A Study On The Genesis Of Bajiazi Gold Ore In Jiapigou,Jilin Province

The Jiapigou ore field is located in Huadian City,Jilin Province,and structurally located in the eastern part of North China Craton,which is a complex area that experienced the formation and evolution of Archean nucleus,Proterozoic rift,and the open and close of paleo-Asian Ocean and the subduction of the paleo-Pacific oceanic plate.Due to the special tectonic systems,a number of gold metallogenic belts were developed in the study area.So far,many research works have done to the NW trend ore body in Jiapigou by researchers,and established the genesis and metallogenic models of the ore deposit.However,the genesis research of NE trend ore body is relatively week.In order to improve the regularity of gold enrichment and mineralization in the Jiapigou metallogenic belt,this thesis chooses the Bajiazi gold ore deposit?nearly NEE trend?as the research object which was served as the representative NE trend gold ore deposit in the Jiapigou ore field.According to the ore deposit geological and metallogenic fuild characteristics and geochronology of the closely related vein rocks,and combined with the results of previous studies,to study the genesis of the ore deposit.The deposit geological research shows that ore body was subjected by the NE trend fault,and overlapped by SN,NW and EW trend fault.The ore veins are mainly developed in the NE trend fault systems.The host rocks consist of Archean supracrustal rocks,Paleozoic intrusions and Mesozoic dykes and so on.Ore body is closely associated by dacitic porphyry,alkali-feldspar granite aplite vein.The mineralization from ore body to wall rocks is gray silicified,high-grade argillization,pyrite sericite,epidotization?chlorite?,potassium and biotitization.Due to the relationship of mineral assemblage,the stages of mineralization can be divided into: ?stage-milky quartz,?stage-quartz-pyrite,?stage-polymetallic sulfide,?stage-quartz carbonate.The petrography features of gangue mineral fluid inclusions in different ore-forming stages show that the types of fluid inclusions in Bajiazi ore deposit are mainly gas-liquid two-phase inclusions?type ?1?,pure liquid inclusions?type ??with small amount of pure CO₂ type inclusions?type ??and CO₂ containing three phase inclusion???.Combined with inclusion microthermometry and Laser Raman analysis results,the fluid in early stage of mineralization is mainly CO₂ in the CO₂-H₂O-NaCl system,in the main stage of mineralization is NaCl-H₂O-CO₂ system and to the late stage is mainly H2 O in the NaCl-H₂O-CO₂ system.The results show that ore-forming fluid is characterized by enriching in CO₂,low salinity and low density NaCl-H₂O-CO₂ system.The vertical perspective,the fluid inclusions homogenization temperatures from-400 m to-680 m height show obvious decrease trend,and ore in the-720 m height lack of fluid inclusions,the homogenization temperature has a tendency to decrease continuously,which indicates that it may be at the end of ore body pinch out lines in 720 m.The features of isotopic geochemistry show that ?18OV-SMOW values range from 8.9 to 13.6‰,?DV-SMOW values at-100—119.1‰.For pyrite,the ?34S range between 1.3 and 8.9‰,and mainly concentrated in 5-9‰.For ore minerals,the 206Pb/204 Pb ratios of Pb isotope range from 15.97 to 16.35,the 207Pb/204 Pb ratios vary from 15.25 to 15.53,and the 208Pb/204 Pb ratios range between 36.90 and 39.37.These characteristics suggest that in the early stage,the ore-forming fluid is mainly magmatic water,and in the late stage of mineralization,the ore-forming fluid was subjected by the influence of atmospheric precipitation gradually.The ore-forming materials were derived from lower crust or I-type enriched mantle source that was subjected by fluid metasomatism which was mainly CO₂.In this study,the dacite porphyry?JBj15D-3?occurred at 208.4±1.1Ma.The alkali-feldspar granite aplite that associated with ore body was emplaced at 186.4±1.8 Ma.Combined with the previous geochronological data in the Jiapigou ore field,we infer that the upper limit of the age of main mineralization would not exceed 186 Ma,which belongs to the Middle Jurassic moneralization.In conclusion,the mineralization of Jiapigou ore deposits were related to the Pacific oceanic plate subducted to the Eurasion Plate,and cause terrane assembly.