Geochemistry and stable isotope composition of the Berkeley pit lake and surrounding mine waters,Butte, Montana |
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| Institution: | 1. Rio Tinto Iron Ore, 152–158 St Georges Terrace, Perth, WA 6000, Australia;2. West Australian Biogeochemistry Centre, School of Plant Biology, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;3. Ecosystems Research Group, School of Plant Biology, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;1. School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, PR China;2. Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-sen University, Guangzhou 510275, PR China;3. School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China;4. CRPG-CNRS, Université de Lorraine, 15 rue Notre-Dame-des-Pauvres BP 20, 54501 Vandoeuvre lès Nancy, France;5. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, PR China;6. Université de Lorraine, INRA, Laboratoire Sols et Environnement, BP 172, 2 avenue de la forêt de Haye, F-54505 Vandoeuvre-lès-Nancy Cédex, France |
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| Abstract: | Samples of mine water from Butte, Montana were collected for paired geochemical and stable isotopic analysis. The samples included two sets of depth profiles from the acidic Berkeley pit lake, deep groundwater from several mine shafts in the adjacent flooded underground mine workings, and the acidic Horseshoe Bend Spring. Beginning in July-2000, the spring was a major surface water input into the Berkeley pit lake. Vertical trends in major ions and heavy metals in the pit lake show major changes across a chemocline at 10–20 m depth. The chemocline most likely represents the boundary between pre-2000 and post-2000 lake water, with lower salinity, modified Horseshoe Bend Spring water on top of higher salinity lake water below. Based on stable isotope results, the deep pit lake has lost approximately 12% of its initial water to evaporation, while the shallow lake is up to 25% evaporated. The stable isotopic composition of SO4 in the pit lake is similar to that of Horseshoe Bend Spring, but differs markedly from SO4 in the surrounding flooded mine shafts. The latter is heavier in both δ34S and δ18O, which may be due to dissolution of hypogene SO4 minerals (anhydrite, gypsum, barite) in the ore deposit. The isotopic and geochemical evidence suggests that much of the SO4 and dissolved heavy metals in the deep Berkeley pit lake were generated in situ, either by leaching of soluble salts from the weathered pit walls as the lake waters rose, or by subaqueous oxidation of pyrite on the submerged mine walls by dissolved Fe(III). Laboratory experiments were performed to contrast the isotopic composition of SO4 formed by aerobic leaching of weathered wallrock vs. SO4 from anaerobic pyrite oxidation. The results suggest that both processes were likely important in the evolution of the Berkeley pit lake. |
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