We present a high-resolution in situ study of oxygen and boron isotopes measured in tourmaline from the world-class San Rafael Sn (–Cu) deposit (Central Andean tin belt, Peru) aiming to trace major fluid processes at the magmatic-hydrothermal transition leading to the precipitation of cassiterite. Our results show that late-magmatic and pre-ore hydrothermal tourmaline has similar values of ${\delta }^{18}$O (from 10.6‰ to 14.1‰) and ${\delta }^{11}$B (from −11.5‰ to −6.9‰). The observed ${\delta }^{18}$O and ${\delta }^{11}$B variations are dominantly driven by Rayleigh fractionation, reflecting tourmaline crystallization in a continuously evolving magmatic-hydrothermal system. In contrast, syn-ore hydrothermal tourmaline intergrown with cassiterite has lower ${\delta }^{18}$O values (from 4.9‰ to 10.2‰) and in part higher ${\delta }^{11}$B values (from −9.9‰ to −5.4‰) than late-magmatic and pre-ore hydrothermal tourmaline, indicating important contribution of meteoric groundwater to the hydrothermal system during ore deposition. Quantitative geochemical modeling demonstrates that the ${\delta }^{18}$O-${\delta }^{11}$B composition of syn-ore tourmaline records variable degrees of mixing of a hot Sn-rich magmatic brine with meteoric waters that partially exchanged with the host rocks. These results provide thus direct in situ isotopic evidence of fluid mixing as a major mechanism triggering cassiterite deposition. Further, this work shows that combined in situ ${\delta }^{18}$O and ${\delta }^{11}$B analyses of tourmaline is a powerful approach for understanding fluid processes in dynamic magmatic-hydrothermal environments.

我们对世界一流的圣拉斐尔锡（–Cu）矿床（中部安第斯锡带，秘鲁）的电气石中测得的氧和硼同位素进行了高分辨率的原位研究，旨在追踪岩浆-水热过渡过程中的主要流体过程。导致锡石沉淀。我们的研究结果表明，晚岩浆和矿石前热液碧玺具有相似的值${\delta }^{\mathrm{18岁}}$O（从10.6‰至14.1‰）和 ${\delta }^{11}$B（从-11.5‰至-6.9‰）。观察到的${\delta }^{\mathrm{18岁}}$O和 ${\delta }^{11}$B变化主要由瑞利分馏驱动，反映了在不断演化的岩浆-水热系统中的电气石结晶。相比之下，与锡石矿共生的同矿热液碧玺的含量较低${\delta }^{\mathrm{18岁}}$O值（从4.9‰到10.2‰）和部分更高 ${\delta }^{11}$B值（-9.9‰至-5.4‰）比晚岩浆和矿石前热液碧玺高，表明陨石地下水在矿石沉积过程中对热液系统具有重要作用。定量地球化学模拟表明${\delta }^{\mathrm{18岁}}$O-${\delta }^{11}$合成矿石电气石的B成分记录了富锡的热岩浆盐水与与母岩部分交换的流水的混合程度的变化。因此，这些结果提供了流体混合的直接原位同位素证据，这是触发锡石沉积的主要机理。此外，这项工作表明，原位结合${\delta }^{\mathrm{18岁}}$O和 ${\delta }^{11}$B电气石分析是了解动态岩浆热液环境中流体过程的有力方法。