Tin, a strategically significant metal, necessitates a comprehensive understanding of its mineralization processes, which hold substantial theoretical and economic value. Primary Sn mineralization is typically associated with highly evolved and relatively reduced granitic magmas. Compared to common granites, Sn- rich granites exhibit complex magmatic evolution, making it challenging to determine their early- stage magmatic compositions using whole rock geochemistry alone. This method yields data primarilyfrom the final stages of magmatic evolution, limiting our capacity to fully grasp the sources and evolution of these magmas. However, mineral phases such as quartz, biotite, and apatite, crystallized within the magma, serve as valuable archives of dynamic compositional changes and fine- scale evolutionary processes that often remain concealed in whole rock analyses. This study presents a comprehensive dataset of major and trace elements (including halogen) for biotites from highly evolved granites in the Zengjialong Sn ore deposit. These biotites exhibit magmatic origins, characterized by low Mg# (5. 70~9. 93, mean=7. 63) and high A/CNK values (1. 77~1. 98, mean=1. 88), consistent with biotites in S- type granites worldwide.These compositional characteristics suggest a metasedimentary origin for the parental magmas. Trace element analysis of biotites shows a systematic increase in Rb, Cs, and Sn, accompanied by concurrent decrease in Pb and K/Rb ratios. This trend suggests that crystal fractionation is dominated by K- feldspar. Furthermore, Sn concentrations in biotites increase fourfold due to progressive fractional crystallization, indicating Sn enrichment in the residual magmas. Biotites also exhibit low Ⅳ(F), Ⅳ(F/Cl), and lg(fHF/fHCl) values, coupled with high Ⅳ(Cl). These values, coupled with the negative correlations between Ⅳ(Cl) and Ⅳ(F/Cl) as well as lg(fH2O/fHF) and lg(fHF/fHCl), suggest a F- rich, Cl- depleted magma system and continuous fluid exsolution during biotite crystallization. Additionally, the biotites in the Zengjialong granite show almost no Fe3+, placing them below the FMQ line in the Fe3+- Fe2+- Mg triangle diagram, indicative of low oxygen fugacity. The biotites in the Zengjialong granite reveal a magmatic system characterized by an enriched source, a high degree of differentiation, low oxygen fugacity, and continuous fluid exsolution. These processes are conductive to Sn enrichment and mineralization. This study demonstrates the utility of biotite geochemistry for distinguishing various magmatic- hydrothermal mineralization systems, including those associated with Cu, Mo, W and Sn.