Isotope-geochemical characteristics of Wenlock carbonate and terrigenous-carbonate deposits of the Subpolar Urals and the southern part of the Chernyshev ridge

Research subject. Wenlock deposits in the Subpolar Urals and southern part of the Chernyshev Ridge. Materials and methods. Carbonate and terrigenous-carbonate rock samples from sections of the Subpolar Urals (outcrops 212, 217) and the southern part of the Chernyshev Ridge (outcrop 479) were studied by isotope analysis of carbonate carbon and oxygen.Results. The isotopic composition of the studied sections varies across a wide range of δ13С (–6.4…–0.05‰) and δ18O (20.0–26.9‰). Therefore, three time intervals characterized by distinctive isotopic characteristics can be distinguished, roughly corresponding to early Sheinwoodian (I), late Sheinwoodian (II) and Homerian (III). Interval I is characterized exclusively by the rocks of outcrop 479 with δ13С (–3.6…–3.0‰) and δ18O (22.4–23.6‰). In Interval II, the average values of isotopic characteristics of outcrop 479 indicate a tendency to weighting carbon (–5.5…–3.5‰) and somewhat heavier oxygen (23.2–25.2‰) isotopes. In outcrop 212, the average isotopic values for carbon and oxygen vary from –2.9 to –1.3‰ and from 21.9 to 24.3‰, respectively. In outcrop 217, the average values are δ13C (–1.8…–0.8‰) and δ18O (22.4–25.4‰). In Interval III, the average values of carbon isotopic composition in outcrop 479 are becoming heavier from –2.5 to –0.7‰. In outcrop 212, the isotopic values of oxygen (21.9–23.1‰) and carbon (–4.9…–2.5‰) tend to become weighting; however, in outcrop 217, the average isotopic values of δ13C (–1.9…–0.5‰) and δ18O (22.3–24.5‰) remain constant. The conducted litho-facial studies showed that the weighting carbon isotopic composition ranging from –6.4…to –3.0‰ is associated with an increase in microbial activity in sediments, the manifestation of vadose-phreatic conditions, and the intake of light carbon dioxide with a flow of terrigenous material from the earth. In the latter case, oxygen isotopic values are also the most lightweight (20.0–23.0‰).Conclusions. The obtained isotopic characteristics of the Wenlock rocks under study indicate the expedience of identifying three time intervals and their correlation with paleogeographic reconstructions of Wenlockian sedimentation in the Timan-northern Ural region.

Native Gold in the Chudnoe Au-Pd-REE Deposit (Subpolar Urals, Russia): Composition, Minerals in Intergrowth and Genesis

Composition of native gold and minerals in intergrowth with rhyolites of the Chudnoe Au-Pd-REE deposit (Subpolar Urals, Russia) was studied using optical microscopy, scanning electron microscopy, and electron microprobe analysis. Five varieties of native gold have been identified, based on the set of impurity elements and their quantities, and on intergrown minerals. Native gold in rhyolites from the Ludnaya ore zone is homogeneous and contains only Ag (fineness 720‰, type I). It is in intergrowth with fuchsite or allanite and mertieite-II. In rhyolites from the Slavnaya ore zone, native gold is heterogeneous, has a higher fineness, different sets and contents of elements: Ag, Cu, 840–860‰ (type II); Ag, Cu, Pd, 830–890‰ (III); Ag, Pd, Cu, Hg, 840–870‰ (IV). It occurs in intergrowth with fuchsite, albite, and mertieite-II (type II), or albite, quartz, and atheneite (III), or quartz, albite, K-feldspar, and mertieite-II (IV). High fineness gold (930–1000‰, type V) with low contents of Ag, Cu, and Pd or their absence occurs in the form as microveins, fringes and microinclusions in native gold II–IV. Tetra-auricupride (AuCu) is presented as isometric inclusions in gold II and platelets in the decay structures in gold III and IV. The preliminary data of a fluid inclusions study showed that gold mineralization at the Chudnoe deposit could have been formed by chloride fluids of low and medium salinity at temperatures from 105 to 230 °C and pressures from 5 to 115 MPa. The formation of native gold I is probably related to fuchsitization and allanitization of rhyolites. The formation of native gold II-V is also associated with the same processes, but it is more complicated and occurred later with a significant role of Na-, Si-, and K-metasomatism. The presence of Pd and Cu in the ores and Cr in fuchsite indicates the important role of mafic-ultramafic magmatism.

Petrochemical features of granite-gneisses of the Nikolaishor massif (Subpolar Urals)

The Nikolaishor massif is located within the heavily eroded gneisses of the Nyarta metamor¬phic complex and is an au-tochthonous massif composed of plagiogranites and kalispartic granites. The conducted petrochemical study of the least modified variety of granites of this massif made it possible to identify the petrochemical features of the studied rock, as well as to determine the geodynamic conditions for the formation of the Nikolaishor massif. According to the results of the silicate analysis, granites are high-alumina leucogranites of the potassium-sodium type. The initial substrate for the stud¬ied rocks was presumably magmatic protolith. Based on the ICP-MS analysis data, various in-dicator ratios were calculated for the granites. The obtained indicators allowed us to conclude that the rocks under consideration were formed from melts enriched mainly with light TR, which belong to a deep crustal type source. Comprehensive study of multivariate discrimi¬nant analysis indicators for the main elements of S. Agrawal, as well as diagrams of J.A. Pearce, D. Papu, N.B. Harris, showed that the rocks of the Nikolaishor massif are syncollisional granites formed in the Late Orogenic time.

Stomatocysts of Chrysastrella paradoxa Chodat (Chrysophyceae) from herb-mosses mire at the western slope of the Subpolar Urals (Russia)

Stomatocysts of Chrysastrella paradoxa was observed in a herb-mosses mire on the territory of “Yugyd va” National Park (the Subpolar Urals, Russia). The morphology of stomatocysts was studied by scanning electron microscopy. This is the second record of Ch. paradoxa in Russia.

A gap in sedimentation in the Silurian section of the Subpolar Urals at the Ludlow-Pridoli boundary

Research subject. The article discusses the results of a new detailed study of a reference section of the Upper Silurian in the Subpolar Urals. This study was undertaken to clarify the existing contradictions concerning the age of the Ludlow-Pridoli boundary deposits and the definition of the Ludlow-Pridoli boundary, which is based on the study of different fauna groups.Materials and methods. The newly collected collections contained more than 100 samples of sedimentary rocks with fossil macro fauna, 22 tests on microfauna, 198 tests on chemical analysis for determining the content of Ba, Sr and δ13C and δ18O isotopes in carbonates. The results of experiments were confirmed by the authors’ bio-sedimentological, paleo-ecological and chemostratigraphic data.Results. The conducted research confirmed the existence of a gap in sedimentation at the end of Ludlow; clarified the thickness of the Sizim stage in the reference section; elucidated its sedimentological and chemostratigraphic characteristics; allowed changes in biodiversity due to a change in the sedimentation regime, paleoecological impact on biota in the late Ludlow and restoration of biota in the early Pridoli to be traced. The study also demonstrated that the time boundaries of the transgressive and regressive stages in the development of the Northern Ural sea basin and the event-stratigraphic boundary of the Ludlow-Pridoli were directly related to the main global events in the Late Silurian (Lau Event, Lower Pridolian Event), the traces of which are preserved in the studied section.Conclusions. The intensification of regressive tendencies across the largest part of the Northern Ural paleobasin in the Late Ludlow, widespread development of microbial biota, cessation of the Silurian reef formation, as well as the extinction of Pentamerida brachiopods – exclusively, indicate a significant ecosystematic restructuring in the late Ludlow. It can be assumed that the absence of a significant positive deviation of the δ13C global Lau Event in this section is associated with the identified gap, the amplitude of which correlates with the Ozarkodina snajdri and Ozarkodina crispa zones located above the Polygnathoides siluricus zone in the conodont sequence of the Upper Ludlow.

Anatomy of intergrowth of florencite and xenotime crystals with oscillatory zoning from veins of the Svodovy Au-REE occurrence, Subpolar Urals

The morphology and anatomy of a paragenetic intergrowth of REE minerals (xenotime and forencite) from veins of Svodovy Au-REE occurrence (Subpolar Urals) are described. The sequence of crystallization of minerals is identifed using the features of internal structure of individuals and aggregates: surfaces of diferent types, sector boundaries and oscillatory, trend and sectorial zoning. Evidence for a HREE-to-LREE directive crystallization of REE phosphates is given. The linear and volumetric rates of face growth are analyzed. The mechanisms of crystal growh and formation of zoning and surfaces of joint growth of sectors of diferent faces are discussed. The crystals are studied using an original method of analysis of mineral aggregates with oscillatory zoning. Figures 9. Tables 2. References 21.