A magmatic-hydrothermal origin for Ag-Pb-Zn vein formation at the Bianjiadayuan deposit, inner Mongolia, NE China: Evidences from fluid inclusion, stable (C-H-O) and noble gas isotope studies

Abstract

Many new discoveries of base and precious metal veins in the Great Hinggan Range district have made this region the most important Ag-Pb-Zn metallogenic belt in northern China. The Bianjiadayuan Ag-Pb-Zn deposit (4.81 Mt @157 g/t Ag and 3.94% Pb + Zn) is one of the new discoveries located in the southern segment of this potentially important but poorly understood metallogenic belt. Vein-type Ag-Pb-Zn ore bodies are predominantly hosted by slate adjacent to a Sn ± Cu ± Mo mineralized porphyry intrusion. Four primary ore stages of arsenopyrite + pyrite + quartz, main sulfides + quartz, silver-bearing sulfosalts + quartz, and boulangerite + calcite are recognized, and a subsequent supergene oxidation stage has also been identified. An alteration assemblage of quartz, sericite, chlorite and epidote is spatially associated with this vein type mineralization. The δDH2O (−139 to −106‰) and δ18OH2O (−16.4 to 12.1‰) isotope compositions obtained from different stage vein quartz and calcite reveal that the early stage ore fluids were dominated by magmatic fluids, whereas the late stage ore fluids contained a significant proportion of meteoric waters, which is consistent with the δ13CPDB (−7.7 to −2.7‰) and δ18OSMOW (−0.4 to 6.4‰) isotope signatures for waning stage calcites reflecting an addition of large volumes of meteoric waters in hydrothermal system. Noble gas isotopes (i.e., 3He/4He ratios of 1.52–3.06 Ra) display evidences of a magmatic fluid responsible for ore formation in the deposit. Fluid inclusion studies suggest that vein formation occurred at progressively lower temperature (310–220 °C) and pressure (100–20 bars). Fluid cooling, phase separation, and mixing with meteoric waters are advocated as the important factors for ore deposition. In combination, available data for the Bianjiadayuan deposit suggest that local Ag-Pb-Zn veins and porphyry type Sn ± Cu ± Mo mineralization are genetically related and formed in one porphyry ore system, which is a useful concept for local and regional Ag-Pb-Zn exploration.