Home – Home – Jornals – Russian Geology and Geophysics 2019 number 6

The Kultuminskoe deposit is located within the Gazimur metallogenic zone in eastern Transbaikalia. Min-eralization is associated with the Middle–Upper Jurassic Kultuma pluton composed of subalkaline rock series ranging from quartz monzonites and quartz syenites to granites and of monzodiorite dikes. Dikes of Late Jurassic age are composed of subalkaline gabbro. Analysis of fractionation trends of major and trace elements suggests that the monzonitoids prevailed in the Kultuma pluton and the dike complex formed through the differentiation of subalkaline basaltic melt from an enriched mantle source. The formation of the gold–copper–iron–skarn and me-dium-temperature veinlet-disseminated polysulfide and epithermal Ag–Te–Bi mineralization as well as iron–magnesia and silica–alkaline metasomatites was a long multistage process during the general evolution of the ore-magmatic system.

Study of granitoids spatially and genetically associated with gold mineralization within the Kara gold ore cluster has provided a new insight into their genesis, association with ore mineralization, and the sources of their ore material. The regional granitoids associated with gold mineralization are part of two individual complexes. One of them, earlier assigned to the Amanan complex, has an isotopic age of 182.9 ± 2.6 Ma and must be related to the subduction zone that existed on the southern margin of the Siberian continent in that period. Its granitoids differ in age and composition from the granitoids of the Amanan complex and must be separated as an independent taxonomic unit after an additional geological study. The second, Amudzhikan-Sretensk, complex has an isotopic age of 151.7 ± 1.9 Ma and might be related to the collision of the Siberian and Mongol–Chinese continents after the closure of the Mongol–Okhotsk ocean. In geochemistry the granitoids of the Amanan(?) complex correspond to adakites and must be considered melting products of the basaltic layer of the oceanic lithosphere. The granitoids of the Amudzhikan-Sretensk complex are similar in geochemistry to sanukitoids, melting products of subcontinental sources contaminated with continental-crust material. The granitoids of both complexes have high contents of gold and must be considered gold-bearing. In the Amanan(?) complex, adakites are the gold-richest rocks (as estimated from the slab melt composition), which in-dicates the primary nature of this gold. In the Amudzhikan-Sretensk complex, the highest contents of gold are specific to primitive sanukitoids, melting products of a mantle source with gold signatures. This suggests the primary nature of gold, whose content is determined by the portion of slab melt in the source of the rock material. The presence of adakites and primitive sanukitoids in the regional granitoid complexes indicates the exist-ence of a subcontinental mantle source with gold signatures during the magma generation. The source formed in the subduction zone that existed on the southern margin of the Siberian continent in the Early Jurassic and was remobilized under collision of the Siberian and Mongol–Chinese continents in the Late Jurassic. This source might have controlled both granitoid magmatism and ore mineralization.

The East Sikhote-Alin volcanic belt extending for ~1500 km is commonly considered a single tec-tonomagmatic structure formed during the Late Cretaceous subduction and the Cenozoic oceanic-slab breakup and active asthenospheric diapirism under transform plate sliding. Based on analysis of the published geological information and the new data on the age and trace-element and isotope compositions of the igneous rocks of the Late Cretaceous Bol’ba Formation, it is shown that the initial stages of volcanism in the southern and northern Sikhote-Alin took place in different geodynamic settings. In contrast to the coeval suprasubductional volcanics of the southern sector (Primorye), the volcanic section of the Bol’ba Formation is dominated by magnesian (Mg# = 26–40) adakites (La/Yb = 19–34) and high-Nb basalts. This igneous rock association and the lead (Δ8/4Рb = 30–46) and neodymium (0.51279–0.51281) isotope ratios of the studied rocks suggest the influence of the «hot» oceanic asthenosphere on magma genesis. The earlier slab breakup north of 48–49 °N was due to the oblique convergence of oceanic and continental lithosphere plates in the Late Cretaceous, accompanied by sinistral shears. The results obtained indicate that the lateral zonation of the eastern Sikhote-Alin is due to different geody-namic settings of formation of its northern and southern sectors rather than variations in its basement composi-tion. In theoretical aspect, the performed research is important for the correct reconstruction of the geologic events in zones of convergence of oceanic and continental plates. It is necessary to take into account not only the general direction of the convergence but also the configuration of the plate boundary.

Manganese silicate rocks together with silicate–magnetite ores and jaspers (Late Anisian–Ladinian) form lenticular or tabular bodies in the Triassic Sikhote-Alin chert formation. The lower part of the formation (Olenekian–Early Anisian) is enriched in clayey and organic matter. Nickel and cobalt compounds and other ore minerals in the Sikhote-Alin manganese silicate rocks belong to two genetic groups including minerals of the valence and ultimately reduced Ni, Co, and other metals. Minerals of the valence species of Ni, Co (sulfoantimonides, sulfoarsenides, sulfides, antimonides, ar-senides, tellurides, and silicates), and other metals (galena, sphalerite, chalcopyrite, arsenopyrite, wolframite, scheelite, molybdenite, cassiterite, stannite, cinnabar, stibnite, boulangerite, jamesonite, bournonite, löllengite, bismuthite, fahlore, altaite, native Sb and Bi, etc.) formed from the protolith material during metamorphism under the same conditions as the rock-forming minerals. The presence of minerals of ultimately reduced Ni, Co (maucherite, native Ni, Ni phosphide, Ni and Co chromides, disordered solid solutions, and intermetallic compounds of Ni with Cu, Zn, Sn, and Pb), and other metals (native Pb, Zn, Fe, Sn, Se, Au, Pt, “cupriferous gold”, and intermetallic compounds of Cu, Sn, Pb, Sb, Al, and Zn) in the manganese silicate rocks is due to the influence of the organic matter of the underlying carbo-naceous silicites. During metamorphism, the most volatile components (first of all, poorly bound water and hy-drocarbons) were released from the heated carbonaceous rocks; as a result, a metal-enriched fluid with highly or ultra-highly reducing properties appeared, which migrated along fractures into other rocks. The manganese silicate rocks are the products of contact metamorphism of siliceous rhodochrosite rocks formed through the diagenesis of biogenic siliceous muds enriched in Mn oxides and organic matter. Erosion of the weathering crust of islands composed of gabbroids of the Kalinovka, Vladimiro-Aleksandrovskoe, and Ser-geevka complexes (in the late Middle Triassic–Late Triassic) played the leading role in the formation of metallif-erous sediments. Manganese silicate rocks localized in the stratigraphic column above the carbonaceous silicites of the Triassic chert formation are enriched in Au (up to 35.38 ppm), Pt (11.27 ppm), and Pd (5.33 ppm). They contain noble-metal minerals and a wide spectrum of native elements and intermetallic compounds. The presence of Au–Pd–Pt–Ni–Co association (typomorphic for basic and ultrabasic rocks) in the Trias-sic protoliths of the manganese silicate rocks and carbonaceous silicites is probably due to the sorption of these elements by Mn and Fe hydroxides and organic matter during the exogenous weathering of the ancient Sikhote-Alin gabbroids.

The method of quasi-equilibrium directional crystallization was used for experimental modeling of the be-havior of noble metals in the presence of Te during the fractional crystallization of Cu- and Ni-rich sulfide mag-ma. The experimental melt contained (mol. %): Fe = 18.5, Ni = 19.1, Cu = 16.7, S = 44.1, and Pt = Pd = Rh = = Ir = Ru = Ag = Au = Te = 0.2, i.e., is similar in composition to the massive pentlandite–bornite ores of plati-num–copper–nickel deposits of the Noril’sk group. The crystallized sample consists of six primary zones differ-ing in chemical and phase compositions. The main minerals crystallizing from the melt include the following sul-fide phases: bornite solid solution (bnss), quaternary solid solution (tss), described earlier in the literature, and three phases (cfpn, cnpn, and npn), which we attributed to pentlandite according to their chemical composition. The primary phases crystallized from the melt decay on cooling with the formation of secondary phases. The cfpn, cnpn, and tss phases decay completely, and the npn and bnss phases, partly. As a result, secondary zoning forms in the sample. Formation of drop-like inclusions of telluride melt was observed in the end zone of the ingot. The obtained data show that pentlandites and tss are the main high-temperature concentrators of PGE, with each of the macrophases showing specific PGE accumulation. Eight types of impurity phases have been detected. They form by different mechanisms: crystallization from sulfide melt of refractory compounds, isolation from telluride melt, and formation through complete or par-tial decay of primary macro- and microphases. A scheme of the zonal structure of the crystallized sample and the evolution of the phase composition during fractional crystallization has been constructed. It clearly demonstrates the intricate formation of primary and secondary major-component and impurity zonings and can be used to explain the nature of the zoned struc-ture of massive PGE-bearing pentlandite–bornite orebodies.

This is a pioneering study on lateral zoning of groundwater chemistry and authigenic mineralogy in the Oxfordian regional reservoir of the Nadym–Taz interfluve. According to thermodynamic calculations, the nonequilibrium–equilibrium water–rock system lacks equilibrium with primary magmatic minerals, such as al-bite, anorthite, and microcline, though the water is moderately saline (up to 63.3 g/L) and has been in interaction with rocks for ~165 million years. Authigenic minerals form continuously and successively (kaolin-ite–montmorillonite–illite–micas–chlorite–albite–microcline) fr om waters that have certain рH and contents of SiO2, Al, Na, K, Ca, and Mg. The equilibrium of groundwater with primary aluminosilicate minerals impinges on a carbonate barrier, and almost all rocks are more or less strongly carbonatized. Authigenic mineral assem-blages from the southern Nadym–Taz interfluve include kaolinite unlike those from the northern part of the re-gion wh ere albitization is more common. Authigenesis generally decays in the eastern direction.

Paleostress inversion may be ambiguous when several markedly different local stress states are inferred for a group of outcrops. Attempts of reconstructing regional stress regimes (compressional, extensional, or strike-slip) by selecting local principal stresses of proximal directions turn out to have poor grounds. Each stress permutation (e.g., extension to compression) attendant with buildup of large irreversible strain (fault slip) requires a 5–6 kbar change in middle-crust horizontal stress and at least 50 Myr stable and uniform loading. Tectonophysical stress reconstructions for present active intracontinental orogens show heterogeneous patterns: Stress directions in uplifts are different from those in large intermontane basins and even in relatively subsided parts of mountain ranges or in adjacent uplifted zones (e.g., a plateau and a range). Paleostresses should be interpreted with reference to present stress fields in the respective areas. It is suggested to reconstruct regional stresses using the approach of L. Sim implying search for “common stress fields”. Another important technique is to trace stress changes in specific structures (large folds etc.) in the course of their evolution. The available data indicate correlation and bipolarity of stress states in large basins and uplifts.

The paper presents results of a seismogeological study based on analysis of seismic data and historical facts about the seismic activity of the Khambin fault zone. According to the data obtained, a genetic type of dislocations on conjugate faults (Gusinoe Ozero and Orongoi paleoseismogenic structures) is related to reverse faults with a strike-slip component. Geophysical studies of the Gusinoe Ozero structure have determined the dip of the fault plane toward the mountain framing of the depression and its outcrop at the bottom of the seismic scarp. The significant seismic potential of the Khambin fault is responsible for the maximum intensity of shocks in the nearby cities and settlements of southeastern Transbaikalia. The seismic fault activity has been confirmed by the historical earthquakes of 1856 and 1885, the M = 5 earthquake that occurred on 2October 1980, and at least two prehistoric earthquakes. The latest of the latter occurred no earlier than ~4 ka and had M = 7.0–7.3, while the earliest was even more intense and took place in the first half of the Holocene, no later than ~6 ka.