Proof of uranyl deposition in unconformity-related uranium deposits, Athabasca Basin, Canada; evidence from synchrotron XAS and XPS analyses of hematite

Except for the recently discovered stable U (super 4+) chloride complex under reduced conditions at high temperatures, genetic models for the formation of uranium deposits had almost invariably invoked the pivotal roles of soluble U (super 6+) species for the transport of uranium in fluids and their reduction to sparingly soluble U (super 4+) as the deposition mechanism. However, the questions of when and how this reduction occurred in most uranium deposits, such as those in the Athabasca basin, Saskatchewan, Canada, are often not clear. The unconformity-related uranium deposits in the Athabasca basin are commonly accompanied by extensive and intensive alteration halos, including hematite-rich alteration or hematitization. Previous U L3-edge X-ray absorption near-edge structure (XANES) studies of uranium-bearing fluid inclusions and thermodynamic modeling demonstrated uranium transport as uranyl (UO (sub 2) (super 2+) ) species in hypersaline fluids in the Athabasca basin. Electron microprobe analyses reveal that hematite inclusions in quartz overgrowths, as well as some disseminated hematite in clay mineral (illite-chlorite) matrices, in both orebodies and associated alteration halos from five uranium deposits (Arrow, Cigar Lake, Key Lake, McArthur River, and Phoenix) in the Athabasca basin contain elevated contents of uranium (up to 2.16 wt.% UO (sub 3) ). Synchrotron U L (sub 3) -edge X-ray absorption spectroscopy (XAS) and U 4fX-ray photoelectron spectroscopy (XPS) analyses show that uranium in hematite occurs dominantly as the uranyl species, providing unambiguous evidence for direct uranyl deposition in the Athabasca basin. However, direct uranyl deposition with hematite during a single episode of hydrothermal alteration can account for only low-grade uranium mineralization. High-grade uranium deposits in the Athabasca basin required multiple episodes of hydrothermal alteration and/or other deposition mechanisms, such as those related to reduction.