Home – Home – Jornals – Russian Geology and Geophysics 2006 number 3

The Sukhoi Log gold deposit is centrally located in the Lena goldfield region, approximately 850 km NE from the city of Irkutsk, and is hosted in Upper Proterozoic marine sandstone, carbonaceous slate and phyllite, metamorphosed to low greenschist facies in an outlying part of the major Akitkan Foldbelt. The disseminated pyritic tabular orebody has no outcrop, is defined solely by assay grades and is located in the axial zone of a large, near-isoclinal, reclining anticline. Highest ore grades occur in pyritic black shale beds, especially where they cross the axial zone and include two elongate higher grade (4-9 ppm gold) cylindrical zones, termed ore pillars, along the gently plunging anticlinal crest. The anticline is exposed E-W over a length of 3 km and plunges at approximately 10

Gold-mercury deposits in Central Asia form a heterogeneous group of gold orebodies spatially and genetically related to different types of endogenous mineralization (ore deposit complexes). Four types of gold-mercury mineralization have been recognized: Au-As-Hg, Au-Sb-Hg, Au-Te-Hg, and Au-Cu-Hg. They are the products of different ore-magmatic systems, formed at their subsurface levels. The classification of these gold-mercury deposits is based on the specific mineral and geochemical compositions of their ores, including the content of Hg in native gold, relationship with different complexes of accessory endogenous mineralization, and physicochemical conditions of their formation.

The problem of the dimensions and boundaries of porphyry Cu-Mo ore association is discussed in terms of the genesis and occurrence of ore-forming porphyry magmatism, geochemistry and zoning of mineralization, and geologic and physicochemical conditions of ore-metasomatic processes. With regard to the common Cu-Mo composition of deposits (though essentially copper and essentially molybdenum deposits were also considered) and their intimate relationship with granitoid porphyry intrusions (of both deep-level basaltoid and shallow-depth genesis in a relatively open environment), all porphyry Cu-Mo deposits can be united into a single porphyry Cu-Mo association, which is subdivided into porphyry Cu, porphyry Mo-Cu, porphyry Cu-Mo, and porphyry Mo types. Defining the boundaries of ore association requires analysis of all products of a porphyry Cu-Mo ore-magmatic system with regard to the common zoning of produced mineralization: Fe-Mo (Cu)-Cu (Mo)-Cu (Au)-Fe (Au)-Pb, Zn-Au (Ag).

Based on results of studying the geochemistry and gold distribution of groundwaters in the Egor'evskoe gold-bearing area, a mechanism of gold mobilization, migration, and accumulation in mountain-forest landscape is proposed. Gold mass transfer is part of an ore-generating process running in a water-rock system. Hydrogenic concentration of gold in placers is equally determined by physicochemical processes of secondary mineral formation and hydrodynamic conditions. The amounts of accumulated gold depend on the gold-producing capacity and volume of waters involved in the mineral formation and on the duration of this process.

New geochemical data are reported for 63 samples of heterochronous Cenozoic basalts taken from separate areas of Mongolia. The rock compositions are strongly heterogeneous indicating lateral and temporary variability of the mantle magma sources. The Taryat-Chulutuin and Dariganga basalts were derived from the most enriched sources. The Early Cenozoic basaltoids of southern Mongolia were formed from melts compositionally close to a depleted mantle. In general, on different geochemical diagrams the compositions of the Mongolian basalts lie within the fields located among the sources of the DM, MORB, EM, PM, HIMU, REC, and OIB types. According to the melt mixture model, the average basaltic magma of Mongolia was generated from a mixture composed of 90% DM melt and 10% enriched magma. Some geochemical parameters (Sm/Nd, La/Yb, etc.) suggest that the mixed melts could have been supplied from different mantle sources. The melting depth corresponds to the region of existence of garnet-spinel lherzolite. Our geochemical data together with the earlier obtained ratios ³He/⁴He evidence the presence of a mantle plume whose composition is close to sources of alkaline magma of oceanic islands.

Offset wavefield continuation (DMO procedure) precedes migration or AVO analysis and simplifies the processing by reducing common-offset seismic gathers to zero offset. Then, the DMO algorithm itself should be quite elegant. Frequency-domain solutions can employ FFT but the available spectral algorithms fail to resolve the true geometry of reflections even in the high-frequency limit. We suggest to solve the DMO equation using the WKB method. The new algorithm includes several transformations of variables applied to reduce Fomel's partial differential equation to an equation resolvable by the classical WKB method. The WKB-based solution was tested in several numerical experiments and showed to resolve the geometry of plane and curved shallow dipping reflectors to a sufficient accuracy.

We model propagation of electromagnetic waves in frequency-dependent media applying the Laguerre transform in time domain. The new algorithm is fourth-order accurate in space and computationally efficient. Maxwell's equations are reduced to a harmonic series of linear algebraic equations in which only the right side depends on the harmonic number and the inverse matrix is the same for all harmonics.
The efficiency of computation for the algebraic equations is improved by fitting the free parameter of the Laguerre transform. The value of this parameter is easy to find and is likewise the same for all harmonics.
The Laguerre scheme provides a better accuracy than the second-order accurate finite-difference solution at large path lengths.
The method is stable both in the region of the wavefield where conductivity approaches zero and the spectral Fourier method is unstable, and in the high-conductivity region where the explicit FDTD code requires a too small time step.

The reported laboratory experiments of cycling measurements investigate strain-amplitude dependence of compressional velocity and attenuation of acoustic waves ( f ~ 1 MHz) in samples of the Nivagal sandstone. Strain-amplitude growth in a range of ε_d ~ (0,3-2) · 10^-6 causes a 0.6% velocity increase and an up to 10% decrease in attenuation. Attenuation curves show a minor hysteresis in a full cycle (A_min → A_max → A_min). Amplitude dependence of velocity and attenuation can be better resolved at higher strains. Attenuation is more sensitive to strain variations than velocity. The revealed relationships are useful as an additional criterion in imaging sandstone formations.

The paper is a sketch of the fifty years-long experience in using wide-angle (postcritical) reflections in DSS. Wide-angle reflections from the Moho are commonly recorded at offsets from 60 to 200 km. Their waveform is variable but correlation against first arrivals is quite stable. Wide-angle reflections are well applicable in seismic exploration.