Moving from rocks to hydrologic systems: Are Cu, Fe, and Zn isotopes fractionated during weathering?
The oxidative weathering of sulfide-rich rocks from mine wastes and from un-mined mineralized regions releases protons and metals into natural waters. The stable isotopic signatures of transition metals like Fe, Cu, and Zn may be used to track these different metal loading sources and to elucidate the mechanisms controlling their mobilities. However, our ability to interpret Fe, Cu, and Zn isotopic measurements in natural waters is limited in part because we do not know if and 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‰ Δ56Fesolution-rock ) and Cu (−0.85 to + 2.0‰ Δ65Cu solution-rock) isotopes and smaller variations in Zn isotopes (0.0 to + 0.2‰ Δ66Znsolution-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. 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 is controlled by the precipitation of Fe(III)-oxide phases, which enriches the solution in the lighter Fe isotopes. This investigation provides a preliminary framework for interpreting the impact of sulfide oxidation reactions on Fe, Cu, and Zn isotopic signatures of natural water systems. ^
Fernandez, Alvaro, "Moving from rocks to hydrologic systems: Are Cu, Fe, and Zn isotopes fractionated during weathering?" (2008). ETD Collection for University of Texas, El Paso. AAI1455879.