The granite bodies associated with mineralization in the eastern Gejiu area are predominantly concealed, with the Baishachong composite granite body being one of the few that are exposed at the surface. Detailed field geological investigations have revealed that the northern part of this granite body consists of pink medium- to- coarse- grained potassium feldspar granite, while the southern part is composed of grayish- white medium- to- fine- grained biotite monzogranite. Moreover, skarn- type tin deposits develop at the contact zone with the surrounding rocks in the southern part. Consequently, it serves as an ideal object for exploring the relationship between magmatic activity and tin mineralization. This paper presents new geochronological, geochemical, and mineralogical studies on the granite of different lithologies and skarn- type ore bodies within the granite body. The crystallization age of the light pink granite is approximately 89 Ma, while the lower intercept age of the grayish- white granite is 80 Ma, and the age of the skarn cassiterite is 84 Ma. Additionally, although all the biotites in the Baishachong granite are annite, the grayish- white granite exhibits more characteristics of regenerative biotite. Therefore, it is inferred that after the emplacement of the grayish- white granite body, magmatic—hydrothermal activity remained intense between 80 and 84 Ma. Although all biotites in the granite body are annite?1, the grayish- white granite primarily contains re- equilibrated biotite. This implies persistent hydrothermal activity lasting approximately 4 Ma post- mineralization?1, as evidenced by mineralogical adjustments in biotite composition reflecting late- stage fluid—rock interactions. The garnets in Baishachong are mainly andradite- dominant andradite -grossular series (And55.58~97.28 Gro 0.16~39.51(Ura+ Pyr+ Spe+ Alm)1.58~5.19), enriched in heavy rare earth elements (HREEs), depleted in light rare earth elements (LREEs), with negative Eu anomalies, and a significant positive correlation between Y and ΣREE, indicating their formation in the early stage of skarn development under relatively closed, low water/rock ratio, and weakly oxidizing neutral fluids. A positive correlation is observed between Al and Fe, as well as Ga and Fe in cassiterite, and the brightness variation of individual cassiterite grains is mainly related to Fe content, suggesting their formation in high- temperature magmatic—hydrothermal fluids with evident coupled substitution of Fe for Sn, it demonstrates that the magmatic fluids forming the skarn are Fe- rich.