Ore-Forming Fluids Constraints On Genesis Of The Qiagong Fe Skarn Deposit,Tibet

To explore the evolution of ore-forming fluids and ore-forming mechanism of the Qiagong Fe skarn deposit in the Gangdese Belt,Tibet,we study the homogenization temperature,salinity,laser Raman spectroscopy and H-O isotopic compositions of major minerals in different stages.In the skarn stage,the main minerals are garnet and pyroxene,and garnets are chosen for inclusion experiments.The homogeneous temperatures of the fluid inclusions that in the prograde stage range from 400 to 550?,and the salinities vary from 15.5 to 20.9 wt%Na Cl eqv.Besides,the salinity of S-type fluid inclusion in this stage is up to 56.5 wt%Na Cl eqv.Both the vapor and the liquid phases in these fluid inclusions are H₂O.At the retrograde stage,the main minerals are epidote,actinolite,chlorite,and magnetite,selecting epidote and actinolite represent this stage for inclusion experiments.The homogeneous temperatures of fluid inclusions during the retrograde stage change from 350 to 420?,and the salinities are primarily concentrated in 14.1?16.6 wt%Na Cl eqv.,partly in 2?8 wt%Na Cl eqv.However,the salinity of type-S inclusion is up to 55.8 wt%Na Cl eqv.Both the vapor and the liquid phases in these fluid inclusions are H₂O.In addition,the liquid phase is also rich in HCO₃^-and CO₃^2-.In quartz-galena stage,the minerals are mainly quartz,galena,and calcite,and quartz is selected for inclusion experiments.The homogeneous temperature range of the fluid inclusions in quartz-galena stage is 238?343?,corresponding to the salinities of 3.1?13.9 wt%Na Cl eqv.Additionally,the total homogenization temperature of CO₂-bearing three-phase fluid inclusions are in 290?310?,corresponding to the salinities of 1.6?11.2 wt%Na Cl eqv.The main minerals are quartz and calcite.during the quartz-calcite stage,the quartz and calcite at this stage are selected for inclusion experiments.,the homogeneous temperatures and salinities of the fluid inclusions vary in 242?360?and 1.7?11.8 wt%Na Cl eqv.Respectively,the component of liquid phase in fluid inclusion is dominated by H₂O,as well as the vapor phase.The H-O isotopes show that the?D_H2O and?¹⁸O_H2O are-106.4‰?-113.2‰and 6.2‰?8.0‰during the prograde stage,and are-84.8‰?-130.1‰and?¹⁸O_H2O is 2.7‰?5.5‰during the retrograde stage,respectively.The?¹⁸O_H2O compositions in the retrograde stage are lower than those in the prograde stage.The?D_H2O and?¹⁸O_H2O are-95.3‰?-103.8‰and-1.6‰?-0.7‰in the quartz-galena stage and are-67.4‰?-101.0‰and-0.8‰?0.6‰in the quartz-calcite stage,respectively.These data indicate that the fluids evolved from high temperature,medium-high salinity to low temperature and low salinity.The ore-forming fluids are mainly derived from magma exsolution in skarn stage.The fluid immiscibility and wall-rock interaction during skarn stage are the main mechanisms of magnetite precipitation.Decreased pressure and temperature during quartz-galena stage might be the prime reasons for precipitation of galena.