Tourmalines in granites and granite-related tin-polymetallic ore deposits of Dachang,Guangxi , belong to schorl-dravite series . Tourmalines from granites contain highΣFeO (14.79%—16.07%) but low MgO (0.92%—2.74%), Whereas tourmalines from orebodies have high MgO (7.71%—10.50%) but lowΣFeO content (0.19%—4.50%). It is obvious that the abundance of MgO becomes increasingly higher with the distance from granite. Such compositional and evolutional features are in accordance with the general law of granitic tourmalines, but different from features of exhalative tourmalines. In Dachang mine, tourmalines can be found in various horizons of the Changpo-Tongkeng deposit and also in different kinds of altered wall rocks, such as Middle Devonian argillutite and Upper Devonian siliceous shale, thick and thin banded limestones, lens-like limestones and marl, making up at least twenty-three kinds of tourmaline-bearing mineral assemblages. In the lattice of tourmaline crystal, Mg and Fe occupy the same Y site; in consequence, when Fe is dominant in hydrothermal solution, it is more easy to form schorl, but when Mg is dominant, dravite is more easy to form. There are many reasons for the high MgO content of tourmaline in the Changpo-Tongkeng deposit:(1) the granites in Dachang are relatively high in Mg compared with other tin-related granites;(2) the deposit is located far away from granite;(3)iron in hydrothermal solution tends to be incor- porated into such sulfides as pyrite and pyrrhotite, and magnesium incorporated into metasomatic minerals like dravite;(4)Mg derived from wall rocks may be another material source for the formation of dravite.