Metallogenic Fluid Characteristics In The Panyanzi Gold Ore Deposit,Kangding County,Sichuan

Pianyanzi gold ore deposit is one of the main deposits in the Kangding gold ore field that is an important part of the Danba-Kangding gold ore belt.Ore bodies are hosted in central section in second layer of sub section of middle section of the upper of Sinian Dengying Formation.The stratiform rich-algal dolomites are the host rock.The auriferous veins are nearly north-south zonal distribution.Sellaite is one of the major gold bearing minerals,especially the granular sellaite.The metal minerals are mainly pyrite,galena,geocronite,bournonite,tetrahedrite;non-metallic minerals are mainly quartz,sellaite,dolomite,sericite,fluorite and calcite,etc.The structure of ores is mainly crystalline structure,metasomatic structure,disseminated structure,vein structure,drusy structure,miarolitic structure,comb structure,etc.Alteration types are mainly pyritization,silicification,sericitization,carbonation,albitization,chloritization.Mineralization process is divided into the gold-pyrite-quartz stage???;the gold-sellaite stage???;the gold-sulfide-sellaite-quartz stage???;the pyrite-dolomite-quartz stage???,in which stage II and III are the main metallogenic stages.The characteristics of fluid are studied by observation of petrographic characteristics,temperature determination,component analysis and thermodynamic calculation of inclusions.The types of fluid inclusions in this deposit are mainly H₂ O inclusions,pure CO₂ inclusions,CO₂-H₂ O inclusions,and halite-bearing inclusions,fluid inclus ion assemblages show the characteristics of fluid immiscibility.The halite-bearing inclusions only appear in stage I,Stage II and III are almost all CO₂-H₂ O inclusions and CO₂ inclusions.The homogenization temperatures ranged from 147.0? to 400?,salinity ranged from 0.43 to 36.4wt%NaCl.From ? to ? stage,the homogenization temperatures,the salinity,pressure and value of pH of fluid inclusions are decreasing,but their oxidation was enhanced.The homogenization temperatures and pressures of CO₂-rich CO₂-H₂ O inclusions vary from 218.6? to 342.3? and from 98.6 MPa to132.6 MPa;The homogenization temperatures and pressures of H₂O-rich CO₂-H₂ O inclusions vary from 216.4? to 315.8? and from 107.6 MPa to 214.7 MPa.The temperature and pressure of these two types of CO₂-H₂ O inclusions are basically consis-tent,indicating that they are immiscible CO₂-H₂ O fluid inclusions assemblage had been trapped in the same period.Mantle hydrotherm enters the secondary fault and interlayer fracture zone along the Guo Dashan fracture and the Pianyanzi fracture.A sudden drop in pressure,reducing the solubility of CO₂ in the ore-forming solution and causing fluid immiscibility.Then the fugacity of CO₂ decreased,the value of pH increased,gold solubility decreased.It led to the reduction of gold solubility in ore-forming fluid,to the formation of the gold.