Geology, Geochemistry And Genesis Of Yangjingou Scheelite Deposit In Hunchun Area, Jilin Province

Yangjingou scheelite deposit, the largest wolfram deposit in northeast China, is located in Yanbian-Xingkai Mesozoic tectonomagmatic belt between North China and Xingkai-Jimusi massif. Ore bodies mainly occurs in the middle and upper segments of early Paleozoic Wudaogou Group and belong to quartz vein scheelite mineralization.The wall-rock of Yangjingou scheelite deposit, Wudaogou Group, are composed of an assemblage of shallow sea facies para rocks, medium-basic volcanics and volcaniclastic rocks.The strata can be divided into three segments which comprise Yangjingou syncline according to lithologic characters.There are two groups of fractures derived from regional fault in the mineral strict,one striking in SN direction and the other extending in NW direction.Both the syncline and the subsidiary fractures dominate the spreading of ore bodies.There develop no magmatic bodies, but various dikes are widely distributed in and around the mine, such as gabbro, diorite-porphyrite, granodiorite and granite-porphyry, especially diorite-porphyrite.The fault F2 divides the Yangjingou scheelite deposit into southern and northern part.The southern ore bodies, striking 340～350°and dipping northeast or southeast with a pitch angle between 65 and 75°, are controlled by NS compresso-shear fractures,while lode ore bodies in the northern part are controlled by NW extensional fractures,striking 280～310°and dipping southeast with a pitch angle between 60 and 80°.Vein-type scheelite mineralization is distinct from wallrocks. Scheelite occurs in 1 to 5 cm wide quartz veinlets, the widest vein can measure up to 4.5 m.There are nearly 60 veins wider than 1m, the longest vein can measure up to 1250m and some may measure up to 500m in depth. 0.5 to 5 cm scheelite crystals in size spread unevenly in quartz veins.Chemical assays from the veins indicate average grades up to 0.44% WO₃.The major metallic mineral in ore is scheelite,which shines light blue on ultraviolet radiation. Other metallic minerals include sulfides such as arsenopyrite, pyrrhotite, pyrite, and chalcopyrite. Gangue minerals mainly include quartz, albite, biotile, chlorite and calcite.Wallrock alteration around quartz veins is intense, including silification, albtization, biotitization, tremolitization, mussization, epidotization, sericitization, actinolization and carbonatation. Four mineralizing stages are confirmed according to inter fingering relations between minerals: silicates stage, oxides stage, sulfides stage and carbonates stage. The scheelite is dominantly deposited during oxides stage.According to studies of main elements, trace elements and rare-earth elements, acid dikes in the area are I type granitoids formed in orogenic settings. They are resulted from the collision between the western margin of Pacific oceanic crust and the eastern margin of Eurasian continental crust during Late Triassic and Middle Jurassic.We confirm mineralizing fluid a compound of magmatic and atmospheric water by H,O isotope studies in quartz.Theδ¹³C values of CO₂ extracted from fluid inclusions in quartz range from -6.5‰to -3.4‰,which is within theδ¹³C range of deep-seated original carbon(from -3.0‰to -7.5‰,Tatlor etc,1986).Further,theδ34S values of sulfides varies little between -0.4‰and +4.7‰and is close to theδ34S values of S from mantle(0±3‰, Chaussidon etc, 1990).From above, it is concluded that the mineralizing fluid must have a deep-seated origin,namely lower crust or upper mantle.Fluid inclusion studies carried out can confirm ore-forming physicochemical conditions.The most abundant fluid inclusions are aqueous ones containing a liquid and a vapor phase.Their measured homogenization temperatures range from 144.7℃to 392.3℃with most between 180℃and 340℃.Temperatures of final melting of ice in aqueous inclusions mainly range from -4.2℃to -1.8℃indicating salinities of 3.0 to 7.0 equivalent weight % NaCl.The density of ore fluid calculated according to the homogenization temperatures and salinities of brine solutions varies between 0.85g/cm³ to 0.95g/cm³(average 0.88g/cm³), corresponding pressure of ore forming is from 5.8 MPa to 88.6MPa and mineralization occurs at a depth between 5.4 and 7.7 km.Based on various data above, we think ore-forming element W stems from magmatic bodies and wallrocks, while Ca comes from the middle and upper segments of Wudaogou Group. Ore fluid is a mixture of magmatic and meteoric water. The blinded deep intrusion provides heat source.The mineralization of Yangjingou scheelite deposit is closely associated to the low lying I type granite of Yanshan Stage,so the mid-epi-depth mesothermal quartz vein scheelite deposit is presumed to form in Stage of Yanshan.The formation of Yangjingou scheelite deposit can be summarized as follows. Ore-forming fluids came from granite of Yanshanian Stage,which derives from Pacific plate subducting to Eurasian plate during Jurassic, flow into the middle and upper segments of Wudaogou epimetamorphized rocks and fill in the gaps to form a quartz vein scheelite deposit.