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

ABSTRACT

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.

KEYWORDS:

• Gold
• mine
• tailings
• yukonite
• sulphide
• vegetation

Acknowledgements

This project was done with the support of Department of Conservation (DOC) and we are grateful for the enthusiastic support of Jim Staton. Kirstine Malloch provided important context and background knowledge on the main Alexander processing sites, and Christine McLachlan’s work on Prohibition tailings gave a useful benchmark for this study. Brent Pooley produced excellent polished thin sections of difficult material. SEM work for the study was carried out at the Otago Micro and Nano Scale Imaging centre (OMNI), University of Otago. We thank two reviewers for their comments.

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

Disclosure statement

No potential conflict of interest was reported by the author(s).