GEOLOGY & GEOPHYSICS, 2001, V 42, N 3, March.
PETROLOGY, GEOCHEMISTRY, AND MINERALOGY
Simulation of physicochemical processes of dissolution, transport, and
deposition of gold in epithermal Au-Ag deposits in
northeastern Russia..
393
Karpov I.K., Chudnenko K.V., Kravtsova R.G., and Bychinskii V.A.
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, <192>empty<169> 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 <$Eroman
{Ag(HS) sub 2 sup -}>. 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.
Gold, epithermal hydrothermal deposits, simulation of physicochemical processes
------------------------------------------------------------------------
Silica-alkaline metasomatism and granitoid magmaism in the Major Sayan Fault zone
409
Savel'eva V.B.
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 fO2 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.
Metamorphism, metasomatism, granites, East Siberia, Major Sayan Fault
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Epigenesis of carbonate rocks in the system orebody-host rocks at stratiform
lead-zinc deposits in the Sakh Republic (Yakutia)
427
Davydov Yu. V.
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.
Epigenesis, lead, zinc, mineralization, stratiform deposit, carbonate rocks,
dolomitization, recrystallization, dissolution, stylolites
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Extrusive subvolcanic rhyodacites of pyrite-bearing formations
434
Belgorodskii E.A.
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.
Rhyodacites, autobrecciation, perlite decomposition, liquation, sulfur-bearing
fluids
------------------------------------------------------------------------
Late Triassic stage of formation of the Mongolo-Transbaikalian alkaline-
granitoid province: data of isotope-geochemical studies.
445
Litvinovsky B.A., Yarmolyuk V.V., Vorontsov A.A., Zhuravlev D.Z., Posokhov V.F.,
Sandimirova G.P., and Kuz'min D.V.
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, 212...5 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 -C) 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.
Rb-Sr and Sm-Nd isotope dates, alkali granites, melt inclusions
PALEOGEOGRSPHY
Evolution of the Riphean-Vendian biolithogenesis settings in the Baikal mountainous area
456
Nemerov V.K. and Stanevich A.M.
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.
Biolithogenesis, lithogeochemistry, microfossils, paleogeographic settings,
geodynamic reconstructions, Late Precambrian, Baikal mountainous area
------------------------------------------------------------------------
Sea water areas of the Uralian region in the Middle and Late Creataceous
471
Amon E.O.
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.
Paleogeography, Middle and Late Cretaceous, sea basins and water areas,
Great Urals
GEOLOGY OF OIL AND GAS
Geologic structure and petroleum potential of the Bajocian-Bathonian deposits
in southeastern West Siberia
484
Kontorovich V.A.
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.
Hydrocarbon migration, Bajocian-Bathonian traps, Jurassic deposits, replacement
zones, Vasyugan Formation, Naunak Formation
------------------------------------------------------------------------
Carbonates in Jurassic deposits in the southeast of the Nyuril'ka depression
(Tomsk Region)
491
Nedolivko N.M., Zhukovskaya E.A., Bazhenov V.A.
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.
Sedimentary rocks, calcite, siderite, dolomite, sedimentogenesis, diagenesis, catagenesis, epigenesis, Jurassic
RECENSION
Petrochemical models.
502
Perchuk L.L.