Abstract

This paper focuses on determining the favorable geological and hydrogeological conditions for forming uranium occurrences in the Froqlos–Jabal Abou Rabah region, Palmyrides, Syria. Eighty rock samples from this area were analyzed by spectrometric gamma-ray technique to determine the radioactive concentrations of eU and eTh, and of %K. Uranium concentrations in shallow and deep groundwaters were also determined. High uranium concentrations were registered owing to the presence of phosphatic outcrops in the study region. The uranium migration trends and its remobilization were analyzed through the analysis of the behavior of eU, eTh, and their ratio. A positive relationship between eU and eU/eTh, and a negative relationship between eTh and eU/eTh in geological formations of different ages supports the secondary remobilization of uranium. The role of groundwater movement in transportation and redeposition of uranium mineralization is discussed: deep waters are less oxidizing than near-surface waters, and do not appear to have significantly remobilized uranium. This indicates that the source of secondary uranium mineralization is surficial or near surficial, and is mainly related to the leaching of outcropping phosphatic layers.

Integration of the results with established radioactive and geological sections reveals four radioactive anomalous zones: north Froqlos, Kherbet Hannora, northwestern flanks of Jabal Abou Rabah (Kherbet Al-Hajar), and the northwestern limit of the Ghuntor depression. The high uranium concentration in north Froqlos is because of its original presence in the phosphate beds and extensive fracturing, which allows groundwaters to penetrate and remobilize the uranium. High uranium concentrations in the other three locations are caused by the presence of evaporites and capillary action, which draws solutions upwards and causes redeposition as surface crusts (gypcrete).

Sommaire

Le présent article traite de l’évaluation de conditions géologiques et hydrologiques propices à la formation de minéralisations uranifères dans la région de Froqlos–Jabal Abou Rabah, dans les Palmyrides, en Syrie. Quatre vingt échantillons de roche provenant de cette région ont été analysés par la méthode de la spectrométrie aux rayons gamma afin de mesurer les valeurs en équivalents d’éléments radioactifs eU, eTh et %K. Les concentrations en uranium dans des eaux souterraines superficielles et profondes ont également été mesurées. Des concentrations fortes en uranium ont été notées dans la région à l’étude; ces dernières sont expliquées par la présence d’affleurements de roches phosphatées. Le comportement migratoire de l’uranium et sa remobilisation ont été étudiés par l’analyse de la variation d’eU, eTh et leur ratio. Une relation positive entre eU et eU/eTh et une relation négative entre eThe et eU/eTh dans les formations géologiques d’âges variés favorise la remobilisation de l’uranium. Le rôle de la circulation de l’eau souterraine dans le transport et la re-déposition de l’uranium est discuté : les eaux profondes sont moins oxydantes que les eaux plus superficielles et ne semblent pas avoir remobilisé significativement l’uranium. Ceci indique que la source de la minéralisation en uranium secondaire est à la surface ou à faible profondeur, et qu’elle est principalement associée au lessivage des affleurements de roches phosphatées.

L’intégration de nos résultats avec ceux en provenance de sections géologiques et radiométriques bien connues fait ressortir quatres zones radioactives anomales: Froqlos nord, Kherbet, Hannora, les flancs nord-ouest du Jabal Abou Rabah (Kherbet Al-Hajar), et la bordure nord-ouest de la dépression de Ghuntor. Les fortes concentrations en uranium à Froqlos nord résultent de sa présence initiale dans les horizons phosphatés, et de la fracturation considérable qui permet à la pénétration des eaux souterraines de remobiliser l’uranium. Les fortes concentrations en uranium des trois autres localités résultent de la présence d’évaporites et à l’action capillaire, qui permet le mouvement des solutions vers la surface et cause leur re-déposition sous forme de croûtes superficielles (gypcrete).

Introduction

An airborne gamma-ray survey was carried out over the Syrian Arab Republic during 1987 as part of a UNDP/IAEA project. About 11 000 line-km of data were recorded along parallel lines over three adjoining rectangular areas (Fig. 1; Riso, 1987; Jubeli, 1990):

  1. Syrian Desert (7189 km line, at 4-km line spacing).

  2. Ar-Rassafeh Badyieh (2240 km line, at 4-km line spacing).

  3. Northern Palmyrides (1600 km line, at 3-km line spacing).

The main purpose of this survey was to assist uranium exploration. The airborne survey did not show strong indications of economically viable uranium deposits, but the highest values of gamma-ray activity, found in central southern Palmyrides, correspond to the locations of phosphatic outcrops. These phosphate deposits have been mined for many years at Khneifis and Al-Sharquieh, located 65 km and 45 km southwest of Palmyra, respectively (Fig. 1).

The study area is important because of its phosphatic outcrops of Cretaceous age, the main source of uranium, and the presence of favorable geological structures for forming uranium traps. This area was not covered by the earlier airborne survey, so a gamma-ray spectrometer technique was used on rock samples from outcrops of different geological units to determine their uranium potential.

The main objectives of this paper are therefore:

  1. To determine the uranium concentrations and other radioactive elements in the geological outcrops in the Froqlos–Jabal Abou Rabah region.

  2. To identify the role of shallow and deep groundwater in the leaching and transporting of uranium.

  3. To apply uranium migration concepts to the analyzed rock samples.

  4. To follow and localize radioactive anomalies in the study region.

  5. To use two known geochemical parameters, the uranium favorability index (UI) and the alteration F-parameter, to estimate their applicability in establishing the degree of uranium remobilization.

  6. To analyze the trends and the behavior of eU, eTh, and their ratios as an indicator of uranium remobilization, to evaluate the redistribution of radioactive elements in the study region.

  7. To define and locate favorable geological structures for further uranium exploration.

Regional Setting

Geology

The study area is located in the central part of Syria, and elevations range from 550 to 1072 m above sea level. The northern part of the area is characterized by rugged mountainous relief, which gradually transforms southward into hummocky uplands and flattened areas.

The Palmyrides are located in central Syria and are subdivided into northern and southern ridges separated by an intermontane basin (Ad-Daww) filled with Neogene–Quaternary deposits. The sedimentary formations exposed in the Palmyrids range in age from Late Triassic to Neogene (Fig. 1).

Cretaceous to recent sedimentary rocks dominate the study area, including limestone, dolomitic limestone, organic limestone, marl, clay, chalky limestone, flint, and gypsum. These facies grade into sandy limestone till (Neogene continental lithofacies) and alluvium, sandstone, and loam; Quaternary to recent sandy loam conglomerate fills wadis and lowlands (Fig. 2). The most distinct and important phenomenon in the study area is the appearance of phosphate beds during the upper Late Cretaceous and Early Paleogene. These phosphate beds, comprising phosphate rocks, phosphate sands, marly phosphates, and glauconite, are the main source of uranium in the study area (Technoexport, 1967).

Drainage patterns represented by dendritic dry wadis are controlled by fold systems and other topographic features that dominate in the study region, and are mainly oriented N–S in the northern part of the region, and SW–NE in the southern part; the pattern has a radial shape in the central part (Jabal Abou Rabah) as shown in Figure 2.

During the rainy season, numerous torrents derived from the north carry clastic materials along ravines and wadis such as W. Murran El-Fawa’reh, W. El Maram, W. Drebil, W. El-Mkemen, and W. Tafha, dropping the detritus in flood plains and lowlands.

Structure

The study area consists of two subbasins, the Froqlos–Al-Bawleyeh trough depression and the Ghuntor depression, both oriented NE–SW. Two sets of NE–SW- and roughly E–W-trending faults are observed in the study area, related to large folds with NE–SW axes (e.g., the Ash-Shomaryeh broad anticline and the Jabal Abou Rabah dome-shaped anticline). These structures give the study area the form of belts of high ridges separated by intermontane basins filled with thick erosional deposits (Technoexport, 1967).

Phosphate Deposits

The geologic column in Figure 3 shows several phosphorite beds ranging in age from Late Cretaceous (Campanian) to Paleogene (Late Eocene; Atfeh, 1967; Jubeli, 1986; Abbas, 1987; Jubeli and Abbas, 1996).

Cretaceous formations mostly contain solid phosphate rocks, whereas the Paleogene units are generally characterized by concretional phosphate associated with glauconite beds (Technoexport, 1967). The Cretaceous phosphate deposits consist of phosphatic microconcretions, oolites, grains, and organic remains, which account for 65% to 80% of the rock (Jubeli, 1998). Depending on the cement composition, the phosphate rocks might be considered siliceous, calcareous, or calcareous-siliceous. In some cases they have phosphate-calcite cement, and, rarely, dolomitic cement. The richest beds (typically 25%–29% P2O5, but locally up to 38.6%) occur in the Upper Cretaceous (Campanian), and have calcareous cement (Technoexport, 1967; Jubeli, 1998).

Apatite is the main phosphate mineral, most commonly present as the carbonate fluorapatite variety francolite; other apatite minerals are found in very rare cases.

Uranium mostly occurs as yellowish secondary minerals such as vanadium uranite (carnotite etc.) within open pores and fractures, and is commonly associated with intense alteration of phosphatic rocks. Some of the uranium mineralization might have formed at the same time as phosphate deposition, or might have developed later as a result of secondary processes caused by surface and subsurface runoff (Jubeli, 1998).

Methodology

Gamma-Ray Spectrometric Technique

As described by Asfahani (2002), the gamma-ray spectrometer consisted of a 12.5% relative efficiency Ge detector, with an energy resolution (full width at half maximum) of 0.998 keV at 122 keV, and 1.88 keV at 1332 keV, coupled to an S-100 spectroscopy system with an 8192 ADC card. The gamma-ray measurements were carried out in a 10 cm-thick lead shield, internally covered with a 1 mm-thick copper sheet. Standard reference materials RGTh and RGU were counted under the same conditions as ore samples to calibrate the spectrometer for energy and efficiency (AQCS, 1995). The spectra were analyzed using the GANAAS software (Gamma and Neutron Activation Analysis Spectrum; Nuclear Analysis Software, 1991).

Gamma-ray spectrometry is an indirect technique which gives the U analysis as eU, equivalent uranium, and is equal to the true uranium concentration only if there is radioactive equilibrium in the rock being analyzed (Asfahani and Abdul-hadi, 2001; Asfahani, 2002).

UA-3 Technique

The main application of the UA-3, Scintrex Uranium analysis is the direct analysis of uranium in natural waters. Immediate and accurate measurements with sensitivity better than 0.05 ppb U can be made. The UA-3 is based on a recently developed electro-optical technique for which patents are pending. The method uses the characteristic fluorescence properties of the uranium ion in solution under certain conditions when irradiated with a very short pulse of ultraviolet light from a nitrogen laser.

Natural water samples were directly analyzed; careful field collection obviated the need for filtering. About 5 ml of the sample and approximately 0.5 ml of the reagent were poured into a quartz sample cell, which was then slid into the optical path of the UA-3 system. After about 10 seconds, the output of the analogue panel meter was read and recorded. Calibration of the instrument is carried out in the same way using standards of known concentration.

Uranium Migration

As described by Abu-Deif et al. (2001), uranium and thorium usually behave isochemically during magmatic processes because of the similarity in their ionic radii. However, during weathering and other crustal processes, U4+ is easily oxidized to U6+, which is soluble in groundwaters, whereas Th remains insoluble in the oxidization zone. Therefore, eTh can be used as a proxy for the original concentration of U in the rock (Aswathanarayana, 1985).

The ratio of eU/eTh is a good index for uranium migration, because it should remain approximately constant in rocks of the same type if migration has not occurred. The half-lives of uranium and thorium are long enough for the measured eU/eTh ratio to be considered to represent the original ratio in the rock (NMA, 1999).

According to NMA (1999), the value of uranium migration (Um) can be computed for a given rock unit by subtracting the original uranium content (Uo) from the present uranium content (Up), such as Um = Up − Uo.

The original uranium concentration (Uo) can be theoretically computed using the equation:

 

Uo=eTh×(regionaleU/eTh)
(1)

where eTh is the average thorium content (in ppm) in a certain rock unit, and the regional eU/eTh is the average regional eU/eTh ratio in different rock units (Abu-Deif et al., 2001; Asfahani et al., 2007).

Knowledge of both Uo and Up allows computation of the extent of uranium migration (Ume%) from the following equation:

 

Ume%=(Um/Up)×100
(2)

The Ume% is a good indicator for determining the migration degree and its direction. If Ume% is positive, then migration is into the unit, whereas if it is negative, migration is out of the unit.

Geochemical Indicators of Uranium Remobilization

The degree of uranium remobilization is reflected in changes of eU/eTh and eU (Fuad et al., 1998). Large positive changes in eU/eTh and eU suggest that U has migrated into the rocks, whereas small or moderate increases of eU/eTh and eU mean that uranium might not have been significantly remobilized, either because of a lack of solutions needed for uranium dissolution, or because the uranium is hosted in resistant lithological units (Asfahani et al., 2008a).

Uranium remobilization can also be indicated by the presence of an inverse relationship between eU/eTh and eTh.

Uranium-enriched zones have been identified through the use of the uranium favorability index (UI) developed by Saunders and Potts (1978):

 

UI=eUeUeTh×eUK%
(3)

The separation of U from Th and K decreases eU/eTh and eU/K% from their mean values (Asfahani et al., 2008a). The degree of rock alteration is provided by using the F-parameter indicator proposed by Efimov (1978). The F-parameter is expressed by the following equation:

 

F=K%eTheU=eUeThK%=K%×eUeTh
(4)

Two specific characteristics about the rock environments can be clarified by using this parameter: the uranium abundance relative to eTh/K%, and the potassium abundance relative to the eTh/eU. Use of the F-parameter is therefore efficient in identifying strong K-alteration zones associated with uranium mineralization.

Statistical Analysis of the Data

The 80 rock samples taken from the study area were subjected to qualitative and quantitative statistical analysis in order to explore the nature and significance of the distribution of radioactive elements in the study region.

Single variate statistics has been applied in order to describe the statistical characteristics of the distribution of each radioactive variable in the outcrops of different ages. These include Min, Max, Mean (), and standard deviation (σ).

The coefficient of variability ( CV%=σX¯×100) is also examined.

Results and Discussion

The main goal of this paper is to determine the uranium potential in the study region. Eighty rock samples and forty-nine water samples (locations are shown in Figure 2) have been analyzed by gamma-ray spectrometry and UA-3 techniques, respectively. Statistical analysis has been carried out as a function of the geological age, as indicated in Table 1.

The eU in outcrops of phosphatic Cretaceous rocks varies between a minimum of 1.66 ppm and a maximum of 132 ppm, with an average of 22.2 ppm and a standard deviation of 33.7 ppm. Four anomalously radioactive zones characterized by high eU were identified: north Froqlos, Kherbet Hannora, the northwestern flanks of Jabal Abou Rabah (Kherbet Al-Hajar), and the northwestern limit of the Ghuntor depression (Figure 4b). The high uranium concentration in north Froqlos is because of the fracturing, which has allowed groundwaters to enter and remobilize the uranium from the phosphate-glauconite-bearing bands and lenses. The high uranium concentrations in the last three locations are caused by the presence of evaporites and capillary action, which draws solutions upwards and causes redeposition as surface crusts (gypcrete). Figure 5 shows the uranium concentration in water samples as a function of depth, where two groups are clearly distinguished.

The first group represents shallow-water samples from wells not exceeding 80 m depth that mostly have uranium concentrations less than 10 ppb. However, some samples have concentrations between 20 and 46 ppb, and these represent wells that have tapped the contact between Paleogene and Neogene rocks. One of these wells from the southern limb of Jabal Ash-Shomaryeh intersected Cretaceous phosphatic beds, which are considered to be the main source of uranium in the study area. These ground waters are responsible for the dissolution of U, and surficial weathering of outcropping phosphatic layers.

The second group represents deep-water samples taken from wells of depths exceeding 150 m. The uranium concentration in these samples does not exceed 10 ppb. This deeper water does not appear to be mobilizing the uranium, perhaps because it is less oxidizing than the near-surface waters. This clearly indicates that the source of uranium is surficial or near surficial, and is mainly related to the leaching of phosphatic outcrops (Asfahani et al., 2008b).

The uranium migration concept described above has been applied in the study area on geological formations of different ages, and the results are shown in Table 2. The uranium and thorium concentrations and their ratios are also used to establish the degree of uranium remobilization.

The original uranium content Uo is computed for the study region according to Equation 1, and is shown in Figure 4a. The present uranium concentration Up is shown in Figure 4b, and a uranium migration rate map has been constructed according to Equation 2 (Fig. 4c). This map shows that the Cretaceous phosphatic outcrops are characterized by an outward migration of uranium (Ume% = −56% in Table 2).

Correlation plots are shown in Figure 6. The relationship between eU and eU/eTh for the Cretaceous phosphatic outcrops is positive (R2 = 0.94), and that between eTh and eU/eTh is inverse (R2 = −0.25), which supports the idea of remobilization of U from its phosphatic source rocks (Fig. 6a). The other Paleogene, Neogene, and Quaternary outcrops are characterized by inward uranium migration, where the values of Ume% are 27%, 68%, and 23%, respectively (Fig. 6b–d). The positive relationships between eU and eU/eTh and the negative relationships between eTh and eU/eTh are also verified, which attests to the remobilization of uranium through such outcrops.

Table 2 also shows the results of UI and F-alteration parameters (Equation 4) for the different geological formations, and a map of the F-alteration parameter is shown in Figure 7. It is interesting to note that the highest values of this parameter correlate with the locations of phosphatic outcrops and the contact between Paleogene and Neogene rocks. A plot of UI as a function of the F-alteration parameter for the different geological units is shown in Figure 8. An inverse relationship is observed between these two parameters, where the phosphatic Cretaceous outcrops are characterized by a high value of the F-alteration parameter (1.17), and a low value of UI (0.022). Such phosphatic outcrops contain a significant amount of uranium and are continuously exposed to leaching and wind erosion processes. This new procedure of parameter plot recently proposed by (Asfahani et al., 2008a) allows a reliable geological interpretation to be obtained.

The phosphatic outcrops are also characterized by high coefficients of variability for U concentrations (CV = 151%), as indicated in Table 1. This CV parameter provides a measure of the homogeneity of the uranium distribution; the higher the coefficient of variability, the lower the homogeneity (Asfahani, 2002). The low homogeneity in the phosphate outcrops likely reflects dissolution and redistribution of uranium.

A geological field check has been carried out in the study area, where more than thirty geological cross sections have been constructed to concentrate mainly on the zones characterized as being important for uranium exploration. The detailed cross sections and available geological maps enable a fence diagram of three geological profiles (I, II, and III) to be established to show the extent of the phosphatic bodies in the study area (Fig. 9). The fence diagram shows the locations of phosphate- and glauconite-bearing strata, and suitable structures for uranium accumulation.

Integration of these results allows the construction of a 3-D model to describe the possible processes that gave rise to the distribution of uranium in the study region (Fig. 10). This model shows the isolines of uranium migration rate, the favorable lithologies, and also structures for uranium accumulation (A, B, C, and D; Figure 10); such structures merit further detailed uranium exploration in order to evaluate their characteristics, particularly at depth.

According to this model, the study area can be divided to the following sectors:

  1. The donor front, which mainly includes the northern part of the study area, where hard and fractured rocks form the core of the Ash-Shomaryeh anticline. The outcropping phosphatic rocks appear on the southern limbs of this anticline, and are the source of uranium. These highlands also supply much of the recent detrital material.

  2. The recipient front, where the main favorable structures for the accumulation of uranium occur. Such structures with specific lithofacies hinder uranium mobility when appropriate conditions exist.

  3. An intermediate area, which possesses all the required lithological and structural properties for the passage of uranium solutions in lateral and vertical directions, and where uranium mineralization is either accumulated inside such structures or is dispersed away.

  4. Tracts where secondary uranium mineralization has been dispersed by both mechanical (wind) and chemical (rain water) effects.

Conclusions

Favorable geological and hydrogeological conditions for forming uranium occurrences in the Froqlos–Jabal Abou Rabah region, Palmyrides, Syria, have been determined. Suitable lithologies and structures for uranium accumulation have been identified by applying different approaches. Uranium remobilization and migration trends have been determined by analysis of the behavior of eU, eTh, and their ratio. Positive relationships between eU and eU/eTh, and negative relationships between eTh and eU/eTh indicate the remobilization of the uranium, and have been verified for geological formations of different ages. Uranium appears to be remobilized mainly by near-surface groundwaters, perhaps because deeper groundwaters are less oxidizing. This clearly indicates that the source of uranium is surficial or near surficial, and is mainly related to the leaching of phosphatic outcrops.

These results have been integrated to form a 3-D model to explain the movement of uranium from its phosphatic sources to its deposition in favorable lithological and structural settings (Fig. 10). Such structures merit further detailed uranium exploration in order to evaluate their characteristics, particularly at depth. The proposed model might be applicable in other similar areas considered to be promising for uranium prospecting.

Acknowledgements

The authors would like to thank I. Othman, General Director of Syrian Atomic Energy Commission, for his permission to publish this paper. The Editor is deeply thanked for important and critical remarks and for his intensive editing. Thanks are also due to the two reviewers Eric Potter and Dave Quirt and to the Associate Editor Eric Grunsky for their reviews of the manuscript. Their suggestions and remarks have considerably improved the paper.