Gejiu, a prominent tin–polymetallic ore district, is distinguished by its diverse mineral complexes. However, the genesis of these complexes and their relationship with mineralization remain inadequately studied. This study utilized whole-rock geochemical analyses to investigate the magmatic sources and petrogenesis of different complex types, aiming to elucidate their implications for tin–polymetallic mineralization. The results indicate that gabbro, monzonite, diorite, and syenite are derived from enriched mantle-derived magmas and have undergone limited crustal contamination. Granites are formed by the mixing of mantle- and crust-derived magmas, involving both physical mixing and chemical diffusion. Major and trace element characteristics suggest that the Gejiu granites predominantly exhibit features of both A-type and I-type granites. Harker diagrams and whole-rock indicators, such as Nb/Ta and Zr/Hf, suggest that granites experienced a two-stage fractional crystallization process, ultimately forming highly evolved biotite monzogranite. Fractional crystallization is the dominant mechanism controlling magmatic evolution, while high-temperature melting and biotite decomposition reactions are critical for the formation of the world-class Gejiu tin deposit.