Tailings typically account for 95 to 99 % of the originally processed ore at base metal mine sites and hence represent a significant waste management challenge for the mining industry. This issue is intensified by modern mining methods that are trending towards the extraction and processing of lower grade, higher volume ore bodies. Planning for tailings storage facility closure presents a challenging task for mine operators and regulators given the requirement for closed facilities to be inert, or if not, stable and contained in perpetuity. To date, much geochemical research has taken place at mine sites characterised by cool climates where precipitation exceeds evaporation, low (less than 2 wt. %) carbonate mineral contents, and typically some years post-closure.
This study examines the evolution of tailings seepage chemistry at one of Australia’s largest and longest operating mine sites located at Mount Isa in North-West Queensland, Australia. There are few analogues in literature for a site of this nature given the tailings facility size (12.3 km2), its location in a semiarid subtropical climate, and specific ore mineralogy. The operational status of the site and 80-year production history allows an exceptional opportunity to examine and compare decommissioned and operational sections of the tailings storage facility.
The study aim is to improve the understanding of the geochemical processes that determine contaminant release or attenuation in sulfidic, high carbonate mine tailings under semiarid, subtropical climatic conditions. The approach involved a three-year field characterisation study, and 14 months of laboratory-based kinetic testing. Analytical methods included the application of stable isotopes (sulfur, oxygen and hydrogen), column leaching and sequential chemical extraction, in addition to more standardised mineralogical and geochemical methods including X-ray diffraction, scanning electron microscopy, mineral liberation analysis, acid-base accounting, elemental analysis, and water chemistry determinations.
Geochemical differences between operational and decommissioned tailings were measured, and attributed to differences in exposure timeframes, ore source (copper or lead-zinc-silver), minerals processing methods, hydraulic fill operations, and waste co-disposal over time.
Tailings seepage was characterised by high concentrations of sulfate (3100 to 8400 mg L-1) and low concentrations of dissolved metals (typically less than 1 mg L-1) in comparison to mine waters from other well-known shale-hosted massive sulfide deposits. Low metal concentrations are principally due to the dissolution of carbonates that maintain tailings seepage between pH 6 and 9, thereby minimising primary mineral dissolution and encouraging metal attenuation through secondary mineral precipitation and adsorption. Sequential chemical extraction results indicate a strong association between potential environmental contaminants and sulfides in operational tailings, and adsorption to and/or co-precipitation with secondary minerals in decommissioned tailings. Processes driving potential environmental contaminant release and attenuation at facility-scale under well-buffered neutral pH conditions are principally related to fluctuations in the degree of tailings saturation, and its subsequent effect on redox conditions, mineral solubility and adsorption.
Sulfur isotopes, when interpreted in conjunction with other conservative geochemical characteristics of seepage, are a useful approach to identify seepage-related waters in semiarid subtropical climates where concentration and dilution effects mean that absolute contaminant concentrations alone may be insufficient to establish contaminant origin (i.e. mine or natural). Sulfur isotopes showed that the dominant source of dissolved sulfate in the tailings seepage is from the oxidation of sulfide minerals. Uncertainty exists in relation to exactly which group of sulfides are represented in δ34Ssulfate tailings seepage compositions: (i) oxidation of historical sphalerite and pyrite in tailings with minor microbial influence; or (ii) oxidation of modern sulfides in tailings with strong microbial influence.
Acid generation and neutralisation rates obtained from column leaching largely supported static acid-base accounting classifications and mineralogical findings. In comparison to decommissioned tailings, operational tailings are typically higher in sulfide and lower in carbonate content, and are potentially acid-forming. Column leaching of intact tailings samples demonstrated that minor decreases in water content (2 to 16 %) were sufficient to increase sulfide mineral oxidation rates and the release of certain potential environmental contaminants including cobalt, copper, manganese and zinc. The reactive nature of surface tailings will be an important consideration for final cover design, particularly under the highly evaporative conditions of Mount Isa.
It is intended that research outcomes will assist in closure planning for Mount Isa and are applicable for consideration in the assessment, design and management of tailings storage facilities at other localities where similar climatic and mineralogical conditions are present. This may include future mine developments in the wider North-West Minerals Province in Queensland, and similar climatic regions within South America and Africa.