Metallogenic Study Of Epithermal Gold Deposits In Chifeng-chaoyang Area,the Eastern Section Of The Margin Of The North China Platform

The research area is located at the border of Chifeng in Inner Mongolia and Chaoyang in Liaoning Province.It is located in the complex tectonic area bordering the Xingmeng orogenic belt in the eastern section of the north margin of the North China platform.The complex structure and magmatism make this area a gold forming area of great concern.At present,more than 20 large and medium–sized gold deposits have been explored,including Zhuanshanzi,Jinchanggouliang,Erdaogou,Nailingou and Xiaotazigou,which are one of the important gold belts in China.In recent years,with the advance of development,resources of some small and medium–sized mineral deposits have been exhausted,so it is urgent to carry out the prospecting work of relevant deep and peripheral gold mines to alleviate the problem of resource exhaustion in the research area.How to solve this existing problem of natural science is extremely urgent.Therefore,this article comprehensively on the basis of previous studies and relying on the China Geological Survey projects,in the typical gold deposit?Zhuanshanzi,Jinchanggouliang and Erdaogou deposit?to carry out the system of the ore deposit geology,fluid inclusions and isotopic geochronology and isotope geochemistry study,so as to provide theoretical basis for further prospecting.The results and progress achieved are as follows:1.The ore types of the typical deposits in the study area are mainly sulphide quartz veins?quartz large veins and veinlets+stockwork?,and a few are alteration rocks.Mineralization generally goes through four stages,namely,the pyrite–quartz stage?early stage of mineralization?,the quartz–pyrite stage?main stage of mineralization?,the quartz–polymetallic sulfide stage?main stage of mineralization?,and the carbonate stage?late stage of mineralization?.Surrounding rock alteration and zonal phenomena are obvious.From the center of ore body to the surrounding rocks on both sides,there are white silicification,pyritized phyllite,cyan silicification,propylitization,and potassic alteration.The metal minerals in the ore are mainly pyrite,chalcopyrite,galena,natural gold,and a small amount of sphalerite and tetrahedrite.2.Research on mineral fluid inclusions of the deposit shows that W–type?aqueous solution?and C–type?CO₂–containing aqueous solution?inclusions are developed in the early stage of metallogenic formation in Jinchanggou gold deposit.The average temperature is 356.4⁴16.8?,the salinity is 3.33¹3.07wt.%,and the density is0.54⁰.73g/cm³.W–type inclusions and C–type inclusions were developed in the main stage of ore formation.The average temperature is 260.5⁴08.9?,the salinity is3.71¹1.48wt.%,and the density is 0.60⁰.84g/cm³.In the late stage of mineralization,W–type inclusion developed,with the homogenization temperature of 202.8³34.2?,salinity of 8.94¹1.11wt.%and density of 0.76⁰.94g/cm³.W–type inclusions were developed in the early stage of Erdaogou gold deposit.The inclusions had homogenization temperature of 334³95?,salinity of 7.72¹1.23wt.%and density of 0.67⁰.74g/cm³.W–type and C–type inclusions were developed in the main stage of ore formation.The homogenization temperature is 214³64?,the salinity is 0.20²3.18wt.%,and the density is 0.59¹.00g/cm³.In the late stage of mineralization,W–type inclusions developed,with average temperature of 172²72?,salinity of 0.35⁵.25wt.%and density of 0.76⁰.93g/cm³.W–type inclusions developed in the early stage of Zhuanshanzi gold deposit,with a homogenization temperature of 284⁴08?,salinity of2.23⁶.58wt.%and density of 0.51⁰.79g/cm³.W–type inclusions developed in the main stage of ore formation,with a homogenization temperature of 200³80?,salinity of0.35³.23wt.%and density of 0.54⁰.89g/cm³.In the late stage of mineralization,W–type inclusions were developed,with homogenization temperature of 145²45?,salinity of 0.35¹.05wt.%and density of 0.81⁰.93g/cm³.The ore–bearing fluids in the early stage of mineralization in the three deposits all belong to the NaCl–H₂O–CO₂ system with medium temperature and intermediate–low salinity.The fluid in the main stage of ore formation evolved into an inmiscible NaCl-H₂O-CO₂ system with moderate temperature and coexistence of medium and low salinity.In the late stage of mineralization,the ore–forming fluid has gradually evolved into a low–temperature and low–salinity NaCl–H₂O system dominated by H₂O.3.The hydrogen–oxygen isotopes of mineral fluid inclusions show that the initial ore–bearing fluids are mainly provided by deep source magma,and mantle source materials are added.The mineralization process is accompanied by the continuous addition of meteoric precipitation.The helium–argon isotope shows that the fluid in the Jinchanggouliang gold deposit is not simply a mixture of mantle fluid and meteoric water,and may the product of formation fluid metasomatism of oceanic crust subduction type EM?enrichment mantle;The sulfur–lead isotopes reveal that the metallogenic materials have the property of crust–mantle mixed source,which is dominated by mantle source.Jinchanggouliang and erdaogou deposits may be derived from the lower crust with?–type enriched mantle.Zhuanshanzi gold deposit ore–forming materials present?–type enrichment mantle.In the process of upward migration of ore–forming fluid,a small amount of ore–forming materials in surrounding rocks were extracted.4.Diagenetic and metallogenic chronology shows that the U–Pb ages of zircons in the surrounding rocks of Zhuanshizi gold deposit range from 252.0±1.5Ma to252.8±3.2Ma,and the isochronic age of sulfide Rb–Sr isotopic in Jinchanggouliang gold deposit range from 127.6±5.5Ma to 127.5±2.0Ma.Based on previous research results,it is concluded that there are at least two gold metallogenic events in the study area,which are late Permian to early Triassic?252Ma²45Ma?and early Cretaceous?140Ma¹26Ma?.5.Geochemical and Hf isotope characteristics of magmatic thermal event elements closely related to mineralization reveal that Zhuanshanzi granite porphyry belongs to a series of rocks with high potassium,calcium and alkali content.And enrichment of large ion lithophile elements,incompatible elements and light rare earth elements,relative loss of heavy rare earth elements and high field strength elements.The rocks are highly differentiated I type granites formed by the magma formed by partial melting of the lower crust through crystallization differentiation.It is under the background of extensional tectonics after the Xingmeng orogenic belt colliding with the North China Plate.Xiduimiangou rock mass?quartz monzonite?belongs to the series of potassium basaltic rocks.And enrichment of large ion lithophile elements,incompatible elements and light rare earth elements,relative loss of heavy rare earth elements and high field strength elements.It shows C type adakitic–like rocks and formed by partial melting of thickened lower crust.As a whole,it was in the extensional period of intense thinning of lithosphere.6.Combining magmatism,fluid origin and evolution with regional tectonic evolution background,it is believed that the mineralization of Zhuanshanzi gold deposit is related to the extension of Xingmeng orogenic belt and North China Plate after collision orogen from late Permian to early Triassic.The ore–bearing fluid reservoir is basaltic magma formed by subduction plate dehydrated metasomatism mantle wedge.The basaltic magma floor penetrated into the deep lower crust and caused partial melting to form acidic magma,and the two mixed to form an initial magma chamber.With the rise of magma,the initial ore–bearing fluid released continued to migrate upward along the ductile brittle faults in different directions formed in the early stage of Zhuanshanzi gold field and extracted the ore–forming materials in the surrounding rocks.The addition of meteoric precipitation led to the strong immiscibility of ore–bearing fluid,which led to the change of physical and chemical conditions of ore–bearing fluid,which led to the formation of Zhuanshanzi gold deposit.The mineralization of Jinchanggouliang and Erdaogou gold deposits is under the background of massive thinning of lithosphere in eastern China during the early Cretaceous.The subduction plates break off,causing the asthenosphere to submerge into the crust and cause partial melting to form the initial magma chamber.The initial ore–bearing fluid released by magma migration continues to migrate upward along the faults formed in the early stage and extracted metallogenic materials from surrounding rocks?metamorphic series and volcanic rocks?.The addition of meteoric water results in strong immiscibility of ore–bearing fluids,resulting in changes in the physical and chemical conditions of the fluids.Gold and other metallic sulfides precipitate to form Jinchanggouliang and Erdaogou gold deposits.