Home – Home – Jornals – Russian Geology and Geophysics 2011 number 2

We report results of experiments in which samples of ultramafic xenoliths from the Udachnaya-Vostochnaya kimberlite pipe (Siberian craton, Yakutia) were heated, in a flow reactor, by flows of reduced gas at temperatures between 800 °C and 1200 °C. The heating tests were preceded by investigation into the gas composition of main phases. When subjected to heating by the reduced gas flows, the samples of metasomatized lithospheric mantle released reduced fluids and formed partial melts, the composition of the latter being controlled by fusibility of rocks ("readiness" of liquid to segregate). The initial compositions of the partially molten rocks turned out to correlate with those of secondary inclusions in matrix minerals and of most fusible minerals in reaction rims and in metasomatic veins, as inferred from the analysis of vitrified melting products and structure of the heated rocks. The effect of partial melting of matrix minerals was to increase the contents of Al (melting of garnet) and Mg (olivine and orthopyroxene rims) in the melts. The experimental melts are difficult to compare with natural felsic and mafic rocks in average compositions. The bulk compositions of fluids in altered ultramafics disagree with the computed equilibrium compositions according to some correlation relations (CO₂ ↔ CO, r⁺).

We have revealed early productive disseminated pyrite-arsenopyrite mineralization with invisible gold in tectonic zones and late nest-veinlet gold-polysulfide mineralization with free gold in zones of silicified rocks at the Suzdal', Zherek, and Bol'shevik deposits in eastern Kazakhstan, localized within black shales. Two varieties of arsenopyrite differing in morphology, chemical composition, and gold contents have been established in them: acicular-prismatic and tabular. Gold was determined by a specially elaborated technique ensuring a detection limit of 30 ppm Au. Acicular-prismatic arsenopyrite is the main ore mineral of the early productive stage of mineralization; it has high gold contents (1400-5360 ppm) and a nonstoichiometric composition (S/As = 1.2) and is slightly depleted in Fe. The absence of correlation between the contents of the main arsenopyrite components and gold and the strongly uneven distribution of gold among the mineral grains and within a grain point to the presence of invisible gold as elemental particles deposited together with arsenopyrite. Tabular arsenopyrite is abundant at the Suzdal deposit, where gold-polysulfide mineralization and argillization are widespread. It has low gold and high antimony contents and a stoichiometric composition. Visible gold usually grows over tabular arsenopyrite.
The isotopic composition of sulfur of acicular-prismatic arsenopyrite and globular-crystalline pyrite formed at the early mineralization stage is characterized by δ³⁴ S = 0.0...-3.3 ‰ and evidences a mantle source of sulfur with a partial borrowing of crustal sulfur. The tabular arsenopyrite and other sulfides of the second productive assemblage show a lighter isotopic composition of sulfur (δ³⁴ S = -7.7...-10.2 ‰), which is due to sulfur fractionation under high oxygen fugacity at the late ore deposition stage. The coexistence of two sets of arsenopyrite of different morphologic varieties and compositions at the deposit point to a long ore deposition, the coexistence of mineralization formed at different stages, and the evolution of physicochemical parameters.

The formation conditions of placers within the Tapsa-Kaakhem gold zone of Tuva have been studied, and the relation between primary and placer gold deposits has been studied. Polycyclicity in the valleys of the region resulted in the formation of gold placers of various morphologies with stratified producing reefs in loose sediments differing in composition and age: Oligocene-Lower Pliocene, Eopleistocene-Lower Pleistocene, Middle Pleistocene, and Upper Pleistocene. A complex relation between placers and their primary sources has been established. In general, the macrocomposition of gold in the producing reefs of different ages reflects the composition of gold from the primary sources. From older to younger producing reefs, gold fineness varies more widely, showing a trend toward smaller values. This suggests that undiscovered primary sources belonging to another association exist in the source area. The same is confirmed by a drastic change in the typomorphic features, grain size, and roundedness of gold in the longitudinal section of the Bai-Syut River valley. The peculiarities of gold from placers formed by the rewashing of glacial drift have been established. In the Kopto placer, nuggets are widespread (>10 mm), which fully retained the shape inherent to vein gold. Apparently, the nuggets were transported to the placer with glacial drift in quartz gravel, which protected them from rounding. Quartz crumbled completely, and gold became free in situ under frost action. The presence of nuggets in the Kopto placer permits predicting a cluster distribution of gold in the known thin quartz veins.

The paper is devoted to monomineralic and multiphase (melt) inclusions in magnetites from the phoscorites of the Loolekop deposit, the Phalaborwa alkaline carbonatite complex, South Africa. Multiphase inclusions vary greatly in phase composition from essentially carbonate to carbonate-silicate-oxide. Their main components are dolomite, calcite, fluorapatite, picroilmenite, phlogopite, magnesite, and brucite. In one multiphase inclusion, a rare carbonate, fairchildite K₂Ca(CO₃)₂, was found in the central part of a large magnetite grain. It coexists with dolomite, picroilmenite, phlogopite, brucite, witherite, and halite. This is the first occurrence of fairchildite in carbonatite complexes and in igneous rocks in general. The composition of this mineral (38.54 K₂O, 23.15 CaO, 1.47 FeO, 0.63 Na₂O, n = 5) is close to the ideal K₂Ca(CO₃)₂. The phase relationships within the inclusion show that potassium-calcium carbonate crystallized after phlogopite and picroilmenite, but before dolomite. Temperature estimations from the pairs magnetite-ilmenite and dolomite-calcite indicate that phoscorites began to form at T > 630-750 °C and the phase K₂Ca(CO₃)₂ is a high-temperature modification (fairchildite) which crystallized directly from melt rather than by solid-phase reactions. However, it can be transformed into a low-temperature modification (bütschliite) with decreasing temperature at 547 °C. Apparently, the extremely low Na content and relatively high K content of the initial calciocarbonatite melt favored the formation of fairchildite in the Phalaborwa phoscorites. According to literature data, if Na₂O dominates over K₂O in the carbonatite melt, nyerereite Na₂Ca(CO₃)₂ is a favorable phase for crystallization, and K incorporates into its structure as a minor component.

The study area covers the central part of the Great Lakes Basin in western Mongolia. A specific feature of the area is abundant evidence of large lakes existed there not long ago. The evidence is barrier beaches, terrace complexes, and large sand massifs. Remote sensing and field studies showed active tectonic processes within the Great Lakes Basin. Numerous paleoseismogenic deformations have been revealed along the periphery and in the water area of paleolake Hyargas Nuur, which was six times larger than the modern lake. The absolute heights of the maximum paleolake phase are 1143 m, which is 115 m higher than the present-day lake level. Tectonic movements led to the elevation of part of the former paleolake water area and barrier beaches to the absolute level of 1175 m. Landslides might have caused tsunami in the paleolake water area.

We report CDP reflection profiling data acquired by Yeniseigeofizika OJSC in the southwestern Siberian craton. The collected reference and regional profiles of a total length of ~5000 km run in W-E and N-S directions and traverse all major structural elements of the craton. The data were processed to obtain important evidence for the sediment structure as a basis for further exploration. The synthesis of the results allowed mapping the Riphean surface, with sedimentary sequences and basement highs. The thicknesses and deformation patterns of rocks were used to distinguish ten zones, which differ in structure and composition as well as in the position with respect to major units of the overlying Vendian-Paleozoic strata. The distinguished zones have been ranked in terms of exploration and licensing priority.

Today, one of the most topical questions in the recent geodynamics of eastern Asia is that concerning the existence of the Amurian Plate and its boundaries. An unambiguous answer is difficult to obtain because seismicity is relatively rare and weak and the plate boundaries are often geomorphologically unclear. One of the methods that can help is satellite geodesy. In the present study, the velocity field of recent horizontal crustal movements was obtained from five repeated GPS observations on the Amur-Zeya geodynamic test ground (Amur Region) in 2001-2007. On this basis, the parameters of the relative rotation of the Eurasian and Amurian plates were calculated. The coordinates of their rotation pole were found to be 122.285°E and 58.950°N, and the angular rotation velocity 0.095 deg/Myr. The resulting kinematic model describes the motion of the Eurasian and Amurian Plates as independent tectonic units. According to statistical analysis, this hypothesis is true at a confidence level of more than 99%. Also, calculations have shown that the eastern boundary of the Amurian Plate passes through a branch of the Tan-Lu fault system. The kinematic model for the Eurasian and Amurian Plates agrees with data on the kinematics of active faults and the modern tectonic stress field at the plate boundary.

The dependence of Stoneley attenuation length on the concentration and composition of salt in pore fluid observed in experiments may be associated with osmosis. The salinity dependence is relevant to methods in which the permeability of porous formations is estimated from its relation ship with the Stoneley attenuation length.