Developments in acid rock drainage prediction: A case study on the abandoned Croydon Au-mines, QLD, Australia

Fifty-one waste-rock samples collected from the abandoned Croydon Au-mines were categorised into ten mesotextural groups (A to J) and characterised by routine and advanced static geochemical tests to identify which groups are currently responsible for acid generation. Traditional net-acid producing potential (NAPP) test procedures confirmed the absence of effective and longer-term neutralising minerals. Advanced NAG tests were performed and the multi-addition NAG test was identified as the most efficient and accurate relative to the single addition NAG and sequential NAG tests. Mesotexture J (massive quartz-pyrite) and H (massive quartz-arsenopyrite-pyrite) were identified as extremely acid-generating with maximum multi-addition NAG values of 446 and 433 kgH2SO4/t respectively. A new classification approach of comparing paste pH values with a site-specific mineralogical ARD Index was trialled, and confirmed the extremely acid generating nature of mesotextures J and H. Future management of the site should focus remediation efforts on the removal or encapsulation of waste rock demonstrating these two mesotextures.