Geochemical and mineralogical characterisation of a tailings-rich sediment bank, King River, Western Australia

The Queen-King River system, Western Tasmania has been severely impacted by historical mining operations (1890s onwards) at the Mt. Lyell Copper mine, Queenstown. For several decades until 1995, riverine processing waste disposal was practiced with approximately 100 Mt of mine tailings and slag materials discharged directly into the Queen River, a tributary of the larger King River. The movement of these wastes has contributed to the genetics of the 2.5 km2 King River delta at Macquarie Harbour, which contains approximately 10 Mt of mine tailings, with a further 10 Mt of fine tailings deposited at the delta front. In this study, we characterised one sediment bank in the King River system (termed Bank D) located approximately 2.5 km from Macquarie Harbour. The objectives were to examine the mine tailings (quantity, degree of oxidation) in the upper 15 cm of the bank and; ii) determine the mobility of elements contained in these surface sediments. In addition basic water quality parameters (pH, EC, dissolved oxygen and redox potential), were analysed in-field using waters collected from an existing nest of piezometers, and a trench dug to 1 m to examine the tailings profile. After drying, sediments (n = 29) were subjected to a range of traditional acid-base accounting tests and mineralogical analyses by X-ray diffractometry and field-emission scanning electron microscopy, and three leach tests. Results showed that the majority of sediments are acid forming with primary pyrite observed and secondary framboidal pyrite actively precipitating. Acidic pH values were measured in pore waters and the King River itself. These sulphides are enriched in Cu and Zn. The sediments also contain iron-oxides enriched in Mn, Zn and Ni which are relatively immobile under these surficial conditions. Our results highlight the need for further characterisation and remediation of these sediments, which, after 98 years since their deposition, continue to pose a geoenvironmental risk to Macquarie Harbour .