Tracking the Fate of Metals in Mining Waste Rock Using M etal Stable Isotopes

The characterization of metal release and attenuation in mining waste rock drainage has traditionally relied upon an array of mineralogical and geochemical techniques. Recent advances in analytical instrumentation now allow robust measurements of minute variations in the isotopic composition of metals in environmental samples, which have become a powerful new tracer of their fate. Here we provide an overview of the application of these “non-traditional” stable isotopes to characterize metal release and attenuation in mining waste rock. First, we briefly review the theoretical framework of stable isotope geochemistry and its application to environmental studies. Second, we present results from an isotopic investigation of Mo and Zn at the Antamina mine, Peru. At Antamina, isotopic compositions of Mo and Zn have been determined in waste rock drainage waters and ore-mineral samples as a first step to determine whether isotope analyses can serve as a tool to track the geochemistry of these metals in waste rock dumps. The range in isotopic compositions measured thus far at Antamina spans 1.88‰ for δ98/95Mo and 0.65‰ for δ66/64Zn. This variability confirms that isotopic fractionation is occurring during metal release and/or attenuation reactions and indicates the potential for metal stable isotopes to track the mobility of metals in mining waste rock dumps. Linking this variability to distinct release and attenuation mechanisms (e.g. dissolution, adsorption and secondary mineral precipitation) will require further investigations to characterize the isotopic signatures of these mechanisms.