Jianwen Yang

Debate continues over the reducing mechanisms for the formation of unconformity‐related uranium (URU) deposits. This paper evaluates, for the first time, the potential of iron‐rich chlorite as a reductant for uranium mineralization using reactive fluid flow modeling method. Our results confirm that Fe²⁺, released from the breakdown of iron‐rich chlorite, can reduce aqueous hexavalent uranium to precipitate economically significant URU deposits similar in size and grade to those formed with CH₄ as the reducing agent. The resulting uranium mineralization tends to occur in the basement and below the downwelling parts of overlying basinal fluid circulation cells, where oxidizing basinal fluid percolates across the unconformity and reacts with upward flowing reducing basement brine. Therefore, the basinal fluid circulation pattern controlled by the permeability of the sandstone aquifer is critical in determining the formation and distribution of URU deposits. When the sandstone layer is more permeable, the simulated uranium deposits become larger in size, and vice versa. If the sandstone permeability is −14 m², no obvious uranium deposits can be formed. In contrast, the permeability of fault zones does not have a significant effect on uranium mineralization, although it does affect fluid flow behaviors within the fault zone itself. We also demonstrate that fault zones do not appear to be a prerequisite for the formation of URU deposits when Fe⁺² serves as a reductant, which highlights important exploration implications. Uranium exploration should, in addition to continuing to target graphitic fault zones, also consider areas where faults and/or graphite units do not exist.

The large-sized Tarangaole uranium deposit and its neighboring Daying and Nalinggou deposits, located in the northeastern margin of the Ordos Basin, constitutes a major uranium resource base in northern China. In order to further clarify the sedimentary material source, uranium source and regional sediment–tectonic setting of the uranium-fed clastic rocks (i.e., Zhiluo Formation(J₂z)) in the district, this paper carried out whole-rock geochemistry, heavy minerals composition and in situ U-Pb dating of detrital zircons for sandstones from the lower section of the Zhiluo Formation. The results have shown that the average chemical differentiation index (CIA) for the host rocks is 73.16 and the chemical weathering degree is moderate. Heavy minerals are mainly composed of ilmenite, garnet, chlorpyrite, zircon, pyrite, apatite, hematite, etc. The U-Pb dating of detrital zircon generally indicates three age peaks, i.e., 260~Ma, 1850~Ma and 2450~Ma, respectively. In conclusion, the source rocks may have been formed at active continental margins, e.g., in a continental margin arc environment. The sedimentary materials mainly come from khondalite series, TTGs, granulite, and mafic–ultramafic intrusive rocks distributed among the Daqing–Ula Mountains and adjacent areas, etc. The Late Paleozoic U-rich intermediate and acidic magmatic rocks spreading over the eastern part of the Ula–Daqing and Wolf mountains have provided the main uranium sources for the formation of major U deposits in the northern Ordos Basin.

Regional paleoregolith is found to exist immediately below unconformities separating basin fills from basement rocks in sedimentary basins. However, the controlling role of paleoregolith on unconformity-type uranium mineralization has not been quantitatively addressed before. Coupled hydrothermal fluid flow and reactive mass transport modeling are therefore performed in this study by using the software TOUGHREACT. The modeling results reveal that preferential flow occurs in the regolith due to its relatively high permeability in comparison with that of the host rocks. The thicker the regolith is, the more concentrated the fluids in the footwall of a fault zone are, leading to more compact and higher-grade deposits therein, and vice versa. Also, displacement of the regolith caused by fault offset plays an important role, as it appears to control the shape of uranium deposits. When the displacement is less than 30 m, the deposits are characterized by a more compact shape. When the displacement is over 60 m, the deposits extend more laterally and even exhibit a ‘discrete’ shape due to the expelling effect of downslope flow that occurs at the fault offset site.

The anionic and cationic species of elements from deeply buried deposits migrate to the near surface driven by various geological forces. The geoelectrochemical exploration method (GEM), derived from CHIM, consists of the application of an electric field to collect these active ions at the designated electrode. Prospecting effects have been investigated by researchers since the coming up of CHIM. However, the cumbersome technical equipment, complex techniques and low production efficiency have restricted its potential application in field geological survey. This paper presents the newly developed CHIM that is electrified by a low voltage dipole. The improved technique allows both anionic and cationic species of elements to be extracted simultaneously in an anode and in a cathode. Compared with the conventional CHIM method, the innovative techniques called dipole geoelectrochemical method are characterized by simple instrumentation, low cost and easy operation in field, and in particular enables simultaneous extraction of anionic and cationic species of elements, from which more information can be derived with higher extraction efficiency. The dipole geoelectrochemical method was proposed and applied in the experiments of the Yingezhuang gold ore from Zhaoyuan, Shandong Province, the 210 gold ore from Jinwozi, Xinjiang Province, and the Daiyinzhang gold polymetallic deposit from Wutaishan, Shanxi Province. There are clearly anomalies above the gold ore body, indicating the effectiveness and feasibility of the improved dipole geoelectrochemical method in both scientific research and mineral exploration. The results of anode extraction in several mining areas have shown good results, indicating that gold may be mainly negatively charged. In fact, many metal nanoparticles, clay minerals, or complexes of metal ions are negatively charged, so they migrate to the anode electrode and enrich.

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.

In this study, a conceptual model is developed based on common features of typical unconformity-related uranium deposits in the Athabasca Basin, Canada. Three reactive flow modeling scenarios are designed to address the effect of fault extension on the formation of uranium deposits. Our results indicate that the location of the fault zone relevant to the unconformity is crucial to the fluid circulation in both the sandstone layer and the basement unit, the temperature distribution, the transport of aqueous components, and the uranium deposition. In particular, this research reveals that the circulating pattern of the basement brine is critical for the ore genesis. The reducing basal brine is capable of carrying aqueous uranium from depth to react with the shallow oxidizing fluid, being percolated to the basement from the overlain sandstone layer, for uranium precipitation. Scenarios 1 and 2, in which the fault zone is mainly in the basement, are in favor of focusing ore-forming hydrothermal fluids into the footwall area in the basement, leading to the formation of uranium deposits therein. Scenario 3, in which the fault zone is mainly in the sandstone layer with a limited extension below the unconformity, is unfavorable for the focusing of fluids, and hence no significant deposits can be formed, except for some minor uranium mineralization occurring in the footwall and other areas in the basement that are spatially associated with the upwelling flow zones in the sandstone layer.