The Physical, Chemical and Mineralogical Properties of Three Cemented Layers Within Sulfide-Bearing Mine Tailings

Journal of Geochemical Exploration 76 (2002) 195–207 www.elsevier.com/locate/jgeoexp

The physical, chemical and mineralogical properties of three cemented layers within sulfide-bearing mine tailings

R.G. McGregora,*, D.W. Blowesb

a XCG Consultants Ltd., 50 Queen Street North, Suite 904, Kitchener, ON, Canada N2H 6P4 b Department of Earth Sciences, University of Waterloo, Waterloo, ON, Canada N2L 3G1 Accepted 22 August 2002

Abstract

+ The oxidation and subsequent dissolution of sulfide minerals within mine tailings impoundments releases H , Fe(II), SO4 and trace elements to the tailings pore water. Subsequent pH-buffering and hydrolysis reactions result in the precipitation of secondary phases such as gypsum, goethite and jarosite. In areas of intense precipitation, cemented layers or ‘‘hardpans’’ often form within the shallow tailings. Three cemented layers within pyrrhotite-bearing mine tailings at the Fault Lake, Nickel Rim and East Mine impoundments located near Sudbury, Canada, were examined. The location of the three cemented layers within the tailings stratigraphy varies as does their location relative to the water table. The morphology, mineralogy and chemical composition of the cemented layers also vary between sites. The bulk density within the three cemented layers all showed an increase relative to the surrounding uncemented tailings ranging from 9% to 29%. The porosity of each cemented layer decreased relative to the surrounding uncemented tailings ranging from an 8% to 18% decrease. The cemented layers also showed relative enrichment of total sulfur, carbon and trace elements relative to the surrounding uncemented tailings. Arsenic concentrations showed an enrichment in the cemented layers of up to 132%, Cd up to 99%, Co up to 84%, Cu up to 144%, Ni up to 693% and Zn up to 145% relative to the surrounding uncemented tailings. All the cemented layers studied show an evolution of the secondary phases with time from a gypsum–jarosite-based cement to a goethite-rich cement. The formation of these layers could potentially have a significant effect on the environmental impacts of sulfide-bearing mine waste. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Cemented layer; Hardpan; Tailings; Geochemistry; Mineralogy

1. Introduction of the sulfide minerals contained within the tailings can result in the generation of low-pH conditions and Mining and metallurgical processing of sulfide- the release of sulfate, ferrous iron and other metals to bearing ores have led to the deposition of sulfide- the tailings pore water. Upon discharge to surface bearing wastes, including mill tailings. The oxidation water bodies, such as rivers and lakes, the ferrous iron and other heavy metals hydrolyse and precipitate out of solution, consuming alkalinity, and possibly low- ering the pH of the receiving water body (Morin et al., * Corresponding author. Fax: +1-519-741-5774. 1988; Blowes et al., 1995, Bain et al., 2000; Johnson E-mail address: [email protected] (R.G. McGregor). et al., 2000).

Previous studies on inactive mine tailings impound- al., 1998; Agnew and Taylor, 2000). Many of the ments have reported the presence of cemented layers above authors have hypothesized that the cemented within the shallow zones of the sulfidic tailings layers act as hydraulic barriers inhibiting transport of (Boorman and Watson, 1976; Kennedy and Haw- pore water, and diffusion barriers inhibiting transport thorne, 1987; McSweeney and Madison, 1988; of pore gases such as O2 and CO2. Laboratory studies Blowes et al., 1991; Tasse et al., 1997; McGregor et of cemented layers have shown that these layers can be

Fig. 1. Map showing locations of the three tailings impoundments. R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207 197 developed in a controlled setting using electrochemical including quartz, chlorite, biotite, calcic plagioclase and/or chemical additives to create a barrier to water and orthopyroxene. and gas migration (Ahmed, 1991, 1995; Chermak and The water table elevation varies from near the base Runnells, 1996; McGregor et al., 1997; Schippers et of the impoundment to 2 m below the base of the al., 1998; Agnew, 1998). impoundment (St-Arnaud et al., 1994). The discontin- This study compares the physical, chemical and uous cemented layer occurs at the surface of the mineralogical characteristics of cemented layers tailings and has undergone severe heaving due to observed at three sulfide-bearing mine tailings freeze/thaw weathering reactions and expansion due impoundments in Northern Ontario, Canada (Fig. 1). to secondary mineral precipitation. The cemented Each cemented layer is located within a different layer is underlain by oxidized tailings indicating that geochemical zone; the surface zone, the oxidized zone sulfide oxidation has occurred below the cemented or the transition zone, within the tailings. layer (Fig. 2). A detailed description of the geochemistry and hydrogeology of the Fault Lake tailings is provided by St-Arnaud et al. (1994) and Woyshner et 2. Site descriptions al. (1995).

2.1. Fault Lake 2.2. Nickel Rim

The Fault Lake tailings impoundment is located 3 The Nickel Rim tailings impoundment is located km north of the Town of Falconbridge, and was used 25 km northeast of Sudbury (Fig. 1). Deposition of the to deposit sulfide-bearing tailings from 1965 to 1978 silty-sand size tailings in a narrow bedrock valley (Fig. 1). The tailings impoundment covers an area of resulted in an impoundment covering approximately 22.2 ha and reaches a maximum depth of approx- 9.4 ha and reaching a maximum depth of 10 m imately 30 m (Table 1). The shallow (<1 m) tailings (Johnson et al., 2000) (Table 1). The tailings contain contain an average of 13.6 wt.% sulfur, mainly in the an average of 3.0 wt.% sulfur, principally as pyrrhotite form of pyrrhotite [Fe1xS]. Trace amounts of chal- in a predominantly silicate gangue (Johnson et al., copyrite [CuFeS2], pentlandite [(Ni,Fe)9S8] and pyrite 2000). Jambor and Owens (1993) identified minor [FeS2] are also present within the tailings. The oxida- amounts of other sulfides within the tailings including tion of pyrrhotite within the shallow tailings has chalcopyrite, pentlandite, pyrite and marcasite. The resulted in a 20-cm thick cemented layer being created principal silicate minerals identified by Jambor and at the surface of the impoundment (Fig. 2). Woyshner Owens (1993) were calcic plagioclase and amphibole et al. (1995) report that as the depth increases, the with clinopyroxene, chlorite, biotite, talc, quartz and sulfur content of the tailings decreases to below 0.9 magnetite. Sulfide oxidation has resulted in low-pH wt.% S. The principal gangue minerals are silicates, conditions and high concentrations of Fe, SO4 and

Table 1 Physical and chemical characteristics of the three tailings impoundments Fault Lake Nickel Rim East Mine Tailings surface area (ha) 22.6 9.4 50 Tailings thickness (m) 30 10 18 Depth to water table (mbgl) 32 0.25–1.15 6.5–10 Average sulfide contenta (as %S) 13.6 2.04 0.75 a Average carbonate content (as %CaCO3) 0.07 0.34 2.19 Dominant sulfide mineral Pyrrhotite Pyrrhotite Pyrrhotite Hardpan depth (mbgl) Surface 0.15 0.25 Hardpan thickness (m) 0.20 0.12 0.08 Geochemical zoneb Surface Oxidized Transition a Arithmatic mean of top 1 m of tailings. b See Fig. 2. 198 R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207

Fig. 2. Plots showing tailings stratigraphy, geochemical zones, sulfur, carbon and calculated porosity values for the shallow tailings at the three tailings impoundments. The hatched areas represent cemented layers, whereas the cross-hatched and white areas indicate unoxidized and oxidized tailings, respectively. other heavy metals in the tailings pore water (Johnson oxidized tailings indicating that sulfide oxidation has et al., 2000).PrecipitationofFe,SO4 and other occurred below the cemented layer (Fig. 2). dissolved constituents has formed a 12-cm thick The thickness of the vadose zone varies throughout cemented layer within the tailings at a depth of 19 the impoundment with a maximum thickness of 1.2 m cm (Fig. 2). The discontinuous cemented layer is being reported by Johnson et al. (2000). The ground- situated above the water table and is surrounded by water flow through the tailings is predominantly R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207 199 horizontal with Johnson et al. (2000) reporting electrode) were made immediately after the extraction groundwater velocities ranging from 4 to 8 m/year. of pore water. The EH values were corrected to the Johnson et al. (2000) provides a detailed description standard hydrogen electrode (SHE). All of the pore- of the hydrogeochemistry of the Nickel Rim tailings. water samples were filtered through 0.45-Am cellulose acetate membranes prior to being split into two sub- 2.3. East Mine samples. The first subsample was acidified to pH<1 with 12 N HCl for cation analysis while the second The East Mine tailings were deposited between sample was left unacidified and used for anion anal- 1946 and 1964 and cover an area of approximately 50 ysis. Carbonate alkalinity was determined in the field ha with a maximum thickness of 18 m (Table 1). The on 3–10-ml samples using a digital titrator and East Mine tailings impoundment is located approx- methyl red/bromocresol green pH indicator. Concen- imately 1 km north of the Town of Falconbridge, trations of Al, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ontario (Fig. 1). The predominantly silicate tailings Ni, Pb, Si and Zn were determined on the acidified contain an average sulfur content of 0.77 wt.% (as S), samples by inductively coupled plasma mass spec- primarily occurring as pyrrhotite. Trace amounts of trometry (ICP/MS). The unacidified subsamples were chalcopyrite, pentlandite and pyrite are also present. analyzed by ion chromatography (IC) to determine the The principal gangue minerals are silicates including concentrations of Cl, NO3,PO4 and SO4. The accu- quartz, chlorite, biotite, calcic plagioclase and ortho- racy of the results was assessed by examination of pyroxene. Layers of concentrated Fe-oxide (predom- blanks and replicates, and by performing charge- inantly hematite) are also present within the shallow balance calculations for all samples. The results of tailings. Intense precipitation of gypsum and jarosite the charge balance calculations were below 10% with hasresultedinan8-cmthickcementedlayerof most being below 5%. Analysis of blank samples tailings at a depth of 25 cm (Fig. 2). indicated no false positive results. Analysis of dup- The vadose zone within the silty-sand size tailings licate sampled indicated reproducibility of results ranges in thickness from 6.5 to 10.2 m. Hydrogeo- within 8%. logical studies of the impoundment indicate that the Geochemical calculations were performed using the impoundment acts as an area of groundwater recharge speciation-mass transfer program MINTEQA2 (Alli- with groundwater flow being downwards and away son et al., 1990). The MINTEQA2 database was from the base of the impoundment at an estimated modified to be consistent with that of WATEQ4F (Ball velocity of 35 m/year. and Nordstrom, 1991). Solubility data for the K, Na and H3O jarosites were changed to the values recommen- ded by Alpers et al. (1989). The revised database also 3. Methods of investigation and interpretation includes calculated or estimated ion-association con- stants for Fe(II), Fe(III), Ca(II) and Al(III) bisulfate 3.1. Pore water complexes not in the original MINTEQA2 database (Nordstrom et al., 1990). Additional solubility data Pore-water samples from the vadose zone were were added to the database for siderite [FeCO3], collected at the three locations, Fault Lake, Nickel NiSO4, dwornikite [NiSO4H2O], NiSO44H2O, Rim and East Mine, using a modified version of the NiSO43Ni(OH)2 and (NiSO4)34Ni(OH)2 (McGregor, technique described by Starr and Ingleton (1992). 1994). Thermodynamic data were also added to the This method uses a 7.6-cm diameter thin-walled database for cobaltocalcite [CoCO3], bieberite aluminium tubing driven into the tailings to obtain a [CoSO47H2O] and heterogenite [CoOOH] (McGre- relatively undisturbed core of the tailings. The cores gor, 1994). were then cut into lengths of 25 cm, and pore water was extracted from each section using a method 3.2. Tailings solids similar to that described by Al et al. (1994). Determi- nations of pore-water pH (Orion Ross 910700 combi- In addition to the cores taken for pore-water nation electrode) and EH (Orion 96-7800-combination analyses, other cores were collected at the three sites 200 R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207 for the testing of the physical and chemical proper- Mineralogical examination, including reflected- ties of the tailings. Samples of the cemented layers and transmitted-light microscopy, simultaneous dif- were also collected at the three locations by excavat- ferential thermal and thermogravimetric analyses ing the overlying tailings and the removing large (DTA, TGA) and X-ray diffraction (XRD) analyses pieces of the cemented layers intact. This method of the cemented samples were conducted at the allowed the acquisition of large samples of cemented Environment Canada Wastewater Technology Centre, layers, which were then isolated from atmospheric the Canada Centre for Mineral and Energy Technol- oxygen, frozen and transported to the laboratory for ogy (CANMET) and Louisiana State University, further testing. The physical properties determined respectively. for the tailings and cemented samples included: the bulk density, particle density, porosity, gravimetric moisture content and air-filled porosity. The bulk 4. Results density and moisture content were estimated using gravimetric techniques. The particle density of the 4.1. Bulk and particle density tailings and cemented layers were measured using a Micromeritics model 1305 multivolume pycnometer. Determinations of bulk and particle density for the Based on the bulk density and particle density, three cemented layers indicate that the bulk and porosity, moisture content and air-filled porosities particle densities within the cemented layers are were calculated. greater than the surrounding uncemented tailings Chemical parameters measured for the tailings and suggesting that secondary-phase precipitates are infill- cemented layer samples included the sulfur and car- ing the tailings contained within cemented layers. bon content which were determined using a LECO The measurement of bulk and particle densities for induction furnace with an infrared detector. Due to the the Fault Lake cemented layer determined that the low content of organic carbon within the tailings bulk density increased from an average of 1.59 g/cm3 (<0.01 wt.%) the carbon within the tailings is assumed (S.D.=0.07, N=8) in the top 1 m of tailings to a to occur as inorganic carbon. The water soluble maximum of 1.86 g/cm3 within the cemented layer fraction of the tailings was determined using the (Table 2). Porosity calculations determined that the methods described by Ribet et al. (1995) and McGre- porosity within the Fault Lake cemented layer gor et al. (1995). The method uses a 1:50 solid to decreased from an average value of 0.51 (S.D.=0.03, liquid ratio continuously shaken for 24 h at room N=8) for the underlying uncemented tailings to a temperature (22F3jC). The extractant is then filtered minimum of 0.44 within the cemented layer (Table 2). using a 0.45-Am membrane. The insoluble sulfate The bulk density within the top 1 m of the fraction was determined using the method outlined uncemented Nickel Rim tailings averaged 1.49 g/ by McGregor et al. (1995). The method uses a 20% cm3 (S.D.=0.08, N=8), whereas the cemented layer (v/v) hydrochloric acid [HCl] solution in a 1:15 solid had a maximum bulk density of 1.64 g/cm3 (Table 2). to extractant ratio to target insoluble sulfate phases Porosity calculations, based on the bulk and particle such as jarosite in addition to weakly crystalline iron densities, determined that the porosity within the (oxy)hydroxides and carbonate phases. The sample is Nickel Rim cemented layer decreased from an average shaken continuously for 20 min at room temperature value of 0.50 (S.D.=0.02, N=8) for the surrounding (22F3jC). The reducible fraction was extracted by uncemented tailings to a minimum of 0.45 within the adding 30 ml of 2 M hydroxylamine hydrochloride cemented layer (Table 2, Fig. 2). [NH2OHHCl] in 25% (v/v) acetic acid [CH3COOH] The bulk density for the shallow East Mine tailings and allowing the mixture to react for 24 h at 95jC range from a minimum of 1.43 g/cm3 near the surface (F4jC) with occasional agitation (Ribet et al., 1995). of the tailings to 1.84 g/cm3 within the cemented layer This extraction targets reducible phases such as crys- (Table 2). The average bulk density of the upper 1 m talline iron and manganese (oxy)hydroxides. The total of tailings was 1.54 g/cm3 (S.D.=0.08, N=10, Table fraction was determined by digesting a sample in 2). Calculated porosity values decreased from an hydrofluoric and nitric acids for a 24-h period. average value of 0.50 (S.D.=0.02, N=10) for the R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207 201

Table 2 Measured particle and bulk densities and calculated porosity values for the upper 1 m of tailings at the three tailings impoundments Site Depth (cm) Particle density (g/cm3) Bulk density (g/cm3) Calculated porosity Fault Lake 0–10 3.30 1.86 0.44 10–20 3.30 1.86 0.44 20–30 3.24 1.54 0.52 30–40 3.29 1.49 0.55 40–50 3.28 1.53 0.53 50–60 3.24 1.63 0.50 60–70 3.21 1.58 0.51 70–80 3.24 1.68 0.48 80–90 3.23 1.58 0.51 90–100 3.19 1.7 0.47 Meana 3.24 1.59 0.51 S.D.a 0.03 0.07 0.03 Nickel Rim 0–10 2.68 1.38 0.49 10–19 2.75 1.36 0.51 19–31 2.96 1.64 0.45 31–40 2.98 1.52 0.49 40–50 3.01 1.54 0.49 50–60 3.08 1.48 0.52 60–70 3.09 1.51 0.51 70–80 3.07 1.55 0.50 80–90 3.05 1.58 0.48 90–100 3.08 1.52 0.51 Meana 2.98 1.49 0.50 S.D.a 0.15 0.08 0.01 East Mine 0–10 2.92 1.45 0.50 10–20 2.95 1.43 0.51 20–25 3.00 1.48 0.51 25–33 2.97 1.84 0.38 33–40 2.98 1.53 0.49 40–50 2.99 1.61 0.46 50–60 2.98 1.49 0.50 60–70 3.01 1.51 0.50 70–80 3.42 1.67 0.51 80–90 3.24 1.62 0.50 90–100 3.13 1.58 0.50 Meana 3.06 1.54 0.50 S.D.a 0.16 0.08 0.02 Bold values indicate cemented layer. a Mean and standard deviation values calculated without using the values for the hardpan layers. surrounding tailings to a minimum of 0.38 within the observations have been made by Boulet and Larocque cemented layer (Table 2). (1998) at the Cleveland mill tailings impoundment in New Mexico, at the Copper Cliff tailings impound- 4.2. Sulfur and carbon contents ment in Sudbury by McGregor et al. (1998) and at the Malartic tailings impoundment in Quebec (Tasse et Determinations of the total sulfur and carbon con- al., 1997). tents within the shallow tailings at the three sites The total sulfur and carbon within the Fault Lake indicate that the total sulfur and carbon concentrations shallow tailings average 13.2 wt.% as S (S.D.=4.62, increased within the cemented layers relative to the N=8) and 0.07 wt.% as CaCO3 (S.D.=0.04, N=8), surrounding uncemented tailings (Fig. 2).Similar respectively (Fig. 2). Total sulfur and carbon concen- 202 R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207 trations within the Fault Lake cemented layer were the East Mine and Fault Lake cemented layers were determined to be 15.2 wt.% as S and 0.13 wt.% as 132% and 42% greater than the average As concen- CaCO3, respectively (Fig. 2). tration within the surrounding uncemented tailings, Total sulfur and carbon within the Nickel Rim respectively. Solid-phase Cd concentrations within shallow tailings average 2.04 wt.% (as S) and 0.34 the three cemented layers ranged from 55% to 99% wt.% (as CaCO3), respectively, in the top 1 m (Fig. 2). greater than the surrounding Cd concentrations Total sulfur and carbon concentrations within the within the uncemented tailings, whereas solid-phase Nickel Rim cemented layer were determined to be Co concentrations within the cemented layers ranged 3.49 wt.% (as S) and 0.45 wt.% (as CaCO3), respec- from 52% to 84% greater than the surrounding tively. The relative increases in total sulfur and carbon uncemented tailings. Similar observations were noted within the cemented layers suggest that a relative for Cu (50–144%), Ni (37–693%) and Zn (44– enrichment of these elements is occurring within the 145%) with the East Mine cemented layer having cemented layer. the greater relative enrichments for all of the trace Analyses of the total sulfur content within the East elements measured. Mine shallow tailings average 0.75 wt.% as S The relative enrichment in total sulfur, carbon and (S.D.=0.48, N=11) with sulfur content increasing with trace elements within the three cemented layers rela- depth (Fig. 2). The total carbon content averages 2.19 tive to the surrounding uncemented tailings suggests wt.% as CaCO3 (S.D.=1.76, N=11) with the carbon that a portion of the cementing secondary phases are content increasing with depth (Fig. 2). Sulfur and composed of sulfur- and carbon-bearing phases. The carbon values within the East Mine cemented layer cementing phases within the cemented layers also act were determined to be 1.00 wt.% as S and 4.0 wt.% as as a sink for trace elements. CaCO3, respectively (Fig. 2). 4.4. Chemical extractions 4.3. Trace metal content 4.4.1. Water soluble fraction Analysis of the trace element content within the Chemical extraction experiments undertaken on the cemented layers indicates that the trace elements cemented solids from the three sites indicate that the analyzed are enriched within the cemented layers solid-phase sulfur and iron is associated with different relative to the surrounding uncemented tailings phases within each cemented layer. The Fault Lake (Table 3). Arsenic (As), in addition to cadmium cemented layer had the smallest fraction of the total (Cd), cobalt (Co), copper (Cu), nickel (Ni) and zinc sulfur associated with the water soluble fraction (Zn), were all found to be enriched within the (0.67%). Within the Nickel Rim cemented tailings, cemented layers relative to the surrounding unce- 13.9% of the total sulfur present within the sample mented tailings. Solid-phase As concentrations within was water soluble, whereas the 39% of the sulfur mass

Table 3 Solid-phase concentrations of selected trace elements within cemented layers and the surrounding uncemented tailings Fault Lake Nickel Rim East Mine Cemented Uncementeda Cemented Uncementeda Cemented Uncementeda As 34 24 <2 <2 51 22 Cd 1.4 0.8 1.4 0.9 37.1 18.6 Co 27 16 35 23 90 49 Cu 176 72 471 313 1712 713 Ni 2940 766 402 294 4684 591 Zn 27 16 137 95 9201 3763 All units are mg/g. a Mean of three samples surrounding cemented layer. R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207 203

Fig. 3. Plots showing extractable and total iron and sulfur for the cemented layers. The four fractions measured included: water soluble, insoluble sulfate, reducible and total fractions. Reducible fraction was not determined for sulfur. The hatched areas represent cemented layers, whereas the cross-hatched and white areas indicate unoxidized and oxidized tailings, respectively.

within the East Mine cemented layer was water jarosite, chemical extractions were carried out using soluble (Fig. 3). Na2CO3. Chemical extractions completed on the Fault Chemical extractions completed on the Fault Lake Lake cemented tailings determined that 2.5% of the cemented solids determined that 0.01% of the total total sulfur present was soluble in Na2CO3, whereas iron within the solids was extracted with distilled water 13.8% of the total sulfur present within the Nickel Rim (Fig. 3). The amount of total iron dissolved within the cemented tailings was soluble in Na2CO3 (Fig. 3). Nickel Rim and East Mine cemented tailings during Chemical extractions carried out on the East Mine the water soluble extractions was below the method cemented tailings determined that 25.3% of the total detection limit. sulfur within the solids was soluble in the Na2CO3 solution (Fig. 3). 4.4.2. Sodium carbonate soluble sulfate fraction Extractions determined that 0.02% of the total iron To estimate the mass of sulfur associated with within the Fault Lake cemented layer was extractable relatively insoluble sulfate-bearing phases such as with Na2CO3, whereas 0.19% of the total iron within 204 R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207 the East Mine cemented tailings was extracted by the hydroxysulfates precipitated from acid mine drainage Na2CO3 (Fig. 3). The amount of iron dissolved by can act as a sink for trace elements and sulfate. Na2CO3 within the Nickel Rim cemented tailings was Mineralogical analysis of the three cemented layers below the method detection limit. also confirmed the presence of the Fe oxyhydroxy- sulfate jarosite [KFe3(SO4)2(OH)6] (Table 4). Geo- 4.4.3. Reducible fraction chemical calculations indicate that the pore water The reducible fraction of the cemented tailings within the three cemented layers is saturated with was estimated using the chemical reductant hydroxyl- respect to K-, Na- and H3O-jarosites indicating a amine hydrochloride and acetic acid. Extractions tendency for these phases to precipitate. The jarosite completed on the Fault Lake cemented tailings indi- present within the cemented layers is most likely the cated that 84.1% of the total iron was susceptible to source of the insoluble sulfur extracted using the dissolution, whereas 54.1% of the total iron within Na2CO3 extractant. the Nickel Rim cemented tailings was reducible using Geochemical modelling of the pore water within the hydroxylamine hydrochloride and acetic acid the cemented tailings suggests that the pore water is (Fig. 3). A maximum of 49.8% of the total iron saturated with respect to the Fe(III) (oxy)hydroxide within the East Mine cemented solids was dissolved phases goethite and ferrihydrite. Mineralogical exami- under these conditions (Fig. 3). nation of the three cemented layers confirmed the presence of goethite (aFeOOH) but failed to detect 4.5. Mineralogical analysis ferrihydrite. Jambor (1997) suggested that ferrihydrite likely contains adsorbed water, and has the composi- Based on mineralogical examination of the three tion FeO[O,(OH)1x5x], where 5 represents a struc- cemented layers, the soluble sulfur fraction is pre- ture vacancy. For the purpose of this paper, any dominantly in the form of gypsum (CaSO42H2O) reference to mineralogical studies of the cemented which was isolated during mineralogical examination layers refers to the Jambor (1997) definition of ferri- of all three cemented layers (Table 4). Geochemical hydrite. The iron oxide phases hematite [Fe2O3] and calculations using MINTEQA2 indicate that the pore maghemite [gFe2O3] were detected within the East water within the cemented layers is saturated with Mine cemented layer and may be the result of dehy- respect to gypsum. Other possible sources of soluble dration of the original ferric iron (oxy)hydroxides that sulfur include melanterite [FeSO47H2O], copiapiate formed the cemented layer. Hematite and maghemite [(Fe,Mg)Fe4(SO4)6(OH)220H2O] and the desorption probably originated as a residue derived from the of sulfate from Fe oxides and oxyhydroxysulfates. smelter and the refinery present at the East Mine site Mineralogical analysis failed to isolate melanterite (Table 4). and copiapiate in the three cemented layers. The mass of adsorbed sulfate was not determined in this study but Webster et al. (1998) showed that Fe(III) oxy- 5. Discussion

The results of this study suggests that each Table 4 cemented layer studied has its own mineralogical, Secondary-phase mineralogy of cemented layers at the three tailings chemical and physical signature which may vary impoundments temporally. The Nickel Rim cemented layer is pre- Phase Chemical formula Fault Lake Nickel Rim East Mine dominantly cemented by Fe(III) oxyhydroxides (i.e. UU U Gypsum CaSO42H2O goethite) and Fe(III) oxysulfates (i.e. jarosite), Goethite a-FeOOH UU U whereas the Fault Lake cemented layer is predom- UU U Jarosite KFe3(SO4)2(OH)6 inantly cemented by Fe(III) oxyhydroxides. The Sulphur S Minor Minor Minor Covellite CuS Trace Trace Not cement within the East Mine cemented layer is identified dominated by the SO4 phases gypsum and jarosite. The formula given is an idealised formula and the phases listed are Based on the age, sulfide content and location of the phases found in the cemented layer. cemented layers, it appears as if the cementing phases R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207 205 evolve over time with the initial cementing phases suggested that jarosite and ferrihydrite are initially being dominated by soluble sulfate phases such as formed under saturated conditions but in lower pH gypsum. The soluble hydrated iron sulfate phase, pore waters, jarosite is preferentially precipitated melanterite, has been shown to be a cementing phase relative to ferrihydrite. in other cemented layers (Blowes et al., 1991; Kami- If jarosite becomes unstable due to geochemical neni and McGregor, 1990). However, melanterite was changes within the tailings or changes in the degree of not detected in the three cemented layers examined in saturation, it may undergo replacement by goethite. this study. The calcium (Ca) within the gypsum is The replacement of jarosite by goethite can be derived from pH-buffering reactions between the pore described by the reaction: water and Ca-bearing carbonate phases such as cal- cite [CaCO ] and dolomite [Ca,Mg(CO ) ], whereas 3 3 2 KFe ðSO Þ ðOHÞ ZKþ þ 2SO2 the SO is derived from the oxidation of sulfide 3 4 2 6 4 4 þ minerals and the mill process water. As the tailings þ 3FeOOH þ 3H ð2Þ are further exposed to infiltration, the gypsum within + 2 + the cemented and uncemented tailings dissolves. This reaction, assuming that the K ,SO4 and H Within the Fault Lake, tailings equilibrium calcula- remain in solution, represents a mass loss of 47–60% tions indicate that the pore water immediately below using densities of 3.25 g/cm3 for jarosite and 3.3–4.3 the cemented layer is undersaturated with respect to g/cm3 for goethite (Berry et al., 1983). This reaction gypsum indicating a tendency for gypsum present appears to be occurring within the Nickel Rim within the tailings immediately underlying the cemented layer as well as the Fault Lake cemented cemented layer to dissolve. McSweeney and Madison layer. McSweeney and Madison (1988), Boulet and (1988) noted the increasing presence of gypsum with Larocque (1998) and Agnew (1998) have suggested depth at the Mineral Point tailings impoundment that a similar type of conversion reaction was occur- suggesting that gypsum within the shallow tailings ring within cemented layers at sites they have studied. is dissolving. These observations suggest that gypsum is a common initial cementing phase. However, with time, the gypsum dissolves and becomes a less 6. Summary important cementing phase within the cemented layers. Agnew and Taylor (2000) observed a similar Cemented layers within the Fault Lake, Nickel Rim mineralogical development and degradation within and East Mine tailings impoundments differ in there cemented layers in mill tailings impoundments in relative location within the tailings stratigraphy as well Australia. as in their physical, mineralogical and chemical proper- As sulfide oxidation reactions continue, dissolved ties. The principal sulfide mineral within all three Fe(II) is released to the pore water. The Fe(II) can tailings impoundments is pyrrhotite. The cemented then be oxidized to ferric iron (Fe(III)) and hydro- layers all have an increase in total C and S concen- lyzed releasing acidity to the pore water through the trations greater than the surrounding uncemented tail- reaction: ings suggesting that the cementing phases are partially composed of C- and S-bearing phases. Mineralogical analysis of all three cemented layers confirms the 4Fe2þ þ O þ 10H O Z4FeðOHÞ þ 8Hþ ð1Þ 2 2 3 presence of the S-bearing phases gypsum and jarosite. Chemical extractions and mineralogical analysis of the As the pore water pH decreases, favorable con- cemented layers indicates that the mass of S- and Fe- ditions for the precipitation of jarosite, natrojarosite, bearing phases within the three cemented layers vary hydronium jarosite and goethite are created. Ahmed between sites with the Fault Lake impoundment having (1995) noted that goethite appeared to be stable in goethite as the predominant cementing phase, whereas unsaturated conditions, whereas jarosite was stable in within the Nickel Rim impoundment the tailings are saturated conditions in pyrrhotite-based cemented cemented by a gypsum–jarosite–goethite combina- layers formed in the laboratory. Nordstrom (1982) tion. The 8-cm thick East Mine cemented layer, which 206 R.G. McGregor, D.W. Blowes / Journal of Geochemical Exploration 76 (2002) 195–207 is situated within the transition zone between the Agnew, M.K., Taylor, G.F., 2000. The development, cycling and oxidized and unoxidized tailings, contains a mixture effectiveness of hardpans and cemented layers in tailings storage facilities in Australia. 4th AMD Workshop, Townsville, Feb. of gypsum, jarosite and goethite, with gypsum being 28–Mar. 2. the dominant cementing agent. Ahmed, S.M., 1991. Electrochemical and surface chemical meth- The cemented layers show enrichment relative to the ods for the prevention of the atmospheric oxidation of sulfide surrounding uncemented tailings with respect to the tailings. Second International Conference on the Abatement of trace elements As, Cd, Co, Cu, Ni and Zn. The East Acid Drainage, Montreal. CANMET, Ottawa, Canada, pp. 305–319. Mine cemented layer showed the greatest enrichment Ahmed, S.M., 1995. Chemistry of pyrrhotite hardpan formation. In: for these trace elements relative to the surrounding Hynes, T.P., Blanchette, M.C. (Eds.), Sudbury ’95—Mining and uncemented tailings, whereas the Fault Lake cemented the Environment. Sudbury, vol. 1. CANMET, Ottawa, Canada, layer had the least amount of enrichment of the three pp. 171–180. cemented layers. These results suggest that cementing Al, T.A., Blowes, D.W., Jambor, J.L., 1994. The pore-water geochemistry of the Cu–Zn mine tailings at Kidd Creek, near Tim- phases have the potential to serve as a sink for dissolved mins, Ontario, Canada. International Land Reclamation Mine trace elements, and that mineralogically immature Drainage Conference and Third International Conference on cemented layers have a greater potential to retain trace the Abatement of Acidic Drainage. U.S. Bureau of Mines Spe- elements. As the mineralogical phases within the cial Publication SP 06A-94, vol. 2. CANMET, Ottawa, Canada, cemented layers mature, there exists a possibility that pp. 208–217. Allison, J.D., Brown, D.S., Novo-Gradac, K.J., 1990. MINTEQA2/ trace elements may be released to the pore water during PRODEFA2. A Geochemical Assessment Model for Environ- the aging process. mental Systems: Version 3.0 User’s Manual. Environmental Re- Based on these observations and measurements, search Laboratory, U.S. E.P.A., Athens, Georgia, USA. naturally occurring cemented layers may act as a sink Alpers, C.N., Nordstrom, D.K., Ball, J.W., 1989. Solubility of jar- for dissolved trace elements. Decreases in the total osite solid solutions precipitated from acid-mine waters, Iron Mountain, CA, USA. Sci. Geol., Bull. 42, 281–298. porosity relative to the surrounding uncemented tail- Bain, J.G., Blowes, D.W., Robertson, W.D., Frind, E.O., 2000. ings may also result in the cemented layers acting as a Modelling of sulfide oxidation with reactive transport at a mine hydraulic and diffusive barrier towards the migration of drainage site. J. Contam. Hydrol. 41, 23–47. infiltrating precipitation and atmospheric gases such as Ball, J.W., Nordstrom, D.K., 1991. User’s manual for WATEQ4F, with oxygen. revised thermodynamic data base and test cases for calculating speciation of major, trace and redox elements in natural water. U.S.G.S., Open File Report 91-183. Berry, L.G., Mason, B., Dietrich, R.V., 1983. Mineralogy: Concepts, Acknowledgements Descriptions, Determinations, 2nd ed. Freeman, New York. Blowes, D.W., Reardon, E.J., Cherry, J.A., Jambor, J.L., 1991. The authors would like to acknowledge the The formation and potential importance of cemented layers in contributions of W. Beck, D. Heagle, A. Williams, inactive sulfide mine tailings. Geochim. Cosmochim. Acta 55, 965–978. R. Caldwell and J. Stegemann to this manuscript. Blowes, D.W., Al, T., Lortie, L., Gould, W.D., Jambor, J.L., 1995. Financial support was provided by Environment Microbiological, chemical and mineralogical characterization of Canada and the Ontario Ministry of the Environment. the Kidd Creek mine tailings impoundment, Timmins area, On- Appreciation is extended to Falconbridge for their tario. Geomicrobiol. J. 13, 13–31. support and access to the field sites. The authors Boorman, R.S., Watson, D.M., 1976. Chemical processes in abandoned sulfide tailings dumps and environmental implications for would like to thank M. Agnew, D. Craw and one northeastern New Brunswick. CIM Bull. 69, 86–96. anonymous reviewer whose comments and sugges- Boulet, M.P., Larocque, A.C., 1998. A comparative mineralogical tions greatly enhanced the manuscript. and geochemical study of sulfide mine tailings at two sites in New Mexico, USA. Environ. Geol. 33 (2/3), 130–142. Chermak, J.A., Runnells, D.D., 1996. 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