ROLE OF VEIN PHASES IN IMMOBILIZING URANIUM AND LEAD RELEASED FROM ALTERED BETAFITE

Plumbobetafite (Ca_0.52U_0.58Pb_0.01)_0.79(Ti_0.96Nb_0.68Ta_0.04Fe_0.32)O₆(OH,F) from Silver Crater Mine in Bancroft, Ontario (age ~1000Ma) occurs with apatite and calcite. Previous chemical analyses suggest that the betafite has undergone primary hydrothermal alteration (>300^oC) and low temperature (<150^oC) weathering based on changes in major cation concentrations in altered versus unaltered areas (Lumpkin and Ewing, 1996). In this study, a 2 mm³-sized betafite crystal was imaged and analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and quantitative compositional data was obtained by electron microprobe analysis (EMPA). An angular network of veins (<5 mm in diameter, 50-400 mm in length; 10’s of veins per mm²) cross-cuts the apatite, while sparser (~10 veins per mm²), linear veins (60-100mm long, 1-10 mm wide) transect the calcite; both minerals are in direct contact with the pyrochlore. The veins demonstrate striking differences with respect to their ability to sequester uranium and lead mobilized by the alteration of the betafite. Specifically, the veins in the apatite host uranium, whereas the veins in the calcite do not. The apatite veins are composed of CaO (7.00-36.77 wt%), P₂O₅ (2.29-20.21 wt%), PbO (0.44-24.71 wt%), FeO (0.08-17.47 wt%), and UO₂ (1.12-10.13 wt%). In contrast, the calcite veins are dominated by FeO (0.81-25.82 wt%), CaO (0.40-21.44 wt%), SiO₂ (0.05-17.23 wt%), and PbO (10.72-12.48 wt%), and additional inclusions in the calcite (~55 x 80 mm²) contain PbO (74.46-79.46 wt%). Complementary transmission electron microscopy (TEM) analysis on the calcite vein phases indicates the presence of PbO nanoparticles in an amorphous Ca-Fe-Si phase. The availability of complexing agents derived from the mineral phases surrounding the pyrochlore during alteration, such as carbonate from the calcite and phosphate from the apatite, may have affected the stability of uranium and lead in solution, thus influencing the presence of lead in both the calcite and apatite veins and the absence of uranium in the calcite alteration products.