39

OCCURRENCE OF URANIUM BEARING MINERALS IN UM SAFI

PYROCLASTICS, CENTRAL EASTERN DESERT, EGYPT.

Ibrahim, M. E.*, Attawiya, M. Y.

* , Osman, A. M.

** and Ibrahim, I. H.

*

* Nuclear Materials Authority, Cairo, Egypt.

** Ain Shams University, Cairo, Egypt.

ABSTRACT:

Um Safi rhyolite (USR) and associated pyroclastic rocks form relatively moderate to high relief (615m) extruded

volcanoclastic and volcano-sedimentary association, serpentinites and ortho-amphibolites with Knife sharp

contact, forming small oblate body striking NW-SE, covering about 0.3 km2. These rocks show different degrees

of subsoildus autometasomatic processes; argillization, greisenization, silicification, hematitization and

fluoritization. USR is originated by a combination of fractional crystallization of lithospheric source and crustal

contamination, whereas the magma was extruded at active continental margin environment

Um Safi pyroclastics (USP) comprise agglomerates and laminated tuffs. The coarsers (agglomerates) show

fragmental structure at the base of rhyolite extrusion whereas the finer laminated tuffs show contorted bands

(0.5-1 cm) of differing shades and occur upward direction. The pyroclastics were erupted with explosive

violence as a turbulent mixture of hot, expanding gases and gas-emitting lava fragments at relatively low

pressures. The pyroclastics could be considered as a good trap for secondary uranium and U-bearing minerals.

The base metal minerals (pyrite, arsenopyrite and corondite) are formed in the deeper part of the epithermal

zone. In the next zone to the top where ascending solutions rise further towards the surface and mingled with the

descending meteoric water, precipitation of secondary uranium mineral (kasolite) and U-bearing minerals

(plumbobetafite, columbite and betafite) occur as a function of oxidation and failing temperature.

Keywords: Um Safi – uranium bearing minerals - pyroclastics.

1. INTRODUCTION

Um Safi volcanic rocks were mapped previously as

felsite by several workers (Akaad and El Ramly,

1963; El Ghawaby, 1966; Akaad et al., 1996;

Abdalla, 2001). Radioactive mineralization of

columbite, uranothorite and zircon were recorded as

fracture filling in the sheared parts of the felsite

rocks at Um Safi (El Ghawaby, 1966). Abdalla

(2001) considered Um Safi as subvolcanic

equivalent for the metaluminous alkali rare metal

granites and is belonging to the post-collision and

orogenically related A2-type granites with only

zircon and uranothorite as radioactive minerals. Um

Safi granitoids (southern part of the studied area)

are composed of a succession of metaluminous

calc-alkaline (older granitoids) and metaluminous

to peraluminous sub-alkaline sub-leucogranites

(younger granites) emplaced during three

successive tectonic events (Ibrahim et al., 2001).

The shear zone (N-S) in muscovite sub-

leucogranites (1.5 km south USR) is considered as

a good trap for uranium mineralizations (zippeite

and beta-uranophane). This work is a contribution

to the understanding of geology, geochemistry and

genesis of secondary uranium and uranium-bearing

mineralization in USP.

2. GEOLOGIC SETTING AND

PETROGRAPHY

USR form relatively moderate to high relief (615m)

extruded the volcano-sedimentary association

(slate, phyllite, Banded Iron Formation and schist),

serpentinites and ortho-amphibolites with Knife

sharp contact, forming small oblated body striking

NW-SE, covering about 0.3 km2 (Fig. 1). The

volcanic rocks of Um Safi are successive sheets of

lavas and pyroclastics, the former being dominant.

The bulk composition of lavas is mainly rhyolitic.

The USR are fine-grained, massive and varies in

colours from buff, yellow or even grey and pink

with dark buff in colour. They show locally

porphyritic and fracturing particularly along the

marginal parts of the extrusion. These rocks show

different degrees of alteration, hematitization,

kaolinitization and silicification so that the colour

sometimes is reddish brown or dark brown due to

the presence of considerable iron oxides and other

opaque minerals. Fluorite veinlets (1 –3 cm thick,

and up to 2 m long) are common through

pyroclastics and made up of violet, green and black

fluorite, quartz and calcite. Quartzification is

clarified by development of many quartz vein,

veinlets and pods in decreasing order. They

crosscut the USR body at its northern part. Some

Egyptian Journal of Geology, v. 46/1, 2002, pp. 39-54

40

quartz veins contain pyrite and arsenopyrite.

Hematitization is manifested by change of USR

colour into red to brown especially along fractures

due to the presence of impregnation of iron oxide.

The USR are invaded by N-S sub-vertically

greisenized microgranite, which shows exfoliated

and cavernous.

The USP consist of detrital materials expelled from

rhyolite volcanics, transported aerially and

deposited upon land surface in lakes or in marine

waters (Heinrich, 1956). They comprise

agglomerates and laminated tuffs. The coarse-

grained agglomerates show fragmental structure at

the base of rhyolite extrusion (0.5-2 m above Wadi

level) whereas the finer-grained laminated tuffs

exhibit contorted bands of differing shades and

occur upward direction (1.5 – 3 m). The

pyroclastics and greisen rocks gain importance due

to their high intensities of radioactivity (114-280

ppm eU and 167-1133 ppm eTh respectively) (Fig.

2). Visible non-radioactive minerals are observed

such as pyrite, arsenopyrite and manganese oxides

or dendrites, as well as, violet fluorite and metallic

black veinlets (N-S trend).

A- Rhyolite is extrusive holocrystalline to

hypocrystalline with aphanitic matrix. The rock is

hard, massive and light coloured. They are often

banded with flow structures, which may appear as

coloured bands strips or lines of spherulites and

spherulitic textures. Breccias, tuffs and welded tuffs

are often associated. They are composed of quartz

and alkali potash feldspars (sanidine) embedded in

cryptocrystalline groundmass and consist of quartz,

sanidine and secondary muscovite. The common

accessories are opaques, apatite and zircon.

Quartz occurs as phenocrysts (0.7×1.2 mm) and

fine-grained groundmass in the matrix. The matrix

quartz is either fine-grained, or intergrown with

alkali feldspar in a complex pattern. Some quartz

forms aggregate of tiny plates and spherulites,

which composed of radiating fibres of feldspar and

cristobalite (Moorhouse, 1959).

Potash feldspar occurs either as subhedral to

anhedral phenocrysts (0.5×0.8 mm) or as fine-

grained groundmass in matrix. Phenocrysts

(sanidine and microperthite) are usually clear and

may be untwinned or twinned according to the

Carlsbad low, sometimes exhibit corrosion effects

due to corrosion by the matrix. Also radial

spherulitic texture appears from central elongated

microperthite phenocrysts. The common alteration

products are kaolinite, sericite and calcite.

Muscovite occurs either as anhedral fine-grained

crystals in groundmass due to alteration products of

potash feldspars, or as veinlets crosscut the other

constituents.

Opaques occur as anhedral crystals or small

veinlets associated with fluorite. Epidote occurs as

small aggregates associated with opaques and

fluorite. Fluorite varies in colours from colourless

to pale violet through deep violet or black. It

usually occurs as small veinlets associated with

opaque minerals. Apatite, allanite and zircon are

present as individual euhedral crystals enclosed

within quartz and feldspar.

B- Pyroclastics are well banded, reddish grey in

colour and composed mainly of crystals and rock

fragments in fine-grained groundmass. The coarser

agglomerates are not well represented for thin

section studies, except forming fine-grained matrix

they may possess. According to Schmid (1981) the

pyroclastics can be classified into fine crystal tuffs

and crystal lithic tuffs.

i. Fine crystal tuffs are fine-grained, laminated or

banded and composed of more than 50% crystal

fragments of quartz, feldspars and epidote set in a

fine-grained matrix of quartz and potash feldspars.

ii. Crystal lithic tuffs are composed of lithic

fragment of rhyolite and quartzite, as well as, sub-

angular to subrounded crystal fragments of quartz

and perthite embedded in fine-grained groundmass

of the same mineral composition. Quartz crystals

are strongly fractured, probably owing to

compaction and cooling of the glass matrix

(Williams and Mc Birnery, 1979). Perthite crystals

sometimes show one side with a crystal face and

elsewhere show irregular or fractured edges. Other

perthite crystals are rounded or irregularly embayed

indicating a high temperature of the environment of

deposition. Ferromagnesian minerals are altered

and commonly bent.

3. GEOCHEMISTRY

3.1. Methodology

Seven samples from fresh volcanics and five

samples from altered rock, as well as, two samples

from greisen were analyzed in the laboratories of

the Nuclear Materials Authority for major elements

by the wet chemical and atomic absorption (with <

1 % error). Some trace elements are determined

using the X-ray fluorescence technique (with 1-5 %

error). The data of chemical analyses and CIPW

normative values are given in Tables 1 and 2. Two

samples from pyroclastic and greisenized rocks

were crushed and their heavy minerals were

separated using Frantz isodynamic separator and

bromoform. XRD and EDAX-SEM techniques

were used for mineral separation by hand picking

under the binocular microscope.

3.2. Fresh Samples

41

The average chemical composition of the studied

fresh volcanic rocks is compared with the average

corresponding published Egyptian and World

rhyolite rocks (Table 3). Generally the values of

major oxide compositions of USR fall between the

corresponding values of the Egyptian and World

rhyolites. It contains nearly similar values of SiO2,

Al2O3, MgO, K2O, CaO and P2O5 with less Fe2O3

and Na2O contents compared with the Egyptian

rhyolite that given by Aly and Moustafa (1984).

Cox et al., (1979) used the total alkalis versus silica

diagram (Fig. 3) for the geochemical classification

of the volcanic rocks. The data points of the studied

fresh volcanic rocks fall in the rhyolite field.

Miyashiro and Shido (1975) proposed the Cr-

FeOt/MgO and Ni- FeO

t/MgO discrimination

diagrams to identify island arc volcanic rocks

through the behavior of Cr and Ni with advancing

fractional crystallization of basaltic magma. The

plots of the examined rhyolite on these diagrams

(Figs. 4&5) show that, they fall mostly within the

island arc and continental margin tectonic setting.

The normalized values of the studied rhyolite rocks

relative to MORB (Hofmann, 1988) are given in

(Fig. 6a). It shows a strong enrichment in all

compatible elements except Sr due to the effects of

assimilation of either crustal materials or oceanic

matter. In comparison with normalized values

related to the bulk continental crust (Hofmann,

1988), the plot in (Fig. 6b) shows that rhyolite is

enriched in Rb, Ba, Zr and Y and depleted in Sr and

Nb, suggesting a significant role of crustal

contamination.

3.3. Altered Samples

According to the normative Q-Ab-Or compositions, the

altered samples could be classified as sodic,

potassic, silicic and greisen as shown in Fig. (7)

after Stemprok (1979). Two altered samples lie

below the granitic eutectic temperature and exhibit

a trend corresponding to crystallization in high PH2O

range (the range is from 0.5 to 3 k bar after Winker

et al., 1975 and closely parallel to the sodic trend).

The other altered samples lie in silicic trend. The

greisenized samples fall close to the greisen trend.

The Na-K variations diagram after (Cuney et al.,

1989) shows five alteration types; Na-

metasomatism, K-metasomatism, silicification,

desilicification and argillization. On figure (8)

argillization and silicification are the main

alteration processes affected the investigated

samples.

Figures (9 a&b) shows that the major oxides of the

altered samples suffered enrichment in MgO, MnO,

CaO, Fe2O3, P2O5, and SiO2 and depletion in K2O,

Na2O, FeO, Al2O3 and, TiO2, than fresh rhyolite

samples, while the trace elements of the altered

samples display enrichment in Th, U, Nb and Zn

and also show depletion in Rb, Ba, Ni, Pb and V

than fresh rhyolite samples.

3.4. Distribution of eU and eTh

The eU-content in fresh rhyolite ranges from 17 to

34 ppm with an average 26 ppm, and the eTh-

content ranges from 32 to 75 ppm with an average

50 ppm, while the eTh/eU ratio is equal 1.9 (Table

4). The eU and eTh values of USR are greater than

values of acidic effusive rocks of Adams et al.,

(1956) while their eTh/eU ratios are lower than

those reported, indicate to uranium enrichment

rather than thorium.

The field radiometric measurements localized three

radioactive anomalies, two within pyroclastics

(includes one tunnel and eleven trenches were

dugged by nuclear materials authority since 1964)

and one within greisenized microgranite.

a- The first one is represented by only one tunnel

(about 1.5 m in width and 6 m in length), which

considered as one of the highst radioactive zones.

The mineralized joint sets in the tunnel strike ENE-

WSW and NE-SW. Kaolinitized, sericitized and

argillized products of USP contain the higher

radioactive values with violet fluorite. The eU-

content ranges from 53 to 563 ppm with an average

of 285 ppm, whereas the eTh-content ranges from

236 to 2229 ppm with an average of 1133 ppm. The

eTh/eU ratio ranges from 2.8 to 4.71 with an

average of 3.9 (Table 4).

b- The second anomaly is represented by trenches.

It is clear that the common mineralized joints,

striking ENE-WSW and NE-SW. The alteration

product; ferrugenation, Mn-dendrites and oxidized

with oxidized sulphide crystals support the

hydrothermal effect. The eU-content (ranges from

132 to 335 ppm with an average of 236 ppm), is

less than the eTh-content (ranges from 157 to 1188

ppm with an average of 905 ppm). The eTh/eU

ratio ranges from 0.5 to 5.8 with an average of 4.1

(Table 4), which is related to mobilization and

redistribution of uranium.

c- The third anomaly is close to greisenized

microgranite, which trend N-S to N20ºE-S20ºW

directions. It is characterized by brownish to

reddish colour (due to ferrugenation) ranging from

4 to 5 m thick and rich by vugs due to oxidized

cubic sulphide crystals. The eU- content (ranges

from 101 to 125 ppm with an average of 114 ppm)

is less than eTh-content (ranges from 161 to 172

ppm with an average of 167 ppm). The eTh/eU

ratio ranges from 1.33 to 1.61 with an average of

1.48 (Table 4).

42

4. MINERALIZATIONS

The mineralizations in Um Safi pyroclastics can be

classified into five categories; 1) U-bearing

minerals (columbite, plumbobetafite and betafite),

2) secondary U-mineral (kasolite), 3) thorium-

bearing minerals (uranothorite and yttrialite), 4)

base-metal minerals (pyrite, arsenopyrite,

cassiterite and corondite) and 5) accessory minerals

(zircon, allanite and fluorite).

i) Columbite [(Fe,Mn)(Nb,Ta,U)2O6] is

isomorphous with tantalite, black in colour and is

an ore of niobium as well as a source of tantalum

(Nb/Ta ratio 4.9). The columbite of Um Safi (Fig.

10a) is radioactive due to presence of moderate

UO2 and ThO2 contents (16.7% and 13.3%

respectively).

ii) Plumbobetafite [(Pb,Ca,U)(Nb,Ti)2O6(OH,F)]

confirmed by EDAX (Fig. 10b). The mineral is

composed mainly of TiO2 (41.3%) and Nb2O5

(20.1%) while UO2 and ThO2 are not common

(1.7% and 2.1% respectively).

iii) Yttrialite [(Y,Th)2Si2O7] is olive-green mineral.

Semi-quantitative analyses (Fig. 10c) indicate that,

it has ThO2 (41.1%) more than Y2O3 (14.3%) and

SiO2 (26.0%).

iv) Uranothorite [(Th,U)O2] is confirmed by

EDAX (Fig. 11a) and contains 40.65% ThO2,

14.0% UO2, 12.0% Y2O3 and 16.0 % SiO2.

v) Arsenopyrite (FeAsS) occurs as silver-white to

steel-grey colour. it is isomorphous with loellingite

mineral and constituting the principle ore of

arsenite. Semi-quantitative analyses of picked

grains were using the EDAX-SEM technique

(Fig.11b ) gives 37.3% As, 35.1% Fe and 25.0% S.

vi) Coronadite (PbMn6O14) is black mineral

associated with iron oxides in ferruginated

pyroclastics (trenches). Semi-quantitative analyses

of picked grains were obtained using the EDAX-

SEM technique (Fig.11c). It contains 67.8% MnO

and 21.5% PbO2 with Pb/Mn ratio equal to 3.2.

vii) Betafite [(Ca,Na,U)2(Ti,Nb,Ta)2O6.(OH)]

occurs as radiated black mineral associated with

quartzo-feldspathic groundmass in pyroclastics

(Fig. 12a).

viii) Kasolite [Pb(UO2)SiO4.H2O] shows dark

yellowish brown colour with radiated or fan likes

shape under Crossed Nicol and considered as

secondary uranium minerals (Fig.12b).

ix) Zircon (ZrSiO4) occurs as euhedral six-sided or

eight-sided form with clusters of opaque inclusions.

It is mainly colourless to pale yellow and associated

with iron oxides and fluorite. Average zirconium

contents is higher in fresh samples (1030 ppm) than

in altered samples (936 ppm). Rankama and

Sahama (1955) stated that thorite is isomorphic

with zircon and it is evident that a large part of

thorium is incorporated in the zircon structure. The

ionic size of zirconium and thorium are not too

unlike and therefore thorium is able to enter the

zircon structure in which it replaces. Two thin

section from greisen were tested for their

radiometrically using SSNDTD type Kodak CN-85

films. The test reveals that zircon in greisen is

radioactivity carrier and show high concentrations

of alpha tracks (Figs. 12c&d).

x) Fluorite (CaF2) minerals possess vitreous luster

and white streak. It is mainly recorded filling

cavities and micro-fractures, which reflect their

secondary origin as resulting from hydrothermal

alteration of rhyolite and/or associated greisen.

Fluorite in USR and greisen samples exhibits a

wide range of colours. Some of them are colourless,

while others are pale rose, pale violet, deep violet

and very deep to blackish violet to black. Some

fluorite grains have various gradations of colours

was separated by hand picking under the

microscope to be tested for their radiometically by

using the Solid State Nuclear Track Detectores

(SSNDTD) type Kodak CN-85 films. This test

reveals that violet and black fluorite grains are

radioactivity carrier and show high concentrations

of alpha tracks (Figs. 12 e&f). Serra (1947) and

Allen (1952) stated that fluorite colour is attributed

to the action of their rare earth elements during the

differentiation of magma or due to the presence of

manganese. Derr et al., (1962) indicated that the

presence of trace and/or rare earth may cause the

different colouration of fluorite. Abdalla (2001)

recorded that Um Safi fluorite is characterized by

considerable enrichment in REE (av. Σ REE = 4179

ppm).

xi) Allanite [(Ce,Ca,La,Y)(Al,Fe)3(SiO4)3.OH] is

brown in colour and pleochroic from pale brown to

dark brown (Fig.12g ). In most cases the allanite

mineral is uranium and thorium carrier but altered

and inverted to an amorphous substance product by

break down of the space lattice by radioactive

emanation (Kerr, 1977).

xii) Cassiterite (SnO2) is a brown or black

tetragonal mineral. It is the principal ore of tin.

They occur in prismatic crystals, massive and form

compact concentric structure (Fig. 12h). Swart and

Moore (1982) suggested that the presence of

coloured zones in the cassiterite crystals is due to

variation in U concentration that range from 0-6

ppm in darker bands.

xiii) Pyrite (FeS2) occurs as well developed cubic

crystals, with pale brass-yellow colour and metallic

luster, often with yellowish brown tarnish

43

associated with goethite. Pyrite occurs either

disseminated in smoky quartz veins or associated

with USP.

xiv) Thorite (ThSiO4) is strongly radioactive

mineral, brown to black tetragonal mineral like

zircon and dimorph with huttonite. It is

isostructural with thorogummite [Th (SiO4)1-X

(OH)4X] and may contains as much as 10%

uranium.

5. SUMMARY AND CONCLUSIONS

The average of eTh/eU ratios are increased from

1.5 in the greisenized microgranite through 1.9 in

the fresh rhyolite to about 4.0 in the pyroclastic

samples (kaolinized and argillite). The lowering of

the eTh/eU ratio indicates redistribution and

localization of secondary uranium mineral

(kasolite). The enrichment of U and Th in

pyroclastics (425-520 ppm eU and 1700-2000 ppm

eTh) may indicate the stabilization of them in late-

to post-magmatic fluids. In the pyroclastics

samples, U and Th are largely incorporated in

uranium bearing-minerals e.g. columbite, yttrialite,

betafite, plumbobetafite and uranothorite. The

higher mobility of U could be retained by Fe- and

Mn-rich solutions and adsorbed by lately deposited

amorphous Fe-oxides and explained its lower Th/U

ratio (Table 2).

The greisen (pneumatolytically altered granitic

rocks) is composed mainly of quartz and mica

(muscovite and sericite) as well as fluorite,

cassiterite, kasolite, allanite, zircon and

arsenopyrite in decreasing order. The lower eTh/eU

ratio (1.6) of greisen rather than tunnel (average

eTh/eU= 3.9) and trenches (average eTh/eU= 4.1)

indicates that the greisen is affected by

hydrothermal-bearing uraniferous solution. The

normal Th/U in the crust is equal to 3.5. So, the

lower ratio may be attributed to the magmatic

activity through the N-S shear zone in muscovite

subleucogranite where their Th/U ratio around 0.2

(Ibrahim et al., 2001).

Kaolinitization, ferrugenation, and argillation

represent the hydrothermal alterations associated

with the high radioactivity values in rhyolites and

greisen. The widespread sericitization through the

rhyolite rocks provides additional evidence of

large-scale movement of solutions through these

rocks. The hexavalent U is ready leached from

rhyolites by dilute acid solutions so some U may

have been transported as UO2(OH)+ and UO2-

complex

The USP appears to have provided less-welded

layers and lenses. This is coincidence with the

similar observation (Smellie, 1982) described from

Duobblon rhyolitic ignimbrites. The leached U

from solutions percolating through fractures,

fissures and permeable bands were re-deposited and

sink. The pyroclastics were erupted with explosive

violence as a turbulent mixture of hot, expanding

gases and gas-emitting lava fragments at relatively

low pressures. The base metal minerals (pyrite,

arsenopyrite and corondite) are formed in the

deeper part in the epithermal zone. In the next zone

to the top where ascending solutions rise further

towards the surface and mingled with the

descending meteoric water, precipitation of

secondary uranium mineral (kasolite) and U-

bearing minerals (plumbobetafite, columbite,

yttrialite, betafite and uranothorite) occur as a

function of oxidation and failing temperature

6. REFERENCES

Adams, J.A., Osmand, J.K. and Rogers, J.I.W., 1956, The geochemistry of thorium and

uranium. In: Physics and chemistry of the Earth.

Pergamon Press, New York, v. 3, p. 298-348.

Akaad, M.K. and El Ramly, M.F., 1963, Geology

and structure of Um Lassaf - Um Nar iron belt,

Eastern Desert of Egypt. Geol. Surv. Egypt, No.

17, 23 p.

Akaad, M.K., Noweir, A.M. and Abu El Ela, A.M., 1996, Geology of Pan-African basement

rocks of the Gabal Al Hadid – Wadi Mubarak

District, E.D., Egypt. Geol. Surv., Egypt. No.73,

78 p.

Allen, R.D., 1952, Variations in chemical and

physical properties of fluorite. Am. Min., v. 37,

p. 901-932.

Aly, M.M. and Moustafa, M.M., 1984, Major

chemistry statistics characterizing common

igneous rocks of Egypt. 9th

Inter. Cong. Stat.

Comp. Sci. Soc. Res., Ain Shams Univ., Cairo.

Abdalla, H.M., 2001, Geochemistry and origin of

rare metal mineralization of Um Safi felsite,

central Eastern Desert, Egypt. Egyptian J. Geol.,

p. 131-149.

Cox, K.G., Bell, J.D. and Pankhurst, R.J., 1979, The interpretation of igneous rocks. London,

Allen and Unwin, 450 p.

Cuney, M., Leroy, J., Volivezo, A., Daziano, C.,

Gambda, B., Zarco, A.J., Morello, O., Ninci, C. and Molina, P., 1989, Metallogenesis of the

uranium mineralized Achala granitic complex,

Argentina: comparison with Hercynian

peraluminous leucogranites of Western Europe.

Proc. Tech. Comm. Meetings, Vienna,

TECDOC-543, I.A.E.A., Vienna, p. 211-232.

Deer, W.A., Howie, R.A. and Zussman, J., 1962, Rock forming minerals. Non silicates, 5,

Longman, London, 371 p.

El Ghawaby, M.A., 1966, Structural and lithologic

controls of localization of radioactive

44

mineralization in a south Qusier area. M.Sc.

Thesis, Ain Shams Univ., Cairo, 100 p.

Heinrich, E.W., 1956, Microscopic petrography.

McGraw-Hill Company, Inc., 295 p.

Hofmann, A.W., 1988, Chemical differentiation of

the earth: The relationship between mantle,

continental crust and oceanic crust. Earth

Planet. Sci., Let., v. 90, p. 297-314.

Ibrahim, M.E., Osman, A.M., Attawyia M.Y. and Ibrahim I.H., 2001, Petrogenesis of

granitoid rocks and origin of uranium

mineralizations of Um Safi area, Eastern Desert,

Egypt. Egyptian J. Egypt, (in press).

Kerr, P.F., 1977, Optical mineralogy. 4th Ed.,

McGraw-Hill Book Inc., 492 p.

Le Maitre, R.W., 1976, The chemical variability

of some common igneous rocks. J. Petrol., v.

17, p. 589-637.

Miyashiro, A.A. and Shido, F., 1975, Tholeiitic and calc-alkaline series in relation

to the behaviors of titanium, vanadium,

chromium and nickel. Am. J. Soc., v. 275, p.

265-277.

Moorhouse, W.W., 1959, The study of rocks in

thin section. Harper and Brothers, New

York, 514 p.

Rankama, k. and Sahama, T.G., 1955, Geochemistry. Chicago Univ. Press, Chicago,

37 p.

Sayyah, T.A. and El Shatoury, H.M., 1973, Some

geochemical features of Wadi Natash volcanics,

Eastern Desert. Egypt. J. Geol., v. 17 (1), p. 85-

95.

Schmid, R., 1981, Descriptive, nomenclature and

classification of pyroclastic deposits and

fragments, recommendation of IUGS

subcommision on the systematics of igneous

rocks. Geology, v. 9, p. 41-43.

Serra, A., 1947, Osservazioni Spettroscopiche Su

fluorine Colarate. Ricerca Sci. Ricostr. Roma, v.

17, 670 p.

Shahien, M.G.M., 1996, Petrographical and

geochemical studies of some volcanics of Sinai

and Eastern Desert, Egypt. Ph.D. Thesis, Cairo

Univ., 218 p.

Smellie, J.A.T., 1982, The mineralogy and genesis

of uranium in rhyolitic ignimbrites of

Precambrian age from Duobblon, Sweden. Min.

Mag., v. 46, p. 187-199.

Stemprok, M., 1979, Mineralization granites and

their origin. Episodes, v. 3, p. 20-24.

Swart, P.K. and Moore, F., 1982, The occurrence

of uranium in association with cassiterite,

wolframite and sulphide mineralization in

South-West England. Min. Magazine, v. 46, p.

211-215.

Williams, H. and Mc Birnery, A.R., 1979, Volcanology: San Francisco, Freeman, Cooper

& Co.

Winkler, H.G.F., Boes, M. and Marcopoulos, T., 1975, Low temperature granitic melts. N. Jb.

Min. Mn., v. 6, p. 245-268.

45

U Nb

Nb

U

Ca

Si

TaFeTiSc

(a)

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Ti

Nb

NbFe

Ta Pb

Si

UCa

(b)

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Th

Si

Y

Fe Th

AlFe

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

(c)

Fig. (10): Semi-quantitative analyses using the EDAX –SEM.

a) Columbite, b) Plumbobetafite and c) Yttrialite.

46

Th

U

CaFe

Si

Y

Fe

Al

(a)

0.70 1.40 2.10 2.80 3.50 4.20 4.90 5.60 6.30

As

S Fe

As

(b)

1.70 2.70 3.70 4.70 5.70 6.70 7.70 8.70 9.70

(c)Mn

Ba

Pb

SiMn

2.00 4.00 6.00 8.00 10.00

Fig. (11): Semi-quantitative analyses using the EDAX –SEM.

a) Uranothorite, b) Arsenopyrite and c) Coronadite.

47

285 236114

1133

905

16755 35 95

0

200

400

600

800

1000

1200

Tunnel

(n=12)

Trenches

(n=9)

Greisen

(n=4)

eU(ppm)

eTh(ppm)

Ra(ppm)

35 45 55 65 75

0

3

6

9

12

15

18

Nephelin

P-N

B+T

P-T

Phonolite

Benmorite

Mugearite

Hawaiite

Basalt

B-A Andesite

DaciteTrachyandesite

Rhyolite

Trachyte

SiO2 (wt)

Na

2 O

+K

2 O

(w

t)

Fig. (2): Bar-diagram showing the average

contents of eU, eTh and Ra (ppm) for the

anomalies sites of Um Safi area. (n= number

of samples)

Fig. (3): Na2O+K2O versus SiO2 diagram for

volcanic rocks, Um Safi area, after Cox et

al., (1979).

0 1 2 3 4 5 61

10

100

1000

FeOT/MgO

Cr

(pp

m)

Abyssal tholeiites

Field volcanic rocks of island arcs and continental

margins

0 1 2 3 4 5 6 7 81

10

100

400

FeOT/MgO

Ni

(pp

m)

MORB

Island arcs

Fig. (4): Cr vs. FeOt/MgO for rhyolite, Um Safi

area, after Miyashiro and Shido (1975).

Fig. (5): Ni vs. FeOt/MgO for rhyolite, Um Safi

area, after Miyashiro and Shido (1975).

0.003

0.01

0.1

1

10

100

1000

Sr K Rb Ba Nb Zr Y

Ro

ck/M

OR

B

( a )

0.1

1

10

100

Sr K Rb Ba Nb Zr Y

Ro

ck/C

.C.

( b )

Fig. (6): Spider diagrams of normalized values of rhyolite, Um Safi area, after Hofmann (1988).

a) related to MORB b) related to bulk continental crust

48

Ab Or

Q

Gre

isen

Sil

icic

Sodic Potassic

P H2O = 1kb

0 1 2 3 4 5 6 70

1

2

3

4

5

6

7

Na (%)

K (

%)

Des

ilici

ficat

ion

K- m

etasomatism

Na - metasom

atism

Argillition

Silicif

icatio

n

Fig. (7): Normative Q-Ab-Or ternary diagram,

showing the alteration of volcanic rocks, Um

Safi area, after Stemprok (1979).

Fig. (8): Na% – K% variation diagram, showing

the alteration types for volcanic rocks, Um

Safi area, after Cuney et al. (1989).

1 313

SiO2

TiO2

Al2O3

Fe2O3

FeO

MnO

MgO

CaO

Na2O

k2O

P2O5

EnrichedDepleted

Fresh

Alt

ered

1 10 3011030

Rb

Sr

Ba

Zr

Y

Nb

Zn

Pb

V

Cr

Ni

Co

Ga

U

Th

EnrichedDepleted

Fresh

Alt

ered

Fig. (9): The enrichment and depletion of major oxides (a) and some trace elements (b) of the

altered relative to the fresh rhyolite samples, Um Safi area.

(a) (b)

49

Table (1): Result of chemical analyses and CIPW normative minerals for

fresh volcanic rocks, Um Safi area.

Fresh volcanic rocks Major

Oxides 1 2 3 4 5 6 7

SiO2 75.07 69.49 71.2 70.51 69.55 70.61 71.16

TiO2 0.13 0.12 0.11 0.12 0.11 0.13 0.11

Al3O3 12.5 14.09 13.85 14.12 14.62 14.29 13.09

Fe2O3 1.76 2.01 1.21 1.61 2.71 2.72 2.45

FeO 0.40 0.36 0.56 0.76 0.86 0.44 0.32

MnO 0.07 0.06 0.09 0.08 0.07 0.08 0.09

MgO 1.01 0.91 0.88 0.96 0.79 1.12 1.11

CaO 1.80 1.36 1.68 1.40 1.12 1.40 1.96

Na2O 3.93 4.63 3.80 3.90 3.75 3.64 4.10

K2O 3.36 4.43 4.55 4.24 4.58 3.79 3.46

P2O5 0.09 0.08 0.10 0.07 0.10 0.08 0.09

L.O.I 0.79 1.99 1.64 1.68 1.14 1.27 1.74

Total 99.91 99.53 99.67 99.54 99.41 99.57 99.68

Trace elements (in ppm)

Rb 298 308 239 259 223 215 54

Sr 12 12 19 11 12 13 13

Ba 494 249 440 466 283 430 350

Zr 876 921 883 1005 1205 1007 1307

Y 282 296 283 321 387 321 420

Nb 32 31 33 30 32 31 30

Zn 23 37 48 31 56 28 41

Pb 78 81 38 36 75 41 595

V 5 5 7 7 5 5 5

Cr 58 97 86 78 120 95 110

Ni 1 3 3 3 5 4 2

Co 4 3 3 3 3 4 4

Ga 52 56 33 38 46 52 39

U* 32 24 25 29 28 17 34

Th* 41 51 44 49 48 32 75

CIPW normative values

Q 34.61 22.49 27.69 27.98 27.38 31.01 29.54

Or 19.85 26.86 27.45 25.65 27.57 22.81 20.9

Ab 33.18 40.12 32.76 33.71 32.25 31.3 35.38

An 6.50 4.65 7.4 6.69 5.06 6.59 7.21

Di 1.47 1.39 0.40 - - - 1.7

Hy 1.84 1.69 2.08 2.47 2.01 2.85 2.05

C - - - 0.72 1.69 1.84 -

Mt 1.14 1.03 1.79 2.39 2.73 1.32 1.03

Hm 0.97 1.35 - - 0.88 1.85 1.79

Il 2.25 0.23 0.21 0.23 0.21 0.25 0.21

Ap 0.20 0.18 0.22 0.16 0.22 0.18 0.20

* = Radiometric analyses

50

Table (2): Result of chemical analyses and CIPW normative minerals for altered volcanic

rocks and greisen, Um Safi area.

Altered volcanic Greisen

Kaolinized Argillite Silicified Ferrugenated Major

Oxides 8 9 10 11 12 13 14

SiO2 70.57 71.11 72.05 76.7 81.5 80.06 69.07

TiO2 0.10 0.11 0.10 0.12 0.13 0.11 0.13

Al3O3 12.19 13.19 11.57 8.94 9.25 11.10 9.43

Fe2O3 3.03 2.18 4.39 3.39 0.81 2.42 10.2

FeO 0.52 0.20 0.28 0.56 0.36 0.80 1.64

MnO 0.14 0.18 0.08 0.08 0.11 0.04 0.18

MgO 2.00 2.80 1.20 0.80 0.80 0.80 0.60

CaO 2.80 1.68 2.24 1.98 1.68 1.12 1.96

Na2O 4.60 0.67 2.76 2.64 0.34 0.31 0.67

K2O 1.22 2.67 1.22 0.93 0.83 1.00 2.37

P2O5 0.10 0.13 0.10 0.10 0.10 0.12 0.15

L.O.I 2.70 3.56 3.94 3.70 3.55 2.08 2.47

Total 99.97 98.48 99.93 99.94 99.46 99.96 98.87

Trace elements (in ppm)

Rb 114 39 112 84 22 507 646

Sr 10 13 14 12 13 12 12

Ba 232 257 226 233 241 98 100

Zr 1193 862 1148 830 855 1133 529

Y 385 277 370 269 274 363 169

Nb 32 32 31 34 35 30 28

Zn 144 80 788 729 248 316 325

Pb 68 44 38 36 49 266 409

V 5 3 4 4 4 4 5

Cr 69 99 78 87 119 86 87

Ni 1 2 2 1 1 2 2

Co 3 4 4 4 3 3 3

Ga 43 34 53 43 30 55 52

U* 105 74 435 426 522 101 125

Th* 249 304 2007 1736 2006 161 172

CIPW normative values

Q 32.26 52.17 46.89 54.68 74.86 72.68 42.93

Or 7.42 16.64 7.52 5.72 5.12 6.04 14.54

Ab 39.97 5.97 24.3 23.18 3.00 2.68 5.87

An 9.24 7.98 10.97 9.6 8.08 4.96 9.18

Di 3.45 - - - - - -

Hy 3.54 7.37 3.13 2.08 2.09 2.04 1.56

C - 6.75 1.92 0.20 5.16 4.83 2.60

Mt 1.89 0.96 0.91 1.78 1.19 2.44 5.70

Hm 1.81 1.63 3.95 2.29 0.02 3.85 17.02

Il 0.20 0.22 0.20 0.24 0.26 0.21 0.26

Ap 0.22 0.30 0.23 0.23 0.23 0.27 0.34

* = Radiometric analyses

51

Table (3): Average of chemical composition of the studied rhyolite in comparison with

some Egyptian and World related rocks.

Rhyolite Major

Oxides 1 2 3 4 5 6

SiO2 71.08 73.22 75.31 75.83 69.7 72.8

TiO2 0.12 0.42 0.23 0.24 0.38 0.28

Al2O3 13.79 12.59 12.55 12.3 14.0 13.3

Fe2O3 2.07 1.60 0.77 1.71 2.26 1.50

FeO 0.53 0.10 0.54 0.59 1.50 1.10

MnO 0.08 n.d 0.04 0.03 0.07 0.06

MgO 0.97 0.81 0.14 0.17 0.99 0.39

CaO 1.53 0.97 0.35 0.56 1.60 1.14

Na2O 3.96 5.38 3.86 3.32 4.20 3.60

K2O 4.06 3.58 4.29 4.12 4.10 4.30

P2O5 0.09 n.d 0.03 0.02 0.09 0.07

n.d= not determined

1-Average of fresh rhyolite of the studied area.

2-Average of rhyolite of Wadi Natash, Eastern Desert (Sayyah and El Shatoury, 1973).

3-Average of rhyolite of Wadi Kid, Sinai (Shahien, 1996).

4-Average of rhyolite of Abu Swayel, South Eastern Desert (Shahien, 1996).

5-Average of Egyptian rhyolite (Aly and Moustafa, 1984).

6-Average of World rhyolite (Le Maitre, 1976).

Table (4): eU, eTh and Ra contents in ppm and K (%), eU/Ra, eU/eTh and eTh/eU ratios of rhyolite and

anomalies.

Radiometric measurements Rock types

eU

(ppm)

eTh

(ppm)

Ra

(ppm)

K(%) eU/Ra eU/eTh eTh/eU

Min. 17 32 12 1.75 1.4 0.4 1.7

Max. 34 75 14 4.29 2.4 0.6 2.2

Rhyolite

(n=6)

Average 26 50 13 2.99 2 0.5 1.9

Min. 53 236 13 0.39 3.7 0.2 2.8

Max. 563 2229 115 3.44 6.9 0.4 4.7 Tunnel

(n=12)

Average 285 1133 55 1.71 5.2 0.3 3.9

Min. 132 157 21 0.19 3.9 0.2 0.5

Max. 335 1188 49 4.4 9.7 2.1 5.8 Trenches

(n=9)

Average 236 905 35 1.6 7.0 0.5 4.1

Min. 101 161 90 2.08 1.09 0.62 1.33

Max. 125 172 97 2.66 1.29 0.75 1.61

An

om

ali

es

Greisen

(n=4)

Average 114 167 95 2.37 1.19 0.68 1.48

52

������� ����� � � ���� �� ������� �� ���������� ������ ��������� ����������� .

������ !��"� ��� *–%����& ' �� ��� *–(��)& ��� ������ ��& **������ ( ������ �*

*������� ������ �*��

**+�� (�& �����

�� �����,����� ���� �� ������ ��-� ������ .�/��0� ����& ��� �� ��� 1�,�2� �3� �����)567� ( ��9 :9;� <9� <��

1���/���)��0� ��-�� (����� ��� ����,���� ����� ��� ��-� ������ :� +����� . =�9> 1��� ?���� <� @��;�� ��A� � � (�,��–

�� � ���A� B�� =���C� .�,D . �9������ E��9� �/��-� ����� ��-��� ?F� G;�� � �9��H������ �9������ 1�9��� �9������ �

�������/��� ��������3��� ���,�� ��� . (� I9� ����� ��-� ���; :� ��/ �)���� <� �*H��� ������� ���� �J���� ��-��� ?F� �2�� ���

���� ����� '�� �*�� <� ���- ������.

K���, ������ ���� �" �����/�� �����,���� ��-� 1���)�0�������� ( K��A� �������)������ .( �9�������0� ��-9� G;��

�3L��� �/��-� K��A� ��J�� ��F ������ ����� �������� ��-� K�&�� <� ����J�� =�,����� E�� . 1M9- (9� �9��- ��-9��� ?F9��

� �A; ��& ���0 :�J� ����� ��H�>� K���� ��H�>� K����� ��A�� ���-� ��F ����,�� �����/�� (��9���� E9�� ���,� ��� E/-�

) �����– �������� �� – ������, ( !��� B�/�� B��� �� �J�& �),0� H��� <� . �9�&� (9� !����� B�� B���� ������ B����� ���

�����)�� ���������� (���� =� �� �� ��� ��H���� ?����� :� ����-���� K�&���� 1������ I�) ����H�, ( (��9����� ��9�������� �9�����

)���������������������, ��������� �� ������� ����)�������� .(