Geochemistry and Stable Isotopes of the Flooded Underground Mine Workings of Butte, Montana

Abstract

Over a century of mining and smelting of the world-class porphyry lode ore deposit at Butte, Montana, has resulted in extensive environmental damage. In addition to its being the location of one of the world’s largest and most acidic mining pit lakes, Butte is host to over 16,000 km of flooded underground mine workings. Of the more than 60 mine shafts that have historically operated in Butte, approximately one dozen are presently accessible for groundwater sampling. The geochemistry of the mine shaft waters is zoned and roughly coincides with a district-wide zonation in hydrothermal alteration and mineralization. Mine waters in the so-called “Central zone” of intense phyllic and advanced argillic alteration have lower pH and very high concentrations of As, Fe, Mn, and Zn, but very low concentrations of dissolved Cu. The scarcity of Cu is attributed to cementation onto scrap iron left in the mines, and/or to replacement of preexisting sulfide minerals below the water table in a manner analogous to supergene enrichment processes. At the other extreme, mine waters in the “Peripheral zone” of weakest alteration have near-neutral pH, low metal concentrations, and contain dissolved sulfide (H2S, HS–). These waters are close to equilibrium with calcite, siderite, crystalline or amorphous MnCO3, and mackinawite (poorly crystalline FeS). A suite of deep groundwater monitoring wells completed in fractured and mineralized Butte Quartz Monzonite, unassociated with the mining complex, shows a similar range in groundwater chemistries to the mine shaft waters, suggesting a fundamental control of bedrock geology on water quality. Based on its isotopic composition, aqueous sulfate in the Butte mine waters was sourced from a combination of pyrite oxidation and leaching of hydrothermal anhydrite associated with early, porphyry-style Cu-Mo mineralization. Aqueous sulfide in the Peripheral zone mine workings is 28 to 50 per mil depleted in 34S relative to coexisting aqueous sulfate, consistent with microbial sulfate reduction. Dissolved inorganic carbon in the majority of the waters sampled appears to have isotopically equilibrated at low temperature with hydrothermal rhodochrosite, an abundant mineral at Butte. Waters with the highest H2S concentrations also have unusually high dissolved inorganic carbon concentrations that are depleted in 13C, consistent with an influx of CO2 from microbial oxidation of organic carbon. The source of organic carbon is not known, but may include timbers used to reinforce the tunnels and stopes. In contrast to the large horizontal gradients in mine water chemistry on a district scale, vertical gradients in chemistry and temperature within each individual shaft at Butte are negligible, possibly due to vertical water circulation. The circulation model is consistent with data on the local geothermal gradient in Butte, and explains why the deepest mine shafts tend to have the warmest water. The Kelley mine has an anomalously warm temperature (~35°C), and some of the excess heat in this mine shaft may have come from pyrite oxidation, a highly exothermic reaction. The flooded underground mine complex of Butte has potential for heat recovery using modern heat pump technology.