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Vol 52, No 3 (2017)
- Year: 2017
- Articles: 5
- URL: https://journals.rcsi.science/0024-4902/issue/view/9860
Article
Criteria for the detection of hydrothermal ecosystem faunas in ores of massive sulfide deposits in the Urals
Abstract
The ore-formational, ore-facies, lithological, and mineralogical-geochemical criteria are defined for the detection of hydrothermal ecosystem fauna in ores of the volcanic-hosted massive sulfide deposits in the Urals. Abundant mineralized microfauna is found mainly in massive sulfide mounds formed in the jasperous basalt (Buribai, Priorsk, Yubileinoe, Sultanov), rhyolite–basalt (Yaman-Kasy, Blyava, Komosomol’sk, Sibai, Molodezhnoe, Valentorsk), and the less common serpentinite (Dergamysh) formations of the Urals (O–D2). In the ore-formational series of the massive sulfide deposits, probability of the detection of mineralized fauna correlates inversely with the relative abundance of felsic volcanic rocks underlying the ores. This series is also marked by a gradual disappearance of colloform pyrite, marcasite, isocubanite, pyrrhotite, and pyrite pseudomorphoses after pyrrhotite; increase of the amount of bornite, fahlores, and barite; decrease of contents of Se, Te, Co, and Sn in chalcopyrite and sphalerite; and decrease of Tl, As, Sb, and Pb in the colloform pyrite. Probability of the detection of mineralized fauna in the morphogenetic series of massive sulfide deposits decreases from the weakly degraded sulfide mounds to the clastic stratiform deposits. The degradation degree of sulfide mounds and fauna preservation correlates with the attenuation of volcanic intensity, which is reflected in the abundance of sedimentary and volcanosedimentary rocks and the depletion of effusive rocks in the geological sections.
173-191
Ferromanganese carbonate metasediments of the Sob area, Polar Urals: Bedding conditions, composition, and genesis
Abstract
The paper presents the results of study of ferromanganese carbonate rocks in the Sob area (Polar Urals), which is located between the Rai-Iz massif and the Seida–Labytnangi Railway branch. These rocks represent low-metamorphosed sedimentary rocks confined to the Devonian carbonaceous siliceous and clayey–siliceous shales. In terms of ratio of the major minerals, ferromanganese rocks can be divided into three varieties composed of the following minerals: (1) siderite, rhodochrosite, chamosite, quartz, ± kutnahorite, ± calcite, ± magnetite, ± pyrite, ± clinochlore, ± stilpnomelane; (2) spessartite, rhodochrosite, and quartz, ± hematite, ± chamosite; (3) rhodochrosite, spessartite, pyroxmanite, quartz ± tephroite, ± fridelite, ± clinochlore, ± pyrophanite, ± pyrite. In all varieties, the major concentrators of Mn and Fe are carbonates (rhodochrosite, siderite, kutnahorite, Mn-calcite) and chlorite group minerals (clinochlore, chamosite). The chemical composition of rocks is dominated by Si, Fe, Mn, carbon dioxide, and water (L.O.I.): total SiO2 + Fe2O3tot + MnO + L.O.I. = 85.6−98.4 wt %. The content of Fe and Mn varies from 9.3 to 55.6 wt % (Fe2O3tot + MnO). The Mn/Fe ratio varies from 0.2 to 55.3. In terms of the aluminum module AlM = Al/(Al + Mn + Fe), the major portion of studied samples corresponds to metalliferous sediments. The δ13Ccarb range (–30.4 to–11.9‰ PDB) corresponds to authigenic carbonates formed with carbon dioxide released during the microbial oxidation of organic matter in sediments at the dia- and/or catagenetic stage. Ferromanganese sediments were likely deposited in relatively closed seafloor zones (basin-traps) characterized by periodic stagnation. Fe and Mn could be delivered from various sources: input by diverse hydrothermal solutions, silt waters in the course of diagenesis, river discharges, and others. The diagenetic delivery of metals seems to be most plausible. Mn was concentrated during the stagnation of bottom water in basin-traps. Interruption of stagnation promoted the precipitation of Mn. The presence of organic matter fostered a reductive pattern of postsedimentary transformations of metalliferous sediments. Fe and Mn were accumulated initially in the oxide form. During the diagenesis, manganese and iron oxides reacted with organic matter to make up carbonates. Relative to manganese carbonates, iron carbonates were formed under more reductive settings and higher concentrations of carbon dioxide in the interstitial solution. Crystallization of manganese and iron silicates began already at early stages of lithogenesis and ended during the regional metamorphism of metalliferous sediments.
192-213
Crystal chemical and geochemical features of genetically different paleozoic dolomites in the Volga–Ural region
Abstract
The comparative analysis of primary sedimentary and secondary catagenetic dolomites revealed that they differ in many typomorphic properties. Sedimentary dolomites are characterized by the pelitomorphic and fine-grained structure, high stoichiometry, and presence of the electron-hole (p-n) centers [SO2]– and [SO3]– in the structure. Secondary dolomites have a fine- to medium-grained structure and distinguished by a lower degree of stoichiometry. Their crystal lattice shows isomorphous replacements Mg ↔ Fe and Mg ↔ Ca. They can be divided into two (massive and porous) varieties. The massive varieties are characterized by the conformal structure and xenomorphic habitus of the closely packed idiomorphic and hypidiomorphic grains. The Ca–Mg and Fe–Mg isomorphism in the massive dolomites is less expressed than in the porous varieties. These specific features of dolomite varieties are related to their different formation settings. The porous dolomites were formed in a free environment that fostered slow growth of the relatively large equant crystals with a high degree of isomorphism, whereas the massive dolomites were formed under “stringent constraints” during a faster growth of crystals.
214-225
Formation mechanism of diapirs of relict lagoonal sediments on the underwater slope of the Curonian Spit
Abstract
Geological-geophysical methods were applied to provide deeper insight into the nature of unique relict lagoonal sediments exposed on the underwater slope of the Curonian Spit. The study of textural features and morphology of the deformed lagoonal sediments indicates a similarity of their formation mechanism with clay diapirism. Analysis of seismoacoustic data and side scan sonar survey allowed us to establish the configuration of paleolagoon and to determine the direction of dune migration in the past.
226-233
History of the geological evolution of the Tsil’ma River basin (midle Timan) in the Devonian
Abstract
The results of bio- and lithostratigraphic studies of the Givetian-Frasnian rocks in the Tsil’ma River basin are reported. They suggest regularities in sedimentation: distinct rhythmicity and similar succession in the structure of formations. We have identified five palynocomplexes that characterize the formations and make it possible to accomplish a confident biostratigraphic subdivision of sections. Their correlation with coeval complexes in the adjacent areas has been accomplished. The results made it possible to unravel specific features of miospore assemblages formed in the continental and coastal-marine facies.
234-248