Contrasting mineralized and barren porphyries in the Zhongdian Arc, insights from biotite and apatite compositions and halogen fugacity

Abstract

AbstractCopper mineralization in the Pulang (PL) porphyry deposit, Langdu (LD) porphyry-skarn deposit and Songnuo (SN) porphyry prospect in northwestern Yunnan, China, is closely related to the emplacement of quartz monzonite porphyries. The chemical compositions of biotite and apatite from those porphyries were analyzed to calculate the halogen fugacity and to constrain mineralized and barren porphyries. Our data show that biotites from the PL deposit have higher MgO, SiO2, TiO2 and F contents than those from the LD deposit or SN prospect. Compared to those in the LD deposit and SN prospect, the Mg (atoms per formula unit (apfu)) and AlVI (apfu) value in biotite is greater, and the F content is greater and the SO3 and Ce2O3/Y2O3 ratio in apatite are lower in the PL deposit. Ti-biotite thermometry and apatite-biotite geothermometry show that the crystallization temperature of biotite from the PL deposit is higher than that from the SN prospect or LD deposit. The results suggest that oxygen fugacity, magmatic sulfur, and H2O contents cannot be used to efficiently distinguish the PL deposit from the LD deposit and SN prospect. However, the halogen chemistry of biotite from the PL deposit is distinctly different from that of the LD deposit or SN prospect according to the lower IV (F), indicating that mineralized quartz monzonite porphyries in the PL deposit formed during the late magmatic stage, which is in contrast to those in the LD deposit and SN prospect. The mineralized porphyries display a remarkable negative linear relationship (r = − 0.96) with the log (f HF/f HCl) and log (f H2O/f HF) ratio, which can be used to distinguish the mineralized and barren porphyries. Compared with other typical porphyry Cu systems, there is a remarkable positive linear relationship between IV (Cl) and log (f H2O/f HCl). In addition, the linear slope and intercept for log (f H2O/f HCl) ratios and the IV (Cl) of biotite from the potassic and phyllic alteration zones are significantly greater than those from other porphyries.