Geochem istry,m ineralogy,and radioactivity of the Abu Furad Area,Central Eastern Desert,Egypt

A.M.El M ezayen·E.K.Abu Zeid·W.S.Hosny·M.G.El-Feky·S.M.Om ar·S.A.Taalab

Abstract Abu Furad is in the Central Eastern Desert,Egypt.Itcomprisesmetasediments,metavolcanics,metagabbros,syn-to late-orogenic granites,and post-orogenic granites,in addition to numerous dykes and veins of different shapes and composition invading all the older rocks cropping out in the study area.Field,petrographic,mineralogical,and chem ical investigations led to the classif ication of Abu Furad granites as quartz diorite,tonalite,granodiorite,and syenogranites.Major oxide and trace element data revealed that the syn-to late-orogenic granites and post-orogenic granites are peralum inous.Syn-to lateorogenic granites originated from calc-alkaline volcanic arc—relatedmagma;while the post-orogenic granites(syenogranite)arehighly fractionated,calc-alkalinegranite from a w ithin plate regime.Radiometrically,the studied quartz diorite,tonalite,and granodiorite had loweruranium and thorium contents and higher eU/eTh ratios than the syenogranites.This may indicate that the syn-to lateorogenic rocks originated from sources depleted in these elements.The average eTh/eU ratio of syenogranites was higher than thatofaveragecontinentalcrust,suggesting that the syenogranites are relatively depleted in U.The studied altered syenograniteswere strongly enriched in U and Th compared to the Earth's crust.On the other hand,the averageofeU in pegmatitesis lower than theglobalaverage for uraniferous pegmatites.The most recorded accessory m inerals in the altered syenogranites were thorite,fergusonite,samarskite,columbite,zircon,monazite,xenotime,apatite,f luorite,sphene,atacam ite,andmalachite,in addition to chromium and nickel inclusions.

Keywords Hydrothermal alterations·Major oxide and trace elements·Radiometry·M ineralogy·Geochem istry·Altered syenogranites·Egypt

1 Introduction

The study area is part of the Central Eastern Desert of Egypt.It covers about 350 km2of crystalline basement rocks from 26°30′20′′to 26°43′32′′N and 33°32′35′′to 33°50′27′′E(Fig.1).The area has previously been studied by several authors.El-Gaby(1975)concluded that the granitic rock of Abu Furad is granodiorite,while Habib(1982)presented the younger Abu Furad granite as a composite pluton representing the transition from the synorogenic granitoids to late-orogenic granites.Ghobrial and Girgis(1982)indicated that the granitoid rocksof theWadi El-Bulah area aremainly quartz diorite,tonalite,and granodiorite.Habib(1987)showed that the ultramaf ic—maf ic rocksexposed in the Pan-African basementbetween Gabal Meatiq and Gabal Abu Furad represent arc ophiolites.Mahmoud(1995)concluded that the older granitoids andyounger granites originatedunder compressional and extensional forces,respectively.Esmail and Moharem(2009)described unzoned and zoned pegmatite pockets as themost important rock types from a radioactive point of view.Azab(2011)studied the petrography,geochem istry,m ineralogy,and radioactivity of the Wadi Safaga area to the north of Gabal Um El-Huw itat.El-Mezayen et al.(2015)studied the petrography and geochem istry of altered granites in the Abu Furad area and documented different degrees of hydrothermal alteration,including silicif ication,hematitization,sericitization,saussuritization,chloritization,andmuscovitization.The aim of the presentstudy was to exam ine the geology and geochem istry of granites in the Abu Furad area,in addition to investigating the radioactivity and m ineralogy of fresh and altered granites and pegmatites.

Fig.1 Locationmap and landsat-7 ETM+band 7,4,2 in R,G,B color channels of Abu Furad area,Central Eastern Desert,Egypt

2 M ethodology

A Nikon polarizing m icroscope equipped w ith an automatic photom icrographic andmechanical stage attachment was used to identifym inerals and textures aswell as study opaquem ineralogy of the rocks under investigation.

Chem ical analyses of whole-rock samples were carried out inACMEanalytical Laboratories of Vancouver,Canada,formajor oxides,trace and rare earth elements by inductively coupled plasma-em ission spectrometry(ICPES)and ICP-mass spectrometry(ICP-MS).Detection lim its for major oxides and trace elements were,respectively,0.001 w t%to 0.04 w t%and 0.01 to 0.5 ppm.Analytical precision,as calculated from replicate analyses,was 0.5%formajor elements and varied from 2%to 20%for trace elements.

M ineral separation and identif ication were carried outat the Laboratories of the Nuclear Materials Authority(NMA).The heavy liquids separation technique using bromoform of specif ic gravity 2.85 gm/cm3was used to concentrate the heavy m inerals.Then,the separation of magnetite was achieved by hand magnet.The heavy mineral fractions were passed through a Frantz isodynam ic magnetic separatormodel L-1,at a side tiltof 5°and forward slope 20°,to separate the remaining magnetite and produce severalmagnetic fractions at 0.2,0.5,0.7,1,and 1.5 amperes.Each of these fractions contained its own characteristic m inerals.M ineral identif ication was performed through X-ray diffraction(XRD)and environmental scanning electron m icroscope(ESEM).

Radiometric analyses(using quantitative gamma-ray spectrometry)were carried out to measure equivalent uranium(eU)and equivalent thorium(eTh)in the f ield by using the Eberline Smart Portable meter(ESP-1),which measures the total gamma ray intensity em itted from rocks in counts per second(cps).Amultichannel analyzer gamma ray spectrometerwasused to determine U,Th,Ra,and K concentrations in the laboratory.Thesystem consists of a NaI(Tl)Bicron scintillation detector connected to a NE-4658 amplif ier and a high voltage—power supply with digital display.

Whole-rock concentrationsof U,Th,and K(w t%)in the studied granites were determ ined by gamma-ray spectrometry.Quantitative determination of these elementswas obtained from a NaI(TI)scintillation detectorhoused at the Geochem ical Exploration Department of the NMA of Egypt.About 350 g of each sample was crushed to grain size 1mm and then packed in plastic containers.The sealed sampleswere then stored forabout4 weeks to allow accumulation of free radon for the purpose of attaining radioactive equilibrium.Twogamma-em ittingsources provided by the International Atom ic Energy Agency(IAEA)were used,namely57Co and137Cs,for f ine and coarse adjustments,respectively.

3 Geologic setting

Granitoid rocks are themost abundant rocks in the Egyptian basement complex and constitute an important rock group that covers vast areas of the Arabian-Nubian Shield in Egypt.They cover about35,000 km2,constituting about 35%of the basement and about 70%of the exposed basement rocks of the Sinai Massif(Bentor 1985).The Egyptian granitoids have attracted the attention of many researchers who have classif ied them according to:(1)Type locality(Shaitian and Gattarian granites),(2)Relative age(older and youngergranites),(3)Dom inantcolor(gray,red,and pink granites),(4)Relation to orogeny(syn-,late-,and post-orogenic granites),and so on.

El-Gaby(1975)divided thegranitoid rocks into:(a)synorogenic granitoids comprising the Shaitian and gray granites aswell as the slightly later phase Aswan granite,and(b)post-orogenic granites comprising the younger pink—red granites.Later,El Gaby and Habib(1982)classif ied the Egyptian granitoids into:

(1)An older,syn-to late-orogenic,calc-alkaline granite series comprising the‘‘old grey granites,''the porphyritic granites of Aswan,and two feldspar‘‘younger granites.''These series correspond to the group A granitoids of El-Shatoury et al.(1984)and encompassboth the G Iand G IIgranites of Hussein et al.(1982).

(2)Ayounger,post-tectonic,alkaline-to-peralkaline granitic series comprising quartz syenite,alaskite,and aegirine-or riebeckite-bearing leucocratic granites.This series corresponds to the group B granitoids of El-Shatoury et al.(1984)and to the G IIIgranites of Hussein et al.(1982).Rogers and Greenberg(1990)classif ied the granites into four categories:late-orogenic(LO),post-orogenic(PO),anorogenic(AR),and ring complex(RC)based solely on tectonic occurrenceand associated rock types.Theyounger granites of Egypt fall in the PO category.

Ahmed and El-Mahallaw i(1995)studied the petrology,geochem istry,and petrogenesis of granitoid rocks sampled from twelve granitoid masses in the northern and central Eastern Desert,and classif ied them into older syn-orogenic(ranging from tonalite to granodiorite composition and locally having trandhjem itic aff inity),and younger granitoidsof phase I(alkaligranites)and phase II(adamellites).

Early ages obtained for the Egyptian post-orogenic granites range from 568 to 597Ma(Fullagar and Greenberg 1978;Hashad 1980);later dating by Hassan and Hashad(1990)proposed a range of 530 to 620 Ma.On the other hand,the syn-to late-orogenic rocks were dated by Hassan and Hashad(1990)to amaximum of 850 Ma and a m inimum of 614 Ma.

The study area contains several rock types,including metasediments,metavolcanics,metagabbros,and granitoids.Granitoid rocks are from various geologic settings and present varied m ineral compositions.They were emplaced over a long time span and include quartz diorite,tonalite,granodiorite,syenogranite,dykes,and veins.A 1:50,000 geologic map of the study area was generated using aerial photographs,Thematic Mapper Lands at images,extensive f ieldwork,and more than one hundred samples collected from the area(Fig.2).

Based on f ield geologic mapping,mode of occurrence,and f ield relationships,themain rock units exposed in the Gabal Abu Furad area,were chronologically arranged as follows:

Metasediments are represented by a small belt of hornblende schists outcropping at the southeastern corner and the west-central region of the mapped area form ing circular outcrops around the El-Bulla granodiorite.Thesemetasedimentsareseparatedfromthesurrounding metavolcanics byasharpandwell-def inedcontact(Fig.3a).

Fig.2 Geologic map of the Gabal Abu Furad area[modif ied after Fow ler et al.(2006)]

Metavolcanics(metadacite)occupy much of the southeastern quarter of themap.These rocks are f ine-to medium-grained and greenish black to reddish black,gray,and pink in color.They are highly fractured,altered and sheared(Fig.3b).

Metagabbros cut across the central part of themapped area.They occur asmassive,gray,compact rock ofmedium to coarse grain size.They are commonly intensely jointed and fractured and are highly altered inmany parts,especially along faultplanes(sericitized and saussuritized).These rocks are frequently cutby quartz veinsand invaded by felsic dykes and pegmatite bodies(Fig.3c).The metagabbros,together w ith the regionally metamorphosed volcanics,represent the earliestmanifestations of islandarc activity in the Arabian-Nubian Shield(Abdel-Rahman 1990;El-Gaby etal.1990).Rb—Srwhole-rock dating of the metagabbrodiorite complexes has yielded early Pan-A frican ages ranging from 980 to 870 Ma(Hashad 1980;Abdel-Rahman and Doid 1987).

Syn-to late-orogenic granites represent the most predom inant lithostratigraphic unitexposed in the study area.They are exposed w idely along Qena-Safaga road and reach batholithic size.In some parts,the batholith is intruded by younger pink granites such as the Gabal Abu Furad,Gabal Umm Taghir El-Foqani,and El-Tahtani.Theserocksexhibitgradationalcontactsw iththe metasediments and the metagabbros and sharp contacts w ith the late-orogenic granites(Fig.3d).Threemain rock types represent the syn-to late-orogenic granites in thestudy area:quartz diorites,tonalities,and granodiorites.These can be differentiated in the f ield according to their color,m ineralogical composition,and visible textural relationships.Generally,they are gray to grayish white and medium-to coarse-grained.The three rock types sometimes show gneissic structure due to the orientation of the maf ic constituents.Manganese dendrites are sometimes observed on the surfaces of the joints and fractures.

Fig.3 Macrophotographs of a Sharp contact betweenmetavolcanics(M.V)and metasediment(M.S).b Highly fractured,altered and sheared metavolcanics(M.V).c Pegmatite bodies invading themetagabbros(M.G).d Sharp contactbetween syn-to lateorogenic granites(LO)and post orogenic granites(PO)

The studied post-orogenic granites are medium-to coarse-grained and distinguished by their pink to red color and high topographic relief.In addition,they are characterized by small outcrops of sem icircular-to-oval outline that rise up to 1032 m above sea level.The late-orogenic granites contain xenoliths of different shapes and sizes from older rocks.The post-orogenic granites are highly fractured and jointed,and altered in places by secondary processes,especially along the fault planes and contacts.

4 Petrography

4.1 Syn-to late-orogenic granites

According to petrographic examinations,syn-to late-orogenic graniteswere classif ied asquartz diorite,tonalite,and granodiorite.The syn-to late-orogenic granite samples were medium-to coarse-grained and whitish pink and grayish white in color,w ith hypidiomorphic granular texture.They were composed primarily of plagioclase,quartz,potash feldspar,hornblende,and biotite.Sphene and iron oxides were present as accessory m inerals,w ith epidote,chlorite,and sericite as secondary m inerals.Plagioclase was observed as tabular euhedral to subhedral crystals,comprising 51.5%to 64.2%of the rock.Plagioclase crystals generally showed pericline tw inning and sometimes normal zoning(Fig.4a).Quartz was found as mediumgrained,subhedral to anhedral crystals.Some quartz crystals showed intergrow th w ith orthoclase,exhibiting granophyric texture.Potash k-feldspars were represented mainlybyorthoclase,orthoclasem icroperthite,andm icrocline.Orthoclase sometimes showed varying degree of kaolinization.They rarely showed clear simple tw inning due to alteration effects(kaolinization and sericitization)(Fig.4b).

Fig.4 Photomicrographs of the syn-to late orogenic granites show ing:a Euhedral zoned plagioclase(PL).b M icrocline crystals associating quartz(QZ)and plagioclase(PL).c Pennenite(Pen)associating epidote.d Allanite crystals associating chlorite(Chl)and plagioclase(PL).e Euhedral sphene crystals associating quartz and opaqueminerals(Fe-ox).f Myrmekitic texture

Hornblende appeared as green anhedral to subhedral crystals.Someof thehornblende crystalshad been partially or completely altered to f laky green chlorite.Some hornblendecrystals showedsimpletw inning,sometimes replaced by biotite and altered to penninite(Fig.4c).Biotite was observed both as separate subhedral crystals and as elongated crystals,greenish brown to brown in color.Biotite f lakeswere slightly to completely altered to chlorite and some crystals were included in allanite(Fig.4d).Sphene was observed as euhedral to subhedral pale brown crystals of irregular and sphenoid shapes associatedw ithbiotite,quartz,andopaque m inerals(Fig.4e).Mostsphene crystalsappeared to have formed as secondary m inerals after ilmenite or titanomagnetite.Epidote occurred as secondary fracture-f ill associated w ith sphene,plagioclase,and chlorite.It was found as small greenish to yellow crystals.Some samples displayed myrmekitic texture(Fig.4f).

4.2 Post-orogenic granites

According to petrographic exam ination,the studied postorogenicgranites were categorizedas syenogranites.Syenogranite samples were generally equigranular and medium-to coarse-grained,w ith hypidiomorphic texture.Potash feldspars,quartz,plagioclase,biotite,and muscovitewereobserved asmainm inerals.Zircon,sphene,and iron oxide were themain accessory minerals.Muscovite,epidote,and clay were found as secondary minerals.

Potash k-feldspars were themost dominantmineral in these rocks,varying from 35%to 50.5%.They were represented by m icrocline and m icrocline perthites.M icrocline appeared assubhedral crystalsand sometimes showed wavy extinction and cross-hatched tw inning.Most of the microcline perthite crystals were f lame type;perthitic texture was also present due to intergrow th between plagioclase and potash feldspar(potash k-feldspar host)(Fig.5a).They formed intergrow th w ith quartz to give micrographic texture(Fig.5b).Myrmekitic texture is shown in Fig.5c.Quartz crystals showed distinct cracking and undulatory extinction,indicating high strain effects(Fig.5d).Plagioclase was observed as subhedral to euhedral crystals,w ith some grains cracked and show ing lamellar and simple tw inning.Biotitewas seen as irregular f lakes with slight alteration to chlorite,especially along grain boundaries.Zircon was recorded as prismatic euhedral to subhedral crystals and usually observed w ith very high relief and high interference color(Fig.5e).Sphene displayed its characteristic sphenoidal shape,wasbrown or yellow ish brown in color,and formed euhedral to subhedral crystals.Epidote was found f illing fractures as small greenish yellowcrystals altered fromplagioclase and associated w ith sphene and chlorite(Fig.5f).

Thegraniticmassshowsdifferentdegreesof hydrothermal alteration,including silicif ication,hematitization,sericitization,saussuritization,chloritization,and muscovitization(El-Mezayen etal.2015).

5 Geochem istry of granitic rocks

The granitic rocks of the study area are syn-to late-orogenic granites and post-orogenic granites.Major oxides(w t%)and trace element contents(mg/kg),along with computed CIPW norms,are given in Tables 1 and 2.Synto late-orogenic granites fell in the tonaliteand granodiorite f ields of Streckeisen's(1976)Ab—Or—An ternary diagram,while the post-orogenic granites displayed composition rangingfromsyenogranite toalkali feldspar granite(Fig.6a).On Irvine and Baragar's(1971)SiO2versus(Na2O+K2O)variation diagram,the studied syn-to lateorogenic granitesand post-orogenic granitic samples fell in the subalkaline f ieldw ith alkalineaff inity(Fig.6b).Maniar and Piccoli's(1989)Al2O3/(CaO+Na2O+K2O)versus Al2O3/(Na2O+K2O)variation diagram shows the syn-to late-orogenic granites samplesmainly in themetalum inous f ield w ith some samples in the peralum inous f ield.

On the other hand,post-orogenic granites samples fell mainly in the peraluminous f ield with some on the contact between the peralkaline andmetaluminous f ields(Fig.6c).The peraluminous nature ismainly attributed to the presence of primary muscovite and normative corundum and absence of any alkaline ferromagnesian minerals,such as aegirine or riebeckite.The plotting of syn-to late-orogenic granites in the metalum inous f ield indicates that magma generation occurred w ithin amature,thickened lower crust.

A log Rb-(Y+Nb)and log Y-Nb binary diagram after Pearce etal.(1984)was used to determ ine tectonic setting of the granitic rocks(Fig.6d);the studied syn-to lateorogenic granites fell in the f ield of volcanic arc granitoids(VAG),while the post-orogenic granites plotted in the within plate granite(WPG)and ocean ridge granite(ORG)f ields.On Mason's(1966)Ba/Rb variation diagram,the syn-to late-orogenic granites and most syenogranite samples plotted around the Ba/Rb line(4.4),while some syenogranitesamples plottedabovetheBa/Rbline(4.4×10-1)due to Rb enrichment(Fig.6e).White and Chappell(1983)diagram shows the studied syn-to lateorogenic granites w ithin the I-type f ield,whereas all syenogranite samples fell in the A-type granite f ield(Fig.6f).

6 Radioactivity

The granitic rocks are strongly enriched in U and Th compared w ith basaltic or ultramaf ic melts(Faure 1986).Radioactive m ineralization formed during the pegmatitic and hydrothermal stages of magmatic differentiation is mostly controlled by the geochemical behavior of U and Th.The large and highly charged ions of these elements prevent their incorporation in the lattices of the m inerals formed during the earlymagmatic stages(Aswathanarayna 1985).

The average U,Th,Ra,and K in syn-to late-orogenic granites,post-orogenic granites,and pegmatite are recorded in Table 3.A comparison of eU and eTh concentrations in the studied syn-to late-orogenic granites alongw ith the corresponding data of Clark(1966)and Rogers and Adams(1969)indicates that the studied syn-to lateorogenic graniteshave lower U and Th contentsand higher eU/eTh ratios than post-orogenic granites.This supports that these rocks originated from sources depleted in U and Th.

Fig.5 Photomicrographsof the postorogenic granitesshowing:a Perthitic texture in syenogranite.b Skeletal crystalofquartz(QZ)enclosed in microcline forming graphic like texture.c Myrmekitic texture in syenogranite.d Undulose extinction of quartz(stretching of quartz).e Zircon(Zr)crystalassociated w ith chlorite(Chl),quartz(QZ)and iron oxide(Fe-ox).f Epidote(Ep)crystalassociated with sphene(Sph),chlorite(Chl)and iron oxide(Fe-ox)

The arithmeticmean of eU in the studied syenogranites was 1.25 mg/kg—lower than that of the orogenic granites of Saudi Arabia(5.6 ppm;Stuckless et al.1984),of common granitic rocks(4.5 ppm)quoted by Killen(1979),and of normal granites(average U 4.75 ppm)as given by Rogers and Adams(1969).Additionally,the average eTh/eU ratio of syenogranites was higher than that of the average continental crust(～3.8;Van Schmus 1995),further supporting that these rocks are depleted in U.Altered granites are considered uraniferous by Darnely(1982)and Assaf et al.(1997),who def ined uraniferous granites as those that contain at least tw ice the Clark value of U(4 ppm;Clark 1966).Relatively high U and Th contents are attributed to the presence of accessory andsecondary m inerals,such as monazite,xenotime,zircon,uranothorite,samarskite,fergusonite,and columbite.

Table 1 Major oxides(w t%),trace elements(mg/kg)and CIPW normsof the investigated syn-to late orogenic granites

The studied altered syenogranites were also strongly enriched in U and Th compared to the Earth's crust(average 1.8 ppm and 7.2 ppm,respectively;Mason and Moore 1982)and the upper continental crust(average 2.7 and 10.5 ppm,respectively;Rudnick and Gao 2003).

On the other hand,the average eU in pegmatites was lower than that of world uraniferous pegmatites(Ford 1982),while the average eTh in pegmatiteswashigher thanthat of world uraniferous pegmatites(Ford 1982),and of Egyptian uraniferous pegmatites(Heikal et al.2001).A eU—eTh variation diagramshows positive correlation between the two radioactive elements in the altered syenogranites(Fig.7a).This could be related to the differentiation trends suggesting the syngenetic origin of radioactivity.The relationship between eU,eTh,and eTh/eU is ill-def ined(Figs.7b,c),suggesting post-magmatic redistribution of U(Charbonneau 1982;El-Mowafy et al.2007).U and Th show signif icant positive correlation with Zr and Hf,indicating that both elements are preferentially hostedinzirconstructure or isomorphous entryof quadrivalent U and Th into zircon(Zr,Hf)SiO4(Fig.8).

Table 2 Major oxides(w t%),Trace elements(mg/kg)and CIPW norms of the investigated post orogenic granites

7 M ineralization

M ineralogical research of the studied altered syenogranites is of great importance.The distribution of chemical elements and the fractionation of some isovalentsw ithin the shear zone aremainly controlled by new ly formedmineral phases.Themost recorded accessory mineralswere:thorite,fergusonite,samarskite,columbite,zircon,monazite,xenotime,apatite,f luorite,sphene,atacam ite,and malachite,in addition to chrom ium and nickel inclusions.

7.1 Zircon-thorite association

ESEM data ref lect the chem ical composition of zircon and its thorite inclusions(Fig.9).Energy dispersive X-ray analysis(EDAX)showed zircon isostructural w ith thorite and a large fraction of the thorium incorporated in the zircon structure as reported by Rankama and Sahama(1955),indicating the presence of intermediate solid solution between zircon and thorite w ith different levels of substitution.EDAX also revealed thorite rich in Fe and consisting essentially of ThO2,ZrO2,CaO,and SiO2.

7.2 Samarskite-(Y)[(Y,U,REE,Ca)(Nb,Ta)O4]

Samarskite group m inerals have the general formula ABO4,where A stands for Y,rare earth elements(REEs),Ca,U,Fe2+,and Fe3+;and B for Nb,Ta,and Ti.Members of the group include samarskite-(Y)(Y,Ca,Fe)NbO4,calciosamarskite(Ca,Y,U)NbO4,and ishikawait(U,Fe,Y,-Ca)NbO4(Hanson etal.1999;Raslan 2008).Samarskite in the Abu Furad altered graniteswasobserved asanhedral to subhedral medium to f ine grains,mainly in greisenized granites and characteristically translucent to opaque yellow ish brown to dark brown(Fig.10).

7.3 Fergusonite(YNbO4)

Fergusonite was observed as sub-rounded to rounded grains,w ith pale to dark yellow or yellow ish brown to darkbrown color(Fig.11)and tabular prismatic shape,as conf irmed by ESEM(Fig.12).

Fig.6 Geochem ical classif ication,magma typeand tectonic setting of the syn-to lateorogenic granitesand postorogenic granites.a Ab—Or—An ternary diagram of Streckeisen(1976).b SiO2 versus(Na2O+K2O)variation diagram of Irvine and Baragar(1971).c A/CNK versus A/NK variation diagram ofManiarand Piccoli(1989).d Y+Nb versusRb variation diagram of Pearceetal.(1984).e BaversusRb variation diagram after Mason(1966).f K2O versus Na2O variation diagram ofWhite and Chappell(1983)

7.4 Columbite[(Fe,M n)Nb2O6]

Columbite occurs as reddish to dark brown and/or tabular,or prismatic crystals(Fig.13).Columbite is considered REEs carrier.

7.5 Zircon(ZrSiO4)

Zircon was observed as bipyramidal and/or shortand long euhedral prismatic crystals,possessing various colors(pale yellow,reddish-brown,reddish-orange,andcolorless)(Fig.14).The euhedral shape of the zircon suggests a magmatic origin.

Table 3 Radiometric of U,Th,Ra(ppm)and K(%),and chem icalmeasurements of(U and Th)for the studied granitic rocks and pegmatite

7.6 M onazite[(Ce,La,Th,Nd,Y)PO4]

In the sampled rocks,monazitewas presentas sub-rounded to rounded grains,with transparent and vitreous luster,generally pale to dark yellow in color.Some grains were brownish-red and reddish-orange,mostly due to staining with iron oxides.EDAX of somemonazitem ineral grains indicated that they are composed mainly of P2O5,La2O3,Ce2O3,Nd2O3,Pr2O3,and ThO2(Fig.15).

7.7 Xenotim e Y(PO4)

Xenotimeoccurred in samplesasprismatic,granular,radial aggregates;brown,brownish yellow and gray in color.EDAX of some xenotime grains in feldspars indicated they are composed mainly of SiO2,Y2O3,K2O,Fe2O3,P2O5,Al2O3,Yb2O3,Er2O3,and MgO(Fig.16).

Fig.7 a Variation diagram of eU versus eTh of the altered granites.b Variation diagram of eU versus(eTh/eU)of the altered granites.c Variation diagram of eTh versus(eTh/eU)of the altered granites

Fig.8 Variation diagrams of eU and eTh w ith Hf and Zr in the altered granite

7.8 Fluorite(CaF2)

Fluorite commonly occurs in hydrothermal,pegmatitic,and pneumatolytic veins;in greissens;in cavities of granites;and occasionally in carbonate rocksand phosphorites.Investigationsofgraniticsamplesunderbinocular m icroscope revealed f luorite varying from colorless to blue,violet,and occasionally black;usually as cubes and sometimes as anhedral f ine crystals(Fig.17).

Fig.9 ESEM photom icrograph and EDAX analysis of zircon and thorite

Fig.10 Photom icrograph show ing samarskite crystals;C.N.

Fig.11 Photom icrograph show ing fergusonite;C.N.

7.9 Apatite

Apatite occurs as an accessory m ineral in almost all igneous rocks.Fluoro-apatite is usually dom inant.Apatite was observed as large prismatic and thick tabular compact crystals(Fig.18).Its presence was conf irmed by XRD(Fig.19).

Fig.12 ESEM photomicrographs and EDAX analysis of fergusonite

Fig.13 Photom icrograph show ing columbite;C.N.

Fig.14 Photom icrograph showing zircon;C.N.

7.10 Cerussite(PbCO3)

Thepresenceof cerussitewasconf irmed by XRD(Fig.19).

Fig.15 ESEM photom icrographs and EDAX analysis ofmonazite

Fig.16 ESEM photom icrographs and EDAX analysis of xenotime

Fig.17 Photom icrograph show ing f luorite crystals;C.N.

7.11 Atacam ite Cu2(OH)3Cl

Atacam itewasobserved as sub-rounded to rounded grains,bright to dark to blackish green in color,EDAX analyses showed composition consisted mainly of CuO,CaO,Cl,and SiO2(Fig.20).

Fig.18 Photom icrograph showing apatite crystals;C.N.

7.12 Sphene(CaTiSiO5)

Rounded to sub-rounded grains of sphene were conf irmed by ESEM and XRD(Figs.21,22).Observed sphene was mainly composed of TiO2,SiO2,CaO,and Fe2O3.

7.13 Phlogopite

Dark brown to black m ica was identif ied using both XRD techniques.The obtained XRD data for pickedm ica f lakes revealed mainly phlogopite-f luor(Fig.23).

7.14 M alachite Cu2CO3(OH)2

Malachite is the predominantcopperore found in the study area.Itwas observed as prismatic,thick,tabular crystals,sometimes rounded to sub-rounded;bright to dark to yellow ish green in color(Fig.24).

7.15 Chrom ium(Cr)

Chrom ium enrichment suggests that U is concentrated in transition zones between granitic mobilizates and metamorphic rocks,particularly serpentinites.Chrom ium was leached from melange ultramaf ics when the Abu Furad granitewasemplacedandunderwentconsequent hydrothermal veining.Sim ilar data in other old goldm ines in the Eastern Desert have been documented by Takla and Hussein(1995)and El-Feky(2000).EDAX indicated some feldspar grains contain spots,mainly composed of Cr2O3,SiO2,A l2O3,and K2O(Fig.25).

7.16 Nickel(Ni)

Granite-related nickel skarn m ineralization has recently been recognized,mainly associated w ith remobilization of nickel from ultramaf ic host rocks during granite intrusion(Hoatson 2005).The nickel content of igneous rocks and the distribution of thismetal in rock-form ingm ineralshave beendiscussedby Vogt(1921).He concluded that,although some nickelmay be present in sulf ides in common igneous rocks,most is contained in silicatem inerals,particularly the ferromagnesianminerals.Feldsparsand the feldspathoids carry littleorno nickel.In the studied altered syenogranite,feldspars contained appreciable amounts of nickel.EDAX revealed some feldspar grains w tih NiO content up to 8.54 w t%(Fig.26).

Fig.19 X-ray diffraction patterns of quartz(Q),f luroapatite(Fl)and cerussite(Ce)

Fig.20 ESEM photom icrographs and EDAX analysis of atacam ite

Fig.21 ESEM photom icrographs and EDAX analysis of sphene

8 Discussion

Abu Furad area granitic rocks include peralum inous,subalkaline,volcanic arc I-type granitoids and w ithin plate A-type syenogranites,probably non-cogenetic,in addition to altered fertile syenogranites andpegmatites.Each granite group may represent individualmagma batches.

Uranium distribution observed in peralum inous granitoids results from f ive main processes:partial melting,magmatic differentiation,late-magmatic,hydrothermal,and meteoric alterations processes(Friedrich and Cuney 1987;Ibrahim et al.2000).The enrichment of U may be related to either crystallization from U-rich magma or magma contaminated by U by deuteric processes before complete consolidation of the older country rocks,or postmagmatic processes by U-rich hypogene and/or supergene f luids.Ranchin(1971)and Pagel(1982)concluded that the peralum inous leucogranites have Ucontent close to 20 ppm;25%to 35%of the U is incorporated mainly in zircon,monazite,and apatite;5%to 6%is in dissem inated and adsorbed form.Uraninitemay account for an average of 60%of the whole rock U.The correlation between U and Th contentsmay indicate either enrichment or depletion of U because Th is chem ically stable during alteration.The altered andm ineralized syenogranites showed positive correlation of U contentw ith Th,indicating U enrichment inm ineralized granitesby post-magmatic processes.This is supported by the ill-def ined relationship between U—Th/U and Th—Th/U.The altered granites had a lower Th/U ratio than the fresh granites.When the Th/U ratio ofmagma is low,excess U is incorporated in low-Th uraninite(Pagel 1981),whereas if the magma has high Th/U ratios,any excess U not substituted in themainminerals or common accessories is incorporated in uranothorite.The average U content of the altered syenogranites was below the Clark value(3to4 ppm),whiletheaveragevaluesof syenograniteswere lower than the Clark value,indicating Umigration during alteration processes(silicif ication,hematitization,sericitization,saussuritization,chloritization,and muscovitization)fromsyenogranites through joints,faults,and other fractures to altered syenogranites.U and Th also showed signif icantpositive correlation w ith Zr and Hf,indicating that both elements are preferentially hostedinzirconstructureor isomorphous entryof quadrivalent U and Th into zircon(Zr,Hf)SiO4.

Fig.22 X-ray diffraction patterns of quartz(Q)and titanite(T)

Fig.23 X-ray diffraction patterns of phlogopite(Ph),clinochlore(Cl)and hematite(He)

Fig.24 Photom icrograph showingmalachite crystals;C.N.

Fig.25 ESEM photom icrographs and EDAX analysis of chromium

Fig.26 ESEM photom icrographs and EDAX analysis of nickel

Accordingly,the origin of secondary U minerals in the Abu Furad area could beexplained as follows:U during the late-magmatic crystallization stage wasmainly trapped in the accessory and secondary m inerals in granites(e.g.zircon,apatite,sphene,and iron oxy-hydroxides).After cooling and solidif ication,the areawasaffected by tectonic events form ing joints,faults,and other fractures,providing pathways for f luids.The area was affected by hypogene f luids(alkaline hypothermal f luids)rich in U,causing hydrothermal alteration of the accessory m inerals and liberating U from their lattices to redeposit in the late brittlefracture zones.The distribution of chemical elements and the fractionation of some isovalentswithin the shear zone aremainly controlled by new ly formedm ineralphases.The most recorded accessory m inerals include:thorite,fergusonite,samarskite,columbite,zircon,monazite,xenotime,apatite,f luorite,sphene,atacam ite,and malachite,in addition to chromium and nickel inclusions.

9 Conclusions

The rocks cropping out in the Abu Furad area are:(1)metasediments,(2)metavolcanics,(3)metagabbros,(4)syn-to late-orogenic granites,and(5)post-orogenic granites.Petrographic and chemical investigations indicated that the syn-to late-orogenic granites are represented by quartz diorite,tonalite,and granodiorite,whereas the postorogenic granitesare represented by syenogranite.Both the syn-to late-orogenic granites and the post-orogenic granites are of peralum inous nature.The syn-to late-orogenic granites are volcanic arc,while post-orogenic granites are from awithin plate granite tectonic environment.The synto late-orogenics are I-type granites,while the syenogranitesare A-type.Altered granitesare considered uraniferous because of their relatively high U and Th contents,which are attributed to the presence of accessory and secondary m inerals.On the other hand,the pegmatites returned lower eU contents than world uraniferous pegmatites,but higher eTh.The eU—eTh variation diagram indicates positive correlation between the two radioactive elements in the altered syenogranites.U and Th showed signif icantpositive correlation w ith Zr and Hf,indicating that both are preferentially hosted in zircon structure or isomorphous entry of quadrivalent U and Th into zircon(Zr,Hf)SiO4.The mainradioactivem ineralsarethorite,fergusonite,samarskite,columbite,zircon,monazite,xenotime,apatite,f luorite,sphene,atacam ite,chrom ium,nickel,and malachite.