Evaluating Controls on Arsenic Geochemistry at the Long Lake Gold Mine in Sudbury, ON

Abstract

The release of As from old mine sites can persist long after cessation of mining activities. This project combines field and laboratory research components at the Long Lake Gold Mine site, near Sudbury, Ontario. The mine was discovered in 1908 and operated intermittently from 1909 until 1939; the mine was later abandoned and is now the responsibility of the Ontario Ministry of Energy, Northern Development and Mines. Arsenic-bearing sulfide-rich tailings were deposited in three topographic depressions near the mill, named TA-01, TA-02 and TA-03. The purpose of this project is to evaluate controls on As biogeochemistry in the Long Lake tailings areas and to provide a detailed geochemical and mineralogical investigation of aqueous- and solid-phase As. During the past 100 years, extensive sulfide oxidation of sulfide minerals in the Long Lake tailings has resulted in acidic conditions and high concentrations of dissolved metals and SO4 in the tailings pore water. Four nests of monitoring equipment were installed within TA-01, to assist in the understanding of the biogeochemical behaviour of As in the tailings and groundwater. Core samples of the sand cap, tailings, and underlying soils were collected for geochemical, mineralogical, and microbiological characterization. Mineralogical and geochemical characterization of the TA-01 tailings showed a zone of sulfide oxidation extending ~0.3-1.0 m below the tailings surface. Arsenic K-edge X-ray absorption near edge structure (XANES) and bulk As K-edge high energy resolution fluorescence detection X-ray spectroscopy (HERFD-XAS) produced results consistent with the mineralogical investigation. Pore water within the near surface tailings was characterized by low pH (2.0-3.9) and elevated concentrations of dissolved metals and SO4. Groundwater was characterized by circumneutral pH values and low concentrations of dissolved metals and SO4. Arsenic concentrations of up to 500 mg L-1 were measured in the tailings pore water and 70 mg L-1 in the underlying aquifer materials. The highest dissolved As concentrations were measured at shallow depths in the tailings corresponding with the lowest pH values and at the depth of the tailings profile near the organic layer interface. The tailings pore water and groundwater were characterized by δ34S-SO4 and δ13C-DIC fractionation indicating the likelihood of dissimilatory sulfate reduction (DSR). Results of this study will be used to inform and complement remediation efforts being undertaken by the Ministry of Energy, Northern Development and Mines. This study will provide information on the nature of mechanisms that affect the release and attenuation of As in over 100 year old sub-aerially deposited sulfide tailings.