Environmental Indicators in Metal Mining

Predictive environmental indicators in mining can be defined as ‘values derived from parameters that provide quantitative information against which some aspects of environmental risks associated with mineral resource development can be measured’. At mine sites, such indicators provide information on conditions and processes that may develop during the mine life cycle and after mine closure. They are measures to forecast changes in the quality of the air, water, land and ecological systems. Predicting environmental risks is typically not an attribute that is strongly embedded into the development of mineral resources. However, a more predictive and proactive approach to early environmental characterization and risk assessment should be used early in the life-of-mine stages, because such an approach supports more effective mineral processing, better storage of waste, improved mine closure outcomes and ultimately reduced financial risks and liabilities to operators and regulators.

Oxidation of sulfide minerals releases sulfuric acid and dissolved metals, with iron sulfides pyrite (FeS₂) and pyrrhotite (Fe_(1−x)S) recognized as the most common acid-forming minerals. Several factors control the oxidation rate including: the oxidant type, sulfide morphology, microbial action, and trace element contents. Whilst metal sulfides such as galena and sphalerite are less acid-forming, they are typically sources of environmentally significant elements such as Cd, Pb and Zn. Common sulfide oxidation reaction products are metal-sulfate efflorescent salts. Dissolution of these minerals is critical to the storage and transport of acids and metals released upon weathering of mineralized rock or mine wastes. Acid formed by sulfide oxidation can be consumed through reaction with gangue minerals. Neutralization is primarily offered by dissolution of carbonate minerals with calcite and dolomite the most effective. Factors affecting carbonate reactivity include: grain size, texture and the presence of trace elements which can influence a mineral’s resistance to weathering. Silicate minerals such as olivine, wollastonite and serpentinite are recognized as effective longer term neutralizers. Lesser neutralizing potential contributions from phyllosilicates, pyroxenes, amphiboles and feldspars have been reported. Micas, clays and organic matter can temporarily adsorb H⁺ ions through cation exchange reactions, with gibbsite and ferric hydroxide recognized as offering neutralizing capacity under acidic conditions. Ultimately, the balance of acid producing and acid consuming chemical reactions will determine the production of acid rock drainage (ARD).

Inadequate prediction of acid rock drainage (ARD) can result in the reputational damage of mine operators, the spending of significant costs for post-closure management and lasting impacts to ecosystems. Instead, accurate prediction of ARD will allow a reduction of environmental risks and associated financial liabilities. At present, the mining industry use a range of static and kinetic chemical tests to measure the balance between the acid generating and acid neutralizing potentials of mine waste materials. The resulting data are used to prepare risk assessments and design waste classification schemes. However, associated with these established tests and practices are several shortcomings including: inadequate sampling; performance of a limited number of tests; late initiation of kinetic trials; classification using restricted waste classification categories; and no consideration given to biological and physical parameters that can influence ARD formation. Therefore, current practices only provide a broad indication of ARD potential over time. Fundamentally, ARD is a multifaceted process controlled by several variables, and therefore new tests and protocols developed in this area must reflect this.

Quantifying the texture, mineralogy and mineral chemistry of rocks in the mine environment is required to predict the value of a deposit and maximize extraction efficiency. Scanning electron microscopy supported by recognition of minerals by characteristic X-ray emissions is the preferred mineral mapping method in the mining industry at present. This system is fully mature and supported by highly optimized software. Laser Raman mapping may compete for some of this space in the future. Very coarse scale mineral maps are possible from drill core images but these cannot be used to measure the key parameters required for most mine planning. Trace elements can be highly concentrated in rare minerals so that they are easy to detect but very difficult to accurately measure due to sampling problems, or they may be very dispersed and difficult to detect at all. There are a range of tools available to support trace element deportment and most studies will need to use more than one methodology. The key new development of the last decade is the emergence of laser ablation inductively coupled plasma mass spectrometry for the measurement of most elements at sub-ppm level. There are still many trace and minor elements for which accurate models of deportment are extremely difficult.

Established protocols for predicting acid rock drainage (ARD) principally utilise geochemical testwork for classifying waste materials. However, ARD formation is dictated by the mineralogy of the sampled material, and indeed textural relationships of both acid forming and neutralizing mineral phases present. In order for such characteristics to be understood, a logical and structured approach to ARD prediction must be adopted. Motivated by this, the three-stage geochemistry-mineralogy-texture-geometallurgy (GMTG) approach was developed. This integrates a range of techniques, with the resulting sample classification based on diverse analyses. This intends to eradicate the possibility of identifying samples as ‘uncertain’ as is the case with established protocols. At stage one, termed ‘pre-screening’ simple tools including paste pH, portable XRF, measurement of sulfur, environmental logging and geometallurgical techniques are used. Samples identified as inert (non-acid forming, non-metalliferous) are not prioritized for further testing. At stage-two termed ‘screening’, established screening tools are used including net acid producing potential (NAPP) and net acid generation (NAG) tests with samples classified by establish criteria. At stage three, termed ‘defining’, only samples identified as acid forming are subjected to in-depth characterization to identify controls on oxidation, as well as advanced geochemical tests to confirm classifications assigned at the conclusion of stage-two. The GMTG approach has potential applications at operations that are at the early life-of-mine (e.g. prefeasibility) and also post-closure phase (e.g. abandoned sites).

Accurate mineralogical identification is critical across the mining value chain from host rock characterization and alteration mapping, to mineral processing and environmental management. Advanced techniques for rapid mineralogical identification in drill core including hyperspectral scanners (e.g., HyLogger™, Corescan™) are becoming increasingly available to the mining industry. However, their high associated costs and logistical problems with deploying to field sites means that these techniques are not yet ubiquitously available. Mineral specific chemical staining techniques for carbonate and feldspar minerals offer a rapid and cheap alternative to hyperspectral scanning and more traditional mineral identification techniques (e.g., X-ray diffractometry, electron microprobe analysis). This contribution provides an updated review of chemical staining procedures for carbonate and feldspar minerals and how they can be used to map the distribution and texture of these minerals in drill core. Used under controlled health and safety conditions, chemical staining of drill core can assist with identification and mapping of these minerals during exploration activities (i.e., resolving alteration assemblages), feasibility studies (i.e., indicating ore grindability and flotation cell pH) and environmental assessments (i.e., domaining short and long term acid neutralization capacity).

Tests currently used by the industry for acid rock drainage (ARD) prediction heavily utilize static geochemical tests. Instead, effective tools which allow for early domaining should be utilized as they can be performed on a greater number of samples, allowing for deposit-wide environmental characterization. These must be simple enough to perform in the core shed or field-laboratory to keep cost and turn-around time to a minimum. Simple field-based pH tests and chemical staining should be performed. In addition, mineralogical characterization methods for drill core materials i.e., an ARD focused logging code and the use of portable instruments (i.e., pXRF, Equotip) should be pursued. This chapter presents several field tools appropriate for ARD prediction. These tools were developed, tested and validated using drill core and waste rock materials obtained from several Australian mines with differing geology, mineralogy and mineralization style. This chapter demonstrates that by utilizing these field based tests, industry has the opportunity to achieve: (i) effective ARD prediction testwork; (ii) detailed deposit-wide characterization, (iii) development of best practice waste management plans; and (iv) identification of the most suitable rehabilitation options.

Automated mineralogy tools are now commonly used during mineral processing for particle characterization to help mine operators evaluate the efficiency of the selected mineral processing techniques. However, such tools have not been efficiently used to assist in acid rock drainage (ARD) prediction. To address this, the computed acid rock drainage (CARD) risk grade protocol was developed. The CARD risk grade tool involves: (1) appropriate selection of samples (i.e., following a geometallurgical sampling campaign); (2) careful preparation of a particle mount sample; (3) analysis on a mineral liberation analyser (MLA) using the X-ray modal analysis (XMOD) function; (4) processing of the XMOD data to produce a whole particle mount backscattered electron (BSE) image and a corresponding image of classified XMOD points; (5) fusion of both images to obtain particle area data; (6) calculation of the CARD risk ratio based on carbonate and sulfide particle areas, relative reactivities (\( {\text{pH}}_{{{\text{CaCl}}_{2} }} - {\text{pH}}_{{{\text{mineral}} + {\text{CaCl}}_{2} }} \)) and acid forming/neutralizing values (calculated based on mineral chemistry and stoichiometric factors, kg H₂SO₄/t); and (7) classification of CARD risk ratios ranging from extreme risk to very-low risk. Testing of the CARD risk grade tool was performed on materials selected from several mine sites representative of both run-of-mine ore and waste. This testing proved that CARD can be effectively used to map ARD risks on a deposit scale and forecast geoenvironmental risk domains at the earliest life-of-mine phases.

Best practice for acid rock drainage (ARD) risk assessment predominately relies on the geochemical properties of sulfidic rocks. Consequently, a plethora of geochemical tests are routinely utilised by the mining industry to predict ARD formation. Due to limitations associated with these tests and their relatively high costs, analysis of recommended best practice sample numbers is rarely achieved, thus reducing the accuracy of waste management plans. This research aimed to address this through identifying potential geometallurgy indicators using drill core samples (n = 70) obtained from the Comstock Chert, a new prospect proximal to Mount Lyell, western Tasmania, Australia. Samples were subjected to a range of mineralogical analyses, routine ARD geochemical tests (i.e., paste pH; acid-base accounting, ABA; net acid generation, NAG), field-based techniques (e.g., portable X-ray fluorescence, pXRF; short-wave infrared spectrometry, SWIR), and geometallurgical analyses (i.e., HyLogger, Equotip). This study demonstrated: (1) HyLogger data allows identification of acid-neutralizing carbonate minerals; (2) Equotip hardness data provide a conservative indication of lag-time to acid formation; (3) CARD risk grading accurately identifies high and low risk ARD domains; and (4) pXRF data provides a sound indication of the abundance of environmentally significant elements. Consequently, the application of geometallurgical techniques to drill core allows the prediction of ARD characteristics that inform waste characterization and management plans.

Management of mine wastes, particularly waste rock, requires careful planning to reduce the likelihood of sulfide oxidation, and generation of ARD. Such a waste management strategy must be based on a thorough understanding of the environmental characteristics of the future waste rock materials. In this study, a waste management strategy for characterizing underground waste rock was developed at a polymetallic mine to determine which materials were appropriate for surficial placement. The criteria for surficial placement set by the regulator were that materials had to be non-acid forming and non-metalliferous. A range of cost-effective field based tools and state-of-the-art laboratory techniques were used on a suite of representative samples collected from the site to determine an appropriate waste management strategy. Ultimately, a modified geochemistry-mineralogy-texture-geometallurgy (GMTG) approach was designed, whereby ARD focused logging and simple pre-screening tools such as paste pH and sulfur analyses were used at stage-one; routine acid base accounting and leachate tests at stage-two, and validation tools including X-ray diffractometry and laser ablation ICPMS at stage-three. Such an approach should be considered for other mine sites at all life-of-mine stages with similar deposit characteristics to ensure correct screening and placement of potentially hazardous waste materials.

pH tests are useful screening tools for assessing the characteristics of first flush waters draining sulfidic rocks and waste materials at mine sites. Rinse and paste pH tests are part of a suite of static tests used in acid-base accounting assessments. This study presents a comparison of eleven different pH tests (e.g., rinse and paste pH tests as well as soil tests of the International Organization for Standardization ISO 10390:2005, American Society for Testing and Materials ASTM D4972-01(2007) and Standards Australia AS4969.2-2008) using three different sulfidic rock samples and the acid-base accounting standard KZK-1. We show that different rinse and paste pH methodologies using different grain sizes and extraction solutions can result in different risk classification for ARD assessments. We suggest pH testing should be standardized in their grain size and solid to solution ratio. pH tests conducted using unweathered materials (e.g., drill core) should be carried out using a 0.01 M CaCl₂ solution.

The original abrasion pH test of Stevens and Carron (1948) is based on grinding and wetting of minerals and subsequent pH measurements of the minerals’ paste and suspensions. Such pH values allow insights into the behaviour of individual minerals upon fluid:mineral reactions in surface environments including mine sites and waste repositories. This study proposes a modified abrasion pH method which includes the use of 0.01 M CaCl₂, an operationally defined grain size (<0.075 mm), and high precision pH measurements. Several mineral specimens (n = 20) were obtained from commercial suppliers for the modified abrasion pH testwork, with the acquired specimens having a range of purities and resultant pH values. The modified abrasion pH testwork demonstrates that small admixtures to monomineralic samples caused significant pH changes. Only pure monomineralic samples provide a true indication of the modified abrasion pH of that mineral type. The NAGpH method aims to oxidize sulfide minerals but the reliability of this method in waste classification can be compromised depending on the sample mineralogy. This study demonstrates that NAGpH measurements of some carbonate minerals led to very alkaline pH values that are possibly due to the formation of Ca and Mg hydroxides. Such reactions do not occur in waste environments and therefore, NAGpH measurements of carbonate-rich wastes may overestimate their acid buffering capacity.

The Iberian Pyrite Belt (IPB) is one of the largest of the world’s massive sulfide provinces. Since the Chalcolithic era, gossans formed from massive sulfide mineralization have been worked for copper, silver and gold. Consequently many historical mine sites have abandoned dumps of gossanous material. One such example is located at Angostura, a historical copper mine which operated from 1906 to 1931. The aims of this study are to determine the mineralogical hosts of environmentally significant elements (As, Ba, Bi, Co, Cu, Hg, Mo, Sb, Se, Ni, Pb, Zn) in gossanous waste rocks dumped adjacent to the Angostura open cut, using geochemical, optical, SEM-MLA and laser ablation techniques. Our findings demonstrate that the gossan materials are enriched in environmentally significant elements with several hosted by iron oxides and iron-oxyhydroxides. Leaching of these gossan materials was performed using three extractants to represent different conditions which may be experienced in a surficial environment (i.e., deionized water, hydrogen peroxide and sulfuric acid). Results from these experiments indicated that under ambient surface conditions all analyzed elements will not be released from their goethite and hematite hosts. However, under ARD conditions, elements such Co, Cu, Pb and Zn will be mobilized.

Predictions on the behavior of environmentally significant elements at mine sites requires the use of advanced laboratory techniques. The aim of this contribution is to demonstrate the use of electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to gain an understanding of likely element behaviour. Sulfidic boulders sampled from an acid rock drainage (ARD) impacted ephemeral stream adjacent to the historical Baal Gammon workings are dominated by chalcopyrite, arsenopyrite, pyrrhotite and lesser pyrite. Micro-analytical investigations using EMPA and LA-ICPMS reveal that chalcopyrite contains significant quantities of Ag, Cd, Sn, In and Zn either substituted directly into the crystal lattice or occurring as discrete sphalerite and stannite inclusions. Arsenopyrite, comprising more than 50 % of some boulders, is most notably rich in Co, Ni, Sb and Se, but it also contains inclusions of sphalerite, chalcopyrite and stannite. By contrast, pyrrhotite contains relatively few trace elements, but it may be a significant contributor to ARD development. The trace element composition of Fe-oxides in the oxidized rinds of these boulders is likely directly influenced by the mineralogy of the sulfidic boulders on which they precipitate. Although significant quantities of As, Bi, Cu, In, Pb and Zn occur in Fe-oxides at Baal Gammon, these elements may be liberated during acid flushing of the ephemeral stream. Consequently, EMPA and LA-ICPMS represent valuable tools for evaluating the source and potential mobility of environmentally significant elements at mine sites.

The Zeehan Pb-Zn field in western Tasmania (Australia) contains over one hundred abandoned and historical mine sites. Combined with a temperate rainforest climate and abundant waste rock material, many sites are affected by acid rock drainage (ARD). The Spray mine, located southwest of the town of Zeehan, was one of the field’s largest historical producers of Pb and Ag. Abandoned in the early 1900s, the site contains numerous adits and waste rock piles which contribute to ARD in the region. The aim of this study was to predict the likely ARD surface water quality, using the major and trace element chemistry of sulfide minerals present within waste materials on site. Major ore sulfides are galena and sphalerite with associated Sb-rich sulfosalt minerals including boulangerite and geocrocite. These minerals contain minor concentrations of Ag, Bi, Cd, In and Sn. Minor arsenopyrite and abundant pyrite (average 7500 ppm As) represent the main repository for As. Siderite is a major gangue mineral, containing slightly elevated In, Pb, Sb and Zn (250–50 ppm). Metals and metalloids (Ag, As, Bi, Cd, Cu, In, Pb, Zn) contained within sulfides and siderite may be mobilized upon mineral dissolution into ARD waters. Consequently, micro-analytical analyses of sulfides and associated gangue minerals can assist in the prediction of aqueous metal and metalloid mobility from sulfidic waste rock piles.

Mineral dusts produced from mining activities pose a risk to human health and the surrounding environment. The particle size distribution of dust is important for determining environmental, occupational health and physiological impacts. Dust is generally thought of as particulates with a diameter of between 1 and 60 μm, but it can be further divided into nuisance dust or total suspended particulates, fugitive dust, inhalable dust, thoracic dust, and respirable dust. This review considers aspects of mineral dust related to the mining of metalliferous ores including: (a) sources of mineral dust at mine sites (i.e. land clearing, drilling and blasting, transport operations, crushing, milling, screening, stockpiles); (b) control measures to reduce dust generation; (c) monitoring techniques; (d) mineral dust characterization to quantify particle concentration, size and morphology and chemical composition; and (e) prediction of mineral dust properties. Predicting the physical and mineralogical characteristics of dust is important for effective dust management and control strategies. At present, there are no appropriate testing procedures available to predict the chemical and mineralogical properties of mineral dust from mining operations. Further work is required to understand mineral fractionation according to grain size and to provide a rapid test methodology that would predict dust composition.

Mine sites operating dust monitoring programs use a variety of techniques to comply with legislative constraints and maintain high standards of environmental and human health protection. One low cost option is the use of dust deposition gauges, which provide information on the total, soluble and insoluble dust fluxes. The aim of this study was to identify methods that could provide information on likely dust sources of samples, which were collected using dust deposition gauges at the Mt Lyell Cu-Au mine, Australia. Elemental analyses of archived dust samples combined with known dust deposition rates allowed quantification of annual metal fluxes. The highest annual metal fluxes were measured for Cu (1–33 g m⁻² year⁻¹) followed by Pb (8–343 mg m⁻² year⁻¹), Cr (3–59 mg m⁻² year⁻¹) and As (1–79 mg m⁻² year⁻¹). X-ray diffractometry and scanning electron microscopy permitted characterization of the mineralogical and morphological properties of dust samples. These analyses revealed that the analysed samples derived from at least four different dust sources. Consequently, geochemical and mineralogical characterization of mineral dust samples combined with a detailed site knowledge allows identification of dust sources at mine sites.

This study presents a novel method to characterize dust particles using a passive dust sampler (PDS). Six different PDS were deployed around six different metal mine sites (Tasmania, Australia) and left in the field for 1 month. Dust particles were analyzed directly on the PDS using a Field Emission Scanning Electron Microscope. Backscattered electron (BSE) images were collected with a resolution of 0.5 μm per pixel and used to characterize the size and composition of dust particles. Those particles >2 μm in diameter were classified according to the range of BSE brightness values, which correspond to mineralogical compositions. Particles were grouped according to BSE brightness and categorized as organic particles, silicates, Fe silicates and oxides, and sulfides. Dust sources with unique particle size:composition relationships were identified at particular mine site domains (e.g. rock crusher, concentrator plant, tailings dam). The documented method can be used to monitor the dispersal of mineral dust and provide information on the mineralogical composition of particle size fractions relevant to occupational health risks at metalliferous mine sites.

Predicting the properties of dust generated at mine sites is important for understanding the impact of dust dispersal to the surrounding environment. This chapter presents a new approach to predicting the mineralogical properties of the PM2.5 and PM10 dust fractions. A purpose-built dust resuspension machine was fitted with a size selective sampler to collect dust fractions. Dust particles were collected onto a polycarbonate filter, which was analyzed using a scanning electron microscope (SEM). Backscattered electron (BSE) maps of the polycarbonate surface were imaged and processed to determine dust properties. For a given population of particles, the BSE brightness distribution of the 2–5 and 5–10 µm size fractions were quantified. The mineralogical composition of the dust size fractions were inferred by the BSE brightness as biogenic particles and sulfates (30–50), silicates (60–100), iron silicates and oxides (110–190), and sulfides (>200). The method was validated by comparing laboratory-generated dust fractions with those collected from dust monitoring stations at a tailings repository site. Similar dust composition and size fractions were observed for both laboratory and field samples. Consequently, the purpose-built dust resuspension device and associated laboratory procedures allow the prediction of mineralogical properties of dust at mine sites.

Knowledge of bioaccessible metals in soils is relevant to the rehabilitation of mine sites and remediation of mined land. Soils of metal mine sites are commonly enriched in metals and metalloids due to mine waste dumping, atmospheric fallout from smelter emissions as well as dust deposition and erosion of particles originating from ore stockpiles, tailings storage facilities, waste rock dumps and exposed mine workings. Upon mine closure, the establishment of an effective and sustainable vegetation community represents an integral part of mine site rehabilitation. Only a vegetated and uncontaminated landscape will lead to site stability, effectiveness of dry covers, minimization of deleterious offsite effects and return of the mined land to a condition that allows a particular post-mining land use. Moreover, plants may represent pathways of metals and metalloids from contaminated substrates into local foodchains. Consequently, a solid understanding of the current and future bioaccessibility of metals and metalloids is of key relevance for assessing mined land for rehabilitation purposes. This paper presents a review of the literature concerning tests that are used to assess and predict the bioaccessibility of metals in contaminated and mining environments.

Successful rehabilitation of mine sites requires a solid knowledge of waste and mine soil properties. In particular, there is a need for a predictive test that allows forecasting of risks to plant growth on contaminated mine soils and waste repositories. In this contribution, a new plant bioaccessibility test is described that meets this need. The methodology for the new test oxidizes the sample with hydrogen peroxide and then extracts the solubilized metals and metalloids from the sample using 1 M ammonium acetate solution. The test is based on aspects of two established tests: (1) the sequential NAG test, which is suitable for sulfidic samples but does not determine metals and metalloids; and (2) the BCR^® extraction, which gives information on the geochemical fractionation of these elements but is not suitable for highly sulfidic samples. Learnings from the method development, a stepwise test protocol and a simplified flow diagram for the test are presented. The proposed new plant bioaccessibility test represents a predictive tool that allows forecasting of metal fractions that may become available to plants upon colonization of sulfidic soils and wastes at mine sites.

Abandoned mine sites with their metal-rich substrates pose significant challenges to naturally colonizing plants. In this study, the abandoned Sn-Cu tailings lagoons at Wheal Maid (Cornwall, UK) have been investigated to establish the bioaccessibility of metals and metalloids (As, Cd, Cu, Pb, Sb, Zn) in exposed tailings and wastes using a new plant bioaccessibility test. Four main substrate types were sampled: (1) mine waste used to construct the lagoons, a relatively uncontaminated material with variable particle size; (2) granular capping material used in the upper lagoon to cover the tailings and relatively uncontaminated; (3) grey tailings a fine to medium grained material with visible sulfides and white secondary salts, extremely high in near-total Zn concentrations; and (4) marbled tailings a fine grained brown/red/yellow material with extremely high near-total As concentrations. The analytical quality of results produced by a new plant bioaccessibility test was monitored using blanks, spiked solutions and repeat analyses. The grey tailings had the highest bioaccessible metal and metalloid content. As this material oxidizes, it will release As, Cd, Cu, Sb, Zn and to a lesser extent Pb in a form which will be more available to plants. This will inevitably delay re-vegetation at the site. The new bioaccessibility test is recommended for sulfidic rocks and waste samples and should be employed at an early mine-life stage to allow appropriate waste classification and improve mine closure outcomes.