24. PB ISOTOPIC COMPOSITIONS OF SULPHIDE MINERALS FROM THE YELLOWKNIFE CAMP: METAL SOURCES AND TIMING OF MINERALIZATION Brian L. Cousens1, Hendrik Falck2, Edmond H. van Hees3, Sean Farrell4, and Luke Ootes2

1. Ottawa-Carleton Geoscience Centre, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6 2. C.S. Lord Northern Geoscience Centre, PO Box 1500, Yellowknife, NT X1A 2R3 3. Geology Department, Wayne State University, Detroit, MI 48202 USA 4. Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5

INTRODUCTION Mineralizing fluids generally pass through a large volume of crust, and as a result the Pb isotopic compositions of the The Yellowknife Volcanic Belt and surrounding metatur- fluids will be averages of the crust traversed by the fluids bidites, commonly referred to as the Yellowknife (including any initial hydrothermal magmatic component). Supergroup, host two producing gold mines, several past- This average allows for reconstruction of the Pb isotopic producing deposits, and many showings and potential pro- evolution of the crust with time, providing that the age of Pb- ducers (Franklin and Thorpe, 1982; Falck, 1992; Thorpe et bearing deposits can be firmly established by independent al., 1992). The host rocks are of late Archean age, but the methods (usually conformable massive sulphide deposits, gold is commonly found in quartz veins and shear zones that summarized in Stacey and Kramers, 1975; Faure, 1986; postdate the deposition of the host rocks (Padgham, 1992). Zartman and Haines, 1988). Pb ratios in a sulphide These veins and shear systems do not include minerals that mineral from a deposit of unknown age can then be plotted provide high-precision geochronological data, such as zircon against the isotopic evolution curve, which gives a “model or monazite (titanite is present but is poor for dating purpos- age” for that mineral. Several models for the isotopic evolu- es). Thus the absolute age of the mineralizing event (or tion of the crust have been proposed, some more complex events) is not well constrained. The goals of this study are to than others, and thus “model ages” differ depending on the utilize of (Pb), a common trace element in model used ( Stacey and Kramers, 1975; Zartman and many sulphide minerals and a major element in galena and Haines, 1988; Kramers and Tolstikhin, 1997). The most sphalerite, to attempt to date gold deposition and to deter- common mineral used to determine model ages is galena, mine what rocks may have been sources of the Pb (and because of its extremely high Pb content, and its common gold?) in these shear zone or quartz-vein systems. occurrence in conformable massive sulphide deposits. Pb Isotope Systematics Some sulphide minerals such as pyrite can include minor amounts of U, and thus U/Pb will be greater than zero and Lead has four isotopes with masses 204, 206, 207, and 208. allow to form new 207Pb and 206Pb, thus 208Pb is produced by the decay of 232Th, whereas 207Pb and increasing 207Pb/204Pb and 206Pb/204Pb as time progresses 206Pb are produced by the decay of 235U and 238U, respec- after crystallization (termed radiogenic ingrowth). Provided tively (Faure, 1986). 204Pb is a stable isotope whose abun- that pyrite occurs with other minerals with different U/Pb dance does not change over time, and for this reason it is (and preferably one mineral with U/Pb = 0), then after any common to refer to the abundance of any other isotope of Pb time, the minerals will lie on a straight line (termed an relative to 204Pb, e.g., 206Pb/204Pb. Lead is a common ele- isochron) in a plot of 207Pb/204Pb vs. 206Pb/204Pb, and the ment in sulphide minerals such as galena, sphalerite, slope of that isochron will be a function of the time that has arsenopyrite, chalcopyrite, pyrrhotite, and pyrite. The Pb is passed since the minerals crystallized (Faure, 1986). Critical inherited from the fluids that precipitate the sulphide miner- to this analysis is knowing that all minerals in the system are als, and at the time of deposition the minerals will therefore cogenetic and that they have remained closed to U or Pb dif- record the isotopic composition of the ore-bearing fluid. fusion since crystallization, either of which can be difficult However, most of these minerals exclude U and Th, such to ascertain. This analysis also assumes that the isotopic that U/Pb and Th/Pb are approximately zero. Thus, the Pb composition of the fluid was constant during the deposition isotopic composition of a sulphide mineral will not change of the minerals being analyzed. appreciably over geological time. Surface weathering has no effect on Pb isotope ratios, and post-crystallization meta- Linear arrays of data points in a Pb-Pb plot could also be morphic or intrusive events may modify isotopic ratios if the result of mixing of Pb with distinct isotopic composi- new Pb is introduced into the sulphide mineral during recrys- tions, either as a result of mixing of Pb from different crustal tallization (Thorpe, 1982). sources as mineralizing fluids traverse the crust, or as a result of a post-crystallization “disturbance event”. In some

Cousens, B.L., Falck, H., van Hees, E.H., Farrell, S., and Ootes, L. 2005: Pb Isotopic Compositions of Sulphide Minerals from the Yellowknife Gold Camp: Metal Sources and Timing of Mineralization; Chapter 24 in Gold in the Yellowknife Greenstone Belt, Northwest Territories: Results of the EXTECH III Multidisciplinary Research Project, (ed.) C.D. Anglin, H. Falck, D.F. Wright and E.J. Ambrose; Geological Association of Canada, Mineral Deposits Division, Special Paper No. p. B.L. Cousens, H. Falck, E.H. van Hees, S. Farrell, and L. Ootes

114o Pb Isotopic Studies in the Slave Province The Slave Province can be divided into eastern and western halves based on the Pb isotope systematics of galenas from several massive sulphide deposits (Fig. 24-1; Thorpe et al., 1992). Galenas from the eastern part of the province are less Nicholas Lake radiogenic than those from the western section, even though N the general character of eastern and western Slave massive Discovery sulphide deposits is similar (Franklin and Thorpe, 1982; Thorpe et al., 1992). This Pb isotope distinction coincides closely, but not exactly, with provinciality in the Nd isotopic composition of granitoid plutons from the Slave Province Viking (Davis and Hegner, 1992). Combined with recent geological, geochemical, and geochronological work in the southern Kansai sample Slave, it is apparent that much of the western part of the o locality province is underlain by basement gneisses and granitoids 63 Burwash Fmn migmatitic older than 3 Ga, whereas the eastern part of the province is sandstone, not ( Dudás, 1989; Isachsen, 1992; Isachsen and Bowring, Clan Lake siltstone 1994; Davis et al., 1996; Isachsen and Bowring, 1997; Granitoids - Kusky, 1989; Yamashita et al., 1998; Bleeker et al., 1999a,b; Prosperous Cousens, 2000;). In the western Slave Province, mineraliz-

Granitoids - ing fluids stripped Pb from older basement rocks that are not Defeat present in the eastern half, and this geological distinction is reflected in the Pb isotopic compositions of sulphide miner- Volcanic Rocks als. The average Pb model age for Slave massive sulphide Granitoids, Meso- deposits is ca. 2670 Ma (Thorpe, 1982). to Late Archean Oro, Homer Pb isotope data are also available for some gold deposits G7 Claims in the Slave Province (Cumming and Tsong, 1975; Thorpe, Crestaurum 1982). Some of these samples have been reanalyzed over the Walsh Lake past fifteen years at the University of Alberta, utilizing Kathleen improved Pb separation techniques and newer mass spec- Cassidy Pt. trometers (R. Thorpe, pers. comm., 1999). Galenas and Supercrest Tom, Ptarmigan sulphosalts from Slave gold deposits plot within the same Giant range as that of massive sulphide deposits, with Pb model ages ranging from 2597 to 2705 Ma (Thorpe, 1982). These data imply a late Archean age for the gold mineralization. However, some anomalous galenas from massive sulphide deposits and some pyrites from gold deposits do not plot McQueen within the range for the majority of the massive sulphide and gold deposits, possibly resulting from remobilization of Pb during a Proterozoic event (Cumming and Tsong, 1975; Thorpe, 1982). Figure 24-1. Map of the Yellowknife area showing sulphide sam- It is important to note that recent studies of metallogeny pling localities (Padgham, 1987). in the Yellowknife area indicate that there are several distinct styles of gold mineralization in the Yellowknife camp (Falck, cases, the slope of the linear array may have no age signifi- 1992; Kerswill and Falck, 1999; Siddorn and Cruden, 2000). cance. In other cases, the slope of the array can be used to Each distinct style of mineralization most likely represents a estimate the time when highly radiogenic Pb was added to an different stage of gold introduction, such that the most eco- older Pb-bearing mineral (Russell et al., 1966; Godwin et al., nomic deposits of gold have the greatest abundance and vari- 1982; Faure, 1986). ety of sulphide minerals (Falck, 1992). None of the mineral- In summary, Pb isotopes in sulphide minerals, particular- izing events are recognized in rocks younger than 2.60 Ga, ly in Pb-rich ores, may record the average isotopic composi- and thus all the events are of Archean age. The complexity tion of the crustal rocks through which the mineralizing flu- of gold mineralization at Yellowknife should be considered ids passed, including any initial magmatic component, and in the interpretation of the Pb isotope data, but to date sam- have the potential to retain some geochronological informa- pling has not systematically covered the various mineral tion (model age or isochron). In the case of shear-zone-host- styles identified by Falck (1992). Nevertheless, isotopic data ed gold deposits, such as those in the Yellowknife area, a Pb have considerable potential to resolve key problems related isotope study represents an opportunity to investigate the ori- to gold metallogeny. gin of the metals found in these deposits.

2 Pb Isotopic Compositions of Sulphide Minerals from Yellowknife: Metal Sources and Timing of Mineralization

a) 17.0 SAMPLE SELECTION AND ANALYTICAL Yellowknife Sulphides

TECHNIQUES Homer Lake Sulphides Gold showings in the Yellowknife area can be divided into 16.5 Slave VMS Sulphides those that include sphalerite and galena and those that do not Slave Granitoids

(Falck, 1992; Kerswill and Falck, 1999). Arsenopyrite, stib- Pb 16.0 nite, pyrrhotite, and pyrite are commonly associated with 204 gold. For this study, several showings and deposits were 46 1 44 208Pb / 204 Pb sampled, and from each site at least one sulphide mineral Pb / 15.5 Crust West/South 42 Slave VMS sample was collected. To successfully use the isochron tech- 2 40 207 nique, at least three different mineral separates are required Mantle 38 1 15.0 from any mineralization event, and for this reason multiple Isochron 36 2 34 samples were collected where possible. Frank Santaguida „„ 32 collected an additional stibnite sample from the Giant mine 12 14 16 18 20 22 24 26 14.5 Eastern Slave VMS workings, and Karen Gochnauer provided additional sam- 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 ples from other showings in the Yellowknife area. 206Pb / 204Pb Sulphide-bearing samples were slabbed with a diamond saw. Each slab was wrapped in a plastic bag, and broken into b) 17.0 small fragments with a rock hammer. Sulphide minerals were carefully separated from the fragments using a binocu- Yellowknife Sulphides 16.5 lar microscope and stored in polyethylene centrifuge tubes. Thorpe/Cumming & Scanning electron microscopy of several sulphide-bearing Tsong Galena Pb Non-galena / sphalerite Cumming & Tsong Best-fit Line rock samples shows that sulphide grains commonly have 16.0 Other Sulphides

204 20 inclusions of other sulphides, so the purity of each mineral Whole-rock Granitoids Crust J Granitoid Ksp/Pl separate is variable. Approximately 50 milligrams of sul- 1 19 Pb / 15.5 H Feldspar Leachates phide was dissolved in warm 1.5N HNO3, and Pb was 18 Sulphides H

207 2 17 extracted using standard anion exchange techniques with HH Mantle „ 1 16 H HBr and HCl (Cousens, 1996). Total procedural blanks for 15.0 Isochron H„ „„„„„ „J„HH„ Pb are < 150 picograms. Samples were loaded onto single Re 2 15 J„H filaments with H PO and silica gel, and were run at fila- 14 3 4 10 15 20 25 30 35 40 45 ment temperatures of 1250 to 1350°C. All Pb mass spec- 14.5 trometer data were corrected for fractionation during the 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 mass spectrometer analysis using NIST SRM981. the aver- 206Pb / 204Pb age Pb isotope ratios measured for this standard over the past seven years (2-sigma errors) are 206Pb/204Pb = 16.890 Figure 24-2. a) 207Pb/204Pb vs. 206Pb/204Pb data for all sulphide 207 204 208 204 mineral samples analyzed in this study. Western and eastern Slave ± 0.012, Pb/ Pb = 15.429 ± 0.014, and Pb/ Pb = VMS (volcanic massive sulphide) data fields and Homer Lake analy- 36.500 ± 0.048. The 0.13 %/amu fractionation factor used is ses from Thorpe et al. (1992), Slave (Yellowknife area) plutons from based on repeat analyses of SRM981 (accepted values from Davis et al. (1996), average continental crust and mantle evolution Todt et al., 1984). curves from Kramers and Tolstikhin (1997) with tick marks every 0.5 Ga. Second-stage, zero-age isochron from Stacey and Kramers Samples of sulphide minerals other than galena or spha- (1975). Lower inset: 208Pb/204Pb vs. 206Pb/204Pb in sulphides from lerite, including arsenopyrite, pyrite, chalcopyrite, and stib- this study. b) Same as panel a, but includes Pb isotopic data for sul- nite, were dissolved in HNO and submitted to the Ontario phide mineral samples from previous studies of gold deposits in the 3 Slave Province. “Yellowknife Sulphides” are from this study, Galena Geological Survey Geochemical Laboratories in Sudbury, data are from Thorpe (1982) and Cumming and Tsong (1975), and Ontario, for trace element analysis by ICP-MS. Only impre- “Other Sulphides” from Cumming and Tsong (1975). Dashed line is cise U, Th, Pb, and base metal concentration data are avail- best-fit line through all data points that are not from galena or spha- able at this time for the sulphide minerals, as a result of par- lerite. Lower inset: Comparison of sulphide data in panel b with present-day Pb isotope ratios in whole-rock granitoids (Cumming tial leaching of U, Th, and other lithophile elements from sil- and Tsong, 1975), granitoids feldspar, and feldspar leachates icate phases adhering to the sulphide grains. In addition, Pb (Yamashita et al., 1999, 2000) from the southwestern Slave concentrations in some samples were above the lab calibra- Province. Best-fit lines through the whole-rock granitoid and tion curve whereas U contents in many samples were close leachate analyses are shown. to detection limits. Pb isotope ratios for all the samples ana- lyzed are listed in Table 24-1, ICP-MS trace element data are PB ISOTOPE RESULTS reported in Table 24-2, and U-Th-Pb-initial Pb isotope ratio Figure 24-2a is a plot of 207Pb/204Pb vs. 206Pb/204Pb, includ- data (based on ICP-MS U, Th, and Pb data) are shown in ing analyses of galena, sphalerite, arsenopyrite, stibnite, Table 24-3. New mineral separates from a subset of samples pyrrhotite, pyrite, and chalcopyrite from this study. Also were reanalyzed for U and Pb concentrations by isotope dilu- shown are fields for volcanic massive sulphide (VMS) gale- tion, as well as for Pb isotope ratios, in order to determine nas from the eastern and western Slave (Thorpe et al., 1992). approximate initial Pb isotopic compositions (Table 24-4), For reference, two Pb isotope evolution curves are shown, because of the imprecision of the ICP-MS U-Pb concentra- one for average upper continental crust (Stacey and Kramers, tion data. 1975) and the other for the upper mantle (Kramers and

3 B.L. Cousens, H. Falck, E.H. van Hees, S. Farrell, and L. Ootes

Table 24-1. Sulphide Lead Isotopic Compositions. 16.8 208 207 206 Sample UTM UTM Pb/ Pb/ Pb/ St + Gal Number Locality Mineral Easting* Northing 204Pb 204Pb 204Pb Giant Stopes 1.24 Ga 16.4 2 AB2172 Cassidy Pt. gl 645000 6938400 33.68 14.77 13.78 Supercrest r = 0.988

Clan Zone 1 Clan Lake sph 640408 6978963 33.82 14.9 13.86 LateSupercrest 96-14 Clan Zone3a Clan Lake ars 640509 6978926 40.16 15.78 18.32

Pb 16.0 Clan Zone 3b Clan Lake ars 640509 6978926 37.98 15.25 16.08 St Clan Zone 3c Clan Lake ars 640509 6978926 34.99 15.04 14.68 204 99-CR-4 Crestaurum sph 635707 6941859 34.24 15.15 14.14 15.6

Pb / 2.31 Ga 99-CR-1 Crestaurum ars 636015 6941768 33.96 15.05 14.05 St 2

207 r = 0.980 99-CR-3 Crestaurum sph 636015 6941768 34.12 15.09 14.13 St + Q5789 Damoti po 592105 7120292 35.99 15.34 15.87 15.2 Au py Giant Galenas, Damoti Vein Damoti py 592105 7120292 34.61 15.17 14.7 Sulphosalts ? DISC 79N Discovery gl 354496 7009467 33.7 14.87 13.88 ? py + St sph 314s G7 Claims sph 34.14 15.14 14.14 14.8 13.0 15.0 17.0 19.0 21.0 23.0 25.0 314p G7 Claims py 33.95 15.07 14.11 206 204 Giant stibnite Giant stope stb 636000 6932200 33.87 15 14 Pb / Pb 11894p Giant 436 py 636000 6932200 34.19 15.06 14.3 Figure 24-3. Pb isotope ratios in sulphides from the Giant mine. A 11896p Giant 429 py 636000 6932200 35.63 15.29 15.8 best-fit regression line is shown for Giant stopes, Supercrest, and Giant VG Giant 914-1 py-sph 636000 6932200 33.93 15.03 14.06 late Supercrest mineralization, with secondary isochron ages shown for the Giant stopes and late Supercrest mineralization (see Faure, 7021p Giant 937#2 py 636000 6932200 34.08 15.07 14.09 1986). Giant galena and sulphosalt analyses from Thorpe (1982) KS 96-10 Giant C-rmp Cpy+py 636000 6932200 33.97 15.03 14.09 and R. Thorpe (pers. comm., 1999). Au = native gold, Gal = galena, KS 96-7 Supercrest stb 636500 6935200 34.2 15.12 14.09 py = pyrite, sph = sphalerite, St = stibnite. KS 96-16 Supercrest stb 636500 6935200 39.01 15.85 19.33 8602 Supercrest stb 636500 6935200 34.09 15.08 14.14 Tolstikhin, 1997). The sulphides have a large range in iso- AB 1610 Supercrest stb 636500 6935200 35.96 15.37 15.96 topic composition, although many mineral analyses overlap KS 96-14a Supercrest gl-stb 636500 6935200 41.49 16.44 22.81 with the western Slave VMS data. Also shown in Figure 24- gl-stb KS 96-14b Supercrest 636500 6935200 44.55 16.57 24.65 2a are three analyses of galenas from a massive sulphide KS 96-14s Supercrest stb 636500 6935200 40.32 16.36 21.94 showing at Homer Lake (Thorpe et al., 1992), near the north "W" Kansai gl 647789 6993083 33.72 14.88 13.88 HF 329p McQueen py 34.16 15.03 14.18 end of the Yellowknife belt, which also overlap with the NU-15-94g Lake gl 361224 7015953 33.7 14.79 13.61 vein/shear sulphides. Feldspar separates from 2.64 to 2.58 Ga Nich Rg Lake gl 361224 7015953 33.76 14.81 13.62 plutons from the Yellowknife area are also similar in com- N89 DH057 Lake ms 361224 7015953 34.04 14.91 13.72 position to the sulphides analyzed here, in particular the N89 DH 058 Lake ars 361224 7015953 33.99 14.89 13.89 feldspar analysis from the ca. 2.59 Ga Prosperous Pluton N89 DH063 Lake py 361224 7015953 34.63 15.14 14.71 (Davis et al., 1996). Sulphides from our study generally plot N89 DH50a Lake ars 361224 7015953 33.77 14.82 13.64 above the Stacey-Kramers curve, requiring a crustal source N89 DH50p Lake py 361224 7015953 33.77 14.8 13.62 for Pb with high 207Pb/204Pb ratios at 2.7 Ga. NU-15-94ms Lake ms 361224 7015953 33.83 14.83 13.64 sph Figure 24-2b compares the isotopic results from this NU-15-95 Lake 361224 7015953 33.93 14.86 13.66 study with data for sulphides from various gold deposits in NU-15-94a Lake ars 361224 7015953 33.89 14.87 13.83 N89 DH035g Lake gl 361224 7015953 33.77 14.81 13.63 the Slave Province (Cumming and Tsong, 1975; Thorpe, Mg Lake gl 361224 7015953 33.73 14.8 13.62 1982). Analyses of galena from Slave gold deposits form a N89 DH50p2 Lake py 361224 7015953 33.75 14.81 13.64 tight array that overlaps with the western Slave VMS data. NL DH39a Lake ars 361224 7015953 33.89 14.88 13.83 Many of the analyses from this study, primarily of galena NL DH43a Lake ars 361224 7015953 35.1 15.32 15.54 and sphalerite, also overlap with the western Slave VMS HF 52 Oro Lake ms 636403 6947288 36.73 15.68 17.69 field. In both this and previous studies (Cumming and Tsong, HF 119s Cassidy Pt. sph 645000 6938400 33.75 14.92 13.9 1975), the Pb isotopic composition of “low-Pb” minerals, AA 4817p Ptarmigan py 644300 6934900 34.12 14.99 14.1 such as arsenopyrite and pyrite, extend to more radiogenic HF 288 Tom Mine sph 644100 6936400 33.75 14.85 13.9 compositions. The three most radiogenic analyses from this 99-VK-2 Viking gl 647308 6999005 33.64 14.83 13.81 study are from the Akaitcho (Supercrest) portion of the Giant VK89-17-39 Viking gl 647308 6999005 33.72 14.86 13.89 mine, as is the most radiogenic analysis of Cumming and Viking Core Viking ars 648369 7000647 34.09 14.95 14.13 Tsong (1975). 99-Vk-1 Viking ars/po 648202 7000324 37.16 15.15 15.58 98-Yk-31a Walsh Lk ars 640953 6942057 33.92 14.96 13.92 Figure 24-3 presents data from the Giant mine. Three 98-Yk-31b Walsh Lk sph 640953 6942057 33.87 14.96 13.93 groups of samples were analyzed: the main stopes (Table 24- Analytical 1, samples 11894, 11896, Giant VG, 7021 py, Giant Stope, Uncertainties 0.05 0.02 .0.02 Giant C-rmp), the Supercrest deposit (samples KS96-7, * All sample sites are within UTM Zone 11 with the exception of the Discovery Mine and 8602, AB1610), and late veins at Supercrest that cross or Nicholas Lake in Zone 12. Samples that lack exact locations are noted by blanks. Samples from mines or large prospects are located according to the location of the mine or core occupy Proterozoic structures (samples KS96-14, KS96-16).

storage area. ars = arsenopyritve, cpy = chalcopyrite, gl = galena, lchte = 2N HNO3 Pyrite and sphalerite from the main stopes fall on a two-point leachate from whole rock powderms = mixed sulphide, po = 8/25/03pyrrhotite, py = pyrite, line, whereas the Supercrest data from stibnite and native sph = sphalerite, stb = stibnite. gold samples fall on a three-point line that corresponds to a secondary isochron age of approximately 2.3 Ga. Galena and sulphosalt analyses from the Giant mine also plot at the non- 4 Pb Isotopic Compositions of Sulphide Minerals from Yellowknife: Metal Sources and Timing of Mineralization

15.8 consistently have low 207Pb/204Pb ratios and define the low end of the main array. This may apply only to this sample set, 15.6 because analyses of galenas and sulphosalts from other gold deposits in the southern Slave Province (particularly the Con 15.4 Damoti (Colomac) Cassidy Point mine) overlap the entire range of the main array (Thorpe, Pb G7 Prospect Clan Lake 1982). Sphalerites also plot within the main array, but span MacQueen Vein Crestaurum 204 15.2 WSV its entire range. The only chalcopyrite analysis in this study Main Oro Lake Discovery Mine Yk also plots within the main array, although chalcopyrites from Pb / Array Kansai 15.0 Plutons Prosperous Point the Crestaurum area and the Akaitcho (Supercrest) deposits

207 Ptarmigan Mine Nicholas Lake West are in some cases more radiogenic (Cumming and Tsong, Tom Mine Viking Mine 14.8 Slave Au 1975). Pyrite, stibnite, and arsenopyrite have variable Pb iso- 2.7 - 2.5 Ga Homer Lake (RT) Walsh Lake (Banting) E Mantle tope ratios that plot both within and to the right of the main 14.6 S 13.0V 14.0 15.0 16.0 17.0 18.0 19.0 array. The lone pyrrhotite analysis plots outside the main 206 Pb / 204Pb array. Thus the two minerals that have Pb as a major con- stituent define the main array. Minerals with lower Pb con- Figure 24-4. Pb isotope ratios in sulphide minerals from all other deposits except the Giant mine. Grey fields are shown for Western tents have variable Pb isotope ratios that may or may not plot (WSV) and Eastern (ESV) Slave VMS deposits, respectively (Thorpe within the main array. et al., 1992), and the white field encloses galena analyses from Western Slave Au (gold) deposits (Thorpe, 1982; Thorpe, pers. TRACE ELEMENT RESULTS comm., 1999). Homer Lake data from Thorpe et al. (1992), and Yk (Yellowknife area) pluton field from Davis et al. (1996). The “main U-Th-Pb Systematics array” includes all sulphide analyses from gold deposits that plot between the WSV and ESV fields. Samples of arsenopyrite, pyrite, chalcopyrite, and stibnite (“low-Pb” minerals), which generally plot outside of the radiogenic end of the main-stope line (arrow in Fig. 24-3; main array, were analyzed for U, Th, Pb, base metal, and Thorpe, 1982). The lines passing through the Giant main lithophile element abundances by ICP-MS (Table 24-2). The stope minerals and the Supercrest minerals almost overlap, goals were two-fold, first to determine if U/Pb ratios in these and thus the two deposits appear to be part of a single system. minerals were high enough to explain their Pb isotope ratios The mixed stibnite and galena separates from the late plotting to the right of the main array, and second to attempt Supercrest vein (Table 24-1, samples KS 96-14 and KS96-16) to correlate base metal abundances with Pb isotope ratios. are extremely radiogenic, and four analyses of different min- Note that these samples were analyzed using a customized eral picks of this sample fall on a line with a secondary technique for which no analytical standards exist, and for isochron age of 1.24 Ga. This young age is consistent with this reason analytical precision and accuracy are impossible the observation that this Pb-Sb mineralization occurs within to assess. These data should be treated as approximate values Proterozoic faults that cut the Supercrest deposit that does only. not contain gold (J. Siddorn, pers. comm., 1999). Based on the U and Pb data in Table 24-2, calculated 238 204 The Pb isotope data for all of the other showings and U/ Pb ratios are less than 2 (in all but the stibnite from deposits investigated are shown in Figure 24-4. Also shown Supercrest sample KS 96-16 with a Pb concentration below is the model isotopic composition of the upper mantle the detection limit of the ICP-MS, which is at odds with the around 2.7 to 2.5 Ga (Kramers and Tolstikhin, 1997), the fields for eastern and western Slave VMS deposits (Thorpe 16.0 et al., 1992), and feldspars from plutonic rocks in the Yellowknife area (Davis et al., 1996). Most of the sulphide 15.8 analyses plot as an array between the upper mantle at 2.7 to Split By 2.5 Ga and the most radiogenic western Slave VMS galenas, 15.6 Mineral Galena which will be referred to as the “main array” from this point Sphalerite on. Notably, galena, sphalerite, and most other sulphide min- Pb 15.4 206 204 Arsenopyrite erals from the Nicholas Lake deposit have lower Pb/ Pb 204 ratios than galena and sphalerite from other Yellowknife area Main 15.2 Array Chalcopyrite deposits, and form a subarray on the left side of the main Pb / Pyrrhotite

array. In contrast, nine analyses from Clan Lake, Nicholas 207 Lake, the Viking mine, and Oro Lake plot above and to the 15.0 Pyrite right of the main array. Two samples from the Damoti Slave Stibnite 14.8 Au Prospect (near the Colomac mine, Indin Lake Belt, north- Mixed Sulphide west of Yellowknife; Table 24-1, samples Q5789 and Damoti 14.6 vein) are also more radiogenic than the main array of sul- 13 14 15 16 17 18 19 20 phides. It is clear that, like the Giant mine, many of the gold 206 204 localities have variable Pb isotopic compositions. Pb / Pb There are some associations between mineral type and Figure 24-5. Pb isotope ratios in different minerals from this study. The field of galena analyses from Slave gold deposits (Slave Au) is isotopic composition (Fig. 24-5). With the exception of the shown in grey (Thorpe, 1982; Thorpe, pers. comm., 1999). The main late Supercrest mineralization at the Giant mine, galenas array is enclosed by the dashed line.

5 B.L. Cousens, H. Falck, E.H. van Hees, S. Farrell, and L. Ootes

Table 24-2. Trace Element Concentrations in Sulphide Mineral Separates. Crestaurum Clan Clan Giant Giant Supercrest Supercrest Giant Supercrest Supercrest Sample 99-CR-1 Zone 3a Zone 3b 11894p 11896p 8602 AB1610 KS 96-10 KS 96-7 KS 96-16 Mineral arsenopyrite arsenopyrite arsenopyrite pyrite pyrite stibnite stibnite chalcopyrite stibnite stibnite Al (ppm) 152 501 14666 1006 576 2409 988 557 309 113 Ca 1796 3372 21764 16996 6738 72549 293471 245663 11018 26540 Cd 36 N.D. N.D. 3 N.D. 12 1 19 2 1 Co >350 139 184 119 69 214 3 2 N.D. 6 Cr 18 17 34 17 8 30 27 28 24 21 Cu 154 24 226 446 112 371 23 877192 999999 83 Fe 16837 268270 199007 469302 129318 298245 13336 299257 5039 1619 Mg 191 398 11659 94010 5474 48090 11687 2777 1515 362 Mn 15 33 582 3768 308 3174 5967 4526 93 403 Mo N.D. 4 2 2 2 N.D. N.D. N.D. N.D. 0 Ni N.D. 48 113 187 32 348 13 64 5 6 Pb 51213 54 48 85 27 6418 3104 23 4546 0 Sb 83 236 1 131 51 1431 213 14 260 768 Sc N.D. 1 10 4 N.D. 10 1 14 1 N.D. Sn N.D. N.D. N.D. N.D. N.D. N.D. N.D. 7 N.D. N.D. Ti N.D. 43 319 N.D. N.D. N.D. N.D. 3 5 5 Tl 9 N.D. 1 2 1 2 21 N.D. 1 123 V N.D. 2 65 9 N.D. 11 N.D. 2 N.D. N.D. W N.D. N.D. N.D. N.D. N.D. N.D. N.D. 22 N.D. N.D. Zn 3630 15 83 587 28 104 104 99 57 40 La N.D. 26.5 169.9 0.98 0.3 2.18 18.72 3.38 N.D. 0.3 Ce N.D. 44.98 289.39 1.88 0.68 5.31 25.42 6.71 0.03 0.45 Pr N.D. 4.97 33.37 0.2 N.D. 0.7 3.1 0.87 N.D. N.D. Nd N.D. 18.83 122.88 1.09 N.D. 4.25 15.52 4.44 0.02 0.3 Sm N.D. 2.86 17.33 N.D. N.D. 1.51 3.52 1.42 N.D. 0.01 Eu N.D. 0.26 0.87 0.28 N.D. 0.73 1.12 0.81 N.D. N.D. Gd N.D. 2.11 10.29 0.89 N.D. 1.83 4.33 2.39 N.D. 0.08 Tb N.D. 0.2 0.93 N.D. N.D. N.D. N.D. 0.42 N.D. N.D. Dy N.D. 1.29 4.75 0.96 N.D. 1.64 2.58 3.26 N.D. 0.03 Ho N.D. 0.18 0.59 N.D. N.D. 0.28 0.41 0.72 N.D. N.D. Er N.D. 0.61 1.7 0.59 0.13 0.96 1.13 2.42 N.D. N.D. Tm N.D. 0.02 0.07 N.D. N.D. N.D. N.D. 0.32 N.D. N.D. Yb N.D. 0.52 0.92 0.63 N.D. 0.89 0.78 2.75 N.D. N.D. Lu N.D. 0.03 0.04 0.07 N.D. 0.08 0.02 0.38 N.D. N.D. Rb N.D. 2.94 101.51 N.D. N.D. 7.62 N.D. 0.18 0.07 0.01 Sr N.D. 1.7 5.5 49.8 N.D. 159.6 119.3 188.7 5.1 23.5 Nb N.D. 0.09 0.03 N.D. N.D. N.D. N.D. N.D. N.D. N.D. Cs N.D. 0.18 4.77 N.D. N.D. 0.84 N.D. 0.2 0.01 N.D. Hf N.D. 1.64 1.24 0.11 0.07 N.D. N.D. 0 0 0 Ta N.D. 0 0 N.D. N.D. N.D. N.D. 0 0 0 Th N.D. 4.97 21.54 1.12 N.D. N.D. N.D. 0.17 0 0 U N.D. 0.78 1.52 0.22 0.1 N.D. N.D. 0.01 0 0.03 Y N.D. 7.09 18.7 6.61 1.81 9.5 17.01 23.49 0.18 0.87 Zr 2.96 171.48 41.39 20.59 2.7 48.32 N.D. 0.51 0.38 1.12 Au (ppb) N.D. 582 94 105 N.D. 2570 215 N.D N.D 39 Ag (ppb) 5069 1874 1616 1854 658 3006 1693 8491 344 N.D All concentrations in weight parts per million, except Au and Ag in weight parts per billion. N.D. = no data

ease with that this sample ran for Pb isotopes on the thermal 24-3, it is evident that U/Pb ratios are extremely small and ionization mass spectrometer). It is clear from inspection of therefore the corrections to measured Pb isotope ratios are Table 24-3 that calculated initial Pb isotope ratios in samples insignificant. Thus, U/Pb and Th/Pb ratios are too low in the from outside of the main array remain to the right of the sulphides to have shifted their initial isotopic compositions main array. We attempted to verify U/Pb ratios in some min- outside of the main array after their formation approximately eral separates by isotope dilution (Table 24-4). As in Table 2.6 billion years ago.

6 Pb Isotopic Compositions of Sulphide Minerals from Yellowknife: Metal Sources and Timing of Mineralization

Table 24-2 continued. Nicholas Nicholas Nicholas Nicholas Nicholas Nicholas Oro Viking Sample NL DH39a NL DH43a N89 DH50a N89 DH057 N89 DH063 NU-15-94a HF 52 Viking Core 99-Vk-1 Mineral arsenopyrite arsenopyrite arsenopyrite mixed sulphide pyrite arsenopyrite arsenopyrite arsenopyrite arseno/pyrrhotite Al (ppm) 10250 4674 5148 4399 226 408 N.D. 1001 25290 Ca >280000 >280000 422 934 608 4629 3206 792392 7553 Cd 1 338 125 1075 N.D. 19 N.D. 82 59 Co 266 75 116 51 83 51 132 127 194 Cr 27 14 12 20 10 20 13 17 80 Cu 175 2203 2693 120 399 14 9 30 142 Fe 436347 207001 337003 215949 56216 319924 589008 272970 205034 Mg 7515 2293 694 1643 285 429 130 1457 20118 Mn 286 146 189 157 4 15 23 40 673 Mo 4 1 4 2 N.D. N.D. 5 4 31 Ni 171 N.D. 17 9 22 194 74 104 204 Pb 157 1917 6986 24993 104 5486 20 27384 67 Sb627109 1 421778 7 Sc 3 N.D. N.D. 1 N.D. N.D. N.D. 1 10 Sn N.D. 2 2 7 N.D. 2 N.D. N.D. 9 Ti 200 N.D. 17 N.D. N.D. N.D. N.D. 20 703 Tl 1 N.D. 1 2 N.D. 2 N.D. 5 1 V 30 N.D. 2 3 N.D. 1 N.D. 6 105 W 47 N.D. 5 221 N.D. 18 N.D. N.D. 52 Zn 145 22661 6010 49688 20 93 6 4037 361 La 32.64 0.29 0.5 4.29 N.D. 4.29 N.D. 0.79 19.78 Ce 72.26 0.6 0.96 7.2 0.29 8.04 N.D. 1.51 43.73 Pr 10.49 N.D. N.D. 0.81 N.D. 0.94 N.D. 0.13 5.55 Nd 47.9 N.D. 0.5 3.43 N.D. 3.99 N.D. 0.71 22.05 Sm 11.11 N.D. N.D. 0.62 N.D. 0.88 N.D. 0.05 4.8 Eu 1 N.D. N.D. N.D. N.D. 0.33 N.D. N.D. 0.79 Gd 7.3 N.D. N.D. 0.43 N.D. 0.6 N.D. 0.02 2.84 Tb 0.68 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.21 Dy 2.93 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 1.72 Ho 0.28 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.07 Er 0.61 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.38 Tm N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0 Yb N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.14 Lu N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0 Rb 75.61 N.D. 5.55 1.44 N.D. N.D. N.D. 0.65 109.47 Sr 3 N.D. N.D. N.D. N.D. N.D. N.D. 9.9 6.9 Nb 0.08 0.03 0.07 0.02 N.D. 0.03 0.05 0.06 0.14 Cs 6.44 N.D. 1.72 0.29 N.D. N.D. N.D. 0.08 7.68 Hf 0.06 0.01 N.D. N.D. N.D. N.D. N.D. 0.02 0.27 Ta N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.01 0 Th 2.5 N.D. N.D. N.D. N.D. N.D. N.D. 0.08 12.71 U 0.62 0.06 0.06 0.21 N.D. 0.05 N.D. 0.03 1.27 Y 8.4 0.21 0.53 0.69 N.D. 0.75 N.D. 0.11 4.42 Zr 125.18 77.67 76.63 36.22 35.64 78.78 154.59 130.38 114.7 Au (ppb) 1243 536 1070 101 419 114 110 3363 14729 Ag (ppb) 2808 6315 4898 42597 1388 17592 415 42131 7457 All concentrations in weight parts per million, except Au and Ag in weight parts per billion. N.D. = no data

In fact, the measured U and Th abundances in some sam- Base Metals ples may be too high. U and Th abundances are positively Other than Pb, base metal abundances in the low-Pb miner- correlated with the lithophile elements Al, Ti, Rb, the rare als do not correlate with Pb isotopic composition. Several of earth elements, and to some extent with Mg, indicating that the arsenopyrite samples (Table 24-2, samples 99-CR-1, NL some U and Th has been derived from silicates adhering to DH43a, N89 DH50a, Viking Core) have both high Zn and Pb the sulphide minerals.

7 B.L. Cousens, H. Falck, E.H. van Hees, S. Farrell, and L. Ootes

Table 24-3. ICP-MS U, Th, and Pb Concentrations and initial Pb Isotope Ratios for “Low-Pb” Sulphide Minerals.

208Pb/ 207Pb/ 206Pb/ 238U/ 235U/ 232Th/ 208Pb/ 207Pb/ 206Pb/ 204 204 204 204 204 204 204 204 204 Sample Site Min Pb Th U Pbm Pbm Pbm Pb Pb Pb Pbi Pbi Pbi 99-CR-1 Crestaurum ars 51213 < 0.1 <0.03 33.96 15.05 14.05 0 <0.001 0 33.96 15.05 14.05 Clan Zone3a Clan ars 54 5 0.8 34.99 15.04 14.68 0.825 0.006 5.43 34.21 14.96 14.25 Zone3b Clan ars 48 21.5 1.5 40.16 15.78 18.32 2.068 0.015 30.29 35.83 15.58 17.25 11894p Giant py 85 1.1 0.2 34.19 15.06 14.3 0.144 0.001 0.76 34.08 15.05 14.23 11896p Giant py 27 < 0.1 0.1 35.63 15.29 15.8 0.205 <0.002 0 35.63 15.27 15.68 8602 Supercrest stb 6418 < 0.1 <0.03 34.09 15.08 14.14 <0.001 <0.001 0 34.09 15.08 14.14 AB1610 Supercrest stb 3104 < 0.1 <0.03 35.96 15.37 15.96 <0.001 <0.001 0 35.96 15.37 15.96 KS 96-10 Supercrest cpy 23 0.2 0 33.97 15.03 14.09 0.024 <0.001 0.42 33.91 15.02 14.07 KS 96-7 Supercrest stb 4546 < 0.1 <0.03 34.2 15.12 14.09 0 <0.001 0 34.2 15.12 14.09 NL DH39a Nicholas L. ars 157 2.5 0.6 33.89 14.88 13.83 0.218 0.002 0.91 33.76 14.86 13.72 NL DH43a Nicholas L. ars 1917 < 0.1 0.1 35.1 15.32 15.54 0.002 <0.001 0 35.1 15.32 15.54 NL DH50a Nicholas L. ars 6986 < 0.1 0.1 33.77 14.82 13.64 <0.001 <0.001 0 33.77 14.82 13.64 N89 DH057 Nicholas L. ms 24993 < 0.1 0.2 34.04 14.91 13.72 <0.001 <0.001 0 34.04 14.91 13.72 N89 DH063 Nicholas L. py 104 < 0.1 <0.03 34.63 15.14 14.71 <0.005 <0.001 0 34.63 15.14 14.71 NU-15-94a Nicholas L. ars 5967 < 0.1 0.1 33.89 14.87 13.83 <0.001 <0.001 0 33.89 14.87 13.83 HF 52 Oro L. ars 21 < 0.1 <0.03 36.73 15.68 17.69 <0.030 <0.001 0 36.73 15.67 17.67 Viking Core Viking ars 27384 0.1 0 34.09 14.95 14.13 <0.001 <0.001 0 34.09 14.95 14.13 99-Yk-1 Viking ars 67 12.7 1.3 37.16 15.15 15.58 1.134 0.008 11.73 35.48 15.04 14.99 Note: In cases where Pb, Th or U are outside of analytical range (< or > symbol), U/Pb and Th/Pb ratios are maximum ratios and calculated initial Pb isotope ratios are too low. "m" = measured (present-day), "i" = initial ratios. Min = mineral. See Table 24-1 for mineral abbreviations and sample locations.

abundances, suggesting that they contain inclusions of spha- Franklin and Thorpe, 1982; Thorpe et al., 1992). The focus lerite. Arsenopyrites with both low Pb and high Pb abun- on galena is primarily due to the high Pb concentration and dances fall within the main array, and an arsenopyrite with resulting minuscule U/Pb ratio in galena, which eliminates almost 2000 ppm Pb plots outside the main array. The most any possible post-crystallization ingrowth of radiogenic Pb. radiogenic arsenopyrite samples have low Pb abundances of It is remarkable that the range of Pb isotope ratios in galenas around 50 ppm. The massive pyrite samples from the Giant and sphalerites from Slave Province gold deposits is nearly mine (Table 24-2, samples 11894py and 11896py) have low the same as that in western Slave VMS deposits (Fig, 24-6). Pb contents but both are relatively rich in Cu, and one pyrite We conclude that Pb in mineralizing fluids that formed layer is relatively rich in Mn, Ni, Zn, and Sb. Yellowknife area gold deposits was derived from the same The stibnites from the Giant mine (Table 24-2, samples sources that contributed Pb to western Slave VMS deposits. 8602, AB1610, KS96-7, KS96-16) are highly variable in The Nicholas Lake deposit is, however, exceptional, in base metal composition, perhaps as a result of inclusions of that galena and sphalerite have lower 206Pb/204Pb ratios and other sulphide or sulphosalt minerals. The stibnites show a generally low 207Pb/204Pb values as compared to other good negative correlation between Pb isotope ratios and Pb Yellowknife area deposits (Fig. 24-6). Following the model concentration, where stibnite with the lowest Pb isotope of Thorpe (1982), the lower Pb isotope ratios are consistent ratios has the highest Pb content. There is also a roughly neg- with an age for the Nicholas Lake deposit slightly older, ca. ative correlation between Pb isotope ratios and Pb/Sb. Either 2850 Ma, than the ca. 2670 Ma age inferred for most Slave the Pb-rich stibnite has crystallized from Pb-rich fluids and massive sulphide deposits. However, mineralized veins at incorporated Pb in its structure, or the Pb-rich stibnite con- Nicholas Lake are hosted by the Nicholas Lake granodiorite tains inclusions of galena or a Pb sulphosalt (but not spha- pluton that has been dated by U-Pb zircon techniques at 2604 lerite - Zn contents in the stibnites are all low) (see Coleman, ± 3 Ma (Ketchum, 2002). The sample collected for dating is 1953, 1957). from the portal entrance on the north side of the pluton, Au abundances in the various minerals are highly vari- somewhat distant from the mineralized zone, but no zonation able, and do not correlate with Pb abundance or Pb isotopic of the pluton is evident and thus the U-Pb zircon age repre- ratios. However, Ag tends to be highest in the more Pb-rich sents a maximum age for mineralization (Ketchum, 2002). It arsenopyrites that also have lower Pb isotope ratios. Thus, is therefore unlikely that the isochron through the Nicholas Ag appears to be following Pb systematics, a common char- Lake sulphides has an age significance. Instead, Pb in acteristic of many mineral deposits. Nicholas Lake galena and sphalerite separates must have been scavenged from a distinctive juvenile (perhaps older?) DISCUSSION source, with a slightly low 206Pb/204Pb ratio compared to the juvenile component in most other main-array sulphides. The Main Array Thorpe (1982) interpreted the variations in Pb isotopic Until this study, Pb isotopic studies in the Slave Province composition of galenas in Slave VMS deposits as a second- have focused entirely on the isotopic composition of galena ary isochron. Another plausible interpretation of the roughly from VMS and gold deposits (R. Thorpe, unpublished data, 8 Pb Isotopic Compositions of Sulphide Minerals from Yellowknife: Metal Sources and Timing of Mineralization

16.5 that would have had Pb isotopic compositions around 2.7 Ga

Nicholas Lake mantle values (see estimates by Yamashita et al., 1999). Clan Lake μ=15 best-fit Granitoids and gneisses of the Central Slave Basement 16.0 Con Mine Slave Complex, which are older than 2.9 Ga, underlie much of the Giant Mine Gneisses central and western Slave (Bleeker et al., 1999a; Isachsen 15.5 Nicholas Lake and Bowring, 1997), and are a possible source of more radi- Sleepy Dragon/ 207 204 Ptarmigan Point Lake granitoids ogenic Pb (higher Pb/ Pb ratios) than the Kam or Pb West Slave VMS μ=11 Banting rocks (Fig. 24-6, granitoid and gneiss fields).

204 15.0 2 Granitoids from the Yellowknife area have been modeled as East Slave Pb / VMS 2 mixtures of these same two components, a juvenile source

207 14.5 similar to Kam volcanic rocks and ancient basement in the Thorpe μ = 8.15 age range of 4.0 to 3.6 Ga (Yamashita et al., 1999), and the 3 Mantle μ = 8.5 mixing of these two components would also explain the 14.0 μ μ Slave VMS 4 Ga Crust range of compositions within the main array (Fig. 24-6). best-fit

13.5 Interpretation of Data from Outside of the 12 13 14 15 16 Main Array 206 Pb / 204Pb Figure 24-6. Pb isotope ratios in sulphides from the main array, In this study, the Giant mine deposit has been investigated in compared to fields for Slave VMS deposits (Thorpe et al., 1992) and the greatest detail. At Giant, there appears to be a broad model 4.0 Ga crust (line with filled circles at ends indicating spread in Pb isotopic compositions amongst the sampled sul- 238U/204Pb (μ) ratios in model) (Yamashita et al., 1999). Thick phide minerals, which include pyrite-stibnite-sphalerite (plus dashed line indicates best-fit through Nicholas Lake data, whereas fine dashed line is best-fit regression line though West Slave VMS galena and sulphosalts, Thorpe, 1982), pyrite-stibnite, stib- samples. Crustal evolution curves are indicated for two models from nite-only clusters (Fig. 24-3), and the Proterozoic galena- Thorpe (1982, solid curve) and upper mantle (dashed curve), where stibnite Supercrest cluster. Note that the four stibnite sam- μ = 238U/204Pb. The intersection of the best-fit line with the crustal ples, (Giant stope, KS96-7, 8602, AB1610) not including the evolution curves defines a model age. Sleepy Dragon and Point Lake granitoid analyses from Davis et al. (1996); Slave 2.7 to 3.1 Ga Proterozoic Supercrest samples, actually bracket the entire gneiss field from Bowring et al. (1990). range of Pb isotopic compositions at Giant. There are at least three interpretations of the spread in iso- linear VMS and gold deposit Pb isotopic data forming the topic compositions. First, the spread may be a result of vari- main array is that it describes mixing of Pb from two crustal able U/Pb in the sulphide minerals, and the samples fall on sources, a juvenile crustal or mantle source with low an isochron with an age of 2.3 Ga. This age is almost 300 Ma 207 204 Pb/ Pb ratios and an ancient, radiogenic crustal source younger than the proposed ages of mineralization based on 207 204 with high Pb/ Pb ratios (Fig. 24-6). This interpretation geological evidence (Kerswill and Falck, 1999; van Hees et mirrors the conclusion that interaction between juvenile and al., 1999; Siddorn and Cruden, 2000), although the only ancient components is recorded in Yellowknife area volcanic event constraining the lower limit of gold mineralization is rocks (Cousens, 2000; Yamashita et al., 2000) and granitoids the 2.19 Ga Dogrib dykes (LeCheminant et al., 1997), which (Davis and Hegner, 1992; Davis et al., 1996; Yamashita et are not mineralized. The overlap in Pb isotopic compositions al., 1999, 2000). In the Yellowknife area, likely sources of of Giant galenas, sphalerites, and sulphosalts in the main juvenile Pb are the Kam and Banting Group volcanic rocks array with dated (ca. 2670 Ma) sulphide deposits in the west-

Table 24-4. Isotope Dilution U and Pb Concentrations and Initial Pb Isotope Ratios.

207 204 206 204 238 204 207 204 206 204 Sample Site Mineral Pb/ Pbm Pb/ Pbm Pb (ppm) U (ppm) U/ Pb Pb/ Pbi Pb/ Pbi Viking Core Viking ars 14.95 14.1 5724 0.011 0.0001 14.95 14.1 NL DH39a Nicholas ars 14.88 13.85 47 0.075 0.0886 14.87 13.81 NU-15-94a Nicholas ars 14.92 13.9 23236 0.122 0.0003 14.92 13.9 N89 DH50a Nicholas ars 14.85 13.67 10195 0.253 0.0014 14.85 13.67 11894p Giant py 15.14 14.87 39 0.24 0.3532 15.11 14.7 11896p Giant py 15.36 16.12 30.8 0.058 0.1132 15.35 16.07 N89 DH063 Nicholas py 15.13 14.68 227 0.02 0.005 15.13 14.68 KS 96-7 Supercrest stb 15.02 14.03 22136 N.D. KS 96-16 Supercrest stb 15.85 19.3 74.9 N.D. 8602 Supercrest stb 15.05 14.13 4078 N.D. AB1610 Supercrest stb 15.44 16.51 393 N.D. Clan Zone3 Clan ars 15.5 17.74 35.6 N.D. HF 52 Oro ms 15.5 16.37 10.6 N.D. N89 DH057 Nicholas ms 14.83 13.68 62357 0.212 0.0002 14.83 13.68 All initial ratios (i) calculated assuming a crystallization age of 2580 Ma. Note that U, Pb, and Pb isotope ratios were determined on the same mineral separate, but not the same separate used to generate data reported in Tables 24-1 to 24-3. N.D. = no data. See Table 24-1 for mineral abbreviations and sample locations. “Lake” has been left off of site names. 9 B.L. Cousens, H. Falck, E.H. van Hees, S. Farrell, and L. Ootes

Crestaurum - sph Nicholas - ms responsible for the spread in Giant mine and Yellowknife

16.0 Crestaurum - arseno Viking - gal area Pb isotope ratios. First, the very radiogenic, late galena

Nicholas - gal Viking - arseno and stibnite from the Giant mine are found in Proterozoic 15.8 Nicholas - sph Clan - sph fault structures. The age of Proterozoic faulting is estab- lished at ca. 1964 Ma (Kusky et al., 1993), postdating the Nicholas - arseno Clan - arseno 2.8 Ga 15.6 Isochron Malley/Mackay event by 200 Ma. Second, it is difficult to Nicholas - py Pb Clan generate a crustal source of Pb as radiogenic as the late gale- 15.4 204 3.25 Ga na and stibnite (or stibnite in the main Giant stopes with Isochron 2.2 Ga 206Pb/204Pb > 19) only 300 Ma after the initial series of 15.2 Nicholas Isochron Pb / Viking Archean gold mineralizing events. Third, no other gold

207 15.0 locality in the Yellowknife area describes an isochron similar Main Array to the 2.3 isochron “age” of the Giant sulphides (Fig. 24-7). 14.8 If the spread in Giant mine analyses is caused by mixing, Nicholas Lake: 3 gal, 4 arseno, 1 sph, 2 py, 2 ms then the late Supercrest galena-stibnite analyses plot at high- 14.6 13.0 14.0 15.0 16.0 17.0 18.0 19.0 er Pb isotope ratios along the best-fit lines connecting the main stope analyses (Fig. 24-2, 24-3). Thus a later 206 Pb / 204 Pb Proterozoic event may be a “disturbance” that has allowed Figure 24-7. Pb isotope ratios in different sulphide minerals from selected deposits. Best-fit regression lines and calculated secondary Pb exchange in some sulphide minerals. Possible igneous isochron ages are shown (see text for discussion). arseno = events include the intrusions of the Athapuscow aulacogen arsenopyrite, gal = galena, ms = mixed sulphide, py = pyrite, sph = on the east Arm of Great Slave Lake (2175 to ~1865 Ma) sphalerite. (Bowring et al., 1984) and the Mackenzie dykes (1270 Ma) (LeCheminant and Heaman, 1989). ern Slave Province also supports an older age for mineral- ization at Giant. U and Pb concentration data from the sul- Other Deposits phides do not substantiate an isochron interpretation. At Nicholas Lake, all but two samples plot within the main The second interpretation is that each cluster of sulphide array, a pyrite and an arsenopyrite (Fig. 24-7). Unlike the minerals in Figure 24-2a represents a separate mineralization case at Giant, these two samples do not plot on a line, and the event that occurred between approximately 2.7 Ga (time of best-fit line does not pass through the Nicholas Lake galenas volcanic activity in the Yellowknife belt) and the late stib- and sphalerites in the main array. No isochron relation exists. nite-galena mineralization at Supercrest. In this case, each cluster of sulphides records the average isotopic composition All of the Crestaurum samples from this study (sphalerite of the crust at the time of each mineralizing event, and the and arsenopyrite) plot within the most radiogenic part of the line joining the various sulphide mineral clusters at Giant has main array (Fig. 24-7), indicating that fluids at Crestaurum no age significance. However, other data from Yellowknife derived most of their Pb from ancient crust (note that none area gold localities (Cumming and Tsong, 1975) fill in the of these samples comes from the main gold zone in the gaps between clusters (Fig. 24-2b), and can be seen to form Crestaurum shear zone, and most sphalerite was deposited as a continuous array extending from the main array towards part of a late stage of mineralization (Falck, 1992)). As noted the Proterozoic galena/stibnite analyses from the Supercrest above, mineralizing fluids at the Con, Giant, and Crestaurum deposit at Giant. mines are extremely similar isotopically and are very likely related. Previous work at Crestaurum has shown that chal- The third possibility is that the minerals that fall outside copyrite (also from a late stage of mineralization) can be of the main array have been isotopically disturbed, and that much more radiogenic (206Pb/204Pb = 17.02, Cumming and they fall on a mixing array between their original Pb isotopic Tsong, 1975) than the sulphides analyzed in this study. composition and that of newly introduced Pb. The problem then becomes identifying the “event” that introduced new Samples from Clan Lake include sphalerite and three Pb, as we discuss below. arsenopyrites. The Clan Lake sphalerite plots within the main array, as do galenas from the area (Thorpe, 1982), but The Yellowknife area is cut by a number a mafic dyke the arsenopyrites all plot outside of the array (Fig. 24-7). swarms, one or more of which may have thermally perturbed Assuming that the arsenopyrites had initial Pb isotope ratios the area. If we return to the ca. 2.3 Ga isochron formed by similar to the sphalerite, then the two less radiogenic sulphides from the Giant mine, the only significant geologi- arsenopyrites fall on a line with a slope corresponding to an cal event in the Slave Province known to have occurred at isochron age of approximately 2.2 Ga. A two-point line pass- around 2.3 Ga is the emplacement of the Malley dykes at ing through the sphalerite and the third, most radiogenic 2232 ± 3 Ma (LeCheminant et al., 1996) followed closely by arsenopyrite indicates a maximum age of 2.8 Ga. The prob- the Mackay dykes at 2210 Ma and the Dogrib dykes at 2188 lem with this analysis is that there is no unequivocal estimate Ma (Bethune et al., 1999). The Malley/Mackay event, pro- of the initial composition of the arsenopyrites, which is nec- posed to be related to rifting events on the east and southern essary to provide the third point on the isochron. The spha- margins of the Slave Province at that time (LeCheminant et lerite sample is not from the same pit as the arsenopyrite al., 1996), was extensive enough to be recorded in recrystal- samples and may not be from the same mineralizing event. lized zircon and rutile ages from granulite xenoliths recov- Even the galena samples from Clan Lake do not have the ered from kimberlites in the Lac de Gras area (Davis, 1997). same isotopic composition as the sphalerite (R. Thorpe, pers. However, three observations argue against this event being comm., 1999). At the Viking mine, two arsenopyrite samples

10 Pb Isotopic Compositions of Sulphide Minerals from Yellowknife: Metal Sources and Timing of Mineralization plot on the 2.2 Ga Clan Lake isochron but two samples of force behind this mineralizing event. Several granulite xeno- galena fall well below it. liths from kimberlites in the Lac de Gras area include zircons By combining all of the Pb isotope data from this study and rutiles that yield Mackenzie dyke ages (Davis, 1997), and previous work (Cumming and Tsong, 1975; Thorpe, supporting the premise that the Mackenzie event was intense 1982), it becomes clear that there is no clustering of Pb iso- enough to drive hydrothermal fluids, scavenging Pb from topic ratios in the radiogenic samples, and that the samples radiogenic granitoids and gneisses, and adding this new radi- form a broad, continuous array between the main array and ogenic Pb to existing Archean sulphide minerals. the late Supercrest galena/stibnite analyses (Fig. 24-2b). Scanning electron microscope images of arsenopyrite- Thus, there is no clustering of data that might indicate that rich samples from various deposits from the Yellowknife several distinct events may have occurred after 2.7 Ga. The area show that arsenopyrite grains are, in general, only best-fit line (r2 = 0.898) through the radiogenic sulphides slightly zoned and show no evidence for a second stage of corresponds to a secondary isochron age of 2.46 Ga (see arsenopyrite growth associated with a postulated Proterozoic Fig. 24-2b) that runs through the middle of the array of late Pb event (Fig. 24-8). However, it is observed that arsenopy- Supercrest analyses. This line is best interpreted as a mixing rite crystals commonly include small, anhedral galena grains line and the 2.46 Ga isochron age has no true age signifi- within fractures. We propose that the galena in fractures was cance, particularly since no geological event can be tied to deposited during the Proterozoic Pb event, and that this late this age (see summary in Bethune et al., 1999). Both end- galena strongly levers the Pb isotopic composition of the members of the mixing array are isotopically heterogeneous, picked mineral grains towards very radiogenic compositions. resulting in the scatter about the best-fit line through the This hypothesis can be tested using the laser-ablation multi- main array and more radiogenic Yellowknife sulphides. collector ICP-MS, which can perform spot Pb isotope analy- The late, Au-free mineralization at Supercrest includes ses of both host and fracture-filling minerals. very radiogenic galena and stibnite that are intimately inter- grown. The analyses plotted in Figure 24-3 are actually mix- The Influence of the Host Rock tures of the two minerals, with the more galena-rich mineral Samples for this study were taken from a variety of host picks having higher 206Pb/204Pb ratios. These sulphides lithologies, including volcanic, plutonic, and sedimentary must represent a distinct fluid depositional event, crystalliz- rocks. Figure 24-9 is a plot of 207Pb/204Pb vs. 206Pb/204Pb ing sulphides with Pb derived from old (Archean), isotopi- for all samples except the late Supercrest mineralization at cally evolved crust. The Pb is too radiogenic to simply be Giant, grouped by host-rock type. Mafic, intermediate, and derived by remobilization of older sulphide mineralization in felsic volcanic rocks of the Kam and Banting groups or the the area. Analyses of feldspar separates from Yellowknife Clan Lake volcanic centre host most of the sulphide samples area granitoids, ranging in age from ~3.4 to 2.4 Ga in age, in this study. These sulphides cover the entire spectrum of Pb and their acid-leachates show that the leachates can have isotopic compositions in the Yellowknife area. Plutonic present-day 206Pb/204Pb ratios in excess of 25 (Yamashita et rocks host sulphides from the Nicholas Lake deposit and the al., 1999, 2000). In addition, 238U/204Pb ratios are common- Viking mine; as previously noted, many Nicholas Lake ly greater than 15 in Yellowknife area granitoids (Yamashita analyses are slightly less radiogenic than all other et al., 1999, 2000; Jackson and Cousens, 2005), allowing for Yellowknife sulphides, but Viking mine samples fall within 206Pb/204Pb and 207Pb/204Pb ratios to reach levels required the range of Pb isotopic compositions defined by the vol- to explain the late Supercrest galena/stibnite compositions at canic-hosted deposits. The Prosperous Point, Cassidy Point, 1.27 Ga. Figure 24-2b (inset) shows a comparison of the sul- and the Discovery mine, as well as Kansai, Tom, and phide data from Yellowknife gold deposits with present-day Ptarmigan deposits are hosted in Burwash turbidites, and all 207Pb/204Pb and 206Pb/204Pb ratios in whole-rock granitoids plot within the lower part of the main array. Thus, only the (Cumming and Tsong, 1975) and granitoids feldspar sepa- turbidite-hosted deposits show any restriction in isotopic rates and acid-leachates (Yamashita et al., 1999, 2000) from composition, but most of the intrusion-hosted deposits over- the southwestern Slave Province. Many of the granitoids lap with them. Note that Pb isotopic composition is more have evolved to very radiogenic compositions subsequent to strongly tied to the sulphide mineral type (Fig. 24-5) than the crystallization, and potentially represent the “enriched” com- host lithology. ponent in sulphides outside of the main array. Note that the The composition of the host rock is proposed to be a fac- present-day granitoid isotopic compositions actually overlap tor in gold deposition at Giant (van Hees et al., 1999) but not those of the sulphides that plot outside of the main array. a factor in gold deposition at Clan Lake (Martel-Amesse et During the Proterozoic, granitoid compositions would have al., 1999). At Clan Lake, vein systems are preferentially plotted to the left of the sulphides, rather than in their pres- developed in the more competent volcanic rocks and rarely ent alignment in Figure 24-2b (inset). The analyzed grani- occur in the sedimentary rocks. At Giant, reactions between toids are thus unlikely to be the enriched crustal end mem- fluids and wall rocks may have been a primary factor in gold ber, even though they would still have been highly radi- deposition, but the loci of gold deposition appear to be with- ogenic compared to the main array during the Proterozoic. in the Campbell and Giant shear zones that serve to focus The Pb isotope systematics of Yellowknife area granitoids fluid flow (Siddorn and Cruden, 2000). The largest deposits deserves further work. (Con, Giant, and Nicholas Lake) all occur in competent vol- The secondary isochron age of 1.24 Ga for the late canic or plutonic rocks that are cut by shear systems. Supercrest galena/stibnite samples (Fig. 24-3) corresponds closely to the age of the Mackenzie dykes (LeCheminant and Heaman, 1989) that may have provided the thermal driving 11 B.L. Cousens, H. Falck, E.H. van Hees, S. Farrell, and L. Ootes

16.0

15.8 Split By 15.6 Host Rock 2.45 Ga Pb Reference

204 15.4 Isochron Main

Pb / 15.2 Array

207 Volcanic 15.0 Sedimentary

14.8 Plutonic

14.6 13 14 15 16 17 18 19 20 206Pb / 204Pb Figure 24-9. Pb isotope ratios in all sulphides from this study except the late Supercrest galena/stibnite mineralization from the Giant mine, subdivided by host rock type. A reference 2.45 Ga secondary isochron is shown based on a best-fit regression line through the sul- phides hosted in volcanic rocks.

source. Two end-member scenarios may explain this obser- vation. First, hydrothermal fluids scavenging metals passed through both juvenile volcanic rocks and older underlying basement before they deposited sulphides, implying that the fluids may have had long and variable pathways such that the proportion of juvenile crust- and basement-derived Pb in the fluids would differ with time. Alternatively, the fluids derived their Pb from a heterogeneous rock that included both a juvenile and an ancient component, and the propor- tions of each component incorporated into fluids varied with time. One rock type in the Yellowknife area that is composi- tionally heterogeneous is the Burwash turbidites, which are composed of detritus from mafic volcanic, felsic volcanic, granitoid, and minor ultramafic rock sources (Jenner et al., 1981; Yamashita and Creaser, 1999). Recent geochemical gl work on the turbidite units of the Burwash show that they are not homogeneous chemically or isotopically, and that the ars > 2.9 Ga Central Slave basement granitoids are not a large gl component in Burwash sedimentary rocks, although an iso- topically enriched component with a low 143Nd/144Nd ratio is present (Yamashita and Creaser, 1999). This enriched component would also have high Pb isotope ratios compared to juvenile crust of the Kam Group. The Prosperous Pluton is an S-type, peraluminous pluton that is proposed to be a melt of Burwash sedimentary rocks related to moderate pressure/temperature metamorphism between 2.63 and 2.58 Ga (Davis et al., 1996). The Prosperous Pluton Pb isotopic composition plots in the mid- dle of the main array. It is plausible that this pluton repre- Figure 24-8. Scanning electron microscope images of arsenopyrite sents an average Pb isotopic composition for the Burwash, grains in Yellowknife area sulphide deposits. a) Zoned arsenopyrite. and that the various detrital components in the Burwash may b) Arsenopyrite grains with galena filling fractures (white). c) Close- up of galena (gl) filling fractures in arsenopyrite (ars) from b above. have Pb isotopic compositions that extend away from this point, to both less (juvenile Kam/Banting) and more radi- Fluid Pathways and Sources of Gold and ogenic (contaminated Kam? (Yamashita and Creaser, 1999; Cousens, 2000)) compositions within the main array. If the Other Metals latter scenario is correct, then the Pb isotopic characteristics The main array (Fig 24-4) is reasonably explained as a of the sulphides provide further evidence that gold at the mixing line between two sources with different Pb isotopic Giant mine was precipitated from -rich hydrothermal compositions, a juvenile source and an ancient crustal

12 Pb Isotopic Compositions of Sulphide Minerals from Yellowknife: Metal Sources and Timing of Mineralization fluids derived from the Burwash turbidites or their deep The Pb in the galena is not remobilized Pb from earlier equivalent (van Hees et al., 1999). mineralizing events, but was scavenged from old, iso- Lastly, it is noteworthy that the pyrrhotite and pyrite sam- topically evolved crust. If the linear array of sulphide ples from the Damoti prospect near Indin Lake are both fair- minerals from the Giant and Supercrest deposits, as dis- ly radiogenic (Fig. 24-4), and a best-fit line passes through cussed above, results from a disturbance event, then this the upper part of the main array. The implication is that the Proterozoic Pb has an average Pb isotopic composition sulphides had initial ratios in the upper part of the array, and that would be a suitable mixing end member. therefore have a large component of old crustal Pb. Yet, iso- 6) The Pb isotopic composition of sulphides is not a direct topic studies of the host rocks at Indin Lake indicate that function of host-rock type, but is a function of the min- they are not underlain by the Central Slave Basement eral analyzed: galena and sphalerite have a much small- Complex (Bleeker et al., 1999a; Pehrsson and Villeneuve, er range of isotopic compositions than arsenopyrite or 1999). If this is so, then the physical presence or absence of stibnite. older basement is not a factor in the Pb isotopic composition 7) Fluids derived Pb either by encountering different of mineralizing fluids, and the older crustal component may crustal rocks during fluid migration, or by passing be derived from sedimentary rocks in the Indin Lake area. through heterogeneous rocks such as the Burwash tur- bidites. The average initial Pb isotopic composition of CONCLUSIONS the Burwash (as measured in K-feldspars from S-type plutons within the Burwash) falls in the centre of the The results of this Pb isotopic study of gold-related sulphide main array of Pb data from galena and sphalerite. This mineralization in the Yellowknife area show that: central position allows for the possibility that detritus 1) Measured Pb isotope ratios in sulphide minerals are vari- from juvenile (mantle-derived Kam/Banting Group vol- able, but with only one exception (Nicholas Lake) gale- canic rocks) and isotopically enriched (crustally con- na and sphalerite analyses plot within, or just below, the taminated Kam Group volcanic rocks) sources may be range for VMS deposits of the western Slave Province. the isotopic end members of the main array. This possi- These sulphides include Pb scavenged by hydrothermal bility is consistent with the proposal that key elements fluids from both juvenile and ancient crustal rocks. The in gold mineralization at Giant, such as arsenic, were slope of a best-fit line through the galena and sphalerite derived from the Burwash sedimentary rocks. analyses corresponds to a late Archean age of mineral- 8) This work confirms that there are several distinct stages ization, between 2.7 and 2.6 Ga. The similar isotopic of gold mineralizing events in the Yellowknife area. characteristics of galena and sphalerite from all deposits However, more work needs to be done to distinguish a within the study area suggest that they are genetically variety of potential processes that are contributing to Pb linked. The exception is the Nicholas Lake deposit, isotopic heterogeneity, including evaluating potential whose galena and sphalerite analyses suggest a slightly sources for primary mineralizing fluids, more detailed distinct, perhaps older volcanic source compared to investigation of multiple mineralization events within other Yellowknife area gold deposits. individual deposits, and determining the importance of 2) Some arsenopyrite, chalcopyrite, pyrite, and stibnite post-crystallization disturbance of Pb within sulphide crystals have more radiogenic Pb isotope ratios than phases. Given that all gold localities in the Yellowknife galena and sphalerite. area include sulphides that plot outside of the main array 3) Sulphosalts, galena, sphalerite, stibnite, and pyrite in defined by galena, it would probably be most profitable gold-bearing veins from the main stopes at the Giant to concentrate future work at either the Giant or Con and Supercrest deposits form a linear array. The array mines where exposure of the various mineralizing may be interpreted as an isochron, corresponding to an events is best. age of 2.3 Ga. Alternatively, the array results from post- crystallization disturbance of the sulphides and addition ACKNOWLEDGMENTS of more radiogenic Pb. This project would not have been possible without the enthu- 4) The spread in Pb isotopic compositions in arsenopyrite, siastic encouragement and assistance of Karen Gochnauer. pyrite, and stibnite from all the deposits in this study We appreciate the efforts of the staff of the Ontario may be due to radiogenic ingrowth of Pb after crystal- Geological Survey Geochemical Laboratories for their cus- lization, may represent multiple fluid deposition events tomized analyses of the sulphide mineral separates. ranging in age from late Archean to middle Proterozoic, Conversations with James Siddorn and John Kerswill were or may result from post-crystallization disturbance that extremely helpful, although they disagree with some of the added younger radiogenic Pb to the sulphides. We interpretations presented here. Samantha Siegel performed favour an interpretation that the more radiogenic sul- mineral separations and the Pb separation chemistry. phides are the result of a Proterozoic disturbance that Reviews by Ralph Thorpe and Jim Scoates have significant- must be younger than 1.9 Ga in age, the age of ly improved the manuscript. This project was funded by a Proterozoic faulting in the Yellowknife area, perhaps research grant from EXTECH III. associated with the Mackenzie igneous event at 1.27 Ga. 5) Galena and stibnite (and chalcopyrite (Cumming and REFERENCES Tsong, 1975)) associated with Proterozoic, gold-free Bethune, K.M., Villeneuve, M.E., and Bleeker, W. mineralization at Supercrest are highly radiogenic, and 1999: Laser 40Ar/39Ar thermochronology of Archean rocks in form an array with a secondary isochron age of 1.24 Ga. the Yellowknife Domain, southwestern Slave Province:

13 B.L. Cousens, H. Falck, E.H. van Hees, S. Farrell, and L. Ootes

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15