Main Features of Formation of the Placers Gold Deposits on the Western Slope of the Middle Urals

Research subject. The article discusses a number of geochemical features (distribution of lanthanides and Th, values of (La/Yb)N and Eu/Eu*) of fine-grained clastic/clay rocks of various Vendian regional stages of the western slope of the Middle Urals. The results were interpreted in the context of data on the composition of the catchment areas of a number of modern rivers and information on the distribution of the same characteristics in suspended matter and modern bottom sediments of the “downstream North Dvina River–White Sea”.Methods and materials. The content of La, Sm, Eu, Gd, Yb, and Th in as many as 200 samples of shales and mudstones of the Serebryanka and Sylvitsa groups served as a research material. The research method was an analysis of the localization of individual and average data points of clayey rocks of the Laplandian, Redkinian, Belomorian and Kotlinian regional stages on paired diagrams (La/Yb)N–Eu/Eu* and (La/Yb)N–Th developed on the basis of analytical data on the composition of bottom sediments of estuarine parts of different categories/classes of modern rivers. Results. It was shown that the sedimentary sequences of the western slope of the Middle Urals are mainly composed of fine-grained clastic material belonging to categories 1 (sediments of world major rivers) and 2 (sediments of rivers draining sedimentary/mixed sedimentary formations). This material is likely to have entered the sedimentation area from the east, from the growing Kadomian Varanger-Kanin-Timan folded-thrust belt/Timan orogen, or/and from the west (Riphean sedimentary sequences of Kama-Belsk aulacogen). In addition, the igneous and metamorphic rocks of the East European Platform basement composing the structures, which could be classified as “igneous/metamorphic terranes” under modern classification, were likely to have made a certain contribution. Conclusions. The obtained results agree well with the conclusions obtained earlier as a result of mineralogical and petrographic studies of the conglomerates and sandstones of the Serebryanka and Sylvitsa groups, as well as with the data from a previous geochemical analysis of clay rocks.

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The first results of isotopic (U-Pb, ID-TIMS) dating of individual zircon grains from dolerite dikes of the Eastern zone of the Urals

LITOSFERA ◽

10.24930/1681-9004-2020-20-2-224-230 ◽

2020 ◽

Vol 20 (2) ◽

pp. 224-230

Author(s):

V. N. Smirnov ◽

K. S. Ivanov ◽

T. V. Bayanova

Keyword(s):

Isotope Geochemistry ◽

Early Carboniferous ◽

Dynamic Mode ◽

Middle Urals ◽

Volcanic Complex ◽

The Urals ◽

Electron Multiplier ◽

The Middle Urals ◽

Eastern Zone ◽

First Results

Research subject. The article presents the results of dating two dolerite dikes differing in geochemical features from a section along the Iset river in the area of Smolinskoe settlement (the Eastern zone of the Middle Urals). Materials and methods. The dating was performed by an U-Pb ID-TIMS technique for single zircon grains using an artificial 205Pb/235U tracer in the laboratory of geochronology and isotope geochemistry of the Geological Institute of the Kola Science Centre of the Russian Academy of Sciences. The lead isotopic composition and uranium and lead concentrations were measured using a Finnigan-MAT (RPQ) seven-channel mass spectrometer in dynamic mode using a secondary electron multiplier and RPQ quadrupole in ion counting mode. Results. The dikes were dated 330 ± 3 Ma and 240 ± 2 Ma. Conclusions. The research results indicate different ages of dolerite dikes developed within the Eastern zone of the Middle Urals. The oldest of the two established age levels corresponds to the Early Carboniferous era. This fact, along with the proximity of the dolerites to the petrochemical features of the basaltoids of the Early Carboniferous Beklenischevsky volcanic complex, allows these bodies to be considered as hypabyssal comagmates of these volcanics. The youngest obtained age level – Triassic – indicates that the introduction of some dolerite dikes was associated with the final phases of the trapp formation developed rarely within the eastern outskirts of the Urals and widely further east in the foundation (pre-Jurassic basement) of the West-Siberian Plate.

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