HR: 0800h
AN: H31B-1304    [Abstracts]
TI: Investigating the Copper Isotope Composition of Red Mountain Creek: a Stream Affected by Acid Mine Drainage
AU: * Kimball, B E
EM: bek151@psu.edu
AF: Pennsylvania State University, Department of Geosciences 315 Hosler Building, Univeristy Park, PA 16802 United States
AU: Mathur, R
EM: mathur@juniata.edu
AF: Juniata College, Department of Geology, Huntingdon, PA 16652 United States
AU: Brantley, S L
EM: brantley@essc.psu.edu
AF: Pennsylvania State University, Department of Geosciences 315 Hosler Building, Univeristy Park, PA 16802 United States
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: Washington State University, Department of Geology, Pullman, WA 99164 United States
AB: Understanding the sources of metals and the processes that affect their transport in watersheds affected by acid mine drainage (AMD) is central to improving stream water quality. Using a new technique to address an old problem, we measured the ⁶⁵Cu/⁶³Cu ratios in filtered (pore size = 0.45μm or 0.22μm) and unfiltered samples of AMD-impacted streamwater collected during low-flow conditions from Red Mountain Creek near Silverton, Colorado. Red Mountain Creek is a small mountain stream receiving metal-rich, acidic drainage from acid-sulfate and quartz-sericite-pyrite alteration zones within dacitic-andesitic lavas and volcaniclastic sediments. We measured δ⁶⁵Cu values [where δ⁶⁵Cu = ((⁶⁵Cu/⁶³Cu_sample/⁶⁵Cu/⁶³Cu_standard) - 1) × 10³] on a multi-collector inductively coupled plasma mass spectrometer; instrumental mass bias was corrected by doping with the Johnson-Mattey Zn solution and bracketing with the NIST976 standard. All samples are enriched in ⁶⁵Cu, with δ⁶⁵Cu values ranging from 1.03 ± 0.10‰ to 3.76 ± 0.10‰ (2σ). Higher values correspond to an inflow emanating from a mineshaft that shows the highest Cu concentration (10.4 mg/L). As Cu becomes less concentrated downstream, the δ⁶⁵Cu values generally decrease. At two of the three sample locations, the filtered samples are more enriched in ⁶⁵Cu than the unfiltered samples, which contain suspended precipitates. These results are consistent with previous batch-leach experiments showing that during dissolution of chalcopyrite (CuFeS₂) and chalcocite (Cu₂S) (with and without Acidithiobacillus ferrooxidans), Cu released into solution by leaching was enriched in ⁶⁵Cu and Cu precipitates were depleted relative to the starting sulfide minerals. This fractionation may indicate that biotic (e.g., microbial metabolism) and/or abiotic processes (e.g., metal sorption and mineral precipitation) induce isotope effects during Cu partitioning. Future measurements of ⁶⁵Cu/⁶³Cu ratios in primary Cu-sulfide minerals and streambed precipitates will be combined with streamwater measurements to help assess fractionation among all Cu-bearing reservoirs in an AMD environment. Such measurements will be used to track Cu through the system using mass balance calculations, thereby giving insight into the mechanisms that control the mobility of Cu, and other metals, in these environments.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0461 Metals
DE: 0463 Microbe/mineral interactions
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: Fall Meeting 2005