Home – Home – Jornals – Russian Geology and Geophysics 2001 number 3

Water-bearing fluids are suggested to result from thermodynamically equilibrium interaction andesite--water in the zone of accumulation of hydrothermal solutions from peripheral magma chambers. Thus, intermediate magma chambers, which are derivates of the primary andesite magma, are assumed to exist. As a result of the interaction, Au and Ag, which are found in andesites and rhyolites in clarke concentrations, are mobilized into a fluid phase with Au enriched by a factor of 200-500 and with Ag enriched by a factor of 100-200. In addition, meteoric waters, infiltrating to the root zone of the hydrothermal block through andesites and coming to the surface through fissure chammels, cannot be a potential source of Au epithermal deposits. However, "empty" Au hydrotherms of meteoric origin can participate in redistribution and remobilization of gold, early deposited, with its removal to the surface at the final stage of evolution of a hydrothermal system. At the final stage, the flow of deep fluids begins to run out. The main dissolved Au mode of occurrence is hydrosulfide [Au(HS)₂]^-. Chloride complexes are subordinate no matter which rock (andesite or rhyolite) is in equilibrium. Silver occurs both as chlorides and as hydrosulfides. However, chlorides are predominant in the rhyolite fluid.

We have studied endogenic processes in the southeastern part of the Major Sayan Fault zone at the boundary of the Siberian Platform and Sayan-Baikal folded region. This is a zone of repeated occurrence of deformations, silica-alkaline metasomatism, and granite formation, which proceeded under moderate pressures no higher than 5.5-6 kbar. Some silica-alkaline metasomatites replacing each other with time include products of high-temperature granitization and quartz-albite-microcline (±magnetite, biotite, riebeckite, aegerine-augite) metasomatites and albitites. Early granitization took place under normal alkalinity with participation of rather reduced fluids, and postmagmatic fluid systems are carbon-enriched. Quartz-albite-microcline metasomatites formed at elevated alkalinity and F and f(O₂) activity. There is a clear difference between granitoids of the early tectonometamorphic cycles and the latest granites of the stage of marginal-suture activity, which are characterized by elevated concentrations of Nb, Y, Be, Sn, Pb, Th, and U.

This paper deals with study of epigenesis (dolomitization, recrystallization, and dissolution) of carbonate rocks at the Mississippi valley-type Pb-Zn deposits in the Sakha Republic (Yakutia). The average Pb + Zn content in carbonate rocks of the Sardana deposit slightly decreases with increasing dolomite content up to 65% and then drastically increases. This dependence is similar to that between the dolomite content and porosity of the Midale beds in the Charles oil field. This suggests that the paragenetic relationship between dolomites and the Mississippi valley-type Pb-Zn mineralization is due to the formation of an additional pore space as a result of epigenetic dolomitization. In ore occurrences of the Kamenka zone, the content of Zn increases with coarsening crystal grains of the host dolomites to 0.2-0.3 mm. Further recrystallization of dolomites does not cause changes in their Zn content. Outside the ore areas of the Sardana deposit, the Pb and Zn contents have been established to be higher in stylolites as compared with the host carbonate rocks by a factor of 390 and 1270, respectively. Metals in stylolite are suggested to be concentrated from a dissolved bed of carbonate rock. Therefore, the initial contents of metals disseminated in the productive member are estimated.

Stable spatial and temporal relationships of pyrite mineralization with acid volcanism make it possible to characterize rhyodacites of producing formations, no matter to which differentiates, of tholeiitic or calc-alkalic magmas, they belong. The rhyodacites of pyrite-bearing formations are distinguished by higher, as compared with average types, contents of iron (FeO* = 3.5-4% and more versus 2-2.5%), high temperatures of homogenization of melt inclusions, and contrast hematite-pyrite zoning of extrusive subvolcanic massifs. The massifs confined to deposit structures are distinguished by a wide development of the phenomena of perlite disintegration, liquation layering, and autobrecciation conjugate with autometamorphism implicately coming to hydrothermal changes of the stage of ore deposition. The extrusive subvolcanic massifs that lie far from ore fields have no such indications of fluid enrichment. In ore fields, the replacement of small-porphyry rhyodacites by large-porphyry and perlite decomposition liquation phenomena is considered evidence of active separation of fluids in connection with the origin of peripheral chambers at interparoxismal stages of ore deposition.

The Mongolo-Transbaikalian alkaline-granitoid province is more than 2000 km long and includes more than 350 plutons of alkali granites and syenites and numerous fields of volcanic derivates of alkaline-granitic magmas. Formation of the major volume of acid magmas in the province was considered to take place continuously from early Permian to early Triassic, about 280-240 Ma ago. The Rb-Sr and Sm-Nd isotope studies of the rocks of the typical trachybasalt-comendite bimodal series of the Tsagan-Hurtei Range showed its Late Triassic age, 2125 Ma. The isotopic Nd-Sm characteristics (_Nd(T) = + 1.9+2.8 both in trachybasalts and comendites) and a regular distribution of REE in basic and acid rocks of the series suggest affinity of these rocks and possible genetic relations of alkaline-granite melts with trachybasaltic magmas. Data on high-temperature homogenization of melt inclusions in phenocrysts of quartz from comendites (to 1100 ^oC) and on the presence of no less than 1 wt.% H₂O and F in these inclusions indicate considerable depths of magma generation or injections of high-temperature basaltic melts into acid magma chambers.

Three major stages of the evolution of sedimentation settings have been refined by lithofacies, geochemical, and microphytological studies. They correspond to the sequential changes of paleobasins: taphrogenic passive margin of the continent, back-arc basin, and foreland. Peculiarities of the paleogeographic zonation of each paleobasin have been determined according to facies analysis of transverse sections. The volume of stagnant conditions tends to increase up the section, reaching its maximum in the pre-Vendian time. On the basis of the reconstructions and actuopaleontological comparison, an ecobiological interpretation of some morphotypes of organic-walled microfossils (OM) has been proposed. Green and brown algae and bacterial associations characteristic of paleobasin zones of various depths have been identified. It has been proven that the appearance of new traits in some OM occurred at the Riphean-Vendian boundary, against the background of a change in the sedimentation pattern. The newly appearing species belong mainly to OM considered to be ancestors of green algae. It is suggested that the formation of the carbon component of the Precambrian aleuropelites was considerably contributed by benthic bacterial associations.

The Middle and Late Cretaceous paleogeography of the Great Urals is considered. Emphasis is placed on the sea basins existing in the region. A nomenclature of the basins is proposed. The temporal variations in their outlines and areas are traced. The positions of straits and the directions of biota migration are outlined.

The geologic structure and petroleum potential of the Bajocian-Bathonian deposits in southeastern West Siberia are considered. Investigation of the geochemistry of biomarker hydrocarbons performed at the Institute of Oil and Gas Geology, Novosibirsk, and the distribution of petroleum fields in the Middle Jurassic deposits suggest that the areas bearing prerequisites for hydrocarbon migration from petroleum-generating rocks of the Bazhenov and Togur Formations into Bajocian-Bathonian traps are most favorable for petroleum deposition. Such are the areas where the marine sediments of the Vasyugan Formation are replaced by the Naunak Formation, deposited under continental conditions, and the Paleozoic-Mesozoic boundary zone is made up of permeable rocks.

We studied the morphology, composition, occurrence, and sizes of deposits of secondary carbonates in Jurassic sedimentary rocks in the southeast of the Nyurol'ka depression (Tomsk Region). We have established a significant difference in the mineralogical and structural compositions of the carbonates as a result of their different genesis and the effect of oil and gas fluids during the formation and destruction of hydrocarbon pools. Diagenetic carbonates have a nearly stoichiometric composition and a nearly ideal structure and are least recrystallized. Carbonates that underwent catagenesis have larger crystals and a more isomorphous and defect structure. In the marginal zones of oil deposits the minerals are most crystallized. Siderite has the most defect structure, and dolomite is similar in structure to calcite, from which it might have formed.