Study On The Geological Characteristics And Enrichment Regularities Of Mineralization Of Changfacou Au-Cu Deposit In Jingyu County, Jilin Province

Changfagou Au-Cu deposit is located in Jingyu County of Jilin Province. It is atypical porphyry Au-Cu deposit in the eastern of Jilin-Heilongjiang. The tectonic positionis located in the high-grade metamorphic zone which is located in north eastern edge ofthe North China plate. It locates to the south of the Dunhua-Mishan fracture, and belongsto the Wangjiadian-Tianhexing polymetallic metallogenic belt.Most of the exposed strata in the area are the Archean Anshan group and Quatenaryloose gravel layer. Magmatic rocks are widely distributed within the mining area.Distribution area of about half of the mining area. Mainly intrusive rocks, scattered smallamount dikes. According to the instrusion age, there are the Archean granite（2585.4±7.4Ma） and diorite（2531.5±8.4Ma）, the Late Triassic granitic porphyry（218±1.4Ma）and the Early Cretaceous quartz porphyry (118.11±0.92Ma). Amongthem, the Early Cretaceous quartz porphyry is the most important host rock ofChangfagou Au-Cu deposit. Geological studies indicate that quartz porphyry rich insilicon and aluminum, belongs to potassium-rich calc-alkaline series to shoshonite rocksequence. Quartz porphyry is the most important ore-bearing rock which is formed in aback-arc extensional environment of the subduction of the Pacific plate.There are three ore blocks in the mining area named-I, II, III block. The orebody areall concealed, and most of the country rock are quartz porphyry. Ore minerals are chalcopyrite, molybdenite, pyrite, pyrrhotite, galena, sphalerite and so on. Ore exturescontains hypidiomorphic-anherdral textures, metasomtic structures, separation of the solidsolution structures, crushing textures. Ore structure contains lumpy structure, veinstructure and disseminated structure. Alteration showed typical surface-type characteristic,from inside to outside consist of potassic alteration zones (potassic)-like phyllitealteration zone (silicification, sericite)-argillaceous alteration zone (clay-based)-propylitization band (carbonation, chlorite, epidotization). The mineralization period canbe divided into four phases: quartz-magnetite mineralization stage, quartz-molybdenitemineralization stage, quartz-polymetallic sulphide mineralization stage, carbonateformation stage. Dajianggang is the newly discovered goldfield, which is located in thewestern ditch of Changfagou. We used several trenchs revealed a number of goldmineralization body, which hosted in typical pyrite phyllic and mainy controlled by faultstructures. Ore minerals are fine-grained pyrite, chalcooyrite, galena, sphalerite, magnetite.Alteration types are mainly silicification, carbonate and so on.The study of the fluid inclusions shows that there are many types of mininginclusions, including gas-liquid two-phase inclusions, three-phase inclusions containingCO2and three-phase inclusions contain NaCl minerals. The results show that from early tolate, the temperature, salinity, density, oxygen fugacity and CO2content of ore-formingfluid decreased. The ore-forming fluid in the main mineralization stage is characterized bymiddle-high temperature and middle-high salinities. The ore-forming pressure can becalculated to almost3to22MPa, the ore-forming depth ranges from0.3to2.2km. Sulfurisotope test results show that the minerals were from multi-source of the mantle and uppercrust pre-forming. The ore-forming fluid of the Dajiagngang deposit is characterized bymiddle temperature、low salinities and low density.According to the dating results, both mineralization and intrusion of the porphyryformed around118Ma, belonging to the late Yanshan. Combined with the evolution of thetectonic setting of the area and geochemical studies, the Changfagou Au-Cu deposit wasformed in a back-arc extensional environment of the subduction of the Pacific plate.Through the comparative analysis by the geological characteristics of Changfagou Au-Cu deposit and several domestic and foreign typical porphyry deposits, we argue that thedeposit genetic type of Changfagou Au-Cu should be set porphyry deposit. According tothe characteristics of deposit geology and mineralization physicochemical conditions ofDajianggang deposit, we initially set its genetic type as mesothermal vein type golddeposit.Mineralization enrichment studies found that ore body overall exist in the quartzporphyry, and corresponding with the surface-type alteration, mineralization showingsimilar zoning. Vertically, shallow part mainly developed galena and sphalerite, while thedeep mainly developed molybdenite and chalcopyritem, deep again developed pyrite andpyrrhotite. Among the whole mining area, due to the damaging effects of NW structure, inthe west part, relatively enrichment of galena and sphalerite mineralization while in thecentral, relatively enrichment of molybdenite and chalcopyritem. Therefore, the centralfocus of the area is the focus rich region of chalcopyrite and molybdenite, and NorthWest part possible formation of lead-zinc body. In addition, Dajianggang deposit in thewest has huge metallogenic potential, need further exploration and research. It issignificantly controlled by NW-trending faults. It is proposed that faults are the mainresearch direction in the future, and recourse the ore along structure, at the same time weshould pay attention to the fracture and displacement effect on ore body by fault structure.