Gold-Bearing Pyrrhotite Ores in East Sayan: Composition and Formation Conditions (by the Example of the Ol’ginskoe Ore Occurrence)

Abstract

Abstract —We present results of study of the chemical composition, formation conditions, and genesis of gold-bearing pyrite–pyrrhotite ores widespread within the ophiolite belts in the southeast of East Sayan. The study was performed at the Ol’ginskoe ore occurrence localized in the Ol’gino gold ore zone. Sulfide mineralization zones are composed of lenticular interbeds and bodies subconfoimable with the bedding of the enclosing schist strata. The ores are carbonaceous siliceous garnet–biotite 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 Р–Т conditions of ore metamorphism (T = 430–540 °C, P ~ 5 kbar) correspond to the lower boundary of the epidote–amphibolite facies. Thermobarogeochemical studies of fluid inclusions in vein quartz showed close temperatures (536–340 °C) but lower pressures, 200–800 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 result 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 gold–sulfide–quartz deposits of the Urik–Kitoi zone in East Sayan.