Rare earth elements (REE) and yttrium in stream waters, stream sediments, and Fe–Mn oxyhydroxides: Fractionation, speciation, and controls over REE + Y patterns in the surface environment

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Geochimica et Cosmochimica Acta 72 (2008) 5962–5983 www.elsevier.com/locate/gca

Rare earth elements (REE) and yttrium in stream waters, stream sediments, and Fe–Mn oxyhydroxides: Fractionation, speciation, and controls over REE + Y patterns in the surface environment Matthew I. Leybourne a,*, Karen H. Johannesson b b

a GNS Science, P.O. Box 30-368, 1 Fairway Drive, Avalon, Lower Hutt, New Zealand Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118-5698, USA

Received 19 October 2007; accepted in revised form 22 September 2008; available online 30 September 2008

Abstract We have collected 500 stream waters and associated bed-load sediments over an 400 km2 region of Eastern Canada and analyzed these samples for Fe, Mn, and the rare earth elements (REE + Y). In addition to analyzing the stream sediments by total digestion (multi-acid dissolution with metaborate fusion), we also leached the sediments with 0.25 M hydroxylamine hydrochloride (in 0.05 M HCl), to determine the REE + Y associated with amorphous Fe- and Mn-oxyhydroxide phases. We are thus able to partition the REE into ‘‘dissolved” (<0.45 lm), labile (hydroxylamine) and detrital sediment fractions to investigate REE fractionation, and in particular, with respect to the development of Ce and Eu anomalies in oxygenated surface environments. Surface waters are typically LREE depleted ([La/Sm]NASC ranges from 0.16 to 5.84, average = 0.604, n = 410; where the REE are normalized to the North America Shale Composite), have strongly negative Ce anomalies ([Ce/ Ce*]NASC ranges from 0.02 to 1.25, average = 0.277, n = 354), and commonly have positive Eu anomalies ([Eu/Eu*]NASC ranges from 0.295 to 1.77, average = 0.764, n = 84). In contrast, the total sediment have flatter REE + Y patterns relative to NASC ([La/Sm]NASC ranges from 0.352 to 1.12, average = 0.778, n = 451) and are slightly middle REE enriched ([Gd/ Yb]NASC ranges from 0.55 to 3.75, average = 1.42). Most total sediments have negative Ce and Eu anomalies ([Ce/Ce*]NASC ranges from 0.097 to 2.12, average = 0.799 and [Eu/Eu*]NASC ranges from 0.39 to 1.43, average = 0.802). The partial extraction sediments are commonly less LREE depleted than the total sediments ([La/Sm]NASC ranges from 0.24 to 3.31, average = 0.901, n = 4537), more MREE enriched ([Gd/Yb]NASC ranges from 0.765 to 6.28, average = 1.97) and Ce and Eu anomalies (negative and positive) are more pronounced. The partial extraction recovered, on average 20% of the Fe in the total sediment, 80% of the Mn, and 21–29% of the REEs (Ce = 19% and Y = 32%). Comparison between REEs in water, partial extraction and total sediment analyses indicates that REEs + Y in the stream sediments have two primary sources, the host lithologies (i.e., mechanical dispersion) and hydromorphically transported (the labile fraction). Furthermore, Eu appears to be more mobile than the other REE, whereas Ce is preferentially removed from solution and accumulates in the stream sediments in a less labile form than the other REEs + Y. Despite poor statistical correlations between the REEs + Y and Mn in either the total sediment or partial extractions, based on apparent distribution coefficients and the pH of the stream waters, we suggest that either sediment organic matter and/or possibly d-MnO2/FeOOH are likely the predominant sinks for Ce, and to a lesser extent the other REE, in the stream sediments. Ó 2008 Elsevier Ltd. All rights reserved.

1. INTRODUCTION *

Corresponding author. E-mail address: [email protected] (M.I. Leybourne).

0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.09.022

Rare earth elements (REE) and Y are of great interest because their unique chemical properties make them espe-

Fractionation and speciation of REE + Y in small catchment streams

cially powerful tracers of fundamental geochemical processes (Hanson, 1980; Henderson, 1984; Banfield and Eggleton, 1987; Elderfield et al., 1990). These properties derive from the fact that, (1) with the exception of Ce, the REEs are generally trivalent in Earth surface systems and are thus chemically fractionated from their nearest neighbors in the Periodic Table (i.e., divalent Ba and tetravalent Hf), and (2) owing to the progressive filling of the 4f electron shell across the REE series, their ionic radii decrease with increasing atomic number (i.e., the ‘‘lanthanide contraction”; Byrne and Kim, 1993; Byrne et al., 1996; Johannesson et al., 1999; Johannesson and Hendry, 2000). This lanthanide contraction imparts subtle and systematic differences in the chemical properties of REEs across the series that are largely predictable, and thus highly useful in studies of those processes that fractionate REEs in the environment. Consequently, the REEs have a long history of use in the study of magma genesis in the Earth’s upper mantle and crust, crustal evolution, and in investigating weathering, crustal denudation, transport of weathering products to the oceans, and for water–rock interactions (Hanson, 1980; Henderson, 1984; Taylor and Mclennan, 1985; Banfield and Eggleton, 1987; Banner et al., 1989; Braun et al., 1990; Bau, 1991; Smedley, 1991; Leybourne et al., 2006b). Moreover, under specific conditions, Ce and Eu can be redox sensitive and thus may record changes in redox conditions. For example, Leybourne et al. (2000) showed that the redox transformation of Ce3+ to Ce4+ may be rapid as anoxic groundwaters discharge to the Earth’s surface. In contrast, under strongly reducing conditions and elevated (hydrothermal) temperature, Eu is present as Eu2+ (Sverjensky, 1984). As a result, hydrothermal fluids and associated sediments from mid-ocean ridge and back-arc spreading centers are characterized by strongly positive Eu anomalies relative to average shale or chondrite (Klinkhammer et al., 1983, 1994a; Michard et al., 1983; Bau and Dulski, 1999). Massive sulfide deposits commonly preserve this strongly positive Eu anomaly of the precursor hydrothermal fluid (Peter et al., 2003; Leybourne et al., 2006b). A number of studies have investigated the fractionation of REEs during chemical weathering and the subsequent transport of the dissolved and suspended REE fractions from the continents to the oceans (Hoyle et al., 1984; Goldstein and Jacobsen, 1988b; Elderfield et al., 1990; Sholkovitz, 1995; Byrne and Liu, 1998; Sholkovitz et al., 1999; Sholkovitz and Szymczak, 2000; Andersson et al., 2001). In general, these studies demonstrate that the heavy REE (HREE) enriched, shale-normalized REE patterns that characterize seawater originate, in part, during chemical weathering of the continental crust and the ensuing transport of REEs in rivers to the ocean (see Elderfield et al., 1990; Johannesson et al., 2006 for review). Many of these studies have typically focused on the lower reaches of large rivers (watersheds encompassing tens of thousands of square kilometers), which tend to provide samples that integrate the effects of chemical and physical weathering and the ensuing riverine transport of REEs in the entire, upstream, watershed (Asmerom and Jacobsen, 1993; Gaillardet et al., 1995, 1997; Dupre´ et al., 1996). In addition, many recent studies have carefully examined chemical

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weathering and REE geochemistry of rivers draining tropical watersheds, whereas others have focused on boreal rivers, all of which are noted for substantial dissolved and colloidal organic matter concentrations (Braun et al., ¨ hlander 1990, 1993, 1998; Dupre´ et al., 1996, 1999; O et al., 1996; Viers et al., 1997; Land et al., 1999; Ingri et al., 2000; Andersson et al., 2001; Pokrovsky and Schott, ˚ sto¨m and Corin, 2003; Tosiani et al., 2004; Pokrov2002; A sky et al., 2005, 2006). These and other studies have clearly demonstrated that REEs are commonly transported in large part within tropical and boreal rivers along with Ferich and/or organic colloids. In contrast, fewer studies have attempted to quantify the relative contributions of chemical and physical weathering processes on the REE concentrations of small catchments and/or clear (i.e., low DOC and suspended loads) headwater streams from temperate regions, despite the fact that such regions are also zones of active weathering (e.g., Tricca et al., 1999; Steinmann and Stille, 2008). Here, we present Y and REE concentrations for stream waters and associated sediments from small, first and second order stream catchments (25–50 km2) in northern New Brunswick, Canada, where suspended sediment loads, including dissolved organic matter, are minor and the dissolved load is thought to predominate (Leybourne et al., 2003). More specifically, we present REE + Y concentrations for what we call the ‘‘dissolved load”, which includes both the dissolved and colloidal fraction in the stream waters (i.e., <0.45 lm), as well as the REE concentrations of the associated bulk stream sediment samples, and for REE extracted from a single leaching experiment of these sediments designed to liberate REEs associated (i.e., adsorbed and/or co-precipitated) with amorphous Fe/Mn oxyhydroxides (Hall et al., 1996; Tricca et al., 1999). The bulk and single-leach Y and REE concentration data for these stream sediments provide the erosional (i.e., detrital) and the hydromorphic (i.e., labile) contributions, respectively, and the application of the single-leach extraction allows an improved understanding of Y and REE partitioning between the detrital, hydromorphic, and ‘‘dissolved” loads (Leybourne et al., 2003). These data are used in conjunction with REE concentration data for local host rocks to evaluate partitioning of Y and REE between aqueous and solid phases. 2. GEOLOGIC SETTING The study area, located in the northwestern part of the Bathurst Mining Camp (BMC), comprises part of the Miramichi terrane of northern New Brunswick, Canada (Fig. 1). The area is underlain by the Cambro-Ordovician Miramichi group, the chiefly Middle Ordovician California Lake and Tetagouche Groups, the Middle to Late Ordovician Fournier Group and the Silurian Chaleurs Group. The Miramichi Group is interpreted as continental margin flysch that conformably to disconformably underlies the California and Tetagouche groups (Van Staal et al., 2003). The latter two groups (i.e., California Lake and Tetagouche groups) are essentially coeval, are composed of a lower, predominantly felsic volcanic part and an upper, predominantly mafic volcanic part, and are interpreted as the initial

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Fig. 1. Location of study area in the Bathurst Mining Camp, New Brunswick, Canada. Main map shows the generalized geology of the study area, after (Van Staal et al., 2003). Also shown are the locations of a number of mines and/or mineralized zones within the study area, particularly the Restigouche Deposit. Individual sample locations visited in the current study are shown as circles that are proportional to the Eu-anomaly of the bulk stream sediment collected at each site. The Eu anomaly is defined here as: Eu/Eu* = EuNASC/(SmNASC*GdNASC)0.5 where the subscript NASC indicates that the sample concentration is normalized to the corresponding North American Shale Composite concentration. It should be noted that both water and sediment samples were collected at each of the sample locations. The Nepisiguit Falls (ON), Flat Landing Brook (OL) and Little River (OLR) formations are part of the Ordovician Tetagouche Group, whereas the Boucher Brook (OBB) and Mount Brittain (OMB) formations are part of the California Lake Group. SDs represents shallow marine rocks of the Silurian Chaleurs Group.

Fractionation and speciation of REE + Y in small catchment streams

deposits of an ensialic back-arc basin (Rogers et al., 2003). Felsic volcanic and associated sedimentary rocks of the Mount Brittain (California Lake Group) and Nepisiguit Falls (Tetagouche Group) formations host volcanogenic massive sulfides (VMS) deposits in the study area (Fig. 1). The California Lake Group is tectonically overlain by mafic volcanic rocks of the Fournier Group, which are interpreted as ocean floor rocks formed in a back-arc basin (Rogers et al., 2003). Shallow marine sedimentary rocks of the Chaleurs Group unconformably overly the Ordovician rocks. Precipitation and river discharge data for the study area indicate that a strong increase in stream discharge typically begins in late April and reaches a maximum in May owing to snow-melt and the resulting spring run-off (Leybourne et al., 2006a). The hydrographic data, combined with geochemical and stable isotopic similarity between shallow groundwaters and surface waters, suggest that shallow groundwater recharge dominantly occurs during snow-melt in the spring (Leybourne et al., 2006a). Comparisons between the major cation and anion concentrations of surface waters in the study area, shallow groundwaters from the Restigouche deposit, and local precipitation indicate that during summer, however, the surface waters primarily reflects baseflow i.e., shallow groundwater discharge (Leybourne, 1998; Leybourne et al., 2003). Stable oxygen and hydrogen isotope analysis of the surface waters and shallow groundwaters at the Restigouche deposit further supports a baseflow source for the stream waters during summer (Leybourne et al., 2006a). 3. METHODS 3.1. Sample collection Surface waters and stream sediments from the active portions of first and second order streams were collected from the study area during July and August 1997 (Fig. 1). In addition, surface waters and stream sediments from around the Restigouche deposit were collected during the summers of 1995 and 1996. Samples were collected at 500 m intervals along stream channels (100 and 200 m intervals around the Restigouche deposit) and were located at least 50 m upstream of roads and culverts in order to minimize anthropogenic contamination (see Leybourne et al., 2003). Where seeps and springs were observed, additional water and sediment samples were collected. All surface water samples were filtered in the field through 0.45 lm filters and into pre-cleaned, 125 ml HDPE sample bottles (e.g., Johannesson et al., 2004), immediately acidified with ultrapure nitric acid (to 0.4%) at the base camp, and stored refrigerated (4 °C) until analysis. Sediments were collected from the active part of the stream in paper bags, air dried, and sieved to 80 mesh (<177 lm). Consequently, all sediment geochemistry results presented here represents the fine sand and smaller components (e.g., silt, clay) of the stream bed sediments. Visual inspection of the stream sediments indicates they contain quartz, weathered alumino-silicate minerals (i.e., clay minerals), some rock fragments, and in some cases a visible coating of what

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appeared to be a Fe-rich, biofilm (Leybourne et al., 2003). More study is necessary to confirm the general mineralogy of the sediments, but it is noteworthy that the overall composition of the sediments appears to be broadly similar to sediments collected from local groundwaters (Leybourne, 2001; Leybourne and Cousens, 2005). 3.2. Stream water analysis Total dissolved Fe concentrations in surface water samples were quantified by inductively coupled plasma emission spectrometry (ICP-ES) at the Geological Survey of Canada. Rare earth elements, Y, and Mn concentrations in surface water samples were measured by ICP-MS (mass spectrometry) following 5 pre-concentration by chelation, using the method of Hall et al. (1995). Briefly, separation and concentration of Y and the REEs were achieved using columns containing a CC-1 chelating resin of macroporous iminodiacetate, which preferentially retains the trivalent REEs relative to alkali and alkali earth metals (Hall et al., 1995). The following isotopes of Y and the REEs were monitored during the ICP-MS analysis in order to minimize isobaric interferences: 89Y, 139La, 140Ce, 141Pr, 144 Nd, 147Sm, 153Eu, 160Gd, 159Tb, 163Dy, 165Ho, 166Er, 169 Tm, 174Yb, and 175Lu. The ICP-MS was calibrated and the sample concentrations verified using a series of REE + Y standards of known concentration (0.1, 2, 10, 100, 250, 500, and 1000 ng/kg). The calibration standards were prepared from NIST traceable standards purchased from High Purity Standards (Charleston, SC). The detection limits for La and Ce were 5 ng/L, 2 ng/L for Gd, and 1 ng/L for the other REE and Y (for details see Leybourne et al., 2000). A subset of the waters were also analyzed for Br and P (expressed as PO4) by ICP-MS, with detection limits of 0.54 lg/L for Br and 1.1 lg/L for PO4 (Leybourne, 1998). 3.3. Stream sediment analysis Total Fe and Mn concentrations in stream sediments were quantified by X-ray fluorescence (XRF) on pressedpowder pellets, whereas Y and REE concentrations were measured by ICP-MS following total dissolution of bulk sediment samples using a multi-acid attack (HF–HClO4– HCl–HNO3) with metaborate fusion of the residue (Leybourne, 1998; Leybourne et al., 2003). In addition, the labile fraction of Y and the REEs associated with the stream sediments were determined by first leaching (i.e., partial extraction) a subset of sediment samples with 0.25 M NH2OH
HCl (i.e., hydroxylamine hydrochloride) in 0.05 M HCl as outlined in Hall et al. (1996). The hydroxylamine hydrochloride leach was used in order to dissolve only the amorphous Fe and Mn oxyhydroxides associated with the stream sediments (crystalline Fe oxides such as goethite, hematite, and magnetite are not dissolved by this extraction; Hall et al., 1996), and REE + Ys loosely bound to clay mineral surface sites and/or organic matter. Briefly, 1 g of sediment sample was added to 20 ml of 0.25 M NH2OH
HCl in 0.05 M HCl. The resultant solution was heated to 60 °C for two hours and vortexed every 30 min,

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following which the samples were centrifuged for 10 min at 2800 rpm. The supernatant was then decanted and filtered through 0.45 lm filters and subsequently analyzed for Y and REEs by ICP-MS as described above for the stream waters. All analyses were performed at the Geological Survey of Canada. The partial leaching method employed in the current study differs from the approach outlined by Tessier et al. (1979), and used previously by Johannesson and Zhou (1999) and Tang et al. (2004), in that it is designed to only reduce, and thus dissolve, amorphous Fe/Mn oxyhydroxides as opposed to both amorphous and crystalline Fe/Mn oxides/oxyhydroxides (Hall et al., 1996). This is chiefly achieved by application of the lower molar HCl concentrations (0.05 M HCl) as compared to the higher HCl concentration (i.e., 0.25 M HCl) recommended by Chao and Theobald (1976) during the NH2OH
HCl leach. Specifically, application of the higher 0.25 M HCl concentrations within the 0.25 M NH2OH
HCl leach appears to also reduce some fraction of the more crystalline Fe oxides (goethite, hematite, magnetite) and possibly attack any labile organic matter fraction within the sediments, thus leading to overestimates of the REE fraction adsorbed to amorphous Fe/ Mn oxyhydroxides (see Hall et al., 1996). Furthermore, Hannigan and Sholkovitz (2001) studied the effects of weathering phosphate-bearing minerals (i.e., apatite) in experimental solutions by employing natural acidic lake waters from Cape Cod (pH 5), and these same lake waters with pH amended to pH 2 with nitric acid. They found that by lowering the solution’s pH, MREE-bearing phosphate minerals could be partially solubilized, imparting MREE enrichments to the weathering solutions. Consequently, it is conceivable that the 0.25 M NH2OH
HCl in 0.05 M HCl leachate solution employed in this study could solubilize some MREEs from phosphate phases within the stream sediments. Therefore, it is important to stress that because all sequential extraction procedures involve operationally defined target mineral phases and are additionally subject to possible artifactual results arising from incomplete dissolution, saturation, and/or re-adsorption of trace elements, the results are at best semi-quantitative (Tipping et al., 1985; Sholkovitz, 1989). One reference standard was included within each group of 20 samples prepared for analysis. For waters, the standard was a field sample collected at the end of the season and prepared in the same manner as the other samples. For the sediments, an in house (i.e., GSC) standard was used. In addition, each group of twenty samples included a field duplicate as well as duplicate analytical analyses. These provided quality controls on accuracy (standards) and precision (duplicates), respectively. Furthermore, for the stream sediments, certified reference materials (JB-1a, GXR-1, LKSD-2, and LKSD-3) were analyzed as unknowns to determine accuracy. For waters, certified reference standards NBS-1640, SLRS-4, and CRM-TMDW were measured to determine accuracy for select analytes other than REEs, and duplicates of unknowns were analyzed to determine precision. Most

waters have ionic balances <5% in error. Full analytical details are given in Leybourne (1998) and Leybourne et al. (1999). 4. RESULTS 4.1. Stream waters The general geochemistry of the stream waters, including pH, major solute compositions, and total dissolved solids (TDS) concentrations, were previously reported (Leybourne, 1998; Leybourne et al., 2000, 2003), and readers are referred to these contributions for detailed discussions of these data. In brief, the majority of the surface waters from the study area are dilute (10 mg/L 6 TDS 6 150 mg/L), circumneutral (6.5 6 pH 6 7.5) CaHCO3-type waters. In addition, Ca-HCO3-SO4 and CaSO4-types waters also occur, and some of the low TDS waters have pH values between 4 and 6 (Leybourne et al., 2003). Concentrations of REEs in stream waters (as RREE, not including Y) range from <5 to 11,540 ng/L, with an average of 253 ng/L (n = 498). Because the stream water samples were filtered through 0.45 lm pore-sized filters, the stream water REE concentrations reflect both colloidal and truly dissolved fractions of REEs in these surface waters (Pokrovsky and Schott, 2002; Pokrovsky et al., 2006). All data are presented in electronic annex-1. Stream water REE + Y concentrations are normalized to the North American Shale Composite (NASC) (Haskin et al., 1968; Gromet et al., 1984) and presented in Fig. 2. Generally, surface waters in the BMC are LREE depleted relative to NASC (i.e., [La/Sm]NASC ratios range form 0.16 to 5.84 for the stream waters, with an average = 0.604, n = 410; Fig. 2 and Table 1, electronic annex-1). In addition, NASC-normalized La values are typically depleted relative to the corresponding NASCnormalized Pr values (e.g., [La/Pr]NASC ranges from 0.42 to 3.13, average = 0.851, n = 408). Surface waters are characterized by strongly negative Ce anomalies; [Ce/Ce*]NASC ranges from 0.02 to 1.25 (average = 0.277, n = 354), whereas Eu is commonly below detection. Cerium anomalies are calculated as Ce/Ce* = CeNASC/ (LaNASC*PrNASC)0.5 and Eu anomalies are calculated as Eu/Eu* = EuNASC/(SmNASC*GdNASC)0.5, where NASC indicates North American Shale Composite normalized values. However, for those stream water samples for which Eu is detectable, Eu anomalies are variable ([Eu/ Eu*]NASC ranges from 0.295 to 1.77, average = 0.764, n = 84). There is no correlation between the Eu anomaly and Ba concentrations, indicating that BaO+ formation in the ICP plasma was insignificant. In addition, Gd is typically slightly enriched over Tb and Dy, relative to NASC (Fig. 2 and Table 1), leading to small enrichments in the middle REE (MREE) with respect to the LREE and HREE for some of the surface waters. All of the surface waters exhibit moderate enrichments in Y with Y/HoNASC ratios ranging from 0.32 to 1.89 (average = 1.14 ± 0.25; n = 98), with 21 waters having Y/HoNASC < 1 (Table 1).

Fractionation and speciation of REE + Y in small catchment streams

B

0.1

Sample/NASC

Sample/NASC

A

0.01

0.001

0.0001

0.1

0.01

0.001

0.0001 La Ce Pr Nd PmSm Eu GdTbDy Y Ho Er TmYb Lu

C

La Ce Pr Nd PmSm Eu GdTbDy Y Ho Er TmYb Lu

D

0.1

10 95th percentile 75th percentile Median 25th percentile 5th percentile

Sample/NASC

1

Sample/NASC

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0.01

0.001

0.1 0.01 0.001

La Ce Pr Nd PmSm Eu GdTbDy Y Ho Er TmYb Lu

Ce/Ce* Eu/Eu* La/Sm La/Yb Gd/Yb Y/Ho

0.0001

0.0001

La Pr Sm Gd Dy Ho Tm Lu Ce Nd Eu Tb Y Er Yb

Fig. 2. Representative Y + REE profiles for surface waters in the Bathurst Mining Camp, normalized to the North America Shale Composite (NASC). (A) shows water samples with large negative Ce anomalies, (B) shows samples with moderately negative Ce anomalies and (C) waters with minor negative Ce anomalies. (D) represents a Box-and-Whisker plot of the surface waters; the box represents the 25th, 50th (notch), and 75th percentile of the data and the whiskers the 5th and 95th percentiles. Full REE + Y data for the surface waters are presented in electronic annex-1.

4.2. Stream sediments: total compositions Total Fe and Mn (i.e., Fe2O3T and MnO) concentrations in stream sediments exhibit similar spatial distribution patterns with high Fe and Mn in stream sediments from the northwestern part of the study area (i.e., near the Restigouche deposit), sediments from streams draining east and west of the deposit, and those draining Miramichi Group sedimentary rocks south of the Restigouche deposit. Stream sediments have variable total REE contents, ranging from 45 to 1031 ppm (average RREE = 240 ppm, n = 451, Y not included). The sediments typically have relatively flat REE + Y patterns when normalized to NASC, especially when compared to the LREE-depleted NASC-normalized REE + Y patterns of the stream waters (Figs. 2 and 3). Nonetheless, subtle fractionation of the REE in stream sediments is apparent from the NASC-normalized plots, including depletions in the LREE (e.g., average [La/ Sm]NASC of 0.778 (range = 0.352–1.12, n = 451; only 19 samples have values >1.00). Moreover, the majority of the stream sediments (>95%) are slightly enriched in the MREE, exhibiting concave-up NASC-normalized patterns (Fig. 3) with [Gd/Yb]NASC > 1.00 (average = 1.42, range = 0.55–3.75, n = 451; only 15 sediments have [Gd/ Yb]NASC < 1; Fig. 4 and Table 1). Most sediments are char-

acterized by negative Ce and Eu anomalies; [Ce/Ce*]NASC ranges from 0.097 to 2.12 (average = 0.799, n = 451) and [Eu/Eu*]NASC ranges from 0.39 to 1.43 (average = 0.802; Fig. 5). In addition, many of the stream sediments are substantially enriched (up to 10 times) relative to NASC. The overall enrichments in the REE in the stream sediments compared to NASC reflect, in part, the elevated REE concentrations of the host lithologies compared to upper continental crust (e.g., see Leybourne, 1998). 4.3. Stream sediments: partial extraction In general, REEs, Fe, and Mn concentrations of the partial extractions (i.e., labile fraction) of the stream sediments differ from those of the bulk stream sediment analyses (Figs. 6 and 7). The partial extractions of the stream sediments exhibit highly variable REE concentrations, albeit, typically lower than those of the bulk sediment. Specifically, RREE ranges from 0.81 to 333 ppm (average RREE = 58 ppm, n = 453) for the partial extracts of the sediments. The partial extractions are also commonly less depleted in the LREE compared to the bulk sediment analyses, with an average [La/Sm]NASC of 0.901 (range = 0.24– 3.31, n = 453; 25% of the partial extractions have [La/ Sm]NASC > 1.00). Similarly, the majority of the partial

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Table 1 Summary statistics for REE in sediments and waters from the BMC, New Brunswick, Canada [Ce/Ce*]NASC [Eu/Eu*]NASC [La/Sm]NASC [La/Yb]NASC [Gd/Yb]NASC [La/Pr]NASC [Gd/Dy]NASC [Y/Ho]NASC Total sediment Mean Standard deviation Minimum Maximum Number <1.00d

0.799 0.214 0.097 2.12 410

0.802 0.15 0.394 1.43 410

0.778 0.135 0.352 1.12 432

0.965 0.266 0.388 3.6 289

1.42 0.342 0.552 3.75 15

0.901 0.079 0.585 1.13 419

1.27 0.146 0.778 2.02 6

1.16 0.108 0.74 1.61 26

Partial sediment Mean Standard deviation Minimum Maximum Number <1.00d

0.681 0.393 0.051 3 384

0.744 0.145 0.046 1.32 435

0.901 0.363 0.241 3.31 337

1.33 0.844 0.274 9.16 151

1.97 0.501 0.765 6.28 3

1.03 0.251 0.494 2.32 271

1.36 0.138 1.03 2.09 0

1.39 0.286 0.73 2.77 20

Total/partiala Mean Standard deviation Minimum Maximum Number <1.00d

1.37 0.453 0.553 4.06 90

1.14 0.923 0.628 18.66 87

0.983 0.395 0.256 3.24 243

0.857 0.39 0.146 4.18 335

0.737 0.151 0.309 1.28 423

0.926 0.222 0.393 1.8 284

0.941 0.101 0.614 1.3 319

0.86 0.141 0.509 1.47 377

Total-partial Mean Standard deviation Minimum Maximum Number <1.00d

0.862 0.18 0.118 1.97 403

0.821 0.163 0.298 1.46 396

0.744 0.2 0.188 1.17 410

0.875 0.25 0.206 2.11 340

1.28 0.344 0.524 3.51 71

0.851 0.141 0.296 1.15 400

1.24 0.174 0.757 2.09 24

1.07 0.134 0.649 1.55 107

Water Mean Standard deviation Minimum Maximum Number <1.00d Number of samples

0.277 0.168 0.02 1.25 353 354

0.764 0.254 0.295 1.77 70 84

0.604 0.401 0.164 5.84 394 410

0.698 0.667 0.145 8.4 253 289

1.5 0.577 0.497 4.73 46 292

0.851 0.225 0.418 3.13 364 408

1.4 0.285 0.635 2.47 27 390

1.14 0.25 0.32 1.89 21 98

Partial/watera Mean Standard deviation Minimum Maximum Number <1.00d Number of pairs

3.21 1.98 0.28 16.79 10 308

1.01 0.297 0.338 2.13 30 59

1.58 0.648 0.246 4.45 44 352

1.99 0.79 0.267 5.49 9 241

1.38 0.341 0.607 2.68 24 242

1.22 0.27 0.554 2.33 56 347

0.999 0.19 0.533 1.6 172 332

1.3 0.425 0.744 3.39 6 65

Felsic and mafic (Restigouche deposit) rocksb Mean 0.902 0.496 Standard deviation 0.028 0.345 Minimum 0.848 0.031 Maximum 0.959 1

0.853 0.161 0.506 1

1.02 0.405 0.293 1

1.2 0.263 0.814 1

0.927 0.08 0.777 1

1.14 0.142 0.855 1

1.14 0.056 1.05 1

Metasedimentsc Mean Standard deviation Minimum Maximum

0.974 0.154 0.415 1.56

1.23 0.338 0.194 2.34

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extractions are MREE-enriched, exhibiting concave-up patterns with [Gd/Yb]NASC > 1 (Fig. 4 and Table 1). The partial extractions are generally more enriched in the MREE than the bulk sediment analyses (average = 1.97,

range = 0.77–6.28, n = 453, and only three samples have [Gd/Yb]NASC < 1.00) (Figs. 3, 4, and 6). Compared to the total stream sediments, Eu and Ce anomalies (both negative and positive) are also generally more pronounced in the

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La Pr Sm Gd Dy Ho Tm Lu Ce Nd Eu Tb Y Er Yb

Fig. 3. Representative NASC-normalized REE + Y profiles for total (i.e., bulk) stream sediments. (A) shows stream sediment (bulk or total analysis) samples with large negative Ce anomalies, (B) shows samples with minor negative Ce anomalies and (C) sediments with minor positive Ce anomalies. (D) represents a Box-and-Whisker plot of the bulk stream sediments: the box represents the 25th, 50th (notch), and 75th percentile of the data and the whiskers the 5th and 95th percentiles. Complete REE data for total (bulk) sediment analyses are presented in electronic annex-2.

partial sediment extracts with [Ce/Ce*]NASC ranging from 0.05 to 3.00 (average = 0.681, n = 453) and [Eu/Eu*]NASC ranging from 0.05 to 1.32 (average = 0.744; Fig. 5). The relationship between the total (bulk) sediment and partial-extraction data differs for different elements (Fig. 7), as shown in lognormal probability plots (Fig. 8). For example, Fe concentrations from total and partial extraction analyses follow essentially sub-parallel trends showing a relatively constant proportion of Fe (about 20%) extracted during the partial leach, regardless of Fe abundance. In contrast, Mn shows a more complex pattern; on average 90% of the Mn is extracted. Lower percentages of Mn are extracted from stream sediments with low and high total Mn concentration compared to sediments with intermediate total Mn concentrations (Fig. 8). Similar plots for individual REEs more closely resemble that for Fe than Mn (Fig. 8). 5. DISCUSSION 5.1. Comparisons of host lithologies The REE + Ys concentrations of felsic and mafic volcanic rocks from the Restigouche deposit, which were presented previously by Leybourne and Cousens (2005), are

typical of metavolcanic and metasedimentary rocks in the camp (Goodfellow et al., 2003; Rogers and Van Staal, 2003; Rogers et al., 2003). Felsic igneous rocks of the Restigouche deposit have relatively flat NASC-normalized REE + Y patterns, and hence, upper continental crustal REE + Y distributions, whereas the mafic rocks exhibit LREE-depleted, shale-normalized patterns and lower overall REE + Y abundances compared to the felsic volcanic rocks, suggestive of a depleted upper mantle source (Saunders, 1984; Leybourne and Cousens, 2005). Host lithologies from the Restigouche deposit exhibit small negative Ce anomalies ([Ce/Ce*]NASC varies from 0.85 to 0.96) and negative Eu anomalies ([Eu/Eu*]NASC = 0.103–0.693) (Table 1). In comparison, mafic dikes and massive sulfide deposits generally have positive, shale-normalized Eu anomalies (0.79 6 [Eu/Eu*]NASC 6 1.52 and 2.17 6 [Eu/Eu*]NASC 6 4.95, respectively). The positive Eu anomalies of the local massive sulfide deposits are typical of massive sulfides from modern settings where the Eu anomalies are thought to reflect the persistence of Eu2+ in the high temperatures, reducing ore-forming hydrothermal fluids (Klinkhammer et al., 1994a,b). Positive Eu anomalies are also characteristic of massive sulfides within the BMC and are thought to reflect feldspar alteration in the ore-generating, hydrothermal fluids (Leybourne et al., 2006b). The least altered

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Fig. 4. Bivariate plots of La/Sm versus Ce/Yb (A and B) and Gd/Yb (C and D) for surface waters, total stream sediment, partial extraction sediment, host felsic volcanic rocks and massive sulfides. Data are normalized to NASC (North America Shale Composite). (A and C) show the raw data, (B and D) show fields covering most data points except extreme outliers. Also shown are values for average metasediments from Goodfellow et al. (2003), volcanic rocks from Leybourne and Cousens (2005) and massive sulfides from Leybourne et al. (2006b).

(based on the CCPI (chlorite-carbonate-pyrite index) and AI (alteration index) (Ishikawa et al., 1976; Large et al., 2001) felsic units from the Restigouche deposit have average RREE = 336 ppm, [La/Yb]NASC = 1.16, [Gd/Yb][La/Sm]NASC = 0.88, and [Eu/Eu*]NASC NASC = 1.24, = 0.41. Two stockwork sulfide samples ([Eu/Eu*]NASC = 0.25 and 0.18) have strong negative Eu anomalies, in contrast to the massive sulfide samples (see Leybourne and Cousens, 2005). 5.2. Detrital versus hydromorphic components The stream sediment geochemistry reflects both the mechanical dispersion (erosion) of catchment host rocks and the hydromorphic (aqueous phase) transport and

transfer of chemical elements from the aqueous phase to the stream sediments (Leybourne et al., 2003). Similar arguments were recently advanced to explain the geochemistry of suspended sediments within groundwaters from the BMC (Leybourne, 2001; Leybourne and Cousens, 2005). Because the hydroxylamine hydrochloride leach only targets amorphous Fe and Mn oxyhydroxides and/or weakly adsorbed REE + Ys (Hall et al., 1996), its application, in conjunction with the total (i.e., bulk) sediment analysis provides a means of investigating the hydromorphic (labile) versus the detrital REE + Y components, respectively, in the stream sediments (Tricca et al., 1999; Leybourne et al., 2003). It should be noted that carbonate minerals are not expected to contribute substantially to the hydroxylamine hydrochloride leachates of the local stream

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[Ce/Ce*]NASC Fig. 5. Bivariate plots of Eu versus Ce anomalies for stream sediments, partial extractions, and waters from the BMC. Also shown are values for average metasediments from Goodfellow et al. (2003), volcanic rocks from Leybourne and Cousens (2005) and massive sulfides from Leybourne et al. (2006b). Ce anomalies calculated as: Ce/Ce* = CeNASC/(LaNASC*PrNASC)0.5 and Eu anomalies are calculated as defined in the caption for Fig. 1. (A) shows the raw data, (B) shows fields covering most data points except extreme outliers.

sediments because: (1) carbonate rocks (as mappable units) are absent from the study region: and (2) the inorganic carbon contents of all stream sediment samples examined (for total digestion) are at or below the detection limits (0.1 wt%) (Leybourne et al., 2003). Moreover, stream water samples are all undersaturated with respect to calcite (Leybourne, 1998), which along with the low inorganic carbon concentrations further supports that carbonate rocks do not significantly contribute to the dissolved or suspended load of streams within the study region. The hydroxylamine hydrochloride leach displays differential selectivity with respect to Fe and Mn in the stream sediments (Fig. 8). Specifically, approximately 20%, on

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average, of the Fe is extracted by the leach, whereas more than 80% of the Mn is extracted (Fig. 8). Hence, the bulk of the Mn in the stream sediments occurs in readily exchangeable (i.e., labile) forms, whereas only about 20% of the Fe associated with the steam sediments is labile. The leach tests therefore suggest that much of the Fe (80%) occurs within the crystalline structure of minerals within the stream sediments or as mature, crystalline Fe oxides (e.g., hematite, magnetite), whereas Mn is chiefly surface adsorbed, forms poorly crystalline Mn oxyhydroxide coatings on stream sediments (Braun et al., 1998; Tani et al., 2003), or is incorporated (i.e., co-precipitated) within the existing Fe oxyhydroxides mineral coatings (Leybourne, 2001). One possible conclusion is that Mn-oxyhydroxide coatings on the stream sediments may be more effective than Fe-oxyhydroxides in scavenging trace metals from solution at the Eh-pH conditions of most surface waters in this study region. Here, scavenging refers to either adsorption and/or co-precipitation of a trace element with the Fe/Mn oxides/oxyhydroxides. However, numerous studies clearly indicate that Fe oxides/oxyhydroxides strongly scavenge REEs and other trace elements from natural waters even when solution pH is less than the pHZPC of these Fe phases (Pokrovsky and Schott, 2002; Quinn et al., 2004, 2006a,b; Pokrovsky et al., 2005, 2006; Tang and Johannesson, 2005). Furthermore, the cumulative frequency plots for REEs (i.e., La and Yb are shown in Fig. 8) are more similar to that of Fe than to Mn, such that although a substantial fraction of the REEs in the stream sediment is labile (20–30%), a greater fraction of the REEs is contained within the crystalline structure of silicate minerals or crystalline metal oxides. Statistical analysis reveals that correlation coefficients between the partial extraction and total sediment compositions are greater for labile elements (mobilized by the hydroxylamine hydrochloride leach) compared to those chiefly contained within minerals of the bulk sediment (Table 2). In the case of REE + Ys, the strong positive correlations between the partial extracts and total sediment REE concentrations, which range from r = 0.682 for Eu to r = 0.846 for Er, further support that an important pool, although not all, of the REE + Ys exists in labile form in these stream sediments (e.g., associated with Fe-oxyhydroxides). The only exception is Ce, which has a weaker correlation, with r = 0.463. However, except for Ce, there is no statistically significant relationship between the REE + Y and Mn concentrations of the partial extract/leachates. The lack of correlation between the REE + Ys and Mn is also underscored by the cumulative plots of the partial extract and bulk sediment concentrations of Mn as compared to individual REEs (e.g., La and Yb; Fig. 8). Again, essentially all of the Mn occurs in readily exchangeable, labile form in these sediments, whereas REEs include both a labile (on average 21–29% of the REE, except Ce = 19% and Y = 31%) and non-labile pool (i.e., occur within the crystalline structure of minerals) within the stream sediments (Fig. 8). Consequently, the distribution of REEs between labile and non-labile phases in these stream sediments is more similar to that of Fe than to Mn

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Fig. 6. Representative NASC-normalized REE + Y profiles for the partial extraction (i.e., amorphous Fe oxyhydroxides; Hall et al., 1996) of the stream sediments. (A–C) represent the same samples as shown in Fig. 3. (D) represents a Box-and-Whisker plot of the bulk stream sediments waters; the box represents the 25th, 50th (notch), and 75th percentile of the data and the whiskers the 5th and 95th percentiles. Complete REE data for the partial extraction sediment analyses are presented in electronic annex-3.

(Fig. 8), suggesting that the readily exchangeable, labile REE pool is chiefly associated with Fe-oxyhydroxides and not Mn-oxyhydroxides. The only statistically significant relationship between a REE and Mn is that between MnO and Ce for the partial extractions, which has a Pearson Product Moment correlation of r = 0.135 (p < 0.001). By comparison, the Pearson Product Moment correlations for partial MnO and the other 13 naturally occurring lanthanides are all around 0.1, and thus not statistically significant. However, statistically significant correlations also exist between partial MnO and the Ce anomaly (Ce/Ce*) and Gd/Yb (r = 0.343; p 6 0.0001, and r = 0.449, p 6 0.0001, respectively). The relationship between Mn, Ce, and the Ce anomaly may reflect microbial and/or abiotic oxidation of Mn(II) and Ce(III), followed by precipitation of Mn(IV) and Ce(IV) oxides, or scavenging of Ce(IV) by precipitating Mn(IV) oxides (Moffett, 1990; De Carlo et al., 1998). Further study of Ce and Mn biogeochemistry in these BMC streams and associated sediments is necessary, however, to sort out the specific processes responsible for these correlations. 5.3. REE fractionation patterns Many of the stream waters and especially the sediments exhibit slight, shale-normalized enrichments in the MREEs relative to the HREEs and, in particular, the LREEs

(Figs. 2, 3, and 6 and Table 1). Recent work suggests that similar REE fractionation patterns in stream and soil waters are primarily the result of apatite weathering, whereas MREE enrichments and negative Ce anomalies in suspended river sediments may reflect an important organic matter component in the sediments (Leleyter et al., 1999; Tricca et al., 1999; Aubert et al., 2001). These arguments for the aqueous phase are further supported by leaching experiments involving the river sediments and acidic solutions (Hannigan and Sholkovitz, 2001). Biogenic apatite commonly exhibits enrichments in the MREE relative to both the LREE and HREE when normalized to shale composites (Wright et al., 1984; Wright et al., 1987; Grandjean and Albare`de, 1989; Grandjean-Le´cuyer et al., 1993; Kemp and Trueman, 2003). In addition, although igneous apatite is typically strongly enriched in the LREE relative to the HREE when normalized to upper mantle proxies such as chondrites, when normalized to upper continental crust proxies like shale composites, it also commonly exhibits enrichments in the MREE. For example, [La/Sm]NASC and [Gd/Yb]NASC ratios for apatite from continental igneous rocks are reported to range between 0.05 to 0.09 and 1.4 to 73, respectively (Hanson, 1980; Gromet and Silver, 1983; Hermann, 2002). Even larger MREE enrichments are reported for apatite from supracrustal rocks of the Archean Isua Belt in southern Greenland (Lepland et al., 2002). If apatite solubility is low in circumneutral

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Fig. 7. Representative REE + Y profiles for the ratio of the REE + Y concentrations for total (bulk) sediment to the partial extractions. (A– C) represent the same samples as shown in Fig. 3. (D) represents a Box-and-Whisker plot of the bulk stream sediments waters; the box represents the 25th, 50th (notch), and 75th percentile of the data and the whiskers the 5th and 95th percentiles.

pH waters (Taunton et al., 2000b), its dissolution rate becomes significant as pH decreases < 7 (Chaı¨rat et al., 2007). As noted above and in previous studies of the Bathurst Mining Camp, although most predominantly of circumneutral pH, a number of surface and groundwaters from the region are acidic (pH < 6; Leybourne et al., 1998, 2000, 2006b; Leybourne and Cousens, 2005). Moreover, apatite dissolution is facilitated in the presence of organic ligands and microbes that produce organic acids such as pyruvate, oxalate, and various fermentation products (Taunton et al., 2000a,b; Welch et al., 2002). Thus, microbially facilitated chemical weathering of apatite, or apatite weathering by acidic waters, within the BMC could potentially impart enrichments in the MREEs to the weathering solutions, hence, to the local surface waters (Hannigan and Sholkovitz, 2001; Ko¨hler et al., 2005). Secondary LREE bearing REE phosphate phases have been identified by SEM in suspended sediments from groundwaters in the BMC (Leybourne, 2001), suggesting that apatite weathering followed by precipitation of secondary REE phosphates (e.g., florencite, rhabdophane) may be an important process occurring with the study area (Banfield and Eggleton, 1987; Taunton et al., 2000a). Formation of secondary REE phosphate replacements of apatite is expected to enrich the weathering solutions in the MREEs relative to the LREEs (Ko¨hler et al., 2005). Under such a scenario, it is reasonable to subsequently expect the weather-

ing solutions to impart similar MREE enrichments to the labile REE fraction of the stream sediments. Comparison of the REE patterns for the total sediment digests and those for the partial extracts reveals that the total sediments are typically enriched in the LREEs and HREEs relative to the MREEs compared to the partial extracts (Figs. 3, 6, and 7). In other words, the partial extracts, which chiefly reflect the labile, amorphous Fe/Mn oxyhydroxides within the sediments, are enriched in the MREEs and depleted in the LREEs and HREEs relative to the total sediment digests. Consequently, in contrast to the conclusions of Leleyter et al. (1999), who suggest that scavenging of REEs in rivers by sediment organic matter chiefly removes MREEs from solution, our data indicate that at least in the case for these New Brunswick streams, amorphous Fe/Mn oxyhydroxides within the stream sediments are an important pool of labile MREEs. Hence, the slight MREE enrichments of NASC-normalized REE patterns for some of the BMC stream waters (Fig. 2) likely reflect buffering of the aqueous REE pool by this labile pool of MREEs in the stream sediments. It is important to note, however, that because we filtered the stream water samples through 0.45 lm pore-size filters, we cannot rule out the possibility that MREE-enriched, organic colloids also contribute to the small MREE enrichments of the stream waters (e.g., see Elderfield et al., 1990; Sholkovitz, 1995).

M.I. Leybourne, K.H. Johannesson / Geochimica et Cosmochimica Acta 72 (2008) 5962–5983

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Fig. 8. Log-normal probability plots of total and partial extraction stream sediments for Fe2O3T, MnO, and selected REE. Also shown are probability plots for both sediment analyses and waters for NASC-normalized REE ratios. For the element plots (A–D), increasing separation between the total and partial analyses indicates a smaller proportion of that species is labile. As total La increases for example, the labile proportion increases. The REE ratio plots (E–H) represent the relative fractionation of the REE between the three pools. For example, for [Ce/Ce*]NASC, surface waters in all cases have larger negative anomalies than the sediments. Most partial extractions have larger negative Ce anomalies than the total sediments.

Table 2 Pearson product–moment correlation between total and partial sediments

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5.4. Europium anomalies

Values that are statistically insignificant at the 99.99% (p < 0.0001) confidence interval (between 0.121 and 0.121) are in italics. Shaded values are simply where the element via total dissolution is the same as by partial leach.

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Because apatite is not commonly enriched in Eu relative to its nearest neighbors in the REE series (i.e., Sm and Gd), it is unlikely that the positive Eu anomalies reported here for some stream sediments and surface waters can be explained by either detrital apatite or chemical weathering of apatite. Nonetheless, it should be noted that Kidder et al. (2003) report positive Eu anomalies for the rims of phosphatic concretions recovered from black shales, which they interpreted to have formed under intense reducing conditions. Thus, an apatite weathering origin for some of the reported Eu anomalies of the stream sediments probably cannot be entirely dismissed. Yan et al. (1999) showed that during a sequential leaching experiment of glacial till samples, the majority of the REEs for a variety of sediment size fractions were hosted in residual minerals (those not dissolved by the sequential leaches) and, further, that these residual minerals exhibit large positive, shale-normalized Eu anomalies. Similarly, weathering of massive sulfides, even under low pH (<3) and oxidizing conditions, did not diminish the large positive Eu anomalies that characterize the massive sulfide deposits in our study region (Leybourne et al., 2006b). Intermediate and felsic volcanic rocks, including those of the study region, generally possess substantial negative Eu anomalies owing to crystal fractionation of feldspars in the melt, whereas mafic rocks typically have smaller or even lack negative Eu anomalies (Hanson, 1980; Saunders, 1984). Thus, one possible explanation for the positive Eu anomalies reported here for some of the stream sediments is preferential mobilization of Eu during weathering of BMC host lithologies (Leybourne and Cousens, 2005; Leybourne et al., 2006b). However, because gossans and massive sulfides are commonly enriched in Eu relative to Gd and Sm when normalized to NASC (Leybourne et al., 2006b), an alternative and more likely hypothesis for the positive Eu anomalies of some stream sediments is that they reflect the contribution of weathering of the gossans and/or massive sulfides. Bau et al. (2004) showed that groundwaters interacting with quartz-rich sediments that also contain plagioclase, K-feldspar and glauconite, largely appear to derive their REE budget from weathering of glauconite, rather than plagioclase or other feldspars. These conclusions are chiefly based on the absence of positive Eu anomalies in the groundwaters, as well as Sr isotope considerations (Bau et al., 2004). Oliva et al. (2004) suggested, based on Ca/Na ratios and Sr isotopic compositions, that surface waters in small granitic catchments are controlled in part by dissolution of Ca-rich phases other than plagioclase (i.e., epidote, prehnite, sphene, apatite). Moreover, geochemical modeling of the major ion composition of surface waters from the vicinity of the Halfmile Lake deposit also supports silicate weathering and dissolution of trace calcite as the principal sources of dissolved solutes in these waters (Leybourne and Cousens, 2005). Furthermore, the Sr isotope compositions of the surface waters in the BMC are consistent with mixing between a mafic igneous rock source (± carbonates) containing low Rb/Sr minerals, such as

Fractionation and speciation of REE + Y in small catchment streams

plagioclase and epidote, and the local felsic rocks (Leybourne and Cousens, 2005). The fact that the stream sediments (and the partial extractions) have small to no negative Eu anomalies compared to most host rocks implies that: (1) Eu is conveyed to the sediments via preferential aqueous transport; (2) the phase(s) that are releasing REEs from the host rocks do not possess a Eu anomaly; or (3) Eu is retained in clay minerals during weathering of host rocks. Inspection of the Eu anomaly versus Ce anomaly plot (Fig. 5) for the surface waters, however, suggests that Eu is relatively more mobile than the other REE (see also Fig. 9 below). Stream

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sediments with large positive Eu anomalies are primarily associated with rocks in the northern part of the study area as well as with the massive sulfide mineralization (Fig. 1). Specifically, stream sediments down stream from the Murray Brook deposit have positive Eu anomalies, likely inherited from the massive sulfides and/or gossan. In contrast, down stream of the Restigouche deposit, stream sediments lack positive Eu anomalies because the associated gossans have negative Eu anomalies. The negative Eu anomalies of the Restigouche deposit gossan likely reflects chemical reworking of the gossan compared to other massive sulfide deposits in the camp (Leybourne et al., 2006b). Comparing the Eu anomaly of the partial extracts to that of the corresponding total sediment analysis provides a means for evaluating the relative mobility of Eu in the local environment (Fig. 9). For example, if the ratio of the Eu anomaly in the partial extracts to the Eu anomaly in the total sediment digests is <1 for sediments where the total sediment digest also has a [Eu/Eu*]NASC < 1, the ratio indicates that the partial extract has a more negative Eu anomaly than the total sediment (Fig. 9). The implication for these samples (the majority of the samples) is that Eu was either less mobile than Sm and Gd during hydromorphic transport, or that Eu has been preferentially sequestered into a different phase than Sm and Gd and is not extracted by the hydroxylamine leach. In contrast, for samples where the ratio of the Eu anomaly of the partial leach to that of the total sediment digest is >1, but total sediment [Eu/Eu*]NASC < 1, the ratio indicates that Eu is preferentially mobilized compared to Sm and Gd. Leybourne and Cousens (2005) suggested preferential mobility of Eu for groundwaters at the Restigouche deposit. They showed, using the REE contents of groundwaters and associated suspended sediments that although these samples typically possessed negative NASC-normalized Eu anomalies, the anomalies were smaller than those exhibited by the host rocks, that is, normalized to least-altered rocks, groundwaters typically possess strong positive Eu anomalies (Leybourne and Cousens, 2005). There are at least two possible explanations to account for the relative positive Eu anomalies in suspended sediments from the mineralized zone. First, Eu is relatively mobile and is readily transferred into solution and subsequently to the suspended sediment. Alternatively, rocks proximal to zones of mineralization are relatively enriched in Eu such that sediment produced from weathering of these rocks also exhibit positive Eu anomalies relative to unaltered rocks in the study region. Compared to unaltered felsic rocks, Eu is preferentially enriched in the groundwaters suggesting that either feldspar weathering is important (Middelburg et al., 1988) and/or that Eu2+ is preferentially mobilized during water–mineral reaction compared to the trivalent REE (Sverjensky, 1984; Bau, 1991). Hopf (1993) noted that, in general, alteration of fresh rhyolite by acidic or alkaline fluids resulted in positive Eu anomalies if the altered rocks were normalized to the unaltered precursor. However, Lentz and Goodfellow (1993) demonstrated that moderately hydrothermally altered rocks from the BMC are depleted in Eu, except for the most intensely altered rocks and the massive sulfides. With the exception of

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carbonate-rich samples and massive sulfides, the more altered rocks in this study typically have negative Eu anomalies when normalized to least altered equivalents. Consequently, the positive Eu anomalies within BMC stream waters and associated sediments likely reflect weathering reactions involving the Eu enriched massive sulfide deposits, as well as preferential mobilization of Eu2+ relative to the more particle-reactive trivalent REEs. 5.5. Cerium anomalies Although the presence or absence of Eu anomalies for the stream waters and associated sediments likely reflects both the relative mobility of Eu as well as a host-rock signal inherited from the positive Eu anomalies of the local massive sulfide deposits, the ubiquitous negative Ce anomalies of the stream waters, which is absent in the local host lithologies, suggests a more complicated origin. The mean (± SD) Ce anomaly (i.e., [Ce/Ce*]NASC) for 451 stream sediment samples from the BMC is 0.799 ± 0.214, which indicates that, on average, Ce in the stream sediments is only marginally fractionated relative to the upper crust or the local host lithologies ([Ce/Ce*]NASC = 0.902 for felsic volcanic rocks and 1.01 for metasedimentary rocks; Table 1). Thus, in general, the stream sediments represent mechanically dispersed host rocks, associated minerals, and weathered residuals. Previous XRD, XRF, and SEM analysis of suspended sediment collected from groundwaters from the BMC indicate a predominance Fe oxide/oxyhydroxidecoated quartz and alumino-silicate minerals, chiefly illite and white mica (± chamosite ± kaolinite; Leybourne, 2001). Nevertheless, a number of these sediments are depleted in Ce relative to La and Pr (Fig. 3). The presence of stream sediments with more negative Ce anomalies than the host metavolcanic and metasedimentary rocks, suggests that other processes in addition to mechanical dispersion have also acted on these sediments. Investigations of the chemical weathering of granitic rocks under oxic conditions indicate that REE-rich accessory phases such as allanite, and perhaps apatite, are vulnerable to dissolution by acidic weathering solutions and that REEs released by these dissolution reactions are commonly reprecipitated as secondary LREE-rich phosphates like florencite and/or rhabdophane (Banfield and Eggleton, 1987; Taunton et al., 2000a,b). As mentioned above, LREE-bearing REE phosphate phases have been recognized in suspended sediments collected from groundwaters of the BMC (Leybourne, 2001). Dissolution of these primary igneous minerals, as well as the formation and subsequent dissolution of the secondary REE phosphates, are likely mediated by microbes by, for example, uptake of phosphate ions from solution, production of organic acids or enzymes that specifically target phosphate by metabolic processes, and/or complexation of REE and Al by organic ligands produced by microbes (Taunton et al., 2000a,b; Brantley et al., 2001; Welch et al., 2002). Stabilization of REE in aqueous weathering solutions as inorganic of organic complexes is expected to facilitate REE transport from the site of active weathering with the percolating fluids (Nesbitt, 1979; Duddy, 1980; Schau and Henderson, 1983;

Braun et al., 1993; Benedict et al., 1997; Brantley et al., 2001). The combined effect of these processes is to mobilize REE in the weathering profile relative to Ce, which is sequestered in essentially insoluble Ce(IV)-rich oxides such as cerianite, CeO2 (Banfield and Eggleton, 1987; Taunton et al., 2000b). Similar observations have been reported in other studies of the geochemical behavior of the REE during chemical weathering (Braun et al., 1990, 1998; Koppi et al., 1996; Nesbitt and Markovics, 1997). The resulting Ce-rich phases are subsequently retained in the weathering profile (Banfield and Eggleton, 1987; Braun et al., 1990; Taunton et al., 2000b) or subjected to limited physical transport in, for example, streams owing to the relatively high density of these heavy minerals (Goldstein and Jacobsen, 1988a; Land et al., 1999). Consequently, one possible explanation for the negative Ce anomalies for some of the stream sediments from the BMC is chemical fractionation of Ce from the other REE during oxic, chemical weathering of the host lithologies followed by the ensuing physical fractionation owing to the limited transport of the resulting heavy Ce-rich oxides. However, this hypothesis cannot be confirmed with our current dataset, and requires more detailed investigations of the stream sediments. The REE profiles of the hydroxylamine hydrochloride leach of BMC stream sediments differ from those of Yan et al. (1999), who performed a similar leach on glacial (i.e., chiefly clay-rich till) sediments collected at depth from a 170 m borehole in Saskatchewan, Canada. The leachates from the clay-rich till samples are enriched in the MREE but do not exhibit negative Ce anomalies. The differences in the REE profiles of the BMC stream sediment leachates and those of the clay-rich glacial till samples of Yan et al. (1999) could reflect, in part, the proposed physical separation of heavy Ce(IV)-rich oxide minerals during transport of the weathering products in BMC streams, as well as the greater hydromorphic REE component of these stream sediments as compared to the clay-rich tills of Saskatchewan. More specifically, REE complexed with strong ligands (e.g., CO32 , organic ligands) in weathering solutions and/ or stream waters are fractionated from the sediment REE pool and subsequently advectively transported by stream waters, whereas chemical diffusion appears to control REE transport in these glacial tills (Johannesson and Hendry, 2000). Groundwater flow velocities are exceedingly slow (0.08 mm/year) in the clay-rich tills, and consequently, diffusive transport processes dominate solute transport (Hendry and Wassenaar, 1999). Although microbially facilitated chemical weathering of crustal rocks can fractionate the REEs, including Ce (Banfield and Eggleton, 1987; Taunton et al., 2000a,b), abiotic processes also appear to drive Ce oxidation reactions and subsequent fractionation of Ce from the REE series. For example, De Carlo et al. (1998) conducted batch experiments using laboratory prepared vernadite (d-MnO2) and amorphous FeOOH at different solution ionic strengths (0, 0.1, and 0.7 molal NaNO3) to investigate REE adsorption and Ce fractionation. These researchers employed cyclic voltammetry using a d-MnO2-coated, Pt electrode to study Ce uptake over a range of electrochemical potentials spanning that of the Ce(III)/Ce(IV) redox couple. Their

Fractionation and speciation of REE + Y in small catchment streams

results strongly suggest that Ce(III) is oxidized by Mn(IV) at d-MnO2 surfaces in the absence of microbes, followed by Ce(IV) adsorption onto d-MnO2. Similarly, Bau (1999) demonstrated that for pH < 5, Ce adsorption onto FeOOH can lead to Ce(III) oxidation and formation of negative Ce anomalies. More specifically, Bau (1999) showed that appKD (where appKD = [REE+Y]Fe oxyhydroxide/[REE+Y]solution) for FeOOH and the REE were greatest for Ce, but only at low pH < 5; experiments at higher pH showed no development of a positive Ce anomaly, although appKD for La become progressively lower relative to Ce and Pr with increasing pH. The rapid rate of Ce(III) oxidation is these Fe-oxyhydroxide experiments and those for d-MnO2, compared to in situ estimates of Ce(III) oxidation rates in seawater (Moffett, 1990) led De Carlo et al. (1998) to argue that Ce(III) oxidation in the oceans may, in part, be abiotically driven. Nonetheless, in situ studies clearly indicate that Ce(III) oxidation is also microbially mediated in the marine environment (Moffett, 1990, 1994a,b). Furthermore, Ohta and Kawabe (2001) suggested, based on experimental precipitation reactions between d-MnO2 and REEs and FeOOH and REEs, that Ce anomalies and LREE fractionation were greater for REE adsorption to Mn- compared to Fe-oxide/oxyhydroxides. These workers also compare their results of Ce(III) oxidation at MnO2 surfaces with the results of Bau (1999). The fact that the majority of the partial extraction-water pairs for sediments and associated stream waters of the BMC have appKD (where (Ce/La) appKD = [REE+Y]partial extract/[REE+Y]solution) > 1, with most pH values >6, is consistent with d-MnO2 being a more important sink for Ce in the stream sediments than Fe(III) oxides/oxyhydroxides. However, close inspection of the total sediment digest REE concentrations for BMC stream sediments normalized to the corresponding partial extract REE concentrations reveal positive Ce anomalies (Fig. 7). These positive Ce anomalies indicate that some sink other than the labile, amorphous Fe/Mn oxyhydroxides in the BMC stream sediments is capturing the bulk of the aqueous, and presumably oxidized Ce from the stream waters. For example, it has been suggested by numerous researchers that organic matter in stream waters and/or sediments is an important sink for Ce (Leleyter et al., 1999; Gruau et al., 2004; Davranche et al., 2008). Moreover, others report that DOC-rich waters typically do not exhibit negative Ce anomalies, presumably due to strong complexing of the REEs, including Ce3+ and Ce4+, by organic ligands, whereas water with low DOC concentrations commonly possess negative Ce anomalies (Dia et al., 2000; Tang and Johannesson, 2003; Gruau et al., 2004; Johannesson et al., 2004; Davranche et al., 2008). Because the lack of Ce anomalies in DOC-rich waters suggests that both Ce3+ and Ce4+ are strongly bound by organic matter, it is reasonable to expect that sediment organic matter may also preferentially scavenge Ce from solution (Leleyter et al., 1999; Davranche et al., 2005). It should be noted, however, that some of the NASCnormalized REE patterns of the partial extracts exhibit positive Ce anomalies (Fig. 6). These same samples have negative Ce anomalies when the total sediment REE concentrations are normalized to the partial extract concentra-

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tions (Fig. 7). Therefore, it would appear that for some of the BMC stream sediments, Fe/Mn oxyhydroxides do contain oxidized Ce scavenged from the ‘‘dissolved” pool. Nonetheless, the majority of our data suggest that another phase besides the Fe/Mn oxyhydroxides (e.g., organic matter) is a stronger scavenger of Ce from the stream waters than these oxyhydroxides. Future investigations should involve closer examination of the REEs in the dissolved, colloidal, and sedimentary organic matter (e.g., biofilms) fractions of these BMC streams. 6. CONCLUSIONS Analysis of roughly 500 stream water samples and their associated bed-load sediments from clear, headwater streams of the Bathurst Mining Camp region in northern New Brunswick, Canada, reveal that the REE + Y can be separated into at least three distinct pools. These include the ‘‘dissolved” (<0.45 lm) fractions, which includes both truly dissolved and colloidal, the labile (hydroxylamine) fraction, and the detrital (total sediment) fraction. These results are broadly in agreement with a number of previous investigations (e.g., Elderfield et al., 1990; Sholkovitz, 1995; Viers et al., 1997; Dupre´ et al., 1999; Pokrovsky and Schott, 2002; Tosiani et al., 2004). However, unlike the majority of these studies, which focused on large rivers and/or rivers draining tropical or boreal environments, our study involved small, headwater streams from a temperate/ maritime climate. Stream waters have LREE depleted, NASC-normalized REE patterns, with substantial negative Ce anomalies and small enrichments in the MREEs relative to the HREEs. The NASC-normalized REE patterns for the total stream sediment digests are generally flat with minor MREE enrichments and exhibit either no Ce anomalies or small negative Ce anomalies. The partial extraction of the stream sediment are commonly more MREE enriched than the total sediment digest and have more pronounced negative Ce and positive Eu anomalies. The data suggest that the two primary sources for the REEs + Y in the streams sediments from the BMC are: (1) mechanical dispersion from local felsic volcanic rocks; and (2) volcaniclastic sediments and water-transported (hydromorphic or labile fraction) REEs + Y that have been partitioned into the sediment phase through adsorption or precipitation. Furthermore, Eu appears to be more mobile than the other REEs, whereas Ce is preferentially removed from solution and accumulates in the stream sediments in a less labile form than the other REE + Y. Despite poor statistical correlations between the REE + Y and Mn in either the total sediment or partial extractions, based on apparent distribution coefficients and the pH of the stream waters, it is most likely that d-MnO2 is the dominant sink for Ce, and to a lesser extent the other REE, in the stream sediments. ACKNOWLEDGMENTS The fieldwork was funded by the Geological Survey of Canada, as part of the EXTECH-II (EXploration TECHnology) program. We thank Wayne Goodfellow in particular for providing the funding for this project. Peter Belanger is thanked for overseeing the geochemical analyses. Gwendy Hall and Judy Vaive are thanked

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for advice on the partial extraction technique. Graeme Phipps is thanked for performing the partial extractions. Mal Gilliss, Chris Turcotte, and Geoff Allaby assisted with collection of the 210-series samples and Craig Scott with some of the MLW series samples. Jan Peter, Wayne Goodfellow, and Dave Lentz are thanked for discussions on the geology of the Bathurst Mining Camp during the course of the project. Very special thanks to our friend and colleague Dan Boyle, who died in 2000, for advice and encouragement throughout the project. Comments from journal reviewers D. Aubert and J. J. Braun, as well as those of the associate editor, J. Schott, helped improve the original manuscript, for which we are very grateful.

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