USE OF CU AND ZN ISOTOPES TO IDENTIFY METAL SOURCES AND ATTENUATION MECHANISMS AT THE FORMER WALDORF MINE, CLEAR CREEK COUNTY, COLORADO

Heavy metals from abandoned mining sites can significantly degrade water quality. However, identifying the metal loading pathways and understanding how the metals are transported and attenuated at these sites can be difficult with traditional geochemical tools. In this study we use stable Cu and Zn isotopes as tools to help distinguish metal sources and sinks at the site of the former Waldorf Mine in Clear Creek County, Colorado. Surface water, pore water, soil, and rock samples were collected from three zones around the former mine site, including the Wilcox mine tunnel/adit, a large waste-rock pile, and a nearby wetland. Water generally flows from the tunnel and waste rock pile through the wetland before intercepting Leavenworth Creek. Samples were analyzed for bulk elemental concentrations at the U.S. Geological Survey in Denver, Colorado. Cu and Zn concentrations in fluids leaving the Wilcox tunnel were around 0.1 mg/L and 7.5 mg/L, respectively and the pH of this water was circumneutral. Fluids in contact with the waste rock pile contained more Cu (up to 1.9 mg/L) and Zn (up to 12.4 mg/L) and a much lower pH (~3.5). These elevated metal loads appear to be attenuated significantly for both Cu and Zn in the wetland. Near surface (i.e, 0-15 cm.) pore waters in the wetland nearest to the waste rock pile contained high concentrations for Cu (up to 1.8 mg/L) and Zn (up to (10.8 mg/L); however, pore waters nearest to Leavenworth Creek do not contain measureable concentrations of Cu and Zn concentrations decrease to about 0.9 mg/L. Cu and Zn isotopic data will allow us to determine the relative impacts of the tunnel and waste rock pile on metal loading in this watershed. Moreover, we can use the isotopes to distinguish between sorption of these metals to the wetland soils vs. dilution from clean groundwater sources (the latter possibility of dilution should not fractionate the isotopes). Zn and Cu isotopes are currently being measured using the Nu Instruments MC-ICP-MS at the University of Texas at El Paso. We will present preliminary isotopic data that document significant metal isotopic variation among the tunnel, waste rock, and wetland sites.