NEUTRAL PORE WATERS AND ARSENIC-RICH HARDPAN STABILITY IN GOLD MINE TAILINGS

Orogenic gold mine tailings with neutral-pH, high As pore waters are common across Nova Scotia, Canada. These tailings contain variable amounts of arsenopyrite, but have sufficient carbonate to neutralize acidity produced from sulfide oxidation. Despite a lack of acidity, water quality is impacted by neutral mine drainage as some metal(loid)s, such as As, remain mobile under circumneutral pH conditions. Arsenic-rich hardpans occur in the weathered tailings at some sites and are particularly well developed in two historical gold districts. This work investigates the role of hardpans in controlling As mobility within tailings pore waters.

Montague and Goldenville are two former gold mines located along the eastern seaboard of mainland Nova Scotia. Typical tailings not associated with hardpan at these sites have near-neutral pH pore waters and lower dissolved As concentrations than locations of well developed hardpan. Hardpan forms where arsenopyrite concentrate was disposed with the tailings and is associated with wide-ranging pH (2.14 to 7.06) and dissolved As concentrations (1.76 to 55.4 mg/L). Sulfide weathering has produced crystalline scorodite, hydrous ferric arsenate (HFA) and hydrous ferric oxides (HFO) as secondary phases cementing tailings into hardpan layers. Where sulfide is nearly depleted hardpan is dispersed, pore water pH is neutral and cements are predominantly HFA and HFO. Where sulfide is still plentiful hardpans are more extensive, pore water pH is acidic, and cements are scorodite and HFA.

To remain a stable form of As attenuation scorodite and HFA require acidic conditions, which is occurring where sulfide persists. Where pore water pH has returned to neutral conditions, hardpan is breaking down and scorodite and HFA cements are incongruently dissolving, releasing As and precipitating HFO. These relationships suggest As sequestration in hardpan cements is not permanent with sulfide availability limiting cement stability. Once the arsenopyrite is completely oxidized these As-bearing cements will dissolve, releasing As back to solution. This mineralogical characterization of hardpan and the corresponding pore waters shows scorodite and HFA cements limit dissolved concentrations of As in tailings pore waters but are not a long-term form of As control.