Geological analogue for circumneutral pH mine tailings: implications for long-term storage,Macraes Mine,Otago, New Zealand

Tailings from the Macraes Au mine cyanidation process are stored in an impoundment about 0.6 km² and 80 m deep whose pH is maintained near 8 by the neutralizing capacity of the gangue minerals. The tailings are sandy (>50 μm particles), have a hydraulic conductivity of about 10⁻² m/day, and contain 0.1–1.0 wt.% S and 0.1–1.5 wt.% graphitic C from the primary deposit. Concentrations of As in the pore water of the mixed tailings, which are a combination of various tailings types, range from 0.1 to 20 ppm, HCO₃^- is 100 to 200 ppm, and dissolved SO₄ is 100–1700 ppm. The mixed tailings will be stored in this impoundment in perpetuity after mining ceases. Confidence in the long-term pH stability of these tailings can be gained from examination of mineralogically and chemically similar geological analogues in the immediate vicinity. A sequence, typically about 5 m thick, of sands and gravels derived from the Macraes mineralized zone 12–28 ka ago contains rounded detrital sulfide mineral grains which are unoxidized despite their close proximity to the surface and the occasional incursion of oxygenated waters. These sediments have a hydraulic conductivity of about 10⁻⁴ m/day. Saturating water pH is currently 7–8. Sands with 0.2–0.8 wt.% organic C host SO₄-reducing bacteria (SRB), and local cementation by authigenic framboidal pyrite has occurred. SRB were found in water-saturated sediments with decreased hydraulic conductivity and alkaline and anoxic conditions. These bacteria are involved in the formation of authigenic framboidal pyrite, reducing the cycling of dissolved Fe in the sediments. Carbon is not a limiting factor in this process as organic matter is present in the sandstone and ground water contains up to 180 ppm HCO₃^-. Comparison of the 28 ka old sediments with the modern tailings suggests that the chemical behaviour of the two will be similar, possibly with the crystallization of authigenic pyrite in the tailings over the long term. As long as the present slightly anoxic and circumneutral pH environmental conditions are maintained in the mixed tailings impoundment, sulfide decomposition and acidification are unlikely.     

Assessment of Hg-contamination in soils and stream sediments in the mineral district of Nambija,Ecuadorian Amazon (example of an impacted area affected by artisanal gold mining)

The Nambija Mineral District (NMD) is located in the southeastern part of Ecuador, east of Zamora (Zamora Chinchípe's country), Ecuadorian Amazon. In this district, Au occurrences have been know since colonial and pre-colonial times, but only after the early 1980s has intensive artisanal Au mining activity been developed. Currently, the different NMD Au occurrences continue to be exploited by artisanal operations and are difficult to control in the study area. The environmental impacts due to Au mining are a consequence of the illegal situation and deficiency in controlling the techniques of ore exploitation. The Au extraction is carried out by outdoor amalgamation, so the indiscriminate use of Hg by artisanal miners, associated with careless methods of tailings disposal, has caused occupational exposure and environmental degradation. The present study evaluated the geochemical dispersion and concentrations of local contamination of metallic Hg in soils, stream sediments and mine tailings in the NMD area. This article aims to contribute to the discussion of environmental changes caused by the artisanal Au mining in the Nambija district. A total of 82 samples (32 soil, 40 stream sediment and 10 mine tailings) were analyzed. The results were compared with the Hg levels in soil and stream sediments considered not to be contaminated in the Nambija mining area and in other areas where Hg is mined in the Amazon basin. In this work, mean total Hg (T-Hg) concentrations of 1.7 μg g⁻¹ in soils and 2.7 μg g⁻¹ in stream sediments have been found. Mercury values in the mine tailing samples revealed values ranging from 89 to 1555 μg g⁻¹. The results found for Hg in the different analyzed materials pointed to contamination of the studied area by this metal, while soil erosion is responsible for an increase in stream sediment's T-Hg concentrations in the different aquatic ecosystems of the Nambija Creek and Nambija River.     

Mobility and fractionation of rare earth elements during supergene weathering and gossan formation and chemical modification of massive sulfide gossan

Primary massive sulfide gossans (MSG) in the Bathurst Mining Camp (BMC), New Brunswick, Canada, are characterized by relative enrichment of Au, Sb, and As, formation of jarosite group minerals (jarosite, plumbojarosite, and argentojarosite) and little or no fractionation in the rare earth elements (REE), including preservation of large positive Eu anomalies (average [Eu/Eu*]_NASC = 4.14 in MSG; 6.61 in massive sulfide mineralization; 0.60 in host rocks). The chemical and mineralogical characteristics of MSG (e.g., Halfmile Lake deposit) imply low pH (<3) and relatively oxidizing conditions during gossan formation; oxidation of a volcanogenic massive sulfide body (comprising pyrite, pyrrhotite, sphalerite, galena, and chalcopyrite) with a falling water table. The lack of light REE or heavy REE fractionation and preservation of positive Eu anomalies characteristic of the original (465 Ma) hydrothermal fluid is consistent with relatively large water-rock ratios during massive sulfide mineralization oxidation, and removal of the REE predominantly as sulfate complexes (LnSO₄⁺, Ln(SO₄)₂⁻). Low pH groundwaters recovered from past producing mines in the BMC display REE patterns reflecting those inferred to have occurred during gossan formation. Gossan at the Restigouche deposit, in contrast to the Halfmile Lake deposit, displays mineralogical and chemical evidence for having been chemically reworked since primary gossan formation. Evidence for chemical reworking includes loss of primary massive sulfide mineralization textures, replacement of plumbojarosite with anglesite, almost complete removal of jarosite minerals, loss of Au, Sb, and As and apparent preferential removal of Eu, resulting in loss of positive Eu anomalies for most samples (average [Eu/Eu*]_NASC = 1.21 in the gossan, with many displaying strong negative anomalies; 3.65 in massive sulfide mineralization; 0.54 in host rocks). Based on geochemical modeling, conditions inferred for the chemical reworking of the Restigouche deposit include near neutral conditions and either relatively oxidizing conditions with Eu²⁺ hosted in a preferentially weathered mineral host (possibly through substitution for Pb in plumbojarosite and beudantite) or cycling between reduced and oxidized conditions during gossan reworking.     

Effects of acid-sulfate weathering and cyanide-containing gold tailings on the transport and fate of mercury and other metals in Gossan Creek: Murray Brook mine,New Brunswick,Canada

Gossan Creek, a headwater stream in the SE Upsalquitch River watershed in New Brunswick, Canada, contains elevated concentrations of total Hg (Hg_T up to 60 μg/L). Aqueous geochemical investigations of the shallow groundwater at the headwaters of the creek confirm that the source of Hg is a contaminated groundwater plume (neutral pH with Hg and Cl concentrations up to 150 μg/L and 20 mg/L, respectively), originating from the Murray Brook mine tailings, that discharges at the headwaters of the creek. The discharge area of the contaminant plume was partially delineated based on elevated pH and Cl concentrations in the groundwater. The local groundwater outside of the plume contains much lower concentrations of Hg and Cl (<0.1 μg/L and 3.8 mg/L, respectively) and displays the chemical characteristics of an acid-sulfate weathering system, with low pH (4.1–5.5) and elevated concentrations of Cu, Zn, Pb and SO₄ (up to 5400 μg Cu/L, 8700 μg Zn/L, 70 μg Pb/L and 330 mg SO₄/L), derived from oxidation of sulfide minerals in the Murray Brook volcanogenic massive sulfide deposit and surrounding bedrock. The Hg_T mass loads measured at various hydrologic control points along the stream system indicate that 95–99% of the dissolved Hg_T is attenuated in the first 3–4 km from the source. Analyses of creek bed sediments for Au, Ag, Cu, Zn, Pb and Hg indicate that these metals have partitioned strongly to the sediments. Mineralogical investigations of the contaminated sediments using analytical scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM), reveal discrete particles (<1–2 μm) of metacinnabar (HgS), mixed Au–Ag–Hg amalgam, Cu sulfide and Ag sulfide.     

Heavy metal contamination of soils and waters in and around the Imcheon Au–Ag mine,Korea

The heavy metal contamination of soils and waters by metalliferous mining activities in an area of Korea was studied. In the study area of the Imcheon Au–Ag mine, soils and waters were sampled and analyzed using AAS for Cd, Cu, Pb and Zn. Analysis of HCO₃⁻, F⁻, NO₃⁻ and SO₄²⁻ in water samples was also undertaken by ion chromatography. Elevated concentrations of the metals were found in tailings. The maximum contents in the tailings were 9.4, 229, 6160 and 1640 mg/kg extracted by aqua regia and 1.35, 26.4, 70.3 and 410 mg/kg extracted by 0.1 N HCl solution for Cd, Cu, Pb and Zn, respectively. These metals are continuously dispersed downstream and downslope from the tailings by clastic movement through wind and water. Because of the existence of sulfides in the tailings, a water sample taken on the tailings site was very acidic with a pH of 2.2, with high total dissolved solids (TDS) of 1845 mg/l and electric conductivity (EC) of 3820 μS/cm. This sample also contained up to 0.27, 1.90, 2.80, 53.4, 4,700 mg/l of Cd, Cu, Pb, Zn and SO₄²⁻, respectively. TDS, EC and concentrations of metals in waters decreased with distance from the tailings. The total amount of pulverized limestone needed for neutralizing the acid tailings was estimated to be 46 metric tons, assuming its volume of 45,000 m³ and its bulk density of 1855 kg/m³.     

Tailings dam seepage at the rehabilitated Mary Kathleen uranium mine,Australia

This study reports on the seepage of metals, metalloids and radionuclides from the Mary Kathleen uranium mill tailings repository. Since rehabilitation in the 1980s, the capped tailings have developed a stratified hydrochemistry, with acid (pH 3.7), saline, metal-rich (Fe, Mn, Ni, U ± As, Pb, Zn), oxygenated (1.05 mg L⁻¹ DO), radioactive waters in the upper tailings pile and near-neutral pH (pH 7.57), metal-poor, reduced (0.08 mg L⁻¹ DO) waters at depth. Seepage (∼0.5 L s⁻¹) of acid (pH 5.5), metal-rich (Fe, Mn ± Ni, U, Zn), radioactive (U-235, U-238, Ra-226, Ra-228, Ac-227) waters occurs from the base of the tailings dam retaining wall into the former evaporation pond and local drainage system. Oxygenation of the seepage waters causes the precipitation of Fe and coprecipitation and adsorption of other metals (U, Y), metalloids (As), rare earth elements (Ce, La) and radionuclides (U-235, U-238). By contrast, alkalis and alkaline–earth elements (Ca, K, Mg, Na, Sr), Mn, sulfate and to some degree metals (U, Zn, Ni), rare earth elements (Ce, La) and radionuclides (U-235, U-238, Ra-226, Ra-228) remain in solution until pH neutralisation and evaporation lead to their precipitation in efflorescences and sulfate-rich evaporative sediments. While the release of contaminant loads from the waste repository through seepage is insignificant (e.g. ∼5 kg of U per year), surface waters downstream of the tailings impoundment possess TDS, U and SO₄ concentrations that exceed Australian water quality guideline values in livestock drinking water. Thus, in areas with a semi-arid climate, even insignificant load releases of contaminants from capped tailings repositories can still cause the deterioration of water quality in ephemeral creek systems.     

Consolidation and incipient oxidation of alkaline arsenopyrite-bearing mine tailings,Macraes Mine,New Zealand

Fine grained (ca. 15 μm), arsenopyrite-bearing mine tailings have been exposed to drying and oxidation for 4 a pending relocation. The tailings are still partly covered by a pond of decanted pore waters. The water table in drying tailings has lowered by 1–3 m and desiccation cracks up to 2 cm wide have formed on the 1 m scale, extending through the unsaturated zone. Tailings in the unsaturated zone have similar pore water contents to saturated tailings: typically 16–32 wt% water. Saturated tailings retain alkaline pH (ca. 10) from the mine cyanidation plant, but pH lowers progressively towards ca. 7 near the surface, or near desiccation cracks, in the unsaturated zone. The redox state of the tailings changes in parallel with pH, with an empirical relationship: Eh(mV)=−55 pH+290. Water in the remnant decant pond reflects this relationship also. Unsaturated tailings have variable but low permeabilities, typically 10⁻³ to 10⁻⁴ m/day, and more permeable horizons have allowed incursion of oxygenated air and/or rain water from desiccation cracks. Sulphide grains in all tailings examined are unaltered. Sulphides and solutions in the tailings are out of thermodynamic equilibrium predicted from the redox–pH conditions, due to kinetic constraints. Incursion of rain water locally facilitates deposition from pore waters of insoluble Fe oxide and arsenate minerals, thus fixing As in the dry unsaturated tailings.     

Mercury mobility in unsaturated gold mine tailings,Murray Brook mine,New Brunswick,Canada

Elevated concentrations of Hg are present (averaging 36 μg/g), mainly as cinnabar, in the Murray Brook Au deposit, located in northern New Brunswick, Canada. After the mined ore was subjected to CN leaching, the tailings were deposited in an unsaturated pile, and 10 a after mine closure an estimated 4.7 × 10³ kg of CN and 1.1 × 10⁴ kg of Hg remain in the pile. Elevated Hg concentrations have been measured in the groundwater (up to 11,500 μg/L) and surface water (up to 32 μg/L) down-gradient of the tailings. To investigate the controls on Hg mobility and leaching persistence, laboratory experiments were conducted using unsaturated columns filled with tailings. Within the first 0.2 pore volumes (PV) eluted, the concentrations of Hg and CN increased to peak concentrations of 12,900 μg Hg/L and 16 mg CN/L, respectively. In the subsequent 0.9 PV, concentrations decreased to approximately 1300 μg Hg/L and 2.8 mg CN/L. Thermodynamic calculations demonstrate that >99.8% of the mobilized Hg in the tailings pore water is in the form of Hg–CN complexes, indicating that Hg mobility to the surrounding aquatic environment is directly dependent on the rate of CN leaching. One-dimensional transport simulations suggest that leached CN can be partitioned into conservative (24%) and non-conservative (76%) fractions. Extrapolation of simulation results to the field scale suggests that CN, and by extension Hg, will continue to elute from the tailings for at least an additional 130 a.     

Natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water: the example of West Bengal and its worldwide implications

In order to investigate the mechanism of As release to anoxic ground water in alluvial aquifers, the authors sampled ground waters from 3 piezometer nests, 79 shallow (<45 m) wells, and 6 deep (>80 m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of As (200–1180 μg L⁻¹) are accompanied by high concentrations of Fe (3–13.7 mg L⁻¹) and PO₄ (1–6.5 mg L⁻¹). Ground water that is rich in Mn (1–5.3 mg L⁻¹) contains <50 μg L⁻¹ of As. The composition of shallow ground water varies at the 100-m scale laterally and the metre-scale vertically, with vertical gradients in As concentration reaching 200 μg L⁻¹ m⁻¹. The As is supplied by reductive dissolution of FeOOH and release of the sorbed As to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (<10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release As to ground water.Arsenic concentrations are high (≫50 μg L⁻¹) where reduction of FeOOH is complete and its entire load of sorbed As is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction is incomplete, the sediments are brown in colour and resorption of As to residual FeOOH keeps As concentrations below 10 μg L⁻¹ in the presence of dissolved Fe. Sorbed As released by reduction of Mn oxides does not increase As in ground water because the As resorbs to FeOOH. High concentrations of As are common in alluvial aquifers of the Bengal Basin arise because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common.     

Hydrogeochemical exploration for gold in the Osilo area,Sardinia, Italy

A study to test the use of hydrogeochemical methods for gold prospecting was carried out in the Osilo area, northern Sardinia. The study area, covering about 30 km² is characterised by Tertiary andesitic rocks. Gold concentrations up to several ppm, associated with abundant pyrite, arsenopyrite, stibnite, tetrahedrite and electrum, and subordinate galena, sphalerite and chalcopyrite, are present in quartz veins associated with a polyphase, incipient and pervasive alteration of the andesitic rocks.Forty-eight water samples (17 streams, 29 springs and 2 boreholes) were analysed for Au and a wide range of major and trace elements, both in solution (< 0.4 μm) and in suspension. Background values for dissolved Au were below the detection limit of the methods used (between 0.3 and 0.5 ng L⁻¹ Au). Gold concentrations in solution up to 3 ng L⁻¹ were found in waters draining the mineralised vein system. The observed dispersion of Au in surface waters was restricted to about 500 m from the auriferous veins. Dissolved Au anomalies do not vary significantly in water samples, taken monthly over a one year period, suggesting that the dispersion of Au is unaffected by seasonal conditions in the Osilo area. For samples where Au was detected both in solution and in suspension, the Au content of the suspended matter was usually lower than that in solution.The best indicators of Au mineralisation, apart from Au itself, both in solution and in suspension, were As and Sb which showed a dispersion clearly related to the known auriferous veins.     

Methylmercury in tailings ponds of Amazonian gold mines (French Guiana): Field observations and an experimental flocculation method for in situ remediation

Sites of monomethylmercury (MMHg) production in Amazonian regions have been identified in hydraulic reservoirs, lake sediments and wetlands, but tailings ponds have not yet received sufficient attention for this purpose. This work evidenced high MMHg production within the water column and the interstitial water of two tailings ponds of French Guiana Au mines located; (i) in a small scale exploitation (Combat) where Hg was used for Au amalgamation, and (ii) in an industrial on-going Au mine (Yaoni) processing without Hg. The (MMHg)_D maximum (2.5 ng L⁻¹) occurred in the oxic water column above the sediment-water interface (SWI) of the most recent tailings pond (Combat), where the substrate was fresh, the redox transition was sharp and the pool of total Hg was large. In the Yaoni pond, the (MMHg)_D maximum concentration (1.4 ng L⁻¹) was located at the SWI where suboxic conditions prevailed. Using the (MMHg)_D concentration as a proxy for Hg methylation rates, the present results show that Hg methylation may occur in various redox conditions in tailings ponds, and are favored in areas where the organic matter regeneration is more active.A 3-month long laboratory experiment was performed in oxic and anoxic boxes filled with high turbidity waters from the Combat Au mine to simulate tailings ponds. Slaked lime was added in an experimental set (2 mg L⁻¹) and appeared to be very efficient for the reduction of suspended particulate matter (SPM) to environmentally acceptable concentrations. However, at the end of the experiment, large (MMHg)_D concentrations were monitored under treated anoxic conditions with the (MMHg)_D maximum located at the SWI above the Fe-reducing zones. No (MMHg)_D was detected in oxic experiments. The use of slaked lime for SPM decantation appears to be an efficient and non-onerous process for Au miners to avoid Hg methylation in tailings ponds when it is combined with rapid drainage of the mine waters. A subsequent human intervention is however necessary for the recovery of soil structure through the cover of dried ponds with organic rich materials and reforestation to avoid the stagnation of rain waters and the occurrence of anoxia.     

Petrochemistry of Granitoids Along the Loei Fold Belt,Northeastern Thailand

Petrochemical characteristics of Permo‐Triassic granitoids from five regions (i) Mung Loei, (ii) Phu Thap Fah – Phu Thep, (iii) Phetchabun, (iv) Nakon Sawan – Lobburi, and (v) Rayong – Chantaburi along the Loei Fold Belt (LFB), northeastern Thailand were studied. The LFB is a north–south trending 800 km fold belt that hosts several gold and base‐metal deposits. The granitoids consist of monzogranite, granodiorite, monzodiorite, tonalite, quartz‐syenite, and quartz‐rich granitoids. These are composed of quartz, plagioclase, and K‐feldspar with mafic minerals such as hornblende and biotite. Accessory minerals, such as titanite, zircon, magnetite, ilmenite, apatite, garnet, rutile, and allanite are also present. Magnetic susceptibilities in the SI unit of granitoids vary from 6.5 × 10⁻³ to 15.2 × 10⁻³ in Muang Loei, from 0.1 × 10⁻³ to 29.4 × 10⁻³ in Phu Thap Fah – Phu Thep, from 2.7 × 10⁻³ to 34.6 × 10⁻³ in Petchabun, from 2.4 × 10⁻³ to 14.1 × 10⁻³ in Nakon Sawan – Lobburi, and from 0.03 × 10⁻³ to 2.8 × 10⁻³ in Rayong – Chantaburi. Concentration of major elements suggests that these intermediate to felsic plutonic rocks have calc‐alkaline affinities. Concentration of REE of the granitoids normalized to chondrite displays moderately elevated light REE (LREE) and relatively flat heavy (HREE) patterns, with distinct depletion of Eu. Rb versus Y/Nb and Nb/Y tectonic discrimination diagrams illustrate that the granitoids from Muang Loei, Phu Thap Fah – Phu Thep, Phetchabun, Nakon Sawan – Lobburi, and Rayong – Chantaburi formed in continental volcanic‐arc setting. New age data from radiometric K‐Ar dating on K‐feldspar from granodiorite in Loei and Nakhon Sawan areas yielded 171 ± 3 and 221 ± 5 Ma, respectively. K‐Ar dating on hornblende separated from diorite in Lobburi yielded 219 ± 8 Ma. These ages suggest that magmatism of Muang Loei occurred in the Middle Jurassic, and Nakon Sawan – Lobburi occurred in Late Triassic. Both Nb versus Y and Rb versus (Y + Nb) diagrams and age data indicate that Nakon Sawan – Lobburi granitoids intruded in Late Triassic at Nong Bua, Nakon Sawan province and Khao Wong Phra Jun, Lobburi province in volcanic arc setting. Muang Loei granitoids at the Loei province formed later in Middle Jurassic also in volcanic arc setting. The negative δ³⁴S_CDT values of ore minerals from the skarn deposit suggest that the I‐type magma has been influenced by light biogenic sulfur from local country rocks. The Au‐Cu‐Fe‐Sb deposits correlate with the magnetite‐series granitoids in Phetchabun, Nakon Sawan – Lobburi and Rayong – Chantaburi areas. Metallogeny of the Au and Cu‐Au skarn deposits and the epithermal Au deposit is related to adakitic rocks of magnetite‐series granitoids from Phetchabun and Nakon Sawan areas. All mineralizations along the LFB are generated in the volcanic arc related to the subduction of Paleo‐Tethys. The total Al (^TAl) content of biotite of granitoids increases in the following order: granitoids associated with Fe and Au deposit < with Cu deposit < barren granitoids. X_Mg of biotite in granitoids in Muang Loei indicates the crystallization of biotite in magnetite‐series granitoids under high oxygen fugacity conditions. On the other hand, low X_Mg (<0.4) of biotite in magnetite‐series granitoids in Phu Thap Fah – Phu Thep and Rayong – Chantaburi indicates a reduced environment and low oxygen fugacity, associated with Au skarn deposit (Phu Thap Fah) and Sb‐Au deposit (Bo Thong), respectively. The magnetite‐series granitoids at Phu Thap Fah having low magnetic susceptibilities and low X_Mg of biotite were formed by reduction of initially oxidizing magnetite‐series granitic magma by interaction with reducing sedimentary country rocks as suggested by negative δ³⁴S_CDT values.     

Nature and practical implications of heterogeneities in the geochemistry of zinc-rich,alkaline mine waters in an underground F–Pb mine in the UK

Water pollution arising from base metal sulphide mines is problematic in many countries, yet the hydrogeology of the subsurface contaminant sources is rarely well-characterized. Drainage water pumped from an active F–Pb mine in northern England has unusual chemistry (alkaline with up to 40 mg.l⁻¹ Zn) which profoundly impacts the ecology of the receiving watercourse. Detailed in-mine surveys of the quantity and quality of all ground water inflows to the mine were made. These revealed major, temporally persistent heterogeneities in ground water quality, with three broad types of water identified as being associated with distinct hydrostratigraphic units. Type I waters (associated with the Firestone Sill aquifer) are cool (<10°C), Ca–HCO₃–SO₄ waters, moderately mineralized (specific electrical conductance (SEC)≤410 μS.cm⁻¹) with <4 mg.l⁻¹ Zn. Type II waters (associated with the Great Limestone aquifer) are warmer (≈15°C), of Ca–SO₄ facies, highly mineralized (SEC≤1500 μS.cm⁻¹) with ≤40 mg.l⁻¹ Zn. Type III waters (in the deepest workings) are tepid (>18°C), of Ca–HCO₃–SO₄ facies, intermediately mineralized (SEC≤900 μS.cm⁻¹) with ≤13 mg.l⁻¹ Zn, and with significant Fe (≤12 mg.l⁻¹) and Pb (≤8 mg/l). Monotonic increases in temperature and Cl⁻ concentration with depth contrast with peaks in total mineralization, SO₄ and Zn at medium depth (in Type II waters). Sulphate, Pb and Zn are apparently sourced via oxidation of galena and sphalerite, which would release each metal in stoichiometric equality with SO₄. However, molal SO₄ concentrations typically exceed those of Pb and Zn by 2–3 orders of magnitude, which mineral equilibria suggest is due to precipitation of carbonate “sinks” for these metals. Contaminant loading budgets demonstrate that, although Type II waters amount to only 25% of the total ground water inflow to the mine, they account for almost 60% of the total Zn loading. This observation has important management implications for both the operational and post-abandonment phases of the mine life cycle.     

Partitioning and mobility of trace metals in the Blesbokspruit: Impact assessment of dewatering of mine waters in the East Rand,South Africa

A suite of trace metals was analyzed in water and sediment samples from the Blesbokspruit, a Ramsar certified riparian wetland, to assess the impact of mining on the sediment quality and the fate of trace metals in the environment. Limited mobility of trace metals was observed primarily because of their high partition coefficient in alkaline waters. Nickel was most mobile with a mean K_d of 10^3.28 L kg⁻¹ whereas Zr was least mobile with a mean K_d of 10^5.47 L kg⁻¹. The overall trace metal mobility sequence, derived for the Blesbokspruit, in increasing order, is: Zr < Cr < Pb < Ba < V < Cu < Zn < Sr < Mn < U < Mo < Co < Ni. Once removed from the solution, most trace metals were preferentially associated with the carbonate and Fe–Mn oxide fraction followed by the exchangeable fraction of the sediments. Organic C played a limited role in trace metal uptake. Only Cu was primarily associated with the organic fraction whereas Ti and Zr were mostly found in the residual fraction. Compared to their regional background, Au and Ag were most enriched, at times by a factor of 20–400, in the sediments. Significant enrichment of U, Hg, V, Cr, Co, Cu and Zn was also observed in the sediments.The calculated geoaccumulation indices suggest that the sediments are very lightly to lightly polluted with respect to most trace metals and highly polluted with respect to Au and Ag. The metal pollution index (MPI) for the 20 sampled sites varied between 2.9 and 45.7. The highest MPI values were found at sites that were close to tailings dams. Sediment eco-toxicity was quantified by calculating the sediment quality guideline index (SQG-I). The calculated SQG-I values (0.09–0.69) suggest that the sediments at the study area have low to moderate potential for eco-toxicity.     

Geochemical controls on arsenic distribution in the Baccu Locci stream catchment (Sardinia,Italy) affected by past mining

The Baccu Locci stream catchment (Sardinia, Italy) is affected by serious As contamination as a consequence of past mining. The presence of both point and widespread sources of contamination (waste-rock dumps and flotation tailings, respectively) strongly affects surface water chemistry, and produces high As concentrations (hundreds of μg l⁻¹) in stream waters. Water chemistry of the Baccu Locci stream changes considerably over a distance of about 10 km as a consequence of various, locally concomitant, processes acting along the stream course: (1) mixing with metal-rich SO₄ waters; (2) dissolution/precipitation of metal-bearing phases; (3) mixing with HCO₃-dominated lake waters; (4) gypsum dissolution coupled with calcite precipitation; (5) mixing with dilute surface and/or ground waters. In contrast to metals (e.g. Pb, Cu, Zn and Cd), whose dissolved concentrations rapidly decrease downstream of the mined area through (co-)precipitation/adsorption mechanisms, As concentrations tend to gradually increase (up to 0.9 mg l⁻¹) along the stream course as far as the alluvial plain, though significant variations are locally observed. This behaviour is mainly due to the higher mobility of As than metals under the near neutral-oxidative conditions occurring in the Baccu Locci stream waters. Results of a leaching test indicate that part of the As contained in the flotation tailings occurs as As(III), which is more mobile and less strongly sorbed than As(V). The As released to the waters by various mechanisms (i.e. release/desorption from the Fe(III)-hydroxides coatings of silicate grains, oxidation of residual arsenopyrite, decomposition of scorodite) tends to remain in solution and to be transported long distances. As a consequence of the widespread presence of highly As-contaminated flotation tailings all over the medium-lower Baccu Locci stream catchment, long-term As contamination is expected.     

Arsenic in surface- and up to 90°C ground waters in a basalt area,N-Iceland: processes controlling its mobility

The concentrations of As in surface- and up to 90 °C ground waters in a tholeiite flood basalt area in N-Iceland lie in the range <0.03–10 μg/kg. With few exceptions surface waters contain <0.5 μg/kg As whereas ground waters generally contain >0.5 μg/kg As. The As content of ground waters increases on the whole with rising temperature. Arsenic is highly mobile in the basalt-water environment of the study area. An insignificant fraction of the As dissolved from the rock is taken up into secondary minerals. Arsenic is less mobile than B but considerably more mobile than Na which has the highest mobility among the major aqueous components. A significant fraction of the As in the basalt occurs in an easily soluble form. The As hosted in the primary minerals is expected to be concentrated in the titano-magnetite. This mineral is stable in contact with both surface- and ground waters and does not, therefore, supply As to the water, explaining the difference in mobility between As and B. Aqueous As concentrations are a reflection of water/rock ratios, i.e. how much rock a given quantity of water has dissolved. This ratio increases with increasing temperature and increasing residence time of the water in contact with the rock. The distribution of As species has been calculated on the assumption of equilibrium at the redox potential retrieved from measurement of aqueous Fe(II) and Fe(III) concentrations. These calculations indicate that pentavalent As is stable in surface waters and in ground waters with an in situ pH of <10 and would occur mostly as H₂AsO₄⁻ and HAsO₄⁻². In higher pH ground waters the concentrations of the arsenite species H₂AsO₃⁻ is significant at equilibrium, up to 65% of the total dissolved As.     

Contrasting dispersion patterns for gold in stream sediments at Timbarra,NSW, Australia

Regional geochemical exploration for granite-hosted Au mineralisation on the Timbarra Plateau has identified poor correlation between bulk cyanide leach (BCL) Au contents in the wet-screened <2000-μm fraction and aqua regia (AR) extractable Au in the wet-screened <180-μm fraction of stream sediments across most parts of the plateau.At James East, there is a weak response to mineralisation in both the BCL and <180-μm fraction Au contents of stream sediments and poor correlation between these fractions. The total Au content of the sediments is primarily contained within the organic-rich <63-μm fraction. There is little visible Au in panned concentrates from stream sediments or adjacent alluvium. Dispersion of Au appears to be predominantly hydromorphic with minor mechanical contributions from a more diffuse palaeo-colluvial source.At the nearby Surface Hill, there is a strong response to mineralisation in stream sediments with contributions to total Au from a wide range of size fractions. The BCL and <180-μm fraction Au contents are closely correlated. Visible Au is present close to bedrock sources in eluvial and alluvial accumulations. Particle size and Au distribution in the stream sediments indicates dispersion of Au to be dominantly by mechanical processes from the proximal part of a colluvial fan on the margin of the plateau.Variations in the concentration, form and distribution of Au in stream sediments at the two sites can be attributed to variations in surficial conditions and the effects of varying amounts of organic material; however, a complimentary factor is the potential loss of colloidal or organic-adsorbed Au through decanting of sieve water during wet-screening of sediments in the field.     

Thermodynamics and organic matter: constraints on neutralization processes in sediments of highly acidic waters

The controls on the internal neutralization of low productivity, highly acidified waters by sulfide accumulation in sediments are yet poorly understood. It is demonstrated that the neutralization process is constrained by organic matter quality and thermodynamic effects which control the relative rates of SO₄ and Fe reduction, and the fate of the reduced Fe and S in the sediments. The investigated sediments were rich in dissolved Fe(II) (0.005–12 mmol l⁻¹) and SO₄ (1.3–22 mmol l⁻¹). The pH ranged from 3.0 to 6.8. Contents of reduced inorganic S (0.1–9.5%), molar C/N ratios of the organic matter (12–80) and metabolic turnover rates (1–110 μeq cm⁻³ a⁻¹) varied strongly. Substantial amounts of Fe sulfides were only found at a simultaneous partial thermodynamic and solubility equilibrium of the involved biogeochemical processes. Sulfide oxidation was apparently inhibited, and SO₄ and Fe reduction coexisted. In this type of sediment increases in C availability cause enhanced neutralization rates. In the absence of a partial equilibrium, the sediments were in a sulfide oxidizing and Fe reducing state, and did not accumulate Fe sulfides. The latter type of sediment will increase neutralization rates in response to decreasing deposition of reactive Fe oxides but not necessarily in response to increases in lake productivity by e.g. fertilization measures.     

The mobility of radium-226 and trace metals in pre-oxidized subaqueous uranium mill tailings

The exchange of ²²⁶Ra and trace metals across the tailings-water interface and the mechanisms governing their mobility were assessed via sub-centimetre resolution profiling of dissolved constituents across the tailings–water interface in Cell 14 of the Quirke Waste Management Area at Rio Algom's Quirke Mine, near Elliot Lake, Ontario, Canada. Shallow zones (<1.5 m water depth) are characterized by sparse filamentous vegetation, well-mixed water columns and fully oxygenated bottom waters. Profiles of dissolved O₂, Fe and Mn indicate that the tailings deposits in these areas are sub-oxic below tailings depths of ∼3 cm. These zones exhibit minor remobilization of Ra in the upper 5 cm of the tailings deposit; ²²⁶Ra fluxes at these sites are relatively small, and contribute negligibly to the water column activity of ²²⁶Ra. The shallow areas also exhibit minor remobilization of Ni, As, Mo and U. The release of these elements to the water cover is, however, limited by scavenging mechanisms in the interfacial oxic horizons. The presence of thick vegetation (Chara sp.) in the deeper areas (>2 m water depth) fosters stagnant bottom waters and permits the development of anoxia above the benthic boundary. These anoxic tailings are characterized by substantial remobilization of ²²⁶Ra, resulting in a relatively large flux of ²²⁶Ra from the tailings to the water column. The strong correlation between the porewater profiles of ²²⁶Ra and Ba (r²=0.99), as well as solubility calculations, indicate that the mobility of Ra is controlled by saturation with respect to a poorly ordered and/or impure barite phase [(Ra,Ba)SO₄]. In the anoxic zones, severe undersaturation with respect to barite is sustained by microbial SO₄ reduction. Flux calculations suggest that the increase in ²²⁶Ra activity in the water cover since 1995 (from <0.5 to 2.5 Bq l⁻¹) can be attributed to an increase in the spatial distribution of anoxic bottom waters caused by increased density of benthic flora. The anoxic, vegetated areas also exhibit minor remobilization with respect to dissolved As, Ni and Zn. The removal of trace metals in the anoxic bottom waters appears to be limited by the availability of free sulphide. Collectively, the data demonstrate that while the water cover over the U mill tailings minimizes sulphide oxidation and metal mobility, anoxic conditions which have developed in deeper areas have led to increased mobility of ²²⁶Ra.