Stratigraphy and mineralogy of tailings at Macraes gold mine, southern New Zealand

ABSTRACT

Processed tailings in the Mixed tailings impoundment at the Macraes gold mine built up a sedimentary pile up to 100 m thick between 1990 and 2013. The tailings consist of sand-dominated fluvio-lacustrine sediments with fine (cm scale) bedding and have a mineralogical stratigraphy that reflects changing mine processes. Most of the tailings (>90%) consist of silicate minerals discarded from the flotation system throughout that time. The deepest tailings (1990–1993) have the lowest contents of sulphides as these were extracted and deposited separately. Between 1993 and 1999, tailings with up to 0.5 wt% sulphide sulphur (as pyrite and arsenopyrite) were deposited. Introduction of a pressure oxidation system in 1999 caused a substantial decrease in sulphide minerals and a parallel increase in ferric oxyhydroxide, ferric sulphate, and ferric arsenate compounds in which trace metals are dispersed. Some primary scheelite reacted during pressure oxidation conditions to form ferric tungstate compounds. Processing of auriferous sulphide concentrate from Reefton, western South Island, from 2007 introduced stibnite to the ore stream, and this was readily oxidised to form insoluble As-bearing ferric antimonate.

KEYWORDS:

• Tailings
• arsenic
• tungsten
• sulphide
• sulphate
• silicate
• autoclave
• oxidation

Acknowledgements

The study would have been impossible without the strong support of OceanaGold Ltd over many years. Oceana staff, especially Simone Cready, Debbie Clarke, Sean Doyle, Gavin Lee, Quenton Johnston, Scott Mossman and Mark Nelson, facilitated data compilation and logistics of site visits and provided many useful discussions on the site. Discussions with Hamish Blakemore on a draft of the ms were also helpful. Enthusiastic research work by Dusk Mains assisted in compilation of some of the data used herein. Two journal reviewers helped substantially to improve the presentation of the ms.

Disclosure statement

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

Data availability statement

All data relevant to this study are included within this paper and within cited references.

Additional information

Funding

This research was funded by Oceana Gold Ltd, the NZ Ministry for Business, Innovation and Employment (MBIE; formerly FRST), and University of Otago.