Fractionation of Fe and Cu isotopes in acid mine tailings : Modification and application of a sequential extraction method

• Sulfidic mine tailings have a high potential for contamination of the environment by triggering acid mine drainage. Hence, it is crucial to understand metal mobilization processes and to develop monitoring tools. Metal isotope fingerprinting as a potential monitoring tool for metal sulfide oxidation processes was in the focus of this study by using stable isotope signatures of Cu and Fe. For Fe, a six-step sequential extraction method was applied, in order to separate potential Fe-bearing minerals (water-soluble, exchangeable fraction, Fe(III)(oxyhydr)oxides, Fe-oxides, sulfides and organic compounds and residual/silicates). For Cu, this method was modified into a four step extraction method (water-soluble, exchangeable fraction, oxalate fraction/bound to Fe-oxides and sulfides/residual). To verify accuracy and precision of the sequential extraction method for metal isotope analysis, isotope fractionation during the extraction procedure was investigated employing minerals for which the mineral composition and the isotopic composition was known. The developed procedure is suitable to separate target minerals with only a small loss in the elemental budget. No significant isotope fractionation was observed during the extraction procedure.Application of this method on two sites of porphyry copper mine tailings in the Atacama Desert in Chile (Chañaral bay) revealed several implications about the mobilization of Fe and Cu in an environmental setting. Iron contents and Fe isotope compositions are homogeneous with depth (0–61 cm; δ56Fe ~0.2–0.3‰) for the bulk and the Fe(hyr)oxide fraction and only the deepest samples at ~60 cm exhibited lower δ56Fe values (~0‰), which are likely related to the occurrence of an alluvium at this depth. The Fe silicate fraction shows higher δ56Fe values (0.6–0.9‰), most likely because of preferential leaching of the light Fe isotopes. This consequently indicates a more pronounced Fe isotope fractionation with depth, as is expected from longer weathering. The Fe sulfide fraction is isotopically lighter compared to the Fe(hydr)oxide fraction, because during sulfide oxidation the heavy Fe isotopes prefer the oxidized forms and oxidative precipitation results in an enriched Fe isotopic signature for Fe(hydr)oxides. The Cu isotope compositions of all bulk samples and individual fractions (except the Cu sulfides) of one site (Ch1) exhibited a decrease of the δ65Cu values from the depth towards the surface, i.e. in agreement with the capillary water rise in the arid climate. A correlation of δ65Cu with pH indicates preferential adsorption of 65Cu on Fe(oxy)hydroxides at site Ch1, which is evident by a change of δ65Cu from 0.5‰ to −0.7‰ in the water-soluble fraction. At another site (Ch12), where pH at depths was potentially not high enough for the formation of Fe-minerals that could adsorb Cu, only minor Cu isotope fractionation was observed in the water-soluble fraction. The Cu sulfide fraction at site Ch1 exhibits higher δ65Cu values with an increase from the bottom (0.42‰) to the surface (0.92‰), which might be related to preferential leaching of the light isotopes, e.g. by microorganisms.

Subject headings

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

Keyword

Applied Geochemistry

Tillämpad geokemi

Publication and Content Type

ref (subject category)

art (subject category)