Layton-Matthews et al 2013 Selenium isotopes Finlayson Lake VMS

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Geochimica et Cosmochimica Acta 117 (2013) 313–331

Multiple sources of selenium in ancient seafloorhydrothermal systems: Compositional and Se, S, and Pb

isotopic evidence from volcanic-hosted andvolcanic-sediment-hosted massive sulfide depositsof the Finlayson Lake District, Yukon, Canada

Daniel Layton-Matthews f,⇑, Matthew I. Leybourne b, Jan M. Peter c, Steven D. Scott a,Brian Cousens d, Bruce M. Eglington e

a Department of Geology, University of Toronto, 22 Russell St., Toronto, Ontario, Canada M5S 3B1b ALS Geochemistry, 2103 Dollarton Highway, North Vancouver, British Columbia, Canada V7H 0A7

c Central Canada Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8d Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S5B6

e Department of Geological Sciences, University of Saskatchewan, Saskatchewan Isotope Laboratory, 114 Science Place, Saskatoon, Canada

SK S7N 5E2f Queen’s University, Department of Geological Sciences and Geological Engineering, Kingston, Ontario, Canada KL7 3N6

Received 18 October 2012; accepted in revised form 1 May 2013; Available online 9 May 2013

Abstract

Volcanic-hosted massive sulfide (VHMS) and volcanic-sediment-hosted massive sulfide (VSHMS; i.e., hosted by both vol-canic and sedimentary rocks) deposits in the Finlayson Lake District, Yukon, Canada, provide a unique opportunity to studythe influence of seafloor and sub-seafloor hydrothermal processes on the formation of Se-poor (GP4F VHMS deposit; 7 ppmSe average), intermediate (Kudz Ze Kayah—KZK VHMS deposit; 200 ppm Se average), and Se-enriched (Wolverine VSHMSdeposit; 1100 ppm Se average) mineralization. All three deposits are hosted by mid-Paleozoic (�360–346 Ma) felsic volcanicrocks, but only the Wolverine deposit has voluminous coeval carbonaceous argillites (black shales) in the host rock package.Here we report the first application of Se isotope analyses to ancient seafloor mineralization and use these data, in conjunctionwith Pb and S isotope analyses, to better understand the source(s) and depositional process(es) of Se within VHMS andVSHMS systems. The wide range of d82Se (�10.2& to 1.3&, relative to NIST 3149), d34S (+2.0& to +12.8& CDT), andelevated Se contents (up to 5865 ppm) within the Wolverine deposit contrast with the narrower range of d82Se (�3.8& to�0.5&), d34S (9.8& to 13.0&), and lower Se contents (200 ppm average) of the KZK deposit. The Wolverine and KZKdeposits have similar sulfide depositional histories (i.e., deposition at the seafloor, with concomitant zone refining). The Sein the KZK deposit is magmatic (leaching or degassing) in origin, whereas the Wolverine deposit requires an additional largeisotopically negative Se source (i.e. ��15& d82Se). The negative d82Se values for the Wolverine deposit are at the extremelight end for measured terrestrial samples, and the lightest observed for hypogene sulfide minerals, but are within calculatedequilibrium values of d82Se relative to NIST 3149 (�30& at 25 �C between SeO4 and Se2�). We propose that the most neg-ative Se isotope values at the Wolverine deposit record the d82Se of the Se-source, and that the wide range in d82Se valuesresults from the combined effects of thermal and chemical degradation and Se-loss from the carbonaceous argillite sourceto a hydrothermal fluid (including magmatic Se i.e., leached and/or magmatic-hydrothermal) with deposition at or nearthe paleoseafloor. Pristine unaltered black shales show little variation in d82Se relative to bulk earth; Se accumulation and

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⇑ Corresponding author.E-mail address: [email protected] (D. Layton-Matthews).


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314 D. Layton-Matthews et al. / Geochimica et Cosmochimica Acta 117 (2013) 313–331

fractionation to more negative isotopic values is interpreted to have been produced by post-sediment deposition, but pre-orestage, reduction of seawater Se within the black shales.� 2013 Elsevier Ltd. All rights reserved.

4 6 8pH

Log

a O

2(aq

)

SO42-

HS-

-20

-50

-40

-30

Log

a O

2(aq

)

-20

-50

-40

-30

HSe-

HSe-HH22Se(aq)Se(aq)

HH22Se(aq)S(aq)

HSeOHSeO33--

200 °C

300 °C 150 °CVolcanogenic Fluid

HSO4-

HH22 (aq)(aq)SeOSeO33

A

B

H2OH2

H2OH2

Fig. 1. Plot of pH vs. log aO2 for major aqueous S and Se speciesunder hydrothermal conditions. (A) Variation of major aqueoussulfur phases at 300, 200, and 150 �C, RS = 0.01 m. (B) Variationof major aqueous selenium species at 300, 200, and 150 �C,RSe = 10�6 m. Note: native selenium is not stable in metal-richfluids at temperatures above 150 �C (Mishra et al., 2011). Hatchedpattern denotes VHMS-forming fluid after Huston et al. (1995).

1. INTRODUCTION

Sulfide ore deposits, particularly those formed fromhydrothermal fluids are among the largest point-sourcereservoirs of selenium on Earth, an element whose geo-chemical behavior is poorly understood. Selenium in sul-fide deposits is typically heterogeneously distributed bothwithin a single ore deposit and within a volcanic-hostedmassive sulfide (VHMS) deposit type (Huston et al.,1995; Hannington et al., 1999a; Layton-Matthews, 2006;Layton-Matthews et al., 2008). Transport of Se withinhydrothermal ore-systems and its subsequent deposition,although poorly understood, is largely related to the sele-nium activity, RS/RSe, pH, temperature and aO2 of thehydrothermal fluid (Huston et al., 1995; Simon and Es-sene, 1996). Selenium transport and deposition followsthe general sulfide–selenide reaction: MeSe2 + H2S(aq) = -MeS2 + H2Se(aq), where Me is a transition metal, forexample Pb in the solid solution between galena (PbS)and clausthalite (PbSe).

Selenide minerals, or Se-rich sulfide end-members of sul-fide–selenide solid solutions, are preferentially and thermo-dynamically predicted to form at higher temperaturesrather than sulfide or Se-poor sulfide end-members(D’Yachkova and Khodakovskiy, 1968). This higher tem-perature stability of Se minerals is consistent with observa-tions of Se-bearing mineralization within modern seafloorhydrothermal vent chimneys (Auclair et al., 1987; Rouxelet al., 2002, 2004), modern sulfide mounds (Hanningtonet al., 1999a), and ancient volcanic-hosted massive sulfide(VHMS) deposits (Huston et al., 1995; Hannington et al.,1999a; Layton-Matthews et al., 2008).

Within modern and ancient seafloor volcanic-dominatedhydrothermal systems, Se may be derived from seawater orleaching of igneous rocks and/or magma degassing. Seawa-ter typically contains low concentrations of Se (170 ng/kg)that may be present as dissolved SeVI, SeIV, Se�II (Fig. 1),or as Se incorporated within organic matter, dependingon the redox conditions (Measures and Burton, 1980). Inanoxic basins, bottom waters and associated anoxic sedi-ments have elevated Se contents (Kluckhohn et al., 1990;Takematsu et al., 1990; Martens and Suarez, 1997; Mer-cone et al., 1999) controlled by selenium reduction withinthe anoxic water column and below the seawater–sedimentinterface. Magmatic sources, such as shallow-level magmasand their derivative rocks in VHMS systems, have low con-tents of selenium (<2 ppm) that nevertheless may contrib-ute Se by hydrothermal fluid leaching or bycontemporaneous magma degassing to a hydrothermal sys-tem, assuming a behavior similar to that of sulfur (e.g.,Hattori, 1993; Sladek and Gustin, 2003; de Ronde et al.,2005).

In this study, we compare three ancient seafloor VHMS(Kudz Ze Kayah—KZK and GP4F) and volcanic-

sediment-hosted massive sulfide (VSHMS; Wolverine)deposits of similar age recently discovered in the FinlaysonLake District, Yukon, Canada. These comprise Se enriched(Wolverine deposit), intermediate Se (KZK deposit), andSe-poor (GP4F deposit) deposits that display variations inSe content at the mineral, mineralization-type and depos-it-type scales (Boulton, 2002; Layton-Matthews, 2006;

D. Layton-Matthews et al. / Geochimica et Cosmochimica Acta 117 (2013) 313–331 315

Bradshaw et al., 2008; Layton-Matthews et al., 2008).Using recent data on the natural selenium isotope variationin modern seafloor hydrothermal sulfide mineralization(Rouxel et al., 2002, 2004) together with Se, S and Pb iso-tope data and bulk Se contents for these deposits, we dis-cuss four possible combined concentration processes andsources for selenium in the Finlayson Lake District depositsand discuss their implications for modern and ancient sea-floor hydrothermal systems.

2. GEOLOGICAL SETTING

The Finlayson Lake District VHMS and VSHMSdeposits occur within the Yukon Tanana Terrane (YTT),a package of poly-deformed metamorphosed sedimentary,volcanic and plutonic rocks that are the products of Mid-Paleozoic continental arc magmatism (Tempelman-Kluit,1979; Mortensen and Jilson, 1985; Mortensen, 1992)(Fig. 2). Syngenetic mineralization occurs in several settingsin the YTT of the Yukon and eastern Alaska (e.g., Hunt,1998; Peter et al., 2007). The Finlayson Lake District is aregionally extensive area of Devonian–Mississippian volca-nic, intrusive, and sedimentary rocks approximately 300 kmlong and 50 km wide, extending from Ross River in thenorth to Watson Lake in the south. The Finlayson Lake

Fig. 2. Tectonic map of the Yukon Territory showing the location ofGordey and Makepeace, (2000).

District is juxtaposed against Proterozoic and other Paleo-zoic strata of the ancient North American continental mar-gin along the Tintina Fault Zone to the southwest andalong the Finlayson Lake Fault Zone to the northeast(Mortensen and Jilson, 1985; Plint and Gordon, 1996,1997; Tempelman-Kluit, 1979) (Fig. 2). The main part ofthe YTT, which underlies most of west-central Yukon, iscontiguous with the Finlayson Lake District portion, afterrestoration of 425 km of Late Cretaceous right-lateral,strike-slip movement on the Tintina Fault (Roddick,1967; Tempelman-Kluit, 1976; Mortensen et al., 1983; Mor-tensen, 1992).

The KZK and GP4F deposits are hosted by the GrassLakes succession, which consists of a �360–356 Ma (Mor-tensen, 1992) felsic volcanic- and sedimentary rock-domi-nated package (Kudz Ze Kayah unit; Murphy, 1998).Coeval felsic porphyritic intrusions of the Grass Lakes suite(Fig. 3) are interpreted to comprise the subvolcanic intru-sive complex for the KZK and GP4F deposits (Pierceyet al., 2003). Rocks within the Kudz Ze Kayah unit areinterpreted to have formed within an ensialic back-arc riftto back-arc basin (Piercey et al., 2003).

In the area of the Wolverine deposit, host rocks are ayounger (�356–346 Ma, Mortensen, 1992; Piercey et al.,2002) succession of felsic volcanic, felsic intrusive and

the Finlayson Lake District (FLD). Geotectonic boundaries from

0 10

km

20

Jules Creekfault

Jules Creekfault

TINTINA FAULT

A

32

4

5

1

1

Post-accretionary intrusions

VHMS deposit

North American continentall margin

Overlap Assemblages

Triassic sedimenatry rocks without conglomerate units

Simpson Lake group

Slide Mountain Terrane

Yukon-Tanana Terrane

Campbell Range Formation

Fortin Creek Group

Money Creek thrust sheet

Big Campbell thrust sheet

Ice

KZKGP4F Wolverine

FyreLake

Hangwall felsic volcaniclastic unit

0

25

25

N S

meters

Massive Sulfide Unit

Cu-rich Massive Sulfide

Strong Footwall sericite & chloritic alteration

Footwall volcaniclastic unit

Open Pit (proposed)

KZK - 414850 mN

50

50

0 meters

Massive Sulfide

Qtz - Feld volcaniclastic Rhyolite

Basalt Fragmental Rhyolite

Carbonate Exhalite

Rhyolite Porphyry

Argillite - Aphyric Rhyolite

MagnetiteExhalite

Carbonaceous Argillte

NE SW Wolverine 16700E

A

C

A

B

DDH WV96-39

B C

Fig. 3. (A) Tectonic and stratigraphic map showing major geotectonic units of the Finlayson Lake District area near the KZK and Wolverinedeposits (from Gordey and Makepeace, 2000); (B) Westward looking geologic cross section through the Wolverine area (after Hunt, 2002;Bradshaw et al., 2008); and (C) Westward looking geologic cross section through the KZK deposit.

Table 1Variation in selenium contents (in ppm) from the GP4F, KZK, and Wolverine deposits by mineralization type.

Type GP4F KZK Wolverine

Low Median High Low Median High Low Median High

1 Footwall mineralization <2 5 (n = 38) 31 <2 108 (n = 104) 897 7 987 (n = 25) 36802 Chalcopyrite–pyrite mineralization <2 18 (n = 17) 78 6 283 (n = 141) 1019 890 2729 (n = 28) 46053 Sphalerite–pyrite mineralization <2 9 (n = 16) 61 <2 171 (n = 470) 753 240 780 (n = 65) 24504 Pyrite breccia N.I. N.I. N.I. <2 82 (n = 76) 280 705 1335 (n = 8) 24405 Pyrite–sphalerite–barite mineralization N.I. N.I. N.I. <2 76 (n = 135) 415 N.I. N.I. N.I.6 Remobilized sulfide mineralization N.I. N.I. N.I. <2 518 (n = 19) 1750 <2 305 (n = 6) 980

N.I. = not intersected.


sedimentary rocks compositionally similar to the Kudz ZeKayah unit. From oldest to youngest, the Wolverine succes-sion consists of: (1) a lower conglomerate unit, (2) a lower

felsic volcanic, volcaniclastic, and felsic porphyry unit, (3)intercalated carbonaceous argillite (“black shale”), felsicvolcanic, volcaniclastic, and felsic porphyritic rocks that

Table 2Se isotopic composition of standard reference materials (non-isotopic standards), provisional Se-standard reference materials (non-isotopicstandards), interlaboratory standards and geological material relative to Merck.

Sample SRM Description Se (lg g�1) Sample mass (mg) d82Se/76Se d80Se/76Se d82Se/78Se

SGR-1 Green River shale 3.5 �1.08a �0.68a

250 �1.0c (n = 3) �0.5c (n = 3) �0.4c (n = 3)GXR-4 Copper mill feed 5.6 �1.29a �0.88a

250 �1.4c (n = 3) �1.0c (n = 3) �1.0c (n = 3)MAG-1 Marine mud 1.16 �1.42a �0.76a

250 �1.2c,e (n = 2) �0.9c (n = 3) �0.7c (n = 3)SCo-1 Shale 0.89 �1.57a N/A �0.92a

250 �1.8c,e (n = 2) �1.4c (n = 2) �0.9c (n = 2)

SRM (provisional)

Canyon Diablo Triolite Meteorite 10 �1.32a N/A N/A�1.10b N/A N/A

250 �1.1c (n = 2) �0.6c (n = 2) �0.6c (n = 2)WMG-1 Mineralized gabbro 153f 250 �1.0c (n = 3) �0.5c (n = 3) �0.4c (n = 3)WMS-1 Massive magmatic sulfide 108 ± 13f 250 �1.0c (n = 3) �0.6c (n = 3) �0.4c (n = 3)WPR-1 Altered peroditite 4 ± 1f 250 �0.9c (n = 3) �0.5c (n = 3) �0.4c (n = 3)

Interlaboratory standards

Merck – Rouxel Nancy Merck split 1 0a N/A 0a

�0.2c (n = 3) �0.1c (n = 3) �0.1c (n = 3)CRPG Nancy split 1 �3.15a N/A �2.13a

�3.1c (n = 3) �1.9c (n = 3) �2.0(n = 3)MH495 Illinois split 1 �4.46a N/A �3.06a

�4.3c (n = 3) �2.6c (n = 3) �2.3c (n = 3)NIST 3149 split NIST Bulk Se standard 1 �1.4a N/A �0.93a,d

�1.5c (n = 3) �0.9c (n = 3) �0.8c (n = 3)

N/A, not available.a Data from Rouxel et al. (2002).b Data from Rouxel et al. (2002), but calculated from d80Se/76Se relative to MH495.c This study.d Calculated from linear mass fractionation law (e.g. Criss, 1999).e Calculated from five replicate digestions.f Provisional value (CCRMP).


host the Wolverine deposit, and (4) a unit comprised ofaphyric rhyolite flows, volcaniclastic rocks, carbonaceoussedimentary rocks, exhalative (Fe formation) sedimentaryrocks, and, at the top of the succession, basalt of MORBaffinity (Piercey et al., 2002). Rocks of the Wolverine suc-cession are interpreted to have formed within an ensialicback-arc rift/basin that evolved to seafloor spreading (Pier-cey, 2001; Piercey et al., 2002).

We have broadly subdivided the sulfide mineralizationin these deposits into six types based on mineralogy, textureand location with respect to footwall alteration (Table 1)(Layton-Matthews et al., 2008). In stratigraphic sequencefrom footwall to hanging wall, these are: Type (1) footwall;Type (2) chalcopyrite–pyrite–pyrrhotite dominant; Type (3)pyrite–sphalerite dominant; Type (4) sulfide breccia; Type(5) barite-rich; and Type (6) remobilized.

3. MATERIALS AND METHODS

3.1. Sampling

Samples used in this study were taken (2001–2003) fromdiamond drill cores from the Teck Cominco Ltd. and Expa-triate Resources Ltd. field core repositories (Layton-Mat-

thews, 2006). Optical microscopy, electron probemicroanalysis (EMPA) and synchrotron radiation X-rayfluorescence microanalysis (SRXRF-MA) were used formineral identification, textural interpretations, and charac-terization of chemical variation. Bulk geochemical and iso-tope data were acquired for subsamples of the samepulverized rock volume.

3.2. Trace elements

Samples were analyzed for trace elements by X-ray fluo-rescence (XRF), Instrumental Neutron Activation Analysis(INAA) and Inductively Coupled Plasma Mass Spectrome-try (ICP-MS) at the Geological Survey of Canada Labora-tories, Ottawa, Canada and at Activation LaboratoriesLtd., Ancaster, Canada. Selenium contents were deter-mined by INAA at the University of Toronto and Activa-tion Laboratories Ltd., and by Continuous HydrideGeneration Inductively Coupled Plasma Dynamic ReactionCell Mass-Spectrometry (CHG-ICP-DRC-MS) at the Uni-versity of Texas at Dallas (Layton-Matthews et al., 2006).The accuracy and precision of bulk selenium analyses wereassessed by replication of several standard reference materi-als with each sample batch (Table 2).

-7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0This studyOther studies

δ82Se

/ 78Se

(‰)

δ82Se/76Se (‰)

Fig. 4. Correlation of 82Se/78Se and 82Se/76Se for a suite of samplesand standard solutions with error bars as presented in Table 2. Thetheoretical relationship between the ratios is given by the dashedline (on the basis of a linear fractionation). Red squares are samplesand standards from this study, whereas black circles are fromprevious studies (Rouxel et al., 2002, 2004). (For interpretation ofthe references to colour in this figure legend, the reader is referredto the web version of this article.)


3.3. Selenium isotopes

Selenium isotope analyses were performed using a quad-rupole Perkin Elmer Sciex Elan 6100 DRC ICP-MS at theUniversity of Texas at Dallas. Analytical procedure and Se-isolation methods together with results of geologically rele-vant standard reference materials are presented elsewhere(Layton-Matthews et al., 2006). Samples and standard ref-erence materials (SRM) were pulverized and subsequentlydigested in perfluoroalkoxy copolymer (PFA) vessels with28.9 M HF, 15.8 M HNO3, and 11.6 M HClO4

(10:10:1 ml). Sealed digestion vessels were then heated toclose to 80 �C for 76 h before evaporation to incipient dry-ness at 70 �C on a hotplate. The temperature of the hotplatewas monitored via a Pt-thermocouple mounted within anempty PFA digestion vessel. Selenium degassing and lossby evaporation was prevented by avoiding the use of HClduring sample decomposition and by maintaining the tem-perature throughout to <80 �C. Following decomposition,6 M HCl (10 ml) was added to the sample residue andimmediately sealed followed by agitation in a water bathat 100 �C for 60 min to quantitatively reduce all SeVI toSeIV. The selenium isotope analytical method involves col-umn separation using Thioglycollic Cotton Fiber (TCF)and analysis via continuous hydride generator introductionto the ICP-DRC-MS. Full instrumental details and figuresof merit are presented in Layton-Matthews et al. (2006).The gas–liquid separator was assembled using an inverted500 ml fluorinated ethylenepropylene (FEP) Nalgene bottlewith a modified PFA cap. A peristaltic pump was used todeliver the acidified sample and reducing agent to the

CHG chamber. Hydride gas was swept using an argon car-rier gas into the ICP torch via 4 mm internal diameter FEPtubing. A modified Scott double-pass spray chamber wasused between the CHG and ICP torch to dampen pulsationfrom the pump of the hydride gas flow to the ICP. Stabilityof the hydride gas was greatly improved by simultaneouslycombining sample and reagent within the CHG chambervia a mixing manifold rather than using a traditional mix-ing coil. For the DRC, an Ar/H2 mixture (99.999% purity,5% H2 95% Ar) was the most efficient reaction gas with low-er memory effects than CH4 or NH4 (Layton-Matthewset al., 2006).

Instrumental drift and mass bias was corrected usingstandard-sample bracketing using linear regression (Rouxelet al., 2002), using the Merck standard (d82SeMerck; i.e.,d82Se = ((82Se/76Sesample � 82Se/76SeMerck)/82Se/76SeMerck) -� 1000). The accuracy and precision of our method usingfour inter-laboratory solutions and the ‘in-house’ Merckstandard we analyzed over 18-months gave a variation of±0.85& d82Se (2r) (Layton-Matthews et al., 2006). Our re-sults of several standards are within analytical error ofother studies (Rouxel et al., 2002, 2004) (Table 2 andFig. 4). Although we report d82Se values in Table 2 relativeto our Merck standard, data in the text, figures and Table 3have been recast with respect to NIST 3149, using the valued82SeNIST = 1.00154�d82SeMERCK + 1.54& (Carignan andWen, 2007).

We also ran a series of experiments to account for iso-baric and polyatomic interferences on the various Se iso-topes (Layton-Matthews et al., 2006). We routinelymonitored 83Kr to determine if Kr in the Ar gas supplywas a potential interference on 78Se, 80Se and 82Se from78Kr, 80Kr, and 82Kr, respectively. No significant interfer-ences were found. 76Ge was found to be quantitatively re-moved during Se capture on the TCF. Most of the As inthe samples was also not retained on the TCF, but ourexperiments found essentially no 75AsH+ forming in theCHG in any case. Finally, we ran a series of experimentsand found that 79BrH+ and 81BrH+ formation wasinsignificant.

3.4. Sulfur isotopes

Total sulfur was determined by the LECO� method atthe Geological Survey of Canada Laboratories, Ottawaand at the G.G. Hatch Isotope Laboratories at the Univer-sity of Ottawa. Samples were weighed into ceramic cruci-bles with tungsten(VI) oxide and placed in a furnace at1350 �C in an oxygen-rich atmosphere. Total sulfur wasdetermined by infrared absorption of evolved sulfurdioxide.

Sulfur isotope measurements of massive sulfide samples,whole rocks and sulfate separates were made at the Univer-sity of Ottawa. Separates of bulk sulfide minerals and bulksulfate minerals were weighed into tin capsules with tung-sten(VI) oxide and loaded into a VarioEL III elementalanalyzer and flash combusted at 1800 �C. Released gaseswere carried by helium through the VarioEL III� elementalanalyzer and cleaned and separated. Liberated SO2 gas wascarried into a DeltaPlus� isotope ratio mass spectrometer


for analysis. The system was calibrated using the interna-tional standards IAEA-S-1, IAEA-S-2, NBS-127, andNBS-123. Analytical reproducibility of the measurementsis ±0.2& (2r). The data are reported in the conventionaldelta notation relative to V-CDT (Vienna-Canyon DiabloTroilite).

3.5. Lead isotopes

Lead isotope ratios were measured using Thermal Ioni-zation Mass Spectrometry (TIMS; Finnigan Triton, Ther-mo Electron Corp.) at the Isotope Geochemistry andGeochronology Research Facility at Carleton University,Ottawa and Multi-Collector ICP-MS (MC-ICP-MS; Finn-igan Neptune, Thermo Electron Corp.) at the Saskatche-wan Isotope Laboratory, University of Saskatchewan,Saskatoon. Sulfide and sulfate separates were crushed andpulverized in an agate mortar, decomposed in 8 N HNO3

and taken to incipient dryness. Decomposed samples weredissolved in HBr for Pb column separation using standardanion exchange techniques (Cousens, 1996). For analysesvia TIMS, instrument mass fractionation was correctedusing NIST SRM 981 (+0.13 AMU-1) (Todt et al., 1984),yielding average ratios of 206Pb/204Pb = 16.891 ± 0.01,207Pb/204Pb = 15.430 ± 0.013, and 208Pb/204Pb = 36.505 ±0.042 (2r). For analyses via MC-ICP-MS, samples wereTl-doped to facilitate corrections for instrumental mass biasbased on an exponential dependence on mass law (e.g., Zhuet al., 2000). Corrections were based on values obtained forseveral aliquots of Tl-doped NIST 982 solution run duringthe same sample sequence. Analytical uncertainties for re-peat analyses of standards are better than 0.01% (2r), buta conservative estimate of 0.05% (2r) has been used forsample analyses based on replicates with varying contentsof Pb, and hence differences in beam intensity. Error corre-lation between 207Pb and 206Pb and between 208Pb and206Pb were calculated from repeat analyses of NIST 982and NIST 981.

4. RESULTS

4.1. Selenium isotopes

Twenty-two samples (16 massive sulfide, 3 carbonaceousargillite, and 3 altered footwall rocks) were analyzed for Seisotopes (Table 3 and Fig. 5). At the GP4F deposit there isa narrow range of d82Se (�0.3& to 0.7&) for Se-bearing(>2 ppm) host rocks and sulfide mineralization, all of whichoverlap within error of the analytical technique (Fig. 6). Atthe KZK deposit, there is a wider and more negative rangeof d82Se for Se-bearing (>2 ppm) host rocks, sulfide miner-alization and coeval carbonaceous argillite. Five samples ofCu-rich and Cu-poor sulfide mineralization range from�3.8& to �0.5& d82Se (Fig. 6). Three samples of carbona-ceous argillite collected from diamond drill holes distalfrom mineralization, but interpreted to be of similar strati-graphic age, give �2.5& to �0.1& d82Se and +3.8& to25.0& d34S (Table 3).

At the Wolverine deposit there is a wide range of d82Sefor massive sulfide mineralization, hydrothermally altered

footwall rocks and carbonaceous argillite (Fig. 6). Ten sam-ples of Cu-rich and Cu-poor sulfide mineralization rangefrom 1.3& to �10.2& d82Se, which represent the isotopi-cally heaviest and lightest samples in our study. One carbo-naceous argillite distal from the coeval Sable zonemineralization (2 km to the southeast of Wolverine deposit)has 0.1& d82Se and +20.4& d34S (Table 3).

4.2. Sulfur isotopes

Thirty-seven samples (21 massive sulfide, 2 barite min-eral separates from massive sulfide, 8 carbonaceous argilliteand 6 altered footwall rocks) were analyzed (Table 3;Fig. 5). The overall range of sulfide d34S values is 0.5& to25&. A narrow range of d34S (+11.1& to +14.6&) wasdetermined for host rocks and mineralization at theGP4F deposit. Within the KZK deposit, there is a muchwider range of d34S for host rocks and mineralization(+3.8& to +25.0& d34S). Ten samples of sulfide rangefrom +9.8& to 13.0& d34S, with two barite separates thathave values of +20.0& and +23.7&. One sample of prox-imal footwall altered rock has a whole rock d34S value of+11.7&. Six carbonaceous argillite samples collected in re-gional diamond drill holes from rocks stratigraphicallyabove the KZK horizon range from +3.8& to 25.0&

d34S. At the Wolverine deposit, five samples from sulfidemineralization have a wide range of d34S from +2.0& to12.2&. In addition, one sample of carbonaceous argilliteand one sample of mineralized and hydrothermally alteredfootwall rhyolite have values of +20.4& and +12.8& d34S,respectively.

4.3. Lead isotopes

Twenty-five samples (6 carbonaceous argillite, 2 foot-wall felsic volcaniclastic, and 17 massive sulfide) from theKZK, GP4F and Wolverine deposits, and 3 massive sulfidesamples from the Fyre Lake Besshi-type VSHMS deposit(Hunt, 2002; Fig. 3a) were analyzed for 208Pb/204Pb,207Pb/204Pb and 206Pb/204Pb (Table 3).

The Pb isotope compositions from the Wolverine depos-it sulfides (n = 6) range from 38.66 to 38.79 208Pb/204Pb,15.69 to 15.74 207Pb/204Pb and 18.83 to 18.86 206Pb/204Pb.These data intersect and lie above the ‘shale growth curve’in 207Pb–206Pb space of Godwin and Sinclair (1982) repre-senting the Pb isotope evolution of upper crustal rocksfor shale-hosted Pb-rich ore deposits in this geological ter-rain (note this curve was created using data for deposits inthe Yukon near the Finlayson Lake District) (Fig. 7). Foot-wall mineralization (Type 1 of Layton-Matthews et al.,2008) has systematically higher 207Pb/204Pb values thanthe main sulfide mass. One carbonaceous argillite sampledistal from coeval Sable Zone mineralization has a Pb iso-tope composition of 38.71 for 208Pb/204Pb, 15.70 for207Pb/204Pb and 18.88 for 206Pb/204Pb (Table 3).

Mineralization from the KZK (n = 12) and GP4F(n = 3) deposits ranges from 38.61 to 38.96 for 208Pb/204Pb,15.67 to 15.78 for 207Pb/204Pb and 18.83 to 18.90 for206Pb/204Pb. These data also lie above the ‘shale curve’(Fig. 7). Copper-rich mineralization (Type 1 of Layton-

Table 3Bulk selenium and Se, S, and Pb isotopic composition of samples from the Finlayson Lake District VHMS and VSHMS deposits.

Deposit Lab number Type d34SCDT S (wt%) 208Pb/204Pb 207Pb/204Pb 206Pb/2 b Se (ppm) d82Se/76SeNIST Se/S � 106

Fire Lake

FL96-13-1 Massive sulfide high Zn 3.7 50.4 38.42 15.64 18.76 BDFL96-13-5 Massive sulfide high Pb 5.1 49.7 38.41 15.63 18.66 BDFL96-43-1 Massive sulfide high Cu 0.5 38.1 38.34 15.62 18.68 BD

GP4F

GP4F-193-1 Laminated sulfide 11.5 47.2 38.75 15.72 18.88 14 0.3 30GP4F-197-1 Mineralized felsic tuff 11.9 1.7 na 8 485POA2986 Altered volcaniclastic 11.3 3.5 38.72 15.71 18.87 23 665POA2988 Banded massive sulfide 11.5 24.3 38.74 15.72 18.88 40 0.7 165POA2989 Proximal alteration w/massive sulfide 11.5 15.3 na 78 �0.3 509POA2991 Proximal alteration 11.1 1.2 na BDPOA2994 Felsic tuff 14.6 4.4 38.67 15.66 18.86 BDPOA2995 Altered felsic tuff 11.3 3.7 na 6 0.0 161

KZK

KZK-107-1 Banded massive sulfide 12.1 47.2 38.81 15.74 18.88 87 �3.8 184KZK-112-1 Proximal alteration w/massive sulfide 11.6 35.9 38.87 15.76 18.90 676 �1.9 1885KZK-112-2 Breccia massive sulfide 10.7 31 38.71 15.71 18.87 580 �0.5 1872KZK-112-3 Proximal alteration w/massive sulfide 11.1 34.2 38.73 15.71 18.87 740 �3.4 2163KZK-112-4 Massive sulfide–barite separate 23.7 14.5 38.89 15.76 18.90 BDKZK-22-1 Proximal alteration w/sulfide 11.2 4.4 na 79 �2.2 1812POA2931 Argillite 15.7 2.3 na BDPOA2938 Argillite 13.6 1 na BDPOA2939 Argillite 15.5 0.6 na 31 5166POA2968 Argillite 14.6 1.2 38.79 15.71 18.99 16 �2.5 1333POA2996 Argillite 25.0 2.4 38.92 15.73 19.44 14 �0.6 583POA3000 Argillite 3.8 0.9 na 6 �0.1POA3006 Proximal alteration w/massive sulfide 11.1 33 38.74 15.71 18.89 330 1002POA3008 Banded massive sulfide 9.8 48.5 38.72 15.71 18.87 380 783POA3009 Proximal alteration w/massive sulfide 13.0 9 38.65 15.67 18.87 180 1991POA3010 Proximal alteration w/massive sulfide 12.5 0.5 38.75 15.72 18.87 1000 204082POA3011 Banded massive sulfide 12.9 40.4 38.74 15.71 18.87 630 1561POA3017 Massive sulfide–barite seperate 20.0 13.4 38.61 15.67 18.83 BDPOA3021 Banded massive sulfide 10.4 34.1 38.96 15.78 18.91 BD

Wolverine

GB-00-005 High Pb, Zn sulfide 12.0 32.4 38.70 15.70 18.83 633 �6.4 1951GB-00-031 Massive sulfide near top 10.8 43.5 38.73 15.71 18.84 217 0.3 499GB-00-045 Banded massive sulfide 7.2 30.3 38.66 15.69 18.85 381 1.3 1256GB-00-055 Banded massive sulfide near base 2.0 41.5 38.77 15.73 18.85 1350 �10.2 3253

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Matthews et al., 2008) has systematically higher 207Pb/204Pbvalues than does Cu-poor mineralization. Carbonaceousargillite (n = 2) stratigraphically above the mineralized se-quence has a Pb isotope composition of 38.79–38.92208Pb/204Pb, 15.70–15.73 207Pb/204Pb and 18.99–19.44206Pb/204Pb. One altered footwall volcaniclastic rocks fromthe GP4F deposit has a Pb isotope composition of 38.67 for208Pb/204Pb, 15.66 for 207Pb/204Pb and 18.86 for206Pb/204Pb.

5. DISCUSSION

Layton-Matthews et al. (2008) made the following con-clusions concerning the variations in Se contents in the Fin-layson Lake District VHMS deposits: (1) Cu-rich (basal)parts of deposits have the highest Se contents; (2) Se-richsulfides are most abundant in footwall stringer mineraliza-tion, but dilution by non-vein material commonly results inlower whole rock selenium contents; (3) hydrothermal min-eralizing fluids at temperatures >250 �C and high Se/S gen-erally precipitated sulfides with decreasing Se contents asfollows; clausthalite-galena� chalcopyrite � sphaler-ite � pyrite > pyrrhotite; (4) hydrothermal mineralizing flu-ids at temperatures <250 �C and low Se/S generallyprecipitated chalcopyrite � sphalerite > pyrite � tetrahe-drite � galena with similar selenium contents; and (5)recrystallization of pre-existing sulfides during zone refiningby a high Se/S mineralizing fluid resulted in increased sele-nium contents of all sulfides and may have resulted in thedeposition of clausthalite. In the following sections, we dis-cuss the implications of the S and Pb isotope data for thegeological and geochemical sources and framework of thethree deposits in the Finlayson Lake District. Subsequently,we discuss the sources of Se, Se enrichment, and controls onthe Se isotope fractionation we measured at each of thedeposits. All three deposits studied here have experiencedlow-grade metamorphism (up to greenschist facies) (Brad-shaw et al., 2008; Layton-Matthews et al., 2008). Giventhe low metamorphic grade and elevated S, Pb and Se con-tents of these deposits, we consider the isotopic values tohave not been perturbed by metamorphism, as shown pre-viously for S, Pb and Re-Os (Yamamoto et al., 1984; Noza-ki et al., 2010).

5.1. Sulfur

Considering the similar geochemical behavior of S andSe (Fig. 1), we also analyzed the S isotope compositionsof the Finlayson Lake District deposit mineralization (Ta-ble 3). Sulfur in VHMS systems is typically derived fromeither seawater and/or igneous reservoirs, but sedimentarysources of sulfur can also contribute to these hydrothermalsystems in some cases (Peter and Shanks, 1992; Shanks,2001). Seawater has experienced large excursions in sulfurisotope composition (i.e., �20&) through time related torecycling via erosion and burial of sedimentary pyrite(Claypool et al., 1980; Petsch and Berner, 1998; Petschet al., 2005), magmatism (Canfield, 2004; Paytan et al.,2004), and redox changes in ocean stratification (Berryand Wilde, 1978; de Graciansky et al., 1984; Jenkyns,

Fig. 5. Histograms of d82SeNIST (upper) and d34S (lower) for allsamples from the Wolverine, KZK, and GP4F deposits. Alsoshown are cumulative frequency curves, subdivided by deposit.


1988; Eastoe and Gustin, 1996). During the formation ofthe Finlayson Lake District deposits, and sequestration of

Fig. 6. Range of d82SeNIST values for the Finlayson Lake District VHMSfor various reservoirs (after Rouxel et al., 2002). Results have been recaWhite bars represent data presented in other studies (Krouse and ThodeRouxel et al., 2002; Wen et al., 2007). Hatched bars represent this study. Vrecast to d82SeNIST.

S in sulfate minerals, in the Lower Mississippian, the sulfurisotope composition of contemporaneous seawater was�+22& (Claypool et al., 1980), consistent with the sulfurisotope composition of barite at KZK (Table 3).

Within sedimented seafloor and sub-seafloor environ-ments, low-temperature (<100 �C) seawater sulfate reduc-tion by S-reducing bacteria, such as Desulfovibrio

desulfuricans, has been identified as a major mechanismfor the formation of sedimentary sulfides (Kaplan et al.,1963). The rates of these biogenic reactions and the degreeto which sulfate reduction sites are in contact with the openocean have pronounced effects on the sulfur isotope compo-sition of precipitated sulfide, with seawater-sulfide isotopicfractionations up to �76& observed (Goldhaber et al.,1977; Zerkle et al., 2009). As a result, marine sulfides havea wide range of d34S values, as do modern seafloor sulfidedeposits in sedimentary environments (Peter and Shanks,1992; Stuart et al., 1994; Zierenberg et al., 1994), albeit withdifferent ranges of d34S: �5& to +15& for sedimentedridges and �55& to �10& for bacteriogenic pyrite in mar-ine sediments. However, the sulfur isotope compositions ofmarine sediments are commonly similar to massive sulfidesin the same setting. For example, Goodfellow et al. (2003)reported sulfur isotopes for pyrite from several black shalesedimentary units in the Bathurst Mining Camp that rangefrom �17& to +27.6&, with an average of 10.9&, similar

and VSHMS deposits compared to compositional ranges reportedst from d82SeMERCK (Layton-Matthews et al., 2006) to d82SeNIST.

, 1962; Rees and Thode, 1966; Hagiwara, 2000; Herbel et al., 2000;ertical grey bar represents bulk earth estimate (Rouxel et al., 2002),

A

B

C

Fig. 7. Plots of Pb isotope ratios for sulfide mineral separates andmassive sulfide samples from the (A) Wolverine, (B) KZK and C)GP4F deposits. The Stacey and Kramers (1975) average continen-tal crust evolution curve is shown in blue and the Godwin andSinclair (1982) “shale curve” of isotopic evolution of upper crustalPb is shown in dashed red. Ellipse represents the error for eachratio rotated to the 204Pb uncertainty trend.


to the average for the massive sulfides in the camp of 12&

(range of �3.1& to +21.6&). Sedimentary pyrite from the

Guaymas Basin has d34S ranging from �9.7& to +11.1&,similar to massive sulfides in the basin (Shanks and Nie-mitz, 1982). In contrast to seawater and sedimentary reser-voirs, magmatic reservoirs are thought to be relativelyconstant through time (d34S �0 ± 3&) and reflect partialmelting of either crustal or mantle reservoirs (Ohmotoand Rye, 1979).

The host sequence of both the KZK and GP4F depositsis dominated by felsic volcanic rocks, and both depositsshow relatively narrow ranges in sulfur isotope compositionof both massive sulfide mineralization and the host rocks(d34S = +9.9& to +14.9&; Table 3). At KZK, hangingwall argillite has a wider range in sulfur isotope composi-tion (+3.8& to +25.0&), and is typical of marine sedimentsin which mixed sulfate-reduction by abiotic and biotic pro-cesses has occurred. Thus, the narrow range of sulfur iso-tope compositions of massive sulfides at KZK and GP4F,together with a lack of correlation with base-metal ratios(e.g., Cu/(Cu + Pb + Zn)), implies a homogeneous mixtureof magmatic and seawater sulfur sources, resulting fromleaching of magmatic S in the reaction zone.

In contrast, Wolverine deposit massive sulfides showmore variation in sulfur isotopic composition (+2.0& to+12.8&; Table 3), similar to in situ laser-SF6 technique sul-fide mineral data (+1.0& to +18.4&) (Bradshaw et al.,2008). Isotopic compositions of individual pyrite grainswithin the hanging wall argillite at Wolverine trend to light-er sulfur isotope values (from 14.3& to �5.7& d34S) consis-tent with biogenic sulfate reduction. However, black shalesfrom the Sable Zone (which is of similar age to the Wolver-ine stratigraphy and is located 2 km along strike to thesouthwest) have sulfur isotope compositions close to coevalseawater (+20.4& d34S). The wide range of sulfur isotopecompositions and the occurrence of the most negative val-ues near the top of the mineralized sequence, together withthe presence of abundant footwall and coeval sedimentaryrocks, likely suggest a sedimentary-biogenic influence onthe sulfur isotope compositions at Wolverine, consistentwith the Se isotope data (see Sections 5.3–5.6 below).

5.2. Lead

Lead isotope ratios have been used as a geochemical tra-cer for elucidating the origin of Pb in VHMS and VSHMSdeposits and as a proxy for the origin of the contained met-als (e.g., Hegner and Tatsumoto, 1987; Fouquet and Mar-coux, 1995; Halbach et al., 1997). Although metamorphicmodification of Pb isotope ratios has been documented(Williams et al., 1984; Compston et al., 1986; Williams,2001), the effect is most evident in Pb-poor mafic-hostedand highly metamorphosed (upper amphibolite-grade)VHMS deposits (Mortensen et al., 2008). The depositsstudied here have high Pb contents and have been subjectedto only relatively low-grade metamorphism (upper green-schist facies), suggesting that the Pb isotope compositionof the sulfide minerals probably has not been modified sincetheir formation.

The least radiogenic sulfides from the Wolverine, KZKand GP4F deposits intersect the shale growth curve of God-win and Sinclair (1982 based on Pb isotope composition of

A

B

Fig. 8. Plots of Pb isotope ratios illustrating multi-stage growthmodels (solid lines) and isochrons (dashed lines) for 207Pb/204Pbmaxima from the KZK and Wolverine deposits. (A) 360 Magrowth models for the KZK deposit for t3 = 1887 Ma (green lines),t3 = 2700 Ma (red lines), and t3 = 3140 Ma (blue lines), respec-tively. (B) 347 Ma growth models for the Wolverine deposit fort3 = 1700 Ma (red lines) and t3 = 3140 Ma (blue lines), respectively.Growth models assume the initial stage (t1 = 4570 Ma, l1 = 7.19)and crust formation (t2 = 3700 Ma, l2 = 9.73) of Stacey andKramers (1975). l = 238U/206Pb. (For interpretation of the refer-ences to colour in this figure legend, the reader is referred to theweb version of this article.)


shale-hosted Zn deposits within the Canadian Cordillera) at�350 Ma (Fig. 7), consistent with the strong influence offootwall sedimentary rocks on the composition of the sul-fides (Godwin and Sinclair, 1982; Pb isotope growth curvesbased on shale-hosted Pb deposits through time) and theage of the footwall intrusions (347.8 ± 1.3 Ma) (Pierceyet al., 2003). Sulfide mineral separates from footwall felsicrocks at Wolverine have both a larger range, and higher207Pb/204Pb values (Fig. 7a), suggesting a mixing trend be-tween coeval black shales and more radiogenic felsic volca-nic rocks and related intrusions. The felsic rocks wouldhave had to have a higher 238U/206Pb value and/or reflectextraction from an older source to generate growth curvesabove the shale growth curve of Godwin and Sinclair(1982; see Fig. 7a). Felsic volcanic rocks dominate the host

stratigraphic sequence at the KZK and GP4F deposits,consistent with the Pb isotope data for these deposits form-ing a cluster extending to higher 207Pb/204Pb values thanWolverine (Fig. 7). In comparison to the felsic-hosteddeposits, the Fyre Lake deposit southwest of Wolverine(Fig. 3) is hosted in mafic volcanic rocks. The Pb isotopecompositions of sulfide minerals at Fyre Lake (Table 3)are consistent with Pb sourced from a mantle magmaticcomponent and an older crustal component (Fig. 7).

The difference between the highest 207Pb/204Pb valuesmeasured at Wolverine and KZK is outside analytical errorand statistically significant (Fig. 7). Three-stage regressiongrowth curves were calculated from the upper crustalgrowth curve using known age constraints for the Wolver-ine (347.8 ± 1.3 Ma) and KZK (�360 Ma) deposits (Pier-cey et al., 2008), and variable 238U/206Pb (Fig. 8). For theKZK deposit, the best model fit for the highest 207Pb/206Pbvalues requires an extraction age of �3140 Ma and238U/206Pb = 11, suggesting that the hydrothermal fluidsthat formed the KZK and GP4F deposits extracted Arche-an Pb from basement rocks in addition to younger Pbsources. The felsic volcanic rocks are the product of partialmelting of much older crust (i.e., as old as �3100 Ma). Pier-cey and Colpron (2009) compiled new and previously pub-lished Nd isotope data for basement rocks of the YTT, andreported clusters of model ages at 1870 and 2720 Ma, withsmaller peaks at 2080 and 2380 Ma. Detrital zircons withinmetasedimentary sequences in the Coast Mountains insoutheastern Alaska are thought to represent the YTTbasement and yield Meso-Archean ages (zircon U–Pb;3340 Ma) (Gehrels and Kapp, 1998). Similarly, the Klatsametamorphic complex within the Finlayson Lake District,approximately 50 km to the southeast of KZK depositand thought to represent blocks of metamorphosed YTTbasement, has also yielded old inherited zircon ages(�2825 Ma) (Devine, 2005).

By contrast, the best model fit for the Wolverine depositis an extraction age of �1700 Ma and 238U/206Pb = 12(Fig. 8). The model extraction ages for lead sources in thehydrothermal reservoirs of the KZK and GP4F(�3140 Ma) and Wolverine (�1700 Ma) deposits are mark-edly different and strongly model dependent. Nevertheless,the isotopic compositions of sources of Pb within thesedeposits were different (Figs. 7 and 8), suggesting that thehydrothermal fluid at the Wolverine deposit interacted withyounger (i.e., black shales in the hydrothermal reactionzone) basement rocks than at the KZK and GP4F deposits.

5.3. Selenium

There are three viable hypotheses that can explain therelationships between Se content, Se/S values and Se isoto-pic compositions for the VHMS deposits in the FinlaysonLake District: (1) Depositional fractionation: process(es)that generate high Se/S values also fractionate Se isotopes,with the lighter isotopes preferentially incorporated in thehigh Se/S rocks at the site of sulfide deposition; (2) Primaryfractionated source: hydrothermal fluids that deposited theSe-rich sulfides with high Se/S values originated from, orinteracted with, a source with highly negative d82Se values;

Fig. 9. Plot of Se vs. Se/S (�106) for sulfide mineralization and argillite samples from the Finlayson Lake District deposits (data fromTable 3). Also included are whole-rock selenium variations and Se/S � 106 for modern sea-floor massive sulfide samples; n = 598, fromHannington et al., (2004).


or (3) Secondary fractionation at the source: the Se-richsource was fractionated by secondary redox processes toisotopically light values.

Within the deposits of the Finlayson Lake District, Seranges from contents similar to modern seafloor sulfidedeposits (�100 ppm; Hannington et al., 2004) to values sim-ilar to the highest Se contents recorded in ancient VHMSand VSHMS deposits (Kidd Creek; Hannington et al.,1999b; Finlayson Lake District; Layton-Matthews et al.,2008) (Fig. 9). Although the Kidd Creek deposit has Se con-tents that range to values similar to the Wolverine andKudz Ze Kayah deposits (Fig. 9), the Finlayson LakeDeposits differ in the abundance of total contained Se (2and 8 Mt of Se for Kudz Ze Kaya and Wolverine, respec-tively; Layton-Matthews et al., 2008); at the Kidd Creek de-posit, the Se-rich ore zones are relatively small (i.e., 0.05 Mtof Se; Hannington et al., 1999b). In the modern oceans, sul-fide mineralization with the highest Se contents and thehighest Se/S values is associated with thick accumulationsof sediment (Fig. 9); the Wolverine deposit plots near themaxima for Se contents and Se/S values. Layton-Matthewset al. (2008) concluded that the bulk Se content of the min-eralization was related to the Se content of host rocks andthe temperature of the ore-forming fluid (Fig. 9). They alsoconcluded that the extreme Se enrichment at the Wolverinedeposit is a function of the abundance of organic-rich(black) shales in the footwall.

5.4. Depositional fractionation

The first hypothesis is that Se becomes fractionated be-cause of processes that occur at the site of sulfide deposition(i.e., from the high temperature (�350 �C) ore-forming flu-ids). These processes include phase separation and zonerefining.

5.4.1. Phase separation (i.e., boiling)

Observed changes (e.g., T, pH, salinity) in end-memberhydrothermal vent fluid compositions over small areas inmodern seafloor hydrothermal systems (Butterfield et al.,

1990, 1994; Charlou et al., 2000; Douville et al., 2002) arestrong evidence of phase separation occurring within someactive seafloor hydrothermal systems. Similarly, there is evi-dence for phase separation (i.e. both vapor-rich and vapor-poor within a fluid inclusion population, Br/Cl > seawater)in fluid inclusions, and this process can dramatically alterthe hydrothermal fluid compositions, which ultimatelymay control the style of modern (Berndt and Seyfried,1990; Lecuyer et al., 1999) and ancient (Cowan and Cann,1988; Nehlig et al., 1998) seafloor hydrothermal systems.Although Se could potentially partition into the vaporphase, as suggested for sulfur (Butterfield et al., 1990;Von Damm, 1990), and perhaps concentrate Se within ahydrothermal reaction zone, at temperatures >400 �C thereshould be no isotopic shift between the two phases (Rouxelet al., 2004). Therefore, such a mechanism cannot explainthe wide and negative range of Se isotopic compositionsmeasured within the sulfide mineralization at the Wolverinedeposit and, to a lesser extent, the KZK deposit. Further-more, at the Wolverine deposit fluid inclusions show no evi-dence of phase separation (Bradshaw et al., 2008),indicating that it did not play a role in Se accumulationand isotopic fractionation.

5.4.2. Zone refining

Fluctuations in hydrothermal fluid composition andtemperature are a strong influence on the style and compo-sition of seafloor sulfide deposits, and early sulfide mineralsare commonly dissolved and redeposited along steep geo-thermal gradients resulting from the mixing of hot ore-forming fluids and cold seawater. This process is referredto as zone refining (e.g., Large, 1977). The most negatived82Se values measured for the Wolverine deposit are largelyrestricted to Cu-rich Type 1 and Type 2 mineralization (seeTable 1) at the base of sulfide lenses, which accounts for asmall amount (5–15 vol.%) of the total mineralization with-in the deposit (Layton-Matthews et al., 2008). Higher in thesulfide lenses the Se isotopic ratios become progressivelyheavier, consistent with the positive correlation betweenSe/S and Se isotopic compositions (Fig. 10). Previous stud-

10 100 1,000 10,000

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ies on the influence of zone refining on metal budgets andsulfur isotopes in ancient VMS deposits have shown thatCu/(Cu + Pb + Zn) decreases and sulfur isotope composi-tions of sulfide minerals become progressively lighter up-wards through the massive sulfide mound (Kajiwara,1971; Gemmell et al., 1998). Although we have shown thatzone refining was operative during sulfide deposition withinthe Finlayson Lake District VHMS deposits (Layton-Mat-thews et al., 2008), Rayleigh distillation-fractionation ef-fects induced by zone refining would have a counter effecton the selenium isotopic distribution. Therefore, zone refin-ing cannot explain the distribution of d82Se.

5.5. Primary fractionated source

If Se is not isotopically fractionated during high-temper-ature sulfide-selenide deposition, the source of Se may havepreviously undergone isotopic fractionation (��10&) dur-ing sediment deposition. If the Se contained within the Wol-verine mineralization is from the host shales, then acomparison with modern and ancient shales should providesupporting evidence.

The limited dataset for modern seafloor sedimentsshows a range of �0.34& to 1.16& d82Se, similar to plank-ton in the modern Pacific Ocean (0.42& d82Se) (Mitchellet al., 2012). Data for Phanerozoic seafloor sediments thathave been filtered to remove those samples affected byweathering and hydrothermal alteration show a similarrange of �0.17& to 2.28& d82Se (Mitchell et al., 2012).These data, together with our current understanding of pri-mary sedimentary processes indicate that it is unlikely thatthe source could contain a sufficiently isotopically depletedSe reservoir to explain the range of Se isotopic values at theWolverine deposit (Fig. 10).

5.6. Secondary fractionation at the source

Given that sediment deposition does not appear to pro-duce the necessary fractionation determined for portions ofthe Wolverine deposit, a third hypothesis is that the Se fromthe Se-rich source was fractionated to isotopically light val-ues during a secondary redox process. Experimental workon the oxidation of reduced Se indicates that there is onlylimited isotopic fractionation (<1& d82Se) (Johnson andBullen, 2004). However, experiments of Se isotope fraction-ation associated with reduction of oxidized Se can result inup to d82Se �15& fractionation (Fig. 6) (Johnson and Bul-len, 2004).

Within ancient shales that have elevated Se contents,such as those documented in the Yutangba deposit of Chi-na, there is a wide range of �12.86& to 7.52& d82Se for theshale and the thermally-maturated kerogen (Wen and Car-ignan, 2011). These authors have invoked secondary redoxprocesses, including redox cycling to explain the wide vari-ation in Se isotopic compositions in this deposit (Wen andCarignan, 2011). Similarly, in modern seafloor hydrother-mal systems a wide range of �4.75& to 0.36& d82Se hasbeen reported for the Lucky Strike sulfide deposits (Rouxelet al., 2004), and a secondary redox process was also in-voked to explain the Se isotopic variations in these deposits.


Rouxel et al. (2004) suggested that a shallow circulation celldeveloped close to the margins of the sulfide mineralization;within this cell, seawater Se(IV) and Se(VI) are reduced andisotopically fractionated. Subsequent mixing with moredeeply sourced hydrothermal fluids allows the isotopicallydepleted Se to circulate through the sulfide mineralization.Although we did not find an isotopically light (negative)Se reservoir within the footwall of the Wolverine deposit,the two afore-mentioned studies indicate that a Se-richand Se isotopically negative source was likely present.

5.7. Implications for Se abundances in volcanogenic massive

sulfide deposits

The proposed environment for the Wolverine deposit(back-arc seafloor volcanism within an anoxic seafloorenvironment) is similar to other ancient VHMS- andVSHMS deposits such as those in the Bathurst MiningCamp and Iberian Pyrite Belt (Goodfellow and Peter,1996; Relvas et al., 2001; Bradshaw et al., 2008). However,we are unaware of any modern examples of such anenvironment.

Fig. 11. (A) Cross-sectional model of the pre-ore stage at the Wolvericonditions undergo Se fractionation to more negative isotopic valuestransport and reduction. (B) Cross-sectional model for the late ore-formLake District showing the hydrothermal fluid flow pathway and interactnegative Se from secondary fractionation (pre-ore stage) is incorporated inmetal distributions through vent-proximal (E–E0) and vent-distal (F–F0)selenium distributions through vent-proximal (A–A0) and vent-distal (B–

Within modern seafloor sulfide environments, theenrichment of Se is strongly influenced by water–rock inter-action, and mineralization with the highest sulfide Se/S arefrom sedimented settings, particularly those with organic-rich sediments (Fig. 9) (Layton-Matthews et al., 2008).The strong positive correlation between Se content, abun-dance of carbonaceous argillite and the negative d82Se val-ues within massive sulfides at the Wolverine depositindicates that sedimented settings are the most likely to hostSe-rich (seleniferous) sulfide deposits. Low-Se (KZK andGP4F deposits) sulfides occur in rock sequences withoutcarbonaceous argillite and display a limited range ofd82Se. As a corollary, very negative d82Se values in modernseafloor sulfides are likely to be found in sediment-domi-nated environments. This conclusion is supported by therange of d82Se from sediment-starved environments suchas Lucky Strike (Rouxel et al., 2004). We suggest that fur-ther work on modern sediment-covered seafloor hydrother-mal systems such as Guaymas Basin, Escanaba Trough andMiddle Valley may yield larger 82Se values, and provideadditional insights into the subsurface negative d82Se reser-voir suggested for the Wolverine deposit.

ne deposit. Here, black shales previously deposited under anoxicfrom secondary shallow seawater recharge through Se oxidation,ing stage of VHMS and VSHMS mineralization of the Finlayson

ion with magmatic volatiles. At the Wolverine deposit, isotopicallyto the hydrothermal fluids. (C) Curves illustrating hypothetical basepositions of a sulfide deposit. (D) Curves illustrating hypotheticalB0) positions of a sulfide deposit. Not to scale.


6. CONCLUSIONS

Evidence presented here and elsewhere (Layton-Mat-thews et al., 2008) indicates that the most likely Se reservoirat the Wolverine deposit is the footwall and coeval carbo-naceous argillites; this reservoir is anticipated to be highlydepleted in d82Se. We also suggest that reactions duringrecharging hydrothermal circulation, as proposed for theLucky Strike sulfides (Rouxel et al., 2004), are unlikelymechanisms to have produced a large and negative d82Sereservoir during global anoxic events.

Selenium in the KZK and GP4F deposits was derived byeither volatile exsolution or leaching from magmatic rocks,whereas selenium in the Wolverine deposit was likelysourced from predominantly sedimentary sources with per-haps a minor magmatic contribution (Layton-Matthewset al., 2008). Based on variations in Pb isotope composi-tions, there are different Pb sources for lead in the KZK,GP4F, and Wolverine deposits. Lead in the KZK andGP4F deposits was leached from far older rocks (Slave Cra-ton age) than the Wolverine deposit.

Type 1 and 2 massive sulfides of the Wolverine deposithave higher bulk Se contents and negative d82Se values,and these likely reflect a thermal control on Se mobilizedwithin the hydrothermal circulation cell. The depositionof Type 1 and 2 massive sulfides at higher temperaturesmay have been a prerequisite for mobilization of isotopi-cally light Se from the source region (Fig. 11).

Prior to our study, such selenium and copper enrich-ments in VHMS deposits were thought to be due to directmagmatic contributions of heat and selenium into VHMSsystems (Huston et al., 1995). Here, we suggest that themagmatically-heated fluid surpassed some threshold tem-perature (perhaps >250 �C), which allowed mineralogically-or biogenically-bound Se to be variably mobilized into thehydrothermal fluid. The magmatic contribution of Se to thehydrothermal system, whether direct (through degassedvolatiles) or by extraction from (dissolution of) magmaticsulfides, was relatively small. Bulk sulfide samples fromVHMS (sediment-starved) deposits contain only a maxi-mum of �1500 ppm Se in Cu-rich samples, whereas sam-ples from the VSHMS (sediment-hosted) Wolverinedeposit contain up to 6000 ppm Se. The large mass(107 kg) of contained Se at Wolverine suggests Se princi-pally was sourced from anoxic marine sediments (carbona-ceous argillite). The Se contents and Se isotopecompositions of the Wolverine sulfides are thus consistentwith mixing two isotopically distinct Se sources (Fig. 11).

Finally, the large negative d82Se values are well withintheoretical values and suggest that more extreme values(perhaps as low as �25& d82Se) are to be expected, partic-ularly as analytical capabilities for difficult samples, such asseawater and porewater, are improved.

ACKNOWLEDGEMENTS

We thank Teck Cominco Ltd., Expatriate Resources Ltd (nowYukon Zinc Corporation) and Pacific Ridge Exploration Ltd. forproviding access to the Finlayson Lake District deposits, samplesand for their interest in this project. This work benefited from dis-cussions with Steve Piercey and Thomas Johnson. Research sup-

port was gratefully received from Natural Sciences andEngineering Research Council (NSERC), NSERC-CollaborativeResearch and Development (CRD), the University of Toronto,the National Science Foundation, the Geological Survey of Can-ada, the Society of Economic Geologists and the MineralogicalAssociation of Canada. This is Geological Survey of Canada con-tribution number 20090191. The paper benefited significantly fromthorough and constructive reviews by Wayne Goodfellow, BruceTaylor, Dave Huston, and Paul Mason, for which we are verygrateful. An anonymous reviewer and Tatsuo Nozaki, as well asAssociate Editor Junichiro Ishibashi are thanked for their con-structive comments that greatly improved the paper.

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