Abstract: The Debao skarn-type copper-tin deposit in the Youjiang Basin is the largest copper deposit in Guangxi province. Little is known about the geochemical characteristics of ore-forming fluids and the occurrence state of tin beyond its chronology and mineralogy. We constrain the characteristics and parameters of mineralizing fluids using microprobe analysis, fluid inclusion petrography and microthermometry, and geochemical modeling. Different from typical skarn-type copper deposits, the garnet in Debao belongs to the end-member of pyrope and grossular and contains a small amount of andradite. Tin occurs mainly in chalcopyrite and pyrite, and its abundance can reach up to 0.11%. The homogenization temperatures of fluid inclusions in garnet are 182- 249 ℃ with an average of 219 ℃ , and those of fluid inclusions in calcite ranges from 109 ℃ to 215 ℃ with an average of 157 ℃ . Therefore, the mineralizing fluids in the early stage of the Debao deposit were interpreted to be of low temperature and mediumlow salinity, while those in the late quartz-carbonate stage are characterized by low temperature and low salinity. Temperature decrease favors precipitation of chalcopyrite. The oxygen fugacity and sulfur fugacity of the Debao deposit constrained from equilibrium phase diagrams of Cu-S-O and Fe-S-O systems under mineralization temperatures are log f_S2 ＞-20, and -44＜log f_O2 ＜-25, respectively. The oxygen fugacity levels of the Debao deposits were higher than those of typical tin deposits and the high log f_O2 fluid environment prevents tin transport in fluids. This is probably one of reasons why cassiterite is rare in the Debao deposit.

Key words: Debao copper deposit, fluid inclusion, metal sulfides, geochemical modeling