The Environmental Legacy of Historic Gold Processing in the Reefton Goldfield

Abstract

Orogenic gold deposits are a source of arsenic (As) and antimony (Sb), in the sulphides arsenopyrite and stibnite respectively. Weathering of these deposits releases As and Sb into the environment but it is ore processing that can concentrate naturally occurring As to Sb to toxic percentage levels. Gold recovery by mercury (Hg) amalgamation and cyanidation results in the addition of metals (zinc (Zn), lead (Pb) and Hg)) and chemicals (cyanide) that can also be concentrated during processing to high levels that exceed national guidelines. Through the practice of crushing and milling during processing, fine (< 500 μm) metal-rich particles are produced that are easily transported by rain splash and wind, and easier to inhale and ingest. This study examines three historic processing sites in the Reefton Goldfield, New Zealand that operated prior to the 1950s when legislation was not in place requiring the remediation of a site after closure. These abandoned processing sites contain historic relics that are of an interest to the Department of Conservation (DOC) to preserve on these heritage sites, as well as containing high metal processing residues that are a potential environmental concern. Of the three sites presented in this study, the toxic As(III) arsenolite in the Edwards roaster at the Alexander processing complex poses the highest hazard. The extreme As levels (in excess of 35 wt% As) are the result of ore roasting to release refractory gold from sulphides, and these extreme As levels are seen at other historic processing sites where the ore was roasted. Crystalline efflorescences inside the Edwards roaster may indicate that As vapours permeated the structure during roasting and are now a point source for As. Ore roasting has also concentrated other metals (Sb, Hg) in the roaster to high levels. High metal levels on the three sites studied are largely sequestered by the formation of secondary iron arsenate and iron oxide minerals. Any dissolved metals leaving the sites are diluted by high regional rainfall. The weathering of sulphides, concentrated by ore processing, has resulted in the localised production of acid but lime added during processing, as well as carbonates in the Greenland Group host rocks, neutralises the acid so there is no acid mine drainage (AMD). There is no significant increase in dissolved metals downstream from the Alexander processing complex, with most metals below detection limits and all metals well below the New Zealand drinking water standards. Previous site investigations into historic processing sites in the Reefton Goldfield, and this study, highlight the importance of undertaking a thorough investigation of metal distribution, speciation and mobility before deciding on a site remediation plan. Ranking of a site for remediation should take into account its land use and future land use, as well as visitor numbers and levels of exposure. On the basis of the processing residue’s mineralogy and geochemistry at the three sites presented in this thesis, it is recommended that due to the remoteness of these sites and low visitor numbers, that they are a low priority for remediation.