Comparison on the Characteristics of Cassiterite-Bearing and Barren Granites in the Beidashan Region, Southern Great Xing'an Range

Recently, many tin polymetallic deposits related to Cretaceous granite have been discovered in the southern part of the Great Xing'an Range. However, not all granites of this period are associated with tin deposits. To obtain a better understanding of the formation conditions of granite associated tin deposits, (Method) in this paper, the cassiterite U-Pb age, zircon U-Pb age, whole rock geochemistry and mineral chemistry analysis of the cassiterite-bearing granites (the Mopanshan area) and barren granites (the Kulongshan area) in the Beidashan pluton were carried out. The geochronology, geochemistry and physical-chemical conditions of magma evolution were compared. The U-Pb weighted average zircon ages of the Kulongshan porphyritic quartz syenite and the Mopanshan biotite granite are 140.2±0.7 Ma and 139.9±0.7 Ma, respectively, and the cassiterite U-Pb weighted average age of the Mopanshan area is 134.9±1.4 Ma. All of them are Early Cretaceous in age. The whole rock geochemical analysis results show that these rocks are rich in silicon (SiO₂=64.96%-76.71%), alkali (Na₂O+ K₂O=8.28%-9.03%), and aluminum (Al₂O₃=12.42%-15.88%). For trace elements, they are relatively enriched in elements such as Th, Pb, and Hf, and deficient in elements such as Nb, Ta, Ti, Sr, and P. For rare earth elements, they show significant enriched HREEs than LREEs, and Eu negative anomaly is unusually obvious. As the content of SiO₂ increases, the content of TiO₂, FeO^T, Al₂O₃, CaO, Na₂O, and P₂O₅ gradually decreases, showing a good negative correlation, suggesting a homogeneous magmatic source. However, the DI value of the Kulongshan sample is 89 and the Sn content is 2×10^-6; the Mopanshan samples have a higher DI value (96-98) and higher Sn content (15×10^-6~36×10^-6). Based on the physical- chemical conditions inverted from mineralogy, the Kulongshan granite has experienced a process of oxygen fugacity increase during cooling, while the oxygen fugacity of the Mopanshan granite has further decreased during the cooling process. In addition, in terms of fluid halogen content, the Ⅳ (F) and Ⅳ (Cl) of biotite indicate the Mopanshan granite (Ⅳ(F)=0.95-1.15; Ⅳ(Cl)=-3.66--3.54) compared to the Kulongshan porphyritic quartz syenite (Ⅳ(F)=1.24-1.28; Ⅳ(Cl)=-2.96--2.52) has a higher concentration of Cl and F. In summary, low oxygen fugacity, high degree of evolution, and high F and Cl abundance, are the main factors affecting enrichment of cassiterite in granite in the Beidashan area.