Prevention of acid mine drainage (AMD) in sulfide-containing tailings
requires the identification of the geochemical processes and element
pathways in the early stages of tailing deposition. However, analyses of
recently deposited tailings in active tailings impoundments are scarce
because mineralogical changes occur near the detection limits of many
assays. This study shows that a detailed geochemical study which
includes stable isotopes of water (delta H-2, delta O-18), dissolved
sulfates (delta S-34, delta O-18) and hydrochernical parameter (pH, Eh,
DOC, major and trace elements) from tailings samples taken at different
depths in rainy and dry seasons allows the understanding of weathering
(oxidation, dissolution, sorption, and desorption), water and element
pathways, and mixing processes in active tailings impoundments.
Fresh alkaline tailings (pH 9.2-10.2) from the Cu-Mo porphyry deposit in
El Teniente, Chile had low carbonate (0.8-1.1 Wt-% CaCO3 equivalent)
and sulfide concentrations (0.8-1.3 wt.%, mainly as pyrite). In the
alkaline tailings water, Mo and Cu (up to 3.9 mg/L Mo and 0.016 mg/L Cu)
were mobile as MoO42- and Cu (OH)(2)(0). During the flotation, tailings
water reached equilibrium with gypsum (up to 738 mg/L Ca and 1765 mg/ L
SO4). The delta S-34 VS. delta O-18 covariations of dissolved sulfate
(2.3 to 4.5% delta S-34 and 4.1 to 6.0 % delta O-18) revealed the
sulfate sources: the dissolution of primary sulfates (12.0 to 13.2%.
delta S-34, 7.4 to 10.9%.delta O-18) and oxidation of primary sulfides
(-6.7 to 1.7%. delta S-34).
Sedimented tailings in the tailings impoundment can be divided into
three layers with different water sources, element pathways, and
geochemical processes. The deeper sediments (> 1 m depth) were
infiltrated by catchment water, which partly replaced the original
tailings water, especially during the winter season. This may have
resulted in the change from alkaline to near-neutral pH and towards
lower concentrations of most dissolved elements. The neutral pH and high
DOC (up to 99.4 mg/L C) of the catchment water mobilized Cu (up to 0.25
mg/L) due to formation of organic Cu complexes; and Zn (up to 130 mg/L)
due to dissolution of Zn oxides and desorption). At I m depth, tailings
pore water obtained during the winter season was chemically and
isotopically similar to fresh tailings water (pH 9.8-10.6, 26.7-35.5
mg/L Cl, 2.3-6.0 mg/L Mo). During the summer, a vadose zone evolved
locally and temporarily up to 1.2 m depth. resulting in a higher
concentration of dissolved solids in the pore water due to evaporation.
During periodical new deposition of fresh tailings, the geochemistry of
the surface layer was geochemically similar to fresh tailings. In
periods without deposition, sulfide oxidation was suggested by
decreasing pH (7.7-9.5), enrichment of MoO42- and SO42-, and changes in
the isotopic composition of dissolved sulfates. Further enrichment for
Na, K, Cl, SO4, Mg, Cu, and Mo (up to 23.8 mg/L Mo) resulted from
capillary transport towards the surface followed by evaporation and the
precipitation of highly soluble efflorescent salts (e.g., mirabilite,
syngenite) at the tailing surface during summer. (C) 2008 Elsevier B.V.
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