Ages, H O C S Pb isotopes, and fluid inclusion study of the Daolundaba Cu–Sn–W–Ag deposit in Inner Mongolia, NE China

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The medium-sized Daolundaba Cu–Sn–W–Ag deposit is located in the southern Great Xing'an Range (SGXR), being one of the representative deposits in this area. The vein orebodies of the deposit occur within the NE-trending fracture zones, which are developed in the Early Permian silty slate and Late Variscan biotite monzogranite. However, its mineralization age remains controversial, which affects the understanding of Sn–polymetallic mineralization in the SGXR. To determine the mineralization age and reveal the sources of fluid and ore-forming material, sericite and monazite dating, fluid inclusion microthermometry, and isotopic analysis were carried out in this study. The ore-forming process at the deposit can be divided into four stages: quartz–fluorite–muscovite–tourmaline–cassiterite–wolframite–arsenopyrite stage (stage I), quartz–fluorite–wolframite–chalcopyrite–arsenopyrite–pyrrhotite stage (stage II), quartz–fluorite–sericite–chalcopyrite–pyrite–pyrrhotite–silver mineral stage (stage III), and quartz–calcite–fluorite–pyrite stage (stage IV). The sericite sample yielded a 40 Ar– 39 Ar plateau age of 138.8 ± 0.7 Ma and an isochronal age of 140.0 ± 1.1 Ma, and the LA-ICP-MS U–Pb age of three monazite samples was 136.0 ± 2.3–134.7 ± 2.8 Ma, indicating that the Daolundaba deposit formed in the Early Cretaceous. Liquid-rich, gas-rich, and daughter mineral-bearing inclusions have been identified in quartz, fluorite, and calcite. Homogenization temperatures and salinities of the fluid inclusions (FIs) in the stage I vary from 309 °C to 389 °C and 6.2 wt% to 46.3 wt% NaCl eqv., respectively. Homogenization temperatures and salinities of the FIs in the stage II vary from 242 °C to 339 °C and 5.3 wt% to 41.4 wt% NaCl eqv., respectively. Homogenization temperatures and salinities of the FIs in the stage III vary from 153 °C to 268 °C and 3.5 wt% to 35.4 wt% NaCl eqv., respectively. There are only liquid-rich inclusions in quartz, fluorite, and calcite of the stage IV; their homogenization temperatures and salinities vary from 114 °C to 188 °C and 2.1 wt% to 7.6 wt% NaCl eqv., respectively. The ore-forming fluids of the Daolundaba deposit are characterized by moderate–high temperature and great fluctuating salinity, belonging to an H 2 O–NaCl ± CO 2 ± CH 4 system. The δ 18 O water and δD values of the stages I and II vary from 2.5 ‰ to 7.2 ‰ and − 127 ‰ to − 106 ‰, respectively; the δ 18 O water and δD values of the stage III vary from − 3.0 ‰ to − 0.3 ‰ and − 126 ‰ to − 94 ‰, respectively; the δ 18 O water and δD values of the stage IV vary from − 10.9 ‰ to − 4.7 ‰ and − 102 ‰ to − 81 ‰, respectively; indicating that the ore-forming fluids of the stages I and II mainly consist of magmatic water, that the ore-forming fluid in the stage III is a mixture of magmatic and meteoric water, and that the fluid in the stage IV is meteoric water. The δ 13 C values for calcite of the stage IV vary from − 9.8 ‰ to − 6.7 ‰, suggesting that the carbon was mainly from a magmatic source. The δ 34 S values range from − 7.4 ‰ to − 1.3 ‰ with an average of − 4.7 ‰. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb ratios of the metallic minerals are in the ranges of 18.293–18.612, 15.519–15.706, and 38.078–38.826, respectively. Both S and Pb isotopic systems indicate that the ore-forming metals primarily came from Mesozoic felsic magma. Fluid boiling and mixing were two main mechanisms for the deposition of ore-forming materials.