<p>The Nong Sua Sn deposit in Thailand is a typical vein-type deposit with three generations of tourmaline (Tur I, II and III). Tur I as disseminations (Tur Ia) or nodules (Tur Ib) in the pegmatite-aplite stock crystallized from the late-magmatic stage. Tur II coexists with quartz in a complex vein system within the stock and is of magmatic-hydrothermal origin. Tur III occurs in cassiterite-tourmaline veins within contact zones between the quartz-tourmaline vein and metasedimentary country rocks (Tur IIIa) or as smaller cassiterite-tourmaline veinlets (Tur IIIb). Both Tur I and Tur II have similar trends in chemical composition with δ¹¹B values from -15.1 ‰ to -11.5 ‰ and from -15.8 ‰ to -11.7 ‰, respectively, which can be explained by a Rayleigh isotope fractionation. The similarity suggests a common magmatic-hydrothermal system from which tourmaline crystallized. Tur III has lower Sn concentrations (22 to 143&#xa0;ppm) and higher Fe³⁺/(Fe²⁺ + Fe³⁺) ratios than Tur I, Tur II, and thus crystallized under more oxidized conditions. Tur III has relatively high δ¹¹B values from -13.3 ‰ to -10.6 ‰ with corresponding δ¹¹B_fluid values from -9.1‰ to -6.4‰ for Tur III and from -14.0 ‰ to -9.0 ‰ for Tur I and Tur II. Thus, oxidizing modified meteoric water with relatively high δ¹¹B values was added to the magmatic-hydrothermal system from which the deposit formed. Precipitation of cassiterite was likely caused by the change of redox state related to fluid mixing. Our study provides new important insights about the evolution of hydrothermal fluids from the pre-ore stages to ore genesis of Sn deposits. Such an evolution of the magmatic-hydrothermal system may be a common process for Sn deposits in the Southeast Tin Province (SEAT) and elsewhere.</p>