Geochemistry, Mineralization, and Fluid Inclusion Study of the Bayan-Uul Porphyry Au-Cu-(Mo) Deposit, Central Mongolia

The Bayan-Uul porphyry Au-Cu-(Mo) deposit occurs within the Mongol-Okhotsk Orogenic Belt, which is a part of the Central Asian Orogenic Belt. To understand geotectonic, petrogenesis, mineralization, and ore-forming fluid evolution of the Bayan-Uul deposit, we report petrographic and geochemical analyses of host rocks, mineralogy of ores, and fluid inclusion characteristics. Based on petrographic and mineralogical analyses, Cu, Mo, and Au mineralization occurs as disseminated and sulfide-bearing quartz-tourmaline veins hosted within granodiorites, monzodiorites, and diorite porphyry and tourmaline breccia. Four main alteration assemblages are identified: potassic, phyllic, argillic, and quartz-tourmaline alteration. The ore mineralogy of quartz-tourmaline veinlets are classified into A-type veinlets (quartz + tourmaline + chalcopyrite + magnetite + pyrite +/- electrum), B-type veinlets (quartz + tourmaline + molybdenum + chalcopyrite + pyrite), and C-type veinlets (quartz + tourmaline + pyrite +/- chalcopyrite). Fluid inclusions are found in quartz-tourmaline veinlets consisting mainly of liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), and solid-bearing multi-phase (S-type) inclusions. The homogenization temperatures for the fluid inclusions in A-type, B-type, and C-type veinlets range from 215 to 490 degrees C, 215 to 500 degrees C, and 160 to 350 degrees C and their salinity varies from 5.4 to 43.5 wt.%, 16 to 51.1 wt.%, and 3.4 to 24.1 wt.% NaCl equivalent, respectively. Coexistance of (L-type), (V-type), and (S-type) inclusions support fluid boiling. The delta 18O values of ore fluids from different mineralizing A-, B-, and C-type veins are 8.7 parts per thousand, 10.9 parts per thousand, and 8.4 parts per thousand, respectively, and the delta 34S values of sulfide minerals range from -1.4 parts per thousand to 5.3 parts per thousand, which indicates magmatic origin.