Mine tailings integrated investigations: The case of Rio tailings (Panasqueira Mine, Central Portugal)

Mining generates huge quantities of waste materials from ore extraction and milling operations, which accumulate in tailings and open impoundments. The risks associated with mine tailings have different nature, namely stability, rupture, surface and groundwater contamination, acid mine drainage (AMD) and precipitation of secondary minerals. In fact catastrophic failure of tailings impoundments has occurred, transporting slurry, dissolved metals and fine reactive metal particles to the environment.Often deposition conditions of old mine tailings are unknown and monitoring is deficient or even inexistent. Thus, in order to prevent accidents it is necessary to establish the internal structure, that is, mapping tailings impoundment, internal sediment structures – limits and stratigraphy – seepage and pathway locations, bedrock depth and topography estimates and actual deposition conditions of the tailings. Owing to the complexity of the problem, only integrated approaches can investigate and monitor comprehensively the characteristics and evolution of mine tailings and its impacts on the environment.As this is a multidisciplinary problem, strategies should consider integrated studies such as Geophysics, Geochemistry and borehole surveys. However data from these sources must be interpreted together, in a GIS database, to provide better models and an enhanced overall knowledge of the studied structures.It is proposed to test and validate such a methodology in a case study, Panasqueira mine, central Portugal and to contribute to the decision-making process of environmental and civil protection agencies with responsibilities in the assessing, prevention, mitigation, and controlling the potential and actual effects of tailings spills on the environment.     

超细尾砂胶结充填体强度计算模型及应用

超细尾砂已成为充填材料的主要来源。为了便于充填料浆配合比设计与强度预测,基于超细料浆微观结构,提出用固体填充率表征充填料浆结构密实程度。选用某矿山超细尾砂进行了63组配合比强度试验。研究结果表明：固体填充率和水灰比与充填体无侧限强度分别呈指数函数和负幂函数关系。采用Pearson理论对试验样本进行相关性分析,结果表明：固体填充率与水灰比相互独立。在此基础上,建立了超细尾砂胶结充填体双变量强度计算模型,模型计算值与试验值的误差在7%以内。进行了超细尾砂胶结体强度随养护时间增长的试验,基于试验数据拟合规律,提出了强度龄期数学模型,建立了三变量强度计算公式。模型能准确预测矿山充填体强度,且能有效指导矿山充填料浆设计。     

Acid neutralization mechanisms and metal release in mine tailings: a laboratory column experiment

Mining and milling of base metal ore deposits can result in the release of metals to the environment. When sulfide minerals contained in mine tailings are exposed to oxygen and water, they oxidize and dissolve. Two principal antagonistic geochemical processes affect the migration of dissolved metals in tailings impoundments: sulfide oxidation and acid neutralization. This study focuses on acid neutralization reactions occurring in the saturated zone of tailings impoundments. To simulate conditions prevailing in many tailings impoundments, 0.1 mol/L sulfuric acid was passed continuously through columns containing fresh, unoxidized tailings, collected at Kidd Creek metallurgical site. The results of this column experiment represent a detailed temporal observation of pH, Eh, and metal concentrations. The results are consistent with previous field observations, which suggest that a series of mineral dissolution-precipitation reactions control pH and metal mobility. Typically, the series consists of carbonate minerals, Al and Fe(III) hydroxides, and aluminosilicates. In the case of Kidd Creek tailings, the dissolution series consists of ankerite-dolomite, siderite, gibbsite, and aluminosilicates. In the column experiment, three distinct pH plateaus were observed: 5.7, 4.0, and 1.3. The releases of trace elements such as Cd, Co, Cr, Cu, Li, Ni, Pb, V, and Zn were observed to be related to the pH buffering zones. High concentrations of Zn, Ni, and Co were observed at the first pH plateau (pH 5.7), whereas Cd, Cr, Pb, As, V, and Al were released as the pH of the pore water decreased to 4.0 or less.     

Experimental studies relating to microfracture in sandstone

A fundamental understanding of the relation between stress concentrations at grain contacts and microfractures in granular aggregates is obtained through two-dimensional photomechanical model studies and is tested through observational studies of experimentally deformed sandstone discs, glass beads, and quartz sand.In uncemented aggregates, the state of stress in each grain is controlled by the manner in which the applied load is transmitted across grain contacts. The angles between lines connecting pairs of contacts and the axis of the principal load acting at the boundaries of the aggregate determine which of all contacts will be most highly stressed or “critical”. Microfractures follow that maximum principal stress trajectory which connects critical contacts, and they propagate through those points where the magnitude of the local maximum stress difference is the greatest. Microfractures, therefore, are extension fractures. It then follows that both the locations and orientations of fractures can be predicted if the state of stress in the grains is known.Positioning of critical contacts depends primarily on sorting, packing, grain shapes, and the boundary load conditions applied to the aggregate. Some critical contacts and, therefore, microfractures tend to join together in a series or “chain”. Orientations of chains are most strongly influenced by the direction of the maximum compressive load at the boundary of the aggregate. A hydrostatic load applied on the boundaries of an aggregate can cause microfracturing within grains. Orientations for microfractures and contact lines are random in poorly sorted aggregates, but they are influenced by packing in well sorted aggregates.Grains of cemented aggregates are more highly stressed at their centers than at contacts. By analogy, microfracture orientations depend strongly on the position of the greatest load axis and only slightly on the low-magnitude stress concentrations at contacts. These microfractures parallel the greatest principal stress trajectory in regions where the magnitude of the maximum stress difference is greatest. These observations lead to the conclusion that fractures in grains of cemented aggregates are also extension fractures and should exhibit a higher degree of preferred orientation than in uncemented counterparts.These conclusions hold when cementing materials have about the same elastic moduli as the grains. Cements may be so weak that the aggregate behaves as if it were uncemented in terms of microfracture fabric, or so stiff that the major part of the load is transmitted by the cement, and the composite is no longer an aggregate in the mechanical sense.     

Influence of hardpan layers on arsenic mobility in historical gold mine tailings

Hardpans, or cemented layers, form by precipitation and cementation of secondary minerals in mine tailings and may act as both physical and chemical barriers. Precipitation of secondary minerals during weathering of tailings can sequester metal(loid)s, thereby limiting their release to the environment. At Montague Gold Mines in Nova Scotia, tailings are partially cemented by the Fe arsenate mineral scorodite (FeAsO₄·2H₂O). Previous studies have shown that the formation of scorodite can effectively limit aqueous As concentrations due to its relatively low solubility (<1 mg/L at pH 3–4) and high As content (43–52 wt.% As₂O₅, this study). Co-existing waters and solids were sampled at Montague Gold Mines to identify present-day field conditions influencing scorodite precipitation and dissolution, and to better understand the mineralogical and chemical relationship between hardpan and tailings. In addition to scorodite, hardpan cements were found to include amorphous Fe arsenate and Fe oxyhydroxide. Nearly all hardpan is associated with historical arsenopyrite-bearing concentrate which provides a source of acidity, As⁵⁺ and Fe³⁺ for secondary mineral precipitation. Pore waters sampled from the hardpan have pH values ranging from 2.43 to 7.06. Waters with the lowest pH values also have the highest As concentrations (up to 35.8 mg/L) and are associated with the most extensive hardpan and greatest amount of weathered sulfide. Samples from areas with discontinuous hardpan and less sulfide have near-neutral pH and lower As concentrations. Detailed petrographic observations indicate that the identity and stability of As-bearing secondary minerals depends on the continued availability of sulfide concentrate. The results of this study are being used to develop remediation strategies for highly weathered, hardpan-bearing tailings that consider the stability of both primary and secondary minerals under various cover scenarios.     

水泥胶结钙质砂地层中单桩竖向承载特性试验研究

钙质砂属于岩土工程中一种特殊的岩土材料,除具有颗粒形状不规则、易破碎等特征,还具有胶结性。针对钙质砂具有胶结性的地质现状,通过室内模型试验研究了胶结钙质砂地层中钢管桩的承载能力、沉降情况及其影响因素,同时与未胶结钙质砂中的桩基承载特性进行了对比。研究结果表明：与未胶结钙质砂中的钢管桩相比,胶结钙质砂的相对密实度对桩基承载力影响程度明显减弱,桩的承载形式依然表现为端承桩,随着钙质砂胶结程度的提升,桩端阻力承载占比越来越高;胶结程度较高的钙质砂地层中桩身侧摩阻力发挥存在异步过程,这是因为桩基沉降时桩身下部破坏砂层形成了更为紧密的新接触面,该接触面对桩身的径向膨胀更为敏感;胶结钙质砂中桩基 q_s-S_u 线没有出现明显的硬化阶段,与未胶结钙质砂地层中桩基的 q_s-S_u多段折线变化规律不同,胶结钙质砂地层中桩基的 q_s-S_u曲线更为接近双曲线线型。     

Plastic compaction of cemented granular materials

We analytically relate hydrostatic stress to strain in a random dense pack of identical spheres cemented at their contacts. The spheres are elastic and the cement is perfectly plastic. This solution for the sphere pack is based on a solution for the normal interaction of two cemented spheres. Initially, the two spheres touch each other at a point. We show that, as loading increases and cement becomes plastic, a finite (Hertzian) direct-contact area between the spheres necessarily has to develop and progress. The stress-strain behavior of the pack depends on the cement's yield limit and on the amount of cement. At the same hydrostatic stress, the deformation of the cemented aggregate is smaller than that of the uncemented one. This difference becomes large as the yield limit increases. We calculate the bulk modulus of an aggregate from the stress-strain curve. In the plasticity domain, the bulk modulus of the cemented aggregate is smaller than that of the uncemented one. The difference between the two may easily reach 50%. Of course, as the cement's yield limit decreases, the aggregate's stress-strain curve and the bulk modulus approach those of the uncemented sphere pack. This theoretical conclusion is qualitatively supported by experiments on epoxy-cemented glass beads. The maximum contact stress in the cemented aggregate may be less than a half of that in the uncemented one. This result explains an experiment where an uncemented glass bead sample failed at a hydrostatic stress of 50 MPa, whereas an epoxy-cemented sample stayed intact.     

Modeling the effect of stratification on cemented layer formation in sulfide-bearing mine tailings

Reactive transport simulations have been applied to investigate possible effects of stratification on the potential of sulfide-bearing mine tailings to form protective cemented layers. The simulations are based on characteristic strata found at a German tailings site, including sulfide-enriched heavy mineral layers, mica-enriched silt layers, and homogeneously mixed layers. The simulated secondary phases (jarosite, gypsum, amorphous ferric arsenate, amorphous Fe hydroxide, alunite, amorphous silica, and kaolinite) are similar to those observed in the field. Using scanning electron microscope analyses of cemented layers, it has been observed that the pore area becomes disconnected if the porosity is decreased to values below 15%, which would indicate a strong decrease in permeability. Stratification was found to play a crucial role in cemented layer formation. Cemented layers are absent or insignificant in systems with a homogeneous distribution of Fe-bearing sulfides. They are extensively developed in systems with (a) an arsenopyrite-rich layer or (b) a mica-enriched layer situated immediately below an Fe-sulfide enriched layer. The modeling results have clearly demonstrated that the key processes operating in scenario (a) are very different from the key processes in scenario (b). In scenario (a), the oxidation of arsenopyrite is followed by the precipitation of amorphous ferric arsenate, which can be solely responsible for significant pore reduction. In scenario (b), the presence of a large amount of reactive aluminosilicates (e.g. biotite and Ca-bearing plagioclase) immediately below the Fe-sulfide rich layer appears to be crucial. Key processes are extensive formation of Acid Rock Drainage (ARD) followed by enhanced (pH-driven) weathering of aluminosilicates, resulting in the accumulation of secondary phases directly below the Fe-sulfide rich layer. In both scenarios, a cemented layer is formed that effectively retards the further downward movement of the oxidation front. The presented details on the role of stratification in the formation of cemented layers could be considered in the construction of mining heaps as a possible measure to stimulate natural attenuation.     

The transfer and fate of Cd, Cu, Pb and Zn from two historic metalliferous mine sites in the U.K.

Monitoring of heavy metal transfers, aerial deposition and fluvial transport to soil, stream sediment and vegetation compartments surrounding two former PbZn mines are reported. Results show that 80–100 a after the closure of the mines relatively large amounts of Cd, Pb, Zn and, in once case, Cu are entering the soil and fluvial systems.Aerial deposition of heavy metals within a 300 m radius of the centre of the tailings heaps is shown to be contributing up to 3.3 kg Cd, 71 kg Cu, 373 kg Pb and 1041 kg Zn annually to the surrounding soil and vegetation compartments. It is shown that more than 4.2 kg Cd/a and 1387 kg Zn/a are being transferred from the tailings heaps via the streams in the form of dissolved load. However, the greatest quantities of Cu (38 kg/a) and Pb (74 kg/a) are transported from the heaps as wash load. Transfers calculated as percentages of the total metal quantities in the tailings heaps highlight the greater relative mobility within the fluvial system of Cd and Zn compared to Cu and Pb. Outside of the tailings heaps, the greatest accumulation of metals is in the surrounding soil, while the smallest was observed to occur in the surrounding mixed grassland vegetation.The most important implications of these findings are in the long-term release and partitioning of pollutant metal species from historic metalliferous mine tailings within the terrestrial and fluvial environments. This information is of major ecotoxicological and agronomic importance and is necessary for the effective reclamation and remediation of such contaminated sites.     

A coupled chemo‐viscoplastic cap model for simulating the behavior of hydrating cemented tailings backfill under blast loading

Although the use of blasting has become a routine in contemporary mine operations, there is a lack of knowledge on the response of cement tailings backfills subjected to sudden dynamic loading. To rationally describe such a phenomenon, a new coupled chemo‐viscoplastic cap model is proposed in the present study to describe the behavior of hydrating cemented tailings backfill under blast loading. A modified Perzyna type of visco‐plasticity model is adopted to represent the rate‐dependent behavior of the cemented tailings backfill under blast loading. A modified smooth surface cap model is consequently developed to characterize the yield of the material, which also facilitates hysteresis and full compaction as well as dilation control. Then, the viscoplastic formulation is further augmented with a variable bulk modulus derived from a Mie–Gruneisen equation of state, in order to capture the nonlinear hydrostatic response of cemented backfills subjected to high pressure. Subsequently, the material properties required in the viscoplastic cap model are coupled with a chemical model, which captures and quantifies the degree of cement hydration. Thus, the behavior of hydrating cemented backfills under the impact of blast loading can be evaluated under any curing time of interest. The validation results of the developed model show a good agreement between the experimental and the predicted results. The authors believe that the proposed model will contribute to a better understanding of the performance of cemented backfills under mine blasting and contribute to evaluating and managing the risk of failure of backfill structures under such a dynamic condition. Copyright © 2015 John Wiley & Sons, Ltd.     

The behaviour of an artificially cemented sandy gravel

The major section of the city of Tehran, Iran has been developed on cemented coarse-grained alluvium. This deposit consists of gravely sand to sandy gravel with some cobbles and is dominantly cemented by carbonaceous materials. In order to understand the mechanical behaviour of this soil, a series of undrained triaxial compression tests and unconfined compression tests were performed on uncemented and artificially cemented samples. Portland cement type I was used as the cementation agent for preparing artificially cemented samples. Uncemented samples and lightly cemented samples (1.5% cement) tested at high confining pressure showed contractive behaviour accompanied with positive excess pore water pressure and a barrelling failure mode. However, cemented samples and uncemented samples tested at low confining pressure (25 and 50 kN/m²) showed dilative behaviour accompanied with negative excess pore water pressure. Shear zones were formed in these samples and a clear peak in excess pore water pressure and stress ratio against strain could be observed. Test results showed that the limiting stress ratio surface for cemented samples is curved and expands as the cementation and density increase. Unconfined compression strength of cemented samples increases with increases in cementation and density as well.     

Contaminant leaching from gold mining tailings dams in the Witwatersrand Basin,South Africa: A new geochemical modelling approach

In this study a new geochemical reaction modelling methodology is used to shed light on the geochemical processes within Witwatersrand tailings impoundments and the evaluation of the geochemical impacts for future mining projects is evaluated. Information from international and local studies on tailings sulphidic tailings impoundments is used to develop a conceptual understanding on a typical Witwatersrand gold tailings impoundment. The tailings impoundments consist of 3 distinct geochemical zones: Oxidation Zone (OZ), a Transition Zone (TZ) and a Reduction Zone (RZ). Individual reaction models are developed for each of the 3 zones. The output of one model is used as an input to the next in spatial order. The results of the final model represent the tailings basal seepage characteristics, which indicate the most likely impacts on groundwater resources. The model results agree with existing information on AMD in the Witwatersrand. The results indicate that the tailings basal seepage is likely to be acidic (pH of ∼3.5), containing elevated concentrations of SO₄ and trace metals (Al, Co, Cr, Cu, Mn, Ni, Zn and U). Predicted Fe concentrations are low, due to the low $f_{{\text{O}}_2}$ in the TZ and RZ and the buffering effect of the precipitation of K-jarosite in the OZ and TZ. This study therefore indicates that the methodology employed produces results that can be correlated to existing information and can thus be used as a methodology in the assessments of impacts from sulphidic tailings material for future mining projects.     

湘西金矿尾矿-水相互作用:2.动力学模拟

从硫化物的氧化、脉石矿物的溶解、氧的扩散、水的流动和溶质质量迁移等过程的耦合作用建立了尾矿-水相互作用的动力学模型。对湖南湘西金矿尾矿库的数学模拟表明：早期尾矿-水的相互作用可以引起酸水的产生和重金属的释放及对环境的污染，影响较大的主要是前30年，随着时间增加，尾矿孔隙水逐渐中性化，污染元素含量显著降低。脉石矿物的溶解和有机物反应可降低尾矿中氧的扩散和产生酸的中和作用。尾矿库上部孔隙水中各组分含量明显高于尾矿库下部，并在约6m深处存在突变带。尾矿库的水文分带是导致地球化学分带的主要原因。     

Trace metal and As solid-phase speciation in sulphide mine tailings – Indicators of spatial distribution of sulphide oxidation in active tailings impoundments

Redistribution of potentially harmful metals and As was studied based on selective extractions in two active sulphide mine tailings impoundments in Finland. The Hitura tailings area contains residue from Ni ore processing, while the Luikonlahti site includes tailings from the processing of Cu–Co–Zn–Ni and talc ores. To characterize the element solid-phase speciation with respect to sulphide oxidation intensity and the water saturation level of the tailings, drill cores were collected from border zones and mid-impoundment locations. The mobility and solid-phase fractionation of Ni, Cu, Co, Zn, Cr, Fe, Ca, Al, As, and S were analysed using a 5-step non-sequential (parallel) selective extraction procedure. The results indicated that metal redistribution and sulphide oxidation intensity were largely controlled by the disposal history and strategy of the tailings (sorting, exposure of sulphides due to delayed burial), impoundment structure and water table, and reactivity of the tailings. Metal redistribution suggested sulphide weathering in the tailings surface, but also in unsaturated proximal areas beside the earthen dams, and in water-saturated bottom layers, where O₂-rich infiltration is possible. Sulphide oxidation released trace metals from sulphide minerals at both locations. In the Hitura tailings, with sufficient buffering capacity, pH remained neutral and the mobilized metals were retained by secondary Fe precipitates deeper in the oxidized zone. In contrast, sulphide oxidation-induced acidity and rise in the water table after oxidation apparently remobilized the previously retained metals in Luikonlahti. In general, continuous disposal of tailings decreased the sulphide oxidation intensity in active tailings, unless there was a delay in burial and the reactive tailings were unsaturated after deposition.     

Early diagenetic phosphate cements in the Albian condensed glauconitic limestone of the Tatra Mountains, Western Carpathians

Early diagenetic phosphate cements are described from the Albian condensed glauconitic limestone of the Tatra Mountains, Western Carpathians with regard to their macro- and micromorphology, distribution, classification, and genesis. The cements occur within stratigraphically condensed semi-pelagic foramini-feral-glauconitic layers and are associated with mature hardgrounds within the Tatra Albian limestone. Phosphate cement fabrics consist of crypto- to microcrystalline carbonate-fluorapatite, and they occur as: (i) rim envelopes, (ii) infillings of intraparticle porosity, (iii) rim cement, (iv) multiple rim cement, (v) palisade fabric and (vi) cluster cement. Micromorphological variability of the cement fabrics results from varying texture of the cemented sediment, the nature of original porosity, as well as from presence of associated microbial fabrics. The microbial fabrics are interpreted as fossilized coccoid cyanobacteria. Phosphate cementation developed under peculiar early diagenetic conditions within semi-closed microenvironments rich in organic matter in the marine phreatic environment. The cementation contributed to the formation of phosphatic fossils and hardgrounds. The accretion of the cements was due to concentration of biologically uptaken phosphorus near the sediment/water interface, enrichment of pore fluids with respect to phosphate, and its precipitation within restricted microenvironments. Phosphate cementation post-dated seafloor formation of pelletal glauconite but predated partial decomposition of organic matter as well as dissolution or neomorphism of aragonite and high-Mg calcite. Phosphate cementation occurred on a carbonate platform following the submersion of Urgonian reefal build-ups. Episodes of phosphate cementation were repeated during the sedimentation of the Tatra Albian limestone as a response to rapid relative sea-level rises and increased influence of nutrient-rich Tethyan waters.     

Formation and composition of cemented layers in low-sulphide mine tailings, Laver, northern Sweden

Cemented layers (hardpans) are common in carbonate or sulphide-rich mine tailings and where pyrrhotite is the predominating Fe-sulphide. Laver, northern Sweden, is an abandoned Cu-mine where the tailings have low pyrrhotite content, almost no pyrite and no carbonates. Two cemented layers at different locations in the Laver tailings impoundment were investigated, with the aim to determine their effects on metal mobility. The cementing agents were mainly jarosite and Fe-oxyhydroxides in the layer formed where the tailings have a barren surface, whereas only Fe-oxyhydroxides were identified below grass-covered tailings surface. Arsenic was enriched in both layers which also exhibit high concentrations of Mo, V, Hg and Pb compared to unoxidised tailings. Sequential extraction indicates that these metals and As were mainly retained with crystalline Fe-oxides, and therefore potentially will be remobilised if the oxic conditions become more reducing, for instance as a result of remediation of the tailings impoundment.An erratum to this article can be found at     

An attempt to use LA-ICP-SMS to quantify enrichment of trace elements on pyrite surfaces in oxidizing mine tailings

Metals released from oxidation and weathering of sulphide minerals in mine tailings are to a high degree retained at deeper levels within the tailings themselves. To be able to predict what could happen in the future with these secondarily retained metals, it is important to understand the retention mechanisms. In this study an attempt to use laser ablation high-resolution ICP-MS (LA-ICP-SMS) to quantify enrichment of trace elements on pyrite surfaces in mine tailings was performed. Pyrite grains were collected from a profile through the pyrite-rich tailings at the Kristineberg mine in northern Sweden. At each spot hit by the laser, the surface layer was analyzed in the first shot, and a second shot on the same spot gave the chemical composition of the pyrite immediately below. The crater diameter for a laser shot was known, and by estimating the crater depth and total pyrite surface, the total enrichment on pyrite grains was calculated. Results are presented for As, Cd, Co, Cu, Ni and Zn. The results clearly show that there was an enrichment of As, Cd, Cu and Zn on the pyrite surfaces below the oxidation front in the tailings, but not of Co and Ni. Arsenic was also enriched on the pyrite grains that survived in the oxidized zone. Copper has been enriched on pyrite surfaces in unoxidized tailings in the largest amount, followed by Zn and As. However, only 1.4 to 3.1% of the Cd and Zn released by sulphide oxidation in the oxidized zone have been enriched on the pyrite surfaces in the unoxidized tailings, but for As and Cu corresponding figures are about 64 and 43%, respectively. There were many uncertainties in these calculations, and the results shall not be taken too literally but allowed the conclusion that enrichment on pyrite surfaces is an important process for retention of As and Cu below the oxidation front in pyrite rich tailings. Laser ablation is not a surface analysis technique, but more of a thin layer method, and gives no information on the type of processes resulting in enrichment on the pyrite surfaces. Although only pyrite grains that appeared to be fresh and without surface coatings were used in this study, the possibility that a thin layer of Fe-hydroxides occurred must be considered. Both adsorption to the pyrite directly or to Fe-oxyhydroxides may explain the enrichment of As, Cd, Cu and Zn on the pyrite surfaces, and, in the case of Cu, also the replacement of Fe(II) by Cu(II) in pyrite.     

Mine drainage from the weathering of sulfide minerals and magnetite

Pyrite and pyrrhotite are the principal minerals that generate acid drainage in mine wastes. Low-pH conditions derived from Fe-sulfide oxidation result in the mobilization of contaminant metals (such as Zn, Cd, Ni and Cr) and metalloids (such as As) which are of environmental concern. This paper uses data from detailed mineralogical and geochemical studies conducted at two Canadian tailings impoundments to examine the mineralogical changes that pyrite, pyrrhotite, sphalerite and magnetite undergo during and after sulfide oxidation, and the subsequent release and attenuation of associated trace elements. The stability of sphalerite in tailings impoundments generally is greater than that of pyrrhotite, but less than pyrite. Dissolved Ni and Co derived from Fe sulfides, and to a lesser extent, dissolved Zn and Cd from sphalerite, are commonly attenuated by early-formed Fe oxyhydroxides. As oxidation progresses, a recycling occurs due to continued leaching from low-pH pore waters and because the crystallinity of Fe oxyhydroxides gradually increases which decreases their sorptive capacity. Unlike many other elements, such as Cu, Pb and Cr, which form secondary minerals or remain incorporated into mature Fe oxyhydroxides, Zn and Ni become mobile. Magnetite, which is a potential source of Cr, is relatively stable except under extremely low-pH conditions. A conceptual model for the sequence of events that typically occurs in an oxidizing tailings impoundment is developed outlining the progressive oxidation of a unit of mine waste containing a mixed assemblage of pyrrhotite and pyrite.     

Prediction of the long-term performance of abandoned lead zinc mine tailings in a Welsh catchment

In this study we investigated the sulphidic mine tailings from Frongoch and Grogwynion, two abandoned lead zinc mines in mid-Wales, UK. Despite falling within the same ore field the mine waste characterisation has identified differences in the tailings from the two sites. Bulk concentrations range from 10 to 52 g kg^− 1 for Pb, 1.1 to 2.9 g kg^− 1 for Zn in Grogwynion and from 1.0 to 130 g kg^− 1 for Pb, 11 to 110 g kg^− 1 for Zn in Frongoch. An experimental (European standard leaching tests TS 14429 and TS 14405) and geochemical modelling approach was used to study the leaching composition as a function of pH and liquid/solid ratio. There was little correlation between the tailings bulk metal concentrations and the leachate composition, but variations in Pb and Zn concentrations were found to be consistent with control of dissolved Pb and Zn by secondary minerals and the mechanisms of dissolution/precipitation/sorption involving them. Specifically, the Grogwynion mine tailings with near-neutral pH have predominantly lead and zinc carbonates controlling Pb and Zn solubility in the leachates, whereas the Pb and Zn concentrations in Frongoch leachates are best modelled with a surface complexation model for metal sorption to oxyhydroxides. The different speciation results in a greater sensitivity of Grogwynion tailings to acidification with a potential release of Pb in solution up to 10 times higher than in Frongoch, despite similar bulk Pb concentrations. At acid pH, Zn is similarly dissolved to a greater extent in Grogwynion than in Frongoch tailings. There was no evidence of sulphide oxidation during the batch and column leaching tests and the suitability of using these European leaching standards for the characterisation of sulphidic mine waste materials for waste management purposes has been considered.