Abstract
The Sandy Flat Mine Site mine site is located in Australia’s Northern Territory (NT), close to the Queensland border at the base of the Gulf of Carpentaria. The key features of the site include two historic Waste Rock Dumps (WRDs), a Tailings Storage Facility (TSF), and the flooded Sandy Flat Pit, all of which are related to open pit mining operations in the 1990s. There are also facilities related to a leaching operation from 2004 to 2008, including several Leach Vats (LVs) and Heap Leach Pads (HLPs) that were not decommissioned by the operator. Local groundwater and Hanrahan’s Creek downstream of the Sandy Flat Mine Site is significantly degraded due to Acid Rock Drainage (ARD) produced by Acid Forming (AF) waste rock, tailings, and residual (leached) ore that remains on site. The Sandy Flat Mine Site has been surrendered by the previous operator(s) and the NT Department of Industry, Tourism and Trade (DITT) are working to address the impacts from the site. Waste rock at the Sandy Flat Mine Site is relatively low sulphide (1-3%) material that is partially to nearly fully oxidized due to exposure to atmospheric conditions since it was deposited in the 1990s. Three types of AF materials (AF-I, AF-II, and AF-III) were identified. Approximately 50% of mine waste samples are depleted entirely of sulphide minerals, implying that most of the original sulfide has been converted to jarosite acidity and titratable acidity. Jarosite acidity represents the dominant acidity type in many samples. AF-I, AF-II, and AF-III materials generate concentrated ARD due to the lack of carbonate minerals and other mineral sources of alkalinity in the local host rock (trachyte) at the site. Further details on the geochemical characteristics of mine waste and the implications for rehabilitation planning are provided below.