Ore-forming fluids and conditions of gold mineralization in the Bangbu orogenic gold deposit, eastern Tibet, China: Constraints from gold fineness, fluid inclusion and thermodynamic simulation

• The early gold has higher fineness (924.5–941.1) than the late gold (881.4–920.2). • Ore-forming fluids contain low temperatures (200–240 °C) and high concentrations of metal elements. • Decrease in temperature and pressure contributes to two stages of gold mineralization, respectively. • Rapid decrease in pressure leads to the lower fineness of late gold. The Bangbu orogenic gold deposit is located in the eastern segment of the Indus–Yurlung Zangbo Suture. Two types of native gold, wiry assemblage of native gold (Au1) in microfractures of pyrite and fine-grained inclusions of native gold (Au2) in pyrite and chalcopyrite, have been recognized in the early banded ore and the late massive sulfide ore, respectively. However, the ore-forming fluids and conditions of two types of gold mineralization remain uncertain. Here, we report field work and results of gold fineness, LA-ICP-MS mapping of pyrite, microthermometry and LA-ICP-MS of fluid inclusions to provide new constraints on this issue. The observation of samples identifies three stages of hydrothermal mineral assemblages: (I) quartz (Qz1) – sericite – pyrite (Py1), (II) quartz (Qz2) – sericite – pyrite (Py2) – native gold (Au1) and (III) quartz (Qz3) – sericite – pyrite (Py3) – native gold (Au2) – chalcopyrite – sphalerite – galena. EPMA analysis determines relatively high gold fineness (924.5–941.1) of Au1 and relatively low gold fineness (881.4–920.2) of Au2. LA-ICP-MS mapping reveals higher contents of Au (>10 ppm) and As (>0.5 wt%) in the cores of Py2 and Py3 than those in their rims. Three types of fluid inclusions, two-phase aqueous inclusions (homogenize to the liquid phase), two-phase CO 2 -bearing aqueous inclusions (homogenize to the liquid phase) and three-phase CO 2 -bearing aqueous inclusions (homogenize to the liquid phase), are identified at room temperature. Homogenization temperatures and salinities of fluid inclusions from three stages of quartz typically range from 200 °C to 240 °C and 4 to 6 wt% NaCl eqv., respectively. LA-ICP-MS analyses of fluid inclusions reveal three types of hydrothermal fluids enriched with different amounts of metal elements, and the highest concentrations of As (1080 ppm), Rb (52 ppm), Ag (6 ppm), Sb (80 ppm), W (42 ppm), Au (4 ppm), Pb (101 ppm), and Bi (5 ppm) are determined in inclusions from the quartz associated with Au2. Based on above results, thermodynamic models of gold solubility and gold fineness are simulated to illustrate the physicochemical conditions of the ore-forming fluids. The analytical results and thermodynamic simulation in this study clarify that two types of native gold may precipitate from the fluids with a decrease in temperature and pressure, respectively. Moreover, the rapid decrease in pressure may result in the lower gold fineness of Au2