Mineralogical control on arsenic release during sediment-water interaction in abandoned mine wastes from the Argentina Puna.

The sulfide-rich residues of La Concordia mine, Argentina Puna, are accumulated in tailing dams that remained exposed to the weathering agents for almost 30years. In such period of time, a complex sequence of redox and dissolution/precipitation reactions occurred, leading to the gradual oxidation of the wastes and the formation of weathering profiles. The sources of arsenic in the wastes were analyzed by XRD and SEM/EDS analysis while a standardized sequential extraction procedure was followed to define solid As associations. In addition, the release of As during sediment-water interaction was analyzed in a period of 10months. The results indicate that primary As-bearing minerals are arsenian pyrite and polymetallic sulfides. As-jarosite and scarce arsenates are the only secondary As-bearing minerals identified by XRD and SEM/EDS. However, the rapid release (i.e., <1h) of arsenic from suspensions of the studied sediments in water, seems to be associated to the dissolution of highly soluble (hydrous)sulfates, as it was determined in samples of the efflorescences that cover the entire site. Contributions from the more abundant As-jarosite are also expected in longer periods of sediment-water interaction, due to its low rate of dissolution in acid and oxic conditions. Finally, near 30% of As remains adsorbed onto Fe (hydr)oxides thus representing a hazardous reservoir with the potential of mobilizing As into porewaters and streamwaters if the acidic and oxidizing conditions that predominate in the region are altered.