A. A. Pek

Problems in estimation of the formation depth of hydrothermal deposits by data on pressure of mineralizing fluids

In this paper, the formation depth of hydrothermal deposits is proposed to be roughly estimated by data on pressure variations in fluid inclusions from maximum Pmax to minimum Pmin values, taking into account the restrictions to the physical limits of this range based on lithostatic and hydrostatic fluid pressure. Under the assumption of a regressive trend in the PT conditions throughout the geological history of the deposits, it is possible to estimate the minimum depth at the beginning of the mineralization process, using a Pmax value and lithostatic fluid pressure gradient (∼260 bar/km), and the maximum depth at the completion of mineralization process, using a Pmin value and hydrostatic gradient (∼100 bar/km). If the mineral deposition depth estimated from the values of Pmax and lithostatic fluid pressure gradient is consistent with the depth estimate from the values of Pmin and hydrostatic fluid pressure gradient, then the resulting depth estimates correspond to the deposit formation depth unchanged throughout the mineral deposition process. In case of the ratio Pmax/Pmin > 2.6–3.0, the deposit is mineralized under variable depth coordinates with an upward movement of mineral deposition level, according to obtained estimates, over a depth range of up to >15 km. In case of the ratio Pmax/Pmin < 2.6, the data on fluid pressure variations does not allow making any definite conclusions on the trends in the deposit formation depth. In deposits with an upward movement of the mineral deposition level, the trends in depth variations based on fluid pressure data are in qualitative agreement with the data on minimum temperature Tmin recorded in deposit fluid inclusions under the assumption of its physical restriction to geothermal field values.

Onset of Fault-Bounded Free Thermal Convection in a Fluid-Saturated Horizontal Permeable Porous Layer

Development of free thermal convection of fluids in permeable rock domain with two parallel vertical faults is considered. Permeability of the faults is much higher than in the enclosing rocks. Upper and lower boundaries of the rock domain are horizontal. The lower boundary of the domain is impermeable. The upper boundary is open. Temperatures at these boundaries are fixed, and the temperature at the lower boundary is higher than the upper one. Three-dimensional model is considered. The conditions for thermal convection onset were obtained by linear stability analysis. The boundary condition for the convection onset is defined as existence of a steady-state solution of the governing equations for the fluid flow and heat transfer with nonzero convection velocities. This takes place when Rayleigh number exceeds certain critical value. These critical Rayleigh numbers are determined for different values of permeability contrast between the faults and the enclosing rocks and different distances between the faults. It is shown that thermal convection in the considered system can develop at Rayleigh numbers which are lower than the critical value for homogeneous rocks. Influence of Rayleigh number on convection cell pattern is analyzed. The role of the thermal convection in the considered systems in formation of ore deposits is examined.

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.

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.

Deposition of finely disseminated gold mineralization in black shales: A hypothesis of microstructural control

The deposition of finely disseminated gold in the deposits hosted in black shales is considered. It is suggested that gold deposition is controlled by microstructure of pore space in host rocks. The pore space structure of tight shales indicates that most pore volume is occupied by nanopores with hundredths of micrometers in characteristic dimension. The balance calculations show that deposition of native gold in nanopore channels of filtration is hampered by shortage of number of atoms necessary to overcome a nucleation threshold of the future gold crystal in the pore volume. When ore-transporting solution meets on its way the cavities (pores, micro- and macrofractures), whose volume is sufficient to overcome the nucleation threshold, the excess content of ore component, which exceeds equilibrium concentration, is released with formation of crystallization centers and further precipitation of gold. The conditions of ore deposition are exemplified in the reference Sukhoi Log deposit hosted in black shales. On the basis on the PT conditions of ore deposition and physical features of fluid heat and mass transfer, it is suggested that ore disseminations were deposited at the early high-temperature stage under a fluid pressure close to lithostatic and at a host rock permeability markedly exceeding its present-day value.

The role of the thermal convection of fluids in the formation of unconformity-type uranium deposits: the Athabasca Basin, Canada

In the global production of uranium, ~18% belong to the unconformity-type Canadian deposits localized in the Athabasca Basin. These deposits, which are unique in terms of their ore quality, were primarily studied by Canadian and French scientists. They have elaborated the diagenetic–hydrothermal hypothesis of ore formation, which suggests that (1) the deposits were formed within a sedimentary basin near an unconformity surface dividing the folded Archean–Proterozoic metamorphic basement and a gently dipping sedimentary cover, which is not affected by metamorphism; (2) the spatial accommodation of the deposits is controlled by the rejuvenated faults in the basement at their exit into the overlying sedimentary sequence; the ore bodies are localized above and below the unconformity surface; (3) the occurrence of graphite-bearing rocks is an important factor in controlling the local structural mineralization; (4) the ore bodies are the products of uranium precipitation on a reducing barrier. The mechanism that drives the circulation of ore-forming hydrothermal solutions has remained one of the main unclear questions in the general genetic concept. The ore was deposited above the surface of the unconformity due to the upflow discharge of the solution from the fault zones into the overlying conglomerate and sandstone. The ore formation below this surface is a result of the downflow migration of the solutions along the fault zones from sandstone into the basement rocks. A thermal convective system with the conjugated convection cells in the basement and sedimentary fill of the basin may be a possible explanation of why the hydrotherms circulate in the opposite directions. The results of our computations in the model setting of the free thermal convection of fluids are consistent with the conceptual reasoning about the conditions of the formation of unique uranium deposits in the Athabasca Basin. The calculated rates of the focused solution circulation through the fault zones in the upflow and downflow branches of a convection cell allow us to evaluate the time of ore formation up to the first hundreds of thousands years.

Possible role of fluids free thermal convection processes in formation of uranium deposits in the Streltsovska ore field (Eastern Trans-Baikal Region)

This paper reports the preliminary results of computer simulation of the free thermal convection of fluids in a residual thermal field of a consolidated shallow magma chamber related to caldera formation. The deeply penetrating thermoconvective circulation of fluids provided conditions for mobilization and transportation of uranium from the vast area under the caldera to the ore deposition sites.

Effect of natural advection on stabilization of contaminant plume in natural traps at underground disposal of liquid wastes

The migration of a contaminant from a zone of injection disposal of hazardous liquid waste in a deep-seated aquifer is considered. Because of its higher density, the polluted groundwater will accumulate under the effect of gravity in aquifer dips (depressions). A 2D-model of variable-density groundwater flow is used to determine the conditions under which the gravity force will prevent polluted groundwater from leaving depressions driven by regional current. As the result, such depressions can serve as natural traps for polluted waters. The required conditions are based on simple analytical relationships, derived from the analysis of a theoretical model of variable-density groundwater flow in an inclined confined aquifer. The obtained technique is used to estimate the efficiency of such a trap at the site of injection disposal of liquid radioactive waste from Mining and Chemical Combine in Krasnoyarsk region. The analytical estimates of the trap with the use of the proposed technique are shown to be in good agreement with the results of numerical simulation of contaminant migration.

Long-Term Solutions to Managing Nuclear Waste in The Russian Federation

The paper deals with some approaches to long-term solutions to managing radioactive wastes (RW) and spent nuclear fuel (SNF) volumes that have been accumulated by enterprises of the Russian Federation (RF) Minatom and RF Ministry of Defence. Short descriptions of the RW and SNF management practices are presented. The paper considers the principle of trend management of high-level waste (HLW), including the separation of fractions containing short-, intermediate-, and long-lived radionuclides; their conversion to stable matrices; and separate disposal of the matrices taking into account safety precautions for the repositories located at various depths and in different geological conditions. The paper also discusses some directions for future work in the fields of novel matrix material development and geophysical-geological research for site selection and characterisation.