The Xiahe-Hezuo Mining area in Gansu Province is one of the important gold deposit dense areas in the West Qinling orogen.The Zaozigou gold deposit is the most famous giant gold deposit in this area.The veined intermediate to acid magmatic rocks are commonly developed in the Zaozigou gold mining area,which are closely related to mineralization.Based on the field investigation of the geological characteristics of the Zaozigou giant gold deposit,the electron microprobe analysis of the biotite was carried out on the veined intermediate to acid magmatic rocks in the mining area.The detailed results show that the FeO_t contents of biotites range from 19.71% to 29.84%,and the MgO contents range from 9.54% to 13.51%.They show the characteristics of both magnesian biotite and iron biotite.The crystallization temperatures of biotite are 776.61℃ to 799.65℃,the crystallization pressures range from 131 MPa to 283 MPa,and the oxygen fugacity lg f_O2 are-14.85 to-13.11.The biotites are characterized by high temperature,low pressure and relatively high oxygen fugacity.The geochemical composition characteristics of biotites show that the veined magmatic rocks in the Zaozigou gold deposit belongs to calc-alkaline granite,which is derived from mixed crust and mantle origin.The mineral chemical characteristics of biotite of the veined magmatic rocks of Zaozigou gold mine and the geological background revealed that the magmatism which the Zaozigou veined magmatic rocks represented contribute to the mineralization of the giant gold hydrothermal deposit.