Search

Thermal convection and fluid mixing are involved in the formation of many mineral deposits, but their relationships and roles in mineralization remain poorly understood because they are generally investigated separately in different studies. Here we present a case study combining thermal convection and fluid mixing to examine the critical factors that controlled the formation of high-grade, world-class unconformity-related uranium (URU) deposits in the Athabasca Basin (Canada). Reactive transport modeling of various scenarios with different degrees of involvement of thermal convection and fluid mixing shows that significant URU mineralization occurs at the intersection of a basement fault with the basin-basement unconformity 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 along the fault, no significant U mineralization occurs either. Furthermore, no significant U mineralization occurs if the U concentration in the basinal fluid is low. We conclude that the exceptionally rich U endowment in the Athabasca Basin is the result of coupling of three critical factors: high-permeability sandstone favoring thermal convection in the basin, ample supply of reducing agents by basement fluids along reactivated basement faults, and abundant U-rich basinal fluid available in the basin sequences. Similar coupling of fluid processes may be essential for the formation of other types of mineral deposits. • Reactive transport modeling reproduces unconformity-related U mineralization. • Thermal convection and fluid mixing must occur simultaneously for U mineralization. • Permeable sandstone is critical for initiating and maintaining thermal convection. • Permeable basement faults linked with regional faults can provide ample reductant. • Availability of U-rich basinal fluids in sandstone is vital for U mineralization. [ABSTRACT FROM AUTHOR]

Uranium (U) and iron (Fe) are generally considered to have contrasting aqueous geochemical behavior, with U dissolution being favored in oxidizing fluids as oxidized U (U6+) and Fe in reducing fluids as reduced Fe (Fe2+) species. However, high concentrations of both U and Fe have been reported for the ore fluids of uranium deposits, suggesting that the two elements can be co-transported in the same fluid. In this study, geochemical modeling, based on recent thermodynamic and experimental data for U and Fe species and fluid inclusion compositions from two types of hydrothermal U deposits (volcanic-related and unconformity-related), was conducted to investigate the nature and conditions of U Fe co-transportation. The results of the modeling indicate that high concentrations of U (>10, up to >100 ppm) and Fe (>1000 ppm) can be dissolved in acidic aqueous fluids (pH < ∼3.3) of moderate to high chlorinity (∼ 55 to 250 g/L Cl), for a wide range of log f O 2 above and below the magnetite-hematite (MH) buffer (from ∼ MH-5 to MH + 30), at temperatures from 150 to 250 °C. There are four different combinations of dominant dissolved U and Fe species depending on the pH and redox conditions: U6+ and Fe3+ species (UO 2 Cl 2 0 and FeCl 3 0) at relatively high f O 2 , U4+ and Fe2+ species (UCl 4 0 and FeCl 4 2−) at relatively low f O 2 , and U6+ and Fe2+ species (UO 2 Cl 2 0 and FeCl 4 2−) or U4+ and Fe3+ species (UCl 4 0 and FeCl 3 0) at intermediate f O 2 conditions. Because the most important U mineral in hydrothermal uranium deposits is uraninite (UO 2), in which U is present as U4+, a necessary condition for U mineralization is that U is either transported as U4+ species or, if transported as U6+ species, precipitation of uraninite is induced by a reducing agent. The recognition that U6+ and Fe2+ species can be co-transported in the same fluid indicates that uraninite can be precipitated without the need to invoke a reducing agent external to the ore fluid. The driver of uranium mineralization in this case is an increase in pH due to fluid-rock interaction that leads to the alteration of the rocks by clay minerals and associated coupled sorption-reduction. These findings provide for a more extensive range of conditions for U transport and deposition than previously imagined and require that interpretation of the conditions of uranium mineralization consider both oxygen fugacity and pH. This study underscores the potential for uranium deposits to occur in geological settings where reducing agents are either absent or insufficiently abundant to account for the observed mineralization, and the possibility for the transport of U in environments that might otherwise be considered unfavorable. [ABSTRACT FROM AUTHOR]

Many paleo-karstic bauxite deposits contain elevated concentrations of Ni and Co. It has been known that Co and Ni are dominantly hosted in sulfides, but it remains unclear how Ni and Co were incorporated into sulfides and what kind of physiochemical conditions were involved. The Maochang bauxite deposit, as one of the large Ni- and Co-bearing paleo-karstic bauxite deposits in Guizhou Province, southwest China, was selected to tackle these questions. Detailed mineralogical and sulfur isotopic analyses using integrated EBSD, EPMA, LA-ICPMS, and LA-MC-ICPMS methods revealed three stages of sulfide mineralization: stage I with pyrite (PyI), stage II with pyrite (PyII), millerite, sphalerite, chalcopyrite and violarite, and stage III with pyrite (PyIII), chalcopyrite and galena. The fine-grained, euhedral PyI has up to 0.79 wt% Ni and up to 0.53 wt% Co. In contrast, PyII, which replaced or overgrew PyI, exhibits high but variable Ni, Co and As contents, and is further divided into three sub-types: Ni-rich PyII-a (up to 16.54 wt% Ni, 0.94 wt% Co, and 0.38 wt% As) and PyII-b (up to 3.30 wt% Ni, 1.41 wt% Co, and 0.36 wt% As), and Co-rich PyII-c (up to 0.99 wt% Ni, 7.57 wt% Co, and 1.81 wt% As). PyIII is Ni- and Co-poor. Ni increases from PyI to PyIIa and PyIIb, and then decreases to PyIIc and PyIII, whereas Co remains at similar levels for PyI, PyIIa and PyIIb, significantly increases to PyIIc, and then drops abruptly to PyIII. δ34S V-CDT values vary dramatically from negative values for PyI (−6.9 to −16.6‰) to positive values for PyII-b (+19.4 to +22.8‰), sphalerite (+17.2 to +19.7‰), and PyIII (+25.3 to +32.3‰). Based on these results, and considering that the protoliths of the bauxites were likely Ni- and Co-rich black shales according to previous studies, a four-step model is proposed to explain the enrichment of Ni and Co in the bauxites: 1) during weathering and bauxite formation on the surface, Ni and Co were leached from the black shale together with sulfate; the metals were dissolved in the solution and partly adsorbed by Fe-oxides/hydroxides under moderate to high f O2 ; 2) during early diagenesis, the sulfate was partially reduced to sulfide via the bacterial sulfate reduction (BSR) process and formed PyI; much of the Ni and Co, together with sulfate, remained in the solution; 3) with increasing burial, temperature increased and f O2 decreased, and the remaining sulfate (with elevated δ34S V-CDT) was reduced to sulfide via the same BSR process and formed PyII; Ni was largely consumed and incorporated into PyIIa and PyIIb, whereas Co remained in the solution until precipitation of PyIIc; 4) by the time of PyIII precipitation, both Ni and Co had been taken by PyII, leaving a Ni- and Co-poor solution and PyIII. Step 1 occurred in a relatively open system, whereas steps 2–4 likely occurred in a closed system. [Display omitted] [ABSTRACT FROM AUTHOR]