Genesis of the Mandongshan gold deposit (Xinjiang, NW China): T-P-fS2 and phase equilibria constraints from the Au-As-Fe-S system

The Mandongshan gold deposit, located in the west Junggar (Xinjiang, China), is hosted in Early Carboniferous basalt and tuff. Ore bodies mainly consist of gold-bearing hydrothermal veins, mineralized basalt and tuff with disseminated sulfides. Hydrothermal process could be identified based on ore petrographic studies: quartz-albite-sulfide-native gold (stage I), arsenopyrite-calcite-native gold (II), and pyrite-calcite (III). Microthermometric and laser Raman spectroscopic analysis of fluid inclusions in quartz demonstrate the ore-forming fluid at stage I was moderate temperature (360-398 degrees C), low salinity (0.5-5.1 wt% NaCI equiv), and H2O-CH4-dominant, which evolved to low-salinity (0.2-1.2 wt% NaCl equiv) with insignificant content of CH4 at 219-267 degrees C (stage III). Contents of ore elements (As, Au, Cu, Sb, Ba, Cs, Rb) in mineralized basalt are higher than those in fresh basalt. Phase analysis for the Au-As-Fe-S system using SUPCRT92 software package with the updated database of slop98.dat indicates that a decrease of H2S activity and oxygen fugacity, caused by phase separation and fluid-rock reactions, induced native gold precipitation at stage I. The decrease of H2S activity triggered by arsenopyrite crystallization caused native gold deposition at stage II. (C) 2016 Elsevier B.V. All rights reserved.