Home – Home – Jornals – Russian Geology and Geophysics 2009 number 7

Hypogene uytenbogaardtite, acanthite, and native gold parageneses have been revealed at the epithermal Yunoe gold-silver deposit, Magadan Region, Russia. Thermodynamic calculations in the system Si-Al-Mg-Ca-Na-K-Fe-Pb-Zn-Cu-Ag-Au-S-C-Cl-H₂O were carried out at 25-400 °C and 1-1000 bars to elucidate the role of hydrothermal solutions in the formation of gold and silver sulfides. Several most probable scenarios for ore-forming processes in the deposit are considered: (1) interaction between cold and heated meteoric waters percolating along cracks from surface to depth and reacting with the host rock-rhyolite; (2) evolution of ascending postmagmatic fluid resulting in chloride-carbonic acid solution, which interacts with rhyolite at 100-400 °C; (3) stepwise cooling of hydrothermal ore-bearing solutions; (4) rapid cooling of ore-bearing hydrotherms on their mixing with cold surface waters. Rhyolite with Pb, Zn, Cu, Cl, S, Ag, and Au clarke contents was taken as an initial host rock. Calculations by model 3 showed the possible formation of uytenbogaardtite and petrovskaite at low-temperature stages. Gold and silver sulfides can be deposited during the mixing of ore-bearing acid chloride-carbonic acid hydrothermal solutions with surface alkaline waters.

The factors determining the REE distribution in natural hydrothermal systems are studied by a numerical experiment. The behavior of REE is examined based on the composition of ore-bearing hydrothermal solutions and the parameters of ore formation processes at different fluorite deposits. These data were obtained in studies of fluid inclusions. Some regularities of the REE behavior during the formation of fluorite deposits have been revealed. It is shown that the REE distribution in fluorites is related mainly to changes in the composition of mineral-producing fluid solution.

The first detailed isotope-geochemical study of carbonate deposits has been performed in the Lower Famennian stratotype section of the northwestern Kuznetsk Basin (Kosoy Utyos), which was localized in the middle latitudes of the Northern Hemisphere in the Late Devonian. The δ¹³ C_карб, δ¹³ C_орг and δ¹⁸ variation curves were constructed for the section deposits. Geochemical and petrographic studies of carbonates allowed allocation of samples that underwent postsedimentation alteration and exclude them from further interpretation. Compared with coeval sections in the other world's regions, the Kosoy Utyos section is characterized by higher δ¹³ C_carb values, up to 5.4 ‰, whereas the maximum value in subequatorial area sections is 4 ‰. The isotope shift amplitude of the studied section reaches 4.6 ‰, which is 1.5 ‰ higher than those in other regions. The δ¹⁸O values are 3 ‰ lower than the ones of the world's coeval sections. The results obtained show that δ¹³C and δ¹⁸O variation trends differ from those of coeval subequatorial sections. The high shift amplitude and maximum δ¹³ C_carb values in the Kosoy Utyos section are due to the shallow-water carbonate sedimentation environments on the Siberian continental shelf and, probably, the lower temperatures of waters in the middle latitudes as compared with the subequatorial areas.

Cenozoic continental rifting in southern East Siberia and northern Mongolia has been associated with subsidence and broadening of rift basins at the account of their mountain borders. This neotectonic trend is, however, superposed with continuous or periodic tectonic inversions in which the basin floor may uplift while marginal fault steps and saddles between basins may subside. Cenozoic geomorphic inversions are expressed in changes of river flow out of Lake Baikal.

A nonlocal dynamic model of petroleum formation is used to explore possible causes and consequences of the nonlinear behavior of large petroleum systems. As a result of this nonlinearity, migration and accumulation of oil starts only after hydrocarbons in a reservoir reach a threshold amount. Significant differences in the amount of in-place resources in macroscopically similar petroleum systems may be associated with minor random changes in initial conditions.

The available data on the composition of the pyrolysis products of kerogen from the Mesozoic carbonaceous strata of the Russian Plate evidence that changes in the contents of total organic carbon (TOC) lead to a regular change of the mechanisms of organic-matter (OM) conservation in sediments. Each mechanism prevails for particular TOC contents. The initial increase in the TOC content of rocks is accounted for by the fact that the higher is the biologic productivity of the basin, the higher is the portion of nonmineralized organic matter. This is due mainly to the mechanism of selective accumulation of the most stable biochemical components such as algaenan. The appearance of H₂S first in the pore waters of sediment and then in the water column increases the degree of preservation of initial OM at the expense of its sulfurization. This process runs first in the lipid and then in the carbohydrate fractions of initial OM.

New data on soft-sediment deformation in Late Pleistocene and Holocene deposits of the northwestern Kola Peninsula (Pechenga River valley) are reported and analyzed in terms of paleoseismicity implications. Soft-sediment deformation is assigned to paleoseismic triggers on the basis of special criteria. One sedimentary section in the Pechenga valley bears signature of several seismic events at the Late Pleistocene-Holocene boundary, constrained by radiocarbon dates. According to the morphology, sizes, and types of seismites, the earthquakes had an MSK-64 intensity at least VI-VII. The observed earthquake-induced deformation may be associated with tectonic subsidence of the Pechenga valley block.

Physical, physicochemical, and mineralogical-petrographic methods have been applied to samples of ophiolite-hosted chromite ore from different deposits and occurrences in the Urals. Temperature dependences of dielectric loss obtained for nine chromite ore samples consisting of 95-98% Cr spinel show prominent peaks indicating a relaxation origin of the loss. The analyzed samples have the loss peaks at different temperatures depending mainly on H = (FeO/Fe₂O₃)*: (FeO/Fe₂O₃)**, where (FeO/Fe₂O₃)* and (FeO/Fe₂O₃)** are, respectively, the ferrous/ferric oxide ratios in the samples before and after heating to 800 °C, and H is thus the heating-induced relative change in the FeO/Fe₂O₃ ratio. These peak temperatures vary from 550 °C (sample 1, high-Cr chromium spinel with more than 52% Cr₂O₃) to 750 °C (sample 2, aluminous and magnesian spinel with less than 30% Cr₂O₃), and H ranges correspondingly from 1.61 to 5.49. The temperature of the loss peaks is related with H as H = 34.30 - 11.52N +? 1.20N², with an error of σ=0.19 ( N = T · 10^-2>, T is temperature in °C).