Home – Home – Jornals – Russian Geology and Geophysics 2010 number 12

The raw-material base of the Russian aluminum-producing industry is considered. The raw materials include common (boxites, nepheline syenites) and uncommon (synnyrites, anorthosites, power-and-heating plant ashes, kaolines) types of ores. With regard to many criteria (reserves and quality of ores, technology of their processing, etc.), the problem of alumina deficit can be solved by mining sillimanite group minerals Al₂SiO₅ (wt.%: Al₂O₃ = 62.9, SiO₂ = 37.1), namely, andalusite, sillimanite, and disthene. Their explored reserves converted to the final product (aluminum) exceed 400 mln tons. This will be enough for more than a hundred years on condition that aluminum will be produced in the present-day output (>4 mln tons in 2008). Almost all deposits can be explored by strip mining, with application of the gravity, flotation, and electromagnetic separation methods for aluminum concentration. The alumina content in concentrates reaches 60-62 wt.%. Only high-quality boxites and the above concentration methods can ensure such a high yield of Al₂O₃. Sillimanite group minerals can be processed together with nepheline ores by sintering or be used for the direct electrothermal production of silumin and aluminum, excluding the alumina production stage. The latter method is the most promising in Russia.

The validity of the Rb/Sr isotopic ages of Cretaceous granitoid complex in the central Chukchi region has been verified. The performed studies showed that the Rb/Sr isotopic ages of late-orogenic granitoids are not reliable. Their variation can be explained in terms of the mixing of primary mantle magmas with continental-crust matter. Most probably, the variation is due to the constant compositions of mixing components and their proportions.

Iron valence state and local environment in a set of fibrous diamonds from Brazilian and Congolese placers were investigated using X-ray absorption and Mossbauer spectroscopies. It is shown that the diamonds could be divided into two main groups differing in the type of dominant Fe-bearing inclusions. In the first group Fe is mostly trivalent and is present in octahedral coordination; diamonds from the second group contain a mixture of Fe²⁺ and Fe³⁺, most likely, with Fe²⁺ in dodecahedral coordination. A few other diamonds contain iron in a more reduced state: The presence of metallic Fe and Fe₃O₄ is inferred from XAS measurements. Spatially resolved XANES and Mossbauer measurements on polished diamond plates show that in some cases the Fe valence state may change considerably between the core and rim, whereas in other cases Fe speciation and valence remain constant. It is shown that Fe valence does not correlate with water and/or carbonate content or ratio, suggesting that iron is a minor element in the growth medium of fibrous diamonds and plays a passive role. This study suggests that, when present, evolution of the C isotopic composition with diamond growth is largely due to changes in chemistry of the growth medium and not due to variations of f _{O 2}

In light of the new clinoform structural model of the Neocomian, introduced at the last stratigraphic meeting, it is suggested to distinguish clinoform units instead of formations in the clinoform zone. Formations (12 in number) remain only west of the central part of the Neocomian basin (east-dipping clinoforms). In the eastern and southeastern zones of the basin, all the formations beyond the clinoform zone, which consist mostly of continental sediments, should be united into the Ust'-Taz Group.

The paper presents the results of a comprehensive investigation into the recent sediments of Proval Bay. This bay formed during catastrophic flooding of a big block of land as a result of an earthquake in 1862. Comparison of the sketch map of the bay for 1862 with its modern map shows that the boundary of the Selenga River delta has shifted considerably eastward. The sedimennts of Proval Bay are sands, silty pelite, and pelitic silt. Terrigenous material is predominant and consists of mineral grains and land plant remains admixed with diatom frustules and sponge spicules. In the southwestern part of the bay, turbidites and a soil layer have been found. The latter was buried when the water level in Lake Baikal rose after the construction of the Irkutsk dam in 1959-1964. In the northeastern part of the bay, a peatlike layer has been found in the bottom sediment core. It formed in Lake Beloe, which existed in the Tsagan steppe before the 1862 earthquake. According to diatom analysis, this lake was shallow and eutrophic. The sedimentation rates in different parts of Proval Bay differ greatly and depend directly on proximity to the Selenga River. Variations in the geochemical indicators which reflect the ratio of organic to clastic components in the bottom sediments of the bay are controlled by temperature and water level variations in Lake Baikal.

A new algorithm is suggested for the joint calculation of three key parameters of reservoir rocks: permeability, electroacoustic constant, and velocity of second bulk compressional wave (slow wave). The algorithm is advantageous in the use of Stoneley waves which are most stable in acoustoelectric log waveform data.

This paper describes a well logging method and device designed to determine radial inhomogeneities in the elemental content of the borehole environment with high spatial resolution. The sounding factor that determines the spatial resolution is the time elapsed from the moment of neutron emission from the device to the moment the device records the gamma rays from neutron inelastic scattering (inelastic gamma rays, IGRs) in the formation. The time interval characterizes the distance to the point of origin of a gamma ray, and the energy of a gamma ray passing through the formation without interaction determines the chemical element involved in inelastic scattering.
Simulations have shown that at each time, the density of inelastic scattering is very well localized in space owing to the small number of fast-neutron scatterings: on average, one to two events. It is the compact localization of inelastic scattering events that provides high radial resolution (and, if necessary, high azimuth resolution) during fast-neutron sounding of formations and measurement of unsteady IGR fluxes. Recording of IGR distributions over time also provides increasing sounding depth because powerful IGR fluxes from nearby regions reach the detector at short times and do not overlap the weaker IGR fluxes from distant regions because the latter reach the detector later.
To evaluate the radial resolution of the method, we calculated the response of the sonde for typical models of a borehole environment which include a borehole, an iron casing, cement, an invaded zone, and an uninvaded rock. The boundaries of spatial inhomogeneities and the elemental content in the regions between these boundaries were determined from time dependences of unscattered spectral lines in IGR spectra for the elements Ca, Si, C, O, and Fe. The results of the numerical simulation indicate a high sensitivity of the measurements to the radial boundaries and an adequate spatial resolution: about 1 cm at a 0.1 ns time sampling of logs. The interfaces between the radial zones are clearly marked in the time distributions by steep fronts with a length of 0.1 ns (at a collimation angle of the source of about 30?) to 0.15-0.4 ns (at an angle of 90?).
A method of solution was formulated for the inverse problem consisting of determining the boundaries of the radial zones and the elemental content in these zones. The problem is solved using a qualitative model of the borehole environment, for example, a "borehole-casing-cement-invaded zone-uninvaded rock" model. The method is based on searching for approximating model curves to measured time distributions of unscattered IGR fluxes jointly for all components of the model. The search is conducted by spatial optimization of the sought parameters - the distances { r_S} from the neutron source to the boundaries of the zones and the concentrations { C } of specified chemical compounds in these zones. The initial approximations for the sought parameters { r_S} and { C } are calculated by linear inversion of logs, which proves to be very accurate because the contribution of singly scattered neutrons to the inelastic scattering density at small times (10 ns) is, on average, 50-90%.
Model curves are calculated by numerical simulation of the transport of neutrons and gamma rays. An appropriate calculation method is the Monte Carlo technique. Since the multiplicity of neutron scattering is low and, for gamma rays, only the unscattered component is of interest, the numerical simulation is a fast process.
The practical implementation of the method requires the use of advanced developments in the design of neutron generators, spectral gamma-ray detectors, and fast analyzers for recording subnanosecond processes. Use of associated-particle neutron generators, Ge semiconductor detectors with electron cooling or LaBr₃ (Ce) and BaF₂ based fast scintillator blocks of high energy resolution will allow the application of the proposed method to logging measurements.

The superdeep North Caspian, South Caspian, and Barents basins have their sedimentary fill much thicker and the Moho, correspondingly, much deeper than it is required for crustal subsidence by lithospheric stretching. In the absence of large gravity anomalies, this crustal structure indicates the presence under the Moho of a thick layer of eclogite which is denser than mantle peridotite. Crustal subsidence in the basins can be explained by high-grade metamorphism of mafic lower crust. The basins produced by lithospheric stretching normally subside for the first ~100 myr of their history, while at least half of the subsidence in the three basins occurred after that period, which is another evidence against the stretching formation mechanism. According to the seismic reflection profiling data, stretching can be responsible for only a minor part of the subsidence in the Caspian and Barents basins. As for the South Caspian basin, there has been a large recent subsidence event in a setting of compression. Therefore, eclogitization appears to be a realistic mechanism of crustal subsidence in superdeep basins.

This is a critical comment on the model of basin formation by eclogitization of mafic crust suggested by E. Artyushkov. The eclogitization model bears uncertainties in average parameters (thickness, density, pressure) of lithospheric mantle, crust, and sediments, which may bias the estimates of subsidence magnitude. Main pitfalls, however, lie in high-pressure petrology: The lithostatic pressure is insufficient for eclogite to form in the lower crust beneath deep basins. It is shown that linear extrapolation of laboratory data on the gabbro-to-eclogite transition onto the field of relatively low pressures and temperatures in the lithosphere is incorrect. The hypothesized role of hot mantle fluids in the gabbro-eclogite transformation appears doubtful in terms of both petrology and kinetics of metamorphic reactions. Eclogite volumes in none of well known eclogitic sites agree with those required for eclogitization-driven subsidence. Artyushkov's criticism of the extension basin formation model is not quite just. There are recent models of a two-layer lithosphere that imply a possibility of brittle and ductile deformation at different crust rheologies. The models we refer to predict most of extension to occur in mantle lithosphere rather than in the crust, this extension being able to produce deep continental basins.