EG0000331 Third Arab Conference on the Peaceful Uses of Atomic Energy, Damascus 9 -13 Dec. 1996 AAEA Uranium Traps in the Phosphate Bearing Sudr Chalk, in Northeastern Sinai, Egypt HA. Hussein, LE.ElAassy, MA. Mahdy, GA. Dabbour, AM. Morsy* and M.Gh. Mansour Nuclear materials Authority, Cairo, Egypt. * Suez Canal University, Ismailia, Egypt. kTjI—uuljjJI pj 0 kl a_5j •r^j " * j *_• ^ 5 ^1 J inj ' I' * -1 J_5J . In iiijVI ^yjl a .Mj-i-ll j .A t II ii . X . ) I . - J . I A < I d -^ >V. • ij a • ,.| I I ^ j * ** I j j * 11 J Abstract The Maastrichtian Sudr Formation in northeastern Sinai is composed of three members, the lower chalk, the middle phosphate and chert-bearing and the upper chalk members. Lemon yellow secondary uranium mineralization, distributed in the lower chalk member and in some phosphate beds from the middle phosphate member are observed. The XRD analyses of some samples from the uranium bearing chalk and the r«v phosphate beds showed the presence of the secondary uranium minerals carnotite, bergenite and upalite. The mode of uranium occurrences could be interpreted as a result of the phosphatic beds decomposition and their subjection to later diagenetic processes. Uranium leaching circulation from phosphate rocks led to the liberation of uranium from the phosphates, and vanadium from the bituminous material and clay minerals. These migrated and were deposited locally and within the underlying chalk beds which acted as a lithologic trap. Introduction The study area is located between longitudes 34° 20" and 34° 40" E and latitudes 30° 15" and 30° 30 N (Fig. 1). Gabal Urayf An-Naqah a doubly plunging fold is located in the central part of the area. The core of this mountain consists of Triassic, Jurassic and Cretaceous rocks which are intensely folded in the ENE-WSW trend. The upper Cretaceous (Maastrichtian) chalk is slightly folded[l] . Sudr Formation is the main phosphate bearing sedimentary horizon in this area. El Aassy, [2] studied a phosphate occurrence east of El Qaa plain area in southwestern Sinai. He subdivided Sudr Formation into three distinct members: the lower chalk member, the middle limestone with chert and phosphate beds intercalations and the upper chalk member. Bartov et al [3], studied Gabal Urayf An Naqah area and subdivided the Sudr Formation into two formations namely: Mishash Formation of Campanian age and Ghareb Formation of Maestrichtian age. Phosphorites of Senonian age were studied in northern Negev just east of the study area, where the yellow uranium mineralization was found close to the phosphorite beds [4]. Sudr Formation has recently drawn the authors attention due to three reasons. First, it represents the host rock of the newly recorded phosphorites [2,5,6]. Second, the discovery of new secondary uranium mineralization in both the chalk and the associated phosphorite beds. Third, this formation includes considerable amounts of bituminous material which may have played a role in the epigenetic processes affecting the area. Thus, this study was carried out in order to investigate the uranium potentiality of Sudr Formation and its suitability as trap for uranium mineralizations. Geologic Setting The sedimentary cover in the studied area ranges from the Anisian (Triassic) Urayf An Naqah Formation to the Middle Eocene Mokattam Formation (Figs. 1 & 2). Triassic and Jurassic rocks are exposed in the core of the doubly plunging Urayf An Naqah anticline. Urayf An Naqah Formation is composed mainly of sandstone interbeds, dark coloured, cross-bedded and of fine to medium grained particles. The sandstone is alternating with fossiliferous limestone and shaly siltstones. Urayf An Naqah Formation is overlain conformably by Abu Nusrah Formation which is related to the lower Carnian-Ladinian (7). Abu Nusrah Formation is composed mainly of carbonate rocks such as fossiliferous limestone, dolomite, and marl interbeds near the top. Qugttrarf PUiiltftM MiMttxm *tn Miymil FmMUtltm ftmtU Mttrfriehlion Cmponion QmiMion-SiiMu (Ml Msfau, Amul Turanian fiSTI Wolo ™J Holal ftmigtjM Ml Holha f«TOlim H«I* Loisit (jrT] B« Triutie u*M E Ur«ri An Neqah Formaljon Fault. rJ ending plw. udion ^^ Bordii Figure (1): Geologic map of the study area (after Geological Survey, 1993, wilh modifications) LJaacsumc, contains Nummullte gixebenjii, pale grey to wbitc* medium bard, highly disturbed with slumping structures. Fig.< 2 )• Cooipil»d li thosUaiigraphJe s#ction,Uray1 An Naqah atma Figure (2) : Compiled lithostratigraphic section, Urayf An Naqah area The Jurassic rocks are represented by the middle Liassic Rajabiyah Formation, composed of limestone, mainly coralline with algal components. The Jurassic rocks are unconformably overlain by the Aptian-Albian Malha Formation, composed of kaolinitic sandstone, with kaolinite and conglomerate interbeds. The section of the Upper Cretaceous rocks is completely exposed even with no missing rock unit. It starts with the Cenomanian Halal Formation at the base which conformably overlies the lower Cretaceous Malha Formation. Turanian Wata Formation is represented mainly by carbonate rocks (limestone and dolomite) with thin sandstone and shale interbeds. The Coniacian-Santonian Matalla Formation conformably overlies the Wata Formation and is composed of limestone, marl and shale. A clastic section of sandstone and shale with phosphatic bed is noticed at the top and may be related to the Campanian Duwi Formation, overlying the Matalla Formation. The Maastrichtian Sudr Formation represents the main target in this study and is composed mainly of carbonate rocks with clastic interbeds and some phosphorite and chert beds. It is conformably overlain by the Paleocene-Lower Eocene Esna Formation which is composed of two members; the lower Esna belonging to the Paleocene, and the upper Esna related to the lower Eocene. The two members are separated by limestone interbed (1 m). The Lower Eocene Thebes or Egma Formations are not recorded in this area. The Middle Eocene Mokattam Formation unconformably overlies Esna Formation and is composed of pale grey to white nummulitic limestone. Lithostratigrapby of Sudr Formation Sudr Formation measures about 120 m in thickness and covers an area of about 400 km2. However, it unconformably overlies the Duwi Formation and is overlain by the Esna Formation, It can be subdivided into three members (Fig. 3) according to its lithology and areal extension. The following is a brief description of each member starting from the oldest. 1. Lower Chalk Member It overlies the Duwi Formation and varies laterally in thickness from 35 to 45 m. Each chalky bed varies from 30 to 70 cm in thickness. It is composed mainly of grey to reddish brown chalk with minor mad interbeds. The top most 4 m are fractured with joint planes in different trends. Visible yellow to canarian yellow secondary uranium minerals were observed with some chert concretions near the top. It also contains black centers of bituminous materials and some with metallic lusters. 2. Middle Phosphate and Chert-bearing Member The middle phosphate and chert-bearing member shows lateral variations either in thickness or in facies. It varies from 25 to 35 m, in thickness, and is well developed in the eastern part of the studied area (Figs.l & 3). The lower 6 to 10 m are composed of chert beds which vary in colour from dark brown to black. They are brecciated at the contact with the underlying unit. This contact is a marker horizon for the presence of the secondary uranium mineralizations. Some chert bands and lenses of light tones are associated with the phosphates, and show patches of the yellow secondary uranium mineralization. The basal brecciated 0.5 m of this chert may indicate an unconformity surface with the lower unit. This chert horizon is overlain by an intercalation of phosphate, marl and clayston (Fig.4). Phosphates are represented by eight beds which vary in thickness from 0.2 to 1.2 m. The two top most phosphate beds are soft to moderately hard, brown and contain relatively high P2O5 content. Visible yellow secondary uranium mineralizations were also noticed in the two upper phosphate beds. Figure (3) : (a) Middle Phosphorite Member overlain by the Upper Chalk Member (U-Ch), Looking E. (b) Intercalation of phosphorite beds with chalk and chalky marl bed which represents the northern wall of the upper photo, Looking N. P f Figure (4): Middle Phosphorite Member with phosphorite and chalk with clay interbeds, Looking SE. Figure (S) : Shark tooth with ferruginous cement around the phosphate pellets U200). 3. Upper Chalk Member This member is equivalent to the Abu Zeneima member and is composed mainly of thick bedded chalk, chalky limestone, marly in some parts, argillaceous and ferruginated. These chalks and marls show some black centers of bituminous material. The thickness of this member ranges from 40 to 50 m. Petrographical and Mineralogical Investigations Petrographically, the phosphatic material in the phosphorites of the Sudr Formation occur in different forms including grains, pellets, and different skeletals such as shark teeth, bone fragments and fish scales. The grains are randomely distributed and are of coarse silt to medium sand size. The grains diameters range from 0.2 to 0.8 mm.Pellets are less abundant and are ovoidal to elliptic in shape. The shark teeth (Fig. 5) are sharply defined in the outer layer and reach 1 cm in length and 2 mm in width. They are highly fractured and filled with secondary calcite. The bone fragments have different shapes with their maximum lengths reaching 1 cm. Three types of cements were observed: phosphatic, calcareous and ferruginous. The phosphatic cement is fine and filling intercavities between the former different allochems. The recorded carbonate cement has different grades of recrystallization while the ferruginous cement (Fig. 5) is present in patches and coating other grains. Mineralogically, secondary uranium minerals in Sudr Formation were observed as tiny aggregates and crustaceous materials in the top of the lower chalk member, as well as in the two upper phosphate beds.