Mine waste materials, produced by several large-scale mines, are abundant in western Tasmania. For example, at the Mt Lyell mine more than 312 Mt of ore at a grade of 1 to 2% Cu were produced during its operation, from Cu-Au VHMS ore bodies around Queenstown. Ore was smelted onsite at Queenstown with 2 Mt of slag dumped on-site. In contrast, the Zeehan ore fields were well endowed with Ag and Pb, with an estimated total ore production of at least 14,000 tonnes of Ag-Pb by 1893. Ore from the Zeehan ore fields was smelted 3 km south of Zeehan leaving over 450,750 t of slag behind. Today, a significant quantity of slag remains at both sites, therefore this study undertaken by Honours student Sarah Gilmour aimed to assess the geoenvironmental risks posed to surface and groundwaters and, through mineral and chemical characterisation, to determine if there is potential for reprocessing these wastes as part of integrated rehabilitation strategies.
The Queenstown slags are granulated and vitreous with a matrix of (Si-Fe-Ca-Mn-Zn-O) glass and Cu-Fe-sulfides. Copper and Zn concentrations are between 0.56 % and 0.61 % and 1.79 % and 2.47 % respectively. Copper is dominantly present as Cu-Fe sulfides (30-40 %) and is present in the glass in minor quantities (0.5 to 2 %). The Cu-Fe-sulfides occur as disseminated spheroidal blebs which mineralogically resemble chalcopyrite. These blebs range in diameter (<1 μm to >500 μm) and are proportionately low in this glass dominated slag. The low content of Cu suggests its efficient recovery by the unique pyritic smelting process employed from 1896 to 1921.
The Zeehan slags are hard and siliceous with a matrix of (Si-Fe-Ca-Mn-Zn-O) glass and euhedral and lath-like crystals of (Ca-Fe) olivine, (Ca-Mn-Zn-Fe) clinopyroxene, Zn-Fe-spinels and Zn-sulfides. At Zeehan the Zn grade varies between 14 % and 18 % with minor Mn, Al and Pb. Zn is dominantly hosted in dendritic Znsulfides and glass but is also elevated in crystalline olivines, pyroxenes and dendritic Fe-spinels.
The sulfide content of the historic slag piles, calculated as maximum potential acidity values (between 84 and 146 kg H2SO4/t for Zeehan and between 26.2 and 10.7 kg H2SO4/t for Queenstown), indicates that both sites have potential AMD risks with the Zeehan site presenting higher concentrations of leached metals. The results from this study suggest identified potential risks associated with these historic wastes with a need for active remediation. Provided the reprocessing and extraction of the Zn from Zeehan (via chemical leaching using H2SO4) and the base metals (via bioleaching using bacteria) from Queenstown is economically viable the new waste produced would theoretically be environmentally benign and therefore suitable as a construction material. Once the slag materials are treated, any associated geoenvironmental risks will decrease and efforts can begin on the rehabilitation of the surrounding soils, water pathways and vegetation that may have been impacted by the dumps.