Abstract: Mineralization of the Kafang tin polymetallic deposit is closely related to granitic magmatism. The chemical composition
of biotite, which is the most common mafic mineral in granite, has been used to reveal petrogenesis and mineralization. Detailed
chemical compositions of biotite from the Kafang granite of the Gejiu tin orefield, a famous tin polymetallic deposit in China, are
presented in this study. Results of electron microprobe analyzer (EMPA) indicate that the biotite has high SiO2, FeOT, Al2O3, K2O
and TiO2, and low MgO, CaO, Na2O and MnO contents, with Fe/(Fe+Mg) values of 0.67-0.83, resembling those of iron biotite.
These characteristics suggest that their host granites were mainly originated from partial melting of crustal materials. Calculation
results of biotite thermometers, Al-in-biotite barometer and oxygen fugacity estimation suggest that they have high crystallization
temperatures of 500-708℃, crystallization pressure of 202-538 MPa with corresponding crystallization depth of 7.64-20.35 km,
and oxygen fugacities from high to low. Integrated with regional geological data, we propose that high iron indexes and from high
to low oxygen fugacities of the biotite from Kafang granite are favored for Sn mineralization. These characteristics could be key
tracers for Sn exploration.

Key words: biotite, granite, mineral composition, petrogenesis, mineralization, Kafang orefield