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Title

Fluid mixing as primary trigger for cassiterite deposition: Evidence from in situ δ18O-δ11B analysis of tourmaline from the world-class San Rafael tin (-copper) deposit, Peru

Authors
Marger, Katharina
Bouvier, Anne-Sophie
Baumgartner, Lukas P.
Rielli, Andrea
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Published in Earth and planetary science letters. 2021, vol. 563, no. 116889
Abstract 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 δ18O (from 10.6‰ to 14.1‰) and δ11B (from −11.5‰ to −6.9‰). The observed δ18O and δ11B 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 δ18O values (from 4.9‰ to 10.2‰) and in part higher δ11B 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 δ18O-δ11B 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 δ18O and δ11B analyses of tourmaline is a powerful approach for understanding fluid processes in dynamic magmatic-hydrothermal environments.
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Structures
Research group Mineral Resources and Geofluids
Project
Swiss National Science Foundation: S19053_169901
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(ISO format)
HARLAUX, Matthieu et al. Fluid mixing as primary trigger for cassiterite deposition: Evidence from in situ δ18O-δ11B analysis of tourmaline from the world-class San Rafael tin (-copper) deposit, Peru. In: Earth and Planetary Science Letters, 2021, vol. 563, n° 116889. doi: 10.1016/j.epsl.2021.116889 https://archive-ouverte.unige.ch/unige:150678

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Deposited on : 2021-03-29

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