Fractionation of Cu, Fe, and Zn Isotopes during the Oxidative Weathering of Sulfide-Rich Rocks

We measured the Fe, Cu, and Zn isotopic compositions of the fluids generated during leaching experiments with pyrite-, chalcopyrite-, and sphalerite-rich rocks and with a sphalerite mineral separate. Our study demonstrates that the oxidative weathering of sulfide-rich rocks can produce substantial variations in Fe (-1.75 to + 1.0‰ Δ ⁵⁶ Fe _{solution-pyrite rock} ) and Cu (0.0 to + 2.0‰ Δ ⁶⁵ Cu _{solution-chalcopyrite rock} ) isotopes and small variations in Zn isotopes (0.0 to + 0.2‰ Δ ⁶⁶ Zn _{solution-sphalerite} ) in the fluid phase relative to the rock. For the Fe and Cu systems we suggest that isotopic fractionation is caused by electron-exchange-driven (e.g., Fe(II)/Fe(III) and Cu(I)/Cu(II) redox) reactions at the surfaces of the sulfide minerals that occur during air and aqueous chemical reactions. Under acidic conditions, these reactions tend to enrich the fluid phase in the heavier Fe and Cu isotopes. However, under circumneutral pH conditions, the Fe isotopic composition in solution was controlled by the precipitation of Fe(III)-oxide phases, which enriched the solution in the lighter Fe isotopes. This investigation provides a preliminary framework for interpreting the impact of sulfide oxidation reactions on the distributions of stable Fe, Cu, and Zn isotopes in natural waters.