The Ulug-Sair Gold Occurrence (Western Tuva, Russia): Mineralogy, Ore Genesis, and S-O Isotope Systematics

Ulug-Sair Au-Bi-Te-Se mineralization is one prospect for native Au in the Western Tuva, and its origin remains debated. Mineralization consists of gold-sulfide-quartz veins in the host sedimentary rocks (conglomerates, siltstones, shales), quartz-tourmaline, and quartz-carbonate-sericite-altered rocks. To determine its origin, we examined the mineralogical-geochemical features, formation conditions, and fluid sources of the Ulug-Sair ore. A mineralogical-geochemical investigation outlines two substages with Au: an early gold-sulfide-quartz with pyrite, chalcopyrite, galena, gold, and electrum; and a late gold-telluride-sulfide-quartz, characterized by the presence of Bi-bearing minerals (AgBiTe, Bi2Te2Se, Cu3BiS3, Bi), tellurides (Au and Ag), Se-tellurides (Ag and Bi), and selenides (Au, Ag, and Hg). The paragenesis of Au-Ag tellurides, and fluid inclusion study data (microthermometry, Raman spectroscopy, LA-ICP-MS, and crush leach analysis (gas and ion chromatography, ICP-MS) in quartz showed that quartz-tourmaline-altered rocks were formed by an aqueous Mg-Na-K-chloride fluid with a salinity of 8-10 wt % NaCl eq. at 325-370 degrees C, whereas the host quartz-carbonate-sericite-altered rocks were formed from CO2-H2O fluid containing CH4 and N-2, with a salinity of 0.18-6.1 wt % NaCl eq. at 200-400 degrees C. Gold-bearing mineral assemblages were formed at P similar to 0.75-1.0 kbar (similar to 2.3-3 km) due to CO2-H2O chloride (Na-K +/- Fe, Mg) fluid with CH4, Na2SO4, and Na2B2O5, and salinities 1.7-12.5 wt % NaCl eq. at temperatures decreasing from 360 up to 115 degrees C (gold-sulfide-quartz veins-360-130 degrees C, and gold-telluride-sulfide-quartz veins-330-115 degrees C), and variable fO(2), fS(2), fSe(2), and fTe(2). Results of the investigation of the isotope 2s composition of S in pyrites indicates the magmatic origin of the fluid (delta S-18(H2S) fluid from -0.4 to +2.5 parts per thousand). The stable O isotope data in quartz indicates that, at an early substage, the formation of ore involved a fluid of magmatic and metamorphic origin (from +8.2 to +11.6 parts per thousand), and, in the later substage, multiple sources of hydrothermal fluids (from +3.1 to +10.4 parts per thousand), including magmatic-derived, metamorphic-derived, and meteoric waters. These data, in conjunction with structurally controlled mineralization, point towards similarities of the Ulug-Sair ore system with orogenic gold deposits.