Ore-forming Fluids Of The Liushaogou Gold Polymetallic Deposit,Liangdang,Gansu Province

Western Qinling region?WQR?is an important gold-polymetallic metallogenic belt in China.The Liushaogou gold polymetallic deposit is one of representative gold deposits in the WQR.In this study,the petrography and microthermometry of the fluid inclusions,and stable isotope compositions were determined to constrain the origin of the ore-forming fluids and metals of the Liushaogou deposit.The Liushaogou deposit is of a quartz vein type gold deposit.Ore minerals are dominantly pyrite and galena,followed by arsenopyrite,sphalerite and chalcopyrite;gangue minerals are mainly quartz and calcite.Based on field evidence,petrographic observations,and cross-cutting relationships,the ore-forming processes of the Liushaogou deposit can be divided into the five stages:barren sulfide quartz stage?I?,quartz-coarse pyrite stage?II?,quartz-fine grained pyrite stage?III?,quartz polymetallic sulfide stage?IV?and carbonate stage?V?.The fluid inclusions in the Liushaogou deposit are mainly liquid-rich,CO₂-bearing,daughter mineral-bearing and gas-rich inclusions.The stage I contains mainly liquid-rich and daughter mineral-bearing fluid inclusions,which yield homogenization temperatures of320-340?and average salinity of 12.9 wt%NaCl equiv..The stage II contains mainly liquid-rich,daughter mineral-bearing and CO₂-bearing fluid inclusions,which show homogenization temperatures of 260-300?and average salinity of homogenization temperatures of 320-340?and average salinity of 12.9 wt%NaCl equiv..The stage III contains liquid-rich,CO₂-bearing and daughter mineral-bearing inclusions;the fluid inclusions display homogenization temperatures of 260-280°C and average salinity of 9.89 wt%NaCl equiv..The stage IV contains mainly gas-rich and liquid-rich fluid inclusions,which show homogenization temperature of240-260°C and average salinity of 4.49 wt%NaCl equiv..The stage V contains only liquid-rich inclusions,which yield homogenization temperature of 220-240?and average salinity of 2.39 wt%NaCl equiv..Fluid inclusion assemblages,microthermometric and stable isotopic data indicate that the early stage fluids in the Liushaogou deposit were magmatic in origin,and characterized by high temperature,high salinity;the late stage fluids were magmatic fluid mixed with meteoric water.Fluid boiling,immiscible and mixing occurred during the stage III and IV,resulting in rapid precipitation of gold and sulfides.