Blowes-Geochemical and Microbiological Characterization Tailings-1998

Geochemical, mineralogical and microbiological characterization of a

sulphide-bearing carbonate-rich gold-mine tailings impoundment,

Joutel, QueÂ bec

David W. Blowes*, John L. Jambor and Christine J. Hanton-FongDepartment of Earth Sciences, University of Waterloo, Waterloo, Ont., Canada, N2L 3G1

Lyne Lortie and W. Douglas GouldEnvironmental Laboratory, CANMET, 555 Booth Street, Ottawa, Ont., Canada K1A 0G1

(Received 16 January 1996; accepted in revised form 31 October 1997)

AbstractÐThe results of an integrated geochemical and mineralogical study conducted at the Agnico-Eagle gold-mine tailings impoundment, Joutel, QueÂ bec, are correlated with bacterial populations deter-mined from an enumeration of 3 groups of Thiobacilli. The tailings were determined to contain ap-proximately 5 wt.% sulphide±S, predominantly as pyrite, and up to 30 wt.% carbonate minerals, chie¯yas dolomite±ankerite and siderite. The objective of the study was to evaluate the potential for the devel-opment of acidic drainage and dissolved-metal migration in carbonate-rich tailings impoundments, andto compare the results of the geochemical and microbiological characterization of the tailings. Sul-phide-oxidation reactions have proceeded to a depth of 20±100 cm below the tailings surface. Pyrrhotiteconsistently shows more alteration than pyrite and arsenopyrite. Pyrrhotite is altered mainly throughthe replacement by goethite. The most abundant Thiobacilli are neutrophilic bacteria of the Thiobacillusthioparus type. The maximum most probable number values for these bacteria occur 20±40 cm belowthe tailings surface, a depth that coincides with the disappearance of oxide coatings. This observation,coupled with the sharp decline in gas-phase O2 concentration, suggests that rapid bacterially-mediatedS±oxidation is occurring at this depth. The pore-water pH throughout the tailings varies between 6.5and 8.5; no low-pH waters were observed in the impoundment. These neutral pH conditions are attrib-uted to the e�ect of acid-consuming carbonate-mineral dissolution reactions, which are also indicatedby increased concentrations of Mg and Ca and alkalinity in the shallow zone of the tailings. As a resultof these acid-neutralization reactions, dissolved metal concentrations are low. # 1998 Elsevier ScienceLtd. All rights reserved

INTRODUCTION

The Agnico-Eagle Au mine, which was in pro-

duction from 1974 to 1994, is near Joutel, QueÂ bec,

approximately 195 km N of Val d'Or (Fig. 1). The

Au occurs within sulphide-rich portions of a pyriti-

ferous stratiform ore deposit (Barnett et al., 1982).

The adjacent Telbel mine, which shared the same

tailings impoundment and ore-processing facilities,

began production in 1985. Gold was extracted from

the ore by ¯otation of the sulphide portion, fol-

lowed by cyanidization and Au reprecipitation. The

mill has the capacity to process about 1600 t/d, but

the recent annual production was 330 000±385 000 t,

grading about 5.8 g/t Au and 1.8 g/t Ag.

Tailings generated at the Agnico-Eagle site were

deposited in a 120 ha elevated impoundment W of

the mine site (Fig. 1). The impoundment is divided

into an older segment in which tailings were depos-

ited from 1974 to 1986, and a newer one in which

deposition occurred from 1986 to 1994. The tailings

contain 4±5 wt.% sulphide±S, mainly as pyrite

[FeS2] and pyrrhotite [Fe(1ÿx)S], with lesser amounts

of arsenopyrite [FeAsS]. The carbonate content of

the tailings is approximately 20 wt.% as CO3, pre-

dominantly as siderite [FeCO3], dolomite±ankerite

[CaMg(CO3)2-Ca(Fe,Mg)(CO3)2], and calcite

[CaCO3]. Sulphide-rich tailings are a potential

source of dissolved metals and acidity that results

from the microbially-catalyzed oxidation of sul-

phide minerals, such as pyrite. This study provides

speci®c information regarding the mineralogical and

microbiological character and interactions that

occur within the Agnico-Eagle tailings.

In this study the results of a 1993 ®eld investi-

gation of the geochemical characteristics of the

older tailings are compared with the results of a

concurrent mineralogical study and microbiological

enumeration (Jambor et al., 1993). To facilitate this

comparison, samples were collected at four piezo-

meter-nest locations on the tailings impoundment.

The primary objectives of the research program

were to determine the potential for release of acidic

Applied Geochemistry, Vol. 13, No. 6, pp. 687±705, 1998# 1998 Elsevier Science Ltd

All rights reserved. Printed in Great Britain0883-2927/98 $ - see front matterPII: S0883-2927(98)00009-2

*Corresponding author.

687

drainage and dissolved metals from the tailingsarea, and to determine the types and numbers ofThiobacilli within the impoundment. These objec-

tives were carried out so that an integrated modelof biotic and abiotic mechanisms within theAgnico-Eagle tailings could be developed.

METHODOLOGY

Microbiological methods

Core samples for microbiological study were collectedusing thin-walled 5.08 cm Al tubing. Thirty-one sampleswere taken at 10±20 cm depth intervals at 4 locations onthe tailings impoundment: AE2, AE3, AE4, and AENT(Fig. 1). Of these, the ®rst 3 are sited on the older portionof the tailings impoundment (deposited before 1986), andAENT is on the new tailings area (deposited since 1986).

The cores were stored and shipped in ice-packed cool-ers, and were delivered to the laboratory at CANMETwithin 24 h of collection. In the laboratory, the Al casingwas cut perpendicular to its long axis and a sample wastaken from the center of the tube. Microbiological enu-meration was conducted using the techniques and media

described by Blowes et al. (1995). Three general physio-logical groups of Thiobacilli were enumerated by the mostprobable number technique (MPN) (Cochran, 1950) inorder to enumerate: (1) neutrophilic Thiobacilli (T. thio-parus and related species), (2) acid-tolerant S oxidizers (T.thiooxidans), and (3) acidophilic Fe oxidizers (T. ferrooxi-dans and related species).

Mineralogical methods

Core samples for mineralogical study were collected at 4piezometer nests on the old impoundment: AE1, AE2,AE3, and AE5 (Fig. 1). All cores were obtained usingthin-walled Al tubes (5.08 cm in diameter), that were cutinto lengths of about 0.5 m after core retrieval. The coreswere kept frozen and were split lengthwise with a bandsaw. One split of each core was extruded from its half-bar-rel onto a plastic sheet and was allowed to thaw and dryat room temperature (25±308C). Similar procedures, whichprevent the loss of pore water and minimize the possibledehydration of secondary precipitates, have been success-fully used in the mineralogical studies of tailings from sev-eral other impoundment sites (e.g., Blowes and Jambor,1990). Prior to removal of the material selected for miner-alogical studies, both the wet and the dried cores were exam-ined for signs of ocherous secondary minerals and

Fig. 1. Plan view of the Agnico-Eagle tailings impoundment.

D. W. Blowes et al.688

precipitates that may have formed during drying. The gen-eral megascopic features were logged. Most of the samplesused for detailed studies consisted of material that had beenremoved from the center of the core along its length. Theresulting samples were then sieved to pass a 300 mm screen.In addition to the selection of these powders, 8 polished thinsections were cut from the ocherous parts of the cores, asthese probably represent the most intensely oxidized portionof the tailings. These thin-section portions were kept intactby impregnation of the core with repeated applications ofcyanoacrylate adhesive prior to thin-section preparation(Jambor, 1994). In all, 65 polished sections and 9 polishedthin sections were examined by optical microscopy usingtransmitted-light and re¯ected-light techniques.

Splits of the screened samples that were used for thepolished sections were used for X-ray di�ractometry.Samples were ground in acetone and the powdered mountswere run with a Rigaku2 di�ractometer using Co X-radi-ation. Sixty-four bulk samples corresponding to those usedfor optical microscopy were processed. Several acid-treatedsamples were run speci®cally for carbonate±mineral determi-nations. The acid treatments involved di�erential dissolutionusing HCl at various strengths to dissolve the carbonate min-

erals, thereby con®rming the identi®cation of carbonate-mineral X-ray peaks by their elimination from di�racto-grams of the acid-treated residues. Acid treatment of calcar-eous samples also simpli®ed the X-ray patterns by reducingthe total number of X-ray peaks.Microbeam analyses were conducted with a JEOL2

820 scanning electron microscope coupled with energy-dis-persion facilities for elemental analyses, and a JEOL2733 electron microprobe with the capacity for energy-dis-persion and wavelength quantitative chemical analyses. Asthe extent of alteration and its intensity in the Agnico-Eagle tailings were found to be limited, quantitative ana-lyses were restricted to the determination of the compo-sitions of the carbonate minerals.

Hydrogeochemical study

Groundwater ¯ow system. A network of 5 piezometernests, including 31 individual piezometers, was installed inthe tailings (Fig. 1; Table 1) to provide hydraulic headand hydraulic conductivity data for use in the delineationof the groundwater ¯ow system. Water samples collected

Table 1. Piezometer information, including hydraulic conductivity values (calculated from piezometer response tests),measured water levels, and hydraulic head values.

HydraulicStatic water level below ground surface Hydraulic head

Well conductivity (m/s) June 13/93 (m) Sept 8/93 (m) June 13/93 (m) Sept 8/93 (m)

AE 1±1 8.3E-08 0.18 0.91 1557.34 1556.61AE 1±2 6.7E-08 0.29 1.39 1557.23 1556.13AE 1±3 3.8E-08 0.36 1.39 1557.16 1556.13AE 1±4 9.7E-09 0.37 1.40 1557.15 1556.12AE 1±6 0.51 1.40 1557.01 1556.12AE 1±9 8.01 2.04 1549.51 1555.48AE 1±11 8.12 2.65 1549.40 1554.87AE 2±1 4.3E-08 0.33 n/a 1556.11 n/aAE 2±2 2.0E-08 0.72 n/a 1555.72 n/aAE 2±3 1.6E-08 1.04 2.06 1555.40 1554.38AE 2±4 5.9E-09 1.02 2.15 1555.42 1554.29AE 2±5 3.1E-09 1.88 2.19 1554.56 1554.25AE 2±6 n/a 4.14 n/a 1552.30AE 3±2 1.1E-07 2.07 2.18 n/a n/aAE 3±3 8.2E-07 2.11 2.62 n/a n/aAE 3±4 3.6E-08 2.11 2.64 n/a n/aAE 3±6 1.8E-08 2.19 2.69 n/a n/aAE 3±8 7.6E-09 7.24 3.14 n/a n/aAE 3±9 9.55 9.26 n/a n/aAE 4±1 1.9E-08 0.52 1.00 1557.05 1556.57AE 4±2 1.6E-08 0.85 1.64 1556.72 1555.93AE 4±3 3.2E-08 0.78 1.67 1556.79 1555.90AE 4±5 9.4E-09 0.92 1.69 1556.65 1555.88AE 4±6 2.5E-09 2.50 1.71 1555.07 1555.86AE 4±7 6.33 5.90 1551.24 1551.67AE 5±1 0.54 0.69 1554.99 1554.84AE 5±2 0.42 2.14 1555.11 1553.39AE 5±3 0.42 1.89 1555.11 1553.64AE 5±4 n/a 1.90 n/a 1553.63AE 5±5 n/a 1.87 n/a 1553.66AE 5±6 n/a 1.92 n/a 1553.61AE 6 1.13 1.77 1553.99 1553.35AE 7 1.65 1.91 1551.88 1551.62AE 8 1.68 1.72 1553.86 1553.82AE 9 1.20 1.73 1555.64 1555.11AE 10 1.76 2.04 1555.38 1555.10AE 11 2.06 n/a 1554.58 n/aAE 12 3.69 3.20 1554.48 1554.97AE 13 2.36 2.30 1554.06 1554.12AE 14 n/a 1.41 n/a 1551.70AE 15 n/a 1.83 n/a 1553.98

Sulphide-bearing carbonate-rich gold-mine tailings impoundment 689

from these piezometers were also used for geochemicalcharacterization of the impoundment. An additional 10piezometers were installed as individual water-table wellsto provide information on the groundwater ¯ow system.These piezometers were either 3.18 cm diameter PVCdrive-point piezometers similar to those used byDubrovsky et al. (1984a,b), or 1.27 cm diameter stainlesssteel or PVC drive-point piezometers similar to thosedescribed by Coggans et al. (1995). All piezometers weredeveloped by repeated bailing, and the hydraulic conduc-tivity of the tailings was estimated by rising-head re-sponse tests, analyzed using the technique of Hvorslev(1951). Hydraulic head measurements were made afterwater levels in the piezometers had stabilized. Theground surface, top of the casing, and location of eachpiezometer and water-table well were surveyed andlocated on the local mine plan.

Vadose-zone sampling. Pore water from the vadose zonewas extracted from sealed core samples collected in7.62 cm diameter thin-walled Al casing. The core sampleswere cut into 25 cm lengths and the ends of the casing®tted with O-ring sealed end-plates, isolating the samplesfrom atmospheric O2(g). Pore water was extracted byslowly applied pressure on the end plates using the tech-nique described by Blowes and Jambor (1990). Determi-nations of pore-water pH (Orion2 Ross 815600combination electrode) and Eh (Orion2 9678BN combi-nation electrode) were made within 1 minute of pore-waterextraction. The performance of the pH electrode was con-®rmed using pH 4.0 and 7.0 standards (traceable toNIST). The performance of the Eh electrode was con-®rmed by comparison to Zobell's solution (Zobell, 1946;Nordstrom, 1977) and Light's solution (Light, 1972). AllEh values were corrected to the standard hydrogen elec-trode (SHE). All samples were ®ltered through celluloseacetate membranes (0.45 mm pore size), then split into twosubsamples. One was acidi®ed to pH <1 with 12 N HClfor cation analysis. The second unacidi®ed sample wasused for anion analysis. All samples were refrigerated untilanalysis. Alkalinity of the samples was determined in the®eld, on 4±10 ml subsamples using a digital titrator anda methyl red-bromocresol green indicator. Samples werereturned to the Water Quality Laboratory at the Univer-sity of Waterloo, where concentrations of Ba, Ca, Cd, Co,Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, and Zn were deter-mined by atomic absorption spectrometry (AAS). TotalAs concentrations were determined by AAS followinghydride generation. Concentrations of SO4

2ÿ, Clÿ, Fÿ,NO3ÿ, and PO4

3ÿ were determined by ion chromatography.Analysis of duplicate and prepared spike samples indicatesthat the estimated error associated with the analytical datais <5% of the measured concentrations.

Saturated-zone sampling. Samples of tailings water werecollected from all piezometer nests. All samples werepassed through 0.45 mm pore-size ®lters, and samples forcation analysis were acidi®ed to pH< 1 with 12 N HCl.All samples were refrigerated until analysis at the Water

Quality Laboratory, University of Waterloo. Chemicalanalyses were performed in the same manner as for thevadose-zone samples. Determinations of pH, Eh, tempera-ture and speci®c conductance were made at each piezo-meter using sealed ¯ow-through cells. Alkalinitydeterminations were made in the ®eld using a digital titra-tor.

Physical properties

The bulk density, particle density, porosity, moisturecontent, and air-®lled porosity were determined at each ofthe locations sampled. The bulk density and gravimetricmoisture content were measured gravimetrically usingsamples collected in 5.08 cm Al casing, and oven dried at808C. The particle density of the dry tailings was measuredusing a Beckman2 Model 930 Air ComparisonPycnometer. Porosity, volumetric moisture content andair-®lled porosity were calculated from bulk density, par-ticle density, and gravimetric moisture-content measure-ments.

Pore-gas sampling

Pore-gas concentrations of O2(g) and CO2(g) weremeasured in the ®eld using a modi®cation of the techniquedescribed by Reardon and Poscente (1984). Narrow diam-eter (0.63 cm) stainless steel tubes were driven into the tail-ings at 10 cm vertical intervals. The sampling tubes wereconnected directly to an O2/CO2 analyzer (Nova2 Model305LBD). This technique provided measurements of gas-phase O2 and CO2 concentrations at the ®eld site.Separate samples were collected for laboratory analysis bygas chromatography. The detection limit for this techniqueis 0.1% O2(g) and 0.1% CO2(g).

RESULTS AND DISCUSSION

The groundwater ¯ow system

For much of the ®nal stages of operation at theAgnico-Eagle mine, tailings deposition was directedfrom a discharge point located near the center of

the impoundment (Fig. 2). This deposition tech-nique resulted in a tailings grain-size distributionthat is di�erent from those observed in most inac-

tive tailings areas. Coarser-grained tailings settlednear the discharge point and are now concentratedin the central portion of the tailings area, in the

Fig. 2. Cross section of the Agnico-Eagle tailings impoundment.


vicinity of piezometer nest AE3 (Fig. 1). Finer-

grained tailings, which settled distally from the dis-

charge point, have accumulated near the impound-

ment margins. The distribution of tailings grain

sizes is re¯ected in the hydraulic conductivity of the

tailings, as estimated from piezometer-response tests

(Table 1). In general, the greatest hydraulic conduc-

tivity values are observed in the central portion

near piezometer nest AE3, at which grain sizes are

coarse.

The ®ne-grained tailings located near the tailings

margins exhibit high moisture contents and have

prevented the extensive drainage of the tailings mar-

gins that has been observed at other tailings areas

(e.g., Waite Amulet, Noranda,z QueÂ ;. Blowes and

Jambor, 1990). As a result, the water table at piezo-

meter nests AE1 and AE2 is within 2 m of the tail-

ings surface. Maintaining high moisture contents is

advantageous because the O2 di�usivity of saturated

tailings is less than that of unsaturated tailings. The

high moisture contents, however, slow the rate of

consolidation and may increase the potential for

uneven settling or dam failure. To prevent these

possibilities, the sides of the tailings dam have been

armored with waste rock from the open pit and

underground operations.

Groundwater ¯ow in the impoundment is di-

rected downward and laterally to the impoundment

edges. The hydraulic conductivity of the tailings

ranges from 2.5�10ÿ9 m/s to 8.2�10ÿ7 m/s.

Vertical velocities were calculated using this range

of conductivity values (Table 2). The measured

hydraulic gradients range from 0.03 to 1.0 for the

vertical component of ¯ow and 6�10ÿ4 to 6�10ÿ3

Fig. 3. Concentrations of dissolved constituents vs depth at piezometer nest AE 1. Filled symbolsmeasured June 1993. Open symbols measured September 1993. Water table indicated by dashed line;

Zn, Cu, Pb, Cd by .; Mn, Ni, Co, Cr by R.




Table 2. Comparison of vertical gradients and calculated velocities.

Depth to discharge-waterrecharge-water

Vertical velocity based ondischarge- and recharge-water

Vertical velocity based on gradients (m/a)

Piezometer nest interface (m) interface (m/a) June/93 Sept/93

0.27 0.31 0.46AE 2 2.04 0.29 1.94 0.26AE 3 2.83 0.40 0.15 0.65AE 4 2.10 0.30 0.51 0.88AE 5 1.17 0.17 ÿ0.50 4.98


for the horizontal component. The greatest vertical

hydraulic gradients were measured at piezometer

nests AE2 and AE5 (Fig. 1).

Rates of groundwater ¯ow can be estimated inde-

pendently by examining the distribution of dis-

solved sulphide-oxidation products using the

approach described by Dubrovsky (1986). Mill dis-

charge-water, slurried into the impoundment within

the tailings, contains high concentrations of Na,

and low concentrations of Ca, Mg, SO4, and dis-

solved metals. Precipitation-recharge water that

entered the impoundment after decommissioning

has been a�ected by sulphide oxidation and by car-

bonate-mineral dissolution. This precipitation-

recharge-water contains high concentrations of SO4,

Ca, Mg, SO4 and other metals, and has lower con-

centrations of Na. The depth of the interface

between the initial mill discharge-water and the




recharge water, which can be observed in Figs. 3±7

and noted in Table 2, varies from <2 m at locationAE5, to13 m at location AE3. Using this interface

depth and the fact that tailings deposition in theold impoundment ceased in 1986, vertical velocities

are calculated that vary from 0.2 m/a to 0.4 m/a.These vertical velocities are similar to those esti-mated from the vertical hydraulic gradient and the

measured hydraulic conductivity (Table 2).

Sulphide±mineral oxidation

The results from optical microscopy indicate thatpyrite is the most abundant sulphide mineral, fol-

lowed by pyrrhotite. Marcasite [FeS2] was noted ina few polished sections but, other than pyrite and

pyrrhotite, the only other sulphide mineral of con-sistent and widespread occurrence is arsenopyrite,

which occurs in trace amounts. No grains of spha-




lerite [ZnS], galena [PbS], or Au were detected in

the mineralogical study.

The oxidation of sulphide minerals is the predo-

minant source of acidity and dissolved metals

characteristic of the acidic drainage associated with

inactive mine wastes. The oxidation of pyrite, the

most abundant sulphide mineral present at the

Agnico-Eagle mine, can be expressed as:

FeS2�s� � 72 O2� g��H2O�l � ÿ4Fe2��aq�

�2SO2ÿ4�aq� � 2H ��aq� �1�

*The Fe(aq)2+ released by sulphide oxidation may be

further oxidized, hydrolyzed, and precipitated as an

amorphous or crystalline ferric oxyhydroxide

through reactions of the form:




Fe2��aq� � 14O2� g� � 5

2H2O�l � ÿ4Fe�OH �3�s� � 2H ��aq��2�

Alternatively, a ferric hydroxysulphate phase, suchas schwertmannite [Fe8O8(OH)6(SO4)] (Bigham etal., 1990, 1994), may precipitate, resulting in the

release of di�erent amounts of H(aq)+ .

Oxidation of pyrrhotite, the second most abun-dant sulphide mineral in the tailings, can be

expressed as:

Fe�1ÿx�S�s� ��2ÿ x

2

�O2� g� � xH2O�aq� ÿ4

�1ÿ x�Fe2��aq�SO2ÿ4�aq� � 2xH ��aq� �3�

The Fe(aq)2+ released by oxidation of pyrrhotite may

be further oxidized and precipitated as shown by

equation 2. The oxidation of pyrite, pyrrhotite, and

Table 3. Saturation indices for the oxide mineralsa and gypsum at piezometer nests AE1 to AE5b.

Well Depth (m)AmorphousFe(OH)3 Goethite Lepidocrocite Jarosite Gypsum

AE1 nestAE1-V1 0.12 4.18 9.51 7.70 4.34 0.04AE1-V2 0.38 2.22 7.55 5.74 0.87 ÿ0.11AE1-V3 0.62 2.60 7.93 6.12 1.80 0.04AE1-V4 0.88 0.60 5.94 4.12 ÿ2.49 0.02AE1-1a 0.92 1.02 6.36 4.54 ÿ2.93 ÿ0.12AE1-1b 0.92 1.43 6.77 4.95 ÿ1.70 ÿ0.14AE1-2 1.91 0.62 5.77 4.14 ÿ4.74 ÿ0.08AE1-3 2.95 3.08 8.18 6.61 1.62 ÿ0.17AE1-4 3.92 3.32 8.42 6.84 3.30 ÿ0.12AE1-6 6.24 4.70 9.84 8.22 2.55 ÿ1.96AE1-9 9.55 0.96 6.11 4.48 ÿ12.15 ÿ2.86AE1-11 10.84 4.30 9.45 7.82 ÿ1.09 ÿ2.64AE2 nestAE2-1 1.19 0.29 5.62 3.81 ÿ4.14 ÿ0.04AE2-2 2.04 2.94 8.12 6.46 1.11 ÿ0.06AE2-3 2.69 3.18 8.31 6.70 2.09 0.09AE2-4 3.89 2.63 7.75 6.15 ÿ0.68 ÿ0.30AE2-5 5.17 0.79 5.92 4.31 ÿ4.63 ÿ0.90AE2-6 5.46 3.54 8.70 7.06 3.58 ÿ0.30AE3 nestAE3-V3 0.62 3.09 8.34 6.61 1.90 0.02AE3-V4 0.88 0.75 5.96 4.27 ÿ5.19 ÿ0.08AE3-V5 1.12 0.51 5.70 4.03 ÿ5.37 ÿ0.12AE3-V6 1.38 2.18 7.34 5.71 ÿ0.68 0.06AE3-V7 1.62 0.21 5.33 3.73 ÿ6.11 0.02AE3-V8 1.92 1.05 6.14 4.57 ÿ3.33 0.04AE3-2 2.18 3.20 8.25 6.72 2.32 ÿ0.23AE3-3 2.83 0.36 5.39 3.88 ÿ5.97 0.02AE3-4 4.54 2.86 7.92 6.38 1.04 ÿ0.08AE3-6 6.04 3.05 8.13 6.57 0.63 ÿ0.19AE3-8 7.93 ÿ0.15 4.96 3.37 ÿ11.10 ÿ1.77AE4 nestAE4-V1 0.10 2.05 7.37 5.57 ÿ1.23 ÿ0.02AE4-V2 0.30 0.55 5.87 4.08 ÿ5.67 0.22AE4-V3 0.50 2.69 8.00 6.21 0.14 ÿ0.09AE4-V4 0.70 1.59 6.89 5.11 ÿ2.23 ÿ0.26AE4-V5 0.90 2.53 7.82 6.05 ÿ0.08 ÿ0.46AE4-1 1.00 2.64 7.93 6.16 1.47 ÿ0.16AE4-2a 2.10 2.97 8.14 6.49 0.34 ÿ0.29AE4-2b 2.10 3.06 8.22 6.58 0.58 ÿ0.36AE4-3 2.73 2.85 8.00 6.37 0.31 ÿ0.09AE4-5 4.87 3.09 8.18 6.61 0.88 ÿ0.31AE4-6 5.73 0.30 5.62 3.82 ÿ6.04 ÿ1.11AE4-7 6.39 ÿ1.07AE5 nestAE5-2 1.17 2.77 7.98 6.29 0.87 ÿ0.32AE5-3 1.96 2.64 7.78 6.16 0.75 ÿ0.12AE5-4 3.23 3.80 8.87 7.32 3.16 ÿ0.32AE5-5 4.06 0.88 5.98 4.40 ÿ4.90 ÿ1.32AE5-6 5.82 1.15 6.25 4.67 ÿ6.36 ÿ0.87

aGoethite: a-FeOOH; lepidocrocite: g-FeOOH; jarosite: KFe3(SO4)2(OH)6; gypsum: CaSO4�2H2O.bSamples designated by V indicate collection from the vadose zone.


Fe(aq)2+ may be catalyzed by autotrophic bacteria of

the Thiobacillus group (Brock et al., 1984). The

acidophilic bacterium Thiobacillus ferrooxidans is

generally considered to be the species responsible

for the generation of acid mine drainage. Other

Thiobacillus species, however, including the acid-tol-

erant mesophilic bacteria Thiobacillus thiooxidans,

which has an observed pH range of growth of 0.5

to 4.0, are also capable of oxidizing reduced S com-

pounds. Thiobacillus thioparus and related species,

which can oxidize sulphide, thiosulphate, and other

reduced S compounds, have an optimum pH range

of 6.0±8.0 (Kuenen et al., 1992; Gould et al., 1994).

Bacterial catalysis accelerates the rate of sulphide

oxidation and may maintain oxidation at rates that

are limited by the physical transport of the oxidant

(O2(g) or Fe(aq)3+ ) through the pore space of the tail-

ings by di�usion or advection, or limitations may

be imposed by oxidized coatings on the mineral

grains.

Oxidation of sulphide minerals is evident by the

brown color of the sur®cial tailings compared to

the dark grey color of the deeper tailings. On the

microscopic scale, oxidation is evident in the

appearance of goethite [a-FeOOH] and other re-

lated Fe oxides or oxyhydroxides. Geochemical cal-

culations conducted using MINTEQA2 (Allison et

al., 1990) indicate that the tailings pore-waters are

consistently supersaturated with respect to goethite

and other ferric oxyhydroxide minerals (Table 3),

suggesting that the goethite rimming the primary

sulphide grains is stable under the geochemical con-

ditions prevalent in the shallow tailings.

Pyrrhotite is commonly considered to be more

susceptible to bacterially-catalyzed oxidation than

pyrite and other sulphide minerals (Brock et al.,

1984). In the vadose zone at the Agnico-Eagle

mine, the alteration rims on pyrrhotite are thick

and indicate more extensive oxidation of this sul-

phide than pyrite, which is surrounded by rare,

narrow alteration rims (Fig. 8). The thickness of the

secondary pyrrhotite alteration rims is indicative of

the extent of oxidation since tailings deposition

ended. As all sulphide minerals at a single location

were exposed to O2 at the same time, the degree of

alteration is indicative of the relative susceptibility

to oxidation. More rapid oxidation of pyrrhotite,

relative to pyrite, inferred from the ®eld obser-

vations is consistent with the results of recent lab-

oratory studies, which also indicate pyrrhotite

oxidizes more rapidly than pyrite (Jambor, 1994;

Nicholson and Scharer, 1994; Mycroft et al., 1995).

Development of partial oxide coatings may change

the rate of oxidation of the residual sulphides, but

the coatings do not change the di�erential in altera-

tion susceptibility.

Less extensive alteration of pyrite than pyrrhotite

has been observed at other locations such as the

Waite Amulet tailings impoundment, QueÂ . (Blowes

and Jambor, 1990; Blowes, 1990) and the Delnite

impoundment near Timmins, Ontario (Jambor andBlowes, 1991; Jambor et al., 1991). At other sites,

the degree of arsenopyrite alteration was observed

to be similar to that observed for pyrite. No alteredarsenopyrite grains were observed during the cur-

rent study, possibly because too few grains of the

mineral were seen.

The depth of active sulphide oxidation is alsoindicated by the depletion of gas-phase O2 in the

vadose zone of the tailings. Pore-gas O2 concen-

trations were measured at 3 locations: AE2, AE3,and AE4 (Fig. 1). High moisture contents at AE1

and AE5 prevented pore-gas sampling. At the 3

sites sampled, pore-gas O2 concentrations decreasesharply from atmospheric concentration

(20.9 vol.%) at the tailings surface to <0.1 vol.%

within the upper 50 cm of the tailings (Fig. 9).Depletion of O2 is accompanied by an increase in

CO2 resulting from acid-neutralizing carbonate-

mineral dissolution reactions. The pore-water pHthroughout the vadose zone is near neutral (Figs.

3±7), indicating that carbonate-mineral dissolution

has been su�cient to consume the acidity generatedby the sulphide-oxidation.

Potential limitations on the rate of sulphide oxi-

dation in mine wastes include an insu�cient supply

of oxidant (O2(g) or Fe(aq)3+ ) or water to the sulphide-

Fig. 8. Unaltered pyrite (py) surrounded by 3 grains ofpyrrhotite showing their greater susceptibility to alteration,here expressed as well-de®ned rims of Fe oxyhydroxides.Polished section, re¯ected plain light, width of ®eld

0.5 mm, site AE 3.


mineral surface, due to physical transport restric-

tions, and slow reaction rates on the mineral surface

(Davis and Ritchie, 1986; Blowes et al., 1991;

Cathles, 1994). Restrictions of the supply of O2(g) to

sulphide surfaces can include slow di�usion of gas-

phase O2 through the partly water-saturated pore

space of the tailings, and di�usion of oxidant

through the hydroxide or oxyhydroxide rims that

surround altered sulphide particles. Oxygen-gas dif-

fusivities are dependent on the porosity and moist-

ure content of the tailings. Numerous empirical

expressions have been developed to relate gas di�u-

sivity of partly-saturated solids to their porosity

and moisture content (Millington and Shearer,

1971; Magnusson and Rassmuson, 1984). Reardon

and Moddle (1985) developed the empirical re-

lationship:

DO2� 3:98 � 10ÿ5

� �eÿ 0:05�0:95

�1:7T 3=2 �4�

where DO2is the oxygen-gas di�usion coe�cient in

cm2/s, e is the air-®lled porosity, and T is the tem-

perature in Kelvin. This equation was based on col-umn studies conducted with partly-saturated milltailings from Elliot Lake, Ont. Oxygen-gas di�usion

coe�cients calculated for the shallow tailings, usingthe ®eld-measured porosity and moisture contentsand the relationship of Reardon and Moddle

(1985), are tabulated in Table 4. The greatest calcu-lated di�usion coe�cients are from the relativelydry tailings present near the tailings surface at

piezometer nest AE3. Increasing moisture contents,and decreasing di�usion coe�cients, are observedwith increased depth.In addition to limits imposed by the di�usivity of

the tailings pore-space, sulphide oxidation may beinhibited by oxidant di�usion through the alterationrims that form on the surfaces of tailings particles.

Inhibition of oxidation by alteration rims has beenobserved in heap-leach piles (Cathles, 1994; Jangand Wadsworth, 1994), waste-rock piles (Davis and

Ritchie, 1986; Ritchie, 1994), and has been inferredin other tailings areas (Blowes and Jambor, 1990;Blowes et al., 1992). The recognition that surface

coatings can inhibit sulphide-mineral oxidation has

Fig. 9. Gas-phase O2 and CO2 concentrations and most probable number (MPN) values forThiobacillus ferrooxidans, Thiobacillus thiooxidans, Thiobacillus thioparus and related bacteria (bacteria

per g of tailings).


resulted in the development of remediation pro-

grams designed to promote the stability and e�ec-tiveness of such coatings (Huang and Evangelou,1994). Extensive alteration rims on oxidized pyrrho-

tite, and to a lesser extent on pyrite, are observedonly in the upper 20±40 cm of the impoundment.Deeper in the tailings, alteration rims are absent,and the rate of oxidation probably is controlled by

the rate of oxidation at the mineral surface.

BACTERIA

Sulphide oxidation in mine wastes is typically cat-

alyzed by bacteria of the Thiobacillus group.Although Thiobacillus ferrooxidans is generally con-sidered to be the microorganism responsible for cat-

alysis of metal sulphide oxidation, other speciesmay play a role in the initial stages of oxidation(Blowes et al., 1995). Most probable number

(MPN) enumeration was conducted for 3 groups of

Thiobacilli: Thiobacillus ferrooxidans and related

species, Thiobacillus thiooxidans, and Thiobacillus

thioparus and related species. Distribution of

Thiobacillus spp. versus depth indicates that the

MPN values of the acidophilic group represented

by T. ferrooxidans and T. thiooxidans are low

throughout the vadose zone, with the exception of

high values detected at the surface at piezometer

nest AE4 (Figs. 9 and 10). The MPN of T. thio-parus and related species, which have an optimum

pH range of 6.0±8.0, are signi®cantly greater than

those of the other species and do not vary signi®-

cantly with depth.

The populations of T. thioparus are 1±6 orders of

magnitude higher than the population of T. thiooxi-dans and T. ferrooxidans at most sampling points.

The pH values of the tailings are generally close to

neutrality, which would tend to favor the growth of

the neutrophilic thiobacilli such as T. thioparus.

At piezometer nests AE1, AE4 and AENT, high

MPN values for T. thiooxidans are observed in the

Table 4. Porosities, moisture contents, and di�usion coe�cients for piezometer nests AE2, AE3, andAE4.

Depth (cm)Total

porosityAir-®lledporosity

%Saturated

Moisture content(Vol.%)

O2 di�usioncoe�cient (m2/s)

AE2 nest0±10 0.50 0.16 68 34 5.0� 10ÿ7

10±20 0.50 0.23 54 27 1.1� 10ÿ6

20±30 0.57 0.19 67 38 7.4� 10ÿ7

30±40 3840±50 0.51 0.15 70 36 4.2� 10ÿ7

50±60 0.52 0.12 76 39 2.4� 10ÿ7

60±70 0.54 0.16 71 38 4.6� 10ÿ7

70±80 0.51 0.13 75 38 2.8� 10ÿ7

80±90 0.57 0.22 61 35 1.1� 10ÿ6

90±100 0.52 0.13 74 38 3.0� 10ÿ7

100±110 0.52 0.17 67 35 5.9� 10ÿ7

110±120 0.49 0.16 66 32 5.1� 10ÿ7

120±130 26AE3 nest0±10 0.57 0.42 26 15 3.8� 10ÿ6

10±20 0.55 0.32 41 22 2.3� 10ÿ6

20±30 0.52 0.40 22 11 3.5� 10ÿ6

30±40 0.53 0.29 44 23 1.9� 10ÿ6

40±50 0.51 0.23 54 28 1.2� 10ÿ6

50±60 0.49 0.19 60 30 7.7� 10ÿ7

60±70 2270±80 0.50 0.32 36 18 2.2� 10ÿ6

80±90 0.50 0.30 41 20 1.9� 10ÿ6

90±100 0.50 0.32 36 18 2.3� 10ÿ6

100±110 0.51 0.21 59 30 9.2� 10ÿ7

AE4 nest0±10 0.52 0.19 63 32 7.7� 10ÿ7

10±20 0.52 0.19 64 33 6.9� 10ÿ7

20±30 0.52 0.19 63 33 7.6� 10ÿ7

30±40 0.54 0.20 63 34 8.2� 10ÿ7

40±50 0.52 0.19 64 34 7.1� 10ÿ7

50±60 0.50 0.17 66 33 5.6� 10ÿ7

60±70 0.50 0.17 65 32 5.7� 10ÿ7

70±80 0.54 0.18 66 36 6.7� 10ÿ7

80±90 0.53 0.21 60 32 9.2� 10ÿ7

90±100 0.51 0.18 64 32 6.8� 10ÿ7

100±110 0.50 0.21 58 29 9.2� 10ÿ7

110±120 0.56 0.13 77 43 2.8� 10ÿ7


upper 1 m of the tailings. At piezometer nest AE4the maximum value is observed at a depth that cor-responds with the sharp decline in gas-phase O2

concentration. Although unaltered sulphide min-erals are present near the tailings surface, wheregas-phase O2 concentrations are greatest, the high-

est MPN values for T. thiooxidans occur 20±40 cmbelow the tailings surface. At location AE4 anincrease in the bacterial population of T. thiooxi-

dans corresponds with the depth of diminishedalteration rims. This observation suggests that thebacterial population of T. thiooxidans thrives where

both gas-phase O2 is high and exposed sulphide sur-faces are available. The occurrence of the exposedsulphide surfaces, the sharp increase in bacterialpopulation, and the sharp decline in gas-phase O2

concentration suggest that the bacterially catalyzedsulphide-oxidation reaction is proceeding rapidly inthe oxidation zone below the occurrence of signi®-

cant alteration rims and above the depth of com-plete O2(g) depletion. As oxidation proceeds, andalteration rims accumulate, the rate of oxidation in

this zone will slow, resulting in O2(g) penetrationinto the deeper tailings.

Acid neutralization mechanisms

The oxidation of sulphide minerals releases H+,SO4

2ÿ, Fe2+, and other metals to the tailings pore-

water. Field measurements indicate that the tail-ings-water pH is near neutral throughout thevadose zone of the tailings (Figs. 3±7). These results

suggest that the acid-neutralizing capacity of the

tailings has been su�cient to consume the acidity

generated by sulphide oxidation. Mineralogical

results indicate that carbonate minerals are abun-

dant throughout the tailings. On the basis of chemi-

cal analyses of tailings solids, X-ray di�ractometry,

and microprobe-determined compositions, the car-

bonate-mineral content of the tailings is estimated

to be about 30 wt.%. These carbonate minerals

include calcite, dolomite-ankerite, and siderite

(Table 5). X-ray di�ractometry estimates of carbon-

ate±mineral abundances suggest that the ratio of

siderite:dolomite-ankerite is approximately 1:1, and

that calcite is <5% of total carbonate. There is

extensive solid-solution substitution in the carbon-

ate minerals at the Agnico-Eagle site, particularly in

dolomite, ankerite, and siderite. Among the substi-

tutions is considerable variation of Mn in siderite

and ankerite-dolomite (Table 5).

At pH above about 6.3, the dissolution of car-

bonate minerals, such as calcite, consumes one mole

of H+ for each mole of calcite consumed:

CaCO3�s� �H ��aq� ÿ4Ca2��aq� �HCOÿ3�aq� �5�

As the pH decreases below approximately 6.3,

carbonic acid [H2CO3] becomes the dominant car-

bonate form and the neutralization reaction may be

described as:

CaCO3�s� � 2H ��aq�FCa2��aq� �H2CO3�aq� �6�

Carbon dioxide gas will be formed from the de-

composition of carbonic acid through the reaction:

Fig. 10. Most probable number (MPN) values for Thiobacillus ferrooxidans, Thiobacillus thiooxidans,Thiobacillus thioparus and related bacteria (bacteria per g of tailings).


H2CO3�aq�FH2O�aq� � CO2� g� �7�

The neutralization of H+ in tailings impoundmentsis commonly accompanied by increased gas-phaseCO2 concentrations (Blowes and Jambor, 1990;

Blowes et al., 1992).The cation composition of the carbonate±mineral

suite also a�ects the potential for acid neutraliz-ation. Base cations, such as Ca and Mg, released by

carbonate dissolution increase the hardness of thetailings water and can participate in mineral precipi-tation reactions with dissolved anions. Because these

metals tend to form relatively soluble salts (e.g.,gypsum [CaSo4�2H2O]), high concentrations of Ca

and Mg remain in solution and are displaced along

groundwater ¯owpaths until they are discharged to

the surface-water ¯ow system. Dissolution of car-

bonate minerals which predominantly contain Ca

and Mg increases the pH and decreases the acid-

generating potential of the pore water.

Dissolution of carbonate minerals containing

multivalent metals such as Fe and Mn also con-

sumes H+. Dissolved Fe(II) and Mn(II) derived

from the dissolution of siderite, ankerite, or ferroan

dolomite, also tend to remain in solution and to be

displaced along the groundwater ¯owpaths.

Discharge of Fe(II)-laden water to oxygenated

bodies of surface water can result in acidi®cation as

Fe(II) is oxidized, hydrolyzed, and precipitated as

shown in equation 2 (Morin and Cherry, 1988).

Table 5. Microprobe analyses of Agnico-Eagle carbonate minerals.

wt% Fe 39.1 40.1 44.4 39.0 38.7 37.6 36.4 37.8 37.7 36.8 44.3 45.2Ca 0.4 0.7 0.2 0.7 0.2 0.8 0.5 0.6 0.5 1.7 0.5 0.5Mg 3.9 2.8 0.8 3.6 4.4 3.9 4.8 3.9 3.7 3.8 0.4 0.4Mn 2.4 2.2 1.5 2.3 1.8 2.5 3.4 2.9 3.6 2.6 2.5 1.7

calc. FeCO3 81.1 83.2 92.1 80.9 80.3 78.0 75.5 78.4 78.2 76.3 91.9 93.8CaCO3 1.0 1.7 0.5 1.7 0.5 2.0 1.2 1.5 1.2 4.2 1.2 1.3MgCO3 13.5 9.7 2.8 12.5 15.3 13.5 16.7 13.5 12.8 13.2 1.4 1.4MnCO3 5.0 4.6 3.1 4.8 3.8 5.2 7.1 6.1 7.5 5.4 5.2 3.6sum 100.6 99.2 98.5 99.9 99.9 98.7 100.5 99.5 99.7 99.1 99.7 100.1

mineral sid sid sid sid sid sid sid sid sid sid sid sid

wt% Fe 44.2 40.3 39.5 37.3 38.2 33.7 32.9 33.6 16.6 14.8 15.0 14.3Ca 0.5 0.4 0.7 0.6 0.3 0.5 0.2 0.8 19.6 20.0 20.3 19.9Mg 0.4 3.2 3.5 4.4 4.3 5.2 6.5 7.2 4.0 4.5 4.1 5.0Mn 1.9 2.3 2.3 2.9 3.5 4.6 5.0 1.2 1.5 2.2 2.0 2.4


mineral sid sid sid sid sid sid sid sid ank ank ank ank

wt% Fe 14.2 13.7 13.7 12.8 12.4 12.4 16.0 14.6 14.2 12.2 11.5 11.6Ca 20.2 19.5 19.5 19.8 20.1 19.7 19.5 19.7 19.4 20.0 19.4 19.5Mg 4.8 5.4 3.3 5.1 5.9 6.1 4.9 5.2 4.4 5.5 5.9 6.0Mn 2.1 1.9 5.9 3.3 2.2 1.8 1.0 2.0 2.5 2.5 2.6 2.8


mineral ank ank ank ank ank ank ank ank ank ank ank ank

wt% Fe 15.8 13.3 10.8 8.0 7.9 4.0 1.9 6.3 1.2 1.2 0.8 0.7Ca 19.8 19.7 19.6 21.7 19.6 22.1 24.1 23.4 38.4 39.7a 40.1a 39.8Mg 4.0 5.5 5.6 8.4 8.6 11.0 10.7 8.3 0.3 0.2 0.1 0.1Mn 1.8 2.5 3.5 0.8 2.6 0.1 0.3 0.2 0.2 0.2 0.2 0.2


mineral ank ank ank dol dol dol dol dol cal cal cal cal

Analyses are of individual grains randomly selected without regard to size or other properties. Analytical conditions:15 kV, 20 nA, 20 s counts; standards were MgAl2O4, MgCO3, Fe2O3, and apatite. Analyst: D.R. Owens.aHigh totals are thought to be due to high analytical values for Ca.


Similarly, the oxidation and precipitation of Mncan occur in the surface-water ¯ow system (Hem

and Lind, 1994).

Controls on the mobility of dissolved constituents

Sulphide oxidation and carbonate dissolutionrelease SO4, Ca, Mg, Mn and Fe to the tailings

pore-water. These reactions have resulted in the

increased concentrations of these constituents in the

shallow tailings-water samples (Figs. 3±7).

Geochemical calculations conducted using the com-

puter program MINTEQA2 (Allison et al., 1990)

indicate that the pore water in the deeper tailings is

generally undersaturated with respect to siderite

and rhodochrosite [MnCO3], and approaches satur-

ation with respect to calcite and dolomite (Table 6).

Water collected from piezometers installed into the

clay underlying the tailings approaches equilibrium

Table 6. Saturation indices for the carbonate mineralsa at piezometer nests AE1 to AE5b.

Well Depth (m) Calcite Dolomite Siderite(C) Siderite(D) Rhodochrosite

AE1 nestAE1-V1 0.12 1.04 1.97 ÿ0.10 ÿ0.65 0.51AE1-V2 0.38 ÿ0.39 ÿ0.01 ÿ1.05 ÿ1.60 ÿ1.48AE1-V3 0.62 ÿ0.52 ÿ0.40 ÿ1.73 ÿ2.28 ÿ1.92AE1-V4 0.88 ÿ0.86 ÿ1.53 ÿ1.17 ÿ1.72 ÿ1.88AE1-1a 0.92 ÿ0.69 ÿ0.56 ÿ1.43 ÿ1.98 ÿ1.47AE1-1b 0.92 ÿ0.71 ÿ0.56 ÿ1.02 ÿ1.56 ÿ1.58AE1-2 1.91 ÿ0.31 ÿ0.84 0.55 ÿ0.04 ÿ1.26AE1-3 2.95 ÿ0.80 ÿ1.65 ÿ2.30 ÿ2.89 ÿ2.20AE1-4 3.92 ÿ1.17 ÿ2.61 ÿ1.64 ÿ2.24 ÿ2.40AE1-6 6.24 ÿ0.77 ÿ1.52 ÿ1.32 ÿ1.91AE1-9 9.55 0.87 1.33 0.44 ÿ0.15 ÿ0.26AE1-11 10.84 0.64 0.95 1.09 0.50 ÿ0.61AE2 nestAE2-1 1.19 ÿ0.01 0.50 0.84 0.28 ÿ1.20AE2-2 2.04 ÿ0.21 ÿ0.25 ÿ1.60 ÿ2.18 ÿ1.70AE2-3 2.69 ÿ0.58 ÿ1.20 ÿ2.52 ÿ3.11 ÿ1.86AE2-4 3.89 ÿ0.71 ÿ1.71 ÿ0.79 ÿ1.38 ÿ1.84AE2-5 5.17 ÿ0.07 ÿ0.26 0.96 0.37 ÿ0.99AE2-6 5.46 ÿ0.16 ÿ0.55 0.23 ÿ0.36 ÿ0.85AE3 nestAE3-V3 0.62 0.56 1.23 ÿ0.62 ÿ1.19 ÿ0.23AE3-V4 0.88 0.46 1.25 ÿ0.20 ÿ0.77 ÿ0.30AE3-V5 1.12 0.28 1.29 1.32 0.74 ÿ0.39AE3-V6 1.38 0.46 1.39 ÿ0.55 ÿ1.14 ÿ0.51AE3-V7 1.62 ÿ0.30 ÿ0.07 ÿ0.13 ÿ0.73 ÿ1.66AE3-V8 1.92 ÿ0.81 ÿ1.14 ÿ1.68 ÿ2.27 ÿ2.30AE3-2 2.18 ÿ0.24 0.02 ÿ1.34 ÿ1.94 ÿ1.09AE3-3 2.83 ÿ1.03 ÿ2.07 ÿ1.84 ÿ2.45 ÿ2.58AE3-4 4.54 ÿ0.92 ÿ2.31 ÿ3.16 ÿ3.76 ÿ2.25AE3-6 6.04 ÿ0.52 ÿ1.37 ÿ2.97 ÿ3.57 ÿ1.79AE3-8 7.93 0.18 ÿ0.28 0.04 ÿ0.55 ÿ0.65AE4 nestAE4-V1 0.10 0.39 0.65 ÿ1.48 ÿ2.03 0.29AE4-V2 0.30 0.61 0.91 ÿ1.21 ÿ1.76 ÿ0.88AE4-V3 0.50 0.68 1.80 ÿ1.12 ÿ1.67 ÿ0.17AE4-V4 0.70 0.22 1.32 ÿ0.17 ÿ0.73 ÿ0.69AE4-V5 0.90 ÿ0.34 0.14 ÿ1.40 ÿ1.96 ÿ1.42AE4-1 1.00 0.11 1.07 ÿ1.65 ÿ2.21 ÿ0.92AE4-2a 2.10 ÿ0.09 ÿ0.39 ÿ3.72 ÿ4.30 ÿ1.17AE4-2b 2.10 ÿ0.16 ÿ0.46 ÿ3.63 ÿ4.22 ÿ1.29AE4-3 2.73 ÿ0.18 ÿ0.50 ÿ3.68 ÿ4.26 ÿ1.24AE4-5 4.87 ÿ0.47 ÿ1.13 ÿ1.97 ÿ2.57 ÿ1.72AE4-6 5.73 0.01 ÿ0.29 1.25 0.70 ÿ0.44AE4-7 6.39 ÿ0.66 ÿ1.92 ÿ1.04AE5 nestAE5-2 1.17 ÿ0.40 ÿ1.02 ÿ3.63 ÿ4.20 ÿ1.19AE5-3 1.96 ÿ0.59 ÿ1.67 ÿ2.38 ÿ2.97 ÿ1.62AE5-4 3.23 ÿ0.92 ÿ2.26 ÿ0.91 ÿ1.51 ÿ1.81AE5-5 4.06 ÿ0.14 ÿ0.37 0.99 0.40 ÿ0.79AE5-6 5.82 0.40 0.33 0.66 0.06 ÿ0.48aSiderite(C): Crystalline, Ksp from Nordstrom et al. (1990) and Ptacek (1992). Siderite(D): Disordered, Ksp from Singerand Stumm (1970). Compositions: calcite CaCO3, dolomite CaMg(CO3)2, siderite FeCO3, rhodochrosite MnCO3.bSamples designated by V indicate collection from the vadose zone.


with respect to calcite, dolomite and siderite, but

remains undersaturated with respect to rhodochro-

site. In the shallow portion of the impoundment,

where the water has been a�ected by sulphide oxi-

dation, many samples approach saturation with

respect to calcite and siderite, and vary from near-

saturation to slightly supersaturated with respect to

dolomite. All but the shallowest samples remain

undersaturated with respect to rhodochrosite. These

observations suggest that the carbonate minerals

dissolve to saturation in the near-surface zone.

Release of Ca, HCO3, and Mg by calcite and dolo-

mite dissolution may favor precipitation of second-

ary dolomite, but because of kinetic hindrances,

dolomite precipitation does not occur and the con-

dition of supersaturation is maintained. Sulphide-

oxidation and acid-neutralization reactions release

SO4 and Ca to the tailings water, resulting in the

precipitation of gypsum. Gypsum occurs through-

out the tailings, and geochemical calculations con-

sistently indicate near-saturation with respect to

gypsum (Table 3).

In addition to Fe and SO4, sulphide oxidation

releases other dissolved elements, such as As, to the

tailings pore-waters. Analyses of tailings-water

samples indicate that concentrations of dissolved As

are consistently <50 mg/l. The concentrations of

dissolved As are less than those observed in an

inactive impoundment of Au-mine tailings near

Timmins, Ont., at which dissolved As concen-

trations exceed 20 mg/l (Blowes, 1990).

Mineralogical examination of core samples collected

from the Agnico-Eagle site indicates that the

arsenopyrite grains are sparse and show little sign

of alteration, suggesting that little As has been

released to the tailings pore-water. Concentrations

of dissolved metals are consistently low and are

restricted to depths near the tailings surface (Figs.

3±7). Maximum concentrations of the dissolved

metals Cu, Ni, Zn, Co, Cr, Cd, and Pb are lower

than observed in base-metal tailings impoundments

at which low-pH conditions prevail (Dubrovsky et

al., 1984a,b; Blowes and Jambor, 1990; Blowes et

al., 1992). The maximum concentrations of these

metals occur within the upper 2 m of the tailings

surface. In the Agnico-Eagle tailings, the maximum

concentrations of Cd, Co, Cr and Cu are consist-

ently <0.2 mg/l, and the concentrations of Ni, Pb

and Zn exceed 0.2 mg/l in only 2 or 3 samples. The

maximum concentration of Zn, 1.83 mg/l, occurs

near the tailings surface at piezometer nest AE1.

The maximum concentration of Ni is <0.5 mg/l,

and the maximum concentration of Pb is 0.24 mg/l.

The adsorption of these metals to ferric oxyhydrox-

ide phases is expected to be extensive under the

neutral to slightly basic pH conditions that are

observed in the tailings (Leckie et al., 1980). It is

probable that adsorption along the groundwater

¯owpath will result in further decreases in the con-centrations of these metals.

CONCLUSIONS

Oxidation of sulphide minerals occurs in theupper 20±100 cm, which is the zone of active oxi-dation in the Agnico-Eagle tailings impoundment.

Sulphide oxidation is indicated by the depletion ofsulphide minerals at the tailings surface, by the for-mation of alteration rims surrounding primary sul-phide particles, and by a sharp decrease in the gas-

phase O2 concentrations. Pyrrhotite is the mostintensely altered sulphide mineral, followed by pyr-ite.

The tailings are rich in carbonate minerals, con-taining calcite, dolomite±ankerite and siderite. Thepore-water pH, measured throughout the tailings

impoundment, is near-neutral. This observationsuggests that the dissolution of the abundant car-bonate minerals has been su�cient to consume the

acidity released by sulphide±mineral oxidation. Theconcentrations of dissolved metals, including Fe, inthe tailings pore-water are low. Because of the highpH conditions present in the impoundment, and

because the concentrations of dissolved Fe haveremained low, there is little potential for net acidgeneration. Acidic drainage is detected in drainage

ditches surrounding the impoundment, but thesource of this acidity seems to be the waste rockused to armor the dam and to enhance impound-

ment stability.The dominant group of sulphide±oxidizing bac-

teria present in the tailings are the neutrophilicThiobacilli such as T. thioparus, which have an opti-

mum pH range of 6.5±8.0. The populations of theacidophilic species such as T. thiooxidans and T.ferrooxidans are much lower. This observation

suggests that the neutrophilic Thiobacilli have amajor role in sulphide oxidation. The maximumMPN values for T. thiooxidans occur 20±40 cm

below the tailings surface, at a depth that coincideswith the depth of active oxidation as indicated bythe sharp decrease in gas-phase O2 concentration.

AcknowledgementsÐWe thank Y. Sylvestre of MinesAgnico-Eagle LimiteÂ e for his assistance. W. D. Robertson,T. A. Al, M. J. Baker, K. J. De Vos, and M. C. Moncurprovided technical assistance in the ®eld. Financial sup-port was provided through grants from Mines Agnico-Eagle LimiteÂ e, the Canada Centre for Mineral and EnergyTechnology (CANMET), and the Natural Science andEngineering Research Council of Canada (NSERC). Themanuscript bene®ted greatly from the comments ofreviewers D. B. Levy and D. Runnells.

Editorial handling:ÐD. D. Runnells


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Blowes-Geochemical and Microbiological Characterization Tailings-1998

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