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Geofísica Internacional (2014) 53-3: 309-319

Original paper

Role of Lithology and Subsurface structures detected by potential Natash area, Eastern Desert, Egypt

Shadia Elkhodary* and Taha Rabeh

Received: September 03, 2013; accepted: October 15, 2013; published on line: July 01, 2014

S. ElkhodaryGeology DepartmentFaculty of ScienceTanta UniversityTanta, Egypt*Corresponding author: [email protected]

Abstract

Wadi Natash area is located in the southern partof the Eastern desert of Egypt. It has a greatimportance for containing accumulations fromthe radioactive minerals of Uranium, Thoriumand Potassium. An integrated potential studywas carried out on the study area with theaim of locating depths to causative bodies withsufficient magnetic susceptibility that mayrepresent magmatic intrusions with relationto the radioactivity location and delineatethe subsurface structures affecting the area.Both magnetic and Bouguer data as well asradiometric data were interpreted rapidlyfor source positions and depths using Eulerdeconvolution, Werner deconvolution and 3Dmodeling techniques. The results deduced fromthe trend analyses show that the major faulttrend affecting the area have NNW-SSE (RedSea–Gulf of Suez trend) direction intersectedby the less predominant NNE-SSW(The Gulf ofAqaba–Dead Sea trend) and WNW-ESE (NajdFault System) fault trends. The causative bodies

were imaged at depths ranging from 0.3 km toabout 1.5 km. The depths along the interpreted markers due to presence of the NNW-SSE faulttrends act as pass channels for the hydrothermalsolutions.

It can be stated that the radioactive mineralaccumulations were caused by the hydrothermalsolutions rich with radioactive minerals as aresult of intruding Natash volcanic to the graniticrocks. The Qouseir clastics and the Nudian sandstone were affected by these solutions and showa positive response for the radioactive minerals.

Key words: magnetic, lithology, radioactiveminerals, 3D magnetic model.

La zona de Wadi Natash se encuentra en laparte sur del desierto del este de Egipto. Tieneuna gran importancia, ya que cuenta con granacumulación de minerales radiactivos, comouranio, torio y potasio. En esta zona se llevó acabo un estudio de potencial integrado, con elobjetivo de localizar a profundidades factibles para representar intrusiones magmáticas

relacionadas con la radiactividad del lugary delinear las estructuras del subsuelo queafectan la zona. Tanto los datos magnéticos yde Bouguer como los datos radiométricos seinterpretaron rápidamente por la posición dela fuente y las profundidades. Lo anterior fuefactible al utilizar la deconvolución de Euler yde Werner, además de técnicas de modeladoen 3D. Los resultados deducidos del análisisde tendencias muestran que la tendencia dela falla principal que afecta a la zona tiene unadirección de NNW-SSE (Mar Rojo-Golfo haciaSuez) y atraviesa por el menos predominanteNNE-SSW (El Golfo de Aqaba-dirección Mar

Muerto) y por WnW-ese (sistema de falla Najd). a profundidades que van de 0,3 kilómetros hasta1,5 km. Las profundidades a lo largo de lasdiscontinuidades se deben a la presencia de lafalla NNW-SSE y actúan como canales de pasopara las soluciones hidrotermales.

Puede afirmarse que las acumulaciones deminerales radiactivos fueron causadas por lassoluciones hidrotermales ricas en mineralesradiactivos como resultado de la intrusiónvolcánica Natash a las rocas graníticas. Losclásticos Qouseir y la piedra de la arena Nudian

se vieron afectados por estas solucionesy muestran una respuesta positiva de losminerales radiactivos.

Key words: magnético, litología, mineralesradiactivos, modelo magnético en 3D.

T. RabehNational Research Instituteof Astronomy and GeophysicsCairo, Egypt

309


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S. Elkhodary and T. Rabeh

310 Volume 53 Number 3

Introduction

The area under study is located in south EasternDesert .It is delineated by longitudes 33° 00 to34° 30’ E, and latitudes 24° 00’ and 25° 00’N(Figure1). Wadi Natash volcanics cropped outin different spots at the eastern part of theinvestigated area as alkaline basalts and somesmall trachytic intrusions which erupted duringthe Upper Cretaceous associated with regionaluplift preceding the northern Red Sea rifting(Figure 1).

This study deals with an integratedinterpretation of the observed aeromagneticand gravity data of Wadi Natash area basedon application of gradient (Euler, Wernerdeconvolution, trend analysis and threedimensional modeling) methods to study theeffect of subsurface structures and lithology in

controlling the radioactive accumulations.

Geologic Setting

The considered area is topographically gradedbetween gentle to rough topography whichtraversed by many wadis trending NE-SW, NW-SE, WNW and E-W controlled by the structuraldirections and the rock types of the area.

The study area is covered with a varietyof basement and sedimentary rocks rangingin age from Precambrian to Quaternary(Figure 1). Precambrian rocks consist of acidicmetavolcanics (MVa), metasediments (MS) the southern eastern part of the area. Thenorthern eastern part of the area is coveredby Upper Cretaceous volcanics (wadi natashvolcanics) and trachyte plugs (Hashad, et al .,1982). Meanwhile, the Upper Cretaceous Nubiansandstones cover the central and western parts

of the area represented by (Abu-Aggag (Ku) andUm Barmil (Kub) Formations). Late cretaceousshall and limestone represented by Dakhla (kud)and Kiseiba (Tpl) occupying the northwesterncorner of the area, (Conoco Report, 1989).

Sahara Swell is that of Wadi Natash in thesouthern Egypt. Natash volcanics crop out inthe study area at the southeastern corner,northeastern, northwestern, and southwesterncorners of the study area. The volcanic rocksand their volcaniclastic sediments are nearlyparallel to the boundary between the (upper and which trends NNW-SSE. This plateau is formed extends laterally for more than 6km. They alsoinclude a number of cones and sills of olivinebasalt. The cones are cone-shaped features,rising up to 26-86 m above wadi level, while sillsare always encountered in the Nubian Formationin the form of flat lying bodies interbededwith the sandstones and measure up to 7min thickness (Hashad, et al., 1982). Ibrahim

of laterites-bearing REEs in Natash area whichform as horizons at the boundary between the

The volcanic sequence is represented by sequences of volcaniclastic sediments. Each of

Figure 1. Location map and surface geologic map ofthe study area after CONOCO (1987).


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Geofísica Internacional

July - September 2014 311

in composition upwards from alkali olivinebasalts through trachybasalts, trachyandesite,to trachytes. The volcaniclastic sedimentscomprise agglomerates and tuffs which containleaf imprints of Cenomanian age (Crawford etal .,1984), which are in good agreements witha 90Ma K-Ar age for the lavas determinedby Ressetar et al., (1981). The age of thesevolcanics suggests that they are not directlyassociated with the actual mid-Tertiary Red Searifting but their alkaline nature implies that theywere involved with a pre-rifting doming process(Crawford, et. al ., 1982).

Structurally, Noweir, et al. (2003) concludedthat the rocks of Wadi Natash have generallinear trend outcrops conformable with thegeneral structure trend. The linear trendsuggests that these volcanic bodies were

brought up along the intersection of the mainNNW to NW fault trend with the E-W faultdirection during the Turonain age.

An aeromagnetic survey conducted by AeroService, (1984) was used to interpret theregional subsurface structures prevailing in thestudy area along with gravity data obtained fromthe 1-arc-minute gravity anomaly (mGal) grid bySandwell and Smith (1997). The aeromagnetic The aircraft was equipped with a tail-stinger ofnon-magnetic plastics of 1.5 meters long at therear of the fuselage. The magnetic data weredigitized by computer digitizing programs andboth Bouguer and the total magnetic anomalymap were obtained (Figures 2 and 3).

The aeromagnetic data was digitized into acomputer using a suitable Surfer Program

Figure 2. Total Intensity aeromagnetic map of study area (after Aero Service, 1984).


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Figure 3. fault trends.

processing was reduction to the magnetic northpole (RTP). This procedure has the advantage ofcompensating for the shift between sources andmagnetic anomalies due to the non-verticality Only induced magnetization was considered.Inclination and declination of the normal conducted in the Fourier domain (Blakely, 1995).The results are presented in (Figure 4) for theaeromagnetic survey. Both the Bouguer anomalyand the RTP aeromagnetic maps (Figures 3 and4) indicate that most anomalies are aligned

to NW, NE, and E-W which may be related tothe Red Sea, and Gulf of Aqaba tectonics. Thenegative anomalies in the central part may bedue to deep depths to the basement rocks orthe lithology of negative magnetic effects.

The RTP aeromagnetic anomaly map (Figure4) shows low and high frequency magneticanomalies distributed along the study area. Theelongated positive magnetic anomalies with amaximum value of 42850 nT were observedover the locations of southwest, west andnorth the study area. They are characterized

by high frequency and high amplitude. Suchmagnetic anomalies are associated with acidicmetavolcanics, metasediments and calc alkalinegranites, Natash volcanic which are mainlycharacterized by high magnetic susceptibilities.These magnetic anomalies are bounded bysteep magnetic gradients, which indicated thepresence of two fault systems, trending in theNW-SE direction (Red sea) and NE-SW (Aqabatrend). The RTP map is marked by negativeanomaly. A strong NE-SW trending anomalytruncated by the E-W trending is observed in thenorth-eastern and central parts of study area.

The variations in gravity anomalies werenormally caused by variations in the density ofsubsurface rocks (Reynolds, 1997), and theyusually indicated faults or lithological contacts.The Bouguer anomaly map of the study area(Figure 3) was generally in good coverage andit showed gravity values between +50 and -30mGal. It is characterized by a group of positiveand negative anomalies of different intensitiesand almost NE–SW, NW-SE and E-W orientation.The strongest positive gravity anomalies (50mGal intensity) lie at the NW and SW corners of


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Figure 4.

the study area which associated with the Natashvolcanic and the granitic rocks and is separatedfrom the negative gravity anomaly by a verysteep gradient. This steep gradient is of NNWtrend and could be interpreted to be a normalfault down-throwing to the east. On the otherhand the eastern side of the concerned areathere is a large high negative anomaly (-30mGalintensity) trending NNE-SSW, associated withthe Qena clastics and the limestone. Thislithological variation on both sides of the faultas well as the magnitude of fault throw beingupthrown to the west is behind the strongdifference in the intensity of both positive and

negative gravity on both sides of the fault.

a) Trend analysis

Other treatments of Potential Field data includetrend analysis to delineate the subsurfacefault trends based on the theory of Grant &West (1965) and on the (1967) was applied to all RTP magnetic and the gravitymaps. The peaks of the gradient curve were

then connected together to show the deducedstructure lines.

Close investigations of these maps (Figures3 and 4) indicate that there are three mainstructural elements which aligned in directionthat bounded both magnetic and gravityanomalies. These elements can be grouped inthe following zones:

1) The NNW-SSE (Red Sea–Gulf of Sueztrend) structural direction as the mainstructural elements occupying the centraland the southern parts of the study area.

2) The NNE-SSW (The Gulf of Aqaba–DeadSea trend) structural direction with lessabundance in both maps. This trendlocated at the northern and southern partsof the area.

3) The WNW-ESE (Najd Fault System)structural direction which lying in thenorthern and southern corners of thestudy area which is the least prevailingfault system.


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b) Werner deconvolution method:

Werner (1953) developed a method foridentifying the geometry of magnetizationcontrasts based on successive determinations depth extent, perpendicular to the measurement source of the magnetic anomaly described by aset of four or more contiguous measurements.The basic expression for the magnetic anomalyof a single dike is:

T x A x x B

x x z

z( )( )

( )=

− +

− +

0

0

2 2 (1)

where x0is the location of the magnetic step, z is

be determined, and x is the location at which wewant to compute the magnetic anomaly. When location, we infer that the model describes themagnetic interface well. This approach was used

Figure 5. Werner de-convolution showing thedepth to the basementrocks and the subsurfacestructures along A3–A3’ (location shown inFigure 4). b) G2-G2’ (location

shown in Figure 3).

by Ku and Sharp (1983), in a method that has

We applied the Werner deconvolutionmethod along set of profiles A-A’ of theaeromagnetic map and G-G’ of the gravitymap Using clustering method. The outcomesfrom application clustering method allow us toshow the interfaces of the subsurface structuresfor the aeromagnetic and gravity profiles.The deduced results (see Figure 5a and 5b)illustrated the exact depth of the magneticand gravity sources where Werner solutionsare clustering to each other. Analysis of these ranges between 1.5 to 0.3km, which is nearlyclose to the depth of the volcanics deducedby Elawadi and Ismail (2006).The shallowestdepths are found at the northern and southernparts of the study area where Natash volcanics

and granitic rocks outcrop while the deepestparts occupy the central and western parts of thearea where the Nubian sandstone and Qouseirclasts are found.


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c) Euler deconvolution method

The Euler deconvolution method delineatesgeological boundaries of magnetization ordensity and gives-estimates of depth to thebasement. Also, this method can be used todetermine the subsurface structures (Grauch,et al ., 2001). Neil (1990) and Neil et al .(1991)applied this technique to magnetic data at Leeds.Few gravity applications have been reported. Inthis study the application of this technique toboth gravity and magnetic data were attempted.The method was originally devised by Thompson 3D gridded data by Reid et al . (1990).

The Euler deconvolution method was interpretations (Figures 3 and 4). The windows

size used in the process was 11 with applyingthe suitable dimensions of coordinates. Thewindow slides and the solutions were calculated.The output of this method was representing bythe horizontal and Euler solutions. Reid (2003)explained the significance of the deducedstructural indices and listed them in Table (1).

Table 1. Structural indices of Euler, after Reid(2003).

Source Type SISphere or compact body at a distance 3cylindrical pipe 2

Thin sheet edge (sill, dike, etc.) 1Fault (small step) 0

deduced subsurface structures. The peaks along structures. It can be noticed, that Euler solutionsare clustering along these structures (Figures6 and 7). Also It can be noticed that the mostsuitable structural indices (SI) representing themagnetic structures are restricted between 2and 4 SI indicating an intruded magnetic bodies.

The results from the both magnetic and Figures 6 and7) show that the Natash volcanic extends to arelatively shallow depth (300-100) meanwhilethe faults which may be the cause of thisvolcanic trending NE-SW with depth reaching

Figure 6. a) 3-D magnetic b) 2-D Euler deconvolutionand horizontal gradient two fault systems alongProfile A2–A2’ (location

shown in Figure 4).


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more than 2km. It is noteworthy that thisgeophysical subsurface interpretation coincideswith the geophysical results by El Gammal etal (2013).

d): 3D magnetic modeling

to a volume v of rock which is magnetized witha dipole moment per unit volume M (Grant andWest, 1965) is:

A(r)= (2)

Where: is a volume integration over the body,r is the distance restricted to the xz plane andr

0 is the distance from point A to the centre of

the magnetic body.

If the magnetic body is not uniformly

magnetized then the extent of the body isbetween y and –y axis and the observationpoints will be made along the x-axis acrossthe middle of the body. Therefore, the threecomponents of the magnetic intensity shallhenceforth be referred to as the magneticanomaly given by equation (2) for the magneticbody is:

Ax = 2M

xP

x + 2M

zQ, A

y = -2M

yR and A

z = 2M

xQ

- 2MzP

z.

Where:

Px = a2U/ax2, P

z = - a2U/az2, R = - a2U/ay2 and

Q = -a2U/axaz2

and U is the Newtonian potential expressed by:

U = 1/2 (3)

Based on Grant and West, (1965) andTalwani (1960) theories, the 3D modelingtechnique was applied to the magnetic datausing software. The technique was applied to the different direction covering almost the area

(Figures 3 and 4). Inspection of the magnetic four bodies representing different lithological showing the highest susceptibility unite (0.01SI) representing Natash volcanic in the northern the prevailing fault trends NNW and NNE.

Figure 7. a) 3-D gravity model deconvolution and horizontalgradient confirming presenceof the two fault systems along

Figure 3).


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of four bodies representing different lithologicaland tectonic units. This model shows the highdensity block (2.6g/cm3) representing Natashvolcanic located at the eastern part of themodeled area. The two fault systems trendingNNW and NNE are clearly found with reachinga depth of more than 2km.

The interpreted structural features coincidewith the Euler deconvolurion of the same trend.

e) Interpreted Structural map with prospectedradioactive areas:

Using the radioactive minerals map (Figure 8)established by Elawadi et al . (2004) we wereable to detect two zones with high concentrationof radioactive minerals accumulations. Theystated that the radioactive anomalies have

been interpreted in younger granites and inadjacent the Nubian sandstones under WadiNatash exhibits of strong radiometric responsesand can be a good target for further mineralexplorations. The radiometric data indicate thatthe granitoid rocks are larger than that presentin the geologic map and indicate presenceof emanations of radioactive minerals fromgranites to the surrounding metamorphic rocksdue to contact metamorphism. These dataindicate that dykes cause a zone of radiometricpotential in the surrounding sedimentary rocks(El Gammal, et al ., 2013).

The regional tectonic framework of thestudied area (Figure 9) was established usingthe integration of all results of interpretationof the aeromagnetic and gravity analyses, in

addition to the horizontal gradient method inwhich the peaks of the curves were plottedand connected together to show the possiblestructural fault lines affecting the study area.Also the 3D modeling to detect the subsurfacestructures and the depth to the basement rocks Field data indicated that the most dominanttrend in the area is of NNW–SSE related to theRed Sea- Gulf of Suez tectonic trend dissectedby the second dominant one trending NNE–SSWrelated to the Gulf of Aqaba–Dead Sea trend andthe WNW-ESE related to the Najd Fault Systemtrend with less contribution in the area underconsideration.

The NNW-SSE fault trends are connectedwith the Red Sea rifting. They occupy thedetected zones of high radioactive mineralsaccumulations (Figure 9). According to Elawadi

et al. (2004), the Qouseir clastics zone ischaracterized by radioactive Uranium minerals.Thus we can conclude that the radioactive zonesare connected with the Red Sea tectonics andthe intruded granitic rocks. The NNW-SSE majorfaults are controlling the hydrothermal solutionsassociated with the intrusion of granitic rocksand may be the cause of the formation ofradioactive minerals.

Conclusion

The Bouguer, Magnetic and RTP anomaliesmaps are used to delineate the subsurfacestructural features prevailed in the study area.They were interpreted using trend analysis, 2DEuler deconvolution, Werner deconvolution and3D modeling techniques. The results show that

Figure 8. Radioelementcomposite image (afterElawadi et al . 2004)showing zones withradioactive minerals

accumulations.


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the area is structurally controlled by major faultstrending in the NNW-SSE direction related to the

Red Sea tectonics. They are intersected by theless predominant fault structures which haveNNE-SSW and WNW-ESE directions.

The study area was affected by mass graniticrock intrusions. The depths to the intruded rocksreach from 300m to 1500m. The Natash volcanicis intruded to granitic rocks and overlyingQousuir clastics and Nubian sandstone.Theintegrated results from gravity, magnetic andradiometry indicate the relationship between theNNW-SSE fault trends and intruded granitic rocksas well as the radioactive mineral accumulationsexists. From the results deduced from all

the interpretations and analyses techniquesapplied to both magnetic and gravity data,it can be concluded that the Qousuir clasticsand the Nubian sandstone rocks were affectedby the intrusion of the Natash volcanic whichis post rifting the Red Sea and it would formradioactive mineral accumulation as a resultof hydrothermal solution. The NNW-SSE faulttrends act as path channels for these solutionsrich with radioactive minerals.

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