Uranium-rich pyrochlores flrom the Iimaussaq complex, South Greenland

Uranium-bearing primary and secondary pyrochlore was found as accessory minerals in pegmatites and mineral veins in the kakortokite rocks of the Ilimaussaq complex in South Greenland. They contain between 7.70 and 17.79 wt.% UO₂ and have the compositional range: (Na_0.08-0.88, Ca_0.90-0.26 Mn_0.00-0.02 REE_0.24-0.30 U_0.11-0.26 [](_0.15-0.63)_Σ2.00 (Nb_1.42-1.67 Ti_0.47-0.30 Zr_0.00-0.02 Fe_0.00-0.10 Si_0.00-0.10)_Σ2 O₆ (F_0.10-0.50 OH_0.17-0.73 []_0.02-0.46)_Σ1 with unoccupied positions in both the A and Y-sites. Deuteric alteration has resulted in the formation of secondary pyrochlores characterized by the almost total removal of Na but without significant change in the content of the other cations present, accompanied, either by depletion of OH in the Y-site or protonation. Its compositional range is: (Na_0.04-0.24 K_0.00-0.05 Ca_0.68.0.21 Mn_0.000.02 REE_0.19-0.31 U_0.11-0.25 []_0.74.1.01)_Σ2 (Nb_1.42-1.69 Ti_0.28-0.46 Zr_0.00-0.01 Fe_0.01-0.05 Si_0.00-0.11)_Σ2 (O_5.78-6.03 []_0.35-0.01) _Σ6 F_0.16-0.44 • nH₂O. Complete silicification (12.7-15.4 wt.% SiO₂) of some pyrochlore crystals is characterized by negative correlation between Si and (Nb+Ti) and has resulted in the following empirical composition (Na_0.04 Ca_0.23 Mn_0.05 Al_0.14 REE_0.37 U_0.10)_Σ0.92 (Si_0.78 Nb_0.95 Ti Fe³⁺_0.07)_Σ2.00O_5.72F_0.07 • nH₂O. An unidentified Nb-U-Pb-REE mineral named Nb3 has the empirical composition: Ca_0.9 Fe_0.5 Mn_0.8 Pb_2.0 REE_1.4 Al_0.5 Si_6.2 U_1.0 (Nb_7.8 Ti_1.9 Zr_0.2)_Σ10.0 F_0.1 (O,OH,H₂O)_n.