Journal of Geochemical Exploration | 2019
Mechanism of gold precipitation in the Gezigou gold deposit, Xinjiang, NW China: Evidence from fluid inclusions and thermodynamic modeling
Abstract
Abstract The Gezigou gold deposit, which is located in west Junggar (Xinjiang, China), is mainly hosted in graphite-bearing siltstone. These ore bodies, which consist of gold-bearing hydrothermal veins and mineralized siltstone with disseminated sulfides, formed in three stages: quartz-sulfides-native gold (I), arsenopyrite-calcite-native gold (II), and quartz-calcite (III) stages. The microthermometric and laser Raman spectroscopic analysis of the fluid inclusions in stage I-quartz demonstrate that the ore-forming fluid was moderate temperature (353–392\u202f°C), low-salinity (2.9–5.3\u202fwt% NaCl equiv.), and H2O-CH4-dominant. The thermodynamic modeling of the Au-Cu-As-Fe-S system performed using the SUPCRT92 software package with the updated database of slop16.dat indicates that a decrease in H2S activity (aH2S) and oxygen fugacity (fO2) were responsible for reducing the solubilities of As and Au, which might be responsible for the coexistence of arsenopyrite and native gold. The decreases in aH2S and fO2 caused by phase separation and fluid-rock reactions induced the precipitation of native gold in the quartz-sulfides-native gold stage. Graphite-bearing siltstone, which may lower the fO2 of ore-forming fluids, is thus an important indicator of prospective gold deposits.