Home – Home – Jornals – Russian Geology and Geophysics 2011 number 3

Diamonds from the Siberian Platform have been studied in terms of their polygenesis, specifically their origin from different types of primary sources. Five types of diamonds have been distinguished, which might have originated from different primary sources, and their distribution over the platform has been shown. The sources of two types of diamonds (kimberlitic and impact) are known. The former originated from Phanerozoic kimberlites, and the latter, from astroblemes. For the other types, nonkimberlitic sources are suggested, including Precambrian ones.

This paper presents the results of the first comprehensive geological, petrographic, mineralogical, geochemical, and geochronological studies of ultramafic and mafic magmatism in poorly explored southwestern Tuva by the example of the Birdag and Khayalyg massifs. These massifs break through ortho- and paraschists, which are presumably Middle Proterozoic. Amphibole gabbros in the Khayalyg massif contain numerous relict host-rock xenoliths. The massifs are dominated by meso- and leucocratic amphibole gabbros with minor ultramafic (serpentinous plagioclase harzburgites and plagioclase lherzolites) and transitional (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbronorites) rocks. The serpentinous plagioclase harzburgites and plagioclase lherzolites occur in the amphibole gabbros of the Birdag massif as small lenticular bodies. The latter are interpreted as xenoliths of ultramafic restites of earlier protrusion rather than mafic-melt differentiates. The wehrlites, olivine clinopyroxenites, hornblendites, and melanocratic olivine gabbronorites forming the outer zones of the xenogenic bodies of ultramafic restites are considered hybrid rocks. They resulted from the contact reactions of mafic melts and their fluids with the xenogenic bodies of ultramafic restites, which were feldspathized during this interaction. In gabbros from both massifs, the chondrite-normalized content of MREE and HREE is lower and that of LREE is higher than those in N-MORB. The plagioclase peridotites, wehrlites, and olivine clinopyroxenites forming xenogenic bodies among the amphibole gabbros of the Birdag massif are richer in REE (especially LREE) than the ultramafic restites in ophiolite associations. This is because they were infiltrated by fluids enriched in these elements during their alteration under the influence of later mafic melts. The studies suggest that the Birdag and Khayalyg ultramafic-mafic massifs result from the spatial coexistence of (1) more ancient small protrusions of ultramafic restites, which occurred as allochthons among Middle Proterozoic(?) metamorphic rocks, (2) later gabbroic intrusions (from 494±16 to 450-447.4±5 Ma), and (3) hybrid transitional rocks (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbros) making up contact-reaction zones along their boundaries.

The physicochemical modeling of mineral formation processes at the Badran subthrust gold-quartz deposit was performed, based on a study of fluid inclusions in quartz by Raman spectroscopy, gas chromatography, thermometry, and freezing. The results show that at stage I, highly productive gold-bearing quartz veins (gray quartz) of the deposit formed from heterogeneous fluid at <320 ?C and 2.0-0.1 kbar with the active participation of CO₂, N₂, and CH₄; the salinity of this solution reached 10 wt.% NaCl-equiv. At stage II (Au-productive), milky-white quartz was produced from the homogeneous medium-chloride-sulfide solution which remained after the heterogenization of the initial fluid, at 300-100 ?C and 0.1 kbar. At stage III (with low Au production), clear quartz formed from homogeneous chloride solutions with salinity of <4.5 wt.% NaCl-equiv. at <200 ?C and <0.1 kbar.
The physicochemical conditions of Au concentration within the complex geochemical system Au-Fe-Cu-Pb-Zn-As-Sb-Hg-Ag-H₂O-Cl-H₂S-CO₂ at the Badran deposit was modeled using the Chiller software. The following models were used: (1) solution-rock interaction and (2) condensation of gas phase (for stage I); (3) simple cooling of medium-chloride-sulfide solution (for stage II); (4) simple cooling and (5) mixing of low-chloride-sulfide solution with acid meteoric waters (for stage III). The models show the sequence of vein formation in the ore-producing system and the host-rock metasomatism in the deep horizons of the deposit.

The Bulka layered intrusion, a standard intrusion of the Bulka complex, is localized in northeastern West Sayan. The layered series is composed of dunites, melanotroctolites, troctolites, olivine gabbro, gabbro, hornblende gabbro, olivine leucogabbro, olivine leucogabbronorites, leucogabbro, hornblende leucogabbro, leucotroctolites, anorthosites, and plagioclase-containing hornblendites. The Mg-number (Mg#) varies from 76 in peridotites (38 wt.% MgO) to 34 in anorthosites (2 wt.% MgO). From bottom to top of the section, the following sequence of crystallization of liquidus minerals is observed: Ol ⇒ Pl ⇒ Cpx ⇒ Opx ⇒ Amph + Mag. The petrochemical features of the massif rocks are consistent with the fractional crystallization of picritic parental magma (24 wt.% MgO, 11 wt.% FeO, Mg# = 80). The compositions of model cumulates determined by fractional crystallization modeling using the COMAGMAT 3.5 computer program at 1-2 kbar, QFM buffer, and 0.5 wt.% H₂O in the melt are similar to those of the intrusion rocks. The REE and trace-element patterns for these rocks show a nonfractionated composition trend with LILE and HFSE depletion. Depleted LILE pattern is similar to that in N-MORB, which evidences that the parental magma was produced through the partial melting of oceanic lithospheric mantle probably similar to the adjacent ophiolites of the Kurtushiba belt. The HFSE depletion of the intrusion rocks might be due to the island-arc processes during the belt formation. The Bulka intrusion formed at the accretion-collision stage of magmatism as a result of the differentiation of picritic melt generated from oceanic lithospheric mantle with superposed island-arc processes.

Literature data on mercury contents in natural waters are reviewed. Soils from natural and natural-technogenic landscapes of southern West Siberia are studied, and the solubility of their mercury in water, ammonium acetate, and hydrochloric extracts is determined. The concentrations of mercury from natural soils in water and ammonium extracts (0.045-0.060 ?g/l) are consistent with its calculated approximate average solubility in unpolluted waters reported in literature.
The average content of mercury in extracts from urbanized soils is higher: Its solubility in water extracts is 5.6-5.9 times higher; in ammonium acetate extracts, 3.8-12.4 times; and in hydrochloric extracts, 1.3-1.6 times. The boundary values of mercury solubility in water and ammonium acetate extracts from the studied soils (0.07-0.10 ?g/l) permit revealing technogenic pollution at its early stages.

Data on lacustrine sediments permitted reconstructing climate and vegetation changes in the steppe zone of southeastern Transbaikalia in the Holocene and the evolution of Lake Zun-Soktui. Also, changes in the size of the lake water plane have been reconstructed. A shallow lake formed in place of an oxbow lake in the middle Boreal phase of the Holocene. In the Atlantic phase, the lake had maximum depth and area. In the late Boreal, early Subboreal, and in the second half of the Subatlantic stage, the lake underwent significant shallowing and sometimes dried up for a short time. In the middle Boreal and the Atlantic phases, the aridization decreased considerably. It has been established that pine forest belts on the Onon high plain before the Subatlantic occupied several times more space than now.

Geomagnetic measurements during the total solar eclipse of August 1, 2008 in Novosibirsk (Klyuchi Observatory, NVS) and at the Burmistrovo station located on the total eclipse axis revealed eclipse-induced changes against the background of the normal daily variation. The main changes are a decrease in the X (north) component and an increase in the inclination I . Similar changes were recorded at the LZH observatory in China. Analysis of data on the eclipse of August 11, 1999 in Europe failed to unambiguously reveal eclipse effects against intense variations of ionospheric origin which occurred at the same universal time (UT). The geomagnetic effect of the solar eclipse may be due to a decrease in the electron concentration in the ionosphere and the corresponding decrease in the ionospheric current density.

Mass measurements of joints in Cenozoic sediments from the Baikal rift have yielded statistical data suitable for processing with a new structural-genetic approach. In the course of processing, the new approach has been methodologically completed with formalized techniques for proceeding from local stress tensor reconstructions to regional-scale stress patterns. The jointing patterns in soft sediments of the area have been recognized to be basically of tectonic origin, and their seeming fuzziness to result from spatial and hierarchic variations during rifting-related stress and strain changes. The patterns vary in time, space, and hierarchy, the three kinds of variability being associated, respectively, with stages of rifting and failure dynamics, with control from the prerift tectonics, and with the existence of six hierarchic levels of stress tensors. For the latter, the complete hierarchy of six stress levels has been recognized for the first time in the Pribaikalian crust.

The eruptive behavior of mud volcanoes has been explored in several forward and inversion models. The forward models, based on classical continuum-mechanics, simulate the nonstationary processes of gas seepage and two-phase gas-mud flow through the conduit of a mud volcano, as well as the quasi-stationary gas frontal drive that pushes mud out, and the subsequent gas escape into the air. Forward modeling is performed with reference to the pressure and temperature dependence of the gas viscosity and compressibility. The inversion for the depth to the mud column base has yielded an unambiguous solution. According to the modeling results, the depths to the mud base and to the gas reservoir are controlled mainly by conduit permeability, while the interval between two successive eruption events depends on the gas/mud viscosity ratio. The modeling has allowed estimates for the mud breccia portion remaining in the conduit by the time of eruption, and for the subsurface volcano thickness inferred using a hydraulic-fracture model of the related gryphon field, and for the rate of gas release at the beginning of an eruption.