Research On Geology,Geochemistry And Metallogenesis Of The Gold Deposits Of The Jiapigou Ore Field In The Continental Margin Of Northeast China

The Jiapigou Gold Ore Field?JGOF?is located in the northwestern region of the Jiapigou-Haigou Gold Metallogenic Belt?JHGMB?between the eastern section of the northern margin of the North China Craton?NCC?and the eastern section of the Central Asian Orogenic Belt?CAOB,which is also called the"Xing?an-Mongolian Orogenic Belt"?XMOB?in China?,approximately 50 kilometers southeast of the Huadian City.As one of the most important gold-producing regions in China,this ore field currently includes more than 20 gold ore deposits and 160 potential ore bodies,including the Xiaobeigou,Sandaocha,Erdaogou,Sidaocha,Bajiazi,and Xiaxitai deposits.In recent years,with the continued exploitation,many new gold deposits have been discovered in the surrounding of the ore field and the Jiapigou-Liupiye area,such as the Dayangcha,Liupiye,Toudaoliuhe,and Binghugou deposits,showing a great prospecting potential in the area.However,some critical problems concerning the gold mineralization of the deposits in this area have remained unresolved,such as the deposit type,diagenetic and metallogenic process,metallogenic epoch and geodynamic setting,thus causing the metallogenic regularity is still unclear.Hence,on the basis of previous studies,and with the support by the project of China Geological Survey?No:12120115034201,0747-1661SITCN130-2?,we select three representative gold deposits?Bajiazi,Xiaxitai,and Sandaocha deposits?to make a systematic research on the geological characteristics of deposits,fluid inclusions,petrogenic and metallogenic geochronology,and deposit geochemistry.The main results achieved in this paper are as follows.1.Geological characteristics of ore field and deposits reveal that the ore bodies are mainly controlled by a series of NW-,NE-NEE-and N-S-striking ductile-brittle and brittle faults that formed during the dextral shearing of the Jiapigou shear zone.The ores are mainly of the pyrite-dominant sulfide quartz vein type and secondarily altered rock type.The gold mineralization can be divided into five stages:?I?milky quartz,?II?beresite,?III?quartz–pyrite,?IV?quartz–polymetallic sulfides,?V?carbonate.Stages III and IV represent the main mineralization stages.From the ore body to the host rock,the alteration zones are defined as follows:a white silicification zone,a silicification–sericitization to?late-stage?carbonatization zone,a navy silicification zone,an epidotization–chloritization zone,and a weak potash feldspathization–biotitization zone.The metallic minerals found within the ores mainly include pyrite,chalcopyrite,sphalerite,galena,and native gold.2.A total of four types of fluid inclusions have been identified in the deposits:NaCl–H₂O?W-type?,CO₂–H₂O–NaCl?C-type?,pure CO₂?PC-type?,and daughter mineral-bearing?S-type?.C-type and W-type fluid inclusions are commonly observed in the early and main mineralization stages in the three deposits,only one S-type fluid inclusion is observed in the main mineralization stage in the Sandaocha deposit,whereas PC-type fluid inclusions can be only observed in the main mineralization stage in the Bajiazi and Sandaocha deposits.The late mineralization stage only developed W-type inclusions.The homogeneous temperatures of the early-stage fluid inclusions from the Bajiazi,Xiaxitai and Sandaocha deposits are 287.5³97.0?,280.5²98.3?,and282.5⁴11.0?,respectively,and the salinities of the early-stage fluid inclusions from the Bajiazi and Sandaocha deposits are 8.82¹9.45wt.%and 4.26¹7.48 wt.%,respectively.The homogeneous temperatures of the main-stage fluid inclusions from the Bajiazi,Xiaxitai and Sandaocha deposits are 183.0²81.0?,197.0²67.8?,and 210.1²87.6?,respectively,and the salinities of fluid inclusions are 3.87¹7.61 wt.%,8.14¹6.71 wt.%,and 2.07¹5.76 wt.%,respectively.The homogeneous temperatures of the late-stage fluid inclusions from the Bajiazi,Xiaxitai and Sandaocha deposits are 119.8¹90?,152.3¹98.0?,and 130.9²09.6?,respectively,and the salinities of fluid inclusions are3.23¹1.34 wt.%,6.45¹2.39 wt.%,and 2.57¹4.04 wt.%,respectively.By combining these new fluid inclusion data with those of previous studies,we propose that the early-stage ore-forming fluids belong to the CO₂-rich CO₂-NaCl-H₂O system and represent moderate-temperature and moderate–low-salinity fluids,and water-rock interaction occurred in the early mineralization stage;the main-stage ore-forming fluids belong to NaCl-H₂O-CO₂ system,undergo fluid immiscibility,and represent moderate–low-temperature and moderate–low-salinity fluids;the late-stage ore-forming fluids belong to the NaCl-H₂O system,and represent low-temperature and low-salinity fluids.3.C-,H-,and O-isotope results show that initial ore-forming fluid consisted of magmatic water produced by the partial melting of lower crustal material containing a mantle component,and mixed meteoric water during the mineralization progresses.S-,Pb-,and Sr-isotope results reveal that metallogenic material was derived predominantly from the mantle containing a crust component,and was most likely derived from a mixed source that had the characteristics of enriched mantle?Type I?.The ore-forming fluid extracted only a small amount of metallogenic material from host rocks during the process of upward migration.4.Metal sulfide Rb-Sr dating results suggest that the metallogenic age of the Xiaxitai deposit is 177Ma¹76Ma.By combining our new data with those of previous studies,we propose that an important gold mineralization occurred at the end of the early Jurassic to middle Jurassic between 178¹70 Ma,which is the transitional period from a compressive regime to an extensional one following the first subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.5.The gold mineralization occurred at the end of the early Jurassic to middle Jurassic is closely related to dioritic magmatism that result in the widespread distribution of diorite dikes in the entire ore field.Zircon U-Pb dating results suggest that the age of the fine-grained diorite is 176.5±2.4Ma.These ore-forming rocks is characterized by quasi-aluminous calc-alkaline and high-K calc-alkaline,and have a low content of REE,significant differences between LREE and HREE;Enrichment of LREE,Th,U,Pb,and large ion lithophile elements Rb,K,Sr,loss of HREE and high strength elements Nb,Ta,P,Ti,etc..Especially the depletion of high strength elements Nb,Ta,and Ti?TNT?,indicating that the dioritic rocks were formed in an island arc or active continental margin environment.Our further research indicates that they were formed in an active continental margin environment that related to the subduction of the Pacific plate.6.Whole-rock geochemistry and zircon Lu-Hf isotope results reveal that the dioritic magmas formed by the mixing of felsic magmas and small amount of mantle-derived mafic magmas.The mantle-derived mafic magmas were derived from melting of depleted lithospheric mantle that had been metasomatized by slab-derived fluids,and the felsic magmas were originated from partial melting of ancient lower crust that caused by the underplating of mantle-derived mafic magmas.7.By combining magmatism,fluid evolution with regional tectonic evolution,the geological process of diagenesis and metallognesis in the study area can be summarized:?1?Middle-Late Triassic?especially the Late Triassic?,the study area was in an extensional tectonic environment that followed the closure of the Paleo-Asian Ocean.During this period,the NW-striking Jiapigou ductile shear zone occurred dextral shearing and was superimposed by brittle deformation,resulting in forming a series of NW-,NE-NEE-and N-S-striking ductile-brittle and brittle faults.These secondary faults supplied the space and channels for magma invasion and ore-forming fluid transportation.?2?During the period from late Early Jurassic to the early Middle Jurassic,the study area experienced a transitional period from a compressive regime to an extensional one following the first subduction of the Paleo-Pacific Plate beneath the Eurasian Plat.Fluid released from subducted oceanic slab was influenced by overlying sediments,and then it migrated upward and reacted on the mantle wedge,resulting in mafic magmas that were characterized by?Type I?enriched mantle properties.During this period,strong metasomatism made the metallogenic material continuously enriched in the mafic magmas.Subsequently,the underplating of mantle-derived mafic magmas caused the partial melting of ancient lower crust and resulted in felsic magmas.Felsic magmas were mixed with small amount of mafic magmas to form dioritic magmas.?3?The dioritic magmas migrated upward to shallow regions of the crust,and then formed dioritic rocks by crystallization differentiation.With the separation of fluid and melt,initial ore-forming fluid was released,and migrated upward along the different directions of secondary faults.Water-rock interaction occurred during this process and resulted in a series of hydrothermal alterations.?4?Fluid immiscibility caused by the addition of meteoric water and/or the rapid decrease in pressure resulted in the escape of volatiles as well as changes in physicochemical conditions of the ore-forming fluid.Subsequently,the stability of the Au-S complex was destroyed,the solubility of gold is reduced,and then the gold mineralization occurred.