Hydrochemistry, mineralogy and sulfur isotope geochemistry of acid mine drainage at the Mt. Morgan mine environment, Queensland, Australia

Mineralogical, hydrochemical and S isotope data were used to constrain hydrogeochemical processes that produce acid mine drainage from sulfidic waste at the historic Mount Morgan Au–Cu mine, and the factors controlling the concentration of SO 4 and environmentally hazardous metals in the nearby Dee River in Queensland, Australia. Some highly contaminated acid waters, with metal contents up to hundreds of orders of magnitude greater than the Australia–New Zealand environmental standards, by-pass the water management system at the site and drain into the adjacent Dee River. Mine drainage precipitates at Mt. Morgan were classified into 4 major groups and were identified as hydrous sulfates and hydroxides of Fe and Al with various contents of other metals. These minerals contain adsorbed or mineralogically bound metals that are released into the water system after rainfall events. Sulfate in open pit water and collection sumps generally has a narrow range of S isotope compositions ( δ 34S = 1.8–3.7‰) that is comparable to the orebody sulfides and makes S isotopes useful for tracing SO 4 back to its source. The higher δ 34S values for No. 2 Mill Diesel sump may be attributed to a difference in the source. Dissolved SO 4 in the river above the mine influence and 20 km downstream show distinctive heavier isotope compositions ( δ 34S = 5.4–6.8‰). The Dee River downstream of the mine is enriched in 34S ( δ 34S = 2.8–5.4‰) compared with mine drainage possibly as a result of bacterial SO 4 reduction in the weir pools, and in the water bodies within the river channel. The SO 4 and metals attenuate downstream by a combination of dilution with the receiving waters, SO 4 reduction, and the precipitation of Fe and Al sulfates and hydroxides. It is suggested here that in subtropical Queensland, with distinct wet and dry seasons, temporary reducing environments in the river play an important role in S isotope systematics.