Home – Home – Jornals – Russian Geology and Geophysics 2013 number 12

The type of convergent boundaries forming in the area of mantle plumes is considered. These convergent boundaries (West Pacific type) are characteristic of the western margin of the Pacific. West Pacific-type boundaries are a regular succession of structures from ocean to continent: island arcs, marginal basins, rift basins, and associated OIB-type volcanics at the continental edge. The convergence zones are up to a thousand kilometers wide. Studies of the history of the part of the Central Asian Fold Belt forming the folded periphery of the Siberian continent have shown that the continent drifted above the African plume or corresponding low-velocity mantle province for most of the Phanerozoic (up to the Early Mesozoic inclusive). This fact determined the West Pacific type of convergent boundaries for the accretionary structures of the Central Asian Fold Belt. The drift of Siberia from African to Pacific province in the Late Cenozoic determined the structure and development of the convergent boundary in the western Pacific, including extensive intraplate magmatism in continental Asia in the Late Mesozoic and Cenozoic.

Regional gravity and magnetic anomalies are interpreted with regard to new geodynamic, geological, and tectonic schemes. Integrated analysis of these data confirms the deep origin of the processes which have created the largest igneous areas and zones of the eastern Central Asian Fold Belt.

We consider the evolution of the tectonic setting and geochemical features of rocks in ultrapure quartz areas of the Baikal region. These are the Chuya uplift of the northern Baikal region, with the Tyya quartz deposit, as well as the Bural-Sar’dag and other deposits in the Gargan block of East Sayan. Similarities are observed in the sedimentation and tectonic evolution of the regions. The conditions of the deposition of quartzites (forearc basins within siliceous-carbonate rocks with slight tholeiitic volcanism and a small amount of terrigenous material) ensure their relative initial purity. Low-grade quartzite-bearing strata are bedded or thrust by collision over an older high-grade basement consisting of a tonalite–trondhjemite–granodiorite association (Gargan block) or active-continental-margin metavolcanics (Chuya uplift). Migmatization of the basement and the intrusion of granitoids therein cause its uplifting, fault displacements, and erosion of the overlying rocks. If the thrusting is associated with retrograde metamorphism of the basement and progressive metamorphism of the cover, backward motion can cause only quartz recrystallization with its purification and the grinding of the other minerals. This leads to the formation of lenses of “superquartzites,” mylonites, cataclasites, and breccias in the autochthon–allochthon suture and within the allochthon.

Natural diamonds from the Ural alluvial deposits have been studied by FTIR spectroscopy. It is shown that these diamonds are similar in some typomorphic characteristics, such as nitrogen content and aggregation state, to the diamonds of the Coromandel (Brazil) and Verkhnee Molodo (Lena region, Yakutia) placers and to the diamonds from kimberlites of the Arkhangel’sk Region but differ significantly in lower contents of hydrogen and higher contents of platelets. The high contents of hydrogen (5–20 cm–1) determined in some diamonds due are to the specific formation of their internal structure and to the presence of inclusions. The nonuniform distribution of nitrogen A- and B-centers throughout the crystal testifies to the zonal structure of diamonds. The temperature conditions of the Ural diamonds formation have been estimated.

Gold-bearing ore occurrences and deposits in jasperoids are objects of a new unconventional type widespread in the West Kalba auriferous belt in eastern Kazakhstan. We present original data on the structural and geologic position and mineralogy of gold-bearing jasperoids of the Baybura ore field, which is a standard object of this type in eastern Kazakhstan. Gold-bearing jasperoids are localized here in shale–carbonate strata of island-arc type (C1) that underwent metasomatism in superintrusive zones of the Kunush complex plagiogranites (C3). Free finely dispersed gold shows typical signs of hypogenous changes. The content of Au varies from 0.1 to 33.5 ppm, and that of Ag, from 0.05 to 2.5 ppm. Based on the data obtained, a geologo-genetic model of the formation of the Baybura gold-bearing jasperoids is proposed. According to the model, the ore deposition is related to the impact of juvenile fluids of plagiogranitic magma on carbonate substrate. The main factors determining the evolution of gold-bearing metasomatites are their localization in zones of high tectonic activity and a drastic change in the acidity–alkalinity regime of solutions in the carbonate environment. Native gold grains are extremely fine and thin and contain a permanent impurity of Hg (0.2–0.7 wt.%). In mineralogical and geochemical features the gold mineralization of the Baybura ore field is similar to that of the Suzdal’ deposit and can be assigned to the Carlin type.

Typomorphic features of native gold and its contents in complex Ti–Zr placers in the southern West Siberian Plain are reported. Three of the placers are of littoral-marine genesis, and two formed under conditions of an alluvial piedmont plain. Native gold from the studied Ti–Zr placers occurs mainly as flattened thin (–0.1 mm) particles which underwent mechanical action. It is marked by a wide fineness variation and the abundance of a very fine (990–1000‰) variety. Most likely, this is chemically transformed clastogenic metal. The gold content of the productive bed (5–30 mg/m ³ native gold and 8–140 ppb bulk gold) is consistent with the dispersion of heavy ore and accessory minerals during mechanical migration in water flows simultaneously with their concentration on geochemical barriers. The native-gold content of complex Ti–Zr placers shows a higher negative correlation with the primary source–placer distance than those of Ti and Zr minerals and a positive correlation with the degree of hydrodynamic reworking (gravity concentration) of transit terrigenous material. On the southern framing of the West Siberian Plain, some regions of northern Kazakhstan are promising for gold of complex Ti–Zr placers as well as fine- and thin-gold placers, gold-bearing weathering crusts, and primary gold deposits.

Quaternary sediments from the borehole DBC-1 drilled in the Darhad Basin, northern Mongolia, have been studied by thermomagnetic (248 samples) and probe microanalysis (9 samples) analyses to determine the origin (cosmic or terrestrial) of native iron. Most of the samples showed extremely low contents of native iron. Only 26 samples have iron in contents sufficient for its reliable identification (10–5–10–3 %). The negligible content of native iron in the Darhad Basin sediments distinguishes them from the earlier studied sediments of different geologic associations of Eurasia and the Atlantic, which we explain by the high sedimentation rate in this basin. However, the bimodal distribution of native-iron contents in the samples with a distinct “zero” mode, similar to that in the objects of Eurasia and the Atlantic, testifies to the predominantly cosmic origin of the native iron.

High-performance algorithms and software have been designed for fast 2D forward modeling and inversion of high-frequency electromagnetic logs to recover the patterns of electrical conductivity and relative permittivity (dielectric constant) of rocks. Forward modeling implies linear representation, with regard to eddy current, of attenuations and phase shifts of electromagnetic fields propagating in 2D conducting formations. In order to improve the modeling quality, the background model is chosen proceeding from the eddy current pattern in a heterogeneous subsurface. The inverse solution is obtained by pseudoinversion of the sensitivity matrix of relative amplitudes and phases. Joint 2D inversion of phase shifts and attenuations improves the reliability of the inferred spatial conductivity distribution and allows estimating permittivity. The reported numerical experiments on inversion of synthetic and field EM logs prove the possibility of estimating the permittivity of complex terrigenous and carbonate reservoirs with water- and oil-filled porosity.

The thermal conductivity of rocks (λ) used previously to estimate heat flow in high-latitude Siberia was assumed to be λ = 1.8–2.0 W/(m·K), according to published evidence, but was almost never measured specially. We measured the thermal conductivity of core samples from boreholes drilled in the northeastern West Siberian Plate and in the Yenisei–Khatanga basin in the 1990s, using two advanced instruments: a comparator and a scanner of thermal conductivity. Altogether there were 305 samples of air-dry sediments (λ _d) from the 1100–4200 m interval of 23 holes, 77 samples of which were then moistened (λ _w). The average thermal conductivities of rocks in dry and wet conditions were found to be λ _d = 1.9 ± 0.2 and λ _w = 2.6 ± 0.1 W/(m·K), respectively. Thus, the true thermal conductivity of sediments, which are saturated with water in this northern area, must be about 20–30 % higher than that assumed in the previous heat flow determinations, and the latter are thus underestimated correspondingly. The updates to the thermal conductivity values imply major revision of the existing ideas of the thermal regime in the Siberian high latitudes. Such a serious revision of geothermal data will obviously require a large amount of additional experimental work.

Groundwater is a very important component of water resources in coastal aquifers in Thoothukudi. It has been established that the groundwaters in the coastal zone of the Zirconium Complex, Pazhayakayal, Thoothukudi district, Tamilnadu, India, are subjected to wave and tidal impact. The groundwater quality was studied by hydrogeological methods, 2D electrical resistivity imaging (ERI) techniques (six profiles), 11 Wenner vertical electrical sounding (VES), and well log analysis. Also, nine geochemical water samples were taken from the study area. The 2D ERI and VES surveys were carried out using WGMD-4 Ltd, Chennai, resistivity meter, multicore cable, and multielectrodes with Wenner array. The collected resistivity data were interpreted using the Res2DINV software. The research shows that the groundwaters are the result of the palaeoriver flow along the Tamirabarani Channel in the western area and of the seawater intrusion in the eastern area. The fresh water is characterized by resistivity of about 10–100 Ohm·m in the study area. The resistivity of 10–50 Ohm·m indicates that the subsurface section is made up of sand, clay, and caliche. Resistivity values of more than 200 Ohm·m are specific for sand dunes. The very low resistivity (<5 Ohm·m) layer might be due to the seawater intrusion in the study area. Six water samples from the well drilled in the coastal area were analyzed, which made it possible to determine the concentrations of major and trace elements in the groundwaters. These data were used to establish the seawater intrusion and coastal environment characteristics in the study area.