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Hydrothermal zones detection using airborne magnetic and gamma ray spectrometric data of mafic/ultramafic rocks at Gabal El-Rubshi area, Central Eastern Desert (CED), Egypt.

INTRODUCTION

Hydrothermal processes can cause different types of change in magnetic mineralogy and radioelement contents in geological units under deformation. They make it possible to apply magnetic and spectrometric geophysical methods as mafic and ultramafic igneous and metamorphic rocks normally contain the greatest quantity of magnetic minerals (high magnetic susceptibilities) and the least abundance of radiometric minerals. Furthermore, airborne geophysical magnetic and gamma ray spectrometric surveys have been used extensively for the definition of mineralization zones and metalliferous deposits [7-17-13-23]. The successes of such geophysical methods are resulting from the unique signatures that are produced by the mineralization zones distinguishing them from the host rocks.

In fact, magnetic anomaly maps can provide important imminent into the subsurface structure of metamorphic areas because of the elevated magnetic response of metamorphic rocks [5]. The interpretation of these maps can give up valuable information on the inner structures possibly associated with magmatic feeding structures and connected to hydrothermally altered terranes [16]. Similarly airborne gamma ray spectrometric survey is used to evaluate variations in the mineral composition, properties of soils and their parent geological material so as to map lateral lithological changes. This method involves the measurement of naturally occurring uranium (U), thorium (Th) and potassium (K), which could be originate as trace elements that present in rock forming minerals and soil profiles. Therefore, hydrothermal alteration reserves (sericite, biotite, K-feldspars and several K-bearing clay minerals) can be identified and mapped by using spectrometric data [5].

Gabal El-Rubshi area, CED, is considered as a rich region which contains familiar mineralization such as magnetite, chromite, copper and talc [12]. Although, geophysical characters of such mineralization are assumed to be similar, it is possible to have some differences. The current study treats with the advantages of airborne geophysical datasets to characterize the hydrothermal alteration zones which can help in indicating areas with potential for the detection of mineral deposits. Accordingly, the present paper aims [1] to identify hydrothermal alteration regions in Gabal El-Rubshi area; [2] to retrieve information about the surface and subsurface geology based on the analysis and explanation of magnetic and gamma ray airborne geophysical data, integrated with accessible geological information.

Geological setting:

There is a general concurrence that many or most of Egyptian ultramafics belong to the Alpine type or orogenic ultramafics characterizing the orogenic belts [12]. A geological map (Fig. 1) produced by Conoco [8] for Gabal El-Rubshi was enhanced in this work, based on new geophysical and extensive field data. Gabal El-Rubshi area is made up of serpentinite (Ser) and associated derivatives, arc volcaniclastic metasediments (Ms), metagabbro-diorite complex (mGb-D), older granites (Og), and pyroxenites (Py). The El-Rubshi ophiolite lies 40 km NE of Quseir City, at Gabal El-Rubshi, as a dismembered ophiolite. Its ultramafic section is well-developed, being dominated by serpentinites, in addition to peridotites, pyroxenites, chromitites, and talc-carbonate rocks [2]. It is emplaced in metasedimentary and metavolcanic rocks (Mv). The pyroxenites occur between the serpentinite and bodies of metagabbros. Along contacts, the pyroxenites are trimmed, serpentinized, and carbonatized [11].

The rocks of Gabal El-Rubshi are highly susceptible to alteration into talc carbonate rocks as a result of CO2 metasomatism [6]. Pyroxenites are mainly fresh but partially serpentinized particularly along shear zones. Pyroxenites are generally occur as the largest mass (13 km2, Fig. 1) which is intruded along the easterly dipping thrust fault operating between El-Rubshi serpentinites and metagabbro-diorite complexes (El-Desoky et al., 2014). The basement rocks are entirely bounded by faults [21]. The Meatiq shear zone is the major fault that bounded Gabal El-Rubshi from the west. It strikes NW - SE and approaches 1 km in width. The contact between the tectonic units of the Meatiq gneiss dome (Gn) and the overlying ED ophiolitic melange is a thrust fault along which the latter is thrust northwestward over the former [11].

MATERIALS AND METHOD

Data acquisition and processing:

The airborne geophysical surveying of the study area was conducted by Aero-Service Division [4] by using the Cesium Vapour magnetometer with a sensitivity of 0.1 nT and a high-sensitivity 256-channel spectrometer (50 l Nal "Tl" crystals). The obtained airborne magnetic and gamma ray spectrometric data were reduced, assembled and finally presented in the form of contour and composite maps at a scale of 1:50.000.

Assuming induced magnetization, a reduction to pole (RTP) filter was applied to the grid of the merged magnetic data. Application of the RTP filter results in magnetic anomalies that are properly positioned over their sources and, most importantly, allows for a more specific interpretation of magnetic source locality relative to geologic features. A range of imaging practices were applied to the airborne magnetic data to visually improve the effects of selected geologic sources using mathematical enhancement techniques including first vertical derivative (FVD) and analytic signal (AS). The vertical derivative map is much more approachable to local influences than to regional effects and therefore tends to give sharper image than the map of the total field intensity. Analytic signal filter is useful as a type of reduction to the pole which was applied to magnetic data collected from the area. This filter gives a representation of the magnetic anomaly of the causal body which depends on the position of the body (horizontal coordinate and depth) and not on its magnetization direction. Structural analysis can be more extended to source body depth investigations. The 3D Euler deconvolution was applied to estimate depths to the magnetic anomalous sources [14]. In this method, depths and horizontal positions are intended by solving linear systems of Euler's homogeneity equations [9], where the knowing parameters are: (i) an anomalous magnetic field; (ii) its gradients along the -x, -y and -z directions; and (iii) a structural index (SI), which is a parameter that provides information about the geometry of the anomalous sources. Solutions were obtained for SI equal to 0. A 7 * 7 window was chosen to approximate each solution associated to each grid cell based on the mean size of the magnetic anomalies of the study area.

In the other hand, the interpretation of spectrometric data is more similar to interpreting the consequences of a conventional geological survey [15]. Airborne gamma ray spectrometric data are classically displayed as images. Total count and individual radioelements are displayed as pseudo-colored images to show the supply of the three radioelements or as abundance ratio images. The abundance ratios, eU/eTh, eU/K and eTh/K, are often more diagnostic of changes in rock types, alteration, or depositional environment than the values of the radioisotope abundances themselves, which are subject to wide variations due to soil cover, etc [15]. The ratio images can be used to emphasize subtle variations in the data. The individual radioelements are combined as false color composites in ternary radioelement image. The ternary map is often used to get an indication of radioactivity distributions and, consequently, enables narrowing down favorable target areas for detailed ground follow up for mineral exploration. The F-parameter of Efimov approach [10], was applied to assess the potential of the gamma ray spectrometric method characterization of areas of change associated with hydrothermal occurrences at Gabal El-Rubshi. The expression for the F-parameter is K* U/Th or K/(Th/U) or U/(Th/K), which can be reflection of as the ratio of K abundance and Th/U or as the ratio of U abundance and Th/K. The F-parameter is an important alteration indicator of rocks. Usually, for unaltered rocks, the F-parameter value is no more than 1.2-1.3, but for altered rocks, it can be 2-5 and sometimes above 10 [1]. The resulting images to be discussed later on comprise colors generated from the virtual intensities of the three components and represents subtle variations in the ratios of the three bands.

Airborne magnetic results:

Figure 2 is a digital elevation model of the area with the high ground located at the western (H1), eastern (H2) and southern part (H3) divided by moderately elevated regions (L2). A low elevated regions (L1) is observed at the northern and northeastern parts of the area. Total magnetic intensity (TMI) anomalies (Fig. 3a) highlight the structural framework and main geological features in the area. The RTP map (Fig. 3b) sharpens the contacts between the magnetic high and low patterns, in which the definitive boundaries of Mv with the surrounding units are well defined and appreciated as compared to Figure 3a. According to the magnetic amplitudes, the RTP anomalies (Fig. 3b) are specified into three main domains (low, medium and high). The high elevations regions, H1 and H3 tend to lie in very high magnetic regions corresponding to the Ser and mGbD units (Figs. 2 and 3b). Strong but discontinuous anomalies are associated with isolated Ser stringers at the southwestern corner of the study area (Fig. 3b). Areas of moderate elevation (L2) is related with moderate to low magnetic anomalies corresponding to the Mv. In the other hand, the Ms unit (Fig. 3b) shows variable magnetic amplitudes at different elevated regions (Fig. 2) within the area.

Figure 4a was generated from the TMI by applying the AS filter which shows the NW-SE trending Mv and Ms magnetic anomalies and several distinct magnetic zones. This filter just like the RTP also delineated the Ser, mGb-D and Mv (not metamorphosed) which showed high magnetic anomalies whereas the metamorphosed Mv, Ms and Gn, recorded the low magnetic anomaly in Fig. 4a. Comparing this map (Fig. 4a), with the TMI (Fig. 3a) and the RTP color contour maps (Fig. 3b), the difference is obvious along the edges of the Mv-Ms contact in the northwestern and the Ser-Mv contact in the north central portion of the current area. The AS amplitude exploits over the edge of the magnetic structures and was used to delineate the boundaries of lithological units (Fig. 4a).

Moreover, the FVD method has helped in attenuating broad, more regional anomalies and enhanced local, more delicate magnetic responses because of being sensitive to shallow magnetic source bodies and contacts. Assessment of the FVD image (Fig. 4b) with the TMI map (Fig. 3a), depicts a clear enhances observed of structural features particularly in the northern and southern regions of the study area. The elongated Mv unit seen in the TMI and RTP maps is rather exposed in the FVD map as a faulted structures trending in the NW-SE direction. The Ser and mGb-D lithological units also have their boundaries well improved by the low magnetic anomalies forming relatively elliptical shapes. As shown on the AS map (Fig. 4a), there are similarities between it and the FVD map (Fig. 4b). The major shown NW-SE directions on the FVD map are represented also on the AS map with high maximum amplitude of 1.6 nT/km.

Airborne gamma ray spectrometric results:

Several distinct mapped spectro-lithologic units are easily distinguishable in the spectrometric data (Fig. 5). As obvious, low values of total gamma radiation count (Fig. 5a) are associated with outcrops of the mafic/ultramafic complexes. Whereas, areas with high values can be allied to the presence of Gn unit. The anomalies associated with the Ser are more challenging to distinguish from the anomalies associated with the mGb-D ones. A number of K anomalies are evident in the spectrometric image (Fig. 5b), which it is essentially common in the felsic Ms as indicated by the elongated high anomalous zone (~30%) at the west part and are low in mafic rocks. Hence, the weak K signature is indicated the Mv ones, except at the NE and south directions which noted moderate K concentration. Figure 5c shows a constructive correlation between low Th values and spatial positions of mafic/ultramafic rocks. Thorium is commonly considered very immobile (22) thus the provinces with low Th concentration suggests that it was mobilized in hydrothermally altered systems. Consequently, the low Th pattern appeared for the Mv unit at the north and south regions (Fig. 5c) indicates the leakage of Th elements by hydrothermal fluids through faults and shears (Fig. 1).

Unlike the K and the Th maps, the U map (Fig. 5d) could not clearly indicate distinct boundary between the Ms-Mv and Ms-Ser at the northwestern and the central parts of the area. Figure 5d is characterized by unusually moderate U signature at the central region of the area that correspond with serpentinites (Ser). Whereas the northeastern parts and the northwestern corner of area are associated with weak U signature (Fig. 5d) which coincide with Gb-D and Mv units. The Ms also produced curiously moderate intensities of immobile and mobile Th (Fig. 5c) and U (Fig. 5d) respectively, even though some high trace of these radioelements can be marked within the formation. These signatures are marked as the result of highly weathered colluvial deposits into the lowlands.

Figure 6 shows an estimate of the original equivalent U concentration (OUC). It is obtained by dividing equivalent Th concentration map by 3.5 (3), which suggests how U was originally dispersed in the area to compare with the present U concentration map (Fig. 5d).

This comparison suggests that U may have been leached and redistributed in mafic/ultramafic rocks along joints, faults and wadies.

Figure 7a represents Th/K ratio concentrations which map some lithologic contrast and enhanced alteration signatures. The increase in K content and decrease in Th/K ratio observed for the Ser and MV units is indicative of hydrothermal alterations. This is because K enrichments are not accompanied by Th during hydrothermal alteration processes (5). Reduction in the Th with increase in K occurring at the contact zone between Ser-Mv and Ser-mGb-D in the middle region (Fig. 7a). Generally, K and other metal constituents are added to the mass rock by hydrothermal solutions, and it is easily observed in mafic/ultramafic units or along lithologic contacts where hydrothermal alteration such as silicification is intensive. The U channel and U/K (Figs. 5d and 7b) ratio are used to determine areas where contents of U are relatively strong. A number of the U intensities are associated within the Ser and Mv. The Ms and some parts of mGb-D units (at the NE direction) showed high K concentration (low U); in addition, low U/K (Fig. 7b) over potassic formations within the concession are likely to be characterized with highly leached soils. The ratio image of U/Th (Fig. 7c) shows that the majority of the area except the central and some regions in the southeastern quarter is dominated by high Th concentration, which highlights Th alteration zones. The mGb-D, Mv and some spots in Ms rock units are seen to record high Th concentration and low U concentration in figure 7c (black polygons).

The F-parameter anomaly values (Fig. 7d) over the area show an obvious contrast that we can discriminate between mafic/ultramafic (blue zones) and Gn unit (from green to pink zones). High F-parameter anomaly values (more than 2) reflect that our area is suffered from high alteration. The potassic alteration metasomatism of mafic/ultramafic rocks can be indicated by F values ranging between 3 and 7 (green and orange anomalies imbedded in blue zone) as shown in figure 7d.

The RGB ternary map of K, U and Th channels (Fig. 8a) provides additional information when compared to individual channel maps (5). The white areas are indicative of high concentration of K, Th and U resulting from Meatiq gneiss and granitic rocks (Fig. 1). Whereas the mafic/ultramafic units appear dark that indicating lower concentrations in K, U, and Th. The magenta shows areas of high K and Th but low U concentrations (Fig. 8a) while greenish colored areas indicate a predominance of U content. The ternary map shows high Th concentration at some spots in the northeastern and central regions of the area. In order to help delineate a hydrothermal alteration zones derived from the spectrometric data in the present study, the ternary image (Fig. 8a) was used to first outline the major spectrometric domains (Fig. 8b) which depict different radioactivity levels. Figure 8b defined 8 spectrometric domains with similar gamma radiometric signatures, classified consistent with the concentration of the radioelements K, eU and eTh. Each radioelement was qualitatively classified into very low, low, medium, high and very high concentration, according to the contribution in the ternary images, defined by these images color.

Structural features:

Specifically, there are strong correlations between structures and major hydrothermal alteration zones (1820). Gamma ray spectrometric method is the most economical technique of providing data on a wide scale of structural trends (19). In our study area, The major gamma ray spectrometric structural features (orange color lines) were interpreted from the total count (TC) image, with illuminations between 45[degrees] and 315[degrees] (Fig. 5a). The structures interpreted through gamma ray spectrometry are shallower than those interpreted from the magnetic data and are coupled with contrasting values of the measured radioelements, which may represent areas of structural weakness. The analysis of this structure shows the prevalence of NW-SE lineaments with subordinate NS and E-W lineaments while NE-SW lineaments existed as minor features (Fig. 9).

Processed magnetic field may uncover or enhance structural features that are not readily observed from total field data. Consequently, the structural features, airborne magnetic lineaments and their different alignments in the study area were deduced from FVD and AS maps (Fig. 4). The extracted airborne magnetic structures (Fig. 9, green and blue color lines) have been examined and correlated using frequency rose diagram for the study area. Two main lineament sets were observed in the study area, with the major directions of the structure sets in NW-SE and E-W directions and a minor set in NE-SW direction (Fig. 9).

In this study, ED method was also used to map the subsurface structures and contacts; it was applied on RTP map to give more correct results. 3D Euler solution for the airborne magnetic data of the study area, at Structural Index (S.I = 0) have been produced (Fig. 10). The results show that, several subsurface faults have been inferred from the modeled airborne magnetic data (Fig. 10) which were not documented in surface geological map (see Fig. 1). However, most of the delineated geological faults were found coincident with the interpreted airborne magnetic faults (green and blue lines, Fig. 9). Moreover, the modeled magnetic data showed that the unreliable faults may extend down for several kilometers. The greatest depth obtained for the modeled faults is more than 1500 m. These deepest faults (blue circles) have NE-SW trend at the northern part of Gabal El-Rubshi (Fig. 10). While, most of the modeled subsurface faults reached 500 m depth.

Discussion:

The airborne geophysical data worked in this study permitted a synoptic vision of the hydrothermal alteration in the lithological units and tectonic structures of Gabal El-Rubshi. By the integration between the interpreted geophysical maps, we can define the following:

> The Ser unit appears in dark tones with green colored areas (Fig. 8a), indicating a relative dominance of U content; although it shows low absolute values for the three channels. This may indicate that a change in chemical conditions led to enrichment in this radiometric value. Such enrichment can be interpreted as evidence for hydrothermal alteration. The majority of this rock unit has an intense magnetic values (Fig. 3b) except at the west and south borders which indicates alteration at the contacts between the different rock units.

> The Mv unit at the northern part has lower radiometric (Fig. 5a) and magnetic (Fig. 3b) values than at the southwestern part. The low magnetic values are related to the metamorphism of this unit into greenschist facies (2). In general, it signifies by a virtual dominance of Th and K or U at some spots; whereas it shows low radiometric values. The increase in Th and K can be interpreted as a proof for alteration through fractures and faults, and weathering products, respectively.

> As similar as the Ser, the mGb-D has a strong magnetic values almost reach to 1364 nT (Fig. 3b) that suddenly decreased to -144 nT at its southern boundary. In the other hand, this lithological unit has a prevailing Th content (Fig. 8b, grey domain) than the other radioelements.

> The strong K concentration and low magnetic values in the area (Fig. 8b, pink domain and Fig. 3b, respectively) associated with the Ms lithological unit which it appears in bright magenta tones (Fig. 8a). It occupies the eastern and western sections of the study area; the variation in U and Th radiation contents is largely due to differing environmental conditions during diagenesis or alteration of these Ms unit. The alteration in this unit may be associated with the occurrence of NE-SW shear zone that is related to regional Meatiq Shear Zone system.

> The appliance of spectrometric method helped in detecting the Py unit at the east to the Ser which indicated by the increase in U content that coincides with the geological map (Fig. 1). Pyroxenites occur as veins and dikes crosscutting ophiolitic upper mantle peridotites. The formation of Py dikes and veins can be attributed to the metasomatism of mantle peridotite by melt or fluid migration resultant from the subducted slab. Another possibility for the formation of Py dikes and veins is through the intrusion of discrete magmas derived from a highly depleted mantle source (11).

> The lineament interpretation has showed that the area is topographically complex, highly dissected (Fig. 9 and 10). Two main surface lineament sets were observed in the study area, with the major orientations of the lineament sets in NW-SE and NE-SW directions and a minor set in N-S and E-W direction. The NW-SE set of fractures is current in most parts of the study area. This set represents the strike slip fractures as parallel faults for example Meatiq Shear Zone (MSZ) which crosses at the west part to the area in a NW-SE direction with relatively 40 km length. The set is related to the extensional phase of ANS evolution started in the late Proterozoic during the final stages of the Pan-African Orogen. The NE-SW lineaments trend set dominate the study area. This trend represents to the tectonic event of the Gulf of Aqaba.

> The 3D Euler solution of the study area shows that the depth of magnetic cause body ranges from hundreds of meters (Fig. 10) to more than 1500 m. The 3D Euler solution reflects the scenery of the basement and its associated structures. Highly faulting plays a significant role in facilitating the alteration processes through the circulation of both hydrothermal solutions and/or meteoric water causing enrichments in elements like U and Th (Figs. 5 and 7).

> From the previous results, the majority of the hydrothermal alteration zones in the study area are indicated at the shear contacts intersection between different lithological units. To generate an approving map of the relation between the structure features and the hydrothermal zones in the area (Fig. 11) by using Arc Spatial Data Modeler (ArcSDM), The first step to map regions with high radioelements values that coincide with elevated mafic/ultramafic rocks. The next step was to draw on the map the interpreted structures and shear zones extracted from magnetic data. Three buffers were created around the interpreted structures. Each buffer is 200 m wide, which delineates areas of 600 m around the structure (Fig. 11).

Conclusion:

The airborne magnetic and gamma ray spectrometric data are helpful in delineating the geologic units and likely hydrothermal alteration zones of the study area. The application of the improvement filtering algorithms such as the RTP and AS to the magnetic data aided in the mapping of the different lithological units in the area. Although changes in the radioelement ratios are slight, they can be detected constant in regional gamma ray data at numerous data points and along closest flight lines. The composite image procedure applied to the radiometric data facilitates the relationship and delineation of lithological units based on faint differences in the radioelements concentrations. The examination of the geophysical datasets in the study area provides fresh information into structural construction. This exposes enhanced structural features that include shear zone, faults, shear and fault junction and fracture systems as magnetic anomalies that essentially trend NW-SE and NE-SW. Evolution of the different lineaments within the area of interest implies that the study area is geodynamically very active and also promising area for mineral resources. On the other hand, the validation of such approach by petrological studies (carrying out geochemical investigations on some collected rock samples) is required to reinforce our preliminary investigation of potential and valuable mineralization within the hydrothermal alteration zones.

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(1) Mohamed Eleraki, (2) Baher Ghieth, (3) Nehal Abd-El Rahman, (1) Sara Zamzam

(1) Geology Department, Faculty of Science, Zagazig University, Zagazig, Egypt,

(2) Exploration Division, Follow Up Department, Egyptian Nuclear Materials Authority, Cairo, Egypt,

(3) Geological Applications and Mineral Resources Division, National Authority for Remote Sensing and Space Sciences, Cairo, Egypt,

Received 12 May 2017; Accepted 5 July 2017; Available online 28 July 2017

Address For Correspondence:

Sara Zamzam, Zagazig University, Geology Department, Faculty of Science, Box., 44519, Zagazig, Egypt. E-mail: sarazamzam87@gmail.com

Caption: Fig. 1: Geological map of Gabal El-Rubshi, Central Eastern Desert, Egypt (After Conoco, 1987).

Caption: Fig. 2: Digital elevation map of the study area.

Caption: Fig. 3: The total magnetic intensity (TMI) of the study area (a) reduced to the magnetic pole, RTP (b).

Caption: Fig. 4: (a) Analytical signal magnetic (AS) and (b) First vertical derivative (FVD) maps of the area.

Caption: Fig. 5: Gamma ray spectrometric images for: (a) total count (TC), (b) potassium (K) concentration, (c) thorium (eTh) concentration and (d) uranium (eU) concentration.

Caption: Fig. 6: The original uranium map (eTh / 3.5) of the study area.

Caption: Fig. 7: Gamma ray spectrometric images for: (a) ratio of eTh/K, (b) ratio of eU/K, (c) ratio of eU/eTh and (d) F-parameter of Efimov contour map of the study area.

Caption: Fig. 8: (a) the Ternary image (RGB=K, eU, eTH) of the spectrometric data and (b) the interpreted spectrometric domains map for the study area.

Caption: Fig. 9: Interpreted structural map extracted from TC (orange lines), AS (green lines) and FVD (blue lines) images with rose diagram for each one.

Caption: Fig. 10: 3D Euler solution from the analysis the airborne magnetic data over the study area, with structural index (S.I= 0).

Caption: Fig. 11: Buffers around structures interpreted from the FVD and AS of the airborne magnetic data.
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Author:Eleraki, Mohamed; Ghieth, Baher; Rahman, Nehal Abd-El; Zamzam, Sara
Publication:Advances in Natural and Applied Sciences
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Geographic Code:7EGYP
Date:Jul 1, 2017
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