Geochemical modeling of diagenesis, hydrothermal alteration, and unconformity-related uranium mineralization in the Athabasca Basin, Saskatchewan, Canada

Abstract

Unconformity-related uranium (URU) deposits in the Athabasca Basin were interpreted to have formed through interactions between oxidizing basinal fluids and reducing basement-derived fluids or basement lithologies under diagenetic-hydrothermal conditions. However, there is controversy regarding whether U is ultimately derived from the basin or the basement, fluid flow mechanisms responsible for metal leaching and transport, and deposition mechanisms of ores. This study uses geochemical modeling to: 1) determine fluid flow patterns responsible for U leaching and transport in the Athabasca basin; 2) recognize critical factors controlling ore deposition near the unconformity intersected by basement faults; and 3) constrain metal sources, fluid migration pathways, and ore precipitation mechanisms for U and associated elements, including Ni, Co, and As, which are anomalously enriched within some URU deposits. Petrographic observation on four drill cores indicates that the top of the sandstone succession below a mud-rich aquitard is characterized by extensive quartz overgrowths, whereas the basal part contains little cement and shows extensive dissolution features. Reactive transport modeling indicates that such a quartz cementation-dissolution pattern can be produced only if the sandstones are sufficiently permeable and thick so that the Rayleigh number exceeds the critical value for thermal convection. The results indicate that thermal convection did occur in the Athabasca Basin and may have facilitated the large-scale circulation of diagenetic fluids to leach and transport U within sandstones. Reactive transport modeling further shows that significant URU mineralization occurs at a fault-unconformity intersection only if thermal convection and basin- basement fluid mixing take place concurrently. If there is no thermal convection in the basin, only sparse U mineralization occurs along the unconformity. If insufficient amount of reducing fluid is provided from the basement fault, no significant U mineralization occurs either. Furthermore, no significant U mineralization occurs if the U concentration in the basinal fluid is low. It is concluded that the formation of URU deposits is the result of coupling of three critical factors: high-permeability sandstone favoring thermal convection in the basin, ample supply of reducing fluids along reactivated basement faults, and abundant U-rich basinal fluids in the basin sequences. Thermodynamic modeling indicates that significant amounts of U can only be transported by highly oxidizing fluids, whereas Ni, Co, and As can be co-transported with U in the same highly oxidizing fluids, or in moderately oxidizing fluids without U. Reaction path modeling further shows that uraninite precipitates before Ni-Co arsenides and sulfarsenides, when ore fluids interact with basement lithologies or mix with reducing fluids. These results confirm the ore precipitation sequences observed in typical URU deposits, and the significance of fluid mixing in ore deposition, and provide theoretical support for crystalline basement rocks as the primary Ni-Co-As source. The thesis concludes that the basin is the primary U source, and thermal convection is vital for leaching of U from the basin and its transport to mineralization sites; the spatial-temporal coupling of thermal convection and basin-basement fluid mixing is key for U deposition and accumulation at fault-unconformity intersections; Ni, Co, and As were leached from basement rocks by percolating basinal brines, and different patterns of fluid flow and fluid mixing result in the co-mineralization of U, Ni, Co, and As in certain deposits, but not in others.