Petrology,geochemistry,radioactivity,and M-W type rare earth element tetrads of El Sela altered granites,south eastern desert,Egypt

A.M.El Mezayen·M.A.Heikal·M.G.El-Feky·H.A.Shahin·I.K.Abu Zeid·S.R.Lasheen

Abstract The southern part of the Eastern Desert of Egypt-about 30 km southwest of Abu-Ramad city-is mainly covered by ophiolitic rocks(Sul Hamed),island arc assemblage,younger granites(muscovite granites of Qash Amer and two mica granites of El Sela),and various acidic and basic dikes.Field,petrological,and geochemical studies indicate that the El Sela shear zone has been subject to hydrothermal and supergene alteration such as kaolinization,albitization,sericitization,and hematitization.It is invaded by quartz ENE-WSW veins associated with hydrothermal alteration accompanied by radioactive mineralization.The investigated younger granitic rocks had very low contents of compatible elements,such as Cr,Ni,and Co;and high contents of incompatible elements,such as Zr,and large ion lithophiles,such as Sr,especially in the El Sela shear zone.Major oxide and trace element analyses revealed calc-alkaline affinity and peraluminous character.These highly differentiated granitic rocks'lower Zr/Hf and higher Y/Ho than the normal ratio are consistent with hydrothermal alteration.Most samples had rare earth element(REE)-patterns with an M-type tetrad effect in thefirst and fourth segmen∑ts and a W-type tetrad in the third segment.The averageREE in the studied granites was lower than the world granite average;the ratio of light to heavy REEs greater.The main radioactive,uranium-bearing,and uraniferous Fe and Mn minerals are uranothorite,autunite,uranophane and autunite as compounds,kasolite,columbite,xenotime,uranophane-bearing zircon and jarosite,silver-bearing pyrite,hematite,and autunite-bearing pyrolusite.

Keywords Geochemistry ·REE-tetrad effect·Radioactive minerals·Radiometric measurements·Qash Amer·El Sela

1 Introduction

The Pan-African orogenic event ended about 615 Ma and subsequent crustal uplifting occurred around 610-550 Ma(Greiling et al.1993;Stern 1994).The Arabian-Nubian shield(ANS)is juvenile Precambrian continental crust linked with a Neoproterozoic ‘‘Supercontinent Cycle''(Worsley et al.1986).This cycle began with the break-up of Rodinia in the early Cryogenian(Meert and Torsvik 2003),continued with the opening and closing of oceanic basins(Stern 1994;Collins and Pisarevsky 2005),and ended with the convergence of East and West Gondwana and the formation of the new supercontinent of ‘‘Greater Gondwana''(Stern 1994,2017).The basement rocks of Egypt represent the northwestern part of ANS in the northern half of the East African Orogen(e.g.Stern 1994).This section separated from the eastern portion in Saudi Arabia during the formation of the Red Sea.The Neoproterozoic crust of the Eastern Desert(ED)of Egypt extends～800 km between the Nile River and the Red Sea,from near Cairo to the Sudan border.The basement complex in ED is divided into three lithologically and structurally distinct domains(Stern and Hedge 1985):North Eastern Desert(NED),Central Eastern Desert(CED),and South Eastern Desert(SED).CED is characterized by low relief,and composed of a late Tonian-Cryogenian(～700-750 Ma)superstructure of ophiolitic crust and arc sequences.SED and CED are the same but SED lacks asphalt roads(impeding access)and lacks interesting deposits such as banded iron formations. ‘‘NED is overwhelmingly composed of a wide range of Ediacaran igneous rocks and associated Hammamat sediments''(Stern 2017,p.4).Granitic rocks constitute about 60%of ED basement outcrops in Egypt.These Egyptian granitic rocks are divided into an older(850-614 Ma)grey or synorogenic granite and younger(610-550 Ma),monzogranite,syenogranite,alkali granite,and post-orogenic granites(Stern and Gottfried 1986;Hassan and Hashad 1990;Beyth et al.1994).The ratio of younger granites to older granites increases northward across ED(Stern 1979;Bentor 1985).This area has been analyzed for geochemistry by El-Nisr et al.(2002)and Khalaf(2005);for uranium mineralization by Rashed(2001),Gaafar(2005),Mira and Ibrahim(2009),Shahin(2011,2014),and Abd El Gawad et al.(2014);and structurally by Ali(2011).The present work includes a detailed inspection of the geology,petrography,uranium mineralization,geochemistry,radioactivity,and the rare earth element(REE)tetrad effect of the younger granitic rocks at Qash Amer and El Sela.

2 Analytical methods

Thirty samples of the younger granitic rocks were prepared as thin sections and examined under a polarizing microscope to determine their mineralogical composition and texture.Representative rock samples were crushed,pulverized using an agate mill,and analyzed for major oxides and trace and rare earth elements in Acme Lab,Vancouver,Canada by inductively coupled plasma emission spectrometry(ICP-ES).Detection limits for trace elements and majoroxides were 0.01-0.5 ppm and 0.001 wt%-0.04 wt%,respectively.The analytical precision,as calculated from replicate analyses,was 0.5%for major oxides and varied from 2%to 20%for trace elements.Some samples were crushed,sieved,and separated to light and heavy fractions by bromoform.Mineral grains were picked from each of the obtained heavy fractions under binuclear microscope.Some of these selected grains were subjected to Environmental Scanning Electron Microscope(ESEM)in the Nuclear Material Authority(NMA),Egypt.Radiometric measurements (using quantitative gamma-ray spectrometry)of eU,eTh,Ra(e),and K%were carried out at NMA using a multichannel analyzer gamma ray spectrometer.

3 Geologic setting

Samples were collected in the southern part of ED of Egypt,about 30 km southwest of Abu-Ramad City,between 22°13′and 22°20′N and 36°08′and 36°20′E.The area is mainly covered by ophiolitic rocks(Sul Hamed,599 m.a.s.l.);an island arc assemblage(502 m.a.s.l.);lateto post-orogenic granites,including muscovite granites(Qash Amer,724 m.a.s.l.)and two mica granites(El Sela,557 m.a.s.l.);and acidic and basic dikes(Figs.1,2).Qash Amer muscovite granites form the highest peak in the study area and occupy its center.They are coarse-grained and range in color from pink to red,and are characterized by fractures,exfoliation,and weathering boulders.The fractures are filled by manganese,which also occurs as disseminated clusters(Fig.3a).

El Sela younger granites occur as low-to high-relief isolated and scattered granitic masses occupying the western part of the mapped area.They range in color from pink to red and in grain size from fine to coarse,with the coarse-grained granites dominant.They are characterized by vertical joints,fractures,cavernous weathering,and exfoliation.They intrude younger metavolcanics and are invaded by different types of dikes and veins that are affected by dextral strike-slip faults(Figs.3b,c).The finegrained granites are exposed as a low pluton dike at the northern periphery of the El Sela granites,intruded by pegmatite and intruding the coarse-grained granites(Fig.3d).These granites are jointed and fractured,and these fractures are filled by iron oxide that contains radioactive minerals(Fig.3e).

Fig.1 Panoramic 3D showing the highest Qash Amer peak

Fig.2 Geologic map modified after Abu El Laban 2002

The El Sela shear zone is dissected by two major faults in the ENE-WSW and NNW-SSE directions.The former trend is usually associated with the major shear zone,extends up to 50 m in width and 1 km length,and is injected by quartz veins.This shear zone is cut and displaced into three parts by two NNW-SSE trending strikeslip faults that extend for a short distance(90 m).Field observations indicated that the granitic rocks have been subjected to different phases of alteration,especially at the El Sela shear zone.They are invaded by ENE-WSW quartz veins that have caused hydrothermal alteration accompanied by radioactive mineralization in the finegrained granites.The alteration manifests as kaolinization,albitization,and hematitization.Secondary uranium mineralization is evident as canary-yellow thin films deposited along small cracks and microfractures(Fig.3f).

4 Petrography

Based on the International Union of Geological Sciences nomenclature of Streckeisen(1976),Qash Amer and El Sela granite samples are recognized as syenogranite(Fig.4).

4.1 Qash Amer syenogranite

Fig.3 Photograph showing the different features in younger granites:a exfoliation and manganese filling fracture in Qash Amer granites.Looking E,b contact between metavolcanics(Mv)and younger granites(Gr),south El Sela.Looking SW,c felsite dike dissected El Sela granites and showing dextral strike-slip fault.Looking N,d contact between fine-and coarse-grained granites in the northern part of El Sela,e finegrained granites filled by manganese and iron oxide and f Secondary uranium minerals in El Sela shear zone

Fig.4 IUGS classification of the studied younger granites(Streckeisen 1976), I=Alkali-feldspar granite, II=Syenogranite,III=Monzogranite,IV=Granodiorite and V=Tonalite

Qash Amer syenogranite is medium-to coarse-grained with hypidiomorphic texture.It consists mainly of potash feldspars,quartz,plagioclase,and mica.Potash feldspars are represented by microcline,microcline perthite,and antiperthite.Microcline occurs as medium anhedral platy crystals showing crosshatched twinning and enclosing quartz.Microcline perthite is present as subhedral crystals altered to kaolinite and sericite(Fig.5a).Antiperthite occurs as platy crystals(up to 5 mm length and 4.5 mm width)between plagioclase and potash feldspar where the host mineral is usually plagioclase(Fig.5b).Quartz occurs as fine-to coarse-grained anhedral crystals that reach up to 3 mm in length and 3.2 mm in width,sometimes fractured and filled by secondary muscovite.Plagioclase(An8-10)is represented by euhedral to subhedral crystals of albite,up to 3.6 mm in length and 2.2 mm in width,partially or completely altered to clay minerals and saussurite,so that twinning is sometimes obscured or evanesced.Some crystals are bent and fractured due to stress.Biotite is less common than muscovite,present as flaky crystals with moderate pleochroism from pale to dark brown,associated with muscovite,and altered to chlorite.Muscovite occurs as primary flaky crystals associated with other constituents,mostly coexisting with biotite and corroded by quartz.It is also present as secondary shreds,including in plagioclase as an alteration product.Accessory minerals-mainly garnet and apatite-occupy about 0.8%of the rock.Garnet occurs in polygonal form,enclosing relics of mica minerals,suggesting that it was formed epigenetically by a metasomatic process.Apatite is found as minute prismatic crystals with grey interference color and included in quartz with six-sided forms.

4.2 El Sela syenogranite

The El Sela syenogranite is medium-to coarse-grained with a hypidiomorphic texture.It is composed mainly of potash feldspars,plagioclase,quartz,and biotite.Potash feldspars occur as subhedral to anhedral crystals of stringand patch-type perthite(Fig.5c).The crystals are mediumgrained with length up to 4 mm and width up to 3.2 mm.The borders of adjacent crystals are characterized by reaction rims occupied by minute crystals of albite.Some crystals enclose fine,zoned plagioclase and biotite poikilitically distributed.Plagioclase(An8-10)occurs as euhedral to subhedral crystals of albite of 3.5 mm length and 1.8 mm width.It exhibits lamellar,pericline,and Carlsbad twinning.Some crystals are characterized by zonation accompanied by selective alteration,especially at their core(saussuritization).Quartz presents as medium anhedral crystals,up to 2.8 mm in width and 2.5 mm in length associated with other constituents.We observed characteristic wavy extinction and occasional corrosion of plagioclase crystals.Biotite-occurring as fine flakes(1 mm length by 0.5 mm width)-is the most common mica mineral and was observed partially or completely altered to chlorite.Muscovite is less common,occurring as irregularflakes associated with biotite orpenninite chlorite(Fig.5d).It was sometimes observed as a secondary mineral filling cracks of plagioclase,quartz,and perthite.Zircon was recorded as minute prismatic crystals with very high relief and interference color of the third order.Some zircon crystals were surrounded by strong pleochroic haloes due to radiogenic effects,included in quartz and coated by iron oxides(Fig.5e).

4.3 El Sela sheared granite(highly altered granites)

The portion of El Sela granites close to the shear zone is highly sheared and subjected to post-magmatic alteration by epigenetic processes(hydrothermal alteration).The main types of alteration observed are kaolinitization,albitization,silicification,and ferrugination.Kaolinitization(argillic alteration)is produced by introducing clay minerals at the expense of the feldspars,where any increase in clay minerals is accompanied by a decrease in feldspars(Fig.5f).Kaolinite is formed by hydration of K-mica or K-feldspar according to the equations of Pirajno(1992):

Albitization(phyllic alteration)of plagioclase is the decalcification of its anorthite content;this alteration is accompanied by saussuritization of the plagioclase and occasionally sericitization of the K-feldspar where the medium is more acidic(low pH)(Fig.5g).Jasinski(1988)pointed out that sericite is formed by acidic solutions between 300 and 350°C.Vladi Marmo(1971)proposed that decalcification of plagioclase is caused by hydrolysis of the plagioclase or introduction of potassium or sodium according to the following equations:

Fig.5 Photomicrograph showing;a kaolinitized microcline perthite;Qash Amer syenogranite;C.N,b Antiperthite including plagioclase;Qash Amer syenogranite.C.N,c string perthite;El Sela syenogranite.C.N,d Biotite completely altered to penninite that filed by secondary muscovite in El Sela syenogranite;C.N,e Metamect zircon embedded in quartz,El Sela syenogranites;C.N,f Kaolinitized microcline in El Sela shear zone;C.N,g Microveinlet of quartz invading sericitzed K-feldspar in El Sela shear zone;C.N and h Iron oxide enriched altered granite with radioactive minerals in El Sela shear zone;C.N

1. Hydrolysis of plagioclase:

2. Introduction of potassium:

3. Introduction of sodium:

Fig.6 EDAX analysis showing:a autunite mineral from El Sela shear zone,b uranophane and autunite mixture from El Sela shear zone,c Uranothorite from northern part of El Sela(trench of Fe and Mn),d kasolite from northern part of El Sela(trench of Fe and Mn)

Silicification results from the addition of secondary silica(SiO2)ortheliberation ofsilicaaccompanying otheralteration processes.Ferrugination occurs when rock is invaded by veinlets of iron oxides(Fig.5h).Ferrugination and silicification along the shear zone are accompanied by higher radioactivity compared with sericitization and kaolinization.This is mainly due to the ability of iron oxides to adsorb uranium minerals from circulating hydrothermal solutions(Hsi and Langmuir 1985).Sweewald and Syfried(1990)suggested that hematitization is caused by strong alkaline hydrothermal solutions between 350 and 500°C and pH＞10.

5 Mineralization

Scanning electron microscope(SEM)and back-scattered electron imaging(BSE)analysis of the heavy minerals indicated presence of the following:

(a) Radioactive minerals

Autuniteis a secondary uranium mineral found in the oxidation and weathering zone,derived from the alteration of uraninite or pitchblende.Under binocular microscope,autunite grains are distinguished by their yellow to pale brown colors(Fig.6a).

Uranophaneoccurs as an alteration product or as a mixing stage between two different phases rich in U,Si,and P(uranophane and autunite)(Fig.6b).

Uranothorite(Th,U)SiO4Thorite,(ThSiO4),is one of the most common thorium silicate minerals.It is mainly composed of ThO2(49%-75%),SiO2(up to 20%),and U(up to 10%).Other elements such as Ca,Mg,Fe,alkalis,Ce,P,Ta,Ti,Zr,Sn,Al,Y,Pb,and Fe3+may be present in small to minor amounts(Heinrich 1958).Several authors have reported the presence of thorite inclusions in rare metalmineralization and accessory heavy minerals separated from some Egyptian pegmatites(Ali et al.2005;Abdel Warith et al.2007;Raslan et al.2010a,b).Thorite is found as an accessory mineral in different rock types and geologic environments,especially pegmatites,granites,quartz-barite veins,syenites,and hydrothermal deposits.It is a primary mineral of thorium and occurs as anhedral opaque mineral grains varying in color from dark brown to black and exhibiting submetallic to greasy luster(Fig.6c).It is found mainly in the metamict state due its high content of radioelements.Uranium content in most mineral grains analyzed exceeded 10 wt%, so it is considered uranothorite.

Fig.7 EDAX analysis showing:a xenotime from northern part of El Sela(trench of Fe and Mn),b columbite from northern part of El Sela(trench of Fe and Mn),c zircon from El Sela shear zone(S.no.80),d uranophane on the surface in zircon from El Sela shear zone(S.no.80),e jarosite from El Sela shear zone(S.no.83)and f inclusion of uranophane on the surface of jarosite from El Sela shear zone(S.no.83)

Fig.8 EDAX analysis showing:a pyrite from El Sela shear zone(S.no.83),b Silver on the surface of pyrite from El Sela shear zone(S.no.83),c pyrolusite from northern El Sela(trench of Fe and Mn),d autunite as inclusions on the surface of pyrolusite from northern El Sela(trench of Fe and Mn),e hematite after pyrite from El Sela shear zone(S.no.83)

Table 1 Complete chemical analysis for the studied granitic rocks

Table 1 continued

Table 1 continued

Kasolite,Pb(UO2)SiO4(H2O),is the only secondary U-mineral formed by circulating meteoric water enriched in silica and Pb leached from metamict U-bearing minerals(Fig.6d).Kasolite is distinguished from the other uranium silicates by its crystal habit and luster.It is a hydrated silicate of lead and hexavalent uranium and is the only uranyl silicate with lead as its major cation.Kasolite is brightly colored(canary and lemon yellow,and brown of different intensities).

(b) Uranium-bearing minerals

Xenotime,(Y,HREE)PO4,is a ubiquitous accessory mineral in granitic rocks of various compositions,granitic pegmatites,migmatites,and low-to high-grade metamorphic rocks(Forster 1998).The mineral tends to incorporate the smaller and heavy REEs(HREEs,Tb-Lu)in addition to yttrium.Xenotime mineral grains are euhedral to subhedral crystals and exhibit yellow to brownish-yellow color.P2O5and Y2O3are the main components of xenotime(Fig.7a).The average contents of the major oxides in xenotime of the studied altered granites were Y2O3(36.6 wt%),P2O5(29.2 wt%),SiO2(15.3 wt%),Yb2O3,F2O3Er2O3,and Tb2O3,(5.11 wt%,4.06 wt%,3.53 wt%,and 1.4 wt%,respectively),demonstrating its greater affinity for HREEs.

Columbite,(Fe,Mn,Mg)(Nb,Ta)2O6,is an ore of niobium as well as a source of tantalum.It occurs as black anhedral crystals with sub-metallic to resinous luster and dark red to black streak.Columbite is frequently considered a carrier of radioactivity(Fig.7b).

Zirconin the samples is characterized by metamictization.The origin of ‘‘Metamict State''is that the internal order of a crystalline form has been destroyed by∝-particle bombardment from radionuclides within the structure(Ali and Lentz 2011).The investigated zircon grains have euhedral form-mostly long prismatic grains.The recorded zircon is yellow or yellowish-red to reddish-brown and sometimes colorless to pale yellow,with most transparent(Fig.7c).Uranophane was observed as inclusions in zircon fractures due to hydrothermal alteration of radioelementbearing zircon(Fig.7d).

(c) Uraniferous iron and manganese minerals

Jarositeis a secondary mineral observed in oxidized portions of sulfide-bearing rocks,typically altering from pyrite,and less commonly as a low-temperature,primary hydrothermal mineral and associated with hematite and pyrite(Kato and Miura 1977).Jarosite is a hydrous sulfate of K and Fe.It is yellow to dark brown(Fig.7e).Uranophane was observed as inclusions on the surface of jarosite(Fig.7f).

Pyriteis the main sulfide mineral observed,and is considered the source of the widely distributed iron minerals in the study area.Pyrite is white to pale yellow(Fig.8a).In the samples,it is partially or completely oxidized to iron oxides such as hematite.This process is considered pseudomorphic desulfurization under oxidizing conditions.Desulfurization of pyrite creates voids that can be refilled by secondary materials such as silver.Silver was observed as inclusions on the surface of pyrite(Fig.8b).

Pyrolusiteis one of the common manganese minerals.It forms under conditions of strong oxidation associated withdeposits formed by circulating meteoric water.Pyrolusite grains are subhedral and black(Fig.8c).In addition,ESEM showed autunite on the surface of pyrolusite(Fig.8d).

Table 2 CIPW norm of the studied granitic rocks

Hematiteis the most widespread iron mineral.It occurs as an alteration product of pyrite and silicate minerals.It is often formed by decomposition of iron-bearing minerals.Hematite is dense brown to black(Fig.8e).

6 Geochemistry

The complete chemical analysis and CIPW norm of the investigated granitic rocks are given in Tables 1 and 2.The younger granitic rocks were subdivided into a)Qash Amer and El Sela plutons and b)highly altered El Sela shear zone.

6.1 Geochemistry of Qash Amer and El Sela granites

Qash Amer and El Sela granitic rocks were determined according to the petrographic description using their modal composition,within the classification of Streckeisen(1976)as syenogranites.Middlemost(1985)proposed the SiO2versus Na2O+K2O diagram to classify granitic rocks.Qash Amer and El Sela granitic rocks plot in the granitefield(Fig.9a).On the normative Or-Ab-An ternary diagram of Streckeisen(1976),they plot in the alkali-granitefield(Fig.9b).Both are calc-alkaline and plot in the extensional suite on the AFM(Na2O+K2O-FeO*-MgO)diagram of Irvine and Baragar(1971)and Petro et al.(1979),and peraluminous based on Shand(1951)(Figs.9c,d).

Trace element patterns of the Qash Amer and El Sela show depletion in Ni and K,and enrichment in Rb,Ga,Th,and U(Fig.9e).The investigated granitic rocks are characterized by a)very low contents of compatible elements(Cr,Ni,and Co),suggesting highly fractionated rocks;and b)fairly high contents of incompatible elements such as Zr,and high concentrations of large ion lithophile elements(LILEs),especially Rb,that measured up to 344.6 and 234.3 ppm in the Qash Amer and El Sela granites,respectively.The average total REE content(Table 1)of the studied Qash Amer and El Sela granites was 29.14 and 65.13 ppm,respectively,suggesting that these granites are very depleted in REEs relative to the international range(250-270 ppm)of Hermann(1970).However,the Qash Amer and El Sela samples returned higher light REEs(LREEs)relative to HREEs in comparison with global values.Chondrite-normalized REE patterns of the studied Qash Amer granites were moderately-fractionated-(La/Yb)N=3.47;while the El Sela,well-fractionated(La/Yb)N=19.LREEs of the Qash Amer granites were slightly fractionated La/Sm=1.55;of the El Sela,wellfractionated La/Sm=6.8.Fractionation of HREEs of both the Qash Amer and El Sela granitic rocks were low(Gd/Yb=1.2 and 1.5,respectively).Normalized REE patterns(Fig.9f)of the investigated Qash Amer and El Sela granitic rocks according to Boynton(1984)show positive Ce anomalies,indicating low O2fugacity at the magma source(Constantopoulos 1988).In addition,the REE diagram shows a negative Eu-anomaly,resulting mainly from plagioclase feldspar fractionation together with low Sr content of the Qash Amer and El Sela granitic rocks(37 and 54 ppm,respectively,on average).

6.2 Geochemistry of the El Sela shear zone

Fig.9 Geochemical variations of the Qash Amer and El Sela granitic rocks.a SiO2versus(Na2O+K2O)variation diagram of Middlemost(1985).B Ab-Or-An ternary diagram of Streckeisen(1976).c AFM diagram of Irvine and Baragar,(1971).d CaO-(Na2O+K2O)-Al2O3 discrimination diagram of Shand(1951).e Chondrite-normalized trace element patterns of Pearce(1982 and 1983).f Chondrite-normalized REE diagram(Boynton 1984)

The altered samples of the El Sela shear zone on the normative Qz-Ab-Or ternary diagram of Stempork(1979)all plotted along the silicic trend(Fig.10a).In the Na%-K%binary diagram after Cuney et al.(1989),sample 80 fell in the K-metasomatism field,sample 82 in desilicification,and sample 84 in Na-metasomatism.Sample 83 plotted in the desilicification field,but near the Na-metasomatismfield,indicating the overprint of different alteration processes on these rocks.High Sr content in samples 83 and 84 is mainly related to albitization as indicated in Fig.10b.Meyer and Hemley(1967)classified K-silicate alteration types into(i)propylitic(containing epidote-chlorite alteration),(ii)sericitic(containing K-feldspars converted into sericite),and(iii)potassic(characterized by the alteration of plagioclase into K-feldspar)subtypes.On the AKF([Al2O3+Fe2O3]-[Na2O+K2O+CaO]-[K2O]-[FeO+MgO+MnO])ternary diagram,all samples fell in the sericite facies(due to sericitization),(Fig.10c).The total alteration of source rock can be determined using the chemical index of alteration(CIA)of Nesbitt and Young(1984).The Al2O3-CaO+Na2O-K2O diagram,shows clear discrimination between slight,moderate,and deep alteration(Fig.10d).The ideal feldspar weathering line(IWL),which parallels the Al2O3,CaO+Na2O trend towards illite,is due to post-depositional K-enrichment of the clay fraction(Fedo et al.1995),suggesting K-metasomatism(Fedo et al.1997;Roser et al.2002).Samples from the El Sela shear zone show strong alteration,plotting along the muscovite-illite trend on the Al2O3-K2O axis,with high Al content.They had high CIA(＞70),indicating lots of K-rich clay.Potassium is mostly associated with potassium feldspar(K [AlSi3AlO8])(Shimmieldand Mowbray 1991),and illite((K,H3O)Al2[(OH)2Si3AlO10])(Yarincik et al.2000).

Fig.10 Geochemical variations of El Sela shear zone granitic rocks.a Qz-Ab-Or of normative ternary diagram of Stempork(1979).b Na%-K binary diagram of Cuney et al.(1989).c AKF ternary diagram of Meyer and Hemley(1967).Where A=Al2O3-(Na2O+K2O),K=K2O and F=FeO+MnO+MgO.d Al2O3-(Na2O+CaO)-K2O ternary diagram Nesbitt and Young(1989).e Chondrite-normalized spider diagram for the investigated granitic rocks.f Chondrite-normalized REEs diagram of Boynton(1984)

Fig.11 Geochemical variations of the Qash Amer and El Sela granitic rocks.a K2O-Na2O binary diagram of Hine et al.(1978).b SiO2-Y binary diagram of Günther et al.(1989).c Log Y versus Nb binary diagram of Pearce et al.(1984).VAG=Volcanic Arc Granites,Syn-COLG=Syn-collision granites,WPG=Within Plate Granites,ORG=Ocean Ridge Granites.d Rb versus Sr diagram of Condie(1973).e Ternary diagram of Qz-Ab-Or of Tuttle and Bowen(1958).f K versus Rb diagram of Shaw(1968)shows the average crustal K/Rb ratio of Taylor(1965)

Similar to the Qash Amer and El Sela granites,trace elemental analysis of the El Sela shear zone revealed negative anomalies in Ni,Cr,and K and enrichment in Rb,Ga,Th,and U(Fig.10e).These granites also contain a)very low amounts of compatible elements(Cr,Ni,and Co);and b)high contents of incompatible elements such as Zr(up to 220.3 ppm),implying an enriched source or extensive liquid evolution;and c)a high concentration of LILEs,especially Ba and Sr(up to 354 and 426 ppm,respectively).The average total REE content(Table 1)of El Sela shear zone samples is 173.23 ppm,suggesting that these granites are depleted in REEs relative to the international range(250-270 ppm)of Hermann(1970).They have high LREEs in comparison with HREEs.Chondrite-normalized REE patterns(Fig.10f)of El Sela shear zone altered granites are highly fractionated(La/Yb)N=11.1.LREEs of the Qash Amer granites and El Sela shear zone are moderately fractionated(La/Sm=2.62)compared with HREEs(Gd/Yb=12.55).

Table 3 Radiometric of[eU,eTh,Ra(eU,ppm)and K%]and chemical measurements of(U and Th)for the studied granitic rocks

7 Discussion

7.1 Topology,tectonic setting and petrogenesis of Qash Amer and El Sela granitic rocks

Fig.12 Variation diagrams of Qash Amer and El Sela granitic rocks Symbols as in Fig.4.a eU versus(eTh/eU).b eTh versus(eTh/eU)and d eU versus eTh

Hine et al.(1978)used the relationship between Na2O and K2O to differentiate between S-and I-type granites.Qash Amer and El Sela granitic samples fell in the I-type field(Fig.11a).Similarly,on the SiO2versus Y binary diagram of Gunther et al.(1989),all samples fell in the I-type field(Fig.11b).Pearce et al.(1984,p.956)classified granitic rocks ‘‘a(chǎn)ccording to their intrusive settings into four main groups-ocean ridge granites(ORG),volcanic arc granites(VAG),within-plate granites(WPG)and collision granites(COLG)…''Our samples occupy VAG,according to a log Y-Nb binary diagram(Fig.11c).The Rb-Sr relationship of Condie(1973),suggested for the determination of crustal depth at which granitic magmas evolved,indicates these granites were emplaced at relatively shallow to moderate depths of 20-30 km(Fig.11d).The Qz-Ab-Or ternary normative diagram of Tuttle and Bowen(1958)suggests that the younger granites formed at pressures of 1-2 kb(Fig.11e).On the K/Rb binary diagram of Shaw(1968),the analyzed samples plot around the crustal line(K/Rb=250)suggested by Taylor(1965)and away from the mantle line(K/Rb=1000)(Fig.11f).This reflects their high K content and suggests their derivation from lower crust materials rather than from the upper mantle.

7.2 Radioactivity

7.2.1 Distribution and geochemistry of uranium and thorium of the studied granitic rocks

Radioelements are mostly found in silicic,potassic rocks;normal values of U and Th in granitic rocks are 4.75×10-6and 17×10-6,respectively(Rogers and Adams 1969).

Average values of eU,eTh,Ra,and K of the studied granitic rocks are given in Table 3.The eU content ranged from 11 to 16 ppm with an average 13.2 ppm,eTh from 10 to 15 ppm with an average 12.8 ppm,Ra(eU)from 10 to 18 ppm with an average 12.8 ppm,eU/eTh from 0.85 to 1.2 with an average 1.04,and eTh/eU from 0.85 to 1.16 with an average of 0.97,all in Qash Amer granites.Qash Amer granitic rocks are enriched in uranium according to Clark et al.(1966)and Rogers and Adams(1969).The eU content of El Sela granites ranged from 6 to 34 ppm,with an average 10.9 ppm;eTh from 15 to 39 ppm with an average 26.3 ppm;Ra(eU)from 3 to 12 ppm with an average 5.8 ppm;eU/eTh from 0.27 and 0.89 with an average 0.41;and eTh/eU from 1.11 to 3.66 with an average 2.7.Rogers and Adams(1969)found higher contents of U and Th in El Sela granites than Clark et al.(1966).The eU content in samples from the El Sela shear zone(mineralized zone)ranged from 9 to 57 ppm with an average 29 ppm,eTh from 0.4 to 20 ppm with an average 10.6 ppm,Ra(eU)from 10 to 82 ppm with an average 33.5 ppm,eU/eTh from 0.45 to 95.0 with an average 28.77,and eTh/eU from 0.01 to 2.2 with an average 0.96.The studied rocks(Qash Amer,El Sela,and El Sela shear zone)are considered uraniferous granites(Darnely 1982;Assaf et al.1997);uraniferous granites are those that contain at least twice the Clark value of uranium(4 ppm,Clark et al.1966).

Fig.13 Variation diagrams of El Sela shear zone:a eU versus(eTh/eU),b eTh versus(eTh/eU)and c eU versus eTh.Symbols as in Fig.7a

The concentration of chemically determined uranium(Uc)and thorium(Thc)of the studied granitic rocks is given in Table 3.Ucin Qash Amer granites ranged from 6.98 to 9.80 ppm with an average 8.34 ppm;Thccontent from 10.3 to 11.45 ppm with an average 10.73 ppm.Uccontent in El Sela granites ranged from 3.7 to 6.4 ppm with an average 4.76 ppm;Thcfrom 19.2 to 24.5 ppm with an average 21.52 ppm.The lower values of Ucin comparison with radiometrically determined values in Qash Amer and El Sela could be related to migration of uranium from these granites to the surrounding rocks.Concentration of Ucin El Sela shear zone samples ranged from 10 to 1305 ppm with an average 386.9 ppm;Thccontent from 16.3 to 19.5 ppm with an average 18.3 ppm.The high contents of Ucin the El Sela shear zone compared to radiometrically determined uranium may be attributed to the presence of recent uranium deposits.

7.2.2 Geochemistry of uranium and thorium in the studied granitic rocks

The geochemical behavior of U and Th in the studied granitic rocks was examined by plotting several variation diagrams.The amount of U remobilization within granitic plutons can be determined by variation diagrams of eU and eTh with their ratios(Charbonneau 1982;El Galy 2007).

The variation between eU with eTh/eU ratios of Qash Amer and El Sela granitic rocks shows negative correlation(Fig.12a),whereas eTh with eTh/eU ratios shows positive correlation(Fig.12b).This suggests that the distribution of radioactive elements is not only magmatic but also due to hydrothermal activity.The relationship between U and Th is helpful in determining if there is an enrichment or depletion of U and Th.A variation diagram of eU and eTh of the studied Qash Amer and El Sela granites displays an ill-defined trend(Fig.12c),indicating hydrothermal influence in the redistribution of these elements.The same conclusion is obtained from the negative correlation seen in the variation diagram of eU with eTh/eU ratios of the El Sela shear zone(Fig.13a);in contrast,eTh with eTh/eU ratios shows positive correlation(Fig.13b).A variation diagram of eU and eTh for El Sela shear zone samples showsnegative correlation (Fig.13c),indicating the important role of hydrothermal solutions in redistribution of these elements.

Distribution maps of eU,eTh,Ra(eU),K,eU/eTh,eTh/eU,and P-factors based on radiometric measurements show the highest eU,Ra(U),K,and eU/eTh concentrated in the north(El Sela shear zone).The highest concentrations of eTh and eTh/eU are in the central and southern parts of the El Sela pluton(Fig.14).

Fig.14 Distribution maps of eU,Ra,K,eU/eTh,eTh,eTh/eU and P-factor based on radiometric measurements

7.2.3 Mobilization and migration of uranium and thorium

1. eU/eTh ratio:eU/eTh ratio is a very important geochemical indicator for U mobilization(Naumov 1959).The eU/eTh ratio for the granitic rocks is commonly about 0.33(Stuckless et al.1977;Boyle 1982;El Galy 2007;El Nahas et al.2011).In Qash Amer granitic rocks,eU/eTh ratio ranged from 0.85 to 1.2 with an average 1.04;in El Sela granites,from 0.27 to 0.89 with an average 0.41;and from 0.45 to 95 with an average 28.77 in El Sela shear zone samples,suggesting a high degree of uranium mobilization and enrichment in the shear zone.

2. Equilibrium P-and D-factors:P-factor(eU/Ra,Hussein 1978)and D-factor(Uc/eU,Hansink 1976)greater than one indicates addition of uranium;less than one,removal of uranium.Addition or removal of uranium suggests certain geological processes such as alteration,which causes disturbance in the equilibrium state.P-and D-factors of the studied granitic rocks are presented in Table 3.P-factor ranged from 0.78 to 1.4 with an average 1.06 in Qash Amer granitic rocks,from 0.91 to 3.77 with an average 1.93 in El Sela granites,and from 0.5 to 1.58 with an average 0.86 in the El Sela shear zone.D-factor ranged from 0.49 to 0.75 with an average 0.63 in Qash Amer granitic rocks,from 0.13 to 0.8 with an average 0.54 in El Sela granites,and from 0.83 to 34.3 with an average 10.1 in the El Sela shear zone.

From the previously calculated P-and D-factors,it can be concluded that there is more than one stage of uranium mineralization;the first occurred in the last 1.5 m.y.(Reeves and Brooks 1978).Cathelineau and Holliger(1987)stated that uranium mineralization is affected by different stages of alteration-such as leaching,mobility,and redeposition ofuranium-thatare affected by hydrothermal solutions and/or supergene fluids,which cause disequilibrium in the radioactive decay series in U-bearing rocks.The later stages of uranium mineralization are represented by recent uranium mineralization evidenced by the large difference between chemically and radiometrically determined uranium.Sadek(2017)concluded from fluid inclusion studies on El Sela granites that the mineralizing fluids were transported mainly through two stages;the first was CO2-rich,at higher temperature(up to 259°C)and high salinity(up to 29 wt%NaCl equ.),suggesting that uranium was transported mainly in the form of uranyl-monocarbonate complex(UO2CO3).Secondary uranium mineralization could have resulted from aqueous mineralized fluids given the temperature gradient away from the heat source(from 198 to 209°C).The fluid/rock interaction and the salinity change(from 15 to 24 wt%NaClequ.)due to post-magmatic mixing ofthe hydrothermal fluids with shallow meteoric water along the fracture system(Gaafar 2005;Shahin 2011)lead to a change in pH and consequent precipitation of secondary uranium as uranophane and autunite.

7.3 The rare earth element tetrad effect

The REE tetrad effect is characterized by a kinked pattern,especially in highly evolved granitic rocks.Both M-type and W-type REE tetrad effects can be observed in a peraluminous melt,with the former in the residual melt phase and the latter in the fluid(Masuda et al.1987).An MW-type of REE tetrad effect has been recorded previously in China(Zhao et al.2008;Cao et al.2013).The REE pattern kinks are characterized by prominent convex and concave tetrads and negative Eu anomalies.Visual inspection suggests that the third and fourth tetrads in most samples are more prominent than the first.The second tetrad is comparatively difficult to recognize due to the anomalous behavior of Eu and the fact that Pm does not occur in nature.Figures 9f and 10f and Table 1 show the strong M-type tetrad effect in the fourth tetrad and strong W-type tetrad effect in the third tetrad observed in samples of Qash Amer and El Sela granitic rocks.In addition,samples 82 and 84 of the El Sela shear zone had an M-type tetrad in the fourth segment and W-type tetrad in the third segment.Sample 83 had a W-type tetrad in the first and third segments.The index of tetrad effect intensity,TE1,4,ranged from 1.11 to 1.5 and from 1.07 to 1.31 in Qash Amer and El Sela granitic rocks,respectively,implying an interaction between melt and water-haloid-rich fluid when these granites crystallized from magma.

MW-type tetrad effect may result from the interaction of aqueous liquids with alkaline rocks(Zhao et al.2008).In Kab Amiri granite,CED,Egypt,Mehdy and El-Kammar(2003)revised the convex(M-type)T1to concave(W-type)T3due to changes in the physico-chemical conditions that prevailed during alkali-metasomatism.Zhao et al.(2010)stated that the peculiar MW-type tetrad effect might be an indicator of Au mineralization of reworked plutons and this vision could be applied in the study area where Au up to 1 ppm was recorded through atomic absorption analyses at NMA.

7.4 Isovalent parameters

Hf and Zr are similar in geochemical behavior,resulting in a small range of ratios in geological materials(Dostal and Chatterjee 2000).In most igneous rocks,Zr/Hf ratios fall in a narrow range of 33-40 and deviation from the range is related to metasomatism or intense fractionation of accessory minerals.Granites with Zr/Hf ratio＜20 have been affected by strong hydrothermal alteration(Irber 1999).The Zr/Hf ratios determined for the Qash Amer granites(15.87-22.5),El Sela granites(22.92-29.73),and El Sela shear zone granites(22.40-30.94)are lower than the norm for geological materials.The chondritic value of Y/Ho ratio is 28.8(Bau 1996).The studied samples returned Y/Ho ratios higher than the chondritic value:Y/Ho ratio of the Qash Amer granites was 31.27-40.00;El Sela granites 35.25-58;and El Sela shear zone granites 34.88-47.These higher ratios suggest complexation with fluorine.Complexation with bicarbonate causes Y/Ho values＜28.This could be indicated by the presence of fluorite and fluoritization accompanied by U mineralization.The chondritic value of Nb/Ta is 17.6±1 according to Jahn et al.(2001);the studied samples had Nb/Ta ratios lower than the chondritic:Nb/Ta of Qash Amer samples was 10.99-13.73;the ElSela 8.67-12.30;and ElSela shearzone 10.44-25.97.Chondritic values of La/Nb and La/Ta ratios are 0.96-1 and 16-18,respectively,according to Jahn et al.(2001).The studied samples returned non-chondritic La/Nb and La/Ta ratios:the La/Nb and La/Ta ratios of the Qash Amer granites were 0.07-0.3 and 0.83-3.5;the El Sela granites 1.4-3.6 and 12.44-44.4;and the El Sela shear zone granites 0.97-1.77 and 13.4-36.28;all respectively.The non-chondritic ratios are additional evidence of hydrothermal activity.

Rb/Sr ratio increases with differentiation;this is due to Sr being depleted in the liquid magma by crystallization of feldspar,while Rb is enriched.This ratio ranged from 2.86 to 18.13 in Qash Amer granite samples,from 2.49 to 14.49 in El Sela samples,and from 0.5 to 4.43 in El Sela shear zone samples.This is probably due to the effect of albitization accompanied by Sr enrichment.Rb,Ba,and Sr are the most useful trace elements in evaluating fractional crystallization in these rocks since their behavior is strongly related to essential minerals such as feldspar(Neiva et al.1987).K,Rb,and Ba have similar ionic radii,as do Ca and Sr.Therefore,Rb and Ba are usually incorporated in K-bearing minerals such as K-feldspar,and Sr is usually found in Ca-bearing minerals such as plagioclase.Rb content increases in the melt fraction during fractional crystallization due to high Rb relative to K resulting in a higher bond energy between K and O than between Rb and O.Thus,K is removed,while Rb tends to concentrate in residual melts,leading to decreasing K/Rb ratios(Ekwere 1985).Ba2+is identical with K in ionic size and incorporated in K+sites of early crystallizing minerals.

High Rb may be associated with K in sericite,and clay minerals may be related to the loss of Ba(Weaver and Pollard 1973).Sericitization and kaolinization associated with mineralization of granitic rocks cause an increase in Rb/Sr ratio and decrease in Ba/Rb.Ba,Sr,and Rb analyses revealed that Qash Amer granites are more fractionated than El Sela granites.Ba ranged from 56 to 154 ppm in Qash Amer and 19-314 ppm in El Sela.In samples 83 and 84,high concentrations of Sr(426 and 378 ppm,respectively,)may be related to Na-metasomatic alteration processes.In addition,high Zr(220.3 ppm)is mainly controlled by the presence of accessory minerals such as zircon.

Granitic rocks are the main host of U mineralization in many parts of the world.Uranium in granites can be divided into two types:primary and secondary(Jiashu and Zehong 1982).The former forms during magma crystallization,whereas the latter is precipitated later from dissolved and transported uranium.The secondary uranium can be subdivided into three types:(a)absorbed uranium in altered minerals such as montmorillonite and chlorite;(b)interstitial uranium,formed as a result of hydrothermal solution migration along mineral boundaries;and(c)uranium in microfractures,emplaced by the circulation of hydrothermal solutions after the deformation of rocks(Ali and Lentz 2011).Most of the Egyptian uranium occurrences in younger granites,such as the Gabal Um Ara(Ibrahim 1986;Abdalla et al.1994)and Gabal Gattar(Roz 1994),are related to fracture-filling;whereas that in Gabal Missikat(Abu Dief 1985),Gabal Erediya(Hussein et al.1986;Abu-Deif 1992;Abdel Naby 2008;El Mezayen et al.2015),and Gabal El Sela(Assaf et al.1996;Ibrahim et al.2005)is the vein type.

Granites associated with uranium and other mineralization have higher Rb/Sr and lower Ba/Rb ratios because of intensive fractionation and increasing volatile activity.The highest Rb/Sr and lowest Ba/Rb are recorded with intensive alteration mostly associated with mineralization(Nockolds and Allen 1953;Taylor 1965;Imeokparia 1981).The uraniferous character of the studied granites is indicated by element contents,including CaO＜0.97%,Zr/Sr＞1.65,Na2O/K2O＜1.11,Rb＞185 ppm,and high Rb/Sr ratios(＞2.5)(Cuney 1984;Assaf et al.1997;Fawzy 2017).

All the studied granitic samples contained CaO＜0.97%and Rb＞185 ppm.The average of Na2O/K2O was 1.1,0.98,and 0.14 for the Qash Amer,El Sela,and El Sela shear zone,respectively.Average Zr/Sr was 2.29,2.44,and 0.79 for the Qash Amer,El Sela,and El Sela shear zone,respectively.Rb/Sr ranged from 2.86 to 18.13 for Qash Amer granites,2.49-14.49 in El Sela granites,and 0.5-4.43 in El Sela shear zone granites.Therefore,the studied granitic rocks are uraniferous.

The alteration processes(e.g.kaolinization,albitization,sericitization,and hematitization)in the El Sela shear zone were induced by hydrothermal solutions.The clay materials are considered a barrier and good host for the supergene mineralizing fluids that move through the fractures and shear zones,due to the ability of impermeable clay to adsorb uranium.U is adsorbed on clay minerals,filling cracks,at pH 6 and 200°C(Langmuir 1978;Giblin et al.1981).

In addition,ferrugination causes an increase in total Fe2O3content at the expense of other oxides.A strong alkaline solution will precipitate Fe+3and U+6within the microfractures in the form of iron oxyhydroxides rich in uranium(Cuney et al.1984;Abaa 1991;Finch 1992;Abd El Gawad et al.2014).

8 Conclusions

Field,petrographical,and geochemical studies suggest the El Sela shear zone has been subjected to hydrothermal and supergene alterations such as kaolinization,albitization,sericitization,and hematitization.These alterations are generally associated with secondary uranium mineralization.Petrographically,the studied granitic rocks are represented by(a)Qash Amer muscovite granites;(b)El Sela two mica granites;and(c)highly altered granites in the El Sela shear zone.The studied Qash Amer and El Sela granitic rocks are characterized by high SiO2(more than 75%),Na2O,and K2O.Highly altered granitic rocks of the El Sela shear zone are higher in Fe2O3T,CaO,and LOI.The LOI reaches up to 6.46 wt%,supporting the role of hydrothermal alteration.The studied granitic rocks are considered uraniferous granites.The eU/eTh ratios in Qash Amer,El Sela granites,and El Sela shear zone granitic rocks of 1.04,0.41,and 28.77,respectively,suggest a high degree of uranium mobilization and enrichment.Most samples show REEs-patterns having M-type tetrad effect in the first and fourth segments and W-type tetrad in the third segment.The average∑REE in the studied granites was lower than the average of the worldwide granites,but with higher LREE contents relative to HREE.