Natural rehabilitation of arsenic-rich historical tailings at the Alexander mine, Reefton, New Zealand

Mine tailings resulting from orogenic gold processing commonly form arsenic (As)-enriched environments. A result of this is that they draw environmental attention in modern gold mining settings and significant effort in long-term rehabilitation. The Alexander River mine processing site in the Reefton area of Westland provides an opportunity to examine the natural rehabilitation of As-enriched tailings that have been left undisturbed for >70 years in a humid environment. These tailings comprise a ∼1 m thick package of finely laminated (mm-scale) sediments that were manually accumulated on a terrace downstream of the gold extraction plant. The tailings sediments are As-rich (up to 5000 mg/kg) due to relict arsenopyrite and As-bearing pyrite. Diagenetic Ca-Fe-arsenate (tentatively identified as yukonite) and As-bearing iron oxyhydroxide have formed during localised oxidation. There was sufficient calcite in the tailings to maintain circumneutral pH in the tailings despite sulphide oxidation. The historical tailing impoundment is now covered by grass, ferns and shrubs, with beech and rimu forest covering several metres at the margins. Leaf analyses indicate that the vegetation is absorbing only minor As. Therefore, the tailings substrate has not halted natural vegetation re-colonisation of this As-rich historical site.