ORIGINAL PAPER

Structural control of El Sela granites and associated uraniumdeposits, Southern Eastern Desert, Egypt

Khaled Gamal Ali

Received: 31 March 2011 /Accepted: 15 November 2011# Saudi Society for Geosciences 2011

Abstract The El Sela area is a part of the basementcomplex of the Eastern Desert of Egypt and the Pan-African Shield. The area comprises outcrops of dismem-bered ophiolites thrust over arc volcano-sedimentary se-quence and intruded by different syn- to post-tectonicgranitoids. Structural analysis of the area enabled theseparation and definition of four structural episodes: (E1)folding–thrusting episode associated with the cratonizationof the arc/inter-arc rock association and the intrusion of thesyntectonic (Older) granites. (E2) Upright folding episodeassociated with the compression and shortening to theENE–WSW direction which is different from the NNW–SSE shortening direction during E1; at the end of E2, latetectonic granites were intruded. (E3) Post-tectonic graniticintrusion episode: two mica granite and granitic dikes wereintruded during this episode. (E4) Fracturing, faulting, andpost-granitic dike extrusion episodes caused different faultsthat took place after cratonization until the present. Thereare three generations of folds during ductile deformation(E1 and E2). The F2 folds are nearly coaxial (along ENE–WSW trend) with the F1 folds. The F3 folding is displayedby folds generally trending NNW–SSE. Therefore, theENE–WSW and NNW–SSE trends can considered aspreexisting discontinuities and mechanical anisotropy ofthe crust in the following structure episodes. Brittledeformation (E3 and E4) reveals the importance of thosetrends which control the multi-injections and many alter-ation features in the study area. During reactivation, asimple shear parallel to the inherited ductile fabrics wasresponsible for the development of mineralized structuresalong the ENE–WSW and NNW–SSE trends. So they can

be considered as paleochannel trends for deep-seatedstructures and can act as a good trap for uranium and/orother mineral resources. Most of the uranium anomalies aredelineated along ENE–WSW and NNW–SSE shear zoneswhere quartz-bearing veins bounded the lamprophyre dikeand microgranites and dissected them in relation to thesuccessive fracturation and brecciation corresponding to therepeated rejuvenation of the structures. Therefore, the struc-tural controls of the uranium mineralizations in the El Selaarea appear to be related to the interaction between inheritedductile fabrics and overprinting brittle structures.

Keywords El Sela . Eastern Desert . Egypt . Uranium .

Granite

Introduction

The Arabian–Nubian Shield (ANS) in NE Africa and WArabia is the largest tract of juvenile continental crust ofNeoproterozoic age on Earth (Patchett and Chase 2002).The arc associations of the Eastern Desert (ED) includevolcano-sedimentary rocks with rare sedimentary ironformations and carbonate beds of a possible back-arcsetting (Sims and James 1984). They are intruded by syn-to late tectonic dioritic–granodioritic plutonites. Thistectonomagmatic cycle ended with cratonization throughthrusting, low-angle shearing, and associated folding andculminated by the intrusion of granodiorites at ~612 Ma asin the Meatiq area (Sturchio et al. 1983; Abdel-Meguid1992). After cratonization, the Neoproterozoic crust wassubjected to regional NW–SE folding and intruded by aflood of large ion lithophile-enriched granites and Dokhanvolcanic flows (El Shazly et al. 1980). Associated molassesediments (Hammamat), mainly derived from volcanic

K. G. Ali (*)Nuclear Materials Authority,Maadi-Kattameya Road, Box 530, Maadi, Cairo, Egypte-mail: khaled_ali66@yahoo.com

Arab J GeosciDOI 10.1007/s12517-011-0489-y

stuff, were deposited in intracratonic basins (Grothaus et al.1979; Abdel-Meguid 1986, 1998). The younger granites(post-tectonic) are considered as the final stage of Pan-African magmatism which ceased at the Precambrian at500 Ma (El Shazly et al. 1980).

The post-tectonic granites are known to host the mostinteresting U mineral occurrences in Egypt (Ammar 1973;El Kassas 1974; Bakhit 1978; Abdel-Meguid 1981, 1986;Salman et al. 1990; Abdel Monem et al. 1998; Gaafar 2000;Ibrahim 2002; Abdel-Meguid et al. 2003; Gaafar 2005; Aliet al. 2005; Ibrahim et al. 2005, 2009; Abu-Deif and El-Tahir 2008). According to their favorability for hostinguranium deposits, the El Sela granites, southern EasternDesert, are related to the high U-favorability category(Abdel-Meguid et al. 2003).

Geological, geochemical, and γ-ray spectrometric datahave been integrated to produce a detailed descriptive studyof the El Sela granites. Two types of uranium deposits havebeen recognized: uranium vein type (Abdel-Meguid et al.2003; Ibrahim et al. 2005, 2009; Gaafar et al. 2006) andsurfacial type. The latter is due to the prevailing arid tosemi-arid weather conditions which have been favorable forforming a pedogenic surfacial uranium deposit in thenorthern part of the area (Ibrahim et al. 2009).

In context, the main aim of this paper was to constrainthe interaction between inherited ductile fabrics and over-printing brittle structures. That is of utmost importance todefine the structural control on the emplacement of the El

Sela granites and the main traps for uranium mineralizationwhich help in constraining the proposed drilling sites in thestudy area.

Geological setting

The El Sela area is bounded by latitudes 22°13′30″–22°19′00″ and longitudes 36°10′00″–36°19′00″ (Fig. 1). Thegranites cover about 80 km2 (representing as a big granitepluton) constituting a part of the Late Precambrian–EarlyPaleozoic Pan-African Orogeny forming the Arabian–Nubian Shield. Geologically, the El Sela area comprisesthe following rock units.

Ophiolitic mélanges

The northeastern part of the Sol Hamed ophiolite complexoccurs in the western part of the Sela area (Fig. 1), whereasthe main sequence of ophiolite mélanges crop out. They arerepresented by serpentinites, metagabbro, and theoiliticbasalt. Serpentinites form conspicuous mountainous ridgeswith steep slopes, while metagabbros are represented bysmall masses, medium-grained, highly altered, deformed,and occasionally foliated. The contact between the serpen-tinites and the metagabbros is occupied by a thrust faultalong which more ductile serpentinites was thrust into thegabbros.

BShear zoneA

GabalEl Sela

50 km

Fig. 1 a Structural map of the Hamisana shear zone, complied from Stern et al. (1990) and Abdelsalam (1994). b Location of the Gabal El Selaarea, southern Eastern Desert, Egypt, and El Sela shear zone (see Fig. 8)

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Metavolcanics

Metavolcanics form a thick sequence of stratified lava flowsinterbanded with their pyroclastics in the southeastern part ofthe area (Fig. 2). They consist typically of 40- to 50-m-thickflows of dark green to gray meta-andesite and, to a lesserextent, of metabasalt, with regular alternations of pale greencolor. It is made up of mafic and intermediate volcanic rocks,metatuffs, and agglomerate. Metatuffs crop out as a thin beltin the NW part of the mapped area (Fig. 2). They have lightgreenish gray to dark gray colors and generally fine- tomedium-grained and foliated rocks. Conglomerate bands are

also recorded which are composed of a grayish green matrixof fine to medium grain enclosing coarse pebbles up to10 cm in diameter composed of immature volcanogenicsediments. The bedding planes of the metatuffs are generallyparallel to the foliation planes.

Syntectonic (older) granites

The syntectonic granites form limited areas at the north-eastern corner and the southern part of the mapped area.They are characterized by low-lying relief and vast sandyplains with scattered blocky outcrops. They are medium- to

Fig. 2 Geological map of the Gabla El Sela area, southern Eastern Desert, Egypt. a Rose diagram of normal faults. b Rose diagram of strike-slipfaults

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coarse-grained and the color range from dark to pale grayaccording to the percentage of mafic minerals. Theyintruded the ophiolitic sequence and metavolcanics andare intruded by the younger granites. They are mainlycomposed of tonalite and granodiorite with gradationalcontacts.

Late to post-tectonic (younger) granites

The younger granites of the El Sela area are represented byGabal El Sela and Gabal Qash Amir. These plutons intruded inthe Sol Hamid ophiolite and calc-alkaline metavolcanics. TheQash Amir granite is located west of Gabal El Sela and isrepresented by an oval body (3×2 km.) surrounded by a vastsand sheet (Fig. 2). It is a strongly weathered, exfoliated, andjointed muscovite monzogranite. These rocks are pale pink,leucocratic, coarse-grained, and composed essentially ofquartz (25–35%), K-feldspar ≥ plagioclase, and muscovite ±garnet. Iron oxides, apatite, allanite, titanite, and zircon arethe main accessories. The low-dipping quartz veins occur atthe southern and eastern margins of the granite body andlocally associated with wolframite ± sulfide. The Gabal ElSela granite displays an elongated belt (9×3 km.) trendingNW–SE. These rocks are pink, slightly leucocratic, medium-to coarse-grained, cavernous, exfoliated, and jointed indifferent directions. They are composed essentially of quartz(25–35%), K-feldspar ≥ plagioclase, and biotite ± muscovite.Iron oxides, monazite, and zircon are the main accessories.Muscovite content is rare and increased toward the southdirection due to the fractionation of the magma. The fine-grained granite facies represent late magma injectionsenriched in uranium which have enhanced the primaryuranium abundance in the vicinity of the structure at amagmatic stage. The Gabal El Sela pluton is affected by twomain sets of faults trending ENE–WSW and NNW–SSE.They show dextral and a sinistral sense of movement,

respectively. Quartz veins and hydrothermal solutions areemplaced along fault zones, especially along the later trends.

El Sela granites are dissected by four different diketypes; they are mostly injected along the ENE–WSWand/orN–S to NNW–SSE directions which represent importanttectonic trends. These dikes were started by the micro-granite and continued in chronological sequence as thefollowing:

(a) Micro-granite dike is a highly radioactive one; itcrosscuts the granitic intrusions along the ENE–WSWand NNW–SSE shear zones (Fig. 3a).

(b) Lamprophyre dikes along the ENE–WSW and NNW–SSE trends; they are of 1- to 3-m thickness and mainlycomposed of large euhedral crystals of alkali feldsparembedded in fine basic amygdaloidal groundmass rich incalcite. It has a higher U content (~40 ppm eU) than theambient granite. Most of the feldspar crystals in thelamprophyre are altered and weathered, leaving theirvugs which were sometimes filled with secondaryuranium minerals.

(c) Quartz dikes are presented as multi-phases of differentcolors within the ENE–WSW and NNW–SSE tectonictrends. The first phase is white, highly brecciatedbarren quartz (1- to 4-m thickness); the second is ahighly radioactive beige to gray jasper which some-times changes to a red color along the fractures byhematitization (20- to 80-cm thickness). These dikesare, strongly jointed, fragmented, brecciated, andcemented by the third black silica phase rich inuranium mineralization or by white silica in otherplaces. Within the ENE–WSW shear zone, the quartzdikes trapped and digested the micro-granite andlamprophyre dikes (Fig. 3b).

(d) Bostonite dikes are present along the N–S and NNE–SSW tectonic trends. They are of 1- to 2-m thickness,

A B

Fig. 3 a Micro-granite injections. b Lamprophyre dike and multi-injections of silica along the ENE–WSW shear zone, Gabal El Sela, southernEastern Desert, Egypt

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fine-grained, massive, and essentially composed ofalkali feldspars, amphiboles, iron oxides, and littlequartz. They are usually fractured, jointed, pale brownto deep red in color, and surrounded by intensively redalteration halos.

Due to the repeated magmatic activities and theassociated fluids, many alteration halos are observed.Hematitization around the bostonite dikes, episyenitiza-tion in the vicinity of the contacts between the twogranite intrusions, and fluoritization (with violet fluorite)were recorded. Also, sericitization (Qz, sericite, andpyrite) is represented as a light yellow bleached zoneof sericitized granite along the central part of the ENE–WSW shear zone.

Structural setting

A systematic structural study was performed to identify anystructural control on the emplacement of the El Selagranites and the main structural control on the developmentof structural trap. To achieve the first aim, the structuralevolution of the study area was investigated from the stagebefore the emplacement of the granitic body. Paleostressorientations were constructed using sets of fault-slip data toachieve the second one. The derived paleostress tensorswere used to identify the extensional and compressionalevents that contributed to increasing the secondary perme-ability, enabling the percolation of hydrothermal fluids andproviding an effective trap for uranium mineralization.

Ductile deformation

Structural analysis of the ductile stage of tectonic deforma-tion in the study area reveals the existence of twodeformation episodes: a folding–thrusting episode (E1) thatwas associated with the cratonization of the arc/inter-arcrock associations and an upright folding episode (E2)following cratonization (Fig. 4). The E1 episode ischaracterized by low-angle thrusting and tight to isoclinalfolding that produced axial plane foliation and stretchinglineations (Fig. 5). The main NE thrust faults, generated atthis stage, define the contact between the dismemberedophiolitic slabs (metagabbros, serpentinites, and amphib-olites) and the arc association volcano-sedimentary rocks.Syntectonic (older) granites were emplaced by imposing aregime of ductile deformation on the rocks of the envelopewhich can provide up to 75% of the required space for thepluton. During emplacement, the metatuffs country rockswere metamorphosed and deformed under ductile condi-tions, which provided most of the space for the pluton(Cobbing 2000).

The area of folded metavolcanics and metatuffs wasdivided into a northern and a southern domain separated bythe axial plane of the major F2 fold to solve this complicatedfolding pattern (McClay 1987). On the southern limb of theF2 fold (Fig. 6a), the data show the bimodality of twosubparallel great circles reflecting the deformation of the twolimbs of an isoclinal tight fold (two limbs of F1) around F3.The elongation of pole concentrations in Fig. 6a is the resultof a later gentle F3 refolding of the F1 fold limbs about theF3 axial plane. Each pole concentration is taken to representa limb of a tight ENE-plunging (12°/N-80°) F1 fold with aSE-dipping axial plane (42°/N-68°). The F3 hinges are foundto define two orientation modes: 70°/N-164° and 30°/N-150°.

The plot of foliation poles for the northern limb of the F2fold does not show the same bimodality as the northern

Fig. 4 Photograph showing the tight nature of F1 and F2 folds andthe open folds of F3, Gabal El Sela area, southern Eastern Desert,Egypt

Fig. 5 Stretched fragments set in highly foliated metatuffs, Gabal ElSela area, southern Eastern Desert, Egypt

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limb due to the tightness of the F1 folds on this limb.The elongation of the polar concentration is again due toopen F3 folding. The intersection of the two poleorientations of the F1 axial plane derived from the dataon the two F2 fold limbs (Fig. 6b) gives an estimate ofthe F2 hinge as 6°/N-74° and the F2 axial plane as 73°/N-78°. The F2 fold is nearly coaxial with the F1 fold,indicating that the compression during this early thrust-ing–folding episode (E1) continued in the same directionto generate approximately coaxial F2 folds. The F1 and F2folds are therefore regarded to have formed during a singleevent and caused the upright parallel coaxial F2 at the endof this episode E1.

The great circle of best fit for the calculated F3 axes,from both limbs of the F2 fold, is shown in Fig. 6c. Thisgreat circle defines the mean axial plane of the F3 folds forthe study area. Its latitude is 89°/N-156°. F3 foldingoccurred by the intracratonic compression of E2, whichacted in a general ENE–WSW trend. It is displayed byfolds generally trending NNW–SSE. The compression andshortening trend during D2 is to the ENE–WSW direction,i.e., quite different from the NW–SE shortening directionduring E1. This means that crustal shortening directionsflipped through about 90°.

The original trend of stretching lineations, N30°W/80°SW,defines the direction of tectonic transport during E1 (Fig. 7a;Shackleton and Ries 1984). The age of this episode can beestimated to be between the formation of the arc/inter-arcrock association and the intrusions of the syntectonic (older)granites. These granites have an age of 660–730 Ma (Stern etal. 1989). They are considered as emplaced at the culmina-tion of the low-angle shearing tectonic event. At the end ofthe E2, numerous plutons of the younger granites in the EDare intruded parallel to the NNW–SSE to NW–SE trend

(Abdel-Meguid 1992). The folding and foliation during theE1 and E2 provided most of the space for the granitic plutonintrusion (Cobbing 2000).

The intersection of the F3 axial plane with the locus planof the stretched lineation is the a-translation axis defined byRamsay (1967) or the movement line as defined by Wiener(1983). The translation a-axis (the maximum slip axis) ofF3 plunges 23°/N-164° (Fig. 7b).

Brittle deformation and paleostress analysis

Paleostress tensors are calculated from fault-slip datasetsaccording to the method of Angelier (1984, 1990, 1994).Assuming that the direction of the maximum shear stress isparallel to the observed striae, the direction of slip on a faultplane depends on the orientation of the maximum (σ1),intermediate (σ2), and minimum (σ3) principal stress axesand on the ratio Φ=(σ2−σ3)/(σ1−σ3). This ratio provides a

CBA

75 27 102

Fig. 6 Stereograms of mesoscopic structural data for Gabal El Selaarea, southern Eastern Desert, Egypt. All stereograms are Schmidt netequal-area lower hemisphere projection. Contoured poles to foliationfrom the northern limb of a regional F1 antiform. The twoconcentrations are limbs of F1 (a) folds. The elongation of the polarconcentrations is due to open F3 refolding of the foliations (contoursat 1%, 2%, 4%, 8%, and 16%). Contoured poles to foliation from the

southern limb of the F2 antiform. F1 folds are tighter, giving a singleelongate (b) concentration. Again, the elongation is due to later F3refolding (contours at 3%, 6%, 12%, and 24%). Intersection of calc F1axial planes from the N and S limbs gives an estimate of theorientation of F2 hinge, as shown. The great circle of best fit for F3hinges gives an estimate of the orientation of the F3 axial plane (c)

A B

b c

a-movementline

Fig. 7 Stereograms of mesoscopic structural data for Gabal El Selaarea, southern Eastern Desert, Egypt. All stereograms are Schmidt netequal-area lower hemisphere projection. a Locus plane of stretchinglineations (23 data points). Average orientation is 80°/N-250°. bTranslation axes a, b, and c

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convenient index to characterize the relationship betweenthe principal stress magnitudes (Angelier 1984). This ratio,Ф, ranges from 0 (meaning that σ2=σ3) to 1 (meaning thatσ1=σ2).

A quality estimator of data dispersion is the average of“ratio upsilon” (RUP; Table 1). The possible values ofestimator RUP range from 0% (maximum shear stressparallel to slip with the same sense) to 200% (maximumshear stress parallel to slip with opposite sense). AverageRUP values below 50% (as in Table 1) generally indicate astrong consistency between the actual fault-slip datadistribution and the computed shear stress distribution(Angelier 1990). Another quality criterion, the averageangle (ANG, in degrees), is the angle between the measured

lineation and the computed slip lineation. The results arequite acceptable for values between 0° and 22° (Angelier1984, 1990).

Paleostress determined from fault-slip data

Paleostress tensor analyses have been conducted on slipsurfaces in the El Sela shear zone and are based oncrosscutting and geometrical relationships between thefaults and dikes. About 283 slip data were measured from24 stations (Table 1 and Fig. 8).

Extensional stress regimes Fault data indicating extensionaldeformation were found at 32 sites (Fig. 8). Generally,

Table 1 Results of paleostresstensor determinations for normalfault systems in the El Selashear zone, Southern EasternDesert, Egypt

σ1, σ2, σ3 are the principal stressaxes; Ф=(σ2−σ3)/(σ1−σ3)Ex extensional system, ANGangle between calculated andmeasured shear, RUP ratio upsi-lon, Conj. No. number of conju-gate fault systems (total=198)

S. no. Type Computed tensor Ф ANG (deg) RUP (%) Conj. No.

σ1 σ2 σ3

01 Ex 356°/72° 92°/2° 183°/18° 0.228 3 11 5

01A Ex 107°/46° 321°/39° 216°/18° 0.156 7 15 4

02B Ex 53°/74° 161°/5° 252°/15° 0.477 6 43 6

02E Ex 303°/60° 41°/4° 133°/29° 0.136 9 19 5

03 Ex 327°/77° 86°/6° 178°/11° 0.359 2 6 5

03A Ex 97°/70° 329°/13° 235°/15° 0.459 3 24 6

05B Ex 331°/62° 208°/16° 111°/22° 0.455 7 34 6

05C Ex 260°/60° 108°/27° 12°/12° 0.375 18 55 9

05D Ex 349°/73° 256°/1° 165°/17° 0.212 2 15 8

05E Ex 57°/76° 279°/11° 187°/9° 0.360 4 17 8

05G Ex 116°/64° 217°/5° 309°/25° 0.074 6 30 5

06 Ex 224°/86° 75°/3° 345°/2° 0.462 5 44 7

07B Ex 52°/65° 270°/20° 174°/14° 0.497 6 37 9

07C Ex 114°/80° 334°/8° 243°/7° 0.276 9 16 6

011A Ex 27°/74° 266°/8° 174°/13° 0.436 4 22 8

011B Ex 56°/70° 301°/9° 208°/18° 0.097 5 17 7

012A Ex 349°/67° 126°/17° 221°/15° 0.486 4 30 6

013A Ex 284°/69° 183°/4° 91°/20° 0.286 2 24 7

014 Ex 269°/81° 129°/7° 39°/6° 0.384 3 15 9

015A Ex 159°/20° 40°/52° 261°/30° 0.924 9 24 4

017 Ex 304°/74° 76°/11° 168°/11° 0.292 3 7 5

018 Ex 71°/61° 246°/29° 337°/2° 0.516 15 42 7

019 Ex 182°/81° 71°/3° 340°/9° 0.493 3 12 5

020 Ex 249°/59° 158°/1° 67°/31° 0.499 4 31 4

021 Ex 291°/10° 188°/52° 29°/36° 0.284 4 27 5

021A Ex 22°/71° 242°/15° 149°/11° 0.312 4 13 5

022 Ex 69°/75° 265°/15° 174°/4° 0.488 5 27 7

022A Ex 186°/79° 331°/9° 62°/6° 0.317 10 26 8

023 Ex 31°/84° 245°/5° 154°/3° 0.404 3 11 6

024A Ex 55°/83° 250°/7° 159°/2° 0.493 3 15 7

025 Ex 109°/63° 276°/27° 9°/5° 0.515 4 14 4

025A Ex 237°/62° 59°/28° 328°/1° 0.351 25 53 5

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normal faults, related to the extensional stress regimes, arecharacterized by discrete slip surfaces. Some of them arecoated with stepped and striated mineralization, e.g., quartz,calcite, gypsum, and epidote. The ENE–WSW faults have azone of cataclasis.

The geometry of the two major fault sets, E–W to ENE–WSW and NW–SE to NNW–SSE, was detected. Oblique-slip faults are also recorded, suggesting a frequentreactivation of earlier weakness planes. Faults recordingE/W- to ENE/WSW-directed extension occurred along theshear zone (Fig. 8). The faults recording NW/SE- to NNW/SSE-directed extension mainly occur in some places wherethe gash fractures along the NW–SE were developed by the

strike-slip faults. High radioactive anomalies are recordedalong those trends, which indicated their importance inlocating the most promising mineralized structures in thearea. From site 013A, the inherited NW/SE-trending dip–slip faults were reactivated during later E/W-directedextension (Fig. 8). This indicates that the E–W extensionis younger than the old NE–SW extensional event.

The ratio of stress differences, Ф, has high values (≥0.5)in three tensors (sites 015A, 018, and 025). Since σ2 is veryclose to σ1, an alternation between dip-slip faulting andstrike-slip faulting modes may have occurred (Fig. 8).Conversely, where the tectonic regime is dominated byextension, a decrease in the ratio Ф results in more irregular

Fig. 8 Lower hemisphere Schmidt projection of fault-slip datacorresponding to the computed stress analysis for extensional phasesin the Gabal El Sela shear zone, Southern Eastern Desert, Egypt. Five-

point star, σ1; four-point star, σ2; three-point star, σ3; computed σ1axes. The number in the circle is the site number

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trajectories of σ3 and local exchange of σ2/σ3; therefore,permutation took place. The Ф has very low values in mostsites, and so permutation is dominant along the shear zones,which plays an important role in increasing the permeabilityalong it.

From the rose diagram of the strike frequencies fornormal faults that formed under extensional regimes, thedominant trends are ENE–WSW and NNW–SSE (Fig. 2).These trends have a genetic relationship by stress permu-tations, as seen before. The same thing for the lessdominant trends, NW–SE and NE–SW, can be noticed.

Compressional stress regimes Generally, the strike-slipfaults related to compressional stress regimes have smoothor shiny fault surfaces. Intensely fractured zones associatedwith these faults vary in thickness from 20 to 600 m. Somefaults have about 60 m offset along a horizontal displace-ment. The evidence for compressional deformation is foundin a dataset of 85 faults collected from all of the 11measured sites and showing a strike-slip displacement or apure reverse fault slip (Table 2). These faults are groupedinto five different compressional regimes: E–W, NE–SW,NNW–SSE, NNE–SSW, and NW–SE (Fig. 9).

The geometry of these fault populations is complex andvaries from site to site. Oblique faulting is common whereslip movements were initiated along preexisting faultplanes. For instance, some NNE/SSW-trending strike-slipfaults have been reactivated into oblique-slip faults. Someextensional veins and normal faults, formed during a N/S-directed extension, have been reactivated as dextral faults(site 016, Fig. 9). Evidence for pure compressional regime(σ3 vertical, with horizontal σ1 and σ2) is only found at one

site (07), whereas there is evidence at other sites for a purestrike-slip regime (σ2 vertical, with horizontal σ1 and σ3).

High values of the stress differences ratio, Ф, normally0.67, 0.9, and 0.87, are calculated at site 09A, i.e., σ2 is veryclose to σ1 (Fig. 9). Therefore, an alternation between strike-slip and dip-slip faulting modes may have occurred.Therefore, NW–SE to NNW–SSE strike-slip faults leftlateral (rose diagram, Fig. 2) have a dip-slip movement inmany cases. At site 05, where thrust faulting has taken place,Ф has the very low value of 0.19 (Fig. 9 and Table 2).

From the rose diagram of strike-slip faults that formedunder compressional regimes, the most dominant trend isNNW–SSE (Fig. 2), which rejuvenated during extensionalregimes. The other dominant trends are ENE–SWS andNW–SE, which represent as two conjugate strike-slipfaults. They also rejuvenated during extensional regimes(transtensional). Sigmoidal shearing was also detectedalong the ENE–WSW and NNW–SSE trends which controlthe injection of the lamprophyre and basic dikes.

The stress permutations induced by stress field changeresulted from the presence of preexisting discontinuitiesand mechanical anisotropy of blocks. In terms of geologicalsignificance, the major causes of stress permutations are theheterogeneity of the brittle deformation and the anisotropyof the mechanical properties that result from fracturing andfaulting (Hu and Angelier 2004).

Therefore, there are two major mineralized structures inthe El Sela area:

1. The ENE–WSW master trend which has multi-injections and many alteration features. The micro-fissuring parallel to the master structures increased the

Table 2 Results of paleostresstensor determinations for strike-slip fault systems in the El Selashear zone, Southern EasternDesert, Egypt

σ1, σ2, and σ3 are the principalstress axes; Ф=(σ2−σ3)/(σ1−σ3)Cp1 pure compression reversesystem, Cp2 strike-slip shearsystem, ANG angle betweencalculated and measured shear,RUP ratio upsilon, Conj. No.number of conjugate fault sys-tems (total=85)

S. no. Type Computed tensor Ф ANG (deg) RUP (%) Conj. No.

σ1 σ2 σ3

07 Cp1 280°/27° 14°/8° 118°/61° 0.704 4 9 4

05 Cp1 323°/6° 232°/15° 74°/74° 0.193 27 65 5

01B Cp2 320°/10° 178°/77° 51°/8° 0.326 6 30 7

02C Cp2 331°/18° 130°/71° 239°/7° 0.464 4 29 7

08 Cp2 95°/6° 201°/68° 3°/21° 0.585 3 11 5

09 Cp2 62°/8° 165°/59° 327°/30° 0.155 7 17 11

09A Cp2 340°/15° 118°/70° 247°/13° 0.905 7 17 4

09B Cp2 16°/16° 168°/72° 284°/8° 0.485 2 18 5

010 Cp2 343°/19° 168°/71° 74°/1° 0.424 4 20 7

012 Cp2 113°/32° 215°/19° 330°/51° 0.438 18 52 5

015 Cp2 291°/4° 173°/81° 22°/8° 0.486 4 43 6

016 Cp2 319°/11° 56°/34° 214°/54° 0.022 6 19 6

017A Cp2 344°/3° 89°/79° 254°/10° 0.354 3 14 6

017B Cp2 170°/1° 74°/80° 261°/10° 0.475 2 15 7

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granitic permeability, playing an important role in thedevelopment of different alteration types along it. Thepermeability increase has promoted fluid percolation,which increases the chance for the ENE–WSW as amineralized trend.

2. The NNE–SSW master trend which developed from thestrike-slip faults (transtensional) and reactivated by

stress permutation phenomena during different exten-sional regimes. Also, this trend has multi-injections andmany alteration types.

The ENE–WSWand NNE–SSWmaster trends are parallelto the axial plan foliation of F2 and F3, respectively, which actas discontinuities and mechanical anisotropy of the crust

Fig. 9 Lower hemisphere Schmidt projection of fault-slip datacorresponding to the computed stress analysis for compressionalphases in the Gabal El Sela shear zone, Southern Eastern Desert,

Egypt. Five-point star, σ1; four-point star, σ2; three-point star, σ3,computed σ1 axes. The number in the circle is the site number

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(Figs. 10 and 11). Therefore, these trends can beconsidered as deep-seated structures deduced from thetectonic map obtained from the interpretation of themagnetic anomaly map as well as the results obtainedfrom the magnetic tectonic trend analysis of Gaafar(2005) and Ibrahim et al. (2005). Therefore, these tendscan act as paleochannels and as a good trap for uranium orother mineral deposits.

Structural history and tectonic interpretation of the ElSela area

The following geological and tectonic episodes wereinferred from the present study and published geochrono-logical data for the surrounding areas (Fig. 12).

Folding–thrusting episode (E1)

This was associated with the cratonization of the arc/inter-arc rock association. Sol Hamed-Onib and Allaqi-Heiani sutures were formed due to the collision betweenGerf and Gabgaba-Gebeit terrains (>715 Ma; Stern et al.1989). Low-angle thrusting ad tight and isoclinal folds ofF1 were formed during this stage (E1a). Also, themetavolcanics, “older and younger metavolcanics,” aresimilar expressions of the ~750-Ma crust-forming event(Ali et al. 2009).

1. The compression during the early folding–thrustingepisode E1a continued in the same direction to generatenearly coaxial folds F2 with F1. Therefore, it isbelieved that the development episode E1a was startedduring E1a and continued during E1b.

2. The F2 folds were formed between the formation of thearc/inter-arc rock association and the intrusion of thesyntectonic (older) granites (660–730 Ma; Stern et al.1989). The intrusion of these granites represents theend of the folding–thrusting episode (E1b).

Upright folding episode (E2)

This was associated with the compression and shortening tothe ENE–WSW direction which is different from theNNW–SSE shortening direction during E1.

1. Initiation of Hamisana shear zone deformation tookplace to form early N-trending upright folds duringE2a (580–660 Ma; Stern et al. 1989). Intensethermal and tectonic activity along HSZ formsNNW-trending upright folds (F3) and ENE-trendingdextral strike-slip faults (550–580 Ma; Stern et al.1989).

2. At the end of this episode (E2b), the first batch fromyounger granites of El Sela area (porphyritic biotitegranite) were intruded (580 Ma; Pecskay et al. 2004).This batch is characterized by the presence of magmaticenclaves and accessory mineral assemblages comprisingmagnetite, apatite, allanite, titanite, and zircon, suggest-ing its relative oxidizing magmatic conditions and itsbelonging to the I-type granitoids. Also, it is a product ofa subduction-related setting and is commonly formedduring the late stage of arc evolution (El Nisr et al.2001).

Post-tectonic granitic intrusion episode (E3)

1. On the other hand, the second granitic batch (equigra-nular biotite of Gabal El Sela and muscovite granite ofGabal Qash Amiri) exhibits A-type affinity. On the

Fig. 10 ENE-WSW shear zone of the Gabal El Sela area, southernEastern Desert, Egypt

Fig. 11 NNW–SSE sigmoidal shearing of the Gabal El Sela area,southern Eastern Desert, Egypt

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basis of petrological and geochemical data, this batchdisplays orogenic features of a post-orogenic environ-ment (E3a; El Nisr et al. 2001). Therefore, this batch ofEl Sela granites can be interpreted as occurring during aprolonged heating event by post-collisional extensionalextension (Xie et al. 2006). It is consistent with theconcept that it represents a continuation of magmatismin a post-orogenic environment, which reactivatesmajor structures.

2. Two mica granite and granitic dikes were intruded atthis stage (E3b, 538 Ma; Pecskay et al. 2004). Normalfault systems were recorded by a delineation of graniticdikes along the ENE and NNW directions. The tectonicmap obtained from the interpretation of the magneticanomaly map as well as the results obtained from themagnetic tectonic trend analysis of Gaafar (2005) andIbrahim et al. (2005) deduced their being deep-seated.The age of this stage is from post-Pan-African (Cambrian)to pre-Hercynian orogeny.

Fracturing, faulting, and post-granitic dike extrusionepisode (E4)

1. Late Carboniferous (Hercynian orogeny) represents anintense period of uplift and erosion caused by the

closure of the Proto-Tethyan Ocean and the LateCarboniferous collision between Africa and Laurussia(Schandelmeier and Reynolds 1997). The impact of thisstage (E4a) in the study area was represented by localshort tenuous period of compression dominated byNNW- to NW/SE-trending σ1 corresponding to thereactivation of the preexisting fabrics.

2. Post-Hercynian orogeny (E4b) was associated with theinitial phase of breaking up of the Pangaea supercon-tinent spanning Late Permian to Middle Jurassic timesand culminated in the development of a new divergent/transform plate boundary between Gondwana andLaurussia (Lambiase 1989; Stampfli et al. 1991;Guiraud and Bellion 1996). The impact of this stage(E4b) in the study area was represented by fracturing,faulting, and dike extrusion.

Structural and metallogenic events

The first structural and Metallogenic event (phase 1) of theEl Sela granitic pluton corresponds to the intrusion of a U-rich microgranitic generally not very thick (1–5 m) aplite,pegmatite, and lamprophyre dikes along the ENE–WSWtrend. The lamprophyre dike extends over more than 5 km

Fig. 12 Tectono-stratigraphic geochronologic classification of rock association of basement rocks in Eastern Desert, Egypt and metallogenicevents (1–5)

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SW from the northern margin of the pluton and correspondsto major radiometric anomalies in the spectrometric maps.

The uranium mineralized structure is localized at thenorthern border of the pluton, close to the contact withthe country rock, along 70° N–75° E fractures parallel tothe dike swarm. Hydrothermal alteration is developedover several meters across the mineralized structure: anexternal envelop involving propylitic (chlorite, epidote)alteration (phase 2) which can be delineated in severallocation along the 70° N–75° E fractures parallel to theshear zone.

Most of the anomalies in the El Sela shear zone arelocated along quartz-bearing veins (three generations, phase3) which bounded the lamprophyre dike and microgranitesand dissected them in relation to successive fracturation andbrecciation corresponding to repeated rejuvenations of thestructures. The first phase is a white, highly brecciatedbarren quartz (thickness, 1–4 m); the second is a highlyradioactive beige to gray jasper which sometimes changesto a red color along the fractures by hematitization(thickness, 20–80 cm). These dikes are strongly jointed,fragmented, brecciated, and cemented by the third blacksilica phase rich in uranium mineralization or by whitesilica in other places. Within the ENE–WSW shear zone,the quartz dikes trapped and digested the micro-granite andlamprophyre dike as lensoidal bodies are completely alteredby phyllic type. Red jasper filling (N170 E 75E) have beenobserved along the north–south faults, up to 1–20 mcrosscutting the main ENE mineralized structure.

However, the major structures are associated with astrong phyllic alteration consisting of the assemblagequartz–sericite–pyrite (phase 4). Each side of the graniteis hydrothermally altered to sericite minerals with variableextents. Enclosing granite resent locally some intensereddening along the wall of the mineralized structure,indicating the percolation of oxidized solution which mayhave been favorable for uranium transportation.

Field measurements in sedimentary fluvial-type calcretedeposits also suggest that present-day groundwater in theseareas (phase 5) may also display the potential to bothdissolve and precipitate uranium in the near surface.Chemical dilatancy and evaporation-driven diffusion thatpromote decomplexing, diffusion, and re-precipitationmechanisms are seen to play integral parts in the continuedchemical re-working and modification of these calcrete-hosted carnotite deposits.

Conclusions and discussion

The El Sela area represents the eastern part of the SolHamed ophiolite-decorated suture which represents theextension of Allaqi-Heiani suture first recognized by

Kroner et al. (1987). The area is composed of dismemberedophiolitic mafic–ultramafic belt northerly thrust over avolcano-sedimentary sequence with a root of tonalities. Thesequence is intruded by younger granites. Recent studiesshow increasingly that most granite plutons are in factcomposed of multiple phase injections of highly variablesizes, not always co-genetic, and each of them may have ahighly variable metallogenic potential (Cuney et al. 1989).Such a conception has a very important consequencebecause the mineralization may be genetically only relatedto a specific granitic intrusion phase within a large granitecomplex, as shown.

The detailed structural study of El Sela area revealsthat a succession of tectonic events controlled thegeometry of the mineralized structures. Ordering andcompilation of the paleostress events associated withductile and brittle deformations has resulted in a historyof four tectonic episodes that affected the study area:(E1) folding–thrusting episode associated with the crato-nization of the arc/inter-arc rock association; (E2) anupright folding episode associated with compression andshortening to the ENE–WSW direction which is differentfrom the NNW–SSE shortening direction during E1; (E3)post-tectonic granitic intrusion episode (two mica graniteand granitic dikes were intruded during this episode);and (E4) fracturing, faulting, and post-granitic dikeextrusion episode causing different faults that took placeafter cratonization until the present.

Structure analysis of the ductile deformation reveals thepresence of three generations of folds during E1 and E2.The F2 folds are nearly coaxial (along ENE–WSW trend)with the F1 folds. F3 folding is displayed by folds generallytrending NNW–SSE. Therefore, the ENE–WSW andNNW–SSE trends can be considered as preexisting dis-continuities and mechanical anisotropy of the crust in thefollowing structure episodes, while the brittle deformationreveals the importance of those trends which control themulti-injections and many alteration features in the studyarea. Also, these trends can be considered as paleochanneltrends for deep-seated structures. Therefore, the ENE–WSW and NNW–SSE trends act as a good trap for uraniumand/or other mineral resources.

Thus, the structural controls of the uranium mineraliza-tion appear related to the interaction between inheritedductile fabrics and overprinting brittle structures. Duringreactivation, a simple shear parallel to the inherited ductilefabrics was responsible for the development of mineralizedstructures. The presence of uranium mineralization ineconomic concentrations may be intercepted if an explora-tion drilling program is performed perpendicular to theENE/WSW-trending mineralized structure or in the inter-section between the ENE–WSW and NNW–SSE trends,which act as litho-structural traps.

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