In Situ Raman Spectroscopic Investigation Of The Transport Of Tungsten In Hydrothermal Fluids

Quartz-vein-type tungsten deposit is a typical hydrothermal deposit,and knowledge on the transport and precipitation of W in hydrothermal fluids is vital towards the re-construction of the mineralization process and the establishment of a sound mineralization model.Most of the relevant studies were focused on the solubility experiments,thermodynamic simulation calculations,and among others.In this case,the hydrothermal W species responsible for the transport of W in ore-forming fluids were investigated indirectly.On the contrary,in situ spectroscopic investigation can reveal the hydrothermal W species from the molecular level.However,due to the limitation of experimental conditions,previous experimental temperatures（200℃）were generally lower than the W-mineralization temperature（300 to 350℃）,which restricted the understanding of the transport of W.In this study,the species of dissolved W were investigated at elevated temperatures and pressure（23～400℃;20～60 MPa）using in situ Raman spectroscopy.The novel fused silica capillary cells allow both in situ optical observation and spectroscopic characterization of the experimental fluid systems over a wide T-P conditions,espcically covering the W-mineralization T-P ranges.The major contributions are summarized below:Firstly,the influences of p H and salinity on the hydrothermal species of W were revealed.In neutral to alkaline solutions,monomeric tungstates([WO₄]^2-and[HWO₄]^-)are predominant W species;their v₁（W=O）modes are centered at～930 cm^-1 and～950 cm^-1,respectively.In addition to[WO₄]^2-and[HWO₄]^-,in weak-moderate acidic solution,polymeric tungstates(v₁（W=O）modes at～960–995 cm^-1)are also important W species;these polymeric tungstates are stable at temperatures up to 300°C.However,only polymeric tungstates are identified as the major W species in strong acidic solution.Fluid salinity exerts a positive effect on the thermal stability of polytungstate species;in moderate acidic solutions,the maximum stable temperature for polymeric tungstate species can reach at least 350°C in the presence of 10 wt%Na Cl.The presence of polymeric tungstates helps to maintain a high W concentration in the fluid.According to the above observations,the polymeric tungstates decompose to form monomeric tungstates with the decreasing fluid acidity and salinity,promoting the precipitation of W from the fluid.Secondly,the influence of CO₂ on the transport and precipition of W was deciphered.CO₂ and its hydrated products([HCO₃]^-and[CO₃]^2-)cannot associate with tungstate,suggesting thatcarbonic tungstate species are not responsible for the transport of W during mineralization.However,the presence of CO₂ could affect the fluid p H,and exert an indirect influence on the species of W.Based on the previous experimental investigations,thermodynamic silulations were conducted to describe the W-mineralization during wall-rock alteration.Results show that CO₂ can promote the mineralization efficiency,because the CO₂-bearing fluid could retain its acidity through the H₂O+CO₂=HCO₃^-+H⁺p H buffer.Continuous release of Fe²⁺and Mn²⁺to the fluid during the wall-rock alteration could be possible only in the presence of sufficient H.Finally,we applied the experimental results to describe the W-mineralization in typical W deposits in the southern Jiangxi Province,and to discuss the geochemical behavior of W in hydrothermal fluid during the mineralizing process.Take the Piaotang deposit as an example,the ore-forming fluids were characterized by H₃BO₃+Na Cl+H₂O+CO₂+CH₄+N₂system,based on in situ Raman spectroscopic analyses of the quartz-hosted fluid inclusions.Vapor-liquid-solid three-phase inclusions yield vapor-liquid two-phase homogenization temperature ranging from 242 to338°C,whereas the total homogenization temperature is higher than450°C.The fluid salinity varies between 1.4 and 7.6 wt%Na Cl equiv.Therefore,the ore-forming fluids are characterized by high temperature,low-moderate salinity,and enrichment of volatile components.During the silicate-oxide stage,the predominated hydrothermal W species are monomeric tungstates due to the high temperature（>300°C）.With the decrease of temperatures,the polymeric tungstates become to be the important species of W in acidic fluids.At the late stage,CO₂ escaped and the fluid p H rised,decreasing the stability of polymeric tungstates.In this case,polymeric tungstates would gradually decompose to form[WO₄]^2-and[HWO₄]^-,which combined with Fe²⁺and Mn²⁺to precipitate wolframite.