Location of Uranium‐Rich Brines Determines the Distribution and Grade of Unconformity‐Related Uranium Deposits

Abstract

Recent fluid inclusion analysis indicates that uranium‐bearing brines were present in both the sandstone and the basement at the time of ore genesis in the Athabasca Basin, Canada. However, the question of how the location of uranium‐rich brines controls the formation of unconformity‐related uranium deposits remains unresolved. In this study, four reactive flow modeling scenarios are designed to address this outstanding issue. Our numerical results confirm that the basement‐hosting uranium‐rich brines are capable of forming high‐grade concentrated uranium deposits in the footwall region of the fault zone below the unconformity interface. In contrast, the sandstone‐hosting uranium‐rich brines lead to the formation of low‐grade sheet‐like uranium mineral precipitates above the sandstone layer rather than in the basement. Therefore, this study reveals that unconformity‐related uranium deposits tend to be formed when the basement unit serves as the main uranium source. Taking the sandstone layer as the main uranium source, no unconformity‐related uranium deposits can be formed since the resultant uranium precipitation has no spatial relation with the unconformity interface.

Effects of Paleoregolith and Fault Offset on the Formation of Unconformity-Type Uranium Deposits

“…For instance, Lin et al [23] examined the effect of fault locations on uranium mineralization. Xu et al [24] confirmed that the location of uranium-bearing brines plays a critical role in ore genesis. However, to the best of our knowledge, no numerical modeling research has been performed to examine the role of paleoregolith in controlling fluid flow and uranium mineralization.…”

“…The top boundary is 3 km below the surface. Previous numerical studies also employed similar layered models, e.g., [24][25][26][27][28]. A paleoregolith layer occurs below the unconformity, which is intersected and offset by a fault zone.…”

“…In addition, the oxygen fugacity f O 2 and pH of different geological units are also provided in Table 3. The data presented in Tables 1-3 are based on previous modeling studies and fluid inclusion reports (e.g., [19,[22][23][24][25][26][27][28][40][41][42]). For instance, a recent fluid inclusion analysis of barren sandstones in the Athabasca Basin [40] indicates that the concentration of aqueous uranium UO 2 2+ ranges from 2.2 × 10 −6 to 9.9 × 10 −5 mol/L, with an average of 2.5 × 10 −5 mol/L, while previous fluid inclusion studies of quartz veins in barren samples coexisting with several uranium deposits in the Athabasca Basin [41,42] report that the UO 2 2+ concentration in the basement brine ranges from 1.0 × 10 −6 to 2.8 × 10 −3 mol/L, and that the average is 1.0 × 10 −4 mol/L.…”

“…In other words, the basement unit represents the main uranium source region. As in previous modeling studies (e.g., [24]), the upper and lower boundaries are assigned constant temperatures of 90 • C and 240 • C, respectively. The side boundaries are presumably impermeable to fluid flow, whereas the top has a constant fluid pressure of 30 MPa.…”

“…The initial conditions include a linear increase in temperature, hydrostatic pressure, and lithostatic pressure from the top to bottom. This paper also assumes that at typical ore-forming temperatures, graphite in the fault zone dissolves to produce aqueous methane, which then reduces aqueous uranium to uraninite (e.g., [24]).…”