Abstract: The Comolong Sn polymetallic deposit located in the Yidun island-arc zone, western Sichuan are believed to have a genetic affiliation to the late Yanshanian Comolong granites. Biotite is identified as an only silicate mineral with volatile components in the granites. The most important observations are that:(1) the chemical compositions and H₂O⁺, F^- and Cl^- in siderophyllite may be used to characterize the rock and ore forming conditions and the nature of magmas; (2) the crystallochemical formula should be revised in that 33.45% of Fe³⁺ turned out by Mossbauer spectra to be aligned in the siderophyllite tetrahedrons; (3) there is a positive correlation between I.S and Q.S volumes of Fe²⁺ and Fe/(Fe+Mg) ratios for siderophyllite, which may indicate the temperatures and oxygen fugacity during the formation of granite masses; (4) Fe²⁺ ordering at M₁ and M₂ sites may reflect the crystallization temperatures and cooling speeds of magmas; (5) the b 0 values for siderophyllite are always changeable as a function of Fe³⁺ contents; (6) the Comolong granites can be assigned, deduced from the chemical compositions of siderophyllite, to the Sn-bearing granites, and (7) the Comolong granites may be postulated to be of crustal derived magmatic origin in the light of infrared absorption spectroscopy and differential thermal analysis.