CHARACTERIZATION OF ARSENIC-HOSTING SOLID PHASES IN TAILINGS DUST FROM GIANT MINE: IMPLICATIONS FOR AIR QUALITY MONITORING AT LEGACY MINE SITES

Dust from the tailings at Giant mine, an abandoned mine in northern Canada, has been a major concern among local residents, particularly those from Ndilo, a community of Yellowknives Dene First Nations located ~2.5km southeast of the site. Historically, roaster waste containing arsenic trioxide was co-deposited with flotation tailings; this is a major concern as arsenic trioxide is the most toxic and bioaccessible solid form of arsenic. The main objective of this research is to characterize the fine fraction of the tailings (particles <63μm in diameter) and dust from Giant mine to identify the As-hosting solid phases therein. Surface tailings material was sampled from within three of the tailings impoundments on site, and sieved to <63μm. From May to July of 2016, a total suspended particulate (TSP) high volume air sampler was set up to continuously sample airborne material coming from the tailings. For all of the surface tailings and dust samples, bulk chemical data was determined via inductively coupled plasma-optical emission spectrometry (ICP-OES) and -mass spectrometry (ICP-MS); mineralogical data via scanning electron microscope (SEM)-based automated mineralogy, and synchrotron-based micro X-ray diffraction (μXRD), micro X-ray fluorescence (μXRF); and oxidation state data for As via X-ray absorption near edge structure (XANES). Bulk chemical data show elevated concentrations of As, Sb, Zn, Pb, and Cu. The As in the tailings is a mix of As^1-, As⁵⁺, and As³⁺, and is more concentrated in the <63μm fraction (3000 – 9300 ppm As). Roaster-generated Fe-oxides (maghemite), Ca-Fe arsenate, and arsenopyrite comprise the majority of As-bearing particles in the surface tailings; Ca-Fe arsenate poses the greatest risk of these three as it is the most bioaccessible. Fe-oxides were the only As-host detected in the dust. Very little arsenic trioxide was actually found in the tailings, and no arsenic trioxide was found in the dust samples. However, previous studies have shown that the soils near the Giant mine tailings contain arsenic trioxide from historic stack emissions, indicating that these soils might actually present a higher risk than the tailings themselves.