Mine Drainage from the Weathering of Sulfide Minerals and Magnetite

Applied Geochemistry 24 (2009) 2362–2373

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Applied Geochemistry

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Mine drainage from the weathering of sulﬁde minerals and magnetite

M.C. Moncur a,b, J.L. Jambor a, C.J. Ptacek a,*, D.W. Blowes a a Department of Earth and Environmental Sciences, 200 University Avenue West, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 b Alberta Research Council, 3608, 33 Street NW, Calgary, Alberta, Canada T2L 2A6 article info abstract

Article history: Pyrite and pyrrhotite are the principal minerals that generate acid drainage in mine wastes. Low-pH con- Available online 17 September 2009 ditions derived from Fe-sulfide oxidation result in the mobilization of contaminant metals (such as Zn, Cd, Ni and Cr) and metalloids (such as As) which are of environmental concern. This paper uses data from detailed mineralogical and geochemical studies conducted at two Canadian tailings impoundments to examine the mineralogical changes that pyrite, pyrrhotite, sphalerite and magnetite undergo during and after sulfide oxidation, and the subsequent release and attenuation of associated trace elements. The stability of sphalerite in tailings impoundments generally is greater than that of pyrrhotite, but less than pyrite. Dissolved Ni and Co derived from Fe sulfides, and to a lesser extent, dissolved Zn and Cd from sphalerite, are commonly attenuated by early-formed Fe oxyhydroxides. As oxidation progresses, a recycling occurs due to continued leaching from low-pH pore waters and because the crystallinity of Fe oxyhydroxides gradually increases which decreases their sorptive capacity. Unlike many other elements, such as Cu, Pb and Cr, which form secondary minerals or remain incorporated into mature Fe oxyhydroxides, Zn and Ni become mobile. Magnetite, which is a potential source of Cr, is relatively stable except under extremely low-pH conditions. A conceptual model for the sequence of events that typically occurs in an oxidizing tailings impoundment is developed outlining the progressive oxidation of a unit of mine waste containing a mixed assemblage of pyrrhotite and pyrite. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction primarily on the mineralogical changes that develop during sulﬁde oxidation, and on the behavior of the principal trace elements that

Pyrite [FeS2] is the most abundant metal sulfide associated with are released by the oxidation of the sulfide minerals. the Earth’s crust (Rimstidt and Vaughan, 2003; Murphy and Stron- gin, 2009) and contributes significantly to the world’s acid drain- 2. Oxidation of the sulfide minerals age through natural weathering or from anthropogenic activities. However, in many mineral deposits, pyrrhotite [Fe S] is also a 1x The oxidation of pyrite, pyrrhotite, sphalerite and magnetite significant contributor of acid drainage. The oxidation of Fe-sulfide may be written as follows: minerals and the subsequent generation of acidity may lead to destructive impacts to receiving ground and surface waters 15 7 2 þ (Blowes and Jambor, 1990; Nordstrom and Alpers, 1999; Johnson FeS2 þ O2 þ H2O ! 2SO4 þ FeðOHÞ3 þ 4H ð1Þ pyrite 4 2 et al., 2000; Levings et al., 2005; Moncur et al., 2006). Low-pH efflu- x 2 2þ þ ent is the primary concern in wastes from sulfide-bearing metallif- Fe1xS þ 2 O2 þ xH2O ! SO þð1 xÞFe þ 2xH ð2Þ 2 4 erous deposits where potentially toxic metals and metalloids are pyrrhotite 2þ 2 solubilized and become mobile in the low-pH pore waters. ZnS þ 2O2 ! Zn þ SO4 ð3Þ sphalerite This paper presents a review of the behavior of pyrite, pyrrho- 2þ 3þ 2þ 3þ 9 tite, sphalerite [ZnS], and magnetite [Fe Fe O4] based on geo- Fe Fe O þ 1=4O þ H O ! 3FeðOHÞ ð4Þ 2 2 4 2 2 2 3 chemical and mineralogical data from over 30 tailings magnetite impoundments studied by the authors worldwide with specific These simplified equations, where Fe(OH) is a replacement for fer- examples from the Sherritt–Gordon tailings in Sherridon, Mani- 3 rihydrite [5Fe O 9H O], do not adequately describe the complex toba, Canada, where detailed mineralogical and geochemical char- 2 3 2 reactions that occur in the oxidation process. For example, the oxi- acterization has been conducted (Fig. 1). The focus of this paper is dation of pyrite involves a series of multiple reactions to achieve the end product in Eq. (1) (Garrels and Thompson, 1960; Nordstrom * Corresponding author. Fax: +1 519 746 3882. and Southam, 1997; Druschel et al., 2004). Reactions involving pyr- E-mail address: [email protected] (C.J. Ptacek). rhotite in Eq. (2) are similarly complex and do not always express

Fig. 1. Map of Canada showing the locations of Sherridon, Manitoba, and other mine sites investigated by the authors that contributed to this manuscript. the amount of acid generated by the mineral (Nicholson and Scha- 2.1. Trace-element mineral associations rer, 1994; Janzen et al., 2000). Crystal-structure vacancies related to the Fe deficiency in pyrrhotite may allow the vacancy-associated Trace elements associated with pyrrhotite are Ni and, to a lesser Fe2+ to waiver between Fe2+ and Fe3+ valence states, thereby poten- extent, Co (Gunsinger et al., 2006b). The same association applies tially destabilizing defect structures that are more easily broken to pyrite, however, pyrite may be arsenical (Paktunc, 2008). The than those of pyrite and oxidize more rapidly than pyrite (Jambor, association of Au with As-bearing pyrite (Cook and Chryssoulis, 2003; Murphy and Strongin, 2009). The presence of Fe3+ in pyrrho- 1990; Fleet et al., 1993; Palenik et al., 2004) results in ineffective tite may increase the rate of oxidation of Fe2+ to Fe3+ which in turn cyanidation unless the pyrite is artificially oxidized (McCreadie will accelerate the oxidation of pyrite and pyrrhotite by Fe3+ et al., 2000). Following ore processing, the arsenical pyrite and through: arsenical residues are commonly stored within tailings facilities, and subsequent dissolution and mobilization of the As is an envi- 3þ 2þ 2 þ ronmental concern. Feð1xÞS þð8 2xÞFe þ 4H2O !ð9 3xÞFe þ SO þ 8H ð5Þ 4 Sphalerite is the principal source of Zn and Cd in mine tailings. 3þ 2þ 2 þ FeS2 þ 14Fe þ 8H2O ! 15Fe þ 2SO4 þ 16H ð6Þ Sphalerite may be present in percentages at sites, where Zn recov- ery was not included in the concentration process, as was the case The oxidation of pyrite and pyrrhotite in Eqs. (5) and (6) is more ra- in the early years of milling at the former Sherritt–Gordon Mine in pid than oxidation by atmospheric O2 in Eqs. (1) and (2) (Nord- Manitoba, Canada (Farley, 1949). Also, Fe typically substitutes par- strom, 1982), and with the dissolution of pyrrhotite, Fe3+ may be tially for Zn in sphalerite (Pring et al., 2008) and can affect the flo- released. tation process potentially decreasing recoveries (Walker, 1930; Although pyrite and pyrrhotite are the primary concerns Gigowski et al., 1991; Zielin´ ski et al., 2000). The oxidation and dis- with regard to the dissolution of Fe-bearing sulfide minerals, solution rates of sphalerite also have been observed to increase sphalerite, magnetite and other Fe-bearing minerals are com- with increasing Fe content in the mineral (Perez and Dutrizac, monly overlooked as potential candidates for contributing to 1991; Weisener, 2002; Blowes et al., 2003). In laboratory studies, mine drainage. Sphalerite is commonly Fe-bearing, magnetite both low-Fe and high-Fe sphalerites have been observed to show contains both Fe(II) and Fe(III), and other minerals that contain a rapid initial dissolution rate with release of Zn from the mineral

Fe (e.g., siderite [FeCO3]; biotite [K(Fe)3AlSi3O10(OH)2]), may surface, followed by a much slower rate of reaction (Buckley et al., provide an additional input to acid generation through the oxi- 1989; Giudici et al., 2002; Weisener et al., 2003; Stanton et al., dation of Fe(II) and hydrolysis of Fe(III) as described by the 2008). reactions: The structure of sphalerite can lead to substitution by a wide variety of ions in percentage amounts, such as Cu, Mn, In, Fe and Cd (de Giudici et al., 2002). Up to 13.4 mol% Cd has been incorpo- 2þ 1 þ 3þ 1 Fe þ O2 þ H ! Fe þ H2O ð7Þ rated in synthetic sphalerite at 600 °C(Maurel, 1978), but in most 4 2 natural occurrences the Cd content is <1 wt.% (Domènech et al., Fe3þ 3H O Fe OH 3Hþ 8 þ 2 ! ð Þ3 þ ð Þ 2002; Moncur et al., 2005; Koski et al., 2008; Pring et al., 2008). The larger atomic radius of Cd relative to that of Zn prefers the where one mole of acid is consumed in Eq. (7), but three are re- wurtzite-type structure (hexagonal ZnS) over that of sphalerite leased in Eq. (8). (Pattrick et al., 1998). 2364 M.C. Moncur et al. / Applied Geochemistry 24 (2009) 2362–2373

Magnetite in sulfide-bearing mine wastes is of interest because oxidation. Due to the lower solubility of galena, the formation of 2+ the mineral forms a solid-solution series with chromite [Fe Cr2O4] secondary anglesite rims slows the progress of further replacement and other members of the spinel group, and can be Cr-bearing but does not completely retard it. Therefore, although galena and (Kadzialko-Hofmokl et al., 2008). Although the common rock- sphalerite are similar in reactivity, galena may persist in a strongly forming minerals such as chlorite, mica, amphibole and pyroxene oxidized environment that has removed sphalerite. Similar alter- may contain trace amounts of Cr (Tischendorf et al., 2001; Jambor ation rim or retardation applies to other minerals, such as pyrrho- et al., 2002), in most of the oxidized tailings impoundments inves- tite and arsenopyrite, except for enargite [Cu3AsS4], where the tigated by the authors, the presence of dissolved concentrations of effects are much less pronounced. Cr have been observed to coincide with the presence of Cr-bearing The limitation concerning the oxidation of primary minerals is magnetite in the tailings (e.g., Moncur et al., 2005; Gunsinger et al., marcasite [FeS2]. Laboratory experiments have shown a similar 2006b). (Wiersma and Rimstidt, 1984) or greater reactivity for marcasite than for pyrite (Rinker et al., 1997; McGuire et al., 2001; Uhlig et al., 2001; Elsetinow et al., 2003) but the reaction rates vary 3. Mineralogical and geochemical behavior widely. The placement of marcasite in Table 1 refers only to primary marcasite of a homogeneous texture and grain size similar The relative resistance of various sulfide minerals to oxidation to that of the associated pyrite. This condition is stressed because in tailings impoundments has been documented previously (Jam- marcasite is commonly a secondary mineral that forms rims and bor, 1994; Jambor and Blowes, 1998; Plumlee, 1999; Gunsinger pseudomorphs after pyrrhotite in the early stage of oxidation et al., 2006a; Koski et al., 2008). An updated version of the relative (Blowes and Jambor, 1990). This type of marcasite is typically het- resistance, as observed by optical microscopy of field samples from erogeneous on a lm or finer scale, and in this form it seems to be a several tailings sites is presented in Table 1. The listing serves as a metastable transitional mineral that very readily alters to goethite guide to the susceptibility of the sulfide mineral to oxidation (Jambor, 2003). where the textures and grain sizes are similar. It is also important The stabilities of the various sulfide minerals can be illustrated to recognize that: (i) a ‘persistence’ effect may be present and (ii) by the sulfide alteration index (SAI), which is a practical method of the order applies to primary minerals. An example of the persis- mapping the changes observed in the appearance and disappear- tence effect would be the opposite behaviors of galena [PbS] and ance of the sulfide minerals through a vertical profile of a tailing sphalerite during oxidation. Although exceptions are common in impoundment. The SAI was first constructed by Blowes and Jambor carbonate-rich environments, where the replacement of Fe-bear- (1990) for a series of cores through the Waite Amulet tailings ing sphalerite is by Fe oxyhydroxides (Bertorino et al., 1995; Fan- impoundment near Noranda (Quebec, Canada). An example of a fani et al., 1997; Boulet and Larocque, 1998) or Fe oxyhydroxides SAI for the tailings from the Sherritt–Gordon mine at Sherridon and sulfur (Jeong and Lee, 2003), in sulfate-rich systems derived (Manitoba, Canada) is described and illustrated in Table 2 and from sulfide mine wastes that generate acid drainage, the dissolu- Fig. 2. At both sites the ore deposit consisted of volcanogenic mas- tion of sphalerite typically occurs without the formation of alter- sive sulfides and the tailings contain pyrite and pyrrhotite. The ation rims that partly or completely pseudomorph the sphalerite. Sherridon site is used to illustrate the mineralogical and geochem- The solubilization of sphalerite typically occurs by particle-size ical changes that typically occur in a sulfide oxidation profile. reduction rather than replacement by secondary minerals. Sphaler- The Sherritt–Gordon mine at Sherridon operated from 1931– ite dissolution and the subsequent precipitation of smithsonite 1932 and from 1937–1951, during which 7.7 Mt of pyritic ore ] has been suggested as a control for dissolved Zn concen- 1 1 [ZnCO3 grading 2.45% Cu, 2.97% Zn, 19.9 g tons Ag and 0.62 g tons Au trations (e.g., Nordstrom and Alpers, 1999; Bain et al., 2001; Malm- were milled, however a Zn concentrate was not produced until ström et al., 2008), however, secondary smithsonite has not been 1942 (Farley, 1949). The sulfide assemblage consisted of mainly identified in sulfide-containing tailings impoundments. The ab- pyrite and pyrrhotite with lesser amounts of chalcopyrite, sphaler- sence of secondary smithsonite in sulfide tailings could be due to ite, and minor amounts of only a few other minerals, including difficulties in identifying minute masses of secondary smithsonite arsenopyrite [FeAsS], cubanite [CuFe2S3] and local occurrences of or due to the high solubility of smithsonite in low-pH waters. galena. The tailings were discharged into two impoundments, the Alternatively, Zn concentrations could also be controlled by the older of the two is briefly referred to in the following; a more de- precipitation of other Zn bearing minerals, such as Fe oxyhydrox- tailed description is in Moncur et al. (2005). ides. The behavior of magnetite seems to be similar to that of The tailings, with groundwater temperatures ranging from sphalerite, but because the magnetite content of most non-calcar- 6.0–15.5 °C, have been oxidizing for over 70 a and have formed eous sulfide deposits is low, the first appearance of magnetite is a continuous ochreous hardpan that is >1 m thick. Near the cen- typically below the zone of intense oxidation. ter of the impoundment the tailings extend to a depth of 7 m The ‘persistence’ effect is demonstrated by galena, when com- and the water table is at a depth of approximately 4 m, however pared to sphalerite, which forms a rim of anglesite [PbSO4] during the top of the hardpan located at 0.90 m marks the zone where the tailings are variably water-saturated. The unsaturated zone Table 1 above the hardpan has a high SAI, is depleted in sulfides and Schematic relative resistance to alteration of sulfides and magnetite in oxidized carbonates, and the pore gas-phase O2 concentrations show a ra- tailings. pid downward decrease to a depth of 0.5 m that reflects O2 con- Pyrrhotite Fe S Low resistance sumption by sulfide oxidation (Fig. 3). In contrast to the deeper 1x ? Galena PbS ? continuous hardpan that begins at 0.90 m, the unsaturated, near- ? ? Sphalerite (Zn,Fe)S ? surface zone contains only thin (1–10 cm) layers of Fe oxyhy- ? Bornite Cu5FeS4 ? ? droxide precipitates lacking lateral continuity, and which can Pentlandite (Fe,Ni) S ? 9 8 ? be categorized as a discontinuous hardpan. The bulk-sample X- Arsenopyrite FeAsS ? ? ? ray data indicate that the zone is rich in gypsum [CaSO 2H O], Marcasite FeS2 ? 4 2 ? Pyrite FeS2 ? jarosite [(K,Na)(Al,Fe) (SO ) (OH) ], native sulfur (from the oxi- ? 3 4 2 6 Chalcopyrite CuFeS ? 2 y dation of pyrrhotite), and goethite [a-FeOOH] (Fig. 4). Optical Magnetite Fe3O4 microscopy revealed that phlogopite [K(Mg,Fe)3AlSi3O10(OH)2] Molybdenite MoS2 High resistance and plagioclase [(Na,Ca)Al(Si,Al)3O8] have been substantially al- M.C. Moncur et al. / Applied Geochemistry 24 (2009) 2362–2373 2365

Table 2 Sulﬁde alteration index (SAI) of the Sherridon tailings (revised after Blowes and Jambor, 1990; Moncur et al., 2005).

Index Alteration 10 Almost complete oxidation of sulﬁdes; traces of chalcopyrite ± pyrite 9 Only sparse pyrite and chalcopyrite; no pyrrhotite or sphalerite 8 Pyrite and chalcopyrite common, but chalcopyrite proportion higher than normal possibly because of pyrite dissolution; no pyrrhotite or sphalerite 7 Pyrite and chalcopyrite proportions normal; pyrrhotite absent but sparse sphalerite present 6 Pyrrhotite absent but sphalerite common 5 Pyrrhotite represented by marcasite pseudomorphs 4 First appearance of pyrrhotite, but only as remnant cores 3 Cores of pyrrhotite abundant 2 Well-developed cores of pyrrhotite, with narrower alteration rims; replacement by marcasite decreasing, and pseudomorphs are absent 1 Alteration restricted to narrow rims on pyrrhotite

Fig. 2. Cross-section through the center of the older Sherridon tailings impoundment, showing the mineralogical changes rated according to a sulﬁde alteration index (SAI) (after Moncur et al., 2005), which is a relative scale of alteration intensity. At SAI = 10, the sulﬁdes have been almost completely obliterated (Fig. 5); at SAI = 3, remnant cores of pyrrhotite are abundant; at SAI = 1, the alteration occurs only as narrow rims on pyrrhotite (Fig. 6e). A continuous hardpan extends from 90 cm to 2 m. The photo to the right of the SAI plot shows fresh exposed tailings on the left adjacent to extensive development of surface blooms of tertiary rozenite (white mineral) that formed shortly after 2+ the Fe -rich continuous hardpan was exposed to O2 (right half of the photo). Analysis of the rozenite showed that Zn is present in trace amounts. The static water table is at approximately 4 m below ground surface.

Vol. % Gas Wt. % Sulfur Wt. % CaCO Θ -n-Saturation Pore Water Density 3 (%) (g cm-3) 0.0 Θ

0.5 O2 CO2 n 1.0 Hardpan Sat. Depth (m) 1.5 Layer

2.0 01020015300.00 0.03 0.05 0501001.0 1.1 1.2

Fig. 3. Variation in O2 and CO2 gas, percent sulfur and CaCO3, volumetric moisture content (H) [where H = (masswater/densitywater)/volumetotal], porosity (n), and saturation, and pore water density, in the Sherridon tailings. The solid horizontal line shows the top of the main hardpan. The static water table is at approximately 4 m below ground surface. Data from Moncur et al. (2005).

tered, and the X-ray data indicate that chlorite and biotite are bite and biotite, by amorphous silica is common in this near-sur- depleted (Fig. 4). Replacement of Al-bearing silicates, mainly al- face zone of the tailings. 2366 M.C. Moncur et al. / Applied Geochemistry 24 (2009) 2362–2373

Fig. 4. Sulﬁde alteration index (SAI) and bulk-sample X-ray diffractometry data for the Sherridon tailings. Plotted points for the diffractometry data represent the X-ray peak height (generally the highest peak in the diffraction pattern of the mineral) as follows: nd – not detected; vw – very low (very weak) peak height; w – weak; m – medium; st – strong. In the plot for goethite, L is for lepidocrocite [cFeOOH], and in the plot for the Fe sulfates, M is melanterite and R is rozenite. The solid horizontal line shows the top of the main hardpan. The static water table is at approximately 4 m below ground surface.

The main continuous hardpan zone extends from 0.9 to 2.0 m. face, Fe3+-rich zone represents a slightly older and more advanced Compared to the ochreous, Fe(III)-rich overlying tailings, the main stage of oxidation which is expected to gradually expand down- hardpan is blackish and solidly cemented; although the same sec- ward approaching the Fe(II)-rich hardpan. Various Fe-sulfate min- ondary minerals that have formed in the near-surface zone are also erals undergo this progression of Fe(II) ? Fe(III) transformation as present. The main hardpan is characterized by an enrichment in oxidation progresses (Nordstrom and Alpers, 1999; Jambor et al.,

Fe(II)-sulfate minerals (melanterite [FeSO47H2O] and rozenite 2000a). [FeSO44H2O]) (Fig. 4), and by decreased porosity (Fig. 3) related to the cementation by secondary minerals. Similar enrichments 5. Geochemical variations with depth of Fe(II)-sulfate minerals in hardpan layers have been observed in the tailings impoundment at the Heath Steele mine, Newcastle, Within the Sherridon tailings, sulfide minerals below the per- NB (Blowes et al., 1992), the Renison Bell Tin Mine, Tasmania, Aus- manent water table do not show signs of alteration (SAI = 0). The tralia (Gilbert et al., 2003) and in mine-waste piles at the Elizabeth unaltered tailings solids average 246 mg g1 Fe, 7.7 mg g1 Zn, Mine, Strafford, VT (Hammarstrom et al., 2005). 2.2 mg g1 Cu, 0.01 mg g1 Ni, 0.08 mg g1 Co, 0.005 mg g1 Cr and 31 mg g1 Al. Pore waters from the unsaturated zone in the 1 1 4. Progression of sulfide alteration Sherridon tailings locally contain up to 280 g L SO4, 129 g L Fe, 55 g L1 Zn, 1.6 g L1 Cu, 0.02 g L1 Ni, 0.11 g L1 Co, The unaltered Sherridon tailings contain up to 60 wt.% of com- 0.003 g L1 Cr, 0.01 g L1 Cd and 7.2 g L1 Al, which in mine-waste bined pyrite and pyrrhotite, with the relative proportion variable settings are among the highest documented values in the world from 1:1 to 1:2, respectively. Mineralogical examination of sam- (Moncur et al., 2005). The pH, Eh and distribution of trace elements ples from the oxidized zone above the main hardpan shows that in the pore waters above and below the permanent water table are the sulfides have been almost completely replaced in the most oxi- illustrated in Fig. 7. High concentrations of Fe, SO4, and many of the dized material near the surface (Fig. 5). The downward progression metals coincide with low-pH conditions above the main hardpan is marked by the initial appearance of pyrite, then by the additional and above the permanent water table. appearance of sphalerite and by marcasite pseudomorphs after Dissolved Cu in the pore waters is derived from chalcopyrite. pyrrhotite (Fig. 6a and b). In other tailings impoundments, Fe oxy- Concentrations are highest at the top of the main hardpan, decreas- hydroxide replacement rims on pyrite are common (Jambor, 2003). ing rapidly to <1.2 mg L1 at depth. The decrease coincides with At the Sherridon tailings, the oxidation of pyrite is marked by the appearance of secondary covellite and melanterite (0.25 wt.% depletion of the mineral. Extensively altered pyrrhotite is first ob- Cu), which appear to act as controls on dissolved Cu. served at the top of the hardpan layer. The thickness of the alter- Dissolved concentrations of Zn and Cd, which are attributed to ation rims on pyrrhotite gradually decreases toward the bottom the dissolution of sphalerite, behave differently. Although Zn was of the hardpan (Fig. 6c and d). The hardpan is a zone of accumula- commonly detected in electron-microprobe analyses of the section of secondary minerals, including small amounts of covellite ondary Fe oxyhydroxides (0.10–0.33 wt.% Zn in six samples), the [CuS]. The accumulation zone forms in advance of the sulfide-oxi- high mobility and only limited attenuation of Zn is reflected by dation front and appears to be migrating downward. The near-sur- the high concentrations in the pore water beneath the main M.C. Moncur et al. / Applied Geochemistry 24 (2009) 2362–2373 2367

Fig. 5. Tailings from the Sherridon sulfide-depleted zone at 20 cm depth. Photo to the left, in plain reflected light, width of field 2.6 mm, shows two adjacent residual grains of chalcopyrite (cp), which are the only remaining sulfides (the whitish grain ‘il’ on the left is ilmenite FeTiO3). The narrow grey rims, as at the arrow, are Fe oxyhydroxides, mainly goethite. Photo to the right is of the same field, but in plain transmitted light. The black areas are Fe oxyhydroxides that have formed as rims and pseudomorphs after Fe sulfides.

Fig. 6. Photomicrographs of the Sherridon tailings; (a–e) are for plain reflected light. (a) Oxidized zone at 60 cm depth, showing complete replacement of pyrrhotite, now represented by blackish pseudomorphs, but with no alteration evident for the whitish grains of pyrite. Width of field is 2.1 mm. (b) Top of the hardpan contact at 90 cm depth, showing unaltered pyrite (py), sphalerite (s, at center), and a grain of chalcopyrite (cp). The zoned grains, as at the arrow, are pseudomorphs after pyrrhotite, wherein the whitish rims are secondary marcasite. Width of field is 2.1 mm. (c) Hardpan contact at 90 cm depth; width of field is 0.625 mm. The white, homogeneous grains are pyrite (py). Pyrrhotite (po, and at the core of the triangular particle at the top) is variably rimmed by marcasite (mr); in the particle at the bottom right no pyrrhotite remains. The large grey grain is sphalerite (sp) containing, at the arrow, an inclusion of former pyrrhotite, now altered to marcasite. (d) Hardpan zone at 116–126 cm depth, showing white unaltered pyrite (py) and weak alteration of pyrrhotite (po) along the rim and basal parting, with a narrow outermost rind of marcasite. Width of field is 0.625 mm. (e) Hardpan zone at 112–118 cm depth, showing cores of pyrrhotite with a thin exterior of marcasite; the intermediate zone is a Fe sulfate, as is indicated by its low reflectance and spectrally high S content. Width of field is 0.625 mm. (f) Basal part of the hardpan zone at 185 cm depth; reflected light, internal reflection, polarizers almost crossed, width of field 0.625 mm. The grains showing the basal parting are marcasite after pyrrhotite, a few remnants of which are present, as at the arrow. The cementing material is yellowish in plain light (whitish in the reproduction), and is apparently a mixture of Fe oxyhydroxide and Fe sulfate. The unaltered grains at the top right and lower left are pyrite (py). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) hardpan, as also observed in other sulfide tailings (Blowes et al., progressively within the hardpan, and are negligible beneath it. 1991; Gunsinger et al., 2006a). Values for Cd, however, are highest The distribution pattern indicates solid-phase attenuation, most within the hardpan, and the distribution is similar to that of dis- likely by sorption or coprecipitation with the Fe oxyhydroxides. solved concentrations of Pb and Cr (Fig. 7). In contrast, dissolved Ni is mobile and largely unattenuated. Dissolved Co and Ni concentrations are derived form the oxida- Chromium has multiple primary sources as it was detected by tion of pyrite and pyrrhotite. Concentrations of dissolved Co, like electron-microprobe analyses in magnetite, biotite and amphibole. 2+ those of Cu, are highest near the top of the main hardpan, decrease As well, almandine garnet [Fe 3Al2(SiO4)3] is present in the tail- 2368 M.C. Moncur et al. / Applied Geochemistry 24 (2009) 2362–2373

Fig. 7. Proﬁles of the water chemistry of the Sherridon tailings from August, 2001. The rectangular enclosure shows the position of the main hardpan, and the dashed line at the bottom of the inverted triangle is the position of the static water table. Data from Moncur et al. (2005).

ings, and the Cr-dominant species uvarovite [Ca3Cr2(SiO4)3]is 6. Generalized paragenesis known to occur in outcrops near the mine (Froese and Goetz, 1981). Although Cr was detected in the Fe oxyhydroxides within Table 3 shows a simplified sequence of the paragenesis that oc- the hardpan, the concentrations of dissolved Cr below the hardpan curs within a sulfide tailings impoundment. The table shows that are not substantially lower than those within it (Fig. 7). most reactions proceed concurrently, however the transformation Small amounts of arsenopyrite are the principal primary source of some minerals is more distinct than others. For example, labora- of As. Speciation of dissolved As concentrations indicates that tory examinations of freshly exposed surfaces of pyrite and other As(III) is more abundant than As(V), but the distribution of both sulfide minerals have shown that the minerals react almost instan- is similar. The downward profile for Astotal is like that of Cu, indi- taneously to produce secondary mineral phases, however the sec- cating distinct attenuation at the top of the hardpan (Fig. 7). ondary minerals at that stage are present in minute amounts and Although a discrete secondary As mineral was not identified in are not observable megascopically or by standard optical micros- the Sherridon tailings, in other impoundments the sorption of As copy (Jambor, 2003). The replacement of pyrrhotite by secondary by Fe oxyhydroxides (Figs. 8 and 9) and substitution in jarosite minerals in Table 3 is the first indicator of alteration in the ‘earliest’ are of common occurrence (Alpers et al., 1994; Jambor, 1999; Jam- stage of the oxidation of a sulfide tailings impoundment. The alter- bor et al., 2000b; Savage et al., 2000; Paktunc and Dutrizac, 2003). ation of pyrrhotite is preceded by the diffusion of S to the grain sur- The tailings profile of pore water pH and the downward mobil- face which forms a S-enriched alteration rim where the Fe has ity of dissolved SO4 and the metal concentrations is well illustrated departed (Pratt et al., 1994a,b; Mycroft et al., 1995). The released in Fig. 7. The oxidation front and the hardpan will continue to Fe oxidizes to form Fe oxyhydroxide and the S-enriched alteration move downward over time. For example, covellite that currently rim typically forms marcasite (Jambor, 1994). In some cases the serves as a solid-phase sink for Cu will be exposed to oxidizing acid boundary between the pyrrhotite core and the marcasite rim is conditions, releasing Cu to the pore waters along with the remobi- also S-enriched and through microprobe analysis, reveals a mineral lization of other temporarily attenuated species bound in soluble that has a composition that is approximately Fe2S3, also observed mineral phases. An example of this type of recycling, specifically by Murphy and Strongin (2009). This Fe sesquisulfide phase is for As, has been documented by Courtin-Nomade et al. (2003) for interpreted by the authors to be less stable than either pyrrhotite As-rich tailings that have oxidized along a sloped topography. Early or marcasite, and it oxidizes much more rapidly producing an inte- attenuation of As occurs in association with amorphous and poorly rior zone that is blackish in reflected light as illustrated in Fig. 6e. crystalline Fe oxyhydroxides, such as ferrihydrite (Jambor and The oxidation of pyrrhotite may also form native sulfur (Fig. 4) and Dutrizac, 1998), and As decreases in the solid phase through Fe(II)-sulfate minerals that may dissolve and crystallize seasonally desorption as goethite becomes the dominant Fe oxyhydroxide as sulfates containing both Fe(II) and Fe(III), as is the case in the mineral phase. Sherridon tailings. M.C. Moncur et al. / Applied Geochemistry 24 (2009) 2362–2373 2369

Fig. 8. Examples of well-developed alteration rims of Fe oxyhydroxide on pyrite in tailings from the Yellowknife gold-producing area, Northwest Territories, Canada. Photos (a) and (b) are backscattered-electron (BSE) images of oxyhydroxide-rimmed pyrite (py) and the corresponding X-ray maps that show the distribution of Fe, As and S. The residual pyrite is outlined well by the maps for S. Note that As is concentrated in the Fe oxyhydroxides and has been sorbed from the pore waters rather than from the low-As host pyrite. The bar scales are 45 lm for (a) and 35 lm for (b).

During the ‘early’ oxidation of pyrrhotite, acid and Fe(III) are re- (Jambor et al., 2000c, 2002; Jurjovec et al., 2002). The silicate min- leased which increases the oxidation of other Fe sulﬁdes and de- erals observed to be the most susceptible to low-pH dissolution creases the pore water pH. The presence of carbonate minerals within oxidized tailings are micas (e.g., biotite) and chlorite (e.g., within the tailings may neutralize the acidity, however if carbonate Sherridon, Fig. 4). In low-pH conditions the incongruent early re- minerals are not present or have become consumed, the pH will lease of K from biotite (Kalinowski and Schweda, 1996; Malm- continue to decrease because most of the common rock-forming ström et al., 1996) provides the main source of K for jarosite minerals have a slow reaction rate and poor neutralizing capacity precipitation (Jambor, 1994). Although other silicate minerals such 2370 M.C. Moncur et al. / Applied Geochemistry 24 (2009) 2362–2373

Fig. 9. (a) BSE image of remnant arsenopyrite (asp) and (b) a pseudomorph of Fe oxyhydroxide after arsenopyrite in tailings from the Yellowknife area, Northwest Territories, Canada. The bar scales are 50 lm and 25 lm, respectively. The black areas within the pseudomorph are voids. The relatively homogeneous distribution of As in the X-ray maps of both (a) and (b) suggests sorption of As rather than the presence of mixtures containing discrete As mineral species. Electron-microprobe analyses of the rims in Fig. 8 gave up to 5.3 wt.% As2O5, and analyses of the rims in Fig. 9 range from 5.6 to 7.1 wt.% As2O5. as K feldspar, muscovite and illite also release K during weathering, tion of fine-grained poorly crystalline Fe oxyhydroxides (Fig. 8 and concentrations are much lower due to the low solubility of the 9; tailings samples from the Yellowknife Au-producing area, minerals (Jambor et al., 2002). As the tailings ‘mature’, pyrrhotite Northwest Territories, Canada) or precipitation of soluble sulfate will be depleted long before pyrite is consumed. During the final minerals. However, the high sorptive and retentive capacities of or ‘late’ stage, the tailings will take on the characteristics of a ma- the Fe oxyhydroxides decline as their crystallinity improves by ture gossan (Blowes et al., 1992) and as the pore water pH rises due increasing the grain sizes and decreasing their surface areas (Cour- to the absence of sulfide minerals, jarosite will become unstable. tin-Nomade et al., 2003), and goethite becomes the dominant Fe During this evolution of tailings weathering, there is a continuous oxyhydroxide because it is a stable phase relative to species such recycling of trace elements often resulting in the temporary forma- as ferrihydrite or lepidocrosite. M.C. Moncur et al. / Applied Geochemistry 24 (2009) 2362–2373 2371

Table 3 Schematic representation of the progressive oxidation of a unit of mine waste containing a mixed assemblage of pyrrhotite (po) and pyrite (py). In the earliest stage of oxidation, the secondary products are formed predominantly from pyrrhotite, with native sulfur and marcasite derived exclusively from pyrrhotite; the Fe oxyhydroxides may include ‘amorphous’ material and ferrihydrite, but typically consist predominantly of very fine-grained goethite with a high sorptive capacity. In the late stage, following consumption of the sulfides, pH will eventually rise and jarosite will therefore also be unstable. Photomicrographs of the Sherridon tailings; (a) earliest to (d) late. (a) Unaltered grains of pyrite, sphalerite (sp) and chalcopyrite (ch) with grains of pyrrhotite rimmed by secondary marcasite (mr). (b) Near the center of the photo is a particle consisting of a core of pyrrhotite, a dark intermediate zone, and a rind of marcasite. The dark zone is vacant, but local areas of Fe sulfate are detectable. Pyrite and sphalerite grains are unaltered. (c) Whitish grains are pyrite and chalcopyrite accompanied by two grains of magnetite (m). On the right is a large particle of goethite. Pyrrhotite is no longer present. (d) On the left is a whitish grain of pitted magnetite and on the right are two particles of compact goethite. Note the absence of sulfide minerals.

Earliest Early Maturing Late Oxidation of Fe sulﬁdes Acceleration Slowing Consumed Native sulfur (from po) – – – Marcasite (from po) – – – Fe oxyhydroxides (ﬁne-gained) Fe oxyhydroxides (coarsening + recycling) Goethite Goethite Fe2+ sulfates (mainly from po) Fe2+ sulfates ? Fe2+ Fe3+ Fe3+ sulfates – – Jarosite (K+ mainly from biotite) Jarosite Jarosite po + py py > po py –

The 4 stages of tailings oxidation presented in Table 3 are not of and the Toxic Substances Research Initiative managed jointly by equal duration. The initial oxidation ‘earliest’ stage is almost Health Canada and Environment Canada. The authors thank Editors instantaneous upon atmospheric exposure, and the effects become David Vaughan and Anne Thompson for organizing this special is- evident in pyrrhotite-rich environments. The progression through sue in honor of the late John L. Jambor. The authors would like to the ‘earliest’ and ‘early’ stages may occur within a few months or express their gratitude to Robert R. Seal and an anonymous re- a few years, whereas, the ‘maturing’ and ‘late’ stages may require viewer for their constructive criticism which greatly improved this many centuries. manuscript and to Lynne Jambor and Mati Raudsepp for their invaluable support and assistance over many years. 7. Conclusions

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