Home – Home – Jornals – Russian Geology and Geophysics 2017 number 9

New characteristics and generalized geostructural, isotope-geochronological, and paleomagnetic data are presented for the first time for the territory of Jeannette Island in the De Long Archipelago. Until recently, this small rock outcrop in the East Siberian Sea, discovered in the late 19th century, has been in effect a blank spot on the geological map. Field work made in 2013 and subsequent laboratory analysis of the obtained sample collection show that the island has a volcanosedimentary section including many thin (up to several meters) dolerite dikes. These dikes are the main target of the study. Newly obtained 40Ar/39Ar isotope-geochronological data indicate that the earliest dike intrusion stage corresponds to the Ediacaran (553.6 ± 10.3 Ma). Several subsequent tectonothermal events can be inferred until and through the Early Carboniferous, which affected the stability of the isotope system in the dolerites. In addition, paleomagnetic data confirm at least one more stage of dike emplacement, in the Early Ordovician, as the coordinates of the virtual geomagnetic poles for the dikes are nearly identical to those of the Ordovician paleomagnetic pole determined for carbonate rocks of Kotelny Island. The study describes the main petromagnetic characteristics and magnetic mineralogy of the studied dolerites, validating the primary thermoremanent nature of the identified characteristic component. The results agree with available information on the paleomagnetism, tectonics, and paleogeography of the New Siberian Islands.

Felsic rocks of the Khamsara terrane in Tyva (southern Siberia) have been studied in terms of major- and trace-element compositions and U-Pb isotope geochronology. The Khamsara Formation rocks have compositions of A 2-subtype within-plate anorogenic volcanics derived from molten continental crust and SIMS U-Pb zircon ages of 463.9 ± 2.8 and 461.7 ± 3.1 Ma of two rhyolite porphyry samples. The Khamsara volcanics are compared with subvolcanic granitoids of the Ognit complex in the Biryusa terrane (Sharyzhalgai basement inlier of the Siberian craton) adjacent to the Khamsara terrane. The volcanics and the granites are coeval and share compositional similarity. The Khamsara Formation is interpreted as a cap complex that marks the upper age bound of the accretionary-collisional events when amalgamated terranes in this part of the Central Asian orogen accreted to the Siberian craton.

We present original data on fluxes of particulate matter through a 1366 m thick water column, measured with sediment traps near the Neutrino Telescope Station in South Baikal. The research was part of a long-term international research project which started in March 1999 in order to investigate current sedimentation in the lake. The total flux of sedimenting particles was considerably higher in 2000 as compared with 1999. This was due to the exceptional growth of siliceous diatoms of the genus Aulacoseira in 2000, leading to the so-called Melosira year. Biogenic silica is predominant in the particulate matter and might reach 56 %, whereas the concentrations of organic carbon and total nitrogen do not exceed 16 % and 1%, respectively.

We present results of study of fluid inclusions in «superquartzites» of the Bural-Sardyk deposit (East Sayan). «Superquartzites» are regarded as a source of extrapure quartz raw material; therefore, the presence of fluid inclusions in them is an important characteristics of their quality. We have first applied not only microthermometry but also LA-ICP-MS to study fluid inclusions in the above rocks. Release of volatiles in quartzite grit was studied by thermal-desorption mass spectrometry. The research has shown the uneven distribution of fluid inclusions in the quartz grains. The inclusions are flattened isometric or rounded (often, of complex morphology), up to 12 μm across. The concentration of salts in the fluid corresponds to 5.5-5.7 wt.% NaCl equiv., Na is the main element in its salt system, and K and Mg are present in background concentrations. According to mass-spectrometric analysis, water is the main volatile component in the «superquartzite» (39 to 124 ppm). Next in abundance are carbon-containing substances (CO, CO2, and hydrocarbons). On stepwise heating of the samples, H2O releases in two temperature regions: at low temperatures (with the maximum release at 400 °C) and at high temperatures (above 1000 °C). The results of the chemical enrichment of «superquartzites» indicate low initial contents of structural impurities, which expands the boundaries for the application of this type of raw material in the production of high-tech quartz products.

Fluid inclusions in quartz from rocks of different ages and with different uranium contents were studied by microthermometry, gas chromatography, and Raman spectroscopy in three areas with uranium mineralization (Skal’noe, Polyarnoe, and Dikoe) in the Litsa ore district of the Kola Peninsula. The studies have revealed differences in the homogenization temperatures, composition of water-salt systems, and salinity of primary and secondary fluid inclusions in the rocks of these areas. Primary fluid inclusions in the rocks of the Skal’noe and Dikoe areas are characterized by a water-salt system with K and Mg (seldom, Na) chlorides and salinity of 0.2 to 13.9 wt.% NaCl equiv. Secondary inclusions are of higher salinity, up to 19.5 wt.% NaCl equiv., and contain Ca chlorides. Primary inclusions in the rocks of the Polyarnoe area show a more complex salt composition with K, Mg, Ca, and, probably, Li chlorides, with salinity varying from 1.7 to 10.4 wt.% NaCl equiv. Salinity of secondary inclusions is higher, up to 16.9 wt.% NaCl equiv. The gas component of fluid in the rocks of the Skal’noe and Dikoe areas is almost pure CO₂ (99.3-100 mol.%); the presence of N₂ (1.1 mol.%) and CH₄ (0.4 mol%.) was revealed in few inclusions only. In the Polyarnoe area, only water inclusions with N₂ were found in the rocks. Thus, the fluids in the rocks of the three areas have mostly a CO₂-H₂O composition with the presence of K, Mg, and Ca chlorides and, seldom, a water-salt composition with the presence of Na salts only. The increased salinity of secondary inclusions and their more complex composition (CO₂ with N₂ and CH₄ impurities) indicate that the fluid became more reducing and might be responsible for the formation of uranium mineralization.

Fluid inclusions, REE and Y patterns, and carbon and oxygen isotope ratios in the minerals of sulfide-carbonate-quartz mineralization of the Orlovka orogenic gold deposit were studied. We have established that fluid inclusions in calcite and quartz homogenize in the same temperature range 217-170 ºC and contain aqueous Mg-K-Na solutions with salinity of 3.0-6.4 wt.% NaCl equiv. According to the results of gas chromatography of inclusions in quartz, the gas phase is a mixture H2O (79-977 ppm) + CO2 (2.64-5.35 ppm) + CH4 (0.002-0.018 ppm) ± N2 (0-1.22 ppm). The REE pattern of calcite shows accumulation of LREE ((La/Yb) N = = 1.28-7.18), (La/Lu) N = 1.10-6.58 (indicating a predominance of REE sorption in the fluid), and weak negative Ce and positive Eu anomalies. Negative Ce anomalies in calcite might be due to the interaction of the fluid with limestones and to the presence of a small amount of meteoric water in it. The positive Eu anomalies reflect the high-temperature environment (>200-250 ºC) that existed in the fluid system before the crystallization of calcite. The δ13CCO2 (-2.0 to 0.9‰) values of the fluid, close to the carbon isotope composition of carbonates of the host rocks (-2.3 to 1.9‰), testify to the metamorphogenic source of carbon. The δ18OH2O values of the fluid depositing quartz (3.1 to 4.5‰) and calcite (4.0 to 4.6 ‰; one sample has 6.6 ‰) suggest that metamorphic water with an impurity of meteoric water prevailed in the fluid system. We propose a model for the gold deposit formation, which takes into account the generation of a mineral-forming fluid at the progressive stage of greenschist dynamometamorphism of the host rocks and the formation of gold mineralization at the regressive stage.

Contents of major impurities (Ag, Cu, and Hg) have been studied in gold from ore deposits of various types: (1) associated with skarns and black shales (Altai-Sayan folded area (ASFA) and North Vietnam), (2) pluton-related porphyry Cu-Mo (ASFA), and (3) volcanic pyritic (Rudny Altai, the Urals, and North Vietnam). Analysis of gold ore mineralization in deposits of these types reveals diverse gold compositions along with diverse compositions of productive mineral associations. Silver is the most abundant impurity in gold from all fields studied, but its contents vary broadly even within a field type. The content of silver in gold depends not only on its abundance in hydrothermal solutions but also on other independent solution parameters: sulfur fugacity, temperature, salt composition, and pH. The regular decrease in native gold fineness from early to late generations in sulfide ore deposits is related to temperature decrease and large-scale sulfide formation. These processes reduce sulfur fugacity in the solutions and favor silver deposition in native gold rather than in sulfides. Gold of later generations is enriched in mercury in many deposits studied, whereas copper gravitates to earlier, high-temperature ones. In addition to deposition temperature, the contents of copper in gold are determined by its content in hydrothermal solutions, as evidenced by the association of copper-rich gold with basic-ultrabasic, skarn, and porphyry copper deposits. The processes causing the deposition of gold of different chemical compositions are complex. They correlate, to an extent, with gold mineralization temperature, whereas the spectrum of impurities often depends on the belonging of a gold deposit to a certain igneous complex.

New experimental data on the temperature and pressure dependences of the viscosity of synthetic hydrous kimberlite melts (82 wt.% silicate + 18 wt.% carbonate; degree of depolymerization: 100·NBO/ T = 313 for anhydrous melts and 100·NBO/ T = 247 for melts with 3 wt.% H2O) were obtained at a water pressure of 100 MPa and at lithostatic pressures of 5.5 and 7.5 GPa in the temperature range 1300-1950 °C. The temperature dependence of the viscosity of these melts follows the exponential Arrhenius-Frenkel-Eyring equation in the investigated range of temperatures and pressures. The activation energies of viscous flow for hydrous kimberlite melts were first shown to increase linearly with increasing pressure. Under isothermal conditions ( T = 1800 ºC), the viscosity of hydrous kimberlite melts increases exponentially by about an order of magnitude as the pressure increases from 100 MPa to 7.5 GPa. The new experimental data on the viscosity of hydrous kimberlite melts (error ±30 rel.%) are compared with similar viscosity data for anhydrous kimberlite and basaltic melts (100·NBO/ T = 51.5) and for hydrous basaltic melts (100·NBO/T = 80). It is shown that at comparable temperatures, the viscosity of hydrous kimberlite melts at a moderate pressure (100 MPa) is about an order of magnitude lower than the viscosity of hydrous basaltic melts, whereas at a high pressure (7.5 GPa), it is more than twice higher. It is first established that water dissolution in kimberlite melts does not affect seriously their viscosity (within the measurement error) at both moderate (100 MPa) and high (7.5 GPa) pressures, whereas the viscosity of basaltic melts considerably decreases with water dissolution at moderate pressures (100 MPa) and remains unchanged at high pressures ( P > 3.5 GPa).

It is shown by numerical modeling that the height of gas flares above underwater methane seeps depends strongly on water parameters. A simulation model of the dynamics of a rising bubble was used. Along with gas exchange through the bubble wall, the model takes into account gas hydrate formation, whose rate is determined by turbulent heat exchange with water. Calculations were performed for depths of 250 to 1500 m with an initial bubble diameter of 0.2 to 1.5 cm. It is shown that at the water temperature of the Arctic seas, rising methane bubbles transform into gas hydrate ice within the first 1-2 m of their path. At the same time, when the water temperature is higher than the hydrate formation temperature near the sea bottom or close to it, the bubbles travel a distance of tens or hundreds of meters before complete dissolution or complete hydration.

Two types of fluctuations, with a short period (SPFs, 3 ± 0.15 years) and with a long period (LPFs, 10-70 years), have previously been found in the secular variation rate (SVR) of the horizontal component H , the vertical component Z, and the inclination I of the geomagnetic field (GMF) . The fluctuations of SVR for the horizontal components of the GMF ( X, Y , and D ) and the periods and amplitudes of their SPFs are estimated in this study. The SVR fluctuations for Y and D are synchronous and have identical phases, and the SVR fluctuations for X are opposite to them in phase, except the areas near the magnetic poles. The SPF periods for X , Y , and D are almost the same (2.8 ± 0.14 years). Jerks in the SVR of Y, especially significant in Europe in 1969 and 1978, were studied. It is shown that jerks are a component of long-period fluctuations in the SVR of Y; they appear at the coincidence of extrema of the same sign in LPFs and SPFs. Jerks need not be considered as a special phenomenon because they can be represented as the sum of quasi-periodic LPFs and SPFs.

Many Permian-Triassic dolerite samples from the Siberian Trap Large Igneous Province exposed to conventional stepwise thermal demagnetization at 250-450 ºC display mid-temperature remanence (MTC) directed opposite to the high-temperature NRM component. Alternating field (A.C.) demagnetization fails to isolate the antipodal component, but it appears during continuous thermal demagnetization, though in a different temperature range. Laboratory experiments and simulations prove that MTC remanence is an artifact resulting from magnetic memory of self-reversing titanomagnetite grains oxidized at low temperature. This effect can interfere with stepwise thermal cleaning and be responsible for misleading patterns of paleomagnetic directions. Given that oxidized titanomagnetite grains are widespread in volcanic rocks, we suggest to identify true paleodirections by combined continuous and stepwise thermal demagnetization. The extension of our model to the case of NRM2 overprint directed at some angle to partially reversed primary NRM1 component accounts for the difference between the results of stepwise and continuous thermal demagnetization observed in samples of the Steens Mountain basalt (USA).

The article deals with a study of the fractal properties of geophysical-data arrays obtained by areal geophysical works over hierarchically organized geologic systems. It is shown by the example of magnetic-survey data in the Pechenga ore region that two-dimensional wavelet analysis is a useful tool for investigating the fractal properties of geophysical-data arrays. The application of wavelet analysis allows detecting the hierarchical structure of organization of the whole ore region, estimating the degree of the system inhomogeneity, and concluding about the local properties of a geologic system, for example, the ore potential of separate sites.