Home – Home – Jornals – Russian Geology and Geophysics 2004 number 2

Another BDP drilling experiment was carried out in the winter of 1999 at 52

Detailed structural and petrological studies of the Shutkhulai metamorphic complex have shown inverse metamorphic zoning and growth of paleotemperatures up the crustal section within a given structure, namely, in the region of transition between gneisses, granite gneisses, migmatites of the upper slab, and metasedimentary deposits of the Oka series (the lower slab). Petrographic study of rocks of the Shutkhulai metamorphic complex, analysis of compositions of rock-forming minerals, and calculation of pressure and temperature of metamorphism, along with structural observations, suggest that structurally the complex is a combination of zones with different parameters of metamorphism not typical of the classic metamorphic zoning: a high-pressure zone (6.2-8.5 kbar) at lower temperatures and a low-pressure zone (3.5-4.2 kbar) at higher temperatures. On the basis of petrochemical study of the rocks making up the Shutkhulai complex, it was concluded that the whole volcanosedimentary unit was subjected to metamorphic transformation.
A qualitative model has been proposed to describe how the Shutkhulai metamorphic complex evolved during a continuous metamorphic process. It is supposed that at the initial stage of its evolution the deep-seated "hot" rocks were thrust over hypsometrically higher "cold" deposits, and the thrust slab exerted the maximum effect on the rocks of the underlying slab. Throughout this stage, metamorphism had an action on the most remote rocks of the transition zone, in which the maximum pressures have been recorded. At subsequent stages of metamorphic evolution, unloading of the overburden caused a drop in pressure in the upper slab and in the adjacent part of the transition zone. The model suggests that the inverted metamorphic isograds persist owing to a high rate of tectonic and metamorphic processes at which the geothermal gradients did not re-establish equlibrium.

Geology and geochemistry of the Karaoyuk and Terekta complexes of the Gorny Altai are described. It has been established that the Karaoyuk complex (D₃) is dominated by subalkalic metadolerites and rhyolites, which form a bimodal association typical of rifting zones. A conclusion is made that the rocks of this complex were formed from initial Cr-Co-Ni magma melts under conditions of back extension at the active continental margin.
The rocks of the Terekta complex (C₁) are represented by metagabbro-dolerites and metadolerites. Despite the most primitive composition of the initial Fe-Ti-V magma, they also formed under extension conditions which are interpreted as extensions in the dome of a granite-gneiss asthenolens.
The distribution patterns of Nd, Eu, and some other elements (Co, Ni, V) suggest the fractionation of the initial magma melts having formed the rocks of the complexes. It is supposed that the dike complexes have their own intrusive "roots" exposed in the Altai geologic structures, which are erroneously considered part of other igneous complexes of the region. The formation of the explored dike complexes is related to the functioning of a mantle column in the zone of back-arc basin on the active continental margin and to tensions above the gently submerging slab during the interaction between the Middle Paleozoic Siberian continent and Irtysh-Zaisan fragment of the oceanic crust of the Paleotethys.

Gold-bearing pyrite from ore metasomatites of an epithermal Au-Ag deposit was studied by Auger and X-ray photoelectron spectroscopy. The study showed the presence of a thin (~ 0.5 mu m) continuous film on the pyrite surface, which contains lithophile elements (Si, Al, K). Its composition varies with depth. The outer layer of the film is compositionally intermediate between sericite and adularia, which are present among the products of hydrothermal alteration of the quartz-feldspar groundmass. Comparison of the Auger spectra of natural and synthetic (hydrothermal) pyrites showed that the initial (juvenile) surface of hydrothermal or metasomatic pyrite is of oxide-sulfide composition and can contain ~10-20 at.% O. The high concentrations of evenly distributed Au on the crystal surface are due to the deposition of gold under the interaction of ore-bearing hydrothermal solution with the "acidic" pyrite surface containing SiO₂ and iron oxides. The chemical composition of the surface is a typomorphic indicator of its interaction with an ore-bearing hydrothermal solution. Along with the high content of "invisible" gold bound with the surface, it serves as a prospecting criterion. The experimental data confirm the presence of stable forms (thin-film precipitates, quasi-phases, or nonautonomous phases) on the mineral surfaces. These forms can be preserved throughout the geologic time and thus bear information about the consequence of physicochemical processes that occurred in the environment.

Au-Ag-PGE mineralization has been discovered in carbonized ultrabasic rocks (up to 9.7 wt.% C) from the Ospa-Kitoi ophiolites (East Sayan). Eight types of gold differing in the proportions of Au, Ag, Cu, and Hg have been recognized. PGE minerals are various Pt and Pd compounds and a Pt-Pd-Sn mineral stoichiometrically corresponding to atokite. Ar-Ar dating of muscovites from granite porphyries showed that carbonization proceeded 690 myr ago or even earlier. The isotopic composition of carbon of carbonaceous matter (CM) points to its mantle or crust-mantle source. The CM of carbonized ultrabasic rocks (CUR) includes graphite, graphitoid, shungite, ultradispersed graphite, and amorphous phase (sp³ hybridization). Gas inclusions in carbonized rocks are dominated by methane (60-100 %) and nitrogen, with CM present on the walls of vacuoles. Carbonization of ultrabasic rocks and formation of noble-metal mineralization occurred either synchronously with serpentinization of ultrabasic rocks or later. The latter process produced awaruite widespread in East Sayan ultrabasic rocks. The geodynamic setting in the study area suggests the formation of CUR in ophiolites with the participation of both a mantle source and carbonaceous black-shale deposits as a source of great amounts of carbon. This setting might occur where a subduction zone is associated with a mantle plume or hot spot.

Placer diamonds from the North (Vishera) and Central (Koivo-Vizhai) Urals, whose primary deposits are still unknown, were studied by IR spectroscopy. There is a relationship between the internal structure of diamonds and the content of structural nitrogen and hydrogen impurities in them. Diamonds from the Vishera and Koivo-Vizhai regions differ significantly from each other in the type of distribution of A-centers and the degree of nitrogen aggregation. This evidences that they formed from different (though of the same type) sources. Comparison of the contents of optically active centers in the studied crystals and rounded diamond crystals from other deposits shows that the Uralian placer diamonds are of kimberlite genesis.

The paper presents the results of SP, TEM, and VLF surveys in the Ol'khon region along two, 6 and 11.5 km, profiles which traverse major two-dimensional tectonic structures. The region is divided into a northwestern and a northeastern tectonic blocks separated by the Kuchulga fault. The northwestern (Chernorud) block is marked by a negative SP anomaly of hundreds millivolt coinciding with a flat electrical conductor detected by TEM soundings at depths between 120 and 450 m. The SP anomaly may be produced by a natural galvanic cell (a geobattery) in which electronic conductive graphite mineralization connects regions of different redox potentials in an electrolytic formation. The tectonic origin of the anomaly remains unclear and may be associated with shearing, mylonitization, and plastic flow in a regional-scale low-angle thrust. The southeastern block confined between the Kuchulga fault and the Baikal shore is geoelectrically uniform and much more stable tectonically than the Chernorud block. Horizontal conductors in the southeastern block were never observed before but may be expected to appear in TEM records from greater-moment sources and/or in MT data.

Efficient two-dimensional inversion of high-frequency electromagnetic data providing reliable estimates of geoelectrical parameters may improve the quality of log data interpretation. We suggest fast linearization algorithms for inversion of relative amplitude and phase difference nonlinear to the magnetic field components, to be used in axisymmetrical models with linear electrical conductivity functions instead of computationally inefficient complete forward solutions. We investigated the behavior of the integrand functions and obtained their asymptotic representation. The amplitude and phase difference estimated using the new linearization algorithm are accurate to at least 1 % over a broad range of model parameters.

Propagation of finite-amplitude waves in friable sands and clays and small-amplitude waves in clays is described by equations of motion using a long-wavelength approximation and differential rather than difference operators. Both linear and nonlinear propagation parameters of P and S waves depend on the microstructure of the medium, and this dependence has been theoretically predicted for the first time for clays of isotropic cellular structure. The energy of seismic waves is much lower than in explosions because of energy transfer into the low-frequency domain. Nonsimultaneous generation of P and S waves in explosions may cause difference in their frequencies. The reported theoretical results agree with the available experimental data.

The volcanogenic rocks of the Kalakan region are represented by: (a) effusions of the Late Mesozoic Vitim Series (trachybasalts), (b) associated subvolcanic intrusions (quartz syenite-porphyries), and (c) teschenites of the Tundak sill. Their ⁴⁰Ar/³⁹Ar ages suggest three stages of Early Cretaceous volcanism: Barremian (121-120 Ma), Aptian (120-114 Ma), and Albian (magmatic episode 1101 Ma). It is proven that the Tundak alkali basalts were separated in time from effusive and subvolcanic magmatites of two preceding stages.