Journal of Geochemical Exploration 143 (2014) 62–73

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Journal of Geochemical Exploration

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Uranium-bearing phosphatized limestones of NW Greece

I.Tr. Tzifas a,b,⁎, A. Godelitsas a, A. Magganas a, E. Androulakaki c,d, G. Eleftheriou c,d,T.J. Mertzimekis a, M. Perraki e

a University of Athens, School of Science, Panepistimioupoli Zographou, 15784 Athens, Greeceb Institute of Geology and Mineral Exploration, Olympic Village, 13677 Acharnae, Greecec National Technical University of Athens, Department of Physics, 15780 Zographou, Greeced Hellenic Centre for Marine Research, Institute of Oceanography, 19013 Anavyssos, Greecee National Technical University of Athens, School of Mining and Metallurgical Engineering, 15780 Zografou, Greece

⁎ Corresponding author. Tel.: +30 2107274689.E-mail address: jtzifas@geol.uoa.gr (I.T. Tzifas).

http://dx.doi.org/10.1016/j.gexplo.2014.03.0090375-6742/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 July 2013Accepted 11 March 2014Available online 22 March 2014

Keywords:UraniumPhosphatized limestonesNW GreeceRadionuclidesGamma-ray spectrometry

Sedimentary Mesozoic rocks from NW Greece (Epirus region), and particularly laminated phosphatized lime-stones, bedded chert-rich limestones and brecciated phosphatized limestones, were examined for their actinidecontent. Gamma-ray measurements using a HPGe detector showed that the above geological materials exhibithigh radioactivity, mainly attributed to the 238U-series. The 238U content (up to 7700 Bq/kg) was determinedby the 1001 keV photopeak of 234mPa, the 238U daughter. Bulk geochemical analyses using ICP-OES/MS showedvariable U concentrations with a notable value of 648 ppm in the case of dark organic-rich material hostedinto the brecciated phosphatized limestones. Relatively high concentrations of Cd, probably related to apatite,were also revealed. On the other hand, the rock is geochemically depleted in LILE (e.g. Cs, Rb, K), as well as inAs, Sb and Se in contrast to “average phosphorite”. Powder-XRD combined with optical microscopy, SEM–EDSand FTIR confirmed abundant apatite, besides calcite, as well as organic compounds (organic matter/O.M.)which should be associated to the high U content. According to Th/Sc vs. Zr/Sc discrimination diagrams theorganic-rich part of the U-bearing phosphatised limestones exhibits a mafic trend, in contrast to the rest of thestudied rocks lying close to typical pelagic sediments. However, Eu/Eu* vs. Ce/Ce* diagrams, in combinationwith SEM–EDS, indicated that the organic-rich part is a typical sedimentary material whereas the organic-poor(and also U-poor) part of the rock is secondary calcite related to surface waters. As far as we know, the studiedrocks fromNWGreece are classifiedas among the richest U-bearing phosphatized limestones and/or sedimentaryphosphorites in the world.

© 2014 Elsevier B.V. All rights reserved.

Introduction

The Epirus region (NW Greece) is generally composed of Mesozoic(250–65 Ma old) sedimentary rocks of the Ionian geotectonic zone ofExternal Hellenides (Fig. 1), derived from the Tethys Paleo-Ocean. Asthese rocks are mostly limestones and shales, which are fundamentallypoor in U (2.2 and 3.7 ppm respectively, Krauskopf and Bird, 1994;Mason and Moore, 1982), no elevated U concentrations would beexpected in the Epirus area. However, it is known, from unpublishedinternal reports by the Greek Atomic Energy Commission (GAEC) andInstitute of Geology and Mineral Exploration (IGME), that in some areasthe natural radioactivity is high due to the presence of phosphate-richsedimentary rocks, i.e. phosphorites (Koukouzas et al., 1978). In

addition, Skounakis (1979), in an unpublished work, reported U-bearing phosphorites and phosphatized limestones.

Phosphorites are marine sediments of biogenic origin, containing 15–20 wt.% P2O5 (Boggs, 2009) and also an average of 120 ppm U (Li andSchoonmaker, 2003). When phosphorites are present in sedimentaryformations they may significantly contribute in radiometric irregularitiesand actinide (likely U) geochemical anomalies. Additionally, they are richin light rare-earth elements/LREE, but not in Th (6.5 ppm) and otherheavy rare-earth elements/HRRE. Moreover, phosphatized limestonescontain less P2O5 than phosphorites, but higher than limestonespossessing an average of about 0.03–0.7 wt.% (Boggs, 2009) and less U.In general, the concentration of U in phosphorites and P-rich sedimentaryformations of various geological ages worldwide, is referred in therange of ca. 3–600 ppm (e.g. Baturin and Kochenov 2001, and referencestherein; Soudry et al., 2002; Bech et al. 2010).

In thisworkwe present, for thefirst time,mineralogical and spectro-scopic data about Mesozoic sedimentary rocks from Epirus, derivedfrom Tethys Paleo-Ocean, hosting unique U-bearing (up to 648 ppm)

Fig. 1. Geological map and stratigraphic column of the U-bearing sedimentary rocks (modified after Marnelis et al., 2007).

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organic-rich phosphorite into phosphatized limestones. In addition,accurate measurements of actinides, and namely U, in Epirus rocksshowing elevated radioactivity, using high-resolution analytical andspectroscopic techniques, are given.

Materials and methods

Samplingwas based on previous unpublished internal reports by theGreek Atomic Energy Commission (GAEC) and Institute of Geology andMineral Exploration (IGME) concerning radiometric irregularities in theMesozoic sedimentary rocks of western and north-western Greece andparticularly in the mountainous Epirus region (Koukouzas et al., 1978).However, the final samples (Fig. 1) were selected by surveying thesuspected sedimentary formations bymeans of a portable radiation de-tector (NaI Canberra-MCB2). Thus, the radioactive specimens con-cerned laminated phosphatized limestones (sample DRYM1) andbrecciated phosphatized limestones partially rich in organic matter(sample PER2A and PER2B). Besides, a non-radioactive sedimentaryrock typical for the area, namely a bedded chert-rich limestone(sample PER1), was also used for comparison reasons (“back-ground” radioactivity towards U – and actinide – content). Thesample DRYM1 (Fig. 2a) was collected in the Drymonas regionfrom Cretaceous horizons which consist mainly of carbonate rocks(limestones rich in phosphorus) alternating with cherts (Vigla Forma-tion). This region geotectonically belongs to the eastern limit of the mid-dle Ionian Zone of External Hellenides (Fig. 1). The samples PER1 andPER2 (Figs. 1 and 2b & c) were collected fromMt. Mitsikeli near Ioanninalake and city, and particularly from the villages of Perivleptos andStoupena. The highly-radioactive rock (PER2) is located approximately10 km N–NW of Ioannina within a brecciated phosphatized limestoneof the uppermost layers of Jurassic Sinion–Pantokrator formation.

Gamma-ray spectrometry was applied for the determination ofnatural and artificial radionuclides activity concentrations in the studiedsamples. The measurements were performed by means of a HPGen-type detector (GC5021 Canberra)with 50%nominal relative efficiencyand 2.3 keV energy resolution at 1.33 MeV, along with a computerizedMCA system (8715 Model and Gamma Vision® software by Canberra)for the data acquisition. A cylindrical lead shield surrounded the

detector in order to reduce the ambient gamma-ray background. Theenergy-dependent detection efficiency was determined using a 152Eureference source. Four samples where properly prepared and measuredfor 24 h each, whereas a phantom sample was also used in order to ex-tract the background contribution from the experimental spectra. Themeasured data were analyzed using the SPECTRW spectrometry soft-ware package (Kalfas and Tsoulou, 2003) and the quantitative analyseswere subsequently performed as described elsewhere (Tsabaris et al.,2007). The 238U activity was measured from spectrum photopeakscorresponding to radionuclides 234mPa at 1001 keV, 234Th at 63.3 keVand 214Bi at 934 keV (Karangelos et al., 2004). Radionuclides 137Csand 40K were measured from 661.67 keV and 1461 keV photopeaks,respectively. The activity concentration (Bq/kg) of 235Uwas determinedfrom the 143.76 keV, 163.15 keV and 205 keV photopeaks where possi-ble (sample PER2B, see Fig. 3), whereas elsewhere from the doublephotopeak at 186 keV assuming secular equilibrium between 238U and226Ra (Tsabaris et al., 2007).

The natural actinide content of the studied rocks (also derived fromgamma-ray spectrometry) was determined by ICP-MS (PerkinElmerSciex Elan 9000) after LiBO2/LiB4O7 fusion and HNO3-digestion of 0.2 gof sample. The major and trace element content of the samples was alsodetermined by ICP-OES and MS; loss on ignition (LOI), total C and Swere also measured using standard methods. The mineralogical studyandphase characterizationwasperformedbyopticalmicroscopy, powderX-ray diffraction/XRD (Siemens D5005 – now Bruker AXS – diffractome-ter), scanning electron microscopy/SEM–EDS (Jeol JSM-5600 equippedwith Oxford EDS) and Fourier-transform infrared spectroscopy/FT-IR(PerkinElmer GX-1 FT-IR).

Results and discussion

Among the sedimentary rocks studied by HR gamma-ray spectrom-etry, the lowest U activity (taken into account as natural radioactivity“background” in the area) corresponds to bedded chert-rich limestones(PER1) and the highest to the organic-rich part of the brecciated phos-phatized limestones (PER2B). The activity concentration results for thecharacteristic radionuclides of each natural radioactivity series, 40Kand the anthropogenic 137Cs are shown in Table 1. The U “background”

Fig. 2. Field images of the studied rocks. DRYM1: laminated phosphatized limestonealternating with cherts (upper); PER1: bedded chert-rich limestone exhibiting Ubackground (middle); PER2A and B: brecciated phosphatized limestone rich in U andorganic matter (lower).

Fig. 3. HR gamma-ray spectra of the studied rocks (upper) and characteristic spectrum ofthe most U-bearing sample (PER2B).

Table 1HR gamma-ray spectrometry data.

Natural decay series Activity (Bq/kg)

DRYM1 PER1 PER2A PER2B

238U 234mPa 269 ± 13 44 ± 2 77 ± 4 7700 ± 425226Ra 215 ± 11 32 ± 2 88 ± 4 6883 ± 345214Pb 208 ± 10 27 ± 1 75 ± 4 7104 ± 615214Bi 191 ± 10 28 ± 1 64 ± 3 6390 ± 320

232Th 208Tl b3(MDA)a235U 235U 10.5 ± 0.5 1.5 ± 0.1 3.6 ± 0.2 322 ± 24

227Th 3.9 ± 0.2 9.3 ± 0.5 336 ± 19223Ra 9.9 ± 0.5 3.1 ± 0.2 342 ± 22219Rn 4.3 ± 0.2 2.7 ± 0.2 393 ± 26

Natural 40K 3.9 ± 0.2 23 ± 1 48 ± 10Anthropogenic 137Cs 2.9 ± 0.2 3.9 ± 1

a Minimum detectable activity.

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sample exhibited 44 ± 2 Bq/Kg concerning 238U and significant 40K(23 ± 1 Bq/Kg). On the other hand, exceptionally high activities of238U and 235U were observed for the sample PER2B (7700 ± 425 Bq/kgfor 238U and 322 ± 24 Bq/kg for 235U). Besides, the organic-poor part ofthe same rock (PER2A) exhibited decreased values (approximately twoorders of magnitude) for the aforementioned natural series. The

laminated phosphatized limestone (sample DRYM1) also showed in-creased activity concentrations compared to the background values. Ac-tivity values were found to be 10.5 ± 0.5 Bq/kg for 235U, 269 ± 13 Bq/kg 238U and 3.9 ± 0.2 Bq/kg 40K (Table 1). It should be emphasized thatthe activities of 232Th series, for all studied rocks, were measured belowthe limit of Minimum Detectable Activity (MDA) 3 Bq/kg.

The U concentrations (in ppm), as calculated from the HR gamma-rayspectra (IAEA, 1989), are shown in Fig. 4. The values are consistent withthe ICP-MS measurements presented in Table 2, revealing 2.9 ppm of U

Table 2Major and trace elements in the studied sedimentary rocks.

Tectonized/re-processedphosphatized limestone

Laminatedphosphatizedlimestone

Beddedchert-richlimestone

Carbonate-richpart

Phosphate/organic-richpart

DRYM1 PER1 PER2A PER2B

MDL wt.% wt.% wt.% wt.%

SiO2 0.01 0.95 82.94 0.25 1.62Al2O3 0.01 0.22 0.35 b0.01 0.56Fe2O3 0.04 0.15 0.29 b0.04 0.10MgO 0.01 0.42 0.08 0.52 0.26CaO 0.01 54.05 8.81 55.62 52.79Na2O 0.01 0.55 0.07 0.02 1.25K2O 0.01 0.06 0.09 b0.01 0.18TiO2 0.01 b0.01 0.02 b0.01 0.03P2O5 0.01 13.19 0.10 0.27 27.53MnO 0.01 0.07 b0.01 b0.01 b0.01Cr2O3 0.002 0.002 0.003 b0.002 0.004LOI 30.1 7.2 43.3 15.4TOT/C 0.02 8.24 1.99 12.53 3.86TOT/S 0.02 0.34 0.03 0.03 1.13

MDL ppm ppm ppm ppm

As 0.5 2.6 1.4 b0.5 1.4Cd 0.1 0.5 0.7 0.2 0.6Co 0.2 3.9 12.9 2.0 7.7Cu 0.1 16.7 20.7 1.3 14.1Cs 0.1 0.2 0.8 b0.1 0.5Mo 0.1 1.0 1.1 b0.1 3.3Ni 0.1 7.9 5.4 b0.1 3.9Pb 0.1 1.4 1.2 0.3 1.0Sr 0.5 1187.8 39.2 173.7 1055.6U 0.1 17.6 2.9 7.2 647.4Zn 1 23 26 12 30Rb 0.1 2.1 1.7 0.6 5.0Ba 1 28 80 3 20Th 0.2 0.2 0.2 b0.2 0.7Ta 0.1 b0.1 0.4 0.1 0.1Nb 0.1 b0.1 0.9 3.9 0.8Hf 0.1 b0.1 b0.1 b0.1 b0.1Zr 0.1 4.0 4.3 b0.1 16.8Ga 0.5 0.9 1.0 b0.5 1.6Au 0.5 0.0076 0.0035 0.0005 0.002Sb 0.1 0.4 0.4 b0.1 0.5Se 0.5 b0.5 b0.5 0.700 b0.5Bi 0.1 b0.1 b0.1 b0.1 b0.1Sn 1 1 b1 b1 b1V 8 108 16 15 65Sc 1 b1 b1 b1 3Be 1 2 b1 b1 b1Hg 0.01 0.09 b0.01 0.01 0.03Ag 0.1 0.2 b0.1 b0.1 b0.1Tl 0.1 b0.1 b0.1 b0.1 b0.1La 0.1 7.9 2.5 4.7 40.2

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in the case of the “background” sample (PER1) and values in the range7.2 ppm to 648 ppm in the case of phosphatized limestones. It is worthnoting that the value of 648 ppm, recorded in the organic-rich partof the and tectonized/brecciated phosphatized limestones (PER2B),represents one of the richest U-bearing phosphatized limestones and/orsedimentary phosphorites in the world (e.g. Baturin and Kochenov2001, and references therein; Soudry et al., 2002; Bech et al. 2010). Thepositive U geochemical anomaly, in comparison to Upper ContinentalCrust (UCC, Rudnick and Gao, 2003), for all studied Tethyan Mesozoicsedimentary rocks from Epirus, as well as the exceptional U anomaly ofsample PER2B, are shown in Fig. 5. It is also obvious that the rocks are con-siderably enriched in Cd, a minor heavy metal which could be associatedto apatites. The U-enrichment of Epirus phosphatized limestones, amongphosphatized limestones and/or sedimentary phosphorites in the world,is also evident in Figs. 6 and 7 including data about the “average phospho-rite” (Li and Schoonmaker, 2003) and characteristic phosphorites andphosphatized limestones from eastern Mediterranean and the rest ofthe world (Baturin and Kochenov, 2001; Bech et al., 2010). At the sametime, it is demonstrated that all phosphorites and phosphatized lime-stones are highly enriched in Cd and Se (Bech et al., 2010). The REE + Ycontent is also high, as well as the content of Cd, due to involvement inthe crystal structure of apatite (Pasero et al., 2010).

The chemical composition of the phosphorites and phosphatizedlimestones depends on conditions during genesis, the lithology, the stra-tigraphy, and the geographical provenance (e.g. Bech et al., 2010). It is no-table that in most of the deposits, which have been studied around theworld, there is almost always a triple sequence of phosphate-containingsediments, SiO2-containing sediments (cherts), and sediments rich in or-ganic matter. It has been recently stated that phosphorite and/or phos-phatized limestone deposits are in fact marine biogenic materials, dueto bacterial activity producing bio-apatite and particularly carbonate apa-tite/francolite (Benmore et al., 1983; Diaz et al., 2008; Goldhammeret al., 2010). Uranium in the ocean, and presumably paleo-ocean, wa-ters (3.2 ppb, Li, 1991) follows anoxic pathways and it is mainly re-moved from the solution by chemical processes taking place at theinterface of organic-rich sediments, whereas reduction of U6+ to U4+

may also take place (e.g. Anderson et al., 1989; Klinkhammer andPalmer, 1991). However, the partitioning of U in different (bio)mineralsand phases constituting marine phosphorites and/or phosphatizedlimestones is not completely resolved.

The studied Epirus U-bearing phosphatized limestones are mineral-ogically rather simple, and the powder-XRD investigation showed onlycalcite (CaCO3) and apatite supergroup minerals (Ca5(PO4)3[F,Cl,OH])in the bulk. In particular, the organic-rich part (PERB), hosting thehighest U concentration (648 ppm), contains also calcite and apatiteas crystalline phases (Fig. 8). The apatite crystals are not visible, for allsamples, under the optical microscope (Fig. 9a-c), due to their “crypto-crystalline” nature (the so-called “collophane”), but some of them are

Fig. 4.Uranium concentrations in the studied rocks as resulted from gamma-ray spectros-copy and ICP-MS analyses.

Ce 0.1 5.6 2.4 3.0 38.3Pr 0.02 1.05 0.42 0.33 4.67Nd 0.3 4.6 1.7 1.3 20.8Sm 0.05 0.89 0.33 0.11 3.40Eu 0.02 0.24 0.07 0.06 0.86Gd 0.05 1.10 0.32 0.20 5.05Tb 0.01 0.17 0.06 0.03 0.68Dy 0.05 0.77 0.37 0.18 4.93Ho 0.02 0.28 0.08 0.03 1.25Er 0.03 0.49 0.20 b0.03 3.15Tm 0.01 0.11 0.02 0.02 0.39Yb 0.05 0.45 0.15 b0.05 2.45Lu 0.01 0.06 0.02 0.02 0.37Y 0.1 10.9 3.1 1.9 69.0

detectable by SEM–EDS (Fig. 9d-f). The phosphatized limestone,DRYM1 (Fig. 9a & 9d), is characterized as micritic and pelmicritic dueto its calcitic and phosphatic grains. The sample PER1 (Fig. 9b & 9e) isa typical chert, consisting mainly of microcrystalline quartz (individual

Fig. 5. UCC (Rudnick and Gao, 2003)-normalized multielement geochemical spidergrams concerning the studied rocks.

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crystals not visible in optical microscope) alternating with calcite-richlayers. The carbonate part of the rock is characterized as biomicriteenriched in radiolarian and diatoms. Moreover, it is observed that inthe examined sample the cracks are filled with diagenetic calcite. In

Fig. 6.UCC (Rudnick and Gao, 2003)-normalizedmultielement geochemical spidergrams conce

addition the radiolaria have been replaced internally by chalcedonyand microcrystalline calcite. The sample PER2 (Fig. 9c & 9f), is a brecci-ated phosphatized limestone rich in organic matter, filling all the dis-continuities and the cracks, and it is therefore optically tectonized/re-

rning the U-bearing samples compared to global phosphorite (Li and Schoonmaker, 2003).

Fig. 7. UCC (Rudnick and Gao, 2003)-normalizedmultielement geochemical spidergrams concerning the U-bearing samples of the present study compared to characteristic phosphoritesand phosphatized limestones from eastern Mediterranean and the rest of the world (Baioumy, 2011; Bech et al., 2010; Imamoglu et al., 2009; Soudry et al., 2002).

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processed. It seems to be a debris flow, matrix supported and poorlysorted monomictic breccia produced by rock fragmentation related tofaulting or other tectonic processes affected by a steep slope submarinearea in a pelagic environment. It has angular sand or gravel to boulder-sized (fine to very coarse) clasts cemented together in amatrix. Thema-trix consists primarily of the cementing material, but it also containssand and/or silt sized clasts cemented together among the coarserclasts. The cement that binds the clasts consists of black organo-phosphatematerial (organic matter+ apatite) aswell as of sparitic cal-cite. The clasts are also rich in fossils and other allochem ingredients likepeloids and extraclasts. Calcite appears as either primary (micritic) or assparite (secondary) as well as biomicrite (rich in biogenic elements). Inaddition, framboidal pyrite aggregates, in fact goethite (FeOOH) pseu-domorphs after – oxidized – pyrite (FeS2), are also visible, specificallyin sample DRYM1, indicating marine Tethyan paleo-environment poorin O2 related to anoxic conditions in the depositional environment. Theabove evidence for anoxic conditions during the formation of Epirusphosphorites and/or phosphatized limestones supports the presence ofU which precipitates in such physicochemical conditions (e.g. Andersonet al., 1989; Klinkhammer and Palmer, 1991). It should be emphasizedthat there are no distinct U minerals or phases, containing U as major el-ement, identified either by optical microscopy or SEM–EDS inmicroscale.Thus, U in the sample possessing the highest content of the actinide ele-ment (PER2B) must be contained as trace element and/or impurity inphosphate and carbonate minerals or, most likely, in organic matter(O.M.) (Fig. 10).

The possible phases hosting U in the studied phosphatized limestones(Fig. 10) may fundamentally be minerals of the apatite supergroup andorganicmatter (O.M., i.e. bitumens). Calcitemay also host traces of the ac-tinide metal. Minor metallic minerals and detrital silicate/aluminosilicatephases are much less probably related to the U presence in the rocks.

There are numerous studieswith regard to the structure of the apatitesupergroup, reporting that the lattice of these phosphateminerals favoursextended trace-element ionic substitutions (e.g. Hughes and Rakovan,2002; Luo et al., 2009; Pan and Fleet, 2002; Pasero et al., 2010; Rakovanet al., 2002). The crystal chemistry of trace metals in the apatite super-groupminerals (Ca5(PO4)3[F,Cl,OH]) is of considerable significance in ge-ology, biology, materials and environmental sciences. Apatite canaccommodate several foreign elements, including actinides, showing en-vironmental concern. Monovalent (Na+, K+), divalent (Sr2+, Pb2+, Ba2+,Mn2+, Cd2+), trivalent (REE3+), as well as tetravalent (Th4+, U4+) andhexavalent cations (U6+) have been reported to substitute into Ca sitesin the apatite structure (Luo et al., 2009; Pan and Fleet, 2002; RakovanandHughes, 2000; Rakovan et al., 2002). According to the reported struc-tural refinements in natural near-end-member fluorapatite, chlorapatite,and hydroxylapatite, there are two Ca-polyhedra in the apatite structurethat are hexagonally disposed about a central [001] hexad (Hughes andRakovan, 2002). The position Ca1 is coordinated to nine O atoms in atricapped trigonal prism. On the other hand Ca2 bonds to six O atomsand one column anion. Thus, the two Ca positions in apatite offer quitedifferent stereochemical environments and are able to accommodate avariety of cations as substituents. In an EXAFS study Rakovan et al.(2002) found U6+ substitutes into the Ca1 site in synthetic apatitegrown at 1350 °C. The substitution causes a decrease in the Ca1\O1and Ca1\O2 bond lengths and EXAFS data indicate a weaker Ca1–O3interaction. They also speculated that a likely distortion is the rotation ofO1 and O2 triads to approximate an octahedral geometry around U6+

(Rakovan et al., 2002). According to the study by Luo et al (2011), alsobased on EXAFS data, U4+may also incorporate in Ca-sites into the struc-ture of apatites. All the above indicate that apatite supergroup mineralscan fundamentally host U, and therefore U concentrated in phosphatizedlimestones and/or sedimentary phosphorites may be partially due to

Fig. 8. Powder-XRD pattern of the U-bearing phosphatized limestone.

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apatite. Furthermore, there is positive correlation between U (in ppm)and P2O5 (in wt.%) for Mesozoic phosphorites from Epirus, Greece(Koukouzas et al., 1978; Skounakis, 1979) and for relevant phosphoritesfrom Negev, Israel (Soudry et al., 2002), as shown in Fig. 11 (data fromthe present study are also included). Previous attempts to explain thepresence of U in sedimentary apatite (e.g. Baturin and Kochenov, 2001)are based on the similarity of elements' ionic radii and also on the as-sumption that U may substitute some Ca in the carbonate apatite/francolite lattice. However, a typical U concentration in pure non-sedimentary apatite (Durango-type fluorapatite) has been reported tobe 8.88 ppm (Trotter and Eggins, 2006). Thus, much higher concentra-tions, and particularly hundreds of ppm, in phosphorites and/orphosphatised limestones, can hardly be explained only on the basis of ap-atite supergroupminerals. At this point, it should also be emphasized thatall the studies about U in apatite structure concern fluorapatites andchloroapatites (Luo et al., 2009; Rakovan et al., 2002) and there are nospecific studies about U in carbonate apatite/francolite ([Ca5(PO4,CO3)3(F,OH)]) occurring, together with fluor- and hydroxy-apatite, inphosphorites and/or phosphatized limestones. Nevertheless, it is hereinargued that in the studied Epirus U-bearing rocks, apatite may contain aportion of U (in the form of both U6+ and U4+), but a high percentageof the concentration of the element should be related to other materials(most likely to the contained, powder XRD-amorphous, organic matter/O.M.).

Calcite, representing the othermajor crystalline phase in the studiedU-bearing rocks, is not a strong candidate for accommodation of high Uamount, either. The average abundance of U in carbonate sedimentaryrocks is 2.2 ppm (Mason and Moore, 1982) whereas the concentrationof U in relevant Tethyan rocks from SE Europe and the Middle East is

reported to be in the range of 1–10 ppm (Ehrenberg et al., 2008).Unusually elevated amounts of U (up to ca. 53 ppm) have beenmeasured in typical marine carbonates originating in the Tethys paleo-ocean (organic-rich calcitic limestone and tectonized/wheathereddolomitic limestone) of Triassic age from Mt. Kithaeron, central Greece(Göttlicher et al., 2009). In the above case, Synchrotron-based μ-XRFand μ-XANES spectroscopy indicated the existence of U4+ in Ca-richregions of the rocks, corresponding to calcite, around distinct Mn-oxidephases. Also, Sturchio et al. (1998), in a similar Synchrotron-basedstudy, had mentioned U4+ in a 35 Ma-old spar calcite (5–35 ppm U)from aMississippi Valley-type zinc ore deposit. As a consequence, calcitein the Epirus U-bearing phosphatized limestones may also contain tracesof U (most likely in the formof U4+)without, however, significant contri-bution to the overall content.

It is therefore reasonable to argue that the majority of U in theorganic-rich part of the phosphatized limestone (sample PER2B) isassociated to the organic matter (O.M.). This geological material ispowder XRD-amorphous, but it can be identified by distinct peaks(Calderon et al., 2011; Ellerbrock andGerke, 2004) in the correspondingFT-IR spectra (Fig. 10). Besides, the CO3

2− and PO43− peaks, corresponding

to calcite and apatite, were also resolved (Pasteris et al., 2004). The broadabsorbance band at ~3451 cm−1 corresponds to O\H stretching vibra-tions in water and hydroxyl groups in various organic and inorganic sub-stances (Ellerbrock and Gerke, 2004; Gerzabek et al., 2006). Minor bandsof low intensity at 2926 and 2854 cm−1 arise fromC\H stretching vibra-tions in aliphatic hydrocarbon. These bands along with the vibrations at2357 cm−1 and 2919 cm−1 indicate the presence of organic material(Calderon et al., 2011). The FT-IR spectrum is also characterized by astrong bandat ~1427 cm−1 (ν3 vibration) and abandofmediumstrength

Fig. 9. Optical microscopic (left) and SEM–EDS (right) images of the studied rocks. a) DRYM1, b) PER1, c)PER2; Cc: calcite, Qz: quartz, Ap: Apatite, and O.M.: organic matter.

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at ~873 cm−1 (ν1 vibration). These absorption bands are related to vibra-tions in the carbonate radical. The band at ~873 cm−1 is specific to calci-um carbonate and is commonly used to determine the presence of calcitein phosphate minerals (Fleet, 2009). Several more or less intensive bandsare caused by the presence of calcite (CaCO3) at 2517, 1428, 874, and711 cm−1. The vibrations at ~477 cm−1 (v2), 579 cm−1 (v4), 603 cm−1

(v4) and 1050 cm−1 (v3) are characteristic of the PO4 groups indicatingthe presence of apatite (Pasteris et al., 2004).

Organic matter in sedimentary rocks, generally known as kerogen(Forsman and Hunt, 1958) is mainly represented by bitumen which iscomposed of asphaltenes + resins and hydrocarbons (Killops andKillops, 2005). Asphaltenes and resins are heavyN, S, O-containingmol-ecules (molecular weight N500), whereas the hydrocarbons are usuallyof lowermolecularweights. The hydrocarbon fractions can be separatedinto aliphatic and aromatic hydrocarbon subfractions, again on the basisof the greater polarity of the aromatics. Traditionally, the aliphatichydrocarbons (or saturates) are described in terms of their paraffinic(acyclic alkane) and naphthenic (cycloalkane) content. Uranium in sed-iments interacts intensively with organic matter and fundamentallyforms compounds with organo-metallic bonding (Parnell, 1988;Rouzaud et al., 1980). According to Leventhal and Daws (1986) insedimentary rocks with low U content, the actinide element is related

to hydroxyls and molecules with both aliphatic and aromatic C-atoms.Otherwise, when the U content is high, the element ismostly associatedto aromatic C-atoms. Therefore when U interacts with organic matter,there are two principal processeswhich can be identified: complexationand reduction (Landais, 1996). During complexation by ionic exchange,the carboxyl functional groups of humic acids, coals, and kerogensare responsible for the complexation of uranium by organic matter(Munier-Lamy et al., 1986). The chemical reactions describing thisphenomenon involve in fact the uranyl cation (UO2

2+) and a dehydroge-nation of the organic matter:

2R\COOH+ UO2+ → RCOO\(UO2)\OOCR + 2H+

The reduction of U6+ to U4+ has been experimentally investigatedby Andreyev and Chumachenko (1964) and Nakashima et al. (1984).They have reported that the following chemical reaction describes theoxidation of organic matter during the reduction of uranium:

2(RH) + UO22+ → 2R° + 2H+ + UO2.

Meunier et al. (1990) have shown that during thermal treatment,natural urano-organic complexes can be destroyed and that U species

Fig. 10. Phases hosting U in the studied U-bearing (648 ppm) phospatized limestone; the FT-IR spectrum of the material indicating the presence of organic matter along with apatite andcalcite.

70 I.T. Tzifas et al. / Journal of Geochemical Exploration 143 (2014) 62–73

can be reduced to U4+ oxides. The UO2+ cationmay then be complexedwith carboxylate groups of the humic acids (Nash et al., 1981). Alterna-tively UO2

2+ ions may combine with electron donor free radicals withinorganic material (Rouzaud et al., 1980). Thus, adsorption and reduction

to precipitate uraninite (UO2) could occur in a single stage. In addition,most of the mineralized carbonaceous materials contain microcrystalsof uraninite, while some other material contains uraniferous organo-metallic complexes that were detectable by SEM–EDS (Rouzaud et al.,

Fig. 11. Correlation of U concentration (Utotal in ppm) and P (P2O5 in wt.%) for Mesozoic phosphorites from Epirus, Greece (Koukouzas et al., 1978; Skounakis, 1979; present study:laminated phosphatized limestones/DRYM1 and brecciated phosphatized limestones/PER2B) as well as Mesozoic phosphorites from Negev, Israel (Soudry et al., 2002).

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1980). Consequently, complex and/or organometallic compounds ofUO2

2+ with carboxylate groups (most likely of humic acids), as well asreduced U in the form of uraninite (Parnell and Eakin, 1987), can beconsidered to be predominantly responsible for the presence of theelement in the Mesozoic U-bearing phosphatized limestones of NWGreece (Epirus). It should be noted that uraninite, if existent, mustoccur in the form of nanocrystals, inasmuch as the SEM–EDS study didnot reveal any U-phase in microscale.

The origin and the formation conditions of the U-bearing brecciatedphosphatized limestones can initially be approached by a Th/Sc vs. Zr/Scdiscrimination diagram, as shown in Fig. 12a. The organic-rich part ofthe U-bearing phosphatized limestones (containing 648 ppm U)exhibits a considerablemafic -fundamentally igneous- trend, in contrastto the rest of the studied rocks (PER1, DRYM1) lying close to typicalpelagic sediments. The organic-poor – calcitic – part (sample PER2A)has not been plotted in the same diagram due to the lack of significantSc content. More information can be obtained by the use of rare earthelements (REE) which are suitable for the study of a wide range ofgeological processes. The geochemical behaviour, and generally thechemistry of REE, has been applied to study the origin of sediments(e.g. Taylor and McClennan, 1985), the chemical weathering (e.g.Hannigan and Sholkovitz, 2001) and the chemical composition of theground water. The enrichment of phosphorites in REE was observedvery early and described in detail by a number of researchers (e.g.Ilyin and Ratnikova 1976; McArthur and Walsh 1984; Piper andMedrano 1995). The negative anomalies of Ce and Euwere used to char-acterize the environment of deposition, and particularly redox condi-tions (Elderfield and Greaves, 1982). The most direct source of REE iswater and all REEs are supposed to replace Ca. The Epirus U-bearingorganic-rich phosphatized limestones illustrate enrichment in light-REE (LREE). The Eu and Ce anomalies, related to oxidation (Ce3+

↔ Ce4+) and reduction (Eu3+ ↔ Eu2+), depend subsequently onredox conditions (Elderfield and Greaves, 1982), and give importantinformation about the depositional environment. Negative Ce anomalieshave been reported to be common in phosphorites (Kidder et al., 2003;McArthur and Walsh, 1984) whereas they have also been implied forEpirus phosphorites (Sovatzoglou-Skounaki and Skounakis, 1989). How-ever, by using a recent Eu/Eu* vs. Ce/Ce* discrimination diagram(Leybourne and Johannesson, 2008), in combination with SEM–EDS, itis proven that the organic-rich part of the U-bearing phosphatized lime-stones is a typical sedimentary material while the organic-poor (andalso U-poor) – calcitic – part of the rock (PER2A) is secondary calcite

related to surfacewaters (Fig. 12b). This observation proves that the stud-ied geological material has been subjected to various surface diageneticand weathering processes. However, these intense natural processeswere not able to completely remove primary U from the sedimentaryrocks, mostly in the form of uranyl carbonate anions (UO2[CO3]34−,Parnell and Eakin, 1987), resulting in long-term immobilization of U,most likely by the organic matter. Thus, it is worthy to make obviousthat the uraniferous, P- and organic matter-rich, carbonate rocks ofNW Greece (Epirus) might also be considered as natural analoguesites of areas with relevant rocks contaminated with actinide, andnamely U, radioactive elements.

Conclusions

Sedimentary Mesozoic rocks from NW Greece (Epirus region), andparticularly laminated phosphatized limestones and brecciatedphosphatized limestones, show, on the basis of field measurements,increased natural radioactivity. On the other hand, neighbouring bed-ded chert-rich limestones show a non-radioactive typical behaviour.

Laboratory HR gamma-raymeasurements showed elevated radioac-tivity (up to 7700 Bq/Kg), mainly attributed to 238U-series, in the case ofdark organic-rich material hosted into the brecciated phosphatizedlimestones. Moreover, bulk geochemical analyses, using ICP-OES/MS,showed high U concentrations with a significant value of 648 ppm.To the best of our knowledge, we herein demonstrate one of the richestU-bearing phosphatized limestone and/or sedimentary phosphorite inthe world.

Moreover, relatively high concentrations of Cd are measured in therocks, probably related to apatite, though lower compared to the “averagephosphorite”. On the other hand, there is a geochemical depletion in LILE(e.g. Cs, Rb, K), as well as in As, Sb and Se. Optical microscopy, togetherwithpowder-XRD, SEM–EDSand FTIR confirmed abundant apatite super-group minerals, besides calcite, as well as organic compounds (organicmatter/O.M.) which should be mainly associated to the high U content.Apatite, and calcite in lesser extent, may also host traces of the actinideelement.

According to a Th/Sc vs. Zr/Sc discrimination diagram the organic-rich part of the U-bearing phosphatised limestones exhibits a mafictrend, in contrast to the rest of the studied rocks lying close to typicalpelagic sediments. The utilization of a recent Eu/Eu* vs. Ce/Ce* discrim-ination diagram, coupled with SEM–EDS, indicated that the aboveorganic-rich (and also U-rich) part is a typical sedimentary material

Fig. 12.Geochemical (a) Th/Sc vs. Zr/Sc (Rollinson, 1996) and (b) Eu/Eu* vs. Ce/Ce* (Leybourne and Johannesson, 2008) discrimination diagrams coupledwith an optical image and SEM–

EDS concerning the origin of the sample PER2B containing 648 ppm U.

72 I.T. Tzifas et al. / Journal of Geochemical Exploration 143 (2014) 62–73

while the organic-poor (and also U-poor) part of the rock is secondarycalcite related to surface waters.

All of the above lead us to argue that, despite various long-term sur-face diagenetic and weathering processes, U still remains immobilizedin NW Greece P- and organic matter-rich carbonate rocks, which, inturn, could also be considered as natural analogue sites of geological envi-ronments contaminated with actinide elements.

Acknowledgements

Many thanks are due to the staff of the Epirus branch (Preveza)of IGME for valuable information concerning the field work andsampling.

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