EPMA Analysis of Hydrothermal Biotite from the Bangpu Porphyry Mo-Cu Deposit of Tibet, China and the Characteristics of Early Ore-forming Fluids

The chemical composition of hydrothermal biotite is important in revealing the physiochemical conditions and evolution of early ore-forming fluids. The Bangpu deposit is a large porphyry Mo(Cu) deposit in the eastern part of the Gangdese metallogenic belt, and hydrothermal biotite is extensive in the potassic alteration zone. Hydrothermal biotite in the potassic alteration zone of dioritic porphyry was analyzed by Electron Microprobe Analyzer (EPMA) and is reported in this paper. Results show that the biotite is characterized by the high content of K and low contents of Na and Ca. The average contents of SiO₂, TiO₂, Al₂O₃, FeO^T, MgO, K₂O are 38.95%, 1.42%, 13.55%, 14.22%, 16.23%, 9.77%, respectively. The anomalously high K/Na values (82.5) may be an important indicator for the porphyry Mo mineralization. Based on chemical components, the temperature and oxygen fugacity of the ore-forming fluids were calculated. Results show that the early ore-forming fluids of the potassic alteration have a high temperature and oxygen fugacity, and the average temperature (458℃) of the deep ore-forming fluids is much higher than that of the shallow ore-forming fluids (366℃) . The decreasing temperature and pressure during the migration of the early ore-forming fluids from deep to shallow resulted in the precipitation of the Mo and Cu sulfides. This research provides an important clue for studying the evolution of ore-forming fluids and the ore-forming mechanism.