Research On Gold Metallogenesis Of The Liupiye Orefield Of The Jiapigou Gold Ore District In Jilin Province

The Liupiye gold orefield is located in the southeast part of Jiapigou Gold ore district in the eastern Jilin Province,which is at the eastern part of the northern margin of the North China Craton.It is a new prospected gold orefield in the Jiapigou gold ore district,including the middle-large Dajia deposit,small-scale Zhemagou deposit and Jiancaogou deposit.Now more than 20 orebodies have been found in this orefield.And it is going to be an another large gold orefield like Jiapigou gold orefield.As the Liupiye gold orefield was found in the 1990s,it has been a hot research for geologists and economic geologists.But due to the late exploration and development,the research on the ore deposit geology and ore genesis is very weak,which seriously restricts the understanding of the metallogenic regularity and deep exploration work in this area.In view of this,this study focuses on the orefield,geological characteristics,fluid inclusion,geochemistry,and geochronology of meso-epithermal magmatic gold ores in the Dajia Deposit and Zhemagou Deposit with the project support of Geological Survey of China and the help of Lipiye Mining Company.And we have achieved results as follw.1.According to the research on geology of ore deposit and mineralography,we conform that the orebodies of the Liupiye orefield are main pyrite-quartz veins,associated with alteratedrock-type orebodies.Four stages of mineralization have been divided for the three deposits:pyrite-sericite stage,quartz-pyrite stage,quartz polymetallic sulfide phase and quartz carbonate stage.The alteration from orebodies to wall rock is white silicification?advanced argillization?beresitization?cyan silicification?epidote-?chlorite??Kfeldsparization+biotitization and the carbonation is usually in the superposition of beresitization zone.The mineralization type is mainly pyrite mineralization,followed by sphalerite,galena,chalcopyrite and trace natural gold.2.Microscopic observation and microthermometry of mineral fluid inclusion in different mineralization stages are shown within the three gold deposits:CO₂-H₂O?C-type?,aqueous?W-type?,and pure CO₂?PC-type?.Microthermometric data for Dajia Deposit indicate that the FIs associated with metallogenesis homogenize at 126³38°C?cluster in a range of 160.5²65.0°C in the main stage?,yielding a salinity of 2.73¹1.95 wt.%NaCl eqv.,and a density of 0.81⁰.96 g/cm³.Microthermometric data for Zhemagou Deposit indicate that the FIs associated with metallogenesis homogenize at 117³28°C?cluster in a range of 143²80°C in the main stage?,yielding a salinity of 2.89¹1.36 wt.%NaCl eqv.,and a density of 0.74⁰.99 g/cm³.Microthermometric data for Jiancaogou Deposit indicate that the FIs associated with metallogenesis homogenize at103³56°C?cluster in a range of 154²73°C in the main stage?,yielding a salinity of2.89¹2.77 wt.%NaCl eqv.,and a density of 0.78⁰.99 g/cm³.3.From mineralized alteration and the raman probe analysis of mineral fluid inclusion,the initial ore-forming fluid for the gold deposits in the Liupiye orefield is medium-to low-temperature,medium-to low-salinity,low density and CO₂-enrich CO₂-H₂O-NaCl.That is to say,it has underwent CO₂ bearing three-phase aqueous fluid,the immiscibility of CO₂ bearing three-phase aqueous fluid and aqueous two-phase fluid,and the unloading of metallogenic elements in the aqueous two-phase fluid.This feature can be used as an important symbol to judge the mineralization.4.The characteristics of H and S,Pb isotopes are further revealed that the initial ore-forming fluids of the three gold deposits are mainly magmatic water.And the cocktail effect of meteoric water and magmatic water is also one of the mineralization of the Liupiye gold oreflield.In addition,the metallogenic material was derived from deep lower crust or I-type enriched mantle formed by metasomatized by CO₂-enrich fluids.At the same time,some minerals in the wall rock may be extracted in the process of rising and migration.5.Study on metallogenic geological features and isotopic chronology of the metallogenic system show that:mineralization of the Liupiye orefield occurred in the early Jurassic?186.8±3.8 Ma?,the magmatism,which is closely related to the mineralization,occurred in the middle Triassic?235.1±6.0Ma?,early Jurassic?186.2±1.3Ma¹83±0.98Ma?,middle Jurassic?173.3±3.1Ma?.Its petrogenesis and mineralization experienced at least 50Ma?173²35Ma?,and the mineralization experienced 8Ma?183¹91Ma?.6.Study on geological and geochemical characteristics closely related to plutonite and dike indicate that magmatic-thermal events before the mineralization are pyrite-bearing diabase and granite,showing a bimodal-like rock association.7.According to study on geochemistry for magmatite and hypabyssal rocks that are associated with the formation of the deposit,showing that there are subalkaline,high potassium ca-alkaline,mafic diabase veins and subalkaline,high potassium ca-alkaline,peraluminous I-type monzonitic granite,indicating bimodal-like magmatism rock assemblages in the extension environment.The diabase holds low SiO₂ and Al₂O₃,higher MgO,and are enriched in LILE?Rb,Th,U and so on?,depleted in Ba,Sr,Eu,Nb,Zr and Ta,showing the character of arc magmatism.We conform that the diabase,associated with the formation of the gold deposits in Liupiye orefield,was possibly derived from decompression melting of an enriched continental lithospheric mantle,which had been previously metasomatized by fluids derived from subducted slabs.The granite complex holds high SiO₂ and Al₂O₃,low MgO,MnO and iron oxide,and are enriched in LILE?Rb,Th,U and so on?,depleted in Ba,Sr,P,Ti,Eu,Nb and Ta,also showing the character of arc magmatism.The granite complex hold lower ¹⁷⁶Hf/¹⁷⁷Hf ratios?0.282341⁰.282504?and?_Hf?t?ratios?-5.5^-11.3?,which was derived from end-product of interaction between magma related subduction and lower mafic crust.Based on all the research progress,we have concluded that the gold mineralization in the Liupiye gold orefield occurred in the late Lower Jurassic.The process of petrogenesis and mineralization can be divided into four stages:?1?In the Triassic,the Jiapigou terrane and the eastern Central Asian Orogenic Belt combined under the paleo Pacific plate subducting under the Eurasia plate;And granitic magma was formed by the partial melting of the lower crust.?2?In the the middle Triassic,basic magma was formed along NW trending structures in the environment of regional extension.?3?In the Early Jurassic,when the large-scale acid magma emplaced,mineral-bearing gabbroic magma rised along the NW trending structures again;When the physical and chemical conditions changed,ore-forming fluids were formed.?4?Further magmatic crystallization and fluid precipitation formed a combination of brittle-ductile shear zone+gabbro-granite+quartz vein-alterated rock type orebodies.