Home – Home – Jornals – Russian Geology and Geophysics 2004 number 10

Volcanogenic hydrothermal and submarine hydrothermal-sedimentary pyrite-polymetallic deposits are known from the Asian part of Russia. They form in island-arc and rifting continental-margin structures and are related to basaltoid or differentiated basaltoid volcanism. The sulfide ores of the recognized groups of deposits are characterized by a sufficiently close composition but differ in contents of gold and silver.
The ores of volcanogenic deposits contain 1.0-1.5 ppm Au and 15-30 ppm Ag, with gold and silver distributed rather irregularly. The maximum contents of gold (up to 7-8 ppm) occur in high-temperature chalcopyrite-pyrite ores of the root zones and in higher-temperature barite-polymetallic ores of subsurface deposition. A correlation between gold and silver has been established.
The ores of large-scale volcanogenic hydrothermal-sedimentary deposits contain 0.09-0.16 ppm Au on average. The content of silver in them reaches 80 ppm, averaging 15-35 ppm. There is no principal difference in levels of gold accumulation in the ores from tuffaceous-terrigene-carbonate rocks and in the ores from pyrrhotite-bearing carbon-carbonate-silica schists.
Mineralization systems that form both hydrothermal and hydrothermal-sedimentary pyrite-polymetallic deposits of massive sulfide ores of close composition are genetically related to occurrences of submarine basaltoid volcanism of different geodynamic settings, and it is unlikely that initially they had radically different contents of gold and silver. Significant differences in sulfur isotopic composition of sulfides are established between volcanogenic hydrothermal and hydrothermal-sedimentary types of massive sulfide ores. Sulfur isotopic composition of volcanogenic hydrothermal deposits is close to the mantle values, whereas sulfur of volcanogenic hydrothermal-sedimentary deposits is of mixed mantle-crustal nature because the sulfates of sea water are involved into hydrothermal system. Low contents of gold in the ores of hydrothermal-sedimentary deposits can be explained either by its partial deposition in deep zones where hydrothermal solutions and sea waters mix or by the fact that, being dissolved in sea water, gold migrated to more distal facies of the basin of ore deposition. Contents of silver in ores of both groups of deposits are kept approximately at the same level, though in some hydrothermal-sedimentary deposits the ores are enriched in silver. The volcanogenic hydrothermal deposits show close patterns of distribution of gold and silver in ores of ancient and recent sulfide mineralization systems.

Thermodynamic computer modeling of the Ag and Sb behavior in hydrothermal process at Ag-Sb deposits has been performed. Parameters of ore-forming fluids, obtained by studying fluid inclusions in minerals of igneous rocks and ores of Sn and Ag deposits, were used as original data for the modeling. Calculations intimate that Ag and Sb occur in concentrated high-temperature acid chloride solutions mainly as chloride species. Computer modeling showed high metal-bearing capacity of natural chloride solutions and their capability for transport of large amounts of Ag and Sb. Temperature regime is the most important factor for ore formation, which controls the deposition of various parageneses at different levels of hydrothermal ore-forming system as temperature decreases.

Methods of scanning and transmission electron microscopy and electron microprobe analysis were used to study magnetites from dolerite-rock salt contacts. It has been established that the first generation of magnetite was formed in the interval 750-800

A series of experiments on melting and crystallization of orendite from the Leucite Hills province (Wyoming, USA) was carried out at atmospheric pressure in air. The liquidus temperature of the melt has been determined, 130010

Using Raman spectroscopy, quenching crusts from mid-ocean ridges have been studied. This experimental method permits identification of the crystalline or amorphous state of a substance. It has been revealed that a decrease in the efficient rate of quenching of matter from periphery to core of a sample leads to the appearance of a disseminated crystalline phase. The characteristic size of the glass structural inhomogeneities, ~2 nm, has been estimated by low-frequency Raman scattering.

In August 2000, an internatonal expedition participating in the COPERNICUS project

New structural and tectonophysical data provide more details on the fault pattern and the stress field of the western Tunka rift in the southwestern flank of the Baikal rift system. The rift valley is controlled mostly by W-E and NE faults, and the rift ridges and basin links are dominated by NW and N-S faults. The fault pattern shows good correlation with the distribution of M3.3 earthquakes. Stress fields reconstructed from fracture measurements and correlated to the available earthquake mechanisms show that the rift currently evolves in a transtension environment.

Isostatic gravity anomalies were inverted under the assumption that compensation of topographic loads is only partly provided by Moho depth variations but is to a large extent due to the geometry of the lithosphere/asthenosphere boundary. This approach allowed us to reveal regional (long-wavelength) gravity lows possibly associated with Late Cenozoic mantle plumes beneath the Baikal rift and mountains in Central Mongolia. The plume tails were simulated in several 3D gravity versions. The position of plumes predicted from gravity inversion is consistent with zones of low seismic velocities in the mantle and with azimuthal anisotropy as the plumes control mantle flow above their tails. The position of most plumes does not contradict the pattern of Late Cenozoic flood volcanism known from geological evidence.

Laboratory experiments on transient electrochemical processes in synthetic and natural ore minerals revealed correlation of their potential response with electrochemistry. The shapes of potential curves correlate with electrochemistry in metals and with stoichiometry and contents of isomorphic impurities in synthetic galena. The geometric diversity of anode and cathode curves for natural minerals is controlled by their composition and physicochemical conditions of formation. Time-dependent polarizability functions record the compositional and electrochemical homogeneity of laboratory samples and in situ orebodies. The transient response of impurity-bearing polarizable conductors contains delays in potential or voltage buildup which produce peaks in polarization curves.