Hirsh, Simon, Mathur, Powell, Megaw, Asael: Province-scale tin and zinc stable isotope fractionation trends in the Bolivian tin belt

Sample solutions of ~200 ppb Zn were doped with 100 ppb Cu (NIST 976) to correct for mass bias with the exponential law. The samples were further corrected with standard-sample-standard bracketing using the AA-ETH Zn isotope standard. We report Sn isotopic compositions of cassiterite and Zn isotopic compositions of paragenetically later, lower-temperature sphalerite from ten Sn and Zn-bearing polymetallic deposits in the Bolivian tin belt.

These deposits represent distinct mineralization styles and metal transport pathways. The Llallagua and Huanuni deposits show significantly heavier Sn isotope compositions compared to all other deposits and contain several distinguishing geological attributes including: genetic association with porphyry phase stocks and/or dikes, vapor-rich and moderately to highly saline fluid inclusions, vertically restricted and/or discontinuous high-grade veins, extensive tourmaline-rich magmatic-hydrothermal breccias, and preferential cassiterite precipitation within oxidized lithologies.

Together, these features suggest that orthomagmatic fluid separation, fluid boiling, and abrupt oxidation contributed to cassiterite precipitation and caused Sn isotope fractionation in higher energy fluid environments. The residual fluids after cassiterite precipitation were depleted in heavy Sn isotopes and propagated along district-scale fluid pathways. The Sn isotopic ranges for cassiterite from the Sayaquira, Colquiri, Colcha, and Monserrat deposits may be a consequence of cassiterite precipitation over longer mineralization length scales with increased meteoric water influx, where distal cassiterite inherited the lighter composition of the evolving ore fluid.

The wide and heavy range of Zn isotopic compositions of sphalerite suggest that early orthomagmatic fluid separation and boiling may have caused significant isotopic fractionation during early fluid transport prior to sphalerite precipitation. Subsequent processes including meteoric water dilution and concomitant fluid pH change likely contributed to the precipitation of isotopically lighter sphalerite in some deposits.