Composition and thermochronology of alkaline granites of Ingur massif: to problem of detection of factors contributing to formation of rare-metal mineralization in alkaline granites of Western Transbaikalia

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Abstract

The article considers the question of what factors contributed to the formation of rare metal mineralization in alkaline granites of Western Transbaikalia. It is based on the results of comparison of petro-geochemical characteristics of alkaline granitoids of nearby ore-bearing Ingur and ore-free Sherbakhtinsky massifs. The rocks of these massifs form a common series of compositions with variations from syenites to alkaline granites (in the Sherbakhtinsky massif) and from alkaline granites to pegmatites (in the Ingur massif). The formation of this series of rocks is associated with a deep differentiation of the original magma common to both massifs, accompanied by a sequential decrease in magnesiality and the accumulation of rare elements (Be, Ta, Nb, Th, U, HREE) in residual melts. They reach the highest values in the pegmatites of the Ingur massif, in which rare metal mineralization appears. Its formation is associated with the fact that, according to thermochronological studies, the Ingur massif for 6 million years was located in the temperature range from 900° to 500°. Such a long stay in the region of high temperatures was accompanied not only by deep differentiation of residual melts, but also stimulated fluid activity, which contributed to the redistribution and accumulation of ore elements in pegmatites.

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About the authors

D. A. Lykhin

Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences

Author for correspondence.
Email: lykhind@rambler.ru
Russian Federation, Moscow

V. V. Yarmolyuk

Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences

Email: lykhind@rambler.ru

Academician of the RAS

Russian Federation, Moscow

A. A. Vorontsov

A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences

Email: lykhind@rambler.ru
Russian Federation, Irkutsk

L. O. Magazina

Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences

Email: lykhind@rambler.ru
Russian Federation, Moscow

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Supplementary files

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2. Fig. 1. Scheme of the geological structure of the Ingursky massif according to [5, 9]. The inset shows the position of the region in the structures of the folded frame of the Siberian platform. 1 - modern river sediments: gravel, sand, clay; 2 - Quaternary river and lake sediments: gravel, sand, clay; 3 - Quaternary basalts; 4 - Early Cretaceous sediments, Turga suite: conglomerates, gravelites, sandstones, oil shales; 5-8 - rocks of the Ingursky massif: 5 - alkaline biotite-riebeckite granites; 6 - subalkaline biotite, alaskite and arfvedsonite granites, 7 - diabase, gabbro-diorite dikes; 8 - pegmatite bodies and their numbers; 9 - faults; 10 - sampling sites and their numbers; 11–16 – conditional for insertion: 11 – alkaline rock massifs and zones of their distribution (Syn – Synnyrskaya, Ud-Vit – Udino-Vitimskaya, Szh – Saizhenskaya, V-S – East Sayanskaya); 12 – granitoids of the Angara-Vitim batholith; 13 – complexes of late Paleozoic marginal belts; 14 – Siberian platform; 15 – paleocontinent; 16 – Paleoasian ocean.

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3. Fig. 2. Photographs of alkaline granites a–c (Vit-1/15) and pegmatites g–m (IGC-2/3, 2/4, 2/6, 4/1), a – with crossed nicols; the rest – in reflected electrons, made on an analytical scanning electron microscope JSM-5610LV with an Oxford INCA 450 X-ray energy-dispersive spectrometer. a – biotite-riebeckite granites with hypidiomorphic-granular structure and euhedral amphibole; b – zircon and ilmenite with an admixture of manganese and inclusions of quartz and pyroxene in granites; c – zircon with an inclusion of apatite and ilmenite with an admixture of manganese and fluorocarbonate intergrown with amphibole; g – zircon with an inclusion of magnetite in biotite; d – intergrowth of magnetite and zircon crystals in biotite; e – graphic intergrowth of rutile, cerium fluocerite and quartz in biotite; g – crystals of zircon, monazite, magnetite in amphibole; h – crystals of magnetite, monazite, ilmenite and zircon in amphibole; i – metamict crystal of unknown thorium mineral with admixture of phosphorus and yttrium intergrown with amphibole; k – crystals of ilmenite and completely disintegrated grain with admixture of Th and REE (presumably the above-mentioned Th mineral) intergrown with amphibole; l – disintegration of unknown mineral with formation of cerium fluocerite with all transition phases, as well as rutile and zircon; m – crystals of zircon, monazite and rutile in albite-feldspar pegmatite.

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4. Fig. 3. Results of 40Ar/39Ar study using the step-heating method of amphibole from granites of the Inguri massif.

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5. Fig. 4. Compositions of rocks of the Inguri massif on petrochemical diagrams. 1 – alkaline biotite-riebeckite, alaskite and arfvedsonite granites; 2 – pegmatites, 3 – compositions of rocks of the Sherbakhtinsky massif according to [3].

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6. Fig. 5. Graphs of the normalized distribution of trace elements according to [11] in granites and pegmatites of the Inguri massif. 1 – granites, 2 – pegmatites, 3 – alkaline granites of the Sherbakhtinsky massif according to [3], 4 – Early Mesozoic alkaline granite-porphyries of Central Mongolia according to [12].

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7. Fig. 6. Distribution of incompatible elements relative to Nb in the rocks of the Inguri massif. Conventional, see Fig. 3. The field of rock compositions of the Sherbakhtinsky massif is highlighted in gray based on data from [3].

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8. Fig. 7. Distribution of petrogenic oxides and incompatible elements relative to the Mg* value in the rocks of the Ingur and Sherbakhtinsky massifs. For the conventional values, see Fig. 3. The field of rock compositions of the Sherbakhtinsky massif according to data from [3] is highlighted in gray.

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