Ore-forming fluids and ore genesis of the large Bayanbaolege Ag polymetallic deposit, Southern Great Xing’an Range, NE China

Abstract

The large Bayanbaolege Ag polymetallic deposit is one of the newly discovered deposits in the Tuquan-Linxi Fe (Sn)-Cu-Pb-Zn-Ag-Nb (Ta) metallogenic belt of the Southern Great Xing’an Range, NE China. Its major ore bodies, sulfide-quartz veins, are distributed within granodiorite porphyry and in adjacent Late Permian strata. According to ore-forming geological conditions, mineralization and alternation characteristics, source and features of ore-forming fluid, as well as metallogenic depth (<1.45 km), the Bayanbaolege deposit belongs to the mesothermal vein type, and has a genetic relationship with the granodiorite porphyry. Three mineralization stages have been recognized, namely, arsenopyrite-pyrite-quartz (stage I), chalcopyrite-pyrite-sphalerite-quartz (stage II) and calcite-galena-silver minerals (stage III). The zircon U-Pb age of the granodiorite porphyry (130 ± 1 Ma, MSWD = 1.8), indicates that the Ag polymetallic mineralization took place in the Early Cretaceous period. Microthermometric measurements and laser Raman analyses of four types of fluid inclusions that formed in the three stages show that the ore-forming fluid belongs to the H2O-CO2-NaCl system, with medium temperatures (mean temperatures, stage I: 368 °C; stage II: 286 °C; stage III: 206 °C) and medium-low salinities (mean salinities, stage I: 2.1–7.6 wt% NaCl equivalent; stage II: 1.6–11.6 wt% NaCl equivalent; stage III: 1.2–8.4 wt% NaCl equivalent). C-H-O isotope data indicate that ore-forming fluid was derived largely from magmatic fluid, with the addition of meteoric water in the later stage.