Mineralogical, Geochemical and Isotopic Compositions of Fracture Coatings in Athabasca Group Sandstones as Records of Elemental Dispersion
The McArthur River unconformity-related uranium deposit (~500 m depth) and the deep Stewardson Lake uranium project (~1100 m depth) are located in the eastern and south-central sections of the Athabasca Basin. By examining fractures and wall rock from McArthur River and comparing this analysis with the chemistry from the Stewardson Lake uranium project, this study determined that fractures can be used to detect dispersion from uranium mineralization at depth. Petrographic, shortwave infrared reflectance spectroscopic (SWIR) and geochemical studies were conducted to identify distinct colors, mineral assemblages, and chemical compositions of fractures. Weak acid leach (WAL) and modified aqua regia (AR) leach analyzed by high-resolution inductively couple plasma mass spectrometry (HR ICP-MS), indicate that brown (Type 1) and white and yellow (Type 2) fractures best record evidence of uranium mineralization at McArthur River through post-mineralization dispersion, as many fractures have low uranium concentrations and low 207Pb/206Pb values. Brown fractures show elevated concentrations of pathfinder elements, Co, Ba, Tl, Mn and radiogenic Pb. The 207Pb/206Pb values of fractures and wall rock reflect post-mineralization dispersion fluids imprinted on wall rock near fractures (low 207Pb/206Pb values; 0.09-0.50), followed by a less radiogenic fluid related to the fracture coatings (higher 207Pb/206Pb values) at McArthur River and Stewardson Lake. Stable isotope results, shown by δ18O and δ2H values, indicate that the footprint of the deposit extends upwards by hydrothermal fluids, through some fractures, including white and yellow fractures rich in fibrous goethite. Continuous leach (CL)-ICP-MS results show an association between Ni, V, and radiogenic Pb with Fe oxide and clay minerals during the 30% HNO3 leach phase for these fracture types. The mineral chemistry of seven fracture types from McArthur River and five fracture types from Stewardson Lake indicates that, although multiple fluid events have affected some fractures, they still preserve evidence of syn-mineralization and post-mineralization dispersion of elements. The mineralogy and geochemistry of fracture fillings indicate syn- and post-mineralization mobilization of pathfinder elements from the McArthur River uranium deposit and Stewardson Lake uranium occurrence at depth, but also provide evidence of a downward migration of elements from the surface, shown by McArthur River fractures.