Geochemical processes influencing arsenic mobility at Bullendale historic gold mine, Otago, New Zealand

Abstract

The historic Bullendale gold mine processed arsenopyrite-rich ore from mesothermal deposits in the South Island of New Zealand from 1864 to 1907. No site rehabilitation was undertaken upon mine closure and processing waste contains arsenic concentration of up to 40 wt%. Originally, all of this arsenic was present as arsenopyrite and with time it has been replaced by secondary arsenic phases. Waste evolution has resulted in a mineralogical gradation of arsenic phases between the ferrihydrite with varying amounts of adsorbed arsenic, amorphous iron arsenates and crystalline kankite, scorodite and zykaite. The stability of the secondary arsenic phases is favoured at low pH (pH 3, <0.1 mg/L) and, at present, reactions associated with iron arsenate dissolution ensure a prevailing pH window of c. 2.5 to 3.5. However, an increase in pH through flooding could destabilise the secondary arsenic minerals resulting in a substantial increase of arsenic solubility (>100 mg/L). In addition to the battery site, an upstream adit is a continuous source of dissolved arsenic to the environment. Due to the alkaline pH conditions in catchment water, arsenic remains mobile and dissolved As of c. 0.03 mg/L is detectable in Skippers Creek more than 2km downstream. Even when this anomaly has been diluted, the arsenic flux of the river is still predominantly controlled by the adit input.