Tzifas, and 13 co-authors, reported in early 2019 that allanite-bearing black coastal sands of Kavala (N. Greece) were studied using a combination of single-crystal XRD, EMPA, μ-XRF, bulk ICP-MS, LA-ICP-MS, μ-XANES and γ-ray spectrometry. The sands are rich in REE (ΣREE+Y: 4010 to 10,810 mg/kg), Th (236–1205 mg/kg) and other critical metals such as Nb, Ta and Co. The main REE- and Th-hosts are allanite and titanite. The allanite unit cell parameters were calculated and its formula was found to be (REE0.470Ca1.499Th0.031) (Ti0.031Fe+3 1.089Al1.880) (Si2.906Al0.094O12) (OH).
The μ-XANES spectra showed that LREE are present in trivalent oxidation state. Analyses of the non-magnetic sand fractions showed higher LREE (12,470 mg/kg) due to accumulation of allanite. The materials showed elevated radioactivity ranging from 885 ± 13 to 3467 ± 20 Bq/kg. The obtained results provide new insights on the provenance of the sands, the abundance of REE, Th, and other immobile elements, and offer new clues for potential exploration and recovery.
The Kavala black coastal sands showed significant enrichment in REE particularly in LREE and lower actinides. Combination of mineralogical and LA-ICPMS techniques demonstrated that allanite, zircons, monazite, apatite and titanites are the main hosting phases, which are responsible for the enrichment. Th and U illustrated also enhanced concentration and were mainly observed in the REE-enriched phases.
The REEs were found in both the magnetic and non-magnetic fractions. This can be attributed to the fact that allanite grains form aggregates with other mineral phases such as magnetite. This observation places serious challenges with regard to the separation and recovery of the REE-rich minerals under investigation. Eu/Eu* - Ce/Ce* bivariate diagrams combined with the authors' mineralogical investigations showed that the black sands must have originated from felsic plutonic rocks around Kavala. Whereas the coastal zone of Kavala is up to 240 km long, the REE potential is accentuated only locally in alluvial accumulations near river mouths, where the black heavy mineral-rich sands alternate with silicate-rich non-mineralized sands.
Monazite is the only mineral used in several cases for REE extraction. However, in the case of Kavala beach sands, monazite participates in small concentrations mainly as fine inclusions in allanite (< 10 μm), indicating its low economic importance. Allanite is the most abundant phase enriched in REEs in the Kavala coastal black sands. However, this phase has never been used for REE extraction on commercial scale. Important additional drawbacks are its high alteration leading to the formation of new phases (e.g., monazite and thorite) and its relatively high Th content.
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