impact of structural lineaments on mineralized occurrences in north abu rusheid-sikait area, south...

The 5th Tunisian Days of Applied Geology, JTGA 2013 227

Impact of Structural Lineaments on Mineralized Occurrences in North Abu Rusheid-Sikait Area, South Eastern Desert, Egypt

Ibrahim Hassan Ibrahim Nuclear Materials Authority, P.O. Box: 530 El-Maadi, Cairo, Egypt

Ibrahim170 @ yahoo.com

ABSTRACT. Abu Rusheid-Sikait area forms part of the Arabo-Nubian basement exposures that situated at the northern peripheral contact between the Central and the South Eastern Desert. Accordingly, its structural pattern is strongly related to that of the late Neoproterozoic pan-African as it represented mainly by polycyclic shear zones mainly coincide with the Eastern Desert Shear Zones (EDSZ) as well as Najd style left lateral strike-slip shear system. The studied structural lineaments are arranged according to two main trend clusters around N-S and NW-SE to WNW-ESE directions representing the main factor controlling the emplacement of the different granitic intrusions in Abu Rusheid-Sikait area. It has been recorded that the northern segment of the biotite granites is separated from the ophiolitic mélange by fractured high strained shear zone trending ENE-WSW forming a zone of mylonitic granite carrying evidences of hydrothermal activities (ferrugination, silicification and kaolinitization). Moreover, most of these lineaments, as obtained from field measurements, carry strike-slip movement criteria associated with oblique slip ones reflecting their reactivation during subsequent cyclic extensional events. It has been found that almost all the recoded mineralizations are related to these extensional events that creating the necessary space either for mineral entrapment and/or the percolation of the hydrothermal carrying solutions along these shear zone. The hydrothermal origin could be accepted for the mineralization within the ENE-WSW shear zone as the entrapped mineralizations include fluorite, kasolite, molybdenite, pyrite and galena in addition to zircon minerals as accessories.

Key words: Abu Rusheid-Sikait; Structural lineaments; Mylonite; Shear zone.

INTRODUCTION

All tectonic structures (discontinuities) are the obvious result of deformational processes that

occurred within a rock volume. Although it is often a matter of scale of observation, the infinite possible

behaviors of rock bodies undergoing deformation can be grossly separated into continuous and

discontinuous ones. According to the relationships between local and temporal stress variations and what

is commonly defined as the regional stress field, tectonic structures like folds are generally defined as

‘ductile’ structures associated to continuum plastic deformation while faults, extensional joints, dykes and

veins are the obvious product of brittle deformation (Caputo, 2005). Many works have been carried out

concerning the tectonic setting of Nugrus-Sikait-Abu Rusheid area with emphasis on studying the ductile

deformation fabrics (e.g. bedding, foliation, lineation, folding and boudin) and the associated phase of

metamorphism, while less attention has been paid to the brittle deformation. Structural lineaments record

the surface expression of almost all brittle structures such as, fractures (faults and joints), dykes and shear

zones. The studying of these brittle discontinuities is an important approach for understanding the tectonic

origin of regional structural and to clarify their impact on mineralized occurrences. In the present work, an

attempt has been made to characterize the significance of these brittle structures on the recorded

228 The 5th Tunisian Days of Applied Geology, JTGA 2013

mineralization in north Abu Rusheid-Sikait area based on satellite imagery analysis, field observations,

structural analysis and spectrometric studies.

1- Overview of the Eastern Desert of Egypt

The Eastern Desert of Egypt (ED) is a part of the Neoproterozoic Arabian– Nubian Shield (Fig. 1).

The Arabian–Nubian Shield consists of Neoproterozoic (1000–542 Ma) crust deformed and

metamorphosed during the East African orogeny (Greiling et al., 1994). The Eastern Desert has been

subdivided into three tectono-stratigraphic domains: South Eastern Desert (SED), Central Eastern Desert

(CED) and North Eastern Desert (NED). These domains (Fig.1) are separated by two ENE-WSW trending

tectonic boundaries (shear zones) among them the northern shear zone extending from Qena to Safaga, -

separating the NED domain from the CED domain, while the second shear zone running from Aswan to

Ras Benas along Marsa Alam-Idfu road separating the CED domain from the SED domain (Stern and

Hedge 1985; El-Gaby et al., 1988). Each domain shows a distinct structural fabrics reflecting continuous

decrease of the ductile deformation from south to north.

The NED document NE-SW main structural trend consistent with the post granitic dykes. The CED

is dominated by a strong NW–SE structural trend expressed in steeply dipping ductile–brittle shear zones

and dissected by ENE deep-seated faults (Bennett and Mosley, 1987; Greiling et al., 1988). Structural

studies document a continuous decrease of crustal shortening (ductile deformation) from south to north

(Fritz et al., 1996; Unzog and Kurz, 2000). This shortening is accommodated by both distinct kinematic

strike-slip faults and large-scale folding. The SED domain mainly contains NW to W and NE trending

thrust duplex foliation belts of metsedimentary and metavolcanic interwoven with ophiolitic nappes within

discrete NW- to NNW-trending, kilometer-scale, shear zones cutting the ophioltic and island arc

metavolcanic/volcaniclastic assemblages. These assemblages are intruded by syn-, late-, and post-tectonic

gabbro/granite complexes.

Most tectonic models for the Eastern Desert relate the older granites to plate convergence and

magma generation above subduction zones and the younger granites to crustal extension related to orogenic

collapse and/or post-orogenic rifting (Stern et al., 1984; Greiling et al., 1994; Moussa et al., 2008). The

development of regional strike-slip shear zones is in some models interpreted to have facilitated

emplacement of the syn-orogenic plutons (Fritz et al., 1996; Bregar et al., 2002).


Fig. 1. (a) Geological sketch map showing the Arabian Nubian Shield. (B) Simplified geological map of the three main Precambrian basement subdivisions of the Eastern Desert in Egypt after Liégeois and Stern, (2010). The tectonic boundaries between Southern Eastern Desert (SED), Central Eastern Desert (CED) and Northern Eastern Desert (NED) are given by Stern and Hedge, (1985). Location of the study area is marked by the white rectangle.

2- Overview of Sikait-Nugrus area

Sikait-Nugrus area is considered as the southeastern extension of the Migif–Hafafit metamorphic

complex of the Eastern Desert of Egypt (part of the Arabian-Nubian Shield). This complex represents one

of three major dome structures in the Eastern Desert. Gabal Meatiq (Loizenbauer et al. 2001), Abu Swayel

(Abd El-Naby and Frisch 2002) and Migif- Hafafit (Fowler and El-Kalioubi, 2002; Abd El-Naby et al.,

2008) that closely linked with the NW-trending Najd Fault style (Stern, 1985), where the eastern and

western margins of these dome are bounded by a set of parallel left lateral strike-slip shear zones and the

northern and southern margins are defined by prominent normal faults (Wallbrecher et al., 1993).

Sikait-Nugrus area lies to the southern contact of the major shear zone known as the Nugrus thrust

fault (Greiling et al., 1988) or the Nugrus strike-slip fault (Fritz et al., 2002) and or Sha’it–Nugrus shear

zone (Fowler and Osman, 2009). This shear zone separates high-temperature metamorphic rocks of the

Hafafit complex in the SW (Hafafit unit) from mainly low-grade ophiolitic and arc volcanic assemblages

to the NE (Nugrus unit) (Bennett and Mosley, 1987). The Hafafit unit consists of Hafafit domes which


include from core to rim granite gneiss of tonalitic and trondhjemitic composition, banded amphibolites

which is overthrusted by ultramafic rocks, alternating bands of biotite and hornblende gneiss, and the

psammitic gneisses at the rim of the domal structure. The Nugrus unit is composed mainly of low-grade

mica schists and metavolcanics and related volcaniclastics. Both units have been intruded by undeformed

leucogranites, especially along thrust zones.

Sikait-Nugrus area is enrichment in various economic mineralization (e.g. Be, Nb, Th, U and REEs),

that make this area as one of the most important and promising areas for different authors (Basta and Zaki,

1961; El-Shazly and Hassan, 1972; Hassan, 1973; Sabet et al., 1976; Hassan et al., 1983; El-Gemmizi 1984;

Hegazy, 1984; Eid, 1986; Hilmy et al., 1990; Takla et al., 1992; El-Maghraby, 1995; Assaf et al., 1998;

Ibrahim et al., 2000; Moghazi et al., 2004). Ibrahim et al., (2004) classified the Abu Rusheid-Sikait granitic

rocks based on the textures and presence of micas into porphyritic biotite granites, deformed biotite granites,

two-mica granites and muscovite granites. It crop out in a belt elongated NW-SE trend. Several types of

mineralization, such as Nb-Ta, zircon, thorite, lithium mica, and secondary uranium minerals are

recognized by Saleh (1997), Abdalla et al. (1998) and Raslan (2005&2008). Ibrahim et al. (2007) recorded

the secondary U-minerals (uranophane, beta-uranophane, kasolite, torbernite, autonite and meta-autonite)

in addition to U-bearing minerals (astrocyanite, betafite and fergusonite) in lamprophyre dykes within the

shear zones in Abu Rusheid area.

GEOLOGIC SETTING

Wadi Abu Rusheid and Wadi Sikait are a tributary of Wadi Nugrus, located at about 97 km SW

from Marsa Alam City, South Eastern Desert. The study area covers about 23 km2 and is situated between

latitude 24°40`- 24°42`N and longitude 34°43`- 34°46`E (Fig. 2). In the study area, rock exposures could

be categorized under two main lithotectonic groups of rock types represented by ophiolitic rocks (oldest)

and intrusive granitic rocks (youngest) following a regional N-S, NW and WNW structural trend parallel

to the prominent fault trend.

The ophiolitic rocks are comprises ophiolitic dismembers assemblage of mountainous size

(serpentinites and metagabbros) thrusted over the ophiolitic mélange which composed mainly of rock

fragments (amphibolite sheets, metagabbros masses, allocthonous serpentinite and related talc carbonate)

embedded in fine-grained matrix of quartzo-feldspathic schist, hornblende biotite schist and garnetiferous

biotite schist. These matrixes are characterized by dark grayish green in colours, bedding, highly foliated

and featured by the frequent presence of macro- and meso-folds. Quartz boudins and pegmatite lenses are

extending parallel to the main foliation. Amphibolites and metagabbros rock fragments are probably related

to the calc-alkaline metagabbros associated with Hafafit gneisses (El-Ramly et al., 1993). The metagabbros

and serpentinites ophiolitic dismembers form fold thrust sheets around Wadi Sikait and Wadi Abu Rusheid

and thrusted over ophiolitic mélange (WNW-ESE and dips 33°/NNE). Stern and Hedge, (1985) assigned an

age between the time of older granitoids emplacement (682 Ma) and that of younger granites intrusion

(565-600 Ma) to this thrust.

The granitic rocks comprise biotite granites (oldest), muscovite granites and alkali feldspar granites

(youngest). Biotite granites are medium- to coarse-grained, reddish pink in color and composed of quartz,


K-feldspars, plagioclases and biotite. Opaques, apatite and zircon are accessories. The contact between the

biotite granites and the ophiolitic mélange is structure contact marked, trending NW-SE and dipping 45°-

55° due NNE and WSW. The rocks are fractured, jointed, exfoliated and the outer margins of have

gneissose textures. Muscovite granites are coarse- to medium-grained, white pinkish in colour and

composed of quartz, K-feldspar, plagioclase and muscovite. Garnet and opaques are accessories. They are

elongated mass emplacement along N-S structural trends between ophiolitic mélange and biotite granites. Alkali

feldspar granites are fine- to medium-grained and composed of quartz, K-feldspar, plagioclases and few

hornblendes. Zircon, apatite and opaques are accessories. They intruded ophiolitic mélange and biotite granites.

They occurred also as offshoots in biotite granites along Wadi Sikait.

In places, narrow elongated mylonitic body, show gneissose structure, trending ENE-WSW with

length exceeding 300 m and a maximum width about 20 m outcropped at the contact between the ophiolitic

rocks and the northern biotite granites. The mylonitic rocks affected with silicification and ferrugination

features attain yellowish to reddish colours and characterized by highly radioactivity. A secondary uranium

mineralization is found in the altered zone of the mylonitic rocks, where it occurs as stains along fracture

surfaces and as acicular crystals filling cavities. Uranium and thorium contents vary from normal values to

43 and 186 ppm, respectively.

The study area is cross-cut by various dykes (aplite, felsites, andesites and dolerites) with different

striking from WNW-ESE to NW-SW then N-S and NE-SW. They cut all the rock types except the muscovite

granites which are mostly cut by quartz veins. The andesite dykes are characterized by columnar joints and

attain to 15-20 m in thickness that parallel to the main WNW sinistral fault. During the fieldworks an old

quarry beryl-bearing quartz vein was observed. The beryl-bearing quartz vein (0.5 to 1.5 m in thick) occurs

along the western periphery of the biotite granites along Wadi Abu Rusheid following the NW structural

trend. Beryl occurs as banded layers within the quartz vein developed by filling of the tensional fractures

along the structural contact between serpentinites and biotite granites. The genesis of beryl-bearing quartz

vein interpreted as a product of the interaction between syntectonic pegmatitic magma or hydrothermal

fluids and the pre-existing basic to ultrabasic rocks (Grundmann and Morteani, 2008).

STRUCTURAL ANALYSIS

For the purpose of the present work, 675 surface structural lineaments have been traced, using

Landsat TM image based on colour differences of contrasting lithological units in north Abu Rusheid-Sikait

area (Fig. 3a). Moreover, detailed field investigations have been carried out on about 25 sites distributed

throughout the study area, where 90 fault-slip data, 60 dykes and 525 fractures have been measured (Fig.

3b). The kinematics of a fault population can be defined by using the fault plane, the corresponding striation,

and the slip vector, measured at several places along a major fault. Based on the structural analysis of minor

fault-slip data among other structural fabrics, we discuss the impact of these discontinuities on the

distribution of the mineralized occurrences among north Abu Rusheid-Sikait area.


Fig. 2: Reconstructed geologic map showing the distribution of the different rock types with respect to the structural elements within north Abu Rusheid-Sikait area.

Surface lineament analysis

Structural lineaments (fractures, faults, joints and dykes) show heterogeneous trend pattern of

deformation reflecting a complex tectonic history accumulated since Precambrian. The identified surface

lineaments in north Abu Rusheid-Sikait area are demonstrated on figure (3a), The commonly used stress

inversion techniques results in the orientation (azimuth and plunge) of the principal stress axes of a stress

tensor as well as a “stress ratio” R = (σ1 − σ2)/(σ1 − σ3), a quantity describing relative stress magnitudes.

The σ1, σ2 and σ3 correspond to maximum, intermediate and minimum stress axes. The shear fractures form

as conjugate sets approximately 30o on either side of the σ1- σ2 plane and the joints formed parallel to σ1

and normal to σ3 (Belayneh and Cosgrove, 2010).

The surface structural lineaments are document multidirectional orientations with two main trend

clusters around the N-S and WNW-ESE directions associated with less dominant E-W, NNW-SSE, NE-

SW, NW-SE, NNE-SSW and ENE-WSW trending ones in decreasing order (Fig. 3b) for the depicted

regional (major) lineaments. The NW-SE to WNW-ESE trending ones delineated the contacts between the


different rock types exposed in the study area following the regional fault trends whereas, the ENE-WSW

trending lineaments represent the mineralized shear zone between the ophiolitic mélange and the northern

segment of the biotite granites in the study area (Fig. 1). It has been found that the NW-SE to WNW-ESE

oriented surface lineaments are corresponding to left lateral strike-slip faults and those of the ENE-WSW

trend represent right and left lateral strike-slip faults.

Dykes trend analysis

Dykes in the study area are manifested as sub-vertical sets of aplite, andesites and dolerites

composition cut through different rock types. The width of dykes varies from 0.5 up to 6 meters and their

length can be traced from few meters to hundreds of meters. Dykes in the study area are arranged according

to WNW-ESE, NW-SE, NNW-SSE, N-S, NE-SW, ENE-WSW, E-W and NNE-SSW trends in decreasing

order with main WNW-ESE, NW-SE, NNW-SSE, N-S oriented clusters (Fig. 3b). The relative age

relationship of these dykes has been adopted from previous studies indicate that the acidic dykes are older

than basic ones and both of them are considered as post granitic dykes. The dykes crosscutting competent

lithologies and unfoliated rocks are attributed to neo-formed fractures initiated perpendicular to the

minimum stress axis related closely to the regional tectonic stress field (Faure et al., 1996).

Paleostress field reconstructed for all dykes is shown in (Fig. 3b). It was obtained from 60 dyke

orientations compiled from existing map and field measurements. Paleostress field reconstruction for NE-

SW trending dykes, determined from 38 dykes, indicates NW-SE striking extension. This trend is

considered the dominant trend of dykes. The E-W and N-S trending dykes determined from 22 dykes,

indicate N-S and E-W extension respectively. Inferred σ3 axes for these dykes are clearly homogeneous

and indicate NW-SE, E-W and N-S striking extension (Fig. 3b).

The structural studies of the deformed and undeformed dykes reveal that they are non-Andersonian

dykes that exploited pre-existing fractures. Dykes that strike parallel to regional foliation are considered to

have been mainly controlled by the pre-existing anisotropy in metamorphic rocks so that their attitude

poorly reflects the paleostress orientation. In contrast, dykes crosscutting competent lithologies and

unfoliated rocks are attributed to neoformed fractures initiated perpendicular to the minimum stress axis so

that their attitude is closely related to the regional tectonic stress field (Faure et al., 1996). Undeformed

dykes display N15°E, N55°E and E-W strikes; whereas the deformed ones show N65°E or N100°E trends

with sinistral shear senses (Figs. 3a&b). These structural data point to reactivation of pre-existing fractures

either as tensional cracks (undeformed dykes) or transtensional shear zones (deformed dykes).


Fig. 3: Structural lineament analysis showing the map of surface traced lineaments as well as the resulted trend frequencies for the distinguished types affecting north Abu Rusheid-Sikait area.

Joint trend analysis

The detailed field observations demonstrates that North Abu Rushied area is dissected by multi-

directional sets of joints dominated by N-S to NNW-SSE, E-W to ENE-WSW, NW-SE and NE-SW trends

clusters as obtained from the quantitative analysis of about field measurements. All joint measurements

(about 525 joints) are constructed as density-contoured lower hemisphere Schmidt stereogram of poles to

joint planes (Fig. 4). Most of these joints are documented as either shear with moderately high dip angle

(65o to 75o) or as almost vertical tensional ones. For each rock type, joint set geometries have been

demonstrated as well as the inferred stress regimes (Fig. 4). Field measurements of joints among the

ophiolitic mélange rocks show multidirectional geometry dominated by moderately dipping NW–SE,


ENE–WSW, N–S, NNE–SSW and E-W orientations in decreasing order. Both biotite and mylonitic

granites documented similar sets of joint oriented due E-W, N-S and NNE–SSW respectively, while those

recorded in the muscovite granites are mainly of N-S dominant orientations. Field measurements delineated

that the ENE-WSW and E-W joint sets are more conspicuous in alkali feldspar granites than in any other

rock type. These results coincide with those obtained from the orientation of the major lineaments either as

subsidiary shears and/or tension shear fractures.

Stress orientations inferred from joint analysis (Fig. 4) reveals a transtensional strike-slip

deformational regimes delineating N-S, NNW–SSE and NW–SE trending extensions that characterized by

oblique to sub-vertical maximum stress axis (σ1) and sub-horizontal minimum stress axis (σ3) oriented due

to N98°E and N65°W. These transtensional strike-slip overall regime is believed to be corresponding to

joint patterns prevailing north Abu Rusheid-Sikait area and responsible for creation of either shear and/or

tension fracture systems.

Fault trend analysis

The geometric characteristics of all major faults measured throughout the study area are presented

in Figures (3b) as frequency rose diagram. Most of these fault population are of strike-slip type and revealed

multi-directional pattern with three major trends; NW-SE, ENE-WSW to E-W and N-S. These faults are

characterized by moderate to steep dip angles (from 65° to 85°) due to the SE, S, NW, E and SW. In fact,

all major faults in the study area are steeply dipping either to the W or to the E and oriented mainly within

eight trend clusters among them N–S, WNW–ESE, NE–SW and NNE-SSW trend clusters are dominant

whereas NNW-SSE, ENE-WSW, NW-SE and E-W trends are less dominant (Fig. 3b) trend. Field

observations confirmed that the NE-SW with the N-S and the WSW-ENE with the ENE–WSW trending

lineaments represent two pairs of conjugate strike-slip fault sets characterized by obvious horizontal

displacements that steeply dipping either to the W or to the E. In addition, sub-vertical to oblique

displacements have been recorded along the N–S, NNE–SSW, WNW–ESE and NE–SW trending

lineaments indicating either neoformed (for sub-vertical) or reactivated (for oblique) normal faults in

response to later on extensional stress fields.

Field measurements depend on analysis of conjugate sets that may belong to extension or

compression regimes depending on their slip directions and fault geometry. From slickensides of minor and

major faults, the fault type can be determined and the corresponding stress tensors can be calculated.

Paleostress reconstruction of brittle deformation is based on the analysis of fault slip data using computer

programs (Delvaux, (1993). These methods depend on determining the best fitting reduced paleostress

tensor for a given fault slip data set. The direction of slip on a fault plane depends on the orientation of the

maximum (σ1), intermediate (σ2), and minimum (σ3) principal stress axes and on the ratio Φ = (σ2- σ3)/(σ1-

σ3).


Fig. 4: Rose diagram of joints in different rocks and density-contoured lower hemisphere Schmidt stereogram of poles to joint planes and the inferred stress regimes, north Abu Rusheid-Sikait area.

Most of the fault populations are of strike-slip type. The strike-slip stress tensors show three

significant strike-slip regimes with vertical to sub-vertical σ2 (Fig. 5), The stress tensors of 1st and the 2nd

ones define two nearly perpendicular transtensional strike-slip stress regime with sub-vertical σ2 and

N103°E trending σ1 for the 1st and N032°E trending σ1 for the 2nd (Figs. 5a&b). The 1st transtensional

strike-slip stress regime inferred from strike slip-fault system with ENE-WSW trending right stepping faults

and WNW-ESE left stepping ones (Fig. 5a) whereas the 2nd is corresponding to N-S trending right stepping

faults and NE-SW left stepping ones (Fig. 5b). Field observation demonstrates that the 1st one is reworked

by the 2nd through σ1-σ3 permutation. The 3rd strike-slip system represents the orientation of principal stress

axes ơ1– ơ3 defined from all faults (Fig. 5c). It defines pure strike-slip deformational stress regime inferred

from strike slip-fault systems with N-S and/or ENE-WSW trending right stepping faults and NE-SW and/or

WNW-ESE left stepping ones respectively inferred from vertical N241°E trending σ2 and horizontal

N066°E trending σ1 associated with horizontal N337°E trending σ3 (Fig. 5c).


Chronology of surface lineaments (Deformation cycles)

Surface lineament sets crossing north Abu Rusheid-Sikait area are correlated to be the relay of cyclic

extensional tectonic events chronologically associated with the different fracturing and dyking prevailing

the study area. Occurrence of dyke's parallel fractures indicates that most of these fracture sets were

generated either as neoformed or reactivated in response to an extensional tectonic regime. Field relations

show that the ENE–WSW and N-S strike-slip faults have dextral senses of movement while the WNW-

ESE and NE-SW faults have sinistral senses of movement. The N-S/NE-SW conjugate set is younger than

the WNW-ESE-WSW/ENE-WSW conjugate set whereas the N-S strike-slip fault displaced the oldest

WSW-ESE.

Accordingly, the evolution of this fracture pattern could be concluded to be resulted from either

displacing or locally reactivating pre-existed late Neoproterozoic structures. The WNW-ESE trend with the

ENE-WSW trend initiate together the 1st order set of shear fractures. The 2nd order set of shear fractures

were developed later on as N-S trending dextral and NE-SW oriented sinistral. The integration of the

analyzed pairs of the conjugate strike-slip faults defines cyclic phases of deformation between parallel NW-

SE to NNW-SSE trending major sinistral strike-slip associated with the development of both 1st and 2nd

order pairs of conjugated shear fractures configuring the surface lineament pattern of north Abu Rusheid-

Sikait area (Figs. 5&6)


Fig. 5: Fault slip data analysis using inversion method for the conjugate shear faults (a&b) and all faults (c) among the study area (obtained from WINTENSOR program of Delvaux, 1993 in lower hemisphere stereogram Schmidt net projection) supported with constructed sketch diagrams showing fault kinematics and the inferred stress fields. (a) WNW–ESE (sinistral) and NE-SW (dextral) old conjugate shear faults. (b) NE-SW (sinistral) and N-S (dextral) young conjugate shear faults. (c) The orientation of principal stress axes ơ1– ơ3 defined from all faults. (Solid lines= fault planes, inward arrows indicate compression, outward arrows indicate tension, circle = ơ1, triangle = ơ 2 and square = ơ 3).


Fig. 6: Modal sketch demonstrates the tectonic domains along left-lateral strike-slip fault segment and the corresponding stress orientations after Segall and Pollard, 1980. (inward arrows=compression & outward arrows=tension). The extension domain (-ve) is of greet important in mining geology especially the mineralized extensional shear zone of U-oxides in granites which is the case in north Abu Rusheid-Sikait area.

MYLONITIC ROCKS AND PETROGRAPHY

The biotite granite pluton crop out in a belt elongated NW-SE trend and characterized by gneissose

structures along the outer margins. A shear zone ENE-WSW (sinistral) is located at the northern segment

of biotite granites and extends for 300 meters and about 20 m in width (Fig. 7a) forming mylonitic granite

rocks. The mylonitic granite rocks preserve a range of microstructures from primary igneous textures, often

with a magmatic flow foliation, through to textures indicative of subsolidus deformation. The intense

mineralized part of the shear-zone varies in width from 1 to 3 meters and in length from 20 to 25 meters

and is encountered at the biotite granites. The mylonitic granite rocks are medium- to fine-grained and

reddish to grayish brown in color. They are usually highly sheared and fractured, and sometimes filled by

veinlets of quartz, calcite, epidote and feldspars. The N-S and ENE-WSW fracturing system increase the

mylonitization, formed shear folding (Fig. 7b) and later affect by ferrugination and silicification. They show

highly brecciation that took place prior to and/or contemporaneous with the hydrothermal solutions.

Sometimes, the original compositions of the mylonitic rocks are obscured and become difficult to be


ascertained because of the high intensity of mylonitization and ferrugination. Ferrugination is the main

alteration features (Fig. 7c) developed within the ENE-WSW shear zone and extend to northern biotite

granites. Silicified mylonite zone is well developed and dissected by quartz veins varying in thickness from

less than 1 cm up to 40 cm, and extends for variable distances, not exceeding 2 m (Fig. 7d).

Silicification and ferrugination process along the mylonitic rocks increase of SiO2 and Fe2O3 and

MnO at the expense of the other major oxides (Helgeson, 1974). Silica content could reach as much as 90%

and formed quartz vein of close-spaced fractures in a network. Quartz veins form where the fluids flow

through larger, open space fractures and precipitate mineralization along the walls of the fracture, eventually

filling it completely. Sweewald and Sayfried (1990) suggested that temperature for ferrugination is varies

between 350o C and 500o C while the temperature for silicification is varying between 300 oC and 400 oC

(Bucanan, 1982).

Fig. 7: (a) Sharp contact between mylonitic gneissic granites against ophiolitic mélange and biotite granites, (b) Close up view showing shear fold associated with parallel left lateral strike-slip fault along mylonitic ductile shear zone, (c) Ferruginated mylonitic zone along shear zone and (d) Silicified mylonitic zone crosscut by quartz veins along shear zone.

Petrographically, the mylonitic rocks along these shear zone are composed mainly of deformed

quartz (ductile-brittle movement), potash feldspars (orthoclase and microcline perthites), plagioclases

(albite) and biotite forming gneissose textures (Fig. 8a). Sericite, kaolinite and epidote are secondary

minerals, whereas zircon, fluorite, pyrite, and other opaques are accessory phases. Quartz shows clear signs

of mylonitization and annealing (Fig. 8b) and occurs as fine subhedral crystals formed around large perthite

crystals. Potash feldspars are represented by string and flame type perthite. Perthites of occur as large

crystals (2–5 mm) within fine-grained quartz and feldspars. They are dissected by irregular quartz veinlets


and are affected by kaolinitization (Fig. 8c). They are cracked and stained by iron oxides gave the rock its

red coloration. Plagioclases occur as subhedral tabular crystals show cloudy appearance due to highly

saussuritization, while others exhibits albite twinning. Highly deformed plagioclases are observed due to

dislocate their lamellae (Fig. 8d). Biotite occurs as flakes usually affected by alteration and replaced by

chlorite and iron oxides.

Opaques are dispersed in the rock, sometimes with the red colour due to the presence of

disseminated pigment of iron oxy-hydroxides. Zircon occurs as aggregates (Fig. 8e), colourless, euhedral

to subhedral prismatic crystals (50–200 μm), which are generally enclosed in biotite and feldspars. Fluorite

is found as subhedral crystals (300-600 μm) with distinct cleavage (Fig. 8f). It varies from violet to purple

in colour; locally observed as small irregular associated with opaques. Most of the fluorite crystals are

usually cracked and fractured. The interstitial fluorite is mostly associated with zircon and feldspars. Pyrite

occurs as euhedral cubic crystals (Fig. 8g) in silicified zone, while along ferruginated zone is dissolved and

secondary carbonate is filling vugs (Fig. 8h).

MINERALIZATION

Two samples from mylonitic rocks along shear zone were crushed and separation of the heavy

fractions at different current intensities. Identified the heavy minerals by using XRD techniques and by the

Environmental Scanning Electron Microscope (model Philips XL30 ESEM) supported by semi-quantitative

energy dispersive spectrometer unit at the Nuclear Materials Authority of Egypt. Zircon (ZrSiO4) occurs

as euhedral eight-sided pyramidal faces at the expense of the prismatic ones (Fig. 9a). The substituting

elements commonly present are Hf, Th and U replacing Zr. Galena (PbS) is characterized by perfect

cleavage parallel to the cubic faces (Fig. 9b). It is produced by ascending solutions emanating from bodies

of igneous rocks. The occurrences of these sulphides minerals induce reducing conditions favorable for

radioactive mineralization. Bornite (Cu5FeS4) is identified by EDX (Fig. 9c) and maybe deposited by

magmatic waters. Molybdenite (MoS2) occurs as fine grains (Fig. 9d) dissemination in fine fissures in

quartz veins. They formed in hydrothermal veins and quartz pegmatites extend far into the post magmatic

stage. Fluorite (CaF2) occurs as colourless to pale violet crystals (Fig. 9e) associated calcite, quartz and

sulphides. Pyrite (FeS2) show well developed cubic crystal (Fig. 9f) and associated fluorite indicated

hydrothermal origin. Kasolite (Pb(UO2)SiO4.H2O) is secondary uranium minerals and occurs as radial

fibrous aggregates of lemon-yellow to brownish yellow colours (Fig. 9g).


Fig. 8: (a) Gneissose texture in mylonitic granites, (b) Mylonitization and annealing quartz in silicified mylonite zone, (c) Kaolinitization in ferruginated mylonite zone, (d) Faulted plagioclase crystal due to cataclastic affect, mylonite granites (e) Zircon aggregates associated with iron oxides, ferruginated mylonite zone (f) Fluorite crystal, silicified mylonite zone (g) Euhedral cubic pyrite crystal, silicified mylonite zone and (f) Secondary carbonate filling the vugs after exsolved pyrite crystals, ferruginated mylonite zone.

The sulphides and iron oxides in the shear zone provide an adequate reducing medium to reduce

mobile U6+ to the insoluble U4+. However, in certain reducing environments, e.g. during the oxidation of


Fe2+ and S2- to ( Fe3+ and S6-), U6+ in the uranyl ion will be reduced to U4+, which results in precipitation of

uraninite primary minerals (Langmuir, 1978; Romberger, 1984). Kasolite along shear zone maybe resulted

from an oxidization product of uraninite or from hydrothermal solution enriched fluorine reacted with

metamictized accessory minerals (Dawood et al., 2010). Depletion of uranium within the shear zone

increased in the vicinity of mixed circulating meteoric (surface) or endogenic (hydrothermal) fluids along

the semi-brittle to brittle fractures (Stuckless and Ferreira, 1976). It can be concluded that, the host high

strain mylonitic rocks themselves are enriched in mineralization due to the circulating hydrothermal

solutions along shear zones in which the segments of high permeability and hydrostatic fluid gradient act

as favourable sites for mineralization (high strained rock cut by brittle shear zone).

SPECTROMETRIC RESULTS

The distribution of natural gamma radioactivity in the various granitic rocks and different contacts

has been measured in the field (using portable GR-512). The term “equivalent” or its abbreviation “e” is

used to indicate that equilibrium is assumed between the radioactive daughter isotope monitored by the

spectrometer and its relevant parent isotope. The results of in situ gamma-ray spectrometric analyses of the

different granitic rocks and the shear zone are summarized in Table (1) and figure (10). Results of

spectrometric data show that, the average eU and eTh-contents increase gradually from the muscovite

granites (5 ppm eU & 10 ppm eTh) to alkali feldspar granites (6 ppm eU & 11 ppm eTh) and biotite granites

(6 ppm eU & 14 ppm eTh) then mylonitic granites along shear zone (8 ppm eU & 22 ppm eTh). The

ferruginated zone show high average radiometric values (43 ppm eU & 186 ppm eTh) than the silicified

zone (33 ppm eU & 134 ppm eTh). The data presented in Table (1) show that, the ferrugination zone was

accompanied by higher intensity of radioactivity rather than the silicification zone as resulted from the high

ability of iron oxides to liberate the radioactive elements from its solutions.

Thorium is typical high field strength elements (HFSE), which are generally considered immobile

during hydrothermal water-rock interaction. Experimentally, thorium may become mobile especially in

high-temperature (magmatic or hydrothermal) environments containing strong complexing agents (Giere,

1993; Keppler, 1993). The fluorite, galena, molybdenite and pyrite in the mineralized shear zone reflect the

important role of fluorite and sulfur as strong agents. These explain that the ferruginated zone in the study

area is enriched by Th-contents related to adsorb thorium elements from their solutions.


Fig. 9: EDX spectrum and XRD-patterns of minerals, a) Zircon, b) Galena), c) Bornite, d) Molybdenite e) Fluorite f) Pyrite and g) Kasolite.


Table 1: eU (ppm), eTh (ppm) and eU/eTh ratio of alkali feldspar granites, muscovite granites, biotite granites and anomalous along shear zone in north Abu Rusheid-Sikait area (n= number of measurements).

Rock Types eU (ppm) eTh (ppm) eU/eTh

Alkali feldspar granites (n=24)

Min. 3 5 0.67 Max. 10 18 0.56

Average 6 11 0.52

Muscovite granites (n=26)

Min. 2 4 0.31 Max. 11 19 1.33

Average 5 10 0.55

Biotite granites (n=32)

Min. 3 5 0.60 Max. 11 20 0.55

Average 6 14 0.45

Anomalies along shear zone

Mylonitic granites (n=32)

Min. 3 7 0.43 Max. 18 48 0.38

Average 8 22 0.37

Silicified zone (n=21)

Min. 7 13 0.54 Max. 77 411 0.19

Average 33 134 0.25

Ferruginated zone (n=23)

Min. 17 23 0.74 Max. 78 461 0.17

Average 43 186 0.23

Fig. 10: Bar diagram show the average contents of eU and eTh for the alkali feldspar granites, muscovite granites, biotite granites, mylonitic granites, silicified zone and ferruginated zone in north Abu Rusheid-Sikait area.


DISCUSSION & CONCLUSIONS

Lineament Controls on Mineralized Occurrences

The area is crossed by N–S to NNW-SSE trending extensional strike-slip fault shear zones with

oblique left-lateral dislocation brittle features and ends on the Nugrus shear zone. In addition, the detailed

field study recorded the development of ENE-WSW to E-W and/or NE-SW array of strike-slip faults with

oblique to dip slip reactivation evidences. The senses of shearing as well as the orientation of these faults

indicate that they represent subsidiary normal shears to the NNW-SSE to NW-SE master shear zones. These

normal shears are overprinted by several post granitic dykes of NNW-SSE to N-S, ENE-WSW to E–W and

NE-SW orientations. There is a definite spatial and temporal association of the recorded mineralization

with extensional tectonics.

The main mineralized zone is hosted in a shear zone adjacent to and/or within the granitic rocks.

The mineralization history could be correlated with the progressive deformation of the shear zone

encountered between parallel left lateral strike-slip fault segments and the granitic intrusion. It has been

found that the mineralized occurrences are just one of a number of intrusive and hydrothermal events

occurring during major extensional left-lateral strike-slip stress regime in which the extensional stress

components transfer along and reactivate the pre-existing ENE-WSW to E-W trending shear fractures where

as the compressional stress components transfer along those trending NNW-SSE to NW-SE and creating

zones of strain shadows and extension relays (Figs. 5&6).

Brittle failure (evidence for which is seen in shear zones) occurred in heterogeneously strained rocks

where nucleation and initial growth of a shear zone network within compositionally and structurally

heterogeneous granitoids took place. Most of these lineaments, as obtained from field measurements, carry

strike-slip movement criteria associated with oblique slip ones reflecting their reactivation during subsequent

cyclic extensional events. Such fracturing provided transient porosity and permeability through which

hydrothermal fluids could migrate, including those responsible for the recorded mineralized occurrences in

Abu Rusheid-Sikait area. They also provided space for minor igneous intrusions, such as dykes of various

types. The depositional, intrusive deformational, and mineralized occurrences recorded in these rocks could

be took place over a relatively brief time interval. The mineralized occurrences are located at the intersection

of the transtensional sinistral shear zones striking between 140o and 150o, which promoted dilation and the

emplacement of the younger granite, with those striking between 080o and 100o. Extension was transferred

to the sinistral shear zones within the trans-tensional stage which promoted connected fracture systems and

localized fluid-flow. This gave rise to the alteration patterns of the deposit and ore deposition in an area

previously affected by the E-W compression.

Migration of fluids in the crust is a prerequisite for many geologic processes such as regional

metamorphism and formation of hydrothermal, magmatic, volcanic systems and ore deposits. The bulk

permeability of rocks is greatly enhanced by fractures, where their geometric properties (attitude, length,

density and aperture) are important to define the hydraulic behaviour of the network (e.g. Davy et al., 2006).

It has been recorded that the northern segment of the biotite granites is separated from the ophiolitic

mélange by fractured high strained shear zone trending ENE-WSW forming a zone of mylonitic granite

carrying evidences of hydrothermal activities (ferrugination, silicification and kaolinitization).


Field observations in the granitic rocks and veins preserved in the shear zone demonstrate a part

of the shear-zone history dominated by trans-tensional left-lateral strike-slip deformation, while dykes as

well as the mineralized occurrences could be formed at the transition stage of the regional structural stress

field from compression to extension and can be used to mark the ending of a deformational shortening-

extension cycle.

Almost all the recoded mineralizations are related to these extensional events that creating the

necessary space either for mineral entrapment and/or the percolation of the hydrothermal carrying solutions

along these shear zone. The hydrothermal origin could be accepted for the mineralization within the ENE-

WSW shear zone as the entrapped mineralizations include fluorite, kasolite, molybdenite, pyrite and galena

in addition to zircon minerals as accessories. Therefore, we conclude that postorogenic veins and dykes

complex not only indicate the ending of an orogenic process, but is also an effective vector for

mineralization allowing a degree of predictability that can assist in exploration targeting.

Fractures are important for both transportation of hydrothermal fluids and precipitation new

minerals along the ENE-WSW shear zone in north Abu Rusheid-Sikait area. Such fracturing provided

transient porosity and permeability through which hydrothermal fluids could migrate, including those

responsible for mineralization as well as the associated U occurrences. The subsolidus reactions between

the hydrothermal solutions and mylonitic granites brought about changes in some elemental concentration.

The hot fluids circulate through huge volumes of fractured rocks dissolving a variety of minerals. These

fluids carry the different elements and metals in solution from both original sources and from leaching out

of some country rocks during upward migration of hydrothermal fluid along fractures. After that the new

minerals (kasolite, fluorite, molybdenite, galena and pyrite) begin to precipitate along the walls of the

fractures extending upwards and outwards.

Regional Lineament Framework

The shear zones in the Pan African basement of the Eastern Desert may be related to compressional

as well as extensional stresses (Greiling et al., 1993). The Sha’it–Nugrus shear zone is distinct between the

South Eastern Desert (Migif-Hafafit gneisses footwall) and the Central Eastern Desert (low-grade

metamorphics hangingwall) (Fowler and Osman, 2009). This shear zone is deformed (regionally and locally

folded and thrust dissected) during later NE–SW compressive tectonism. Syn-kinematic granitoid intrusion

is featured along the Sha’it–Nugrus shear zone and has been dated at ~600 Ma. Accordingly, the north Abu

Rusheid-Sikait area lies to the southern contact of Sha’it–Nugrus shear zone, located between the Central

and Southern Eastern Desert boundary and affected by NE–SW compressive tectonism followed by granitic

intrusions along NW regional trends.

Accordingly, the structural lineaments along north Abu Rusheid-Sikait area are mainly fractures

(joints and faults), dike and shear zones. The evolution of all these fractures is either displaced or locally

reactivates the pre-existing late Neoproterozoic structures. Two major sets (WNW-ESE and N-S) of

lineaments are common with joints, dykes and faults are identified and mapped in the study area. The

lineaments are related to ESE-WNW to NE-SW and ENE-WSW compression with contemporary NNE-

SSW, NW-SE and NNW-SSE extension. Most of the fractures are extensional, which were subsequently

reactivated into strike-slip faults and accommodated the emplacement of granitoid rocks in the study area.


The biotite granite pluton in the study area is crop out in a belt elongated NW-SE and may be formed

the incremental injection of magmas into active NW-SE Sha’it–Nugrus shear while the muscovite granite

injected along N-S local active shear. The outer margins of biotite granite intrusion have mainly gneissose

structures and parallel to the main foliation of the ophiolitic rocks. These gneissose structures may be

formed during the crystallized and cooling of the magmatic melt under directed pressure or during the

process of the magma’s movement, as a result of which there is parallel arrangement of the mica and

feldspars. Continuous regional stress field variations can be induced by magma intrusion as suggested by

Vigneresse et al. (1999). Accordingly, this gneissose structure give evidences of intensive shearing

attributed to the compression inferred from the cyclic pulses of biotite granitic intrusions.

The main mineralized zone (ferrugination and silicification) is hosted by a shear zone within the

high strained mylonitic granites between the ophiolitic mélange and the northern segment of biotite granites

in north Abu Rusheid-Sikait area. The mineralized occurrence is elongated parallel to regional and local

structural trends. Mylonitic rock which has a well-developed fracture system may serve as an excellent host

rock. Fault zones are excellent places for fluids to circulate and precipitate mineralization. Faulting may

develop breccia and gouge, which is often a good candidate for replacement style mineralization. The form

of mineralization and alteration associated with faults is highly variable, include massive to fine-grained,

networks of quartz veinlets, and occasionally vuggy textures along mylonite rocks. Hydrothermal fluids

circulated along fractures and faults (channelways), it usually obvious because precipitated minerals and

altered wallrocks remains as evidence.

ACKNOWLEDGEMENTS

The author is very grateful to Prof. M.S. Mostafa for the valuable suggestions, fruitful discussions and constructive review of the manuscript.

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