Waste Rock Piles Research Articles

Moving towards sustainable mine waste rock management through automated mineral and geochemical analysis

Mining activities generate vast amounts of waste rock (WR); if this waste is sulphide- and sulphosalt-bearing, it can pose a risk to ecosystems due to contaminated mine drainage. Strategic long-term planning is necessary to prevent and mitigate adverse environmental impacts. This study includes a detailed characterization of WR from a mine in northern Canada and defines the diameter of physical locking of sulphides and sulphosalts (DPLS) that can be used to separate the WR into two fractions according to the geochemical reactivity. An automated mineralogy system (QEMSCAN®) and computed tomography (CT) were used to quantify the liberation degrees of acid-generating (sulphides) and neutralizing minerals (carbonates). Chemical and mineralogical analysis indicated that sulphides, mostly pyrite (FeS2) and gersdorffite (NiAsS) (dominant sulphides), were enriched in the fine- to mid-sized fractions compared to the coarse fractions. These minerals were more liberated within the fine fractions and their liberation degree was considered negligible at sizes >2.5 mm. Consequently, 2.5 mm was defined as the critical diameter of sulphide reactivity (DPLS) for the studied WR. Furthermore, kinetic tests using humidity cells were conducted to confirm this result and to assess the geochemical behaviour of the total fraction (< 10 mm), the coarse fraction (> 2.5 mm) and the fine fraction (< 2.5 mm). Sulphides associated with the coarse fraction were not significantly exposed to weathering conditions; sulphides were associated with non-sulphide gangue minerals, which was confirmed by QEMSCAN® and computed tomography. Moreover, mineral weathering occurred primarily in the fine fraction (< 2.5 mm) due to the higher available exposed surface area of the minerals. The geochemical analysis of humidity cells leachate revealed that the pH values remained within the range of 7.07 and 8.61, and that the instantaneous metal concentrations (Fe, Cu, and Ni) were consistently below environmental limits during the154- day test period. However, As concentrations from fine and total fractions exceeded environmental standards. Sample reactivity showed that the fine fraction controlled the geochemical behaviour of the studied WR, where the fine fraction was 1.5 times more reactive than the total fraction and 10 times more reactive than the coarse fraction which represents about 60 wt% of the total sample. Consequently, screening waste rock in accordance with the DPLS could prove to be an effective method for managing WR to prevent contaminated drainage. This approach may allow a reduction in the amount of WR to be stored in surface piles (which need to be managed) and will reduce the costs associated with the reclamation of waste rock piles.

Dissolved thiolated arsenic formed by weathering of mine wastes

Aqueous thiolated arsenic (As) species play an important role in the biogeochemical cycling of As in wetlands and hydrothermal systems. Although mine wastes such as tailings ponds and waste rock piles may harbor similarly sub-oxic and neutral to alkaline conditions that favor the formation and mobility of thio-As species, quantitative data on their existence in these systems is lacking. We conducted laboratory column experiments under contrasting redox conditions with waste rock from the Antamina mine, Peru, and processed tailings from Montague, Nova Scotia, Canada. Dissolved As concentrations between 1 and 7000 μg/L were recorded in drainages across these mine waste types, with up to 13 μg/L As present in thiolated form, predominantly monothioarsenate. Higher percentages of thio-As species (up to 5%) were observed in drainages from enargite-rich materials compared to arsenopyrite-bearing materials (<0.5%). The lower abundance of dissolved thio-As in the arsenopyrite-rich mine waste samples is attributed to their partially oxidized nature and reduced mineral reactivity under the experimental circumneutral drainage pH, the difference in S [-II/0]-to-As molar ratios compared to the enargite-rich mine waste samples, as well as the oxidation of di- and tri-thiolated As species by dissolved Fe. Overall, our results demonstrate that aqueous thiolated As species may occur in mine wastes with different As-bearing minerals and could play an important role in governing the mobility and fate of As in these systems.

Assessing the importance of drainage layers over geomembrane liners within engineered cover systems: Seven years of field monitoring at three mine waste rock piles

Geomembrane liners are commonly subject to defects that can allow water leakage through these openings and significantly undermine their performance. While defects are difficult to prevent and characterize, minimization of potential leakage can still be attained with appropriate lateral drainage over the liner. The objective of this study is to assess the performance of different drainage layer materials within HDPE-lined cover systems at three mine waste rock piles (WRPs) in the Sydney Coalfield, Nova Scotia, Canada. Natural soil, clean gravel and a geocomposite net were employed at the Summit, Victoria Junction and Franklin WRPs, respectively. Extensive field monitoring of precipitation (PPT), moisture content within the cover materials, and head of water above the liner, was performed. The head of water and moisture was consistently high at Summit throughout the year, while consistently low at Victoria Junction and Franklin, with the geocomposite net being just as effective as the gravel layer. Using typical assumptions of defect size/number, leakage rates at Victoria Junction and Franklin remained very low due to limited water head and flow gradient. Therefore, while the prevention of defects is highly challenging at HDPE-lined cover systems, potential defect leakage can be proactively controlled during cover system design with the implementation of adequate drainage layers above the liner.

Delineation of the spatial variability of carbonate and sulfide minerals in mine waste-rock piles using geostatistics

Social and environmental consequences associated with sulfide-bearing mine wastes (i.e., waste rock and mill tailings) and acid mine drainage constitute growing threats to sustainable mining and development, biodiversity of ecosystems, and global water quality. Spatial heterogeneity of geochemically important parameters in waste-rock piles is well-known and is important to incorporate in environmental impact assessments; however, quantitative characterization of spatial heterogeneity remains under investigation. Here, we show the integrated results from trans-disciplinary waste-rock studies at three mine sites in northern Canada, conducted over the past 20 years. These studies focus on diverse projects, including an experimental-scale test pile study, characterization of medium-scale stockpiles, and an examination of an operational-scale pile. Through geostatistical analyses, we observed that spatial heterogeneity of geochemically important parameters, including the abundances of carbonate and sulfide minerals, in field-scale waste-rock piles can be represented by random distributions using log-normal scaling. Our findings are foundational for forecasting waste-rock weathering and for environmental risk assessments, and can be applied to enhance the development of long-term waste-rock management strategies and improve reclamation measures of waste-rock piles throughout the world.

Geochemical controls on mobilization of metals from a 100-year-old waste rock pile and implications for selection of cover amendments

Reclamation of mine waste rock piles typically consists of constructing a cover with amendments to improve conditions for vegetation. However, cover amendments have potential to mobilize metals in waste by introducing new chemicals and altering pH and redox conditions. This study evaluates metal phases in a 100-year-old waste rock pile with high metals content (3.5% lead by weight, 0.8% zinc, and 0.75% copper) and the potential for these metals to be mobilized by several cover materials and amendments (topsoil, spent brewery grain, biochar, compost, commercial soil media, and phosphate). Laboratory testing indicates that metals have weathered from their initial metal sulfide phases (galena, sphalerite, chalcopyrite), and are now also present as sulfates, phosphates, carbonates, and phases associated with manganese/iron oxides. Sequential extraction tests demonstrated that the largest extractable fraction of metals is associated with manganese/iron oxides (37% of lead by weight, 22% of copper, and 26% of zinc), suggesting an environmental risk should geochemically reducing conditions develop and mobilize metals in the pile after cover construction. Testing of specific cover materials demonstrated that metals mobilization also occurs from low pH (as with spent brewery grain), formation of stable aqueous metal–organic complexes (as with spent brewery grain and compost), and ligand exchange (as with phosphate amendment). Results of this study demonstrate the importance of identifying metal phases present in a waste rock pile prior to selecting cover amendments.

Improved imaging efficiency of large mine waste rock piles with integrated electrical and electromagnetic geophysical techniques

Acid mine drainage (AMD) can emanate from mine waste rock piles (WRPs) for extended periods, resulting in severe environmental contamination. Since the generation and release of AMD is dependent on the volume and composition of acid-generating waste rock, effective characterization of WRPs is necessary for successful long-term AMD management. Conventional methods such as coring and test pits are invasive and only provide sparsely distributed point information in WRPs which are typically large and highly heterogeneous. Geoelectrical methods such as direct current (DC) resistivity, time-domain induced polarization (TDIP) and electromagnetics (EM) can provide non-invasive and continuous information of WRPs. However, due to the enormous scale of typical WRPs, improved strategies are needed for more efficient imaging. The objective of this study was to demonstrate the improved efficiency of integrated imaging techniques with rapid but low-resolution EM, and laborious but high-resolution DC and TDIP, to characterize the acid-generating regions of a coal mine WRP in Nova Scotia, Canada. EM was first employed to rapidly image the entire pile footprint and locate zones with high conductivity. Focused 3D DC and TDIP surveys were then performed over these zones, which were <10% of entire WRP, to better characterize the associated waste rock composition. DC and TDIP images indicate coincident and continuous areas of low resistivity (<15 Ohm·m) and high chargeability (>15 mV/V), which are indicative of generated AMD (leachate) and stored AMD (sulfides), respectively. This interpretation is validated by corresponding geological and geochemical measurements within the WRP. This study demonstrates the value of integrating complementary electrical and electromagnetic methods for more efficient characterization of large mine WRPs, with EM rapidly locating zones of interest, and simultaneous DC and TDIP imaging then discriminating between areas that generate AMD and areas that are affected by AMD flow paths.

Heavy metal release and acid generation during the oxidation of sulfide waste rock induced by ferric iron

As aby-product of waste rock from sulfide ore, Fe3+ is one of the important factors that promote acid production and heavy metals release from waste rock pile fields. In this paper, the dynamic leaching tests were conducted to investigate the behavior of acid generation and heavy metal release from the sulfide waste rock of Dexing copper mine under different Fe3+ concentrations. The Sulphur content in the waste rock samples was 3.56 % and the net acid production potential (NAPP) reached 47.95 kg·t−1 H2SO4. Under the oxidation of 600 mg·L-1 Fe3+, the pH value of the leaching system decreased to 5.53, which is 1.85 units lower than that of the control check group (CK). However, the quantity of SO42- in the leachate was reduced by 41.81 % compared to the CK group, as the jarosite precipitation was more likely to form under high Fe3+ conditions. The oxidation of Fe3+ effectively accelerated the release of heavy metals from the waste rock. In addition, the heavy metals release quantity gradually increased with the increase of the Fe3+ concentration, with the maximum release quantities of Cu, Pb, Zn, and As of 569.69, 333.75, 222.69, and 31.11 μg·kg−1, respectively, which increased by 27.66 %-129.60 % compared to the CK group. The release of SO42-, Cu, Pb, and Zn was controlled by the combined internal diffusion and the chemical reaction, while the release of As was best fitted to the interfacial chemical reaction-controlled model. The SEM analysis showed that the waste rock near the top of the column was strongly corroded, resulting in the increase of the specific surface area of the waste stone by 24.48 %. The re-precipitation reaction of heavy metals mainly occurred near the bottom of the leaching column. Leaching under Fe3+ oxidation caused a decrease in the proportion of residual heavy metals in the waste rock, which will lead to the easier release of heavy metals.

Gamma-ray spectrometry applied in the identification of potential acid mine drainage generation zones in waste rock pile with uranium ore and associated sulfides (caldas, Brazil)

Metal mining deeply modifies local environmental dynamics and produces large volumes of waste rock and residues, which is expected to be carefully disposed in order to prevent soil, water, and environment degradation. The Osamu Utsumi mine was a pioneer uranium ore exploitation site in Brazil, located in the State of Minas Gerais. The open pit and the high concentration of sulfide in waste rock piles are nowadays a dispendious concern to the Nuclear Industries of Brazil (INB, in Portuguese), due to sulfide oxidation and the resultant acid mine drainage (AMD). The objective of this research is to analyze the correlation between gamma-ray spectrometry data and the genera- tion of AMD in a waste rock pile, locally known as BF-04. Potassium concentrations can be used to distinguish zones of rock depletion, lowest concentrations, and radioelement sedimentation. The radiometric data acquisition combined with previous studies indicates that high uranium and tho- rium concentrations are associated with high sulfide zones. The mineralized uranium ore includes sulfides that are rapidly destroyed by surface hydration conditions, responsible for generating acid mine drainage. In this sense, superficial zones in the waste rock pile with high levels of eU and eTh are areas exposed to oxidation and hydration of sulfides, and, therefore, their recognition and neutralization can contribute to decrease AMD generation in the study site and areas with similar characteristics. The gamma-ray spectrometry results can subsidize more effective and low-cost reme- diation techniques.

Geophysical Diagnosis of Diversion Channel Infiltration in a Uranium Waste Rock Pile

Geophysical Diagnosis of Diversion Channel Infiltration in a Uranium Waste Rock Pile

Diavik Waste Rock Project: Simulation of the geochemical evolution of a large test pile using a scaled temperature and sulfide-content dependent reactive transport model

The Diavik Waste Rock Project (DWRP) project included four principal components focused on the development of techniques for assessing the environmental impacts of waste rock at mine sites. These components were small-volume laboratory experiments, intermediate- and large-volume field experiments, and assessment of the operational-scale waste-rock stockpiles, which facilitated characterization of waste-rock weathering at different scales. The heavily instrumented large-scale field experiments (test piles) were constructed to replicate, as closely as practicable, the temperature, water flow, and gas transport regimes of a waste-rock pile that is exposed to annual freezing and thawing cycles and to facilitate characterization of the long-term weathering of a low-sulfide waste rock. An integrated conceptual model of sulfide-bearing waste-rock weathering, developed at the small scale, was applied to assess the capacity of the conceptual model to capture the geochemical evolution of the waste rock at the large field-scale test-pile experiment. The integrated conceptual model was implemented using reactive transport code MIN3P, taking into account scale-dependent mechanisms. The test-pile mineralogy was similar to the small-scale laboratory experiments and included low-sulfide waste rock with an S content of 0.053 wt% (primarily pyrrhotite). The flow regime of the test pile was simulated using parameters measured as part of other DWRP investigations, including temporally variable infiltration estimates that represented the measured precipitation events at the site. The temporally and spatially variable temperature of the test pile was interpolated from values measured using instrumentation installed at the beginning of the experiment and was included in the simulation to refine the temperature dependence of the geochemical reactions. To allow continuous, multi-year simulation, freezing was also simulated to represent the conditions experienced at the test-pile experiment. Normalized root mean square error analysis of the large-scale field experiment simulation results indicated most parameters compare well to measured daily mass flux (i.e., the fraction of the range of annual values encompassed in the residual was less than 0.5 for SO4, Fe, Ni, Si, Ca, K, Mg, Na, and pH and 1.0 or less for all parameters except Cu). The method of using an integrated conceptual model developed from the results of humidity cell experiments to implement a mechanistic approach for assessing the primary geochemical processes of waste-rock weathering on a large scale was shown to provide reasonable results; however, the results are specific to the study site and the approach requires application to various sites under different geological and climatological conditions to facilitate further refinement.

Assessment of released natural radionuclides by waste rock pile and mining pit associated with a uranium mine at Caldas, Minas Gerais, Brazil.

The Ore Treatment Unit was a uranium mining company that is currently being decommissioned. The local rainfall index makes it necessary to release effluents into the environment. After releasing, the wastewater is available for unrestricted use. Current study aims to use national and international recommendations to assess the radiological potability of released effluents at one of the three points of company's interface with the environment. Twenty-four samples of water were collected and activity concentrations (AC) were obtained by gross alpha count, gross beta count, and for arsenazo spectrophotometry. Statistical analysis techniques were applied to the data with the purpose of understanding the results for the soluble, particulate, and total fractions. The mean AC for effluents were 3.580, 0.082, 0.103, 0.063, and 0.090Bq L-1 for Unat, 226Ra, 210Pb, 232Th, and 228Ra, respectively, for the total fraction. The analysis of variance pointed to Unat as a critical radionuclide, since it presented more than 90% of the total AC released into the environment. Pearson's R2 pointed to soluble fraction as a major contributor to the total AC released. The guidance level proposed by WHO was used to assess the radiological potability of the effluents. The results obtained indicated the need for trigger other analyses. Committed effective dose was estimated due to the unrestricted use of effluents and the value obtained, 0.23mSvyear-1, was below the maximum allowed limit. Finally, the radiotoxicity of the released effluent was evaluated and the value obtained was ~ 50% of the maximum allowed limit. In conclusion, the present study showed that the level of radioactivity released into the environment by the Ore Treatment Unit does not present a radiological risk to the surrounding population.

Characterization of heavy metal migration, the microbial community, and potential bioremediating genera in a waste-rock pile field of the largest copper mine in Asia

Long-term metal(loid) pollution in waste rock piles is an underestimated environmental problem during ore production. To investigate the factors affecting the distribution of metal(loid)s, various physicochemical and microbial properties were analyzed in 70 soil and water samples taken from a waste rock pile field. The results were analyzed using different methods, such as interpolation and multivariate statistical analyses. The results showed that the distribution of metal(loid)s in the topsoil (0–20 cm) was characterized by large variations in Cu (38.33–5834.18 mg/kg), Cd (0.14–29.29 mg/kg), Cr (7.08–326.16 mg/kg), and As (3.68–91.06 mg/kg). The mean pH value of the soil in the waste rock pile field was 3.85, while it was 5.03 outside of the rock pile. The Shannon–Wiener diversity and Chao's richness indices decreased from 5.46 and 1855.62 outside of the rock pile to 3.95 and 817.82 inside the rock pile, respectively. Multivariate statistical analysis indicated that significant spatial heterogeneity in the soil properties led to differences in the metal(loid) fractions, and the residual fraction of different metal(loid)s varied from 20.69% to 99.98%. Acidification of the media and increasing the number of acidophilic bacteria mobilized Cu and Cr but did not affect As. Mobilization of Cu/Cr iron oxides and organic matter (OM) on As was detected in situ compared with previous studies. The results of this study can provide useful reference data for the remediation of metal(loid)–contaminated soil in acidic waste rock pile fields.

Hydraulic conductivity testing and destructive sampling of field-scale mine waste test piles

A commingled waste rock and tailings test pile and a waste rock test pile were evaluated to determine saturated hydraulic conductivity and destructively sampled to measure dry density. The commingled test pile contained a mixture of filtered tailings and waste rock blended to isolate waste rock particles as inclusions within the tailings matrix. Test piles were constructed in the shape of truncated 5 m high pyramids with 25 m base sides and flat 5 m × 5 m top surfaces, and instrumented to monitor water content (and additional geochemical indicator parameters) within the test pile and seepage from the base of the pile. Piles were decommissioned after 26 months of operation. Saturated hydraulic conductivities were measured using sealed double-ring infiltrometers (2.4 m square outer ring and 1 m square inner ring). Tensiometers and embedded water content sensors were used to measure progression of the wetting front, and the final location of the wetting front in the commingled test pile was directly measured during decommissioning. Field-measured saturated hydraulic conductivities were compared to laboratory-measured results intended to simulate the test piles. Despite having a lower average density, the commingled waste rock and tailings had a hydraulic conductivity approximately 2.5 times lower than the waste rock.

Assessment of hydrogeochemical behavior of layered waste rock stockpiles: A meso-scale laboratory experiment

Large quantities of waste rocks are removed during ore extraction particularly in open pit mining and stored at the surface in piles. When these materials contain reactive minerals and are exposed to surface environmental conditions, they can generate contaminated mine drainage. A new approach of construction of waste rock piles using inclined compacted benches was developed in order to control water infiltration and minimize contamination. An experimental waste rock pile was built on the Lake Tio mine site to evaluate the performance of this approach under field conditions. To address all concerns related to the limitation of the field experiment, laboratory infiltration tests were conducted inside a steel and plexiglass box filled with three layers of different materials including a flow control layer (consisting of two compacted sand and crushed anorthosite sub-layers) and a layer of coarser materials (hemo-ilmenite waste rock containing sulphides and the non-reactive anorthosite waste rock). These infiltration tests were carried out testing different rates of precipitation (8 mm/day, 17 mm/day and 27 mm/day) at different slopes (0, 3, 5 and 7%). The effects of precipitation rate and box slope on the flow and quality of the infiltrated water in the system were studied in detail. Results show that the inclination angle of the box and the applied rainfall rate largely govern the diversion of water down the slope of the infiltration box. The results also show that the distribution of infiltration within the system was dependent on rainfall intensity. Based on the results of infiltration tests, it was found that nickel and sulfate loadings are influenced by the applied rainfall rates and a better diversion of water by the flow control layer improves the drainage water quality. These results suggest that nickel and sulfate loadings are mainly controlled by the capillary barrier effects (CBEs), flow paths and the flow rate. The findings are useful for improving the use of a flow control layer as a waste rock management and reclamation method.

Evaluating dual-domain models for upscaling multicomponent reactive transport in mine waste rock

Reactive transport models have proven abilities to simulate the quantity and quality of drainage from mine waste rock. Tracer experiments indicate the presence of fast and slow flow regimes in many heterogeneous waste-rock piles. Although multidomain models have been developed specifically for systems with such distinctive hydrodynamics, there have been limited applications of multidomain reactive transport models to simulate composite drainage chemistries from waste-rock piles to date. This work evaluated the ability of dual-domain multicomponent reactive transport models (DDMRTMs) to reproduce breakthrough curves of conservative (chloride) and reactive (molybdenum) solutes observed at a well-characterized experimental waste-rock pile at the Antamina Mine, Peru. We found that the DDMRTM simulations quantitatively matched eight-year-long records of conservative transport through the waste-rock pile when parameterized mainly with field-measured properties obtained from the site and limited calibration. The DDMRTM model also provided a reasonable match to field observations of the reactive solute. The limited calibrated parameters are physically realistic, corroborating the ability of these multidomain models to reproduce the complex reactive-transport processes governing polluted rock drainage from large-scale waste-rock piles.

Sphalerite weathering and controls on Zn and Cd migration in mine waste rock: An integrated study from the molecular scale to the field scale

Degraded water quality and environmental impacts caused by weathering of sulfide-bearing mine wastes are a legacy remaining at many historical mine sites. Release and mobilization of toxic metals (e.g., Zn and Cd) in mine drainage are often associated with weathering of primary sulfide minerals and precipitation and dissolution of secondary minerals. This study aims to couple field-scale measurements of physicochemical parameters with μm- and nm-scale mineralogical characterization to investigate sphalerite weathering and controls on Zn and Cd migration in an uncovered mine waste-rock pile. Elevated concentrations of Zn and Cd, potentially harmful to the receiving environment, were detected in waste-rock pore water and seepage. A suite of analytical techniques, including scanning electron microscopy – energy dispersive X-ray spectroscopy (SEM-EDS), scanning transmission electron microscopy (STEM) with EDS, electron microprobe analysis (EMPA), and synchrotron-based micro X-ray fluorescence (μ-XRF) mapping and micro X-ray absorption near edge structure (μ-XANES) spectroscopy were utilized in a high-resolution investigation of elemental- and secondary-mineral associations with sphalerite grains. EMPA and μ-XRF elemental maps and STEM investigations show depletion of Zn, Fe, S, and Cd, local enrichment of Cu, and distinct dissolution pits at the margins of sphalerite grains. Using μ-XANES, the speciation of solid phases containing Zn, Cu, Fe, and S indicates the dominance of primary sphalerite with microscopic inclusions of chalcopyrite, along with sparse occurrence of secondary Cu-bearing sulfides formed during weathering. These results suggest sphalerite oxidation leads to congruent dissolution without formation of distinct secondary-mineral coatings. Sphalerite weathering significantly contributes to elevated aqueous concentrations of Zn and Cd. Field-scale observations and mineralogical investigations indicate common occurrence of Fe-oxyhydroxides in the waste-rock pile. Seepage chemistry and surface complexation modeling results suggest that pH-dependent variations in reactions on the surfaces of the Fe-oxyhydroxides control aqueous concentrations of Zn and Cd, and the Zn/Cd ratio in the seepage. This work highlights the importance of understanding the mechanisms of sulfide-mineral oxidation and subsequent surface precipitation and adsorption, such that appropriate remediation action can be implemented to limit mobilization of metals to sensitive ecosystems.

Mineralogy-dependent sulfide oxidation via polysulfide and thiosulfate pathways during weathering of mixed-sulfide bearing mine waste rock

Tracking the S oxidation pathway in sulfide-bearing mine waste-rock piles is complicated by variations in water content, O2 and Fe3+ concentrations, microbial diversity, mineralogy, the occurrence of a variety of S species associated with incomplete oxidation, and nonlinear coupling between physicochemical processes. Synchrotron-based S K-edge X-ray absorption near edge structure (XANES) spectroscopy facilitates the identification and quantification of S species (i.e., S2−, S22−, S8, S2O32−, and SO42−) in a weathered mine waste-rock pile containing pyrite, sphalerite, galena, pyrrhotite, and chalcopyrite. Mineralogy-dependent polysulfide and thiosulfate pathways both affect the S oxidation within the waste-rock pile. Currently, the polysulfide pathway, with S8 as the intermediate S species, generates high concentrations of dissolved metals (from the rapid oxidation of monosulfides including sphalerite, galena, pyrrhotite, and chalcopyrite). As these monosulfides are depleted, the thiosulfate pathway of pyrite oxidation, including intermediate oxidation products (i.e., S8 and S2O32−), will become the dominant oxidation pathway, resulting in the potential for generation of additional acidic drainage. This information highlights the importance of identifying the sulfide-mineral oxidation reaction pathways when attempting to estimate acid generation potential, and when developing remediation strategies for the storage and management of sulfide-bearing waste rock.

Environmental radioactivity in gold mining in Burkina Faso and potential recycling of mining waste rocks

ABSTRACT Naturally occurring radionuclides have been reported to occur in high concentrations in gold deposits and with other metal ores in Africa. These radionuclides can be released during mining operations and ore processing, causing harmful radiation exposure of mine workers and nearby populations, and may render the mining waste rocks unusable. One large gold mine site in Burkina Faso, near Mana Village, was assessed for naturally occurring radionuclide levels. The area was surveyed for ambient radiation dose rate. Samples from waste rock and sludge from ore milling were analysed by alpha spectrometry, confirming the presence of naturally occurring radionuclides but in relatively low concentrations. It was concluded that mine workers are not exposed to radiation doses significantly above the unmodified natural radiation background. Furthermore, waste rock piles from mining operations can be recycled and used in road construction.

Exploration geochemistry of surficial media over the high-grade McArthur River uranium deposit, Saskatchewan, Canada

ABSTRACT An orientation survey using surficial media was performed over the high-grade McArthur River unconformity-related U deposit (Saskatchewan, Canada) to test whether or not secondary dispersion of elements related to the ore body or alteration zone can be detected at the surface more than 500 m above the deposit. Organic-rich Ah-horizon soils, Fe-rich B-horizon soils, C-horizon soils, tree cores of Jack pine (Pinus banksiana), and glacially dispersed boulders of Manitou Falls Formation sandstone that host the U deposit were collected in four sampling grids near the mine site. Two of the grids overlaid the trace of the P2 fault that hosts the deposit and extends nearly to the surface, one grid overlaid both the P2 fault and one of the high-grade ore bodies (Zone 4), and one grid was located 2.5 km away from the ore body surface trace in the barren hanging wall of the P2 fault. The grid overlying the Zone 4 ore body had the highest proportion of samples with elevated U and low 207Pb/206Pb ratios, the latter indicative of radiogenic Pb from a high-U source, measured in two size fractions of Ah-horizon soils using Na pyrophosphate leach, pine tree cores using total digestion, and sandstone boulders using 2% HNO3 leach. A handful of pathfinder elements, such as As, Co, Ni, and Pb, are variably associated with the U and radiogenic Pb. Sandstone boulders with an assemblage of dravite + kaolinite ± illite, determined using shortwave infrared (SWIR) spectroscopy and matching the alteration mineralogy in the Manitou Falls Formation above the U deposit, were prevalent in the grid above the Zone 4 ore body and in the adjacent grid in the direction of glacial dispersion. A coarse fraction of the B-horizon soils, leached with 5% HNO3, highlighted the grid above the Zone 4 ore body to a lesser extent, whereas HNO3 leaches and aqua regia digests of C-horizon soil separates did not highlight the P2 fault or ore body trace due to influence by parent till mineralogy. Results of environmental monitoring at the mine site, which was active at the time of sampling, suggest that dust containing U, Pb, and radionuclides from waste rock piles and a ventilation shaft could influence A-horizon soil geochemistry near the mine site, and that U and radiogenic Pb anomalies in B- and C-horizon soils near the water table are close to a treated mine effluent discharge point. However, older trees that record elevated U and radiogenic Pb in annual rings that pre-date mining activity, and alteration mineralogy and geochemistry of boulders that are less susceptible to the influences of mining activity, add confidence that the geochemical anomaly in diverse surficial media above the Zone 4 ore body represents secondary dispersion from the underlying U deposit.

Prediction of acid rock drainage in waste rock piles part 2: Water flow patterns and leaching process

In waste rock piles, the leaching process involved in acid rock drainage is mainly controlled by water flow. This paper (Part 2) investigates the effects of heterogeneities on the water flow patterns by applying probability density functions to hydrogeological properties. In this study, a piecewise constant distribution is proposed to describe the permeability inside waste rock piles, which reflects the effect of both finer and coarser pores. Compared with uniform water flow obtained from traditional homogeneous modeling, various water flow patterns and their pathways inside waste rock piles can be simulated by the proposed model. In addition, the leaching process is also investigated by coupling the calculated water flow with the geochemical reaction based on the water film model proposed in part 1. For demonstration, these models are integrated and applied to the full-scale waste rock pile at Equity Silver mine in British Columbia, Canada. Because the iron loading is highly correlated to the acidity at this site, it is found that the fluctuation of annual lime consumption for neutralization at this site can be well predicted by the integrated model. In addition, the results indicate that waste rock piles with different spatial patterns of permeability distribution, but with the same probability density function, may have different water flow patterns and spatial distributions of iron concentrations inside the pile. However, the total water flow discharge rate and iron loading profiles from the pile are almost the same on the temporal scale.