Mineral chemistry of magmatic and hydrothermal biotites from the Bangpu porphyry Mo (Cu) deposit, Tibet

Abstract

The Bangpu deposit, located in the eastern part of the Gangdese porphyry copper belt, Tibet, is a porphyry Mo (Cu) deposit in a continental collisional orogenic belt. The Mo (Cu) mineralization occurs mainly in monzogranite porphyry, whereas the Cu (Mo) mineralization occurs mainly in diorite porphyry. Most magmatic and hydrothermal biotites from the Bangpu deposit are Mg-rich, plotting in the phlogopite and Mg-biotite compositional fields. The Bangpu magmatic biotite has high BaO (>0.1 wt%) and TiO2 (>3 wt%), while the hydrothermal biotite has low BaO (<0.1 wt%) and TiO2 (<3 wt%). Hydrothermal biotite from the Bangpu Mo (Cu) mineralization system has higher Al2O3, SiO2, MgO, K2O, Na2O and F contents and lower TiO2, FeOtot, MnO and Cl contents than hydrothermal biotite from the Cu (Mo) mineralization system. Crystallization temperatures range from 757 °C to 827 °C and 687 °C to 740 °C for magmatic biotite in monzogranite porphyry and biotite monzogranite, respectively. Hydrothermal biotite geothermometry yields a temperature range from 362 °C to 547 °C for the Mo (Cu) mineralization system and 302 °C to 410 °C for the Cu (Mo) mineralization system. Calculated oxygen fugacity, relative F and Cl contents, and fugacity ratios indicate that the Cu (Mo) and Mo (Cu) mineralization systems formed from different magmatic-hydrothermal fluids. The Mo (Cu) mineralization system formed from F-rich high oxygen fugacity (above NNO) magmatic-hydrothermal fluids favorable for extracting Mo from the melt and transporting Mo, whereas the Cu (Mo) mineralization system formed from Cl-rich high oxygen fugacity (above NNO) magmatic-hydrothermal fluids favorable for extracting Cu from the melt and transporting Cu. The Bangpu porphyry Mo (Cu) deposit is the result of two different superimposed magmatic-hydrothermal fluids.