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REE enrichment and implications to REE mineralogy in the Tošići-Dujići bauxite deposit, Dalmatia Inland, Croatia

Introduction: Bauxite is a rock used primarily as raw material in production of aluminium. Along with major mineral constituents like gibbsite, boehmite, hematite, goethite, anatase and kaolinite, bauxite contains various metals like rare earth elements (REE), which are either associated with major mineral phases or are contained in accessory mineral phases. REE are typically enriched in the Mediterranean bauxites, sometimes reaching several thousand mg/kg in deeper horizons. This can trigger REE authigenic mineralization. An easy identification of REE minerals (minerals containing REE as major constituents) in bauxites is hampered by their low content, small size and dispersion throughout the samples. Objectives: The bauxite deposit in Tošići-Dujići, Dalmatia Inland, Croatia, shows an enrichment of REE, reaching up to 3500 mg/kg in its deeper horizons. Such a REE content outreaches an average bauxite REE abundance of the area several times. Also, there is a significant enrichment of heavy REE (HREE), which is in contrast to usual prevalence of light REE (LREE) in the local bauxites. Thus, it is interesting to investigate origin of such a REE distribution and relate it to REE mineralogy in the rock. The research was funded by European Institute of Innovation and Technology (EU body supported from Horizon 2020 research and innovation programme) through KAVA 17089 REEBAUX. Materials & methods: The bauxite samples were collected at six horizons of the former bauxite exploitation pit at the locality. All samples were analyzed for major and trace elements using ICP-MS after lithium borate sample decomposition. The samples were also treated by aqua regia and solute analyzed by ICP-MS, so that a fraction of REE in residual mineral phases could be determined. A correlation analysis of total and individual REE abundances with major chemical constituents was performed in order to determine REE preferences toward major or refractory mineral phases in the samples. Major minerals in the samples were determined by X-ray powder diffraction. The samples of two horizons with the highest REE concentrations were prepared for SEM-EDS analysis in order to identify occurrence of REE minerals. SEM analysis was performed by JEOL JSM-6510 LV and Oxford INCA X-act EDS system. Results: REE abundances in the analyzed samples range from 959-3496 mg/kg. XRD yields boehmite, gibbsite, hematite, goethite, anatase and kaolinite as major mineral phases. Around 32-58% of REE are extracted from the samples by aqua regia (LREE 34-64%, HREE 44-67%). REE are positively correlated with P as well as HREE, while LREE are mostly correlated to Ca. LREE are most likely associated with carbonates, and HREE with phosphates, being also confirmed by the leaching data. SEM-EDS analysis confirms the occurrence of monazite-(Ce), xenotime- (Y) and florencite-(Ce). Occurrence of florencite-(Ce) is an indication of authigenic REE mineralization in the samples. Conclusion: The SEM-EDS analysis provides an evidence on the occurrence of REE minerals in the Tošići-Dujići bauxite deposit. As supported by the correlation among REE and major constituents, the finding proves general criteria for occurrence of authigenic REE minerals in bauxites, although appearance and mineral morphology also suggest detrital origin of some REE minerals.

REE enrichment, REE minerals, bauxite, Croatia

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