"-

s

(G~o'i1rClphic pOle)

South magnetic pol~ (£·-90)

\

Figure 3-2 Variation of geomagnetic and geographic components

TiOz

After Sch6n (1996)

F v = vertical component of total field vector

i = angle of magnetic inclination, and 8 = angle of magnetic declination.

The magnitude (F) of the total magnetic field vector (or total field intensity) is:

F

=

(F\rrF2v)112 == (F2~F2E+?v)1l2

3.2.4 Magnetic susceptibility and remanence

The magnetic properties that need to be considered in magnetic investigations of are bodies are magnetic susceptibility arid remanent magnetization. Remanent and induced magnetism both contribute to static field magnetic anomalies. Practically all the constituents that give a high magnetization 10 rocks are ferromagnetic minerals. Bleil and Petersen (1982) reported that the most important and abundant group of ferromagnetic minerals in rocks are iron fu'ld iron-titanium oxides; iron oxyhydroxides and iron sulphides are significant, but not abundant.

The