Continuous leach inductively coupled plasma mass spectrometry:applications for exploration and environmental geochemistry

W.R. MacFarlane1, T.K. Kyser1, D. Chipley1, D. Beauchemin2 & C. Oates3

1Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario, Canada, K7L 3N6(e-mail: macfarlane@geol.queensu.ca)

2Chemistry Department, Queen’s University, Kingston, Ontario, Canada, K7L 3N63Anglo American plc, 20 Carlton House Terrace, London SW1Y 5AN, UK

ABSTRACT: Continuous leach inductively coupled plasma mass spectrometry(CL-ICP-MS) is a new analytical technique developed to address some of theshortcomings of bulk leach techniques while overcoming the uncertainties associatedwith interpreting selective extraction data. It provides information on the specificgeochemical sites and mineral phases from which elements are being released usingreal-time data generated by continually analysing progressively reactive solutionsfrom water to 30% nitric acid as they are pumped through a sample directly into ahigh resolution ICP mass spectrometer. Mineral breakdown reactions can bemonitored from the major elements released, thereby eliminating uncertaintiesrelated to host phase/trace element associations. By comparing major and minorelement release patterns, trace elements can be reliably assigned to host phases.Results from single mineral phases, mixtures of mineral phases, and natural oresamples indicate that the release of elements from specific minerals is not obscuredin more complex samples and that reprecipitation and back reactions are not aconcern with this method. Scanning electron micrograph (SEM) examination of thereaction products has been used to verify which phases react and to support theCL-ICP-MS data interpretation. Results for natural soil samples indicate that ‘false’mobile element anomalies can be identified using CL-ICP-MS and underscore theimportance of understanding where trace elements reside in samples used forenvironmental studies or mineral exploration.

KEYWORDS: sequential extraction, high-resolution ICP-MS, exploration geochemistry, environmental

contamination, mobile elements

INTRODUCTIONAs the number of undiscovered ore deposits occurring inoutcrop and under shallow cover dwindles, explorationists haveturned their attention to more challenging targets, such asmineralization buried under hundreds of metres of cover (e.g.Kelly et al. 2003). Although elemental anomalies have beenreported in soil samples over mineralization buried at 500 m ormore (Goldberg 1998), many conventional geochemical explo-ration techniques are unable to detect the low levels of elementsthat have migrated from blind deposits (Goldberg 1998; Xueqiu1998). As such, more sophisticated techniques are required todetect these anomalies.

Bulk leach techniques that provide total or near totalelemental concentrations use strong reagents like aqua regia todissolve mineral phases such as phosphates, carbonates, organicmatter and oxides that may contain mobile elements whileleaving behind the silicates that are not likely to be involved intrapping mobile elements (Marin et al. 1997; Kelly et al. 2003).These techniques can, however, produce false anomalies or failto detect real ones because they are a better reflection of thecomposition of the soil grains themselves, rather than themobile elements that are attached to their surfaces. Moreover,

even in samples where aqua regia digests only that portion ofthe sample that is likely to contain mobile elements, dis-tinguishing between anomalies and background can be difficult(Goldberg 1998).

A multitude of selective extraction schemes (SESs) have beendeveloped to map specifically that fraction of an element thatwas previously mobile, but has since been trapped by a specificphase in a geochemical barrier (e.g. Tessier et al. 1979;Ochsenkuhn-Petropulu et al. 1996; Marin et al. 1997; CardosoFonseca & da Silva 1998; Mann et al. 1998; Xueqiu 1998;Gomez Ariza et al. 2000; Wenzel et al. 2001; Williams & Gunn2002). The theory is based on the observation that ‘pathfinder’elements migrate away from point sources such as buriedmineralization and may be subsequently immobilized by specificphases in the near-surface environment (Goldberg 1998; Hall1998). The mechanisms currently being investigated by whichelements can be transported from mineralization at depthinclude groundwater movement (Bradshaw et al. 1974; Winter &Pfannkuch 1984; Clark et al. 1990), fault-and-fracture-facilitatedgas transport (Lovell et al. 1983; Malmqvist & Kristiansson1985; Klusman 1993; Hall et al. 1997; Wang et al. 1997),diffusion down a concentration gradient (Smee 1983), ‘fast-ion

Geochemistry: Exploration, Environment, Analysis, Vol. 5 2005, pp. 123–134 1467-7873/05/$15.00 � 2005 AAG/ Geological Society of London

migration’ (Goldberg 1998), and ionic migration under anelectrochemical field (Smee 1983; Govett et al. 1984; Govett &Atherden 1987; Hamilton 1998). Given that elements migratingfrom mineralization at depth must, by definition, be mobile, themobile portion of an elemental concentration should thereforebe a more sensitive exploration tool (Hall 1998; Mann et al.1998).

Selective extraction schemes attempt to determine the parti-tioning of metals into various phases of the sample, thusproviding insight into their provenance. The fundamentalassumption of such a scheme is that each reagent attacks onlyspecific phases such that mobile pathfinder elements can betargeted (Cardoso Fonseca & da Silva 1998). In practice, thistype of specificity is difficult to achieve because the mechanismsand phases that trap mobile elements are so varied (CardosoFonseca & da Silva 1998; Hall 1998). In addition, trappingmechanisms depend on the interaction between elements andthe geochemical barriers they encounter and can includeprecipitation as soluble phosphates and sulphates, adsorptiononto mineral surfaces (especially oxyhydroxides of Mn and Fe),occlusion within mineral grains, adsorption onto organic matterand soil colloids, chelation, complexation, co-precipitation andsubstitution within clay minerals (Goldberg 1998; Hall 1998).

As a result of the complexities associated with both releaseand trapping of pathfinder elements, SESs are normally modi-fied for each study (Gomez Ariza et al. 2000). Experimentalconditions such as reagent type and concentration, sample-to-reagent volume ratio, temperature and contact time areroutinely varied to optimize SES methods for specific sampletypes (Gomez Ariza et al. 2000), specific phases (Krasodebska-Ostrega et al. 2001) or specific analytes (Wenzel et al. 2001).Some of the more popular methods include the enzyme leachtechnique for elements bound to amorphous Mn-oxide species(Clark 1993), the mobile metal ion (MMI) technique for weaklybound metals (Mann et al. 1998), the acidified acetate-bufferedextraction for elements bound to carbonates (Chao 1984;Kersten & Forstner 1989; Hall et al. 1996a), the hydroxylaminehydrochloride leach for elements bound to Mn-oxide species(Chao 1972; 1984) or Fe-oxide species (Chao & Zhou 1983),and the sodium pyrophosphate leach for elements bound toorganic material (McKeague et al. 1971; Hall et al. 1996b).Although widely accepted, the SES recommended by theEuropean Communities Bureau of Reference (BCR procedure)illustrates the fundamental difficulty with SES data. Althoughthe BCR procedure has been shown to be reproducible enoughfor evaluation of trace element mobility in environmentalimpact studies, it does not determine the specific phasesresponsible for their release (Marin et al. 1997; Beauchemin et al.2003). As such, the phases releasing trace elements are ‘opera-tionally defined’ as those species likely to react with the reagentsin each step of the protocol (Tessier et al. 1979; Marin et al.1997). Given these complexities, the assignment of traceelements to specific host phases is viewed with scepticism bysome (Chao 1984; Kheboian & Bauer 1987; Cardoso Fonseca &da Silva 1998).

Redistribution of analyte between the leach solution andremaining solid can be an additional factor affecting SES results.Analyte loss can occur when elements released from the sampleare re-adsorbed, precipitated, or complexed at new geochemicalsites (Hall 1998). This may lead to misinterpretation of selectiveextraction data because metal levels found in the extracts maynot be representative of metal concentrations in the phasesattacked (Rendell & Batley 1980; Gomez Ariza et al. 1999). Asa consequence, selective extraction may not be suitable fordistinguishing the phase associations of trace metals in somenatural soils (Kheboian & Bauer 1987).

The end result is a plethora of SESs that utilize a largenumber of reagents, are complicated, time-consuming, and canbe expensive. Coupled with their uncertain reliability, SESs canrepresent a less than ideal option for some environmental andexploration studies.

This paper describes an analytical technique that addressesthe shortcomings of bulk leach techniques while overcomingmany of the uncertainties associated with selective extraction.Continuous leach inductively coupled plasma mass spec-trometry (CL-ICP-MS) is a technique first described byBeauchemin et al. (2003) where the sample is leached continu-ously from solutions that are injected directly into a high-resolution ICP mass spectrometer (HR-ICP-MS). The real-timedata generated provide reliable information on the specificgeochemical sites and mineral phases from which elements arebeing released. This paper demonstrates that CL-ICP-MShas the potential to generate information that is normallyunavailable using existing extraction techniques.

INSTRUMENTATION AND METHOD

Metal concentrations are measured with a Thermo FinniganElement magnetic sector inductively coupled plasma massspectrometer because the detection of some critical elementsrequires mass resolution greater than 300, which is typical ofquadrupole ICP-MS systems. For example, all Fe isotopes sufferfrom spectroscopic interferences from abundant polyatomicions such as argides, nitrides and oxides, as does the singleisotope of Mn. High resolution ICP-MS was used to minimizethese interferences and others on isotopes of other elements(Chipley et al. 2003). Three mass resolutions are used tominimize spectral interferences (low, medium and high)with nominal M/�M at 5% peak height of 300, 3000 and7500 (respectively). Table 1 summarizes the HR-ICP massspectrometer operating parameters and Table 2 lists the iso-topes measured using each resolution. Tune parameters wereoptimized to produce maximum sensitivity, flat-topped low

Table 1. HR-ICP-MS operating and acquisition parameters.

Tune parameters

sample gas flow c. 1 l/minauxiliary gas c. 0.9 l/mincool gas 14 l/minguard electrode in

Low resolution acquisition parameters

mass window 80%samples/peak 10sample time 0.00011 msdetection mode bothintegration window 50%runs/passes 70/6

Medium resolution acquisition parameters

mass window 150%samples/peak 15sample time 0.0015 msdetection mode bothintegration window 50%runs/passes 70/6

High resolution acquisition parameters

mass window 150%samples/peak 12sample time 0.0015 msdetection mode bothintegration window 50%runs/passes 70/6

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resolution peaks, and narrow apical medium and high resolutionpeaks using a tuning solution of 1 ppb each of Fe, In and U.Multiple isotopes of most elements are determined with mul-tiple resolutions to monitor for interferences. Sample solutionsare introduced into the ICP mass spectrometer using a1.4 ml/min glass Meinhard concentric nebulizer and a Scottdouble-pass spray chamber. The rate of sample solution uptakeis controlled by a peristaltic pump set at 100 µl/min. The verylow flow-rate was chosen to maximize time resolution allowingall elements to be determined in each 50 µl of leach solution.Although such a low flow-rate may starve the 1.4 ml/minnebulizer, sensitivity did not appear to be a problem. The useof micro-nebulization, although advantageous with respect tosensitivity, would likely be incompatible with the high totaldissolved solids (TDS) encountered with this technique.

The nitric acid used for the continuous leach is sub-boilingdistilled in Teflon, diluted with deionized water (18 M�) andspiked with 1 ppb In as an internal standard to correct data forfluctuations in sample flow-rate, matrix effects and instrumentsensitivity. Nitric acid is used as a standard reagent forCL-ICP-MS because it is a low-interference medium, will attackmany resistant phases when applied in high concentration andwill allow Cl contents to be determined.

Sample preparation involves selecting the finest fractionavailable from the minimum amount of crushing. This allowsthe sample to be disaggregated into individual crystals or grainsbut does not alter the fundamental components of the samplethemselves. Exposing the interior of sample grains or crystalsmay lower the sensitivity of the continuous leach technique bycombining mobile elements trapped on surfaces with elementscontained within the constituent particles. It may also allowfluid inclusions to contribute elements to the results. Forlithified samples, this is achieved using a single blow from aheavy hammer covered in clean plastic. The finest fraction (<70mesh, 0.210 mm, fine sand) liberated using this technique isthen selected for analysis. Soil samples are sieved to <150 mesh(0.111 mm, very fine sand).

Approximately 50 mg (� 2 mg) of the homogenized sampleis packed in a short length of Teflon tubing (1/8 inch i.d.)

plugged at both ends by quartz wool (Fig. 1). A weight of 50 mgwas chosen as a standard sample size because it providesadequate reproducibility while minimizing TDS and keeping thesignal below instrumental maxima. The latter point becomescritical when analysing soils from arid regions which containlarge amounts of highly soluble salts. This column is thenconnected in line between the nebulizer of the HR-ICP massspectrometer and a peristaltic pump that delivers progressivelymore reactive solutions (deionized water (18 M�), 1% HNO3,10% HNO3, 30% HNO3) at a rate of 100 µl/min through thesample and into the mass spectrometer (Fig. 1). Each leachsolution is analysed for c. 40 min while continually cyclingbetween low, medium and high resolution. With a scan time ofc. 0.5 min, all elements are measured in each 50 µl of leachsolution. All analyses were carried out at room temperature,which was held at 22�C.

Elemental data are normalized to the In signal duringanalysis. Isotope data are then used to rule out spectroscopicinterferences and to calculate the total response of each elementbased on natural isotopic abundances. Data for each elementare then plotted as normalized instrument response in countsper second (c.p.s.) versus time in minutes. To simplify theinterpretation process, a multiplication factor is applied to theelemental data to bring all response curves to a common set ofaxes for ease of comparison. Replicate analyses of soil, ore andbackground samples indicate that the rate of reaction of thephases involved is virtually the same for duplicates, although theconcentration of the elements released can vary by several percent depending on how much of each phase the samplecontained.

For this study, a variety of sample types were analysed inorder to develop the method. A suite of mono-mineralicsamples (Table 3) selected from the collection of the MillerGeological Museum at Queen’s University was used to con-strain the behaviour of individual phases in the leach solutions.Each mineral was crushed in a hammer mill and then powderedin a clean quartz mortar and pestle, analysed by X-ray diffrac-tion (XRD) to verify its mono-mineralic composition and byCL-ICP-MS to obtain major and trace element release patternsfor individual phases. These samples were ground more aggres-sively than the lithified material described earlier because thesignature of the phase itself was being investigated rather thatthe signal from its surface. Mixtures of these pure phases wereprepared to simulate natural soils and ores and to evaluatematrix effects, interferences and re-adsorption. A suite of eightsamples from exotic-type mineralization in northern Chile wasalso analysed to evaluate element mobility and to identifycharacteristic suites of pathfinder elements. These samples,along with the single phases and synthetic mixtures, wereexamined under the SEM after each stage of leaching toconfirm the reactions deduced from continuous leach data. Toproduce sample material for each stage of leaching, fourcolumns were loaded for each sample. The columns werethen subjected to the continuous leaching procedure off-line,removing one column after each stage of leaching. The sampleswere then removed from the columns, dried in an oven, and

Table 2. Isotopes measured in low, medium, and high resolution during CL-ICP-MS.

Low Medium High

59Co 7Li 7Li85Rb 10B 11B86Sr 7B 31P87Sr 31P 32S90Zr 37Cl 34S98Mo 43Ca 55Mn115In 44Ca 56Fe138Ba 55Mn 75As200Hg 56Fe 77Se202Hg 60Ni 78Se203Tl 61Ni 99Ru204Pb 63Cu 101Ru206Pb 64Zn 103Rh207Pb 66Zn 115In208Pb 73Ge 133Cs209Bi 76Se 185Re232Th 77Se 187Re235U 81Br 188Os238U 90Zr 189Os

92Zr 191Ir115In 193Ir133Cs 194Pt

195Pt197Au

Fig. 1. Apparatus for continuous leach (CL)-ICP-MS.

Continuous leach ICP-MS 125

mounted for SEM analysis. A suite of soil samples collectedover a suspected Irish-type Pb–Zn deposit was analysed usingthe CL-ICP-MS technique. Partial extraction data indicatedpathfinder element anomalies that were proven ‘false’ in asubsequent drilling programme. The two data sets werecompared to assess the validity of the two methods.

RESULTS

Single phases and artificial mixtures

Dominantly single mineral phases were analysed by CL-ICP-MSto verify the rates of reaction between individual mineral phasesand the solutions used. While not absolutely mono-mineralic,they reflect the rate at which typical minerals will react with thesolutions and verify whether incongruent dissolution or backreactions are possible.

Measurement by CL-ICP-MS of the breakdown of themanganese oxide phases pyrolusite, an amorphous Mn oxide,and manganite, a crystalline Mn oxide, shows limited release ofMn during leaching with water, but significant releases of Mnwith 10% and 30% HNO3 leaching (Figs. 2a, 3a). Aberrantreleases of Mn in the initial water wash most likely representparticles physically liberated from the sample surfaces duringsample preparation. This phenomenon occurs in all samples fora wide range of elements regardless of mineralogy and, whenconfined to the first few scans of the analysis, does not appearto be significant in terms of the overall mineral breakdownsequence.

Pyrolusite has a similar rate of reactivity with the leaches asmanganite, a more crystalline Mn oxide (Figs. 2a, 3a). The lesserbut significant release of Fe from these relatively pure samplesunderscores the intimate association of Fe and Mn oxidephases, although the Mn/Fe ratio for the sample of manganiteis 10 whereas it is only 5 for the less crystalline pyrolusitesample. Furthermore, almost identical release patterns for Mnand Fe in pyrolusite (Fig. 2a) indicate that the Mn and Fe oxideconstituents are equally reactive. In contrast, release of Fe doesnot parallel the release of Mn in the manganite sample (Fig. 3a),consistent with distinct phases reacting.

Release of trace elements such as Co, Ba and Rb fromamorphous pyrolusite parallel the release patterns for Mn andFe (Fig. 2b). Two distinct groups of trace elements are releasedfrom the manganite sample, and these correspond to the releaseof either Mn or Fe. Elements such as Pb, Th and U are releasedearly and decline quickly in 10% and 30% HNO3 (Fig. 3b),corresponding to the release of Fe from dissolving Fe oxides(Fig. 3a). In contrast, Ba, Sr and Zr show more constant ratesof release in 10% and 30% HNO3 (Fig. 3c) and correspondmore to the release rate of Mn from the amorphous Mn oxidephase pyrolusite (Fig. 3a).

Gypsum (CaSO4.H2O), another common mineral in ores andsoils, shows limited release of Ca and S during leaching withwater, consistent with its limited solubility in water (Fig. 4a).With 1% HNO3, both elements are released initially and thenthe rate of reaction declines simultaneously for both. Calciumand S show excellent correlation in their release patterns asexpected because they are both being released from the same

Table 3. Summary of the attributes of single phases used in this study.

Group Mineral Chemical Formula Property

Mn Oxides Manganite MnO(OH) CrystallinePyrolusite MnO2 Amorphous

Fe Oxides Hematite Fe2O3 CrystallineGoethite FeO(OH) Amorphous

Sulphates Gypsum CaSO4?2H2O HydrousBarite BaSO4 Anhydrous

Phosphates Apatite Ca5(PO4) 3(F,Cl,OH)Carbonates Calcite CaCO3

Malachite Cu2CO3(OH) 2

Silicates Quartz SiO2

Clays Illite Mixed D=1.50–1.60Hydroxides Atacamite Cu2Cl(OH)3

Fig. 2. Fe and Mn (a) and selected trace element (b) release patternsfrom the Mn oxide phase pyrolusite (amorphous form) measured byCL-ICP-MS during leaching with deionized water, 1% HNO3, 10%HNO3, 30% HNO3. Multiplication factors are applied to bringelements to a common scale (e.g. Mn X2), and MR indicates theresolution (i.e. medium resolution).

W.R. MacFarlane et al.126

phase congruently. The initial release of Ca and S in 1% HNO3,followed by decrease in the rate and the subsequent resurgencein reaction rate in 10% HNO3 leach are consistent with bothsurface and pH-controlled reaction kinetics wherein the changein concentration is exponential with time (Rose et al. 1979;Krauskopf & Bird 1995).

The steady decline in Ca and S between the 10% and 30%HNO3 leaches indicates that the phase has been largelyexhausted. Trace elements such as Sr, Ba and even Rb showidentical release patterns to Ca and S with releases in 1% and10% HNO3 followed by a gradual decline throughout reactionwith 30% HNO3 (Fig. 4b). Once again, good correlation isexpected as these trace and major elements are being releasedby the breakdown of the same mineral phase.

Other nearly ‘mono-mineralic’ phases examined with CL-ICP-MS included quartz, apatite, goethite, hematite and illite. Aswith the minerals discussed above, none of these was found tobe pure and elements not normally in their structure werereleased during leaching. Nevertheless, the release of the majorelements is an indication of the reactivity of these phases duringCL-ICP-MS. Leaching of quartz indicated that Ge was initiallyreleased in 1% HNO3 and showed a constant rate of reactionwith increasing acid strength. Apatite showed rapid concurrentrelease of P and Ca beginning with 1% HNO3 and decliningrapidly to background at the end of leaching with 10% HNO3.Release of U, Th, Ba and Pb paralleled that of Ca and P.Elements released from leaching of goethite and hematite weredominated by the reaction rates of Fe and Mn oxides, whichbegan with the 1% HNO3 leach and increased with 10% and30% HNO3 leaches. Although releases of Fe and Mn weresimilar in these minerals, the Fe/Mn ratios were distinct foreach leach solution, similar to that observed for manganite. Illitewas the only mineral that reacted significantly with the water

Fig. 3. Fe and Mn (a) and selected trace element (b, c) releasepatterns from the Mn oxide phase manganite (MnO, crystalline form)measured by CL-ICP-MS during leaching with deionized water, 1%HNO3, 10% HNO3, 30% HNO3. Multiplication factors are appliedto bring elements to a common scale (e.g. Mn X2), and LR, MRindicates the resolution (i.e. low and medium resolution).

Fig. 4. Ca and S (a) and selected trace element (b) release patterns forgypsum (CaSO4.2H2O) measured by CL-ICP-MS.

Continuous leach ICP-MS 127

leach, releasing U and Cl. This sample was located near anunconformity-type uranium deposit in Australia, so the U andCl were likely mobile elements adhering to the surface of theclay mineral. All other elements showed minimal release untilthe 30% HNO3 leach whereupon Fe and Ca were released.These were likely released from hematite and goyazite, respect-ively, which occur as trace authigenic phases in the sandstonesfrom which the illite was extracted.

Individual minerals show characteristic release patterns ofmajor and trace elements so that it is possible to identifywhich minerals are reacting in a mixture. To verify this, amixture of calcite (CaCO3), barite (BaSO4) and malachite(Cu2(CO3)(OH)2) in equal weight proportions were leached(Fig. 5a, b). As with individual phases, initial scavenging of smallparticles with water results in high initial concentrations of mostelements whereas the absence of release of major elements inthe water leach is consistent with slow rates of dissolution ofthese phases with water. Limited release of Cu, Ba and Cacharacterize initial breakdown during 1% HNO3 leach (Fig. 5a).Release of Ca begins in the 1% HNO3 leach indicating attack onthe calcite in the mixture, which is accelerated by the 10%HNO3 leach. Release of Cu shows that malachite is alsodissolved primarily by 10% HNO3 and the near-perfect corre-lation of peak shapes for Cu and Ca indicates that these two

elements are being released at similar rates from two phases thathave similar reactivity in HNO3. However, the constant andsignificant release of Cu combined with the much smallerrelease of Ca during the 30% HNO3 leach indicates thatmalachite is still present whereas calcite was largely removedfrom the sample by 10% HNO3 (Fig. 5a). Both Cu and Cacontents decline near the end of 30% HNO3 leach indicatingthat most of the carbonate and malachite have dissolved. Incontrast, Ba contents increase throughout the 30% HNO3leach, consistent with barite as the most resistant of the threephases. The much lower signal for S relative to Ba (Fig. 5a, b)reflects both the reduced sensitivity of high mass resolutioncompared to low resolution as well as the enhanced sensitivityof heavy relative to lighter elements by ICP-MS.

Trace element data for the mixture show two distinct groups(Fig. 5b). Elements such as Pb show release most similar to Cafrom calcite whereas elements such as Sr showed continuedincrease in reaction rate through 30% HNO3, indicating releasefrom barite. Thus, release of both major and trace elementsfrom mixtures of minerals having different reaction rates duringleaching can be discerned using CL-ICP-MS.

Exotic-type ores from northern Chile

Eight samples of ore from exotic-type Cu deposits fromnorthern Chile (Munchmeyer 1996) were characterized withSEM analysis (Fig. 6) before and after each stage of leaching andcompared to CL-ICP-MS results (Fig. 7). These samples werecollected from the central ore zone of an exotic-type depositand consist of ore minerals filling void spaces in a gravel- tocobble-sized channel fill unit. The clastic material is derivedfrom the metamorphic basement on which they developedand comprises a propylitic assemblage of plagioclase, calcite,chlorite, epidote and pyrite. Exotic-type ores are derived fromthe weathering of porphyry Cu deposits and consist of a varietyof oxidized Cu and Mn minerals, mainly silicates, chlorides,oxides and carbonates. In addition, there is a plethora of minorphosphates and sulphates. These minerals impregnate thegravels and underlying bedrock of palaeoaquifers that carriedCu-rich fluids derived from adjacent porphyry Cu environments(e.g. Munchmeyer 1996).

Prior to leaching, petrographic and SEM analyses showed avariety of intergrown phases of variable grain sizes in the oresamples including crandallite-group minerals (Cu-bearing phos-phates), Cu- and Mn-bearing carbonates and oxides, gypsum, aswell as barren Fe- and Mn-oxides and silicates (Fig. 6a). Thecomplexity in the mineral chemistry precluded identification ofall the phases, although the relative proportions of majorelements could be identified using SEM–energy dispersivespectroscopy (EDS).

During the water leach, soluble phosphates (crandallite-group minerals), Mn oxides and Cl salts are dissolved releasingP, Mn, Ca and Cu (Fig. 7a, b). SEM analysis confirms that manyof the phosphates present in the original sample are not presentfollowing the water leach (Fig. 6b) whereas gypsum, carbonatesand silicates remain. Other elements released during leachingwith water include Ba, Sr and U (Fig. 7c, d), which match therelease pattern of P, primarily from phosphates. Release of Zn,Hg and Co (Fig. 7c, d) parallel those of Mn and Ca.

Low count-rates for most major and trace elements duringleaching with 1% HNO3 indicate minimal dissolution becausemost of the highly soluble phases have been removed and theresidual phases are not yet susceptible to reaction. Minordissolution of Fe and Mn oxide phases does occur to someextent, releasing Cu and minor Co (Fig. 7b, c). By the end of the1% HNO3 leach, phases containing Cu have been removed

Fig. 5. Cu, Ba and Ca (a) and selected trace element (b) releasepatterns for a 1:1:1 mixture of calcite (CaCO3), barite (BaSO4) andmalachite (Cu2(CO3)(OH)2) measured by CL-ICP-MS. Each phase inthe mixture was confirmed as nearly mono-mineralic by XRDanalysis.

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along with much of the carbonate (Fig. 6c). With moreaggressive solutions during 10% and 30% HNO3 leach, Fe andMn oxides are dissolved with increasing vigour (Fig. 7b). After

10% HNO3 leaching, no Cu-bearing phases remain (Fig. 6d).Leaching with 30% HNO3 leaves only residual oxides andsilicates (Fig. 6e).

Fig. 6. SEM images of an exotic-type ore from northern Chile after each stage of the continuous leach process: (a) pristine sample showingoriginal mineralogy and texture, (b) after deionized water leach showing sample mineralogy after removal of water-soluble phases such as saltsand phosphates, (c) after 1% HNO3 leach, (d) after 10% HNO3 leach, (e) after 30% HNO3 leach. All images were acquired in backscatter mode.Scale bar is 100 µm.

Continuous leach ICP-MS 129

Observations from the SEM on the run products support theCL-ICP-MS data on the dissolution of various phases duringeach stage of leaching. In addition, no evidence of reprecipita-tion or back reaction was observed in any of the SEM photosgenerated for any sample at any stage of leaching.

Irish-type Pb–Zn prospects

Five soil samples from a geochemical survey to find Irish-typePb–Zn deposits at depth demonstrate another application ofdata from CL-ICP-MS. Extraction using dilute aqua regiaindicated anomalous concentrations of Cu, Ag and Zn overseveral areas, but subsequent drilling encountered no minerali-zation. The purpose of CL-ICP-MS was to constrain, in bothbackground and anomalous samples, the phase distribution ofpathfinder elements such as Ba, Pb, Cu, Hg and Zn.

Irish-type Pb–Zn deposits contain sediment-hosted massivesulphides that are classified between syngenetic shale-hostedSEDEX deposits and epigenetic carbonate-hosted MVT de-posits. They occur when metal-bearing hydrothermal fluidsencounter clean, non-argillaceous carbonates with good poros-ity and permeability. Fluid migration is structurally controlledwith the best mineralization occurring at the point of maximumnormal fault displacement. The most abundant sulphide min-erals are sphalerite and galena (Hitzman & Large 1986; Hitzman1998).

Leaching of anomalous samples with water releases solublechloride salts and phosphates as shown by releases of P, Ca, Cland Na (Fig. 8a, b). Trace elements associated with these phasesin anomalous samples include Cu, Ag, Hg and Zn (Fig. 8c, d).Similar major elements, along with Mn, are leached by waterfrom barren samples (Fig. 9a, b). Trace elements released duringwater leaching of background samples (Fig. 9c, d) are identicalto those from the anomalous samples, except for the lower Agcontents.

Reaction with Fe and Mn oxides in both the anomalous (Fig.8b) and background (Fig. 9b) samples is minimal with the 1%HNO3 leach, but is significant with 10% and 30% HNO3. TheFe/Mn ratios of the oxides that react with HNO3 solutions aresimilar for both background and anomalous samples andconstant with continued leaching. Only minor amounts ofpathfinder elements are released with Fe and Mn in thebackground samples (Fig. 9c, d), whereas Cu, Ag and Pb areassociated with the release of Fe and Mn in the anomaloussamples (Fig. 8c, d).

CL-ICP-MS data indicate that no anomaly is present in thissurvey because all samples contain nearly equivalent mobilepathfinder element concentrations in the most labile fraction.Anomalies identified using other methods are unlikely to bederived from a deposit at depth because the anomalousconcentrations in pathfinder elements reside in residualminerals and are not hydromorphic.

Fig. 7. Release patterns measured by CL-ICP-MS for an exotic-type ore from northern Chile for the elements: (a) Ca, P, Cl; (b) Cu, Fe, Mn;(c) U, Hg, Co; (d) Sr, Zn, Rb.

W.R. MacFarlane et al.130

DISCUSSION

The rate at which minerals react with fluids depends on a varietyof factors that include surface area, surface structure, defects,solubility, diffusion of components through the mineral struc-ture and pH of the solution. Many dissolution reactions haverates that decrease exponentially with time as the surface-area-to-volume ratio of the mineral increases, although the rates canbe complex. Data from CL-ICP-MS analysis of single mineralphases exemplify the exponential decay patterns as seen in theelement profiles of most samples. Most dissolution reactionsproceed at an initially rapid rate (Krauskopf & Bird 1995) asleached or altered surface layers or defects in the crystal surface(which are generally more reactive) begin to dissolve. As thesehighly reactive sites are consumed and less altered, moreordered mineral is exposed, the reaction slows from its initiallyrapid rate and the elemental releases seen in CL-ICP-MSdiminish. As the next more acidic leach solution is introduced,this process is repeated with the next most reactive set ofsurface sites. This model of surface- and pH-controlled disso-lution reaction kinetics is consistent with element releasepatterns generated by CL-ICP-MS.

Analysis of dominantly mono-mineralic phases by CL-ICP-MS is an essential step in calibrating the new methodbecause it verifies the rate of reaction for each phase. The rateof release of Fe and Mn from pyrolusite in 1% HNO3 (Fig. 2a)is similar to release from crystalline manganite (Fig. 3a),suggesting that reactivity is not proportional to crystallinity for

the oxides. Similarly, the release of Fe from goethite andhematite is similar despite their difference in crystallinity deter-mined from XRD. In each of the oxide phases studied, therewere significant Fe and Mn contents during reaction withHNO3 solutions. Despite variable rates for Fe and Mn releaseduring leaching of the oxides, Fe/Mn ratios were relativelyconstant for the pyrolusite (Fig. 2a), goethite and hematiteanalysed, but variable for the manganite (Fig. 3a). The mangan-ite variability is most likely the result of inclusions of Fe oxiderather than a reflection of incongruent dissolution because theFe and Mn react similarly. Some trace elements in the mangan-ite have reaction rates identical to the Fe phase (Fig. 3b)whereas others, such as Sr, Ba and Zr (Fig. 3c), appear to bemore similar to the release of the Mn phase. Similar reactionrates of trace and major elements can be attributed to therelease of these from a single phase because it is unlikely thatreaction rates of a phase dissolving would be so similar to therelease of trace elements from another phase that was notreacting.

The rate of reaction of gypsum is such that initial dissolutionbegins in 1% HNO3 and slows exponentially with time (Fig. 4a).This pattern repeats with each subsequent solution. The ratesof release of Sr, Ba and Rb are identical to those of Ca and S(Fig. 4b). Rates of reaction during CL-ICP-MS for barite, calciteand malachite in an artificial mixture are not significant untilleaching with 10% HNO3 (Fig. 5a). None of these phasesshowed any significant release of major or trace elements

Fig. 8. Release patterns measured by CL-ICP-MS for a soil sample that has anomalous concentrations of Zn, Pb and Ag in aqua regia digestscollected above a suspected Irish-type Pb–Zn deposit for the elements: (a) Na, Cl, Ca; (b) P, Fe, Mn; (c) Cu, Ag, Hg; (d) Zn, Zr, Pb.

Continuous leach ICP-MS 131

during leaching with water, and only gypsum began reactingsignificantly with 1% HNO3.

Continuous leaching of soil samples collected for explorationof Irish-type deposits indicates that Cu, Ag and Zn contents,which can be anomalous using partial dissolution by weak aquaregia, occur in water-soluble salts in both anomalous andbackground samples, but also in residual oxide phases (Cu andAg) only in anomalous samples (Fig. 8a–d). Soluble phosphatesand chlorides are the hydromorphic phases that control therelease of these elements in both background and anomaloussamples, but only anomalous samples have elevated contents ofthese elements in less soluble Fe and Mn oxides and sulphides.These phases reacted with the aqua regia used for the conven-tional analysis, thereby producing apparent anomalies in someof the pathfinder elements. Background samples contain Fe andMn oxides, but no associated metals that would indicateIrish-type mineralization.

SEM analysis confirms CL-ICP-MS interpretations of therelative reaction rate of the minerals in these samples (Fig.6a–e). It further confirms that the dissolution rates identified bysingle-phase trials are not substantially altered when measuredin natural mixtures. Additionally, continual delivery of freshreagent to the sample has overcome reprecipitation problemsthat have plagued some other methods (Hall 1998), in thatdissolution products are immediately removed from the system

and analysed. The risk of an element reprecipitating and beingassigned to the wrong phase later in the leach is minimized.

CL-ICP-MS analyses of samples of ore from exotic-typemineralization show significant release of Ca, Cl and Ba saltsand minor phosphates during the water wash that continuesinto the 1% HNO3 leach (Fig. 7). The elements Sr, Rb (notshown), Br (not shown), U, Co, Hg, Cu and Zn are stronglyassociated with these phases and, therefore, considered to bemobile from these ores. The mobile and easily leached elementsin samples from two separate exotic-type deposits are similarand variations in the release of elements during leaching of theore reflect differences in mineralogy and grain size. Dissolutionof Fe and Mn oxides is not significant until 1% and 10% HNO3leach, releasing Zr, Tl, Th, Bi, Mo, Pb (not shown) and Rb (notshown) during the 30% HNO3. These elements would likely beresidual rather than mobile from exotic-type mineralization.

APPLICATIONS

The advantage of CL-ICP-MS is the generation of real-timedata not available using conventional geochemical techniques.By observing element concentrations as reactions proceed inreal time, ambiguities related to sequential leaches and lowresolution limitations of bulk leaches are simultaneously over-come. As a consequence, CL-ICP-MS has applications in

Fig. 9. Release patterns measured by CL-ICP-MS for a barren soil sample collected adjacent to a suspected Irish-type Pb–Zn deposit for theelements: (a) Na, Cl, Ca; (b) P, Fe, Mn; (c) Cu, Ag, Hg; (d) Zn, Zr, Pb.

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exploration geochemistry for identifying the pathfinder elementsuite from a known ore type, as was done for exotic-type ores.Results from the Irish-type Pb–Zn prospect highlight anotheruse of this technique for exploration geochemistry. The siteswithin a soil or rock sample that host pathfinder elements canbe identified and the mobile fraction of pathfinder elements canbe determined without being diluted by the breakdown ofresidual phases using CL-ICP-MS, so that smaller anomalies canbe detected.

The CL-ICP-MS method also has great potential as a tool forenvironmental science. Mapping of subsurface contaminationcould be carried out by identifying suites of mobile elements byanalysing soils from contaminant sites. This could minimize theneed for expensive and disruptive drilling and trenching. Inaddition, evaluation of contaminant mobility under a variety ofsoil conditions could allow a remediation specialist to moreeffectively allocate resources.

CONCLUSIONS

CL-ICP-MS is a novel analytical technique that can provideinformation on the specific phases and mineralogical sites fromwhich trace elements are being released. Concentration profilesare obtained from samples during leaching with deionized waterand 1%, 10% and 30% HNO3 while continuously monitoringanalyte signals by high resolution ICP-MS. Compared to batchextraction procedures normally used, the approach involvesminimal sample preparation, reduces contamination because theleaching is performed in a closed system and allows correlationof reaction rates for major and trace elements. However,analysis time is long and the spectra obtained can be complex.Continuous on-line monitoring also allows a greater resolutionof the various phases reacting with a given reagent. It cantherefore be used to design effective leaching strategies and totrace isotopic compositions.

Special thanks to Dave Lawie and Anglo American plc for providingsample material and scientific support for this project. Funding forthis project was provided by Queen’s University, Anglo American,NSERC CRD, NSERC MFA, and NSERC Discovery grants to KurtKyser and Diane Beauchemin.

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