Ore Genesis And Tectonic Setting Of Toudaochuan Gold Deposit In Central Jilin Province,Northeast China

The Toudaochuan gold deposit is located in Yongji County,Jilin Province,and tectonically situated at the intersection of the northern margin of the North China Plate and the southeastern segment of the Xing'an Mongolia Orogenic Belt.This deposit was discovered and developed earlier in central Jilin,but theoretical research is still weak.Based on the collection of regional and mining area data on geology,deposit and mineral exploration,field investigation,test analysis and comprehensive research have been carried out.This paper has ascertained the geological background,geological characteristics,ore-forming geological conditions,ore-forming physical and chemical conditions,sources of ore fluids and material of the Toudaochuan gold deposit.Moreover,the genetic type and the metallogenic mechanism have been further identified.According to the diagenetic age,geochemical characteristics of the metallogenic intrusions and the latest results of regional tectonic evolution,this paper has defined the metallogenic age,revealed the nature of the magma source region,and discussed the metallogenic tectonic setting.The ore bodies of the Toudaochuan deposit,hosted in the metamorphic volcanic rocks of the Yufutun Formation,are mainly controlled by the NNE-trending faults and characterized by sulfide-quartz veins and quartz-calcite veins.The metallic minerals in primary gold ore mainly include chalcopyrite,pyrite,sphalerite,hessite,petzite and natural gold;non-metallic minerals include quartz,potassium feldspar,chlorite,epidote,plagioclase,and calcite.The ore structures mainly include massive,veinlet,and miarolitic structures.The ore textures include euhedral-subhedral-anhedral granular texture,metasomatic relict texture,metasomatic corrosion texture and exsolution texture.Such alteration as silicification,chloritization,epidotization,sericitization,and carbonation are commonly developed in ore bodies and wall rocks,and the silicification,epidotization are closely related to the gold mineralization.According to crosscuting relationship of different veins,the mineral assemblage,intergrowth relationship of minerals,ore textures and structures,as well as wall-rock alterations,the hydrothermal mineralization process can be slassified into three stages,namely,quartz-pyrite stage,quartz-gold polymetallic sulfide stage,and quartz-carbonate stage.Among them,the quartz-gold polymetallic sulfide stage is the major gold mineralization stage.The fluid inclusion petrography,microscopic thermometry and laser Raman spectroscopy analysis of different metallogenic stages of the Toudaochuan gold deposit show that the early stage(quartz-pyrite stage)mainly developed liquid-rich phase(L type)and CO₂-rich(C2 type)inclusions,belonging to the CO₂-H₂O-Na Cl system with medium temperature and low salinity.The major mineralization stage(quartz-gold polymetallic sulfide stage)mainly contained liquid-rich phase(L type),gas-rich phase(V type),gas-pure phase(PV type),CO₂-bearing(C1 type)and CO₂-rich(C2 type)inclusions,of which L type,C1 type and C2 type are the most developed.Fluid immiscibility occurred at this stage,and the fluid belonged to the CO₂-H₂O-Na Cl system with medium-low temperature and medium-low salinity.Only liquid-rich(L type)inclusions developed in the quartz of late stage(quartz-carbonate stage),and the fluid had evolved into low temperature,low salinity H₂O-Na Cl system.From the early mineralization stage to the late mineralization stage,the range of?¹⁸O_H2Ovalues are from 3.64‰?5.44‰,2.20‰?3.54‰,to 0.50‰?1.00‰,and the range of?D values are from-84.7‰?-66.3‰,-83.4‰?-80.8‰,to-89.0‰?-84.3‰,respectively,indicating that the ore-forming fluid was mainly derived from magma and that the amount of meteoric water gradually increased during the main and late stages.The salinity and temperature of the ore-forming fluids showed a decreasing trend from the early mineralization stage to the late mineralization stage.The fluid immiscibility and the mixing of meteoric water may be the main mechanism of Au-rich mineralization.The?³⁴S values of sulfides in the ore are all negative,ranging from-7.5‰to-1.6‰,indicating that they are mainly derived from magma,with a small amount of formation sulfur mixed.The²⁰⁶Pb/²⁰⁴Pb values are 18.477?18.579,the²⁰⁷Pb/²⁰⁴Pb values are 15.567?15.582,and the²⁰⁸Pb/²⁰⁴Pb values are 38.286?38.324,indicating a crust-mantle mixed source.By comparing with typical quartz-vein type gold deposits such as the Wulong gold deposit in eastern Liaoning and the Linglong gold deposit in Jiaodong area,and to combine all the evidence,this study suggests that the Toudaochuan gold deposit belongs to the mesothermal magmatic hydrothermal quartz-vein type gold deposit,and has a close temporal,spatial and genetic relationship with granite porphyry.The LA-ICP-MS zircon U-Pb weighted mean ages of the metallogenic granite porphyry are 117.6±0.8 Ma,and that of the diorite-porphyrite which cut the ore bodies are115±2.9 Ma.These isotope dating results indicate that the diagenesis and mineralization took place in the early Cretaceous.The major elements geochemistry of the metallogenic granite porphyry show that the Si O₂content is 75.81%?76.91%,the Al₂O₃content is12.10%?12.60%,the Na₂O content is 3.72%?3.81%,the K₂O content is 4.11%-4.39%,the Mg O content is 0.06%?0.13%,and the Ca O content is 0.17%?0.22%,which imply that the metallogenic granite porphyry belongs to the high potassium calc-alkaline series,weakly peraluminous with the characteristics of high silicon,rich alkali and poor calcium and magnesium.The total amount of rare earth elements(?REE)is 187.76×10^-6?215.65×10^-6.The chondrite normalized REE curve is relatively flat,light rare earth elements are slightly enriched,while heavier rare earth elements are slightly depleted,with no obvious fractionation.Also,it has obvious negative Eu anomalies and weakly positive Ce anomalies(?Eu=0.06?0.07,?Ce:1.30?1.54),showing obvious"Swallow distribution"characteristics.Trace elements are generally enriched in large ion lithophile elements such as Rb,Th and U,but depleted in P,Ti,Ba,Sr,Eu and other elements.Based on the above geochemical characteristics,the granite porphyry is determined to be A-type granite.Combined with the comparative study of the regional granites in the same period and the diagrams of tectonic environment discrimination,it can be believed that the granite porphyry in the mining area may be derived from the partial melting of the young lower crust related to the mantle material.And the granite porphyry and associated Toudaochuan gold deposit were formed in the lithosphere extensional tectonic setting related to the subduction of the Pacific plate to Eurasia Plate.