Genesis of the Wurinitu W-Mo deposit, Inner Mongolia, northeast China: Check for Constraints from geology, fluid inclusions and isotope systematics

The Wurinitu W-Mo deposit is located in the Erlian-East Ujimqin metallogenic belt in the Great Xing'an Range, NE China. Tungsten and molybdenum mineralization generally occurs as veins in quartz hosted by the Middle to Lower Ordovician wall rocks or as disseminations in the Early Cretaceous monzogranite intrusions. Fluid inclusion studies and multiple isotope analyses, provide systematic evidence to determine the evolution of the hydrothermal fluids, the source of metals and fluids, as well as mineral precipitation mechanisms, which are significant for the magmatic-hydrothermal system but are poorly understood. Three types of fluid inclusion assemblages (FIAs), including L (liquid-rich two-phase FIAs), V (vapor-rich two-phase FIAs), and L-V (liquid-vapor two-phase FIAs), have been identified in quartz veins from the W-mineralization (I), Mo-mineralization (II), and quartz - carbonate (III) stages at Wurinitu. The ore-forming fluids evolved from a high- to moderate-temperature and moderate- to low-salinity H2O-NaCl +/- CO2 system in the W- and Mo- mineralization stages to a low-temperature and low-salinity H2O-NaCl system in the quartz-carbonate stage; they were trapped at temperatures of similar to 365 degrees C, similar to 266 degrees C, and similar to 198 degrees C. The results of hydrogen and oxygen isotope analyses (delta O-18(H2O) = -4.0 to +7.7 parts per thousand; delta D-H2O = -116 to -92 parts per thousand) indicate that the stage I fluids dominantly originated from magmatic water, and the stages II and III show addition of meteoric water. Sulfur (delta S-34(VCDT) = 1.9-4.0 parts per thousand) and lead isotope values (Pb-206/Pb-204 = 18.099-18.728, Pb-207/Pb-204 = 15.527-15.616, and Pb-208/Pb-204 = 38.019-38.465) suggest that the ore metals in the hydrothermal fluids were derived from the granitic magma. All these observations reveal that (1) the Wurinitu W-Mo deposit is a magmatic-hydrothermal ore system, (2) W- and Mo- mineralization occurred under lithostatic pressure of 203-294 bars, and hydrostatic pressure of similar to 102 bars, respectively, and (3) fluid boiling and mixing with meteoric water might be the key factors controlling the large-scale ore deposition at Wurinitu.