URANYL DEPOSITION IN UNCONFORMITY-RELATED DEPOSITS IN THE ATHABASCA BASIN: EVIDENCE FROM URANIUM SPECIATION IN HEMATITE AND GOETHITE
Date
2023-08-28
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Thesis
Degree Level
Masters
Abstract
With the exception of the recently discovered stable U4+ chloride complex at high temperatures under reduced conditions, genetic models for the formation of uranium deposits had almost invariably invoked the pivotal roles of soluble U6+ species for the transport of uranium-bearing fluids and their reduction to sparingly soluble U4+ as the deposition mechanism. However, the questions of when and how this reduction in most uranium deposits such as those in the Athabasca basin (Canada) occurred are often not clear. The unconformity-related uranium deposits in the Athabasca basin are commonly accompanied by extensive and intensive hematite-rich alteration halos. Previous U L3-edge XANES studies of uranium-bearing fluid inclusions and thermodynamic modeling demonstrated uranium transport as uranyl (UO22+) species in hypersaline fluids in the Athabasca basin. Electron microprobe analyses reveal that hematite inclusions in two early generations of secondary quartz overgrowth, as well as some disseminated hematite in clay mineral (illite-chlorite) matrices, in both orebodies and associated alteration haloes from five uranium deposits (Arrow, Cigar Lake, Key Lake, McArthur River, and Phoenix) in the Athabasca basin, contain anomalously high contents of uranium (up to 2.16 wt.% UO3). Synchrotron U L3-edge X-ray absorption spectroscopy (XAS) and U 4f X-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. This mechanism of direct uranyl deposition with hematite and other minerals such as quartz (or their amorphous or microcrystalline precursors) during a single episode of hydrothermal alteration can account for low-grade uranium mineralization, but high-grade uranium deposits in the Athabasca basin required multiple episodes of hydrothermal alteration and reduction-induced deposition mechanisms. In addition, synchrotron U L3-edge XAS analyses show that uraniferous goethite associated with boltwoodite in late alteration assemblages of mineralized metapelites at the Eagle Point deposit is also dominated by the uranyl species, supporting its roles in late uranium remobilization.
Description
Keywords
Uraniferous hematite, adsorbed uranyl, X-ray absorption spectroscopy, uranium deposits, deposition mechanism
Citation
Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geology