Grains of pyrochlore and secondary phases from tailings of Silver Crater Mine in Bancroft, Ontario (Canada) have been studied to understand the alteration processes, redox conditions, and retention of pyrochlore-derived species (U, Ti, Nb, Pb, Ta, REE) in near-field environments. Alteration processes are documented by the formation of two types of co-existing secondary veins associated with primary apatite and calcite: (i) amorphous Fe-rich veins, 46-75wt.% of FeO, and ~500ppm of UO2, and (ii) crystalline calcite-rich veins, found in fractures and penetrating the pyrochlore. Based on electron microprobe analysis (EMPA), the chemical composition of the pyrochlore is: (Ca0.84U0.35Fe0.20Na0.09Pb0.04Ln0.04Mn0.03Sr0.01Th0.01Mg0.01)1.62 (Nb1.00Ti0.87Ta0.10Si0.02)2.0O6.5F0.14. Elemental mapping revealed that migration of liberated U, Pb, Nb, Ta, Ti, and REE, is confined to the secondary veins of Fe-rich and calcite-rich compositions. Transmission electron microscopy (TEM), high-angle annular dark-field scanning TEM (HAADF-STEM), energy dispersive spectroscopy (EDS), and electron energy loss spectroscopy (EELS) results showed that pyrochlore contains nanoparticulate inclusions of uraninite, galena, and magnetite, while secondary veins host betafite, magnetite, Pb0, cerusite, and 10Å mica nanoparticles (NPs). Randomly oriented uraninite NPs, 15nm in size, concentrate around pores, 50-100nm in size, in the pyrochlore. In the Fe-rich veins, HAADF-STEM images revealed that U, Pb, Nb, and Ti were sequestered in the form of spherical betafite NPs, <800nm in size, with composition: (Ca1.1Fe0.35Pb0.28U0.09)1.83(Ti1.56Nb0.44)2.0O6.1. The association of betafite NPs, magnetite, and Pb0 NPs in Fe-rich and calcite-rich veins indicates reducing conditions during alteration of pyrochlore and immobilization of pyrochlore derived elements. This observation combined with identification of nanoscale galena and magnetite in pyrochlore, and the association of Pb0 and Fe3O4 in veins, indicate relatively low fS2 and fO2 conditions during pyrochlore alteration. In spite of prolonged exposure (≥20years) to atmospheric conditions, pyrochlore and betafite NPs retained <25wt.% and <6wt.% of UO2, respectively; and no secondary uranyl phases were observed. The alteration of pyrochlore most likely began with metamictization, followed by volume swelling, fracturing and surface interactions with fluids that caused mobilization of major and minor elements. The occurrence of amorphous Fe-rich material on the surface of the pyrochlore suggests that amorphous gels could form in Fe-rich environments as an alteration product of crystalline waste forms. The nano-geochemical complexity of the samples investigated here suggests that there is a significant nano-scale component to the sequestration of actinides during the alteration of natural and likely synthetic materials.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology