Home – Home – Jornals – Russian Geology and Geophysics 2007 number 3

New data on the geochemistry and isotopic composition of chloride brines of the Siberian Platform are presented. The distribution of stable isotopes (²H, ¹⁸O, and ³⁷Cl) in brines of the Tunguska, Angara-Lena, western part of the Yakutian and Olenek artesian basins and ⁸⁷Sr/⁸⁶Sr in brines of the western part of the Olenek artesian basin was studied in the context of the problem of genesis of highly mineralized groundwaters. Results of the study and comparative analysis of the geochemical and isotopic peculiarities of the Siberian Platform brines conform to the theory of brine formation through the interaction of connate waters with enclosing rocks.

We discuss the redox environments and the compositions of bottom sediments and sedimentary pore waters in the region of a hydrothermal vent in Frolikha Bay, Lake Baikal. According to our results, the submarine vent and its companion nearby spring on land originate from a common source. The most convincing evidence for their relation comes from the proximity of stable oxygen and hydrogen isotope compositions in pore waters and in the spring water. The isotope composition indicates a meteoric origin of pore waters, but their major- and minor-element chemistry bears imprint of deep water which may seep through permeable faulted crust. Although pore waters near the submarine vent have a specific enrichment in major and minor constituents, hydrothermal discharge at the Baikal bottom causes a minor impact on the lake water chemistry, unlike the case of freshwater geothermal lakes in the East-African Rift and North America.

The Middle Cenozoic evolution of magmatism in the Schmidt Peninsula between 37 and 25 Ma began with eruptions of subalkaline and moderately alkaline andesite, latite, trachyandesite, and trachyrhyolite lavas and ended with subvolcanic intrusions of highly alkaline strongly undersaturated essexites. According to trace-element data, magmatism evolved from melting of a mantle source in the zone of ocean-continent plate convergence to small-degree partial melting in lithospheric mantle at the final stage. This succession is generally typical of Late Cenozoic continental-margin magmatism in southeastern Russia. The similarity in the Middle and Late Cenozoic stages of magmatism is evidence for their individual significance.

We suggest a new approach to study apparently chaotic jointing near faults of different geometries. It implies matching jointing stereograms to standard patterns developed for strike-slip, reverse, thrust, and normal faulting. The standard patterns correspond to joint triplets that regularly initiate in small rock volumes under second-order stress fields during the formation of fault zones. The approach was tested for the cases of failure in bedrock and unconsolidated sediments and showed up as an efficient tool for investigating the origin, structure, and evolution of faults.

This study is concerned with the eolian deposits on the Baikal shore. It has been established that (1) the eolian sands are distinctly divided into two sequences: Khuzhir (Late Pleistocene) and Peschanka (Holocene); (2) the ilmenite enrichment of the sands is due to the removal of sand grains (containing mainly light minerals) by strong winds up the shore, first, from a dewatered beach zone, and then, from zones of psammitic-material transition. This leads to the concentration of ilmenite and other heavy minerals, first, within the beach zone, and then, within the zones of prevailing eolian-sand transition. The accumulation-transition zones are most enriched in heavy minerals, including ilmenite; (3) the ilmenite content in the study Ti-ore placer deposit shows a high correlation ( R = 0.89) with the total content of heavy minerals.

Using the ICP-MS method we have studied the isotope systematics of Sr and Nd as well as trace element composition of a representative collection of kimberlites and related rocks from the Siberian Platform. The summarized literature and our own data suggest that the kimberlites developed within the platform can be divided into several petrochemical and geochemical types, whose origin is related to different mantle sources. The petrochemical classification of kimberlites is based on persistent differences of their composition in mg# and in contents of indicator oxides such as FeO_tot, TiO₂, and K₂O. The recognized geochemical types of kimberlites differ from one another in the level of concentration of incompatible elements as well as in their ratios.
Most of isotope characteristics of kimberlites and related rocks of the Siberian Platform correspond to the earlier studied Type 1 basaltoid kimberlites from different provinces of the world: Points of isotopic compositions are in the field of primitive and weakly depleted mantle. An exception is one sample of the rocks from veins of the Ingashi field (Sayan area), which is characterized by the Sr and Nd isotopic composition corresponding to Type 2 micaceous kimberlites (orangeites).
The most important feature of distribution of isotopic and trace-element compositions (incompatible elements) is their independence of the chemical rock composition. It is shown that the kimberlite formation is connected with, at least, two independent sources, fluid and melt, responsible for the trace-element and chemical compositions of the rock. It is supposed that, when rising through the heterogeneous lithosphere of the mantle, a powerful flow of an asthenosphere-derived fluid provoked the formation of local kimberlite chambers there. Thus, the partial melting of the lithosphere mantle led to the formation of contrasting petrochemical types of kimberlites, while the geochemical specialization of kimberlites is due to the mantle fluid of asthenosphere origin, which drastically dominated in the rare-metal balance of a hybrid magma of the chamber.

Isoferroplatinum mineral assemblage has been first discovered in the Simonovsky Brook gold placer localized in the Egor'evskoe gold-bearing district (Salair Range). It includes isoferroplatinum (Pt₃Fe) grains with inclusions of cuprorhodsite CuRh₂S₄, braggite (Pt,Pd)S, vysotskite PdS, high-Pt rhodarsenide (Rh,Pd,Pt)₂ As, keithconnite Pd₃Te, hollingworthite RhAsS, Cu and Fe sulfides, basic plagioclase, magnetite, epidote, quartz, kaolinite, and leucoxene. The assemblage might have been produced from small gabbroid bodies widespread in the northwestern Salair region, including those of the Uralian platinum-bearing type, or from gabbroids of other complexes.

Microdiamonds (crystals smaller than 1 mm) of octahedral and cubic habit from Udachnaya kimberlite pipe (Yakutia) have been compared in order to distinguish genetic features inferred from carbon isotopic composition and impurity defects. Microdiamonds of cubic habit from the Udachnaya kimberlite pipe have a fibrous internal structure and a high content of nitrogen impurity (400-3000 ppm). Octahedral microdiamonds from the same deposit are distinguished by a low nitrogen content of 0 to 500 ppm and zoning structure. The isotopic composition of carbon (δ¹³C is -4.7