Genesis and tectonic setting of the Bulage Pb-Zn deposit, Inner Mongolia, China: Evidence from geology, fluid inclusions, EMPA, H-O isotope systematics, zircon U-Pb geochronology, and geochemistry

The recently discovered Bulage Pb-Zn deposit is located in the central part of the Erlianhot-Dongwuqi Metallogenic Belt (eastern Central Asian Orogenic Belt) in Inner Mongolia, China. The ore bodies of the deposit are hosted in alkali feldspar granite and in dacitic crystal tuff within Unit 2 of the Carboniferous Baoligaomiao Formation. The Bulage deposit is a low to moderate temperature hydrothermal vein-type deposit and is controlled by fractures. The hydrothermal mineralization can be divided into three stages: early (quartz + specularite +/- magnetite), middle (quartz + sphalerite + galena +/- chalcopyrite +/- pyrite), and late (calcite +/- quartz + sphalerite + galena). The deposit contains liquid-rich two-phase (VL-type) and pure-liquid (L-type) fluid inclusions. The initial ore-forming fluids are attributed to the low to moderate temperature and low salinity of a NaCl-H2O system. Hydrogen and oxygen isotopic compositions of quartz and calcite indicate that the ore-forming fluids were dominated by meteoric water with minor contributions of magmatic water. The quartz diorite, monzonite porphyry, and alkali feldspar granite in the deposit yield LA-ICP-MS zircon U-Pb ages of 305-300 Ma. These high-K calc-alkaline I-type granitoids are enriched in large-ion lithophile elements (LILEs; e.g., Rb, Cs, and K) and light rare earth elements (LREEs) and are depleted in high-field-strength elements (HFSEs; e.g., Nb, Ta, and Ti) and heavy rare earth elements (HREEs). The primary magma of these granitoids was derived from the partial melting of lower crust. The parental magma of the quartz diorite may have provided part of the ore-forming material and acted as a heat source for mineralization. Therefore, we infer that the Bulage Pb-Zn deposit was formed as a result of the southward subduction of the Paleo-Asian Ocean beneath the North China Craton during the Late Carboniferous.