Fluid inclusion study of the Baiyinnuo' er Zn-Pb deposit, south segment of the Great Xing' an Mountain, northeastern China

The Baiyinnuo' er deposit is the largest Zn-Pb deposit in northern China, which is related to the Yanshanian magmatic intrusions emplaced in limestone of the Early Permian Huanggangliang Formation. The hydrothermal evolution can be generally divided into three stages. The pre-ore stage (P-stage), with dominant mineral assemblages of pre-ore garnet and pyroxene, is characterized by coexisting brine and vapor-rich fluid inclusions, indicating an immiscible condition and, using the homogenization temperatures (470 degrees C) and salinities of the brine (similar to 44% NaCleqv), trapping pressure can hence be estimated to be similar to 400 bars, equivalent to a depth of similar to 1.5km. In some stockwork quartz, halite-saturated inclusions homogenized by halite dissolution are abundant with minimum trapping pressures ranging from several hundred bars to 3000 bars, interpreted as the result of over-pressure caused by the continued exsolving fluid from the magma chamber. In the syn-ore stage (S-stage), hydrous minerals and sulfides are widespread, and liquid-rich inclusions coexisted with vapor-rich inclusions are observed in sphalerite as well as quartz and calcite. An average trapping pressure of similar to 150 bars, which is estimated by the average homogenization temperature of similar to 350 degrees C and salinity of similar to 7% NaCleqv, corresponds to a formation depth of 1.5km, the same as the pre-ore stage. The post-ore stage (L-stage) is characterized by calcite (-quartz-fluorite) veinlets with little sulfides. In this stage, CaCl2-bearing fluid is recognized and Ca/Na increases dramatically as the temperature decreases, suggesting a non-magmatic origin and most likely from the sedimentary limestone. Fluid inclusion studies show that the mineralization-related fluid is of magma origin. Prograde minerals formed during the P-stage fluid immiscibility while the sulfide deposition occurred during the S-stage fluid boiling. The fluids responsible for P- and S-stage evolved through different cooling paths.