Geological Characteristic And Evolution Of Ore-forming Fluid Of Galale Cu-Au Skarn Deposit,Tibet

The Galale copper-gold deposit is a typical skarn deposit and located in southern margin of the west of Bangong Co-Nujiang Metallogenic Belt.At present,the study of this deposit is mainly focused on mineralogy and chronology,but less on ore-forming fluids.This paper have explored the evolution of ore-forming fluids and the precipitation mechanism of minerals of Galale copper-gold deposits by fluid inclusion petrography,microthermometry and laser raman analysis.According to mineral composition,metasomatic alteration and the sequence relationship,the mineralization of this deposit can be divided into dry skarn stage and wet skarn stage,quartz-iron-copper-sulfide stage and carbonate stage.Mineralization mainly occurs in quartz-iron-copper-sulfide stage.In dry skarn stage,inclusions in garnet mainly include V type,L type,L-V type,V-L type and S type.The gas compositions of inclusions are H₂ O and CO₂,and the daughter minerals are chalcopyrite and hematite.Microthermometry shows that the homogenization temperature of fluid inclusions in garnet range from 423?to 596?,and salinities range from 10.1 to 18.8%wt.NaCleq and 45.2 to 53.1%wt.NaCleq,respectively.Inclusions in epidote from wet skarn phaseinclude V type,L-V type,V-L type.The composition of gas and liquid of inclusions are bothH₂ O.The homogenization temperature of fluid inclusions in epidote range from 275? to 428 ?,and salinities range from 12.1 to 20.8%wt.NaCleq,respectively.Fluid inclusions in quartzand calcite from iron-copper-sulfide stage are V type,L type,L-V type,V-L type and S type,S type are less and gas phase composition of inclusions are diverse,mainly including CO₂?N₂?C3H6?SO₂?H₂S.The transparent mineral is calcite.The homogenization temperature of fluid inclusionsrange from 133? to 339?,and salinities range from 2.4 to 15.6%wt.NaCleq and 30.1 to 34.3%wt.NaCleq,respectively.The homogenization temperature of fluid inclusions in calcite from carbonate stage range from 118? to 183?,and salinities range from 0.18 to 6.88%wt.NaCleq,respectively.From early stage to late stage,the temperature and salinity of ore-forming fluid of Galale deposit were gradually decreased.The characteristics of fluid inclusions of Galale depositshows thathigh salinity fluid formed in the skarn stageby boiling carries a large number of ore-forming elements and migrates in high salinity fluids.Because of the decrease in temperature and release of pressurecaused by boilingin iron-copper-sulfide stage,the gas?CO₂?SO₂?N₂?escapes.Meanwhile,the concentration of metal elements in a fluidhas been enhanced.The pH value and reducibility both increased,the high valence of S?+4 or +6?are reduced and formed a large amount of S^2-.For all these reasons,the solubility of Cu and Au decreases,leading to precipitation and enrichment of the sulfide containing copper and gold complex.