1Jackson, Laura, 2K ng, Lexi, 1Parbhakar-Fox, Anita, 3Meffre, Sebastien
1The W.H. Bryan Mining & Geology Research Centre, Sustainable Minerals Institute, The University of Queensland, Brisbane, Australia; 2RGS Environmental, Brisbane, Australia; 3 The school of Natural Sciences, The University of Tasmania, Hobart, Tasmania
The Rosebery Pb-Zn-Cu-Ag mine, 3 km north west of Rosebery, Tasmania, Australia has been in operation since 1936. During this time >17 Mt of tailings were deposited in Bobadil Tailings Storage Facility, which opened in 1974 and reached capacity in 2018. Historically the materials contained in the Bobadil tailings are known to be endowed in ecotoxic metals including Pb, Zn, Cu, As and Mn, as would be expected based on the ore mineralogy (i.e., sphalerite, galena, pyrite). To assess the risks posed, samples were collected from 10 trenches (52 samples) and 4 cores the upper 2 m across the accessible parts of the TSF and detailed geochemical and mineralogical studies (e.g., acid base accounting (ABA), X-ray diffractometry, sulphide alteration index (SAI), mineral liberation analysis (MLA), scanning electron microscopy, laser ablation ICPMS) undertaken to assess the viability of reprocessing as a means to reducing environmental risks associated with the facility, and extend the mine life.
Eleven facies (A to K) were visually defined in these sampled tailings, ranging from oxidised hardpan (i.e., Facies K) to fresher sulphide dominated tailings (Facies A). Despite this visual heterogeneity, ABA results classified all samples as potentially acid forming (PAF) with total sulphur ranging from 3.8 to 13.8 %. The inherent acid neutralising potential (ANC) was low across all facies (0.5 to 1.9 % carbon) and is complimentary to the measured tailings mineralogy which reported a low abundance of carbonates (<2 % calcite). Sulphide alteration index (SAI) values confirm most tailings as un-oxidised to partially armoured. When SAI values are screened against paste pH values, these materials classified as PAF with a lag time to AMD generation anticipated. MLA results reported >89 % of pyrite as liberated and where locked, mineral associations were dominantly with muscovite and quartz. To determine the tenor and deportment of precious, base and critical metals in the pyrite and sphalerite LA-ICP-MS analysis reported trace metals (e.g., Co, Ni, Cd and Bi) in pyrite were considered low, whist in sphalerite bivalent metals including Cd and Mn were notably high. Only two Au inclusions were identified in MLA-SEM images. Due to the homogenous, trace element free and highly liberated pyrite particles these tailings could be amenable to reprocessing and desulphurisation. The remaining gangue tailings have the potential to be reused into products such as ceramics, road base and industrial building materials. With additional analysis of tailings at depth, a robust retreatment framework could be redeveloped to help remove the requirement to maintain and manage a large tailings facility in perpetuity.
Lexi is a geochemist at RGS Environmental. She has completed an Honours degree in Environmental Geochemistry at the University of Tasmania. Her Honours study involved the characterisation of mineralogy and geoenvironmental behaviour of mine waste (tailings) as part of the rehabilitation plan for a tailings storage facility in Western Tasmania.