Fluid Evolution Characteristics Research Of The Hongqiling Sn-W Polymetallic Deposit In Southern Hunan

The Hongqiling Sn-W polymetallic deposit is located on the north edge of the Nanling metallogenic belt,on the northeast side of the Dongpo ore field,and is one of the typical vein Sn-W-Pb-Zn deposits in southern Hunan.Three mineralization stages are identified based on mineral assemblage characteristics and the crosscutting relationship of vein bodies:Sn-W mineralization stage（S1）,Pb-Zn mineralization stage（S2）,and post mineralization stage（S3）.EPMA and trace element analysis are performed on wolframite,cassiterite,sphalerite,chalcopyrite,and pyrrhotite from the Hongqiling deposit in this paper.Simultaneously,microscopic thermometry for fluid inclusions of wolframite and gangue minerals quartz and fluorite,as well as H-O-S isotope analysis,were performed in order to reveal the nature and evolution process of ore-forming fluid during mineral formation and to investigate the mineral precipitation mechanism.The following is the main understanding gained from research:（1）The EPMA test reveals that the main metal mineral in the mining area—wolframite—is rich in Fe overall and evolves from deep to shallow.The wolframite type has a middle intensive type,indicating that it formed from a deep source magmatism.The trace element test reveals that the wolframite in the Hongqiling mining area has high Nb,Ta,and Sc content,and the trace element content varies with distance from the deep rock mass in different stages of evolution.The REE distribution pattern is generally left dipping LREE deficient,and wolframite samples near the pluton side in the deep are consistent with the distribution pattern of Qianlishan granite,indicating that the early W forming fluid came from Qianlishan’s granitic magma.The variable and stable valence elements in another high-temperature mineral—cassiterite show two occurrences of first rising and then falling,implying that the magmatic hydrothermal fluid may have two pulsating exsolutions.A large amount of trace elements are added to the ore-forming fluid each time the magma hydrothermal solution dissolves.However,as high-temperature minerals crystallize further,these elements enter the mineral phase in large quantities,resulting in a decrease in the element content in the ore-forming fluid,which leads to a decrease in the trace element content in thesubsequently crystallized sulfides.（2）Examinations of inclusions in S1 wolframite and coexisting quartz using infrared and microthermometry show that the mineralizing fluid likewise belongs to the Na Cl-H₂O system.The Th of inclusions in wolframite is～40°C higher than that of coexisting quartz.Moreover,the fluid experienced a decrease in temperature accompanied by nearly constant salinity,which indicates that wolframite precipitation is due to fluid mixing and simple cooling,and the precipitation is earlier than quartz.The ore fluid has low salinity,low density,and a wide temperature range,as per our microthermometric data:the S1 stage has homogenization temperatures（Th）of 236-377.6°C（average 305.3°C）and salinity of 3.5-10.7 wt.%Na Cleqv;the S2 stage has Th of 206.5-332°C（average 280.7°C）and salinity of 1.6-5.1 wt.%Na Cleqv;and the S3stage has Th of 170.9-328.7°C（average 246°C）and salinity of 0.2–5.9 wt.%Na Cleqv.According to laser Raman probe analysis,H₂O dominates the fluid inclusions in the S1and S2 stage quartz,with CO₂ and trace N₂ following close behind.Based on the results of the aforementioned investigation,the fluid inclusions in quartz,fluorite,and calcite are mainly H₂O-Na Cl vapor-liquid two-phase.（3）In situ S and H-O isotope data show that the samples haveδ³⁴S=-2.58‰to1.84‰,and the ore fluids haveδD=-76.6 to-51.5‰（S1 and S2）,andδ¹⁸Ofluid=-6.6to-0.9‰（S1）and-12.9 to-10.2‰（S2）.All these indicate that the mineralizing fluid was derived from the granitic magma at Qianlishan,with substantial meteoric water incursion during the ore stage,due to its unique clastic rock surroundings,Hongqiling offers excellent room for magma upwelling and external fluid mixing.Such fluid mixing and subsequent cooling are most likely the primary controls for ore deposition.