Source, mobility and fate of critical Be, Bi, F and W from historical sulfidic-oxidic skarn tailings : Re-mining as remediation method?

Abstract: There is a potential risk that geochemical cycles of several critical metals will be affected in the pristine environment, when mining of these metals increases to meet the demand in green technology. The identification of critical metals is based on the economic importance and vulnerability to supply restrictions. In the past, naturally low concentrations in the environment, and instrumental analysis with higher detection limits, has limited research regarding several of these critical metals. However, to understand their geochemical behavior and potential environmental impact are of high importance to ensure a responsible development of mine waste- and water management.       Skarn ores can contain high amounts of Fe-sulfides, carbonates and fluorite, together with enriched concentrations of critical metals such as Be, Bi and W. Nevertheless, little attention has been paid to mine drainage from skarn tailings and their environmental impact, compared to tailings from sulfidic deposits. At Yxsjö mine site, Sweden, skarn tailings enriched in the major elements C, F, S (1.0, 1.9 and 1.2 wt.%.) and Be, Bi, and W (average 280, 500 and 960 ppm, respectively) were deposited in Smaltjärnen repository (1918-1963). The tailings were stored in ambient conditions until 1993 when the tailings were covered by sewage sludge. In-between 1969-1989, tailings were discharged into Morkulltjärnen repository, which was covered with sewage sludge and partly water saturated directly after closure. This thesis focuses on the Smaltjärnen tailings. The element distribution  in the tailings were identified by combining 1) total concentrations of nine targeted minerals from rock drilled cores, 2) total concentrations of 99 samples from four intact tailings cores, and 3) environmental mineralogy (EM) conducted on one of the cores. The environmental mineralogy included paste-pH, sequential extractions, optical microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), Raman vibrational spectroscopy and X-ray diffraction (XRD). Environmental mineralogy was also used to reveal geochemical processes affecting the mobility of elements in the tailings. Monthly water samples (May-October, 2018) were taken in three groundwater wells in the tailings, and at five surface water locations downstream the tailings. At three surface water locations, the diatom taxonomy response to the water quality was used to evaluate the impact on ecosystems. The quality of the mine drainage was compared to surface water downstream Morkulltjärnens repository and to a reference point. The overall results were used to evaluate the need for remediation, and particularly, the possibility to use re-mining as remediation method.        The Smaltjärnen tailings contained 88 wt.%. of Ca-rich silicates accompanied by minerals such as calcite [CaCO3], fluorite [CaF2], monoclinic and hexagonal pyrrhotite [Fe1-xS)], danalite [Be3(Fe4.4Mn0.95Zn0.4)(SiO4)3.2S1.4], scheelite [CaWO4] and bismuthinite [Bi3S2] (average 5.7, 3.6, 2.4, 0.3, 0.1 wt.%. and 0.1 wt.%., respectively). Both pyrrhotite and danalite had oxidized in the upper parts of the tailings down to 2.5m depth, and calcite had partly neutralized the acid produced resulting in a pH decrease from 8 to 4 in the upper parts of the tailings. Weathering of danalite was intensified by the more acidic conditions (pH<6), in which Be hydrolyzes. The lowered pH enabled dissolution of fluorite, resulting in severely high concentrations of F in the groundwater (average 73 mg/L) and surface water (average 1.6 mg/L). In the uppermost tailings, secondary gypsum [CaSO4], Al-complexes and hydrous ferric oxides (HFO) had formed.       The geochemical behavior of Be was complex in the tailings and in surface water downstream the tailings. According to the sequential extraction, Be released from danalite in the upper most tailings were present in water soluble phases, as exchangeable phases and had co-precipitated with Al- and Fe-oxyhydroxides. A strong correlation between Be, Ca and S in water soluble phases and in the surface water downstream the tailings indicated that Be partly substituted for Ca in secondary gypsum. In two groundwater wells, secondary precipitates of a white sludge containing Be, Al, F and Zn were found, indicating that Be was partly removed from the groundwater by Al-complexes. In the third groundwater well, the globally highest dissolved concentrations of Be were measured (average 4.5 mg/L), and in the surface water the concentrations (average 41 µg/L) were well above thresholds values for aquatic organisms (1 µg/L). In these pH-conditions (average 5.7-6.5) and oxygenated waters, Be is expected to precipitate as Be(OH)2 if complexing ligands are absent. A strong correlation between dissolved Be and F was found in the surface water, indicating that Be-fluorocomplexes had formed.       Bismuth and W have previously been considered as relatively immobile elements. However, the results showed that both Bi and W had partly been mobilized from their primary minerals (bismuthinite and scheelite) in the tailings. Weathered bismuthinite and scheelite grains with rims of goethite and water soluble phases of Bi and W were found in the deeper tailings with pH>7. The release of WO42- was hypothetically attributed to anion exchange with CO32- on surfaces of scheelite. Because, at the same depth where W was mobilized, solid C was accumulated and secondary orthogonal calcite was frequently detected with Raman spectroscopy. Bismuth was scavenged in the tailings by exchangeable phases and co-precipitation with HFO in the upper-most tailings. In the groundwater, Bi was just above the detection limit in all groundwater wells, while W was found in elevated concentrations. In the surface water, Bi and W were transported in the particulate phase together with Fe, and settled in the sediments a few 100 meters from the tailings outlet.       Surface water downstream Morkulltjärnen had a near-neutral pH (average 6.6) and of all elements analyzed, only dissolved W (average 1.1 µg/L) were high compared to threshold values (0.8 µg/L) and the reference sample. Dissolved Be, Ca, F and S from Smaltjärnen, and dissolved W from Morkulltjärnen were found in elevated concentrations more than 2 km from the mine site. Along this distance, metal tolerant diatom species (Achnanthidium minutissimum group II and Brachysira neoexilis, respectively) were dominant (>50%), indicating a negative impact on ecosystems. The mine drainage from Smaltjärnen had a larger negative impact on the diatom taxonomy with higher abundance of metal tolerant species, lower richness and evenness, more than 1% of deformed valves and the taxonomy was affected by the lower pH, compared to diatoms downstream Morkulltjärnen repository.       In conclusion, pyrrhotite oxidation was the direct or indirect cause of Be, Bi, F and W mobilization in the Smaltjärnen tailings, resulting in low quality mine drainage. The oxidation rate decrease with time, but weathering of the Smaltjärnen tailings is expected to be ongoing for hundreds of years since only a small part had weathered during the 50-100 years of storage. The low water quality and negative impact on diatoms, stress the need for remediation. Low concentrations of Be, Bi, Ca, F, Fe and S, accompanied by a near-neutral pH (average pH 6.6) downstream Morkulltjärnen, suggested that cover and water saturation could inhibit sulfide and danalite oxidation, and indirectly prevent fluorite weathering. However, high concentrations of dissolved W downstream Morkulltjärnen displayed that cover and water saturation can increase the mobility of W in the Smaltjärnen tailings, which needs to be taken into consideration. This thesis shows the importance of understanding the complex mineral and element matrix in skarn tailings before choosing remediation technique. Re-mining could be a beneficial remediation method since most W were found in intact scheelite grains. However, more research regarding the mineral processing and metallurgy is needed to ensure a sustainable extraction technique that separates sulfides, carbonates, danalite and fluorite, and deposits them in a proper way.