Sandstone Type Uranium Occurrences in Western Murzuk Basin, Libya

MINING GEOLOGY, 38(2), 115•`119, 1988

H. S. ASSAF*, K. HANGARI*, H. S. AGELI*, B. O. MEHDI** and F. S. BAKHIT***

Abstract: This study is concerened with the uranium occurrences in western flank of the Murzuk basin, southwest Libya. Numerous uranium occurrences have been recognized in sedimentary rocks of the Zarzaitine Formation of Triassic period. These occurrences were investigated by geological, radiometrical, mineralogical and geochemical methods to reveal the occurrence and geological setting of these uranium mineralizations, especially the relationship with sedimentary environment of the host rock. These occurrences belong to a stratabound uranium mineralization in sedimentary rocks of the Zarzaitine For- mation. Five sedimentary cycles were recognized in the formation, and lithology of the continental sediments consist of coarse to fine grained clastic materials. Beta-uranophane, carnotite and tyuyamunite are important ore minerals, however, tetravalent uranium mineral has never been identified in these occurrences. The uranium mineralization is associated with geochemical anomalies of vanadium, molybdenum, selenium, iron and sulfur. Uranium and these associated elements seem to be supplied from Hoggar granite massif belt.

1. Introduction 2. Geology

The uranium minerals in the Zarzaitine For- 2.1 Regional setting mation were first discovered in 1973 (ASSAFet The uranium occurrences, herein refered to al., 1974) in sandstone of the Zarzaitine For- as 'Al Awaynat-Serdles occurrences', are on mation at the Al Awaynat-Serdles area (Fig. 1). the western flank of Murzuk basin, southwest This discovery gathered attention to the Zar- zaitine Formation which is exposed along the western flank of Murzuk basin. Since then the entire area has been subjected to an intensive programme of geological and radiometric in- vestigation (ASSAF et al., 1976; Geoinstitut, 1979; 1981; 1982). These investigations have resulted in not only confirming the Zar- zaitine Formation as a favourable horizon for uranium mineralization but also discovering additional subsurface uranium occurrences at a depth of 30-60 m.

Received on February 14, 1987, accepted on January 25, 1988 * Atomic Energy , Tripoli, Libya. ** Industrial Research Centre , Tripoli. *** Nuclear Materials Corporation , Cairo, Egypt. All the correspondence should be addressed to ; Dr. F. S. Fig. 1 Generalized geological map of the Murzuk BAKHIT, 18A El Badow Street, Heliopolisse, Cairo, basin, Libya, showing the location of studied area Egypt. (after DETLEF BUSCHE, 1978).

115 116 H. S. ASSAF, K. HANGARI, H. S. AGELI, B. O. MEHDI and F. S. BAKHIT MINING GEOLOGY:

100 m thick in the northern part of the area and decreases to about 50 m in its southern part. The formation overlies the Dembaba For- mation unconformably with 0.8 m thick local basal conglomerate and is unconformably overlain by the Taouratine Formation. Macro fossils are not found in the Zarzaitine Forma- tion. The formation was eroded locally in Cenozoic era and is overlain partly by younger fluvial sediments and fossil wood. Well developed upward-fining sedimentation cycles characterize the Zarzaitine Forma- tion. Five such cycles have been observed and individual cycles vary in their thickness from few metres to as much as 20 m (Geoinstitut, 1979). Within these sequences three main rock types have been recognized, these are from conglomerate through sandstone to clay and siltstones. They are intergradational and Fig. 2 Geological map of the Al Awaynat-Serdles vary widely in thickness and composition area (after JAKOVLJEVIC,1984) and the locations of along strike and dip. Some units pinch out uranium occurrences. completely within few hundred meters. The conglomerate is the least abundant rock type and commonly occurs as lenticular beds, 0.2- Libya. They occur in the sediments of the Zar- 0.4 m thick. It is yellow and brownish red in zaitine Formation. These sediments are of con- its color and is highly friable. The pebbles in tinental origin and lie unconformably on the the conglomerates are rounded and consist weathered and eroded marine sediments of of limestone, claystone with minor amounts Dembaba Formation of Upper Carboniferous of quartzites and igneous rock fragments in a period. They are overlain by the continental ferruginous clayey and carbonate matrix. sediments of Taouratine and Messak Forma- The sandstone is fine to coarse grained and tions of Jurassic and Cretaceous ages respec- the color varies from greyish yellow, grey tively (Fig. 2). The Zarzaitine Formation has green to brownish red. It is usually finely been mentioned by many previous authors as laminated and, occasionally, cross-bedded. a part of the "Continental Post Tassilian The sand grains are angular to subangular, Group" whose age probably extends from Per- moderately to well sorted and are constituted mian to Lower Cretaceous (KILIAN, 1931; DE of quartz (41-98%), feldspars (2-20%), mica LAPPARENT et al., 1948; BANERJEE, 1980). flakes (0-4%), and minor amounts of lithic The area is a part of the North African plat- fragments of quartzite and chert. The matrix, form and has simple structure with persistent which consists of 20-80% of the sandstone, is and barely perceptible (1-2•‹) easterly dips. ferruginous clay and calcareous material. Ac- The beds are cut by a few N-S and NE-SW cessory minerals are tourmaline, epidote, zir- trending normal faults. con and apatite. The calcite content of the 2.2 Local geology sandstone reaches up to 23% (ASSAF, 1976; The Zarzaitine Formation in the Awaynat- Geoinstitut, 1979, 1981, 1982). Serdles area is distributed in a narrow norther- The siltstone-claystone is predominantly ly-trending belt, 6-30 km wide, and the brownish red, but is localy greyish green in distribution extends along the western flank of color. It is finely laminated and consists of the Murzuk basin. The formation is about ferruginous clayey material. It contains dis- 38(2), 1988 Sandstone Type Uranium Occurrences in Western Murzuk Basin, Libya 117 persed, rounded to angular quartz and rarely concentration of uranium occurs most com- feldspars grains less than 0.04 mm in diame- monly in sediments with organic matter ter. Unidentified opaque mineral is observed and/or fossilized logs. rarely (ASSAF, 1976; Geoinstitut, 1979, 1981, 3.2 Subsurface mineralization 1982). The following regional features are Subsurface mineralizations at Al Awaynat pointed out as the characteristics of the Zar- and Tamulilt occurrences are encountered at zaitine Formation in the area: depth ranging between 30 and 60 m below the (1) The clastic materials are mostly supplied surface. The following preliminary character- mainly from quartz-rich sedimentary rock istics are obtained from bore hole exploration: with subordinate contributions from igneous • Uranium mineralization is exclusively con- and metamorphic rocks and limestone. fined to greyish, fine to medium grained sand- (2) The clastic materials of the Zarzaitine stone with abundant fossil plant material and Formation were transported and deposited in dispersed pyrite grains. fluvial and continental environments. ¥ The individual ore body is small, 1-2 m (3) The source areas, as indicated by the thick, and has uranium content in the range be- available paleocurrent data lies to the south tween 30 and 300 ppm. and southwest direction, a position which coin- ¥The role of organic matter in the deposition cides with the Hoggar craton (ASSAF, 1976, and localization of uranium is undisputable. 1980; EL MAXHROUF,1979; Geoinstitut, 1982). The higher uranium concentrations were commonly recognized in portions of the sandstone 3. Uranium Occurrences with high organic matter content. More than 60 radioactive anomalies were ¥The uranium concentration tends to in- discovered through detaild geologic and crease southwards. The most significant radiometric surveys in the Zarzaitine Forma- uranium concentrations, 200 and 323 ppm U, tion. Some of them were with visible showings were found in two sites, 500 m apart, in of uranium mineralization. These anomalies Tamulilt area. and showings are distributed along the strike ¥No uranium mineral has been identified in for approximately 100 km in length. The most the subsurface mineralized zones. significant occurrences of uranium mineraliza- 3.3 Mineralogy and geochemistry tion are discovered in two areas. These two The principal uranium minerals are beta- areas are informaly called Al Awaynat and uranophane, carnotite and tyuyamunite. Tamulilt occurrences (Fig. 2). Limited number These hexavalent uranium minerals were iden- of exploration drillings have resulted in in- tified only in the surface mineralized zone. tersecting minor subsurface extension of They occur either as thin films stained on the uranium mineralization in both areas as will walls of cracks and joints in host rocks, be mentioned later. coatings on detrital grains and wood frag- 3.1 Surface mineralization ments; or as discrete grains and aggregates The surface uranium mineralization is within pore spaces of the sediments. Black hosted in grey colored, fine-grained sandstone tetravalent uranium minerals have not yet and conglomerate. The mineralization occur been identified in either surface or subsurface as single or clustered small bodies. They are ar- zones. ranged in a trend parallel to stratification of Radiometric assay for these ore samples host rocks and paleo-stream channels. An in- from both surface and subsurface mineralized dividual ore body is lenticular mass with zones revealed that these occurrences are essen- elongated or amoeba-like outline, from 2 to tially in radioactive equilibrium. The ratios of 45 m in length and width and up to 1.8 m in U/eRa of analyzed samples from these occur- thickness. The uranium content within the ore rences are close to unity and range between body is extremely variable, which ranges from 0.79 and 1.3. 37 ppm to about 10.000 ppm U. The higher Pyrite is an important accompanying 118 H. S. ASSAF, K. HANGARI, H. S. AGELI, B. O. MEHDI and F. S. BAKHIT MINING GEOLOGY:

minerals, indigenous to the host sandstone, by the action of groundwater. After oxidation and dissolution of these minerals, uranium was deposited in the hexavalent form in other parts of the formation (ASSAF, 1976; Geoinstitut, 1979). (2) Uranium was leached from the granitic mass of the Hoggar craton and epigenetically introduced into the sedimentary units by the action of groundwater (Geoinstitut, 1982). However, it was revealed on the basis of the latest exploration and studies, that the epigenetic source hypothesis seems to be accep- table. Arguments in favour of this origin are: • All mineralized bodies are confined to permeable channel structures within sandstone bed with abundant organic materials and the host sandstone is sandwitched between impermeable siltstone-claystone. • Uranium concentration is associated with V, Mo, and Se, which is suggestive of the epigenetic introduction into the mineralized spaces. • No detrital uranium minerals have been detected in the sedimentary sequence which Fig. 3 Relation between uranium and associating encloses the host sandstone. Also the mode of heavy minerals in the sequence is too small to elements like vanadium, molybdenum , and selenium in mineralized zone. These contents were derive the uranium. determined spectrometrically by ASSAFet al. (1976) • The mineralized bodies are occasionally pre - and Geoinstitut (1981, 1982). sent transgressive to bedding. On the basis of the available geological data given in this paper, the following genetic mineral in these occurrences and iron and model is suggested. Favourable conditions for sulfur are the useful indicator elements of the dissolution and transportation of uranium geochemical exploration. Anomalies of V, Mo could be happened in a period between the end and Se are also recognized in the zone of the of the Triassic age and initial stage of the uranium mineralization (Fig. 3). These sedimentation of the Taouratine Formation. elements have been distributed in a similar At that time, the granitoid of the Hoggar trend with uranium, indicating their genetic craton cropped out in the southern and relationship. The association of these elements southwestern peripheries of the Murzuk basin. can be used as diagnostic geochemical anoma- Under this condition, uranium and other lies and path finders for uranium exploration elements were leached out by groundwater in Al Awaynat-Serdles area. from the granitic masses of the Hoggar craton 3.4 Origin through weathering. The uranium is trans- Following two hypotheses are suggested for ported to the permeable members of the host the origin of the uranium occurrences in Al rocks and moved down through the flanks of Awaynat-Serdles area: the basin towards the center. Precipitation (1) The uranium was derived from detrital and fixation of uranium and other associated uranium minerals and/or accessory heavy elements occurred in the sediments where the 38(2), 1988 Sandstone Type Uranium Occurrences in Western Murzuk Basin, Libya 119 organic matters are abundant. cad. Sci., Paris, 227, 1106•`1108.

The surface uranium mineralization could DETLEF BUSCHE (1978): On the Origin of the Msak and have been formed through mobilization and Hamadat Manghini Escarpment. The 2nd Sym- precipitation by the action of surface water on posium on the Geology of Libya, Vol. 3. (M. J. the weathered and exposed portions of Zar- SALEM and M. T. BUSREWIL ed.), Academic Press, zaitine Formation. This may be happened dur- 1980. EL MAXHROUF, A. A. and EL GHOUNDI, A. R. (1979): ing the rainy period of Tertiary time. Uranium activities in Libyan Arab Jamahirya,

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Ohanet et Bourahet (Sahara Central). C.R. de Ac-

リビア,ムルズク盆地西部の砂岩型ウラン鉱床

H.S.アサフ・K.ハンガリ・H.S.アゲリ・B.O.メディ・F.S.バクヒット

要旨:リビア南西部ムルズク盆地の西縁に分布する,三ウラン鉱物としては β－ウラノフェン,カルノータイ

畳紀のザルザイティン層中には無数の砂岩型ウラン鉱床ト,テュヤムナイトが重要で,4価のウラン鉱物は見つ

の鉱徴が見られる.同層中には5回の堆積サイクルがあかっていない,鉱化に伴いパナジン,モリブデン,セレ

り,粗粒から細粒のいずれも陸成の堆積物がみられる. ン,鉄,およびイオウの濃集がみられる.これらの元素

ウランの鉱徴はその中の有機物に富む粗粒砂岩中にみらおよびウランはホッガー花崗岩マッシフより,地下水に

れ,古流系のチャンネル堆積物中に限られている. より溶出沈澱してできたものと考えられる.