Assessment of Image Ratio Technique for Gold Exploration in Arid Region Using Landsat ETM+7: Limitations and Possible Source of Misinterpretations

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1 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. Abdegalil 1, Abu Elhassan Musa 2 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 ironbearing 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 Page 17 International Journal of Geosciences and Geomatics, Vol. 4, Issue 1, 2016

2 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' ' 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. Page 18 International Journal of Geosciences and Geomatics, Vol. 4, Issue 1, 2016

3 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 Page 19 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 International Journal of Geosciences and Geomatics, Vol. 4, Issue 1, 2016

4 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 Page 20 International Journal of Geosciences and Geomatics, Vol. 4, Issue 1, 2016

5 similar hues to oxidation process of hydrothermally altered rocks. A. Fig. 6: Hydrothermally alteration zone based on image processing and ground observation. 4. Conclusion and recommendations: 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]. 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 Page 21 International Journal of Geosciences and Geomatics, Vol. 4, Issue 1, 2016

6 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, 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., Remote Sensing Geology. 2nd ed. Berlin Heidelberg: Springer-Verlag. [4] Crosta, A., Moore, J. McM, 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 [5] Madani, A., Abdel Rahman, E.M., Fawzy, K.M., Emam, A., 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] Ramadan, T.M., Kontny, A.M., Mineralogical and structural characterization of alteration zones detected by orbital remote sensing at Shalatein District, SE Desert, Egypt. J. African Earth Sci [7] Jin, X., Paswaters, S., Cline, H., A comparative study of target detection algorithms for hyperspectral imagery; In:Algorithms and Technologies for Multispectral, Hyperspectral, and Ultraspectral Imagery XV. Proceedings of SPIE W W12. [8] Bodruddoza, Mia, Yasuhiro, Fujimitsu, 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 [9] Mwaniki, M.W., Moeller, M.S., Schellmann, G., 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: /isprsarchives-xl-7-w3-897 [10] Kenea, N.H., 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., Mapping gossans in arid regions with Landsat TM and SIR-Cimages:The Beddaho alteration zone in northern Eritrea. J. African Earth Sci. 30(4) [12] Elkhidir, S. O., 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., 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., Landsat ETM+7 Digital Image Processing Techniques for Lithological and Structural Lineament Enhancement: Case Study Around Page 22 International Journal of Geosciences and Geomatics, Vol. 4, Issue 1, 2016

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