The Xiongcun copper (gold) ore district in Tibet is located on the southern margin of the middle Gangdise metallogenic belt, mainly composed of No.Ⅰ, No.Ⅱ, and No.Ⅲ orebodies and several mineralized bodies. In this paper, the modes of occurrence and composition of hydrothermal biotite in the potassium silicate alteration zone of No.Ⅰ and No.Ⅱ orebodies were systematically studied by means of microscopic identification and electron microprobe analysis. Biotite of No.Ⅰ orebody in the Xiongcun ore district is mainly phlogopite and magnesium biotite while biotite of No.Ⅱ orebody is mainly magnesium biotite. Biotite of both orebodies is characterized by low Ti (TiO₂<3%) and high Al (Al₂O₃>15%), with high MgO content, Mg/Fe value>0.5, and K/Na value greater than 10, which shows a good correlation with mineralization. The average crystallization temperature of hydrothermal biotite in No.Ⅰ orebody is 470℃, and the oxygen futility is between NNO and HM. The average crystallization temperature of hydrothermal biotite of No.Ⅱ orebody is 234℃, and the oxygen futility is between NNO and FQM, indicating that No.Ⅰ orebody was formed in a hydrothermal system with high temperature and oxygen futility, while No.Ⅱ orebody was formed in a hydrothermal system with relatively low temperature and oxygen futility. The Ⅳ(F) value of hydrothermal biotite in No.Ⅰ orebody ranges from 0.61 to 2.72, with an average value of 1.26. The Ⅳ(Cl) value is from -5.49 to -4.53, with an average value of -5.03. The Ⅳ (F/Cl) value is from 5.63 to 7.89, with an average value of 6.29. The Ⅳ(F) value of hydrothermal biotite in No.Ⅱ orebody ranges from 1.83 and 3.32, with an average value of 2.66. The Ⅳ(Cl) value is from -5.64 to -4.89, with an average value of -5.31. The Ⅳ (F/Cl) value is between 7.14 and 8.68, with an average value of 7.97. These data indicate that No.Ⅰ and No.Ⅱ orebodies were formed in the Cl-rich hydrothermal system, and that the hydrothermal solution of No.Ⅱ orebody was richer in Cl and poorer in F than that of No.Ⅰ orebody. Metallic elements such as Cu and Au were easily extracted and moved in the form of metal complex by Cl-rich fluid. In the process of fluid migration, changing physicochemical conditions would reduce the solubility of metal complex and led to sulfide precipitation.