Vertical gradients in geochemistry of flooded mine shafts in Butte, Montana.

摘要

The flooded mines of Butte, Montana include over 15,000 km of vertical shafts, horizontal drifts and stopes, interconnected to each other in varying degrees. The main objective of this study is to document the presence or absence of vertical gradients in chemistry, temperature, and stable isotopic composition of mine water in the flooded shafts. Mine shafts that were successfully sampled for this thesis include the Anselmo, Steward, Kelley, Granite Mountain, and Pilot Butte shafts of the so-called "East Camp" mine workings, the Travona shaft of the "West Camp" mine workings, and the Orphan Girl and Marget Ann shafts of the "Outer Camp" mine workings. At each of the above shafts, a vertical profile of environmental parameters including temperature, SC, pH, and ORP was obtained by lowering a datasonde through the top 300 feet of the water column. Water samples at various depths to a maximum of 1200 ft below static water level (BSWL) were brought to the surface with either a bailer or a "Kemmerer" point sampler. The East and West Camps are hydrologically separated, and contain water with very different chemistry. Whereas the majority of the East Camp waters are slightly acidic with elevated heavy metal concentrations (including Fe, Mn, and Zn), the West Camp waters are pH-neutral and enriched in hydrogen sulfide (H2S), with extremely low metal concentrations. Water in the Outer Camp is more similar to the West Camp in most respects. The Orphan Girl shaft contains the highest concentration of H2S (up to 10 mg/L) of any of the mine waters of Butte.;The results of this study show surprisingly few changes in chemistry, temperature, and isotopic composition with depth BSWL in most of the mine shafts. Some of the shafts showed systematic changes in pH and Eh in the top 300 ft BSWL, but these changes can be explained by inwards diffusion of O2 and outwards diffusion of CO2 towards the top of the water column. Likewise, there were virtually no vertical changes in the stable isotopic composition of water or dissolved sulfate in any of the mines, however, horizontal changes were large.... Horizontal variations in the stable isotopic composition of SO4 are attributed to three processes: 1) oxidative dissolution of sulfide minerals, which produces SO 4 with a S isotopic composition similar to the precursor sulfides; 2) dissolution of hydrothermal sulfate minerals (gypsum, anhydrite) present in altered and mineralized bedrock; and 3) bacterial sulfate reduction.;The lack of any discernible gradients in temperature and salinity (i.e., SC) in most of the mine shafts implies that the shaft waters are circulating in a vertical direction. The Kelley Mine is an exception to this rule, as it does show gradients in chemistry and temperature between 300 and 1200 ft BSWL. The Kelley mine water is anomalously warm (34°C) at the surface, and becomes even warmer (> 38°C) at depth. It is speculated that the high temperature of the Kelley Mine water is partly due to pyrite oxidation, a strongly exothermic reaction. This is consistent with the fact that the Kelley Mine has by far the lowest pH and highest dissolved Fe and SO 4 concentrations of any of the mine shafts in the Butte complex.;Keywords: mine water, geochemistry, stable isotope, bacterial sulfate reduction