Indicators of gold deposit near Khartoum, Sudan: Satellite gravity and Landsat 8 OLI remote sensing data analysis

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1 American Journal of Earth Sciences 2014; 1(2): Published online April 20, 2014( Indicators of gold deposit near Khartoum, Sudan: Satellite gravity and Landsat 8 OLI remote sensing data analysis Khalid A. Elsayed Zeinelabdein 1, Eiman A. Mohamed 2 1 Department of Geology, Faculty of Petroleum and Minerals, Al Neelain University, Sudan 2 Remote Sensing and Seismological Sciences Authority, National Centre for Research, Sudan address kalsayed2001@yahoo.com(k. A. Elsayed Zeinelabdein), ebabdalla@hotmail.com(e. A. Mohamed) To cite this article Khalid A. Elsayed Zeinelabdein, Eiman A. Mohamed. Indicators of Gold Deposit Near Khartoum, Sudan: Satellite Gravity and Landsat 8 OLI Remote Sensing Data Analysis,American Journal of Earth Sciences. Vol. 1, No. 2, 2014, pp Abstract This study represents a reconnaissance investigation to elucidate the mineral potentiality of the Sabaloka Inlier. The Sabaloka Inlier is well known by its geological importance to the geologists and earth scientists in general. Many researches were conducted in order to investigate its geological setting and tectonic significance. However, studies concerning the assessment and evaluation of its mineral potentiality are uncommon. Hence, the objectives of the present study are to carry out general prospecting in the area and to search for possible mineral deposits through an integrated analysis of satellite gravity and Landsat 8 OLI data. The major lithological units in the area include granulite, gneisses and migmatites, older granitoids, Sabaloka Igneous Complex, Nubian sediments and superficial deposits. The structural history of the area is complex and reveals several phases of superimposed folding and faulting of different styles. The relative gravity highs within the study area are caused by the Seleitat younger granite ring and other shallow intrusion in NE the area. Quartz veins and felsite dykes that came in the later stages of the igneous activity may qualify the area prospective with respect to gold deposits. Standard colour composites of Landsat 8 OLI image clearly discriminated a large number of quartz veins and felsite dykes in the northwestern portion of the area. Band ratioing technique disclosed the presence of alteration zones displayed in yellow to yellowish orange in the mineral composite and in neon red to yellowish brown in the hydrothermal composite. These zones are mainly located in the central and northwestern parts of the area. The majority of the lineaments are parallel to the Keraf Shear Zone. Shearing criteria can be detected from the satellite image as well as from rock outcrops. The study area may represent a new mining district due to the shearing affected most of the rocks. The geochemical analysis proved the existence of gold in the area, though the determined values are unsatisfactorily low. This necessitates detailed ground survey to be carried out in order to fully recognize the mineralpotentiality of the area. Keywords Sabaloka Inlier, Gold Deposits, Landsat 8 OLI Images, Satellite Gravity 1. Introduction The Sabaloka Inlier is well known by its geological importance to the geologists and earth scientists in general. Many researches were conducted in order to investigate its geological setting and tectonic significance. However, studies concerning the assessment and evaluation of its mineral potentiality are restricted to [1]. Hence, the objectives of the present study are to carry out general prospecting in the area and to search for possible mineral deposits through an integrated analysis of satellite gravity and Landsat 8 OLI data. The study also aims at geochemically analyze some ore samples from the outlines target ones. The study area represents a part of the Sabaloka Inlier,

2 34 Khalid A. Elsayed Zeinelabdein and Eiman A. Mohamed: Indicators of Gold Deposit Near Khartoum, Sudan: Satellite Gravity and Landsat 8 OLI Remote Sensing Data Analysis which is located north of Khartoum. It is bounded by the following UTM coordinates: Easting and Northing (Fig. 1). This area is characterized by semi-desert climatic conditions with average annual rainfall ranges between mm. The highest daily mean temperature is about 43 C during the summer period from may to october, and the lowest mean temperature is about 16 C during winter period from december to Februrary. The vegetation cover is represented by small grasses and pushes dominant in most of the area. 3. Methodology 3.1. Materials The present study made use of the available data, which include: - Landsat 8 OLI image, path 173 and row 49, acquired on April 24 th 2013, downloaded from the United States Geological Survey (USGS) in digital format. - Satellite gravity gridded data covering the study area in Excel spread sheets format. - Geochemical analysis for Au determination. - Geological map of Sudan at the scale of 1:2,000,000 obtained from the Geological Research Authority of Sudan [6] Methods Fig 1.A map showing the location of the study area. 2. Geological and Structural Settings Sabaloka is one of a large group of anorogenic igneous complexes in Sudan which is characterized by predominance of acidic rocks, shallow level of emplacement, the common presence of ring structures, and a tendency to include peralkaline variants[2]. The basement complex of Sabaloka inlier is encircled by the Cretaceous Sandstone [3]. The major lithological subdivisions include granulite, gneisses and migmatites, older granitoids, Sabaloka Igneous Complex, Nubian sediments and superficial deposits [2], [4]. The basement complex includes metamorphic rocks of high amphibolite facies and numerous lenses and bands of rocks of granulite facies.the gneiss assemblage contains evidence that an early granulite facies metamorphism was followed by retrogression into the amphibolite facies and widespread migmatization [2]. The structural history is complex and reveals several phases of superimposed folding, however, three phases of folding were identified and appear to have affected the basement rocks.the earliest phase is a horizontal deformation expressed by isoclinal recumbent folding. The second phase of deformation appears to be sub- horizontal and is indicated by recumbent minor folds which fold the early foliation. The third phase of folding is characterized by large scale open folds plunging gently to the east[5]. Digital image processing of remotely-sensed data is adopted for providing general idea about the lithological cover and to map possible alteration zones occur in the study area. Standard colour compositing and band ratioing are among the main techniques utilized in the present study. Geophysical investigations through the analysis of satellite gravity data were carried in order to acquire information about the subsurface geological structure. Lineament mapping and the subsequent structural analysis were performed to detect any structural control on mineralization, if any. A one-day reconnaissance field trip was conducted, where representative chip samples from the suspected targets were collected. Geochemical analysis was conducted for a limited number of samples to determine their gold content. 4. Results and Discussions 4.1. Satellite Gravity Interpretation The gravity point data were interpolated to create gravity surface and consequently a contour map presented in Figure (2). From this figure, it can easily be observed that the gravity lows are clustered in the southern and northwestern parts of the map area. These gravity lows are caused by the sedimentary rocks cover these parts (Khartoum Basin to the south and Shendi Basin to the north); the only exception is the Sabaloka Igneous Complex, where the low gravity anomaly may be attributed to the thick crust due to acid volcanic activity in this part of the area. On the other hand, the gravity highs are found in the southwestern, central and northeastern sectors of the map area. The northeastern positive anomaly reaches more than +5 mgal. Discussion of this anomaly is out of the scope of the present study. The remaining relatively high anomalies are circular in shape with increasing values towards the center. The central anomaly is additionally characterized by zigzagging contours in its northern portion. The southeastern gravity high is principally caused by the Seleitatyounger granitering intrusion.by analogy, the central gravity high may represent a similar igneous ring

3 American Journal of Earth Sciences 2014; 1(2): intrusion. If this suggestion is true, then the quartz veins and felsite dykes that came in the later stages of the igneous activity may qualify the area quite prospective as far as gold is concerned. Fig 3b. Landsat OLI colour composite, RGB = 765 (yellow points are locations of the collected samples). Fig2.Satellite regional gravity map of the study area Landsat 8 OLI Image Processing According to outcome of the preliminary geophysical investigation, digital image processing of Landsat 8 OLI image was carries out to enhance the lithological discrimination to detect any intrusive rocks and to map the alteration zones in the study area. Two standard colour composites were created, namely: OLI 752 and 765 in RGB, respectively (Fig 3). It is clear from these images that the area is approximately flat and no distinctive igneous intrusion appears at the surface. Some igneous bodies are less frequently scattered in the image area that are sheared in the north-south direction. Sedimentary cover is restricted to the north eastern part of the image. The drainage system seems to be structurally controlled by faults and shear zones running generally to the north towards the River Nile. In the northwestern portion of the image, a large number of quartz veins and felsite dykes can easily be discerned. Felsite dykes are known to abide gold in Sudan [7] and they are regarded as a new style of gold mineralization. Alteration zone mapping was conducted through ratioing digital image processing technique to acquire a comprehensive clue about the mineral potentiality of the area. To this end, two ratio colour composite images were prepared: band ratio color composite obtained by assigning the band ratios: OLI 6/7, 4/2, 5/4; and 6/7, 6/5, 4/2 to RGB, respectively (Fig. 4). These ratio colour composites are termed the mineral composite and hydrothermal composite, respectively[8]. On the first ratio colour composite, the alteration appeared in yellow to yellowish orange. These zones are concentrated in the central and northwestern parts of the image area. In the second ratio colour composite, the alteration zones are displayed in neon red to yellowish brown and located in places where were previously mapped by the mineral composite. Distinct alteration colours appear on both the mineral and hydrothermal composites in the northeastern sector of the image area. It is worthmentioning that these coloursdo not represent altered rocks; instead, they are mainly due to the presence of iron-rich sedimentary rocks. Fig 3a. Landsat OLI colour composite, RGB = 752 (yellow points are locations of the collected samples). Fig4a. Landsat OLI band ratio colour composite, RGB = 6/7, 4/2, 5/4. (yellow points are locations of the collected samples).

4 36 Khalid A. Elsayed Zeinelabdein and Eiman A. Mohamed: Indicators of Gold Deposit Near Khartoum, Sudan: Satellite Gravity and Landsat 8 OLI Remote Sensing Data Analysis occur along the Keraf Shear Zone in the River Nile State along with its extension in the Butana area, where intensive artisanal mining activities take place. Comparatively, the study area may represent a similar mining district. However, detailed ground survey is required Geochemical Analysis Fig4b. Landsat OLI band ratio colour composite, RGB = 6/7, 6/5, 4/2. (yellow points are locations of the collected samples) Lineament Mapping and Structural Analysis Lineaments may representthe surface expressions of rock fractures. They have therefore been used in mineral exploration [9]. Accordingly,the structural analysis of lineaments is an important step in acquiring a basic knowledge about the structural evolution of an area. For that reason, a lineament map was prepared through the spatial enhancement and interpretation of the Landsat 8 OLI image (Fig. 5). The outcome of the digital processing and the consequent image interpretation pointed out to the existence of possible mineralized zones. As mentioned in the methodology section, a one-day reconnaissance field trip was conducted in order to collect chip samples from certain points, determined prior to the field. Unfortunately, not all the points are accessible, 11 out of 15 point were found actually reachable. A number of 11 samples were collected and sent to Rida Mining Co. for analysis. Table (1) shows the results of these analyses. Table (1).Geochemical analysis for gold determination. Sample No. Au (ppm) Sample No. Au (ppm) UA UA UA UA UA UA UA UA UA UA UA The above table shows the gold content of the analyzed samples that ranges from 0.03 to 0.15 g/ton. These values are unsatisfactorily low. From another point of view, they indicate the presence of gold in the study area. Since, the samples were collected from surface outcrops, which are generally subjected to intensive weathering, the collection and analysis of samples from the subsurface may provide high gold content and hence, qualify the area quite prospective. 5. Conclusions Fig 5.Lineament map of the study area. From the lineament map in Figure (5), it is apparent thatthe majority of the lineaments have the N-S direction parallel to Wadi ElHawadof [10] to the east, which is parallelto the KerafShear Zone to the north. Sign of shearing cannot be overlooked and the shearing criteria can be observed from the satellite image as well as from rock outcrops. The sheared rocks are of great importance due to their mineral potentiality. Gold deposits are well known to This study represents a reconnaissance investigation to elucidate the mineral potentiality of the Sabaloka Inlier. The relative gravity highs within the study area are caused by the Seleitat younger granite ring and other shallow intrusion in NE the area. Quartz veins and felsite dykes that came in the later stages of the igneous activity may qualify the area prospective with respect to gold. Standard colour composites of the enhanced Landsat 8 OLI image clearly discriminated a large number of quartz veins and felsite dykes in the northwestern portion of the image area.band ratioing technique disclosed the presence of alteration zones displayed in yellow to yellowish orange in the mineral composite and in neon red to yellowish brown in the hydrothermal composite. These zones are concentrated in the central and northwestern parts of the area. The ironrich sedimentary rocks in NE of the area causes similar colours as those of the alteration zones.the majority of the lineaments have the N-S direction parallel to the Keraf Shear Zone.Shearing criteria can be observed from the

5 American Journal of Earth Sciences 2014; 1(2): satellite image as well as from rock outcrops. The study area may represent undiscovered mining district due to the shearing affected most of the rocks.although the determined values are unsatisfactorily low, the geochemical analysis proved the existence of gold in the area. However, this necessitatesdetailed ground survey to be carried out. Acknowledgments The authors wish to thank Switch Co., Sudan for financing the field work and the geochemical analysis of chip samples. The United States Geological Survey(USGS), is highly appreciated for making the Landsat 8 OLI images available.special thanks are due to the editor and anonymous reviewer for their valuable comments. References [1] B. E. Khalil. Geochemical evidence of zoning in wolframcassiterite deposit, Sabaloka, Sudan. Abstracts, Third conference on African Geology, Khartoum, [2] D. C. Almond. The Sabaloka igneous complex, Sudan. Philosophical Transactions of the Royal Society of London, Series A, A287, , [3] A. Kröner, R. J. Stern, A. S. Dawoud, W. Compston, and T. Reischmann. The Pan-African continental margin in northeastern Africa: evidence from a geochronological study of granulites at Sabaloka, Sudan. Earth and Planetary Science Letters, 85, , [4] A. S. Dawoud and A. Sadig. Structural and Gravity Evidence for an Uplifted Pan-African Granulite Terrain in the Sabaloka Inlier, Sudan. Journal of African Earth Sciences, Vol. 7, No. 5/6, pp , [5] A. S. Dawoud. A Geological and Geophysical Study of the Area South-East of Qerri Station, Eastern Sabaloka, North Khartoum Province, Sudan. Thesis, Univ. Khartoum, [6] Geological Research Authority of Sudan (GRAS). Geological map of Sudan, [7] A. H. El-Nadi, K. A. Elsayed Zeinelabdein and I. S. Babiker. Gold mineralization potentialities in the area between Wadi Umm Beckol WadiAkasha, Northern Sudan. (unpub. report), [8] ERDAS IMGINE, ErdasImaginefield guide. [9] L. C. Rowan and P. H. Wetlaufer. Iron-observation band analysis for the discrimination of iron- rich zones. U.S. Geological survey, Type III Final Report Contract S AG, [10] A. E. M. Elsheikh, K. A. Elsayed Zeinelabdein, S. O. H. Elkhidir, A. E. Ibrahim. The geometrical configuration of the newly discovered Abu Deleig Sedimentary Sub-basin, Central Sudan, using remote sensing, structural analysis and geophysical survey. Arabian Journal of Geosciences. Arab J Geosci. 7: , DOI /s