Spectrometric and Geochemical Characteristics of Um Doweila Bostonite, Southeastern Desert, Egypt.

M.E. Ibrahim, G.M. Saleh, I.H. Ibrahim, M.S. Mostafa, M.S. Azab, M.E. Darwish, H.M. Asran, and T.A. Lasheen

Nuclear Materials Authority

P.O. Box: 530 El Maadi, Cairo, Egypt.

ABSTRACT: Um Doweila bostonite (UDB) represents one of the uranium occurrences related to per-

alkaline volcanics and located to the south of Wadi (W.) Allaqi, Egypt. UDB is emplaced in the NE-SW direction for about 11 km long and varies from 1-10m in width crosscutting three country rocks; volcanogenic metasediments, meta-andesite and marble with sharp contacts. Um Doweila area is characterized by two semi-perpendicular dominant trends (NE-SW and NW-SE) resulting from an extensional phase of deformation prior in age to the bostonite emplacement.

The mineralogical study of the UDB samples reveals the presence of uranophane, beta-uranophane, wolframite (hueberite), monazite, zircon, rutile and opaques whereas pyrite is common in volcanogenic metasediments. UDB is affected by common proximal hydrothermal minerals; sericite, chlorite, pyrite and ankerite. The detailed spectrometric survey for UDB reveals the coincidence of eU/K and eU contour maps especially along the volcanogenic metasediments (reduced regime). The U migrated from the central zones of UDB in close contact to the fault zones (channel-ways) to the peripheries outward and relatively concentrated at the NE and SW upper and lower corner respectively.

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INTRODUCTION Episodic emplacement of alkaline magmatism is common in NE Africa. Over 130 ring

complexes and ring dykes were emplaced during the time span between closing of Island arc activity at the end of the Pan-African orogeny and opening of the Red Sea (1-2). Alkaline rocks often in the form of ring complexes, isometric stock-like masses and ring dykes comprise the Phanerozoic alkaline province in southern Egypt. The province occurs in Pan-African domains of gneisses, metavolcano-sedimentary sequence and older granitoids. The province embraces a wide time span from the lower Paleozoic to Late Cretaceous with peaks centered tentatively around Siluro-Devonian, Permo-Triassic, Early and Late Cretaceous (3).

The Eastern Desert of Egypt is considered to be one of the densest areas of continental dyke swarms on the earth (4). However, there is a significant controversy about the relative age of different dyke suites where mafic dykes cutting felsic dykes are as common as felsic dykes intruding mafic dykes. Another controversy is about the availability of a magma over such a broad zone and about the stress field history at the time of dyke intrusions. The new volcanics in the basement rocks of Egypt were characterized by most eruptions of small cluster plugs related to lava flows (5). The Um Doweila bostonite (UDB) represents an excellent example of the new volcanics related to the basement rocks of Egypt. The geologic aspects of the study area were discussed by some authors (6-10).

The present research aims to clarify the distribution of radioelements, mineralogical and the geochemical characteristics of UDB dyke.

GEOLOGIC SETTING The study area is bounded by latitudes 22o 15` and 22o 20` N and longitudes 33o 27` and

33o 29` E in the southern Eastern Desert of Egypt. Um Doweila Bostonite (UDB) extends in the NE-SW direction for about 11 km with 1-10 m width crosscutting three country rocks; volcanogenic meta-sediments (chlorite schist, phyllite and pebbly quartz phyllite), meta-andesite and marble with sharp contacts (Fig. 1). The northeastern and southwestern parts of the UDB are characterized by fine- to medium-grained, equigranular rocks, low relief and 8 Km long whereas the middle part of UDB (3 km long) is characterized by porphyritic texture high relief and low intensity of radioactivity compared with the other parts. The UDB is characterized by its reddish brown colour due to high content of iron oxides especially along fractures as well as manganese dendrites along joint surfaces. Silicification ferrugination, fluoritization and kaolinization processes are predominating features. due to hydrothermal solutions. UDB is mainly composed of quartz, microperthite, alkali amphiboles (riebeckite and rare arfvedsonite) set in a fine cryptocrystalline groundmass showing trachytoid and flow textures. Zircon, apatite, fluorite and opaques are accessories.

The fracture framework of the study area is characterized by two semi-perpendicular dominant trends (NE-SW and NW-SE) resulting from an extensional phase of deformation prior in age to the emplacement of bostonite within the NE-SW main fracture trend (Fig. 2). These two main fracture trends are associated with two minor NNW-SSE to N-S and ENE-WSW to E-W trends. In fact, these fracture systems are recorded cutting among the country rocks (volcanogenic metasediments, meta-andesite and marble), whereas the Nubian sandstones are deposited later than these trends.

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The NE-SW and NW-SE tensional fractures have been reactivated as strike-slip faults (sinistral and dextral respectively) under N-S (semi-meridional) followed by a ENE-WSW trending regional compressional stresses creating zones of ductile deformation characterizing coupling tectonic environment (Fig. 2) which played an important role in the metamorphism of the country rocks. The metamorphism in the country rocks is of weak to very weak type that occurred under moderate pressure with depletion in silica and enrichment in CaO and MgO restricted to marble zone. Thus, the area under investigation could be considered as a large scale disjointed schistosity characterizing the coupling tectonic environment (11-12).

MINERALOGY In the present study, identification of uranium and associated minerals has been carried

out by means of binocular microscope XRD (Model PW 1830) and environmental scanning electron microscope (ESEM) supported by qualitative energy dispersive spectrometer (EDS) unit at the Nuclear materials Authority of Egypt. The mineralogical study of the UDB samples reveal the presence of uranophane, beta-uranophane, wolframite (hueberite) monazite, zircon, rutile and opaques whereas, pyrite is a common in volcanogenic metasediments.

1- Uranophane and β-uranophane are the most abundant secondary U-minerals. Uranophane [Ca (UO2)2(SiO3)2(OH)2X5H2] is orange in colour ranges from lemon-yellow to straw yellow. ESEM investigation shows that the uranophane crystals are 250-290 mm in length with a clearly identifiable prismatic shape and a nearly square cross-section. They show one perfect cleavage parallel to the elongation of the crystal and traces from another (Figs. 3A&B). β-uranophane is dimorph of uranophane. Some uranophane crystals show secondary enrichment of Fe (4.6%). This could be attributed to Fe for Ca in a later phase of fluids mineralization interaction. This interaction is at least one of the reasons for the altered shape of the uranophane crystal. This is in agreement with (13-14), who reported appreciable amounts of Fe in the chemical analyses of uranophane.

2- ESEM images and semi quantitative microanalyses of pyrite (FeS2) were carried out and illustrated in (Fig. 3C). The semi-quantitative analyses for the pyrite flame gave S=52.3%, Fe= 47.5% and Pb=4.9%. Pyrite forms euhedral crystals up to 0.25 cm long and is commonly found in vugs. Pyrite itself is typically replaced and resorbed by later deposited sulfides such as bismuthinite, galena and chalcopyrite (14).

3- Wolframite (Huebnerite) ranges in colour from black to reddish brown. SEM image (Fig. 3D) shows enrichment in W= 76.78% and Mn=21.13%. (15) indicates that W ranges in composition from 40% to 78% in Wolframite.

4- Monazite is a chief accessory mineral in the UDB volcanic rocks. It varies in size from less than 30 µm up to 50 µm in diameter. The elements of Fe, Si, Al, La, Ce, Pr (LREE), Ni and Cu have been reported in substantial amounts, but have been ascribed to impurities (Fig. 3E). U/Th ratio equal to 0.9 manifesting enrichment of U relative to thorium in original magma.

5- Rutile occurs as small elongated or semi-rounded grains intergrown with ilmenite and attributed to high temperature oxidation of ilmenite. The ESEM analyses show that rutile consists essentially of Ti=96.91%, Si=1.48% and Al=1.38% (Fig. 3F).

6- Zircon forms subhedral to euhedral grains, varies in size from less than 40µm up to 150µm in diameter. Zircon is partially altered to amorphous thorite and xenotime, probably due

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to the effects of radioactivity of U and Th in it (16). The data indicates the predominance of Zr (71%) but depleted in U and Th and Zr/Hf ratio= 47 (Fig. 3G).

RESULTS OF CHEMICAL ANALYSES The average major oxides of the analyzed thirty seven samples reflects the effect of the

country rocks in the chemical composition of UDB, where UDB across marbles are enriched in CaO, MgO, Cu, Ni and Sr and depleted in SiO2, Zn, Nb and Zr (Tables 1&2) compared with UDB across both volcanogenic metasediments and meta-andesites.

The alteration box plot is used to discriminate between geochemical trends of diagenetic alteration from those hydrothermal one. The data are plotted in common proximal hydrothermal minerals; sericite, chlorite, pyrite, dolomite and ankerite which lies on the right hand CCPI axis and the upper AI axis (17) (Figs. 4&5). The samples are plotted in trends 3,4 and 5 in decreasing order which represented by chlorite + sericite + pyrite and carbonate.

The most prominent geochemical characteristics of altered samples include substantial variations in the concentrations of Na, Ca, Mg and Fe Figure (6) (18). Increasing alteration intensity is associated with a coupled gradual decrease in Na2O and CaO. The Na2O+CaO values of the studied altered samples range from 1-6 wt% reflecting plagioclase destruction and formation of muscovite/sericitization, whereas the MgO+FeO values is commonly between 10 and 20wt% indicating intense pyritization and chloritization (Fig.6).

SPECTROMERTIC DATA In UDB area, the gamma ray spectrometry measurements were carried out using a GS-256

spectrometer with a 7.62x7.62 cm2 sodium iodide (Thalium) [Na1(Ti)] crystal detector.

Detailed spectrometric studies were carried out along NW-SE profiles at a grid pattern of 10x10m. Results of the gamma-ray spectrometric survey of UDB are illustrated in the form of eTh, eU, K, eU/eTh, eU/K & U mobilization contour maps (Figs. 7&8). The equivalent thorium map (Fig. 7A) shows high eTh content at the middle part of the UDB dyke cutting both marble and meta-andesite. The equivalent uranium map (Fig. 7B) shows high eU content (20 up to 570 ppm) close to the volcanogenic-metasediments as country rocks for UDB, especially at the NE and SW parts of UDB area. The K% contour map (Fig. 7C) coincides with the Th contour map. The eU/eTh ratios map (Fig. 7D), indicates an increase from 2 to 7 in the UDB passing through the volcanogenic-metasediments reflecting an increase of uranium than thorium during the hydrothermal activities ascending through the foliation and fractures planes of the host rock. The eU/K contour map (Fig. 8A) is in agreement with the eU contour map and shows high eU/K ratio up to 400. The overlap between the high eU/K and the eU anomalies is helpful in delineating the U enrichment zones within the UDB.

The remobilization of uranium contour map (eU-eTh/3.5) enables the delineation within the limit between the negative contours (leaching) and positive contours (deposition). The high level up to 400 ppm (Fig. 8B) forming semi-rounded zone in the NE corner. The U is migrated from the central zone of UDB close to fault zones (channel ways) to outward the periphery.

CONCLUSION Um Doweila peralkaline bostonite (UDB) could be considered the longest U-bearing dyke

in the Eastern Desert of Egypt. It extends in the NE-SW direction for about 11 km and ranges

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from 1-10 m in width and emplaced across three country rocks; volcanogenic meta-sediments, meta-andesite and marble with sharp contacts.

The NE-SW and NW-SE tensional fractures have been reactivated as strike-slip faults (sinistral and dextral respectively) under N-S (semi-meridional) followed by a ENE-WSW trending regional compressional stresses creating zones of ductile deformation characterizing coupling tectonic environment.

The metamorphism in the country rocks is of weak to very weak type that occurred under moderate pressure with depletion in silica and enrichment in CaO and MgO restricted to marble zone.

The mineralogical study of the UDB samples reveal the presence of uranophane, beta-uranophane, wolframite, monazite, zircon, rutile and opaques whereas pyrite is a common in volcanogenic metasediments. UDB is affected by common proximal hydrothermal minerals; sericite, chlorite, pyrite and ankerite.

The detailed spectrometric survey revealed coincidence of eU/K and eU ratios along the country volcanogenic metasediments rocks (reduced regime). The U migrated from the central zones of UDB close to fault zones (channel-ways) inward to the peripheries outward and concentrated at the low relief NE and SW upper and lower corner.

REFERENCE [1] Vail, J. (1985): Ring complex and related rocks in Africa. J. Afriaca. Earth Sci., 8, 19-40 pp. [2] Bouden, P., (1985): Magmatic evolution and mineralization in the Nigerian Younger granite

Province In: Evans, A. M. (Edd.): Metallization associated with acid magmatism, London, Wiley, 51-61p.

[3] Hashad, A. H. and El Ready, M.W.M. (1979): Geochronology of the an orogenic alkalic rocks, South Eastern Desert, Egypt. Annal. Geol. Surv. Egypt, X., 81-101.

[4] Vail, J. (1970): Tectonic control of dykes and related irruptive rocks in Eastern Desert, Africa. In: Clifford T.N. & Gass I. G. (edd.) African magmatism and tectonic, Oliver & Boyd, Edinburgh, 337-354.

[5] El Ramly, M.F. (1972): A new geological map for the basement rocks in the Eastern and Southwestern Desert of Egypt, Scale 1: 1000,000. Annal Geological Surv. Egypt, II, 1-18p.

[6] Krs, M. (1972): Technical report on follow-up geophysical survey in the period 1968-1972. Internal report, Aswan Mineral Surv. Project, Geol. Surv., Egypt.

[7] Amin, H.A. (1973): Geological survey of uranium bearing dyke at Umm Doweila area. Internal report, Geol. Surv., Egypt, Cairo.

[8] Hussein, A.A. and Abu Zied, K.M. (1974): On the geology and geochemistry of the radioactive dyke of Umm Doweila, South Eastern Desert, Egypt Internal report, Geol. Surv.,

[9] Hussein, H.A., Mansour, S.I., Kamel, A.F. and El Ready, M.W. (1988): Distribution of radioactivity in Um Doweila dyke, South Eastern Desert, Egypt, 4th Conf. Nuc. Sc. Appl., 201-212. 4

[10] Gado, H.S.S. (1993): Geochemistry and radioactivity of Umm Doweila area, South Eastern Desert, Egypt, M.Sc. Thesis, Ain Shams Univ., Egypt, 168p.

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[11] Gratier, J. P. (1984): La déformation des roches par dissolution-cristallisation. Aspects naturels et expérimentaux de ce fluage avec transfert de matiére dans la croute supérieure. Thése, Univ. Scient. Et Méd. Grenoble.

[12] Bjornerud, (1989): Forme et orientation des indicateurs cinématiques dans les zones à déformation cisaillante. J. Strust. Geol., 11 (8), 1045-1049.

[13] Frondel, C. (1958): Systematic mineralogy of uranium and thorium. USA Geological survey. Bulletin 10 64, 400p.

[14] Heinrich, E. W. (1958): Geology of radioactive raw materials. Mc Graw-Hill, New York. 654p.

[15] Higgins, N.C. (1980): Fluid inclusions evidence for the transport of tungsten by carbonate complexes in hydrothermal solutions. Canadian Jour. Earth Sci., V. 17, 823-830.

[16] Deer, W.A., Howie, R.A. and Zussman, J., (1992): An introduction to the rock forming minerals. 2nd Edition, Longman Sci. Tech., London.

[17] Large, R.R., Allen, R.L., Blacke, M.D., and Herrmann, W., 2001, Hydrothermal alteration and volatile element halos for the Rosebery klens volcanic-hosted massive sulfide deposit, Western Tasmania, Economic Geology, 96, 1055-1072.

[18] Le Maitre, R.W., Bateman. P., Dubek, A., Keller, J., Lameyre, J., Le Bas, M.J., Sabine, M.A., Schmid, R., Wooley, A.R., and Zanettin, B. (1989): A classification of igneous rocks and glossary of terms: Recommendations of the International Umnion of Geological sciences subcommisson on the systematics of igneous rocks: Oxford, Black-well Scientific publications, 193p.

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TABLES AND FIGURES:

Table ( 1 ): Average of chemical analyses of UDB in country rocks.

UDB across meta-andesite (n= 9) SiO2 Al2O3 Fe2O3 TiO2 MgO CaO Na2O K2O

Min. 55.20 7.30 13.58 0.40 0.50 0.84 0.41 2.79 Max. 67.00 15.50 19.36 1.51 1.60 3.10 4.86 4.40

Average 60.72 12.06 16.44 0.70 0.89 1.66 1.71 3.56 UDB across marble (n= 7)

SiO2 Al2O3 Fe2O3 TiO2 MgO CaO Na2O K2OMin. 42.40 7.70 15.97 0.15 0.44 0.73 0.41 0.47Max. 64.30 18.40 21.56 0.85 7.26 8.70 5.39 3.90

Average 54.88 10.91 17.27 0.42 2.24 4.33 1.88 2.71UDB across volcanogenic metasediments (n= 21)

SiO2 Al2O3 Fe2O3 TiO2 MgO CaO Na2O K2OMin. 57.06 7.40 9.18 0.23 0.16 0.39 0.47 0.52Max. 72.20 13.17 22.50 0.67 1.60 3.09 6.10 4.00

Average 63.98 10.44 16.04 0.44 0.96 1.19 1.77 3.00n= number of analyzed samples

Table (2) Average of trace elements (ppm) of UDB. S.No Cr Co Ni Cu Zn Zr Rb Y Ba Pb Sr Ga V Nb

1 47 7 25 29 492 295 117 236 146 86 20 31 47 2862 53 7 40 113 291 1675 118 99 161 39 135 26 103 1693 43 7 15 26 414 2063 165 158 82 63 19 37 35 296

1= UDB across volcanogenic metasediments, 2= UDB across marble and 3= UDB across meta-andesite

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Fig. 1: Geological map of Um Dowelia area, south Eastern Desert, Egypt.

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Fig. 2: Structural framework of Um Dowelia area, South Eastern Desert, Egypt.

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A B

C D

E F

G

Fig. (3): Environmental Scanning Electron Microscope (ESEM) showing uranophane (A), β-uranophane (B), pyrite (C), wolframite (D), monazite (E), rutile (F) and zircon (G) grains.

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Fig. (4): Discrimination diagram separate the hydrothermal from the diagenetic trends (17).

Fig. (5): An alteration box plot with vectors for various alteration minerals versus diagenetic trajectories (17) for UDB dyke in the different hosting rock types.

Fig. (6): Diagram showing alteration vectors for the hydrothermal alterations in UDB dyke in the different hosting rock types (18) .

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A B

C DFig. (7): Detailed contour maps showing eTh (A), eU (B) in ppm, K% (C) and eU/eTh (D), for

UDB.

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A BFig. (8): Detailed contour maps showing eU/K (A) and U mobilization (B) for UDB.