Distribution and speciation of uranium in pristine Tethyan phosphorites, Ionian Zone, Albania: Insights from synchrotron XRF/XANES analyses

Abstract

Tethyan phosphorites are globally the most important source of phosphate fertilizers and occasionally contain elevated contents of uranium (U) and rare-earth elements (REE). Extensive research has been conducted to elucidate Tethyan phosphorite occurrences, geochemistry, and formation mechanisms. However, the distribution and primary speciation of uranium in Tethyan phosphorites, during sedimentation and early diagenesis, remain unclear due to the scarcity of pristine phosphorites. This contribution integrates field and laboratory studies of Tethyan phosphorites of the Ionian Zone from Albania, including petrography, paragenetic relationships, chemical compositions, and bulk and microbeam synchrotron U L₃-edge X-ray absorption near-edge structure (XANES) and microbeam synchrotron X-ray fluorescence (μsXRF) mapping. These data provide a molecular-level understanding of the distribution, speciation, and enrichment mechanisms of uranium in pristine (primary) phosphorites. The phosphorites in the Ionian Zone of Albania occur as stratiform beds reaching several meters thick, lateral extension of up to 200 m, and U concentrations of up to 210 ppm. The phosphorites show three stages of mineral assemblages: primary mineralization dominated by carbonate-rich fluorapatite, organic matter, and phosphatized thin bivalve shells; followed by vein-filling calcite and pyrite; and finally, pyrite oxidation to iron oxides during surface weathering. The μsXRF mapping reveals positive correlations between U vs. P, S, and Sr, while bulk and microbeam U L₃-edge XANES data indicate dominant U⁴⁺ species in carbonate-rich fluorapatite. The results collectively suggest that uranium occurs as U⁴⁺ in pristine carbonate-rich fluorapatite, formed in a low-energy, oxygen-minimum marine environment. These findings not only enhance our understanding of the formation mechanisms of uranium-bearing Tethyan phosphorites but also provide a baseline for determining uranium enrichment mechanisms in marine phosphorites worldwide.