Geoenvironmental characterisation of historical mine tailings: a multidisciplinary approach

The Mount Lyell Copper Mine was the largest copper mining company in Western Tasmania, Australia, operating from 1893 until 1994. Commencing in 1916, mine waste (i.e., topsoil, pyritic tailings, smelter slag, runoff from waste rock and effluent from mine workings) was disposed into adjacent waterways, initiating a long and ongoing history of environmental degradation in the Queen?King River system. In this study, water and sediments were collected along the Queen-King Rivers to assess the transport and fate of these legacy mine waste materials. To resolve the thickness, internal structure, and composition of the legacy mine wastes which have been deposited in a delta at the mouth of the King River, geophysical surveys were conducted in parallel with mineralogical and geochemical sampling. This study investigates the long-term environmental impacts on the Queen-King River system from legacy mine waste management practices to assist on future rehabilitation of these sites, including assessing the feasibility of metal recovery from the river and tailings.
The long-term hydrochemical and environmental impacts of mine waste disposal in the Queen-King River was investigated using nearly 20 years of water quality data (i.e., pH, redox potential, electrical conductivity, and dissolved metal concentration). Seepage from historical workings and waste from the Mount Lyell mine was found to influence water pH at all sample sites below the confluence of Haulage Creek. Downstream of this confluence pH was consistently less than 3.5 and downstream of Lynchford to Macquarie Harbour pH was consistently less than 5. Local baseline waters ranged in pH from 5.1 to 8.4. Dissolved Al (3.7-181.5 mg.L^-1 ), Co (0.04-3.3 mg.L^-1 ), Cu (0.7-71.0 mg.L^-1 ), Fe (3.0-1202.4 mg.L^-1 )and Zn (0.2-13.7 mg.L^-1 ) immediately downstream of Haulage Creek were measured above Australian and New Zealand Environment and Conservation Council guidelines for freshwater (ANZECC, 2000). Concentrations of dissolved Cu, Fe and Zn remained high along the length of the river system to its outflow in Macquarie Harbour (approximately 22 km).
Mineralogy of sediments in the Queen-King River system is comprised of quartz, muscovite, chlorite, pyrite (highly concentrated in the King River delta), chalcopyrite and secondary minerals including goethite (particularly at and downstream of Haulage Creek), ferrihydrite and other Fe-bearing and authigenic minerals (abundant in the upper oxidized layer of the delta). Inclusions of minerals such as barite and galena, and minor molybdenite were also observed. Environmental tests on mining-impacted sediments (i.e., paste pH and net acid generation testing) indicate potential for acid and metalliferous drainage (AMD) and metal(loid) leaching to the Queen-King Rivers and Macquarie Harbour from the riverine and deltaic sediments.
The deltaic sediments are enriched in pyrite (up to 18 wt%). Oxidation of pyrite has created an oxidized upper horizon at the delta (10-30cm depth), where authigenic minerals (e.g., Fe-oxyhydroxides and sulphates) prevail. Furthermore, river freshwater infilling, wave motion and tidal fluctuations are oxidizing these pyrite-rich deltaic sediments inward from Macquarie Harbour and the King River. Concentrations of other metals, such as Fe (up to 27 wt%), Cu (up to 7700 ppm) and Co (up to 780 ppm) are elevated in delta sediments. Copper was found to be hosted in chalcopyrite and Co is primarily hosted in pyrite. The prevalence of sulfide minerals such as pyrite and chalcopyrite in the sediments of the King River contribute to a high acid-forming potential (paste pH 3.3 to 6.8 and NAG pH 4.3 to 1.8) with risk of metal leaching (e.g., Cu, Co, Ni and Zn) of sediments in the delta. This observation is supported by measured pore water chemistry (pH 3.1 to 6.3) that indicates high contents of dissolved heavy metals, notably Fe, Cu, Co and Zn. Metals leached into pore water are sourced from pyrite (i.e., Co, Ni and As), chalcopyrite (i.e., Cu and Ag), molybdenite (i.e., Mo) and galena (i.e., Pb). Authigenic mineral (i.e., Fe-oxyhydroxides, Fe-sulphates and silicate mixtures) present mainly at the oxidized profile, provide effective sinks for these metal(loids), causing enrichment of these mineral phases at the surface of the delta.
The internal structure of the delta is complex and consists of several interbedded layers, as inferred by geophysical surveys (i.e., electromagnetics, seismic refraction, and resistivity). Dry unconsolidated tailings are suggested between 0-2 m depth. Saturated unconsolidated tailings are indicated from 2-5 m depth. Salt-water intrusion is recognized at nearly 4 m and the transition into natural sediments/bedrock is expected at approximately 12 m. This corresponds to nearly 10Mt of tailings stored in the delta, which represents up to 4083 t of Co and approximately 45,439 t of Cu in tailings.
his research presents a detailed study of the Queen-King River system at a micro (e.g., mineralogy, geochemistry and element deportment) and macro (e.g., internal structure of the King River delta) scale, and emphasizes high mass loads in the Queen-King Rivers, high metal concentrations in sediment and water as well as enrichment at the King River delta. These findings indicate that legacy mine wastes deposited within Queen-King River system pose a long-term environmental hazard, although the waste stored at the delta is naturally cocooned by surface oxidation of tailings and high pH seawater. High metal concentrations (e.g., Cu, Co) of tailings at the King River delta could represent a potential source of valuable material that may be metallurgically recovered, resulting in both environmental and economic benefits.