Guffey, and 6 co-authors, reported in 2018 that the Millennium deposit, a c. 650-m deep monomineralic uranium deposit, is located in the southeastern Athabasca Basin, Saskatchewan, a region containing numerous high-grade unconformity-type U deposits. 3D modelling of the whole-rock lithogeochemistry of sandstones above the deposit reveals a distinct footprint with select major and trace elements showing increased concentrations towards mineralization.
Molar Mg/K ratios increase from background levels 10 km north of the deposit, along the B1 conductive trend, whereas Mo, Co, Rb, and Ga exhibit elevated concentrations immediately above the deposit, extending vertically from the unconformity to surface. Lead, Ag, Bi, Sb, REE, and Y exhibit elevated concentrations up to 650 m above the main mineralized body. Increasing Mg/K values indicate the transition from diagenetic to hydrothermal alteration with shifts from illitic (K-dominant) to chloritic and dravitic (Mg-dominant) alteration, with the latter increasing more proximal to mineralization.
Trace element enrichment patterns highlight that fractures and faults were conduits for fluid flow from the basement into the basin, both during ore formation and through tectonically driven post-depositional remobilization. Key indicators such as molar element ratios (Mg-K-Al) and trace elements related to redox reactions provide scalable vectors at the Millennium deposit that are likely applicable to similar unconformity-type U deposits elsewhere.
The authors concluded that the Millennium deposit is an unconformity-type U deposit with the majority of mineralization located greater than 650 m below the surface, c. 150 m below the unconformity, but with lenses extending to the unconformity itself. Despite these depths and the simple-type mineralization (monomineralic pitchblende), a unique geochemical signature is present in the Manitou Falls Formation sandstones that extends hundreds of metres vertically and horizontally from the deposit that may reflect ore forming processes and associated fluid-rock interaction.
Molar element ratios of Mg and K to Al exhibit significant changes within 10 km of the deposit, notably within the upper lithofacies, that signify hydrothermal host-rock alteration consistent with the formation of chlorite (Al-Mg-sudoite) and alkali-deficient dravite, minerals known to be associated with unconformity-type U deposits. Along the B1 conductive trend the molar ratios Mg/K increase with proximity to the deposit. Within 1.5 km of the deposit, several trace elements, including U, REE, and Y, form haloes of elevated to anomalous concentrations that intensify with proximity to the unconformity and the deposit location.
These geochemical distributions likely signify primary dispersion along fluid pathways involved in the mineralizing system, in particular, Mo, Co, Rb, and Ga, which form haloes shaped as a vertical plume extending from the unconformity to upper lithofacies. This suggests that vectoring toward a deep, monomineralic U deposit is possible using whole-rock geochemistry from overlying sandstones.
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