Fluid inclusion characteristics and geological significance of the Xi'ao copper-tin polymetallic deposit in Gejiu, Yunnan Province

摘要

The newly discovered Xi'ao Cu-Sn polymetallic deposit is located in the inner alteration zone of the Laochang-Kafang granite edge in Gejiu. Yunnan Province. The mineralization process of this deposit can be divided into four stages. Cassiterite and chalcopyrite mainly formed at stages Ⅱ and Ⅲ, respec-tively. Fluid inclusions at different stages were studied by petrography, microthermometry, and laser Raman spectrometry. Five fluid inclusion types are observed. Microthermometry results revealed that the ore-forming fluid evolved from high temperature with low to intermediate salinity to low temperature with low salinity. At stage Ⅰ, the fluid was under near-critical conditions and then separated into coexisting liquid and vapor phases at stage Ⅱ. Finally, the fluid was boiling and immiscible at stage Ⅲ. A meteoric fluid mixing process was also observed at this stage. Laser Raman spectra identified CH_4 at the skarn and mineralization stages. Combined with other geological features, the deposit was concluded to have formed under a relatively reduced environment. The CO_2 component abruptly increased at stage III and probably originated from the exterior. Fluorine may play an important role in mineralization. Cassiterite and chalcopyrite were mainly deposited at 340-440 and 300-400 ℃ under pressures of 18-50 and 12-36 MPa, respectively. Boiling and immiscibility are probably the main reason for mineral deposition. The deposit of cassiterite and chalcopyrite showed similar salinities but higher temperatures than other mineralization types of deposits in the Gejiu area. This rinding indicated that the study deposit probably represents the high-temperature part of the Gejiu mineralization system, thereby benefiting mineral explorations in the Gejiu area.