V. I. Velichkin

Genesis and formation conditions of deposits in the unique Strel’tsovka Molybdenum-Uranium ore field: New mineralogical, geochemical, and physicochemical evidence

The ambiguity of genetic interpretations of uranium ore formation at Mo-U deposits of the Strel’tsovka ore field led us to perform additional geochemical, mineralogical, and thermobarogeochemical studies. As a result, it has been established that closely related U and F were progressively gained in the Late Mesozoic volcanic rocks from the older basic volcanics (170 Ma) to the younger silicic igneous rocks (140 Ma). The Early Cretaceous postmagmatic hydrothermal epoch (140–125 Ma) is subdivided into preore, uranium ore, and first and second postore stages. The primary brannerite-pitchblende ore was formed in association with fluorite. At the first postore stage, this assemblage was replaced by a U-Si metagel, which was previously identified as coffinite. The metagel shows a wide compositional variation; its fine structure has been studied. The preore metasomatic alteration and related veined mineralization were formed under the effect of sodium (bicarbonate)-chloride solution at a temperature of 250–200°C. The uranium ore formation began with albitization and hematitization of rocks affected by supercritical fluid at 530–500°C; brannerite and pitchblende precipitated at 350–300°C. The chondrite-normalized REE patterns of pitchblende hosted in trachybasalt, trachydacite, and granite demonstrate a pronounced Sm-Nd discontinuity and a statistically significant tetrad effect of W type. These attributes were not established in REE patterns of rhyolites derived from the upper crustal magma chamber. This circumstance and a chronological gap of 5 Ma between silicic volcanism and ore formation do not allow us to suggest that uranium was derived from this magma chamber. According to the proposed model, the evolved silicic Li-F magma was a source of uranium. U4+, together with REE, was fractionated into the fluid phase as complex fluoride compounds. The uranium mineralization was deposited at a temperature barrier. It is suggested that hydromica alteration and the formation of molybdenum mineralization were genetically unrelated to the uranium ore formation.

Iron redistribution during postore stage at uranium deposits of the Khiagda ore field, Vitim district

The localization conditions of mineralization at uranium deposits of the Khiagda ore field related to the economic sandstone type in paleovalleys are considered in this paper. These conditions allow different genetic treatments. Uncertainty in genetic estimates is caused by localization of orebodies in both gray sedimentary rocks at the boundary with bleached rocks and in the bleached rocks themselves. The results of mineralogical and geochemical studies have shown that bleaching is related to iron redistribution at the postore stage. Obvious evidence in favor of this conclusion has been obtained from statistical processing of quantitative soluble iron determinations in various geochemical types of host rocks. The postore bleaching is superposed on earlier oxidized and ore-bearing gray rocks, obliterating the coloration inherent to redox ore-controlling zoning.

Estimation of the time of magma chamber solidification beneath the Strel’tsovka caldera and its effect on the nonstationary temperature distribution in the upper crust, the eastern Transbaikal region, Russia

The hydrothermal Mo-U deposits of the Strel’tsovka ore field, unique in reserves, are localized in the Late Mesozoic caldera of the same name. The consideration of geochemical processes that controlled uranium transfer by ore-bearing fluids and its precipitation in orebodies has shown that a nonstationary temperature distribution could have exerted a substantial effect on ore formation. The temperature field in the Strel’tsovka caldera, which was caused by a shallow-seated magma chamber that existed beneath the caldera by the onset of the ore stage, was simulated by mathematical modeling. A one-dimensional nonstationary model of conductive heat transfer taking into account the latent heat of magmatic melt crystallization was used. The problem was solved with the finite difference method. It has been established that, at optimal parameters of the model, the magma chamber would have completely crystallized in 56 ka; the maximum estimate is 133 ka. Three million years after emplacement of the granitic intrusion, the related thermal anomaly in the upper crust should have disappeared. The results obtained indicate that granitic melt of this chamber could not have been a source of uranium-bearing solutions that formed deposits 5 Ma after the cessation of magmatic activity.

Domal structures and hydrothermal uranium deposits of the Erzgebirge, Saxony, Germany

The domal structures of the Erzgebirge in Saxony, Germany, which control localization of the Variscan hydrothermal U, Sn, W, and base-metal deposits, started to form in the Late Riphean in the course of folding, regional metamorphism, and granitization and were eventually created at the end of the Paleozoic, when an enormous mass of granitic melts was emplaced and cooled. As is illustrated by the world’s largest Schlema-Alberoda vein uranium deposit, the fractures that determined the localization of ore-bearing veins were formed in the outer contact zones of the Variscan granites related to domal uplifts. Beyond the zones of contact metamorphism, the orientation of ore-bearing fractures changes, their morphology simplifies, the amplitude of displacements decreases, and most fractures pinch out. Such a zoning is explained by development of faults around growing granitic domes.

A comprehensive study of the spread of radioactive contamination in the geological medium near Lake Karachai, Chelyabinsk oblast

The spread of radioactive contamination in the subsurface medium near Lake Karachai is considered. The complexity of this process requires a comprehensive approach to its study. The source of radioactive contamination is overviewed. The map of faults in subsurface medium is considered in order to determine the prevailing direction of contaminated groundwater flow. Photometry in observation wells has been used for structural geological estimation of transport properties of the shallow aquifer, where contaminated groundwater is moving. This study was carried out along with hydrochemical logging, which makes it possible to estimate the dynamics of contamination of subsurface medium and vertical distribution of groundwater contamination. Special attention is paid to transport of radionuclides in the form of radiocolloid particles. Groundwater samples were taken from various depths corresponding to different contamination levels near Lake Karachai. The depth intervals of sampling were determined from the data of hydrochemical logging. Ultrafiltration through membranes with a specific pore size in combination with gamma spectrometry was used to characterize radionuclide transfer with colloidal particles differing in size. The local elemental composition of the radiocolloid surface was studied by Auger spectroscopy. The chemical composition and structure of radiocolloid particles were determined by X-ray photoelectron spectroscopy with consecutive etching of the particle surface by argon ions for a certain depth. The data obtained indicate that radiocolloid particles are heterogeneous and an organic shell consisting of humic and fulvic acids and technogenic organic compounds coat their surface.

Model of heat and mass transfer by fluid during formation of Mo-U deposits in the Strel’tsovka ore field, eastern Transbaikal region: Forced convection of solutions generated by a deep source

The Strel’tsovka and Antei uranium deposits located in the Strel’tsovka caldera are unique in ore resources. According to the considered mathematical model, the uranium source of these deposits was related to the middle-lower crustal silicic magma chambers or had mantle origin. Boundary conditions of the model are based on modern views of physicochemical conditions of hydrothermal process in the Strel’tsovka ore field and factors governing ore deposition therein. Modeling results are consistent with morphology of orebodies and ultimate uranium resources of the deposits and thus confirm indirectly that the physicochemical parameters of the ore-forming system are coherent. The maximal duration of uranium ore deposition is estimated at 500 ka.

A model of radionuclide transfer by groundwater on the territory of the Russian Research Centre Kurchatov Institute

The convective transfer of 137Cs and 90Sr by groundwater on the territory of the Russian Research Centre Kurchatov Institute (RRCKI) was modeled. Geological data on the RRCKI site and possible sources of radionuclides show that the uppermost aquifer, composed of Quaternary sediments, is the most probable region of spreading of radioactive contamination. Since the lateral migration of radionuclides is predominant, a 2D horizontal model was used for the forecasting of spreading of radioactive contaminants in the subsurface medium. Transient or long-term repositories of radioactive materials at the RRCKI site (concentrated sources) and aquifer rocks contaminated in the course of removal of radionuclides from these repositories (distributed source) are responsible for groundwater pollution. The initial 137Cs and 90Sr distributions used in the forecasting of radionuclide migration were determined from their contents in core samples taken from wells drilled in contaminated areas of the RRCKI site. The zone of radionuclide migration is limited by poorly permeable moraine loam from below and by the water table from above. To determine the upper and lower boundaries, these surfaces were mapped in plan view. The data of meteorological observations over a long period were used to map the intensity of precipitation in the studied territory. The density of rocks in the uppermost aquifer and partition coefficients of radionuclides between rocks and groundwater were estimated from the data of laboratory examination of the core samples. The available data on the permeability of rock samples and the results of test pumping out of some wells were involved in the consideration. The results obtained and the data on the water table allowed us to define a gauge problem for determining the distribution of the filtration coefficient in the uppermost aquifer. This problem was solved taking the intensity of precipitation into account. The properties of the uppermost aquifer and the initial radionuclide distribution therein were used as initial data for modeling 137Cs and 90Sr migration on the territory of the RRCKI over 50 years.

Safety assessment concept for repositories of spent nuclear materials in crystalline rocks

The methodological issues in safety assessment of repositories of spent nuclear materials (high level wastes and untreatable irradiated fuel) in crystalline rocks are discussed, and a concept of such assessment is proposed. The problem is urgent because of the need for scientific confirmation of the safety assessment procedure for such repositories in Russian territory.The concept develops the provisions of Russian normative base in the field of handling of radioactive wastes of this type, taking into account the world experience. The concept is based on theoretically sound principles to be followed in safety assessment for geological repositories. It takes into account the results of analysis of a vast literature on this issue published both in Russia and abroad. Radiological safety criteria and methods for improving the reliability of prediction calculations for determining safety indicators are characterized. The arguments that supplement radiological indicators are shown to be of importance for substantiating the safety of the anticipated repository. The regular implementation of the assessment procedure at each completed stage in the implementation of the project of terminal geological isolation of wastes is regarded as a means for quality control of the performed works.

Deposits of the Hiagda Uranium Ore Field, Buryatia: Formation Conditions and Ore Control Factors

The paper concerns the geological–structural conditions of ore localization, mineralogy and chemistry of ore, and geochemical features of the ore-bearing medium at exogenic–epigenetic paleovalley deposits of the Hiagda ore field. The general localization regularities and ore control factors of uranium mineralization can be used for mining, exploration, and refining the prospecting criteria and indications of this type of deposits.

Experimental study of crystalline-rock permeability: Implications for underground radioactive waste disposal

New methods and approaches to studying the permeability of crystalline rocks based on physical and numerical experiments are considered along with the results obtained by them for rock samples taken from the likely sites of HLW and SNF repositories or depositories, including metavolcanites from the area of PA Mayak and granitoids from the Krasnoyarsk MCC area. The comparative analysis of the obtained results served as a basis for proposing a system of methodological procedures for studying the permeability of crystalline rocks in sections of boreholes drilled in areas of likely allocation of HLW and SNF depositories and repositories.