ration 92 (2007) 97–110www.elsevier.com/locate/jgeoexp

Journal of Geochemical Explo

Mineralogical and geochemical study of element mobility at thesulfide-rich Excelsior waste rock dump from the polymetallic

Zn–Pb–(Ag–Bi–Cu) deposit, Cerro de Pasco, Peru

Jochen Smuda a,d,⁎, Bernhard Dold a,1, Kurt Friese b,2,Peter Morgenstern c,3, Walter Glaesser d,4

a Centre d'Analyse Minérale, Anthropole, University of Lausanne, 1015 Lausanne, Switzerlandb UFZ-Umweltforschungszentrum, Department Lake Research, Brueckstr. 3a, D-39114 Magdeburg, Germany

c UFZ-Umweltforschungszentrum, Department Chemical Analytic, Permoserstraβe 15, D-04303 Leipzig, Germanyd University of Leipzig, Institute of Geophysics and Geology, Talstr. 35, D-04211 Leipzig, Germany

Accepted 8 August 2006Available online 2 October 2006

Abstract

We present a mineralogical and geochemical study of the sulfide-rich waste rock dump Excelsior (size: 94 ha) originating from theCerro de Pasco mine (altitude: 4300 m a.s.l.), Central Andes, Peru. The aims of this study were: to characterise (1) the secondarymineral assemblage and (2) the acidmine drainage (AMD) from this waste rock dump. (3) This informationwas used to create amodelof the element transport in and out of the waste rock dump under the highly variable mountain climate with high precipitation duringthe wet season and high evaporation during the dry season. The main ore minerals found in the polymetallic Pb–Zn–(Ag–Bi–Cu)deposit are pyrite, sphalerite, galena and enargite with minor finds of tennantite, covellite and cerrusite. Gangue was dominantlyquartz±hematite±siderite±muscovite. The waste rocks had a high potential of acid generation with low neutralization potential(estimated N60 wt.% pyrite, b5 wt.% calcite/dolomite) with an already acidic environment (average paste – pH 2.8). Gypsum,different types of jarosite and a variety of Fe-sulfates (e.g. melanterite (Fe(SO4)·7H2O) and rozenite (Fe(SO4)·4H2O)) were thedominant secondary minerals. Less frequent secondary minerals were: Fe(III)-hydroxides (schwertmannite (Fe16O16(OH)12(SO4)2),fibroferrite (Fe(SO4)(OH)·5H2O)), and Mg-, Mn- and Zn-sulfates (e.g. starkeyite (Mg(SO4)·4H2O), mallardite (Mn(SO4)·7H2O),goslarite (Zn(SO4)·7H2O), respectively).

For the primary mineral assemblage, X-ray fluorescence analyses displayed average concentrations of the heavy metals: Cu 0.1wt.%, Zn 1.1 wt.%, Pb 1.2 wt.% and Cd 40 mg/kg and of the metalloid As 0.15 wt.%. In secondary minerals an average enrichmentof Cu: 0.8 wt.%, Zn: 2.9 wt.%, As: 0.27 wt.% and Cd: 71 mg/kg was observed. Effects of rain events on the waste rocks were simulatedby water-leach tests of (1) solid samples from the top, representing waste rocks leached by infiltrating rainwater and (2) solid samplesaround the base, representing waste rocks affected by outcropping AMD generated in the waste rock dump. The leachates showedstored acidity (pH 1.2–5.6 of the leachates) and high solubility of secondary minerals (average electrical conductivity: 9.7 mS/cm andaverage concentrations for Fe: 928 mg/L, Zn: 315 mg/L, Cu: 7.5 mg/L, As: 4.1 mg/L, Cd: 1.59 mg/L, and Pb: 0.31 mg/L). The AMD

⁎ Corresponding author. Centre d'Analyse Minérale, Anthropole, University of Lausanne, 1015 Lausanne, Switzerland. Fax: +41 21 692 4315.E-mail addresses: Jochen.Smuda@unil.ch (J. Smuda), Bernhard.Dold@unil.ch (B. Dold), Kurt.Friese@ufz.de (K. Friese),

Peter.Morgenstern@ufz.de (P. Morgenstern), Walter.Glaesser@ufz.de (W. Glaesser).1 Tel./fax: +41 21 692 4324.2 Tel./fax: +49 391 8109 150.3 Tel./fax: +49 341 235 2625.4 Tel./fax: +49 341 973 2809.

0375-6742/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.gexplo.2006.08.001

98 J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

output at springs at the base of the waste rock dump is chemically similar to the leachates (acidic pH (2.78–5.10), oxidizing Eh (319–684 mV)), but higher charged with maximum concentrations for Fe: 5640 mg/L, Zn: 3000 mg/L, Cu: 161 mg/L, As: 8.0 mg/L, Cd:6.6 mg/L, and with higher electrical conductivity (19–26 mS/cm).

The data suggest that AMD formation is strongly controlled by the local climate. During the dry winter season, high evaporation ofoutcropping pore solutions and subsequently precipitation of efflorescent salts resulted in a heavy metal-enrichment at the base of theExcelsior waste rock dump. During the wet summer season, rain events caused the dissolution of most efflorescent salts, removed theenrichment at the base and resulted in a washout of acid solutions rich in Fe, Mn, Zn, Cu, Cd, As and S.© 2006 Elsevier B.V. All rights reserved.

Keywords: Acid mine drainage; Waste rock dump; Climate; Efflorescent salts; Heavy metal-sulfates; Leach test

1. Introduction

Sulfidic mine waste dumps are an important source ofmetal contamination in the mining environment. Whileacid mine drainage (AMD) in mine tailings has beenstudied extensively (e.g. Jambor, 1994; Dold andFontboté, 2001), there are still few works on the subjectof AMD formation in sulphidic waste rock dumps.Sracek et al., 2004, studied AMD and secondary min-erals from a waste rock dump with a low pyrite content.They pointed out that physical and geochemical pro-cesses within a waste rock dump are interconnected anddepend strongly on climate, waste rock texture and sta-bility. Recently, the importance of climate for the AMDgeneration due to formation and dissolution of water-soluble secondary minerals at mining sites has beenrecognised. Gierè et al. (2003) studied the role ofsecondary mineralogy in the control of the migration ofheavy metals and As at a relatively small sulfide-richwaste rock dump in Russia. Beside a characterisation ofthe secondary mineral assemblage of sulfidic wastedumps, their analyses of climatic influences on the sec-ondary minerals suggested that AMD output is stronglycontrolled by rain events. Frau (2000) reported thathighly water-soluble secondary melanterite from pyritealteration can store acidity and heavy metals during dryseasons which are then released due to the dissolution ofthis mineral during wet seasons.We studied a sulfide-richwaste rock dump at the elevated plain of the CentralAndes of Peru to characterise these secondary minerals,their water-solubility and the element outflow from thiswaste rock dump under the high precipitation–high evap-oration mountain climate.

Water-leach tests are useful to simulate rain eventsand evaluate the solubility of secondary minerals andthe element/acid liberation. Hammarstrom et al. (2005)showed with this type of leach tests that efflorescentsalts from different mining waste locations in the easternUS dissolve easily in water, producing acidic, metal-

loaded solutions. In this study, we focused on differ-ences between solid samples and leachates from sam-ples from the upper terraces of the waste rock dump andfrom samples from its base. The geochemical and min-eralogical differences between these samples were usedto evaluate water pathways and effects of the climate onelement transport and fixation. Geochemical data fromthe outcropping AMD displayed the subsequent transportof elements out of the waste rock dump system.

2. Description of the studied waste rock dump

The open pit and underground mine of Cerro dePasco (Volcan Corporation S.A.A.) is located in theSierra Central of Peru at an altitude of 4300 m above seaterrace (Fig. 1A). Since the 16th century, Cu and Aghave been exploited from the Cerro de Pasco ore body. In1963, the exploitationmoved to Pb–Zn–Ag. The studiedExcelsior waste rock dump operated from 1943 until2000 initially as a dump for rocks with non-economicCu – and later for non-economic Zn and Pb concentra-tions. The waste rock dump is located in the SW of themine in the bottom of a valley and occupies an area of94 ha, containing 26,400,000 m3 of broken rocks. Itpartly overlies the downstream Quiulacocha tailingsimpoundment (ca. 60 ha covered, 114 ha not covered).The bedrock of the deposition zone consists of Devonianphyllites and shales with low hydraulic conductivity.

2.1. Ore and regional geology

The polymetallic Zn–Pb–(Ag–Bi–Cu) Cerro dePasco deposit represents an economically vital poly-metallic source with an estimated total amount of100 Mt at about 7 wt.% Zn, 2 wt.% Pb and 3 oz/t Ag(Baumgartner et al., 2003). Hydrothermal solutionsassociated with volcanic activity formed as the firststage of sulfide deposition an extensive replacementbody of quartz–pyrite in Triassic–Jurassic limestone/

Fig. 1. (A) Location and geological setting of the Cerro de Pasco mine area, Peru. (B) Topographical map of the Excelsior waste rock dump with thesampling points.

99J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

dolomite which represents 90 vol.% of the entire orebody (Einaudi, 1977). At the contact zone between thehost rock and the quartz–pyrite body, carbonates of thehost rock formation were altered to ankerite and siderite.Tertiary volcanic rocks (quartz monzonite/tuff) whichpartially overlaid the host rock formation were altered incontact with this quartz–pyrite body to muscovite–quartz–pyrite. The second stage of sulfide depositionformed the main Zn–Pb ore in the form of pipes withdominantly sphalerite–galena and minor tennantite,marmantite and chalcopyrite. Veins rich in enargite,luzonite, galena and minor tennantite represent a thirdstage of sulfide deposition. The final hydrothermalactivity resulted in local leaching and formation of amineral assemblage of pyrite–hematite–realgar associ-ated with a high Ag-mineralization. The oxidation of thetop of the sulfide body formed a gossan which containsquartz and hematite/goetite as major phases (Petersen,1965).

2.2. Mining and deposition processes

The waste rocks deposited at the Excelsior wasterock dump had an average size of 10 cm and are up to2.5 m in diameter; the fraction of grains smaller than2 mm were estimated as about 10 vol.%. The waste rockimpoundment consisted of three terraces constructed bythe downpour of fractured rocks at the slopes of thedifferent terraces with an uncontrolled mixing ofdifferent rock types. As result, cross-bedding of thewaste rocks and a gradation of rocks with big boulders atthe base and fine grain material at the top were observed(Fig. 2A). The deposition commenced in the northernpart with waste rocks from the Cu-ore and advancedwith the later deposition of waste rocks from the Zn–Pbore successively to the south and from the 1st terrace tothe 3rd terrace, respectively. The northern part and lowerterraces were therefore exposed to alteration for a longerperiod than the southern parts and the top terrace. The

Fig. 2. Schematic cross-section of the Excelsior waste rock dump: (A) structure; (B) main water pathways.

100 J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

surfaces of the terraces were mainly floored with a 10 to30 cm thick cover of rugged pyrite-rich waste rocks toprevent heavy mine equipment from getting stuck. Themaximum height of the dump is 55 m; the slopeinclination is mainly between 33° and 36°.

2.3. Climate and hydrology

Based on data from the local meteorological stations(Cerro de Pasco and Upamayo), the climate at Excelsiorwaste rock dump is characterised by an average precip-itation of 1025 mm/year (mainly in the summer period),an average evaporation of 988 mm/year and an averageannual temperature of 4.2 °C.

Fig. 3. Outcrop at the south-western base of the Excelsior waste rock dump (ssample) and oxidizing primary rocks (pyrite, volcanic rocks; AR sample).suggested the control of solution flow by the cross-bedding in the waste roc

The hydrological system of the Excelsior waste rockdump received water mainly from rain. Two water path-ways were observed in the waste dump (Fig. 2B): (1)Surface run-off at the waste dump slopes after rainevents; and (2) water infiltration into the dump. In thewaste rock dump, the main direction of water infiltrationwas suggested to be vertical. At different positions of thewaste rock dump, base material was removed whichformed overhangs. It was observed that the precipitationof efflorescent salts at these outcrops was favoured atdiscrete cross-bedding layers (Fig. 3). This observationsuggested a certain influence on the water infiltration bythe cross-bedding. A broad spring zone existed at thesouth-western limit of the Excelsior waste rock dump

ampling point CPE-30, Fig. 1B) with efflorescent salts (melanterite; ESThe preferential formation of ES stalactites at grain-size boundariesk dump.

101J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

(water sampling points CPE-W1-3, Fig. 1B). Thesewater outcrops were explained by the difference ofhydraulic permeability between the coarse waste rockdump material (high water permeability, not watersaturated) and the underlying sandy/silty tailings mate-rial (low water permeability, water saturated). Similar toan aquitard, the underlying tailings impoundmentseemed to retard infiltration through its fine material.This led to a horizontal water transport with spring-likeoutcrops at the southern base of the dump. The AMDfrom the springs was channelled and conducted to alagoon on the southern part of the Quiulacocha tailingsimpoundment.

3. Methods

3.1. Sampling and field methods

The field campaign was carried out during the dryseason 2003 (May to July). Water and solid samplingwas undertaken focussing on the element output of thewaste rock dump. A total of 68 mineral and rocksamples at 53 sampling points and 4 water samples at4 sampling points were taken from the surface of theExcelsior waste rock dump (Fig. 1B). The mainly ver-tical water pathway suggested an element transport fromthe top to the base. Therefore, solid samples were clas-sified as: (1) altered rock samples from the surface of thetop terraces (altered primary minerals with a loweramount of mainly in situ-formed secondary mineral),termed “AR top” samples in this study; (2) altered rocksamples from the base of the Excelsior waste rockdump, termed “AR base” samples (Fig. 3); (3) precip-itated secondary mineral samples (efflorescent salts)from the surface of the top terraces, termed “ES top”samples and (4) precipitated secondary mineral samples(efflorescent salts) from the base, termed “ES base”samples (Fig. 3, centre).

3.1.1. Water samplingAt water sampling points, temperature, pH (WTW®

pH meter), Eh (WTW® Eh meter) and electrical conduc-tivity were measured immediately on-site. The sampleswere filtered through a 0.2 μm cellulose membrane filterand split into two sub-samples. One was acidified topHb2 with HNO3 for cation analysis. The second un-treated sample was stored b4 °C for anion analysis.

3.1.2. Solid samplingSolid sampling points were described in the field by

rock type, alteration degree and, if possible, a descriptionand identification of mineral content. The paste pH of the

sample was taken according to MEND (1991) with aWTW® pH meter. Samples were air-dried (b35 °C) andhomogenised by crushing/grounding with an agate mill.Humid ES samples were sealed in PET-sampling bottlesdirectly on-site to prevent the alteration of minerals.

3.2. Geochemical methods

3.2.1. X-ray fluorescence spectroscopyFor trace element analysis of solids, aliquots (4 g) of

78 samples were dried at 105 °C, ground with an agateball mill (Retsch®), mixed with Hoechst® wax, pressedto pellets and analysed using energy-dispersive X-rayfluorescence (EDXRF). The measurements were per-formed with the XRF-spectrometer XLAB 2000®(SPECTRO® Instruments) running the software pack-age X-LAB Pro 2.2. The calibration of the spectrometerwas based on certified and measured data of about50 reference materials including iron-rich soils, riverand lake sediments and geological materials. In cases inwhich the analysed samples were strongly enriched withheavy metals (e.g. Fe, Cu, Zn and Pb), the samplematerial was diluted with SiO2 powder (Riedel-de-Haen®) to adjust the metal concentration to within theworking ranges of the available calibration.

For major element analysis, a further part of thesample powder was diluted with (Li2B4O7) (1 g sample+7 g (Li2B4O7)) to prepare glass discs produced by fusion.The measurements were performed using the wave-length dispersive X-ray fluorescence (WDXRF) spec-trometer S4 Pioneer® (Bruker-axs®), equipped with a4 kW Rhodium X-ray tube. The spectrometer wasoperated in vacuum conditions and the 34mm collimatormask was utilised in conjunction with the analysingcrystals OVO55, Ge, LIF100 and LIF110. The calibra-tions of the WDXRF spectrometer for the majorelements were adjusted by certified reference materials,CANMET-LKSD1-LKSD4 (lake sediments), CAN-MET-STSD1-STSD4 and GBW07309-11 (stream sedi-ments), NIST-SRM2689 and NIST-SRM2691 (coal flyashes) using the software package SPECTRAplus®.

3.2.2. Leach tests and water analysisThe German standard method DIN38414 (1984) was

used to analyse the solubility of waste rock minerals inwater. The chemical properties of the leachates and thewater-soluble fraction from solid samples from theupper terraces and the base of the Excelsior waste rockdump were compared to analyse enrichment and fixa-tion of elements.

From the base of Excelsior waste rock dump, 9 ARsamples with similar distances between the sampling

Table 1AMD-relevant characteristics of rock types at the Excelsior waste rockdump

Rock type Quartz–pyriterocks

Volcanicrocks

Carbonaticrocks

Volume share 70–80% 15–20% ca. 5%AMD-neutralprimary minerals

Quartz,hematite

Quartz,muscovite

Quartz

Acid-producingprimary minerals

Pyrite Pyrite Not present

Alteration degree(estimated in field)

Low (b10%) Very high(N90%)

High(N50%)

Acid-consumingprimary minerals

Not present Albite Dolomite

Liberated elements(major fraction)

Fe, Zn, Cu, Mn, S Fe, S Ca, Mg

Liberated elements(minor fraction)

Pb, As, Cd K, Ca

Secondary minerals Gypsum,jarosite,plumbojarosite,hydroniumjarosite, metalsulfates, Fe(III)hydroxides

Hydroniumjarosite,jarosite,kaolinite,Fe sulfates,Fe(III)hydroxides

Gypsum, Mgsulfates

102 J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

points were chosen. From the top of the Excelsior wasterock dump, 10 AR samples were chosen. From eachsample, pre-dried (b35 °C) and milled aliquots of 10 gwere mixed with 100 ml of deionised water (pH 7, Eh of0.45 V, not buffered) and shaken for 24 h at 100 rpm.The leachates were filtered through 0.2 μm cellulose–nitrate filters. The conductivity and pH of the leachateswere measured immediately. The leached solid sampleswere dried (b35 °C) and weighed to calculate the per-centage of soluble phases.

Major elements of leachates and water samples fromthe Excelsior waste rock dump were analysed using ionchromatography (Dionex® DX-120). Trace elementswere analysed using ICP–MS (Hewlett-Packard® 4500,Institute F.-A. Forel, University of Geneva, Switzerland)and ICP–AES (Spectro® Spectroflame® P/M, UFZUmweltforschungszentrum, Leipzig, Germany). Leachtests for 9 ES samples (5 samples from the upper terrace,4 samples from the base of the Excelsior waste rockdump) were performed to confirm the high water-solu-bility of pure efflorescent salt samples (water solublefraction between 89.2 and 94.4 wt.%). Due to their highwater-solubility, XRF data of ES samples were usedinstead of leach test data for the above mentioned anal-ysis of element enrichment and fixation.

3.3. Mineralogical methods

3.3.1. Optical microscopyPolished sections and polished thin sections of selec-

ted bulk samples were prepared using epoxy resin toembed the loose-packed samples and analysed usingtransmitted light and reflected light microscopy.

3.3.2. X-ray diffraction analysisAll samples were analysed as bulk samples with the

aid of X-ray diffraction (XRD), using a SiemensD5000® diffractometer equipped with monochromatedCuKα (λ=1.54056 Å) X-radiation. Conserved humidES samples were grounded with an agate mortar andanalysed immediately to prevent the alteration of min-erals, e.g. by the loss of crystal water. Scan settings were0.05° 2θ step size and 2 s counting time per step. Peakinterpretation was performed using the program suiteDIFFRAC AT®.

3.3.3. Differential X-ray diffraction analysesThe poorly crystalline Fe(III) hydroxide schwertman-

nite was detected in a two-step sequential extractionprocedure and differential X-ray diffraction (DXRD) asdescribed by Dold (2003). The diffractometer settingswere 0.05° 2θ step size and 20 s counting time per step.

The milled samples were treated in the first step withdistilled water for 1 h and in the second step in darknesswith 0.2 M ammonium oxalate at pH 3, for 15 min (afterDold, 2003). Scans were done before and after eachtreatment step. Scans from the ammonium oxalate-treated samples were intensity-corrected and then sub-tracted from the scans of the water-treated samples. Theleachates from the sequential extraction were filtered(0.2 μm) and analysed for element concentrations asdescribed in Leach tests and water analysis (3.2.2).

4. Results

4.1. Mineralogy

At the Excelsior waste rock dump, three differenttypes of waste rocks were found (Table 1): (1) sulfide-rich rocks from the quartz–pyrite ore body and its Fe(III)-oxide-rich oxidation products near the surface, (2)sericitised monzonites/tuff and (3) dolomitic rocks fromthe host rock formation. These rocks were characterisedby their different contents of acid-producing/acid-con-suming minerals and their different degree of alteration.

4.1.1. Quartz–pyrite rocksThe main waste rock at the Excelsior waste rock

dump belonged to the quartz–pyrite body. Quartz andpyrite as major phases dominated the mineral

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assemblage in AR samples. The ore minerals sphalerite,tennantite, cerussite, galena and coronadite were iden-tified as minor and trace phases and anglesite as aprimary and possibly secondary trace mineral in theExcelsior waste rock dump. A different alterationintensity of small pyrite grains (up to 2 mm) wasobserved in thin sections. Intact, idiomorphic pyritecrystals were found in close proximity to strongly alteredpyrite. Pyrite grains greater than 2 mm showed alterationrims of varying thickness (up to 1.5 mm) consisting ofmicrocrystalline Fe(III) hydroxides. Massive pyriterocks in the field showed superficial alteration withalteration rims of less than 1 mm. Due to the observedoxidation grades, a long-term AMD potential by thepyrite content must be assumed. A minor fraction ofwaste rocks from the quartz–pyrite body belonged tothe oxidized top of the quartz–pyrite body in situ atthe deposit. Rocks from this gossan contain quartz andhematite/goetite as major phases.

4.1.2. Volcanic rocksVolcanic rocks (quartz monzonite/tuff) in contact

with the ore body could originally contain up to 15 vol.%pyrite in disseminated distribution due to the sericiticalteration during the sulfide deposition (personal com-munication, Volcan geologists). This contributed a fur-ther important quantity of pyrite to the system inaddition to the pyrite from the ore body. Pyrite crystalsin volcanic rocks are small in size (b1 mm), the rockmatrix has a high porosity. At the Excelsior wasterock dump volcanic rocks were observed to be alreadystrongly altered. Boulders of volcanic rocks of up to1 m in diameter had already completely decomposedinto white, loose material with grain sizes of severalmm and a strongly acidic paste pH (1.31–2.58, n=6).Microscopic and XRD studies showed that the mainprimary mineral fractions of these rocks were quartz(∼20 vol.%) and altered relicts of albite (NaAlSi3O8)(∼5 vol.%). Agglomerates of secondary minerals (mi-crocrystalline Fe(III) hydroxides, muscovite, kaoliniteand gypsum) made up the major fraction (N70 vol.%).From the original pyrite content, only relicts remained,contributing less than 1 vol.% of the total volcanicrock mineral assemblage. These observations pointed tothe interpretation that due to the small crystal size andthe high rock porosity, pyrite in the volcanic rocksoxidized much faster than massive pyrite from thequartz–pyrite body. This resulted locally in stronglyacidic environment where volcanic rocks were depos-ited. AMD from these areas was suggested to acceleratethe further alteration of other waste rocks in the wasterock dump.

4.1.3. Carbonatic host rocksLess than 5 wt.% of the waste rocks belonged to the

host rock formation and contained quartz, dolomite andsiderite as major mineral phases. While dolomite canneutralize protons, siderite may additionally produceprotons through the hydrolysis of liberated Fe(III),which can oxidize pyrite. This fact reduces the neutral-ization capacity of this type of carbonatic waste rocks.

At the surface of the Excelsior waste rock dump,dolomite was still present at some sites. This suggestedthat rain infiltrated rapidly into the Excelsior waste rockdump and that acid solution could not migratehorizontally on the surface. Inside the Excelsior wasterock dump, carbonates were assumed to be mostlydissolved due to acid pore water. The generally lowproportion of carbonatic rocks on the waste rock as-semblage and the mix of acid-consuming and acid-producing carbonates in this rock type indicated that theExcelsior waste rock dump has an extremely low acidneutralization capacity. This led to an acidic environ-ment with an average paste pH of 2.8. The pH dependedstrongly on the presence of dolomite; values variedbetween pH 0.8 and 4.7 for waste rock areas withoutdolomite (48 sampling points) and between pH 5.1 and7.3 for areas with dolomite (6 sampling points).

4.1.4. Low soluble secondary mineralsSolid solutions of secondary hydronium jarosite

((H3O)Fe3(SO4)2(OH)6) and K-jarosite (KFe3(SO4)2(OH)6) were observed in most AR samples. The X-raypatterns of solid solutions of alkali jarosites as pointedout by Dutrizac and Jambor (2000) are very similar tothe X-ray pattern of hydronium jarosite ((H3O)Fe(SO4)2(OH)6). However, the small amounts of feldspars andother alkali-bearing minerals in the Excelsior waste rockdump resulted in a low availability, especially of Na, andwere likely to favour the formation of hydroniumjarosite instead of the thermodynamically morefavoured solid solution of alkali jarosites. The XRDidentification of hydronium jarosite also in samples withNa concentrations below detection limit supported thisinterpretation. In samples rich in Pb, the XRD inter-pretations suggested the presence of solid solutionsbetween jarosite, hydronium jarosite and plumbojarosite(PbFe6(SO4)4(OH)12), possibly associated with beudan-tite (PbFe3(AsO4)(SO4)(OH)6).

Goethite as a minor phase was found in samples withthe typical altered rock–mineral association pyrite–(hydronium/plumbo–) jarosite–gypsum.

Some AR samples were collected at superficial waterpathways. These samples were characterised by smallgrain sizes (b1 mm), quartz as the major primary

Table 3XRF data from AR and ES samples from the upper terraces (top) andfrom the base (base) of the Excelsior waste rock dump: averageconcentrations± standard deviation of selected major and traceelements

Element AR (top)(n=17)

AR (base)(n=32)

ES (top)(n=5)

ES (base)(n=12)

Na2O (wt.%) 0.15±0.15 0.30±0.32 0.49±0.28 0.62±0.46K2O (wt.%) 1.41±0.78 1.40±1.92 0.18±0.40 0.27±0.38CaO (wt.%) 1.94±4.23 4.86±4.62 0.35±0.41 3.69±5.98Al2O3 (wt.%) 6.23±2.60 5.93±4.44 1.09±1.53 0.92±0.84SiO2 (wt.%) 28.16±10.45 32.51±15.00 2.81±4.53 3.77±4.23MgO (wt.%) 0.67±0.34 1.53±1.96 2.02±1.33 2.57±1.98Mn (wt.%) 0.23±0.23 0.65±0.74 1.14±0.59 2.73±3.05Fe2O3 (wt.%) 26.95±9.47 22.38±12.09 21.48±5.92 18.79±12.44S (wt.%) 7.44±2.81 6.83±4.48 11.99±1.25 11.38±2.45Cu (wt.%) 0.11±0.21 0.08±0.20 1.27±2.36 0.81±2.79Zn (wt.%) 0.87±0.57 1.19±1.13 2.94±1.31 3.40±2.90Pb (wt.%) 1.10±0.82 1.14±2.28 0.10±0.22 0.13±0.18Cd (mg/kg) 23±15 48±83 77±62 80±88As (mg/kg) 2030±1433 1205±1052 896±561 2043±3694

Table 2Major secondary Fe-, Mg-, Zn- and Pb-sulfates of the Excelsior wasterock dump

Fe-sulfates Mg-sulfates Zn-sulfates Pb-sulfates

Melanterite(FeSO4·7H2O)

Epsomite(MgSO4·7H2O)

Goslarite(ZnSO4·7H2O)

Anglesite(PbSO4)

Siderolite(FeSO4·5H2O)

Starkeyite(MgSO4·4H2O)

Gunningite(ZnSO4·2H2O)

Fibroferrit (Fe(SO4)(OH)·5H2O)

Kieserite(MgSO4·H2O)

Changoite(Na2Zn(SO4)2·4H2O)

Rozenite(FeSO4·4H2O)

Szomolnokite(FeSO4·H2O)

Romerite (Fe3(SO4)4·14H2O)

Copiapite (FeFe4(SO4)6(OH)2·20H2O)

104 J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

mineral fraction and a high content of secondary Fe(III)hydroxides (up to 60 vol.%). Fe(III) hydroxidesappeared as light yellow to dark brownish agglomerates.A clear identification for these samples by XRD was notpossible. One sample from the top of the waste rockdump (CPE-61(AR), Fig. 1B) and one from the base ofthe waste rock dump (CPE-32(AR)), both with a highcontent of ‘poorly crystalline Fe(III) hydroxides’, wereselected for DXRD analyses. In both samples, schwert-mannite (Fe16O16(OH)12(SO4)2) was identified as mainsecondary Fe-mineral.

4.1.5. Water-soluble secondary mineralsGypsum, Mg-sulfates and heavy metal sulfates were

the major phases in ES samples. This mineral grouprepresented the water-soluble efflorescent salts precipi-tated from oversaturated pore solutions. The mostfrequent metal sulfates (Table 2) were Fe-sulfates(melanterite, rozenite, less frequently romerite, sidero-lite, copiapite, szomolnokite and fibroferrite) followedby Mg-sulfates (epsomite, starkeyite and kieserite). Inaddition to these minerals, Zn-sulfates (goslarite,gunningite, changoite) and Mn-sulfates (mallardite,MnSO4·7H2O) could be found at some sites.

Cu concentrations reached up to 10 wt.% in ESsamples. An identification of Cu-minerals by means ofXRD or optical microscopy was not possible. Thissuggested that Cu was not precipitated as a Cu-mineralbut substituted metals in other sulfates (mainly melan-terite) as also described from other sites (Jambor et al.,2000).

Two different types of gypsum appeared: (1) crystalsof between 0.05 and 0.4 mm with irregular shapes and

marginal intergrowth with microcrystalline Fe(III)hydroxides, and (2) very small idiomorphic crystals atthe boundary to submicroscopical size, possibly result-ing from fresh precipitation from pore water and/or asresult of precipitation during the drying process aftersampling.

4.2. Geochemical results

4.2.1. Solid sample analyses

4.2.1.1. Major elements. Si, Fe and S from the quartz–pyrite body dominated the element composition ofthe AR samples average concentrations (Table 3) withaverage concentrations of 28.2±10.5 wt.% SiO2 in ARtop samples/32.5±15.0 wt.% SiO2 in AR base sam-ples, 27.0±9.5 wt.% Fe2O3 in AR top samples/22.4±12.1 wt.% Fe2O3 in AR base samples, 7.44±2.81 wt.% Sin AR top samples/6.83±4.48 wt.% S in AR base sam-ples. The volcanic rocks also contribute to these elements.Fe and S are the major elements in the ES samples(average concentrations: 21.5±5.9 wt.% Fe2O3 in topsamples/18.8±12.4 wt.% Fe2O3 in base samples, 12.0±1.3 wt.% S in top samples/11.4±2.6 wt.% S in basesamples). Fe and S concentrations were slightly higherin top samples than in base samples, while Si concentra-tions showed an opposite trend. These trends resultedprobably from the flooring of the top terraces with pyrite-rich rocks. Aluminium in AR samples had its originmainly in muscovite from altered volcanic rocks. A sig-nificant Al concentration in ES samples is only found inimpure samples with high primary mineral content. This

Table 4Heavy metal and arsenic concentrations (ICP–MS) of sequentialextractions for the Fe(III) hydroxides

Typesample

As(mg/L)

Cd(mg/L)

Cu(mg/L)

Fe(mg/L)

Mn(mg/L)

Pb(mg/L)

Zn(mg/L)

Water leachCPE-32

(AR)0.26 0.02 0.31 84.5 45.72 0.003 32.52

CPE-61(AR)

b0.06 0.02 0.02 2.6 11.21 0.009 8.20

NH4-ox. AC leachCPE-32

(AR)73.74 b0.01 0.12 803 0.26 0.044 0.73

CPE-61(AR)

b0.06 b0.01 0.02 922 0.22 0.005 0.45

Sampling locations: see in Fig. 1B.

105J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

suggested that high Al contents were mainly related toprimary silicates. The concentrations for Na and K inAR samples were low due to the low content of primaryNa and K-minerals in the primary mineral assemblage(average concentrations: 0.15±0.15 wt.% Na2O for ARtop samples/0.30±0.32 wt.% Na2O for AR basesamples; 1.41±0.78 wt.% K2O for AR top samples/1.40±1.42 wt.% K2O for AR base samples). While Nawas enriched in ES samples (0.49±0.28 wt.% Na2O forES top samples/0.62±0.46 wt.% Na2O for ES basesamples), K showed a contrary trend (0.18±0.40 wt.%K2O for ES top samples/0.27±0.38 wt.% K2O forES base samples). This suggested that K was strongerfixed in the system, probably due to the formation ofK-jarosite.

Ca and Mg from the dolomitic ore host rock hadlower average concentrations in AR samples from theupper terraces (1.94±4.23 wt.% CaO in top samples/4.86±4.62 wt.% CaO in base samples, 0.67±0.34 wt.%MgO in top samples/1.53±1.96 wt.% in base samples).The reason for this observation was also the flooring ofthe top terraces with pyrite-rich, Ca-/Mg-mineral-poorrocks. Ca is not enriched in ES samples while Mgshowed enrichment (average concentrations: 0.35±0.41 wt.% CaO in top samples/3.69±5.98 wt.% CaOin base samples, 2.02±1.33 wt.% MgO in top samples/2.57±1.98 wt.% MgO in base samples).

4.2.2. Heavy metals and arsenic

4.2.2.1. Copper, zinc and cadmium. AR samplescontained a higher average Cu concentration in top sam-ples (0.11±0.21 wt.% Cu) than in base samples (0.08±0.20 wt.% Cu) due to enargite–tennantite veins in thequartz–pyrite body (Einaudi, 1977) and the flooring of theterraces with pyrite-rich rocks. Cu is enriched in ESsamples (average concentrations of 1.27±2.36 wt.% intop samples/0.81±2.79 wt.% in base samples).

The primary mineral assemblage of the waste dumpdisplayed a high average concentration of Zn (0.87±0.57 wt.% for AR top samples/1.19±1.13 wt.% for ARbase samples), due to the main Zn minerals sphalerite(ZnS) and marmatite ((Zn,Fe)S) from the ore body. In ESsamples, Zn was enriched in the form of efflorescent salts(averages: 2.94±1.31 wt.% Zn in top samples/3.40±2.90 wt.% Zn in base samples). Especially at overhangswhich formed a protection against rain at the base ofExcelsior waste rock dump, maximum Zn concentrations(up to 8.5 wt.% Zn, sample CPE-31(ES)) were found.

Cadmium occurred as a substitute in sulfide miner-als of the ore body (e.g. in sample CPE-2(AR) with∼15 wt.% sphalerite, 295 mg/kg Cd; and CPE-12(AR)

with∼10 wt.% tennantite ((Cu,Fe)12As4S13), 381mg/kgCd). The average Cd concentrations in the ES samplesshowed enrichment (77±62 mg/kg Cd for top samples/80±88 mg/kg Cd for base samples) in comparison tothe AR samples (averages: 48±83 mg/kg Cd in topsamples/23±15 mg/kg Cd in base samples). Cadmiumwas suggested to be incorporated into or coprecipitatedwith metal sulfates (e.g. in the sample CPE-30b(ES)with rozenite, starkeyite and 300 mg/kg Cd; sampleCPE-52(ES) with szomolnokite and 192 mg/kg Cd), asdescribed from other sites (Alpers et al., 1994).

4.2.2.2. Lead. The sulfide ore mineral galena PbS andits alteration product anglesite (PbSO4) (as primarymineral and also as freshly formed secondary mineral)represented the lead content in the Excelsior waste rockdump (average of 1.10±0.82 wt.% Pb for AR topsamples/1.14±2.28 wt.% Pb for AR base samples). Inthe zones where secondary efflorescent salts particularlyprecipitated (mainly at the base of the waste rock dump),the average Pb concentration was nearly one tenth ofthat in the AR samples (average of 0.10±0.22 wt.% Pbfor ES top samples/0.13±0.18 wt.% Pb for ES basesamples). The low concentration of Pb in the ESsamples pointed to the low solubility of the mineralanglesite and the low mobility of Pb in the sulfate-richExcelsior waste rock dump environment.

4.2.3. ArsenicSources for arsenic at the Excelsior waste rock dump

were mainly the arsenic-rich pyrite, arsenopyrite(FeAsS) and enargite (Cu3AsS4)/luzonite (Cu3AsS4)from the ore body, but also realgar (As4S4) and orpiment(As2S3), which were part of the supergene mineralassemblage of the Cerro de Pasco mine (Einaudi, 1977).

106 J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

AR samples displayed higher average concentrations intop samples (2030±1433 mg/kg As) than in base sam-ples (1205±1052 mg/kg As) due to the flooring of topterraces with pyrite-rich rocks. Sequential extraction ofan Fe(III) hydroxide-rich AR sample from the base of theExcelsior waste rock dump (CPE-32(AR)) showed ahigh concentration for As (up to 73.7 mg/L) in the Fe(III)hydroxide leach step (Table 4). This confirmed theknown sorption capacity of Fe(III) hydroxides for As(Dold and Fontboté, 2001; Fukushi et al., 2003; Lebrunet al., 2004). In a second AR sample (CPE-61(AR)) fromthe top terrace of the Excelsior waste rock dump,schwertmannite was also identified. In contrast to thefirst mentioned sample, this sample contained a lowconcentration of As in the Fe(III) hydroxide leach step

Table 5Water analyses of leachates from AR sample and AMD water samples: pH, E(SF)/total dissolved solids (TDS) of selected elements; average and standard

Type/Sample

pH Eh

(mV)EC(mS/cm)

As(mg/L)

Ca(mg/L)

Cd(mg/L)

Cu(mg/L)

Fe(m

LeachatesAR (top)CPE-44 2.43 n.m. 4.0 0.92 105 0.47 0.50 4CPE-48 1.20 n.m. 5.4 3.30 60 0.26 0.00 6CPE-49 2.56 n.m. 4.6 0.74 539 0.57 4.80 5CPE-50 2.32 n.m. 5.2 14.81 66 0.38 10.92 12CPE-54 2.33 n.m. 3.5 0.02 83 0.25 0.00 1CPE-55 2.17 n.m. 5.0 4.63 259 0.47 22.45 8CPE-56 2.74 n.m. 4.0 0.08 501 0.50 0.00CPE-57 2.32 n.m. 6.3 11.99 219 0.31 0.00 19CPE-59 2.33 n.m. 14.5 33.27 360 4.49 35.76 74CPE-62 2.32 n.m. 7.0 7.07 164 0.89 13.86 17Average 2.27 6.0 7.68 236 0.86 8.83 15S.D. 0.41 3.2 10.35 178 1.29 12.19 21

AR (base)CPE-2 4.02 n.m. 3.6 0.08 567 14.81 15.55CPE-7 5.60 n.m. 2.4 bdl 635 0.46 bdlCPE-11 3.79 n.m. 8.7 bdl 517 1.76 1.51 2CPE-15 2.82 n.m. 5.2 0.10 566 0.92 4.67 2CPE-18 5.47 n.m. 3.2 bdl 564 0.34 bdlCPE-24 2.88 n.m. 9.7 bdl 504 0.58 3.53 1CPE-28 2.91 n.m. 3.0 0.14 523 1.26 16.67 14CPE-30 2.77 n.m. 8.3 0.12 608 0.04 0.71CPE-35 3.20 n.m. 5.1 0.26 382 1.40 11.78 3Average 3.72 5.5 0.08 541 2.40 6.05 2S.D. 1.12 2.8 0.09 73 4.69 6.77 4

All leachatesAverage 2.96 5.7 4.08 541 1.59 7.51 9S.D. 1.09 8.3 8.29 73 3.35 9.84 16

AMD waterCPE-W1 4.86 359 21.2 2.81 411 0.92 1.08 36CPE-W2 4.94 336 19.0 2.09 559 0.57 1.13 31CPE-W3 5.10 319 23.3 1.98 694 3.58 1.28 56CPE-W4 2.78 684 26.0 7.99 614 6.60 161.1 16

Abbreviations: top: samples from the surface of the upper terraces of the Exwaste rock dump; S.D.: standard deviation; n.m.: not measured; bdl: below

(b0.06 mg/L). The low As concentration most probablyresulted from As-poor sulfides in this sampled area.

ES samples displayed a high average concentration ofarsenic and enrichment at the base of the Excelsior wasterock dump (averages: 896±561 mg/kg As for ES topsamples/2043±3694 mg/kg As for ES base samples),suggesting that mobile arsenic can be fixed when metalsulfates precipitate, as observed at otherwaste rock dumps(Gierè et al., 2003).

4.2.4. Water leach testsAfter 24 h of stirring, all leachates possessed acidic pH

values varying between 1.2 and 5.6 for the AR samples(Table 5). The water-soluble fraction of the leached ARsamples represented between 2.6 and 39.6 wt.% and led in

h, electrical conductivity (EC), pH, ICP–MS data and soluble fractiondeviation of the different sample categories

g/L)K(mg/L)

Mg(mg/L)

Mn(mg/L)

Na(mg/L)

Pb(mg/L)

S(mg/L)

Zn(mg/L)

SF(wt.%)

52 2.64 70 75.6 7.24 0.06 917 167 6.377 4.86 69 91.1 6.90 1.85 1110 103 9.841 4.49 91 90.4 8.15 0.03 1620 152 5.925 4.14 88 145.2 6.98 0.02 1650 127 21.357 6.60 74 71.8 8.61 0.05 540 86 5.972 7.57 6 69.1 6.28 0.16 1250 130 8.449 7.64 104 186.9 7.80 0.11 720 146 7.468 7.32 7 29.3 11.58 0.28 1890 116 14.100 7.38 145 85.3 5.43 0.08 10660 1389 39.620 6.76 116 226.1 8.36 0.03 2468 290 13.206 5.94 77 107.1 7.73 0.27 2283 271 13.264 1.76 44 60.3 1.67 0.56 3000 397 10.5

76 0.4 95 n.m. 1.2 2.27 1024 447 27.40.8 2.1 29 n.m. 0.8 0.35 535 107 2.651 bdl 657 n.m. 0.9 0.07 2110 708 12.763 bdl 52 n.m. bdl 0.14 1085 322 5.40.1 3.6 263 n.m. 3.1 bdl 820 68 4.641 bdl 987 n.m. 0.5 0.06 2074 491 1352 bdl 178 n.m. bdl 0.22 1928 611 3.344.9 13.0 31 n.m. 0.8 0.06 634 18 11.850 bdl 194 n.m. bdl bdl 1139 521 6.486 2.1 276 0.8 0.35 1261 366 9.754 4.3 330 1.0 0.73 616 250 7.8

28 2.1 276 107 0.8 0.31 1261 315 11.581 4.3 330 70 1.0 0.63 617 330 9.2

TDS85 19.2 3158 2258 73.8 1.17 8100 1845 4.752 25.0 3549 1752 121 0.55 6820 1218 5.640 27.7 4716 2467 8.2 1.28 9730 2302 5.232 3.1 7792 2625 bdl 0.14 9800 3000 7.5

celsior waste rock dump; base: samples from the base of the Excelsiordetection limit. Sampling locations: see Fig. 1B.

107J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

high electrical conductivity in the leachates (between 2.4and 14.5 mS/cm). All leachates showed high concentra-tions of heavy metals and also of Ca (average: 236±178 mg/L for AR top samples/541±73 mg/L for AR basesamples), Mg (average: 77±44 mg/L for AR top samples/276±330 mg/L for AR base samples), As (average: 7.7±10.4 mg/L for AR top samples/0.08±0.09 mg/L for ARbase samples) and S (average: 2283±3000 mg/L for ARtop samples/1261±616 mg/L for AR base samples). Thehigh concentrations of Ca and Mg represented the contentof water-soluble secondary gypsum and Mg-sulfates (e.g.starkeyite, MgSO4·4H2O).

The dissolution of Fe sulfates generated high Feconcentrations (average: 1506±2164 mg/L for AR topsamples/286±454 mg/L for AR base samples) in theleachates. The liberated Fe(II) can be oxidized to formFe(III) which is an effective sulfide oxidizer (Singer andStumm, 1970). This process is supposed to enhance thesulfide oxidation at the Excelsior waste rock dump.

The average concentrations of Cu and Zn in leachateswere 8.8±12.2 mg/L for AR top samples/6.1±6.8 mg/Lfor AR base samples and 271±397 mg/L for AR topsamples/366±250 mg/L for AR base samples, respec-tively. The high concentrations displayed clearly thatZn- and Cu-containing secondary sulfates dissolverapidly in contact with water and liberate these metals.

Pb showed low concentrations in the leachates(average 0.27±0.56 mg/L for AR top samples/0.35±0.73 mg/L for AR base samples) due to the alreadydiscussed low solubility of anglesite.

The concentrations of arsenic in the leachates variedgreatly between samples from different locations andshowed lower concentration in base sample leachates(average: 7.7±10.4 mg/L for AR top samples/0.08±0.09 mg/L for AR base samples). Fixation of As on Fe(III)hydroxides lowered possibly the concentration of As inpore solutions at the base of the waste dump. At the toplocal low As concentrations seemed to result from lowconcentrations of As in the surrounding primary minerals,as shown by the Fe(III) hydroxide leach test. Samples withhigh arsenic concentrations in the leachates (up to 33.3 mg/L, sample CPE-59(AR)) had a very low paste pH (1.1–1.7)in the field, suggesting a high pyrite oxidation in thesezones. This oxidation process liberated As which was fixedin the sulfides and resulted in the observed highconcentration of As in the leachates.

The leachates had an average Cd concentration of 0.9±1.3 mg/L for AR top samples/2.4±4.7 mg/L for AR basesamples. This observation confirmed the suggestion thatCd was enriched at the base of the Excelsior waste rockdump in water-soluble secondary minerals, as discussedwith the XRF data from solid samples.

Element concentrations varied widely, demonstratingthe different content and composition of water-solublesecondary minerals in the samples.

4.2.4.1. AMD from the Excelsior waste rock dump.AMD samples originated from the broad spring zone at thesouth-western base of the Excelsior waste rock dump(CPE-W1 to 3, Fig. 1B) and from a small water outcrop atthe north-western base (CPE-W4). CPE-W1 to 3 werelinked together as a broad spring zone at the most recent,southern part of the Excelsior waste rock dump base. AMDfrom this zone was characterized by an acid pH (range:4.86–5.10), high electrical conductivity (range: 19.0–23.3 mS/cm), a slightly oxidizing environment (range Eh:319–359 mV) and a high charge of metals (Table 5). Thetotal flow rate of this spring zone is estimated to be around10 L/s.

CPE-W4 is an isolated, small water outcrop at thenorth-western, oldest area of the Excelsior waste rockdump. The more acid pH (2.78), higher Eh (684 mV)and higher electrical conductivity (26.0 mS/cm) incomparison with the broad spring zone in the SWseemed to result from the longer exposition of the wasterocks in this area and the therefore more advancedoxidation of sulfides. The high concentration of Cu inthe water sample CPE-W4 (161.1 mg/L) possiblyresulted from the source rocks in this area: the oldestwaste rocks from the historically first mined Cu-rich andsurface-near ore of the Cerro de Pasco deposit.

The trends of mineral solubility and element mobilityfrom the leach tests were confirmed by the heavy metalconcentrations in the AMD waters: Cu 1.1–1.3 mg/L atCPE-W1 to 3, 161.1 mg/L at W4, Zn 1218–2302 mg/Lat W1 to 3, 3000 mg/L at W4, Fe 3685–5640 mg/L atW1 to 3, 1623 mg/L at W4, Cd 0.9–3.6 mg/L at W1 to3, 6.6 mg/L at W4. The metal concentrations werehigher in comparison with the average concentration ofthe AR leaching tests. This observation was explainedby the high interaction of pore water with altered wasterocks during the slow infiltration.

The AMD effluents also contained high concentra-tions of As (CPE-W1 to 3: 2.0–2.1 mg/L As, CPE-W4:8.0 mg/L As). Arsenic appeared to be not completelyretained by Fe(III) hydroxides during the flow path inthe Excelsior waste rock dump, probably due to its highconcentration.

Manganese showed high concentrations in leachates(data only available for AR top sample leachates: aver-age 107±60 mg/L Mn). The sequential extraction dataof the water extraction step (Table 4, 11.2–45.7 mg/LMn) and the water data (Table 5, 1752–2625 mg/L Mn)also displayed high concentrations of Mn. Source of

108 J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

Mn seemed to be the dissolution of Mn-sulfates (e.g.mallardite, Mn(SO4)·7H2O).

5. Discussion

The comparison of the XRF, XRD, leach test andAMD data was used to identify source rocks of AMDand trace back the transport of acids and heavy metalsand their retention at the Excelsior waste rock dump inform of freshly precipitated secondary minerals at theExcelsior waste rock dump.

The heavy metals Zn, Cd, Mn as well as the alkalineearth metals Ca and Mg showed higher concentrationsin AR samples from the base of the Excelsior wasterock dump than from its top terraces. The leach testsand XRF data from ES samples confirmed that thesemetals were enriched in ES samples, especially at thebase of the waste rock dump. This enrichment wasexplained by the transport of AMD from the top to thebase of the waste rock dump, leaching of heavy metalsinside of the waste rock dump by the acidic solutionsand the subsequent precipitation of metal sulfides dueto evaporation of outcropping AMD during the dryseason.

Cu concentrations were lower in AR samples andtheir leachates from the base of the Excelsior waste rockdump than from the top. In ES samples, Cu is enrichedin comparison to AR samples. This suggested that in thedry season, secondary Cu-bearing sulfates precipitatedclose to sites with high Cu concentration in alteringpyrite-rich primary rocks.

Water-soluble Fe-sulfates were the most abundantheavy metal sulfates at the waste rock dump. AMD andall leachates had high Fe and S concentrations indicatinghigh water-solubility of Fe-sulfates and mobility for ironand sulfate, when water as solvent and transport mediumwas available, e.g. during the rainy season. AR samplesfrom the top had higher concentrations of Fe and S dueto flooring with pyrite-rich waste rocks. Leachates fromAR samples and ES samples from the top terraces alsohad higher concentrations of leachable Fe and S incomparison with base samples and their leachates. Thissuggested that water-soluble Fe-sulfates precipitated inthe dry season near to the oxidizing Fe-sulfides anddepended in their concentration on the local concentra-tion of Fe-sulfides. A possible explication for the lowerFe concentrations in leachates from samples from thebase of the waste rock dump was the precipitation of notwater-soluble secondary minerals like schwertmanniteand jarosite in this zone.

The heavy metal Pb showed no trends of enrichment.Pb had in samples from the top and from the base similar

concentrations, most probably due to the low solubilityof primary and secondary anglesite.

Potassium concentrations in AR samples from top andbase samples were nearly the same, while AR samplesfrom the base had higher sodium concentrations than ARsamples from the top. The higher content of albite-richvolcanic rocks at the base was suggested to be the sourceof the higher concentrations of Na. Altering volcanicrocks were most probably also the reason of Naenrichment in ES samples at the base of the Excelsiorwaste rock dump. High concentrations in AMD samplesdisplayed that both alkali metals were mobile after thealteration of its source rocks. Na and K concentrations inleachates fromAR base samples were much lower than inleachates from AR top samples. This observationsuggested that at the base of the waste rock dump K andNa were at least partially fixed in freshly precipitated,water-insoluble secondary minerals, e.g. jarosites.

The high arsenic concentration in AR samples fromthe top resulted from the flooring of top terraces withpyrite-rich, arsenic-containing waste rocks. The arsenicenrichment in ES samples from the base suggestedtransport of dissolved arsenic, while the lower concen-trations in leachates from base samples contradicted themobility of As. These observations and the high Asconcentrations in AMDwaters at the base suggested thatAs was mobile and transported to the base afterliberation by the oxidation of As-containing sulfides.During the evaporation of pore water at the base of thewaste rock dump arsenic appeared to be fixed inminerals with low water-solubility, e.g. due to adsorp-tion on freshly precipitated Fe(III) hydroxides.

6. Conclusion

At the Excelsior waste rock dump, pyrite from twodifferent sources (ore body/volcanic rocks) was presentas an acid producer in a high concentration (estimatedN60 wt.% of the total mineral assemblage). Ore sulfides(Zn, Pb)were found in local concentrations up to 10wt.%.Minerals with a capacity of neutralising AMD wereonly minor or trace fractions (b5 wt.% carbonates).Sulfide oxidation took place producing a strongly acidicenvironment (average paste pH 2.8). Pyrite from theCerro de Pasco quartz–pyrite body (N60 wt.% of thetotal mineral assemblage) oxidized slowly, producing along-term risk of AMD. Pyrite from volcanic wasterocks represented ca. 1 wt.% of the total mineral as-semblage of the Excelsior waste rock dump. This pyritehas due to small crystal size and high rock porosity ahigh oxidation rate and liberated acids and Fe(III)which could accelerate further pyrite oxidation. The

Fig. 4. Model of climatic effects on the formation and dissolution of efflorescent salts and the transport and mobility of acid, metal-rich solutions at theExcelsior waste rock dump: (A) dry season; (B) rainy season.

109J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

current study allowed the development of a schematicmodel of the seasonal variation in secondary mineralformation and dissolution and subsequent elementoutflow from the Excelsior waste rock dump (Fig. 4).(A) High evaporation during dry seasons concentratedthe acidic, metal-rich pore solutions near to the surfaceof the Excelsior waste rock dump up to supersaturationand caused the successive precipitation of efflorescentmetal salts. The formation of secondary gypsum, Fe(III)-hydroxides, Fe-sulfates (partly Cu-bearing) andminor quantities of Mg-, Zn- and Mn-sulfates wasobserved. At the base of the Excelsior waste rock dumpa major precipitation zone of efflorescent salts wasfound during the dry season as result of outcropping,evaporating pore solutions. (B) Rainwater dissolvedduring the wet season water-soluble secondary mineralsprecipitated during the dry period, infiltrated and trans-ported acids and mobile elements into the waste rockdump where they react with primary minerals and leachheavy metals (Zn, Cu, Cd) out of primary sulfides. Theheavy metals Fe, Mn, Zn, Cu, Cd (as cations), Caand Mg (as cations) and the anion SO4

2− were mobilein the Excelsior waste rock dump system. Arsenic wasonly partly retarded in the system by adsorption on Fe(III) hydroxides, AMD waters still contained up to7.99 mg/L As. The mobility of Pb was limited by thelow solubility of the Pb mineral anglesite. Most of theheavy metal sulfides precipitated during the dry seasonseemed to be dissolved during the wet season and leftthe Excelsior waste rock dump as AMD. As result, theformation of an enrichment of heavy metal as efflo-rescent salts at the base of the Excelsior waste rockdump was limited to the dry season.

Twomain pathways were assumed for the outcroppinghighly mineralized acid solutions: (1) outcrop at a broad

spring zone at the base of the waste rock dump where theAMD was channelled and conducted to a lagoon on theQuiulacocha tailings impoundment, and (2) infiltrationfrom the waste rock dump into the underlying parts of theQuiulacocha tailings impoundment.

Due to the high quantity of sulfides (N60 wt.%) inand the low neutralization potential of the waste rocks(b5 wt.% carbonates) as well as the slow oxidation ofthe pyrite content, a long-term AMD potential for theExcelsior waste rock dump must be assumed.

Acknowledgements

We thank A. Cornejo and J. Rosa del Castillo (Cen-tromin Peru) for their permission, access to theirproperties and also their interest and support. Specialthanks are due to the management and geologists and allstaff involved in this project from VOLCAN S.A.A. fortheir interest, the access to the properties, their logisticsupport and their collaboration, especially V. Gobitz,F. Grimaldo, L. Osorio and W. Hevedia. We thankP. Schreiter, P. Schreck, W. Schmitz and G. Kommichau,University of Leipzig, Germany, as well as H.-R. Pfeiferand L.Dufresne, University of Lausanne, Switzerland, foranalytical support and helpful discussions. For the ICP–MS analyses, we give many thanks to P.-Y. Favarger fromthe Institute F.-A. Forel, Geneva, Switzerland. We thankR. Wennrich (UFZ Leipzig-Halle, Germany) for hisanalytical support. For their support in Peru in the fieldwork, sample preparation and analytical approaches,we also would like to thank R. Mucho and R. Paredes(INGEMMET Peru) and J. Torrejón (GEPASAC Peru).

The project was supported by the German NationalAcademic Foundation and the Swiss National ScienceFoundation.

110 J. Smuda et al. / Journal of Geochemical Exploration 92 (2007) 97–110

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