Arsenic mineralogy of near-surface tailings and soils; influences on arsenic mobility and bioaccessibility in the Nova Scotia gold mining districts

The mineral form, grain size and texture of As-bearing particles are important factors influencing the risk to human health associated with exposure to As-contaminated soils, sediments and mine wastes. Mining of arsenopyrite-bearing gold ores in Nova Scotia in the late 1800s and early 1900s has left a legacy of weathered, As-rich tailings deposits in more than 60 gold districts across the province. Fourteen samples of near-surface tailings and one of soil from several former gold mines frequented by the public were sieved to <150 mu m and characterized using conventional mineralogical techniques (XRD, microscopy and EPMA) and synchrotron micro-analysis (mu -X-ray diffraction, mu -X-ray fluorescence and mu -X-ray absorption spectroscopy). Two high-As (>20% As) mill concentrates exposed at the surface within the tailings deposits are dominated by a single As mineral, fine-grained scorodite (FeAsO (sub 4) .2H (sub 2) O) in one case, and massive unweathered arsenopyrite in the other. In the tailings (0.7 to 7% As), scorodite and amorphous hydrous ferric arsenate (HFA) are the most common As-bearing major components, occurring as discrete grains or grain coatings on gangue minerals. Other major As phases identified in the tailings include As-bearing amorphous hydrous ferric oxyhydroxides (HFO), kankite (FeAsO (sub 4) .3.5H (sub 2) O), pharmacosiderite [KFe (sub 4) (AsO (sub 4) ) (sub 3) (OH) (sub 4) .6-7H (sub 2) O], yukonite [Ca (sub 7) Fe (sub 12) (AsO (sub 4) ) (sub 10) (OH) (sub 20) .15H (sub 2) O], amorphous Ca-Fe arsenates, and arsenopyrite. Minor or trace constituents include: As-bearing ferric oxyhydroxides with up to 10% As (HFO, goethite, lepidocrocite and akaganeite), As-bearing sulfates (jarosite [(K,Na,H (sub 3) O)Fe (sub 3) (SO (sub 4) ) (sub 2) (OH) (sub 6) ], tooeleite [Fe (sub 6) (AsO (sub 3) ) (sub 4) (SO (sub 4) )(OH) (sub 4) .4H (sub 2) O]) and realgar (As (sub 4) S (sub 4) ). Arsenic-bearing HFO (2.5% As) and goethite (0.08% As) were identified in the single B-horizon soil sample. This study is part of a broader coordinated effort by a multi-department federal and provincial advisory committee formed to coordinate the study of ecosystem and human health risks associated with historical gold mine sites in Nova Scotia. Our study shows that (i) the mineralogy of As in weathered tailings is highly variable, with aggregates of more than one As-bearing phase common in a given sample, and (ii) major differences in As mineralogy in the tailings are mainly controlled by factors that influence the weathering history (e.g., presence or absence of mill concentrates, degree of water saturation, and abundance of relict carbonate minerals). The variable solubility of these primary and secondary As-bearing minerals influences both the environmental mobility and the bioaccessibility of As in near-surface tailings and soil samples.