SURFICIAL WEATHERING OF IRON SULFIDE MINE TAILINGS UNDER SEMI-ARID CLIMATE, PART 1: MINERAL WEATHERING TRAJECTORY

Mine wastes introduce anthropogenic weathering profiles that - in arid and semi-arid regions - remain unvegetated for decades after mining cessation. As such, they are vulnerable to wind and water dispersion of particulate matter to adjacent ecosystems and residential communities. In sulfide ore tailings, propagation to depth of the oxidative weathering front controls the speciation of major and trace elements and, in turn, the time-dependent change in tailings capacity to support microbial and vegetative colonization. To investigate the relationship between the gossan oxidative reaction-front propagation and molecular speciation of redox-active iron and sulfur a semi-arid climate, we studied tailings at the Iron King Mine and Humbolt Smelter Superfund Site in Arizona (USA) where the processing of sulfidic ore produced tailings with high surficial accumulations of toxic metal(loid)s including Pb and As. Here, we report a multi-method data set resolving the coupling of iron (Fe) and sulfur (S) geochemistry in the top 2 m of the tailings through the use of complementary wet chemical and synchrotron-based investigation, including X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). A distinct redox gradient was observed within the top 0.5 m and XAS and XRD data indicate that the near-complete solid phase transformation of ferrous sulfides to iron (oxyhydr)oxides and (hydroxy)sulfates accompanied by negligible mass depletion of Fe and S concentration with depth (180-210 and 100-120 g kg^-1, respectively). Initial weathering products, ferrihydrite and gypsum, transform to schwertmannite, then jarosites with an accompanying decrease in pH (7.3 to 2.3). Interestingly, ferrihydrite is observed even in the lowest pH samples, indicating the persistence of metastable minerals in these semiarid tailings. Goethite, a thermodynamically stable solid phase weathering product of the secondary iron phases, was not detected in any sample. The resulting sharp geochemical speciation gradients have important implications for the mobility and bioavailability of co-associated toxic metal(loid)s.