INTRODUCTION
Phosphorites, or phosphate rocks, are sedimentary rocks rich in phosphatic minerals, mainly the apatite group ones (Tucker & Jones, 2023 and references therein). From deposition to diagenesis the primary apatite can undergo various cation and anion substitutions (Tab. 1; Jarvis et al., 1994).
| Constituent ion | Substituting ion |
|---|---|
| Ca2+ | Na+, K+, Ag+, Mg2+, Sr2+, Ba2+, Cd2+, Mn2+, Zn2+, Bi3+, Sc3+, Y3+, REE3+, U4+ |
| PO43+ | CO32-, SO42-, CrO42-, CO3•F3-, CO3•OH3-, AsO43-, VO43-, SiO44- |
| F- | OH-, Cl-, Br-, O2- |
Carbonate fluorapatite is the main species reported in sedimentary phosphorite deposits, characterised by the partial substitution of carbonate ions into the apatite phosphate sites (Jarvis et al., 1994).
Phosphate rocks are the primary source of phosphates and elemental phosphorous, both of which are critical raw materials with little to no substitutes. These resources are extensively used in agriculture and the chemical industry. More than 80% of the production is used by the fertilizer industry, because phosphorus is an essential nutrient for all living organisms and cannot be replaced by other substances (Zhang et al., 2006).
Phosphorites also represent important geological archives, capturing time intervals characterised by significant oceanographic, climatic, and biotic events, often linked to episodes of climate warming, accelerated hydrological cycle driving continental weathering, increased organic productivity, reduced sedimentation rates, perturbation of the carbon cycle, and specific redox conditions at the sediment-water interface (Föllmi, 1996; Birch, 2005; Abed, 2013; Pufhal & Groat, 2017). The sedimentological context in which phosphorites form typically includes marine shelf environments, epicontinental seas, and oceanic upwelling zones, where microbial processes, such as decomposition of organic matter, play a fundamental role in phosphate precipitation and diagenetic alteration (Glenn & Arthur, 1990; Föllmi, 1996; Birch, 2005).
The accumulation of phosphate minerals is strongly influenced by seawater chemistry, continental nutrient input, microbial activity, and fluctuations of sea-level and ocean circulation patterns (Föllmi, 1996). Birch (2005) emphasised that phosphorite deposits often develop through repeated cycles of dissolution and reprecipitation under diagenetic conditions, progressively enriching primary sediments in phosphate content and potentially incorporating trace elements such as uranium and rare-earth elements.
Phosphate ore deposits are rare and geographically unevenly distributed due to the complex set of conditions necessary for their formation. While most of the global phosphate reserves are located in the Mediterranean Basin and Eastern Atlantic passive margin, particularly in Morocco and Western Sahara, China remains the dominant producer worldwide. Numerous, some of which world-class, phosphorite deposits, hosted within sedimentary successions of Jurassic to Cretaceous age, distributed across North Africa, Middle East and Southern Europe, constitute the Mediterranean Phosphogenic Province (Notholt, 1985). Due to their significant economic importance, the North African and Middle Eastern deposits are well studied and extensively documented in published literature (e.g. Soundry et al., 2002; Pufahl et al., 2003; Edelman Furstenberg, 2008; Schneider et al., 2012; Abed, 2013; Soundry et al., 2013; Salama et al., 2015; Garnit et al., 2017; Amireh et al., 2018; Zarasvandi et al., 2019; Gundogar et al., 2022; Ghasemian et al., 2022; Levy et al., 2023). Conversely, published studies on phosphorite deposits in Southern Europe are limited. Phosphorite ores have been recorded in Greece, Italy and Albania (Serjani, 1991,1996; Stamatakis, 2004; Föllmi et al., 2008; Scopelliti et al., 2010; Tzifas et al., 2014; Föllmi et al., 2015; Mongelli et al., 2018). This study investigates two scarcely-studied uranium-bearing phosphorite deposits, occurring in the Lower Jurassic marine carbonate rock succession of Southern Albania. A multi analytical approach was applied to the characterisation of these sites, to provide detailed mineralogical, sedimentological and geochemical information on the Jurassic uranium-bearing phosphorites from the Ionian Zone in Albania. This investigation further explores their formation processes, palaeoenvironmental context, and potential economic significance, contributing to the broader understanding of phosphorite genesis within the Mediterranean region and beyond.
GEOLOGICAL SETTING
The formation and geologic history of the Albanian phosphorite deposits are closely related to the geodynamic evolution of the Albanian sector of the Alpine-Himalayan orogenic belt. Albania lies within the Dinaric-Albanid-Hellenic sector of the Alpine-Himalayan orogen (Velaj, 2015) (Fig. 1a), resulting from the collision of Gondwana derived fragments with Eurasian continental lithosphere (Rosenbaum et al., 2002). This sector is part of the Greater Mediterranean region, whose evolution is attributed to processes of accretion, subduction, and back-arc extension involving fragmented microplates and the opening and closure of the Alpine Tethys between Africa and Europe (Angrand & Mouthereau, 2021). Albania geology and topography are both defined by and evidence of these processes. The Albanides are structurally divided into two major palaeogeographic domains: Internal and External Albanides, with further subdivisions of these macro areas (Nieuwland et al., 2001; Frasheri et al., 2009; Velaj, 2015). The Internal Albanides, consisting of Triassic and Jurassic rocks, comprise the Mirdita ophiolite and the Korabi Zone and the sedimentary basin successions that developed on top of these folded units. The External Albanides are composed of syn-rift and post-rift carbonate deposits covered by flysch successions and can be divided into three westward thrust zones, from East to West: the Krasta-Cukali Zone, the Kruja Zone and the Ionian Zone.
- a) Geologic sketch map of Albania and location of the study areas in the Ionian Zone (redrafted after Velaj, 2015). b) Detailed geologic maps of Fushëbardha, extracted from the geological map of Gjirokastra 1:50000 after Gucaj, 2015. c) Bogaz extracted from the geological map of Konispoli 1:50000 (after Gucaj, 2015) phosphorite areas.
A regional tectonic map of Albania a) alongside detailed local geological maps and stratigraphic legends for the Zhulati b) and Shkalla c) areas.
The studied deposits structurally belong to the Ionian Zone, further subdivided in Berati, Kurvaleshi and Cika belts. In the Ionian Zone succession, the oldest recognised sedimentary deposits are Triassic evaporites, covered by Upper Triassic to Lower Jurassic platform carbonates and Middle Jurassic to Eocene pelagic carbonates, overlain by Oligocene to Burdigalian foredeep siliciclastic flysch deposits (Nieuwland et al., 2001).
The phosphorite deposits in the Ionian Zone occur within the Jurassic and Cretaceous carbonate successions closely associated with carbonate strata (Fig. 2), either alternating as phosphorite laminae or beds or embedding carbonate clasts. Significant occurrences of Jurassic phosphorites have been identified and studied by Albanian researchers particularly in areas such as Bogaz, Fushëbardha (Fig. 1b,c), and other localities within the Ionian Zone, where these phosphatic horizons display distinct petrographic, mineralogical, and geochemical characteristics, indicative of their unique depositional settings and diagenetic histories (Rakaj et al., 1995; Xhomo et al., 2002). The stratigraphic framework suggests that these phosphorite layers reflect distinct sedimentary cycles associated with regional sea-level fluctuations and episodic phosphogenic events (Papa, 1993; Xhomo et al., 2002). The Jurassic phosphorites located at Fushëbardha (Fig. 1b) and Bogaz (Fig. 1c) are the subject of this study.
- Generalised stratigraphy of the Ionian Zone sedimentary succession (modified after Serjani, 1991).
A stratigraphic column showing Mesozoic-Cenozoic eras, lithologies, and thicknesses, with a detailed inset log of phosphatic and chert layers.
MINING POTENTIAL
Based on the evaluation of 12 known phosphorite deposits in the Ionian Zone of Albania (10 deposits from Upper Cretaceous and 2 deposits from Lower Jurassic described in this study), the total phosphorite geological reserves in Southern Albania were estimated at approximately 57 million tons, with additional potential for expansion beyond the studied areas (NANR, 2025). Regarding the geological reserves of uranium-bearing phosphorites in Southern Albania have been historically estimated at about 1,000,000 tons in Fushëbardha and 1,557,000 tons in Bogaz, with reported contents ranging 3-14 wt% P2O5 and 0.005-0.045 wt% U3O8 (Geological Survey of Albania, 2020). In 1984, a beneficiation study (Demi et al., 1984) was carried out on low-grade phosphorites from Fushëbardha; however, the tested samples were not considered representative of the entire deposit. The reviewers of that study suggested extending such investigations to the Bogaz occurrence, which was regarded as potentially more favourable. Nevertheless, due to the socio-political transformations taking place in Albania during the late 1980s, these recommendations were never implemented.
MATERIALS AND METHODS
Fieldwork was carried out in two main phosphorite localities of Southern Albania: Bogaz and Fushëbardha. Both localities are situated within the Ionian tectonic zone and are historically known for their phosphate-bearing sedimentary rocks. At Bogaz (N 39° 43’ 10”, E 20° 7’ 32”) six samples were collected from outcropping phosphorites, at the entrance and inside the tunnels of an abandoned phosphate mine located within the Jurassic to Cretaceous marine carbonate deposits. Fushëbardha (N 40° 5’ 59”, E 19° 59’ 37”) is characterised by highly fractured rocks of the Jurassic portion of the stratigraphic succession and only small, centimetre- to decimetre-size, but abundant nodules of phosphates were recovered. The old mining galleries are now completely closed for safety reasons, and samples were collected from the outcropping rocks. Sample preparation and analyses took place at the Earth Science departments of the University of Milan and the Polytechnic University of Tirana.
Seven 30 μm-thick thin sections (one for each Fushëbardha sample, five for Bogaz), five 100 μm-thick thin sections (two Fushëbardha, three Bogaz) and eight 25mm polished sections were prepared. These sections were analysed under transmitted polarised light with the Zeiss Axioscope 5 petrographic microscope equipped with the 2.5x, 5x, 10x, 20x objective lenses and the Axiocam 305 colour kit camera for image acquisition at the University of Milan. Polished sections were examined with the petrographic microscope Nikon Eclips 50i POL (episcopic/discopic illumination) with 4x, 10x, 20x, 50x and 100x objectives and photos were taken with the Deltapix camera 20Mpcs at the Polytechnic University of Tirana.
In the field, radioactivity was measured using a FNIRSI GC-01 Geiger counter capable of measuring γ-rays, β-rays and X-rays. The cumulative equivalent dose was 0.00μSv-500.0mSv, the energy range 48keV-1.5MeV < 930% and the sensitivity was 80CPM/μSv. Measuring points were chosen within less than 200 m from each other and the detector was put in direct contact with the rock. The results were registered in the UTM WGS 84 zone 34N coordinate system only when the reading fluctuation remained below 2%. The data were elaborated with Geosoft Oasis Montaj to compose maps of radioactivity anomalies. The powerful environment provided by the software allows modelling, mapping and interpretation of geophysical, geochemical and geological data. The grid was created with the kriging method and was integrated with orthophotos to produce the anomalies map of the study area.
Scanning Electron Microscope (SEM) was used for back-scattered electron imaging of polished carbon-coated thin sections, secondary electron imaging of gold-coated pristine freshly broken rock fragments and acidified rock samples, and for preliminary chemical analysis through energy-dispersive X-ray spectroscopy (SEM-EDS). The used SEM is a JSM-IT 500 SEM at the University of Milan.
Powder X-ray diffraction (XRD) spectra were obtained for four samples with a Philips X’Pert MPD high temperature chamber powder diffractometer, with a starting angle of 5°, end angle 70°, step size of 0.033° and time per step of 60 seconds. The bulk mineralogy was determined using the Panalytical X’Pert High Score software for qualitative analysis of crystalline phases, integrated with additional mineral phase identification and semi-quantitative analyses using the DIFFRAC.EVA with reference to the International Centre for Diffraction Data PDF-4+ database.
Electron Micro-Probe Analyses (EMPA) were carried out with a JEOL JXA-8200 Super Probe (JEOL Ltd., Akishima, Japan), equipped with 5 wavelength-dispersive spectrometers (WDS) and one silicon drift detector energy-dispersive spectrometer (EDS). The parameters used: 15kV accelerating voltage, 15 nA sample current on brass, 20 seconds of counting time for peaks and 10 seconds for the background. The elements analysed were F, Mg, Zr, Nd, Th, Si, La, Dy, U, P, Ce, Yb and Ca. To identify the accessory phases that fall outside of the selected chemistry EDS was used. A series of natural minerals were used as standards. Detection limits are approximately: 3 ppm for F, 8 ppm for Mg, 134 ppm for Zr, 137 ppm for Nd, 147 ppm for Th, 318 ppm for Si, 224 ppm for La, 156 ppm for Dy, 343 ppm for U, 0.5 ppm for P, 170 ppm for Ce, 524 ppm for Yb and 0.3 ppm for Ca.
RESULTS
Lithology, composition and texture
The Bogaz samples (Fig. 3) are carbonate breccias, composed of angular limestone clasts, ranging between 2 mm and 3 cm in size, embedded in a dark grey to black phosphatic matrix. The clasts are uniform in texture and predominantly consist of peloidal skeletal packstone and packstone to grainstone including calci-mudstone intraclasts, bioclasts with micrite envelops, oncoids, benthic foraminifera (Siphovalvulina, textulariids), green dasycladacean algae (Palaeodasycladus mediterraneus), incertae sedis as Thaumatoporella parvovesiculifera, echinoid spines and gastropod fragments. Other clasts are peloidal wackestone with cortoids and millimetre-size fenestrae and dissolution vugs lined by rims of early marine radiaxial fibrous calcite cement followed by meteoric phreatic scalenohedral and burial blocky sparite cements. The adjacent clasts display similar textures and components as they belonged to the same lithified limestone before being fragmented. The phosphatic material surrounding the clasts in the intergranular space, in fenestral porosity, within fractures preceding the vein-filling sparite cement consists of authigenic calcium phosphate cement. Both lithified limestone clasts and the surrounding calcium phosphate cement are cross-cut by fractures filled by blocky sparite or dolomite (Fig. 4).
- a) Panoramic view of Bogaz Jurassic stratigraphic succession, the mine entrance and waste piles. b) Detail inside the mine gallery where samples were collected. c) Phosphate mineralisation inside the gallery showing the carbonate breccia with dark grey/brown phosphate cement in the intergranular space. d-e) Detail of hand-specimens of carbonate breccia samples with angular clasts and calcium phosphate cement represented by the dark material surrounding the angular clasts.
Five fieldwork photos labeled a to e showing a mountain outcrop, an underground mine tunnel, rock walls with a hammer, and a brecciated hand sample.
- Photomicrographs of Bogaz phosphorites and carbonate microfacies: a) Detail of the fluorapatite mineral cement surrounding the breccia carbonate clasts made of packstone and packstone/grainstone with peloids, oncoids, intraclasts and benthic foraminifera. b) Detailed view of the benthic foraminifer possible Siphovalvulina. c) Dissolution vug filled by phreatic meteoric scalenohedral calcite cement on the rim followed by phosphatic cement at the void core. d) Carbonate breccia with clasts of peloidal packstone/grainstone with benthic foraminifera with fluorapatite cement cross-cut by calcite veins.
Four thin-section photomicrographs labeled a to d of sedimentary carbonate rocks showing microfossils, peloids, matrix, and micrometer-scale bars.
At Fushëbardha (Fig. 5) the light brown to beige carbonate strata are associated with centimetre- to metre-thick levels of brown phosphorites. The limestone texture is a packstone to grainstone containing thin-shelled bivalves (former Posidonia, attributed to Bositra-type pelagic bivalves) with micrite matrix and calcite spar cement in shelter porosity, and scattered pellets of calcium phosphate. The phosphatic horizons are nodular packstone of phosphate pellets and intraclasts in a calcium phosphate matrix with rare remains of fish scales, cross-cut by sparite filled fractures (Fig. 6).
- a) Panoramic view of Fushëbardha outcrops showing the bedded Jurassic succession. b-c) Detailed view of the outcrop with dark colour phosphorites forming nodules and lenses within carbonate bed facies. d) Hand specimen of massive phosphorite cross-cut by calcite-filled veins.
Four geological photos: a) hillside outcrop, b and c close-ups of weathered, fractured strata with a rock hammer, d) cut rock sample with a scale bar.
- Photomicrographs of Fushëbardha phosphorites and carbonate microfacies: a,b,c,d) Carbonate microfacies with former Posidonia, Bositra-like, thin shelled bivalves in beds associated with carbonate micrite matrix and sparite cement, calcite-filled fractures and nodules of phosphatic material. e,f,g,h) Massive phosphorite lens with nodules and matrix showing different colours of apatite, cross-cut by fractures filled by blocky mosaics of equant calcite cement.
Eight photomicrographs a to h, of thin-section rock samples under a microscope, showing microstructures, fractures, and calcite veins with scale bars.
Mineralogy
A combination of carbonate-rich fluorapatite (Ca0.83(PO4)0.48F0.17O0.08) and carbonate-fluorapatite (CFA) (CaF(Ca,C)4((P,C)(O, OH, F)4)3), associated with variable proportion of calcite, were proposed by the PDF4+database for samples B1 and F1 (Fig. 7a, 7c). Sample BT1 showed a combination of fluorapatite and carbonate-fluorapatite (Fig. 7b), while mineralogical analysis of sample F2 provided dominant calcite, quartz and minor carbonate-fluorapatite corresponding to the thin-shelled bivalve packstone/grainstone of Fushëbardha (Fig. 7d). Due to the possible presence of diverse carbonate-rich fluorapatite mineral phases, further in the text these fluorapatite minerals will be referred to under the general acronym of CFA.
- Powder XRD spectra: a) Bogaz B1: abundant calcite with carbonate rich fluorapatite (Ca0.83(PO4)0.48F0.17O0.08) and carbonate-fluorapatite (CaF(Ca,C)4((P,C)(O, OH, F)4)3). b) Bogaz BT1: abundant calcite with carbonate-fluorapatite and fluorapatite. c) Fushëbardha F1 phosphorite facies: carbonate rich fluorapatite (Ca0.83(PO4)0.48F0.17O0.08) and carbonate-fluorapatite (CaF(Ca,C)4((P,C)(O, OH, F)4)3), with subordinate calcite. d) Fushëbardha F2 Bositra-like facies: calcite with minor quartz and CFA.
Four Powder XRD spectra labeled a to d shows mineral composition counts versus 2 theta for samples B1, BT1, F1, and F2 with color-coded peaks.
Calcium phosphate micro-structures and chemical composition
The SEM analysis of Bogaz samples shows that the CFA cement consists of tabular euhedral crystals with hexagonal basal cross-section, around 10 μm wide and 1 μm high, forming random aggregates surrounding the carbonate clasts or 5 to 15 μm long fibrous crystals forming radial rosettes or isopachous rims around carbonates clasts (Fig. 8a-b). Thin sections highlighted variations in the CFA cement, with at least two distinguishable phases (Fig. 8c-d). Semi-quantitative analysis with EDS showed abundant organic matter as carbon associated with both calcite and CFA. The primary trace minerals found are cassiterite and pyrite.
- a) and b) SEM images of Bogaz showing euhedral hexagonal habit CFA arranged in radial rosette structures. c) and d) Back-scattered electron SEM images of Bogaz thin sections showing darker grey colour calcite and two lighter grey CFA cement colours. e) Secondary electron SEM images of Fushëbardha samples showing possible EPS filamentous structures in CFA crystal substrate. (f) Prismatic crystal with hexagonal base of CFA with remnants of segmented filamentous bacteria emerging from a tubular hole within the CFA crystal (red arrow).
Six scanning electron micrographs labeled a to f showing microcrystalline structures, clastic textures, pores, and micrometer scales.
At Fushëbardha CFA prisms with hexagonal base, 5-10 microns wide, and 20 microns high are associated with filamentous structures and organic films with alveolar structures (Fig. 8e-f) resembling microbial biofilm Extracellular Polymeric Substances (EPS)Abundant organic carbon was found associated with every measure and abundant pyrite and subordinate quartz were found associated with the carbonate facies.
EMPA analyses provided fundamental quantitative chemical data on the CFA composition, associated trace minerals, trace elements and uranium distribution. The fluorine and calcium concentrations in CFA show an excess when compared with a stoichiometric fluorapatite while phosphorous is slightly in defect (Tab. 2), giving a CaO and P2O5 negative correlation. Among the trace minerals found, the most abundant were iron hydroxides, followed by fewer barite, zircon and native copper crystals; a single crystal of bastnäsite (REE fluor-carbonate) was identified (Fig. 9). No uranium minerals were detected and uranium was only found in CFA, with a patchy distribution from below detection limit up to 1867 ppm. The contents of Nd, La, Dy and Ce are very low (up to 0.1 %), those of Zr are slightly higher (ZrO2=0.22%) in the sample where zircon crystals were found. In this sample, Yb contents are also higher (Yb2O3 varies from 0.1 to 0.26%).
| FB1-C2-5 | FB1-C2-6 | FB1-C2-7 | FB3-C1-4 | FB3-C1-5 | FB3-C1-6 | F1-C4-4 | F1-C4-5 | F1-C4-6 | F2-C2-4 | F2-C2-5 | F2-C2-6 | BOG1-C1-1 | BOG1-C1-2 | BOG1-C1-3 | BOG2-C3-1 | BOG2-C3-2 | BOG2-C3-3 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| F | 5.16 | 4.93 | 4.70 | 4.84 | 4.80 | 4.82 | 5.13 | 4.87 | 4.73 | 5.00 | 4.26 | 4.26 | 4.90 | 4.80 | 5.17 | 4.69 | 4.34 | 4.57 |
| MgO | 0.16 | 0.22 | 0.16 | 0.28 | 0.28 | 0.31 | 0.21 | 0.26 | 0.28 | 0.26 | 0.31 | 0.22 | 0.25 | 0.29 | 0.19 | 0.41 | 0.36 | 0.45 |
| ZrO2 | 0.22 | 0.04 | 0.06 | B.d.l. | B.d.l. | 0.26 | B.d.l. | B.d.l. | 0.18 | 0.11 | 0.06 | B.d.l. | B.d.l. | 0.10 | B.d.l. | B.d.l. | B.d.l. | B.d.l. |
| NdO3 | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | 0.05 | 0.03 | 0.05 | 0.05 | 0.05 | B.d.l. | 0.05 | B.d.l. | 0.10 | B.d.l. | B.d.l. | 0.05 |
| ThO2 | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | 0.04 | B.d.l. |
| SiO2 | 0.43 | 1.67 | 0.42 | B.d.l. | 0.06 | 0.04 | B.d.l. | 0.04 | B.d.l. | 0.06 | 0.17 | 0.16 | B.d.l. | B.d.l. | B.d.l. | 0.61 | 0.80 | 1.74 |
| La2O3 | B.d.l. | B.d.l. | 0.06 | B.d.l. | 0.09 | B.d.l. | 0.03 | B.d.l. | 0.06 | 0.04 | 0.03 | 0.06 | B.d.l. | 0.03 | B.d.l. | B.d.l. | 0.05 | B.d.l. |
| Dy2O3 | 0.03 | B.d.l. | B.d.l. | 0.07 | 0.03 | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | 0.07 | B.d.l. | B.d.l. | 0.12 | B.d.l. | B.d.l. | 0.05 | B.d.l. |
| UO2 | 0.03 | 0.08 | 0.07 | 0.03 | B.d.l. | B.d.l. | B.d.l. | B.d.l. | B.d.l. | 0.05 | 0.06 | 0.12 | B.d.l. | B.d.l. | B.d.l. | 0.15 | 0.17 | 0.15 |
| P2O5 | 32.51 | 32.07 | 31.59 | 31.75 | 31.54 | 29.47 | 32.25 | 30.54 | 31.24 | 30.97 | 26.96 | 29.24 | 30.68 | 30.45 | 31.08 | 31.74 | 31.12 | 31.19 |
| Ce2O5 | 0.06 | 0.04 | 0.04 | 0.08 | 0.18 | B.d.l. | B.d.l. | 0.07 | 0.11 | B.d.l. | B.d.l. | 0.10 | B.d.l. | 0.08 | B.d.l. | 0.07 | 0.08 | 0.04 |
| Yb2O5 | B.d.l. | B.d.l. | 0.21 | B.d.l. | B.d.l. | 0.06 | B.d.l. | B.d.l. | B.d.l. | 0.15 | 0.07 | B.d.l. | 0.11 | 0.08 | B.d.l. | 0.20 | B.d.l. | 0.26 |
| CaO | 51.19 | 49.99 | 50.88 | 52.75 | 52.19 | 52.55 | 53.11 | 52.57 | 52.57 | 52.87 | 52.93 | 52.42 | 48.94 | 48.55 | 49.24 | 51.82 | 51.34 | 50.74 |
| Total | 89.79 | 89.04 | 88.19 | 89.80 | 89.18 | 87.50 | 90.79 | 88.39 | 89.23 | 89.56 | 84.98 | 86.57 | 84.94 | 84.50 | 85.78 | 89.68 | 88.34 | 89.19 |
- Back-scattered electron SEM images of the accessory minerals (a-d Fushëbardha, e-f Bogaz): a) Iron hydroxide with relict framboidal structure of possible pyrite, now iron hydroxide. b) Barite crystal. c) Iron hydroxide crystals near the contact between calcite and CFA. d) Bastnäsite. e) Detrital zircon crystal. f) Native copper.
Six SEM micrographs a to f showing mineral phases including Cal, CFA, Fe-hydroxide, Barite, bastnäsite, Zircon, and Native copper with scale bars.
Radioactivity maps
Different studies collected radiometric anomalies data in Albania but they had mainly a regional character (Vukzaj et al., 2018) or the measurements were sporadic and did not deliver radioactivity maps of the deposits (Ciko et al., 1999). To increase the performance of U-bearing phosphorite sampling, the mapping of radioactivity anomalies precedes the geological sampling. The measurements covered the surface of Jurassic deposits at the North-West of Fushëbardha village and the right riverbank of Pavllo river at Bogazi locality.
Background value ranges are 0.18-0.2μS/h for Fushëbardha and 0.15-0.18μS/h for Bogaz. The radioactivity at Fushëbardha phosphorite deposits varies from 1 to 1.4 μS/h, but the highest value is 1.6 μS/h at the phosphorite outcrops. The radioactivity values detected at Bogaz are 1.5 to 1.86 μS/h. The highest values were found at the phosphorite outcrops and near the mine galleries. Elaborating the data, two maps of radioactivity were produced, one for Fushëbardha and one for Bogaz. The anomaly maps combined with orthophotos are shown in Figure 10 and Figure 11.
- Radioactivity anomalies at Fushëbardha Jurassic phosphorite deposit. The highest values of radioactivity are surveyed at the top of the hills where the main outcrops are mapped.
A radioactivity map of the Fushbardha area showing color-coded radiation levels overlaid on a satellite view, with a legend and survey points.
- Radioactivity anomalies at Bogaz phosphorite deposit. The highest values of radioactivity are surveyed around the outcrops and exploration adit stocks, but downhill extensions are also observed.
A radioactivity map of the Bogaz area showing color-coded radiation levels overlaid on a satellite view, with a legend and survey points.
DISCUSSION
Origin of Lower Jurassic phosphorites in Southern Albania
Most of the bioavailable phosphate in the marine environment is quickly incorporated into the primary producers in the photic zone. Through organic matter, phosphate, is eventually transported to the deep ocean where it can be oxidised and re-transported to the surface to repeat the cycle (Föllmi, 1996). Ocean drilling has shown that phosphate can be mobilised up to 200-300 m below the sea floor, with the theoretical limit imposed by the ability of microbial activity to break down organic matter set to 500 m (Föllmi, 1996). It is only when other geological, chemical and biological conditions co-exist that deposition of phosphate minerals becomes possible. The most important characteristics for phosphorite development are thought to be the presence of high biological productivity due to upwelling and organic matter decomposition on the sea floor in sub-oxic conditions (Rao et al., 2002). Phosphorous is released below the sediment-water interface by the decomposition of organic matter, where it can reach sufficiently high concentrations in pore waters for the precipitation of CFA (Filippelli, 2008). In cases where the detrital input is low, the phosphorite will form as thin laminae and sparse nodules, while with periodic disruption, winnowing and increasing environmental reworking in high-energy environments, abundant nodules to massive phosphorite deposits form (Filippelli, 2011; Crosby & Bailey, 2012). The best economically significant phosphate deposits, especially the oldest, consist of densely packed phosphate nodular grains that show evidence of repeated phosphorisation and growth in multistage events during reworking and diagenesis (Jarvis, 1992; Hiatt & Budd, 2003; Pufahl et al., 2003; Soundry et al., 2013).
Regarding this case study from Southern Albania, the aforementioned distinctive geological, biological and chemical conditions occurred during the Early Jurassic in the area of interest. At the time, the extensional tectonics associated with the Alpine Tethys rifting, produced a horst and graben seafloor morphology with structural highs separated by basinal areas, thus favouring the upwelling of nutrient-rich waters on top of shallower highs (Karakitsios, 1995). The Bositra-type fossil assemblage at Fushëbardha can be related to deposition of the Toarcian age Posidonienschiefer (Röhl et al., 2001; Caswell & Coe, 2013). This information, combined with the scarce nodular nature of the phosphate mineralisation, proving minor to absent reworking, allows to restrict the timing of phosphorite formation to the same age of the carbonate deposition. The necessary redox conditions during this time span were provided by the early Toarcian Oceanic Anoxic Event (T-OAE). Whereas in the northern European basins extensive euxinic conditions were predominant, in the Southern Alpine Tethys—Mediterranean area the event was expressed by a generalised decline in oxygen levels without reaching euxinic or anoxic conditions (Jenkyns, 1985; Remirez & Algeo, 2020 a,b; Gambacorta et al., 2024).
Phosphorite deposits at Bogaz post-date a process of tectonic, hydraulic or hydrothermal brecciation that formed the intergranular pore space for the phosphorous-rich diagenetic fluids that later precipitated the CFA cement. The brecciated carbonate host-rock can be dated to the Pliensbachian-Sinemurian stages on the basis of the microfossil assemblage of benthic foraminifera (Siphovalvulina) and dasyclad algae (Palaeodasycladus mediterraneous). Given the fact that the carbonate clasts consist of lithified cemented limestone cross-cut by burial calcite cementfilled fractures and these clasts were cemented by CFA, the phosphate mineral precipitation must have taken place after the formation of Fushëbardha primary phosphorites in the early Toarcian, post T-OAE and deposition of Fushëbardha phosphorites or later.
The SEM imaging of the Fushëbardha samples provided clear evidence of the presence of microbial remains, in the form of filaments and extracellular polymeric substances (EPS), while the Bogaz samples did not show any evidence of remnant organic material. Tectonic or hydrothermal hydraulic brecciation allowed for later circulation of secondary fluids, enriched in phosphorus from the stratigraphically above phosphorites, that migrated through the already lithified and then fractured stratigraphically lower carbonate clasts, precipitating CFA in the interparticle space. The phosphatic material in both cases exhibits typical authigenic textures such as radial crystal growths, radial rosette structures, suggesting precipitation from phosphate-saturated pore fluids (Abed, 2013).
Uranium-bearing phosphorites and environmental implications
Uranium is an important tracer inherited from the organic matter and is incorporated in the apatite structure through substitution between Ca2+ and U4+, but the highly volatile nature of U with changing oxidising conditions during diagenesis, giving rise to U6+ which is more easily transported by fluids, often results in a heterogeneous distribution within phosphorite deposits (Altaschuler et al., 1958; Compton et al., 2000; Zanin & Zamiralova, 2007; Liu & Zhou, 2017). The average content of U in phosphorites of the Mediterranean region is 120 ppm (Notholt, 1985; Abed & Sadaqah, 2013; Abed, 2013). The brecciation in the Bogaz deposits would have not only facilitated the migration of phosphate-rich fluids but also produced a favourable environment for uranium mobilisation at higher valency (U6+) (Gabitov et al., 2024). The increased concentration of uranium within the brecciated mineralisation, particularly in areas where pyrite oxidation and fluid acidification occurred, could also constitute a further process of uranium remobilisation and enrichment in phosphorite systems (Hadad et al., 2023).
The negative correlation between CaO and P2O5 (Fig. 12) and the F exceeding 2 wt% well align with the CFA chemistry (Jones et al., 2002; Brookfield et al., 2009; Soundry et al., 2013).
- Negative correlation between CaO and P2O5. The measured data are compared with a stoichiometric fluorapatite and a CAF.
A scatter plot of P2O5 versus CaO weight percent shows a negative linear correlation for Fushëbardha, Bogaz, C F A, and Apatite samples.
The higher concentration of uranium in the secondary phosphorite mineralisation in brecciated limestone can be explained by the pyrite oxidation and acidification of diagenetic fluids that help the leaching of uranium from the overlying primary layered phosphorites, and infiltration and precipitation of fluorapatite at lower stratigraphic levels (Hadad et al., 2023). Uranium mobilisation resulted in its very heterogeneous distribution at different scale as shown from EMPA analyses of CFA at the lowest micro-scale and from U anomalies mapping at the largest outcrop scale.
From a sustainability and raw materials management perspective, the EU Critical Raw Materials Act (EC, 2023) recognizes phosphate rock and phosphorus as critical resources, but extraction must comply with Best Available Techniques (BAT) under the Industrial Emissions Directive 2010/75/EU. This requires integrated environmental management plans, including radioactivity monitoring, dust suppression, controlled waste disposal, and water treatment systems (EC, 2010) if future exploration and extraction activities will be undertaken.
The uranium content of the studied phosphate ores raises concerns about environmental radioactivity hazards. In general, the whole phosphate fertilizers production chain is subject to a high environmental risk related to radioactivity. Contamination can occur at the mine due to exposure and removal of U-rich rocks (e.g., Banzi et al., 2000; Khater et al., 2004; Abbadi, 2005; Bolivar et al., 2009), during processing of phosphate rocks by the fertilizers industry (e.g., Guerrero et al., 2020; Nabil et al., 2022; Tayar et al., 2024) and as a consequence of the use of phosphate fertilizers that can pollute soil, waters and crops (e.g., Ali & Awad, 2015; Hamed et al., 2025). Radioactivity is mainly due to the decay chain of 238U resulting in high concentration of the 222Rn radioactive isotope. As a consequence, recording ambient air radioactivity is the most important measurement for the assessment of the hazard. The International Commission for Radon Protection (ICRP) recommends for radon exposure, mainly based on application of the optimisation principle with an appropriate reference level, an annual dose in the range of 1–20 mSv. The Commission considers that a value of the order of 10 mSv annual dose should remain a benchmark for setting a reference level for radon exposure (ICRP, 2014). Banzi et al. (2000) found at the Minjingu phosphate mine in Tanzania a range of ambient air radioactivity over 5 years of 1.375-1.475 μSv/h with an average of 1.415 μSv/h. Contamination is much higher in mining tunnels like at the Abu-Tartor phosphate mine in Egypt, where radioactivity ranges between 0.69 and 81.99 mSv/y, with an average of 26.90 mSv/y, exceeding even the highest of the ICRP reference doses (Khater et al., 2004). Khelifi et al. (2016) studied the exposure of workers at two mines of the Gafsa phosphate mining district, Tunisia, where the annual average radioactivity ranges between 0.2 and 0.5 mSv/y. They calculated an annual average dose for different workers in different workplaces at the mine and found values ranging between 0.09 and 1.24 mSv/y, well below the ICRP commission suggested upper threshold of 20 mSv/y for workers as a 5 years average (ICRP, 2007). Radioactivity ranges at Albanian studied sites are comparable to those detected at Minjingu, with values slightly lower at Fushëbardha and slightly higher at Bogaz. It is not possible to compare such values with ICRP commission recommended doses as this would require a specific study with several measurements throughout the year aimed to assessing the annual dose. A simple operation to transform the hourly value to an annual one assuming constant exposure would lead to the following ranges: 8.76-12.26 mSv/y at Fushëbardha and 13.14-16.29 mSv/y at Bogaz. These values are much higher than those detected at Tunisian mines, where, anyway, the workers exposure results far below the ICRP threshold. As a whole, our results show that Albanian phosphate mining sites radioactivity environmental hazard has to be severely addressed as part of any mining feasibility study.
While the recorded in-situ gamma dose rates remain within the natural background range typical for some mineralised areas (IAEA, 2003), prolonged occupational exposure in future mining or processing scenarios could exceed recommended limits if not mitigated. Notably, similar phosphorite horizons in Greece have reported uranium levels in the range of 300–650 ppm (Papastefanou et al., 2001), highlighting a regional context of elevated natural radioactivity that warrants harmonised cross-border environmental monitoring.
Potential environmental risks also include the mobilisation of uranium and associated trace elements (e.g., rare earth elements) during weathering. Radioactivity maps clearly show an extension of anomalies along the watercourse, indicating the leaching of radioactive elements and their concentration in weathering products. The alteration of pyrite-bearing phosphorites may also produce acid mine drainage, facilitating the leaching of uranium and heavy metals into surrounding ecosystems (Hudson-Edwards et al., 2011), a potential risk particularly relevant for Bogaz where brecciation and pyrite oxidation are observed, and for the nearby Pavllo River, which could be a potential pathway for contaminant dispersion.
The EU Water Framework Directive 2000/60/EC and Groundwater Directive 2006/118/EC set strict thresholds for uranium in drinking water (0.03 mg/L; WHO, 2017), emphasizing the need for groundwater protection in areas of phosphate exploitation.
CONCLUSIONS
The multi analytical approach applied in the present study sheds light on the genesis of scarcely-known phosphorite deposits from the Southern Europe Phosphate District. The two studied deposits: Fushëbardha and Bogaz, in spite of their proximity and occurrence within Lower Jurassic marine carbonate deposits (Toarcian and Pliensbachian, respectively), show quite different sedimentological and tectonic contexts that led to the formation of the calcium phosphate-rich deposits. At Bogaz the mineralisation postdates sedimentation and is closely related to a tectonic and/or hydrothermal brecciation event that created the secondary porosity where carbonate-rich fluorapatite CFA precipitated from diagenetic fluids. At Fushëbardha CFA occurs mostly as a primary sedimentary deposit related to oxygen-depleted seafloors associated with the Toarcian Oceanic Anoxic Event, probably on structural highs formed due to extensional tectonics. The high uranium contents are primarily related to the U-rich organic matter source; anyway, enrichments are higher at Bogaz where acidic diagenetic fluids were more effective in leaching U.
The present study confirms that while some general features, such as sub-oxic conditions in marginal subtidal depositional settings, are shared by phosphate-rich rocks, several specific factors, even in deposits associated in space and time, affect their primary depositional to secondary diagenetic genesis. This results in a diversity of textural, mineralogical and geochemical characteristics that are site specific. From a radioactivity assessment stand point a longer measurement time frame should be employed in a possible future mining feasibility study, so as to give comparable data with recommended levels of exposure.