The sul
fi
de-rich residues of La Concordia mine, Argentina Puna, are accumulated in tailing dams that remained
exposed to the weathering agents for almost 30 years. 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 forma-
tion of weathering pro
fi
les. The sources of arsenic in the wastes were analyzed by XRD and SEM/EDS analysis
while a standardized sequential extraction procedure was followed to de
fi
ne solid As associations. In addition,
the release of As during sediment
–
water interaction was analyzed in a period of 10 months. The results indicate
that primary As-bearing minerals are arsenian pyrite and polymetallic sul
fi
des. As-jarosite and scarce arsenates
are the only secondary As-bearing minerals identi
fi
ed by XRD and SEM/EDS. However, the rapid release
(i.e.,
b
1 h) of arsenic from suspensions of the studied sediments in water, seems to be associated to the dissolu-
tion of highly soluble (hydrous)sulfates, as it was determined in samples of the ef
fl
orescences that cover the en-
tire 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