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

The oxidative weathering of sulfide-rich rocks releases protons and metals into solution.  The stable isotopic signatures of transition metals like Fe, Cu, and Zn may be used to track different anthropogenic and natural metal loading sources and to elucidate the mechanisms controlling metal transport.  However, our ability to interpret Fe, Cu, and Zn isotopic measurements in natural waters is limited because we do not have a complete understanding of how these metal isotopes are fractionated during the oxidative weathering of sulfide minerals.  In order to address this problem, we performed leaching experiments with pyrite-, chalcopyrite-, and sphalerite-rich rocks and with a sphalerite mineral separate and measured the Fe, Cu, and Zn isotopic compositions of the fluids.  Our study demonstrates that the oxidative weathering of sulfide-rich rocks can produce substantial variations in Fe (-1.75 to +1.0 ‰ Delta⁵⁶Fe_{solution-rock}) and Cu (0.0 to +2.0 ‰ Delta⁶⁵Cu_{solution-rock}) isotopes and smaller variations in Zn isotopes (0.0 to + 0.2 ‰ Delta⁶⁶Zn_{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 mineral surfaces that occur during air and aqueous chemical interactions.  During leaching, these surface 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 is controlled by the precipitation of Fe(III)-oxide phases, which enriches the solution in the lighter Fe isotopes.  Based on these results we present a conceptual framework for interpreting the impact of sulfide oxidation reactions on Fe, Cu, and Zn isotopic signatures of natural water systems.