Fingerprinting two metal contaminants in streams with Cu isotopes near the Dexing Mine, China

Transition metal isotope signatures are becoming useful for fingerprinting sources in surface waters. This study explored the use of Cu isotope values to trace dissolved metal contaminants in stream water throughout a watershed affected by mining by-products of the Dexing Mine, the largest porphyry Cu operation in Asia. Cu isotope values of stream water were compared to potential mineral sources of Cu in the mining operation, and to proximity to the known Cu sources. The first mineral source, chalcopyrite, CuFeS2 has a 'tight' cluster of Cu isotope values (-0.15 parts per thousand to +1.65 parts per thousand; +0.37 +/- 0.6 parts per thousand, 1 sigma, n = 10), and the second mineral source, pyrite (FeS2), has a much larger range of Cu isotope values (-4 parts per thousand to +11.9 parts per thousand; 2.7 +/- 4.3 parts per thousand, 1 sigma, n = 16). Dissolved Cu isotope values of stream water indicated metal derived from either chalcopyrite or pyrite. Above known Cu mineralization, stream waters are approximately + 1.5 parts per thousand greater than the average chalcopyrite and are interpreted as derived from weathering of chalcopyrite. In contrast, dissolved Cu isotope values in stream water emanating from tailings piles had Cu isotope values similar to or greater than pyrite (>+6 parts per thousand, a common mineral in the tailings). These values are interpreted as sourced from the tailings, even in solutions that possess significantly lower concentrations of Cu (<0.05 ppm). Elevated Cu isotope values were also found in two soil and two tailings samples (delta Cu-65 ranging between +2 to +5 parts per thousand). These data point to the mineral pyrite in tailings as the mineral source for the elevated Cu isotope values. Therefore, Cu isotope values of waters emanating from a clearly contaminated drainage possess different Cu isotope values, permitting the discrimination of Cu derived from chalcopyrite and pyrite in solution. Data demonstrate the utility of Cu isotopic values in waters, minerals, and soils to fingerprint metallic contamination for environmental problems. (C) 2015 Elsevier B.V. All rights reserved.