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Can. J. Earth Sci. 40: 77–97 (2003) doi: 10.1139/E02-094 © 2003 NRC Canada

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Neodymium isotope geochemistry of felsic volcanicand intrusive rocks from the Yukon–Tanana Terranein the Finlayson Lake Region, Yukon, Canada1

Stephen J. Piercey, James K. Mortensen, Robert A. Creaser

Abstract: Devonian–Mississippian felsic rocks from the Finlayson Lake region have variable geochemical and Nd isotopiccharacteristics that provide insights into the tectonic and metallogenic evolution of the Yukon–Tanana terrane (YTT),and the northern Cordillera. Late Devonian (�365–360 Ma) calc-alkaline and tholeiitic arc felsic rocks in the mafic-dominatedFire Lake unit yield εNd350 = –4.8 and +0.1, respectively, and have 1.49–1.94 Ga depleted mantle model ages (TDM).Devonian–Mississippian (�360–356 Ma) felsic volcanic (Kudz Ze Kayah unit, Wolverine succession) and intrusiverocks (Grass Lakes suite) associated with volcanogenic massive sulphide (VMS) deposits have εNd350 = –7.8 to –9.5with TDM = 1.59–2.25 Ga. A granitoid sample from the Early Mississippian (�350–345 Ma) Simpson Range plutonicsuite has εNd350 = –12.9 and TDM = 2.01 Ga, similar to previously reported values for this suite. The VMS-associatedGrass Lakes suite of granitoids has higher high field strength element (HFSE) and rare-earth element (REE) contents,and higher Zr/Sc, Zr/TiO2, Nb/La, and Zr/La values relative to the Simpson Range plutonic suite; these geochemicalfeatures are similar to coeval VMS-associated felsic volcanic rocks in the Kudz Ze Kayah unit. The identification ofsimilar HFSE–REE-enriched felsic volcanic and subvolcanic intrusive rocks may aid in delineating prospective regionsfor VMS mineralization in the YTT and other continental-margin arc to back-arc environments. The geochemical andNd isotopic data for these YTT felsic rocks suggest that they reflect episodic mid-Paleozoic arc (Fire Lake unit; SimpsonRange plutonic suite) and back-arc magmatism (Kudz Ze Kudz unit; Wolverine succession) built upon a transitionalbasement with variable, but significant, influence from evolved (Proterozoic) crustal materials.

97Résumé : Des roches felsiques (Dévonien–Mississippien) de la région du lac Finlayson ont des caractéristiques géochimiqueset isotopiques Nd qui donnent des aperçus de l’évolution tectonique et métallogénique du terrane de Yukon–Tanana etde la Cordillère septentrionale. Des roches felsiques, calco-alcalines et d’arc tholéiitique, datant du Dévonien tardif(~365–360 Ma), dans l’unité de Fire Lake, dominée par des roches mafiques, ont donné εNd350 = –4,8 et +0,1, respectivement;elles ont des âges modèles pour le manteau appauvri (TDM) de 1,49–1,94 Ga. Les roches felsiques, volcaniques (unité deKudz Ze Kayah, succession Wolverine), datant du Dévonien–Mississippien (360–356 Ma), et les roches intrusives (suitede Grass Lakes) associées au gisements de sulfures massifs d’origine volcanique (VMS) ont des valeurs εNd350 = –7,8à –9,5 avec TDM = 1,59–2,25 Ga. Un échantillon de granitoïde provenant de la suite plutonique Simpson Range duMississippien précoce (~350–345 Ma) a des valeurs εNd350 = –12,9 et TDM = 2,01 Ga, ce qui est semblable à ce qui aété rapporté antérieurement pour cette suite. La suite de granitoïdes de Grass Lakes, associée aux sulfures massifsd’origine volcanique, a des teneurs en éléments à champ électrostatique élevé (HFSE) et d’éléments de terres rares(REE) supérieures à celles de la suite plutonique de Simpson Range; les valeurs de Zr/Sc, Zr/TiO2, Nb/La et Zr/Lasont aussi plus élevées. Ces caractéristiques géochimiques sont semblables à celles des roches volcaniques felsiquesassociées aux VMS contemporains dans l’unité de Kudz Ze Kayah. L’identification de roches volcaniques felsiquesenrichies en éléments de terres rares et en HFSE et de roches intrusives sub-volcaniques pourrait aider à délimiter desrégions propices à une minéralisation VMS dans le terrane de Yukon–Tanana et dans d’autres environnements d’arc demarge continentale à d’arrière arc. Les données géochimiques et isotopiques Nd pour ces roches du terrane de Yukon–Tananaportent à croire qu’elles sont le reflet d’un magmatisme d’arc épisodique du Paléozoïque moyen (unité de Fire Lake,suite plutonique de Simpson Range) et d’un magmatisme d’arrière-arc (unité de Kudz Ze Kayah; succession Wolverine)

Received 9 October 2001. Accepted 10 October 2002. Published on the NRC Research Press Web site at http://cjes.nrc.ca on29 January 2003.

Paper handled by Associate Editor L. Corriveau.

S.J. Piercey2,3 and J.K. Mortensen. Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, The University ofBritish Columbia, Vancouver, BC V6T 1Z4, Canada.R.A. Creaser. Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, ABT6G 1E3, Canada.

1Mineral Deposit Research Unit (MDRU) Contribution P-140.2Present address: Mineral Exploration Research Centre (MERC), Department of Earth Sciences, Laurentian University, Ramsey LakeRoad, Sudbury, ON P3E 2C6, Canada.

3Corresponding author (e-mail: [email protected]).

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édifiés sur un socle en transition qui subissait une influence variable, mais significative, des matériaux évolués de lacroûte (Protérozoïque).

[Traduit par la Rédaction] Piercey et al.

Introduction

Devonian–Mississippian felsic igneous rocks are widespreadthroughout the pericratonic terranes of the Cordillera in bothAlaska and Canada, but the relative importance of crust versusmantle sources, and (or) varying nature of basement contri-butions and types in their genesis is poorly constrained. Felsicigneous rocks of the Yukon–Tanana terrane (YTT) exhibitstrong evidence for the incorporation of significant quantitiesof evolved continental material. For example, most felsicvolcanic and intrusive rocks are characterized by inheritedProterozoic zircon (Mortensen 1992a, 1992b; Grant 1997),and have feldspar Pb-isotopic values that lie on the “shalecurve” (Mortensen 1994), a highly radiogenic Pb-isotopecrustal growth curve (µ = 12.16) for the Cordilleranmiogeocline (Godwin and Sinclair 1982). Furthermore, spatiallyassociated syngenetic volcanogenic massive sulphide (VMS)and sedimentary exhalative (SEDEX) sulphide mineralizationexhibit evolved Pb-isotopic signatures (Mortensen 1994 andunpublished data). The limited Nd isotopic data that is presentlyavailable for igneous rocks within the YTT point to asubstantial influence from evolved crustal material(Mortensen 1992a, 1992b; Stevens et al. 1996; Grant 1997;Creaser et al. 1997). Similarly, sedimentary rocks from theYTT show evidence for mixing between juvenile andevolved sources but with contributions from old continentalcrustal material (Creaser et al. 1997). There is, however, apaucity of Nd isotopic data on stratigraphically and temporallyconstrained felsic rocks within YTT, and the relative rolesthat continental crust, oceanic crust, mantle, and subductedslab have played in the genesis of YTT felsic rocks is poorlyunderstood.

Recent studies have been aimed at answering the latterquestions, but have concentrated primarily on the geochemistryof felsic volcanic rocks (Piercey et al. 1999, 2001a; Grant1997). This has resulted in only a rudimentary understandingof the geochemistry of YTT felsic intrusive rocks and thegeochemical, isotopic, and petrological relationships betweenintrusive and extrusive phases (e.g., Stevens et al. 1996;Grant 1997; Piercey et al. 2001a; Colpron 2001). Given theimportance of subvolcanic intrusive complexes in generatingand maintaining VMS hydrothermal systems (Campbell etal. 1981; Lesher et al. 1986; Galley 1996; Large et al. 1996),and the the recent VMS discoveries in felsic volcanic rocksof the YTT (e.g., Piercey et al. 2001a), understanding therelationships between extrusive and coeval subvolcanic intru-sive rocks is of paramount significance for the localization andexploration for new VMS occurrences.

The main goals of this paper are to use the geochemical,and in particular Nd-isotopic data, to elucidate the relativeroles of mantle and continental crust in the genesis of felsicintrusive and extrusive rocks of the YTT in the FinlaysonLake region. The excellent stratigraphic and temporal controlon the felsic volcanic and intrusive rocks in this area(Murphy 1998; Murphy and Piercey 1999a, 1999b, 2000)

makes it possible to obtain insight into the stratigraphic andsecular variations in the geochemical and isotopic characteristicsof felsic rocks, which in turn provide insight into the tectonicsetting and evolution of the YTT in the Finlayson Lake region.Finally, many felsic rocks in this district are spatially associatedwith VMS mineralization, and the identification of whichsubvolcanic intrusions are coeval with VMS-associated volcanicrocks is of exploration significance and constitutes the finalgoal of this paper.

Geological setting

The YTT in the Finlayson Lake region consistspredominantly of mid- to late-Paleozoic volcanic, intrusiveand sedimentary rocks within three unconformity-boundedsuccessions, the Grass Lakes, Wolverine Lake, and CampbellRange successions (Murphy and Piercey 1999a, 1999b, 2000;Piercey et al. 2001a). The Grass Lakes succession consistsof a lowermost unit of quartz-rich metaclastic rocks, oflikely continental affinity (e.g., Grant 1997), which is overlainby the �365–360 Ma (Mortensen 1992a) mafic volcanic-dominated Fire Lake unit. The Fire Lake unit is overlain bythe 360–356 Ma felsic volcanic and sedimentary rock-dominatedKudz Ze Kayah unit and unit 4 (Figs. 1, 2; Murphy andPiercey 1999a, 1999b, 2000; Mortensen 1992a. 1992b; Grant1997). Details on the petrology and geochemical attributesof the Kudz Ze Kayah unit can be found in Piercey et al.(2001a).

The Grass Lakes succession is intruded by voluminousgranitic intrusions of the Grass Lakes suite, which are interpretedto be the plutonic equivalent of the Kudz Ze Kayah unit(Figs. 1, 2). The Grass Lakes suite is characterized by foliatedperaluminous, K-feldspar porphyritic to megacrystic granitethat intrudes both the Kudz Ze Kayah unit and unit 4,(Figs. 1, 2). In general, the Grass Lakes suite forms intrusivebodies with bulbous forms that in places have dyke- and sill-likeapophyses and underlie large tracts of the Finlayson Lakeregion (Fig. 2; Murphy 1998; Murphy and Piercey 1999a,1999b, 2000). These intrusions are relatively unaltered andconsist predominantly of augen-shaped K-feldspar phenocryststo megacrysts in a foliated siliceous matrix, with minorplagioclase feldspar and quartz phenocrysts. Most feldsparsare unaltered but some show minor sericite alteration. Structuralfabric development in the granitoids has resulted in many ofthe samples having recrystallized quartz and minor biotiteand muscovite along fabric planes. Locally, the intrusionsare massive to weakly foliated, but in other areas, they havebeen overprinted by a strong Cretaceous ductile fabric (cf.Murphy 1998), and they have a strongly developed foliationand, in places, well-developed shear bands. The Grass Lakessuite granitoids are spatially associated with the Kudz ZeKayah and GP4F deposits, (Figs. 1, 2; see later in the text)and Mortensen (1992a) obtained a concordant 360 ± 1 MaU–Pb zircon age on a Grass Lakes suite intrusion, overlapping

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the ages for the volcanic rocks of the Kudz Ze Kayah unit(Mortensen 1992a; Piercey and Mortensen, unpublished data).

The Wolverine Lake succession unconformably overliesthe Grass Lakes succession and is a felsic volcanic andsedimentary rock-dominated succession that is younger andless deformed than the Kudz Ze Kayah unit (Murphy andPiercey 1999a, 1999b) and hosts the Wolverine VMS deposit.Felsic volcanic rocks occur throughout the unit, includingporphyritic to non-porphyritic types below the Wolverine deposit(unit 5f/qfp, unit 6-hw) and aphyric rhyolitic rocks above

the deposit (unit 6-hw; Figs. 1, 2; cf. Piercey et al. 2001a).High-level intrusions in the immediate subvolcanic environmentoccur within the Wolverine Lake succession in the immediatefootwall to the Wolverine VMS deposit (Piercey et al. 2001b);however, there are no mid-crustal level (�2–3 km depth)intrusions of the scale of the Grass Lakes suite or SimpsonRange plutonic suite within the Wolverine Lake succession.These features suggest that the Wolverine Lake successiondoes not have an exposed subvolcanic intrusive complex.Capping the entire Wolverine Lake succession are massive

Fig. 1. Geological map of the Finlayson Lake district illustrating the relationships of granitoid intrusive rocks to volcanic rocks andvolcanic-hosted massive sulphide (VMS) deposits. Map modified after Murphy and Piercey (1999a, 1999b, 2000).

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basalts, volcaniclastic rocks, and lesser sedimentary rocks(�200 m–thick unit; Bradshaw et al. 2001). Field andgeochronological constraints show that the Wolverine Lakesuccession is younger than the Grass Lakes succession;current U–Pb zircon ages suggest it ranges from 356 ± 1 Ma(Mortensen 1992a) to 346.6 ± 2.2 Ma (Piercey 2001).

The Simpson Range plutonic suite is located within thesouthwestern portion of the Finlayson Lake region (Fig. 1)and in the hanging wall of the Money Creek thrust (MCT;Murphy and Piercey 2000). These granitoids are composedof massive to foliated granitoid batholiths and apophysesthat include hornblende-granodiorite, biotite-monzogranite,and K-feldspar porphyritic granite (Mortensen 1992a; Grant1997; Piercey and Murphy 2000). Intrusions of the SimpsonRange plutonic suite are relatively pristine and are lessstrained than the Grass Lakes suite granitoids; however, inthe immediate hanging wall of the MCT the rocks arestrongly foliated and have been involved in late Paleozoicthrusting along the Money Creek thrust fault (Fig. 1;Murphy and Piercey 2000). The Simpson Range plutonicsuite is also distinct from the Grass Lakes suite in containingabundant hornblende and biotite. The Simpson Rangeplutonic suite is not known to be associated with significantVMS mineralization, and current U–Pb zircon ages suggestranges from �358 to �345 Ma (Mortensen 1983, 1992a;Grant 1997; Mortensen unpublished data). Although theseages are coeval with the Wolverine Lake succession, theSimpson Range plutonic suite granitoids have differing geo-chemical systematics than the felsic rocks in the WolverineLake succession and are interpreted to be arc-relatedgranitoids (Grant 1997; see later in the text), whereas theWolverine succession is interpreted to represent back-arcmagmatism (Piercey et al. 2001a).

The rocks of the Finlayson Lake district have been subjectedto various degrees of deformation, but most of the deformationpostdates the Devonian–Mississippian formation of the rocksof this study. In the Late Pennsylvanian, rocks of the FireLake unit were displaced �30 km from the west-southwestto their present position (Figs. 1, 2; Murphy and Piercey2000; Murphy 2001; Murphy et al. 2002). The dominantfabric in the Finlayson Lake region is related to Cretaceousductile deformation and low displacement southwest-vergentfolding and thrusting (Murphy 1998). A Mississippian eventof uncertain kinematics has affected the Grass Lakes succession;however, this will be addressed in the discussion in the contextof the regional evolution of the YTT in the Finlayson Lakeregion.

Lithogeochemistry and isotope geochemistry

Felsic volcanic and intrusive rocks were collected during1 : 50 000 scale regional mapping and from diamond drillcore. The sample suite provides a comprehensive stratigraphicand temporal distribution of felsic rocks that can be used toassess any stratigraphic and secular variations in Nd isotopegeochemistry. Although an attempt was made to collect leastaltered samples through field screening, thin section evaluation,and mobile element geochemistry, in some cases alterationwas inevitable, especially in the volcanic rocks (see Pierceyet al. 2001a). Most geochemical interpretations are thereforebased on immobile major and trace elements and Nd-isotopicdata. Samples analyzed were �1 kg or larger in size hadweathered edges removed by a diamond saw, whereas drillcore samples had sufficiently fresh surfaces. Samples werepulverized in a steel jaw crusher with most subsequentlypulped and powdered in a ceramic mill. Some samples were

Fig. 2. Schematic cross section of the Money Creek thrust illustrating the relationships of felsic intrusive suites in the Finlayson Lakeregion to the volcano-sedimentary units and volcanogenic massive sulphide (VMS) deposits. Figure modified after Murphy and Piercey(2000). SRPS, Simpson Range plutonic suite.

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pulped in a Cr-steel mill and others in a tungsten carbidemill. Samples that have been crushed by Cr-steel likely haveexcess Cr values, whereas those crushed by tungsten carbidelikely have excess Ta. Given these constraints, we havedenoted samples crushed by these methods in Table 1 andcontaminant elements (Cr, Ta) are not used for in this paper.Powders from the aforementioned steps were used in allanalytical techniques.

Major elements were determined on fused beads by X-rayfluorescence (XRF), with the exception of H2O and CO2,which were analyzed by infrared spectroscopy, and FeOanalyzed by modified Wilson titration. Samples analyzed fortrace elements were totally dissolved using a combination ofnitric, perchloric, and hydrofluoric acids, with a lithiummetaborate flux if any residual material existed after the firstacid attack. These solutions were then analyzed for traceelements using a combination of inductively coupled plasmaemission spectrometry (ICP–ES: Ba, La, Pb, Sc, Sr, V, Y,Yb, and Zr if > 100 ppm) and inductively coupled plasmamass spectrometry (ICP–MS: remaining rare-earth elements(REE), Cs, Rb, Th, U, Ga, Hf, Ta, and Zr if < 100 ppm).Further details on the methodology can be obtained from theGeological Survey of Canada at << http://132.156.95.172/chemistry >>. Details on the precision and accuracy of thesemethods have been previously reported in Piercey (2001)and Piercey et al. (2001a).

Neodymium isotopic geochemistry was completed at theUniversity of Alberta Radiogenic Isotope Facility usingthermal ionization mass spectrometry (TIMS), following themethods of Creaser et al. (1997). Values for the GeologicalSurvey of Japan (GSJ) Shin Etsu Nd isotope standardyielded an average 143Nd/144Nd = 0.512106 with an analyticaluncertainty of ±0.000012 (1σ), which is interpreted to be theminimum uncertainty estimate of the 143Nd/144Nd for anyparticular sample (Creaser et al. 1997). Neodymium isotopedata are presented relative to a value of 143Nd/144Nd = 0.512107for the Geological Survey of Japan Shin Etsu Nd standard,which is equivalent to 0.511850 for the La Jolla standard(Tanaka et al. 2000). Initial 143Nd/144Nd ratios and εNd werecalculated at 350 Ma the approximate age of all of the samplesin this study, and to facilitate comparison to other datagenerated in the YTT (Grant 1997; Creaser et al. 1997).

Geochemical data presented in this paper include samplesfrom the Grass Lakes suite and Simpson Range plutonicsuite, and for the sake of completeness we have includeddata from Piercey et al. (2001a) for felsic volcanic rocks forwhich Nd isotopic data are presented (Tables 1, 2). Neodymiumisotopic data are presented for the latter intrusions (n = 2),and from tholeiitic and calc-alkalic felsic volcanic rocksfrom the Fire Lake unit (n = 2), felsic rocks in the Kudz ZeKayah unit (n = 2), and from the unit 5f/qfp (n = 2), unit 6-fw(n = 2), and aphyric rhyolites of unit 6-hw (n = 1); thesedata are presented in Table 3. In addition, geochemical andNd isotopic data for the Simpson Range plutonic suite fromGrant (1997) are compiled and compared throughout this paper.

Geochemical attributes of the intrusive rocks

The main geochemical attributes of the felsic volcanicrocks have been previously described in Piercey et al. (2001a)and are not repeated herein. The geochemical data for felsic

volcanic Nd samples is shown for comparison to data for theintrusions on Figs. 3–6. The geochemical data and key elementratios for the felsic intrusive rocks are presented in Tables 1and 2. The Grass Lakes suite is characterized by high SiO2contents and granitic Zr/TiO2 ratios (Tables 1, 2; Fig. 3a).The molar Al/(Ca + Na + K) and Al/(Na + K) ratios(A/CNK and A/NK, respectively) for the Grass Lakes suiteare broadly peraluminous but are variable and some of thehigher values likely reflect increased Al at the expense ofCa–Na–K due to alkali mobility during hydrothermaldestruction of feldspar (Table 2). The variable Al2O3/Na2Oratios and alkali contents (Tables 1, 2) suggest that there hasbeen alkali and major-element mobility during alteration/metamorphism. The immobile high field strength element(HFSE) contents of the Grass Lakes suite granitoids are highand they plot in the within-plate (A-type) field on the Nb–Yplot of Pearce et al. (1984) (Fig. 4a). The HFSE-enrichmentis also reflected in the high Zr contents and high Ga/Al ratiosof Grass Lakes suite granitoids, as they plot on the edge ofthe A-type and I- and S-type granite fields in Fig. 4b. Theelevated HFSE contents of the Grass Lakes suite granitoidsare also reflected by their Zr–Nb systematics as the GrassLakes suite are displaced towards higher Zr and Nb contentsrelative to the Simpson Range plutonic suite (Fig. 5a; Table 1).The Nb/La and Zr/La ratios of the Grass Lake suite granitoids(and Simpson Range plutonic suite) provide a good indicatorof the amount of HFSE and REE enrichment present in theGrass Lakes granitoids, and it is notable that they trendtowards higher average Nb/La (0.65) and Zr/La (5.6) valuesrelative to the Simpson Range plutonic suite (Fig. 5b–5c;Table 2). Piercey et al. (2001a) illustrated HFSE/compatibleelement ratios (Zr/TiO2, Zr/Sc), and compatible element ratios(Ti/Sc) were important in discriminating betweenVMS-associated and VMS-barren felsic rocks in the YTT.The first two ratios were interpreted to measure theHFSE-enrichment (Zr) of the felsic rock relative to the degreeof compatible-element (Sc, TiO2) fractionation; whereas theTi/Sc ratio was interpreted to reflect the crustal source of therocks (e.g., Lentz 1999). Similar to Piercey et al. (2001a),these ratios discriminate between the VMS-associated GrassLakes suite with high Zr/TiO2, Zr/Sc, and Ti/Sc values, fromthe Simpson Range plutonic suite, with lower values forthese ratios. Furthermore, the values for the Grass Lakesgranitoids overlap the fields for the VMS-relatedHFSE-enriched felsic volcanic rocks of the Finlayson Lakeregion (Fig. 5d–5f). The primitive-mantle-normalized plot ofthe Grass Lakes suite granitoids also illustrates the afore-mentioned features (Fig. 6d).

The Simpson Range plutonic suite has lower SiO2 contentsand Zr/TiO2 ratios compared to the Grass Lakes suite (Tables 1,2; Fig. 3a). The A/CNK and A/NK ratios have values thatare transitional between peraluminous and metaluminousgranitoids, but there are variations in the alkali element contents(Tables 1, 2). The Al2O3/Na2O ratios of the samples arerelatively constant, but the minor variability likely reflectsalkali-element mobility; one sample (P99-104) has a veryhigh Al2O3/Na2O ratio (�40), suggesting that it is intenselyaltered (Table 2), and this is also reflected on the primitivemantle-normalized plot in which the REE appear to be mobile(Fig. 6). The HFSE contents of the Simpson Range plutonicsuite are lower than those of the Grass Lakes suite and they

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Sample: P98-39* P98-25^ P98-40^ P98-42^ P99-135 P99-15 P99-24 P99-82

Rock type: KFG KFG KFG KFG KFG KFG HbGd HbD

Suite or unit: GLS GLS GLS GLS GLS GLS SRPS SRPS

Easting (m): 430100 426250 429750 432000 435650 426810 428487 429853

Northing (m): 6806650 6807250 6811450 6807050 6798300 6799611 6793907 6787687

SiO2 (wt.%) 74.4 75.6 76.0 72.5 73.2 77.5 64.8 61.3

TiO2 0.29 0.16 0.27 0.58 0.26 0.08 0.43 0.52

Al2O3 13.4 12.6 12.6 12.5 13.5 13.0 13.0 15.1

Fe2O3T 2.6 2.0 1.2 3.9 2.1 0.8 5.3 7.7

Fe2O3 0.8 0.3 0.5 0.8 0.9 0.4 1.7 2.7

FeO 1.6 1.5 0.6 2.8 1.2 0.4 3.6 5.0MnO 0.01 0.01 0.01 0.07 0.01 < 0.01 0.12 0.16

MgO 1.1 0.7 0.9 1.2 0.4 0.8 2.4 3.2

CaO 0.7 0.2 0.3 0.4 0.5 0.3 3.6 3.7

Na2O 3.2 3.3 3.3 4.1 2.8 1.8 2.1 2.3

K2O 3.0 4.4 4.0 3.0 6.3 4.8 4.3 3.2

P2O5 0.15 0.09 0.16 0.09 0.10 0.10 0.10 0.16

H2O 1.2 0.7 0.9 1.0 0.6 1.3 2.3 3.0

CO2 0.1 0.2 0.1 0.1 < 0.1 0.1 1.8 0.6

Sc (ppm) 4.1 3.3 3.8 8.6 5.5 2.9 15.0 25.0

V 21 < 5 25 25 10 < 5 83 149Rb 88 170 90 92 300 140 130 100Cs 1.2 6.3 0.4 0.5 13.0 3.5 2.9 0.7Ba 1000 215 1498 567 610 390 1400 870Sr 46 30 53 38 60 38 250 200Ga 19 17 16 19 22 19 14 17Ta 1.4 3.0 2.6 3.0 2.2 2.3 0.9 1.1Nb 19.0 21.0 19.0 30.0 30.0 20.0 11.0 12.0Hf 4.4 3.5 4.3 12.0 6.9 3.7 4.8 5.3Zr 180 100 173 528 240 100 150 170Y 30 41 29 47 63 36 17 21Th 16.0 18.0 15.0 21.0 29.0 15.0 17.0 60.0U 2.5 3.2 2.6 5.1 6.1 2.8 4.0 2.4La 39.0 30.0 39.0 45.0 64.0 18.0 27.0 112.0Ce 84.0 65.0 82.0 100.0 140.0 40.0 53.0 200.0Pr 9.1 7.4 9.5 11.0 15.0 4.4 5.6 16.0Nd 33.0 26.0 34.0 43.0 55.0 16.0 20.0 46.0Sm 6.5 6.0 6.5 8.4 11.0 4.1 3.6 5.5Eu 1.1 0.4 1.1 1.2 0.6 0.3 0.6 0.8Gd 5.8 5.4 5.0 7.2 9.9 4.5 3.0 4.0Tb 0.92 1.00 0.84 1.30 1.80 0.90 0.47 0.60

Dy 4.9 6.6 4.6 7.7 10.0 6.0 2.8 3.4

Ho 0.98 1.30 0.92 1.60 2.00 1.20 0.61 0.67Er 2.6 3.7 2.5 4.5 5.4 3.2 1.7 1.9Tm 0.44 0.56 0.37 0.70 0.83 0.50 0.27 0.32Yb 3.0 3.7 2.4 5.1 5.2 3.4 1.9 2.3Lu 0.43 0.49 0.33 0.73 0.78 0.45 0.32 0.37

Table 1. Geochemical data for Yukon–Tanana terrane felsic (+intermediate) intrusive and volcanic rocks from the Finlayson Lake region.

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Sample P99-88 P99-108A P99-115 P99-118 P99-104 P99-45a P99-39a P98-KZK2*a

Rock type HbD ShGD KFG KFG ShGD RHY RHY QFP-I

Suite or unit SRPS SRPS SRPS SRPS SRPS FLU-CAR FLU-TR KZK

Easting (m) 436450 433541 433459 434180 437911 424931 426200 414600

Northing (m) 6787380 6784256 6781430 6782820 6785126 6793923 6794195 6814400

SiO2 (wt.%) 51.2 69.5 76.6 66.4 65.2 72.2 71.4 75.60

TiO2 0.59 0.32 0.04 0.53 0.82 0.26 0.33 0.29

Al2O3 15.7 14.6 12.4 14.2 16.8 14.2 12.7 12.30

Fe2O3T 10.0 2.0 1.2 5.1 6.7 2.6 2.9 1.90

Fe2O3 3.0 0.9 1.0 2.5 3.0 2.1 2.6 0.70

FeO 7.0 1.1 0.2 2.6 3.7 0.4 0.3 1.10MnO 0.18 0.05 0.03 0.06 0.22 0.03 0.17 0.01

MgO 7.6 1.5 0.2 1.9 2.6 1.0 1.3 0.79

CaO 7.7 1.5 0.4 2.0 0.0 0.8 2.4 0.23

Na2O 3.5 2.6 3.2 2.8 0.4 3.1 0.0 1.10

K2O 1.2 5.9 5.1 3.8 3.3 2.9 1.1 6.71

P2O5 0.10 0.10 0.02 0.15 0.08 0.06 0.06 0.09

H2O 3.0 1.3 0.6 2.4 4.2 2.2 4.3 1.00

CO2 0.1 0.3 <0.1 1.0 0.1 0.9 2.8 0.10

Sc (ppm) 28.0 5.3 2.0 11.0 25.0 7.4 13.0 4.7

V 259 33 5 74 103 < 5 8 10Rb 44 170 260 150 150 72 33 210Cs 1.4 2.0 2.8 2.8 7.0 7.2 1.9 1.20Ba 1500 4200 230 1100 3400 470 260 580Sr 460 200 34 440 51 100 150 20Ga 15 17 12 15 19 16 17 19.0Ta 0.3 1.3 2.3 1.3 0.8 1.1 0.5 1.7Nb 5.4 17.0 14.0 17.0 13.0 19.0 9.8 25.0Hf 1.9 4.4 4.1 4.6 1.8 5.0 3.5 5.9Zr 75 140 100 150 110 200 200 230Y 14 27 19 19 7 25 44 33Th 2.9 24.0 35.0 25.0 4.5 10.0 2.4 23.0U 0.8 5.1 3.2 4.8 2.6 2.1 0.4 6.10La 12.0 64.0 24.0 58.0 3.0 35.0 21.0 42.00Ce 24.0 100.0 47.0 100.0 7.3 67.0 47.0 110.00Pr 2.9 11.0 4.9 11.0 1.0 7.2 6.5 10.00Nd 12.0 38.0 15.0 37.0 4.6 27.0 29.0 37.00Sm 2.5 6.2 2.9 5.6 1.2 4.8 8.0 7.20Eu 0.7 1.2 0.2 1.0 0.0 0.9 1.9 0.51Gd 2.4 5.0 2.5 4.2 1.2 4.4 8.1 6.20Tb 0.39 0.78 0.46 0.59 0.21 0.67 1.30 1.00

Dy 2.3 4.3 2.7 3.2 1.4 3.9 7.5 5.90

Ho 0.46 0.84 0.59 0.62 0.30 0.81 1.50 1.20Er 1.2 2.3 1.8 1.6 1.0 2.3 4.1 3.20Tm 0.20 0.37 0.34 0.25 0.16 0.40 0.65 0.54Yb 1.3 2.5 2.8 1.7 1.2 2.9 4.4 3.60Lu 0.21 0.41 0.50 0.28 0.22 0.45 0.63 0.52

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Sample P98–52^a P98–102*a P98–145*a P99-WV-4La, b P98–69*a P99-WV-3Da, b

Rock type FPRI RHY FT FT QFP APRHY

Suite or unit KZK WV-5F WV-5F WV-6-FW WV-6-FW WV-6-HW

Easting (m) 393911 439390 434900 439536 433790 440016

Northing (m) 6811945 6807680 6809300 6811509 6815820 6810931

SiO2 (wt.%) 67.2 79.9 75.1 69.3 75.7 73.0

TiO2 0.65 0.15 0.14 0.39 0.41 0.61

Al2O3 15.9 10.6 13.6 14.0 13.9 11.9

Fe2O3T 3.9 0.1 3.1 3.9 0.2 2.6

Fe2O3 2.2 0.0 2.1 — 0.0 —

FeO 1.5 0.3 0.9 — 0.2 —MnO 0.04 <0.01 <0.01 <0.01 <0.01 0.02

MgO 0.6 0.0 1.0 0.6 0.1 1.3

CaO 0.3 0.1 0.3 0.6 0.1 0.6

Na2O 0.3 1.4 0.1 1.4 1.3 0.2

K2O 9.5 7.1 5.2 5.8 7.1 2.2

P2O5 0.21 0.03 0.07 0.14 0.05 0.04

H2O 1.9 0.2 2.0 <0.1 0.8 3.1

CO2 0.2 0.1 0.1 0.3 0.1 0.3

Sc (ppm) 8.8 1.9 4.2 8.2 5.9 22.0

V 26 <5 <5 19 9 179Rb 190 180 150 140 150 130Cs 2.9 3.9 0.6 2.0 2.0 9.5Ba 1700 470 1600 2600 1400 9531Sr 55 28 15 73 45 320Ga 23 15 24 24 21 22Ta 3.2 1.6 2.3 2.1 2.0 0.8Nb 42.0 19.0 29.0 36.0 34.0 12.0Hf 13.0 4.8 4.5 12.0 11.0 4.4Zr 590 160 140 470 500 160Y 37 19 65 54 30 26Th 45.0 27.0 22.0 33.0 31.0 8.2U 4.4 3.6 3.4 5.6 2.3 2.5La 124.0 23.0 38.0 94.0 110.0 25.0Ce 220.0 52.0 79.0 200.0 200.0 64.0Pr 23.0 5.0 9.4 23.0 23.0 6.7Nd 86.0 18.0 34.0 82.0 79.0 25.0Sm 15.0 3.5 8.3 15.0 12.0 5.3Eu 2.2 0.2 0.3 1.7 0.9 0.5Gd 12.0 3.0 8.7 12.0 8.1 4.6Tb 1.60 0.49 1.60 1.70 0.99 0.71

Dy 7.5 3.0 9.6 9.2 4.8 4.3

Ho 1.40 0.64 2.10 1.90 0.93 0.95Er 3.8 1.9 5.8 4.7 2.6 2.6Tm 0.55 0.35 0.95 0.72 0.44 0.44Yb 3.8 2.4 5.9 4.7 2.9 2.9Lu 0.59 0.35 0.81 0.75 0.45 0.49

Note: All samples from Universal Transverse Mercator (UTM) Zone 9V, North American Datum (NAD) 27. *, crushed in Cr-steel mill; ^, crushed intungsten carbide; all others crushed in ceramic mill. Rock type: KFG, K-feldspar porphyritic granite; HbGd, hornblende granodiorite; HbD, hornblende diorite;ShGD, sheared granodiorite; RHY, rhyolite flow; FPRI, feldspar porphyritic high-level intrusion; FT, felsic tuff, QFP, quartz–feldspar porphyritic rhyoliteflow; APRHY, aphyric rhyolite flow. Suite/unit: GLS, Grass Lakes suite; SRPS, Simpson Range plutonic suite; FLU-CAR, Fire Lake unit calc-alkalinerhyolite; FLU-TR, Fire Lake unit tholeiitic rhyolite; KZK, KZK unit; WV-5f/qfp, Wolverine succession, unit 5f/qfp; WV-6FW, Wolverine succession, unit6, footwall to Wolverine deposit; WV-6HW, Wolverine succession, unit 6, hanging wall to Wolverine deposit.

aCompiled from Piercey et al. (2001a); Ni, Cr, Cu, Pb, Zn, Bi, Cd, In, Sn, Mo, As, Ag, and Tl were analyzed but are not included in this table as theyare below or close to detection limits. These data are available from the senior author upon request.

bSample P99-WV-4L is from drill hole W95-03 at depth 158.2 m. Sample P99-WV-3D is from drill hole W95-04 at 99.0 m depth.

Table 1. (concluded).

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Piercey et al. 85

Sample: P98–39* P98–25^ P98–40^ P98–42^ P99–135 P99–15 P99–24 P99–82

Rock type: KFG KFG KFG KFG KFG KFG HbGd HbDSuite or unit: GLS GLS GLS GLS GLS GLS SRPS SRPSA/CNKa 1.36 1.19 1.23 1.16 1.09 1.48 0.89 1.08A/NKa 1.57 1.24 1.30 1.25 1.18 1.59 1.61 2.08Al2O3/Na2O 4.19 3.82 3.82 3.05 4.82 7.22 6.19 6.57Ti/Sc 424 291 426 404 283 165 172 125Zr/Sc 43.9 30.3 45.5 61.4 43.6 34.5 10.0 6.8Zr/Nb 9.5 4.8 9.1 17.6 8.0 5.0 13.6 14.2Zr/TiO2 621 625 641 910 923 1250 349 327Zr/La 4.6 3.3 4.4 11.7 3.8 5.6 5.6 1.5Nb/Y 0.63 0.51 0.66 0.64 0.48 0.56 0.65 0.57Nb/La 0.49 0.70 0.49 0.67 0.47 1.11 0.41 0.11104 × Ga/Al 2.7 2.6 2.4 2.9 3.1 2.8 2.0 2.1La/Ybnb 9.3 5.8 11.7 6.3 8.8 3.8 10.2 34.9T-ZrSat 824 762 814 915 828 781 755 780

Sample P99–88 P99–108A P99–115 P99–118 P99–104 P99–45 P99–39 P98-KZK1*Rock Type HbD ShGD KFG KFG ShGD RHY RHY RHYSuite/Unit SRPS SRPS SRPS SRPS SRPS FLU-CAR FLU-TR KZKA/CNKa 0.75 1.09 1.08 1.15 3.98 1.47 2.28 1.22A/NKa 2.24 1.37 1.15 1.64 3.98 1.73 10.76 1.29

Al2O3/Na2O 4.49 5.62 3.88 5.07 42.00 4.58 — 37.33

Ti/Sc 126 362 120 289 197 211 152 377Zr/Sc 2.7 26.4 50.0 13.6 4.4 27.0 15.4 59.3Zr/Nb 13.9 8.2 7.1 8.8 8.5 10.5 20.4 7.6

Zr/TiO2 127 438 2500 283 134 769 606 941

Zr/La 6.3 2.2 4.2 2.6 36.7 5.7 9.5 5.5Nb/Y 0.39 0.63 0.74 0.89 1.86 0.76 0.22 0.75Nb/La 0.45 0.27 0.58 0.29 4.33 0.54 0.47 0.60104 × Ga/Al 1.8 2.2 1.8 2.0 2.1 2.1 2.5 2.5La/Ybnb 6.6 18.4 6.1 24.5 1.8 8.7 3.4 16.7T-ZrSat 642 776 755 785 830 839 872 810

Sample P98–52^ P98–102* P98–145* P99-WV-4L P98–69* P99-WV-3DRock Type FPRI RHY FT FT QFP APRHYSuite/Unit KZK WV-5f/qfp WV-5f/qfp WV-6FW WV-6FW WV-6HWA/CNKa 1.40 1.04 2.15 1.46 1.38 3.05A/NKa 1.47 1.06 2.34 1.64 1.41 4.36

Al2O3/Na2O 53.00 7.57 136.00 10.00 10.69 59.50

Ti/Sc 443 473 200 285 417 166Zr/Sc 67.0 84.2 33.3 57.3 84.7 7.3Zr/Nb 14.0 8.4 4.8 13.1 14.7 13.3

Zr/TiO2 908 1067 1000 1205 1220 262

Zr/La 4.8 7.0 3.7 5.0 4.5 6.4Nb/Y 1.14 1.00 0.45 0.67 1.13 0.46Nb/La 0.34 0.83 0.76 0.38 0.31 0.48104 × Ga/Al 2.7 2.7 3.3 3.2 2.9 3.5La/Ybnb 23.4 6.9 4.6 14.3 27.2 6.2T–ZrSat 941 796 833 926 933 863

Note: *, crushed in Cr-steel mill; ^, crushed in tungsten carbide; all others crushed in ceramic mill. Rock types and suites/units as in Table 1. Eu/Eu* =Eupm / (Gdpm × Smpm)0.5, where pm = primitive mantle normalized to values of Sun and McDonough (1989). T-ZrSat, zircon saturation temperature ofWatson and Harrison (1983).

aMolar Al/(Ca+Na+K) and Al/(Na+K) ratios.bNormalized to chondritic values of Sun and McDonough (1989).

Table 2. Key element ratios for the YTT felsic intrusive rocks.

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plot in the field for volcanic arc (I-type) granites (Fig. 4a)and have low Zr contents and Ga/Al ratios (Tables 1, 2),consistent with an I-type affinity (Fig. 4b). The lower HFSEcontents of the Simpson Range plutonics suite are reflectedby their lower Zr–Nb contents (Table 1) and Nb/La andZr/La values (Nb/Laavg = 0.35; Zr/Laavg = 5.7; Figs. 5a–5c;Table 2). Furthermore, similar to VMS-barren felsic volcanicrocks from the district, the Simpson Range plutonic suitehave lower Zr/TiO2, Zr/Sc, and Ti/Sc ratios when comparedto the Grass Lakes suite (Fig. 5; Table 2). The primi-tive-mantle-normalized plot of the Simpson Range plutonic

suite granitoids have broadly similar characteristics, andwith the exception of the altered P99-104, all the granitoidshave strong negative Nb and Ti anomalies typical ofarc-derived granitoids (Fig. 6e). The light REE (LREE)-en-richment is variable (Fig. 6e) but appears to be broadly cor-related with incompatible Zr, Nb, and Ga (Table 1),suggesting that the LREE-enrichment may be related to frac-tionation.

Neodymium isotopic systematicsNeodymium isotopic data for the felsic rocks of the

Finlayson Lake region are presented in Table 3 and Fig. 7.Felsic rocks from the Fire Lake unit have εNd350 = +0.1 to –4.8and TDM = 1.49–1.94 Ga (Fig. 7; Table 3). Felsic volcanic(Kudz Ze Kayah unit, Wolverine succession) and intrusive(Grass Lakes suite) rocks associated with VMS deposits inthe district have evolved Nd isotopic characteristics withεNd350 = –7.8 to –9.5 and TDM = 1.59–2.25 Ga (Fig. 7; Table 3).The VMS-barren Simpson Range plutonic suite sample has avalue of εNd350 = –12.9 and TDM = 2.01 Ga (Fig. 7; Table 3),which is similar to previously reported data for the SimpsonRange plutonic suite εNd350 = –7.4 to –12.7 and TDM =1.23–2.10 Ga (Grant 1997; Fig. 7).

Some workers have suggested that Nd-isotopic signaturescan be fractionated during hydrothermal alteration andmetamorphism (e.g., Prior et al. 1999), two processes thathave affected the rocks of this study to a certain degree. It isnotable, however, that the differences in the εNd350 values ofrocks from the same unit are negligible (Table 3). This suggeststhat the isotopic signatures of the felsic rocks have not beenfractionated during alteration and metamorphism and that theεNd350 values of the rocks reflect their source characteristics.

Discussion

Role of crustal material in genesis of felsic rocks in theFinlayson Lake region

The presence of evolved εNd350 and fSm/Nd values in allYTT felsic rocks in the Finlayson Lake region point to a

Sample Rock type Suite or unit

143 3NdNd144

147SmNd144 εNd350

a fSm/Nda εNd0

a TDMb

Sm (ppm) Nd (ppm)

P99-45 CAR FLU 0.512207 (8) 0.1159 –4.8 –0.41 –8.4 1.49 4.99 26.05P99-39 TR FLU 0.512578 (9) 0.1680 +0.1 –0.15 –1.2 1.94 8.22 29.59P98-KZK2 FPRI KZK 0.512068 (8) 0.1224 –7.8 –0.38 –11.1 1.82 6.86 33.91P98-52 FPRI KZK 0.511986 (8) 0.1011 –8.5 –0.49 –12.7 1.59 15.25 91.25P98-102 RHY WV-5f/qfp 0.512065 (8) 0.1208 –7.8 –0.39 –11.2 1.80 3.39 16.95P98-145 FT WV-5f/qfp 0.512125 (8) 0.1471 –7.8 –0.25 –10.0 2.36 7.33 30.15P99-WV-4L FT WV-6FW 0.512009 (7) 0.1055 –8.2 –0.46 –12.3 1.62 15.01 86.07P98-69 FPI WV-6FW 0.511994 (7) 0.0930 –7.9 –0.53 –12.6 1.47 12.03 78.24P99-WV-3D APHRY WV-6FW 0.512107 (6) 0.1244 –7.1 –0.37 –10.4 1.80 5.40 26.22P99-24 HbGd SRPS 0.511777 (6) 0.1092 –12.9 –0.44 –16.8 2.01 3.35 18.53P98-25 KFG GLS 0.512014 (7) 0.1361 –9.5 –0.31 –12.5 2.25 5.32 23.65P99-WV-10A BAS WV-6HW 0.513071 (15) 0.2315 +6.9 0.18 +8.5 — 3.05 7.95

Note: Rock types and suites/units as in Table 1. CAR, calc-alkaline rhyolite; TR, tholeiitic rhyolite; FPRI, feldspar (+quartz) porphyritic rhyoliticintrusion (high level); FPI, feldspar porphyritic rhyolitic intrusion (high level); BAS, basalt (mid-ocean ridge basaltic (MORB) end-member representativeof depleted mantle at 350 Ma; see Piercey et al. 2002). FLU, Fire Lake unit. Estimated 143Nd/144Nd uncertainties, in brackets, at the 2σ level.aCalculated using 143Nd/144Nd of chondrite uniform reservoir (CHUR) = 0.512638 and 147Sm/144Nd = 0.1966 (Hamilton et al. 1983).bCalculated using the values of 143Nd/144Nd = 0.513163 and 147Sm/144Nd = 0.2137 for the DM reservoir (Goldstein et al. 1984).

Table 3. Neodymium isotopic data for YTT felsic rocks in the Finlayson Lake region.

Fig. 3. Modified Winchester and Floyd (1977) plot of Pearce (1996)for YTT intrusive and volcanic rocks from the Finlayson Lake region.GLS, Grass Lakes suite; SRPS, Simpson Range plutonic suite;SRPS-G, Simpson Range plutonic suite from Grant (1997), FLU-TR,Fire Lake unit tholeiitic rhyolite; FLU-CAR, Fire Lake unitcalc-alkaline rhyolites; KZK-F, Kudz Ze Kayah unit felsic rocks;WV-5-F, Wolverine succession, unit 5 felsic rocks; WV-6-FW,Wolverine succession, unit 6, footwall felsic rocks from the Wolverinedeposit; WV-6-HW, Wolverine succession, unit 6, hanging wallaphyric rhyolite from the Wolverine deposit.

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significant role for crustal material in their genesis. Thesedata in conjunction with previous evidence from Nd–Sr–Pbisotopic data and U–Pb zircon inheritance (Mortensen 1992a,1992b; Grant 1997), and Pb-isotopic data on syngenetic felsicVMS-related sulphides (Mortensen 1994, and unpublisheddata) all support a significant role for crustal materials in thegenesis of the felsic rocks from the Finlayson Lake region.Notably, however, there are variations in the amount ofcrustal influence as indicated by the Nd data. In particular,the tholeiitic and calc-alkaline felsic volcanic rocks in theMoney Creek thrust sheet have higher values than all otherrocks in the district. The chondritic nature (εNd350 = +0.1)of the tholeiitic felsic rocks and the slightly negative(εNd350 = –4.8) values for the calc-alkalic felsic rocks suggestthat these rocks have seen less crustal influence than manyother rocks in the region (Table 3; Fig. 7). These data suggestthat the Fire Lake unit rocks may have been built upon acomposite basement of oceanic and continental material(e.g., Piercey et al. 2001a, 2001c). For example, the FireLake unit felsic rocks are interlayered with island-arctholeiitic and calc-alkalic mafic rocks with little evidence forcrustal contamination (Piercey 2001). Furthermore, elementratios interpreted to reflect the crustal source of the FireLake unit felsic rocks (Nb/Ta, Ti/Sc; see Piercey et al. 2001afor a detailed review) suggest that these felsic rocks werederived from melting a mixed mafic–felsic (or sedimentary)substrate (Piercey et al. 2001a). In contrast, all other felsicrocks, regardless of age or stratigraphic position, have verysimilar, evolved Nd isotopic signatures (Table 3; Fig. 7).The presence of strongly negative εNd350 and old TDM agesall suggest influence from a crustal source with an extendedhistory of LREE-enrichment (Table 3), typical of continentalbasement, or sedimentary rocks derived from such a source.

In an attempt to quantify the potential contribution ofcrustal material to the genesis of the felsic rocks, we havecalculated mixing lines (Langmuir et al. 1978) to potentialcrustal contributors. The assumption in these calculations isthat the granitoids originate, initially, by basaltic underplatingof continental crust and that the magmas are basaltic-crusthybrids (e.g., Huppert and Sparks 1988). The choice of basalticend members comes from model values of depleted mantle(DM) at 350 Ma (εNd = +9.5; Goldstein et al. 1984), and

mid-ocean ridge-type-basalt (MORB) from the Wolverinesuccession (εNd = +6.9; Piercey 2001), which likely representsthe local DM reservoir at this time (e.g., Piercey et al. 2002).The crustal end members are less well defined, as there is nocrystalline basement exposed within the YTT (Mortensen1992a).

Possible end members that may reflect the basement toYTT are the sedimentary lithologies within the lowermoststratigraphic assemblage of YTT. The evolved YTT sedimentaryrocks provide the best present estimate of rocks derived fromthe basement to YTT and as such are chosen as end members.The sedimentary dataset includes YTT sedimentary rocksfrom the Teslin zone (Creaser et al. 1997) and those fromthe Finlayson Lake region (Grant 1997). The data reportedby Creaser et al. (1997) are not from the YTT in theFinlayson Lake region but are used because they provide themost comprehensive dataset for crustally derived rocks in theYTT, and because they have similar isotopic and geochemicalattributes to sedimentary rocks in the Finlayson Lake region(e.g., Grant 1997). Their dataset includes three sedimentarygroups with variable contributions from juvenile to evolvedcrustal material, from most juvenile to most evolved beingNI, NII, and NIII. Samples from the Finlayson Lake regionare from quartz-rich metaclastic rocks of unit 1 (GrS), thelowermost sedimentary rocks of the Grass Lakes succession.These data are shown on mixing lines in Fig. 8.

In all of the plots in Fig. 8, mixing lines were calculatedfor the isotopic data plotted against isotopic and trace elementratios that reflect increasing crustal influence (147Sm/144Nd,Zr/Yb, Th/Yb, and La/Yb). Key features of note on theseplots are the overlap of the felsic samples of the FinlaysonLake region with the NI, NII, and GrS, suggesting that thesignatures of these contaminants are too juvenile and withoutenough crustal residence to explain the isotopic attributes ofthe felsic rocks. Mixing lines to the NIII reservoir oftenencompass the felsic dataset but do not yield unique mixinglines in which all felsic data lie upon the line indicative of aspecific sedimentary contaminant (Fig. 8). It is possible thatthe sedimentary material from the YTT is not representative ofthe crust that these felsic rocks have been derived from. Inparticular, these felsic rocks were likely formed at high levelsin the crust (e.g., garnet not stable in residue; e.g., Lesher et

Fig. 4. Discrimination plots of the felsic intrusive rocks in relation to volcanic rocks from the district, including the Nb–Y plot (a) ofPearce et al. (1984) and the Ga/Al–Zr plot (b) of Whalen et al. (1987). Only samples with MgO < 3% and SiO2 > 64% are includedin the plots. Symbols and abbreviations as in Fig. 3.

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Fig. 5. Key high field strength element (HFSE) and rare-earth element (REE) plots for the intrusive rocks of the YTT in the FinlaysonLake region. In (a) the Zr–Nb plot of Leat et al. (1986) illustrates the higher Zr and Nb contents of the Grass Lakes suite in relationto the Simpson Range plutonic suite and how they overlap with the HFSE-enriched volcanic rocks of the Kudz Ze Kayah unit andWolverine succession. The Zr–TiO2 (d) and Zr–Sc (e) plots illustrate the higher Zr/TiO2 and Zr/Sc ratios of the Grass Lakes suite relativeto the Simpson Range plutonic suite. Although there is overlap, the Grass Lakes suite have average Ti/Sc (f), Nb/La (b), and Zr/La (c) ratiosthat are higher than those of the Simpson Range plutonic suite. Symbols and abbreviations as in Fig. 3.

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Piercey et al. 89

al. 1986; Piercey et al. 2001a); however, their upper crustalsource may not be reflected by the sedimentary lithologiesof the YTT, and as such these end members might not beappropriate for the mixing calculations.

Another notable feature of these felsic rocks is the lack ofunique mixing lines between εNd350 and the HFSE–REE ratios,reflecting a possible decoupling of the HFSE and REE fromthe Nd-isotopic signatures. For example, it is notable that theSimpson Range plutonic suite, Grass Lakes suite, and all felsicvolcanic rocks, with the exception of the Fire Lake unit,

have εNd350 values that are similar (Table 3), suggesting deri-vation from a common crustal reservoir. In contrast, thesesame felsic rocks have very distinctive HFSE–REE systematics(Figs. 3–6; Tables 1, 2). We suggest that the decoupledHFSE–REE systematics may be due the HFSE–REE beingcontrolled by accessory mineral phases (e.g., zircon, titanite,magnetite–ilmenite, monazite) either during continental crustalpartial melting or fractionational crystallization. For example,fractionation of accessory mineral phases could strongly affectthe trace element budget of the rocks (e.g., Bea 1996a, 1996b)

Fig. 6. Primitive mantle-normalized trace-element plots of (a) Fire Lake unit felsic volcanic rocks; (b) Kudz Ze Kayah unit felsic rocks;(c) Wolverine succession felsic rocks; (d) Grass Lake suite granitoids; and (e) Simpson Range plutonic suite granitoids. Samples in (a)to (c) are those with Nd isotopic analyses (data from Piercey et al. 2001a). Primitive mantle values from Sun and McDonough (1989).Symbols as in Fig. 3.

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but would have very minimal effect on the Nd-isotopicsignatures of the rocks. Another possibility, and one that theauthors favour in light of recent work (Piercey et al. 2001a),is that the variation in HFSE–REE contents reflects varioustemperatures of crustal fusion, and by association variabledegrees of efficiency of accessory mineral dissolution duringcrustal melting (Fig. 9; Watson and Harrison 1983; Creaseret al. 1991; Bea 1996a, 1996b; Watson 1996; Lentz 1999;Piercey et al. 2001a). Although calculated zircon saturationtemperatures can be affected by alkali-element mobility(Watson and Harrison 1983), the hypothesis that HFSE–REEvariation is due to variable temperatures of crustal fusion ispartly supported by the calculated zircon saturation temperatures(Table 2).

In contrast to HFSE–REE systematics, the Nd isotopicdata reflect the felsic magma source regions, and thus providethe best estimates of crustal involvement. In an attempt toprovide a quantitative assessment of crustal influence, theneodymium crustal index (NCI) of DePaolo et al. (1992) isemployed. The NCI is defined as

[ ] [ ]NCI = Nd Nd Nd Ndrock MC CC MCε ε ε ε− −/

where εNdrock is the εNd of the sample in question, εNdMC isthe εNd of the mantle component, and εNdCC is the εNd ofthe crustal component. The index assumes that the felsicrocks are mantle–crust hybrids (as earlier in the text), themost likely mechanism for generating granitic and felsicvolcanic rocks (e.g., Huppert and Sparks 1988). In thecalculations, the mantle end member is assumed to be theDM reservoir (εNd350 = +9.5), and the crustal reservoirs aretaken to be the average values of the NI, NII, NII, and GrSYTT sedimentary rocks of Creaser et al. (1997) and Grant

(1997). Results of the calculations are presented in Table 4and indicate that NI, NII and GrS samples are too juvenile toexplain the εNd values in the Finlayson Lake felsic rocks.Particularly, many of the calculations using these crustal endmembers require over 100% crustal Nd contributions(i.e., NCI > 1). The NIII sedimentary rocks have signaturesthat are sufficiently evolved to be a possible crustalcontributor. Calculations using this end member indicate thatthe Fire Lake unit felsic rocks in the Money Creek thrusthave 30–45% crustal Nd contributions, whereas all otherrocks of the district require 53–71% crustal Nd contributions.These results support the hypothesis that there are (were)differences between the basement in the Money Creek thrustand elsewhere in the Finlayson Lake region.

Tectonic significanceGeological, geochemical and isotopic data in this and other

papers (Grant 1997; Murphy and Piercey 1999a, 1999b, 2001;Piercey et al. 2001a) illustrate that there was a complex andepisodic history of arc magmatism and back-arc basin generationin the YTT of the Finlayson Lake region. In the Devonian–Mississippian (�365–360 Ma), calc-alkaline and tholeiiticfelsic volcanism were occurring concurrently with mafic arcmagmatism recording east-dipping subduction (present-daycoordinates; cf. Mortensen 1992a) with variable mantle versuscrustal influence (e.g., Piercey 2001). The variation in theNd isotopic signatures and the less evolved nature of theFire Lake unit felsic volcanism suggest that this arc systemwas likely built on transitional basement, partially floored byoceanic and continental (or continent-derived) crust (cf. Grant1997; Piercey et al. 2001a, 2001c).

At �360 Ma, arc volcanism was disrupted by arc rifting

Fig. 7. εNd350 versus 147Sm/144Sm for felsic rocks from the Finlayson Lake region. Shown for comparison are Nd isotopic data forYTT sedimentary rocks from the Teslin Zone (NI, NII, and NIII from Creaser et al. 1997), and unit 1 metasedimentary rocks of theYTT in the Finlayson Lake region (GrS from Grant 1997). DM = depleted mantle, CHUR = chondritic uniform reservoir. Symbols andabbreviations as in Fig. 3.

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and the commencement of ensialic back-arc basin formation(Fig. 10). This activity is recorded in the non-arc suites ofmafic rocks in the Fire Lake unit (Piercey 2001), the occurrenceof HFSE–REE-enriched felsic volcanism in the Kudz ZeKayah unit (Piercey et al. 2001a), and the HFSE–REE-enrichedgranitoid magmatism in the Grass Lakes suite (Fig. 10). Thevery evolved εNd signatures and Proterozoic TDM agessuggest that this back-arc system was centered on either old(Proterozoic) continental crust or rocks (sedimentary?) derivedfrom Proterozoic basement. This period of arc rifting andfelsic magmatism coincides with the formation of the KudzZe Kayah and GP4F VMS deposits (Fig. 10). This event isalso coeval with rifting, alkalic magmatism, and VMS depositformation (Wolf, MM deposits) in the Cassiar terrane(Mortensen and Godwin 1982; Mortensen 1982; Holbek andWilson 1998); rifting, clastic sedimentation, and VMS andSEDEX activity in the Selwyn basin and Kechika trough(Gordey et al. 1987; Irwin and Orchard 1989; Paradis et al.1998); and initial opening of the Slide Mountain Ocean(Nelson 1993; Nelson and Bradford 1993; Creaser et al.1999). These features point to a coupled tectonic and

metallogenic history for both the YTT and the NorthAmerican margin in the mid-Paleozoic (e.g., Paradis andNelson 2000).

The temporal extent of back-arc formation within theKudz Ze Kayah unit is uncertain, but there was a disruptionof back-arc spreading at �357 Ma (Murphy and Mortensen,unpublished data) in which there was uplift, deformation(Murphy 1998), and formation of an unconformity betweenthe Kudz Ze Kayah unit and the Wolverine Lake succession(Fig. 10; Murphy and Piercey 1999a, 1999b). At �356–346 Ma,there was a return to back-arc basin magmatism (Fig. 10;Mortensen 1992a; Piercey 2001). The persistence of similargeochemical and Nd-isotopic signatures in the lower WolverineLake succession to those from the Kudz Ze Kayah unit suggeststhat the back-arc magmatic system continued to produce felsicmagmatic rocks by high-temperature crustal melting thatwas indistinguishable from earlier Kudz Ze Kayah unitback-arc activity (Fig. 10). In the upper Wolverine Lake suc-cession, however, felsic rocks differ significantly from thosethat underlie Wolverine deposit horizon. At this transition, thereis a shift from HFSE-enriched felsic rocks below the deposit

Fig. 8. Nd isotopic trace-element mixing lines calculated for (a) 147Sm/144Nd, (b) La/Yb, (c) Th/Yb, and (d) Zr/Yb. Notable in all plotsis that the NI, NII, and GrS appear too juvenile to explain the Nd isotopic signatures of the rocks. Mixing lines to the NIII sedimentaryrocks provide mixing lines that encompass the felsic dataset but do not yield unique mixing lines. It is likely, however, that the lack ofunique mixing lines with Zr, Th, and La likely reflect the melt kinetic control on these elements during crustal melting rather than attributesof the crustal source (see discussion in text), as is reflected by the Nd isotopic data. Graticules on the mixing lines are every 20%.Sources of data are Creaser et al. (1997) and Grant (1997); DM, depleted mantle (Hamilton et al. 1983). Symbols as in Fig. 3 andsolid circle is a Wolverine succession basalt sample (Piercey et al. 2002).

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to HFSE-depleted above the deposit (Piercey et al. 2001a),yet both footwall and hanging wall have similar εNd350 values(Table 3), suggesting a common crustal source. The contrastingHFSE–REE characteristics between the two suites likelyreflect temperature effects and the kinetic efficiency ofcrustal melting, with the lower HFSE magmas reflectinglower temperature melts still within an evolving back-arc basin(Piercey 2001; Piercey et al. 2001a; Watson and Harrison1983; Bea 1996a, 1996b; Watson 1996). The capping of theWolverine succession by MORB lavas with εNd350 = +6.9(Piercey 2001; Piercey et al. 2002) suggests that felsicvolcanism in the succession eventually abated and the regionunderwent significant crustal attenuation and ultimatelysea-floor spreading occurred (Fig. 10).

The Wolverine Lake succession back-arc magmatism isbroadly coeval with the arc magmatism within the SimpsonRange plutonic suite, and it is likely that this suite of intrusions

represent the temporally coeval arc to the Wolverine back-arc(Fig. 10). The low HFSE contents of the granitoids, coupledwith their negative Nb anomalies on primitive mantle-normalizedplots (Fig. 6) are consistent with their formation within anarc environment. The presence of arc-derived rocks ofsimilar age and younger to the Simpson Range plutonicsuite in the Little Salmon succession of the Glenlyon regionof the Yukon (Colpron 2001) and in the Teslin zone ofsoutheastern Yukon (Stevens et al. 1996) suggest that coevalEarly- to mid-Mississippian arc magmatism was outboard(west) of the Finlayson Lake region (Fig. 10). The westwardmigration of arc magmatism from the Finlayson Lake regionin the Devonian–Mississippian to the western portions of theterrane in the Early Mississippian likely reflects slab rollback, which in turn induced back-arc spreading in theWolverine Lake back-arc (Fig. 10). Following arc and back-arcmagmatism in the Wolverine Lake succession, arc magmatismappears to have been primarily focussed in the more westernparts of the terrane for most of the Mississippian (Nelson etal. 2000; Colpron et al. 2001).

Implications for VMS mineralizationAs noted by many workers, significant VMS mineralization

is commonly associated with sizable subvolcanic intrusivecomplexes (Campbell et al. 1981; Cathles 1983; Galley1996; Large et al. 1996; Whalen et al. 1998). Furthermore,identifying which subvolcanic intrusive complexes are coevalwith VMS-bearing volcanic rocks is of significant importancein delineating prospective VMS-bearing successions. Basedon age and stratigraphic constraints, the subvolcanic GrassLakes suite granitoids are not coeval with the Fire Lake unit,but are likely coeval to slightly younger than the Kudz ZeKayah unit (Mortensen 1992a; Murphy and Piercey 1999a,1999b; Piercey and Mortensen, unpublished data). TheHFSE–REE systematics of the Grass Lakes suite granitoidsare very similar to the Kudz Ze Kayah unit volcanic rocks(Figs. 3–6) with similar Nb/La, Zr/La, Zr/Sc, Zr/TiO2, andTi/Sc ratios, and HFSE–REE contents (Figs. 3–6). The GrassLakes suite is also temporally equivalent to the Kudz ZeKayah unit (Mortensen 1992a) and has a similar εNd350value as the felsic volcanic rocks of the Kudz Ze Kayah unit,supporting the hypothesis that it is the subvolcanic intrusivesystem to the Kudz Ze Kayah unit. The HFSE–REE charac-teristics of the Simpson Range plutonic suite diverge fromthe Grass Lakes suite and are characterized by lowerHFSE–REE contents, lower Zr/Sc and Zr/TiO2 values, andlower average Nb/La and Zr/La ratios (Fig. 5; Table 2).Furthermore, significant quantities of VMS mineralizationhave yet to be found associated with the Simpson Rangeplutonic suite, although several small VMS occurrences havebeen previously reported (e.g., the Kneil occurrence). Sincethere are abundant K-feldspar porphyritic to megacrystic (or“augen”) bearing granites within the YTT in both Yukon andAlaska (Mortensen 1992a), identification of granitoids withHFSE-enrichment; high Zr/Sc, Zr/TiO2, Zr/La, and Nb/Lavalues; and evolved isotopic signatures is of paramountsignificance in delineating potentially fertile versus barrenVMS environments in the YTT.

The elevated HFSE and REE contents of the Grass Lakessuite are virtually identical to coeval VMS-associated felsicvolcanic rocks in the Kudz Ze Kayah unit (Figs. 5–6). This

Fig. 9. Schematic diagram to illustrate the differences in HFSE–REEcontents in different magma batches due to differential fusiontemperatures of a common crustal source and their relationshipto volcanogenic massive sulphide (VMS) mineralization. Withincreasing temperature (T), the efficiency of dissolution ofHFSE–REE-enriched accessory mineral phases increases andmelts derived from higher temperature melting contain higherabundances of HFSE and REE. By virtue of their higher temperaturederivation, HFSE–REE-enriched granitoid subvolcanic intrusivecomplexes can drive more vigorous and longer lasting hydrothermalsystems (Campbell et al. 1981; Cathles 1983; Galley 1996). Thisappears to be the case in the Finlayson Lake as theHFSE–REE-enriched Grass Lakes suite granitoids (hightemperature) are associated with VMS deposits, whereas thelower HFSE–REE-bearing Simpson Range plutonic suite is largelybarren of associated mineralization.

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association of VMS deposits with HFSE–REE enriched felsicmagmatism also holds true for the Wolverine succession inthe district (Piercey et al. 2001a), and for similar environmentsin the Bathurst Mining camp (Whalen et al. 1998; Lentz1999). The common association of VMS deposits withHFSE–REE-enriched felsic magmatism in these districts impliesa link between the petrogenesis of these felsic rocks and oredeposit formation. The low values of Sc and TiO2 likely reflectthe fractionation of phases such as magnetite (± titanite; e.g.,Lentz 1999; Piercey et al. 2001a) within high-level subvolcanicmagma chambers. Lesher et al. (1986) have previously arguedthat extensive crystal fractionation is typical of felsic rocksthat have formed at high levels in the crust, and the low Scand TiO2 imply that the Grass Lakes suite has undergoneextensive fractionation at high levels in the crust.

Accompanying the low Sc and TiO2 are high values ofHFSE that lead to high Zr/Sc and Zr/TiO2 ratios. The elevatedHFSE and REE contents of the Grass Lakes suite, and otherVMS-associated felsic volcanic rocks, likely reflecthigh-temperature melting of continental crustal material(Fig. 9; Watson and Harrison 1983; Creaser et al. 1991; Bea1996a, 1996b; Watson 1996). Many workers have shownthat the bulk of the HFSE and REE in the continental crustreside in accessory mineral phases (Fig. 9; Bea 1996a,1996b). Furthermore, these workers have illustrated that theHFSE and REE budgets of felsic melts derived from themelting of continental crust is proportional to the tempera-ture and the efficiency whereby these phases are dissolvedduring the crustal melting episode (Fig. 9; Watson and Har-

rison 1983; Creaser et al. 1991; Bea 1996a, 1996b; Wat-son 1996). In particular, all other things being equal, thehigher the temperature of melting, the more HFSE andREE will be present in the subsequent melt (Fig. 9; Watsonand Harrison 1983; Creaser et al. 1991; Bea 1996a, 1996b;Watson 1996). By association it can be implied that HFSE-and REE-enriched felsic melts likely reflect a higher temper-ature origin than those with lower HFSE and REE contents(Fig. 9; Watson and Harrison 1983; Creaser et al. 1991;Lentz 1999; Piercey et al. 2001a).

The combination of subvolcanic intrusions with evidencefor high-level fractionation and high-temperature originsprovides two key ingredients for prospective VMS environ-ments. High-temperature magmas in subvolcanic intrusivecomplexes at high levels in the crust proximal to VMSenvironments will result in hydrothermal systems that canrun longer and more vigorously than those associated withlower temperature and deeper subvolcanic complexes(Fig. 9; Cathles 1983; Barrie et al. 1999). The results of thispaper suggests that the Grass Lakes suite of intrusions werelikely significant contributors to the heat budget and providedthe thermal energy to form the hydrothermal systems thatled to the formation of VMS deposits in the Kudz Ze Kayahunit (Kudz Ze Kayah and GP4F deposits; Fig. 9). Granitoidswith similar HFSE–REE systematics and evolved Nd, Sr, Pb,and O isotope systematics have also been identified as beingcogenetic with VMS mineralization in the Bathurst Miningcamp (Whalen et al. 1998). It is possible that high-temperature,HFSE–REE-enriched felsic magmatism with evolved isotopic

Sample Rock type Suite/unit NI NII NIII GrS

P99-45 CAR 2-FLU 1.34 0.86 0.45 0.75P99-39 TR 2-FLU 0.88 0.56 0.30 0.49P98-KZK2 QFPI KZK-U 1.63 1.04 0.55 0.91P98-52 FPI KZK-U 1.69 1.08 0.57 0.94P98-102 RHY WS-5 1.62 1.04 0.55 0.91P98-145 FT WV-5 1.63 1.04 0.55 0.91P99-WV-4L FT WV-6-FW 1.66 1.06 0.56 0.93P98-69 FPI WV-6-FW 1.64 1.05 0.55 0.91P99-WV-3D APHRY WV-6-FW 1.56 1.00 0.53 0.87P99-24 HbGd SRPS 2.10 1.34 0.71 1.17P99-25 KFG GLS 1.78 1.14 0.60 0.99SG94-54 ShGd SRPS 1.78 1.14 0.60 0.99SG94-88A ShGd SRPS 1.96 1.25 0.66 1.09SG94-73 HbGd SRPS 2.07 1.32 0.70 1.15SG94-90B HbGd SRPS 2.09 1.33 0.70 1.16SG94-2A BtMg SRPS 1.99 1.27 0.67 1.11SG94-50 BtMg SRPS 1.64 1.05 0.55 0.91SG94-5B ShGd SRPS 1.59 1.01 0.54 0.88

Note: Rock types and suites/units as in Tables 1 and 3. QFPI, quartz–feldspar porphyritic intrusion;ShGd, sheared granodiorite; BtMg, biotite magnetite. The calculations are based on a mantle endmember (MC) with a composition similar to the DM reservoir (εNd350 = +9.49) and crustal endmembers (CC) using the average values for the NI (εNd350 = –1.15), NII (εNd350 = –7.16), NIII (εNd350

= –22.01), and GrS (εNd350 = –9.60) YTT sedimentary rocks (Creaser et al. 1997; Grant 1997). Notableis that the NI, NII, and GrS reservoirs are too juvenile to explain the Nd isotopic attributes of the felsicrocks from the Finlayson Lake region, as many require greater than 100% crustal Nd contributions (i.e.,NCI > 1). The NIII sedimentary rocks appear to be the only suitable end member; however, it ispossible that older Proterozoic basement might lie at depth and be a more suitable end member. Allsamples prefaced with SG94 are from Grant (1997).

Table 4. Neodymium crustal index (NCI) calculations (DePaolo et al. 1992) for the felsicrocks of the YTT in the Finlayson Lake region.

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signatures may be a common feature to prospective felsicsuccessions in bimodal silicaclastic VMS environments(e.g., Whalen et al. 1998; Lentz 1999; Piercey et al. 2001a).

Conclusions

(1) The Nd isotope geochemistry of felsic rocks from theFinlayson Lake region illustrate that the felsic rocks arevariably influenced by evolved (Proterozoic) continentalcrust or sedimentary rocks derived from such a Proterozoic

crust. Isotopic characteristics vary stratigraphically andspatially within the same stratigraphic unit, with rocksin the Fire Lake unit in the Money creek thrust havingsignificantly less crustal influence, whereas most rocksin the Kudz Ze Kayah unit, Wolverine succession,Grass Lakes suite, and Simpson Range plutonic suitehave significant contribution from evolved (Proterozoic?)sources.

(2) Geochemical data on the felsic intrusive rocks in theFinlayson Lake region are divergent with the Grass

Fig. 10. Schematic diagrams to illustrate the tectono-magmatic evolution of the YTT in the Finlayson Lake region. Arc magmatismwithin the Fire Lake unit is disrupted at �360 Ma by rifting of the Fire Lake arc and the subsequent formation of the Kudz Ze Kayahback-arc rift/basin (a). Within this basin basaltic underplating during rifting led to the generation of high-temperature (high-T) meltsderived from continental crust that led to the formation of the Kudz Ze Kayah felsic volcanics, and the coeval subvolcanic intrusionsof the Grass Lakes suite of granitoids (a). During this rifting event the Kudz Ze Kayah and GP4F deposits formed. Following theKudz Ze Kayah event magmatism was disrupted �357 Ma (Mortensen and Murphy, unpublished data) by a brief episode of deformation,uplift, and erosion (Murphy 1998) to form an unconformity atop the Grass Lakes succession, which was subsequently followed by continuedfelsic volcanism in the Wolverine back-arc (b). The Wolverine back-arc phase involved pre-Wolverine deposit felsic volcanism similarto the Kudz Ze Kayah volcanics (b). Following deposit formation magmatism shifted to more HFSE–REE-depleted felsic volcanism,which in turn was followed by MORB-type mafic magmatism, which is interpreted to represent the onset of sea-floor spreading withinthe Wolverine back-arc basin (b). MSL, mean sea level.

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Lakes suite granitoids displaying significantly higherHFSE and REE contents and Zr/Sc, Zr/TiO2, Nb/La,and Zr/La ratios than the Simpson Range plutonic suite.The Grass Lakes suite is geochemically and geochro-nologically similar to VMS-associated felsic volcanicrocks of the Kudz Ze Kayah unit, suggesting that it isthe subvolcanic heat pump to the VMS hydrothermalsystems within this unit.

(3) It is notable that, with the exception of the Fire Lakeunit felsic rocks, all other felsic rocks have similarevolved Nd isotopic attributes but with different HFSEand REE systematics. The apparent decoupling of theHFSE–REE from the isotopic data suggests that differentfelsic suites were derived from a common crustal sourceregion, but the HFSE and REE budget of the rocks wasgoverned by the efficiency of dissolution of continentalcrust (e.g., Watson and Harrison 1983; Bea 1996a,1996b; Watson 1996). The higher HFSE–REE-bearingfelsic rocks likely reflect higher temperature melting ofa common continental, or continent-derived, substrate;VMS mineralization in the Finlayson Lake region ispreferentially associated with HFSE- and REE-enrichedfelsic magmatism.

(4) The geochemical and isotopic data presented in thisstudy illustrate that the Finlayson Lake region recordsepisodic mid-Paleozoic arc and back-arc magmatismthat was built upon a transitional basement consisting ofboth ocean and continental (or continent-derived) crust.

Acknowledgments

Don Murphy, Suzanne Paradis, and Jan Peter are thankedfor continued collaboration and discussions regarding researchin the YTT. Discussions with Rob Carne, Maurice Colpron,Peter Holbek, Paul MacRobbie, Harlan Meade, Don Murphy,Terry Tucker, and Bill Wengzynowski are gratefully acknowl-edged. Don Murphy is especially thanked for a thoroughreview of a previous draft of this manuscript. Formal journalreviews by Marc Laflèche, Ross Stevenson, and Joe Whalenand comments by Associate Editor Louise Corriveau aregreatly appreciated. This project is funded by the YukonGeology Program (D.C. Murphy); Geological Survey ofCanada and Ancient Pacific Margin NATMAP Project (S.Paradis); Atna Resources and Expatriate Resources; a NaturalSciences and Engineering Research Council (NSERC) ofCanada operating grant (J.K. Mortensen); and an NSERCpost-graduate scholarship, a Geological Society of AmericaStudent Research Grant, and the Hickok-Radford Fund ofthe Society of Economic Geologists (S.J. Piercey). TheRadiogenic Isotope Facility at the University of Alberta issupported in part by an NSERC Major Facilities AccessGrant. During the final assembly of this manuscript SJP wassupported by an NSERC operating grant and a grant fromthe Laurentian University Research Fund.

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