Sources and circulation of water and arsenic in the Giant Mine,Yellowknife, NWT,Canada

Recovery of gold from arsenopyrite-hosted ore in the Giant Mine camp, Yellowknife, NWT, Canada, has left a legacy of arsenic contamination that poses challenges for mine closure planning. Seepage from underground chambers storing some 237,000 tonnes of arsenic trioxide dust, has As concentrations exceeding 4000 ppm. Other potential sources and sinks of As also exist. Sources and movement of water and arsenic are traced using the isotopes of water and sulphate. Mine waters (16 ppm As; As^V/As^III ≈ 150) are a mixture of two principal water sources – locally recharged, low As groundwaters (0.5 ppm As) and Great Slave Lake (GSL; 0.004 ppm As) water, formerly used in ore processing and discharged to the northwest tailings impoundment (NWTP). Mass balance with δ¹⁸O shows that recirculation of NWTP water to the underground through faults and unsealed drillholes contributes about 60% of the mine water. ;emsp]>Sulphate serves to trace direct infiltration to the As₂O₃ chambers. Sulphate in local, low As groundwaters (0.3–0.6 ppm As; δ³⁴S_SO4  ～ 4 ‰ and δ¹⁸O_SO4  ～ ? 10 ‰) originates from low-temperature aqueous oxidation of sulphide-rich waste rock. The high As waters gain a component of ¹⁸O-enriched sulphate derived from roaster gases (δ¹⁸O_SO4  = + 3.5 ‰), consistent with their arsenic source from the As₂O₃ chambers. High arsenic in NWTP water (～ 8 ppm As; δ¹⁸O_SO4  = ? 2 ‰) derived from mine water, is attenuated to close to 1 ppm during infiltration back to the underground, probably by oxidation and sorption by ferrihydrite.