Geochemistry of Scorodite-Bearing Hardpan and Its Significance to Arsenic Cycling at a Former Gold Mine

Cemented layers, or “hardpans,” in arsenic-rich mine tailings are being studied at Montague Gold Mines in Nova Scotia to better understand their role in the natural attenuation of arsenic. Hardpan consists of secondary minerals cemented together and may sequester metals in the tailings.  At Montague, some tailings are cemented by the iron arsenate mineral scorodite (FeAsO₄∙2H₂O).  The precipitation of scorodite forms a solid shown to efficiently limit arsenic as a mobile phase. Moreover, scorodite has a low bioaccessibility and solubility relative to other products of arsenopyrite weathering. Characterizing the hardpan will lead to greater understanding of the risk to human health associated with publicly-accessible historic mine sites such as Montague.

Areas of extensive hardpan show higher arsenic concentrations (35,800 μg/L) and lower pH values (3.78). Pore waters from these spots plot within the scorodite stability field. However, pore waters from locations with dispersed hardpan have more alkaline pH (7.06) and lower arsenic concentrations (1760 μg/L), placing them outside the stability field even though scorodite has been observed there. It is suggested that incomplete incongruent dissolution of scorodite to some form of iron oxyhydroxide is happening at the higher pH areas.

High dissolved concentrations reflect ongoing oxidation of arsenopyrite (FeAsS) in the tailings, providing a source of acidity, iron and arsenic to waters. Under some conditions secondary phases will precipitate and limit the concentration of dissolved arsenic. Conversely, cements may be breaking down and releasing elements back into solution. Precipitation and dissolution reactions involving scorodite occur over a wide pH range at Montague as predicted from experimental work. Future work will investigate variations in hardpan mineralogy and evidence for scorodite precipitation or dissolution. Results will contribute to understanding whether the cements in these hardpan phases are sufficiently stable to persist and sequester arsenic.