20_2

Volume 20, 2, 2001

Geodynamics and Geophysics
I.I.Abramovich, S.D. Voznesenskiy and N.G. Mannafov. Geodynamic evolution and metallogeny of the Okhotsk-Kolyma segment of the Okhotsk-Chukotka volcanogenic belt 3
A.G. Nurmukhamedov. The geoelectrical section in the upper part of the earth's crust along the Nizhnyaya Oblukovina-Andrianovka profile (Kamchatka) 13
V.B. Kaplun. Results of geoelectrical monitoring by the method of magnetotelluric sounding (Zabaikalskoye settlement, Khabarovsk Territory) 24
F. I. Manilov, Yu. F. Manilov and V. A. Makhinina. Structural peculiarities of the upper consolidated crust in Priamurye and adjacent areas from results of gravity field reduction 34

Petrography and Geochemistry
S.V.Yefremov and D.V.Dudkinskiy. Petrography, geochemistry, and the origin model for Cretaceous volcanites of Telekay district, Central Chukotka 43
A.Yu. Antonov. Geochemistry of Au, Ag, Ge,Ga in the Quaternary volcanic rocks of the Kurile island arc 62

Metallogeny
E.L. Shkolnik, L.B. Khershberg, Ye.V. Mikhailik, O.V. Chudaev, M.M. Zadornov, M.Ye. Melnikov and V.A. Polinovskiy. Deposition conditions, formation regularities, and peculiar concentrations of cobalt- rich ferromanganese crusts on the guyots of Magellan seamounts (Pacific Ocean) 73
A.N.Kalyagin, P.Ya.Tishchenko, A.Yu. Gukov, T.I. Volkova, L.N. Kurilenko and R.V. Chichkin. The nature of the ferromanganese formations of the Laptev Sea 87

Reviews
V.G.Moiseenko, V.A. Stepanov. The book "Platinum metal deposits in Russia" 97
V.I.Goncharov. Gold ore giants of Russia and the World 100
L.V.Eirish. The book "The BAM (Baikal-Amur Railroad) gold ore deposit" 101
V.G.Moiseenko, A.A.Sidorov. The origin of the Earth, and the nature of its endogenic activity 102

Anniversaries
Lev Isaakovich Krasny (90th birth anniversary) 105

I.I.Abramovich, S.D. Voznesenskiy and N.G. Mannafov
Geodynamic evolution and metallogeny of the Okhotsk-Kolyma segment of the Okhotsk-Chukotka volcanogenic belt

A great deal of factual data has been investigated as part of the Okhotsk-Kolyma geological maps at 1:500 000 compiled in the last few years. This provides a possibility to specify the geodynamic history and metallogenic potential of the Okhotsk-Kolyma segment of the Okhotsk-Chukotka volcanogenic belt. The volcanogenic belt development can be regarded as a result of another stage of the Koni-Murgal subduction zone from the Late Jurassic when the Kula plate velocity rose strongly. As a consequence the tectonic stress increased to high values, the lithosphere permeability reduced significantly. And therefore the whole frontal part of the belt remained avolcanic. Only in the belt's rear part volcanogenic molasse accumulated. After the Benioff zone's jumping oceanward in the Late Jurassic - beginning of the Early Cretaceous the volcanic activity embraced the belt including the territory under the Okhotsk sea. The two-stage belt history is reflected in its petrochemical and metallogenic zonation.
The assumption that ore components being under the effect of thermo- and barogradient fields migrate within the asthenosphere in the period of tectono-magmatic activity agrees satisfactorily with the time-space regularities of ore deposits distribution throughout the belt. This allows us a to make an estimate of the probability of discovering new different types of ore deposits in the region of interest. Massive sulfide-polymetallic deposits can be keyed in the amagmatic extensions of the transtension structures in the deep rear of the belt. Some local permeability zones and structural traps are a good reason for the prospects of the mercury mineralization in the region. Intense subduction was not favourable for separation of gold from silver within the mantle fluid flows, resulting in the deposit composition. A possibility to discover deposits of one or another gold/silver ratio can be inferred from petrochemical features of the host rocks. Structures like Taryn subvolcano and the Kuidusun volcanic field are the surface expressions of paleoasthenospheric traps. The points where they are intersected by translithospheric faults are most promising with reference to tin deposits.

A.G. Nurmukhamedov
The geoelectrical section in the upper part of the earth's crust along the Nizhnyaya Oblukovina-Andrianovka profile (Kamchatka)

The results of magnetotelluric sounding along the sublatitudinal profile crossing the central part of Kamchatka Peninsula are offered. Basing on the analysis of polar diagrams, amplitude and phase curves with reference to different directions, it is shown that regional transverse curves should preferably be used to study the upper part of the section. A geoelectrical section through the upper part of the earth's crust is suggested, and stratigraphic reference of the distinguished geoelectrical horizons is given.

V.B. Kaplun
Results of geoelectrical monitoring by the method of magnetotelluric sounding (Zabaikalskoye settlement, Khabarovsk Territory)

This paper offers the results of statistical analysis of magnetotelluric monitoring conducted at one site during 9 months. Involving results of previous investigations, a longitudinal l and a transverse amplitude curve have been constructed in a period range of lgT = -0.6¸4.0 [s]. A geoelectrical model has been computed on the basis of these curves. Conductive layers in the lowermost strata of the earth's crust (30-35 km) and upper mantle (200-250 km) have been distinguished. The third conductive layer is inferred at depths of 100-120 km identified with the asthenosphere.
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DF. I. Manilov, Yu. F. Manilov and V. A. Makhinina
Structural peculiarities of the upper consolidated crust in Priamurye and adjacent areas from results of gravity field reduction

Gravity field map with elimination of contrast density heterogeneities of the upper crust is compiled for the southern part of the Russian Far East and Northeast China bounded by the longitude 120°E and latitude 42-62°N. This increased efficacy of gravimetry for deep geophysical studies due to more clearly displayed deep crustal peculiarities. The scheme is constructed for deep-seated faults distinguishable on the map compiled. The structural features of the crust in the Songliao, Amur-Zeya, Middle Amur and Uchur-Maya sedimentary basins are revealed.

S.V.Yefremov and D.V.Dudkinskiy
Petrography, geochemistry, and the origin model for Cretaceous volcanites of Telekay district, Central Chukotka

The petrography and the geochemistry of Cretaceous volcanites of the Telekay district have been examined, and an origin model has been suggested in this paper. According to these data, volcanites are divided into three genetic groups: derivatives of the initial magma - andesites and latites; derivatives of the crustal magma -rhyolites; volcanites, which originated due to contamination of the initial magmas by crustal material - dacite, trachydacite. According to their geochemical peculiarities, volcanites of first and third groups correspond to derivatives of the shoshonite-latite and high-K calc-alkaline series. The rhyolites are high-K calc-alkaline and are similar to the Telekay granites, which are widely spread in this district.
The model of volcanites origin consists of three stages. The first stage includes intrusion of the initial melt of andesitic or latite composition into granitoid plutons of the Telekay complex. The second stage involves fusion of the granitoids in the thermal field of the initial magma and formation of rhyolite melts. The third stage implies the formation of dacitic and trahydacitic magmas owing to contamination of the initial melts by crustal materials.
The model was examined using petrographical, geochemical, mathematical, and thermodynamical methods. The obtained data correlate well with the evolution of the natural composition volcanites.

G.A.Yu. Antonov
Geochemistry of Au, Ag, Ge,Ga in the Quaternary volcanic rocks of the Kurile island arc

Au, Ag, Ge, and Ga distribution was investigated in volcanites of different acidity from 50 surface and submarine Quaternary volcanoes arcoss and along the entire strike of the Kurile island arc (IA). The average Au-concentrations (as opposed to Ag) in the basalts gradually decrease from the frontal zone to the rear volcanic zone of the Kurile IA, while in the andesites they increase. In the basalts it is most likely conditioned by the maximum saturation of the mantle magma-generating substrate in its frontal zone. The "reverse" zoning in the Kuril andesites can be explained only in the case when most of these rocks are the result of a mixture of the mantle basaltoid and crustal salic magmas. This indicates a greater Au-saturation of the crustal magma-generating substrate in the IA rear volcanic zone. Basalts of all zones in the "bend" part of the Kurile IA center are maximally Au-saturated.
The maximum average variations of Au-concentration are often found in the basalts, less often - in andesibasalts, still less often - in andesites. The composition of volcanites depends on the Au- and Ag-saturation of the initial magmas and on their saturation in volatiles (primarily, S and Cl) . A decrease in magma alkalinity assists emanation accumulation and intensity of Au- and Ag removal from the melt with volatiles, and determines its increased ore-generating potential of these elements. A great number of causes stipulated almost uniform Ge- and Ga dispersion at all stages of magma evolution.
Thus, the frontal zone of volcanism of the Kurile island arc and also the entire transverse "bend" zone of the IA center are most favorable for ore occurrences. The noticeable Ag-Ge-enrichment and Au-impoverishment of North Kurile volcanic rocks as opposed to South Kurile rocks reflect a different Ag-Ge specialisation of these regions.

IE.L. Shkolnik, L.B. Khershberg, Ye.V. Mikhailik, O.V. Chudaev, M.M. Zadornov, M.Ye. Melnikov and V.A. Polinovskiy
Deposition conditions, formation regularities, and peculiar concentrations of cobalt-rich ferromanganese crusts on the guyots of Magellan seamounts (Pacific Ocean)

Dimensions, morphology, and depth levels of cobalt-rich ferromanganese crusts (FMC) are analyzed on the basis of the detailed studies of the guyots of Magellan Seamounts in the Pacific. Hydrological and hydrochemical environment, substrate, and bacterial biota are responsible for the ore deposition process. Regional and local relations between some kinds of FMC in the guyot orebodies are discussed.

A.N.Kalyagin, P.Ya.Tishchenko, A.Yu. Gukov, T.I. Volkova, L.N. Kurilenko and R.V. Chichkin The nature of the ferromanganese formations of the Laptev Sea

In sediments of the Laptev Sea unknown earlier ferromanganese formations have been found. On the basis of structural-textural external signs they were in five groups: 1) tube- and spindle-shaped pseudomorphoses over and within invertebrates; 2) nuclear and non-nuclear nodules; 3) flagellum- and tube-like skeletons of polychates; 4) flat and flattened bark-like nodules and crusts; and 5) micronodules. All types of ferromanganese formations are sorted in three main genetic series: eigenferrous formations of autochthonous (polychates, goethite micronodules) and allochthonous (nuclear nodules) nature; ferromanganese nodules formed under mild hydro-geodynamic conditions at the boundary of the “sediment-seawater” geochemical barrier; and ferromanganese formations produced under conditions of the changing physico-chemical medium. Ferromanganese formations of allochthonous type have signs of littoral zones. They contain both bivalent and trivalent iron and have a low degree of oxidation of manganese in comparison with the autochthonous type formations. Manganese minerals with a moderate degree of oxidation are represented by vernadite and buserite. Such features of iron and manganese indicate different conditions of their formation and occurrence. The main distinctive feature of ferromanganese mineralisation of the Laptev Sea is the redox barrier – a water layer enriched in oxygen, and reduced sediments. This barrier provides favorable conditions for bacterial formation of ferromanganese ores. Understanding of the genesis of ferromanganese formations should be found in the study of organic matter treatment by bacteria.