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The Study of Iron Ore Prospect using

2-D Resistivity and Induced Polarization

(IP) Method

Rosli Saad

Senior Lecturer (Dr.), Geophysics Section, School of Physics, Universiti Sains

Malaysia, Penang, Malaysia; e-mail: [email protected]

Imran Adli

Undergraduate Student, Geophysics Section, School of Physics, Universiti Sains

Malaysia, Penang, Malaysia; email: [email protected]

Ahmad Sayful Mohamad

Undergraduate Student, Geophysics Section, School of Physics, Universiti Sains

Malaysia, Penang, Malaysia; email:[email protected]

ABSTRACTIron ore has very high potential to be explored in Malaysia. For this study, 2-D resistivity andinduced polarization method are used. 2-D resistivity method is just a guide in order to

identify the iron ore bodies whereas induced polarization method is used to differentiate well

between ground water and iron ore deposits beneath ground. A total of six survey lines with

total length of 2.3km conducted for the study which divided into 3 different lots, each lotconsist of 2 survey lines. Pole-dipole array is used with 5 m electrode spacing. The results

show that area is underlain by a thick alluvium with chargeability value of 0.1-3msec which

makes this area has iron ore prospect. The thickness of alluvium beneath the ground surface is

up to 130m and the bedrock for this area lies at 50-130m with chargeability >5msec. Thearea has high potential for iron ore mining and dominated with alluvium with chargeability

rate of

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Table 1: Major iron compounds (Lindgren and Waldemar, 1933)

Iron ore can be divided into two types which are high-grade and low-grade. High-grade iron

ore means the presence of iron in compound is high. It is refers to compound contains more than60% of Fe, and probably comes from deposits of massive hematite formed by in situ enrichment

of iron, most commonly a banded iron formation. Low-grade iron ore is a naturally formed iron-

rich rock usually contains of 25-30% of Fe. The economical iron ore that usually extracted aremagnetite and hematite. The majority of iron ore deposits worldwide usually contains low-grade

iron ore due to presence of other minerals mainly silica. According to United States Geological

Survey (USGS), the iron ore output of 3,465,895 tons in 2010 from Malaysia is increased by

more than 20% compare to a year before output which is 1,470,186 tons. This shows that demand

of iron ore is rising yearly among industry players and iron ore is among the hottest commodity

besides oil, gas and gold.

THEORY OF 2-D RESISTIVITY

Resistivity method is an electrical geophysical imaging method to measure subsurface apparentresistance distribution by injecting DC current into the ground using 2 current electrodes (C and

C). A potential difference is measured using 2 potential electrodes (P and P). The resistivity

method is used in the study of horizontal and vertical discontinuities in the electrical properties of

the ground, detection of three-dimensional bodies of anomalous electrical conductivity (Kearey et

al., 2002), locating subsurface cavities, mineral and groundwater exploration. Besides that, it is

used extensively in borehole logging in oil exploration. Pole-dipole array used remote electrode,

C which located at infinity or as far as possible perpendicular to survey lines. To get a good data,

this array need forward & reverse measurement (Figure 1).

Figure 1: The forward and reverse pole-dipole array

Name Formula %Fe

Hematite FeO 69.9

Magnetite FeO 74.2

Goethite/Limonite HFeO ~63

Siderite FeCO 48.2

Chamosite (Mg,Fe,Al)Si,Al)(OH) 29.61

Pyrite FeS 46.6

Ilmenite FeTiO 36.81

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The main parameter measured from 2-D resistivity is resistance of rocks or minerals. The

resistivity of material is defined as the resistance in ohms between the opposite faces of a unit

cube of the material (Kearey et al., 2002). Every rocks and minerals will have their own

resistance values. Sedimentary rocks tend to have low resistivity due to high water content and

more porous compare to igneous and metamorphic rocks (Table 2).

Table 2: Resistivity values of some common rocks and soil materials.Material Resistivity (m)

Alluvium 10 80

Sand 60 1000

Clay 1 100

Groundwater(fresh) 10 100

Sandstone 8 4x10

Shale 20 2x10

Limestone 50 4 x10

Granite 5000 1,000,000

THEORY OF INDUCED POLARIZATION (IP)

Induced polarization, IP is an electrical geophysical method to measure the effect on current

flow of charge storage beneath ground. IP is the most useful geophysical methods in mineral

exploration due to its ability to detect disseminated minerals. The basic concept is overvoltage

effect (Figure 2). This overvoltage effect occurred due to characteristic of ground which acts as

capacitor when DC current is switched off, voltage measured from 2 potential electrodes is not

suddenly drop to zero but takes finite time to decay with time. After few moments, DC current is

switched on and voltage measured also takes same finite time to reach a steady value. There are

two main mechanisms of rock polarization which are grain polarization and membrane

polarization (Milsom, 2003). Overvoltage effect is contributed by minerals which are goodconductors where its magnitude depends on both magnitude of impressed voltage and mineral

concentration. It is most pronounced when the mineral is disseminated throughout the host rock

as the surface area available for ionicelectronic interchange is then at a maximum. The effect

decreases with increasing porosity as more alternative paths become available for the more

efficient ionic conduction (Kearey et al., 2002).

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Figure 2: The voltage versus time graph shows overvoltage effect when current isswitched off

The main parameter measured from IP method is chargeability of minerals and rocks.

Chargeability is defined as ratio of the area under the decay curve to the potential difference

measured before switching the current off. True chargeability is the ratio of the over- or

secondary voltage, Vs, to the observed voltage, Vo (Seigel, 1959). Chargeability of minerals and

rocks are shown in Table 3.

Table 3: Chargeability of common minerals and rocks (Telford et al., 1990)Injection current = 3s

Integration time = 0.02s 1s

Mineral Chargeability (ms)

Pyrite 13.4

Chalcocite 13.2Copper 12.3

Graphite 11.2

Chalcopyrite 9.4

Bornite 6.3

Galena 3.7

Magnetite 2.2

Malachite 0.2

Hematite 0.2

METHODOLOGY

Two electrical imaging methods which are 2-D resistivity and IP are chosen for this study. 2-

D resistivity method is used to identify the ore bodies from the resistivity value contrast whereas

IP method is used to differentiate between ground water with iron ore by differentiating the

chargeability of subsurface. Six survey lines were carried out with each lot contains 2 survey

lines. Total length of 6 survey lines is 2.3km (Table 4). The orientation of survey lines for Lot 1

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and 2 are almost west-east while for Lot 3, the survey is about south-north. Pole-dipole array is

used for the survey with 5m minimum electrode spacing.

Table 4: Length of survey lines at study area

STUDY AREA

The study area is located at north Peninsular Malaysia, 15km from Baling town and 16km

from Sik town (Figure 3). Total area of studied area is 28 acres which consist of 3 lots of land.

The study area is covered by old rubber estate plantation, secondary and primary jungle.

Figure 3: Orientation of survey lines and location of study area.

Line

name

Lot Distance(m)

L1 10

400

L2 10

400

L3 20

400

L4 20

400

L5 30

400

L6 3 0300

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RESULTS AND DISCUSSION

The resistivity results of Line 1-6 shows the area is underlain by a thick alluvium withresistivity value of 10-800m. The alluvium covers a depth of up to 130m. The alluvium can be

divided into two main zones which are hard layer (800m - 1500m) and boulders or hard

material (>1500m). The bedrock with resistivity value of >800m is expected to lies at 50-

130m depth. The IP results of Line 1-6 shows the area is suggested underlain by a thick alluvium

with chargeability value of 0.1-3msec. The alluvium covers a depth up to 130 m. The bedrockwith chargeability value of >5msec is expected to lies between 50-130m depth (Figure 4-6).

The potential area for iron exploration with estimated iron ore average grade of 20-40% is the

area with chargeability value of 0.1-3msec. This chargeability range is acceptable due to

chargeability of hematite and magnetite is 0.2msec and 2.2msec respectively.

Figure 4: Resistivity section (left) and IP section (right) of Lot 1

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CONCLUSION

Alluvium with chargeability rate of

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ACKNOWLEDGEMENTS

The authors would like to thank all member of Geophysics Section from Universiti Sains

Malaysia whom involved in this study.

REFERENCES

1. Kearey, P., Brooks M. and Hill I. (2002) An Introduction to GeophysicalExploration, 3rd edition, Blackwell Science, pp 183-203.

2. Griffith, D. H. and Barker R.D. (1993) Two dimensional resistivity imaging andmodeling in areas of complex geology, Journal of Applied Geophysics, Vol. 29,pp 211-226.

3. Hutchinson (2009) Geology of Malaysia, Geological Society of Malaysia.4. Keller, G.V. and Frischknecht F.C. (1996) Electrical methods in geophysical

prospecting, Pergamon Press Inc., Oxford.

5. Lindgren and Waldemar (1933) Mineral Deposits, 4th edition, New Tork:McGraw-Hill, pp 129.

6. Loke, M.H. and Barker R.D (1996) Rapid least-squares inversion of apparentresistivity pseudosection using a Quasi-Newton method, Geophysical Prospecting,

Vol. 44, pp 131-152.

7. Telford, W.M. and R.F. Sheriff (1990) Applied Geophysics, 2nd edition, CambridgeUniversity Press

8. Wait, J. R. (1958) Discussions on a theoretical study of induced electricalpolarization, Geophysics, Vol. 23, pp 144-154.

9. Zonge, K.L., Sauck, W.A. and Sumner, J.S. (1972) Comparison of time, frequency,and phase measurements in induced polarization, Geophysical Prospecting, Vol. 20,

pp 626-648

10.Milsom, J. (2003) Applied Geophysics, 3rd edition, John Wiley & Sons Ltd., pp83-126

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