A CONTRIBUTION TO THE GEOCHEMISTRY OF EL-SIBAIYA PHOSPHORITES , NILE VALLEY , EGYPT

El-Sibaiya phosphorites represent a part of the Middle East to North Africa phosphogenic province. They encountered in the Gabal (G.) Duwi Formation of Late Cretaceous to Paleogene age. The studied samples have been collected from the eastern and western sides of the Nile River. Major elemental analysis of the collected samples declared positive correlations between P 2 O 5 and CaO and F. These correlations indicate that these elements are contained in the apatite phase. On the other hand, P 2 O 5 shows negative correlations with other major elements such as Al 2 O 3 , SiO 2 , Fe 2 O 3 and MgO. Some trace elements such Cr, Sr and V show positive correlation with P 2 O 5 . The average of the total rare earth elements (∑REEs) in the studied phosphorites is 262ppm. Chondrite-normalized patterns of ∑REEs of the studied phosphorites show a good coherence with the world average of shale suggesting that ∑REEs in the studied phosphorites were derived from the associated clastic debris through the diagenetic remobilization. The week negative Ce and Eu anomaly of the studied samples and the marked enrichment in LREEs emphasize that the studied phosphorites subjected to a later post-depositional modification. Laser ablation analysis showed that apatite mineral of the studied phosphorites is enriched in some elements such as Ca, F, Cr, Sr, V, Zn, Ba and REEs. This explains the major role of substitution processes of these elements within apatite lattice. The uranium average in El-Sibaiya East phosphorites is 81.6 ppm, while in El-Sibaiya West is 51.1ppm. Apatite minerals play an important role in the accumulation of U within the studied phosphorites.


INTRODUCTION
Phosphorites are one of the important economic deposits in the world as they are the raw material for many important industries.They represent the host rock for some strategic elements especially REEs, uranium, thorium and fluorides.
The geochemistry of phosphorites has been widely studied due to their economic importance (Jarvis et al., 1994).The chemi-cal composition of apatite mineral is highly variable because its crystal structure allows a variety of substitutions (McClellan and Lehr, 1969).Va r iability in chemical composition of apatite may reflect difference in original composition, modification during diagenesis, or modification during weathering (Jarvis et al., 1994).Some of previous studies on the Nile Va l ley phosphorites are summarized within ISSN 2314-5609 Germann et al. (1984) stated that the Egyptian phosphorites are mainly composed of phosphatic minerals francolite and dahllite and non-phosphatic varieties represented by dolomite, calcite, quartz, kaolinite and illite, as well as the gangue minerals pyrite, gypsum and anhydrite.Jarvis et al. (1994) concluded that Rare earth elements (REEs) concentrations with their characteristic patterns and Ce and Eu anomalies in the marine phosphorites are useful indicators of their marine depositional environments.Picard et al. (2002) reported that phosphorites commonly display highly variable REEs patterns due to the variations in the composition and amount of associated detritus, depositional environment, sea-and pore-water redox, pH, age and water depth.Baioumy et al. (2007) recognized that the Egyptian phosphorite occurs mainly as carbonate fluorapatite (francolite).Baioumy (2011) stated that the negative Ce and Eu anomalies of the upper Cretaceous phosphorites in Egypt reflect their formation under reducing conditions with some evidences of post-depositional modifications of these elements.
This work aims to study the mineralogy and geochemistry of El-Sibaiya phosphorites.The importance and possible controlling factors of the elemental variability among the studied phosphorites are also discussed.To achive this objective, a combination of mineralogical and geochemical analyses has been carried out.
The Duwi Formation represents the establishment of the first fully marine condition that extended across Egypt during the major transgression in Late Cretaceous time (Glenn and Arthur, 1990).
The studied phosphorites occur along the eastern side (El-Sibaiya East) and the western side (El-Sibaiya West) of the Nile River (Fig. 1).This Formation is mainly composed of phosphorite beds intercalated with shale, marl, oyster limestone and chert bands (Fig. 2).
The studied phosphorite succession has been subdivided into three horizons: A, B and C (Fig. 2).

SAMPLES AND METHODS
Eleven bulk phosphorite samples were collected from the economic bed (horizon A) of the Duwi Formation from El-Sibaiya East and El-Sibaiya West.The main characteristic dspacing of francolite is (2.79-2.80Å, 2.69-2.70Åand 2.62-2.63Å) have been detected in the patterns of the analyzed samples (Fig. 3).Carbonate-hydroxyl apatite (dahllite) Ca 5 [(PO 4 ,CO 3 ) 3 |(OH,O)] can uncommonly be detected in some samples.
The petrographic studies declared that the studied phosphorites are composed mainly of phosphatic and non-phosphatic constituents (Fig. 4).The phosphatic constituent represents 65 to 80% by volume of the whole studied rock, they are classified into phosphatic mudclasts (collophane grains) and phosphatic bioclasts (fish bones and sharks teeth).Detrital quartz grains, carbonates as shell fragments as well as iron oxides are the main nonphosphatic constituents of the phosphorites.

Geochemistry
The studied phosphorites were examined through detailed chemical investigations to determine the major, trace and rare earth elements composition and declaring the interrelationships between them.The resultant data are listed in Tables (1-6).
The major oxides (P 2 O 5 , CaO, Fe 2 O 3 , MgO, SiO 2 , Al 2 O 3 , Na 2 O, K 2 O, TiO 2 and F) were estimated using the conventional wet chemical techniques of Shapiro and Brannock (1962), with some modifications given by El-Reedy (1984).The concentrations of major oxides were measured using UNICAM spectrophotometer, JENWAY flame photometer and SCHOTT titrimetric technique using EDTA.
The X-ray fluorescence technique was used to determine some trace elements content (Cr, Ni, Cu, Zn, Zr, Rb, Y, Ba, Sr and V) within the studied samples.This technique was carried out using PHILIPS Unique-II spectrometer equipment with the automatic sample changer PW 1510, (30 positions).It is connected to a computer system using X-40 program for spectrometry.
X-ray diffraction technique (XRD), was used to identify the unknown minerals using PHILIPS PW 3710/31 diffractometer, scintillation counter, Cu-target tube and Ni filter at 40 kV and 30 mA.These analyses were carried out through the laboratories of the Egyptian Nuclear Materials Authority (NMA).
The rare earth elements content were determined using the ICP-MS and laser ablation at the laboratories of Senckenberg Research Institute in Dresden, Germany.This technique was carried out using Excimer-Laser (UP 193, New Wave) and ICP-MS (Element 2XR, Ther-moFisher)

Mineralogy and Petrography
The mineralogy of El-Sibaiya phosphorites was mainly investigated by X-ray diffraction (XRD), the obtained data revealed that the examined samples are composed essentially of phosphate and non-phosphate minerals.
Carbonate-flourapatite (francolite) Ca 5 [(PO 4 ,CO 3 ) 3 |(F,O)] is the most abundant   The high SiO 2 ratio can be attributed to the presence of quartz grains, silica cement and/or clay in the bulk samples.

Aluminum oxide
Al 2 O 3 in the eastern side phosphorites has an average of about 3.48%.It decreases to be 1.29% in the western sidephosphorites (Tables 1&2).This is due to the associated clay minerals which identified by XRD analysis.

Magnesium oxide
MgO content in the eastern side phosphorites has an average of about 3.01%, whereas in the western side it decreases to 2.75%.Part of this ratio may occur as a main constituent in some clay minerals as montmorillonite and kaolinite.

Sodium and potassium oxides
Na 2 O content of the eastern side phosphorites has an average of about 1.25%, that decreases to 0.85% in the western side phosphorites.K 2 O content has an average of about

Phosphorus and calcium oxides
Phosphorus contained mainly in apatite.P 2 O 5 average content in the studied samples are 27.60% and 23.12% for the eastern side and western side, respectively (Tables 1&2).
Calcium occurs as phosphates, carbonates and/or sulfates.CaO content in the studied samples is nearly the same along the two sides of El-Sibaiyawith an average of about 40%.

Fluorine
Fluorine average content is 2.25 % for the eastern side phosphorites and 2.19% for the western side phosphorite (Tables 1&2).

Iron oxide
The total iron in the studied phosphoriteswas determined as Fe 2 O 3 .Fe 2 O 3 content in the eastern side phosphorites has an average of about 4.12%, while in the western sidedecreased to be 2.98% (Tables 1&2)..

Silicon oxide
SiO 2 content in the eastern side phosphoriteshas an average of about 9.98%.Whereas Table2: Major oxides concentrations of the western side phosphorite 0.28% for the eastern side phosphorites decreasing to 0.11% in the western side phosphorites (Tables 1&2).
K 2 O is essential constituent of the clay minerals where it is principally associated with aluminum in illite and/or illite-smectite mixedlayer clay minerals.

Chromium and vanadium
Chromium and vanadium have the same geochemical behavior in the studied phosphorites.McKelvey et al. (1953) suggested that Cr and V are shared between apatite and clays.
Chromium content in the studied samples is nearly the same along the two sides with an average of about 127ppm (Tables 3&4).
The phosphorites of the eastern side contain vanadium with an average of about 151 ppm, whereas in the western side vanadium has an average of about 104ppm.

Nickel
Nickel content in the studied samples is nearly the same along the two sides withan average of about 14ppm.The average of Cr/  Ni ratio in the studied phosphorites reaches nearly up to 9.14 suggesting their derivation from mafic volcanic sediments (Tables 3&4).

Copper
Copper content in the studied samplesis nearly the same along the two sides havingan average of about 13ppm (Tables 3&4).

Zinc
Zinc content in the studied phosphorites of the eastern side has an average of about 144 ppm decreased to be 134ppm in the western side (Tables 3&4).

Zirconium
•Zirconium content in the studied samplesis nearly the same along the two sides having an average of about 14ppm.

Barium
•Barium content in the studied phospho- rites is nearly the same along the two sides having an average of about 700 ppm.

Strontium
Strontium is strongly enriched in marine phosphorites, its average content in the eastern side phosphorites is 1496 ppm which decreases to 1240 ppm in the western side phosphorites (Tables 3&4).

Inter-elements relationships
The statistical treatment of analytical data of the studied phosphorites show positive correlation between P 2 O 5 and other elements that located inside the apatite lattice such as CaO, F and Sr as expected.Meanwhile P 2 O 5 shows negative correlation with some other elements that located outside the apatite lattice such as Fe 2 O 3 , SiO 2 and Al 2 O 3 .
P 2 O 5 shows positive correlation with some trace elements such as Cr, Sr and V.This positive correlation with P 2 O 5 indicate the substitution of these elements in the apatite structure.

Rare earth elements
In the present work, ten representative bulk samples (five from each side) are subjected to chemical analysisusing the ICP technique to determine the concentrations and average abundances of the REEs (Table 5).
Total rare earth elements (ΣREEs) content in the eastern sidehave an average of about 274.8 ppm decreased to be 249.7ppm in the western side.Light rare earth elements (LREEs) represent more than 85% of the total rare earth elements with an average of about 244.7 ppm and 216.4 ppm for the eastern side and western side respectively.On the other hand, heavy rare earth elements (HREEs) represent less than 15% of (ΣREEs) with an average of about 46.6 ppm and 33.2 ppm for the eastern side and western side respectively.
The enrichment of the RREs in the studied phosphorites can be affiliated to the Sea  (1993), apatite and phosphate-rich faunal remnants (like fish teeth)that constitute these phosphorites can store high quantities of REEs due its unique crystallochemical properties.Altschuler et al. (1967) attributed the variations in the rare earth elements content within the different types of phosphorites to the variation in depositional conditions.

Chondrite-normalization patterns
The REEs pattern (Fig. 14 1-Studied phosphorites may reflect post depositional modification of the REE's distribution in these deposits (Baioumy, 2011).The slight negative Ce and Eu anomalies can reveale their marine origin as well as the prevailing reducing conditions during their formation.
2-The pattern of the studied phosphorites is phoriteswith the world average shale (Piper 1974) that normalized to the chondrite REEs concentrations (Nakamura, 1974) can declare that: The pattern exhibitsthe LREE's enrichment relative to the HREE's and slight negative Ce and Eu anomalies.The LREE's enrichment in the Fig. 14: Chondrite normalized pattern of REEs from El-Sibaiya phosphorites with world average of shale (Piper,1974).
in relative harmony with that of world average of shale.This may suggest that the REE's were derived from the associated clastic debris by diagenetic remobilization as mentioned by (McArthur and Walsh 1985).

Laser ablation ICPMS
Ten composite phosphorite samples (five samples from each side) were analyzed using the ICPMS laser ablation.Ten spots were taken in every sample exactly in collophane pellets to cover their chemical composition (Fig. 15).
The data obtained from the laser ablation technique (Table 6) can be used to illustrate the role of apatite mineral in the adsorption and accumulation of most of some elements which reflect the geochemical characteristic and peculiarities of the whole rock, where they show various elemental concentrations that mainly depend on their location either inside or outside the apatite lattice.Some major elements especially Ca and F occur in high concentrations where they mostly locate within the apatite lattice and reveale positive correlation with phosphorus.On the other hand, Fe, Mg, Si, Al, Na, K and Ti are recorded in low concentrations which indicate that these elements locate outside the apatite lattice, as they have a negative correla-tion with P 2 O 5 .Some trace elements also vary in their concentrations, where Cr, Zn, Ba, Sr and V are occurred in high content, whereas Ni, Cu, Zr, Rb and Y show lower content (Table 6).
The analyzed apatite minerals revealed higher concentration of REEs relative to the non-phosphatic minerals.

RADIOACTIVITY
Marine phosphorites are considered as a good source of uranium where, they contain a value between 50-200 ppm, which represents 15 to 60 times greater than that of crustal abundances (Adams et al., 1959 andFinch et al., 1973).In the present work, uranium and thorium contents within the studied phosphorites samples along the both sides of the Nile Va l ley were radiometrically measured and listed in Table ( 7).
The uranium content in El-Sibaiya East phosphorites ranges from 55.0 to 100.0 ppm with an average value of 81.6 ppm, while thorium content in these samples attains values between 1.0 to 7.0 ppm, with an average value of 3.1 ppm (Table 7).
On the other hand, the uranium content in El-Sibaiya West phosphorites ranges from  adsorption of uranium and/or this may be post depositional leaching processes of uranium from the iron minerals.Cheney et al. (1979) recognized that the phosphatic deposits have a higher CaO/P 2 O 5 ratio than that in the pure fluorapatite, where as the CaO in the francolite-type apatite remains nearly constant, whereas the P 2 O 5 varies with the substitution of carbonate and fluoride from 1.32 in pure fluorapatite to 1.62 in highly substituted francolite.

DISCUSSION
CaO/P 2 O 5 ratio for the studied phosphorites 1.59; this relatively high ratio can refer to the presence of free carbonates and possibly to some non-phosphatic minerals such as gypsum or any clay fraction.Some of the studied phosphorites exhibit F/P 2 O 5 ratio reaches about 0.09.This ratio is close to the ideal one (0.089) recorded by Rooney and Kerr (1967) for pure fluorapatite.The apparent excess of F/P 2 O 5 in some samples can be directly governed by the structural CO 2 content, owing to the replacement of PO 4 by CO 3 + F (McArthur, 1985), or both 30.0 to 73.0 ppm with an average value of 51.1 ppm, while the amount of thorium varies between 1.0 to 5.0 ppm, with an average value of 2.6ppm (Table 7).

Uranium / Element Relationships
In the present work, uranium content in the studied area shows positive variations with P 2 O 5 , CaO and Sr with the correlation coefficient (r) 0.71, 0.70 and 0.51, respectively for El-SibaiyaEast and (r) 0.63, 0.52 and 0.51 respectively for El-Sibaiya West.On the other hand, it shows negative relationships with Fe 2 O 3 , SiO 2 and Ni with correlation coefficient (r) -0.56, -0.84 and -0.50 respectively for El-Sibaiya East and (r) -0.74 and -0.54 respectively for .
The positive correlation between U and P 2 O 5 , CaO and Sr reflects the role of apatite in the accumulation of U within the studied phosphorites.Also, this confirmed by the obtained results of laser ablation technique.These results are agreed with that of McKelvey and Carswell (1956).On the contrary, the negative correlation between U and Fe 2 O 3 reflects that iron do not represent any role in    , 1969).
Minor fluorine may be associated with the organic matter and other constituents (Price and Calvert, 1978).
Iron in these deposits as elicited in the petrography present in the form of hematite, goethite and/or pyrite.Cook (1972) reported that iron can introduce into the apatite lattice by weathering.McConnell (1973) believes that Si +4 may occur in the apatite structure as a substitution for phosphorus.Sharafeldine (1999) reported that the SiO 2 content in the bulk Egyptian phosphorite rocks reaches up to 15.40%, which is higher than those in the phosphatic pellets (2.17%).
Aluminum is reported by Cook (1972) as a major element tied primarily to the detrital contaminates in phosphorites located outside the apatite lattice.
Sodium content in the Egyptian phosphorites is mainly related to the presence of the halite mineral (Lehr et al. 1967).Soliman et al. (2006) concluded that the elements V, Cr, Ni, Zn, Mo and Cu in phosphaticrich shale are more enriched in clay fractions, organic matter and sulfides than in phosphatic components.

54
AHMED M. BISHADY et al.Garver and Royce (1993) suggested that where Cr and Ni concentrations are anomalously high, Cr/Ni ratio from 1.2 to 1.6 should be expected if these elements were derived from ultramafic rocks.Higher ratios are probably indicative of derivation from mafic volcanic rocks.
Copper is mainly associated with organic matter, where it has a high affinity to humic materials (Mohamed, 1979).Mudroch (1983) reported that Cu and some other trace elements are commonly associated with the clay and silt size sediments as well as sulfides.
Zinc content can be compared with the world average (195 ppm) mentioned by Altschuler (1980) who concluded that the high content of Zn is attributed to the sulphate phase admixed in phosphorites.Tooms et al. (1969) stated that Zr concentration in phosphorites may be governed by the ability of apatite lattice to assimilate it.Pacey (1984) attributed the impoverishment of Zr in some phosphorites to its occurrence in extraneous detrital phases substituting for Ti in titaniferous minerals such as ilmenite.Arrhenius (1963) mentioned that Ba occurs in the lattice of pelagic fish bone apatite.El-Kammar (1970) reported a positive correlation between Ba and Al in the Nile Va l ley phosphoritesand attributed this to its partial association with the clay fraction.Strotium content is extremely higher than the world average (70 ppm) in phosphorites mentioned by Altschuler (1980), who also stated that Sr, Ba and Zn reveal a marked affinity towards fixation in the apatite crystal lattice.Dabous (1981) attributed the strong positive correlation between P 2 O 5 and CaO in the Nile Valley phosphorites to the presence of calcitic materials as cementing material.
The substitution of some elements in the apatite structure is reported by several authors especially McConnell (1952), Gulbrand-sen (1966), Cook (1972) and Prevot and Lucas (1980).These authors recognized that V substitutes easily P as vanadate and occasionally reaching commercial concentration.Nickel behaves like V in the studied deposits, but its incorporation into apatite seems unlikely, which may illustrate its lower content in the present phosphorites.The analyzed samples show high values of Ba which positively correlates with CaO and P 2 O 5 .In spite of the larger ionic radius of Ba 2+ relative Ca 2+ , it may be accommodated within the carbonate fluorapatite lattice.
Also, the organic materials play a major role in the adsorption and accumulation of most of these trace elements within the studied phosphorites.According to Gulbrandsen (1966) and Mohamed (1979) Ni, Cr, V and Zn are thought to be associated with organic matter.The enrichment of some trace elements in phosphorites may be due to the reduction conditions induced by the organic matter (Toom et al., 1969).
Rare earth elements (REEs) concentrations with their characteristic patterns and Ce and Eu anomalies in marine phosphorites are useful indicators of marine depositional environments (Jarvis et al., 1994).Phosphorites commonly display highly variable REEs patterns due to the variations in the composition and amount of associated detritus, depositional environment, sea-and pore-water redox, pH, age and water depth (Picard et al., 2002).El-Kammar et al. (1979) cited that, REEs content of the Nile Va l ley phosphorites reflects the terrigeneous supply of the depositional sea rather than its depth.Soliman (2003) reported that the concentration of REEs in phosphorites are related to some types of their components (fish debris).From the radioactive view point, the stud-

Fig. 1 :
Fig. 1: Key map showing the location of the studied area &ŝŐ͘ϯ

Fig. 17 :
Fig.17: C o rrelation between uranium and Fe 2 O 3 (%) in the studied area Germann et al. (1987) confirmed the relative REEs enrichment of Abu Tartur phosphorites in the Western Desert compared to the Nile Valley and Red Sea regions.

Table 3 :
Trace elements concentrations of the eastern side phosphorite W = Western side

Table 4 :
Trace elements concentrations of the western side phosphorite

Table 6 :
Average complete chemical analyses using laser ablation ICPMS