Geologiya I Geofizika, 2002, V 43, N 1, January.
PETROLOGY, GEOCHEMISTRY, AND MINERALOGY
YAKUTIAN KIMBERLITE PROVINCE: POSITION IN THE STRUCTURE OF THE SIBERIAN
CRATON AND COMPOSITION OF THE UPPER AND LOWER CRUST
O. M. Rozen, V. P. Serenko, Z. V. Spetsius, A. V. Manakov, and N. N. Zinchuk
3
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.
Lower crust, upper crust, xenoliths in kimberlites, collision,
isotope geochemistry, Archean, Proterozoic, Siberia
FLUID CONVECTION IN SEDIMENTARY-BASIN RESERVOIRS ON THERMAL ACTION
OF DIKES AND SILLS
O. P. Polyanskii and V. V. Reverdatto
27
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 resrvoir 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.
Fluid flow, convection, sedimentary basin, numerical modeling
SUPERSUBDUCTION ANATECTIC GRANITOIDS OF THE URALS
G. B. Fershtater, F. Bea, N. S. Borodina, E. A. Zin'kova,
P. Montero, and E. S. Shagalov
42
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=0,7-1,0 Ptot) 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.
Subduction, anatexis, fluid, magmatism, substratum, restite, melt,
Urals
SEPARATION OF FORMS OF GOLD OCCURRENCE IN PYRITES BY STUDYING
STATISTIC SAMPLES OF ANALYTICAL DATA
V. L. Tauson, O. I. Bessarabova, R. G. Kravtsova, T. M. Pastushkova,
and N. V. Smagunov
57
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
exclusivey 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
microparticles and, possibly, more complicated metastable Au-bearing
microphases on the surface.
Gold, forms of occurrence, mineral synthesis, crystals, pyrite,
specific surface
GEOLOGY OF OIL AND GAS
REGIONAL MODEL FOR GEOLOGIC STRUCTURE AND PETROLEUM POTENTIAL
OF NEOCOMIAN DEPOSITS IN SOUTHEASTERN REGIONS OF WEST SIBERIA
V. A. Kontorovich
68
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.
Neocomian deposits, seismogeological model, Neocomian petroleum
potential, southeastern West Siberia
GEOPHYSICS
VECTOR MAGNETIC SURVEY IN PROSPECTING OF IRON-ORE DEPOSITS
A. V. Ladynin, A. N. Vasilevskii, A. F. Pavlov, and A. A. Popova
79
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.
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