Home – Home – Jornals – Russian Geology and Geophysics 2021 number 10

In the northern Ladoga area, the age of the Sortavala Group rocks in the southeast of the Raahe-Ladoga zone of junction of the epi-Archean Fenno-Karelian Craton and the Paleoproterozoic Svecofennian province, their relationship with dome granitoids, the age of the provenances, and the time of metamorphic processes were estimated. The study was focused on the Nd isotope composition of rocks, the geochemical and isotope-geochronological parameters of zircon from the granite-gneisses of the Kirjavalakhti dome, the basal graywackes of the lower unit and the trachytes of the middle unit of the Sortavala Group, and the plagio- and diorite-porphyry dikes cutting the volcanosedimentary units of this group. The new isotope-geochemical data show a Neoarchean age of the granitoids of the Kirjavalakhti dome (2695 ± 13 Ma) and their juvenile nature (ε_Nd( T ) = +1.5). The granitoids underwent tectonometamorphic transformations (rheomorphism) in the Paleoproterozoic (Sumian) (2.50-2.45 Ga), which are recorded in the U-Th-Pb isotope system of the rims of the ancient cores of zircon crystals. The volcanosedimentary complex of the Sortavala Group formed on the heterogeneous polychronous (3.10-2.46 Ga) continental crust of the epi-Archean Fenno-Karelian Craton. With regard to the errors in determination of the age of clastic zircon, the minimum concordant U-Th-Pb ages of 1940-1990 Ma of detrital zircon from volcanomictic graywackes of the Pitkyaranta Formation can be taken as the upper age bound of terrigenous rocks, which agrees with the maximum age of the Sortavala Group rocks estimated from the U-Th-Pb (SIMS) age of 1922 ± 11 Ma of the Tervaoya diorites (Matrenichev et al., 2006). According to the proposed new tectonic model, the accumulation of the volcanosedimentary complex of the Sortavala Group, its metamorphism, erosion, and overlapping by the Ladoga Group turbidites had already occurred in the pericratonic part of the epi-Archean Fenno-Karelian Craton by the time of the Svecofennian continent-island arc collision, subduction, and formation of bimodal volcanoplutonic complexes of the young Pyhäsalmi island arcs and felsic volcanics of the Savo schist belt (1920-1890 Ma).

The Lahroud Ophiolite in northwestern Iran contains extensive zones of Paleozoic ophiolite as remnantsof the Paleo-Tethys oceanic crust. The principal rock units are gabbro overlain by pillowbasalt, which is intruded by granites and interbedded with pelagic sedimentary units including radiolariancherts. Geochemistry and radioisotope studies, supported by Nd, Sm, Sr, and Pb isotope data, indicatethat the Lahroud Ophiolite originates from a within-plate basaltic mantle source. The isotope studiesshow that the basalts are derived from Indian-type oceanic mantle sources. The radiogenic data indicatethe involvement of subduction-related terrigenous materials in the source magma. All the rocks are geochemically cogenetic and were generated by fractionation of a melt with a composition of average E-MORB with a calc-alkaline signature. Two ⁴⁰Ar/³⁹Ar ages, 343 ± 3 Ma for muscovite minerals and 187.7 ± 7.7 Ma for glasses, suggest that metamorphic and basaltic rocks formed during the Late Paleozoic to Early Jurassic, respectively. Microfossil studies show the presence of Paleozoic biostratigraphy. The crystallization process and rifting into the oceanic crust in the Lahroud Ophiolite probably began in the Carboniferous, with volcanic activity continuing during the late Triassic.

The paper concerns the sediment sequence, which is widespread in the Yenisei valley and in the Tuva and Minusa depressions and also present in the valleys of the southern Chulym plain. The sediments of this sequence were previously described as “Neogene mud-shedding”, as well as moraines, alluvial fan deposits, alluvium of Middle Pleistocene high terraces, and lacustrine sediments. The giant ripple marks on the Upper Yenisei terraces was commonly interpreted as ribbed moraines; however, in recent studies, these ridges have been repeatedly referred to as marks of giant current ripples. Besides, some recently published papers provide description of geology of this sequence fragments suggesting its deposition by cataclysmic floods. Geomorphological analysis of the area shows Pleistocene glaciers to have been localized within the medium-high mountainous areas. The glaciers did not reach the Tuva and Minusa depressions and occupied large areas only in the Todzha basin and on the periphery of the Darkhat basin, forming a glacial dam at its outlet, which resulted in glacial-dammed lakes filling the basin completely. These lakes outburst, and the resultant flooding led to the deposition of megaflood sediments, which we refer to here as the Upper Yenisei sediment sequence. A detailed analysis of its facies architecture revealed similarity of these sediments to those of the Sal’dzhar and Inya sequences in Gorny Altai. Most of the Upper Yenisei megaflood sediments are localized in topographic lows of the Tuva and Minusa depressions. Beyond the Altai-Sayan mountainous area, the megaflood sediments of the Upper Yenisei sequence compose high terraces of the Yenisei, Chulym, Chet’, and Kiya rivers in the southern Chulym plain. The formation of Upper Yenisei sequence dates to the first half of the Late Pleistocene, inasmuch as it contains inset alluvial sediments of the second terrace of the Yenisei River. The available data allow suggesting that the Upper Yenisei sequence formed in the first Late Pleistocene regional glaciation. The Sal’dzhar sequence in Gorny Altai and the fourth terrace of the Ob’ River on the Fore-Altai plain are stratigraphic analogs of the Upper Yenisei sequence. The Upper Yenisei and Sal’dzhar sequences can thus be considered future regional markers serving as a link for the local stratigraphic schemes of the Altai-Sayan mountainous area and adjacent West Siberian plains. The results obtained call for verification by geochronological dating, first of all, by modern luminescence dating methods covering a wider chronological interval than radiocarbon dating.

We consider the geologic structure of the Nerunda gold ore field located in the Nerunda-Mama ore district in northern Transbaikalia. Gold-quartz low-sulfide formation and ore-bearing carbonate-terrigenous strata and intrusive complexes are briefly described. An ore complex of beresite-listvenite metasomatites hosting carbonate-quartz veins and vein-veinlet zones is characterized. Two stages of ore formation have been recognized. Anomalous geochemical associations and the composition of ore mineralization typical of these stages have been established. Mineralogical and geochemical studies of gold-bearing metasomatites of the Nerunda ore field were carried out. The known geochemical and mineralogical search criteria used for the assessment of the erosion zone level of gold deposits were applied to the geologic conditions of the Nerunda ore field and the Nerunda-Mama gold ore district as a whole. The emphasis was made on the express assessment of the erosion zone level at the early stage of prospecting. We draw a conclusion about the gold potential of the poorly studied ore objects at depth and give guidelines for the following geological exploration.

The compositions of volatile components in cordierite, tourmaline, and quartz from pegmatites of the Kuhilal deposit were studied by pyrolysis-free gas chromatography-mass spectrometry (GC-MS), IR and Raman spectroscopy, and microthermometry, and their comparative analysis was performed. Capillary GC-MS was applied to determine the component composition and relative contents (rel. %) of volatiles from different zones of crystals and fractions of cordierite. It has been established that water and carbon dioxide prevail among them. Hydrocarbons are predominantly aliphatic, cyclic, and oxygenated. Heterocyclic, nitrogenated, and sulfonated compounds are present in gas-liquid inclusions in tourmaline and quartz, and volatile components are localized in both structural cavities and nonstructural positions in cordierites.

The paper presents results of a petrogeochemical and isotope-geochronological study of the granite-leucogranite association of the Pavlov massif and felsic volcanics from the Elash graben (Biryusa block, southwest of the Siberian craton). A characteristic feature of the granite-leucogranites is their spatial and temporal association with vein aplites and pegmatites of the East Sayan rare-metal province. The U-Pb age of zircon from granites of the Pavlov massif (1852 ± 5 Ma) is close to age of the pegmatites of the Vishnyakovskoe rare-metal deposit (1838 ± 3 Ma). The predominant biotite porphyritic granites and leucogranites of the Pavlov massif show variable alkali ratios (K₂O/Na₂O = 1.1-2.3) and ferroan (Fe^*) index and peraluminous composition; they are comparable with S -granites. The studied rhyolites of the Tagul River (SiO₂ = 71-76%) have a low ferroan index, an increased K₂O/Na₂O ratio (1.6-4.0), low (La/Yb) _n = 4.3-10.5, and a clear Eu minimum (Eu/Eu^* = 0.3-0.5); they are similar to highly fractionated I -granites. All the coeval Later Paleoproterozoic (1.88-1.85 Ga) granites and felsic volcanics of the Elash graben have distinct differences in composition, especially in the ferroan index and heavy REE content, owing to variations in the source composition and melting conditions during their formation at postcollision extension. A wide range of isotopic parameters of granites and felsic volcanic rocks (ɛ_Nd from +2.0 to -3.7) and zircons (ɛ_Hf from +3.0 to +0.8, granites of the Toporok massif) indicates the heterogeneity of the crustal basement of the Elash graben, which formed both in the Archean and Paleoproterozoic.

The mineralogy and contents of major and trace elements (including REE+Y) in bulk samples and separate size fractions of caviar-like ironstones from the Kamysh-Burun deposit (Kerch iron province) are studied to estimate the contributions of different REE+Y species to the total budget. The analyzed ore samples contain MREE adsorbed on Fe³⁺-(oxy)hydroxides, as well as LREE authigenic phosphates. The predominant rhabdophane-type (Ce(PO₄)nH₂O) phases are enriched in La, Pr, Nd, and Ca, depleted in Ce, and free from Th. The REE carriers belong to solid solution series of two main types: LREE(PO₄)·nH₂O - (Ca,Ce,Th)(PO₄)·H₂O (rhabdophane-like phase and brockite) or LREE(PO₄)·nH₂O - (Ca,U,Fe³⁺)((PO₄),(SO₄))·2H₂O (rhabdophane-like phase and tristramite). REE phosphates occur most often in the ≤ 0.25 mm fractions of ironstones, where average and maximum ΣREE contents ( X_av = 606-1954 ppm; X_max = 769-3011 ppm) are comparable with the respective amounts in the Chinese industrial clay-type REE deposits. The Kerch ores are commercially attractive unconventional resources of highly demanded Pr and Nd: they can be extracted at relatively low costs, due to high Pr/Ce and Nd/Ce ratios, while low Th and U reduce the environmental risks from stockpiled wastes.