Abstract: The hydrogeological functioning of four different areas in a complex evaporite-karst unit of predominantly aquitard behavior in S Spain was investigated. Environmental dating tracers (3H, 3He, 4He, CFC-12, SF6) and hydrochemical data were determined from spring samples to identify and characterize groundwater flow components of different residence times in the media. Results show a general geochemical evolution pattern, from higher (recharge areas) to lower positions (discharge areas), in which mineralization rises as well as the value of the rCl−/SO42−, evidencing longer water-rock interaction. Ne values show degassing of most of the samples, favored by the high salinity of groundwater and the development of karstification so that the concentration of all the considered gases were corrected according to the difference between the theoretical and the measured Ne. The presence of modern groundwater in every sample was proved by the detection of 3H and CFC-12. At the opposite, the higher amount of radiogenic 4He in most samples also indicates that they have an old component. The 3H/3He dating method does not give reliable ages as a consequence of degassing and the large uncertainty of the 3He/4He ratios of the sources for the radiogenic Helium. The large SF6 concentrations suggest terrigenic production related to halite and dolomite. Binary Mixing and Free Shape Models were created based on 3H and CFC-12 data to interpret the age distribution of the samples. Two parameters (GA50 and >70%) were proposed as an indicator of that distribution, as they provide further information than the mean age. Particularly, GA50 is derived from the median groundwater age and is presented as a new way of interpreting mixed groundwater age data. A greater fraction of old groundwater (3H and CFC-12 free) was identified in discharge areas, while the proportion and estimated infiltration date of the younger fractions in recharge areas were higher and more recent, respectively. The application of different approaches has been useful to corroborate previous theoretical conceptual model proposed for the study area and to test the applicability of the used environmental tracer in dating brine groundwater and karst springs.

Abstract: The Nalinggou–Daying uranium metallogenic belt is situated at the northern Ordos Basin, China. Petrographical, mineralogical and geochemical techniques were used to study the ore-bearing sandstones and host rocks in the Nalinggou–Daying uranium metallogenic belt. The present study shows that uranium minerals, i.e., coffinite, pitchblende, and brannerite, are mostly disseminated around pyrite and detrital particles. The ore-bearing sandstones are enriched in organic matter, with which this reductive environment influenced uranium leaching. The carbonate concentration of the uranium ores is markedly higher than that of the host rocks, and intense carbonatization occurs in the ore-bearing sandstones. In this case, the usage of the classical in-situ leach uranium mining technique by injecting H2SO4 + H2O2 solution produces calcium sulfate precipitate, which can lead to blocking of the ore-bearing strata. For this reason, laboratory and field uranium mining tests were conducted using CO2 + O2 in-situ leaching technology and were demonstrated to be successful, illustrating that this approach is technically feasible. Inhibiting ore bed blockage and increasing the amount of injected O2 are important for uranium leaching in this setting.

Abstract: In Situ Recovery (ISR) is the most important process of uranium production in the world (50%). It consists of an injection of a leaching solution into a permeable mineralized aquifer (sandstone), pumping of the solution after dissolution of the ore minerals and recovery of the uranium from the pregnant solution in a surface plant. In this context, the fate of swelling clay minerals such as smectites is of main importance due to their role in the mobility of cationic elements by diverse geochemical processes such as ion-exchange reactions or dissolution. The present study details analysis of dioctahedral smectites before and after in-situ leaching by acidic (H2SO4) ISR solutions. Samples were collected from two sedimentary basins hosting some of the main uranium roll front deposits exploited by acidic ISR (Tortkuduk deposit, Shu-Saryssu basin, Kazakhstan, and Dulaan Uul and Zoovch Ovoo deposits, Sainshand basin, Mongolia). Scanning Electron Microscope and X-Ray Diffraction analysis revealed that dioctahedral smectite is a ubiquitous mineral in all analyzed samples, before and after acidification, and revealed a difference of crystal chemistry of the smectites between deposits of Kazakhstan (beidellite type) and Mongolia (montmorillonite type). Chemical analysis and semi-quantification of the smectites before and after acidification also revealed a difference in chemical reactivity, with a higher dissolution of montmorillonite layers compared to beidellite ones, and the importance of ion-exchange reactions. These findings are consistent with literature data obtained on model systems. The persistence of dioctahedral smectites after several years of acidification is crucial for the understanding of geochemical processes during uranium production or remediation of the aquifers. Finally, based on the analysis of samples from U-deposits hosted in both sedimentary basins, a schematic model of the impact of acid solutions on dioctahedral smectite was proposed.