Study on fluid inclusions of the Xishanwanyangchang silver ore deposit in volcanic rocks, Inner Mongolia

The Xishanwanyangchang silver ore deposit in volcanic rocks, lied in mid-west area of Inner Mongolia, is located in the northwest of the North China Plate. Hanging wall rock of the silver deposit is Permian biotite adamellite, and footwall rock is Cambrian onalite. Silver polymetallic ore-bodies are lentiform or stratiform occurring in the rhyolite, rhyolite porphyry, and rhyolite breccia of lower series of Cretaceous Bainvyangpan Formation (K1bn) as well as contact zone of the volcanic rock and granite. The ores mainly have euhedral to subhedral granular, anhedral granular, metasomatie-relict and cataclastic textures and brecciated, disseminated, vein and massive structures. The mineralization mainly is ferritization, pyritization, fluoritization and mineralization associated with lead-zinc. The sulfide mineral mainly consists of pyrite, and a small amount of argentite, acanthite, sphalerite and galena. The hydrothermal ore-forming process includes three stages, i. e. the early, middle and late stages, characterized by quartz-pyrite stage, quartz-polymetallic sulfides stage and quartz-carbonate stage, respectively. The main types of wall rock alteration include quartzitification, sericitization, chloritization, pyritization, limonitization, kaolinization, carbonatization. The gangue minerals are mainly composed of quartz, opal, chalcedony, fluorite, potash feldspar and a small amount of sericite. In quartz phenocrysts of rhyolite, three types of fluid inclusions can be observed, i. e. NaCl-H2O solution, the fluid inclusion containing halite daughter mineral and the fluid inclusion containing chalcopyrite daughter mineral. In early stage, homogeneous temperatures of fluid inclusions are mainly above 330°C , with fluid salinities ranging from 0.35% to 5.86% NaCleqv, and the liquid-rich fluid inclusions are mainly observed, with high emperature and low salinity. In middle stage, homogeneous temperatures of fluid inclusions are mainly ranging from 250°C to 330°C , with fluid salinities ranging from 0. 35% to 31. 90% NaCleqv, and the fluid inclusions containing halite daughter mineral and the fluid inclusions containing chalcopyrite daughter mineral can be observed except liquid-rich fluid inclusions, characterized by the daughter mineral-bearing inclusions of high salinity coexisting with the liquid-rich fluid inclusions of low salinity. In late stage, homogeneous temperatures of fluid inclusions are mainly ranging from 169°C to 250°C , with fluid salinities ranging from 0.71% to 32.66% NaCleqv, and the fluid inclusions containing halite daughter mineral can be observed except liquid-rich fluid inclusions, characterized by the fluid inclusions containing halite daughter mineral of high salinity coexisting with the liquid-rich fluid inclusions of low salinity. The fluid inclusions containing chalcopyrite daughter mineral of high salinity in middle stage are resulted from boiled fluid, and the fluid inclusions containing halite daughter mineral of high salinity in late stage are preferably interpreted as being generated directly from a water-saturated crystallizing magmatic melt. The mineralization should be triggered by the temperature dropping and fluid mixing. Metal precipitation is mainly due to mixing between metal-bearing, high-salinity fluid and cold groundwater in the place where ores were deposited, resulting in enrichment and mineralization of the metal elements.