- © 2014 Mineralogical Society of America
The case of the Giant mine illustrates how a large, long-lived Au mine has resulted in a complex regional legacy of As contamination and an estimated remediation cost of almost one billion Canadian dollars (AANDC 2012). The mine, located a few km north of the city of Yellowknife on the shore of Great Slave Lake (Figs. 1, 2) produced more than 7 million troy ounces of Au (approximately 220 tonnes) from a largely underground operation. Giant mine was the largest producer in the Yellowknife greenstone belt, which produced more than12 million troy ounces (~370 tonnes) in total (Bullen and Robb 2006). Arsenopyrite-bearing Au ore was roasted from 1949 to 1999 as a pretreatment for cyanidation (Fig. 3a). Poor or nonexistent emission controls in the early years resulted in the release of an estimated 20,000 tonnes of roaster-generated As2O3 to the surrounding environment through stack emissions (CPHA 1977; Wrye 2008). Over the lifetime of the mine, however, most of the As2O3 (237,000 tonnes) was stored in underground chambers (Fig. 3b) and is a now an ongoing source of As to groundwater and surface water (INAC 2007; Jamieson et al. 2013). Other roaster products include As-bearing maghemite and hematite (calcine) were deposited with tailings and re-mobilized into creek and lake sediments. Under reducing conditions, post-depositional remobilization of As associated with roaster-generated Fe oxides results in release of As to sediment pore water and reprecipitation of some As as a sulfide phase (Fawcett and Jamieson 2011). However, As(III) in maghemite and As2O3 persists in the oxidizing conditions of near-surface tailings and soils (Walker et al. 2005; Jamieson et al. 2013).
Ore roasting increases the solubility, toxicity, and bioaccessibility of As by converting sulfide-hosted As to oxide-hosted As. At Giant, …