Home – Home – Jornals – Russian Geology and Geophysics 2002 number 12

Fluid inclusions in minerals from more than 22 Au-Hg deposits (Central Asia, Urals, East Sayan, Gorny Altai, northeastern Russia, Mongolia, etc.) have been examined by thermobarogeochemical methods (thermo- and cryometry, Raman spectroscopy of gas phase). It has been established that Au-Hg deposits are low-temperature (280-50 ^oC) hydrothermal objects which formed in subsurface (volcanogenic-hydrothermal) or shallow-depth (plutonogenic-hydrothermal) conditions, under pressures lower than 500-600 bars. The volcanogenic-hydrothermal Au-Hg deposits were formed with participation of weakly concentrated (10-0.5, less frequently to 14 wt.%) chloride or chloride-bicarbonate-sodium hydrothermal fluids with low-density N₂-CO₂ (CH₄) gas phase. The ore-forming fluids of plutonogenic-hydrothermal deposits are characterized by wider variations in concentration (from 0.2 to 25 wt.%), complex salt composition (NaCl, CaCl₂, FeCl₂, KCl, etc.), and dense highly CO₂ gas phase (CO₂ N₂ CH₂); in salt composition and reduced character they similar to magmatic fluids. The characteristic feature of the fluid regime of ore-forming systems of volcanogenic- and plutonogenic-hydrothermal Au-Hg deposits is the heterogenous state of ore-forming fluids. In particular ore districts, the physicochemical parameters of formation of volcanogenic- and plutonogenic-hydrothermal Au-Hg deposits are generally close to the parameters of formation of similar Au-Sb, Sb and Sb-Hg deposits. The results obtained from the thermobarogeochemical study stress the polygene nature of Au-Hg deposits.

Mantle sources of Late Cenozoic alkali-basalt lavas in the vast territory of East Sayan have been investigated based on space and time variations of Pb, Sr, and Nd isotopic ratios. The evolution of volcanism is interpreted in the context of interaction of a plume-like mantle thermal anomaly and a moving lithospheric plate. Volcanism initiated 22-20 myr ago in the northeastern part of the Riphean Tuva-Mongolia massif (Urik segment), propagated westward through its northwestern part, and focussed within the Caledonian East Tuva zone in the past 2 Ma. Between 22 and 12 Ma, basaltic lavas contained a predominant common component of a deep convecting mantle material (probably, rising from greater depths) with high Pb and Nd and low Sr isotopic ratios. The interaction of this material with the lithosphere produced a shallower lens with lower Pb and Nd and higher Sr ratios. This component first appeared at 12-9.5 myr and became part of all later volcanic products. The role of lithospheric material varied in space and time: 8/4Pb = 60-93 were in the range of the DUPAL anomaly in the Tuva-Mongolian basalts, below this range (20-54) in the East Tuva lavas, and intermediate (50-63) on the periphery of the basaltic massif.

Biogeochemical studies have been carried out on seven lakes of the Altai Territory, situated in different landscape-geochemical zones. The basic contents of heavy metals in the biological objects determining the biogeochemical background have been estimated. These contents serve as references in monitoring of increasing element concentrations during the man-induced pollution of lakes. The method of cluster analysis was used to carry out a computer-aided classification of correlations between chemical compositions of biological objects, bottom sediments, and regional soils. It has been established that the accumulation of trace elements in the living matter differs considerably from their accumulation in bottom sediments. The bottom sediments accumulate chiefly Cr, Ni, and Co, whereas the living matter accumulates Hg, Zn, Cd, and Mn. There are zones of local pollution in Lake Bol'shoe Yarovoe with elevated concentrations of mercury in biological objects and bottom sediments, resulting from industrial waste of a chemical plant in the town of Yarovoe.

The Mid-Tien-Shan ophiolites make up several blocks in the Chatkal Ridge, tectonically related to the type sections of the region. The study was carried out at the most representative, Karaterek, ophiolite complex, which forms a nape lying on the Silurian volcanogenic-terrigenous deposits and Middle Carbonaceous olistostrome. The Karaterek Massif is tectonically layered and is in inverse setting. It contains (1) a complex of the melanocratic basement represented by pyroxenites, antigorite serpentinites, gabbro-amphibolites, and actinolite schists and (2) volcanogenic-sedimentary cover, whose bottom is made up of apovolcanic chlorite-epidote and other schists with members of quartz-sericite and siliceous schists. Above lie phyllites, tuff sandstones, and cherts. A complex of the conodonts Drepanoistodus sp., Paracordylodus sp., and Periodon cf. aculeatus have been documented in the cherts from the Arenigian-lower Middle Ordovician, which is the first reliable dating of the ophiolites of the region. In age and composition, the Karaterek ophiolites may, probably, be compared with the Ordovician ophiolites from the eastern Urals.

A digital elevation model (DEM) is applied to morpho-tectonic analysis of the northern mountain surroundings of Lake Telestkoye. The investigation into the neo-tectonic structure of the region and vertical offset of block movements implies detection of peneplain remnants, digital elevation modelling, and interpretation of aerial and satellite imagery. Lineaments expressed in the surface topography and/or distinguished on aerial and satellite images were divided into three groups. Those of the first two groups are different kinds of normal faults, and the third-group lineaments without geomorphically expressed vertical component have been interpreted as eroded structures of the pre-Cenozoic basement. Structures of the three types often belong to single lineament, which may indicate reactivated and stable neotectonic segments of the old basement faults. The revealed neo-tectonic blocks are rarely larger than a few kilometers across. Most of normal faults occur on the sides of the Teletsk and the adjacent Kamga and Koldor grabens, where vertical offset of recent block movements reach their maximum of 400-1200 m; in other grabens, the offset is mostly under 100-300 m. The surface topography of the region is strongly controlled by tectonics, evident from long V-shaped river valleys along zones of weakness. Some valleys are up to 600-800 m deep, and the total topographic gradient is 1600 m. In addition to tectonic factors, intense erosion is caused by recent lowering of the Lake Teletskoye level which is the regional base level of erosion.

To predict the petroleum potential, we used stratigraphic, tectonic, lithologo-facies, and geochemical criteria. We have considered their significance and estimated the petroleum potential from a total of criteria. The structures of the Toarcian, Pliensbachian, and Hettangian-Sinemurian regional reservoirs of the Lower Jurassic have been characterized. The low petroleum potential of the reservoirs is due to the following factors: intricate structure, low and very low values of capacity-filtration parameters of reservoir rocks, widespread sealing beds of poor and very poor quality, low oil-generating potential of oil source rocks, and a high degree of catagenetic transformation of OM, as well as nearly ubiquitous saturation of reservoirs with formational waters.

Time-domain electromagnetic (TEM) soundings were performed in the vicinity of a plant in Saratov town, Russia, to detect hydrocarbon pollution of groundwater and to trace possible pollution flows in drainage zones. The presence of hydrocarbons in rocks is indicated by high electrical resistance and induced polarization (IP) effects found in many TEM curves. Calculations show that the polarizability is the ratio of the excess electrical conductance to the electrical conductance in pores. Relaxation time was found to be controlled mainly by the radius of pores in water-saturated sediments. IP effects can be recorded by TEM soundings in wet sediments with imperfect phase relationships if the pore radius is the in the range of 10

Differential-normalized electrical measurements (DNEM) have shown their efficiency in petroleum prospecting. The method implies detection of oil and gas deposits from polarization anomalies in haloes around rocks altered under the effect of the hydrocarbon pool. Polarization in the geo-electric section is determined layer-by-layer using space and time derivatives of the TEM process and differential-normalized parameters.