Assessment of Biotite-Based Thermobarometers in Porphyry Systems: A Case Study from the Julong Cu–Polymetallic District of Tibet, China

In porphyry systems, the physicochemical properties of ore-related intrusions critically influence both metallogenic fertility and the resulting metal assemblages. Biotite is a widespread magmatic mineral capable of recording subtle changes in physicochemical parameters throughout the evolution of porphyry systems. Throughout the years, several biotite thermobarometers have been proposed; however, the most appropriate combination for application to porphyry systems remains uncertain. In the Julong Cu–polymetallic district, where magmatic biotite is pervasive in the ore-related intrusive suite, we integrate available temperature, pressure, and oxygen fugacity data to assess which combination of biotite-based thermobarometers best captures the physicochemical condition of the magma. Our results demonstrate that the structural formula recalculation method of Li et al. (2020), when combined with the thermometer of Li and Zhang (2022) and the barometer of Uchida et al. (2007), yields the most accurate reconstruction of magmatic conditions in porphyry systems. In the Julong district, this integrated approach reveals that the earliest granodiorite crystallized at depths of ~3.2–6.3 km, under strongly oxidizing conditions (~NNO+3 to HM) and in the presence of elevated volatile concentrations. A slightly younger monzogranite formed at ~4.4 km depth, recording lower oxygen fugacity conditions (~NNO+2 to NNO+4). Its elevated F concentration and lower oxygen fugacity suggest a genetic link to the fractional crystallization of magmatic phases, especially magnetite. On the other hand, the ore-related monzogranite porphyry (~NNO+3 to HM) shares the oxidized signature of the granodiorite and was emplaced at ~3.4 km depth. Its low log(fH2O/fHCl) value reflects elevated HCl activity, conducive to the efficient magmatic transport of Cu and Mo.