Characterisation and bioavailability of metals in historic mine wastes, Waiuta, Westland

Abstract

New Zealand has a long history of mining which has left a legacy of abandoned, contaminated mine sites with no accountability to the previous mining companies. The Prohibition Gold Mine, near Waiuta, Westland, was operational from 1938 to 1951 and is now administered by the Department of Conservation (DoC). An exclusion zone was in place during this study around the processing and roasting plant, where extremely high arsenic concentrations (up to 40 wt%) had been identified, due to the historical roasting of arsenopyrite rich sulphide ore. Prohibition Mine had been listed as New Zealand’s most contaminated site and was remediated in 2016.

Following the discovery of a tailings impoundments at the site, DoC initiated further investigation of the wider site, outside of the exclusion zone, to extend knowledge of the wider site for management and potential remediation. A FP-XRF survey to delineate and quantify areas with elements of environmental concern, arsenic (As), copper (Cu), mercury (Hg), lead (Pb) and zinc (Zn), which were elevated above industrial standards, was undertaken on site. The survey identified five hotspots with elevated metals and six types of mine waste with varying metals and metal concentrations. Samples were collected for further analysis by XRF, XRD and SEM to determine the mineralogy and stability of the samples. The largest quantity of mine waste at the site was tailings material. A large proportion of the material was clastic grains which have been unchanged in the >70 years since being deposited from the roaster and silicate tailings, indicating that they are stable in the current environment. There is a much smaller proportion of iron-oxide cements which are post-depositional, less stable and display visible mobility within the substrate in photomicrographs. Bioavailability and mobility of the arsenic was investigated in this study through leaching experiments; both standing water and phosphorus amended, as well as the examination of arsenic uptake in plants. The concentration of arsenic in the mine wastes, even outside the exclusion zone, is elevated and therefore the bioavailability of the As is of concern due to its transferability to the biosphere and potential affect on flora and fauna as well as human health. Following remediation, the site was reopened to the public in December 2016. It is recommended that the un-remediated grass areas to the south of the assay office, as well as the western side of the cableway, where the cupels are located, be remediated due to the high As and Pb concentrations, including the removal of the cupels, as these are easy to collect and keep as souvenirs due to their small size. These areas are very accessible to the public and could potentially pose a risk to public health. The other areas investigated and identified as having high metal levels have limited leaching, therefore have low geochemically mobility and are reasonably inaccessible to the public.