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Russian Geology and Geophysics

2019 year, number 5

GOLD-BEARING PYRRHOTITE ORES IN EAST SAYAN: COMPOSITION AND FORMATION CONDITIONS (by the example of the Olginskoe ore occurrence)

B.B. Damdinov, L.B. Damdinova, S.M. Zhmodik*, and A.G. Mironov

Geological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Sakhyanovoi 6a, Ulan Ude, 670047, Russia

*V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences,
pr. Koptyuga 3, Novosibirsk, 630090, Russia
Keywords: Gold, pyrrhotite ores, composition, formation conditions, genesis, East Sayan

Abstract

We present results of study of the chemical composition, formation conditions, and genesis of gold-bearing pyritepyrrhotite ores widespread within the ophiolite belts in the southeast of East Sayan. The study was performed at the Olginskoe ore occurrence localized in the Olgino gold ore zone. Sulfide mineraliza-tion zones are composed of lenticular interbeds and bodies subconformable with the bedding of the enclosing schist strata. The ores are carbonaceous siliceous garnetbiotite schists variably enriched in sulfide minerals, mainly pyrrhotite, with impurities of other silicate minerals: tremolite, chlorite, albite, plagioclase, diopside, and epidote. In addition, sphalerite, chalcopyrite, pyrite, and arsenopyrite are present in small amounts in the ores. The established Ж conditions of ore metamorphism (T = 430540 , P ~ 5 kbar) correspond to the lower boundary of the epidoteamphibolite facies. Thermobarogeochemical studies of fluid inclusions in vein quartz showed close temperatures (536340 ) but lower pressures, 200800 bars, which indicates that the quartz vein formation was related to a pressure drop. The predominant salts of the fluid inclusions are Fe and Mg chlorides with Na and K impurities. The geologic location, structures, textures, and mineral, geochemical, and isotope compositions of the studied sulfide ores indicate their formation in submarine deep-water environments as a re-sult of the activity of hydrothermal systems, analogues of black smokers, later subjected to metamorphism. During metamorphism, primary ores underwent mineral and chemical transformations: pyrrhotitization of pyrite, appearance and coarsening of native gold particles, and redistribution of components (Mn, As, etc). The low contents of gold and some ore-forming elements (Zn, Cu, and Pb) in pyrrhotite ores testify to the removal of these elements with a metamorphogenic fluid. The removed ore-forming components might have been a source of material for later goldsulfidequartz deposits of the UrikKitoi zone in East Sayan.

DOI: 10.15372/RGG2019028