Study On Genesis And Mineralization Of Xiahuolong Au Deposit In Jilin Province

Xiahuolong gold deposit is a medium-sized hydrothermal deposit discovered and proved in Ji'an area of Jilin Province in the early liberation period.It occurs in the area of metamorphic rock series of Ji'an Group of Lower Proterozoic and Mesozoic magmatic emplacement.Since its discovery,it has been concerned and studied by geologists,but up to now,there has been a great controversy about the genetic type of the deposit.Therefore,on the basis of previous studies,this paper has carried out the research work of deposit geology,fluid geology,diagenetic and metallogenic chronology,element and isotope geochemistry.Some scientific problems,such as the restriction of magma on mineralization,origin and evolution of ore-forming fluid,source of ore-forming materials,metallogenic mechanism and mineralization,have been discussed.The main achievements and progress are as follows:(1)The geological study of the deposit shows that the ore bodies of the deposit are gold bearing siliceous altered rock type and reticular quartz vein type,and the mineralization mainly occurs in the secondary fault system of NW trending faults.The host rocks are paleoproterozoic Ji'an Group metamorphic Marine calcareous clastic rocks.The alteration types of surrounding rock are skarn,silicification,carbonation,cyanophanerization and sericite,and the mineralization is mainly pyrite,chalcopyrite,galena,sphalerite and silver-gold.The metallogenic process can be divided into prograde skarn stage(I),retrograde skarn stage(II),sulfid-rich quartz-fine-grained pyrite-Cu-Au polymetallic stage(III),quartz-coarse-grained pyrite-Pb-Zn polymetallic stage(IV),quartz-carbonate stage(V),which has the metallogenic characteristics of metasomatic hydrothermal deposit.(2)Fluid geology reveals that the quartz-sulfide stage,gas-rich phase,liquid-rich phase and pure gas/liquid phase inclusions are developed.The homogenization temperatures are 273?367?,201?346?and 120?196?from early to late stage,and the corresponding salinitions are 4.01%?8.67wt.%Na Cl_eqv.,1.73%?5.99wt.%Na Cl_eqv.and 0.87%?3.85wt.%Na Cl_eqv.,respectively.Combined with laser Raman probe analysis and H-O isotope characteristics,it is further concluded that the ore-forming fluid is H₂O-Na Cl-CO₂±CH₄ system in general,and the ore-forming fluid in quartz-sulfide stage has been added into atmospheric precipitation,and the ore-forming fluid has experienced multiple mixing and water-rock reaction during the whole ore-forming process.The fluid property is an evolution process from medium high temperature and medium low salinity to low temperature and low salinity.(3)Zircon U-Pb isotopic dating of monzonite and associated diorite porphyrite in the mining area reveals that the zircon u-pb isotopic dating of medium-grain monzonite and diorite porphyrite dike in the mining area is 1910?2484Ma,and the zircon U-Pb isotopic dating of diorite porphyrite dike is 124.1±1.9Ma,indicating that the mineralization of the deposit is related to the magmatism of diorite porphyry which formed in the Early Cretaceous.(4)The element geochemical characteristics of diorite porphyrite show that it is high-k calc-alkaline rock series,enriched in LREE and large ion lithophile elements,relatively deficient in Nb,Ta,Ti and other high field strength elements,Eu anomaly is not obvious,REE distribution model is right-leaning type.The initial magma has the characteristics of magma produced by partial melting of mantle wedge metasomatism from plate subduction fluid,including shallow emplacing(<1.5km),low differentiation(DI=55.87?65.30),reduction and high water content(?FMQ=-8.94?+3.76;log(?O₂)=-7.71?-26.72)lay the foundation for the formation of the deposit.(5)The geochemical characteristics of S and Pb isotopes show that the sulfur source is the mixed source of magmatic sulfur and stratigraphic sulfur.The lead belongs to abnormal lead,and the original lead age is calculated to be 2700 Ma,and the strata contribute greatly to the source of lead.Combined with previous research results,it is revealed that the ore-forming materials are mainly derived from magmatism and Proterozoic strata.Based on the above studies on mineralization and progress,we believe that the Xiahuolong gold deposit is a contact metasomatic hydrothermal deposit,which is the product of the diorite magma emplacement and the metamorphic marine calcareous clastic rock of the Paleoproterozoic Ji'an Group.The mineralization is suitable for the continental margin extension environment after the subduction of the paleo-Pacific plate in the Early Cretaceous of the Mesozoic.