Geochemical characteristics of biotite and apatite from the Zhongdian porphyry Cu deposit belt in northwest Yunnan, China: An indicator of halogen fugacity and mineralization

Abstract

Abstract Copper mineralization at the Pulang (PL) super large porphyry deposit, Langdu (LD) medium-sized porphyry-skarn deposit and Songnuo (SN) porphyry prospect, Zhongdian Island, northwest Yunnan, China, is closely related to the emplacement of quartz monzonite porphyries. The chemical composition of biotite and apatite from those porphyries were analyzed to constrain the halogen fugacity and its control on mineralization. Our electron microprobe data show that biotites from PL deposit have higher MgO, SiO2, TiO2 and F content than that from LD or SN. In comparison to LD and SN, PL also display higher Mg (apfu) value and lower AlⅥ (apfu) in bitotite and higher F content but lower SO3 content and Ce2O3/ Y2O3 ratio in apatite. Application of Ti in biotite thermometry and apatite-biotite geothermometer shows that the crystallization temperature of biotite from PL is higher than that from SN or LD. The results suggest that oxygen fugacity and the magmatic sulfur and H2O contents couldn’t efficiently distinct PL from LD and SN. However, the halogen chemistry of biotite from PL is distinctly different from that from LD or SN by its lower IV (F), indicating that mineralized quartz monzonite porphyries from PL is formed in the late magmatic stage than that from LD and SN. And the mineralized porphyries display a remarkable negative linear (r = -0.96) in the ratio of log (f HF/f HCl) and log (f H2O/f HF), which can effectively distinguish PL from LD and SN. Compared with other porphyry Cu systems, only in mineralized systems, there is a remarkable positive linear relationship between IV (Cl) and log (f H2O/f HCl) ratios. Besides, the linear slope and intercept for log (f H2O/f HCl) ratios and IV (Cl) of biotite from potassic and phyllic alteration zone is significantly higher than those from other parts of the porphyries.