Fluid inclusion study on the Shisiwan gold deposit in Bainaimiao ore field, Inner Mongolia, China
内蒙古白乃庙矿田十四万金矿床流体包裹体研究

The Shisiwan gold deposit is loacted in Xuniwusu gold mineralization belt in Bainaimiao ore field.The quartz veins are controlled by NE-trending faults which are the secondary structure of the EW-trending ductile-shear zone.The ore-forming process can be divided into three stages.The barren quartz veins were formed in early stage and then structurally deformed.The early stage fluid inclusion types are aqueous,carbonic-aqueous and carbonic.The homogenization temperature of early-stage fluid inclusions ranges 260 ～420℃,with average salinity of 6.78% NaCl eqv.The middle stage mineral paragenesis is sulfides+calcite+chlorite+sericite+fine-grained quartz,infilling fissions of early-stage quartz.The middle stage minerals do not show structurally deformation and contain aqueous and carbonic fluid inclusions.The homogenization temperature of middle-stage fluid inclusions ranges 140～260℃,with average salinity of 7.22% NaCl eqv.The late-stage calcite veins contain aqueous fluid inclusions yielding homogenization temperatures of 140～180℃,with average salinity of 2.15% NaC1 eqv.Laser Raman spectra indicates that gases in fluid inclusions are major of CO_2,CH_4and N_2.In general,the ore-forming fluid evolved from carbonic to aqueous,i.e.from CH_4+CO_2+H_2O,through to NaClH_2O+CH_4,and to NaCl-H_2O.Fluid immiscibility occurred in both early and middle stages and resulted in release of CO_2and CH_4.Calcite precipitation in middle stage reduced the activity of CO_2 and thus induced precipitation of sulfides and gold.Therefore,the study in both fluid inclusions and ore geology suggests that the Shisiwan deposit is a gold deposit of orogenic class.The ore-forming fluids are interpreted to have derived form metamorphic devolatilation of the Xuniwusu formation during late Permian-early Triassic collisional orogeny between the Siberia and North China continents.