Home – Home – Jornals – Russian Geology and Geophysics 2003 number 10

The Nazarovskoe deposit lies in the Ozernoe ore cluster within a relic of the Late Cambrian volcanosedimentary rocks in the field of Late Paleozoic granitoids. It is confined to the most elevated tectonic block of the Ozernoe relic and is situated above the crest-like uplift of leucocratic granites. Its main mineralization is represented by multistage stratiform deposits of recrystallized zinc-pyrite ores and Fe-Mn-oxide ores that occur in tuffaceous terrigene carbonate rocks skarned and altered to a varying degree. There is a vestige of hydrothermal-sedimentary ore deposition related to Lower Cambrian volcanism. Gold mineralization is found chiefly in skarned ore deposits and within partly hydrothermally altered country rocks. Gold occurs in various mineral assemblages and displays different structural relationships. According to mineralogical data, its higher contents in pyrite, pyrrhotite (to 10 ppm), and sphalerite (to 3.6 ppm) are due to a mechanical impurity. Gold is directly correlated with bismuth and silver. It was deposited at the stage of recrystallization of stratiform ores during the contact metamorphism as well as at later stages in connection with the formation of galenite mineralization and quartz-carbonate veinlets. The composition of native gold at the deposit ranges from high-fineness gold to kuestelite. The gold mineralization is younger than the stratiform sulfide ores. The main portion of gold was delivered by hydrothermal solutions along the subconcordant fault zone and is related to granitization of Lower Cambrian ore-bearing volcanosedimentary rocks. The magmatogenic hydrothermal component in the deposit ores has been established from such typomorphous elements as Mo, Co, Bi, and Be.

The composition, structure, and conditions of formation of the tentatively rich Fedorovskoe-1 primary gold deposit are considered. The deposit was discovered at the site where a gold placer had been exploited for 165 years in a gold-rich Siberian province. It was revealed and investigated using complex geologo-mineralogical, geophysical, and geochemical methods. We have first studied the mineral composition of ores and the structure of the ore field and determined the deposit formation parameters. Based on the results obtained and the earlier established regularities of the composition of gold-ore mineralization and its distribution in the regional ophiolites, new primary gold deposits are predicted in the South Siberian province.

Numerical modeling based on the Selektor-C software was used to analyze the thermodynamic conditions of eclogitization of basic rocks in the Earth's crust section of the Siberian Platform, which is associated with the coronite formation in differentiated basic rocks of the Yenisei Range. Schematic facies of development of equilibrium associations of metamorphic minerals have been constructed for initial compositions of layered basic rocks in the presence of a fluid of varying composition, and the behavior of the density of apobasitic rocks has been shown as a function of T and P. It is found that local variations of coronite composition are controlled first of all by the amount and composition of the fluid. An assemblage of ilmenite, K-feldspar, and biotite in coronas unambiguously indicate the presence of a water-bearing fluid at pressures of less than 9 kbar and temperatures of more than 750

Based on geological, petrochemical, and geochronological data, the postcollisional nature of granitoids of the Primorsky complex in western Cisbaikalia is substantiated. The age of the Primorsky complex granites determined by the U-Pb zircon method is 1859 16 Ma. These granitoids are similar in petrochemistry to A-type granites and, partly, to classical Early Proterozoic rapakivi.
We have established that in geologic setting, age, and composition the Primorsky complex granitoids are similar to other Early Proterozoic postcollisional granitoid complexes on the periphery of the Siberian craton - Tarak, Sayan, Shumikha, and Kodar. Some petrochemical differences between these granitoid complexes are due to regional variations in the compositions of the sources of granites and the conditions of magma crystallization rather than to different geodynamic settings of the granite formation.
The conclusion is drawn that the Early Proterozoic postcollisional granitoids spread throughout the southern flank of the Siberian craton originated during the completion of accretion of large lithospheric blocks and the formation of the Siberian craton in the period 1.84-1.88 Ga.

Sedimentary apatites subjected to high temperatures under catagenesis and hydrothermal effect are shown to have a reduced Sr content. This is the case in both ancient (Cambrian and Precambrian) and relatively young (Late Jurassic and Neogene) phosphorites. It is not ruled out that possible variations in Sr content in the waters of sea sedimentation basins affect the Sr content in phosphorites, but this hypothesis is as yet unjustified by factual data.

Distributions of Be, B, Cd, Cu, Zn, Pb, Mo, Cr, Ni, Co, Sb, Mn, Mg, and Hg have been studied in bottom sediments of 67 lakes, oxbows, and man-made ponds of the Altai Territory. The water bodies occur in different landscape-climatic zones and are fed by both flood and ground waters. The main factor of the formation of geochemical composition of sediments is the natural supply of trace elements with solid and liquid runoffs from the water-catchment areas, which results in different levels of concentration of elements in the bottom sediments of water bodies situated in different zones. The vertical section of the sediments of some lakes distinctly shows an increase in concentrations of the most of heavy metals from bottom to top. In some cases this phenomenon is due to natural factors, while in the others, man-induced ones. Though of the West Siberian regions the Altai Territory occupies one of the last places in the amount of industrial atmospheric wastes, the cumulative effect of global, regional, and local sources has resulted in a 1.5-3-fold increase in the contents of Cd, As, Sb, Pb, Zn, and Hg in the sediments for the last 100 years.

This paper reports new paleomagnetic data of dike series of the ophiolites from the Srednyaya Ters' Massif, Kuznetsk Alatau. Detailed paleomagnetic analysis supported by petrological and geochemical data permitted us not only to establish what position was occupied by this ophiolite association in the past but also to roughly estimate the age of separate parts of its section. Emphasis is placed on the discrepancy between paleomagnetic directions for dikes of different generations, which provided a possibility of their paleomagnetic dating. It has been established that the dike series is represented by subintrusive bodies of basic rocks (gabbros, gabbro-diabases, diabases, and diabase porphyries) of the first generation formed under the conditions of the island-arc system of the Paleoasian ocean, and by a set of dikes of second generation. The subintrusive bodies formed in the Vendian-Early Cambrian, whereas paleomagnetic data suggest that the dikes cutting hyperbasites and gabbros and, likely, the dike-sill complex are younger than of Early Cambrian age.

Size distribution of oil and gas pools in poorly explored provinces is analyzed using a new maximum likelihood estimator of parameters in a truncated Pareto distribution. The estimator quality is tested through Monte-Carlo imitation.

Maxwell's equations simulating 1D wave propagation in frequency-dependent media are derived using a robust and computationally efficient algorithm of the Laguerre transform approximation of fourth-order accuracy in space. Maxwell's equations reduce to a harmonic series of linear algebraic equations, and the convolution of the relaxation functions reduces to simple superposition of the harmonics. The equations have a single inverse matrix for all harmonics and each harmonic is obtained by multiplication of this matrix by the right term.
With a proper choice of modeling parameters, the Laguerre scheme is superior to many FDTD codes as it provides a better than second-order accurate approximation in time and shows a slower error accumulation. Compared to the Fourier transform, the Laguerre solution is as accurate but times less expensive.
The new algorithm is applicable without significant limitations to Maxwell's equations for two- and three-dimensional attenuating media.

The TEM problem for a current loop centered on the axis of a conducting cylindrical layered magnet is solved using uniform codes that couple Tikhonov's method for horizontally homogeneous half-space and separation of variables. The transient response is investigated as a function of model parameters simulating a multistring well casing.

Nucleation of large (M = 5-6) earthquakes in the Baikal basin was investigated from data on the M = 1.1 M = 2.2 and 2.2 < M < 3.3 foreshock activity. The seismic process was constrained between the first foreshocks and the decay of aftershocks on the basis of specially introduced parameters that represent the behavior of groups of small shocks associated with the main event. These parameters, including grouping radius (r_c), distance from grouping center (C) to a M = 5-6 epicenter, scatter (r_b), frequency, and apparent migration velocity, were applied to analyze the foreshock and aftershock activity of the 25 February 1999 earthquake in southern Lake Baikal. The seismic process was the most active in the epicentral zone 20-30 km offshore near the Listvyanka bay. The behavior of foreshocks changed 15 days before the main shock. Their grouping center migrated with time approaching the epicenter of the future event. Short-term precursors were observed only before the 25 February 1999 earthquake in South Baikal basin, and other offshore events in Central and North Baikal were not preceded by detectable foreshock activity. Small events related with nucleation of larger earthquakes have a number of specific features and may be useful in earthquake prediction.