Brief Study On The Fluid Inclusion Characteristics Of The Yanzigou Gold Deposit In Danba,Sichuan Province

Tectonic position of the study area was located at the east margin of the passive continental of Yangzi Plate.Strata of studying area belongs to Danba deep detachment-superimposed tectonic slices unit,outcrop is Devonian Weiguan Group.Yanzigou gold deposit situated the central of Yongxi fornix,the west of Tonglufang anticline,a special place where the territorial deep-great faults of Donggu and Lianghekou and Maoniuhe is intercorss and tectonic deformation is intricate and intense.These tectonic elements macroscopily controls the establishment of the gold deposit.The ore bodys in Yanzigou gold deposit mainly occur in the black carbonaceous slate and the gray-green quartz sericite phyllite of Devonian Weiguan Group.Ore mining area is divided into three blocks-Mozigou ore block,Nichonggou ore block and Yanzigou ore block.According to the composition of the ore minerals,the ore can bedivided into quartz vein gold ore and carbonaceous slate type layered gold ore which has a dense network of quartz veins and pyrite.The metal minerals are mainly sulfide in hydrothermal stage,pyrite framboid in the sedimentary diagenetic period,hydrothermal over-growth zoning pyrite in different ore-forming stage.Ore structure is brecciated,veiny,ribbon,earthy and the texture is automorphic-hypautomorphic granular,over-growth zoning,allotriomorphic granular,replacement remnant and pseudomorphic.Gold mineral is mainly occured in pyrite,chalcopyrite and limonite as inclusion gold,fissure gold.Wall-rock alteration is rich,the main alteration is siliconization,beresitization,epidotization,ferritization,etc.According to the geological setting and geochemical characteristics,metallogenic evolution of the deposit is divided into hydrothermal sedimentation period,hydrothermal stage,supergene stage.Hydrothermal stage is the mainly metallogenic period,combined with the Ore-controlling structure's deformation sequence,mineralization,alteration and mineral assemblage,it can be divided further into quartz-pyrite stage,goldsulfide-quartz stage,quartz-calcite stage.The fluid inclusions in quartz at the hydrothermal mineralization period are divided into three categories: H₂O inclusions,CO₂ inclusions and CO₂-H₂O inclusions,and the CO₂-H₂O inclusion of them is the main type.The homogenization temperatureof H₂O inclusions and CO₂ inclusions is 393?,pressure is 1148.5¹79.0MPa.CO₂-H₂O inclusions whose homogenization temperature is 276.3³59.8?,salinity is0.83³.52wt%NaCleqv,density is 0.68⁰.925g/cm3,pressure is 160.3¹81MPa,pH is5.427⁵.620,Eh is 0.0502^-0.0583.The metallogenic fluid is acidic NaCl-CO₂-H₂O,with high temperature,high pressure,low salinity,low density.When heated,the homogeneous phase state of H₂O-rich CO₂-H₂O inclusions is H₂O phase,while that of CO₂-rich CO₂-H₂O inclusions is the CO₂ phase.Moreover,they have the same homogeneous temperature and pressure.This indicates that they are caught in CO₂ and low-salt water immiscible fluid during the same period.When immiscible fluid occurred,a large amount CO₂ overflowed,the fugacity of O₂ decreased,and pH increased.It led to the solubility of gold in ore-forming hydrothermal reduction,rapid precipitation of gold,and the formation of deposits.