Sulfur speciation and the sources of water-soluble sulfate in three
oxidizing sulfidic mine tailings impoundments were investigated by
selective dissolution and stable isotopes. The studied tailings
impoundments-Piuquenes, Cauquenes, and Salvador No. 1-formed from the
exploitation of the Rio Blanco/La Andina, El Teniente, and El Salvador
Chilean porphyry copper deposits, which are located in Alpine,
Mediterranean, and hyperarid climates, respectively. The water-soluble
sulfate may originate from dissolution of primary ore sulfates (e.g.,
gypsum, anhydrite, jarosite) or from oxidation of sulfide minerals
exposed to aerobic conditions during mining activity. With increasing
aridity and decreasing pyrite content of the tailings, the sulfur
speciation in the unsaturated oxidation zones showed a trend from
dominantly Fe(Ill) oxyhydroxide fixed sulfate (e.g., jarosite and
schwertmannite) in Piuquenes toward increasing presence of water-soluble
sulfate at Cauquenes and Salvador No. 1. In the saturated primary zones,
sulfate is predominantly present in water-soluble form (mainly as
anhydrite and/or gypsum). In the unsaturated zone at Piuquenes and
Cauquenes,the delta(34)S(S04) values ranged from +0.5 parts per thousand
to +2.0 parts per thousand and from -0.4 parts per thousand to +1.4
parts per thousand Vienna Canyon Diablo Troilite (V-CDT), respectively,
indicating a major sulfate source from pyrite oxidation
(delta(34)S(pyrite) -1.1 parts per thousand and -0.9 parts per
thousand). In the saturated zone at Piuquenes and Cauquenes, the values
ranged from -0.8%. to +0.3 parts per thousand and from +2.2 parts per
thousand to +3.9 parts per thousand, respectively. At Cauquenes the 34 S
enrichment in the saturated zone toward depth indicates the increasing
contribution of isotopically heavy dissolved sulfate from primary
anhydrite (similar to+10.9%o). At El Salvador No. 1, the
delta(34)S(SO4) average value is -0.9 parts per thousand, suggesting
dissolution of supergene sulfate minerals (jarosite, alunite, gypsum)
with a delta(34)S similar to -0.7 parts per thousand as the most
probable sulfate source. The gradual decrease Of delta(18)O(S04) values
from the surface to the oxidation front in the tailings impoundments at
Piuquenes (from -4.5 parts per thousand to -8.6 parts per thousand
Vienna Standard Mean Ocean Water, V-SMOW) and at Cauquenes (from -1.3
parts per thousand to -3.5 parts per thousand) indicates the increasing
importance of ferric iron as the main electron acceptor in the oxidation
of pyrite. The different delta(18)O(SO4) values between the tailings
impoundments studied here reflect the local climates.