Authigenic iron oxyhydroxide rims attenuate deleterious element fluxes during sulphide oxidation in historical gold mine tailings

Authigenic iron oxyhydroxide rims attenuate deleterious element fluxes during sulphide oxidation in historical gold mine tailings | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Authigenic iron oxyhydroxide rims attenuate deleterious element fluxes during sulphide oxidation in historical gold mine tailings Steve Jason Chingwaru, Bjorn Heyden, Margreth Tadie This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6884307/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Historical gold tailings pose environmental hazards globally, leading to acid mine drainage and the release of harmful elements due to the oxidation of unrecovered sulphides. The metastable secondary iron oxyhydroxides formed during this process influence the mobility of these elements, although the specific mass flows between the sulphide cores, iron oxyhydroxide rims and the environment remain poorly understood. Using a gold tailings storage facility in the Klerksdorp goldfields (South Africa) as a natural laboratory, this study investigates the role of secondary iron oxyhydroxide rims in controlling the mobility of Co, Ni, As, Pb, Zn, Au, and Cu after prolonged exposure of tailings to surface conditions. The release versus retention of these deleterious elements is characterised and quantified using a multi-method approach including LA ICP-MS, automated mineralogy, and wet chemistry. Although iron oxyhydroxide rims make up less than 1% of the mineralogy, they strongly retain As, Ni, Cu, and Zn fluxes that emanate from the precursor sulphides. In contrast, Co, Au, and Pb show limited compatibility (< 3% deportment) with these rims, suggesting their preferential mobilisation into the environment. The measured immobilisation of some of these elements is insufficient to meet environmental standards, highlighting the need for direct remedial measures, including metallurgical reprocessing and effective capture mechanisms for deleterious elements. These strategies are essential for reducing environmental risks and can simultaneously recover valuable (and critical) metals such as Au, Cu, Co, and Ni. Earth and environmental sciences/Environmental sciences Earth and environmental sciences/Solid earth sciences Physical sciences/Chemistry Physical sciences/Engineering/Chemical engineering Gold tailings Deleterious elements Iron oxyhydroxide Oxidation Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction The global minerals sector generates billions of tons of mine waste (e.g., tailings) annually 1 . While modern tailings storage facilities (TSFs) are engineered to mitigate environmental risks, many historical TSFs were poorly designed and now serve as significant point sources of contamination. In South Africa, the world-class Witwatersrand goldfields have been mined continuously since 1886, yielding over 52,000 tons of gold and more than 6 billion tons of sulphide-rich mine waste 2 – 4 . These sulphides, predominantly composed of pyrite (FeS₂), host significant concentrations of deleterious trace metals and metalloids e.g., As, Cd, Co, Se, Zn 5 and readily oxidise upon exposure to meteoric fluids and surface conditions. This oxidation leads to the formation of metastable iron oxyhydroxide and, to a lesser extent, sulphate minerals, releasing acidity (H⁺), sulphate (SO₄²⁻), and trace elements collectively known as acid mine drainage (AMD) 6 . In the early stages of oxidation, sulphide minerals are preferentially replaced by iron oxides along cracks and grain boundaries, forming characteristic rim textures (Fig. 1 a). As oxidation progresses, sulphides are completely replaced by authigenic iron oxyhydroxides, forming gossans 7 . The adverse environmental and human health effects of AMD are well-studied 8 , 9 , with case studies documented in the Witwatersrand goldfields, e.g. 10 , 11 and numerous other sulphide-bearing ore deposits worldwide, e.g. Avarenga et al . 12 and Konanç & Değermenci 13 . Despite extensive research on AMD and the established role of iron oxyhydroxides in trace element sequestration through adsorption and isomorphic substitution 14 , 15 . Studies on their effectiveness in mitigating harmful element fluxes from sulphide oxidation remain limited 16 . The historical tailings of the Witwatersrand goldfields, subjected to prolonged weathering, provide an ideal natural laboratory for investigating the retention and release of deleterious elements during the sulphide-to-oxide transition. This research employs in-situ laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS), which offers high sensitivity for trace element detection. Unlike controlled lab-scale studies that examine relatively constrained geochemical systems, this approach captures the complexity of natural weathering processes and provides empirical insights into trace element mobility during sulphide oxidation. 2. Materials and methods 2.1 Study areas, sample preparation and preconcentration Approximately 150 kg of tailings material was collected from eleven boreholes that were sampled from the oxidised zone (10 m depth) of a 3,95 km 2 TSF in the Klerksdorp Goldfields. Further details of the sampling site and the sample collection and preparation protocols are provided in the supplementary material (SM) and by Chingwaru et al. 17 . To concentrate a significant amount of sulphide and oxide minerals (the heavy fraction; density > ~ 2.95 g/cm³) for analysis, a gravity separation pre-concentration process was employed. This comprised a sequence of steps entailing natural settling, decantation, super-panning and heavy liquid separation. Aliquots of both the bulk tailings sample and the heavy mineral fraction were mounted into 30 mm epoxy resin mounts (duplicate samples prepared of each) and polished to a 1 µm finish. 2.2. Mineralogical and chemical analyses The bulk and heavy mineral fraction mounts underwent initial evaluation using reflected light optical microscopy, whereafter their modal mineralogy was analysed using a quantitative evaluation of materials by scanning electron microscopy (QEMSCAN) at the University of Cape Town. The Zeiss EVO MA15VP scanning electron microscopy (SEM), located at the Central Analytical Facility (CAF) at Stellenbosch University, equipped with an Oxford Instruments Wave Dispersive X-ray Spectrometer (WDS) and appropriate standards, was used to determine major and minor element concentrations in the pyrite cores and iron oxyhydroxide rims. These data were complemented with measurements of trace element (e.g., As, Au, Co, Cu, Pb, Zn) concentrations analysed using LA ICP-MS with a 193 nm Excimer laser from Applied Spectra coupled with an Agilent 7700 Q ICP-MS with quality control and calibration done with relevant standards processed in (Laser Ablation Data Reduction) LADR software (see SM). Sulphide grains with oxidised rims ≥ 10 µm wide were selected for in-situ ablation analysis (10–20 µm spot sizes; Fig. 1 a), resulting in 111 ablation spots between the pyrite core and adjacent iron oxyhydroxide alteration rim. To determine acid-leachable element deportment, duplicate 1 g composite tailings aliquots were analysed for As, Au, Cu, Fe, Ni, Pb and Zn using microwave-assisted aqua regia digestion at 60°C for 6 hours. Measurements on an Agilent 7900 ICP-MS were calibrated with certified reference materials WQB-1 and PTM-1a. Gold was included because of its economic significance 18 , 19 and because of its potential toxicity in nanoparticle form 20 . 2.3. Calculations To evaluate element mobility during the oxidation of sulphides, an oxidative loss coefficient (LE) was calculated using Eq. 1 , following the approach of Diao & Wen 21 and Lu et al. 16 . Where C py represents the average element concentration in unaltered pyrite cores, and C ox represents the average element concentration in the surrounding iron oxyhydroxide rims, both determined via LA-ICP-MS analysis. The LE calculation is based on the following assumptions: 1) The total iron mass remains constant during pyrite oxidation, as Fe is a major component of pyrite and is considered immobile during weathering 16 . The oxidation of pyrite follows the reaction: 4FeS 2 + 15O 2 + 14H 2 O → 4Fe (OH) 3 + 8H 2 SO 4 (2) This reaction suggests that iron oxyhydroxide is the primary product of pyrite oxidation, however, natural systems are complex, and the absolute mineralogy of the oxidation rims is likely a mixture of poorly crystalline Fe phases (see SM for further details). In calculating the LE values for each pyrite core-rim pair (Fig. 2 ), the unaltered pyrite core is assumed to represent the chemistry of the rim, before its transformation into iron oxyhydroxide. A positive LE value indicates element leaching into the surrounding TSF, while a negative LE value signifies local element enrichment within the iron oxyhydroxide rim. Mass balance calculations were conducted to determine the elemental distributions (deportment) for metals/metalloids of interest among minerals present in the TSF material. These calculations were performed using Eq. 2, modified after Chingwaru et al . 18 : 3 Where m total represents the total mass of the element in the tailings sample, calculated from the bulk sample mass and total metal concentration obtained via aqua regia digestion and ICP-MS analysis. m phase represents the mass of the element hosted within specific mineral phases, determined using automated mineralogy and mineral chemistry data from LA-ICP-MS ablation analysis. 3. Results & Discussion 3.1 Redistribution of trace elements during sulphide oxidation The QEMSCAN analyses indicate that pyrite is the dominant sulphide mineral, constituting up to 1.3% of the bulk sample mineralogy. In contrast, other identified sulphides, including arsenopyrite, galena, chalcopyrite, sphalerite, pentlandite, and pyrrhotite, are present only in minor amounts, collectively accounting for approximately 0.2% of the modal mineralogy (see SM). Among these, pyrite is the only sulphide exhibiting notable oxidation along its rims (Fig. 1 a). Electron microscopy confirms that these oxidation rims consist of iron oxyhydroxides/oxides, though their precise mineralogical composition remains less well-constrained. They likely comprise a mixture of Fe-oxyhydroxide polymorphs (e.g., goethite) and poorly crystalline species (e.g., ferrihydrite). The trace element concentrations within the iron oxyhydroxide rims exhibit high variability, with mean values and standard deviations consistent with previous studies 17 : Co (1211.58 ± 2556.56 ppm), Ni (1182.10 ± 2251.07 ppm), As (713.18 ± 469.11 ppm), Pb (418 ± 2870.84 ppm), Zn (18.22 ± 57.15 ppm), Au (4.51 ± 28.86 ppm) and Cu (28.24 ± 51.14 ppm) (Fig. 1 b). These baseline concentrations, combined with LA-ICP-MS analyses of adjacent iron oxyhydroxide rims (Fig. 1 a), were used to calculate the oxidative loss coefficient (LE) (Eq. 1 ; Fig. 2 ). The LE values proxy for the relative mobility or retention of deleterious elements during sulphide oxidation. The oxide rims exhibit significantly lower concentrations and consequently, positive LE values for Co (227.70 ± 509.91 ppm), Au (0.31 ± 0.46 ppm), and Pb (53.15 ± 89.43 ppm) compared to their respective concentrations in pyrite cores (Fig. 1 b). In contrast, the oxide rims show elevated concentrations of Cu (262.57 ± 352.48 ppm) and Zn (531.28 ± 1986.84 ppm) which are notably higher than the corresponding cores (negative LE values in Fig. 2 ), whereas the concentrations of Ni (890.09 ± 1535 ppm) and As (505.57 ± 1144.16 ppm) show only slight, insignificant levels of relative enrichment in the rims. Pyrite, a primary mineral that likely crystallised at least 2.7 billion years ago, incorporates trace elements within its crystal structure through direct substitution (e.g., Ni²⁺, Co²⁺ for Fe²⁺; As⁻ for S⁻), coupled substitution reactions (e.g., Au⁺ + Bi³⁺ → 2Fe²⁺), or nanoscale inclusions (e.g., Pb as galena) 5 . In contrast, authigenic oxide rims consist of small crystallites with a high surface-area-to-volume ratio. Their trace element concentrations are controlled by surface adsorption and ionic substitution within the mineral structure. Elements such as Au and Pb are poorly retained due to their significant ionic size and charge mismatches with Fe³⁺ crystallographic sites in tetrahedral and octahedral coordination 22 , 23 (Fig. 2 ), as well as their limited adsorption affinity for Fe oxyhydroxide surfaces. Under ideal conditions, Co can substitute into goethite at levels up to ~ 8 mol% 24,25 ; however, this degree of incorporation was not observed in our samples. This may be due to its reduced affinity for Fe oxyhydroxide surfaces when competing with other ions such as As⁵⁺ 26 , 27 . The remaining elements exhibited negative LE values, indicating preferential retention within Fe oxyhydroxide rims rather than environmental mobility. Cu and Zn are strongly retained, likely through the formation of inner-sphere complexes, which are highly ph-dependent 28 , 29 . Retention may also be enhanced by the high Mn concentration in the rims (~ 0.4 wt.%; SM S3), which can modify the crystal structure and charge balance, necessitating divalent ion incorporation for charge compensation and stabilisation 30 . Ni and As, representing cationic and anionic substituents in pyrite, also remain in the solid phase rather than mobilising into the environment. Ni²⁺ exhibits a relatively small ionic radius difference from Fe³⁺ (~ 7%; Fig. 2 ), and up to 75% of total Ni in natural goethite has been shown to substitute directly for Fe 24 . In contrast, As does not replace Fe in iron oxyhydroxides due to significant differences in ionic radius and charge (Fig. 2 ). Instead, it binds strongly to these minerals through surface complexation and co-precipitation 32 . Under oxidising conditions in the tailings, As⁵⁺ adsorption is favoured, forming stable inner-sphere complexes with surface hydroxyl groups 32 , 33 . Over time, these adsorbed As⁵⁺ species become structurally incorporated into the mineral matrix, further enhancing their retention. 3.2 Mass balance calculations for the TSF The geochemical distribution and behaviour of deleterious elements within pyrite and its associated iron oxyhydroxide alteration rims (Section 3.1 ) emphasise the importance of understanding the overall mineralogical deportment in tailings. The elemental deportment is crucial for assessing the environmental mobility and availability of these elements. Total elemental concentrations from aqua regia digestible phases, along with concentrations by mineral phase (Fig. 2 ), were used in mass balance calculations to determine the deportment of deleterious elements (Fig. 3 ). A major proportion of deleterious elements are hosted within the tailings sulphide minerals (Fig. 3 ). Pyrite hosts significant proportions of As (26%), Co (39%), Ni (25%), Au (45%), and Zn (29%), but only minor amounts of Cu (0.5%) and Pb (3%). Other sulphides, mainly chalcopyrite, galena, and arsenopyrite, contain substantial amounts of Cu, Pb, and As, respectively. Although representing as little as 1% of the total mineralogy, the iron oxyhydroxide rims host about 14% of As, 5% of Co, 3% of Cu, 15% of Ni, 0.3% of Pb, and 12% of Zn (Fig. 3 ). Given that most of this deleterious element concentration likely derives from the precursor pyrite, our study underscores the natural attenuation process operating in the evolving Witwatersrand TSF system, whereby the authigenic rims are responsible for significant deleterious element sequestration. The mass fraction of deleterious elements not accounted for by these mass balance calculations (up to 40%) is attributed to discrete phases of unknown speciation. Further studies are required to elucidate a more comprehensive deportment for each deleterious element of interest. 4. Scientific, Environmental, and Economic Significance The multi-scaled analytical approach, integrating LA ICP-MS (low detection limits, small spot size, and simultaneous trace element measurement) with bulk mineralogical techniques (QEMSCAN and wet chemistry), provides critical insights into the role of authigenic iron oxyhydroxide rims in attenuating deleterious element fluxes in a TSF from the historical Witwatersrand goldfields at summarised in Fig. 4 . Assuming conservation of Fe during pyrite oxidation, the rims are predicted to sequester up to 15% of the initial As, Cu, Zn, and Ni contained within the precursor pyrite (Fig. 4 ). However, the lack of speciation data for both the iron oxyhydroxide rims and associated contaminants limits the ability to assess sequestration longevity. For example, previous studies indicate that metals such as Ni, Zn, and Cd can become irreversibly bound to goethite within 2 to 120 days, with some fraction structurally incorporated over time 34 . The ultimate long-term stability of this contaminant sink will depend on factors such as bond strength, structural transformations, ageing of iron oxyhydroxides, cationic substitutions affecting solubility, and ambient TSF physicochemistry (e.g., pH, redox potential, ionic strength). The formation of thick iron oxyhydroxide rims (Figs. 1 & 4 ) also plays a secondary role in controlling oxidation and contaminant release by limiting oxygen diffusion to the sulphide cores 35 , 36 . As the ambient geochemistry dictates iron oxyhydroxide stability and associated sorption-desorption reactions, our measured concentrations and mass balance calculations are most relevant to the sampled upper 10 meters of the TSF. At greater depths, if reducing conditions develop as depicted in Fig. 4 , rim stability may decrease, potentially releasing incorporated elements. This suggests a depth- and spatially dependent control on trace metal concentrations and speciation within the TSF, e.g. 37 , 38 . Understanding this variability could inform targeted remediation strategies whereby oxidised zones are managed differently to reduced zones (where contaminants remain in the sulphide host), and to zones of intermediate redox potential (where the adsorption mechanisms and efficacies of developing oxide rims will be nuanced). From an economic perspective, 45% of the Au in the sample material is associated with sulphides, with only a small fraction sequestered by iron oxyhydroxides (LE = 93; Fig. 2 ). This suggests that Au, along with Co and Pb, is mobilised and potentially accumulates in deeper reduction zones within the TSF. Such redistribution will influence the Au grade if TSFs are considered secondary resources for sustainable metal recovery 18 . 5. Conclusions The study presents a practical methodology for obtaining empirical measurements of acid mine drainage fluxes from historical gold TSF, accounting for the natural attenuation by authigenic oxide rims. These findings underscore the importance of a targeted rehabilitation strategy to mitigate environmental risks and realise the economic potential of such tailings. Developing tailored metallurgical reprocessing flowsheets focused on recovering gold, sulphides, and iron oxyhydroxides is essential for a full recovery of associated deleterious elements. This deportment-informed approach not only safeguards ecosystems and public health but also facilitates the recovery of critical metals like Cu, Co, and Ni, which are required for the green energy transition. Declarations Acknowledgements This research was funded by the DST-NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis (CIMERA) and the Society of Economic Geologists (SEG) Tim Nutt Fund. Thank you to the analytical staff at CAF, Stellenbosch University and the QEMSCAN unit, University of Cape Town. We further extend our thanks to the mining house Harmony Gold Limited for sample donations. Data Availability The datasets generated in this study (see methodology for the techniques used to generate the datasets) are available on the Zenodo research repository at DOI: 10.5281/zenodo.15588324 is assessable using the link: https://zenodo.org/records/15588324?token=eyJhbGciOiJIUzUxMiJ9.eyJpZCI6ImQzMDE2OWY2LWMyMzYtNGQ0Zi1hZGIyLWI2MTcyMjI3OGNjMCIsImRhdGEiOnt9LCJyYW5 kb20iOiI5MWRhNWIwNTgxNzYwZGU4 MzUwYWE1NDRhN2ZlMDE1OCJ9.ncCKCG-vebkEOK6bizjQT1N352ryJ4gAKxza8 L3wIl8Nyasc2qxRqjhdqgGEuvc3-ezdxWnqvM5c1K5q4Awxxw . References Araujo, F. S. M., Taborda-Llano, I., Nunes, E. B. & Santos, R. M. Recycling and Reuse of Mine Tailings: A Review of Advancements and Their Implications. Geosciences (Switzerland) 12 , (2022). Frimmel, H. E. A Giant Mesoarchean Crustal Gold-Enrichment Episode : Possible Causes and Consequences for Exploration. Society of Economic Geologists 18 , 209–234 (2014). Handley, M. Where is all the gold? 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Environmental Science and Technology 58 , 2007–2016 (2024). Huminicki, D. M. C. & Rimstidt, J. D. Iron oxyhydroxide coating of pyrite for acid mine drainage control. Applied Geochemistry 24 , 1626–1634 (2009). DeSisto, S. L., Jamieson, H. E. & Parsons, M. B. Influence of hardpan layers on arsenic mobility in historical gold mine tailings. Applied Geochemistry 26 , 2004–2018 (2011). Ali, J. D. et al. Occurrence and mobility of thiolated arsenic in legacy mine tailings. Science of the Total Environment 929 , 172596 (2024). Rodríguez-Pacheco, R., Butlanska, J. & Oliva-González, A. O. Geotechnical and Hydrogeological Zonation of Tailings Storage Facilities: Importance for Design, Construction, Operation, and Closure. Minerals 15 , 105 (2025). Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6884307","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":473267808,"identity":"b48f60fc-2087-4868-bd03-268af9f265c3","order_by":0,"name":"Steve Jason Chingwaru","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6ElEQVRIiWNgGAWjYHACNoYECIPxAZg6QIIWZgPitcAYEkRpkW9gfvbg4Y7axP7ZZ8yqecoY5PhuJDB+5sGjxeAAm7lB4pnjiTPO5Zjd5jnHYCx5I4FZGq8WBh42icS2Y7kNZ3jMbvO2MSRuuJHAgFeLfANUy3yglmKglnqgFubf+LQwHABrqcndANTCDNSSYHAjgQ2/ww6zmQG1HKjfeIatWHLOOQnDmWcetlnOweew9uZnkj/b6ozlzjBv/PCmzEae73jy4Rtv8DmMGUwehvLAUcPYgE8DDNTBtBCjeBSMglEwCkYaAAAek0jTGDg/OQAAAABJRU5ErkJggg==","orcid":"","institution":"Stellenbosch University","correspondingAuthor":true,"prefix":"","firstName":"Steve","middleName":"Jason","lastName":"Chingwaru","suffix":""},{"id":473267810,"identity":"5084228e-62b8-496e-b948-48c680792485","order_by":1,"name":"Bjorn Heyden","email":"","orcid":"","institution":"Stellenbosch University","correspondingAuthor":false,"prefix":"","firstName":"Bjorn","middleName":"","lastName":"Heyden","suffix":""},{"id":473267811,"identity":"0d953662-e0b5-4564-9245-0bd3b7b5d5f5","order_by":2,"name":"Margreth Tadie","email":"","orcid":"","institution":"Stellenbosch University","correspondingAuthor":false,"prefix":"","firstName":"Margreth","middleName":"","lastName":"Tadie","suffix":""}],"badges":[],"createdAt":"2025-06-13 03:38:23","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6884307/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6884307/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":84994496,"identity":"71b1146b-8fd0-413b-a861-41e407e1dc7c","added_by":"auto","created_at":"2025-06-19 15:58:55","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":544016,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a)\u003c/strong\u003eProvides a zoomed-in view of the grain after ablation of the pyrite core and iron oxyhydroxide rims. \u003cstrong\u003e(b)\u003c/strong\u003e The spider diagram illustrates the average concentration of trace elements in the two distinct mineral phases (n = 111).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6884307/v1/66cf341abc212c0ca35a8a39.png"},{"id":84994498,"identity":"cea4ea48-ce17-425a-9cd8-d48451e1b10f","added_by":"auto","created_at":"2025-06-19 15:58:55","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":804553,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of the ionic radii (in angstroms, Å) and charge of the elements analysed in this study relative to Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e in octahedral coordination \u003csup\u003e31\u003c/sup\u003e. Stippled and shaded areas encompass ±15% of the ionic size and ±1 valence for Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e (typical coordination environments given by inset ball-and-stick diagrams). The colour bar provides the average calculated oxidative leaching coefficients for the respective elements for pyrite core measurements paired with the associated iron oxyhydroxide rim.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6884307/v1/6e2f5d0aea9f0677b82d46b4.png"},{"id":84995018,"identity":"fe909929-1724-4c36-b412-206515545c06","added_by":"auto","created_at":"2025-06-19 16:06:55","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":377186,"visible":true,"origin":"","legend":"\u003cp\u003eA stacked percentage bar chart displaying the bulk deportment of elements across each mineralogical fraction. Other sulphides include arsenopyrite, galena, sphalerite, chalcopyrite, pentlandite, and pyrrhotite. Discrete phases refer to the proportion of the element not accounted for during the mass-balance calculations. This proportion is predicted to reside in mineral phases not evaluated using the LA ICP-MS (e.g., silicates, carbonates, phosphates, etc.).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6884307/v1/0c3ab408e1083adf81c8946d.png"},{"id":84994502,"identity":"aea6d464-8e6e-458c-a94c-29d443954fd7","added_by":"auto","created_at":"2025-06-19 15:58:55","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":619991,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical summary of the role of authigenic iron oxyhydroxide rims in selectively attenuating deleterious elements within the upper 10 metres of the oxidising zone in historical gold tailings\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6884307/v1/a309b32853328d09267f2433.png"},{"id":92501040,"identity":"2addc474-ec41-40e2-94fe-dc3c32d2084a","added_by":"auto","created_at":"2025-09-30 11:24:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2969458,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6884307/v1/e8d444df-d084-4a3a-ac2b-3f4346d15127.pdf"},{"id":84994501,"identity":"d3e33f6d-cb06-4b95-b38b-2f01c2e148f9","added_by":"auto","created_at":"2025-06-19 15:58:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":1064171,"visible":true,"origin":"","legend":"","description":"","filename":"SRSupplementaryInformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6884307/v1/7a6ff01be47db3de3ad86f36.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Authigenic iron oxyhydroxide rims attenuate deleterious element fluxes during sulphide oxidation in historical gold mine tailings","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe global minerals sector generates billions of tons of mine waste (e.g., tailings) annually \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. While modern tailings storage facilities (TSFs) are engineered to mitigate environmental risks, many historical TSFs were poorly designed and now serve as significant point sources of contamination. In South Africa, the world-class Witwatersrand goldfields have been mined continuously since 1886, yielding over 52,000 tons of gold and more than 6\u0026nbsp;billion tons of sulphide-rich mine waste \u003csup\u003e\u003cspan additionalcitationids=\"CR3\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. These sulphides, predominantly composed of pyrite (FeS₂), host significant concentrations of deleterious trace metals and metalloids e.g., As, Cd, Co, Se, Zn \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e and readily oxidise upon exposure to meteoric fluids and surface conditions. This oxidation leads to the formation of metastable iron oxyhydroxide and, to a lesser extent, sulphate minerals, releasing acidity (H⁺), sulphate (SO₄\u0026sup2;⁻), and trace elements collectively known as acid mine drainage (AMD)\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn the early stages of oxidation, sulphide minerals are preferentially replaced by iron oxides along cracks and grain boundaries, forming characteristic rim textures (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). As oxidation progresses, sulphides are completely replaced by authigenic iron oxyhydroxides, forming gossans \u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. The adverse environmental and human health effects of AMD are well-studied \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, with case studies documented in the Witwatersrand goldfields, e.g. \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e and numerous other sulphide-bearing ore deposits worldwide, e.g. Avarenga \u003cem\u003eet al\u003c/em\u003e.\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e and Konan\u0026ccedil; \u0026amp; Değermenci \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eDespite extensive research on AMD and the established role of iron oxyhydroxides in trace element sequestration through adsorption and isomorphic substitution \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. Studies on their effectiveness in mitigating harmful element fluxes from sulphide oxidation remain limited \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. The historical tailings of the Witwatersrand goldfields, subjected to prolonged weathering, provide an ideal natural laboratory for investigating the retention and release of deleterious elements during the sulphide-to-oxide transition. This research employs in-situ laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS), which offers high sensitivity for trace element detection. Unlike controlled lab-scale studies that examine relatively constrained geochemical systems, this approach captures the complexity of natural weathering processes and provides empirical insights into trace element mobility during sulphide oxidation.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1 Study areas, sample preparation and preconcentration\u003c/h2\u003e\n \u003cp\u003eApproximately 150 kg of tailings material was collected from eleven boreholes that were sampled from the oxidised zone (10 m depth) of a 3,95 km\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e TSF in the Klerksdorp Goldfields. Further details of the sampling site and the sample collection and preparation protocols are provided in the supplementary material (SM) and by Chingwaru \u003cem\u003eet al.\u003c/em\u003e \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. To concentrate a significant amount of sulphide and oxide minerals (the heavy fraction; density \u0026gt; ~ 2.95 g/cm³) for analysis, a gravity separation pre-concentration process was employed. This comprised a sequence of steps entailing natural settling, decantation, super-panning and heavy liquid separation. Aliquots of both the bulk tailings sample and the heavy mineral fraction were mounted into 30 mm epoxy resin mounts (duplicate samples prepared of each) and polished to a 1 µm finish.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2. Mineralogical and chemical analyses\u003c/h2\u003e\n \u003cp\u003eThe bulk and heavy mineral fraction mounts underwent initial evaluation using reflected light optical microscopy, whereafter their modal mineralogy was analysed using a quantitative evaluation of materials by scanning electron microscopy (QEMSCAN) at the University of Cape Town. The Zeiss EVO MA15VP scanning electron microscopy (SEM), located at the Central Analytical Facility (CAF) at Stellenbosch University, equipped with an Oxford Instruments Wave Dispersive X-ray Spectrometer (WDS) and appropriate standards, was used to determine major and minor element concentrations in the pyrite cores and iron oxyhydroxide rims. These data were complemented with measurements of trace element (e.g., As, Au, Co, Cu, Pb, Zn) concentrations analysed using LA ICP-MS with a 193 nm Excimer laser from Applied Spectra coupled with an Agilent 7700 Q ICP-MS with quality control and calibration done with relevant standards processed in (Laser Ablation Data Reduction) LADR software (see SM). Sulphide grains with oxidised rims ≥ 10 µm wide were selected for in-situ ablation analysis (10–20 µm spot sizes; Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea), resulting in 111 ablation spots between the pyrite core and adjacent iron oxyhydroxide alteration rim.\u003c/p\u003e\n \u003cp\u003eTo determine acid-leachable element deportment, duplicate 1 g composite tailings aliquots were analysed for As, Au, Cu, Fe, Ni, Pb and Zn using microwave-assisted aqua regia digestion at 60°C for 6 hours. Measurements on an Agilent 7900 ICP-MS were calibrated with certified reference materials WQB-1 and PTM-1a. Gold was included because of its economic significance \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e and because of its potential toxicity in nanoparticle form \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3. Calculations\u003c/h2\u003e\n \u003cp\u003eTo evaluate element mobility during the oxidation of sulphides, an oxidative loss coefficient (LE) was calculated using Eq. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, following the approach of Diao \u0026amp; Wen \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e and Lu \u003cem\u003eet al.\u003c/em\u003e\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eWhere C\u003csub\u003epy\u003c/sub\u003e represents the average element concentration in unaltered pyrite cores, and C\u003csub\u003eox\u003c/sub\u003e represents the average element concentration in the surrounding iron oxyhydroxide rims, both determined via LA-ICP-MS analysis. The LE calculation is based on the following assumptions: 1) The total iron mass remains constant during pyrite oxidation, as Fe is a major component of pyrite and is considered immobile during weathering \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. The oxidation of pyrite follows the reaction:\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003e4FeS\u003csub\u003e2\u0026nbsp;\u003c/sub\u003e+ 15O\u003csub\u003e2\u003c/sub\u003e + 14H\u003csub\u003e2\u003c/sub\u003eO\u0026nbsp;→\u0026nbsp;4Fe (OH)\u003csub\u003e3\u003c/sub\u003e + 8H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (2)\u003c/h3\u003e\n\u003cp\u003eThis reaction suggests that iron oxyhydroxide is the primary product of pyrite oxidation, however, natural systems are complex, and the absolute mineralogy of the oxidation rims is likely a mixture of poorly crystalline Fe phases (see SM for further details). In calculating the LE values for each pyrite core-rim pair (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e), the unaltered pyrite core is assumed to represent the chemistry of the rim, before its transformation into iron oxyhydroxide. A positive LE value indicates element leaching into the surrounding TSF, while a negative LE value signifies local element enrichment within the iron oxyhydroxide rim.\u003c/p\u003e\n\u003cp\u003eMass balance calculations were conducted to determine the elemental distributions (deportment) for metals/metalloids of interest among minerals present in the TSF material. These calculations were performed using Eq. 2, modified after Chingwaru \u003cem\u003eet al\u003c/em\u003e. \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e:\u003c/p\u003e\n\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003cbr\u003e\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\n\u003c/div\u003e\n\n\u003cp\u003eWhere m\u003csub\u003etotal\u003c/sub\u003e represents the total mass of the element in the tailings sample, calculated from the bulk sample mass and total metal concentration obtained via aqua regia digestion and ICP-MS analysis. m\u003csub\u003ephase\u003c/sub\u003e represents the mass of the element hosted within specific mineral phases, determined using automated mineralogy and mineral chemistry data from LA-ICP-MS ablation analysis.\u003c/p\u003e"},{"header":"3. Results \u0026 Discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Redistribution of trace elements during sulphide oxidation\u003c/h2\u003e \u003cp\u003eThe QEMSCAN analyses indicate that pyrite is the dominant sulphide mineral, constituting up to 1.3% of the bulk sample mineralogy. In contrast, other identified sulphides, including arsenopyrite, galena, chalcopyrite, sphalerite, pentlandite, and pyrrhotite, are present only in minor amounts, collectively accounting for approximately 0.2% of the modal mineralogy (see SM). Among these, pyrite is the only sulphide exhibiting notable oxidation along its rims (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Electron microscopy confirms that these oxidation rims consist of iron oxyhydroxides/oxides, though their precise mineralogical composition remains less well-constrained. They likely comprise a mixture of Fe-oxyhydroxide polymorphs (e.g., goethite) and poorly crystalline species (e.g., ferrihydrite). The trace element concentrations within the iron oxyhydroxide rims exhibit high variability, with mean values and standard deviations consistent with previous studies \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e: Co (1211.58\u0026thinsp;\u0026plusmn;\u0026thinsp;2556.56 ppm), Ni (1182.10\u0026thinsp;\u0026plusmn;\u0026thinsp;2251.07 ppm), As (713.18\u0026thinsp;\u0026plusmn;\u0026thinsp;469.11 ppm), Pb (418\u0026thinsp;\u0026plusmn;\u0026thinsp;2870.84 ppm), Zn (18.22\u0026thinsp;\u0026plusmn;\u0026thinsp;57.15 ppm), Au (4.51\u0026thinsp;\u0026plusmn;\u0026thinsp;28.86 ppm) and Cu (28.24\u0026thinsp;\u0026plusmn;\u0026thinsp;51.14 ppm) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eThese baseline concentrations, combined with LA-ICP-MS analyses of adjacent iron oxyhydroxide rims (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea), were used to calculate the oxidative loss coefficient (LE) (Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The LE values proxy for the relative mobility or retention of deleterious elements during sulphide oxidation. The oxide rims exhibit significantly lower concentrations and consequently, positive LE values for Co (227.70\u0026thinsp;\u0026plusmn;\u0026thinsp;509.91 ppm), Au (0.31\u0026thinsp;\u0026plusmn;\u0026thinsp;0.46 ppm), and Pb (53.15\u0026thinsp;\u0026plusmn;\u0026thinsp;89.43 ppm) compared to their respective concentrations in pyrite cores (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). In contrast, the oxide rims show elevated concentrations of Cu (262.57\u0026thinsp;\u0026plusmn;\u0026thinsp;352.48 ppm) and Zn (531.28\u0026thinsp;\u0026plusmn;\u0026thinsp;1986.84 ppm) which are notably higher than the corresponding cores (negative LE values in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), whereas the concentrations of Ni (890.09\u0026thinsp;\u0026plusmn;\u0026thinsp;1535 ppm) and As (505.57\u0026thinsp;\u0026plusmn;\u0026thinsp;1144.16 ppm) show only slight, insignificant levels of relative enrichment in the rims.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePyrite, a primary mineral that likely crystallised at least 2.7\u0026nbsp;billion years ago, incorporates trace elements within its crystal structure through direct substitution (e.g., Ni\u0026sup2;⁺, Co\u0026sup2;⁺ for Fe\u0026sup2;⁺; As⁻ for S⁻), coupled substitution reactions (e.g., Au⁺ + Bi\u0026sup3;⁺ \u0026rarr; 2Fe\u0026sup2;⁺), or nanoscale inclusions (e.g., Pb as galena) \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. In contrast, authigenic oxide rims consist of small crystallites with a high surface-area-to-volume ratio. Their trace element concentrations are controlled by surface adsorption and ionic substitution within the mineral structure. Elements such as Au and Pb are poorly retained due to their significant ionic size and charge mismatches with Fe\u0026sup3;⁺ crystallographic sites in tetrahedral and octahedral coordination \u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), as well as their limited adsorption affinity for Fe oxyhydroxide surfaces. Under ideal conditions, Co can substitute into goethite at levels up to ~\u0026thinsp;8 mol% \u003csup\u003e24,25\u003c/sup\u003e; however, this degree of incorporation was not observed in our samples. This may be due to its reduced affinity for Fe oxyhydroxide surfaces when competing with other ions such as As⁵⁺ \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe remaining elements exhibited negative LE values, indicating preferential retention within Fe oxyhydroxide rims rather than environmental mobility. Cu and Zn are strongly retained, likely through the formation of inner-sphere complexes, which are highly ph-dependent \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Retention may also be enhanced by the high Mn concentration in the rims (~\u0026thinsp;0.4 wt.%; SM S3), which can modify the crystal structure and charge balance, necessitating divalent ion incorporation for charge compensation and stabilisation \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNi and As, representing cationic and anionic substituents in pyrite, also remain in the solid phase rather than mobilising into the environment. Ni\u0026sup2;⁺ exhibits a relatively small ionic radius difference from Fe\u0026sup3;⁺ (~\u0026thinsp;7%; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), and up to 75% of total Ni in natural goethite has been shown to substitute directly for Fe \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. In contrast, As does not replace Fe in iron oxyhydroxides due to significant differences in ionic radius and charge (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Instead, it binds strongly to these minerals through surface complexation and co-precipitation \u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Under oxidising conditions in the tailings, As⁵⁺ adsorption is favoured, forming stable inner-sphere complexes with surface hydroxyl groups \u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Over time, these adsorbed As⁵⁺ species become structurally incorporated into the mineral matrix, further enhancing their retention.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Mass balance calculations for the TSF\u003c/h2\u003e \u003cp\u003eThe geochemical distribution and behaviour of deleterious elements within pyrite and its associated iron oxyhydroxide alteration rims (Section \u003cspan refid=\"Sec8\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e) emphasise the importance of understanding the overall mineralogical deportment in tailings. The elemental deportment is crucial for assessing the environmental mobility and availability of these elements. Total elemental concentrations from aqua regia digestible phases, along with concentrations by mineral phase (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), were used in mass balance calculations to determine the deportment of deleterious elements (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA major proportion of deleterious elements are hosted within the tailings sulphide minerals (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Pyrite hosts significant proportions of As (26%), Co (39%), Ni (25%), Au (45%), and Zn (29%), but only minor amounts of Cu (0.5%) and Pb (3%). Other sulphides, mainly chalcopyrite, galena, and arsenopyrite, contain substantial amounts of Cu, Pb, and As, respectively. Although representing as little as 1% of the total mineralogy, the iron oxyhydroxide rims host about 14% of As, 5% of Co, 3% of Cu, 15% of Ni, 0.3% of Pb, and 12% of Zn (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Given that most of this deleterious element concentration likely derives from the precursor pyrite, our study underscores the natural attenuation process operating in the evolving Witwatersrand TSF system, whereby the authigenic rims are responsible for significant deleterious element sequestration. The mass fraction of deleterious elements not accounted for by these mass balance calculations (up to 40%) is attributed to discrete phases of unknown speciation. Further studies are required to elucidate a more comprehensive deportment for each deleterious element of interest.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Scientific, Environmental, and Economic Significance","content":"\u003cp\u003eThe multi-scaled analytical approach, integrating LA ICP-MS (low detection limits, small spot size, and simultaneous trace element measurement) with bulk mineralogical techniques (QEMSCAN and wet chemistry), provides critical insights into the role of authigenic iron oxyhydroxide rims in attenuating deleterious element fluxes in a TSF from the historical Witwatersrand goldfields at summarised in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Assuming conservation of Fe during pyrite oxidation, the rims are predicted to sequester up to 15% of the initial As, Cu, Zn, and Ni contained within the precursor pyrite (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). However, the lack of speciation data for both the iron oxyhydroxide rims and associated contaminants limits the ability to assess sequestration longevity. For example, previous studies indicate that metals such as Ni, Zn, and Cd can become irreversibly bound to goethite within 2 to 120 days, with some fraction structurally incorporated over time \u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. The ultimate long-term stability of this contaminant sink will depend on factors such as bond strength, structural transformations, ageing of iron oxyhydroxides, cationic substitutions affecting solubility, and ambient TSF physicochemistry (e.g., pH, redox potential, ionic strength).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe formation of thick iron oxyhydroxide rims (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e \u0026amp; \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) also plays a secondary role in controlling oxidation and contaminant release by limiting oxygen diffusion to the sulphide cores \u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. As the ambient geochemistry dictates iron oxyhydroxide stability and associated sorption-desorption reactions, our measured concentrations and mass balance calculations are most relevant to the sampled upper 10 meters of the TSF. At greater depths, if reducing conditions develop as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, rim stability may decrease, potentially releasing incorporated elements. This suggests a depth- and spatially dependent control on trace metal concentrations and speciation within the TSF, e.g. \u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. Understanding this variability could inform targeted remediation strategies whereby oxidised zones are managed differently to reduced zones (where contaminants remain in the sulphide host), and to zones of intermediate redox potential (where the adsorption mechanisms and efficacies of developing oxide rims will be nuanced).\u003c/p\u003e \u003cp\u003eFrom an economic perspective, 45% of the Au in the sample material is associated with sulphides, with only a small fraction sequestered by iron oxyhydroxides (LE\u0026thinsp;=\u0026thinsp;93; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This suggests that Au, along with Co and Pb, is mobilised and potentially accumulates in deeper reduction zones within the TSF. Such redistribution will influence the Au grade if TSFs are considered secondary resources for sustainable metal recovery \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThe study presents a practical methodology for obtaining empirical measurements of acid mine drainage fluxes from historical gold TSF, accounting for the natural attenuation by authigenic oxide rims. These findings underscore the importance of a targeted rehabilitation strategy to mitigate environmental risks and realise the economic potential of such tailings. Developing tailored metallurgical reprocessing flowsheets focused on recovering gold, sulphides, and iron oxyhydroxides is essential for a full recovery of associated deleterious elements. This deportment-informed approach not only safeguards ecosystems and public health but also facilitates the recovery of critical metals like Cu, Co, and Ni, which are required for the green energy transition.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was funded by the DST-NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis (CIMERA) and the Society of Economic Geologists (SEG) Tim Nutt Fund. Thank you to the analytical staff at CAF, Stellenbosch University and the QEMSCAN unit, University of Cape Town. We further extend our thanks to the mining house Harmony Gold Limited for sample donations.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets generated in this study (see methodology for the techniques used to generate the datasets) are available on the Zenodo research repository at DOI: 10.5281/zenodo.15588324 is assessable using the link: https://zenodo.org/records/15588324?token=eyJhbGciOiJIUzUxMiJ9.eyJpZCI6ImQzMDE2OWY2LWMyMzYtNGQ0Zi1hZGIyLWI2MTcyMjI3OGNjMCIsImRhdGEiOnt9LCJyYW5\nkb20iOiI5MWRhNWIwNTgxNzYwZGU4\nMzUwYWE1NDRhN2ZlMDE1OCJ9.ncCKCG-vebkEOK6bizjQT1N352ryJ4gAKxza8\nL3wIl8Nyasc2qxRqjhdqgGEuvc3-ezdxWnqvM5c1K5q4Awxxw .\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAraujo, F. S. M., Taborda-Llano, I., Nunes, E. B. \u0026amp; Santos, R. M. 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Geotechnical and Hydrogeological Zonation of Tailings Storage Facilities: Importance for Design, Construction, Operation, and Closure. \u003cem\u003eMinerals\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 105 (2025).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Gold tailings, Deleterious elements, Iron oxyhydroxide, Oxidation","lastPublishedDoi":"10.21203/rs.3.rs-6884307/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6884307/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHistorical gold tailings pose environmental hazards globally, leading to acid mine drainage and the release of harmful elements due to the oxidation of unrecovered sulphides. The metastable secondary iron oxyhydroxides formed during this process influence the mobility of these elements, although the specific mass flows between the sulphide cores, iron oxyhydroxide rims and the environment remain poorly understood. Using a gold tailings storage facility in the Klerksdorp goldfields (South Africa) as a natural laboratory, this study investigates the role of secondary iron oxyhydroxide rims in controlling the mobility of Co, Ni, As, Pb, Zn, Au, and Cu after prolonged exposure of tailings to surface conditions. The release versus retention of these deleterious elements is characterised and quantified using a multi-method approach including LA ICP-MS, automated mineralogy, and wet chemistry. Although iron oxyhydroxide rims make up less than 1% of the mineralogy, they strongly retain As, Ni, Cu, and Zn fluxes that emanate from the precursor sulphides. In contrast, Co, Au, and Pb show limited compatibility (\u0026lt;\u0026thinsp;3% deportment) with these rims, suggesting their preferential mobilisation into the environment. The measured immobilisation of some of these elements is insufficient to meet environmental standards, highlighting the need for direct remedial measures, including metallurgical reprocessing and effective capture mechanisms for deleterious elements. These strategies are essential for reducing environmental risks and can simultaneously recover valuable (and critical) metals such as Au, Cu, Co, and Ni.\u003c/p\u003e","manuscriptTitle":"Authigenic iron oxyhydroxide rims attenuate deleterious element fluxes during sulphide oxidation in historical gold mine tailings","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-19 15:58:50","doi":"10.21203/rs.3.rs-6884307/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7cc7ceaf-5d3d-4dbb-8e11-0e320febfe15","owner":[],"postedDate":"June 19th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":50262742,"name":"Earth and environmental sciences/Environmental sciences"},{"id":50262743,"name":"Earth and environmental sciences/Solid earth sciences"},{"id":50262744,"name":"Physical sciences/Chemistry"},{"id":50262745,"name":"Physical sciences/Engineering/Chemical engineering"}],"tags":[],"updatedAt":"2025-09-30T11:23:55+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-19 15:58:50","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6884307","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6884307","identity":"rs-6884307","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}