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 Yxsjöberg, Sweden, were chemically and isotopically (δ³⁴S and δ¹⁸O) characterised to reveal sulphate (SO₄²⁻) sources, sulphide oxidation reaction pathways and historical environmental conditions in the tailings. δ³⁴S was additionally used to trace the weathering of danalite [(Fe,Mn,Zn)₄Be₃(SiO₄)₃S], 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 pore-waters 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 δ¹⁸O_SO4 values (−3.0 to +0.1‰) whereas the underlying lower OZ (LOZ) showed S accumulation and lower δ¹⁸O_SO4 values (−4.6 to −4.2‰). The higher δ¹⁸O_SO4 suggested the role of atmospheric oxygen, O₂ (as oxidant), contribution of evaporated rainwaters and/or evaporation in the upper zones of the tailings. The lower δ¹⁸O_SO4 values were indicative of ferric iron (Fe³⁺) as oxidant and the possible incorporation of ¹⁶O into SO₄²⁻ during its formation, most probably from snow melt or depleted rainwater. The δ³⁴S_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 δ³⁴S fractionation (+0.2 to +1.9‰) between SO₄²⁻ in the OZ and pyrrhotite, as well as the low δ¹⁸O_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 δ¹⁸O_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 δ³⁴S_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 δ³⁴S_SO4 (−0.8 to +0.6‰) and δ¹⁸O_SO4 (−1.8 to −1.0‰) signatures corresponded to SO₄²⁻ from a mixture of pyrite, pyrrhotite and chalcopyrite oxidation by O₂ at the LOZ (i.e. oxidation front). Negative δ³⁴S 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 δ³⁴S_SO4 (−2.6 to −1.8‰) and δ¹⁸O_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 δ³⁴S and δ¹⁸O as tracers of geochemical processes and environmental conditions that have existed in the tailings.