Assessment of Image Ratio Technique for Gold Exploration

__________________________________________________________________________________________

Page | 17 International Journal of Geosciences and Geomatics, Vol. 4, Issue 1, 2016

Assessment of Image Ratio Technique for Gold Exploration in Arid Region

Using Landsat ETM+7: Limitations and Possible Source of Misinterpretations

Esamaldeen Ali 1, Mohamed Y. Abdegalil1, Abu Elhassan Musa2

1. Faculty of Petroleum & Minerals, Al Neelain University, Box: 12702, Khartoum, Sudan

2. Member of National Geological Council, Khartoum, Sudan

Keywords: Landsat ETM+7, Gold exploration, arid region, image ratio; alteration

Abstract—In modern exploration program, the use of

multispectral satellite images is one of the standard

procedures during the early stages of gold exploration,

due to its high efficiency and low cost. The main goal

of the present study is to assess the efficiency of image

ratio technique for gold mineralization in arid region.

The area around the third cataract, northern state,

Sudan has been selected for the current purpose.

Landsat ETM+7 images have been used in the current

study. The patterns recognized in the digitally

processed satellite images showed a clear

discrimination of granitic rocks, Sandstone from mafic

metavolcanics, as well as delineate the structural

features in the region. The spectral properties of band

ratios have successfully been used to delineate

alteration zones. Common false color composite ratio

images have been applied by combining band ratio

images in RGB. From these entire analyses through

visual interpretation and ground truthing the results

show that more than 60% of the image mask with iron

alteration due to the dominantly ferruginous

Sandstone formation in the study area and addition to

clay-rich wadies deposits. From field observation very

narrow hydrothermally alteration zone has been

detected along highly sheared metavolcanics and their

margin with syn-orogenic granites in the central part

of the study area. This study clearly concluded that

nevertheless, the significance of image ratio technique

for gold exploration in early stage of exploration

program, care should be taken during image

interpretation in such geological condition.

Introduction:

Hydrothermal alteration is defined as the reflection

of response of pre-existing rock-forming minerals to

physical and chemical conditions different than those,

under which they originally formed, especially by the

action of hydrothermal fluids [1].This alteration can

produce distinctive assemblages of minerals that vary

according to the location, degree and longevity of

those flow processes. However, certain minerals

associated with hydrothermal processes, such as iron-

bearing minerals (e.g., goethite, hematite, jarosite and

limonite) and hydroxyl bearing minerals (clays and

sheet silicates) show diagnostic spectral features that

allow their remote identification [2,3].

In modern exploration programs remote sensing

technique plays a very significant role in early stage of

exploration strategy. This technique rapidly can help to

delineate potential area from non-interesting areas,

which can guides to effectively reducing the costs of

exploration in further stages. However, the use of

multispectral satellite images especially, Landsat is one

of standard procedures during the early stages of

mineral exploration, due to its high efficiency and low

cost. It is very successful and has important role in

recognized hydrothermally alteration and detected

__________________________________________________________________________________________


effective features on mineralization such as contacts,

linear structures and shear zones. In fact, there are

many studies around the world related to

hydrothermal alteration mapping using multispectral

satellite images especially, Landsat and Aster [4-9].

In Sudan (arid region), several scholars used different

multispectral Landsat for lithological, structural

mapping and delineate potentially gold mineralization

areas [10-16]. However, the main goal of the present

study is to delineate alteration zone related to gold

mineralization in the area around the third cataract,

northern state, Sudan (Fig.1). Hence, availability of

Landsat satellite ETM+ has been used for particular

application. The area under investigation is covered by

scene p175r46, acquired on 1 Sep 2000 which is free of

cloud cover. The spectral bands of Landsat 7 have a

spatial resolution of 30 meters (visible, NIR, SWIR); 60

meters (thermal); and 15 meters of panchromatic

band. In this study the six none thermal bands (1-5 &

7) were used for the spectral analysis, whereas the

panchromatic band was used for the visual

interpretation of the topographical differences in the

area.

Fig. 1: Location map of the study area

1. Digital Image Processing for lithological

discrimination:

Before starting digital image processing (DIP) the

image is pre-processed for atmospheric scattering.

Then, the subsetting has been applied to the target

area using geographic coordinates (latitudes 20° 00'-

19° 40' N and longitudes 30° 16'-30° 46' 50" E), with

total area of approximate 1985 Km2. (Fig.1). Linear

contrast enhancements and decorrelation stretch

transformation of RGB combinations were applied to

the Landsat imagery to delineate lithological units.

However, RGB combinations involving bands from

each spectral region (i.e. visible, mid infrared and

SWIR) were found to have most contrast on lithological

features than individual bands. The procedure of DIP

for lithological discrimination is out of the scope of the

current study. These techniques with different color

composite images (Fig. 2 & 3) are produced suitable

differences in tones among different lithology (i.e.

basement, Phanerozoic cover and alluvial deposits).

However, accuracy of the outcomes of above

mentioned methods were checked the comparison

with the field observations and the final geological

map is carried out as shown in figure (4). As seen in

Figure (2) Linear contrast stretch shows low contrast

among basement lithology (e.g. different granitic

units), while increasing contrast between sedimentary

and crystalline rocks over the whole area. Recent

sediments, including wadi fill and alluvial fans of

various geological times scale were readily

distinguishable. The River Nile deposit appears light

blue in the image and can be easily separated from

younger alluvial fans. In Figure (3) the basement

geology is more distinguishable, where sharp

boundary is well defined between circular-shaped

basic volcanics and cretaceous sandstone formation as

well as syn-orogenic granites and sedimentary rocks.

The syn-orogenic granites in the central part of the

image have low relief topography with bright yellow

hues due to the overlain sandy superficial deposits.

__________________________________________________________________________________________


Fig.2: Linear contrast enhancement of color

composite image of bands 7, 4 & 1 in

RGB

Fig. 3: Decorrelation stretched color composite

image of bands 7, 4 & 1 (RGB)

Fig. 4: Regional geological map based on digital image processing followed by ground trothing

2. Band ratio technique for gold exploration

The band ratio technique is based on highlighting the

spectral differences that are unique to the materials

being mapped. Identical surface materials can give

different brightness values because of the topographic

slope and aspect, shadows, or seasonal changes in

sunlight illumination angle and intensity. Bands ratios

are known for enhancement of spectral contrasts among

the bands considered in the ratio operation and have

__________________________________________________________________________________________


successfully been used in mapping of alteration zones.

From the theoretical point of view and mineral’s spectral

properties of bands, particular band ratio has identity to

recognize particular minerals which have been used in

geology applications [17, 18] (Table 1).

Table 1: Selective band ratio for particular mineral

detection

Band

ratio Uses

3/1 Iron oxide

5/1 Magnetite (Fe+2 & Fe+3) content

5/7 Hydroxyl bearing minerals and clay

minerals

3/4*5/4 Metavolcanics

5/4 Fe-rich aluminum silicate minerals

3/4 *4/5 Limonite

Band rationing and combinations with most contrast

were also investigated whereby, for bands ratios

involving geology, it was observed that contrast

increased with use of bands in different spectral

regions. Thus, for the current purposes, common false

color composite ratio images have been applied by

combining band ratio images in RGB: Abram’s ratio

image (3/1:5/4:5/7) by Abram et al. [19]; Sabin’s ratio

image (5/7:3/5:3/1) )[20] and Sultan ratio image

(5/7:5/1:3/4*5/4) by Sultan et al. [21]. The results of

image ratios have been subjected to simple linear

stretching to enhance contrast level (Fig.5A, 5B & 5C).

3. Results and Discussion:

From the results of mineral exploration technique the

first false color composite ratio image (Fig. 5A)

illustrated clay minerals displayed as red and light violet

hue due the presence of hydrated minerals and the

altered iron-oxide as light blue due to the existence of

ferrous iron oxides, whereas Fe-rich aluminum silicate

minerals (medium relief appearance) appear with dark

green hue. The ratio of band 5/4 gives difference

between iron oxides dominance and hydroxyl due to the

strong absorption of band4.

Using ratio image (5/7:3/5:3/1), we obtain altered clay

minerals as red; ferrous oxide as green and ferric iron

oxides as blue color (Fig. 5B). As it is clear from this

image, the iron-rich altered rocks appear in light blue

hue. However, the delineation of this altered zone is

impossible. This is because this color is very common in

the image and many rock types are displayed in the

same color. Figure (5C) displays minerals containing

hydroxyl minerals, ferric and ferrous iron oxides show

respectively deep violet , green and blue color using

Sultan’s ratio (5/7:5/1:3/4*5/4).

From these entire analyses we found that more than

60% of the images mask with iron alteration and

partially with clay-rich altered zones. However, by using

the results of lithological discrimination and delineation

of the hydrothermal altered rocks through ground

investigation, the results show that small narrow

hydrothermal alteration zone along the highly sheared

metavolcanic rocks and their margin with syn-orogenic

granites in the central part of the image. These

metavolcanics situated as sandwich unit between

different types of syn-orogenic granites (Fig. 4 & 6).

However, the northeastern and southeastern part of the

image is almost covered by cretaceous sandstone

formation. By this result, we can enable to delineate the

clay-rich altered rocks avoiding the areas of river banks

and wadies. Thus, this study clearly concluded that

nevertheless, the significance of band ratio technique in

the early stage of mineral exploration, care should be

taken during image interpretation in such geological

condition (e.g. arid region). This due to the two facts:

first, alluvial deposits along wades and river banks have

same spectral signatures as clay-rich altered zone due to

the high deposition of clays by transportation agencies.

Second is iron cap of sandstone formation which has

__________________________________________________________________________________________


similar hues to oxidation process of hydrothermally

altered rocks.

A.

B.

C.

Fig. 5: False color composite ratio images in RGB: A.

Abram’s ratio (3/1:5/4:5/7)[19]; B. Sabins ratio

(5/7:3/5:3/1) [20]; C. Sultan ratio image

(5/7:5/1:3/4*5/4) [21].

Fig. 6: Hydrothermally alteration zone based on image

processing and ground observation.

4. Conclusion and recommendations:

The band ratios images are known for enhancement of

spectral contrasts among the bands considered in the

ratio operation and have successfully been used in

mapping of alteration zones. In this study, band ratio

technique has been investigated using Landsat

multispectral data (ETM+) for gold exploration in arid

environment. The result of image processing technique

has been checked with ground truthing. Although, the

significance and powerful of medium resolution Landsat

satellite images in recognized hydrothermally alteration

zones, the study show that more than 60% of the images

mask with iron alteration and partially with clay-rich

altered zones due to the iron cap and kaolinitization of

dominantly sandstone formation in the study area. In

addition to, the existing of alluvial deposits along wades

and river banks. Nevertheless, from these entire

analyses through visual interpretation and ground

truthing the results indicate small narrow hydrothermal

alteration zone along the highly sheared metavolcanics

rocks and their margin with syn-orogenic granites in the

central part of the study area. These metavolcanics

situated as sandwich unit between different types of

syn-orogenic granites.

Finally, we can recommended by attention to the results

that nevertheless remote sensing techniques used as a

fast and cheap tool for exploration, attention should be

taken in such geological condition due the occurrence of

__________________________________________________________________________________________


erruginous sandstone strata and clay-rich wadies

deposits.

Acknowledgement:

The Authors would like to show their sincere gratitude

to all staff of Manakib mining company who have

arrange the field accommodation and transportation

during the field work.

References:

[1] Eoin, H., Macdonald, 2007. Handbook of gold

exploration and evaluation. Woodhead Publishing

Limited, Abington Hall, Abington, Cambridge CB21

6AH, England, pp.664

[2] Haldar, S. K. (2013). Mineral Exploration: Principles

and Applications. British Library Cataloguing in

Publication Data, pp.333

[3] Gupta, R., 2003. Remote Sensing Geology. 2nd ed.

Berlin Heidelberg: Springer-Verlag.

[4] Crosta, A., Moore, J. McM, 1989. Enhancement of

Landsat The matic Mapper imagery for residual soil

mapping in SW Minais Gerais State, Brazil: A

prospecting case history in Greenstone belt terrain.

International Proceeding s of the Seventh Erim

Thematic Conference: Remote Sensing for

Exploration Geology, pp. 1173–1187.

[5] Madani, A., Abdel Rahman, E.M., Fawzy, K.M., Emam,

A., 2003. Mapping of the hydrothermal alteration

zones at Haimur Gold Mine Area, South Eastern

Desert, Egypt using remote sensing techniques. The

Egyptian J. Rem.Sens. Space Sci. 6 47–60.

[6] Ramadan, T.M., Kontny, A.M., 2004. Mineralogical

and structural characterization of alteration zones

detected by orbital remote sensing at Shalatein

District, SE Desert, Egypt. J. African Earth Sci. 40 89–

99.

[7] Jin, X., Paswaters, S., Cline, H., 2009. A comparative

study of target detection algorithms for

hyperspectral imagery; In:Algorithms and

Technologies for Multispectral, Hyperspectral, and

Ultraspectral Imagery XV. Proceedings of SPIE 7334

73341W1–73341W12.

[8] Bodruddoza, Mia, Yasuhiro, Fujimitsu, 2012.

Mapping hydrothermal altered mineral deposits

using Landsat 7 ETM+ image in and around Kuju

volcano, Kyushu, Japan. J. Earth Syst. Sci. 121, No. 4,

pp. 1049–1057

[9] Mwaniki, M.W., Moeller, M.S., Schellmann, G., 2015.

A comparison of Landsat 8 (OLI) and Landsat 7

(ETM+) in mapping geology and visualising

lineaments: A case study of central region Kenya.

36th International Symposium on Remote Sensing

of Environment, Berlin, Germany,

doi:10.5194/isprsarchives-XL-7-W3-897

[10] Kenea, N.H., 1997. Digital enhancement of Landsat

data: spectral analysis and GIS integration for

geological studies of the Derudeb area, southern

Red Sea Hills, Sudan. Ph.D thesis Berliner geowiss.

[11] Abdelsalam, M.G., Stern, R.J., Berhane, W.G., 2000.

Mapping gossans in arid regions with Landsat TM

and SIR-Cimages:The Beddaho alteration zone in

northern Eritrea. J. African Earth Sci. 30(4) 903–916.

[12] Elkhidir, S. O., 2006. Remote sensing and GIS

applications in geological mapping, prospecting for

mineral deposits and ground water exploration-

Berber sheet area, northern Sudan. Ph.D. Thesis

((unpublished).

[13] Babikir, I. A. A., 2006. Digital Image Processing of

Landsat7 data and GIS Application for Geological

Investigation in Jebel Erba Area, Red Sea Hills, NE

Sudan. Ph.D. Thesis (unpublished). [14] Ali, E.A., El

Khidir, S.O., Babikir, I.A.A., Abdelrahman, E.M.,

2012. Landsat ETM+7 Digital Image Processing

Techniques for Lithological and Structural

Lineament Enhancement: Case Study Around

__________________________________________________________________________________________


Abidiya Area, Sudan. The Open Remote Sensing

Journal, 2012, 5, 83-89.

[15] Khalid, A. Elsayed Zeinelabdein, Abdel Halim H. El

Nadi, 2014. The use of Landsat 8 OLI image for the

delineation of gossanic ridges in the Red Sea Hills of

NE Sudan. American Journal of Earth Sciences 2014;

1(3): 62-67

[16] Khalid, A. Elsayed Zeinelabdein, 2010. Sudan

experience in using remote sensing for mineral

prospecting. Proceedings of the 11th Arab

International Conference on Mineral Resources,

Arab Industrial Development and Mining

Organization, Tripoli, Great Jamahiriya. p. 636-644,

Oct. 25-27.

[17] Sabine, C., 1999. Remote Sensing Strategies for

Mineral Exploration. In: A.N. Renz (Ed.), Remote

Sensing for Earth Sciences: Manual of Remote

Sensing. John Wiley & sons, Inc., 375-447.

[18] Crippen, E.R., 1989. Selection of Landsat TM band

and band-ratio combinations to maximize Lithologic

information in color composite displays. Proceed.

7th Them. Conf. Remote Sens. Exp. Geol., Calgary,

Alberta, Canada, 2, 917-921.

[19] Abrams, M.J., Brown, D., Lepley, L., Sadwski, R.,

1983. Remote Sensing for porphyry copper deposits

in Southern Arizona. Elpaso Texas. Econ. Geol., 78,

591-604.

[20] Sabins, F. Floyd, 1999. Remote Sensing for Mineral

Exploration. Ore Geol. Rev., 14. Elsevier Science.

[21] Sultan, M., Arvidson, R.E., Sturchio, R.C. and

Guiness, E. A., 1987. Lithologic mapping in arid

regions with Landsat Thematic Mapper data:

Meeting dome, Egypt. Geol. Soc. America Bull., 99,

748-762.

Documents

Assessment of Image Ratio Technique for Gold Exploration