Home – Home – Jornals – Russian Geology and Geophysics 2024 number 3

On 21-25 November, 2022, Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences (Irkutsk), organized an All-Russian anniversary conference celebrating 65 years to the date of the Institute foundation and 105 years to the birth of its first director, Lev Vladimirovich Tauson, Full Member of the Russian Academy of Sciences. The results reported at the conference encompass a wide range of research fields in modern geochemistry, including isotope geochemistry of igneous, metamorphic, and sedimentary rocks in various geodynamic settings; chemistry of ore-magmatic systems and modern methods of mineral exploration; environmental geochemistry, geoecology, and paleoclimate; laboratory modeling and thermodynamic calculations of natural and production-related processes and materials; advanced analytical methods and information technologies for geosciences. The conference presentations pay tribute to Lev Tauson whose academic carrier, as well as all creative activity, had been closely related with the development of the Institute of Geochemistry. The preface paper provides a review of topics discussed at the conference concerning various geodynamic and geochemical problems, including sources of material, petrogenesis, and metallogeny.

Integrated geological data, petrogeochemical characteristics of magmatites of the Kurung complex (the Ket-Kap-Yuna igneous province of the Aldan Shield), and previously published isotopic data along with the results of geochronology and geochemistry research provide evidence for this complex evolving in the Late Cretaceous in a setting of riftogenesis (final phase) of continental margins. Similar to the Late Jurassic-Early Cretaceous polyformational volcano-plutonism of the Aldan Shield, the Late Cretaceous alkaline magmatism in the Ket-Kap-Yuna province is associated with the tectonomagmatic activation which manifested itself mainly as continental margin rifting associated with deep reorganization of the southwestern, southern and southeastern framing of the Siberian craton triggered by gravitational sliding of the lithospheric plates. Results of the petrogeochemical analysis of the studied magmatites revealed that the heterogeneous rock groups identified within this rock complex are not related to each other by fractional or other type of differentiation, thus suggesting their possible derivation as a result of fluid syntexis-type interaction between mantle-derived alkali-basite melt and felsic crustal melts. Alkali-basite parent melts are shown to have formed during partial melting of variably enriched mantle which is widely spread beneath the Aldan Shield and is varied in composition (from BSE enriched to nearly EM-I). Origination of such deep-seated melts (magma sources) was associated with the rifting processes (strike-slip tectonics) triggered by the setting of gravitational sliding of lithospheric plates. The formation of alkaline-salic magmas of the Kurung complex is associated with large-scale selective assimilation of crustal material by high-temperature fluidized mantle magmas during their ascent to the surface, possibly as a result of fluid syntexis of alkaline-basite mantle magmas and crustal smeltings formed under their influence.

We have first determined the petrogeochemical, Sm-Nd isotope, and U-Th-Pb geochronological (detrital-zircon LA-ICP-MS) characteristics of terrigenous rocks from a thick sequence in the south of the Sangilen block, which is part of the Central Asian Orogenic Belt (CAOB). The rocks are heterogeneous in facies composition: From west to east, there is a transition from graywacke sandstones with an admixture of pyroclastics to much more silicic lithoid arenites. Geochronological data on detrital zircons from the predominant graywacke sandstones mark the following intervals of concordant ages (Ma): 787-907, 1870-2236, 2613-2725, and 2900-2980. With regard to the oldest determined age (early Cambrian, 520 Ma) of the igneous rocks intruding these sandstones, the possible period of accumulation of terrigenous sequences in the south of the Sangilen block is estimated at 790-520 Ma. The obtained geochemical, Sm-Nd isotope, and U-Th-Pb geochronological data point to Neoproterozoic and early Precambrian island arc and continent-marginal complexes as probable provenances for the terrigenous deposits of the “southern band”. The relative proportions of rocks of these complexes in the deposits changed with distance from the sedimentary basin (from west to east, in modern coordinates): The portion of early Neoproterozoic juvenile rocks decreased, and the portion of early Precambrian metamorphic complexes increased. The Paleoproterozoic and Mezoarchean detrital zircons found in the terrigenous rocks of the Sangilen block could not originate from any known Precambrian complexes of the Tuva-Mongolian microcontinent. This gives grounds to regard other craton blocks of the CAOB as their sources.

Primary melt and fluid inclusions in quartz and dalyite of calcite-quartz carbonatitoids of the Murun massif have been studied. They contain CO₂, N₂, CH₄, C₂H₆, C₃H₈, H₂, and H₂S as well as sulfates and hydrocarbonates as solid daughter phases. The inclusion solutions contain HS- and chlorides. In addition, disordered carbon and bitumen are present. It is concluded that the magmatic fluid phase that was in equilibrium with quartz was characterized by a predominantly H₂O-CO₂-H₂S composition and a reduced state. This explains the appearance of sulfides at the magmatic stage of crystallization of calcite-quartz carbonatitoids of the Murun massif. We substantiate the hypothesis that the sulfates, hydrocarbons, and H₂ detected in the inclusions form at decreasing temperature as a result of shift of the redox equilibrium.

Multidirectional dynamics of changes in the chemical composition of the water of the only runoff of Lake Baikal, the Angara River, has been established. The cyclic-wave change in the interannual chemical composition of the Baikal water has been revealed, which indicates a long-term memory effect in the chemical time series of elements. Positive and negative temperature and chemical trends in the state of the water in the source of the Angara are observed, which are due to climate changes, the confinement of Baikal to the rift zone, and the composition of the enclosing rocks. It is shown that the current concentrations of monitored elements in the Angara source water depend also on their past concentrations but can change dramatically in the case of catastrophic environmental disturbances caused by natural and anthropogenic disasters (fires, drought, floods, earthquakes, etc.). The relationship between the elevated concentrations of uranium in the water and the seismic process in the Baikal area has been traced. The data obtained are based on the long-term (1950-2021) monitoring of the chemical composition of water in the Baikal ecosystem (Baikal, its tributaries, and Angara source).

Understanding the main events of platinum-group element (PGE) ore formation is impossible without analysis of the sources and behavior of major ore-forming components, namely, platinum, osmium, sulfur, and copper, which are important indicators of magmatic and hydrothermal processes. In contrast to the Re-Os isotope system, the radiogenic Pt-Os isotope system, as well as stable isotopes of Cu and S in PGE deposits, are still relatively understudied. Our comprehensive research is aimed at filling this gap. The paper presents data for the Guli massif of ultramafic and alkaline rocks and carbonatites in Polar Siberia and on the zonal Nizhny Tagil and Svetly Bor clinopyroxenite-dunite massifs in the Middle Urals, which include: (1) the contents of the highly siderophile elements (HSE) in whole rocks and platinum-group minerals (PGM), (2) the Re-Os and Pt-Os isotope systematics of chromitite, Os-Ir alloys, and Ru-Os sulfides, (3) the sulfur isotope composition in Ru-Os and Ir-Rh sulfides in primary and secondary PGM assemblages, and (4) the copper isotope composition in Pt-Fe minerals from chromitites and placers. The research was performed using scanning electron microscopy, electron probe microanalysis, and high-precision isotope-geochemical analysis. The high-precision Re-Os and Pt-Os isotope data show that the HSE contents in chromitites and PGM of the Guli massif were controlled by the composition of the mantle source that evolved with near-chondritic time-integrated Re/Os and Pt/Os ratios, which are also typical of the sources of most komatiites and abyssal peridotites. The δ⁶⁵Cu values of the studied samples of ferroan platinum and isoferroplatinum are identical within the analytical uncertainty and are close to 0‰, which is typical of high-temperature Cu-containing minerals. The sulfur isotope compositions of the Ir-Rh sulfides of the kashinite-bowieite series and of the Ru-Os sulfides of the laurite-erlichmanite series in the primary PGM assemblages indicate that the source of sulfur has a chondritic isotope composition, which is in agreement with the osmium isotope composition of the Ru-Os sulfides and Os-Ir alloys. The heavy sulfur isotope composition (δ³⁴S = 5.6 ± 1.5‰) of As-containing erlichmanite is consistent with its secondary origin. The new data on the isotope compositions of osmium, copper, and sulfur can be used as new important parameters that characterize the sources of PGE mineralization.

Here we report results of microforms’ studies of native gold and its alloys in igneous rocks, modified to varying degrees by secondary processes. We discuss the composition and occurrence of both the deep-seated magmatic gold-bearing alloys and the products of their transformation under conditions of the upper Earth’s crust. Gold-bearing Kamchatka adakites and ankaramites, Ildeus massif mafic-ultramafic intrusions and adakites from the Stanovoy fold system as well as dacites from the Bolivian Andes were formed during melting of either the suprasubduction mantle wedge or the subducted oceanic crust. In depleted peridotites from the Avachinsky Volcano in Kamchatka as well as suprasubduction ophiolites from Polar Urals, Eastern Sayan and the Western Mediterranean Betic-Rifean belt, the gold-bearing mantle was hybridized by subduction-related melts and high-temperature fluids. Volcanic rocks associated with the Lesser Khingan Fe-Mn deposits and Zolotaya Gora Au deposit in Southern Urals as well as Taragai ultramafic rocks in the South Khingan Range display subduction-related geochemical characteristics. Gold-bearing trachytes in the Virginian Appalachians (USA) represent felsic differentiates of mafic intraplate magmas. We propose that one of the principal forms of gold transport into the upper crustal environments is represented by Cu-Ag-Au alloys, which precipitated from mantle-derived silicate melt enriched in chalcophile and siderophile elements. Such Cu-Ag-Au alloy-rich magmatic rocks can either constitute primary sources of precious metals in the mantle-crust system or serve as geochemical precursors to the formation of native gold assemblages in epithermal and mesothermal ore deposits. Presence of magmatic gold particles in subduction-related igneous rocks and mantle restites hybridized by subduction-derived melts and high-temperature fluids suggest the existence of gold-rich horizons in the Earth’s mantle at depths comparable to typical depths of generation of primary convergent zone and some within-plate magmas.

The Ugakhan gold ore deposit is located within the Lena gold ore province, the largest one in Russia. It belongs to the group of deposits of the Sukhoi Log genetic type. We present results of petrological, mineralogical, and isotope-geochemical study of gold mineralization at the deposit. A scheme of the sequence of mineral formation at the deposit has been developed, which includes five stages: (1) the early (syndiagenetic) stage, when framboid pyrite I enriched in Au, Ni, Co, and As formed; 2) the stage of catagenesis of ore-bearing sediments, with recrystallization of early pyrite I and crystallization of pyrite II, also with elevated Au, Ni, Co, and As contents; (3) the stage of progressive metamorphism, with the formation of ore pyrrhotite from a water-CO₂ fluid with a high content of H₂S; (4) the ore formation stage, marked by an assemblage of pyrite III, galena, sphalerite, chalcopyrite, and native gold at the deposit, which was synchronous with regressive metamorphism in the region; (5) crystallization of post-ore euhedral coarse-grained pyrite IV. The geochemical and isotope (δ³⁴S and Pb-Pb) data rule out the input of mineral-forming components from an additional (external) source during the hydrothermal-metasomatic transformation of ore-bearing rocks. The δ³⁴S values in the early morphotypes of pyrite in the deposit ores vary from +5.7 to +9.1‰ and are close to the δ³⁴S values of the barren rocks of the Buzhuikhta Formation (+4.2 to +16.4‰). The Pb-Pb isotope characteristics and regularities of variations in Pb isotope composition established for gold mineralization indicate a predominant inflow of lead from Neoproterozoic metasedimentary strata. The mineral and geochemical specifics of the Ugakhan deposit are consistent with the concept of the metamorphic origin of gold deposits of the Sukhoi Log type, which confirms that the rocks of the Buzhuikhta Formation are promising for new gold ore objects.

The paper presents results of research into the conditions of formation of ores in the poorly studied Algama ore cluster (Bodorono deposit and Dyvok ore occurrence) located at the junction of the Aldan Shield and the Stanovoi area. We have established that Bi and Se minerals (bismuthinite, lillianite, native bismuth, tellurobismuthite, tetradymite, hedleyite, pilsenite, and laitakarite) are present in the ores of the Bodorono deposit. Two successive productive stages of mineral formation have been distinguished: Au-polymetallic and Au-Bi-Te. The corresponding minerals are products of the evolution of a hydrothermal system, during which a gradual decrease in fluid temperature (from 300 to 145 °C) and salinity (from 5 to 1.9 wt.% NaCl equiv.) took place. The fineness of native gold gradually increases from early (~840‰) to late (~940‰) stages and changes in passing from simple sulfides to sulfosalts. The evolution of the ore system is accompanied by a change in the composition of the vapor phase of fluid inclusions from CH₄-CO₂ to CO₂ with an impurity of N₂ and CH₄. The results of ⁴⁰Ar/³⁹Ar dating of pre-ore metasomatites point to ore-forming processes at the Bodorono deposit ca. 150 ± 1.8 Ma. Analysis of the isotopic composition of lead in galena shows the leading role of the ancient crustal source of ore matter. The calculated isotopic composition of oxygen (δ¹⁸O_H2O) in ore-bearing quartz varies from 1.0 to 7.3‰, which corresponds to an aqueous fluid of a mixed source. The Dyvok ore occurrence differs from the Bodorono deposit in the mineral composition of ores and the physicochemical parameters of ore formation. Four mineral stages have been established within the ore occurrence: gold-arsenopyrite-pyrite-quartz, pyrite-chalcopyrite-sphalerite, quartz-boulangerite, and telluride. The telluride stage is represented by hessite, altaite, volynskite, merenskyite, melonite, and rucklidgeite. Gold-bearing mineralization formed from a fluid of medium salinity (0.9-9.2 wt.% NaCl equiv.) with a predominance of CO₂ and an impurity of CH₄ in the vapor phase at moderate temperatures (310-360 °C). The calculated values of δ³⁴S and δ¹⁸O varied from 2.2 to 3.0‰ and from 0.6 to 12.0‰, respectively. The ⁴⁰Ar/³⁹Ar age of gold mineralization is 124.0 ± 1.5 Ma, which corresponds to the stage of tectonomagmatic activity in the Aldan Shield.