We present a combined study of oxygen and boron isotopes (δ18O and δ11B) measured in situ in tourmaline from the giant San Rafael tin deposit (Central Andean Tin Belt, Southeast Peru) to trace fluid processes during the magmatic-hydrothermal transition leading to tin mineralization. The results show that magmatic and pre-ore hydrothermal tourmaline has similar δ18O-δ11B values, which are consistent with crystallization in a continuously evolving magmatichydrothermal closed system. In contrast, syn-ore hydrothermal tourmaline has δ18O-δ11B values defining a linear array interpreted as the result of mixing with external fluids, thus indicating an opening of the hydrothermal system during the ore stage. Quantitative fluid modeling shows that a Rayleigh fractionation process in a closed system reproduces closely the δ18O- δ11B compositions of magmatic and pre-ore tourmaline, whereas mixing of a hot and saline (500°C, 45 wt% NaCl eq) tin-rich magmatic fluid with a heated meteoric fluid (250°C, 0 wt% NaCl eq) explains the observed δ18O-δ11B compositions for syn-ore tourmaline.