Evolutionary historical model of the Dukat silver giant

There are good grounds to believe that the inherited development of ore bodies controls the most important regularities in the location of deposits, especially of large ones, and also explains quite clearly the relatively selective ore occurrence in different magmas. Giant silver deposits were identified among both Paleozoic bodies and their regenerated Late Mesozoic vein-disseminated analogues. Jointly they represent a unified polychronic ore formation range. The performed investigations of mineral composition and the isotopic Sm-Nd system of granitoids in the Dukat ore field and their gold- and silver-bearing melanocratic inclusions established the possible occurrence of the Paleozoic juvenile continental crust of the Siberian Craton in their composition and confirmed our complicated scheme of oregenesis at the Dukat silver giant. The occurrence of high-grade gold in the melanocratic mineral aggregates of granitoids in the Dukat ore field should be regarded as a feature of hypabyssal gold-bearing zones being of high commercial interest.     

Gold deposits in dikes of the Yana-Kolyma belt

The ore-bearing dikes widespread in the Yana-Kolyma metallogenic belt vary in composition from granite porphyry to lamprophyres, gabbromonzonite, and dolerite. The dikes make up suites tens of kilometers in extent and up to 8–10 km wide or fields a few thousand square kilometers in area. Previously, the ore mineralization was traditionally referred to the gold-quartz type superimposed on dikes. However, isotopic and thermobarogeochemical data indicate that the mineralization is close to an intrusion-related gold deposit. A deep magma chamber could have been a source of sulfur in sulfides from beresites and quartz veins hosted in the Kolyma dikes. The deposits considered in the paper are permissive for large economic reserves with low and medium Au grades.     

Seasonal effects in the ionosphere-thermosphere response to the precipitation and field-aligned current variations in the cusp region

The seasonal effects in the thermosphere and ionosphere responses to the precipitating electron flux and field-aligned current variations, of the order of an hour in duration, in the summer and winter cusp regions have been investigated using the global numerical model of the Earths upper atmosphere. Two variants of the calculations have been performed both for the IMF B_y < 0. In the first variant, the model input data for the summer and winter precipitating fluxes and field-aligned currents have been taken as geomagnetically symmetric and equal to those used earlier in the calculations for the equinoctial conditions. It has been found that both ionospheric and thermospheric disturbances are more intensive in the winter cusp region due to the lower conductivity of the winter polar cap ionosphere and correspondingly larger electric field variations leading to the larger Joule heating effects in the ion and neutral gas temperature, ion drag effects in the thermospheric winds and ion drift effects in the F2-region electron concentration. In the second variant, the calculations have been performed for the events of 28–29 January, 1992 when precipitations were weaker but the magnetospheric convection was stronger than in the first variant. Geomagnetically asymmetric input data for the summer and winter precipitating fluxes and field-aligned currents have been taken from the patterns derived by combining data obtained from the satellite, radar and ground magnetometer observations for these events. Calculated patterns of the ionospheric convection and thermospheric circulation have been compared with observations and it has been established that calculated patterns of the ionospheric convection for both winter and summer hemispheres are in a good agreement with the observations. Calculated patterns of the thermospheric circulation are in a good agreement with the average circulation for the Southern (summer) Hemisphere obtained from DE-2 data for IMF B_y < 0 but for the Northern (winter) Hemisphere there is a disagreement at high latitudes in the afternoon sector of the cusp region. At the same time, the model results for this sector agree with other DE-2 data and with the ground-based FPI data. All ionospheric and thermospheric disturbances in the second variant of the calculations are more intensive in the winter cusp region in comparison with the summer one and this seasonal difference is larger than in the first variant of the calculations, especially in the electron density and all temperature variations. The means that the seasonal effects in the cusp region are stronger in the thermospheric and ionospheric responses to the FAC variations than to the precipitation disturbances.     

Geochemical peculiarities of ores from the largest Natalka gold deposit in Northeastern Russia

This study of the behavior of trace and rare earth elements in ores from the Natalka gold deposit allows us to draw several conclusions. It is suggested that ore formation is related to the regional metamorphism of the host terrigenous carbonaceous rocks, which could be the major source for trace and rare earth elements. Minor enrichment of the Natalka ores in W is evidence of the contribution of magmatic fluid, which could be superimposed on early quartz veins, in ore formation. Our results support the metamorphic–magmatic model of formation of economic gold–quartz deposits of the Yana–Kolyma Belt. The similarity of metasomatites of the Natalka deposit with disseminated gold–sulfide refractory ores from the Nezhdaninskoe and Bakyrchik deposits points to the possible presence of such ores in the Natalka deposit. Our data are important for forecasting regional metallogenic reconstructions, search, and evaluation of gold deposits.     

Tectono-metallogenic settings of the formation and convergence of disseminated Au–sulfide mineralization

Large Au-sulfide deposits (GSDs) of disseminated ores occur worldwide in metallogenic provinces of various ages (from Precambrian to Pliocene). The studies performed showed that the great genetic diversity of GSD is determined by the similar oregenesis conditions that appear in different tectono-metallogenic settings (TMSs). A GSD is included in a certain TMS by the respective changes in the mineral and geochemical assemblages of ores. However, in the majority of cases, the GSDs of different TMSs are convergent (quasi-identical) in the texture, structure, and mineral composition of ores. All types of the above TMSs are found in Russia, which allows forecasting the discovery of new GSDs in each setting.