Home – Home – Jornals – Russian Geology and Geophysics 2001 number 6

The paper presents the structure and peculiarities of the software for processing the thermodynamic information stored in the REETHERM databank. The databank stores initial data for thermodynamic calculations modeling geochemical fractionation of rare-earth elements (REE) which involves hot fluids. The software establishes relations between the following databases: BASE1 (equations of formation of REE complexes), BASE2 (indices of the HKF equation, which provides the base for the algorithm for calculating the thermodynamic indices of dissolved REE compounds), and BASE3 (references). The software is written in the Delphi Client/Server language, and the data are presented as dBase files. They allow operations with information networks, including data export-import. The thermodynamic data stored in REETHERM include indices of the HKF equation for major geochemically important types of inorganic REE complexes, as well as data on ligands and background components of natural fluids. The article contains tables and figures illustrating the effect of temperature on the stability of hydroxyl, carbonate, and bicarbonate monoligand REE complexes in the range from 25 to 350 ^oC. The potential of the bank for correction of stability constants with regard to changes in fluid's dielectric constant caused by the presence of significant concentrations of carbon dioxide and other nonpolar gases is illustrated by the example of carbonate complexes.

The REE composition of uraniferous alluvial deposits in the Malinovka paleovalley of the Chulym-Yenisei depression is studied. The contents of elements are determined by the INAA and X-ray fluorescent methods and by emission and X-ray spectroscopies. The minerals are identified by X-ray, luminescent, and laser analyses.
According to clarke of concentration, the elements follow the sequence: Se (3600), U (250), W (185), As (47), Mo (13), V (6.3), Hf (2.1), Ge (1.8), Y (1.3), Sc (1.2), Zr (1.2), REE (0.9), and Th (0.4). Such elements as U, Mo, Se, V, Hf, W, As, and Y are characterized by an extremely uneven distribution in the deposits (V > 70%). The ore-bearing zone shows three types of elevated concentrations of elements: primary clastogene (W, Ge), primary sorption-biogenic (U, Se, Mo, V, As), and secondary redeposited infiltrogenic (elements of the first two types plus Hf, Sc, and REE). The high content of W is explained by its supply from primary deposits of the Altai-Sayan region, and the high content of Ge is due to the presence of fragments of Ge-bearing coals.

Studies of the contents and distribution of gold and silver and their composition and parageneses at the barite-polymetallic and pyrite-polymetallic deposits of northwestern Rudny Altai show that ore concentrations of these elements formed during the generation of deposits of pyrite-polymetallic ore formation related to the Middle-Upper Devonian basalt-rhyolite volcanism.
The concentrations of gold and silver in the deposits of different mineral types and their distribution in orebodies are extremely uneven. The barite-polymetallic deposits show the highest concentrations of gold and silver and various mineral forms of their occurrence. The total contents of gold and silver at these deposits range from 4.3 to 6.3 ppm and from 43 to 82.8 ppm, respectively. Gold is characterized by a varying composition with fairly high contents of Ag (to 72%) and Hg (to 27%). A specific feature of these deposits is a multistage development of ore formation processes and the presence of several generations of gold-silver minerals. The most productive were the late stages, when barite and barite-polymetallic ores and veined essentially copper ores of gold-bornite-chalcopyrite-pyrite association formed.
The pyrite-polymetallic deposits are characterized by rather low contents of gold (0.25-1.05 ppm) and silver (12-62.7 ppm). Gold-silver minerals are typically represented by one generation corresponding to the main stage of formation of pyrite-polymetallic ores and show an increased fineness (Au – from 74.75 to 89.61%, Ag – from 25.58 to 8.32%).
The most productive barite-polymetallic deposits occur at the bottom of the section of the Devonian volcanosedimentary series. Pyrite-polymetallic deposits with elevated contents of gold are also localized at this level. This is especially typical of the deposits in the Zmeinogorsk ore region.

As opposed to many other features, REE distribution in plagioclase (Pl) has been studied poorly. First data on this subject were published about 30 years ago, and since then about 300 analyses of REE in Pl have been reported by different authors. This study is an attempt to summarize those data by putting them in a unified database and paying attention to the most important features of lanthanide geochemistry in this mineral. The samples of Pl analyzed for REE were taken from a wide variety of petrographic rocks: from meteorites and lunar mafites to mafic-ultramafic rocks from ophiolitic and other complexes as well as basalts, dacites, rhyiolites, and some metamorphites. The total REE content in Pl varies from initial to several tens of parts per million; light elements and Eu dominate with a drastically subordinate role of heavy lanthanides. The REE concentration in a mineral demonstrates a certain dependence on its general composition and the type of component rocks. The Pl samples from the rocks of ophiolitic complexes and some meteorites are the most depleted in REE. The REE-enriched samples are from acid and normal effusive rocks, some lunar and earth gabbroids, and metamorphic rocks. The chondrite-normalized REE patterns in Pl always have a negative slope, and relevant (La/Yb)n values vary in a wide range, from 5 to 200. The patterns show a Eu maximum; its intensity is expressed as Eu*=2Eu_n/(Sm+Gd)_n varies from 6 to 145 on the average. High Eu* values are characteristic of the mineral from the rocks formed under reducing conditions, e.g., from meteorites. The REE distribution coefficients in the Pl-clinipyroxene system reflect the behavior of lanthanide fractionating during joint crystallization of minerals. In the Pl-melt system the coefficients normally decrease from La to Lu (except for Eu) and from acid to magnesial rocks. Using these coefficients, we can probabilistically calculate the REE compositions of model magmatic melts. The small changeability of the coeffiicents of interphase REE distribution calculated for Pl samples from the same object can be used as a criterion for the system equilibrium. Mechanism and forms of REE inclusion in the Pl structure remain to be studied. Heterovalent isomorphism of Ca²⁺ and REE³⁺ with possible participation of Na₁₊, Si₄₊, Al₃₊, and Sr₂₊ seems to be likely.

Granitoids of Western Sangilen occur mostly in high-temperature zones (sillimanite and hypersthene-K-feldspathic) of the Lower Ordovician low-pressure granulite metamorphism. According to the degree of displacement, the granitoids are subdivided into the following groups: autochthonous (leucosome of migmatites), paraautochthonous (small bodies to a few tens of square meters in area), and allochthonous (rather large intrusive massifs). Analysis of the chemical composition, geologic setting, and age of these groups suggest that they resulted from granite-forming processes which took place in the Ordovician. It is beyond doubt that metamorphism and granite formation are the consequence of the same major thermal event.
According to mineralogic and petrochemical features, the studied granites form a sequence: from paraautochthonous hyperaluminous S-granites to allochthonous weakly hyperaluminous (Khorumnug complex), subaluminous (Ulor complex), and transitional subaluminous-metaaluminous A-granites (Ukhadag and Matut complexes). In this sequence the index of alumina content of rocks decreases and alkalinity increases. Nevertheless, all granitoids have common geochemical features: elevated contents of Sr and Ba and reduced contents of Rb, U, and Th. These properties are also peculiar to metapelitic rocks, which, along with the similarity of correlation between oxides in the S- and A-granites and spatial coincidence of their occurrences, suggests that all granitoids of Sangilen resulted from melting of the same metasedimentary substrate. In this case generation of A-granites required supply of some amount of alkalies, ⁸⁶Sr, etc. into the crust. The supply can be related to gabbros and diorites of elevated alkalinity which exist in the region.

We have studied the geochemistry of elements in trachybasalts, trachyandesite-basalts, trachyrhyolites, comendites, and alkali and subalkalic granitoids of age J₃-K₁ making up a bimodal volcanoplutonic association in one of the depressions of the Late Mesozoic Transbaikalian rift system (TRS). The petrogeochemical features of the rocks from different regions are compared. Except for some local peculiarities, the parameters of the studied trachybasalts and andesite-basalts may be considered typomorphic for all TRS basaltoids of age J₃-K₁. Using the strontium isotope method, we have established that the upper-crust matter was assimilated by trachybasaltic magma and have estimated its content in the rocks.
The acid volcanic rocks of the West Usugli association are shown to be comagmatic with granitoids of a pre-Tulurian magmatic complex. The source of their substance differed from that of basaltoids, and basic and acid magmas evolved independently of each other. At the same time, the geochemical specificity of the acid members of the association (high concentrations of F, K, Rb, REE, and other lithophile elements in which associated basaltoids are enriched) is related to the effect of fluids from the below-lying trachybasalt layer. The strontium isotope compositions of acid rocks somewhat disagree with isochron models, which confirms the hypothesis that they received Rb at the late stages of their formation.

On the basis of analysis of the available data of drilling, geophysical survey, and previous geological and structural models, a tectonic map was compiled for the basement of the central, Mid-Ob', part of the West Siberian Plate. This map shows outlines of large tectonic features where they come to the surface of the basement, indicates formational composition and shape of the objects mapped, and tentatively displays the vertical sequence of their occurrence and stratigraphic volume. Substance composition and geometry are used to compile the legend and to qualitatively differentiate the modern structure of the objects under study irrespective of the existing global concepts as well as to compile sets of tectonic maps made at different scales but using a common procedure of separation of the mapped tectinic units of different ranks.

The Oka plateau exists as a moderately elevated (2400 m) intermontane area bounded by the major East Sayan dome, the Main Sayan Fault, and the Baikal Rift system, in the zone of the rift strike break. The plateau, which has a block structure, and its surroundings show specific features of recent geodynamics, including seismicity and distribution of Cenozoic basalts. These specific features are related to the position of the region over a hotspot swell along 100 E, which corresponds to a column of low-density material connecting a sublithospheric asthenolith with the lower mantle.

The paper presents a theoretical description of induced polarization decay in ion-conductive rocks, which develops the steady-state model of Friedrichsberg Sidorova based on the diffusion nature of induced polarization. Grain contacts and pore space filled with interstitial liquid are regarded as narrowing and broadening current conduits that are characterized by different transport numbers. An analytical solution of the unsteady diffusion equation yielded an expression describing time constants of induced polarization for a model in which wide pores are much longer than the narrow ones. Experimental transient characteristics obtained for sands with different average grain sizes are consistent with the suggested theory.
According to theoretical and experimental results, the time constant of induced polarization decay for pure sand is proportional to the square of its average grain size.

Investigation of the relationship between strong earthquakes and the planetary precession within the limits of the rotational earthquake model yielded the following results. (1) The planetary precession is caused by zero oscillations generated by the seismotectonic process in the upper lithosphere. (2) Chandler's frequency splits into f_{ch 1} = 0.835 year^-1 and f _{ch 2} = 0.860 year^-1, as the values of zero frequencies for the N-S (Pacific) and W-E (Alps Himalayas) oriented seismic belts are different. (3) The distribution of radius values of pole paths according to their number has a regular component 0''.05, which is equal to the seismotectonic energy released in the foci of strongest earthquakes. The relation between the solar activity and seismotectonics is hypothesized to be the result of interaction between the yearly precession component and two Chandler's frequencies.