The genesis of the supergene REE-fluorocarbonate and uranyl mineralization in the Abu Rusheid area of the South Eastern Desert of Egypt

The Abu Rushied area is an interesting occurrence of metamorphic-related rare earth element (REE) fluorocarbonate and uranyl mineralization within shear zones in the psammitic gneiss. The shear zones appear to play a significant role in the mineralization and enrichment of REE and U. The study combined field observations, petrographic analysis, Secondary Electron Imaging (SEI), and quantitative Electron Probe Micro Analyses (EPMA) to investigate the mineralogy and geological controls on the REE and U mineralization. The newly discovered REE-fluorocarbonate minerals bastnäsite-La and synchysite-Y are associated with uranyl phosphate (autunite) and uranyl silicate minerals (uranophane and soddyite). The bastnäsite-La is enriched in La2O3 (21.21 wt% on average) and Ce2O3 (3.58 wt% on average), as well as Y2O3 (13.23–15.91 wt%). However, it lacks significant Nd2O3 (< 1 wt%), and contains a certain amount of F (5.28 wt%, on average). Representative formulae for Abu Rushied bastnäsite-La is: (La0.240Ce0.041Pr0.130Nd0.001Sm0.035 Eu0.008Gd0.016Dy0.028Ho0.012Er0.008Tm0.003Yb0.013Lu0.006Y0.239)Σ0.78(CO3)(F1.026,Cl0.008)Σ1.03. The synchysite-Y is less abundant than the bastnäsite-La in the mineralized zones. However, the synchysite-Y is enriched in the heavy rare earth elements (HREEs) and yttrium (Y), containing 51.37–55.91 wt% HREE + Y. In contrast, the light rare earth element (LREE) contents are much lower, ranging from 2.65 to 5.26 wt%. Notably, the synchysite-Y lacks detectable amounts of neodymium (Nd), europium (Eu), and terbium (Tb). An expanded representative formula for Abu Rushied synchysite-Y is: Ca0.623(La0.013Ce0.011Pr0.017Sm0.018Gd0.020Dy0.106Ho0.019Er0.049Tm0.009 Yb0.061Lu0.018Y0.778)Σ1.12(CO3)2(F0.718,OH0.282) Σ1.0. A two-stage model, with an initial hydrothermal alteration phase followed by supergene mineral precipitation, provides a plausible explanation for the observed enrichment of REEs and U within the shear-zone hosted mineralization. The structural permeability of the shear zones and potential for clay mineral formation could have been important factors in concentrating and precipitating the REE-fluorocarbonates and uranyl minerals.