Study On Ore-forming Fluids Of Typical Skarn Tungsten Deosit In Eastern Tianshan,NW China

The East Tianshan is located in the southern margin of the Central Asian orogenic belt.In recent years,many skarn-tpe tungsten deposits have been discovered in the Precambrian massif of the East Tianshan.In order to reveal the characteristics of ore-forming fluids,determine the genesis of the deposit and explore the regional metallogenic regularity of this type of deposit,this paper takes two typical skarn deposits,Xiaobaishitou tungsten deposit and Heiyanshan tungsten deposit,as examples,carries out a systematic study in deposit geology,geochemistry and metallogenic chronology,and discusses the regional metallogenic regularity of skarn tungsten deposits and obtains the following understandings.Xiaobaishitou Tungsten deposit was formed in Triassic.The orebodies of the Xiaobaishitou Deposit occur in the skarn zone where the granite body contacts the surrounding rock and the ductile shear zone near the contact zone.The metallogenic process of the deposit can be divided into dry skarn stage,wet skarn stage,oxide stage,sulfide stage and carbonate stage.The types of fluid inclusions include W,C,PC and S types.The homogenization temperature of fluid inclusions ranges from 124 to297?and salinity from 1.0 to 39.7%.The fluids are medium-low temperature and medium-high salinity fluids.H-O isotope study shows that the ore-forming fluid is a mixture of magmatic water and atmospheric precipitation water.Temperature reduction and fluid mixing are the main metallogenic mechanism of the Xiaobaishitou tungsten deposit,while fluid immiscibility is the second one.Zircon U-Pb age of biotite monzonitic granite in Heiyanshan tungsten deposit is326.9±1.6 Ma,indicating that the deposit was formed in Late Carboniferous.Orebody occurs in skarn contact zone formed by biotite monzonite and strata of Xingxingxia Group contacting or in fracture of rock mass of contact zone.The ores are mainly banded,sparsely disseminated and veinlet disseminated scheelite ores.The metallogenic stages are divided into skarn stage,quartz-scheelite-sulfide stage and quartz-carbonate stage.The main type of fluid inclusions is W-type,with homogenization temperatures ranging from 136 to 338?and salinity from 2.7%to13.4%.The fluids are medium to high temperature and medium salinity fluids.H-O isotope study shows that the ore-forming fluid is a mixture of magmatic water and atmospheric precipitation water.Temperature reduction and fluid mixing are the main metallogenic mechanisms of the deposit.The formation of skarn-tpe tungsten deposit in the eastern Tianshan area is closely related to the long-term and multi-stage magmatic activities in the area.Multi-stage magmatism provides heat source,water source and some material source for mineralization.Volatile components in hydrothermal solution provide necessary conditions for the occurrence of double metasomatism in contact zone,and further enrich tungsten elements in formation.In the process of hydrothermal metasomatism of marble,Ca²⁺concentration increases and fluid PH value decreases.With the addition of atmospheric precipitation,fluid temperature decreases and pressure decreases,which makes scheelite precipitate.