The Ak-Sug Porphyry Copper-Gold-Molybdenum Deposit, East Sayan: Noble Metal Mineralization, PT-Parameters, and Composition of Ore-Bearing Fluid

Ore mineralization of the Ak-Sug Porphyry Copper-Gold-Molybdenum deposit formed during three stages: 1) porphyry-copper mineralization with simple sulfides in quartz-sericite and quartz-sericite-chlorite metasomatites, 2) subepithermal Au-Bi-Te-Pd-quartz mineralization in quartz-sericite metasomatites, and 3) intermediate-sulfidation Au-Ag mineral assemblages with selenides, tellurides, and Sb and As sulfosalts in argillisites. Fluid inclusion studies (microthermometry, Raman spectroscopy) of quartz and mineral thermometry (an assemblage of Au and Ag tellurides) showed that porphyry copper and subepithermal mineralization precipitated from hydrocarbon-aqueous-chloride (Na-K +/- Fe) fluid with salinity of 20.1-32.8 wt % NaCl eq. at 435-375 degrees C and hydrocarbon-aqueous-chloride (Na-K +/- Fe +/- Ca +/- Mg) fluid with salinity of 7.5-15.0 wt % NaCl eq. at 415-325 degrees C, respectively. The epithermal mineral assemblages precipitated at similar to 0.55 kbar from hydrocarbon-aqueous-chloride (Na-K +/- Fe +/- Ca +/- Mg) fluid with salinity of 1.4-12.6 wt % NaCl eq. at 370-200 degrees C. The latest low-temperature (240-190 degrees C) and diluted (3.5-4.9 wt %) fluids are characterized by variations in Na and K chlorides; Fe2+, Fe3+, Ca, and Mg carbonates; and Na, K, and Mg sulfates. The S isotopic composition of the fluid of different mineral assemblages varies from -2.7 to +0.3 parts per thousand and suggest that they are derivatives of a single porphyry system. The delta 18O values of the fluid of porphyry copper (7.4 parts per thousand) and subepithermal (7.0 parts per thousand) stages indicate its magmatic genesis, whereas those of the epithermal stage (from +1.2 to +7.2 parts per thousand) are evident of mixing of magmatic fluid and meteoric waters (from +0.4 to +5.7 parts per thousand). Our isotopic data, combined with mineralogical-geochemical peculiarities and formation conditions of ores, provide tracing the principles of the evolution of mineral assemblages, temperatures, composition, and fluid salinity at the Ak-Sug deposit upon the transition from porphyry copper to epithermal stage.