| 1. |
- Hällström, Lina, et al.
(författare)
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Geochemical characterization of W, Cu and F skarn tailings at Yxsjöberg, Sweden
- 2018
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Ingår i: Journal of Geochemical Exploration. - : Elsevier. - 0375-6742 .- 1879-1689. ; 194, s. 266-279
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Tidskriftsartikel (refereegranskat)abstract
- Little attention has been paid to tailings from skarn ore deposits and their environmental impact, even though they can contain elevated concentrations of elements of potential concern together with sulfides and fluorite. Historical skarn tailings at Yxsjöberg, Sweden, containing e.g. Be, Bi, Cu, F, Sn, S, W, and Zn were geochemically characterized as a first step to evaluate the environmental impact and the potential to re-mine the tailings. The tailings were deposited between 1897 and 1963 in the Smaltjärnen Repository without dams or a complete cover, and have been in contact with the atmosphere for >30 years. Four vertical cores throughout the tailings were taken and divided into 134 subsamples, which were analyzed for total concentrations and paste pH. Selected samples from different depths were mineralogically characterized using optical microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), Raman vibrational spectroscopy, and X-ray diffraction (XRD). Minerals, hand-picked from drilled rock cores, were analyzed for the element content, and a modified Element to Mineral Conversion (EMC) that pinpoints the quantitative distribution of elements between the minerals in the tailings was carried out. The average concentrations of Be, Bi, Cu, Sn, Zn, W, F and S in the tailings were 284, 495, 946, 559, 301, and 960 ppm, and 1.9 and 1.2 wt%, respectively. The tailings has reached a late stage development due to pyrrhotite oxidation resulting in low pH (<4) in the uppermost tailings, and formations of secondary minerals such as gypsum, hydrous ferric oxides (HFO) and orthogonal calcite. Secondary pyrite and magnetite, formed from monoclinic pyrrhotite was detected, and different weathering rates of secondary pyrite, hexagonal and monoclinic pyrrhotite was indicated, with secondary pyrite as the most stable and monoclinic pyrrhotite as the least. The rare and easily-weathered mineral danalite (Fe4Be3(SiO4)3S) was found in the drilled rock cores and by XRD in the tailings. However, the mineral could not be found by optical microscopy or SEM-EDS. This suggests that the mineral has been weathered to a great extent, which poses a high risk of releasing elements of potential concern to the groundwater since danalite contains approximately 40% of the total Be and Zn concentrations in the tailings. Fluorine was mainly found in fluorite, Cu in chalcopyrite, and Bi in bismuthinite; which all showed signs of weathering in acidic condition in the uppermost part, subsequent with decreased concentrations, followed by accumulation peaks deeper down in the tailings correlated with Al. Tungsten was mainly found in scheelite; most grains were unweathered, but a few grains had altered rims or HFO on the mineral surfaces. Tin was mainly found in ferrohornblende, hedenbergite and grossular. Beryllium, Cu, F, and Zn has high potential to be released to the surrounding environment from the Smaltjärnen Repository, while W, Bi and Sn are relatively stable in the tailings. Most of the scheelite is intact and re-mining could, therefore, be a suitable remediation method that would both reduce the environmental impact and simultaneously support the supply of critical raw materials in the EU.
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| 2. |
- Hällström, Lina, et al.
(författare)
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Metal Release from Acidic and Near-Neutral pH-Conditions in Historical W, Cu and F Skarn Tailings at Yxsjöberg, Sweden
- 2018
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Ingår i: 11th ICARD | IMWA | WISA MWD 2018 Conference. - 9780620806503 ; , s. 351-356
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Konferensbidrag (refereegranskat)abstract
- Weathering products stored in the pore water and/or as easily soluble salts in historical skarn tailings containing Be, Bi, Cu, W, and Zn, were released in water soluble fraction in the upper-most acidic tailings, at the visual oxidation front (1.5m), and/or below 2.5m (pH>7). Thus, there is a risk that these metals can pollute receiving waters by neutral mine drainage. Re-mining the tailings could be a suitable remediation method that would both reduce the environmental impact and simultaneously support the supply of metals.
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| 3. |
- Hällström, Lina, P.B
(författare)
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Geochemical Characterization of Historical W, Cu and F Skarn Tailings at Yxsjöberg, Sweden : With focus on scheelite weathering and tungsten (W) mobility
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Little attention has been paid to tailings from skarn ore deposits and their environmental impact, even though they can contain elevated content of elements of potential concern. Historical skarn tailings from a former scheelite mine at Yxsjöberg, Sweden, containing e.g. Be, Bi, Cu, F, Sn, S, W, and Zn were geochemically characterized as a first step to evaluate the potential environmental impact and if re-mining of the tailings can be a remediation option. Beryllium, Bi, F, and W are considered as elements of potential concern, and are at the same time listed by the European Commission (2017) as critical raw materials. Scheelite is considered as a relatively stable mineral but most research has been focused on extraction processes. A few laboratory studies have shown weathering of scheelite by artificial groundwater, where the release of WO42-was hypothesized to be due to anion exchange by CO32-. Thus, the release of W from scheelite should be favorable in skarn tailings due to the presence of carbonates.The tailings at Yxsjöberg were deposited between 1897 and 1963 in the Smaltjärnen Repository without dams or a complete cover, and have been in contact with the atmosphere for more than 30 years. Four vertical cores (P2, P4, P5, and P7) throughout the tailings were taken and divided into 134 subsamples, and analyzed for their total chemical composition and paste pH. Selected samples from different depths were mineralogically characterized using optical microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), Raman vibrational spectroscopy, and X-ray diffraction (XRD). Minerals, hand-picked from drilled rock cores, were analyzed for the element content, and a modified Element to Mineral Conversion (EMC) in combination with a 7-step sequential extraction of the tailings was used to pinpoint the quantitative distribution of elements between the minerals in the tailings. Scheelite weathering and secondary sequestration of W was studied in detail in one representative core. The release of elements to groundwater in the tailings and downstream surface water was measured monthly during 2018 to especially examine the mobility of W.The average concentrations of Be, Bi, Cu, Sn, Zn, W, F and S in the tailings were 280, 490, 950, 560, 300, and 960 ppm, and 1.9 and 1.2 wt.%, respectively. The uppermost tailings have been exposed to the atmosphere for more 30 years, with intensive pyrrhotite oxidation and carbonate dissolution, resulting in low pH (<4) condition, release of elements, and subsequent formations of secondary minerals such as gypsum and hydrous ferric oxides (HFO). Two different deposition periods were identified in one vertical profile. The later period of deposition extended from the ground surface down to 3.5 m, and the older from 3.6 to 6 m. Both periods had unoxidized tailings in the bottom and oxidized sulfides, depletion of calcite and HFO formations upwards.Tungsten was primarily found in scheelite and the abundance was 0.1 wt.% in the tailings at Yxsjöberg. Accumulation peaks of W were found with depth in P2, P4 and P7. In the upper peak of P4, 30% of the total W was associated with secondary HFO. This indicates that weathering of scheelite and secondary capturing of W had occurred in the tailings. At present time, a peak of W in water soluble phases where coinciding with accumulated C in the solid phase at 2.5m depth. Carbonate ions were released when calcite was neutralizing the acid produced in the upper oxidized tailings and the released CO32-was transported downwards to conditions with pH >7. There, it mainly had precipitated as secondary calcite, but small parts are hypothesized to have exchanged WO42-on the surface of scheelite,releasing WO42-to the pore water. In the oxidized tailings of the older and deeper tailing,goethite was found as rims around scheelite grains. This is probably due to Fe ions attracted to the negative surface of scheelite. This is suggested to decrease further weathering of scheelite, and also to some extent decrease the release of WO42-to the groundwater from above lying layers through adsorption.The changes in geochemical conditions in the tailings have decreased the water quality in receiving waters of Smaltjärnens Repository with increased concentrations of Ca, F, Fe,Mn, and SO42-. pH was on average 6.3 in the groundwater in the tailings and 5.7 in the surface water 300 m downstream. EC was on average 2.6 mS/cm in the groundwater and 131 μS/cm in the surface water. Low concentrations of dissolved W was found in the groundwater (max: 20 μg/l) in the tailings and in the downstream surface water the W concentration (max: 0.2 μg/l) was 20 times larger than the reference sample. These results show that scheelite is releasing W to a limited degree to downstream waters and are therefore a potential resource to re-mine to support the supply of critical raw materials in the EU.Beryllium, F, and Zn were released to the downstream surface water from the Smaltjärnen Repository, and to a higher degree than W. The rare and easily-weathered mineral danalite (Fe4Be3(SiO4)3S) contained approximately 40% of the total Be and Zn concentrations in the tailings and is suggested to be the major source to the release of Be and Zn. Fluorine was mainly found in fluorite which showed signs of weathering in the acidic condition in the uppermost oxidized tailings, subsequent with decreased content in the tailings. The mobility of these elements will be further studied.
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| 4. |
- Salifu, Musah, et al.
(författare)
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Strontium (87Sr/86Sr) isotopes: A tracer for geochemical processes in mineralogically-complex mine wastes
- 2018
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Ingår i: Applied Geochemistry. - : Elsevier. - 0883-2927 .- 1872-9134. ; 99, s. 42-54
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Tidskriftsartikel (refereegranskat)abstract
- Interpretation of geochemical data based primarily on elemental concentrations often lead to ambiguous results due to multiple potential sources including mineral weathering, atmospheric input, biological cycling, mineral precipitation and exchange processes. The 87Sr/86Sr ratio is however not fractionated by these processes. In this study, Sr isotope (87Sr/86Sr) ratios have been coupled with chemical data of Sr and Rb-bearing minerals, tailings and leachates (water-soluble) to gain insight into the geochemical processes occurring within the Yxsjöberg Cu-W mine tailings, Sweden. The tailings have been exposed to oxidizing conditions resulting in three geochemical zones namely (i) oxidized, (ii) transition and (iii) unoxidized zones. Leachates from the oxidized zone are acidic (pH = 3.6–4.5) and contain elevated concentrations of metals (e.g. Fe, Cu and Zn) and SO4. The low pH has also led to subsequent weathering of most silicates, releasing Al, Ca, Mg and Na into solution. The 87Sr/86Sr ratio in the tailings ranges from 0.84787 to 1.26640 in the oxidized zone, 0.92660–1.06788 in the transition zone, whilst the unoxidized zone has values between 0.76452 and 1.05169. For the leachates, the 87Sr/86Sr ratio ranges from 2.44479 to 5.87552 in the oxidized zone, 1.37404–1.68844 in the transition zone and 1.03697–2.16340 in the unoxidized zone. Mixing (between mineral weathering and atmospheric sources) was identified as the major process regulating the Sr composition of the tailings and leachates. The highly radiogenic signatures of the leachates in the oxidized zone suggests weathering of biotite, K-feldspar and muscovite. Despite the very radiogenic signatures in the oxidized zone, increments in Ca/K ratios, Be, Ce, Tl, Al, Fe and SO4 concentrations in the water-soluble phase were recorded in its lower parts which suggests the dissolution of amphibole, pyroxene, plagioclase, fluorite, gypsum, Al and Fe –(oxy) hydroxides as well as cation exchange by clay minerals. Presence of clay minerals has led to the partial retainment of radiogenic 87Sr/86Sr resulting in increased 87Sr/86Sr in the solid tailings material at these depths. The 87Sr/86Sr ratios of the water-soluble phase in the transition zone is similar to that of helvine and could indicate its dissolution. In the upper part of the oxidized zone, the 87Sr/86Sr ratios and trends of Be, Ca, SO4, Tl and Zn in the water-soluble phase suggest the dissolution of gypsum which precipitated from a leachate with the isotopic signature of helvine. In the lower part of the unoxidized zone, elevated concentrations of W were recorded suggesting scheelite weathering. But the 87Sr/86Sr ratios are higher than that expected from dissolution of scheelite and indicates additional processes. Possible sources include biotite weathering and groundwater. This study reveals that when interpreting geochemical processes in mine waste environments, 87Sr/86Sr should be considered in addition to chemical constituents, as this isotopic tracer offers better insights into discriminating between different solute sources.
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