Geochemistry and stable isotope composition of the Berkeley pit lake and surrounding mine waters, Butte, Montana

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 SO 4 in the pit lake is similar to that of Horseshoe Bend Spring, but differs markedly from SO 4 in the surrounding flooded mine shafts. The latter is heavier in both δ 34S and δ 18O, which may be due to dissolution of hypogene SO 4 minerals (anhydrite, gypsum, barite) in the ore deposit. The isotopic and geochemical evidence suggests that much of the SO 4 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 SO 4 formed by aerobic leaching of weathered wallrock vs. SO 4 from anaerobic pyrite oxidation. The results suggest that both processes were likely important in the evolution of the Berkeley pit lake.