MOVING FROM ROCKS TO HYDROLOGIC SYSTEMS: ARE CU, FE, AND ZN ISOTOPES FRACTIONATED DURING WEATHERING?

Stable transition metal isotopic signatures in hydrologic systems are a potentially useful tool to track the behavior of metals released during the weathering of metal-rich rocks in watersheds impacted by acid rock drainage. One unanswered question, however, is whether the weathering of these rocks/minerals results in measurable isotopic fractionations of the metal isotopes. To explore this uncertainty, we investigated Cu, Fe, and Zn isotope fractionations during experimental dissolution of rock samples collected from a number of distinctly mineralized and altered zones within the Prospect Gulch watershed near Silverton, Colorado. We performed a series of variable-pH and pH-stat dissolution experiments (pH approx. 2) in acidic media (HCl, HNO₃, and H₂SO₄ acids) to approximate acidic drainage. Experimental solutions were sampled as a function of time and analyzed for their bulk elemental compositions using an ICP-OES technique. Cu and Zn were readily leached from samples of the anthropogenic mine-tailings scattered throughout the watershed, whereas, these metals were relatively insignificant in leach fluids from the mineralized country rock. However in pH-stat experiments (between 4 to 200 hours), Fe was readily leached from all rocks in the watershed in nearly equal amounts, including samples from the mine-tailings, and propolitic-, quartz-sericite-pyrite-, and clay-rich pyrite- altered volcanoclastic country rock. All of these rocks contained disseminated pyrite. Chosen samples from each experiment were prepared for isotopic analysis by isolating Cu, Fe, and Zn from matrix elements using an anion-exchange chromatography technique. Preliminary isotopic data, measured using the U.S. Geological Survey's Nu Plasma® multicollector ICP-MS in Denver, Colorado, suggests that the first Zn leached from sulfide-rich rocks is heavier (by about 0.2 per mil relative to JMC) than the Zn isotopic signatures of the rocks themselves. As leaching progresses, the isotopic signatures in solution begin to mirror those of the rock. Additional Cu, Fe, and Zn isotopic analyses are underway.