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Russian Geology and Geophysics

2007 year, number 10

1.
Fine gold particles and gold dust in alluvial autochthonous placers in southern West Siberia

G.V. Nesterenko, V.V. Kolpakov
Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Fine gold particles; alluvial placers; southern West Siberia
Pages: 783-798

Abstract >>
This paper deals with the behavior of fine gold particles (0.25-0.1 mm) and gold dust (<0.1 mm) during the formation of alluvial placers in denudation areas in southern West Siberia. Native gold particles with a low settling velocity, <5-10 cm/s, such as dust (<0.1 mm) and fine flat particles, are transported with a river stream beyond denudation areas. This easily migrating gold participates in the formation of lithochemical flows, which are an important indicator of gold mineralization. It occurs in the upper beds of alluvial section. Heavier fine gold particles are partly retained at the site of river activity, and the rest are partly redeposited in cooler parts of placers, being graded according to their settling velocity, as evidenced from the direct relationship between the flatness and size of gold particles. Fine gold particles do not form large accumulations in alluvial autochthonous placers. The main typomorphic features of fine gold particles and gold dust are their abundance in ore sources, compositional similarity to larger gold fractions, and high migration ability. In contrast to morphological features of gold, these ones remain stable during the river drift.
																								



2.
Genesis of gold and silver sulfides at the Ulakhan deposit ( northeastern Russia )

N.E. Savva a , G.A. Pal'yanova b
a Northeastern Research Institute, Far East Branch of the RAS, 16 ul. Portovaya, Magadan, 685000, Russia
b Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090
Keywords: Uytenbogaardtite; oxidation zone; thermodynamic modeling; Au(Ag)-containing pyrite
Pages: 799-810

Abstract >>
Geology and mineralogy of the Ulakhan Au-Ag epithermal deposit (northeastern Russia, Magadan Region) are considered. A four-stage scheme of mineral formation sequence is proposed. Concentrations of Au and Ag in minerals of early and late parageneses were determined. It has been established that uytenbogaardtite is associated with native gold and hypergenesis stage minerals - goethite, hydrogoethite, or limonite replacing pyrite. The compositions of uytenbogaardtite (Ag3AuS2), acanthite (Ag2S), and native gold were studied. The composition of the Ulakhan uytenbogaardtite is compared with those of Au and Ag sulfides from other deposits. Thermodynamic calculations in the system H2O-Fe-Au-Ag-S-C-Na-Cl were carried out, which simulate the interaction of native gold and silver with O2 and CO2-saturated surface waters (carbonaceous, sulfide-carbonaceous, and chloride-sodium-carbonaceous) in the presence and absence of acanthite and pyrite at 25
																								



3.
Petrogenetic analysis of fluid and melt inclusions in minerals from mantle xenoliths from the Bele pipe basanites ( North Minusa depression )

A.V. Golovin, V.V. Sharygin
Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Fluid and melt inclusions; mantle xenoliths; basanites; melts
Pages: 811-824

Abstract >>
Results of study of different types of inclusions in minerals from mantle xenoliths from the Bele pipe basanites are presented. Two groups of inclusions were recognized in the host minerals according to their genesis. The first group includes single, apparently primary, fluid inclusions. They were discovered only in orthopyroxenes and consist of CO2 (95 mol.%) and N2 (5 mol.%). These inclusions had partly leaked. The densities of two least leaked inclusions from different xenoliths are 1.05 and 1.14 g/cm3, and their trapping pressures are estimated at >8.5 and 12 kbar, respectively. The second group includes syngenetic secondary fluid, melt, and crystalline inclusions. In composition the secondary fluid inclusions differ from the primary ones in higher concentrations of N2 (up to 7 mol.%). Their maximum density is 0.57 g/cm3, which corresponds to 2.4-2.6 kbar and 1100-1200
																								



4.
Paleomagnetism of Upper Jurassic basalts from Transbaikalia: new data on the time of closure of the Mongol-Okhotsk Ocean and Mesozoic intraplate tectonics of Central Asia

D.V. Metelkin a, b , I.V. Gordienko c , V.S. Klimuk c
a Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
b Novosibirsk State University, 2 ul. Pirogova, Novosibirsk, 630090, Russia
c Geological Institute, Siberian Branch of the RAS, 8 ul. Sakh'yanovoi, Ulan Ude, 670047, Russia
Keywords: Paleomagnetism; bimodal magmatism; Mongol-Okhotsk Ocean; intraplate strike-slip motions; Transbaikalia
Pages: 825-834

Abstract >>
The Upper Jurassic basalts (150-160 Ma) described as the Ichetui Formation over the territory of the Tugnui, Margintui, and Maly Khamar-Daban volcanic structures have been studied paleomagnetically. It is shown that natural remanent magnetization still contains a component which may reflect the geomagnetic field direction at the beginning of the Late Jurassic. This is supported by reversal and conglomerate tests. Calculation of mean paleopole gives: Plat = 63.6
																								



5.
Spherulites and spherulitic tactoids of anthraxolite

Yu.M. Dymkov a , N.P. Yushkin b , A.I. Zubov a , E.G. Pavlov c , A.S. Savel'ev b , V.N. Filippov b
a All-Russian Research Institute of Chemical Technology, 33 Kashirskoe shosse, Moscow, 115409, Russia
b Institute of Geology, Komi Scientific Center, Uralian Branch of the RAS, 54 ul. Pervomaiskaya, Syktyvkar, 167610, Russia
c Russian State Geological Survey University, 23 ul. Miklukho-Maklaya, Moscow, 117997, Russia
Keywords: Anthraxolite; spherulite; spherulite; polymer; tactoid
Pages: 835-843

Abstract >>
A pebble of anthraxolite found in the basin of the Kendei River, a tributary of the Lena River, was examined under polarization metallographic and electron microscopes to get an insight into their structure and ontogeny. A polished section of the mineral demonstrates evident anisotropy in polarized reflected light and finely sculptured microrelief of growth and induction surfaces of spherulites on the fracture (fractograms). The morphogenetic inspection and study of ontogenetic signs of origination, growth, phase transformations, and other changes of anthraxolite revealed a growth mechanism typical of spherulitic dendrites owing to low-angle splitting of radial subindividuals. Layered gravity structures (`mineralogical levels') have been found in crystallization voids. They are composed of an ordered even-grained aggregate of spherical crystals of anthraxolite with the tactoid texture of precious opal. The tactoid mineralogical levels of anthraxolite are characterized by the block structure of microspherulite aggregates, signs of deformation, twinning and geometrical selection of blocks, and elements of recrystallization. Growth-oriented zones of regular nets with endotaxial inclusions of graphite inside the cells have been observed on induction surfaces in spherulitic dendrites. Their appearance is considered an indicator of phase transformations. Supramolecular structures and their effect on the morphology of anthraxolite are discussed in short.
																								



6.
Resistivity pattern of crust and upper mantle in Southern Urals

A.G. Diakonovaa, K.S. Ivanovb, P.F. Astafiev a , V.S. Vishnev a , A.D. Konoplin a
a Institute of Geophysics, Uralian Branch of the RAS, 100 ul. Amundsena, Ekaterinburg, 620016, Russia
b Institute of Geology and Geochemistry, Uralian Branch of the RAS, 7 Pochtovyi per., Ekaterinburg, 620151, Russia
Keywords: Electromagnetic sounding; simulation; model; tectonics; structural geology
Pages: 844-850

Abstract >>
A resistivity model of the southern Urals to depths of 120 km was obtained by numerical simulation of natural- and controlled-source EM soundings at 160 kHz to 4 · 10-4 Hz. The structure of crust and upper mantle was imaged along a transect running ~800 km across the East European Platform, the Ural foredeep, and the Ural mountains. The new data on geology and tectonics of the southern Urals enlarge the knowledge gained through URSEIS-95 reflection profiling along one of best representative cross-orogen profiles. We discovered a large conductor traceable to depths at least 100-120 km at the junction between the East European Platform and the Ural foredeep. It indicates that the Ural foredeep originated in a weak tectonic zone at the platform edge. The Ural orogen is imaged as a nearly bivergent structure to depths of 70-80 km producing a mosaic pattern of conductors rooted deep beneath the Magnitogorsk greenstone province and the granitic belt of the central East Ural uplift where it is 150 km wide at a depth of ~120 km. We interpret the discovered deep roots in the context of the geological history of the Urals.
																								



7.
Resistivity logging in a multi-scale medium with lognormal conductivity

E.P. Kurochkinaa, O.N. Soboleva b , M.I. Epovc
a Institute of Thermal Physics, Siberian Branch of the RAS, 1 prosp. Akad. Lavrentieva, Novosibirsk, 630090, Russia
b Institute of Computing Mathematics and Mathematical Geophysics, Siberian Branch of the RAS, 6 prosp. Akad. Lavrentieva, Novosibirsk, 630090, Russia
c Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Resistivity logging; lognormal conductivity distribution; subgrid modeling; fractality
Pages: 851-856

Abstract >>
We derived equations of effective conductivity coefficients to estimate mean current density and its variance in resistivity logging problems for isotropic multi-scale porous media with lognormal conductivity distribution. The equations have quite a simple form if the medium parameters satisfy refined Kolmogorov's similarity hypothesis. The results of the refined perturbation theory obtained using subgrid modeling are compared with the results of numerical simulation and the conventional perturbation theory.
																								



8.
Quality of chrysotile asbestos and high-temperature conductance of the host ultramafics: possible linkage

V.V. Bakhterev
Institute of Geophysics, Uralian Branch of the RAS, 100 ul. Amundsena, Ekaterinburg, 620016, Russia
Keywords: Serpentinization of ultramafics; electrical resistance; high temperature; activation energy; tenacity of chrysotile asbestos
Pages: 857-862

Abstract >>
The paper reports physical, physicochemical, and mineralogical-petrographic studies of serpentinite samples from the Bazhenovo deposit of chrysotile asbestos. The samples differ in mineralogy, physical and physicochemical parameters, and in quality (fiber tenacity) of the hosted chrysotile asbestos. The experiments allowed estimating the temperature dependence of electrical resistance of serpentinites in a range of 20 to 800