pdf № 275, November

DOI: 10.19110/2221-1381-2017-11-3-8

A new model of geological structure for the Southern zone of the Early Proterozoic Pechenga-Varzuga Greenstone Belt (PVB) is proposed. In the article arguments are given that at the final stages of evolution in the Southern zone a young Svecofennian-Vepsian Volcanic Belt was formed, in many respects analogous to the volcanic belts of the Phanerozoic. This new approach to the PVB structure is important from both a geological and a metallogenic point of view, since deposits of a wide range of ore-bearing elements are associated with volcanic belts: Au, Ag, PGE, Sn, Bi, Sb, Pb, Zn, etc. The model is based on the results of the author’s mapping of various volcanic-tectonic structures of the region. In the Southern-Pechenga zone of PVB, an early group of linear Svecofennian volcano-tectonic structures and a later group of subisometric Vepsian volcanic structures were identified. In the Southern-Imandra-Varzuga zone, the volcano-tectonic Svecofennian structures are represented by the Early-Svecofennian linear and Late-Svecofennian subisometric varieties.

Keywords: Pechenga-Varzuga belt, Southern zone, volcanic belt, volcano-tectonic structure, Early Proterozoic, Svecofennian, Vepsian.

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DOI: 10.19110/2221-1381-2017-11-9-14

On the basis of qualitative interpretation of the gravitational field additional data were obtained, confirming the north-western («out-Uralian») orientation and sharply concordant interrelationships of the early structural assemblages of the Nerkayu eclogite-schist rock association with the rocks related to Uralian orogeny («uralids»). The features of the deep structure of the Nerkayu segment of the earth’s crust have been established for the first time, and the interrelation of density inhomogeneities in the middle and upper crust has been revealed. New arguments are presented in favor of the concept concerning the Nerkayu eclogite-schist complex as a tectonically displaced fragment of the Lower Precambrian crystalline basement, which is involved in the structure of Uralian orogeny rocks («uralids»).

Keywords: Subpolar Urals, Nerkayu eclogite-schist complex, deep structure, petrophysical characteristics, density inhomogeneities.

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DOI: 10.19110/2221-1381-2017-11-15-21

During the geomorphological studies in the mountainous part of Bashkortostan, new data were obtained on the existence of two denudation levels corresponding to the geomorphological cycles of the Urals development in the Paleogene and Neogene, as well as at least four levels of the high-altitude planation in the Quaternary period. Presumably, the Kaga structural-denudation depression, the hypsometric position of which is 540—560 m, corresponds to the level of Miocene planation, and the pediments and pediplains of the upper tier of the relief (with absolute marks of 600—640 m) characterize the level of Oligocene planation. They are confined to the weathering crust with brown iron ore, gold deposits. A hypothesis is advanced that frost weathering and mountain glaciation in the Pleistocene and Holocene were the most important factors for the formation of flat tops of high mountain ranges and slope terracing. The obtained information is necessary for tectonic reconstructions, geological mapping, searches for ore-bearing weathering crusts, restoration of the history of development of the western slope of the Southern Urals.

Keywords: leveling plane, pediment, pediplane, altiplanation terrace, mountain glaciation, Ural.

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DOI: 10.19110/2221-1381-2017-11-22-27

For the first time the mineralogical characteristics of fine native gold from ores of Novobobrovskoe rare-metal and rare earth deposit are presented. We studied the gold using microprobe technique and Raman spectroscopy. The gold finding in the ore (angular particle with dimensions 0.3 ´ 0.2 ´ 0.1 mm) is characterized by increased concentrations of silver (16—31 wt. % Ag), gradually increased to the periphery. Minerals of ore associations, gothite, monazite and quartz, were found on the surface of gold cuts by means of electron microprobe analysis and Raman spectroscopy. These minerals are typical for the ores of Novobobrovskoe deposit. Determined levels of concentration of impurity elements in monazite — cerium, neodymium, lanthanum, praseodymium, samarium (dominants: cerium — close to two tens wt. % Ce₂O₃ and neodymium — first ten wt. % Nd₂O₃), thorium (6—8 wt. % ThO₂) и strontium (2—3 wt. % SrO) are identical for chemical contents of monazites of this deposit.

Keywords: Native gold, thorium-bearing Ce-monazite, hydrothermal ore veins, Novobobrovskoe rare-metal and rare earth deposit, Middle Timan.

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DOI: 10.19110/2221-1381-2017-11-28-42

For the first time zircons from conglomerate-breccia bed of the Ichetyu occurrence were analyzed with local dating (U-Pb-method, SHRIMP-II and SIMS). The wide range of age values within the interval from 3283 to 706 Ma was determined. The high-yttrium and mid-yttrium varieties were marked among zircons, yttrium contents correlate with heavy rare earth elements and phosphorus. The obtained material corresponds to the dating results and geochemistry of zircons selected from underlying titanium ores of Pizhemskoe deposit. This fact proves a single zircon source for both industrial sites. In addition zircon could originate from different depth levels of underlying crystal basement.

Keywords: zircon, U-Pb-age, trace element composition, Ichetu occurrence, Middle Timan.

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