Ore-forming Fluids Of The Baocun Gold Deposit,Tongling,Anhui Province

The Baocun deposit is a typical skarn gold deposit in Tongling district.A detailed field investigation,fluid inclusions and stable isotope study has been carried out on the Baocun gold deposit,with focus on an analysis of the ore-forming fluids and origin of the Baocun gold deposit.The Au-orebodies of the Baocun deposit occur mainly as lenticulars or veins along the contact zone between the Baocun pluton and carbonate rocks.Ore minerals are dominantly pyrite,pyrrhotite,chalcopyrite,galena,sphalerite,natural bismuth,stibnite and natural gold;gangue minerals mainly include garnet,diopside,epidote,sericite,chlorite,quartz and calcite.Natural gold is granular and intergrown with bismuth-bearing minerals.Based on mineral texture,assemblage and cross-cutting relationship,the ore-forming processes of the Baocun gold deposit can be divided into the skarn,barren sulfide quartz,quartz-pyrite,quartz-chalcopyrite,bismuth-gold polymetallic,and carbonate stages.Gold was mainly precipitated in the bismuth-gold polymetallic stage.The fluid inclusions in the Baocun deposit are mainly daughter mineral-bearing,liquid-rich and gas-rich inclusions.Microthermometric data show that the homogenization temperatures of fluid inclusions,from skarn stage to carbonate stage,are concentrated in the range of490~519,370~390,351~369,310~325,275~310,122~232?,respectively;the salinities are concentrated in the range of 10.8~55.8,9.8~44.5,9.6~43.8,7.3~40.3,6.5~35.34,0.5~5.5wt%NaCl equiv.,respectively;the densities are concentrated in the range of 0.41~0.49,0.68~0.65,0.76~0.65,0.78~0.8,0.81~0.84,0.88~0.97g/cm~3,respectively.Fluid inclusion assemblages and stable isotopic data indicate that the early stage fluids(e.g.,the skarn,barren sulfide quartz,quartz-pyrite,and quartz-chalcopyrite stages)in the Baocun deposit were magmatic in origin,whereas the late stage fluids(e.g.,bismuth-gold polymetallic,and carbonate stages)were the mixture of magmatic and meteoric water.Comprehensive analysis suggested that fluid mixing and multi-stage boiling occurred during fluid evolution of the Baocun deposit.The early boiling led to the enrichment of ore-forming materials;and the late boiling induced rapid precipitation of gold and sulfides(e.g.,pyrite,pyrrhotite,chalcopyrite,galena,sphalerite);and the fluid mixing of magmatic and meteoric water resulted in bismuth and gold precipitation.