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Russian Geology and Geophysics

2002 year, number 1


O. M. Rozen, V. P. Serenko, Z. V. Spetsius, A. V. Manakov, and N. N. Zinchuk
Keywords: Lower crust, upper crust, xenoliths in kimberlites, collision, isotope geochemistry, Archean, Proterozoic, Siberia
Pages: 6-26

Abstract >>
The Siberian craton has been tectonically regionalized. Comparison of isotopic ages shows that this craton formed in the late Paleoproterozoic (2.0 ÷1.8 Ga ago) after Archean microcontinents had collided and amalgamated. The microcontinents formed 3.5, 3.1, and 2.5 Ga ago at the cost of the sialic matter released from the mantle. On collision they coincided along the sutures and transformed into tectonic blocks (granite-gneiss and granite-greenstone terranes). High-temperature metamorphism of terranes and granite formation in zones of collision proceeded simultaneously, which is a necessary consequence of thermal relaxation in the thickened crust of the collision prism. The first-rank structures are superterranes: Tunguska, Aldan, Stanovoy, Anabar, and Olenek tectonic provinces. The Yakutian kimberlite province lies within the Anabar and Olenek superterranes.
The core of the wells drilled on the closed area of the Yakutian kimberlite province contains granulite complexes of the Magan and Daldyn terranes and granite-greenstone complexes of the Markha terrane (Anabar superterrane). Also, it demonstrates the Kotuikan zone of collision and accompanying zones of faults made up of granitoids in association with blastomylonites and cataclasites of amphibolite facies. The crust of the Yakutian province formed as part of the crust of the Siberian craton 1.8 Ga ago, when a collision orogen appeared. After it had been eroded, a peneplain formed, upon which Early Riphean clastics began to accumulate (1.65 Ga ago).
Crustal inclusions in kimberlites characterize a drastic lateral inhomogeneity of the lower crustal horizons. Their composition in the Daldyn and Magan granulite-gneiss terranes (Muna and Mirny kimberlite fields, respectively) corresponds to the metabasite-plagiogneiss formation of the Anabar Shield, with inclusions captured at a depth of 10-20 km. In the Markha granite-greenstone terrane, the inclusions in the Nakyn kimberlites characterize the upper-crust granite-gneiss complex of amphibolite facies (0-10 km). The lower-crust granulite complexes of this terrane have no analogs on the day surface: the Daldyn field - 50% metabasites dredged from 20-30 km and the Alakit field - more than 80% metaterrigenous and metacarbonate rocks coming from 10-30 km. In the pipes of the Birekta granite-greenstone terrane (Obnazhennaya and Slyudyanka pipes), metabasites make up more than 60% of crustal xenoliths, and the basement of such terranes is supposedly enriched in basites.
Spatial distribution of kimberlites reveals no direct relationship with the upper- and lower-crust complexes, but it can be inferred indirectly, from relationships between structures of the crust and lithospheric mantle. This, in turn, will advance the frontiers of the diamondiferous area in the Yakutian kimberlite province.


O. P. Polyanskii and V. V. Reverdatto
Keywords: Fluid flow, convection, sedimentary basin, numerical modeling
Pages: 27-41

Abstract >>
Flow regimes of pore fluid were studied, with the vapor-water phase transition near solidifying intrusions of dikes and sills in a sedimentary basin taken into account. The study was based on a computer program simulating a one- or two-phase flow of the vapor-water mixture depending on the phase state of the fluid. Under consideration is a real section of the Yenisei-Khatanga basin, northwest of the Siberian Platform. Two types of models were used: (1) intrusion of a sill into the basement of the basin, beneath a reservoir bed, and between two reservoirs; (2) the same situations but with a vertical dike and off-branching sill. Some numerical experiments were carried out to study the effect exerted on convection by pore fluid of a single sill or a dike combined with sill that are intruded into the sediment. The calculated results permit prediction of the beginning of convection depending on type of intrusion, its location in the section of sedimentary basin, initial temperature, and physical parameters of rocks. Patterns of evolution of temperature field and velocities of fluid flows in the sediment around solidifying intrusions have been obtained. The problem is important for predicting the behavior of both water and hydrocarbon fluids in basins where trap magmatism is expressed.


G. B. Fershtater, F. Bea, N. S. Borodina, E. A. Zin'kova, P. Montero, and E. S. Shagalov
Keywords: Subduction, anatexis, fluid, magmatism, substratum, restite, melt, Urals
Pages: 42-56

Abstract >>
The most intense supersubduction anatexis in the Urals occurred in the late Early Carboniferous (340-320 Ma ago). It is characterized by high water saturation (PH20 = 0,7-1,0Ptot) of the generated melts, caused by additional supply of water into the zone of anatexis. Anatexis occurs in the zone of stability of main hydroxyl-bearing minerals - biotite and hornblende accumulated in restite.
Anatectic melt is either of tonalite or granodiorite composition. This composition of melt is due to a basite substratum whose degree of melting provides about 40% of melt sufficient for separation from the substratum.
Outmelting of granitoid melts is accompanied by water basite magmatism. The products of this magmatism are represented by high-Sr hornblende gabbros, which are the source of heat and matter (substratum) for anatexis. Gabbroids and products of crystallization of anatectic melt share the mineral composition: Hbl + Bt + An20-45 + Ep Kfs Q + Sph + Ap + Ilm Mt
Prolonged basite magmatism inecreased the crust thickness from below, thus causing its underplating in a suture megablock, in the adjacent island-arc zones, and in the regions of development of supersubduction tonalite-granodiorite massifs in the continent-marginal zones.


V. L. Tauson, O. I. Bessarabova, R. G. Kravtsova, T. M. Pastushkova, and N. V. Smagunov
Keywords: Gold, forms of occurrence, mineral synthesis, crystals, pyrite, specific surface
Pages: 57-67

Abstract >>
It is shown on the basis of experimental data that, to define a structural component of Au impurity, it must be isolated from the total concentration of uniformly distributed Au, chiefly in its sorption form. For this purpose the curve "content of the uniformly distributed Au form vs. mean specific surface of crystal" is to be extrapolated to the domain of such values at which the effect of the surface is negligibly small. In studying gold impurity in pyrite the optimal specific surface is 6 cm2/g. This approach can also be applied to natural minerals. By the example of pyrites from Au-Ag deposits in northeastern Russia the problem of "invisible" gold has certainly been solved: Its appearance in pyrite is exclusively due to sorption rather than to Au incorporation into mineral structure. Comparison of experimental and natural data shows that the mechanism of impurity absorption is the same in both cases and it is involved with the active role of the crystal surface and surficial defects. The high concentrations of "invisible" gold in pyrite are caused by its sorption of intermediate compounds and complexes containing Au and its accessory elements (first of all, As). On decomposition they leave Au0 micro particles and, possibly, more complicated metastable Au-bearing micro phases on the surface.


V. A. Kontorovich
Keywords: Neocomian deposits, seismogeological model, Neocomian petroleum potential, southeastern West Siberia
Pages: 68-77

Abstract >>
In the latitudunal Ob' region and in northern West Siberia, the most petroleum-promising Neocomian deposits have been explored for decades.
In southeastern West Siberia, where hydrocarbon deposits are largely concentrated in the Upper Jurassic, the Neocomian geologic structure has never been modeled on the basis of complex data on seismic prospecting and deep drilling.
As part of the present study, we performed complex interpretation of materials of regional seismic prospecting and well survey, whose results were the first step to a regional seismogeological model for the structure of Neocomian deposits in southeastern West Siberia.
This work also addresses the questions related to estimation of the Neocomian petroleum potential in southeastern West Siberia. In particular, analysis of geological and geophysical materials suggests that the formation of hydrocarbon deposits is favored largely by tectonically active zones, with fractures crossing the Jurassic and Cretaceous deposits and capable to play the role of channels for migration of hydrocarbons from oil-producing rocks of the Bazhenov Formation to younger horizons.


A. V. Ladynin, A. N. Vasilevskii, A. F. Pavlov, and A. A. Popova
Keywords: Vector magnetic survey; geomagnetic field (GMF): vector modulus, declination, inclination, variations in GMF; fluxgate theodolite, mira azimuth, survey network, measurement precision, survey efficiency, estmation of parameters of anomalous bodies
Pages: 78-89

Abstract >>
An effective and precise technique for vector magnetic survey has been developed for searching and prospecting for iron-ore deposits, using equipment including a fluxgate theodolite to measure declination and inclination and a proton (or quantum) magnetometer to measure the modulus of geomagnetic field vector. A high efficiency of survey, comparable with the performance of M-27 magnetometer component surveys, is reached by optimization of declination and inclination measurement with a fluxgate theodolite and by minimization of the number of astronomical measurements of the geographical azimuth of mira (profile). The accuracy of angular measurements corresponds to the requirements of quality of measured diurnal variations in the geomagnetic field. The actual accuracy of all components of magnetic induction vectors under the conditions of strongly differentiated magnetic field of iron-ore deposits depends significantly on the accuracy of tying the measurement points in plan and on the precision of coincidence of the fluxgate magnetometer with the sensor of the module magnetometer. Analysis of accuracy of vector measurements will be the subject of a special paper. Here, only tentative data are given.
Our technique is tested on the small Samson magnetic deposit in the Khakass Autonomous Region. It appeared to be reliable in estimation of orebody parameters from results of interpretation of vector magnetic anomalies by selection of 2D models for types of beds varying in magnetization, size, position in plan, and depth. The reliability of the estimates of parameters of anomalous objects is illustrated by variants of selection based on various elements of anomalous field. A way is directed to construct an algorithm for solving inverse problems of vector magnetometry for complicated 3D bodies, taking into account mutual magnetization of their parts by own fields of the deposit, which is the subject of a special study.