Genesis of the Mandongshan gold deposit (Xinjiang, NW China): T-P-ƒS2 and phase equilibria constraints from the Au-As-Fe-S system

- The similar precipitation mechanism is responsible for the coexistence of arsenopyrite and native gold in the Mandongshan gold deposit.
- The low fO2 is consistent with the fact that arsenopyrite is the dominant sulfide in the Mandongshan gold deposit.
- Decrease of aH2S and fO2 is the major mechanism causing native gold precipitation.

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 °C), low-salinity (0.5-5.1 wt% NaCl 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 °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.

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