Ore Genesis and Hydrothermal Evolution of the Baiyinnuo'er Zinc-Lead Skarn Deposit, Northeast China: Evidence from Isotopes (S, Pb) and Fluid Inclusions

The Baiyinnuo'er zinc-lead deposit (32.74 Mt at 5.44% Zn, 2.02% Pb, and 31.36 g/t Ag), located in the south segment of the Great Xing'an Range, is the largest Zn-Pb deposit in northern China. Skarn and orebodies mainly occur between the different units of the Huanggangliang Formation, or within the contact zone between the intrusions and Permian marble. Several phases of igneous rocks exposed within the mining areas, and among them the Yanshanian plutonic rocks, which intruded into limestone of the early Permian Huanggangliang Formation, are interpreted to be the source of ore, since their Pb isotope compositions (Pb-206/Pb-204 = 18.25-18.35, Pb-207/Pb-204 = 15.50-15.56, and Pb-208/Pb-204 = 38.14-38.32) are highly consistent with the sulfides, including sphalerite, galena, and chalcopyrite (Pb-206/Pb-204 = 18.23-18.37, Pb-207/Pb-204 = 15.47-15.62, and Pb-208/Pb-204 = 37.93-38.44). Sulfur isotope values of the sulfides give a narrow delta S-34 interval of -6.1 to -4.6 parts per thousand (mean = -5.4 parts per thousand, n = 15), suggesting the ore-forming fluid is of magmatic origin. Three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observation, which are the preore stage (garnet-clinpyroxene-wollastonite-magnetite +/- sulfides), the synore stage (sulfides-epidote-quartz-calcite +/- garnet), and the postore stage (calcite-chlorite-quartz-fluorite). Several fluid evolution episodes can be inferred from microthermometric results at the Baiyinnuo'er Zn-Pb deposit: 1. Immiscibility: Preore-stage coexistence of halite-beating brine inclusions (S-1-type, similar to 44 wt % NaCl equiv) and vapor-rich fluid inclusions (V-type) sharing the same homogenization temperatures (similar to 470 degrees C) confirms that fluid unmixing occurred under lithostatic pressures of similar to 400 bars (similar to 1.5 km), and the brine is considered to account for most prograde skarn minerals (e g, clinopyroxene). 2. Overpressure trapping: Preore-stage brine inclusions homogenized by halite dissolution (S-2-type) postdated the immiscible assemblages. This type of inclusions is characterized by high but variable (minimum) trapping pressures (150-3,000 bars) relative to S-1-type inclusions and can be explained as a result of entrapment under overpressuring condition. 3. Boiling: The presence of both vapor and liquid inclusions (i.e., V- and L-type) in the same assemblages within synore-stage quartz, calcite, and sphalerite indicates the occurrence of fluid boiling (similar to 350 degrees C), at hydro-static pressures of similar to 150 bars, and depth of similar to 1.5 km), which resulted in large-scale mineralization in the Baiyinnuo'er Zn-Pb deposit. 4. Mixing with external fluids: Fluid inclusions scattered within postore-stage calcite or secondary trails in synore-stage minerals show low homogenization temperatures (<260 degrees C) and both decreasing (for L-type) and increasing (for CaCl2-bearing inclusions, i.e., Lc-type) trends for salinities as homogenization temperatures decrease, implying addition of both meteoric water (low-temperature, low-salinity) and basinal brines (low-temperature, Ca-rich), respectively. Systematic fluid inclusion studies also indicate that the mineralization-related fluid is of magmatic origin. Prograde minerals formed during the preore-stage fluid immiscibility while sulfides deposition occurred during the synore-stage fluid boiling. Mixing with external fluids began as the hydrothermal system cooled to <300 degrees C, when the main metal precipitation process had ended.