9. Regional Correlations, Tectonic Settings, and Stratigraphic Solutions in the Yellowknife Greenstone Belt and Adjacent Areas

Home , Abitibi greenstone belt, Snare River

9. REGIONAL CORRELATIONS, TECTONIC SETTINGS, AND STRATIGRAPHIC SOLUTIONS IN THE YELLOWKNIFE GREENSTONE BELT AND ADJACENT AREAS FROM GEOCHEMICAL AND SM-ND ISOTOPIC ANALYSES OF VOLCANIC AND PLUTONIC ROCKS Brian Cousens1, Hendrik Falck2, Luke Ootes2, Val Jackson2, Wulf Mueller3, Patricia Corcoran4, Craig Finnigan5, Ed van Hees6, Cathy Facey7, and Alberto Alcazar7

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. Dépt. des Sciences de la Terre, Université du Quebec à Chicoutimi, Chicoutimi, QC G7H 2B1 4. Dept. of Earth Sciences, University of Western Ontario, London, ON N6A 5B7 5. Dept. of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 6. Geology Dept., Wayne State University, Detroit, MI 48202 7. Ottawa-Carleton Geoscience Centre, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6

INTRODUCTION (Padgham and Fyson, 1992; Bleeker, 2003). Thus the origin and evolution of the Slave province appears to be dramati- The Canadian Shield consists of a number of Archean cra- cally different from that of the Superior province. The small tonic fragments sutured by Proterozoic collisional belts. volume of volcanic greenstone belts limits the extent to How did these ancient cratonic fragments originate, and do which we can use the geochemistry of volcanic rocks to infer we see evidence that tectonic processes similar to modern tectonic setting(s) craton-wide, but also enhances the impor- plate tectonics existed during their creation? The best stud- tance of such studies to understanding the evolution of the ied of these cratonic fragments, the Superior province, is Slave. subdivided into long, curvilinear belts based on the relative abundances of volcanic and plutonic versus metasedimenta- The Yellowknife greenstone belt (YGB) is one of sever- ry rocks. This belt-like structure of lithotectonic assemblages al small greenstone belts exposed in the southern part of the was interpreted to signify that the Superior craton grew by Slave Province. This greenstone belt is extremely well pre- the addition of successively younger, allochthonous island served, has well-exposed contact relationships, and has suf- arc, oceanic, and forearc sedimentary terranes, punctuated fered primarily low-grade, greenschist facies metamor- by mantle plume events (e.g., Langford and Morin, 1976; phism. The YGB has been mapped in considerable detail, Thurston, 1991; Kimura et al., 1993), supporting a plate tec- has been the subject of major and trace element geochemical tonic model for Archean crustal growth that is similar to studies, and is well dated by modern U-Pb zircon techniques. modern plate tectonics. However, geological studies over the Nevertheless, only a handful of modern rare earth element past decade suggest that lithotectonic assemblages may com- and neodymium (Nd) isotopic analyses exist for volcanic monly be autochthonous in origin (as summarized in and plutonic rocks in the Yellowknife area (e.g., Dudás, Thurston, 2002). 1989; Davis and Hegner, 1992; Yamashita and Creaser, 1999). Radiogenic isotopic data, in conjunction with field Critical to models of the evolution of the Superior (and observations, and major and trace element geochemistry, other) craton are the field relationships and geochemistry of have the potential to help resolve stratigraphic and tectonic volcanic, plutonic, and sedimentary rocks in greenstone belts problems, and to distinguish between different tectonic sce- that retain a record of their tectonic setting. Based on the narios for the origin of the volcanic rocks. geochemical characteristics of modern volcanic rocks in different plate tectonic settings, the geochemical signatures of This geochemical study began in 1996, soon after the ancient volcanic rocks have been used to distinguish completion of the first U-Pb zircon geochronological study between continental, oceanic (spreading centre, oceanic of the YGB and the recognition that the volcanic sequence plateau, seamount), back-arc, or volcanic arc origins for was underlain by older continental crust (Isachsen et al., greenstone belts in Canada and worldwide (e.g., Condie and 1991; Isachsen, 1992; MacLachlan and Helmstaedt, 1993; Baragar, 1974). Isachsen and Bowring, 1997). Several geologic problems were identified that could be addressed by a detailed geo- The geology of the Slave province contrasts strongly with chemical study. Firstly, what is the geological relationship that of the Superior province. It lacks the belt-like structure between the 2.7 Ga Kam and 2.66 Ga Banting groups of the of the Superior, is dominated by metasedimentary and plu- YGB, and what tectonic setting is inferred from their geo- tonic rocks, includes only a small volume of volcanic green- chemical systematics? Secondly, within the Kam Group, stone belts, and includes three to four rifted margins

Cousens, B., Falck, H., Ootes, L., Jackson, V., Mueller, W., Corcoran, P., Finnigan, C., van Hees, E., Facey, C., and Alcazar, A. 2005: Regional correlations, tectonic settings, and stratigraphic solutions in the Yellowknife greenstone belt and adjacent areas from geochemical and Sm-Nd isotopic analyses of volcanic and plutonic rocks; Chapter 9 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. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

what magmatic evolution trends can be identified from the ed to be related to synvolcanic dykes in the overlying Kam base of the volcanic pile to its top and what processes are Group, imply that the volcanic rocks of the Kam Group were responsible? Does older basement play a role? Thirdly, with- fed through and deposited over the Bell Lake Group in the poorly studied Banting Group, are putative Banting (MacLachlan and Helmstaedt, 1995). Further evidence for rocks exposed along the western shore of Yellowknife Bay older basement rocks beneath the YGB comes from xeno- really part of the Banting Group? Are felsic dykes that cut liths of tonalitic gneiss in a diatreme at the Con mine south the Kam Group feeders to the Banting Group (Helmstaedt of Yellowknife, which have discordant U-Pb zircon ages of and Padgham, 1986; Isachsen, 1992)? What geochemical 3040 to 3300 Ma (Nikic et al., 1980). similarities exist between the Banting Group and some other To the west, the YGB is intruded by the Western Plutonic 2.66 Ga volcanic complexes in the southwestern Slave Complex, which includes from south to north the Defeat Province? Fourthly, are other small greenstone belts within Plutonic Suite, the Duckfish Granite, and the Anton the immediate Yellowknife area related to YGB volcanism? Complex (Henderson, 1985). Precise U-Pb zircon ages from these granitoids range from 2608–2641 Ma (Henderson, Regional Geological Setting 1985; Henderson et al., 1987; Dudás et al., 1990; van The surface geology of the YGB is shown in Figure 9-1, and Breemen et al., 1992). Much of the Anton Complex is intru- the regional stratigraphy (with U-Pb zircon ages) is dis- sive into the YGB and the underlying gneissic played in Figure X-X in Falck et al. (2005). The belt has basement/cover group in the Dwyer Lake area, and may be been subdivided into two groups, the dominantly mafic Kam an early deformed part of the Defeat Plutonic Suite Group and the unconformably overlying, more felsic-domi- (Helmstaedt and Padgham, 1986; MacLachlan and nated Banting Group (Helmstaedt and Padgham, 1986). Helmstaedt, 1995). To the east, the YGB is conformably Precise (analytical uncertainties of +/- 1 to 4 Ma) U-Pb zir- overlain by the Duncan Lake Group, including the Walsh con crystallization ages from Kam Group felsic volcanic Lake and Burwash formations, a thick pile of greywacke and rocks range from 2722 to 2701 Ma, although some cherty mudstone turbidites thought to be basin-fill sediments felsic tuffs of the lower Kam Group include inherited zircons (Henderson, 1985; Helmstaedt and Padgham, 1986). The as old as 2820 Ma (Isachsen, 1992). Banting Group crystal- Kam and Banting groups are unconformably overlain by the lization ages are ~2660 Ma (Isachsen, 1992). The metavol- conglomerates and sandstones of the Jackson Lake canic supracrustal package forms a steeply dipping homo- Formation (Henderson and Brown, 1966; Helmstaedt and cline, such that the stratigraphic “way-up” is to the southeast. Padgham, 1986). Recent study of the Jackson Lake con- The rocks are metamorphosed to greenschist grade, but glomerates shows that they occur along the trace of the metamorphic grade increases to amphibolite in proximity to Yellowknife River Fault Zone (YRFZ, Falck et al., 2005, younger intrusions. The belt was subsequently dismembered Fig. X-X) that separates the Kam Group from the by Proterozoic faulting into four major blocks (Helmstaedt Banting/Duncan Lake groups, and represent post-faulting and Padgham, 1986). deposition of coarse sediments within low topography in the More abundantly felsic volcanic complexes of ca. 2.66 Ga fault zone (Martel et al., 2001; 2002). age are common in the southwestern Slave Province, includ- The YGB stratigraphic sequence of Mesoarchean gneissic ing the Banting Group, the Clan Lake complex north of basement, a cover group consisting of post-2.93 Ga Yellowknife, volcanic rocks in the Russell Lake area west of quartzites, banded iron formation, and minor metavolcanic Yellowknife, and the southern Beaulieu/Cameron River rocks, and an overlying ca. 2.7 Ga greenstone belt has been greenstone belts east of Yellowknife (Henderson, 1985; proposed to be present throughout parts of the central Slave Hurdle, 1985; Lambert, 1988; Mortensen et al., 1988). Province (Bleeker and Ketchum, 1998). The gneissic grani- Despite the large number of papers on the geology, toid basement and overlying cover sequence are referred to geochronology, and geochemistry of the YGB published as the Central Slave Basement Complex and Central Slave over the past two decades, the exact relationship between the Cover Group (Falck et al., 2005, Fig. X-X). Regional Pb 2.7 and 2.66 Ga volcanic packages remains poorly under- (sulphides) and Nd (plutons) isotopic studies of the Slave stood. Province have demonstrated that much of the western Slave Basement rocks to the YGB have been recognized, which (including the YGB) is underlain by basement rocks > 2.8 include metasedimentary, metavolcanic, and plutonic rocks Ga in age, whereas the eastern Slave lacks such an ancient exposed in the area around Dwyer Lake, Bell Lake, and basement (Bowring et al., 1989; Dudás, 1989; Davis and Nelson Lake at the north end of the belt (Helmstaedt and Hegner, 1992; Thorpe et al., 1992). Padgham, 1986; Isachsen, 1992; MacLachlan and Helmstaedt, 1993; Isachsen and Bowring, 1997; Bleeker and SAMPLING AND ANALYTICAL METHODS Ketchum, 1998). Originally termed the Dwyer Formation or Volcanic and plutonic rocks from the YGB area were col- Dwyer Group (Helmstaedt and Padgham, 1986), these rocks lected between 1996 and 2001. Many of the felsic volcanic have been split into two lithostratigraphic packages: gneissic rock samples were collected from the same locations as plutonic rocks with U-Pb zircon ages of greater than 2.93 Ga, those sampled for U-Pb zircon geochronology (Isachsen, which are found along the eastern margin of the plutonic 1992). Potential basement granitoids of the Central Slave Anton Complex; and overlying quartzites, felsic volcanic Basement Complex and Central Slave Cover Group were rocks, and banded iron formation that are greater than 2.8 Ga sampled at Dwyer, Bell, and Nelson lakes to the north, in in age, termed the Bell Lake Group (Isachsen and Bowring, many cases from outcrops that were previously dated 1997). Amphibolite dykes in the Bell Lake Group, interpret- (Isachsen, 1992; Isachsen and Bowring, 1997).

2 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses

Map 1

Yellowknife Greenstone Belt Post-volcanic granitoids 1 CANADA 7 Jackson Lake Fm. Dwyer Lake 1 6 Burwash Fm. USA 5 Walsh Lake Fm. Duckfish 6 Lake BANTING GROUP 7 b. Prosperous Granite Likely Slave Lake 4 a. Ingraham Chan Province Lake Prosperous Lake 3 Duck Fm. Chan Banting Lake KAM GROUP Oro 5 e. Kamex Fm. Lake d. Yellowknife Bay Fm. 2a Lake 2 c. Townsite Fm. b. Crestaurum Fm. Map 2 a. Chan Fm. Lake 1 CSBC/CSCG 2b 4b Appendix 4a sh contact 6-1 Wal 6 fault Vee 2c Map 1 N Lake

2d 74a Map 2 2b 6 Mafic 2c 2e Giant Au mine Volcanic 62 o 30' 2d Felsic Slemon GS L ake Volcanic Western 2a Plutonic 2b Greywacke/ Granodiorite 4 Mudstone Complex 2c 6 fault SN1-4 Con 3 Duck Kam Au mine Lake 2d 3 Lake

Russell 3 Lake 2d Kam Point 6 RU1-2 Yellowknife 7 Bay

5 km 2e

Sub Islands Drill Sites 6

4 Central Bay Drill Sites West Mirage Islands 4? 114 o 30'

Figure 9-1. Map of the Yellowknife greenstone belt (Map 1) and Russell Lake /Snare River (Map 2) area with sample locations indicated by stars (maps modified from Padgham, 1987a; Jackson, 2001). “GS” indicates location of Giant Section. Stars in Yellowknife Bay indicate the locations of drill cores sampled as part of this study. See Appendix 6-1 of Jackson and Cousens (2005) for Bell Lake and Clan Lake locations. Some sample sites lie outside area of Map 1. CSBC = Central Slave Basement Complex; CSCG = Central Slave Cover Group; Fm. = Formation 3 B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

500 J Nd isotopic analyses were performed on the crushed pow- ƒF 97SRC224 (57.3; -5.2) „ ders. Between 100 and 200 milligrams of sample were J 97SRC255 (78.6; -4.9) spiked with a mixed 149Sm-148Nd spike and then dissolved H 97SRC300A (53.6; +1.2) 100 in Savillex Teflon beakers. Nd and Sm were separated fol- ƒ F CSBC DL-1 (76.2; -3.0) „ ƒ lowing standard cation exchange techniques (Cousens, H F „ „ CSBC DL-3 (76.6; -3.4) F „ƒ 1996). Total procedural blanks for Nd are less than H 147 144 J H F „ƒ ƒ CSBC RL-1 (77.3; -5.3) 400 picograms and are insignificant. Sm/ Nd ratios are H ƒ J J J JHF J reproducible to better than 1%. Samples were loaded with F ƒ J J J J J 10 H „ J J J „ HF HNO3 or H3PO4 on one side of a Re double filament assem- H „H bly, and run in a Finnigan MAT261 thermal ionization mass ƒ ƒ H H F H H „ ƒ„ H H H spectrometer at temperatures of 1780 to 1820°C. Isotope F F 146 144

Rock / Primitive Mantle „F ratios are normalized to Nd/ Nd = 0.72190. Analyses of ƒ F F F ƒ„ „F F „ „ ƒ„ the United States Geological Survey standard BCR-1 yield 1 ƒ ƒ ƒ 143 144 J Nd = 29.02 ppm, Sm = 6.68 ppm, and Nd/ Nd = 0.5 0.512668 ± 20 (n=4). Seventy-one runs of the La Jolla stan- Th Nb La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu dard average 143Nd/144Nd = 0.511877 ± 18 (2-sigma, Figure 9-2. Primitive mantle-normalized (Sun and McDonough, September 1996 – May 2002). The isotopic data are listed in 1989) incompatible element patterns for Central Slave Basement T Complex (CSBC) granitoids from the Bell Lake and Yellowknife area. Appendix 9-1. Initial εNd values are calculated using avail- 2940 Brackets in legend indicate the SiO2 content (wt %) and εNd able U-Pb zircon ages (Isachsen, 1992; Ketchum, 2002). value for each granitoid. Granitoids that pre-date Kam volcanism are potential con- taminants of Kam magmas, and thus εNd values at 2700 Ma Drill core samples from southern Yellowknife Bay were are of greatest interest. Epsilon values at time T are calculat- collected from the Giant mine core storage area in 1999 ed using the following relation: (Cousens and Falck, 2001). Drill logs indicate that the major T 143 144 T 143 144 T lithologies intersected by drilling southeast of the Sub εNd =[( Nd/ Ndsample / Nd/ NdCHUR )-1] *10000 Islands include mafic to intermediate lavas, gabbro sills, and where CHUR is the Chondrite Uniform Reservoir and T is rare turbidites. No felsic volcanic rocks were encountered. generally the time the rock was formed. The uncertainty in These lithologies are consistent with those of the Kamex the initial εNd values is ± 0.8 epsilon units, based on the Formation exposed in the Sub Islands and at Kam Point analysis of duplicate samples. At 2.7 Ga, depleted upper (Henderson and Brown, 1966). In contrast, rock types drilled mantle is proposed to have had an εNd between +2.5 and in central Yellowknife Bay include mafic to intermediate +3.0 (Machado et al., 1986; DePaolo, 1988). volcanic rocks, high-Mg basalts, and numerous felsic volcanic and intrusive units. The abundance of felsic units is GEOLOGICAL AND GEOCHEMICAL typical of the Banting Group. The rare high-Mg basalt units DATA underlie the felsic volcanic units of the Central Bay grid, but Central Slave Basement Complex And Central are not typical of either the Banting or Kam groups. Slave Cover Group The sample locations are listed in Appendix 9-1 and shown in Figure 9-1 (stars). Gneissic granitoids from the Central Slave Basement Complex are commonly often mildly foliated, consisting of The volcanic and plutonic rocks were slabbed, crushed in quartz, plagioclase, and potassium-feldspar-rich layers sepa- a Chipmunk jaw crusher, and then ground to a fine powder rated by wispy biotite-chlorite-magnetite septa. Feldspar and in an agate ring mill. Major element oxides and some trace quartz grains are typically anhedral and interlocking. These element (Ba, Cr, Ni, Zr, Y, Nb, Sr, and Rb) abundances were gneissic granitoids include mostly discordant zircons with determined by fused disk x-ray fluorescence spectroscopy at 207Pb/206Pb ages of more than 2900 Ma, but a sample of either the Ontario Geological Survey Geochemical Anton mylonite gneiss from Bell Lake yielded concordant Laboratory in Sudbury or the Department of Earth Sciences zircons with an age of 2947 ± 1.3 Ma (Isachsen, 1992; at the University of Ottawa. Loss-on-ignition (LOI) was Isachsen and Bowring, 1997). Six samples of Central Slave determined as volatile loss at 1000°C. S and CO were ana- 2 Basement Complex intrusions were collected, including a lyzed by combustion at the Ontario Geological Survey. For diorite, a granodiorite, and four granites (Appendix 9-1). some samples, Co, Cu, Ni, Sc, V, and Zn were analyzed by inductively coupled plasma – optical emission spectroscopy With the exception of sample RL-1 (Appendix 9-1), the at the Ontario Geological Survey. For all samples, the rare Central Slave Basement Complex granitoids are peralumi- earth elements, Th, U, Hf and Ta were determined by acid- nous with K2O/Na2O >1. Three gneissic granitoids from digestion inductively coupled plasma-mass spectrometry Bell and Dwyer lakes (see Appendix 9-1 and Fig. 9-2, sam- (ICP-MS) at the Ontario Geological Survey. Nb contents ples DL-1, DL-3, RL-1) have very similar primitive-mantle were also determined by ICP-MS to take advantage of the normalized (pmn) incompatible element patterns, and are lower detection limits for this element compared to XRF. A characterized by enrichment in Th and the light rare earth set of representative analyses is shown in Table 9-1. The pre- elements (REE), negative Nb anomalies, small negative Eu cision of the data were estimated using results for duplicate anomalies, and strong depletion in the heavy REE relative to runs of several samples and blind standards included in the primitive mantle (Fig. 9-2, normalized to primitive mantle, sample set, and are listed in Table 9-1. Sun and McDonough, 1989). All of the granitoids have pro- nounced negative Nb (Fig. 9-2), as well as Sr, P, and Ti

4 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses

Table 9-1. Representative geochemical and isotopic analyses, Yellowknife greenstone belt. Sample 01-YK-22 97-YK-11 97-YK-7 97-YK-1 GS-18 98-YK-17 98-YK-19 01-YK-33 Precision Group Lower Kam YkBay Fm YkBay Fm Townsite Kamex Fm Banting Banting Duck Fm Type Ranney Ch pillow pillow felsic tuff Bode Tuff pillow felsic tuff pillow

SiO2 69.13 46.57 49.72 65.70 70.22 53.39 64.86 51.51 0.5 TiO2 0.27 1.15 1.15 0.77 0.38 1.39 0.44 1.52 2.0 Al2O3 17.96 14.27 14.41 14.21 12.89 16.62 14.39 14.65 1.2 t Fe2O3 0.98 14.94 15.04 6.00 1.06 10.83 4.45 10.98 1.6 MnO 0.01 0.25 0.24 0.07 0.05 0.24 0.06 0.31 3.4 MgO 0.78 6.04 5.67 2.64 0.71 4.14 0.72 3.91 1.7 CaO 0.66 11.74 8.42 3.38 4.05 6.77 4.20 6.35 2.0

Na2O 0.16 2.04 3.65 4.19 0.70 3.13 2.94 3.13 2.7 K2O 5.49 0.26 0.06 0.54 4.33 0.10 2.15 0.22 3.5 P2O5 0.05 0.10 0.09 0.21 0.11 0.12 0.13 0.36 5.0 LOI 2.40 2.45 0.72 1.21 4.70 4.30 5.10 6.10 6.5 Total 97.88 99.81 99.17 98.92 99.20 101.02 99.44 99.04 S 0.00 0.02 0.09 0.06 0.03 0.01 0 0.01 10

CO2 0.44 1.50 0.22 0.12 2.80 0.71 3.21 3.25 10 1 H2O 1.96 0.95 0.50 1.09 1.90 3.59 1.89 2.85 Nb2 23 4125481411 Zr 129 78 77 258 130 79 148 219 4 Y 5 27 27 38 10 27 12 36 6 Sr 25 180 68 141 20 145 219 271 2 Rb 94 7 7 23 130 bdl 55 4 10 Ba 186 56 70 391 410 <10 526 13 7 Cr 25 184 188 20 6 238 21 161 6 Co bdl 46 51 14 bdl 51 7 35 16 Cu 6 147 142 30 bdl bdl bdl 53 10 Ni 11 86 89 21 bdl 116 37 115 17 Sc 3 35 36 11 4 bdl bdl 23 5 V 31 318 323 81 34 356 63 179 4 Zn 8 108 115 88 15 84 73 129 3 La3 6.57 3.98 2.76 38.23 14.22 4.69 40.92 14.30 3 Ce 14.61 10.78 8.38 76.47 28.97 12.28 76.02 35.32 3 Pr 1.69 1.69 1.37 8.83 3.29 1.93 8.58 4.97 4 Nd 5.80 8.60 6.94 31.84 12.20 9.97 30.09 21.63 2 Sm 0.93 2.79 2.45 6.52 2.39 3.05 4.58 5.50 2 Eu 0.36 1.04 0.83 1.49 0.73 1.08 1.09 1.83 5 Gd 0.65 3.84 3.41 6.60 2.05 4.00 2.91 6.09 2 Tb 0.09 0.66 0.63 1.03 0.29 0.72 0.38 1.03 2 Dy 0.51 4.40 4.30 6.15 1.47 4.69 2.07 6.24 6 Ho 0.11 0.97 0.96 1.27 0.29 1.04 0.35 1.28 2 Er 0.33 2.83 2.75 3.61 0.71 2.94 1.05 3.72 7 Tm 0.05 0.41 0.41 0.54 0.11 0.41 0.15 0.55 7 Yb 0.33 2.59 2.50 3.47 0.62 2.52 0.93 3.40 3 Lu 0.05 0.38 0.37 0.56 0.09 0.36 0.15 0.54 3 Th 1.34 0.48 0.48 15.61 5.20 0.44 11.73 1.60 5 U 0.44 0.12 0.12 4.42 1.42 0.12 2.97 0.47 5 Hf 3.53 0.80 1.00 6.60 3.19 1.06 3.25 5.21 5 Ta bdl 0.23 0.22 1.00 bdl 0.25 0.42 0.73 14 Age (Ma)4 2850 2705 2705 2705 2700 2660 2660 2670 Nd5 5.91 7.88 6.58 31.79 9.98 11.14 22.12 21.89 143Nd/144Nd 0.51082 0.51274 0.51286 0.51131 0.51127 0.51271 0.5109 0.512 0 εNd -35.37 2.07 4.34 -25.82 -26.63 1.45 -33.98 -12.50 Sm5 0.89 2.58 2.26 6.5 2.23 3.58 3.27 5.72 147Sm/144Nd 0.0915 0.198 0.2072 0.1237 0.1353 0.1944 0.0894 0.1581 143Nd/144Nd 0.5091 0.50921 0.50916 0.50911 0.50886 0.5093 0.50933 0.50921 T εNd 3.3 1.6 0.7 -0.4 -5.3 2.2 2.8 0.8 TDM 2833 3494 3017 3537 3118 2699 3027 t 1 2 Total iron expressed as Fe2O3. H2O = LOI-CO2. LOI = loss-on-ignition when powder fused. Nb through Zn in ppm by fused disk XRF. 3 La through Ta by ICP-MS, in ppm. 4 Age assumed in initial ratio calculation. 5 isotope dilution measurement. See full data table in Appendix 9-1. Elemental precision estimates based on duplicate analyses, in percent of average values for each element. Isotope dilution Nd and Sm precision ~ 1%. Uncertainty in 143Nd/144Nd is +/- 0.00002, based on reproducibility of Nd metal standard and duplicate runs of samples. 147 144 T Precision in Sm/ Nd ~ 0.5% and εNd values +/- 0.8 epsilon units, based on duplicate runs of samples. 5 B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

anomalies (not shown, see Appendix 9-1). In contrast, the present. Secondary carbonate in the groundmass is common. diorite (sample 97SRC300A) and granodiorite (sample The cherty tuff horizons in the Kam Group usually are very 97SRC224) are not as light REE depleted, lack an Eu anom- quartz-rich with rare quartz or feldspar phenocrysts, aly, and are less depleted in the heavy REE. Sample although some tuffaceous units include well-preserved crys- 97SRC255, a sheared granitoid from south of Bell Lake, has tal tuffs with lozenges of quartz-feldspar porphyry interbed- a dramatically different REE distribution: the pattern is ded with finely laminated cherty tuffs. Crystal-rich tuffs near roughly flat with the exception of a large negative Eu anom- the top of the Crestaurum Formation are well exposed in the aly and a high at Th. Trapper Lake area. The granitoids vary considerably in initial isotopic compo- The volcanic rock samples collected have been variably sition. Assuming a minimum crystallization age of 2940 Ma, altered by greenschist facies metamorphism. Elements such T the gneissic granitoids have had εNd of -3.0 to -5.3 and have as K, Rb, Sr, and U are mobile under conditions of seafloor depleted mantle model (TDM) ages of 3.4 to 3.5 Ga. The and greenschist metamorphic conditions, and previous work T diorite has a positive εNd of +1.2. Their εNd 2700 values on the YGB has shown this to be the case for rocks of the (i.e., at the time of Kam Group volcanism) range from Kam Group (Cunningham and Lambert, 1989). Given the -6.2 to -8.6. The gneissic granitoids have the highly negative observation that the rare earth elements (REE) and the high- εNdvalues at 2700 Ma typical of continental crust formed field-strength elements (Th, Nb, Ta, Zr, Hf, Ti) remain rela- during the early Archean (e.g., Theriault and Tella, 1997). tively immobile in rocks metamorphosed to greenschist A felsic volcanic unit within the Central Slave Cover facies (Brewer and Menuge, 1998; Humphris and Group at Dwyer Lake, commonly termed the “Dwyer Thompson, 1978; Ludden et al., 1982), the discussion of the metarhyolite” (sample K92-94, from MacLachlan and data will focus on these elements. All data plotted in the fol- Helmstaedt, 1993), has been dated at between 2821 and 2853 lowing figures are recalculated to 100% on an anhydrous Ma (see Falck et al., 2005, Fig. X-X) (Isachsen, 1992; basis. 2850 Ketchum, 2002). The sample has an εNd value of -1.4 The range of rock types found in all formations of the and a TDM of 3.3 Ga. This metavolcanic rock is quite simi- Kam Group vary only slightly (Fig. 9-3a). Mafic rocks are lar in trace element composition to felsic flows and tuffs of primarily subalkaline basalts and basaltic andesites in the the Crestaurum and Townsite formations (see the following Chan, Crestaurum, and Yellowknife Bay formations. The section). Kamex Formation includes a greater proportion of evolved basalts and andesites than the underlying formations. The Kam Group Andesites form only a small proportion of the volcanic and The Kam Group has been the subject of prior field and geo- intrusive package. Tuffaceous units in the Kam Group are chemical studies that define the basic geochemical charac- primarily dacites and rhyolites. Basaltic rocks all plot in the teristics of the belt (Jenner et al., 1981; Goodwin, 1988; Mg-rich tholeiitic basalt or Fe-rich tholeiitic basalt fields in Cunningham and Lambert, 1989; MacLachlan and Figure 9-3a, and the majority of the more evolved volcanic Helmstaedt, 1995). The ~10 km-thick Kam Group is divided rocks straddle the tholeiitic-calc-alkaline boundary or plot into five conformable formations, which from base to top are within the calc-alkaline field. In a plot of TiO2 vs. the Chan, Crestaurum, Townsite, Yellowknife Bay, and MgO/FeO, however, most of the intermediate rocks also fol- Kamex (Fig. 9-1; Falck et al., 2005, Fig. X-X) (Helmstaedt low a tholeiitic trend (Cousens, 2000a, Fig. 5B). and Padgham, 1986; Padgham, 1987a; Falck et al., 1999). Figure 9-4 presents bivariate diagrams demonstrating the All but the Townsite formation are composed dominantly of variation of key trace elements with MgO. Ni and Cr con- mafic volcanic rocks and sills. The age of the Chan and tents decrease with decreasing MgO content. With the lower Crestaurum formations is uncertain: whereas they are exception of a Townsite gabbro unit, the maximum Ni con- commonly considered to be part of the ca. 2.7 Ga Kam tent in Kam Group volcanic rocks is 260 ppm, considerably sequence, the apparent older age of the Crestaurum lower than the most unfractionated modern mid-ocean ridge Tuff/Ranney Chert (Falck et al., 2005, Fig. X-X) and the dis- lavas (Basaltic Volcanism Study Project, 1981). V concen- covery of banded iron formation in the Crestaurum trations increase with decreasing MgO until MgO drops Formation raises the possibility that they are part of the below 7%, where V follows two trends: evolved lavas in the Central Slave Cover Group (Falck et al., 2002; Ootes and Chan Formation and synvolcanic gabbros in the Crestaurum Lentz, 2002). Formation maintain high V concentrations, whereas Although primary minerals are rarely preserved in the Townsite, Kamex, and Crestaurum evolved lavas and tuffs greenschist to amphibolite-facies volcanic rocks of the YGB, are rapidly depleted in V as MgO decreases. Concentrations primary structures and textures are generally well preserved of incompatible elements, such as Ba, Zr, and La, tend to be (Helmstaedt et al., 1986). In thin section, mafic rocks of the higher at a given MgO content in the Townsite and Kamex YGB are altered to an assemblage of chlorite, amphibole, formations than in the Chan, Crestaurum, and Yellowknife and variably albitized plagioclase feldspar, with minor epi- Bay formations (Fig. 9-4). There is a consistent increase in dote. In some samples, secondary carbonate is visible both La and Th concentrations, as well as La/Sm ratios, with filling fractures and disseminated in the groundmass. Most decreasing MgO content in all formations of the Kam Group. of the pillow lavas are aphyric or include trace amounts of Primitive mantle-normalized (Sun and McDonough, 1989) olivine or plagioclase phenocrysts. Felsic volcanic rocks in incompatible element patterns for mafic to intermediate vol- the Kam Group are commonly feldspar or quartz-feldspar canic rocks of the Kam Group are shown in Figure 9-5. In all porphyritic with a fine-grain matrix. Rarely, biotite is also five formations, patterns for mafic to intermediate rocks

6 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses

250 1000 FeO+Fe2O3+TiO2 Ni Cr a) Kam Group TB Chan 200 800 Tholeiitic Fe Crestaurum 150 A 600 Townsite D 100 Yellowknife Bay 400 R TB B Mg Kamex A 50 200 D CSBC R Calc-alkalineFeO+Fe O +TiO 2 3 2 0 0 Al O MgO 2 3 V La 60

Banting 400 b) Banting Group Tholeiitic TB Fe West Yellowknife Bay 40 A 300 D #9 Dyke/Ryan Lake TB 200 R 20 B Mg BK UK 100 D A R Calc-alkalineFeO+Fe2O3+TiO2 0 0 Al2O3 MgO Chan Th La / Smpmn Crestaurum c) Other Volcanic 4 Tholeiitic 30 Townsite Complexes TB Yellowknife Bay A Fe Clan Lake Kamex 3 D 20 TB Russell/Snare 2 R B Mg Duck Fm. A 10 D 1 R Calc-alkaline

Al2O3 MgO 0 0 0246810 024681012 Figure 9-3. Jensen (Jensen, 1976) classification diagrams for: MgO (wt%) a) Kam Group and Central Slave Basement Complex (CSBC) granitoids; b) Banting Group; and c) Clan Lake Complex, Russell Lake Figure 9-4. Harker diagrams for Kam Group volcanic rocks. Ni, Cr, /Snare River area, and Duck Formation. Thicker black curved line V, La, and Th concentrations in weight parts per million (ppm), MgO separates tholeiitic from calc-alkaline lavas. TB = tholeiitic basalt, in weight percent. Fe = high iron, Mg = high Mg, B = calc-alkaline basalt, A = andesite, D = dacite, R = rhyolite, BK = basaltic komatiite, UK = ultramafic komatiite.

T range from light rare earth element (REE) depleted with no +2.5 and -0.5. εNd correlates negatively with SiO2 content Nb anomaly to slightly light REE enriched with a negative and La/Sm (Fig. 9-6a). An exception to this is the T Nb anomaly. As the La/Sm ratio increases, so too does the Crestaurum Tuff/Ranney Chert, which has positive εNd but size of the negative Nb anomaly. Andesites and dacites from high La/Sm and SiO2. The felsic rocks of the Kam Group fall T the Kamex Formation, including a pillowed andesite from into two groups, one with εNd between +0.5 and -1.0, and T the Giant Section (location “GS” in Fig. 9-1), have elevated the other with εNd < -2. The former includes the felsic light REE and Th abundances, but large depletion in Nb, flows, tuffs, and felsic intrusions of the Townsite Formation, compared to Kamex basaltic lavas and gabbroic sills (Fig. 9- as well as andesites through dacites of the Kamex T 5e,f). Felsic rocks (thick lines) from the Crestaurum, Formation. The latter, highly negative εNd group includes Townsite, and Kamex formations are extremely similar, with the felsic tuffs near the top of the Crestaurum Formation and the exception of the Crestaurum Tuff/Ranney Chert (CT/RC) the felsic tuffs of the Giant Section (Kamex Formation). and the Bode Tuff, displaying strong enrichment in the light REE, large negative Nb anomalies, and variable negative Eu The Banting Group anomalies (Fig. 9-5b,c,f). The Crestaurum Tuff/Ranney The Banting Group is composed mainly of calc-alkaline Chert tuffs and the rhyodacite boulder from the Bode Tuff intermediate to felsic pyroclastic, volcanic, and sedimentary have lower REE, Nb, and Th abundances compared to all rocks, with subordinate pillow lavas and mafic sills. Banting other felsic tuffs from the Kam Group, but are also light REE rocks, as well as felsic dykes cutting the Kam Group and enriched and depleted in Nb (Fig. 9-5b,f). Two sandstone interpreted to feed the Banting Group, have been dated by U- samples collected from the same locality as the Bode Tuff Pb zircon techniques at between 2658 ± 2 and 2664 ± 1 Ma (the Giant Section) have normalized patterns that are very (Isachsen, 1992). Although the Kam and Banting groups similar to that of the boulder in the Bode Tuff (Fig. 9-5f). now form the contiguous YGB, the original contact and pet- T εNd in Kam Group lavas range from +4.0 to -5.5, and the rogenetic relationships between the two groups are unclear. majority of the mafic to intermediate lavas vary between The contact between the Kam and Banting groups is faulted,

7 B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

Figure 9-5. Primitive mantle-nor- 300 300 malized (Sun and McDonough, a) Chan Formation Lavas d) Yellowknife Bay Formation Lavas 1989) incompatible element 100 100 plots for: a) the Chan Formation; b) Crestaurum Formation; c) Townsite Formation; d) Yellowknife Bay Formation; 10 e) Kamex Formation, except for 10 the Giant Section; and f) the Giant Section. Thick dashed lines indicate felsic rocks, thin lines are mafic to intermediate 1 1 rocks. Grey field in (e) shows range of patterns for gabbro sills 3000.4 3000.4 in Kamex Formation. CT/RC = b) Crestaurum Formation Lavas e) Kamex Formation Lavas Crestaurum Tuff / Ranney Chert 100 and Tuffs 100

10 10 Rock / Primitive Mantle 1 1 CT/RC Kamex Formation Sills 3000.4 3000.4 c) Townsite Formation Gabbros f) Kamex Formation, 100 and Tuffs 100 Upper Giant Section

10 10

pillow andesite Bode Tuff 1 mafic sill 1 sandstone 0.4 0.4 Th Nb La Ce Pr NdPmSmEu Gd Tb Dy Ho Er Tm Yb Lu Th Nb La Ce Pr NdPmSmEu Gd Tb Dy Ho Er Tm Yb Lu

and commonly obscured by an infilling of younger con- a) 5 glomerates belonging to the Jackson Lake Formation (Falck, 1990). With the exception of one sequence of rocks exposed 4 BT just north of Yellowknife that is proposed to be transitional CT/RC between Kam and Banting volcanism (the Giant Section, see pmn 3 Discussion below), the only physical link between the Banting and Kam groups are felsic dykes (#9 dykes, La / Sm 2 Chan Henderson and Brown, 1966) of Banting age that crosscut Crestaurum 1 the Kam Group and are lithologically similar to Banting Townsite Yellowknife Bay 2-sigma quartz-feldspar porphyries (Bailey, 1987; Padgham, 1987b). Kamex 0 45 55 65 75 85 -6 -4 -2 0 2 4 6 The Banting Group is split into the Ingraham and T SiO2 (wt%) ε Prosperous formations (Fig 9-1; Falck et al., 2005, Fig. X-X) Nd (Helmstaedt and Padgham, 1986; Bailey, 1987; Padgham, b) 9 Ingraham/Prosperous 8 1987a). The Ingraham Formation is divided into a southern West Yellowknife Bay Shot Lake Member and a northern Greyling member. In 7 #9 Dykes / Ryan Lake addition to the Ingraham and Prosperous formations exposed 6 north of the Giant mine, there are many localities south of 5 pmn the Giant mine where faulting and the lack of exposure make 4 3

establishing the stratigraphic relationships difficult. These La / Sm localities include the Old Town Peninsula, Latham Island, 2 Mosher Island, Jolliffe Island, an island south of Kam Point, 1

and the West Mirage Islands (Bailey, 1987; Padgham, 1987a; 0 40 50 60 70 80 -6 -4 -2 0 2 4 6 Relf, 1987). Rocks exposed above the Kam Group at the T SiO2 (wt%) ε Giant Section, adjacent to the Giant gold mine (location GS Nd T T in Fig. 9-1), were interpreted to be transitional to the Banting Figure 9-6. εNd vs. SiO2 and εNd vs. La/Sm (primitive mantle normalized, pmn) in: a) volcanic rocks of the Kam Group; and b) the Group based on the greater abundance of felsic volcanic Banting Group. BT = Bode Tuff; CT/RC = Crestaurum Tuff/Ranney rocks and “mattress” textures in pillow lavas (Padgham, Chert.

8 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses

1987a; Falck, 1990). Drilling under the north end of 300 Ingraham/Prosperous 1200 Yellowknife Bay by the Giant mine and beneath central NiWest Yellowknife Bay Cr Yellowknife Bay has also intersected ash flow tuffs consid- #9 Dykes / Ryan Lake 1000 ered to be similar to rocks of the Banting Group (Baragar, 200 800 1975; van Hees et al., 1999; Cousens and Falck, 2001). These proposed Banting Group equivalents will be referred 600 to collectively as the West Yellowknife Bay suite. 100 400 In general, rocks of the Banting Group are more deformed than rocks of the Kam Group, resulting in flat- 200 tened pillows and sheared felsic units. Thus primary textures 0 0 are more difficult to discern than in rocks of the Kam Group. V La Felsic rocks in the Banting Group include massive quartz- 80 feldspar and feldspar porphyries, feldspar crystal tuffs, ash- 400 flow tuffs with preserved fiamme, and volcaniclastic sedi- 60 mentary rocks. Secondary carbonate in the groundmass is 300 common. 40 200 In a Jensen plot (Jensen, 1976), the rocks of the Ingraham and Prosperous formations range in composition from subal- 100 20 kaline basalt through rhyolite (Fig. 9-3b). Pillow lavas from the Shot Lake member are basalt to basaltic andesite in com- 0 0 position and plot in the tholeiitic field. Pillow lavas in the Th La / Smpmn Greyling member, one unit from the Prosperous Formation, 8 a hornblende-plagioclase flow and a pillow lava from the 30 6 West Mirage Islands, are all andesites. In a plot of TiO2 vs. MgO/FeO (not shown), Banting mafic through intermediate 20 rocks follow a tholeiitic fractionation trend (Cousens et al., 4 2002, Fig. 2B). All of the volcaniclastic units and the quartz 10 porphyries are dacites and rhyolites. Andesites through rhy- 2 olites fall within the calc-alkaline field. Rocks of the West Yellowknife Bay suite also includes tholeiitic basalts, but 0 0 02468 0246810 more evolved rocks are transitional to calc-alkaline in com- MgO (wt%) position. The #9 dykes and various phases of the Ryan Lake Figure 9-7. Harker diagrams for Banting Group volcanic rocks. Ni, Pluton also range from mafic to felsic compositions that Cr, V, La, and Th concentrations in weight parts per million (ppm), straddle the tholeiitic to calc-alkaline boundary. In the MgO in weight percent. Banting Group, rock compositions are more evenly distrib- uted over the range from basalt to rhyolite compared to the Ingraham and Prosperous formations, the West Yellowknife Kam Group. Bay suite (including the West Mirage Islands) and the #9 Figure 9-7 shows the variations in trace element concen- dykes and Ryan Lake Pluton are shown in Figure 9-8. tration with decreasing MgO content in Ingraham and Basaltic pillow lavas from the Shot Lake Member of the Prosperous formation volcanic rocks. Ni and Cr contents Ingraham Formation (Fig. 9-8a, “pillow lavas”) have flat generally decrease with decreasing MgO concentrations. V patterns that commonly lack negative Nb anomalies. Gabbro follows two trends, generally increasing in concentration sills from the Ingraham Formation are slightly light rare with decreasing MgO until an MgO content of ~4%, below earth element enriched (La/Smpmn ~ 1.5) with large negative which V concentrations range from 150 to 50 ppm. La, Th, Nb anomalies (Fig. 9-8a, “sills”). Andesitic to dacitic pillow lavas of the Ingraham Formation are moderately light rare and the La/Smpmn ratio exhibit a similar divergence at ~4% MgO: all three are uniformly low above 4% MgO, then earth element enriched (La/Smpmn ~ 4) with prominent neg- jump significantly below 4% MgO. ative Nb anomalies plus enrichment in Th. Crystal tuffs and metasedimentary rocks from the Prosperous Formation have Also shown in Figure 9-7 are trace element variations in patterns much like andesites of the Ingraham Formation with the West Yellowknife Bay suite, the #9 dykes, and the Ryan enhanced light rare earth element enrichment and depletion Lake Pluton. All three groups roughly follow the same trends in Nb. All andesitic to dacitic rocks are commonly more with decreasing MgO concentrations as the Ingraham and depleted in the heavy rare earth elements than the basaltic Prosperous formations. At MgO > 4%, concentrations of Ni rocks, and have a concave-up pattern from Gd through Lu. and Cr in the West Yellowknife Bay suite lavas are in the low Felsic volcaniclastic rocks and lavas from the Ingraham and end of the range for the Ingraham and Prosperous rocks, and Prosperous formations have very similar incompatible ele- V varies to a greater degree. Between 6 and 4% MgO, La, ment patterns, with large negative Nb anomalies and signif- Th, and La/Smpmn ratios are slightly higher in the West icant depletion of the heavy rare earth elements (1–3 times Yellowknife Bay lavas than in the Ingraham and Prosperous primitive mantle). Banting Group lavas lack negative Eu formations. anomalies. Primitive-mantle-normalized (Sun and McDonough, 1989) incompatible element patterns for rocks of the 9 B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

300 a) 8c). The tonalitic marginal phase of the pluton has REE patterns with slightly shallower slopes than the monzonitic inte- Ingraham / Prosperous 100 rior phase and the #9 dykes, but all are characterized by strong light REE enrichment, Nb and heavy REE depletion, Felsic and no Eu anomaly. Rocks T Nd isotope ratios, expressed as εNd values, are plotted 10 versus SiO2 and La/Smpmn in Figure 9-6b. Ingraham and Prosperous formation lavas, West Yellowknife Bay suite, and #9 dyke/Ryan Lake Pluton rocks all exhibit strong correla- Pillow tions between La/Smpmn and SiO2. With only three excep- Lavas tions, basalts through rhyolites of the Ingraham and Sills T 1 Prosperous formations have εNd values that range between +4 and -1. Rocks of the West Yellowknife Bay suite have the 0.4 T same range of εNd . The #9 dykes and the tonalities through T b) monzonites of the Ryan Lake Pluton exhibit a range of εNd West Yellowknife Bay Suite from +2 to -2. None of these assemblages displays a correla- 100 T T tion between εNd and SiO2; εNd are generally > -1 throughout the range from basalt through rhyolite. The Clan Lake Complex 10 The Clan Lake volcanic complex is located north of the YGB and east of the Bell Lake volcanic complex (Jackson and Cousens, 2005). Earlier mapping indicated that the volcanic rocks intrude the Burwash Formation (Hurdle, 1985; Hurdle, 1987) and the felsic lavas have been dated by the U-Pb zir- 1 Rock / Primitive Mantle con technique at 2661 ± 1 Ma (Mortensen et al., 1992). The 0.4 stratigraphic sequence from bottom to top consists of basaltic to andesitic volcanic rocks, dacitic volcanic rocks, c) #9 Dykes, Ryan Lake Pluton andesitic to dacitic volcanic rocks, dacitic to rhyolitic tuffs, 100 and an upper suite of basaltic to andesitic volcanic rocks. Sample locations are shown in Jackson and Cousens (2005, Appendix 6-1). The lavas range from basalt to rhyolite in composition (Fig. 9-3c). Extended rare earth element patterns for Clan Lake lavas show that basalts have flat patterns 10 with little or no Nb depletion whereas the rhyolites are highly enriched in the light REE and very depleted in the heavy REE, with no negative Eu anomaly (Fig. 9-9a). The dacite Ryan Lake mafic has a pattern intermediate between the basalts and the rhyo- Ryan Lake felsic T 1 lites. εNd values increase with increasing La/Sm and SiO2, #9 dykes from -1.4 in the basalt to +2.0 in the most evolved rhyolite T 0.4 (Fig. 9-10). The negative εNd values in the mafic rocks are Th Nb La Ce Pr NdPmSmEu Gd Tb Dy Ho Er Tm Yb Lu unusual. Figure 9-8. Primitive mantle-normalized (Sun and McDonough, The Duck Formation 1989) incompatible element plots for: a) volcanic and intrusive rocks from the Ingraham and Prosperous formations of the Banting Group; The Duck Formation consists of a suite of volcanic rocks and b) volcanic rocks from the West Yellowknife Bay suite; and c) #9 gabbroic intrusions located on the southeastern shores of dykes and intrusive phases of the Ryan Lake Pluton. Thin lines = mafic to intermediate rocks; thick lines = felsic rocks. Yellowknife Bay (Fig. 9-1; Easton et al., 1982). The igneous package intrudes the Burwash Formation and is cut by gran- Volcanic rocks of the West Yellowknife Bay suite (Fig. 9- itoids of the Defeat Plutonic Suite (Falck et al., 2005, Fig. X- 8b) also have systematically different trace element patterns X). The Duck Formation occupies the same stratigraphic position as the Clan Lake Complex, but differs from the Clan depending on the SiO2 content of the rock. Mafic to intermediate rocks are slightly light REE enriched with small but volcanic package in that it lacks felsic volcanic rocks (Fig. variable negative Nb anomalies. Felsic rocks are strongly 9-3c). light REE enriched and Nb depleted, with heavy REE abun- In addition to samples of Duck Formation lavas and sills dances less than four times primitive mantle. Patterns for the onshore, drill hole M95-31 along the southeastern edge of mafic and felsic rocks cross at Eu, and there are no Eu anom- Yellowknife Bay (Fig. 9-1) intersected a dacite flow that alies in the patterns. may be part of the Duck Formation. Duck volcanic rocks Quartz-feldspar porphyry #9 dykes and tonalitic through plot along the tholeiitic basalt/calc-alkaline basalt boundary, monzonitic phases of the Ryan Lake Pluton, the proposed whereas the dacite falls in the calc-alkaline field (Fig. 9-3c). source of the #9 dykes, all have similar REE patterns (Fig. 9- Figure 9-9c shows extended rare earth element patterns for 10 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses

300 samples was collected from the Russell Lake area (Fig. 9-1; a) Henderson, 1985; Jackson, 1999). The volcanic rocks form a Clan Lake Complex 100 homoclinal lens between metasedimentary rocks and are intruded by granitoids along its southeastern base. Rare tops indicators consistently show that the volcanic rocks face to the northwest. The felsic volcanic rocks are mostly fragmen- tal, although shearing has obscured primary textures in many 10 flows. Mafic to intermediate flows are pillow lavas, and most if not all of the volcanic rocks were deposited sub- aqueously. A feldspar-phyric rhyolite tuff from the top of the basalt Russell Lake sequence was dated by U-Pb zircon techniques 1 dacite at 2658 ± 1 Ma (Mortensen et al., 1992). Sample locations rhyolite are indicated in Figure 9-1. 0.4 Four samples were collected in the area where the Snare 300 River empties into the west side of Russell Lake, including a b) 2-metre thick felsic tuff bed, a felsic dyke, a gabbroic dyke, 100 Russell Lake / Snare River and a fine-grain pillowed flow (samples Sn-1 to Sn-4, respectively; Appendix 9-1). Two more samples were collected from the east side of Russell Lake, consisting of a quartz-bearing and quartz-free massive flow (Ru-1 and Ru-2, 10 respectively; Appendix 9-2). The volcanic rocks range from basalt to rhyolite in composition (Fig. 9-3c), and the mafic to

Rock / Primitive Mantle Ru-1 intermediate rocks fall just within the calc-alkaline field. Ru-2 Figure 9-9b shows normalized incompatible element patterns 1 basalt Sn-3 for the Russell River/Snare Lake sample set. The basaltic pil- andesite Sn-4 Sn-1 rhyolites low lava, Sn-3, has a flat pattern (La/Ybpmn = 1.5), whereas Sn-2 the andesite dyke, Sn-4, is light REE enriched with a nega- 0.2 300 tive Nb anomaly. All of the felsic lavas are light REE enriched but are depleted in the heavy REE. Sn-1 and Sn-2 c) 100 Duck Formation have distinctly lower REE abundances than Ru-1 and Ru-2. Note the small positive Eu anomalies in the felsic rocks, a feature that has not been seen in felsic rocks elsewhere in the T Yellowknife area. Calculated εNd values in the basaltic flow and andesite dyke are +2.2 and +2.8, respectively. ε T 10 Nd ranges from +2.4 to +3.1 in the felsic rocks (Fig. 9-10).

5 1 gabbro sills pillow lavas Clan Lake dacite (drill) Russell/ 4 Snare 0.2 Th Nb La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu Duck Figure 9-9. Primitive mantle-normalized (Sun and McDonough, 1989) incompatible element plots for: a) Clan Lake Complex; pmn 3 b) Russell Lake/Snare River area; and c) Duck Formation. La / Sm Duck lavas and gabbros. Duck samples of this study split 2 into two groups, pillow lavas that have small negative Nb anomalies and gabbro sills that lack Nb anomalies. Two previously published Duck analyses also split into these two groups (Jenner et al., 1981). The dacite has a much steeper 1 REE pattern than the pillow lavas and gabbro sills, lacks a T Eu anomaly, and is depleted in the heavy REE. εNd range from -0.2 to +1.6 in the Duck Formation, and the Duck gab- T 0 bro sills and pillow lavas overlap in εNd (Fig. 9-10). The T -6 -4 -2 0 2 4 6 dacite drill core sample has an εNd of +1.5. T Russell Lake/Snare River Complex εNd T Figure 9-10. εNd vs. La/Sm (primitive mantle-normalized, pmn) in To further evaluate the geochemical signatures of ca. 2.66 Ga rocks of the Clan Lake Complex, Russell Lake/Snare River area, and felsic complexes in the south Slave Province, a small suite of Duck Formation.

11 B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

Defeat RL-12 (-1.2) 800 NL-4, not shown in Fig. 9-11), extending the range for Defeat Yk-41 (+1.6) Defeat plutons in the Yellowknife area (+0.5 to -1.2, Defeat MV96-5 (-0.5) Yamashita et al., 1999) and falling in the middle of the range Defeat RL-13 (+0.8) for Slave plutons of this age (see Davis and Hegner, 1992; Pud Stock Yk-25 (+0.2) 100 Dudás et al., 1991). The Pud Stock, the felsic intrusion at Duckfish Yk-27 (-0.5) Con (sample 01-Yk-24), and the microgranite pod (sample Aplite pod Yk-29 (-7.8) 01-Yk-26) adjacent to the Negus quartz vein all have simi- Prosperous Yk-42 (+4.8) lar, Defeat trace element and isotopic characteristics (same as Pud Stock in Fig. 9-11; see Appendix 9-1). 10 The Duckfish Granite is distinct from the Defeat plutons in terms of its magnetic and radiometric signature, but not in

Granitoid / Primitive Mantle terms of its geochemistry. Although the Duckfish Granite is

1 K-rich, it is still only slightly peraluminous and is similar in major element composition to Defeat plutons. Its trace element signature is distinct only in that the middle REE are

0.2 depleted relative to the heavy REE (i.e., Dy/Ybpmn < 1; sam- Th Nb La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ple 01-Yk-27, Fig. 9-11). The degree of middle REE deple- Figure 9-11. Primitive mantle-normalized (Sun and McDonough, tion increases with increasing SiO2 and decreasing P2O5. 1989) incompatible element plots for postvolcanic plutons of the Isotopically, the two Duckfish Granite samples (01-Yk-27, Defeat Plutonic Suite, Duckfish Granite, and Prosperous Granite. T Patterns in grey are from Yamashita et al. (1999). Numbers in brack- 01-Yk-28; εNd = -0.3 and -0.5) fall within the range of ets are εNdT values. Defeat plutons. However, the 2611 Ma aplite pod (sample 01-YK-29) Postvolcanic Granitoids adjacent to the Duckfish Granite is not part of the Duckfish Granitoids in the Yellowknife area can be split into three body. Its trace element pattern strongly parallels that of groups: The Central Slave Basement Complex (discussed Defeat granitoid (sample RL-12) from Bell Lake (Jackson earlier), the Western Plutonic Complex (Defeat Plutonic Suite, and Cousens, 2005), having low La/Smpmn and La/Ybpmn, ca. 2630 Ma), and the Prosperous Granite (ca. 2600 Ma). and high Nb/Lapmn. But what is most distinctive is its T During the course of this study, samples of various grani- extremely negative εNd of -7.8 (duplicate value = -10.3). toids were collected primarily to determine their isotopic This value is somewhat more negative than Central Slave character. Basement Complex granitoids at 2.6 Ga and all Slave plutons analyzed to date (Davis and Hegner, 1992; Yamashita et Samples of Defeat plutons were taken at Bell Lake, from al., 1999). The aplites are associated with a large magnetite- a quarry west of Yellowknife, and from drill cores beneath carbonate alteration system that is clearly visible on aero- the southern part of Yellowknife Bay. The Pud Stock, a fel- magnetic maps of the area. This alteration may be responsi- sic breccia body from the Con mine, and a fine-grained ble for anomalously high values of Mo (450 ppm), Cu granitic pod near the Negus quartz veins are also of Defeat (96 ppm), W (32 ppm), and Sb (8 ppm) in the aplite bodies, age (2634 ± 10 Ma for Pud Stock; J. Mortensen and P. and may also have severely altered the Nd isotope systemat- Strand, pers. comm., 1993). The Prosperous Granite was ics of these bodies. sampled east of Yellowknife on the Ingraham Trail. The anomalously magnetic Duckfish Granite, dated at 2608 Ma, The Prosperous Granite intrudes the Burwash Formation. was sampled near its margins, as was a 2611 Ma (U-Pb zir- The Prosperous Granite sample 98-Yk-42 analyzed here and con and Re-Os molybdenum), molybdenite-bearing, aplite that analyzed by Yamashita et al. (1999) are very similar dyke cutting Lower Kam metavolcanic rocks not far from chemically. Rare earth patterns are actually sigmoid in the Duckfish margin (Ootes et al., 2002). shape, with large negative Eu anomalies (Fig 9-11). Prosperous Granites are enriched in Nb, Th, and P O rela- The Defeat and Prosperous samples are largely medium- 2 5 tive to the REE. A large felsic dyke cutting Duck Formation K granitoids, as are most granitoids with SiO < 75 wt % in 2 basalts and gabbros between Kadaicha Bay and Duck Lake the Slave Province (Davis and Hegner, 1992). The Duckfish (sample 01-Yk-38, see map by Easton et al., 1982) has an Granite samples range from high-K to medium-K, whereas REE pattern that is almost identical to that of Prosperous the nearby aplite dyke (01-Yk-29) is distinctly low-K in Granites. Isotopically, the Prosperous Granite is heteroge- composition. All Defeat, Duckfish, and Prosperous samples neous. Our analysis of sample 98-Yk-42 yields an ε T of are mildly peraluminous, with the exception of the aplite Nd +4.8, whereas previous analyses of samples from Prosperous dyke adjacent to the Duckfish Granite. Three of the Defeat granitoids are +2.0 (Dudás et al., 1991) and -0.8 (Yamashita suite samples (RL-13, 98-Yk-41, MV96-5) and the Pud et al., 1999). Although we encountered no analytical prob- Stock sample (98-Yk-25) have REE patterns typical of lems during sample processing and mass spectrometry, our Defeat plutons, with moderate La/Yb , low heavy REE pmn ε 2600 value is high compared to previous analyses of the abundances, and no significant negative Eu anomaly (Fig. 9- Nd Prosperous Granite. 11; see also Yamashita et al., 1999). Three Defeat samples (e.g., Figure 9-11, RL-12; MV96-6, NL-4 not shown in figure) are distinctive, having much higher heavy REE abundances and large negative Eu anomalies. Defeat plutons have 2630 εNd values of +1.6 (sample 98-Yk-41) to -1.9 (sample 12 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses

DISCUSSION a) 4

2 Crestaurum Tuff/ Group-wide Geochemical Relationships and Ranney Chert

Chan Origin of the Kam Group 0 The volcanic rocks of the various formations in the Kam Crestaurum AFC2 Townsite Group have many geochemical similarities. Firstly, all mafic T -2 Nd Yellowknife Bay ε to intermediate rocks follow a tholeiitic or transitional to Kamex -4 calc-alkaline fractionation trend. Secondly, all formations BT CSBC AFC1 within the Kam Group include basaltic rocks with flat to -6 slightly light REE-depleted incompatible element patterns, Bulk -8 Mixing as expected for melts of Archean depleted upper mantle and CSBC similar to modern mid-ocean-ridge basalts (MORB). -10 Thirdly, the formations include some lavas that are enriched 01234567 in the light REE, similar to enriched (E-) MORB that are less La/Smpmn commonly found along modern mid-ocean ridges (Sun and McDonough, 1989). However, the Kam REE-enriched lavas b) 4 also have negative Nb anomalies, which are virtually 2 unknown in E-MORB. Note that light REE enriched, Nb- depleted lavas are found in the Chan Formation, which was 0 previously thought to include only REE-depleted MORB- -2 like basaltic rocks (MacLachlan and Helmstaedt, 1995). T Kam

Nd Group Fourth, the volcanic rocks of the Kam Group have variable ε Banting Group T -4 #9 Dykes / εNd values, the mafic lavas generally having positive values Ryan Lake ClanLake and the felsic rocks negative values. -6 Russell/Snare The major, trace element, and isotopic characteristics of Duck -8 Kam Group volcanic rocks are inconsistent with an origin CSBC from a single, homogeneous source. Figure 9-6a demon- -10 T 0123456789 strates the negative correlation of εNd with SiO2 and La/Smpmn La/Smpmn for volcanic rocks of the Kam Group. At least for T the mafic to intermediate rocks, this correlation could be Figure 9-12. εNd vs. La/Sm (primitive mantle-normalized, pmn) in: interpreted to result from melting of a heterogeneous mantle, a) volcanic rocks of the Kam Group; and b) Banting Group, Clan Lake Complex, Russell Lake/Snare River area, and Duck Formation. with locally variable histories of light to middle REE enrich- (a) Bulk Mixing curve indicates assimilation of bulk Central Slave ment. However, the correlation of isotopic composition and Basement Complex (CSBC) granitoid into mantle-derived melt (Townsite basalt), with tick marks at 10% increments. The “AFC 1” La/Smpmn with SiO2 suggests that the isotopic and trace element compositions of the volcanic rocks are also effected by and “AFC 2” curves are two assimilation-fractional crystallization models for the Kamex Formation (Bode Tuff, “BT”) and Townsite crustal contamination (e.g., Harris, 1989). For mafic to felsic Member, respectively, where parental basaltic magmas are simulta- volcanic rocks of the Crestaurum and Yellowknife Bay for- neously fractionating and assimilating granitoids (stars) with 2700 ε 2700 = -8 and -2, respectively. Tick marks on the assimilation- mations, the contaminant has an εNd of less than -4. Nd Central Slave Basement Complex gneissic granitoids, with fractional crystallization curves indicate F (mass magma remaining / 2700 mass initial magma) from 1 to 0.1, in increments of 0.1 (for details, εNd values of -6 to -9, are possible candidates for the see Cousens, 2000a). (b) Dashed field outlines range of Kam Group contaminant. The felsic rocks of the Townsite member imply rocks from (a). Grey curves are the same Bulk Mixing and AFC 2700 curves from (a). a contaminant with a less negative εNd , although given the gap in SiO2 between the mafic and felsic rocks (few rocks of intermediate composition) it is also possible that the lant to the parental liquid. The assimilation-fractional crys- felsic rocks are unrelated petrogenetically to the mafic rocks. tallization model is more realistic, as it describes the chemi- T cal evolution of a magma as it crystallizes in a magma cham- The decrease in εNd and increase in La/Smpmn coincide ber and concurrently assimilates crustal material along with a decrease in Nb/La, reflecting the progressive increase chamber walls. The change in concentration of a trace ele- in the size of the negative Nb anomaly in Kam Group vol- ment in the evolving magma depends on fractionation of that canic rocks. Assimilation of light rare earth element-rich but element between crystallizing minerals and magma (distri- Nb-poor granitoid material (Fig. 9-2) would impart this sig- bution coefficient, D), the ratio of the rate (mass/unit time) nature on more evolved mafic to intermediate rocks, and of assimilation to crystallization (r), the initial concentration would also produce the fanning of incompatible element pat- of the element in the parent magma and in the assimilant, and terns (Fig. 9-5) in mafic to intermediate rocks of the Kam the mass fraction of magma remaining after a certain period Group. of time (mass magma/mass initial magma, F). 143Nd/144Nd To test this hypothesis, two examples of assimilation- in the magma is not changed during fractional crystalliza- fractional crystallization model curves (for details, see tion, since minerals do not fractionate the heavy isotopes DePaolo, 1981) and a bulk-mixing curve for a basaltic par- from the magma, but assimilation of crustal material may ent magma and Central Slave Basement Complex gneissic dramatically change isotope ratios in the evolving magma if granitoid assimilant are shown in Figure 9-12a. The bulk- there is a difference in isotopic composition between magma mixing curve is calculated by incrementally adding assimi- and wall rocks. The “AFC 1” model in Figure 9-12a assumes

13 B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

a fractionating assemblage of olivine, clinopyroxene, Fe-Ti and 4) a continental rift (summarized in Henderson, 1985). oxides and plagioclase in the proportions 20:30:5:45%, bulk The geochemistry of the volcanic rocks of the Kam Group D values of 0.06 and 0.12 for La and Sm, respectively has important implications for the tectonic setting in which (Green, 1994), an initial composition of a Yellowknife Bay these rocks were erupted. In their analysis of the geochem- Formation depleted tholeiite (sample Yk-7; εNd = +1.5), an istry of the YGB, Cunningham and Lambert (1989) conclud- average Central Slave Basement Complex gneissic granitoid ed that geochemical diagrams used to classify the tectonic 2700 contaminant (εNd = -8), and an r value of 0.6. The “AFC setting of Cenozoic volcanic rocks (e.g., Pearce and Cann, 1” curve follows the data array for some Kamex and 1973) produce inconsistent results for the Archean Crestaurum Formation rocks, whereas the bulk-mixing curve Yellowknife rocks. To some extent this is due to element generally passes below the Kam data array. The Townsite, mobility during greenschist facies metamorphism. Even so, Chan, some Crestaurum, and Kamex data arrays require an Kam Group volcanic rocks generally plot in the ocean floor 2700 assimilant with a less negative εNd , approximately -2 and basalt field and Banting Group rocks fall in the convergent -4, respectively. The “AFC 2” curve in Figure 9-12a demon- margin field (Cunningham and Lambert, 1989). strates how evolved rocks from the Townsite Formation It has been proposed that greenstone belts in the southern could be derived from a Niven member basaltic parent (εNd Slave province are slivers of oceanic crust trapped between 2700 = +3) with a granitoid assimilant having an εNd value of two colliding cratons, the Anton Terrane to the west and the -2, assuming the same fractionating assemblage, D values, Contwoyto/Hackett River terrane to the east (Kusky, 1989; and value of r as the “AFC 1” model. Assimilation-fraction- 1990). The “oceanic crust” could actually consist of any vol- al crystallization modeling generally supports the hypothesis canic pile trapped between the colliding terranes, and such that the intermediate to felsic Kam Group volcanic rocks rocks could include mid-ocean-ridge basalts, oceanic plateau were derived from mafic parent magmas with depleted man- basalts, intraplate seamount lavas, island-arc lavas, or a tle incompatible element patterns and isotopic compositions, back-arc complex. A mid-ocean ridge, oceanic seamount, or but were modified by interaction with older basement rocks oceanic plateau (see Mahoney, 1987; Sun and McDonough, prior to their eruption. This interaction produces the fan-like 1989) setting for the Kam Group appears unlikely based on set of incompatible element patterns in the Kam Group (Fig. T the large variations in La/Smpmn and εNd with SiO2 and the 9-5). abundant evidence for an underlying (or at least nearby) cra- Based on new field mapping, textural analysis, and geo- ton (>2800 Ma detrital zircons in felsic cherty tuffs). It is chemical analysis, Finnigan and Duke (2005) propose an also unlikely that Kam Group volcanism was linked to a alternative origin for some Townsite Formation felsic rocks. mantle plume in an intraplate environment, since Archean Newly recognized feldspar and quartz-feldspar porphyry mantle plumes are proposed to produce komatiites (e.g., intrusives dominate much of the Niven and Brock lenticles Herzberg, 1995) but komatiites are rare in the YGB and in (small diapirs in the Townsite Formation, Falck et al., 2005, other Slave greenstone belts (Helmstaedt et al., 1986). Fig. X-X). The intrusive porphyries are proposed to have Modern oceanic island arcs, such as the Mariana and been generated by in situ partial melting of wet mafic lava Tonga-Kermadec arcs, generally erupt submarine and sur- flows of the Crestaurum Formation during intrusion of the face lavas that follow a tholeiitic fractionation trend (e.g., Townsite gabbroic sills. The Nd isotopic data from Townsite Ewart and Hawkesworth, 1987; Lee et al., 1995). However, felsic rocks do not distinguish between an assimilation-frac- rocks with more than 62% SiO2 are rare and the basalts and tional crystallization and a basalt partial-melting model, due basaltic andesites have ubiquitous negative Nb (and other T to the low εNd values in upper Crestaurum Formation mafic high field strength elements) anomalies coupled with large to intermediate rocks that differ only slightly from those in ion lithophile element (LILE) enrichment. As in the other Townsite felsic rocks. purely oceanic settings, the source of the > 2.8 Ga zircons in The geochemical evidence for interaction between mafic the Crestaurum Tuff and Niven member felsic rocks of the magmas and continental basement rocks supports the field Kam Group remains problematic: older basement rocks must interpretation that the décollement between the basement be present to provide the mixed zircon populations found in rocks and the greenstones in the central Slave Province these units. No such older basement exists beneath modern involves only limited transport of the upper plate green- island arcs. stones (Bleeker and Ketchum, 1998; Bleeker et al., 1999b). One modern tectonic environment where lavas are domi- The basement granitoids are clearly excellent candidates for nantly submarine but can form in proximity to a continent being the contaminant in the intermediate to felsic volcanic with older basement rocks is a back-arc basin. Excellent rocks at Yellowknife, suggesting that they underlay the modern examples include the Japan Sea (Tamaki, 1988) and greenstones at the time of eruption. the Woodlark Basin (Taylor et al., 1995). Miocene lavas from the Japan Sea back-arc are tholeiitic, submarine basalts Tectonic Setting of the Kam Group that range in composition from normal MORB-like basalts to Proposed origins for greenstone belts of the southern Slave light REE and LILE-enriched basalts with small negative Nb Province include: 1) an island arc (Folinsbee et al., 1968; anomalies (Cousens et al., 1994; Pouclet et al., 1995). Condie and Baragar, 1974); 2) slivers of oceanic crust Radiogenic isotope ratios covary strongly with light REE trapped in a suture zone during the accretion of two terranes enrichment, in the same manner as volcanic rocks from the (e.g., Kusky, 1989; 1990); 3) a back-arc basin intruded and Kam Group. One question that remains is, if the Kam Group overlain by later arc volcanism of the Banting Group is the result of back-arc extension then where is the remnant (Helmstaedt and Padgham, 1986; Helmstaedt et al., 1986); of the associated volcanic arc? No arc-related volcanic or

14 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses plutonic rocks of Kam age are found in the southern Slave 500 province (van Breemen et al., 1992), with the exception of 40867f minor calc-alkaline volcanic rocks in parts of the Cameron 01-Yk-30 and Beaulieu River volcanic belts (Dostal and Corcoran, 100 40871f

1998). It is possible that the arc sequence has been removed 40872f by postorogenic processes, but it is remarkable that there are no plutons preserved in the Western Plutonic Complex that Bode Tuff can be related to a putative Kam-age arc complex. Sandstone Finally, it is proposed that Kam Group volcanism repre- 10 Sandstone sents volcanic activity at the margin of an intracratonic basin (Henderson, 1985). Many Mesozoic flood-basalt suites associated with the opening of the modern Atlantic Ocean are Rock / Primitive Mantle submarine to subareal, tholeiitic basalt sequences that are as thick as 5 km (e.g., Harris, 1989; Robillard et al., 1992; 1 Saunders et al., 1997). These volcanic suites overlie continental basement, commonly include minor felsic rocks, and 0.3 customarily exhibit geochemical evidence for assimilation of Th Nb La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu older basement rocks, producing correlations between light Figure 9-13. Primitive mantle-normalized (Sun and McDonough, REE enrichment, isotope ratios, and SiO . The mantle 1989) incompatible element plots for 2604 Ma quartz-feldspar por- 2 phyry intrusions in the Chan Formation (open symbols), a rhyodacite sources of young flood-basalt suites include depleted mantle, boulder from the Bode Tuff, and sandstone layers from the Giant mantle plumes, and enriched subcontinental lithosphere, all Section from above and below the Bode Tuff. Intrusions with open tri- with different trace element and isotopic characteristics. It angle symbols closely resemble the Bode Tuff and sandstone pat- could be argued that the light REE-enriched, Nb-depleted terns. lavas of the Kam Group were derived from juvenile, enriched lithospheric mantle (modified by addition of subduction- felsic rocks in the Kam Group (-5.3). Sandstones collected T stratigraphically proximal to the Bode Tuff have virtually related fluids) with an εNd value only slightly lower than T identical incompatible element patterns and εNd values of that of depleted mantle at 2.7 Ga. However, the observed cor- T T +0.3. These εNd are distinct from those of cherty tuffs else- relation between εNd and SiO2 is more difficult to explain in this way (e.g., Harris, 1989; Saunders et al., 1997). where in the Kam Group. Thus detritus supplied to the base of the Kamex Formation was distinctly different from the The submarine setting, tholeiitic fractionation trend, and surrounding mafic lavas and underlying felsic tuffs/intru- evidence for ancient, continental crustal contamination of sions. mantle-derived magmas leads us to favour a continental margin rift setting for the Kam Group. A swarm of small quartz-feldspar porphyry intrusions, dated at 2706 +/- 2.5 Ma (see Falck et al., 2005, Fig. X-X), Geology and Geochemistry of the Giant Section cut the Chan Formation in the vicinity of Homer Lake and Chan Lake (samples 01-Yk-30, 40867f, 40871f, 40872f in The Giant Section, a cross-section through the upper part of Fig 9-13). Whereas two of the intrusions (samples 40867f, the Kam Group near the Giant mine, presents several geo- 40871f) have REE patterns identical to Crestaurum or logical problems. First, the transition from the Yellowknife Townsite formation felsic tuffs, two of the intrusions (sam- Bay to the Kamex Formation is beautifully exposed at the ples 01-Yk-30, 40872f) analyzed here have incompatible Giant Section (Falck, 1990; Falck et al., 1999). The Bode element patterns similar to the Giant Section sandstones and Tuff is a marker horizon that delineates the boundary Bode Tuff boulders, although incompatible element abun- between the Yellowknife Bay and Kamex formations (“BT”, T dances are slightly lower in the intrusions (Fig. 9-13). εNd Falck et al., 2005, Fig. X-X). Earlier work has shown that the values in the intrusions range from -16 to +0.3. Although the Bode Tuff is geochemically anomalous, and clasts in the tuff U-Pb zircon ages for the rhyodacite boulder of the Bode Tuff have no known extrusive or intrusive equivalent in the YGB and these intrusions differ slightly, the intrusions represent (Cousens, 1997). Where do the clasts come from? A second possible igneous parental material for the distinctive sand- problem, that geological observations suggest that the upper- stones of the lower Kamex Formation. The intrusions appear most Giant Section is a transitional sequence between the to have breached the bedrock surface and the resulting extru- Kam and Banting groups, is discussed in the next section. sive tuffs and volcaniclastic rocks were re-sedimented The Bode Tuff is a distinctive reverse-graded, conglom- downslope to the south, in the area of the present-day Giant eratic, interflow sediment that includes rounded clasts of Section (Falck, 1990). rhyodacite porphyry (Padgham, 1987b; Falck, 1990). U-Pb zircon analyses of two boulders from the Bode Tuff and two The Banting Group: What is Banting and fine-grained Bode Tuff samples at Kam Point yield the same What is Not? age, within error, of 2702 ± 2 Ma (Isachsen, 1992). The rhyodacite clast analyzed here (sample GS-18) is distinct from The Banting Group traditionally includes the Ingraham and felsic units in the Upper Kam and Townsite Member in its Prosperous formations north of Yellowknife Bay. Felsic “#9” relatively steep incompatible element pattern, lower overall dykes cutting the Kam Group and the West Yellowknife Bay incompatible element abundances, and lack of a negative Eu suite as defined here have been proposed to be part of the T Banting Group, as well as the upper parts of the Giant anomaly (Fig. 9-5f). It also has the most negative εNd of all 15 B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

400 appear to cement the relationship between the #9 dykes and the Banting Group. In addition, the geochemical similarity of the Ryan Lake Pluton and volcanic rocks of the Banting Group suggests that this pluton is older than all other dated 100 Banting felsic Kam felsic granitoids of the Western Plutonic Complex (recently con- firmed by U-Pb and Re-Os geochronology (Ootes et al., 2002)). The interpretation that the uppermost rocks of the Giant Section are part of the Banting Group is based on geological

10 observations, including the increased abundance of interflow

Rock / Primitive Mantle sediments that are rare in the Kam Group but common in the Kam mafic Banting Group, the selvage morphology of pillow lavas, and the presence of felsic ignimbrite flows in the uppermost part of the section (Padgham, 1987b; Falck, 1990). The upper Banting mafic Giant Section mafic sill has a flat incompatible element pat-

1 tern (Fig. 9-5f), like basalts in the Kam Group and the Kam Th Nb La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu sill to the south, but unlike the Shot Lake gabbro sills found Figure 9-14. Summary and comparison of primitive mantle-normal- on nearby Old Town Peninsula and Navigation Island (Fig. ized (Sun and McDonough, 1989) incompatible element patterns for 9-8a). The andesite lava at the Giant Section has a highly mafic and felsic rocks of the Kam and Banting groups (including #9 T dykes, Clan Lake, and Russell/Snare area samples). Note the lack negative εNd (-3.2) that is atypical of samples from the of negative Eu anomalies and strong heavy REE depletion in Banting Group. The felsic ignimbrites have negative Eu Banting felsic rocks compared to Kam felsic rocks. anomalies, lack heavy rare earth element depletion, and have T εNd values of less than -1, which are more common in the Section (Bailey, 1987; Falck, 1990; Padgham, 1987b; Relf, Kam than in the Banting Group. The evidence indicates that 1987). Does the geochemistry of these units support this the proposed “Banting” rocks of the Giant Section are part of stratigraphic assignment within the YGB? the Kam Group, and in particular, the new Kamex Formation Figure 9-14 is a compilation of incompatible element pat- (Falck et al., 2005). terns in mafic and felsic volcanic rocks of the Kam and Banting groups. Mafic rocks in the two groups have over- Origin of the Banting Group lapping middle to heavy REE abundances, and differ slight- The Banting Group is commonly referred to as predomi- ly in light REE abundances. It has already been noted that nantly felsic and calc-alkaline in composition (Helmstaedt basalts with flat REE patterns occur in both the Kam and and Padgham, 1986; Padgham, 1987a; Goodwin, 1988; Banting groups. Felsic rocks of the Kam and Banting groups Cunningham and Lambert, 1989), and has been interpreted are, on the other hand, consistently different in several as a volcanic arc sequence based on these two characteristics respects. Compared to Kam felsic rocks, Banting felsic rocks (Helmstaedt and Padgham, 1986). However, mafic rocks fol- have similar light REE abundances, but lack negative Eu low a tholeiitic fractionation trend. Together, the mafic and anomalies and are heavy REE depleted. Thus the REE pat- evolved rocks do not follow the usual calc-alkaline trend terns of felsic rocks can be used as a stratigraphic fingerprint observed in modern volcanic arc settings (Irvine and of Kam versus Banting volcanism. Baragar, 1971), and a plausible interpretation is that the Crystal tuffs from Jolliffe Island, the felsic ignimbrite unit mafic and felsic suites are not directly related petrogeneti- from Latham Island, and felsic rocks drilled beneath cally. Yellowknife Bay all carry the diagnostic trace element pat- Mafic through intermediate rocks from the Banting terns of Banting felsic rocks, in particular a strong heavy Group are very similar chemically to mafic through interme- REE depletion (Fig. 9-8). Gabbros from Navigation Island, diate rocks of the Kam Group. The mafic pillow lavas of the Old Town, Mosher and Jolliffe islands have incompatible Banting Group Shot Lake member have incompatible ele- T element patterns nearly identical to those of gabbros intrud- ment patterns and εNd values typical of depleted mantle at ing the Shot Lake member, suggesting a geochemical link to 2.7 Ga, and thus at least some of the most primitive magmas the Banting Group (Fig. 9-8). from the Banting Group are melts of depleted upper mantle. No felsic volcanic rocks have been analyzed from the More evolved basaltic andesites to andesites have correlated T West Mirage Islands. Mafic to intermediate rocks have La/Smpmn, SiO2, and εNd values that are indicative of incompatible element patterns and isotopic ratios that over- crustal contamination, much like rocks of intermediate com- lap with mafic to intermediate rocks of the Kam and Banting position in the Kam Group and other mafic to intermediate groups, so no definitive stratigraphic assignment can be volcanic belts in the western Slave Province (Fig. 9-12b; made. Further sampling of the West Mirage Islands, focusing Yamashita et al., 2000). on felsic rocks that are more diagnostic of Kam versus Felsic rocks in the Banting Group do not consistently T Banting volcanism, is required. have more negative εNd than Banting mafic rocks, but The #9 dykes are geochemically and isotopically identi- instead are generally positive. Thus, Banting felsic rocks cal to felsic units of the Banting Group, as are samples of could not have evolved by the same fractional crystallization dykes from the marginal phase of the Ryan Lake Pluton (Fig. – crustal contamination mechanisms that were paramount in 9-8c). Along with the common U-Pb zircon age, this would the Kam Group (compare Fig. 9-12a,b). An alternative

16 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses model for the origin of Archean (and younger) granitoids and 100 10% Melt felsic volcanic rocks is partial melting of the continental 25% garnet hornblendite crust (mafic igneous or Al-rich sedimentary protoliths) fol- 25% Melt 10% Melt 10% garnet hornblendite lowed by fractional crystallization (Martin, 1987; 25% Melt

Drummond and Defant, 1990; Zegers and van Keken, 2001; 10% Melt hornblendite Barth et al., 2002; Selbekk et al., 2002). Consistent with this 25% Melt model, Banting Group felsic rocks have the geochemical Rapp AB-1 Melt characteristics of Archean tonalite-trondhjemite-granodior- 10 ite/dacite suites that originate by partial melting of a mafic protolith (Martin, 1987; Drummond and Defant, 1990; Zegers and van Keken, 2001). Partial melting (10–30%) of a Range of garnet-bearing amphibolite at lower crustal to upper mantle Banting Group pressures produces a dacitic to rhyolitic melt with a distinc- Felsic Rocks tive depletion in the heavy rare earth elements compared to / Primitive Mantle Felsic Volcanic melts of upper mantle peridotite. 1 Figure 9-15 displays the primitive mantle-normalized La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu rare earth element patterns of Banting Group felsic rocks and Figure 9-15. Primitive mantle-normalized rare earth element pat- compares them with patterns generated in model melts from terns for felsic rocks of the Banting Group (shaded field) and calcu- garnet amphibolite and amphibolite protoliths with the aver- lated partial melts of average Kam Group basaltic andesite (see age rare earth abundances of Kam mafic to intermediate details in Cousens et al., 2002). Calculated patterns are for 10 and 25% melting of potential protoliths with garnet-free and garnet-bear- lavas. Assuming batch melting and appropriate partition ing hornblendite residues (modeled after Martin, 1987; Drummond coefficients (Martin, 1987; Cousens et al., 2002), the rare and Defant, 1990). Banting Group felsic rock patterns are most sim- earth element patterns of Banting Group rocks are very similar to melts from ~10% garnet hornblendite (filled triangles). The pat- ilar to those of melts of garnet-bearing (10–25%) amphibo- tern of an experimentally derived dacite melt of a garnet-bearing protolith (inverted open triangle, Rapp et al., 1999) is also comparable lite. In addition to their similarity to model melts of garnet to those of Banting samples. amphibolite, rare earth element patterns for Banting felsic rocks are very similar to those of experimentally-derived directly exposing the overlying mafic rocks to the convect- dacitic melts from a garnet-bearing mafic protolith (Rapp et ing upper mantle (Zegers and van Keken, 2001). A second al., 1999). model is that of direct melting of a subducting slab to pro- T Positive εNd values in Banting felsic magmas indicate duce primary felsic magmas commonly referred to as that they were derived by melting of relatively juvenile crust, adakites (Defant and Drummond, 1990; Drummond and perhaps hydrated basaltic rocks that underplated the crust Defant, 1990; Martin and Moyen, 2002). Subducted oceanic during Kam magmatism. The lack of negative Eu anomalies crust converts to amphibolite and then to eclogite at increas- in Banting felsic rocks indicates that feldspar was not a ing depth of penetration into the mantle, and if the crust is major fractionating phase, that is consistent with a dacitic young and warm enough, it can melt beneath an arc. For primary melt that did not fractionate much further prior to example, many dacites from Mount St. Helens in the eruption. In contrast, felsic rocks of the Kam Group do have Cascade Range, Washington, are depleted in the heavy rare negative Eu anomalies, indicating that Kam felsic magmas earth elements compared to dacites from nearby Mount experienced a significant amount of plagioclase fractiona- Adams, and are postulated to be melts of the subducting Juan tion as they evolved from basaltic parental magmas. This de Fuca plate (Defant and Drummond, 1993). In this model, conclusion mirrors that of previous studies of the felsic rocks the Banting Group may represent melts of relatively hot, of the Banting Group (Goodwin, 1988; Cunningham and Archean subducting oceanic crust. The model melts shown Lambert, 1989), with the additional constraint that the in Figure 9-15 could be produced in either of these tectonic T crustal source must be juvenile in order for εNd to be gen- scenarios. erally greater than zero. To distinguish between the subduction and crustal melt- The petrogenesis of the Banting Group differs fundamen- ing models for the Banting Group, we must return to the tally from Kam Group volcanism in that the mafic and felsic geology of the Yellowknife area. Although it is possible that volcanic rocks of the Banting Group are not part of a genet- Banting rocks are related to melting of a subducting slab in ically related sequence, i.e., they are decoupled in their ori- an arc setting, there is no geological evidence for an oceanic gin and evolution. What tectonic models allow for the coin- or continental arc in the Yellowknife region. Outcrops of cident formation of these two magma types? First, the Banting-age rocks are scattered over the south Slave rather Banting Group may have been erupted within a rift environ- than arranged in curvi-linear belts, there are no accretionary ment, like the Kam Group, but in the case of the Banting wedge complexes, and no belts of subvolcanic plutons of Group, the rate of lithospheric extension was lower and Banting age exist. In addition, the metamorphic geology of mafic magmas were generally unable to reach the surface. the YGB region is not consistent with an arc setting (J. This would have allowed mafic magmas to pond, perhaps at Thompson, pers. comm., 2001). For these and other reasons the crust-mantle interface, and by cooling and crystallizing (see the Summary section of this paper), we do not support a they released heat to melt the lower crust. The process of modern plate tectonic subduction model for the Banting heating the lower crust could also be explained by delamina- Group. tion of the largely eclogitic, lowermost layer of mafic crust,

17 B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

Kam versus Banting Volcanism Beneath Basaltic rocks in both groups range from flat to slightly light Yellowknife Bay REE-enriched patterns with a small negative Nb anomaly. But it is the felsic rocks that are diagnostic, having low Detailed geochemical studies of volcanic rocks in the Kam heavy REE abundances, lacking a negative Eu anomaly, and and Banting groups have shown that mafic rocks of both T having positive εNd values. Thus the Clan Lake complex groups are tholeiitic and follow similar magma evolution likely has the same origin as the Banting Group, where fel- trends (summarized earlier). Mafic volcanic and gabbroic sic rocks are partial melts of a juvenile garnet amphibolite rocks of the Sub Islands and Central Bay drill sites (Fig. 9- located in either the lower crust or a subducted (more cor- 1) also follow a tholeiitic trend. True basalts from both the rectly an “overridden”) slab and mafic rocks are predomi- Sub Islands and Central Bay areas, including the two high- nantly melts of the upper mantle. Mg basalts, have flat to slightly light rare earth element- enriched incompatible element patterns with little or no defi- The Duck Formation ciency in Nb, as do true basalts from the Kam and Banting Although the Duck Formation occupies the same strati- groups. Volcanic rocks with more than 52% SiO2 from the Sub Islands area are all significantly light rare earth element graphic position as the Clan Lake Complex (Falck et al., enriched with large negative Nb anomalies, and with the 2005, Fig. X-X), it differs from the Clan volcanic package in exception of sample CE06-17m (see Appendix 9-1), these that it is not dominated by felsic volcanic rocks. more evolved rocks also have higher abundances of all the Duck volcanic rocks are tholeiitic or transitional to calc- incompatible elements. Although the intermediate rocks alkaline (Fig. 9-3c), as are mafic rocks in the Clan Lake from the Sub Islands area have incompatible element pat- Complex and Banting Group, and both compatible and terns much like intermediate pillow lavas of the Greyling incompatible element variations parallel those in the Banting member of the Banting Group, heavy rare earth element Group (see Fig. 9 in Cunningham and Lambert, 1989). abundances are higher and more closely resemble basaltic Differences between Banting and Duck mafic to intermedi- andesites to andesites of the upper Yellowknife Bay and ate rocks are that Duck lavas and gabbros tend to have high- Kamex formations (Cousens, 2000a; Cousens and Facey, er TiO2 and Zr, but lower Y abundances compared to Banting 2000). Isotopically, there is no difference between the mafic rocks of similar SiO2 content, and Duck rocks have lower rocks of the two drill areas, and all fall within the range of Ba/Zr and Ba/Nb ratios than Banting lavas. Pillow lava pat- the Kam and Banting groups. terns resemble basaltic andesite patterns from both the Kam Felsic volcanic rocks, specifically rhyolites, are restricted and Banting groups, but the gabbros have patterns similar to to the Central Bay area, and all have incompatible element modern enriched (E-) MORB (Fig. 9-9c). Duck Formation rocks have sources with distinct incompatible element char- patterns overlapping those of felsic rocks of the Banting T Group. They are characterized by heavy rare earth element acteristics but variable εNd . The Duck gabbros are the only depletion, which suggests that these volcanic rocks were par- rocks from the YGB that may have an enriched (E-MORB) tial melts of a garnet-bearing crustal source. Isotopically, rather than depleted (N-MORB) upper mantle source. T Central Bay rhyolites have positive εNd and plot in the The dacite flow sampled by drilling near the east shore of range of felsic rocks from the Banting Group. In contrast, Yellowknife Bay has an incompatible element pattern that is felsic rocks of the Kam Group generally have ε T < 0. indistinguishable from those of Banting Group felsic rocks, Nd T The high-Mg basalts from the Central Bay have higher and has an εNd of +1.5 (Fig. 9-9c, 9-10). If this flow is part of the Duck volcanic centre, then this sample appears to link SiO2 and lower MgO concentrations than true ultramafic rocks from the Bell Lake area (Cousens, 2000b). incompati- Duck volcanism with that of the Banting Group. ble element patterns indicate that these are high-degree melts Alternatively, dacite flows of the Banting Group in central of depleted upper mantle. Yellowknife Bay actually extend as far east as drill hole M95-31 (southeasternmost sample locality, Map 1, Fig. 9-1). One intrusive rock from the Central Bay area, sample MV95-41-536 (Appendix 9-1), may be a Dogrib or Indin Russell Lake / Snare River Volcanic Rocks dyke of early Proterozoic age. It is a basalt dyke with a light rare earth element enriched, incompatible element pattern Russell Lake/Snare River felsic lava REE patterns have the that includes a small negative Nb anomaly. If this basalt is curvature between Gd and Lu and overall low heavy REE 2665 Ma in age, it has a very high ε T of +6.5, but more abundances that are characteristic of Banting-type felsic Nd 2658 importantly has an early Proterozoic depleted mantle model rocks. εNd values are positive for all lava types, which is also consistent with Banting-type volcanism. Based on this age (TDM) of 2223 Ma. The isotopic data point to a Proterozoic age for this rock. admittedly small data set, we conclude that volcanism at Russell Lake forms part of a south Slave-wide magmatic In conclusion, bedrock samples from beneath south event. This event at ca. 2.66 Ga resulted in localized partial Yellowknife Bay can be split into rocks with Kam Group melting of lower crustal rocks beneath the southwestern affinities around the southwestern shore (Sub Islands) of Slave Province, as well as melting of the depleted upper Yellowknife Bay and rocks with Banting Group affinities mantle to produce basaltic magmas. Although occurrences of beneath the central bay north of the West Mirage Islands. ca. 2.66 felsic volcanic rocks are confined to the southwestern Slave, tonalite-trondhjemite-granodiorite-type plutonic Clan Lake Complex rocks of this age are scattered throughout the Slave Province, Overall, the geochemical signature of the Clan Lake vol- leading to the possibility of a pan-Slave lower crustal canic pile is very similar to that of the Banting Group. reworking episode.

18 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses

Yellowknife Area Granitoids Yamashita et al. (2000) further propose that the Yellowknife-Winter Lake-Point Lake string of greenstone The range of ε T values in Defeat samples suggests that the Nd belts may represent a single back-arc basin that formed to the granitoids include variable proportions of mantle- and west of the proposed Hackett River volcanic arc in the east- crustal-derived components. There is a positive correlation ern Slave (Kusky, 1989). Much more detailed work is between the size of the negative Eu anomaly (Eu*: the meas- required to evaluate this proposal. ured Eupmn divided by the extrapolated value between Sm T Another greenstone belt that has been studied in detail is and Gd in Fig. 9-11) and εNd values, consistent with increasing assimilation of older basement rocks as the gran- the Indin Lake belt northwest of Yellowknife (Pehrsson, itoid magmas evolved (see also Davis and Hegner, 1992). 1998; Pehrsson and Villeneuve, 1999). Volcanism in the Indin belt is split into three lithostratigraphic groups, the The Prosperous Granite is proposed to be a partial melt of > 2.67 Ga Hewitt Lake, the ca. 2.67 Ga Leta Arm, and the Burwash sedimentary rocks at ca. 2.6 Ga (e.g., Boyle, 1961; < 2.65 Ga Chalco Lake groups. The Hewitt Lake package is Yamashita et al., 1999). This hypothesis is supported by its remarkably similar geochemically to the Kam Group. The peraluminous composition, and the presence of muscovite Leta Arm Group includes both tholeiitic and calc-alkaline and garnet as crystallizing phases in the granite. ε 2600 val- Nd suites, the latter of which has no equivalent in the ues of Burwash Formation turbidites, proposed to be the Yellowknife area. Although Chalco Lake felsic rocks have source materials for the Prosperous Granite, range from +2.4 the characteristic of Banting-type felsic rocks, Chalco Lake to -2.3 (Yamashita and Creaser, 1999), whereas the range for volcanic rocks and intrusions were emplaced between 2647 Prosperous Granite samples is +4.8 to -0.8 (this study, Dudás and as late as 2609 Ma, which postdates Banting volcanism. et al., 1991; Yamashita et al., 1999). We note that Nd concen- Note that without geochronological controls, it may be dan- trations in all Prosperous Granites are low (< 1 ppm) and that gerous to correlate units of greenstone belts outside of the ε 2600 values are variable, as would be expected in anatec- Nd Yellowknife area to stratigraphic units within the YGB. tic melts derived from an isotopically heterogeneous source. COMPARISONS TO OTHER SLAVE SUMMARY – NEW KNOWLEDGE CON- GREENSTONE BELTS CERNING BEDROCK GEOCHEMISTRY AND GEOLOGICAL SETTINGS Rare earth element and Nd isotopic data from other Slave greenstone belts are sparse. Mafic, intermediate, and felsic Prior to this series of studies, the geology, primary rock volcanism in the Hanikahimajuk Lake area of the Point Lake types, general geochronological sequence, and major ele- greenstone belt is similar in age and geochemistry to the ment characteristics of the Yellowknife greenstone belt had Kam Group of the YGB (Yamashita et al., 2000). The been outlined, and several models for the origin of the belt Yamashita et al. (2000) study identified two suites of mafic were proposed. A summary of the geochemical characteris- rocks, type 1 and type 2 with mid-ocean ridge and island arc tics of all of the areas discussed in this paper is shown in tholeiite affinities respectively, both of which are variably Table 9-2. contaminated by Central Slave Basement Complex tonalites. In particular, the basic petrologic differences between the Yellowknife mafic to intermediate lavas, with the exception Kam and Banting groups were known prior to this work but of some Kamex Formation andesites, follow the same mix- their relationship was uncertain. At the time that this work ing trend with basement tonalities as the Yamashita et al. began, a ‘back arc-arc’ pair relationship was the model to (2000) type 1 mafic volcanic rocks (shown in Fig. 9 in test, as was the ‘autochthonous – allochthonous’ relationship Yamashita et al., 2000). In the basal part of the Point Lake between the greenstone belt and surrounding rocks. It is belt, three lava types have been identified (Corcoran and clear that the Kam Group represents a large outpouring of Dostal, 2001). The Peltier Formation includes crustally-con- primarily mafic magmas within a submarine, rift environ- taminated tholeiitic rocks similar to Yamashita et al.’s (2000) ment. However, the setting of this rift is confined by the types 1 and 2, but also includes distinctive, type 3, heavy presence of inherited zircons in tuffaceous units and Nd iso- REE-depleted mafic to intermediate volcanic rocks that have tope ratios that decrease with increasing magma evolution undergone no contamination. These heavy REE-depleted from basalt to rhyolite, which demand the presence of older lavas appear to require a garnet-bearing source; their moder- continental basement beneath the belt. Interaction between ate MgO, high Sr/Y and La/Ybpmn are consistent with a fel- upper mantle melts and basement rocks is recorded in vol- sic adakite melt that has undergone reaction with mantle canic rocks throughout the 10 km-thick volcanic package. In peridotite to produce an andesite melt. Corcoran and Dostal turn, these isotopic characteristics (combined with field (2001) propose a Yamato Basin-like, back-arc origin for this mapping and U-Pb geochronology of MacLachlan, 1993; part of the Point Lake belt. Yamashita et al. (2000) favour a MacLachlan and Helmstaedt, 1995; Bleeker et al., 1999a; model of continental extension to produce type 1 magmas, as 1999b) show that Kam Group magmas were emplaced we propose for the Kam Group, followed by arc volcanism through and upon existing Central Slave basement, leaving to produce type 2 magmas, similar to the model proposed for little doubt that the YGB is autochthonous. the Chan Formation of the northern YGB (MacLachlan and We have shown that the ‘calc-alkaline’ label attached to Helmstaedt, 1995). In the case of the YGB, there are no true the Banting Group is a red herring: mafic rocks follow a calc-alkaline rocks (type 2 of Yamashita et al. (2000) or type tholeiitic fractionation trend, similar to the Kam Group, 3 of Corcoran and Dostal (2001)) and thus no convincing whereas felsic rocks are melts of juvenile mafic crust and are evidence of “arc” volcanism. unrelated directly to the mafic rocks. The contact between the Banting and Kam groups is everywhere unconformable, 19 B. Cousens, H. Falck, L. Ootes, V. Jackson, W. Mueller, P. Corcoran, C. Finnigan, E. van Hees, C. Facey and A. Alcazar

Table 9-2. Summary of geochemical characteristics of major geological units, Yellowknife greenstone belt. Geological Unit Age (Ma) Description Prosperous Pluton ca. 2596 Peraluminous, muscovite-garnet granite, low REE abundances, flat to sigmoidal REE pattern, negative Eu anomaly, enriched in Nb, Th, P, εNd +5 to -1 Western Plutonic Complex 2641-2620 Mildly peraluminous, moderately light REE-enriched, low heavy REE, generally (Defeat Plutonic Suite) no Eu anomaly but exceptions exist, εNd +2 to -2 Duck Formation ca. 2660? pillow lavas: slightly light REE-enriched, small negative Nb anomaly, εNd > 0 gabbros: slightly light REE-enriched, no Nb anomaly, E-MORB pattern, εNd > 0 Correlative: Clan Lake complex? Banting Group 2678-2658 Mafic rocks: flat to slightly light REE-enriched incompatible element patterns, εNd > 0 Felsic rocks: strongly light REE-enriched and heavy REE-depleted, negative Nb anomaly, no Eu anomaly, εNd > 0 Includes: #9 quart-feldspar porphyry dykes cutting Kam Group and Ryan Lake Pluton; West Yellowknife Bay islands; central Yellowknife Bay (drill core) and likely the Mirage Islands Correlatives: Clan Lake complex; Russell Lake-Snare River volcanic complex

Kam Group 2720-2702 Mafic rocks: flat to slightly light REE-enriched incompatible element patterns, εNd > 0 Felsic rocks: Moderately light REE-enriched, flat heavy REE pattern, negative Nb and Eu anomalies, εNd < 0 Includes: Giant Section, Bode Tuff (reworked felsic intrusives), Kamex Formation Correlative: Bell Lake complex Central Slave Basement >2900 Gneissic granitoids: peraluminous, enriched in light REE, Th, low heavy REE, Complex εNd << 0 at 2700 Ma, model ages ca. 3500 Ma

including the Giant Section where a transitional sequence of kled across the southwestern Slave Province and may be the rocks was proposed previously. The #9 feldspar porphyry product of a widespread tectonothermal event in this part of dykes that cut the Kam Group are identical chemically to fel- the Slave Province. Mafic lavas in the Banting Group are sic rocks of the Banting Group, and a large number of these submarine pillow lavas, and the Banting Group and its dykes radiate from the Ryan Lake Pluton that also has a equivalents are associated with thick metasedimentary units Banting geochemical signature and age (Ootes et al., 2002). (e.g., Burwash Formation turbidites), consistent with an Putative Banting-like rocks exposed along the west shores of extensional setting that might arise due to crustal thinning, Yellowknife Bay and beneath central Yellowknife Bay are heating, and melting. Our reconnaissance studies of the Clan indeed southern equivalents of the Ingraham Formation of Lake and Russell Lake/Snare River complexes show that the Banting Group. they too have Banting-like geochemical and isotopic charac- The geochemistry of felsic rocks in the Banting Group teristics. Further study of 2.66 Ga felsic volcanism through- can be explained by either a subduction or crustal melting out the southern Slave province (e.g., southern Cameron- model, which utilize similar source rocks (subducted ocean- Beaulieu belt) is required to further establish this crustal ic crust vs. mafic lower crust) but in very different tectonic melting event hypothesis, as is a geochemical study of the settings. What other evidence is there to support either Western Granodiorite Complex to find plutonic equivalents model? The convergent margin models assumes that plate of the Banting Group besides the Ryan Lake Pluton. subduction occurred during the Archean as it does today. In One as yet unresolved question is the exact depositional the convergent margin model, it would be expected that relationship between the Banting and Kam groups. Was the mafic rocks interbedded with the felsic lavas would have a Banting Group deposited unconformably over the Kam subduction zone signature, but instead Shot Lake pillow Group in its present geographic position? The occurrence of basalts have a depleted mantle source with no subduction the #9 dykes cutting the Kam Group below the Kam-Banting signature at all. Unlike modern arcs, the Banting Group and contact is evidence that the two rock groups have always other 2.66 Ga felsic complexes in the Slave province are not occupied their current relative geographic position. large-scale linear magmatic systems with associated However, the dykes cannot be seen to cross the contact. In metasedimentary accretionary prisms, subvolcanic plutonic addition, our data from Clan Lake and Russell Lake/Snare belts, and evidence for crustal uplift due to plate collision. It River complexes show that other Banting-like volcanic com- is also remarkable that ALL Banting felsic rocks have an plexes exist in close proximity to the main YGB, and poten- adakite composition, unlike modern arcs where adakites are tially the #9 dykes may have fed the Clan Lake (or some relatively rare, but the small size of the Banting Group and a other?) felsic volcanic centre. Unfortunately, the #9 dyke – higher Archean geothermal gradient to promote slab melting Banting Group relationship cannot give us unequivocal may explain this. Thus, there is little geological evidence to information concerning motion along the Yellowknife River support an arc origin for 2.66 Ga magmatic rocks in the Fault Zone. Slave province. Instead, 2.66 Ga felsic complexes are sprin- 20 Regional Correlations, Tectonic Setting and Stratigraphic Solutions from Sm-Nd Isotopic Analyses

ACKNOWLEDGEMENTS Brewer, T.S. and Menuge, J.F. 1998: Metamorphic overprinting of Sm-Nd isotopic systems in Our geochemical work in the Yellowknife area was inspired volcanic rocks: The Telemark Supergroup, southern by Bill Padgham, Al Donaldson, and Larry Aspler. Many Norway; Chemical Geology, v. 145, p. 1-16. thanks to Christine Vaillancourt, Bill Fyson, Kate Condie, K.C. and Baragar, W.R.A. Maclachlan, Bill Davis, Sally Pehrsson, Carolyn Relf, Joe 1974: Rare-earth element distributions in volcanic rocks from Heimbach, and other Yellowknife geo-types for field assis- Archean greenstone belts; Contributions to Mineralogy and tance, samples, and discussion of Yellowknife geology. Petrology, v. 45, p. 237-246. Donna Switzer, Jason Rickard, Muy Ngo, Brenda Obina, Corcoran, P.L. and Dostal, J. Samantha Siegel, Cathy Channing, Al Alcazar, and Michèle 2001: Development of an ancient back-arc basin overlying conti- Burkholder provided superb isotope lab assistance. Formal nental crust: The Archean Peltier Formation, Northwest reviews by George Jenner and Steve Piercey are appreciated Territories, Canada; Geology, v. 109, p. 329-348. and have improved the manuscript. Geochemical work was Cousens, B.L. supported by contracts through Indian and Northern Affairs 1996: Magmatic evolution of Quaternary mafic magmas at Long Valley Caldera and the Devils Postpile, California: Effects of Canada, Geology Division, Yellowknife, a Natural Science crustal contamination on lithospheric mantle-derived magmas; and Engineering Research Council Research Grant to Brian Journal of Geophysical Research, v. 101, p. 27673-27689. Cousens, and grants from the EXTECH III program. 1997: An Isotopic and Trace Element Investigation of Archean Supracrustal Rocks of the Yellowknife Volcanic Belt, Slave FIGURE CAPTIONS: Province, Northwest Territories; Indian and Northern Affairs Canada, DIAND Geology Division, EGS, 1997-08, 39 p. 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