Stable and radiogenic isotopes as tracers for geochemical processes in mineralogically-complex mine waste environments : Insights from 13C, 2H, 18O, 34S and 87Sr/86Sr

Sammanfattning: Mining and its related activities generate large volumes of mine wastes such as tailings that can have negative environmental implications. One of such mine wastes of potential environmental concern is the historical Yxsjöberg Cu-W-F skarn tailings in Sweden, which encompasses a complex mineralogy including sulphides, carbonates, silicates, oxides / wolframite and halides. Some of these minerals contain high contents of potentiallytoxic elements such as Be, Bi, Cu, F, Zn and W; hence posing a significant threat to surrounding soils, aquatic ecosystems and drinking water quality due to their weathering. Potential remediation strategies for this site require a detailed understanding of the mobilization and transportation of contaminants in and from the tailings to the surrounding environments. Therefore in this present work, chemical and isotopic (18O, 13C, 34S, 87Sr/86Sr) composition of minerals, tailings and water-soluble (WS) fractions of the tailings were used to gain comprehensive insights into geochemical processes including mineral weathering and precipitation, trace elemental sources and sulphide oxidation reaction pathways within the tailings. Furthermore, chemical composition and water mixing analyses, aimed at quantifying the elemental contributions of the tailings and tailings groundwater to the downstream surface waters were carried out using 18O, 2 H and 87Sr/86Sr isotope data of ground and surface waters collected during 6 different sampling campaigns from May to October 2018. Subsequently, the consistency of the mixing analyses was evaluated via a simple model. Biogeochemical processes regulating the stable carbon isotope signatures of dissolved inorganic carbon; DIC (δ13CDIC) of the ground and surface waters were also investigated. Three distinct geochemical zonation namely; oxidised (OZ), transition (TZ) and unoxidised zones (UZ) based on pH, elemental contents and colour, were observed in the tailings as a result of their long term storage and exposure to oxidising conditions. The OZ was characterised by a low pH (3.6 - 4.5), depletion of S as well as Be, Co, Cu and Zn in both the bulk tailings and WS fractions, particularly in the upper OZ (UOZ). On the other hand, the WS fractions of the lower OZ (LOZ), which included the oxidation front, recorded elevated concentrations of these trace metals and SO4 2- .Mineralogical observations and elemental contents of the tailings as well as the 34S and 18O signatures of SO4 2- ( 18OSO4 and 34SSO4) of the WS fractions in the OZ, TZ and upper UZ (UUZ) pointed to the historical and extensive weathering / oxidation of danalite and pyrrhotite by Fe3+ (i.e. at pH ≤ 3) in the UOZ, particular during their early stages after deposition when the sulphide surfaces were fresh. Very radiogenic 87Sr/86Sr ratios coupled with elevated concentrations of silicate-associated elements such as Al, Fe, K and Mg in the WS fractions of the OZ indicated the weathering of biotite, K-feldspar and muscovite. Weathering of Ca-bearing minerals as well as the dissolution of secondary minerals (e.g. gypsum) in the LOZ resulted in high Ca/K ratios. In the TZ and UUZ, the WS fraction 87Sr/86Sr and 34SSO4 values as well as consistent peaks of Ca, Be, Mn, SO4 2- and Zn suggested the dissolution of gypsum with a similar isotopic composition as danalite. Danalite was weathered in the OZ and hence Be was assumed to have been mobilised from this zone and trapped secondarily in gypsum in the UUZ. The 18OSO4 and 34SSO4 signatures of the WS fractions in the middle UZ suggested their mobilisation from the current oxidation front and represented mixed signals from the incomplete oxidation of pyrite, pyrrhotite and chalcopyrite via atmospheric oxygen (O2), resulting in the potential formation of intermediate sulphur species such as elemental S. Negative 18OSO4 and 34SSO4 signatures of the lower UZ were attributed to their probable release from processes such as carbon-bonded sulphur mineralisation and hydrogen sulphide (H2S) oxidation in the forests surrounding the mine site. Recorded negative δ13C signatures of some carbonates (average = -2.7 ‰) in the tailings compared to that of primary calcite (δ13C= +0.1 ‰)signaled the precipitation of secondary carbonates. The δ13C values of these secondary carbonates were ascribed to a mixture of C sources from atmospheric CO2, degraded organic matter and primary calcite dissolution in the tailings. The water mixing analyses indicated that the elemental contributions of the tailings groundwater to the downstream surface waters were small (1 -17 %), resulting in low dissolved concentrations of various elements in the latter relative to that of the former and thus a low negative environmental impact. The results of the mixing analyses and the model suggested that the low elemental concentrations of the surface waters were due to various potential retention mechanisms such as precipitation, sorption and reductive processes within the tailings and at the outlet of the tailings. The δ13CDIC values of the groundwater samples were attributed to mixed C signals from the primary calcite and potential secondary carbonate dissolution in the tailings as well as degradation of the vegetation and sewage sludge on the tailings, the peat underneath the tailings and the surrounding forests. The δ13CDIC signatures of the downstream surface waters seemed to be dependent on the climatic seasons and groundwater contributions. However, limitations with respect to unavailable data on DIC concentrations as well as a myriad of potential biogeochemical processes that could influence the DIC pool and δ13CDIC values of the surface waters made it difficult to pinpoint the major regulating process (es) of the δ13CDIC signatures. Nonetheless, the results of this study shows that the use of the 18O, 34S, 87Sr/86Sr and to a lesser extent 13C isotopes, coupled with the chemical and mineralogical data offer better insights into discriminating between various elemental sources and related geochemical processes, especially in mineralogical-complex setting such as skarn tailings. Furthermore, the results of the mixing analysis and the model data emphasises the importance of a thorough understanding of the hydrogeochemical processes along groundwater flow paths, as these processes can modulate the amount of dissolved elements reaching the surface waters.