| 1. |
- Alakangas, Lena
(författare)
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Sulphide oxidation, oxygen diffusion and metal mobility in sulphide-bearing mine tailings in Northern Sweden
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Large quantities of sulphide-bearing mining wastes produced from ore processing are deposited throughout the world. Sulphide oxidation in the wastes may release acidic water with high concentrations of metals to the environment. Remediation strategies are usually site specific, since the physical and chemical properties of the wastes vary. Therefore, sulphide oxidation, oxygen diffusion and metal mobility in unoxidised and oxidised, remediated and unremediated wastes have been studied in the present work. The efficiency of different cover systems on unoxidised tailings from Kristineberg, were studied in pilot-scale test cells (5*5*3 m3)under field conditions. Clayey till, sewage sludge, apatite and Trisoplast were used as sealing layers and unspecified till as a protective cover. In one cell tailings were left uncovered. Unoxidised tailings in the test-cells in the initial stage after deposition showed relatively low sulphur release (600- 800 mg/l)in leachate waters, which probably was an effect of high moisture content in the tailings prior to deposition. Near-neutral pH found in the leachates was an effect of neutralisation by carbonate minerals present and lime (Ca(OH)2) added prior to deposition. Similar sulphur concentrations were found also in the uncovered tailings. The sulphide oxidation rate increased with time in the uncovered tailings, and decreased in the covered. The lowest oxygen concentrations were observed below the cover system with sewage sludge, which was the most effective barriar against oxygen in a short-term perspective. The oxygen fluxes through the clayey till and apatite layers were within the same magnitude and varied between 0.5 and 4 mole/year,m2. The Trisoplast layer seemed to have failed as a barrier against oxygen. Tailings studied at field scale at Laver and Kristineberg had oxidised for more than 50 years. The tailings at Kristineberg have high pyrite content (c.25% and 50%) and those at Laver have low grade of pyrrhotite (2-3%). The Laver tailings are unremediated, while at Kristineberg the tailings were remediated in 1996. The transport of metals in the drainage water at Laver decreased during a study period of 8 years. The transport of dissolved sulphur indicated a declining trend of sulphide oxidation rate in the tailings, which was confirmed by oxygen measurements in the tailings and weathering rate estimations. The decline was considered to be natural as a result of the increased distance that oxygen has to travel to reach unoxidised sulphide grains. The major part of the amounts of metals released by sulphide oxidation were secondarily retained in the tailings, and to a small extent in layers cemented by jarosite and Fe-(oxy)hydroxides. Sequential extraction of these layers showed that metals such as Cu and Pb were mostly associated with crystalline Fe-(oxy)hydroxides. Most important retention mechanism was, however, sorption onto minerals surfaces below the oxidation front. The studied Impoundment 1 at Kristineberg was remediated by two different methods; on one part a dry cover consisting of a sealing layer and a protective cover were applied, and the groundwater table was raised and a single dry cover applied on the other part. When the groundwater table was raised in oxidised tailings, secondarily retained metals such as Fe, Mg, Mn, S and Zn were remobilised resulting in increased concentrations in the groundwater. The concentrations declined with time, due to dilution by inflowing uncontaminated water. Decreased concentrations of Fe, Mg, Mn, S and Zn were observed also in the groundwater below the dry cover as the amount of percolating water decreased. The concentrations of trace elements such as Cd, Co, Cr, Cu, Ni and Pb were almost depleted in the groundwater, since these metals were retained within the tailings by mechanisms such as co-precipitation, precipitation and sorption. Analysis of pyrite grains by LA-ICP-SMS showed that pyrite surfaces were important for retention of As and Cu, in particular, but also for Cd and Zn. This study shows that the physico-chemical conditions expressed by pH and redox potential have a large impact on element mobility’s. For example, As was mobilised as a result of remediation, while the concentrations of most metals decreased in the drainage waters.
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| 2. |
- Hamberg, Roger
(författare)
-
Cementation of cyanidation tailings – effects on the release of As, Cu, Ni and Zn
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Knowledge about mineralogy and chemical composition in sulfidic tailings is essential to predict how tailings management may affect the future leachate quality. At a gold mine in the north of Sweden, gold was extracted from inclusions in arsenopyrite and pyrrhotite by the use of cyanide. Sulfides in the ore dissolved to a large extent during the cyanide leaching process causing sulfide-related elements such as As, Cu, Ni and Zn to be mobilized to a various extent. In a subsequent water treatment process, a significant proportion of As and Cu was captured in secondary formed Fe-precipitates. Large proportions of water-soluble Ni- and Zn-species in tailings suggested that this treatment was insufficient to reduce the mobility of Ni and Zn. Maintaining oxidized, neutral conditions is of major importance for the immobility of As, Cu, Ni and Zn during further management of the cyanidation tailings (CT).Part of the CT were planned to be managed in underground cavities by the use of a cemented paste backfill (CPB) -application. In CPB, a monolithic mass is formed as tailings are mixed with small proportions (4-7 weight %) of pozzolanic materials and backfilled into underground excavated areas. Using a CPB-application may decrease the sulphide oxidation rate, reducing exposure of mineral surfaces to oxygen and increasing water saturation levels within the material. In this study, CT was mixed with binders (1-3 wt. %) for the formation of a low-strength (0.2 Mpa) CT-CPB-mass. These mixtures were stored at moisturized conditions and subsequently subjected to oxidized and flooded conditions in a laboratory-based study. During short-term storing, high water saturation levels were preserved in the CT-CPB-mixtures, but, sulfide oxidation still progressed, and the release of Zn, Cu, and Ni was still lower compared to that in CT. The opposite was true for As, probably due to a desorption from Fe-precipitates. The desorbed As was subsequently incorporated into less acid-tolerant species (i.e. Ca-arsenates and As bonded to cementitious phases) in the CT-CPB:s, that readily dissolved and released more As compared to that in CT.A complete flooding of CPB-filled workings may take a long time to be reached. During this transition period, zones with low levels of water saturation forms in the CPB-monoliths, which could increase the sulphide oxidation rate, lower pH and dissolve the cementitious binders. In this study, strength decreased along with the water saturation levels in the CPB-mixtures, due to a more extensive pyrrhotite oxidation. A minimal proportion (1 wt. %) of binders did not suppress Cu and As leaching during flooding, but Ni and Zn-leaching were still lower than from CT. In the CT-CPB:s, proportions of As, Cu, Ni and Zn associated with cementitious phases increased in tandem with the fraction of binders. Using higher binder proportions in the CPB, as water saturation levels were lowered, substantially increased the Zn-release while there was an insignificant change in the As-release, and substantially lower Cu- and Ni-release. Pyrrhotite oxidation proceeded in the CT-CPB-mixtures independent of water saturation level. So, increasing binder proportion in a CPB does not necessary mean that trace metals are more stabilized, due to the formation of acid-intolerant fractions. Results from this study, pinpoints the importance of having knowledge about trace element distribution and mineral assemblage in tailings before management methods are chosen and implemented.
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| 3. |
- Hällström, Lina
(författare)
-
Source, mobility and fate of critical Be, Bi, F and W from historical sulfidic-oxidic skarn tailings : Re-mining as remediation method?
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 4. |
- Karlsson, Teemu
(författare)
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Geochemical and Mineralogical Characterization of Waste Rocks for Preliminary Mine Drainage Quality Prediction
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Acid or neutral rock drainage (ARD or NRD) with its attendant elevated concentrations of harmful elements presents one of the main challenges related to the management of waste rocks. Low-quality drainage is a particular issue with respect to mineral deposits containing sulfide minerals, of which pyrite and pyrrhotite are especially prone to produce acidic drainage when exposed to oxygen and water. The generation of low-quality drainage depends primarily on the composition of mine waste, in particular the proportions of acid-producing and neutralizing minerals, as well as the abundance of harmful elements bound to leachable mineral phases. To mitigate adverse environmental impacts, it is important to characterize waste rocks at an early phase of any given mining project. Early-phase characterization is needed in designing appropriate waste facilities, water treatment and closure techniques, and to investigate the potential possibilities for utilization of waste material. Several methods have been developed for characterizing waste rocks and for predicting their potential for generating low-quality drainage. These methods include static and kinetic testing, geochemical extractions, geochemical modelling, and the use of analogs from similar, older, mine waste sites. Geochemical extractions and static tests, such as acid-base accounting (ABA) and net acid-generation (NAG) tests, are commonly used for preliminary screening, and in selecting suitable samples for further testing. The assortment of these preliminary characterization methods should be expanded and their performance in ARD and element mobility prediction investigated further, to improve the accuracy of drainage quality prediction.The objective of this study has been to enhance waste rock management by developing tools for preliminary waste rock characterization and drainage quality prediction. An additional objective has been to improve the capacity for using geochemical and mineralogical data that have already been obtained during early phases of a mining project, and to provide general information about Finnish waste rock characteristics, so as to highlight the need for regional scale waste rock management and investigations. Accordingly, this study is based on waste rock and drainage samples collected from 19 Finnish active and closed mine sites, with the aim of assessing and comparing the performance of different methods for the preliminary prediction of drainage quality. The investigated acid potential (AP) methods included the ABA test in accordance with the established standard EN 15875, the single-addition NAG test as presented in the AMIRA guidebook, and an additional calculation based on SEM mineralogy. Furthermore, the suitability of seven different sulfur analytical methods for AP assessment was evaluated. The assessed methods for element mobility prediction included single-addition NAG test leachate analysis, as well as aqua regia (AR) and hydrogen peroxide ammonium citrate (HA) extractions, which are commonly used in mineral exploration to determine the concentrations of valuable elements bound to sulfide minerals.Based on the results, pyrrhotite was found to be the main sulfide mineral contributing to AP in the waste rocks at the investigated sites, with pyrite being the next in importance. The abundance of sulfide species other than pyrite in the waste rocks led to the realization suggested that the appropriate factor for defining the AP, based on multiplying the S content, should instead take into consideration the dominant sulfide species, rather than assume that all S is pyritic. Silicate minerals, especially biotite, were found to be important contributors to the neutralization potential (NP). The results suggested that the AP and NP calculations based on the SEM mineralogy serve as a useful tool in the ARD prediction, as they also reveal the source minerals for AP and NP. However, it is recommended to make use of EDS spectral data to verify that the S concentration calculated by modal mineralogy is in accord with total S based on the EDS sum spectra. The AR-extractable S concentration appeared to be a useful discriminant for determining the S concentration for the AP calculation, as it does not leach baryte, thus more accurately representing the S-content in sulfide.The most abundant harmful elements in the investigated waste rocks were Co, Ni, Cu, and Cr, whereas in the waste rock pile drainages the most prominent elements were Ni, Co, Zn and Cu. Results indicated that the use of the Finnish PIMA values (the threshold values defined in the Government Decree 214/2007 on the Assessment of Soil Contamination and Remediation Needs) in the waste rock characterization should be reassessed, especially for Cr, for which concentrations often exceeded the PIMA threshold values, even though they were not elevated in the corresponding drainage waters. Based on the measured drainage water concentrations, the AR and HA extraction methods appeared to be effective in the prediction of preliminary ARD quality. The AR extraction realistically reflected the abundances of elements that are likely to occur in elevated concentrations in the waste rock drainage water. However, this method overestimates contaminant mobilities in some circumneutral drainage cases, and the mobility of Cr in general. The HA extraction also performed well in the prediction of harmful element mobilities, with the exception of Al mobility in acidic drainage systems. The HA extraction appeared to be a more sulfide specific method compared with AR, which is evident for example, in better prediction of Cr mobility. The single-addition NAG test leachate analysis performed well in assessing the mobility of harmful elements, but only when the test leachate was acidic, as the elements appeared to co-precipitate when the NAG test leachate pH rises above 3-6.
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| 5. |
- Nyström, Elsa
(författare)
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Preventing Acid Rock Drainage Formation from Sulfidic Waste Rock Using Secondary Raw Materials
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the central and most challenging environmental problems related to mining is acid rock drainage (ARD) formation. The drainage is characterized by low pH and elevated concentrations of sulfate, metals, and metalloids formed when sulfide-bearing minerals are subjected to oxygen and water. Current remediation solutions, including active and passive techniques, have been developed to reduce ARD's negative impact. However, these treatments require continuous maintenance with an incessant addon of chemicals, energy consumption, not to mention long-term monitoring, until sulfide oxidation has ceased. Once it has been initiated, ARD formation could last for hundreds to thousands of years, making these approaches costly and unsustainable. A more strategic and environmentally sustainable approach would focus on preventing sulfide oxidation rather than treating its symptoms. This thesis explores five different secondary raw materials (SRM) (denoted below) for their use as amendments to prevent pyrite oxidation during storage. A combination of several mineralogical and geochemical methods was used to assess the materials' ability to maintain circumneutral pH when leaching pyritic waste rock (> 60% pyrite) in small-scale test cells (10L) to promote HFO precipitation on the pyrite surfaces. The oxidation of waste rock resulted in a drainage characterized by low pH (<2) and extensive element mobilization of up to 80% of the original content during the first two years. The results highlight the importance of trace element characterization and the need for early preventive measures to hinder or reduce the risk of acid drainage formation that requires active and costly long-term treatment. Conversely, adding 1-5 wt.% SRM to the waste rock created drainages with circumneutral pH and substantially lower sulfate and metal concentrations. However, not all materials could maintain circumneutral pH for an extended time, such as blast furnace slag (air-cooled and granulated) and cement kiln dust. These materials either require larger volumes of water to dissolve or contain minerals that allowed the material to harden upon water contact, inhibiting its neutralization capacity. Biomass bark ash showed a similar but less extensive, hardening effect resulting in a better ability to maintain circumneutral pH for more than two years despite its small addition (1-2.5wt.%). A similar ability was observed for lime kiln dust (5 wt.%). Conversely to lime kiln dust, the ash contained high soluble elements of potential concern, and its usage should be questioned despite only a temporary increase of elements through wash-out. However, the correlation between the amount of bark ash added and the timespan of circumneutral pH was not linear, resulting in the risk of prematurely declining pH if too little is added. Conversely, adding too much bark ash increases the risk of material hardening. One major concern with this treatment method is that it can inflate secondary minerals formation, leading to latent acidity and element release through their dissolution in changing geochemical conditions, such as wet or dry coverage measures. However, the addition of small amounts of SRM (1-4% of the waste rock's net neutralizing potential) to the waste rock dramatically improves the overall drainage quality without increasing the total amount of secondary minerals formed compared to no addition. In general, the type of secondary minerals formed on the waste rock without SRM treatment was considered less stable in an oxidizing environment than those formed through SRM treatment, suggesting that not treating the waste rock is inferior to SRM treatment both before and after covering measures.In conclusion, this thesis's results show that using small amounts of SRM can prevent oxidation during at least two years, likely due to HFO formation on the reactive surfaces. Consequently, it can substantially limit the need for treatment measures, both before and after remediation, decreasing the overall need and cost for chemicals, energy, and long-term monitoring, stressing the need for applying preventive measures during the storage time from mining to remediation. However, secondary minerals' long-term stability needs further evaluation and understanding before this method can be applied on a larger scale.
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| 6. |
- Salifu, Musah
(författare)
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Stable and radiogenic isotopes as tracers for geochemical processes in mineralogically-complex mine waste environments : Insights from 13C, 2H, 18O, 34S and 87Sr/86Sr.
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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