Geochemistry of Flooded Underground Mine Workings Influenced by Bacterial Sulfate Reduction |
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| Authors: | Amber J Roesler Christopher H Gammons Gregory K Druschel Harry Oduro Simon R Poulson |
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| Institution: | (1) Department of Chemistry and Geochemistry, Montana Tech of The University of Montana, Butte, MT 59701, USA;(2) Department of Geological Engineering, Montana Tech of The University of Montana, Butte, MT 59701, USA;(3) Department of Geology, University of Vermont, Burlington, VT 05405, USA;(4) Department of Geological Sciences and Engineering MS-172, University of Nevada-Reno, Reno, NV 89557, USA |
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| Abstract: | Unlike the majority of the water in the flooded mine complex of Butte Montana, which includes the highly acidic Berkeley pit
lake, groundwater in the flooded West Camp underground mine workings has a circum-neutral pH and contains at least 8 μM aqueous
sulfide. This article examines the geochemistry and stable isotope composition of this unusual H2S-rich mine water, and also discusses problems related to the colorimetric analysis of sulfide in waters that contain FeS(aq) cluster compounds. The West Camp mine pool is maintained at a constant elevation by continuous pumping, with discharge water
that contains elevated Mn (90 μM), Fe (16 μM), and As (1.3 μM) but otherwise low metal concentrations. Dissolved inorganic
carbon in the mine water is in chemical and isotopic equilibrium with rhodochrosite in the mineralized veins. The mine water
is under-saturated with mackinawite and amorphous FeS, but is supersaturated with Cu- and Zn-sulfides. However, voltammetry
studies show that much of the dissolved sulfide and ferrous iron are present as FeS(aq) cluster molecules: as a result, the free
 concentration of the West Camp water is poorly constrained. Concentrations of dissolved sulfide determined by colorimetry
were lower than gravimetric assays obtained by AgNO3 addition, implying that the FeS(aq) clusters are not completely extracted by the Methylene Blue reagent. In contrast, the clusters are quantitatively extracted
as Ag2S after addition of AgNO3. Isotopic analysis of co-existing aqueous sulfide and sulfate confirms that the sulfide was produced by sulfate-reducing
bacteria (SRB). The H2S-rich mine water is not confined to the immediate vicinity of the extraction well, but is also present in flooded mine shafts
up to 3 km away, and in samples bailed from mine shafts at depths up to 300 m below static water level. This illustrates that
SRB are well established throughout the southwestern portion of the extensive (>15 km3) Butte flooded mine complex. |
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| Keywords: | Mine water Hydrogen sulfide Iron sulfide clusters Bacterial sulfate reduction Geochemistry Arsenic Rhodochrosite |
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