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The Dahutang tungsten polymetallic ore field is located north of the Nanling W–Sn polymetallic metallogenic belt and south of the Middle–Lower Yangtze River Valley Cu–Mo–Au–Fe porphyry‐skarn belt. It is a newly discovered ore field, and probably represents the largest tungsten mineralization district in the world. The Shimensi deposit is one of the mineral deposits in the Dahutang ore field, and is associated with Yanshanian granites intruding into a Neoproterozoic granodiorite batholith. On the basis of geologic studies, this paper presents new petrographic, microthermometric, laser Raman spectroscopic and hydrogen and oxygen isotopic studies of fluid inclusions from the Shimensi deposit. The results show that there are three types of fluid inclusions in quartz from various mineralization stages: liquid‐rich two‐phase fluid inclusions, vapor‐rich two‐phase fluid inclusions, and three‐phase fluid inclusions containing a solid crystal, with the vast majority being liquid‐rich two‐phase fluid inclusions. In addition, melt and melt‐fluid inclusions were also found in quartz from pegmatoid bodies in the margin of the Yanshanian intrusion. The homogenization temperatures of liquid‐rich two‐phase fluid inclusions in quartz range from 162 to 363°C and salinities are 0.5wt%–9.5wt% NaCl equivalent. From the early to late mineralization stages, with the decreasing of the homogenization temperature, the salinity also shows a decreasing trend. The ore‐forming fluids can be approximated by a NaCl–H2O fluid system, with small amounts of volatile components including CO2, CH4 and N2, as suggested by Laser Raman spectroscopic analyses. The hydrogen and oxygen isotope data show that δDV.SMOW values of bulk fluid inclusions in quartz from various mineralization stages vary from –63.8‰ to –108.4‰, and the δ18OH2O values calculated from the δ18OV‐SMOW values of quartz vary from –2.28‰ to –7.21‰. These H–O isotopic data are interpreted to indicate that the ore‐forming fluids are mainly composed of magmatic water in the early stage, and meteoric water was added and participated in mineralization in the late stage. Integrating the geological characteristics and analytical data, we propose that the ore‐forming fluids of the Shimensi deposit were mainly derived from Yanshanian granitic magma, the evolution of which resulted in highly differentiated melt, as recorded by melt and melt‐fluid inclusions in pegmatoid quartz, and high concentrations of metals in the fluids. Cooling of the ore‐forming fluids and mixing with meteoric water may be the key factors that led to mineralization in the Dahutang tungsten polymetallic ore field.

Skarn Sn-polymetallic deposits, located in the southern Great Khingan Range, can be divided into Sn–Fe and Sn–Pb–Zn deposits. By systematically studying the geochemical characteristics of source granitoid and deposits, the ore-forming mechanisms were established, and the differences in ore-forming processes between Sn–Fe and Sn–Pb–Zn deposits are discussed. The main findings are as follows: (1) these two deposits were formed in the Late-Yanshanian period; (2) the source granitoid evolved at an early stage in a reducing environment, while the oxygen fugacity increased at a late stage through the influence of a deep-seated fault; (3) fine-grained syenogranite from Dashishan showed a higher degree of evolution than the syenogranite from Damogutu; (4) the Damogutu Sn–Fe and Dashishan Sn–Pb–Zn deposits shared a source of ore-forming fluid, and Fe, Sn, Pb, and Zn all derived from Late-Yanshanian granitoids; and (5) the ore-forming fluid experienced a continuous evolution process from the magmatic to hydrothermal stage, and the magmatic–hydrothermal transitional fluid played a very important role in skarnization and mineralization.

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