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Stable sulphur and oxygen isotopes as indicators of sulphide oxidation reaction pathways and historical environmental conditions in a Cu-W-F skarn tailings piles, south-central Sweden
Stockholm University, Faculty of Science, Department of Geological Sciences.
Number of Authors: 42019 (English)In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 110, article id 104426Article in journal (Refereed) Published
Abstract [en]

Improved remediation strategies or predictive modelling of acid mine drainage (AMD) sites, require detailed understanding of the sulphide oxidation reaction pathways, as well as pollutant-source characterisation. In this study, ore minerals, solids and water-soluble fractions of an oxidising Cu-W-F skarn tailings in Yxsjoberg, Sweden, were chemically and isotopically (delta S-34 and delta O-18) characterised to reveal sulphate (SO42-) sources, sulphide oxidation reaction pathways and historical environmental conditions in the tailings. delta S-34 was additionally used to trace the weathering of danalite [(Fe,Mn,Zn)(4)Be-3(SiO4)(3)S], a rare and unstable sulphur-bearing silicate mineral containing high concentrations of beryllium (Be) and zinc (Zn). Eighteen subsamples from a drill core of the tailings were subjected to batch leaching tests to obtain water-soluble fractions, which reflected both existing porewaters and easily-soluble secondary minerals. The tailings were categorised into three geochemical zones: (i) oxidised zone (OZ), (ii) transition zone (TZ) and (iii) unoxidised zone (UZ), based on prevailing pH, elemental concentrations and colour. The upper OZ (UOZ) showed a sharp depletion of sulphur (S) and relatively higher delta O-18(SO4) values (3.0 to +0.1%) whereas the underlying lower OZ (LOZ) showed S accumulation and lower delta O-18(SO4) values (4.6 to 4.2%). The higher delta O-18(SO4) suggested the role of atmospheric oxygen, O-2 (as oxidant), contribution of evaporated rainwaters and/or evaporation in the upper zones of the tailings. The lower delta O-18(SO4) values were indicative of ferric iron (Fe3+) as oxidant and the possible incorporation of O-16 into SO42- during its formation, most probably from snow melt or depleted rainwater. The delta S-34(SO4) values in the OZ (+2.3 to +2.4%) suggested SO42- from pyrrhotite oxidation in the UOZ which has been subsequently mobilised to the LOZ. Low delta S-34 fractionation (+0.2 to +1.9%) between SO42- in the OZ and pyrrhotite, as well as the low delta O-18(SO4) values in the LOZ suggested the complete oxidation of pyrrhotite by Fe3+, signalling that previously, a low pH (<3) prevailed in the tailings. Mineralogical observations confirmed that pyrrhotite was completely oxidised in the UOZ, with the formation of hydrous ferric oxides (HFOs) coatings. The observed current high delta O-18(SO4) and pH (3.9-4.5) values in the UOZ were attributed to decreased oxidation rate and silicate buffering, limiting the availability of aqueous Fe3+ and subsequent formation of HFOs. The delta S-34(SO4) signatures of the water-soluble SO42- in the TZ and UUZ suggested the dissolution of gypsum which precipitated from a leachate from the weathering of danalite in the UOZ. In the middle UZ, the delta S-34(SO4) (0.8 to +0.6%) and delta O-18(SO4) (1.8 to 1.0%) signatures corresponded to SO42- from a mixture of pyrite, pyrrhotite and chalcopyrite oxidation by O-2 at the LOZ (i.e. oxidation front). Negative delta S-34 fractionation values (3.0 to 1. 6%) between these minerals and the water-soluble SO42- were attributed to the potential formation of intermediate S species, due to the partial oxidation of the sulphides. Consequently, the S accumulation in the LOZ could be due to the likely formation of the intermediate S species and secondary pyrite identified in this zone. The lower UZ coincided with the groundwater table and registered consistent negative delta S-34(SO4) (2.6 to 1.8%) and delta O-18(SO4) (7.6 to 4.4%) values. These signatures were hypothesised to be controlled by SO42- from the mineralisation of organic S in peat underneath the tailings and/or H2S oxidation, with possible contribution from sulphide oxidation in the tailings. This study highlights the usefulness of delta S-34 and delta O-18 as tracers of geochemical processes and environmental conditions that have existed in the tailings.

Place, publisher, year, edition, pages
2019. Vol. 110, article id 104426
Keywords [en]
Skarn tailings, Yxsjoberg, Water-soluble sulphate, Danalite weathering, Sulphur isotopes, Carbon-bonded sulphur
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:su:diva-176594DOI: 10.1016/j.apgeochem.2019.104426ISI: 000494711200001OAI: oai:DiVA.org:su-176594DiVA, id: diva2:1377003
Available from: 2019-12-10 Created: 2019-12-10 Last updated: 2019-12-10Bibliographically approved

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