Cassiterite deposition triggered by fluid mixing: evidence from in-situ ?18O-?11B analysis of tourmaline from the San Rafael tin deposit, Peru

We present a combined study of oxygen and boron isotopes (delta O-18 and delta B-11) 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 delta O-18-delta B-11 values, which are consistent with crystallization in a continuously evolving magmatic-hydrothermal closed system. In contrast, syn-ore hydrothermal tourmaline has delta O-18-delta B-11 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 delta O-18-delta B-11 compositions of magmatic and pre-ore tourmaline, whereas mixing of a hot and saline (500 degrees C, 45 wt% NaCl eq) tin-rich magmatic fluid with a heated meteoric fluid (250 degrees C, 0 wt% NaCI eq) explains the observed delta O-18-delta B-11 compositions for syn-ore tourmaline.