Abstract. The paper reports on the results of studies of ore-bearing rocks of the Ugakhan gold deposit (Bodaybo district): metasandstones, metasiltstones and carbonaceous shales. The rocks consist of quartz, feldspar (albite, orthoclase), Fe-Mg chlorite, mica (muscovite, sericite) and carbonates (calcite, dolomite, ankerite) and accessory titanite, rutile, tourmaline, zircon and apatite. All rocks contain fragments of microfossils exhibiting striking concentric zonation with alternated dark (carbonaceous matter) and light (carbonate-mica material) layers. In a range from metasandstones to carbonaceous shales, the rocks are characterized by an increase in mica amount and the content (up to 3%) of carbonaceous matter, as well as the formation of regeneration rims around relic tourmaline and zircon. The REE mineralization includes silicates (REE-bearing epidote, thorite), fuorocarbonates (bastnesite) and phosphates (monazite, xenotime, ankylite), which are closely related to U minerals (uraninite, cofnite). Bastnesite, ankylite and thorite formed due to the decomposition of earlier REE-bearing epidote, whereas monazite and xenotime are the products of decomposition of apatite. Uraninite formed during lithifcation of matrix of carbon-bearing rocks and is replaced by cofnite. The thermal analysis of carbonaceous matter and the formation temperature of chlorite calculated using chlorite geothermometer (296–371 °С) indicate the transformation of rocks under conditions of sericite-chlorite subfacies of greenschist facies of metamorphism.

Keywords: REE, Th and U minerals, microfossils, thermal analysis, metasedimentary rocks, gold deposit, Bodaybo region.

Received 24.11.2020, accepted 22.06.2021

E.V. Shepel, South Urals Federal Research Center of Mineralogy and Geoecology UB RAS, Institute of Mineralogy, Miass, Chelyabinsk district, 456317 Russia; liena-safna@mail.ru

N.R. Ayupova, South Urals Federal Research Center of Mineralogy and Geoecology UB RAS, Institute of Mineralogy, Miass, Chelyabinsk district, 456317 Russia; 

M.A. Rassomakhin, South Urals Federal Research Center of Mineralogy and Geoecology UB RAS, Institute of Mineralogy, Miass, Chelyabinsk district, 456317 Russia; 

P.V. Khvorov, South Urals Federal Research Center of Mineralogy and Geoecology UB RAS, Institute of Mineralogy, Miass, Chelyabinsk district, 456317 Russia

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