Uranium mineralization in the Judean Desert and in the northern Negev, Israel

Ore Geology Reviews, 4 (1989) 305-314 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

305

URANIUM MINERALIZATION IN THE JUDEAN DESERT AND IN THE NORTHERN NEGEV, ISRAEL S. ILANI and A. STRULL Mineral and Energy Resources Division, 30 Malkhei Yisrael St, Jerusalem 95501 (Israel) (Received December 21, 1987; revised and accepted July 15, 1988)

Abstract Ilani, S. and Strull, A., 1989. Uranium mineralization in the Judean Desert and in the northern Negev, Israel. Ore Geol. Rev., 4: 305-314. Secondary, yellow uranium minerals, formed by weathering, soil formation or sedimentation processes are widespread in arid areas of the northern Negev and the Judean Desert, Israel. They occur in scattered exposures as much as several hundreds of square meters in extent over an area of 700 square kilometers. The uranium content in hand specimens ranges from 200 to 3900 ppm. In most samples uranium is in disequilibrium with respect to its daughters. Geologic considerations suggest that the uranium is leached out of the Senonian phosphorites which are exposed over much of the area and from the metamorphosed phosphorites of the Hatrurim formation (Senonian to Palaeocene) and migrate towards the local base levels, mainly towards the Dead Sea Rift Valley. The uranyl ion together with ions of vanadate, phosphate and carbonate form various uranium minerals mainly meta-autunite, meta-tyuyamunite and carnotite. The secondary uranium minerals are associated with gypsum which forms thin veins and is found also as matrix in pedogenic layers that range from 0.2 to 1.5 meters in thickness. This gypsum-rich pedogenic layer, like calcrete, might be a lithological marker and trap, especially under reducing conditions.

Introduction

Two distinct types of uranium mineralization occur within the northeastern Negev and the Judean Desert in central Israel. Primary uranium mineralization is represented by uranium enrichments in phosphorites of Senonian age as well as by uranium anomalies associated with epigenetic iron-oxide veins. Uranium concentrations within the large phosphorite deposits reach 230 ppm and average approximately 100 ppm (Gross, 1977; Nathan and Shiloni, 1977; Nathan et al., 1979; Shiloni, 1984). The uranium concentrations within the iron-oxide veins, which are of considerably smaller extent, reach 2000 ppm in hand specimens. Both of 0169-1368/89/$03.50

these primary types of mineralization have been the subject of extensive research (on phosphates: Mazor, 1963; Nathan and Shiloni, 1977; Nathan et al., 1979; Starinsky et al., 1982; Stein et al., 1982; Avital et al., 1983; on iron-oxide veins: Ilani et al., 1982, 1984, 1987). Less well studied are the occurrences of secondary uranium mineralization of Quaternary age which were formed by weathering and soil formation processes under arid conditions. The secondary uranium minerals are associated with gypsumrich crusts and veins in the soils. The presence of meta-tyuyamunite and other unidentified yellow uranium minerals occurring as tiny aggregates and veinlets within the Senonian (Mishash Formation) rocks was first reported

© 1989 Elsevier Science Publishers B.V.

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by Sass (1956, unpubl, rep. ). Meta-autunite and meta-tyuyamunite were identified by Gross (1977) within the Hatrurim Formation. Dan and Smith (1981) and Dan (pers. commun., 1984) reported the observation of pedogenic secondary uranium mineralization associated with gypsum-rich layers, crusts and fissure veins within the Judean Desert. Gross and Ilani (1987), studied the mineralogical composition of the secondary uranium occurrences in the Judean Desert and in the northern Negev. Until now the reported occurrences were of small extent. During a recent survey, more mineralization was encountered over an area of 700 square km within the northern Negev and the Judean Des-

ert (Fig. 1). The occurrences found todate are relatively small and scattered (up to a few hundreds of square meters).

Geographic and geologic background The area of study is situated between the Negev and the Judean Desert bordering by the fault escarpment of the Dead Sea Rift Valley in the east. The climate is noted for its aridity and strong evaporation with annual precipitation less than 200 mm. Rocks ranging from Upper Cretaceous marine sedimentary rocks to Quaternary continental sediments are exposed in the area. A generalised stratigraphic section is presented in Fig. 2. Limestones and dolomites

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study in the laboratory. X-ray diffraction analysis was used to identify the uranium minerals. A scanning electron microscope (JEOL-840) was used in order to study the paragenesis and the relations of the secondary uranium minerals to the other minerals such as gypsum and calcite. Elemental analyses were carried out by atomic absorption spectrometry (Perkin-E1mer A.A.-460). Uranium was determined directly using the delayed neutron activation (DNA) technique (Amiel, 1962). The equivalent-uranium (eU) was determined by gammaspectrometry which monitors the 214Bi daughter. A Bicron, 5" × 5" well-type NaI crystal was used for these measurements.

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of Cenomanian to Turonian age of the Judea Group are overlain by the chalks, marls, cherts, porcelanites and phosphorites of Senonian to Early Eocene age of the Mount Scopus Group. These are overlain by Neogene to Quaternary conglomerates. In places, a metamorphosed facies, the Hatrurim Formation ("Mottled Zone") is present in place of the normal Senonian to Neogene sequence (Kolodny et al., 1971; Gross, 1977). S a m p l i n g and analytical methods A field reconaissance survey was carried out using a portable gamma-ray spectrometer (SRAT SPP-2). Where anomalous radiometric signals were encountered, samples of the mineralization and the host rocks were collected for chemical, petrographic and mineralogic

Secondary uranium mineralization occurs as two main types: (1) associated with Senonian phosphorites; and (2) in soils. In both types the secondary uranium mineralization is usually associated with gypsum.

(1) Secondary uranium mineralization associated with phosphorites The phosphorites are enriched in uranium and are an obvious source for secondary type uranium minerals. The association of yellow uranium minerals with phosphorites is common in the northern Negev and the Judean Desert. In fact, wherever the phosphorites occur, secondary uranium minerals may be found. Meta-autunite and meta-tyuyamunite are often observed on fresh faces of phosphorite quarry walls. Secondary uranium minerals occur in the phosphorites of the Mishash Formation as well as in the metamorphosed phosphorites, carbonates and clays of the Hatrurim Formation (Figs. 1 and 3). In the latter case, secondary uranium minerals are especially abundant in the metamorphic phosphorite layer, but they also occur in lesser amounts throughout the entire metamorphic sequence. The uranium content

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in the samples from the phosphatic bituminous chalk at the top of the Mishash Formation is 80-570 ppm. The uranium content in the samples from the metamorphosed phosphorite at the base of Hatrurim Formation is 50-890 ppm. The uranium content in the samples from the metamorphic chalk of Ghareb Formation is somewhat less and ranges from 20-200 ppm. Uranium minerals are observed as: (a) tiny veinlettes up to 1 mm in thickness and several centimeters in length, filling joints and fractures within the host rocks; and (b) aggregates of tiny plates up to 1 cm in diameter on gypsum crusts. In all these occurrences the uranylphosphate meta-autuniteII Ca (U02) 2 (PO4) 2"2-6H20 and the uranyl-vanadate metatyuyamunite Ca (UO2) 2 (V04) 2"3-5H20 are always present. The latter mineral reflects the high vanadium content of the phosphorites. Of lesser importance and occurring only sporadically are the uranyl-phosphates saldeite Mg (UO2) 2 (PO4) 2"8-10H20, autunite Ca (UO2) 2 (P04) 2"8-12H20, phosphuranylite Ca (U02) 4 (P04) 2 (OH) 4" 7H20 and the uranyl vanadates carnotite K2 (UO2) 2 (V04) 2"3H20, tyuyamunite Ca (U02) 2 (VO4) 2"5-8H20, rauvite Ca (UO2) 2VloO2s" 16H20, bayleyite Mg2 (UO2) (CO3)3" 18H20 and zellerite Ca (UO2) (CO3) 2"5H20 (Gross and Ilani, 1987 ).

(2) Secondary uranium mineralization in soils The occurrence of secondary uranium minerals with soils especially lithosols, is common throughout the geologic section especially near the phosphorites (Fig. 2). Secondary uranium minerals have been found in lithosols covering phosphorites, chalks and cherts of the Mount Scopus Group as well as the carbonates of the Judea Group and in one instance in the Neogene conglomerate (Figs. 4 and 5). In all the lithosol occurrences, the uranium mineralization is associated with gypsum. It appears that the aridity of the region is a strong factor in the preservation of the secondary uranium minerals, for the secondary mineralization is found only in the regions below the 200 m m isohyth where the leachingprocesses are weak (Dan and Smith, 1981 ). This area is characterized by high temperatures most of the year and consequently a high ratio of evaporation to rainfall. In this arid area, the soil layers are best developed at the base of the slopes and on the plains. Here the washed out carbonate is concentrated at depth of 20 to 40 cm whereas the gypsum precipitates at depths of 60 to 100 cm, forming in places gypsum-rich layers (Fig. 4a) (Dan and Smith, 1981 ). The gypsum-rich layer extending in places over some hundreds of

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oped and are often partly eroded, so that the gypsum-rich layer is exposed. This layer contains yellow uranium minerals, mainly metatyuyamunite and meta-autunite. The content of uranium in the hand specimens is as much as 200 ppm. The gypsum is also found within the matrix of a polymictic, well-rounded to subangular, conglomerate of Neogene (Pliocene ? ) age (Fig. 4b). This conglomerate unconformably overlies the Menuha chalk and the Mishash chert and is located about 50 m above the present dry river bed. The gypseous matrix found in a layer up to 1.5 m thick, from about 0.5 m from the surface down to a depth of 2 m, contains yellow uranium minerals - - mainly meta-tyuyamunite and meta-autunite. The minerals are found growing on the gypsum crystals and included within the gypsum matrix as well as coatings on the polymictic pebbles of the conglomerate. The uranium content in hand specimens is up to several hundreds ppm. Secondary uranium mineralization was also found at the Makhtesh Qatan Anticline in the northern Negev in lithosols covering dolomites of Cenomanian age. On the moderate slopes of these sites, in a lithosol layer which is up to 30 cm thick, uranium enriched gypsum crusts and veins are developed, filling fractures in the dolomites. The content of the uranium in hand samples is up to 3900 ppm. Unlike the Judean Desert occurrences, in the Makhtesh Qatan

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square meters is often enriched in yellow uranium minerals, mainly meta-tyuyamunite, meta-autunite and carnotite. Bayleyite, tyuyamunite and rauvite are less common (Gross and Ilani, 1987). The content of uranium in hand specimens is as much as 500 ppm. In the extremely arid areas where the annual rainfall is less than 100 mm, the soils are less well develMishash Menuha Fm.

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Secular disequilibria studies By monitoring the uranium content directly by delayed neutron activation (DNA) and indirectly by the 214Bi y-ray emitting isotope, an estimate of the degree of secular equilibria within the samples may be obtained. Samples of the primary low grade uranium phosphorites as well as secondary mineralization from both the phosphorite association and

311 the pedogenic association were analyzed both by DNA and gamma-ray spectroscopy. Most of the analyses of the phosphorites yielded results within the statistical error of measurements by both techniques indicating that the phosphorites are close to secular equilibrium (Shiloni, in prep. ). This is consistent with the stratigraphic position of the phosphorite layers (Senonian). On the other hand, the secondary uranium minerals exhibit varying degrees of secular disequilibrium. In most cases there is an apparent considerable excess of uranium (U) measured by DNA compared to that by gamma-ray spectroscopy (eU), perhaps indicating that the growth of the daughter products had not advanced sufficiently to reach secular equilibrium. These samples are mainly less than 300,000 years old. The secondary uranium minerals associated with the metamorphosed phosphorites (Fig. 6, group B ), for example, indicate a wide range of disequilibria values. Four samples show an excess of daughter products relative to the parents. This may indicate that uranium was or is being leached out. Nine samples are near equilibrium, indicating that the mineralization is over 300,000 years (at least four half-lifes of 2:~°Th, 75,000 years and not considering initial 2:~4U/~;~sU disequilibrium). Ten samples are clearly out of equilibria in favour of the parent. Most of the samples of secondary uranium mineralization from the metamorphic phosphorites and the bituminous chalk of the Mishash Formation exhibit secular equilibrium or minor excess of 23sU whereas most of the samples of secondary uranium mineralization from the pedogenic occurrences and the metamorphosed chalk of the Ghareb Formation are not in equilibrium. There is an excess of the parents. This may indicate the relatively recent age and the dynamic nature of the mineralization processes, provided that the mobility of radium or the radon gas is not considerable. As the latter information is not well known, and as there is probably some degree of radon gas leakage, absolute ages can not be assessed. However, there

is a high probability that these samples are younger then 300,000 years and have undergone both weathering and formation processes to the present. Discussion

The main questions that we would like to address in regard to the uranium mineralization are: (1) What is the age of the mineralization? (2) What is the source of the uranium and how far is it dispersed? (3) What is the mechanism of transportation and what are the conditions by which the uranium concentrates? (4) In what geological conditions does the uranium concentrate? (5) What is the importance of the process ? (1) The time required for the formation of secondary uranium minerals must be less than that for the formation of the aridic soils. The formation age of the aridic gypsum-rich soils in the area is probably younger than the younger stages of Lower Palaeolithic and LevaloisoMousterian when the whole Near East was subjected to humid conditions which changed the Nubian aquifer (Issar, 1983) and formed Lake Lisan in the Dead Sea Graben (Begin et al., 1974) and lakes in the Arab Desert, such as the E1 Jafr Basin, Jordan and in Saudi Arabia (Huckriede and Wiesemann, 1968). It is most probable that these humid conditions caused washing and leaching processes and gave rise to an environment wherein gypsum-rich pedogenic layers with secondary uranium minerals could not be formed or preserved. Following this humid period, a period of extreme aridity prevailed which is the "post Lisan arid phase" (Huckriede and Wiesemann, 1968). According to Kaufman (1971), Lake Lisan formed some 60,000 years B.P. and lasted till about 18,000 years B.P. According to Druckman et al. (in: Begin et al., 1985) the upper age limit of Lake Lisan at elevation - 190 m is about 14,600 +_240 years B.P. The water level of Lake Lisan according to Begin et al. (1985), shows a drastic drop at about 11,000 years B.P. This might im-

312

ply that the "post Lisan" aridic period started some 11-14,000 years B.P. It is most probable that this is the period when the aridic soils and associated secondary uranium mineralization were formed. (2) Based on geochemical and mineralogical studies, the source of the uranium is most probably the phosphorites. Mineralogical results show that the secondary uranium minerals are mostly uranium-vanadates (Gross and Ilani, 1987). Vanadium is also enriched in the phosphorites of the Negev (Nathan et al., 1979). Moreover, near the phosphorites, the secondary uranium minerals are mostly uranyl-phosphates and uranyl-vanadates. The maximum distance between the phosphorites and the pedogenic occurrences is less then 3 km which might be the maximum dispersion distance of uranium, but it does not exclude the possibility that part of the uranium migrated further eastwards towards the Dead Sea Rift Valley. (3) It appears that the uranyl ion as well as the vanadate, phosphate, carbonate and sulphate ions are dispersed by surface rainfall waters and perhaps also by shallow groundwaters. The rainfall waters which filtrate downward, are enriched with salts, including NaC1, which is common at the surface in this arid area (Gilat, 1980). These relatively salty waters which are better agents of solubility and extraction than fresh waters (Rose et al., 1979), interact with the phosphorite layers and leach out the uranyl ion and the other ions. The uranyl ion UO2 occurs as a surface-adsorbed species on the apatite (Haynes, 1981). These waters carry the leached ions probably as uranyl-carbonate U02(CO3) or uranyl-phosphate UO2(HPO4) (Langmuir, 1978), in the streams and in the subsurface towards local base levels in the area. On their route through faults and joints in the rocks these salty meteoric waters emerge at the surface in places due to capillary action. The strong evaporation near the surface leads to re-precipitation of various minerals. According to present field and pet-

rographic observations, gypsum precipitates in the early stages; followed by uranyl-vanadates and the other uranium minerals which are found growing on the gypsum. Nevertheless, part of the gypsum-rich layers did not contain yellow uranium mineralization. Uranyl-sulfate minerals were not found, which may be due to the fact that SO~- was precipitated as gypsum and was not subsequently available for the formation of the secondary uranium minerals. (4) The secondary uranium mineralization concentrates mostly in synclines and small grabens, as well as in faults which are found between the main phosphorite exposures and the Dead Sea Rift Valley. The synclines and grabens serve as local base levels for the meteoric waters carrying the uranium complexes. The major base level of the area is the Dead Sea Rift Valley which became activated since the late Miocene-Early Pliocene. (5) It is most probable that the uranium in the study area was dispersed in the past 3 million years, when the phosphorite outcrops were subjected to weathering processes and were trapped partly at the local base levels. It is interesting to note that studies in Egypt reach similar conclusions. Hassan et al. (1983) reported on secondary uranium mineralization occurrences in extensive areas in Egypt. There, phosphorites crop out in highlands which drain into depressions such as in the Bahariya Oasis, where secondary uranium minerals were found in surface deposits. The authors relate these occurrences to calcrete-type deposits. Economic uranium-ore bearing calcretes in Western Australia and Namibia (Carlisle, 1983) develop under an arid climate in the capillary zone along the axes of large stable drainage systems having low gradients. Carnotite, the only ore mineral, results from: (1) evaporative concentration of U, V and K; (2) destabilization of uranyl carbonate complexes consequent to evaporative and common-ion precipitation of Ca-Mg carbonate; (3) oxidation of V 4+ to V 5+ in upwelling groundwaters. Carnotite in gypcrete is economically significant but less

313

common. The richest concentrations occur where groundwaters rise towards the evaporative zone. Carlisle (1983) divided these uranium occurrences into two types: upper pedogenic calcrete, lying above the water table, and underlying non-pedogenic calcrete deposits that reside near or beneath the local water table. The secondary uranium mineralization found in the study area might be similar to the pedogenic type.

Results and conclusions Study of the secondary uranium mineralization in the Judean Desert and in the northern Negev reveals that the uranyl ion is leached out from the phosphorites of the Mishash and Hatrurim formations and migrates towards the local base levels. The uranyl ion together with ions of vanadate, phosphate and carbonate form in places various secondary uranium minerals associated with gypsum. The latter forms pedogenic layers (gypcrete) which in places are spread over some hundreds of square meters and reach thicknesses of 0.2-1.5 m. The study indicates that the region might contain widespread and thicker gypsum-rich layers, enriched in uranium secondary minerals. This might be found especially in places close to or under the local perched water table. The gypsum-rich pedogenic layer might be a lithological trap, especially in places were the local water table reaches this layer and where reducing conditions can prevail as in synclines and near to major faults. The Hatrurim areas contain numerous occurrences of uranium and therefore the gypsum-rich layers in these areas should be of particular interest.

Acknowledgement The authors are grateful to Prof. Y. Dan, H. Koyumdjisky and S. Nissim (the Vulcani Institute of Agricultural Research, Israel) and Prof. M. Magaritz (the Weizmann Institute, Israel)

for initially introducing us to pedogenic occurrences in the Judean Desert. Thanks to Dr. G. Steinitz, Dr. J. Kronfeld, Dr. Y. Nathan, Dr. A. Ayalon, Dr. Y. Bartov and Dr. H. Zafrir for their helpful comments and for reviewing the manuscript, and to Y. Assael, U. Vulkan, Dr. E. Neeman, G. Lifschitz and A. Hadar (the Exploration Department, Soreq Nuclear Research Center) for their assistance in the chemical analyses and in the field work. To M. Dvorachek for his help in carrying out the SEM analyses. Thanks to S. Levy and A. Peer for drafting the figures and to B. Katz for editing the manuscript.

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