Home – Home – Jornals – Russian Geology and Geophysics 2012 number 7

The Pb-Zn deposits in the Lo Gam structure, northeastern Vietnam, account for >80% of all the Pb and Zn resources of Vietnam. All the deposits make up four isolated ore districts (Thai Nguyen, Cho Don, Cho Dien, Na Son), which can be combined in one metallogenic zone extending for >100 km from southeast to northwest. The Pb-Zn deposits in all the ore districts show some similarity to stratiform (Mississippi-type) deposits: confinement to Devonian carbonate sediments; localization at the intersection of faults of different orientations; vein and stockwork (pocket-vein-disseminated) morphology of the mineralized zones; evidence for hydrothermal-metasomatic formation (carbonate-rock marbleization, quartz-carbonate veins, etc.); and low and moderate mineralization temperatures (<250?C). On the other hand, some differences from stratiform deposits are observed: widespread occurrence of Permo-Triassic igneous rocks in the above ore districts; absent tabular orebodies, which are typical of stratiform deposits; large set of trace elements (In, Bi, Sb, Au, Ag, Cu, Cd) not typical of stratiform deposits; and an endogenic primary source, as evidenced by the isotope composition of sulfur (δS³⁴ = 2.68 ‰), which is close to the meteoritic one, and a set of trace elements, which are mainly of deep genesis. All this indicates that the above Pb-Zn deposits within the carbonate units are low- and moderate-temperature hydrothermal-metasomatic products associated with active magmatism which took place in this region in the Permo-Triassic.
The differences in the mineral composition of the deposits, as well as in the trace-element set and contents at different deposits, clearly indicate an intricate ore formation process and the relation of the deposits with magmatism of different compositions. The simple mineral composition and the limited set of trace elements (Cd, Ag, Sb, As) at the Lang Hich deposit are closer to the characteristics of stratiform deposits. Also, manifestations of magmatism are almost absent here. On the contrary, unusually high In (75.8), Sn (307.5), Cu (1080), Ag (157.7), Bi (99), and As (13,650) contents were observed at the deposits of the Cho Don and Cho Dien districts, with widespread granitoid magmatism in the Phia Bioc complex. Rare-earth mineralization (orthite) and high Mo, Re, and Rb contents at the deposits of the Na Son district are probably due to the widespread occurrence of stratified alkaline volcanics and their subvolcanic analogs, which belong to the Pla Ma complex (ξ γPZ₂pm).

The Lukinda dunite-troctolite-gabbro massif in the Selenga-Stanovoy superterrane on the southeastern framing of the Siberian Platform was earlier considered Precambrian. The performed ⁴⁰Ar/³⁹Ar dating of the massif plagioclase yielded an Early Permian age (285±7.5 Ma). The main specific petrochemical features of the intrusion rocks during their crystallization differentiation are an increase in SiO₂ and CaO contents and a decrease in FeO_tot content, with TiO₂ content remaining low and showing minor variations. A specific geochemical feature of the Lukinda massif ultrabasite-basites is a slight domination of LREE over HREE, with (La/Yb)_N = 1.0-8.2. The depletion of the massif rocks in LILE (except for Sr and Ba), REE, and HFSE suggests that the massif formed on an active continental margin.

Polished sections of ferromanganese crusts on underwater rises in the Sea of Japan were studied on a JXA8100 electron microprobe. Mineral phases of REE have been detected. They have a Ln₂O₃ chemical composition, where Ln is La-Ce-Pr-Nd, La-Ce-Nd, or, much more seldom, La-Ce and La-Ce-Pr. With regard to the same chemical composition of REE grains in the ore crusts and basalts from Medvedev Volcano, it has been concluded that these REE were supplied during postvolcanic gas-hydrothermal processes.

We consider the formation of the Dal'nii eluvial gold placer (Bol'shoi Anyui ore-placer district, western Chukchi Peninsula), related to the Dal'nyaya gold-bearing porphyry Mo-Cu occurrence. The Dal'nii placer is located within the transition between the Kur'ya Ridge and Anyui basin, which has been relatively stable at the recent (Pliocene-Quaternary) tectonic stage. Minor recent uplift determined the slight denudation of interfluves, the leading role of eluvial processes in the formation of a loose cover on them, and the preservation of the relict matter of pre-Pliocene chemical-weathering crusts (including the oxidized zones of orebodies) in present-day eluvium. The Dal'nii placer consists of relict weathering crust placers altered by recent eluvial processes in different degrees. Therefore, it is relatively rich in metal, whereas the primary lode contains mainly fine-grained gold, which is almost not released from ore under periglacial lithogenesis in present-day interfluves. We suggest calling this genetic type of placers "residual-eluvial." The primary lodes being highly eroded (during the formation of residual concentrations, which serve as an intermediate reservoir for these placers), residual-eluvial placers or their parts might not be directly related to specific orebodies at the present-day level of erosional truncation.

The first data on the silver content in volcanics of the West Siberian Plate are presented, and data on basalts of the Siberian Platform are supplemented. The silver contents in all studied rocks do not depend on the fractionation of initial melts and contamination of the host rocks and average 0.07-0.10 ppm. The high silver contents can be associated only with sulfide formation.

In this paper we present the results of generalization of paleomagnetic data for the territory of the Siberian craton and its folded framing that were obtained during the last fifteen years. We propose a new version of the apparent polar wander path of the Siberian continental plate, including the interval from the Mesoproterozoic-Neoproterozoic boundary up to the end of the Mesozoic. The constructed path forms the basis for new concepts of the tectonics of the Siberian paleocontinent and the paleooceans that surrounded it. We present a series of paleotectonic reconstructions based on paleomagnetic data, which not only displays the paleogeographic position of the Siberian continent but also reveals the features of the tectonic evolution of its margins during the last billion years. In particular, it has been established that large-scale strike-slip motions played an important role in the tectonic regime of the continental plate at all stages of its development.

The shape of a plume conduit produced by melting solid paraffin block above a local heat source was studied experimentally as a function of the relative thermal power of the plume Ka = N/N ₁, where N is the power of the plume source and N1 is the power corresponding to the amount of heat transferred by conduction through the plume conduit to the surrounding solid paraffin block. The limiting power of the plume source at which the plume erupts at the Earth's surface (N _lim1 = (1.35-1.60) · 10¹⁰ W) and the power at which the mushroom-shaped plume head formed at the base of the refractory layer (N _lim2 = (1.78-1.90) · 10¹⁰ W) with no horizontal mantle flow were determined. The dependence of the diameter of the base of the plume on the Ka number was established. The Ka value and the diameter of the plume base were determined for the Hawaiian and Iceland plumes, for the plume responsible for the formation of the Tunguska syneclise and for the McKenzie and Central Atlantic continental plateau-basalt provinces, and for the Ontong Java and Manihiki oceanic lava plateaus.

Dielectric spectra of water-containing rocks forming a mineral framework of the oil-saturated bed (clay minerals amount to <16%) were measured in the frequency range from 500 to 15 GHz and in the temperature range from 20 to 80?C. A physical model for computing spectra of the dielectric permittivity and effective conductivity of the studied rocks has been elaborated. Its input parameters are water content (salt solution) and temperature. The errors of the computed spectra relative to the measured ones are estimated.

We formulate the fundamentals of the geothermal method for determining the hydrate saturation of bottom sediments. According to laboratory experiments (A.A. Trofimuk Institute of Petroleum Geology and Geophysics, A.V. Nikolaev Institute of Inorganic Chemistry), detecting gas hydrates in bottom sediments requires measurement of thermal conductivity at least twice at one bottom site, using a cylindric probe with different heater power values. Changing the latter permits controlling gas hydrate stability and instability. A low-power probe does not destroy gas hydrates and permits measuring the true thermal conductivity of the sediments. Increasing heater power destroys gas hydrates near the probe and drastically increases effective thermal conductivity. Comparison between true and effective thermal conductivity clearly shows the presence of gas hydrate in the sample or their absence from it. A technique was proposed for the quantitative interpretation of changes in the temperature field of a cylindrical probe. It permits quite a precise determination of the mass of gas hydrate that decomposed in the layer surrounding the probe over a certain period. Also, it permits a rough estimation of the gas hydrate content in the sediments. Thermal conductivity can be measured in the field with submersible multichannel thermoprobes, which are commonly used to study the heat flow through the bottom of water basins. Now it is important to perform field experiments, so that we gain the necessary experience with the geothermal method.

This study provides a compilation of the paleointensity records for the Cretaceous period derived from sediments of the Russian Plate and adjacent areas. The paleoinetensity values were calibrated using the laboratory redeposition experiments. Remarkable differences in the relative paleointensity variations were detected at the Berriasian-Early Barremian, Late Barremian-Santonian, and Early-Late Maastrichtian boundaries. In the Berriasian-Early Barremian interval, the paleointensity varied stochastically, with the amplitude of about 1 Н ₀ and the mean value of 0.63 Н ₀ ( Н ₀ is the present-day geomagnetic field intensity assumed to be 40 μT). The records for the Barremian-Santonian give a picture of the geomagnetic field with alternating high- and low-amplitude features. The mean paleointensities remain constantly high (being on average 0.87 Н ₀), and intervals of low-amplitude variation alternate with the pronounced bursts (3.5 Н ₀). The Late Maastrichtian interval is characterized by high-amplitude paleointensity variations (4 Н ₀) and a sharp drop toward the end of the interval. All records show remarkable similarities near the boundaries between geological time intervals, which are an increase in the amplitude and mean values of intensity at the end of intervals followed by a decrease toward the beginning of the subsequent interval.