Home – Home – Jornals – Russian Geology and Geophysics 2012 number 9

The origin of gold and silver deposits in the Southern Kamchatka ore district is considered in terms of a quantitative model of the dynamics of volcanogenic orthomagmatic fluid systems (VOFSs). This model takes into account structural, fluid dynamic, and thermophysical features of phase evolution in hydrothermal fluid systems differing in geometry and structural conditions of the discharge on the surfaces of volcanic edifices. It is shown that VOFSs forming sulfide-rich gold and silver deposits have no stationary impermeable caps in their discharge areas. Rather, for the most part, narrow regions of junction of phase fronts form in their interiors and migrate to the surface of volcanic edifices.
Three geothermal system types are predicted by the example of plane and conical fluid conductance zones with a cap horizon: (I) where the shallow decompression boiling zone does not arise at all because of large lateral heat loss, (II) where subsurface decompression boiling zones appear at the beginning of the heat wave formation and then such a zone is practically confined to the cap rocks, and (III) where a quasi-stationary decompression boiling zone forms after an initial instability period or approach to a thermal equilibrium. Fluctuations or oscillations of decompression boiling zone fronts within a range of depths can exist in type III systems.

The microstructure and magnetic properties of accessory Fe-Cr-spinels from the Kytlym massif of the Urals platinum-bearing belt were studied. Atypical Fe-Cr-spinels in the form of magnetic microareas in grains of primary nonmagnetic Fe-Cr-spinel have been revealed for the first time in the bed dunites of the Kytlym multiphase concentrically zoned massif, North Urals. These spinels are responsible for the magnetic properties of the dunites.
It has been established that the microareas are separations in solid solution Fe²⁺ (Cr_{2- x} Fe_x ³⁺)O₄, which are enriched in Fe³⁺ and are probably an intermediate product of the transformation of primary accessory Fe-Cr-spinel during the formation of the dunite massif. These are magnetic microphases with a particular chemical composition, cation distribution, and corresponding reversed crystal lattice, which determine the main magnetic properties of the microarea: the magnitude and direction of magnetization vector and Curie temperature.
The formation of this earlier unknown type of magnetic Fe-Cr-spinel is probably conjugate with the formation and concentration of PGE mineralization in the bed dunites of the Kytlym platinum-bearing massif.
The presence of such magnetization carriers in rocks and ores must be taken into account in geophysical research at the Urals chromite and platinum-chromite deposits.

The zoning of massive orebodies at Cu-Ni sulfide deposits such as Noril'sk is commonly explained by fractional crystallization of sulfide magmatic melt. On the theoretical description of fractionation of its components, the results of mineralogical studies of orebodies are usually interpreted using the Rayleigh equation or its modification. But this equation is not applicable to describe crystallization of multicomponent melt and cocrystallization of several phases. In this work we present strict equations describing the distribution of components in a directly crystallized sample.
We analyzed the influence of phase reactions on the successive formation of phases during crystallization and on the formation of primary zoning in the sample. This approach permits one to compute the component distribution curves and the crystallization paths by the quantitative phase diagram model. An experimental study of fractionation in the systems Fe-Ni-S and Cu-Fe-S was carried out. They can be regarded as systems modeling formation of Ni- or Cu-rich sulfide ores. Such studies also yield qualitative and quantitative information about the phase diagrams of geochemical systems.
We demonstrated that directed crystallization can be applied to determine the equations of phase reactions and the dependence of partition coefficients on the melt composition and to construct the paths of crystallization and evolution of the tie-line position during one-phase and cotectic crystallization. By the example of the system Fe-Ni-S, all possible types of sample zoning after fractional crystallization are shown.
The main regularities of fractionation have been formulated, which are also applicable to multicomponent systems, e.g., Cu-Fe-Ni-S, which is widely used on the modeling of formation of zonal Cu-Ni sulfide ores.

The lower Katun' area has a complex neotectonic framework, with the largest fault zone of Katun' consisting of several en-echelon graben segments. Late Pleistocene sediments that fill the Katun' Fault bear signature of earthquake-induced soft-sediment deformation (seismites). Deformation due to seismic triggers can be discriminated from nonseismic one on the basis of special features and be related to prehistoric earthquakes according to a number of criteria. The observed deformation inside and outside burial mounds of the Chultukov Log-1 group may result from an earthquake that occurred at the end of the first millennium BC. Fault scarps in Late Pleistocene sediments, as well as deformed Iron Age tomb patterns, indicate that the Katun' lower reaches can have experienced past earthquakes of intensity at least 5 or 6 and magnitudes from 4.5 to 6.0.

The paper presents magnetobiostratigraphic data on the Cretaceous sediments stripped by well 8 in the southern side of the Om' basin within the Om'-Lar'yak facies zone (southern West Siberia). The biostratigraphical data show that the sediments under study formed in the Albian-Maastrichtian. The component analysis of natural remanent magnetization, based on thermal demagnetization and demagnetization with an alternating magnetic field, revealed the characteristic magnetization component. This confirms the paleomagnetic data used to compile the Cretaceous paleomagnetic section of the well. A paleomagnetic description was obtained, and a magnetobiostratigraphic key section of the Cretaceous sediments of the well was compiled on the basis of comprehensive data. It comprises five Upper Cretaceous regional horizons and same-named formations (Pokur, Kuznetsovo, Ipatovo, Slavgorod, Gan'kino), which have not been studied paleomagnetically at all in West Siberia. The magnetobiostratigraphic section comprises Albian-Maastrichtian stratigraphic units (43.5 Myr) and consists of three magnetozones. For example, the Pokur, Kuznetsovo, and Ipatovo Formations (total thickness 210 m), which show normal polarity with small reversed-magnetization horizons, form one long normal-polarity zone, N (al-st). The Slavgorod and Gan'kino Formations (total thickness 75 m), separated by a sedimentation gap, form two reversed-polarity magnetozones, R ₁ (km) and R ₂ (mt).
Reference datums (paleontologically well-constrained magnetozones) were used to correlate the magnetobiostratigraphic section with the common magnetostratigraphic and magnetochronological scales. The long normal-polarity magnetozone N (al-st), spanning the Albian, Cenomanian, Turonian, Coniacian, and Santonian, matches the Dzhalal hyperzone and chron C34 (~112-83.6 Ma). The reversed-polarity zones, spanning most of the Campanian ( R ₁ (km) (Slavgorod Formation) and Maastrichtian R ₂ (mt) (Gan'kino Formation)), match chrons C33(r) and C31(r) in the absolute chronology (~83.6-80 and 71-68.5 Ma, respectively) with a gap two chrons long (C33(n) and C32).

The Gunzhin Fault is a tectonic structure in the southern Hentiyn arched uplift (central Mongolia). A fragment of this fault, exposed by two paleoearthquakes, is traced northeastward for ~25 km from the northern periphery of Ulaanbaatar. The first comprehensive seismogeological studies here were aimed at determining the seismic potential of seismogenic structures on the basis of the dislocation parameters. Judging by the strain amplitudes and length, earthquakes with a magnitude of >7.0 might be generated here. The paleoearthquake was dated by the radiocarbon method at 7800-4667 B.C. Seismogenic zones with a high seismic potential were identified in central Mongolia for the first time. Dextral strike-slip faults were observed here among active NE-trending faults, suggesting the presence of E-W compressive stress.

The potential field data are considered the main supporting factor in the geophysical exploration process for detecting and evaluating the subsurface structures. In this respect a detailed land magnetic survey was performed in the area that was subjected before to seismic investigations. The main target of this study is to detect the deeper subsurface structures and to investigate possible relations of these structures with earthquake activity.
The RTP aeromagnetic map was used for detecting the regional extension of the interpreted structures from the land magnetic survey. The interpretations were performed by RTP land and aeromagnetic maps using filtering technique, least-squares separations, tectonic trend analysis, spectral analysis, Werner method, and Euler and two-dimensional techniques. The results show that the main dominant tectonic trends are 35?N- 45?W, 45?N-65?E, E-W, and Aqaba trends.
Moreover, two seismic lines WQ85-31B and 127 were interpreted, and the location of these lines was matched with the deduced tectonic map. The results show that there is a great matching between the location of the faults deduced from both the geomagnetic and seismic data. The obtained results show a complete agreement with the well logging data.
Furthermore, these structures are correlated with the recorded earthquake activities by National Egyptian Seismological Network (ENSN). The correlation implies that the studied area is more stable than other adjacent areas in the northern parts of Egypt close to the Mediterranean Sea and Nile River delta.

The main natural and human-induced radiation factors were assessed on the basis of long-term targeted radioecological studies in the Central Ecological Zone of the Baikal Natural Territory as a world heritage site. We identified areas with a problematic level of different radiation parameters determining the current radiation environment. Such areas should be taken into account in the development and implementation of nature management plans in the Baikal region, including ecotourism.

Synthesis of the available stratigraphic data on the Baikal basin sediments exposed around the lake and their correlation with offshore lake sediments and with onshore sections in the Baikal Foredeep allows a new perspective of the Baikal rift history. The basin sediments on the Baikal shore comprise three tectonic-lithologic-stratigraphic complexes (TLSC), which correspond to three seismic stratigraphic sequences (SSS) in the lake sediments and to three complexes in the Baikal Foredeep.
The oldest unit, TLSC-1, has a particular lithology being deposited in an environment which never repeated in the later history of the area. This proves the validity of its lithostratigraphic correlation with Masstraichtian-Early Oligocene sediments of the Baikal Foredeep constrained by biostratigraphy. Further support comes from isotope dating, paleontology, and other evidence.
Unlike seismostratigraphy-derived models, SSS-1 is correlated in the new model with TLSC-1 rather than with the Tankhoi Formation (which actually represents TLSC-2), and the onset of rifting is placed at the Late Cretaceous-Paleogene rather than the Oligocene (or Miocene). Thus, the Baikal rifting began prior to the India-Eurasia collision, and the first rifting pulse originally had other causes. This inference agrees with fission-track apatite thermochronology indicating Cretaceous ages of samples from the Barguzin rift basin on the northeastern flank of the rift system.
Rifting was developed successively in three different tectonic settings and was driven by different geodynamic mechanisms at each stage. First it was a passive response to Late Cretaceous-Eocene distributed continent-wide extension in Asia (purely passive rifting). At the second stage spanning Late Oligocene-Early/Late Pliocene time, the area was subject to compressive impact from the India-Eurasia collision, which propagated from the southwest since the Eocene and reached the region about 30 Ma to take control over its geodynamics (conventionally passive "impactogenic" rifting). Finally, the Pliocene-Quaternary evolution has been driven by extension from a local source associated with hot mantle material rising to the base of the rifted crust (active rifting).
The major rifting stages are further subdivided into substages: two substages in the second stage with the boundary at ~10 Ma (Middle-Late Miocene) and three substages in the third stage, with boundaries at 1.0-0.8 and 0.15-0.12 Ma. The stages and substages of rifting are separated by events of tectonic activity and stress reversal when additional compression produced folds and shear structures. The events that mark the stage boundaries show up as gaps, unconformities, and deformation features in the deposition patterns.
Thus, the three units of synrift sediments composed of eluvium, early molasse, and late molasse, respectively, were deposited during preorogenic, early orogenic, and postorogenic stages of rifting driven by passive (first purely passive and then "impactogenic") and active mechanisms.
The new model of the Baikal rift history agrees with data obtained by different other methods.

The problem of the Late Meso-Cenozoic tectonics and geodynamics of the southern framing of the Siberian Platform is discussed. This area abounds in Late Meso-Cenozoic structures composed of sedimentary deposits. Analysis of voluminous geologo-stratigraphic, geophysical, and geochronological (track dating) data showed that these structures, including the Baikal Rift Zone, resulted fr om the far-range impact of two collisions related to the prolonged convergence of the North China and Indian continents with Eurasia, which took place in the Late Jurassic-Paleocene and Cenozoic, respectively. The synchronous occurrence of two great tectonic events led to the formation of a complex structure and a sedimentary basin, which are of different geodynamic nature and cannot be united into the same sequence of Late Cretaceous-Cenozoic formation of the Baikal Rift Zone.
The Baikal system of shears and conjugate rifts formed in the Pliocene-Quaternary as a result of the far-range impact of the Indo-Eurasian collision. The latter led to the accumulation of the upper layered undeformed seismostratigraphic complex in all three Baikal basins. The sediments in Central Baikal are up to 3 km thick, and in the Selenga River valley they reach 5-6 km in thickness. The active intracontinental rift structures are characterized by zonal sedimentation. The middle layered deformed seismostratigraphic complex 1-1.5 km thick is recognized in all three Baikal basins and is similar to the Upper Oligocene-Lower Pliocene sediments of lacustrine-littoral, deltaic, and lacustrine facies widespread throughout the Baikal and Altai-Sayan regions and in Mongolia, wh ere they formed in the environments of large lake systems. As a result of the deformations caused by the Indo-Eurasian collision, the Upper Oligocene-Lower Pliocene sediments were involved in the Pliocene-Quaternary ramp and inilateral-ramp structures in intermontane basins of the Altai-Sayan region and in Mongolia. The lower seismically transparent seismostratigraphic complex occurs only in South Baikal and Central Baikal. It is 1 km thick in the east and up to 4-5 km thick in the west. The complex is a fragment of the Late Cretaceous-Paleogene lacustrine-river sediments of the large fore-Baikal piedmont basin, which formed at the final stage of evolution of the vast Mongolo-Okhotsk orogen.