Geochemical and hydrological evolution of mine impacted waters at the Argyle Diamond Mine, Western Australia

The lamproite ore body at the recently closed Argyle diamond mine is geochemically benign, but the surrounding sedimentary sequence contains two thin pyritic black shale beds that are prone to acidification. Although the waste rock is on average net neutralizing, some localized acidification and sulfate release was first observed in 1999 after 15 years of open pit mining. Contact and receiving water quality have been actively managed and monitored for 20 years to further limit the already low environmental risks posed by nitrate, nickel, magnesium and sulfate. All four solutes exhibited increasing concentrations across the monitoring network during or shortly after the period of peak waste rock production from 1999 to 2004. Since open pit mining and associated rock blasting ended in 2013, concentrations of nitrate have declined by 50%-90% at most locations. Nickel concentrations have also declined by 30% at the two acidic waste rock seeps which account for 80% of the dissolved nickel load. Sulfate and magnesium concentrations generally peaked between 2010 and 2013 and have not exhibited any significant increasing or decreasing trends since then. Annual offsite sulfate release since 2014 has averaged about 0.4% of the total sulfur inventory stored on site. Future sulfate concentrations and loads are predicted to gradually decline as the remaining sulfur inventory is depleted. Even during the 2005 to 2008 period, when all solute release rates were near their maximum, the receiving aquatic ecosystems remained intact with only subtle changes to some invertebrate communities. It is clear that peak release rates have already passed. The geochemical evolution at Argyle is much more rapid than at most other large mines. This is likely due to several factors including: 1. the low mean sulfur concentrations in the black shales; 2. the short geochemical lag time in the reactive units; 3. the high annual rainfall; and 4. a black shale segregation program that was initiated in roughly 1996.