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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
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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.
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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
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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
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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
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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.
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